4.1. Kraft Lignins
Ma et al. reported the complete ethanolysis of Kraft lignin by the use of α-MoC1−x
/AC (280 °C) and the main products were phenols, phenyl and C6
alcohols and C8
]. The addition of H2
gas in the reaction instead of the inert atmosphere had a negative impact on the formation of liquid products, increasing the alcohols and decreasing the ester yield. Comparing ethanol, methanol, isopropanol and water, ethanol was the most effective solvent providing more liquid products. In a following paper, the effect of catalyst properties was examined in the depolymerization of lignin under supercritical ethanol [111
]. The activity sequence of the catalysts was: carbide > metal > nitride > oxide and the main products were esters, alcohols and aromatics with different ratios depended on the catalyst. Kraft lignin ethanolysis was also studied by the same group using alumina-supported molybdenum catalysts at 280 °C, reduced at different temperatures [112
]. The depolymerization resulted in C6
alcohols (mainly hexanol), C8
esters (2-hexanoic acid ethyl ester), monophenols (2-methoxy-4-methyl phenol), benzyl alcohols (O-methyl benzyl alcohol) and arenes (xylene). The increase in the reduction temperature from 500 to 750 °C resulted in a gradual increase of total production yield reaching 1390 mg/g lignin but further increase to 800 °C, decrease the yield. The same trend was exhibited in each group of products. The inferior activity of the material reduced at 750 °C was correlated with the metallic phase of Mo and the lower activity of the material reduced at 800 °C with the collapse of porous structure due to sintering phenomena. The effect of the reaction time was also studied. When the reaction time increased from 4 to 6 h the product yield increased dramatically from 142 to 1390 mg/g lignin but when the time was prolonged to 10 h, the product yield decreased.
The conversion of Kraft lignin into monomeric alkyl phenols was achieved over Cu/Mo-ZSM-5 catalysts at 220 °C using hydrogen produced by reforming and water gas shift reactions taking place in the water/methanol solvent system and in the presence of NaOH which was used for enhanced lignin solubility [113
]. If no NaOH, methanol or water was used in the reaction, low conversions were observed, despite the fact that in the presence of methanol or water high selectivity to phenol was observed but low monomeric product yield. The optimum water: methanol ratios for the production of monomeric products were determined to be 1:1 and 3:1. The first ratio (1:1) led to 95.7 wt.% conversion and 70.3% selectivity for phenol, 3-methoxy and 2,5,6 trimethyl phenol. The molecular weight of EtOAc soluble products were 286.7 g/mol. In all reaction systems, almost no char was observed, with the exception of the experiment in the absence of NaOH which resulted in high char formation (20.4 wt.%). The proposed mechanism consisted of four steps and is shown in Figure 7
The reductive depolymerization of kraft lignin was also examined in a water–ethanol mixture 50/50 (v/v) with formic acid as an in-situ hydrogen source, by the use of Ni-based catalysts compared to 5% Ru/C [114
]. The effectiveness of the formic acid is evident even in the absence of catalysts (89 wt.% yield of liquid depolymerized lignin at 200 °C). The catalysts resulted in decrease in the molecular weight of the products but also in an unfortunate increase of solid residue due to condensation reactions evoked by the acidic properties of the supports. 10% Ni/Zeolite led to a slight increase of the yield up to 93.5 wt.%, decrease of Mw to 3150 g/mol and 9.3 wt.% solid residue. The ability of formic acid to depolymerize Kraft lignin in the absence of any other catalyst was also reported by the same group in a previous publication [115
]. Under the optimum operating conditions of ca. 300 °C, 1 h, 18.6 wt.% substrate concentration, 50/50 (v/v) water–ethanol medium containing formic acid (FA) with FA-to-lignin mass ratio of 0.7, lignin (Mw ~10,000 g/mol) was effectively de-polymerized towards a liquid product (DL, Mw 1270 g/mol) at a yield of ~90 wt.% and <1 wt.% yield of solid residue (SR). Higher acidity caused condensation of the intermediate products. Higher temperatures or prolonged reaction times resulted also in repolymerization. Formic acid as hydrogen source has been also used by Liguori and Barth for the depolymerization of Kraft lignin to phenols in water and with Pd-Nafion SAC-13 as catalyst at 300 °C [116
]. The main products obtained were guaiacol, pyrocatechol and resorcinol. Nafion SAC-13 acted as a Brønsted acid, activating the lignin aryl ether sites and promoting the hydrogenolysis to phenols.
