Dehydration and Dehydroxylation of Layered Double Hydroxides

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Dehydration and Dehydroxylation of Layered Double Hydroxides: New Insights from Solid-State NMR and FT-IR Studies of Deuterated Samples Guiyun Yu, Yahui Zhou, Rong Yang, Meng Wang, Li Shen, Yuhong Li, Nianhua Xue, Xuefeng Guo, Weiping Ding, and Luming Peng J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 May 2015 Downloaded from http://pubs.acs.org on May 19, 2015

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The Journal of Physical Chemistry

Dehydration and Dehydroxylation of Layered Double Hydroxides: New Insights from SolidState NMR and FT-IR Studies of Deuterated Samples Guiyun Yu,†,‡ Yahui Zhou,† Rong Yang, † Meng Wang,† Li Shen,† Yuhong Li,† Nianhua Xue,† Xuefeng Guo,† Weiping Ding,† and Luming Peng†,*

†

Key Laboratory of Mesoscopic Chemistry of MOE and Collaborative Innovation

Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡

School of Chemical and Biological Engineering, Yancheng Institute of Technology,

Yancheng 224051, China

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ABSTRACT. The dehydration and dehydroxylation processes (25 ~ 375 °C)

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of a

variety of Mg and Al containing layered double hydroxides (LDHs), (Mg1xAlx(OH)2(A

n-

)x/n·yH2O, where x = 0.197 ~ 0.273 and An- = NO3-, ClO4-, Cl- or CO32-)

were carefully investigated by using high resolution solid-state 1H and 27Al solid-state NMR, as well as FT-IR spectroscopy on deuterated samples. Dehydration is found to occur at a lower temperature (usually below 150 °C) than dehydroxylation (usually above 150 °C), the latter consisting of two sub-processes. Definitive evidences show that the dehydroxylation of Mg2 AlOH species starts at a lower temperature than Mg3OH, however, the temperature ranges for the two processes overlap significantly. Furthermore, dehydration and dehydroxylation are affected by both the nature of the charge compensating anions in the interlayer and the ratio of the intralayer cations (Mg/Al). The fact that FT-IR spectra with higher resolution can be obtained on deuterated samples implies this approach can be readily used for in-situ study of LDHs.


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INTRODUCTION The general formula of layered double hydroxides (LDHs) can be written as [M2+13+ nxM x(OH)2(A )x/n·yH2O],

where a fraction (x%, 17 ≤ x ≤ 33) of divalent cations

M2+(e.g., Mg2+), in a brucite-like environment are substituted by trivalent cations M3+(e.g., Al3+). LDHs are a class of very important inorganic supramolecular materials, in which the compositions can be tuned for a variety of practical applications.1-4

In

particular, LDH and/or mixed metal oxide derived from calcination of LDH, can be used as catalysts, adsorbents, as well as fire resistant materials.5-7 Since these applications often require elevated temperature, it is crucial to understand the detailed structural information in the thermal evolution of LDHs. Thermogravimetric methods have been the key technique to extract this information.8-14 According to these data, the thermal evolution of LDH is believed to follow three stages in general: loss of water in the interlayer region (dehydration), loss of OH- groups bound to the intralayer cations (dehydroxylation) and loss of interlayer anions. However, these methods can hardly provide direct structure information at atomic scale and there is still debate on the details of thermal evolution of the LDHs, especially in the dehydroxylation processes. For example, on the basis of the in-situ TG/DTA data of model compounds Al(OH)3 and Mg(OH)2, Yang and coworkers concluded that OH- groups bonded with Al3+ disappear first at a lower temperature range of 190 ~ 280 °C, while OH- species connected to Mg2+ start to disappear at a higher temperature range of 280 ~ 405 °C in a thermal treatment of Mg and Al containing LDHs.10 In contrast, Zhang and coworkers assigned the lower temperature 3 ACS Paragon Plus Environment


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event at 300 ~ 370 °C to dehydroxylation of Mg3OH (OH- groups connected to three Mg2+ ions) and the high temperature one at ~ 410 °C to that of Mg2AlOH (OH- groups connected to two Mg2+ ions and one Al3+ ion) based on their TG-DTA-MS data.9 Solid-state NMR spectroscopy represents a powerful tool that can be used to study the local structure of various functional materials. Recently, significant progress on the structure information of LDHs has been made by applying this technique.15-21 Sideris and coworkers demonstrated that signals due to interlayer water, Mg3OH and Mg2AlOH species can be distinguished in 1H solid-state NMR spectra acquired at an ultrahigh magic angle spinning (MAS) rate (> 40 kHz).20 25Mg, 17O and 71Ga solid-state NMR spectroscopy can also be applied to extract valuable information on the local environments of cations in a variety of LDHs.15, 18, 21 More recently, we have shown that high resolution 1H NMR spectra of LDHs can be obtained at a much slower MAS rate (~ 5 kHz) after the samples being simply deuterated, which allows convenient investigations of internuclear distances and detailed cation ordering information with dipolar recoupling NMR schemes.16 FT-IR spectroscopy is another important characterization method that can provide rich information on the functional groups in LDHs.10, 22-25 Different OH- groups (either in the hydroxide sheets or in the interlayer water) and interlayer anions can be distinguished in FI-IR spectra and this method can be conveniently applied in the in-situ conditions. However, previous studies showed that different OH- vibrations due to interlayer water, Mg2AlOH and Mg3OH could not be resolved, limiting its applications in the structural studies of LDHs.


