Interfacial catalysts for sustainable chemistry: advances on atom and energy efficient glycerol conversion to acrylic acid

Abstract

Conversion of glycerol to acrylic acid represents an emerging application in transforming waste biomass to valuable products in chemical industry. However, achieving almost perfect atomic efficiency and intrinsic zero-waste generation still remain a grand challenge to meet stricter regulations on energy consumption. Interfacial catalysis plays a central role in bridging nanostructured catalysts with green chemistry. Despite a few preliminary attempts in academia and industry, the application of interfacial catalysts remains limitedly successful. In this review, we first describe morphological & electronic uniqueness of three important interfacial catalysts, (I) bimetallic nanoclusters, (II) metal–acid composites and (III) Brønsted/Lewis acid hybrids. The resulting catalytic properties for C–H and C–O bond cleavage during oxidation and dehydration of glycerol to acrylic acid will then be critically discussed, to reveal the key advances and challenges in this area. As oxidation and dehydration represent two important reactions in industry, the outcome of this work is to provide insightful information developing cost-effective catalytic materials for various other sustainable chemical applications.

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1. Acrylic acid: a key essential for sustainable chemical industry

1.1 Background

Acrylic acid (AA), a C₃ carboxylic acid, is an important commodity intermediate in modern chemical industry. AA is also known as 2-peropenoic acid as there is a C=C bond existing in the molecular structure. It has a density of 1.062 g cm⁻³ at 20 °C and is miscible with water in all ratios. As a key building block, AA can be transformed into a variety of downstream products (Fig. 1).^1,2 More importantly, AA is used as a performance enhancer in superabsorbents, BPA-free plastics (BPA: bisphenol a), and synthetic rubbers as well as coatings, textiles and detergents.

Fig. 1 Acrylic acid as building blocks for various applications.

The market share for various consumers’ product is also presented in Fig. 1. Superabsorbers and detergents are the most popular applications, which were projected to a global consumption of more than 1 million tons in 2020, with 15–20% annual growth. Other applications include phosphate-free washing agents, coatings for printed circuit boards.^2–4

AA can be manufactured from fossil- and bio-based feedstocks (Fig. 2).^5,6 AA is conventionally produced by catalytic oxidation of propylene in a two-step conversion route, involving production of acrolein or a modified Reppe process from acetylene (Scheme 1).^6–9 AA can also be produced from hydrolysis of acrylonitrile. For propylene-based route, in the first step, propylene can be partially oxidized into acrolein over Mo-based catalyst materials at temperatures in the range of 310–400 °C with 0.1–0.3 MPa of steam vapor and air, the product of which is then fed to a second reactor to produce AA at 260–300 °C. The overall conversion of propylene is 95% with yield to AA of 80–90%. Catalysts used include Bi, Co, Fe, Mo based heterogenoues materials for the first stage, while second-stage catalysts are mixed metal oxides containing Mo and V.

Fig. 2 Manufacture of acrylic acid and esters from fossil-based (red) and bio-derived (green) feedstocks.

Scheme 1 Propylene-based and acetylene-based routes.

Walter Reppe, the founder of modern acetylene chemistry, discovered the reaction involving Ni carbonyl promoted acetylene and water/alcohols reaction to give AA and derivatives.¹⁰ Acrylonitrile route is often considered as modified propylene oxidation route. The yield of acrylonitrile is lower than that of AA from propylene oxidation.

AA has been produced in large quantities, for example in European Union (EU) and North America.¹⁰ The market for downstream products is very dynamic, as production worldwide for AA has reached 6.5 million tons in 2019 with a strong growth rate of 5% per annum. Actually, market reports on global consumption have suggested that the demand for AA and its derivatives has already exceeded 8.0 million tons in 2019. From the perspective of life cycle analysis, propylene-, acetylene- and acrylonitrile-based routes all demand significant energy input for cracking, partial oxidation and ammoxidation reactions. Although currently dominant, the strong interests worldwide seeking for more “greener” feedstocks and processes motivated researchers to explore more opportunities.^6,11

1.2 Bio-derived acrylic acid: scope of the review

Replacing propylene and acetylene with bio-oxygenates has been emerging as a promising route to manufacture AA from renewable resources.^6,11 Although extensive attention being paid for bio-AA synthesis, up to date, there are only two review articles discussing the progress on manufacture of AA from biomass as the starting material. Ueda and colleagues summarized the key challenges in converting glycerol to acrylic acids via multiple routes, including (i) glycerol–acrolein (red route in Fig. 3), (ii) glycerol–allyl alcohol (purple route in Fig. 3) and (iii) glycerol–lactic acid routes (green route in Fig. 3).¹² The most recent review from Zhou's group critically revised the highlights in catalyst perfomances for (i) glycerol–acrolein route.⁵

Fig. 3 Conversion of glycerol to AA via multiple transformation routes.

1.2.1 Atom and energy economics for route (iii). As all three routes start with glycerol and end with AA as the final product, one would expect similar atom efficiency and energy input for all three routes. However, detailed inspection of the three routes reveals that, route (iii) shows several possible atomic and energetic advantages over the other two.^3,13,14 Conversion of glycerol to lactic acid generally needs much lower temperature (90–200 °C) compared with the first step involved in (i) and (ii).^15–18 More importantly, removal of H species from glycerol molecules obviously undergoes varied reaction pathways for the three conversion routes.^19–22 While (i) and (ii) often involves hydration as the first step to lose H species in the form of water molecules,^{4,7,9,12,23,24} (iii) oxidation of glycerol can occur in the absence of O₂, thus producing valuable H₂ as an important co-product.^7,8,25,26 Obviously, oxidative reaction of glycerol in the presence of O₂ still occur under much milder condition (<130 °C) compared with other two routes (>260 °C). This is the main advantage for synthesis of AA from glycerol via lactic acid as key intermediate.^27–30 Dehydration of lactic acid to AA requires inexpensive heterogeneous catalysts, rather than homogeneous or noble metal species.^8,25

With global efforts in developing poly-lactic acid for bio-degradable polymers, bans on single-use fossil-derived plastics have become stricter in the past few years. Thus one can predict the strong growth in the market of lactic acid as a cost-effective intermediate for sustainable production of AA in the coming future.^{2,6,8,23,29,31} Therefore, this review article will be particularly focused on selective synthesis of AA from glycerol via lactic acid as the key intermediate, with focused discussion on catalyst design and molecular catalysis involved in oxidation of glycerol to lactic acid, and dehydration of lactic acid to AA as final product.

2. Fundamentals on interfacial catalysis

2.1 Interfacial catalysis: morphological & electronic uniqueness

According to the brief summary on catalyst design, hybridized materials have been studied for glycerol conversion to lactic acid and subsequent dehydration of lactic acid to AA. Specifically, supported metallic catalysts, mixed metal oxides and metal–acid composites, with 2D or 3D morphology (Fig. 4) are commonly used for conversion of glycerol to lactic acid.^27,28,32 For dehydration of lactic acid, mostly widely studied candidates are modified zeolites, phosphate and sulfates (Fig. 5).^33–35 This is because each of the reactions actually involves multiple elementary steps. Among those above-mentioned hybridized materials, three key interfacial structures are crucial for tunable activity, selectivity and durability of solid catalysts (see Fig. 6 for conceptual description).

Fig. 4 Hybridized catalysts for glycerol conversion to lactic acid, (a) dehydrogenation step, (b) dehydrogenation–dehydration step.

Fig. 5 Dehydration of lactic acid to acrylic acid over hybridized catalysts.

Fig. 6 Conceptual description for three types of interfacial catalysis.

2.1.1 Metal–metal interface. Morphological and electronic coupling at metal–metal interfaces contributes to tunable activity for C–H bond cleavage. The hybridization of d orbitals could alter surface electronic density and number of valence electrons for active sites. Furthermore, non-noble Cu, Co and Ni have been often selected as the second metal/promoters, which show strong electronic donor properties.^36–38 For metal–metal interfaces, as the number of metal neighbors or the coordination number of the surface metal changes, the bandwidth of the d-DOS (density of states) also changes.^39–46 The general rule is that, higher coordination number contributes to broader band (Fig. 7(a)). Broader band may partially lower the overall energy level of unoccupied orbitals below Fermi level. The lowered electron orbitals will not easily be accessible to reactants. Thus, formation of metal–H and metal–O bond can be affected during activation of glycerol molecule. In particular, the consequence of a nearly constant d-band filling is an upshift in the d-band center, leading to a more catalytically reactive surface, as more unoccupied orbitals interact with substrate molecules for selective activation of chemical bonds. Therefore, upshift of d-band center could result in improved activity for metal catalysts when activating oxygenated molecules.^42,43,45

Fig. 7 Electronic reconfiguration for three interfacial catalysts: (a) Metal–Metal & Metal–Acid Interface (b) Modified Lewis Acidic Centers.

This is the reason that bimetallic catalysts often display relatively better activity compared with monometallic candidates. The morphological effect is also important for redox reactions. But it seems that very few studies were focused on this aspect. At present, application of bimetallic catalysts is only primarily on oxidation of glycerol to glyceraldehyde, after which acidic or basic promoters are needed to facilitate dehydration reactions.

