Sonwabo E.
Bambalaza
ab,
Henrietta W.
Langmi
*a,
Robert
Mokaya
c,
Nicholas M.
Musyoka
*a,
Jianwei
Ren
ad and
Lindiwe E.
Khotseng
b
aHySA Infrastructure Centre of Competence, Energy Centre, Council for Scientific and Industrial Research (CSIR), PO Box 395, Pretoria 0001, South Africa. E-mail: hlangmi@csir.co.za
bFaculty of Natural Science, University of the Western Cape, Bellville, Cape Town 7535, South Africa
cSchool of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
dMechanical Engineering Science Department, University of Johannesburg, Johannesburg, South Africa
First published on 12th November 2018
We report a rare case whereby a metal–organic framework (MOF), namely UiO-66, is compacted at high pressure (∼700 MPa or 100000 psi) resulting in densification and improved total volumetric hydrogen storage capacity, but crucially, without compromising the total gravimetric uptake attained in the powdered form of the MOF. The applied compaction pressure is also unprecedented for MOFs as most studies have shown the MOF structure to collapse when compacted at very high pressure. The UiO-66 prepared in this study retained ∼98% of the original surface area and microporosity after compaction at ∼700 MPa, and the densified pellets achieved a total H2 uptake of 5.1 wt% at 100 bar and 77 K compared to 5.0 wt% for the UiO-66 powder. Depending on the method used to compute the volumetric uptake, the densified UiO-66 attained unprecedented volumetric capacity at 77 K and 100 bar of up to 74 g L−1 (13 g L−1 at 298 K) compared to 29 g L−1 for the powder (6 g L−1 at 298 K) using a conventional method that takes into account the packing density of the adsorbents, or 43 g L−1 (compared to 35 g L−1 for the powder at 77 K and 100 bar) based on a method that uses both the single crystal and skeletal densities of MOFs. However, regardless of the difference in the calculated values according to the two methods, the concept of UiO-66 compaction for improving volumetric capacity without compromising gravimetric uptake is clearly proven in this study and shows promise for the achievement of hydrogen storage targets for a single material as set by the United States Department of Energy (DOE).
The potential use of MOFs for material-based on-board hydrogen storage systems requires the selection of candidate MOFs that meet certain universally accepted targets. An example of such targets for 2020, as offered by the United States Department of Energy (DOE), require that the system has a gravimetric uptake of 4.5 wt% H2 and a volumetric capacity of 30 g L−1 H2.11 While some MOFs have been reported to attain total gravimetric H2 uptake exceeding 10 wt%, their respective volumetric H2 capacities seldom reach 30 g L−1. Indeed, many different types of MOFs have been assessed in efforts to reach these targets such as in the study by Goldsmith et al.12 who explored the relationship between total gravimetric uptake and total volumetric H2 capacity for a large number of MOFs. It was found that most MOFs have relatively low total volumetric densities and only a few were within the 2017 DOE target at the time (5.5 wt%; 40 g L−1) for a single storage material. The results indicated that a maximum volumetric H2 capacity of about 55 g L−1 was reachable, but that any further changes in materials properties to increase the total gravimetric H2 uptake resulted in a reduction in the total volumetric H2 capacity. This means that the surface area of the best performing MOFs should be limited to those that do not compromise the total volumetric H2 storage capacity of the MOF. Therefore, for H2 storage applications using MOFs, the focus needs to shift towards the direction where both the total volumetric H2 storage capacities and surface areas are simultaneously improved while maintaining the crystal structure of the MOF material.
As an alternative, the compaction of powdered samples into shaped bodies such as monoliths has been implemented to improve the total volumetric H2 capacity of MOFs. However, in most cases this improvement in volumetric capacity is always accompanied by a reduction in gravimetric uptake.13–16 Most of the previous studies on gas adsorption showed that the total volumetric H2 capacity of adsorbed gas is related to the density of the adsorbent and that by using shaping strategies such as compaction, the relatively low densities of MOFs can be increased in order to improve their total volumetric H2 storage capacity.13–15 Indeed, a study by Ardelean et al.16 reported the compaction of MIL-101 (Cr) powder to achieve envelope densities close to the crystal density of MIL-101 (Cr) and the resultant “monoliths” were shown to have an impressive total volumetric H2 capacity of 40 g L−1.
