The methanol-to-hydrocarbons process was first discovered and investigated by Chang et al. [1, 2] in the 1970s. After several decades of research,the methanol-to-olefins (MTO) process has been developed as an alternative non-petrochemical route to the production of light olefins from coal via methanol,and this technique is currently employed on an industrial scale [3, 4].
Fundamental research into the MTO reaction mechanism has proceeded simultaneously with this industrial utilization [5, 6],and more than 20 distinct reaction mechanisms have been proposed to explain the direct formation of C-C bonds from C1 species such as methanol or dimethyl ether (DME) [7-11]. However,these direct C-C formation mechanisms are unlikely,due to the high associated energy barriers [12-14]. Dahl et al. [15-17] have put forward an alternative,indirect C-C formation mechanism,the so-called hydrocarbon-pool mechanism,that bypasses these high energy barriers,based on computational simulations and experimental data,and this mechanism is now generally accepted [18-23].
Because it is an excellent catalyst for the MTO reaction to produce light olefins,the MTO mechanism over ZSM-5 zeolite has been intensively investigated by many researchers [24-32]. In 2006,Svelle et al. [33] found inconsistencies in the 13C contents of ethene and other higher alkenes when carrying out 12C/13C-methanol switch trials over ZSM-5,and proposed that ethene formation was generated by a different mechanism than the other,higher alkenes. Soon after,they proposed a dual-cycle mechanism consisting of aromatic and alkene cycles to explain the inconsistent ethene and higher alkene data [34]. Other studies have found similar evidence of a dual-cycle mechanism and examined the properties of the two cycles in MTO reactions over different zeolites [35-37].
Scheme 1 summarizes the dual-cycle mechanism,which can be regarded as the combination of an aromatic cycle and an alkene cycle. In the aromatic cycle,for both paring and side-chain routes,methylcyclopentenyl (MCP+) and polymethyl- benzenium (polyMB+) cations are the primary active intermediates [24, 25, 38]. In the case of the alkene cycle,involving alkene methylation and cracking,C3 to C7 straight chain alkenes predominate [39-41].
The formation and reaction of MCP+ and polyMB+ cations and the deprotonated forms of these same compounds (Scheme 2) represent the key evidence for the aromatic cycle. Using solid-state magic angle spinning (MAS) NMR,several highly active carbenium cations,including MCP+ and polyMB+,have been successfully detected in zeolites. The generation and transformation of these species has been studied in detail so as to elucidate their important roles in the MTO reaction,using several zeolites such as ZSM-5,SSZ-13,SAPO-34 and Beta [24, 37, 38].
In the case of zeolites that do not favor the generation and accumulation of bulky cyclic species,such as ZSM-22,very low amounts of methylcyclopentadienes and aromatic species are generated compared with the amounts produced by SSZ-13,SAPO-34 and Beta zeolites. However,the MTO reaction still proceeds over ZSM-22,and primarily produces C3-C7 straight chain alkenes. In addition,12C/13C-methanol switch experiments indicate very low participation of bulky cyclic species in the reaction. The MTO reaction over ZSM-22 leads to a very high 13C content in the resulting alkenes,and so can be considered to be almost completely dependent on the alkene cycle to convert methanol and produce olefins [40, 41].
The dual-cycle mechanism is not just a simple addition of two cycles,but rather an integrated collective in which the two cycles significantly affect one another. As an example,the olefin products from the aromatic cycle can function as active intermediates during the alkene cycle. Another connection between the two cycles is the hydrogen transfer reaction,which is a secondary reaction in the MTO process and primarily converts higher olefins in the alkene cycle to hydrogen-rich alkanes and hydrogen-poor cyclic species such as methylcyclopentadienes and aromatics in the aromatic cycle [34, 42, 43].
In the present study,methanol conversion was performed over ZSM-5 zeolite at 300 °C as a means of studying the dual-cycle mechanism of the MTO reaction. Analyses of the effluent products as well as the species retained in the zeolite,in addition to 12C/13C-methanol switch experiments,were conducted to assess the relative predominance of the aromatic cycle or alkene cycle mechanism at different WHSV values and contact times. The data indicate that varying the reaction conditions during the MTO reaction over ZSM-5 can emphasize one reaction route over another.
The ZSM-5 zeolite used in this work was an NKF-5 zeolite in the H-form,purchased from the Catalyst Plant of Nankai University. This material was applied as the catalyst for MTO reactions after being calcined at 550 °C for 3 h.