2.1.2 Metal–acid interface. Metal–acid interface is one of the most commonly studied subjects in the past few decades, particularly in the areas of petroleum and coal refining. Different from the case with metal–metal interface, Lewis acid often show strong electronic withdrawing capability from metals, altering the d-band center of metallic sites.³⁹ Strong metal–acid interaction actually induces the formation of additional interfaces. This is because as metallic sites display more cationic nature, the coordination number of metal centers is actually enhanced, leading to broader d-bandwidth.⁴⁶ For biomass conversion, the electronic interaction at metal–Lewis acid interfaces alters the intrinsic activity of metal sites for C–H bond cleavage reaction of glycerol. Different from the case with metal–metal interfaces, the presence of Lewis acidic centers induces the formation of metallic cations (Fig. 7(a)) with strong electronic withdrawing capabilities against oxygenates. Therefore, the additional interfaces are often found to promote tandem catalysis for C–H and C–O bond cleavage during glycerol conversion. Interestingly such combination is often useful for enhanced oxidation reactions, therefore there should exist alternative mechanism for metal–Lewis acid catalyzed C–H cleavage.

Nevertheless, the presence of Lewis acidic sites also contributes to dehydration and structural rearrangement reactions. This is the main advantage for metal–acid systems over bimetallic catalysts. However, to our best knowledge, such hybridized materials have not been utilized in dehydration of lactic acid or one-pot oxidation–dehydration of glycerol to AA. Existing studies are still focused on electronic coupling enhancement for C–H cleavage reaction.

2.1.3 Brønsted/Lewis acid interface. Growing trends to substitute metallic catalysts with nonmetallic catalysts have motivated researchers to explore Brønsted acids as effective alternatives. As a matter of fact, synthesis of predominant Brønsted acidic sites is difficult as most acidic materials have both Brønsted and Lewis acidic sites.^47–49 Interestingly, such formation is beneficial for tandem C–H cleavage and dehydration reactions during glycerol conversion to lactic acid.

Brønsted/Lewis acid interface is crucial for dehydration of lactic acid to AA, which will be heavily discussed in later sections. Brønsted acids can exchange metallic cations during activation of lactate molecules, generating stabilized intermediate species, while Lewis acid can promote formation of C=C bond under elevated temperatures.^50–58

We should not neglect the band gap shift induced by the presence of heteroatoms, such as non-metallic P, B, S as well as metal cations in modified Lewis acid catalysts.⁵⁹ The presence of impurities in metal oxides (often characterized as Lewis acids) upshifts Fermi levels (Fig. 7(b)), and generates more reactive surface due to enhanced electronic conductivity of semiconductor materials.

2.2 Interfacial catalysts for glycerol conversion to lactic acid

Synthesis of lactic acid from glycerol involves catalytic C–H bond cleavage as key reaction step (see Fig. 4(a)),^27,28,32 which has been recognized extensively. There exist some controversies over the role of alkaline promoters during C–H bond cleavage reactions.^31,60–65 To our best knowledge on experimental and computational studies, alkaline species act as catalysts for C–H cleavage, under hydrothermal conditions. However, when reaction temperature is lower than 250 °C, alkaline promoters have often been to be effective in facilitating metal-catalyzed C–H cleavage reactions.⁶⁶

2.2.1 Bimetallic catalysts. It is not surprising to find in literature that, Pt and Au are most commonly used materials for dehydrogenation of glycerol to glyceraldehyde.²³ In general, many bimetallic catalysts show enhanced activity for C–H bond cleavage of glycerol to glyceraldehyde. Examples include supported PtAu, PtCo and PtSn catalysts.⁶⁶

2.2.2 Metal–acid composites. For non-noble metal catalysts, Cu-based materials seem to outplay other candidates, mainly in terms of selectivity. According to literature survey, immobilization of Cu species on metal oxides (metal–acid interface) and low dimensional supports, and morphological manipulation are widely reported in existing literature. Examples include two-dimensional CuNi on layered double hydroxides (LDH), Cu/LDHs, Cu/CoO and Cu/graphene oxide.⁶⁷ Overall high lactic acid selectivity of 90+% can be achieved over Cu catalysts under inert conditions.

2.2.3 Brønsted/Lewis acid composites. Lewis acid promoters have also been found to be effective for facilitating tandem C–H cleavage and Cannizzaro rearrangement reactions (Fig. 4(b)).^13,23 Mixed metal oxide with dual-acid interfacial catalysis play key roles in tandem conversion of glycerol to lactic acid. The most promising example presented in literature is heteropoly acid.⁶⁸ However, it is necessary to point out that, limited attention has been paid to base-free synthesis.⁶⁹ As Lewis acids and relevant hybridized materials can effectively facilitate tandem oxidation, dehydration and Cannizzaro rearrangement reactions, increasing research efforts should be dedicated to this area to obtain environmental ways of producing lactic acid.⁷⁰

2.3 Interfacial catalysts for dehydration of lactic acid to acrylic acid

The first case on dehydration of lactic acid to AA was dated back to 1960s using metal phosphates and sulfates as catalysts.^2,33 Even till today, extensive efforts have been put on developing more efficient phosphate and sulfate-based materials for enhanced performances on dehydration of lactic acid.^50–58 But good selectivity toward dehydration against coking is still difficult to achieve to satisfy the need of commercial implementation.

2.3.1 Cation mediated acidic catalysts. Up to today, main research focus has been primarily focused on modified zeolite and hybridized phosphate materials with remarkable performances (Fig. 5), examples of which include Co/Cu/Fe modified beta zeolites,¹⁵ alkali/alkaline earth modified beta zeolite,^22,71,72 LiNO₃ modified ZSM-5,⁷³ Na₂HPO₄ and rare earth metal cation modified NaY,^47–49 phosphate and sulfate modified MCM-41,⁷⁴ NaNO₃ modified SBA-15,⁷⁵etc.^33–35 Ionic liquids have also been studied for this reaction.^76–78 Clearly, modification of interfacial electronic coupling effect, between cation–silica, cation–cation and cation–phosphate/sulfate is important to enhance chemoselectivity for dehydration reactions over other side reactions.

The general strategy for synthesizing effective dehydration catalysts involves modification of surface Brønsted and Lewis acidity/basicity. Because, according to mechanistic study, strong acidic sites often induce decarbonylation and coke formation above 370 °C, leading to irreversible deactivation of catalysts and poor product yield. Modified phosphates and sulfates are also among most popular candidates, as altering the stoichiometry of the crystal often leads to the formation of vacant O or cationic sites.

3. Synthesis of lactic acid from glycerol: chemistry & catalysis

Catalytic conversion of glycerol to lactic acid is achieved through oxidation in the presence of O₂, or non-oxidative dehydrogenation in aqueous medium.^79–83 This area has been extensively reviewed in the past decade, as it provides an important alternate to replace glucose-based biological conversion route (Fig. 2 and 8).^{7,23,29–31,66,84} As already mentioned, catalytic dehydrogenation (C–H bond cleavage) of glycerol to lactic acid generates H₂ as a valuable co-product. Current efforts are clearly classified into two categories, (a) continuing development of active and durable noble and non-noble catalysts for dehydrogenation reactions in alkaline medium, (b) catalyst and solvent engineering in exploring more effective C–H cleavage and subsequent structural rearrangement in base-free medium.^32,85–88

Fig. 8 Oxidation of glycerol via hydrothermal, oxidation and non-oxidative dehydrogenation routes (permission obtained from ref. 60).

3.1 Noble metal catalysts

Rational control of desorption step of glyceraldehyde is key for tunable selectivity for lactic acid against glyceric acid (a deep oxidation product).^89–91 Experimental work has indicated that, formation of glyceric acid from glyceraldehyde is a competitive reaction with isomerization of glyceraldehyde to dihydroxyacetone, which is the precursor for lactic acid. Current efforts have been focused on the influence of metal composition on tunable C–H cleavage selectivity. Therefore, the fundamentals on tuning adsorption strength of glyceraldehyde on metallic sites, and isomerization rate over acidic sites are crucial for enhanced selectivity towards lactic acid.^36–38 Unfortunately in most cases, base promoters are still needed for enhancing isomerization pathway.^92–94 Lewis acids, such as oxidized carbon, carbon nitride, TiO₂, can also play an important role, because the electronic withdrawing capabilities of those support could alter electronic density over Pt and Au sites, thus shifting reaction pathways to lactic acid.^94–96

3.1.1 Metal–acid composites. Metal–support (Lewis acid) interaction is key to enhance C–H cleavage of glycerol. Representative examples for Pt and Au catalysts are presented in Table 1. The electronic interaction between Pt and supports was also investigated by Yang and Jin recently (Fig. 9(a–h)).⁶⁰ They found that the electronic coupling effect between PtCo clusters and CeO₂ support contributes to red shift of binding energy for Pt species, with two-fold enhancement in activity for dehydrogenation of glycerol. Co and CeO₂ both play as electronic mediators for Pt sites. As a result, electronic density at Pt sites was enhanced in the presence of Co₂⁺ and Ce³⁺. The selectivity for lactic acid is about 80% with 1,2-propanediol (1,2-PDO) and ethylene glycol (EG) as main co-products at 90+% conversion level of glycerol. In another study, Pescarmona and colleagues prepared near atomically dispersed Pt⁰/Pt⁴⁺/Pt²⁺ catalysts on ZrO₂ supports (Fig. 9(l)). Such material displays remarkable conversion (96%) and lactic acid yield (95%) at 160 °C and 2.0 MPa N₂.⁹⁷ Clearly, the Lewis acidic properties of ZrO₂ support promote dispersion of Pt elements with electronic deficient active centers. They optimized valence states of Pt nanoparticles, and found that more Pt⁰ over Pt⁴⁺/Pt²⁺ components favor activity enhancement with almost unchanged selectivity towards lactic acid. The remarkable selectivity of lactic acid is possibly due to the addition of H₂ scavenger (cyclohexene). Because H₂ formed in situ might also facilitate transfer hydrogenolysis thus formation of lactic acid could be restrained. The presence of cyclohexene consumes H₂ generated from C–H cleavage of glycerol, to enhance selectivity towards lactic acid. Catalyst stability still needs to be improved in this area.

Fig. 9 Catalyst structures for conversion of glycerol to lactic acid, (a–h) electronic coupling in Pt–Co/CeO₂ hybrids,⁶⁰ (i and j) PtAu@Ag nanoparticles,⁹⁸ (k) AuPt//H-beta zeolite,⁹⁹ (l) Pt/ZrO₂ catalyst.¹⁰⁰

Table 1 Performances of noble metal nanocatalysts for glycerol conversion to lactic acid

#	Catalyst	Reaction condition	TOF (h^−1)	X (%)	S (%)	Feature	Ref.
1	AuPt/USY	160 °C	—	80	60	Twinned particles	ref. 99
		0.3 MPa O2 (RT)				2–4 nm
		Base free
2	PtCo/CeO2	200 °C	1534	82	80	Electronic coupling in Pt–Co	ref. 60
		1.0 MPa N2 (RT)
		NaOH
3	Pt/ZrO2	160 °C	995	96	95	Pt^0/Pt^4+/Pt^2+ formation due to electronic withdrawing of ZrO2	ref. 97
		2.0 MPa N2
		NaOH
4	PtAu@Ag	0.5 M KOH, N2	—	100	18	Displacement of Ag with Au and Pt creates more surface defects	ref. 98
5	AuCu/CeO2	250 °C		87.2	76.5	AuCu nanorod	ref. 91
		H2
		NaOH

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3.1.2 Bimetallic catalysts. For compositional and morphological control, Heeres and colleagues reported AuPt/USY catalysts for direct glycerol oxidation to lactic acid in base-free medium.⁹⁹ They found that the formation of bimetallic AuPt nanoparticles actually induces twinned boundaries, which could be active sites for oxidation reactions. The influence of surface acidity on the synergistic performances needs further attention in this field.

3.2 Non-noble metal-based catalysts

Inexpensive materials include heteropoly acids,¹⁰¹ ZrO₂,¹⁰² ZnO,¹⁰³ Cu-based metal oxides^104–106 and bimetallic nanoparticles,^107–110 Co-based catalysts,^111,112 Ni-based catalysts,^113–115 alkaline metal oxides.¹¹⁶ The main strategy for designing non-noble catalysts includes manipulation of structural morphology and incorporation of metal with Lewis/Brønsted acidic/basic sites.

3.2.1 Bimetallic catalysts. Recently, Ni and Cu based metal catalysts are among most popular candidates in literature (Table 2). While Cu materials display high chemoselectivity for C–H bond cleavage, the choice of selecting Ni-based catalysts was inspired by their dual functionalities for both dehydrogenation combined with remarkable activity in catalyzing hydrogenation reactions as significantly cheaper alternatives to noble metals.¹¹⁵ Pescarmona and colleagues found that ceria support tends to atomically disperse bimetallic NiCo species thus catalytic performances can be enhanced. The remarkable selectivity towards lactic acid over NiCo catalysts is partially due to the presence of H scavengers in the system.¹¹⁵ Unfortunately, most current studies have not paid sufficient attention to the electronic coupling behaviors for non-noble metal catalysts.

Table 2 Performances of Ni and Cu catalysts for synthesis of sodium lactate

#	Catalyst	Reaction condition	TOF (h^−1)	X (%)	S (%)	Feature	Ref.
1	NiCo/CeO2	160 °C	13.3	91	85	Atomically dispersed NiCo	ref. 115
		2.0 MPa N2
		NaOH
2	CuBr2	155 °C	—	72	90	Nanobranch CuBr2	ref. 109
		NaOH
3	Cu	230 °C	—	71	69	Cu nanoparticles	ref. 117
		NaOH
4	CuO/ZrO2	180 °C	—	67	85	Weak Cu^2+ and ZrO2 interaction	ref. 118
		NaOH				CuO particles enters the pores of ZrO2 support

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3.2.2 Metal–acid composites. Cu-Based materials are regarded as one of the most promising inexpensive candidates, because of the poor activity for C–O cleavage but high chemoselectivity for C–H bond rupture. It seems that current interests on developing Cu catalysts, are focused on tuning Cu-promoter/support interaction. For example, Li and colleagues reported branched CuBr₂ catalysts with about 90% selectivity towards lactic acid during glycerol conversion.¹⁰⁹ The morphology is key for tunable Cu–Br interaction. Zavrazhnov and colleagues conducted vigorous kinetic modeling on Cu catalyzed glycerol dehydrogenation in base medium.¹¹⁷ They found that glycerol molecule and NaOH tend to be activated in different mechanism, thus a combined Power law, Eley–Rideal and Langmuir–Hinshelwood model best represent the trend in experimental studies. In another study, Yin and colleagues investigated the kinetics of Cu catalyzed glycerol hydrogenation to lactic acid in base medium.¹¹⁹ They found that the electronic transfer between Cu and ZrO₂ support is critical for tunable reaction rates. Liu and Dong also observed similar electronic transfer effect for CuO/ZrO₂ catalysts.¹¹⁸ Manfro and colleagues studied Cu–MgO interaction and confirmed that electronic donating properties of MgO stabilize CuO particles.¹²⁰ Although Cu dispersion is as low as <10%, the strong electrostatic force enable excellent performances in continuous production of lactic acid (selectivity: 85% at 180 °C).

3.3 Synthesis of lactic acid in base-free medium

Currently, fermentative production of lactic acid has the advantage of very high enantiomeric purity of the product. But the need for disposing calcium sulfate co-produced in equal amounts with lactic acid, has posed a major drawback of existing biotechnological processes. Therefore, chemical synthesis of lactic acid in base-free medium show both technological and economic advantages over the biological route. Two main classes of hybridized catalyst materials, including heteropoly acids and metal–acid hybrid materials, will be critically discussed in this part.

3.3.1 Brønsted/Lewis acid/base. Wang and colleagues reported fabrication of trifunctional polyoxometalate materials for the formation of lactic acid from glycerol in neutral environment (Fig. 10).⁶⁸ They used lysine and H₃PMo₁₂O₄₀ to form hybrid materials, which incorporate acid–base pairs in catalyst structures. Experimental results confirmed that the activation energy for acid catalyzed C–H bond cleavage is approximately 120–149 kJ mol⁻¹, while basic sites is critical for promoting dihydroxyacetone dehydration to pyruvaldehyde (E_a: 27–36 kJ mol⁻¹). The same group also reported graphene oxide supported lipid hybridized H₃PMo₁₂O₄₀ catalysts for glycerol conversion to lactic acid.¹²¹ Remarkable TOF values in the range of 200–258 h⁻¹ and 80% selectivity to lactic acid (conversion: ∼90%) were documented at 60 °C. The synergy between acidic and basic sites contribute to the enhanced activity during glycerol conversion.

Fig. 10 Heteropoly acid catalysts. (a) Self-assembly of lysine and H₃PMo₁₂O₄₀ to hybrid micelles (permission obtained from ref. 68), (b) synthesis of graphene oxide supported H₃PMo₁₂O₄₀ catalysts (permission obtained from ref. 121).

The authors also ascribed the unique performances with tunable hydrophobicity of graphene oxide. The surface hydrophobicity also influences the uptake of glycerol and O₂ molecules, which leads to tunable activity for glycerol oxidation in neutral medium. They modified the surface properties by varying lipid content on graphene support. Medium hydrophobicity with water contact angle of 90° show maximum uptake of glycerol and O₂ molecules and best catalytic activity among other lipid-doped materials (conversion of glycerol: 97%, selectivity of lactic acid: 93%, Fig. 11). Therefore, it is clear that proper hydrophobicity could enhance adsorption of both reactants on catalyst surface, thus enhancing the reaction rates for activation of C–H and O=O bonds. For surface characterization using IR also demonstrated that the formation of covalent amide bonds between ethyl and carboxylic groups is key for enhanced oxidation performances of polyoxometalate catalysts.

Fig. 11 Hydrophilicity of Ag modified H₃PMo₁₂O₄₀ catalysts (refer to ref. 122 for more details, permission obtained from ref. 122).

