Molecular-sieving Separation of p-Xylene with Metal-organic Frameworks

Metal-organic frameworks (MOFs)1),2) have recently attracted much attention because of their unique properties such as crystalline and microporous structure, high thermal stability up to around 400 °C, and easy modification of pore structure and chemical properties3). Generally, MOFs consist of a multi-valent metal cation or cluster at the corner position and dior tri-carboxylic acids as the bridging unit (linker)2). MOFs with desired physical and chemical properties may be designed and synthesized by combining various metal cations and linkers. MOFs with unique properties have high potential for various applications such as adsorbents, membranes, sensors, catalysts, and others. In particular, MOFs are expected to act as lightweight hydrogen storage materials4)~6) due to their extremely high specific surface areas reaching 6000 m2 g1. Recently, MOFs have been widely studied as catalysts7),8). MOFs have also been evaluated for the separation of aromatics9),10) and aliphatic hydrocarbons11),12). p-Xylene is an important raw material for terephthalic acid, the co-monomer for polyethylene terephthalate. Nowadays, p-xylene is produced by separation from the xylene mixture by adsorption with zeolite13) or crystallization14). In the former method, xylenes diffuse into the micropores of zeolite and separation of p-xylene is based on the adsorption stability in the micropores. Multiple adsorption columns are needed for effective separation, and the feed flow is changed by the operation of many valves systematically (pseudo-transfer bed separation). Consequently, the separation process is complicated and the equipments are costly. In contrast, separation by crystallization necessitates heating and cooling of the vessel and so the energy consumption is high. Therefore, a new process to separate p-xylene at lower cost is very desirable. MOFs have been investigated for the separation of xylenes15)~25). In most cases, xylenes diffused into the micropores and separation was governed by the diffusion rate based on the molecular size of xylenes or by interactions between the MOF surface and xylenes. The separation mechanism is similar to that of the conventional process using zeolite, so the required volume of adsorbent is large because all the xylene substrate [Regular Paper]