Let’s talk about what most would consider a pretty easy metal-organic framework – MOF-5. Before we jump into this specific compound, it would probably benefit everyone why. Most of the research centered around these compounds came at the result of zeolite work, as porous materials with a lot of functionality (water purification, catalysts for cracking — but mostly in detergents). Unlike the adsorbing qualities of zeolites, MOFs are generally studied because of their ability to trap gas, such a H2 and CO2 — so these buggers can be used in gas separation, purification and sensing selective gases. The nice thing about such an experimental science, there is much to be learned — varying ligand, metal, and pore size to study their effectiveness in action.

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MOF-5 ZnO4 polyhedra with 1,4-benzenedicarboxylates

Typically, MOFs are synthesized by mass transfer techniques or traditional hydrothermal and solvothermal methods, which take days and even weeks for mass transfer reactions. Although recently these have become an attractive prospect for microwave irradiation — with the hope of faster reaction times and more uniform structures (the idea would be to increase the surface area and compound which would trap CO2). A current method (Chemical Engineering Journal 2010) for synthesis and evaluation: 2.93 g of zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O and 0.55 g of terephthalic acid (H2 BDC) in 50mL of NMP were mixed together and transferred to sealed Teflon pressure vessels followed by heating for 30min under irradiation. Two methods: 1) first included a ramp to 105C and holding the temperature, 2) ramping to 105C followed by a shallow ramp to 130C. As a comparison, MOF-5 was also prepared by conventional relux over a 4h period. The morphologies from method 1 and conventional reflux is shown in the SEM images below. Higher quality crystals were formed under microwave conditions.

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a) MW 105C 30 min b) relux 4 h

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Take a look through the XRD patterns to recognize impurities formed under reflux conditions and how MOF-5 is conditioned following the reaction methods (washing and solvent exchanges). What remains is — which one traps CO2 more efficiently?

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Surface area and CO2 absorption properties

The mw method produced MOF-5 with 12x the surface area and 3x the CO2 absorption power. Honestly for me, the most interesting part of the article is in the application. Several cycles of capture and release and recapture gives you an idea how good the thought process and application can be for breakthrough materials in chemistry.