Esben Taarning

Conversion of Sugars to Lactic Acid Derivatives Using Heterogeneous Zeotype Catalysts

Approaching Lactate Inorganically Conversion of biomass to value-added chemical compounds currently relies in large part on fermentation. For full-scale displacement of petroleum as the chemical industrys primary feedstock, alternative conversion technologies will be necessary. Holm et al. (p. 602) have found that Lewis acidic zeolite derivatives suspended in methanol can catalyze the selective conversion of glucose, fructose, and sucrose sugars to methyl lactate, a versatile synthetic intermediate for commercial products. The catalysts were easily separated from product mixtures and proved robust over six reaction and regeneration cycles. Lewis acid catalysis offers an alternative to fermentation in converting sugars to a commercial chemical feedstock. Presently, very few compounds of commercial interest are directly accessible from carbohydrates by using nonfermentive approaches. We describe here a catalytic process for the direct formation of methyl lactate from common sugars. Lewis acidic zeotypes, such as Sn-Beta, catalyze the conversion of mono- and disaccharides that are dissolved in methanol to methyl lactate at 160°C. With sucrose as the substrate, methyl lactate yield reaches 68%, and the heterogeneous catalyst can be easily recovered by filtration and reused multiple times after calcination without any substantial change in the product selectivity.

Beyond Petrochemicals: The Renewable Chemicals Industry

The use of renewable resources has attracted significant attention in recent years for many different reasons. 2] Renewable resources include electricity made from kinetic energy stored in wind, potential energy stored in water, thermal energy stored as heat underground and as solar influx in the form of electromagnetic radiation, and energy stored in chemical bonds in the case of biomass. Although renewable resources have been used for various purposes for centuries, there is currently a significant focus on expanding and optimizing this use in the form of new technologies fit for the 21st century. The use of biomass as a resource has developed rapidly in recent years, and it will become an important contributor to our available resources in the future. Biomass sets itself aside from the other renewable resources, since the energy it contains is stored as chemical bonds. This characteristic allows biomass to be used for several purposes apart from electricity and heat generation, such as the production of liquid fuels and chemicals. Indeed, biomass is the only renewable source of useful carbon atoms. Although biomass is annually renewable, it is still a scarce and limited resource, especially when produced in a sustainable manner, and it is important to use it in the most efficient way. This Essay argues for the production of select chemicals, thereby effectively replacing petroleum, as an efficient use and illustrates some of the current efforts that are made in the chemical industry towards adoption of biomass as a feedstock. Availability of Biomass Resources

The Renewable Chemicals Industry

The possibilities for establishing a renewable chemicals industry featuring renewable resources as the dominant feedstock rather than fossil resources are discussed in this Concept. Such use of biomass can potentially be interesting from both an economical and ecological perspective. Simple and educational tools are introduced to allow initial estimates of which chemical processes could be viable. Specifically, fossil and renewables value chains are used to indicate where renewable feedstocks can be optimally valorized. Additionally, C factors are introduced that specify the amount of CO2 produced per kilogram of desired product to illustrate in which processes the use of renewable resources lead to the most substantial reduction of CO2 emissions. The steps towards a renewable chemicals industry will most likely involve intimate integration of biocatalytic and conventional catalytic processes to arrive at cost-competitive and environmentally friendly processes.

Zeolite-catalyzed biomass conversion to fuels and chemicals

Heterogeneous catalysts have been a central element in the efficient conversion of fossil resources to fuels and chemicals, but their role in biomass utilization is more ambiguous. Zeolites constitute a promising class of heterogeneous catalysts and developments in recent years have demonstrated their potential to find broad use in the conversion of biomass. In this perspective we review and discuss the developments that have taken place in the field of biomass conversion using zeolites. Emphasis is put on the conversion of lignocellulosic material to fuels using conventional zeolites as well as conversion of sugars using Lewis acidic zeolites to produce useful chemicals.

Sn-Beta catalysed conversion of hemicellulosic sugars

Conversions of various pentoses and hexoses into methyl lactate has been demonstrated for the Sn-Beta catalyst. It is found that pentoses are converted to methyl lactate in slightly lower yields (∼40%) than what is obtained for hexoses (∼50%), but higher yields of glycolaldehyde dimethyl acetal are observed for the pentoses. This finding is in accordance to a reaction pathway that involves the retro aldol condensation of the sugars to form a triose and glycolaldehyde for the pentoses, and two trioses for hexoses. When reacting glycolaldehyde (formally a C2-sugar) in the presence of Sn-Beta, aldol condensation occurs, leading to the formation of methyl lactate, methyl vinylglycolate and methyl 2-hydroxy-4-methoxybutanoate. In contrast, when converting the sugars in water at low temperatures (100 °C), Sn-Beta catalyses the isomerisation of sugars (ketose–aldose epimers), rather than the formation of lactates.

Aerobic oxidation of aldehydes under ambient conditions using supported gold nanoparticle catalysts

A new, green protocol for producing simple esters by selectively oxidizing an aldehyde dissolved in a primary alcohol has been established, utilising air as the oxidant and supported gold nanoparticles as catalyst. The oxidative esterifications proceed with excellent selectivities at ambient conditions; the reactions can be performed in an open flask and at room temperature. Benzaldehyde is even oxidised at a reasonable rate below −70 °C. Acrolein is oxidised to methyl acrylate in high yield using the same protocol.

Oxidations of amines with molecular oxygen using bifunctional gold–titania catalysts

Over the past decades it has become clear that supported gold nanoparticles are surprisingly active and selective catalysts for several green oxidation reactions of oxygen-containing hydrocarbons using molecular oxygen as the stoichiometric oxidant. We here report that bifunctional gold–titania catalysts can be employed to facilitate the oxidation of amines into amides with high selectivity. Furthermore, we report that pure titania is in fact itself a catalyst for the oxidation of amines with molecular oxygen under very mild conditions. We demonstrate that these new methodologies open up for two new and environmentally benign routes to caprolactam and cyclohexanone oxime, both of which are precursors for nylon-6.

Oxidation of glycerol and propanediols in methanol over heterogeneous gold catalysts

Aerobic oxidation of glycerol over metal oxide supported gold nanoparticles in methanol results in the formation of dimethyl mesoxalate in selectivities up to 89% at full conversion. The oxidative esterification takes place in methanol, acting both as solvent and reactant, and in the presence of base. Thus, it constitutes a direct transformation of the glycerol by-product phase from biodiesel production or from glycerol obtained e.g. by fermentation. Au/TiO2 and Au/Fe2O3 was found to have similar catalytic activity, whereas Au/C was inactive. 1,2-Propanediol was oxidized to methyl lactate with a selectivity of 72% at full conversion, while 1,3-propanediol yielded methyl 3-hydroxypropionate with 90% selectivity at 94% conversion. Methyl 3-hydroxy propionate can be easily converted into methyl acrylate, which is then a green polymer building block.

Design of a Metal‐Promoted Oxide Catalyst for the Selective Synthesis of Butadiene from Ethanol

The synthesis of buta-1,3-diene from ethanol has been studied over metal-containing (M=Ag, Cu, Ni) oxide catalysts (MO(x)=MgO, ZrO2, Nb2O5, TiO2, Al2O3) supported on silica. Kinetic study of a wide range of ethanol conversions (2-90%) allowed the main reaction pathways leading to butadiene and byproducts to be determined. The key reaction steps of butadiene synthesis were found to involve ethanol dehydrogenation, acetaldehyde condensation, and the reduction of crotonaldehyde with ethanol into crotyl alcohol. Catalyst design included the selection of active components for each key reaction step and merging of these components into multifunctional catalysts and adjusting the catalyst functions to achieve the highest selectivity. The best catalytic performance was achieved over the Ag/ZrO2/SiO2 catalyst, which showed the highest selectivity towards butadiene (74 mol%).

Selective Production of Aromatics from Alkylfurans over Solid Acid Catalysts

Solid acid catalysts were studied at temperatures near 523 K for the production of benzene, toluene, and p‐xylene by the reaction of ethylene with furan, 2‐methylfuran, and 2,5‐dimethylfuran, respectively, through the combination of cycloaddition and dehydrative aromatization reactions. Catalysts containing Brønsted acid and Lewis acid sites (i.e., WOx–ZrO2, niobic acid, zeolite Y, silica–alumina) were more active than catalysts containing predominantly Lewis acid sites (γ‐Al2O3, TiO2), which indicates the importance of Brønsted acidity in the production of aromatics. Microporosity is not required for this reaction, because amorphous solid acids and homogeneous Brønsted acids demonstrate significant activity for p‐xylene production. The production of p‐xylene from 2,5‐dimethylfuran proceeded at higher rates compared with the production of toluene and benzene from 2‐methylfuran and furan, respectively. Both WOx–ZrO2 and niobic acid demonstrate superior activity for aromatics production than does zeolite Y. WOx–ZrO2 demonstrates a turnover frequency for p‐xylene production that is 35 times higher than that demonstrated by zeolite Y. In addition, mesoporous materials such as WOx–ZrO2 offer higher resistance to deactivation by carbon deposition than do microporous materials. Results from Raman spectroscopy and the trend of turnover frequency with varying tungsten surface densities for a series of WOx–ZrO2 catalysts are consistent with previous investigations of other acid‐catalyzed reactions; this suggests that the high reactivity of WOx–ZrO2 is mainly associated with the presence of subnanometer WOx clusters mixed with zirconium, which reach a maximum surface concentration at intermediate tungsten coverage.