近日，浙江工业大学、桂林理工大学、青阳非金属矿研究院、浙江大学等科研人员合作，采用原位高温和低温傅里叶变换红外光谱、原位高温和低温X射线衍射法，研究了钠基蒙脱石在温度为20~800 ℃和-150~20 ℃范围内的化学键演变，以及在20~700 ℃和-268~20 ℃范围内的晶格涨缩特征。研究发现，在20~700 ℃范围内时，蒙脱石中的羟基含量随着温度升高而降低，而在-150~0 ℃范围内蒙脱石中羟基含量基本保持不变。蒙脱石片层“破碎”边缘形成的羟基与表面吸附的水分子有关，其含量随颗粒尺寸的减小而增加。蒙脱石层间水分子与其硅氧四面体可以通过Si2O-H2O键发生作用；蒙脱石中的水分子（包括表面吸附水和层间水）在原位加热室（密闭环境中）至100 ℃、或在20 ℃的真空中可被全部移除。随着温度的上升，钠基蒙脱石从20 ℃下的层间单层水分子态转变为100 ℃下的脱水状态，然后至700℃下的脱羟基状态，其对应的层间距由1.27收缩至0.97 nm，再收缩至0.96 nm；当钠基蒙脱石从20 ℃降温至-268 ℃时，其层间距增加约0.03 nm，呈现负膨胀现象。有关工作以“Evolution of Chemical Bonding and Crystalline Swelling−Shrinkage of Montmorillonite upon Temperature Changes Probed by in Situ Fourier Transform Infrared Spectroscopy and X‑ray Diffraction”为题发表在美国化学会的Langmuir期刊上。
据悉，美国化学会的有关表面化学领域经典优质期刊 “朗缪尔（Langmuir），刊名取自科学家欧文·朗缪尔（Irving Langmuir，1881年1月31日－1957年8月16日）。欧文·朗缪尔是美国化学家、物理学家，1932年诺贝尔化学奖得主他也因为在表面化学上的工作被授予1932年诺贝尔化学奖。 他是第一个成为诺贝尔奖得主的工业化学家，美国新墨西哥州索科罗附近的“朗缪尔大气实验室（Langmuir Laboratory for Atmospheric Research）”以他的名字命名，而美国表面化学的研究期刊也命名为“朗缪尔（Langmuir）“。
Clay minerals are distributed in Earth’s crust and troposphere and in Martian crust where temperature varies. Understanding the changes of chemical bonding and crystalline swelling–shrinkage of montmorillonite (Mnt) upon temperature changes is fundamental for studying its surface reactivity and interaction in specific surroundings. However, such an issue remains poorly understood. Here, in situ high- and low-temperature Fourier transform infrared (HT- and LT-FTIR) spectroscopy and X-ray diffraction (HT- and LT-XRD) were performed to study the evolution of chemical bonding and crystalline swelling–shrinkage of sodium-montmorillonite (NaMnt) upon temperature changes. The FTIR results show that the hydroxyl content in NaMnt decreased when the temperature increased from 20 to 700 °C, while it is independent of temperature from 0 to −150 °C. The formation of hydroxyls at the “broken” layer edges of NaMnt is related to adsorbed water molecules on the surfaces, and its content increased when the particle size of the NaMnt decreased. The water molecules in the interlayer space of NaMnt could bond to the tetrahedral sheet of NaMnt through Si2O–H2O bonds. HT- and LT-XRD results reveal that all of those water molecules in NaMnt were removed after heating to 100 °C in the heating chamber. The NaMnt was transformed from a state of monolayer interlayer water molecules at 20 °C to a dehydrated state at 100 °C, and then to a dehydroxylated state at 700 °C. Accordingly, the basal spacings of NaMnt were changed from 1.27 to 0.97 nm and then to 0.96 nm, respectively. When NaMnt was cooled from 20 to −268 °C, a crystalline swelling of NaMnt occurred with an increase of 0.03 nm of basal spacing. This work demonstrates that high/low temperature has a remarkable effect on the hydroxyls and the water molecules in NaMnt, which in turn affects its swelling–shrinkage performance. These findings provide some in-depth understanding of the changes of chemical bonding and crystalline swelling–shrinkage of montmorillonite upon temperature changes and the reasons behind these, which might be helpful for the design of engineering Mnt in high-/low-temperature applications.
