Shusuke Okada

Active Site Design in a Core–Shell Nanostructured Catalyst for a One‐Pot Oxidation Reaction

Hydrogen peroxide (H2O2) is not only an environmentally friendly oxidizing agent, but also a highly selective oxidant used in the manufacturing of numerous organic and inorganic compounds. The commercial production of H2O2 by the sequential hydrogenation and oxidation of an alkyl anthraquinone requires a great deal of toxic solvent and a large energy input due to the multistep reactions. The direct synthesis of H2O2 from H2 and O2 over supported Pd-based catalysts has attracted significant attention recently, because it would minimize adverse environmental effects and result in lower cost. The key problem is to stabilize the resulting H2O2, because H2O2 simultaneously undergoes hydrogenation and/or decomposition to water by the same catalysts employed for its formation. This undesirable feature causes a slow H2O2 formation rate and prevents the accumulation of high H2O2 concentration. The addition of an acid or halide to the reaction medium is useful to achieve high selectivity; however, the additional operation for removal of the acid or halide becomes an even more troublesome issue. To overcome this problem, the utilization of in situ generated H2O2 from H2 and O2 to subsequently oxidize organic reactants in the same reaction vessel (one-pot oxidation reaction) could be an alternative strategy. It is possible to use the unstable H2O2 immediately without isolation/purification steps, which would contribute to saving energy and time, as well as avoid the risk of transportation of the concentrated H2O2. We predict that the key to construct a one-pot oxidation system is the efficient dispersion of H2O2 generated on Pd nanoparticle (NP) sites to the neighboring catalytically active oxidation sites before any undesirable conversion to water can occur. Therefore, the primary requirement is the precise architecture of the two types of active sites. There have been recent reports on one-pot oxidation using in situ generated H2O2. The most convenient method is a physical mixture of both components. A combination of Ti-containing zeolite (TS-1) and supported Pd NPs has also been developed; however, all such catalyst systems still suffer from low utilization efficiency of diluted H2O2, because the random location of both types of active sites resulted in undesirable H2O2 decomposition. Considering this, we present a new type of core–shell structured catalyst to enable the one-pot oxidation of sulfide to sulfoxide with high efficiency and selectivity, in which a uniform SiO2 core supporting Pd NPs was covered with a Ti-containing mesoporous silica shell (Pd/SiO2@TiMSS). Although the inner Pd NPs are located at the boundary of the core–shell structure, reactants can penetrate to the Pd NPs through the mesoporous shell. The Ti-oxide moieties located within the mesoporous region are catalytically active toward sulfide oxidation with H2O2 and are considered to be highly dispersed at the atomic level. It is expected that the H2O2 generated on the inner Pd NP sites could selectively interact with the Ti-oxide moieties within the mesopores before dispersion to the solvent can occur, which would ultimately enhance the oxidation activity. The procedure for the synthesis of Pd/SiO2@TiMSS is schematically illustrated in Figure 1. Colloidal SiO2 NPs (average diameter: 280 nm: see the Supporting Information, Figure S1) were firstly prepared by the Stçber method. The Pd NPs were then successfully deposited on the surface of SiO2 spherical NPs by reduction of a PdCl2 precursor using Sn anchored to the hydroxyl groups of SiO2 as reducing agent in the presence of sodium formate. The Pd NPs supported on SiO2 were further coated with Ti-containing mesoporous silica shell using cetyltrimethyl ammonium bromide (CTAB) as structure directing agent (SDA), tetraethoxysilane (TEOS) as a silica source, and tetrapropyl orthotitanate (TPOT) as a Ti source. The final core–shell structure, Pd/SiO2@TiMSS (Pd: 1.0 wt%, Ti: 0.6 wt % determined by ICP analysis), was obtained after calcination at 823 K in air to remove the SDA (Figure 2). To elucidate the effect of the Pd NPs location on enhancement of one-pot oxidation using in situ generated H2O2, two reference samples, SiO2@Pd(R)/TiMSS and SiO2@Pd(S)/TiMSS, were synthesized. In the case of the SiO2@Pd(R)/TiMSS, in which (R) indicates random, the Pd NPs were deposited after calcination, which allowed the [a] S. Okada, Dr. K. Mori, Dr. T. Kamegawa, Prof. Dr. H. Yamashita Graduate School of Engineering Osaka University, 1-2 Yamadaoka, Suita Osaka 565-0871 (Japan) Fax: (+81) 6-6879-7457 E-mail : yamashita@mat.eng.osaka-u.ac.jp [b] Prof. Dr. M. Che Institut Universitaire de France and Laboratoire de R activit de Surface Universit Pierre et Marie Curie-Paris 6 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101241.

Hydrophobic Modification of Pd/SiO2@Single‐Site Mesoporous Silicas by Triethoxyfluorosilane: Enhanced Catalytic Activity and Selectivity for One‐Pot Oxidation

To enhance the catalytic activity in a selective one-pot oxidation using in-situ generated H(2)O(2), a hydrophobically modified core-shell catalyst was synthesized by means of a simple silylation reaction using the fluorine-containing silylation agent triethoxyfluorosilane (TEFS, SiF(OEt)(3)). The catalyst consisted of a Pd-supported silica nanosphere and a mesoporous silica shell containing isolated Ti(IV) and F ions bonded with silicon (SiF bond). Structural analyses using XRD and N(2) adsorption-desorption suggested that the mesoporous structure and large surface area of the mesoporous shells were retained even after the modification. During the one-pot oxidation of sulfide, catalytic activity was enhanced significantly by increasing the amount of fluorine in the shell. A hydrophobic surface enhanced adsorption of the hydrophobic reactant into the mesopore, while the less hydrophobic oxygenated products efficiently diffused into the outside of the shell, which improved the catalytic activity and selectivity. In addition, the present methodology can be used to enhance the catalytic activity and selectivity in the one-pot oxidation of cyclohexane by using an Fe-based core-shell catalytic system.

Synthesis of Pd nanoparticles on heteropolyacid-supported silica by a photo-assisted deposition method: an active catalyst for the direct synthesis of hydrogen peroxide

A simple and unique methodology to synthesize active Pd nanoparticles using Cs2.5H0.5PW12O40 heteropolyacid-supported silica (CsHPA/SiO2) under UV-light irradiation has been developed. By the photo-assisted deposition (PAD) method, a Pd2+ precursor in aqueous solution can be deposited and easily reduced by the [PW12O40]4− species generated by photolysis of [PW12O40]3− species in the presence of iPrOH as a sacrificial reagent, to afford nanosized Pd0 metal particles (Pd/CsHPA/SiO2). No direct Pd metal deposition was observed on either pure SiO2 without the CsHPA phase under UV-light irradiation or on CsHPA/SiO2 without UV-light irradiation. The catalytic activity for the direct synthesis of hydrogen peroxide (H2O2) using H2 and O2 gases under atmospheric pressure was strongly dependent on the preparation conditions, variation of CsHPA content, and the type of catalyst support. In addition, the Pd/CsHPA/SiO2 catalyst prepared by the PAD method exhibited significantly better activity than the conventionally prepared impregnation catalyst. The Pd surface area from CO adsorption measurements, and also the suppression of undesired side reactions were determined as the crucial factors to achieve efficient catalytic performance.