Guang Xiong

Preparation of highly dispersed desulfurization catalysts and their catalytic performance in hydrodesulfurization of dibenzothiophene

Abstract Micro-mesoporous ZK-1 molecular sieves with different Si/Al ratios were used as supports for binary Co-Mo hydrodesulfurization (HDS) catalysts. The CoMo/ZK-1 catalysts were prepared using an over-loading impregnation method, and characterized using N2 physisorption, X-ray diffraction, temperature-programmed NH3 desorption, temperature-programmed reduction (TPR), ultraviolet-visible diffuse reflectance spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The results show that the CoMo/ZK-1 catalysts have high surface areas (∼700 m2/g), large pore volumes, and hierarchical porous structures, which promote the dispersion of Co and Mo oxide phases on the ZK-1 supports. The TPR results show that the interactions between the Co and Mo oxide phases and the ZK-1 support are weaker than those in the CoMo/γ-Al2O3 catalyst. The HRTEM results show that the CoMo/ZK-1 catalysts have better MoS2 dispersion and more active edge sites. The catalysts were tested in HDS of dibenzothiophene. Under mild reaction conditions, the activity of Co and Mo sulfides supported on ZK-1 was higher than those of Co and Mo sulfides supported on ZSM-5, AlKIT-1, and γ-Al2O3.

The effect of acid treatment on the active sites and reaction intermediates of the low-cost TS-1 in propylene epoxidation

This study investigated the roles of the different titanium species of low-cost TS-1 in propylene epoxidation and the influence of acid treatments. The low-cost TS-1 zeolites with different titanium species were synthesized using the hydrothermal method and characterized by ultraviolet (UV)-Raman, X-Ray Diffraction (XRD), ultraviolet visible diffuse reflectance spectroscopy (UV-Vis), N2 physical absorption and NH3 temperature programmed desorption (NH3-TPD) techniques. The roles of the different titanium species in the low-cost TS-1 samples were investigated by gas chromatography-Raman spectrometry (GC-Raman) during the propylene epoxidation process. The framework titanium species was found to be active for propylene epoxidation, while the extra-framework Ti species were found to be harmful for propylene epoxidation due to their acidity. The extra-framework Ti species can be removed by acid treatments. It was found that the acid treatment of the as-synthesized TS-1 is more effective since the amorphous TiO2 was not transformed into anatase TiO2 upon calcination. Strong acids, such as HCl, HNO3 and H2SO4, are more efficient for the elimination of the extra-framework TiO2, but do not significantly affect the framework titanium species. After treatment with an acid of a suitable concentration, the amount of the active intermediate species Ti–OOH(η2) can be increased, and a higher conversion of propylene and yield of PO can be achieved.

Crystallization mechanism and catalytic performance of TS-1 synthesized by an aerosol-assisted method

TS-1 was synthesized by an aerosol-assisted method. SiO2–TiO2 amorphous powder was prepared by an aerosol process from an aqueous solution of silicon and titanium sources under acid conditions, which can make titanium ions highly distributed without complex operations. The powder was crystallized with a given amount of template agent (25% TPAOH) under autogenous pressure. Without adding extra water, the crystallization system maintained a high concentration of template agent, which can significantly reduce the amount of template and the discharge of polluted water. TS-1 with different Si/Ti ratios and different crystallization times were synthesized from the SiO2–TiO2 amorphous powder by the aerosol-assisted method. The structure, composition and morphology of TS-1 were characterized by X-ray diffraction, UV-vis spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectrometry (XRF) and electron microscopy. The crystallization mechanism of TS-1 has been studied at different crystallization times. The results indicated that the crystallization process followed a liquid-phase mechanism. The incorporation of the Ti species into the framework was delayed as compared to that of the Si species. Furthermore, TS-1 exhibited good catalytic performance in propylene epoxidation with hydrogen peroxide.

Aerosol-assisted synthesis of hierarchical porous titanosilicate molecular sieve as catalysts for cyclohexene epoxidation

Hierarchical titanosilicate composites (HTS) have been synthesized by an aerosol spray method, by which TS-1 nanoparticles were assembled with cetyl trimethyl ammonium bromide and tetraethylorthosilicate/tetra-n-butyl titanate to form a hierarchical porous composites with a core–shell structure. The structure, porosity, morphology and composition of the materials were characterized by X-ray diffraction, nitrogen adsorption isotherm, Raman spectroscopy, electron microscopy and X-ray fluorescence spectroscopy. The hierarchical porous molecular sieve combines the advantages of TS-1 and Ti-KIT-1, and shows high activity and selectivity to epoxide in cyclohexene epoxidation with H2O2. The yield of cyclohexene oxide on HTS is much higher than those on TS-1 and Ti-KIT-1. The selectivities to cyclohexene oxide and 1,2-cyclohexanediol can be adjusted by varying the titanium contents of the HTS samples.

Facile synthesis and catalytic applications of micro-mesoporous molecular sieve ZK-1

A micro-mesoporous molecular sieve ZK-1 was synthesized at 170xa0°C by a facile one-step hydrothermal method. The structure of ZK-1 can be controlled by the crystallization time. ZK-1(I), the KIT-1 containing ZSM-5 building units in the pore walls, is formed during 2–6xa0h. ZK-1(II), A mixture of ZSM-5 and KIT-1, is formed after 8xa0h. The framework structure, porosity, morphology and acidity of the obtained materials were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Transmission electron microscopy, N2 adsorption, X-ray fluorescence, NH3-temperature programmed desorption and 27Al Magic angle spinning NMR spectroscopy. The samples exhibit good steam stability and better catalytic performances in the isomerization of o-ethyltoluene than KIT-1, suggesting their promising applications in the conversion of large molecules.

Enhancing hydrothermal stability of nano-sized HZSM-5 zeolite by phosphorus modification for olefin catalytic cracking of full-range FCC gasoline

In this study, phosphorus modification by trimethyl phosphate impregnation was employed to enhance the hydrothermal stability of nano-sized HZSM-5 zeolites. A parallel modification was studied by ammonium dihydrogen phosphate impregnation. The modified zeolites were subjected to steam treatment at 800 °C for 4 h (100% steam) and employed as catalysts for olefin catalytic cracking (OCC) of full-range fluid catalytic cracking (FCC) gasoline. X-ray diffraction, N2 physical adsorption and NH3 temperature-programmed desorption analysis indicated that, although significant improvements to the hydrothermal stability of nano-sized HZSM-5 zeolites can be observed when adopting both phosphorus modification strategies, impregnation with trimethyl phosphate displays further enhancement of the hydrothermal stability. This is because higher structural crystallinity is retained, larger specific surface areas/micropore volumes form, and there are greater numbers of surface acid sites. Reaction experiments conducted using a fixed-bed micro-reactor (catalyst/oil ratio = 4, time on stream = 4 s) showed OCC of full-range FCC gasoline—under a fluidized-bed reaction mode configuration—to be a viable solution for the olefin problem of FCC gasoline. This reaction significantly decreased the olefin content in the full-range FCC gasoline feed, and specifically heavy-end olefins, by converting the olefins into value-added C2–C4 olefins and aromatics. At the same time, sulfide content of the gasoline decreased via a non-hydrodesulfurization process. Nano-sized HZSM-5 zeolites modified with trimethyl phosphate exhibited enhanced catalytic performance for OCC of full-range FCC gasoline.

Construction of an operando dual-beam fourier transform infrared spectrometer and its application in the observation of isobutene reactions over nano-sized HZSM-5 zeolite

An operando dual-beam Fourier transform infrared (DB-FTIR) spectrometer was successfully de-veloped using a facile method. The DB-FTIR spectrometer is suitable for the real-time study of the dynamic surface processes involved in gas/solid heterogeneous catalysis under real reaction condi-tions because it can simultaneously collect reference and sample spectra. The influence of gas-phase molecular vibration and heat irradiation at real reaction temperatures can therefore be eliminated. The DB-FTIR spectrometer was successfully used to follow the transformation of isobutene over nano-sized HZSM-5 zeolite under real reaction conditions.