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Here we use high resolution 1H and 27Al MAS NMR, as well as FT-IR spectroscopy to study the dehydration and dehydroxylation pathways of six deuterated LDHs with different interlayer anions and different Mg/Al ratios. Our data provide definitive evidences on the order of dehydroxylation processes and the factors related to the thermal stabilities of OH- groups, which are critical in controlling the desired properties of LDH materials. Furthermore, we show higher resolution FT-IR spectra can be obtained on deuterated LDHs and different hydroxyl species arising from interlayer water, Mg2AlOH and Mg3OH can now be distinguished, thus in-situ FT-IR spectroscopy can now be readily applied to study the important cation ordering information in real time.

EXPERIMENTAL SECTION Material Preparation. Six layered double hydroxides with various Al concentrations and anions were synthesized by using a co-precipitation method.26-28 a. MgAl-LDH-NO3. In a typical synthesis, stoichiometric amounts of Mg(NO3)2·6H2O and Al(NO3)3·9H2O (Al molar percentages, or Al3+%, n(Al)/(n(Mg)+n(Al)) = 27.5, 25.0 and 20.0 %) were dissolved in distilled water ([Mg2+] + [Al3+] = 1 M) to achieve the desired Mg/Al ratio. This solution and 2 M NaOH aqueous solution were then added drop wise at a rate of 0.5 mL·min-1 into a flask under the protection of N2 at room temperature. During the reaction, the mixture was vigorously stirred and a constant pH of 10 was maintained by adding either HNO3 or NaOH (0.5 M) solution. After that, the white precipitates were placed in in a Teflon hydrothermal autoclave at 180 °C for 48 5 ACS Paragon Plus Environment


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h. The resulting material was filtered, washed thoroughly with deionized and decarbonated water and vacuum-dried overnight at room temperature. b. MgAl-LDH-Cl. The sample was prepared similarly as above. An aqueous solution A containing stoichiometric amounts of AlCl3 and MgCl2 (Al3+%

= 20.0 %) was used.

c. MgAl-LDH-CO3. The as-synthesized MgAl-LDH-NO3 (Al3+%

= 19.7 %) material

was mixed with 150 mL of 0.01 M Na2CO3 aqueous solution and heated at 100 °C for 2 h for ion exchange. The ion exchange process was repeated twice and the products were washed and vacuum-dried. d. MgAl-LDH-ClO4. The alcoholic HClO4 solution prepared by diluting 60 % HClO4 (0.35 g) with 10 mL ethanol was added dropwise to a vigorously stirred suspension of MgAl-LDH-CO3 (0.7 g) in 90 mL ethanol under N2 flow (500 mL·min−1). The mixed suspension was stirred for 0.5 h under N2 flow. The resulting suspension was washed with ethanol and vacuum-dried. Deuteration: The deuteration of LDHs involves mixing the specific LDH sample with D2O and stirring the mixture at 80 °C under the protection of N2then drying the solid material under vacuum at 25 °C . Thermal treatment: The deuterated LDHs were heated at a ramping rate of 2 °C/min to the specified temperature and were kept at that temperature for 1 h under vacuum before it was cooled down and stored in a dry N2 glovebox for further investigations. The deuterated LDH samples (MgAl-27.3-NO3, MgAl-25.1-NO3, MgAl-19.7-NO3, MgAl-25.1-ClO4, MgAl-19.8-Cl and MgAl-19.7-CO3) were denoted according to their Al3+ molar percentages based on the ICP results, as well as the charge compensating 6 ACS Paragon Plus Environment

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anions in the interlayer regions. The Al3+ molar percentages for the initial and deuterated MgAl-LDH are the same, indicating that no selective dissolution occurs after deuteration. Characterization. XRD analysis was performed on a Philips X’Pro X-ray diffractometer using Cu Kirradiation (=1.54184 Å) operated at 40kV and 40 mA. The data were collected between 5- 80° (2θ). Fourier transform infrared (FT-IR) spectra were collected on a Thermo Nicolet iS10 spectrophotometer. The spectra were obtained in the range of 4000–600 cm-1. The Mg and Al molar percentages were determined by inductively coupled plasma (ICP) emission spectroscopy (Perkin-Elmer ICP OPTIMA5300DV). The TG profile of each sample was collected on a thermal analysis system (NETZSCH STA 449C) in N2 atmosphere at a ramping rate of 10 °C/min from 30 to 700 °C. All of the MAS NMR experiments were performed on a Bruker Avance III 400 spectrometer equipped with 3.2 and 4.0 mm MAS probes at room temperature. All 1H and 2H MAS NMR spectra were collected with a rotor-synchronized spin echo sequence (/2 –  –  – – acq,  = one rotor period) at a MAS rate of 20 kHz with the recycle delay set to 1 s. Quantification of different H species was performed by using 1

H spin echo NMR data and the results are consistent with the values obtained with

single pulse NMR data.

