Synthesis of Mg-Al Mixed Oxides with Markedly High Surface Areas from Layered Double Hydroxides with Organic Sulfonates.

Mg-Al mixed oxides with record-high surface areas and basic site concentrations were synthesized from Mg-Al layered double hydroxides with interlayer isethionate (Ise) or 3-hydroxy-1-propanesulfonate (HPS). Anion exchange of interlayer CO3 2- in synthetic hydrotalcites with the organic sulfonates induces disorders in layer stacking as characterized by powder X-ray diffraction and enables facile delamination in water. Thermal treatment of materials anion-exchanged by Ise (MgAl-Ise) and HPS (MgAl-HPS) in N2 and H2 resulted in the formation of Mg-Al mixed oxides with marked enhancement in Brunauer-Emmett-Teller (BET) surface area relative to those treated in air. Treatment in a flow of H2 is particularly effective, doubling the surface area of mixed oxides derived from MgAl-Ise relative to those obtained in a flow of N2. A higher degree of disorder in layer stacking in MgAl-HPS than MgAl-Ise resulted in the formation of Mg-Al mixed oxides with higher surface areas than those from MgAl-Ise. As a result, thermal activation of MgAl-HPS in a flow of H2 yielded Mg-Al mixed oxides with the highest BET surface area (410 m2 g-1) and CO2 uptake (1.6 mmol g-1 at 25 °C and 100 kPa) in all samples. These values are significantly higher than those obtained from the initial hydrotalcites as well as those reported in the literature with similar Mg-Al ratios. Investigation of the thermal activation steps by thermogravimetric analysis and mass spectrometry indicates that the key factors to achieve high surface area and CO2 uptake are to weaken interactions between layers by inducing stacking disorders and to facilitate the removal of interlayer sulfonates by preventing the formation of sulfates from them via thermal activation under a reducing environment.

Introduction

Delamination of lamellar materials forms nanosheets, which offers an opportunity to synthesize high-external-surface-area materials with highly accessible active sites.[1] A general challenge for this route is the restacking of nanosheets upon drying and calcination.[2] Layered double hydroxides (LDHs) consist of brucite-like positively charged layers and interlayer anions and possess the general formula of [M1–2+M3+(OH)2][A]·mH2O, where M2+, M3+, and A represent divalent and trivalent metal cations and interlayer anions, respectively.[3] These materials serve as precursors to materials used for a wide range of applications such as catalysts and adsorbents.[4−16] Anion exchanges of interlayer carbonates in Mg–Al LDHs with isethionate (HO(CH2)2SO3–, Ise)[17] and chlorides in Zn–Al LDHs with dodecyl sulfate (CH3(CH2)11SO4– for Zn–Al LDHs)[18] are known to allow facile delamination in water and butanol, respectively. However, calcination of the resulting solids in a flow of air to obtain mixed metal oxides converts interlayer organic sulfur species to inorganic sulfate species.[19] The interlayer sulfates graft on the surface of LDHs during calcination, bridge neighboring nanosheet layers, and form metal salts.[19,20] Consequently, the resultant mixed oxides possess only low porosity. In addition, the removal of such sulfate species requires heat treatment at temperatures ∼900 °C, where metal oxides inevitably sinter and transform into different phases such as spinel.

We have recently reported that thermal activation of Mg–Al LDH possessing Ise (MgAl–Ise) in a flow of N2 facilitates the removal of interlayer Ise and enhances the Brunauer–Emmett–Teller (BET) surface area of the resultant mixed oxides by up to 8 times relative to that treated in a flow of air.[21] In this work, we report two new findings: (1) anion exchange of synthetic hydrotalcite (MgAl–CO3) with 3-hydroxy-1-propanesulfonate (HO(CH2)3SO3–, HPS) induces a higher degree of disorder in layer stacking than that with Ise. (2) Thermal activation in a flow of H2 enhanced the BET surface area of Mg–Al mixed oxides derived from MgAl–LDHs containing organic sulfonates relative to those obtained using a N2 flow. Combining these two findings led to the successful synthesis of Mg–Al oxides with a record-high BET surface area of 410 m2 g–1 and a CO2 adsorption capacity of 1.6 mmol g–1 at 25 °C and 100 kPa. Both of these properties significantly exceed the corresponding values for Mg–Al mixed oxides obtained from conventional MgAl–CO3 used in this work as well as those reported in the literature.

