Influence of Coke Deposition on Selectivity in Zeolite Catalysis

Sep 2, 1997 - In addition, it is difficult to analyze the effect of coke formation on the selectivity ..... Chen, D.; Rebo, H. P.; Moljord, K.; Holmen...
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Ind. Eng. Chem. Res. 1997, 36, 3473-3479

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Influence of Coke Deposition on Selectivity in Zeolite Catalysis D. Chen,† H. P. Rebo,† K. Moljord,‡ and A. Holmen*,† Department of Industrial Chemistry, Norwegian University of Science and Technology, N-7034 Trondheim, Norway, and SINTEF Applied Chemistry, N-7034 Trondheim, Norway

The selectivity of a complex reaction depends on a number of factors, such as the reaction mechanism, operating conditions, catalyst properties and catalyst deactivation. The present work discusses how the selectivity of a complex reaction depends on the formation of coke. For zeolite catalysts, changes in selectivity can be a result of intrinsic selectivity effects or shape selectivity effects. A method is suggested to analyze a complex reaction system with deactivation caused by coke formation, and different cases of selectivity change during deactivation of zeolites are discussed. Transition-state shape selective deactivation is proposed as a mechanism in addition to the deactivation mechanisms suggested by Guisnet and Magnoux (Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1). By variation of the space velocity, the selectivities to the main products are measured as a function of conversion (optimum performance envelopes). Selectivities at given conversions can then be compared for results obtained with different contact time and with varying degree of catalyst deactivation (space velocity loops). Selective or nonselective deactivation is thereby distinguished. This type of selectivity plot is applied to two different types of reaction, i.e. ethene oligomerization over HZSM-5 and methanol conversion to light olefins (MTO) over SAPO-34. The selectivity of ethene oligomerization was affected only by the decrease in conversion due to coke formation; hence, this is an example of nonselective deactivation. Selective deactivation was found for methanol conversion over SAPO-34. 1. Introduction The formation of heavy secondary products referred to as coke is a main cause of deactivation of acidic zeolite catalysts. The effect of coke formation on catalyst activity for different reactions has been extensively investigated by different methods. Reviews concerning zeolite deactivation have been presented by Guisnet and Magnoux (1989), Bhatia et al. (1989-90), and Bibby et al. (1992). Some important problems have been resolved, e.g. the interdependency of coking and deactivation rates, the role of zeolite pore structure in determining the chemical composition of the coke (Magnoux et al., 1987) and the poisoning of acid sites by coke (McLellan et al., 1986). However, the interpretation of experimental results in order to understand selectivity changes due to coke deposition is less clear. Shape selectivity effects are important in determining the product distribution in zeolite catalysis, and the selectivities depend on a number of factors. Three types of shape selectivity have been identified (Weisz, 1980), i.e. reactant shape selectivity, product shape selectivity, and transition-state shape selectivity. The relative importance of these types depends on the zeolite pore structure and the shape and sizes of the reactant and product molecules and might be influenced by coke formation. The change in selectivity due to coke formation has been explained by changes in the diffusivity of the molecules or in the zeolite pore volume, with the formation of the coke (Dadyburjor, 1992; Rene´ Bos et al., 1995; Xu et al., 1995). These interpretations were based on observed selectivities with time on stream. It should be noticed that changes in selectivities due to the formation of coke for a complex reaction are normally coupled with a decrease in conversion. The various possible shape selective effects cannot be distinguished only by a time-based analysis. Even though the effect of coke formation on selectivity is generally a very important topic for zeolite applica† ‡

Norwegian University of Science and Technology. SINTEF Applied Chemistry. S0888-5885(97)00022-5 CCC: $14.00

tions, few theoretical analyses are available. Froment and Bischoff (1962) discussed the selectivity problem for deactivation in Wheeler’s type II and III reactions, using rate constants which were dependent on the amount of coke on the catalyst. This discussion is useful for the design and optimization of reactors. However, for the reverse problem, i.e. how to extract information about the effect of coke on shape selectivity from experimental data, the theoretical and experimental aspects are not that well developed. The present work deals with the investigation of the influence of coke on the selectivities for Wheeler’s type III reaction. Focus is put on the development of a method to specify different parameters that determine selectivity changes with coke formation. A type of selectivity plot is proposed and applied to ethene oligomerization over HZSM-5 and methanol conversion to light olefins (MTO) over SAPO-34, based on kinetic data obtained in the TEOM (tapered element oscillating microbalance) reactor. Although both reactions have been studied extensively (O’Connor and Kojima, 1990; Rene´ Bos et al., 1995), the importance of shape selectivity effects and the role of coke formation regarding reaction selectivity is still not clarified for these two reactions. 2. Methodology Wheeler (1951) classified multiple reaction networks into three general types in order to discuss the effect of diffusion on selectivity. Type I selectivity involves the relative rate for two different simultaneous reactions on the same catalyst. Type II selectivity is concerned with parallel reactions for one single reactant, while type III selectivity is valid for consecutive reactions. Rickert and Wei (1968) developed selectivity models for eight systems based on Wheeler’s basic types of reactions, using a time dependant deactivation function. They defined two concepts for deactivation: Nonselective deactivation, where the deactivation functions are identical for different elementary reactions, and selec© 1997 American Chemical Society

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tive deactivation, where each reaction has different deactivation functions. However, this definition is not identical to Wheeler’s definition for poisoning, where selective or nonselective deactivation describes the effect of poisoning only for a simple reaction (Wheeler, 1951). In most cases the data for catalyst activity are instantaneous values measured at some definite reaction time. Initial conversions and yields on fresh catalysts are often used to characterize the reaction network. However, extrapolation to zero time for obtaining initial data might be accompanied with large uncertainty for a fast deactivating process. Campbell and Wojciechowski (1970) developed an integral method to avoid extrapolation, in which the time averaged yields were plotted against time averaged conversion in a selectivity plot. The catalyst-to-oil ratio was a fixed parameter when applied to catalytic cracking. In this way a system of constant catalyst-to-oil loops was generated. This system of loops can be enveloped by a single curve which corresponds to the selectivity curve for that product and is called the optimum performance envelope (OPE). The OPE also describes the selectivity pattern of the same system without catalyst decay. This condition is in practice approximated by runs at short time on stream. Then, the OPEs are used to determine whether the observed reaction products are primary or secondary and stable or unstable (Best and Wojciechowski, 1977) and to calculate the initial selectivities by determining the slopes of these curves at zero conversion. However, it has been pointed out (Weekman, 1969) that such a technique can give distorted results due to the so-called smoothing effect of time averaging. In addition, it is difficult to analyze the effect of coke formation on the selectivity in detail by this averaging technique. The idea presented in this paper is to use instantaneous yields and conversions instead of time averaged data and to keep the conception of the optimum performance envelope. At a certain space velocity the instantaneous conversion decreases with time on stream or coke content. By plotting the instantaneous yields versus conversions for different coke contents, a system of loops, called a selectivity plot, can be generated with space velocity as a fixed parameter. Comparing the different space velocity loops with the OPE makes it possible to specify the effect of coke formation on the reaction mechanism. Consider the Wheeler’s type III reaction: k1

k2

A 98 B 98 D

(1)

It is assumed that B and D are products and that coke will be formed from both B and D or from D only or that D is the coke itself. This pattern would be valid for the main and coking reactions in many industrial processes. To simplify the problem, first order reactions and uniform coke distribution through a static catalyst bed at isothermal conditions are assumed. For a first order reaction, the rate constants k can be expressed as a function of the coke content C on the catalyst:

k1 ) k10 exp(-R1C)

(2)

k2 ) k20 exp(-R2C)

(3)

In general, coke formation might change the density of acid sites, the distribution of acid site strength, and the pore structure in zeolite catalysts. As a result, the

Figure 1. Theoretical yields versus the instantaneous conversions for the reaction A f B f D, where k1 ) 1, k2 ) 2, R1 ) 0.2 are assumed. The solid lines give the optimum performance envelope (OPE) curves. Nonselective deactivation (R2/R1 ) 1) is shown by the following: b, B; 2, D; (- ‚ -) selective deactivation (e.g. R2/R1 ) 1.5).

