Significance of mesoporous crystals for catalytic application

Studies in Surface Science and Catalysis 148 Terasaki (Editor) 9 2004 Elsevier B.V. All rights reserved.

163

Significance of mesoporous crystals for catalytic application John Meurig Thomas ~'b and Robert Raja c

aDepartment of Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. E-mail" [email protected] bDavy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W 1S 4BS, U.K. CDepartment of Chemistry University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K. E-mail" [email protected] ABSTRACT Ordered mesoporous silicas offer unprecedented opportunities not only in the preparation and clarification of the mode of action of well-defined, single-site catalytically active centres on high-area solid surfaces, they also afford means of designing and producing new, high-performance heterogeneous catalysts, some of which are already of considerable commercial significance. The large, adjustable pores of these ordered silicas, with their high surface density of pendant silanol groups, readily permit bulky organometallic precursors to be uniformly anchored on their inner walls and, by subsequent gentle thermolysis, to be converted to well-defined active sites, the process of conversion as well as the ensuing catalysis being followed by in situ X-ray absorption and FTIR spectroscopy thereby elucidating the precise mechanism of the catalysis. A particular example is the TiW-centred epoxidation of olefins, a process that is already of significance in sustainable development, since the unsaturated components (e.g. fatty acid esters) of sunflower and soya bean oils (and other plant sources) may be converted industrially to useful products (by HO-Ti-(OSi-)3 active centres). Bulky mixed-metal carbonylate precursors (typified by [RuloPt2C2(CO)28]2-) may also be readily converted to nanoparticle (1 to 1.5

164

nm dia.) catalysts at the interior surfaces of mesoporous silicas. These exhibit exceptionally high activities and selectivities in low-temperature hydrogenations, many of which may be carried out under solvent-free conditions. A potentially important conversion of muconic acid (derived from plant sources) into adipic acid can be effected on Ru~0Pt2 nanoparticle hydrogenation catalysts. The adjustable curvatures and diameters of mesoporous silica permit asymmetric organometallic catalysts to be anchored in a spatially constrained manner, thereby leading to enhanced performance enantioselective catalysts, again of potential commercial significance. In view of some of the practical disadvantages (e.g. stability, cost, decomposition products and other factors {see Kleitz et al, Micropor. Mesopor. Mat., 44 (2001) 95}) of organic-template-derived mesoporous silicas, there is merit in using (for potential industrial applications), less wellordered mesoporous silicas, also of sharply defined pore diameters, for enantioselective hydrogenations. Specialized techniques of high-resolution electron microscopy (e.g. scanning transmission electron tomography and high-angle annular dark field imaging) as well as in situ XAFS studies conducted in parallel with XRD measurements are required fully to characterize most of our catalysts. 1.

INTRODUCTION

In the context of catalysis, the full impact of the enfergence of mesoporous silicas that occurred in the early 1990s is best appreciated if one first briefly recalls the quite exciting situation that prevailed at that time concerning zeolitic and other microporous solids belonging to the aluminosilicate and aluminophosphate families. Unlike bulk metal, alloy and other binary solid catalysts, openstructure aluminosilicates (embracing natural and especially synthetic zeolites) as well as open-structure aluminophosphates (A1POs), particularly framework-substituted variants (MA1POs where M - Mg, Co, Mn, Zn, ...) are prime examples of uniform heterogeneous catalysts. Not only are the active sites in these catalysts distributed in a spatially uniform manner [1], they also conform to Langmuir's classic assumption in that the energy released upon the uptake of adsorbate species is constant up to monolayer coverage. This energetic uniformity is seen vividly in the calorimetric work cited in Fig. 1. where there is a constancy in the heat of adsorption of pyridine on the Brrnsted acid sites (of pentasil zeolitic catalysts) up until all

165

the sites are neutralized (i.e. up to monolayer coverage of the catalytically active acid centres). Such constancy is never seen with metal catalysts because even a single-crystal face has a number of closely-spaced, distinct energetic sites; moreover the various individual sites are so close together in a metal surface that mutual repulsion causes a diminution in heat of adsorption with increasing coverage. Calorimetric measurements (see Fig. 1.) again reveal the nature of the surface, which is clearly not uniform energetically nor spatially. In zeolitic (and MA1PO) catalysts, the active sites are so far a p a r t - and the higher the Si/A1 ratio in a pentasil, acidic zeolite, the greater the separation distance between equivalent -Si-O(H)-A1- that they effectively behave as "single-site" catalysts. Likewise, in the shapeselective, Br0nsted acid catalysts of the MA1PO-18 family [2], for example, where the active sites are the protons loosely attached to the oxygens adjacent to the doubly-charged M H ions (typically Co I~, Zn II, Mg u, etc) that occupy a small percent of the sites normally occupied by framework AIm ions, there is again spatial and energetic uniformity in the active sites. And even for other types of zeolitic catalysts, such as "ship-in-bottle" (or "tea bag") ones (where for example Cu-perchlorophthalocyanine [3] entities are incarcerated in zeolite Y or cobalt salophen [4] are encapsulated in zeolite Y, used respectively for the aerial oxidation of methane to methanol and primary or secondary alcohols to aldehydes or ketones), the notion of well-separated "single-site" active centres remains valid. A l t h o u g h - up until the early 1990s - microporous zeolites and MA1PO-type catalysts, because of their open structures, offered ready access of many reactants to, and egress of products away from, the active sites, their pore diameters (always less than 10 A) were so small that they prevented any progress to be made in catalytically converting molecules other than those of modest size and shape. Strenuous efforts were made up to the 1990s, using bulky organic templates- many designed with inventive ingenuity- to generate larger-pore zeolitic and MA1PO-type catalysts. But this endeavor, now seems, in retrospect, to have been chimerical. No one succeeded to generate stable (de-templated) microporous catalysts possessing pore dimensions much greater than 10 A. On reading the work of U.S.-based workers at the Mobil Co., on surfactant-mediated synthesis of the M41S family [5] and also of the Japanese (Tokyo-Waseda) groups on folded sheet silicates (FSM) derived from the synthetic mineral kanemite [6], one sensed immediately that the stage was set for a dramatic surge in the teeming availability of new types of heterogeneous catalysts that could not have hitherto been contemplated. It was not simply a case of the door being opened to enable one to process,

166

catalytically, in a petrochemical sense, bulkier hydrocarbons for cracking and reforming than had been possible hitherto with the so-called "large-pore" zeolites typified by faujasite, one could now also entertain many other challenging and important possibilities. For example, one had previously yearned to conduct subtle, elegant, possibly enantioselective conversions, or to oxidize as well as to hydrogenate selectively molecules of importance in fine-chemical, agrochemical and pharmaceutical contexts. P

e -D i

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-6

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Typical locations of the BrSnsted acid sites (-=AIO(H)Si---) are indicated by filled circles

jmm-m---m-m 9 m-m--m

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9

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Apparent coverage (ML)

Fig. 1. (Top) In the acidic zeolitic catalyst H-ZSM-5 (HxSin-xAlxO2n, with Si/A1 ranging from 10 to 500), the active centres are widely separated and are never less than 0.6 nm from one another. In this uniform catalyst, each active site has the same enthalpy of adsorption of pyridine up to their total coverage (after Parrillo et al, Appl. Catal. A., 110 (1994) 67). (Bottom fight) The (111) face of a fcc metal has approximately ten times as many adsorption sites {of three distinct kinds, top (T), bridged (B), hole (H)} per unit area than any surface of H-ZSM-5. This energetic non-uniformity is reflected in the decline of the enthalpy of adsorption with coverage. (Bottom left data refer to CO adsorbed on Pt{ 111 }, after Yeo et al, J. Chem. Phys., 106 (1997) 393).

167

There was also the intriguing thought that, with such well-ordered, well-defined, mesoporous silicas, the large internal areas of which were replete with silanol groups, it should at least be possible to compare the catalytic performance of a single-site homogeneous catalyst with that very same catalytically active site when anchored (and hence rendered heterogeneous) on the surface of a mesoporous silica. (This hope was indeed later fulfilled as described in Section 5 below). But numerous other thoughts occurred to one. Indeed, when, in 1994, Galen Stucky, Ferdi Schtith and co-workers published [7] in Nature their work on mesoporous silica the Editor of that journal asked me (JMT) to write [8] an accompanying News and Views, an excerpt from which I reproduce below: "The prospect of producing open-structure networks of a wide variety of inorganic materials, with apertures in the range 20 to 200 ,~ diameter, is brought a stage nearer by the work of Galen Stucky, Ferdi Schiith and their co-workers, reported on page 317 of this issue. Such mesoporous ordered solids are likely to be of great practical value in a host of applications in the physical and biological sciences, in engineering and conceivably in medicine. Their mode of formation also sheds new light on the important phenomenon of biomineralization." 2.

