Effective Combination of the Metal Centers in MOF-Based Materials toward Sustainable Oxidation Catalysts

Abstract

A successful encapsulation of Keggin-type polyoxomolybdate (H₃[PMo₁₂O₄₀], PMo₁₂) into metal-organic framework (MOF) materials with an identical framework but distinct metal centers (ZIF-8 with Zn²⁺ and ZIF-67 with Co²⁺) was accomplished by a straightforward room-temperature procedure. The presence of Zn²⁺ in the composite material PMo₁₂@ZIF-8 instead of Co²⁺ in PMo₁₂@ZIF-67 caused a remarkable increase in the catalytic activity that achieved a total oxidative desulfurization of a multicomponent model diesel under moderate and friendly conditions (oxidant: H₂O₂ and solvent: ionic liquid, IL). Interestingly, the parent ZIF-8-based composite with the Keggin-type polyoxotungstate (H₃[PW₁₂O₄₀], PW₁₂), PW₁₂@ZIF-8, did not show the relevant catalytic activity. The ZIF-type supports present an appropriate framework to accommodate active polyoxometalates (POMs) into their cavities without leaching, but the nature of the metallic center from the POM and the metal present in the ZIF framework were vital for the catalytic performance of the composite materials.

Keywords: heterogeneous catalysis; nanocomposite material; oxidative desulfurization; polyoxometalates; zeolitic imidazolate frameworks.

Figure 1

Schematic representation of the in situ sustainable preparation of the PMo₁₂@ZIF composite materials.

Figure 2

FTIR spectra (a) and Powder XRD patterns (b) obtained for ZIF-8, ZIF-67, and POM@ZIF composite materials (PMo₁₂@ZIF-8, PW₁₂@ZIF-8, and PMo₁₂@ZIF-67).

Figure 3

SEM micrographs and corresponding EDS spectra recorded for ZIF-8 (above, left), PMo₁₂@ZIF-8 (above, right), PW₁₂@ZIF-8 (middle), ZIF-67 (below, left), and PMo₁₂@ZIF-67 (below, right).

Figure 4

N₂ adsorption–desorption isotherms at −196 °C of both MOFs and POM@MOF composite materials: (a) ZIF-8 and ZIF-8 based materials; (b) ZIF-67 and ZIF-67 based materials. Filled and unfilled symbols relate to adsorption and desorption processes, respectively.

Figure 5

(a) Catalytic behavior of homogeneous POMs, PMo₁₂ and PW₁₂, and supporting MOFs, ZIF-8 and ZIF-67. Assays were performed at 70 °C in a model fuel/[BMIM]PF₆ biphasic system with 3 μmol of POM or 15 mg of MOF. ODS reaction starts at 0 min with the addition of aq. H₂O₂, after a 10 min extraction step. (b) Desulfurization profile of a multicomponent model fuel using PMo₁₂@ZIF-8, PW₁₂@ZIF-8, and PMo₁₂@ZIF-67 catalysts, [BMIM]PF₆ extraction solvent, H₂O₂ oxidant, at 70 °C. (c) Recycling desulfurization process for ten consecutive cycles (2 h each reaction) using PMo₁₂@ZIF-8 catalyst.

Figure 6

Powder XRD patterns obtained for recovered materials after catalysis (AC): (a) ZIF-67(AC) and PMo₁₂@ZIF-67(AC); (b) ZIF-8(AC) and PW₁₂@ZIF-8(AC); (c) PMo₁₂@ZIF-8, PMo₁₂@ZIF-8(AC) and PMo₁₂@ZIF-8(AC3) (AC3 meaning after the 3rd catalytic cycle). (d) ³¹P MAS NMR spectra obtained for PMo₁₂@ZIF-8 and PMo₁₂@ZIF-8-AC.

Figure 6

Powder XRD patterns obtained for recovered materials after catalysis (AC): (a) ZIF-67(AC) and PMo₁₂@ZIF-67(AC); (b) ZIF-8(AC) and PW₁₂@ZIF-8(AC); (c) PMo₁₂@ZIF-8, PMo₁₂@ZIF-8(AC) and PMo₁₂@ZIF-8(AC3) (AC3 meaning after the 3rd catalytic cycle). (d) ³¹P MAS NMR spectra obtained for PMo₁₂@ZIF-8 and PMo₁₂@ZIF-8-AC.