catalyst is deactivated and the different rate constants are affected through their R values, which express the sensitivity to coke for each reaction. The higher the R value, the faster the activity decreases for this reaction with increasing coke content. The change in conversion of reactant A due to coke formation depends mainly on the R1 value, while the selectivity depends not only on the R1 value but also on the R1/R2 ratio. The R1/R2 ratio is related to the deactivation mechanism. A selectivity change caused by a change in acid density, acid strength distribution, or conversion can be classified as an intrinsic selectivity change, while a selectivity change caused by a change in catalyst pore structure can be classified as a shape selectivity change. For the Wheeler’s type III reaction network, the theoretical OPE curves are presented in Figure 1. The OPE curve for component B goes through zero, which is not the case for component D. B and D show the typical patterns for primary and secondary products, respectively. In the case of nonselective deactivation, i.e. R1/R2 ) 1, the product selectivities are a function of the conversion (XA) only. The space velocity loops are identical to the OPE, as shown in one of the cases in Figure 1. Figure 1 clearly shows that nonselective deactivation favors the formation of primary products and suppresses the secondary products. For selective deactivation, the product yields deviate from the OPEs with decreasing conversion due to coke formation (Figure 1). The deviation depends on the nature of the deactivation, i.e. on the R1/R2 ratio. 3. Experimental Section The operation of the TEOM microbalance is based on the relationship between the natural frequency of the oscillating tapered element containing the catalyst and its mass. The TEOM reactor has not only excellent sensitivity, detecting a mass change as low as a few micrograms, but also a very short response time which is particularly suitable for transient studies. The experimental setup has been described in detail elsewhere (Chen et al., 1996a,b). Helium was used as the carrier gas, as the diluting gas for the reaction mixture, and as the purge gas for the microbalance. Air was used for calcination as well as for regeneration of the catalyst. The product composition was analyzed by an on-line GC

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(HP5890) using FID and a GS-Q capillary column (30 m, 0.543 mm). The tapered element was loaded with 5-30 mg of zeolite particles (0.07-0.3 mm) together with quartz particles. Quartz wool was used on the top and at the bottom of the catalyst bed to keep the catalyst particles firmly packed. Quartz particles of identical size as the catalyst particles were installed between the catalyst and the quartz wool to minimize radial temperature and concentration gradients. The template-free ammonium form of the ZSM-5 zeolite CBV 8020 with a Si/Al ratio of 39, average crystal size of 0.3-0.6 µm, and a surface area of 450 m2/g was obtained from PQ Zeolites. The sample was calcined in situ at 500 °C for 12 h in diluted air (10% O2). Calcined SAPO-34 with a unit cell composition of (Si2.88Al18P15.12)O72 was obtained from SINTEF-Oslo and dried in situ at 500 °C for 6 h. The ethene oligomerization was performed at 450 °C and 1 bar and with a mole fraction of ethene in the feed equal to 0.3. In order to obtain the selectivity plots, the space velocity was varied from 19 to 250 (g/(g of cat.)‚h). The first GC analysis was taken after 5 min and thereafter every 1 h. The MTO reaction was studied at 425 °C, WHSV ) 57-384 (g/(g of cat.)‚h), using a methanol partial pressure of 0.08 bar. Due to rapid catalyst deactivation, the MTO reaction was studied using 3 min interrupted pulses with GC analysis taken after 2 min for each pulse (integrated pulse method). The time between each pulse was about 40 min, allowing for completion of the GC analysis of each pulse. The reactant mixture of methanol and helium was replaced by pure helium between the 3 min pulses. It was found by preliminary experiments that the aging in helium did not influence the amount or the deactivating effect of coke significantly. 4. Results and Discussion 4.1. Nonselective Deactivation: Ethene Oligomerization. At low conversions, C3-C7 olefins were the main products whereas the amount of light alkanes and aromatics was small. Oligomerization of ethene was the major reaction occurring at low conversions, accompanied by some cracking of the heavy oligomers, isomerization, and alkylation of the cracking products. Aromatization was important at long contact time. No heavy products with more than nine carbon atoms were detected. Quann et al. (1988) established that, at olefin aromatization conditions, the olefin isomers are in equilibrium. Therefore, isomers of the same number of carbon atoms can be lumped together. For simplicity the C5-C7 fractions were lumped together, as were all aromatics and paraffins. Since the deactivation for ethene oligomerization was quite slow, as shown in Figure 2 for WHSV ) 52.5 g/(g of cat.)‚h, the initial activity at TOS ) 5 min can be considered to be identical with that for the fresh catalyst. Yield-versus-conversion curves were obtained by varying the space velocity from 19 to 250 g/(g of cat.)‚h. Typical plots for different products at constant temperature are shown in Figure 3. The initial yields of products at different initial conversions obtained at different space velocities and otherwise identical conditions are plotted as solid lines (named OPE curves in Figure 3a-d). As the time on stream increased for a certain space velocity (e.g. WHSV ) 52.5 g/(g of cat.)‚h