The challenge, however, is that the application of external pressure onto nanostructured porous materials has been shown to typically result in the reduction of surface area and may also induce the amorphisation of MOF crystals.17–19 Another response of porous materials to external applied pressure includes anisotropic changes in crystal topology, such as seen in the MIL family of MOFs (e.g. MIL-53 and MIL-101),20 which may result in widening of pores. These structural changes can have a direct impact to gas adsorption and it is generally observed in MOFs that upon compaction, the surface area reduces and results in a diminished total gravimetric H2 uptake.17,18,21–23 In order to counteract this inverse relationship between gravimetric H2 uptake and compaction (i.e. densification), the use of highly robust MOFs which show high resistance to applied pressure is desirable for MOF-based materials that retain their initial surface area and micropore volumes upon compaction at high pressure.
The textural properties of MOFs may also be modified by applying simple adjustments to the MOF synthesis conditions.24–28 One of the extensively investigated methods is the synthesis of MOFs with defects or missing organic linkers in their crystal lattice, which results in the presence of open metal sites within the MOF crystal structure.29–33 At the open transition metal (TM) centres there exists an opportunity for enhanced TM to gas interactions that may improve gas uptake as described in previous studies.34–36 The compaction of MOFs has been shown to lead to increased volumetric H2 capacities and in this study we aim to investigate the effects of compaction at significantly higher pressures (∼700 MPa or 100000 psi) than previously reported for UiO-66,17 and possibly improve the volumetric H2 capacity for UiO-66 towards the DOE target of 30 g L−1.
(1) |
The total volumetric H2 capacity was calculated from the gravimetric data using two approaches. The most commonly reported approach uses the packing density of the MOF material to calculate the total volumetric capacity as shown in eqn (2) and (3). The second approach comes from a recent study by Ahmed et al.37 who highlighted some recommendations made by Parilla et al.38 where the total volumetric H2 capacity was calculated from gravimetric data (excess H2 uptake in wt%) using both the single crystal and skeletal densities of MOFs according to eqn (4).
vθExc = θExc × dMOF | (2) |
vθT = θT × dMOF | (3) |
(4) |
In Fig. 2a the N2 adsorption isotherms for both powder and compacted UiO-66 show type I adsorption and in Table 1 it can be seen that the compacted UiO-66 showed a less than 2% decrease in BET surface area (1707 m2 g−1) compared to UiO-66 powder (1737 m2 g−1), and ∼5% reduction in micropore surface area and micropore volume. Such a result is unusual for MOFs, as given the nature of their highly porous crystal structure, it can be expected that they show high compressibility, and negative response to applied pressure. Wu et al.,44 have reported that the minimum shear modulus for UiO-66 is ca. 13.7 GPa due to the strong Zr–O bonding in its crystal lattice, and is much higher than the value for most MOFs such as ZIF-8 (ca. 2.7) and MIL-53 (Ga) (ca. 0.16 GPa).45 Therefore, due to the little changes in crystal structure at ∼665 MPa, it can be expected that UiO-66 crystals would retain most of their textural properties at applied compaction pressures below the minimum shear modulus, as shown in this study. The total pore volume (Table 1), on the other hand, was found to differ more significantly with UiO-66 powder having 0.96 cm3 g−1 whereas the UiO-66 pellet was 0.81 cm3 g−1, a 16% reduction in total pore volume. This can be attributed to the significant reduction in macropores that are due to interparticle voids that are removed upon compaction as a result of the close packing of the particles (Fig. S4, ESI†).