Results and Discussion

Anion Exchange and Delamination

Anion exchange of MgAl–CO3 (scanning electron microscopy (SEM) image shown in Figure S1 in the Supporting Information) with Ise or HPS white powder solids. The powder X-ray diffraction (PXRD) pattern characterizing MgAl–CO3 (Figure ) exhibits sharp (00l) reflections characteristic of the layered double hydroxide phase. This diffraction pattern shows well-separated (110) and (113) peaks in the mid-2θ region, indicating the absence of turbostratic disorder.[22] After anion exchange by Ise and HPS, the (003) reflection shifted to lower 2θ angles, indicative of the expansion of the interlayer space, 1.1 nm for MgAl–Ise and 1.2 nm for MgAl–HPS, by accommodating larger molecules (∼0.7 nm for Ise and ∼0.8 nm for HPS) than CO32– (∼0.3 nm). The PXRD data characterizing both MgAl–Ise and MgAl–HPS show significantly broader (003) reflections and asymmetric 0kl reflections in the mid-2θ region with the extent being higher for the latter sample. These results indicate the turbostratic disorder of Mg–Al double hydroxide layers and higher extent of such disorder for MgAl–HPS than MgAl–Ise. The higher extent of disorder in MgAl–HPS may be caused by a longer alkyl chain length of HPS than Ise, which presumably weakens interactions between nanosheets in MgAl–HPS relative to those in MgAl–Ise. The small shoulder at a 2θ value of approximately 11° in the PXRD data for MgAl–HPS may be caused by unexchanged layers. The Fourier transform infrared (FT-IR) spectrum characterizing MgAl–CO3 (Figure ) shows a band at 1374 cm–1, corresponding to interlayer CO32–.[3] After anion exchange with Ise and HPS, the band nearly disappeared for Ise and the relative intensity became reduced for HPS. In addition, new intense bands appeared at approximately 1190 and 1042 cm–1, which are assigned to νas(SO3–) and νs(SO3–) of organic sulfonates, respectively.[17] A weak band at about 1420 cm–1 can be assigned to the bending mode of CH2.[17] These data indicate the near-complete anion exchange for MgAl–Ise and some unexchanged carbonates remaining in MgAl–HPS. Ion chromatography analysis show that sulfur contents in MgAl–Ise and MgAl–HPS were 8.56 and 5.45 wt %, respectively. Thermogravimetric (TG) analysis data in Figure show that water contents in MgAl–Ise and MgAl–HPS, which had been determined by the weight loss below 200 °C, were 2.5 and 6.2 wt %, respectively. On the basis of these results coupled with the IR data indicating a fraction of unexchanged CO32– in MgAl–HPS, the chemical formulae for these samples are represented as [Mg3Al(OH)8(HOC2H4SO3)](0.5H2O) for MgAl–Ise and [Mg3Al(OH)8(HOC3H6SO3)0.6(CO3)0.2](1.4H2O) for MgAl–HPS.

Figure 1

PXRD patterns for MgAl–CO3 (bottom), MgAl–Ise (middle), and MgAl–HPS (top).

Figure 2

FT-IR spectra for MgAl–CO3 (bottom), MgAl–Ise (middle), and MgAl–HPS (top).

Figure 3

TG profiles for MgAl–CO3 (bottom), MgAl–Ise (middle), and MgAl–HPS (top) recorded in flowing air or N2.

MgAl–LDH (Mg/Al atomic ratio = 3) bearing Ise is known to readily delaminate in water, yielding transparent colloidal suspensions that exhibit Tyndall effects.[17] Iyi et al. demonstrated the successful delamination of the MgAl–LDH layers by showing that solids recovered from such a suspension consisted of delaminated nanosheets approximately 2 nm in thickness through atomic force microscopy (AFM) characterization.[17] They showed further that dispersion of MgAl–LDH with an Mg/Al ratio of 2 in water yielded a suspension with lower transparency, which had been caused by fewer degrees of delamination (thicknesses of nanosheets were in the range of 2–6 nm as characterized by AFM). Dispersing our MgAl–Ise and MgAl–HPS samples, which had been prepared from MgAl–CO3 with an Mg/Al ratio of 3, in decarbonized water at 0.1 mg-solid (mL-water)−1 yielded similar transparent solutions with no visible solids and the solutions exhibited Tyndall effects (Figure S2 in the Supporting Information). Thus, we infer successful delamination of MgAl–LDHs layers in water.

