Article pubs.acs.org/JPCC
Pressure-Induced Insertion of Ammonia Borane in the Siliceous Zeolite, Silicalite-1F Jason Richard,†,‡ Sonia León Cid,†,‡ Jérôme Rouquette,† Arie van der Lee,‡ Samuel Bernard,‡ and Julien Haines*,† †
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, Université de Montpellier, 34095 Montpellier Cedex 5, France IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM), Université de Montpellier, Place E. Bataillon, F-34095, Montpellier, France
‡
S Supporting Information *
ABSTRACT: A combination of Raman spectroscopy and X-ray diffraction was used to investigate the insertion of ammonia borane in the 5.5 Å diameter pores of the hydrophobic, all-silica zeolite, silicalite-1F in the pressure range up to 4.8 GPa. Insertion and nanoconfinement result in the appearance of new Raman modes, especially in the N−H stretching region and significant changes in the intensities and pressure dependencies of a large number of other modes. Orientational disorder of the −BH3 and −NH3 groups persists to higher pressures in nanoconfined as compared to the bulk ammonia borane. The structure of the recovered sample was determined by single crystal X-ray diffraction. Each pore in the unit cell was found to contain between 2 and 3 molecules of ammonia borane forming single-molecule chains due to the spatial constraints. In situ, high pressure, X-ray powder diffraction indicated that the compressibility of the ammonia borane-silicalite-1F composite is three times lower than that of empty silicalite-1F due to pore filling. These results show that silicalite-1F can be a suitable nanoscaffold for this important chemical hydrogen-storage material.
1. INTRODUCTION Ammonia borane is the III−V analog of ethane, with the carbon atoms being replaced by boron and nitrogen (Figure 1). This gives rise to a polar B−N bond and partially charged hydrogen atoms, which form intermolecular dihydogen bonds. As a consequence of these intermolecular interactions, ammonia borane is a soft solid under ambient conditions, whereas ethane is a gas. There has been great interest in ammonia borane as a potential material for chemical hydrogen storage for onboard applications as it contains 19.6% hydrogen by mass.1−3 This hydrogen is released thermally in three successive steps. Solventmediated insertion and nanoconfinement of ammonia borane in mesoporous silica, boron nitride nanopolyhedra and metal− organic frameworks, which act as nanoscaffolds, has been found to modify the hydrogen release behavior in terms of onset temperature and the nature of the eventual byproducts.4−13 In contrast to solvent-mediated insertion, the use of physical pressure has the potential to produce a higher degree of pore-filling, such as that is observed in the case of H2O in the superhydration of zeolites14,15 and porous aluminophosphates,16,17 while avoiding impurities coming from the solvent. Pressure has been used to insert a large variety of atoms and simple molecules in zeolites such as Ar, Xe, H2O, CO, CO2, C2H4, C2H2, and C2H6O2 among others.14,15,18−28 This strongly modifies the mechanical behavior and phase stability of the host zeolite. © 2016 American Chemical Society
Figure 1. Molecular structure of ammonia borane and crystal structure of silicalite-1F (orthorhombic form).
Pressure can also tune framework−guest interactions and confinement effects. In the present work, pressure is used to insert ammonia borane in the all-silica zeolite silicalite-1F and this process is investigated by Raman spectroscopy and X-ray diffraction. Silicalite-1F with the MFI (Mobile-five) structure type has a three-dimensional pore system consisting of linear pores in the [010] direction and sinusoidal pores in the ac plane Received: February 29, 2016 Revised: April 11, 2016 Published: April 12, 2016 9334
DOI: 10.1021/acs.jpcc.6b02134 J. Phys. Chem. C 2016, 120, 9334−9340
Article
The Journal of Physical Chemistry C with diameters of 5.5 Å × 5.4 Å and 5.5 Å × 5.1 Å, respectively29 (Figure 1), which are larger than the van der Waal’s diameter of the ammonia borane molecule, thereby allowing it to enter the pores. Silicalite-1F was chosen for this study as it is highly hydrophobic, such that the pores are free of water molecules, which could react with ammonia borane. In addition, wellcharacterized, high-quality single crystals are readily available for this siliceous zeolite.
Boehler−Almax diamonds. The DAC was mounted on a goniometer head on the Oxford Diffraction Gemini 4-circle diffractometer with the Sapphire CCD detector placed at 65.00 mm from the sample. Mo Kα radiation was used and acquisition was performed over a 6° range in ϕ to improve powder averaging. Calibration using LaB6 and integration of the images was performed using FIT2D.35 The unit cell parameters were obtained using full-profile fitting with the program Fullprof.36
2. EXPERIMENTAL METHODS 2.1. Preparation of the Silicalite-1F−Ammonia Borane Mixtures. The ammonia borane used in this study was obtained from Aldrich (97% purity), and the calcined, hydrophobic silicalite-1F single crystals obtained by the fluoride method were obtained from the SOMEZ. A first sample was prepared by heating finely ground silicalite-1F single crystals and ammonia borane powder at 381 K for 8 min under an argon atmosphere in order to obtain a very intimate mixture. All manipulations were carried out under inert conditions. In particular, ammonia borane and the hydrophobic silicalite-1F single crystals were handled in an argon-filled glovebox (MBraun MB200B; O2 and H2O concentrations kept at
