Shutao Xu

Heterogeneous Ceria Catalyst with Water-Tolerant Lewis Acidic Sites for One-Pot Synthesis of 1,3-Diols via Prins Condensation and Hydrolysis Reactions

The use of a heterogeneous Lewis acid catalyst, which is insoluble and easily separable during the reaction, is a promising option for hydrolysis reactions from both environmental and practical viewpoints. In this study, ceria showed excellent catalytic activity in the hydrolysis of 4-methyl-1,3-dioxane to 1,3-butanediol in 95% yield and in the one-pot synthesis of 1,3-butanediol from propylene and formaldehyde via Prins condensation and hydrolysis reactions in an overall yield of 60%. In-depth investigations revealed that ceria is a water-tolerant Lewis acid catalyst, which has seldom been reported previously. The ceria catalysts showed rather unusual high activity in hydrolysis, with a turnover number (TON) of 260, which is rather high for bulk oxide catalysts, whose TONs are usually less than 100. Our conclusion that ceria functions as a Lewis acid catalyst in hydrolysis reactions is firmly supported by thorough characterizations with IR and Raman spectroscopy, acidity measurements with IR and (31)P magic-angle-spinning NMR spectroscopy, Na(+)/H(+) exchange tests, analyses using the in situ active-site capping method, and isotope-labeling studies. A relationship between surface vacancy sites and catalytic activity has been established. CeO(2)(111) has been confirmed to be the catalytically active crystalline facet for hydrolysis. Water has been found to be associatively adsorbed on oxygen vacancy sites with medium strength, which does not lead to water dissociation to form stable hydroxides. This explains why the ceria catalyst is water-tolerant.

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.

A top-down approach to prepare silicoaluminophosphate molecular sieve nanocrystals with improved catalytic activity.

Silicoaluminophosphate SAPO-34 molecular sieve nanocrystals have been prepared by a post-synthesis milling and recrystallization method, which is further proven to be universally applicable to other SAPO molecular sieves. The obtained SAPO-34 with reduced Si enrichment on the external surface shows considerably improved catalytic performance in the MTO reaction.

Dual template-directed synthesis of SAPO-34 nanosheet assemblies with improved stability in the methanol to olefins reaction

A spherical self-assembly of SAPO-34 nanosheets was hydrothermally synthesized by using a quaternary ammonium-type organosilane surfactant [3-(trimethoxysilyl) propyl]octadecyldimethylammonium chloride (TPOAC) as the mesoporogen and a part of the silica source, and diethylamine (DEA) as the microporous template. The prepared materials were well characterized by XRD, XRF, SEM, TEM, N-2 adsorption-desorption, NH3-TPD, NH3-adsorbed IR and pyridine-adsorbed IR measurements. It revealed that the morphologies, compositions and acidity of the SAPO-34 products changed significantly with the increased TPOAC/TEOS ratio, showing that the TPOAC/TEOS ratio had a significant impact on the crystallization process of SAPO-34. Solid state C-13, Si-29 NMR and TG-DTA analyses were further conducted to investigate the status of TPOAC in the final products. A possible crystallization process involving TPOAC was proposed. The catalytic performances of the synthesized SAPO-34s were evaluated using the MTO reaction. The optimal SAPO-34 nanosheet assemblies with a hierarchical porous structure displayed a remarkably enhanced catalytic lifetime and high yields of light olefins.

Polystyrene sulphonic acid resins with enhanced acid strength via macromolecular self-assembly within confined nanospace

Tightening environmental legislation is driving the chemical industries to develop efficient solid acid catalysts to replace conventional mineral acids. Polystyrene sulphonic acid resins, as some of the most important solid acid catalysts, have been widely studied. However, the influence of the morphology on their acid strength--closely related to the catalytic activity--has seldom been reported. Herein, we demonstrate that the acid strength of polystyrene sulphonic acid resins can be adjusted through their reversible morphology transformation from aggregated to swelling state, mainly driven by the formation and breakage of hydrogen bond interactions among adjacent sulphonic acid groups within the confined nanospace of hollow silica nanospheres. The hybrid solid acid catalyst demonstrates high activity and selectivity in a series of important acid-catalysed reactions. This may offer an efficient strategy to fabricate hybrid solid acid catalysts for green chemical processes.

A novel solvothermal approach to synthesize SAPO molecular sieves using organic amines as the solvent and template

A novel solvothermal synthesis route designated as aminothermal synthesis, in which organic amines are used as both the dispersing medium and the template, is developed for the synthesis of SAPO molecular sieves. Three synthetic systems based on triethylamine (TEA), diethylamine (DEA) and TEA-DEA binary mixture were studied. SAPO-34 with good crystallinity and high yield (ca. 90%) was successfully synthesized from TEA and TEA-DEA systems. By adjusting the crystallization temperature, SAPO-18 was for the first time obtained using TEA template. In the case of DEA, a small-pore molecular sieve DNL-6 with RHO topology was synthesized. In an attempt to make the synthetic process environmentally benign and reduce the waste emission, organic liquid was collected after the synthesis of SAPO-34 and reused in next crystallization. It was found that the crystalline product and yield were almost the same as those synthesized with fresh amines. Moreover, CO2 and CH4 were chosen as the probe molecules to investigate the adsorption properties of products. SAPO molecular sieves synthesized by aminothermal method exhibited good adsorption capacities and high CO2/CH4 ratios.

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.

Multimodal Zr-Silicalite-1 zeolite nanocrystal aggregates with interconnected hierarchically micro-meso-macroporous architecture and enhanced mass transport property

Hierarchical porous architecture with interconnected trimodal micro-meso-macroporous systems constructed from uniform zeolite Zr-doped silicalite-1 nanocrystals has been prepared. The synthesis has been made by using glycerin as a reaction medium via a quasi-solid-state crystallization of hierarchically meso-macroporous zirconosilicate precursor under the effect of the structure directing agent TPAOH. The presence of glycerin is crucial in the synthesis systems to maintain the porous hierarchy. The pores inter-connectivity, Zr location in the framework, the acidity and the catalytic activity have been studied by laser-hyperpolarized (129)Xe NMR spectroscopy, UV-visible spectroscopy, temperature-programmed desorption of ammonia and the catalytic isopropylbenzene cracking probe reaction, respectively. The products possess well-defined macrochannels interconnected with mesopores located in the macropore walls, which in turn have been constructed from microporous MFI-type zeolite units. (129)Xe NMR study indicated that the hierarchically micro-, meso-, macro-pore systems are homogeneously distributed throughout the final materials and well interconnected, which is important for molecular diffusion. The TPD-NH(3) investigation revealed that the hierarchically micro-meso-macroporous materials constructed from zeolite Zr-Silicalite-1 nanocrystals present strong acidity.