27

Al MAS NMR spectra were obtained with a single pulse

sequence employing a short excitation pulse of 0.3 s (~flip angle) with 1H decoupling during acquisition at a MAS rate of 20 kHz. 2D

27

Al 3QMAS NMR

(acquired with the z-filter sequence29) and 1H-27Al HETCOR NMR spectra were recorded at a spinning speed of 14 kHz. 1H/2H and

27

Al shifts were referenced to 7

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external standards of H2O and 0.1 M Al(NO3)3 aqueous solution at 4.8 and 0.0 ppm, respectively. The 1H/27Al TRAPDOR (TRAnsfer of Population in DOuble Resonance) experiments were carried out at a spinning speed of 5 or 10 kHz with an rf field of 60 kHz for

27

Al irradiation.30 The amounts of hydrogen were determined via integrating

the spectral intensities from 10 to -4 ppm. The simulation of the 27Al and 2H MAS NMR were performed using DMFIT package.31

RESULTS AND DISCUSSION Structural and Compositional Analysis. The X-ray diffraction (XRD) patterns of the six as prepared deuterated LDHs are shown in Figure S1, which confirm that all of the products possess hydrotalcite-like structure indexed with R3m rhombohedral symmetry and no unwanted impurity phase is present. The compositions of the LDHs examined by elemental analyses, as well as the lattice parameters c (=3d003) and a (=2d110) calculated on the basis of the 2 angles of (003) and (110) peaks, are shown in Table S1 and S2, respectively. The parameter a is more closely related to the molar percentage of trivalent cation Al3+. When more Al3+ ions are incorporated, due to the smaller diameter of Al3+ (0.50 Å) compared to Mg2+ (0.65 Å), the d110 spacing decreases, manifested as the (110) peak shifting to a smaller 2angle. The decrease of d110 also implies the increase of attractive interaction within the layer.25, 32, 33 On the other hand, for the LDHs with the same Al3+ content but different interlayer anions, the change of the parameter a is slim.


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The gradual increase of parameter c with the Al3+ molar percentage, in NO3-containing LDHs (Table S2) can be ascribed to the combined effects of the increasing attractive interaction between the layers and interlayer anions, as well as the arrangement changes of NO3- in the interlayer space.25 For the LDHs with the same Al3+ molar percentage but different interlayer anions, the parameter c is dependent on the characteristics of the anions and their orientations.13 The smaller parameter c and the interlayer spacing in CO32--containing LDHs compared to NO3--containing LDHs, indicate different anion packing arrangement,18 possibly a result of the hydrogen bonding between CO32- and hydroxyl groups from the LDH layers. The largest parameter c can be found in ClO4-containing LDH, which can be associated with the largest ionic radius of ClO4- (4.72 Å). The 27Al MAS NMR spectra of six deuterated LDHs are shown in Figure S2. A single sharp peak can be observed at around 9 ppm with a set of spinning sidebands for all samples. This peak arises from 6-coordinated Al ions surrounded solely by Mg ions in octahedral geometry in the first cation coordination shell.15, 16, 20 The small changes in the shape and position of the peaks in different samples may be attributed to the slight distortion of the surrounding Mg atoms, or the subtle changes in crystallinity and/or the strength of hydrogen bonding between the Al(OH)6 and interlayer species.18 No peak in the frequency of approximately 70 ppm due to 4-coordinated Al ion is observed, consistent with the high purity of the LDHs prepared. Figure 1 shows the 1H MAS NMR spectra of six deuterated LDHs at a moderate spinning speed of 20 kHz. Three peaks at 4.8, 3.4 ~ 2.3 ppm and 0.7 ~ 1.1 ppm can be 9 ACS Paragon Plus Environment


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resolved, corresponding to water in the interlayer regions, Mg2AlOH and Mg3OH species, respectively.15, 18-20 The molar percentages of Mg2AlOH and Mg3OH species extracted from NMR data agree well with the results obtained from ICP measurements (Table S3). With the increase of Al molar percentage (19.7 ~ 27.3 %), the peak of Mg2AlOH shifts to higher frequencies, consistent with the stronger acidity at higher Al content. The shift also makes the peak owing to Mg2AlOH overlap more with the peak due to interlayer water, decreasing the spectral resolution at relatively high Al molar percentages.20

2.3

MgAl-19.7-NO3 2.6

MgAl-19.8-Cl

2.7

MgAl-19.7-CO3

2.9

MgAl-25.1-ClO4

3.4

MgAl-25.1-NO3 MgAl-27.3-NO3

10

8

6

4

2 ppm

0

-2

-4

Figure 1. The 1H MAS NMR spectra of deuterated MgAl-LDHs with different Al molar percentages and different anions acquired at 9.4 T.

The FT-IR spectra of different deuterated LDHs are shown in Figure 2. The broad and almost featureless absorption region at around 3400 ~ 3600 cm-1 for all of the samples can be attributed  stretching. As observed previously, it is difficult to distinguish different OH and H2O species in this region, presumably due to the strong 10 ACS Paragon Plus Environment

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association effects between Mg2 AlOH, Mg3OH and interlayer water molecules.34 In contrast, the absorption region of OD stretching at around 2550 ~ 2710 cm-1 typically shows a broad peak at approximately 2607 ~ 2665 cm-1 with two shoulder peaks, one at higher and one at lower wavenumbers. The shoulder adsorption at 2550 ~ 2590 cm1

can be ascribed to the stretching vibration of D2O, while the main peak and its higher

wavenumber shoulder (2690 ~ 2705 cm-1) can be tentatively assigned to Mg2AlOD and Mg3OD stretching vibrations, respectively, according to the FT-IR data of model compounds deuterated Mg(OH)2 and bayerite Al(OH)3 (Figure S3 and additional discussion in the Supporting Information). The higher resolution observed can be ascribed to both the weaker association between OD groups and D2O, owing to the lower extinction coefficient of the O-D bands than the O-H bands, and restricted light scattering in the region which leads to less noisy spectra.35 The interlayer anions can also be distinguished with FT-IR spectroscopy and the bands at 1073, 1325 and 1381 cm-1 (Figure 2 and Table S4) can be assigned to the stretching vibrations of ClO4-, NO3and CO32- respectively.22, 36, 37



MgAl-19.7-NO3 2693

MgAl-19.8-Cl MgAl-19.7-CO3

Transmittance(%)

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MgAl-25.1-ClO4

2720

2661

2693 2693

2617

2670

2607

1381

2240

2717

MgAl-25.1-NO3 2532 2670

2665 MgAl-27.3-NO3

2530

2632 1208

2693

1073

2612

2536

1325

3500

3000

2500

2000

1500

1000

Wavenumber(cm -1)

2750

2500 2250 Wavenumber(cm -1)

Figure 2. FT-IR spectra of deuterated LDHs with different Al molar percentages and anions.

Dehydration and Dehydroxylation of LDHs. NO3--containing LDHs sample. Figure 3 shows the room temperature 1H MAS NMR, FT-IR and 27Al MAS NMR spectra of deuterated MgAl-27.3-NO3 heated at different temperature in comparison to the as prepared sample. After the sample was heated to 150 °C (Figure 3a) and kept at the temperature for 1 h, the peak of interlayer water disappeared (4.7 ppm), indicating the water species in the interlayer region escaped first upon heating. This observation is consistent with our DTA data (Figure S4). The molar percentage of Mg2 AlOH groups extracted from 1H NMR (80 %, Figure 4) is just slightly smaller the value at room temperature (82 %) (Table S5) while the resolution of the two peaks becomes slightly 12 ACS Paragon Plus Environment

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worse. The proportion of Mg2 AlOH decreases lightly to 78 % after a thermal treatment at 220 °C for 1 h. With a higher thermal treatment temperature, the molar percentage of Mg2 AlOH decreases rapidly and this value is only 38 % at 300 °C and 20 % at 375 °C, respectively (Figure 3a). The data shows that the thermostability of Mg2 AlOH is worse than that of the Mg3OH in the hydroxide layer, which is consistent with results obtained by Yang10 and Kameda.38



b

a

as prepared

as prepared

2693

150

150 oC

220

Transmittance(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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oC

250 oC

300 oC

2612 2536

o

220 oC 250 oC

1470 300 oC 375 oC

375 oC

10

8

6

4 2 ppm

0 -2

-4

3000

2500

1500

1000

Wavenmuber(cm-1)

74

c 375 oC 300 oC 250 oC 220 oC 150 oC as prepared

1500

1000

500

0

ppm

-500

-1000

-1500

Figure 3. Room temperature 1H MAS NMR (a), FT-IR (b) and 27Al MAS NMR spectra (c) for as prepared MgAl-27.3-NO3 and the samples heated at different temperature.

The results were further supported by FT-IR spectroscopy (Figure 3b). After a thermal treatment at 150°C, the stretching vibration at 2536 cm-1 due to D2O disappears, again proving the removal of water in the interlayer regions of the LDH. With increasing thermal treatment temperature, both the intensities of the peaks at 2693 and 2612 cm-1 decrease, however, the intensity of the band at 2612 cm-1 decreases more and the absorption peak at 2693 cm-1 becomes the major band in the spectra at 300 °C or 14 ACS Paragon Plus Environment

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higher. On the basis of this data, the assignment of the two peaks at 2693 and 2612 cm1

to Mg3OD and Mg2AlOD species, respectively, is confirmed. FT-IR data can also provide the structure information of the interlayer anions.

There is little change in the prominent absorption at 1325 cm-1 due to 3 vibrational mode of NO3- when the temperature increases from room temperature to 220 °C, which proves that arrangement of NO3-anions in the gallery space are relatively unperturbed and their D3h symmetry is kept in spite of the removal of all the interlayer water. However, a shoulder peak at ~1470 cm-1, which is due to NO3- anions in lower C2 symmetry in the interlayer,39 starts to appear 250 °C or higher. The increase in the intensity of this band along with the increase of the temperature indicates there are more NO3- anions with lower symmetry after thermal treatment at higher temperature.25, 39 27

Al MAS NMR spectroscopy was used to further investigate the change of LDH

structure at various temperatures (Figure 3c). The higher intensities of the satellite transitions after the sample being heated at 150 °C, as compared to the as prepared sample, can be ascribed to decreased mobility of the water within the layers.18 The majority of the signal at 9 ppm in the sample heated at 150 °C can be attributed to 6coordinated Al species in the hydroxide sheets, however, a very weak shoulder peak at around 74 ppm due to 4-coordinated Al starts to appear. The intensity of this new peak becomes more significant and continues to increases after the samples being heated at 220 °C and higher temperatures, coinciding with the decrease of the molar percentage of Mg2AlOH observed in 1H NMR and FT-IR, indicating a fraction of Al ions become 4-coordinated in the dehydroxylation of Mg2AlOH.40, 41 15 ACS Paragon Plus Environment


100 MgAl-NO3-27.3 MgAl-NO3-25.1 MgAl-ClO4-25.1 MgAl-NO3-19.7

90 80 Mg2Al-OH(%)

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70 60 50 40 30 20 10 0

50

100

150

200

250

300

350

400

Temperature(oC)

Figure 4. The molar percentage of Mg2AlOH obtained from line fits of 1H MAS NMR spectra of LDHs at different temperature.

