Materials Today Advances 19 (2023) 100390 Contents lists available at ScienceDirect Materials Today Advances journal homepage: www.journals.elsevier.com/materials-today-advances/ Hf/porphyrin-based metal-organic framework PCN-224 for CO2 cycloaddition with epoxides Sergio Carrasco a, b, *, Gisela Orcajo a, Fernando Martínez a, Inhar Imaz c, Safiyye Kavak d, Daniel Arenas-Esteban d, Daniel Maspoch c, e, Sara Bals d, Guillermo Calleja a, Patricia Horcajada b, ** a Chemical and Environmental Engineering Group, ESCET, Universidad Rey Juan Carlos, C/ Tulipan s/n, 28933, Mostoles, Madrid, Spain b Advanced Porous Materials Unit (APMU), IMDEA Energy Institute, Avda. Ramo n de la Sagra 3, E-28935, Mo stoles, Madrid, Spain c Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, Barcelona, 08193, Spain d EMAT and NANOlab Center of Excellence, University of Antwerp, Groenenborgerlaan 171, Antwerp, 2020, Belgium e ICREA, Pg. Lluís Companys 23, 08010, Barcelona, Spain a r t i c l e i n f o a b s t r a c t Article history: Herein, we describe for the first time the synthesis of the highly porous Hf-tetracarboxylate porphyrin- Received 17 April 2023 based metal-organic framework (MOF) (Hf)PCN-224(M) (M ¼ H2, Co2þ). (Hf)PCN-224(H2) was easily and Received in revised form efficiently prepared following a simple microwave-assisted procedure with good yields (56e67%; space- 6 June 2023 time yields: 1100e1270 kg m3$day1), high crystallinity and phase purity by using tri- Accepted 7 June 2023 Available online 19 June 2023 fluoromethanesulfonic acid and benzoic acid as modulators in less than 30 min. By simply introducing a preliminary step (10 min), 5,10,15,20-(tetra-4-carboxyphenyl)porphyrin linker (TCPP) was quantitatively metalated with Co2þ without additional purification and/or time consuming protection/deprotection Keywords: MOF steps to further obtain (Hf)PCN-224(Co). (Hf)PCN-224(Co) was then tested as catalyst in CO2 cycload- Hafnium dition reaction with different epoxides to yield cyclic carbonates, showing the best catalytic performance Porphyrin described to date compared to other PCNs, under mild conditions (1 bar CO2, room temperature, 18 PCN e24 h). Twelve epoxides were tested, obtaining from moderate to excellent conversions (35e96%). CO2 Moreover, this reaction was gram scaled-up (x50) without significant loss of yield to cyclic carbonates. Cycloaddition (Hf)PCN-224(Co) maintained its integrity and crystallinity even after 8 consecutive runs, and poisoning was efficiently reverted by a simple thermal treatment (175  C, 6 h), fully recovering the initial catalytic activity. © 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction application in real-world systems is an urgent need in the short to mid-term [6,7]. CO2 emissions are one of the most concerning challenges that Metal-organic frameworks (MOFs) are crystalline porous coor- our society is facing nowadays [1e4]. Although the transition of the dination polymers consisting of polydentate organic linkers and existing infrastructure from carbon-based sources to cleaner al- metal cations, that have proven to be useful as catalysts due to the ternatives would be ideal, many of the proposed technologies are presence of accessible coordinatively unsaturated metal sites (CUS) not yet sufficiently developed to facilitate large-scale imple- [8,9]. In addition, their high regular micro- and meso-porosity en- mentation [5]. Besides more severe legislations, the discovery of ables the fast diffusion of different chemical species to these new materials with suitable sorption/catalytic properties for numerous catalytic active sites. However, some limitations remain as a challenge, mainly reproducibility and stability, in terms of synthesis and performance, respectively [10,11]. MOFs based on tetravalent cations and polycarboxylate * Corresponding author. porphyrin linkers have particularly attracted the attention of the ** Corresponding author. E-mail addresses: sergio.carrasco@imdea.org (S. Carrasco), patricia.horcajada@ scientific community as potential catalysts for CO2 transformation imdea.org (P. Horcajada). because of the presence of two potential catalytic sites [12]: i) the https://doi.org/10.1016/j.mtadv.2023.100390 2590-0498/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 structural metal cluster based on M4þ cations showing an gravimetry (weight loss) and PXRD (structural integrity). Thermal extraordinary oxophilicity [13,14]; and ii) an extra divalent cation stability was monitored by TGA and variable-temperature XRD. BET that can be allocated in the void corresponding to the polypyrrole surface area was determined by nitrogen sorption and isosteric macrocyclic ring of the linker. The synergetic effect between the heat of adsorption was estimated from the CO2 isotherms at Brønsted and Lewis acid character of i) and ii), respectively, has different temperatures. Particle size and morphology were resulted in outstanding conversions and selectivity towards cyclic observed by scanning electron microscopy (SEM) and the pore size carbonates in CO2 cycloadditions under mild reaction conditions and cluster location determined from MOF nanocrystals using [14e16]. high-angle annular dark-field scanning transmission electron mi- In 2012, Hong-Cai Zhou reported the synthesis of the Porous croscopy (HAADF-STEM). Using (Hf)PCN-224(Co) as model catalyst, Coordination Network (Zr)PCN-222 (MOF-545) and, one year later, CO2 cycloaddition conditions were optimized (mol% MOF and (Zr)PCN-224, both based on Zr4þ and tetrakis(4-carboxyphenyl) cocatalyst, time and gas pressure) using epichlorohydrin. Other porphyrin TCPP [17,18]. These two MOFs differ in the coordination PCN materials were also prepared here following other procedures of the structural metal, 8- or 6- for (Zr)PCN-222 and (Zr)PCN-224, already described in the literature: (Zr,Hf)PCN-222(H2) and (Zr) respectively, resulting in remarkably different structures and PCN-224(H2) [12,18]. Twelve different epoxides were then tested in physicochemical properties. (Zr)PCN-222 shows a hexagonal such reaction to obtain the corresponding cyclic carbonates, which morphology with the 3D Kagome-like topology, 1D channels with 2 are relevant green solvents, monomers to obtain polycarbonates, pore sizes of 1.2 and 3.2 nm, and a BET surface area of ca. drug precursors or even electrolytes in lithium batteries, among 2000 m2 g1. (Zr)PCN-224 presents a cubic structure with 3D other applications [27,28]. Finally, MOF recyclability was evaluated nanochannels (1.9 nm), a larger BET surface area (ca. 2600 m2 g1), upon different thermal regeneration treatments. and a higher chemical stability of up to pH 11 vs. pH 7 found in (Zr) PCN-222. These MOFs have been successfully proposed as catalysts in a 2. Methods wide variety of reactions, including CO2 cycloaddition with epox- ides to obtain cyclic carbonates [12], CO2 photoreduction to yield 2.1. Synthesis of (Hf)PCN-224(H2) fuels [19], photo-oxidative condensation for the preparation of imines [20], or photocatalytic hydrogen