Methanol Conversion to Light Olefins over SAPO-34. Sorption

The effects of adsorption and diffusion of the reactants on methanol to olefins (MTO) and propene conversion over SAPO-34 have been studied in an osci...
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Ind. Eng. Chem. Res. 1999, 38, 4241-4249

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Methanol Conversion to Light Olefins over SAPO-34. Sorption, Diffusion, and Catalytic Reactions D. Chen,† H. P. Rebo,† K. Moljord,‡,§ and A. Holmen*,† Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway, and SINTEF Applied Chemistry, N-7464 Trondheim, Norway

The effects of adsorption and diffusion of the reactants on methanol to olefins (MTO) and propene conversion over SAPO-34 have been studied in an oscillating microbalance reactor. The adsorption parameters of methanol and propene at reaction conditions (698 K) were determined by a pulse method, and the results were identical to the values obtained by extrapolation from low temperatures (323-398 K). Inverse uptake diffusion times were calculated from adsorption data at low temperatures, and these results were dependent on the temperature and the adsorbed amount. The inverse steady-state diffusion times calculated from the inverse uptake diffusion times were independent of the temperature and the adsorbed amount. The influence of diffusion on the reaction rates was estimated on the basis of the inverse steady-state diffusion time, using the Weisz-Prater criterion. The methanol conversion over SAPO-34 was influenced by diffusion of the reactant, while the propene conversion was not. A kinetic study revealed that both the rate constant and the site coverage of propene were much lower than that of methanol at 698 K. The deactivation behavior during the MTO reaction over SAPO-34 was studied by measuring both the adsorbed amount of methanol and the conversion at different coke contents. Catalyst deactivation was proposed to be due to a decreasing number of sites available for adsorption at high coke contents and a lower diffusivity, hence a lower effectiveness factor due to coke deposition. 1. Introduction The catalytic conversion of methanol to lower olefins (MTO) is a way of converting natural gas and coal to chemicals via methanol.1 The process originates from Mobil’s process for converting methanol to gasoline (MTG) using ZSM-5,2 in which olefins were shown to be reaction intermediates. As a consequence of the discovery of aluminophosphate molecular sieves,3 attention has been given to the use of SAPO-34 as a catalyst for MTO, due to its narrow pores (0.43 nm) extending in three dimensions and its mild acidity.4 High selectivities to ethene and propene, unfortunately accompanied by fast catalyst deactivation due to coke formation,5-7 have been reported. Numerous approaches have been taken to study the reaction mechanism8,9 as well as the selectivities and catalyst deactivation for the MTO reaction.5,6,10-12 Unfortunately, most of these approaches involve conversion levels close to 100% due to the high MTO reaction rate, making it difficult to avoid the effects of secondary reactions. In addition, as a result of the very fast deactivation, the main reactions are difficult to decouple from the coking reactions themselves. The reaction mechanism, the effect of adsorption and diffusion, and secondary reactions, including coke formation, are aspects of the MTO reaction that are still not resolved. * To whom correspondence should be addressed. Phone: +47 73 59 41 51. Fax: +47 73 59 50 47. E-mail: holmen@ chembio.ntnu.no. † Norwegian University of Science and Technology. ‡ SINTEF Applied Chemistry. § Present address: Statoil Research Center, Postuttak, N-7005 Trondheim, Norway.

Evidently, fundamental research devoted to the study of the MTO reaction requires some compromises in terms of experimental complexity. A new microbalance (TEOM ) tapered element oscillating microbalance) has been developed and applied to the study of catalytic reactions involving coke formation and deactivation,13,14 sorption at reaction conditions,15 adsorption and diffusion in zeolites,16-19 and kinetics of gas-solid reactions.20,21 The present study focuses on the sorption, diffusion, and catalytic reactions during MTO over SAPO-34. This work is part of a research program aiming at a more fundamental understanding of the MTO reaction through the study of the kinetics and reaction mechanism, including catalyst deactivation by using the TEOM reactor. Kinetic studies measuring the gas-phase conversion give indirect information about the concentration of intermediates on the surface and the activity of the catalytic sites. Zeolitic catalysts with well-defined active sites have high adsorption strength and capacities, and reactants are considered to lose their gas-phase characteristics inside the zeolite pores. For a detailed understanding of reactions in zeolites, it is important to consider the actual concentration of reactants in the zeolite pores as well as the number and activity of the available sites. This is also essential for an understanding of the effect of coke formation on the activity of zeolites. Reactant adsorption in zeolites is usually studied at low temperature and then extrapolated in order to estimate the adsorption parameters at reaction conditions. For coked samples, the TEOM reactor has proved to be an excellent tool for studying in situ the effect of

