Sustainable pathways to chemicals and fuels from lignocellulose via catalytic cleavage and coupling reactions
Sun, Zhuohua
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Sun, Z. (2018). Sustainable pathways to chemicals and fuels from lignocellulose via catalytic cleavage and coupling reactions. Rijksuniversiteit Groningen.
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Complete Lignocellulose Conversion with Integrated Catalyst
Recycling
Lignocellulose, the main component of agricultural and forestry waste, harbors tremendous potential as a renewable starting material for future biorefinery practices. However, this potential remains largely unexploited due to the lack of comprehensive strategies that derive substantial value from all its main constituents. In this chapter, we present such a catalytic strategy, which is able to transform lignocellulose to a range of attractive products. At the center of our approach is the flexible use of the non-precious metal catalyst (Cu20-PMO) in two distinct stages of a lignocellulose conversion process that enables integrated catalyst recycling through full conversion of all process residues.
This chapter has been published as: Z. Sun, G. Bottari, A. Afanasenko, M. C. A. Stuart, P. J. Deuss, B. Fridrich & K. Barta, Nat. Catal. 2018, 1, 82–92.
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2.1 Introduction
Lignocellulose is a non-edible, renewable starting material consisting of lignin, cellulose and hemicellulose, that harbors significant potential for the sustainable production of chemicals and fuels.1,2 Yet, in order to unlock this potential, fundamentally new catalytic methods3 and innovative biorefinery approaches are desired that are able to accommodate the structural complexity of lignocellulose and derive value from all its major components. 4
2.1.1. Typical biorefinery approaches and the fate of lignin
In a typical biorefinery, lignocellulose is first separated to its constituents by pre-treatment.1 This approach, however, is energy intensive5, and predominantly focuses on producing high quality cellulose.6,7 In this pretreatment or fractionation process, all components of the lignocellulose are affected and both the chemical structure and polymeric nature are altered after the treatment.4 Moreover, under these processing conditions the lignin component is structurally modified.8,9 As shown in Figure 2.1, classic pulping and carbohydrate hydrolysis methods due to the use of harsh acidic and alkaline conditions evoke cleavage of β-O-4 ether bonds10, hereby generating reactive lignin fragments that are prone to subsequent irreversible repolymerisation.11 Compared to native lignin structure, the isolated lignin contains much more stable C-C bonds, rendering its further catalytic valorization very challenging.4,12–14 This remains true despite impressive advances regarding the selective conversion of lignin model compounds15,16 and depolymerization of organosolv lignin.17–21
Figure 2.1 Schematic illustration of lignin degradation (O-linkages represent ether bonds). Adapted from ref. 11. Copyright 2018 Royal Society of Chemistry.
2.1.2. Catalytic lignocellulose fractionation
Recently, elegant research by Abu-Omar, Rinaldi and Sels has focused on lignocellulose fractionation in the presence of heterogeneous catalysts (namely RCF process).22–29 These methods are more extensively discussed in Chapter 1, hold much promise for the selective production of aromatic monomers from the lignin fraction, they leave a significant portion of the renewable carbon equivalents unutilized and mixed with the catalyst. Thus, in these systems it is (mainly) the cellulose part that is tedious to valorize and catalyst recycling has been identified as a key challenge.4,30
2.1.3. The development of the ‘LignoFlex’ process
In this chapter a new catalytic strategy that inspired by the RCF process will be described. The goal of this novel approach is to achieve the catalytic conversion of both the lignin as
43 well as the (hemi)cellulose fraction of lignocellulosic biomass to value added chemicals and allow for integrated, efficient catalyst recycling. In order to achieve complete lignocellulose conversion and efficient catalyst reuse, we have envisioned a novel process that relies on the use of Cu20-PMO catalyst which was prepared by calcining 20% copper-doped hydrotalcite and has been used for one-pot catalytic conversion of lignocellulose in supercritical methanol condition.31 At the central of this approach is the flexible use of a Cu20-PMO catalyst in a two-step lignocellulose conversion process (Scheme 2.1). In the first step, native lignin was converted to a single aromatic alcohol in excellent selectivity (>90%) at mild condition. In the second step, the unreacted (cellulose rich) residues are fully converted to alcohol rich liquid solutions, thereby liberating the catalyst for reuse and offering a distinct advantage over existing systems.4
Scheme 2.1 Proposed strategy for the complete lignocellulose conversion to aromatic and aliphatic alcohols through the flexible use of Cu20-PMO under mild (Step 1) and supercritical conditions (Step 2).
