In Situ Generation of Brønsted Acidity in the Pd-I Bifunctional Catalysts for Selective Reductive Etherification of Carbonyl Compounds under Mild Conditions

Abstract

Selective synthesis of ethers from biomassderived carbonyl compounds is an important academic and industrial challenge. The existing processes based on strong acid or metallic catalysts cannot provide high selectivity to ethers due to the occurrence of side reactions. Hereby we propose a Pd-I bifunctional heterogeneous catalyst for the selective reductive etherification of aldehydes with alcohols. Extensive catalyst characterizations uncovered the presence of iodine species on the surface of Pd nanoparticles. Heterolytic dissociation of hydrogen on the I-Pd surface sites leads to the "in situ" generation of a Brønsted acid, which promotes the reaction toward the corresponding ethers with extremely high selectivity under very mild reaction conditions.

Dual Metal–Acid Pd-Br Catalyst for Selective Hydrodeoxygenation of 5-Hydroxymethylfurfural (HMF) to 2,5-Dimethylfuran at Ambient Temperature

“…Signal of oxidized Pd 2+ in PdO can be observed before and after modification by Br due to the partial oxidation of surface Pd atoms. Notably, an additional peak appears at much higher binding energy at 338.0 eV in the Pd 3d XPS after removing of H 2 [36] . This difference can be attributed to the higher However, the addition of H 2 in the FTIR cell results in the appearance of the Brønsted acidity.…”

Lignin Compounds to Monoaromatics: Selective Cleavage of C−O Bonds over a Brominated Ruthenium Catalyst

“…This reaction requires acid sites. The low selectivity to ether products indicates the absence of acid sites over both Ru/C and Br‐Ru/C catalysts [17a] . According to our previous reports, Pd‐I and Pd‐Br could generate in situ Brønsted acid sites by heterolytic dissociation of H 2 [17] .…”

Catalytic Reductive Alcohol Etherifications with Carbonyl‐Based Compounds or CO2 and Related Transformations for the Synthesis of Ether Derivatives

“…In 2019, two more Pd‐catalyzed strategies for the synthesis of furfuryl alkyl ethers were reported (Scheme 10). In the first of them, Ordomsky, Khodakov and co‐workers undertook a superficial modification of Pd/Al 2 O 3 catalyst by treatment with small amounts of Et−I (Scheme 10, see also Scheme 9M) [94] . The novel designed bifunctional heterogeneous Pd−I catalyst showed high efficiency in HMF and FF reductive etherifications using excess of aliphatic alcohols at mild conditions.…”

“…[93] Finally, in 2019 a Pd−I bifunctional catalyst was developed by Khodakov, Ordomsky and co‐workers for the mild and aromatic ring tolerant reductive etherification (Scheme 9 M, see also Scheme 10 ). [94] In this work, the authors defined that heterolytic dissociation of H 2 can occur on the surface of alumina‐supported Pd NPs modified with iodine species. This process in‐situ generates Brønsted acid sites that are fully responsible of the etherification.…”

“…In the first of them, Ordomsky, Khodakov and co‐workers undertook a superficial modification of Pd/Al 2 O 3 catalyst by treatment with small amounts of Et−I (Scheme 10 , see also Scheme 9 M). [94] The novel designed bifunctional heterogeneous Pd−I catalyst showed high efficiency in HMF and FF reductive etherifications using excess of aliphatic alcohols at mild conditions. Finally, Guan and co‐workers presented the Pd/SiO 2 ‐catalyzed reductive etherification between furfural and ethanol to give furfuryl ethyl ether.…”