Selective Removal of Surface Acidity in ZSM-5 Zeolite Using (NH4)2SiF6 Treatment

Ammonium hexafluorosilicate, (NH4)2SiF6, has been found to be partially selective for removing surface acidity in zeolite ZSM-5. Under static digestion conditions using HZSM-5 extrudate, the zeolite was found to lose approximately 30% of its acid sites as determined by NH3 desorption and hexane cracking results. However, tetrapropylammonium desorption and α-naphthyl red adsorption studies showed up to a 50% reduction in surface acid sites. This surface deactivation was conclusively demonstrated in propene oligomerization catalytic studies. Use of fluorosilicate-treated HZSM-5 in the surface acidity-sensitive reaction gave substantial viscosity index (VI) improvements in the C20+ fraction due to more linear oligomerized products. It is believed that the static digestion procedure necessary to treat the HZSM-5 catalysts in this study allowed for increased reaction between the zeolite surface and the SiF62− anion. If optimized, this method may be a useful technique to prepare catalysts for surface acidity-sensitive hydrocarbon conversion reactions.