Conversion of Oxygenates to Chemicals over Mixed Metal Oxide Catalysts

Rebecca Ann Long Baylon

ZnxZryOz catalysts were developed and used to convert small oxygenates to chemicals with high selectivity and stability. They were synthesized using (1) a hard-template method and (2) an incipient wetness method. These procedures produced catalysts with (1) Bronsted and Lewis sites and (2) with only Lewis sites. For the ethanol to 1,3-butadiene conversion, activity testing showed that the hard-template ZnxZryOz gave higher selectivity to 1,3-butadiene. NH3 temperature programmed desorption and IR studies using pyridine as a probe molecule showed that the strong acidic sites led to undesired dehydration to ethylene. To minimize this, Na doping was used to suppress acidity and achieve up to 47% selectivity to 1,3-butadiene selectivity. Na doped ZnxZryOz catalysts showed that weak/medium acidic sites are sufficient for the latter dehydration steps. Strong acidic sites are not desired due to their role in catalyzing ethanol dehydration to ethylene. For the ethanol to isobutene conversion, the redesigned ZnxZryOz catalyst with only Lewis acid/base pairs, performed with higher selectivity (~ 85% selectivity) and stability since Bronsted sites were responsible for undesired coking and C4 isomerization. This redesigned catalyst showed that Lewis acid/base pairs are capable of cascade aldolization of ethanol to isobutene reaction. Space velocity studies showed that acetone to isobutene was the rate determining step. Raman spectroscopy revealed that Zn was incorporated into the ZrO2 lattice making a solid solution rather than a separate zincite phase. Comparison to ZrO2 activity, which favors decomposition products (CO2 and CH4), shows Zn modifies the ZrO2‘s acid-base pairs so that cascade reactions are favored. Not only is this ZnxZryOz catalyst capable of ethanol conversion to acetone but also ketonization of carboxylic acids such as acetic acid. This agrees well with our proposed reaction pathway in which ethanol is converted to acetone via acetaldehyde’s oxidation to acetic acid and acetic acid’s ketonization to acetone. Longer carboxylic acids and ketones were used as feedstocks to confirm. Propanoic acid yielded C6 olefins and mixed feeds yielded C5 olefins due to cross reactions. Multiple ketones, varying mixed feed ratios and Zn/Zr ratios also confirmed the proposed pathway that achieves a controllable range of C3-C6 olefins.

Conversion of Oxygenates to Chemicals over Mixed Metal Oxide Catalysts

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