Electrochemical hydrogenolysis of carbonyl groups in aldehydes and ketones using zinc cathodes

Abstract

Electrochemical cells and methods for the reduction of aldehydes and ketones via selective hydrogenolysis of their carbonyl bonds are provided. The electrochemical cells and methods use zinc-containing electrocatalytic cathodes that promote electrochemical hydrogenolysis of the carbonyl bond of the aldehyde or ketone over hydrogenation in an acidic electrolyte solution. As a result, the carbonyl-groups of the aldehydes and ketones are reduced into alkyl groups with high relative selectivities.

1 . A method for the electrochemical reduction of an aldehyde or a ketone in an electrochemical cell comprising:
 a cathode comprising zinc in a catholyte solution, the catholyte solution having a pH of 3 or less and comprising one or more carbonyl compounds selected from aryl-aldehydes, aryl-ketones, alkyl-aldehydes, alkyl-ketones, or a combination thereof, and an acid, wherein at least one of the one or more carbonyl compounds is not an aryl-aldehyde in which a carbonyl carbon is directly bonded to a furan ring;   an anode in an anolyte solution comprising a chemical species to be oxidized; and   an electrical circuit connecting the cathode and the anode;   the method comprising:   applying an electrical potential difference between the cathode and the anode that induces the electrochemical reduction of a carbonyl group in the one or more aryl-aldehydes, aryl-ketones, alkyl-aldehydes, or alkyl-ketones into an alkyl group via hydrogenolysis at the cathode, wherein the selectivity for the hydrogenolysis of the carbonyl group is higher than that of the hydrogenation of the carbonyl group, and also induces the electrochemical oxidation of the chemical species to be oxidized at the anode.   
     
     
         2 . The method of  claim 1 , wherein the catholyte solution comprises at least one of the aryl-aldehydes or at least one of the aryl-ketones. 
     
     
         3 . The method of  claim 2 , wherein the aryl-aldehyde, the aryl-ketone, or the combination thereof is derived from lignocellulosic biomass. 
     
     
         4 . The method of  claim 1 , wherein the catholyte solution is an aqueous solution. 
     
     
         5 . The method of  claim 4 , wherein the catholyte solution has a pH of two or less. 
     
     
         6 . The method of  claim 5 , wherein the catholyte solution has a pH of less than 1. 
     
     
         7 . The method of  claim 1 , wherein the catholyte solution is made by forming a first solution comprising the one or more carbonyl compounds and adding the acid to the first solution. 
     
     
         8 . The method of  claim 1 , wherein the catholyte solution comprises water and an organic co-solvent. 
     
     
         9 . The method of  claim 1 , wherein the catholyte solution is a non-aqueous solution. 
     
     
         10 . The method of  claim 9 , comprising acetonitrile as a non-aqueous solvent. 
     
     
         11 . The method of  claim 9 , wherein the acid has a pKa of less than 1. 
     
     
         12 . The method of  claim 1 , wherein the zinc of the cathode is present as metallic zinc (Zn 0 ). 
     
     
         13 . The method of  claim 1 , wherein the cathode comprises a zinc compound comprising Zn 2+  ions. 
     
     
         14 . (canceled) 
     
     
         15 . The method of  claim 1 , wherein the electrochemical reduction has a relative selectivity for the hydrogenolysis of the carbonyl group over hydrogenation of the carbonyl group of at least 80%. 
     
     
         16 . The method of claim  14 , wherein the electrochemical reduction has an absolute selectivity for the hydrogenolysis of the carbonyl group of at least 50%. 
     
     
         17 . The method of  claim 12 , wherein electrochemical reduction has a Faradaic efficiency for the hydrogenolysis of the carbonyl group of at least 20%. 
     
     
         18 . The method of  claim 17 , wherein the catholyte solution is an aqueous solution having a pH of 2 or less. 
     
     
         19 . The method of  claim 18 , wherein the electrochemical reduction has an absolute selectivity for the hydrogenolysis of the carbonyl group of at least 50%. 
     
     
         20 . The method of  claim 2 , wherein the at least one aryl-aldehyde or the at least one aryl-ketone comprises a phenyl ring directly bonded to a carbonyl carbon of the aryl-aldehyde or the aryl-ketone. 
     
     
         21 . The method of  claim 1 , wherein the catholyte solution further comprises water. 
     
     
         22 . The method of  claim 1 , one or more carbonyl compounds selected from aryl-aldehydes, aryl-ketones, alkyl-aldehydes, alkyl-ketones, or a combination thereof, and an acid, wherein at least one of the one or more carbonyl compounds is not an aryl-aldehyde in which a carbonyl carbon is directly bonded to a furan ring makes up at least 50 mol. % of aromatic compounds present in the catholyte solution prior to the application of the potential difference. 
     
     
         23 . The method of  claim 1 , one or more carbonyl compounds selected from aryl-aldehydes, aryl-ketones, alkyl-aldehydes, alkyl-ketones, or a combination thereof, and an acid, wherein at least one of the one or more carbonyl compounds is not an aryl-aldehyde in which a carbonyl carbon is directly bonded to a furan ring are not derived from lignocellulosic biomass. 
     
     
         24 . The method of  claim 23 , one or more carbonyl compounds selected from aryl-aldehydes, aryl-ketones, alkyl-aldehydes, alkyl-ketones, or a combination thereof, and an acid, wherein at least one of the one or more carbonyl compounds is not an aryl-aldehyde in which a carbonyl carbon is directly bonded to a furan ring comprise p-anisaldehyde, 4-acetamido benzaldehyde, 4-acetoxybenzaldehyde, p-tolualdehyde, benzaldehyde, acetophenone, 4-hydroxyphenylacetone, or 3-(3-(trifluoromethyl)phenyl)propanal.