Synthesis of an extra-large molecular sieve using proton sponges as organic structure-directing agents

Abstract

The synthesis of crystalline microporous materials containing large pores is in high demand by industry, especially for the use of these materials as catalysts in chemical processes involving bulky molecules. An extra-large-pore silicoaluminophosphate with 16-ring openings, ITQ-51, has been synthesized by the use of bulky aromatic proton sponges as organic structure-directing agents. Proton sponges show exceptional properties for directing extra-large zeolites because of their unusually high basicity combined with their large size and rigidity. This extra-large-pore material is stable after calcination, being one of the very few examples of hydrothermally stable molecular sieves containing extra-large pores. The structure of ITQ-51 was solved from submicrometer-sized crystals using the rotation electron diffraction method. Finally, several hypothetical zeolites related to ITQ-51 have been proposed.aluminophosphate | extra-large microporous materials | zeolite synthesis

Synthesis of highly stable metal-containing extra-large-pore molecular sieves

“…[6] This material showed high hydrothermal stability after calcination procedures but, low Brønsted acidity due to the preferential distribution of the silicon species as "silicon islands" instead of isolated Si tetrahedron. [6] The synthesis of SAPO materials with isolated Si species within the framework is a very challenging issue, since the SiO 4 tetrahedral units tend to coordinate with other SiO 4 tetrahedral units forming these undesired Si-rich areas.…”

“…[6] This material showed high hydrothermal stability after calcination procedures but, low Brønsted acidity due to the preferential distribution of the silicon species as "silicon islands" instead of isolated Si tetrahedron. [6] The synthesis of SAPO materials with isolated Si species within the framework is a very challenging issue, since the SiO 4 tetrahedral units tend to coordinate with other SiO 4 tetrahedral units forming these undesired Si-rich areas. [11] Thus, specific host-guest organic-inorganic interactions between the OSDA and the crystalline structure are required not only to direct the formation of the desired crystalline structure with a specific framework but, also, to preferentially place the silicon atoms in framework isolated units.…”

“…[1b] Having that in mind, we have recently proposed the use of a bulky aromatic proton sponge molecule as OSDA [1,8-bis(dimethylamino)naphthalene, DMAN, see Figure 1], [6] since this organic molecule presents the proper physico-chemical properties, [7] including the suitable size and rigidity, to direct the crystallization of extra-large pore zeolites. In this sense, the DMAN molecule has allowed the synthesis of a new silicoaluminophosphate (SAPO), named ITQ-51, which presents extra-large 16-ring pores (~9 Å) in its crystalline structure, and even more important, shows very high hydrothermal stability after calcination treatments.…”

“…In this sense, the DMAN molecule has allowed the synthesis of a new silicoaluminophosphate (SAPO), named ITQ-51, which presents extra-large 16-ring pores (~9 Å) in its crystalline structure, and even more important, shows very high hydrothermal stability after calcination treatments. [6] Up to that moment, only one zeolite, the germanosilicate ITQ-40, was described in the literature containing 16-ring pores in its framework, [8] but its structure collapsed after calcination and exposure to ambient conditions. This lack of crystallized zeolites containing 16-ring pores reveals the difficulties of finding appropriate OSDAs able to stabilize this type of channels.…”

“…[10] *Author for correspondence (acorma@itq.upv.es, mmoliner@itq.upv.es ). Unfortunately, the silicoaluminophosphate form of the ITQ-51 shows low Brønsted acidity because the isomorphic substitution of Si atoms within the SAPO material resulted in the formation of large "silicon islands" in the zeolitic frameworks, [6] and it has been broadly described that these silicon-rich areas show very low Brønsted acidity. [11] However, other elements, such as transition metals, could also be isomorphically substituted within the neutral AlPO frameworks in addition to silicon atoms, resulting in the formation of MeAlPO materials.…”