Tuning the Catalytic Properties of UiO-66 Metal-Organic Frameworks: From Lewis to Defect-Induced Bronsted Acidity

[EN] The Lewis/Bronsted acidity and catalytic properties of UiO-66-type metal-organic frameworks are studied in the context of tunable acid catalysts based on the presence of linker defects that create coordinatively unsaturated Zr4+ centers. Fourier transform infrared spectroscopy of adsorbed CO and direct pH measurements are employed to characterize hydrated and dehydrated UiO-66 containing different number of Zr4+ sites associated with defects. These sites can strongly polarize coordinated water molecules, which induces Bronsted acidity in the hydrated material. Upon dehydration of the solid, the coordinated water molecules are removed, and the underlying coordinatively unsaturated Zr4+ cations become exposed and available as Lewis acid sites. Herein we show, for various acid-catalyzed reactions, how it is possible to shift from a Bronsted acid to a Lewis acid catalyst by simply controlling the hydration degree of the solid. This control adds a new dimension to the design and engineering of MOFs for catalytic applicationsFinancial support by the Spanish Government is acknowledged through projects MAT2017-82288-C2-1-P and the Severo Ochoa program (SEV-2016-0683)Cirujano, FG.; Llabrés I Xamena, FX. (2020). Tuning the Catalytic Properties of UiO-66 Metal-Organic Frameworks: From Lewis to Defect-Induced Bronsted Acidity. The Journal of Physical Chemistry Letters. 11(12):4879-4890. https://doi.org/10.1021/acs.jpclett.0c00984S48794890111

Conversion of Levulinic acid into chemicals: Synthesis of biomass derived levulinate esters over Zr-containing MOFs

Zr-containing metal-organic frameworks (MOFs) formed by either terephthalate (UiO-66) or 2-aminoterephthalate ligancls (UiO-66-NH2) are active and stable catalysts for the acid catalyzed esterification of levulinic acid with EtOH, n-BuOH and long-chain biomass derived alcohols, with activities comparable (in some cases superior) to other solid acid catalysts previously repotted. The effect of functional group substitution at the ligand benzene ring, alcohol chain length, particle size and contents of defects on the catalytic activity of the MOFs are studied in detail. In UiO-66-NH2, a dual acid-base activation mechanism is proposed, in which levulinic acid is activated on Zr sites and the alcohol at the amino groups of the ligand. Large variations of the catalytic activity from batch to batch suggest that the active sites are located at defect positions associated to ligand deficiency of the solid. Particle size seems to have a minor impact only in UiO-66-NH2, in which diffusion of levulinic acid is somehow hindered due to the amino groups present in the linkers.Financial support from the Consolider-Ingenio 2010 (project MULTICAT, CSD-2009-0050), the SEVER OCHOA program (project SEV-2012-0267), and the Spanish Ministry of Science and Innovation (project MAT2011-29020-C02-01) is gratefully acknowledged.García Cirujano, F.; Corma Canós, A.; Llabrés I Xamena, FX. (2015). Conversion of Levulinic acid into chemicals: Synthesis of biomass derived levulinate esters over Zr-containing MOFs. Chemical Engineering Science. 124:52-60. doi:10.1016/j.ces.2014.09.047S526012

Cu-MOFs as active, selective and reusable catalysts for oxidative C O bond coupling reactions by direct C H activation of formamides, aldehydes and ethers