3.3.2 Metal–acid composites. Huang and Wang correlated Brønsted and Lewis acidity of Ag modified H₃PMo₁₂O₄₀ catalysts for glycerol oxidation. It was found that Brønsted acid facilitate oxidation of glycerol to dihydroxyacetones and pyruvaldehyde, while balanced Brønsted and Lewis acidity contributes to the formation of lactic acid.¹²² The concept of metal–Lewis acid pair catalysts was also studied by Shishido and colleagues for Pt/Nb₂O₅ catalysts (Fig. 12(a)).¹²³ The acidic sites were measured by layer structured Nb₂O₅ materials. They found that layered Nb₂O₅ has larger amounts of Lewis and Brønsted acid sites per weight than conventional Nb₂O₅ and Nb(OH)₅. The strong dual-acid materials promote dehydration and rearrangement reactions during glycerol conversion. The experimental results at different temperature further confirm that, rearrangement of pyruvaldehyde in neutral condition needs to overcome much higher activation barriers, because conversion of dihydroxyacetone at 90 °C does not give lactic acid as the main product. But glycerol conversion at 150 °C show 80% selectivity toward lactic acid in one pot. Although detailed information on metal–acid interaction is still not clear, the tandem catalysis at metal and acid interface is key for improved yield of lactic acid in alkali-free medium.

Fig. 12 TEM images of solid catalysts for tandem glycerol conversion to lactic acid. (a) Layered Nb₂O₅ catalyst (permission obtained from ref. 123), (b) nanobranched CuBr₂ catalysts (permission obtained from ref. 109), (c) AuPd/CNT catalyst (permission obtained from ref. 124), (d) Au–Sn-USY catalysts (permission obtained from ref. 69), (e) Sn-MCM-41 (permission obtained from ref. 70), (f) Au/CuO catalyst (permission obtained from ref. 125), (g) Ag/Al₂O₃ catalyst (permission obtained from ref. 126).

3.4 Oxidation of glycerol to alkyl lactate

Crude glycerol is produced with approximately 30–60% water. Most attention is being paid to conduct oxidation in aqueous medium. Conversion of glycerol in neutral condition requires sophisticated design of metal–acid pair catalysts, however, often leading to poor chemoselectivity towards lactic acid. One pot conversion of glycerol to lactate esters has attracted increasing attention recently, as production of methyl and ethyl lactate from glycerol actually need formation of lactic acid followed by esterification processes.

This topic is still new in this area. At present, the main strategy for one pot glycerol conversion to alkyl lactate is hybridization of metallic nanoparticles with Lewis acid in methanol or ethanol solvent (Table 3).^127,128 Lewis acidity is necessary to transform pyruvaldehyde into lactic acid. In this context, modified zeolite catalysts are extensively investigated in this area. USY and Beta are most popular candidates for supports as tunable Lewis acidity can be easily achieved by doping Sn and other hetero atoms.^125,129,130 According to the characterization results, the Lewis acidity caused by the presence of Sn and other heteroatoms leads to coordinated intermediate complexes, which would promote isomerization reactions. This is the key that, weak Lewis acidic centers are favorable for the formation of dehydrated species, which are the precursors for lactate in neutral medium. The formation of such coordinated species is critical for improved selectivity for lactate. Noble metals such as Au and Pd are commonly used to provide metallic centers for oxidation step (glycerol to glyceraldehyde), while dehydration of glyceraldehyde and condensation between lactic acid and alcohols occur in the presence of acidic sites.^131–133

Table 3 Oxidation of glycerol to alkyl lactate

#	Catalyst	Reaction condition	TOF (h^−1)	X (%)	S (%)	Feature	Ref.
1	Au/Sn-USY	Methanol	94.6	88	89	Electron sufficient SnOx depress Au nanoparticles	ref. 69
		0.5 MPa O2
		160 °C
2	AuPd/CNT	Methanol	—	55	75	Synergy between AuPd and acidic sites	ref. 124
	Sn-MCM-41	0.6 MPa O2
		140 °C
3	Au/CuO	Methanol	208	74	62	Au and CuO interaction contributes to α-bond activation of O–H group	ref. 70
	Sn-MCM-41	3.0 MPa O2
		140 °C
4	Au/TiO2	Methanol		55	62	—	ref. 70
	Sn-MCM-41	3.0 MPa O2
		140 °C
5	Au/CuO	Methanol	—	86	60	Au–CuO interaction leads to lower Cu^2+ reduction temperature	ref. 125
	Sn-Beta	1.5 MPa O2
		90 °C

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3.4.1 Metal–acid composites. Lu and colleagues studied Au/CuO catalysts with Sn-Beta catalysts for effective oxidation of glycerol.¹²⁵ They used copper acetate as precursor to prepare thin CuO layers. H₂-TPR characterization suggested that Au–Cu²⁺ strong interaction leads to unusual electronic transfer from Au to Cu²⁺. Such interaction weakens Cu–O bond strength, thus reduction temperature of CuO decreases from 330 °C to 279 °C. TEM images further confirmed that such strong electrostatic interaction contributes to finely dispersed Au particles of approximately 3–4 nm. Catalytic conversion of glycerol at 90 °C and 1.5 MPa shows that the synergistic performances induced by Au–Cu²⁺ strong interaction results in a four-fold enhancement of oxidation activity. The synergy between Au and Sn-Beta zeolite is also important, as dihydroxyacetone is the major by-product during oxidation, indicating that isomerization to form pyruvaldehyde is slow in this system (glycerol conversion: 86%, selectivity of lactic acid: 60% at 90 °C).

Based on the above-mentioned experimental results, it is clear that conversion of glycerol to lactate in alcoholic medium also undergoes three steps, dehydrogenation of glycerol, dehydration and structural rearrangement. While the first reaction is conducted over metallic catalysts, the latter two need acidic catalytic materials. Pérez-Ramírez and colleagues investigated the possibility of glycerol oxidation to lactate over one catalyst.¹²⁶ Interestingly, oxidation of glycerol to pyruvaldehyde seems to be independent on Lewis acidity of alumina support. It is the Ag–Al₂O₃ interaction that induces the formation of active Brønsted and Lewis acidic sites for tandem conversion of glycerol. Poor selectivity of lactate is achieved due to low density of Lewis acidic sites on Al₂O₃ surface.

3.4.2 Bimetallic and metal–acid composites. Combining the advantages of bimetallic catalysts with metal–acid pairs could synergistically promote oxidation and dehydration step during glycerol conversion. Pescarmona and colleagues reported AuPd/CNT + Sn-MCM-41 catalyst admixtures for one pot glycerol conversion to methyl lactate (Fig. 12(c)).¹²⁴ They found that well-dispersed bimetallic AuPd catalysts with small particle size facilitate primary oxidation, while surface Brønsted acid group promote dehydration of glyceraldehyde to pyruvaldehyde. Lewis acidic Sn-MCM-41 accelerates rearrangement of pyruvaldehyde with methanol to methyl lactate (glycerol conversion: 88%, selectivity of lactic acid: 89% at 160 °C). Yang and colleagues designed Sn-USY supported Au catalysts. They found that framework Sn leads to formation of Lewis acidic sites in USY structure. Acidity measurement suggested that Au nanoparticles tend to locate at extra-framework of Sn sites, which receive electrons USY. The electron sufficient environment depresses aggregation of Au nanoparticles. The role Sn in the framework of was also studied in details by Huang and colleagues.¹³⁴ They investigated the role of Brønsted and Lewis acidity induced by incorporation of Sn on dehydration of dihydroxyacetone to pyruvaldehyde. It is shown that structural Sn species enhances the amounts of Brønsted acidity, which is beneficial for dehydration of dihydroxyacetone. But Lewis acidic sites are critical for rearrangement of pyruvaldehyde to lactate. NMR analysis demonstrates that extra-framework Sn species contribute to the formation of Lewis acidic sites. Therefore, effective dehydration and rearrangement of dihydroxyacetone can be achieved at 65 °C with 90% selectivity to ethyl lactate in ethanol medium.

3.5 Summary and outlook

Nanostructured and metal–acid hybrid catalysts are among most popular candidates for catalytic synthesis of lactic acid from glycerol. It is important to mention that, global incentive for sustainable conversion technologies for biomass to renewable chemicals has driven the research efforts from synthesis of lactate salts in base medium, towards base-free oxidation of glycerol in either aqueous or alcoholic medium. Undoubtedly, existing efforts for improving production rates of lactate salts in base medium are clearly shifting towards non-noble metal catalysts such as Ni and Cu. However, the durability of these inexpensive materials is not yet to be fully investigated in literature. Based on experimental studies, it indicates that electronic coupling effect between metal and promoters rather than size of metal particles, is key to contributing enhanced performances for C–H bond cleavage.

(i) Base-free synthesis of lactic acid in aqueous medium suggests that heteropoly acids and metal–acid hybrid catalysts show promising performances. Fundamental understanding on the nature of Brønsted and Lewis acidic sites is crucial for successful implementation of this technology in industry. At present, experimental studies on catalyst design are primarily focused on morphological features of metallic sites, with limited attention paid to metal–acid interaction in hybridized materials. Clearly, more research efforts need to be paid on this topic in future studies.

(ii) Another unaddressed issue which has bottlenecked this area for decade, is how to achieve asymmetrical synthesis of L-lactic acid over D-lactic acid, as the former has wider applications in polymer industry. As for chemical synthesis of lactic acid, this issue seems to be difficult to be addressed using conventional solid catalysts. Alternative strategies such as immobilization of bio-active or organometallic species and formation of unique active sites should be considered in this field for future studies.