Thermal Decomposition Steps of the Mg–Al LDHs Characterized by Thermogravimetric (TG) Analysis and Mass Spectrometry (MS)

TG analysis identifies several key steps in thermal decomposition of LDHs.[23] The data characterizing MgAl–CO3 heated in air (Figure , bottom) show weight losses associated with desorption of solvent molecules present on the external surface and gallery space at <200 °C,[24] dehydroxylation of the surface hydroxyls at 200–400 °C, and decomposition to mixed metal oxides at >400 °C. The TG data recorded in flowing air and N2 nearly overlap each other, indicating the absence of effects of the gas atmosphere. In contrast, the TG profiles characterizing MgAl–Ise and MgAl–HPS exhibit significantly different weight changes above 300 °C when they were heated in air or N2 (Figure , middle and top). The profiles of MgAl–Ise and MgAl–HPS heated in air show a steep weight loss at >900 °C, whereas those of MgAl–Ise and MgAl–HPS heated in N2 show the absence of such weight loss in this temperature region; the latter profiles show that the corresponding weight loss occurs at <600 °C. The TG profiles obtained in air and N2 resulted in the same final weight loss at 1200 °C, indicating the formation of the same final phase at this temperature. When MgAl–Ise and MgAl–HPS are heated in flowing air, reactions and 2 are assumed to proceed, both of which lead to the liberation of SO3,[25] respectively.The weight losses associated with these conversions are calculated as 54 and 52 wt %, respectively. These results are consistent with the TG experiments that show a weight loss of approximately 51 wt %.

It is known that interlayer sulfates in LDHs graft between layers and form metal sulfates upon calcination in air and that decomposition of metal sulfates and desorption of sulfur oxides occurs at ∼900 °C.[19] Thus, the results suggest that thermal treatment in N2 prevents the formation of sulfates from Ise and facilitates the removal of sulfur species at a relatively low temperature.[21] The TG profile for MgAl–HPS in N2 shows a much steeper weight loss at 400 °C than that of MgAl–Ise in N2, suggesting that the removal of HPS may be more facile than that of Ise because of the higher disorder in the structure of MgAl–HPS. Both profiles for MgAl–Ise and MgAl–HPS in N2 show only small weight losses above 600 °C, indicating that the majority of sulfur species can be removed at temperatures below 600 °C.

Because our TG instrument has a limited material capability to conduct experiments in H2 at elevated temperatures, similar thermal activation experiments were conducted in flowing 10% H2 in helium, air, or helium, which can be considered as equivalent to N2 in this experiment, in a quartz tubular reactor containing MgAl–Ise while the exhaust from the reactor was monitored by a mass spectrometer. The MS data recorded in flowing air (Figure ) show mass signals ascribed to sulfur oxides at 400, 600, and ∼1000 °C. In contrast, the MS data collected in flowing helium and 10% H2 in helium show mass signals only at 300–550 °C. In addition, the MS spectrum recorded in helium shows a small shoulder at approximately 500 °C, whereas that in 10% H2 in helium shows near absence of such a shoulder. The results confirm that removal of sulfur species is more facile in reducing gases like H2 or inert gases like helium and N2 than oxidizing gases like air, and that H2 facilitates removal of such species than an inert gas.

Figure 4

Mass spectra (m/z 64, SO2) of the effluent gases from the tubular reactor containing MgAl–Ise in flowing air (green), helium (blue), or 10% H2 in helium (red). The data were normalized with respect to the helium signal (m/z 4).

Grafting of Sulfates on LDH Layers Characterized by PXRD

Some papers[19,20] reported a larger contraction in basal spacing and smaller gallery height of LDHs possessing interlayer sulfates than those possessing other anions like CO32– when they were heated to 200 °C. Researchers attributed these differences to the grafting of sulfates on the surface of LDH layers. Because our TG data show different weight changes above 300 °C under different gas atmospheres, MgAl–Ise and MgAl–HPS samples were treated in a flow of air, N2, or 10% H2 in helium for 3 h and the resultant materials were characterized by PXRD. Samples treated in air (MgAl–Ise-A300 and MgAl–HPS-A300) show smaller basal spacings than other samples (Figure ). If the thickness of the brucite layer (4.77 Å) is subtracted from the basal spacing for MgAl–HPS-A300, a gallery height of 3.03 Å can be estimated. This value is much smaller than the size of a sulfate ion (4.8 Å), indicating that grafting of sulfates had occurred at 300 °C under an air flow. In contrast, the samples treated in N2 and H2 show larger gallery heights. In particular, MgAl–HPS treated in 10% H2 in helium (MgAl–HPS-H300) shows the largest gallery height in all samples. Subtraction of the thickness of the brucite layer from the basal spacing for this sample gives the gallery height of 4.8 Å, which is essentially the same as the size of a sulfate ion. These results indicate that thermal activation in H2 and N2 is effective to minimize irreversible grafting of sulfates on LDH layers, which presumably led to more facile removal of sulfur species in H2 and N2 than in air.

