Letter pubs.acs.org/JPCL
Probing Local Structure of Layered Double Hydroxides with 1H SolidState NMR Spectroscopy on Deuterated Samples Guiyun Yu,†,‡ Ming Shen,# Meng Wang,† Li Shen,† Wenhao Dong,† Sheng Tang,† Li Zhao,† Zhe Qi,† Nianhua Xue,† Xuefeng Guo,† Weiping Ding,† Bingwen Hu,*,# and Luming Peng*,† †
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡ School of Chemical and Biological Engineering, Yancheng Institute of Technology, Yancheng 224051, China # Physics Department & Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, China S Supporting Information *
ABSTRACT: By using a simple and efficient deuteration process, 2H has been successfully introduced into layered double hydroxides (LDHs). Due to significantly less 1H−1H homonuclear dipolar coupling, high-resolution 1H solid-state NMR spectra can now be obtained conveniently at medium to low spinning speed to extract the information of cation ordering in LDHs. Furthermore, we show that double-resonance experiments can be applied easily to investigate internuclear proximities and test possible cation-ordered superstructure models. This approach can be readily extended to LDHs with different compositions to explore the local structure and the key interactions between the cations in the layer and interlayer anions.
SECTION: Spectroscopy, Photochemistry, and Excited States
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upon thermal treatment of Al-containing LDHs, it cannot give intralayer cation distribution information.18,19 Recently, Sideris and co-workers showed that ultrafast magic angle spinning (MAS) 1H NMR spectroscopy could be applied to distinguish the signals arising from Mg3OH, Mg2AlOH, and interlayer water.14 The quantitative 1H NMR results indicated a nonrandom Mg/Al distribution and the avoidance of Al−O− Al linkages, comparable to the well-known “Lowenstein’s rule” in zeolites. This leads to a “honeycomb” ordering of the 33% Al-substituted LDH, where only Mg2AlOH species exist. On the basis of the results obtained from 1H double quantum (DQ) MAS NMR spectroscopy, however, Cadars et al. argued that even within a largely ordered structure, a small concentration (∼10%) of defects, for example, MgAl2OH, existed in Al-rich LDHs.16 This matter was further investigated by the 25Mg NMR spectroscopy at an ultrahigh magnetic field, and the results supported the conclusion that the Mg/Al distribution was nonrandom and the presence of MgAl2OH defects in LDHs prepared by Cadars was likely due to a different synthesis method.14,15 More recently 17O NMR experiments were also successfully applied to study the cation arrangements in LDHs and confirm the conclusions obtained
he general formula for layered double hydroxides (LDHs), or “hydrotalcite-type” materials, is 3+ n− M2+ 1−xMx (OH)2(Ax/n)·yH2O, where a fraction (x%, with x varied from 17 to 33) of divalent cations M2+, for example, Mg2+, in a brucite-like environment are replaced by trivalent cations M3+, for example, Al3+. The resulting mixed metal hydroxide layers possess positive charges; therefore, charge compensation and exchangeable anions (An−), along with water molecules, are present between the layers (Figure S1, Supporting Information).1−3 LDHs are very important inorganic supramolecular materials, in which the compositions can be controlled for a variety of applications including environmental applications,4,5 catalysis,6 photochemistry,7−9 electrochemistry,10,11 and biomedical sciences.12 To obtain LDH materials with desired properties requires detailed information on the local structure, such as the M3+/M2+ intralayer cation ordering and the supramolecular host−guest interactions between the hydroxide layer cations and interlayer anions.13 Clear and convincing evidence of cation arrangement, however, has just been found experimentally,14−17 with mostly solid-state NMR techniques. Solid-state NMR spectroscopy is a powerful tool that investigates the local structure of a wide range of functional materials at the atomic/molecular scale. Although 27Al NMR spectroscopy can readily distinguish six- and four-coordinated Al ions and has been used to track Al coordination changes © 2014 American Chemical Society
Received: November 20, 2013 Accepted: January 2, 2014 Published: January 2, 2014 363
dx.doi.org/10.1021/jz402510a | J. Phys. Chem. Lett. 2014, 5, 363−369
The Journal of Physical Chemistry Letters
Letter
