Comparative Study on the Chloromethane to Olefins Reaction over

Sep 26, 2014 - ABSTRACT: Conversion of chloromethane to olefins over SAPO-34 and HZSM-22 catalysts has been investigated at various temperatures...
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Comparative Study on the Chloromethane to Olefins Reaction over SAPO-34 and HZSM-22 Ling-tao Kong, Ben-xian Shen,* Ji-gang Zhao, and Ji-chang Liu* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237 People’s Republic of China S Supporting Information *

ABSTRACT: Conversion of chloromethane to olefins over SAPO-34 and HZSM-22 catalysts has been investigated at various temperatures. SAPO-34 showed higher activity and better stability than HZSM-22 catalyst. Ethylene and propylene were the main products over SAPO-34, whereas the main products over HZSM-22 were propylene, butylene, and C5+ alkanes in this conversion. Polymethylated benzenes and methylnaphthalene were the main components of coke species confined in the spent SAPO-34 catalyst. However, long-chain alkanes were the main species trapped in the spent HZSM-22 catalyst in the initial period, and many polymethylated benzenes emerged with growing time on stream. The differences in selectivity and coke species from both spent catalysts suggested that the conversion of chloromethane to olefins was greatly influenced by the topology structure of zeolite catalyst and that olefin methylation and cracking might be an important route followed over the HZSM-22 catalyst.

1. INTRODUCTION Due to excessive reliance on the declining reserves of crude oil and the increasing demand of bulk chemicals, the conversion of methane to high-value chemicals, as an alternative nonoil route, has been an extremely significant research field.1,2 Meanwhile, methane is the major component of the abundant natural gas, biogas, landfill gas, etc. However, to date, the only industrialized process to convert methane to hydrocarbons involves the production of syngas followed by Fischer−Tropsch synthesis or methanol to hydrocarbons, whereas syngas production is an expensive process that consumes great amounts of energy.3 Among the various approaches to activate methane,2−8 Taylor and co-workers reported a promising path by which methane was first converted to monosubstituted methyl halides followed by conversion of methyl halides to hydrocarbons using HZSM5 catalyst under relatively mild conditions in 1988.9,10 Considerable research works have been devoted to the transformation of methyl halides to hydrocarbons via zeolite catalysts over the years.11−23 The conversion and product distribution in the methyl halides to hydrocarbons reaction are greatly influenced by the topology structure and acidity of different zeolite catalysts. HZSM-5 catalyst employed in the reaction of methyl halides to hydrocarbons showed higher catalytic activity than the others, whereas the selectivity of light olefins was lower than that of SAPO-34 catalyst. Svelle and coworkers14−16 performed comparative studies on the conversion of methanol and methyl halides to olefins over zeolite materials from the perspective of experiment and computation, and the results demonstrated that the conversion of methyl halides to olefins over SAPO-34 has an extraordinary resemblance to the methanol to olefins (MTO) reaction according to the product distribution and the coke species retained in the catalyst. However, the conversion level of methanol in the MTO process is higher than that of methyl halides, which could be the reason that the methanol molecule is adsorbed more © 2014 American Chemical Society

strongly with the acid sites from zeolite materials than the methyl chloride molecule due to the existence of hydrogen bonds, leading to a higher coverage concentration of methanol molecules and a higher subsequent reaction rate.14−16,22 Su et al. systematically investigated the chloromethane to olefins reaction over various metal-modified SAPO-34 molecular sieves and proposed that the chloromethane to olefins reaction also followed the hydrocarbon pool (HP) mechanism over SAPO34.17−19 McFarland et al. found that the yield of light olefins from methyl bromide can be improved by the incorporation of Co into the SAPO-34 catalyst.20 In brief, due to suitable CHA cages with eight-member ring openings and relatively mild acidity, the SAPO-34 molecular sieve has been the most promising catalyst in the conversion of methyl halides to olefins. It has been suggested that the conversion of methyl halides to light olefins also follows the HP mechanism as MTO reaction.17 In recent years, massive contributions have elucidated the mechanism in MTO reaction, whereas the exact route in the formation of the initial C−C bond is still debatable due to the uncontrollable and complicated secondary reactions.24 Moreover, the increasing experimental and computational research works are inclined to support the HP mechanism, which was first proposed by Dahl and Kolboe.25,26 According to the HP mechanism in the methanol to hydrocarbons process, hydrocarbon pool intermediates retained in the catalyst are methylated by methanol, and then light olefins are produced from the intermediates by elimination reaction in an indirect way. Then numerous investigations indicated that polymethylated benzenes (polyMBs) acted as the Received: Revised: Accepted: Published: 16324

