Amino-functionalized carbon nanotubes as metal-free catalysts for N-formylation of amines with CO2

The N-formylation reaction of amines by CO2 hydrogenation is a promising method for converting CO2 into chemical products with industrial application value. However, most of the currently developed catalysts are metal-homogeneous catalysts, which contain metal pollution and are not easy to recycle. Thus, we modified carbon nanotubes with N-containing amino compounds and prepared a polyethylenimine (PEI) grafted carbon nanotubes metal-free catalyst, which was applied to the N-formylation reaction of CO2 and aniline. By adjusting the grafting concentration and the type of amino compound, the acid-base properties and surface defects of the catalyst were regulated. The CNT-0.5PEI catalyst had the optimal catalytic activity after 12 h of reaction at 80 degrees C, with a conversion rate of 95.41 % for aniline and a selectivity of 94.04 % for the target product N-formylaniline. It reveals that its catalytic performance can be comparable to the reported homogeneous catalysts. Our research found that increasing the surface defects and basic sites of carbon nanotubes was conducive to the enrichment and activation of CO2, thereby enhancing the catalytic activity. The experimental and calculation results indicate that the reaction has relatively low activation energy under the catalysis of CNT-0.5PEI, and the catalyst has the strongest adsorption capacity for CO2 and aniline reactants. In addition, the CNT-0.5PEI displays a wide range of substrate applicability and excellent cyclic stability. Therefore, cheap and environmentally friendly metal-free modified carbon nanotube catalyst is an ideal substitute for scarce and polluting metal homogeneous materials. It has important reference significance for the efficient conversion and utilization of CO2 catalyzed by metal-free catalyst.