Aerogel catalysts

Applied Catalysis, 72 (1991) 217-266

217

Elsevier Science Publishers B.V., Amsterdam

Review

Aerogel catalysts G.M. Pajonk Laboratoire des Matdriaux et Procddds Catalytiques, Universitd Claude Bernard Lyon I, ISM, 69622 Villeurbanne Cddex (France), tel. (+ 33) 72448252, lax. ( + 33) 78892583 (Received 17 October 1990)

CONTENTS 1.

Introduction ............................................................................................................................... 218

2.

Preparation ofaerogel catalysts ................................................................................................220 2.1.

Simple inorganic oxide aerogels ......................................................................................221

2.1.1.

Silica (supports, spillover) ........................................................................................221

2.1.2.

Alumina (supports, spillover) ...................................................................................223

2.1.3.

Zirconia (supports, spillover, isomerization) ..........................................................224

2.1.4.

Titania (supports, photocatalysis) ..........................................................................224

2.1.5.

Thoria (oxidation of acids or esters conversion into ketones) .............................. 225

2.1.6.

Chromia (conversion of alcohols to amines, oxidation of acetaldehyde in acetic acid) ............................................................................................................................ 225

3.

2.1.7.

Ferric oxide (oxidation of acetaldehyde into acetic acid) ...................................... 225

2.1.8.

Molybdena (supports) ...............................................................................................225

2.1.9.

Other single oxide aerogels .......................................................................................225

Binary aerogels ........................................................................................................................... 226 3.1.

4.

5.

B i n a r y o x i d e - o x i d e aerogels ............................................................................................226

3.1.1.

Nickel oxide based catalysts ......................................................................................227

3.1.2.

Other binary aerogel catalysts ...................................................................................227

3.2.

Ternary oxide aerogel catalysts .......................................................................................228

3.3.

M e t a l o n oxide aerogels ................................................................................................... 229

Physico-chemical properties ..................................................................................................... 231 4.1.

Apparent densities ........................................................................................................... 231

4.2.

Porous volumes ................................................................................................................. 232

4.3.

Surface areas ..................................................................................................................... 237

4.4.

Particle size distribution .................................................................................................240

4.5.

XRD structures ................................................................................................................ 241

4.6.

Solid state chemical reactivity ........................................................................................241

Putting aerogel catalysts into service .......................................................................................244

0166-9834/91/$03.50

© 1991 Elsevier Science Publishers B.V.

218 6.

Catalysis by aerogels ..................................................................................................................

247

6.1.

Early work of the 1930s ...................................................................................................

247

6.2.

More recent work .............................................................................................................

248

6.2.1.

Partial oxidation of organic reactants ......................................................................

6.2.2.

Hydrogenation reactions ...........................................................................................

250

6.2.2.1.

Selective conversion ofcyclopentadiene into cyclopentene .............................

6,2.2.2.

Hydrogenation of toluene ....................................................................................

251

6,2.2.3. 6.2.2.4.

Hydrogenation of benzene ................................................................................... Hydrogenation o f n i t r o b e n z e n e into aniline ......................................................

252 253

6.2.2.5. Hydrogenation with aerogels activated by hydrogen spillover ......................... 6.3. Ethylene polymerization .................................................................................................

7. 8.

248 250

253 254

6.4.

Carbon monoxide or carbon dioxide-hydrogen reactions ............................................

254

6.5.

Nitric oxide reduction by a m m o n i a ................................................................................

256

Miscellaneous reactions and new developments ..................................................................... Conclusions .................................................................................................................................

259 262

References ...................................................................................................................................

263

Abstract Aerogel catalysts are prepared by the sol-gel method associated with the supercritical drying procedure. T h e resulting catalysts, in the form of simple or mixed oxides and supported metals exhibit interesting high surface areas and large pore volumes chiefly in the range of the macropores. T h e i r very good resistance to heat t r e a t m e n t s allow t h e m to be used for all types of catalysed reactions up to 450-500 ° C. Aerogel catalysts show in general greater activities and selectivities t h a n the corresponding xerogels. T h e i r stability with time on stream is also remarkable.

Keywords: aerogels, catalyst preparation (sol-gel m e t h o d ) , pore structure, selectivity (several reactions ), stability.

1. I N T R O D U C T I O N

Historically, the first materials called aerogels were made in the early 1930s by Kistler [ 1 ]. Immediately after their invention, Kistler thought of potential applications including catalysis [2 ]. However it was only in the late 1960s that these types of materials were rediscovered at least for practical application in particle physics and astrophysics. This was undoubtedly due in part to a more convenient way to synthesize aerogels, especially silica aerogels by starting with organosilicon compounds, which were developed at the University Claude Bernard, Lyon [3 ]. Interestingly, the first contemporary large scale application of silica aerogels took place in high energy particle physics in the form of Cerenkov detectors. Currently great attention is focused on their use as thermal (transparent) and acoustic insulators. Many papers have been published on these topics. In the 1980s two specific symposia were organized on aerogels [4,5] and two exten-

219

sive review papers [6,7] appeared. These symposia and papers primarily focused on aerogel's physical properties and not the chemical ones. This is indeed rather puzzling, because aerogels, like silica for example, exhibit many (sometimes very unusual) other properties of interest for chemistry in general such as:

(1) (2) (3) (4) (5)

very high porosities, and consequently very low apparent densities, possibility to preform the material into: fine powder, lumps or monoliths, monoliths can be produced transparent in the UV-visible range, extremely low thermal conductivity coupled with their very specific acoustic properties (sound attenuation), (6) high specific surface areas, (7) very good textural stability during heat treatment at high temperatures, (8) excellent properties as gel to glass precursors, (9) though aerogels are almost always amorphous solids or a mixture of one amorphous phase with another one more or less crystallized, they can also easily be completely crystallized. In 1934 Kistler [ 2 ] pointed out that the interest in using aerogels as catalysts is due to the qualities mentioned in points (1), (6) and (7) indicated above. But there are counterpoints indicated by point (5) and partly by (1) and (2) since heat transfer problems and pressure drops can develop and severely limit their practical use. As it will be described below, considerable progress has been only recently made to overcome these drawbacks. Apart from the works of Kistler or Foster and Keyes [ 1,8 and 9 ], the literature before 1973 mentions very few references of aerogel catalytic applications with the exception of Riess [ 10] who studied some textural and thermal properties of silica aerogel S and Santocel. Once again he underlined the very good capacity of these materials to resist thermal treatment compared to more classical forms of silica gels (xerogel). It is only since 1974 that a significant number of catalytic results began to be published in the literature. The first of this group of papers concerned a special means of activating an alumina aerogel for ethylene hydrogenation into ethane [11 ]. Soon after, a small review dealing with the preparation methods of aerogel catalysts was presented by the author at the First International Symposium on the Preparation of Heterogeneous Catalysts in Brussels in 1975 [12]. In this review [12] aerogel catalysts for partial oxidation reactions of olefins, hydrogenolysis of ethylbenzene, selective hydrogenation of cyclopentadiene were depicted and corresponded to catalytic tests performed in fixed bed microreactors only. Taking advantage of their great specific areas, their high selectivities, their open loose textures and interesting thermal properties beside their good stabilities with time on stream, aerogel catalysts were evaluated in more severe conditions such as integral fixed or mobile beds in a very large range of reac-

220

tions: nitroxidation of aliphatic and aromatic hydrocarbons into unsaturated nitriles (mono o r / a n d di-nitriles), hydrogenations of nitrobenzene into aniline and toluene into methylcyclohexane, spillover investigations, isomerization of methylcyclopropane, Fischer-Tropsch synthesis, methanol synthesis, polymerizations of ethylene or propylene, deNOx reactions, catalytic radiative panels and methane oxidative coupling. Due to their extremely large porosities, aerogels are also worthy of investigation as hydrotreatment catalysts in HDN and HDS processes. This review will be organized as follows: first, we shall survey the many methods of preparation, secondly, the physico-chemical properties will be described with special consideration for catalysis, thirdly, the chemical engineering problems which must be solved in order to put aerogels into service will be discussed and finally some examples of application in catalysis will be given. The conclusion will also deal with considerations concerning their possible future. No further information will be given about the specific means of drying gel, i.e. the hypercritical drying conditions. Those interested in this point may look at a short review recently published and dealing with the topic of drying gels in order to conserve their textural properties [13]. 2. P R E P A R A T I O N OF A E R O G E L CATALYSTS

The first catalyst obtained as an aerogel was described in the literature by Kistler et al. [2] soon after the discovery of the hypercritical drying method [ 1 ]. It consisted of a simple oxide, namely thoria. The principle of preparation was based on the sol-gel process, starting with thorium nitrate and ammonia both dissolved in water. At that time it took no less than eleven days to make a few grams of thoria aerogel! This is obviously a time consuming method principally because solvent exchange took a very long time to achieve, in this precise example acetone was exchanged first with water and then with alcohol over a period of 12 days. Moreover nothing was said about the length of the washing period of the precipitate to get rid of nitrate and ammonium ions. Thus washing and solvent exchange periods were major disadvantages preventing any further development of such kinds of catalysts, until the disclosure of an entirely new way of making aerogels, through the sol-gel process, but using organic derivatives (mainly alkoxides, acetates or acetylacetonates) that are generally soluble in non-aqueous solvents such as alcohols, benzene, acetone, etc. This breakthrough was made in Lyon by Nicolaon and Teichner in 1968 [3], for silica first, and then extended very rapidly for other simple inorganic oxides as well as for binary and even ternary mixed oxides [ 12,14-16 ]. The new method was also applied to the fabrication of metals supported on oxide in one step [11,17]. As a general feature, the aerogel preparation procedure combines the fantastic flexibility of the sol-gel process by retaining the textural properties of

221

the wet gel, leading to particularly dry solids in a highly divided state as heterogeneous catalysis may require. In this new preparation method, one must note that water is now no longer a solvent but only a reactant, since it is well known that water in excess exerts a detrimental effect upon the textural properties of the aerogels due to its very high critical temperature of 374 °C as well as its high critical pressure of 219 atm. In these critical conditions and, of course, above (called hypercritical conditions), water peptization ability is enormous and finally leads to recrystallized solids exhibiting rather poor textural qualities (see for instance alumina aerogels below). This section describes the main methods to obtain aerogels catalysts and also cryogels which are presently under development [ 13,18,19 ]. The preparation of all the various types of aerogels is beyond the scope of this review, since it is restricted to those used in catalysis. References 4 and 5 are particularly suitable to provide a good deal of data on the subject in general.

2. I. Simple inorganic oxide aerogels 2.1.1. Silica (supports, spillover) Aerogels of this class were made as supports for active phases in catalysis or even as catalysts per se. As is well known, silica is one of the most important supports because of its chemical inertness. Nonetheless as will be seen later, it can be transformed into a real catalyst via a hydrogen (or oxygen) spillover activation procedure also developed in this laboratory [20]. Briefly, the preparation of a silica aerogel can be extended to other simple oxides with the appropriate precursors and dispersion media. In general the sol-gel chemistry leading to catalysts can be represented by the following steps: (i) hydroxylation (hydrolysis) of the metal alkoxide M-OR+H20~M-OH+ROH (ii) formation of hydroxy bridges (olation reaction ) M-OH + M O H X (X-- H or alkyl) ~ M-OH-M + XOH (iii) formation of oxygen bridges (oxolation reaction) M-OH + MOX-~ M O M + X O H (X = H or alkyl ) These reactions are nucleophilic additions or substitutions which can be easily acid or base catalysed [21]. For instance, silicon tetramethoxide is dissolved in methanol and the solution obtained hydrolysed by the stoichiometric amount of water, either at a pH > 7, neutral or < 7 [ 14,15 ]. Excess water is always detrimental to the textural properties of the aerogel. To summarize, the sol-gel system involves first a hydrolysis reaction followed by a condensation reaction, both reactions can

222

be base or acid catalysed if necessary. In general volatile acids or bases are preferred, such as acetic acid or ammonia as catalysts. The secondary products of the hydrolysis-condensation reactions, being methanol and water, are easily evacuated in hypercritical conditions in an autoclave [3 ]. The main parameters which govern the properties of the aerogel are well identified and simple: the concentration of the silicon organo-compound dissolved in methanol, the ratio of the number of water molecules added to that of the silicon precursor and finally the pH of the gelification. The next section of the review describes in more detail the influences of all known parameters on the properties of the aerogels. With this method, it took only a few hours to make an aerogel instead of a few weeks as mentioned for instance in Peri's paper [22] where the substitution of water by methanol, according to the Kistler's recipe, took at least three weeks (see below). Another silicon precursor, silicon tetraethoxide dissolved in ethanol as described by Foster and Keyes and by Peri [9,22], can be transformed into an aerogel of silica when hydrolysed (in the presence of HCI) and exchanged by methanol and subsequently autoclaved. It is obvious that this tedious method has been abandoned in favour of the one proposed by Nicolaon and Teichner [3 ]. Silicon tetraethoxide is directly dissolved in ethanol and the alcogel is evacuated in the autoclave with respect to that solvent. Generally speaking, the organic metal derivatives are often metal alkoxides [21] which are commercially available. Concerning silica, it was found that the hydrolysis rate of the silicon alkoxide is acid catalysed while the polymerization is base catalysed [4,5,14,15]. Alcohol interchange can affect strongly both hydrolysis and condensation steps as shown by Chen et al. [23] who described the properties of silica prepared as xerogel with Si (OCH3) 4 dissolved in methanol on the one hand and in ethanol on the other hand. The silica obtained with the parent alcohol exhibited a specific area twice that of the other sample. As was already shown by Peri [22] for silica at least, the hypercritical drying method giving the aerogel does not modify the specific area of the final silica and by comparison with the corresponding conventional silica xerogel the above results of Chen et al. [23 ] seem also to be valid for aerogels. As classification of the reactivities of metal alkoxides as a function of their electrophilic character towards nucleophilic reactions has shown, silicon alkoxides are much less reactive than those corresponding to the block, d, f and even p groups [21,24,25], it is not surprising that silica condenses with some difficulties when it is compared to alumina, titania, zirconia etc. where a wet gel is very rapidly obtained at room temperature for instance. Alcohols usually used as solvents have high critical constants (methanol for examples has a Tc = 242 ° C and Pc = 79 atm) thus it is interesting to try low Tc medium such as carbon dioxide (Tc = 31 ° C, Pc = 74 atm) as some authors have done [26-30]. But in this case a solvent exchange has to be carried out

223 between alcohol and carbon dioxide because it is not possible directly to prepare a gel in liquid carbon dioxide. Silica aerogels are materials which are very stable when treated at high temperature even in air. For example a silica aerogel still exhibits a surface area of 800 m2/g and a pore volume of 16.5 g / c m 3 (measured by mercury penetrometry) when treated thermally in air at 500 ° C. This stability is very interesting for practical catalytic applications since most of them do not proceed at higher temperatures. A silicagel was also obtained using a freeze-drying method coupled with the sublimation of the solvent (the resulting solid is called a cryogel) [18,19,31] which as for aerogels, is a drying method avoiding interface meniscii between liquid and vapor in the capillaries.

2.1.2. Alumina (supports, spillover) Aluminas are well known and widely used in catalysis for their acidic properties and also in the form of (presumably) inert carriers to support metals or again as a component of bifunctional catalysts. The sol-gel chemistry of alumina is quite different from that of silica on the basis of the corresponding reactivity of the alkoxides. The general reaction scheme already mentioned holds again, but now it must be noted that aluminum alcoholates are not always soluble in their parent alcohols, so interchange with another alcohol or solvent can be the rule. Nevertheless it is always possible to prepare the alumina (wet) alcogel in a heterogeneous medium which consists of either a solution of the aluminium alkoxide in an organic solvent (not necessarily an alcohol) not miscible with water or the dispersion of the alkoxide in its parent alcohol which for example is miscible with water [32]. For instance it is possible to dissolve aluminium s-butylate or aluminium isopropylate in benzene and then add water and finally evacuate benzene in hypercritical conditions, or to disperse the aluminium sbutylate in an alcohol (s-butanol for example ), add water and evacuate the alcogel in hypercritical conditions in the autoclave. The three methods give the same textural properties for the alumina aerogels obtained. A thorough study published in ref. 32 shows that surstoichiometry in water with respect to the aluminum derivative is a factor that decreases both surface areas and pore volumes and increases crystallinity (in the form of beohmite ). Thus it is possible to obtain A1203 aerogels as amorphous or crystalline solids. However it is always possible to get an amorphous alumina aerogel even in the presence of a large excess of water provided that, before autoclaving, the alcogel is carefully washed with alcohol from its excess water. In general, the preferred aluminum precursors are aluminium s-butylate or aluminium isopropylate. Alumina aerogels are very resistant to thermal treatment in vacuum or in air (to a smaller degree). T h e y may exhibit a surface area of 460 m2/g when

224

heated in vacuum or 239 me/g in air at 600 ° C. They remain amorphous even at temperatures higher than 1000 ° C when in vacuum, while they become crystallized at 1000 ° C in air. These aerogels are more thermally stable than amorphous xerogels of high surface area [33] or than less developed alumina gels as recently published by Ishiguro et' al. [ 34 ].

2.1.3. Zirconia (supports, spiUover, isomerization) Zirconia is very interesting from a catalytic point of view because it develops a unique surface chemistry involving both redox and acid-base functions [35 ]. Therefore zirconia may find many applications as a catalyst or as a support of active phases [3]. Thus zirconia aerogels may gain increased interest in the near future. As zirconium isopropylate is not soluble in its parent alcohol, it is dissolved in benzene or dispersed in isopropanol which results in a heterogeneous process which can be applied to get the zirconia aerogel [14,15,32]. With substoichiometric amounts of water, the aerogel is amorphous, whereas, in the opposite conditions of hydrolysis, zirconia adopts the monoclinic structure. The resistance of zirconia aerogels towards heat treatments is in general less than that of alumina aerogels. Nevertheless they still exhibit a surface area of 204 m2/g under vacuum at 500°C and close to 60 m2/g in air. Their structure changes from amorphous to monoclinic when they are thermally treated in air at 350 ° C and in vacuum at 400 ° C. Zirconium precursors can be either isopropylate (not soluble in its parent alcohol ) or propylate which can be dissolved in propanol for example.

2.1.4. Titania (supports, photocatalysis) Titania is widely involved in photocatalysis or in catalysis where it is generally used as a reducible support. The ability to be a reducible support has lead to the well documented S M S I effect when a group VIII metal is dispersed on titania [36,37]. Titania can be formed from solutions of titanium butylate in butanol or propylate in propanol. These solutions and heterogeneous dispersions of titanium alcoholates in benzene can be hydrolysed [32]. Contrary to zirconia or alumina aerogels, titania is always obtained as anatase whatever the amount of water used for hydrolysis. If the alcogel is carefully washed from its unreacted water by anhydrous butanol, then an amorphous titania aerogel can be prepared. This is of importance since it has been shown for TiO2 that the larger the ratio of water to alkoxide the greater the surface areas. Hence it is possible to get amorphous titania aerogels (or anatase) developing high surface areas. This point of amorphous versus crystallized aerogel will be discussed below when solid state chemistry is considered for multiphase aerogels containing at least one amorphous phase [38,39].

225

2.1.5. Thoria (oxidation of acids or esters conversion into ketones) Thoria aerogels were the first aerogels fabricated for use as catalysts by Kistler's procedure [ 1,2,8] involving an inorganic thorium chemical, namely thorium nitrate, and ammonia followed by washing, solvent exchange and finally autoclaving. 2.1.6. Chromia (conversion of alcohols to amines, oxidation of acetaldehyde in acetic acid) Formerly chromium nitrate [ 9 ] or chromium chloride [40 ] were the precursors used to make chromia aerogels according to Kistler's method. Presently, chromia aerogels can be obtained by a much more simpler method involving chromium III acetate preformed with Rashig alumina rings as described by Armor and Carlson [41]. It is also claimed by these authors that despite its powerful oxidizing properties CrO3 could be chosen as the chromia precursor, which then gave a high surface area sample of about 452 m2/g without any support. 2.1.7. Ferric oxide (oxidation of acetaldehyde into acetic acid) As already mentioned for chromia aerogels, first inorganic and then organic iron precursors were chronologically involved. For instance the ferric oxide aerogel prepared by Foster and Keyes [9 ] was derived from ferric chloride and dried by Kistler's procedure which was also the method used by Kearby et al. [40 ]. At the present time iron (III) acetylacetonate is dissolved in methanol and is serving as the corresponding aerogel precursor [42-44]. 2.1.8. Molybdena (supports) Molybdenum dioxide aerogels are of interest in electrocatalysis and were developed in this laboratory by reacting molybdenyl (VI) acetylacetonate dissolved in methanol with ammonia and evacuated in the autoclave filled with hydrogen [12,32,45]. Using the aerogel method it was possible to make a high surface area and good electrical conducting molybdenum dioxide. 2.1.9. Other single oxide aerogels Easily reducible oxides, such as NiO, CuO, PbO, V205 etc., are difficult to prepare according to the present method since the reducing medium of the autoclave coupled with the high temperatures and pressures lead to the formation of the metals in most cases. Nevertheless these difficulties can be in principle overcome by drying the wet gels with respect to carbon dioxide or by going through the freeze-drying method in order to get cryogels. The third way to prepare highly divided reducible aerogel oxides is also to reoxidize the hypercritical dried reduced oxides like vanadia, chromia, nickel oxide, copper oxide, lead oxide [46]. A brief survey of the catalytic literature clearly shows that biphase catalysts

226 are the most widely described, thus it is now time to turn to the preparation of binary aerogel contacts such as oxide-oxide and metal-oxide ones. 3. BINARYAEROGELS A discussion of the importance of supported catalysts is beyond the scope of this review, the present author examined this point several years ago [47] and drew the attention to the "moving" concept of what can be considered as a single, pure support because it is now well established that a carrier is not thoroughly inert towards the supported phase. For instance, the SMSI effect has clearly demonstrated this poin¢ of view. Thus we prefer to talk about binary systems in general rather than the relatively vague supported ones. The aerogel method (as well as the cryogel one) allows the preparation of metal dispersed on an oxide and binary oxides in the same way as simple oxides (or metals). In fact the literature shows that multicomponent aerogels have been tested as catalysts as early as 1937 by Foster and Keyes [9] and Kearby et al. [40]. But of course it was again Kistler's method which served to make these aerogels so that a new impetus was really given only in 1976 with a series of papers from this laboratory [ 12,48,49 ] in particular.

3.1. Binary oxide-oxide aerogels Binary mixed oxide-oxide aerogels that are principally NiO, CuO, Th02, Cr203, PbO and Fe203 oxides associated mainly with A1203, Si02, Zr02 and MgO are described. Commercially available metal alkoxides, acetates or acetylacetonates work well taking into account as discussed above that heterogeneous sol-gel precursor systems compose suitable reactant mixtures (see above). The principle of the synthesis is to prepare the co-gelling of both metal derivatives in appropriate organic solvents (or dispersing agents) by reaction with water in stoichiometric amounts, while establishing the required pH with the use of volatile bases and acids. The hypercritical drying is operated with respect to the solvent (dispersant) exhibiting the highest critical temperature in the case when two different solvents or dispersants have been selected. Kearby et al. [40] gave some details on the preparation of Cr203-A12Q, Fe203-A1203, SIO2-A1203 and A1203-TiO2 all based on the use of chlorides and ethylene oxide with the exception of silica which was derived from silicon tetraethoxide. A ThO2-A1203 aerogel was also made starting with a separate stabilized colloidal solution. In all cases except for the Si02-A1203 mixed aerogel, alumina was always the major component. Each of these gels were dried in hypercritical conditions with respect to methanol according to the Kistler's method [ 1 ], i.e. after solvent exchange. The binary oxide-oxide aerogels describedbelow were obtained by using the

227

method developed in this laboratory by Nicolaon and Teichner [3 ] and extended by Gardes et al. [48,49 ] and Astier et al. [ 12 ].

3.1.1. Nickel oxide based catalysts (a) NiO-A1203 (partial oxidation of olefins-nitroxidation of hydrocarbons into nitriles) Nickel acetate was dissolved in methanol while aluminium s-butylate was solubilized in s-butanol. The water required to hydrolyse both components was poured into the first solution. Then the two solutions were mixed under stirring and the alcogel was finally evacuated in hypercritical conditions with respect to s-butanol ( Tc = 261 ° C, Pc-- 41.4 atm) according to the rule given above concerning mixtures of solvents or dispersing agents. More or less acidic media were achieved with acetic acid [48]. A variant of these conditions was proposed later by Abouarnadasse et al. [50] consisting in preparing a nickel acetate and aluminium s-butylate isopropanolic solution. Recently NiO-A1203 cryogels were prepared using the cryodessicating-subliming method of Klvana et al. [18]. The analysis of their textural properties showed that they were, to some extent, less highly divided than the corresponding aerogels. In the case of the preparation of the cryogel, nickel acetate was dissolved in ethylene glycol and a solution of aluminium s-butylate in t-butanol was used. Improvement of the method is now in progress. (b) NiO-Si02 (nitroxidation of hydrocarbons into nitriles) Sayari et al. [51 ] and later Abouarnadasse et al. [38,39] described the preparation of these kinds of aerogels. They were obtained from the hydrolysis of solutions of nickel acetate and silicon tetramethoxide both dissolved in methanol. (c) NiO-MgO (nitroxidation of hydrocarbons into nitrile, ammonia synthesis) In the fabrication of these catalysts, magnesium methylate with nickel acetate in solution in methanol were used as described in the papers of Marinangelli et al. [52] and Sayari et al. [51]. 3.1.2. Other binary aerogel catalysts (a) V2Os-MgO (ammonia synthesis ) A catalyst of this formula was obtained by hydrolysis of a methanolic solution of vanadium (III) acetylacetonate and magnesium methylate [52]. (b) Cr203-A1203 (nitroxidation of hydrocarbons into nitriles) Sayari et al. [53] have made such catalysts starting with methanolic solutions of chromium (III) acetylacetonate and aluminium s-butylate dissolved in s-butanol. (c) PbO-A1203 (nitroxidation of hydrocarbons into nitriles) This type of aerogel catalyst, first prepared by Abouarnadasse et al. [54],

228

involved a hot solution of lead acetate in methanol which was further reacted with aluminium s-butoxide dissolved in isopropanol, and water. (d) PbO-Zr02 (nitroxidation of hydrocarbons into nitriles) New aerogels were recently manufactured by Manzalji and Pajonk [55] starting with solutions of lead acetate in methanol and zirconium isopropoxide in isopropanol. (e) Fe203-A12Q (Fischer-Tropsch synthesis) The first Fe203-A1203 aerogels were prepared by Bianchi et al. [42] from the combination of two solutions of iron (III) acetylacetonate dissolved in sbutanol and of aluminium s-butylate also in s-butanol. A variant of this method was later proposed by Blanchard et al. [44] in which, in a first step, the alumina aerogel is prepared and then it is redispersed in a methanolic solution of iron (III) acetylacetonate, and finally the dispersion is subjected to hydrolysis followed by drying in hypercritical conditions. Another approach utilized by these authors was by simply impregnating a preformed alumina aerogel by iron (III) acetylacetonate in methanol. T h e n a simple decomposition followed when the solution was heated to hypercritical conditions in the autoclave (here the hydrolysis of iron when the solution was acetylacetonate was no longer carried out). (f) Fe203-Si02 (Fischer-Tropsch synthesis) Silicon tetramethoxide dissolved in methanol was combined with the same iron solution described just above [42]. Once again, the same modifications of the process as those described above for Fe203-A1203 have been carried out [44]. (g) TiC14-A1203 (ethylene polymerization) Fanelli et al. [56] impregnated TIC14 on alumina aerogels either directly from the vapor phase (CVD) or from a solution of TiCl4 in heptane. They also synthesized alumina aerogels from homogeneous or heterogeneous hydrolysis of an aluminum precursor (methanol, isopropanol, or s-butanol). It appeared that changing s-butanol for methanol before the hypercritical step had some beneficial influence on the properties of the final catalyst (see below). (h) CuO-A1203 (n-butane and n-butene partial oxidation) Starting with copper acetate dissolved in methanol and aluminium s-butylate dissolved in s-butanol, Centi et al. [57] prepared a selective oxidation catalyst.