MOFs with Cu2+ centers linked to four nitrogen atoms from azaheterocyclic compounds, i.e., pyrimidine [Cu(2-pymo)2] and imidazole [Cu(im)2], are active, stable and reusable catalysts for oxidative C O coupling reactions by direct C H activation of formamides, aldehydes and ethers. The measured catalytic activities are clearly superior to other homogeneous cupric salts, especially for the [Cu(im)2] MOF. The previously reported activity of the Cu2+ centers for cumene oxidation allows the use of the MOF as a bifunctional catalyst for olefin epoxidation with O2. The overall catalytic process consists of a cascade reaction in which the Cu-MOF first produces cumyl hydroperoxide and then the same Cu2+ centers catalyze the oxidative C O coupling reaction using the generated hydroperoxide as the oxidant.Financial support from the Consolider-Ingenio 2010 (project MULTICAT), the Severo Ochoa program, and the Spanish Ministry of Science and Innovation (project MAT2011-29020C02-01) is gratefully acknowledged. Prof. Eduardo Palomares is gratefully acknowledged for providing the Cu-exchanged zeolites.Luz Minguez, I.; Corma Canós, A.; Llabrés I Xamena, FX. (2014). Cu-MOFs as active, selective and reusable catalysts for oxidative C O bond coupling reactions by direct C H activation of formamides, aldehydes and ethers. Catalysis Science and Technology. 4(6):1829-1836. doi:10.1039/c4cy00032cS182918364

Bridging homogeneous and heterogeneous catalysis with MOFs: Cu-MOFs as solid catalysts for three-component coupling and cyclization reactions for the synthesis of propargylamines, indoles and imidazopyridines

[EN] Copper-containing MOFs are found to be active, stable and reusable solid catalysts for three-component couplings of amines, aldehydes and alkynes to form the corresponding propargylamines. Two tandem reactions, including an additional cyclization step, leads to the effective production of indoles and imidazopyridines. In particular, the lamellar compound [Cu(BDC)] (BDC = benzene dicarboxylate) is highly efficient for the preparation of imidazopyridines, although a progressive structural change of the solid to a catalytically inactive compact structure is produced, causing deactivation of the catalyst. Nevertheless, the phase change can be reverted by refluxing in DMF, which recovers the original lamellar structure and the catalytic activity of the fresh material. The use of [Cu(BDC)] for this reaction also prevents the formation of Glaser/Hay condensation products of the alkyne, even when the reaction is performed in air atmosphere. This is a further advantage of [Cu(BDC)] with respect to other homogeneous copper catalysts, for which the use of an inert atmosphere is necessary.Financial support by Ministerio de Educacion y Ciencia e Innovacion (Project MIYCIN, CSD2009-00050; PROGRAMA CONSOLIDER.INGENIO 2009), Generalidad Valenciana (GV PROMETEO/2008/130) and the CSIC (Proyectos Intramurales Especiales 20108 01020) is gratefully acknowledged. CSIC and Fundacion Bancaja are gratefully acknowledged for a research contract to I.L.Luz Mínguez, I.; Llabrés I Xamena, FX.; Corma Canós, A. (2012). Bridging homogeneous and heterogeneous catalysis with MOFs: Cu-MOFs as solid catalysts for three-component coupling and cyclization reactions for the synthesis of propargylamines, indoles and imidazopyridines. Journal of Catalysis. 285(1):285-291. https://doi.org/10.1016/j.jcat.2011.10.001S285291285

Conversion of levulinic acid to gamma-valerolactone over Zr-containing metal-organic frameworks: Evidencing the role of Lewis and Bronsted acid sites

[EN] Zr-containing UiO-66 and MOF-808 are evaluated for converting levulinic acid (LA) into gamma-valerolactone (GVL) through various routes: (i) Step-wise esterification of LA to n-butyl levulinate (nBuL) and Meerwein-PonndorfVerley (MPV) reduction to GVL; (ii) One-pot two-steps esterification with n-butanol followed by MPV reduction with sec-butanol; and (iii) direct conversion of LA into GVL through a tandem reaction. Selection of this multistep complex reaction evidences the participation of the different acid sites (Lewis or Bronsted) of the material in each individual step: Bronsted-induced acid sites catalyze esterification reaction efficiently, while Lewis acid sites are the preferred sites for the MPV step. Sulfation of MOF-808 is used to enhance the Bronsted acidity of MOF-808, which improves the performance for esterification. However, the sulfate groups introduced are detrimental for the MPV step, since they reduce the intra-pore space available to form the required bulky transition state. These results evidence the need to find the best equilibrium between Bronsted and Lewis acid sites to optimize the outcome of this multistep reaction.Financial support by the Spanish Government is acknowledged through projects MAT2017-82288-C2-1-P and the Severo Ochoa program (SEV-2016-0683). Aula-CEMEX is also acknowledged for a fellowship to JMG.Guarinos, J.; Cirujano, F.; Rapeyko, A.; Llabrés I Xamena, FX. (2021). Conversion of levulinic acid to gamma-valerolactone over Zr-containing metal-organic frameworks: Evidencing the role of Lewis and Bronsted acid sites. Molecular Catalysis. 515:1-11. https://doi.org/10.1016/j.mcat.2021.111925S11151

Facile "Green" Aqueous Synthesis of Mono- and Bimetallic Trimesate Metal-Organic Frameworks

[EN] Various isoreticular monometallic (Co2+, Ni2+, Cu2+, and Zn2+) and bimetallic (Co-Ni, Co-Zn, Mn-Ni) trimesate MOFs have been prepared by a fast (10 min) and green synthesis method from aqueous solutions, at room temperature and ambient pressure. A combined XRD and SEM/EDX analysis clearly revealed bimetallic compounds form true solid solutions rather than a simple physical mixture of pure-phase monometallic compounds. Moreover, a detailed evaluation of the evolution of cell parameters with the composition provides strong evidence indicating a preferential occupation of one crystallographic position (bidentate terminal sites) by Co2+ (or Mn2+) ions. This leads to a precise and predictable array of metal ions in the framework, which can be finely tuned by changing the overall composition of the bimetallic MOF. Implications are envisaged in the design and catalytic properties of well-defined single-site catalysts.This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 641887 (project acronym: DEFNET) and the Spanish Government through projects MAT2017-82288-C2-1-P and Severo Ochoa (SEV-2016-0683). The Microscopy Service of the Universitat Politecnica de Valencia is gratefully acknowledged for the electron microscopy measurements.Nowacka, AE.; Briantais, P.; Prestipino, C.; Llabrés I Xamena, FX. (2019). Facile "Green" Aqueous Synthesis of Mono- and Bimetallic Trimesate Metal-Organic Frameworks. Crystal Growth & Design. 19(9):4981-4989. https://doi.org/10.1021/acs.cgd.9b00237S4981498919

Selective Aerobic Oxidation of Cumene to Cumene Hydroperoxide over Mono- and Bimetallic Trimesate Metal Organic Frameworks Prepared by a Facile Green Aqueous Synthesis

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Sustainable Chemistry & Engineering, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acssuschemeng.8b06472"[EN] Co-Ni and Mn-Ni bimetallic trimesate MOFs prepared by a fast aqueous synthesis method are excellent and reusable catalysts for the selective aerobic oxidation of cumene to cumene hydroperoxide (CHP). Isolation of Co2+ (or Mn2+) in an inert Ni-BTC framework is a good strategy to optimize CHP selectivity above 90%: since only Co2+ sites catalyze CHP decomposition, a drop of the CHP selectivity is observed as the cobalt content in the bimetallic MOF increases. The statistical probability of having isolated Co2+ sites is calculated as a function of the total cobalt content of the bimetallic compound, assuming homogeneous distribution of Co2+ ions in the Ni-BTC framework and preferential occupation of terminal sites. Thus, in our best sample, with a Co:Ni ratio of 5:95, 73% of the total Co2+ ions are isolated so that CHP decomposition/overoxidation processes at the surface of the catalyst are not likely to occur before CHP desorption. This can explain the excellent CHP selectivity (91%) attained over this material. This "site isolation" effect is further supported by similar findings on Mn-Ni bimetallic compounds.This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 641887 (project acronym: DEFNET) and the Spanish Government through projects MAT2017-82288-C2-1-P and Severo Ochoa (SEV-2016-0683).Nowacka, AE.; Briantais, P.; Prestipino, C.; Llabrés I Xamena, FX. (2019). Selective Aerobic Oxidation of Cumene to Cumene Hydroperoxide over Mono- and Bimetallic Trimesate Metal Organic Frameworks Prepared by a Facile Green Aqueous Synthesis. ACS Sustainable Chemistry & Engineering. 7(8):7708-7715. https://doi.org/10.1021/acssuschemeng.8b06472S770877157

