Abstract
The catalytic performance of pristine and modified CNTs for catalytic oxidation of butane/butene to corresponding alkenes has been studied in the present work, owing to its great potential in petro-industry and considerably growing interest in metal-free catalysis. For comparison, the catalytic activity using other materials for catalytic oxidation reaction, such as activated carbons, diamond-like carbon and metal oxides, has also been investigated under the same reaction conditions. A comparative investigation on the catalysts before and after reaction has been performed by using a series of joint experimental techniques in catalysis, for example, TEM, TPO, TPD, NH3-TPD, XPS and IR. The detailed knowledge on the chemical nature of surface functionalities has been achieved and, based on the analysis of activity-surface functionalities relationship, the reasonable reaction model has been proposed accordingly.
It has been found that the pristine CNTs display high activity but low selectivity for catalytic oxidation of butane to target products, butene and butadiene. The oxidation treatment is able to improve the catalytic performance of CNTs. A better catalytic performance can be further achieved by using phosphoric modified oxidized CNTs. In addition, molecule grafting as a new catalytic modification method has firstly been applied to modify CNTs and, significantly, the as-modified CNTs display an active and stable catalytic performance even after 40 hours reaction. The grafting modification can effectively immobilize small molecules, like 2-furoic acid and methyl cyclopentanone-2-carboxylate, to the carbon defects on the surface of CNTs. The surface investigation suggests that a variety of moieties remain on the surface of CNTs after reaction process, thus indicating that the selective and stable catalytic performance could be attributed to the existence of the grafted functional groups.
Two kinds of reaction pathways, i.e., the total oxidation and selective oxidation, participate in catalytic oxidation of butane. The former one can be related with the non-dissociative oxygen molecules, which are chemisorbed and activated on the surface of CNTs. The latter one should be correlated with quinone groups, generated via dissociative chemisorption of gaseous oxygen. The characterization supports the non-competitive adsorption model: hydrocarbons molecules are preferably adsorbed by the quinone groups and oxygen molecules are adsorbed on the π-electron-rich surface of CNTs, forming electrophilic O22- and O2- species. The following dissociation of O2 species could occur on the carbon defects, resulting in the generation of active sites for catalytic oxidation. The oxidation treatment significantly improves the catalytic performance by generating the oxygenated surface groups acting as active sites for catalytic oxidation of butane. However, the majority of oxygen species generated via oxidation do not involve in the catalytic oxidation of butane, which has been removed during the reaction process. The improvement in catalytic performance by using the phosphoric acid modified CNTs can be attributed to the inhibition of combustion of butane. The reasonable elementary steps proposed in present work include the adsorption of hydrocarbons and dehydrogenation on the quinone groups, the recombination of hydroxyl groups and following regeneration of quinone groups via dissociative chemisorption of gaseous oxygen. The carbon oxides form as byproducts from the combustion of hydrocarbons.