3.2. Ternary oxide aerogel catalysts A third component has been introduced into binary catalysts in order to develop new functionalities in general. Few difficulties are encountered when preparing ternary aerogels [12,48]. Kearby et al. [40 ], for example, published the preparation of a ternary aero-

229

gel catalyst A1203-Cr203-ThO2. Aluminum and chromium chlorides were reacted with ethylene oxide to produce a gel (aquagel) then a sol of stabilized thoria was added to the previous cogel. The fundamental principles for making ternary aerogel catalysts are again given in the previously cited papers of Gardes et al. [48] and Astier et al. [ 12 ] which describe NiO-SiO2-A1203 catalysts using nickel acetate methanolic solutions and aluminum s-butylate and tetramethoxy silane both dissolved in sbutanol. Other aerogels of the same type were developed by Sayari et al. [51] namely NiO-A1203-MgO, NiO-SiO2-MgO and thoroughly studied by XRD methods to characterize their structures and by adsorption of ammonia to measure their acidities. Additional ternary oxide systems described in the literature include three other series of catalysts all containing NiO depicted by Zarrouk et al. [43] for NiO-Fe203-A1203, NiO-V20~-MgO by Marinangelli et al. [52 ] and lastly NiO-A1203 aerogels promoted either by MgO or Fe203 by Rahman et al. [58]. In this case ferric acetylacetonate was dissolved in methanol in the presence of ammonia in order to ease its solubilization. Cr203-A1203MgO aerogels were synthesized by Sayari et al. [53] while Cr203-Fe203 containing A1203 or MgO were described by Willey et al. [59 ]. All of these catalysts were tested in nitroxidation or partial oxidation reactions (see below ) with the exceptions of the Cr203-A1203-MgO which was used in the reduction of nitric oxide by ammonia, and the NiO-V2Os-MgO which was tested for ammonia synthesis in its reduced form. Baraton et al. [60,61 ] have recently studied the surface groups of a cordierite-like aerogel (2MgO-2 A12Q-5Si02) [62 ] with the assistance of Fourier transform infrared methods. The cordierite was made out of tetraethoxysilane in ethanol, aluminium s-butylate in s-butanol and magnesium nitrate solutions as precursors. The final aerogel developed a surface area in the range of 400 m2/g even after having been heat treated in air at 600 ° C. TGA analysis showed that the nitrate dissociated during pyrolysis [62 ]. When the precursors used for binary and ternary aerogels have similar rates of hydrolysis and/or condensation the resultant aerogels generally consist of homogeneous mixed oxides. They exhibit properties of homogeneity and ultrafine mixing. On the other hand, when the relevant hydrolysis and/or condensation rates are dissimilar the aerogels obtained are not homogeneous mixed oxides. Usually the first hydroxide which gels is "coated" by the other (s) which is (are) formed later in the sol-gel process or during the drying stage. It appears, therefore, that the choice of the precursors and the conditions for performing the sol-gel conversion (pH in particular) is of paramount importance for fabrication of catalysts with tailor-made physico-chemical properties. 3.3. M e t a l on oxide aerogels

As long as alcohols are used for solvents or dispersing agents their chemical reducing properties, combined with the (hyper)critical temperature and pres-

230

sure conditions, are able to reduce in the autoclave more or less the so-called reducible oxides (NiO, CuO, etc. ), which are co-gelled with their carrier. This direct in situ reduction in one step can be improved by filling the autoclave with hydrogen instead of nitrogen before proceeding to heat the vessel [ 12,49 ]. Nickel, palladium, iron and copper based aerogels have been extensively described in the literature. The early Pt-SiO2 and Ni-SiO2 catalysts were obtained by Foster and Keyes [9] who used Kistler's method of synthesis. Since the 1970s the nickel based catalysts are prepared as described by Gardes et al. [49] consisting of Ni-A12Q, Ni-Si02, Ni-SiO2-A1203, Ni-SiO2-MgO, i.e. with the precursors which have already been quoted in the previous sections. Cu-A1203 catalysts were first obtained in a one-step process according to Pajonk et al. [ 17 ] and in this case the remaining copper oxide was reduced by a stream of hydrogen at 200°C in a second step. A second method was also developed by Taghavi et al. [63] for copper on SiO2 or MgO supports and consisted in impregnating the SiO2 or MgO aerogels by the copper derivative (cupric tetraminohydroxide in alcohol). Klvana et al. [64] have prepared a mixed Cu-SiO2 aerogel in a one step process with copper acetate as a precursor. A series of nickel on MoO2 aerogel catalysts with nickel acetate and Mo (VI) acetylacetonate as precursors which showed good electrical conductivity were described by Astier et al. [ 12 ]. Pt-MoO2 was also prepared by Astier et al. [65 ] by impregnation of an aerogel of MoO2 by a solution of chloroplatinic acid in methanol and thus duplicating one of the two methods early developed by Taghavi et al. [63 ]. Two Pd-A1203 aerogels were described in a paper by Armor et al. [66]. The first one was made from palladium acetate dissolved in warm acetone and aluminium s-butylate dissolved in s-butanol. Water in the required amounts was added in the form of a solution in methanol. The second one differed from the first one in that neither butanol nor methanol were used. Instead isopropanol was the common solvent in this case. No pretreatment of the Pd-A1203 was necessary after its evacuation in the autoclave since X-ray photoelectron spectroscopic (XPS) analysis showed that only Pd ° was present in the Pd-A1203 aerogel catalyst. This is not in general the case with commercial Pd-A1203 catalysts where Pd 2+ is detected. In another work methanol synthesis was carried out with a carbon dioxide and hydrogen mixture (instead of the conventional carbon monoxide and hydrogen one) on Cu-ZrO2 aerogels prepared by Pommier and Teichner [67] and on Cu-ZrO2-A1203 obtained by a two-step procedure. Cu-ZnO-A12Q ternar:~ aerogels were also made by a two-step process. The procedure was as follows: first an alumina aerogel was made and it was impregnated by a solution containing copper and zinc acetate in methanol and finally re-evacuated in hypercritical conditions with respect to methanol [67]. T h u s it is obvious that practically all known classical xerogel catalysts can be also dried in the form of aerogels and for, a few of them, as cryogels at the

231 present time. It is worth mentioning that hypercritical drying with respect to carbon dioxide is under development and the same is also true concerning the cryogel drying system which presents the advantage that it can be used in principle with water as the solvent or as the dispersing medium. 4. PHYSICO-CHEMICALPROPERTIES The literature gives very few physico-chemical properties. This laboratory has, however, carefully studied a large variety of aerogel catalysts from this point of view and the main results will be summarized below as published by our several teams. Properties such as BET surface areas, metallic areas, porosities (pore volumes and their distributions), apparent densities, and crystallinity are considered in this section in relation to the many parameters of preparation of the solids which are mainly: the concentration of the precursors in their solutions or dispersions, the amount of water for hydrolysis, the pH of gelification, and the nature of the precursors and solvents. Aerogel catalysts are characterized by large surface areas, very high pore volumes (including micro-meso and macro-pores) and, consequently, low to very low apparent densities. Beside these common properties they are most often amorphous. Indeed these types of catalysts complete the range of the properties exhibited by the catalysts obtained through the sol-gel procedures but leading to xerogels or through the flame process giving fumed solids or aerosols. In general, xorogels develop total pore volumes of the order of 1 cm 3 at most, while the corresponding aerogel catalysts exhibit total pore volumes one order of magnitude greater than the former ones as a rule. Roughly speaking, xerogel catalysts are very often crystallized and similar aerogels can be amorphous or crystallized or mixed, quite at will, with the exception of silica. Very low apparent densities, high porosity and good thermal insulating properties may cause problems in chemical reactors as it will be examined below.

4.1. Apparent densities Chavarie et al. [68] have measured the so-called "bulk" densities of a series of aerogels, simple or binary, made in this laboratory while Armor et al. [69] have given data for several approaches to make Cr203 aerogels as well as for palladium on A1203 [70]. For these latter aerogels, apparent densities range from 0.003 to 0.5 g/cm 3. Table 1 reports the data of Chavarie et al. [68] and Armor et al. [69]. This table shows that these aerogel catalysts (or supports) are characterized

232 TABLE 1 A p p a r e n t densities of some aerogel catalysts Aerogel composition

A p p a r e n t density in g / c m 3

Reference

Cr203 (ex C r Q ) C r 2 Q (ex acetate in isopropanol) Cr203 (ex acetate in m e t h a n o l )

0.15-0.38 a 0.54-0.54 a 0.16-0.21 ~ 0.15 b 0.054 0.04-0.06 c 0.06-0.08 c 0.06-0.08 c 0.06-0.08 c 0.06-0.08 c 0.05-0.06 c 0.03-0.26

69 69 69

Cr203 (ex n i t r a t e ) Fe203-A1203 Cu/AI203 A1203 Ni-A1203 Ni-SiOe Si02 TIC14 on A1203

71 72 68 68 68 68 68 56

aIn the presence of water as wetting agent and redried. bSame as in a but m e t h a n o l replaces water. cCompacted by tapping.

by very unusual low apparent densities and that their porosities (or porous volumes) are very important, as can be seen in the next section. 4.2. Porous volumes

Only data corresponding to the most representative aerogels in catalysis will be given. Table 2 shows the results obtained by mercury penetrometry (VpHg) while Table 3 shows the influence of some preparation parameters on the pore volumes. Generally, xerogels do not exhibit pore volumes as measured by mercury intrusion exceeding 1 cm3/g. Table 2 indicates that, again, aerogels are nonconventional solids from this point of view, developing much higher pore volumes and therefore porosities as high as 95-98%. Gardes et al. [48,49] have studied very extensively the influence of many preparation parameters of nickel and nickel oxide supported aerogels. For NiA1203 they have found that the solvent, the nature of the precursor (isopropoxide, s-butoxide) for aluminium exerted a limited influence on the pore volume~while the quantity of water used for hydrolysis, the nickel amount or its precursor (nickelocene, nickel acetate, nickel acetylacetonate) have no influence at all. Ternary aerogels Ni-MgO-A12Q have porous volumes depending significantly on their nickel contents but were insensitive with respect to the amounts of MgO incorporated into the catalyst [49]. NiO-A1203 aerogels showed porosities varying with the amounts of water,

233 TABLE 2 Pore volumes of representative aerogel catalysts (or supports ) Aerogel

Pore volumes VpH~ in cm3/g

NiO-Al203 NiO-SiO2-A1203 Ni-A1203 Ni-AI~O3-MgO Pd-AI203 Cr203 TIC14 on A1203 MoO2-Ni MoO2 A1203 A1203 SiO2 Zr02 NiO-Fe203-A1203

Lowest

Highest

6.2 4.4 6.9 7.8 2.8 1.3 1.8 0.7 0.8 3.4 4.5 2.7 2 2.7

15.4 17.1 18.1 18.5 6.5 3.7 6.6 a 1.3 1.3 8.6 17.3 18 14 13

Reference

48 48 49 49 66 69 56 45 45 73 32 32 32 43

aThe samples were obtained by impregnation of precalcined A1203 at 700 ° C. TABLE 3 Palladium-alumina aerogel surface properties (from ref. 66) Catalyst

Palladium (wt.%)

Surface area (m2/g)

Pore volume c (cm3/g)

5711 a 5735 a 5736 b

5 2 5

329 642 134

4.8 6.5 2.8

aS-butanol-methanol-acetone-water. bIsopropanol-acetone-water ( + minor in situ butanol). CThe pore distribution curves gave the traditional aerogel profile. A large change in volume occurred as soon as pressurization began, followed by a slow, steady uptake of mercury. No major breaks in the pore size distribution were observed.

the nickel content, and the acidic pH of preparation. For each parameter a maximum porous volume was recorded. Figs. 1 and 2 show the evolution if the porosities measured by physical adsorption (VpN) and mercury porosimetry (Vp~) as a function of water used for hydrolysis and acidities, respectively. A}~ding silica to the NiO-A1203 aerogels led to constant porous volumes provided that its amount was not below 20% in weight as shown in Fig. 3. The porous volumes were found to be independent of the nickel concentration but they depended slightly on the amount of water used for hydrolysis.

