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.
Due to the burgeoning world population, demands for energy and chemicals are sharply increasing. Therefore, traditional nonrenewable fossil resources, particularly coal and petroleum, are going to be run out, and their concomitantly environmental and climatological impacts are also urgently needed to be addressed in the meantime [
Lactic acid (LA, 2-hydroxypropanoic acid), one of the great appeals among carbohydrate-derived platform molecules, is an important feedstock for the production of alkyl lactates, biodegradable plastics such as polylactic acid (PLA), and other valuable chemicals under suitable reaction conditions in the assistance of catalytic functionalities. Specially, PLA polymer bearing the advantages of biodegradability, compostability, and biocompatibility could be utilized in a wide range of applications such as eco-friendly packages. In addition, carbon neutral balance is accepted when PLA is disposed to release CO2 and water. LA was firstly found in 1780 by the Swedish chemist Scheele in acid milk [
It is estimated that LA demand in 2020 will be above 600000 tons [
Conventional fermentative method for producing LA.
Chemical catalysis (homogeneous or heterogeneous), being considered to be the formidable strategy to transform cellulosic biomass into value-added chemicals with acceptable selectivity, is rising progressively [
Currently, many research groups are employing the chemocatalysts to synthesize lactic acid (in water) or lactates (alcoholic solvents). The main emphasis of this review is to depict the state-of-the-art development of LA or lactates production from sugars and real lignocellulosic biomass resources, with chemocatalysis especially heterogeneously catalysts which have tremendous advantages (i.e., recyclable, reusable, and environmentally benign). Furthermore, structure-function relationships, reaction mechanisms, and guidance on designing heterogeneous acid catalysts for LA or lactates production are also discussed, accordingly.
The hydrothermal process, one of the most potential approaches, is used in the conversion of biomass into valuable resources, since water can serve as a reaction medium bearing special properties when treated in the high temperature and pressure [
Catalytic transformation of lactic acid from different feedstocks by alkaline.
Entry | Substrate | Catalyst | Reaction conditions | Yield of LA (%) | Reference |
---|---|---|---|---|---|
1 | Glycolaldehyde | NaOH | 300°C, 10 min | 28.1 | [ |
2 | Glucose | NaOH | 300°C, 1 min | 27 | [ |
Ca(OH)2 | 300°C, 1 min | 20 | |||
3 | Glycerin | KOH | 300°C, 80 min | 90 | [ |
4 | Glucose | NaOH | 300°C, 1 min | 27 | [ |
Glucose | Ca(OH)2 | 300°C, 1 min | 20 | ||
Cellulose | Ca(OH)2 | 300°C, 90 s | 19.2 | ||
Starch | Ca(OH)2 | 300°C, 90 s | 18.7 | ||
5 | Cellulose | NaOH + Ni | 300°C, 1 min | 34.1 | [ |
6 | Glucose | NaOH + CuO | 188°C, 0.15 min | 59 | [ |
7 | Cellulose | NaOH + Zn/Ni/C | 300°C, 5 min | 42 | [ |
8 | Glucose | NaOH + NiCl2 | 300°C, 1 min | 25 | [ |
9 | Glucose | Na2SiO3 | 300°C, 1 min | 30 | [ |
10 | D-Glucose | Hydrotalcite | 50°C, 8 h | 20.3 | [ |
11 | D-Glucose | KOH/Al2O3 |
180°C, microwave 3.1 W·g−1, 40 min | 75 | [ |
D-Mannose | 41 | ||||
D-Fructose | 36 | ||||
D-Ribose | 43 | ||||
D-Arabinose | 35 | ||||
D-Sucrose | 23 | ||||
12 | Hydroxyacetone | Pt/MgO-Al2O3-800 | 40°C, 6 h | 100 (selectivity) | [ |
13 | Glucose | [IMEP]Cl, NaOH | 100°C, 30 min, N2 | 63 | [ |
[IMEP]Cl, KOH | 65.5 | ||||
14 | Glucose | Ba(OH)2 | 250°C, 3 min | 57 | [ |
15 | Glucose | Ba(OH)2 | Ball milling (solvent-free) | 35.6 | [ |
16 | Glucose | Ba(OH)2 | 25°C, 48 h | 95.4 | [ |
Fructose | 83.5 | ||||
17 | Corn cobs | Ca(OH)2 | 300°C, 30 min | 44.76 | [ |
18 | Bread residues | Ca(OH)2 | 300°C, 30 min | 73 | [ |
19 | Rice straw | NaOH, NiO nanoplates | 260°C, 2 h | 58.81 | [ |
20 | Alginate | CaO | 200°C, 6 h | 14.66 | [ |
Proposed retro-aldol of glucose and fructose by complexation with Ca2+.