Fig. 2 Textural properties for UiO-66 before and after compaction at 665 MPa: (a) N2 isotherms at 77 K; (b) pore size distribution in the micropore region (PSD < 20 Å). |
Sample | Surface areaa (m2 g−1) | Pore volumeb (cm3 g−1) | Packing density (g cm−3) | Skeletal density (g cm−3) | Gravimetric H2 uptake (wt%) | Volumetric H2 capacity (g L−1) | Totalf | ||
---|---|---|---|---|---|---|---|---|---|
Excess | Total | Excessd | Totale | ||||||
a Values in parenthesis are micropore surface area and percentage micropore surface area of the total surface area. b Values in parenthesis are micropore volume and percentage micropore of the total pore volume. c Tapped density of UiO-66 powder. d Excess volumetric capacity calculated from the packing density as per eqn (2). e Total volumetric capacity calculated from the packing density as per eqn (3). f Total volumetric H2 capacity calculated using the single crystal (1.24 g cm−3) and skeletal densities of UiO-66 as reported by ref. 37. | |||||||||
UiO-66 powder | 1737 (1559, 90%) | 0.96 (0.60, 63%) | 0.57c | 1.65 | 2.1 (0.4) | 5.0 (1.1) | 12 (2) | 29 (6) | 35 (7) |
UiO-66 pellet | 1707 (1484, 87%) | 0.81 (0.57, 70%) | 1.45 | 1.78 | 2.7 (0.3) | 5.1 (0.9) | 39 (4) | 74 (13) | 43 (6) |
The pore size distribution curves shown in Fig. 2b are consistent with reported literature with pores of size ∼6, 8 and 11 Å, which correspond to the free diameters in tetrahedral cages, triangular windows and octahedral cages of UiO-66, respectively.46,47 There was also evidence of a broadened pore size distribution towards higher values, with peaks at ∼14 Å up to ∼18 Å which was an additional sign of UiO-66 crystals with defects and possibly open Zr metal sites. The observation of retention of UiO-66 crystal structure after compaction up to 665 MPa as evident from the X-ray diffraction patterns in Fig. 1 was also translated in the textural properties of powdered and compacted UiO-66 forms (Fig. 2). A noticeable feature of the UiO-66 prepared in this study is that the surface area and pore volumes obtained are amongst the highest values reported for UiO-66 prepared using organic acid modulators.31,48 Shearer et al.31 reports an in-depth description on the role of monocarboxylic organic acids such as formic acid in the formation of defects in UiO-66, which results in increased BET surface areas compared to non-modulated (defect-free) UiO-66. The defects in crystals suggest the presence of open metal sites which can enhance the binding affinity of gas molecules at low gas pressures and may result in enhanced gas uptake than would be expected, especially within open metal centres contained in highly microporous environments such as found in MOFs.34,36
The trend in the total H2 uptake (wt%) data shown in Fig. 3a is consistent with that observed for the surface area for the powdered and compacted UiO-66. The powder and compacted UiO-66 have relatively similar hydrogen uptake, which means that there was no compromise of total H2 uptake after compaction of UiO-66 powder at 665 MPa pressure. There were, however, slight differences in their adsorption isotherms as it can be seen that the excess H2 uptake in UiO-66 powder reaches a maximum at relatively low pressure (20–30 bar) after which there is a decrease up to 100 bar. This is the typical behaviour in physisorption-based gas uptake due to rapid adsorption onto the adsorbent surface at low surface coverage (high number of available binding sites) followed by a higher desorption to adsorption rate upon saturation of the surface excess (i.e. excess surface coverage ≈ 1).49 In the excess H2 isotherm for compacted UiO-66, the pressure at which maximum uptake is reached was much higher (>50 bar), and also there was no significant decrease up to 100 bar. The uptake of the H2 is reversible (Fig. S5, ESI†) as the adsorption and desorption isotherms overlap. This is the expectation for physisorption based uptake and is also an indication of the robustness of our H2 sorption measurements.