THE PRACTICAL ADVANTAGES OF MESOPOROUS SILICA

Although the theme of this Symposium encompasses mesoporous crystals in general, it is prudent to emphasize that, in a catalytic context, mesoporous s i l i c a - rather than any other mesoporous s o l i d - has, so far, had the profoundest consequences. This is partly because silica itself has most of the attributes of an ideal catalyst support. The merits of having available a higharea silica with well-defined pore dimensions are obviously attractive features to the catalyst merchant. In addition, owing to its excellent thermal and chemical stability, ease of handling and profusion of exposed silanol groups, silica is ideal for the heterogenization of molecular catalysts. Moreover, silica as a support has a rather rigid structure and does not swell in solvents, so that it may conveniently be used at both high and low temperature and at high pressure, in sharp contrast to most organic supports (e.g. styrenes). Its very inflexibility and non-compressibility makes silicatethered (i.e. anchored) catalysts suitable for use in continuous-flow reactors, which is a potentially key factor in chemical engineering situations. As has been recognized by others, site-isolation of anchored molecular catalysts (or

168

of any active catalyst derived from a bulky anchored precursor) can be more carefully defined on silica than on a flexible polymer backbone, but the catalyst (or ligand) loading must not be too high so as to maintain the condition of active site isolation. Another key advantage possessed by silica is that a number of the elements of the Periodic T a b l e - not just obvious candidates like germanium, carbon, aluminium or gallium but also a range of transition-metal ions may be substitutionally incorporated in place of silicon into its structure, thereby transforming it into an active catalyst of a kind governed by the nature of the substituting element. This was obvious [8] from the outset: "The feasibility of synthesizing mesoporous silicas with some redox element such as titanium incorporated into their inner surfaces is an idea that has occurred to several groups [9-11]. After all, the remarkable properties of titanium silicalite, in which TiIv ions are inserted into microporous silica thereby converting it into a highly selective oxidation catalyst for the generation of catechol and hydroquinone from phenol is already [12] harnessed industrially by the Enichem Company in Italy, where such work first began. Corma and his colleagues [10], using their Ti-incorporated mesoporous silica (pore diameter approximately 20 ~) have selectively oxidized rather large organic molecules such as norbornene." But what was perhaps most attractive of all (as judged in early 1990s) about mesoporous s i l i c a - provided it possessed the necessary mechanical strength and thermal stability in air and in contact with solvents - was the high surface concentration of pendant silanol groups (in the range 1 to 2 OH per 1 nm2). This meant that a clear route, easily trodden by preparative chemists, existed for introducing an almost limitless range of large organic, and in particular organometallic moieties into the inner walls of high-area "crystalline" silica supports. By appropriate modification of their conditions of preparation laid down by the American and Japanese instigators of mesoporous crystals, it soon became possible to prepare mesoporous silica-alumina as was done early on by Bellussi et al [13]. And indeed it proved relatively easy, by judicious choice of the alkoxide of a particular element, to incorporate traces of transition-metal ions (notably titanium [9-11], but also chromium, vanadium [14] and other ions) into the walls of MCM-41 silicas. Such preparative ease stimulated a number of interesting catalytic ventures.

169

3.

SOME INITIAL CATALYTIC MESOPOROUS SILICA

APPLICATIONS

OF

Two preparative approaches were pursued: (i) the incorporation [14] during growth of 'redox' or 'acid' catalytically active sites into the (thin) walls of the mesoporous silica; and (ii) post growth grafting (or anchoring or tethering, the terms are used as synonyms here) of the catalytic centre on to the inner surfaces of the mesoporous silica. The catalytic activity of mesoporous silica-alumina (i.e. acidic solids) of the MCM-41 type reported by Bellussi et al [13] generated some early optimism because of their encouraging performance in propene oligomerization (to produce gasoline and middle distillates with unique selectivity towards C9 and C~2 hydrocarbons, as this is a significant component of industrial catalysis). Soon, however, the limitations in stability and general mechanical and thermal ruggedness of these filigree versions of (MCM-41) mesoporous catalysts became apparent. It is now known that comparatively little real headway has been made in the last decade on mesoporous silica-aluminas in high-temperature catalytic applications, where hydrothermal stability of the catalyst is a key requirement [15]. For milder applications, however, such as low-temperature alkylations [ 16], mesoporous silica-aluminas are viable replacements for mineral acids as industrial Brtinsted acid catalysts. The catalytic activity, particularly in selective oxidation, of Ti TM ions embedded within the walls of MCM-41 mesoporous silicas was studied in several early investigations [9-11]. And modest catalytic activity, but very high selectivities were reported, as illustrated in Fig. 2. A significant advance was made when Ti TM active centres were grafted on to the inner surfaces of mesoporous silicas using an organometallic precursor, in particular titanocene dichloride (Ti(Cp)2C12) as was done by Maschmeyer et al [17]. The key steps in the introduction of the isolated, single-site, active centers on to the inner walls of MCM-41 are shown in Fig. 3. The detailed course of this "heterogenization" of a Ti rv active centre was followed by in situ X-ray absorption spectroscopy combined with in situ Xray diffractometry [ 18]. (The former to track the precise environment of the Ti, its valence state and its extent of coordination to neighbouring atoms; the latter to record whether the crystallinity of the mesoporous host is retained). Two important points emerged from this work. First, the "halfsandwich" Ti TM compound, where one of the cyclopentadienes is retained, serves as a device to secure 'single-site' Ti TM active centres.

170

~

Fig. 2a. Computer-graphic representation of a titanium-containing mesoporous silica catalyst which selectively oxidizes 2,5-di-tert.butyl phenol into the corresponding quinone in the presence of H202 (after Thomas [8]). Fig. 2b. Illustration (after Thomas and Greaves, Science, 265 (1994) 1675) showing ease of titanium-catalyzed epoxidation of cyclohexene.

171

triethylamine

Ti(cP)2 surface grafted

l ,.et. amine

Ti(cP)2CI 2 inside MCM-41

0

,

9

2

,, t . , ' . ;

oT

9

,wq

q

H-O-Ti(OSi)3

cp lost to give Ti(cp) on surface

Fig. 3. The grafting of TirV-centred active sites to the inner walls of mesoporous silica occurs via the interaction of titanocene dichloride (Ti(Cp)2CI2) and pendant silanol groups. A half-sandwich surface compound (bottom fight) forms as an intermediate. (Colour code: Ti, O, Ci, H = white).

@ Ti - R'OH

@0

1"1~ intermediate

0 " 0 9

HOOR'

- R'OH

112 intermediate

Fig. 4. Mechanism of epoxidation of cyclohexene by an alkyl hydroperoxide (HOOR') catalyzed by the TirV-active centre tripodally bound to silica. In situ XAFS and DFT calculations indicate two plausible surface intermediates (111 and 1"12)(Thomas et al. [33]).

172

Table 1 Cyclohexene epoxidation at 298 K in acetonitrile [32] Catalyst Ti'I'MCM41 Ti'I'MCM41 Ti'~MCM41 Ti---~MCM41 Ti---~MCM41 Ti---~MCM41 MCM41 MCM41 MCM41

Oxidant Cyclohexene Otherlb] Selectivity TOF[c] oxide (%)[al products(%) w.r.t, epoxide h"1 TBHP 13.5 0.7 95.2 16.4 MPPH 36.3 0.3 99.2 37.3 air 0 0 0 0 TBHP 1.8 0.1 94.4 1.5 MPPH 3.3 0.5 88.3 2.5 air 0 0 0 0 TBHP 0 0.7 0 MPPH 0 0 0 air 0 0 0 -

[a] Percentages given are based on the amount of cyclohexene converted to cyclohexene oxide. [b] Other products derived from cyclohexene and in the case of MPPH reactions from benzyl radicals. [c] Number of moles of epoxide produced per mole TiTMper hour. Since this half-sandwich is an essential feature of the process of heterogenization there is no possibility for the formation of dimeric (or higher) Ti TM compounds as is invariably the case when so-called isolated Ti centres are prepared via aqueous solutions. Second, the EXAFS results (together with in situ FTIR) proves beyond doubt that the catalytic site is Ti TM in tetrahedral coordination and that the Ti TM ion is tripodally connected, via oxygens, to the surface of the silica as represented in Fig. 4. (It is noteworthy that our EXAFS results ruled out a previously proposed [20] "titanyl" (>Ti=O) structure for the active site). If we symbolize the catalyst in which Ti TM centres are embedded (during growth) as Ti---~MCM-41 and the catalyst in which the Ti TM centres are grafted to the walls as TiI"MCM-41, we may compare their catalytic activity with one another, and with the Ti/SiO2 catalysts used by the Shell Co. to epoxidize propene to propylene oxide [21,22]. If, furthermore, we compare the catalytic performance of these two TiIV-centred, mesoporous catalysts in a typical epoxidation reaction, where cyclohexene is epoxidized either by tert. butyl hydroperoxide (TBHP) or by 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH), the results (Table 1) unmistakeably reveal that the 'grafted' Ti TM centre is superior, by a factor of ten or so, in its activity. (In fact, by comparing the Til'MCM-41 with any other TilV-centred catalyst,

173

including the industrially used preparation, we find that the grafting method yields the best ever recorded catalytic performance). 3.1.

Other initial ventures

Three other early endeavours to capitalize upon the merits of the availability of mesoporous silicas are outlined in this section: (i) the design of a cobalt oxo-centred catalyst grafted on to silica to achieve the selective, low-temperature oxidation of cyclohexane to cyclohexanone [23]; (ii) sulfonic acid functionalized mesoporous silicas as catalysts for condensation and esterification reactions [24]; and (iii) the production of organic bases (as mild catalysts) attached to the inner walls of mesoporous silica via alkylsiloxy chains. The catalytic activation, especially partial oxidation, of alkanes constitutes one of the major challenges of present-day chemistry; and the conversion of cyclohexane to cyclohexanone is among the principal target reactions since the latter is used as a feedstock in several industrial processes, including the production of nylon from e-caprolactam and adipic acid [2529]. Maschmeyer et al [23] took as a point of departure the fact that several oxo-centred trimeric cobalt (III) acetates (coordinated with pyridine [30]) exhibit considerably more activity in selectively oxidizing the tertiary C-H bond in adamantane than their dimeric analogues. They therefore grafted the following species: Co3(~3-O)(OAc)5(~t2-OH)(py)3 on to the inner walls of MCM-41 and monitored changes in its structure (by in situ EXAFS and XRD [18]) during its use in the oxidation of cyclohexane with tert. butyl hydroperoxide (to yield tert. butanol and a mixture of cyclohexanone and cyclohexanol). 29Si MASNMR spectroscopy was also used to identify precisely the nature of the immobilization of the catalyst. Interesting results were obtained" there was appreciable catalysis, during the course of which EXAFS studies revealed a significant change in the structure of the oxocentred Co m t-rimer. By functionalizing mesoporous silica with sulfonic acid groups, Van Rhijn et al [24] produced catalytic materials that were very effective for the formation of bisfurylalkanes and polyol esters. An outline of the nature of these catalysts is shown in Fig. 5.