The powder X-ray diffraction (XRD) pattern of the ZSM-5 zeolite was obtained using a PANalytical X’Pert PRO X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å),operating at 40 kV and 40 mA. N2 adsorption-desorption data were acquired with a Micromeritics ASAP 2020 physical adsorption analyzer at -196 °C. The crystal size and morphology of the zeolite were assessed by field emission scanning electron microscopy (FE-SEM,Hitachi,SU8020),and the chemical composition was determined using a Philips Magix-601 X-ray fluorescence (XRF) spectrometer.
The 29Si,27Al and 1H MAS NMR spectra of the ZSM-5 zeolite were recorded on a Bruker Avance Ⅲ 600 spectrometer using a 4-mm MAS probe,at respective resonance frequencies of 119.2,156.4 and 600.13 MHz. 29Si MAS NMR spectra were acquired at a spinning rate of 6 kHz using high-power proton decoupling,accumulating 1024 scans with a π/4 pulse width of 2.5 μs and a 10-s recycle delay. Chemical shifts were referenced to 4,4-dimethyl-4-silapentane sulfonate sodium salt (DSS) at 0 ppm. 27Al MAS NMR spectra were obtained at a spinning rate of 12 kHz using a single pulse sequence,and 600 scans were accumulated with a π/8 pulse width of 0.75 μs and a 2-s recycle delay. Chemical shifts were referenced to (NH4)Al(SO4)2•12H2O at -0.4 ppm. Prior to 1H MAS NMR experiments,the zeolite was dehydrated at 400 °C at a pressure below 10-3 Pa for 20 h. 1H MAS NMR spectra were acquired using a single pulse sequence with a π/4 pulse of 2 μs and a 10-s recycle delay,with adamantane (1.74 ppm) as the chemical shift reference. The sample was weighed to allow for quantification of the Brönsted acid site density,and the 1H MAS NMR spectrum was resolved using the Dmfit software package in conjunction with Gaussian-Lorentz line shapes,using adamantane (1.74 ppm) as the quantitative external standard,with a spectrum acquired under the same NMR conditions [37, 44, 45].
The ZSM-5 zeolite was pressed,broken into pieces and then sieved to obtained particles with sizes ranging from 40 to 60 mesh. Prior to the reaction,a quantity of the catalyst (50 to 100 mg) was loaded into a stainless steel tube reactor with an inner diameter of 5 mm and activated at 500 °C under flowing N2 (25 mL/min) for 40 min,after which the reactor was heated to 300 °C in preparation for the reaction. 13CH3OH was fed into the reactor by passing a carrier gas (25 to 129 mL/min) through a saturator containing methanol held at 5 to 14 °C,giving WHSV values of 2 to 12 h-1. All reactions were performed under atmosphere pressure. The effluent exiting the reactor was maintained at 200 °C and subsequently analyzed via online gas chromatography (GC),using a PoraPLOT-Q capillary column and a flame ionization detector (FID) in conjunction with mass spectrometry (MS,Agilent 7890B/5977A). The extent of methanol conversion and the selectivities for various products were calculated on a CH2 basis,considering DME as a reactant [46, 47].
In the 12C/13C-methanol switch experiments,12C-methanol was initially fed into the reactor for some time,after which the feed flow,which had passed through a saturator containing 12C-methanol,was quickly switched to pass through a saturator holding 13C-methanol and this scenario was maintained for an additional 0.5-3 min [46]. The isotopic distributions of the effluents were analyzed using the same online GC-MS instrumentation described in Section 2.2. Following the reaction,the reactor was rapidly quenched with liquid nitrogen to stop the reaction and the catalysts were discharged for analysis,as described in Section 2.4.
After the reaction,the catalyst was quickly cooled,discharged from the reactor,transferred into a Teflon vial,and dissolved in 20 wt% HF over 1 h. Following complete dissolution,the solution was neutralized with 7 wt% aqueous KOH,and CH2Cl2 was added to the vial to extract the chemical species retained in the catalyst. The CH2Cl2 phase was subsequently analyzed by GC-FID-MS (Agilent 7890A/5975C) using an HP-5 capillary column [48, 49]. Hexachloroethane was employed as an internal standard for quantification and the NⅠST11 mass spectral library was used to determine the identities of the various analytes.
