RESEARCH ARTICLE www.advmat.de Monolithic Zirconium-Based Metal–Organic Frameworks for Energy-Efficient Water Adsorption Applications Ceren Çamur, Robin Babu, José A. Suárez del Pino, Nakul Rampal, Javier Pérez-Carvajal, Philipp Hügenell, Sebastian-Johannes Ernst, Joaquin Silvestre-Albero, Inhar Imaz, David G. Madden, Daniel Maspoch, and David Fairen-Jimenez* 1. Introduction Space cooling and heating, ventilation, and air conditioning (HVAC) accounts for roughly 10% of global electricity use and are responsible for ca. 1.13 We are currently living in one of the warmest periods in recorded climate his- gigatonnes of CO2 emissions annually. Adsorbent-based HVAC technologies tory. With these increases in global temper- have long been touted as an energy-efficient alternative to traditional atures, the demand for heating, ventilation, refrigeration systems. However, thus far, no suitable adsorbents have been and air conditioning (HVAC) has steadily developed which overcome the drawbacks associated with traditional sorbent increased over the past three decades. materials such as silica gels and zeolites. Metal–organic frameworks (MOFs) Worldwide, there are currently about 1.6 bil- lion air conditioning (AC) units in use, con- offer order-of-magnitude improvements in water adsorption and regeneration suming over 2000 terawatt-hours (TWh) or energy requirements. However, the deployment of MOFs in HVAC roughly 10% of the 21 000 TWh of electricity applications has been hampered by issues related to MOF powder consumed globally in 2016.[1] The increase processing. Herein, three high-density, shaped, monolithic MOFs (UiO-66, in demand for cooling can be attributed UiO-66-NH2 , and Zr-fumarate) with exceptional volumetric gas/vapor uptake to a wide variety of applications, includ- are developed—solving previous issues in MOF-HVAC deployment. The ing domestic and office AC units, server farm cooling, and industrial heat exchang- monolithic structures across the mesoporous range are visualized using ers. In 2017, there were approximately 8 small-angle X-ray scattering and lattice-gas models, giving accurate million data centers around the world that predictions of adsorption characteristics of the monolithic materials. It is also consumed 416.2 TWh of electricity.[2] This demonstrated that a fragile MOF such as Zr-fumarate can be synthesized in equated to 2% of global electricity consump- monolithic form with a bulk density of 0.76 gcm−3 without losing any tion of which 38% of this consumption was consumed by AC systems.[3] The high en- adsorption performance, having a coefficient of performance (COP) of 0.71 ergy consumption of existing HVAC tech- with a low regeneration temperature (≤ 100 °C). nologies comes with a significant penalty in terms of their environmental footprint. C. Çamur, R. Babu, N. Rampal, D. G. Madden, D. Fairen-Jimenez J. A. Suárez del Pino, J. Pérez-Carvajal[+] , I. Imaz, D. Maspoch The Adsorption & Advanced Materials Laboratory (A2ML) Catalan Institute of Nanoscience and Nanotechnology (ICN2) Department of Chemical Engineering & Biotechnology CSIC and the Barcelona Institute of Science and Technology Campus UAB University of Cambridge Bellaterra, Barcelona 08193, Spain Philippa Fawcett Drive, Cambridge CB3 0AS, UK J. Pérez-Carvajal[+] E-mail: df334@cam.ac.uk Laboratoire de Physique de l’Ecole Normale Supérieure-ENS Université PSL CNRS Paris 75005, France The ORCID identification number(s) for the author(s) of this article P. Hügenell, S.-J. Ernst can be found under https://doi.org/10.1002/adma.202209104 Fraunhofer-Institute for Solar Energy Systems (ISE) Heidenhofstr. 2, 79110 Freiburg, Germany [+] Present address: Instituto de Ciencia de Materiales de Madrid, J. Silvestre-Albero Consejo Superior de Investigaciones Científicas (ICMM-CSIC), 28049 Laboratorio de Materiales Avanzados Madrid, Spain Depto. de Química Inorgánica © 2023 The Authors. Advanced Materials published by Wiley-VCH Universidad de Alicante GmbH. This is an open access article under the terms of the Creative San Vicente del Raspeig E-03690, Spain Commons Attribution License, which permits use, distribution and D. Maspoch reproduction in any medium, provided the original work is properly cited. ICREA DOI: 10.1002/adma.202209104 Pg. Lluís Companys 23, Barcelona 08010, Spain Adv. Mater. 2023, 35, 2209104 2209104 (1 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de The demand for AC units is expected to triple to 5.6 billion due to their characteristic Type V, S-shaped isotherms (Figure by 2050 as global populations rise and the demand for HVAC S1, Supporting Information).[18–26] Studies on MOF-based adsor- increases.[4] The rising demand for HVAC in the coming decades bent refrigeration have seen significant improvements in COP will require a significant shift in terms of energy efficiency to en- (e.g., MIP-200 = 0.81)[23] when compared with zeolites[27] and able sustainable cooling in the future. silicates.[11] Similarly, the use of MOFs has seen dramatic re- Traditional HVAC technologies employ energy-intensive de- ductions in the heat requirements for adsorbent regeneration humidification processes that require incoming air to be cooled (≤100 °C) compared to zeolites.[28] to the dew point of moisture (ca. 5–7 °C) to enable condensation. The integration of adsorption materials in AHT devices is only As an alternative to the traditional chiller technology, adsorption- possible after some specific shaping processes to obtain opti- driven heat transformation (AHT) systems have long been touted mum performance.[27,29] Since MOFs are traditionally produced for energy-efficient cooling systems, raising the evaporation tem- as powders, their shaping is crucial for integration into indus- perature to ambient temperatures (ca. 20–25 °C), which in turn trial processes.[26] Numerous shaping techniques have been re- can improve the coefficient of performance (COP) of the en- ported to produce MOFs as granules, pellets, and foams.[30,31] tire system.[5] The AHT process of heating and cooling is based However, the loss of surface area and porosity during densifica- on an adsorption–desorption cycle of a working fluid that con- tion and shaping (between 14% and 64%) compared to powders sists of four steps: isosteric heating (B→C), isobaric desorption remains a major issue in MOF deployment in the industry.