Zhijie Wu

Mercaptosilane-Assisted Synthesis of Metal Clusters within Zeolites and Catalytic Consequences of Encapsulation

We report here a general synthetic strategy to encapsulate metal clusters within zeolites during their hydrothermal crystallization. Precursors to metal clusters are stabilized against their premature colloidal precipitation as hydroxides during zeolite crystallization using bifunctional (3-mercaptopropyl)trimethoxysilane ligands. Mercapto (-SH) groups in these ligands interact with cationic metal centers, while alkoxysilane moieties form covalent Si-O-Si or Si-O-Al linkages that promote zeolite nucleation around ligated metal precursors. These protocols led to the successful encapsulation of Pt, Pd, Ir, Rh, and Ag clusters within the NaA zeolite, for which small channel apertures (0.41 nm) preclude postsynthesis deposition of metal clusters. Sequential treatments in O(2) and H(2) formed small (approximately 1 nm) clusters with uniform diameter. Titration of exposed atoms with H(2) or O(2) gave metal dispersions that agree well with mean cluster sizes measured from electron microscopy and X-ray absorption spectroscopy, consistent with accessible cluster surfaces free of mercaptosilane residues. NaA micropore apertures restrict access to encapsulated clusters by reactants based on their molecular size. The ratio of the rates of hydrogenation of ethene and isobutene is much higher on clusters encapsulated within NaA than those dispersed on SiO(2), as also found for the relative rates of methanol and isobutanol oxidation. These data confirm the high encapsulation selectivity achieved by these synthetic protocols and the ability of NaA micropores to sieve reactants based on molecular size. Containment within small micropores also protects clusters against thermal sintering and prevents poisoning of active sites by organosulfur species, thus allowing alkene hydrogenation to persist even in the presence of thiophene. The bifunctional nature and remarkable specificity of the mercapto and alkoxysilane functions for metal and zeolite precursors, respectively, render these protocols extendable to diverse metal-zeolite systems useful as shape-selective catalysts in demanding chemical environments.

Synthesis and Catalytic Properties of Metal Clusters Encapsulated within Small-Pore (SOD, GIS, ANA) Zeolites

The synthesis protocols for encapsulation of metal clusters reported here expand the diversity in catalytic chemistries made possible by the ability of microporous solids to select reactants, transition states, and products on the basis of their molecular size. We report a synthesis strategy for the encapsulation of noble metals and their oxides within SOD (Sodalite, 0.28 nm × 0.28 nm), GIS (Gismondine, 0.45 nm × 0.31 nm), and ANA (Analcime, 0.42 nm × 0.16 nm) zeolites. Encapsulation was achieved via direct hydrothermal synthesis for SOD and GIS using metal precursors stabilized by ammonia or organic amine ligands, which prevent their decomposition or precipitation as colloidal hydroxides at the conditions of hydrothermal synthesis (<380 K) and favor interactions between metal precursors and incipient aluminosilicate nuclei during self-assembly of microporous frameworks. The synthesis of ANA requires higher crystallization temperatures (~415 K) and high pH (>12), thereby causing precipitation of even ligand-stabilized metal precursors as hydroxides. As a result, encapsulation was achieved by the recrystallization of metal clusters containing GIS into ANA, which retained these metal clusters within voids throughout the GIS-ANA transformation.

Preparation of a Cu–Ru/carbon nanotube catalyst for hydrogenolysis of glycerol to 1,2-propanediolviahydrogen spillover

A Cu–Ru nanoparticle catalyst supported on carbon nanotubes was prepared by a chemical replacement reaction between Cu metal nanoparticles and Ru3+ cations. The as-prepared catalyst was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, H2 and CO chemisorption, and transmission electron microscopy. The results showed that the highly dispersed Ru clusters were present on the external surface of the Cu particles. These tiny Ru clusters did not activate glycerol to carry its hydrogenolysis, but instead activated and generated active hydrogen, which was transferred to the Cu surface nearby viahydrogen spillover. The Cu–Ru catalyst exhibited selectivity for 1,2-propanediol that was as high as Cu metal, and much higher hydrogenolysis activity than pure Cu metal because of the hydrogen spillover effect, which benefited from the Ru clusters.

Synthesis of nickel nanoparticles supported on metal oxides using electroless plating: Controlling the dispersion and size of nickel nanoparticles

Nickel nanoparticles supported on metal oxides were prepared by a modified electroless nickel-plating method. The process and mechanism of electroless plating were studied by changing the active metal (Ag) loading, acidity, and surface area of metal oxides and were characterized by UV-vis spectroscopy, transmission electron microscopy, scanning electron microscopy, and H(2) chemisorption. The results showed that the dispersion of nickel nanoparticles was dependent on the interface reaction between the metal oxide and the plating solution or the active metal and the plating solution. The Ag loading and acidity of the metal oxide mainly affected the interface reaction to change the dispersion of nickel nanoparticles. The use of ultrasonic waves and microwaves and the change of solvents from water to ethylene glycol in the electroless plating could affect the dispersion and size of nickel nanoparticles.

