Gisela Orcajo

Effect of Ion-Exchange Modification on Hydrogen and Carbon Dioxide Adsorption Behaviour of RhoZMOF Material

Zeolite-like metal-organic frameworks (ZMOFs) can adsorb hydrogen and carbon dioxide because of their negatively charged structures, being able to be ion exchanged with metal cations to further increase the electrostatic field inside the framework cavities. Ion exchange of RhoZMOF with alkaline and alkaline-earth metal cations was performed up to stoichiometric levels with one single ion-exchange step. Screen effect caused by water-coordinated molecules around the exchanged cations hindered the hydrogen uptake, although slightly higher heats of hydrogen adsorption were observed for Na+, Li+, Mg2+ and Ca2+ exchanged samples. Conversely, the same exchange treatment produced a very interesting enhancement in CO2 adsorption of RhoZMOF at 303 K. Moreover, because there are no water molecules surrounding the metallic cations, RhoZMOF material was exchanged with an Li+–crown–ether complex in the nitrobenzene/acetonitrile mixture, free of water. Nitrogen and hydrogen adsorption isotherms at 77 K showed higher specific surface area and hydrogen adsorption capacity than the material with hydrated cations.

A Recyclable Cu-MOF-74 Catalyst for the Ligand-Free O-Arylation Reaction of 4-Nitrobenzaldehyde and Phenol

The activity and recyclability of Cu-MOF-74 as a catalyst was studied for the ligand-free C–O cross-coupling reaction of 4-nitrobenzaldehyde (NB) with phenol (Ph) to form 4-formyldiphenyl ether (FDE). Cu-MOF-74 is characterized by having unsaturated copper sites in a highly porous metal-organic framework. The influence of solvent, reaction temperature, NB/Ph ratio, catalyst concentration, and basic agent (type and concentration) were evaluated. High conversions were achieved at 120 °C, 5 mol % of catalyst, NB/Ph ratio of 1:2, DMF as solvent, and 1 equivalent of K2CO3 base. The activity of Cu-MOF-74 material was higher than other ligand-free copper catalytic systems tested in this study. This catalyst was easily separated and reused in five successive runs, achieving a remarkable performance without significant porous framework degradation. The leaching of copper species in the reaction medium was negligible. The O-arylation between NB and Ph took place only in the presence of Cu-MOF-74 material, being negligible without the solid catalyst. The catalytic advantages of using nanostructured Cu-MOF-74 catalyst were also proven.

New URJC-1 Material with Remarkable Stability and Acid-Base Catalytic Properties

Emerging new metal-organic structures with tunable physicochemical properties is an exciting research field for diverse applications. In this work, a novel metal-organic framework Cu(HIT)(DMF)0.5, named URJC-1, with a three-dimensional non-interpenetrated utp topological network, has been synthesized. This material exhibits a microporous structure with unsaturated copper centers and imidazole–tetrazole linkages that provide accessible Lewis acid/base sites. These features make URJC-1 an exceptional candidate for catalytic application in acid and base reactions of interest in fine chemistry. The URJC-1 material also displays a noteworthy thermal and chemical stability in different organic solvents of different polarity and boiling water. Its catalytic activity was evaluated in acid-catalyzed Friedel–Crafts acylation of anisole with acetyl chloride and base-catalyzed Knoevenagel condensation of benzaldehyde with malononitrile. In both cases, URJC-1 material showed very good performance, better than other metal organic frameworks and conventional catalysts. In addition, a remarkable structural stability was proven after several consecutive reaction cycles.

Transition metal inclusion in RhoZMOF materials

Porous RhoZMOF is a negatively charged zeolite-like MOF material susceptible to post-synthesis modification by an ion-exchange process replacing as-synthesized organic cations with transition metals. This material exhibiting open transition metal sites is a very attractive candidate for industrial applications related to clean energy technologies, like electrochemistry and gas storage. In the present work, partial or total Co2+, Ni2+ and Cu2+ inclusions have been successfully carried out in the RhoZMOF structure by ion-exchange treatment. Particularly, complete exchange of HDMA+ with Co2+ could be reached at room temperature after 12 h of contact time. For Cu2+ exchange, a rapid transformation of the RhoZMOF structure into a new Cu-rich, crystalline and practically non-porous phase was observed at room temperature; its appearance can be avoided and the desired structure can be preserved when ion-exchange treatment is carried out at −20 °C for 6 h, reaching a partial ion-exchange degree of 30%. Moreover, for Ni2+ ion-exchange treatment at 25 and 0 °C, a covering of an amorphous Ni-rich organometallic phase over the crystals was detected by SEM analysis; its appearance can be prevented by reducing the amount of HDMA+ in the media by developing the ion-exchange process in two steps: exchanging HDMA+ with Na+ and Na+ with Ni2+. The results provide a post-synthesis modification route for obtaining porous MOF materials containing open transition metal sites accessible for redox catalysis and selective gas adsorption.

Influence of chemical composition of the open bimetallic sites of MOF-74 on H2 adsorption

The success of using physisorbent metal–organic framework materials for hydrogen storage at near-to-room temperatures pass through increasing their adsorption enthalpies. The inclusion of open mixed-metal adsorption sites is one of the most interesting strategies to improve the interaction forces between H2 and solids. In this work, equimolar mixed metals of Co, Ni, Cu and Zn have been achieved in MOF-74 structure which is known for exhibiting the highest concentration of open metal sites per unit volume among MOF materials. Homogenous distribution of both metals on the solid surface was confirmed in all cases. Textural properties were correlated to the average molecular weight of the unit cell of the samples. Finally, hydrogen adsorption properties were related to the chemical composition of the inorganic clusters in the materials, outstanding the Ni50Co-MOF-74 material with the most effective bimetallic mixture, where high partial charge and polarizability properties synergistically work for enhancing the affinity to hydrogen molecules.