[32,33] (C→D), isosteric cooling (D→A), and isobaric adsorption (A→B) These reductions in performance can be associated with the in- (Figure 1). In the adsorption cycle (Figure 1, Step 1 and Step 2), corporation of dead volume through the use of binders and/or the effective cold is produced during the evaporation of the work- the structural collapse of the MOF structure and porosity as a ing fluid (Qev ) as the process is endothermic. Concurrently, the result of the high mechanical pressure.[26,34–36] Even more im- useful heat (Qads ) is released by the adsorption of the produced portant, pelletized MOFs using lower mechanical pressures to vapor. As the adsorbent will be saturated after a while, the des- avoid pore collapse suffer from low bulk densities due to parti- orption of the working liquid—in other words, the regeneration cle packing inefficiencies. In contrast, the recent development of of the adsorbent—will be required. In the desorption cycle (Fig- so-called monolithic MOFs (mono MOFs)[37] has led to a paradigm ure 1, Step 3 and Step 4), the energy (Qdes ) is used to desorb the shift in terms of MOF shaping. We have reported the sol–gel working fluid at a temperature (Tdes ). The desorbed working fluid synthesis of several mono MOFs (mono UiO-66, mono HKUST-1, and condenses by producing useful heat (Qcon ).[6–8] For chilling appli- mono ZIF-8) [32,37,38] as shaped bodies up to about 1 cm3 in size with- cations here, the facile regeneration of adsorbent materials at low out using supports, applied mechanical pressures, or binders. temperature (Tdes ≤ 100 °C) and a step between 10% and 30% RH The synthesis of mono MOFs represents a viable route towards in- are a critical requirement to develop AHT systems with optimum dustrial production of MOFs in high-density, shaped bodies while efficiency.[9,10] preserving the full adsorption performance of the material. The Commercially available AHT pumps currently employ tradi- development of mono MOFs has already given way to materials that tional porous adsorbents such as silicas and zeolites. While these demonstrate benchmark performance in terms of natural gas materials work on this application, their limitations in terms and hydrogen storage.[32,37,39] of tunability limit their performance and potential for further In this contribution, we investigate the water adsorption per- development. Additionally, many traditional adsorbents require formance of mono MOFs for HVAC applications. We have previ- high temperatures for regeneration (e.g., Tdes >180 °C for zeo- ously demonstrated that Zr-based MOF UiO-66 (UiO = Univer- lites), which in turn can dramatically reduce the COP—silica sitetet i Oslo), is amenable to high-density mono MOF formation. gel = 0.35 and zeolite 4A = 0.56—of the AHT system.[11,12] Herein, we extend our sol–gel approach to synthesize and assess The lack of suitable adsorbents is currently the major limiting the water adsorption performance of three monolithic Zr-MOFs, factor in the development of solid adsorbent dehumidification mono UiO-66, mono UiO-66-NH2 , and mono Zr-fumarate, comparing systems. them to their powder and pelletized forms. Among these three As an alternative to traditional adsorbents, MOFs[13,14] have materials, we have already published mono UiO-66 and mono UiO- garnered significant attention recently for water adsorption ap- 66-NH2 for gas storage applications whereas mono Zr-fumarate has plications. There are currently more than 100 000 MOF[15,16] been synthesized for this study. These materials feature large structures in existence versus ca. 1000 silica’s and zeolites; their hydrophilic channels in their crystal structure, which leads to tunable nature enables precise control of material design at the outstanding water-adsorption properties. This includes a high molecular level. This, in addition to their extremely high pore vol- uptake of water in a low relative pressure range, characteristic umes, can enable the design of materials with ultra-high poros- Type V (i.e., S-shaped) isotherms facile desorption at fairly low ity and optimal MOF–water interactions, making them ideally temperatures, and high COP for chillers.[25,40] Then, we evaluate suited for overcoming issues related to water adsorption capac- the performance of the six adsorbents (powders and monoliths) ities and regeneration energies. The use of MOFs for water cap- from single-component water adsorption isotherms, adsorption ture and dehumidification applications has only recently been enthalpy (ΔHads ), and COP calculations, kinetics studies, and investigated, partly due to the development of MOFs with supe- adsorption–desorption cyclability testing. Finally, using a lattice- rior thermochemical and hydrolytic stability.[17] To date, MOFs gas model based on the combination of small-angle X-ray scatter- have displayed benchmark performance for a wide variety of wa- ing (SAXS) and grand canonical Monte Carlo molecular simula- ter adsorption applications, including atmospheric water harvest- tions, we reveal the monolithic structure of the materials across ing, adsorbents-based chillers, and adsorbent-based heat pumps the mesoporous range. Adv. Mater. 2023, 35, 2209104 2209104 (2 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 1. Operation principle of an AHT pump. Isosteric diagram of an AHT pump cycle of the adsorption (Step 1 and Step 2) and the desorption (Step 3 and Step 4) cycles where the operating temperature and pressure of the evaporator are fixed at Tev = 10 °C and Pev = 1.23 kPa, and the operational temperature and pressure of the condenser are fixed at Tcon = 30 °C and Pcon = 4.25 kPa. In Step 1 (isobaric adsorption, A→B), the working fluid (water) is heated from 10 °C (Tev ) to 30 °C (Tcon ), using energy equivalent to Qev coming from the surroundings (effective cooling). Here, the adsorbent loading increases from Wmin to Wmax , while the temperature of the adsorbent reduces from 80 (T2 ) to 30 °C (Tcon ), giving out energy equivalent to Qads . In Step 2 (isosteric heating, B→C), the adsorbent is heated from 30 (Tcon ) to 50 °C (T1 ), using energy equivalent to QBC , while at the same time, pressure increases from Pev to Pcon ; adsorbent loading remains unchanged at Wmax . In Step 3 (isobaric desorption, C→D), the adsorbent is heated from 50 (T1 ) to 100 °C (Tdes ), using energy equivalent to Qdes or, in other words, Qreg , while at the same time, the adsorbent loading reduces from Wmax to Wmin , and an energy equivalent to Qcon is given out to the surroundings (effective heating). In Step 4 (isosteric cooling, D→A), the adsorbent is cooled from 100 (Tdes ) to 80 °C (T2 ), giving out energy equivalent to QDA , while at the same time, the pressure reduces from Pcon to Pev ; adsorbent loading remains unchanged at Wmin . Adv. Mater. 