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2.2. Synthesis of SAPO-34. SAPO-34 catalyst was synthesized according to the previous literature.47 The molar composition of the precursor mixtures was 1.0 Al2O3:1.0 P2O5:0.60 SiO2:2.0 TEA:0.20 TEAOH:60 H2O. In a typical synthesis, 15.37 g of orthophosphoric acid was diluted by 35 g of deionized water with vigorous stirring, and then 9.89 g of pseudoboehmite was mixed stepwise. After stirring, 8.00 g of colloidal silica was added to this mixture, which was then stirred for a further 2 h before 7.85 g of TEAOH and 13.65 g of TEA were added. The precursor gel was transferred into Teflon-lined stainless steel autoclaves with a capacity of 0.2 L and aged at ambient temperatures for 24 h and then crystallized at 200 °C for 24 h under autogenous pressure. Finally, the SAPO-34 mixture was centrifuged, washed, dried at 100 °C, and then calcined at 550 °C for 6 h to remove the template. The ZSM-22 raw powder was purchased from Nanjing JiCang nNanotechnology Co., Ltd. The purchased ZSM-22 raw powder was first calcined at 550 °C for 8 h to remove the template. Those calcined samples were ion exchanged with 3 × 6 h using 0.5 M NH4NO3 solution and then calcined at 550 °C for 4 h to proton form. 2.3. Chloromethane to Olefins Reaction Test. The catalytic performances of the obtained SAPO-34 and HZSM-22 samples were evaluated by the chloromethane to olefins reaction at 400, 450, and 500 °C operated under near atmospheric pressure in a stainless steel fixed-bed reactor (10 mm i.d.), respectively. Catalyst (0.72 g; 40−60 mesh) was filled, and the weight hourly space velocity (WHSV) was 1.89 h−1. The catalyst was pretreated in dry nitrogen at 500 °C for 60 min. Chloromethane was fed with the nitrogen as diluent, and the rate of chloromethane/nitrogen was 1:5 in volume flow. The products from the reactor were analyzed by using a gas chromatograph equipped with a flame ionization detector. A capillary column (50 m × 0.32 mm i.d. fused-silica PLOT/ Al2O3) was used for the gas phase product separation. 2.4. Carbon Deposit Component Analysis. Organic intermediate species confined in the discharged catalysts, which were quenched immediately at different given reaction times over the chloromethane to olefins process, were analyzed following the procedures as described in previous literature.48 Spent SAPO-34 and HZSM-22 catalysts (0.2 g) were dissolved by 1 mL of 40% hydrofluoric acid solution in the Teflon tubule, respectively. After complete dissolution, the organic phase was extracted by 2 mL of dichloromethane, and then the organic extract was neutralized using 1 mL of 0.5 M sodium carbonate solution. Finally, the organic phase was analyzed by using an Agilent 6890-5973N GC-MS with HP-5 MS capillary column (30 m × 0.25 μm × 0.25 μm). 2.5. Characterization. The powder XRD patterns of all calcined samples were recorded on a Rigaku D/MAX-γB X-ray diffractometer with Cu Kα1 radiation (γ = 0.154056 nm), which was operated at 40 kV and 100 mA. The crystal morphology images of the samples sprayed gold were obtained by the scanning electron microscope on an FEI Nova NanoSEM450 with an acceleration voltage of 5 kV. The nitrogen adsorption isotherm was measured at −196 °C using the ASAP2020 autoadsorption analyzer. Prior to all measurements, all samples were degassed at 300 °C under vacuum for 8 h. The total surface area was calculated according to the BET isothermal equation, whereas the mesoporous volume was evaluated using the BJH and t-plot method. Chemical compositions of the tested catalysts were determined by inductively coupled plasma atomic emission spectroscopy

