J. Phys. Chem. C 2008, 112, 7731–7734
7731
Cu(I)-Catalyzed Epoxidation of Propylene by Molecular Oxygen Wenming Zhu, Qinghong Zhang, and Ye Wang* State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, P. R. China ReceiVed: January 31, 2008; ReVised Manuscript ReceiVed: March 6, 2008
We found that a simple K+-modified CuOx-SiO2 catalyst prepared by a sol-gel method was efficient for the epoxidation of propylene by oxygen. The catalyst did not require prereduction, and propylene oxide (PO) could be formed with a selectivity of 78% under oxygen-rich conditions. We demonstrated that Cu(I) formed during the reaction over the K+-modified CuOx-SiO2 catalyst was the active site for propylene epoxidation by oxygen. Cu(I) was proposed to account for the activation of oxygen, generating an active oxygen species for propylene epoxidation, while K+enhanced the PO selectivity probably by neutralizing the Lewis acid sites on catalyst surface. The enhanced dispersion of copper species due to the interaction with K+ may also contribute to the increase in PO selectivity. Propylene oxide (PO) is one of the most important synthetic intermediates in the chemical industry. Currently, PO is produced mainly through chlorohydrin and organic hydroperoxide processes. However, the chlorohydrin process faces serious environmental problems and the organic hydroperoxide process produces large amounts of coproducts.1 The catalytic epoxidation of propylene by a “green” oxidant has attracted much attention.1–3 Using H2O2 or H2-O2 gas mixture, PO selectivity of >90% can be obtained over a proper catalyst such as TS-1 or Au-Ti-based catalyst.4,5 However, the high cost of H2O2 or the low efficiency of H2 limits the commercialization of these processes.1 The catalytic epoxidation of propylene by O2 is the most desirable route for PO production. However, there has been less success in developing effective catalysts for propylene epoxidation by O2 although ethylene epoxidation by O2 has been commercialized for several decades using modified Ag catalysts.1–3 Many research efforts have also been paid to Ag-based catalysts in propylene epoxidation by O2, but PO selectivity can hardly exceed 50% even at very low propylene conversions.3,6 The main difficulty for propylene epoxidation is believed to arise from the high reactivity of the allylic C-H bonds in propylene. Through surface chemistry studies, Lambert et al.7 have suggested that Cu is more selective than Ag for the epoxidation of larger alkenes with allylic C-H bonds by preadsorbed oxygen atoms. The lower basicity of the oxygen atoms on metallic Cu may favor its epoxidation selectivity.8 On the other hand, Monnier and Hartley9 have pointed out that Cu(0) is difficult to function as a true epoxidation catalyst because it may be readily oxidized under reaction (oxidative) atmosphere into Cu2O or CuO, which is believed to be inactive or nonselective. Actually, only a few Cu-based catalysts including NaCl-modified VCe1-xCux oxide, NaCl-modified Cu/ SiO2 and Cu/SiO2 have been reported for propylene epoxidation, and Cu(0) has been proposed as the active phase for all of these catalysts.10 PO selectivities of 40-50% could be obtained over these catalysts only at very low propylene conversions (1% dramatically decreased PO selectivity to
