Synthesis Of Zeolite NaA Membranes On Surface Of Pt/Al₂O₃ Particles And Application In Shape Selective Oxidation Reaction

Zeolite NaA membranes were synthesized on the surface of spherical Pt/Al₂O₃ particles by using hydrothermal and microwave heating methods.The effects of composition of materials,alkalinity,synthesis time and temperature on zeolite membranes formation were investigated and discussed.The zeolite NaA crystals and the surface morphology of membranes were characterized by using XRD and SEM techniques.A model oxidation reaction of a mixture of CO,n-butane and air was used to evaluate the performance of zeolite membranes and demonstrate the permselectivity of reactant molecules on continuous membranes.As indicated by XRD and SEM,the two-stage synthesis was available for improving the integrity of zeolite NaA membranes in hydrothermal synthesis.The XRD results demonstrated that the synthesized membranes show the feature of pure zeolite NaA crystals,and the increase of synthesis times benefit to form continuous compact membranes as demonstrated by the SEM photographs.The activity of CO oxidation on NaA/Pt/Al₂O₃ after two-stage synthesis could reached 94%at 210℃,and the permselectivity of CO on the continuous compact zeolite membranes was almost 100%. It was apparently suggested that the reactants CO and O₂ can diffuse and permeate through the pores of zeolite NaA membranes and the oxidation takes place on Pt/Al₂O₃, but the reactant molecule n-butane more than CO and O₂ in molecular diameter can not reach to the catalytic species on NaA/Pt/Al₂O₃.The results showed that the membranes possess the excellent integrity,exhibiting molecular sieving.The experimental results also showed that continuous compact zeolite NaA membranes could successfully deposit on the surface of Pt/Al₂O₃ by microwave heating. Microwave heating method could effectively shorten the synthesis time compared with hydrothermal method under the same synthesis condition.In order to improve membranes quality,employing two-stage synthesis is necessary,which is proved to restrain transformation of zeolite NaA and increases intergrown degree of membranes in gel reactant system for short synthesis time.XRD and SEM results demonstrated that there existed continuous compact zeolite NaA membranes with the average thickness of 21μm on the surface of Pt/Al₂O₃ particles after two-stage microwave heating with synthesis time for 20 min each time.The activity of CO oxidation could also reached 94%at 210℃,and the selectivity to CO was almost 100%on NaA/Pt/Al₂O₃.