Pretreatment of HZSM-5 with organic alkali and cobalt: Application in catalytic pyrolysis of lignin to produce monocyclic aromatic hydrocarbons

Abstract

In this work, we adopted organic alkali and cobalt pretreated HZSM-5 zeolites as catalysts for monocyclic aromatic hydrocarbons production via catalytic fast pyrolysis of lignin in a two-stage fixed bed reactor. For the treatment, four kinds of organic hydroxides (tetraalkylammonium hydroxides, denoted as TMAOH, TEAOH, TPAOH, and TBAOH) were chosen as the desilication agent, and cetyltrimethylammonium bromide (CTAB) was introduced as assistant to control the pretreating process. In addition, cobalt modification was further conducted over the optimized alkali-treated sample for acidity adjustment. The treated zeolites were systematically characterized by XRD, SEM, NH3-TPD, and N2-adsorption/desorption techniques. The characterization results were analyzed together with the catalytic testing data to reveal the structure-function relationship of the catalysts in catalytic pyrolysis of lignin. As it turned out, the HZSM-5 zeolite pretreated by 0.5 mol/L of TPAOH in the presence of CTAB exhibits a coordinated micropore/mesopore proportion, proper average mesopore size, and a successful preservation of acidic properties. The addition of CTAB in the alkali treatment process was not only beneficial to preventing the over-treatment of HZSM-5, but could also regulate the mesoporous structure of zeolite. Eventually, the highest production of monocyclic aromatic hydrocarbons (31.8%, relative content) was achieved by the TPAOH pretreated HZSM-5 (2C-0.5TPHZ) at 500 °C, far exceeding that of the parent HZSM-5 (17.3%, relative content). Further modification of the 2C-0.5TPHZ sample with 1 wt% of cobalt could enhance the acidity of the alkali-treated zeolite, leading to a further improvement of the monocyclic aromatic hydrocarbons production to 38.3%. The reusability of the pretreated sample was further evaluated by cyclic reaction for 5 times, which demonstrated its reactivity stability for monocyclic aromatic hydrocarbons production.