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Abstract
This study is dedicated to the comparative investigation of the catalytic activity of layer-like Faujasite-type (FAU) zeolite X obtained from three different synthesis routes (additive-free route, Li2CO3 route, and TPOAC route) in a liquid-phase Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate to ethyl trans-α-cyanocinnamate. It is shown that the charge-balancing cations (Na+ and K+) and the morphological properties have a strong influence on the apparent reaction rate and degree of conversion. The highest initial reaction rate could be found for the layer-like zeolite X synthesised by the additive-free route in the potassium form. In most cases, the potassium-exchanged zeolites enabled higher maximum conversions and higher reaction rates compared to the zeolite X catalysts in sodium form. However, very thin crystal plates (below 100 nm thickness), similar to those obtained in the presence of TPOAC, did not withstand the multiple aqueous ion exchange procedure, with the remaining coarse crystals facilitating less enhancement of the catalytic activity.
Highlights
The Knoevenagel condensation is a special case of aldol condensation and one of the most established carbon coupling reactions for the synthesis of fine chemicals and biologically active substances [1,2]
These materials synthesised in the absence of any morphology modifying additive were further referred to as additive-free layer-like zeolite X, according to the nomenclature suggested by Reiprich et al [8]
We present the results of our comparative investigations on the catalytic activity of layer-like zeolite X obtained from the three above-mentioned synthesis routes in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate
Summary
The Knoevenagel condensation is a special case of aldol condensation and one of the most established carbon coupling reactions for the synthesis of fine chemicals and biologically active substances [1,2]. Besides the additive-free synthesis route, which yields quite coarse layer-like zeolite X, the addition of morphology modifying agents (MMAs) can be used to achieve layer-like zeolites, which can facilitate even higher specific external surface areas, larger mesopore volumes, thinner layer-like crystals, and different contents of the zeolitic intergrowth structure EMT (Elf Mulhouse Chimie Two) [9,10,11] Such MMAs can be, e.g., inorganic salts as reported for the addition of Li2CO3 or organic mesoporogenes such as 3-(trimethoxysilyl)propyl dimethyloctadecylammonium chloride (TPOAC) or 3-(trimethoxysilyl)propyl dimethylhexadecylammonium chloride (TPHAC) [9,10]. The thickness of their single layers can be controlled via a variation of the synthesis gel composition [8]
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