Congbiao Chen

The intrinsic effects of shell thickness on the Fischer–Tropsch synthesis over core–shell structured catalysts

A series of core–shell catalysts with cobalt nanoparticles coated by silica shells were prepared to provide an insight into the effects of the shell thickness on the Fischer–Tropsch synthesis. The catalysts displayed uniform silica shell thicknesses in the range of 4.3–18.2 nm as ascertained by TEM. From the H2 chemisorption results, increasing the shell thickness did not reduce the number of active sites due to the similar active cobalt surface areas. Even though the reducibility determined by H2-TPR decreased rapidly with the increase in shell thickness, the catalytic activity was not evidently reduced. The hydrocarbon products shifted to shorter chains and the C15–C18 selectivity had a volcano-type dependence as the shell thickness increased, which is probably because thicker shells contribute to more severe diffusion limitations of the reactants.

Studies of Cobalt Particle Size Effects on Fischer–Tropsch Synthesis over Core–Shell‐Structured Catalysts

A series of core–shell‐structured catalysts that consist of different‐sized Co3O4 nano‐particles and silica shells were prepared by an in situ coating method. The reduced catalysts displayed uniform core sizes that ranged from 5.5–12.7 nm as ascertained by TEM, which concurred well with XRD analysis. The BET results revealed the highly mesoporous nature of the silica shell, which contributes to the facile access of the reactant gas to the active sites on the core particles. The degree of reduction of the calcined catalysts studied by H2 temperature‐programmed reduction was enhanced with increased Co particle size. In the Fischer–Tropsch synthesis, a volcano‐like curve was plotted as the CO conversion and Co‐time‐yield revealed a rapid growth if the particle size increased from 5.5 to 8.7 nm and then decreased with further increased particle size to 12.7 nm, which is an effect of the combination of Co dispersion and reducibility. However, the turnover frequency remained invariant for catalysts with particle sizes larger than 8.7 nm. If we consider the product selectivity, generally, larger particles led to a longer chain length of hydrocarbons with a larger chain‐growth probability. The selectivity towards methane decreased and the corresponding heavy hydrocarbons (C19+) increased continuously with the increase of particle size. The catalyst with a particle size of 8.7 nm exhibited the highest selectivity and the maximum space‐time‐yield towards middle distillates (C5–C18) because of the modest chain‐growth probability.

Effects of macropores on reducing internal diffusion limitations in Fischer–Tropsch synthesis using a hierarchical cobalt catalyst

Internal diffusion limitations in Fischer–Tropsch catalysts strongly affects their catalytic activities and product selectivities. Large pellet catalysts demonstrate especially severe internal diffusion limitations in fixed bed reactors. In order to overcome this problem, macropores were introduced into cobalt catalysts, and the resulting effects on reaction activity and selectivity were studied. Meso–macroporous silica (S1) with mesoporous walls was prepared by a sol–gel process and was used to prepare the Co/S1 catalyst. A bimodal mesoporous silica (S2) support with an equivalent mesopore diameter to the S1 support was also prepared for comparison. The effects of internal diffusion limitations in the S1 and S2 supports with different pellet sizes on FT synthesis were investigated. The results showed that the macropores played an important role in reducing internal diffusion limitations, especially for large pellet catalysts.

The one-step oxidation of methanol to dimethoxymethane over sulfated vanadia–titania catalysts: influence of calcination temperature

Sulfated vanadia–titania catalysts were prepared by the rapid combustion method and calcined at different temperatures. The influence of calcination temperature on the physicochemical properties of the catalysts was characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H2-TPR-MS), thermogravimetry (TG) and temperature programmed desorption of ammonia (NH3-TPD) techniques. The catalytic activities were evaluated by the partial oxidation of methanol to dimethoxymethane (DMM). The results showed that vanadia and sulfate were highly dispersed as the catalysts were calcined at 723 and 773 K. The reducibility of the highly dispersed vanadia was stronger than the aggregated vanadia. And the larger number of acidic sites was related to the higher dispersion of sulfate. Moreover, the higher dispersion of vanadia contributed to higher methanol conversion, and the stronger reducibility combined with the larger number of acidic sites led to high DMM selectivity. As a result, the catalysts calcined at 723 and 773 K presented higher methanol conversion and DMM selectivity than those calcined at 673 K or above 823 K.

Effect of hierarchical meso–macroporous structures on the catalytic performance of silica supported cobalt catalysts for Fischer–Tropsch synthesis

A series of meso–macroporous silica supports with the same macroporous diameter but different mesoporous diameters were prepared by introducing phase separation into a sol–gel process and used to prepare cobalt catalysts for Fischer–Tropsch synthesis. The mesoporous diameter could be controlled in the range 6.5–35.0 nm while the macroporous diameter was kept at approximately 500 nm. The mesoporous porosity of the meso–macroporous silica supports greatly influenced the size, reducibility and dispersion of cobalt nanoparticles, and therefore resulted in different catalytic performances for Fischer–Tropsch synthesis. The meso–macroporous catalyst with an appropriate mesoporous size of 8.5 nm displayed a higher catalytic activity due to the best combination of the Co dispersion and reduction degree. The product distribution strongly depended on the mesoporous diameter due to the following two reasons: 1) the difference in the H2/CO ratio on the active sites due to the diffusional limitations of CO in the mesopores; 2) the Co crystallite size effect. In addition, large pellet catalysts (800–1700 μm) exhibited similar product distributions to small pellet catalysts (180–250 μm), which indicated that the macropores played an important role in reducing internal diffusion limitations for large pellet catalysts.

Elucidating the nature and role of copper species in catalytic carbonylation of methanol to methyl acetate over copper/titania–silica mixed oxides

In this study, a series of copper/titania–silica mixed oxide (Cu/TS) catalysts with different copper contents were prepared by a sol–gel method. The catalytic activity was evaluated by halide-free methanol carbonylation to methyl acetate (MA). The properties of the catalysts were mainly characterized by N2 adsorption–desorption, X-ray diffraction, dissociative N2O chemisorption, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia and in situ Fourier transform infrared. The results show that the crystal size and aggregation degree of Cu increase with increasing Cu content. In addition, the amount of surface Cu+ firstly increases and then decreases, and the maximum is 1.560 mmol g−1 for the 10.24 Cu/TS catalyst. Both the amounts of adsorbed CO and surface Lewis acid sites are found to be proportional to the amount of surface Cu+ species. The catalytic performance shows that the space time yield (STY) of MA is also strongly related to the amount of surface Cu+ species, and the 10.24 Cu/TS catalyst has a maximum of 1.770 mol h−1 kgcat−1. It is found that the surface Cu+ species act not only as metal sites which adsorb CO but also as Lewis acid sites which promote the adsorption of methanol (methoxy). Furthermore, the activation of CO is the major factor for the MA synthesis.

Silicon carbide supported cobalt for Fischer–Tropsch synthesis: probing into the cause of the intrinsic excellent catalytic performance

The thin SixOy layer on a SiC surface is changed to Al2O3 to form Al2O3@SiC. Co/Al2O3@SiC shows distinct different catalytic behaviour with Co/SiC, indicating that the SixOy layer on the surface of SiC plays a great role in the intrinsic excellent catalytic performance of Co/SiC.

Exposure of (001) planes and (011) planes in MFI zeolite

This communication provides an insight into the exposure of (001) and (011) planes in MFI zeolite and the objective of controlling MFI crystal shapes between the coffin and octagonal shapes is achieved by adjusting the amount of template TPAOH in MFI synthesis.