The powder XRD pattern of the catalyst is provided in Fig. 1 and demonstrates the high purity and crystallinity of the ZSM-5 zeolite. The SEM image in Fig. 2 indicates a hexagonal morphology and an average crystal size of approximately 1.5 μm.
The 29Si MAS NMR spectrum obtained from the ZSM-5 (Fig. 3(a)) indicates Si atoms in several different chemical environments: Q3,Si(1Al) and two nonequivalent crystallographic Q4 sites [45]. The framework Si/Al ratio,calculated by deconvolution of the 29Si MAS NMR spectrum according to Loewenstein’s rule [50],was 19.0,reasonably close to that determined from XRF (17.9). The 27Al MAS NMR spectrum (Fig. 3(b)) shows that the majority of the Al atoms were in the framework tetrahedral coordination state. The 1H MAS NMR spectrum (Fig. 3(c)) exhibits peaks at 5.7,3.8,2.5 and 1.7 ppm,respectively attributed to bridge hydroxyl groups with hydrogen bonding and acting as Brönsted acid sites,bridge hydroxyl groups acting as Brönsted acid sites,aluminum hydroxyl groups with hydrogen bonding,and silicon hydroxyl groups [45, 51]. The Brönsted acid site density was determined to be 0.77 mmol/g by quantifying the amounts of both types of bridge hydroxyl groups based on their signal intensities,using adamantane as an external standard. Based on N2 adsorption-desorption data,the BET surface area and t-plot micropore volume of the ZSM-5 sample were 355 m2/g and 0.124 cm3/g,respectively.
Fig. 4 summarizes the effluent analysis data acquired during MTO reactions over the ZSM-5 at 300 °C. The methanol WHSV values 2,6 and 12 h-1 correspond to reactions performed with catalyst-reactant contact times of 156,32 and 16 ms,respectively. At the lowest WHSV of 2 h-1,the methanol conversion was only 1.4% at a time on stream (TOS) of 5 min,and the products were primarily C2 to C6,along with a substantial amount of methane. At TOS = 20-30 min,the reaction resulted in 100% methanol conversion and high selectivities for the relatively heavier C4 to C8 components and xylene. However,after reaction of 30 min,the methanol conversion gradually declined from 100% to approximately 55% at TOS = 255 min,accompanied by a decrease in the selectivities for C4 to C7 and an increase in the selectivities for ethene,propene and trimethylbenzene. At higher WHSV values of 6 and 12 h-1,the reaction exhibited an induction period and the methanol conversion increased in an “S” curve,which is typical of an autocatalytic reaction. Careful comparison of the data indicates that at higher WHSV values the induction period was increased,to 55 min at 6 h-1 and 105 min at 12 h-1,and the maximum conversion at each of these WHSV values dropped,from 74% to 50%. At the beginning of the induction period,the hydrocarbon products were primarily low-carbon species,such that only methane was detected at TOS = 5 min at 6 and 12 h-1,suggesting the deposition of hydrogen-poor species such as aromatic compounds in the zeolite pores. As the reaction proceeded,similar to the results obtained at 2 h-1,heavier species emerged among the effluents,and greater selectivity was observed for ethene,propene and various aromatics. After the induction period,the reaction seems to have stabilized,and the conversion and product selectivities remained relatively constant.
Comparing the reactions at the three different methanol WHSV values,there is a negative correlation between the maximum methanol conversion and the WHSV. Ⅰt is easy to understand that when using a constant amount of catalyst,as more methanol is supplied,there will be greater amounts of unconverted methanol and thus a lower maximum conversion rate. With regard to the selectivities for the gas phase products,although reactions of the three different WHSV values all produced primarily ethene,propene,butene and aromatics during the stable reaction time period,slight differences were still evident. During the low WHSV (2 h-1) reaction,more ethene and aromatics were generated,while the higher WHSV (6 or 12 h-1) reactions resulted in higher selectivities for propene and butene. In addition,there was less formation of propane and other alkanes during the high WHSV reactions. The hydrogen transfer index (HTI) values,defined as the C3H8/C3H6 selectivity ratios,are plotted as functions of TOS in Fig. 5. These data indicate the significantly promoted hydrogen transfer reaction at a low WHSV and a long contact time,and in contrast,the hydrogen transfer reaction is largely suppressed at high WHSV reactions. These variations in the selectivity imply that the reactions follow different routes when applying different methanol WHSV values.