2.2 Results and discussion
2.2.1 Mild depolymerization of pine lignocellulose
To obtain aromatics directly from lignocellulose, we started our investigations by treating pine lignocellulose over a Cu20-PMO catalyst in hydrogen at 140–220 °C. The results are summarized in Table 2.1. The products consisted of a clear, colourless methanol solution (Figure 2.2a) and a solid residue containing unreacted lignocellulose and catalyst.
To our surprise, at 180 °C the small molecule fraction of the liquid phase contained predominantly one aromatic compound, 4-propanolguaiacol (1G), which could be isolated (Entry 10). The remarkably clean reaction produced a very clean GC-FID trace (Figure 2.2b), remarkably, showing that in our system 1G was obtained in excellent (> 90%) selectivity with a non-noble-metal catalyst under relatively mild conditions (180 °C). GPC trace (Figure 2.2c) and also 2D-HSQC image (Figure 2.2a) of the same solution confirmed the formation of 1G as the main product. Decreasing the temperature to 140 oC, only a small amount of monomers (4 mg) could be obtained and increasing the temperature to 220 oC lead to the low
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selectivity of 1G. Time course study (Figure 2.3) showed that the yield of 1G increases linearly before 18h and then keeps constant after further reactions. The reaction could also be upscaled using 10 g pine lignocellulose (Entry 11) with almost the same yield and selectivity. Further, pre-extracted organosolv lignin underwent depolymerization under standard catalytic conditions, but resulted in lower monomer yield (Entry 12). This is due to the modification of the native lignin structure during organosolv processing resulting in fewer cleavable β-O-4 linkages.10
Figure 2.2 Analysis of products after catalytic conversion of pine lignocellulose by a. 2D NMR spectrum; b. GC-FID and c. GPC chromatograms (For reaction condition see Table 2.1, Entry 10). 0 5 10 15 20 25 30 35 0 10 20 30 40 50 Yi el d % Time (h) 1G 2G 3G
Figure 2.3 Product formation profiles in reactions using pinewood. Reaction conditions see Table 2.1, Entries 4-9.
45 Table 2.1 Mild depolymerization (Step 1) of pine lignocellulose at different reaction
conditions Entry Substrate (mg) Time(h) Temperature (oC) Methanol-solubles (mg)b Methanol-insolubles (mg)c Mass Balance (%) Monomers (mg)d Monomer Yield (%)e 1G 2G 3G Sum 1a 1000 18 180 70 790 86 25 3 1 29 10 2 1000 18 140 10 880 89 4 0 0 4 1 3 1000 18 220 140 680 82 22 12 2 36 13 4 1000 6 180 40 830 87 16 2 0 18 6 5 1000 2 180 30 870 90 14 1 0 15 5 6 1000 4 180 40 870 91 15 2 0 17 6 7 1000 12 180 70 800 87 22 2 1 25 9 8 1000 24 180 90 780 87 25 3 1 29 10 9 1000 36 180 90 770 86 25 3 1 29 10 10f 2000 18 180 140 1800 97 44(40)g 4 0 48 9 11h 10000 18 180 930 8730 97 239(220)g 25 12 276 10 12i 200 18 180 80 100 90 5 2 1 8 4 13 500 18 180 40 370 82 9 1 0 10 6
Reaction conditions: Cu20-PMO 200 mg, methanol 10 mL, 180 oC, H2 40 bar, 18 h, 3,5-dimethylphenol 20 mg as an internal standard. a. Average yields of repeated 3 reactions are given. b. Weight of methanol soluble products excludes the weight of internal standard. c. Weight of methanol insoluble solid residue excludes the weight of the catalysts. d. Determined based on GC-FID measurement using calibration curves and internal standard. e. Monomer yield = weightmonomers /weightlignin f.400 mg Cu20-PMO was used. g. Weight of isolated pure product. h. Cu20-PMO 1 g, methanol 100 mL. i. Organosolv lignin extracted from pine lignocellulose was used as substrate.
