Solvolysis of wood and pure cellulose by anhydrous hydrogen fluoride vapor

Biomass 12 (1987) 227-246

Solvolysis of Wood and Pure Cellulose by Anhydrous Hydrogen Fluoride Vapor G r e g o r y L. R o r r e r , S a m i S. A s h o u r , M a r t i n C. H a w l e y Department of Chemical Engineering, Michigan State University, East Lansing, MI 48824-1226, USA

and Derek T. A. Lamport MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, M1 48824, USA (Received 6 October 1986; revised version received 12 February 1987; accepted 13 February 1987) ABSTRACT The solvolysis of bigtooth aspen wood and pure cellulose (filter paper) by anhydrous hydrogen fluoride (HF) vapor was investigated. Each substrate was exposed to a 100% HF vapor stream at 0.101 MPa and 27°C to 77°C for a given reaction time. The water-soluble reaction products were then post-hydrolyzed in 2 N trifluoroacetic acid for 1 h at 121°C. With this twostep hydrolysis, maximum glucose yields of 85% from wood and 94% from pure cellulose were obtained. HPLC analysis of the water-soluble reaction products obtained before post-hydrolysis revealed that glucose and glucosyl fluoride repolymerized to oligomers during vapor-phase HF solvolysis of cellulose. The oligosaccharide product distribution was dependent on reaction time, temperature and sample water content. Key words." Hydrogen fluoride, solvolysis, cellulose.

1 INTRODUCTION T h e use of anhydrous hydrogen fluoride (HF) to convert lignocellulose to glucose and lignin offers high glucose yields, ambient processing conditions, and near 100% H F recovery. A comparison of the H F hydro227 Biomass 0144-4565/87/S03.50- © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

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G. L. Rorrer, S. S. Ashour, M. C. Hawley

lysis of lignoceUulose with other lignocellulose hydrolysis processes has been made by Hawley et al.1 Anhydrous HF in either the liquid or vapor phase can hydrolyze cellulose. Recent laboratory-scale studies have characterized the basic reaction chemistry 2-7 and the kinetics 8-9 of the liquid-phase HF solvolysis of wood and pure cellulose. However, all pilot-scale studies have focused on the vapor-phase HF solvolysis of lignocellulose, because less HF is required for processing and the HF adsorbed on the lignocellulose is more easily recovered. Most recently, researchers at Canertech of Canada,~°,l ~ Biotechnical Institute of Denmark, l 2.~3 and A. G. Hoechst ~4 studied the pilot-scale, vapor-phase HF solvolysis of various lignocellulosic substrates. Although the configuration of each pilot-scale process was slightly different, all reported that glucose recoveries ranged from 80% to 90%, and 99% of the HF used for processing was recovered and recycled. Despite the success of these pilot-scale studies, fundamental reactionrate and sugar-yield data for the vapor-phase HF solvolysis of lignocellulose are still scarce. Fredenhagen and Cadenbach ~5 first studied solvolysis of cellulose by HF vapor and determined that if the vapor temperature was low enough, then a liquid adsorption phase would be formed on the substrate. They reported that at ambient HF vapor temperature, one part HF to one part cellulose (by weight) was loaded onto the substrate, and after a 15 minute reaction time, the maximum watersoluble carbohydrate recovery ranged from 85% to 95%. Hoch and Bohnuek, ~6 and later Sarkov et al. ~7 studied the vapor-phase HF solvolysis of pine wood. Both concluded that much less HF is required for the vapor-phase process than the liquid-phase process, and 75% to 80% of the original polysaccharides in pine wood were rendered water-soluble following hydrolysis by HF vapor. Recently, Rorrer et al. TM studied the kinetics of the solvolysis of wood (bigtooth aspen) by HF vapor on a laboratory scale. Experimentally, a single 'bone-dry' wood chip (0.4 to 0-5 mm thickness) was exposed to an HF vapor stream of known HF partial pressure (0.02 to 0.101 MPa diluted in nitrogen) and temperature (28°C to 108°C) for a given reaction time. The water-soluble reaction products were 'post-hydrolyzed' in dilute acid to recover glucose and xylose from 'reversion' oligosaccharides. Maximum glucose yields of 80% to 85% and xylose yields of 70% to 80% were obtained (after posthydrolysis) from wood chips exposed to 100% HF vapor stream at 28°C for at least 3 min. The glucose yield versus time curve was sigmoidal, decreased nonlinearly with decreasing HF partial pressure, and decreased with increasing temperature, indicating that physical adsorp-

