Physico-Chemical nanomaterials science

The catalytic activity of phosphotungstic acid-carbon nanocomposites in the dehydration reaction of bioethanol

Soloviov V.1, Diyuk V.E.1, Vakaliuk A.V.1, Tsapyuk G.G.1, Mischanchuk O.V.2, Mariychuk R.3, Lisnyak V.V.1

1 Taras Shevchenko National University of Kyiv, 62a, Volodymyrska Str., Kyiv 01601, Ukraine, E-mail: vadiga@ukr.net
2 O.O. Chuiko Institute of Surface Chemistry, the National Academy of Science of Ukraine, Gen. Naumova St., 17, General Naumov Str., 03164 Kyiv, Ukraine
3 Prešov Universirty in Prešov, Ul. 17 Novembra, 1, 080 01, Prešov, Slovakia

Significant production volumes, ease of production, and low cost of bioethanol make it a valuable fuel additive and raw material for preparing important organic substances: diethyl ether, ethylene, ethylene oxide, propylene, butadiene, acetaldehyde, vinyl acetate, etc. Heteropolyacids are the most active catalysts for ethanol dehydration. They differ from other catalysts by significant acidity and high stability. Due to the low specific surface area, in the production of acid-based catalysts, heteropolyacids are usually loaded onto various carriers to increase their catalytic activity by increasing dispersion. 
In this work, we investigated the activity and selectivity of nanocomposite catalysts. They were prepared using phosphotungstic acid (PTA) H3PW12O40•nH2O and various carbon materials, including activated carbon, carbon fibers, and carbon nanotubes. These catalysts were examined in the gas-phase reaction of ethanol dehydration and characterized by different methods: SEM, adsorption of N2, TG/DTG, TPD MS, XRD, and FTIR.
We showed that the obtained nanocomposites are active catalysts in bioethanol dehydration. In the temperature range of 120170 °C, the main product of dehydration is diethyl ether, and its maximum yield is 66%. In the temperature range of 190–230 °C, bioethanol can be dehydrated over catalysts to ethylene with 100% selectivity. Increasing the PTA content from 0.005 to 0.01 mmol per gram of support increases the yield of diethyl ether and decreases the temperatures of the formation of dehydration products. The temperature window of the diethyl ether formation expands by 1030 °C when carbon fibers are used as a carrier. We prove that the use of oxidized carbon solids leads to a significant increase in the catalytic activity of the obtained composites, regardless of the content of the applied PTA. In the study of adsorption-desorption of ethanol, we found that oxygen-containing functional groups are surface centers of PTA immobilization and ethanol adsorption. Oxidation of the carrier surface is a method that improves the absorption of alcohol in the temperature range of 110150 °C, increases the rate of dehydration, and increases the productivity of catalysts.