To explore the effects of the Al molar percentage and more structural information during dehydration and dehydroxylation, 1H NMR, FT-IR and

27

Al NMR data of

deuterated MgAl-25.1-NO3 and MgAl-19.7-NO3 samples heated at different temperatures in comparison to the as prepared samples were obtained (Figure 5 and 6). Similar as MgAl-27.3-NO3, after a thermal treatment at 150°C, the signal owing to interlayer water (4.7 ppm) disappears in the 1H NMR spectrum of MgAl-25.1-NO3 (Figure 5a). For MgAl-19.7-NO3, a small fraction of water can be observed after thermal treatment at 150 °C (Figure 6a). It can be explained that it requires a higher temperature for water molecules to escape from the smaller interlayer space in LDHs 16 ACS Paragon Plus Environment

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with a lower Al molar percentage (also see additional discussions in the Supporting Information). The peaks due to Mg2AlOH and Mg3 OH species are found at 2.9 and 1.0 ~ 1.2 (fitted by 1 or 2 peaks) ppm,19 respectively. The ratios of the Mg2AlOH and Mg3OH (n(Mg2AlOH)/n(Mg3OH)) obtained from 1H NMR are the same as the as prepared samples for both MgAl-25.1-NO3 and MgAl-19.7-NO3 at 150 °C, consistent with the observations for MgAl-27.3-NO3, indicating that no dehydroxylation occurs at 150 °C (Table S6 and S7). n(Mg2AlOH)/n(Mg3OH) decrease with the increase of the temperature for MgAl-25.1-NO3 and MgAl-19.7-NO3, again suggesting that Mg3OH species are more stable than Mg2AlOH and it is independent of the Al molar percentages.



a

b as prepared 2670

as prepared

2622

150 oC

Transmittance(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 oC

220 oC

250 oC

2530

220 oC 250 oC 300 oC 1470

375 oC

300 oC 375 oC

10

8

6

4

2

0 -2

ppm

3000

-4

2500

1500

Wavenumber(cm-1)


1500

1000

500

0

-500

-1000

-1500

ppm

Figure 5. Room temperature 1H MAS NMR (a), FT-IR (b) and 27Al MAS NMR spectra (c) for as prepared deuterated MgAl-25.1-NO3 and the samples heated at different temperature.


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b

a

as prepared as prepared 2714

150

oC

2661 2620

150 oC

Transmittance(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


220 oC

250

oC

300

oC

220 oC 250 oC 1495

300 oC 375 oC

375 oC

10

8

6

4 2 ppm

0 -2

3000

-4

2500

1500

1000

Wavenumber(cm-1)


1500

1000

500

0

-500

-1000

-1500

ppm

Figure 6. Room temperature 1H MAS NMR (a), FT-IR (b) and 27Al MAS NMR spectra (c) for as prepared deuterated MgAl-19.7-NO3 and the samples heated at different temperature.

The determination of the temperature ranges that dehydroxylation of the Mg3OH and Mg2 AlOH species occur requires measuring the amounts of Mg3OH and Mg2 AlOH species in a temperature range. The decrease of the sample mass due to the loss of water in dehydration and dehydroxylation processes was taken into account according to the TGA results, and 1H NMR spectroscopy was then able to measure the absolute amounts 19 ACS Paragon Plus Environment


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of different hydroxyl sites (Figure 7 and Table S8, details in the determination of the relative amounts of Mg2AlOH and Mg3OH species can be found in the additional experimental details section in the supporting information). After a thermal treatment at 150 °C, the amounts of Mg3OH and Mg2 AlOH keep the same while the total amount of H decreases, indicating the occurrence of only dehydration process. The amount of Mg2AlOH decreases significantly at 220 °C, while there is no change for the amount of Mg3OH until 250 °C. At a temperature of 250 to 300 °C, dehydroxylation of both Mg3OH and Mg2AlOH species occur. After a thermal treatment at 375 °C, there is only a small amount of Mg2AlOH left, however, the amount of Mg3OH is still significant. These NMR data provide definitive evidences for the order of the dehydration and dehydroxylation processes. The dehydration is completed at 150 °C for NO3-containing LDHs and no dehydroxylation takes place at this temperature. The dehydroxylation of Mg2AlOH starts at a lower temperature (lower than 220 °C for NO3-containing LDHs) than Mg3OH (~ 250 °C for NO3--containing LDHs) while the temperature range for the two dehydroxylation processes significantly overlap, in contrast to the conclusions that the temperature ranges for the two dehydroxylation are clear cut obtained from some of the previous thermogravimetric analysis.10,38


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100

Total H Mg2Al-OH Mg3-OH

80

60 H%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

20

0 0

25

150

200

250

300

350

400

Temperature(oC)

Figure 7. The amounts of different H species in MgAl-25.1-NO3 after thermal treatments at different temperatures. The total amount of H in the as prepared sample is set to be 100 %.

The FT-IR spectra of MgAl-25.1-NO3 and MgAl-19.7-NO3 also show that the signals due to Mg3OD, Mg2AlOD and D2O can be resolved (Figure 5b and 6b). The disappearance of the band at 2530 and 2620 cm-1 after thermal treatments of the samples at 150 °C can be ascribed to the removal of interlayer water. According to the change in the intensities of the bands, the thermal stability of Mg3OD species are found to be stronger than Mg2AlOD. These conclusions are the same as the ones obtained from MgAl-27.3-NO3. The FT-IR spectra also show effects of Al molar percentages on the vibrational modes of the interlayer anions in the LDH samples after being heated. For example, after a thermal treatment at 250 °C, the shoulder peak at 1470 cm-1 in the IR 21 ACS Paragon Plus Environment


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spectra of MgAl-25.1-NO3, which is due to NO3- in C2v symmetry, is larger as compared to MgAl-27.3-NO3. This is because there are less anions and thus more free interlayer space in MgAl-25.1-NO3 and it is easier for NO3- to form low-symmetry structure (as compared to D3h mode with high symmetry) after thermal treatment. It can also be connected to the “stick-lying” model of NO3- in the interlayer region.25 With even less Al concentration, however, the shoulder peak at ~1500 cm-1 becomes weaker at 250 °C in MgAl-19.7-NO3 as compared to MgAl-27.3-NO3 and MgAl-25.1-NO3. Although there are less interlayer NO3- ions in MgAl-19.7-NO3, the decrease in the interlayer spacing with the decreasing Al makes it more difficult for NO3- anions to form lowsymmetry structure, leading to so-called “flat-lying” model25, 39, in which NO3- anions lie in the middle of the interlayer and the NO3- plane is parallel to the hydroxide layers. The 27Al NMR data of NO3--containing LDHs with less Al (MgAl-25.1-NO3 and MgAl-19.7-NO3) show that no 4-coordinated Al ion can be observed up to 150 °C (Figure 5c and 6c), indicating only dehydration takes place in these conditions, while dehydroxylation starts to occur in MgAl-27.3-NO3 at 150 °C. The relative concentrations of 4-coordinated Al increase with the increase of temperature (≥ 220 °C), suggesting the dehydroxylation of Mg2AlOH which leads to the conversion of 6coordinated Al ions to 4-coordinated Al ions. At 375 °C, the proportion of 4coordinated Al ions obtained from

27

Al NMR data (n(4-coordinated)/(n(4-

coordinated)+n(6-coordinated), ~ 42%, Figure S5, Table S9 and Figure S8) is similar to that of -l2O3,40 implying complete dehydroxylation of Mg2AlOH species. These observations agree well with the 1H NMR (Table S8) and FT-IR data. 22 ACS Paragon Plus Environment

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On the basis of the spectroscopic results from MgAl-27.3-NO3, MgAl-25.1-NO3 and MgAl-19.7-NO3, which have the same interlayer anion and different Al molar percentages, it can be found that the dehydration temperature is slightly higher (> 150 °C) for low Al sample MgAl-19.7-NO3 with smaller interlayer spacing. At the same time it is noticeable that 4 coordinated Al ions can be observed clearly for the high Al sample MgAl-27.3-NO3 after 150 °C thermal treatment, while this peak is extremely small or absent for the samples with less Al (MgAl-25.1-NO3 and MgAl-19.7-NO3), indicating the dehydroxylation temperature for high Al LDH sample can be slightly lower. These results imply that the Al concentration in LDHs is an important factor in controlling the two processes.

ClO4--containing LDHs sample. Figure 8 shows the 1H NMR, FT-IR and27Al NMR spectra of as prepared MgAl25.1-ClO4 and the samples heated at different temperature. For the as prepared sample, three hydroxyl signals for Mg3OH (0.8 ppm), Mg2AlOH (2.8 ppm) and interlayer water (3.5 ppm) can be readily distinguished (Figure 8a). The 1H NMR spectra of MgAl-25.1ClO4 heated at higher temperatures (150 ~ 375 °C) follow a similar trend as observed for the NO3--containing LDHs. For example, the interlayer water is removed first at a temperature lower than 150 °C and the fraction of Mg2AlOH declines gradually as thermal treatment temperature is increased in the range of 250 ~ 375 °C (Figure 8a and Figure 4). It is noticeable that the temperature for dehydroxylation is about 70 ~ 100 °C higher than those of NO3--containing LDHs. These observations may be associated with 23 ACS Paragon Plus Environment


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the formation of hydrogen bonding between oxygen ions in the perchlorate groups and the OH groups, as well as the stronger coulombic interactions between the layers and the anions in the ClO4--containing LDHs than that of NO3--containing LDHs. 42,43 The order of dehydration and dehydroxylation of MgAl-25.1-ClO4 upon thermal treatment was further investigated with FT-IR spectroscopy (Figure 8b) and the observations are consistent with 1H NMR results. The variation in absorption band is similar with that of NO3--containing LDHs at a temperature of 150 °C. At a temperature of 150 ~ 250 °C, there is no significant changes in either the band at 2717 cm-1 attributing to Mg3OD or the band at 2665 cm-1 due to Mg2AlOD, indicating the thermostability of the hydroxyl groups is significantly enhanced by the presence of ClO4-. Both the intensities of the absorption bands arising from Mg2AlOD and Mg3OD groups decrease after a thermal treatment at 300 °C, and the former decreases more significantly. The FT-IR spectrum of the as prepared deuterated MgAl-25.1-ClO4 show a strong 3 mode at 1073 cm-1 due to ClO4- with Td symmetry in the interlayer region,22 while the absence of 1 mode at lower wavenumbers indicates the structure of ClO4- is not distorted. This sharp absorption peak at ~1073 cm-1 does not change much at a temperature of 150 ~ 250 °C, indicating that arrangement of ClO4- in the interlayer space are still associated with Td symmetry. The 3 mode shows degeneracy and moves from 1073 to 1098 cm-1 while a new small 1 stretch mode can be seen at ~1011 cm-1 after thermal treatment at 300 °C, suggesting the structure of ClO4- is now distorted, i.e., the symmetry of the ClO4- anion has been lowered from Td to C3 or C3v. When the temperature reaches 375 °C, the 3 band is split into two bands at around 1098 and 1220 24 ACS Paragon Plus Environment

