A Study On Au/Ti-MCF Catalysts For The Propylene Epoxidation In The Presence Of H₂ And O₂

Direct epoxidation of propene using O2 and H2 with gold nanoparticles deposited on Ti-containing supports opens an attractive route for the propene oxide?PO?production since the pioneer work of Haruta and co-workers.This catalytic process possesses tremendous industrial andacademic significance and also is more environmental benign than the current commercial routes?chlorohydrin and hydroperoxide processes?.It has been reported that in this process the PO selectivity is higher than 90%at low conversion?<10%?under mild conditions.Nevertheless,the catalysts suffered from several drawbacks such as the lower Au utilization efficiency,high preparation cost,and rapid catalyst deactivation.In this work,a novel Ti-containing mesocellular silicious foams?Ti-MCF?supported Au catalyst was synthesized and applied in propene epoxidatin in the presence of H2 and O2.The structures and catalytic performance of the Au/Ti-MCF catalysts were investigated in detail.Major achievements are summarized below:A post-grafting method was employed to incorporateTi into the frameworks of mesoporous MCFs,yielding a novelTi-MCF composite with large mesopores,big specific surface areaand 3D foamed structure.Characterizations includingN2 physisorption and HR-TEM indicated that a series of Ti-MCFswere successfully fabricated with adjustableaverage pore diameterin the range of 8-24 nm.As revealed by UV-Vis and Raman measurement,most of the incorporated Ti species were present as tetrahedrally coordinated Ti?IV?species isolated in silica matrix.In order to improve the utilization efficiency of HAuCl4,a two-step synthesis strategy combining surface amino-functionalization and chemical reduction deposit was designed and adopted to prepare Au/Ti-MCF catalysts for the propylene epoxidation.Greatly enhanced Au uploading efficiency was achieved following this strategy.A nomial 0.5 wt%Au/Ti-MCF-40 catalyst possessed an actual Au loading of 0.44 wt%as determined by ICP analysis,while a nomial 4 wt%Au/Ti-MCF-40-DP from conventional deposit-precipitation method only had an Au loading of 0.11 wt%.Under413 K and 10000 mL h^-1 g-cat^-1,the 0.5 wt%Au/Ti-MCF-40catalyst exhibited excellent catalytic performance with propene conversion of 1.68%,of 95.9%and H2 efficiency of 22.8%,while the counterpart from deposit-precipitation method only showed propylene conversion of 0.63%and propene oxide selectivity of 86.2%.These results suggest that this two-step synthesis methodcombining surface amino-functionalization and chemical reduction deposit is an feasible,efficient and cost-effective Au catalystpreparation method for gas-phase propene epoxidation.Catalytic stability is crucial for Au catalysts for gas-phase propene epoxidation.Long-term catalytic performances of 0.5%Au/Ti-MCF-40 and 4%Au/TS-1-DP catalyst were compared under indentical reaction conditions.Under 413 K and 10000 mLg-cat^-1h^-1,the deactivation rate of 0.5%Au/Ti-MCF40 catalyst was slow and the catalytic activity only degraded by a factor of 13%for a time on stream of 1400 min,whereasthe 4%Au/TS-1-DP catalyst considerably deactivated in 300 min.Calcination at 573 K in air on the deactivated Au/TS-1-DP catalyst partiallyrestored the catalytic ability suggesting the blocking of active sites by deposits was the main cause for catalyst deactivation.NH3-TPD,TG,BET,HR-TEM characterization results suggest that the greatly enhanced catalytic stability of Au/Ti-MCF catalysts is probably arised from the unique porous structure,weakened surface acidity and the enhanced surface hydrophobicityby surface functionalization contributing to the fast desorption of propene oxide products and inhabitationof active site blocking.