High Removal Efficiency and Regeneration Property of Formaldehyde Capture by Ti 4+ -based Porous Coordination Polymer Yasuko Mito-oka, 1 Yuki Sawada, 1 Tadao Masumori, 1 Satoshi Horike, 2 Hiroshi Kitagawa, 3,4 and Susumu Kitagawa* 2,4 1 Research Center, Toyobo Co., Ltd., 2-1-1 Katata, Otsu, Shiga 520-0292 2 Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510 3 Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502 4 Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501 (E-mail:kitagawa@icems.kyoto-u.ac.jp) The formaldehyde capture process is investigated by a titanium ion (Ti 4+ )-based porous coordination polymer (PCP), MIL-125-NH 2 . This framework contains aromatic amino groups on the pore surface, which effectively capture formaldehyde. The removal efficiency and regeneration properties of MIL- 125-NH 2 are better than the conventional amine-impregnated activated carbon, and the material does not degrade even after >140 adsorption/desorption cycles. The development of solid adsorbents for volatile organic compounds (VOCs) is important for air purification. Among the various VOCs, the presence offormaldehyde in adhesives, paints, or preservatives is a major problem, it being partially responsible for the sick building syndrome. Formaldehyde can be captured and removed by porous materials such as activated carbon, amine-impregnated porous materials, and rare-metal- supported catalysts 13 and photocatalysts. 46 However, these conventional materials have certain drawbacks. Although activated carbon and amine-impregnated activated carbon (here- after amine-impregnated AC) are inexpensive, they adsorb other substances apart from formaldehyde, and some of the materials gradually degrade after long-term use of amine-impregnated AC. Furthermore, amine-impregnated ACs are difficult to reuse, because their amino moieties are reactive to VOCs, and form imine bonds with aldehydes. 7,8 Rare-metal-supported catalysts are expensive, and they work at >100 °C to obtain sufficient removal capacity. Similarly, photocatalysts require an external energy source. Therefore, it is important to develop formalde- hyde adsorbents that provide sufficient capacity without con- suming considerable amounts of energy as well as stability and recyclability. Porous coordination polymers (PCPs) or metalorganic frameworks (MOFs) consist of metalions and bridging ligands. They have a large surface area and as a result, a significant gas adsorption potential. 912 Theirhigh adsorption and thermal/ chemical stability are significant advantages for VOC capture. Furthermore, it is possible to control the interaction with the VOC molecules to optimize the capture potential. 1317 In this research, we studied an amino-decorated PCP, MIL-125- NH 2 , 18,19 as a formaldehyde adsorbent. The results indicate that aromatic amino groups in the pores effectively capture formal- dehyde under humid conditions. Figure 1 presents the crystal structure of titanium tereph- thalate ([Ti 8 O 8 (OH) 4 (bdc) 6 ] or MIL-125), which is composed of Ti 4+ ions and 1,4-benzenedicarboxylic acid (H 2 bdc). Titanium is a non-toxic metal, suitable for practical applications. MIL-125 has a 3D pseudocubic structure consisting of cyclic octameric inorganic sub-units of edge- and corner-sharing TiO 5 (OH) octahedra connected through the bdc ligands. It has BET surface area of 1550 m 2 g ¹1 and high hydro and thermal stabilities. MIL-125-NH 2 is a derivative of MIL-125 obtained from the substitution of H 2 bdc by 2-amino-1,4-benzenedicarboxylic acid. The BET surface area of MIL-125-NH 2 is 1540 m 2 g ¹1 , which is comparable to that of MIL-125, with the other properties being similar. We synthesized MIL-125-NH 2 according to the reported procedure, and characterized the product by powder X-ray diffraction (PXRD, Figure S1), thermogravimetric analysis (Figure S2), and N 2 gas adsorption (Figure S3). The water- adsorbing behavior of MIL-125-NH 2 shows a steep uptake of gas at relative humidity (RH) = 20% at 25 °C, indicating hydrophilicity. 20 To investigate its formaldehyde removal performance, amine-impregnated AC was also employed. The amine-impregnated AC consists of sulfanilic acid and coconut- shell-activated carbon; the BET surface area is 1550 m 2 g ¹1 with a pore volume of 0.7 cm 3 g ¹1 . Breakthrough curve measurement is a typical method to evaluate formaldehyde removal efficiency with porous adsorb- ents. A schematic representation of the breakthrough curve measurement procedure and the formaldehyde breakthrough curves are shown inFigure 2. The measurement details are provided in the figure caption. We monitored the concentrations offormaldehyde in the inlet and outlet gases (Figure 2a) to estimate the removal efficiency (R eff ) defined as given below: Figure 1. Crystal structure of MIL-125. TiO 5 (OH) is shown as sky- blue octahedra; gray and pink represent C and O, respectively. H atoms are omitted. CL-150845 Received: September 4, 2015 | Accepted: September 28, 2015 | Web Released: October 3, 2015 1694 | Chem. Lett. 2015, 44, 1694–1696 | doi:10.1246/cl.150845 © 2015 The Chemical Society of Japan