2023, 35, 2209104 2209104 (3 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 2. Zr-based MOFs UiO-66, UiO-66-NH2 , and Zr-fumarate. a) The secondary building unit (SBU) of the UiO-family and ligands (fumaric acid, terephthalic acid, and 2-aminoterephthalic acid, respectively, from left to right) employed. b) Zr-fumarate structure (purple, zirconium; cyan, oxy- gen; gray, carbon; and white, hydrogen). c–f) Optical images of mono UiO-66 (c), mono UiO-66-NH2 (d), Zr-fumarate gel (e), and mono Zr-fumarate (f) (scale bars = 0.5 cm). g) Optical image of the gram-scale (335 g) mono Zr-fumarate product in a 500 mL bottle. h) Experimental PXRD pattern of mono Zr-fumarate (red) compared to the simulated one (black). 2. Synthesis, Characterization, and Scale-Up sorption isotherms at 77 K show similar microporosity and small changes in the mesoporosity for three different batches (Figure Zr-based MOFs such as the UiO-family are promising mate- S2, Supporting Information). Then, this procedure was found to rials for HVAC applications due to their high thermal and be scalable, whereby a 1 L batch reaction was found to produce chemical stability, and large porosity. UiO-type MOFs consist ca. 67 g (≈75% yield) of mono Zr-fumarate. To confirm the scala- of Zr-oxo clusters linked together by carboxylate-based ligands bility of the process, we reproduced the 1 L batch reaction five (L), such as terephthalic acid (BDC), 2-aminoterephthalic acid times, which resulted in 335 g (Figure 2g). Powder forms of UiO- (ABDC) and fumaric acid (FA), to give the overall composition of 66, UiO-66-NH2 , and Zr-fumarate (powd UiO-66, powd UiO-66-NH2 , [Zr6 O4 (OH)4 (L)6 ]n (Figure 2a,b). Herein, we have studied three and powd Zr-fumarate) were synthesized using similar procedures different sets of samples: monolithic (mono), standard powders but including fast overnight drying at 100 °C under vacuum. (powd), and pelletized (pell) samples. mono UiO-66 and mono UiO- To achieve an extensive comparison, we used the already pub- 66-NH2 were synthesized via previously reported methods.[37] In lished physical properties for the pelletized form of UiO-66, UiO- a typical synthesis, MOF primary nanoparticles are successfully 66-NH2 , and Zr-fumarate (pell UiO-66, pell UiO-66-NH2 , and pell Zr- densified using mild drying conditions to yield centimeter-scale fumarate).[34,35] We utilized powder X-ray diffraction (PXRD) to monoliths (Figure 2c,d). The new monolithic material, mono Zr- confirm the structures of all synthesized materials. Figure 2h fumarate, was produced by using the same procedure previ- and Figure S3, Supporting Information, show the PXRD pat- ously described for powder form[41] (Figure 2e). After the col- terns for the mono MOFs produced, displaying a clear Scherrer loidal solution was formed, we washed the material several times line broadening, caused by the non-convergence of the diffraction through centrifugation by using acetone and ethanol. Then, the line in nano-sized particles. Field electron scanning electron mi- collected Zr-fumarate gel was dried at ambient conditions slowly croscopy (FE-SEM) further confirmed that the produced mono Zr- overnight or longer depending on the amount of the gel to form MOFs comprise densified MOF nanoparticles with particle sizes mono Zr-fumarate (Figure 2f). Before scaling up this procedure, of 20 ± 4.5, 19 ± 3.6, and 25 ± 5 nm for mono UiO-66, mono UiO- we checked the reproducibility of the different batches by com- 66-NH2 , and mono Zr-fumarate, respectively (Figure S4, Support- paring the porosity of the produced mono Zr-fumarate. The N2 ad- ing Information). Figure S5, Supporting Information, shows the Adv. Mater. 2023, 35, 2209104 2209104 (4 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Table 1. Experimental and simulated textural, physicochemical, and water adsorption properties of monolithic (mono), and powdered (powd) samples, as well as molecular simulation (sim) for the Zr-MOFs, studied. Pelletized (pell) data is taken from the literature. Materials SBET a) SBET (v)b) W0 c) Vtot d) 𝜌b e) H2 O uptake at 25 °C [m2 g−1 ] [m2 cm−3 ] [cm3 g−1 ] [cm3 g−1 ] [g cm−3 ] RH [%] = 10 RH [%] = 30 RH [%] = 90 [g g−1 ] [g cm−3 ] [g g−1 ] [g cm−3 ] [g g−1 ] [g cm−3 ] mono UiO-66 1223 905 0.47 1.59 0.74 0.07 0.05 0.20 0.15 0.41 0.30 powd UiO-66 1223 635 0.50 2.05 0.52 0.07 0.03 0.20 0.10 0.42 0.21 35 pell UiO-66 1459 627 0.54 – 0.43 – – – – – – sim UiO-66 1120 1413 0.39 – 1.62 – – – – – – mono UiO-66-NH2 988 1027 0.38 1.47 1.04 0.10 0.10 0.18 0.19 0.48 0.50 powd UiO-66-NH2 1094 645 0.42 0.94 0.59 0.11 0.07 0.20 0.13 0.27 0.17 35 pell UiO-66-NH2 816 408 0.33 – 0.50 – – – – – – sim UiO-66-NH2 897 1063 0.39 – 1.28 – – – – – – mono Zr-fumarate 854 649 0.33 0.90 0.76 0.15 0.12 0.26 0.21 0.39 0.31 powd Zr-fumarate 868 469 0.33 1.39 0.54 0.09 0.09 0.21 0.14 0.44 0.26 26,34 pell Zr-fumarate 569–479 – – – – – – – – – – sim Zr-fumarate 821 1429 0.28 – 1.74 – – – – – – a) BETarea (SBET ); b) Volumetric BET area (SBET (v)); c) Micropore volume (W0 ) obtained at P/P0 = 0.1; d) Total pore volume (Vtot ) obtained at P/P0 = 0.98; e) Bulk density (𝜌b ) quantified using Hg-porosimetry. thermogravimetric analysis (TGA) of the mono MOF samples, with The use of modulators is related to an increase in the num- monolith decomposition occurring at temperatures higher than ber of framework defects in Zr-MOFs, forming narrow meso- 250 °C. pores due to missing linkers or metal clusters.[45] To examine Figure S6, Supporting Information, shows the experimental the missing linkers in the structures of mono Zr-MOFs, we used N2 adsorption isotherms at 77 K for monolithic and powder X-ray photoelectron spectroscopy (XPS) to quantify Zr/C ratio samples as well as obtained from grand canonical Monte Carlo for each material, CHN combustion through inductively cou- (GCMC) simulations. Table 1 shows the BET areas (SBET ) of pel- pled plasma-optical emission spectrometry (ICP-OES) to quan- letized samples taken from the literature[26,34,35] and synthesized tify atomic Zr%, C%, and N%, and nuclear magnetic resonance samples calculated using the extended Rouquerol’s criteria im- (1 H-NMR). Figures S15–S17, Supporting Information, show the plemented in BETSI,[42] (Figures S7–S13, Supporting Informa- XPS results; Table S4, Supporting Information, show the main tion) as well as the micropore (W0 ) and total (Vtot ) pore volumes, results. Together, we observed an increase in the Zr/C ratio for bulk (mercury) density, and water uptakes at different relative mono UiO-66, mono UiO-66-NH2 , and mono Zr-fumarate, which are humidities.[43] There are no significant differences between pow- 0.11, 0.11, and 0.13, respectively, compared to Zr/C ratios of der and monolithic materials in the micropore region and in defect-free structures, that is, 0.06, 0.06 and 0.12, respectively. terms of the calculated SBET values. However, compared to the Literature attributed these changes to missing linker defects due GCMC isotherms, the experimental samples have higher SBET to the C decrease in the structure.[44,46] Table S5, Supporting In- and W0 than the theoretical maximum for defect-free sim UiO- formation, shows the CHN combustion analysis from ICP-OES, 66, sim UiO-66-NH2 , and sim Zr-fumarate. This is the result of the which verifies these findings. From the 1 H-NMR studies, we also presence of modulators in the synthesis and missing linkers found that the molar ratios of acetate/ligand in the structures in the final structures. Indeed, calculated defect-free SBET using were 0.13, 0.16, and 0.13 for mono UiO-66, mono UiO-66-NH2 , and GCMC simulations are also close to the original report of UiO-66 mono Zr-fumarate, respectively. These results are similar to those (Lillerud and co-workers, 1187 m2 g−1 ),[44] and reports on UiO- previously reported for powder samples (Figure S18, Supporting 66-NH2 (Farha and co-workers, 800 m2 g−1 )[45] and Zr-fumarate Information).[47,48] (Yaghi et al. and co-workers, 690 m2 g−1 ).[40] For all the materi- We then analyzed the packing efficiency of the monolithic als, we obtained high gas uptakes below 0.1 P/P0 , indicating the and powder materials, measuring their bulk densities (𝜌b ) presence of extensive microporosity. We also found N2 uptake through mercury intrusion porosimetry, a technique based on increases at higher relative pressures, indicating the existence Archimedes’ principle that can also probe the macro and meso- of mesoporosity. The NLDFT pore-size distributions (PSD) high- porosity of the samples; Table 1 shows the densities and Fig- light the substantial micropore volume, whereas the BJH analysis ure S19, Supporting Information, shows the mercury intru- confirms significant volumes of wide mesoporosity, varying be- sion pore size distributions. From the comparison of mono MOFs tween 10 and 30 nm for the monoliths and 10 and 15 nm for pow- and powders, the critical advantage of monolithic materials is ders encountered due to the inherent assembly of the nanoparti- their high bulk density, which translates to higher volumetric cles (Figure S14, Supporting Information). SBET , pore volumes, and adsorption capacities compared to their Adv. Mater. 2023, 35, 2209104 2209104 (5 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de traditional powder counterparts. Even though the synthe- used to create a 3D reconstruction of the monoliths. Figure 3g sized powder samples have similar gravimetric SBET values to and Figure 3h show the S2 (r) of the target and reconstructed monolithic materials, mono UiO-66, mono UiO-66-NH2 , and mono Zr- medium for both materials; Figure 3j,k shows the 3D reconstruc- fumarate show 28%, 49%, and 30% higher volumetric N2 gas tion of the lattice-gas models for the activated mono UiO-66 and adsorption capacity and, therefore, volumetric SBET , than their mono Zr-fumarate, respectively—for each structure, the gray vox- powder counterparts (Figure S6d–f, Supporting Information). As els are the cells occupied by the material and white voxels cor- we have explained elsewhere, the high density of the monoliths respond to the porosity of the material. Figure 3i,l shows the arises from the efficient packing of MOF primary particles within comparison of experimental and GCMC simulated N2 isotherms the monolithic macrostructure.[32,33] On the other hand, the in- obtained on different models for UiO-66 and Zr-fumarate, re- terparticle space present within the body of powders but also of spectively. GCMC simulations on a perfect structure can only pellets reduces the bulk density of the materials. Densification capture, as expected, the microporosity of both MOFs—this ap- and pelletization of UiO-66, UiO-66-NH2 , and Zr-fumarate pow- proach works well for structures with perfect packing. But, in the ders have been previously reported under a variety of pressures case of the monoliths, we see an additional slope in the isotherm with and without binders.[34,35] In our case here, as a result of the associated with the porosity that arises due to the random pack- pelletization, these materials show higher bulk densities. How- ing of the particles—this extrinsic porosity is not accessible us- ever, the use of pressure usually brings together a reduction in ing GCMC simulations alone. To describe the isotherm for the the gravimetric BET area.