All-Silica SSZ-74 Synthesized in Fluoride or Fluoride-Free Media: Investigation on Organic Structure-Directing Agent’s Locations Inside Pores

Abstract

Pure silica SSZ-74 zeolite (-SVR type) was synthesized in fluoride or fluoride-free media using the hydroxide salts of the symmetric 1,6-bis(N-methylpyrrolidinium)hexane and asymmetric 1-methyl-1-[6-(trimethyl-azaniumyl)-hexyl]pyrrolidinium diquats as organic structure-directing agents (OSDA). A structural study using the Rietveld method revealed unexpected orientation of the OSDAs inside the micropores of the zeolite. Unlike the original structural elucidation, which proposed the OSDA's location inside the channel parallel to c-axis in a distorted conformation, in both samples, the OSDAs are here located elsewhere, inside the undulated connecting-channels along the [110] direction and in a more favorable linear conformation. This surprising result was confirmed on an energy point of view by computational method together with solid-state NMR experiments. Moreover, the impact of a thermal treatment on this particular structure 1 containing ordered vacancies was studied by high-temperature powder X-ray diffraction under different atmospheres.

Zeolite Phase Selectivity Using the Same Organic Structure-Directing Agent in Fluoride and Hydroxide Media

“…Both results are in agreement with experiments (Table 2). The fluoride energetic terms, ( 4) and (7) in Table 1, have a larger weight in MEL than in MTW, and this justifies why MEL is the product in fluoride media. MEL contains 4/96 OSDA−F molecules/SiO 2 , a larger proportion than in MTW (2/56), hence contributing to the larger stability of the ( 4) and ( 7) contributions.…”

“…Also, in the synthesis procedure, tetramethylammonium cations were also needed as the cotemplate to obtain UFI, 46 and we attribute this to an additional stabilization of TMA cations inside [4 5 5 4 6 4 8] cavities of UFI. When fluoride is present, the Coulombic terms E(SDAF) and E(z-SDAF), columns ( 4) and (7) in Table S4, give a larger stability to LTA, since lta and D4Rs are more efficiently packed and are closer to each other, without [4 5 5 4 6 4 8] cavities present in UFI.…”

“…The fluoride route remains academically important, since it contributes to the synthesis of new structures and to understanding better the outcomes and intricacies of the zeolite synthesis, but the inherent danger and environmental disadvantage of using HF makes this route commercially inconvenient. In just a few cases, structures first synthesized in fluoride were later synthesized in hydroxide media, such as BEC (silicogermanate), ITQ-21 (silicogermanate), STF (pure silica), IWV (aluminosilicate), SVR (aluminosilicate and pure silica), and STW (pure silica), this being a two-step process that may allow introduction of new zeolites in industrial applications, through the more environmentally safe synthesis in hydroxide media.…”

The Directing Role of Aluminum in the Synthesis of PST-21 (PWO), PST-22 (PWW), and ERS-7 (ESV) Zeolites

“…While the topic of Si, Al ordering in a zeolite is a topic widely studied, as well as the role of OSDAs in the synthesis of zeolites, ,,− the reasons why (or if) the aluminum contributes to stabilize such a zeolite phase is a less explored aspect, and this is the aim of the present study. Following the idea of our previous work regarding the roles of fluoride and aluminum as SDAs in the synthesis of zeolites, we now try to compare the role of aluminum and fluoride on the synthesis of recently synthesized aluminosilicate zeolites PST-21 (PWO), PST-22 (PWW), and ERS-7 (ESV) by selecting certain specific cases in which different zeolites are obtained using the same OSDA when the synthesis gel has a silicate or aluminosilicate chemical composition.…”