4. Dehydration of lactic acid to acrylic acid: chemistry & catalysis

4.1 Design strategy for dehydration catalysts

Different from the reaction involving dehydrogenation of glycerol, conversion of lactic acid to AA does not need a metal catalyst to facilitate the formation of C=C bond. The main reactions for conversion of lactic acid include dehydration, decarbonylation and decarboxylation. Decarboxylation of lactic acid to acetaldehyde and CO₂ is the main side reaction. Since the first study in 1958 by Holmen and colleagues, various sulfate, phosphate and modified zeolite based catalysts have been proposed and studied in details.^135–138 Acidic sites are necessary for dehydration reactions, strong acidic sites, however often leads to C–C cleavage side reactions. Therefore, the main focus on the rational design of catalysts is to achieve medium or weak acidity of solid catalysts, in order to get good selectivity towards dehydration rather than decarboxylation products.

Tunable acidity is also crucial for improving catalyst stability. This is because decarboxylation induced C–C cleavage produces coke precursors such as acetaldehyde, which might undergo condensation reactions and eventually form aromatics on acidic sites, completely inhibiting dehydration reactions.^139–141 Therefore, weakly acidic sites seems to be beneficial to minimize the competing reactions that destroy carboxylic group in lactic acid and maintain long-term stability under elevated reaction temperature (e.g., >350 °C). Furthermore, it is important to emphasize that, basic sites are also needed for selective dehydration reaction. Basic sites could potentially protect the carboxylic group, while the acidic sites activate the α-hydroxyl for the dehydration reaction.^19,142–144 This is the underlying reason that, phosphate anions, alkaline, alkaline-earth metal cations are often incorporated into the framework of acidic catalysts.

Therefore, in this section, we will discuss the rational design of dehydration catalysts from the perspective of the nanostructure for acidic/basic sites and molecular catalysis involving C–H and O–H bond activation during dehydration processes.

4.2 Dehydration of alkyl lactate

As already mentioned in the previous sections, oxidation of glycerol in aqueous medium leads to the formation of lactic acid or lactate salts, while lactate esters can be formed in alcoholic medium.^145–149 Direct dehydration of lactate esters represents an atomic economical way to produce acrylic esters. Alkali–metal-exchanged zeolite materials, such as NaY is among the most extensively investigated materials for dehydration of methyl lactate to methyl acrylate.^136,150,151 Modified NaY materials show improved performances, as metallic cations often alter acidic centers thus reaction pathways for decarbonylation and coking can be restrained.¹⁵² Notably, the use of lactate esters as substrates also avoid the unwanted self-oligomerization by induced by lactic acid.¹⁵³ Mechanistic studies demonstrated that the presence of metallic cations can generate Brønsted acidity in situ, facilitating dissociative adsorption of lactate and generating surface methoxy groups.^153,154 Besides Y zeolites, phosphate salts are often hybridized with metal oxide to enhance the dispersion of dehydration sites (see Table 4 for summary of catalyst performances).^155,156

Table 4 Oxidation of glycerol to alkyl lactate

#	Catalyst	Reaction condition	TOF (h^−1)	X (%)	S (%)	Feature	Ref.
1	Poisoned NaY	330 °C	—	38	95	Poisoned Brønsted acidic sites by pyridine	ref. 153
		30 wt% aqueous ML
2	NaY	330 °C	80	∼13	50	Significant decarbonylation on NaY	ref. 157
		30 wt% aqueous ML
3	NaH2PO4/SiO2	340 °C	—	80	29	Chain of polyphosphate key for selectivity	ref. 156
		Pure ML
4	K2HPO4-Al2(SO4)3/MCM-41	410 °C	—	98	68	Formation of K2SO4 and AlPO4 phase on MCM-41	ref. 74
		60 wt% aqueous ML
5	CaPOH	380 °C	—	100	35	–OH in polyphosphates as active sites	ref. 159
		30 wt% aqueous ML

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4.2.1 Modified zeolites (medium/weak acid composites). Early studies on dehydration of lactate were focused on tunable surface Brønsted and Lewis acidity by introducing various sodium and potassium salts into the framework of zeolites. Huang and colleagues studied the influence of various KX (X: Cl, Br, I) on surface acidity and basicity.¹⁵¹ They found that KI/NaY materials display the strongest basicity among other samples, because loss of I⁻ species during calcination promotes the formation of basic K₂O species. Such active sites interact strongly with O species and form a cyclic transition state with lactate molecule.¹⁵¹ As an alternative reaction mechanism, in situ FTIR studies in literature also revealed that the presence of cations such as K⁺ and Na⁺ mediated the Brønsted acidity and inhibited decarbonylation pathways by forming relevant lactate salts on catalyst surface.^150,151,157

The strong hydrophilicity of parent and modified NaY materials may have detrimental influence on chemoselectivity and catalyst stability. The formation of H₂O during dehydration may block the weak Lewis acidic sites thus inhibiting further turnovers, in particular with the fact that lactate esters are much more hydrophobic in comparison with sodium lactate (Fig. 13(a)).¹⁵⁴ Based on the experimental studies, it is possible that, weak Lewis acidic sites may selectively promote dehydration reactions, while strong Lewis and Brønsted acidity could accelerate decarbonylation, proton transfer and coke formation due to strong hydrophilicity nature. The presence of Na⁺ and K⁺ is important to prevent the reaction pathways for the formation of hydrophobic site due to strong adsorption of ester species.

Fig. 13 (a) Proposed mechanisms for dehydration and decarbonylation of methyl lactate (redrawn from ref. 153 and 154), (b) formation of PO₄ species (redrawn from ref. 156), (c and d) structure of CaP hydroxyapatite catalysts for dehydration and decarbonylation reactions (redrawn from ref. 138).

4.2.2 Phosphates (cation-acid composites). Clearly, traditional approaches using NH₃ and CO₂ desorption measurement attempting to finely tune surface properties, cannot reveal the nature of reaction on the level of molecular catalysis.¹⁵⁸ To overcome the intrinsic issues caused by the blockage of water molecules, Loridant and colleagues prepared alkaline earth phosphates as inexpensive catalytic materials for conversion ethyl lactate.¹⁵⁹ Surface characterization show that orthophosphates and pyrophosphates leads to weakened PO–H bond. Experimental studies confirm that alkaline earth mono and dihydrogenophosphates or polyphosphates are active sites for dehydration reactions. Such unique structural properties tend to enhance hydrolysis of ethyl acrylate into AA and ethanol, over in situ formed Brønsted acidic sites, preventing polymerization (coke formation) and C–C cleavage side reactions. With regard to the possible rate limiting step for conversion of alkyl lactate, the formation of sodium lactate in situ is not believed to be the rate limiting step. Acid catalyzed dehydration or desorption of acrylate are highly possible to be the slowest step, although the dynamic equilibrium between sodium and alkyl lactates determines the chemoselectivity toward AA or ester.¹⁵⁴

The nature of Na⁺ and phosphate anions for activation of C–OH groups was studied by Wang and colleagues.¹⁶⁰ Proton transfer involved between the reaction molecules and the active sites is supposed to be confined to the terminal P–OH and P=O groups of the terminal (HO-PO₂Na) units. It is strongly believed that the terminal and internal P–OH groups could be two key sites for H transfer reactions. The “methoxyl” further donates one proton to the P=O group and dissociates from the catalyst surface, which regenerates the active site and leads to the formation of acrylate. Wang and colleagues also found that the loading of NaH₂PO₄ influence the surface acidity on SiO₂, because the extent of condensation of phosphate determine the surface ratio of silica surfaces, which could catalyze lactate into decarboxylation products (Fig. 13(b)).¹⁵⁶ This is because strong acidity of phosphate promotes C–C cleavage rather than dehydration reactions. This is the main reason that excess Brønsted and Lewis acidity is not favorable for dehydration reactions. Excess coverage of phosphate species however could lead to poor surface area and pore volume.

Wang and Tan studied K₂HPO₄ and Al₂(SO₄)₃ modified MCM-41 materials. The two precursors react with each other and form AlPO₄ and K₂SO₄ phases on the surface of support.^74,161 They found that the acid/base density is strongly dependent with K₂HPO₄/Al₂(SO₄)₃ ratio. Reaction between the two precursors generates H⁺ as strong Brønsted acidic species, and formation of K₄P₂O₇ through condensation as medium basic sites. The presence of AlPO₄ and K₂SO₄ phases alleviate the total acidity and basicity. The optimal ratio for acid/base sites is 1.9, which are different from other reports, indicating dehydration of methyl lactate involves cation induced formation of methyl group and lattice oxygen facilitated H transfer reactions. Therefore, the activation energy might be reduced on the surface of bifunctional materials. Chang, Halligudi and Han reported Ca₃(PO₄)₂–Ca₂(P₂O₇) nanocomposites for dehydration of methyl lactate (Fig. 14(b)).⁵⁶ Different from zeolite or silica supported phosphates, Ca₃(PO₄)₂–Ca₂(P₂O₇) show both acidity and basicity. This could mediate some of the side reactions due to the uncontrollable formation of acidity sites on catalyst surface. They found that ratio of Ca₃(PO₄)₂/Ca₂(P₂O₇) affects the acidity–basicity, which is key for enhanced properties of dehydration reactions.

