Facile synthesis of hierarchical NaX zeolite from natural kaolinite for efficient Knoevenagel condensation

Abstract

Zeolite catalysts have found extensive applications in the synthesis of various fine chemicals. However, the micropores of zeolites impose diffusion limitations on bulky molecules, greatly reducing the catalytic efficiency. Herein, we explore an economic and environmentally friendly method for synthesizing hierarchical NaX zeolite that exhibits improved catalytic performance in the Knoevenagel condensation reaction for producing the useful fine chemical 2-cyano-3-phenylacrylate. The synthesis was achieved via a low-temperature activation of kaolinite and subsequent in-situ transformation strategy without any template or seed. Systematic characterizations reveal that the synthesized NaX zeolite has both inter-crystalline and intra-crystalline mesopores, smaller crystal size, and larger external specific surface area compared to commercial NaX zeolite. Detailed mechanism investigations show that the inter-crystalline mesopores are generated by stacking smaller crystals formed from in-situ crystallization of the depolymerized kaolinite, and the intra-crystalline mesopores are inherited from the pores in the depolymerized kaolinite. This synthesis strategy provides an energy-saving and effective way to construct hierarchical zeolites, which may gain wide applications in fine chemical manufacturing.

1. Introduction

Recently, zeolites have attracted great attentions in the synthesis of a wide range of fine chemicals, such as flavors, fragrances, pharmaceutical intermediates, detergents and pigments [1,2]. Traditionally, fine chemicals are synthesized via the homogeneous catalysis system [3,4]. However, the separation and recovery of the homogeneous catalysts are difficult and expensive, because they are typically dissolved in the reaction mixture [5]. Moreover, some homogeneous catalysts, such as liquid acid and base, may generate hazardous waste or emit harmful byproducts, leading to environmental concerns [6,7]. Zeolites are known to have several advantages in fine chemical synthesis as a heterogeneous catalyst [8,9]. They are often able to be easily separated from the reaction mixture, allowing for their recovery and reuse, which can reduce costs and increase efficiency. Additionally, zeolites are commonly considered as environmentally friendly catalysts, as they are non-toxic and their production does not generate hazardous waste. Their porous structure and shape selectivity make them highly selective, resulting in high yields of desired products and reduced waste. Furthermore, zeolites are also stable under a wide range of reaction conditions, making them suitable for industrial processes.

In these years, there has been a significant interest in the zeolite-catalyzed Knoevenagel condensation reaction for the synthesis of fine chemicals such as pharmaceuticals, agrochemicals, and natural products [10,11]. Knoevenagel condensation reaction is a versatile and efficient method for the formation of C–C bonds and the introduction of various functional groups into organic molecules [12]. Therefore, the Knoevenagel condensation reaction is a valuable tool for the synthesis of complex organic molecules and has significant applications in the pharmaceutical and fine chemical industries [13–15]. NaX zeolites with basic sites from the sodium cations in zeolite channel, can effectively catalyze the Knoevenagel condensation reaction under the mild conditions [16,17]. However, the Knoevenagel condensation involves bulky molecules, and the micropore in conventional NaX zeolite brings about serious steric hindrance and diffusion limitation of bulky molecules, which greatly decrease the catalysis efficiency [18,19]. To tackle the issue above, scientists have been exploring the hierarchical zeolites with multi-modal pore structures to overcome the drawback of conventional zeolites. Hierarchical zeolites have a more open pore structure than conventional zeolites, enabling better accessibility of bulky reactants to active sites and reducing diffusion limitations [20–22]. This makes them promising materials for heterogeneous catalysis applications. For instance, Verboekend et al. [19,23] reported a ten-fold increase in catalytic activity of hierarchical NaX zeolite compared to the microporous NaX zeolite in the Knoevenagel condensation of benzaldehyde with malononitrile. The enhanced activity was attributed to the improved accessibility of reactants to active sites in hierarchical zeolites. Grass et al. [18] de