Importance of Crystalline and Microporous Structures of MOFs for Application to Petrochemical and Related Processes

Abstract

Metal-organic framework (MOF), also known as Porous Coordination Polymer (PCP), has crystalline and microporous properties similar to zeolites, which are the basis for various applications1),2). MOF was first reported in the early 1990’s3) and the number of studies on MOFs has increased dramatically. Zeolite was first recognized in 1756 and was widely studied in the early 20th century. Zeolites consist of MO4 tetrahedrons (M: typically Si or Al) and therefore are rigid, inorganic polymer materials4). Zeolites have the additional important specific property of ion-exchange. Consequently, zeolites have been used as solid acid catalysts and detergent additives as “softener.” In contrast, MOF typically consists of metals as the corner cation or cluster and organic “linkers.” MOFs contain organic components, so the thermal stability of MOFs is lower than that of zeolites, although some MOFs are stable up to around 400 °C5). The number of review papers on MOFs continues to increase yearly, with presently more than ca. 400 review papers per year. On the other hand, the number for zeolites reached the highest of 250 in 2008, and since has been decreasing. More than 20,000 types of MOFs have been reported6), whereas only 242 types of zeolites are known4). MOFs are crystalline and microporous, with very high specific surface areas up to 8000 m2 g1 by the BET method7). The pore size ranges from a few angstroms to more than 10 Å (1 Å=1010 m) depending mainly on the size of the linkers. Generally, a larger linker gives a larger pore size. MOFs have been evaluated for gas adsorption8) and separation9), separation of heavy metals10), sensors11), thermaland photo-catalysis12),13), optics14), drug delivery15), electro-chemistry16), biomedical and bioimaging17), and other functions. MOFs have also been used as the carbon source for thermaland electro-catalysts after thermal decomposition18),19). The microporous structure of MOFs is the most important characteristic for industrial uses. However, the cost of MOFs is also very important. The pore size can be controlled by adopting specific linkers usually with functional groups, but the cost tends to drastically increase and makes the use of MOFs difficult. Therefore, balancing the cost and performance becomes crucial. [Review Paper]