Homochiral Porous Framework as a Platform for Durability Enhancement of Molecular Catalysts

Self-quenching and vulnerability of active sites are major issues posed for practical applications of highly efficient chiral organometallic catalysts. Here, we demonstrate an effective strategy to address these challenges by constructing them into homochiral porous frameworks, which renders them with extraordinary resistance against deactivation yet fully retains the intrinsic catalytic activities and selectivities under heterogeneous systems. Representatively, after partial metalation of the porous chiral phosphoramidite ligand-based frameworks (Phos-HPFs) with Rh species, the afforded catalysts exhibit dramatically enhanced durability while maintaining the activity and selectivity of the homogeneous counterparts in asymmetric hydrogenation of olefins. The rigid framework of Phos-HPFs can isolate the active sites, thus preventing the self-quenching from forming coordinatively saturated complexes, while the active sites surrounded by dense free chiral ligands in Phos-HPFs can inhibit them from decomposing into metallic particles. Our work thereby highlights the advantages of HPFs for the deployment of catalysts, which offers an opportunity for enhancing the utilization efficiency rather than merely having the benefits of easy separation and recycling of the chiral catalysts

Porous Polymerized Organocatalysts Rationally Synthesized from the Corresponding Vinyl-Functionalized Monomers as Efficient Heterogeneous Catalysts

Porous polymerized organocatalysts (PPOs) have been successfully synthesized from the corresponding vinyl-functionalized monomers under solvothermal conditions. These PPOs have high surface areas, large pore volumes, hierarchical porosity, and good stability. Utilized as a typical organocatalyst, the porous polymerized 2,2,6,6-tetramethylpiperidine-1-oxyl (PPO-TEMPO) with stable free radicals shows high activities and excellent recyclabilities in selective oxidations of a variety of alcohols to the corresponding aldehydes or ketones. This synthesis method may open a new door for developing heterogeneous catalysts with high activity and good recyclability in the future

Design and Synthesis of Mesoporous Polymer-Based Solid Acid Catalysts with Excellent Hydrophobicity and Extraordinary Catalytic Activity

Novel excellent hydrophobic- mesoporous- polymer-based solid acid catalysts have been successfully synthesized by copolymerization of divinylbenzene (DVB) with sodium <i>p</i>-styrene sulfonate (H-PDVB-<i>x</i>-SO₃H's) under solvothermal conditions. N₂ isotherms and TEM images showed that H-PDVB-<i>x</i>-SO₃H's have high BET surface areas, large pore volumes, and abundant mesoporosity; CHNS element analysis and acid–base titration technology showed that H-PDVB-<i>x</i>-SO₃H's have adjustable sulfur contents (0.31–2.36 mmol/g) and acidic concentrations (0.26–1.86 mmol/g); TG curves showed that H-PDVB-<i>x</i>-SO₃H's exhibited much higher stability of the active site (372 °C) than that of the acidic resin of Amberlyst 15 (312 °C); contact angle and water adsorption tests showed that H-PDVB-<i>x</i>-SO₃H's exhibited excellent hydrophobic properties. Catalytic tests in esterification of acetic acid with cyclohexanol, esterification of acetic acid with 1-butanol, and condensation of benzaldehyde with ethylene glycol showed that H-PDVB-<i>x</i>-SO₃H's were more active than those of Amberlyst 15, SO₃H-functionalized ordered mesoporous silicas, and beta and USY zeolites, which were even comparable with that of homogeneous H₂SO₄. The superior hydrophobicity of solid acid catalysts would be favorable for achieving excellent catalytic performance because water usually acts as a byproduct in various acid-catalyzed reactions, which can easily poison the acid sites and result in opposite reactions. Synthesis of porous solid acid catalysts with good hydrophobicity would be very important for their applications

Aluminum Fluoride Modified HZSM-5 Zeolite with Superior Performance in Synthesis of Dimethyl Ether from Methanol

A series of HZSM-5 catalysts modified with various loadings of aluminum fluoride (AlF₃) were prepared from a mechanical mixture route. Combined characterizations of X-ray diffraction, Fourier transform infrared (FT-IR), ²⁷Al, ²⁹Si, ¹⁹F MAS NMR, N₂ sorption, and NH₃-tempeature-programmed desorption (NH₃-TPD) techniques show that the structure, texture, and acidity of HZSM-5 catalysts can be adjusted with the loading of AlF₃. A suitable amount of AlF₃ modification (2 wt %) could increase the framework aluminum content and the surface area of HZSM-5. However, when the loading of AlF₃ came to 3 wt % or more, the contrary results were obtained, which could be ascribed to the dealumination of the zeolitic framework. The catalytic activities for dehydration of methanol to dimethyl ether (DME) show that suitable amount of AlF₃-modified HZSM-5 exhibited much higher activity and better stability than parent HZSM-5. The combination of “tunable” synthesis and “superior” properties is very much valuable in the academic and industry

Excellent Performance of One-Pot Synthesized Cu-SSZ-13 Catalyst for the Selective Catalytic Reduction of NO_<i>x</i> with NH₃

