Jinzhe Li

Direct observation of cyclic carbenium ions and their role in the catalytic cycle of the methanol-to-olefin reaction over chabazite zeolites.

Carbenium ions in zeolites: Two important carbenium ions have been observed for the first time under working conditions of the methanol-to-olefins (MTO) reaction over chabazite zeolites using (13) C NMR spectroscopy. Their crucial roles in the MTO reaction cycles have been demonstrated by combining experiments and theoretical calculations.

Synthesis of mesoporous ZSM-5 catalysts using different mesogenous templates and their application in methanol conversion for enhanced catalyst lifespan

In this work, two kinds of mesoporous ZSM-5 were synthesized successfully using a hydrothermal methodology by utilizing different soft templates, namely, dimethyl octadecyl [3-(trimethoxysilyl)propyl]ammonium chloride ([(CH3O)3SiC3H6N(CH3)2C18H37]Cl, TPOAC) and hexadecyl trimethyl ammonium bromide (C16H33(CH3)3NBr, CTAB). The obtained mesoporous ZSM-5 samples were compared with conventional ZSM-5, and the effects of different surfactant usages during the synthesis of mesoporous ZSM-5 on the physicochemical and catalytic properties were systematically investigated. Multiple techniques, such as XRD, SEM, N2 adsorption techniques, HP 129Xe NMR, 27Al MAS NMR, 29Si MAS NMR, and 1H MAS NMR, were employed for the characterization. Although the synthesized mesoporous ZSM-5 samples had equal surface areas, they presented different relative crystallinities, morphologies, pore-size distributions, micropore–mesopore interconnectivity, framework atom coordination states and acidities. When using these synthesized ZSM-5 samples as catalysts for methanol conversion, the mesoporous ZSM-5 templated with TPOAC exhibited an extremely long catalyst lifespan compared to conventional ZSM-5, while mesoporous ZSM-5 templated with CTAB showed no advantage in prolonging the catalyst lifetime during the reaction. The differences in the catalytic lifespan and the reduction of coke deposition were correlated to the variation of acidity and porosity with the mesopore generation in the ZSM-5 catalysts by the usage of different structure-directing agents. Compared to the mesopore structure-directing agent, CTAB, with the use of TPOAC as the template and part of the Si source, mesoporous ZSM-5 could be synthesized with good mesopore–micropore interconnectivity, which accounted for the improved catalytic performance in the reaction of methanol conversion.

Generation of diamondoid hydrocarbons as confined compounds in SAPO-34 catalyst in the conversion of methanol

Formation of adamantane hydrocarbons and their confinement in SAPO-34 caused the long induction period and the quick catalyst deactivation in methanol conversion. Via ship-in-a-bottle synthesis, adamantane and methyladamantanes could be produced from methanol conversion in the cage of 8-ring SAPO catalysts under very mild reaction conditions.

Elucidating the olefin formation mechanism in the methanol to olefin reaction over AlPO-18 and SAPO-18

The mechanism of the methanol to olefin (MTO) reaction over AlPO-18 (without Bronsted acid sites) and two SAPO-18 (with different Bronsted acid site densities) catalysts has been investigated. The Bronsted acid site density of AlPO-18 and SAPO-18 catalysts was determined by 1H MAS NMR spectroscopy. Methanol conversion over the catalysts showed that the catalytic activity of the catalysts was strongly influenced by their Bronsted acid site density. Using 13C magic angle spinning (MAS) NMR, we directly observed the pentamethylcyclopentenyl cation (pentaMCP+) over SAPO-18 under real MTO reaction conditions, but no carbenium ion was detected over AlPO-18. Furthermore, analysis of confined organics by 13C MAS NMR and GC-MS clearly demonstrated that higher Bronsted acid site density improved the formation and accumulation of some important and reactive hydrocarbon pool species, such as pentaMCP+ and polymethylbenzenes. With the aid of the 12C/13C-methanol switch technique, the detailed olefin formation mechanism was elucidated. During the MTO reaction, light olefin generation over SAPO-18 mainly followed the aromatic-based hydrocarbon pool mechanism; however, the olefin methylation and cracking mechanism accounted for the production of light olefins over AlPO-18.

Coke Formation and Carbon Atom Economy of Methanol‐to‐Olefins Reaction

The methanol-to-olefins (MTO) process is becoming the most important non-petrochemical route for the production of light olefins from coal or natural gas. Maximizing the generation of the target products, ethene and propene, and minimizing the production of byproducts and coke, are major considerations in the efficient utilization of the carbon resource of methanol. In the present work, the heterogeneous catalytic conversion of methanol was evaluated by performing simultaneous measurements of the volatile products generated in the gas phase and the confined coke deposition in the catalyst phase. Real-time and complete reaction profiles were plotted to allow the comparison of carbon atom economy of methanol conversion over the catalyst SAPO-34 at varied reaction temperatures. The difference in carbon atom economy was closely related with the coke formation in the SAPO-34 catalyst. The confined coke compounds were determined. A new type of confined organics was found, and these accounted for the quick deactivation and low carbon atom economy under low-reaction-temperature conditions. Based on the carbon atom economy evaluation and coke species determination, optimized operating conditions for the MTO process are suggested; these conditions guarantee high conversion efficiency of methanol.