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cm-1 and the 1 band becomes stronger. These observations should be related to the collapse of the hydroxide layers and the further dehydroxylation of ClO4--containing LDHs at these temperatures. The

27

Al MAS NMR data (Figure 8c) further support the results from 1H NMR

and FT-IR spectroscopy. Only 6-coordinated Al ions are present in the as prepared sample and the samples calcined at relative low temperatures. A small NMR signal due to 4-coordinated Al ions start to appear in the sample heated at 250 °C and the fraction of this signal grows up with the increase of the temperature, consistent with the expected dehydroxylation process in this temperature range. This observation also shows 27Al NMR is a sensitive method to monitor if dehydroxylation takes place.



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Figure 8. Room temperature 1H MAS NMR (a), FT-IR (b) and 27Al MAS NMR spectra (c) for as prepared MgAl-25.1-ClO4 and the samples calcined at different temperature.

Cl--containing LDHs samples 1

H NMR spectra for as prepared deuterated MgAl-19.8-Cl in comparison to the

samples calcined at different temperature are shown in Figure 9a. Three peaks due to interlayer water and two types of hydroxyl sites can be clearly distinguished. However, after the sample was heated at high temperature, even at 50 °C, only broad and 26 ACS Paragon Plus Environment

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overlapping resonances due to hydroxyl groups were observed. 1H-27Al TRAPDOR NMR experiments30 were performed on Mg-Al-19.8-Cl after calcination at 50 °C to help spectral assignments (Figure 9b). The peaks at 2.9 and 0.7 ppm have a larger reduction in intensity from 1H-27Al TRAPDOR experiment, which can be assigned to Mg2AlOH species with a short H-Al distance and the resulting large dipolar coupling. The relatively smaller TRAPDOR effects are found for the peaks at 1.3 and -0.7 ppm and these two peaks can be assigned to Mg3OH species, in which the nearest Al ions are not in the first but in the second/third coordination shell of the hydroxyl groups. The different chemical shifts of Mg2 AlOH or Mg3OH species must be related to different local environments, possibly different hydrogen bonding. The room temperature 1H NMR data of samples calcined at different temperature can be analyzed according to this assignment (Figure 9a). At 150 °C, the water molcules in the interlayer regions are completed removed. The increasing fraction of Mg3OH species compared to Mg2AlOH species along with the increase of the temperature (> 220 °C) is similar to the observations in NO3-- and ClO4--containing LDHs, confirming the thermal stability of the Mg3OH groups is stronger. This is further supported by

27

Al NMR results of

samples after calcination (Figure 9c). Only 6-coordinated Al ions can be observed and there is little change in the spectra of the samples calcined at 150 °C, compared to the as prepared sample, while after thermal treatment at 220 °C or higher, 4-coordinated Al ions appear, again confirming the temperature range of dehydration (< 150 °C) and dehydroxylation processes (≥ 220 °C).



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a

b

Mg2AlOH H2O

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Mg3OH

control

as prepared Al irradiation difference

50 oC

10

5

0 ppm

-5

-10

150 oC

c

* 200 100 0 -100-200 ppm

220 oC as prepared 50 oC 150 oC

250 oC

220 oC

*

250 oC

300 oC

10 8

300 oC

6

4

2

ppm

0 -2

-4

1500

1000

500

0 ppm

-500

-1000

-1500

Figure 9. (a) Room temperature 1H MAS NMR spectra for as prepared deuterated MgAl-19.8-Cl and the samples calcined at different temperature. (b) Room temperature 1

H-27Al TRAPDOR NMR spectra of deuterated MgAl-19.8-Cl calcined at 50 °C at a

spinning rate of 10 kHz. Al irradiation time: 0.1 ms. (c) Room temperature 27Al NMR spectra of deuterated MgAl-19.8-Cl at different temperature. The changes in the 1H NMR of hydroxyl sites at relatively low temperature (< 220 °C) may be related to the more disordered arrangement of Cl- anions in the interlayer. According to the work from Kirkpatrick and coworkers, the Cl- anions are 6-coordinated, hydrogen bonded to 2 interlayer water molecules and 4 OH groups of the hydroxide layers (2 each on top and bottom layers) at room temperature.44 The narrow linewidths observed in the as prepared sample (top of Figure 9a) are considered to be related to the dynamic averaging bonding associated with highly mobile water and chloride ions. For the partially or fully dehydrated samples, the Cl- ions are 28 ACS Paragon Plus Environment

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regarded to hydrogen bond more rigidly in the interlayer with 6 OH groups (3 each on top and bottom layers) in a trigonal prism arrangement,42, 45, 46 leading to much wider signals.

CO32--containing LDHs samples The 1H NMR spectra of Mg-Al-19.7-CO3 calcined at different temperature are similar to the data of Mg-Al-19.8-Cl (Figure 10a). The 1H NMR spectrum of the sample heated at 50 °C show overlapping and broader resonances. At least 4 peaks can be observed in the frequency region for Mg2AlOH and Mg3OH species. Similar TRAPDOR spectra were collected on this sample as MgAl-19.8-Cl heated at 50 °C (Figure 10b). Therefore, the peaks at 3.9 and 0.5 ppm with a larger TRAPDOR fraction are assigned to Mg2AlOH species, while the peaks at 2.2 and -0.4 ppm associated with a smaller TRAPDOR effect are assigned to Mg3OH sites. After the sample is calcined at 150 °C, the intensity of the peak at 3.9 and 0.5 ppm due to Mg2AlOH decreases significantly, while the intensities of the other two peaks do not change much. This observation suggest that dehydroxylation already occurs at 150 °C and it is further supported by the 27Al NMR data, in which 4-coordinated Al ions start to show up in the spectrum of the sample calcined at 150 °C (Figure 10c).

