(682b) Maximizing Metal Functionality of Ppm Pt in HZSM-5 for Efficient Light Alkanes Dehydroaromatization

Maximizing the metal functionality in the acidic zeolite is critical to the design of efficient bi-functional catalytic materials with minimized noble metal loading for cost-efficient chemical syntheses. To develop an efficient catalyst for light alkanes dehydroaromatization (DHA), Pt is incorporated into the acidic HZSM-5 zeolite, leading to the synergized metal/acid functionalities. However, the Pt species in HZSM-5 are likely to be located at the Brønsted acid sites, forming [Pt_m-H_n]ⁿ⁺ adduct with positive Pt charges. However, increased electron deficiency is not desired for DHA because it stabilizes the platinum-ethylene complex, leading to the formation of heavier oligomers and accelerating catalyst deactivation. According to our recent studies, the lifespan (stability) of Pt/ZSM-5 catalyst during ethane DHA depends highly on the loading of Pt. While the lifespan of the 0.05wt% Pt/ZSM-5 catalyst is about 15 h, the 0.01wt% Pt/ZSM-5 catalyst was completely deactivated within only 4 h, which was further decreased to 1 h when decreasing the Pt loading to 0.005 wt%.

We show that the activity and stability of 0.005-0.01 wt% (50-100 ppm) Pt/HZSM-5 during ethane DHA can be significantly improved by tuning the coordination structure of the Pt species from [Pt_m-H_n]ⁿ⁺ adduct to [Pt₁-Zn_n]^δ+ hybrid cluster. The specific activity of BTX is up to 8.2 mol/g_Pt/min (or 1592 min^-1) over the 10 ppm Pt-Zn₂/HZSM-5 catalyst. The deactivation constant for the [Pt₁-Zn_n]^δ+ cluster is decreased by a factor of 120 in contrast to that of the [Pt_m-H_n]ⁿ⁺ counterpart. With the concept of tuning the Pt metal functionality by coordinating isolated Pt to metal/metal oxo clusters, we show that it remains high potential to maximize the metal functionality of Pt (or other PGMs) – lowering Pt loading to ppm level – in the acidic zeolites for efficient DHA of the stable light alkanes.