(415b) The Coupling of Ethanol to Butanol Over Calcium Hydroxyapatites

The use of corn-based bioethanol as a domestic and renewable transportation fuel alternative has led to a recent surge of ethanol production in the United States. This increased availability combined with decreasing market prices has made ethanol an economically viable platform feedstock for higher-value fuels and chemicals, such as butanol. Butanol is an important industrial chemical commodity and has recently generated interest as a potential gasoline fuel additive because of its high energy density and ability to improve fuel economy for biofuel blended gasoline.

The catalytic conversion of ethanol to butanol is thought to occur via the Guerbet reaction – a well-known industrial route for higher alcohol synthesis that ultimately couples two short-chain alcohol molecules to produce a longer chain saturated alcohol. The most commonly-accepted path for this reaction involves an initial dehydrogenation of ethanol to form acetaldehyde, which then undergoes an aldol condensation reaction followed by hydrogenation of the resulting unsaturated condensation products. Recent reports have demonstrated that the highest activity and butanol selectivity in ethanol coupling reactions can be obtained over hydroxyapatite (HAP) catalysts [1]; however the active site on these materials is unknown, which complicates catalyst design and optimization. This work explores the surface acid and base sites on calcium hydroxyapatite to determine their influence on catalytic performance during the Guerbet coupling of ethanol.

Ethanol conversion studies were performed at atmospheric pressure in a gas-phase, fixed-bed reactor system equipped with an on-line gas chromatograph. Acid and base sites of the catalytic materials were characterized using adsorption microcalorimetry of probe molecules such as NH₃ and CO_2, as well as diffuse reflectance FT-IR spectroscopy (DRIFTS) during stepwise temperature programmed desorption (TPD) and during reaction at elevated temperatures. The DRIFTS of ethanol on HAP revealed the presence of adsorbed ethanol. Results from TPD indicated that the ethanol is weakly held to the HAP surface compared to that on MgO, which is consistent with the affinity of the surface for other adsorbates as probed by adsorption microcalorimetry [2]. Results from characterization and reactivity testing will be used to determine important structure-function relationships for HAP materials.   

References

[1] Tsuchida, Takashi, Shuji Sakuma, Tatsuya Takeguchi, and Wataru Ueda. "Direct synthesis of n-butanol from ethanol over nonstoichiometric hydroxyapatite." Industrial & engineering chemistry research 45, no. 25 (2006): 8634-8642.

[2] Birky, Theodore W., Joseph T. Kozlowski, and Robert J. Davis. "Isotopic transient analysis of the ethanol coupling reaction over magnesia." Journal of Catalysis 298 (2013): 130-137.