Figure 2. Conversion and coke formation during ethene oligomerization over HZSM-5 versus time on stream at WHSV ) 52.5 g/(g of cat.)‚h, PC2H4 ) 0.3 bar, and 450 °C: 9, conversion (wt %); b, coke (wt %).

in Figure 2), the conversion and the amount of coke on the catalyst changed. Product yields obtained at constant space velocities but with different degree of deactivation are plotted versus the corresponding conversion in the same figures, where different symbols correspond to different space velocities. For fresh catalysts, the selectivity of a complex reaction depends on the conversion. At low conversion, the yield of propene, butenes, and C5-C7 hydrocarbons increased linearly with the conversion, and all these lines went through zero. This pattern clearly indicates that these products are primary products (Best and Wojciechowski, 1977). On the contrary, only small amounts of aromatics and paraffins were formed at low conversion and the OPE curve did not go through zero. These components can therefore be considered as secondary products. At higher conversion, the selectivity to primary products decreased while the selectivity to secondary products increased. The reaction can therefore be divided into the following steps:

ethene f C3-C7 olefins f aromatics + paraffins Coke formation evidently resulted in selectivity changes as well as reduction in the conversion. By comparing the product yields at a given conversion level for fresh catalysts at different space velocities (OPEs) and for coked catalysts by varying the degree of deactivation (space velocity loops), it is possible to eliminate the influence of change in conversion. It is also possible to distinguish between selective and nonselective deactivation. The fact that no deviation in yields from the OPE was observed for the coked zeolite at different space velocities indicates nonselective deactivation in the case of ethene oligomerization over HZSM-5. This means that the main reactions deactivated with the same rate, i.e. all Ri are identical, and all the observed changes in the product selectivities caused by coke formation were simply a result of the decrease in conversion. In general, two important conclusions can be drawn for nonselective deactivation of Wheeler’s type III reaction: (1) reactions A f B and B f D are based on the same type of mechanism, i.e. they need the same number and strength of active sites, and (2) the product and the transition-state shape selectivities are not affected by coke formation, or the product selectivities are not controlled by the shape selectivities. Although there are different types of reactions involved in ethene oligomerization over HZSM-5, it is

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Figure 3. Yields of (a) propene, (b) butene, (c) C5-C7, and (d) aromatics and paraffins during ethene oligomerization over HZSM-5 at PC2H4 ) 0.3 bar and 450 °C. Lines: OPE curves. Symbols represent experimental results at different WHSV (g/(g of cat.)‚h): 200 (0); 83 (*), 52.5 (2), 41 (×), 19 ((). For identical symbols, different degrees of conversion refer to different coke contents.

