Carbon monoxide-driven reductive organic transformations and precious metals recycling
Herein we present the efforts made toward employing the homogeneously-catalyzed Water-Gas Shift Reaction (WGSR) to drive essential reductive chemical transformations, and selectively extract rhodium from solid waste. The generation of rhodium hydride from the reaction of rhodium carbonyl with water has been achieved at temperatures as low as room temperature when the reaction was performed in a solution of simple tertiary amines in polar solvents. We investigated the mechanism by which this reaction proceeds and studied the effect of the reaction parameters on the ability of rhodium to deliver its hydride to different reducible functional groups e.g. activated alkenes, aldehydes, and ketones. We demonstrated the ability of the WGSR to drive the reductive alkylation of several classes of activated methylene compounds at room temperature. Under catalysis by rhodium trichloride (2–3 mol %), carbon monoxide (10 bar), water (2–50 equiv), and tertiary methyl /ethyl amines (2.5–7 equiv), the scope has been successfully expanded to cover a wide range of alkylating agents, including aliphatic and aromatic aldehydes, as well as cyclic ketones, in moderate to high yields. This method is comparable to, and for certain aspects, surpasses the established reductive alkylation protocols. The reductive amination of aldehydes has been demonstrated to be feasible under the same conditions with the exception of furfurals which undergo a newly-identified, reductive Piancatelli rearrangement to yield 2-enoneamines. A novel Pd/Rh dual‐metallic cooperative catalytic process has been developed to effect the reductive carbonylation of aryl halides in moderate to good yield. In this reaction, water is the hydride source, and CO serves both as the carbonyl source and the terminal reductant through the water–gas shift reaction. The catalytic generation of the Rh hydride allows for the selective formation of highly hindered aryl aldehydes that are inaccessible through previously reported reductive carbonylation protocols. Moreover, aldehydes with deuterated formyl groups can be efficiently and selectively synthesized using D2O as a cost‐effective deuterium source without the need for presynthesizing the aldehyde. Addition of an electrophile e.g. allyl acetate to the reaction resulted in the formation of aryl-vinyl ketones through the WGSR-driven reductive carbonylative coupling. We developed a mild and selective method for rhodium recovery that relies on the use of carbon monoxide to extract rhodium nanoparticles on various supports in polar solvents. Unlike the traditional recycling technologies, this method operates at low temperature and does not require strong acids. Moreover, the CO-induced leaching is complimentary to leaching by acids in terms of selectivity toward rhodium versus other precious metals and results in metal recovery in the form of reduced metallic clusters. The method performs best on freshly reduced surfaces and can be promoted by the addition of tertiary amines. Besides CO gas, formic acid can also be used as a leachant by decomposition to produce CO by Rh catalysis. The concept of the CO-induced leaching could be applied to the extraction of rhodium from catalytic convertors and nuclear waste or utilized to modify rhodium nanoparticle size and composition
Water-Gas Shift, rhodium carbonyl hydride, reductive carbonylation, alkylation, amination, rhodium recycling.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign