D.S. van Es

Reaction Pathways for the Deoxygenation of Vegetable Oils and Related Model Compounds

Vegetable oil-based feeds are regarded as an alternative source for the production of fuels and chemicals. Paraffins and olefins can be produced from these feeds through catalytic deoxygenation. The fundamentals of this process are mostly studied by using model compounds such as fatty acids, fatty acid esters, and specific triglycerides because of their structural similarity to vegetable oils. In this Review we discuss the impact of feedstock, reaction conditions, and nature of the catalyst on the reaction pathways of the deoxygenation of vegetable oils and its derivatives. As such, we conclude on the suitability of model compounds for this reaction. It is shown that the type of catalyst has a significant effect on the deoxygenation pathway, that is, group 10 metal catalysts are active in decarbonylation/decarboxylation whereas metal sulfide catalysts are more selective to hydrodeoxygenation. Deoxygenation studies performed under H2 showed similar pathways for fatty acids, fatty acid esters, triglycerides, and vegetable oils, as mostly deoxygenation occurs indirectly via the formation of fatty acids. Deoxygenation in the absence of H2 results in significant differences in reaction pathways and selectivities depending on the feedstock. Additionally, using unsaturated feedstocks under inert gas results in a high selectivity to undesired reactions such as cracking and the formation of heavies. Therefore, addition of H2 is proposed to be essential for the catalytic deoxygenation of vegetable oil feeds.

Isohexide Derivatives from Renewable Resources as Chiral Building Blocks

The combination of rapidly depleting fossil resources and growing concerns about greenhouse gas emissions and global warming have stimulated extensive research on the use of biomass for energy, fuels, and chemicals.[1] Although biobased chemicals have the potential to reduce the amount of fossil feedstock consumed in the chemical industry today, the most abundant type of biobased feedstock, that is, carbohydrates, is often unsuitable for current high-temperature industrial chemical processes. Compared to hydrophobic aliphatic or aromatic feedstocks with a low degree of functionalization, carbohydrates such as polysaccharides are complex, overfunctionalized hydrophilic materials. One approach to overcome these drawbacks is to reduce the number of functional groups, resulting in more stable, industrially applicable bifunctional biobased building blocks,[2] such as furan-2,5-dicarboxylic acid,[3] levulinic acid,[4] and isosorbide.[5] Isosorbide (1,4:3,6-dianhydrosorbitol) is a rigid bicyclic diol that is derived from sorbitol and can ultimately be obtained from glucose-based polysaccharides such as starch and cellulose.[6] Apart from isosorbide, in which the hydroxyl groups on C2 and C5 are in the exo and endo positions, respectively, two other isohexides are known (Figure 1): the symmetrical endo-endo isomer isomannide (1,4:3,6-dianhydromannitol) and the exo-exo isomer isoidide (1,4:3,6-dianhydroiditol), derived from d-mannitol and l-iditol, respectively.

Renewable Rigid Diamines: Efficient, Stereospecific Synthesis of High Purity Isohexide Diamines

We report an efficient three-step strategy for synthesizing rigid, chiral isohexide diamines derived from 1,4:3,6-dianhydrohexitols. These biobased chiral building blocks are presently the subject of several investigations (in our and several other groups) because of their application in high-performance biobased polymers, such as polyamides and polyurethanes. Among the three possible stereo-isomers, dideoxy-diamino isoidide and dideoxy-diamino isosorbide can be synthesized from isomannide and isosorbide respectively in high yield with absolute stereo control. Furthermore, by using this methodology dideoxy-amino isomannide-a tricyclic adduct-was obtained starting from isoidide in high yield. Our improved synthetic route is a valuable advance towards meeting scale and purity demands for evaluating the properties of new biobased performance materials, which will benefit the development of these plastics.

Concurrent formation of furan-2,5- and furan-2,4-dicarboxylic acid: unexpected aspects of the Henkel reaction

The concurrent formation of furan-2,5- and furan-2,4-dicarboxylic acid under solvent free conditions via a disproportionation reaction is described. By reacting potassium-2-furoate at 260 °C in the presence of 22 mol% of (Lewis acidic) catalysts like CdI2 or ZnCl2, potassium-2-furoate is disproportionated to furan and furandicarboxylic acids. Besides furan and furan-2,5-dicarboxylic acid (2,5-FDCA) as the main products, furan-2,4-dicarboxylic acid (2,4-FDCA) is also formed as a by-product. Experimental evidence has been obtained that, under the reaction conditions applied, 2,5-FDCA and 2,4-FDCA are formed by separate reaction pathways. Selectivity towards the different FDCA isomers is affected by the type of catalyst used. Single-crystal X-ray analysis shows that 2,4-FDCA has a more ‘linear’ character compared to 2,5-FDCA and hence is structurally more comparable to terephthalic acid (TA), making it an interesting monomer for synthetic polyesters.

