Zeolitic coatings and their potential use in catalysis

Mlaof%ROUS AM

MESOPOROUS MAT!RI&

Microporousand MesoporousMaterials21 (1998) 213-226

Zeolitic coatings and their potential use in catalysis J.C. Jansen a-*, J.H. Koegler a, H. van Bekkum a, H.P.A. Calis a, CM. van den Bleek a, F. Kapteijn a, J.A. Moulijn a, E.R. Geus b, N. van der Puil ’ a Delft University of Technology, Faculty

of Chemical

Technology and Materials Science, Julianalaan 136, 2628 BL, De& Netherlands b Shell, SRTCA, Posthus 38000, 1030 BN, Amsterdam, Netherlands ’ ABB Lummus Global Inc. 1515 Broud Street, Bloomfield, NJ 07003-3096. USA

,Abstract

The formation of zeolitic coatings and their properties, such as the thickness, continuity and orientation of the crystals, are related to the presence and macro-organization of a precursor phase. Based on this view, preshaped zeolitic coatings can be prepared which may be either active catalysts themselves or an inert thin membrane on an existing catalyst. They can be applied in low pressure drop reactors, in adsorption units, in catalytic distillation units and in integrated reaction separation systems as zeolitic membranes. Coating preparation and particular aspects of the performance of the zeolitic coatings in a membrane configuration, in selective catalytic conversion of NOx, as an inert membrane on a catalyst phase, and model structured catalyst systems are discussed. 0 1998 Elsevier Scien.ce B.V. All rights reserved. Keywords: Membrane; Composite catalyst; Permeation; Synthesis of coatings; Hydrogenation; Alkylation

1. Introduction 1.1. General

In an increasing number of papers the development and interest in the use of zeolitic coatings has become evident [l-3]. The zeoliteesupport composite leads to a performance that is beyond the limitations of unsupported zeolites, by adding mechanical strength, and, depending upon the particular use, improved heat transfer or catalytic activity. Frequently studied and applied zeolites like A, Y, Ferrierite, Mordenite, MFI and Beta have been prepared on different supports [4,5]. * Correspondingauthor. 1387-181 l/98/$19.00 6 1998 Elsevier Science B.V. All rights reserved PII: S1387-181 I (97)00061-9

In general, a zeolite coating on any support can be made in a direct one-step synthesis, or by applying an existing zeolite phase, being either large single crystals [6] or tiny crystallites [7,13], to the support. This paper will be mainly concerned with direct synthesis methods. The compatibility between the zeolite synthesis conditions and the

support material is desirable, although it may be hard to achieve [9, lo]. In Table 1 various types of

support are given together with their physical and chemical properties. A few of the given supports are inert and have no interaction with the zeolite solution or the zeolite crystal phase. Preferable are supports with hydrophilic properties resulting in good wetting with the zeolite synthesis mixture, and having interaction

potential

with the zeolite

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J.C. Jansen et al. / Microporous and Mesoporous Materials 21 (1998) 213-226

Table 1 Supports currently used in the preparation of zeolite coatings Support material

Nature

Surface areaa

Sphereslextrudates a-Al,O, YAW,

hydrophobic hydrophilic

high high

Single crystal wafers

Si TiO, Sapphire (wAl,O,)

relative stability at high pH

-t

hydrophilic hydrophilic hydrophobic

+

Amount of surface OH-groupsb

References

low high

[%I11 [51

high medium low

Plates/disks

Stainless steel Quartz Vitreous glass Pressedcarbon

+

high

+

high high high low

hydrophilic

high

+

low

[IS]

hydrophobic hydrophilic hydrophilic

medium

+

medium

low medium high

II91

+ +

hydrophilic hydrophilic hydrophilic hydrophobic

low low low

Foams

a-A&O, Fibers

Carbon Vegetal Inorganic

WI PI 1

Inerts

Gold Teflon

hydrophobic hydrophobic

1221 ~23I

a Low: < 1 m2 g-i; medium: l-10 m* g ‘; high: > IO m2 g-i, blot: 2.50Hnm-2.

crystal surface, such as anchoring, grafting, tethering, van der Waals, hydrogen bridge or chemical reaction bonding, or physical interactions based on electrical and/or surface tension forces. However, those types of support have the tendency to be less stable under the zeolite synthesis conditions. It is important to choose synthesis conditions such that dissolution of the support is controlled or negligible with respect to the time needed to form a crystalline phase. This may either be obtained by using mild conditions, or by the formation of a protective silica gel on the support, thus preventing further attack. An example of the first procedure is an alumina porous support tube which was used as one of the nutrient sources, thus no alumina was added to the solution, for the formation of a ZSM-5-type coating on/in the tube to prepare a membrane [ 111. An example of

the second procedure is an MFI-type layer on Si-wafer where a coating of oriented crystals is obtained [12,13]. When the support is inert the interaction with the zeolite synthesis mixture is poor. However, the compatibility in the system can be improved, as shown with a pressed activated carbon disk to which a montmorillonite binder phase was added [171. The formation of zeolite coatings is clearly based on the knowledge and experience of regular zeolite synthesis from solution. In general, in a direct coating synthesis, the synthesis mixture used has an excess of nutrients with respect to the mass needed for the coating in a relatively large liquid volume. This often results in abundant crystallization/precipitation phenomena [6] and in crystals, generally differing in size, that are carried