10.1021/ie9807046 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/09/1999

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coke deposition on the adsorption and reaction simultaneously.15,16 Diffusion usually plays an important role for catalytic reactions in zeolites. Very little information about the effect of diffusion of reactants and products on the reactions in SAPO-34 has been found in the literature. However, a great deal of experimental and theoretical effort has been made to obtain diffusivities in zeolites in general, and an extensive review has been made by Ka¨rger and Ruthven.22 Previous works16,17,19 have shown that the TEOM is one of the more promising tools for studying diffusion in zeolites. The present approach uses the TEOM reactor to deconvolute the MTO reaction mechanism into reaction, adsorption, and mass transfer phenomena and to study the effect of coke formation on the adsorption during reaction. The adsorption, diffusion, and reaction of propene are also studied in order to understand the importance and mechanism of secondary reactions of olefins. 2. Experimental Section The experimental setup including the TEOM reactor is designed for transient (pulse) as well as for continuous experiments. The setup is similar to what has been described previously.14,16 Methanol was introduced into an evaporator by a high-pressure liquid micro-mass-flow controller. Part of the helium was introduced at the entrance of the evaporator in order to carry the liquid methanol. The diluted methanol vapor was fed to the reactor through a carefully controlled four-way valve. A second line was connected to this four-way valve, where the flow rate of pure helium was kept exactly similar to the total flow rate in the parallel line. The pressure of helium in this line was identical to that in the reactor in order to minimize the interference in the mass signal during a switch. Calcined SAPO-34 with a unit cell composition of (Si2.88Al18P15.12)O72 was obtained from SINTEF Applied Chemistry. The synthesis of SAPO-34 is similar to what was reported by Dahl et al.23 The catalysts were characterized by liquid N2 adsorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), and temperature programmed desorption (TPD) of ammonia.24 No crystalline impurity was observed by XRD. From the SEM images the SAPO-34 crystals appeared as cubes varying in size, but all were less than 2 µm. No amorphous phase was observed. A pore volume of 0.26 cm3/g and a BET surface area of 615 m2/g were obtained from N2 adsorption. By NH3 TPD the strong acid sites were measured to be 1.08 mmol/g, which is in good agreement with Grønvold’s result.24 A catalyst density of 1480 kg/m3 was reported by the producer (SINTEF). The catalyst particles (52-140 mesh) were dried in situ at 500 °C for more than 3 h. Quartz particles (52-140 mesh) were placed between the quartz wool and the catalyst particles in order to avoid temperature gradients and to improve the distribution of the flowing gas in the catalyst bed. The MTO reaction was studied continuously as well as at transient conditions using 3-min, 9-s, and 6-s pulses. Most of the MTO experiments were performed at 698 K and a methanol partial pressure of 7.2 kPa. Mass changes caused by adsorption, coking, and desorption were recorded continuously with different time intervals; the fastest was every 0.11 s. The reactor

effluent for the 3-min pulses was analyzed by GC after 2-min on stream. The effect of the partial pressure on the main reactions and the deactivation over SAPO-34 was studied by 9-s pulses, where the reactor effluent was analyzed after 10 s on steam. The space velocity was adjusted to obtain similar degrees of conversion. The higher the partial pressure, the higher the space velocity had to be to keep the conversion constant. The results show that the coke formation depends on the total amount of methanol fed per catalyst mass. Short pulses were necessary simply to obtain a low increase in coke content from each pulse. Adsorption of methanol and propene was carried out in the TEOM at partial pressures of 0.5-10 kPa and temperatures of 323-393 K with a catalyst loading of 5.2-7.6 mg. Before each set of experimental runs (increasing sorbate partial pressures at a given temperature), the catalyst was regenerated in air for 2 h and then kept in helium for 30 min. High sensitivity of the TEOM (about 2-5 µg) makes it possible to measure a small mass change due to adsorption at reaction conditions. With controlled partial pressure 6-s pulses of methanol/helium were used to measure the adsorption of methanol at reaction conditions. Consecutive 6-s and 3-min pulses were used to study the effect of coke formation on the adsorption of methanol. Due to the operating principle of the TEOM, the mass of the gas occupied in the void volume of the tapered element affects the change in vibrational frequency of the reactor. A mass change proportional to the density difference between the two gas mixtures is detected when switching from one gas to another at isothermal conditions. The determination of the adsorbed amount or coke formation is done by correcting this density change. The mass change due to a density change can be measured in a quartz particle bed at identical condition.15 The density change at identical purge flow rate and total gas flow rate through the sample cell can also be estimated by eq 1, where M2 is the average

