Zhuopeng Wang

Hydrothermal synthesis of zeolites with three-dimensionally ordered mesoporous-imprinted structure.

Zeolites are microporous materials with pores and channels of molecular dimensions that find numerous applications in catalysis, separations, ion exchange, etc. However, whereas uniformity of micropore size is a most desirable and enabling characteristic for many of their uses, in certain cases, for example in reactions involving bulky molecules, it is a limitation. For this reason, synthesis of hierarchical zeolites with micro- and mesoporosity is of considerable interest as a way to control molecular traffic for improved catalytic and separation performance. Herein, we report a general synthesis route for the confined synthesis of zeolites within three-dimensionally ordered mesoporous carbon templates by conventional hydrothermal synthesis. Various zeolites, including BEA, LTA, FAU, and LTL, with three-dimensionally ordered mesoporous-imprinted structure have been synthesized by this approach. It is expected that these hierarchical zeolite materials will provide building blocks for thin-film and other syntheses and may provide a basis for quantitatively studying the mass-transfer limitation on the catalytic performance of zeolite catalysts.

Production of p‐Xylene from Biomass by Catalytic Fast Pyrolysis Using ZSM‐5 Catalysts with Reduced Pore Openings

Pores for thought: Chemical liquid deposition of silica onto ZSM-5 catalysts led to smaller pore openings that resulted in >90% selectivity for p-xylene over the other xylenes in the catalytic fast pyrolysis of furan and 2-methylfuran (see scheme). The p-xylene selectivity increased from 51% with gallium spray-dried ZSM-5 to 72% with a pore-mouth-modified catalyst in the pyrolysis of pine wood.

A structural resolution cryo-TEM study of the early stages of MFI growth

Room-temperature aging of zeolite precursor silica sol was followed by SAXS and cryo-TEM. Cryo-TEM imaging of zeolite materials with structural resolution is demonstrated. The results suggest the formation of predominantly amorphous aggregates before MFI crystallization.

Rapid synthesis of Sn-Beta for the isomerization of cellulosic sugars

Because of their unique catalytic activity and excellent hydrothermal stability, zeolites, aluminosilicate molecular sieves, have been extensively used in petrochemical processing and the production of high-value chemicals and biofuels from naturally abundant biomass. Instead of being used as solid Bronsted acid catalysts, molecular sieves containing tetrahedrally coordinated Ti and Sn have been explored as solid Lewis acid catalysts for redox reactions. Ticontaining, high-silica molecular sieves with the zeolite beta topology (Ti-Beta) and MFI topology (TS-1) have been employed for various selective oxidation reactions, such as olefin epoxidation, selective oxidation of alcohols, hydroxylation of phenol and ammoximation of cyclohexanone. Sn-Beta, a tin-containing molecular sieve with the zeolite beta topology, has been used in the Meerwein–Ponndorf– Verley (MPV) reduction of aldehydes and ketones, the Meerwein– Ponndorf–Verley–Oppenauer (MPVO) oxidation of alcohols, and the Baeyer–Villiger oxidation reaction. Recently, due to its particular Lewis acidic properties, Sn-Beta has been shown to catalyze the isomerization reactions of triose sugars (dihydroxyacetone and glyceraldehyde), pentose sugars (xylose and xylulose) and hexose sugars (glucose and fructose) with activities that are comparable to biological processes. In particular, it has been revealed that Sn-Beta is a water tolerant Lewis acid catalyst, and can catalyze the isomerization reactions in aqueous phase at low pH, which is most likely due to its hydrophobic nature derived from the high-silica microporous structure. Because of its unique properties, Sn-Beta has also been used for the one-pot synthesis of 5-(hydroxymethyl)-furfural (HMF), an important precursor for the production of renewable polymers and biofuels, from glucose by combining with a homogeneous acid catalyst (HCl) in a biphasic system. Although Sn-Beta has shown promising catalytic properties, its industrial applications and related researches in academia have been hindered by the difficulties in synthesizing this material, particularly the use of hydrofluoric acid and the long crystallization time. In general, active Sn-Beta is synthesized using the fluoride anion as a mineralizing agent under near-neutral conditions, with a crystallization time of around 40 days as reported by Moliner et al. The long crystallization time could be due to the relatively low supersaturation degree, and the limited nucleation caused by the fluoride anion and the neutral pH used in the synthesis. To reduce the crystallization time, a seeded growth method was applied to the synthesis of Sn-Beta. However, it still requires from 22 days to 30 days to accomplish the synthesis. In this study, we report that the morphology and dispersion status of zeolite seeds in the synthesis gel can substantially affect the crystal growth kinetics of Sn-Beta. By uniformly distributing crystalline zeolite beta seeds in the synthesis gel, high-quality Sn-Beta can be synthesized in only 2 days with a nearly complete conversion (.90%) of the provided silica source. The Sn-Beta catalyst synthesized by this approach is highly active for the isomerization of triose (C3), pentose (C5) and hexose (C6) sugars. In contrast to the previous seeded growth method, in this study 200 nm crystalline zeolite beta nanocrystals were used as seeds and added to the synthesis mixture as a suspension. Crystalline zeolite beta nanocrystals (Si : Al = 23) were prepared according to the previous literature. In order to avoid the irreversible aggregation caused by calcination and drying, dealumination of the zeolite seeds was carried out by directly treating the stable seed solution with a concentrated nitric acid solution. The dealuminated zeolite beta seeds were collected by centrifugation and thoroughly washed with deionized water until the pH of the supernatant was close to neutral. The final concentration of the obtained seed solution was adjusted to 0.145 g mL by dispersing the seeds in deionized water. During the whole process, no drying or calcination was performed on the sample, which enabled us to prepare a stable suspension with welldispersed dealuminated zeolite seeds. The crystallinity of the seeds showed no sign of a significant change after the dealumination process, as illustrated by the XRD patterns (Fig. S1, ESI{). After the dealumination, no detectable Al was found in the seeds by elemental analysis. Details of the seed synthesis and the dealumination process can be found in the supplementary information.{ For the synthesis of Sn-Beta, a clear synthesis solution was made by adding tetraethylorthosilicate (TEOS) to tetraethylammonium hydroxide solution (TEAOH). Tin(IV) chloride was first dissolved in deionized water before being added to the prepared clear solution. The resulting solution was stirred in a hood until the ethanol generated from the hydrolysis of TEOS was completely evaporated. Next, HF was added with stirring, and the solution turned into a dry gel at this stage. Finally, the suspension containing dealuminated zeolite seeds (4.1 wt% seeds with respect to the silica content in the Chemical Engineering, University of Massachusetts, 159 Goessmann Lab, 686 North Pleasant Street, Amherst, MA 01003, USA. E-mail: wfan@ecs.umass.edu; Fax: 1 413 545 3540; Tel: 1 413 545 1750 { Electronic Supplementary Information (ESI) available. See DOI: 10.1039/ c2ra21381h { These authors contributed equally to this work. RSC Advances Dynamic Article Links