1G: 4-Propanolguaiacol, 2G: 4-Propylguaiacol, 3G: 4-Ethylguaiacol.
2.2.2 Proposed mechanism of lignin depolymerization - mechanistic studies with lignin β-O-4 model compounds
In order to determine the reactivity of the lignin β-O-4 linkage, we have studied the reactivity of simple β-O-4 model compounds. First, compound S1 (2-(2-Methoxyphenoxy)acetophenone) was reacted under reaction conditions that correspond to the ones used in the actual lignin depolymerization experiments. Product formation profiles were recorded by running a number of separate experiments for various reaction times (Figure 2.4). These revealed a very efficient cleavage of the β-O-4 linkage, to yield guaiacol (b) as expected, whereby the acetophenone (a) underwent rapid hydrogenation to 1-phenylethanol (c) under the reaction conditions. Further conversion of 1-1-phenylethanol (c) to ethylbenzene (d) likely via dehydration/hydrogenation or direct hydrogenolysis was also observed. Rapid hydrogenation of S1 to the corresponding alcohol S2 (2-(2-Methoxyphenoxy)-1-phenylethanol) was also observed as expected under these reaction conditions. Thus, to clarify the reactivity of S2 in this system, product formation profiles were recorded when S2 was directly used as substrate (Figure 2.5), which yielded the same products as compound S1, guaiacol (a) and 1-phenylethanol (c), and ethyl-benzene (d). In order to prove that a hydrogen neutral cleavage of the β-O-4 linkage via dehydrogenation/hydrogenation, a sequence of reaction steps also reported in the
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literature32, S2 was treated over Cu20-PMO under N2 atmosphere. Indeed, the formation of acetophenone (a) and guaiacol (b) was observed as expected (Scheme 2.2).
0 2 4 6 8 10 0 10 20 30 40 50 60 70 80 90 100 Yi el d (% ) Time (h) b c d S2
Figure 2.4 Product formation profiles in reactions using the β-O-4 model compound S1. Reaction conditions: Cu20-PMO (0.1 g), S1 (0.5 mmol), methanol (10 mL), 3,5-dimethylphenol (20 mg), 180 oC, H2 (40 bar). (Full conversion of S1 was observed within 1h)
0 2 4 6 8 10 12 14 16 0 10 20 30 40 50 60 70 80 90 100 C o n ve rsi o n o r Yi e ld % Time (h) b c d S2
Figure 2.5 Product formation profiles in reactions using the β-O-4 model compound S2. Reaction conditions: Cu20-PMO (0.1 g), S2 (0.5 mmol), methanol (10 mL), 3,5-dimethylphenol (20 mg), 180 oC, H2 (40 bar).
47 Scheme 2.2 Cleavage of β-O-4 dimer S2 under N2 with Cu20-PMO catalyst. Reaction condition: Cu20-PMO (0.1g), S2 (0.5 mmol), methanol (10 mL), 3,5-dimethylphenol (20 mg), 180 oC, N2 (40 bar), 6 h.
Based on these results we proposed the following reaction pathways (Scheme 2.3) involving in the cleavage of simple lignin model compounds S1 and S2. S1 can be quickly converted to S2 by hydrogenation reaction under H2, and S2 may be converted to S1 by dehydrogenation reaction in the condition without H2. The β-O-4 linkages in both S1 and S2 can be cleaved by Cu20-PMO and form b and a/c. Compound c could be also obtained by hydrogenation of a. Further conversion of c to d via dehydration/hydrogenation or direct hydrogenolysis is also possible.
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Scheme 2.4 The reactivity of model compound S3 (β-Hydroxypropiovanillone) with Cu20-PMO catalyst. Reaction conditions: Cu20-Cu20-PMO (0.05 g), S3 (0.25 mmol), methanol (10 mL), 3,5-dimethylphenol (20 mg), 180 oC, H2 (40 bar), 0.5 h.
