کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
60885 | 47551 | 2015 | 7 صفحه PDF | دانلود رایگان |
• Acid strength effects on acid–base cooperativity are systematically investigated.
• A controlled spatial arrangement of acid and base sites was achieved.
• H-bond donors outperform stronger acids as base promoting sites.
• Changing a methyl to an ethyl group decreases the activity.
• Rational catalyst design proposes a methyl amino alcohol as an optimal catalyst.
Weak acids are known to enhance the activity of amines in aldol condensation reactions on silica-based catalysts. The effects of acid strength and arrangement of the promoting site with respect to a secondary amine have been investigated in the aldol condensation of 4-nitrobenzaldehyde with acetone. Changing the substituent of this secondary amine from a methyl to an ethyl group decreases the activity. An intramolecular OH function provided by a primary alcohol incorporated on the β-carbon of the amine substituent exhibits a similar cooperativity as an intermolecular OH function provided by neighboring surface silanols. A maximum activity was achieved when the secondary amine with the same primary alcohol-containing substituent was surrounded by surface silanols, indicating the potential advantage of simultaneously activating both reactants by the formation of a hydrogen bond in contrast to the consecutive activation when there is only one promoting site in the vicinity of the amine. Changing the alcohol to stronger acids resulted in a reduced cooperativity with increasing acid strength. After removing the silanols from the surface, the activity of the catalysts which exhibit an intramolecular cooperativity retained about 68–83% of their activities while the activity of the conventional secondary amine was reduced by a factor of four compared to its intermolecularly cooperative counterpart.
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Journal: Journal of Catalysis - Volume 325, May 2015, Pages 19–25