Mechanistic insights into the production of methyl lactate by catalytic conversion of carbohydrates on mesoporous Zr-SBA-15

Homogeneous and Heterogeneous Catalysis of Glucose to Lactic Acid and Lactates: A Review

The current societal demand to replace polymers derived from petroleum with sustainable bioplastics such as polylactic acid (PLA) has motivated industry to commercialize ever-larger facilities for biobased monomers like lactic acid. Even though most of the lactic acid is produced by fermentation, long reaction times and high capital costs compromise the economics and thus limit the appeal of biotechnological processes. Catalytic conversion of hexose from biomass is a burgeoning alternative to fermentation. Here we identify catalysts to convert glucose to lactic acid, along with their proposed mechanisms. High Lewis acidity makes erbium salts among the most active homogeneous catalysts, while solvent coordination with the metal species polarize the substrate, increasing the catalytic activity. For heterogeneous catalysts, Sn-containing bimetallic systems combine the high Lewis acidity of Sn while moderating it with another metal, thus decreasing byproducts. Hierarchical bimetallic Sn-Beta zeolites combine a high number of open sites catalyzing glucose isomerization in the mesoporous regions and the confinement effect assisting fructose retro-aldol in microporous regions, yielding up to 67% lactic acid from glucose. Loss of activity is still an issue for heterogeneous catalysts, mostly due to solvent adsorption on the active sites, coke formation, and metal leaching, which impedes its large scale adoption.

Bifunctional Au-Sn-SiO2 catalysts promote the direct upgrading of glycerol to methyl lactate

Valuable alkyl lactates can be obtained from (waste) glycerol, through a two-step process that entails (i) the oxidation of glycerol to dihydroxyacetone (DHA) catalyzed by support Au nanoparticles and (ii) a rearrangement of DHA with an alcohol effectively catalyzed by Sn-based heterogeneous catalysts. To solve selectivity and processing issues we propose to run the process as a cascade reaction, in one step, and with a single bifunctional catalyst. Tackling the challenge associated with the preparation of such bifunctional catalysts, here, an aerosol-assisted sol-gel route is exploited. The catalysts feature small Au nanoparticles (3-4 nm) embedded at the surface of mesoporous Sn-doped silica microspheres. The preparation successfully leads to insert both active sites in their most active forms, and in close proximity. With the bifunctional catalysts, the yield for the final product of the cascade reaction (methyl lactate) is higher than the DHA yield when only the first reaction is carried out. This highlights a beneficial substrate channeling effect which alleviates side reactions. Interestingly, the bifunctional catalysts also markedly outcompeted mechanical mixtures of the corresponding monofunctional Au- and Sn-based catalysts. Thus, the spatial proximity between the two active sites in bifunctional catalysts is identified as a key to stir the cascade reaction towards high lactate yield.

Production of Methyl Lactate with Sn-USY and Sn-β: Insights into Real Hemicellulose Valorization

Potassium exchanged Sn-β and Sn-USY zeolites have been tested for the transformation of various aldoses (hexoses and pentoses), exhibiting outstanding catalytic activity and selectivity toward methyl lactate. Insights into the transformation pathways using reaction intermediates-dihydroxyacetone and glycolaldehyde-as substrates revealed a very high catalytic proficiency of both zeolites in aldol and retro-aldol reactions, showcasing their ability to convert small sugars into large sugars, and vice versa. This feature makes the studied Sn-zeolites outstanding catalysts for the transformation of a wide variety of sugars into a limited range of commercially valuable alkyl lactates and derivatives. [K]Sn-β proved to be superior to [K]Sn-USY in terms of shape selectivity, exerting tight control on the distribution of produced α-hydroxy methyl esters. This shape selectivity was evident in the transformation of several complex sugar mixtures emulating different hemicelluloses-sugar cane bagasse, Scots pine, and white birch-that, despite showing very different sugar compositions, were almost exclusively converted into methyl lactate and methyl vinyl glycolate in very similar proportions. Moreover, the conversion of a real hemicellulose hydrolysate obtained from Scots pine through a simple GVL-based organosolv process confirmed the high activity and selectivity of [K]Sn-β in the studied transformation, opening new pathways for the chemical valorization of this plentiful, but underutilized, sugar feedstock.