Solvolysis of wood and pure cellulose

229

tion of HF vapor onto the lignocellulosic matrix is prerequisite to cellulose solvolysis. The two major requirements for effective acid-hydrolysis of lignocellulose are high glucose yield under favorable hydrolysis conditions and efficient acid recovery/recycle. In this paper, the solvolysis of lignocellulose by HF vapor is studied with emphasis on the first requirement, high glucose yield under favourable hydrolysis conditions. Previous work on this subject shows that three fundamental questions still need to be answered. What is the glucose yield versus time curve for the solvolysis of wood versus pure cellulose by 100% HF vapor as a function of reaction temperature? What is the maximum glucose yield attainable from the vapor-phase HF solvolysis of pure cellulose relative to cellulose in wood? What is the 'reversion' oligosaccharide product distribution before post-hydrolysis and what reaction parameters is it dependent upon? The answers to each of these questions are essential for comparing the reaction rates and glucose yields of the vapor-phase process with the more well-studied liquid-phase process. Ultimately, these results can also be used to more quantitatively assess the feasibility of vapor-phase HF solvolysis of lignocellulose relative to other lignocellulose hydrolysis technologies. The objective of this investigation is to determine the glucose yield and oligosaccharide product distribution for the vapor phase HF solvolysis of wood and pure cellulose as a function of time and temperature under carefully controlled, laboratory-scale conditions. Experimentally, cellulose is hydrolyzed to glucose in two steps. First, a single wood or pure cellulose 'chip' is exposed to a 100% HF vapor stream for a given time and temperature, then immersed in water to stop the reaction. The water-soluble reaction products are then mildly 'post-hydrolyzed' in dilute acid to recover glucose from 'reversion' oligosaccharides. Water-soluble reaction products both before and after post-hydrolysis are analyzed by HPLC.

2 MATERIALS AND METHODS 2.1 Hydrogen fluoride Anhydrous hydrogen fluoride (HF) of 99.9% purity was obtained in liquid form from Matheson Gas Products. The HF liquid was contained in a sealed steel cylinder (1.6 kg HF capacity) and stored at room temperature under its own vapor pressure (107.5 kPa). For safety con-

230

G. L. Rorrer, S. S. Ashour, M. C. Hawley

siderations, the HF cylinder was kept in a fume hood. The normal boiling point of HF is 19.54°C.

2.2 Sample preparation Bigtooth aspen (Populus grandidentata Michx) and Whatman filter paper No. 3 respectively served as the lignocellulose and pure cellulose substrates. The composition of bigtooth aspen is given in Table 1. Roughlychipped bigtooth aspen was microtomed with the grain of the wood into shavings of 0.4 to 0"5 m m thickness. The shavings were cut perpendicular and parallel to the grain of the wood into square wafers of 1 cm per side. Whatman filter paper No. 3 (99% pure crystalline cellulose of ca. 0.5 mm thickness) was cut into square wafers of 1-5 cm per side, similar to the wood wafers. The wood and pure cellulose wafers were dried and stored in vacuo at room temperature over phosphorous pentoxide. The typical dry weight was 30 mg for each wood chip and 50 mg for each 'pure cellulose' (filter paper) chip. TABLE 1

Composition of Bigtooth Aspen (Dry Basis) Component

Wt (%)

Cellulose Hemicellulose Lignin Extractives Ash

50"0 29.0 16.9 4.0 0' 1

2.3 Vapor-phase HF solvolysis A single wood or pure cellulose wafer is exposed to a 100% HF vapor stream within the apparatus shown in Fig. 1. The apparatus is designed to contain the HF vapor yet still provide easy access to the sample. HFwetted parts are constructed of the HF-resistant materials monel and Teflon. The entire apparatus is contained within a fume hood. Dried wood or pure cellulose wafers were weighed to precision of 0.1 mg. The HF vapor flowrate and temperature were set and allowed to stabilize. A single wood or pure cellulose wafer was loaded upright onto a Teflon sample holder and inserted into the reactor at the sample access port. The Teflon sample holder kept the sample aligned perpendicular to

