Hong Je Cho

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

Ultra-selective high-flux membranes from directly synthesized zeolite nanosheets

A zeolite with structure type MFI is an aluminosilicate or silicate material that has a three-dimensionally connected pore network, which enables molecular recognition in the size range 0.5–0.6 nm. These micropore dimensions are relevant for many valuable chemical intermediates, and therefore MFI-type zeolites are widely used in the chemical industry as selective catalysts or adsorbents. As with all zeolites, strategies to tailor them for specific applications include controlling their crystal size and shape. Nanometre-thick MFI crystals (nanosheets) have been introduced in pillared and self-pillared (intergrown) architectures, offering improved mass-transfer characteristics for certain adsorption and catalysis applications. Moreover, single (non-intergrown and non-layered) nanosheets have been used to prepare thin membranes that could be used to improve the energy efficiency of separation processes. However, until now, single MFI nanosheets have been prepared using a multi-step approach based on the exfoliation of layered MFI, followed by centrifugation to remove non-exfoliated particles. This top-down method is time-consuming, costly and low-yield and it produces fragmented nanosheets with submicrometre lateral dimensions. Alternatively, direct (bottom-up) synthesis could produce high-aspect-ratio zeolite nanosheets, with improved yield and at lower cost. Here we use a nanocrystal-seeded growth method triggered by a single rotational intergrowth to synthesize high-aspect-ratio MFI nanosheets with a thickness of 5 nanometres (2.5 unit cells). These high-aspect-ratio nanosheets allow the fabrication of thin and defect-free coatings that effectively cover porous substrates. These coatings can be intergrown to produce high-flux and ultra-selective MFI membranes that compare favourably with other MFI membranes prepared from existing MFI materials (such as exfoliated nanosheets or nanocrystals).

Base free, one-pot synthesis of lactic acid from glycerol using a bifunctional Pt/Sn-MFI catalyst

Under base free and mild reaction conditions, 80.5% selectivity of lactic acid (LA) was achieved at 89.8% conversion of glycerol using a bifunctional Pt/Sn-MFI catalyst. In the tandem reaction pathway, selective oxidation of glycerol to glyceraldehyde (GLA) and dihydroxyacetone (DHA) was cascaded with Lewis acid catalyzed isomerization of GLA/DHA into LA.

Lewis acid zeolites for tandem Diels–Alder cycloaddition and dehydration of biomass-derived dimethylfuran and ethylene to renewable p-xylene

Lewis acid zeolites including Zr-, Sn-, and Ti-BEA were examined for tandem [4 + 2] Diels–Alder cycloaddition of 2,5-dimethylfuran (DMF) and ethylene to oxanorbornene with subsequent dehydration to produce biorenewable p-xylene. Zr-BEA (Si/Zr = 168) exhibited superior performance with improved recalcitrance to deactivation, which was attributed to its low activity for the hydrolysis of DMF to 2,5-hexanedione and subsequent condensation. Zr-BEA also achieved the highest selectivity to p-xylene of 90% at 99% conversion of DMF. For low catalyst loading within a three-phase reactor, the reaction rate to form p-xylene was linearly proportional to the number of Lewis acid sites, while high catalyst loading exhibited zero order dependence on Lewis acid sites. A maximum achievable reaction rate was shown to be consistent with a transition in rate-limiting reactions from dehydration of oxanorbornene, the Diels–Alder product, to the Diels–Alder cycloaddition of DMF and ethylene.

Photocatalytic degradation of 17α-ethinylestradiol (EE2) in the presence of TiO2-doped zeolite.

Current design limitations and ineffective remediation techniques in wastewater treatment plants have led to concerns about the prevalence of pharmaceutical and personal care products (PPCPs) in receiving waters. A novel photocatalyst, TiO2-doped low-silica X zeolite (TiO2-LSX), was used to study the degradation of the pharmaceutical compound, 17α-ethinylestradiol (EE2). The catalyst was synthesized and characterized using XRD, BET surface analysis, SEM-EDAX, and ICP-OES. The effects of different UV light intensities, initial EE2 concentrations, and catalyst dosages on the EE2 removal efficiency were studied. A higher EE2 removal efficiency was attained with UV-TiO2-LSX when compared with UV-TiO2 or UV alone. The EE2 degradation process followed pseudo-first-order kinetics. A comprehensive empirical model was developed to describe the EE2 degradation kinetics under different conditions using multiple linear regression analysis. The EE2 degradation mechanism was proposed based on molecular calculations, identification of photoproducts using HPLC-MS/MS, and reactive species quenching experiments; the results showed that oxidative degradation pathways initiated by hydroxyl radicals were predominant. This novel TiO2-doped zeolite system provides a promising application for the UV disinfection process in wastewater treatment plants.

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.

Renewable p‐Xylene from 2,5‐Dimethylfuran and Ethylene Using Phosphorus‐Containing Zeolite Catalysts

p‐Xylene is a major commodity chemical used for the production of polyethylene terephthalate, a polymer with applications in polyester fibers, films, and bottles. The Diels–Alder cycloaddition of 2,5‐dimethylfuran and ethylene and the subsequent dehydration of the cycloadduct intermediate is an attractive reaction pathway to produce renewable p‐xylene from biomass feedstocks. However, the highest yields reported previously do not exceed 75 %. We report that P‐containing zeolite Beta is an active, stable, and selective catalyst for this reaction with an unprecedented p‐xylene yield of 97 %. It can catalyze the dehydration reaction selectively from the furan‐ethylene cycloadduct to p‐xylene without the production of alkylated and oligomerized products. This behavior is distinct from that of Al‐containing zeolites and other solid phosphoric acid catalysts and establishes a commercially attractive process for renewable p‐xylene production.

Efficient mechano-catalytic depolymerization of crystalline cellulose by formation of branched glucan chains

Selective hydrolysis of cellulose into glucose is a critical step for producing value-added chemicals and materials from lignocellulosic biomass. In this study, we found that co-impregnation of crystalline cellulose with sulfuric acid and glucose can greatly reduce the time needed for ball milling compared with adding acid alone. The enhanced reaction time coincides with the rapid formation of branched α(1→6) glycosidic bonds, which have been shown to increase water solubility of β(1→4) glucan oligomers. Co-impregnation of glucose was crucial for the rapid formation of the α(1→6) branches, after which a carbon-based catalyst can rapidly hydrolyze the water-soluble glucan oligomers to 91.2% glucose yield faster than conventional approaches.

Adsorption and reaction properties of SnBEA, ZrBEA and H-BEA for the formation of p-xylene from DMF and ethylene

The adsorption and reaction properties of H-BEA, SnBEA, ZrBEA and siliceous BEA were examined to understand the reaction of 2,5-dimethylfuran (DMF) with ethylene to form p-xylene. Temperature-programmed desorption (TPD) of diethyl ether, DMF, 2,5-hexanedione and p-xylene on each of the zeolites demonstrated that the Bronsted sites in H-BEA are more reactive than the Lewis sites in SnBEA and ZrBEA and tend to promote the oligomerization of DMF and 2,5-hexanedione, even at 295 K; however, the adsorbed 2,5-hexanedione is converted to DMF at both Lewis- and Bronsted-acid sites. H-BEA, SnBEA and ZrBEA all catalyzed the reaction to p-xylene with high selectivity in a continuous-flow reactor, with all three catalysts showing rates that were first order in both DMF and ethylene. H-BEA was found to deactivate rapidly due to coking, while ZrBEA and SnBEA were both stable. The implications of these results for practical applications are discussed.