234

Vp

J

=cn'~/g

cm3/g

"15 1,8

1

3 (suo)3

,,, + 2 ~ H 3 C O O 1 2 ~,

Fig. 1. NiO-AI203 aerogels: evolution of the micro, meso and macroporosities as a function of the amounts of water.

,v'p Cm3/g

V'DN

1 5

\, 0.5

r,,:H~,COOH

Fig. 2. Same as in Fig. 1 but as a function of the amounts of acetic acid.

Armor et al. [66], in their study of Pd-A1203 aerogels, gave data linking the pore volumes to the amount of palladium and the solvent used as shown in Table 3. However, Cr203 aerogels prepared as described by Armor et al. [69] had pore volumes nearly independent of the chemical nature of the chromium precursor as shown in Table 4. Astier et al. [ 45 ] described the influence of the concentration of acetylace-

235

Ip ' C m ~j,g

-10

-8

0J ~6

0,6

r

5

1' 0

;'.

:~0

AI203

Fig. 3. Si02-A1203 aerogels: porous volumes as a function of the amount (in percent) of A1203. TABLE4 Surface areas and pore volumes of various chromium (III) oxides {from ref. 69 ) Synthetic route a

Surface area (m2/g)

Pore volume (cm3/g)

Cr(III) + u r e a Cr (CH3COO)3.H20 CrO3 Cr (CH3COO)3.H20 b

785 516 528 650

3.7 2.7 3.6 1.3

~Prepared by hypercritical solvent removal at 275 ° C. bSuspension heated to 275 °C and then cooled to room temperature without venting the fluid. TABLE5 Surface areas and pore volumes of MoO2 aerogels (from ref. 65 ) Sample

Concentration in Ac. Ac. Mo per litre of methanol

Surface area (m 2/g)

Pore volume (cm 3/g )

M1 M2 M3 M4

28.5 57.5 74 85

144 168 113 79

1.12 0.86 1.20 1.32

tonate of molybdenum(VI) for MoO2 aerogels as well as that of the MoO2 percentages in MoO2-Ni aerogel catalysts as given by Tables 5 and 6, respectively.

236 TABLE 6 Surface areas and pore volumes of MoO2-Ni aerogels (from ref. 45) Catalyst

Percentage of MoO2 on Ni(% )

Surface area (m2/g)

Pore volume (cm3/g)

Surface area a (mZ/g)

NiM Mo 0.5 Ni M Mo 2 Ni M Mo 5 Ni M Mo 10 Ni M Mo 20 Ni M

0.5 2 5 10 20

1.0 0.7 1.3 2.6 10.3 14.6

0.78 0.78 0.75 0.75 0.98 1.28

16 33 94 69

aCalculated surface areas of pure MOO2.

C,~

Alp

(wt~ A lo(]

Fig. 4. Alumina aerogels from AIP-methanol-water: pore volume against composition of the starting mixtures for different fixed amounts of AIP and water-to-methanol ratios. (C) ) Pore volume (cm:~ g - l ) ; ( • ) pore volume (cm 3 g - l ) peak at optimum water input. Autoclave, 300 cm3; liner, 220 cm 3. (AIP: aluminium isopropoxide ). The water-to-AIP ratios are expressed in mole per mole along the AB abcissa. Amounts of AIP are indicated along the CA line.

237 Alumina aerogels have been extensively described by Teichner et al. [32] and more recently by Armor and Carlson [ 73 ]. The first group of authors concluded that the pore volumes were independent of the water to aluminium sbutoxide ratio, provided it was stoichiometric or substoichiometric. The greatest pore volumes were obtained for the smallest concentrations of aluminium precursor involved. T h e y also indicated that calcination in air or in vacuum decreased porosity at temperatures higher than 600 ° C. This behavior was also recorded by the second group of researchers who concluded that porosity is much more developed when aluminium isopropoxide was chosen as the precursor when the water to alkoxide ratio was high and when a two-step procedure was used (the procedure consisted of first, preparing a mixture of methanol and aluminium isopropoxide and then adding water). Fig. 4 summarizes their findings. Silica was thoroughly studied [32 ] and it was shown that operating in acidic or neutral medium led to the highest pore volumes, even for high water to tetramethoxysilane ratio (exceeding the stoichiometric ratio). Volumetric percentages of Si (OCH3) 4 in methanol between 10 to 20% gave the largest porosity. Finally, SiO2 aerogel still maintained a high porosity when heat treated in air at temperatures not in excess of 600°C. 4.3. Surface areas

It is well known that the aerogel method provides solids exhibiting unusual high surface areas (as measured by the classical B E T method with nitrogen at 77 K) of the order of many hundreds of m 2 per gram. These high areas are conserved even after calcination treatments up to 500 ° C. This means that, in principle, the actual specific area of a working aerogel catalyst is still as high as a few hundreds m 2 per gram! Table 7 gives the lowest and the highest specific areas for some of the most frequent aerogel catalysts cited in the literature. Obviously the values indicated in this table are, with a few exceptions, much higher than those belonging either to xerogels or fumes. As shown above for pore volumes, the surface areas of aerogel catalysts remain very stable with relatively high temperature calcination treatments (see Table 8 for SiOe, AleO3 and ZrO2 [32] ). Silica is remarkably stable, whereas AleOa or ZrOe maintain surface areas in the order of 100 me/g at about 500 ° C when treated in air or in vacuum. Armor and Carlson [ 73 ] report a result of 482 me/g for an A1203 aerogel calcined at 800 °C in air for eight hours. An extensive study of the influence of many preparation parameters on the surface areas of nickel based aerogel catalysts can be found in the papers from Gardes et al. [48,49 ] and Sayari et al. [51 ]. Increasing the water to nickel oxide

238 TABLE7 BET surface areas of aerogel catalysts Aerogel

Surface a r e a s ( m 2 / g )

Reference

Lowest to highest Pt-A1203 Pure Cu Pd-A1203 Cr203 TiCl4-Al203 Cr203-AI2O 3

PbO-A1203 FezO,~-A1203 Fe203-SiO2 NiO-Al203 NiO-SiO2 NiO-MgO CuO-Al~O~ CuO-Zr02 CuO-ZnO ZnO-AlzO3 Ni-A120:~ Ni-SiO2 Cu-A12Oz Fe-A1203 Fe-SiOz MoO2 MoO2-Ni NiO-SiO2-Al203 NiO-MgO-A1203 NiO-SiO2-MgO NiO-AlzO:wFe20.~ Fe203-CrzO3-A120.a CuO-ZnO-MgO CuO-ZnO-MoOz CuO-ZnO-A1203 CuO-ZnO-ZrO2 Ni-AI20.~-MgO Ni-SiO2-A1203 A1203 SiOz ZrO2 TiOz

450 ( 120 )a 0.23 134-642 516-785 171-501 200-474 84-373 230-570 690-760 270-710 655-733 75-171 369 219 12 383 150-650 (7-53)a 620 (356) a 660 (30) a 355 750 79-168 0.7-15 30-890 194-305 391 235-421 300-700 259 6-17 311-319 150-161 220-950 ( 11-83 )" 480-730 (4.6-68) a 123-616 366-1004 138-387 96-107

76 74 66 or Table 3 69 or Table 4 56 53, 75 38, 39 43 44 48 51 51 67 67 67 67 49 64 17 42 42 45, Tables 5, 6 45, Table 5 48 51 51 43 59 67 67 67 67 49 49 32 32 32 32

"The metallic surface areas are given in me/g of metal in parentheses.

239 TABLE 8 Surface area in function of the temperatures of treatment in air (A) or in vacuum (V) for some simple aerogels (from ref. 32) Aerogel

Temp. in °C

Surface area (m2/g) (A)

SiO2

RT 300 500

860 1004 800

-

A1203

RT 400 600

464 405 239

464 512 461

Zr02

RT 300 600

317 254 38

317 329 84

700

Ni =0,6 i N~ =0,6 AI÷Si I AI+Mg

600

i

Surface area (m2/g) (V)

500

c~E 4.00

300

200 i

100

0,5 A1

~ i

0,5 A~

AI÷Si

I

AI+McJ

0

Fig. 5. Surface areas of NiO containing aerogels in function of the support compositions.

plus aluminium s-butoxide ratios resulted in a decrease of the surface areas as well as the increase in nickel content (see below, the structure section). Increasing acidic pH supplied by acetic acid also exerts a detrimental effect on this textural property. Fig. 5 gives the BET areas for two types of aerogel with their respective nickel-to- (aluminium plus silicon) and nickel-to- (aluminium plus magne-

240 TABLE 9

Surface areas of NiO-A1203 (NA), Fe203-A1203 (FA) and NiO-FezO3-A1203 (NAF) aerogel catalysts as a function of their atomic compositions (from ref. 43 ) Atomic percent composition

Symbols

Surface area (m2/g)

% A1

% Ni

83.3 66.6 55.5 50.0 37.5

16.7 33.3 44.5 50.0 62.5

0 0 0 0 0

NA NA NA NA NA

0.2 0.5 0.8 0.1 1.6

540 455 310 265 -

0 0 0 0 0 0

2 10 20 50 70 90

FA FA FA FA FA FA

0.02 0.11 0.25 1.00 2.33 9.00

570 480 400 350 290 230

0.25 1 2 5 10 15 20

NAF NAF NAF NAF NAF NAF NAF

98 90 80 50 30 10 49.75 49 48 45 40 35 30

50 50 50 50 50 50 50

% Fe

0.25 1 2 5 10 15 20

421 285 284 272 268 235

slum) ratios. Both series held a constant value of 0.6 with respect to the atomic composition of the corresponding supports aluminium-to- (aluminium plus silicon ) and aluminium-to- (aluminium plus magnesium), respectively. It is seen that for this series of the ternary aerogels depicted in this figure, that for a given atomic ratio of nickel to the sum of the cations of the support, the more acidic the carrier the greater the surface area. This rule applies again with AleQ or MgO as simple supports [51]. The influence of composition for binary Fe203-Al:O3, NiO-A1203 and ternary NiO-Fe2Q-AIzQ aerogels upon the surface areas are indicated in Table 9. As a general rule the higher the amount of the crystallized phase (s) contained in the aerogel, the lower the surface area. In this table the crystalline phases detected by XRD analysis are NiO or Fe203 or NiO plus Fe203 while alumina remained amorphous (see below). 4.4. Particle size distribution

A series of ten different samples of aerogel catalysts was analysed by a laser particle granulometer (dry). The results clearly showed that all samples ex-

241 TABLE 10 Mean particle size of aerogel catalysts The measurements were carried out on a Sympatec laser granulometer by courtesy of Socidt~ Prodemat, Villeurbanne, France AerogeP

Mean particle size

(~m) A1203 MgO Zr02 Cu-MgO Si02-A1203 NiO-Ti02 b NiO-A12Q PbO-MgO PbO-ZrO~b

5 7 5 9 6 1.5 5 5 1.5

"All samples were made at the LMPC. bThese samples showed particle size smaller than 0.5 #m.

hibited a monomodal distribution (volumetric mode) of particle sizes. Thus, it is also possible to prepare very homogeneous particles by the autoclave method. The particle dimensions were always comprised in the range i to 9 pm as shown by Table 10. Fig. 6 gives a very representative size distribution pattern for NiO-A1203 aerogel. 4.5. X R D structures

Silica and alumina engaged in multicomponent catalysts are always amorphous as shown by XRD analysis [32 ]. Pure silica aerogel is also always amorphous. Table 11 contains the data concerning the crystallinity of the most frequently encountered catalysts. In general, crystallinity is developed by the use of large amounts of water with respect to the hydrolysis reaction, and also by increasing the metal oxide content in binary or ternary aerogels catalysts containing SiO2, A1203. The presence of poorly crystallized NiO or Ni A1204 precursors in binary aerogels is of great importance for nitroxidation reactions for instance (see below ). 4.6. Solid state chemical reactivity

Silica or alumina which remains amorphous in multiphase catalysts such as the binary aerogels NiO-SiO2 and NiO-A1203 contributes to the formation of

242

Volumic 1"

'

,

distribution .

-;

.

.

.

.

.

.

.

.

,

.

.

.

.

.

,

.

.

•

~ r ~ - v

.

.

.

.

.

.

.

SO

.

4~ o., ....... i-.--i.-i--i.i-~ii-; .......-...i..;:.:,.i.; ~': ~.;.....;:-;-.i-i-;i:i;.......i-.--;.-;-;.i.':;i.

ii:i !iili/ iiiiiii iiiii!i!

....... ;i i;:'"...... i :, !ii?!!i : ! ii:iiii : o.,

40

::? ?i~ i i i i::iiil : "/ii!i!'$".~, i ! ii!ii!i

i i i iii::i i if!i!!!

:

i

: : :::::~

I : : : : : : : ;,

: : :::'::

4.J .¢-4

: : : ::::

.......i----i--i-}-i-ii{-.~.......i--~--!--i-{-i/:-}!-4"---i---i---i-!-!-i-!'i! .......'!---~-~":-.~':.:;

:y~

:o

~9 q~ 2S

¢:

20

:

C

.......

