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Influence of acidic (HPO) and alkaline (NaOH) additives on the catalytic reductive fractionation of lignocellulose Tom Renders, Wouter Schutyser, Sander Van den Bosch, StevenFriso Koelewijn, Thijs Vangeel, Christophe M. Courtin, and Bert F. Sels ACS Catal., Just Accepted Manuscript • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 10, 2016
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ACS Catalysis
Influence of acidic (H3PO4) and alkaline (NaOH) additives on the catalytic reductive fractionation of lignocellulose Tom Renders,a Wouter Schutyser, a Sander Van den Bosch,a Steven-Friso Koelewijn,a Thijs Vangeel,a Christophe M. Courtin,b and Bert F. Sels a* a
Center for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
b
Center for Food and Microbial Technology, KU Leuven, Kasteelpark Arenberg 22, 3001 Leuven, Belgium
ABSTRACT: Reductive catalytic fractionation of lignocellulose is a promising ‘lignin-first’ biorefinery strategy wherein lignin is solvolytically extracted from the cell wall matrix and simultaneously disassembled, resulting in a stable lignin oil and a solid carbohydrate-rich residue. Herein, we report on the different influence of acidic (H3PO4) and alkaline (NaOH) additives on the Pd/C-catalyzed reductive processing of poplar wood in methanol (MeOH). It was found that the addition of small quantities of H3PO4 results in three rather than two product streams, since under acidic conditions both delignification and alcoholysis of hemicellulose are promoted, leaving behind a cellulose-rich pulp. The simultaneous acidcatalyzed fractionation of the carbohydrates into separate cellulose and hemicellulose streams provides opportunities for more efficient down-stream conversion as processing parameters can be tailored to the needs of both streams. Alkaline conditions on the other hand also enhance delignification, but additionally cause (i) the formation of other lignin products than under neutral and acidic conditions, (ii) a hampered degree of lignin depolymerization and (iii) substantial loss of cellulose from the pulp. Further on, a modified process descriptor (LFFE: lignin first fractionation efficiency) was applied to evaluate the fractionation efficiency of lignocellulose in its three major constituents. According to this new efficiency measure, mildly acidic conditions performed best.
KEYWORDS: lignocellulose, biorefinery, lignin chemistry, heterogeneous catalysis, hydrogenolysis, fractionation INTRODUCTION The goal of the ‘bio-refinery’, in analogy to the petrorefinery, is to fractionate a raw renewable carbon resource into specific and purified product mixtures, creating valuable streams which can be processed further by the chemical industry.1-4 The most abundant and probably most promising sustainable resource is lignocellulose, comprising three major biopolymers: cellulose, hemicellulose and lignin.5-7 Many biorefinery schemes focus on (hemi)cellulose valorization, while the lignin fraction is regarded as a waste product that can be implemented in low-value applications (binders, dispersants, emulsifiers etc.) but is mostly burned for energy recuperation.8, 9 In pursuing more added-value and making the future biorefinery more profitable, lignin valorization recently got into the spotlight.10-19 However, selective conversion is hampered by lignin’s complex chemical structure and its tendency to repolymerize. Nevertheless, continuous progress is being made leading to auspicious reductive,20-22 oxidative,23, 24 and bio-catalytic25, 26 lignin valorization approaches. Catalytic reductive lignocellulose fractionation One promising biorefinery strategy is the catalytic reductive fractionation of lignocellulose, wherein woody or
herbaceous biomass is processed at elevated temperatures (423 K – 523 K) in an organic solvent (e.g. methanol) in presence of a heterogeneous redox catalyst under hydrogen atmosphere (Figure 1).20-22, 27-34 During this process, lignin as present in the matrix is extracted through thermally-induced solvolysis and is simultaneously disassembled via hydrogenolysis. This results in a select number of phenolic monomers, dimers and short oligomers. Repolymerization is inhibited in this ‘ligninfirst’ concept since reactive intermediates are quenched by reductive stabilization, leading to monomer yields close to the theoretical maximum.20, 29 These monomers are potential intermediates for the production of sustainable polymer building blocks (e.g. alternative alkylphenols, bisphenols, caprolactone),35-37 commodity chemicals (e.g. phenol, propylene, methanol),38 or biofuels (e.g. C6-C18 alicyclic compounds).29, 31, 39-45 Besides the acquired lignin oil, almost all (hemi)cellulose sugars are retained in a carbohydrate-rich solid residue which is suitable for further chemocatalytic20, 28, 46-51 or biocatalytic21, 31 valorization. Regarding this biorefinery approach, the chemical structure of the obtained phenolics is strongly affected by the choice of redox catalyst in combination with the applied hydrogen pressure. By tuning these parameters,
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Figure 1. Schematic representation of the catalytic reductive fractionation of lignocellulose. Lignin is solvolytically extracted from the matrix and instantly depolymerized with a redox catalyst via hydrogenolysis. The result is a (hemi)cellulose-enriched pulp in addition to a lignin product oil. Several studies were conducted in our group regarding this process.20, 27, 28 Here, we report on the effects of acidic (H3PO4) and alkaline (NaOH) additives on this ‘lignin first’ biorefinery strategy, with focus on the fate of both the lignin fraction as well as the carbohydrate fractions. The simultaneous fractionation of the carbohydrates into a separate cellulose residue and soluble hemicellulose stream could be advantageous for further down-stream carbohydrate valorization.
n-propyl-, n-propanol-, or n-propenyl-substituted guaiacol and -syringol can be selectively acquired.22, 27 Furthermore, also the choice of solvent plays a critical role as it determines to which degree delignification takes place and to what extent the carbohydrate fraction remains untouched.28 To quantify this balance, a new efficiency measure was recently introduced, denoted as LFDE (lignin first delignification efficiency), which is defined as the degree of delignification times the retention of carbohydrates. Methanol and ethylene glycol were identified as the best performing bio-derivable solvents according to this new empirical descriptor.28 Nonetheless, either long reaction times or high temperatures are required in order to achieve a near complete delignification, implying a large energy input and a high operating pressure. Furthermore, retention of both cellulose and hemicellulose in the carbohydrate pulp is not always desired since efficient bio- or chemocatalytic conversion of these carbohydrate fractions often requires different processing conditions. Also, certain pulp applications prefer a pure cellulose substrate. In this regard, removal of hemicellulose from the matrix in the form of a third product stream can be interesting (Scheme 1). To enhance lignin and/or hemicellulose removal at lower temperature, acidic or alkaline additives are often employed in biomass pretreatment and fractionation processes, including paper manufacturing and bio-ethanol production,52-58 though their use is fairly unexplored within the context of reductive lignocellulose processing.29, 33 Acid-catalyzed lignocellulose processing Dilute aqueous acids (e.g. H3PO4,59-62 H2SO4,63-68 and organic acids69-71) are known to catalyze the depolymerization of hemicellulose by hydrolysis of
glycosidic bonds, making the cellulose in the recalcitrant cell wall more accessible. Dilute acid pretreatment is therefore often employed prior to enzymatic cellulose hydrolysis and fermentation.72-74 Related to dilute acid pretreatment is (aqueous) organosolv fractionation, typically executed in mild-acidic water-organic solvent mixtures. Besides hemicellulose removal, also lignin is extracted from the matrix by dissolution in the organic solvent. The result is (i) a cellulose-enriched pulp, (ii) soluble sugar products derived from hemicellulose and (iii) a solid lignin residue obtained after precipitation from the extraction liquor.65, 66, 69, 70, 75, 76 Another emerging technology is γ-valerolactone (GVL)-assisted processing. This fractionation technique uses a mixture of GVL, water and dilute H2SO4 to completely solubilize the lignocellulosic biomass, including the lignin fraction. As demonstrated by Dumesic et al., the carbohydrates are hydrolyzed and the lignin fraction can be precipitated afterwards, generating a residue closely resembling the native lignin structure.64, 77, 78 Base-catalyzed lignocellulose processing Alkaline regimes (e.g. NaOH,79-81 Ca(OH)2,82-84 and ammonia85-90) are known to promote delignification by breaking ester linkages between lignin and carbohydrates combined with lignin fragmentation, dissolution and/or repolymerization.57 For this reason, many traditional pulping processes are executed with alkaline additives, for example kraft pulping (NaOH/Na2S) and soda pulping (NaOH/anthraquinone).10, 91-93 More recently, several pretreatment processes involving ammonia were developed mainly because of its ease of recuperation by simple evaporation. Some examples are AFEX (ammonia fiber expansion)89, ARP (ammonia recycle percolation)87
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and SAA (soaking in aqueous ammonia).90 Although bases strongly augment lignin removal, partial solubilization of (hemi)cellulose together with biomass swelling and crystallinity alteration of cellulose are possible side effects.57, 58, 94, 95 In this contribution, the effects of two frequently used pretreatment agents, H3PO4 and NaOH, were investigated on the catalytic reductive fractionation of poplar (Populus sp.) and pine wood (Pinus sp.) in MeOH. An extensive qualitative and quantitative analysis of the complete lignin product fraction (monomers, dimers, oligomers) was performed, together with determination of the retention of pentose and hexose carbohydrates in the pulp. The results reveal characteristic effects of acidic and alkaline processing conditions on the major lignocellulose constituents. To evaluate the fractionation of lignocellulose in its three major constituents, a new, modified version of the LFDE descriptor is introduced. This measure is termed lignin first fractionation efficiency (LFFE) and is defined as a product of three terms: the degree of delignification, the removal of hemicellulose sugars and the retention of cellulose (~glucose) in the pulp. EXPERIMENTAL Chemicals & materials For a list of all used chemicals and materials as well as a more complete description of the experimental procedures, the reader is kindly referred to the ESI†. Sawdust Dry poplar and pine wood were milled and sieved to obtain a sawdust fraction with a size of 250-500 µm. Next, a twostep extraction procedure was executed utilizing a Soxtec 2055 Avanti apparatus to remove any extractives like fats, waxes etc.96 Porous thimbles with ~2.5 g sawdust were completely submersed in 70 mL of a boiling 2:1 (v/v) toluene:ethanol mixture for 15 min, followed by a standard Soxhlet extraction step in which the thimbles were kept above the boiling liquid for 3 h. After extraction, the samples were dried overnight at 353 K. To determine the Klason lignin content, a standard two-step acid (H2SO4) hydrolysis procedure was followed, according to Lin & Dence (ESI†).97 Catalytic reaction In a typical reaction, 2 g of pre-extracted sawdust was loaded in a 100 ml stainless steel batch reactor (Parr Instruments & Co.), together with 40 mL MeOH, 0.2 g Pd/C (5 wt%) and NaOH or H3PO4. The reactor was sealed, flushed with N2 and pressurized with 2 MPa H2 at room temperature (RT). The mixture was stirred at 750 rpm and the temperature was increased to 473 K (~ 15 K min-1). After the pre-set reaction time, the reactor was cooled and depressurized at RT. The reactor content was quantitatively collected by washing with ethanol, filtered and washed to separate the solid residue from the liquid product mixture. Alkaline product mixtures were acidified with a slight excess of dilute HCl to neutralize phenolate species.98
Lignin product analysis MeOH was evaporated from the liquid product mixture, followed by a threefold liquid-liquid extraction with dichloromethane (DCM) and water to separate solubilized carbohydrate derivatives, acids and salts from the more apolar lignin hydrogenolysis products. Next, also DCM was evaporated yielding a viscous lignin-derived oil which was further dried overnight at 353 K. The weight of the obtained oil was used to determine the degree of delignification (based on the weight of the Klason lignin content). To analyze the products, the oil was resolubilized in ~7 mL DCM together with a weighed amount of 2isopropylphenol serving as internal standard. The monomers were quantified by gas chromatography (quantification explained in