A combination of a supported metal catalyst (Ru/C) with various MgO based catalysts was studied in the depolymerization of kraft lignin in supercritical ethanol as hydrogen donor solvent [117
]. In the absence of Ru/C, the most active catalyst proved to be MgO/ZrO2
with the highest bio-oil yield (47.7 wt.%) consisting mainly of phenolic compounds, followed by MgO/C (43.3 wt.%) and MgO/Al2
(42.1 wt.%), as can be observed in Table 2
. The number of base/acid sites of the MgO based catalysts was found to be correlated with the catalytic activity: MgO/ZrO2
which possessed the highest number of base sites and the less acid sites, exhibited the highest catalytic activity. In contrast, the catalyst with the highest acidity, i.e., MgO/Al2
, was the less active one. The addition of Ru/C led to higher bio-oil yield in the range of 70.9–82.7 wt.%, rich in higher alcohols and aliphatic esters, while the highest bio-oil yield (88.1 wt.%) and the lower solid residue (8.8 wt.%) was observed for Ru/C when used alone. The unexpected increase of catalytic bio-oil molecular weights, compared to the non-catalytic bio-oil, was attributed to the higher concentrations of heavy compounds. The addition of hydrogen gas in the experiment catalyzed by Ru/C + MgO/ZrO2
did not improve the bio-oil yield (76.9 wt.%). Upon replacement of Kraft lignin with organosolv lignin, the Ru/C + MgO/ZrO2
resulted in lower yield and lower molecular weight of bio-oil, with a higher yield of aromatic monomer. The higher depolymerization efficiency of organsolv lignin was attributed to the less condensed structure and the absence of catalyst-poising sulfur.
Esposito et al. synthesized two different nickel-based materials, TiN-Ni and TiO2
-Ni and tested their activity in the hydrogenolysis of Kraft lignin in various alcohols, in a flow reactor system under relatively mild temperature and pressure conditions, considering mainly the effect of alcohols on lignin solubility, without discussing their potential hydrogen donating function [118
]. Higher catalytic activity exhibited by the TiN-Ni, was attributed to the better dispersion of Ni in TiN phase as well as to the more favorable titanium oxidation state, i.e., being (III) in TiN compared to (IV) in TiO. Substituted phenols (3.2 wt.%) and aromatic fragments (60 wt.%) with small molecular weights were obtained for TiN-Ni.
Supercritical water/isopropanol systems were applied for the depolymerization of Kraft lignin over Fe on Rh/La2
]. Different ratios of water and isopropanol were used to adjust the optimum in situ H2
production, which was correlated with the hydrogen donating capability of water and isopropanol and the relative amount of Fe in the catalyst. Gradual increase in water content led to a gradual decrease in H2
selectivity. Considering the products, the increase in water content, resulted in a progressive increase of aromatics and aliphatic acid/esters while a sharp decrease in hydrogenated cyclics was observed. The products distribution obtained at the different ratio of isopropanol/water can be seen in Figure 8
. Apart from the liquid products, similar trends were observed in the gas products.
Singh et al. studied the depolymerization of Kraft lignin in methanol at 220 °C by the use of homogeneous (NaOH) and heterogeneous catalysts (HZM-5 and iron turnings from lathe machining) [120
]. Compared to the non-catalytic experiment, which resulted in 73.6 wt.% depolymerization yield, the homogeneous NaOH led to 68.5 wt.% yield, 5.1 wt.% monomeric compounds and 100–1000 g/mol molecular weight distribution. The heterogeneous HZSM-5 resulted in higher depolymerization yield of 85.1 wt.%, with lower monomers amount 4.2 wt.% and the same molecular weight distribution while the iron turnings resulted in lower depolymerization (44.4 wt.%) with 1.7 wt.% monomers and 100–2000 g/mol molecular weight distribution. In all cases, the main products were alkyl substituted phenols. It was suggested that methanol, in addition to being a solvent in the reaction, acted also as a hydrogen donor. The hydrogen released due to thermal reforming of methanol-induced hydrogenolysis of the ether linkages in lignin resulting in lignin depolymerization and demethoxylation.
4.2. Soda Lignins
Hensen et al. carried out a thorough investigation of the reductive depolymerization of soda lignin by the use of CuMgAlOx
catalysts. In a first article, the group examined the influence of solvent, reaction time, and catalyst [121
]. The most effective combination proved to be CuMgAlOx
with ethanol at 300 °C, resulting in 17 wt.% monomers yield, comprising of aromatic products with small amounts of furans, hydrogenated substituted cyclic compounds and deoxygenated aromatics, as can be observed in Figure 9
. The Cu content of 20 wt.% in the CuMgAlOx
mixed oxide was found to induce the highest activity for Guerbet, esterification and alkylation reactions, resulting in higher monomers yield and low repolymerization [122
]. Both the non-catalytic experiment and the use of methanol resulted in low monomer yields (5 and 6 wt.%) and in the latter case the products were mainly methylated phenols and guaiacol-type compounds. Copper in MgAlOx
improved the monomer production and the formation of deoxygenated aromatics compared to the MgAlOx
, and PtMgAlOx
catalysts. Considering the repolymerization via the phenolic hydroxyl groups, the authors suggested that ethanol not only acts as a hydrogen-donor solvent, but also as a capping agent and formaldehyde scavenger, as shown in Figure 10
]. In a following paper, they discussed the influence of reaction temperature on the products [124
]. At lower temperatures in the range of 200–250 °C, recondensation reactions are dominant whereas at higher temperatures of 380–420 °C, the char formation due to carbonization played a major role. At the intermediate range of 300–340 °C, the depolymerization of lignin is enhanced, resulting in the formation of reactive phenolic intermediates which can be protected by alkylation, Guerbet and esterification reactions. CuO in the parent CuMgAlOx
catalyst favors the alkylation reactions while its progressive reduction to Cu may lead to the undesirable increased hydrogenation of the aromatic ring.
Soda lignin depolymerization was performed over metals catalysts supported on ZSM-5 zeolite in supercritical ethanol at 440 °C [125
]. When comparing the transition metals Ni, Co and Cu at 10 wt.% loading on ZSM-5 with Si/Al2
of 200, the 10% Cu/ZSM-5(200) catalyst showed the highest yield of monoaromatic compounds (15.3 wt.%). Changing the metal loading from 10% to 5 and 30% a small decrease in monoaromatic compounds was observed. In order to find the optimum Si/Al2
ratio of ZSM-5, Si/Al2
was varied from 30 to 200. The highest yield 98.2 wt.% of monoaromatic compounds was obtained over 10 wt.% Cu/ZSM-5(30) due to the higher acid density. The beneficial effect of Cu in the depolymerization was confirmed by the experiment conducted in Cu-free ZSM-5 (30) which resulted in 89.4 wt.% monoaromatic compounds. The authors suggested that the in situ produced hydrogen atoms were adsorbed onto the surface of Cu leading to cleavage of ether bonds and thus promoting the depolymerization.
4.3. Alkali Lignins
In the hydrogenolysis of alkali lignin in supercritical ethanol, Zhou et al. found that CuNiAl-hydrotalcite was more active catalyst than Ni/ZSM-5 or Ru/C, resulting in 49.5 wt.% bio-oil yield at 290 °C [126
]. The strong basic sites of hydrotalcite, compared to the acidic ZSM-5 and carbon, inhibited the recondensation of reactive compounds of bio-oil. The addition of phenol as co-solvent to ethanol (phenol/lignin=0.8), increased bio-oil yield to 72.3 wt.%. Phenol is suggested to promote the hydrogenolysis due to the enhanced solubilization of lignin and the capping agent action favoring the formation of mono-phenolics compounds and suppressing repolymerization. Higher amounts of phenol resulted in lower bio-oil yields and increased molecular weights, phenomena which attributed to secondary repolymerization reactions. Also, the addition of hydrogen gas did not enhanced the bio-oil yield (70.3 wt.%). The optimum temperature and time considering the bio-oil yield were determined to be 290 °C and 3 h.
The effective activity of nickel-based catalysts was also reported by Li et al., in the hydrogen transfer conversion of alkali lignin using isopropanol/water solvent [127
]. Alkali lignin showed high conversion (93%) over Raney Ni catalysts at 180 °C, superior than with Pd/C catalyst which led to low conversion and liquefaction rates. Lower conversion was observed for Klason lignin due to its more condensed nature, attributed to the high acid concentrations in the Klason lignin preparation process.
The synergistic activity of formic acid and Pd/C was examined in the catalytic depolymerization of alkali lignin in subcritical water [128
]. When the reaction was contacted without formic acid and Pd/C at 265 °C for 1 h, the liquid products yield was 58.2 wt.% and the solid residue 30.6 wt.%. The addition of formic acid slightly increased the liquid products to 61.6 wt.% but extremely decreased the solid residue to 0.64 wt.%. The addition of Pd/C catalyst either in the presence of formic acid or not, resulted in lower liquid products (45.8 and 41.3 wt.%) and higher solid residues (16.3 and 54.4 wt.%). The products yields from lignin depolymerization in all reaction systems are shown in Table 3
. The catalyst favored the conversion of formic acid and production of H2
via reforming and water–gas shift reactions and promoted the repolymerization reactions. Significant differences are observed in the composition of liquid products. In the absence of formic acid and Pd/C, the main compound was guaiacol, while in the presence of formic acid or both formic acid and Pd/C, catechol was the main compound. Pd/C can catalyze the hydrogenolysis of the aryl–O ether bond resulting in significant yield of phenol and char formation.
4.4. Organosolv Lignins
Toledano et al. studied the hydrogenolysis of organosolv lignin from olive tree prunings under microwave irradiation, using a variety of hydrogen donor solvents without any H2
]. In a first paper, they examined the activity of metallic (Ni, Ru, Pd, Pt) catalysts supported on Al-SBA-15 and found that 10% Ni/Al-SBA-15 with tetralin as solvent provided improved bio-oils with 17 wt.% yield, consisting of simple phenolics, including mesitol and syningaldehyde, as well as, a small amount of esters. The product distribution obtained over Ni, Ru, Pd, Pt catalysts supported on Al-SBA-15 can be seen in Table 4
. The other metals exhibited lower activity with high remaining lignin due to repolymerization phenomena [129
]. In a further work, the same group carried out detailed research on the solvent effect on the hydrogenolysis of organosolv lignin from olive tree prunings with 10% Ni/Al-SBA-15 at 150 °C [130
]. The most effective solvent, as shown in Figure 11
, was proved to be formic acid resulting in high bio-oil yield (28.89 wt.%), no biochar and a wide variety of phenolics compounds. Formic acid was noted to exhibit additional acidolytic properties for the depolymerization of lignin. Less efficient solvents were glycerol and tetralin with the later leading mainly to phthalates. The authors highlighted the unexpected behavior of isopropanol which in the case of the catalytic experiment, it was proven less efficient compared to the blank experiment due to the dehydrogenation of isopropanol to acetone over the Ni/Al-SBA-15 catalyst.
