Catalytic conversion of Zhaotong lignite thermal dissolution derived oil to cycloalkane fuels

Low-rank coals are abundant and widely distributed in the world. However, their utilization is hindered by inherent characteristics such as high oxygen and moisture content, along with low calorific value. This study investigates the preparation of cycloalkane fuels from lignite, aiming to achieve clean and efficient utilization of these resources while expanding the supply of cycloalkane fuels, thereby enhancing energy security. Specifically, Zhaotong lignite thermal dissolution derived oil was utilized as the raw material for catalytic alkylation-hydrodeoxygenation process. Initially, phenolic model compounds were employed to probe the catalytic activities of various catalysts in the alkylation-hydrogenation of phenols. The optimum reaction conditions for catalytic hydrodeoxygenation and alkylation reactions were explored systematically. Then, the thermal dissolution derived oil was alkylated under the optimal reaction conditions. Following alkylation, NiFeAlO_x is used as the catalyst for catalytic hydrogenation and deoxygenation to remove oxygen-containing functional groups from the alkylation products, thereby improving the stability and quality of the final hydrocarbon products. The results demonstrated that the Hβ zeolite catalyst, characterized by a silicon-aluminum ratio of 25, exhibited high catalytic activity for alkylation of phenols, the alkylation products are mainly ortho position products and cyclohexanol dehydration is the initial step of the reaction and the resulting cyclohexene is the alkylation reagent. In addition, the growth of alkyl side chains increases the spatial site resistance, leading to a decrease in the conversion of the reactants, and the interactions between phenolic compounds inhibit the alkylation reaction. The characterization of the molecular sieves indicated that Hβ molecular sieves have the largest specific surface area and mesopore volume and suitable acid centre sites for the alkylation reaction. The active components of NiFeAlO_x exist in the form of Ni and FeNi₃, and the introduction of Fe not only increases the dispersion of Ni and the content of oxygen vacancy but also enhances the acidity and H₂ activation ability. In addition, electrons migration from Fe to Ni results in the formation of electron-rich Ni species and electron-deficient Fe species. The catalyst with Ni/Fe/Al ratio of 3∶0.5∶2.5 displayed the highest performance for hydrodeoxygenation of phenols. Under the conditions of 180 ℃, using n-hexane as solvent in nitrogen atmosphere for 10 h, the phenolic compounds conversion exceeding 80%, coupled with bicyclic product selectivity exceeding 90%. According to GC/MS analysis, approximately 60% of the phenolic monomers present in the pyrolysis derived oil were successfully converted into bicyclic products under these conditions. Subsequent processing revealed that after alkylation with Hβ zeolite and hydrodeoxygenation with NiFeAlO_x, the relative content (RC) of phenolic compounds decreased significantly from 35.38% to 9.74%. Concurrently, the RC of alkanes surged from 8.52% to 42.18%, with monocyclic and dicyclic alkanes contributing 16.63% and 21.26%, respectively. After alkylation-hydrodeoxygenation of Zhaotong lignite thermal dissolution derived oil, phenolic compounds can be converted to monocyclic alkanes and bicyclic alkanes, with bicyclic alkanes dominating the content. These results indicate that phenols in thermal dissolution derived oil can be converted into dicyclic alkanes through alkylation-hydrogenation deoxygenation process, which provides a possibility for producing high-density liquid fuel from the soluble portions of lignite.