Pengyu Niu,Hongjuan Xi,Jun Ren,Minggui Lin,Qiang Wang,Litao Jia,Bo Hou,Debao Li
Highly siliceous ZSM-22 was synthesized under hydrothermal conditions with ZSM-22 seeds. Characterization by FT-IR and 1H and 29Si NMR spectroscopy confirms that due to the existence of defective sites in the siliceous ZSM-22, acidic silanol groups including vicinal silanol and silanol nests are detected. Acidic silanol groups are considered as the acid sites of siliceous ZSM-22. As a result, siliceous ZSM-22 shows much lower acid site density and lower acid strength than silica–alumina ZSM-22 which contains hydroxyl groups bridging Si and Al as acid sites. In n-dodecane isomerization, Pt/siliceous ZSM-22 shows much higher isomer selectivity than Pt/silica–alumina ZSM-22. Furthermore, a lower loading amount of Pt on siliceous ZSM-22 (about 0.2 wt% Pt) can lead to high isomer selectivity. Owing to the acidic silanol groups and the low acid site density of siliceous ZSM-22, a good balance between metal and acid function in Pt/siliceous ZSM-22 is achieved, which results in a lower allowable Pt loading value and higher isomer selectivity.
A core–shell ZSM-22 (Z@S) was synthesized via epitaxial growth. The characterization results of Z@S validated that the acid sites located on the external surface of core ZSM-22 were passivated by the epitaxial shell of highly siliceous ZSM-22 and the acid strength of ZSM-22 remained unchanged. For comparison, highly siliceous and boron-containing ZSM-22 were also synthesized and characterized. In n-dodecane hydroisomerization, highly siliceous ZSM-22 almost without acid sites at the external surface achieved a much higher iso-dodecane selectivity than boron-containing ZSM-22 with weak acid sites. Z@S with strong acid sites and passivated surface acidity showed high activity and favored pore mouth catalysis but exhibited a lowered iso-dodecane selectivity. The reduced external surface acid sites enhanced the diffusion of n-alkane into the micropores, leading to severe cracking reaction caused by acid sites on the internal surface. Further modifying Z@S via micropore blockage could greatly inhibit cracking reaction inside the micropores and confine isomerization at the pore mouth of ZSM-22. This new ZSM-22 catalyst achieved higher activity and a higher iso-dodecane yield than catalysts with weak acidity.
Single atom catalysts have been recently reported as the efficient catalysts for hydroisomerization compared to the conventional nanoparticle catalyst. Despite the fascinating advantage in enhancing the metal activity or reducing the catalyst cost, the long-term stability of the single atom catalyst seemed not that satisfactory, especially when reacting under atmospheric pressure. Herein, Pt single atom catalysts were combined with Cu nanoparticles by different means to explore a new way to improve the catalytic performance. Cu nanoparticles were located away from Pt sites to form a binary catalyst, or acted as the support of Pt atoms to form a single atom alloy (SAA) catalyst. With the binary catalyst Pt 1 –0.1Cu@CS, the conversion of the n -heptane after time on stream of 50 h increased to 70.3 %, higher than Pt 1 @CS (47.3 %). Besides, excessive Cu nanoparticles in the binary catalyst were found rather harmful to the catalysis instead. However, the SAA catalyst Pt 1 Cu@SAPO-11 showed a poor activity in hydroisomerization compared to Pt 1 @CS, indicating that the alloy structure should go against the activation of reactant. The roles of Pt atoms and Cu nanoparticles in different catalysts were discussed in detail. The reaction mechanism of hydroisomerization was refined with the different functions of the dual metal sites. This work might be instructive for the design of single atom catalysts combined with metal nanoparticles.
危险品化学品经营许可证（带存储）