Catalytic Transfer Hydrogenation of Furfural to Furfuryl
Alcohol under Mild Conditions over Zr-MOFs: Exploring the Role of
Metal Node Coordination and Modification
posted on 2020-03-06, 14:37authored byAnil H. Valekar, Minhui Lee, Ji Woong Yoon, Jaesung Kwak, Do-Young Hong, Kyung-Ryul Oh, Ga-Young Cha, Young-Uk Kwon, Jaehoon Jung, Jong-San Chang, Young Kyu Hwang
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-d8 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.