evident from these observations that they require the same catalyst acidity. Shape selectivity normally plays an important role in the selectivity of reactions catalyzed by zeolites. At least, the transition-state shape selectivity is known to be important for olefin reactions over HZSM-5 (Chen et al., 1994). The nonselective deactivation observed during ethene oligomerization over HZSM-5 indicates that the shape selectivity was not influenced by coke formation. However, the diameter of the product molecules varies over a large range in ethene oligomerization, e.g. from the linear olefins and paraffins to the aromatics. The difference in diffusivities between these molecules can be of several orders of magnitude (Chen et al., 1994), and a large difference in the effect of coke deposition on the selectivities of the different sized molecules is expected for reactions controlled by products diffusion. According to the observation of nonselective deactivation, it can probably be concluded that the selectivities to products in ethene oligomerization over HZSM-5 are not controlled by product shape selectivity, even for coked samples. The pore volume of HZSM-5 decreased with increasing coke content during ethene oligomerization (Chen et al., 1996b). Coke in the pores of HZSM-5 during olefin aromatization has been shown to be aromatic in nature (Dimon et al., 1993). The main coke components located in the pores of HZSM-5 were alkylaromatics or polyaromatics with a diameter of about 8-8.5 Å. HZSM-5 has a three-dimensional pore structure without cavities, and coke is expected to form at the channel intersections. The channel intersection of HZSM-5 has a maximum diameter of about 9.6 Å, and it can be proposed that when a coke molecule is formed at an

intersection, this intersection is deactivated. Even though the reactant ethene might diffuse into the pores and reach the intersections, the reaction might not proceed because of steric hinderance of intermediates formed via bimolecular reactions. This can explain the observation that the residual activity for ethene oligomerization decreased faster than the accessible volume for ethane adsorption over HZSM-5, when ethane and ethene have a similar kinetic diameter (Chen et al., 1996b). This suggests that transition-state shape selectivity plays an important role in the deactivation but not in the product selectivities of a zeolite with channel intersections. This can also explain why no additional transition-state shape selectivity induced by coke deposition existed for coked samples. It should be pointed out that the explanation given above would represent an extension of the deactivation mechanism concept proposed by Guisnet and Magnoux (1989). The deactivation of all zeolites has been considered to occur in three ways depending on the coke content: (a) Limited access for the reactant molecules to the active sites (site coverage); (b) blockage of the access to the active sites in the cavities (or in the channel intersections) where coke molecules are located; (c) blockage of the access to the active sites in the cavities (or in the channel intersections) where there are no coke molecules. Magnoux et al. (1989) also observed a relatively larger decrease in activity than in pore volume in n-heptane cracking over H-offretite, which was explained by limitations in diffusion of the reactant. This behavior is not likely in the present work because of the small size of the ethene molecule. Therefore, an additional

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deactivation mechanism has to be considered where the reactant molecules can get access to the active sites in partly coked cavities (or channel intersections), but the reaction might be influenced by spatially constraining the formation of large intermediates. This type of deactivation can be defined as transition-state shape selective deactivation. It is interesting that nonselective deactivation also was found during the methanol to gasoline (MTG) reaction over HZSM-5 (Sedran et al., 1990), where each step is affected by deactivation through the same R values. 4.2. Selective Deactivation: MTO Reaction. The conversion of methanol over SAPO-34 was controlled to be less than 100% by choosing high WHSV values. Yield-versus-conversion plots for DME, olefins, and coke were obtained as described above and are shown in Figure 4a-c. Dimethyl ether (DME) appears to be an unstable primary product (Figure 4a), while all the olefins from ethene to C6 were identified as stable secondary products, which was indicated by linear OPE curves for the olefins which did not go through zero conversion. This means that the secondary reactions of olefins can be considered to be negligible in the presence of oxygenates. Based on the experimental results, a simplified reaction model is suggested here:

MeOH f DME f olefins f coke The OPE curves are difficult to obtain experimentally for fast deactivating processes. The integrated pulse method can be used, but the effects of catalyst aging during exposure to the carrier gas between each pulse must be examined. This effect was found to be negligible for MTO over SAPO-34. Initial conversions and yields used to generate OPE curves can be obtained by the first pulse, and this pulse must be short enough to obtain data at a sufficiently low coke content. TOS ) 2 min was selected for the MTO reaction. Although this treatment is not very rigorous, it does not lead to large errors with respect to the selectivity plots in this case, since the data at low coke contents do not deviate significantly from the OPE curves (Figure 4a-c). For high coke contents, the yield of DME, olefins, and coke deviated significantly from the OPE curves, as shown in Figure 4a-c. At identical conversion, the yield of olefins for the coked catalysts was lower than for the fresh catalysts, while the yield of DME was higher. At relatively low coke contents, the yields of DME were reduced with coke formation, i.e. with decreasing conversions, and these yields were slightly lower than the OPE curve. These observations indicate selective deactivation for the MTO reaction. The reason for selective deactivation can be rather complex, and three possible cases have to be considered: (1) the presence of two reaction steps requiring a different number or different strength of acid sites; (2) domination of the product shape selectivity (the selectivity change is related to changes in the ratio of diffusivity of component B and D); (3) domination of the transition-state shape selectivity and the different sizes of the intermediates of the reactions A f B and B f D. The deactivation will then be faster for a reaction with larger intermediates than for a reaction with smaller intermediates. The transition-state shape selectivity is normally considered to be an important property of the medium pore size zeolite ZSM-5, but this type of shape selectivity might also be dominating at high coke contents, even for zeolites with large cavities. However,

Figure 4. Yields versus conversion of methanol during MTO over SAPO-34 at PMeOH ) 0.072 bar and 425 °C: (a) DME; (b) olefins; (c) coke. Symbols with dashed lines represent experimental results at different WHSV (g/(g of cat.)‚h): 384 (b); 253 (×), 114 (+); 82 (2); 57 ((). Solid lines: OPE curves.

the effect is probably only significant for site coverage deactivation, not for pore blocking. The first case is caused by intrinsic selectivity changes, and the two latter are caused by changes in shape selectivity due to coke formation. The higher selectivity to DME at high coke contents can probably be explained by a change in the acid strength distribution. Two types of acidic sites in SAPO-34 have been found by ammonia TPD experiments: strong acid sites (desorption temperature 385 °C) and weak acid sites (desorption temperature 165 °C) (the number of strong acid sites is 1.06 mmol/(g of cat.)). The acid strength distribution for strong acid sites is

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essentially uniform (Nawaz et al., 1991). SAPO-34 has the chabazite structure, which has three cavities per unit cell (Anderson et al., 1990). From the composition of the unit cells, the number of cavities has been calculated to 1.37 mmol of cavities/g. Assuming 100% crystallinity and a homogeneous distribution of the acid sites leads to 0.8 strong acid sites per cavity (Grønvold, 1994). In other words, only 80% of the cavities contain strong acid sites. DME formation can be catalyzed by rather weak acid sites, while the olefin formation from DME seems to need stronger acid sites. It is reasonable to assume that coke is mostly deposited in the cavities containing strong acid sites and that the reaction in these cavities deactivates with coke formation. The deactivation rate for the reaction over weak acid sites might be lower, and a higher selectivity to DME will result from coke formation. This coincides with the results from porosity experiments. It was found that 20% of the pore volume was remaining at 19 wt % coke, when the yield of olefins was close to zero and the methanol conversion to DME was relatively high. However, the mechanism of transition-state shape selective deactivation might also result in the increased DME/methanol ratio with coke formation. The formation of olefins from oxygenates might go through relatively larger intermediates than the DME formation. As a consequence the formation of olefins deactivates faster than the formation of DME with an increasing amount of coke on the catalyst. Methanol and DME are normally considered to be at chemical equilibrium over HZSM-5 in the MTG reaction (Sedran et al., 1990). However, based on the experimental results on SAPO-34 (Figure 4a), it can be concluded that the ratio of DME to methanol does not correspond to the ratio predicted at chemical equilibrium. It was found that the chemical equilibrium can only be reached at close to 100% conversion and only for fresh catalysts. As coke formed in the pores of SAPO-34, the methanol to DME ratio shifted from the ratio at chemical equilibrium. This indicates that DME diffusion plays a role in this reaction. At low coke contents, the yield of DME decreased with coke formation. This has been explained by the effect of DME diffusion in the pores which was confirmed by studying the conversion of DME on SAPO-34 with different crystal sizes (Chen et al., 1997b). However, the effect of the external surface has also to be considered. If DME is formed mostly at the external surface, a higher DME selectivity at higher coke contents will result. DME can diffuse into the pores and be converted to olefins, which is confirmed by separate experiments with DME conversion over SAPO-34 (Chen et al., 1997a). It is reasonable to assume that the DME diffusion is affected more by coke deposition than the methanol diffusion due to the larger diameter of the DME molecule. As more coke is formed, less and less DME diffuses into the pores to react and more and more DME remains as product. However, this mechanism has been examined by pretreating the external surface in order to eliminate the acid sites (Niekerk et al., 1996; Chen et al., 1997a). It was found that the modified sample reacted in a way very similar to the unmodified sample, and only a slightly longer lifetime was found for the modified sample. In addition, nonselective reactions at the external surface were not observed; i.e. hydrocarbons heavier than C6 and aromatics did not exist in the product. These observations

