Study On Support Modification Of Propylene Direct Vapor Phase Hydrogen Oxidation Au/TS-1 Catalyst

Propylene oxide?PO?is an important chemical intermediate.Different production processes will have different impacts on production indicators such as economic benefits and environmental protection requirements.Direct gas phase epoxidation of propylene is a new type of propylene oxide production process that has been researched in recent years.The by-product is only water and environmentally friendly.However,there are still many problems in the Au/TS-1 catalyst of the reaction,such as low activity of the catalyst,poor stability,and low utilization rate of hydrogen.In this paper,the support of nano-gold catalyst supported on the titanium-silica molecular sieve was studied.First,starting from the acidity and alkalinity of the support,the N-TS-1 molecular sieve was successfully prepared by nitriding in a high temperature NH₃ stream.The nitridation temperature and nitridation time control the type and content of nitrogen species.The N species will attract Au to enrich the surface of the molecular sieve,which will increase the Au capture rate?from 1.03 to 2.25%?.The increase in the gold loading increases the propylene conversion of the propylene gas phase epoxidation reaction?from 3.7 to 7.4%?,resulting in an increase in the corresponding PO production rate(from 78.7 to 138.4 g_POh^-1kg^-1_Cat).Nitriding reduces the acidity of the surface of the molecular sieve and reduces the acid strength to inhibit the deposition of PO on the surface of the support,slowing down the deactivation caused by the formation of oligomers or isomerization on the surface of the adsorbed PO.Then,in order to overcome the problem that the carbon deposit generated by the reaction of the TS-1 molecular sieve is easy to block the microporous channel,we introduced a green and inexpensive surfactant F127 and successfully synthesized the crystallinity by steam-assisted crystallization.The mesoporous distribution is uniform?3.3 nm?of hierarical MTS-1 crystal.The amount of F127 added has an important influence on the structural properties of the MTS-1.The effective introduction of mesopores in the hierarical MTS-1improves the diffusion coefficient,which inhibits the formation and evolution of heavier carbonaceous deposits and improves the stability of the catalyst in gas phase propylene epoxidation reaction.Finally,we synthesized TS-1-S by adding urea as a crystallization modifier and using one-step hydrothermal synthesis.The addition of an appropriate amount of urea in the sol?n?urea?/n?SiO₂?=0.1⁰.15?facilitates the polymerization of the oligomeric silicate and the titanate,promoting the entry of titanium into the molecular sieve framework.The molecular sieve has an improved performance in catalyzing the gas phase epoxidation of propylene.When an excess of urea?n?urea?/n?SiO₂?=0.35?is added to the synthesis system,urea inhibits the growth of the crystal in the b-axis direction,obtaining a sheet-like TS-1-S.In the catalytic gas phase propylene epoxidation reaction,the diffusion of reactants and product molecules in the interior is facilitated,and the mass transfer ability is better.However,excessive urea adversely affects the entry of titanium into the molecular sieve framework and it is easy to form anatase TiO₂,which seriously affects the selectivity and rate of PO formation.