......

........... -iiiii ............... C.

4-~...~~...~ ~. t O .......~,....~._~..~.~-~i~..~.-..,-.~...~.:~:-.i.....~.-~ .......~..-?i-~..~i .......;..-.~-.

o.~

ol lO

] ] :! iil]], ,/': i:;i:~::]:: i i ili::::~.'~:.: i iiii[ii

-t

0 10

10

] :: ::N~i::i :: i iiii.~_ 1

i

:: ] i:: !ill i iiiiii

2 10

IO

0 3

Fig. 6. Particle size distribution of aerogel. The complete analysis of aerogel powder catalysts by the author is under way (to be published). T A B L E 11 Crystalline structure of aerogel catalysts Aerogel

X R D detected phase

Reference

A1203 ZrO2 Ti02 NiO-AI203 Cr203-AI203-MgO Fe20,~-AI203 Fe203-NiO-AI203 Fe203-SiO2 PbO-A1203 NiO-Si02 NiO-MgO NiO-MgO-AI203 NiO-A1203-SiOz

amorphous or boehmite amorphous or monoclinic anatase or amorphous NiO, Ni A1204 Cr203 Fe203 Ni A1204, Ni Fe204 F%O4, Fe20~ Pb (OH)2, PbO1.37, Pb304 amorphous NiO NiO amorphous

32 32 32 69 53 43 43 44 77 38, 39 51 51 51

243

o

i

b c

I

I

I

I

900

800

700

600

I/~(cm -I )

Fig. 7. Infra-red spectra of catalyst N i O - S i 0 2 (NS III) as a function of the temperature of heating in oxygen for 24 h: (a) 440°C, (b) 500°C, (c) 730°C. T A B L E 12 Evolution of the structure as recorded by X R D of a NiO-A1203 aerogel as a function of heat treatment in oxygen (from ref. 78) Temperature (°C) 400 530 600 700 800 900

. I2~ I4~ 2.83 1.05 0.731 0.324 0 0

Surface area (m2/g) 224 144 117 89.5 63 47

amorphous silicate or spinel of nickel at very low temperatures in contrast with conventional xerogels. Fig. 7 shows the IR spectra of a NiO-SiO2 treated in oxygen at increasing temperatures, indicating the presence of an amorphous nickel silicate at relatively low temperatures. The IR bands concerned are located at 665-710 cm-1 [38,39]. Sayari and Ghorbel [78] have clearly demonstrated that a pure Ni A1204 spinel, very well crystallized, is formed at 700 ° C exhibiting a high surface area as indicated by Table 12 where the ratio of the intensities of the X-rays 200 and 400, belonging respectively to NiO and to Ni A12Oa are given when a N i O A1203 aerogel is progressively heated in oxygen from 400 ° C up to 900 ° C.

244

A survey of the literature shows indeed that all of the aerogel catalysts are usually used at temperatures in the vicinity of 400 °C (for activation at least). So a particularly high reactivity of the solid state between the amorphous phase (SiQ, A1203, etc.) and the crystalline one (s) exists giving more or less new and more organized compounds in the catalysts. As will be seen later these new combinations are generally responsible for the stability of the catalysts with time on stream. This is specifically demonstrated for nitroxidation catalysts. 5. P U T T I N G AEROGEL CATALYSTS INTO SERVICE

As obtained from the autoclave, the aerogel catalysts are generally in the form of fine powders or lumps which are fragile, loose and difficult to handle in reactors. Moreover they are also very good thermal insulators when they contain refractory oxides such as A1203, MgO, Zr02 and chiefly Si02 [4-7]. For their use in fixed bed reactors, severe pressure drops occur as well as mass and heat transfer limitations. Therefore, Armor and Carlson [41 ] proposed to preform the catalysts in the autoclave with encasing devices such as open-form supports: cordierite honeycombs, Rashig rings, wire mesh screens, Fiberfrax, vermiculite, boiling stones, glass and metal tubings etc. They succeeded in fabricating Cr203 on ceramic Rashig rings or embedded into alundum boiling stones. They also formed PdSiO2 encased by cordierite. They called this form of aerogels 'pelletized" ones. Their general procedure consisted of encasing the aerogel at the step of the alcogel precursor in the autoclave. Another procedure adopted by Klvana et al. [ 79 ] involved mixing the aerogel powder with steel wool as an open form. A Ni-SiO2 catalyst in a powder form was thoroughly studied for its hydrodynamic properties and the following was found [ 79 ]: the pressure drop versus time on stream curves for different fixed gas superficial velocities showed that the curves passed through a maximum and then stabilized at an irreversible steady-state. At steady-state, the behavior obeys the Blake-Kozeny equation [80]. It was confirmed that a decrease in the porosity of the bed resulted in an increase of the pressure drop. At steady-state, the pressure drop varies with the temperature of the heat treatments in nitrogen or hydrogen according to the curves shown in Fig. 8 where it is clear that a maximum occurred for both gases at 250°C followed by a rapid decline to zero at 400°C where the two curves merge. No satisfactory explanation has been found yet, but further studies are in progress in order to clarify these very unusual hydrodynamic properties. The practical conclusion drawn is that a treatment at 400 °C develops the permeability of the solids to a very high level. Also, the same observations applied to pure SiO2 and A1203 aerogels. The same Ni-SiO2 catalyst was then tested in the hydrogenation of toluene into methylcyclohexane in the gas phase at about 100 ° C. Strong heat transfer limitations were observed (the temperature gradient recorded was of 6-7 K)

245

2000

H2

1500

1000

500 400 300 200 100 0 100

200

300

400 T°C

Fig. 8. Influence of the temperature and the nature of the gas on the pressure drop on a Ni-Si02 aerogel catalyst bed with an initial apparent density da -- 0.2437 g / c m 3, and a gas velocity w = 10-3 m/s: hydrogen (curve a), nitrogen (curve b).

as shown by the percentage of conversion against contact time curves given in Fig. 9. To eliminate this limitation, a "dilution" of the aerogel was made in dry conditions by the addition of stainless steel twigs. As indicated by the straight lines of Fig. 9 the mass transfer limitation with "undiluted" Ni-SiO2 aerogel catalyst was also very important. Aerogels, despite their very small particle dimensions ( < 10/~m), can be nevertheless perfectly fluidized as was demonstrated recently [68,72,81,83,84]. According to Geldart's classification [85], these small particles develop very strong interparticle forces of Van der Waals origin, which prevent them from fluidization, they are class C powders [86,87 ]. In fact aerogels do not fluidize at their corresponding minimum fluidization gas velocity as predicted by the theory based on their particle dimension. In order to fluidize they require a much higher superficial velocity, the threshold being called the minimum clustering velocity. At minimum clustering velocities, aerogel particles rearrange in clusters reaching dimensions of the order of a few millimeters, and take on characteristics of group A powder particles [85 ] as indicated by Figs. 10 and 11 respectively. These clusters are of a dynamical nature for they return to the fine powders of glass C when this critical gas superficial velocity is reduced to lower values.

246

:oci 15C~C

i

13C°C BO

70 4£

50

I !O°C

-- 4c

x

4"150

3C

4' 150 ]

2C

/

~

~

90°C

_.*.9o . . . . . . . . .

0,1

0,5

1,0

1,5 W _ x103 FT

Fig. 9. Conversion of toluene in methylcyclohexane (X in %) against W / F T in g min/mol at four reaction temperatures on a sample of Ni-Si02 aerogel diluted by the stainless steel wool. The crosses and corresponding figures represent the conversions observed at the same reaction temperatures with the sample of aerogel not diluted.

II111 IIIII

__! ,,,,, IIII~ X /y

'ii

111[III

,,

Low velocity ~4ormal fluidization oreferent*al ,~egligeable interchannel ing ~articulate forces

Pl uq ri se Vet I hiOh

r'Chunk~"

i n t e r o a r t ~culate

Forces

Fig. 10. Descriptive hydrodynamic behaviour of fluidized aerogels: (m) Particle including bulk porosity.

247

'°~ I \ [',1/ \~ ' , rl

', ~ ~'~

-- ~ "

"~ ~

I: ,,\

I1',\

0

. I0

\

I ~,d.,t

\

/clusters \

','\\

o

I ~

\

I

®

\

I-~-

~oler~s

|

I~|S

r--,, '

~

I00

I000

Particle diameter

dp

(~m)

Fig. 1 i. Fluidizability diagram according to Geldart.

All aerogels give similar results i.e. they all develop a reversible rearrangement into fluidizable chunks. It was also demonstrated that mixing aerogels in the non fluidizable form with alumina particles of much higher density eases the fluidizability of SiO2 aerogels for instance [88] and makes it possible to circulate these solids in appropriate reactors [72]. Some of the factors governing the fluidizability of Ni-SiO2 aerogel catalysts have been discussed by Lauga et al. [84 ] and Klvana et al. [ 83 ]: decreasing the nickel percentage is detrimental, and increasing the bed porosity improves the fluidization. Also, high gas humidity together with addition of alumina both diminish the interparticle forces and thus give more fluidizable catalytic beds. Li et al. [72] have shown that Fe2Oa-SiO2 and NiO-SiO2 aerogels possess the hydrodynamic behavior of Geldart's group A or B particles in fast circulating fluidized bed conditions. The properties described above open the field of investigation of aerogels for application to high exo- or endo-thermic catalytic reactions. Some examples are given in the next section. 6. C A T A L Y S I S

BY AEROGELS

6.1. Early work of the 1930s As soon as aerogels were produced by Kistler's method, they were immediately applied to catalysis in the field of organic chemistry. At that time the

248 conversions of organic acids such as acetic, phenylacetic and propionic acids to acetone, ethylbenzylketone and diethylketone, respectively, were of some industrial interest. Kistler et al. [2 ] underlined the fact that in the form of aerogels thoria was superior in selectivity and durability with time on stream and equal in conversion when compared to all other catalysts known at that time (different forms ofthoria xerogels). Swann et al. [8] soon extended this work to higher ketones by reacting aliphatic esters on thoria aerogels. They published results concerning ethyl laurate conversion in order to obtain laurone and claimed that they had better results with thoria aerogels than with conventional thoria or phosphorous pentoxide. Partial oxidation of acetaldehyde into acetic acid with air was investigated on numerous simple or mixed aerogels by Foster and Keyes [9]. They ran this selective oxidation reaction on silica, Pt-SiO2, Ni-SiO2, Cr203, Fe203 aerogel catalysts as well as on the corresponding ordinary gels. They found that pure silica xero- and aero-gels behave similarly but their best catalyst was the P t SiO2 aerogel. A mixed chromia-alumina aerogel catalyst gave good results for the conversion of alcohols into amines as described in the paper of Kearby et al. [40]. This catalyst was the best among a variety of other aerogels, including silica, alumina, chromia, thoria etc. Butyl, amyl and benzyl alcohols were tested for their conversions into their respective monoamines which were preferentially researched as the reaction products. More than thirty years elapsed between this last work and the recent catalytic studies carried out with aerogels which resumed approximately around the 70s. 6.2. More recent work 6.2.1. Partial oxidation of organic reactants The partial oxidation of isobutylene into methylacrolein was observed on NiO-A12Q and NiO-SiO2-A1203 aerogel catalysts [12]. The yields in methacrolein and acetone were 60 and 23% respectively for catalysts activated at 300°C with dry nitrogen. A pretreatment with water vapor and oxygen improved these yields up to 67 and 25%, respectively. This result emphasized the potential of using NiO as a partial oxidation active catalyst when engaged with A1203 or SIO2-A1203 [12]. Diisobutylene was also formed in the presence of the NiO-SiO2-Al~Q aerogel. Even isobutane and propane were transformed into acetone (selectivity of 30% at 260°C with isobutane and 100% with propane) [89]. A parent reaction, namely nitroxidation of aliphatic and aromatic hydrocarbons, was extensively developed in this laboratory. A short review was recently published by the author [77] while an extensive one is in preparation [90]. The reaction consisted of reacting nitric oxide with the hydrocarbon to convert

249

it into the corresponding saturated and unsaturated nitriles. Various mixed aerogels were involved in this synthesis, among the most studied of these were those based upon NiO or PbO combined with A1203 or silica [38,39,54,91,92]. Schematically a nitroxidation reaction can be represented as follows: ( A o r A' )---CH3 + ~ N O - , (A or A' )--C-=N+3H20 +~N2 2 where A represents an aliphatic hydrocarbon such as propene, isobutylene, isobutane, propane, while A' stands for toluene, the xylenes and the monotolunitriles. As a general result the conversion into the nitriles shows a global selectivity in the range of 80-90%. Table 13 gives some data for each aliphatic hydrocarbon studied. Fig. 12 shows the results recorded with the ternary NiO-SiO2-A1203 and NiO-MgO-A1203 aerogel catalysts in the selective conversion of isobutylene into methacrylonitrile, as a function of their composition expressed as atomic ratios of the cations included in the solids [91]. It has been also shown that the pure NaA1204 spinel (see Table 12 ) develops the same selectivity in the formation of methacrylonitrile (from isobutene) compared to the NiO-A1203 catalyst not pretreated at temperatures above 400°C [78,91]. The nickel spinel (ex-aerogel) catalysts showed the highest activity expressed per m 2 of catalyst. This phase was suspected of intervening selectively in the catalytic conversion of the substrate and of contributing the TABLE 13 Selectivities (S % ) into nitriles, at steady state, in the conversion of aliphatic hydrocarbons Hydrocarbon

Aeroge!

