Selective Formation of Linear Alkanes from n‑Hexadecane Primary Hydrocracking in Shape-Selective MFI Zeolites by Competitive Adsorption of Water

MFI type zeolites have been known for decades for their strong tendency toward gas formation in hydrocracking. Our strategy for selectivity control in hydrocracking turns strong adsorption in micropores, responsible for secondary cracking, to an advantage. Enhancing diesel yield is a challenging goal in hydrocracking catalysis. Additionally, linear alkanes increase the diesel fuel cetane number and, consequently, effect a dramatic reduction in exhaust emissions. This study demonstrates for a set of Pt on mesoporous MFI zeolite catalysts that the gains in activity and selectivity attributed to enhanced mass transport are but modest by comparison to the effects of the competitive adsorption of water. Water suppresses secondary cracking, and primary cracking is now reported over MFI zeolites. Furthermore, competitive adsorption of water in shape-selective MFI zeolites facilitates desorption of the primary n-C₁₆ cracking products and suppresses subsequent isomerization resulting in high yields of linear alkanes. The selectivity for linear alkanes from hydrocracking of n-hexadecane over Pt/MFI nanosheets reaches 80% at 80% conversion.