[34,35] In comparison to the pelletized monolith across the entire p/p0 range sampled, GCMC com- samples, mono UiO-66, mono UiO-66-NH2 , and mono Zr-fumarate ex- puted isotherms must be complemented with isotherms calcu- hibit exceptional bulk densities (Table 1). lated using the lattice-gas models. Hence, to model the trajectory of the system in the grand canonical ensemble, we employed ki- netic Monte Carlo simulations to obtain the nitrogen adsorption 3. Small-Angle X-ray Scattering and Lattice-Gas isotherms. From the mono Zr-fumarate and mono UiO-66 isotherms Models generated, we found an excellent agreement between experiment and theory within the high-pressure region (p/p0 > 0.6) of the To fully understand the porous texture and structure of our adsorption isotherm, providing complementary data to the pure monoliths as well as the contributions to the total gas uptake GCMC-calculated isotherms. This demonstrates the applicabil- arising from inter-particle mesopores, we used small-angle X-ray ity of lattice-gas models in capturing the accessible extrinsic, scattering (SAXS) and lattice-gas models. Figure 3a–f shows the inter-particle mesoporosity of mono MOFs. Collectively, these re- experimental SAXS curves, the fits based on a spherical form fac- sults suggest that the hierarchical porosity of mono Zr-MOFs can tor model, and the particle size distributions; Table S3, Support- be accurately described computationally across the micro- and ing Information, summarizes the results. Since the Guinier re- mesoporous range, enabling robust future predictions of adsorp- gions of these materials—the region which describes the radius tion characteristics. of gyration (Rg ) of the particles—lie outside the range probed, we used instead a simplified power law based on Porod’s law given 4. HVAC Applications by Equation (1): With the materials characterized, we moved to the determination ( ) ( )−P I Q = B Q (1) of the water adsorption properties of the powdered and mono MOF materials. We initially performed single-component water ad- where B is the prefactor and P is the power-law exponent, fit- sorption isotherms. The powdered MOFs illustrated character- ted to the slope of I(Q) in the low-Q region. The power-law expo- istic water adsorption isotherms similar to those previously re- nents for mono UiO-66 and mono Zr-fumarate are 3.035 and 3.111, ported in the literature for each material. Importantly, mono MOFs respectively, indicating the presence of surface fractals, whereas retained high water uptake associated with the powdered variants for mono UiO-66-NH2 the power-law exponent is 2.895, indicating of each material (Figure 4a).[40] The narrow and hydrophilic pores the presence of mass fractals. Importantly, the particle size distri- in mono Zr-fumarate give way to a steep increase in the water up- butions for mono UiO-66 and mono UiO-66-NH2 are similar, exhibit- take in the 5%–30% relative humidity (%RH) range of the Type V ing two main particle size populations of mean diameters ≈80 isotherm. Increasing the length of the linker through isoreticular and ≈160 Å. In addition, a small volume fraction of the parti- chemistry increases the hydrophobicity of the mono MOF, leading cles in mono UiO-66-NH2 exhibits a mean diameter of 234 Å. For to a shift of the inflection point (𝛼) from ca. 8% RH to ca. 25% mono Zr-fumarate, we see one main population of particles with RH for mono UiO-66. In comparison, the functionalization of the a mean diameter of 76 Å, and an equivalent volume fraction of ligand with a hydrophilic functional group, such as –NH2, leads particles having mean diameters of 167, 252, 339, and 424 Å. to strong attraction between –NH2 and water molecules since the Anticipating the performance of the different samples on wa- –NH2 group serves as an H-bond donor. We have explored before ter adsorption, as described below, we decided to focus on the top- the impact of the host-guest and guest-guest interactions in the two performing materials for the lattice-gas models, and hence shape of the isotherms;[48] in this case, it involves the change in excluded UiO-66-NH2 . Lattice-gas models have been widely used the isotherm’s shape from Type V (i.e., S-shaped) to a combined to investigate the nature of adsorption hysteresis for fluids Type I (i.e., from convex to concave) + Type IV, and the shift of the trapped in interconnected void spaces of porous glasses.[46,47] Us- step found to 2%–16% RH for mono UiO-66-NH2 .[20,49] In addition ing the SAXS data for mono UiO-66 and mono Zr-fumarate, a two- to the increased interaction, the addition of the –NH2 functional point correlation function S2 (r) was generated, which was then group can also impart strain on the structure, giving way to linker Adv. Mater. 2023, 35, 2209104 2209104 (6 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 3. a–l) SAXS experimental data, SAXS fitting, and particle size distributions for mono UiO-66, mono Zr-fumarate, and mono UiO-66-NH2 (a–f) + Lattice-gas model reconstruction and molecular simulations for mono Zr-fumarate and mono UiO-66 (g–l). Experimental SAXS data is shown in red (circles) and fitting is shown in black (line) for mono UiO-66 (a), mono UiO-66-NH2 (b), and mono Zr-fumarate (c). Particle size distribution is shown in blue (line) for mono UiO-66 (d), mono UiO-66-NH2 (e), and mono Zr-fumarate (f). Comparison of S2 (r) functions of the target (gray line) and reconstructed medium (black dashed line) for mono Zr-fumarate (g) and mono UiO-66 (h). Reconstructed 3D realization of mono UiO-66 (j) and mono Zr-fumarate (k) defined on a bcc lattice with periodic boundaries; gray (white) voxels represent the solid (void) phase. The size of the system is 60 × 60 × 60 pixels for the 3D realization. Comparison of the experimental adsorption isotherms to simulated adsorption isotherms of N2 at 77 K for UiO-66 (i) and Zr-fumarate (l); monolith—squares (blue for UiO-66 and orange for Zr-fumarate); powder—diamonds (gray); kMC simulations within the lattice-gas model—circles (yellow), and GCMC simulations—triangles (green). Adv. Mater. 2023, 35, 2209104 2209104 (7 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 4. a,b) Comparison of gravimetric (a) and volumetric (b) water adsorption isotherms of mono UiO-66 (blue squares), mono UiO-66-NH2 (green triangles), and mono Zr-fumarate (red diamonds) at 25 °C. Water adsorption isotherms for corresponding powdered materials are represented by black circles. Adsorption and desorption isotherms are represented by closed and open markers, respectively. rotation and an overall reduction of the pore size, hence increas- (1.9 × 10−4 g g−1 min−1 ) than powd UiO-66-NH2 (1.7 × 10−4 g ing the hydrophilicity of the material.