Figure 5

PXRD patterns for MgAl–Ise and MgAl–HPS treated at 300 °C for 3 h in a flow of air (green), N2 (blue), and 10% H2 in helium (red). The values show the basal spacings.

Porous Properties

Surface Area

After thermal activation above 300 °C, all LDHs were converted into mixed oxides (or a mixture of metal oxides and sulfates). The PXRD data for MgAl–CO3 and MgAl–Ise treated in air or N2 were reported previously[21] and those for MgAl–HPS are shown in Figure S3 in the Supporting Information. All of the PXRD data characterizing samples that were treated at 400–600 °C show two broad peaks that are indicative of periclase phase. However, the samples exhibit significantly different porous properties. N2 adsorption data show that MgAl–Ise calcined in air at 400 °C (MgAl–Ise-A400) possesses a low BET surface area (Figure d, BET plots are shown in Figure S4 in the Supporting Information). Replacing air with N2 during heat treatment significantly enhanced the surface area probably because a higher fraction of sulfur species was removed during heat treatment, as described in the preceding section. Using H2 instead of N2 during thermal activation led to a further increase in the surface area to approximately 350 m2 g–1. Increasing the duration of H2 treatment of MgAl–Ise from 3 to 6 h yielded Mg–Al mixed oxide (MgAl–Ise-H400-H400) that possesses nearly the same BET surface area (357 m2 g–1, adsorption isotherm shown in Figure S5 in the Supporting Information) as MgAl–Ise-H400, showing that treatment for longer than 3 h gives no enhancement in the surface area.

Figure 6

N2 adsorption isotherms collected at −196 °C (a–c) and BET surface area (d) characterizing samples thermally activated at 400 °C in flowing air (green), N2 (blue), and 10% H2 in helium (red). The solid and open symbols in (a–c) correspond to adsorption and desorption branches, respectively. The insets in (a–c) represent pore size distributions determined by applying the data in adsorption branch to the Dollimore–Heal method.

Calcination of MgAl–HPS in air yielded Mg–Al mixed oxides (MgAl–HPS-A400) with a BET surface area of 125 m2 g–1. This value is much higher than that for MgAl–Ise-A400, which may be enabled by a higher degree of stacking disorder in MgAl–HPS as characterized by PXRD (Figure ). Changing the gas atmosphere from air to N2 in the thermal activation resulted in a significant increase in BET surface area, which exceeds that of MgAl–Ise-H400. Furthermore, the sample obtained by the thermal activation in H2 (MgAl–HPS-H400) exhibited the highest BET surface area of 410 m2 g–1. This value is ∼150 m2 g–1 higher than mixed oxides derived from the original MgAl–CO3 and those reported in the literature[5,26−29] (Table ). Thus, the results demonstrate the significant effects of the combination of stacking disorder of Mg–Al double hydroxide nanosheets that had been induced by anion exchange by Ise and HPS, the different reactivity of these interlayer sulfonates in different gas atmosphere, and the proper conditions of thermal activation on the porous properties of Mg–Al mixed oxides.

Table 1

Comparison of BET Surface Areas with Literature Dataa

sample	anion	SBETb (m2 g–1)	qCO2c (mmol g–1)	remarks
MgAl–HPS-H400	HPS (HO(C2H4)3SO3–)	410	1.6	this work
MgAl–HPS-N400	HPS (HO(C2H4)3SO3–)	383	1.2	this work
MgAl–Ise-H400	Ise (HO(C2H4)2SO3–)	345	1.4	this work
MgAl–Ise-N400	Ise (HO(C2H4)2SO3–)	200	0.63	this work
MgAl–CO3-A400	CO32–	258	1.0	this work
MgAl–CO3-H400	CO32–	236	1.2	this work
MgAl–CO3-N400	CO32–	268	1.2	this work
ref (27)	C4H9O–	332		synthesized by a sol–gel method in 1-butanol
ref (28)	CO32–	277		synthesized by a microwave method
ref (26)	CO32–	<200		synthesized by a coprecipitation method
ref (29)	CO32–	143		synthesized by a coprecipitation method
ref (5)	CO32–	238		synthesized by a coprecipitation method

Data for Mg–Al mixed oxides with Mg/Al ratio of approximately 3 and thermally treated at either 400 or 500 °C.