Proposed pathway of formation of LA from glucose by alkaline hydrothermal reaction.
To keep up Jin group’s works concerning the conversion of carbohydrates biomass into LA, they added Ni as cocatalyst in the LA production started from cellulose under hydrothermal conditions with NaOH [
As mentioned above, solid alkaline catalyst bearing low corrosiveness seems to be a better choice, when employed in the production of LA under hydrothermal process. In this regard, versatile activated hydrotalcites were utilized as alkaline catalysts to convert glucose in a flow reactor into LA, giving 20.3% LA yield at 50°C [
Unfortunately, the aforementioned hydrothermal process almost adopted relatively high reaction temperatures (i.e., 300°C), thus limiting their promising industrial applications to some extent. For the sake of overcoming this challenge, Wang et al. used the polymeric catalyst (polymerization of imidazole and epichlorohydrin, [IMEP]Cl) as weak Lewis acid along with NaOH/KOH to transform glucose into LA [
In spite of the LA production using Ba(OH)2 investigated by Jin group before, Esposito and Antonietti also researched the LA manufactured from glucose with Ba(OH)2 in detail [
Proposed pathway for the conversion of glucose to lactic acid with the base at room temperature under nitrogen (Path I general base catalyst route; Path II: Ba(OH)2 catalytic-complex route).
As discussed above, sugars can be transformed into LA with acceptable yield under alkaline hydrothermal process. Nonetheless, LA production directly from real biomass is highly more commended without hesitation. As shown in Table
Proposed reaction pathway for catalytic hydrothermal conversion of sodium alginate into lactic acid with hydrated CaO catalyst.
As discussed above, with the addition of alkaline catalysts especially under hydrothermal reaction conditions, LA could be produced with relatively acceptable yield, to some extent. However, acid solution was always needed to employ to neutralize the base, along with hydrolysis the possible lactates to acquire the final product, pure LA. Therefore, acidic catalysts appear to be a better choice to be used in the production of LA and lactates, starting from different types of feedstocks (i.e., C6, C3 sugars, and so on), through several specific catalytic reactions. Specifically, Lewis acidic sites are regarded as the key role in the transformation of LA or lactates from different carbohydrates even cellulose. In addition, from the environmental point of view, heterogeneously acidic catalysts which are less corrosive and can be recycled from the products for reutilization are the main discussed objects regarding the LA preparation in the following two parts, accordingly.
DHA or GLY is being deemed as the key intermediate with respect to transforming saccharides into LA. As a consequence, illuminating the LA production directly from simply C3 sugars is important to further understand the reaction of converting C6 and cellulose to LA. Generally, 100 (DHA) and 120 (GLA) kJ/mol are demanded to be energetically favored, in order to be isomerized to LA [
Proposed reaction pathway for converting C3 sugars to LA in aqueous with AlIII salts catalyst.
It is worth noting that during the bioprocessing of biomass upgrading, tunable acidic, thermal stable, and shape-selective zeolites materials are considered to be the most promising heterogeneously solid acidic catalysts with superior catalytic performance [
As a consensus, effective LA production from hexoses is more preferable compared to C3 sugars. In a similar manner, the novel well-aligned Nb2O5 nanorod owning highly single crystallinity was also designed to produce LA from glucose [
Catalytic transformation of lactic acid from C6 sugars and cellulose by acids.
