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Ind. Eng. Chem. Fundam. 1983, 22, 426-430
Reaction Pathway Analysis of Thermal and Catalytic Lignin Fragmentation by Use of Model Compounds Stephen J. Hurfft and Mlchael 1. Kleln' Department of Chemical Engineering and Center for Catalytic Science and Technology. UnlversHy of Delaware, Newark. Dekware 19711
Hydrodeoxygenation (HDO) of the lignin model compounds anisole (methoxybenzene)and guaiacol (2-methoxyphenol) over a Co0-Mo03/y-A1203hydrotreating catalyst was studied from 523 to 598 K at 34.5 bar hydrogen pressure. Anisole's prlmary HDO reaction was demethylation.to phenol, whlch In turn underwent subsequent dehydroxybtion and hydrogenation reactions to benzene and cyclohexane. Arrhenlus parameters for anlsole disappearance were [log A (cm3/g of cat. s), E' (kcal/moJ)] = [11.0 k 2.8, 29.7 f 7-11. The primary guaiacol HDO demethylation reaction to catechol (1,2dihydroxybenzene) was over 30 times faster than anisole demethylation to phenol at 523 K; secondary catechol HDO yielded phenol. The experimental results are used in analysis of the thermochemical conversion of lignin to low molecular weight products: HDO of lignin or lignlnderlved phenolics should increase the yields and reduce the complextty of the phenolic product spectra observed from lignin pyrolysis.
Introduction The thermal or catalytic fragmentation of low-rank coal and woody biomass (lignin, cellulose, hemicellulose) resources to fuel and chemicals is complicated by their rather appreciable contents of oxygen. Hydroprocessing lignin to aromatics or monohydroxy1 phenols, for example, would involve hydrodeoxygenation (HDO) of methoxyl- and hydroxyl-substituted aromatics, a reaction that has been little studied and for which reaction pathway and kinetic information is scarce. The present report is of a reaction engineering analysis of lignin hydroproceasing as discerned through the catalytic reactions of two lignin model compounds, anisole (methoxybenzene) and guaiacol (2-methoxyphenol). Since lignin is both a major component of biomass and also an evolutionary precursor to coal, insights revealed here should be applicable to utilization of biomass and low-rank coals. Lignin is the phenolic copolymer that results from the enzyme-initiated, dehydrogenative polymerization of coniferyl, coumaryl and sinapyl alcohols, shown in Figure 1 (Glasser and Glasser, 1974; Harkin, 1967; Freudenberg and Neish, 1968). Although the methoxyl substituent on each coniferyl and sinapyl alcohol monomer unit survives the polymerization unchanged, the phenolic oxygen of each monomer may appear in the polymer as either a free hydroxyl or in one of only a few types of ether bonds, namely one of an a- or P-aryl alkyl ether, a diary1 ether, or a phenylcoumaran moiety. Lignin pyrolysis, appreciable by 733 K, includes fragmentation of the thermally scissile a-and @-arylalkyl ether bonds and the attendant formation of single-ring phenolics related to the three monomer alcohols; secondary thermolyses of these primary phenolics broaden the distribution of observed phenolic products considerably. Thus each of alkyl-substituted phenols, cresols, guaiacols, catechols, and dimethoxyphenols are found in lignin pyrolysis product spectra (Iatridis and Gavalas, 1979; Chan and Kreiger, 1981; Kirshbaum and Domburg, 1970; Connors et al., 1980; Allan and Matilla, 1971), and Domburg et al. (1971), Kirshbaum et al. (1976) and Jegers (1982) have respectively identified over 30,24, and 20 different phenols from lignin pyrolysis. Unfortunately, no single phenol dominates, and even in a favorable case where the total 'E. I. du Pont de Nemours Company, Parkersburg, WV. 0 196-431318311022-0426$Q~,5010
