Confining isolated atoms and clusters in crystalline porous materials for catalysis

[EN] Structure-reactivity relationships for nanoparticle-based catalysts have been greatly influenced by the study of catalytic materials with either supported isolated metal atoms or metal clusters comprising a few atoms. The stability of these metal species is a key challenge because they can sinter into large nanoparticles under harsh reaction conditions. However, stability can be achieved by confining the nanoparticles in crystalline porous materials (such as zeolites and metal-organic frameworks). More importantly, the interaction between the metal species and the porous framework may modulate the geometric and electronic structures of the subnanometric metal species, especially for metal clusters. This confinement effect can induce shape-selective catalysis or different chemoselectivity from that of metal atoms supported on open-structure solid carriers. In this Review, we discuss the structural features, synthesis methodologies, characterization techniques and catalytic applications of subnanometric species confined in zeolites and metal-organic frameworks. We make a critical comparison between confined and non-confined isolated atoms and metal clusters, and provide future perspectives for the field.We are grateful for financial support from the European Research Council (grant ERC-AdG-2014-671093, SynCatMatch) and the Spanish Government through the Severo Ochoa Program (SEV-2016-0683).Liu, L.; Corma Canós, A. (2021). Confining isolated atoms and clusters in crystalline porous materials for catalysis. Nature Reviews Materials. 6(3):244-263. https://doi.org/10.1038/s41578-020-00250-32442636

Synthesis and characterization of new metal-carbon catalysts for hydrogenation of D-Glucose

Ph.DDOCTOR OF PHILOSOPH

Nanostructured catalytic films for multiphase microstructured reactors

The application of microstructured catalytic reactors for gas-liquid reactions requires the development of new techniques for the incorporation of highly active catalytic thin films onto their microstructured surfaces. These catalytic thin films may have open porosities even up to 50%. Their application limits the pressure drop over the microreactor in comparison to micro packed beds, it enhances the catalyst accessibility, and it may significantly reduce mass transfer limitations. In this PhD thesis, ordered mesoporous silica and titania films with a thickness of 100 to 400 nm were developed via an evaporation induced self assembly method on different substrates (glass, silicon, titanium). These polymer templated meso¬porous silica films have a narrow pore size distribution and were synthesized with a wide range of pore sizes (2 to 8 nm). A new calcination protocol was developed which allows the complete removal of the surfactant at mild conditions. The thermal and hydro¬thermal stability of the films that were obtained with an ionic surfactant was improved by pH adjust¬ment during hydrolysis and by Al incorporation. Microwave assisted hydro¬thermal synthesis of these ordered microporous films was also investigated in an attempt to reduce the synthesis time from several days to less than 10 hours. The obtained thin films have been loaded with polymetallic nanoparticles with a size of 1 to 3 nm to specifically activate a selected functionality of complex organic molecules. Methods for the deposition and the stabilization of bi-metallic and tri-metallic clusters by adsorption onto the mesoporous thin films have been investigated. A "one pot" sol-gel synthesis of the mesoporous films with embedded colloidal nano¬particles was developed which eliminates an additional impregnation step and produces a uniform distribution of the active components throughout the mesoporous films. Various experimental techniques such as ellipsometric porosimetry, XRD, 2D SAXS, XPS, SEM, and TEM have been applied to obtain insight in the physical and chemical phenomena that determine the performance as well as the stability of the thin films. The activity and the selectivity of the resulting catalytic thin films have been investigated in the batch and in the continuous mode in the hydrogenation of citral and phenylacetylene. The latter was done in a 10 m long micro capillary (i.d. 250 µm) with a catalytic thin film deposited onto its inner channel wall surface. It was shown that the selectivity towards the target product can be changed by varying the metal ratio in the bimetallic nanoparticles. The high stability of these catalytic thin films allows their further implementation in fine chemicals synthesis using microstructured reactors

Ruthenium

Ruthenium is a precious metal not widely known to non-scientists. It is a target of much research, however. It is used in computer hard drives, the tips of fountain pens, and as a catalyst to purify car exhaust, among other uses. This book presents information and research on the properties and applications of ruthenium, including potential uses in phytochemical functions and anticancer activity

Zeolite encapsulated metal complexes as heterogeneous catalysts for oxidation reactions

>Magister Scientiae - MScThis study describes the synthesis and characterisation of Cu(II) and V(IV) complexes of tri- and quadridentate ligands L1 and L2 formed by condensation of ethylenediamine with acetylacetonate in 1:1 and 1:2 molar ratio, respectively. Encapsulation of these metal complexes in the nanocage of zoilite-Y generates new heterogeneous catalysts. These catalysts were synthesized employing the flexible ligand method encapsulation technique.The structures of these encapsulated complexes were established on the basis of various physico-chemical and spectroscopic studies. The results indicated that the complexes did not hinder or modify the framework or structure of the zeolite, confirming successful immobilization of Schiff-bases through the voids of zeolite Y.These encapsulated complexes were screened as heterogeneous catalysts for various oxidation reactions such as such as phenol, benzene, styrene and cyclohexene using a green oxidant (H2O2).For comparison, the corresponding neat complexes were screened as potential homogeneous catalysts for these oxidation reactions. The results proved that the corresponding homogeneous systems described here represent an efficient and inexpensive method for oxidation of phenol, benzene, styrene and cyclohexene, having advantages over heterogeneous catalysis are its high activity and selectivity and short reaction times. Its major problem is its industrial application regarding principally the separation of the catalyst from the products.The size of the substrate has a significant effect on the conversion by encapsulated complexes such as in styrene oxidation. Therefore, it was established that steric effects of the substrates play a critical role in the poor reactive nature of the encapsulated complexes.In general, the percentage conversion decreased upon encapsulation of complexes in zeolite Y. All catalysts studied proved to be potential catalysts for the various oxidation reactions.It has been shown in this study that encapsulation can effectively improve product selectivity but requires a longer reaction time in most cases for maximum activity.Furthermore,oxovanadium complexes were more reactive than copper-based catalysts in all oxidation reactions tested in this study.A reaction mechanism study revealed that the activity of the encapsulated and neat complexes occurs through either formation of peroxovanadium (V) or hydroperoxidecopper(II) intermediate species.The studies in this thesis, therefore, conclude that the Cu(II) and V(IV) complexes encapsulated in Y-zeolite are active heterogeneous catalysts for the selective oxidation of various substrates. Encapsulation of the metal complexes in the super cages (-cages) of the zeolite matrix has the advantages of solid heterogeneous catalysts of easy separation and handling, ruggedness, thermostability, reusability (regeneration of the deactivated catalysts) as well as share many advantageous features of homogeneous catalysts

