Synthesis and characterization of metals doped on fibrous silica zeolites for benzene alkylation with methanol

Abstract

Benzene alkylation with methanol (benzene methylation) offers an alternative process to produce toluene, which is an important chemical intermediate in petrochemical industries. However, the existing catalysts have low performance for toluene yield due to their intrinsic micropores and high acidity. In this research, zeolite catalysts (ZSM-5, Y and Beta) with fibrous morphology (HFZ, HFY and HFB) were prepared to study their properties and catalytic activity’s relationship with benzene methylation. The fibrous ZSM-5 (HFZ) was further modified using different concentrations of silica source, tetraethylorthosilicate (TEOS). Then, different transition metals (TMs) such as cobalt (Co), titanium (Ti) and manganese (Mn), as well as various Mn loading (1-10 wt%) were loaded on HFZ catalyst using impregnation method to enhance the benzene methylation performance. The catalysts were characterized using X-ray diffraction, nitrogen physisorption, field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared (FTIR) spectroscopy, pyridine adsorbed FTIR, 2,6-lutidine adsorbed FTIR, Raman spectroscopy and thermogravimetric analysis. The catalytic testing was conducted at 300 – 400 ºC under atmospheric pressure. The toluene yield for different fibrous zeolite catalysts and concentration of TEOS was found in the order: 1.0HFZ (63.1 %) > 0.5HFZ (57.5 %) > 1.5HFZ (55.7 %) > HFBEA (50.9 %) > HFY (50.4 %) at 300 ºC. This result could be attributed to the adequate mesoporosity and Brönsted acid sites of the 1.0HFZ, thus decreasing the diffusion limitation and side reactions. For different TMs loaded on HFZ, the Mn/HFZ outperformed Co/HFZ and Ti/HFZ. Among the Mn loadings (1 – 10 wt%), the 5Mn/HFZ reached the highest toluene yield of 69.6 % at 350 ºC, carrier gas flowrate of 20 cm3s-1 and benzene: methanol ratio of 1. In addition, the 5Mn/HFZ possessed outstanding stability over 72 h time on stream, as compared to pristine HFZ with activity loss of 10.8 % for toluene yield. The in-situ FTIR study corroborated that Lewis acid sites originated from Mn are beneficial for toluene formation by enhancing the benzene ring stabilization and adsorption during the alkylation reaction and inhibit the side reaction. The optimum toluene yield predicted by response surface methodology was 68.8 % at reaction temperature of 361 ºC, carrier gas flowrate of 19.0 cm3s-1 and benzene:methanol ratio of 1.45. Based on the above observations, this study highlights the potential role of fibrous silica ZSM-5 and Mn catalysts in the benzene methylation reaction, particularly in the production of toluene.