phenolics yield might approach 30%, pyrolysis would more likely generate 30 different phenols each in 1%yield than one single phenol in 30% yield. Model compound pyrolyses (Klein, 1981; Shaposhnikov and Kosyukova, 1965; Kravchenko et al., 1970; Connors et al., 1980; Kislitsyn et al., 1971; Friedlin et al., 1949; Obolentsev, 1946)have revealed that the reactions of lignin aromatic methoxyl groups contribute to this complexity. For example, anisole pyrolysis a t 723 K yields products including phenol, 0-cresol, xylenols, benzene, benzaldehyde, and toluene (Klein, 1981; Friedlin et al., 1949; Shaposhnikov and Kosyukova, 1965; Obolentsev, 1946). Too, a relevant empirical observation is that the pyrolysis product spectra of guaiacol, veratrole, and 2,6-dimethoxyphenol, all compounds containing at least two ortho oxygen-containing substituents, contain coke, but the product spectra of the singly oxygenated anisole and phenol do not (Klein, 1981). Since the vast majority of lignin's phenolic units resemble guaiacol, veratrole, or 2,6-dimethoxyphenol, it would seem likely then that the occurrence in lignin of ortho oxygen-containing substituents contributes to coke formation during ita pyrolysis. AU of the foregoing suggests that removal of at least one oxygen-containing substituent during the thermochemical conversion of lignin would be desirable. Phenyl-oxygen bond cleavage can be accomplished easily at 500-650 K using traditional hydroprocessing catalysts under a partial pressure of hydrogen gas. These HDO reactions include both hydrogenation and dehydroxylation steps, and the oxygen is ultimately removed as water. Thus HDO of the prototype reactant phenol yields benzene plus water and cyclohexanol as primary products, with both of these, and especially the latter, capable of secondary reaction to cyclohexane (and water) under high hydrogen pressure (Weisser and Landa, 1973). HDO of cresol is similar. HDO of oxygen-containing heterocyclics such as benzofuran (Rollman, 1977; Landa et al., 1969) and dibenzofuran (Hall and Cawley, 1939; Krishnamuthy et al., 1981) occurs both via direct oxygen extrusion and through hydrogenated intermediates. Krishnamurthy et al. (1981) also observed parallel pyrolytic reactions. Catalytic oxygen removal from lignin and lignin-derived phenolics should evidently occur at temperatures well below those required for thermal fission of phenyl-oxygen bonds. However, the literature contains little information 0 1983 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 22, No. 4, 1983 427
CONIFERYL
COUMARYL
SINAPYL
~ICINNAMYL ALCOHOL MONOMERS
GUAIACOL bl LIGNIN MODEL
ANISOLE
COMPOUNDS
Figure 1. Lignin monomer alcohols and related model compounds. Table I. Anisole and Guaiacol Hydrodeoxygenation Catalyst Composition and Properties as Determined by Supplier catalyst: CoO-MoO,/yAl,O,, HDS-16A supplier: American Cyanamid composition, wt % COO 5.6 MOO, 11.2 Na,O 0.03 Fe 0.04 surface area, m2/g: 176 pore volume, cm3/g: 0.50
pertaining to the HDO of lignin-related compounds and, certainly, no application of this information in the analysis of lignin conversion processes. This motivated the present investigation of anisole and guaiacol catalytic HDO reaction paths.
Experimental Section Anisole and guaiacol hydrodeoxygenation reactions were studied at temperatures ranging from 523 to 598 K and at a hydrogen pressure of 34.5 bar. Here we delineate the details of materials, reactor, procedures, and analysis methods employed. Materials. The reactants, sulfiding chemicals [carbon disulfide and a custom mixture of hydrogen sulfide (10 mol %) and hydrogen (go%)], gas chromatography standards, and carrier gases were all commercially available and used as received. The catalyst was a Co0-MoO3/y-A1,O, hydrodesulfurization catalyst supplied by American Cyanamid (AERO HDS-16A, MTG-53-0731)with the catalyst composition and properties listed in Table I. This standard commercial catalyst was received as 1/16-in. extrudates which were ground and sieved to 80-100 mesh prior to use. Fresh catalyst was sulfided prior to each experiment as a 40 cm3/min flow of hydrogen su1fide:hydrogen (1:9 volumetric ratio) passed over it while being heated to and held for 2 h at 673 K. The catalyst was allowed to cool to room temperature gradually, at which point it was introduced into the reactor injection device (see below) along with n-hexadecane solvent. Reactor. The reactor of choice was a standard l-L 316 stainless steel batch autoclave (Autoclave Engineers, Inc.) equipped for reactants injection, sampling, agitation, pressure measurements, and temperature control to within f l K at 704 K. A system for injection of a slurry of cold catalyst, reactant, and carbon disulfide (to maintain the catalyst in its sulfided form) diluted in n-hexadecane solvent into the preheated reactor allowed for precise definition of time zero. Incorporation of a stainless steel filter (0.5 pm pore size) into a sampling line prevented the removal of catalyst particles during sample collection and