HP intermediates in MTO reaction over zeolite catalysts studied so far.27−32 More recently, the research works from Svelle and coworkers showed that a dual-cyclic catalytic mechanism in MTO reaction over ZSM-5 could exist.33−35 They found that ethylene was mainly produced from the aromatic based HP route, whereas higher olefins, such as propylene and butylene, might be formed by the olefin methylation and cracking cycle, which was first reported by Dessau.36−45 In other words, the aromatic based HP mechanism could not be the sole reaction route occurring for the production of light olefins in MTO reaction. Lately, the contributions involving olefin methylation and cracking route have been widely investigated over HZSM22(TON) catalyst in MTO reaction.38−43 The aromatic based HP mechanism could be suppressed largely in MTO reaction over HZSM-22 catalyst, which could not provide sufficient room for the accommodation of bulk polyMBs as the HP active species.44 Liu et al. and Svelle et al. found that HZSM-22 showed comparable activity as SAPO-34 catalyst and relatively high selectivity of C3+ products in MTO reaction. Therefore, it has been implied that the olefin methylation and cracking cycle was the main route of methanol conversion over HZSM-22. However, the chloromethane to olefins reaction over HZSM22 catalyst has not yet been investigated until now. In the present work, our aim is to further understand the process of chloromethane to olefins over zeolites with different topologies. In addition, it is useful in the rational design of catalysts for controlling the selectivity of the desired products in chloromethane to olefins reaction by investigating the effect of catalysts with different topologies on product distribution. Hence, the catalytic conversions of chloromethane to olefins and the speciation of the occluded coke over SAPO-34 and HZSM-22 have been studied in detail under various temperatures.

2. EXPERIMENTAL SECTION 2.1. Zeolite Structure. The topology structures of SAPO34 and HZSM-22 zeolites are provided in Figure 1,46 and their

Figure 1. Topology structures of SAPO-34 and HZSM-22.

structural properties are summarized in Table S1 in the Supporting Information. SAPO-34 is a silicoaluminophosphate molecular sieve with large CHA cages that are interconnected by eight-membered rings in three-dimensional mode, and the pore mouth of CHA cage is 0.38 × 0.38 nm in diameter. HZSM-22 (TON structure, Si/Al = 35) is an alumiunosilicate molecular sieve incorporated by unidimensional noninteracting 10-ring channels with diameters of 0.46 × 0.57 nm. 16325

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50 nm. Yang et al.49 reported the MTO reaction over nanoSAPO-34 catalysts. It has been found that the nanosize catalyst notably enhanced the lifetime of SAPO-34 and lowered the coke formation rate, whereas the product distributions in MTO reaction were basically identical over SAPO-34 catalyst with different particle sizes. Hence, the particle size of HSAPO-34 catalyst has an extremely limited influence on the product distribution in the chloromethane to olefins reaction. Nitrogen adsorption and desorption isotherms are shown in Figure S1 of the Supporting Information, and the textural properties of SAPO-34 and HZSM-22 catalysts are summarized in Table S1. The overall acidity properties of the SAPO-34 and HZSM-22 samples were estimated by temperature-programmed desorption of ammonia and the ammonia desorption profiles as presented in Figure 4 and Table 1. The deconvolution was

(ICP-AES) using an Agilent 725ES instrument. Thermal analyses were measured on an SDT Q600TGA analyzer. The temperature was increased to 800 °C under flowing air (60 mL/min) at a constant ramping rate of 10 °C/min. NH3-TPD experiments were measured using the ASAP2920 equipped with a thermal conductivity detector (TCD). Before the measurement, 100 mg of sample was degassed in a He stream (20 mL/min) at 500 °C for 1 h. After cooling to 100 °C, the sample was put in a mixed gas flow of 10% NH3 and 90% He (50 mL/min) for the adsorption of the NH3 subsequently, and then the sample was exposed to flowing He for 30 min to remove weakly adsorbed NH3. Finally, a TPD profile was obtained when the sample was heated from 100 to 800 °C at a ramping rate of 10 °C/min under flowing He (50 mL/min).

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. XRD patterns of SAPO-34 and HZSM-22 catalysts are shown in Figure 2. The XRD

Figure 4. NH3-TPD curves of the SAPO-34 (A) and HZSM-22 (B) samples. Figure 2. Powder XRD patterns for SAPO-34 (A) and HZSM-22 (B).