Solvolysis of wood and pure cellulose

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the vapor-stream and collected the products of the reaction. After a specified reaction time, the sample holder and HF-reacted sample were removed from the reactor and immediately immersed in 2 ml of chromatography-grade water (room temperature) in order to stop the reaction and dissolve t h e water-soluble reaction products from the insoluble residue. The water contained a known inclusion of myoinositol (typically 10.0 mg), which served as an internal standard to account for sugar losses incurred during sample processing and chromatographic analysis. The sample holder was washed with an additional 2 ml of water, with the washings added to reaction products. The reaction mixture (water, sugars, and residue) was then thoroughly mixed to release all water-soluble reaction products into solution. The sample holder was carefully dried before reloading. The procedure was repeated with a new sample at different HF solvolysis times until the desired amount of yield versus time data were obtained. In all experiments, the HF flowrate was 1000 ml min-l, metered by a Brooks AR-MITE 37 stainless-steel rotameter at the 0"101 MPa reactor pressure and 27°C HF vapor delivery temperature. At this flowrate, HF vapor was f o o d e d relative to the substrate in a ratio of at least 500 to 1 (by weight) per minute. The HF vapor temperature in the

232

G. L. Rorrer, S. S. Ashour, M. C. Hawley

reactor was set at either 27°C, 50°C, or 77°C. At each temperature the reaction time was varied from 10 seconds to 8 minutes.

2.4 HF neutralization When the HForeacted lignocellulose is immersed in water, glucosyl fluoride and other fluoridated sugars are hydrolyzed to release free sugars and HF into solution. Any HF that is freely adsorbed on the HF-reacted substrate is also released into solution. Calcium carbonate powder was added (in 5 mg increments) to the sugar product solution to precipitate the dissolved HF to calcium fluoride. The neutralized sugar product solution was centrifuged from the insoluble residue (lignin, unreacted cellulose, calcium carbonate, calcium fluoride), decanted, and frozen for storage.

2.5 Post-hydrolysis In earlier liquid-phase studies, it was reported that glucose and glucosyl fluoride repolymerized to oligomers in HF solution. 2-7 Thus, the neutralized sugar product solution was mildly hydrolyzed in dilute acid to recover glucose from these 'reversion' oligosaccharides. Typically, a 50 /A aliquot of the sugar product solution (containing 0 to 150/~g sugar) was blown-down to dryness over nitrogen at 50°C, and then hydrolyzed in 200 ~1 of 2 N trifluoroacetic acid (TFA) for 1 h at 121°C. At these post-hydrolysis conditions, the hydrolysis of oligomers to glucose was complete and the acid-catalyzed degradation of glucose to hydroxymethylfurfural (HMF) was negligible. The post-hydrolyzed sample was blown-down to dryness over nitrogen at 50°C in order to evaporate the volatile TFA (b.p.--72.4°C). The sample was then redissolved in 50/A chromatography-grade water for sugar yield determination via carbohydrate HPLC.

2.6 Sugar chromatography The water-soluble products of the vapor-phase HF solvolysis of wood and pure cellulose were analyzed both before and after post-hydrolysis by High Performance Liquid Chromatography (HPLC). Glucose and other monomeric wood sugars obtained after post-hydrolysis were separated on a BIO-RAD Aminex HPX-87P carbohydrate analysis column at 85°C. A 'Microguard' deashing column (BIO-RAD Laboratories) was plumbed in front of the analysis column to remove any anions or particulates present in the 50 ~1 injected sample. The mobile phase

Solvolysis of wood and pure cellulose

233

was degassed HPLC-grade water at 0.6 ml min-~. Column effluent sugars were detected by their refractive index change relative to HPLCgrade water. Sugar yields were calculated by the 'internal standard' method, using myo-inositol as the reference/standard. Retention times for glucose, xylose, and myo-inositol were 14.0 minutes, 15.0 minutes, and 21.5 minutes, respectively. Water-soluble reversion oligosaccharides, cello-oligomers, and free sugars obtained before post-hydrolysis were separated on two BIORAD Aminex HPX-42A oligosaccharide analysis columns connected in series. The two-column arrangement was used to improve the separation of the oligomers of 5 to 8 glucose residues in size, because a single column could only effectively separate oligomers of 5 or less glucose residues in size. Column analysis conditions are the same as those described for the monomer sugar analysis column.

