Alberto Villa

Covalent Triazine Framework as Catalytic Support for Liquid Phase Reaction

An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active phase causes deactivation. In this work, a covalent triazine framework (CTF) as support for Pd nanoparticles is compared to activated carbon (AC), the typical support used in liquid phase reactions. The results indicate that the presence of the N-heterocyclic moieties on the surface of the frameworks is beneficial for improving the stability of Pd nanoparticles during the liquid phase glycerol oxidation. Pd/CTF showed better activity and in particular better stability when compared to Pd supported on activated carbon (AC).

Selective Oxidation of Glycerol under Acidic Conditions Using Gold Catalysts

H-mordenite-supported PtAu nanoparticles are highly active and selective in the oxidation of glycerol under acidic conditions, which allows the direct preparation of free acids (see picture). The high selectivity for C{sub 3} compounds results from the negligible formation of H{sub 2}O{sub 2}, in contrast to PtAu nanoparticles supported on activated carbon.

Pd‐modified Au on Carbon as an Effective and Durable Catalyst for the Direct Oxidation of HMF to 2,5‐Furandicarboxylic Acid

Mixed noblility: We show that the modification of a gold/carbon catalyst with platinum or palladium produces stable and recyclable catalysts for the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA): the support and nanoparticle chemistry directly mediate the selective oxidation of terminal hydroxyl groups in bio-derived HMF. This finding is a significant advance over current conversion technology because of the technological importance of FDCA.

Glycerol oxidation using gold-containing catalysts

Glycerol is an important byproduct of biodiesel production, and it is produced in significant amounts by transesterification of triglycerides with methanol. Due to the highly functionalized nature of glycerol, it is an important biochemical that can be utilized as a platform chemical for the production of high-added-value products. At present, research groups in academia and industry are exploring potential direct processes for the synthesis of useful potential chemicals using catalytic processes. Over the last 10 years, there has been huge development of potential catalytic processes using glycerol as the platform chemical. One of the most common processes investigated so far is the catalytic oxidation of glycerol at mild conditions for the formation of valuable oxygenated compounds used in the chemical and pharmaceutical industry. The major challenges associated with the selective oxidation of glycerol are (i) the control of selectivity to the desired products, (ii) high activity and resistance to poisoning, and (iii) minimizing the usage of alkaline conditions. To address these challenges, the most common catalysts used for the oxidation of glycerol are based on supported metal nanoparticles. The first significant breakthrough was the successful utilization of supported gold nanoparticles for improving the selectivity to specific products, and the second was the utilization of supported bimetallic nanoparticles based on gold, palladium, and platinum for improving activity and controlling the selectivity to the desired products. Moreover, the utilization of base-free reaction conditions for the catalytic oxidation of glycerol has unlocked new pathways for the production of free-base products, which facilitates potential industrial application. The advantages of using gold-based catalysts are the improvement of the catalyst lifetime, stability, and reusability, which are key factors for potential commercialization. In this Account, we discuss the advantages of the using supported gold-based nanoparticles, preparation methods for achieving highly active gold-based catalysts, and parameters such as particle size, morphology of the bimetallic particle, and metal-support interactions, which can influence activity and selectivity to the desired products.

Triazine‐Based Polymers as Nanostructured Supports for the Liquid‐Phase Oxidation of Alcohols

A covalent triazine framework (CTF) was used as support for palladium nanoparticles (NPs) and Pd/CTF was applied as the catalyst in the selective oxidation of benzyl alcohol. N groups in the CTF appeared more efficient than those created on carbon nanotubes (CNTs) by NH(3) /high-temperature treatment in stabilizing Pd NPs against growth during the immobilization step. This assured a high metal dispersion, which led to a highly active and stable catalyst in the alcohol oxidation reaction. Indeed, Pd on the CTF was more stable in recycling than Pd on activated carbon (AC) and on nitrogen-doped CNTs, particularly avoiding leaching of Pd NPs. Moreover, Pd on the CTF was less sensitive than Pd on AC to the decrease of reactant concentration. This in turn led to a higher selectivity to benzaldehyde (98 %) with a considerable activity (turnover frequency 1453 h(-1) ).

Gold Sols as Catalysts for Glycerol Oxidation: The Role of Stabilizer

Gold nanoparticles (Au NPs), stabilized by polyvinylalcohol (PVA), tetrakishydroxypropylphosphonium chloride (THPC), and citrate, have been synthesized and tested in the liquid‐phase oxidation of glycerol. THPC‐stabilized Au NPs have been found to exhibit remarkable catalytic activity (TOF=2478 h−1) in comparison to PVA‐ and citrate‐stabilized nanoparticles (TOF=715 and 160 h−1, respectively). The catalytic results can be explained in terms of particle size but aging treatment also revealed the influence of the protective agent. PVA as stabilizer produced small and quite stable gold nanoparticles differently from the THPC system. However PVA also limits the accessibility of reactant to the active site.

