Effect of metal modification of HZSM-5 on catalyst stability in the shape-selective methylation of toluene

Shape-selective methylation of toluene (T) with methanol (M) was carried out in a continuous flow fixed-bed reactor over modified HZSM-5 catalysts. The effect of metal modification on the catalytic performance, especially the catalyst stability, was investigated. Various techniques including X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM), were used to characterize the catalysts. The results showed that P-Mg modified catalysts with 3 wt% SiO₂ exhibited excellent stability (more than 500 h), but lowered p-xylene selectivity (less than 90%). Upon further increasing the amount of SiO₂, the catalyst deactivated in a relatively short time on stream, although the para-selectivity was significantly increased up to 99.5%. The high p-xylene-selectivity catalyst modified further with Pt, Pd, Co, Ni metal demonstrated excellent stability, even after 100-400 h on stream, and the selectivity for p-xylene of more than 98.3% with toluene conversion of 20.0% was achieved. Hence, loading metals with good hydrogenation properties, such as Pt, Pd, Co and Ni, was an effective method to improve the catalyst stability.