Synthesis of ZSM-5 using different silicon and aluminum sources nature for catalytic conversion of lignite pyrolysis volatiles to light aromatics

Highlights

• ZSM-5 with different Si and Al sources was used for reforming of lignite volatiles.

• Si and Al sources affect the activity of ZSM-5 for lignite volatiles reforming.

• ZSM-5 synthesized with TEOS and ASO give highest BTEXN yield.

• BTEXN formation is associated with both porous and acid properties of ZSM-5.

• Al sources mainly affect strong acid sites by changing crystallinity of zeolite.

Abstract

ZSM-5 zeolites with different physicochemical property were synthesized using different silicon (Si) and aluminum (Al) sources by hydrothermal method with controlling Al and Si addition. Different crystal shapes were observed from three Si sources. Al sources can influence crystal formation during crystallinity. Al nature causes different amorphous species which affect zeolite activity by blocking the accessible pores. A feasible mechanism of Si and Al sources for crystallization of zeolite was proposed. The activity for lignite volatiles-to-aromatics (LVTA) reaction was investigated to obtain light aromatics. Zeolite using tetraethylortho silicate and aluminum sulfate octahydrate as Si and Al sources shows large surface area and more accessible pores which cause excellent BTEXN yield (23.7 mg/g). The tar yield in the process of LVTA reaction is associated with both porous and acid properties while excessive acidity may cause the formation of coke on the zeolite which causes the blockage. ZSM-5 with optimum and accessible acid sites and abundant pores give a high tar yield and low coke yield. This work provides a ZSM-5 catalyst with suitable acidity and pore property for LVTA reaction.

Introduction

Many upgrading methods like catalytic cracking and hydrodeoxygenation of lignite pyrolysis tar are being developed [1], [2], [3], [4], [5]. In recent years, in situ upgrading of coal/biomass pyrolysis volatiles over ZSM-5 was widely reported for conversing raw materials into light aromatics such as benzene, toluene, ethylbenzene, xylene, and naphthalenes (BTEXN) [3], [6], [7], [8], [9]. The excellent catalytic performance of ZSM-5 in this reaction is attributed to its unique structure, thermal stability, acidity and shape selective property. The point of view of turning hierarchical pore structure, loading activated metal and controlling acidity by post-treatment methods were largely absorbed by the academics to enhance BTEXN yields [10], [11]. They are passionate about and pursuing the structure-activity relationship between the physicochemical properties of modified ZSM-5 and product selectivity. However, the physicochemical and catalytic properties influenced by the hydrogel chemistry (sources of silica and alumina, template, cations and anions, molar ratios of SiO₂/Al₂O₃, OH/SiO₂, etc.) and crystallization conditions (temperature, aging, agitation, etc.) are still unclear in this reaction. As we known, the acidity of ZSM-5 plays a key role in reaction pathway and product distribution. Its catalytic properties of ZSM-5 are influenced by crystal size [12]. However, the acidity, micropore diffusivity and thermal/hydrothermal stability may be substantially different if the crystal morphology (shape and size) of ZSM-5 is changed [13]. Ren et al. [14] reported that controlling the added amount of Al source as-synthesized ZSM-5 with different acidic amounts and confirmed that the range of SiO₂/Al₂O₃ ratio from 25 to 50 is beneficial for in situ upgrading lignite-volatiles to BTEXN. Schmidt et al. [12] found that zeolite with small crystal size has larger external surface area and short diffusional path compared to large crystals. This means that the small crystals of ZSM-5 has more active sites and higher catalytic activity and thus can improve coke-deactivation selectivity. Therefore, designing the crystal morphology of ZSM-5 is an important step to optimize its activity, shape selectivity, anti-deactivation and longevity for in situ upgrading process of lignite-volatiles. Liu et al. [13] reported that the different Al sources can influence the crystallinity and crystal size of nano-size ZSM-5. Mohaned et al. [15] showed that the sample prepared by fumed silica has highest crystallinity among the three silica-made samples. The solubility of silica source significantly affects the crystal size during crystallization [16].

Herein, we prepared several ZSM-5 with different crystal morphology by turning the source of Si and Al during the starting synthesis. Subsequently, these zeolites were used for in situ upgrading volatiles from lignite pyrolysis to produce BTEXN. Insights into the effect of different Si and Al sources on the crystal size, morphology, surface area and acidity of ZSM-5 were discussed. These physicochemical properties of ZSM-5 influenced by initial parameters were established to analyze reforming reaction of volatiles.