[28] g−1 min−1 )—in other words, the monolithic material adsorbs Table 1 compares the gravimetric and volumetric water up- more water per minute than the powder gravimetrically and take of the materials at 10%, 30%, and 90% RH at 25 °C. The volumetrically (Figure 5a,b). The kinetics of adsorption is highly gravimetric water adsorption isotherms of the powder and mono- dependent not only on the interaction between water and the lithic samples display similar shapes and uptakes. The excep- internal surface with the specific ligands but also on the packing tion is mono UiO-66-NH2, which exhibits extensive water uptake of the primary particles, and therefore the intrinsic and extrinsic at higher relative humidity compared to powd UiO-66-NH2 (Fig- porosity of the monoliths. Importantly, the impact of the ligand ure 4a). This large water uptake can be attributed to the addi- effect, and in this case the hydrophilic –NH2 groups, is better tional meso-/macroporosity in the monolithic variant observed observed in the monoliths than in the powdered MOFs. We hy- in the N2 adsorption isotherm (Figure S6a, Supporting Infor- pothesize this is due to the denser packing of the MOF primary mation). In contrast to the gravimetric data, the monoliths ex- particles where the water molecules must travel through the hibit much higher volumetric adsorption performance compared intrinsic and extrinsic pore network during the adsorption and to their powder counterparts thanks to their combined high 𝜌b desorption processes. In contrast, in the case of the powder, wa- and SBET (Figure 4b). mono UiO-66, mono UiO-66-NH2 , and mono Zr- ter goes through the interstitial voids between the particles and fumarate showed 50%, 46%, and 50%, higher volumetric wa- is less exposed to the molecular design of the internal porosity. ter capacity at 30% RH than their powdered forms, respectively In the case of UiO-66, saturation takes place at 40% RH. −4 (Table 1). mono UiO-66 showed slower adsorption kinetics (1.3 × 10 g g−1 −1 −4 −1 −1 We also looked at adsorption kinetics while collecting the min ) than the powder version (1.7 × 10 g g min ), whereas water adsorption isotherms. Figure S20, Supporting Infor- desorption kinetics were similar. The slower kinetics might be mation, shows the adsorption and desorption kinetics of the caused by the densely packed hydrophobic nature of the ligand powder and monolithic materials; we also converted this data to and pores. However, when looking at the adsorption gradients volumetric units for comparison. powd UiO-66-NH2 with its –NH2 and the amount of water adsorbed per minute, mono UiO-66 shows hydrophilic groups were faster than mono UiO-66-NH2 to complete similar results (1.0 × 10−4 g cm−3 min−1 ) to that of powd UiO-66 one adsorption–desorption cycle (4000 min). Indeed, mono UiO- (0.9 × 10−4 g cm−3 min−1 ) due to the higher volumetric water up- 66-NH2 showed slower desorption kinetics than powd UiO-66-NH2 take between 0 and 40% RH (Figure 5c,d). For, Zr-fumarate, al- (6250 min). However, when comparing the amount adsorbed though equilibration for mono Zr-fumarate is slower (2.0 × 10−4 g at a given time and calculating the gradients, mono UiO-66-NH2 g−1 min−1 ) than for the powder (2.8 × 10−4 g g−1 min−1 ) in the 0– reached saturation pressure at ca. 30% RH slightly faster 30% RH region, both versions took the same time to complete Adv. Mater. 2023, 35, 2209104 2209104 (8 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 5. a–f) Gravimetric (first column) and volumetric (second column) water adsorption kinetics up to defined saturation pressure for powder(powd UiO-66-NH2 (a), powd UiO-66 (c), powd Zr-fumarate (e)) and monolith (mono UiO-66-NH2 (b), mono UiO-66 (d), mono Zr-fumarate (f)) Zr-MOFs. one adsorption and desorption cycle (4500 min). Again, when We also used the variable-temperature isobars of each looking at the amount of water adsorbed per minute, volumetri- mono MOF to calculate the COP for cooling. The COP for cooling cally, mono Zr-fumarate (1.7 × 10−4 g cm−3 min−1 ) shower steeper is defined as the useful produced energy divided by the energy gradients than powd Zr-fumarate (1.5 × 10−4 g cm−3 min−1 ) (Fig- required, namely the ratio of vaporization heat (Qev ) to regenera- ure 5e,f). All in all, mono Zr-fumarate (4500 min) was the fastest to tion heat (Qreg ) (Equations S11–S16, Supporting Information).[51] finish one adsorption–desorption cycle compared to mono UiO-66 To evaluate both Qev and Qreg , we calculated the heat capacity (6250 min) and mono UiO-66-NH2 (6200 min) (Figure S20, Sup- (cp ) of the monoliths as a function of the temperature of des- porting Information). orption (Tdes ) through a differential scanning calorimeter (Fig- The water adsorption properties of the mono MOFs prompted ure S22, Supporting Information). To determine the volumet- us to further examine these materials for adsorption chiller ap- ric working capacities (Δw) of the working pairs mono MOF/water plications at low regeneration temperatures (<100 °C). This is pairs, we analyzed the isosteric cycle diagrams of the adsorption, essential for the utilization of modest energy sources such as in- air-conditioning cycle separately. Importantly, Δw is defined as dustrial waste heat and solar heat instead of electricity.[50] Initially, the difference between the maximum (wmax ) and minimum wa- variable temperature water adsorption isotherms (25 and 40 °C) ter uptakes (wmin ). Figure 6b–d shows the cycle diagrams for the (Figure S21, Supporting Information) were collected to calculate monolithic MOF/water pairs, shown by the points from I to IV, the enthalpy of adsorption, ΔHads , using the Clausius–Clapeyron and calculated using the water adsorption isobars in a range of equation (Equation S16, Supporting Information). Figure 6a vapor pressure values (0.7, 1.2, 2.4, 3.7, and 5.6 kPa) and under shows its evolution with the vapor uptake. First, mono UiO-66 variable Tdes (Figure S23, Supporting Information) with the oper- displays a high ΔHads in absolute terms at low coverage ational temperature of the cycle evaporation (Tev ) fixed at 10 °C, (−60 kJ mol−1 ), dropping to −35 kJ mol−1 at an uptake value of and the temperatures of adsorption (Tad ) and condensation (Tcon ) ca. 10 wt.% before stabilizing at ca. −40 kJ mol−1 from 15 wt. both fixed at 30 °C. In this cycle, the working capacity (Δw) de- % until saturation. In contrast, mono UiO-66-NH2 displays a high pends on the Tdes . Once we had determined the Δw, Tdes , cp , and ΔHads in absolute terms of (−108 kJ mol−1 ), confirming our ΔHads parameters, we calculated the COP as a function of Tdes . previous observation on the shape of the isotherm. Again, the Figure 7a–c shows the volumetric Δw and COP as a function high ΔHads can be attributed to the presence of hydrophilic – of Tdes , with COP values of 0.66, 0.40, and 0.71 for mono UiO-66, NH2 functional groups within the material.