Date Published: December 01, 2016
Publisher: International Union of Crystallography
Author(s): Enrico Mugnaioli, Mauro Gemmi, Marco Merlini, Michele Gregorkiewitz.
Octahedral molecular sieves (OMS) attract increasing interest in the search for novel electrode materials for energy storage and water desalination. While a nanometric particle size is desirable for such applications, this makes ordinary single-crystal characterization difficult and many OMS structures are still waiting for elucidation. Here we present the long awaited structure of a well known material, (Na,□)5[MnO2]13, resolved by a combination of electron diffraction tomography, dynamical scattering theory and X-ray powder Rietveld refinement. A new type of tunnel structure was found, able to explain previously reported electrochemical properties. This structure also suggests a possible mechanism for topotactic transformations between different manganese oxide OMS frameworks.
(Na,□)5[MnO2]13 belongs to an emergent group of compounds which is now usually referred to as octahedral molecular sieves (OMS; Suib, 2008 ▸), in allusion to their open framework structures resembling zeolite molecular sieves, the well known tetrahedral counterpart. Zeolites are widely used in chemical processes (ion exchange, shape selective catalysis, semipermeable membranes etc.; Breck, 1974 ▸; Gorgojo et al., 2008 ▸) and their unique properties can be explained in terms of crystal structure: there are presently 231 different framework topologies (cf.http://www.iza-online.org) and efforts are ongoing to find new frameworks and applications (Cundy & Cox, 2003 ▸; Camblor & Hong, 2010 ▸; Bellussi et al., 2012 ▸).
(Na,□)5[MnO2]13 was prepared in a two-step procedure similar to that used by Lan et al. (2011 ▸) for the synthesis of manjiroite (Na-hollandite). A solution of 0.8 g NaOH in ∼ 20 ml deionized and freshly boiled water is added slowly, using a magnetic stirrer, to a solution of 1.97 g of MnCl2·4H2O in ∼ 30 ml of deionized water. The brown precipitate is filtered and washed with deionized water until the effluent reaches pH = 7 and subsequently dried at 363 K for 24 h. For the second step, a small quantity (0.1–0.2 g) of the dry powder is mixed with 4 g NaNO3 and heated in a porcelain crucible at 778 K for 24 h. The product of this reaction, mainly (Na,□)5[MnO2]13, is a dark brown powder (Fig. 1 ▸) which was isolated from NaNO3 through washing with water and filtration. Reagents were MnCl2·4H2O (Panreac, PRS), NaOH (Baker Analyzed) and NaNO3 (Merck Suprapur).
(Nax□1 − x)5[MnO2]13 was synthesized using a new and facile procedure which yielded nanorods with the Na load x = 0.80. The long-awaited crystal structure of this material has been resolved and shows a novel OMS framework containing three distinct types of tunnel, which differs radically from the previously assumed romanèchite framework containing only one type of tunnel. A particularly interesting detail of the new framework is the existence of MnO5 square pyramids which, on oxidation from Mn3+ to Mn4+, may act as centres for nucleophilic attack from a nearby under-shared oxygen. This mechanism is likely to play a fundamental role for both synthesis and electrochemical behaviour of manganese-based OMS structures.
References cited in the supporting information include: Armstrong et al. (1998 ▸), David (2001 ▸), Drits et al. (2007 ▸), Jeong & Manthiram (2001 ▸), Kim et al. (2012 ▸), Sauvage et al. (2007 ▸), Tian & Billinge (2011 ▸).