∆m ) VeP(M2 - M1)/RT

(1)

molecular weight of the feed mixture, M1 is the molecular weight of the inert gas, ∆m is the mass change detected in the TEOM due to the density change, Ve is the “effective volume”,13 T is the temperature, and P is the pressure. The estimated effective volume does not depend on the temperature and the composition of the reactant mixture but was found to be dependent on the flow rate of the purge gas. Diffusion of purge gas into the tapered element (TE), damping effects, and possible temperature differences between the in and out sides of the TE (high sensitivity of the oscillating frequency to temperature) are possible reasons for the effect of the flow rate of the purge gas on the estimated effective volume. 3. Results and Discussion 3.1. Adsorption, Desorption, Reaction, and Coke Formation during 3-min Pulses. The catalyst lifetime in MTO at the reaction conditions used in the present work is almost equal to the time required for one GC analysis, about 30 min. Hence with our experimental setup continuous experiments have to be repeated a number of times to obtain sufficient data of conversions and selectivities against coke content or

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Figure 1. Transient mass response (Mt, g) during four subsequent pulses (3 min) of methanol on 5.5 mg of SAPO-34 at T ) 698 K, WHSV ) 385 h-1, and PMeOH ) 7.2 kPa. For comparison the mass response from a pulse (3 min) on quartz is shown.

time on stream. The experimental technique with 3-min pulses, which reduces the number of necessary experiments significantly, was therefore used in the present approach. It has been proven that the cumulative coke content and the conversion were almost identical for both continuous and pulse experiments.25 Interrupted (pulse) experiments made it possible to carry out several time-consuming GC analyses during the same run. Typical mass response curves are shown in Figure 1. A rapid increase was observed initially caused by both adsorption and gas density change in the microbalance reactor. The density change was accounted for by subtracting a blank run with inert quartz particles, as shown in Figure 1. The weight increase following the initial period was caused by coking and was almost linear with time on stream. After switching back to pure He after 3 min, the weight curves decreased first rapidly due to the density change from the reactant mixture to pure helium and followed by a slow decrease caused by desorption and diffusion of products out of the pore system. However, the effect of cracking of coke precursors in the zeolite cages on the slow decrease in weight should be considered. Anyhow, the fact that the rate of coke formation was identical for different pulse lengths (from 6 s to 3 min) as well for continuous experiments25 showed that the effect of cracking could be neglected. After about 30 min, the mass response curves were stable, and the permanent mass increase during the 3-min pulses, which is defined as coke in the present work, could be obtained. By assuming a linear change in the weight with time on stream during the 3-min pulses, the coking rate could easily be calculated. Figure 1 shows that the desorption/diffusion rate was lower than the adsorption/diffusion rate of methanol on SAPO-34, probably because of the larger molecular size of the products. This indicates that the rate of desorption/diffusion might influence the overall reaction rate. Due to the very rapid coke deposition, the adsorption process was coupled with coke formation and could not be distinguished from it. Hence, it was difficult to obtain the absolute adsorption amount from these mass response curves. As mentioned above, the mass decreases were mainly caused by the desorption of products but could also partly be due to reactant desorption. However, the desorption of products can be measured at a conversion

Figure 2. Transient mass response curves for a 6-s pulse of methanol over the catalyst bed (A), over a quartz bed (C), and the corrected curve for the mass response over SAPO-34 (B) (catalyst loading, 5.5 mg) at T ) 698 K, WHSV ) 283 h-1, and PMeOH ) 7.2 kPa. Table 1. Desorbed Amount after the First Pulse during the MTO Reaction over SAPO-34 at 698 K, a Methanol Partial Pressure of 7.2 KPa, and Different Space Velocities, thus Different Conversions of Oxygenates (methanol and DME) WHSV(g/gcat.‚h)

384

253

114

82

57

conversion (wt %) amount desorbed (g/(gcat.‚%))

