Surface modification of nano-sized HZSM-5 and HFER by pre-coking and silanization

摘要

Deposition of carbonaceous and siliceous materials was used to enhance the selectivity of HZSM-5 and HFER by passivating non-selective acid sites present on the external surface of zeolite crystallites.The nature of carbonaceous deposits formed during the pre-coking treatment was studied by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-TOF MS)after zeolite dissolution by hydrofluoric acid.Molecular weights up to 1300 Da were observed for coke species present on the pre-coked HZSM-5 sample to which carbon numbers n_C ~ 100 can be assigned.In terms of inactivation of surface acidity,samples modified by pre-coking were found to be more effective compared to those by one-cycle silica deposition,as evidenced by ~(31)P MAS NMR of adsorbed tributylphosphine oxide as the probe molecule.Accordingly,pre-coked HZSM-5 yielded a higher shape selectivity during xylene isomerization,i.e.,a reduced formation of undesirable toluene and trimethylbenzenes.For skeletal isomerization of n-butene on HFER,silanization caused a decrease in isobutene formation due to the deposition of siliceous materials within the channels.Pre-coking of HFER showed no specific effect on isobutene selectivity and,accordingly,these polyaromatic carbonaceous deposits cannot be assigned to specific catalytic sites.