Continuous Henry reaction to a specific product over nanoporous silica-supported amine catalysts on fixed bed reactor

Abstract We report a method for continuously producing the nitroaldol, the nitrostyrene, or the Michael product by performing the Henry reaction over a fixed bed reactor that is packed with primary or secondary amine-functionalized nanoporous materials. The % conversion of the reactants as well as the % selectivity to the particular product were found to be strongly dependent on the residence time of the reactants in the reactor (weight hourly spatial velocity or WHSV) as well as the type of reactant, catalyst and reaction temperature used. When a 0.08 M p-hydroxybenzaldehyde solution in nitromethane was passed over the fixed bed reactor containing primary amine-functionalized mesoporous silica catalyst by postgrafting in toluene (AP-T) at 90 °C with WHSV of 0.20, the reactor continuously and selectively generated for hours the p-hydroxy-β-nitrostyrene product with 100% selectivity at 31% reactant conversion (or with 90% selectivity at 88% reactant conversion for WHSV of 0.10). The remaining 12% product in the latter case was the Michael product. The corresponding primary amine-functionalized sample prepared by postgrafting of 3-aminopropyltrimethoxysilane (APTS) in isopropanol (AP-I) also gave similar results with slightly higher efficiency and selectivity to p-hydroxy-β-nitrostyrene. When the same reactant solution was passed over the bed-reactor packed with secondary amine grafted mesoporous silica catalyst by postgrafting in toluene (MAP-T) with WHSV of 0.25 at 90 °C, the reactor also continuously produced selectively the p-hydroxy-β-nitrostyrene product but less efficiently; i.e. with 91% selectivity at 21 reactant conversion for WHSV of 0.20 (or with 85% selectivity at 34% reactant conversion for WHSV of 0.10). Here also, the remaining product was the Michael addition product. Increasing the reaction temperature of the reactor containing the primary amine catalyst to 150 °C at WHSV of 0.10 for p-hydroxybenzaldehyde reactant led to the reversal of the product type from being 90% p-hydroxy-β-nitrostyrene to >85% Michael product with ∼100% reactant conversion. Raising the reaction temperature of the reactor containing a secondary amine catalyst for p-hydroxybenzaldehyde reactant also increasingly favored the formation of the Michael product. When changing the reactant to 0.08 M p-nitrobenzaldehyde, the reactor packed with secondary amine catalyst resulted in the nitroalcohol product with 90% selectivity at 40% reactant conversion for WHSV of 0.15. These results indicate that higher WHSV lead to greater selectivity to a particular product; however, lower WHSV and higher temperatures favor greater reactant conversion reaching as high as ∼100% in all the cases although they can be accompanied by less % selectivity. By simply adjusting the WSHV's or the temperatures to optimum values, one of the products can be exclusively generated in a continuous manner. The continuous reactor and the catalysts were proven to catalyze the reactions and give the respective product(s) continuously for several days. This method can be used as a route for the mass production of industrially and pharmaceutically important p-substituted nitroalcohol, nitrostyrene, or Michael addition product with high selectivity, by simply packing mesoporous catalysts within a fixed bed reactor.