Chengyi Dai

Hollow zeolite encapsulated Ni–Pt bimetals for sintering and coking resistant dry reforming of methane

Highly dispersed Ni–Pt bimetallic nanoparticles encapsulated in hollow silicalite-1 single crystals (1.5Ni–0.5Pt@Hol S-1) were a superior catalyst for sintering and coking resistant dry (CO2) reforming of CH4. Large Ni particles loaded on the surface of solid silicalite-1 crystals triggered coke formation, which simultaneously degraded the catalytic activity of small Ni particles. With Ni encapsulated in hollow crystals, the small Ni particles inhibited coke formation. The encapsulating shell prevented coke formed outside from degrading the activity of nickel on the inside, leading to stable high activity even in the presence of carbon. Compared with single metals (Ni or Pt), 1.5Ni–0.5Pt@Hol S-1 enhanced the dispersion of nickel and platinum. In the dry reforming of methane, the 1.5Ni–0.5Pt@Hol S-1 catalyst operated stably under high gaseous hourly space velocity (GHSV = 72 000 ml g−1 h−1) without any inert gas. Only 1.0 wt% carbon deposition was observed by thermogravimetric analysis (TGA) after 6 h of the reaction. Hollow zeolite crystals can reliably support coke resistant catalysts for dry reforming of CH4 and multi-metallic catalysts with well-dispersed nanoparticles.

Pd and Pd–CuO nanoparticles in hollow silicalite-1 single crystals for enhancing selectivity and activity for the Suzuki–Miyaura reaction

Pd and Pd–CuO nanoparticles were successfully encapsulated in hollow silicalite-1 single crystals by tetrapropylammonium hydroxide (TPAOH) hydrothermal treatment with an “impregnation-dissolution-recrystallization” process. The size and number of particles in the hollow zeolite depended mainly on the nature of the metal. For palladium, the palladium nanoparticles easily aggregated into larger particles in the hydrothermal process, which displays excellent substrate selectivity for the meta- and para-substituted aryl bromides in the Suzuki–Miyaura reaction. For Pd–CuO binary metals (oxide), introducing copper oxide prevents aggregation of palladium, which shows about 3 times higher activity than encapsulated single Pd catalyst for the above reaction. The strategy using a hollow zeolite crystal as a support is a more reliable method for preparing multi-metallic (oxide) catalysts with well-dispersed nanoparticles.

Hollow Alveolus-Like Nanovesicle Assembly with Metal-Encapsulated Hollow Zeolite Nanocrystals

Inspired by the vesicular structure of alveolus which has a porous nanovesicle structure facilitating the transport of oxygen and carbon dioxide, we designed a hollow nanovesicle assembly with metal-encapsulated hollow zeolite that would enhance diffusion of reactants/products and inhibit sintering and leaching of active metals. This zeolitic nanovesicle has been successfully synthesized by a strategy which involves a one-pot hydrothermal synthesis of hollow assembly of metal-containing solid zeolite crystals without a structural template and a selective desilication-recrystallization accompanied by leaching-hydrolysis to convert the metal-containing solid crystals into metal-encapsulated hollow crystals. We demonstrate the strategy in synthesizing a hollow nanovesicle assembly of Fe2O3-encapsulated hollow crystals of ZSM-5 zeolite. This material possesses a microporous (0.4-0.6 nm) wall of hollow crystals and a mesoporous (5-17 nm) shell of nanovesicle with macropores (about 350 nm) in the core. This hierarchical structure enables excellent Fe2O3 dispersion (3-4 nm) and resistance to sintering even at 800 °C; facilitates the transport of reactant/products; and exhibits superior activity and resistance to leaching in phenol degradation. Hollow nanovesicle assembly of Fe-Pt bimetal-encapsulated hollow ZSM-5 crystals was also prepared.

A facile strategy for enhancing FeCu bimetallic promotion for catalytic phenol oxidation

The mesoporous ZSM-5 zeolite obtained from alkaline treatment was found to be a superior support of bimetallic FeCu, minimizing the nanoparticle size, enhancing the bimetallic interaction, and promoting catalytic oxidation of phenol. The physicochemical characteristics of the as-prepared FexCuy/ZSM-5 samples were evaluated by XRD, TEM, Ar adsorption, H2-TPR, and 57Fe Mossbauer spectroscopy which revealed a strong bimetallic interaction. Meanwhile, phenol oxidation was applied as a probe reaction under mild conditions. By supporting FeCu bimetallic oxides on mesoporous ZSM-5, the obtained Fe5Cu5/ME displayed the highest activity, which can be attributed to both the minimized nanoparticle size and the enhanced bimetallic interaction. The mesoporous ZSM-5 support used in this work was obtained from alkaline treatment, which led to a rough mesoporous surface. This surface sufficiently enhanced the dispersion and prohibited metal migration, therefore preventing nanoparticle aggregation and enhancing the bimetallic interaction. The strategy of using mesoporous ZSM-5 obtained from alkaline treatment as a support is a reliable method for preparing multi-metallic catalysts with well-dispersed nanoparticles.

