Restricting Polyoxometalate Movement Within Metal-Organic Frameworks to Assess the Role of Residual Water in Catalytic Thioether Oxidation Using These Dynamic Composites

Abstract

Comprised of high valent metal centers connected via oxide bonds, polyoxometalates (POMs) have a rich history in redox reactions. These discrete metal oxide clusters often are immobilized on heterogeneous supports to increase stability and allow for recyclability for catalytic reactions. One such support is the metal-organic framework (MOF), NU-1000. When POMs are installed into NU-1000, they are first sited in the mesoporous channel. Upon application of heat and removal of some physisorbed water, the POMs migrate to the microporous channel, where some active sites are blocked by the MOF linkers, resulting in lowered catalytic activity. To restrict this movement from the mesopore, we report here the use of two MOFs, naphthalene dicarboxylate-modified NU-1000 (NU-1000-NDC) and NU-1008, as supports for the Keggin-type polyoxometalate (POM), H 3 PW 12 O 40 . Upon the application of heat, these frameworks were found to hinder or prevent the movement of the POM between channels by blocking the aperture(s) connecting the mesoporous and microporous channels. The composite POM@MOF materials were used for the oxidation of a mustard gas simulant, 2-cholorethyl ethyl sulfide, using H 2 O 2 as the oxidant. The POM@MOF catalysts exhibit enhanced reactivity when compared to the POM or MOF alone, more than doubling the initial rates. The activity trend in PW 12 @NU-1000-NDC matched that in PW 12 @NU-1000, where the scCO 2 -activated sample poised the POM in the mesopores to give greater substrate accessibility and enhance the reaction rate compared to the heated sample where the POMs are poised in the micropores. Contrary to these observed trends, PW 12 @NU-1008 prohibits POM migration and performs superior when water has been removed at elevated temperatures as the active sites are more accessible in the mesoporous channel.

H5PV2Mo10O40 Polyoxometalate Encapsulated in NU-1000 Metal–Organic Framework for Aerobic Oxidation of a Mustard Gas Simulant

“…The composite material PV 2 Mo 10 @NU-1000 was synthesized in a similar manner to that for PW 12 @NU-1000. ,, Briefly, NU-1000 − was added to an aqueous solution of H 5 PV 2 Mo 10 O 40 and allowed to sit for 3 days. After, the solid was washed rigorously with water and ethanol before activation via supercritical CO 2 drying, yielding PV 2 Mo 10 @NU-1000-scCO 2 (Figure left; see Supporting Information).…”

Pyrene-based metal organic frameworks: from synthesis to applications

“…Therefore, the selectivity of the reaction is highly important. Two different POMs, H 3 PW 12 O 40 and H 5 PV 2 Mo 10 O 40 , have been encapsulated in NU-1000 (named as PW 12 @NU-1000 245,247,248 and PV 2 Mo 10 @NU1000, 244 respectively) for the oxidation of CEES using hydrogen peroxide (H 2 O 2 ) as the oxidant. Results showed that while 100% conversion of the parent agent is successfully done, the problem is the selectivity toward the desired non-toxic oxidation product.…”

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

“…In this section, we will discuss how MOFs have been enlisted to support POMs and to provide reactant accessibility. [170][171][172][173][174][175] Steric encapsulation of POMs relies on size matching between the MOF cavity and POM, where the MOF pore is big enough to accommodate a POM, but the apertures/windows are small enough to prevent the POM from leaving. 176 To this end, Mukhopadhyay et al 177 prepared a composite by growing ZIF-8 around the pre-formed POM (Fig.…”

“…23). 170,172 Siting in the hexagonal mesopore can be considered a kinetically determined location, while the trigonal micropore is the thermodynamically favoured location. Interconnecting the meso-and micro-pores are smaller pores that can distort sufficiently to permit the observed POM migration.…”