Development of covalent triazine frameworks as heterogeneous catalytic supports

Covalent triazine frameworks (CTFs) are established as an emerging class of porous organic polymers with remarkable features such as large surface area and permanent porosity, high thermal and chemical stability, and convenient functionalization that promotes great potential in heterogeneous catalysis. In this article, we systematically present the structural design of CTFs as a versatile scaffold to develop heterogeneous catalysts for a variety of chemical reactions. We mainly focus on the functionalization of CTFs, including their use for incorporating and stabilization of nanoparticles and immobilization of molecular complexes onto the frameworks

Selective Heterogeneous Catalytic Hydrogenation of Nitriles to Primary Amines

Primary amines are important intermediates, especially in the area of pharmaceutical, plastic and agrochemical industry. The heterogeneous catalytic hydrogenation of nitriles is one of the most frequently applied process for the synthesis of diverse amines. However, the control of the selectivity is a critical issue in this reaction. Over the past decade, many studies have been reported using heterogeneous transition metal catalysts for the selective reduction of nitriles to the corresponding primary amines. The type of the catalysts, especially, the chemical nature of metals in the catalysts is one of the most important factors to influence the selectivity, the reaction rate, and possibly also the reaction pathway and the deactivation of the catalyst. Thus, this review focuses on the heterogeneous transition metal catalysts and summarizes the recent developments achieved in the selective catalytic hydrogenation of nitriles to primary amines

Palladium and gold catalysts for sustainable chemical processing

The main focus of this thesis is the investigation of sustainable routes for the production of commercially important higher and functionalised aliphatic and aromatic amines through the application of (oxide and carbon) supported palladium and gold catalysts. In the hydrogenation of butyronitrile as a model aliphatic nitrile, unsupported Pd promoted the formation of primary and secondary amines. The acid-base character of the support and available surface reactive hydrogen are critical catalyst variables. The greater acidity of Pd/C (relative to Pd/Al2O3) resulted in the predominant formation of the tertiary (tributyl-) amine where spillover hydrogen serves to elevate hydrogenation rate. The combination of Ba with Pd (supported bimetallic) proved effective in promoting hydrogenation activity with 100% selectivity to the secondary amine, which is attributed to a decrease in acidity and modification to Pd dispersion that enhances surface hydrogen. The feasibility of an alternative route for the synthesis of higher aliphatic secondary and tertiary amines from primary and secondary amine feedstock has been demonstrated. Control of contact time is key where the use of a multiple catalyst beds in series facilitates higher yields. This is accounted for in terms of surface reaction mechanism. This configuration was also efficient for the synthesis of benzylamine from benzonitrile. Hydrogenation selectivity was further assessed by considering the reduction of functionalized nitroarenes (p-chloronitrobenzene (p-CNB) and p-nitrobenzonitrile (p-NBN)). It is shown that the redox nature of the support has a direct impact on the activity and selectivity response. The formation of Pdδ+ (on carbon) activates the nitro group with subsequent C-Cl bond scission with the formation of p-chloroaniline (p-CAN) and aniline (AN). The occurrence of Pdδ- (on SiO2 and Al2O3) favours interaction via the aromatic ring that activates both –NO2 and –Cl for attack generating AN and nitrobenzene. The formation of a PdZn alloy (established by XPS analysis) in addition to Pd0 selectively activates the –NO2 group and promotes the sole formation of p-CAN at all levels of conversion (and close to 100%). Exclusive conversion of p-NBN to p-aminobenzonitrile was achieved over a series of oxide (CeO2, Fe2O3, Fe3O4, TiO2, ZrO2 and Al2O3) supported (1 mol %) Au catalysts. Hydrogen uptake is structure sensitive and favoured by smaller nano-scale metal particles with a consequent increase in activity. Reaction over Au/TiO2 delivered the highest specific hydrogenation rate, which is explained on the basis of –NO2 activation at the metal-support interface that is facilitated by Nδ+ interaction with electron rich gold (Auδ-, demonstrated by XPS). This effect is shown to extend to TiO2 supported Ag and Pd. Supported Au is also effective in the selective hydrogenation of benzaldehye in liquid phase operation using water as a green solvent. 100% yield of the target benzyl alcohol was attained over Au/Al2O3 whereas Pt/Al2O3 generated toluene and benzene as significant (hydrogenolysis) by-products. Solvent effects were evaluated where a direct correlation between selective hydrogenation rate and dielectric constant is demonstrated and ascribed to competitive adsorption, which was more severe for less polar alcohol solvents. Solvation by polar water facilitated benzaldehyde activation. The same activity and selectivity trends were found to also apply to continuous gas phase reaction. The results presented in this thesis demonstrate, for the first time, direct participation of the support in the catalytic hydrogenation of aliphatic nitriles over Pd-based catalysts. This can be harnessed to enhance amine production in a sustainable continuous flow gas operation process. Moreover, secondary and tertiary aliphatic amines can be selectively produced from the correspondent primary and secondary amines over Pd in continuous mode. The use of reducible supports can result in the formation of an alloy phase and surface defects with beneficial selectivity and activity effects in the production of functionalized amines. The selective catalytic action of supported Au catalysts has been established in achieving 100% yield of benzyl alcohol (from benzaldehyde) using water as a benign solvent

Recent advances on the utilization of layered double hydroxides (LDHs) and related heterogeneous catalysts in a lignocellulosic-feedstock biorefinery scheme

