Weikun Lai

Insight into the effect of non-stoichiometric sulfur on a NiMoS hydrodesulfurization catalyst

The findings in this study provide new insight into the NiMoS model, revealing that there is a sulfur dynamic equilibrium between the NiMoS edge and the gas phase. Since the evolution of non-stoichiometric sulfur proceeds rapidly at the initial stage of hydrodesulfurization reaction, the sulfur dynamic equilibrium does not draw so much attention. The results indicate that excess sulfur on the Ni–Mo–S edge can be reduced by hydrogen to form SH groups and release H2S, which lead to a low sulfur-covered NiMoS edge and a significant increase in coordinatively unsaturated sites (CUS), resulting in an outstanding HDS activity. Apart from elucidating the effect of non-stoichiometric sulfur on the NiMoS structure, the relationship among H2S partial pressure, Sx and HDS activity has been quantitatively studied. It is expected that these results will be used to deepen the understanding of HDS reaction over promoted MoS2 catalysts, as well as to guide the research on ultra-deep HDS of fuel oils.

One pot synthesis of NiMo–Al2O3 catalysts by solvent-free solid-state method for hydrodesulfurization

A simple and solvent-free solid-state method was used to prepare NiMo–Al2O3 hydrodesulfurization (HDS) catalysts using Ni(NO3)2·6H2O, (NH4)6Mo7O24·4H2O, and AlCl3·6H2O as the solid raw materials and polyethylene glycol (PEG) as an additive. The effects of PEG addition on the precursor thermal decomposition, catalyst properties and dibenzothiophene (DBT) HDS activity were investigated. The as-prepared catalysts were characterized by nitrogen adsorption–desorption measurements, powder X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The results showed that an increase in PEG addition dramatically increases specific surface area and pore volume of the catalyst, and improves Mo sulfidability and active MoS2 dispersion by blocking the aggregation of metals, and consequently increases the number of HDS active sites. However, excess PEG leads to the decrease in specific surface area and pore volume attributed to the metal sintering caused by the strong heat release during thermal decomposition. As a result, dibenzothiophene HDS activity enhanced with increasing PEG addition and peaked at NiMoAl-15 (15% weight ratio of PEG to alumina), which exhibited a significantly higher activity as compared to the NiMo/Al2O3 catalyst prepared by wetness co-impregnation.

Synthesis of a multi-branched dandelion-like SAPO-11 by an in situ inoculating seed-induced-steam-assisted conversion method (SISAC) as a highly effective hydroisomerization support

A dandelion-like SAPO-11 with multiple branches was fabricated by seed-induced steam-assisted conversion (SISAC), which combined an in situ inoculating seed method (in this case, the initial gel was pre-crystallized at 433 K for 24 h) and steam-assisted conversion (SAC) process. The dandelion-like SAPO-11 was fully crystalline and exhibited more acidity, external surface and mesopore volume compared with conventional hydrothermally crystallized SAPO-11, confirmed by Ar adsorption–desorption, pyridine-adsorbed infrared (Py-IR), 2,6-dimethylpyridine-adsorbed infrared (2,6-DMPy-IR) and NH3 temperature-programmed desorption (NH3-TPD). After loading Pt nanoparticles by incipient wetness impregnation, the morphological modification (benefiting from smaller nanobranches and introducing more external surface active sites) leads to enhanced heptane hydroisomerization activity of the dandelion-like Pt/SAPO-11 as great as 15.4% higher in comparison with a conventional Pt/SAPO-11 catalyst.

Quantitative relationship model between support properties and dibenzothiophene hydrodesulfurization conversion over NiMo/Al2O3

A “property index” has been developed, which is a supergeometric average of the support properties. Based on the relationship between the catalytic reaction conversion and the apparent rate coefficient (k), a quantitative relationship model was established between the support properties and the conversion of dibenzothiophene (DBT) hydrodesulfurization (HDS) reaction by building a correlation between “property index” and the apparent rate coefficient. Moreover, an analysis method about the contribution rate of the property variables to catalytic activity was provided. The model parameters were estimated by fitting a series of the experimental data of the DBT HDS conversion over the catalysts with different support properties. Finally it has been shown that for the proposed model, the total average relative deviation was 1.43% and the prediction deviation was 2.21%.

Direct synthesis of 3-picoline from 2-methylglutaronitrile over supported PdZn catalyst: promoting effects of Zn

The promotion effect of transition metal on the catalytic performance of a Pd catalyst for the direct synthesis of 3-picoline from 2-methylglutaronitrile was investigated. The PdZn/SiO2–Al2O3 catalysts with appropriate Zn/Pd molar ratio exhibited a high 3-picoline yield. Kinetic and mechanism analysis revealed a kinetically relevant step of dehydrogenation in 3-methyltetrahydropyridine to 3-picoline involving the bound of 3-methyltetrahydropyridine to a Zn site and dehydrogenation by a vicinal Pd site to form 3-picoline. Such synergy between Zn and Pd sites accounted for the observed superiority of PdZn catalysts in direct synthesis of 3-picoline.

Internal defects-oriented dissolution: controllable evolution of hollow ZSM-5 nano-structures

A hollow ZSM-5 nano-structure with a shell thickness of ca. 40 nm is successfully formed in 40 min via ammonia leaching under mild conditions. It is supposed to be crucial to control the release of hydroxyl ions for intact hollow structure construction. Management of the dissolution procedure enables visualization of the internal architecture, providing fundamental knowledge on the primary subunits and potential diffusion barriers of MFI-type single nanocrystals. The dissolution process can be extended to prepare hollow zeolite encapsulated catalysts with better performance in shape-selective hydrogenation reactions.