Clays are naturally occurring minerals and distributed in the Earth’s crust and troposphere, even in Mars’s Noachian terrains and Gale crater. (1−3) As one of the hydrous phases, clay minerals are transformed from the chemical reaction between water and nominally anhydrous materials, (4) so that water molecules and hydroxyls are generally distributed in their interlayers and crystal lattices, respectively. The physicochemical state of water molecules in clays changes with temperature, causing crystalline swelling–shrinkage, which influences their surface reactivity and further the chemical processes in sediments and soils. (5−8) For example, the guest organic molecules in soil could react with water molecules in clays through chemisorption or physisorption on the clay surfaces. (9) In engineering fields, clays have been applied as radiation and permeable barriers, (10) catalysts and catalyst carriers, (11) lubricants and coating additives, (12) etc. They are often served at varying high and low temperatures. Taking a clay-based nuclear waste barrier as an example, the temperature of nuclear waste is generally up to room temperature and even might be higher than 450 °C, (13) which has an effect on the shrinkage-swelling of clay and may further affect the barrier property of the whole engineering structure. In this context, understanding the evolution of chemical bonding and crystalline swelling–shrinkage of clays upon changes in temperature is a foundational issue in clay structural mineralogy and engineering applications.
Within the 2:1 layer type clay minerals, montmorillonite (Mnt) is the most commonly used and studied mineral in scientific and industrial fields. It contains an octahedral sheet sandwiched by two tetrahedral sheets. The octahedral sites of Mnt are occupied by Al, with some of them substituted by Fe and Mg. (14,15) Because of the isomorphous substitution of cations in the octahedron, the Mnt layers carry negative charges balanced by interlayer hydrated cations. Those interlayer hydrated cations, and chemical bonding on the layers of Mnt, especially for hydroxyls, are dramatically related to temperature, which could further cause crystalline swelling–shrinkage. (7,16−20) This topic has been investigated and discussed by many geological, chemical, and material scientists. (21−23) Among those works on chemical bonding in Mnt at various temperatures, ex situ Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) methods were generally employed. Most of them investigated the Mnt structure by heating and then cooling to room temperature. However, it is difficult for such an ex situ characterization method to reveal its actual physicochemical state at a certain temperature because the data is generally obtained at room temperature. Furthermore, its complete evolution with temperature might not be accurately understood. Therefore, it is highly desired to unveil the evolution of chemical bonding and crystalline swelling–shrinkage of Mnt on temperature using an in situ technology.
In recent decades, the advances of in situ characterization methods have made it more reliable to understand the effect of temperature on structural changes of Mnt when compared with an ex situ calcination and freezing method. Using in situ XRD, many works revealed that the swelling–shrinkage of Mnt is related to relative humidity, temperature, layer charge, and interlayer cations. (24−27) Morodome and Kawamura (18) systematically studied the swelling–shrinkage behavior of Mnt through precise control of the relative humidity and temperature. They found that the basal spacing decreased with decreasing relative humidity for NaMnt and CaMnt and decreased with temperature increasing from 50 to 150 °C for CaMnt. Zampori et al. (28) reported that the basal spacing of CaMnt continuously decreased from 25 to 850 °C. Hong et al. (27) examined the basal spacing of NaMnt from 25 to 700 °C through an in situ heating method and suggested that it shrunk from 1.262 to 1.002 nm from 25 to 200 °C but increased by 0.015 nm from 500 to 700 °C. However, these findings on crystalline swelling–shrinkage of Mnt at varying temperatures are controversial. In addition, how low temperature affects the evolution of chemical bonding and further influences the crystalline swelling–shrinkage of Mnt remains poorly understood.
In this work, the chemical bonding and the crystalline swelling–shrinkage of NaMnt were investigated by in situ high- and low-temperature Fourier transform infrared (HT- and LT-FTIR) spectroscopy and X-ray diffraction (HT- and LT-XRD). The temperature range for FTIR analysis of NaMnt was from 20 to 800 °C and from 0 to −150 °C. For in situ varied temperature XRD characterization of NaMnt, the temperature was set in the ranges from 20 to 700 °C and from 20 to −268 °C. Besides the pristine NaMnt, the testing samples contained ball-milled NaMnt and acid/alkali-etched NaMnt for better discrimination of different chemical bonds in NaMnt. This work provides a foundation for understanding the correlation of the chemical bonding and crystalline swelling–shrinkage of Mnt with temperature.
Pristine montmorillonite (Mnt) was collected from Heishan, Liaoning Province, China. It was purified by sedimentation at room temperature. The chemical composition (wt %) of Mnt is as follows: SiO2 71.96%, Al2O3 14.70%, Fe2O3 1.58%, MgO 5.54%, CaO 1.64%, TiO2 0.11%, Na2O 3.76%, and K2O 0.10%. As sodium ions are the interlayer ions of Mnt, the purified montmorillonite is labeled as NaMnt hereafter. Sulfuric acid (95.0–98.0%) and sodium hydroxide (analytical grade) were provided from Xilong Chemical Co., Ltd., China.