generation [21], among The procedure was adapted from the literature including some others [13,22]. Unfortunately, when catalytic mechanism involves minor modifications [12]. Briefly, HfOCl2$8H2O (263.0 mg, the CO2 fixation to the structural metal cluster, the integrity of PCNs 0.642 mmol) and benzoic acid (2.49 g, 20.43 mmol) were mixed in a is usually compromised [23,24]. Carrasco et al. prepared (Zr)PCN- 30-mL a microwave vial with 8 mL DMF. The cluster formation was 222 for the CO2 cycloaddition of epoxides and aziridines to obtain performed at 140  C for 5 min under stirring (600 rpm, 0 bar). After the corresponding cyclic carbonates and oxazolidinones [12]. They cooling, the previous cluster solution was poured in another vial observed that the conversion was maintained during 4 consecutive containing TCPP (99.3 mg, 0.125 mmol) and 4 mL of DMF, and the runs, but crystallinity and particle size significantly decreased upon new mixture was homogenized in an ultrasonic bath for 1 min. successive catalytic cycles. Thus, the preparation of more stable and Trifluoromethanesulfonic acid (0.25 mL, 2.82 mmol) was rapidly active MOFs is in strong demand. added and the vial was immediately closed. The MOF synthesis was Intrigued by (Zr)PCN-222 limitations, we speculated that performed at 150  C for 20 min under stirring (600 rpm, 3.9 bar). decreasing the cluster coordination could enhance the catalytic The minimum required power (within 0e15 W) was applied to activity. PCN-224 structure showing hexa-coordinated clusters maintain the temperature in both steps. Crystals were collected by should be more active than PCN-222 showing octa-coordinated centrifugation (10000 rpm, 10 min) and washed with absolute clusters, providing additional space for catalysis [18]. Besides, ethanol (3x30 mL, 10000 rpm, 10 min). The solid was finally despite its lower cluster coordination, PCN-224 shows an improved resuspended in 5 mL ethanol and dried in a glass vial at 100  C for stability over PCN-222 counterpart [17]. In addition, replacing Zr4þ 12 h, obtaining 184.2 mg of (Hf)PCN-224(H2) (C144H78N12O64Hf12, by Hf4þ must enhance the acidity of the cluster and the affinity 67% yield based on metal basis). towards CO2 because of the higher oxophilicity of the latter cation, improving the overall catalytic performance (bond dissociation energies of 801 kJ mol1 for HfeO and 790 kJ mol1 for ZreO) [14]. Thus, in that sense, (Hf)PCN-224 emerges as a potential catalyst 2.2. Synthesis of (Hf)PCN-224(Co) with outstanding properties for this chemical reaction. To date, however, its synthesis has not been reported and only one example The procedure was similar to that described for the non- of (Hf)PCN-222 has been described, demonstrating the scarce metalated counterpart but including a previous additional step attention paid to Hf in favor of Zr when considering this MOF family and other minor modifications. TCPP (195.1 mg, 0.247 mmol) and [25]. anhydrous CoCl2 (170.2 mg, 1.310 mmol) were mixed in a 30-mL In this work, we report the synthesis of (Hf)PCN-224 over- microwave vial together with 6 mL DMF. Metalation was con- coming some of the synthetic issues classically found in sol- ducted at 175  C for 10 min under stirring (600 rpm, 1.9 bar, 20 W). vothermal methods by using microwave radiation (i.e. faster In parallel, the cluster solution was prepared using HfOCl2$8H2O reactions, higher efficiency, phase selectivity, lower cost and easier (532.0 mg, 1.300 mmol) and benzoic acid (5.10 g, 41.76 mmol) with morphological control) [12,26]. Special attention was paid to the 6 mL DMF at 140  C for 5 min (600 rpm, 0 bar). After cooling, cluster nature and coordination effect of the modulator during the protocol solution was poured on the metalation solution and then, tri- optimization. The synthesis of (Hf)PCN-224(Co) upon linker met- fluoromethanesulfonic acid was added (0.6 mL, 6.77 mmol) to alation with Co2þ was also studied until high phase purity was finally conduct the polymerization at 150  C for 30 min under observed. The novel Hf-porphyrin structure was resolved by syn- stirring (600 rpm, 6.0 bar). Crystals were washed and activated chrotron XRD and the chemical stability evaluated for both meta- following the above-mentioned procedure, obtaining 318.1 mg of lated and non-metalated MOFs in different aqueous (pH 1e14) and (Hf)PCN-224(Co) (C144H72N12O64Co3Hf12, 56% yield based on metal organic media by UVeVis spectroscopy (linker releasing), basis). 2 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 2.3. Stability tests counterparts, were prepared according to previously reported protocols in the case of PCN-222 series [12], or by simply replacing 10 mL of different solvents were added to 10 mg (Hf)PCN- hafnium precursor by ZrOCl2$8H2O in the procedure described 224(H2) in 30 mL glass vials under ultrasonic treatment (5 min, herein to obtain (Zr)PCN-224(H2,Co). Similarly, CoCl2 was 40  C) until homogeneous suspensions were obtained. Vials were substituted by the corresponding Ni2þ, Cu2þ and Zn2þ chlorides to placed in an orbital shaker (200 rpm, 24 h, room temperature) and yield (Hf)PCN-224(Ni, Cu or Zn). For comparison purposes, these materials were recovered by centrifugation and washed twice with materials were subjected to the same reaction conditions as absolute ethanol (13000 rpm, 8 min). After drying (100  C, 24 h), described above for (Hf)PCN-224(Co): epichlorohydrin (0.02 mL, weight losses were gravimetrically calculated in an analytical bal- 0.255 mmol), PCN (1 mol%, different amounts according to their ance and crystallinity evaluated by PXRD. Different amounts of molecular weight), TBAB (7.5 mol%, 2.95 mg), CO2 (1 bar) for 24 h. NH4OH 28% wt., NaOH 2 M, HCl 37% wt. or PBS 0.5 M were used to For scaling-up, similar reaction conditions were applied performing control pH in aqueous media (1e13) and two different polymer the reaction with the following amounts: epichlorohydrin (1.0 mL, concentrations were tested: 10 and 1 mg mL1. Polymers were 12.8 mmol), (Hf)PCN-224(Co) (1 mol%, 225.0 mg), TBAB (7.5 mol%, subjected to an ultrasonic treatment under harsh conditions (1 h, 154.4 mg), CO2 (1 bar) for 24 h. The product was extracted with 60  C) and further washed and dried following a similar procedure ethyl acetate (x2, 15 mL), the solvent evaporated under reduced as that described above. pressure and finally purified by column chromatography (SiO2; hexane/ethyl acetate 8:2). The substrate scope was evaluated for 12 2.4. Evaluation of linker releasing by UVeVis spectroscopy epoxides: 1,2-epoxypropane (propylene oxide), 1,2-epoxybutane (butylene oxide), 1,2-epoxypentane (pentene oxide), 1,2- Initially, 10 mg of MOFs were suspended in 1 mL of each solution epoxyhexane (hexene oxide), 1,2-epoxytetradecane, 1-chloro-2,3- at the corresponding pH. Suspensions were treated in the ultra- epoxypropane (epichlorohydrin), 2,3-epoxy-1-propanol (glycidol), sonic bath for 1 h at 60  C and centrifuged afterwards (13500 rpm, 3,3-dimethyl-1,2-epoxybutane, 1,2-epoxy-2-methylpropane, 3,4- 5 min). 