Conversion of glycerol to aromatic hydrocarbons over modified HZSM-5 catalysts

Abstract

The conversion of glycerol to aromatic hydrocarbons was investigated over modified HZSM-5 zeolites. The catalytic activity was carried out in a continuous flow fixed-bed reactor at 400 ºC, 300 psig and H₂/feed molar ratio of 10. The contact time or W/F was varied from 0.07 to 1 h to study the reaction pathway. The effect of SiO₂/Al₂O₃ ratios of HZSM-5 was also studied. Silylation of HZSM-5 catalysts using tetraethyl orthosilicate (TEOS) via chemical liquid deposition (CLD) was studied for improving p-xylene selectivity, Parameters in preparing catalysts i.e. TEOS loading and CLD cycle number were varied to obtain the optimal condition. The conversion of glycerol yielded a variety of aromatics, mainly xylenes and toluene. It was found that propenal, acetol and acetaldchyde were formed as primary product from glycerol via acid-catalyzed dehydration and dissociation respectively. Olefin products from carbonyl bond dissociation of acetaldehyde and oxygenate pool (propenal, propanal, propen-2-ol and methanol) tended to react each other to produce aromatics via oligomerization and dehydration. HZSM-5 with SiO₂/Al₂O₃ ratio 30 showed the highest aromatics yield. The aromatics yield decreased with the increase of SiO₂/Al₂O₃ ratio (from 30 to 280) except HZSM-5 with SiO₂/Al₂O₃ ratio of 23 which showed low aromatics yield due to the strong adsorption of water on the surface acidity. The selectivity of p-xylene in xylenes increased with increasing TEOS amount and CLD cycle number. The highest p-xylene selectivity in xylenes was found on 2x0.5CLD20/HZSM-5(30), prepared by a two cycle of 0.5 ml of 20 vol.% TEOS in cyclohexane.