[20,47,48] Conversely, mono UiO-66-NH2 , and mono Zr-fumarate, respectively. The maxi- the ΔHads of mono Zr-fumarate remained around −50 kJ mol−1 , mum working capacities at Tdes of 100 °C were 0.08, 0.12, and showing a value closer to that of the enthalpy of evaporation of 0.15 cm3 cm−3 . Despite its hydrophilic nature, mono UiO-66-NH2 water, and making it an excellent candidate for this application. has the lowest COP value (0.40) and working capacity (0.12 at Adv. Mater. 2023, 35, 2209104 2209104 (9 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 6. a) Enthalpy of adsorption as a function of loading for mono UiO-66 (blue triangles), mono UiO-66-NH2 (green squares), and mono Zr-fumarate (red diamonds). b–d) Isosteric cycle diagram for the working pairs mono UiO-66/water (b), mono UiO-66-NH2 /water (c), and mono Zr-fumarate/water (d) calculated for an air-cooling cycle including the volumetric working capacity calculated considering the bulk density of the mono MOFs. The colored dots on the isosteric diagrams represent the certain temperatures (from 30 to 100 °C see Figure S23, Supporting Information) for the indicated water uptake in a range of vapor pressure values (0.7, 1.2, 2.4, 3.7, and 5.6 kPa) found by using the isobars (Figure S23, Supporting Information) Points A–D show the flow of the process, where (B to C) is the isosteric heating, (C to D) is the isobaric desorption, (D to A) is the isosteric cooling, and (A to B) is the isobaric adsorption. 100 °C). This behavior in mono UiO-66-NH2 can be attributed to the phase of Zr-fumarate (Figure 7f and Figure S25, Supporting high –NH2 …. H2 O interactions as illustrated by the shape of the Information). isotherm and the ΔHads calculations, resulting in high regenera- Finally, we compared the performance of our mono MOFs tion requirements (Figure 6a). The working capacity of mono UiO- within the benchmark powder materials found in the literature 66 at low temperatures (0.03 and 0.05 cm3 cm−3 for Tdes of 60 and such as Ni2 Cl2 BBTA and Co2 Cl2 BBTA,[53,54] Zr-fumarate [52], 70 °C, respectively) is slightly higher than that of mono UiO-66-NH2 Al-fumarate,[55] MIP-200,[23] SAPO-34,[27] MIL-160,[56] and CAU- (0.03 and 0.04 cm3 cm−3 for Tdes of 60 and 70 °C, respectively) 10.[57] Among these benchmarks, Al-fumarate and SAPO-34 have but, overall, it suffers from its hydrophobic nature, showing been reported in pelletized form with a density of 0.61[55] and the lowest working capacity of the three materials at 0.70[58] g cm−3 , respectively. The other materials have not been 100 °C (0.08 cm3 cm−3 ) (Figure 7a). The mono Zr-fumarate mate- shaped and/or the bulk densities of the shaped bodies have not rial shows the best performance with a working capacity of 0.15 been reported in the literature. The powder materials cannot be cm3 cm−3 at 100 °C (Figure 7c), with a remarkably high COP, used in AHT systems therefore shaping and densification are equivalent to its powder form.[52] crucial. It has been reported that bulk density yields 50% lower To examine the stability of mono Zr-fumarate for HVAC appli- than the theoretical crystal density after pelletization for several cations, we carried out cycling tests over 50 consecutive water MOFs.[39,59,60] Therefore, we divided the crystal densities by half adsorption–desorption cycles (Figure 7d and Figure S24, Sup- to estimate the pellet densities, enabling us to compare the vol- porting Information). The sample was cycled between 50 and umetric capacity of the benchmark materials with that of our 120 °C in a saturated water atmosphere to promote fast adsorp- mono MOFs and compacted MOF pellets. We estimated the pel- tion and desorption. The gravimetric uptake was found to de- let densities of the benchmark materials based on their crys- crease from 0.25 to 0.20 g g−1 (ca. 20%) over 45 cycles, reach- tal densities which are 1.16 g cm−3 for MIP-200, 1.10 g cm−3 ing later a plateau. We extended it to 50 cycles (i.e., 5 additional for Ni2 Cl2 BTDD, 0.65 g cm−3 for Co2 Cl2 BTDD, 1.07 g cm−3 for ones) to make sure that the uptake is stable at 0.20 g g−1 (Fig- MIL-160 and 1.15 g cm−3 for CAU-10. To do the comparison, ure 7e). The mono Zr-fumarate was subsequently analyzed using first, we plot the gravimetric water sorption isotherm at 25 °C PXRD and 77 K N2 adsorption. The BET area decreased by 10% (Figure 8a) and then the volumetric water sorption isotherm at after the 50 cycles, whereas the PXRD confirmed the crystalline 25 °C (Figure 8b) based on the reported or the estimated pellet Adv. Mater. 2023, 35, 2209104 2209104 (10 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 7. a–c) COP for cooling and volumetric working capacity (Δw) as a function of Tdes for mono UiO-66 (a), mono UiO-66-NH2 (b), and mono Zr-fumarate (c). d) mono Zr-fumarate adsorption/desorption cycle testing performed over 50 consecutive cycles with regeneration at 120 °C. e) mono Zr-fumarate adsorption/desorption cycle plot zoomed in between cycles 41st–50th with regeneration at 120 °C. f) 77 K N2 adsorption/desorption isotherms for pristine (black stars) and post-water adsorption (gray circles) mono Zr-fumarate. Figure 8. a) Gravimetric and b) estimated volumetric water adsorption performance comparison of mono Zr-MOFs with powder benchmark adsorbents, measured at 25 °C. c) Corresponding COP values for chiller applications of mono Zr-MOFs compared with powdered benchmark materials as a function of Tdes (Tev = 10 °C and Tcon = 30 °C). densities belonging to benchmark powder materials and the bulk mance. The high bulk densities of mono MOFs provide greater vol- density of our mono MOFs. Our monolithic materials have excel- umetric adsorption capacities when compared to their powdered lent volumetric sorption capacities, even when compared to the and pelletized counterparts. Here, we have synthesized monoZr- estimated volumetric capacity of the benchmark pellet materials, MOFs, introducing an improved version of existing materials in which may be lower than their estimated bulk density. Especially, terms of their shaped performance. Our monoliths can be syn- mono Zr-fumarate displayed comparable COP for cooling to bench- thesized on a multigram scale, preserving the adsorption capabil- mark adsorbents for AHT systems (Figure 8c), being an excellent ities after shaping. Moreover, we have visualized the monolithic candidate with its green synthesis method to be commercialized structures across the mesoporous range using SAXS and lattice- for HVAC applications. gas models, giving accurate future predictions of the adsorption characteristics of the monolithic materials. This has allowed us 5. Conclusion to examine their performance for HVAC applications, comparing them to their experimental performance. We have previously reported how mono MOFs can form high- Among mono Zr-MOFs studied, mono Zr-fumarate stood out density shaped bodies whilst retaining their adsorption perfor- in overcoming the traditional limitations of MOF shaping Adv. Mater. 