Effects of alkali-treated hierarchical HZSM-5 zeolites on the production of aromatic hydrocarbons from catalytic fast pyrolysis of waste cardboard

The effects of hierarchical porous HZSM-5 zeolites on improving the yield of aromatic hydrocarbons during catalytic fast pyrolysis (CFP) of waste cardboard (WCB) were investigated. Desilicated HZSM-5 zeolites were successfully prepared by alkaline treatment using NaOH solution with different concentrations (0.1 M–0.7 M). A Py-GC/MS apparatus was adopted to carry out the CFP experiments. Results showed that under mild alkaline treatment (NaOH concentration ≤ 0.3 M), the micropores and crystallinity of HZSM-5 were preserved, while the mesopores and weak acid sites increased. The desilicated HZSM-5 zeolites increased the total yield of pyrolytic products from WCB. With regard to the product distribution, carbon yield of aromatics increased dramatically, while that of sugars, carbonyl compounds and other oxygenates decreased obviously. Specifically, HZSM-5 treated by 0.3 M NaOH solution (HZ-0.3 M) was proved to be the most effective catalyst, resulting in a 44% increase of carbon yield of aromatics and an 82% increase of carbon yield of BTX (benzene, toluene and xylenes) comparing to the parent HZSM-5. In addition, the amount of coke deposited on HZ-0.3 M decreased by 29% compared to that on the parent HZSM-5. The retained micropores and the created mesopores of desilicated HZSM-5 zeolites contributed to the enhancement of catalytic performance. However, harsh alkaline treatment of HZSM-5 zeolites (NaOH concentration ≥0.5 M) resulted in the grave collapse of zeolitic crystallinity and the obvious loss of micropores. As a result, the catalytic activities of HZSM-5 zeolites had been weakened.