Differential adsorption energies of ethane, ethylene, and acetylene on the metal-organic framework (MOF) UiO-66 (Zr) and UiO-66 (Zr) functionalized with –NH2 and -NO2 were measured using adsorption calorimetry with a Tian-Calvet calorimeter. Measurements were taken on dehydroxylated samples at both 293K and 195K at coverages ranging from 0.05 mmol/g to 1.2 mmol/g. In comparing the MOFs, the adsorption energies for UiO-66 and UiO-66-NH2 were similar for all three gases; however, the adsorption energies for UiO-66-NO2 were over 20% higher. The results suggest that –NO2 functionalization may have a significant impact on the confinement effects and Van der Waal’s interactions of the adsorbates with the MOF, which is consistent with the larger size of –NO2. In comparing the adsorbates, the adsorption energies from highest to lowest were acetylene &gt; ethane &gt; ethylene. The results suggest that kinetic diameter may be the dominating factor for adsorption energy differences between ethane and ethylene; however, molecular polarity is likely dominating the adsorption behavior of acetylene since acetylene has the smallest kinetic diameter of the three adsorbates. Finally, the adsorption energies at 293K steadily decreased with increasing coverage suggesting that the MOFs have multiple types of adsorption sites. Calorimetry measurements were taken at 195K from 0.05mmol/g to 7.8 mmol/g to more clearly define the partition function between the two types of adsorption sites. The low temperature calorimetry results more clearly distinguished the adsorption energy of the two types of adsorption sites and the inflection point in the adsorption energy versus coverage curve corresponds to the distribution of small and large pores in the UiO-66 framework. Very high heats at low coverages indicate a high energy adsorption site; We are exploring unsaturated metal sites as a possible explanation for this behavior.

Calorimetric adsorption study of light hydrocarbons on functionalized UiO-66 MOF

2019 Spring National Meeting

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