Based on this reactivity pattern, in the lignin reactions, the formation of model compound S3 (β-Hydroxypropiovanillone) or/and the corresponding alcohol would be liberated upon the cleavage of the lignin β-O-4 linkage. Thus, we have also investigated the reactivity of S3 (Scheme 2.4) under the reaction conditions to prove the feasibility of the formation of 1G which was found as main product in our lignin depolymerization reactions. Indeed, treating S3 under the reaction conditions that were also used for lignin depolymerization, compound 1G and the corresponding 2G were found already after 30 minutes. Based on the reactivity of acetophenone above, 1G was formed from S3 via a rapid hydrogenation and dehydration/hydrogenation or direct hydrogenolysis. This experiment has also shown that the –OH group alpha to the aromatic ring was more prone to dehydration or direct hydrogenolysis under these reaction conditions, explaining the retention of the gamma –OH in 1G in most lignin depolymerization runs. This experiment also explains the formation of 2G, which is formed from 1G likely via direct (and slower) hydrogenolysis. During these experiments the corresponding unsaturated intermediates were not observed, suggesting that the direct hydrogenolysis pathway operates, however their formation cannot be excluded at this point.
In order to further confirm the formation of 1G, more complex lignin β-O-4 model compounds S4 and S5 with –CH2OH moiety were then studied. As shown in Scheme 2.5, products with –CH2CH2CH2OH as the side chain were both observed with high selectivity.
49 Scheme 2.5 The reactivity of model compound S4 and S5 using Cu20-PMO catalyst at 1 hours and 6 hours. Reaction condition: Cu20-PMO (0.1g), S4 or S5 (0.5 mmol), methanol (10 mL), 3,5-dimethylphenol (20 mg), 180 oC, H2 (40 bar).
Based on these studies, we propose that depolymerization of native lignin proceeds via scission of the β-O-4 linkage through a series of dehydrogenation, hydrogenolysis and hydrogenation events involving a ketone intermediate. This is in agreement with existing two-step methods that achieve efficient depolymerization of organosolv lignin to aromatics through selective pre-oxidation followed by reductive cleavage. For example, Stahl and co-workers33 have achieved very efficient lignin depolymerization by first oxidize the secondary alcohol in the β-O-4 linkage to corresponding ketone using catalytic amount of 4-acetamido-TEMPO/HNO3/HCl under aerobic conditions. This oxidization step activated the linkage for the desired C−C or C−O bond scission in the second depolymerization step. Westwood and co-workers34 also presented a new approach based on the same methodology but using molecular oxygen as the oxidant and catalytic amounts of 2,3-dichloro-5,6-dicyano- 1,4-benzoquinone (DDQ) and tert-butyl nitrite (tBuONO). Upon preoxidation, Zn/NH4Cl was applied in the second step at 80 °C.
In the present study of using Cu20-PMO catalyst, this sequence of steps occurs over a single, multifunctional catalyst, providing 1G in superior selectivity.
2.2.3 Control experiments using pine lignocellulose
To demonstrate the clear advantage of the applied catalytic conditions, control reactions were performed. Control experiments were operated in a 25 mL Parr reactor and the full processing scheme of obtained products is shown in Scheme 2.6. Methanol soluble products from 4 different reactions are compared carefully by NMR and GPC analysis.
50 All methanol solubles Methanol solution (~10ml) Methanol soluble products Transferred to centrifuge tube and dry Concentrate by solvent evaporation DCM soluble products DCM insoluble products Add DCM, sonicate and centrifuge NMR analysis GPC analysis Solvent remove Solvent change to THF
Scheme 2.6 Processing steps carried out during fractionation of methanol soluble products obtained after control reactions and one catalyzed reaction using pine lignocellulose.
Figure 2.6 Comparison of the products after depolymerization of pine lignocellulose at different reaction conditions by 2D-HSQC NMR and GPC. General reaction conditions: pine lignocellulose (1 g), methanol (10 mL), 180 oC, H2 (40 bar), 6 h.
Indeed, while the 2D NMR spectrum of a sample obtained on standard catalytic treatment (Cu20-PMO and H2) was assigned to the main product 1G (Figure 2.2a) without any lignin linkages and control reactions without catalysts, hydrogen or copper all delivered brown solution of organosolv lignin, as evidenced by 2D NMR analysis that showed all relevant lignin linkages with varying contents. Thus, in this case, lignin was extracted from the lignocellulose substrate but was not depolymerized or partially depolymerized in control reactions. This was also confirmed by comparing the gel permeation chromatography (GPC) traces (Figure 2.6b).
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2.2.4 Full material utilization integrated with catalyst recycling
Figure 2.7 Complete conversion of pine lignocellulose to aromatic and aliphatic alcohols through the flexible use of Cu20-PMO with integrated catalytic recycling. Reaction conditions: Step 1: 0.2 g Cu20-PMO, 0.5 g pine lignocellulose, 10 mL methanol, 180 oC, H2 40 bar, 18 h. Step 2: 6 mL methanol, 320 oC, 6 h for 1st and 2nd cycle, 8 h for 3rd to 5th cycle. aMonomer yield = weight monomers / weight lignin. b Conversion based on the weight of the remaining solid residue.
In contrast to typical depolymerization procedures, which generally result in complex product mixtures, the great advantage of the mild and selective catalytic method developed here is that only a few aromatic compounds are obtained and single product can be easily separated. Undesired side reactions, such as overreduction of the aromatic rings or recondensation of reactive fragments formed during lignin depolymerization, are minimized under these conditions. However, this method leaves linkages other than the β-O-4 intact,
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thus a fraction of lignin is unconverted. Additionally, the whole of the (hemi)cellulose portion (~60% by mass), which makes up a significant quantity of renewable carbon equivalents, remains unutilized. These solid residues stay mixed with the heterogeneous Cu20-PMO catalyst, making catalyst recycling very challenging. To overcome this challenge, we envisioned liberating the catalyst through conversion of the solid residues to valuable products. This catalyst-recycling step relies on the unique reactivity of Cu20-PMO in supercritical methanol at 300–320 °C, whereby a fraction of the solvent undergoes in situ methanol reforming to create reaction conditions suitable for the complete conversion of lignocellulose to small molecules. We anticipated that the unreacted solid residues formed in Step 1 could also be converted, assuming that no catalyst deactivation took place during this initial processing step.
Indeed, excellent conversion of the reaction solids was achieved by simply heating the residue from the step 1 to 320 °C in freshly added methanol. The composition of the obtained clear, colourless methanol solutions containing mainly aliphatic alcohols, small amounts of ethers and esters, as well as minor amounts of alkyl-phenols (Figure 2.7). Recycling experiments comprising both Step 1 and Step 2 were carried out using pine lignocellulose to obtain aromatic and aliphatic alcohols, respectively. Characterization of the catalyst after the first such cycle showed regularly distributed Cu nanoparticles of 20–50 nm, characteristic for an active catalyst (Figure 2.8b). Indeed, full lignocellulose conversion was maintained for a total of 10 runs (5 mild, 5 supercritical). A small decrease in 1G yield in the fifth mild run and a change in product composition in the fifth supercritical run was observed and accordingly, aggregation of magnesium and copper was observed after a total of 10 runs (Figure 2.8d and e). Elemental analysis of the liquid samples after the first and the fourth mild and supercritical runs showed minimal leaching of Cu, Mg or Al (Table 2.2).
Table 2.2 Leaching tests during catalyst recycling for Cu20-PMO catalyst.
Cycles Cu (mg/L) Mg (mg/L) Al (mg/L) Cu (%)a Mg (%) Al (%) 1m 1 4 2 0.03 0.08 0.09 1s 2 11 2 0.08 0.28 0.11 4m 3 2 2 0.10 0.04 0.09 4s 29 3 4 1.13 0.08 0.21 1mb 3 17 3 0.10 0.35 0.13
a. percentages refer to the total initial mass of the individual element in the catalyst. b. Separate experiment carried out for 36 hours reaction time.
53 Figure 2.8 TEM images of (a) fresh Cu20-PMO catalysts, (b) Cu20-PMO catalyst after one mild (180oC) and one supercritical (320oC) run, (c) elemental mapping of Cu20-PMO catalyst after one mild (180oC) and one supercritical (320oC) run, (d) Cu20-PMO catalyst after five mild (180oC) and five supercritical (320oC) runs, (e) elemental mapping of Cu20-PMO catalyst after five mild (180oC) and five supercritical (320oC) runs.
2.2.5 Demonstrating the generality of the method using lignocellulose from different plant sources
After obtaining 1G from pine lignocellulose in excellent selectivity, the generality of the approach was demonstrated using a variety of wood types and catalyst recycling was integrated (Figure 2.9). High aromatic monomer yields were achieved in most cases, especially with poplar (36%), beech (31%) and maple (30%) lignocellulose. Interestingly, all product mixtures contained typically two, and a maximum of three main products, the type of which depended on the native structure of each lignin. We found alcohol 1S as the main product when starting from poplar, beech or maple lignocellulose. Predominantly 2S was obtained from oak and mainly 1G from pine and cedar lignocellulose. Interestingly, we were able to isolate both 1G (22 mg) as well as 1S (31 mg) as pure compounds from a run with maple wood (1 g). The composition of the obtained methanol solutions (SMix1–SMix8) was largely similar irrespective of the starting material (all containing mainly cellulose), with slight differences originating from the lignin structure of the original wood samples.
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Figure 2.9 Complete conversion of various lignocelluloses to aromatic and aliphatic alcohols through the flexible use of Cu20-PMO under mild (Step 1) and supercritical conditions (Step 2). a, Process steps including catalyst recycling. b, Results upon full conversion of various types of lignocellulose.
In order to better understand the relationship between the native lignin and obtained monomers, we further analyzed the structure of isolated lignin from each wood in detail. As shown in Table 2.3, different lignocellulose contains different amount of lignin. Walnut shell has the highest lignin content of more than 50% and poplar has only 18.6% lignin. Hardwoods contain both Syringyl and Guaiacyl unites and softwoods like pine and cedar contain only Guaiacyl unite. A good correlation was observed between the syringyl/guaiacyl (S/G) ratio measured in the starting lignin and the S/G ratio of the corresponding aromatic products.
55 Table 2.3 Structure of organosolv lignins obtained from various sources displaying the corresponding syringyl/guaiacyl ratios and corelations with the S/G ratios of the monomeric products obtained upon depolymerization.
Lignocellulose Lignin Content (%)a Lignin composition (%)b S:G S G H Crude lignin Lignin monomers Pine 28.6 0 100 0 0.0 0.0 Walnut 50.8 65 27 8 2.4 1.7 Poplar 18.6 57 43 0 1.3 1.8 Oak 30.2 71 29 0 2.5 2.4 Beech 18.8 73 27 0 2.8 3.0 Maple 26.0 65 35 0 1.9 1.9 Alder 23.0 61 39 0 1.5 1.1 Cedar 35.1 0 100 0 0.0 0.0
a. Determined by ABSL method (chapter 2.4.7). b. Determined by 1H-13C-HSQC (peak area integrations of the corresponding signals).
2.3 Conclusions
This Chapter described the development of the LignoFlex process for complete conversion of lignocellulose. The unique reactivity of the noble-metal-free catalyst (Cu20-PMO) plays the key role in both mild (Step 1) and severe (Step 2) depolymerization of lignocellulose and enables the conversion of process residues that would otherwise block the catalyst to aliphatic small molecules, thereby enabling catalyst recycling. Mechanism studies by using lignin model compounds showed the unique activity and selectivity of Cu20-PMO for lignin depolymerization via cleavage of the most-abundant β-O-4 linkage. Control reactions suggested that copper and hydrogen are both important for this process. This catalyst also performed good stability during recycling test and wide applicability for conversion of different feedstocks. . A good correlation was also found between the syringyl/guaiacyl (S/G) ratio of native lignin and the corresponding aromatic products.
The catalytic strategies developed in this Chapter produce substantial value from both lignin and the (hemi)cellulose components of lignocellulosic biomass. The overall approach embraces the complexity of the renewable starting material and provides aromatic and aliphatic alcohol intermediates for further direct transformations. In the following Chapters, I will present several possible applications of the produced aromatic and aliphatic alcohols.
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2.4 Experimental procedure
2.4.1 Preparation and characterization of PMO catalysts
The HTC (hydrotalcite) catalyst precursors were prepared by a co-precipitation method. The catalyst prepared in this procedure is named as Cu20-PMO in which 20% of the Mg2+ ions were replaced with Cu2+ ions in a 3:1 Mg/Al hydrotalcite precursor.
