UiO系列結構是鋯金屬有機框架結構中較為新穎的一類材料，具有優異的熱穩定性、化學穩定性、高機械性質以及高比表面積等特性，使其成為氣體貯存以及氣體分離研究之熱門材料。UiO-66是UiO系列中的一員，其結構以鋯金屬離子做為中心，搭配對苯二甲酸(benzene-1,4-dicarboxylic acid)作為有機配位基。研究顯示，於UiO-66製程中加入酸性改質劑(acid-modulator)可以誘導其晶體結構中鍵連缺陷(missing-linker defect)的產生，此結構性缺陷可以增加材料的孔洞性(porosity)以及自由體積(free volume)，進而提升其氣體吸附性質。 本研究使用蒙地卡羅法(Monte Carlo simulations)討論完美結晶以及含鍵連缺陷之UiO-66結構的二氧化碳氣體吸附機制。結果顯示，於高壓下，材料中的自由體積為影響氣體吸附性質之主因，然而低壓下，氣體吸附行為卻是由等溫吸附熱(isosteric heat)所主導，使得高壓下完美結晶之UiO-66擁有最低之二氧化碳吸附量，然而在低壓下，卻有出最好之吸附表現。此外，藉由密度泛函理論(density functional theory)計算結構對於單一氣體分子之吸附能(adsorption energy)，用以輔助解釋低壓下之氣體吸附行為。實驗上，我們以甲酸(formic acid)作為改質劑，合成不同缺陷程度的改質樣品，並以熱重分析(thermogravimetric analysis)量化樣品中的缺陷，二氧化碳吸附之實驗結果與模擬計算上有相同的趨勢，為此我們確信本研究以模擬作為研究手法對氣體吸附機制的解釋。此研究之特色在於結合實驗與模擬之手法，系統性地探討鋯金屬有機框架結構對於二氧化碳氣體吸附之行為與機制。 ;The UiO family, a series of relatively new zirconium-based MOF, has attracted much attention due to its high thermal, chemical, and mechanical stability. Coupled with the high surface area, these features make it a suitable material for gas storage and separation. UiO-66, one such material with benzene-1,4-dicarboxylic acid (BDC) as its linker, possesses good crystallinity. Recent studies have shown that the missing-linker defect can be introduced in the crystal lattice by use of modulators. Such structural disorder is believed to enhance the porosity and free volume so as to improve gas adsorption performance. In this work, Monte Carlo simulations and density functional theory (DFT) are applied to explain adsorption behavior of ideal and defect-containing UiO-66. Under high pressure, the accessible free volume is the main factor of adsorption behavior in the framework, while the isosteric heat (or adsorption energy) plays the dominant role at low pressure. Because of the different mechanism of gas adsorption under different pressure, defect-free UiO-66 shows the lowest CO2 adsorption capacity under high pressure but the highest uptake under low pressure. The adsorption energy was further quantified by DFT in the single gas adsorption condition as an aid to explain the adsorption behavior under low pressure. In our experimental works, different defect-level samples had been tuned by varying the concentration of formic acid (FA) in the synthesis process and quantified by thermogravimetric analysis (TGA). Same features of the CO2 adsorption mechanism were found, confirming the influence of adsorption energy and capacity seen in simulations. This combined study gives us a systematic discussion in understanding gas adsorption behavior in zirconium-based MOFs.
