Selective Styrene Oxidation Catalyzed by Phosphate Modified Mesoporous Titanium Silicate

Selective oxidation of organics over an efficient heterogeneous catalyst under mild liquid phase conditions is a very demanding chemical reaction. Herein, we first report the modification of the surface of mesoporous silica MCM-41 material by phosphate for the efficient incorporation of Ti(IV) in the silica framework to obtain highly ordered 2D hexagonal mesoporous material STP-1. STP-1 has been synthesized by using tetraethyl orthosilicate, triethyl phosphate, and titanium isopropoxide as Si, P, and Ti precursors, respectively, in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB) under hydrothermal conditions. The observed specific surface area and pore volume of STP-1 were 878 m2g−1 and 0.75 ccg−1, respectively. Mesoporous STP-1 has been thoroughly characterized by XRD, FT-IR, Raman spectroscopy, SEM, and TEM analyses. Titanium incorporation (Ti/Si = 0.006) was confirmed from the EDX analysis. This mesoporous STP-1 was used as a heterogeneous catalyst for the selective oxidation of styrene into benzaldehye in the presence of dilute aqueous H2O2 as an oxidizing agent. Various reaction parameters such as the reaction time, the reaction temperature, and the styrene/H2O2 molar ratio were systematically studied in this article. Under optimized reaction conditions, the selectivity of benzaldehyde could reach up to 93.8% from styrene over STP-1. Further, the importance of both titanium and phosphate in the synthesis of STP-1 for selective styrene oxidation was examined by comparing the catalytic result with only a phosphate-modified mesoporous silica material, and it suggests that both titanium and phosphate synergistically play an important role in the high selectivity of benzaldehyde in the liquid phase oxidation of styrene.

Speciation of Oxygen Functional Groups on the Carbon Support Controls the Electrocatalytic Activity of Cobalt Oxide Nanoparticles in the Oxygen Evolution Reaction

The effective use of the active phase is the main goal of the optimization of supported catalysts. However, carbon supports do not interact strongly with metal oxides, thus, oxidative treatment is often used to enhance the number of anchoring sites for deposited particles. In this study, we set out to investigate whether the oxidation pretreatment of mesoporous carbon allows the depositing of a higher loading and a more dispersed cobalt active phase. We used graphitic ordered mesoporous carbon obtained by a hard-template method as active phase support. To obtain different surface concentrations and speciation of oxygen functional groups, we used a low-temperature oxygen plasma. The main methods used to characterize the studied materials were X-ray photoelectron spectroscopy, transmission electron microscopy, and electrocatalytic tests in the oxygen evolution reaction. We have found that the oxidative pretreatment of mesoporous carbon influences the speciation of the deposited cobalt oxide phase. Moreover, the activity of the electrocatalysts in oxygen evolution is positively correlated with the relative content of the COO-type groups and negatively correlated with the C═O-type groups on the carbon support. Furthermore, the high relative content of COO-type groups on the carbon support is correlated with the presence of well-dispersed Co₃O₄ nanoparticles. The results obtained indicate that to achieve a better dispersed and thus more catalytically active material, it is more important to control the speciation of the oxygen functional groups rather than to maximize their total concentration.

Chromium-Based Complexes Bearing N-Substituted Diphosphinoamine Ligands for Ethylene Oligomerization

A range of novel N-substituted diphosphinoamine (PNP) ligands Ph₂PN(R)PPh₂ [R = F₂CHCH₂ (1); R = Me₂CHCH₂ (2); R = Me₂CHCH₂CH₂ (3)] have been synthesized via one-step salt elimination reaction. The ligand-coordinated chromium carbonyls [Ph₂PN(R)PPh₂]Cr(CO)₄ (4-6) were further synthesized, and X-ray crystallography analysis of complex 6 revealed the κ²-P,P bidentate binding mode of Cr center and the molecular structure of PNP ligand 3. Then the catalytic ethylene oligomerization behaviors of PNP ligands 1-3 bridging chromium chloride complexes {[Ph₂PN(R)PPh₂]CrCl₂(μ-Cl)}₂ (7-9) were further discussed in depth. Experimental results showed that complex 7 with the strong electron-withdrawing F₂CHCH₂ group can promote the nonselective ethylene oligomerization, while both complex 8 and complex 9 with the electron-donating Me₂CHCH₂ and Me₂CHCH₂CH₂ groups can significantly enhance the selective ethylene tri/tetramerization. The good catalytic activity of 198.3 kg/(g Cr·h), the selectivity toward 1-hexene and 1-octene of 76.4%, and the low PE content of 0.2% were simultaneously achieved with the Al/Cr molar ratio of 600 using the complex 8/MMAO system at 45 °C and 45 bar. These excellent results were mainly attributed to the fact that the β-branching of bridging ligand 2 increased the steric bulk of the N-moiety for complex 8.