Additional lewis acid sites of protonated fibrous silica@BEA zeolite (HSi@BEA) improving the generation of protonic acid sites in the isomerization of C6 alkane and cycloalkanes

Abstract

This paper describes a study of the nature of metal-free protonated fibrous silica BEA (HSi@BEA) and its ability to generate protonic acid sites originating from molecular hydrogen and from C6 alkanes and cycloalkanes. HSi@BEA was successfully synthesized by a microemulsion system and subsequently applied in n-hexane, cyclohexane and methylcyclopentane isomerization. The HSi@BEA consisted of microspheres with a bicontinuous concentric lamellar structure morphology. Its core was mainly composed of an aluminosilicate framework of BEA, while the lamellar structure consisted of silica. The particle size of HSi@BEA was in the range of 400 nm to 700 nm with pore diameters between 3.6 nm to 5.6 nm and 15 nm to 25 nm. 27 Al MAS NMR results showed that the additional silica lamellar structure of the HSi@BEA increased the extra-framework aluminum (AlOH) which acts as Lewis acidic sites in the catalyst. FTIR results revealed that these additional Lewis acid sites in the metal-free catalyst generated a high amount of protonic acid sites by playing a role as electron acceptors after the dissociation of H2 and C6 alkane and cycloalkanes. Significantly, the participation of more protonic acid sites enhanced the catalytic performance of the C6 alkanes and cycloalkanes, n-hexane, cyclohexane and methylcyclopentane, at a low reaction temperature of 423 K. The presence of a silica lamellar structure resulted in a two to threefold higher isomer yield of n-hexane, cyclohexane and methylcyclopentane compared to bare HBEA catalyst which mainly produces cracking products. In addition, a kinetics study showed that the activation energy could be lowered by two to three times at elevated temperatures in the range of 423 K to 623 K.