performed to separate the overlapped desorption peaks of ammonia.50 Both catalysts exhibit two evident ammonia desorption peaks. The ammonia desorption peaks at low temperature (∼170 °C) for SAPO-34 and HZSM-22 catalysts reveal the desorption of NH3 molecules with weak acid sites stemmed from surface hydroxyl groups. The ammonia desorption peak at high temperature (∼400 °C), usually assigned as strongly acid sites, is associated with Bronsted acid sites and Lewis acid sites. The relatively small desorption area of HZSM-22 indicates less acidity than from SAPO-34 catalyst. Sastre et al. reported that the similar product distribution has been observed in MTO reaction over SAPO-34 catalysts with different silicon contents.51 Liu and co-workers found that SAPO-34 molecular sieves with different acidity properties in the chloromethane to olefins reaction have great effects on the activity and coke content, whereas the product distributions were basically identical.52 Therefore, the different topology structures of HSAPO-34 and HZSM-22 catalysts with different acidity levels could have a more important effect on the product distribution in the chloromethane to olefins reaction. 3.2. Catalytic Performance. The conversion of chloromethane to olefins versus time on stream (TOS) over SAPO-34 and HZSM-22 catalysts under different temperatures is shown in Figure 5. At the beginning of the chloromethane to olefins reaction, SAPO-34 and HZSM-22 present high chloromethane conversion with the first sampling at a reaction time of 5 min, which indicates their high initial activities. The conversion over SAPO-34 at the tested reaction temperatures range gradual

pattern of the synthesized SAPO-34 sample indicates typical patterns of CHA-framework SAPO-34 materials with pure crystallinity in Figure 2A, which is in good agreement with those reported for SAPO-34 samples in previous literature.47 The HZSM-22 molecular sieve also exhibits a typical pattern of TON topology structure, which is in accordance with those reported results without any presence of impurity phase in the literature.42,45 The SEM images of SAPO-34 and HZSM-22 catalysts are displayed in Figure 3. The SAPO-34 particles are cubic-like morphology of about 1.5 μm in length. The HZSM-22 sample exhibits needle-shaped particles with an average size of 1 μm ×

Figure 3. SEM images of SAPO-34 (A) and HZSM-22 (B). 16326

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Table 1. NH3-TPD Acid Amount Calculation of SAPO-34 and HZSM-22 Catalysts acidity (mmol NH3/g solid) sample

weak and medium acid sites(100−300 °C)

strong acid sites (300−550 °C)

total amount

SAPO-34 HZSM-22

0.407 0.142

0.152 0.051

0.559 0.193

about 12% at 400 °C. Interestingly, the selectivities of butylene (15%) and C5+ alkanes (22%) were remarkable in chloromethane to olefins reaction over HZSM-22. It is commonly believed that the conversion of chloromethane to olefins over SAPO-34 also follows the HP mechanism. Moreover, polymethylated benzenes were expected as the important HP intermediates in this reaction.17,21 The general consensus is that the formation of light olefins by the aromatic based HP mechanism has been inhibited, whereas the olefin methylation and cracking route is favored to occur in MTO reaction over HZSM-22 catalyst. Hence, the priorities in the high selectivity of propylene and C5+ alkanes in chloromethane to olefins reaction over HZSM-22 catalyst might stem from the methylation of alkenes and subsequent cracking reactions. When this reaction was performed at 450 and 500 °C, the selectivity of propylene was gradually decreased while the selectivity of ethylene was rising over SAPO-34 catalyst. Meanwhile, the selectivity of methane was obviously increased, especially at 500 °C; the promotion in the selectivity of methane was outstanding and became the main product with time on stream over SAPO-34. It is easy to note that with the elevated reaction temperature over HZSM-22 catalyst, the selectivities of propylene, butylene, and C5+ alkanes were also decreasing while the selectivity of methane was increased. Therefore, the formation of methane is greatly influenced by the growing reaction temperature. Methane was expected as a primary product formed directly from methanol in MTO reaction.53 Su et al. reported that methane might stem from the cracking reaction in chloromethane to olefins reaction.54 In summary, it is hard to confirm the exact formation route of methane until now. Furthermore, the initial selectivity of ethylene was improved as the reaction temperature rose and gradually dropped to about 10% with time on stream over HZSM-22 catalyst, which indicated that a high reaction temperature is conducive to the formation of ethylene. 3.3. GC-MS Analysis of Coke Species. The differences in product selectivity over SAPO-34 and HZSM-22 catalysts have been clearly observed, and from them it can be concluded that the topological structure of catalyst has a significant influence on the exact reaction mechanism of chloromethane to olefins. The coke species trapped in spent SAPO-34 and HZSM-22 catalysts with different reaction times were detected by GC-MS, and the results are shown in Figure 7. These results including polymethylated benzenes, methylnaphthalene, and phenanthrene as the predominant components of the HP intermediate species over SAPO-34 catalyst at 450 °C are well in line with previous literature.15,19 With time on stream at 450 °C, the amount of HP species was rapidly increasing, and many macromolecular organic species appeared over SAPO-34 catalyst, such as phenanthrene and pyrene. The mole ratio of propylene to ethylene with the first sampling at a reaction time of 5 min was the highest and gradually decreased in Figure 6, which could be why the diffusivity of ethylene is slightly superior to that of higher alkenes such as propylene in the gradually blocked three-dimensional channels of SAPO-34 with rapidly increasing coke by product shape selectivity.41,55

Figure 5. Chloromethane conversion versus time at different temperatures over SAPO-34 and HZSM-22 catalysts. SAPO-34: (■) 400 °C; (●) 450 °C; (▲) 500 °C. HZSM-22: (△) 400 °C; (▽) 450 °C; (○) 500 °C (WHSV = 1.89 g CH3Cl/g catalyst/h; T = 400, 450, or 500 °C; atmospheric pressure; feed concentration = CH3Cl 16.7% mol).

declines with prolonged reaction time due to the coverage of acid sites and blockage of pore channels by coke.18−21 Clearly, HZSM-22 catalyst displays faster activity loss than that from SAPO-34 with time on stream. On the one hand, the onedimensional 10-member ring channels with nonintersecting HZSM-22 are more easily blocked by coke than SAPO-34 with three-dimensional cross pore channels. On the other hand, this could be ascribed to the lower acidity levels than those of SAPO-34 catalyst, and the acid sites are mainly distributed in the external surface of HZSM-22 catalyst.45 Therefore, the acid sites from HZSM-22 catalyst are much more readily covered by coke species. Figure 6 displays the product distribution of chloromethane to olefins over SAPO-34 and HZSM-22 catalysts at different temperatures. The products, including methane, ethylene, propylene, butylene, and so on, were identical in general, whereas the selectivity of main products was obviously different in this reaction over SAPO-34 and HZSM-22 catalysts. Ethylene and propylene were the main products over SAPO34, whereas propylene, butylene, and C5+ alkanes were the predominant products over HZSM-22 in chloromethane conversion, which is similar to the methanol conversion over HZSM-22 catalyst reported by Teketel.39 No liquid products were observed over both catalysts under this tested temperature range, and hydrogen chloride as the product is not plotted in the product distribution. When this reaction was performed at 400 °C, the selectivity of propylene gradually declined from 65% at the initial stage with time on stream and stabilized at 45% finally, whereas the selectivity of ethylene rapidly ascended and steadied at 35% after 60 min over SAPO-34 catalyst. However, over HZSM-22 catalyst, propylene was the dominating product with approximately 44% selectivity, whereas the selectivity of ethylene was 16327

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Figure 6. Product distribution of chloromethane to olefins over SAPO-34 and HZSM-22 at various reaction temperatures (400, 450, 500 °C): (■) methane; (●) ethane; (▲) ethylene; (▼) propane; (◆) propylene; (△) butylene; (▽) C4 alkane; (☆) C5+ alkane.

occur as the supplementary route over HZSM-22 catalyst in chloromethane to olefins conversion. The component evolution of the trapped organic species from both spent catalysts at 3 min TOS over different temperatures is depicted in Figure 8. The components of organic intermediate species over SAPO-34 were basically unchanged with elevated reaction temperature, whereas the amount of HP intermediate species was increasing according to the intensity of GC-MS peaks, which implied the reaction rate was obviously enhanced due to the elevated temperature. It is well consistent with the increasing selectivity of methane with the elevated reaction temperature over SAPO-34 in Figure 6, based on the balance of hydrogen molecules. In other words, the elevated temperature in chloromethane conversion was beneficial to the production of methane.

Meanwhile, this result may imply that during the initial stage and the steady period of this conversion over SAPO-34 catalyst, the formation routes of olefin might be diverse. When this conversion of chloromethane to olefins was performed over HZSM-22 catalyst, the organic intermediate species were predominant long-chain alkanes, and a handful of xylene also appeared at 3 min in Figure 8. With the growing time on stream, many polymethylated benzenes, methyl naphthalene, and so on were detected in this spent HZSM22 catalyst, which indicated that the actual reaction route over HZSM-22 is different from the route over SAPO-34. With combinations of the above-mentioned product selectivity, the methylation and cracking reactions might be the important route over HZSM-22 catalyst in this initial reaction stage. Moreover, with the increasing emergence of organic polymethylated benzenes, the aromatic based HP mechanism could 16328

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Figure 8. Composition evolution of the trapped organic species in the spent SAPO-34 (A) and HZSM-2 2(B) catalysts at 3 min TOS over different temperatures.

benzenes were emerging by oligomerization, hydrogen transfer, and so forth. 3.4. TG Analysis. The coke amounts of the discharged SAPO-34 and HZSM-22 catalysts with different TOS are shown in Table 2. The coke amount of SAOP-34 catalyst was

Figure 7. Evolution of the organic carbon deposit species retained in the spent SAPO-34 (A) and HZSM-22 (B) catalysts at different reaction times.

