A functionalized Hf(iv)–organic framework introducing an efficient, recyclable, and size-selective heterogeneous catalyst for MPV reduction

Abstract

UiO-66 (UiO = University of Oslo) metal–organic frameworks (MOFs) exhibit high thermal and chemical stability, large surface area, and defined pore shape and size. On the other hand, Meerwein–Ponndorf–Verley (MPV) reduction is a selective hydrogenation reaction of carbonyl group through hydrogen transfer between ketones (aldehydes) and alcohols. Herein, in this work, a Hf(IV)-based UiO-66 MOF called CSMCRI-KNC (CSMCRI-KNC = Central Salt and Marine Chemicals Research Institute-Krishnath College) was synthesized using a 2-((2-hydroxy-4-methoxy-benzyl)amino) terephthalic acid linker via the solvothermal procedure. The guest-free form of CSMCRI-KNC (called CSMCRI-KNC′) was successfully employed as a reusable heterogeneous catalyst for the hydrogen-transfer reaction between carbonyl compounds and alcohol (isopropanol). The catalyst showed a high yield, high conversion, broad substrate scope, and high stability on the MPV reduction reaction. Interestingly, the catalyst was size-selective too. The catalytic performance of CSMCRI-KNC′ was compared with its Zr analog, UiO-66-NH₂(Hf), UiO-66-NH₂(Zr), UiO-66(Hf), and UiO-66(Zr). Interestingly, our developed catalyst showed much better activity compared to the others. The systematized research discovered that the excellent performance of CSMCRI-KNC′ predominantly originated from the more positively charged Hf(IV) species of [Hf₆(OH)₄(O)₄]¹²⁺, which could boost the activation of the carbonyl group and instantaneously endorse the hydrogen-transfer process assisted by the available basic sites on CSMCRI-KNC′. The incorporation of the functional group also contributed to the excellent catalytic performance of CSMCRI-KNC′. Notably, the preparation of CSMCRI-KNC′ avoided the utilization of an expensive organic linker, and the prepared CSMCRI-KNC′ showed superior or similar catalytic activity than most reported Zr/Hf-containing catalysts, meaningfully empowering it to be a more practical option in the field of catalysis, and hydrogen transfer reactions.