Non-oxidative reactions of propane on Zn/Na-ZSM5

Abstract

Propene formation rates during propane conversion at 773 K on Zn/Na-ZSM5 are about ten times higher than on Zn/H-ZSM5 catalysts with similar Zn content. The total rate of propane conversion is also higher on Zn/Na-ZSM5 by a factor of four. Propane reactions lead to high propene selectivities (>50%) as protons are replaced by Na cations in Zn/H-ZSM5 catalysts. The titration of acid sites with Na⁺ cations decreases the rate of acid-catalyzed chain growth reactions and the selectivity to C₆–C₉ aromatics. X-ray absorption studies at the Zn-K edge showed that aqueous ion exchange of Na-ZSM5 with Zn cations leads to isolated (ZnOH)⁺ species located at cation exchange sites. Unlike Zn species in Zn/H-ZSM5 (<1.0 wt.%), high temperature condensation reactions of (ZnOH)⁺ species with neighboring zeolite OH groups are less likely to occur in Zn/Na-ZSM5 and most Zn species remain as (ZnOH)⁺. Temperature programmed reduction studies show that Zn species in Zn/Na-ZSM5 reduce at lower temperatures than the (O^-–Zn²⁺–O^-) species present in Zn/H-ZSM5. D₂ exchange with surface OH groups showed that some protons are formed during ion exchange. Higher deuterium contents in products of C₃H₈–D₂ mixtures on Zn/Na-ZSM5 suggest that (ZnOH)⁺ species in Zn/Na-ZSM5 catalyze rate-determining hydrogen desorption steps during propane conversion more effectively than (O^-–Zn²⁺–O^-) sites present in Zn/H-ZSM5. The presence of (ZnOH)⁺ species and a lower acid site density in Zn/Na-ZSM5 leads to much higher propane conversion rates than on Zn/H-ZSM5. As the acid site density decreases, propene aromatization rates decrease, which leads to less hydrogen to be disposed by a more efficient hydrogen recombinative desorption species (ZnOH)⁺.

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