According to the indirect C−C bond formation mechanism,the MTO reaction requires hydrocarbon-pool species to perform the methanol conversion to olefins. The organic species retained in the catalyst therefore play a very important role in the overall reaction. During the induction period,the gradual generation and accumulation of organic species in the zeolite transforms the original material to an efficient working catalyst. Following this induction period,the methanol conversion reaches a maximum and almost all the methanol conversion and olefin production occurs with the aid of these retained organic species,acting as important intermediates. As such,the mechanism of the MTO process is closely associated with the hydrocarbon-pool species in the zeolite,and the analysis of these retained organic species should provide helpful information regarding the effects of the various WHSV values on the reaction mechanism.
During the induction period,the concentrations of retained active species were increased,leading to the maximum conversion at the end of the induction period. At this point,there should be few retained inert organic species on the highly reactive catalyst and so,following methanol conversion at TOS = 20 min (WHSV = 2 h-1),TOS = 55 min (WHSV = 6 h-1) and TOS = 105 min (WHSV = 12 h-1),the discharged catalysts were dissolved in HF and the organic species were extracted with CH2Cl2 and analyzed via GC-MS. Fig. 6 show that the primary retained species at the different WHSV values were polymethylbenzenes,and in each case there is a similar distribution,with pentamethylbenzene (pentaMB) and tetramethylbenzene (tetraMB) as the main aromatics together with relatively small amounts of hexamethylbenzene (hexaMB),trimethylbenzene (triMB) and xylene.
These polymethylbenzene species,which are important active retained organic species in the MTO reaction,are generated via secondary reactions of olefin products,including oligomerization,cyclization and hydrogen transfer [24, 34]. Although these aromatics are generated and accumulated as the reaction proceeds at all WHSV values,there are still variations in the rate of aromatics accumulation. When the hydrogen transfer reaction is pronounced,the aromatics tend to be generated more rapidly and accumulate in the zeolite to a greater extent as the methanol is consumed,and vice versa. Thus the accumu-lation rate of aromatics,to some extent,reflects the extent of secondary reactions,especially the hydrogen transfer reaction,just as in the HTI comparison in Fig. 5.
The results in Fig. 6 show the retained organic species accumulated in the zeolite. However,the amounts of aromatic species were obtained while applying different WHSV values and reaction times,meaning that the catalysts were exposed to different amounts of methanol feed. For this reason,the peak integration values of several aromatics were used to determine the average accumulation rates at various methanol feeds,with the results provided in Fig. 7. These data allow the accumulation rates from reactions with different WHSV values to be directly compared. From Fig. 7,it can be obtained that those reactions with lower WHSV and longer contact time exhibit much higher aromatic accumulation rates. As noted,the generation and accumulation rates of these aromatics can be used to assess the extent to which the hydrogen transfer reaction occurs. The rapid accumulation of aromatics demonstrates the rapid progression of the hydrogen transfer reaction at the lowest WHSV and longest contact time. In contrast,a high WHSV and short contact time obviously restrict the generation of aromatics and severely suppress the hydrogen transfer reac-tion.
In addition to polymethylbenzene species,MCP+ ions are also important active intermediates in the MTO reaction over ZSM-5. Haw et al. [24] and Wang et al. [30, 32] demonstrated the existence of several alkyl isomers of MCP+ and their deprotonated neutral form,MCP. In the present study,MCP species were detected during the GC-MS analysis of retained species (Fig. 6),although the concentrations of these compounds were very low and sometimes obscured by the high concentrations of methylbenzenes. To remove the interference of the methylbenzenes,extracted ion chromatograms were obtained during analysis of the retained species,at m/z = 94,108,122 and 136,representing dimethylcyclopentadiene (diMCP),trimethylcyclopentadiene (triMCP),tetramethylcyclopentadiene (tetraMCP) and pentamethylcyclopentadiene (pentaMCP) and their isomers,respectively. These are presented in Fig. 8.
From Fig. 8,it is evident that there are four types of retained MCP species obtained at different WHSV values and that their distributions are very similar. However,there are some small variations in the amounts. In the reaction with a low WHSV (2 h-1),the peak areas of almost all the MCP species in the zeolite are greater than those obtained using higher WHSV values (6 or 12 h-1),even though the reaction time at the low WHSV was 20 min,and therefore much shorter than the 55- and 105-min time spans of the high WHSV reactions. Thus,the average rate at which MCP species accumulated when employing a low WHSV and a long contact time definitely exceeded those in the reactions with a high WHSV and short contact time. Similar to the polymethylbenzenes data,the generation and accumulation of these MCP species were also affected by the extent of the hydrogen transfer reaction. Judging from both the total amounts and the above estimates of the average accumulation rates at various methanol feeds,the formation of MCP species is even more sensitive to the WHSV and the catalyst-methanol contact time than is the formation of polymethylbenzenes.
