Tailoring molecular sieve properties during SDA removal via solvent extraction

Abstract

Pure-silica molecular sieves with the MFI topology and zincosilicates, aluminosilicates, borosilicates and pure silicates with the *BEA topology are synthesized and subjected to solvent extraction treatments in an effort to remove the organic structure-directing agents (SDAs) from the micropores. For both molecular sieve topologies, the amount of SDA that can be removed by extraction is found to be dependent on the size of the SDA and the strength of interaction of the SDA with the molecular sieve framework. Furthermore, the potential for extraction of SDAs from the micropores of the material is shown to correlate well with the temperature at which the SDA combusts in thermogravimetric analyses experiments. For materials with SDAs that are small relative to the size of the micropores, the fraction of SDA that can be removed is found to correlate well with the fraction of the SDA that decomposes below 400°C in the materials studied here. SDA that burns or decomposes at temperatures exceeding this value is strongly bound to the framework via ionic charge-balancing interactions. The ease of liberation of charge-balancing tetraethylammonium (TEA) cations from the various metallosilicates is shown to be Zn>B>Al, following the reverse trend of known Bronsted acidity of the various types of sites. It is shown that this tightly bound SDA is removed by extraction under conditions that simultaneously hydrolyze part of the framework. For example, TEA cations charge-balancing boron atoms in the silicate framework are removed with concomitant hydrolysis of the B–O–Si bonds, releasing the tightly bound TEA cation with subsequent desorption of the boron and TEA from the molecular sieve pores. A borosilicate with the *BEA topology synthesized with TEA fluoride as an SDA is shown to be a precursor to a variety of molecular sieves as was previously demonstrated for the zincosilicate with the *BEA topology, CIT-6.