Ball Milling of the NaMnt
As the hydroxyls can generate at the “broken” layer edges of Mnt, fabricating more layer edges might help understand the types and stability of hydroxyls in Mnt. Ball milling is considered an effective method to break up and exfoliate Mnt layers to produce more “broken” layer edge bonds. (29,30) In brief, 80.0 g of NaMnt was mixed with 80.0 mL of deionized water at room temperature and stirred for 2 h to obtain a NaMnt slurry. Then, 20 mL of NaMnt slurry was introduced to a 120 mL nylon tank (inner diameter of 52.8 mm) and ball-milled using zirconia balls (15 balls with a diameter of 10.1 mm together with 90 balls with a diameter of 5.6 mm) for 2 h in a planetary ball miller (QM-3SP4, Nanjing NanDa Instrument Plant, China). The rotational speed was set to 300 rpm, and the direction of rotation was reversed every 30 min. After the ball milling, a milled slurry was obtained and then dried at 80 °C to constant weight. The product is marked as NaMnt-BM hereafter.
Acid/Alkali Etch of Mnt
To better investigate silicon–oxygen groups in Mnt, acid-etched Mnt was prepared and used as a contrast sample. Acid-etched Mnt was prepared as follows. 160.0 g of NaMnt was dispersed in 1.6 L of deionized water at 80 °C and stirred for 2 h to obtain a NaMnt dispersion. After standing for 24 h, the upper 492.0 mL dispersion was transferred into a flask. Then, 34.0 mL of sulfuric acid and 132.4 mL of deionized water were added to the dispersion, followed by stirring at 80 °C for 4 h. The resulting product was obtained by centrifugation at 4000 rpm for 5 min and dried at 80 °C to constant weight. The acid-etched NaMnt was ground and denoted as HMnt.
To prepare alkali-etched Mnt, following the above procedure, 160 mL of NaMnt dispersion was mixed with 7.7 g of NaOH and then reacted under the same conditions. The prepared alkali-etched Mnt was denoted as AMnt and was also used as a contrast sample to explore the types and stability of silicon–oxygen groups in Mnt.
In Situ High- and Low-Temperature Fourier Transform Infrared Spectroscopy
In situ high-temperature Fourier transform infrared (HT-FTIR) spectra of NaMnt were measured with a Nicolet iS50 FTIR spectrometer equipped with a continuum microscope. An Instec HS1300 heating chamber with CaF2 windows was used for heating. The heating procedure was set from 100 to 800 °C with an increment of 100 °C at a heating rate of 15 °C/min. The sample was tested after a dwell time of 5 min. In addition, to investigate the adsorption kinetics of water molecules in NaMnt, a pressed NaMnt slice was first placed in an oven at 140 °C for 4 h and then transferred to the FTIR spectrometer immediately for rapid testing.
In situ low-temperature Fourier transform infrared (LT-FTIR) spectroscopy of NaMnt was carried out on a Bruker Vertex 70 FTIR spectrometer equipped with a variable temperature chamber, which has a function of heating and cooling. The cooling procedure was programed from 0 to −150 °C at a decrement of 50 °C with a cooling rate of 5 °C/min. The sample was tested after 5 min dwell time. In addition, to better study the hydroxyls in NaMnt at low temperature, a comparison program, heated to 200 °C before cooling from 0 to −150 °C, was set after the first whole cooling procedure.
For in situ HT-FTIR and LT-FTIR characterizations, the samples were pressed to be self-supporting slices without any additive. The slices were detected at ambient conditions for 64 scans at a resolution of 4 cm–1 for HT-FTIR and 0.4 cm–1 for LT-FTIR. FTIR background spectra were collected before sample testing at each temperature. In addition, conventional KBr-FTIR of samples based on a KBr pellet pressing method was performed on a Thermo Nexus 470 Fourier transform infrared spectrometer.
In Situ High- and Low-Temperature X-ray Diffraction
In situ high-temperature X-ray diffraction (HT-XRD) of NaMnt was performed with a PANalytical X’Pert PRO MPD diffractometer equipped with a heating chamber containing a sample holder and a carbon film window allowing the penetration of X-rays. For comparison, the NaMnt was first tested at 20 °C inside the heating chamber and then tested at 20 and 100 °C and at every 100 °C step increase from 100 to 700 °C inside the heating chamber in air conditions. The heating rate was 10 °C/min, and the dwell time was 3 min at every temperature. After the sample was detected at 700 °C, it was allowed to cool to 20 °C and was then tested inside and outside (in the air) the heating chamber. Additionally, an extra pristine NaMnt was tested in a vacuum inside the heating chamber at 20 °C.
In situ low-temperature X-ray diffraction (LT-XRD) of NaMnt was obtained using a Rigaku SmartLab X-ray powder diffractometer equipped with a temperature-dependent chamber containing a copper sample holder. A beryllium film window was installed on the chamber, allowing the penetration of X-rays. The sample was tested in a vacuum inside the chamber at temperatures from 0 to −268 °C with a decrement of 50 °C. The cooling rate was 5 °C/min, and the dwell time was 5 min at every temperature. Before testing under low temperatures, the NaMnt was tested at 20 °C inside and outside the chamber, respectively.