1 mL of DMF were added to each material, vortexed and epoxy-1-butene (butadiene monoxide), 1-allyloxy-2,3- sonicated for 1 min, and then centrifuged under similar conditions epoxypropane (allyl glycidyl ether) and 1,2-epoxyethylbenzene in order to wash TCPP remaining stack to the polymer. Both (styrene oxide) under previously optimized conditions: epoxide aqueous and organic fractions were mixed (2 mL in total) to be (different volumes, 0.255 mmol), (Hf)PCN-224(Co) (1 mol%, analyzed by UVeVis spectroscopy. Only samples treated at pH ¼ 12 4.5 mg), TBAB (7.5 mol%, 2.95 mg), CO2 (1 bar) for 24 h. For recy- were conveniently diluted (1/40) with H2O:DMF 1:1, v/v prior to clability tests, the material was washed with absolute ethanol (x2, their measurement. MOFs were then washed twice with 2 mL 2 mL) after extraction with CDCl3 and dried at 100  C for 12 h. ethanol and then dried at 100  C for 12 h. The final weight of the Similar amounts of epichlorohydrin and TBAB were added in each material was used to determine the total amount of material lost cycle until run 7. After this run, the regeneration of the catalyst was during the overall process by gravimetry. This procedure was per- evaluated upon different thermal treatments: 150, 175 and 200  C formed by triplicate for each polymer and pH value. UVeVis mea- for 6 and 12 h, and then subjected to a new catalytic cycle under surements were performed inside quartz cuvettes within similar reaction conditions. 800e300 nm wavelength range, after supernatant filtration with a 0.22 mm nylon filter. To evaluate the linker releasing, calibration 3. Results and discussion using porphyrins was performed in a mixture of H2O:DMF 1:1, v/v within 0.25e9.50 and 0.05e20.50 mM concentration ranges for In this work we firstly optimized the microwave-assisted syn- TCPP(H2) and TCPP(Co), respectively. thesis of (Hf)PCN-224(H2), focusing in the nature of the modulator. Afterwards, the preparation of (Hf)PCN-224(Co) was also optimized 2.5. Catalytic tests until phase purity was fulfilled. All the reactions were placed inside a stainless-steel reactor 3.1. Synthesis and characterization without stirring or additional solvents at room temperature. After purging the system with a continuous flow of CO2 (2 min, 2 bar), the 3.1.1. Optimization of synthetic protocol outlet valve was closed setting the desired pressure by controlling Note that several authors claim that the procedure to obtain Hf- the inlet of the reactor. After catalysis, 2,3,5,6- based MOFs is as simple as replacing Zr by Hf precursors, ascribed tetrachloronitrobenzene (10e20 mol% respect to epoxide) was to the same d0 electronic configuration and having a similar radius added as NMR internal standard to the reaction vessel. Extraction because of the lanthanide contraction in the case of Hf4þ [29]. was performed with CDCl3 (x3, 1.5 mL) and centrifuged in poly- However, we observed that this statement could not be directly propylene tubes (13000 rpm, 5 min). The supernatant was filtered applied, probably due to the higher acidity of Hf4þ compared to (nylon, 0.22 mm) prior NMR measurements. For a fixed amount of Zr4þ, resulting in amorphous polymers, even further enhanced epichlorohydrin (0.02 mL, 0.255 mmol), different amounts of under microwave irradiation due to the excessive polarization of catalyst (Hf)PCN-224(Co) (0, 0.25, 0.5, 1 and 2 mol%, 0e9.1 mg) and the electronic cloud [30]. Thus, synthetic conditions (nature and co-catalyst TBAB (2.5, 5.0, 7.5 and 10 mol%, 1.0e4.0 mg) were tested amount of modulators, temperature, and synthesis time) should be for a fixed time (6 h) and temperature (24  C) at two different CO2 carefully optimized to produce a well crystallized and pure (Hf) pressures (1 and 12 bar). Different reaction times were evaluated (1, PCN-224 material. 3, 6, 9, 15, 20 and 24 h) after finding the optimal conditions: In a typical microwave procedure [12], the metal cluster is epichlorohydrin (0.02 mL, 0.255 mmol), (Hf)PCN-224(Co) (1 mol%, initially formed using the salt precursor in presence of benzoic acid 4.5 mg), TBAB (7.5 mol%, 2.95 mg) and CO2 (1 bar). Different blank (140  C, 5 min). After the cluster stabilization, the MOF polymeri- and reference tests (see Table S7) were performed under different zation takes place under a second microwave irradiation step conditions for a fixed amount of epichlorohydrin (0.02 mL, (150  C, 20 min) by adding the TCPP linker and a modulator (see 0.255 mmol) and time (24 h). Supporting information; SI). Herein, we evaluated the effect of Different PCN materials, i.e. (Zr)PCN-222(H2), (Zr)PCN-224(H2), different acids as modulators (trifluoromethanesulfonic, meth- (Hf)PCN-222(H2) and their corresponding Co2þ-metalated anesulfonic, trifluoroacetic, formic, acetic, propionic; Figure S2) and 3 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 their efficiency to replace the benzoate coordinated to the cluster to richer in (Hf)PCN-224(Co) (Figure S4). The highest purity obtained start nucleation and promote crystal growth. Tri- in this test was 90% of (Hf)PCN-224(Co) (samples E and F with 4.0 & fluoromethanesulfonic acid (pKa ¼ 14.7) was selected as the 4.5 mmol of trifluoromethanesulfonic acid, respectively). However, optimal candidate providing excellent crystallinity, morphology adding more than 5 mmol of the acid resulted in a higher (Hf)PCN- and yields (64 ± 4%, metal basis). Such a strong acid was required to 222(Co) content, being possible to obtain this phase pure with break the HfeO bond established with benzoate moieties and Hf4þ, 5.5 mmol (sample denoted H; Table S2). Under such conditions, a without effectively coordinating the cation because of the steric higher rearrangement between linkers and clusters was observed hindrance of eCF3 moiety and its poor donating ability, allowing because of the lower deprotonation rate, explaining the low yields the preferential interaction with the carboxylate units of the TCPP (18%, related to (Hf)PCN-222 phase) observed and the higher co- linker, more similar in form and acidity (pKa ¼ 3.20e3.75) to the ordination achieved. original benzoate molecules (pKa ¼ 4.20) [31]. It should be noticed So that favor higher nucleation rates and forcing a faster poly- that using DMF as the synthesis solvent is essential both to solu- merization, while increasing the reaction yield, we decided to bilize the TCPP linker and to modulate the acidity-basicity of the double the concentration of every reagent keeping intact the overall system, thus controlling the deprotonation and coordina- amount of solvent (12 mL DMF). For this experiment, we selected tion rate of TCPP to Hf4þ, regulated in return by the presence of the the sample D (3.4 mmol trifluoromethanesulfonic acid; Table S2, trifluoromethanesulfonic acid interacting with the solvent, as Figure S4) as a balance between purity (85% (Hf)PCN-224(Co)) and already proposed by Forgan in different systems [32]. This process mass obtained. After this optimization, we were able to prepare occurred here in a fast and defective manner, critical