Cuiyu Yuan

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.

Synthesis of mesoporous ZSM-5 using a new gemini surfactant as a mesoporous directing agent: A crystallization transformation process

A new gemini surfactant, [C18H37(CH3)2–N+–(CH2)3–N+–(CH3)2C18H37]Cl2 (C18-3-18), has been successfully used as the mesopore directing agent in the hydrothermal synthesis of mesoporous ZSM-5 (MZSM-5). The synthesis of MZSM-5 was realized with a low temperature crystallization process at 130 °C. The amount of C18-3-18 used in the synthesis affected the relative crystallinity and the textural properties of the obtained MZSM-5. Detailed investigation showed that the formation of MZSM-5 followed a crystallization transformation process. The use of C18-3-18 resulted in the formation of mesoporous material during the early stage of the synthesis, which was converted into MZSM-5 crystals templated by tetrapropylammonium bromide to form the MFI phase. As the synthesis proceeded, the MZSM-5 crystals aggregated into particles by weak interactions. This work shows that C18-3-18 can be used as a mesopore directing agent, which could provide a route for the synthesis of other mesoporous zeolites.

Methanol-to-olefin induction reaction over SAPO-34

The methanol-to-olefin induction reaction over the SAPO-34 was performed using a fluidized-bed system. We found that the whole induction period could be divided into three reaction stages. Further investigation of the reaction kinetics revealed that this induction reaction behavior was different from that over H-ZSM-5 catalyst. Compared with the H-ZSM-5, the generation of initial active centers is easier over SAPO-34 because of its limited diffusivity and the spatial confinement effect of the cages. However, the autocatalysis reaction stage is difficult over SAPO-34 because of the continuous formation of inactive methyladamantanes.