J. C. Jansen et al. / Microporous and Mesoporous Materials 21 (1998) 213-226

onto the coating due to unavoidable crystallization throughout the solution [14]. Therefore, the synthesis time is often a much more important parameter than in a regular zeolite synthesis. To control the coating configuration and its properties such as continuity, degreeand direction of crystal orientation, and thickness (i.e. a crystal film thinner than 1 pm or a layer thicker than 1 pm), an understanding of the general theory of heterogeneous nucleation and crystallization on the support is needed [24]. 1.2. Membranes Zeolite membranes are being developed with the ultimate target to achieve unique separation on a molecular scale in a single step [25]. Membranes are used on a more regular scale to control the feed of components, keep components separated in the reactor or separate a product from a mixture of products. Thus catalytic membrane reactors are sought in which a zeolite membrane configuration is united with a catalytic conversion process to improve the reactor performance. Catalytic membrane reactors belong to the category of so-called multifunctional reactors. Isomerization processes are an example of potential applications. In each of these processes,e.g. Butamer, Hysomer, HOTPenex, the thermodynamic equilibrium leads to a mixture, whereas only one of the components is desired [26]. Therefore, these processesare combined with a separation unit being either a distillation or a sorption/desorption processwith zeolites. Distillation often requires a large number of trays and is, therefore, energy intensive, whereas the discontinuous operation of a sorption’desorption process is an important drawback. However, in an integrated catalytic zeolite membrane the required isomer yield and purity may be obtained in a single-step continuous process. Other examples are off-gases being flared, but from which it can be attractive to recover hydrogen and other components. In a hybrid process of Air Products [27], refinery gas containing Cl to C4 and hydrogen is treated. By using a carbon membrane which has similar hydrophobic properties as zeolite silicalite-1, the C3 and C4 are removed and the hydrogen is subsequently recovered in high

215

purity by means of a PSA unit with zeolites. Membrane distillation might be another example. The permeation flux of the lighter hydrogen components all pass through a maximum, at quite distinct temperatures. Operation at various temperature levels enables the permeation of the heaviest components first at the highest temperatures. Extensive exploration has been carried out on the preparation and testing of MFI-type membranes [ 31. Besidesacademic studies on the formation and performance of zeolite A membranes [28,29], there is one case in which the preparation of a small unit working on a zeolite A pervaporation membrane for H,O-ethanol separation has been corrmrercialized [ 301. Ferrierite-type zeolite membranes have been synthesized on alumina supports by impregnating the porous support with the synthesis gel phase. The system shows separation factors for benzene-p-xylene of 100 [I]. In another effort a gel phase on a porous support, mounted at a certain elevation in the autoclave, is converted into zeolite by evaporating water and neutral template from the bottom of the reactor [ 11. To avoid gravity-induced settlement of a gel phase or crystal phases an experiment has been carried out with the porous support on top of the synthesis mixture; this resulted in a polycrystalline zeolite film [31]. In an interesting study a single crystal of Ferrierite is prepared in a membrane configuration [32]. IJnfortunately, intrinsic to the framework topology, the tablet-shaped crystal form only contains ten-ring and eight-ring pores parallel to the large crystal faces. Separation factors for n-butane-isobutane mixtures of 116 are consistent with data measured on an MFI-type membrane [33] made by a crystallization/precipitation method 1341. 1.3. Catalyst coatings Besides the synthesis of membranes, zeolitic coatings have been developed for prestructured catalyst systems.Advantages are the separation of the small zeolite catalyst particles from the process fluid, as well as the low pressure drop across the reactor. Feasible applications for this type of structured

216

J. C. Jansen et ul. i Microporous and Mesoporous Materials 21 (I 998) 213-226

catalyst, mainly using MFI- or BEA-type zeolites, are those in which advantages in terms of pressure drop, catalyst efficiency or selectivity, or improved mass and heat transfer, pay back the extra costs of the structured catalyst packing compared with random packings of particles in a fixed bed reactor. Examples for which this is expected are flue gas treatment (e.g. deNOx [35]), selective oxidation and (de-)hydrogenation (e.g. ethyl benzene dehydrogenation), trickle flow processes(e.g. hydrodesulfurization) and catalytic distillation (e.g. MTBE, ethyl tert-butyl ether (ETBE) [36] and EB [ 51 synthesis). The work described in this paper comprises the developments in the preparation of zeolitic coatings and the exploration of the potential use of zeolitic coatings in the area of separation and. in particular, in catalysis.

2. Discussion of coating preparation

In principle there are three fundamental approaches to forming a zeolite coating on a support, as depicted in Fig. 1. Examples of these approaches are given below.

ZEOLITE possible approaches

liquid phase

I

2. I. First upproach

In the most commonly used method an inert support is immersed in a zeolite synthesis mixture. Table 2 lists several synthesis conditions that result in different zeolite coatings. Using mixture ALfrom Table 2, a coating with a thickness of 50 pm of randomly oriented elongated prismatic crystals with a large crystal size distribution is obtained that forms a continuous phase, as is also shown in membrane tests [34,39]. Synthesis mixture B yields crystals uniquely oriented perpendicular to the support surface with a relatively small crystal size distribution [ 371. When synthesis mixture C, given in Table 2, is used without Na ions. an oriented film of cubicshaped crystals with a relatively small crystal size distribution and a film thickness of less than 1 urn is obtained [38]. In all cases a gel phase was developed on the support, but in the first case, when Na ions were present, discrete sol particles aggregated into a discontinuous phase from which nucleation and crystallization starts in all directions, as depicted in Fig. 2(a). Without Na ions, in the third case,a coalesced,continuous gel phase is developed in which little or no template is present. The nucleation and crystallization take

COATINGS II

Ill

----)

coating e support B Fig. 1. Three possible approaches to Ihrm a zeolite coating on a support: 1 nutrient sources in the liquid phase: II nutrient isources provided by the liquid phase and the support; III nutrient sources present within the support.

