Synthesis, Characterization And Membrane Reactor Applications Of MFI Type Zeolite Membrane

Recently, zeolite membranes have attracted considerable interests in bothindustrial and academic sectors. They can be applied to high-temperature andhigh-pressure conditions. Zeolite membranes offer high permeance and excellentseparation properties because of the molecular sieving pore size, uniform pore sizedistribution, and excellent adsorption properties of zeolites. The zeolite membranescan also be used in membrane reactors by taking advantages of both separation andcatalytic properties. On the other hand, the disproportionation of converting toluene top-xylene and benzene is one of the most important processes of shape-selective MFItype zeolite. Although the p-selectivity of the reaction can be more than 90%, thehigh-selectivity is at cost of low conversion (only 20-30%), a great deal of reactantshave to be recycled in the reactor.In this paper, the modeling analysis of toluene disproportionation to p-xylene inan MFI type zeolite membrane reactor was reported. The model calculation revealedthat high p-selectivity (>99%) could be achieved for the situation where touluene wascompletely converted completely by using reactant and product regions. Such analysisprovided guidelines for experimental efforts in development of the MFI type zeolitefor effective formation of p-xylene.In respect of the zeolite membrane preparation, silica gel was selected as thesilica source to synthesize the 70 nm-sized silicalite-1 zeolite seeds. MFI type zeolitemembranes were synthesized by the template-free secondary growth method. Wefound the growth mechanism of multilayer zeolite membranes and two synthesisrecipes by which could grow different morphological single-layer zeolite membranescould be grown. The key of the synthesis of multilayer zeolite membranes was thecontact condition of synthesis sol with the seed layer, if the seeds layers were coveredby the silica gel in the synthesis solution, the upper seeds would not be able to growoutwards to form a single-layer membrane, and the silica gel that was adsorbed intothe seed layers would infill the boundaries of the crystal grains, and repair the gaps.Single gas permeation was adopted to test the MFI type membrane, the results showedthat the membrane made by the template-free secondary growth method could notdistinguish between n-butane and i-butane even after the calcination at 450℃.The catalytic membrane reactor equipped with the modified ZSM-5 zeolitemembrane was evaluated by the disproportionation of toluene. The results showedthat if there were gaps in the membrane, toluene would mainly diffuse to the permeateside through the gaps, and the toluene conversion was reduced to about 1%. Therewas no use for the ion exchange of MgP2+P and for the chemical liquid deposition of Sito plug up the gaps, but the modification method could reduce the pore opening of thezeolite, leading to increasing the p-xylene selectivity up to 90%. The tolueneconversion with no gap Mg-HZSM-5 zeolite membrane made in this paper could beup to 31.1%, where the p-selectivity was 62.5%.