Table 2. TG Analysis for the SAPO-34 and HZSM-22 Catalysts with Different TOS

The organic intermediate species from the spent HZSM-22 were mainly long-chain alkanes, which are different from those from spent SAPO-34 catalysts in the initial reaction stage. Meanwhile, the components of organic intermediate species over HZSM-22 catalyst were also unchanged with the elevated reaction temperature, and few polymethylated benzenes as HP intermediate species were detected by GC-MS. The kinetic diameters of o-xylene and hexamethylbenzene were calculated at approximately 0.62 and 0.72 nm, respectively.44 Although the pore mouth of the HSAPO-34 cage is 0.38 × 0.38 nm in diameter, the interior diameter of the CHA cage is approximately 12.7 × 0.94 nm. The formation of polymethylated benzenes could occur in SAPO-34 catalyst by ship-inbottle. Nevertheless, HZSM-22 catalyst incorporated by onedimensional noninteracting 10-ring channels with the diameter of 0.46 × 0.57 nm cannot provide enough room for the formation of bulky polymethylated benzenes,40−43 which is well correlated with Figure 8. This suggests that the formation of bulky aromatics over HZSM-22 catalyst was suppressed largely in the initial reaction stage. With the further conversion of chloromethane over HZSM-22, more and more polymethylated

coke/catalyst (wt %) TOS (min) (at 450 °C)

TOS (min)

SAPO-34

HZSM-22

3 20 180

0.83 1.38 4.39

0.56 1.53 1.74

obviously greater than that from HZSM-22 catalyst at 180 min TOS, which could be attributed to the three-dimensional topology structure with CHA cage and the more acid sites of SAPO-34. The coke amounts were less at 3 min TOS for SAPO-34 and HZSM-22 catalyst. However, interestingly, the coke amount of HZSM-22 catalyst at 20 min TOS was 1.53%, which was close to the 1.74% at 180 min TOS. This suggested that most of the coke from HZSM-22 has been formed in the initial stage (0−30 min), which could be why the acid sites of HZSM-22 catalyst are mainly distributed in the external surface, which was much more readily covered by coke species. Hence, the conversion of chloromethane over HZSM-22 catalyst was 16329

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rapidly falling in the initial reaction stage, as depicted in Figure 5.

4. CONCLUSIONS SAPO-34 and HZSM-22 have been employed to comparatively investigate the conversion of chloromethane to olefins over different reaction temperatures. According to the differences in gas-phase product selectivity and the component of carbon deposit over SAPO-34 and HZSM-22 catalysts, the conversion of chloromethane to olefins reaction is strongly influenced by the zeolite topology structure. It has been found that ethylene and propylene are the main products over SAPO-34 catalyst at 400 and 450 °C. Meanwhile, the trapped intermediate species from SAPO-34 are polymethylated benzenes as in the previous literature. Furthermore, propylene was the dominating product with approximately 44% selectivity, whereas the selectivity of ethylene was about 12% with TOS at 400 °C. Meanwhile, large amounts of butylene and C5+ alkanes were formed over HZSM-22 catalyst in this reaction. Moreover, in the beginning stage of this conversion over the spent HZSM-22 catalyst, the long-chain alkanes were the main components of the retained intermediate species, and then numerous polymethylated benzenes subsequently emerged. In the initial stage of the chloromethane to olefins reaction, the aromatic based HP mechanism could be largely suppressed on the basis of the evolution of coke species over HZSM-22 catalyst. The olefin methylation and cracking route might be a more important reaction route to be followed over HZSM-22 catalyst in the chloromethane to olefins reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Nitrogen isothermal adsorption−desorption curves of calcined SAPO-34 and HZSM-22 samples; structural parameters of SAPO-34 and HZSM-22 catalysts; NH3-TPD analysis of the discharged SAPO-34 and HZSM-22 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(B.S.) E-mail: [email protected]. Phone: +86-21-64252916. Fax: +86-21-64252851. *(J.-c.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank the analysis and test center of the State Key Laboratory of Chemical Engineering in East China University of Science and Technology.

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REFERENCES

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