The dual-cycle mechanism for MTO reactions over ZSM-5 was first proposed by Svelle et al. [33] and Bjørgen et al. [34] in 2006,and consists of aromatic and alkene cycles. The aromatic cycle involves MCPs and aromatics as active intermediates,while the alkene cycle primarily consists of alkene chain generation in conjunction with alkene methylation and cracking. The alkene products from the aromatic cycle can be regarded as intermediates that subsequently promote the alkene cycle. Simultaneously,the hydrogen transfer reaction transforms the higher chain alkenes generated in the alkene cycle to aromatics,which in turn promotes the aromatic cycle. In this way,the two cycles affect and interact with one another.
12C/13C-methanol switch experiments are widely used in the study of the MTO reaction to track the destination of 13C atoms from methanol and so to elucidate the reaction route during methanol conversion. This technique also allows one to distinguish the active organic species and to determine their roles in the overall reaction. In this study,12C/13C-methanol switch trials were carried out with different WHSV values,and the total 13C contents of the effluent alkenes and retained organics after the switch experiments are shown in Fig. 9.
For the reaction performed at a low WHSV (2 h-1 in Fig. 9(a)),the 13C contents of the C3-C6 alkenes and xylene were approximately 30% at a switch time of 2 min,while the 13C content of the ethene was very high (up to 70%). In the case of the retained organic species,the pentaMB had the highest 13C content,70% at a switch time of 2 min,with 13C contents of about 50% for the tetraMB,hexaMB,tetraMCP and pentaMCP. With the exception of the triMB,the 13C contents of the retained organic species were close to those of the effluent species under low WHSV (2 h-1).
The data obtained from the reactions with higher WHSV values (6 h-1 in Fig. 9(b) and 12 h-1 in Fig. 9(c)) indicate that,although the pentaMB still had the highest 13C content (approximately 50%) among the retained species,the 13C contents of all the retained species were lower compared with those obtained at a WHSV of 2 h-1. At the same time,with the exceptions of ethene and xylene,the 13C contents of the effluent alkenes increased substantially,from 30%-40% at a WHSV of 2 h-1 to 70%-90% at WHSV values of 6 and 12 h-1. These opposing changes resulted in pronounced differences in the 13C contents of the effluent alkenes and retained compounds at WHSV values of 6 and 12 h-1. In addition,the ethene 13C content of 65% was significantly lower than the 70%-90% levels in the C3-C6 alkenes in the reactions performed at WHSV values of 6 or 12 h-1,which is the opposite to the results obtained at the low WHSV.
Typically,a higher 13C content in an organic species indicates greater participation of this species in the reaction process and demonstrates that the conversion route that produces this species is highly active. In the case of the low WHSV reaction,the 13C contents of the effluents and retained organic species were similar,meaning that the production of the effluent alkenes was strongly linked to the presence of the retained species. Considering the participation of retained species in the aromatic cycle of the dual-cycle mechanism,it is evident that the aromatic cycle was undoubtedly predominant during the low WHSV reaction. The alkene cycle is typically responsible for the generation of propene,butene and other higher alkenes. Conversely,the production of ethene is almost completely dependent on the aromatic cycle,and so the high 13C content of the ethene also demonstrates the high activity of the aromatic cycle during the low WHSV reaction.
The higher WHSV (6 or 12 h-1) reactions generated alkenes with 13C contents that were higher than those of the retained organic species. Therefore,the aromatic cycle only contributed partly to the MTO activity and the alkene cycle must also have proceeded during methanol conversion at the same time. The alkene cycle utilizes gas phase C3 to C7 alkenes as active intermediates and does not required the retained species in the catalyst,and so the difference in the 13C contents of the effluent alkenes and retained species obviously indicates the important role of the alkene cycle in the higher WHSV reactions. In addition,the lower 13C content of ethene compared to those of the higher alkenes also suggests the major role of the alkene cycle. In general,during the MTO reaction over ZSM-5,low WHSV conditions favor the aromatic cycle while high WHSV conditions promote the alkene cycle.
