Acid Properties of Nanocarbons and Their Application in Oxidative Dehydrogenation

Carbon is emerging as an important metal-free catalyst for multiple types of heterogeneous catalysis, including thermocatalysis, photocatalysis, and electrocatalysis. However, the study of mechanisms for carbon catalysis has been impeded at an early stage due to the lack of quantitative research, especially the intrinsic kinetics (e.g., intrinsic TOF). In many carbon-catalyzed reactions, the surface oxygenated groups were found to be the active sites. Recently, we have shown that these oxygenated groups could be identified and quantified via poisoning by small organic molecules; however, these small molecules were toxic. As most of the oxygenated groups are acidic groups, they could also be identified and quantified with respect to the acid properties. More importantly, the method based on acid properties is very green and environmentally benign, because only inorganic bases are added. In this work, the acid properties of carbon nanotubes (CNTs) treated by concentrated HNO₃ were thoroughly studied by mass titration and Boehm titration. The two titration methods were also compared to the conventional methods for acidity analysis including NH₃ pulse adsorption, NH₃-TPD, and FT-IR. Boehm titration was very effective to quantify the carboxylic acid, lactone, phenol, and carbonyl groups, and the findings were consistent with the results from XPS and NH₃ pulse adsorption. These CNTs were applied in the oxidative dehydrogenation (ODH) of ethylbenzene, and the activity of these catalysts exhibited a good linear dependence on the number of carbonyl groups. The value of TOF for the carbonyl group obtained from Boehm titration was 3.2 × 10^–4 s^–1 (245 °C, atmosphere pressure, 2.8 kPa ethylbenzene, 5.3 kPa O₂). For better understanding the acidity of nanocarbon, these CNTs were also applied in two acid-catalyzed reactions (Beckmann rearrangement and ring opening), and a good linear relationship between the conversion and the number of acidic sites was found