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

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 nC∼100nC∼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 31P 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.