Sara Iborra

Conversion of biomass platform molecules into fuel additives and liquid hydrocarbon fuels

In this work some relevant processes for the preparation of liquid hydrocarbon fuels and fuel additives from cellulose, hemicellulose and triglycerides derived platform molecules are discussed. Thus, it is shown that a series of platform molecules such as levulinic acid, furans, fatty acids and polyols can be converted into a variety of fuel additives through catalytic transformations that include reduction, esterification, etherification, and acetalization reactions. Moreover, we will show that liquid hydrocarbon fuels can be obtained by combining oxygen removal processes (e.g. dehydration, hydrogenolysis, hydrogenation, decarbonylation/descarboxylation etc.) with the adjustment of the molecular weight via C–C coupling reactions (e.g. aldol condensation, hydroxyalkylation, oligomerization, ketonization) of the reactive platform molecules.

Converting carbohydrates to bulk chemicals and fine chemicals over heterogeneous catalysts

In this Critical Review, we discuss how carbohydrates can be transformed into a variety of chemicals through heterogeneous catalysis. We focus particularly on oxidation, reduction and dehydration of hexoses, as well as one-pot reactions of di- and polysaccharides. Most of the reactions involve heterogeneous catalysts, although some interesting homogeneously catalyzed processes are also included.

Biomass into Chemicals: Aerobic Oxidation of 5‐Hydroxymethyl‐2‐furfural into 2,5‐Furandicarboxylic Acid with Gold Nanoparticle Catalysts

5-hydroxymethyl-2-furfural is selectively converted into 2,5-furandicarboxylic acid (99 mol % yield) in water, under mild conditions (65-130 degrees C, 10 bar air) with gold nanoparticles supported on nanoparticulate ceria (Au-CeO(2)) and on titania (Au-TiO(2)); the former being more active and selective. A reaction mechanism is established and the rate-limiting step of the reaction is the alcohol oxidation of 5-hydroxymethyl-2-furancarboxylic acid into 2,5-furandicarboxylic acid. The effects of pressure, temperature, substrate-to-catalyst ratio, and amount of base are studied to find the most suitable reaction conditions. The absence of metal leaching is verified by chemical analysis of the reaction solution and by confirming that the reaction does not occur after catalyst removal by filtration in hot water. Substrate degradation is strongly diminished and the catalyst life increased by performing the reaction in two temperature steps.

Homogeneous and heterogeneous catalysts for multicomponent reactions

Organic synthesis performed through multicomponent reactions is an attractive area of research in organic chemistry. Multicomponent reactions involve more than two starting reagents that couple in an exclusive ordered mode under the same reaction conditions to form a single product which contains the essential parts of the starting materials. Multicomponent reactions are powerful tools in modern drug discovery processes, because they are an important source of molecular diversity, allowing rapid, automated and high throughput generation of organic compounds. This review aims to illustrate progress in a large variety of catalyzed multicomponent reactions performed with acid, base and metal heterogeneous and homogeneous catalysts. Within each type of multicomponent approach, relevant products that can be obtained and their interest for industrial applications are presented.

Designing the adequate base solid catalyst with Lewis or Bronsted basic sites or with acid-base pairs

Abstract A series of heterogeneous catalysts with Lewis and Bronsted basic sites, and acid–base bifunctional pairs has been used in order to perform organic reactions. By changing the chemical composition and activation conditions it is possible to have predominantly Lewis or Bronsted base catalysts within a large range of well defined basicities. This allows to select the most appropriate catalyst for a specific reaction. Thus, MgO, calcined hydrotalcites, rehydrated hydrotalcites and grafted quaternary organic ammonium hydroxides on MCM-41, have been used as catalysts in Knoevenagel condensation, aldolization and Michael additions. Catalysts containing mild acid–base pairs such as those existing in amorphous aluminophosphates (ALPOs) allow to achieve high selectivities with still very reasonable activities.

Aldol Condensations on Solid Catalysts: A Cooperative Effect between Weak Acid and Base Sites

An amorphous aluminophosphate (ALPO) catalyst containing weak acid and base centers can carry out the aldol condensation of heptanal with benzaldehyde at much higher rates and selectivities than conventional solid acid (amorphous or crystalline aluminosilicates) or base catalysts (MgO, hydrotalcites, KF/Al2O3, nitrurated ALPO). With the weak acid-base catalyst, the reaction occurs through a bifunctional acid-base mechanism that involves the activation of benzaldehyde, by protonation-polarization of the carbonyl group on the acid sites, and the attack of the enolate heptanal intermediate generated on the basic sites. With this type of bifunctional acid-base catalyst, compounds with weak basicities are already able to undergo the reaction with a much higher selectivity than those obtained on stronger acid or base catalysts.

Heterogeneized Brönsted base catalysts for fine chemicals production: grafted quaternary organic ammonium hydroxides as catalyst for the production of chromenes and coumarins

Abstract A solid Bronsted base catalyst with a uniform distribution of basic sites with high base strength has been obtained by anchoring tetraalkylammonium hydroxide on the surface of MCM-41. It is highly active and selective for carrying out Knoevenagel and aldol condensation as well as Michael additions under mild reaction conditions. Salicyaldehyde and diethylglutaconate react on the catalyst to give the corresponding chromene and coumarine. While the phenoxide anion is the common intermediate, the cyclisation to chromene occurs very fast on the anchored tetraalkylammonium hydroxide. A high selectivity to chromenes is then obtained, being the catalyst of potential interest for the production of chromenes for thermochromic, photochromic and pharmaceutical materials.

One-step synthesis of citronitril on hydrotalcite derived base catalysts

Abstract Citronitril was obtained starting from ethyl cyanoacetate and benzylacetone using solid base catalysts. Hydrotalcites were found to carry out this reaction efficiently. The hydrotalcite catalyst was optimized from the point of view of its textural properties, composition ,i.e. Ca, Mg, and Zn as divalent cations, and Al/(Al+Mg) in the case of the magnesium-aluminium hydrotalcites. The influence of water, reaction temperature and polarity of the solvent on conversion and selectivity was studied.

One-pot synthesis of phenols from aromatic aldehydes by Baeyer–Villiger oxidation with H2O2 using water-tolerant Lewis acids in molecular sieves

Abstract Sn-Beta zeolite is employed as catalyst in the Baeyer–Villiger oxidation of aromatic aldehydes. Aldehydes involving alkoxy substituents are oxidized to the corresponding formate ester which are hydrolyzed to the corresponding phenols. Choosing the adequate reaction conditions the main product can be preselected. Dioxane solvent and a hydrogen peroxide deficit give predominantly the ester whereas ethanol solvent or aqueous acetonitrile favor the phenol product. Double bonds in alkyl side chains do not react and the corresponding unsaturated phenols are obtained with very high chemoselectivity. Bronsted sites are active for the Baeyer–Villiger oxidation of aromatic aldehydes with H2O2 provided that the molecule does not contain olefinic groups. In any case, the selectivity of Bronsted acid zeolites is lower than that of Sn-Beta.

Photobiocatalytic chemistry of oxidoreductases using water as the electron donor

To date, water has been poorly studied as the sacrificial electron donor for biocatalytic redox reactions using isolated enzymes. Here we demonstrate that water can also be turned into a sacrificial electron donor to promote biocatalytic redox reactions. The thermodynamic driving force required for water oxidation is obtained from UV and visible light by means of simple titanium dioxide-based photocatalysts. The electrons liberated in this process are delivered to an oxidoreductase by simple flavin redox mediators. Overall, the feasibility of photobiocatalytic, water-driven bioredox reactions is demonstrated.