Highly Efficient Ultralow Pd Loading Supported on MAX Phases for Chemoselective Hydrogenation

Palladium is one of the most efficient metals for the hydrogenation of organic compounds. However, when molecules, such as nitroaromatics, with several reducible functionalities, are hydrogenated, Pd, like any other very active metal, such as nickel or platinum, often behaves unselectively. One strategy to render Pd more selective is to choose the proper support. Herein, we show that MAX phase powders of Ti 3 SiC 2, Ti 2 AlC, or Ti 3 AlC 2 can chemoselectively hydrogenate 4-nitrostyrene to 4-aminostyrene, with 100% selectivity, at around 3–4% conversion. To boost the latter, we loaded Ti 3 SiC 2 with 0.0005 wt % Pd and increased the conversion to 100% while maintaining the 4-AS selectivity at >90%. By optimizing the Pd loading, we were also able to increase the turnover frequency 100-fold relative to previous literature results. The identification of this highly efficient and chemoselective system has broad implications for the design of cost-effective, earth-abundant, nontoxic, metal catalysts, with ultralow noble metal loadings.