BET surface area.

CO2 uptake at 25 °C and 100 kPa.

Porous Structure

Although understanding of thermal activation behavior of LDHs is important,[26] exact mechanism of the genesis of pores upon thermal activation of LDHs generally remains elusive and the porosity of thermally activated layered hydroxides is not well understood. Characterization by field emission scanning electron microscopy (FE-SEM) shows that MgAl–CO3-H400 retained the sand-rose morphology, whereas MgAl–Ise-H400 and MgAl–HPS-H400 lost such morphology and sheets seem to have curled and bent as shown in Figure S6 in the Supporting Information. These changes were caused presumably by stacking disorders of layers in the latter samples. The adsorption isotherm data characterizing samples obtained from MgAl–Ise and MgAl–HPS in N2 and H2 flow exhibit type-IV isotherms like those characterizing samples derived from MgAl–CO3 (Figure a–c), indicating that the samples are micro-mesoporous like many other Mg–Al oxides derived from LDHs. The data show type H3 hysteresis,[30] suggesting that slit-shaped pores formed between platelike particles. Because the difference in the shape of the adsorption isotherms resides mostly in the low-pressure region (relative pressure <0.1) and the mesopore size distributions are similar among samples except for MgAl–Ise-A400 (Figure , insets), we infer that the enhanced surface area originates mostly from the increase in the number of micropores. Because interlayer sulfonates caused the significant difference in porous properties, we infer that these micropores originate from spaces formed between nanosheets.

CO2 Adsorption

To examine potential application of mixed oxides derived by the current method to CO2 adsorbents, catalysts, and catalyst supports, CO2 uptakes were measured at 25 °C. Mg–Al mixed oxides derived from MgAl–CO3 show CO2 uptakes at 1.0–1.2 mmol g–1 (Figure and Table ). Both MgAl–Ise-A400 and MgAl–HPS-A400 exhibit lowered CO2 uptakes because of their low surface areas and the existence of sulfate species that block basic sites. MgAl–Ise-N400 and MgAl–HPS-N400 exhibit significantly higher CO2 uptakes than those obtained by calcination in air because of the higher fractions of the removal of sulfur species. However, areal CO2 densities, which were calculated by dividing each CO2 uptake by the corresponding BET surface area, were both 1.9-CO2 nm–2. This value is less than the value for MgAl–CO3-A400 (2.4-CO2 nm–2), suggesting the partial blocking of basic sites by sulfur species. In contrast, both MgAl–Ise-H400 and MgAl–HPS-H400 exhibit an areal CO2 density of 2.4-CO2 nm–2, demonstrating the beneficial effects of thermal activation in a reducing environment. The high CO2 uptakes by MgAl–HPS-H400 relative to those for mixed oxides derived from MgAl–CO3 suggest that some of the micropores created between nanosheets may be accessible more readily by CO2 (kinetic diameter = 3.3 Å) than N2 (kinetic diameter = 3.6 Å). MgAl–HPS-H400 exhibited the highest CO2 uptake because of its high surface area, suggesting prospective applications to catalysts, catalyst supports, and adsorbents.

Figure 7

(a) CO2 adsorption isotherms collected at 25 °C for mixed oxides derived from MgAl–CO3 (▲), MgAl–Ise (■), and MgAl–HPS (●) at 400 °C in flowing air (green), N2 (blue), and 10% H2 in helium (red) for 3 h. (b) CO2 uptakes by samples in (a) at 25 °C and 100 kPa.

