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 method followed by wet impregnation for the N-methylation of aniline to N,N-dimethylaniline (NNDMA) via imine intermediates. Pt/M-ZrO₂ exhibits excellent catalytic activity with a TOF 156 times higher than that of its counterpart, Pt supported on tetragonal ZrO₂ (Pt/T-ZrO₂) under identical reaction conditions, affording a nearly quantitative yield of NNDMA under mild conditions (30 °C, 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 Pt/M-ZrO₂ significantly contributes to the facile activation of H₂ and CN of imine intermediates, followed by hydrogenation to form NNDMA. H₂-diffuse reflectance Fourier transform infrared (H₂-DRIFT) measurements, colour-changing tests 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 (LAS) facilitate CN 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.