J. C. Jansen et al. / Microporous and Mesoporous Materials 21 (1998) 213-226

nutrients

SO, TPABr

217

t.rplUl.~

events

TPA

(a) NaoH

TPA SiO, TPAOH (b) Fig. 2. Influence of gel phase organization on zeolite orientation in coating formation Arrows indicate the crystal growth direction. (a) Discontinuous gel phase with zeolite nuclei on its surface resulting in a randomly oriented zeolite layer. (b) Continuous gel phase with zeolite nuclei on its surface resulting in a randomly oriented zeolite layer through preferential growth along the gel surface.

Fig. 3). It is well known (e.g. see Ref. [41]) that high nucleation and crystallization rates are achieved when high supersaturations are used. This results in a large number of small crystallites showing roughened growth. In this case a continuous layer can be attained, as shown in Fig. 3 using mixture D in Table 2, where the normal crystal morphology is lost and a grain structure is obtained [ 12,131. The indication from this case study is that the organization of precursor phases, which are based on the synthesis compositions, controls the crystal orientation. Thus the formation of a continuous layer is, in general, accomplished by growth of grains that are small and, depending on the highest concentratio,n prevailing, irregularly shaped. An additional advantage of using small crystallites is that the resulting monolayered coating is very thin.

place at the interface between the template and silica nutrient phases and thus the crystal orientation is set from the start, since the viable nuclei have their fastest growing directions along this interface [40]. This is illustrated in Fig. 2(b). When the synthesis mixture is relatively diluted in TPA and OH-, as in the second case (mixture B), the nucleation and crystallization kinetics of both gel and zeolite formation are greatly reduced and are more likely to be influenced by the support. When gel formation on the support is retarded, the bulk of the nutrient pool is still in the liquid phase; then prismatic crystals grow preferentially towards the solution, perpendicular to the support. The discontinuity in the oriented crystal layer is caused by pinholes that are difficult to close when relatively large crystals with well-developed crystal faces are present, as prepared with mixture C (see Table 2 MFI-type layer as a function of synthesis mixture and support type Mixture

Support

Synthesis formulation

A B C D E

steel steel Si-wafer Si-wafer fused quartz

IO SiO,:l NaZO:15TPA,O:l670 H,O; IO SiO,:O.55TPA,O: 1100H,O; IO SiOz:1.5TPA,O:42OOH?O; IO SiOz:1.5 TPA,O: 1050H,O; 10 SiO,: 1.5TPA,O: 1670H,O;

(=24-120 h; r=4 h; r= 12 h; t=2.5 h; t=20 h;

T= T= T= T= T=

IWC. 160°C. 165°C. 165°C. 165°C.

Coating properties

Ref.

50 urn, random crystals crystals I to support crystals(]to support crystals](to support 20 urn, crystals =to support

I341 [371 [381 [12,13] [ 161

218

J. C. Junsen et ul. : Microporous

O-1

2

and Mesoporous

3

4

5

Materiu1.s 21 (1998)

6

Fig. 3. Pinhole size versus crystal size in a silicalite-I

A model zeolite membrane study of interest was carried out recently using semiconductor technology [25,42]. A suitable silicon nitride support has been made in a few steps, as outlined in Fig. 4, starting from a silicon( 100) wafer of 0.5 mm thickness. This wafer is coated with a 0.5 urn layer of SIN by means of low pressure chemical vapor deposition. With a photoresist-etching step (1 M NaOH at 80°C for 5-6 h) the silicon phase is selectively removed in the center part of the square wafer, and a thin SiN layer is obtained. A silicalite-1 synthesis mixture is placed in contact with the exposed SIN at 180°C in an autoclave for

7

213-226

8

9

coating.

2 to 3 h. Finally, the SIN layer is dissolved in 85% H,PO, at 157°C and a thin membrane of TPA-silicalite-1 is achieved, supported only sideways by the silicon wafer matrix. 2.2. Second approach

As an interface between two nutrient phasescan induce orientation of crystals, a study was undertaken to prepare a thin oriented crystal laye~r/film of MFI [ 161 using the second approach from Fig. 1. Thus the support provides the silica source and the solution phase provides the template and the mineralizing agent, Suitable supports are quartz, vitreous glass or a silicon wafer with a 100 nm thermal oxide layer of silica obtainled at 1200K. The support is immersed in a liquid that only contains TPAOH in solution. The mixture (E) is given in Table 2; however, the relative amount of liquid with respect to the substrate surface is of importance here. A first result, shown in Fig. 5. indeed shows very thin oriented MFI crystals. The kinetics of gel phase formation must preferably be slower than the kinetics of crystallization in order to obtain prismatic crystal growth along the support surface; thus the formation of this zeolite coating is still controlled by the precursor phase organizations. 2.3. Third approach

Fig. 4. Sequence of preparation steps to obtain a silicon--silicon nitride composite with a zeolite layer.

Probably, the thinnest coating can be formed by means of the third approach given in Fig. 1. In

1. C. Jansen et al. 1 Microporous and Mesoporous Materials 21 (I998) 213-226

219

formation of MFI-type tectosilicate via van der Waals interactions between TPA and (alumino)silicate from a clear solution has been extensively studied recently [45,46].