Platinum catalysed aerobic selective oxidation of cinnamaldehyde to cinnamic acid

Aerobic selective oxidation of allylic aldehydes offers an atom and energy efficient route to unsaturated carboxylic acids, however suitable heterogeneous catalysts offering high selectivity and productivity have to date proved elusive. Herein, we demonstrate the direct aerobic oxidation of cinnamaldehyde to cinnamic acid employing silica supported Pt nanoparticles under base-free, batch and continuous flow operation. Surface and bulk characterisation of four families of related Pt/silica catalysts by XRD, XPS, HRTEM, CO chemisorption and N2 porosimetry evidence surface PtO2 as the common active site for cinnamaldehyde oxidation, with a common turnover frequency of 49,000 ± 600 h−1; competing cinnamaldehyde hydrogenolysis is favoured over metallic Pt. High area mesoporous (SBA-15 or KIT-6) and macroporous-mesoporous SBA-15 silicas confer significant rate and cinnamic acid yield enhancements versus low area fumed silica, due to superior platinum dispersion. High oxygen partial pressures and continuous flow operation stabilise PtO2 active sites against in-situ reduction and concomitant deactivation, further enhancing cinnamic acid productivity

Catalysis with Ruthenium for Sustainable Carbon Cycles

Nestled between the noble and non-noble metals in the periodic table, ruthenium, one of the transition metals, offers a combination of intriguing properties. Due to its variable oxidation states and its ability to form complexes with various Lewis base compounds, ruthenium, has been widely used in the field of catalysis. Its application has led to groundbreaking breakthroughs in a variety of chemical transformations and has attracted considerable attention in both academic research and industrial applications. Ruthenium catalysis is a dynamic and rapidly evolving field, with ongoing efforts to further advance the efficiency and selectivity of these catalysts. Importantly, in the context of sustainability, ruthenium-based catalysts play an important role in promoting green chemistry practices. Because ruthenium catalysts are highly efficient, only small amounts of the element need to be used. Recovery rates at the end of catalyst life are typically very high, minimizing the need to mine fresh ore. The use of ruthenium catalysts promotes the utilization of renewable resources in various chemical transformations, is at the heart of the realization of new energy-related processes, and by enabling efficient and highly selective chemical transformations reduces waste and harmful emissions. These aspects reinforce the metal’s importance in the quest for a more sustainable future

Porous hybrid materials for heterogeneous catalysis and gas storage

A series of new Ru(diphosphine)(diamine)Cl2 complexes with siloxy pendant groups was synthesized and immobilized on mesoporous silica nanoparticles (MSNs) with the hope of generating highly active heterogeneous catalysts by taking advantage of the very large channel diameters (~2-5 nm) and short diffusion lengths for the substrates as a result of nanoparticle sizes of ~300-1000 nm. Upon activation with base co-catalysts, these new Ru complexes were highly active for homogeneous asymmetric hydrogenation of ketones and racemic a-branched arylaldehydes with enantiomeric excess (ee) up to 94 and 99%, respectively. These Ru complexes were readily immobilized onto several types of MSNs via the siloxy functionalities and the immobilized Ru precatalysts were highly active for the asymmetric hydrogenation of ketones with up to 82% ee and a-branched arylaldehydes with ee’s of up to 97%. Highly porous and robust metal organic frameworks (MOFs) were also synthesized for hydrogen storage and for potential use as asymmetric catalysts. 4,8-connected MOFs of the scu topology based on copper paddlewheels and aromatic-rich octa-carboxylic acid bridging ligands were synthesized in order to overcome the tendency of MOFs to undergo framework distortion upon solvent removal. The rigidified MOFs are capable of storing up to 2.5 wt% of H2 at 1 bar (77 K), and 5.5 wt% of H2 at 30 bar (77 K). A series of homochiral porous MOFs were synthesized using bridging ligands containing the chiral BINAP oxide functionalities. The easily accessible catalytic sites make these MOFs interesting candidates for applications in heterogeneous asymmetric catalysis

Hydrogenation and Hydrogenolysis with Ruthenium Catalysts and Application to Biomass Conversion

With the rising emphasis on efficient and highly selective chemical transformations, the field of ruthenium-catalysed hydrogenation and hydrogenolysis reactions has grown tremendously over recent years. The advances are triggered by the detailed understanding of the catalytic pathways that have enabled researchers to improve known transformations and realise new transformations in biomass conversion. Starting with the properties of ruthenium, this chapter introduces the concept of the catalytic function as a basis for rational design of ruthenium catalysts. Emphasis is placed on discussing the principles of dissociative adsorption of hydrogen. The principles are then applied to the conversion of typical biomolecules such as cellulose, hemicellulose and lignin. Characteristic features make ruthenium catalysis one of the most outstanding tools for implementing sustainable chemical transformations