Preparation of aromatic and bio-char by pyrolysis of biomass and plastics catalyzed by modified HZSM-5

Metal oxides are widely used in the formulations of the catalysts for catalytic pyrolysis of biomass, due to the large specific surface area, pore size, and suitable acidity. HZSM-5 was the best mesoporous catalyst for aromatics synthesis because of the best acidity. However, there are some problems, such as low target yield, catalyst coking and deactivation with solo the catalysts. In order to improve the quality of bio-oil, a catalytic co-pyrolysis vapor upgrading of bamboo sawdust and LDPE was conducted by a fixed bed reactor, further to investigate effect of bamboo to LDPE ratio (1:0, 2:1, 1;1, 1:2, 0:1) of pyrolysis and catalyst pyrolysis on product yield. This study also revealed the effect of metal oxide (including HZSM-5, CaO, MgO, CeO2, La2O3 and SnO2), the dual-catalyst bed of HZSM-5 and base catalysts, HZ to MgO ratio (HZ: MgO=5:1, 2:1, 1:1, 1:2, 1:5) and combination mode (layered mode and mixed mode) on the promotion of aromatic hydrocarbons and bio-char. In addition, an investigation was made to explore the synergistic effect, reaction mechanism, and process optimization. Experimental results illustrated that: the addition of LDPE and metal oxide can effectively promote the conversion of biomass, thereby to reduce the yield of bio-oil (from 36.82% to 9.76%~23.96%), and further to enhance the quality of bio-oil, and the graphitization degree of bio-char, indicating an obvious synergistic effect. Hydrogen-rich fragments that derived from LDPE can promote the Diels-Alder reactions of furans, and participate in the hydrocarbon pool reactions of non-furanic compounds. As a result, a high yield of hydrocarbons was achieved. Scum as the hydrogen donor, showed a significant synergistic effect with biomass to enhance the production of bio-oil and aromatic hydrocarbons, when the H/Ceff value exceeded 1. The maximum yield of aromatic hydrocarbons (89.32%) was obtained, when the optimal biomass to LDPE ratio was 1:2. MgO catalyst can facilitate the formation of phenol and alkylphenol. The MgO-based catalysts suppressed the formation of ethylene glycol, furans, carboxylic acids, and the aldehydes like acetaldehyde or hydroxyl acetaldehyde, but did not show much effect on the formation of hydroxyl acetone and acids. The formation of hydroxyl acetone was generally promoted by the oxide catalysts, as ketones were more stable than the aldehydes. CaO can effectively transform acids into ketones, as well as improved the yield of olefins. Moreover, La2O3 and SnO2 can exhibit better effect on furans formation. The mixed mode can effectively improve the yield of aromatics, compared with the layered mode. Ex-situ catalytic pyrolysis using HZ mixed MgO with ratio of 2:1 increased the aromatic hydrocarbon yield to 84.99% and mono-aromatic selectivity to 60.09%. Meanwhile, the contents of toluene and xylene were 25.97% and 16.91%, respectively, at the bamboo to LDPE ratio of 1:1, pyrolysis temperature of 450°C, catalytic temperature of 550°C, and the feedstock to catalyst of 1:2. The mixed mode can effectively promote the selectivity of benzene, toluene, and xylene, whereas, the layered mode can effectively promote the formation of alkylbenzene. The addition of MgO obviously inhibited the formation of PAHs. The findings can provide an efficient method to produce high-quality biofuels from renewable biomass resources. Normally, different kinds of metal oxides had different effects on the catalytic pyrolysis of biomass and plastic, with the different ability of anti-carbon deposition. These factors should be fully considered in the development of metal oxide based catalysts.