Entry | Substrate | Catalyst | Reaction conditions | Yield of LA (%) | Reference |
---|---|---|---|---|---|
1 | Glucose | Nb2O5 nanorod | 250°C, 4 h | 39 | [ |
2 | Fructose | Sn(IV) organometallic complexes | 190°C, 0.5 h | 63 | [ |
3 | Sucrose | Zn-Sn-Beta | 190°C, 2 h | 54 | [ |
4 | Glucose | Pb-Sn-Beta | 190°C, 2 h | 52 | [ |
5 | Fructose | MIL-100 (Fe) | 190°C, 2 h | 32 | [ |
6 | Levoglucosan | La(OTf)3 | 250°C, 1 h | 75 | [ |
Glucose | 74 | ||||
Xylose | 61 | ||||
7 | Cellulose | AlW | 190°C, 24 h | 28 | [ |
ZrW | 19 | ||||
8 | Cellulose | ZrW | 190°C, 24 h | 3 g/L−1 | [ |
Pine wood sawdust | 190°C, 8 h | 1.2 g/L−1 | |||
9 | Xylose | ZrO2 | 200°C, 40 min | 42 | [ |
Xylan | 200°C, 90 min | 30 | |||
10 | Cellulose | ZrO2 (monoclinic) | 200°C, 6 h | 21.2 | [ |
11 | Cellulose | 10%ZrO2-Al2O3 | 200°C, 6 h | 25.3 | [ |
12 | Cellulose | Er(OTf)3 | 240°C, 30 min | 89.6 | [ |
13 | Cellulose | ErCl3 | 240°C, 30 min | 91.1 | [ |
14 | Cellulose | Er-K10 | 240°C, 30 min | 67.6 | [ |
15 | Cellulose | Er/deAl |
240°C, 30 min | 58 | [ |
16 | Cellulose | Pb(NO3)2 | 190°C, 2 h | 71 | [ |
17 | Cellulose | VOSO4 | 180°C, 2 h | 54 | [ |
18 | Fructose | AlCl3 + SnCl2 | 190°C, 2 h | 90 | [ |
Glucose | 180°C, 2 h | 81 | |||
Cellulose | 190°C, 2 h | 65 | |||
19 | Cellulose | NbF5-AlF3 | 180°C, 2 h | 27.3 | [ |
20 | Cellulose | Nb@CaF2 | 180°C, 2 h | 15.4 | [ |
21 | Xylose | LaCoO3 perovskite | 200°C, 1 h | 38 | [ |
Glucose | 200°C, 1 h | 40 | |||
Cellulose | 240°C, 1 h | 24 |
In addition to these functional materials aforementioned for LA transformation from hexoses, Huang et al. employed the solid Lewis acidic material (Table
The direct transformation of cellulose being the main component of lignocellulosic biomass into valuable chemicals such as LA is highly desired in making important contributions to biomass-based renewable biorefinery [
In fact, the yield of LA is not very high when using the aforementioned catalysts from cellulose. With respect to this regard, Wang et al. designed the erbium- (Er-) based Lewis acid catalyst for the production of LA directly from cellulose, along with high yields (Table
In addition to Dong group, Wang et al. also researched the LA production from cellulose with a high yield using Lewis metal inorganic salts, and detailed reaction mechanism was also demonstrated (Table
Proposed reaction pathways of the transformation of cellulose in the presence and absence of Al(III)-Sn(II) catalyst.
Proposed reaction mechanism of the isomerization and retro-aldol fragmentation with Al(III)-Sn(II) catalyst.
As shown above, niobium-based catalysts can catalyze C3 and C6 sugars into LA (Section
Proposed redox reaction pathway of LA production from glucose using LaCoO3.
Bio-based methyl/ethyl lactates (ML, EL), nontoxic liquids owning high boiling points, are being served as the potential value-added compounds with a high extent of functionality especially in green alternative solvents [
Generally, C3 sugars were usually employed to be the substrates for the production of lactates to serve as the model reaction, with the final aim of using lignocellulose biomass directly. Derived from the relevant studies of mechanism, as illustrated in Figure
Proposed mechanism for the conversion of DHA to ethyl lactate.
Proposed mechanism of ML formation by Sn-Beta.
As discussed above, tin-based acids catalysts showed excellent reactivity towards lactates formation. According to these results, tin ion-exchanged montmorillonites [
Compared to trioses, the transformation of hexoses into lactates through chemocatalysis is more preferable. Some relatively typical homogeneous Lewis acidic catalysts were studied with respect to converting C6 sugars into lactates, such as SnCl4 [
Catalytic production of lactates from C6 sugars and cellulose via acid-catalysis.