the ESI†) using an Agilent 6890 Series chromatograph equipped with a HP5-column and a flame ionization detector (FID). Quantification of the dimers was similar to the monomers, yet trimethylsilylation with N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) was applied to increase the volatility of the compounds. To qualitatively verify the degree of lignin depolymerization, the molar mass distribution was studied with gel permeation chromatography (GPC). An aliquot sample of the product oil was solubilized in THF. Measurements were performed on a Waters E2695 separations module equipped with a M-gel column (3 µm, mixed), using THF as eluent (1 mL min-1) and UV-detection at 280 nm. Carbohydrate analysis The sugar retention was based on the amount of sugars in the lignocellulose substrates and in the carbohydrate pulp after reaction, using a standard total sugar procedure, adapted with hydrolysis conditions for cellulose-rich materials (ESI†).69, 99, 100 In addition, X-ray diffraction spectroscopy (XRD) was used to indicate the degree of cellulose crystallinity in the pulps, while scanning electron microscopy (JEOL JSM-6010 JV microscope ) was used to get insight in the micromorphology of the sawdust particles. RESULTS & DISCUSSION H3PO4 and NaOH were tested in different concentrations (1.25 g L-1 to 5 g L-1) in the catalytic reductive fractionation of poplar wood to study the influence of the apparent pH on the process. Although H2SO4 is the most employed inorganic acid in biomass pretreatment, utilization of H3PO4 effectuates milder and less corrosive process conditions. Reactions were performed for 3 h at 473 K with 2 g pre-extracted poplar sawdust, a fast-growing energy crop relevant to the biorefinery concept.101, 102 Methanol was used as a solvent to extract lignin from the matrix, while Pd/C (5 wt%) under hydrogen atmosphere (2 MPa) was used as catalyst to instantly convert the extracted lignin to monomers, dimers and short oligomers. The choice for methanol and Pd/C is motivated in our previous study.27
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Figure 2. Delignification (right axis) and phenolic monomer yield (left axis) for the reductive fractionation of poplar sawdust with varying concentrations of H3PO4 or NaOH. Reaction conditions: 2 g pre-extracted poplar sawdust, 40 mL MeOH, 0.2 g Pd/C (5 wt%), 2 MPa H2 at room temperature, 473 K, 3 h. A benchmark reaction at 523 K (neutral) was performed as a reference for comparison.
Delignification & monomer analysis After reaction, the complete product mixture was filtered to separate the catalyst and the carbohydrate pulp from the solubilized lignin products. Next, a lignin product oil was prepared by evaporation of the reaction solvent followed by liquid-liquid extraction with water and dichloromethane (DCM) to separate the lignin products from other, more polar, solubilized compounds (sugars, acidic or base additive). The DCM-phase was subsequently evaporated to yield the lignin oil. For alkaline reactions, the filtered solution was acidified with dilute HCl prior to solvent evaporation to neutralize hydrophilic phenolate species. The degree of delignification is determined by the mass ratio of the lignin product oil and the measured Klason lignin content (ESI†), and is depicted in Figure 2 for processing in presence of different acid and base concentrations. Delignification increases with increasing acidity or basicity ranging from 56% (neutral) to 96% (5 g L-1 H3PO4) and 85% (5 g L-1 NaOH). Processing in neutral or acidic media resulted in a yellowish to brown product, whereas the lignin oil obtained from alkaline fractionation was deep black for all NaOH concentrations (Figure S1). Besides the degree of delignification, Figure 2 shows the respective monomer yields, determined by gas chromatography (GCFID). The total monomer yield from the reaction in neutral conditions equals only 26 C-mol% of the Klason lignin content. 86% of the monomers are composed of 4-npropanolguaiacol (1) and –syringol (2) whereas a smaller fraction is composed of the n-propyl-substituted analogues (3, 4), in agreement with earlier work.27 With increasing acidity, the total monomer yield strongly increases until a plateau close to the theoretical maximum (~42%) is reached, with most of the monomers bearing a n-propanol substituent (83%). Concurrently, also minute quantities (