Cu-SSZ-13 samples prepared by a novel one-pot synthesis method achieved excellent NH₃–SCR performance and high N₂ selectivity from 150 to 550 °C after ion exchange treatments. The selected Cu_3.8-SSZ-13 catalyst was highly resistant to large space velocity (800 000 h^–1) and also maintained high NO_<i>x</i> conversion in the presence of CO₂, H₂O, and C₃H₆ in the simulated diesel exhaust. Isolated Cu²⁺ ions located in three different sites were responsible for its excellent NH₃–SCR activity. Primary results suggest that the one-pot synthesized Cu-SSZ-13 catalyst is a promising candidate as an NH₃–SCR catalyst for the NO_<i>x</i> abatement from diesel vehicles

Transesterification Catalyzed by Ionic Liquids on Superhydrophobic Mesoporous Polymers: Heterogeneous Catalysts That Are Faster than Homogeneous Catalysts

Homogeneous catalysts usually show higher catalytic activities than heterogeneous catalysts because of their high dispersion of catalytically active sites. We demonstrate here that heterogeneous catalysts of ionic liquids functionalized on superhydrophobic mesoporous polymers exhibit much higher activities in transesterification to form biodiesel than homogeneous catalysts of the ionic liquids themselves. This phenomenon is strongly related to the unique features of high enrichment and good miscibility of the superhydrophobic mesoporous polymers for the reactants. These features should allow the design and development of a wide variety of catalysts for the conversion of organic compounds

Product Selectivity Controlled by Zeolite Crystals in Biomass Hydrogenation over a Palladium Catalyst

This work delineates the first example for controlling product selectivity in metal-catalyzed hydrogenation of biomass by zeolite crystals. The key to this success is to combine the advantages of both Pd nanoparticles (highly active sites) and zeolite micropores (controllable diffusion of reactants and products), which was achieved from encapsulation of the Pd nanoparticles inside of silicalite-I zeolite crystals as a core–shell structure (Pd@S-1). In the hydrogenation of biomass-derived furfural, the furan selectivity over the Pd@S-1 is as high as 98.7%, outperforming the furan selectivity (5.6%) over conventional Pd nanoparticles impregnated with S-1 zeolite crystals (Pd/S-1). The extraordinary furan selectivity in the hydrogenation over the Pd@S-1 is reasonably attributed to the distinguishable mass transfer of the hydrogenated products in the zeolite micropores

Selective Conversion of Cyclohexene to 2‑Methoxycyclohexanol over Molybdenum Oxide on Beta Zeolite

Selective transformation of olefins is an important route to produce cyclic monoether glycol products, but the current catalysts still suffer from insufficient activity, selectivity, and durability. Herein, we reported that molybdenum oxide on siliceous beta zeolite was highly efficient for the selective conversion of cyclohexene with a trans-2-methoxycyclohexanol yield as high as 95% in the presence of methanol and tert-butyl hydroperoxide as an oxidant. This reaction included the epoxidation of cyclohexene and subsequent solvolysis. Mechanistic studies revealed that the siliceous beta zeolite benefited the formation of abundant monomeric MoOx species, which was efficient for both the epoxidation of cyclohexene and subsequent solvolysis. Furthermore, this catalyst was also active for the conversion of other olefins, achieving the corresponding β-alkoxyalcohols

Origin of the Low Olefin Production over HZSM-22 and HZSM-23 Zeolites: External Acid Sites and Pore Mouth Catalysis

The methanol-to-olefin (MTO) reaction was studied over two one-dimensional 10-ring zeolites with similar pore structures: ZSM-22 (TON) and ZSM-23 (MTT). It was found that the initial low but detectable production of olefins over both zeolites was catalyzed by external and/or pore mouth acid sites through hydrocarbon pool mechanism. Evidence is listed as follows: The uncalcined HZSM-22 had similar MTO activity as the calcined catalyst. The HZSM-22 zeolite with smaller crystal size had higher MTO activity. Both zeolites treated by HNO₃ to selectively leach the external acid sites showed significantly reduced initial MTO activity. Both zeolites coked through catalytic cracking of 1,3,5-triisopropylbenzene showed significantly lower initial MTO activity. This conclusion may also be suitable for other one-dimensional zeolites with pore size below 5.7 Å in the MTO reaction

Strong Metal–Support Interactions Achieved by Hydroxide-to-Oxide Support Transformation for Preparation of Sinter-Resistant Gold Nanoparticle Catalysts

The strong metal–support interactions (SMSI) are well-known but crucial for preparation of supported metal nanoparticle catalysts, which generally occur by reduction and oxidation under harsh conditions. Here, we delineate the example of constructing SMSI without reduction and oxidation, where the key is to employ a hydroxide-to-oxide support transformation. The covering of Au nanoparticles by oxides, electronic interaction, and changes in CO adsorption tests of the catalyst are identical to those of the classic SMSI. Owing to the SMSI with oxide barriers on the Au nanoparticles, the supported Au catalysts are sintering-resistant at high temperatures, which benefit long-life reactions, outperforming the conventional supported catalysts