174

OH I

O-S-O I 1. neutral H202 2.0.2 M H2SO4 3. rinsing

Ib=

(CH2)3 Si

Fig. 5. Van Rhijn et al's method [24] of producing sulfonic acid functionalized mesoporous silica catalysts for condensation and esterifications. One of the reactions that they catalyse is

~0~-~ 2

Me

+ Me\ / c ---0 Me

Me

Me

Me which is the formation of 2,2-bis(5-methylfuryl) propane. (A bisfurylalkane of this kind is a key intermediate for macromolecular chemistry). Neither the acidic forms of zeolite Y or zeolite Beta are of any use for this reaction: they each yield tarry oligomeric products, which promptly deactivate the zeolitic catalysts. It seems that the hydrophobic (functionalized) surface of the mesoporous silica prevents too strong an adsorption and oligomerization of 2-methylfuran, while its larger dimension facilitates product desorption. The MCM-SO3H (coated) catalyst of Van Rhijn achieves greater than 80 percent conversion to the bisfurylalkane with 95 percent selectivity towards the desired product. By using the procedure outlined in Fig. 6., amine or diamine functions may be directly grafted to the mesoporous silica. The Knoevenagel condensation (of active methylene compounds of the type Z-CH2-Z') with aldehydes or ketones yields olefinic products (such as R'RC=CZZ ' ) using these amine-functionalized silicas [31 ].

175 (RO)sSi/~/~NH2

G

/

Fig. 6. Brunel et al [31] converted micelle-templated silicas (MTS) into catalysts rich in amine or diamine functions.

4.

ILLUSTRATIVE CASE HISTORIES" A SUMMARY

Here we deal first (Section 5) with TiW-catalyzed (mesoporous solids) for selective oxidation, focussing mainly, but not exclusively, on epoxidation. Apart from shedding much light on the principles of catalytic action, it transpires that these TiIV-centred catalysts play an increasing role in sustainable development in that they can convert abundantly available feedstocks such as fatty acid methyl esters (obtained from plant sources exemplified by sunflower oil and soya bean oil) as well as the vast family of naturally-occurring terpenes into desirable products for the polymer, fabrics and foodstuffs industries. We then describe some other transition-metal-ion (mesoporous silica) catalysts, which again exhibit good performance, and which are also examples of "single-site" heterogeneous catalysts. In Section 6 we focus on the scope offered by mesoporous silicas to design and produce novel enantioselective catalysts, which are of great commercial potential. Here we show how chemical advantage of the concavity of the mesopores may be exploited to enhance the enantioselectivity of a chiral catalyst grafted on to the walls of the mesoporous silica. Finally, in Section 7 we summarize the great advantages offered by mesoporous silicas as support for bimetallic nanocatalysts that are extremely active in a variety of selective hydrogenation reactions. Again it will emerge that sustainable development looms significantly with these catalytic variants since some of the intermediate products of biocatalytic conversion of corn and other plant material may, by selective hydrogenation, be converted to bulk chemicals such as adipic acid, which has wide use in nylon and other textile manufacture and in the foodstuffs industry. In

176

addition, many of the selective hydrogenations, leading to commercially important products, may be effected in a solvent-free fashion, a procedure that is environmentally benign. 0

TIW-CENTRED SELECTIVE OXIDATION CATALYSTS

As explained elsewhere [17,19,32-36] our in situ (XAFS aided by FTIR and UV-Vis) studies of the TiW-centred active site at the internal surfaces of mesoporous silica (grafted via Ti(Cp)2C12) produced an unambiguous picture both of the tetrahedrally coordinated metal ion and paved the way to a deeper understanding of the mechanism of the epoxidation of alkenes by peroxidic reagents. Moreover, by knowing precisely the atomic environment of the active centre, it became possible to boost further its catalytic activity. For example, one of the three silicons in mesoporous silica to which the Ti w is linked, tripodally via oxygen atoms, may be replaced by germanium, thereby boosting the catalytic performance [37]. Furthermore, owing to our knowledge of the atomic environment of the TiW-centred active site in the heterogeneous catalyst, soluble molecular analogues - the so-called silsesquioxanes also possessing well-defined (single) Ti TM active sites (see Fig. 7 . ) - could be prepared, and their catalytic performance directly compared with their heterogeneous analogues. Very seldom, if ever, is it possible to make a direct comparison of the catalytic performance of a particular active site which has essentially the same atomic architecture in the heterogeneous and homogeneous case. This comparison, (facilitated by the use of pre-edge and near-edge X-ray absorption spectroscopy and by molecular dynamics calculations) provides [38] quantitative information pertaining to the tetrahedrally coordinated active site (see Table 2 and Fig. 7). Table 2 Comparison of the performance of insoluble heterogeneous, single-site Tiw/SiO2 epoxidation catalysts with their homogeneous soluble molecular analoguestal Homogeneous Catalysts

Heterogeneous Catalysts

TOF (h~)

Til"sio2 Til"MCM41 Til'Gel"MCM41

52 26 34 40

18

[c-CsH9)7Si7012Ti(OSiPh3)] [c-CsH9)7Si7OI2Ti(OGePh3)]

[a] See Ref [38] for reaction conditions.

177 Ge

.

Ti

0.8 [

.

0.7

-o- Til"Gel"MCM-41 -o- Ti'I'MCM-41 -x- T

9 o O

Si

/

~0.5

O H

0.4 --Si

1

~ 0.3

Si

,~ 0.2

R--Si'O~r ~ \,~ _J'~-S' ~ -O"~XTi / ""~ o . . x , . o

R

I~-I0"1 o:

R

.. 0

100

.

.

200

300

400

Time I min

Fig. 7. (Top left) The performance of a TiIV-centred catalyst grafted on silica (Til"MCM41) is less than that of a grained catalyst in which one of the three silicons (in HOTi(OSi)3) is replaced by Ge (Til"Gel"MCM-41). Both are superior to an ordinary Til"SiO2 catalyst. The activity (Table 2) of the heterogeneous catalysts may be directly compared with analogous homogeneous catalysts (prepared from an appropriate silsesquioxane; R (_cC5H9)) (bottom let~) {see [38] and S. Krijnen et al, Angew. Chem. Int. Ed. Engl., 37 (1998) 356; Phys. Chem. Chem. Phys., 1 (1999) 361 }.

Attfield et al [39] showed that grafting Ti-(OSiPh3)4 onto the internal surface of MCM-41 (without further calcination) produces an epoxidation catalyst with high activity and high selectivity. This arises because the presence of the phenyl groups stabilizes the catalytic Ti TM centres towards attack from atmospheric moisture. Interestingly, the elegant work of Tilley and his colleagues [40-43], who have pioneered the so-called molecular precursor strategy for control of catalyst structure (to arrive, as with the Ti(Cp)zC12 precursor at well-spaced, single sites) also found that when they grafted-OSi(OtBu)3 groups on tO their SBA-15 specimens of mesoporous silica (without calcination) they too observed enhanced stability in their Ti TM catalytically active sites. (We shall return to Tilley's method of preparing highly effective, atomically dispersed active sites on mesoporous silica below not least because it is applicable to other transition-metal-ions besides titanium- but it is instructive to emphasize here the advantages of using tris(tert-butoxy)siloxy titanium complexes to generate single-site catalysts). Note, for example, that in the molecular entity Ti[OSi(OtBu)3]4 there is -

178

already built into this precursor the stoichiometry and environment (i.e. tetrahedrally coordinated Ti surrounded by four OSi- groups) that is desired in the ultimate active catalyst. (These complexes react with the pendant silanol groups of MCM-41 or SBA- 15).

5.1.

Grafted TiIV-centred catalysts for the epoxidation of fatty acid methyl esters [44]

Epoxidized fatty acids and their derivatives have been used for many commercial applications such as plasticizers and stabilizers in chlorinecontaining resins, as additives in lubricants, as components in thermosetting plastics, in urethane foams and as wood impregnants. Vegetable oils and fats are renewable sources of two popular unsaturated fatty methyl esters" methyl(Z)-9-octadecanoate (methyl oleate, structure 1 in Fig. 8. and methyl-(E)-9octadecanoate, methyl elaidate, structure 2). In the past, an environmentally unfriendly "peracid" method was used to epoxidize the naturally occurring unsaturated compounds.

Fi~; 8. Both methyl oleate (1) and methyl elaidate (2) are completely epoxidized using Ti'*-grafted catalysts and tert. butyl hydroperoxide (3_) as oxidant (atter Guidotti et al [44]).

179

Now, however, as Ravasio and her coworkers have shown [44], the TiW-grafted active site on mesoporous silica (via Ti(Cp)2(C1)2 [17]) is an excellent and environmentally friendly method for converting the fatty acid methyl esters (FAME) into their epoxides. These workers have recently shown that the TiW-grafted catalyst also effectively converts the (doubly) unsaturated components of soya bean oil into useful epoxides- another important step towards sustainable development.

5.2.