Fig. 3 Gravimetric H2 uptake (wt%) for powder and compacted UiO-66 up to 100 bar measured at (a) 77 K and (b) 298 K. The plots are for excess (open symbols) and total (closed symbols) H2 uptake. |
Due to the compaction, it can be expected that access to binding sites/pores for H2 molecules would be more difficult due to the significant reduction in voids or macropores in-between UiO-66 particles resulting in a reduced grain boundary between the UiO-66 crystallites. As a result, the rate at which H2 molecules diffuse into the UiO-66 framework becomes reduced and higher pressures are required to reach maximum excess surface coverage and also the rate of desorption of H2 molecules within the pores becomes restricted by the close packing. The H2 adsorption at 298 K up to 100 bar in Fig. 3b were found to be higher for the UiO-66 powder compared to compacted UiO-66. As expected, physical adsorption of H2 in porous materials becomes lowered as the temperature increases, and higher pressures are required to observe improved adsorption.49 The difference observed for powdered and compacted UiO-66 can be resulting from kinetic effects as the diffusion of H2 in compacted UiO-66 would be slower compared to UiO-66 powder and because of the significantly reduced H2 adsorption at 298 K compared to 77 K, the excess H2 adsorption in compacted UiO-66 would only match that of powdered UiO-66 at pressures higher than 100 bar. The respective uptake values given in Table 1, comparing UiO-66 powder and UiO-66 pellet, show that at least 90% of the UiO-66 textural properties are retained upon compaction at 665 MPa applied pressure; the major difference is in their packing and skeletal densities, which influence the calculation of the total volumetric H2 capacity as given in eqn (3) and (4). As expected the packing density increased after compaction, however, it was interesting that the skeletal density also increased. The UiO-66 prepared in this study was a polycrystalline powder made up of different crystallite sizes (Fig. S1, ESI†). The compaction of UiO-66 powder at sufficiently high pressure would thus result in the reduction of the grain boundary between the crystallites, reducing its volume in the densified state. This may result in some micropores to be inaccessible under non-equilibrium gas uptake measurements such as those used for helium pycnometry and result in higher skeletal density results.
For the purpose of this study, the volumetric H2 capacity was calculated using two methods. Fig. 4a and b shows data obtained using eqn (4) taking into account the recommendations made by Parilla et al.38 for calculations based on gravimetric measurements of H2 uptake at high pressure. In Fig. 4a, the volumetric H2 capacity (at 77 K) for compacted UiO-66 exceeds that of powdered UiO-66 from 25–100 bar by up to ∼26%. Given that both the textural properties and the total gravimetric H2 uptake up to 100 bar were not compromised after compaction, the significant difference in volumetric H2 capacity can therefore be attributed to the improvement of the UiO-66 density after compaction. Indeed previous studies13,15,16 have largely reported on the improvement of volumetric H2 capacity of MOFs due to compaction but there have been seldom reports where the MOF gravimetric H2 uptake is also not compromised with compaction. Furthermore, the ratio of the volume of H2 adsorbed to the volume of H2 in the gaseous/bulk phase (Vad/Vbulk) can serve as an indication of the efficiency of H2 storage in the MOF compared to that of an empty container/cylinder. Under the specified isotherm conditions (T = 77 K, 0 < P < 100 bar), the H2 density (g L−1) increases linearly with pressure and Vad/Vbulk ≈ 1 as calculated using the ideal gas law, i.e., no adsorption would be expected in an empty container. In the presence of UiO-66, as shown in Fig. S6 (ESI†), the Vad/Vbulk ratio rapidly increases at pressures below 5 bar peaking at 80 cm3 cm−3 and 40 cm3 cm−3 for powdered and compacted UiO-66, respectively. This means that at pressures below 5 bar, UiO-66 stores at least 40 times more H2 by volume compared to the amount stored in an empty container/cylinder under the same conditions. As the pressure increased from ∼5 bar up to 100 bar, the Vad/Vbulk ratio decreases sharply initially and then steadily towards Vad/Vbulk ≈ 1. The rapid increase in adsorbed H2 at low pressure can be attributed to the readily available adsorption sites (such as pores and pore walls). As these spaces/sites become filled with mono- and/or multilayers of H2 molecules, the uptake of more H2 becomes restricted and may lead to an equilibrium state of adsorption being reached (i.e., saturation). Interestingly, from ∼30 to 100 bar, Vad/Vbulk for compacted UiO-66 is greater than that of the powdered UiO-66, showing more uptake of H2 at elevated pressures. This correlates with the excess adsorption isotherm obtained for compacted UiO-66 in relation to the UiO-66 powder, i.e., the densified sample takes up more excess H2. Clearly, the significant increase in the UiO-66 density after compaction at 665 MPa translates to a rise in the calculated total volumetric H2 capacity. Similar to the gravimetric H2 uptake, the volumetric capacities were also found to be significantly lower at 298 K compared to 77 K, and the UiO-66 powder had slightly higher uptake capacity than compacted UiO-66. This observation suggests that the adsorption onto compacted UiO-66 is significantly affected by kinetic effects under non-equilibrium H2 uptake conditions. Therefore, given that the surface area and pore volumes are effectively retained after compaction, it can be expected that at higher H2 adsorption pressures (greater than 100 bar), the room temperature volumetric hydrogen uptake capacity of compacted UiO-66 would exceed that of UiO-66 powder in a manner similar to that observed at 77 K.
Fig. 4 Volumetric H2 isotherms for powdered and compacted UiO-66 up to 100 bar: (a and b) volumetric H2 capacity/density (g L−1) at 77 K and 298 K obtained using eqn (4), and (c and d) volumetric H2 capacity/density at 77 K and 298 K obtained using eqn (3). |
We also report in Table 1 and Fig. 4c, d the total volumetric H2 capacity calculated using the packing density of powdered and compacted UiO-66, which is currently the most commonly used method of calculating the total volumetric capacity.13,16,50–53 The results show that there is a remarkable increase in total volumetric H2 capacity from 29 to 74 g L−1 (6 to 13 g L−1 at 298 K) due to the improved packing density of UiO-66 after compaction at 665 MPa as calculated using eqn (3). The results however, do show a significant improvement of the total volumetric capacity due to compaction regardless of the mathematical model for conversion of gravimetric data into volumetric values.
We also take note of the results obtained at 77 K and 25 bar (Table S2, ESI†) based on a recent study by Balderas-Xicohténcatl et. al.55 showing that the volumetric H2 capacity in MOFs follows a linear relationship with the MOF's volumetric surface area (m2 mL−1), similar to Chahine's rule. In our results, the compaction of UiO-66 powder increased the packing density from 0.57 to 1.45 g mL−1 and thus increasing the volumetric surface area from 990 to 2475 m2 mL−1. We see that multiplying the volumetric surface area by the slope 1.9 × 10−2 m2 mL−1 (derived from calculations made by Balderas-Xicohténcatl et. al.55) we obtain a calculated theoretical total volumetric capacity of 18 and 47 mg mL−1 (g L−1) for powder and compacted UiO-66, respectively. It is noteworthy that such a calculation yields virtually identical total volumetric capacities at 77 K and 25 bar, as calculated in our work described above using the packing density in eqn (3), for both powder and compacted UiO-66. The calculations and derivations made by Balderas-Xicohténcatl and co-workers were based on many different types of MOFs and also took into consideration the effect of compacting MOFs to increase packing densities. The strong agreement between our results to their estimations serves to validate our data and calculations and moreover, more generally, a strong indicator of the significant influence of packing density on volumetric H2 capacity in MOF powders and other related porous materials.
Footnote |
† Electronic supplementary information (ESI) available: Six additional figures (consisting of SEM images, TGA analysis, comparative nitrogen isotherms and pore size distribution curves, and adsorbed H2 volume fractions), and two tables (consisting of comparative textural properties, packing density, and H2 uptake). See DOI: 10.1039/c8ta09227c |
This journal is © The Royal Society of Chemistry 2018 |