Organosolv lignin from switchgrass had been successfully depolymerized in ethanol, 20 wt.% Pt/C and formic acid as hydrogen donor molecule at 350 °C for the production of phenol and substituted phenols with higher H/C and lower O/C molar ratios [131
]. In a similar reaction system, organosolv lignin was depolymerized using isopropanol as solvent, Ru/C as catalyst and formic acid as hydrogen donor at 400 °C [132
]. The catalytic experiments resulted in 71.2 wt.% lignin oil, negligible solids formation and significant amount of water (9.6%) due to the decomposition of formic acid. The activity of the catalyst was confirmed by the experiment conducted in the absence of Ru/C which led to only 18 wt.% conversion and a large amount of solids derived either from the unconverted lignin or repolymerization reactions. The effective conversion of lignin was further compared with the hydrotreatment experiment, conducted in the absence of solvent (only Ru/C + H2
gas), exhibiting lower oil yield (63.1 wt.%) compared to the use of isopropanol as solvent. Considering the chemical composition of derived oils, the major products were ketones (methyl isobutylketone) followed by aromatics, catechols and alkylphenolics. In an attempt to improve the catalytic reaction system, methanol and ethanol were also tested as solvents. Methanol gave almost similar yield (68.4 wt.%) with isopropanol but higher amounts of alkylphenolics and aromatics whereas ethanol resulted in lower yield (63.4 wt.%).
The effect of biomass feedstock and isolation method of lignin on the depolymerization in supercritical ethanol and formic acid, in absence of catalyst, at 250–350 °C was also investigated [133
]. The reaction was performed in lignin derived from oak (hardwood) and pine (softwood), isolated as ethanosolv, formasolv and Klason types. Regardless the isolation method, all lignins exhibited bio-oil and conversion yields above 90 wt.% with low solid residue <2.5 wt.% at 350 °C. At this temperature, the combination of ethanol with formic acid facilitated the hydrogen production which quenched the radicals, suppressing the repolymerization. For the hardwood type biomass, at lower reaction temperature (250 °C), the ethanosolv and the formasolv lignin exhibited lower bio-oil yield, 68.0 and 77.5 wt.%, respectively, while Klason lignin bio-oil yield dramatically decreased to 19.3 wt.% due to the abundance of C-C recalcitrant bonds formed in the Klason process. With regard to bio-oil composition, at 350 °C, linear and branched short-chain oxygenated species from the decomposition of ethanol and formic acid, monoaromatic species and long chain fatty acid alkyl esters from the esterification of woody biomass with ethanol were produced. At lower reaction temperature, where the deoxygenation/hydrogenation reactions are limited, carbonyl or double bond-containing monoaromatics were formed. With regard to softwood biomass, the ethanosolv and formasolv lignins exhibited 97.1 and 99.4 wt.% conversion and 88.1 and 90.7 wt.% bio-oil yield. Again, the Klason lignin depolymerization resulted in lower conversion (95.1 wt.%) and bio-oil yield (81.7 wt.%). At the low temperatures of 250–300 °C, the most important parameter was suggested to be the relative abundance of ether linkages in the lignin structure.
The catalytic activity of Cu based porous metal oxides towards the depolymerization of organosolv lignin extracted from candlenuts was examined in supercritical methanol at 310 °C [134
]. Taking into consideration lignin conversion, the following rank was determined: Cu20
PMO > Cu20
PMO > Cu20
PMO (PMO stands for porous metal oxide). For the best catalyst Cu20
PMO, lignin conversion reached 48.3% after 1 h reaction. Due to the lower methanol reforming ability of lanthanum, the reaction was also carried out for higher times and the conversion reached 98% after 6 h. Under the same reaction conditions, the activity Cu-free porous metal oxides were also examined. [Mg/Al]PMO exhibited the highest lignin conversion of 74.6% for 5 h reaction. Comparison of CuPMOs to Cu-free analogs showed that Cu promotes higher yields of methanol-soluble products and suppresses re-condensation reactions. The Cu20
PMO variant was suggested as the most effective catalyst in terms of limiting over-reduction of aromatic intermediates due to the lower methanol-reforming of lanthanum, thus regulating the in situ production of hydrogen.