Grafted TiW-centred catalysts for the epoxidation of terpenes [45] Major sources of terpenes (which are natural products, the structure of which is built up from isoprene units) are balsams, natural resins and essential oils, but they are also by-products of lemon- and orange-juice production as well as of the pulp and paper industries. Some terpenes, notably (-)-a-pinene and (+)-limonene are among the more readily (naturally) available optically active substances and are therefore used for the syntheses of other optically active products. Here again it is obvious that catalysts capable of efficiently functionalizing terpenes are of value in the context of sustainable development. The work of Ravasio and her collaborators [45] has shown that the Til"MCM-41 (grafted) catalyst (derived from Ti(Cp)2(C1)2 [17]) is particularly good in epoxidizing such important terpenes as ot-terpinol, carveol and limonene (see Table 3) under mild conditions (i.e. at ca 85 ~ using tert. butyl hydroperoxide, TBHP, in CH3CN). Indeed, in harmony with earlier work on the epoxidation of cyclohexene [32] also using TBHP, the Til'MCM-41 surpasses the activity of the sol-gel grown Ti~MCM-41 by a factor of ten in the case of the ~t-terpinol, the main constituent of pine oil. Table 3 Turnover frequency (TOF) of terpene epoxidation on Ti---~MCM-41 and Til"MCM-41 Substrate ot-terpineol carveol limonene T = 85~

TOF (h- 1) Ti---~ 2 15 4

Ti~" 20 33 20

CH3CN solvent; 30 % wt catalyst; TBHP:terpene mole ratio = 1

Judging by the results of other workers who have compared the catalytic performance of Til"MCM-41 (i.e. the gratted variety) with that of

180

the Ti---~MCM-41 (sol-gel preparation) there is no doubt of the superiority of the former. Thus, in their study of the hydroxylation of benzene in the liquid phase (using aqueous H202) He et al [46] found both higher activity and enhanced selectivity to phenol (as well as greater chemical stability) with the grafted catalyst. 5.3.

Other transition-metal ion, single-site catalysts supported on mesoporous silica Shortly after the titanocene method of introducing isolated TiW-centred active sites at the surfaces of mesoporous silica was introduced [17], the method was applied with success to the production of molybdenum and vanadyl centres (also on to MCM-41)- see Fig. 9. Mo w active centres on silica are good catalysts for the oxidative dehydrogenation of methanol to formaldehyde [47]. Likewise vanadium (Vv) centres on mesoporous silica are good catalysts for the epoxidation of alkenes and for oxidation of alkanes to alcohols and ketones [48]. Maschmeyer et al [49] subsequently used other Ti-containing precursors to produce novel siliceous high-area supports, such as TUD-1, in which both mesopores and micropores were present. These materials were prepared (without any involvement of micelles or alkylammonium ions as templates) using metal-complexes of a benign kind.

\osi

sio

sio*"

osi

M= Mo~C ~

oHt

Cp2MC~

M= Ti ,,_ M~

mesoporous silica

~ 101

M SiO''"/ ~OSi SiO

M

SiO

SiO~*""/x'"OSi SiO

l'~s

OSi

.MI

i

9 SiO

Fig. 9. Single-site selective oxidation catalysts on mesoporous silica may be formed from their parent cyclopentadiene analogues (see Refs. [17], [46] and [47]).

181

A different approach, alluded to earlier, was pioneered by Tilley et al [40-43, 50-57] in which a molecular precursor route is taken to arrive at a series of active catalysts on mesoporous (and certain other) supports. The metal ions in question cover those of Ti, Cr, Fe and vanadyl. And the essence of their preparation is that the desired atomic environment aimed at in the final catalyst (e.g. Ti-(OSi)4 or Ti-(OSi)3) is already present in the socalled thermolytic molecular precursor. Thus, by taking as the precursor (iprO)Ti[OSi(OtBu)3]3 the environment ultimately achieved in the single-site catalyst is Ti-(OSi)3, and from the precursor Ti[OSi(OtBu)3]4 it is Ti-(OSi)4 [40]. Typical supports used by Tilley were the high-area mesoporous silicas MCM-41 and SBA-15, the latter being distinctly more thermally stable (owing to its thicker walls) even though their activity was roughly equal [40]. o

o

II

Ii

!.

?

o l..-"" OH

OH silica

l

.

HOtBu

OH

i

s,,i;; .... OH ]

+

M[OSi(OtBu)~n

surface

l

"HOSi(O~}u)3

'Buo

'BuOI~.Si/O" M[OSi(O~u)3]n.1 I OH O OH OH l ~HI O~ OH

M[OSi(OtBu)3]~l

/

OH

O

OH

OH

,fill I

" H20' "CH2=CMe2

I

I A

OH

Fig. 10. The Tilley method [40,43] of preparing single-site catalysts on mesoporous silica via thermolytic molecular precursors such as M[OSi(OtBu)3]n.

182

A general picture is given in Fig. 10. The precursor is bonded to the hydroxyl groups of the surface of the silica via protonolysis reactions. For the case of an alkoxy(siloxy) species of the type M[OSi(OtBu)3]n, where M Ti, Fe, Cr, ..... this surface-attachment chemistry occurs with loss of HOtBu or HOSi(OtBu)3, to result in bonding to the surface through M-O-(surface) or Si-O-(surface) linkages, respectively. Calcination then leads to the highly dispersed supported metal of nominal composition MOx(n-1)SiO2. A typical situation, relevant to the case of isolated Fe atoms at some silica surfaces (namely xerogels, but applicable in principle to mesoporous silica) is shown in Fig. 11. The activities and selectivities of this catalyst for selective oxidation of three reactants with H202 are also shown [58]. (tBuO)3SiO~, ~OSi(OtBu)3 < ~ /Fe 9" (tBuO)3SiO ~ ' O ' ~ t.../

OH OH OH OH j OH OH OH I

= i

i

=

-HOSi,OtBu)3

SBA-15 silica surface 1.0 OH nm "2

molecular precursor and spectroscopic model

tBuO

'Buo I.

~

OtBu

/. o'Bu tBu~'O/~l(.,Ire-/S'-O~)\OtBu "o" \

OH o.~S i

OH OH

~

"O

,,Si~ O ~- O

. CH2=CMe2

well-defined, isolated sites 0.23 Fe nm "2

isolated, pseudo-tetrahedral O-Fe(OSiO3) sites selective oxidation catalysts for various organic compounds with H202: selectivity

Q

~

~--OH

O

--(30

TOF, mol (tool Fe)"1$.1

100%

2.5 x 10.3

99%

6.2 x 10"4

100%

1.2 x 10.2

Fig. 11. Single-site 'Fe' catalysts on silica exhibit good activity and selectivity [56,58]

Nowotny et al [59], extending the work of others [60] on rhodiumcatalyzed hydroformylations (in which an alkene and a mixture of CO and H2 are catalytically converted to an aldehyde), compared the behaviour of Rh(II) dinuclear complexes when they were separately grafted on to ordinary silica

183 and on to MCM-41 mesoporous silica.

The dinuclear complex was

[Rh2(~t-PC )2(~t-O2CR)2] where ~t-PC is a bridging ortho-metalated arylphosphane ligand (see Fig. 12.). The performance of the immobilized catalysts 3 and 4 (Fig. 12.) was studied using styrene and 1-decene. The chemoselectivity towards the formation of aldehyde products was nearly quantitative in all the experiments employing styrene. Some catalyst leaching took place from each support, and the drop in activity was appreciably less in the case of the complex gratted inside the MCM-41.

OH3

I _..Rh~

I

I j.O I ..ah

CH=

Rh~ I ~ O ,--~-'-~T~i ,,ah toluene / HOAc

CH3

~

CH3

A

1

endo-X

R

X

Ph

2

I Si02 E0~~0 / Si(CH2)2

3

I MCM-41-E~O~SilCH2)2 0"

4

Fig. 12. This dinuclear Rh compound galled on silica (see text) smoothly hydroformylates styrene to its linear aldehyde.

To summarize, we show in Fig. 13. the many oxidative reactions (of considerable industrial significance) of unsaturated and saturated hydrocarbons that may be effected by transition-metal ion, single-site catalysts supported on mesoporous silica.

184

O

+ TBHP~

~

C

~ 0

~HC=CH.--~----I~ z i~ H + HaOz

~~H202

CO+ H2

-I- TBHP.~---...ID- ~ 0

O + l'IzOz~ ~OH

~L j~~'--~ + TBHP O (CH2)'O ~ ~"~C8H'170 --O-"~ ( C H , ) 7 . ~ ~

CH3OH~

HCHO~

H H H I I I H--C--C~C~H ~ t

I

CeH,,

Mesoporous Silica

Oz

H ~#.=m~mm=,m"~~ I "~_/H 1 H--C--C--C,. 1

I

H H H highselectivity

I I "H

H H

moderate yields OH

/ '

j~"

I P 0

0

,T Y~ H--C--C--CmH ~ l I I H H H

(lowyields)

~ § Hz02~[~

Y /H H--C~C"--C \H i i H H

+T B H P~

~0

~ _ , 0~+H~02

high activity poor selectivity

Fig. 13. A selection of the important selective oxidations that may be effected by a range of metal-centred, single-site catalysts grafted on mesoporous silica.

@ DESIGNING CHIRAL CATALYSTS CONFINED WITHIN MESOPOROUS SILICA: THEIR SUPERIOR PERFORMANCE RELATIVE TO HOMOGENEOUS ANALOGUES In the pharmaceutical and agrochemical industries, as well as in the expanding fields of fine chemicals generally [61], which encompasses fragrances and flavors, there is a growing demand for enantiomerically pure products, driven in part by ever-more exacting legislation and in part by stringent scientific criteria. To date, the asymmetric catalysts employed both on the laboratory and industrial scale have been homogeneous, largely because these possess well-defined, single-site active centres. No one doubts that, from the standpoints of ease of separation of products and regeneration of the catalyst, heterogeneous asymmetric catalysts would be far superior to their homogeneous counterparts. The cost alone of the sophisticated chiral ligands often exceeds that of the noble metal employed, so that catalyst

185

recovery is of cardinal importance for the application of enantioselective metal-centred catalysis to large-scale processes (particularly in continuousflow reactors) [62]. The problem, however, has been that, hitherto, almost all attempts to heterogenize (by immobilization on an appropriate support) homogeneous chiral catalysts has led to poor performance, principally because a spectrum [63] of different kinds of active site was generated by the very act of heterogenization. We recognized [64-67] quite early on that mesoporous silica, because of its large pores and profusion of functionable, pendant silanol groups presented unprecedented opportunities for designing powerful new types of chiral catalysts in which advantage could be taken of the spatial restrictions (for prochiral reactants) that exist after grafting and confining asymmetric (homogeneous) catalysts in the pores and channels of mesoporous silica. In other words, with such supports, no longer would a spectrum of active sites result: single sites would prevail.

6.1.

Strategic principle

The large-diameter channels of MCM-41 family (see Fig. 13.) prompted us to graft quite sizeable chiral metal complexes and organometallic moieties on to the inner walls of these high surface area solids (see section 3.1 and ref. [23] above) by a variety of ways that included functionalizing pendant silanols with organic groups such as alkyl halides, amines, carboxylates and phosphanes. This opened the way to the preparation of novel catalysts consisting of quite large (surface) concentrations of accessible, well-spaced, and structurally well-defined active sites (As outlined in Section 9 below, the whole panoply of in situ and ex situ techniques of characterization, embracing spectroscopy, resonance and diffraction of diverse kinds could and were deployed for such purposes [67,68]). One expected, and we did indeed find, as shown in Sections 6.2 and 6.3, that such heterogeneous solid catalysts behave at least as efficiently as their homogeneous counterparts and sometimes with far superior enantioor regioselectivity. Various kinds of organometallic, chiral catalysts may be tethered to the inner walls of a mesoporous silica employing the strategy illustrated in Fig. 14. The key features here are the reactant's (i.e. the substrate's) interaction with both the pore walls and the chiral directing group. The confinement of the reactant (substrate) within the mesopore should lead to a larger influence of the chiral directing group on the orientation of the substrate (reactant) relative to the reactive catalytic centre when compared to the situation in solution.

186

Tether of variable length

Mesoporous silica

i~ ~, .

"ChiralSpace"

Through-Space Interactions

Fig. 14. Schematic representation of the confinement concept in which the substrate is incarcerated in the cavity of a chiral modified mesoporous host and leads to chiral heterogeneous catalysis [65].

6.2.

Proof of principle in allylic amination

To test the idea encapsulated in Fig. 14., we first decided to investigate the allylic amination reaction between cinnamyl acetate and benzylamine. This reaction has two possible products: a straight-chain one (which is favored as a result of the retention of the delocalized n system) and a chiralbranched one (Scheme 1):

Ph/.....~-.-.,.../OAc +

PhCH2NH2

Ph/,.,,,,~'-.,,,./N HCH2Ph THF, 313 K

..{.

[cat]* PhCH2NH

H

Scheme 1. The allylic amination of cinnamyl acetate and benzylamine

The aim of the reaction is to produce (with an effective chiral catalyst) the greatest possible yield of the branched product with the highest possible

187

enantiomeric excess (ee). Three related chiral catalysts were chosen: one homogeneous; another the same homogeneous catalyst grafted on to a convex, non-porous silica surface (such as the commercial product known as Cabosil); and yet another the same chiral homogeneous catalyst grafted on to the inner walls of mesoporous (MCM-41) silica.

R O/Sk~

o/

I ~ Si"'O"V

7

Ph% t Ph

Ph% ~,Ph

/ S i\

/ S iN

I

Fe

? ?Si ?Si x ..Si_ ? ~Si O-,o "O',o "~

\"~0

/

R : CH2CH2CH2Br

.,Ip~

Me\

MezN~r

N Ph-, F'2 Me - r e

I

/

ph, pF'~I_ e 7 ,

c~" \ ~

Ph-,P"

'

tt

Me

Ph2 3

,/

"f" ,

/

,

t7"~1"/~" N ~

N* Ph- P ~

!

.

/

"'"

""

""

Me

Me ,

,

~

9CABOSIL . . . - / " ./. s ~ . / ~ , N . . , , . v1~ ~ oPh2Pl] L e Me

" .-" f'"'/ . /'/ 7,

.-

/

/

/ //

,"

/

7

-

Ph2 5

." iv

4

Fig. 15. Sequence of steps showing the immobilization of N-[l',2-bis(diphenylphosphanyl)ferrocenyl]-ethyl-N,N'-dimethylethylenediamine (2) in its chirally constrained and unconstrained (5) states. R - (CH2)3Br

188

We demonstrated [69] that a chiral ligand derived from 1,1'bis(diphenylphosphino)ferrocene (dppf) bonded to an active metal centre (Pd II) and tethered, via a molecular link of appropriate length, to the inner walls of a mesoporous silica (MCM-41 of c__aa30 A diameter) yields a degree of catalytic regioselectivity as well as an e__gethat is far superior to either the homogeneous counterpart or the Cabosil-bound catalyst (We chose a chiral chelate based on dppf for several reasons: first, its planar chirality never undergoes racemization; second, it is synthetically very accessible; and third, dppf possesses functionalities suitable for reaction with pore-bound tethers). Care was taken to ensure that all activity is confined to the internal surface of the mesoporous silica. This was achieved by selectively deactivating the external surface of the support. Our overall approach to the comparisons between the three systems is summarized in Fig. 15. from which it is seen that the mesoporous framework was first treated with Ph2SiC12 to deactivate the exterior walls of the MCM-41sample. The interior walls of this material were then derivatized with 3-bromopropyltrichlorosilane to give the "prepared" MCM-41 designated I in Fig. 15.

dppf [_c_(CsH9)7Si7012]

IFe IPd OCI Osi dppf-diaminePd-catalyst

OH

Conv (%)

StraightChain (%)

Branched (%)

ee (%)

Homogeneous

76

99+

Tethered-Silica

98

98

2

43

MCM-41-Confined

99+

50

50

95

Fig. 16. Whereas the homogeneous dppf chiral catalyst (top left) yields no branched product (and no enantioselectivity) and the non-porous-silica tethered dppf catalyst yields but a small amount of the branched product, the spatially constrained form produces a substantial branched form and a high value of ee.

189

The ferrocenyl-based ligand (S)-l-[(R)-l',2-bis(diphenylphosphino)ferrocenyl]ethyl-N,N'-dimeth-ylethylenediamine 2, was prepared by literature methods. On treatment of the activated MCM-41 with 1 with an excess of 2, the chiral catalytic precursor 4 is produced, and this, on reaction with PdCI2CH3CN gives the required catalyst 6. A separate related procedure yielded the Cabosil-supported catalyst 7. The grafted chiral catalyst 6 was fully characterized by MASNMR and EXAFS spectroscopy [67,68]. The mesopore-confined catalyst showed an enantioselectivity for superior to that of both the homogeneous (only linear product) and Cabosil-tethered analogue as shown in Fig. 16.

6.3.

Exploiting confined hydrogenations

chiral

catalysts

for

enantioselective

Having established the principle for the case of allylic amination, we then proceeded to take advantage of asymmetric catalysts grafted inside mesoporous silica to a number of industrially important hydrogenations.

6.3.1. Conversion of ethyl nicotinate to ethyl nipecotinate [70] 0

0

2H2

~

H2

PdJC ....

ethyl nicotinate

0

~

O ~

1,4,5,6-tetrahydronicotinate

Pd/C

chiralmodzfier

ethyl nipecotinate

Scheme 2. The two-step hydrogenation of ethyl nicotinate to ethyl nipecotinate Ethyl nipecotinate is an important intermediate in biological and medicinal transformations. Previous efforts to hydrogenate enantioselectively an aromatic ring such as that in ethyl nicotinate had resulted [71 ] in values of e_~ethat were less than 6 percent, but a two-step (Scheme 2) process (using a cinchonidine modified Pd catalyst supported on carbon) raised the e~ to 19 percent at a conversion of 12 percent [72]. Using our Pd(dppf)-chiral catalyst confined within the 30 A pores of mesoporous MCM-41 we achieved (in a single-step) conversions in excess of 50 percent with an e~ of 17. The Pd(dppf) catalyst was also grafted to the vertex of an incompletely condensed silsesquioxane cube [73] (compare Fig. 7 above), the idea here being to

190 create a soluble (homogeneous) analogue of our confined chiral catalyst attached to a (O--Si--O)n framework. This catalyst resulted in a racemic product, thereby proving the chiral advantage achieved by confinement of the Pd(dppf) active centre inside siliceous mesopores (Fig. 17.).

II1

'

Me

cAaos,t. "'~Si

.

/

ph2

dpp f-ferrocenyl-

Substrate

t

Conv

ee

diamine-Pd-Catalyst

(reactant)

(h)

(%)

(%)

Homogeneous (I)

ethyl

72

15.9

nicotinate

120

27.2

Tethered-Silica (II)

Tethered to MCM-41and confined inside mesopore (Ill)

,l'll

ethyl

72

,2.6

2

nicotinate

120

19.2

2

ethyl

48

35.5

i7

72

53.7

20

nicotinate

O P d ~o c N 0 Cl C) H OP OSi 0 Fe 0

0

Fig. 17. The dppf catalyst confined within mesoporous silica yields substantial ee of the desired nipecotinate product. Neither the homogeneous chiral catalyst not its nonconstrained, tethered form (on Cabosil) yield any significant ee of the nipecotinate.

