Grafting of hydrophilic ethylene glycols or ethylenediamine on coordinatively unsaturated metal sites in MIL-100(Cr) for improved water adsorption characteristics Martin Wickenheisser a , Felix Jeremias b , Stefan K. Henninger b , Christoph Janiak a,⇑ a Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf, 40204 Düsseldorf, Germany b Dept. Thermally Active Materials and Solar Cooling, Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany article info Article history: Received 25 April 2013 Received in revised form 4 July 2013 Accepted 10 July 2013 Available online 23 July 2013 Keywords: Metal–organic framework Grafting Ethylene glycols Ethylenediamine Water adsorption Heat transformation abstract Grafting of activated MIL-100(Cr) with EG (ethylene glycol), DEG (diethylene glycol), TEG (triethylene glycol) and EN (ethylenediamine) leads to a decrease of the BET surface area and pore volume. Yet, water adsorption isotherms of the modified compounds MIL-100(Cr)-EG, MIL-100(Cr)-DEG and MIL-100(Cr)-EN show a favored uptake of water at lower partial pressures and no loss of total water uptake capacity com- pared to non-modified MIL-100(Cr). The reduction in surface area is offset by an increased hydrophilicity and an advantageous smaller pore size for the adsorption of water. MIL-100(Cr) is therefore very prom- ising as a water sorption material, e.g., for heat-transformation applications. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Since the past decade, tremendous research progress has been made in the utilization of metal organic frameworks (MOFs) [1– 7], e.g., in gas storage [8–14], gas separation [8–10,15–20] and catalysis [21–26]. This class of compounds is based on metal clus- ters or metal ions linked by organic ligands forming a three-dimen- sional network possessing unique properties like huge surface areas and large pore volumes due to their tunable compositions. In recent years several groups have proposed that MOFs can be used as materials for reversible adsorption and desorption of water for possible use in heat transformation processes [27–36]. The ba- sic principle for thermally driven adsorption chillers or heat pumps is shown in Fig. 1 [37,38]. The working fluid, e.g., water, is ex- changed reversibly between the evaporator/condenser and the adsorbent, e.g., a MOF-material. The whole process can be split into a production and a regeneration cycle. During the production cycle (Fig. 1b) water is evaporated taking up evaporation enthalpy at a low temperature level. The evaporation enthalpy is useful cold in the cooling case, or taken from the environment during heat pump mode. The water vapor is then adsorbed in the porous material, releasing heat at a medium temperature level. The heat is used in the heat-pump case, or rejected to the environment during cool- ing mode. In the regeneration cycle (Fig. 1a) the saturated porous material is simply heated to release the adsorbed water. The re- quired driving heat can be obtained at low cost, e.g., by a solar thermal facility, district heating, or excess heat from power-heat cogeneration plants. The water vapor is condensed at a medium temperature level and the heat of condensation is used or simply released to the environment in a cooling application [29]. Inorganic porous compounds like silica gels, aluminophosphates or zeolites have already been studied as water adsorbents for ther- mally driven adsorptions chillers or heat pump applications [39], but they have several disadvantages [40,41]. Zeolites have a high affinity to water and already adsorb at a low relative pressure of PP 0 1 = 0.001–0.01, but they require high desorption temperatures (typically over 300 °C) and have a low water loading lift [34]. Silica gels are less hydrophilic than zeolites which lead to lower desorp- tion temperatures (typically approx. 100 °C) but also to a low water loading within the cycle [34]. Therefore, the development of new porous materials for water adsorption/desorption processes is an active research topic [37,38,41–43]. Purely inorganic materials like silica gels or zeolites are also not too versatile in terms of their chemical composition and tunable pore sizes compared to MOFs. Several MOFs have already shown higher water uptake capacities 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.07.024 ⇑ Corresponding author. Tel.: +49 2118112286. E-mail addresses: stefan.henninger@ise.fraunhofer.de (S.K. Henninger), jania- k@uni-duesseldorf.de (C. Janiak). Inorganica Chimica Acta 407 (2013) 145–152 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica