Densification and shaping of pure Cu-BTC powders using a solid-state chemical transformation

MOFs are a class of porous crystalline materials whose unique properties have led to applicability in several fields ranging from gas adsorption to drug delivery. Despite their high potential, MOFs are usually found as fine powders, a property that can limit their use in industrial applications. Here, a novel approach is proposed to form densified Cu-MOF (Cu-BTC) powders and monoliths using 1,2-ethanedisulfonic acid (EDSA) as a densification agent. A MOF/EDSA mixture was heated to ∼150 °C; the molten EDSA not only promotes the growth of larger MOF crystallites, but also stimulates condensation reactions between the carboxylate-based MOF ligands, further binding the particles together. When this reaction was done in a stainless-steel die under pressure MOF-based monoliths could also be formed. Notably, using this approach, the MOF had a higher density, significantly improving the volumetric CO2 adsorption capacity. We believe this contribution provides the basis for future work wherein the intrinsic MOF particle surfaces can be selectively engineered to improve their properties towards shaping for industrial applications.


Introduction
Climate change represents one of the most significant challenges of the 21st century [1].A significant driver of climate change is the excessive emission of greenhouse gases since the time of the industrial revolution [2].Amongst numerous greenhouse gases, CO 2 is often considered as the most problematic one for long term effect [3].Today, there is a great need for technologies which can rapidly and selectively extract CO 2 from the atmosphere with minimal economic and ecological cost with several different processes and materials proposed to tackle the CO 2 problem [3,4].
In recent years, metal-organic frameworks (MOFs) have emerged as a class of materials with vast potential to selectively adsorb CO 2 from the atmosphere or other gas mixtures [4].MOFs are 3-dimensional, crystalline, and porous materials comprised of inorganic metal nodes and organic linkers [5,6].Tuning of the MOF structure by reticular chemistry has allowed for the formation of a large database of synthesized MOFs with excellent gravimetric CO 2 adsorption characteristics [7].A key roadblock, however, is that most MOFs are after synthesis loose polydisperse, polycrystalline powders having low densities [8].These drawbacks severely limit the utility of MOFs for adsorption applications from an engineering perspective.A potential solution is the compaction of MOFs to form granules or monoliths [9].Albeit that this process increases the density of the material, it can come with the risk of damaging the framework, thereby reducing the accessible surface area and consequently the CO 2 adsorption capacity [9].To note, an increase in density leads to better volumetric CO 2 adsorption capacities which is desirable to for instance reduce the size of the adsorbent bed.
Hence, in this study, we report a simple synthetic method to increase the density of polycrystalline MOF powders.Cu-BTC is a MOF composed of metal coordination polymers having Cu acting as joints and benzene-1,3,5-tricarboxylate (BTC) ligand as the linkers.We chose Cu-BTC as the starting point for our experiments as it is easily synthesized and one of the most widely studied materials to date.The method developed utilizes 1,2ethanedisulfonic acid (EDSA, pK a = −2) as a solid acid which we suppose has two consequences on the MOF structure: the highly acidic nature of EDSA is supposed to promote (a) redissolution-precipitation of the MOF crystals leading to larger particles; (b) reactions between terminal -OH groups on the external MOF surface leading to the formation of bridges between the MOF crystals binding them together.Post treatment, the MOF powders retain their crystallinity and porosity but show an increased density likely due to a combination of the two effects of the EDSA on the MOF powder.We further, show that this effect occurs in another chemically diverse MOF namely Cr-BDC, indicating that this process is potentially generalizable.

Method Synthesis of Cu-BTC
The synthetic method for making Cu-BTC was adapted from a previous report [10].Blend 60.85 g of BTC in 1350 ml of ethanol and stir (200-300 rpm) for 30 min.In a separate container, blend 50 g of Cu (OH)2 in 500 ml distilled water.Add the Cu containing solution to the BTC containing solution and stir for 24 h at a stirring speed of 300 rpm and at room temperature.Filter the solution and rinse the filter cake with ethanol (1500 ml) and let it dry for another 24 h on filter paper.Finally, the solids were transferred to a vacuum desiccator and let dry for another 24 h.The samples were always stored under vacuum to minimize exposure of the Cu-BTC powder to moisture.
Synthesis of modified Cu-BTC with a loading of 0.4 mol mol −1 of EDSA (Cu-BTC-0.4)200 mg of the Cu-BTC powder was ground with 30 mg of EDSA with a mortar and pestle.The ground solids were then transferred into a 50 ml round bottom flask.The flask was evacuated till ∼5 mbar and heated to 150 °C for 24 h.The solids were washed with ethanol (5x, 40 ml) and dried in a vacuum oven under reduced pressure.The as-synthesized solids were stored in a vacuum desiccator for further use.
Synthesis of modified Cu-BTC with a loading of 0.8 mol mol −1 of EDSA (Cu-BTC-0.8)200 mg of the Cu-BTC powder was ground with 60 mg of EDSA with a mortar and pestle.The ground solids were then transferred into a 50 ml round bottom flask.The flask was evacuated till ∼5 mbar and heated to 150 °C for 24 h.The solids were washed with ethanol (5x, 40 ml) and dried in a vacuum oven under reduced pressure.The as-synthesized solids were stored in a vacuum desiccator for further use.
Synthesis of Cu-BTC-mono 400 mg of the Cu-BTC powder was ground with 120 mg of EDSA with a mortar and pestle.The ground solids were transferred into an 8 mm die equipped with a punch.The powder was lightly pressed under the weight of the punch (∼0.7 N).The die was transferred into an isothermal oven and heated to 150 °C for 24 h.The monolith was isolated from the die, soaked in ethanol (100 ml) for 12 h.Subsequently, the monolith was dried in a vacuum oven under reduced pressure and stored in a vacuum desiccator for further use.

Results and discussion
Characterization of Cu-BTC and densified Cu-BTC powders The selected MOF, Cu-BTC (BTC 3− = benzenetricarboxylate), is comprised of copper paddlewheels interlinked by benzene tricarboxylate linkers (BTC) forming a cage like structure with pores between 1-1.5 nm [10,11].The Cu-BTC powder was synthesized according to a reported method [10] and subsequently characterized.The surface area is around 1400 m 2 g −1 with a pore volume of around 0.7 ml g −1 and an apparent density of 0.13 g ml −1 .Given this, the synthesized powder was ground using a ceramic mortar and pestle with a defined amount of EDSA (0.4 mol mol −1 MOF and 0.8 mol mol −1 MOF) and then loaded into a round bottom flask and heated to 150 °C for 24 h.It should be noted that the EDSA has a melting point between 110 °C-120 °C and hence, the selected temperature was meant to be above its melting point all while well within the temperature range where the Cu-BTC is stable.
Post treatment, the powder was washed with ethanol (5x, 40 ml) and dried at room temperature.The assynthesized powders, hereby referred to as Cu-BTC-0.4and Cu-BTC-0.8for the two EDSA loadings, showed good crystallinity (figure 1(a)) and porosity (figure 1(b)) and a 2-fold increase in density for the samples (table 1).It is noted that at higher molar loadings of EDSA (with respect to the Cu-BTC), the crystallinity of the parent Cu-BTC is lost.The micropore volume, calculated from nonlocal density functional theory (NLDFT) simulations for pores smaller than 2 nm, has a commensurate decrease along with a decrease in BET surface area.On the other hand, the macropore volume calculated with the BJH method increased for the treated samples; this could be due to the increased number of interparticle macropores because of the effect of EDSA on the MOF particles.Further, FTIR spectroscopy (figure 1(c)) shows presence of new peaks which can be assigned to ester-like and ether-like linkages.This can be ascribed to condensation reactions between the surface carboxylates.Further confirmation of this is seen via XPS, where we see a new peak due to C-O-C and C=O-O-C groups in the C 1 s spectrum (figure 1(d)).
Although modifications of the surface carboxylate groups have been previously reported, such intra-particle condensation reactions between MOF particles are being reported for the first time here.It should also be noted that, these reactions are expected to mainly occur on the surface of the MOF particles due to the presence of condensable groups: sulfonic acid, carboxylate, or hydroxy groups.TGA analyses of the powders indicates an increase in the decomposition temperature of Cu-BTC lending further credence to the occurrence of the interparticle condensation reactions.SEM imaging shows some morphological difference in the shape of the Cu-BTC particles from octahedra to more irregularly shaped polyhedral (Figure S1).Interestingly, the number average  particle size of the particles decreases in the following order: Cu-BTC Cu-BTC-0.4> Cu-BTC-0.8(figure 2).Further, the percent of particles below a threshold value of 300 nm shows an increase from around 30% for Cu-BTC to around 95% for Cu-BTC-0.8.In contrast, a sharpening of the peaks corresponding to the (222) and (220) planes in the PXRD measurement indicates an increase in the overall crystallite size (Table S2).Both results could indicate an atomic Ostwald ripening process which may be occurring due to the acidity of the EDSA.Interestingly, the treatment has a direct impact on the thermal stability of the powders.The thermal decomposition temperature of the powders increases on treatment with the EDSA as measured by TGA measurements (Figure S3).

Characterization of densified monoliths
After observing that a treatment with EDSA of Cu-BTC powder led to the densification of the powder, we worked to make monolithic Cu-BTC to take advantage of this process.For this, a slight alteration of the recipe used to prepare MOF powders was done.The Cu-BTC powder was ground with EDSA to yield a solid mixture which was loaded into a stainless steel the die and pressed lightly under the weight of the punch that goes with the die (∼0.7 N).After the die was heated to 150 °C like the conditions used for treating the Cu-BTC powders.It is important to note that under the same conditions the ground Cu-BTC powder does not form a monolithic part which can be taken out of the die (Figure S4).The densification process occurs to a greater extent in the case of the monolith, likely due to the compaction with the punch, with the final monolith density reaching around 1 g/ml.The EDSA/Cu-BTC ratio was kept at the value of 0.8 mol EDSA / mol Cu-BTC.Optical and SEM imaging of the monolith indicated the presence of tiny macropores and meso-/micropores showing hierarchical porosity (Figure S5).The temporal dependance of the densification process was checked by carrying out the monolith formation process for 6, 12, 24 and 72 h.Notably, the density and correspondingly the volumetric CO 2 adsorption (at 313 K and 0.15 bar) of the monoliths peak at ∼1 g/ml with a compression time of 24 h (figure 3(B)).In the same respect, this sample also had a significantly improved volumetric CO 2 adsorption capacity at 313 K and 0.15 bar (figure 3(A)) when compared to the parent powder.Longer reaction times are not beneficial, supposedly due to the MOF degradation upon exposure to a temperature of 150 °C.For all samples treated with EDSA, the volumetric CO 2 adsorption capacity is higher than the Cu-BTC powder, highlighting a beneficial effect of this treatment.Further, the isosteric heat of adsorption (Q st ) of CO 2 calculated between a temperature range of 20 °C-60 °C (Figures S6, S7 Breakthrough CO 2 capture performance of the monoliths Finally, the separation of CO 2 from gas mixtures was assessed for the monolith, Cu-BTC-0.8-mono-24h, and compared to the Cu-BTC powder via TGA and breakthrough measurements.For breakthrough, CO 2 was first separated from a gas mixture containing 85% N 2 and 15% CO 2 both with and without humidity using a home-built setup (figure 4).The performance of the parent powder had a breakthrough separation time of approximately 12 min g −1 in dry conditions (figure 4(A)) while the monoliths showed a separation time of 7 min g −1 (figure 4(B)).Last, both materials are shown to be highly cyclable, with up to 10 CO 2 adsorption (40 °C) and desorption (120 °C) cycles in a stream of pure CO 2 (figure 5).Although the gravimetric capacity of the monolith is lower, the volumetric working capacity is expectedly higher than the parent Cu-BTC-powder.

Extension of strategy to Cr-BDC
The densification strategy was also extended to another MOF system, namely Cr-BDC, which was chosen because of its mesoporous structure and its interest in several different applications.Interestingly, similar trends in the  PXRD patterns of the as-synthesized and treated Cr-BDC are observed as in the case of Cu-BTC.This includes a new peak appearing at around 2-3 degrees due to the surface modification (Figure S8).Further, particle size distributions obtained from SEM images show a similar decrease in the average particle size as observed for Cu-BTC (Figure S9).Thus, we can conclude that the EDSA leads to a similar densification process in case of Cr-BDC.

Conclusion
In conclusion, we utilized EDSA, an acid reagent, to densify a polycrystalline Cu-BTC powder.We believe that the reaction proceeds via two mechanisms occurring in parallel.The first one proceeds via an Ostwald ripening of the Cu-BTC particles.The second mechanism proceeds via condensation reactions between the uncoordinated terminal surface groups.We also show that this chemical densification process can be carried out to make robust, monolithic Cu-BTC structures with good volumetric CO 2 capture performance.Finally, we envisage that this methodology can be further extended beyond Cu-BTC and Cr-BDC to other MOFs thereby making accessible a new chemical way to densify MOF monoliths which constitutes ongoing work.We hope that densification strategies like this one, will the future lead to effective ways to shape fine MOF powders into larger aggregates that have both optimum volumetric and gravimetric capacities.

Figure 2 .
Figure 2. Particle size distributions for the synthesized Cu-BTC (red), Cu-BTC-0.4(blue) and Cu-BTC-0.8with the ANOVA average particle sizes plotted as a function of the molar ratio of EDSA to Cu-BTC.

Figure 3 .
Figure 3. (A) Volumetric CO2 adsorption isotherms for the monolithic Cu-BTC compared to the synthesized Cu-BTC powder along with representative images for the two samples; (B) Variation of the density (black) and volumetric CO2 capacity at 313 K and 0.15 bar as a function of the reaction time; (C) Isosteric heat of adsorption of the Cu-BTC-mono (blue) as compared with the synthesized Cu-BTC-powder (black).

Figure 4 .
Figure 4. Breakthrough plots for Cu-BTC-powder (A) and Cu-BTC-0.8-mono-24h (B) in dry conditions.All experiments were carried out in a mixed gas stream containing 15% CO2 and 85% N2.Note: For figure 4(A), the spheres in black represent N2 and the spheres in gray CO2.For figure 4(B), the spheres in dark blue represent N2 and the spheres in light blue represent CO2.