Mononuclear Zeolite-Supported Iridium: Kinetic, Spectroscopic, Electron Microscopic, and Size-Selective Poisoning Evidence for an Atomically Dispersed True Catalyst at 22 °C

Abstract

This work addresses the question of what is the true catalyst when beginning with a site-isolated, atomically dispersed precatalyst for the prototype catalytic reaction of cyclohexene hydrogenation in the presence of cyclohexane solvent: is the atomically dispersed nature of the zeolite-supported, [Ir­(C₂H₄)₂]/zeolite Y precatalyst retained, or are possible alternatives including Ir₄ subnanometer clusters or larger, Ir(0)_<i>n</i>, nanoparticles the actual catalyst? Herein we report the (a) kinetics of the reaction; (b) physical characterizations of the used catalyst, including extended X-ray absorption fine structure spectra plus images obtained by high-angle annular dark-field scanning transmission electron microscopy, demonstrating the mononuclearity and site-isolation of the catalyst; and the (c) results of poisoning experiments, including those with the size-selective poisons P­(C₆H₁₁)₃ and P­(OCH₃)₃ determining the location of the catalyst in the zeolite pores. Also reported are quantitative poisoning experiments showing that each added P­(OCH₃)₃ molecule poisons one catalytic site, confirming the single-metal-atom nature of the catalyst and the lack of leaching of catalyst into the reactant solution. The results (i) provide strong evidence that the use of a site-isolated [Ir­(C₂H₄)₂]/zeolite Y precatalyst allows a site-isolated [Ir₁]/zeolite Y hydrogenation catalyst to be retained even when in contact with solution, at least at 22 °C; (ii) allow a comparison of the solid–solution catalyst system with the equivalent one used in the solid–gas ethylene hydrogenation reaction at room temperature; and (iii) illustrate a methodology by which multiple, complementary physical methods, combined with kinetic, size-selective poisoning, and quantitative kinetic poisoning experiments, help to identify the catalyst. The results, to our knowledge, are the first identifying an atomically dispersed, supported transition-metal species as the catalyst of a reaction taking place in contact with solution