Ligand Fluorination to Optimize Preferential Oxidation of Carbon Monoxide by Water-Soluble Rhodium Porphyrins

Authors

Justin C. Biffinger, U.S. Naval Research Laboratory, Washington, DCFollow
ShriHarsha Uppaluri, University of Nebraska-Lincoln
Haoran Sun, University of South Dakota
Stephen G. DiMagno, University of Nebraska-LincolnFollow

Date of this Version

2011

Comments

Published in ACS CATALYSIS Volume: 1 Issue: 7 Special Issue: SI Pages: 764-771 DOI: 10.1021/cs2001187

This article is a U.S. government work, and is not subject to copyright in the United States.

Abstract

Catalytic, low temperature preferential oxidation (PROX) of carbon monoxide by aqueous [5,10,15,20-tetrakis- (4-sulfonatophenyl)-2,3,7,8,12,13,17,18-octafluoroporphyrinato]- rhodium(III) tetrasodium salt, (1[Rh(III)]) and [5,10,15, 20-tetrakis(3-sulfonato-2,6-difluorophenyl)-2,3,7,8,12,13, 17,18-octafluoroporphyrinato]rhodium(III) tetrasodium salt, (2[Rh(III)]) is reported. The PROX reaction occurs at ambient temperature in buffered (4 ≤ pH ≤ 13) aqueous solutions. Fluorination on the porphyrin periphery is shown to increase the CO PROX reaction rate, shift the metal centered redox potentials, and acidify ligated water molecules. Most importantly, β-fluorination increases the acidity of the rhodium hydride complex (pK_a = 2.2 ± 0.2 for 2[Rh-D]); the dramatically increased acidity of the Rh(III) hydride complex precludes proton reduction and hydrogen activation near neutral pH, thereby permitting oxidation of CO to be unaffected by the presence of H₂. This new fluorinated water-soluble rhodium porphyrin-based homogeneous catalyst systempermits preferential oxidation of carbonmonoxide in hydrogen gas streams at 308 K using dioxygen or a sacrificial electron acceptor (indigo carmine) as the terminal oxidant.