S % in the main nitrilea

Total S % in all nitriles

Reference

Propene

NiO-A1203 Cr203-A1203-MgO NiO-Al20a NiO-SiO2-AleQ PbO-A1203

90 74 78 76 87

(C3H3N) (C3H3N) (C3H3N) (C3H3N) (C3H3N)

95 89 87 85

58 53, 97 93, 95 93, 94, 95 77

Isobutylene

NiO-Al203 NiO-Si02 NiO-MgO Cr203-A1203 Cr203-Al203-MgO

86 79 58 76 74

(C4H3N) (C4H3N) (C4H3N) (C4H3N) (C4H3N)

95 86 64 85 89

96 91 96 53, 97 53, 97

Isobutane

Cr203-A1203 NiO-A1203 Fe203-A1203 NiO-A1203-Fe203

51 40 39 45

(C4H3N) (C4H3N) (C4H3N) (C4H3N)

81 63 56 70

78 43 43 43

aThe main nitrile is indicated in parentheses.

250 I

~

=Q.6

,

N-9~ =0.6

I

.._.~.~.~:----: 0

05

~

0.5

A~5, AI

I

~,1AI *Mcj

0

Fig. 12. Isobutylene conversion: selectivities into methylacrylonitrile as a function of the compositions of the ternary aeroge! catalysts.

high selectivities of nickel containing aerogels in combination with badly crystallized NiO a n d / o r spinel species. No correlation was found between the acidity of the aerogel catalysts and their aptitude to yield methacrylonitrile. Aromatics are also selectively converted into nitriles over Cr203-A1203, NiOA1203, NiO-SiO2 and PbO-A12Q aerogels as indicated in Table 14. Again, with aromatics it is clearly shown that the total selectivities into nitriles is very high and attains a mean value of 80%. Notice that the same catalysts are good contacts for aliphatics as well as for aromatics, which is not observed with the conventional ammoxidation processes. The deactivation of NiO-A1203 based systems was investigated recently in two papers by R a h m a n et al. [58,100] and suggestion of additives proposed to improve their stability in the conversion of propylene. These additives were MgO or Fe203. A CuO-A1203 aerogel catalyst was tested in the partial oxidation of n-butane and l-butane with air and showed a selectivity of 20% for furane formation and 12% for crotonaldehyde at 310°C when n-butane was the feed. W h e n 1butene was the feed it gave furane (S-- 22% ), acetic acid (S --- 14% ), methylvinylacetone (S = 6% ) and crotonaldehyde (S = 15% ) [57 ].

6.2.2. Hydrogenation reactions 6.2.2.1. Selective conversion of cyclopentadiene into cyclopentene. This reaction was first studied on a Cu-A1203 aerogel catalyst by Pajonk et al. [17] in a differential, static fixed bed reactor. It gave a selectivity of 100% in the cyclene and exhibited the highest turnover frequency of all the other classical types of catalysts investigated [63 ]. Soon after, this work was extended to an integral fixed bed flow reactor by Chaouki et al. [ 101] and then to a fluidized bed re-

251 T A B L E 14 Selectivities (S % ) in aromatic nitriles Aromatic hydrocarbon

Aerogel

S % in the main nitrile

Reference

Toluene

NiO-A120~ NiO-SiO2 Ni-AI204 PbO-A1203

91 91 86 93

(CsHsCN) (CsHsCN) (CsH~N) (CsH~N)

50, 38, 38, 38,

p-Xylene

Cr2Oa-Ale03

13 2 51 16

(TN)a-85 (PN) b (BN) c (TN)a-24 ( P N ) b (BN) c

99

PbO-A1203 a

98 39 39 39

54

o-Xylene

PbO-AI203 e

86 (OTN)[-8 (BN) c

54

m-Xylene

PbO-AI203 g

68 ( M T N ) h - l l (BN) c

77

OTN MTN h PN b

PbO-AI203 PbO-AI20~ PbO-A1203

67 (BN) c 58 (BN) c 30 ( T N ) a

92 92 84

~Terephthalonitrile. bParatolunitrile. CBenzonitrile. aTerephthalonitrile was formed at 713 K with S = 4 3 % , [25 (BN) c and 18 (PN)b]. ePhthalonitrile was obtained at 713 K with S = 20%, [44 (OTN)[ and 20 (BN)C]. rOrthotolunitrile. gIsophthalonitrile is also formed at 713 K with S = 13%, [68 ( M T N ) b and 11 (BN)C]. hMetatolunitrile.

actor described by the same authors [102 ]. In all cases the selectivity was 100% into cyclopentene, whatever the conversions, or the type of the reactors. Fig. 13 briefly summarizes the necessary conditions to fluidize the Cu-A1203 aerogel catalyst and Fig. 14 gives the results obtained when such conditions are achieved in the reactor at 220 ° C.

6.2.2.2. Hydrogenation of toluene. Toluene was chosen to store hydrogen for clean vehicle engines by means of its catalytic hydrogenation into methylcyclohexane on a Ni-SiO2 aerogel catalyst between 95 and 150°C in fixed bed and in fluidized bed conditions [64,79,83 ]. The procedures adopted to eliminate the transfer limitations as well as the pressure drops encountered in the fixed bed reactor were given above. Again, the selectivities to methylcyclohexane were 100% for all the reaction conditions. With the previous reaction this clearly gives evidence that aerogel catalysts

252

~grces .. ~,,

c

,/~

/ !

/

Fig. 13. Balance between the forces exerted on a particle of aerogel catalyst and its dimension for fluidization: (FA) interparticle force, (FG) drag force, (PA) apparent weight of the cluster, (Dpc) diameter of the cluster. (Dc is a critical dimension).

30

2o

10 Nodel

[ 0

0

& plug flow. .

~0 20 Exp. conversion (Z)

30

Fig. 14. Calculated versus experimen~l conversions of cyclopentadiene into cyclopentene in fluidized bed conditions at 200 ° C.

can be utilized in fluidized bed reactors for exo- (and endo-) thermic conversions.

6.2.2.3. Hydrogenation of benzene. In this case, a MoO2 aerogel was chosen as a support for the preparation of nickel Mond-MoO2 and Pt-MoOe catalysts in

253 order to hydrogenate benzene into cyclohexane at temperatures varying from 50 ° up to 100 ° C [ 45,65 ]. A synergetic effect occurred for the Ni-MoO2 catalyst with respect to pure nickel, taking the form of an electronic interaction, while with the P t - M o 0 2 the reaction appeared to be structure sensitive due to the strong interaction between the platinum species and the MoO2 which is known also to be a good electrical conductor. According to the classification proposed by Che and B e n n e t t [ 103 ] this reaction develops a negative or antipathetic structure sensitivity on Pt-MoO2. This catalytic behaviour is contrary to t h a t generally found with more classical platinum supported catalysts.

6.2.2.4. Hydrogenation of nitrobenzene into aniline. Armor et al. [66] reported this reaction which was carried out in a slurry reactor containing a Pd-A12Q aerogel dispersed in ethanol. As Table 15 shows, the aerogel catalyst was better t h a n the commercial catalysts also tested. Conversions of nitrobenzene and selectivities into aniline were both 100%. The authors emphasized t h a t it was not necessary for the Pd-Al2Oa catalyst to be activated, in contrast with commercial catalysts, i.e. once it was removed from the autoclave it was immediately operative. Commercial catalysts require a pretreatment with formaldehyde or hydrogen at high temperatures in order to develop their catalytic activities.

6.2.2.5. Hydrogenation with aerogels activated by hydrogen spillover. Refractory oxides such as SiO2, A1203, MgO and ZrO2 are generally considered to be inactive in hydrogenation reactions. Nevertheless, this laboratory has shown that a hydrogen spillover activation procedure carried out at 430 °C on refractory TABLE 15 Hydrogenationof nitrobenzene to aniline with various palladium/A1203catalysts at 25 °C and 1 atm of hydrogenpressurea (from ref. 66) Catalyst

Catalyst number

Surfacearea (m 2 g )

Rateb mmol h- l

5% Pd/AI203powder (Alpha) 5% Pd/A1203aerogel (MeOH)¢ 2% Pd/Al203 aerogel (MeOH)~ 5% Pd/Al203 aerogel (i-PrOH)u 5% Pd/Al20~powder (Engelhard) 5% Pd/AI203 xerogel

( ¢ 4332 ) (¢5711) (~5735) ( ¢ 5736) (~4341) ( ¢ 3991 )

88 328 642 134 84 422

1.6 1.9 2.1 2.7 2.1 0.6

aEthanol (25 cm3) containing nitrobenzene (0.5 cm3) at stir rate of 300 rpm with a flowrate of hydrogenof 17.5 cm3min-1. bCalculated from the plot of the amount of nitrobenzene remainingvs time {2.4% C6H.~NO2was initiallypresent); 5 mg of catalyst. ~MeOH:prepared in methanol. di-PrOH: prepared in isopropanol.

254

oxides when activated by a hydrogen donor in an hydrogen atmosphere in a specially designed reactor is a very efficient means to create active hydrogenating centers at their surfaces [20,104]. The corresponding aerogels were very interesting because they retained their particular high specific areas once they were activated by this spillover process. Molecules such as ethylene, acetylene, but-l-ene, benzene, the cyclohexadienes and cyclohexene were hydrogenated at fairly low temperatures (100 °200°C). Even a paraffin like n-heptane could be hydroconverted at 270 °C on a silica aerogel activated by hydrogen spillover [105]. The main reaction products were methane, ethane, heptene, the heptadiene (s), benzene and toluene when the feed contained hydrogen. When hydrogen was replaced by helium, the composition of the reactants changed to methane, heptene, the heptadiene(s), ethane, acetylene, toluene and benzene. The authors suggested that, beside hydrogenation active sites, acidic sites were simultaneously created at the surface of the aerogel. 6.3. Ethylene polymerization Fanelli et al. [55 ] described the behaviour ofTiC14-A1203 aerogels under low pressure conditions. They found high molecular weight polyethylene along with a broad molecular weight distribution together with a low melt index. Moreover the catalytic activities were found to be correlated with the reciprocal of the bulk (apparent) densities as shown in Fig. 15. Porous volumes seemed also to be correlated with activity, however, the surface areas were not. The aerogel catalysts were superior to the corresponding xerogels impregnated catalysts. The aerogel catalysts were as active as the well known Ziegler MgO-TiC14 contact, i.e. they were better than the former TiC1Jalkyl3A1 systems. The salient feature of this study was that the aerogel texture was not sensitive to the impregnation step during preparation, indeed the bulk (apparent) densities remain the same before and after the impregnation by TIC14. 6.4. Carbon monoxide or carbon dioxide-hydrogen reactions Fischer-Tropsch and methanol synthesis catalysts were also prepared by the aerogel technique. Fe203-SiO2 aerogel catalyst exhibited much higher productivities (300 times more) at steady state than the non supported Fe20• or the classical xerogels [106]. Moreover the mixed aerogel did not deactivate by carburization or through the formation of inactive carbon species. The authors disclosed that it was the oxidized form which was the best way to obtain such high activities in hydrocarbons, and that only the aerogel was able to maintain an adequate

255

240 r =0.98

200 °

160

~120 <

801 40 olI

3

5 7 9 (Bulk Density) ~cm;'g

11

13

240

2001 •~

r =0,87

160 t

'-=~ 120 i

400-] 1

3

5

P o r e V o l u m e , cm~/g

24G~'

.1

r = 0.82

2001 160-~ "~ 120<

40" 0 100

(3

200

300

S~r~:e ~ •g PE/g ( c ~ ) - ~ m

400

.~

m2/g

(c~.)

Fig. 15. Correlation of catalyst activity with physical properties for TiC14-AI203 ethylene polymerization aerogel catalysts.

256

state of oxidation with time on stream [ 107 ]. The same trend was also recorded when alumina replaced silica [44]. Table 16 gives the selectivities recorded for aerogel and xerogel catalysts of the same gross chemical compositions. The reaction mechanism was found to be of the redox type involving the dissociation of carbon monoxide and the formation of an active carbon species when hydrogen is present in the feed [ 10 ]. Binary and ternary aerogels containing copper catalysts were tested in the synthesis of methanol with carbon dioxide and hydrogen feeds by Pommier and Teichner [67]. As with Fischer-Tropsch reactions, they found that preoxidized aerogels were more efficient in the synthesis than the pre-reduced ones. Tables 17 and 18 summarize the main results observed with the binary and ternary copper aerogel catalysts, respectively. The results shown in Table 16 indicate that pure Zr02 aerogel is also a good catalyst for the methanol synthesis. Tables 16 and 17 also show that the presence of zirconia is beneficial for the formation of methanol and that it may be due to an interaction between copper and zirconia of an electronic nature (copper may transfer an electron to zirconium ions).

6.5. Nitric oxide reduction by ammonia Iron oxide-chromia on A1203 or MgO aerogels were evaluated as catalysts of the selective reduction of nitric oxide by ammonia into nitrogen and water [59]. The reaction mechanism of this reduction was investigated on Fe203Cr203-A1203 (the binary catalysts Fe203 (or Cr203 ) on A1203 were less active) T A B L E 16 Selectivities (S % ) into various hydrocarbons at 250 ° C, hydrogen-to-carbon monoxide ratio = 9 at atmospheric pressure (from ref. 44) Nature of the solid

SiFel0 (A) a (2D) preoxidized h l F e l 0 ( h ) a (2D) preoxidized SiFel0 (X) b unreduced AlFel0 (X) b unreduced aaerogel form. bxerogel form.