Gold catalyzed liquid phase oxidation of alcohol: the issue of selectivity.

Commercial carbon nanotubes (CNTs) and carbon nanofibers (CNFs) modified in various ways at the surface have been used as supports for gold nanoparticles (AuNPs) in order to study their influence on the activity/selectivity of catalysts in the aqueous oxidation of alcohol. Particularly oxidative treatment was used to introduce carboxylic functionalities, whereas subsequent treatment with NH3 at different temperatures (473 K, 673 K and 873 K) produced N-containing groups leading to an enhancement of basic properties as the NH3 treatment temperature was increased. The nature of the N-containing groups changed as the temperature increased, leading to an increase in the hydrophobicity of the support surface. Similar Au particle size and similar textural properties of the supports allowed the role of chemical surface groups in both the activity and the selectivity of the reaction of glycerol oxidation to be highlighted. An increase of basic functionalities produced a consistent increase in the activity of the catalyst, which was correlated to the promoting effect of the basic support in the alcoholate formation and the subsequent C-H bond cleavage. The selectivity towards primary oxidation products (C3 compounds) was the highest for the catalysts treated with NH3 at 873 K, which presented the most hydrophobic surface. The same trend in the catalyst activity has been obtained in the aqueous benzyl alcohol base-free oxidation. As in the case of glycerol, the increasing of basicity and/or hydrophobicity increased the consecutive reactions.

New challenges in gold catalysis: bimetallic systems

Since the discovery of the peculiar catalytic activity of gold catalysts, it became clear that gold could play a fundamental role also as a modifier. Indeed, more active catalysts such as those based on Pd or Pt showed specific and sometimes unexpected properties when modified with gold. This paper reviews advances in the field of Au-based bimetallic catalysts with particular attention to their preparation, characterization and catalytic activity. AuPd catalysts, the most widely studied, have been chosen as an example to show how the different obtained morphologies (alloy, core–shell, decorated particles) can contribute to the catalytic activity.

Gold Colloids: From Quasi-Homogeneous to Heterogeneous Catalytic Systems

Ruby red colloids of gold have been used for thousands of years and in the past have attracted much attention due to their optical properties. Surface plasmon resonance (SPR) bands are responsible for gold colloid colors and typically appear for nanometer-sized gold nanoparticles (GNPs). These lie in the visible range and their position (and intensity) depends on the size, distribution of size, and shape of GNPs but also on their interaction with other materials (i.e., support). Scientists consider colloids as quasi-homogeneous systems, but because of their intrinsic thermodynamic instability, they need different capping agents providing sufficient stability. The strength and the nature of the interaction between the protective (or functionalizing) molecule and the GNP surface is of utmost importance. It can determine the catalytic properties of the nanoparticles, as they mainly interact with the active sites, thus interfering with reactant. Therefore, the protective layer should contribute to the colloid stability, but at the same time, it should not be irreversibly adsorbed on the active site of the GNP surface providing convenient accessibility to reactant. From a catalytic point of view, the milder the interaction is between the particle surface and the capping agent, the more the activity increases. Unfortunately, the reaction conditions often do not allow the required stability of GNPs, which constitutes a fundamental prerequisite for stable catalytic activity. Anchoring GNPs on suitable supports can circumvent the problem, and this technique is now considered a valuable alternative to classical methods to produce highly dispersed gold catalysts. In this Account, we describe the advantages in using this technique to produce gold heterogeneous catalysts of high metal dispersion on a large variety of supports with the possibility of tuning to a large extent the size and (even partially) the shape of GNPs. We also review our recent progress on the sol-immobilization technique. Specifically, we highlight how, depending on its nature, the protective agent not only mediates the activity of GNPs in alcohol oxidation process but also actively participates in the anchoring process and to the stability of GNPs depending on the support surface. We can also use the modification of the metal surface operated by the capping agent to prepare bimetallic species and influence the surface potential, which modifies the intrinsic activity of the GNP. In conclusion, this technique implies many contributions (sometimes not yet clarified factors) that are not simply concerning dimension and dispersion of GNPs or type of support. Chemists should make careful selection of the protective agent and reaction parameters depending on which support is used in which reaction.

Gold on carbon: one billion catalysts under a single label

Despite the wide use of carbon materials as supports for heterogeneous catalysis, generic labels are often used to describe the catalysts, i.e. Au/C, making comparisons between different Au/C catalysts difficult even for the same application. A variety of structures and chemically modified surfaces are in fact available, making gold nanoparticles supported on carbonaceous supports extremely versatile catalysts.