Metal nanoparticles have received intense scientific attention in the field of catalysis. Precise engineering of nanomaterials' size, shape and surface composition, including adsorbed capping ligands, is of utmost importance to control activity and selectivity, and distinguish colloidally prepared metal nanoparticle catalysts from traditional heterogeneous catalysts. The interface between the material and the reaction medium is where the key interactions occur; therefore, catalysis occurs under the influence of capping ligands. In this Perspective review, we focus on the choice of capping ligands (or stabilizing agents), and their role and fate in different steps from preparation to catalysis. Evaluating the influence of the ligands on the catalytic response is not trivial, but the literature provides examples where the ligands adsorbed on the nanoparticle surface dramatically change the activity and selectivity for a particular reaction, while acting either as a dynamic shell or a passivation coating. Steric and electronic effects resulting from the presence of adsorbed ligands have been proposed to influence the catalytic properties. Attempts to remove the capping ligands are discussed, even though they are not always successful or even necessary. Finally, we outline our personal understanding and perspectives on the use of ligands or functionalized supports to tune the activity and selectivity of supported metal nanoparticles.

Abstract:The oxidation of bio-based molecules in general, and of carbohydrates and furanics in particular, is a highly attractive process. The catalytic conversion of renewable compounds is of high importance. Acids and other chemical intermediates issued from oxidation processes have many applications related, especially, to food and detergents, as well as to pharmaceutics, cosmetics, and the chemical industry. Until now, the oxidation of sugars, furfural, or 5-hydroxymethylfurfural has been mainly conducted through biochemical processes or with strong inorganic oxidants. The use of these processes very often presents many disadvantages, especially regarding products separation and selectivity control. Generally, the oxidation is performed in batch conditions using an appropriate catalyst and a basic aqueous solution (pH 7-9), while bubbling oxygen or air through the slurry. However, there is a renewed interest in working in base-free conditions to avoid the production of salts. Actually, this gives direct access to different acids or diacids without laborious product purification steps. This review focuses on processes applying gold-based catalysts, and on the catalytic properties of these systems in the base-free oxidation of important compounds: C5-C6 sugars, furfural, and 5-hydroxymethylfurfural. A better understanding of the chemical and physical properties of the catalysts and of the operating conditions applied in the oxidation reactions is essential. For this reason, in this review we put emphasis on these most impacting factors.

A comparative study of the promotion effect of weak and strong bases in the aqueous oxidation of benzyl alcohol by Au/TiO2 showed that better conversion and aldehyde selectivity can be obtained using weak bases.

We report herein HMF (5-hydroxymethylfurfural) and furfural oxidation to 2,5-furandicarboxylic (FDCA) and furoic acids, respectively, in water, under base-free conditions and using supported gold nanoparticles. Prepared catalysts showed high catalytic activity under environmentally friendly conditions. Especially, the use of base-free conditions enables getting rid of the most usually needed neutralization step generating salts that must be further handled/eliminated. We showed that the conversion and the selectivity to desired products depend on the basicity of the support. To this respect, the Au/MgO catalyst was the most active sample (100% and 90% yields to furoic acid and 2,5furandicarboxylic acid, respectively), although a partial leaching of Mg was observed (89 ppm). Novel Au/MgF 2 -MgO catalysts were also very active (97% yield of furoic acid) with a final pH of 7.5. High yields of acid were also obtained (78%) with an acid final pH of 3.8, actually enabling the isolation of products under their acid form directly from the reaction medium without a neutralization step.

Furfural (FF) is a strategic product for the development of highly valued chemicals from biomass. The oxidation product of FF, furoic acid (FA), is an important precursor for the synthesis of green esters, such as methyl furoate. Taking into account issues with the direct furfural oxidation, furfural derivatives, such as alkyl furoates, can be easily prepared via oxidative esterification.Here, Au nanoparticles that were immobilized on alkaline-earth metal oxide supports were studied for the oxidative esterification of furfural while using alcohol as both reactant and solvent. The formation of esters is favored by the presence of basic sites on catalyst surface, resulting in high selectivity, preventing the formation of the acetal as a by-product. The Au/MgO sample provided up to 95% methyl furoate (MF) yield, a fast reaction rate, and high performance for furfural:Au molar ratios between 50 and 300. Furthermore, this catalyst was stable during reuse, since both the selectivity and the activity were maintained after four cycles. Oxidative esterification products were achieved in the presence of other alcohols, leading to the formation of esters of up to C 5 (isopentyl furoate) with high selectivity (>99%). Linear and branched esters were formed, but the long-chain linear alcohols resulted in higher yields, such as n-butyl furoate in 94% yield.Catalysts 2020, 10, 430 2 of 14 considered to be one of the most important platform compounds in biorefineries of the future [8][9][10]. Several products that are obtained via HMF oxidation are of great interest for the polymer industry because their derivatives can be used as C6 monomers to replace petrochemical monomers [1-10]. For example, 2,5-furandicarboxylic acid (FDCA) is a monomer in the production of polyethylene 2,5-furandicarboxylate (PEF), being a green alternative to polyethylene terephthalate (PET) [11][12][13]. The direct use of FDCA in the industry is difficult due to its low solubility in most commonly used solvents. The furan-2,5-dimethylcarboxylate ester (FDMC) is actually more suitable for the subsequent polymerization reaction, thanks to its better solubility. For this reason, the development of catalytic systems that are capable of producing FDMC directly from the HMF has received much attention today. One alternative is the production of FDCA can be obtained from furoic acid (furfural oxidation product) via the Henkel reaction [13]. However, the use of Au catalysts can also be a viable alternative. FDCA can be obtained from furoic acid (furfural oxidation product) via the Henkel reaction [13]. Recently, Au catalysts have been successfully applied for the oxidative esterification of alcohols [14-17] and they have also been explored for the esterification of furfural and HMF [18][19][20][21][22][23][24][25][26][27][28]. The first example of HMF esterification to produce dimethyl-2-furoate (FDMC) required the use of a base (NaOMe) [29]. Although other catalytic systems have then been reported, there are few examples of successful Au catalytic systems for t...

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A novel ionic metathesis catalyst, [Ru(1‐CH3‐4‐CO2Py+)2(IMesH2)(CH‐2‐{2‐PrO}‐5‐NO2C6H3)][OTf−]2 (3 b; IMesH2=1,3‐dimesitylimidazolin‐2‐ylidene, Py=pyridine), has been prepared. 3 b and the Grubbs–Hoveyda catalysts [Ru(1‐CH3‐4‐CO2Py+)2(IMesH2)(CH‐2‐{2‐PrO}C6H4)][OTf−]2 (3 a) and [RuCl(1‐CH3‐4‐CO2Py+)(IMesH2)(CH‐2‐{2‐PrO}C6H4)][OTf−] (5 a) were used for ring‐opening metathesis polymerization (ROMP) reactions under both homogeneous and biphasic liquid–liquid conditions by using the ionic liquid 1‐butyl‐2,3‐dimethylimidazolium tetrafluoroborate ([BDMIM+][BF4−]) and toluene. All catalysts were active in the ROMP of norborn‐2‐ene‐based monomers, with cis‐cyclooctene and dicyclopentadiene providing good yields under homogeneous conditions and complex 5 a the most active catalyst. With all catalysts, the use of a chain transfer agent (CTA) allowed for the synthesis of polymers with low metal contents between 10 and 80 ppm, corresponding to a ruthenium removal of 98–99.8 % without any additional purification step. In addition, the use of a CTA allowed for recycling experiments under biphasic conditions, in which 3 a and 5 a were particularly active for several cycles.

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