Selectivities (%)

C1

C2

C3

C4

C5

C6

C02

EC > 1

44.5

22.5

16,5

7.5

4.5

1.5

3

52.5

46

20

18.5

6.5

4.5

2.5

2

52

44.5

25

14

6

2

-

8.5

47

55

25.5

11

2

-

7

38.5

257

o

o

-,~ ,.~ •.~ ~ o

O O r..)

t

8 R

o o 0oo ooo

ooo ooo

r.)r.)o

~

coc~c~

¢o

rD

t~

o

o

~

0 0 0

0 0 0 0 0 0

0 0 0 0 0 0 0

r,..)Or~

r-..)rD

O

r.,)

coco

eoc, o

~c~

~d~ ~ d ~dd ~d d

~rD

r~

o8~

c'oc'~0

~ gd d ~E

o

-2 ,g

< r.r.1

~

.< o

?

?

?

?

~8

258

o 0

%

o

o

0

0 0 0

0

0

0 0 0 0 0 0

0

0

0

0

0 0 0 0 0 0 0 0 0 0

0

0 0 0

0

0

ooo

0

"4

o

o

o

o~03

0

~

o

~

~

0 , ~

0

r~

~ ~-~ 0 -~.~ ~'~ ,'~ ~ ..~ 0"3

o

o

'oo

78

CO

,-~

~

oo

oo

~-~

0

0

0

0

0

0

0

0

259 TABLE 19 Comparison of initial rates for the reduction of nitric oxide by ammonia for various supported catalysts at 250°C (from ref. 110) Catalyst

Rate of nitric oxide reduction (/lmol/s g)

10.0% platinum on carbon aerogel 10.0% platinum on carbon aerogel 0.7 % platinum supported on silica 5.6% V20~ supported on titania

2.00 0.00 1.94 0.69

in the oxidized form which was also a prerequisite for a good stability with time on stream. The highest activity occurred at 640 K (conversion of nitric oxide for a 525 ppm of nitric oxide and ammonia mixture in air: 48% ). A redox mechanism was proposed to explain the fact that oxygen significantly enhanced the reaction rate at temperatures between 45 and 750 K. IR investigations were performed on the iron oxide (or chromia) on alumina and on the ternary catalyst and helped explain the reaction mechanism. Ammonia adsorbs onto a fully oxidized site and reduces the metal ions, then nitric oxide is chemisorbed and decomposed, finally the catalyst is reoxidized by oxygen [109]. The active sites seem to be the chromium and iron ions partially in a reduced state, which favours a redox mechanism. The highest activity was that exhibited by a Cr203-AI203 oxidized aerogel while the highest conversion (60%) was recorded with the ternary aerogel catalyst. Very recently, Zhao et al. [110 ] prepared platinum on carbon aerogels from the reaction between resorcinol and formaldehyde in the presence first of sodium carbonate, then chloroplatinic acid dissolved in dimethylformamide was added and finally evacuated as an aerogel in the autoclave. This procedure is close to that of Pekala and Kong [111]. The BET surface area of a 10% (w/ w) platinum on carbon was 765 m2/g. The catalytic test performed at 250°C showed that this aerogel catalysis has a greater activity than a conventional platinum (0.7% in weight)-SiO2 or V205 (5.6% in weight deposited on TiO2) one. Table 19 shows some of these results. 7. MISCELLANEOUS REACTIONS AND NEW DEVELOPMENTS

Ammonia synthesis was carried out on a new series of aerogels at 400 ° C with the stoichiometric synthesis feed, under atmospheric pressure [52]. Binary catalysts such as V/MgO, Ni/MgO, Fe/A1203 and Fe/SiO2 aerogels and a ternary aerogel V-Ni/MgO were studied. Table 20 shows the results and points out that the best catalyst was the ternary one. In Table 21 is a comparison between the activities of the triply

26O T A B L E 20

Comparison of catalytic activities of various aerogel catalysts for ammonia synthesis (from ref. 52)

Catalysts

3.1% 3.0% 5.7% 5.0% 3.1% 3.1% 2.6% 2.5%

V/MgO Ni/MgO Fe/A1203 Fe/Si02 Ni+3.3% V/MgO Ni+3.3% V/MgO V + 11.6% P t / M g O V / M g O mixed with 10.2% P t / M g O

Rate (moles NH3) (h gme~l)

Temperature reaction

1.41" 10-~ 1.0" 10 - s 1.42.10 -5 2.64-10 -4 1.82"10 -4

S V ( h - 1) a

( °C )

Hydrogen to nitrogen ratio

400 400 400 400 400 390 400

3 3 3 3 3 1.5 3

2.500 2.500 2.500 2.500 2.500 1.875 2.500

400

3

2.500

aSpace velocity. T A B L E 21

Comparison of the activity of the ternary aerogel catalyst with that of an industrial triply promoted iron catalyst in ammonia synthesis (from ref. 52 ) Catalyst

Temperature of reaction ( ° C )

Hydrogen to nitrogen ratio

Z (% NH3)

S V ( h - 1)

3.1% N i + 3 . 3 % V / M g O

400 400

3 3

0.053 0.048

2.500 30.000

Triply promoted iron

promoted iron catalyst and the ternary aerogel working under the same conditions. The last catalyst was very stable with time on stream. From Table 21 it can be seen that taking into account the space velocities (S,V.), the percentage of ammonia (Z) produced by the aerogel is of the same order of magnitude as the one given by the commercial catalyst (at S V = 2500 the commercial catalyst percentage would be 0.166 to be compared to 0.053 for the aerogel when one applies the well known rule Z 2 × S V = constant for a promoted iron catalyst) [112]. The hydrogenolysis of ethylbenzene into benzene, toluene and methane was performed at 335 ° C on a Ni-A12Oa aerogel in a fixed bed flow reactor [ 12 ]. No cyclohexane was detected, and only minute traces of ethane were registered. Table 22 gives data for the aerogel catalysts as well as for pure nickel (exoxalate ). A titania aerogel was tested in the photocatalytic partial oxidation of paraffins and olefins by Formenti et al. [ 113 ]. TiO2 aerogel was under the form of porous anatase and with isobutane it gave acetone with a selectivity of 58%.

261 TABLE 22 Surface properties of the catalyst Ni A2 and those of pure metallic nickel and their catalytic activities (from ref. 12) Sample

% Ni by weight

Surface area (m2/g)

Metal surface area (m2/g)Ni

Da (A)

SBb (% )

SENc (% )

Ni A2 Ni

28.8 100

650 1.7

31 1.7

70 4000

18 14

1 6

aMean particle diameter determined by magnetic measurements. bSelectivity in the formation of benzene. CSelectivity in the rupture of aromatic nucleus.

But here it seems that porosity is not a positive factor in order to develop the best catalytic properties which are found with the various non porous titanias for the same reactions. A Zr02 aerogel prepared in this laboratory was studied as a catalyst for hydrogenation and/or isomerization reaction of ethylene, methylcyclopropane and 1-butene after having been activated either thermally or by the hydrogen spillover procedure [114 ]. The Zr02 sample was able to hydrogenate ethylene and but-l-ene at 200°C when it was activated by spillover [104]. A detailed study of the hydrogenation and isomerization of 1-butene conducted between room temperature and 200 ° C indicated that the reaction mechanism was analogous to that proposed by Gerberich and Hall [115 ]. A conversion of 90% and a c i s - t o - t r a n s ratio of 0.6 were recorded when this aerogel was thermally activated under vacuum [ 114 ]. Recently Droege et al. [116 ] prepared a series of mixed silica aerogels containing lanthanum and niobium oxides, exhibiting BET surface areas of 700 to 800 m2/g before reaction. These catalysts were used at 800 °C for the oxidative coupling conversion of methane by oxygen with a methane-to-oxygen ratio of 3 : 1 at atmospheric pressure. For La-SiO2 aerogels, a methane conversion of 20% was recorded and the selectivity to C2 was 30%. Nb-Si02 were as active as the La-Si02 catalysts but less selective in C2 showing selectivities only in the range of 8%. Ternary aerogels La-Nb-SiO2 gave results similar to those recorded with La-SiO2 aerogels (selectivity in C2 40% and methane conversion 13% ). A new way of making silica aerogels using a sonochemical technique was reported by De La Rosa-Fox et al. [117]. This process does not require the presence of a solvent any longer. The tetraethoxysilane and water acidified mixture was simply submitted to the action of ultrasound and the sonogel was finally evacuated in hypercritical conditions. It is likely that this new approach can be also extended to the preparation of other aerogel catalysts.

262 8. CONCLUSIONS

The sol-gel technique provides a unique method in order to develop very highly divided materials that can find multiple applications in the ever growing domain of catalysis. The conventional ways used to dry these gels lead to the xerogels, but as it was clearly shown by Glaves et al. [118] the drying step is critical if one wishes to obtain such divided solids. Using an in situ NMR method these authors showed clearly that the pore structure and the surface area of a still wet gel collapsed gradually during the drying treatment. For example a silicagel xerogel sample lost 1300 m2/g of surface area during drying from the stage where it was wet (1500 m2/g) to that where it was completely dried (200 m2/g).

The aerogel method seems to be a unique one since it gives solids with large developed textural properties which can be conserved at the temperatures that are generally selected for catalytic reactions. As Armor et al. [66] suggested, aerogels should be evaluated in hydrotreatment processes such as hydro-denitrogenation and -desulphurization because of their very large pore volumes and good stabilities. Aerogels can possibly find applications as components in solid superacid catalysts as Yamaguchi [ 119 ] recently pointed out the advantage of using large surface area supports for superacids. Aerogels (and probably cryogels too) provide a way to make catalysts to specifications. In particular metal-on support catalysts prepared under this form are very attractive since the sol-gel step permits one "to pour the metal out as a thin film or in any shape that is wanted" according to Droege [120]. At the macroscopic level, aerogels present a monodisperse distribution of particle dimensions which can be reorganized into "dynamical" clusters which are then easily fluidized so that their uses in real conditions are now envisaged and not only restricted at the laboratory scale. At the microscopic level, the mixed aerogels or their parent gels represent very homogeneous combinations originating from chemical interactions at the molecular scale in the liquid phase (or dispersed in a liquid phase) that can be viewed as "frozen" at the solid state. Aerogels are, in this respect, a sort of a three dimensional dry picture of the sol-gel product with all their catalytic potentialities intact and available for revelation. Because aerogels exhibit high solid state chemical reactivities, they can also constitute another (and unusual) source of precursors in order to synthesize new catalysts with large surface areas and pores at relatively lower temperatures than the conventional sintering of mixtures of dry powders. Now, returning to Kistler's original paper where he wrote: "the ability to form an aerogel is a general property of gels. It seems that if there are cases in which it proves impossible to convert a normal gel into an aerogel, these cases

263

will be exceptions" [1], it is the author's wish that this quote may also be repeated about aerogels as catalysts. ACKNOWLEDGEMENTS

The help of Prof. R.J. Willey is gratefully acknowledged by the author.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

S.S. Kistler, J. Phys. Chem., 36 (1932) 52. S.S. Kistler, S. Swann and E.G. Appel, Ind. Eng. Chem., 26 (1934) 388. G.A. Nicolaon and S.J. Teichner, Bull. Soc. Chim. Fr., (1968) 1906. J. Fricke (Editor), Aerogels, Springer Verlag, Berlin, Heidelberg, New York, 1986. R. Vacher, J. Phalippou, J. Pellous and T. Voignier (Editors), 2nd Int. Symposium on Aerogels (ISA 2), Rev. Phys. App., 24 (1989). R.J. Ayen and P.A. Jacobucci, Rev. Chem. Eng., 5 (1988) 157. H.D. Gesser and P.C. Goswami, Chem. Rev, 89 (1989) 765. S. Swann, E.G. Appel and S.S. Kistler, Ind. Eng. Chem., 26 (1934) 1014. H.D. Foster and D.B. Keyes, Ind. Eng. Chem., 29 (1937) 1254. H.E. Riess, Adv. Catal., 4 {1952) 87. G.E.E. Gardes, G.M. Pajonk and S.J. Teichner, J. Catal., 33 (1974) 145. M. Astier, A. Bertrand, D. Bianchi, A. Chenard, G.E.E. Gardes, G. Pajonk, M.B. Taghavi, S.J. Teichner and B. Villemin in B. Delmon, P.A. Jacobs and G. Poncelet {Editors), Studies in Surface Science and Catalysis, Vol. 1, Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 315. G.M. Pajonk, Rev. Phys. App., 24 (1989) 13. S.J. Teichner in J. Fricke (Editor), Aerogels, Springer Verlag, Berlin, 1986, p. 22. S.J. Teichner, Rev. Phys. App., 24 (1989) 1. G.M. Pajonk and S.J. Teichner in J. Fricke (Editor), Aerogels, Springer Verlag, Berlin, 1986, p. 193. G. Pajonk, M.B. Taghavi and S.J. Teichner, Bull. Soc. Chim. Fr., (1975) 983. D. Klvana, J. Chaouki, M. Repellin-Lacroix and G.M. Pajonk, Rev. Phys. App., 24 (1989) 29. G.M. Pajonk, M. Repellin-Lacroix, S. Abouarnadasse, J. Chaouki and D. Klvana, J. Non Cryst. Solids, 121 (1990) 66. W.C. Conner, G.M. Pajonk and S.J. Teichner, Adv. Catal., 34 (1986) 1. C. Sanchez and J. Livage, New J. Chem., 14 (1990) 513. J.B. Peri, J. Phys. Chem., 70 (1966) 2937. K.C. Chen, T. Tsuchiya and J.D. Mackenzie, J. Non Cryst. Solids, 81 (1986) 227. J. Livage, Solid State Chem., 64 (1986) 322. J. Livage, C. Sanchez, M. Henny and S. Doeuff, Solid State Ionics, 32/33 (1989) 633. P.H. Tervari, A.J. Hunt and K.D. Lofftus, Mater. Lett., 3 (1985) 363. P.H. Tervari, A.J. Hunt and K.D. Lofftus in J. Fricke (Editor), Aerogels, Springer Verlag, Berlin, 1986, p.31. C.J. Brinker, K.J. Ward, K.D. Keefer, E. Holupka, P.J. Bray and R.K. Pearson, in J. Fricke (Editor), Aerogels, Springer Verlag, Berlin, 1986, p. 57. C.P. Cheng, P.A. Jacobucci and E.N. Walsh, U.S. Patent, 4 619 908 {1986). F. Graser and A. Strange, U.S. Patent, 4 667 415 (1987).

