Synthesis of mesoporous sodalite using various organic templates as catalyst in knoevenagel condensation reaction of 2-nitrobenzaldehyde and ethyl acetoacetate

Abstract

Sodalite is a type of zeolite having ultramicropore size and high thermal stability with strong basic property. However, microporous sodalite has several drawbacks in the catalyst application due to its small pore size (2.8 Å) and low surface area. To overcome these disadvantages microporous sodalite was modified to have mesoporous structure, while still maintaining its active sites suitable for catalyst applications. In this study, mesoporous sodalite was synthesized using various organic templates as structure directing molecules and used as a base catalyst in the Knoevenagel condensation reaction. A hierarchical mesoporous sodalite has been synthesized hydrothermally at 150ºC for crystallization using various mesotemplates. Results from XRD and FTIR spectroscopy showed that all templates being used in the synthesis produced sodalite phase. The sodalite synthesized using organosilane template and a dual template containing a mixture of tetrapropylammonium hydroxide and cetyltrimethylammonium bromide, have shown mesoporosity, as proven by nitrogen adsorption analysis. The field emission scanning electron microscopy (FESEM) micrographs of the mesoporous sodalite obtained showed spherical morphology in the size range ~ 10-16 nm. 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy showed the sodalite framework has a Si/Al ratio equal to 1, while 27Al MAS NMR spectrum exhibited the unsymmetrical tetrahedral Al. The synthesized sodalite samples which were modified with metal ions (K+, Cs+) enhanced the sodalite basicity. Results from Hammet indicators and temperature programmed desorption of CO2 studies revealed that potassium mesoporous sodalite prepared using dual template has the highest strength and amount of basic sites. The reactivity for all of the prepared sodalite samples was evaluated in the Knoevenagel condensation of 2-nitrobenzaldehyde and ethyl acetoacetate, producing 4-(2-nitrophenyl)-but-3-en-2-one as the main product, and trans-2-nitrocinnamic acid as the side product. All synthesized mesoporous sodalites were more reactive than the microporous sodalites counterpart, producing > 70% selectivity for the main product. Based on the conversion of reactant, the potassium mesoporous sodalite using dual template has shown the most active catalyst which gave 95% conversion at 150oC and 6 hours reaction using 0.2 g catalyst. Result from the experimental study was in accordance with the response surface methodology (RSM) findings for the optimum reaction parameters which were at 150oC, 6 hours reaction time, 0.2 g catalyst for 97.1% conversion. This study has proven that besides basicity, hierarchical mesoporosity of sodalite is important for the enhancement of the reactivity of the sodalite catalyst for reactions involving larger molecules such as in the Knoevenagel reaction.