Catalysis of Organic..

100 Propylene Partial OxidationThe most valuable product for propylene partial oxidation is PO. Increasingtemperature usually enhances the PO yield and propylene conversion, furtherincreasing temperature causes PO yield and selectivity to decrease. High POselectivity was only observed at low propylene conversion. The highest reported POyield was reported at the level of 2 -3 % (3). A better understanding of themechanism of the reaction is needed for the design of a selective catalyst to give highPO yields. We have employed in situ infrared (IR) spectroscopy to study the natureand structure of adsorbed species and mass spectrometry (MS) for product formationprofile during partial oxidation. Our objective is to determine the reaction pathwayfor the formation of specific oxygenated products including propionaldehyde,acetone and PO from propylene partial oxidation over Au-based catalysts.Experimental SectionTwo types of Au (nominal 2 wt%) catalyst were prepared using two different goldprecursors as well as support (9). The specific procedure for preparing Au/TiO x -SiO 2 involved (i) dispersion of Silica gel (Alfa Products) in methanol/titanylacetylacetonate (Sigma-Aldrich), (ii) calcination of resulting powder obtained in firststep at 800°C for 4 h, (iii) deposition-precipitation of Au on the supports, (iv) dryingat 100°C for 2 h and calcination at 300 °C for 4 h (10). The catalyst prepared isdenoted as Au/TiO x -SiO 2 because the stoichiometry of oxygen associated with Ti onthe SiO 2 is not known.Preparation of Au/TiO 2 -SiO 2 involved (i) intermixing of Silica gel (AlfaProducts) and TiO 2 (Degussa) in appropriate amount keeping Ti/Si atomic ratio at2/100, (ii) introduction of powdered mixture obtained in first step into a solution ofAuCl 3 at 70 °C, adjusting the pH to 7, (iii) drying at 80 °C for 12 h and calcination at400 °C for 5 h. The catalysts prepared were characterized by DR (diffusereflectance) UV-VIS (Hitachi).The experimental reactor system consists of three sections: (i) a gas meteringsection with interconnected 4-port and 6-port valves, (ii) a reactor section includingan in situ diffused reflectance infrared Fourier transform spectroscopy reactor(DRIFTS, Thermo-Electron) connected to tubular quartz reactor, (iii) an effluent gasanalysis section including a mass spectrometer (11).Temperature-programmed reaction (TPR) studies of partial oxidation ofpropylene was carried out by flowing C 3 H 6 (Praxair), O 2 (Praxair), H 2 (Praxair) andAr (Praxair) through DRIFTS and stainless steel tubular reactor (3/8” OD) loadedwith catalyst. Feed gas at 40 ml/min and 1 atm consists of C 3 H 6 (10%), O 2 (10%),H 2 (10%) and Ar (70%) for temperature program reaction studies. Prior to eachexperiment, the catalyst was pretreated in H 2 (10 vol%) and O 2 (10 vol%)simultaneously at 250°C. Temperature was monitored with a K type thermocoupleconnected to an omega temperature controller.During TPR with Ar/C 3 H 6 /H 2 /O 2 (28/4/4/4 ml/min), the composition of thereactor effluent was monitored by Omnistar MS (Pfeiffer Vacuum). The m/e signalsmonitored by the mass spectrometer were 2 for H 2 , 18 for H 2 O, 41 for propylene, 32