Isomerization and Cracking of Hexane over Beta Zeolites Synthesized by Dry Gel Conversion Method

Beta zeolite (BEA) samples with different SiO2/Al2O3 ratios were synthesized by the dry gel conversion method (DGC) using tetraethyl ammonium hydroxide (TEAOH) as a structure directing agent (SDA). Catalytic properties of BEA for the isomerization and cracking of hexane were evaluated by comparing with the typical zeolites, mordenite (MOR), ZSM-5 (MFI), and ZSM-22 (MWW) with SiO2/Al2O3 ratios of 30-40. The combined selectivities for branched alkanes (2- and 3-methylpentanes, and 2,2- and 2,3-dimethylbutane (b-C6), 2- and 3-methylbutanes, 2,2-dimethylpropane (b-C5), and 2-methylpropane (b-C4)) were highly dependent on the type of zeolites, and decreased in the order: BEA > MWW >> MFI. BEA had the highest activity and selectivity for the isomerization, and MFI had the highest activity and selectivity for cracking to lower alkanes and alkenes. These differences were ascribed to the differences in pore structure, acid properties, and reaction parameters, resulting in the high performance for the isomerization and cracking over BEA with wide channels and weak acidity. The effects of SiO2/Al2O3 ratio of BEA were examined in the isomerization of hexane. The combined selectivity for branched alkanes was almost constant with increase in the SiO2/Al2O3 ratio; however, the selectivity for b-C5 and b-C4 decreased with the increase in the ratio, resulting in increased selectivity for b-C6. These findings indicate that the selectivity for the isomerization was not affected by acid amounts; however, the cracking of isomerized products was enhanced by the increased acid amounts. Catalytic activity and selectivity for branched alkanes remained in the similar level during the reaction using catalysts with the same acid amounts. These results suggest that the acid sites on BEA with different SiO2/Al2O3 ratio prepared by DGC method have uniform and discrete activity with selectivity for the isomerization.