3. Poteotial applications in catalysis 3.1. MFI membrane perjormance

As separation plays a role in catalysis, membrane performance of zeolite coatings is discussed in the framework of this contribution. In particular, the permeation and separation behavior of MFI-type zeolite as membrane material will be elaborated. Most permeations are studied on unary and binary systems of gaseous hydrocarbons and permanent gases. Most experiments are carried out with a Wicke-Kallenbach-type cell. On one side the membrane is exposed to the feed mixture and on the other side a sweep gas is used to remove the permeated gas(es) and analyze the mixture by means of a mass spectrometer or gas chromatography. Some of the membranes have been used for more than 2 years without loss of properties [39], having been exposed to temperatures between 200 and 700 K and to pressure differences up to 10 bar over the membrane. For the permeation and separation behavior of

20 pm Fig. 5. Oriented silicalite-1 crystal layer grown from an amorphous quartz substrate using only TPAOH in the synthesis solution.

this case, the Ostwald rule of successive phase transformations is valid [43]. As an example, the procedure depicted in Fig. 6 can be mentioned. A mesoporous precursor phase is constructed via a sequenceof treatments which has the same chemical bulk composition as ZSM-5, but with a lower density. Upon hydrothermal treatment ultrasmall 2-D nucleated tectosilicate entities are generated in the wall of the mesoporous material, as indicated by n-hexane cracking [44]. Apparently, the mesoporous precursor phase reorganizes into tectosilicate around TPA molecules. The mechanism of

/

MCM-41 ( Si/AI =24) calcination ion exchange with TPA heat treatment (glycerol)

J 2-D tectosilicate Bronsted acidity

1 hexane cracking Fig. 6. Example of direct partial conversion of the support to a zeolitic coating using a template-exchanged mesoporous aluminosilicate with a bulk chemical composition identical to ZSM-5.

J. C. Jansrn et ul. i Microporous and Mewporous Materiuls 21 (I 998) 213.. 226

220

Flux (mmol~m-~~sl] ;

‘O-

t

60-

ui 1=: E”

50

IOOkPa

s :

s-l]

25

20

20

H, (95 kPa)

15 cp 0 T

Flux [mmol-m-2

25

singI*camp.

40-

15 1.0

10 30-

0.5

5 n-C, (5 kPs)

20c

g

n-C4

lo-

0

T/K

200

0

400

2w

time [s]

(4

Flux [mmol m%-‘]

seleobvlty 40,

15r

400

time [s] [-]

Fig. 7. Permeation for light alkanes through a silicalite-1 membrane as a function of temperature at 100kPa feed pressure.

silicalite-1 membranes the following factors are of importance. Permeation of molecules with kinetic diameters clearly smaller than the channel aperture of silicalite-1 is controlled by the adsorption characteristics and the intrinsic diffusivity; it is thus highly dependent on operating conditions. Permeation of molecules of the size of the pore apertures, such as SF,, isobutane. iso-octane and benzene derivatives [39,47], is possible to some extent; this is because of the flexibility of the zeolite framework and results in activated permeation behavior. Very large molecules. such as triethylamine. 1,3,5-tri-isopropylbenzene and perfluorobutylamine, do not permeate; hence they are perfectly suitable for testing the quality of the zeolitic coating as a membrane [48,49]. For the separation of binary mixtures the component fluxes are determined by diffusivity, competitive adsorption and zeolite loading [39]. These characteristics can result in extraordinary permeation fluxes. As a function of temperature they increase, pass through a maximum and decrease, go through a minimum and increase again, as shown in Fig. 7. This behavior has been modeled based on the Maxwell -Stefan theory for multicomponent diffusion [50]. Strongly adsorbing components can virtually block the permeation of weakly adsorbing species. almost independent of the size of the permeating molecule. If the difference in adsorption decreases, the relative concentrations become important in the permeation rates. Thus, separation selectivities are strongly dependent on the

300

(b)

400 500 Temperature

600 [K]

-300

400 500 600 Temperature [K]

Fig. 8. (a) Fluxes of n-butane and H, permeation at ambient. Left: single component; right: binary mixture. (b) Fluxes and selectivities of n-butane (50 kPa) and isobutane (50 kPa) permeation. Lines: single component; symbols: binary mixture:.

operation conditions, explaining the sometimes different results reported in the literature. The HZ-n-butane mixture as a function of temperature [51] is a nice example. Fig. 8(a) shows the difference between single component and binary mixture diffusion at room temperature. Here, nbutane adsorbs so strongly that hydrogen has no chance of entering the pores. With increasing temperature the flux of n-butane increases due to the increasing diffusivity, but at higher temperatures the flux decreases due to a faster decreasing occupancy in the silicalite-1. At this point H, has a chance to enter the pores and starts to pemleate. Above 550 K the hydrogen permeates much ;i‘aster than the n-butane due to the higher diffusivily. In this case a reversal of the permeation selectivity occurs. Shape selectivity is observed for binary mixtures of small and large molecules, i.e. those that fit easily in the silicalite-1 pores, and for larger molecules having a kinetic diameter comparable with the zeolite pores [ 521. Here also a strong temperature dependence exists. Permeation of mixtures

