Theoretical study on the feasibility of intramolecular hydrogen transfer reaction of 5-hydroxymethylfurfural

Highlights

• Intramolecular hydrogen transfer reaction (IHTR) mechanism within 5-HMF is proposed.

• The feasibility of IHTR reaction on Cu, Pd and Cu₃Pd₂ was studied by DFT.

• The favorable pathways for IHTR reaction on different catalysts were determined.

• Cu₃Pd₂ is predicted to be the most active catalyst for IHTR reaction of 5-HMF.

Abstract

The intramolecular hydrogen transfer reaction (IHTR) of 5-hydroxymethylfurfural can potentially reach a 100% atomic economic efficiency, which is of great significance to the utilization of biomass raw materials. In this work, the feasibility of IHTR on Cu, Pd and Cu₃Pd₂ bimetallic catalysts was investigated by density functional theory (DFT) calculations. Eight intramolecular hydrogen-transfer pathways, consisting of four elementary reactions each, are proposed. The elementary reactions were categorized and discussed as follows: C-H bond cleavage, C₂(H)=C₃ hydrogenation, O-H bond cleavage and C₂=C₃(H) hydrogenation. The relevant adsorption, activation and reaction energies were calculated and compared. Results reveal that the C-H bond is not easy to break on Cu(111) . On the surface of Pd(111), both the O-H bond cleavage and C=C bond hydrogenation reactions require higher energy barriers to overcome. Cu₃Pd₂(111) demonstrates high activity for both hydrogenation and dehydrogenation reactions. The reaction steps of the lowest energy path on Cu₃Pd₂(111) involve C-H bond cleavage, C₂=C₃(H) hydrogenation, O-H bond cleavage and C₂(H)=C₃ hydrogenation. The rate-limiting step is the cleavage of the C-H bond, and its activation barrier is 0.89 eV. As a result, the Cu₃Pd₂(111) catalyst is predicted to be the most active catalyst for intramolecular hydrogen migration, while the Cu(111) surface is also conducive to the IHTR reaction with slightly higher energy barrier. The calculation results provide valuable theoretical basis for the optimization of experimental research.

Introduction

In the 21^st century, the traditional chemistry industry is facing challenges from the increasingly stringent requirements toward human-sustainable development. The egress to meet these challenges would be to develop and apply green-chemical techniques [1], [2], [3]. One of the twelve criteria of green chemistry is atom economy [4], which refers to the concept of maximizing the number of reactant atoms that are transformed into the target product. The atom economy of a chemical reaction is measured by the percentage of atoms that were converted; it was first proposed by Trost in 1991 [5]. The ideal atomic economy reaction would imply a hundred percent conversion of the atoms in the reactants into that of the products, no by-products nor waste are produced, and thus, zero waste discharge is achieved [6].

5-hydroxymethylfurfural (5-HMF) is an important platform chemical [7,8] and fine-chemical raw material [9], [10], [11], [12] which can be used to prepare a variety of products via oxidation, hydrogenation, or condensation reactions. For example, the hydroxymethyl branched-chain dehydrogenation of 5-HMF is often conducted to produce 2,5-furandiformaldehyde (DFF). For the traditional liquid-phase dehydrogenation reaction of alcohols, unsaturated hydrogen acceptors such as styrene [13], cyclohexene [14], acetone [15] are usually added. The problem is that, in the liquid-phase dehydrogenation reaction, the removed hydrogen from adding equimolar hydrogen acceptors is not effectively used [16,17]. Instead, if the dehydrogenation product DFF was used as the hydrogen acceptor, the problem of inefficient utilization of the hydrogen atoms can be safely and effectively addressed. The reaction we propose is called 5-HMF intramolecular hydrogen transfer, which is a reaction step in the green synthesis process of adiponitrile proposed by Wang et .al [18]_, which yields an atomic economic reaction of 100%.

The development on intramolecular hydrogen transfer reaction (IHTR) is as follows. Dai [19] discovered that during the synthesis of p-xylene from 4-methyl-3-cyclohexene-1-carbonylacetaldehyde (4-MCHCA), the intermediate 4-methylbenzaldehyde produced from the dehydrogenation of 4-MCHCA was immediately converted into p-xylene and water through in-situ hydrogenation without the addition of an external hydrogen source. In another study, Zaccheria et al. [20] investigated the dehydrogenation reaction of an alcohol to ketone. In the absence of a hydrogen acceptor, dihydrochalcone can be obtained directly from carveol suggesting that carveol act as both the hydrogen donor and acceptor. These reports demonstrate the possibility of intramolecular hydrogen transfer reaction. The IHTR of 5-HMF was first proposed by Wang's research group [1], yet its reaction mechanism has not been studied.

The intramolecular hydrogen transfer reaction of 5-HMF includes two processes: the dehydrogenation of hydroxymethyl, and the C=C hydrogenation of the furan ring, which would require a bifunctional catalyst. On one hand, Lin et al.[21], [22], [23], [24], [25], [26], [27], [28] have discovered that Cu-based catalysts are usually very active towards the dehydrogenation of alcohols into ketones. On the other hand, there are studies [29], [30], [31], [32] showed that Pd-based catalysts played a significant role in promoting the hydrogenation of furan ring. Therefore, a copper and palladium-based alloy can be conceived as a potential bifunctional catalyst for the intramolecular hydrogen transfer reaction of 5-HMF. It is reported by Son et al. that Cu₃Pd₂ (Cu₆Pd₄) is a good catalyst for furan ring hydrogenation reaction [33]. However, the research on this specific composition is scarce, especially for the theoretical calculation research [34]. Considering that the intramolecular hydrogen transfer reaction in this work involves the hydrogenation step of the furan ring, therefore, Cu₃Pd₂ bimetallic catalyst is selected in this study.

Herein, density functional theory (DFT) is used to study the feasibility of intramolecular hydrogen transfer process of 5-HMF on copper and palladium-based bifunctional catalysts. The proposed reaction mechanism of intramolecular hydrogen transfer in 5-HMF involves the displacement of two H atoms from the 5-HMF branch to the C=C of the furan ring near the hydroxymethyl branch. The reaction kinetics of the proposed Pathways were modelled on Cu (111), Pd (111), and Cu₃Pd₂ (111) and calculated based on four reaction types: O-H bond breakage, C₂(H)=C₃ hydrogenation, C-H bond breakage and C₂=C₃(H) hydrogenation. The comparison of the reaction energetics revealed that Cu₃Pd₂(111) was the most-suitable candidate surface for intramolecular hydrogen transfer. This study on atomic economy is conducive to the sustainable development of mankind.