by Sideris and co-workers.17 Both 25Mg and 17O are quadrupolar nuclei with low gyromagnetic ratios and low natural abundances; thus, the 25Mg or 17O NMR approach is less routine, and it is more convenient to probe the local structure with 1H NMR spectroscopy. Owing to large 1H−1H homonuclear dipolar couplings present in these systems,20 CRAMPS (combined rotation and multiple-pulse sequence) experiments or ultrafast spinning rates (i.e., >40 kHz) were required to ensure enough resolution of the major resonances in 1H NMR.14−16,21 However, CRAMPS is not quantitative by its nature, and cation-ordering information can hardly be obtained. It is also more challenging to reintroduce dipolar coupling at ultrafast spinning to perform double-resonance experiments (e.g., transfer of population in double resonance (TRAPDOR)),22,23 and thus, it is more difficult to investigate the fundamental and key interactions between cations and anions that define the supramolecular chemistry in LDHs, hindering the exploration of the structure and properties of LDHs. In addition to preparing solid samples for high-resolution neutron diffraction studies, deuteration has long been used to decrease the effective 1H−1H dipolar coupling by increasing the average proton internuclear distance and thus improve the resolution in 1H MAS NMR.24−26 Surprisingly, this approach has never been used to study LDHs to date. Herein, we show that deuteration of LDH samples can be achieved with a simple procedure, and the structure of LDHs can be explored at a much slower spinning rate with 1H MAS NMR spectroscopy to extract the cation-ordering information. Double-resonance experiments, in combination with numerical simulations, can be conveniently performed to explore the internuclear proximities, which is critical to understand the structure and interactions in LDHs. The NO3−-containing Mg/Al LDH samples were prepared via a coprecipitation method. Powder X-ray diffraction (XRD) was used to verify that the as-prepared samples were LDHs without any unwanted phases. The obtained materials with different Al molar percentages exhibit characteristic diffraction peaks of LDH (Figure S2, Supporting Information). The deuteration of the NO3−-containing LDH involves mixing the sample with D2O at 80 °C and drying the solid samples under vacuum at 60 °C. XRD and FT-IR spectroscopy were used to characterize the LDHs after deuteration. The XRD pattern of the Mg/Al LDH with a 25.1% Al molar percentage (denoted as MgAl-25.1) after deuteration (Figure S3b, Supporting Information) resembles the pattern before deuteration (Figure S3, Supporting Information), implying that the structure of the LDH is preserved in the process. Careful examination reveals that the peak positions after deuteration are associated with a larger d spacing (inset of Figure S3, Supporting Information). The lattice parameter c calculated based on the XRD data, which corresponds to three times the distance between the two consecutive layers, is 24.23 Å for MgAl-25.1 before deuteration, while this value is increased to 26.63 Å for the deuterated sample, indicating subtle structure changes after deuteration. The FT-IR spectra of MgAl-25.1 before deuteration (Figure S4a, Supporting Information) show a strong OH absorption band at around 3400−3600 cm−1. This band becomes much weaker, and a strong OD vibration peak at about 2500−2600 cm−1 can be observed in the sample after the deuteration process (Figure S4b, Supporting Information).27 The presence of the strong OD peak indicated that deuterium has been introduced into LDHs in the deuteration process, as D2O in the
interlayer region and/or Mg3OD and Mg2AlOD species in the hydroxide layer. If assuming the intensity of the band corresponding to NO3− does not change after deuteration, the deuterium molar percentage in the deuterated sample can be calculated as 80%. 27 Al MAS NMR spectroscopy was used to further investigate the structure of the as-prepared and deuterated LDHs. A sharp resonance at 9 ppm with a set of spinning sidebands over more than 5000 ppm is observed for MgAl-25.1 before deuteration (Figure S5a, Supporting Information), while no other signal appears, indicating that all of the Al ions are in the sixcoordinated environment in LDHs. The spectrum does not change much for the deuterated LDHs (Figure S5b, Supporting Information), suggesting that Al ions stay six-coordinated and the structure is maintained after deuteration. Careful examinations of the 27Al NMR spectra reveal that the intensities of the spinning sidebands are slightly stronger for the deuterated sample. Because H/D exchange does not involve any changes in the Al’s first coordination shell that defines the quadrupolar coupling, it can be tentatively ascribed to the incorporation of deuterium into the LDH structure, which reduces the 1H−27Al dipolar coupling. After exposing to 75.8 RH (relative humidity) for 200 h, the 27Al NMR spectrum (Figure S5c, Supporting Information) resembles the sample before deuteration, indicating that the 1H−27Al dipolar coupling has been reintroduced due to the replacement of deuterium by a proton. The improvement of the spectral resolution by deuteration can be clearly seen by comparing the 1H MAS NMR spectra of as-prepared and deuterated NO3−-containing Mg/Al LDHs at a moderate spinning speed of 20 kHz. At low Al molar percentages (i.e.,