Mnt could adsorb and reserve water molecules in its galleries (Figure 1A) and could dissociate water molecules to produce H+. (31) Water molecules have an influence on surface reactivity of Mnt, further accelerate the chemical processes in sediments and soils, (7,8) and are the earliest component removed when heating. (32) Thus, the effect of temperature on water molecules in Mnt was first studied herein.
The 3542–3100 and 1700–1580 cm–1 regions of the HT-FTIR and LT-FTIR spectra of NaMnt were selected and enlarged to investigate the evolution of water molecules during heating from 20 to 800 °C and cooling from 0 to −150 °C (Figure 2). In the 3542–3100 cm–1 region of the HT-FTIR spectrum at 20 °C (Figure 2Aa), the band at 3420 cm–1 is attributed to stretching vibration (ν1) of water molecules in NaMnt, while the ν1 vibration of water molecules appears at 3410 cm–1 in the LT-FTIR spectra (Figure 2C). This suggests that a red-shift of the stretching frequency of water molecules in NaMnt happened at low temperatures. Such a red-shift might be ascribed to the breaking of H bonds of water molecules caused by the thermal fluctuations. (33) Additionally, the infrared absorption around 1640 cm–1 (1626–1652 cm–1) in HT-FTIR spectra and that at 1649 cm–1 in LT-FTIR spectra are observed. This belongs to the bending vibration (δ) of the molecular water in NaMnt. (34−36)
When the NaMnt was heated to 100 °C in the heating chamber, the integral areas of the overlapped peaks in the 3542–3100 and 1700–1580 cm–1 regions (Figure 2A,B, marked by the light pink box) greatly decreased. As a result, the ν1 and 2δ vibrations at 3420 and 3240 cm–1 in the spectrum could not be seen at 100 °C (Figure 2Ab). Interestingly, for the δ vibration of molecular water in NaMnt at 100 °C, a small infrared absorption was detected at 1628 cm–1, which is off the center of the peak at 1640 cm–1 at 20 °C in the HT-FTIR spectrum (Figure 2Ba,b, marked in a light pink box). This indicates that a small amount of water molecules remained in the NaMnt at 100 °C. This slight infrared absorption was ascribed to the adsorbed water molecules on the surfaces of NaMnt. It is evidenced by the reappearing infrared absorption at 1628 cm–1 when the NaMnt was cooled to 20 °C from 800 °C (Figure 2Bj). In addition, the removal of water molecules from NaMnt was confirmed by the ca. 0.97 nm basal spacing of NaMnt at 100 °C from the HT-XRD result (Figure 3Bb), suggesting that no water molecules remained in the NaMnt interlayers.
A comparison experiment (a NaMnt slice predried in an oven at 140 °C and then tested by FTIR) has been implemented to study the adsorption kinetics of water molecules in NaMnt. Two strong infrared absorptions at 3420 and 3240 cm–1 of water molecules in predried NaMnt were detected, suggesting that the water molecules could re-enter the NaMnt after drying at 140 °C. This result reveals the reversibility of adsorption of water molecules in Mnt at 20 °C and desorption at 140 °C.
As the temperature of NaMnt rose further in the heating chamber, the intensity of infrared absorptions of water molecules in NaMnt at 3420 and 1628 cm–1 reduced further from 100 to 200 °C, and those absorptions were completely invisible from 300 to 800 °C (Figure 2Ad–i,Bd–i). This suggests that the water molecules have been completely removed in this temperature range.
With the temperature of NaMnt decreasing from 0 to −150 °C, the frequencies of the stretching vibration (3410 cm–1) and bending vibration (1649 cm–1) of water molecules in NaMnt did not shift. However, the relative intensities of their bending and stretching vibrations at subzero temperatures are different from those at 20 °C. Specifically, the relative intensity of the bending vibration of water molecules in NaMnt decreased with the decrease in temperature from 0 to −150 °C. Moreover, it is higher than their stretching vibration at subzero temperatures, while it is lower than the stretching vibration at 20 °C (Figure 2Ca–d,Da–d). This indicates that the reduction in the intensity of the stretching vibration of water molecules in NaMnt caused by the temperature drop is greater than that of the bending vibration. This phenomenon is more prominent for the NaMnt tested from 0 to −150 °C after heating to 200 °C in the temperature chamber. The intensity of the stretching vibration of water molecules in NaMnt is much lower than that of the bending vibration (Figure 2Ce–h,De–h). As the NaMnt was heating to 200 °C and then cooling to 0 to −150 °C in the same chamber, the removed water molecules at 200 °C re-entered into NaMnt when the temperature decreased. Thus, the stretching and bending vibrations of water molecules in NaMnt were reidentified.
The bands at 3240 cm–1 in HT-FTIR spectra and at 3238 cm–1 in LT-FTIR spectra are the Fermi resonance (2δ, coupling of fundamental stretching vibrations and the bending overtone) of water. (25,37) Previous works reported that the intensity of the Fermi overtone is more prominent at lower temperatures. (38−40) They made this discovery by studying the Fermi overtone of water molecules at temperatures above 2 °C. Herein, it is found that the intensity of the Fermi overtone could approach the intensity of the fundamental stretching vibrations of water molecules in NaMnt at −150 °C, as pointed by red arrows in Figure 2De,h. The vibration intensity of the Fermi overtone of water molecules in NaMnt increased by about 1.6 times in the FTIR spectrum as the temperature decreased from 0 to −150 °C.
Effect of Temperature on Crystalline Swelling–Shrinkage of Mnt
Changes in temperature lead to the evolution of water molecules in NaMnt. Further, the change of water molecules in NaMnt results in the variation of chemical bonding and crystalline swelling–shrinkage of NaMnt. Thus, the structure of NaMnt at varying temperatures was analyzed by in situ HT-XRD and LT-XRD, and its chemical bonding and crystalline swelling–shrinkage were elucidated.
In situ HT-XRD of NaMnt was tested from 20 to 700 °C, and patterns are shown in Figure 3A. In addition, after heating to 700 °C, the NaMnt was cooled to 20 °C and tested inside and outside the heating chamber, separately. As the heating chamber is equipped with a carbon film window allowing the penetration of X-rays, a reflection at the 2θ 6.91° (insert in Figure 3A) was detected. Thus, all of the patterns collected in the heating chamber contain an additional background peak. This peak is overlapped with the (001) reflection of NaMnt at 2θ 6.95° (basal spacing of 1.27 nm) in the heating chamber at 20 °C (Figure 3Ak). A shoulder peak accounting for the (001) reflection of NaMnt can be distinguished at 2θ 7.31° (Figure 3Ba). Interestingly, when the temperature rose to 100 °C in 22 min, the (001) reflection of NaMnt appeared at 2θ 9.15° with a basal spacing of 0.97 nm (Figure 3Bb). This attribution can be confirmed by the appearance of the (001) reflection of NaMnt tested outside the heating chamber (without a carbon film background) after cooling from 700 to 20 °C (Figure 3Aj). It is noteworthy that the basal spacing of NaMnt is close to the thickness of its single TOT (tetrahedron–octahedron–tetrahedron) layer (ca. 0.96 nm). (41,42) This suggests that complete dehydration of NaMnt occurred when heating to 100 °C.
The process of complete dehydration in the NaMnt interlayer is shown in Figure 3D. In addition to the temperature, the loss of water molecules can be ascribed to the low relative humidity in the heating chamber. Specifically, with the temperature increased to 100 °C, the relative humidity in the heating chamber decreased to a low level, which led to the loss of water molecules. This result is consistent with the previous reports (18,43) that the basal spacing of Mnt is dependent on the relative humidity: the basal spacing of Na-montmorillonite could decrease from 15.8 to 9.6 nm with the relative humidity decreasing from 80% to 0%. (25) The basal spacing of NaMnt at 0.97 nm in a vacuum at 20 °C (Figure 2Am) also supports this finding. It is also in accordance with the HT-FTIR result that the stretching and bending vibrations of water molecules in NaMnt almost completely disappeared in the spectrum when the temperature increased to 100 °C (Figure 2Ab,Bb).
As the temperature of NaMnt continued to rise from 100 to 700 °C in the heating chamber, the intensity of the (001) reflection decreased (Figure 3Bb–i), and the reflection slightly shifted to a higher 2θ. A similar result is found for the (110) reflection of NaMnt at 2θ 35.02° (Figure 3Cb–i). This indicates that, after removing interlayer water molecules at 100 °C, loss of structural hydroxyls in NaMnt occurred with increasing temperature. Consequently, the distance between adjacent crystal faces in NaMnt went closer, and a reduction of the order of the atoms in NaMnt took place during the heating period.
In situ LT-XRD was used to ascertain the crystalline swelling–shrinkage of NaMnt under low temperatures (Figure 4A). The selected (001) reflection of NaMnt was enlarged in Figure 4B. It shows that the (001) reflection of NaMnt appears at 2θ 6.94° (basal spacing of 1.27 nm) inside the temperature chamber (Figure 4Ba) at 20 °C, while it shifted to 2θ 9.03° with a basal spacing of 0.98 nm (Figure 4Bb). The evolution of basal spacing of NaMnt could be ascribed to the vacuum inside the temperature chamber. The monolayer interlayer water molecules in NaMnt were removed in a vacuum. (44) The result is similar to the basal spacing of NaMnt of 0.97 nm in the heating chamber at 20 °C (Figure 3Am).
