via protective desiliconization and external surface modification
Aromatic hydrocarbons are widely employed as fuel additives and in the production of polymers, medicine and dyestuffs. Prevous studies have shown that aromatics have become scarce as a result of the increasing shortage of petroleum reserves worldwide. Therefore, there is a need to explore alternative routes for aromatic hydrocarbons generation. Recent research reports have highlighted the transformation of methanol to aromatics (MTA). This approach introduced an alternative nonpetroleum route for the production of fuels and chemicals from a renewable energy source. This is due to the fact that methanol is readily accessible from extensive resources, such as coal, natural gas, and bio-masses.
Recently, studies has shown that HZSM-5 molecular sieve is the main catalyst for MTA process. To be specific, the three-dimensional frame of HZSM-5 is a distinctive class of crystalline aluminosilicate with two micropore systems, that is, sinusoidal channels and straight channels which are suitable for shape-selective catalysis of aromatics. Unfortunately, when HZSM-5 is used as a catalyst for the conversion of methanol to aromatics (MTA), its single microscope system often results in severe diffusion limitations. Consequently, this causes a decrease in the reaction rate and a change of selectivity to aromatics. To address this, Lanzhou University of Technology Researchers: Chunhui Cheng, Prof. Guixian Li, Dr. Dong Ji, and Professor Yu Zhao, together with Prof. Jianyi Shen from the Nanjing University, proposed to study in depth the regulating hierarchical structure and acidity of HZSM-5 for methanol aromatics, with the aim being to design a novel approach. Their original research article is now published in the research journal Microporous and Mesoporous Materials.
In their approach, an ingenious protective desiliconization method with a mixed solution of sodium hydroxide and tetrapropylammonium hydroxide as a desiliconization agent was developed to create hierarchical porous systems without destruction of the main structure of the molecular sieve. In addition, hierarchical porous systems combining with multiple levels of porosity were shown to offer superior improvements of catalytic properties that integrate the catalytic features of micropores with enhanced accessibility and transportation of the auxiliary mesopore within a unitary system.
The authors reported that although the desiliconization greatly increased mesoporous volume, it also sharply increased the acidity of the catalyst for the strong acid sites, leading to affect the distribution of aromatics and accelerate the carbon deposition rate. Thus, the deposition of inert SiO₂ on the catalyst surface had to be carried out subsequently to passivate the surface acidity and adjust the orifice of the catalyst. Accordingly, a hierarchical porous HZSM-5 molecular sieve catalyst ([email protected]+TP) with suitable acidity was precisely prepared, which exhibited the high selectivity to benzene, toluene and xylenes (BTX) with the excellent anticoking ability in the MTA reaction.
In Summary, the study reported a composite material, [email protected]+TP, amalgamating a hierarchical porous system and a suitable acidity which was successfully fabricated by protective desiliconization and subsequent silanization modification. Based on this information, the pore architecture and acidity of zeolites were among the aspects reported to influence product distributions and catalyst lifetime in the MTA transformation reaction. Remarkably, the simulation results were in good agreement with experimental data, further confirming that the suitable acid site and the mesoporous connectivity in the core were crucial for a high yield of aromatics and high throughout diffusion. In a statement to Advances in Engineering, authors explained that through their study, a comprehensive understanding of the hierarchical structure and acidity of HZSM-5 for methanol was achieved. They further added that their results for the [email protected]+TP catalyst, which showed the most stable selectivity of aromatics and a long catalytic lifetime, was an ideal multifunctional microreactor model for highly efficient catalysis.
Chunhui Cheng1, Guixian Li1, Dong Ji, Yu Zhao*, Jianyi Shen. Regulating hierarchical structure and acidity of HZSM-5 for methanol to aromatics via protective desiliconization and external surface modification. Microporous and Mesoporous Materials 312 (2021) 110784