The oxidative esterification of glycerol to methyl glycerate in methanol using gold on oxidic supports: an insight in product selectivity

Gold nanoparticles on different oxidic supports (TiO2, Al2O3 and ZnO) have been studied for the oxidation of glycerol in methanol, using molecular oxygen as the oxidizing agent in a batch set-up. The main oxidation products are methyl glycerate and dimethyl mesoxalate in over 95% selectivity at high glycerol conversion, indicating that C–C bond scission occurs at a significantly lower extent compared to glycerol oxidations in water. The product selectivity is a function of the support. Highest selectivity (82% at 72% conversion) to methyl glycerate is observed in the case of Au/TiO2 as the catalyst. The use of a base is not essential for the glycerol oxidation reaction to occur, although for TiO2 and Al2O3 higher initial activities are found in the presence of sodium methoxide. Au/ZnO gives comparable activity and selectivity both in the presence and absence of a base. Oxidation experiments with reaction intermediates indicate that oxidation of methyl glycerate to higher oxygenates does not occur to a significant extent in methanol. An alternative pathway for the formation of dimethyl mesoxalate involving dihydroxyacetone is proposed.

Towards sugar-derived polyamides as environmentally friendly materials

As part of our ongoing study investigating isohexide-based polyamides, we have synthesized isosorbide(bis(propan-1-amine)) (DAPIS) and studied its reactivity in the polymerization towards fully biobased polyamides. Polycondensation of nylon salts with various contributions of DAPIS afforded a family of homo- and copolyamides, which were characterized using complementary spectroscopic techniques. The chemical structure of the materials was determined by FT-IR, 1D and 2D liquid-state NMR spectroscopy, whilst the supramolecular arrangement, conformational changes upon heating, and molecular mobility of the polymers were investigated by solid-state 13C{1H} Cross-Polarization/Magic-Angle Spinning (CP/MAS) NMR and 13C{1H} Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) experiments. The abundance of the different DAPIS conformers was determined by DFT-D computational methods. The thermal properties of the polyamides were tested for polymers with different amounts of isohexide units in the backbone by DSC and TGA, demonstrating that the increasing amounts of isohexide diamines efficiently decrease their melting points and slightly decrease their thermal stability. The relaxation processes of the isohexide-derived polyamides were studied by DMTA.

Base-free selective oxidation of pectin derived galacturonic acid to galactaric acid using supported gold catalysts

Agricultural residues like sugar beet pulp (SBP) are an interesting feedstock for the production of 2nd generation bio-based chemicals and materials. The pectin fraction of SBP is rich in galacturonic acid (GalA), a C6 sugar acid. The oxidation of this uronic acid at C1 yields galactaric acid (GA), which has several industrially interesting properties. It was previously shown that the Au catalysed oxidation of uronic acids under basic conditions is highly effective, yet leads to the co-production of salts. Hence, here we report for the first time on the oxidation of an acidic carbohydrate substrate, GalA, at its autogenic pH (2.2) in water, using carbon supported gold nanoparticles, under mild conditions in the presence of molecular oxygen. The comparison of the Au/C catalyst prepared by a colloidal deposition method with benchmark commercially available metal oxide supported gold catalysts shows that under acidic conditions, the Au/C catalyst is more active and more selective than Au/TiO2, and more stable than Au/Al2O3. The difference in selectivity is attributed to the H2O2 mediated chain scission reaction of the substrate (GalA) which is observed only in the case of metal oxide supported Au catalysts. The Au/C catalyst shows 100% GA selectivity at 76% GalA conversion (333 K, 21 h batch time) and a GA yield of up to 95% was obtained at 353 K. Detailed characterization of the fresh and spent Au/C catalysts by ICP-OES, TEM and XPS analyses showed no gold leaching, particle sintering or change in metal composition. The Au/C catalyst was fully regenerated by a mild alkaline wash, and used in five consecutive runs without any significant decrease in activity or selectivity. Labelling experiments with 18O2 and H218O2 revealed that under base-free conditions, the oxygen incorporated in the aldaric acid originates from the solvent water.

From batch to continuous: Au-catalysed oxidation of D-galacturonic acid in a packed bed plug flow reactor under alkaline conditions

Currently biomass based conversions are often performed in batch reactors. From an operational and economic point of view the use of a continuous plug flow reactor is preferred. Here we make a back to back comparison of the use of a batch and plug flow reactor for the oxidation of (sodium)-galacturonate to (disodium)-galactarate using a heterogeneous Au-catalyst. We will show that the use of a three phase plug flow reactor results in enhanced O2 mass transfer which resulted in a 10–40 fold increase in productivity (up to 2.2 ton m−3 h−1). However, the product selectivity slightly dropped from >99 mol% in batch (controlled pH) to 94 mol% in packed bed (uncontrolled pH). Both reactors suffer from the low solubility of the reaction product. We will show that this solubility is the most significant challenge for performing this oxidation on industrial scale.