Elsevier

Inorganica Chimica Acta

Volume 359, Issue 8, 15 May 2006, Pages 2513-2518
Inorganica Chimica Acta

Polymeric δ-MgCl2 nanoribbons

https://doi.org/10.1016/j.ica.2006.01.044Get rights and content

Abstract

δ-MgCl2 has relevant applications in the field of electrochemical energy storage and Ziegler–Natta catalysis. Here, we clarify the short-range structural peculiarities that make the disordered phase δ-MgCl2 extremely chemically active relative to the higher lattice energy phases, α-MgCl2 and β-MgCl2. X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and nuclear magnetic resonance (NMR) results are included. These findings, demonstrate the existence of [MgCl2]n nanoribbons and active nanosurfaces in δ-MgCl2 and provide new insight about the nature of the bonding in the allotropic forms of MgCl2.

Graphical abstract

δ-MgCl2 has relevant applications in the field of electrochemical energy storage and Ziegler–Natta catalysis. Here, we clarify the short-range structural peculiarities that make the disordered phase δ-MgCl2 extremely chemically active relative to the higher lattice energy phases, α-MgCl2 and β-MgCl2.

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Introduction

Compared to the large degree of attention given to lithium-ion systems for electrochemical energy storage applications, relatively little interest has been devoted to magnesium ion-conducting polymer electrolytes [1]. In spite of the potential advantages envisaged as performance, cost and safety, the preparation of magnesium polymer electrolytes has been considered very difficult due to the high lattice energy of magnesium salts [2], [3]. However, electrolytic complexes based on polyethylene glycol and δ-MgCl2 have been proposed [4] not only for usage in magnesium batteries but also for potentiometric pH sensors [5]. The δ-MgCl2 phase is well known as a unique support for the active titanium halide center in the Ziegler–Natta catalysis [6] and is characterized by high crystallographic disorder [4]. The aim of the current study is the understanding of the structure–property relationship of δ-MgCl2 at the nano-scale to target energy storage and catalytic applications of this material.

Previous X-ray and FT-IR studies of crystalline MgCl2Bx adducts (B = Lewis Base like ethyl formate, ethyl acetate, ethyl p-methoxybenzoate, ethanol, etc.) have provided evidence for MgCl2 polymeric chains [7], [8], [9], [10], [11]. These studies suggest that δ-MgCl2, which can be obtained by the complete elimination of B from the adducts (x  0), is the disordered array of a large number of covalent [MgCl2]n polymeric chains [8], [9], [10], [11], [12]. The hypothesis of existence of [MgCl2]n polymeric chains certainly claims further experimental evidence. In spite of that, such indications of covalency in δ-MgCl2 are in line with the fact that the crystal (α-form or β-form) requires an analysis beyond the Born–Mayer rigid ionic picture [13]. The first-order version of this model does not account for the relatively large number of reported ‘covalent’ effects in this and other supposedly ionic systems [14]. In order to explain these and similar observations an extended ionic model was proposed which takes into account polarization effects [14], [15]. In contrast with this strictly ionic interpretation, calculations of the electronic and structural properties of β-MgCl2 reveal partial covalency with a surprising contribution from the d-orbitals [16]. Clearly, the dilemma ‘ionic versus covalent’ in compounds like MgCl2 has not been fully resolved.

A summary of the structural characteristics of α- and β-MgCl2 phases is relevant for the following discussion. The common α-form [17] of MgCl2 exhibits the prototype structure of CdCl2 [18]. This phase is characterized by a distorted cubic close packing of Cl atoms (…ABCABC…), as shown in Fig. 1a. Two outerlying Cl layers sandwich a single interstitial plane of Mg atoms (Cl–Mg–Cl triple-layer). The crystallographic parameters are: R3¯m, a = b = 3.596 Å, c = 17.590 Å; Mg 0, 0, 0; Cl 0, 0, 0.25784(8). The β-form [19] consists of a layered structure with hexagonal close packing of Cl atoms (…ABABAB…), according to the structural prototype CdI2 [18]. The crystallographic parameters are: P3¯m1, a = b = 3.641(3) Å, c = 5.927(6) Å; Mg 0, 0, 0; 2Cl ± (1/3, 2/3 0.23). The triple-layer structures are identical in both α- and β-phases.

In this paper, a detailed spectroscopic investigation is carried out on δ-MgCl2 in comparison with α-MgCl2 by means of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and nuclear magnetic resonance (NMR). New compelling evidence has been found in favor of the existence of [MgCl2]n nanoribbons and active nanosurfaces in δ-MgCl2 that sheds light on the ‘covalent’ aspects of MgCl2 chemistry.

Section snippets

Reagents

The δ-MgCl2 phase in this study was synthesized from metallic magnesium (99+%) and 1-chlorobutane (Aldrich) [4], [12], [20]. α-MgCl2 (99.99%, Aldrich) is used for comparison. All transfer and handling operations were performed in an inert atmosphere (Ar or N2). Cl K-edge XAS measurements were performed δ- and α-MgCl2. Samples were measured in fluorescence at the X15B beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL).

Instrumentation

35,37Cl NMR measurements were

Results and discussion

The direct synthesis of δ-MgCl2 from magnesium metal and 1-chlorobutane adopted in this investigation yields a higher disordered phase than that obtained via thermal treatment under reduced pressure of the adducts MgCl2Bx, [7], [8], [9], [10], [11] or alternatively, by milling α-MgCl2 (or β-MgCl2) [19]. The structure of δ-MgCl2 in comparison with α-MgCl2 can be elucidated by the analysis of previous XRD data [4], [7], [8], [9], [10], [11], [19], [20]. Fig. 2a shows the XRD pattern of α-MgCl2.

Concluding remarks

Taken together, these results indicate that the disordered phase δ, when compared to the crystalline phase α, is characterized by a more evident covalent character, demonstrating the existence of [MgCl2]n polymer chains. The unique chemical reactivity of δ-MgCl2 and its role in the Zigler–Natta catalysis and the preparation of magnesium ion conducting polymer electrolytes for magnesium batteries can find a natural framework of interpretation in terms of these MgCl2 nanoribbons and the active

Acknowledgements

This work is dedicated to Prof. S. Bresadola and Prof. R. Zannetti of the University of Padova, Italy, now retired, after a life spent in the development and transmission of scientific knowledge. The authors wish to thank G.G. Amatucci of Rutgers University, New Jersey, for providing supporting data, J. Dannenberg and H. Matsui of Hunter College of CUNY, New York, for useful discussion and D.L. Akins of City College of CUNY, New York, for instrumental assistance.

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