Organosolv lignin isolated from eucalyptus was depolymerized in water to syringol monomers over β-CaP2
]. Despite that eucalyptus is a hardwood feedstock, the use of phosphate catalysts selectively produced only syringol with yield of 8.47% over β-CaP2
and 6.67 % over CoP2
. The hydrogenolysis of organosolv lignin from hybrid poplar in supercritical ethanol at 320 °C, by the use of amorphous B-containing FeNi alloyed catalysts, resulted in lignin depolymerization and the production of deoxygenated aliphatic side chains [136
4.6. Enzymatic and Acid Hydrolysis Lignins
The role of formic acid in the reductive depolymerization of lignin has been studied in the work of Oregui-Bengoechea et al. [138
]. In the hydrogenolysis of enzymatic hydrolysis eucalyptus lignin over NiMo/sulfated alumina and ethanol at 320 °C, the synergistic action of formic acid and catalyst was shown by the increased oil yield (38.4 wt.%) compared to the uncatalyzed experiment (23 wt.%) and the experiment where gaseous H2
had been used instead of formic acid (19.7 wt.%). All the derived oils contained methoxy-, hydroxyl- and alkyl- substituted benzenes. The authors suggested that the formic acid is involved in lignin depolymerization via a formylation-elimination-hydrogenolysis mechanism, which includes also the catalytic decomposition of formic acid that provides molecular H2
for the hydrogenolysis reaction (Figure 12
). The important role of the solvent was discussed and ethanol proved to be more effective (oil yield 38.4 wt.%) than methanol (23.8 wt.%) or isopropanol (21.7 wt.%), as suggested in similar studies [130
Similar observations about the enhanced depolymerization in the presence of formic acid and ethanol had been reported also by Kristianto et al. for the concentrated acid hydrolysis lignin from fruit bunch palm oil [139
]. From the catalytic activity screening of 5% Pd/C, 5% Ru/Al2
, 5% Ru/C and 10% Ni/C at 300 °C it was shown that the most active catalyst was 5% Ru/C with 31 wt.% bio-oil yield and 47.1 wt.% solid residue. The addition of formic acid increased the bio-oil yield to 62.9 wt.% and decreased the solid residue to 19.8 wt.%, making formic acid better hydrogen donor than the external gas. The products obtained from the reaction were phenol and its derivatives whose amount increased with increase in formic acid/lignin ratio and reaction time. Small amounts of phenolic compounds with ethyl and ester groups were observed due to alkylation and esterification reactions of phenol intermediates with ethanol.
The synergistic effect of Raney Ni and zeolites catalysts had been investigated in the depolymerization of enzymatic hydrolysis lignin from bamboo residues [140
]. In a methanol/water reaction mixture at 250 °C, when Raney Ni was combined with zeolite catalysts, the yield of mono-phenols significantly increased from 12.9 wt.% (Raney Ni) and 5 wt.% (zeolite) to 27.9 wt.% (HUSY and Raney Ni). The optimum ratio of zeolite: Raney Ni was determined to be 8:4. The authors proposed that Raney Ni could act as lignin cracking and methanol reforming catalyst and zeolite as Brønsted and/or Lewis solid acid essential for ether solvolysis and dehydration. Furthermore, zeolites can act as a blocking agent to prevent reactions between the original lignin and the unstable lignin fragments.
4.8. Selection and Design Criteria for an Effective Catalyst in Reductive Depolymerization of Lignin Using Hydrogen Donors
As discussed in the previous sections, the catalysts which have been widely studied in the reductive depolymerization of lignin by the use of hydrogen donors are based on noble (Pd, Ru, Pt) or transition metals (Ni, Cu) supported on carbon, zeolites and silica materials due to their known ability to catalyse the cleavage (hydrogenolysis) of C-O and C-C bonds, the hydrodeoxygenation of oxygenated compounds and the hydrogenation of aromatic double bonds. A summary of the most representative catalytic systems is shown in Table 6
. However, the specific mechanisms of the in situ hydrogen production has been scarcely discussed with general reference to reforming of alcohols and related water gas shift reaction as well as decomposition of formic acid. Furthermore, while the terms “transfer hydrogenation” or “hydrogen transfer” have been used in some cases, there was no systematic effort to elucidate the reaction pathways involved in relation to the catalyst properties and the experimental conditions.
