Catalytic Transfer Hydrogenation of Furfural to Furfuryl Alcohol under Mild Conditions over Zr-MOFs: Exploring the Role of Metal Node Coordination and Modification

Abstract

The catalytic transfer hydrogenation (CTH) reaction is considered as a potential route for upgrading bio-based carbonyls to their corresponding alcohols. Herein, a series of Zr-based metal−organic frameworks (Zr-MOFs) containing various types of metal node to ligand coordinations were synthesized and tested for CTH of furfural (FUR) to furfuryl alcohol (FOL). It was found that metal node coordination plays a more important role than porosity in Zr-MOFs. MOF-808 (synthesized using a scaled-up approach to achieve a higher batch yield), with the lowest metal node to ligand coordination (coordination number 6), was found to be the most active catalyst among the various tested Zr-MOFs. Furthermore, M-MOF-808, modified by simple methanol activation (M), outperformed the pristine MOF-808 in CTH of FUR to FOL even at 30 °C in the presence of 2-propanol (IPA) as the hydrogen source. The simple modification of the metal node in the Zr-MOF changed the acid− base properties of the MOF-808 surface through the development of coordinatively unsaturated sites (CUS), hydroxyl and methoxy groups in the framework of the Zr-MOF, which probably help to facilitate the adsorption of FUR and IPA onto the metal node surfaces of the catalyst. To evaluate the versatility of methanol activation in CTH, further substrates, including other types of biomass and representative carbonyl compounds over M-MOF-808, were tested. To demonstrate heterogeneous catalysis, the catalyst was recycled for five consecutive cycles, with little loss after the first cycle, and was fully characterized to observe any changes in its structure. Mechanistic insights were provided by isotopically labeled 2-propanol-d 8 experiments, indicating FUR reduction through transfer hydrogenation. Finally, the reaction mechanism for CTH of FUR to FOL was proposed in detail using density functional theory (DFT) calculations over metal node modified model systems of a 6-connected Zr-MOF.

Chemoselective Transfer Hydrogenation of Flavoring Unsaturated Carbonyl Compounds over Zr and Hf‐based Metal–Organic Frameworks

“…Acid–base sites in the Zr‐MOFs were already proved as an important aspect of the CTH reactions. These sites in Zr‐MOFs are generated from the metal node, which possesses ZrOZr bonding and was well discussed in our previous work 28,29 . Therefore, acid and base sites were measured using NH 3 and CO 2 temperature‐programmed desorption (TPD), respectively (Supporting information Figure S1).…”

“…Consequently, Zr‐containing MOFs have recently been tested for CTH of various carbonyl compounds 25–27 . Our research group has also contributed to the successful transformation of various biomass‐derived carbonyl compounds using series of Zr‐based functional and metal node‐modified MOFs under mild reaction conditions 28,29 . Acid–base properties originated from metal nodes regarded as active sites for the CTH reaction using Zr‐MOFs.…”

Modified Metal–Organic Frameworks as Efficient Catalysts for Lignocellulosic Biomass Conversion

“…The increased number of active sites enhances the reactivity regardless of the surface area. In addition to the surface area and CN, the activation of metal nodes also influences the reactivity 26,30 . MOF‐808, which had the best catalytic activity, was immersed in methanol and activated at the boiling point of methanol.…”

“…FAL, 1.0 g (10.4 mmol); 2‐propanol, 25 g (416 mmol); catalyst, 0.1 g; temperature, 82 °C (reflux) (Reprinted with permission from ref. 30. Copyright 2020, American Chemical Society).…”

“…This includes the post‐treatment of MOFs, as well as defect engineering during synthesis of MOF. Crucially, the metal nodes are often used as Lewis acid sites for catalytic reactions, 27,28 and the reduction in the CN increases the possibility that the metal ion will bind to the reactants in the catalytic process 29–32 . In other words, the catalytic reaction can proceed faster when the metal centers in MOFs have lower CNs.…”