3 RESULTS AND DISCUSSION 3.1 Action of H F vapor on w o o d and filter paper structure

It was observed that the overall wood chip structure remained intact upon exposure to HF vapor, whereas the pure cellulose (filter paper) chip 'melted' to a glutinous drop. At complete cellulose conversion all the products of the vapor-phase HF solvolysis of pure cellulose were soluble in water, whereas for wood, a brown lignin shell retaining the overall structure of the original wood chip sample remained insoluble. 3.2 G l u c o s e yield versus time

Glucose yield versus time curves for the solvolysis of bigtooth aspen and pure cellulose (filter paper) by a 100% HF vapor stream at 27°C, 50°C, and 77°C, are presented in Figs 2, 3, and 4, respectively. All glucose yields shown were determined after post-hydrolysis of the water-soluble reaction products. The maximum theoretical glucose yield from bigtooth aspen was taken as 3-44 mmol g-~, an estimation based on previous material balance studies for the liquid-phase HF solvolysis of bigtooth aspen at 0°C. s The maximum theoretical glucose yield from pure cellulose (filter paper) was taken as 6.17 mmol g-~ based on the assumption that the cellulose is pure glucan. Glucose yields from pure cellulose, a substrate of known glucan composition, were higher than those from bigtooth aspen, a substrate of estimated glucan composition. Maximum glucose yields from pure cellu-

G. L. Rorrer, S. S. Ashour, M. C. Hawley

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lose consistently reached 90% to 94%, whereas glucose yields from bigtooth aspen consistently reached only 80% to 85%. The glucose yield versus time curve from both bigtooth aspen and pure cellulose was sigmoidal and decreased with increasing temperature.

Solvolysis of wood and pure cellulose

235

The fact that glucose yield versus time data from pure cellulose followed the same trends with time and temperature for as cellulose in wood further strengthened the supposition that HF must first physically adsorb onto the substrate, break hydrogen bonds joining the cellulose chains, and then cleave the fl-l,4 glycosidic linkage joining the glucose residues that constitute the cellulose chain.~8

3.3 Xylose yield Xylose production from the vapor-phase HF solvolysis of hemicellulose in bigtooth aspen followed the same trend with temperature as for glucose production from cellulose in bigtooth aspen. However, the rate of xylose production was faster than the rate of glucose production, because hemicellulose is amorphous, whereas cellulose in wood is primarily crystalline. After post-hydrolysis, the maximum xylose recovery was 70% of theoretical (the maximum theoretical xylose yield from bigtooth aspen was taken as 1.4 mmol g-1).8 However, the maximum xylose yield was 80%, provided a correction was made for xylose degradation during post-hydrolysis. The hydrolysis-loss (correction) factor was defined as the ratio of xylose amount before posthydrolysis to the xylose amount after post-hydrolysis. The hydrolysis-loss factor was determined to be 1.14 (average after five assays), based on a control experiment wherein 140 #g xylose and 160 /~g myo-inositol (internal standard) were reacted in 200/A of 2 N TFA at 121 °C for 1 h. Since the focus of this study was on glucose yield, xylose yield will not be considered further here.

3.4 Possible glucose yield losses The solvolysis of pure cellulose by HF vapor and the subsequent diluteacid post-hydrolysis of the water-soluble reaction products did not yield the maximum theoretical level of glucose, although maximum glucose recoveries of 90 to 94% were consistently obtained. To account for the loss, we first searched for the usual products of acid-catalyzed glucose dehydration, levoglucosan and HMF. Levoglucosan and HMF in standard solution were successfully separated and detected via carbohydrate HPLC on a BIO-RAD HPX-87P column at the operating conditions given in Section 2. The retention times for levoglucosan and HMF were 28.5 minutes and 33'0 minutes, respectively. HMF was not detected in the water-soluble reaction products of the vapor-phase HF solvolysis of cellulose at complete cellulose conversion, either before or after post-hydrolysis. However, levoglucosan was detected in small

236

G. L. Rorrer, S. S. Ashour, M. C. Hawley

amounts (0.5 to 1.0% by weight) in the water-soluble reaction products, both before and after post-hydrolysis. Post-hydrolysis did not appear to hydrolyze levoglucosan back to glucose significantly, but quantification was difficult because the amount of levoglucosan in the reaction products was near the quantifiable lower limit of our present carbohydrate HPLC. Levoglucosan was also detected in trace amounts in the reaction products of the liquid phase HF solvolysis of cellulose? The glucose degradation that occurred during post-hydrolysis was not the result of acid-catalyzed glucose dehydration. When 160 ktg glucose and 160/~g myo-inositol (internal standard) were reacted in 200/A of 2 N TFA at 12 I°C for 1 h, a net loss of 3% glucose occurred (average after 5 assays), with the dried reaction products possessing a faint yellow-red color. This result confers with the work of Ruiter and Burns, '9 who reported a net 3% glucose loss at the same hydrolysis conditions. Ruiter and Burns attributed the loss to the formation of a glucosyl trifluoroacetic ester. They reported that the glucose/glucosyl trifluoroacetic ester equilibrium shifted back toward glucose by a secondary hydrolysis in 0.25 N TFAat 121°C for 1 h.

