Improving the conversion of petroleum coke to solid acid catalysts – effectiveness of hydrogen peroxide and impact of silica contamination

Abstract

Petroleum coke (petcoke) samples containing silica sand (SiO₂) in the concentrations of ∼50 and 7 wt% were functionalized with nitric acid and hydrogen peroxide to produce carbon-based solid acid catalysts by converting the inherent sulfur species in petcoke to sulfonic acid sites (–SO₃H). The effect of SiO₂ content and functionalization agent on the physical, surface, and acidic properties of the prepared materials were evaluated, and their catalytic activities were assessed in the esterification reaction of octanoic acid and methanol at 60 °C, yielding 17–54% of ester after 6 h. The functionalization with H₂O₂ is more environmentally friendly than with HNO₃ and produces more active catalysts (ester yields of 54% and 45%, respectively). The activities of the prepared materials were more dependent on having fewer oxygen-containing carbon groups (related to total acidity) on the surface than on a higher concentration of sulfonic acid sites. Thus, the improved activity of the catalyst prepared with H₂O₂ was due to a lower total acidity (∼1 mmol g⁻¹) than the catalyst prepared with HNO₃ (∼5 mmol g⁻¹). The stabilities of petcoke-derived catalysts were poor, and regeneration was not possible, which is a common problem for carbon-based solid acid catalysts. The petcoke with a higher content of SiO₂ resulted in better esterification performance. The silica sand promoted the reduction of particle size during the ball-milling process, leading to reduced diffusion limitations and the generation of silanol groups that are hydrogen-bond donors, helping in the esterification reaction.