Energy & Fuels 1997, 11, 247-248
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Investigation of Reaction Pathways Involved in Lignin Maturation A. C. Buchanan, III,* Phillip F. Britt, and John A. Struss Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6197 Received August 13, 1996. Revised Manuscript Received October 7, 1996 The organic transformations associated with the genesis of fossil fuels such as kerogen, petroleum, and coal continue to be intensely investigated. Simulated maturation studies1 are being employed to explore the importance of reacting medium effects (open vs confined pyrolysis),1a,g the role of water,1a,b the influence of sediment composition (e.g. mineral catalysis),1c,d,h etc. Hatcher and co-workers have presented detailed descriptions of the structural changes that occur in lignin at various stages of the coalification process, and very specific solid-state organic reactions have been proposed.1e-g Using a solid-state lignin model compound, we now provide the first direct evidence that cleavage of the prominent β-O-4 aryl ether linkages (Ar′-CH2CH2-O-Ar) in lignin can occur in the solid state with re-formation of 1,2-diarylethane linkages (Ar′-CH2CH2-Ar-OH) at relevant maturation temperatures (150 °C) if interdispersed clay minerals are present as catalysts. However, the selectivity of this process relative to competing acid-catalyzed pathways, such as aromatic dealkylation, is found to be a sensitive function of the reaction temperature over the 150-300 °C range. The silica-immobilized lignin model compound, ≈PhCH2CH2OPh-o-OCH3 (or ≈PPE-o-OMe, where “≈” represents the Si-O-C surface linkage),2,3 shown in Scheme 1 contains the guaiacyl unit that is abundant in gymnospermous lignin.1e,f The reaction of ≈PPE-oOMe at 150-300 °C was examined in the presence of interdispersed inorganic solids that included silica (Aerosil 200), silica-1% alumina (Aerosil Mox-170), acid-exchanged clay (montmorillonite K-10), and three natural clays as shown in Table 1.4 ≈PPE-o-OMe is unreactive at 150 °C in the presence of silica, but reacts readily in the presence of silica-alumina and clays having roughly comparable surface areas that function as acid catalysts (Table 1).5 The product slate shown in Scheme 1 is consistent with the occurrence of acidcatalyzed reactions involving carbocation intermediates.6 Guaiacol (o-MeOC6H4OH) is the dominant prod(1) (a) Mansuy, L.; Landais, P. Energy Fuels 1995, 9, 809. (b) Lewan, M. D. In Organic Geochemistry; Engel, M. H., Macko, S. A., Eds.; Plenum Press: New York, 1993; Chapter 18. (c) Smith, J. W.; Batts, B. D. Org. Geochem. 1992, 18, 737. (d) Ohta, K.; Venkatesan, M. I. Energy Fuels 1992, 6, 271. (e) Hatcher, P. G. Org. Geochem. 1990, 16, 959. (f) Hatcher, P. G.; Faulon, J. L.; Wenzel, K. A.; Cody, G. D. Energy Fuels 1992, 6, 813. (g) Behar, F.; Hatcher, P. G. Energy Fuels 1995, 9, 984. (h) Hayatsu, R.; McBeth, R. L.; Scott, R. G.; Botto, R. E.; Winans, R. E. Org. Geochem. 1984, 6, 463. (i) Botto, R. E. Energy Fuels 1987, 1, 228. (j) Carrado, K. A.; Hayatsu, R.; Botto, R. E.; Winans, R. E. Clays Clay Min. 1990, 38, 250. (2) The synthesis of the precursor phenol, p-HOPhCH2CH2OPh-oOCH3, has been reported,3a as well as detailed procedures for covalent attachment of model compounds to a silica surface.3 Underivatized phenol was removed by Soxhlet extraction with benzene. (3) (a) Britt, P. F.; Buchanan, III, A. C.; Thomas, K. B.; Lee, S.-K. J. Anal. Appl. Pyrolysis 1995, 33, 1. (b) Britt, P. F.; Buchanan, III, A. C. J. Org. Chem. 1991, 56, 6132. (c) Buchanan, III, A. C.; Britt, P. F.; Thomas, K. B.; Biggs, C. A. Energy Fuels 1993, 7, 373.
S0887-0624(96)00127-2 CCC: $14.00
Table 1. Effect of Additives on ≈PhCH2CH2OPh-o-OCH3 Reaction at 150 °C additive
surf areaa (m2 g-1)
convrnb (%)
SiO2 SiO2-1% Al2O3 montmorillonite K-10 Na montmorillonite Ca montmorillonite kaolin
200 170 183 79 78 90
200: 1), but aromatic dealkylation becomes increasingly competitive at higher temperatures. At 300 °C, cleavage (8) Olah, G. A.; Head, N. J.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 1995, 117, 875.
of the ether linkage is favored by only ca. 5:1. However, in vitrinite formation from gymnospermous lignin, there should be a high degree of selectivity for cleavage of β-aryl ether linkages before elevated maturation temperatures are experienced. Interestingly, demethylation of ≈PPE-o-OMe to form 3, which is also observed as a key step in the conversion of lignin into lignites,1e-i does not occur to a significant extent in this model compound. Formation of 3 never accounts for more than 1% of the products despite the substantial degree of cleavage of the β-O-4 ether linkage. This may be a consequence of the need to form a less stable methyl cation in the proton-catalyzed cleavage of the methyl ether.7 The results suggest that cleavage of the Ar-O-CH3 ethers in lignin occurs at a much slower rate than does cleavage of the β-O-4 ether linkages or involves an alternative reaction pathway or catalyst. In summary, this model compound study provides new insights into the reaction pathways involved in lignin maturation. Clays are shown to behave as acid catalysts that produce facile, selective cleavage of relevant β-aryl ether linkages in the solid state at 150 °C. However, aromatic dealkylation becomes kinetically competitive at higher temperatures. Demethylation of aryl methyl ethers was found not to be competitive under the conditions investigated. Acknowledgment. We thank M. E. Sigman for the surface area measurements. Research performed at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DE-AC0596OR22464. EF9601273