With a decrease in temperature from 20 to −268 °C, the (001) reflection of NaMnt shifted from 2θ 9.03° to 8.75°, meaning that the basal spacing of NaMnt increased by about 0.03 nm (from 0.98 to 1.01 nm) (Figure 4C). Interestingly, the reflection around 2θ 10.5° in the XRD pattern of NaMnt widens gradually from 50 to 200 °C (the inset in Figure 4C). Such results indicate that a decrease in temperature led to a decrease of the stacking regularity of layers and an expansion of the crystal lattice of NaMnt.
Effect of Temperature on Structural Hydroxyls in Mnt
The hydroxyls in Mnt are solid acids, and its content and stability determine its surface reactivity, especially for the hydroxyls at the layer edges. (45) As the hydroxyls are distributed in the layers and at the “broken” layer edges of NaMnt, fabricating more layer edges might be favorable to reveal how temperature affects the relative content of structural hydroxyls in NaMnt. Thus, in the present experiment, NaMnt and ball-milled NaMnt (NaMnt-BM) were tested using HT-FTIR in the temperature range 20–700 °C.
As shown in Figure 5A, the structural hydroxyls in NaMnt were detected around 3612–3638 cm–1 as an overlapped peak. Although the metal–OH stretching vibration was widely accepted, the interpretation of the overlapped peak remains controversial. Kuligiewicz et al. (46) reported that the peak at ∼3622 cm–1 is assigned to the stretching modes of structural hydroxyls in Mnt, but the peak at 3638 cm–1 cannot be resolved. By contrast, Yeşilbaş, et al. (47) thought that the band at ∼3622 cm–1 is ascribed to the water molecules bounded with siloxane groups in Mnt. Based on the adsorption at 3623 cm–1 in the HT-FTIR spectra of NaMnt at 600 °C, it is better ascribed to the stretching modes of structural hydroxyls in NaMnt as molecular waters have been removed at 600 °C. Kloprogge et al. (48) reported that the peak at 3638 cm–1 is attributed to the linkage of the hydrogen bonding to siloxane groups in rectorite. Kloprogge et al. (49) reported that the band at ∼3638 cm–1 is attributed to the metal–OH in the Mnt octahedral layer. Comparing NaMnt with NaMnt-BM in the HT-FTIR spectra in this work, it is found that the adsorption band at ∼3638 cm–1 is observed only at 20 °C for NaMnt-BM, while this band can be distinguished in the wide temperature range 20–500 °C for NaMnt. In this context, the band at ∼3638 cm–1 is the combination of structural hydroxyls and the linkage between interlayer water molecules and Si2O groups (Si2O–H2O) in NaMnt at 20 °C. In contrast, it is also most possibly related to the hydroxyls associated with metal–O in NaMnt when the temperature is higher than 100 °C.
The adsorption at 3623 cm–1 in the HT-FTIR spectrum of NaMnt at 600 °C (Figure 5Ag) might be assigned to the hydroxyls bonded with Al atoms, because the [AlO6] octahedron is more stable than other octahedrons. (50,51) The hypoactive vibration at 3612 cm–1 is most likely ascribed to those hydroxyls bonded with Mg and Fe atoms. They were removed when the temperature increased to 600 °C. On this basis, the whole band around 3612–3638 cm–1 is ascribed to the structural hydroxyl adsorption in NaMnt, resulting from the different strength and distance between hydrogen and oxygen atoms coordinated with different metal atoms (metal = Al, Fe, and Mg) when the temperature is higher than 100 °C.
In addition, the band at ∼3638 cm–1 decreased sharply in intensity at 100 °C for NaMnt-BM, indicating that the hydroxyls bonded with Fe and Mg were easier to remove in NaMnt-BM than in NaMnt (Figure 5B). The NaMnt-BM having been ball-milled, leading to the breaking and exfoliation of the layers, (29,30) implies that dehydration and dehydroxylation in small NaMnt particles are easier than in large ones as the temperature rises from 100 to 700 °C.
To reasonably calculate the relative content of hydroxyls in Mnt, the baseline in the 3520–3725 cm–1 region of HT-FTIR spectra of NaMnt and NaMnt-BM was first subtracted, and then, the integral areas of the adsorption peaks in this region were calculated to represent the content of hydroxyls. Since the water molecules were removed at 100 °C in the heating chamber, the adsorption peaks in the 3520–3725 cm–1 region of HT-FTIR spectra are attributed to structural hydroxyls of NaMnt. Thus, the relative content of hydroxyls in NaMnt and NaMnt-BM at 100 °C was defined as 100%, separately. Such values at each temperature were obtained (Table 1). When the temperature increases to 200 °C, the hydroxyl loss in NaMnt is 10.6%, while it is 13.8% in NaMnt-BM. Such hydroxyl loss could be ascribed to the removal of hydroxyls at “broken” layer edges of NaMnt.
Table 1. Relative Content of Hydroxyls in NaMnt and NaMnt-BM from 100 to 700 °Ca
Method for calculating hydroxyl content in Mnt: first, subtract the baseline in the 3520–3725 cm–1 region of HT-FTIR spectra of Mnt, and then, calculate the integral areas of the adsorption peaks in the region. Such calculated integral areas represent the content of hydroxyls in Mnt. NaMnt-BM: ball-milled NaMnt.
