46919-AC5
Mechanistic Understanding of Au and Au Alloy/TS-1 Propylene Epoxidation Catalysts
Propylene oxide (PO) is a high value-added industral chemical. However, the major PO production routes, the chlorohydrin and hydroperoxide processes, involve mutiple stages and require additional separation and/or purification units that increase the cost. Therefore, a single-step, direct catalytic partial oxidation of propylene to PO using molecular oxygen has long been desired. The discovery by Haruta and coworkers that nanoscale gold particles on titania supports provide a highly selective (~ 99%) route to vapor phase propylene oxide production using a mixture of propylene, oxygen, and hydrogen under ambient pressure has opened a new page in the PO production. The hypothesis that the role of hydrogen is to form HOOH, which is the electrophyllic oxidant has recently been supported by direct observation of HOOH formation on Au/TiO2 systems. . Early work in our laboratory demonstrates that TS-1 (titanium silicate -1) as the support for nano-gold particles enhances the propylene conversion to around 8% and provides stability for at least 40 hours, while keeping the selectivity as high as 80%. However, this improvement in PO catalytic performance is still short of the goal of PO selectivity of 90% at a propylene conversion higher than 10% and hydrogen greater than 50%. Our strategy to enhance the PO conversion/selectivity and H2 efficiency is based on two different approaches: introducing other supports to alleviate possible mass transfer issue occurred inside TS-1 and exploring the possibility of enhancing activity by alloying Au with other metals. Details are discussed below Au-Pd Catalysts - Recently Hutchings et al. have shown that Au-Pd catalysts prepared by coimpregnation methods possessed significantly higher activity in liquid phase H2O2 synthesis than either pure gold catalyst or pure palladium catalysts. Larger Au-Pd alloy particles were associated with higher H2O2 activity. DFT calculations, however, suggest that Au-Pd alloy trimers are energetically stable with or without oxygen adsorption and represent a potential catalyst for H2O2 formation and increased PO conversion. To follow this hypothesis we will prepare Au-Pd/TS-1 catalysts with Au/Pd ratio 1/1 will be prepared by both coimpregnation and deposition/precipitation methods to evaluate which method gives the best performance. Then, the effects of Au/Pd ratio, metal loading, and the preparation conditions will be systematically investigated. MTS-9 and Hollow TS-1 Supports - To alleviate the possible mass transfer issue caused by small pore (~5 Å) in microporous materials such as TS-1, different kinds of mesoporous titanium silicates such as Ti-MCM-41,Ti-MCM-48 have been reported in the literature as supports for gold catalysts. However, the PO production rate of neither Au/Ti-MCM-41 nor Au/Ti-MCM-48 exceeded that of Au/TS-1. The lower oxidation ability and/or PO production rate might be attributed to the different titanium coordination environments in both Ti-MCM-41 and Ti-MCM-48 since they consist of amorphous material versus TS-1 which has a crystalline structure with Ti in tetrahedral coordination. Recently Xiao et al. successfully synthesized ordered mesoporous (MTS-9) titanosilicate, which exhibits similar phenol hydroxylation activity to that of TS-1. Therefore, we are attempting to synthesize MTS-9 and expect a higher PO production rate with this support. On the other hand, Wang et al. successfully synthesized a so-called Hollow TS-1 via a dissolution–recrystallization process. The hollow TS-1 was reported to possess the same chemical composition and approximately the same size as the original TS-1 crystals but with large intra-crystalline voids inside the TS-1 particles. These materials will also be examined as another opportunity to alleviate the possible mass transfer issues in the epoxidation reaction. Current Experiments – We have constructed a new PO reaction system consisting of a flow control panel facilitating five controllable flow channels via mass flow controllers, a reactor system in which a recycle reactor and plug flow reactor are available, and a GC system in which two different columns (Chromosorb 102 packed column and Supelcowax 10 capillary column) are used simultaneously to analyze reactor effluents. Initial tests with Au/TS-1 demonstrate ability to produce PO and qualitatively detect the other components of the reactor effluent. GC calibration is in progress. We have successfully prepared TS-1, as evidenced by the characteristic MFI structure in the XRD pattern as well as the UV-vis absorption spectrum that indicates the presence of the tetrahedral Ti in TS-1. The preparation of MTS-9 is in progress, with successful synthesis expected shortly.
