Catalytic fast co-pyrolysis of lignocellulosic biomass and polypropylene over bimetallic catalysts to promote the formation of hydrocarbons

Abstract

Lignocellulosic biomass is considered a renewable resource with great potential to replace fossil energy, which has a relatively low effective H/Ceff ratio. As a result, its thermochemical conversion products are of low calorific value and quality. Polypropylene (PP) is rich in hydrogen and contains little oxygen. Catalytic fast co-pyrolysis (co-CFP) of cotton stalks (CS) and PP significantly improved bio-oil quality. It enables the efficient and clean utilization of CS and PP, while enhancing the concentration of BTEX (Benzene, toluene, ethylbenzene, xylene) and other liquid hydrocarbons in bio-oil. In this study, bimetallic catalysts are prepared by alkali-modified HZSM-5 and zirconium dioxide (ZrO₂) loaded with Ni and Mo, respectively. The impact of different ratios on the product is explored, revealing their synergistic effects in the co-CFP process. XRD, XPS, TEM, and other methods characterize the morphology and physicochemical properties of the catalysts. The co-CFP of CS and PP by Py-GC/MS and thermogravimetric experiments is investigated. The experimental conditions were as follows: pyrolysis temperature of 600 °C, raw material mixing ratio of 1:1, and feedstock-to-catalyst ratio of 1:1. In the catalytic stage, when the Ni:Mo ratio was set to 2:1, the relative hydrocarbon content in the Ni-Mo/ZrO₂ catalyst reached a maximum value (75.2 %) at Ni:Mo= 2:1. The relative hydrocarbon content initially decreased and then increased with decreased Ni/Mo ratio and reached the minimum at 1:2. The relative hydrocarbon content in the Ni-Mo/HZSM-5 catalyst reaches a maximum value (60.9 %) at Ni:Mo= 2:1. Ni-Mo/HZSM-5 catalyst has more substantial selectivity for aromatic hydrocarbons. The synergistic effect between metals is explored using reaction kinetics based on thermogravimetric experiments. Based on thermogravimetric experiments, the synergistic effects between metals were analyzed using reaction kinetics. Specifically, this involves effectively reducing the activation energy of the reaction, thereby promoting the formation of hydrocarbons. For the NiMo/ZrO₂ catalyst, the activation energy generally decreases with increasing Mo content (from 38.51 to 29.47 kJ/mol). In contrast, when HZSM-5 is used as the support material, the activation energy decreases with increasing Mo loading and decreasing Ni loading (from 12.31 to 8.43 kJ/mol). The right amount of metal loading introduces the catalyst's active site, and the synergy between the metals further improves the bio-oil quality. The activation energy is lowest when Ni:Mo = 2:1. The change trend is consistent with the content of hydrocarbons.