Insight into the performance of different Pt/KL catalysts for n-alkane (C6–C8) aromatization: catalytic role of zeolite channels

Abstract

Zeolite channel architecture is vital to catalytic activity and product distribution during naphtha reforming, which can be effectively utilized by designing the locations of active sites. Herein, uniformly dispersed Pt clusters were deposited on KL zeolite using atomic layer deposition (ALD) to elucidate the catalytic role of the zeolite channel architecture in n-alkane (C6–C8) aromatization. Various characterizations supported that Pt/KL-x catalysts with different Pt cluster positions (namely at the zeolite orifice and within the zeolite channels) were obtained successfully, and Pt clusters preferred to occupy the sites within the zeolite channels for Pt/KL-2 and Pt/KL-3 catalysts with prolonged diffusion time of Pt precursors (60 and 180 seconds, respectively) in ALD treatment. As a result, the aromatic selectivity of the Pt/KL-2 catalyst was up to 81.6%, 55.3% and 46.5% for n-hexane, n-heptane and n-octane aromatization, respectively. Comparatively, the selectivity was only 44.4% (n-hexane), 34.1% (n-heptane) and 34.8% (n-octane), respectively, for the Pt/KL-1 catalyst with Pt clusters at the zeolite orifice due to the Pt precursor diffusion time being limited to 5 seconds. This supported that Pt clusters inside the zeolite channels could facilitate n-alkane aromatization owing to the effective utilization of the channels. The superiority of the channel architecture for n-alkane aromatization decreased as the alkane size increased from C6 to C8. DFT calculation was used to further investigate the relationship between the catalytic role of the channels and carbon numbers, and indicated that primary dehydrogenation of larger-sized alkanes became difficult owing to the minor catalytic role of the channels. The cleavage of primary C–H bonds in n-octane adsorbed on a Pt cluster within the channels had an energy barrier of 1.05 eV (TS), which was higher than those for n-heptane (0.85 eV) and n-hexane (0.3 eV) dehydrogenation. Briefly, the KL zeolite channel architecture was favourable for n-alkane reforming, while the facilitating effect becomes less efficient with increasing alkane size. Our work could ultimately contribute to better understanding of the catalytic role of zeolite channels for n-alkane reforming with various carbon numbers, and understanding the different reaction behaviours of Pt/KL catalysts during C6–C8 dehydrocyclization.