Effect of morphology and particle size of ZSM-5 on catalytic performance for ethylene conversion and heptane cracking

Abstract

To determine the effect of morphology and particle size of ZSM-5 (particle type ZSM-5) on catalytic performance (catalytic activity, product distribution, and catalyst lifetime), ZSM-5 with varying particle size and a ZSM-5 nanosheet were prepared to study the conversion of ethylene and cracking of n-heptane. The physicochemical properties were also investigated by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, ²⁹Si MAS, and ²⁷Al 3Q MAS nuclear magnetic resonance.

The rate of ethylene conversion was found to increase with decreasing ZSM-5 zeolite particle size, while the product distribution was independent of the particle size and morphology of the catalyst. In n-heptane cracking, the ZSM-5 nanosheet showed lower catalytic activity than particulate ZSM-5, and the distribution of hydrocarbon products was independent of the morphology of ZSM-5. In both reactions, the effects of morphology and particle size on catalytic performance will be discussed.

Introduction

Proton-exchanged zeolites, such as H-ZSM-5, are widely used in the petrochemical and fine chemical industries as shape-selective solid acid catalysts for various hydrocarbon conversions [1], [2], [3]. As catalysts with a well-defined pore system, zeolites allow control of the product distribution according to shape-selectivity effects. However, as the zeolite particles are larger than the micropores, the diffusion rates of reactant molecules within the zeolite crystals are lower than the intrinsic reaction rates. This means that generally the crystal size of zeolite influences the reaction rates and selectivity [4]. Moreover, as the effective Brönsted acid sites for catalytic reactions are distributed on the internal surfaces of the main channels, the pore mouths are easily clogged due to coke deposition, thus shortening the lifetime of the catalysts. This adverse effect on the catalytic activity of the zeolites can be overcome by reducing the zeolite crystal size, which led to the idea of the synthesis of zeolite nanocrystals. Zeolite nanocrystals with smaller crystallite size and higher external surface area exhibit shorter diffusion path lengths, thus improving the molecular diffusion. Furthermore, the density of pore openings in nanocrystalline zeolites is found to be higher than in microcrystalline zeolites. Thus, zeolites with nanocrystals are effective in decreasing the diffusion resistance and preventing pore clogging in gas–solid and liquid–solid heterogeneous catalytic reactions [4].

Among many kinds of zeolites, H-ZSM-5 is a versatile catalyst for many industrial applications [5], [6], [7]. ZSM-5 is a three-dimensional 10-membered ring (MR) zeolite composed of straight and zig-zag channels with a pore diameter of 0.56 nm [5]. Though ZSM-5 is successfully employed in many catalytic processes, there is still a need to increase the efficiency, lifetime, and molecular diffusion. In recent years, many attempts have been made to prepare ZSM-5 nanocrystals whose activities were studied for various catalytic reactions. For example, Serrano and coworkers carried out the cracking of polyethylene on nanocrystalline and microcrystalline ZSM-5 zeolites. They found that nanocrystals, with their higher surface area and more active sites, showed better catalytic performance than microcrystals [8]. Wang and coworkers reported the catalytic performance of both nano- and microscale ZSM-5 in different reactions: toluene disproportionation, toluene alkylation with methanol and trimethyl benzene cracking [9]. A superior reactivity was found over the nanoscale zeolite due to the increased pore openings and high accessibility of acid sites.

Choi and coworkers reported two-dimensional ZSM-5 nanosheets having a thickness of 2 nm [10], [11]. These nanosheets with large external surface areas showed enhanced activity in the transformation of bulky organic molecules and in gasoline to olefin conversions. As a catalyst for surface reactions, the H-ZSM-5 nanosheet outperformed particulate H-ZSM-5 [10].

Since zeolites are shape-selective catalysts, it is very important to know the effects of their crystallite size and pore structure on product selectivity. In most studies, product selectivity is explained by the zeolite pore size; however, pore size changes with particle size and morphology [10]. For example, a ZSM-5 nanosheet exhibits a mesoporous structure with a pore diameter in the range of 2–8 nm, which is much higher than the pore diameter of conventional ZSM-5 (<2 nm). Zhu and coworkers reported a micro- and mesoporous structure for nanocrystalline (40 nm) ZSM-5 samples, in which a wide pore size distribution was observed [12]. Grieken and coworkers reported the change in the pore size distribution of ZSM-5 with respect to crystallite size [13]. This means that the product selectivity for a particular reaction over a particular zeolite is not constant. Rather, it varies with the zeolite pore size. Meanwhile, Corma et al. pointed out the importance of the possibility of controlling product selectivity not only by the pore size but also the volume of cavity of zeolite [14].

Recently, we have found that specific carbenium cations produced on H⁺-exchanged zeolite can be recognized by the shape of the zeolite cavity and that the selectivity for reaction products depends on the volume of the zeolite cavity, not on the entrance pore diameter of the zeolite [15]. For example, in the conversion of C₂H₄ using H⁺-exchanged zeolites with 8-, 10-, and 12-MR, the selectivity for C₃H₆ depended on the volume of the zeolite cavity, not on the entrance pore diameter. In this case, the volume of the zeolite cavity yielding C₃H₆ with the highest selectivity is almost the same as the volume of octyl carbocations.

These results prompted us to investigate the effect of pore structure, morphology, and particle size of ZSM-5 zeolite on the rate of C₂H₄ conversion, C₃H₆ selectivity, C₃H₆ production rate, and catalyst lifetime in the conversion of C₂H₄ to C₃H₆. In the present investigation, a series of H-ZSM-5 zeolites with particle sizes in the range 0.13–13 μm and an H-ZSM-5 nanosheet (thickness: 1.8 nm) were prepared. The former ZSM-5 will be referred to as the particle type ZSM-5.

This study focuses on the H-ZSM-5 nanosheet, whose physical properties are completely different from the particle type H-ZSM-5. A detailed investigation into product selectivity and catalytic activity with respect to particle size and structure of H-ZSM-5 nanosheet is presented in what follows.