Template-regulated distribution of isolated SiO4 tetrahedra in SAPO zeolites
image: Top 10 stable Si-distribution models for SAPO-35 zeolites prepared (A) with methylpiperidine (NMP) and (B) with hexamethyleneimine (HMI). These models were obtained from high-throughput computations on 44,697 enumerated SAPO-35 models. Template molecules were omitted for clarity. view more
Credit: ©Science China Press
Zeolites are among the most important heterogeneous catalysts in chemical industry. The active sites in zeolites can be generated around specific TO4 tetrahedra in zeolite frameworks. For example, the AlO4 tetrahedra in aluminosilicate zeolites provide negative charges to the zeolite frameworks, which afford Brønsted acidity favoring important catalytic reactions. The distribution of AlO4 tetrahedra determines the locations of active sites and the chemical properties of aluminosilicate zeolites. Similarly, in silicoaluminophosphate (SAPO) zeolites, the SiO4 tetrahedra are responsible for the Brønsted acidity, as well as their chemical performance. Regulating the concentration and the distribution of active TO4 tetrahedra in zeolite frameworks plays an important role in the development of high-performance zeolite catalysts. For aluminosilicate zeolites, the distribution of AlO4 tetrahedra can be regulated by several approaches, such as the introduction of framework heteroatoms, the usage of alcoholic additives and the employment of different organic amines as the template molecules.
Compared to AlO4 tetrahedra in aluminosilicate zeolites, the distribution of SiO4 tetrahedra in SAPO zeolite framework is more difficult to control. Suppressing the formation of Si islands and regulating the distribution of isolated SiO4 tetrahedra in SAPO zeolite frameworks is formidably challenging. This is because a large amount of Si in the synthetic system tends to form Si islands in SAPO zeolites, while a small amount of Si may slow the growth of SAPO zeolites and lead to a low product yield or the formation of impurities. Meanwhile, the isolated SiO4 tetrahedra are usually distributed among all possible T-sites, and their exact locations cannot be unambiguously determined via conventional X-ray diffraction techniques. As a result, the effects of the distribution of isolated SiO4 tetrahedra on the chemical properties of SAPO zeolites have not yet been identified so far.
In response to this challenge, recently, the research team led by Professor Jihong Yu from Jilin University proposed the template-regulated distribution of isolated SiO4 tetrahedra in a SAPO zeolite unraveled by high-throughput computations combined with various state-of-the-art characterization techniques. Using a new template molecule, N-methylpiperidine (NMP), SAPO-35 zeolites have been synthesized with LEV-type framework topology. Compared to the SAPO-35 zeolites prepared with the conventional templates, e.g., hexamethyleneimine (HMI), those prepared with NMP showed excellent thermal stability when comparing samples of the same or similar chemical composition. With the aid of high-throughput computations based on 44,697 SAPO-35 structure models combined with solid-state 29Si MAS NMR and synchrotron X-ray diffraction, the mysterious distributions of isolated SiO4 tetrahedra were unraveled, which resulted from the different structure-directing effects of distinct template molecules and were responsible for the property improvement of SAPO zeolites. This work reveals that the distribution of isolated SiO4 tetrahedra in SAPO zeolites can be regulated by the host-guest interactions provided by template design, opening a new avenue to the rational tuning of the chemical properties of SAPO zeolites.
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See the article:
Unraveling templated-regulated distribution of isolated SiO4 tetrahedra in silicoaluminophosphate zeolites with high-throughput computations