Fig. 14 (a) NaY catalyst (permission from ref. 47), (b) Ca₃(PO₄)₂–Ca₂(P₂O₇) nanocomposites (permission from ref. 56), (c) Na₂HPO₄ modified NaY catalyst (permission from ref. 21), (d) lanthanum phosphate nano-rods (permission from ref. 50), (e) hydroxyapatite catalyst (permission from ref. 139).

4.3 Dehydration of lactic acid

4.3.1 Modified zeolites (weak acid/base pairs). Direct dehydration of lactic acid or sodium lactate have also been widely studied over modified acidic catalysts. The most general strategy for improving chemoselectivity toward dehydration rather than decarbonylation and decarboxylation is doping foreign ions to modify the nature of acidic sites in zeolites.^{48,49,52,73,162,163} Performances of various solid catalysts for dehydration of lactic acid are presented in Table 5. For example, Zhang and co-workers studied phosphate doped ZSM-5 zeolite for 97% conversion of lactic acid to 78% selectivity of AA at 350 °C.¹³⁵ It is interesting to find that, while NaOH treated ZSM-5 materials show poor surface area due to pore blockage. But subsequent impregnation of Na₂HPO₄ hardly causes substantial damage to the structure of ZSM-5. Structural analysis showed that surface acidity has been mediated owing to the presence of Na₂HPO₄ species. However, experimental results showed that selectivity of AA is not dependent with the modified ratio of medium/weak sites. New mesopores were also created by NaOH treatment, which may favor the incorporation of Na₂HPO₄ into the inner wall. Therefore, the chemoselectivity has been shift from more decarbonylation products to AA (dehydration). But the underlying reason still need further experimental investigations. The role of Na₂HPO₄ species on modifying surface acidity and basicity has also been investigated in details by Ji and Au.^21,47 The presence of Na₂HPO₄ species on NaY surface almost completely remove acidity sites. In addition, the presence of Na⁺ on the surface facilitate the in situ generation of sodium lactate during dehydration step, thus internal –OH and terminal C–H group can be activated on relatively basic sites. As a result, selectivity for acetic aldehyde is minimized from 22% to <5%, with increased AA formation from 35.5% to 71% at 340 °C.

Table 5 Dehydration of lactic acid to AA

#	Catalyst	Reaction condition	X (%)	S (%)	Feature	Ref.
1	Phosphate doped ZSM-5	350 °C	97	78	Phosphate immobilized within mesopores	ref. 135
		30 wt% aqueous LA
2	Ba/NaY	370 °C	100	45	Medium basicity key for high selectivity	ref. 142
		38 wt% aqueous LA
		LHSV: 3 h^−1
3	KCaNa/NaY	350 °C	100	83	Ion exchange of Ca^2+ with proton leads to formation of CaCO3 during catalyst regeneration	ref. 19
		20 wt% aqueous LA
		LHSV: 3 h^−1
4	K/NaY	320 °C	99	50	K^+ as electronic modulator	ref. 22
		29 wt% aqueous LA
		LHSV: 3 h^−1
5	Na2HPO4/NaY	340 °C	93	77	Na2HPO4 effectively remove acidity sites	ref. 47
		34 wt% aqueous LA
6	Na2HPO4/NaY	340 °C	78	72	HPO4^2− adjacent to Na^+ interact with OH entities	ref. 21
		34 wt% aqueous LA
		LHSV: 2.27 h^−1
7	KNa/ZSM	360 °C	96	80	Charge compensation of K^+ in AlO4 anions modify surface basicity	ref. 20
		36 wt% aqueous LA
		WHSV: 2.1 h^−1
8	K/ZSM-5	365 °C	95	76.5	Balanced Lewis acidity/basicity	ref. 72
		40 wt% aqueous LA
		WHSV: 2.1 h^−1
9	CsNa/Beta-Y	360 °C	97	70	Reduction of acidity by Cs^+ and Na^+	ref. 71
		35.7 wt% aqueous LA
		WHSV: 2.1 h^−1
10	NaNO3-SBA-15	370 °C	91	41	NaNO3 (104) and (113) facets	ref. 75
		34 wt% aqueous LA
11	Hydroxyapatite	350 °C	∼90	65–73	Aliphatic and aromatic growth of coke on catalyst surface	ref. 139
		40 wt% aqueous LA
12	Hydroxyapatite	375 °C	100	60	Ca/P ratio determines morphology and selectivity	ref. 138
		50 wt% aqueous LA
		WHSV: 3 h^−1
13	Hydroxyapatite	375 °C	100	74	Both Na^+ content and Ca/P ratio determine morphology and selectivity	ref. 55
		50 wt% aqueous LA
		WHSV: 3 h^−1
14	Hydroxyapatite	360 °C	74	62	Ca/P ratio determines the ratio of acidity/basicity and selectivity	ref. 33
	(Ca(NO3)2–(NH4)2HPO4	35.7 wt% aqueous LA
		WHSV: 2.1 h^−1
15	Hydroxyapatite doping Na^+	350 °C	90	78	Na^+ and Ca^2+ as charge compensating pairs for H^+	ref. 35
		38 wt% aqueous LA
16	Hydroxyapatite	380 °C	76	45	—	ref. 143
	Ca10(PO4)6(OH)2Sr10(PO4)6(OH)2	38 wt% aqueous LA
17	Hydroxyapatite	400 °C	99.7	76	—	ref. 51
	Ba2P2O7	38 wt% aqueous LA
18	Sr3(PO4)2 + H3PO4	400 °C	90	72	Sr3(PO4)2 + H3PO4 show acid–base pair features	ref. 144
		20 wt% aqueous LA
19	BaSO4	400 °C	100	82	Acidity/basicity ratio: 5/7 show optimal performance	ref. 165
		20 wt% aqueous LA

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It is generally accepted that phosphate anions can act both Lewis acidic and basic sites for liberating water molecules during dehydration reactions. But alkali- and alkaline earth-metal cations modified zeolite catalysts often exhibit different kinetic behaviors for activation of lactic acid. This is because that cations tend to coordinately modify the preexisting Brønsted acidic sites by altering polarity of adjacent Al³⁺ in framework. The relatively large cation replaces H⁺ and Na⁺ in the framework is enthalpy favorable. But such modification unavoidably changes the surface electronic density at lattice O. Therefore, Huang and colleagues also demonstrated that the nature of cation species is also critical for improving chemoselectivity for dehydration reactions. They modified NaY with various alkaline earth metal ions including Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺.¹⁴² They have ascribed the varied performances for dehydration of lactic acid to surface basicity after incorporating those cations. The amphoteric properties of lattice oxygen determines the nucleophilic nature of catalysts. CO₂ desorption profiles indicated that medium basicity sites (240–320 °C) should be the main active sites for the formation of AA. Since oxygen sites provide main basicity, it is found that cation-oxygen distance increases with cation radius: Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺. In addition, the presence of cations with much larger radius than H⁺, could modify the electronic density of framework O close to the extra-framework cations, inducing weakened bonding between lactic acid and acidic sites.¹⁹ Therefore, the modified chemisorption properties of zeolite catalysts show preferably good selectivity for dehydration reactions rather than decarbonylation reactions catalyzed by strong acids.

Xu's group studied the influence of alkaline cations on surface basicity of ZSM-5 and beta zeolites (Fig. 15(a)).^20,71 They also confirmed that the presence of relatively larger cations such as K⁺ improves both conversion and selectivity for AA. The presence of K⁺ could interact with surface silanol group and adsorbed water, leading to displacement of protons with K⁺, contributing to much higher basicity compared with original silanol sites. This is the main difference between silica supported weak Lewis acidic catalysts and phosphate materials. However, one might ignore the fact that the type of cages in aluminsilicates also affects the charge compensation when alkaline cations are presents in the framework.²⁰ Therefore, a balance should be achieved for weakly basic sites and sufficient charge compensation in AlO₄⁻ anions to maintain good selectivity toward dehydration rather than C–C cleavage. They also studied the influence of electrostatic potential of Cs⁺ and Na⁺ modified beta zeolite on acidity/basicity and chemoselectivity of AA. Overall medium and weak acidic and basic sites are beneficial for selective formation of AA.

Fig. 15 Influence of surface properties on catalytic performances for dehydration of lactic acid. (a) Influence of electrostatic potential on acidity/basicity and selectivity of AA (permission obtained from ref. 71), (b and c) surface ratio of acidity/basicity for hydroxyapatite on activity and product selectivity (permission obtained from ref. 33). (d and e) Acidity/basicity ratio on product selectivity KNa-Beta and KNa-ZSM-5 zeolites.

Similar observation was also achieved by Chen and colleagues.⁷² In their work, ZSM-5 zeolites were modified with alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺). As radius of cations increases, acidity of ZSM-5 decreases significantly. Among all modified ZSM-5 samples, the surface basicity only increases slightly when doping alkali metal cations. KZSM-5 catalyst displays 91% conversion of lactic acid at 350 °C with 70.3% AA selectivity. Compared the acidity and basicity before and after ion exchange with alkali metal cations, it is found that Brønsted acidic sites almost completely disappear, while Lewis acidity and basicity are enhanced slightly. Based on the above mentioned studies, it is clear that the strong adsorption of lactate species could potentially shift the dynamic equilibrium towards kinetically unfavorable state,¹⁴¹ when electronic reconfiguration of lactic acid molecule inhibit decarbonylation due to weakened acidic sites.

