Single Crystals of ZSM‐5/Silicalite Composites

Abstract

The processes of the formation of dialkylbenzenes from monoalkylbenzene, such as disproportionation and alkylation, are among the most important in the chemical industry. These processes were carried out using solid acid catalysts in earlier times. The activity of these catalysts, such as silica±alumina, was low. Since the late 1960s, ZSM-5 catalysts have been extensively studied because of their much higher selectivity for para-isomers, which are the most valuable compounds for commercial use. However, para-selectivity significantly decreases because of the acid sites on the external surface and the size of the pore openings. Hence, surface modification and pore size control have been proposed in order to enhance the selectivity.[1±9]On the other hand, zeolites have been studied for membrane separation techniques, [10±13] as well as for catalysis, for a long time. In the last decades, combined chemical reactors with zeolite membranes have been of great interest and a lot of applications have been reported.[14±21] Compared with conventional chemical reactors, membrane reactors have great advantages, such as higher selectivity and/or yield of products, simplification of processes, and inhibition of catalyst poisoning. However, the main issue of membrane reactors is the lower permeation rate than the reaction rate. Due to the low permeation flux, a significantly large membrane area and thin zeolite membrane are required. However, the synthesis of large zeolite membranes without any defects, like pinholes or cracks, and a method of controlling the membrane thicknesshave not yet been established. Recently, we have proposed a particle level membrane reactor. A catalyst particle has been coated with a permselective membrane. [22,23] The platinum-loaded TiO 2 particles (particle size = 0.6 mm) coated with a silicalite-1 membrane showed high product selectivity in the hydrogenation of hex-1-ene (1-Hex) and 3,3-dimethylbut-1-ene (3,3-DMB) mixtures. The 1-Hex/3,3-DMB selectivity was 20 because of selective permeation of 1-Hex through the silicalite-1 membrane.[23] Silica± alumina catalyst particles (particle size = 1 mm) were coated with silicalite-1 membranes (silicalite/silica±alumina) and used for the disproportionation of toluene to produce xylene isomers. The silicalite-1 coatings on catalyst particles enhanced the para-selectivity [22] because of selective removal of p-xylene through the silicalite-1 membrane. Toluene conversion, however, significantly decreased from 1.5 to 0.08 % with the coating because the thickness of the silicalite-1 membrane was large (40 lm), which limited the diffusion of the products. In addition, the catalytic activity of silica±alumina was not very high.To solve these problems, in this study we have developed a novel composite catalyst consisting of a zeolite crystal with an inactive thin layer. A silicalite-1 layer was grown on protonexchanged ZSM-5 crystals (silicalite/H-ZSM-5). The conventional zeolite films on porous supports as well as on the particles [22,23] mentioned above have co...

Zeolite Membranes in Catalysis—From Separate Units to Particle Coatings

“…The proposed model for the formation of the silicalite-1 layer on ZSM-5 crystals is illustrated in Figure 11 [269]. Silicalite-1 crystals grow on the ZSM-5 crystal surface perpendicular to the a and c axes under hydrothermal conditions at 453 K for 24 h without agitation.…”

“…Silicalite-1 crystals grow on the ZSM-5 crystal surface perpendicular to the a and c axes under hydrothermal conditions at 453 K for 24 h without agitation. In this context, the benefit of silicalite zeolite membrane coated on H-ZSM-5 crystals was evidenced for the selective p-xylene formation in alkylation of toluene with methanol [246,247,269]. The obtained para-selectivity of up to 99.9% was attributed to the suppressed further isomerization of p-xylene since the silicalite coating reduced the acid sites on the external surface of the H-ZSM-5 catalyst.…”

Fixed-Bed Adsorption Separation Of Xylene Isomers over SiO2/Silicallite-1 Core-Shell Adsorbents

“…Zeolite core-shell composites are novel class of materials for their potential application in the field of catalysis, adsorption, and storage and controlled release [11][12][13][14][15]. The application of these materials in adsorption based separation process has not been fully explored.…”

Morphology Control of Silicalite/HZSM-5 Composite Catalysts for the Formation of Para-Xylene

“…However, the secondary isomerization of p-xylene into its isomers, o-; m-xylenes is one of important factors that reduces the selectivity. Pretreatment or external surface modification methods to deactivate external acid sites or to narrow the pore-mouth openings of ZSM-5 catalytic particles have found to enhance the paraselectivity such as the impregnation of phosphorous, MgO, boron [2,3], chemical vapor deposition (CVD) [4][5][6][7], chemical liquid deposition (CLD) [5,[8][9][10][11][12], and others [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Recently, Breen had reported an impregnation of HZSM-5 with boric acid, giving more than 99.9% of paraselectivity with the toluene conversions of 3.5-7.9% [11].…”