Preparation of Carbon-Based Solid Acid Catalyst from High-Sulfur Petroleum Coke with Nitric Acid and Ball Milling, and a Computational Evaluation of Inherent Sulfur Conversion Pathways

A series of petroleum coke (petcoke)-derived solid acid catalysts were prepared via nitric acid treatment with or without ball milling pretreatment. The inherent sulfur in petcoke was converted to sulfonic groups, which were active sites for the esterification of octanoic acid and methanol at 60 degrees C, with ester yields of 14-43%. More specifically, samples without ball milling treated at 120 degrees C for 3 h had a total acidity of 4.67 mmol/g, which was 1.6 times that of the samples treated at 80 degrees C, despite their -SO3H acidities being similar (similar to 0.08 mmol/g). The samples treated for 24 h had higher -SO3H (0.10 mmol/g) and total acidity (5.25 mmol/g) but not increased catalytic activity. Ball milling increased the defects and exposed aromatic hydrogen groups on petcoke, which facilitated further acid oxidation (0.12 mmol -SO3H/g for both materials and total acidity of 5.18 mmol/g and 5.01 mmol/g for BP-N-3/120 and BP-N-8/90, respectively) and an increased ester yield. DFT calculations were used to analyze the pathways of sulfonic acid group formation, and the reaction pathway with NO2 center dot was the most thermodynamically and kinetically favourable. The activities of the prepared catalysts were related to the number of -SO3H acid sites, the total acidity, and the oxygen content, with the latter two factors having a negative impact.

Automated summary

A series of solid acid catalysts derived from petroleum coke were prepared using nitric acid treatment with or without ball milling pretreatment. The catalysts exhibited high activity for the esterification of octanoic acid and methanol, with the activity dependent on the number of -SO3H acid sites, the total acidity, and the oxygen content.