Phosphorus-modified b-axis oriented hierarchical ZSM-5 zeolites for enhancing catalytic performance in a methanol to propylene reaction

Highlights

• A b-axis oriented hierarchical ZSM-5 (Z5-C) zeolite is synthesized using glucose as additive.

• The Z5-C catalyst exhibits a high propylene selectivity of 53.01 % and a long lifetime of 45 h;

• An appropriate phosphorus modification increases the lifetime of the Z5-CP2 catalyst to 79 h.

Abstract

In this work, a b-axis oriented hierarchical ZSM-5 zeolite was prepared with glucose as an additive and further modified by phosphorus to improve the propylene selectivity and prolong the lifetime of the catalyst in a methanol to propylene (MTP) reaction. The physiochemical properties were characterized by XRD, low-temperature N₂ sorption, SEM, TEM, XPS, solid-state NMR, FTIR, and NH₃-TPD analyses. The b-axis oriented ZSM-5 (Z5-C) zeolite exhibited a microsized hexagonal lamellar structure with a thickness of approximately 220 nm. Analyzing the MTP reactions revealed that the newly prepared Z5-C catalyst exhibited a higher propylene selectivity of 53.01 % and a longer lifetime of 45 h compared to those of a conventional nanosized ZSM-5 catalyst (propylene selectivity of 43.97 % and lifetime of 10 h). In addition, the phosphorus modification remarkably increased the lifetime of the b-axis-oriented ZSM-5 catalyst to 79 h.

Introduction

Methanol to hydrocarbon (MTH) conversion reactions have received wide attention due to the extensive sources of raw materials, such as coal and renewable biomass, as a replacement for conventional crude oil [1]. With the ever increasing demands of light olefins, including ethylene, propylene, and butene, the methanol to propylene (MTP) reaction is considered a promising alternative approach for propylene production to fill the demand gap from steam cracking naphtha [2]. ZSM-5-based catalysts, which possess a unique tri-dimensional pore structure and surface acidity, have been successfully applied in a commercial MTP process by Lurgi company [3]. However, improving the propylene selectivity and prolonging the catalytic lifetime of these ZSM-5 catalysts are the major goals for MTP technology at present [[4], [5], [6]].

To date, fabricating hierarchical structures, modifying crystal morphology and size, and adjusting the acid properties of ZSM-5 zeolites are the main strategies to improve the catalytic performance in MTP reactions [[7], [8], [9], [10], [11]]. It is recognized that microporous ZSM-5 with auxiliary mesopores and nanometer-sized crystals are beneficial for shortening the path length of molecular diffusion and enhancing accessibility to the internal surface acid sites. For instance, Li et al. [12] prepared an interconnected hierarchical ZSM-5 by a post dealumination-realumination method to improve the lifetime and propylene selectivity of the MTP catalyst.

For crystal sizes, it is not always a benefit toward improving the performance of the MTP reaction if the crystal sizes of ZSM-5 zeolite become too small [[13], [14], [15]]. Light olefins and coking precursors are more likely to adsorb on the acid sites of the highly exposed external surface of these nanosized crystals, although the catalytic lifetime increases in most cases [[16], [17], [18]]. In addition, small crystals decrease the shape-selective effect of micropores and lower propylene selectivity [19]. For example, Ryoo’s group synthesized ZSM-5 nanosheets with a thickness of 2 nm along the b-axis dimension, and the large number of acid sites on the external surface on these zeolites suppressed catalyst deactivation during a methanol to gasoline (MTG) conversion [20,21]. However, these acid sites on the external surface of nanosheets contributed little to improving the MTH conversion, as reported by Kim et al. [22]. Sugimoto et al. [13] found that the propylene selectivity over microsized ZSM-5 with large crystals of 3−4 μm was 8.1 % higher than that on nanosized ZSM-5 with crystal sizes less than 200 nm. A recent study by Wu et al. [14] also showed that ZSM-5 zeolite with an appropriate crystal size of approximately 150 nm promoted propylene selectivity and extended the catalytic lifetime compared with those of ZSM-5 zeolites with other crystal sizes.

Recent studies have indicated that propylene is primarily produced in nonintersecting straight channels parallel to the b-axis of ZSM-5 zeolite, where an alkene-based cycle is active [[23], [24], [25]]. However, the relatively long diffusion path of zig-zag channels parallel to the a-axis contributes more to deactivation [18,26]. Thus, an innovative solution is to design a microsized ZSM-5 zeolite that is thin along the b-axis direction; in this case, both shortening the diffusion path and strengthening the shape-selective effect of the straight channels are considered. For example, Xiao’s group reported a one-pot growth of TS-1 crystals with controllable b-oriented lengths, which were synthesized with the aid of urea as an organic additive and a fluorinated surfactant [27]. Zhang et al. [28] synthesized a hexagonal lamellar ZSM-5 zeolite with a thin b-axis dimension of less than 450 nm under a neutral fluoride medium, and this catalyst performed with relatively high propylene selectivity (45.1 %) and a long lifetime in the MTP reaction.

In this work, we demonstrate a facile hydrothermal crystallization method to synthesize a microsized b-axis oriented ZSM-5 zeolite (Z5-C) in the presence of environmentally friendly glucose. Z5-C exhibits superior catalytic performance in the MTP reaction with much higher propylene selectivity and a longer catalytic lifetime than conventional nanosized ZSM-5 (Z5-A). In addition, a phosphorus modification further prolongs the catalytic lifetime of the b-axis-oriented ZSM-5 zeolite (Z5-CP2). The high selectivity for propylene and extended lifetime of Z5-CP2 were related to its thinness along the b-axis direction, hierarchical structure, and appropriate surface acidity. Therefore, our work provides a new route for the facile synthesis of highly efficient MTP catalysts in the future.