posted on 2024-02-10, 00:34authored bySiya T. Hulushe, Gareth M. Watkins, Setshaba D. Khanye
Herein, we present a copper(II) metal–organic
framework, [Cu2(btec)(OH2)4]·2H2O (1) [(btec)4– = 1,2,4,5-benzenetetracarboxylate],
that undergoes
single-crystal-to-single-crystal transformations into two anhydrous
phases 2′ and 2″ with the
chemical formula [Cu2(btec)], triggered by two-step dehydration at 403 and 433 K, respectively.
After immersion in water for 3 days at room temperature, 2′ transformed into [Cu2(btec)(OH2)] (3), while both 2′ and 2″ took
1 week to revert to 1. Dynamic vapor sorption studies
validated water-induced reversible structural transformations at 70%
relative humidity (RH). According to single-crystal X-ray diffraction
(SC-XRD), the local coordination geometry of the Cu2+ ion
in 2′ changed from a saturated octahedron
to a coordinatively unsaturated square-based pyramid in 3, manifested by changes in color and dimensionality. From a topological
point of view, all of the scaffolds show a binodal (3,6)-connected kgd topology with the point symbol {43}2{46}. In addition, the materials were thoroughly characterized
using routine spectroscopic data and various analytical techniques.
The catalytic activity of the microporous materials in the liquid-phase
oxidation of styrene in acetonitrile, using 30% (wt) H2O2 as the oxidant, was investigated. The excellent performance
of the monohydrous phase 3 was shown to be superior to
the pristine framework and the anhydrous counterparts, as evidenced
by a good turnover number (TON) and turnover frequency (TOF) = 82.6
and 21.0 h–1, respectively. Within 4 h, the substrates
were catalytically oxidized to the desired products with up to 67%
conversion and 100% benzaldehyde selectivity. It is worth noting that
the accessible active metal sites and higher surface area enhanced
the catalytic properties of 3. Furthermore, the maintenance
of catalytic efficiency over five cycles and reusability are illustrated
and discussed in terms of the structural differences of the microporous
frameworks. Thus, a preliminary reaction mechanism for the selective
oxidation of styrene is proposed. This study not only provides a fascinating
example of MOF chromism achieved by thermal activation and rehydration
but also sheds some light on the relationship between pore-surface-
or metal-engineered sites in MOFs and their heterogeneous catalytic
performances.