Although the production of alkenes is the main reaction during the MTO process,secondary reactions,such as the hydrogen transfer reaction,inevitably occur. In the dual-cycle mechanism (Scheme 1),the hydrogen transfer reaction connects the alkene cycle and the aromatic cycle via the generation of aromatics,MCPs and other hydrogen-poor species that function as active intermediates in the aromatic cycle. To a certain degree,then,secondary reactions (especially the hydrogen transfer reaction),are necessary for the aromatic cycle to proceed. The alkene products generated via the aromatic cycle in turn act as the intermediates for the alkene cycle and promote the activity of the alkene cycle as well. Thus the interaction between the aromatic and alkene cycles jointly promotes the overall MTO reaction.
In Section 3.2,MTO reaction results showed that the initial methanol conversion increased with increasing TOS and that the maximum methanol conversion was obtained at a WHSV of 2 h-1,with the lowest value at a WHSV of 12 h-1. However,when the WHSV is raised,it should be considered that the methanol feed into the catalyst bed is increased. Under these conditions,catalyst penetration may be observed if the methanol feed cannot be completely converted,and the absolute amount of converted methanol will be uncertain,especially in the induction period. To exclude the effects of variations in the methanol feeding rate,the absolute amount of converted methanol normalized to the unit mass of catalyst was calculated for each trial,based on the methanol conversion and the actual WHSV of each reaction. These values were used to assess the methanol conversion capacity under different WHSV conditions (Fig. 10).
Fig. 10 shows that,after the induction period,the conversion capacity of the ZSM-5 catalyst increases as the WHSV is increased,exhibiting the opposite trend to the lowered conversion values observed in Fig. 4. During the induction period,the conversion capacities gradually increase in all the reactions,reflecting the autocatalytic nature of the process. However,in the initial period of each reaction,the sharp increase in the conversion capacity at the low WHSV shortens the induction period.
In the case of the low WHSV reaction,the long contact time favors the hydrogen transfer reaction and promotes the rapid generation of retained organic species. The accumulation of these active aromatics is thus accelerated and the fresh zeolite is quickly transformed to a working catalyst. When employing the low WHSV,the reaction primarily proceeds via the aromatic cycle and exhibits high methanol conversion capacity in the initial reaction period. The reactions at higher WHSV values show the opposite behavior. The shorter contact times limit the extent of the hydrogen transfer reaction and suppress the generation of aromatics,lowering the conversion capacity during the initial reaction period. However,as the reaction proceeds after the induction period,a higher methanol conversion capacity is obtained,which can be attributed to the greater amount of methanol feeding and more reactants reacting with active intermediates. At the same time,the enhanced methanol conversion capacity brings about more olefin generation,and thus drives the alkene methylation and cracking reactions for methanol conversion. As demonstrated by the 12C/13C- methanol switch experiments,the alkene cycle,followed by methanol conversion,is improved at higher WHSV values and lower catalyst-feedstock contact times.
MTO reactions were performed while varying the methanol WHSV value. The existence of a dual-cycle mechanism for the MTO reaction over ZSM-5 at 300 °C was confirmed by the analysis of effluent products,confined organic species and 12C/13C-methanol switch experiments. Ⅰt was determined that hydrogen transfer reactions could be adjusted by varying the catalyst-feedstock contact time and,in this manner,the methanol conversion reaction route (that is,whether by the aromatic cycle or alkene cycle) can be regulated. In the case of a reaction with a low WHSV,the longer time span over which the methanol contacts the catalyst bed benefits the hydrogen transfer reactions of alkene products and promotes the generation and ac-cumulation of aromatics and MCPs on the ZSM-5 catalyst. This in turn promotes the aromatic cycle and generates ethene and aromatics among the effluents. In contrast,higher WHSV values are associated with shorter contact times that reduce the extent of the hydrogen transfer reaction and restrict the accumulation of MCPs and aromatics species. This also lowers the reactivity of the aromatic cycle and the main route of the alkene cycle in the MTO process,the C3 to C7 alkenes,shows more involvement in the reaction and also displays high selectivities among the effluents. Variations in the methanol-catalyst contact time also affect the accumulation of organic species and this effect further alters the dual-cycle mechanism. These findings should lead to the development of useful strategies for manipulating the MTO reaction to generate alkenes and aromatic products over the ZSM-5 catalyst.
2016,
Vol. 37 Issue (8): 1413-1422 
PDF (776 KB)
2016,
Vol. 37 Issue (8): 1413-1422 PDF (776 KB)