Catalytic conversion of asphaltenes to BTXN using metal-loaded modified HZSM-5

HZSM-5 zeolites with Si/Al ratios of 20, 35, 50 and 65 were prepared by the directing crystallization pro-cess of silicalite-1 seeds. The influence of Si/Al ratios on the production of benzene, toluene, xylene and naphthalene (BTXN) originated from asphaltenes catalytic pyrolysis was explored by adopting Py-GC/MS. Modified Z5-50 zeolites were prepared by various metal ions (Ni2+, Mo6+, Fe3+, and Co2+) with different loading rates (3% (mass), 5% (mass), 7% (mass), and 9% (mass)) and the physical and chemical properties of these zeolites were characterized by XRD, SEM, ICP-OES, XPS, NH3-TPD, FTIR, Py-IR and N2 adsorp-tion-desorption isotherm. In addition, they were employed to catalyze the conversion of asphaltenes pyrolysis production to BTXN using Py-GC/MS. Results show that the highest relative content of aromat-ics has been obtained over HZSM-5 with Si/Al ratio of 50 (Z5-50), reaching 61.87%. Besides, the loading of Ni, Mo, Fe, and Co on Z5-50 leads to an increase of acid strength and provides new active sites. The rel-ative content of BTXN increases by 3.17% over 3Ni-Z5, which may be ascribed to that Ni promoted the conversion of polycyclic aromatic hydrocarbons (PAHs) to monocyclic aromatics due to the cracking of aliphatic side chains of PAHs and the decrease of phenolic activation energy. While under the catalysis of 5Mo-Z5, the relative content of aromatics and BTXN augmented by 5.75% and 4.02%, respectively. In addition, the highest relative content of aromatics reaches 70.09% when the loading rate of Fe was 7% (mass), and the relative content of BTXN increases from 25.87% to 29.42%. The results demonstrate that the active sites provided by different metal species expressed diverse effects on BTXN. Although the Bronsted/Lewis acid ratios of HZSM-5 modified by metal decreased, the acid strength and the relative content of BTXN both increased, which illustrated that there is a synergistic catalysis with the Bronsted acid sites and Lewis acid sites provided by metal species. In general, the performance of the cat-alyst is affected by the pore structure, acidity and metal active sites. Moreover, the possible formation mechanism of BTXN derived from asphaltenes catalytic pyrolysis was proposed on the basis of structural features and catalytic performances of a series of zeolites.(c) 2021 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.

Automated summary

HZSM-5 zeolites with different Si/Al ratios were studied for their catalytic performance in the conversion of asphaltenes pyrolysis production to BTXN. The results showed that HZSM-5 with Si/Al ratio of 50 exhibited the highest relative content of aromatics. The addition of different metal ions (Ni2+, Mo6+, Fe3+, and Co2+) led to an increase in acid strength and the formation of new active sites on the Z5-50 zeolites. The study also revealed that the active sites provided by different metal species had diverse effects on the production of BTXN.