Since the water molecules were removed at 100 °C in the heating chamber, the integral area of the adsorption peak in the 3520–3725 cm–1 region of HT-FTIR spectra of NaMnt and NaMnt-BM at 100 °C was defined as 100%.
When the temperature increased to 300 °C, NaMnt and NaMnt-BM had an approximate relative content of hydroxyls of about 83%. With the temperature increased to 400, 500, and 600 °C, the relative content of hydroxyls in NaMnt-BM decreased by 5.0%, 12.9%, and 10.4% more than that in NaMnt, respectively. When the temperature of the NaMnt increased from 600 to 700 °C, the relative content of hydroxyls in NaMnt decreased from 51.2% to 6.8%, while it decreased from 40.9% to 4.3% in NaMnt-BM. Considering that no other types of chemical bonding formed after ball milling (no new chemical bond vibration appeared in the FTIR spectrum) and that the removed hydroxyls during heating are structural hydroxyls, the hydroxyl loss could be ascribed to the removal of hydroxyls at “broken” layer edges of NaMnt. This result suggests that more hydroxyls formed at the “broken” layer edges and layer surfaces of NaMnt-BM and confirms the easier loss of hydroxyls in small particles. This is in agreement with the report that the content of hydroxyls increased as the particle size decreased. (52)
As for the content of hydroxyls of NaMnt at temperatures from 0 to −150 °C, no significant change in hydroxyl content was observed (Figure 5D). Comparing the adsorption peaks in LT-FTIR spectra of NaMnt at 0 °C before and after dehydration at 200 °C (Figure 5Da,e), about 7% of the hydroxyl content in NaMnt is removed (calculated from the integral areas of the adsorption peaks). This result confirms that water molecules contribute to the formation of hydroxyls at the “broken” layer edges of NaMnt.
The LT-FTIR spectra of NaMnt in the 600–1000 cm–1 region and the magnifications of the selected regions are shown in Figure 6. To better study the hydroxyls at low temperature, the NaMnt was first heated to 200 °C and then tested from 0 to −150 °C at every 50 °C reduction in the temperature chamber. As a few oxygen atoms in Mnt sheets connect with hydrogen atoms forming hydroxyls, the change of hydroxyls on the temperature might affect the vibration strength of the strong covalent bonds.
Al2OH, FeAlOH, and MgAlOH are the three main types of vibrational bands related to hydroxyls in Mnt sheets (Figure 1BII–IV), with the wavenumber at 915, 886, and 843 cm–1, respectively. With the temperature decreased from 0 to −150 °C, no significant change in these three vibrational bands, including the strength and location, was observed (Figure 6B,C). In contrast, the NaMnt was tested from 0 to −150 °C after heating to 200 °C, and the vibrational intensity of the three bands decreased while the vibrational frequency was maintained. This indicates that the dipole moment between metal–O decreased. The reason might be that the heating of NaMnt to 200 °C caused the loss of the part of hydroxyls on those bonds. Thus, the distance of the metal–O decreased. Consequently, the decrease in the interatomic distance resulted in the decrease in dipole moment between metal–O. This result also confirms that the structural hydroxyls in Mnt could be partly removed at 200 °C. The crystalline shrinkage of Mnt is also related to the loss of hydroxyls when increasing temperature.
The AlOSi (Figure 1BV) vibrational band in Mnt was tested at around 626 cm–1. With the temperature decreased from 0 to −150 °C, the vibrational intensity of AlOSi in Mnt decreased (Figure 6Ea–d). A similar result was obtained for the NaMnt tested from 0 to −150 °C after heating to 200 °C (Figure 6 Ee–h). The decrease in temperature might have led to the reduction of the distance of atoms between Al–O–Si, thereby resulting in the decrease of the dipole moment. Quartz is an impurity in NaMnt. The Si–O vibrational bond of quartz was also detected (Figure 6C). It shows that its vibrational intensity and frequency of Si–O in quartz are independent of temperatures from 0 to −150 °C.
Effect of Temperature on Structural Silicon–Oxygen Groups in Mnt
Silicon–oxygen groups in Mnt were also studied by LT-FTIR and KBr-FTIR. Acid/alkali-etched Mnt was prepared and used as contrast samples. For the NaMnt at low temperatures, the two peaks at 1128 and 1003 cm–1, corresponding to the perpendicular and in-plane stretching vibration of Si–O, (53,54) respectively, are overlapped (Figure 7A), while they appear at 1093 and 1041 cm–1 for the NaMnt at 20 °C. With the temperature decrease from 0 to −150 °C, the strength of the in-plane stretching vibration of Si–O decreased. The strength of the in-plane stretching vibration of Si–O is higher than its perpendicular stretching vibration at 0 °C (Figure 7Aa), while they are close at −150 °C (Figure 7Ad). This phenomenon can be attributed to the decrease of the dipole moments in the Si–OH bond due to the decrease in hydroxyl strength. That is because the formation of hydroxyls in NaMnt is related to interlayer water molecules. (33,55,56) The decrease in temperature led to the breaking of Si2O–H2O bonds in NaMnt, further decreasing the dipole moments of Si–O.
