Zeolite Synthesis in Fluoride Media: Structure Direction toward ITW by Small Methylimidazolium Cations

Abstract

Pure silica ITW zeolite can be synthesized using 1,2,3-trimethylimidazolium and 1,3-dimethylimidazolium cations and fluoride anions as structure-directing agents (SDAs). Similarly to the previously reported 1,3,4-trimethylimidazolium, the dimethyl cation can also produce the zeolite TON, but this higher framework density phase finally transforms in situ into ITW. The structures of the as-made and calcined phases prepared with the new cations show a unit cell doubling along z, and the refined structures are reported. Periodic Density Functional Theory calculations provide the energies of the six SDA-ITW and SDA-TON zeolites, and their relative stabilities fully agree with the experimental observations. Structure-direction in this system is discussed from experimental and theoretical results that give strong support to the idea that strained silica frameworks are made possible in fluoride media by decreasing the covalent character of the Si-O bond. This decreased covalency is enhanced with the 1,2,3-trimethyl isomer, which is shown to be the strongest SDA for ITW and, at the same time, is the more hydrophilic of the three SDAs tested. Our observations with the three SDAs agree with the so-called Villaescusa's rule, i.e., the low framework density phase is favored at higher concentrations, but at the same time question the supersaturation hypothesis that has been proposed to explain this rule, since here the low-density phase is the most stable one.

Synthesis of Pure Silica MWW Zeolite in Fluoride Medium by Using an Imidazolium‐Based Long Dication

“…The morphologies of the obtained samples were investigated by field‐emission scanning electron microscopy (FE‐SEM), using a Philips XL30 S‐FEG. Multinuclear magic‐angle spinning (MAS) NMR spectra of as‐synthesized samples were obtained at room temperature on a Bruker AV‐400‐WB equipment and the details can be found in our previously reported work . Fourier Transform infrared spectra were obtained with a Bruker IFS 66/S spectrophotometer.…”

“…Multinuclear magic-angle spinning (MAS) NMR spectra of as-synthesized samples were obtained at room temperature on aB ruker AV-400-WB equipment and the details can be found in our previously reported work. [31] Fourier Transform infrared spectra were obtained with aB ruker IFS 66/S spectrophotometer.N itrogen adsorptiondesorption measurements were carried out in aM icromeritics ASAP 2010 equipment at the N 2 liquefaction temperature of 77 K. The calcined samples were outgassed under vacuum at 120 8C.…”

On the Porous Silicate HPM‐5

“…A synthesis mixture with the composition 0.3ROH/0.05KOH/0.01Al 2 O 3 /1.0SiO 2 /15.0H 2 O [R = 1,2,3‐trimethylimidazolium (123TMI + )] was prepared by combining aluminum hydroxide [Al(OH) 3 · H 2 O, 85 %, Aldrich], tetraethyl orthosilicate (TEOS, 98 %, Aldrich), potassium hydroxide (KOH, 85 %, Aldrich), ROH, and deionized water. The iodide salt of 123TMI + was prepared according to the procedures described elsewhere and then transformed into an aqueous solution of its hydroxide form . In a typical synthesis, aluminium hydroxide was mixed with an appropriate amount of water in a solution of ROH and stirred at room temperature for 1 h. After adding KOH, the resulting solution was stirred for an additional hour.…”

“…On the other hand, if a combination of structure‐directing effects proves to be effective in crystallizing a given zeolite structure, one possible strategy in the search for further new zeolites could be to hamper one particular structure‐directing effect, while allowing the rest to act. For instance, the synthesis of the D4R‐containing zeolite ITW apparently needs a combination of fluoride (for the D4R unit) and an organic cation based on the charged imidazolium ring with not exceedingly large substituents (1,2,3‐ or 1,3,4‐trimethylimidazolium, 1,3‐dimethylimidazolium, 1‐ethyl‐2,3‐dimethylimidazolium, and even 2‐ethyl‐1,3,4‐trimetylimidazolium) . The crystallization of ITW, which works best for a composition of pure silica, could be frustrated by working in the absence of fluoride (to hopefully avoid D4R and hence ITW) using hydroxide instead.…”

Unseeded hydroxide-mediated synthesis and CO2 adsorption properties of an aluminosilicate zeolite with the RTH topology

“…14 Since the first work by Zones in early 1980s, 15,16 imidazolium derivatives have often been used as OSDAs for the synthesis of novel zeolite structures, due to the well-defined shape and rigidity of the ring, accessibility in price, and diversity, which can be secured by easily attaching various groups into both N atoms of the imidazole ring. 23 Since the ITW structure contains double 4-ring (d4r) units 24 that for a pure-silica composition are only accessible through the use of fluoride anions, 25,26 we were interested in the structure-directing ability of 123TMI + in the absence of fluoride and in the presence of Al for charge balance in the crystallized product. 22 On the other hand, 1,2,3-trimethylimidazolium (123TMI + ), a relatively simple but rigid organic cation, has been known as a selective OSDA leading to the crystallization of ITQ-12 (ITW) in fluoride media across wide water contents (H 2 O/SiO 2 = 6.5-24.5).…”