60th Annual Report on Research 2015

Prior to this awarded grant, we explored the use of two MOFs, (V) MIL-47 and (Ti) MIL-125, as heterogeneous catalysts for the oxidation of thiophenic compounds. We found that while the V-based MIL-47 displayed high activity, it was unstable under reaction conditions. The Ti-based MIL-125, however, although not as active as MIL-47, was stable even under harsh conditions. The lower activity of MIL-125 was attributed in part to its small pore size that could not accommodate bulky thiophenic complexes such as dibenzothiophene (DBT). Therefore, we proposed to synthesize a mesoporous analogue of MIL-125 (this ACS PRF grant). We accomplished this task through the use of a synthetic procedure (vapor-assisted crystallization, VAC, Figure 1) that was inspired by our previous experience with zeolite synthesis. In this method, a dried precursor powder containing MOF precursors and an ionic surfactant templating agent (cetyltrimethylammonium bromide) were subjected to VAC to facilitate the formation of hierarchically mesoporous/microporous MIL-125 similar to steam-assisted crystallization of zeolites.

Figure 1. Formation of hierarchically microporous and mesoporous MIL-125 using the vapor-assisted crystallization method developed in the Hicks Lab.

The newly synthesized mesoporous MIL-125 (meso-MIL-125) was fully characterized using a number of techniques. N₂ physisorption analysis was used to determine the meso-MIL-125 contained significant amounts of both microporosity and mesoporosity while XRD diffraction patterns showed the new material was composed of crystalline MOF. The meso-MIL-125 material was then tested as a catalyst in the oxidation of DBT by tert-butyl hydroperoxide (TBHP) and compared to the microporous analogue MIL-125 (micro-MIL-125). Catalysis data (Figure 2) was fit to pseudo-first-order kinetics and it was discovered the meso-MIL-125 exhibited a much faster reaction rate than its microporous analogue. Furthermore, normalization of reaction rate data on accessible surface area and thiophene adsorption studies allowed us to suggest the enhancement in reaction rate was due to greater accessibility of DBT to catalytically active sites through the incorporation of mesoporosity.

Figure 2. Fitting of experimental data to a pseudo-first-order rate model in the oxidation of DBT by TBHP using (blue circles, ●) micro-MIL-125 and (black squares, ■) meso-MIL-125 as catalysts.

We also explored the synthesis of novel MOFs for achieving the ultimate goal of creating new sulfur oxidation catalysts. Notably, there have been few reports that detailed the synthesis of new Ti-based MOFs except for functionalized MIL-125 analogues, to the best of our knowledge. Therefore, we have benchmarked a series of MOF-74 structures that can be constructed by the use of various metals (e.g., Zn, Co, Ni, and Mg), while providing Lewis acidic metal sites to potentially interact with both the sulfur compound and an organic oxidant. For this study, we have solvothermally synthesized new Zn-based metal-organic frameworks (Zn-MOFs) using two pyridine-based carboxylate linkers (pyridine (PDC) and bipyridine (BPDC)) to generate unique pore topologies (Figure 3). As expected, these Zn-MOFs (ZnMOF-PDC and ZnMOF-BPDC) provided coordinatively unsaturated Zn²⁺ sites with square pyramidal geometry, leading to an enhanced interaction with two reactant gases (CO₂ and acetic acid), as evidenced by Raman and in situ DRIFT spectra, respectively. We believe these MOF structures can be extended to create several iso-structural MOFs, wherein open metal sites that are active for sulfur oxidation (i.e., Ti, and V) can be incorporated. The synthesis of these promising sulfur oxidation catalysts is our on-going research. By exhibiting these new metal-ligand combinations for MOF construction we have laid experimental groundwork that we believe will lead to the creation of new sulfur oxidation catalysts.

Figure 3. Schematic representation of the synthesis of pyridine-based Zn-MOFs.

Of additional note, we have pyrolyzed the pyridinedicarboxylate-containing Zn-based metal-organic framework (ZnMOF-PDC) to form a nanoporous carbon incorporating N dopants (Figure 4). The resulting N-doped carbon can be potentially applicable as a metal-free catalyst for various liquid-phase oxidation reactions due to its Lewis basicity (e.g., oxidation of cyclohexane, benzyl alcohol, and styrene). Therefore, in this study, we have optimized the ZnMOF-PDC structure prior to pyrolysis to provide a large amount of Lewis basic N dopants accessible to the reactants after pyrolysis. The optimal ZnMOF-PDC was obtained by using N-heterocycle additives (i.e., pyridine and 4,4-bipyridine) to control the amount of coordinated DMF in the base ZnMOF-PDC structure, thereby increasing its thermal stability during pyrolysis. This aided in reducing the loss of the N source from the ZnMOF-PDC materials during the thermal transformation and generated porous carbons with a greater amount of accessible Lewis basic N dopants, as demonstrated by their enhanced abilities for capturing the reactant gas (i.e., CO₂). Through the results of this project, we were able to demonstrate enhanced control over the properties of MOF-templated materials. This will potentially allow us to better optimize the properties of our catalytic materials for the sulfur oxidation.

Figure 4. Schematic representation of the pyrolysis of Zn-MOFs for the formation of N-doped carbon.

Overall, this funded ACS PRF Doctoral New Investigator Grant has had a significant impact on the PIs research direction and research support.  The grant has provided the PI an opportunity to study new MOF materials as catalysts and adsorbents, and synthesis methods to create enhanced catalysts – both of which are areas of research that will remain priorities in the PIs group.  Thus far, three publications have been accepted, one is in review, and one is in preparation.  Furthermore, this grant has funded one graduate student fully who has since successfully defended their PhD and one graduate student partially. 

Publications Acknowledging ACS PRF to Date

1.     N.D. McNamara and J.C. Hicks*, Chelating Agent-Free Vapor-Assisted Crystallization Method to Synthesize Hierarchical Microporous/Mesoporous MIL-125 (Ti), ACS Appl. Mater. Interfaces 2015, 7, 5338-5346.

2.     J. Kim, A.G. Oliver, G.T. Neumann, and J.C. Hicks*, Zn-MOFs containing pyridine and bipyridine carboxylate organic linkers and open Zn²⁺ sites, Eur. J. Inorg. Chem., 2015, 18, 3011-3018.

3.     J. Kim, A.G. Oliver, and J.C. Hicks*, Enhanced CO₂ capture capacities and efficiencies with N-doped nanoporous carbons synthesized from solvent-modulated, pyridinedicarboxylate-containing Zn-MOFs, CrystEngComm, 2015. DOI: 10.1039/C5CE00828J.