Ti3C2 MXene as heterogeneous catalyst for carbodiimides guanylation with over 102 h−1 turnover frequencies

A series of Ti₃C₂ samples has been prepared from Ti₃AlC₂ by NH₄F-HCl etching, followed by subsequent post-treatments, such as DMSO expansion, ultrasound exfoliation, thermal annealing at 500 °C, surface modification by ammonium persulfate, tetrabutylammonium hydroxide and hydrochloric acid. These Ti₃C₂ samples have been tested as heterogeneous catalysts for guanylation of symmetrical carbodiimides by aromatic and secondary aliphatic amines, observing the formation of the corresponding guanidines, accompanied by dialkylureas. The initial reaction rates between N,N’-diisopropylcarbodiimide and p-toluidine correlate linearly with the density of weak strength acid sites of the Ti₃C₂ sample measured by NH₃ thermo-programed desorption. This has led us to propose these weak acid sites as active centers of the reaction. Based on the population of weak acid sites, the estimated turnover numbers and frequencies were calculated to be 101 and 114 h⁻¹ for the most active Ti₃C₂ sample. These values rank Ti₃C₂ among the most active noble metal-free solid guanylation catalysts reported so far. Upon reuse, the Ti₃C₂ sample undergoes gradual deactivation, characterization of the four-times used Ti₃C₂ sample indicates that the crystallinity is preserved. Catalyst deactivation derives from deposition of the reaction products on the surface and/or from the increase of the oxygenated surface functional groups upon reuse. Deactivation can be partially reversed by thermal product desorption. DFT calculations indicate that the rate determining step in the reaction mechanism is the cycloaddition between the carbodiimide and the deprotonated toluidine anchored to the Ti₃C₂ surface. The present results illustrate the potential that MXenes have as solid catalysts for organic reactions, particularly Ti MXenes to promote reactions requiring amine activation, such as the present guanylation reaction.