for the sta- pure (Hf)PCN-224(Co) in a good yield (55.4 ± 0.8%, metal basis) and bilization of hexa-coordinated clusters, suggesting that the lower crystallinity. This fast and efficient microwave method allowed to coordination mode of (Hf)PCN-224 can be achieved under kineti- produce 1270 kg m3 day1 of (Hf)PCN-224(Co) and 1100 kg cally controlled conditions favored by microwave heating [15]. This m3 day1 of (Hf)PCN-224(H2). Space-time yields (STY) surpassed acid-base kinetically controlled equilibrium, responsible for the those reported for other MOFs being produced by industry during linker deprotonation and coordination, was essential as any other the early 2010s (e.g. (Al)MIL-53, (Cu)HKUST-1, (Fe)MIL-100 and (Zn) acid with higher pKa (methanesulfonic, trifluoroacetic, formic, ZIF-8 with 160, 225, 20 and 100 kg m3 day1, respectively) [34]. acetic, propionic) resulted in either amorphous solids or mixture of Note that current STY values achieved by microwave reactions (Hf)PCN-222 and (Hf)PCN-224 phases (Figure S2) decreasing the could even reach 15000 kg m3 day1 (Al)MIL-53 [35], empha- polymerization rate and the amount of the material obtained, sizing the interest of microwave-assisted reaction from an indus- supporting our previous evidence. trial point of view. After the optimization of (Hf)PCN-224(H2), we further investi- PXRD patterns of (Hf)PCN-224(H2) and (Hf)PCN-224(Co) were gated the possibility to directly obtain the metalated MOF by simply compared to that described for (Zr)PCN-224 (Figure S5), showing a introducing an additional microwave-assisted step, as reported perfect matching among them. Indeed, Le Bail analysis revealed a elsewhere [12]. So, (Hf)PCN-224 was metaled with Co2þ since the good fitting between the proposed structure and the experimental mentioned work also demonstrated the outstanding performance PXRD pattern (Figure S6). Note that because of the inherent ca- of (Zr)PCN-222(Co) in CO2 cycloaddition compared to other meta- pacity of microwave heating to yield small micro- and even nano- lated homologues (Ni2þ, Cu2þ and Zn2þ), supporting the interest in metric crystals, the crystalline structure could not be resolved by this cation. Thus, the first step consisted of the Co2þ metalation of conventional single crystal XRD methods, thus synchrotron radia- the TCPP(H2) linker using a large stoichiometric excess (1:5) of tion was used. Here, despite the low quality of the diffraction CoCl2 in presence of DMF (175  C, 10 min), required to ensure a datasets, the obtained model confirmed that the structure of (Hf) quantitative metalation of the linker. The role of the solvent in this PCN-224 is isoreticular to that previously described for (Zr)PCN- process was critical to introduce divalent cations inside the 224 (see Supporting information; SI for further structural infor- porphyrin by deprotonating its NeH moieties. Surprisingly, unlike mation: Table S3, Figure S7). in the Zr-based analogues (i.e. (Zr)PCN-222(H2)/(Zr)PCN-222(Co)), where both polymers were purely obtained independently on the 3.1.2. Physicochemical characterization use of TCPP(H2) or TCPP(Co), here (Hf)PCN-224(Co) was obtained A full characterization (morphological, textural, compositional, mixed with (Hf)PCN-222(Co) (aprox. 82 vs. 18% based on PXRD, etc.) of (Hf)PCN-224(H2) and (Hf)PCN-224(Co) was carried out, Table S2, Figure S3) using the same composition than that resulting paying special attention on their thermal and chemical stability. in pure (Hf)PCN-224(H2). Using 4 equiv. in excess of Co2þ, while Morphology and crystal size significantly differed between both having a negligible impact on the (Zr)PCN-222 structure, revealed the metalated and non-metalated MOFs. As observed by field to be of great importance for the (Hf)PCN-224 formation. We emission scanning electron microscopy (FE-SEM), (Hf)PCN-224(H2) speculated that, together with the higher Brønsted acid character of crystals nucleated and grew as pseudo-spherical particles Hf4þ- compared to Zr4þ- cluster, Co2þ was also involved in the (540 ± 80 nm, n ¼ 100 Fig. 1a and Figure S11a), whereas (Hf)PCN- above-mentioned equilibrium participating as a Lewis acid coor- 224(Co) showed perfectly defined cubic crystals with truncated dinating triflate anions from the media and significantly modifying edges (3.0 ± 0.7 mm, n ¼ 100 Fig. 1b and Figure S11b). In the first the apparent pH of the mixture [33]. To confirm this hypothesis, case, nucleation might be the most important process in detriment different amounts of trifluoromethanesulfonic acid were tested for of growth, aspect typically observed for crystals resulting from a fixed pre-polymerization mixture (Table S2. 0.64 mmol Hf4þ, kinetically controlled microwave reactions [36]. In contrast, ther- 20.3 mmol benzoic acid, 0.124 mmol TCPP, and 0.62 mmol Co2þ in modynamically stable phases are favored in solvothermal condi- 12 mL DMF). The amount of MOF significantly decreased by tions, like in the case of (Zr)PCN-222, where growth was typically increasing the amount of the strong acid (from 160 to 55 mg using considered the controlling process of the crystallization, and thus 1.1 or 5.5 mmol of acid, respectively). At lower acid concentrations providing larger well-faceted cubic microcrystals [12]. In order to than that of the standard mixture optimized for (Hf)PCN-224(H2) support this hypothesis, we performed the solvothermal synthesis (2.8 mmol), we observed an enrichment of (Hf)PCN-222(Co) phase of (Hf)PCN-224(H2) by using the similar pre-polymerization by PXRD (Figure S3; see samples denoted A and B in Table S2 for mixture than that used in the microwave protocol (120  C for compositions) indicating, as expected, a higher coordination of the 16 h without stirring vs. 150  C for 20 min under stirring), obtaining cluster. As the acid concentration increased, the final polymer was well-defined cubic crystals in the micrometric size range 4 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 Fig. 1. FE-SEM images of: a) spheric-shaped (Hf)PCN-224(H2) crystals at x10000; b) FE-SEM images of truncated (Hf)PCN-224(Co) crystals in [011], [101], [110] and their corre- sponding inverse directions at x3000 magnification (inset: x15000). c) High resolution HAADF-STEM image of a (Hf)PCN-224(Co) particle oriented along the [100] zone axis. Fast Fourier transform (FFT) of the image, confirming the crystal structure (top-right), and the theoretical model for PCN-224 structures superimposed on a magnified area indicated by orange dotted rectangle (bottom-right). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article). (2.5 ± 0.8 mm; Figure S12a), one order of magnitude larger than (ATR-FTIR, Figure S14), which band assignment can be found in those particles obtained under microwave heating, confirming thus Table S4. Upon the linker coordination, the broad band of OeH that the growth process is favored over nucleation under sol- around 3000 cm1 in free TCPP(H2) dramatically decreased its in- vothermal conditions. HAADF-STEM image acquired at low electron tensity, indicating the coordination with the metal cluster in both dose (181.35 e A2) of a particle oriented along the [100] zone axis MOFs. Besides this band attributed to oligomeric acid species [37], from the (Hf)PCN-224(Co) sample (Fig. 1c) shows a very ordered the single band of monomeric COOH units at 3640 cm1 dis- porous structure with an average pore size of 1.9 nm (Figure S13), as appeared after polymerization. This evidence was additionally described in literature for PCN-224 structures [18], allowing the supported by the hypsochromic shift of C]O and CeO bands (1685 visual confirmation of the PCN-224 structure. and 1278 cm1 in TCPP(H2) to ca. 1730 and 1305 cm1 in MOFs, Further, the larger crystal size of the metalated (Hf)PCN-224(Co) respectively) and a significant decrease in their intensities, in comparison with the non-metalated could be related with the revealing the vibrational constraint of carboxylate moieties inter- presence of Co2þ during polymerization, which seems to favor acting with the metal ion units [20]. Another evidence of this growth instead nucleation, despite the microwave-assisted syn- interaction relied on the presence of the HfeO band located at thesis. In fact, this extent was confirmed replacing Co2þ by Cu2þ to 667 cm1 for both MOFs, negligible in TCPP(H2) [38]. In addition, yield (Hf)PCN-224(Cu), further used for catalytic comparison pur- the sharp band at 1002 cm1 in (Hf)PCN-224(Co) indicated the poses, where micrometric cubes of similar size to those observed interaction between Co2þ and the N of pyrrol moieties [12]. The for (Hf)PCN-224(Co) were obtained (Figure S12b). This aspect spectrum of the metalated MOF lacked typical NeH bands that can affected not only the crystal dimensions but also their poly- be found in TCPP(H2) and (Hf)PCN-224(H2) at ca. 3320 (stretching), dispersity, as different growth rates resulted in a wide variety of 960 (bending in plane) and 700-710 cm1 (bending out of plane), particle sizes in the case of metalated MOFs (Figure S11c). supporting the full metalation of (Hf)PCN-224(Co). Materials were also spectroscopically characterized by attenu- The relevant textural properties of both MOFs were measured by ated total reflection e Fourier transform infrared spectroscopy sorption isotherms of N2 and CO2. First, N2 sorption measurements 5 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 at 77 K provided type-I isotherms, typical of microporous materials plasma - optical emission spectroscopy (ICP-OES, Co2þ exp.: (Figure S8), with pore volumes and sizes of 1.57 ± 0.09 and 3.3 ± 0.2%; theo.: 3.3%). Thermal stability was further investigated 1.58 ± 0.08 cm3 g1 (single point method), and 2.2 and 2.1 nm (DFT by variable temperature VT-PXRD (Figure S16). Last patterns method) for the non-metalated and Co-metalated MOFs, respec- showing measurable diffraction peaks were found at 340 and tively. On the other hand, the branch at p/p0 ¼ 0.05e0.16 was used 400  C for (Hf)PCN-224(H2) and (Hf)PCN-224(Co), respectively. for the calculation of the Brunauer-Emmett-Teller (BET) surface These values were 60e70  C lower than those observed in TGA, area, obtaining 2408 ± 13 and 2218 ± 5 m2 g1 (Figure S9). Differ- suggesting a prior cluster discoordination before linker decompo- ences between both values can be ascribed to the smaller particle sition. After 570  C, the unique presence of HfO2 was observed size observed for (Hf)PCN-224(H2) and (Hf)PCN-224(Co) (Figure S17). (540 ± 80 nm vs. 3.0 ± 0.7 mm). Note that these values agree with The structural stability of (Hf)PCN-224(H2, Co) was studied in those observed for the Zr4þ counterpart in previous publications the presence of different solvents (1 mg mL1, 24 h) by PXRD [18,39,40]. (Fig. 2). We observed an outstanding stability in all the tested The adsorption/desorption capacity of both materials towards organic solvents regardless their nature, including typical media for CO2 was also tested by measuring the corresponding isotherms at catalytic reactions, demonstrating the broad applicability scope of 273 and 293 K (Figure S10). (Hf)PCN-224(H2) and (Hf)PCN-224(Co) this material. Both polarity and solvent molecule size were some- were able to adsorb up to 2.43 and 2.24 mmol g1 (at 273 K and how affecting the cluster coordination in different manners [43,44]. 760 mmHg), and 1.57 and 1.41 mmol g1 (at 293 K and 760 mmHg), (Hf)PCN-224(H2,Co) easily coordinate OH groups or water mole- respectively. These values compared well to other PCN materials cules taking the place of bidentate linkers without affecting the such as their Zr counterparts: (Zr)PCN-222(H2) and (Zr)PCN- Hf6(O/OH)8 core [45]. For instance, considering protic polar sol- 222(Co) are able to adsorb 2.51 and 2.55 mmol g1 (at 273 K and vents such alcohols, the trend of losing MOF weight was clearly 760 mmHg), and 1.67 and 1.72 mmol g1 (at 293 K and 760 mmHg) defined: methanol>1-propanol>1-butanol (Figure S18), according [12]. In the case of (Hf)PCN-224, the non-metalated MOF offered to the length of the alkyl chain and supporting this evidence. higher CO2 adsorption capacity since its more available space Finally, hydrolytic instability of many MOFs is a limiting factor expressed by larger surface-to-volume ratio. Note that this phe- for their application. Thus, the stability of (Hf)PCN-224(H2) in nomenon cannot be explained in the basis of CUS availability aqueous media (10 mg mL1) at different pH values was explored (416 mmol g1 coming from Hf4þ and 436 mmol g1 from Hf4þ and (Figure S21). Weight losses were below 20% within the pH range Co2þ in the non-metalated and metalated counterparts, respec- 1e11, maintaining the structural integrity, which is in agreement tively). The smaller particle size of (Hf)PCN-224(H2) (540 ± 80 nm) with previous observations for (Zr)PCN-224 [18]. The MOF held its resulted in a higher exposure of CUS than in the case of (Hf)PCN- crystallinity even after using commercial HCl (37% wt.) but with 224(Co) (3.0 ± 0.7 mm) for the same amount of material. However, higher weight losses (23%). At pH 13 or using NH4OH (28% wt.), the affinity for the gas was found to be slightly higher in the latter structure was seriously damaged resulting in amorphous materials (23.3 ± 0.8 vs. 21.3 ± 0.6 kJ mol1; estimated by the isosteric heat of with weight losses higher than 60%. (Hf)PCN-224(H2) was found to CO2 adsorption from the isotherms at different temperatures using be stable in deionized water at 1 and 10 mg mL1 concentrations the Clausius-Clapeyron equation) [41], because of the presence of (Figure S22). Crystallinity was unaltered using a phosphate buffer Co2þ. Independently on the metalation, these values were signifi- solution at low concentration (PBS 0.01 M, pH 7.4), but not using a cantly higher than those reported for (Zr)PCN-222(H2 and Co) of higher salt content (PBS 0.5 M, pH 7.4) where 16% of the initial 17.4 and 17.6 kJ mol1, respectively, and within the range of the best material was lost, becoming totally amorphous. On the other hand, performing and well-stablished