J. C. Jansen et al. i Mirroporous

and Mrsoporous

Materials

21 (I 9%) 213. 226

221

of n-butane and isobutane are given in Fig. 8(b). A comparison is made with single-component diffusion. Calculation of the ideal selectivity (based on single-component permeation) shows a dacreasing trend, whereas the selectivity for the mixture remains fairly high up to around 450 K under the prevailing conditions. These examples indicate the great importance of measuring the permeation of mixtures for determination of selectivities, as the single-component permeation measurements may lead to false conclusions. 3.2. Exhaust gas treatment Zeolite coatings of MFI, MOR and BEA were applied to structured supports consisting of metal and ceramic monoliths, metal static mixers and ceramic foams. Zeolite coveragesof up to 30gm-’ of support were obtained, which translates to reactor loadings of zeolite of up to 100 kg mm3 of reactor. Although this loading is considerably lower than the loading obtained in a traditional bed of catalyst particles, the advantages of pressure drop, efficiency and selectivity are expected to be suthciently high to justify the application of this concept in various practical processes.Two possible applications will be discussed in more detail. The first one is the selective catalytic reduction of nitrogen oxides, using ammonia as the reducing agent, over a metal monolith coated with a tilm of Cu-exchanged MFI crystals. Cu-ZSM-5 has been reported to be an active catalyst for the selective catalytic reduction of NOx from industrial flue gases[35]. Therefore, a monolith-type structured metal packing, consisting of AISI 316 stainless steel wire gauze, was coated with an in situ grown binderless layer of ZSM-5 ( Si/Al = 44). Scanning electron microscope (SEM ) pictures of the coated wire gauze are shown in Fig. 9. The zeolite coverage of the metal wire gauze was 2.5 g mW2of wire. After synthesis, the packing was activated through calcination, ion exchange with an aqueous copper acetate solution, and again calcination, yielding an exchange level of 227%. The catalyst packing was tested for deNOx activity and selectivity at 350°C and atmospheric pressure, using a simulated flue gas (N2 with 0 to 15%

Fig. 9. SEM image of an AISI 316 stainless steel wire gaque coated with a layer of axially grown ZSM-5 crystals.

HZO, 5% Oz and 20-200ppmv NO) and NH, concentrations ranging from stoichiometric to a large excess. At a space velocity of 1340kg,,, kg,-,~,ice h- ’ an NOx conversion of about 80% was achieved, which was only weakly dependent on the water and ammonia concentrations. The performance of this reactor concept was compared with traditional deNOx reactors filled with randomly packed beds of standard vanadiaj titania catalyst particles. Based on the turnover frequency, the zeolites are 30 times more active. However, on a per kilogram basis a zeolite contains fewer active sites than a traditional amorphous catalyst, owing to the relatively limited number of exchange positions in a zeolite. Yet the activity of the zeolite expressed per unit reactor volume is most relevant. In those terms, the zeolite film tested on the structured packing shows the same activity as the traditional packed bed reactor with catalyst particles. However, it should be taken into account that the structured packing has a pressure drop that is one to two orders of magnitude lower than the pressure drop across a packed bed reactor. Furthermore, the packing tested had a zeolite coverage of only 2.5 g me2 of surface; this can be increased easily by a factor of ten, presumably resulting in a ten times higher activi,ty per unit volume. It can, therefore, be concluded

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XC. Jansen et ul. / Microporous and Mesoporous Materials 21 (I 998) 213-226

that zeolitic coatings on structured packings have a strong potential. 3.3. ETBE catalytic distillation The second example of a zeolite coating on a structured support is the use of a distillation column packing coated with a film of H-BEA for the synthesis of ETBE from ethanol and isobutene. Synthesis of BEA resulted in zeolite coverages of up to 14.5g m-’ of support. In a monolith with a channel diameter of 2 mm this corresponds to a zeolite loading of N 30 kg mm3 of packing. After calcination, ion exchange with an aqueous ammonium nitrate solution and calcination again, a catalytic site concentration of 1.8 mmol H+ g-’ of zeolite was obtained. The packing was used as a reciprocating stirrer in an autoclave into which ethanol and isobutene were introduced. Typical reaction conditions were 90°C and autogeneous pressure. During the reaction, sampleswere drawn and analyzed on ethanol conversion, ETBE production and production of the by-products diethyl ether, tert-butanol and di-isobutene. The performance of the binderless H-BEA coating was compared with the performance of a commercial ion exchange resin catalyst, H-BEA powder and a binderless H-MOR coating on a monolithic support. It was found that the selectivity of the H-BEA coatings was at least as good as that of H-BEA powder and the commercial catalyst (seeFig. 10). Whereas under comparable conditions the turnover frequency of the H-BEA powder was four times higher than that of the commercial catalyst, the turnover frequency of the H-BEA coating seemed to be lower. Further research will be carried out based on the above promising results. 3.4. inert membraneon catalyst The feasibility regarding the coverage of a thin silicalite-1 coating on a classic catalyst system was tested in a model study [53]. The support was a TiO, wafer on which a Pt phase comprised of particles of 50 nm was dispersed by a spin coating technique. This system was immersed in a silicalite-I synthesis mixture and coated with crys-

6il

7b

8b TlrC]

9il

160

40

Fig. 10. Comparison of the selectivity to ETBE versus temperature of commercially available catalysts and supported zeolite coatings.