39.0 1.9

59.6 1.6

80.1 1.8

89.5 1.8

92.5 1.8

close to 100%, where all reactant molecules are converted to products. The amount of product desorbed can be determined from the mass response curves by subtracting the mass change due to coke formation and density change. The desorbed amounts obtained during the first 3-min pulses for different space velocities are listed in Table 1. As shown in Table 1, the desorbed amount was relatively constant, regardless of the conversion. This can be explained by strong methanol adsorption on the active sites and by the fact that the desorbed amount of methanol is relatively small. 3.2. Measurement of Adsorption during Reaction. The adsorption parameters could not be directly obtained from the mass response curves for the 3-min pulses in Figure 1, due to the fast coke formation. It is therefore necessary to use very short pulses to minimize the coke formation. The response from pulses of different lengths was compared, and 6-s pulses were selected for measuring adsorption during the MTO reaction. A typical mass response curve for a 6-s pulse is shown in Figure 2. Curve B in Figure 2 shows the adsorption and desorption with time on stream, which was obtained from the raw data (A) after correction for the mass response of a pulse over a nonadsorbing quartz bed at identical conditions (C). Figure 2 shows that the rate of desorption is very slow. The concentration of methanol in the pores was estimated from the adsorbed amount. Assuming that the methanol partial pressure of 7.8 kPa is low enough to operate in the Henry’s law region, the Henry’s law constant under reaction conditions was calculated to be 0.035 mmol/(g‚kPa). The adsorption measurements at reaction conditions were compared to the adsorption parameters found at low temperatures. The isotherms for methanol adsorption over SAPO-34 at 348, 373, and 398 K were measured in the TEOM and are shown in

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Figure 3. Methanol adsorption isotherms over SAPO-34 at different temperatures: (, 348 K; 2, 373 K; 9, 398 K. Lines: predicted by the Langmuir model.

Figure 4. Dimensionless Henry’s law constant (K ) bqS) versus inverse temperatures: b, methanol; 9, propene.

Table 2. Saturation Concentration in Adsorbed Phase qS and Langmuir Constant b for Given Temperature (°C) in the Langmuir Model (Equation 2) Based on the Results Given in Figures 4 and 6 methanol qS (mmol/g) b (kPa-1)

propene

75 °C

100 °C

125 °C

50 °C

75 °C

100 °C

4.71 1.69

3.79 1.16

2.84 0.67

1.29 1.83

1.11 1.35

1.02 0.80

Figure 3. In none of the adsorption experiments was dimethyl ether (DME) detected by the GC analysis. The data were fitted by the ideal Langmuir model (eq 2),

( )

1 1 1 1 ) + q qS bqS p

(2)

and the estimated parameters are listed in Table 2. In eq 2 q is the adsorbed concentration (mmol/g), qS is the saturated concentration (mmol/g), p is the partial pressure (kPa), and b is the Langmuir constant (kPa-1). It has to be pointed out that the ideal Langmuir equation did not fit the experimental data exactly, as shown in Figure 3. This is probably caused by a different adsorption mechanism at low and high methanol partial pressures.26 However, the simple Langmuir equation can still be used as an approximate description of the methanol adsorption isotherm, as was also done by Kmiotek et al.27 for methanol adsorption on HZSM-5. Such a fit is also useful as a first estimate of the Henry’s law constant (K ) bqS). The K values obtained were then converted to a dimensionless form (moles of sorbate per unit volume in the adsorbed phase, divided by moles of sorbate per unit volume in the fluid phase), where the measured pore volume of SAPO-34 of 0.26 cm3/g was used.24 Figure 4 illustrates the dimensionless K plotted against the inverse of the absolute temperature. The consistency between the adsorption parameters measured during reaction and the values obtained at low temperature is good, implying that reasonable adsorption parameters could be measured by 6-s pulses at reaction conditions. Propene adsorption on SAPO-34 was studied in a similar way. Since the coke formation during propene conversion over SAPO-34 at identical reaction conditions is much slower than during the MTO reaction, 1-min pulses were used to study the sorption and reaction of propene.

Figure 5. Isotherms for adsorption of propene over SAPO-34 at different temperatures: (, 323 K; 2, 348 K; 9, 373 K. Lines: predicted by the Langmuir model.