Isobutane aromatization over a complete Lewis acid Zn/HZSM-5 zeolite catalyst: performance and mechanism

In short-chain alkane aromatization catalyzed by metal-modified HZSM-5, the metal cations acting as Lewis acid sites are considered to be the active centres for the dehydrogenation. However, other possible effects of the metal cations during the reaction have been neglected. In this study, a combination of experiments and DFT calculations has been carried out to investigate the complete effects of the Zn on ZSM-5 in isobutane aromatization. Zn8.47/HZSM-5 (Zn/Al > 1) with weak acidic properties was prepared as a Lewis acid sites-dominated catalyst. It was found that the (Zn–O–Zn)2+ Lewis acid sites acted as the main active sites and no bridged hydroxyl groups (Bronsted acid sites) were present. In isobutane conversion, it was surprisingly found that Zn8.47/HZSM-5 showed better catalytic performance at low temperature than HZSM-5 (Bronsted acid sites-dominated catalyst) and Zn2.34/HZSM-5 (Bronsted and Zn-Lewis acid sites co-existing catalyst). DFT calculations and operando dual-beam Fourier transform infrared spectrometry (DB-FTIR) characterization were employed to understand how the isobutane aromatization could be catalyzed on the Lewis acid site (Zn–O–Zn)2+ exclusively without the participation of bridged hydroxyl groups. It was directly observed over DB-FTIR that the Lewis acid-type Zn/HZSM-5 enjoyed a faster aromatization rate and higher stability than HZSM-5 under real aromatization conditions. This firstly testified that the complete reaction of isobutane aromatization could be exclusively catalyzed by (Zn–O–Zn)2+ Lewis acid sites without the participation of bridged hydroxyl groups. The [Zn-(OH)−]+ species could be produced in situ and acted as the Bronsted acid sites. Owing to the weak acidity of [Zn-(OH)−]+, the Lewis acid sites played a key role during the aromatization reactions via the carbanionic mechanism.

The Crucial Role of Skeleton Structure and Carbon Number on Short-Chain Alkane Activation over Zn/HZSM-5 Catalyst: An Experimental and Computational Study

For the initial activation of short-chain alkanes over Zn/HZSM-5 catalyst, the impact of branching degree and carbon numbers of reactants on the competition between dehydrogenation and cracking was systematically studied by experiment and calculation. The experiments were carried out on fixed-bed flow micro-reactor over HZSM-5 and Zn/HZSM-5 catalysts, by using n-butane/i-butane and propane/n-hexane as reactants with different branching degree and carbon numbers. Compared with HZSM-5, Zn/HZSM-5 obviously accelerated the cracking of C–H bond of short-chain alkanes and increased the selectivity of BTX aromatics. The selectivity to hydrogen produced from n-butane and n-hexane was higher than i-butane and propane, respectively. On the contrary, the selectivity to methane was correspondingly lower, i-butane and propane with high percentage of terminal carbons effectively suppressed the dehydrogenation. The key point to decide the reaction process through dehydrogenation or cracking is the initially activated sites of reactant. To verify this conclusion, theoretical calculations were carried out. The results showed that the (Zn–O–Zn)2+ Lewis acid sites of Zn/HZSM-5 accelerated the cracking of C–H and C–C bond simultaneously. Namely, if the initial activation occurred on the terminal carbons of reactant, the subsequent reaction would be kinetically competitive between cracking and dehydrogenation. Cracking would inevitably occur (thermodynamically favorable), and then the by-products of methane and ethane were produced in large amount. If the initial activation occurred on the internal carbons, the dehydrogenation reaction is kinetically favorable, which is beneficial to reducing the dry gas production. Therefore, cracking is more favorable than dehydrogenation for smaller alkanes and branched alkanes with high percentage of terminal carbons.Graphical AbstractThe percentage of terminal carbons of short-chain alkanes determines the probability of dehydrogenation route over Zn/HZSM-5. The activation of the internal carbon via the dehydrogenation route is more favorable for the normal alkane with higher carbon number during the first stage of conversion suppressing the formation of the by-products of methane and ethane.