Selective conversion of cellulose into bulk chemicals over Brønsted acid-promoted ruthenium catalyst: one-pot vs. sequential process

Acid hydrolysis and hydrogenation/hydrogenolysis reactions can be combined for catalytic conversion of cellulose into renewable biorefinery feedstocks by using two heterogeneous catalysts: sulfonic acid (–SO3H) functionalized mesoporous silica (MCM-41) and Ru/C. The combination is suitable for a one-pot tandem process to convert cellulose into alkanediols (mainly propylene glycol and ethylene glycol), yet deactivation of the sulfonic acid (–SO3H) functionalized mesoporous silica occurred rapidly after only one reaction cycle because of an irreversible change in the mesoporous structure and loss of acid groups. However, much better selectivity of hexitol or γ-valerolactone (GVL) can be obtained in a sequential tandem process by hydrogenating the hydrolysis products, glucose and levulinic acid (LA). A similar irreversible deactivation of acid catalyst also occurred when it involved the hydrogenolysis of glucose into alkanediols. When the sulfonic acid-functionalized mesoporous silica is filtered, and the hydrolysis products of cellulose are directly used in the hydrogenation reaction without further purification, a better selectivity and stability of hexitol production can be obtained. Under such conditions, the lifetime of the catalyst system can be significantly extended, up to 6 times the original durability of the acid-functionalized silica.

Synthesis and characterization of a porous amorphous Ni–B catalyst on titania by silver-catalyzed electroless plating

An amorphous Ni–B/TiO2 catalyst has been synthesized by silver-catalyzed electroless nickel plating. The amorphous structure of Ni–B nanoparticles was characterized by X-ray diffraction (XRD) and selected area electron diffraction (SAED). A transmission electron micrograph (TEM) of the Ni–B/TiO2 showed particles with size ranging from 30 to 50 nm were homogeneously dispersed over TiO2 support. High-resolution transmission electron microscopy (HRTEM) indicated that the Ni–B nanoparticles presented a porous and flower-like morphology, which was different from the solid sphere of conventional Ni–B particles prepared by impregnation–reduction method. To investigate the formation process of the porous Ni–B particles, the diffusion of nickel nuclei and the growth of Ni–B particles were characterized. The as-prepared porous Ni–B/TiO2 catalyst exhibited superior catalytic activities in the hydrogenation reactions to those of catalysts prepared by conventional methods.

Fe3O4@SiO2@Pd-Au: a highly efficient and magnetically separable catalyst for liquid-phase hydrodechlorination of 4-chlorophenol

A magnetic core–shell nanocomposite, Fe3O4@SiO2@Pd-Au, was synthesized by reducing palladium and gold cations previously bound to the amine ligand-modified surface of silica-encapsulated magnetic iron oxide (Fe3O4) nanoparticles, and served as a highly efficient and easily-recyclable catalyst for liquid-phase hydrodechlorination of 4-chlorophenol under mild conditions.

Facile fabrication of mesopore-containing ZSM-5 zeolite from spent zeolite catalyst for methanol to propylene reaction

A facile and eco-friendly approach for the synthesis of mesopore-containing ZSM-5 zeolite with small crystal size was proposed by subtly making use of a coke-deposited spent zeolite catalyst. When used in the methanol to propylene reaction again, the refabricated ZSM-5 catalyst exhibited a much higher propylene selectivity and a longer catalytic lifetime.

Synthesis by Replacement Reaction and Application of TiO2‐Supported Au–Ni Bimetallic Catalyst

A TiO2‐supported Au on Ni bimetallic nanoparticle catalyst is prepared by electroless nickel plating and the sequential chemical replacement reaction of supported Ni nanoparticles with Au3+ ions. The size of nickel nanoparticles decreases and highly dispersed gold clusters form on the external surface of the nickel nanoparticles during replacement and sequential thermal treatment. Hydrogen chemisorption and X‐ray photoelectron spectroscopy show the interface interaction between Au atoms and the surface of nickel nanoparticles, optimization of which can enhance hydrogen adsorption and chlorobenzene hydrodechlorination activity and stability. The surface enrichment of gold species on the Au–Ni bimetallic nanoparticles benefits the removal of Cl species from the surface of active sites during hydrodechlorination and then promotes the stability of the catalyst. A series of Au–Ni bimetallic nanoparticles with different particle sizes are synthesized by using different‐sized nickel particles from electroless plating as precursors. The larger Au–Ni nanoparticles exhibit higher hydrodechlorination activity and stability, suggesting that chlorobenzene hydrodechlorination on Au–Ni nanoparticles is structure sensitive. These findings hold promise for a simple route to design and synthesis of bimetallic systems as highly active and stable catalysts for the catalytic reactions.

Facile control of inter-crystalline porosity in the synthesis of size-controlled mesoporous MFI zeolites via in situ conversion of silica gel into zeolite nanocrystals for catalytic cracking

We report here a strategy for the facile synthesis of hierarchical MFI zeolite nanocrystals with controllable inter-crystalline mesopores by a one-step hydrothermal synthesis method using silica gel as the silica source and tetrapropyl ammonium as the microporous template without any other mesoporous templates or zeolite seeds. Powder X-ray diffraction results show the MFI structure with high crystallinity for all as-prepared zeolites. Scanning electron microscope characterization shows that 400–1000 nm zeolite aggregates are composed of the assembly of ~100 nm zeolite nanocrystals. Transmission electron microscopy results indicate the formation of inter-crystalline mesopores in the aggregated nanocrystals among the interspace of zeolite nanocrystals. The high mesopore volume (0.13 cm3 g−1) and external surface area (93 cm2 g−1) of the aggregated MFI zeolites are observed by N2 sorption measurements. The inter-crystalline porosity of MFI zeolites varies with the change in aggregation and the size of zeolite nanocrystals by changing the sodium concentration or the type of sodium salt in aluminate–silicate gels during hydrothermal crystallization. The mesoporous MFI zeolite aggregates exhibit similar light olefin selectivities and remarkably enhanced lifetime in the catalytic cracking of hexane compared to the highly dispersed MFI zeolite nanocrystals.