The method for measurements of propene adsorption at low temperature was also different from the measurements of methanol adsorption. While a methanol adsorption equilibrium was reached after a short period, an apparent adsorption equilibrium was not observed during propene adsorption. After a rapid initial mass increase, the mass continued to increase relative slowly with time on stream. This is in good agreement with the observation for olefin adsorption over HZSM-5,28 where the results were explained by irreversible adsorption. Olefins are considered to be adsorbed as carbenium ions on zeolites, and oligomers can be formed on the zeolite surface at ambient temperature via carbenium ions.29 The mass increase at longer time periods thus corresponds to the coke formation from oligomers. Heating in an inert gas was sufficient to desorb completely what was adsorbed. For propene adsorption measurement 50-s pulses were used to minimize the coke formation. The isotherms for propene at 323, 348, and 373 K are shown in Figure 5, and the estimated parameters of the ideal Langmuir model are listed in Table 2. The dimensionless Henry’s law constant is also plotted against the inverse of temperature in Figure 4. It is evident from Figure 4 that Henry’s law constant at reaction temperature can be extrapolated from low-temperature values. Figure 6 presents the predicted isotherm at the

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Figure 6. Adsorption of methanol (q, mmol/gcat.) over SAPO-34 versus the partial pressure of methanol (p, kPa) at 698 K. Experimental results: 2, 9-s pulses; b, 6-s pulses. Line: predicted by the Langmuir model.

reaction temperature and shows that the predicted adsorbed amount is close to the measured mass increase during the first 6-s pulses at different partial pressures of methanol. However, the mass increase during the first pulses of the integrated 9-s pulses, which were measured at different partial pressures and space velocities but at identical conversion, were 20-40% higher than the equilibrium adsorption amounts predicted by the Langmuir model. This observation suggests that some coke was formed during the 9-s pulses. 3.3. Diffusion and Reaction in SAPO-34. Data for the adsorption of methanol and propene are used to estimate the diffusivities in SAPO-34. The pulse shape is normally affected by the void volume and the backmixing in the reactor, in the valves, in the connection of lines and also by the valve response time. No attempt was made to estimate diffusivities from the mass response curves at the reaction temperature, due to the difficulty in generating well-defined pulses. It took about 5 s to reach the desired concentration after an injection of a pulse at the conditions used in the present work. Nevertheless, the TEOM is suitable for measuring the diffusivities at relatively low temperatures.16,17,19 The effects of bed diffusion in the diffusion measurements have been studied by varying catalyst loading from 5 to 20 mg at 298 K and a methanol partial pressure of 7.7 kPa. The measured methanol diffusivity was found to increase with decreasing sample size, indicating a significant effect of bed diffusion. Two additional experiments have been done on the 5 mg catalyst sample with hourly methanol space velocities of 0.6 and 1.5. Identical methanol diffusivities were obtained, indicating that the bed diffusion and the thermal effect can be neglected for 5 mg catalyst samples. A typical uptake curve for methanol diffusion in SAPO-34 at 303 K and a methanol partial pressure of 1.8 kPa is shown in Figure 7. The transient diffusion equation30 for a slab geometry (eq 3) for sorbate uptake

mt me

)1-

8

∞

∑

1

π2n)0(2n + 1)2

(

exp -

)

(2n + 1)2π2DCt 4L2

(3)

was found to give the best fit to the experimental data, where mt is the adsorbed amount at time t (s), me is the

Figure 7. Uptake curve for methanol over SAPO-34 at 303 K and a methanol partial pressure of 1.8 kPa: O, experimental data; line, predicted by eq 3.

Figure 8. Inverse uptake and steady-state diffusion times over SAPO-34 versus inverse temperature at different partial pressures. Methanol: 4, 0.75 kPa; 0, 1.48 kPa; O, 2.92 kPa; Propene: *, 0.62 kPa.

adsorbed amount at infinite time, L is the half-length of the slab, and DC is the uptake diffusivity. The zeolite samples studied in the present work have a certain crystal size distribution, meaning that the absolute diffusivity is difficult to extract from the uptake curves. However, the inverse characteristic diffusion time (DC/L2) is a more precise parameter in a reaction/ diffusion model. Therefore, only the inverse characteristic diffusion time was reported in the present work. Figure 8 shows that the estimated inverse characteristic uptake diffusion times depend on the partial pressure and the temperature. The uptake diffusivities or diffusion times are estimated from uptake curves based on the concentration in the adsorbed phase, in contrast to the reaction rate in a diffusion/reaction equation which is based on the gas-phase concentration. The uptake diffusivity is therefore multiplied by a Henry’s law constant to transform

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to the effective diffusivity based on the gas-phase concentration.31 A similar approach was used by Garcia and Weisz,32 where the “steady-state diffusivity” was defined. If all the molecules internally sorbed are assumed to be equally mobile, the definition of the steady-state diffusivity is given by eq 4, where DSS is

