1. Introduction
Metal–organic frameworks (MOFs) are a group of materials represented by a variety of structures and chemistries affecting their properties and applications [1, 2]. Even though they are perfect crystals, this perfection is not always beneficial for the target performance, especially in catalysis. Therefore, in recent years, research has been carried out with focus on increasing the defects populations in otherwise perfect MOFs [3]. The defects in MOFs can arise from crystal imperfections caused mainly by partial ligand replacement [4, 5], missing clusters [6], heterogeneity of ligands [7, 8] or a use of modulators during the MOF synthesis [9, 10].
UiO-66 is a well-studied MOF, which exhibits a high surface area, chemical, mechanical and thermal stability [11-13]. The synthesis of this MOF is relatively easy and enables the tunability of its structural defects [4-10]. These defects, besides being known to increase pore volumes and surface areas, also change pores’ morphology/distribution and affect the adsorption of gases such as CO2[14-18], H2O [19, 20], H2 [15, 21-23], C2H6 [24-27] or C2H4 [25].
Adsorption of CO2 on UiO-66 has been extensively studied in search of sufficient sequestration media. Both experimental [15-17] and theoretical approaches have been explored [14, 17]. At 0.1 MPa and 0°C, adsorbed amounts of ~ 2 mmol/g have been reported [15, 16]. While the heat of adsorption measured by adsorption methods was between 22 and 28 kJ/mol [15, 16] and increased with the introduction of defects [16], results of infrared spectroscopy at variable temperature suggested that the heat of CO2 adsorption by hydrogen bonding reaches 38 kJ/mol, and 30.2 kJ/mol - by dispersive interactions [14]. Recently, we have reported that defects in UiO-66, synthesized in the presence of nanocellulose crystals of various chemistries, not only increased the amount of CO2 adsorbed by about 30 % at 0°C, compared to UiO-66 (3.25 mol/g as compared to 2.5 mmol/g), but also led to an increase in the heat of adsorption from 20 to 27 kJ/mol [28].
In the case of H2 adsorption on UiO-66, the amount adsorbed at – 196°C and 0.1 MPa increased from 1.25 % to 1.50 % upon changing the solvent from ethanol to DMF used in its synthesis [21]. This also resulted in an increase in the homogeneity of the crystals (well defined sizes of 150-200 nm in DMF) and in an increase in the surface area from 784 m2/g in ethanol to 1358 m2/g in DMF. In this case, the lower surface area of the sample synthesized in ethanol was linked to impurities due to polymerization products of benzenedicarboxylate (BDC). Nanosized UiO-66 of an enhanced porosity due to the more effective removal of solvent and unreacted BDC from the pores had a surface area of 1434 m2/g and a H2 uptake of 1.50 % [15]. Ren et al. studied adsorption of hydrogen in UiO-66 synthesized with various amounts of formic acid as a modulator [23]. They found that the increase in the amount of modulator increased the size of the crystals and the surface area to 1367 m2/g. On this material, 1.5 % of H2was adsorbed at – 196°C and 0.1 MPa.
Although the separation of alkanes from alkenes is an industrially important process, UiO-66 has rather rarely been used in this application, likely owing to the lack of unsaturated metal sites, which would provide the specific interactions/π-complexation, as in the case of Cu-BTC (benzenetricarboxylate) [24]. Nevertheless, Qian at al. [24] reported that the number of carboxyl groups in the linkers of UiO-66 affected the C2H6/C2H4separation. When 1, 2, 4 BTC (in UiO-66-COOH) was used as a linker, there was no difference between the amount adsorbed of both species at 25°C, and about 1.3 mmol/g were reported. Increasing the number of carboxylic acid in the linker (1, 2, 4, 5-BTEC; in UiO-66-(COOH)2) increased the amount adsorbed to 1.8 mmol/g and the surface was slightly more favorable for adsorption of C2H6 compared to that of C2H4 ( ~ 0.1 mmol/g more of C2H6 was adsorbed at 0.1 MPa). The latter happened despite the smaller surface area of UiO-66-(COOH)2 (622 m2/g compared to 713 m2/g for UiO-66-COOH). This trend was linked to smaller pore sizes in UiO-66-(COOH)2 compared to those in UiO-66-COOH. The heats of C2H4adsorption (Qsto) showed the homogeneity of the surface sites (flat) and Qsto increased from 24.7 kJ/mol for UiO-66-COOH to 27.4 kJ/mol for UiO-66-(COOH)2 [24]. The effect of UiO-66 topology on the adsorption of ethane and ethylene was also studied by Li and coworkers [25]. On their UiO-66 with a surface area of 1014 m2/g, 67 % more of C2H6 than that of C2H4 was adsorbed, and the heats of adsorption were 26 and 24 kJ/mol, respectively. The difference in the heat of adsorption was linked do the difference in the polarity of these two molecules. The adsorption of ethane at 30°C on UiO-66 was studied by Llewellyn and coworkers [26], who measured a capacity of 1.7 mmol/g and a heat of adsorption of ~32 kJ/mol on a material having a surface area of ~ 1050 m2/g.
Considering that a small alteration in the porosity of UiO-66 and defects in their structure were reported to affect the gas adsorption behavior, the objective of this paper is to investigate the effects of the addition of nanographite (~ 1 to 6 wt. %) with particle sizes of about 19 µm on the porous properties and the gas adsorption behavior of the final UiO-66/nanographite composites. Since the nanographite particles are much larger than crystals/aggregates of UiO-66 [23], they are expected to work as platforms for the deposition of the latter. Owing to the lack of functional groups on the basal planes of nanographite flakes [29], the UiO-66 deposition might occur mainly via dispersive forces and due to the affinity of the organic linker (BTC) for the graphite surface. This is expected to create either an interface or defects in the MOF phase and, as a consequence, to increase the porosity. Although composites containing UiO-66 and graphite oxide have been synthesized and used as CO2 adsorbents, resulting in an increased storage capacity [17, 18], their mechanism of formation was different than that expected here due to the lack of oxygen groups on nanographite used in our approach. These defects can be either of chemical or physical in nature and their geometric dimensions should be rather small and of the order of the fraction of a nanometer. Therefore, to test the extent of the approach applied, the surface accessibility of the synthesized materials has been investigated with regard to adsorption of N2, CO2, H2, C2H6 and C2H4. Even though nitrogen is commonly used to evaluate the porosity [30], despite its known kinetic limitations to access very small pores, we have also measured the adsorption of hydrogen at – 196°C. CO2 was not used for the textural characterization owing to is possible specific interaction with the surface of adsorbents [31].The isosteric heats of adsorption were calculated for CO2, C2H6 and C2H4 from their adsorption isotherms measured at least at two temperatures. The heats of adsorption of C2H6 and C2H4 were analyzed in details since the larger sizes of these molecules than other chosen by us as the porosity probes might affect the surface interactions. Moreover π bond of C2H4 might affect the strength of adsorption if acidic groups are on the accessible surface [32]. The adsorption of several small molecules on UiO-66/nanographite composites with different contents of nGr allowed understanding the effect of nanographite on the final performances of the materials.