Layered double hydroxides (LDHs) and derived materials have been widely used as heterogeneous catalysts for different types of reactions either in gas or in liquid phase. Among these processes, the valorization/upgrading of lignocellulosic biomass and derived molecules have attracted enormous attention because it constitutes a pivotal axis in the transition from an economic model based on fossil resources to one based on renewable biomass resources with preference for biomass waste streams. Proof of this is the increasing amount of literature reports regarding the rational design and implementation of LDHs and related materials in catalytic processes such as: depolymerization, hydrogenation, selective oxidations, and C-C coupling reactions, among others, where biomass-derived compounds are used. The major aim of this contribution is to situate the most recent advances on the implementation of these types of catalysts into a lignocellulosic-feedstock biorefinery scheme, highlighting the versatility of LDHs and derived materials as multifunctional, tunable, cheap and easy to produce heterogeneous catalysts

Synthesis of flow‐compatible Ru-Me/Al2O3 catalysts and their application in hydrogenation of 1-iodo-4-nitrobenzene

The development of an active, selective, and long-term stable heterogeneous catalyst for the reductive hydrogenation of substituted nitrorarenes in continuous operation mode is still challenging. In this work, Ru based nanoparticles catalysts promoted with different transition metals (Zn, Co, Cu, Sn, or Fe) were supported on alumina spheres using spray wet impregnation method. The freshly prepared catalysts were characterized using complementary methods including scanning transmission electron microscopy (STEM) and temperature programmed reduction (TPR). The hydrogenation of 1-iodo-4-nitrobenzene served as model reaction to assess the catalytic performance of the prepared catalysts. The addition of the promotor affected the reducibility of Ru nanoparticles as well as the performance of the catalyst in the hydrogenation reaction. The highest yield of 4-iodoaniline (89 %) was obtained in a continuous flow process using Ru-Sn/Al2O3. The performance of this catalyst was also followed in a long-term experiment. With increasing operation time, a catalyst deactivation occurred which could only briefly compensate by an increase of the reaction temperature

Reduction of nitro compounds using 3d-non-noble metal catalysts

The reduction of nitro compounds to the corresponding amines is one of the most utilized catalytic processes in the fine and bulk chemical industry. The latest development of catalysts with cheap metals like Fe, Co, Ni, and Cu has led to their tremendous achievements over the last years prompting their greater application as "standard" catalysts. In this review, we will comprehensively discuss the use of homogeneous and heterogeneous catalysts based on non-noble 3d-metals for the reduction of nitro compounds using various reductants. The different systems will be revised considering both the catalytic performances and synthetic aspects highlighting also their advantages and disadvantages

Elucidation of single atom catalysts for energy and sustainable chemical production: Synthesis, characterization and frontier science

The emergence of single atom sites as a frontier research area in catalysis has sparked extensive academic and industrial interest, especially for energy, environmental and chemicals production processes. Single atom catalysts (SACs) have shown remarkable performance in a variety of catalytic reactions, demonstrating high selectivity to the products of interest, long lifespan, high stability and more importantly high atomic metal utilization efficiency. In this review, we unveil in depth insights on development and achievements of SACs, including (a) Chronological progress on SACs development, (b) Recent advances in SACs synthesis, (c) Spatial and temporal SACs characterization techniques, (d) Application of SACs in different energy and chemical production, (e) Environmental and economic aspects of SACs, and (f) Current challenges, promising ideas and future prospects for SACs. On a whole, this review serves to enlighten scientists and engineers in developing fundamental catalytic understanding that can be applied into the future, both for academia or valorizing chemical processes

Silica-supported Fe/Fe–O nanoparticles for the catalytic hydrogenation of nitriles to amines in the presence of aluminium additives

The hydrogenation of nitriles to amines represents an important and frequently used industrial process due to the broad applicability of the resulting products in chemistry and life sciences. Despite the existing portfolio of catalysts reported for the hydrogenation of nitriles, the development of iron-based heterogeneous catalysts for this process is still a challenge. Here, we show that the impregnation and pyrolysis of iron(II) acetate on commercial silica produces a reusable Fe/Fe-O@SiO2 catalyst with a well-defined structure comprising the fayalite phase at the Si-Fe interface and alpha-Fe nanoparticles, covered by an ultrathin amorphous iron(III) oxide layer, growing from the silica matrix. These Fe/Fe-O core-shell nanoparticles, in the presence of catalytic amounts of aluminium additives, promote the hydrogenation of all kinds of nitriles, including structurally challenging and functionally diverse aromatic, heterocyclic, aliphatic and fatty nitriles, to produce primary amines under scalable and industrially viable conditions.Web of Science51292

Synthesis and catalysis of chemically reduced metal–metalloid amorphous alloys

This is the published version. Copyright 2012 Royal Society of ChemistryAmorphous alloys structurally deviate from crystalline materials in that they possess unique short-range ordered and long-range disordered atomic arrangement. They are important catalytic materials due to their unique chemical and structural properties including broadly adjustable composition, structural homogeneity, and high concentration of coordinatively unsaturated sites. As chemically reduced metal–metalloid amorphous alloys exhibit excellent catalytic performance in applications such as efficient chemical production, energy conversion, and environmental remediation, there is an intense surge in interest in using them as catalytic materials. This critical review summarizes the progress in the study of the metal–metalloid amorphous alloy catalysts, mainly in recent decades, with special focus on their synthetic strategies and catalytic applications in petrochemical, fine chemical, energy, and environmental relevant reactions. The review is intended to be a valuable resource to researchers interested in these exciting catalytic materials. We concluded the review with some perspectives on the challenges and opportunities about the future developments of metal–metalloid amorphous alloy catalysts