Most experimental studies have been focused on dehydration of commercial lactic acid rather than crude lactic acid. Jaimes and colleagues investigated the performances of several modified NaY catalysts for conversion of lactic acid derived from sugarcane molasses.¹⁶⁴ Lactic acid with 152 g L⁻¹ concentration was derived for dehydration reactions. KBr/NaY, KI/NaY and Ca₃(PO₄)₂/NaY display different performances, where KI/NaY and Ca₃(PO₄)₂/NaY both show good tolerance to impurities in crude lactic acid. KI/NaY catalyst show a 90% conversion of lactic acid and 15% selectivity for AA.

4.3.2 Modified mesoporous catalysts (Brønsted/Lewis acids/bases). For neutral silica materials, doping alkali cations has also been widely accepted for an effective strategy. This is because post-treatment of impregnated alkali-doped silica enable generation of strong basic oxide species such as Na₂O and K₂O for dehydration reactions.⁷⁵ Previous results have already confirmed that sodium ion in catalyst structure can be transferred to lactate intermediates to facilitate dehydration reactions.

4.3.3 Modified hydroxyapatite catalysts (Lewis acids). Similar to Ca₃(PO₄)₂–Ca₂(P₂O₇) materials, where both acidity and basicity coexist, calcium hydroxyapatite materials often contain both weak acidic and medium basic sites in crystal phases, therefore regarded as promising candidates for dehydration reactions. Because weak acidity and medium basicity could coordinately accelerate dehydration reactions. In a typical stoichiometric hydroxyapatite structure, O²⁻ atoms in –OH groups often interact strongly with Ca²⁺ cations to form a triangle structure. However, excess or substoichiometric doping of cations create defected sites to maintain overall charge balance in the distorted framework. Missing cations often lead to oxygen defects in such triangle structure, which display a Lewis acid-type properties. In general, calcium modified hydroxyapatite materials display a Ca/P atomic ratio of 1.67 as single crystal. Therefore, two strategies have often been applied, one involving structural defects with non-stoichiometric cation content, while the other focused altered acid/base ratio due to doping of foreign cations with larger radius.

Alkaline-earth phosphate catalysts materials were studied for dehydration of lactic acid by Loridant and colleagues.¹³⁷ They compared the performances of CaHPO₄ and SrHPO₄ at 170–380 °C. While dilactide is the main product in the absence of solid catalysts, AA, acetaldehyde and CO are main products over the phosphate materials. Similar to the case of alkaline metal cation modified catalysts, IR spectra of used catalysts suggested that there also exists a cation transfer from surface of catalysts to lactate molecule, promoting dehydration reactions. But how to control the purity of phosphate species still remains a grand challenge. In another study, Dongare and Umbarkar showed that the content of Na⁺ and ratio of Ca/P significantly affect the selectivity for dehydration reaction.^55,138 With Ca/P ratio decreasing from 1.9 to 1.3, hydroxyapatite rod morphologies become more favorable. Their studies employed FTIR spectra to probe the surface active sites for C–OH bond activation. Typically, the detection of P–O stretching for PO₄³⁻ species and –OH groups confirm the existence of hydroxyapatite species.¹³⁸ Conversion of lactic acid reached 100% at 375 °C with 60% selectivity for AA. Again, carbonyl stretching peaks for calcium lactate was observed in IR spectra, suggesting that Ca²⁺ interact with lactic acid and form seven member ring to activate internal –OH group in the molecule of lactic acid. Mechanistic studies further show that phosphate groups tend to activate internal –OH while Ca²⁺ active carboxylic group. Therefore, optimal ratio of Ca/P is important.

Clearly, the ratio of Ca/P reflects the surface oxygen deficiency in hydroxyapatite catalysts. Xu's work on hydroxyapatite also demonstrated that the ratio of Ca/P is important to influence the ratio of surface acidity/basicity.³³ The correlated the catalytic activity and selectivity for AA and acetic aldehyde with acidity and basicity. When Ca/P ratio is 1.62, hydroxyapatite catalyst displays optimal performances with 74% conversion of lactic acid and 71% selectivity for AA. The volcano-type dependence for the production rate of AA and acidity/basicity ratio reveals the nature of cooperative acid–base catalysis for selective dehydration.

Matsuura and Yanagisawa reported a series of selective dehydration hydroxyapatite catalysts containing tunable Ca/P ratios.³⁵ They used NaOH as precipitation agent and modified the ration of Ca/P by different precursor concentrations. However, one might ignore the fact that both Na⁺ and Ca²⁺ are present in the structure of hydroxyapatite catalysts. They found that the cooperative catalysis by both of the cations induces the occupation of Na₊ at Ca²⁺ deficient sites. This structural evolution is strongly believed to promote charge compensate interstitial H⁺, which further favors attachment of the proton to internal –OH to form H₂O molecules. Such behaviors are not well reflected in basicity measurement. But acidity profiles agree very well with the ratio of Na⁺/Ca²⁺ in catalysts. The catalyst gave 90% conversion of lactic acid and 78% selectivity for AA, which is almost the highest value for hydroxyapatite-type catalysts.

Onda studied different types of hydroxyapatite catalysts with various cations and anions.¹⁴³ The comparison of Ca₁₀(PO₄)₆(OH)₂, Sr₁₀(PO₄)₆(OH)₂, Pb₁₀(PO₄)₆(OH)₂, Ca₁₀(VO₄)₆(OH)₂ and Sr₁₀(VO₄)₆(OH)₂ catalyst showed that basic catalysts are favorable for conversion of lactic acid. But Ca₁₀(PO₄)₆(OH)₂ and Sr₁₀(PO₄)₆(OH)₂ with balanced acidic and basic sites show much higher yield of AA (conversion: 72–76%, selectivity: 38–45%). The performances of Sr-based phosphate catalysts were studied by Tang and colleagues.¹⁴⁴ They showed that Sr₃(PO₄)₂ catalysts immobilized with small amounts of H₃PO₄ display best performances. It is important to mention that Sr₃(PO₄)₂ materials display weak acidity and poor conversion of lactic acid. But combined Sr₃(PO₄)₂ with H₃PO₄ show both acidity and basicity exhibit 90% conversion and 70% selectivity for AA.

Other phosphate catalysts, including lanthanum phosphates were also synthesized with controlled morphologies for this reaction. Wang and colleagues prepared lanthanum phosphate nano-rods using butylamine as template agent. They found that the concentration of the template has s strong influence on the agglomeration of lanthanum phosphate nano-rod. It is also proposed that butylamine can react with surface P–OH group on the nano-rod. The electrostatic interaction between butylamine and lanthanum phosphate causes the formation of La₂O₃ phase. Such surface coordination also induces electronic transfer between O 2p and La 4f orbitals. Such electronic reconfiguration also leads to tunable acidities. For example, as ratio of butylamine/lanthanum phosphate increases, concentration of acidity sites decrease significantly.

The morphological evolution of lanthanum phosphate catalysts also contributes to enhanced surface basicity. IR spectra suggests that –OH species attached on phosphorous atoms show OH⁻ properties in the lattice, which intrinsically enhance surface basicity for dehydration reactions. Based on the experimental studies on hydroxyapatite catalysts, the cations such as Ca²⁺ and La³⁺ tend to coordinate with lactate species to avoid unwanted activation of carboxylic group for C–C cleavage, thus adjacent acidic –OH group could selectively promote dehydration reactions.^33,138,144

4.3.4 Other novel catalysts. Other anion species such as sulfates also show good promotional effect for dehydration reactions. Tang and colleagues studied the influence of interplanar spacing of (121) on surface acidity and basicity of BaSO₄.¹⁶⁵ They prepared the materials using various precipitation agents such as aqueous ammonia, ethylenediamine and butylamine. The sample prepared by aqueous ammonia show acidity/basicity ratio of approximately 5/7, which show 100% conversion of lactic acid with 82% at 400 °C. The authors ascribed such remarkable performance to lattice defects in (121) facet. The escaped cations are electro-positively charged thus often acting as Lewis acid, while electro-negatively charged vacancies can donate electrons and act as basic sites for catalytic reactions. Low acidic/basic site ratio, although contributing to high conversion of lactic acid, also unfavorably promoting decarbonylation reactions (possibly following alternative mechanism from acid-catalyzed decarbonylation).⁵⁴ Again, this work demonstrated that balanced acidity and basicity is crucial for high yield of AA. The types of precipitation agent, pH values of precipitation and calcination temperature are critical factors altering surface acidity and basicity ratios.

Bouwman and coworkers studied a very interesting system, where catalytic cracking of lactide and poly(lactic acid) is conducted under mild conditions, with 58% yield of AA (Fig. 16).¹⁶⁶ Conventional processes for dehydration of lactic acid is carried out at 300–400 °C, which often induces side reactions such as decarbonylation, thus leading to poor selectivity toward AA. Nucleophile agents such as tetrabutylammonium bromide (TBAB) is used as promoter, with sulfonic acid or 3-bromopropionic acid is used as acidic catalyst and create a water-free environment (130–150 °C).

