Directing the Cleavage of Ester C−O Bonds by Controlling the Hydrogen Availability on the Surface of Coprecipitated Pd/Fe₃O₄

Abstract

The ready availability of esters from natural sources makes them promising substrates for the sustainable production of key intermediates of the chemical industry. In this context, methods to selectively cleave the alkoxy or ester C−O bond to obtain either alkane+acid or alcohol+aldehyde were investigated. Using Pd/Fe3O4(r) [(r)=reduced] as the catalyst of choice and either 2‐propanol—as indirect H source—or molecular hydrogen—as ample H source—the hydrogen coverage on the surface of the catalyst was controlled. This enabled selective cleavage of either the alkoxy C−O bond or the ester C−O bond in aromatic esters as shown for the hydrogenolysis of 2‐phenylethylacetate as model substrate. Noteworthy, hydrogenation of the aromatic ring was not observed. Thus, an exceptionally high chemoselectivity of Pd/Fe3O4(r) to hydrogenolysis of the ester group is combined with the option to readily direct the regioselectivity.

Upgrading Lignocellulosic Biomasses: Hydrogenolysis of Platform Derived Molecules Promoted by Heterogeneous Pd-Fe Catalysts

“…The conversion growths by increasing the reaction temperature and, at 240 • C, BPE is fully converted (100% conversion) into phenol and toluene as the only reaction products (100% aromatic yield). The low tendency of the co-precipitated Pd/Fe 3 O 4 catalyst to hydrogenate the aromatic ring was previously observed [270] and confirmed with crosscheck experiments with phenol and toluene.…”

Catalytic Transfer Hydrogenolysis as an Effective Tool for the Reductive Upgrading of Cellulose, Hemicellulose, Lignin, and Their Derived Molecules

“…In the case of benzyl phenyl ether (BPE), authors proposed that the CTH process is very sensitive to the steric hindrance of the involved H-donor system, postulating a reaction mechanism in which the H-transfer from the alcoholic solvent and the C-O bond cleavage occur in a single step (Scheme 7). The same authors previously presented the bimetallic Pd/Fe3O4 system as a very efficient catalyst in the cleavage of the C-O bond in aromatic ethers and esters for the production of arene derivatives in presence of 2-propanol as a solvent/H-donor [126,127]. In the case of benzyl phenyl ether (BPE), authors proposed that the CTH process is very sensitive to the steric hindrance of the involved Hdonor system, postulating a reaction mechanism in which the H-transfer from the alcoholic solvent and the C-O bond cleavage occur in a single step (Scheme 7).…”

Pd/Fe3O4 Nanofibers for the Catalytic Conversion of Lignin-Derived Benzyl Phenyl Ether under Transfer Hydrogenolysis Conditions

“…-The direct co-electrospinning of the two solutions containing the metal solutions of the corresponding inorganic Pd 2+ and Fe 3+ precursors followed by calcination and H2 reduction (Pd/Fe3O4[cnf] catalyst). This type of heterogeneous Pd/Fe3O4 catalyst has been successfully used in the cleavage of C-O and C-C bonds of lignocellulosic derived molecules under catalytic transfer hydrogenolysis (CTH) conditions [19][20][21]. In CTH reactions, the use of an indirect H source such as 2-propanol, ethanol, methanol, or formic acid allows the lysis of carbon-carbon or carbon-heteroatom bonds in lignocellulosic biomasses and in their derived model molecules, thus lowering their oxygen content [22][23][24].…”