6.3.2 Other enantioselective hydrogenations using confined, chiral diamino-type ligands [74-76] Comparatively few reports have hitherto been published in which Rh ~ or Pd" asymmetric complexes without phosphane ligands have been used to activate hydrogen, but a growing number employing nitrogen-containing ligands has appeared of late for the purpose of enantioselective conversions [77]. The chiral ligands shown in Fig. 18. have been used by us with Rh t as the metallic

192

The catalyst itself is pseudo-square-planar where the Rh I is bonded to 1,5cyclooctadiene (COD). The hydrogenations (Scheme 3) investigated by us were:

~, / / \

H

Ph

CO2H

I-t2[cat]

, . _

y -

Ph

H~ /CO2H / \ ,

Ph

Ph

OH

Ph/

~(OMe

I[ 2 [cat]

~

OMe Ph

0 O

Scheme 3. Schematic representation of the hydrogenations of E-ot-phenylcinnamic acid and methyl benzoylformate.

By grafting the R h I chiral complex on both a concave silica (using MCM-41, 30 A diameter) and a convex silica (a non-porous silica) we established beyond doubt that the spatial restrictions imposed by the concave surface at which the active centre was located enhances the enantioselectivity of the catalyst- see Fig. 20.

silica

concave

193

silica

convex

Fig. 20 a. Graphical model (to scale) showing the constraint at the catalyst (see Fig. 19) when anchored on a concave silica in contrast to the situation on a convex (Cabosil) surface.

100-

8O

"5 60

-~,,,,,,,,,~,

~

#,,,,,,,,, '

!!l,! !lI!

ii II I

ose,e~~ iI

t.-

40

2O r

I/

0

Heterogeneous (concave)

Heterogeneous (convex)

Fig. 20 b. The chiral diamine organometallic catalyst constrained at a concave silica surface surpasses the performance (selectivity and ee) of the same catalyst (shown in Fig. 20 a.) attached to a convex, non-porous silica.

194

(C).

A deeper analysis of the beneficial use of asymmetric organometallic catalysts constrained within mesoporous silica [78] Convinced of the merit (in enantioselective syntheses and other organic processes) of constraining asymmetric organometallic catalysts within siliceous nanopores (so as to increase the interaction between the pore wall and the active centre and hence to restrict access of reactant to the catalyst) we embarked on a systematic study [78] in which a range of porous silicas were investigated. In each of these there is a very narrow spread of pore diameter. Rather than employing as porous siliceous supports the organic-template derived (MCM-41 or SBA-15) varieties, a set of commercially available desiccant silicas having narrow pore size distributions (Fig. 21.) (designated Davison 923, 634 and 654). (These are made by reacting sodium silicate with a strong mineral acid (usually sulfuric acid); the pore-size being controlled by gel time, final pH, temperature, concentration of reactants, etc). Compared to MCM-41 type silicas they are much lower in cost, more thermally and mechanically stable, less susceptible to structural collapse and available in a range of granularities. They also have some intersecting pores that facilitate the diffusion of the reactant species to the immobilized catalyst. The average diameters [79] of the pores of these silicas is, respectively, 38, 60 and 250 .A,, and their respective surface areas are 700, 500 and 300 m2g-l.

:

:

:

:

:

: ~(~: Ol~s~rptiOnPori~Volu~haplot

. . . . . . . . . . . . . . . . . . . . . .[

;.. . . . . . . . . . . . .

.~,dVll~ll

'

D

; " ' ; " ; " ~ . 9 9. . . . . . . . . . . . . . . . . . . . . . . .

.

.

.

.

.

.

.

.

.

.................~.............................................. ',~ii ......................... .

I

.

"

.

.

" '

Ave[agePorte Oi~lm0te~r=~!!:.3~8 !A~ ..................

[ .... c. . . . . . . . . . . . . . ~ . . . . . . . . . ~. . . . . . . i . . . . . ~. . . ... . .i . - .- - i.. . . i . . i . . . . . . . . . . . . . . . . . . . . . . . . .

.

.

.

i.

.

o)

E

.................a.!! .... ............_i~i iiiiii! ....... !ii!! .... ......................... i 1

i

-

.................,i ..............

......i.....iiH!!i ........................

! a 0

10

i

:-i-

!iii

~,- ~ i-:i:

Pore DiameterA

i

;~i ~- '- :-i' 100

Fig. 21. Pore-size distribution curve for the mesoporous sample (Davison 923) which has a value of 38 A mean pore diameter.

196 Table 4 Asymmetric hydrogenation of methyl benzoylformate Catalyst

~~

Cat

Et~ / ' ~ ~N R~ " ~

.

Homo Het

.

NH2

Rh(COD)AEP

H2 Ph

t~/

~

HN

~

R

82.6 93.3

542 153

82-~ 77

Dav. 634 I (60 A)

0.5 2.0

67.1 93.9

440 I 65 154 61

Dav. 654

0.5 2.0 2.0

44.6 86.1 69.9

292 141 60

Rh(I)

,

Het

Dav. 923 (38 A)

Rh(I)

Day. 634 (60 A)

0.5 1.0

59.7 75.5

458 290

68 73

Dav. 654 (250 A)

0.5 38.8 2.0 , 83.1 0.5 i 46.2

298 159 145

0 4 53 I

i i

l

0.5 2.0

'

(60 A)

Pd(II) i

' Het

Pd(allyl)PMP

/) ~

MCM-41 i Pd(II) (30 A) ~ , Catalyst Recycled

,

63.0 91.5

i

0.5'60.7 2.0 i 86.9 0.5 96.0

Day. 654 (250 A)

Homo

Pd ~

0.5 ' 92.8 " 643 ! 85 2.0 95.8 166194

Rh(I)

Dav. 923 (38 A) Dav. 634

]

0 0 0

0.5 t 7 7 . 7 1 5 9 6 ' 5 0 2.0 98.1 188 , 79

Rh(I)

~ ~

Rh(COD)PMP

ee

/

0.5 2.0

Dav. 923 (38 A)

Rh(I)

Homo Het

\--

0

Rh(I)

-

Rh(COD)DED

TOF th-l~ 46

,

Homo

NH2 Ph

,nv Conv

t

(h) I 2.0 I

62

(250 A) / N

Metal

Silica (pore dia.)

0,.

!

436 159

72 78

420 65 151 i 59 i 264 55 !

i

0.5 2.0

89.8 98.9

542 149

62

2.0

100

151

66

67

substrate -- 0.5 g; solvent =- 30 ml; catalyst (homogeneous) ~- 10 mg; (heterogeneous) = 50 mg; H2 - 20 bar; T = 313 K; TOF = [(mOlsubstrate-com,erted)(mOlcomplex(maorPd)/siliea)-lhl]

197

In their homogeneous form, only the Rh(COD)PMP and Pd(allyl)PMP exhibit any significant enantioselectivity (ee) under the reaction conditions (see Table 4) employed by us, whereas the other two homogeneous catalysts (namely Rh(COD)AEP and Rh(COD)DED) did not display any significant ee. This is probably because the bulkiness of PMP in comparison to AEP and DED exerts further spatial congestion in the vicinity of the active centre. Table 4 summarizes the results with all four chiral catalysts and shows that, as expected from arguments given above, chiral restriction does indeed boost the ee values in a manner that logically reflects the declining influence of spatial constraint in proceeding from the 38 A to the 60 A to the 250 A pore-diameter silica. For the heterogeneous catalysts the trend with Rh(COD)PMP is mirrored by both AEP and DED ligands and it is clear that even when some of the asymmetric catalysts exhibit significant ee's under homogeneous conditions, their performance is much enhanced when immobilized in a constrained environment It is also noteworthy that the noncovalent method of anchoring the organometallic catalyst does not lead to facile leaching when the catalyst is recycled. Further experimental details are given in patents [82] recently filed by German industry. Although, in general, enzymes are at present more widely used industrially than asymmetric transition-metal complexes for enantioselective catalytic conversions involving pharmaceuticals and agrochemicals, the latter are of increasing importance and are more generally applicable especially for reactions that cannot be catalyzed enzymatically. In commenting on our earlier preliminary work on constrained catalysts, the authors [61 ] of a recent comprehensive text on the role of heterogeneous catalysts in the production of fine chemicals remarked: "This approach seems to hold considerable

promise for meeting the future challenge of developing robust, recyclable catalysts for asymmetric syntheses." We believe that this view is vindicated by our subsequent endeavours. In particular, the convenience of using a noncovalently grafted chiral catalyst has obvious practical merit. 0

7.1.

NANOPARTICLE BIMETALLIC HYDROGENATION CATALYSTS SUPPORTED ON MESOPOROUS SILICA

Physical and chemical characteristics Finely divided metallic and bimetallic particles ranging in size from a few to several thousand atoms have long played an important part in laboratory and industrial catalysis. Two main problems concerned with their preparation and properties were recognized early on: first, the difficulty in preparing 'mono-disperse' nanoparticles (i.e. where they are all of the same

198

size and contain the same number of atoms), and second, their propensity to sinter on the underlying support when subjected to modest temperatures. Traditional methods for preparing nanoparticle catalysts have generally involved the procedure of so-called 'incipient wetness" a solution of the appropriate salt, containing the metal that is ultimately desired as a nanoparticle, is allowed to be absorbed and/or adsorbed (depending on the porosity) by the high-area support, typically alumina, silica or silica-alumina. After thorough drying, reduction and judicious heat-treatment the supported nanoparticles appear. But this method almost invariably produces a distribution of particle sizes, and very often the nanoparticles contain several hundred or so atoms.

