Exploring the Correlation between Structure Properties and Methane Adsorption of Metal-Organic Frameworks

本論文利用理論模擬方法建構與計算實驗上現有38組金屬有機框架材料(MOFs)之密度(D)、體表面積(VSA)、質量表面積(GSA)、比孔洞體積(SPV)、孔隙率(VF)、孔洞尺寸(PS)等各項結構參數，並探討MOFs結構參數與65bar下甲烷氣體單位體積吸附量(cm3(STP)/cm3)之關聯性。 我們發現孔隙率、孔洞尺寸及質量表面積此三項結構參數的調控會影響甲烷吸附量，其中孔洞尺寸尤其重要，因為具有高吸附量之MOFs材料的孔洞尺寸都偏小，最佳的孔洞尺寸範圍落在5~13Å，而MOFs的材料孔洞尺寸的改變源於所使用配位分子的結構大小，含三反應官能基的配位分子在反應官能基方向以苯環或碳鏈延長會使得孔洞尺寸超過最佳範圍，所以此類MOFs較不具有設計上的發展性。然而含四反應官能基的配位分子加入1~2個苯環環長度的延長都仍可維持最佳孔洞尺寸範圍，或是在配位分子中引入氮環(嘧啶)可以增加配位分子的柔軟性，提高甲烷吸附的效果，且此類MOFs都可達到240 cm3(STP)/cm3左右的甲烷吸附量，使得其在設計與發展上相當具有潛力，有機會突破現有MOFs材料的最高甲烷吸附量。 最後藉由定量構效關係(Quantitative Structure−Property Relationship, QSPR)得到VF、DP、GSA與甲烷吸附量之關係式: ，根據此關係式，我們預測新型MOFs材料若能有效控制小尺寸的孔洞尺寸及搭配合適的質量表面積，並最大限度的提升孔隙率，則65bar下的甲烷吸附量有機會超過300 cm3(STP)/cm3，接近美國能源局制定的標準。In this study, we employ simulation methods to build and calculate density(D)、volumetric surface area(VSA)、gravimetric surface area(GSA)、specific pore volume (SPV)、void fraction(VF) and pore size(PS) of thirty-eight types of metal-organic frameworks (MOFs) and explore the correlation between these structural properties and methane volumetric adsorption capacity at 65bar and 298K in MOFs. We find that VF、PS and GSA mainly correlate methane adsorption capacity, especially of pore size. MOFs with high methane adsorption capacities have smaller pore size and the optimal pore size is in the range 5-13 Å.The geometry size of organic linker mainly affect MOFs’ pore size. We also find that expanding tritoptic carboxylate linkers with benzyl or alkyl chains will makes MOFs’ pore size beyond optimal range. Consequently, MOFs with tritoptic carboxylate linkers are hard to enhance methane volumetric adsorption capacity through modifying its organic linkers. However, expanding tetratopic carboxylate linkers with one to two benzyl or alkyl chains still maintains MOFs’ pore size in optimal range. In addition, introduction of pyrimidine groups into the tetratopic carboxylate linkers can increase the dynamic freedom of linkers and thus improve adsorption performance. This kinds of MOFs can reach methane volumetric adsorption capacities up to about 240 (cm3(STP)/cm3) and thus are promising candidate MOFs materials.We build multilinear regression models for predicting the methane adsorption capacity at 65bar and yield following reasonable model: According to this model, small pore size、suitable gravimetric surface area and high void fraction are necessary structural conditions for high methane volumetric adsorption capacity at 65bar in MOFs