In a typical procedure, a solution containing AlCl3·6H2O (12.07 g, 0.05 mol), Cu(NO3)2·2.5H2O (6.98 g, 0.03 mol) and MgCl2·6H2O (24.40 g, 0.12 mol) in deionized water (0.2 L) was added to a solution containing Na2CO3 (5.30 g, 0.05 mol) in water (0.3 L) at 60 oC under vigorous stirring. The pH was kept between 9 and 10 by addition of small portions of a 1 M solution of NaOH. The mixture was vigorously stirred at 60 oC for 72 h. After cooling to room temperature, the light blue solid was filtered and resuspended in a 2 M solution of Na2CO3 (0.3 L) and stirred overnight at 40oC. The catalyst precursor was filtered and washed with deionized water until chloride free. After drying the solid for 6 h at 100 oC, 15.07 g of the hydrotalcite (HTC) was obtained. Before use, 4 g of hydrotalcite was calcined at 460 oC for 24 h in air and 2.5 g of Cu20-PMO can be obtained.
The Mg/Al-PMO catalyst (without any dopants) was prepared using AlCl3·6H2O (12.07 g, 0.05 mol) and MgCl2·6H2O (30.50 g, 0.15 mol) following the same procedure as above and 15.3 g Mg/Al-HTC was obtained. Before use, 3 g of this hydrotalcite was calcined to yield about 2 g Mg/Al-PMO catalyst.
Powder X-ray analysis was performed on a Bruker XRD diffractometer using Cu Kα radiation and the spectra were recorded in the 2θ angle range of 5°-70°. Elemental analysis was performed on a Perkin Elmer instrument (Optima 7000DV). The textural characterization was achieved using conventional nitrogen adsorption/desorption method, with a Micromeritics ASAP 2420 automatic analyzer. Prior to nitrogen adsorption, the samples were outgassed for 8 h at 250 oC. The Barrett−Joyner−Halenda (BJH) method was used for the calculating of pore volume and average pore size. TEM measurements were performed on a FEI Tecnai T20 electron microscope. Elemental distribution was measured in STEM mode using a HAADF detector and an X-max 80 EDX detector (Oxford instruments).
2.4.2 Mild depolymerization of lignocellulose
The mild depolymerization of pine lignocellulose was carried out in a 25 mL or 100 mL high pressure Parr autoclave with an overhead stirrer. Typically, the autoclave was charged with Cu20-PMO catalyst (0.2 g), pine lignocellulose (1 g), 3,5-dimethylphenol (20 mg as internal standard) and methanol (10 mL). The reactor was sealed and pressurized with H2 (40 bar) at room temperature. The reactor was heated to 180 oC and stirred at 400 rpm for 18 h. After reaction, the reactor was cooled to room temperature. Then 0.1 mL solution was collected with syringe and injected to GC-MS or GC-FID after filtration with a PTFE filter (0.42 µm). After that the solution and solids were transferred into a 50 mL centrifuge tube. The solid was separated from the reaction solution by centrifugation and subsequent decantation,
57 additionally washed with methanol (2 × 40 mL), and dried overnight in the desiccator under vacuum. All the solution was collected in a round bottom flask and the solvent was removed. The remaining products were dried with rotary evaporation under vacuum.
Analysis of the liquid sample was performed on a Hewlett Packard 6890 series equipped with a HP-5 capillary column and a flame ionization detector (FID). The following operating conditions were used: injection temperature of 300 oC, column temperature program: 40 oC (5 min), 10 oC /min to 280oC (6 min), detection temperature of 300 oC. Quantification of the lignin monomers was performed as follows: Sensitivity factors of the products were obtained by calibration with authentic standards. Identification of lignin monomers was first performed on GC-MS and then confirmed by comparing with authentic standards.
For the reaction described in Table 2.1, Entry 10, the methanol soluble part was purified by column chromatography on silica gel with pentane: ethyl acetate (4:1) as eluent. This afforded dihydroconiferyl alcohol (1G) as colorless oil (40 mg).
