Pushing the Limits on Metal–Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for o-Xylene Isomerization and Disproportionation

Abstract

Acid-catalyzed skeletal C-C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal-organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), P NMR, N physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WO clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WO-ZrO, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal-organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization.

POM@MOF Hybrids: Synthesis and Applications

“…However, it is important to note here that a stabilizing effect of the POM on the MOF will only be obtained when the shape, size, and symmetry of the POM match the MOF host [135]. This stabilizing effect even allowed the application of POM@MOF catalysts in aggressive reactions, as was demonstrated in the very nice work of Hupp, Farha, and Notestein [133]. In this study, the Zr-based MOF, NU-1000, was loaded with H 3 PW 12 O 40 for its use in the strong acid-catalyzed reaction of o-xylene isomerization/disproportionation at 250 • C (see Figure 11).…”

“…Phosphotungstic acid encapsulated in NU-1000 for its use in the aggressive hydrocarbon isomerization reaction. Reprinted with permission from[133]. Copyright (2018), American Chemistry Society.…”

Single‐Site, Single‐Metal‐Atom, Heterogeneous Electrocatalyst: Metal–Organic‐Framework Supported Molybdenum Sulfide for Redox Mediator‐Assisted Hydrogen Evolution Reaction

“…The node of the parent framework presents labile protons in the form of ligated hydroxo and aqua groups . The protons are readily displaceable upon introducing reactive metal precursors (molecular complexes) either through a vapor‐phase (in an atomic layer deposition‐like process) or condensed‐phase reaction …”

Encapsulating Keggin‐H 3 PW 12 O 40 into UIO‐66(Zr) for manufacturing the biodiesel

“…Even early, G. Ferey et al have attempted to load the heteropoly acid into MIL-101 super-cage when they reported the MIL-101 [12]. Afterwards, the HPA@MOF composites have continuously being reported on the basis of the MOF type, such as KKUST-1 [13], MIL-101 [14], IRNENU [15], HKUST-1(MOF-199) [16,17], MIL-100 [18], MIL-53 [19], ZIF-8 [20,21], NU-1000 [22,23], UIO-66(Zr) [24,25] and so forth. Among these, UIO-66(Zr) and UIO-66(Zr) analogies which are formed by the Zr 6 (OH) 4 O 4 (CO 2 ) 12 cluster, have drawn more attentions due to the extraordinary chemical and thermal stability [26].…”