Suppression of platinum sintering on Pt-M/ZSM-22 (M=Ce, La, Re) catalyst for n-dodecane isomerization
Small Pt particles (about 1.2 nm) were highly dispersed on ZSM-22 zeolites through strong electrostatic adsorption (SEA) method and exhibited promoting effect on the catalytic performance in n-dodecane isomerization in comparison with the conventional catalyst. Several promoters (Ce, La, Re) were introduced to further improve the aggregation resistance of the small Pt particles. A series of characterizations including H2-TPR, XPS and STEM as well as DFT calculation provided the solid evidences that the strong interaction between Pt and promoter species not only effectively hinders the Pt aggregation but also alters the electron state of Pt sites. Moreover, Ce and La promoters also lead to the slightly improved amount of B acid sites determined by Py-FTIR. Thus, improved activity and selectivity were achieved on Ce-modified catalyst due to the better balanced bifunctionality between the slightly increased acidity of zeolites and electron-deficiency of Pt. The electron-rich Pt sites are responsible for the significant increase of CH4 selectivity and the decrease of total isomer selectivity in n-dodecane isomerization. In addition, high-temperature reduction test revealed the higher isomerization activity and selectivity (about 10% higher than unmodified catalyst) of Ce-modified catalyst owing to the strong bond of Pt-O-Ce, further confirming the positive effect of Ce promoter on the sintering resistance of small Pt particles.