materials for the selective tap water (sulfate content <2$104 M, total hardness <5$104 M in adsorption and separation of CO2, such as modified silica gel par- terms of CaCO3, pH 6.8) was well-tolerated because of the lower salt ticles (MCM-41-100, 0.66 mmol g1, 19.8 kJ mol1), zeolites (Na-4A, content (12% weight loss and still crystalline). This lower stability in 1.61 mmol g1, 25.2 kJ mol1) and other MOFs [42], revealing (Hf) saline media has been reported before for other MOFs [46], but PCN-222 (H2,Co) as promising CO2 adsorbents. smartly exploited for purposes different from catalysis, such as drug Thermogravimetric analysis provided relevant information on delivery [23,47]. PXRD patterns at different pH values revealed the the composition and thermal stability of both materials in com- excellent structural stability of both MOFs even at low concentra- plement with other techniques (energy-dispersive X-ray spectros- tion (1 mg mL1, Figure S23), with a linker releasing below 2% copy X-EDS, thermal-dependent XRD). In general, decomposition of within pH 7e11 monitored by UVeVis spectroscopy (Table S6). (Hf)PCN-224(H2) occurred faster than for (Hf)PCN-224(Co), sug- Structural differences between (Hf)PCN-224(H2) and (Hf)PCN- gesting that the presence of cobalt within the linker enhanced its 224(Co) were remarkably found at pH 1 and 12 based on a peak integrity (Figure S15) [12,20]. The main weight losses were broadening effect (Figure S23a), suggesting a higher structural observed at 395 and 470  C, respectively, corresponding to the stability of (Hf)PCN-224(Co), similar to that observed at high tem- decomposition of the organic linker. In general, three significant perature. Further details concerning the chemical and structural weight variations can be observed in PCN materials. The first (up to stability of these materials can be found in the Supporting 100  C), similar in both MOFs (2 wt%), corresponded to small sol- Information. vent molecules loaded within the porous structure, mainly ethanol used in the washing step. This was followed by the loss of eOH 3.2. Catalysis groups coordinated to metal clusters in a broad temperature range (150e300  C) and cannot be separated from the last and most Cycloaddition of CO2 with epoxides was selected as our catalytic important weight loss, the abovementioned linker decomposition. model reaction test because of the easy procedure, the clearance Final residue was 48 and 52 wt% for (Hf)PCN-224(H2) and (Hf)PCN- without the need of a solvent and the practical interest of cyclic 224(Co), respectively, in agreement with the theoretical amount of carbonates [27]. In addition, when performed under mild condi- residual oxides after decomposition in air regarding their molecular tions (room temperature), it results in negligible secondary sub-/ formula: 49.1% (HfO2) and 51.8% (HfO2 þ CoO). The experimental by-products such as diols or polymers observed at higher temper- Hf/Co elemental ratio measured by X-EDS (12.6 ± 0.6) in (Hf)PCN- atures [48e50]. In summary, it is considered one of the most effi- 224(Co) matched well with the expected value (12.1), and the cient ways for CO2 chemical fixation, being less energetically quantitative metalation was confirmed by inductively coupled demanding than other conversion reactions such as 6 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 Fig. 2. PXRD patterns of: a) (Hf)PCN-224(H2) and b) (Hf)PCN-224(Co) after treatment in different solvents (at room temperature, 24 h, 1 mg mL-1, 200 rpm). hydrogenations [10,13,27]. However, this reaction requires the conversion (entries 2 and 6), (Zr)PCN-224(Co) and (Hf)PCN-224(Co) presence of a co-catalyst responsible for the epoxide ring opening, resulted in 90 and 96% conversion (entries 4 and 8), respectively. typically halide anions in the form of tetrabutylammonium salts. First, it was evident that the CUS accessibility to hexa-coordinated We proposed here (Hf)PCN-224(Co) as a promising candidate in clusters of PCN-224 is enhanced when compared to the octa- CO2 cycloadditions because of its higher chemical stability and coordinated clusters found in PNC-222 [18]. Further, the higher availability of CUS compared to other well-known PCNs [12,18,51]. oxophilicity of Hf4þ significantly increased the conversion These previous works also demonstrated the superior catalytic compared to Zr4þ in the case of PCN-224 [14]. Intriguingly, the activity of Co2þ-metalated PCNs compared to other metal ions and difference between both ions in terms of acidity was not evident in the non-metalated counterparts. The optimized conditions of the the case of the PCN-222 structure when is metalated with Co. This catalytic process can be found in SI. fact indicates a cooperative catalytic effect between the Lewis acid Under optimized conditions (0.25 mmol epichlorohydrin, 1 mol character of Co2þ and the Brønsted acid character of Zr4þ/Hf4þ % MOF, 7.5 mol% TBAB, 1 bar CO2, 24 h, 24  C; Figure S27), the cluster exclusively on PCN-224. Indeed, these results could be ex- catalytic performance of (Hf)PCN-224(Co) was compared to that of pected considering that the optimization was performed using (Hf) other non-metalated and Co2þ-metalated PCNs, as well as other PCN-224(Co). Different structures (not only in terms of cluster co- (Hf)PCN-224(M) metalated counterparts, here prepared (M ¼ Ni2þ, ordination but also pore size, accessibility, and polar nature of Cu2þ, Zn2þ; Table 1). No significant differences were found in terms cavities) and, more importantly, additional CUS modified the of conversion between all the non-metalated MOFs (i.e. (Zr)PCN- overall time to ensure the complete diffusion of reagents to cata- 222(H2), (Zr)PCN-224(H2), (Hf)PCN-222(H2) and (Hf)PCN-224(H2), lytic sites and every step involved in the catalytic cycle: epoxide 56e59%, entries 1, 3, 5 and 7), suggesting that both Zr4þ and Hf4þ coordination, ring-opening, CO2 addition and cyclic carbonate structural ions contributed similarly to CO2 cycloaddition under the release. Kinetic equilibrium was achieved for (Hf)PCN-224(Co) after optimized conditions for (Hf)PCN-224(Co), independently on their 24 h (Figure S28b). cluster coordination and Brønsted acid character. In contrast, it Comparatively, to obtain similar epichlorohydrin conversions seems of great importance when MOFs were Co2þ-metalated. using (Zr)PCN-222(Co) as that found here for (Hf)PCN-224(Co) While (Zr)PCN-222(Co) and (Hf)PCN-222(Co) provided 74 and 73% (95%: 1 mol% MOF, 7.5 mol% TBAB, after 20 h), it was required to harden the reaction conditions (89%: 2 mol% MOF, 10 mol% TBAB, after 18 h) [12]. Other authors have described a similar behavior on Table 1 this basis for hexa- and octa-coordinated clusters in PCNs [15], and Conversions for different PCN materials in CO2 cycloaddition of epichlorohydrin tested in this work.a dodeca-coordinated cluster in UiOs [14], for the CO2 photoreduc- tion and glycerol acetalization, respectively. In the former work, Jin Entry MOF Metal Conversion (%) concluded that 6-linker connected systems based on M4þ should be 1 (Zr)PCN-222 (H2) 59 regarded as missing linker defective systems, compared to ideal 12 2 (Co) 74 linkers connected to M4þ6-nodes. This was in agreement with the 3 (Zr)PCN-224 (H2) 57 4 (Co) 90 larger BET surface area and lower diffusion energy barriers found 5 (Hf)PCN-222 (H2) 57 for PCN-224 compared to PCN-222, showing 3D and 1D channels, 6 (Co) 73 respectively [18,52]. In the case of non-metalated counterparts, the 7 (Hf)PCN-224 (H2) 56 cooperative effect between both cations did not exist. 