Table 3. Weisz Numbers (W ) tD/tR) for the MTO Reaction and Propene Conversion over SAPO-34, tR ) CsS/(rAGS)obs ) 1/kobs and tD ) L2/DeA

DSS ) DCC∞/C0

Table 4. Turnover Frequencies (TOF), Site Coverages (θ), and Rate Constants (k) during the MTO Reaction and Propene Conversion over SAPO-34 at 698 K and a Partial Pressure of 7.2 kPa

(4)

the steady-state diffusivity, C∞ is the concentration inside the pores at infinite time, and C0 is the concentration of probe molecules in the gas phase. The steadystate diffusivity is then identical to the effective diffusivity in Post’s approach.31 Due to the large adsorption capacity of zeolites, the steady-state diffusivity could be 2-5 orders of magnitude larger than the uptake diffusivity. For example, the inverse characteristic uptake diffusion time estimated from the uptake curve shown in Figure 8 was 0.035 s-1, whereas the inverse characteristic steady-state diffusion time was 879 s-1 by using C∞/C0 ) 24 978. In agreement with the results of Garcia and Weisz32 and Rebo et al.,17 the steady-state diffusion over SAPO34 was found to be a nonactivated process, as shown in Figure 8. It means that the steady-state diffusivity can be directly put into the Weisz-Prater criterion at reaction conditions. The applicability of measured steadystate diffusivity in predicting diffusion effects on reactions has been proven previously.19 It was found19 that the diffusivity of methanol estimated from the reaction rates on the differently sized crystals at high temperatures was identical to the steady-state diffusivity measured at low temperatures. The olefin formation from methanol is a complex reaction. If DME and all hydrocarbons including coke are treated as a product, the MTO reaction can be considered as a simple reaction: MeOH f products. The classical Weisz-Prater criterion33 is then used to predict the importance of diffusion during the conversion of methanol over SAPO-34. The Weisz number (W) was defined as follows:

W ≡ ηφ2 ) (rA)obsFSL2/DeACsS

(5)

where (rA)obs is the apparent reaction rate, FS is the density of the catalysts (14 800 kg/m3), DeA is the effective diffusivity, L is the half-length of the crystal, CsS is the concentration at the crystal surface, φ is the Thiele modulus, and η is the effectiveness factor which can be calculated by eq 6. When the Weisz number is

η)

tanh φ φ

(6)

known, the effectiveness factor can also be estimated from eq 5. The Weisz number can be described as a dimensionless ratio of two time constants W ) tD/tR, where tR ) CsS/(rAFS)obs ) 1/kobs is a measure of the characteristic reaction time of the reactant and tD ) L2/ DeA is the intracrystalline characteristic diffusion time in the crystal with a characteristic half-length L. The effective diffusivity DeA is considered to be identical to the measured steady-state diffusivity DSS. It was assumed that the effect of intercrystalline diffusion was negligible, since small particles were used. W . 1 implies that the reaction rate is controlled by diffusion, while W < 1 implies that the reaction is free of diffusion limitations.

methanol propene

tR (ms)

tD (ms)

W

η

0.37 41.23

1.11 1.56

3.00 0.04

0.33 0.99

MTO propene conversion

TOF (s-1)

θ

k (s-1)

1.16 0.01

0.23 0.06

5.04 0.17

The kinetic experiments were performed at 698 K and a reactant partial pressure of 7.2 kPa for both the MTO reaction and the propene conversion. The measured characteristic reaction time of reactants, the diffusion times tD, the calculated Weisz numbers, and the effectiveness factors are presented in Table 3. The effectiveness factors in Table 3 show that the methanol conversion is influenced by methanol diffusion, whereas propene conversion is not limited by propene diffusion. However, it cannot simply be concluded that the propene conversion is free of diffusion limitations, because the diffusion of products might also affect the reaction rate, considering multicomponent diffusion. 3.4. Kinetics. The TEOM reactor provides a promising way to simultaneously study the adsorption and reaction in a catalytic system, since it permits measuring rapid and small changes in the catalyst mass.15 The reaction rate can be described by the following equation:

TOF ) kθ

(7)

where TOF ) r/NS is the turnover frequency, r is the reaction rate (mol/(gcat.‚s), k is the rate constant (1/s), θ ) N/NS is the site coverage, N and NS are concentrations of intermediates on the surface and the total concentration of active sites (mol/gcat.), respectively. The number of active sites was assumed to be equal to the number of strong acid sites as determined by ammonia TPD experiments. The result was 1.08 mmol/g determined by the amount of NH3 desorbed above 553 K. The turnover frequency (TOF), site coverage, and rate constant for the MTO reaction and propene conversion are all listed in Table 4. Table 4 shows a rather low surface coverage in SAPO-34 for the MTO reaction, implying that only a fraction of the acid sites takes part in the reaction simultaneously. The apparent reaction rate for propene conversion over SAPO-34 is much lower than for the MTO reaction. As shown in Table 4, the site coverage and the rate constant were lower for propene than for methanol. In fact, the site coverage of propene was about four times lower than for methanol. The low propene reactivity over SAPO-34 explains why the secondary reactions of olefins have little importance for the product selectivities during MTO over SAPO-34. Figure 4 shows that the difference in the adsorbed concentration of methanol and propene becomes smaller at higher temperatures, suggesting that the secondary reactions of olefins will become relatively more important at higher temperatures.

Ind. Eng. Chem. Res., Vol. 38, No. 11, 1999 4247 Table 5. Ratios between the Henry’s Law Constant for Propene (Kpropene) and Methanol (Kmethanol) and the Coke Selectivities during MTO at Different Temperatures temp (°C) Kpropene/Kmethanol coke selectivitya (wt %)

425 0.51 1.2

500 0.56 2.0

550 0.60 4.5

a Coke selectivity (wt %) ) [coke content (g/g cat.)]/[the amount of hydrocarbon formed (g/gcat.) × 100. A coke content of 18 wt % was used in the calculation.

Table 5 shows that the coke selectivities increased significantly with increasing temperature. The coking rate depends on the intrinsic rate constant of coke formation and the concentration of coke precursors inside the pores, where carbenium ions or adsorbed olefins inside the pores in SAPO-34 can be considered as the major coke precursors during MTO.25 The rate constant of coke formation is expected to increase with increasing temperature. In addition, the ratio of adsorbed olefins to adsorbed methanol, as indicated by the ratio of adsorbed propene to adsorbed methanol in SAPO-34 (Table 5), increased with increasing temperature, and will partly contribute to the higher coke selectivity at the higher temperature. It is expected that the relative adsorption of methanol and olefins depends on the hydrophilic nature of the catalysts. A modification of SAPO-34 to give an increased hydrophilicity could possibly reduce the coke deposition. Decreasing of the coking rate of SAPO-34 by the addition of water has been observed by Marchi and Froment.6 The authors suggested that the relatively strong water adsorption and hence lower adsorbed concentration of olefins is the main reason for lower coke deposition. 3.5. Effect of Coke Formation. In general, coke deposition in the zeolite pores influences the accessibility of active sites by blocking the sites directly and by blocking the pores, thus affecting the diffusion, the adsorption, and the reactions. If the reaction rate is influenced by diffusion, the rate is given by eq 8. The

TOF ) kθη

(8)

effect of coke deposition on the rate constant k depends on the acidic properties of the catalyst, that is, both the number and the strength of the acid sites. If there is more than one active site in each cavity or the acid strength distribution is nonuniform, the specific activity for each active site is most likely dependent on the coke deposition. The number of strong acidic sites of SAPO34 is 1.08 mmol/gcat. measured by NH3 TPD. SAPO-34 has the chabazite structure, which has three cavities per unit cell.24,34 From the composition of the unit cells, the number of cavities has been calculated to be 1.37 mmol of cavities/g. Assuming 100% crystallinity and a homogeneous distribution of the acid sites gives an estimate of 0.8 strong acid sites per cavity, which is in good agreement with Grønvold’s results.24 On the basis of the results of Nawaz et al.11 the distribution of the strong acidic sites is rather uniform. The reaction rate constant k for each active site can then be assumed to be constant, regardless of the number of active sites inside the crystals. Coke deposited inside the cavities blocks the active sites, hence lowering the number of active sites available, and consequently the concentration of the adsorbed reactant. Coke deposition also decreases the diffusivity due to pore blocking, and both effects could affect the measured reaction rate. The deactivation rate can be

Figure 9. Relative activity and adsorption of methanol versus coke content on SAPO-34 at WHSV ) 283 h-1, 698 K, and PMeOH ) 7.2 kPa: 9, relative adsorption (K/K0); b, relative conversion (X/X0). K0 and X0 are the initial Henry’s Law constant and initial conversion, respectively.

