Energy & Fuels 2009, 23, 2235–2241
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Energy Dispersive X-ray Fluorescence Analysis of Sulfur in Biomass J. Michael Robinson,* Staci R. Barrett, Kevin Nhoy, Rajesh K. Pandey, Joseph Phillips, Oscar M. Ramirez, and Richard I. Rodriguez Department of Chemistry, The UniVersity of Texas of the Permian Basin, 4901 East UniVersity BouleVard, Odessa, Texas 79762 ReceiVed October 22, 2008. ReVised Manuscript ReceiVed January 26, 2009
Development of commercially viable chemical transformations of biomass components into platform chemicals is hampered in many cases by poisoning of the metal catalysts assisting these conversions. Sulfur contained in the small amount of protein in biomass is one type of catalyst poison. An energy dispersive X-ray fluorescence (ED-XRF) spectroscopy method was developed to analyze low ppm level sulfur (S) in biomass feedstocks and in subsequent residues from pretreatment reactions. Calibrations utilized disks made with portions of S-containing standards milled with microcrystalline cellulose. These mixtures represented a method of “standard additions” that gave excellent linear plots from ∼2 to 2250 ppm and simultaneously determined that Avicel contained 23 ((5) ppm S. Accuracy of this method was verified by two external laboratories each using a different method of analysis, UVF and ICP-AES. A minimum detection limit of 2 ppm was determined for this method with an 86 ppm standard. Representative biomass feedstocks and pretreatment residues were analyzed for S.
1. Introduction Although the major components of renewable biomass (hemicellulose, cellulose, and lignin) are sought as individual platforms for conversion to chemicals and fuels, it is usually the minor components derived or liberated during pretreatment fractionation that interfere with subsequent process steps. For example, pretreatment reactions in the cellulosic ethanol process liberate phenols and other moieties that interfere with and thus must be removed before fermentation with biochemical catalysts. In chemical paths to platform chemicals and fuels, the development of commercially viable transformations of carbohydrates is hampered by poisoning of the metal catalysts that are used to assist these conversions. In an analogous fashion, deleterious moieties must also be removed before successful chemical catalysis processes for biomass conversions can become sustainable. In liquid phase processes, certain metal cations1 and sulfur, as mercaptan groups,2 are poisonous to high surface area metal catalysts. Gasification of biomass may leave the majority of S in the ash, but corrosion from volatilized SO2 is also of concern.3 Metal ions can be analyzed by a number of quite sensitive instrumental methods and can be removed from solutions by ion exchange or other selective methods. With few exceptions,4,5 the sulfur content in biomass has not been studied at the low level required for most feedstocks. * Corresponding author. Phone: +1-432-552-2237; fax: +1-432-5522236; e-mail:
[email protected]. (1) Arena, B. J. Appl. Catal. A 1992, 87, 219–229. (2) Elliott, D. C.; Peterson, K. L.; Muzatko, D. S.; Alderson, E. V.; Hart, T. R.; Neuenschwander, G. G. Appl. Biochem. Biotechnol. 2004, 115, 807–825. (3) Obernberger, I.; Brunner, T.; Ba¨rnthaler, G. Biomass Bioenergy 2006, 30, 973–982. (4) Saarela, K.-E.; Harju, L.; Lill, J.-O.; Rajander, J.; Lindroos, A.; Helselius, S.-J. Holzforschung 2002, 56, 380–387. (5) Elliot, D. C.; Neuenschwander, G. G.; Hart, T. R.; Butner, R. S.; Zacher, A. H.; Engelhard, M. H.; Young, J. S.; McCready, D. E. Ind. Eng. Chem. Res. 2004, 43, 1999–2004.
Sulfur contained in the naturally occurring protein in biomass is much lower in the larger woody fraction (∼0.3%) than in the leaves and bark or in herbaceous biomass (4-6%) plants. Specific amino acid content of these proteins also varies, including those amino acids with S-containing functional groups, such as cysteine, with a highly reactive mercaptan functional group. As a result, S content varies widely with type of plant, soil nutrients, and many other factors.6 Sulfur in SO4-2 salts is not deemed a catalyst poison but does contribute to the total sulfur that might be measured, and may lead to an overestimation of the poisoning capacity. Determination of sulfur in plant material has been performed by numerous methods that usually require total decomposition of the sample. Current biomass databases list S content typically to only one significant figure if below 0.1%. These results are reported in the percent range presumably due to both the limitations of combustion data and lack of previous interest in sulfur as a poison for catalytic processes. Rapid and accurate X-ray methods determine total sulfur more conveniently in a variety of biomass materials such as: medicinal plants7 (1592-14 497 ppm), pine roots (472-990 ppm),8 rice flour9 (NIST SRM 1568: 1200 ppm), and milk powder (1176-2370 ppm).10 None of these X-ray analyses for S required the low level (