Direct Mechanism of the First Carbon–Carbon Bond Formation in the Methanol‐to‐Hydrocarbons Process

In the past two decades, the reaction mechanism of C-C bond formation from either methanol or dimethyl ether (DME) in the methanol-to-hydrocarbons (MTH) process has been a highly controversial issue. Described here is the first observation of a surface methyleneoxy analogue, originating from the surface-activated DME, by in situ solid-state NMR spectroscopy, a species crucial to the first C-C bond formation in the MTH process. New insights into the first C-C bond formation were provided, thus suggesting DME/methanol activation and direct C-C bond formation by an interesting synergetic mechanism, involving C-H bond breakage and C-C bond coupling during the initial methanol reaction within the chemical environment of the zeolite catalyst.

A low-temperature approach to synthesize low-silica SAPO-34 nanocrystals and their application in the methanol-to-olefins (MTO) reaction

A low-temperature strategy is developed for the synthesis of low-silica SAPO-34 with a tunable Si content and relatively uniform Si distribution in the crystals, which is hitherto difficult to achieve. It is demonstrated that a lower crystallization temperature and a silicon source with relatively low reactivity are important factors leading to successful synthesis. The crystal size of SAPO-34 could be effectively decreased to 200 nm through a seed-assisted approach. The local atom environments of low-silica samples are investigated by solid state MAS NMR, which confirms the unique existence of Si(4Al) species in the framework. The obtained low-silica SAPO-34 exhibits excellent catalytic performance in the MTO reaction and the occurrence of catalyst deactivation varies with the acid properties. Through optimizing the Si content of the samples, a long catalyst life and a high initial/maximum selectivity to ethylene plus propylene could be achieved simultaneously over SAPO-34 with a Si content of Si/(Si + Al + P) = 0.047. This result would be valuable for the improvement of the catalytic properties of SAPO-34 used for a commercial MTO fluidized-bed reactor. However, SAPO-34 with a very low Si content (e.g. Si/(Si + Al + P) = 0.039) exhibits a shortened catalyst life due to the insufficient Bronsted acid sites in spite of the high selectivity to light olefins.

Methanol to hydrocarbons reaction over HZSM-22 and SAPO-11: Effect of catalyst acid strength on reaction and deactivation mechanism

The conversion of methanol to hydrocarbons has been investigated over HZSM-22 and SAPO-11. Both of these catalysts possess one-dimensional 10-ring channels, but have different acidic strengths. Comparison studies and C-12/C-13 isotopic switching experiments were conducted to evaluate the influence of the acidic strength of the catalyst on the conversion of methanol, as well as its deactivation mechanism. Although the conversion of methanol proceeded via an alkene methylation-cracking pathway over both catalysts, the acidity of the catalysts had a significant impact on the conversion and product distribution of these reactions. The stability of the catalysts varied with temperature. The catalysts were deactivated at high temperature by the deposition of graphitic coke on their outer surface. Deactivation also occurred at low temperatures a result that the pores of the catalyst were blocked by polyaromatic compounds. The co-reaction of C-13-methanol and C-12-1-butene confirmed the importance of the acidity of the catalyst on the distribution of the hydrocarbon products

Methanol conversion on ZSM-22, ZSM-35 and ZSM-5 zeolites: effects of 10-membered ring zeolite structures on methylcyclopentenyl cations and dual cycle mechanism

ZSM-22, ZSM-35 and ZSM-5, aluminosilicate zeolites possessing 10-membered ring channels, have been used in the present study as the catalysts of the MTO reaction. The diversities in dimensions and connection types of the 10-membered ring channels of the three zeolite catalysts make their performances in the MTO reaction quite different. As the key active species involved in the hydrocarbon-pool mechanism in the MTO reaction, methylcyclopentenyl cations (MCP+) and methylbenzenes have been captured by 13C MAS NMR and GC-MS over the three zeolite catalysts during methanol conversion. The comparative studies of the retained organics generation over the zeolite catalysts indicate that due to the spatial confinement effects of the inorganic frameworks, the retained organic species generated in the catalysts during the MTO reaction are influenced by both their sizes and amounts. A detailed analysis of the confined organic species showed the formation of MCP with varied methyl substitutions over the three zeolites. 12C/13C-methanol switch experiments were employed to investigate the reaction route for product generation. The differences in the participation levels of the methylbenzene and methylcyclopentadiene over the three zeolite catalysts imply that the formation and function of the organic species formed in the 10-membered ring channel were impacted by the chemical environment of the zeolites, and the methanol conversion that occurred in the 10-membered ring channels of the three zeolites also followed different reaction routes.

Direct observation of methylcyclopentenyl cations (MCP+) and olefin generation in methanol conversion over TON zeolite

The mechanism of the methanol to olefin (MTO) reaction over H-ZSM-22, a TON-type zeolite without cavities or channel intersections, has been investigated in the temperature range of 250–350 °C. For the first time, an induction period in low-temperature methanol conversion and the methylcyclopentenyl cation (MCP+) formed during this period have been observed directly and successfully. 13C magic angle spinning (MAS) NMR, 13C-labeling experiments and theoretical calculations have been employed to confirm the important active intermediates during methanol conversion at 300 °C. The reactions performed at different temperatures were comparatively studied and the differences in the reaction route for alkene formation from methanol conversion and the modes of H-ZSM-22 catalyst deactivation were revealed.