Algae from Cr-containing infiltrate bioremediation for valorised chemical production - Seasonal availability, composition, and screening studies on hydrothermal conversion

Integrating bioremediation of toxic wastewater with value-added production is increasing interest, but - due to some essential problems - it is hardly applied in industrial practice. The aim of the study was an annual observation of the taxonomic and biochemical composition of various Cr-resistant algal communities grown in the existing Cr-containing infiltrate treatment system, selection of the most suitable algal biomass for infiltrates bioremediation and chromium-loaded algae conversion under mild subcritical conditions. Considering continuous availability and relatively constant chemical composition, Cladophora sp. was selected for utilisation in the chromium bioremediation system, simultaneously as a waste biomass source suitable for hydrothermal conversion. Screening studies conducted in a continuous pilot plant confirmed the possibility of selective extraction of saccharides and their separation from the metals remaining in the solid residual. The negligible concentration of metals in the obtained sugar-rich aqueous phase is essential for its further use in biotechnological processes.

Enhanced catalytic activity of layered double hydroxides via in-situ reconstruction for conversion of glucose/food waste to methyl lactate in biorefinery

Conversion of food waste into valuable chemicals under mild conditions has attracted increasing attention. Herein, a series of nano-sized MgAl layered double hydroxides (LDHs) were firstly developed as solid base catalyst for the methyl lactate (MLA) production directly from glucose/food waste. Glucose, which could be easily obtained from cellulose or starch-rich food waste via hydrolysis, was thus selected as the model compound. It is inspiring to find that the metal hydroxide layer in prepared LDHs was highly stable and suitable enlarged interlayer distance was reconstructed owing to in-situ intercalation of formed aromatics during the reaction, which was demonstrated by ²⁷Al magic angle spinning nuclear magnetic resonance and time-of-flight secondary ion mass spectrometry analysis. As a result, in-situ activation of the catalysts along with gradually enhanced catalytic activity was obtained in the recycling runs and the highest MLA yield of 47.6% from glucose was achieved over LDHs (5:1) after 5 runs at 150 °C. Most importantly, the scope was further extended to other typical substrates (e.g. Chinese cabbage and rice) and the results demonstrated the effectiveness of present conversion system for real food waste.

Catalytic Conversion of High Fructose Corn Syrup to Methyl Lactate with CoO@silicalite-1

Methyl lactate (MLA), a versatile biomass platform, was typically produced from the catalytic conversion of high-priced fructose. High fructose corn syrup (HFCS) is a mixture of glucose, fructose, water, etc., which is viewed as an economical substitute for fructose to produce MLA due to the much lower cost of separation and drying processes. However, the transformation of HFCS to MLA is still a challenge due to its complex components and the presence of water. In this work, the catalytic conversion of HFCS to MLA over CoO@silicalite-1 catalyst synthesized via a straightforward post citric acid treatment approach was reported. The maximum MLA yield reached 43.8% at 180 °C for 18 h after optimizing the reaction conditions and Co loading. Interestingly, adding extra 3% water could further increase the MLA yield, implying that our CoO@silicalite-1 catalyst is also capable for upgrading wet HFCS. As a result, the costly drying process of wet HFCS can be avoided. Moreover, the activity of CoO@silicalite-1 catalyst can be regenerated for at least four cycles via facile calcination in air. This study, therefore, will provide a new opportunity to not only solve the HFCS-overproduction issues but also produce value-added MLA.

Emerging heterogeneous catalysts for biomass conversion: studies of the reaction mechanism

The development of efficient catalysts to break down and convert woody biomass will be a paradigm shift in delivering the global target of sustainable economy and environment via the use of cheap, highly abundant, and renewable carbon resources. However, such development is extremely challenging due to the complexity of lignocellulose, and today most biomass is treated simply as waste. The solution lies in the design of multifunctional catalysts that can place effective control on substrate activation and product selectivity. This is, however, severely hindered by the lack of fundamental understanding of (i) the precise role of active sites, and (ii) the catalyst-substrate chemistry that underpins the catalytic activity. Moreover, active sites alone often cannot deliver the desired selectivity of products, and full understanding of the microenvironment of the active sites is urgently needed. Here, we review key recent advances in the study of reaction mechanisms of biomass conversion over emerging heterogeneous catalysts. These insights will inform the design of future catalytic systems showing improved activity and selectivity.

