Metal Oxide-Derived MOF-74 Polymer Composites through Pickering Emulsion-Templating: Interfacial Recrystallization, Hierarchical Architectures, and CO2 Capture Performances

Currently, metal–organic framework (MOF)–polymer composites are attracting great interest as a step forward in making MOFs a useful material for industrially relevant applications. However, most of the research is engaged with finding promising MOF/polymer pairs and less with the synthetic methods by which these materials are then combined, albeit hybridization has a significant impact on the properties of the new composite macrostructure. Thus, the focus of this work is on the innovative hybridization of MOFs and polymerized high internal phase emulsions (polyHIPEs), two classes of materials that exhibit porosity at different length scales. The main thrust is the in situ secondary recrystallization, i.e., growth of MOFs from metal oxides previously fixed in polyHIPEs by the Pickering HIPE-templating, and further structure-function study of composites through the CO2 capture behavior. The combination of Pickering HIPE polymerization and secondary recrystallization at the metal oxide–polymer interface proved advantageous, as MOF-74 isostructures based on different metal cations (M2+ = Mg, Co, or Zn) could be successfully shaped in the polyHIPEs’ macropores without affecting the properties of the individual components. The successful hybridization resulted in highly porous, co-continuous MOF-74–polyHIPE composite monoliths forming an architectural hierarchy with pronounced macro-microporosity, in which the MOF microporosity is almost completely accessible for gases, i.e., about 87% of the micropores, and the monoliths exhibit excellent mechanical stability. The well-structured porous architecture of the composites showed superior CO2 capture performance compared to the parent MOF-74 powders. Both adsorption and desorption kinetics are significantly faster for composites. Regeneration by temperature swing adsorption recovers about 88% of the total adsorption capacity of the composite, while it is lower for the parent MOF-74 powders (about 75%). Finally, the composites exhibit about 30% improvement in CO2 uptake under working conditions compared to the parent MOF-74 powders, and some of the composites are able to retain 99% of the original adsorption capacity after five adsorption/desorption cycles.


M 2+ -derived MOF-74 powders
Zn-MOF-74 1 : 0.5 g of Zn(NO 3 ) 2 .6H 2 O (1.6 mmol) was dissolved in 1 ml of deionized water. After 15 minutes of continuous stirring, a solution containing 0.1 g of DHBDC (0.5 mmol) dissolved in 20 ml of DMF was added. The reaction mixture was then transferred to the glass vial and heated at 100 °C for 20 h. After decanting the hot mother liquor and rinsing with DMF, the product was immersed in 30 ml of MeOH, which was replaced with fresh solvent every day, for three days. The final product was then immersed in MeOH until further research was performed.
Co-MOF-74 2 : 0.3 g of Co(ac) 2 .4H 2 O (1.5 mmol) was dissolved in 3.4 ml of deionized water. After 15 minutes of continuous stirring, 0.1 g of DHBDC (0.5 mmol) dissolved in 6.7 ml of THF was added. The reaction mixture was then transferred into a Teflon-lined autoclave and heated at 110 °C for 72 h. After cooling down to room temperature, the Co-MOF-74 was obtained by filtration. To extract solvent molecules from the pores, Co-MOF-74 was immersed in 30 mL of methanol, which was replaced daily with fresh solvent, for three days. The final product was then filtrated and dried at ambient conditions. Mg-MOF-74 3 : 0.03 g of DHBDC (0.2 mmol) was dissolved in a mixture of 5.5 ml of MeOH and 4.5 ml of DMF. After 10 minutes of continuous stirring, 0.1 g of Mg(NO 3 ) 2 .6H 2 O (0.4 mmol) was added to the solution. The reaction mixture was then transferred into a Teflon-lined autoclave and heated at 120 °C for 22 h. The reaction mixture was decanted and the final product was soaked three times with DMF and MeOH (1:1). The final product was stored in MeOH prior the further studies.