tallites within 5 h after activation of this mixture at 165°C. A coverage of more than 95% of the Pt phase by zeolite was obtained according to SEM observations. However, based on the test reactions carried out (vide infra), the Pt layer seemsto be completely covered with the zeolite phase. Thus a molecular shape-selective catalyst is achieved almost without external surface activity. Catalytic tests were carried out in a batch gas-phase reactor. Pt-TiO, catalysts, as well as silicalite-1 -Pt-TiO, composites, were tested. The reaction chosen was the hydrogenation of 3,3-dimethyl-1-butene and I-heptene at 100°C in single component and competition experimen.ts.A hydrogen/olefin ratio varying from 17 to 100 was used in the feed. The reaction took place in a 10% H,-90% Ar or a 50% H,-50% Ar mixture at atmospheric pressure. The results of the competitive hydrogenation of 1-heptene and 3,3-dimethyl- 1-butene over both catalysts are given in Figs. 11 and 12 respectively. Fig. 12 indicates that the composite appears to be stable under reaction conditions for at least 30 h. As can be concluded from the figures, the activities decreased significantly in the experiment with the zeolite coating. The initial conversion rates have decreased from 10m6to 10m9mol h-l. The observed rate selectivity rA/rS is 37, as calculated on the basis of initial rates of conversion, and is of the same order as those for optimized Pt-containing ZSM-5 catalysts in the conversion

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J. C. Jansen et al. / Microporous and Mesoporous Materials 21 (I 998) 213-226

3.5. Fixed bed alkylation catalyst

::

60

.E ?

40

8

20 oc 0

3

2

I

time (h) Fig. 11. Competitive hydrogenation of l-heptene (A ) and 3,3-dimethyl-l-butene (0) at 100°C over Pt--TiO, catalyst.

100

,

I 7s

-

so

-

25

-

,

Zeolite H-BEA-type coatings were synthesized on macroporous WA&O, spheres (see Fig. 13), as well as on macroporous extrudates [ 551. The features of the BEA-coatings on alumina, as measured, are given in Table 3. The extrudates allow a relatively high loading of BEA, but are less stable in the zeolite synthesis mixture. Tests were carried out in two types of reactor. One type was a fixed bed reactor with a volume of 10 ml, operated at 185°C and a pressure of 500 psi (34.45 bar) in the liquid phase. As a reaction, the transalkyl.ation of benzene with diethylbenzene was studied using a benzeneto diethylbenzene ratio of 3: I.. The catalyst was loaded between glass beadsand quartz wool. The fixed flow was set at 55 g h-‘, while the amount of catalyst was varied between 1.6 and 10 g. As a reference, catalytic tests were done with commercial BEA and USY extrudates. The second type of reactor was a differential lixed bed reactor which was operated at 190°C and 350 psi (24.1 bar)

0 0

5:

10

20

15

time

2s

30

(h)

Fig. 12. Competitive hydrogenation of I-heptene ( LI ) and 3,3-dimethyl-I-butene (0) at 100°Con silicahte-I-Pt--TiO, catalyst for 30 h, given as conversion (%) as a function of time.

of linear and branched olefins. An example is the competitive hydrogenation of 1-hexene and 4,4-dimethyl-1-hexene at 100°C [54], where a selectivity of 25 was measured for small crystals of ZSM-5. However, the zeolite-coated catalyst system gives both hydrogenation and double-bond isomerization products: actually, 40% 2- and 3-h heptenes at 25% conversion. Based on silicahte-1 permeation experiments, it is expected that the amount of hydrogen reaching the catalytic sites at 100°C is small, which is due to the lower adsorption strength of hydrogen compared with that of hydrocarbons. The relatively low hydrogen to olefin ratio at the Pt sites may explain the relatively high amounts of isomers versus the hydrogenation products.

Sum Fig. 13. SEM of an H-BEA coating on macroporous a-Al,O, spheres. Table 3 Details of BEA-coated u-alumina supports a-Alumina

Spheres Extrudates

Size (mm) Primary particles (urn) Surface area of support (m2g ‘) Surface area of support + BEA (m2g ‘) Surface area of BEA (m’ g- ‘) BEA loading (%)

1 3-10 0.36 27.9 20.0 4.1

1.4x5 0.75-1.0 12.8 97.3 60.5 16.7

J. C. Jansen et ~1. / Microporous and Mrsoporous Materials 21 (1998) 213-726

724

Table 4 Rate constants for the transalkyiation

of benzene with diethylbenzene,

and for the alkylation

of benzene with ethene

Catalyst Beta, bulk DEB conversion kobs.corikm3 g&i b ’ )

r-Al,O,

extrudate-beta

a-Al,O,

sphere-beta

1.26 x IO 2

3.54xlO-3

1.29x

3.54x

3.71 x 10 3 kl"t~InSLC(cm”G,:

s ‘) 3.88 x 10 3 0.96

‘1

lo-"

0.98

IO 3

1.00

Benzene alkylation k obs.corr (cm3 d

k Intr.c.IF(cm3 g,Z sm‘) '1

0.96x

s '1 2.70 x 10 ’ 0.34 0.78

at a recirculation rate of 200 g min ‘. In this reactor the liquid-phase alkylation of benzene with ethene was studied. The feed contained 0.45 mol% ethene dissolved in benzene. The observed and intrinsic rate constants per gram of catalyst in the transalkylation reaction are summarized in Table 4. A first-order dependence on the concentration of benzene and a diffusivity of approximately 9.6 x 10-l’ m7 SK’ were assumed. The values of the intrinsic rate constants of the coatings and the bulk catalysts at 185°C are comparable, suggesting that similar acid sites are present in both types of catalyst and that no structural changes are induced by fixation to the support. At 185°C the observed rate constant per gram of zeolite of the thin film on the extrudate is higher than the observed rate constant for the bulk catalyst. by a factor of three. The observed rate constants of the coatings on the spheres and the bulk catalyst have similar values. The effectiveness factor in all experiments is close to or equal to unity, which means that, under the present conditions, the intrinsic reaction rates are small compared with diffusion rates; thus the observed reaction rate per gram of catalyst of the coated samples cannot exceed that of the bulk catalysts. The results of the benzene alkylation to ethylbenzene are shown in Table 4. The intrinsic rate constants here are also comparable with the intrin-