The intensities of the two

peaks due to Mg3OH start to decline at 250 °C, indicating dehydroxylation of the Mg3OH species occur. Therefore, the thermal stability of Mg3OH is also higher than Mg2AlOH in Mg-Al-19.7-CO3. The lower frequency peaks due to Mg2AlOH (0.5 ppm) and Mg3OH (-0.4 ppm) species with weaker intensity are probably related to the more 29 ACS Paragon Plus Environment


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complicated local environments and disorders of anions in the interlayer galleries.42, 45, 46

The hydroxide layers of partly or fully dehydrated LDHs may have greater affinities

for bivalent anions compared to monovalent anions, leading to different chemical environments. Also the hydroxyl groups close to the CO32- should be associated with higher basicity, resulting in a more negative shift.

a

b Mg2AlOH control Mg3OH

H2O

Al irradiation

as prepared

difference 15

10

5

50 oC

0

ppm

-5

-10

-15

c 150 oC as prepared

220 oC

50 oC 150 o C

250 oC

220 o C 250 o C 300 o C

300 oC 10

8

6 4 2 ppm

0 -2 -4

1500 1000 500

0 -500 -1000 -1500 ppm

Figure 10. (a) Room temperature 1H MAS NMR spectra for as prepared deuterated MgAl-19.7-CO3 and the samples calcined at different temperature. (b) Room temperature 1H-27Al TRAPDOR NMR spectra of deuterated MgAl-19.7-CO3 calcined at 50 °C at a spinning rate of 10 kHz. Al irradiation time: 0.1 ms. (c) Room temperature 27

Al MAS NMR spectra for as prepared deuterated MgAl-19.7-CO3 and the samples

calcined at different temperature.


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Comparing the data for MgAl-19.7-NO3, MgAl-19.7-CO3 and MgAl-19.8-Cl, as well as the data for MgAl-25.1-NO3 and MgAl-25.1-ClO4, which have the same or very similar Al molar percentages and different charge compensating anions, the differences in the temperature range for dehydroxylation can be very large (> 70 °C between MgAl25.1-NO3 and MgAl-25.1-ClO4), while dehydration process is not complete only for NO3--containing LDH at an Al molar percentage of 19.7 %.

Thus, the dehydration

and dehydroxylation processes are complicated issues and worth further detailed investigations.

CONCLUSIONS The dehydration and dehydroxylation processes of LDHs with different anions (NO3-, ClO4-, Cl- or CO32-) in the interlayer and / or different Al molar percentages (19.7 ~ 27.3 %) have been successfully investigated with 1H MAS NMR, FT-IR and

27

Al

MAS NMR spectroscopy on deuterated samples. The results prove that the dehydration takes place at a lower temperature (usually < 150 °C) than dehydroxylation (> 150 °C). The temperature for dehydration is mainly controlled by the interlayer spacing, and influenced by the interaction between the anions and water molecules in the interlayer regions at the same time. Definitive spectroscopic evidences from also 1H NMR, FTIR and 27Al NMR also show that the dehydroxylation of Mg2AlOH occurs at a lower temperature along with the formation of 4-coordinated Al ions, as compared to the dehydroxylation of Mg3OH in all of the six LDHs studied. The specific temperature range for the two dehydroxylation processes overlap significantly and are dependent on 31 ACS Paragon Plus Environment


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the nature of the interlayer anions and the Al molar percentages. In particular, the high resolution FT-IR spectra obtained from deuterated LDHs for the first time imply that in-situ FT-IR spectroscopy can be readily applied to follow the structure changes of LDHs in a thermal treatment in real time. This combined spectroscopic approach can be further extended to study the thermal behaviors of other LDHs and provide insights for rational design of LDHs and related materials with desired properties.

ASSOCIATED CONTENT

Supporting Information. Additional discussion and characterization including 27Al MAS NMR and fitting results, FT-IR, thermal decomposition spectra, TRAPDOR NMR spectra, 1H27Al HETCOR NMR spectra, 27Al 3QMAS NMR spectra, 2H MAS NMR data, elemental analyses, lattice constants as well as summaries of chemical shifts and parameters of quadrupolar interactions. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Phone: +86-83686793. Email: [email protected].

Funding Sources This work was supported by the National Basic Research Program of China (Grant 2013CB934800), the National Natural Science Foundation of China (NSFC) (Grants 32 ACS Paragon Plus Environment

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21222302, 21073083 and 20903056), NSFC - Royal Society Joint Program (Grant 21111130201), Program for New Century Excellent Talents in University (NCET-100483), the Fundamental Research Funds for the Central Universities (Grant 1124020512).

Notes The authors declare no competing financial interests.

References

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Table of Contents

The detailed information of dehydration and dehydroxylation of layered double hydroxides can be extracted with solid-state NMR and FT-IR spectroscopy on deuterated samples.


Dehydration and Dehydroxylation of Layered Double Hydroxides

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