Fluoride-free synthesis of a Sn-BEA catalyst by dry gel conversion

Sn containing molecular sieves with BEA topology (Sn-BEA) are active Lewis catalysts for a large variety of redox reactions. However, the synthesis of Sn-BEA often requires the use of toxic chemicals (e.g. hydrofluoric acid). In this study, we demonstrated that an active Sn-BEA catalyst can be directly synthesized in a non-fluoride medium via a dry gel conversion method with the aid of seed crystals. It was shown that seeding of zeolite BEA crystals can facilitate crystal growth and lead to BEA topology with framework Sn. Furthermore, it was found that ion-exchange with ammonium ions is indispensable to retain the crystal structure during calcination. The catalytic activity of the Sn-BEA catalyst was evaluated by the isomerization of glucose and reaction of pyruvaldehyde in the aqueous phase, and compared with the conventional hydrophobic Sn-BEA synthesized in a fluoride medium. The catalytic activity of the Sn-BEA synthesized in the absence of fluoride was found to be lower than that of the conventional Sn-BEA for catalyzing the isomerization of glucose in the aqueous phase, which could be attributed to its hydrophilic surface. For the reaction of pyruvaldehyde in the aqueous phase, the Sn-BEA synthesized in this study showed similar activity to the conventional hydrophobic Sn-BEA, indicating that the catalytic activity of Sn-BEA catalysts largely depends on the type of reactions. The current work provides the first example of direct preparation of a Sn-BEA catalyst in a caustic medium.

Direct, single-step synthesis of hierarchical zeolites without secondary templating

Hierarchical ZSM-5 was directly synthesized by controlling the nucleation, growth and template-free self-assembly of zeolite precursors formed in the initial stage of the zeolite crystallization process. The facile synthesis results in spherical particles primarily composed of a micro-mesoporous core of aggregated 30–50 nm MFI nanocrystals, and a thin crystalline shell. MFI nanocrystal assembly coupled with a possible dissolution–crystallization mechanism results in a template-free route to broadly distributed mesopores and, thereby, a ca. 25 nm diffusion length in a micrometer-sized particle. The materials exhibit enhanced mass transport and superior catalytic activity for bulky molecules.

The effects of ZSM-5 mesoporosity and morphology on the catalytic fast pyrolysis of furan

ZSM-5 catalysts with different morphologies were synthesized and evaluated for the catalytic conversion of furan in a fixed bed reactor to provide insights into the rational design of zeolite catalysts for catalytic fast pyrolysis (CFP). The effects of mesoporosity and morphology of ZSM-5 catalysts on the production of aromatics and olefins as well as catalyst deactivation were investigated. The results suggest that increasing mesoporosity and decreasing crystallite size can increase furan conversion and affect selectivity to products. Improved selectivities to benzene, toluene, xylene and olefins were achieved with mesoporous ZSM-5 and 100 nm ZSM-5 compared to 800 nm ZSM-5. Coke formation on zeolite catalysts during the reaction of furan was also largely reduced (up to 65%) by introducing mesoporosity. It was observed that coke is mainly formed and accumulated inside the micropores of ZSM-5 catalysts rather than on the external surface or within the mesopores. Characterization of mass transport in the coked zeolite samples using cyclohexane as a probe molecule suggested that coke blocks micropores, leading to a decrease in micropore volume during the catalyst deactivation process. However, due to the three-dimensional pore structure of ZSM-5, the mass transport properties of mesoporous ZSM-5 do not exhibit an apparent change. Catalyst deactivation was mainly due to the coverage of active sites by coke, rather than the blockage of the transport pathways by coke.