Tailoring Pt locations in KL zeolite by improved atomic layer deposition for excellent performance in n-heptane aromatization

Abstract

New evidences have been collected to demonstrate that Pt localization plays a crucial role in modulating the catalytic performance of Pt/KL catalyst for n-heptane aromatization. Here, an improved atomic layer deposition (ALD) technique was applied to deliberately tune the locations of Pt nanoparticles (NPs) in the Pt/KL catalysts. Both scanning transmission electron microscopy (STEM) and CO diffuse reflectance infrared Fourier transform (CO-DRIFT) spectra suggest that Pt NPs can be gradually introduced into the depths of KL zeolite channels by the increment of exposure time of Pt precursor from 1/12 to 20 min in one ALD cycle. Moreover, once KL channels are blocked by some encapsulated species (AlOx as example), Pt NPs can only be localized on the orifices or external surface and grow larger. As a result, surprisingly high aromatics selectivity (84.3%) in n-heptane aromatization is performed for a Pt/KL catalyst with Pt dominantly localized in the depth of zeolite channels even at a low temperature of 420 °C. Comparatively, the aromatics selectivity decreases to 31.5% and heptene (mainly 1-C7=) selectivity is up to 51.3% for a Pt/KL catalyst with some of Pt deposited on the orifices or external surface of KL. A suggested mechanism of n-heptane cyclization based on density-functional theory (DFT) calculations was provided to delineate the optional reaction pathways on different Pt localizations. Pt sites inside the KL zeolite channels are more preferable for heptene cyclization than those near the zeolite orifices both from thermodynamically and kinetically. A heptene molecule absorbed on Pt inside zeolite cage was found to curve itself, reducing the reaction barrier for the breakage of CH bond to be only 1.37 eV, therefore activating the 6th carbon atom (the rate-determining step). The barrier comparison for that on Pt near the orifices is 1.84 eV. The calculated mechanism broadens the fundamental understanding of the role of zeolite channels for alkane reforming reactions.