Impact of heteroatom addition into mesoporous silica for water adsorption in the low partial pressure range

Mesoporous silicas are known to be high-performing water adsorbents in high humidity levels due to their large pore volumes. However, for low humidity conditions, these materials typically present a less expressive performance, which is a drawback for many applications. In the present report, mesoporous silica SBA-15 was functionalized with Al, Ti, Zr and Li in order to improve their performance in this condition. The influence of functionalization in porosity, morphology and acidic sites was investigated. Samples with an increased number of acidic sites and with higher microporosity when compared to pure silica were produced. This was responsible for their enhanced performance for water adsorption in low moisture conditions. Sample functionalized with zirconium in SBA-15 synthesis improved the water adsorption capacity of pure silica by three times, reaching up to 127 g kg⁻¹ at a relative pressure of 0.2 and 570 g kg⁻¹ close to saturation pressure. This sample was found to be a promising material to be applied in processes which require high adsorption capacities in both low and high water partial pressure ranges. Moreover, the understanding of the mechanisms behind the heteroatom functionalization can be applied to any silica material in order to enhance its attractiveness towards any polar molecule.

Efficient catalytic conversion of microalgae residue solid waste into lactic acid over a Fe-Sn-Beta catalyst

Microalgae residue was efficiently converted into lactic acid with a high yield (33.9%) under mild reaction conditions (210 °C, 2 h) over a Fe-Sn-Beta catalyst. Under the action of homogeneous H₃O⁺ and distinct Lewis acid sites on the catalyst, the production of lactic acid from microalgae residue underwent three main reaction steps: hydrolysis, isomerization, and retro-aldol condensation. Results demonstrated that the lipid component had a strong inhibitory effect on the production of lactic acid due to the formation of aromatics, esters, and complex nitrogenous heterocyclic compounds, which covered or poisoned the Lewis acid sites of the catalyst. The protein component acted as a chemical buffer that enhanced the production of lactic acid by controlling the release of monosaccharides from the carbohydrate fraction of microalgae and maintaining the catalytic activity of the catalyst. Thus, microalgae residue demonstrated great promise for the production of value-added chemicals.

One-pot conversion of dihydroxyacetone into ethyl lactate by Zr-based catalysts

Efficient strategies for producing bio-based reagents from sustainable biomass are highly attractive for cost-effective sustainable manufacturing. In this study, a series of eco-friendly Zr-based catalysts (basic zirconium carbonate, zirconium dioxide and zirconium hydroxide) were investigated for the efficient conversion of dihydroxyacetone to ethyl lactate in a one-pot system, in which basic zirconium carbonate exhibited the best performance with 100% dihydroxyacetone conversion and 85.3% EL (ethyl lactate) yield at 140 °C, 4.0 h and 1.0 MPa N2. The improved activity of basic zirconium carbonate could be attributed to the synergistic effect among acid and base active sites. Furthermore, this low-cost catalyst shows improved thermochemical stability and recyclability under optimal conditions, where no significant decrease in activity was observed after three runs. This catalytic process could be identified as a promising alternative to produce ethyl lactate from renewable biomass and its derivatives.

A smart use of biomass derivatives to template an ad hoc hierarchical SAPO-5 acid catalyst

A smart design of hierarchical SAPO-5 acid catalyst using biomass derived monosaccharides as sustainable and low-cost mesoporogens has been developed. The hierarchical SAPO-5 was characterized by several physico-chemical techniques to elucidate structure-properties relationships and was tested as a catalyst in the MW-assisted glucose transformation in 5-HMF using γ-valerolactone (GVL) as green solvent.

Heterogeneous Catalytic Synthesis of Methyl Lactate and Lactic Acid from Sugars and Their Derivatives

Recent developments in sugar transformations to methyl lactate and lactic acid are critically summarized. The highest yield of methyl lactate from glucose obtained over Sn(salen)/octylmethyl imidazolium bromide catalyst was 68 % at 160 °C whereas the highest yield of lactic acid of 58 % was achieved over hierarchical Lewis acidic Sn-Beta catalysts at 200 °C under inert atmosphere. In addition to the desired products also humins are formed in water whereas in methanol alkyl glucosides- and -fructosides as well as acetals were generated, especially in the presence of Brønsted-acidic sites. The main challenges limiting the industrial feasibility of these reactions are incomplete liquid phase mass balance closure, complicated product analysis and a lack of kinetic data. In addition to reporting optimized reaction conditions and catalyst properties also catalyst reuse and regeneration as well as kinetic modelling and continuous operation are summarized.