8 (Co) 96 On the other hand, it is not a coincidence that most works have 9 (Ni) 53 10 (Cu) 55 paid more attention to cobalt instead other divalent cations 11 (Zn) 61 regarding metal-based Lewis acid catalysis using metalated por- a Reaction conditions: 0.25 mmol epichlorohydrin, 1 mol% MOF, 7.5 mol% TBAB, phyrins in Zr-based PCNs [12,18,51,53]. The higher efficiency of 1 bar CO2, 24 h, 24  C. Co2þ has been explained as a multifactorial effect, but all of them 7 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 agree on the compromise between a moderate Lewis acid character yield using internal standard). Turnover number (TON) and turn- and the convenient cation size, able of shifting out from the over frequency (TOF) of epichlorohydrin converted into the corre- porphyrin plane [54e56]. The out-of-plane distortion together with sponding cyclic carbonate were 83 and 4 h1, respectively. The the lower steric hindrance of reagents arranged around Co2þ scope of the reaction was then evaluated (Scheme 1). Epoxides compared to bigger cations are behind this relevant catalytic effect. bearing linear alkyl chains of different lengths were well tolerated To probe this explanation, the epichlorohydrin conversion was with excellent and good yields, from propylene oxide (R ¼ eCH3, studied using (Hf)PCN-224(M) (M ¼ Ni2þ, Cu2þ, Zn2þ) under opti- 92%) to 1,2-epoxyhexane (R ¼ -(CH2)3CH3, 67%). However, moder- mized conditions for (Hf)PCN-224(Co). Interestingly, conversions ate yields were observed in the case of longer alkyl chains (1,2- (53e61%, Table 1, entries 9e11) were like that obtained using (Hf) epoxytetradecane, R ¼ -(CH2)11CH3, 34%). Interestingly, a linear PCN-224(H2) as catalyst (56%), suggesting a negligible effect from relationship with good fitting was found between conversion and the divalent cation located in the center of the porphyrin ring. This number of C atoms in eR (Figure S29a), suggesting diffusion issues aspect was also observed before for (Zr)PCN-222(M), where (Zr) of the epoxide throughout the pores when increasing its size. This PCN-222(Ni and Cu) gave even lower yields (38 and 42%, respec- trend was extended for the rest of substrates in terms of molecular tively) than the non-metalated MOF (58%) in the CO2 cycloaddition weight and length (Figure S29b-c), with the remarkable exception with 1,2-epoxynonane [12]. A similar trend was noted here of styrene oxide and allyl glycidyl ether (19 and 53% yield, (Co2þ>Zn2þ>H2>Cu2þ>Ni2þ), independently on the MOF structure. respectively). As all the substrates tested in this experiment were Other works describing organometallic complexes based on planar selected to enter inside pores (1.9 nm), we hypothesized that ligands coordinated to divalent cations, i.e. M(salphen) catalysts different hydrophobic and electrostatic interactions may occur (M ¼ Ni2þ, Cu2þ, Zn2þ), for CO2 cycloadditions with epoxides have between these two molecules and the MOF skeleton because of the reported similar results [57]. Nevertheless, the inverse catalytic presence of benzene rings and the oxygen of the ether group activity was observed in Diels-Alder reactions, where the ability of together with the double bond, respectively. These interactions Cu2þ and Ni2þ to form square planar complexes were critical to were not remarkable for the rest of substrates, lacking benzene observe higher conversions, while Co2þ was the least efficient units, and showing one single O atom in the form of eOH (glycidol, cation. Thus, we suspected that the preferential spatial disposition 77% yield) or only one double bond (butadiene monoxide, 65% of the former cations inside the porphyrin, located in the same yield). plane as N atoms of the pyrrole rings, could be behind the low One of the main issues observed for (Zr)PCN-222(Co) was its activity of (Hf)PCN-224(Ni2þ, Cu2þ and Zn2þ) in CO2 cycloadditions limited stability after consecutive catalytic runs [12]. This fact was compared to (Hf)PCN-224(Co) since negligible out-of-plane explained by the presence of TBAB as co-catalyst and the low sta- distortions. bility of PCN-222 materials in basic media. To avoid this limitation, The reaction was successfully 50-times scaled-up (from 0.26 to we evaluated the recyclability of (Hf)PCN-224(Co), exhibiting a 12.80 mmol) under similar reaction conditions. After purification higher chemical resistance and catalytic activity than the PCN-222 by column chromatography, yield (1.45 g, 83%, isolated yield) did counterpart. Unlikely (Zr)PCN-222(Co), (Hf)PCN-224(Co) particles not significantly differ from the low scale reaction (86%, 1H-NMR were not damaged after 7 consecutive runs, maintaining an Scheme 1. Reaction scope.a a Reaction conditions: 0.25 mmol epoxide, 1 mol% (Hf)PCN-224(Co), 7.5 mol% TBAB, 1 bar CO2, 24 h, 24  C. Conversions and yields (in parentheses) determined by 1H-NMR spectroscopy calculating the ratio Hc/d/(Ha/b þ Hc/d) and using 2,3,5,6-tetrachloronitrobenzene as internal standard, respectively. 8 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 performed at 150, 175 and 200  C for 6 and 12 h (Figure S32). Interestingly, we recovered the initial catalytic activity in the cycle 8 after a simple thermal treatment at 175  C for 6 h (82.4 ± 1.2% vs. 81.0 ± 1.3% in cycle 1). According to these results, we proposed that this thermal treatment should be applied after 3 consecutive cat- alytic runs to restore the initial performance of this MOF catalyst. As far as we know, despite their catalytic potential and the ev- idences provided in this work, Hf porphyrin-MOFs have never been studied for CO2 cycloadditions (Table S8). Although Zr porphyrin MOFs have demonstrated to work well, they usually requires from higher catalysts loadings [12], CO2 pressures [18] and/or tempera- tures [58] than those described here. In addition, their recyclability has not been explored beyond the 4th catalytic run in all these cases, usually limiting the substrate scope to 1e5 epoxides. A similar epichlorohydrin conversion to that reported here was observed when a doubly Zn2þ/Co2þ-metalated (Zr)PCN-224 was pyrolyzed at 950  C, and the resulting carbon-based material was used as cata- lyst at 50  C for 20 h [59]. Other Hf-based MOFs lacking porphyrin linkers (e.g., (Hf)VPI-100(Cu), (Hf)PCN-777 and (Hf)MOF-808) can achieve very high conversions for the same substrate (>96%) but only under more severe conditions (e.g. 80e90  C and even, 10 bar CO2) [60,61]. We suspect that the widespread attention paid to Zr porphyrin MOFs, although showing lower catalytic performance than Hf porphyrin MOFs, relies on their easier preparation under solvothermal conditions, while well-stablished synthetic protocols have not been reported yet for the hafnium counterparts. 