25-

o

~Pt

15

0

10 5 0

4_

0

1

2

3

4

5

T

,..,w

,

6

7

8

-~

9

10

Position (nm~

Fig. 23. (Top left) Scanning transmission electron tomograph of mesoporous silica containing Ru~0Pt2 nanoparticle catalysts. (Top right) HAADF images of Rul0Pt2 nanoparticles with the electron-induced X-ray emission peaks (shown in bottom fight) of three individual particles each consisting of 12 atoms (and each weighing 2 zeptograms {2 x 10 -2! g}). Each nanoparticle is separately imaged and its precise composition may be determined from the X-ray spectra. A bimetallic system, involving say, ideally nanoparticles of Cu and Ru in intimate contact with one another may prove very difficult to prepare in

199

this way. In recent years, other workers have used mixed-metal cluster compounds as precursors. Thus Nashner et al [83] used the cluster [PtRus(CO)6] on carbon supports, but they found that the PtRu5 'cores' aggregated to produce relatively uniform nanoparticles ranging in diameter from c___aa8 to 23 ~. And with precursors such as [Re6C(CO)I8{p3-Re(CO)3} {p3-Ir(CO)2}] 2- the resulting material formed separated nanoparticles, the original metal ratio of Re and Ir having been lost. As soon as mesoporous silicas became available, with their pores large enough readily to allow access to quite bulky mixed-metal carbonylate (typically [Ru12C2(CO)32Cu4C12] 2- or [Pd6Ru6(CO)24]2-) and with their internal surfaces rich in silanol groups, it immediately became apparent [84] that a reliable method of introducing well-defined, mono-disperse, uniformly distributed (spatially) nanoparticle bimetallic catalysts consisting of 4 or 6 or 12 or 16 metal atoms (in specific ratios such as RusPt pr Ru~0Pt2) was open to us. Full details are contained in Refs. [84-88]. The nature of the bonding of these nanoparticles, in precise atomic detail, is determined from XAFS spectroscopy, and scanning transmission electron microscopy yields their spatial distribution within the pores. Electron-induced X-ray emission (on individual nanoparticles) reveals [89] the atomic ratio of the constituent elements and scanning transmission electron tomography shows the morphology of the nanoparticles within the nanopores- see Fig. 23. O

[PPN]*[Ru 12Cu4C2(CO)32CI2]2"[pPN]§

Ru

O cu

OP

ON Oc O o O si

io

i

M ~C-~O ..... H~O

I

\ Si

Fig. 24. (Left) A single mesopore replete with its pendant silanol group, with which the carbonyl groups of the mixed-metal carbonylate carbonylate (such as that shown on the fight with its molecular cation, PPN) may form hydrogen bonds of the kind schematized here.

200

If one of the components of the bimetallic nanoparticles is chosen carefully, its oxophilicity (e.g. of Cu or of Pd) secures the bimetallic entity firmly to the support. This endows the nanoparticles with a far greater resistance to sintering when the system is heated- see Fig. 24.

7.2.

High-performance nanocatalysts for single-step hydrogenations

After demonstrating [90,91 ] that we could routinely prepare bimetallic catalyst particles that are (i) so small (1 to 1.5 nm diameter) that essentially all the atoms in them are exposed to reactant species; (ii) so firmly anchored (via S i - O - M bonds with M - Cu, Ag, Pd, etc) to the walls of the mesopores that their tendency to sinter and coalesce is minimal; and (iii) distributed in a spatially uniform manner so as to provide easy diffusional access of reactants to, and egress of products from, the nanoparticles, we proceeded to carry out a number of commercially relevant hydrogenations. Many of these could be effected in a solvent-free, single-step fashion, features that are invaluable in the context of clean technology and green chemistry. These nanoparticle bimetallic catalysts exhibit exceptional activity and are highly selective (as is shown below). Because of their minute size (which confers novel electronic properties upon them) each a t o m - and almost all are at the surfaces of the nanoparticles - is coordinatively unsaturated. Secondly, as was realized earlier by Sinfelt [91 ], bimetallic clusters are vastly superior catalytically to their monocomponent counterparts. This is well illustrated in our work on the hydrogenation of 1-hexene, 1-dodecene or naphthalene, where we examined the activity of bimetallic PdRu and monometallic Ru and Pd nanoparticles, derived from Pd6Ru6, Ru6 carbonylate clusters and a Pd complex, respectively. The turnover frequencies displayed by the Pd6Ru6 nanocluster (in hexene hydrogenation) was a factor of ten or more in excess of those for monometallic Ru or Pd clusters. It is not yet c l e a r - more theoretical study is required- why the synergy between Pd and Ru is so pronounced. But it is relevant to note the well established ability of Ru to activate molecular hydrogen and of Pd to activate the olefinic bond. Bimetallic nanoparticles such as Ru6Pd6, Ru6Sn, Ru~0Pt2, Ru~0Pt, Rul2Cu4 and Rul2Ag4, anchored within mesoporous silica all exhibit high activities and frequently high selectivities, depending upon the composition of the nanoparticles, in a number of single-step (and often solvent-free) hydrogenations at low temperatures (333 to 373 K). The selective hydrogenation of polyenes (such as 1,5,9-cyclododecatriene and 2,5norbornadiene) are especially efficient. Good performance is found with these nanoparticle catalysts in the hydrogenation of dimethyl terephthalate (DMT) to 1,4-cyclohexane dimethanol (CHDM) and of benzoic acid to

201

cyclohexane-l-carboxylic acid, and also in the conversion of benzene to cyclohexene, the latter being an increasingly important reaction in the context of the production of Nylon. Table 5 Single-Step, Highly Active and Selective Nanoparticle Catalysts for the Hydrogenation of Some Key Organic Compounds Catalyst Pd6Ru6/SiO2 Ru6Sn/SiO2 Cu4RuI2/SiO2 Ag4Rul2/SiO2 Pd6Ru6/SiO2 Ru6Sn/SiO2

~-~____ ('-~ ~--~ ~---J+ ~ ~ ~---. ~ . . ~ ~-L.~ ~_~ .~~~LL_ ~ ~

-

t h 8 8 8 8 8 8

Pd6Ru6/SiO2 Ru6Sn/SiO2

f~

-

811176 8 10210

RusPtl/SiO2 Rul0Pt2/SiO2 Pd6Ru6/SiO2

Reaction

f~ ~

/"~ ]~

RusPtl/SiO2 Ru10Pt2/SiO2 Pd6Ru6/SiO2

RusPh/SiO2 Pd6Ru6/SiO2 RusPt~/SiO2 Rul0Pt2/SiO2 Pd6Ru6/SiO2

f~ + ~

-

~~.~~

,~o-c~c-o~ o o HO~~cH,oH

RusPtl/SiO2 Rul0Pt2/SiO2 Pd6Ru6/SiO2 RusPti/SiO2 RuioPt2/SiO2

Solvent

(~coo. l (~coo, ~ ~

24 24 24

167 317 126

Precursorfor caprolactam and nylon

hexadecane hexadecane

24 24

792 1660

~

hexane hexane

24 24

453 512

Paints, waxes, lacquers, polishes and as substitutes for turpentine, Precursor for nylon and ecaporolactam

(C~ooH

C2HsOH C2HsOH C2HsOH

5 5 5

912 965 1012

~ ~

§

H~176 ~ "cooH

Coatings, lactones, polymers 62625 Starting 6 1 7 9 0 materialin 6 3 2 1 6 productionof K-A oil 4 155 Polyester 4 714 fibres, 8 125 polycarbonates polyurethanes

C2HsOH C2HsOH C2HsOH

___. ~ ~ . . . , ,./-~"-'~

oH ~

C2HsOH C2HsOH C2HsOH

TOF Commercial (hl) Significance 2012 Polymer 1 9 8 0 intermediates, 690 ketonesand 465 polyesters 5 3 5 0 Laurolactam, 1 9 4 0 copolyamides, nylon intermediates

Nylon 6,6, gelatins, jams, polyurethanes, lubricants

Catalyst = 20 mg; H2 pressure = 20 bar; TOF = [(mOlsubstr)(mOlcluster)-~h-l]; Mesoporous SiO2 used here is of the MCM-41 type.

Note:

202

Table 5, above, highlights some of these conversions, as well as the remarkably high turnover frequencies (TOF) and an indication of the commercial potential of the reactions for which we have found viable catalysts.

7.3

Adipic acid from sustainable sources via a mesopore-supported Rul0Pt2 nanoparticle catalyst [92] Muconic acid (see Fig. 25.) may be readily produced from corn using a biocatalyst (devised by J.W. Frost [93]). We have discovered a catalyst (nanoparticle of Ru~0Pt2 generated by gentle thermolysis after introducing the carbonylate precursor [RuIoPt2C2(CO)28]2- into mesoporous silica [92]) that is superior to all others (Rh/A1203, Pt/SiO2, Pd6Ru6/SiO2) in converting this acid, in a single-step, to the desired adipic acid. Adipic acid is a major stepping-stone in the production of Nylon and other fabrics and foodstuffs. Hitherto, it has been produced (by a sequence of environmentally aggressive steps [90]) from fossil-fuel sources, in particular benzene (which is converted to cyclohexane that is oxidized to cyclohexanone and cyclohexanol and these are then transformed to adipic acid). It is clear that bimetallic nanocatalysts, allied to the appropriate biocatalyst, have a major role to play in future environmentally benign industrial processes. Very recently [94] a highly selective nanoparticle colloidal catalyst, supported on mesoporous silica, has been developed to hydrogenate phenol preferentially to cyclohexanone (with cyclohexanone:cyclohexanol ratios in excess of 100). This is deemed a major step forward in the context of industrial catalysis.