Hydrotreating of Methyl Esters to Produce Green Diesel over Co- and Ni-Containing Zr-SBA-15 Catalysts

This work explores the performance of Zr-SBA-15 as a support for the conversion of methyl esters into green diesel. Thereby, a series of SBA-15 samples incorporating different amounts of Zr into the walls have been synthesized and impregnated with Ni and Co as hydrogenating species. All materials have been characterized, pelletized and tested in the hydrotreating of methyl esters using a fixed-bed tubular reactor at 300 °C, 30 bar of H2 and a continuous flow of 0.1 mL/min for 6 h. Co and Ni phases are present both over the surface and within the pores of the support. Interactions between the metals and the Zr species are more pronounced for Co-containing samples, leading to smaller Co particles at low Si/Zr ratios. Materials having higher Zr contents resulted in more methyl ester conversion, although an adequate metal-support combination was required to promote the hydrodeoxygenation (HDO) route. Co/Zr-SBA-15 materials exhibited more conversion (>90%) and higher yields of C11-C20 hydrocarbons (>60%) than their Ni-based counterpart catalysts. Specifically, Co/Zr-SBA-15(17) sample showed remarkable behavior, favoring the HDO pathway (n-C18/(n-C18 + n-C17) > 0.5), while keeping the methyl esters’ conversion close to 100% along the time on stream. These results evidence a synergistic effect between Co and Zr.

Zr-Incorporating SBA-15 for conversion of the ethanol–acetaldehyde mixture to butadiene

Zr incorporation into SBA-15 enhanced the BD yield due to Zr–O–Si bond formation.

Oxidative desulfurization of dibenzothiophene catalyzed by molybdenum dioxide immobilized on zirconia-modified silica

MoO₂/ZrO₂–SiO₂ catalysts show high activity in the oxidative desulfurization reaction, and the desulfurization rate can reach 99.96%.

Seed-assisted hydrothermal synthesis of Sn-Beta for conversion of glucose to methyl lactate: effects of the H2O amount in the gel and crystallization time

The crystallization rate, morphology, Sn state and microenvironment and catalytic performance for the conversion of glucose to methyl lactate of Sn-Beta are significantly affected by the water amount in the synthesis gel and the crystallization time.

Overview of the recent advances in lignocellulose liquefaction for producing biofuels, bio-based materials and chemicals

The concerns over the increasing energy demand and cost as well as environmental problems derived from fossil fuel use are the main driving forces of research into renewable energy. Lignocellulosic biomass comprised of cellulose, hemicellulose, and lignin is an abundant, carbon neutral, and alternative resource for replacing fossil fuels in the future. Solvent liquefaction of lignocellulosic biomass is a promising route to obtain biofuels, bio-based materials, and chemicals using a range of solvents as reaction media under moderate reaction conditions. Recently, several researchers have considered novel approaches for enhancing the process efficiency and economics. This review article reports the state-of-the-art knowledge of lignocellulose liquefaction in the recent three years with the main focus on the feedstock, liquefaction technology, target products, and degradation mechanism of each biomass component. This review is expected to provide an important reference for research into the solvent liquefaction of lignocellulose in the near future.

D-Excess-LaA Production Directly from Biomass by Trivalent Yttrium Species

D-lactic acid (D-LaA) synthesis directly from actual biomass via chemocatalytic conversion has shown high potential for satisfying its enormous demand in widespread applications. Here we report yttrium (Y(III))-species-catalyzed conversion of xylose and raw lignocelluloses to LaA with the highest yield of 87.3% (20% ee to D-LaA, ee%=(moles of D-LaA - moles of L-LaA)/(moles of D-LaA + moles of L-LaA) × 100). Combining experiments with theoretical modeling, we reveal that [Y(OH)₂(H₂O)₂]⁺ is the possible catalytically active species, enabling the unconventional cleavage of C3-C4 in xylulose and the subsequent dehydration of glyceraldehyde to pyruvaldehyde (PRA). The distinct interactions between hydrated-PRA and [Y(OH)₂(H₂O)₂]⁺ species contribute to the formation of different enantiomers, wherein H-migration via re-face attack leads to L-LaA and that via si-face attack yields D-LaA. The lower strain energy barrier is the origin of excess D-enantiomer formation.

