Synergistic mechanism between Brønsted acid site and active cerium species in hydride transfer reaction over CeY zeolites

Abstract

In this study, cyclohexene was used as a representative of olefin and catalyzed by CeY zeolites in a fixed-bed reactor under mild conditions, and the influence of Ce species in hydride transfer reaction over CeY zeolites was evaluated. CeY zeolites show more excellent hydride transfer properties than HY zeolite. Based on the results of almost identical Brønsted acid properties but not the product distributions for 0.075CeY and 0.075CeY(DC) samples, it should be suggested that the Brønsted acid strength and density are not the deciding factors to the hydride transfer reaction. A unique band at 1442 cm⁻¹ in situ FTIR spectroscopy spectra are assigned to pyridine complexes bonded to a class of active Ce species that could reversibly migrate from the core of SOD cages to its 6-rings mouth towards the supercages. These results provide valuable information that these active Ce species should play a synergistic role with the Brønsted acid sites in enhancing the hydride transfer reaction with a bimolecular mechanism over CeY zeolites.

Introduction

Hydride transfer reaction plays a very important role in influencing both product distribution and catalyst stability in the fluid catalytic cacking (FCC) process.^1,2 About 30 years ago, Scherzer,³ and Biswas and Maxwell⁴ have confirmed that more aromatics and paraffin generated in gas oil cracking by using the zeolite catalysts. Since then, it is generally accepted that the hydride transfer reaction can reduce the alkene content affecting the product distribution.5, 6, 7 The FCC gasoline is by far the main blending component in the gasoline pool, up to over 70 vol%. To meet the China VI gasoline standards (GB 17930-2016),⁸ legislate strict concentration of olefins in gasoline, it is of great significance to regulate and utilize the hydride transfer reaction in FCC processes.

Generally, the hydride transfer reaction in the catalytic cracking process is a bimolecular reaction via the Rideal mechanism.^9,10 The protonation of one reactant molecule (usually an olefin molecule) on a Brønsted acid site is the first step of a hydride transfer reaction; then a hydrogen is transferred as a negative species from another reactant in the gas phase (this step is the rate-controlling step) to form a paraffin molecule and new carbenium ion, respectively.

Rare earth (RE) containing Y zeolites (REY) has always been the major active component for fluid catalytic cracking catalysts.11, 12, 13, 14, 15, 16 Sedran and coworkers evaluated the influence of different rare earth ions on hydride transfer over Y zeolite and their result showed that the relative importance of the hydride transfer reactions increased linearly as a function of both Brønsted acidity of the catalysts and ionic radius of the rare earth element considered.¹¹ Akzo Nobel reduced the olefin content of gasoline by 5 vol%–10 vol% by using the strategy of increasing the rare earth content of the FCC catalyst to enhance the hydride transfer activity.¹⁶ Cheng and Rajagopalan studied the transformation behaviors of cyclohexene on REY zeolites. They proposed that the introduction of rare earth elements improved the acid density and thermal stability of the zeolites, which facilitate the hydride transfer reaction.¹⁷ It could be assumed, therefore, that a certain degree of catalytic control could be exerted on the hydride transfer reactions through the regulation of structural properties of rare earths in REY zeolites.

Back to the basics, the migration, chemical transformation, and structure identification of rare earth ions in REY zeolites have been extensively investigated.^18,19 However, insufficient attention has been devoted to study on the nature of the influence of rare earth species on catalytic performance. Chemical speciation and location of RE species in REY zeolites, which depends on the degree of ion exchange and calcination conditions, have always been a noteworthy and controversial subject in the field of FCC catalysts and other applications of REY zeolites.19, 20, 21, 22, 23, 24, 25, 26, 27 Currently, the commonly recognized possible Ce species in calcined Ce zeolites mainly include: Ce³⁺, Ce(OH)²⁺, Ce(OH)₂⁺, Ce₂(OH)₄⁺, Ce₂O₂²⁺, Ce₄O₄⁴⁺, [Ce₂O₂]⁴⁺, CeO₂, etc. Among them, Ce(OH)²⁺ and Ce₂O₂²⁺ are the most likely species in CeY zeolites with relatively low ion exchange, which are appropriate for FCC catalysts or adsorbents.^23,24 Our research group has been paying attention to this issue for nearly a decade, focusing on the effect of Ce species structure on the selective adsorption desulfurization performance of CeY zeolites. An excellent adsorbent has been developed by the controlling preparation of the effective cerium (III) hydroxylated species as adsorption active sites in CeY zeolite.¹⁹

The present study aimed to elucidate the synergistic mechanism between the Brønsted acid site and the active Ce species in a hydride transfer reaction over CeY zeolites. Experiments were carried out in a fixed-bed reactor under very short contact time under mild conditions and using cyclohexene as representative olefin species in gasoline. The results obtained allow proposing that the regulation of the cerium species in CeY zeolites should be a promising strategy to control the performance of the hydride transfer reactions.