4. Conclusions Fig. 3. Recyclability test. a) SEM images (x10000) of (Hf)PCN-224(Co): a) as- synthesized; b) after 7th cycle. c) Conversion (left y-axis) and EDX ratio (right y-axis) In this work, we presented for the first time the assembly of of Hf/Co (pink) and Co/Cl (green) found every catalytic cycle up to run 7. Conditions: 0.25 mmol epichlorohydrin, 1 mol% MOF, 7.5 mol% TBAB, 1 bar CO2, 15 h, 24  C. (For metalated and non-metalated Hf based PCN-224 through a simple interpretation of the references to colour in this figure legend, the reader is referred to and efficient microwave-assisted synthesis with remarkable phase the Web version of this article). purity and good yields (>55%, STY>1100 kg m3 day1). The in- fluence of different acids as modulators was investigated, concluding that a strong acid such as trifluoromethanesulfonic acid excellent crystallinity (Figure S30) and well-defined size and shape was required to allow the coordination of the porphyrin linker and (Fig. 3a and b). However, significant differences were interestingly thus the MOF formation. Metalation was successfully performed in found in terms of conversion between cycle 1 (81.0 ± 1.3%) and 4 a 3-step, 1-pot fashion without further purification and tedious pre- (74 ± 3%), after which catalytic activity decreased a 9% compared to synthetic linker modifications, enabling the presence of additional the original conversion. Further, performance decreased a 17% after coordinatively unsaturated sites (CUS). These materials showed run number 7 (Fig. 3c). Despite its excellent stability, we observed a excellent textural properties and outstanding thermal (>340  C) progressive catalyst poisoning confirmed by X-EDS. Hf/Co and chemical stabilities in different aqueous media (pH 1e11) and elemental ratio was similar upon consecutive catalytic cycles organic solvents, preserving their structural integrity even under (12.5 ± 1.1 after MOF synthesis and 11.5 ± 1.6 after 7 cycles), while harsh conditions (ultrasonic bath, and in the presence of HCl for 1 h Co/Cl ratio dramatically decreased for the same steps (12.2 ± 1.1 and 60  C). and 0.74 ± 0.11, respectively). The higher Cl content was related to As catalyst in CO2 cycloaddition reactions with epoxides, (Hf) the presence of remaining epichlorohydrin molecules attached to PCN-224(Co) displayed the best performance of all the porphyrin- CUS, as previously demonstrated by ATR-FTIR (Figure S19). Similar based MOFs reported to date owing to: i) its exceptional stability; CeH stretching bands from the epoxide, with their corresponding ii) the lower cluster coordination (6) compared to typical M4þ- shifts, were observed after cycle 7 (Figure S31), suggesting this carboxylate clusters (12 or 8 such as in UiO-66 or PCN-222, interaction. respectively), allowing for faster reagents diffusion and interac- We speculated that this effect was not mentioned nor observed tion with CUS; iii) the higher oxophilicity of the structural Hf4þ before in CO2 cycloadditions because: i) the typical epoxide used for compared to Zr4þ; and iv) the synergy between the Brønsted and catalysis optimization is propylene oxide, which had a negligible Lewis acid character of Hf4þ cluster and Co2þ, respectively. Besides, impact on CUS poisoning (Figure S20); ii) the presence of Cl favored we evaluated 12 different epoxides, the largest scope tested to date the Hf/Co(MOF)-O(epoxide) interaction evidenced by ATR-FTIR with MOFs in this reaction, and the recyclability of the material up (Figure S19) with significant CeH band shifts; and iii) this is the to unprecedented 8 cycles under mild conditions (room tempera- first work where a PCN-based catalyst was reused up to 7 consec- ture, 1 bar CO2, solvent-free reaction) with negligible loss of the utive catalytic runs (Fig. 3). Thus, the model epoxide demonstrated initial catalytic activity upon a simple thermal regeneration treat- to be extremely challenging for recyclability tests. So, the regen- ment (175  C, 6 h). eration thermal treatment (100  C for 24 h) demonstrated to be We expect that this work will provide a better understanding on effective to fully remove propylene oxide but not epichlorohydrin, the synthesis of Hf-based porphyrin MOFs, the importance of not only because the higher interaction of the latter with the MOF, cluster stabilization prior linker coordination and the possibilities but also due to its higher boiling point. From these results, we of microwave-assisted reactions in such MOF synthesis. Because of evaluated this aspect through different regeneration tests their potential, and the current limited scientific production in this 9 S. Carrasco, G. Orcajo, F. Martínez et al. Materials Today Advances 19 (2023) 100390 regard, this study paves the way for further producing and applying Abbreviations these MOFs not only for CO2 cycloadditions with epoxides as acid catalysts, but also for other interesting catalytic applications, such MOF metal-organic framework as photo-/electro-chemical reductions to obtain green fuels, the PCN porous coordination network development of optical sensors, drug delivery systems and pho- TCPP 5,10,15,20-(tetra-4-carboxyphenyl)porphyrin to(thermal) cancer therapy, among others. ATR-FTIR attenuated total reflection e Fourier transform infrared X-EDS energy-dispersive X-ray spectroscopy ICP-OES inductively coupled plasma - optical emission Author contributions spectroscopy TGA thermogravimetric analysis Conceptualization, S.C. and P.H.; synthetic methodology and PXRD powder X-ray diffraction physicochemical characterization, S.C., G.O., G.C. and P.H.; TEM TBAB tetrabutylammonium bromide characterization, S.K., D.A.E., S.B; structural characterization, I.I. and CUS coordinatively unsaturated sites D.M.; catalytic reactions, S.C., G.O., F.M., G.C.; resources, S.C, P.H., FE-SEM field emission e scanning electron microscopy G.O., F.M., G.C.; writing, review and editing, all. All authors have BET Brauner-Emmet-Teller read and agreed to the published version of the manuscript. UiO Universitetet i Oslo HAADF-STEM high-angle annular dark-field scanning transmission Funding sources electron microscopy H2020-MSCA-COFUND GOT Energy Talent (ref. 754382), “Comunidad de Madrid” and European Regional Development References Fund-FEDER 2014-2020-OE REACT-UE 1 for their financial support [1] F. Mehmood Mirza, A. Sinha, J. Rehman Khan, O.A. Kalugina, M. Wasif Zafar, to VIRMOF-CM project associated to R&D projects in response to Impact of energy efficiency on CO2 Emissions: empirical evidence from COVID-19, H2020-MSCA-ITN-2019 HeatNMof (ref. 860942), M- developing countries, Gondwana Res. 106 (2022) 64e77, https://doi.org/ ERA-NET C-MOF-cell (grant PCI2020-111998 funded by MCIN/AEI 10.1016/j.gr.2021.11.017. [2] W.F. Lamb, M. Grubb, F. Diluiso, J.C. 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