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.