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.

Niobium: The Focus on Catalytic Application in the Conversion of Biomass and Biomass Derivatives

The world scenario regarding consumption and demand for products based on fossil fuels has demonstrated the imperative need to develop new technologies capable of using renewable resources. In this context, the use of biomass to obtain chemical intermediates and fuels has emerged as an important area of research in recent years, since it is a renewable source of carbon in great abundance. It has the benefit of not contributing to the additional emission of greenhouse gases since the CO2 released during the energy conversion process is consumed by it through photosynthesis. In the presented review, the authors provide an update of the literature in the field of biomass transformation with the use of niobium-containing catalysts, emphasizing the versatility of niobium compounds for the conversion of different types of biomass.

Mesoporous Sn-In-MCM-41 Catalysts for the Selective Sugar Conversion to Methyl Lactate and Comparative Life Cycle Assessment with the Biochemical Process

The use of biomass for the production of energy and higher added value products is a topic of increasing interest in line with growing environmental concerns and circular economy. Mesoporous material Sn-In-MCM-41 was synthesized for the first time and used as a catalyst for the transformation of sugars to methyl lactate (ML). This catalyst was characterized in depth by various techniques and compared with Sn-MCM-41 and In-MCM-41 catalysts. In the new Sn-In-MCM-41 material, both metals, homogeneously distributed throughout the mesoporous structure of MCM-41, actuate in a cooperative way in the different steps of the reaction mechanism. As a result, yields to ML of 69.4 and 73.9% in the transformation of glucose and sucrose were respectively reached. In the case of glucose, the ML yield 1.5 and 2.6 times higher than those of Sn-MCM-41 and In-MCM-41 catalysts, respectively. The Sn-In-MCM-41 catalyst was reused in the transformation of glucose up to four cycles without significant loss of catalytic activity. Finally, life cycle assessment comparison between chemical and biochemical routes to produce ML allowed us to conclude that the use of Sn-In-MCM-41 reduces the environmental impacts compared to Sn-MCM-41. Nevertheless, to make the chemical route comparable to the biochemical one, improvements in the catalyst and ML synthesis have to be achieved.

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.

Spiky-shaped niobium pentoxide nano-architecture: highly stable and recoverable Lewis acid catalyst

Niobium pentoxide particles with a complex three-dimensional (3D) nanostructure consisting of a spiky structure have been developed as recyclable and recoverable Lewis acid catalysts. The morphology of the niobium pentoxide was successfully controlled from 1D to 3D via a bridging-ligand-assisted hydrothermal treatment, without changing the crystal structure. Compared with dispersed one-dimensional (1D) niobium pentoxide nanorods with a major-axis length and minor-axis length of 20 nm and 5-8 nm, respectively, the spiky-shaped niobium pentoxide composed of 300 nm spherical cores and nanorods with a minor-axis length of 5 nm maintained its surface nanostructure even after calcination at 400 °C in air. The 400 °C-calcined spiky particles exhibited the highest production rate of 2-((4-methoxyphenyl)amino)-2-phenylacetonitrile (0.115 mmol m^-2) in a Strecker reaction, resulting in a nanoscale and ordered surface structure of spiky particles that simultaneously exhibit high specific reactivity and high structural stability. Acid site analysis and Raman spectroscopy revealed that stable nanorods that grew in the (001) orientation functioned as Lewis acid catalysts and that the origin of the acidity was a flexible Nb-O polyhedral structure in the single-nanoscale (<10 nm) niobium oxide rods. This study proposes that the spiky-shaped niobium pentoxide exhibits sintering resistivity and high activity and has potential applications as a recoverable and recyclable solid acid catalyst.

