Influence of basicity on 1,3-butadiene formation from catalytic 2,3-butanediol dehydration over γ-alumina

- We studied 2,3-butanediol dehydration over two commercial forms of alumina.
- Calcination temperature and sodium content cause the 1,3-butadiene selectivity difference.
- 3-Buten-2-ol is the intermediate of 2,3-butanediol dehydration to 1,3-butadiene.
- Acetone selectivity increased as the sodium loading increased.
- BDO dehydration and acetone formation mechanism is proposed based on DFT calculations.

The direct catalytic conversion of 2,3-butanediol (BDO) to 1,3-butadiene (BD) was studied over two commercial forms of alumina (denoted as F200 and SCFa) at temperatures between 240 °C and 450 °C. Even though these two catalysts are both high surface area forms of γ-alumina, they gave remarkably different results, with SCFa giving higher BD selectivities at all experimental conditions. The difference is attributed to the higher surface area of F200, which means a greater number of acid sites that can convert BDO to methyl ethyl ketone (MEK). NH3 and CO2-TPD results supported this conclusion by showing that the two forms of alumina had different acid/base properties. Experimental results also showed that BD selectivity was improved by increasing temperature, increasing residence time and co-feeding water. The residence time study combined with density functional theory (DFT) calculations proved that 3-buten-2-ol (3B2OL) is an important intermediate in the conversion of BDO to BD. BD selectivity decreases over sodium modified alumina SCFa. It is hypothesized that on sodium-modified alumina, 3B2OL is dehydrogenated to form methyl vinyl ketone (MVK) as opposed to dehydration to BD. Basic sites catalyzed the retro-aldol condensation of MVK, which produces acetone and formaldehyde via cleavage of the CC bond. This is in agreement with DFT calculations showing that the proposed pathway for acetone formation is more energetically favored on Na-modified γ-Al2O3 (1 1 0) surface compared to the pristine (1 1 0) surface.

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