Catalytic properties of pristine and defect-engineered Zr-MOF-808 metal organic frameworks

[EN] Various defect-engineered Zr-trimesate MOF-808 compounds (DE-MOF-808) have been prepared by mixing the tricarboxylate ligands with dicarboxylate ligands; viz. isophthalate, pyridine-3,5-dicarboxylate, 5-hydroxy-isophthalate, or 5-amino-isophthalate. The resulting mixed-ligand compounds, MOF-808-X (X = IP, Pydc, OH or NH2) were all found to be highly crystalline and isostructural to the unmodified MOF-808. Pristine MOF-808 showed better catalytic performance than a UiO-66 reference compound for the Meerwein-Ponndorf-Verley (MPV) reduction of carbonyl compounds. This was attributed to a higher availability of coordinatively unsaturated Zr4+ sites (cus) in MOF-808 upon removal of formate ions. Meanwhile, cus in UiO-66 are only located at defect sites and are thus much less abundant. Further improvement of the catalytic activity of defect-engineered MOF-808-IP and MOF-808-Pydc was observed, which may be related with the occurrence of less crowded Zr4+ sites in DE-MOF-808. The wider pore structure of MOF-808 with respect to UiO-66 compounds translates into a sharp improvement of the activity for the MPV reduction of bulky substrates, as shown for estrone reduction to estradiol. Interestingly, MOF-808 produces a notable diastereoselectivity towards the elusive 17--hydroxy estradiol.This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 641887 (project acronym: DEFNET). Financial support from the Spanish Ministry of Economy and Competitiveness (program Severo Ochoa SEV20120267), the Spanish Ministry of Science and Innovation (project MAT2014-52085-C2-1-P), and the German Research Foundation (project KA 1698/19-1) is also gratefully acknowledged. The Microscopy Service of the Universitat Politecnica de Valencia are gratefully acknowledged for the SEM images.Mautschke, H.; Drache, F.; Senkovska, I.; Kaskel, S.; Llabrés I Xamena, FX. (2018). Catalytic properties of pristine and defect-engineered Zr-MOF-808 metal organic frameworks. Catalysis Science & Technology. 8(14):3610-3616. https://doi.org/10.1039/c8cy00742jS3610361681

AgBTC MOF-Mediated Approach to Synthesize Silver Nanoparticles Decorated on Reduced Graphene Oxide (rGO@Ag) for Energy Storage Applications

[EN] Nanowires of silver-based metal-organic framework (MOF) (AgBTC, BTC = 1,3,5-benzenetricarboxylate) were grown onto graphene oxide layers to generate GO-AgBTC nano composites. Thermal treatment of these composites in inert atmosphere produced reduced graphene oxide (rGO) decorated with well-dispersed and homogeneous silver nanoparticles (rGO@ Ag). The easy and scalable synthesis of AgNPs via MOF-mediated synthesis was achieved by the thermal decomposition of the AgBTC directly onto the rGO surface. The structure, morphology, and electrochemical properties of this novel material were investigated by XRD, Raman, TGA, FE-SEM, TEM, cyclic voltammetry, and galvanostatic charge and discharge experimental techniques. The results showed improved capacitive features for rGO@Ag. Specific gravimetric capacitance measured by galvanostatic charge- discharge yielded a value of 151.97 F/g at the current density of 0.5 A/g, pointing out that MOF-mediated synthesis offers a facile method to generate rGO electrodes decorated by uniformly distributed nanoparticles for energy storage devices.The authors are grateful for Grant PID2020-112590GB-C21 funded by MCIN/AEI/10.13039/501100011033.Barjola, A.; Rapeyko, A.; Sahuquillo, O.; Llabrés I Xamena, FX.; Giménez Torres, E. (2023). AgBTC MOF-Mediated Approach to Synthesize Silver Nanoparticles Decorated on Reduced Graphene Oxide (rGO@Ag) for Energy Storage Applications. ACS Applied Energy Materials. 6(18):9159-9169. https://doi.org/10.1021/acsaem.2c038729159916961

Pd@UiO-66-Type MOFs Prepared by Chemical Vapor Infiltration as Shape-Selective Hydrogenation Catalysts

[EN] Host-guest inclusion properties of UiO-66 and UiO-67 metal-organic frameworks have been studied using ferrocene (FeCp2) as probe molecule. According to variable-temperature solid-state H-1 and C-13 CP-MAS-NMR, two different environments exist for adsorbed FeCp2 inside UiO-66 and UiO-67, which have been assigned to octahedral and tetrahedral cavities. At room temperature, a rapid exchange between these two adsorption sites occurs in UiO-67, while at -80 degrees C the intracrystalline traffic of FeCp2 through the triangular windows is largely hindered. In UiO-66, FeCp2 diffusion is already impeded at room temperature, in agreement with the smaller pore windows. Palladium nanoparticles (Pd NPs) encapsulated inside UiO-66 and UiO-67 have been prepared by chemical vapor infiltration of (allyl)Pd(Cp) followed by UV light irradiation. Infiltration must be carried out at low temperature (-10 degrees C) to avoid uncontrolled decomposition of the organometallic precursor and formation of Pd NPs at the external surface of the MOF. The resulting Pd-MOFs are shape selective catalysts, as shown for the hydrogenation of carbonyl compounds with different steric hindrance.Financial support from the Consolider-Ingenio 2010 (project MULTICAT), the Severo Ochoa program, and the Spanish Ministry of Science and Innovation (project MAT2011-29020-C02-01) is gratefully acknowledged. C. R. is grateful for a graduate student fellowship awarded by the Cluster of Excellence RESOLV (EXC 1069) funded by the German Deutsche Forschungsgemeinschaft (DFG). This project has further received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skolodowska-Curie grant agreement, number 641887.Luz Mínguez, I.; Roesler, C.; Epp, K.; Llabrés I Xamena, FX.; Fischer, RA. (2015). Pd@UiO-66-Type MOFs Prepared by Chemical Vapor Infiltration as Shape-Selective Hydrogenation Catalysts. European Journal of Inorganic Chemistry. 23:3904-3912. https://doi.org/10.1002/ejic.201500299S3904391223Corma, A., García, H., & Llabrés i Xamena, F. X. (2010). Engineering Metal Organic Frameworks for Heterogeneous Catalysis. Chemical Reviews, 110(8), 4606-4655. doi:10.1021/cr9003924Farrusseng, D., Aguado, S., & Pinel, C. (2009). Metal-Organic Frameworks: Opportunities for Catalysis. Angewandte Chemie International Edition, 48(41), 7502-7513. doi:10.1002/anie.200806063Gascon, J., Corma, A., Kapteijn, F., & Llabrés i Xamena, F. X. (2013). Metal Organic Framework Catalysis: Quo vadis? ACS Catalysis, 4(2), 361-378. doi:10.1021/cs400959kLlabres i Xamena, F., & Gascon, J. (Eds.). (2013). Metal Organic Frameworks as Heterogeneous Catalysts. Catalysis Series. doi:10.1039/9781849737586Li, B., Wang, H., & Chen, B. (2014). Microporous Metal-Organic Frameworks for Gas Separation. 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