ARTICLE pubs.acs.org/IECR
Thermal Dehydration and Vibrational Spectra of Hydrated Sodium Metaborates Amy M. Beaird,† Ping Li,‡ Hilary S. Marsh,†,§ W. A. Al-Saidi,‡ J. Karl Johnson,‡ Michael A. Matthews,† and Christopher T. Williams*,† †
Department of Chemical Engineering, University of South Carolina, 301 Main Street, Swearingen Engineering Center, Columbia, South Carolina 29208, United States ‡ Department of Chemical and Petroleum Engineering, Room 1249 Benedum Hall, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ABSTRACT: Sodium metaborate hydrates are a class of compounds represented by the stoichiometry NaBO2 3 xH2O. Recently, sodium metaborate has received attention as the byproduct of sodium borohydride hydrolysis, a reaction that is under consideration for hydrogen storage applications. The aim of this work was to understand the disposition of water in the crystal structure of hydrated sodium metaborates and to characterize the thermal stability and dehydration of the various hydrated species to optimize hydrogen storage efficiency as well as recyclability of the borate. Observations from a suite of analytical techniques including thermal analyses (thermogravimetric analysis/differential scanning calorimetry), X-ray diffraction, and Raman spectroscopy were correlated to characterize the dehydration mechanism of commercially available sodium metaborates, with an emphasis on the dihydrate (x = 2). A transformation from tetrahedrally coordinated boron to trigonal boron occurs when NaB(OH)4 (x = 2) is heated between 25 and 400 °C. The first dehydration to Na3[B3O5(OH)2] (x = 1/3) releases 5 mol of water between 83 and 155 °C. The final mole of water is released between 249 and 280 °C, and Na3B3O6 (x = 0) is formed. Raman spectra are reported for x = 2 and 1/3 for the first time. First-principles density functional theory was used to compute Raman spectra of the x = 1/3 and 2 material in order to assign the modes. We found reasonably good agreement between the experimentally measured and calculated vibrational frequencies.
1. INTRODUCTION Sodium borates are commercially important as additives in photographic developers, detergents, glasses, and adhesives.1 The ability of boron to coordinate with oxygen both tetrahedrally (BO4) and trigonally (BO3) lends itself to unique chemistries for a variety of applications.2 Sodium metaborate hydrates are a class of compounds represented by the stoichiometry NaBO2 3 xH2O. Recently, sodium metaborate has received attention as the byproduct of sodium borohydride hydrolysis, a reaction that is under consideration for hydrogen storage applications NaBH4 þ ð2 þ xÞH2 O f NaBO2 3 xH2 O þ 4H2
ð1Þ
The “excess hydration factor” (x) of sodium metaborate plays a key role in the hydrogen storage density because it represents excess water that is unutilized in the hydrolysis reaction.3 Generally, this reaction is performed near room temperature in aqueous solution with a heterogeneous catalyst or acid accelerators, and most studies report an excess hydration factor of x = 2.3,4 Thus, the gravimetric energy efficiency [(kg of H2/(kg of reactants)) 100%] is reduced from an ideal (x = 0) value of 10.92 to 7.34 wt % H2.5 Furthermore, the regeneration of NaBH4 from the NaBO2 byproduct may be affected by the hydration state.68 Therefore, to optimize hydrogen storage efficiency as well as recyclability of the borate, an understanding of both the thermal stability and dehydration mechanisms of hydrated sodium metaborates is required. Previous studies9 have identified four dominant hydration states (x = 4, 2, 1/3, 0) that precipitate from saturated aqueous borate solutions at temperatures between 25 and 400 °C (Table 1). r 2011 American Chemical Society
Nies and Hulbert9 identified a 1/2 hydrate, but Corazza et al.10 proved by X-ray diffraction that the structure corresponds to a 1/3 hydrate. This misnomer has been propagated throughout the NaBH4 hydrolysis and other borate literature. Our results also confirm the 1 /3 stoichiometry as will be discussed in more detail. As demonstrated in our previous work,3 the species corresponding to x = 4 does not appear at conditions of applied utility; therefore, we consider x = 2, 1 / 3 , and 0 in this paper. The aim of this work was to understand the disposition of water in the crystal structure of hydrated sodium metaborates and to characterize the thermal stability and dehydration of the various hydrated species. Observations from a suite of analytical techniques including simultaneous thermogravimetric analysis/ differential scanning calorimetry (TGA/DSC), X-ray diffraction (XRD), and Raman spectroscopy were correlated to characterize the dehydration mechanism of commercially available sodium metaborate dihydrate. First-principles density functional theory (DFT) was employed to compute Raman spectra of the x = 1/3 and 2 species to assign the vibrational modes.
2. EXPERIMENTAL METHODS 2.1. Thermal Dehydration Study. Commercial samples of NaBO2 3 2H2O (Rio Tinto, Boron, CA) were subjected to Received: November 21, 2010 Accepted: April 27, 2011 Revised: April 24, 2011 Published: May 26, 2011 7746
dx.doi.org/10.1021/ie102345j | Ind. Eng. Chem. Res. 2011, 50, 7746–7752
Industrial & Engineering Chemistry Research Table 1. Nomenclature and Temperature Ranges for Which the Predominant Sodium Metaborate Hydration States Precipitate from Saturated Aqueous Solutions sodium metaborate
NaBO2 3 xH2O
precipitation
x
temperatures (°C)
tetrahydrate
4
dihydrate
2
54110
1
1
110350 350þ
/3 hydrate anhydrate
/3 0