3.5 HF-catalyzed glucose-lignin condensation In general, the maximum glucose yield obtained from the vapor-phase HF solvolysis of cellulose in wood (estimated as 80 to 85%) was lower than the maximum glucose yield obtained from the vapor-phase HF solvolysis of pure cellulose (90 to 94%). In this study, the maximum theoretical glucose yield from bigtooth aspen was taken as 3.44 mmol g- 1 dry wood, an estimation based on previous material balance studies for liquid-phase HF solvolysis of bigtooth aspen at 0°C. s,9 Despite the uncertainty of the glucan composition in bigtooth aspen, a 10% difference in normalized maximum glucose yield from bigtooth aspen relative to pure cellulose does suggest that glucose-lignin condensation is occurring, catalyzed by HF adsorbed on the wood. Circumstantial evidence already exists for glucose-lignin condensation during liquid-phase HF solvolysis of wood. Defaye et al., 6 reported that during the liquid-phase HF solvolysis of birchwood at 28°C, the glucose yield passed through a maximum of 95%, and then slowly decreased to a limiting value of 80% with increasing reaction time. However, at 0°C, a maximum yield of 95% was attained and remained steady with increasing reaction time. Defaye hypothesized that HF was catalyzing the reaction of glucose (or glucosyl fluoride) with functional groups (as yet unknown) on lignin. This reaction was temperature dependent, because at 28°C the reaction proceeded much more rapidly

Solvolysis of wood and pure cellulose

237

than at 0°C, where no perceptible condensation occurred. As an aside note, liquid HF also catalyzed the condensation of lignin onto itself.6,2° We attempted to recover oligosaccharides trapped in the waterinsoluble, HF-reacted lignin residue. Dried lignin samples from wood chips reacted with HF vapor to maximum glucose yield were hydrolyzed in 2 N TFA at 121°C for 1 h. This procedure yielded less than 2% additional glucose, indicating that oligomer-lignin adducts were probably not significantly formed. If glucose-lignin adducts were formed, the reaction mechanism probably involved the direct attack of glucosyl fluoride with the functional groups constituting lignin. 3.6 Comparison of maximum glucose yields from liquid- and vaporphase HF solvolysis

Glucose yields from liquid-phase HF solvolysis of bigtooth aspen are slightly higher than glucose yields from vapor-phase HF solvolysis of bigtooth aspen (Table 2). The difference was probably due to two factors: (1) lower temperatures were used for previous liquid-phase studies relative to the present vapor-phase studies, and so the (hypothesized) rate of glucose-lignin condensation would be correspondingly lower; and (2) the use of a hydrolysis-loss factor for post-hydrolysis, TABLE

2

Comparison of Typical Maximum Glucose Yields from Liquid-Phase and Vapor-Phase HF Solvolysis of Wood and Pure Cellulose

Substrate

HF phase (anhydrous)

Solvolysis conditions

Average maximum glucose yield",h

Temp. (°C)

Time (min)

Uncorrected (%)

Corrected (%)'

1 27 50

>/15 >i 3 /> 7

-92 90

97 95 93

Filter paper

liquid vapor

Bigtooth aspen

liquid

- 4

vapor

4 27 50

>/35

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93

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-82 82

92 84 84

"Maximum theoretical glucose yield from filter paper taken as 6' 17 mmol g-~; maximum theoretical glucose yield from bigtooth aspen taken as 3.44 mmol g- ~.~.'~ hLiquid-phase yield and kinetic data from Selke. ~,9 'Hydrolysis-loss correction factor for glucose during post-hydrolysis taken as 1.03.

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which was used in previous liquid-phase studies, but not in this present vapor-phase study (although for comparison, a hydrolysis-loss factor for glucose (1.03) was taken into account in the glucose yield calculations given in Table 2).