However, for the NaMnt tested at −150 °C after removing the molecular water at 200 °C in the temperature chamber (Figure 7Ah), the in-plane stretching vibration of Si–O is still higher than its perpendicular stretching vibration. This result provides another piece of evidence for the linkage between interlayer water molecules and Si2O groups (Si2O–H2O).
For the chemical attack of Si–O in Mnt, the perpendicular Si–O stretching in NaMnt appears to be more sensitive to alkali than the in-plane stretching vibration, evidenced by the intensity of the perpendicular Si–O stretching vibrational band at 1093 cm–1 decreasing sharply for AMnt (Figure 7Bb). Another possibility for the weakening of absorption at 1093 cm–1 is the decrease of quartz content due to alkali attack, since such absorption is contributed by the perpendicular Si–O on the layers of AMnt and the Si–O in quartz. In addition, the in-plane stretching vibration of Si–O shows a red-shift from 1041 to 1038 cm–1, while it shows a blue shift from 1041 to 1045 cm–1 for HMnt (Figure 7C). This is a result of many factors, including the interaction between the siloxane groups and the interlayer hydrogen ion and the framework after the dissolution of ions from the octahedron and tetrahedron.
This work has revealed that upon temperature changes the chemical bondings in NaMnt, including those interlayer water molecules, the sructural hydroxyls and the silicon-oxygen groups, varied and the NaMnt underwent crystalline swelling or shrinkage. It is found that the hydroxyl content in NaMnt decreased when the temperature increased from 20 to 700 °C, while it is independent of temperature from 0 to −150 °C. The hydroxyls in NaMnt associated with Al2O are more stable than those with FeAlO and MgAlO in the octahedral sheet of NaMnt from 20 to 700 °C. Since the particle size of Mnt decreased after milling, more hydroxyls formed at the “broken” layer edges. Moreover, it is found that the formation of hydroxyls at the “broken” layer edges of NaMnt is related to adsorbed water molecules on the surfaces. The water molecules in the interlayer spaces could bond to the tetrahedral sheet of NaMnt through Si2O–H2O bonds, and those water molecules in NaMnt could be removed in a confined environment at 100 °C. A crystalline shrinkage of NaMnt occurs on heating from 20 to 700 °C, while a crystalline swelling occurs on cooling from 20 to −268 °C. These findings may be helpful to link the degree of hydrocracking and aromatization of soil organic matter with the relative content of hydroxyls and water molecules in Mnt during historical evolution, to design Mnt-derived materials with enhanced properties, and to study the structure of Mnt-based materials after high-/low-temperature service.
Chunhui Zhou - Research Group for Advanced Materials & Sustainable Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, 18 Chao Wang Road, Hangzhou 310014, China; Qing Yang Institute for Industrial Minerals, Industry Park, You Hua Township, Qingyang 242804, China; Engineering Research Center of Nonmetallic Minerals of Zhejiang Province, Zhejiang Institute of Geology and Mineral Resources, 58 Ti Yu Chang Road, Hangzhou 310007, China; https://orcid.org/0000-0001-5733-9011; Email: firstname.lastname@example.org
Cunjun Li - Research Group for Advanced Materials & Sustainable Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, 18 Chao Wang Road, Hangzhou 310014, China; College of Materials Science and Engineering, Guilin University of Technology, 12 Jian Gan Road, Guilin 541004, China; Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources, 12 Jian Gan Road, Guilin 541004, China; https://orcid.org/0000-0002-7519-7990
C.L.: Conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, software, visualization, and writing─original draft. Y.X.: Data curation and writing─review and editing. Y.Y.: Methodology and writing─review and editing. L.W.: Funding acquisition, resources, and writing─review and editing. C.Z.: Conceptualization, formal analysis, resources, supervision, writing–review and editing, and funding acquisition.
The authors declare no competing financial interest.
The work was supported by the National Natural Science Foundation of China (22072136; 42062003; 41672033), the Guangxi Natural Science Foundation (2019GXNSFBA245052), the Scientific Research and Technology Development Project of Fang Cheng Gang (Fangke AB21014002), the Engineering Research Center of Nonmetallic Minerals of Zhejiang Province (ZD2020K05), and the Projects from Qing Yang Institute for Industrial Minerals (KYY-HX-20220336; KYY-HX-20170557). The authors are grateful to Mrs. Liying Chen from State Key Laboratory of modern optical instruments, Zhejiang University, for her help in high-temperature FTIR characterization of Mnt.