Carbon Nanotube-Based Solid Sulfonic Acids as Catalysts for Production of Fatty Acid Methyl Ester via Transesterification and Esterification

A series of polymer-carbon nanotube composite materials (CNT-P-SO3H) were prepared by covalent grafting of multiwalled carbon nanotubes (CNTs) with sulfonic acid-functionalized polymers (P-SO3H) including poly(3-vinyl-1-sulfonic acid imidazolium chloride)-grafted multiwalled CNTs (CNT-PVSAIC), poly(4-vinyl-1-sulfonic acid pyridinium chloride) -grafted multiwalled CNTs (CNT-PVSAPC), and poly(4-styrenesulfonic acid) -grafted multiwalled CNTs (CNT-PSSA). Such a functionalization method provides a facile route to obtain various polyelectrolyte brushes on the surfaces of CNTs in order to improve the dispersibility and modulate the acidity of CNTs to selectively introduce functional groups and densely create active sites over CNTs for potential catalytic applications. Both CNT-PVSAIC and CNT-PVSAPC consist of cationic polyelectrolyte chains functionalized by sulfonic acid groups, whereas CNT-PSSA is composed of anionic polymer brushes grafted by sulfonic acid groups. The physicochemical properties of CNT-P-SO3H were analyzed by BET, TGA, XRD, FT-IR, XPS, Raman, and HRTEM techniques. The resulting CNT-P-SO3H materials exhibit excellent catalytic activity as CNT-based solid acids in liquid phase transesterification of triglycerides with methanol and esterification of oleic acid with methanol, which are typical model reactions for biodiesel production. The outstanding catalytic performance of the CNT-P-SO3H catalysts is attributed to the combination of the mesoporous structure together with a well extended P-SO3H coating over the outer surface of the CNTs, providing the formation of a dense but uniform surface distribution of active sites.