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Peroxo-heteropoly compound PO4[W(O)(O2)2] was synthesized on calcium-deficient hydroxyapatite using a reaction of surface [HPO4]2− groups on hydroxyapatite with a Na2[W2O3(O2)4] aqueous solution. The vibration of [HPO4]2− at 875 cm−1 became very weak, and the vibration of the peroxo-oxygen bond [O–O]2− at 845 cm−1 appeared in the FT-IR spectrum of the solid product, indicating that PO4[W(O)(O2)2] was formed on the surface of hydroxyapatite. The formed solid sample was further reacted with PdCl2(PhCN)2 in an acetone solution to fix PdCl2 between the O sites on the hydroxyapatite. Elemental analyses proved that the resultant solid contained 1.2 wt.% Pd, implying that PdCl2 molecules were immobilized on the surface of hydroxyapatite. The hydroxyapatite-based hybrid compound containing Pd and PO4[W(O)(O2)2] was used as a heterogeneous catalyst in a methanol solvent for propylene epoxidation by molecular oxygen in an autoclave batch reaction system. A propylene conversion of 53.4% and a selectivity for propylene oxide of 88.7% were obtained over the solid catalyst after reaction at 363 K for 8 h. The novel catalyst could be reused by a simple centrifugal separation, and the yield of propylene oxide did not decrease after the reaction for five runs. By prolonging the reaction time to 13 h, the highest yield of propylene oxide at 363 K over the solid catalyst was obtained as 53.8%, which was almost the same as that of the homogeneous catalyst containing PdCl2(PhCN)2 and [(C6H13)4N]2{HPO4[W(O)(O2)2]2} for the propylene epoxidation. Methanol was used as a solvent as well as a reducing agent in the propylene epoxidation by molecular oxygen. Small particles of Pd metal were formed on the surface of the hybrid solid catalyst during the reaction, and acted as active species to achieve the catalytic turnover of PO4[W(O)(O2)2] in the propylene epoxidation by molecular oxygen in methanol. [ABSTRACT FROM AUTHOR]

Abstract: Ti- and Al-containing hexagonal mesoporous silicas (Ti-Al-HMS) were synthesized through a sol–gel reaction using dodecylamine as a surfactant. All Ti4+ ions could enter into the HMS framework until Ti/Si=4mol% and all Al3+ ions could enter into the HMS framework until Al/Si=1mol% in Ti-Al-HMS after calcination at 923K for 4h. The Al3+ ions at the extra-framework of HMS are much stronger acidic sites than the Al3+ ions in the intra-framework of HMS. In a fixed-bed catalytic system, Ti-Al-HMS (Ti/Al/Si=4/1/100 (molar ratio)) showed 47.8% of propylene conversion and 30.6% of selectivity for propylene oxide (PO) for the propylene epoxidation by molecular oxygen at 523K. The simultaneous existence of Ti and Al in the framework of HMS is very important for improving the PO yield. An epoxide-like intermediate formed from Ti site, Al site and adsorbed O2 was proposed for explaining the synergy effect of Ti and Al in Ti-Al-HMS for the propylene epoxidation by molecular oxygen. The Ti4+ ions at the extra-framework had little effect on improving the PO yield, and the Al3+ ions at the extra-framework greatly decreased the selectivity for PO, because their strong acidity promoted the oligomerization-cracking of propylene. The sol–gel method is more effective than the impregnation method for preparing a catalyst containing Ti and Al for the propylene epoxidation by molecular oxygen. Ti-Al-HMS deactivated rapidly for the propylene epoxidation at 523K due to the carbonaceous deposits on the catalyst surface. The deactivated catalyst could be regenerated by calcinating in air at 923K for removing the carbonaceous deposits. Adding a little amount of hydrogen in the feed gas obviously decreased the speed of catalyst deactivation but could not completely suppress the catalyst deactivation. The amounts of the carbonaceous deposits were 2.6wt% without H2 and 0.9wt% with H2 in the feed gases over Ti-Al-HMS after 3h on stream at 523K. The KNO3 additive could improve the selectivity for PO and the Ca(NO3)2 additive could improve the propylene conversion over Ti-Al-HMS for the propylene epoxidation by molecular oxygen. [Copyright &y& Elsevier]

Abstract: The catalytic system containing Pd(OAc)2 and peroxo-heteropoly compound [(C6H13)4N]3{PO4[W(O)(O2)2]4} (denoted by THA-PW4) in methanol showed 81.6% selectivity for propylene oxide at a propylene conversion of 42.7% using molecular oxygen as an oxidant in an autoclave reactor at 373K for 6h, whereas, Pd(OAc)2 or THA-PW4 alone showed low conversions. The catalytic system containing Pd(OAc)2 and THA-PW4 in methanol is reusable for propylene oxidation by means of vacuum distillation after reaction. XRD patterns and Pd K-edge EXAFS indicate that Pd0 species formed by the reduction of Pd(OAc)2 with methanol acts as an active species in propylene epoxidation with molecular oxygen. FT-IR spectra of Pd¿THA-PW4 before and after reaction proved that the peroxy oxygen bonds of THA-PW4 could be regenerated in methanol medium by molecular oxygen in the presence of Pd, but could not be regenerated in acetonitrile medium. It is likely that methanol molecule reacts with oxygen molecule over Pd0 species to form a peroxy intermediate HOCH2OOH, which regenerates the peroxy oxygen bonds of THA-PW4 and achieves catalytic turnover for propylene epoxidation. Because the peroxy intermediate HOCH2OOH is not stable and finally decompose to COx and H2O, a part of methanol is co-oxidized. Moreover, hydrogen peroxide also probably formed in situ in the catalytic system during the reaction and plays an important role to regenerate the peroxy oxygen bonds of THA-PW4. [Copyright &y& Elsevier]

Ti- and Al-containing MCM-41 (Al-c-MCM-41 and Al–Ti-c-MCM-41) have been synthesized and tested for direct oxidation of propylene with molecular oxygen. The reaction occurred over Al- and Ti-containing MCM-41 like materials. Al–Ti-c-MCM-41 was found to be more effective than Al-c-MCM-41, while no product was formed for MCM-41 alone. The catalyst performances of Al–Ti-c-MCM-41 at 523 K were dependent on the Si/Al2 and Si/Ti ratios and the highest PO yield was 3% when Si/Al2 of 150 and Si/Ti ratios of 100 were used. A postulated scheme containing Al and Ti species is proposed. [Copyright &y& Elsevier]

The catalyst system consisting of Pd, Ti-modified MCM-22, and methanol was found to catalyze the epoxidation of propylene with molecular oxygen without hydrogen and PO was formed with a PO yield above 19.5% and 458 turnovers based on palladium. The catalyst performance of Pd is affected by the properties of Ti-modified MCM-22 such as BET surface area and the amount of NH3 adsorption. The catalyst material consisting of palladium and Ti-modified MCM-22 can be recovered and used 3 times without reduction in the PO yield. It is assumed that not only palladium but also peroxy intermediates derived from methanol/oxygen play an important role in the oxidation of propylene. [Copyright &y& Elsevier]