Aluminophosphates and Silicoaluminophosphates Zeolites tailored by different Imidazolium Cations and their evaluation as catalysts for CO2-Epoxide Cycloaddition

Abstract

In this work, we study the structure-directing effect of three different cations based on 1,2-dimethylimidazolium moieties linked by a linear bridge of n = 3, 4 or 5 methylene groups (3BDMI, 4BDMI, and 5BDMI, respectively), for the synthesis of silicoaluminophosphate and aluminophosphate microporous molecular sieves. These cations allowed the crystallization of three different structures (AFI, SFO and SAS), presenting different specificity for these phases. They all yielded zeolite AFI, which is considered a default structure; nevertheless, 3BDMI and 4BDMI cations (in AlPO and SAPO compositions) tend to favor the crystallization of PST-27, a monoclinic version of the AFI structure. The crystallization of the SAS and SFO phases depended on the mineralizing agents (fluoride or hydroxide), and was only afforded in SAPO composition; when the synthesis was carried out in hydroxide medium, the three OSDA induced the crystallization of the SAS framework. In contrast, the SFO zeolite was yielded under fluoride medium, and only with 3BDMI and 4BDMI cations. In an attempt to understand the structure-directing behavior of these cations, molecular simulations were carried out to study the locations and conformations of the dications in each of these frameworks. In the case of SAS, cavities were filled with two imidazolium rings of consecutive cations, which site stacked to each other stabilized through π···π interactions between imidazolium rings. In the SFO framework, the organic dications arranged in a zig-zag like conformation, siting their imidazolium rings in consecutive lateral side-pockets. On the other hand, the materials obtained were applied as catalysts for the cycloaddition of CO₂ in epoxide reactions, exhibiting interesting catalytic activities.