Influence of Carbon Monoxide in Chemoselective Heterogeneous Catalytic Reactions PDF Download

Author: Daniel R. Marple
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Languages : en
Pages : 20

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Book Description
The activity for the hydrogenation of crotonaldehyde and selectivity to butyraldehyde and crotyl alcohol was studied in the absence and presence of a small molecule, carbon monoxide at low pressures (19.1, 10, and 2 mTorr). The addition of small amounts of carbon monoxide to the reaction (9 Torr crotonaldehyde, 160 Torr H2, 373 K) decreased conversion and decreased the overall selectivity to crotyl alcohol, while exerting very little influence on the turnover frequency for butyraldehyde. It was determined that the active sites responsible for the formation of crotyl alcohol are also responsible for the formation of n-butanol because the selectivity to both the unsaturated alcohol and saturated alcohol decreased with increasing CO partial pressure or increasing time-on-stream with a constant pressure of carbon monoxide. This is direct evidence that CO competes for the active sites responsible for crotyl alcohol and n-butanol formation. It was also found that carbon deposits from extended time-on-stream experiments (~20 h) inhibit the formation of crotyl alcohol. The addition of CO shortened the time to achieve steady-state by 25% for the formation of butyraldehyde and 31% for the formation of crotyl alcohol which was presumably due to elimination of the most active sites for decomposition of crotonaldehyde. The apparent activation energies are as follows: 41.8 kJ/mol for the formation of butyraldehyde in the absence of CO, 76.0 kJ/mol for the formation of butyraldehyde in the presence of 19.1 mTorr CO, and 33.1 kJ/mol for the formation of crotyl alcohol in the absence of CO. Increasing the temperature from 353 to 373 K increased the TOF for butyraldehyde by ~2 fold (from 0.3 - 0.54 x 10-3 s-1 to 0.6 - 1.2 x 10-3 s-1), increased the TOF for crotyl alcohol ~1.2 times (from 0.12 - 0.20 x 10-3 s-1 to 0.14 - 0.24 x 10-3 s-1), and decreased the selectivity to crotyl alcohol from 28% to 18%. The TOF decreased to its steady- value faster when more CO was added for both butyraldehyde and crotyl alcohol. It was found that the Pt(3.6 nm)/SBA-15 (3.6 nm represents the average particle size) catalyst changed during use and an amorphous Pt/SiO2 catalysts worked initially, but then stopped producing crotyl alcohol. Currently, we do not known reason for this behavior. Further research should focus on surface studies of the Pt(3.6 nm)/SiO2 catalyst during reaction, varying the partial pressure of carbon monoxide to compare the activity and selectivity of Pt/SiO2 versus Pt/SBA-15 catalysts, and comparing the effects of varying amounts of carbon monoxide on Pt catalysts of varying nanoparticle size.