Drastic Enhancement of the CO 2 Adsorption Properties in Sulfone- Functionalized Zr- and Hf-UiO-67 MOFs with Hierarchical Mesopores Pantelis Xydias, Ioannis Spanopoulos, Emmanuel Klontzas, George E. Froudakis, and Pantelis N. Trikalitis* Department of Chemistry, University of Crete, Voutes 71003, Heraklion, Greece * S Supporting Information ABSTRACT: The sulfone-functionalized Zr- and Hf- UiO-67 metal-organic frameworks with hierarchical mesopores were successfully synthesized using the ligand 4,4′-dibenzoic acid-2,2′-sulfone, with acetic acid or HCl as the modulator. Compared to UiO-67, the zirconium solid shows a remarkable 122% increase in CO 2 uptake, reaching 4.8 mmol g -1 (17.4 wt %) at 1 bar and 273 K (145% at 298 K) and more than 100% increase in CO 2 / CH 4 selectivity. C urrently, an enormous research activity is devoted to porous metal-organic frameworks (MOFs) or porous coordination polymers mainly because of the prospect of finding application in important technological sectors related to energy and the environment. 1 In particular, the reduction of greenhouse gas emissions and especially CO 2 is currently a major target worldwide, and highly porous sorbents like MOFs hold great promise as cost-effective alternatives to the existing technolo- gies. 1b,2,3 A particular MOF that has captured a great deal of attention because of its high thermal and chemical stability is the zirconium-based, 12-connected Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 (bdc) 6 (UiO-66) microporous material, made with terephthalate dianions (bdc 2- ). 4 This material shows interesting CO 2 sorption properties especially for low-pressure and ambient-temperature applications that can be further improved with functionalized H 2 bdc linkers 5 and, in particular, with those containing the highly polar sulfonic acid group, as has been demonstrated both theoretically 5b and experimentally. 5c The isostructural analogue UiO-67, based on the linker 4,4′ -biphenyldicarboxylate (bpdc 2- ), because of its larger pore size and increased pore volume, is considered as a very promising material for CO 2 adsorption. 6 However, related studies are limited, 7 and to the best of our knowledge, there is no report on functionalized UiO- 67. We report here the synthesis and characterization of the sulfone-functionalized analogue of UiO-67(Zr), using the ligand 4,4′-dibenzoic acid-2,2′-sulfone (H 2 bbs), in the presence of acetic acid (AcOH) or HCl as the modulator, denoted as 1 AcOH and 1 HCl , respectively. We have also isolated the hafnium analogues, denoted as 2 AcOH and 2 HCl . For comparison purposes, we synthesized Zr-UiO-67 using the ligand 4,4′-biphenyldicar- boxylic acid (H 2 bpdc), denoted as 3 AcOH and 3 HCl . Remarkably, compared to the parent, high-quality nonfunctionalized solid 3 AcOH , 1 AcOH shows a 122% increase in CO 2 uptake at 1 bar and 273 K (145% at 298 K), reaching 4.8 mmol g -1 (2.88 mmol g -1 at 298 K), with high selectivity toward CH 4 and N 2 . In terms of key structural features, we provide a direct proof, using 1 H NMR spectroscopy, for the presence of AcO - anions coordinated to the Zr 6 clusters in 1 AcOH . Moreover, both AcOH and HCl induce in 1 AcOH and 1 HCl hierarchical mesopores. Below, we present and discuss a detailed structural and gas sorption study of the zirconium-based solids, although key findings for the hafnium analogues are also reported. Because of the lack of suitable single crystals, the ideal structure of 1 shown in Figure S2 was built using a ligand replacement methodology starting from the known structure of UiO-66. 4 The powder X-ray diffraction (PXRD) patterns of high-quality 1 AcOH and 2 AcOH are shown in Figure 1. All compounds are isostructural, and the PXRD pattern can be indexed to the cubic system with refined unit cells of 26.61(2), 26.54(3), and 26.87(2) Å for 1 AcOH , 2 AcOH , and 3 HCl , respectively. The smaller unit cell of 1 AcOH and 2 AcOH compared to 3 HCl is due to the shorter length of the bbs 2- linker caused by the bending of the two aromatic rings. It is worth noticing that, despite its large deviation from linearity (163°), this linker is capable of forming an isostructural solid with UiO-67. Argon sorption isotherms at 87 K for 1 AcOH , 1 HCl , and 3 HCl are shown in Figure 2, from which important porosity data were determined and are summarized in Table S1 in the Supporting Information (SI). Accordingly, both 1 AcOH and 1 HCl show almost identical Brunauer-Emmett-Teller (BET) areas, 1442 m 2 g -1 (Langmuir 1597 m 2 g -1 ) and 1456 m 2 g -1 (Langmuir 1601 m 2 g -1 ), respectively, which are lower compared to 3 HCl (BET area 1996 m 2 g -1 and Langmuir 2253 m 2 g -1 ) presumably because of Received: September 25, 2013 Figure 1. Powder X-ray diffraction pattern of 1 AcOH (left) and 2 AcOH (right). Insets: representative SEM images. Communication pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic402430n | Inorg. Chem. XXXX, XXX, XXX-XXX