Facile synthesis of zeolite-encapsulated iron oxide nanoparticles as superior catalysts for phenol oxidation

Meso-ZSM-5 modified by polyethyleneimine has been found to be an excellent support for iron oxide with improved physicochemical properties of iron oxide particles including size and chemical state. The resulting ZSM-5 encapsulated iron nanoparticles exhibit superior catalytic activity for phenol oxidation.

Facile one-step synthesis of hierarchical porous carbon monoliths as superior supports of Fe-based catalysts for CO2 hydrogenation

A versatile strategy involving one-step desilication of coke-deposited spent zeolite catalyst was successfully developed to prepare hierarchical porous carbon monoliths (HPCMs). Such a strategy avoids the use of hard or soft templates and carbon sources, eliminates high temperature carbonization, simultaneously minimizing the emissions from processing spent catalysts. The resulting carbon exhibits a controlled morphology such as three-dimensional networks, hollow spheres or nanosheets, a high degree of graphitization and a multi-level porous structure. Its mesopore (2–50 nm) surface area can reach 522 m2 g−1 and both mesopore and macropore (50–350 nm) volumes are more than 1.0 cm3 g−1. Such hierarchical porous carbon was found to be a superior support for minimizing the nanoparticle size and enhancing the synergism of the Fe–K catalyst for promoting CO2 hydrogenation. Using such a catalyst results in increased conversion of carbon dioxide and enhanced selectivity of high value olefins (C2–4) and long-chain hydrocarbons (C5+).

Mesostructure-tunable and size-controllable hierarchical porous silica nanospheres synthesized by aldehyde-modified Stöber method

Mesostructure-fine-tuned and size-controlled hierarchical porous silica nanospheres were synthesized by aldehyde-modified Stober method in the TEOS–CTAB–NH3·H2O–aldehyde system. The samples were characterized by XRD, N2 adsorption–desorption isotherms, SEM, TEM and TG analysis. The results indicate that the particle size of the micro/mesoporous silica nanospheres synthesized with acetaldehyde as a co-solvent can be controlled from 40 to 850 nm by regulating the molar ratio of acetaldehyde to water and the initial pH of the synthesis solution. When propionaldehyde or butyraldehyde was used as a co-solvent, hierarchical porous silica nanospheres with large cone-like cavities and small mesopores in the cavity wall were synthesized; the diameter of the flower-like nanospheres is less than 130 nm. The hierarchical pore structure of the flower-like silica nanospheres can be fine-tuned by controlling the polymerization of butyraldehyde by the synthesis temperature from 27 to 100 °C, both the depth and opening diameter of the cone-like cavities can be fine-tuned from 40 to 2 nm; simultaneously, the small mesopores templated by CTAB become more ordered.

Evolution of iron species for promoting the catalytic performance of FeZSM-5 in phenol oxidation

Facile modification of FeZSM-5 utilizing an organic directing agent (ODA) for iron species evolution toward better Fenton activity was developed. Pristine FeZSM-5, prepared hydrothermally using tetrapropylammonium bromide as the ODA, was thermally treated in nitrogen or air atmosphere for comparison. The as-prepared FeZSM-5 were evaluated by XRD, SEM, TGA, Ar physical adsorption, H2-TPR, UV-Vis, XPS and Mossbauer spectra, which confirmed well-dispersed iron species and partial transformation of framework iron to non-framework iron and Fe(III) to Fe(II) during thermal treatment in N2. The phenol oxidation reaction was applied as a probe reaction and phenol conversion can be improved from 44% to 90% within 60 minutes. By thermal treatment in N2 atmosphere, the ODA plays an important role in modifying non-framework iron species from Fe(III) to Fe(II) ions which is desirable for improving catalytic activity in Fenton-like systems. The strategy utilizing the thermal reduction effect of the ODA on iron species is a reliable method for preparing well-dispersed and valence tunable iron oxide nanoparticles encapsulated in ZSM-5.

Precise control of the size of zeolite B-ZSM-5 based on seed surface crystallization

A series of boron-containing ZSM-5 (B-ZSM-5) catalysts with particle sizes from ∼153 nm to ∼14.2 μm are synthesized by regulating the addition of silicalite-1 seeds. The B-ZSM-5 particle size with seed addition (D), the seed addition amount (x), the seed particle size (d) and the B-ZSM-5 particle size without seed addition (D0) are related with by the new function D3 = d3D03/[xD03 + (1 − x)d3], and the particle size of B-ZSM-5 has been precisely controlled and predicted in both micro (TPABr as template) and nano (TPAOH as template) synthesis systems. The function also shows the evolution of B-ZSM-5 particle size versus the seed amount and size, which helps guide the synthetic choice of the seed amount used or decreasing the seed size to decrease the B-ZSM-5 size for a specific synthesis system. Furthermore, the effect of particle size on the catalytic performance of B-ZSM-5 for the methanol to propylene (MTP) reaction is also investigated. The addition of 1 wt% ∼74 nm seeds to the synthesis system improves the resulting catalyst lifetime from 46 h to 794 h.