show that reactions at the external surface are not significant. SAPO-34 consists of cavities (1.1 × 0.65 nm) and narrow pores (0.43 nm) extending in three dimensions. Aromatic molecules are too big to diffuse out of the pores and are trapped in these cavities, since the molecular diameters are larger than the pore diameter. This leads to fast coke formation and deactivation. The coke selectivity was found to be lower on coked samples than on fresh samples at the same conversion level. This might result from the reduced space in the cavities of SAPO-34 due to coke formation which might be suppressed by coke formation itself. This also indicates that coke formation on SAPO-34 during the MTO reaction is a transition-state shape selective reaction. 5. Conclusions The selectivity changes during catalyst deactivation result from intrinsic selectivity and shape selectivity changes caused by coke formation. A method to specify the type of selectivity change is suggested, where the yields at different conversions obtained at different space velocities are compared to the yields obtained at various degrees of deactivation. Nonselective or selective deactivation can thereby be distinguished, and information about the deactivation mechanism can be obtained. The zeolite pore structure appears to be important and determines whether the deactivation is selective or nonselective. It was found that the selectivity changes with coke formation during ethene oligomerization over HZSM-5 were caused by changes in conversion only. This is a typical example of nonselective deactivation in zeolite catalysts. A transition-state selectivity mechanism was proposed for the deactivation of ethene oligomerization over HZSM-5 to explain the selectivity behavior. Selective deactivation was demonstrated for the MTO reaction over SAPO-34. The reason for this selective deactivation can be quite complex. The coke deposition resulted in changes not only in intrinsic selectivity but also in shape selectivity. The coke formation during MTO was found to be a transition-state shape selective reaction. Acknowledgment The support from the Norwegian Research Council and from Norsk Hydro ASA is gratefully acknowledged. Literature Cited Anderson, M. W.; Sulikowski, B.; Barrie, B. J.; Klinowski, J. In situ solid-state studies of the catalytic conversion of methanol on the molecular sieve SAPO-34. J. Phys. Chem. 1990, 94, 2730. Best, A. N.; Wojciechowski, B. W. On identifying the primary and secondary products of the catalytic cracking of cumene. J. Catal. 1977, 47, 11. Bhatia, S.; Beltramini, J.; Do, D. D. Deactivation of zeolite catalysts. Catal. Rev.sSci. Eng. 1989-90, 314, 431. Bibby, D. M.; Howe, R. F; McLellan, G. D. Coke formation on highsilica zeolite. Appl. Catal. 1992, 93, 1. Campbell, D. R.; Wojciechowski, B. W. Selectivity of aging catalyst in static, moving and fluidized bed reactors. Can. J. Chem. Eng. 1970, 48, 224. Chen, N. Y.; Degnan, T. F.; Smith, C. M. Molecular transport and reaction in zeolites, design and application of shape selective catalysts; VCH Publishers: New York, 1994. Chen, D.; Grønvold, A.; Rebo, H. P.; Moljord, K.; Holmen, A. Catalyst deactivation studied by conventional and oscillating microbalance reactors. Appl. Catal. 1996a, 137, L1.

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Received for review January 9, 1997 Revised manuscript received May 9, 1997 Accepted May 13, 1997X IE9700223

X Abstract published in Advance ACS Abstracts, July 15, 1997.