1H NMR (400 MHz, CDCl 3): δ 6.73 (d, J = 7.8 Hz, 1H), 6.59 (d, J = 8.1 Hz, 2H), 3.77 (s, 3H), 3.57 (t, J = 6.4 Hz, 2H), 2.62 - 2.44 (m, 2H), 1.84 - 1.67 (m, 2H). 13 C NMR (101 MHz, CDCl3): δ 146.41, 143.66, 133.70, 120.87, 114.26, 111.00, 62.23, 55.83, 34.45, 31.73.
HMRS (ESI) calculated for C10H14O3 [M+H]+: 181.08592, found 181.08698.
2.4.3 Conversion of methanol insoluble residues in supercritical methanol
The solid residue obtained after the mild depolymerization reactions described above was further treated according to the following procedure: The solid residues containing unreacted lignocellulose mixed with the Cu20-PMO catalyst were placed in 10 mL Swagelok stainless steel microreactors. Typically, the lignocellulose solids originating from conversion of 1 g lignocellulose were first separated to 4 equal parts based on weight and then transferred to 4 identical microreactors. 3 mL methanol was then added to each reactor and they were sealed and placed into a pre-heated aluminum block preheated to 320 oC. After the indicated reaction time, the microreactors were rapidly cooled in an ice-water bath and the contents of the reactors were quantitatively transferred to a centrifuge tube. The liquids were separated by centrifugation and decantation and subsequently analyzed by GC-MS-FID. The remaining solids were dried in a desiccator under vacuum for overnight until stable weight.
2.4.5 Control experiments using pine lignocellulose
Control experiments were performed in a 25 mL Parr reactor following the procedure described in Chapter 2.4.2. After reaction the methanol soluble part was concentrated to 10 mL by evaporating part of the solvent and then transferred to a 15 mL centrifuge tube in which methanol was fully removed and the solids were dried in a desiccator until stable weight. Then solids were washed with 10 mL DCM (dichloromethane) and the suspensions were additionally treated in an ultrasonication bath to help solubilization. After
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centrifugation, the DCM soluble fraction was transferred to a round bottom flask. This process was repeated two times to remove all the DCM soluble products. The DCM was then removed from the combined washings. The DCM soluble products were analyzed by 2D NMR using CDCl3. After solvent exchange to THF, these samples were additionally analyzed by Gel Permeation Chromatography (GPC).
2.4.6 Catalyst recycling tests and leaching tests
Catalyst recycling tests were carried out using pine lignocellulose. In a typical run 0.5 g pine lignocellulose and 0.2 g Cu20-PMO, 10 mL methanol was used (Step 1, mild). After that the solid residues were separated and fresh methanol was added and the mixture was heated to 320 oC (Step 2, supercritical). After that the content of the reactor was transferred to a centrifuge tube, and the methanol solution was separated from the solid (catalyst) by centrifugation and subsequent decantation. The solid was additionally washed with methanol (2 × 20 mL), then with acetone (1 × 20 mL), and dried overnight at room temperature in vacuum. The solid (catalyst) was then used in the next run as the same procedure (Step 1 and Step 2). The products from Step 1 and Step 2 were both analyzed by GC-FID or GC-MS-FID.
Leaching tests: 1 mL of the solution obtained after reaction was transferred to a 10 mL glass vial, and the solvent was removed under vacuum. Aqua Regia was then added to the vial and the concentration of each metal was measured by Perkin Elmer instrument (Optima 7000DV).
2.4.7 Determination of lignin content
Lignin content was determined by the acetyl bromide method (ABSL)35,36. The dried lignocellulose chips (between 2 and 5 mg) were added to 10 mL glass vials with 2.5 mL of 25% acetyl bromide in acetic acid. The vials were tightly sealed with Teflon lined caps. Then they were stirred overnight at room temperature until the wall tissue of lignocellulose completely dissolved. The samples were then transferred to 50 mL volumetric flasks containing 2 mL NaOH (2 M). The tubes were rinsed with acetic acid to complete the transfer. Then 0.35 mL of freshly prepared hydroxylamine hydrochloride (0.5 M) was added to the volumetric flasks which were then made up to 50 mL with acetic acid and inverted several times. The absorbance of the solutions was recorded at 280 nm with UV/Vis spectrophotometer (Model DU730, Beckman Coulter, Brea, CA). Lignin content was then calculated based on the calibration curve made with organosolv lignin from Sigma Aldrich. The results are shown in Table 2.3.
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