2023, 35, 2209104 2209104 (11 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 23, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202209104 by Readcube (Labtiva Inc.), Wiley Online Library on [26/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de techniques. This method demonstrates that producing densely Received: October 3, 2022 packed bodies is possible even for fragile MOFs that cannot Revised: March 7, 2023 be shaped by applied pressure without a significant decrease[34] Published online: April 21, 2023 in performance due to the pore collapse. mono Zr-fumarate also showed excellent performance with exceptional volumetric per- formance and high COP for cooling (0.71 at 100 °C), being com- parable with the reported benchmark materials as shown above (Figure 8). [1] B. Dean, J. Dulac, T. Morgan, U. Remme, B. Motherway, The Future High gravimetric uptake is crucial to be used in real-life appli- of Cooling: Opportunities for Energy-Efficient Air Conditioning In- cations as much as high volumetric capacity. Not only for, for ex- ternational Energy Agency, 2018, https://iea.blob.core.windows.net/ ample, vehicular applications (such as an HVAC system in a car) assets/0bb45525-277f-4c9c-8d0c-9c0cb5e7d525/The_Future_of _ where the space is limited but also for any other system where Cooling.pdf . the capex and opex can be minimized with smaller and more [2] Y. Li, X. Wang, P. Luo, Q. Pan, Energies 2019, 12, 1494. [3] J. Ni, X. Bai, Renewable Sustainable Energy Rev. 2017, 67, 625. compact systems. Thus, the volumetric capacity of the MOF is [4] B. J. van Ruijven, E. De Cian, Nat. Commun. 2019, 10, 2762. critical. On top of that, even though MIP-200 and Co2 Cl2 BTDD [5] S. Cui, M. Qin, A. Marandi, V. Steggles, S. Wang, X. Feng, F. Nouar, seem to have higher COP values due to their high gravimetric C. Serre, Sci. Rep. 2018, 8, 15284. uptake, they are still synthesized only as powders, which cannot [6] E. Hastürk, S. -J. Ernst, C. Janiak, Curr. Opin. Chem. Eng. 2019, 24, 26. be used in industrial applications at this stage. Also, when select- [7] X. Liu, X. Wang, F. Kapteijn, Chem. Rev. 2020, 120, 8303. ing optimal materials, the synthesis process of the selected mate- [8] J. Pérez-Carvajal, G. Boix, I. Imaz, D. Maspoch, Adv. Energy Mater. rial needs to be considered. The chemicals used in the synthesis 2019, 9, 1901535. procedure have an important effect on industrialization in terms [9] C. A. Balaras, G. Grossman, H.-M. Henning, C. A. Infante Ferreira, of cost, efficiency, and toxicity of the process. Therefore, mono Zr- E. Podesser, L. Wang, E. Wiemken, Renewable Sustainable Energy Rev. fumarate demonstrates that mono MOFs hold great promise for ap- 2007, 11, 299. [10] H. - M. Henning, Appl. Therm. Eng. 2007, 27, 1734. plications in next-generation, energy-efficient, adsorption-driven [11] M. B. Elsheniti, O. A. Elsamni, R. K. Al-dadah, S. Mahmoud, E. HVAC technologies. Elsayed, K. Saleh, in Sustainable Air Conditioning Systems (Eds.: C. Ghenai, T. Salameh), IntechOpen, London, UK 2018, Ch. 4. Supporting Information [12] A. Kumar, D. G. Madden, M. Lusi, K. - J. Chen, E. A. Daniels, T. Curtin, J. J. Perry IV, M. J. Zaworotko, Angew. Chem., Int. Ed. 2015, 54, 14372. Supporting Information is available from the Wiley Online Library or from [13] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O. the author. M. Yaghi, Science 2002, 295, 469. [14] S. Kaskel, Angew. Chem., Int. Ed. 2010, 49, 9565. [15] P. Z. Moghadam, A. Li, S. B. Wiggin, A. Tao, A. G. P. Maloney, P. A. Acknowledgements Wood, S. C. Ward, D. Fairen-Jimenez, Chem. Mater. 2017, 29, 2618. This project has received funding from the European Research Coun- [16] P. Z. Moghadam, A. Li, X. - W. Liu, R. Bueno-Perez, S. - D. Wang, S. cil (ERC) under the European Union’s Horizon 2020 Research and In- B. Wiggin, P. A. Wood, D. Fairen-Jimenez, Chem. Sci. 2020, 11, 8373. novation Programme (NanoMOFdeli), ERC-2016-COG 726380, Innovate [17] N. C. Burtch, H. Jasuja, K. S. Walton, Chem. Rev. 2014, 114, 10575. UK (104384) and EPSRC (EP/S009000/1). N.R. acknowledges support [18] S. M. Towsif Abtab, D. Alezi, P. M. Bhatt, A. Shkurenko, Y. from the Cambridge International Scholarship and the Trinity Henry- Belmabkhout, H. Aggarwal, Ł. J. Weseliński, N. Alsadun, U. Samin, Barlow Scholarship (honorary). D.F.-J. thanks the Royal Society for fund- M. N. Hedhili, M. Eddaoudi, Chem 2018, 4, 94. ing through a University Research Fellowship. The XPS facility and the [19] D. Fröhlich, S. K. Henninger, C. Janiak, Dalton Trans. 2014, 43, 15300. Tecnai F20 TEM are supported through the Cambridge Royce facili- [20] F. Jeremias, V. Lozan, S. K. Henninger, C. Janiak, Dalton Trans. 2013, ties grant EP/P024947/1 and Sir Henry Royce Institute—recurrent grant 42, 15967. EP/R00661X/1. J.S.A. would like to acknowledge financial support from [21] N. S. Hanikel, M. Prévot, F. A. Fathieh, E. Kapustin, H. Lyu, H. J. Wang, Ministerio de Ciencia e Innovación (Project PID2019-108453GB-C21). N. Diercks, T. M. Grant Glover, O. Yaghi, ACS Cent. Sci. 2019, 5, 1699. Note: The formatting of the header row of Table 1 was corrected on [22] S. Chaemchuen, X. Xiao, N. Klomkliang, Yusubov, M. S. F. Verpoort, June 8, 2023, after initial publication online. Nanomaterials 2018, 8, 661. [23] S. Wang, J. S. Lee, M. Wahiduzzaman, J. Park, M. Muschi, C. Martineau-Corcos, A. Tissot, K. H. Cho, J. Marrot, W. Shepard, G. Conflict of Interest Maurin, J. - S. Chang, C. Serre, Nat. Energy 2018, 3, 985. C.Ç., R.B., and D.F.-J. have financial interests in the start-up company Im- [24] M. H. Bagheri, S. N. 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