Recrystallisation of mesoporous SBA-15 into microporous ZSM-5

SBA-15 was impregnated with tetrapropylammonium hydroxide and sodium aluminate and subsequently recrystallised under autogenous pressure in glycerol containing different amounts of water. Under the studied conditions, the formation of ZSM-5 was observed only in the presence of glycerol containing 30 or more wt.% of water. In the particular case of glycerol containing 30 wt.% of water, ZSM-5 structural units were observed by FTIR spectroscopy in a still X-ray amorphous solid after 12 h. However, when ZSM-5 could be detected by X-ray diffraction, scanning electron microscopy showed that the SBA-15 particles and the ZSM-5 crystals are segregated. Al-27 MAS NMR of the X-ray amorphous sample containing ZSM-5 structural units demonstrates that aluminium is mainly in tetrahedral co-ordination. Nevertheless, the observed chemical shift was lower than that observed for crystalline ZSM-5, thus indicating that the aluminium sites in this amorphous recrystallised sample are different from the aluminium sites in ZSM-5. The activity of the predominantly amorphous recrystallised samples in the isomerisation of m-xylene was lower than that of samples containing predominantly crystalline ZSM-5. For the latter, an enhanced selectivity towards p-xylene was observed, indicating that the reaction occurs on Bronsted sites in the shape-selective micropores of the ZSM-5 structure. For the predominantly amorphous recrystallised samples an o/p-xylene ratio higher than one indicates that in this case the isomerisation occurs by a bimolecular transalkylation on non-selective sites. However, the activity of the SBA-15 samples recrystallised in the presence of aluminium. is lower than that over SBA-15 recrystallised under comparable conditions, but in the absence of aluminium. This indicates that the enhanced activity of the latter might be due to an enhanced hydrophobicity caused by dehydroxylation of the pore surface during the recrystallisation process which could be observed by FTIR spectroscopy. A higher hydrophobicity could enhance the adsorption of the organic substrate and this concentration effect would be responsible for the enhanced activity observed on the recrystallised samples without or low ZSM-5 content. Taking into account that dehydroxylation of the pore surface might be achieved by much simpler thermal treatment and that the ZSM-5 crystallised as segregated phase, the studied recrystallisation method seems not able to yield improved catalysts for the transformation of bulky molecules as it was expected. (c) 2006 Elsevier Inc. All rights reserved.