IO -'

0.10 0.96

3.02x10-' 0.40 0.76

sic rate constants of the bulk zeolites. Here, a firstorder dependence on the ethene concentration and an ethene diffusivity of 3.1 x lOma m2 s-l were assumed. Although the intrinsic reaction rates in the alkylation are about a factor of 100 higher than in the transalkylation. the effectiveness factors for the bulk catalysts are still high: between 0.72 and 0.83. The large number of meso- and/or macropores in the beta catalysts creates a high intrinsic accessibility, which reduces the influence of diffusion limitation on the observed rate of reaction. The beneficial qualities of the zeolite coatings, which in principle can lead to a higher activity per gram of catalyst, are not apparent in this specific case under the present conditions. However, theoretical calculations [5] show that, in principle, the catalytic performance of zeolite coatings can exceed that of bulk catalyst at high intrinsic reaction rates and relatively low diffusivities (see Fig. 14). Thus, optimization of the reactor performance is needed with respect to the specific catalytic reaction, involving the zeolite coating thickness and the zeolite catalyst type.

4. Conclusions Coatings of different zeolite types can be prepared, with variable thickness, continuity and, in the case of silicalite-1, crystal orientation. The

J. C. Jansen et al. i Microporous and Mesoporous Materials 21 (1998) 213. 226

mw”T--YT

--‘---.-mrry-

log (intrinsic

activity)

Fig. 14. Calculated observed rate constants in a first-order reaction as a function of the intrinsic rate constant for different zeolite catalyst shapes.

synthesis is a function of the precursor phase on the support. Zeolite membranes are promising candidates for difficult gas separations, as indicated by membrane tests carried out for MFI-type zeolites. An inert zeolite membrane on a Pt-catalyst demonstrated shape- and regio-selective effects in hydrogenation test reactions. The first tests of structured zeolite BEA catalysts are reported, in this particular case not directly showing a higher activity per gram of catalyst of a coated system compared with a packed bed system. In structured catalysts the advantages of reduced pressure drop and improved heat transfer are expected to be sufficiently high to justify the application of this concept in various practical processes. Acknowledgement

The authors are thankful to Dr. J.M. van de Graaf, Dr. 0. Oudshoorn and Mr. M. Janissen for providing data and for helpful discussions.

225

[3] J.C. Jansen, E. Coker. Curr. Opinion Solid State Mater. Sci. 1 (1996) 65. [4] N. Nishiyama, K. Ueyama, M. Matsukata, Microporows Mater. 7 ( 1996) 299. [S] N. van der Puil, Development and catalytic testing of zeolitic coating, Thesis, Delft University of Technology, 1997. [6] E.R. Geus, Preparation and characterization of composrte inorganic zeolite membranes with molecular sieve prop=erties, Thesis, Delft University of Technology, 1993. [7] J. Hedlund, B. Schoeman, J. Sterte, Chem. Commun. (1997) 1193. [X] S. Mintova. J. Hedlund, B. Schoeman, V. Valtchtrv, J. Sterte, Chem. Commun. (1997) 15. [9] S. Mintova, V. Valtchcv, L. Koustantinov, Zeolites I7 ( 1996) 462.

[lOI V. Valtchev, S. Mintovd, L. Konstantinov, Zeolites I5 (1995) 679. [ 111 B.-L. Su, Ch. Provoost. in: R. Brunesen (Ed.), Proc. 4th Workshop of the ESF network catalytic membrane reactors, Oslo, 1997,p. 99. [I21 J.H. Koegler, H.W. Zandbergen, J.L.N. Harteveld, M.S. Nieuwenhuizen, J.C. Jansen, H. van Bekkum, in: J. Weitkamp et al. (Eds), Studies in Surface Science and Catalysis, vol. 84A, Elscvier. Amsterdam, 1994, p. 307. [I31 J.H. Koegler, H. van Bekkum, J.C. Jansen. Zeolites in press. I141 J.H. Koegler, A. Arafht, H. van Bekkum, J.C. Jansen, m: H. Chon et al. (Eds), Studies in Surface Science and Catalysis, vol. 10%. Elsevier, Amsterdam, 1997,p. 2163. [ISI H.P. Calis, A.W. Gerritsen, C.M. van den Bleek, C.11. Legein, J.C. Jansen, H. van Brkkum. Can. J. Chem. Eng. 73 (1995) 120. J.H. Koegler, D. van Scherpenzeel,J.C. Jansen, G.M. van Rosmalen. in preparation. R. van der Vaart, H. Bosch, K. Keizer. T. Reith, Microporous Mater. 9 (1997) 203. G. Seijger, Zeolite covered ceramic foams, Masters Thesis, Delft Liniversity of Technology, 1997. S.P.J. Smith, V.M. Linkov, R.D. Sanderson, L.F. Petrik. C.T. O’Connor. K. Keiser, Microporous Mater. 4 (1995) 385.

1201S. Mintova, V. Valtchev, Zeolites 16 ( 1996) 31. PII V. VaItchcv, J. Hedlund, B.J. Schoeman, J. Sterte, S. Mintova, Microporous Mater. 8 ( 1997) 93.

WI S. Feng, T. Bein, Science 265 ( 1994) 1839. v31 M.W. Anderson, K.S. Pachis. .I. Chi. W. Carr, J. Malcr.