Supercritical alcohols can donate hydrogen in the form of molecular hydrogen, hydride, or protons, with the hydride deriving from α-hydrogen and the proton from the alcohol hydroxyl, forming simultaneously electron-deficient hydroxylalkylation species, alkoxide ions and aldehydes [142
]. Similarly, decomposition of formic acid in supercritical water conditions leads to in situ H2
formation. However, the relatively high temperatures, e.g., >280 °C, at which most of the lignin hydrogenolysis studies in supercritical solvents have been conducted, may also induce repolymerization-condensation reactions of the initially formed monomer phenolics, especially in the presence of acidic catalysts. Thus, optimization of the overall system, i.e., type of solvent/hydrogen donor—reaction temperature—catalyst properties, is required in order to achieve high yields of liquid products enriched in monomers.
With regard to the catalyst properties, three main interactive criteria should be considered: (i) effect on the in situ hydrogen production mechanism, (ii) high hydrogenation reactivity and facilitated activation of the lignin C-O or C-C bonds, and (iii) stabilization of reactive intermediates via alkylation or other reactions. The first criterion is related with the catalyst properties that should be tailored towards enhanced reforming and WGS reaction activity and/or transfer and stabilization via surface intermediates of hydride (H:) or protons (H+) by selecting appropriate metals and supports. In this latter case, which is less discussed in the literature, an effective catalyst or catalyst support surface would comprise of Lewis acid sites and Brønsted or Lewis basic sites that can attract H: and H+ from alcohols, respectively, thus initiating the steps of hydrogen transfer and lowering the overall reaction required temperature. Materials with such properties can be various transition metal oxides or mixed oxides, such as ZrO2, TiO2, MgO, etc and their modified/doped analogues, as well as metal-modified zeolites and other aluminosilicates. The surface acid-base properties of these materials can be tuned by selecting the appropriate composition. The second criterion refers to the intrinsic (de)hydrogenation and redox activity of noble or transition metals which includes the dissociative adsorption of molecular H2, as two hydrogen atoms, which are available to participate in the various hydrogenation or hydrogenolysis pathways. The hydrogen atoms on the noble/transition metals may interact directly with the abundant ether bonds in the lignin fragments or with a double bond (at deeper hydrogenation conditions) or they can interact via spill-over phenomena with a nearby sorbed intermediate, such as an ether bond reacting with the Brønsted acid sites of the support of the hydrogenating metal. Thus, the synergistic action of the metal with the support could also lead to facile hydrogenation/hydrogenolysis reaction reducing further the required reaction temperature. The third criterion is related with the effect of the catalyst on the reactions occurring between the alcohol solvent and the formed reactive intermediates in lignin hydrogenolysis with the aim to inhibit their repolymerization, one representative example being that of favoring their alkylation with methyl or ethyl moieties (from methanol or ethanol respectively) in the presence of metal oxides such as SiO2-Al2O3, CuO, TiO2, etc. A balance between hydrogenation and alkylation may also be desirable, in order to limit repolymerization and not lose the aromatic nature of the obtained monomers, thus pointing to metal oxides (mainly being used as supports) that cannot be easily reduced in situ in the presence of hydrogen.
Along these lines and in addition to the more classical hydrogenation catalysts, e.g., noble or transition metals on carbon, zeolites, silica, etc., more “sophisticated” multifunctional catalytic formulation have been recently reported, as described in the sections above, including TiN-Ni and TiO2
-Ni, FeNiB alloys, Cu based materials such as Cu20
PMO (porous metal oxides) and CuMgAlOx
, Fe on Rh/La2
, and others. With regard to catalyst recovery from batch reactor systems, new magneticcatalytic formulations with weakly acidic Brønsted-type centers, such as Fe3
@Re and Co@Nb2
which exhibited promising behavior in the reductive depolymerization of lignin using gaseous H2
, could be also effectively used in catalytic transfer hdyrogenolysis reactions [143