Zeolite Synthesis in FluorideMedia: Structure Directiontoward ITW by Small Methylimidazolium Cations

다국어 초록 (Multilingual Abstract)

<P>Pure silica ITW zeolite can be synthesized using 1,2,3-trimethylimidazoliumand 1,3-dimethylimidazolium cations and fluoride anions as structure-directingagents (SDAs). Similarly to the previously reported 1,3,4-trimethylimidazolium,the dimethyl cation can also produce the zeolite TON, but this higherframework density phase finally transforms <I>in situ</I> into ITW. The structures of the as-made and calcined phases preparedwith the new cations show a unit cell doubling along <I>z</I>, and the refined structures are reported. Periodic Density FunctionalTheory calculations provide the energies of the six SDA-ITW and SDA-TONzeolites, and their relative stabilities fully agree with the experimentalobservations. Structure-direction in this system is discussed fromexperimental and theoretical results that give strong support to theidea that strained silica frameworks are made possible in fluoridemedia 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 observationswith the three SDAs agree with the so-called Villaescusa’srule, i.e., the low framework density phase is favored at higher concentrations,but at the same time question the supersaturation hypothesis thathas been proposed to explain this rule, since here the low-densityphase is the most stable one.</P><P><B>Graphic Abstract</B>
<IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-4/ja209832y/production/images/medium/ja-2011-09832y_0002.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja209832y'>ACS Electronic Supporting Info</A></P>