Regulating hydrogen spillover in Pt/M-ZrO2 catalysed reductive N-methylation of aniline under mild conditions.

Abstract

Amphoteric metal oxides, such as ZrO2, which possess both acidic and basic surface functionalities, are beneficial for multistep reactions that involve the activation of various molecules. In line with this, deciphering these active species in the support is crucial for designing efficient catalysts for the reductive N-methylation of aniline, which involves formaldehyde and H2 as the C1 source and reductant, respectively. Pt supported on monoclinic ZrO2 (Pt/M-ZrO2) is synthesised via a hydrothermal followed by wet impregnation approach for the N-methylation of aniline to N,N-dimethylaniline (NNDMA) via imine intermediates. Pt/M-ZrO2 exhibits an excellent catalytic activity with 75 times higher TOF than its counterpart, Pt supported on tetragonal ZrO2 (Pt/T-ZrO2) under identical reaction conditions, giving a nearly quantitative yield of NNDMA under mild conditions (30 ℃, 5 bar H2). NH3- and CO2-temperature programmed desorption (TPD) studies reveal that Pt/M-ZrO2 possesses 2.2 and 1.7 times higher densities of acidic and basic sites, respectively, compared to Pt/T-ZrO2. The coexistence of a balanced amount of these active species on the Pt/M-ZrO2 significantly contributes to the facile activation of H2 and C=N of imine intermediates, followed by hydrogenation to form NNDMA. The H2-diffuse reflectance Fourier transform infrared (H2-DRIFT), colour-changing test with WO3, and poisoning studies demonstrate that Pt0 sites facilitate dissociative H2 adsorption, whereas basic sites promote hydrogen spillover to the M-ZrO2 support surface, and Lewis acidic sites facilitate C=N bond activation and subsequent hydrogenation. This study highlights catalytic discrepancies in hydrogen spillover on the surface of Pt/M-ZrO2, due to the strong metal-support interaction facilitated by the amphoteric nature of the M-ZrO2 support. These findings highlight the crucial role of acidic-basic functionalities in the hydrogen spillover mechanism, providing valuable insights into the rational design of efficient catalysts, particularly for activating molecules, such as H2, during the reductive N-methylation reaction.