Catalytic upgrading of coal pyrolysis volatiles by Ga-substituted mesoporous ZSM-5.

• Dual-templating synthesis of gallium-substituted mesoporous ZSM-5 (Ga-MZ5). • Ga-MZ5 exhibited superior performances with high aromatic-yield and effective phenol-reduction in CFP. • Dehydrogenation/cracking ratio decided by zeolite acidity. • Deoxygenation of phenols dominated by non-framework Ga x O y species. • Diffusion superiority contributed by hierarchical meso-/microporosity. Five MFI-type zeolite catalysts, which were aluminosilicate ZSM-5 (Z5), gallium-substituted silicalite-1 (Ga-S1), mesoporous ZSM-5 (MZ5), gallium-substituted mesoporous silicalite-1 (Ga-MS1) and gallium-substituted mesoporous ZSM-5 (Ga-MZ5), were hydrothermally synthesized with combined using of TPAOH and TPOAC as micropore and mesopore structure-directing agents respectively, followed by the application in the catalytic upgrading of volatiles from low-rank coal fast pyrolysis. The obtained results indicated that significant improvements with the yield of high value-added products of BTEXN (benzene, toluene, ethylbenzene, xylene and naphthalene) increased from 5.4 mg.g−1 to 11.9–26.4 mg.g−1, as well as the content of light fractions (<300 °C) in coal tars increased from 51% to 65–88% were realized by using the representative MFI-type zeolite catalysts in contrast to non-catalyst pyrolysis. Moreover, the possible mechanism for catalytic upgrading over the best-performing catalyst of gallium-substituted mesoporous ZSM-5 (Ga-MZ5) with the highest yield of aromatics and effective reduction of phenols was proposed on the basis of systematic comparison of structural features and catalytic performances of various MFI-type zeolite catalysts and fundamental study of the synergetic effects dominated by acidity, ratio of Brønsted/Lewis acid sites, intra-/extra-framework gallium species and hierarchical meso-/microporosity. [ABSTRACT FROM AUTHOR]