Effect of metal precursors on the performance of Pt/ZSM-22 catalysts for n-hexadecane hydroisomerization

Abstract

The effects of Pt precursors on the performance of Pt/ZSM-22 catalysts for n-hexadecane hydroisomerization were investigated with 0.5wt.% platinum loading using three platinum precursors: Pt(NO3)2, H2PtCl6, and Pt(NH3)4Cl2. The catalyst derived from H2PtCl6 exhibits the highest platinum dispersion and nPt/nA value and displays better isomerization activity and selectivity. Three effects can be traced back to the precursors: the valence state of Pt, the location of Pt (in cationic or anionic group), and the existence of chloride. The predominant effect is that of valence states of Pt in precursors, which play a crucial role in determining the textural properties (both particle size and crystal structure) of catalysts. Different crystal orientations of Pt0 are found on the reduced samples, with Pt(IV) precursors oriented along the [011] crystallographic axis, while those with Pt(II) precursors are oriented along the [001] axis. The former favors smaller Pt particles and exposes more active Pt{111} corner sites, while the latter exhibits larger Pt particles and exposes more Pt{100} facets, as revealed by combining HRTEM with CO-IR studies. Moreover, Pt{111}/ZSM-22 shows higher isomerization selectivity than Pt{100}/ZSM-22. The other effect is location of Pt in precursors. Compared with the sample using Pt(NH3)4Cl2 precursor, the catalyst with (NH4)2PtCl4 precursor exhibits smaller particles and higher platinum dispersion and facilitates isomerization reactivity. The existence of chloride is also a nonignorable effect. The samples containing chloride ions without amine groups in precursors favor platinum dispersion due to avoiding autoreduction by NH3 during the calcination process.