Progress in catalyst developments for CO<sub>2</sub> hydrogenation coupled with <italic>N</italic>-alkylation of arylamines and nitroarenes

Abstract

N -alkyl aromatic amines are widely used as important intermediates in the fields of petrochemical industry, rubber industry, dye preparation, drug research and development and fine chemical synthesis. The traditional synthesis of N -alkylarylamines has many problems, such as high price of raw materials, corrosion of reaction equipment and easy to cause environmental pollution. In recent years, great progress has been made in the research of catalysts for coupling CO2 hydrogenation with N -alkylation of arylamines and nitroarenes to prepare N -alkyl aromatic amines. The use of abundant greenhouse gas CO2 as an alternative C1 resource for N -alkylation reaction gives water as the only by-product, which makes the reaction greener and environmentally friendly. In 2013, Beydoun and co-workers first reported that a homogeneous ruthenium complex ([Ru(triphos)(tmm)]) in trifluoroimide (HNTf2) can effectively catalyze CO2 hydrogenation coupled with N -alkylation of secondary amines and primary amines, giving good product yields. Subsequently, heterogeneous catalytic CO2 hydrogenation coupled with N -alkylation using supported Pd, Rh, Pt, Au, and Cu catalysts and those with Zn and Ga as promoters have been extensively studied. Several catalytic systems showed high selectivity in preparation of N -alkylation products under relatively mild conditions. In addition, hydrogen silane and borohydride were used as strong reducing agents to replace H2, achieving high efficiency in N -alkylation of arylamines or nitroarenes, although there exist disadvantages of high toxicity and non-recyclability in these systems. It was found that the factors affecting the selectivity of N -alkylation products mainly include active metals, carrier materials, and reaction conditions (temperature, pressure, CO2/H2 ratio, and CO2/H2 feed sequence). It mainly manifested that: (1) Active metals with moderate reducing properties can avoid side reactions of aromatic ring hydrogenation; (2) introduction of basic sites in the carrier material can achieve effective adsorption of CO2 and aniline; (3) when the reaction raw material is aniline or nitrobenzene, compared with MA, the reaction requires an increase in temperature, H2/CO2 ratio and reaction time; and (4) a catalyst with high selectivity to the target product should perform well in CO2 hydrogenation and N -alkylation but control the deep reduction of products. Thus, to improve the activity and selectivity of known catalytic system and design a new catalyst, it is of importance to elucidate the reaction mechanism. For instance, with the controlled experiments, in-situ infrared spectroscopic studies and other characterization techniques, the results indicated that the essential intermediates may include HCO*, HCOO–, and HCOOH when CO2 hydrogenation coupled with N -alkylation of aniline. Considering the kinetics, activating CO2 molecule or increasing the nucleophilicity of nucleophiles C- or N- will contribute to the formation of C–N or C–C bonds. Further, the obtained experimental results showed that the recycle stability of catalysts for CO2 hydrogenation coupled with N -alkylation is mainly affected by carbon deposition and sintering of active sites. However, the deactivated catalysts can be readily regenerated by calcination or reduction. Using bimetal catalysts can markedly improve the catalyst performance and stability. At present, improving the catalyst performance and selectivity to N -alkylation products with mild reaction conditions become the issues to focus on. In addition, selective conversion of CO2 into other functionalized N -alkyl compounds remains to be further deepened and improved. Nevertheless, the selective conversion of CO2 to C2 compounds for coupling with N -alkylation of arylamines or nitroarenes is a challenging topic. We also notice that use of MOF (metal-organic framework) materials as catalysts for CO2 hydrogenation to synthesize methanol and ethanol has achieved significant progress. MOF materials and photo-thermal synergistic catalysis may be applied to coupl CO2 hydrogenation with N -alkylation of aromatics. This paper reviews the research progress of N -alkylation of arylamines and nitroarenes coupled with CO2 hydrogenation, with emphasis on homogeneous and heterogeneous catalysts for the alkylation of aniline, alkylaniline and nitrobenzene coupled with CO2 hydrogenation. The structure-performance relationship, influencing factors and catalytic mechanism of the catalysts are discussed. The development prospect of catalysts in this field is also prospected.