3.7 Reversion oligosaccharide profiles from pure cellulose Previous studies for the liquid-phase HF solvolysis of wood and pure cellulose revealed that glucosyl fluoride and glucose repolymerized to oligomers in HF solution.2-7 The oligosaccharide product distribution shifted toward higher oligomers with increasing reaction time, temperature and initial cellulose concentration in liquid HF. The glycosidic linkage composition was randomized, with the a-l,6 linkage predominant. If the liquid-phase reaction was terminated by dilution of HF in water followed by calcium carbonate neutralization, then the D.P. (degree of polymerization) of the oligosaccharide product distribution ranged from 1 to 6, with the average D.P. depending on solvolysis conditions. However, if the HF liquid was removed by evacuation at 100°C, the D.P. of the oligomers increased (although the oligomers still remained watersoluble) with the average D.P. centered around 6. In light of these previous liquid-phase results, it was desirable to characterize the watersoluble oligosaccharide product distribution of the vapor-phase HF solvolysis of cellulose obtained before post-hydrolysis as a function of solvolysis conditions. Chromatograms representing the oligosaccharide product distribution of the vapor-phase HF solvolysis of pure cellulose at 27°C, 50°C and 77°C are presented in Figs 5, 6, and 7, respectively. Glucose yields before and after post-hydrolysis for the samples profiled in Figs 5 to 7 are given in Table 3. The oligosaccharide product distribution was dependent on temperature and reaction time. When the HF vapor-stream temperature was increased, the oligomer product distribution dramatically shifted toward oligomers of greater than 8 glucose residues in size. The time course of the oligosaccharide product distribution at 27°C revealed that initially the oligomer profile was moving toward higher oligomers, whereas at long times (near complete conversion) the oligomer profile was shifting back toward glucose (Fig. 5). Such behavior suggests that water-soluble, r - l , 4 linked cello-oligomers were initially present, indicating that the cellulose-cleavage mechanism by HF vapor is random and not endpreferred. Coupled to this observation was the appearance at long reaction times of 'rider peaks' on the main peaks of oligomers of two or more glucose residues. Evidently, an array of oligomers of the same size but of different glycosidic linkage composition exist.

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TABLE 3 Glucose Yields from Vapor-Phase HF Solvolysis of Pure Cellulose Before and After Post-Hydrolysis for Selected Samples

Glucose yield (%)

Reaction time

Reaction temperature

(min)

(°C)

Before post-hydrolysis

After post-hydrolysis

1.50 2.50 4.00 6"00 1.50 2.50 6"00 6"00 1'50 2.00

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1.2 7'9 13.5 14.5 0"3 1.0 2.4 1.1 7.0" 14.9"

48.3 88"0 93"5 91.0 8"0 31.2 84.3 40.0 78.0 88.4

a 1 part water to 1 part dry cellulose.

Ideally, it would be desirable to isolate each oligomer and determine its glycosidic linkage composition via Hakomori methylation 2~ and gas-liquid chromatography of the alditol-acetate derivatives 22 of the methylated, hydrolyzed products. However, such a time-consuming and detailed analysis was beyond the scope of this paper. Further, since the oligosaccharide product distribution was perceived to be complex and not fully resolved by our present chromatographic analysis, the oligomer product distribution (beyond glucose) was not quantified.

Solvolysis of wood and pure cellulose

241

Evidently, the extent of reversion for the vapor-phase HF solvolysis of cellulose was much higher than for the liquid-phase HF solvolysis of cellulose. During the solvolysis of cellulose by HF vapor, glucosyl fluoride, glucose, and oligomers resided on the reacting substrate. In contrast, for the solvolysis of cellulose by HF liquid, glucosyl fluoride, glucose, and oligomers were diluted in HF solution. Since the concentration of glucosyl fluoride and glucose in the reaction medium was higher for the vapor-phase process than the liquid-phase process, the extent of reversion was correspondingly greater. The extent of reversion was also increased as the result of reaction temperature, which was always higher for the vapor-phase process than for the liquid-phase process. 3.8 Reversion oligosaccharide profile from bigtooth aspen The oligosaccharide product distribution for the vapor-phase HF solvolysis of bigtooth aspen exhibited nearly identical trends to those from pure cellulose. The oligosaccharide product distribution of the xylose residues (from hemicellulose) complicated the oligosaccharide profile and thus made the chromatogram difficult to interpret. However, a sample chromatogram is provided in Fig. 8, where the oligosaccharide product distribution for the vapor-phase HF solvolysis of bigtooth aspen at 27°C was profiled on a BIO-RAD HPX-42A oligosaccharide analysis column and BIO-RAD HPX-87P monosaccharide analysis column connected in series. 3.9 Effect of sample water content Mort and Parker 7 reported that the addition of water to the liquid HF/ cellulose reaction mixture at the initiation of the reaction dramatically 27°C (D ^

