posted on 2020-06-12, 12:07authored byPeng He, Yunlei Chen, Jack Jarvis, Shijun Meng, Lijia Liu, Xiao-Dong Wen, Hua Song
The effects of Zn–Pt interaction and Pt dispersion over
a uniform compact cylindrical shape ZSM-5 (UZSM-5) on the catalytic
octane aromatization performance are investigated. The comparison
between different Pt- and Zn-modified ZSM-5 catalysts demonstrates
the significance of ZSM-5 morphology and, more importantly, the metal
distributions on it. For the UZSM-5 support, Pt atoms prefer to occupy
the sites within its inner pores, resulting in high selectivity to
xylenes during the octane aromatization. The Zn deposit in inner pores
and higher dispersion of Pt lead to the spillover of Pt sites to the
external surface, which is critical for the activation of octane to
produce reaction intermediates that are further converted to aromatics
over the inner pore catalytic sites. These effects are evidenced by
a diffuse reflection infrared Fourier transform spectroscopy study
of CO adsorbed on the catalyst surface. In situ X-ray absorption fine
structure spectra are collected to probe the coordination number and
the chemical environment of Pt and Zn atoms in the catalysts during
the octane aromatization reaction. Pt and Zn are well dispersed and
stable during the reaction, and a partial reduction of Pt during the
reaction is observed. A theoretical study using the density functional
theory method predicts that the reaction and transition-state intermediates
upon octane activation are better stabilized by Pt(111) of Pt external
surface sites with a smaller activation barrier, indicating their
significance in C–H activation. This hypothesis is further
evidenced by comparing the octane aromatization performance of various
modified catalysts through varying Zn loading, blocking inner pores,
and covering the external catalytic sites with SiO2.