Michiel Dusselier

Lactic acid as a platform chemical in the biobased economy: the role of chemocatalysis

Upcoming bio-refineries will be at the heart of the manufacture of future transportation fuels, chemicals and materials. A narrow number of platform molecules are envisioned to bridge natures abundant polysaccharide feedstock to the production of added-value chemicals and intermediate building blocks. Such platform molecules are well-chosen to lie at the base of a large product assortment, while their formation should be straightforward from the refined biomass, practical and energy efficient, without unnecessary loss of carbon atoms. Lactic acid has been identified as one such high potential platform. Despite its established fermentation route, sustainability issues – like gypsum waste and cost factors due to multi-step purification and separation requirements – will arise as soon as the necessary orders of magnitude larger volumes are needed. Innovative production routes to lactic acid and its esters are therefore under development, converting sugars and glycerol in the presence of chemocatalysts. Moreover, catalysis is one of the fundamental routes to convert lactic acid into a range of useful chemicals in a platform approach. This contribution attempts a critical overview of all advances in the field of homogeneous and heterogeneous catalysis and recognises a great potential of some of these chemocatalytic approaches to produce and transform lactic acid as well as some other promising α-hydroxy acids.

Fast and Selective Sugar Conversion to Alkyl Lactate and Lactic Acid with Bifunctional Carbon–Silica Catalysts

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Brønsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Brønsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Brønsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Brønsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Brønsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.

Shape-selective zeolite catalysis for bioplastics production

Synthesizing more sustainable plastics Zeolites can help synthesize cheaper plastic precursors from biologically sourced feedstocks. Producing sustainable plastics must compete with more cost-effective petrochemical-based synthesis routes. Dusselier et al. developed a zeolite-based strategy to catalyze the transformation of microbially produced lactic acid into lactide, a difficult-to-synthesize precursor of biodegradable polylactic acid plastics. The selectivity of nearly 80% is based on active site spatial confinement in the zeolite micropores. This step substantially simplifies current high-cost synthesis routes and generates nearly zero waste using current reactor technologies. Science, this issue p. 78 Zeolite catalysts improve the synthesis of the precursors to biodegradable plastics. Biodegradable and renewable polymers, such as polylactic acid, are benign alternatives for petrochemical-based plastics. Current production of polylactic acid via its key building block lactide, the cyclic dimer of lactic acid, is inefficient in terms of energy, time, and feedstock use. We present a direct zeolite-based catalytic process, which converts lactic acid into lactide. The shape-selective properties of zeolites are essential to attain record lactide yields, outperforming those of the current multistep process by avoiding both racemization and side-product formation. The highly productive process is strengthened by facile recovery and practical reactivation of the catalyst, which remains structurally fit during at least six consecutive reactions, and by the ease of solvent and side-product recycling.

Tuning the acid/metal balance of carbon nanofiber-supported nickel catalysts for hydrolytic hydrogenation of cellulose.

Carbon nanofibers (CNFs) are a class of graphitic support materials with considerable potential for catalytic conversion of biomass. Earlier, we demonstrated the hydrolytic hydrogenation of cellulose over reshaped nickel particles attached at the tip of CNFs. The aim of this follow-up study was to find a relationship between the acid/metal balance of the Ni/CNFs and their performance in the catalytic conversion of cellulose. After oxidation and incipient wetness impregnation with Ni, the Ni/CNFs were characterized by various analytical methods. To prepare a selective Ni/CNF catalyst, the influences of the nature of oxidation agent, Ni activation, and Ni loading were investigated. Under the applied reaction conditions, the best result, that is, 76 % yield in hexitols with 69 % sorbitol selectivity at 93 % conversion of cellulose, was obtained on a 7.5 wt % Ni/CNF catalyst prepared by chemical vapor deposition of CH(4) on a Ni/γ-Al(2)O(3) catalyst, followed by oxidation in HNO(3) (twice for 1 h at 383 K), incipient wetness impregnation, and reduction at 773 K under H(2). This preparation method leads to a properly balanced Ni/CNF catalyst in terms of Ni dispersion and hydrogenation capacity on the one hand, and the number of acidic surface-oxygen groups responsible for the acid-catalyzed hydrolysis on the other.

Will Zeolite‐Based Catalysis be as Relevant in Future Biorefineries as in Crude Oil Refineries?

Transition from petroleum- to biomass-based fuel economy will require new conversion strategies. In a petroleum refinery, particular hydrocarbon fractions from crude oil are catalytically converted into high-grade fuels. Certain zeolite catalysts are performing exceptionally well. Unlike petroleum fractions, biomass-derived compounds have a high oxygen content requiring low-temperature catalytic aqueous phase processes for selective conversion and stability of zeolite catalysts in hot liquid water. It will be shown that recent developments in zeolite synthesis and modification allow adapting zeolite properties to achieve selective conversion of biomass compounds/fractions as well.

Mechanistic Insight into the Conversion of Tetrose Sugars to Novel α‐Hydroxy Acid Platform Molecules

α‐Hydroxy acids (AHAs) such as lactic acid are considered platform molecules in the biorefinery concept and have high‐end applications in solvents and biodegradable polyester plastics. The synthesis of AHAs with a four‐carbon backbone structure is a recently emerging field. New biomass‐related routes towards their production could stimulate their practical use in new polyester plastics. Herein, we report the unique catalytic activity of soluble tin metal salts for converting tetroses, namely erythrulose and erythrose, into new four‐carbon‐backbone AHAs such as methyl vinylglycolate and methyl‐4‐methoxy‐2‐hydroxybutanoate. An in situ NMR study together with deuterium labeling experiments and control experiments with intermediates allowed us to propose a detailed reaction pathway.

Tailoring nanohybrids and nanocomposites for catalytic applications

Research on and development of inorganic–organic nanohybrids and nanocomposite materials has attracted increasing attention in recent years. Synthetic strategies for such materials vary from grafting or co-condensation of Si and C sources to the impregnation of silica with polymers. Nanohybrids, prepared using organosilanes, and nanocomposites, obtained by hard or soft templated synthesis, are discussed. Various strategies will be presented that demonstrate how additional carbon properties can be exploited maximising the activity, selectivity and stability of composite materials as solid catalysts. Composite materials allow for the extensive engineering of a catalyst enabling careful tuning of the type, amount and position of active sites, as well as the porosity and hydrophilic nature of the final catalyst. These materials not only combine the advantages of silica (e.g. thermal stability, rigidity, ordering) and carbon (e.g. flexibility, ductility) but also allow their synergetic action in various catalytic applications.

Lipoteichoic acid is an important microbe-associated molecular pattern of Lactobacillus rhamnosus GG

BackgroundProbiotic bacteria are increasingly used as immunomodulatory agents. Yet detailed molecular knowledge on the immunomodulatory molecules of these bacteria is lagging behind. Lipoteichoic acid (LTA) is considered a major microbe-associated molecular pattern (MAMP) of Gram-positive bacteria. However, many details and quantitative data on its immune signalling capacity are still unknown, especially in beneficial bacteria. Recently, we have demonstrated that a dltD mutant of the model probiotic Lactobacillus rhamnosus GG (LGG), having modified LTA molecules, has an enhanced probiotic efficacy in a DSS-induced colitis model as compared to wild-type.ResultsIn this study, the importance of D-alanylated and acylated LTA for the pro-inflammatory activity of LGG was studied in vitro. Purified native LTA of LGG wild-type exhibited a concentration-dependent activation of NF-κB signalling in HEK293T cells after interaction with TLR2/6, but not with TLR2 alone. Chemical deacylation of LTA interfered with the TLR2/6 interaction, while a moderate effect was observed with chemical dealanylation. Similarly, the dltD mutant of LGG exhibited a significantly reduced capacity to activate TLR2/6-dependent NF-κB signalling in a HEK293T reporter cell line compared to wild-type. In addition, the dltD mutant of LGG showed a reduced induction of mRNA of the chemokine IL-8 in the Caco-2 epithelial cell line compared to wild-type. Experiments with highly purified LTA of LGG confirmed that LTA is a crucial factor for IL-8 mRNA induction in Caco-2 epithelial cells. Chemical dealanylation and deacylation reduced IL-8 mRNA expression.ConclusionsTaken together, our results indicate that LTA of LGG is a crucial MAMP with pro-inflammatory activities such as IL-8 induction in intestinal epithelial cells and NF-κB induction in HEK293T cells via TLR2/6 interaction. The lipid chains of LGG LTA are needed for these activities, while also the D-alanine substituents are important, especially for IL-8 induction in Caco-2 cells.

Selective Catalysis for Cellulose Conversion to Lactic Acid and Other α-Hydroxy Acids

This review discusses topical chemical routes and their catalysis for the conversion of cellulose, hexoses, and smaller carbohydrates to lactic acid and other useful α-hydroxy acids. Lactic acid is a top chemical opportunity from carbohydrate biomass as it not only features tremendous potential as a chemical platform molecule; it is also a common building block for commercially employed green solvents and near-commodity bio-plastics. Its current scale fermentative synthesis is sufficient, but it could be considered a bottleneck for a million ton scale breakthrough. Alternative chemical routes are therefore investigated using multifunctional, often heterogeneous, catalysis. Rather than summarizing yields and conditions, this review attempts to guide the reader through the complex reaction networks encountered when synthetic lactates from carbohydrate biomass are targeted. Detailed inspection of the cascade of reactions emphasizes the need for a selective retro-aldol activity in the catalyst. Recently unveiled catalytic routes towards other promising α-hydroxy acids such as glycolic acid, and vinyl and furyl glycolic acids are highlighted as well.

CO2 reverse selective mixed matrix membranes for H2 purification by incorporation of carbon–silica fillers

By filling a PDMS top layer with porous carbon–silica microspheres, a defect-free mixed matrix membrane was created with notable CO2 reverse selective separation properties. For the separation of CO2 over H2 at room temperature and 10 bar inlet pressure, these membranes demonstrate high CO2 gas fluxes up to 3 × 10−7 mol cm−2 s−1, in combination with ideal separation factors in the range of 6 to 9. The present separation data signify an important step forward in the removal of CO2 from H2 using a reverse selective separation strategy. Moreover, they elucidate the potential of such mixed matrix membranes in the emerging field of CO2 separation.