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G. L. Rorrer, S. S. Ashour, M. C. Hawley

shifted the reversion oligosaccharide product distribution toward glucose. Since the reversion oligosaccharide product distribution for the vapor-phase HF solvolysis of cellulose at 50°C was centered at oligomers of greater than 8 glucose residues in size, Mort's result temptingly suggested that water be added to the cellulose substrate just prior to cellulose solvolysis by HF vapor in the attempt to shift the oligosaccharide product distribution back toward glucose. In a preliminary set of experiments, 50/~1 of HPLC-grade water at room temperature was carefully pipetted onto a preweighed, 'bone-dry' cellulose chip of ca. 50 mg (1 part water to 1 part dry, pure cellulose by weight). The 'wetted' pure cellulose chip was then reacted with a 100% HF vapor stream at 50°C using the same procedure as the 'dry' cellulose experiments. Not expectedly, the addition of water dramatically shifted the oligosaccharide product distribution back toward glucose, as shown by the chromatogram in Fig. 9 (see Fig. 6 for comparison). Glucose yields for these samples before and after post-hydrolysis are given in Table 3. The addition of water also increased the glucose production rate (Table 4). Evidently, the hydrolysis of glucosyl fluoride to glucose by water in the 'wetted' substrate competed with the reaction of glucosyl fluoride with glucose or oligosaccharides to higher oligomers. 3.10 The rate processes solvolysis of cellulose

of liquid-phase versus vapor-phase HF

Evidently, liquid-phase HF solvolysis and vapor-phase HF solvolysis of lignocellulose are governed by different rate processes. For the liquidphase process, the cellulose is first dissolved in liquid HF and then

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5

1'0

~

0.

O 13"

15

2'0

s°°c fi

120

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25

30

35

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Fig. 9. Oligosaccharide profile (before post-hydrolysis) of 'wetted' pure cellulose (1 part water to 1 part dry cellulose by weight) reacted with a 100% HF vapor stream at 50°C and 1"0 atm.

Solvolysis of wood and pure cellulose

243

TABLE 4 Effect of Sample Water Content on the Glucose Yield for the Vapor-Phase HF Solvolysis of Pure Cellulose at 50°C

Reaction time (min)

Glucose yield (%) Dry a cellulose

Wed' cellulose

0'5

2

4

1'0

5

29

1'5 2'0

12 20

78 88

"Glucose yields interpolated from data in Fig. 3. h1 part water per 1 part dry cellulose (by weight).

solvolytically cleaved to glucosyl fluoride in HF solution, where the rate of glucose production increases with increasing temperature by Arrhenius' Law. However, for the vapor-phase process, HF vapor must first physically adsorb onto the cellulose before reacting. Since the heat of physical adsorption is always negative, the equilibrium loading of HF vapor on cellulose will decrease with increasing temperature by Le Chatelier's Principle and thus the glucose production rate will decrease with increasing temperature. 3.11 Future research

The vapor-phase HF solvolysis of lignocellulose is a promising biomassconversion technology worthy of continued research and development. The results of this study suggest three avenues for future research. First, the hypothesis of glucose-lignin condensation during vapor-phase HF solvolysis of lignocellulose must be tested. It is proposed that the maximum glucose yield from the vapor-phase HF solvolysis of pure cellulose be compared with the maximum glucose yield from vapor-phase HF solvolysis of cellulose/lignin substrates of known composition. The amount of glucose yield loss of the cellulose/lignin composite substrate relative to pure cellulose substrate could gauge the glucose yield lost to HF-catalyzed glucose-lignin condensation. Second, the adsorption/ desorption isotherm of HF vapor onto lignocellulose must be precisely determined on a laboratory scale. Toward this end, we have set up an experiment to measure the in situ gravimetric uptake of HF on a single lignocellulose chip as a function of time and reaction conditions. This

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G. L. Rorrer, S. S. Ashour, M. C. Hawley

data will be valuable for determining the optimum conditions for HF loading and recovery. Finally, the kinetics and product distribution of the reversion process must be more precisely characterized and quantified by improving the oligomer separation and determining the glycosidic linkage composition in each oligomer. Once the reversion process has been fully characterized, techniques for minimizing or inhibiting the extent of reversion can be identified, and routes for the enzymatic hydrolysis of oligomers to glucose or direct fermentation of oligomers to useful products can be explored.

4 CONCLUSIONS This study has shown that 100% anhydrous hydrogen fluoride (HF) vapor at ambient temperature and pressure rapidly hydrolyzed lignocellulose (wood) to water-soluble carbohydrates and water-insoluble lignin. However, glucose and glucosyl fluoride repolymerized to higher oligomers during vapor-phase HF solvolysis, with the oligosaccharide product distribution dependent on reaction time, temperature, and sample water content. Dilute-acid post-hydrolysis (2 N TFA at 121 °C for 1 h) of the water-soluble products readily recovered glucose from these 'reversion' oligosaccharides. The maximum glucose yield from vapor-phase HF solvolysis of pure cellulose (after post-hydrolysis) was 90 to 94% of theoretical, assuming that filter paper is pure glucan. Acid-catalyzed glucose dehydration to HMF or levoglucosan during vapor-phase HF solvolysis of cellulose was negligible. The maximum glucose yield from vapor-phase HF solvolysis of bigtooth aspen wood (after post-hydrolysis) was 0.50 to 0.53 g glucose per gram of dry wood (2-75 to 2.92 mmol g-l). On a normalized basis, the maximum glucose yield from bigtooth aspen was estimated as 80 to 85% of theoretical, assuming that the glucan composition of bigtooth aspen is 3.44 mmol g-1, an approximate value determined by previous material balance studies for liquid-phase HF solvolysis of bigtooth aspen wood at 0°C. HF-catalyzed glucose-lignin condensation was hypothesized to account for the difference in the maximum percent glucose yield from bigtooth aspen wood versus pure cellulose, despite the uncertainty of the glucan composition of bigtooth aspen. However, this hypothesis must be tested by reacting composite cellulose/lignin substrates of known composition with HF vapor. The results of these experiments will positively determine if maximum glucose yields from pure cellulose are higher than those from bigtooth aspen.

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245

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation under Contract CPE-8315703. Portions of this work were presented at Energy from Biomass and Wastes XI, Orlando, FA, 19 March 1987.

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12. Bentsen, T. (1982). Hydrolyse af halm via forbehandling med vanfri fluorbrinte, Medelser fra. Bioteknisk Institut. Afdelingfor BioteknoL, 1-2, 10-14. 13. Reffstrup, T. & Kau, M. (1985). Hydrogen fluoride aided hydrolysis of lignocellulosic biomass: development of a fast continuous process. In: New approaches to research in cereal carbohydrates, eds R. D. Hill and L. Munck, Elsevier Science Publishers B.V., Amsterdam, pp. 313-7. 14. Franz, R., Erckel, R., Riehm, T., Worule, R. & Deger, H. (1982). Lignocellulose saccharification by HF. In: Energy from biomass, second E.C. conference, eds A. Strub, P. Chartier and G. Schleser, Applied Science Publishers, London, pp. 873-8. 15. Fredenhagen, K. & Cadenbach, G. (1933). Der abbau cellulose durch fluorwasserstoff und ein neues verfahren der holzverzuckerung durch hochkonzentrierten fluorwasserstoff. Angnew. Chem., 46, 113-7. 16. Luers, H. (1938). Das holzverzuckerungsverfahren mit fluorwasserstoff von Hoch und Bohunek. Holz Roh und Werkstoff, 1,342-4. 17. Sarkov, V. I., Bolotova, A. K. & Boiko, T. A. (1972). Wood hydrolysis by anhydrous hydrogen fluoride. Komplek. Pererab. Rast. Syr., 39, 39-49. 18. Rorrer, G. L., Hawley, M. C. & Lamport D. T. A. (1986). Reaction rates for gas-phase hydrogen fluoride saccharification of wood. I &EC Prod. R &D, 25, 589-95. 19. Ruiter, J. M. & Burns, J. C. (1986). Rapid determination of cell-wall monosaccharides in flaccidgrass. J. Agric. Food Chem., 34, 780-5. 20. Smith, J. J., Lamport, D. T. A., Hawley, M. C. & Selke, S. M. (1983). Feasibility of anhydrous hydrogen fluoride to 'crack' cellulose. J. Appl. Poly. Sci: Appl. Poly. Symp., 37, 641-51. 21. Hakomori, S. (1964). A rapid permethylation of glycolipid and polysaccharide catalyzed by methylsulfinyl carbanion in dimethyl sulfoxide. J. Biochem., 55, 205-7. 22. Albersheim, P., Nevins, D. J., English, P. D. & Karr, A. (1967). A method for the analysis of sugars in plant cell-wall polysaccharides by gas-liquid chromatography. Carb. Res., 5,340-5.