allowed for the rapid quench of small liquid aliquots. Experimental Procedures. Typical experimental operation began by connecting the injection device, filled with sulfided catalyst and a solution containing 5.0 wt % reactant, 0.5% CS2and 34 cm3of n-hexadecane carrier oil, to the reactor, which contained 350 cm3 of n-hexadecane solvent and a hydrogen gas phase. After pressure testing, the agitated reactor contents were raised to 10 K above the desired reaction temperature at 20.7 bar. This compensated for the addition of cold feed and catalyst at the beginning of each experiment. Opening a ball valve connecting the injection device, pressurized to 90 bar, to the reactor allowed complete transfer of the injector’s contents in about 2 s and defined time zero. The pressure was quickly adjusted to 34.5 bar. Samples were removed before time zero (for analysis of thermal cracking of the carrier oil) and at intervals ranging from 1min at the early stages of reaction to 100 min after 8 h or more of reaction. Product Analysis. Reaction products were analyzed by either a Perkin-Elmer Model 3920B or a HewlettPackard Model 5880 gas chromatograph, both of which were equipped with flame ionization detectors and electronic integrator units. In both instances two types of gas chromatographic columns were used. The first, used for separation of benzene, cyclohexane, anisole, and phenol, was a 6 ft X l/g in. stainless steel column packed with 10% Carbowax 20M on 8O/lOO Chromosorb WAW. The second column was used for analysis of guaiacol, catechol, o-cresol, and methylcatechol, these compounds being obscured on the first column due to interference with the n-hexadecane solvent peak; it was a 50 m X 0.25 mm i.d. borosilicate glass WCOT capillary column with SE-54 as the stationary phase. Products were identified by gas chromatography with standard coinjection. Separate measurement of product response factors, ai, in mol of ilarea of i, relative to one for the external standard Tetralin, allowed determination of quantitative product yields. The foregoing allowed calculation of a material balance index, MBI, defined by eq 1and a measure of the present capabilities for product identification and quantification m
MBI =
is1
Ci/Co
(1)
where m = number of identified products and Co = initial concentration of reactant (mol/g of solution). Equation 1shows MBI to be an aromatic or naphthenic ring balance, which was useful in two ways. First, defined in this matter, MBI often had a value of 1 (100% closure) even though light products’ yields (e.g., methane, water) and hydrogen consumption could not be measured. This allowed development of the reaction pathways shown below. Second, at very high reactant conversions MBI was often less than unity, which suggested that either formation of additional aromatics and/or naphthenes or decomposition of these kinds of products occurred. Additional experimental details are available (Hurff, 1982).
Results Anisole and guaiacol hydrodeoxygenation (HDO) reactions were studied over the experimental conditions listed in Table 11. In the discussion to follow we consider the products’ temporal variations, likely reaction pathways, and kinetics for HDO of both reactants. Anisole Hydrodeoxygenation. Anisole HDO yielded phenol, benzene, and cyclohexane as major products along with lesser amounts of o-cresol, toluene, and methylcyclohexane; these latter minor products were quantified
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Ind. Eng. Chem. Fundam., Vol. 22, No. 4, 1983
Table 11. Anisole and Guaiacol Hydrodeoxygenation Experimental Grida init. concn, temp, holding reactant mol/g of oil K time, min 3.6 x 3.8 x 3.8 x 4.0 x
anisole anisole anisole guaiacol
10-5 10-5 10-5 10-5
598 548 523 523
Co0-MoO3/yA1,O3 catalyst; 34.5 bar hydrogen pressure.
‘0
100
200
T~,$~,,l,
400
I
400 600 500 1200
500
600
Figure 2. Temporal variation of major products’ yields from anisole hydrodeoxygenationat 523 K and 34.5 bar pressure: (0) anisole; (A) phenol; (+) benzene, (X) cyclohexane).
