An Efficient Synthesis Strategy for Metal-Organic Frameworks: Dry-Gel Synthesis of MOF-74 Framework with High Yield and Improved Performance

Vapor-assisted dry-gel synthesis of the metal-organic framework-74 (MOF-74) structure, specifically Ni-MOF-74 produced from synthetic precursors using an organic-water hybrid solvent system, showed a very high yield (>90% with respect to 2,5-dihydroxyterepthalic acid) and enhanced performance. The Ni-MOF-74 obtained showed improved sorption characteristics towards CO2 and the refrigerant fluorocarbon dichlorodifluoromethane. Unlike conventional synthesis, which takes 72 hours using the tetrahydrofuran-water system, this kinetic study showed that Ni-MOF-74 forms within 12 hours under dry-gel conditions with similar performance characteristics, and exhibits its best performance characteristics even after 24 hours of heating. In the dry-gel conversion method, the physical separation of the solvent and precursor mixture allows for recycling of the solvent. We demonstrated efficient solvent recycling (up to three times) that resulted in significant cost benefits. The scaled-up manufacturing cost of Ni-MOF-74 synthesized via our dry-gel method is 45% of conventional synthesis cost. Thus, for bulk production of the MOFs, the proposed vapor-assisted, dry-gel method is efficient, simple, and inexpensive when compared to the conventional synthesis method.

The mixture was placed in a holed polymer mesh pouch made from Fluorinated Ethylene Propylene polymer (FEP) and it was placed in a Teflon-lined stainless steel autoclave. 10 ml of solvent mixture of THF: H 2 O (1:1) or DMF: EtOH: H 2 O, was put at the bottom of the reactor.
The entire assembly was heated at 110 °C for different time span (72h, 48h, 24h and 12h) for THF-water system while 24h heating for DMF-ethanol-water system respectively. After the reaction, compound was collected and washed thoroughly with THF (2 -3 times) to remove unreacted starting material. The solvent was collected as colorless liquid.

Conventional solvothermal synthesis of Ni-MOF-74 (Ni-MOF-74(CS))
0.374 g (1.5 mmol ) of nickel(II) acetate tetrahydrate and 0.148 g (0.75 mmol) of H 2 DHTA (2,5dihydroxyterepthalic acid were dissolved in 10 mL of a mixed solvent (THF:H2O = 1:1) and the mixture was placed in a Teflon-lined stainless steel autoclave. The entire assembly was heated at 110 °C for 72h. After the reaction, compound was collected and soaked in methanol for 3 days with replacing fresh methanol every 24h.
10 ml of solvent mixture of THF: H 2 O (1:1) was put at the bottom of the reactor. The entire assembly was heated at 110 °C for 72h. After the reaction, compound was collected and washed thoroughly with THF (2 -3 times) to remove unreacted starting material. The solvent was collected as colorless liquid.
Section II: Sorption studies.

Gas adsorption studies (CO 2 and R12)
For BET surface area measurements, the nitrogen adsorption and desorption isotherms were measured in a Quantachrome instrument at 77K using liquid nitrogen. Before the adsorption experiment, the MOF samples were activated at 200 °C for 12h under vacuum. The CO 2 sorption experiments were also performed using the Quantachrome instrument. The fluorocarbon (R12) adsorption, desorption measurements were performed using Intelligence Gravimetric Analyzer (IGA) instrument. Prior to measuring the sorption studies, the sample was placed in a container of the IGA chamber and the weight of the sample was recorded before activation. The temperature of the furnace was increased up to 200°C under vacuum at a rate of 5°C/min to remove the trapped solvent molecules. The sample was cooled to RT, its dry mass was set, and the experimental temperature 25°C was maintained by the IGA water bath. The static mode of the IGA was used to measure the sorption studies. The pressure points were set beforehand using the IGA software. The pressure was maintained at the set point by active computer control of the inlet/outlet valves throughout the duration of the experiment. Weight increases resulting from adsorption at each pressure step were plotted against the pressure.           Here we introduce a simplified cost analysis for the Ni-MOF-74 synthesis which elucidates the cost advantage of DGC method. The solvent mixture can be reused for subsequent DGC runs, by switching the pouch bag with fresh precursor mixture. We believe that integrity of the solvent mixture will be retained up to 3 runs, hence we cost compare the DGC method with conventional solvothermal synthesis with 3 cycles of the synthesis. Table S1 illustrates the scaled up cost from the cost of the materials used in this run using DGC method while Table S2 using conventional synthesis. Prices of the materials used in this context are bulk priced from the commercial vendors. The quantities of the reactants and solvents are based on the Dietzel procedure. 1 Table S1. Cost of Ni-MOF-74 synthesis using dry-gel method.