Advances in catalytic routes for the production of carboxylic acids from biomass: a step forward for sustainable polymers

Polymers are ubiquitously present in our daily life because they can meet a wide range of needs and fields of applications. This success, based on an irresponsible linear consumption of plastics and the access to cheap oil, is creating serious environmental problems. Two lines of actions are needed to cope with them: to adopt a circular consumption of plastics and to produce renewable carbon-neutral monomers. This review analyses the recent advances in the chemocatalytic processes for producing biomass-derived carboxylic acids. These renewable carboxylic acids are involved in the synthesis of relevant general purpose and specialty polyesters and polyamides; some of them are currently derived from oil, while others can become surrogates of petrochemical polymers due to their excellent performance properties. Polyesters and polyamides are very suitable to be depolymerised to other valuable chemicals or to their constituent monomers, what facilitates the circular reutilisation of these monomers. Different types of carboxylic acids have been included in this review: monocarboxylic acids (like glycolic, lactic, hydroxypropanoic, methyl vinyl glycolic, methyl-4-methoxy-2-hydroxybutanoic, 2,5-dihydroxypent-3-enoic, 2,5,6-trihydroxyhex-3-enoic acids, diphenolic, acrylic and δ-amino levulinic acids), dicarboxylic acids (2,5-furandicarboxylic, maleic, succinic, adipic and terephthalic acids) and sugar acids (like gluconic and glucaric acids). The review evaluates the technology status and the advantages and drawbacks of each route in terms of feedstock, reaction pathways, catalysts and economic and environmental evaluation. The prospects and the new research that should be undertaken to overcome the main problems threatening their economic viability or the weaknesses that prevent their commercial implementation have also been underlined.

Efficient synthesis of niobium pentoxide nanowires and application in ethanolysis of furfuryl alcohol

Nb2O5 nanowires with high specific surface area and crystallinity were prepared by using ammonium oxalate and an acetic acid solvent system. The nanomaterial was applied in ethanolysis of furfuryl alcohol (FA), and the yield of the product, 2-(ethoxymethyl)furan (FEE), achieved was up to 79.6%. Compared to mesoporous Nb2O5 materials and other porous materials, the residence time of FEE on the surface of the catalyst is shorter, and the yield of ethyl levulinate (EL) is lower. Furthermore, a high temperature calcination treatment can change the acid sites and acidity type distribution on the nanowire surface. By XRD, NH3-TPD, IR, and TG-DTA determination methods, it was found that the weak and medium-strong acid sites on the surface of Nb2O5 nanowires were reduced after a 300 °C treatment, and the amount of strong acid was relatively higher. According to the catalytic performance test data and acidity determination, it was concluded that more weak acid and medium-strong acid sites improve the conversion of furfuryl alcohol to FEE, and the strong acid sites promote further conversion of FEE to EL.

Direct conversion of cellulose into isosorbide over Ni doped NbOPO4 catalysts in water

Ni doped NbOPO₄ catalysts were used efficiently for the one-pot conversion of cellulose to isosorbide under aqueous conditions.

Low Temperature Dehydration of Glycerol to Acrolein in Vapor Phase with Hydrogen as Dilution: From Catalyst Screening via TPSR to Real-Time Reaction in a Fixed-Bed

Temperature programmed surface reaction (TPSR) was developed as a method for rapid screening of catalysts. In this study, a series of acid catalysts was screened for the low-temperature dehydration of glycerol to acrolein via TPSR. Results suggested that most catalysts show activity of glycerol conversion to acrolein at a greatly different temperature range. HY, SiO2 supported H4SiW12O40 (STA/SiO2), SO42−/ZrO2, and SO42−/TiO2 were observed to be efficient for the conversion of glycerol into acrolein at 210 °C, which was significantly lower than that generally reported (250–340 °C). Moreover, high selectivity of acrolein was gained at 85% and 86% over SiW/SiO2 and SO42−/TiO2, respectively. A new style catalyst, ZnCl2/SiO2, was also found to be highly selective to acrolein and evaluated in a conventional fixed-bed reactor. Especially, stability tests showed that the catalyst life was up to 300 h with no clear deactivation on ZnCl2/SiO2 with hydrogen as dilution.