only for HDO at 523 K, and amounted at most to 18% of total products’ weight. Temporal variations of major products’ yields from anisole HDO at 523 K are shown in Figure 2, a plot of anisole, phenol, benzene, and cyclohexane yields, in moles of product per initial anisole moles, vs. reaction time, in minutes. Trend lines shown in Figure 2 are for illustrative purposes and do not represent best-fit parameters. Inspection of Figure 2 reveals the following reaction network information. First, anisole conversion was essentially unity at 300 min reaction time, at which point material balance closure, including minor product proportions, was 100% . This indicates that all important products were accounted for at 523 K and 300 min. Second, the predominant product, phenol, appeared with an initial slope that closely matched the negative of anisole decomposition slope in Figure 2, while the other major products, benzene and cyclohexane, appeared with initial slopes of essentially zero. This suggests that phenol was a primary anisole HDO product, whereas benzene and cyclohexane were secondary products. It is also noteworthy that phenol attained maximal proportions at about 140 min reaction time. Third, cyclohexane yields were always higher than benzene yields at 523 K. Finally, material balance closure fell to about 95% at 600min reaction time, by which time anisole had been completely converted, suggesting that only minor proportions of reaction products were unaccounted for at 523 K. Conversion of anisole at 548 K was essentially complete after 50 min reaction time. Phenol attained maximal proportions at about 15 min, and its secondary decomposition was complete by 300 min reaction time. Cyclohexane and benzene yields asymptotically approach 0.53 and 0.23, respectively. The minor products, 0-cresol, toluene, and methylcyclohexane, were not quantified at 548 K. The proportions of these minor products reached 18% of total products at 523 K, which, if added to the observed 75% material balance at 548 K, would indicate closure in excess of 90%. Thus it is likely that either these products’ proportions were more appreciable at 548 K than at 523 K or that a small but significant proportion of secondary
bMe
bH
Figure 3. Preliminary anisole and guaiacol hydrodeoxygenation reaction networks.
reaction products went unidentified. Reaction of anisole at 598 K was rapid and essentially complete in less than 5 min reaction time. Phenol proportions peaked in just a few minutes time and its secondary decompostion was complete by 200 min. Cyclohexane and benzene yields at 200 rnin were approximately 0.40 and 0.05, respectively, both of these being smaller than observed at 548 K. This information, coupled with the observation of an apparent cyclohexane maximum at about 200 min reaction time, suggests that secondary decomposition of cyclohexane occurred at 598 K. Too, material balance calculations, including only identified major products, showed closures of about 80% for short reaction times where anisole conversion was complete and about 50-60% at longer reaction times of about 200 min where not only anisole but also phenol was completely consumed. Anisole Reaction Pathways. The foregoing allowed development of the preliminary anisole HDO network shown in Figure 3. HDO at 523 K showed phenol to be the only major primary product, with benzene and cyclohexane both appearing apparently simultaneously as secondary products. Since cyclohexane proportions were always higher than those of benzene, the scheme accounted for formation of these compounds through parallel phenol decomposition pathways and not a series of sequential steps (e.g., benzene hydrogenation). HDO results at 598 K suggested secondary decomposition of cyclohexane to occur, and since Kalechits (1961) had observed methylcyclopentane formation from cyclohexane, a decomposition reaction of cyclohexane was included in Figure 3. The anisole HDO network of Figure 3 thus includes primary reaction of anisole to phenol, which in turn undergoes competitive reactions to benzene and cyclohexane, the latter also capable of decomposition. Although not quantified or important in the present work, note that a formally analogous pathway involving: (1)anisole isomerization to o-cresol, (2) o-cresol competitive reactions to toluene or methylcyclohexane, and (3) decomposition of methylcyclohexane, can be written. Pseudo-first-order rate constants of k (cm3/g of cat. s) = 0.0763,0.603, and 2.78 for anisole decomposition at 523, 548, and 598 K, respectively, yield apparent Arrhenius parameters of [log A (cm3/g of cat. s), E* (kcal/mol)] = [11.0 f 2.8, 29.7 f 7.11. It should be emphasized that the batch autoclave reactor used did not permit discrimination between reaction rate decreases due to reactant conversion and due to possible catalyst deactivation. Guaiacol Hydrodeoxygenation. Guaiacol HDO at 523 K yielded catechol, phenol, benzene, and cyclohexane as major products and also minor products including 0-cresol, toluene, methylcyclohexane, and methylcatechol. An average material balance closure of about 80% at moderate guaiacol conversion levels suggesta that additional products were likely formed also. The temporal variations of major products’ yields are shown in Figure 4. Inspection of Figure 4 shows that guaiacol conversion was essentially unity after 200 min
Ind. Eng. Chem. Fundam., Vol. 22, No. 4, 1983 420 Table 111. Yields of Guaiacols, Catechols, and Phenols from Kraft Lignin Pyrolysis at 673 K and 7.5 and 60 min Holding Time (Jegers, 1982)
0 BLY IACOL 0 U.TECHOL
APHENOL t BENZENE X CYCLOHEXANE
product yield, w t % of original lignin
Product no.
C i 0.601
-
product guaiacol 4-methylguaiacol 4-ethylguaiacol catechol 4-methylcatechol 4-ethylcatechol
1
,
