Catalytic conversion of a biofuel to hydrocarbons: effect of mixtures of HZSM-5 and silica-alumina catalysts on product distribution

摘要

The potential for producing hydrocarbons from the conversion of biofuels has been the focus of attention in recent years. In a preliminary study, we observed that it was possible to produce various types of liquid hydrocarbons and also to dramatically change the hydrocarbon content from aromatic to aliphatic by mixing silica-alumina and HZSM-5 catalysts in different proportions. In the present work, an in-depth study was undertaken in order to investigate the effect of various mixture compositions of silica-alumina and HZSM-5 on the yield and selectivity for liquid hydrocarbons. The biofuel used in the present study was produced by the rapid thermal processing of maple wood. The runs were performed in a fixed-bed microreactor operating at atmospheric pressure, 1.8-7.2 WHSV and 330-410℃. It was interesting to observe that for all catalyst mixtures, the optimum yields of organic liquid product (OLP) and total hydrocarbons were obtained at 370℃. The HZSM-5 content (H_f) of the catalyst mixtures ranged between 0 and 40 wt.%. The catalysts were thoroughly characterized by the following techniques: X-ray powder diffraction, temperature-programmed desorption with ammonia, FT-IR and NMR spectroscopy and measurement of their BET and pore sizes. The yield of OLP increased with H_f and ranged between 13 and 27 wt.% of the biofuel feed. Aliphatic hydrocarbons were the main products (37-77 wt.% of OLP), followed by aromatic hydrocarbons (2-38 wt.% of OLP). At low H_f (below 10 wt.%X the main effect of HZSM-5 was to increase the extent of cracking and thereby increase the aliphatic hydrocarbon production. At H_f > 10, a combination of cracking followed by shape selectivity resulted in the production of aromatic hydrocarbons at the expense of aliphatic hydrocarbons. The results were analyzed statistically in order to determine which factors (namely, HZSM-5 content in the catalyst (H_f), space velocity, temperature and their interactions) were mainly responsible for the formation of OLP and its hydrocarbon content. The results showed that all three factors affected the OLP yields rather significantly. However, the aliphatic hydrocarbon yield was mostly affected by the space velocity and H_f, and the aromatic hydrocarbon yield was significantly affected by temperature and H_f. A regression surface response model was used to relate the yields of these products with the above- mentioned factors.