% trans, transmuconic acid

adipic acid

Rul0Pt2 mesoporous silica

9 Ru 9 Pt

00 Si tt OC

Fig. 25. Schematic diagram illustrating the process of converting trans, trans-muconic acid, derived from glucose, to adipic acid, which is used in the manufacture of nylon. The catalyst is composed of bimetallic Ru~0Pt2nanoparticles anchored via two Pt-O and one Ru-O bonds (established by X-ray absorption) to mesoporous silica (pore diameter 3nm).

203

SO

MESOPOROUS AND NON-POROUS SILICA AS CATALYST SUPPORTS- A COMPARISON

Industrial chemists and laboratory researchers have recognized the great advantages of silica (outlined in Section 2 above) for over a hundred years, but it was not until the early 1970s that organometallic chemists began to explore the silica surface as a possible rigid ligand for their deeper study of catalysis and practical exploitation, especially in polymer science [95]. Twenty or so years ago, Basset and co-workers began a sustained, systematic and highly successful series of studies of surface organometallic chemistry [96-98] in which, inter alia, the transfer of concepts and practices of molecular organometallic chemistry were made to well-defined surfaces, socalled Aerosil silica. Organometallic compounds, especially metal carbonyls, anchored to silica surfaces have also been much investigated by Gates and co-workers [96]. Aerosil silica (produced by the Degussa Co.) has a surface area of some 200 mZgl, and, depending upon the temperature of its formation or subsequent treatment, it may have a variable amount of pendant silanol groups. The surfaces of Aerosil silicas heated to 700 ~ or so are richer in siloxane linkages ( - S i - O - S i - ) than in silanols (=-SiOH). Basset quotes [98] a silanol surface concentration of 0.7 + 0.2 per nm 2 (which is equivalent to 0.23 OH g~ when treated at 700 ~ The first key difference between mesoporous silica and Aerosil, therefore, is the far greater availability of pendant silanols in the former. With a surface area of 800 to 1000 mag~ typical mesoporous silicas have approximately ten times as many pendant silanol groups available for creating single-site, immobilized organometallic catalysts than a typical Aerosil. On the other hand, the thermal and general mechanical stability and lesser aptitude to collapse of Aerosil surpasses that of the MCM-41 family, and, to a lesser degree, of the SBA-15 family. But the mesoporous silica (prepared by the Davison division of the W.R. Grace Co.) matches and often exceeds the thermal and mechanical stability of Aerosil- see Section 6.3 C above). The opportunities for exploring single-site heterogeneous catalysis (of silica-functionalized surfaces) for both Aerosil and mesoporous silica are about e q u a l - and both are very considerable. Basset et al. have already demonstrated [97] the especial merits of using non-porous silica for a range of pure and applied catalysis, embracing inter alia olefin polymerisation, olefin metathesis and even low-temperature hydrogenolysis of alkanes (catalyzed by a tripodally grafted Zr centre to the silica surface [98]). The quintessential difference between our own work on mesoporous silica and the previous work on the non-porous silica is that we have the

204

supreme advantage of being able to exploit the (adjustable) pore diameter (and hence the concavity of the surface) so as to achieve enhanced enantioselectivity using immobilized, organometallic chiral catalysts. In addition, we are also able to capitalize upon the greater surface concentration of silanols (with all that that permits for creating single-site heterogeneous catalysts). Mesoporous silicas have made it possible to winnow the grain of understanding from the chaff of overwhelming (often confusing) evidential fact, thereby deepening our knowledge of the fundamentals of heterogeneous catalysis [99] whilst at the same time opening up important new practical applications of the phenomenon. The preparative breakthroughs that led to the ready production of various kinds of mesoporous silica proved crucial. But equally important have been the techniques of catalyst characterization- some well-established but many of them new - that have placed this area of single-site, heterogeneous catalysis on such a firm platform. Our final section outlines these techniques. 9.

SUMMARY OF THE TECHNIQUES USED CHARACTERIZE MESOPOROUS CRYSTALS CATALYTIC SIGNIFICANCE

A

TO OF

Adequate descriptions are already available [100] concerning the standard methods of characterizing mesoporous (and microporous) solids. These embrace the use of gas adsorption isotherms, low-angle and ordinary X-ray diffraction, scattering methods (of neutrons) and the most popular forms of optical, scanning and transmission electron microscopy. Here we are concerned only with those techniques, not in widespread use as yet, that we ourselves have developed principally to elucidate the nature of nanoporous and nanoparticle catalysts. Ex situ methods include all the conventional, as well as some less commonly used spectroscopic procedures: FTIR; Raman; UV-Vis (Diffuse-reflectance); mass spectrometry; ESR; multi-nuclear (solid-state) MASNMR (especially of ~H, 2H, ~3C, 27A1, 29Si and 3~p); X-ray diffractometry and conventional transmission electron microscopy. We have relied heavily both on conventional high-resolution (HR) transmission electron microscopy [101-104] (TEM) (see Fig. 26) and on high-resolution (HR) scanning transmission electron microscopy (STEM) with their allied technique of electron-induced X-ray emission (for analytical

205 powers down to the zeptogram (10 .2] g) level) [105-107]. The STEM approach readily enables images to be recorded at high-angles and with an annular dark-field (HAADF), where so-called Z-contrast prevails [ 105-107]. (At high scattering angles, Rutherford scattering d o m i n a t e s - intensity is thus proportional to the square of the atomic number, Z). The HAADF method readily identifies (see Fig. 23.) a few isolated atoms (of relative high Z) on a light background, such as s i l i c a - one atom of platinum produces an electron intensity equal to that of four hundred oxygen atoms.

7% thiolsiloxane la 3d 'FDU-5' 'STA- 1 1 ~

,.~,.~ ...... O,5

1

1

2.0

2 "l'he~

FDU-5 STA-11

2.5

3.0

2-5% thiol p 6mm SBA-15

D. Zhao et al. Angew. Chem. Int. Ed. 41 (2002) 3876 A.E. Garcia-Bennett (PhD Thesis, St. Andrews, 2002)

Fig. 26. HRTEM is indispensable in identifying (and solving [108]) the structure of new large-pore mesoporous crystals. The precise phase that forms depends of the conditions of crystallization. With 7 percent thiol-siloxane the STA-11 phase (identical to FDU-5) forms space group Ia 3d; with 2 to 5 percent thiol, the hexagonal phase (p 6mm), i.e. SBA-15, is formed. (Courtesy of Dr. P.A. Wright, St. Andrews University) HRSTEM and HRTEM are uniquely well-suited to explore both the average structure and local infractions or other structural irregularities in the bulk and at the surface of mesoporous silica [102,103] (see Figs. 23. and 26). In the hands of Terasaki et al [108-110], the conventional, transmission electron microscope, through the method of electron crystallography, is

206

capable of solving, de novo and in atomic detail, the structure of new microporous crystals. Tomography is also feasible using STEM HAADF imaging [107,111,112]. In general, this entails reconstructing the three-dimensional structure of an object from an angular series of two-dimensional images (projections). It has enabled us to determine the three-dimensional distribution of bimetallic nanocatalysts within mesopores of silica, and for the elemental composition of each nanoparticle to be evaluated- see Fig. 23. In situ methods of characterizing catalysts have been evolved by us over the years - see Refs. [67,68] - and the most important tool deployed by us for this purpose is the combined use of X-ray absorption spectroscopy (XAFS) (embracing near-edge, i.e. XANES as well as extended-edge, i.e. EXAFS structures) and X-ray diffraction (XRD) (see Fig. 27). When the XAFS is recorded in an energy-dispersive mode (as indicated in Fig. 27.) rapid measurements are possible (giving rise to the acronym QUEXAFS - quick EXAFS [113,114]). The great merit of XAFS is that it can conveniently identify the immediate chemical environment of all elements {with Z above 10}. Bond distances and coordination n u m b e r s - as well as valence states and the degree of flexibility or rigidity of the local structure are retrievable this way. The bonus offered by combining XAFS with XRD is that the entire structural integrity of the mesoporous crystal may be directly assessed [113,114]. (This combination, XAFS-XRD, proved invaluable in tracking the local environment of Co I~ ions prior to and during crystallization of a microporous solid from a nutrient gel [115].) Curved pos,l,on-sens,t,ve detector

Curved energy-dispersing crystal

""

(forXRD)

_.2.._;:..:--_-- : - Photodiode array

(for XAFS)

Fig. 27. A set-up of this kind [ 18] enables the determination of both the immediate atomic environment of the catalytically active site (from XAFS) and the long-range structural integrity of the mesoporous crystal (from XRD) to be recorded in parallel under in situ conditions of catalysis.

207

In the actual testing of catalytic performance use is made of minirobots for the rapid transfer of minute (0.1 ~tl) aliquots of the reactantsproducts from high-pressure, PEEK-lined stainless-steel cells to either GC or LC-MS analysers. This enables kinetic data on conversion and growth and decay of intermediates to be rapidly determined.

We acknowledge the support of the EPSRC (for a rolling grant to JMT), to the Royal Commission for the Exhibition of 1851 (for a Research Fellowship to RR), to Bayer and BP (for financial support) and to all our colleagues, especially Prof. B.F.G. Johnson and Dr. P.A. Midgley, for their cooperation. REFERENCES

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