Scandium triflate catalyzed acetylation of starch at low to moderate temperatures☆
Introduction
There is increasing interest in discovering new biobased materials to replace petroleum-based polymers (Gross & Kalra, 2002). Starches, in particular, have attracted considerable interest in diverse applications such as biodegradable plastics, coatings, adhesives, foams, flocculants, dispersing agents, sequestering agents (Belard et al., 2005, Davis and Song, 2006, Knill and Kennedy, 2005, Roper, 2002, Zobel and Stephen, 2006). To improve functionality and economics, new methods of chemically modifying starch are needed to control substitution patterns and to reduce energy inputs and byproducts (BeMiller, 1997).
Currently, most starch esters produced commercially are low DS products made by aqueous suspension reaction of starch, anhydrides and NaOH to maintain pH 7–9 (Jarowenko, 1986, Rutenberg and Solarek, 1984). Some starch acetates of moderate to high DS are also made by this process (Tessler and Billmers, 1996, Xu et al., 2004). Large amounts of wastewater and sodium acetate byproduct are produced, resulting in high cost. There is no control of substitution pattern.
There have been many studies of preparation of starch esters in non-aqueous systems including heating in acetic acid (Shogren & Biswas, 2006), formic acid (Aburto, Alric, & Borredon, 1999), pyridine, DMSO, and dimethylacetamide (Rutenberg & Solarek, 1984). Catalysts used have included minerals acids and bases (Jarowenko, 1986), Lewis acids such as I2 (Biswas, Shogren, & Willett, 2005), and enzymes such as lipases and proteases (Bruno et al., 1995, Qiao et al., 2006, Rajan and Abraham, 2006). Starch esters specifically substituted at the 2 (Dicke, 2004, Klohr et al., 2005) and 6 (Chakraborty, Sahoo, Teraoka, Miller, & Gross, 2005) positions have been prepared. The drawbacks of enzyme catalysis are that the starch must be dissolved in polar solvents such as DMSO which usually severely decrease the activity of the enzyme. Alternatively, lipase catalyzed acylation of a suspension of starch nanoparticles in toluene is facile (Chakraborty et al., 2005) but this requires a major alteration in starch structure to first form nanoparticles.
Strong Lewis acid catalysts such as scandium (III) triflate (Sc(OTf)3) have been shown to rapidly catalyze ester formation between alcohols and anhydrides (Ishihara, Kubota, Kurihara, & Yamamoto, 1996) as well as between diols and diacids to form polyesters (Takasu, Oishi, Iio, Inai, & Hirabayashi, 2003). Catalysis occurs at ambient or slightly elevated temperatures and specificity for primary versus secondary alcohols has been shown under certain conditions (Procopiou, Baugh, Flack, & Inglis, 1998). Although Sc(OTf)3 is expensive, it is active at low levels (∼0.1 mol%) and can be easily recovered and reused making it a potential “green” catalyst.
The purpose of this study was to determine the potential of Sc(OTf)3 as a catalyst for the preparation of starch acetates under mild conditions. The effects of temperature, time, reactant (acetic anhydride versus acetic acid), reactant concentration, catalyst concentration and starch gelatinization on DS were studied. Bulk versus surface DS were measured using NMR and XPS, respectively.
Section snippets
Materials
Normal corn starch (pure food grade) was purchased from A.E. Staley (now Tate & Lyle, Decatur, IL) and had a moisture content of 9.6%. The starch was dried in a vacuum oven at 110 °C for 3 h before use. Glacial acetic acid, acetic anhydride (99+%) and scandium (III) trifluromethanesulfonate (99%) were purchased from Aldrich (Milwaukee, WI).
Acetylation of starch
Reactions were performed in 10 ml glass Reacti-Vials (Pierce, Rockford, IL) with teflon screw caps. Starch (1 g), acetic anhydride (0.63–1.89 g, 1–3 mol/mol
Results and discussion
Compositions of starch acetates prepared by reaction of granular (native) corn starch with acetic anhydride (3 mol/mol glucose) and Sc(OTf)3 (1 mol% based on starch) at room temperature are shown in Table 1. DS values as determined by 1H NMR were very small (<0.02), indicating very little total acetylation. After reaction, however, the starch granules were difficult to disperse in water, suggesting some surface reaction. This was confirmed by XPS spectra of corn starch before and after reaction (
Acknowledgements
The author thanks Elizabeth Krietemeyer for preparation of starch acetates and Drs. David Weisleder and Karl Vermillion for NMR characterization. XPS was carried out in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the US Department of Energy under Grant DEFG02-91-ER45439. The help of Dr. Rick Haasch in performing these experiments is gratefully acknowledged.
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