264 31 E. Degn Egebert and J. EngeU, Rev. Phys. App., 24 (1989) 23. 32 S.J. Teichner, G.A. Nicolaon, M.A. Vicarini and G.E.E. Gardes, Adv. Colloid Interface Sci., 5 (1976) 245. 33 G. Tournier, M. Lacroix-Repellin, G.M. Pajonk and S.J. Teichner, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Studies in Surface Science and Catalysis, Vol. 31, Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 333. 34 K. Ishiguro, T. Ishikawa, N. Kakuta, A. Ueno, Y. Mintarai and T. Kamo, J. Catal., 123 (1990) 523. 35 K. Tanabe, Mater. Chem. Phys., 13 (1985) 347. 36 R.T.K. Baker, S.J. Tauster and J.A. Dumesic (Editors), ACS Symposium Series, Strong Metal-Support Interactions, Am. Chem. Soc., Washington, D.C., 1986, p. 298. 37 Z. Paal and P.G. Menon (Editors), Hydrogen Effect in Catalysis, Marcel Dekker, New York, 1988. 38 S. Abouarnadasse, G.M. Pajonk and S.J. Teichner, App. Catal., 16 {1985) 237. 39 M.J. Phillips and M. Ternan (Editors), Proc. 9th Int. Congr. Catal., The Chemical Institute of Canada, Ottawa, 1988, Vol. 4, p. 1936. 40 K. Kearby, S.S. Kistler and S. Swann, Ind. Eng. Chem., 30 {1938) 1082. 41 J.N. Armor and E.J. Carlson, App. Catal., 19 {1985) 327. 42 D. Bianchi, H. Batis-Landoulsi, C.O. Bennett, G.M. Pajonk, P. Vergnon and S.J. Teichner, Bull. Soc. Chim. Fr., (1981) 345. 43 H. Zarrouk, A. Ghorbel, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1982) 71. 44 F. Blanchard, B. Pommier, J.P. Reymond and S.J. Teichner, in G. Poncelet, P. Grange and P.A. Jacobs (Editors), Studies in Surface Science and Catalysis, Vol. 16, Preparation of Catalysts III, Elsevier, Amsterdam, 1983, p. 395. 45 M. Astier, A. Bertrand and S.J. Teichner, Bull. Soc. Chim. Fr., (1980) 191. 46 G.M. Pajonk and B. Pommier, to be published. 47 G.M. Pajonk and S.J. Teichner in L. Cerveny (Editor), Studies in Surface Science and Catalysis, Vol. 27, Catalytic Hydrogenation, Elsevier, Amsterdam, 1986, p. 277. 48 G.E.E. Gardes, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1976) 1321. 49 G.E.E. Gardes, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1976) 1327. 50 S. Abouarnadasse, G.M. Pajonk, J.E. Germain and S.J. Teichner, Appl. Catal., 9 (1984) 119. 51 A. Sayari, A. Ghorbel, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1981) 7. 52 R. Marinangelli, G.M. Pajonk and S.J. Teichner, in Proc. 7th Simposio Iberoamericano de Catalisis, La Plata, Argentina, 1980, p. 126. 53 A. Sayari, A. Ghorbel, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., {1981) 220. 54 S. Abouarnadasse, G.M. Pajonk and S.J. Teichner, in Actas Xe Simposio Iberoamericano de Catalisis, Merida, Venezuela, 1986, Vol. II, p. 615. 55 T. Manzalji and G.M. Pajonk, submitted paper. 56 A.J. Fanelli, J.V. Burlew and G.B. Marsh, J. Catal., 116 (1989) 318. 57 G. Centi, F. Trifiro, A. Vaccari, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1981) 1. 58 M. Rahman, R.J. Willey and S.J. Teichner, Appl. Catal., 36 (1988) 209. 59 R.J. Willey, J. Olmstead, V. Djuhadi and S.J. Teichner, in M.M.J. Treacy, J.M. Thomas and J.M. White (Editors), Microstructure and Properties of Catalysts, Mat. Res. Soc., 1988, p. 359. 60 M.I. Baraton, T. Merle-Mejean, P. Quintard and V. Lorenzelli, Rev. Phys. App., 24 (1989) 239. 61 M.I. Baraton, T. Merle-Mejean, P. Quintard and V. Lorenzelli, J. Phys. Chem., 94 (1990) 5930. 62 H. Versteghem, A.R. Di Giampolo and A. Dauger, J. Mater. Sci. Lett., 6 (1987) 1187. 63 M.B. Taghavi, G.M. Pajonk and S.J. Teichner, J. Colloid Interface Sci., 71 (1979) 45.

265 64 D. Klvana, J. Chaouki, D. Kusohorsky, C. Chavarie and G.M. Pajonk, Appl. Catal., 42 ( 1988 ) 121. 65 M. Astier, A. Bertrand and S.J. Teichner, Bull. Soc. Chim. Fr., (1980) 218. 66 J.N. Armor, E.J. Carlson and P.M. Zambri, Appl. Catal., 19 (1985) 339. 67 B. Pommier and S.J. Teichner in M.J. Phillips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis, The Chemical Institute of Canada, Ottawa, 1988, Vol. 2, p. 610. 68 C. Chavarie, K. Dobson, R. Clift and J.P.K. Seville, Proc. 37th Cong. Chem. Eng., Montreal, Canada, 1987, p. 110. 69 J.N. Armor, E.J. Carlson and W.C. Conner, React. Solids, 3 (1987) 155. 70 J.N. Armor and E.J. Carlson, U.S. Patent, 4 469 816 (1984). 71 E.J. Carlson, J.N. Armor, W.J. Cunningham and A.M. Smith, U.S. Patent, 4 828 818 (1989). 72 J. Li, R. Legros, C.M.H. Brereton, J.R. Grace and J. Chaouki, Powder Technol., 60 (1990) 121. 73 J.N. Armor and E.J. Carlson, J. Mater. Sci., 22 (1987) 2549. 74 J.N. Armor, E.J. Carlson and G. Carrasquillo, Mater. Lett., 4 (1986) 373. 75 H. Zarrouk, A. Ghorbel, G.M. Pajonk and S.J. Teichner, in Proc. 9th Simp. Iberoamericano de Catalise, Lisbon, Protugal, 1984, p. 339. 76 M. Lacroix, G. Pajonk and S.J. Teichner, in T. Seiyama and K. Tanabe (Editors), Studies in Surface Science and Catalysis, Vol. 7, New Horizons in Catalysis, Elsevier, Amsterdam. 77 G.M. Pajonk, in G. Centi andF. Trifiro, (Editors), Studies in Surface Science and Catalysis, Vol. 55, New Developments in Selective Oxidation, Elsevier, Amsterdam, 1990, p. 229. 78 A. Sayari and A. Ghorbel, React. Kinet. Catal. Lett., 15 (1980) 401. 79 D. Klvana, G.M. Pajonk, C. Chavarie and G. Pepin in Proc. VIth Int. Symp. Heterog. Catalysis, Sofia, Bulgaria, 1987, Vol. 1, p. 54. 80 D. Kunii and O. Levenspiel, in "Fluidization Engineering", J. Wiley, New York, 1969. 81 C. Chavarie, G.M. Pajonk, D. Klvana, J. Chaouki and S.J. Teichner, in Proc. 2nd World Cong. Chem. Eng., Montreal, Canada, 1981, Vol. III, p. 82. 82 J. Chaouki, C. Chavarie, D. Klvana and G.M. Pajonk, Powder Technol., 43 (1985) 117. 83 D. Klvana, J. Chaouki, C. Lauga, C. Chavarie, D. Kusohorsky and G.M. Pajonk, in Fluidization VII, Eng. Found Ed., New York, 1989, p. 211. 84 C. Lauga, J. Chaouki, D. Klvana and C. Chavarie, Powder Technol., 1990, accepted paper. 85 D. Geldart, Powder Technol., 7 (1973) 285. 86 J. Visser, Powder Technol., 58 (1989) 1. 87 G. Molerus, Powder Technol., 33 (1982) 81. 88 C. Brereton, J. Chaouki, J.R. Grace, R. Legros and J. Yeung, in Proc. 37th Congr. Chem. Eng., Montreal, Canada, 1987, p. 113. 89 G. Matis, F. Juillet and S.J. Teichner, Bull. Soc. Chim. Fr., (1976) 1633. 90 G.M. Pajonk, J. Chem. Phys., in press. 91 A. Sayari, A. Ghorbel, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., (1981) 16. 92 S. Abouarnadasse, G.M. Pajonk and S.J. Teichner in "Heterogeneous Catalysis and Fine Chemicals" M. Guisnet et al. Eds, Elsevier, 1988, p. 371. 93 F. Zidan, G. Pajonk, J.E. Germain and S.J. Teichner, Bull. Soc. Chim. Fr., (1977) 1021. 94 F. Zidan, G. Pajonk, J.E. Germain and S.J. Teichner in "Proceed 7th Int. Vac. Cong. and 3rd Int. Conf. on Solid Surf." Vienna, Austria, 1977, p. 853. 95 F. Zidan, G. Pajonk, J.E. Germain and S.J. Teichner, J. Catal., 52 (1978) 133. 96 A. Sayari, A. Ghorbel, G.M. Pajonk and S.J. Teichner, Bull. Soc. Chim. Ft., (1982) 39. 97 A. Sayari and A. Ghorbel, in Proc. Conf. Chem. Kuwait, 1983, p. 303. 98 S. Abouarnadasse, G.M. Pajonk, S.J. Teichner and J.E. Germain, Can. J. Chem. Eng., 62 (1984) 521. 99 S. Zine, A. Sayari and A. Ghorbel, Can. J. Chem. Eng., 65 (1987) 127. 100 M. Rahman, R.J. Willey and S.J. Teichner, Rev. Phys. Appl., 24 (1989) 7.

266 101 102 103 104 105 106 107 108

109 ll0 111 112 113 114 115 116 117 118 119 120

J. Chaouki, C. Chavarie, D. Klvana and G.M. Pajonk, Appl. Catal., 21 (1986) 187. J. Chaouki, C. Chavarie, D. K1vana and G.M. Pajonk, Can. J. Chem. Eng., 64 (1986) 440. M. Che and C.O. Bennett, Adv. Catal., 36 (1989) 55. G.M. Pajonk and A. E1 Tanany, in Proc. 12th Simposio Iberoamericano de Catalise, Rio de Janeiro, IBP, Brazil, 1990, Vol. 2, p. 341. M. Lacroix, G.M. Pajonk and S.J. Teichner, J. Catal., 101 (1986) 314. F. Blanchard, B. Pommier, J.P. Reymond and S.J. Teichner, J. Mol. Catal., 17 (1982) 171. B. Pommier, J.P. Reymond and S.J. Teichner, Z. Phys. Chem., 144 (1985) 203. B. Pommier, J.P. Reymond and S.J. Teichner in S. Kaliaguine and A. Mahay (Editors), Studies in Surface Science and Catalysis, Vol. 19, Catalysis on The Energy Scene, 1984, p. 471. R.J. Wiley, H. Lai and J.B. Peri, J. Catal., accepted paper. J.L. Zhao, R.J. Willey and R.W. Pekala, Presented at the MRS Full Symposium, Boston, Mass., U.S.A., Nov. 1990. R.W. Pekala and F.M. Kong, Rev. Phys. App., 24 (1989) 33. A. Nielsen, Adv. Catal., 5 (1953) 1. M. Formenti, F. Juillet, P. Meriaudeau and S.J. Teichner, Bull. Soc. Chim. Fr., (1972) 69. G.M. Pajonk and A. E1 Tanany, to be published. H.R. Gerberich and W. K. Hall, J. Catal., 5 (1966) 99. M.W. Droege, L.M. Hair, W.J. Pitz and C.K. Westbrook, Lawrence Livermore National Laboratory UCRL 100768 preprint, June 1989. M. De La Rosa-Fox, L. Esquivias and J. Zarzycki, Rev. Phys. App., 24 (1989) 233. C.L. Glaves, J. Brinker, D.M. Smith and P.J. Davis, Chem. Mater., 1 (1989) 34. T, Yamaguchi, Appl. Cat., 61 (1990) 1. M. Droege, Chem. Eng., (1990) 11.