Metal oxide (MO)-derived MOF-74 powders
ZnO-MOF-74: 0.1 g of DHBDC (0.5 mmol) was dissolved in a mixture of 10 ml DMF, 0.5 ml iPrOH and 0.5 ml of deionized water. After 10 minutes of continuous stirring, 0.1 g of ZnO (1.0 mmol) was added to the solution. The reaction mixture was then transferred to the glass vial and heated at 100 °C for 20 h. The final product was then filtrated, washed with DMF and dried at ambient conditions. MgO-MOF-74: 0.03 g of DHBDC (0.2 mmol) was dissolved in a mixture of 9.0 ml of DMF, 0.6 ml of EtOH and 0.6 ml of deionized water. After 10 minutes of continuous stirring, 0.01 g of MgO (0.2 mmol) was added to the solution. The reaction mixture was then transferred into a Teflon-lined autoclave and heated at 120 °C for 26 h. The final product was then filtrated, washed with mixture of EtOH and DMF (1:1) and dried at ambient conditions.
Oleic acid (OA) coated nanoparticles. The surface of ZnO, Co 3 O 4 and MgO nanoparticles (NPs) was modified with oleic acid (OA). Nanoparticles were suspended in the solution of OA and ethanol (the mass fraction of OA was 15 wt% to NP) by sonication. Subsequently, the suspension was mixed with magnetic stirrer for 48 h at room temperature and dried in the oven at 50 °C overnight. The amount of OA attached onto the NPs' surface was determined by TGA.

Metal oxide-based PH composite monoliths:
Water-in-oil (W/O) HIPEs were used to obtain PDCPD-based PH composites. The DCPD monomer (1.30 g), Pluronic® L121 (0.065 g), toluene (50µL) and OA-coated NPs (between 10 and 30 wt% according to the DCPD) were placed in a 3-neck round-bottomed flask equipped with a mechanical stirrer and a dropping funnel. The mixture was stirred at 400 rpm for 5 min and upon continuous stirring at 25°C deionized water (5.5 mL) was added drop-wise over about 1 h. Afterwards, the initiator M2 (1.3 mg, 0.0007 mmol in respect to DCPD) dissolved in toluene (0.25 mL) was added and the emulsion was stirred for further 5 min. Subsequently, the emulsion was transferred to an appropriate mould (i.e. glass vials). The filled moulds were transferred into a preheated oven operating under air. Curing of the emulsions at 80 °C for 4 h resulted in the formation of white rigid monoliths in all cases. The specimens were purified by Soxhlet extraction with acetone for 24 h and subsequently dried in a desiccator under vacuum (10 mbar) until the weight was constant.

MOF-based PH composite monoliths:
MOF-based PH composite materials were then solvothermally crystallized from metal oxidebased PH composite precursors.

Zn-MOF 74-based PH composites:
For the recrystallization of ZnO-based PHs, 0.05 g DHBDC (0.3 mmol) was dissolved in the mixture of three solvents -5.0 ml of DMF, 0.3 ml of iPrOH and 0.3 ml of deionized water. After 15 minutes of continuous stirring, a piece of ZnObased PH monolith (0.3 g) was added to the mixture. The reaction mixture was then transferred into a Teflon-lined stainless-steel autoclave and heated at 110 °C for 48 h. After the solvothermal treatment, the recrystallized composite was rinsed with acetone and dried at ambient conditions.

Co-MOF 74-based PH composites:
For the recrystallization of Co 3 O 4 -based PH, 0.06 g of DHBDC (0.4 mmol) was dissolved in mixture of 10 ml of DMF and 10 ml of EtOH. After 15 minutes of continuous stirring, a piece of Co 3 O 4 -based PH monolith (0.3 g) was added to the mixture, which was transferred into a glass vial and heated at 150 °C for 120 h. After the solvothermal treatment, the recrystallized composite was rinsed with acetone and dried at ambient conditions.

Mg-MOF 74-based PH composites:
For the recrystallization of MgO-based PH, 0.03 g of DHBDC (0.2 mmol) was dissolved in a mixture of 4.5 ml of DMF and 5.5 ml of MeOH. After 15 minutes of continuous stirring, a piece of MgO-based PH monolith (0.3 g) was added to the mixture. The reaction mixture was then transferred into a Teflon-lined stainless-steel autoclave and heated at 150 °C for 48 h. After the solvothermal treatment, the recrystallized composite was rinsed with acetone and dried at ambient conditions. Figure S1. TGA of the MO after surface modification by OA