Figure 10. Desorption of products and conversion of methanol versus coke content on SAPO-34 at WHSV ) 114 h-1, 698 K, and PMeOH ) 7.2 kPa: (, amount of products desorbed; b, methanol conversion.

derived from eq 8 and is presented in eq 9, where

N η r ) r0 N0 η0

(9)

subscript zero refers to zero coke content. For reactions without diffusion limitation, a deactivation rate (r/r0) should be equal to the decreasing rate of the adsorbed amount (N/N0). When the deactivation rate is faster then the decreasing rate of the adsorbed amount (r/r0 > N/N0), the effect of diffusion on the deactivation must be considered. The changes in adsorption and conversion of methanol are illustrated in Figure 9, at WHSV ) 283 h-1 and a methanol partial pressure of 7.2 kPa, while the changes in desorption of products and conversion of methanol are presented in Figure 10, at WHSV ) 114 h-1 and identical methanol partial pressure. The amount of desorbed products is rather constant up to 13 wt % coke, after which it decreases significantly with the coke content. Figure 9 shows that the conversion and adsorption of methanol on SAPO-34 decreased linearly with the coke content (coke < 12 wt %) but that the conver-

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sion decreased more than the adsorption capacity, similar to what was observed by Hershkowitz15 in catalytic cracking of decane over zeolite Y. However, one should keep in mind that a basic requirement for the application of eqs 8 and 9 is the uniform distribution of coke in the reactor. A uniform coke distribution can be assumed at WHSV ) 283 h-1. As only a small change in the adsorption capacity was caused by coke formation, the significant decrease in conversion was most likely caused by the decrease in diffusion of methanol, hence a lower effectiveness factor. This observation is consistent with the results obtained from the study of differently sized SAPO-34 crystals.19,20 4. Conclusions The adsorption of reactant, reaction, desorption of products, and coke formation were studied as a function of coke content in an oscillating microbalance reactor under reaction conditions during the MTO reaction and propene conversion over SAPO-34. The amount of reactant adsorbed at reaction conditions could be extrapolated from low-temperature experiments quite well. The methanol adsorption changed only slightly, while the conversion changed significantly with coke formation, probably because of a lower reactant diffusion rate due to coke formation in the catalyst. Both the reactivity and site coverage of methanol were much higher than for propene, explaining why the secondary reactions of olefins were less important during the MTO reaction at these reaction conditions. Diffusions of methanol and propene were studied by an uptake method at relatively low temperatures. The inverse characteristic uptake and steady-state diffusion times were estimated from experimental data. The uptake diffusion times depend on the temperature and the partial pressure of methanol or the adsorbed amount, while the steady-state diffusion time is independent of the temperature and the adsorbed amount. The steadystate diffusion times were used to estimate the importance of diffusion during the reactions. The methanol conversion was found to be influenced by the methanol diffusion, while the propene conversion was not influenced by propene diffusion. Acknowledgment The support by the Norwegian Research Council and Norsk Hydro ASA is gratefully acknowledged. Literature Cited (1) Vora, B. V.; Marker, T. L.; Barge, P. T.; Fullerton, H. E.; Nilsen, H. R.; Kvisle, S.; Fuglerud, T. Economic Route for Natural Gas Conversion to Ethylene and Propene. Stud. Surf. Sci. Catal. 1997, 107, 87. (2) Chang, C. D. Methanol Conversion to Light Olefins. Catal. Rev.sSci. Eng. 1984, 26, 323. (3) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. Silicoaluminophosphate Molecular Sieves: Another New Class of Microprous Crystalline Inorganic Solids. J. Am. Chem. Soc. 1984, 106, 6092. (4) Froment, G. F.; Dehertog, W. J. H.; Marchi, A. J. Zeolite Catalysis in the Conversion of Methanol into Olefins. Catalysis 1992, 9, 1. (5) Liang, J.; Li, H.; Zhao, S.; Guo, W.; Wang, R.; Ying, M. Characteristics and Performance of SAPO-34 Catalyst for Methanolto-Olefin Conversion. Appl. Catal. 1990, 64, 31. (6) Marchi, A. J.; Froment, G. F. Catalytic Conversion of Methanol to Light Alkenes on SAPO-34. Molecular Sieves. Appl. Catal. 1991, 71, 139.

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Received for review November 9, 1998 Accepted August 18, 1999 IE9807046