Fig. 16 Plausible mechanism of degradation of lactide to AA.¹⁶⁶

4.4 Mechanistic studies

Xu and colleagues conducted kinetic modeling and found that the in situ generation of Brønsted acid sites on NaY leading to acetaldehyde and coke formation, also unfavorably causing suppression of activity of Lewis acidity sites for lactate dehydration.¹⁵⁴ The presence of surface H₂O is critical for the generation of Brønsted acid. Future improvement of NaY catalysts for dehydration of lactate should be focused on restraining the in situ generation of Brønsted acid using effective inhibition agents.^157,158 Control studies on surface poisoning of acidic sites with pyridine species suggest that, inhibition of bulky acid/base complexes on NaY surface prevent coking reactions.¹⁵³

Traa and colleagues proposed an alternative mechanism to understand the possible cause for side reactions on alkaline cation modified NaY catalysts (Fig. 17(a)).¹⁴¹ They found that high oxygen charge on NaY surface could induce a considerable number of lattice oxygen sites to act as nucleophile agents. Therefore, the Brønsted acidic sites in the zeolite and protons could form a dynamic equilibrium for dehydration and decarbonylation reactions. Relatively less negatively charged oxygen species could contribute to inhibition of decarbonylation reactions. Highly negative charged oxygen sites tend to activation carboxylic group of lactic acid, where medium strong nucleophilic tends to selectively activation -OH and terminal C–H group in the molecule of lactic acid.

Fig. 17 (a) Dynamic equilibrium of activated lactic acid on acid/base catalysts,¹⁴¹ (b) schematic reaction mechanism for methyl lactate conversion to sodium polyphosphate catalyst,¹⁶⁰ (c) proposed deactivation mechanism on hydroxyapatite catalyst.¹³⁹

Huang and Wang studied the promotional effect of K⁺ cations on dehydration reactions using in situ characterization techniques.¹⁶⁷ IR spectra revealed that C=O stretching mode of lactate salts is the evidence of α-OH activation, agreeing well with DFT results.¹⁶⁸ The dried sample of lactic acid in KBr wafer also leads to the formation of potassium lactate, as confirmed in IR spectra. However, there also exist a dynamic equilibrium of lactate salt and lactic acid during direct dehydration of potassium lactate. The rehydration of potassium lactate is key for the formation of lactic acid, which might undergo polymerization and decarbonylation reactions.

Wang and Tan studied the possible deactivation mechanism on hydroxyapatite catalysts (Fig. 17(c)).¹³⁹ They found the formation of coke undergoes two stages: aliphatic carbon and aromatic carbon growth. Fundamental understanding on the deactivation mechanism on molecular level is very difficult. They employed NMR method to identify carbon species on spent catalysts, in order to probe plausible mechanism for the formation of coke on catalyst surface. The evolution of signal strength for aliphatic and aromatic carbon nuclei suggest that aliphatic species are the precursor for aromatic cokes. However, whether the coke is formed preferably on acidic and basic sites are still not clear. Surface characterization of spent catalyst suggested that the formation of coke does not chemically alter phase purity, but partially blocking both acidic and basic sites. But measurement of acid–base strength indicated that strong acidic and basic sites both contributes to decarbonylation reactions, which provide precursors for coke formation.

Wang and Borgna studied the possible deactivation mechanism of KCa modified NaY zeolites.¹⁹ It is found that ion exchange of Ca²⁺ during activation of lactate molecule leads to phase segregation of Ca²⁺ from bulk phase, leading to the formation of CaCO₃ during catalyst regeneration. Such structural evolution eventually cause performance deterioration. Therefore, finding an effective approach to stabilize the cations in zeolite framework should be the focus of future study.

It is also important to know that the dimer of lactic acid may cause ambiguous detection of AA, resulting in overestimation.¹⁶⁹ The degradation of lactide is found to follow this mechanism: (a) activation of lactide involves a carbo-cationic intermediate. An E₂ mechanism with bromide ions acting as a weak base is more likely to be plausible, leading to the formation of hypothetical oxiranone intermediate. (b) Once dehydration reaction occurs, transformation of one equivalent of lactide into two AA, in nature, is catalytic cracking by H⁺ and Br⁻ ions. The key step is involving nucleophile attack of carbon atom in aprotic environments. We need an acidic and strong leaving group, thus Br⁻ ion is more suitable than Cl⁻ ion for this reaction.

4.5 Summary

Numerous literature results have already confirmed the critical role of acid/base pairs in selective dehydration of lactic acid to AA. Experimental studies on the mechanistic aspect of dehydration reactions demonstrated the essence of cations in activating lactate molecules for C–O bond cleavage against C–C bond rupture. With respect to surface characterization, the classic characterization technique such as TPD and IR is still heavily used to estimate the surface acidity and basicity of catalytic materials. Kinetic studies revealed that medium acidity and basicity is optimal for catalytic activity and selectivity toward AA. However, there is lack of fundamental understanding on the nature of acidic and basic sites at lattice level. Surface engineering to precisely control the number of acidic and basic sites still remains a grand challenge. Therefore, continuing efforts should be dedicated to tailored structures for both acidic and basic sites to achieve optimal activity and selectivity.

5. Concluding remarks and outlooks

In the context of searching resource-sufficient technologies for sustainable manufacture of acrylic acid and derivatives, it is strongly believed that glycerol can serve as a promising starting material over crude oil and shale gas feedstocks. Considering the chemistry for aqueous medium oxidation and dehydration reactions over energy intensive gas-phase oxidation and dehydration, glycerol–lactic acid route seems to outstand other conversion routes in terms of atom economics. In this review article, the reaction chemistry involved in glycerol oxidation to lactic acid and further dehydration of lactic acid to acrylic acid, has been critically discussed with various conceptual design of hybridized catalyst materials.

For glycerol conversion to lactic acid, catalytic C–H bond activation of glycerol and structural rearrangement of intermediates to form lactic acid are found to be metal-determined and acid/base promoted reactions, respectively. The morphological and electronic effects in nanoparticle catalysts are key for improving activity and selectivity towards C–H cleavage in alkaline medium. Alternatively, metal–acid/base interaction is crucial for tandem glycerol conversion to lactic acid in base-free medium under mild conditions. This part of study requires extensive efforts for further enhancing the overall activity for both dehydrogenation and structural rearrangement reactions.

Dehydration of lactic acid is not a new topic. But the competing reactions between dehydration and decarboxylation at elevated temperatures demand sophisticated design of acidic and basic sites on the surface of solid catalysts. Experimental investigations have revealed the mechanistic aspects on molecular level. However, engineering the lattice structures of Lewis acid/base sites still remains largely unsolved.

Future work should be particular focused on the follow areas (key points summarized in Fig. 18):

Fig. 18 Development of hybridized materials for effective glycerol conversion to AA.

5.1 Replacing noble-based materials with Brønsted acid composites, or metal–acid/base hybridized materials

This is the key to achieve atom economical conversion of glycerol to lactic acid, which is still in the process of commercialization competing with glucose fermentation route. Gaining fundamental understanding on acid-promoted tandem oxidation and dehydration represents the key breakthrough on the development of 2^nd generation of glycerol conversion route in base-free medium, more importantly, alleviating the operation and disposal costs due to unfavorable generation of waste salts during acidification of lactates obtained in alkaline medium.

5.2 Lattice engineering for precise control of acidic and basic sites with novel solid acid/base hybridized materials

Despite of decade efforts in synthesizing zeolites, phosphates and sulfates with strong surface acidity, even up to today, rational design of such materials with well-defined Lewis acidity and basicity is still one of the key challenges plaguing catalyst industry. Tailored surface acidity and basicity at lattice level is important to manufacture cost-effective catalysts for dehydration reactions. Computational studies recently revealed that, surface strain is the fundamental driving force to tune the electronic density on acidic or basic sites. Therefore, strain engineering approaches need to be reconsidered for future design of acid/base interfacial catalysts.

5.3 Developing nanostructured solid materials durable in aqueous medium

Long-term stability is still an unsolved issue, as in most experimental studies with continuous reactor, the catalyst life seem to be no less than 50–100 h on stream. Therefore, developing water-tolerant solid materials, via surface modification techniques, could potentially address this issue. In addition, other engineering aspects, such as interfacial mass transfer, surface wetting behaviors and void ratio of catalyst bed also affect the durability of solid catalyst materials.

5.4 Technological and economic consideration

To commercialize the glycerol-to-AA technology, secure and stable supply of crude glycerol and cost-effective power purchase agreement should be considered as priority. As scale of plant plays a key role in determining the economics of glycerol conversion technologies, one should also consider the potential quality of bio-based AA compared with fossil-derived AA. This is because small amounts of unfavorable impurities might be detrimental for current polymer plants.

Author contributions

X. J. drafted the manuscript. K. M. prepared some of the figures. G. Z., M. L. collected literature materials. Y. S., Z. S. and C. Y. provided comments.

Conflicts of interest

The authors declared no competing interests.

Acknowledgements

This work is supported by National Natural Science Foundation of China (22078365, 21706290), Natural Science Foundation of Shandong Province (ZR2017MB004), Innovative Research Funding from Qingdao City, Shandong Province (17-1-1-80-jch), “Fundamental Research Funds for the Central Universities” and “the Development Fund of State Key Laboratory of Heavy Oil Processing” (17CX02017A, 20CX02204A) and New Faculty Start-Up Funding from China University of Petroleum (YJ201601059).

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