Entry | Substrate | Catalyst | Reaction conditions | Yield of lactates (%) | Reference |
---|---|---|---|---|---|
1 | Glucose | SnCl4-NaOH | 160°C, 2.5 h | 47 | [ |
Fructose | 57 | ||||
Sucrose | 51 | ||||
2 | Fructose | InCl3·4H2O-SnCl2, NaBF4 | 160°C, 10 h | 72 | [ |
3 | Glucose | ZnCl2 | 200°C, 3 h | 47.7 | [ |
Fructose | 51.7 | ||||
Sucrose | 47.5 | ||||
4 | Sucrose | Sn-beta | 160°C, 20 h | 68 | [ |
Glucose | 43 | ||||
Fructose | 44 | ||||
5 | Xylose | Sn-beta | 140°C, 20 h | 42 | [ |
6 | Glucose | Sn-MCM-41 | 160°C, 20 h | 43 | [ |
7 | Sucrose | K-PT-Sn-beta | 170°C, 16 h | 71 | [ |
8 | Xylose | Zr-SBA-15 | 240°C, 6 h | 41 | [ |
Fructose | 44 | ||||
Glucose | 38 | ||||
Sucrose | 40 | ||||
9 | Glucose | Hierarchical Sn-Beta | 160°C, 10 h | 58 | [ |
10 | Inulin | Sn-SBA-15 | 160°C, 20 h | 57 | [ |
11 | Sucrose | Sn-beta-H | 160°C, 20 h | 72.1 | [ |
12 | Fructose | Sn-beta-9h | 160°C, 10 h | 47 | [ |
Mannose | 39 | ||||
Sucrose | 57 | ||||
13 | Fructose | Sn-Si-CSM-773–20.4 | 155°C, 20 h | 17 | [ |
Glucose | 32 | ||||
Sucrose | 45 | ||||
14 | Sucrose | ZIF-8 | 160°C, 24 h | 42 | [ |
15 | Glucose |
|
160°C, 6 h | 34 | [ |
16 | Cellulose | SnCl2·2H2O-ZnCl2 | 210°C, 4 h | 32.1 | [ |
Sugarcane bagasse | 190°C, 6 h | 31.2 | |||
17 | Cellulose | Zr-SBA-15 | 240°C, 10 h | 28.1 | [ |
18 | Cellulose | Zr-SBA-15 | 260°C, 2 h | ∼33 | [ |
19 | Cellulose | Ga-doped Zn/H-nanozeolite Y | 270°C, 5 h | 57.8 | [ |
Glucose | 270°C, 1 h | 64.0 | |||
Fructose | 270°C, 1 h | 67.3 |
Alternatively, heterogeneously solid Lewis acidic catalysts seem to be a better choice due to their high activity and recyclability. With regard to transformation of lactates from C6 sugars, Taarning’s work is regarded as the most valuable and pioneering research without hesitation (Table
However, the aforementioned studies investigated the tin-based zeotypes for efficient production of lactates, and little attention was paid to the systematic investigation of kinetic and mechanistic understanding in the Sn-Beta-catalyzed lactates course. With regard to this, Tosi et al. designed the relatively detailed kinetic analysis of fructose, glucose, and sucrose transformation to ML through typical Sn-Beta [
Generally, the direct utilization of cellulose as feedstocks for the production of lactates is being deemed to be a milestone, especially through the versatile chemocatalysis. With respect to this research, limited studies regarding the efficient transformation of lactates from cellulose via chemocatalysis can be presented, because of the very complex reactions and the inherent rigidity of cellulose. For the homogeneous catalyst used in lactates production from cellulose, SnCl2·2H2O-ZnCl2 was utilized as an efficient catalyst to ML production with 32.1% yield in methanol, at relatively mild reaction conditions with 210°C for 4 h [
Nonetheless, the aforementioned catalysts not presented high yields of lactates from cellulose directly. With respect to getting a high productivity of lactates from cellulose, Verma et al. designed the Ga-doped Zn/H-nanozeolite Y catalysts, to be served the most efficient materials so far for converting cellulose directly into ML with 57.8% yield at 270°C, 5 h in supercritical methanol [
Proposed reaction pathway for the conversion of methyl lactate from cellulose using Ga-doped Zn/HNZY.
Catalytic transformations of valuable organic acids such as lactic acid, levulinic acid, and amino acid from renewable carbon resources including polysaccharides, lignin, and their derivatives is of high interest for a sustainable chemical industry in the future [
From the environmental-friendly point of view, heterogeneously solid acidic catalysts which are less corrosive and can be recycled from the reaction medium for reutilization are considered to be the better choice currently. Sn-based zeotype catalysts bearing strong Lewis acidities have demonstrated excellent performance for the transformation of sugars to lactates. However, long synthesis time especially Sn- More attention is recommended to pay on the synthesis of active, selective and durable solid acidic catalysts for the efficient transformation of cellulosic biomass into LA and lactates The design of novel multifunctional (i.e., controllable active sites, strong Lewis acidic functional groups with weak Brønsted acidic sites, and acid-base bifunctional sites) heterogeneous catalysts is highly appreciated It should be reinforced to design the mesoporous nanocatalysts, bearing large surface area along with large pore size, in order to render reactants contact with the active sites easily More studies are demanded to propose a facile method that can prepare the target catalysts in view of large-scale and low-cost For lactates production, the recyclability of methanol or ethanol solvent should be taken into consideration, which is likely to affect the whole process economics, in order to intensify the sustainable process It is indispensable to be devoted into an insightful understanding in terms of the reaction mechanism and structure-properties of the catalysts, which is helpful to understand the reaction pathways and the better design of catalysts
The authors have no conflicting interests to declare.
This work was financially supported by the Natural Science Foundation of China (21576059 and 21666008), the Key Technologies R&D Program of China (2014BAD23B01), and Chinese State Scholarship Fund (No. 201706670012).