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Y(III) exhibited outstanding efficiency for biomass conversion to lactic acid

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A high yield of D-excess-lactic acid with 20% ee value was obtained from xylose

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Reaction mechanism was successfully revealed by isotopic labeling and DFT study

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The lower strain energy induced the enantioselective formation of D-lactic acid

Catalytic Upgrading of Biomass-Derived Sugars with Acidic Nanoporous Materials: Structural Role in Carbon-Chain Length Variation

Shifting from petroleum-based resources to inedible biomass for the production of valuable chemicals and fuels is one of the significant aspects in sustainable chemistry for realizing the sustainable development of our society. Various renowned biobased platform molecules, such as 5-hydroxymethylfurfural, furfural, levulinic acid, and lactic acid, are successfully accessible from the transformation of biobased sugars. To achieve the specific reaction routes, heterogeneous nanoporous acidic materials have served as promising catalysts for the conversion of bio-sugars in the past decade. This Review summarizes advances in various nanoporous acidic materials for bio-sugar conversion, in which the number of carbon atoms is variable and controllable with the assistance of the switchable structure of nanoporous materials. The major focus of this Review is on possible reaction pathways/mechanisms and the relationships between catalyst structure and catalytic performance. Moreover, representative examples of catalytic upgrading of biobased platform molecules to biochemicals and fuels through selective C-C cleavage and coupling strategies over nanoporous acidic materials are also discussed.

Advances in porous and nanoscale catalysts for viable biomass conversion

Solid catalysts with unique porosity and nanoscale properties play a promising role for efficient valorization of biomass into sustainable advanced fuels and chemicals.

Chemocatalytic Production of Lactates from Biomass-Derived Sugars

In recent decades, a great deal of attention has been paid to the exploration of alternative and sustainable resources to produce biofuels and valuable chemicals, with aims of reducing the reliance on depleting confined fossil resources and alleviating serious economic and environmental issues. In line with this, lignocellulosic biomass-derived lactic acid (LA, 2-hydroxypropanoic acid), to be identified as an important biomass-derived commodity chemical, has found wide applications in food, pharmaceuticals, and cosmetics. In spite of the current fermentation of saccharides to produce lactic acid, sustainability issues such as environmental impact and high cost derived from the relative separation and purification process will be growing with the increasing demands of necessary orders. Alternatively, chemocatalytic approaches to manufacture LA from biomass (i.e., inedible cellulose) have attracted extensive attention, which may give rise to higher productivity and lower costs related to product work-up. This work presents a review of the state-of-the-art for the production of LA using homogeneous, heterogeneous acid, and base catalysts, from sugars and real biomass like rice straw, respectively. Furthermore, the corresponding bio-based esters lactate which could serve as green solvents, produced from biomass with chemocatalysis, is also discussed. Advantages of heterogeneous catalytic reaction systems are emphasized. Guidance is suggested to improve the catalytic performance of heterogeneous catalysts for the production of LA.

Direct conversion of C6 sugars to methyl glycerate and glycolate in methanol

The present work deals with the one-pot conversion of C6 sugars to methyl glycerate and glycolate via a cascade of retro-aldol condensation and oxidation processes catalyzed by using MoO₃ as the Lewis acid catalyst and Au/TiO₂ as the oxidation catalyst in methanol. Methyl glycerate (MGLY) is the product of C6 ketose (fructose), while methyl glycolate (MG) is produced from C6 aldose (mannose, glucose). It is found that a good one-pot match between two reactive processes is the key to the production of MGLY and MG with high yield (27.6% MGLY and 39.2% MG). A separated retro-aldol condensation and oxidation process greatly decreases their yields, and even no MGLY can be obtained in this separated process. We attribute this to high instability of glyceraldehyde/glycolaldehyde and their different reaction pathways which mainly depend on whether acetalization of retro-aldol products (glyceraldehyde and glycolaldehyde) occurs with methanol or not. This result opens a new prospect on the accumulation of C3 products other than lactate from biomass-derived carbohydrates.

Methyl glycerate (MGLY) and methyl glycolate (MG) are directly produced in maximum yield by the one-pot conversion of hexose, and the formation of MGLY and MG experience different reaction routes.