Green synthesis of ZSM-5 using silica fume and catalytic co-cracking of lignin and plastics for production of monocyclic aromatics

Abstract

ZSM-5 with hierarchical pore structure was synthesized by a simple two-step hydrothermal crystallization from silica fume without using any organic ammonium templates. The synthesized ZSM-5 were oval shaped particles with a particle size about 2.0 lm and weak acid-dominated with proper Brønsted (B) and Lewis (L) acid sites. The ZSM-5 was used for catalytic co-cracking of n-octane and guaiacol, low-density polyethylene (LDPE) and alkali lignin (AL) to enhance the production of benzene, toluene, ethylbenzene and xylene (BTEX). The most significant synergistic effect occurred at n-octane/guaiacol at 1:1 and LDPE/AL at 1:3, under the condition, the achieved BTEX selectivity were 24% and 33% (mass) higher than the calculated values (weighted average). The highest BTEX selectivity reached 88.5%, which was 3.7% and 54.2% higher than those from individual cracking LDPE and AL. The synthesized ZSM-5 exhibited superior catalytic performance compared to the commercial ZSM-5, indicating potential application prospect.

1. Introduction

Monocyclic aromatic hydrocarbons (MAHs) such as benzene, toluene, ethylbenzene and xylene (BTEX) are important industrial chemicals, and are widely used in the production of chemical fiber, plastic and rubber [1,2]. By far, the production of BTEX mainly depends on petroleum resources via naphtha catalytic reforming and pyrolysis gasoline hydrogenation approaches [3,4]. However, the increasing shortage of oil resource and demand for BTEX necessitates developing an alternative source to produce BTEX. As the sole renewable carbon resource, biomass has attracted much attention of researchers [5,6]. Strenuous efforts have been made to produce BTEX via rapid pyrolysis and catalytic cracking of biomass [7,8]. Nevertheless, due to the complicated chemical structure and high oxygen content (35%–50% (mass)) in biomass, the rapid deactivation of catalysts due to coking and low selectivity of target products could be the bottleneck problem countered in the large-scale application. Catalytic cracking of biomass are prone to produce toxic polycyclic aromatic hydrocarbons (PAHs) and phenolics [9,10]. To resolve this issue, many studies carried out catalytic co-cracking of a co-feedstock of biomass and plastics (as a hydrogen-rich material), demonstrating a notable effect on enhancing BTEX production compared to the individual cracking of biomass [11,12].

It is well known that biomass consists of cellulose, hemicellulose and lignin. Every year about 50 million tons of lignin waste residue is produced from pulp engineering, which becomes a cheap under-utilized raw material [13]. Pyrolysis and co-pyrolysis are suitable for lignin conversion due to its highly complex and recalcitrant features. Even though pyrolysis of lignin is difficult (due to easily forming coke) but lignin has the highest theoretical value to obtain BTEX due to its rich phenyl in its chemical structure. Therefore, a lot of research work for improving BTEX production were conducted on catalytic co-cracking of lignin and plastics. Zhang et al. [14] conducted catalytic co-cracking of black-liquor lignin and different plastics, found that co-cracking with polystyrene produced the maximum aromatics yield (55.3%), while co-cracking with polyethylene produced the maximum olefin yield (13%). Bu et al. [15] reported that the addition of low-density polyethylene (LDPE) into the pyrolysis process of microwave-torrefied lignin enhanced aromatic hydrocarbon production (yield increased from 1.94% to 22.83%), promoted thermal degradation of lignin and improved the reaction rate. Development of novel catalysts and research on pyrolysis process for lignin are now in full swing.

ZSM-5 tends to be common catalyst used in the catalytic cracking of biomass due to its uniform micropores, tunable acidity and high hydrothermal stability [16]. However, ZSM-5 is generally prepared by pure chemicals, e.g. tetraethyl orthosilicate and sodium silicate as the silica sources, and aluminum sulfate and sodium aluminate as the alumina sources, as well as organic structuring agent (OSDA). This increases the manufacturing cost and causes environmental pollution (due to the released toxic waste during the removal process of templates). Hence, it is necessary to find a ZSM-5 synthesis method with low-cost materials and without using an organic template. New synthesis pathway of ZSM-5 have been developed using low-cost fly ash, kaolin and rice husk ash [17–19]. However, these methods could not avoid the use of the expensive OSDA.