Yanfei Zhang

Preparation of TiO2 hollow nanofibers by electrospining combined with sol–gel process

TiO2 hollow nanofibers have been fabricated by directly annealing electrospun polyvinylpyrrolidone (PVP)/Tetra-butyl titanate (TBT) composite nanofibers. In this approach, PVP/TBT composite fibers were first synthesized by electrospinning PVP/TBT solution, and then calcined at high temperature with an appropriate heating rate to form hollow TiO2 nanofibers. During the heat treatment, the solvent composition, the amount of TBT and the heating rate have important influences on the morphologies of the TiO2 nanofibers. Morphologies of TiO2 could be tuned from solid nanofibers to belts, hollow nanofibers and rods by controlling the appropriate preparation conditions. The crystal structure, morphology, surface composition and specific surface area of the TiO2 hollow nanofibers were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Brunauer–Emmett–Teller analysis, respectively.

Preparation of SrAl2O4:Eu2+, Dy3+ fibers by electrospinning combined with sol-gel process

One-dimensional SrAl(2)O(4):Eu(2+), Dy(3+) fibers were fabricated by a simple electrospinning combined with sol-gel process. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and photoluminescence were used to characterize the fibers. The results show that the phase structure of SrAl(2)O(4):Eu(2+), Dy(3+) belongs to a monoclinic one, the composite fibers and fibers calcined at high temperature remain the original one-dimensional texture, and the SrAl(2)O(4):Eu(2+), Dy(3+) was a green emission.

Single-crystalline nanowires of SiC synthesized by carbothermal reduction of electrospun PVP/TEOS composite fibres

Nanowires of SiC were synthesized by carbothermally reducing PVP/TEOS composite fibres obtained by electrospinning. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) indicated that the SiC nanowires are single crystalline in nature. Both Fourier-transformed infrared spectroscopy and HRTEM indicated that a thin layer of SiO2 was formed on the outer surface of the nanowire as a result of post-heat treatment for the removal of residual carbon. Such SiO2 layer protects the inner SiC fibre from further oxidation. The formation mechanism of single-crystalline SiC nanowires was proposed based on our understanding and characterizations. The growth of the nanowire is believed to be along the (111) of its cubic cell.

One pot synthesis of hierarchically macro/microporous ZSM-5 single crystals

Hierarchical aluminosilicate-based MFI zeolite single crystals with intracrystalline macropores were fabricated in one pot for the first time. The strategy involved the outward dissolution of the aluminosilicate precursor and simultaneous recrystallization of Si and Al species under the guidance of the organic template tetrapropylammonium bromide. The interesting outward dissolution of the aluminosilicate precursor from core to shell led to the formation of intracrystalline macropores. The Si/Al ratios of these macro/microporous ZSM-5 single crystals could be easily tuned in a wide range by varying the aluminum and alkali contents of the aluminosilicate precursors. The hierarchical ZSM-5 zeolite with highly interconnected macropores exhibited high performance in the 1,3,5-triisopropylbenzene cracking reaction due to the improved intracrystalline diffusion. This study provides a new strategy to fabricate highly efficient zeolites with hierarchical porosity in a simple and cost-effective way.

Understanding of the dissolution–crystallization fabrication strategy towards macro/microporous ZSM-5 single crystals

In this study, detailed investigations about a novel dissolution–crystallization synthetic route for the fabrication of hierarchically macro/microporous ZSM-5 single crystals were conducted. During the fabrication, pre-prepared mesoporous aluminosilicate spheres (MASS) acted not only as raw materials but also as quasi “scaffold templates” simultaneously to construct additional macropores. Factors like the amount of alkali sodium hydroxide (NaOH), crystallization temperature and Si/Al ratios of MASS could affect the successful fabrication of such hierarchical zeolites via varying either the dissolution behaviors of precursors or the crystallization of zeolite crystals. Generally, only with suitable amounts of alkali NaOH that matched the Si/Al ratios of precursors and at appropriate crystallization temperature could the balance between the dissolution and crystallization of MASS precursors be achieved and thus the intracrystalline macropores be constructed. Owing to the introduction of rich intracrystalline macropores, the diffusional properties of the hierarchical ZSM-5 crystals were significantly enhanced, which has also been well proved through the toluene adsorption experiment with an intelligent gravimetric analyzer. Thus, with a similar Zn-modification, all the three hierarchical samples with different Si/Al ratios exhibited a much longer lifetime and higher aromatics selectivity in the MTA reactions than the conventional microporous sample.

A new insight into the reaction behaviors of side-chain alkylation of toluene with methanol over CsX

Side-chain alkylation of toluene with methanol is a potential technology to realize the one-step production of styrene under mild conditions. Herein, a series of cesium X zeolites with different cesium contents were prepared through an ion-exchange method. The performance of the CsX zeolites was extensively investigated in the side-chain alkylation of toluene and methanol. The acid–base properties of the involved CsX were characterized and the results showed that the acid–base properties present regular changes with varied cesium contents. Based on the full understanding of the acid–base properties of the involved CsX, the reaction behaviors of the main and side reactions in the side-chain alkylation of toluene with methanol over CsX were investigated and new insights were revealed. It has been found that the crucial steps for the main reactions were the activation of toluene and the formation of formaldehyde from dehydrogenation of methanol when the cesium exchange degree of CsX was lower and higher than 32.3%, respectively. The hydride transfer reaction between styrene and carbon deposition is responsible for the formation of ethylbenzene over CsX with lower cesium contents, while the transfer hydrogenation of styrene with methanol and formaldehyde was the main path for ethylbenzene formation over CsX with higher cesium contents. And it was found that the existence of Bronsted acid sites accounts for the formation of xylene over CsX. These new findings will give a new insight into the reaction behaviors of the side-chain alkylation of toluene with methanol over CsX and may also provide a potential modification method for high-efficiency catalysts.