Search

Allylation using allylic alcohols is environmentally friendly because water is the only by-product. Allylic alcohols are one of the most unreactive allylating agents for homogeneous Pd-catalyzed allylation; however, they can undergo electrophilic activation with acidic active species. Heterogeneous catalysts are known for their unique catalytic performance due to the concerted effect of the immobilized metal center and support material surface. Our group has developed a mesoporous-silica-supported Pd complex catalyst that promotes the Tsuji–Trost allylation of dicarbonyl compounds with allylic alcohols through the concerted effect of Pd and surficial silanol groups. In this work, to enhance the catalytic activity of the supported Pd complex for allylation with allylic alcohols, the acidity of the support was increased by doping the silicate backbone with aluminum. Several spectroscopic techniques, such as Pd K-edge X-ray absorption fine structure (XAFS), solid-state NMR, and pyridine adsorption FT-IR measurements, were applied to confirm the structure of the catalyst. The Pd complex catalyst immobilized on an Al-doped support showed enhanced catalytic activity in allylation owing to the activation of the allylic alcohol by both the Al Lewis acid sites and silanol groups. The site-selective immobilization of the Pd complex through a silane-coupling reaction on Brønsted acid sites (Si–O(H+)–Al) enhances the concerted effect between the Pd complex and Lewis acidic Al sites on the MS surface.

The direct coupling of methane from natural gas and shale gas is an essential chemical process to produce C2 products which are substrates for polymers, resins, and various functional organic compounds. An appropriate catalyst is necessary to achieve high selectivity to convert methane to the target products. In this study, we investigated the ability of in situ-generated bimetallic Ru–Sn particles to catalyze the non-oxidative coupling of methane to C2 species at 500 °C and observed C2 products’ selectivity of >99% in hydrocarbons. The addition of Sn to Ru resulted in the suppression of coke formation while maintaining the conversion rate of methane to hydrocarbons. X-ray absorption fine structure analysis revealed the mechanism of the in situ formation of Ru–Sn bimetallic particles on the surface of the support. Ru(IV) is initially reduced to Ru(0) particles under methane flow at 400–500 °C. Then, Sn(IV) species around the Ru particles are reduced and dispersed on the outer surface of the Ru particles at 500 °C, although the Sn species were not reduced without Ru at 500 °C. These results point out that Sn(IV) is reduced by active hydrogen atoms generated by the reaction between Ru particles and methane molecules. Hydrogen spills over the surface and reduces the Sn(IV) species. This in situ-formed Ru–Sn bimetallic species showed good performance for non-oxidative coupling of methane.

Allylation using allylic alcohols is environmentally friendly because water is the only by-product. Allylic alcohols are one of the most unreactive allylating agents for homogeneous Pd-catalyzed allylation; however, they can undergo electrophilic activation with acidic active species. Heterogeneous catalysts are known for their unique catalytic performance due to the concerted effect of the immobilized metal center and support material surface. Our group has developed a mesoporous-silica-supported Pd complex catalyst that promotes the Tsuji–Trost allylation of dicarbonyl compounds with allylic alcohols through the concerted effect of Pd and surficial silanol groups. In this work, to enhance the catalytic activity of the supported Pd complex for allylation with allylic alcohols, the acidity of the support was increased by doping the silicate backbone with aluminum. Several spectroscopic techniques, such as Pd K-edge X-ray absorption fine structure (XAFS), solid-state NMR, and pyridine adsorption FT-IR measurements, were applied to confirm the structure of the catalyst. The Pd complex catalyst immobilized on an Al-doped support showed enhanced catalytic activity in allylation owing to the activation of the allylic alcohol by both the Al Lewis acid sites and silanol groups. The site-selective immobilization of the Pd complex through a silane-coupling reaction on Brønsted acid sites (Si–O(H+)–Al) enhances the concerted effect between the Pd complex and Lewis acidic Al sites on the MS surface.

In this study, a novel Rh–iodide complex was synthesized through a surface reaction between an immobilized Rh cyclooctadiene complex and alkylammonium iodide (N+I−) on SiO2. In the presence of ammonium cations, the SiO2-supported Rh–iodide complex could be effectively used for the one-pot synthesis of various silylcarbonate derivatives starting from epoxy olefins, hydrosilanes, and CO2. The maximum turnover numbers (TONs) for the hydrosilylation reaction and the CO2 cycloaddition were 7600 (Rh) and 130 (N+I−), respectively. The catalyst exhibited much higher performance for hydrosilylation than solely the Rh complex on SiO2. The mechanism of the Rh-catalyzed hydrosilylation reaction and the local structure of Rh, which is affected by the co-immobilized N+I−, were investigated by using Rh and I K-edge XAFS and XPS. Analysis of the XAFS profiles indicated the presence of a Rh−I bond. The Rh unit was in its electron-rich state. Curve-fitting analysis of the Rh K-edge EXAFS profiles suggests dissociation of the cycloocta-1,5-diene (COD) ligand from the Rh center. Results from spectroscopic and kinetic analyses revealed that the high activity of the catalyst (during hydrosilylation) could be attributed to a decrease in steric hindrance and the electron-rich state of the Rh. The decrease in the steric hindrance could be attributed to the absence of COD, and the electron-rich state promoted the oxidative addition of Si−H. To the best of our knowledge, this is the first example of a one-pot silylcarbonate synthesis as well as a determination of a novel surface Rh–iodide complex and its catalysis.