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Energy & Fuels 2007, 21, 3102–3108
Oxygen Substituent Effects in the Pyrolysis of Phenethyl Phenyl Ethers Phillip F. Britt, Michelle K. Kidder, and A. C. Buchanan, III* Chemical Sciences DiVision, Oak Ridge National Laboratory, P.O. Box 2008, MS-6197, Oak Ridge, Tennessee 37831-6197 ReceiVed June 22, 2007. ReVised Manuscript ReceiVed August 21, 2007
The dominant structural linkage in the abundant renewable energy resource, lignin, is the arylglycerol-βaryl ether commonly referred to as the β-O-4 linkage. The simplest representation of this linkage, unadorned by substituents, is phenethyl phenyl ether (PhCH2CH2OPh; PPE). Our prior detailed kinetic studies of the pyrolysis of PPE at 330–425 °C showed that a free-radical chain mechanism is involved that cycles through radicals formed at both the R- and β-carbons. The previously unrecognized competing path involving rearrangement of the β-carbon radical by an O,C-phenyl shift accounts for ca. 25% of the products. In this paper, we explore the effect of aromatic hydroxy and methoxy substituents on both the rate and product selectivity for the pyrolysis of these more complex lignin model compounds. The pyrolysis reactions are conducted in biphenyl solvent at 345 °C and low conversions to minimize secondary reactions. The pyrolysis rates were found to vary substantially as a function of the substitution pattern. The rearrangement path involving the β-carbon radical and O,C-phenyl shift was found to remain important in all the molecules investigated, and the selectivity for this path also showed a dependence on the substitution pattern.
Introduction Biomass continues to draw increasing attention as a possible renewable source of transportation fuels and as a feedstock for chemical production, and the status is described in recent review articles.1,2 Lignin is the second most abundant natural biopolymer found in vascular plants and is also a byproduct of the pulping process in paper mills. However, lignin is underutilized because the decomposition of the complex, irregular, highly branched polymer structure yields mixtures of products and biooils that need further purification or upgrading before use. There is still considerable potential for the production of phenols and other chemicals from the thermochemical conversion of lignin and woody biomass.3–5 Hence, an improved understanding of the depolymerization process is needed to affect better control over the selective production of higher value chemicals. Fundamental studies of the thermochemical reactions of model compounds that represent structural elements in lignin have been providing important insights toward this end.3,6–16 * Corresponding author. E-mail:
[email protected]. (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. ReV. 2006, 106, 4044. (2) Mohan, D.; Pittman, C. U., Jr.; Steele, P. H. Energy Fuels 2006, 20, 848. (3) Amen-Chen, C.; Pakdel, H.; Roy, C. Bioresour. Technol. 2001, 79, 277. (4) Lignin: Historical, Biological, and Materials PerspectiVes; Glasser, W. G., Northey, R. A., Schultz, T. P., Eds.;ACS Symposium Series 742; American Chemical Society: Washington, DC, 2000. (5) DeVelopments in the Thermochemical ConVersion of Biomass; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic: London, 1997. (6) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. J. Anal. Appl. Pyrol. 2000, 54, 153. (7) Kuroda, K.; Nakagawa-izumi, A. Org. Geochem. 2006, 37, 665 and references therein. (8) Drage, T. C.; Vane, C. H.; Abbott, G. D. Org. Geochem. 2002, 33, 1523. (9) McDermott, J. B.; Klein, M. T.; Obst, J. R. Ind. Eng. Chem. Process Des. DeV. 1986, 25, 885.
Figure 1. (a) Principal precursors for lignin formation. (b) Dominant β-O-4 aryl ether linkage in lignin.
Lignin precursors are believed to be p-coumaryl, coniferyl, and sinapyl alcohols (Figure 1a) that undergo enzyme-initiated, dehydrogenative, free-radical copolymerization to form a heterogeneous, three-dimensional polymer containing a myriad of (10) Klein, M. T.; Virk, P. S. Ind. Eng. Chem. Fundam. 1983, 22, 35. (11) Britt, P. F.; Buchanan, A. C., III; Thomas, K. B.; Lee, S.-K. J. Anal. Appl. Pyrol. 1995, 33, 1. (12) Britt, P. F.; Buchanan, A. C., III; Malcolm, E. A. J. Org. Chem. 1995, 60, 6523. (13) Britt, P. F.; Buchanan, A. C., III; Cooney, M. J.; Martineau, D. R. J. Org. Chem. 2000, 65, 1376. (14) Britt, P. F.; Buchanan, A. C., III; Malcolm, E. A. Energy Fuels 2000, 14, 1314. (15) Kidder, M. K.; Britt, P. F.; Buchanan, A. C., III Prepr. Pap.—Am. Chem. Soc., DiV. Fuel Chem. 2002, 47 (1), 387. (16) Kidder, M. K.; Britt, P. F.; Buchanan, A. C., III Energy Fuels 2006, 20, 54.
10.1021/ef700354y CCC: $37.00 2007 American Chemical Society Published on Web 09/25/2007
Pyrolysis of Phenethyl Phenyl Ethers
Energy & Fuels, Vol. 21, No. 6, 2007 3103
Scheme 1.
radical (polar effect). We are currently employing computational methods to provide a more quantitative understanding of the reactivity of polar radicals in these hydrogen transfer reactions. We recently found that if the rate of the O–C phenyl shift is decreased, e.g., by surface confinement in a nanoporous silica, the rearrangement path is hindered and the R/β-selectivity is increased.19 This is a consequence of the fact that, if the rearrangement step is not fast compared with hydrogen transfer steps, the β-radical can convert to the thermochemically more stable R-radical by bimolecular hydrogen transfer with PPE. The fact that the R/β-selectivity showed no clear dependence on PPE concentration supports the thermochemical kinetic analysis that the O–C phenyl shift is relatively fast for PPE in fluid phases. In this study, we examine the effect of aromatic hydroxy and methoxy substituents on PPE pyrolysis to see whether this competitive free-radical pathway remains operative for these more complex lignin model compounds as well as to determine the influence of the substituents on both the pyrolysis rate and product selectivity. Unfortunately, substituents can potentially affect all of the steps in the reaction mechanism (Scheme 2) through electronic and steric factors. As an example, a substituent could alter the rate of initial O–C homolysis impacting the overall pyrolysis rate. The product selectivity could also be altered by substituent effects, for example, by changes in the selectivity for hydrogen abstraction at the R- and β-carbons or through changes in the rates of the O–C phenyl shift and radical scission steps. Thus, a systematic study of the impact of substituents on reaction rate and product selectivity is needed. The β-O-4 aryl ether model compounds investigated in this study are shown in Scheme 3 along with the predicted major reaction products assuming similar behavior to PPE.20
linkages.3,4,6,17 The proportions of these monomers vary between lignins found in hardwoods, softwoods, and grasses. The dominant backbone linkage in lignin is the β-O-4 linkage (Figure 1b) that accounts for 46–60% of the interunit linkages. The simplest model compound that represents this linkage is phenethyl phenyl ether (PhCH2CH2OPh; PPE). Our approach has been to understand the pyrolysis chemistry of PPE in detail12 and then to systematically probe the influence of complexities such as restrictions on diffusion,14 the presence of acid catalysts,11 and the influence of substituents.11,13–16 Since the thermochemical conversion of lignin has been examined under a range of conditions (e.g., temperature, heating rate, residence time, pressure), which leads to different products, we have also explored the pyrolysis of PPE under both conventional (low temperature, long residence time)12 and flash (high temperature, low pressure, short residence time)13 pyrolysis conditions. Flash vacuum pyrolysis (500 °C,
