Shape selectivity of ZSM-5 zeolite modified with chemical vapor deposition of silicon and germanium alkoxides

Abstract

The shape selectivity of ZSM-5 zeolite was modified with the chemical vapor deposition of silicon and germanium ethoxides. Both Si and Ge modifications were carried out by using one and three CVD cycles. Modified zeolites were characterized with FTIR and MAS NMR spectroscopy. The deposition of alkoxides took place mainly on the external surface and therefore no significant changes in strong Brønsted acidity were observed. This was confirmed by IR and 1H MAS NMR spectroscopy as well as with the catalytic cracking of n-hexane over modified zeolites in which no significant loss of activity compared with unmodified zeolite was detected (except in the case of triply silane deposited ZSM-5). The reduction of channel dimensions of modified zeolites was concluded from the reduced chemisorption capacity of pyridine. In the conversion of methanol to hydrocarbons the selectivity of modified zeolites towards aliphatic, linear and small olefinic products increased and the formation of aromatic and branched hydrocarbons decreased. In the cracking of n-hexane the effect of modifications on shape selectivity was less notable. The passivation of external surface was characterized by the cracking of 1,3-di-isopropyl benzene. Both Si and Ge modifications decreased the outer surface acidity but the passivation was concluded to be more complete in silane deposited ZSM-5 zeolites.