Chem. 2 (1992) 255. v41 J.c’. Jansen, D. Kashchiev, A. Erdem-Senatalar, in: J.c‘.

References [ 1) N. Nishiyama, Synthesis of zeolitic membranes by vaporphase transport method and their permeation properties, Thesis, Osaka University, 1997. [2] S. Mintova, B. Schoeman, V. Valtchev, .I. Sterte, S.Y. MO. T. Bein. Adv. Mater. 9 (1997) 58.5.

Jansen et al. (Eds), Studies in Surface Science and Catalysis, vol. 85, Elsevier. Amsterdam, 1994,p. 215. PI M J. den Exter, Exploratory study of the synthesis and properties of 6-, 8- and IO-ring tectosilicates and their potential application in zeolite membranes, Thesis. Delft University of Technology, 1996. [261 S.T. Sie, in: J.C. Jansen. M. StGcker, H.G. Karge, J. Weitkamp (Eds). Advanced Zeolite Science ilnd

226

[27] [28] [29] [30] [31] [32] [33]

[34] [35] [36]

[37] [38] [39] [40]

J. C. Jansen et al. / Microporous and Mesoporous Materials 21 (19%‘) 213 226

Application, Studies in Surface Science and Catalysis, vol. 85, Elsevier, Amsterdam, 1994,p. 587. M.B. Rao, S. Sircar, J. Membrane Sci. 85 (1993) 253. T. Masuda, H. Hara, M. Kouno, H. Kinoshita. K. Hashimoto, Microporous Mater. 3 ( 1995) 565. P. Alderliesten, Inorganic membranes, Internal communique ECN-B-96-033, Netherlands Energy Research Foundation ECN, 1996. T. de V Naylor, in: Int. Congress on Membranes and Membrane Processes,Yokohama, 1996. Y. Yan, M.E. Davis, G.R. Gavalas, Ind. Eng. Chem. Res. 34 (1995) 1652. J.E. Lewis, G.R. Gavalas, M.E. Davis, AICHE J. 43 (1997) 83. J.M. van de Graaf, F. Kapteijn, J.A. Moulijn, in: A.B.J. Kemperman, G.H. Koops (Eds), Euromembrane ‘97, Third Int. Symp., Dinkeldruk. Oldenzaal. 1997, p. 242. E.R. Geus, M.J. den Exter, If. van Bekkum, W.J.W. Bakker, J.A. Moulijn, Microporous Mater. 1 ( 1993) 131. M.C. Campa. V. Indovina. G. Minelli. G. Moretti, I. Pettiti, P. Porta, A. Riccio, Catal. Lett. 23 (1994) 141. M. Janissen, Feasibility of binderless zeolitic coatings for the liquid-phase synthesis of ethyl rert-butyl ether Masters Thesis. Delft University of OfTBE), Technology, 1997. W.P. Wolterbeek. Masters Thesis, Delft University of Technology, 1993. J.C. Jansen, W. Nugroho, H. van Bekkum. in: R. von Ballmoos, J.B. Higgins, M.M.J. Treaty (Eds). Proc. 9th Int. Zeolite Conf., Montreal, 1992, p. 247. W.J.W. Bakker. F. Kapteijn. J. Poppe, J.A. Moulijn, J. Membrane Sci. 117 (1996) 57. J.H. Koegler, H. van Bekkum. J.C. Jansen, Zeolites in press.

[41] D. Kashchiev, J. Cryst. Growth 40 (1977) 29. [42] H. Schellevis, Masters Thesis, Delft University of Technology, 1996. [43] R.M. Barrer. in: M.L. Occelli, H.E. Robson (Eds), Zeolite Synthesis, ACS Symposium Series. vol. 398. ACS, Washington, DC, 1989.p. 1I. [44] K.R. Kloetstra, H. van Bekkum, J.C. Jansen Chem. Commun. in press [45] S.L. Burkett, M.E. Davis. Chem. Mater. 7 (1995) 920. [46] P.P.E.A. Demoor, T.P.M. Beelen, R.A. van Santen, Microporous Mater. 9 ( 1997) 117. [47] C.D. Baertsch, H.H. Funke. J.L. Falconer, R.D Noble, J. Phys. Chem. 100 ( 1996) 7676. [48] T. Sano. M. Hasegawa, Y. Kawakami. Y. Kyozumi, H. Yanagista. D. Kitamoto. F. Mizukami. in: J.Wcitkamp et al. (Eds), Studies in Surface Science and Catalysis, vol. 84, Elsevier, Amsterdam, 1994.p. 1175. [49] Y. Yan. M. Tsapdtsis. G.R. Gavalas, ME. Davis, J. Chem. Sot. Chem. Commun. ( 1995) 227. [50] F. Kapteijn, W.J.W. Bakker, J.A. Moulijn, Microporous Mater. 3 ( 1994) 227. [51] F. Kapteijn, W.J.W. Bakker, J.W. van de Graaf, G. Zheng, J. Poppe, J.A. Moulijn. Catal. Today 25 (1995) 21.3. [52] H.H. Funke, M.G. Kovalchick. J.L. Falconer, R.D. Noble, Ind. Eng. Chem. Res. 35 ( 1996) 1575. [53] N. van der Puil. E.J. Creyghton. EC. Rodenburg, S.T. Sie, H. van Bekkum, J.C. Jansen, J. Chem. Sot. Faraday Trans. 92 ( 1996) 4609. [54] R.M. Dessau, J. Catal. 89 ( 1984) 520. [55] N. van der Puil, F.M. Dautzenberg. H. van Bekkum, J.C. Jansen. in preparation.