Application Of Zeolite NaA Membranes In Selective Oxidation Reaction

Zeolites,types of aluminosilicates,possess regular pore structures. Due to their pore/window sizes ranging from 0.3 to 0.8 nm,zeolites exhibit molecular-sieving properties in general.The basic concept of the membrane reactor is a coupling of a catalyst with a membrane,implying that reaction and separation could be carried out simultaneously.The membrane reactor could break through the limits of chemical equilibrium and thus increase the yield.Zeolite membranes coated on catalyst particles could result in shape-selective catalysis for either reactant or product molecules.In terms of the window-opening pore size(about 0.4 nm),zeolite NaA membranes may be applicable in separation of some small molecules such as H₂,CO,O₂,etc.,from large molecules such hydrocarbons,and a molecular-sieving effect can be expected.In the open literature,some work has been conducted on zeolite NaA membranes used for water removal and gas/vapor separation,but the use of NaA membranes to prepare composite zeolite catalysts has been very limited. Therefore,studies on zeolite membrane coated on catalyst particles have important research values and broad application prospects.In this thesis,zeolite NaA membranes were synthesized on the surface of spherical Pt/Al₂O₃ particles using a hydrothermal method.The effects of composition of materials,alkalinity,synthesis time and temperature on the formation of zeolite NaA membranes onto the catalyst particles were investigated in detail.The crystalline of NaA and the morphology of membranes were characterized using XRD and SEM techniques,respectively.A model oxidation reaction of a mixture of carbon monoxide,ethene and air was used to evaluate the performance of the synthesized composite catalysts.Inherently,the permselectivity of reactant molecules on continuous membranes was demonstrated.Defect-free NaA membranes could be successfully coated onto Pt/Al₂O₃ particles by a two-step synthesis method,confirmed by XRD and SEM.The synthesized membranes show the feature of pure zeolite NaA crystals characterized by the XRD technique.The two-step synthesis resulted in the formation of continuously compacted NaA membranes,as demonstrated by the SEM images.The conversion of CO during the oxidation over the composite catalyst obtained by the two-step synthesis was complete at 180℃while the conversion of ethane was very limited, resulting in a selectivity of 96%for CO.This is due to the facts that the reactants of CO and O₂ can easily permeate through the coated NaA membranes and the oxidation can take place on Pt/Al₂O₃.Although the kinetic diameter of ethene(0.39 nm)is smaller than the window-opening size of NaA(0.4 nm),ethene molecules cannot freely permeate through the defect-free NaA membranes due to their strong mass transfer limitations.The current investigation shows that a selective oxidation of CO in the presence of ethene can be achieved over the zeolite NaA composite catalysts.