Thermochimica Acta 587 (2014) 1–10 Contents lists available at ScienceDirect Thermochimica Acta j ourna l h om epage: www.elsevier.com/locate/tca Unusual adsorption behavior of volatile hydrocarbons on MOF-5 studied using thermodesorption methods Wacław Makowski a,∗ , Maria Ma ´ nko a , Piotr Zabierowski a , Kinga Mlekodaj a , Dorota Majda a , Janusz Szklarzewicz a , Wiesław Lasocha a,b a Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Kraków, Poland b Polish Academy of Sciences, Institute of Catalysis and Surface Chemistry, Niezapominajek 8, 30-239 Kraków, Poland a r t i c l e i n f o Article history: Received 27 January 2014 Received in revised form 18 March 2014 Accepted 15 April 2014 Available online 23 April 2014 Keywords: MOF-5 Porosity QE-TPDA Thermodesorption Hydrocarbons a b s t r a c t Adsorption of n-alkanes (C 5 –C 10 ), isooctane, benzene, toluene and p-xylene on MOF-5 was studied by means of quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) and temperature programmed desorption (TPD). Unusual thermodesorption profiles showing increasing complexity with the size of a probe molecule have been observed. Number of peaks in the ther- modesorption profile increased from one for pentane or benzene to five for decane. Considerable adsorption–desorption hysteresis was found for isooctane. These unusual thermodesorption profiles were discussed in comparison to those obtained for zeolites and ordered mesoporous silicas. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Metal–organic frameworks (MOFs) are a new class of porous materials, interesting not only because of their unique structures and properties, but also due to their numerous potential applica- tions, including catalysis [1], gas separation [2] and storage [3–5]. Microporous Zn 4 O(BDC) 3 , known as MOF-5 [6], is a prototypic structure in the family of isoreticular metal–organic frameworks (IRMOF-1). Primitive cubic lattice of MOF-5, comprising Zn 4 O clus- ters in the nodes and 1,4-benzenedicarboxylate anions (BDC) as their linkers, contains two types of nearly spherical cavities of about 1.2 and 1.5 nm in size, interconnected by 0.8 nm windows [7]. This void space remains stable after evacuation of a solvent and is acces- sible for adsorption of guest molecules of suitable size and shape. MOF-5 has been extensively studied as a prospective adsor- bent for storage and/or separation of H 2 , CH 4 and CO 2 , therefore characterization of its adsorptive properties was focused mostly on these gases [8,9] and on N 2 or Ar used for analysis of its porosity [10]. Limited results concerning adsorption of other compounds, including volatile hydrocarbon, in the micropores of MOF-5 has been published so far. Higher than expected values of the adsorp- tion enthalpy of n-alkanes and aromatic hydrocarbons, obtained in ∗ Corresponding author. Tel.: +48 12 6632245; fax: +48 12 6340515. E-mail addresses: makowski@chemia.uj.edu.pl, waclaw.makowski@gmail.com (W. Makowski). an inverse gas chromatography study by Luebbers et al. [11], were attributed to structural defects present in MOF-5. Bimodal distri- bution of adsorption energy of benzene revealing heterogeneity of the adsorption sites in MOF-5 was reported recently by Shim et al. [12]. An inconsistency concerning porosity and adsorptive properties of different materials with the apparent MOF-5 structure has been reported [13]. Much lower adsorption capacity and pore volume found for the MOCP-L synthesized by Huang et al. [14], than for the original IRMOF-1 [10], was attributed to Zn(OH) 2 species present inside the cavities [13] or ZnO microcrystals formed outside the micropores [15]. Only limited formation of the double interpene- trated framework was found [13]. The aim of this work was a study on adsorption of several volatile hydrocarbons on MOF-5 by means of the quasi-equilibrated tem- perature programmed desorption and adsorption (QE-TPDA). This technique, recently developed for characterization of porous mate- rials [16–20] has been successfully applied in studies on zeolites, mesoporous silicas and their carbon replicas. It allows determi- nation of the adsorption enthalpy and entropy [17], micro- and mesopore volume, as well as assessment of the pore size, including calculation of the mesopore size distribution [19]. 2. Experimental There are several methods for MOF-5 synthesis described in lit- erature [13,14,21–23]. The main differences between them concern http://dx.doi.org/10.1016/j.tca.2014.04.016 0040-6031/© 2014 Elsevier B.V. All rights reserved.