Yongying Chen

Effect of surface proton exchange on hydrodesulfurization performance of MCM-41-supported catalysts

Abstract Ni-Mo, Co-Mo, Ni-W and Pt were supported over siliceous MCM-41 (Si-MCM-41) or the proton-exchanged Si-MCM-41 (H + -MCM-41) in order to investigate the effect of support surface modification on the performance of the catalysts. The prepared catalysts were evaluated by the hydrodesulfurization (HDS) of dibenzothiophene (DBT). It is indicated that the supported Ni-Mo sulfides exhibited the highest HDS activity regardless of the support, and that Ni-W sulfides showed the highest hydrogenation activity. The H + -MCM-41-supported Ni-Mo, Ni-W or Pt catalyst performed much better in HDS of DBT than the Si-MCM-41-supported counterpart. From the relative selectivity of cyclohexylbenzene (CHB) to biphenyl (BP) ( S CHB / S BP ), it could be concluded that the higher HDS performance of H + -MCM-41-supported catalysts may be attributed to the enhanced hydrogenation activity. Nevertheless, a little difference in HDS activity was observed for Co-Mo sulfides supported on Si-MCM-41 or H + -MCM-41. Since the cleavage of sulfur atoms mainly takes the route of hydrogenolysis in HDS of DBT catalyzed by Co-Mo sulfides, the improvement in hydrogenation activity was not significant for Co-Mo sulfides when Si-MCM-41 was ion-exchanged with HNO 3 . TPR study showed that the profiles of Ni-Mo/Si-MCM-41 and Ni-W/Si-MCM-41 changed markedly after the proton exchange of Si-MCM-41 while the profile of Co-Mo/Si-MCM-41 changed a little. It may be concluded that the removal of sulfur from DBT and hydrogenation take place on separate active sites, and that the spillover of the dissociated hydrogen species over the surface is essential to the hydrogenation pathway. Hydroxyl groups on H + -MCM-41 favors the spillover of the hydrogen species, enhancing the hydrogenation activity of its supported catalysts. On the other hand, Na + cations on Si-MCM-41 may “trap” the spillover hydrogen species, lowering the hydrogenation activity of its supported catalysts.

Facile Preparation of Ni2P with a Sulfur-Containing Surface Layer by Low-Temperature Reduction of Ni2P2S6

Preparation of Ni2P by temperature-programmed reduction (TPR) of a phosphate precursor is challenging because the P-O bond is strong. An alternative approach to synthesizing Ni2P, by reduction of nickel hexathiodiphosphate (Ni2P2S6), is presented. Conversion of Ni2P2S6 into Ni2P occurs at 200-220 °C, a temperature much lower than that required by the conventional TPR method (typically 500 °C). A sulfur-containing layer with a thickness of about 4.7 nm, composed of tiny crystallites, was observed at the surface of the obtained Ni2 P catalyst (Ni2P-S). This is a direct observation of the sulfur-containing layer of Ni2P, or the so-called nickel phosphosulfide phase. Both the hydrodesulfurization activity and the selective hydrogenation performance of Ni2P-S were superior to that of the catalyst prepared by the TPR method, suggesting a positive role of sulfur on the surface of Ni2P-S. These features render Ni2P-S a legitimate alternative non-precious metal catalyst for hydrogenation reactions.

Hydrodesulfurization of Dibenzothiophene Over Ni-Mo Sulfides Supported by Proton-Exchanged Siliceous MCM-41

Strong acid sites on the surface of mesoporous MCM-41 were generated by ion-exchanging siliceous MCM-41 with dilute HNO3 solution (0.5 M). The XRF determination indicates that most of the sodium cations contained in MCM-41 can be removed by the proton exchange, and dealuminization was observed during the proton exchange. The acidity of the mesoporous materials was characterized by means of NH3-TPD and the Hammett indicators. It is revealed that new strong acid sites (-5.6 > H0 > -8.2) were generated after the first 2 h of ion exchange and that the following ion exchanges had little effect on the acidic properties. XRD patterns of the mesoporous materials indicate that the structure of siliceous MCM-41 was improved by HNO3 ion exchange. When Ni-Mo sulfides were supported on the prepared solid acid (H+-MCM-41), high performance in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was observed. However, the HDS activity was decreased while the selectivity of biphenyl (BP) was increased, when H+-Si-MCM-41 was ion exchanged with Na2CO3 aqueous solution. TPR profiles of the supported Ni-Mo oxides reveal that the acidic properties of the supports greatly influence the hydrogenation activities of the bimetallic oxides. The high performance of H+-MCM-41-supported Ni-Mo catalysts may be attributed to the enhanced hydrogenation activity. The introduction of Na cation into the support led to the decrease of the HDS activity due to the poor hydrogenation ability of the supported bimetallic oxides. The HDS activity is well correlated with the low H2 consumption temperature in the TPR profiles.

The Effect of CeO2 on the Hydrodenitrogenation Performance of Bulk Ni2P

Bulk Ni2P and CeO2-containing bulk Ni2P (Ce–Ni2P(x), where x represents the Ce/Ni atomic ratio) were prepared by a co-precipitation method followed by an in situ H2 temperature-programmed reduction procedure. The catalysts were characterized by XRD, CO chemisorption, TEM, N2 adsorption–desorption, XPS and X-ray absorption spectroscopy (XAS). Their hydrodenitrogenation performances were studied using quinoline (Q) and decahydroquinoline as the model compounds. Both the hydrogenation and C–N bond cleavage activities of Ni2P were improved by the introduction of CeO2. CeO2 mainly accelerated the denitrogenation of Q to propylcyclohexane rather than to propylbenzene. XRD and XPS measurements revealed that the Ce species in Ce–Ni2P(x) were mainly in the oxide form and both Ce4+ and Ce3+ species coexisted on the surface of the catalysts. Addition of CeO2 significantly decreased the particle size of Ni2P, resulting in increased specific surface areas and CO uptakes, possibly due to the strong interaction between the Ce species and Ni2P. At a Ce/Ni atomic ratio higher than 0.25, segregation of CeO2 took place. XAS results of the passivated catalysts showed that CeO2 not only affected the oxidability of Ni2P but also led to the formation of metallic Ni. The promoting effect of CeO2 was discussed by considering the electronic interactions between Ce species and Ni2P as well as the presence of the amorphous Ni and low valence Ce3+ species.

Photocatalytic degradation of methylene blue by MoO3 modified TiO2 under visible light

Abstract MoO 3 /P25 catalysts were prepared by an impregnation method. The catalysts were characterized by X-ray diffraction, ultraviolet-visible spectrophotometry, Fourier transform infrared spectroscopy, and laser Raman spectroscopy, and their photocatalytic activty was evaluated by the degradation of methylene blue dye under visible light. The monolayer dispersion threshold of MoO 3 on P25 was around 0.1 g/g. The strong interaction between the monolayer-dispersed tetrahedral-coordinated molybdenum oxide species and P25 led to a decrease in the band gap of P25, thus increasing the visible light absorption of the catalyst. Crystalline MoO 3 was formed on catalysts with a MoO 3 /P25 mass ratio above 0.1. In these cases, the visible light absorption of the catalysts decreased with increasing MoO 3 content. The band gap of the catalyst was not the only factor affecting its photocatalytic activity for the degradation of methylene blue under visible light. MoO 3 /P25 with the MoO 3 to P25 mass ratio of 0.25, which possessed not only suitable band gap but also a certain amount of crystalline MoO 3 , showed the best catalytic performance.

Creation of Oxygen Vacancies in MoO3/SiO2 by Thermal Decomposition of Pre-Impregnated Citric Acid Under N2 and Their Positive Role in Oxidative Desulfurization of Dibenzothiophene

The XPS, XANES, and UV–Vis results revealed that oxygen vacancies were formed in the surface of MoO3/SiO2 by impregnation of the catalyst with citric acid followed by a temperature-programmed heating under N2 atmosphere. The presence of the induction period during the oxidative desulfurization (ODS) of dibenzothiophene over these Mo-based catalysts using cumene hydroperoxide as the oxidant suggests that the reaction would follow a free radical mechanism. Both the higher ODS activity and the faster deactivation of the modified catalysts give evidences that the oxygen vacancies should possess a high ODS activity.Graphical Abstract

Hydrodesulfurization of dibenzothiophene over Mo-based catalysts supported by siliceous MCM-41

Deep hydrodesulfurization (HDS) catalysts were prepared by depositing Co−Mo or Ni−Mo species over siliceous MCM-41. The extremely high surface area of MCM-41 favors the dispersion of the active species, resulting in a very high reactivity in desulfurizing dibenzothiophenes. A maximum HDS activity was observed at Co/Mo or Ni/Mo molar ratio of 0.75 for the supported catalysts, higher than that of the conventional γ-Al 2 O 3 supported catalysts. A 35 S isotope tracer technique was used to investigate the HDS reaction mechanism. It was revealed that sulfur atoms retained on the surface could be removed only by the introduction of a sulfur-containing compound, indicating that sulfur atom exchange between sulfur-containing compounds and the active sites is involved in the HDS reaction. Accordingly, a reaction mechanism for HDS is proposed.

Hydrodenitrogenation of Quinoline and Decahydroquinoline Over a Surface Nickel Phosphosulfide Phase

The hydrodenitrogenation (HDN) of quinolone (Q) and decahydroquinoline (DHQ) over a Ni2P catalyst prepared by the reduction of a conventional phosphate precursor and a surface nickel phosphosulfide phase (Ni2P–S) obtained by the reduction of Ni2P2S6 were studied. A reaction network of the HDN of Q over Ni2P was proposed. Both the hydrogenation and C–N bond cleavage activities of Ni2P were enhanced after the introduction of sulfur. The surface sulfur species of Ni2P–S changed significantly after the HDN of DHQ. The thiolate or thiol species were detected as the major sulfur-containing species in the surface of the spent Ni2P–S catalyst.Graphical Abstract