Conclusions

In this work, a new strategy to synthesize Mg–Al mixed oxides with high surface area and concentration of basic sites is provided. Anion exchange of MgAl–CO3 with Ise and HPS caused expansion of interlayer with disordered stacking of Mg–Al double hydroxide nanosheets, and HPS caused a higher degree of disorder in layer stacking as indicated by the broader XRD reflections. The resultant materials, MgAl–Ise and MgAl–HPS, can be dispersed in water. However, subsequent drying and calcination in air yielded MgAl mixed oxides with low surface areas because of the decomposition of interlayer sulfonates to sulfates, which graft between layers and form metal sulfates as indicated by the PXRD data. TG data confirm that the removal of such sulfates requires temperatures ∼900 °C. In contrast, thermal treatments of MgAl–Ise and MgAl–HPS in N2 and H2 facilitate the removal of interlayer sulfur species at temperatures <600 °C. Consequently, treatments in N2 and H2 led to marked increases in BET surface area. Thermal activation in H2 is particularly effective, enabling 2-fold enhancement in BET surface area of Mg–Al mixed oxides relative to those obtained in flowing N2 in the case of MgAl–Ise. When the thermal activation was applied to MgAl–HPS, it yielded Mg–Al mixed oxides with the highest BET surface area and CO2 uptake at 25 °C and 100 kPa in all samples. Further tailoring of the nature of interlayer anions and thermal treatment conditions is anticipated to lead to enhance these properties, which may open new applications of these materials in the future.

Experimental Section

Materials

Hydrotalcite, MgAl–CO3 (Kyoward 500PL, Mg3Al(OH)8(CO3)0.5·mH2O,) was obtained from Kyowa Kagaku Kogyo Co., Ltd. An SEM image of this material is shown in Figure S1 in the Supporting Information, showing a sand-rose morphology. The chemical formula of MgAl–CO3 can be represented as [Mg3Al(OH)8(CO3)0.5·2.4H2O], as reported previously.[21] Isethionic acid ammonium salt and 3-hydroxy-1-propanesulfonic acid sodium salt were purchased from Sigma-Aldrich, Japan, and used as received. Ethanol (99.5%) and methanol (99.8%) were purchased from Wako Pure Chemical Industries, Ltd.

Anion Exchange of MgAl–CO3

MgAl–CO3 was anion-exchanged using ammonium salt of Ise according to the literature method.[17] MgAl–CO3 (400 mg) was dispersed in 140 mL of ethanol in a 300 mL flask. To this solution was added 380 mg of isethionic acid ammonium salt in 60 mL of ethanol, which corresponds to a isethionates/carbonates molar ratio of 2. The slurry was heated under a continuous flow of N2 at 70 °C. After the slurry was stirred for 2 h, the solids were collected by filtration and freeze-dried at a pressure of 0.3 hPa and at a temperature of −10 °C overnight. The anion-exchanged sample is abbreviated as MgAl–Ise. Anion-exchange experiments with HPS were conducted by the same procedure as that for MgAl–Ise except for using 440 mg of the HPS salts in methanol and heating the slurry at 60 °C for 3 h. The anion-exchanged sample is abbreviated as MgAl–HPS.

Thermal Activation Procedure

A photo of the experimental setup was provided in our previous study.[21] A MgAl–LDH sample (≈100 mg) was loaded on a ceramic boat. The ceramic boat was loaded in a quartz tube reactor (2.5 cm diameter and 65 cm length), and each end of the tube was fitted with an Ultra-Torr-type fitting that is connected to a gas line and exhaust line. MgAl–LDH samples were heated to 300–600 °C at a ramp rate of 20 °C min–1 in flowing air, N2, or 10% H2 in helium at a flow rate of 100 mL min–1 and held at the designated temperature for 3 h. Then, the reactor was allowed to cool to ambient temperatures. In some experiments, samples were treated again at the same temperature. Samples are identified by a code of the form MgAl-Y-Zx-Zx or MgAl-Y-Zx, where x, Y, and Z stand for the temperature of thermal treatment, type of anions (CO3, Ise, or HPS), and type of gases (A = air and N = N2, H = H2), respectively. For example, MgAl–Ise heated at 400 °C sequentially in flowing H2 for 3 h and then in flowing H2 for an additional 3 h is denoted as MgAl–Ise-H400-H400. On the other hand, when the same sample was heated only in flowing H2 at the same temperature for 3 h, it is denoted as MgAl–Ise-H400.

Characterization

Elemental analysis for S was performed at the Global Facility Center of Hokkaido University, using a Dionex ICS1600 ion chromatography system. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku RINT Ultima IV equipped with a Cu Kα source and a D/teX Ultra detector. Data were recorded from 5 to 80° 2θ at a scan rate of 10° min–1. IR spectra of solid samples were collected on a JASCO Fourier transform spectrometer (FT/IR-6100) equipped with an MCT detector cooled to 77 K by liquid N2. Powder samples were pressed between KBr plates using a JASCO engineering Tablet Master series and loaded into the sample chamber of the instrument. Data were recorded under vacuum in transmission mode with a spectral resolution of 4 cm–1. Each spectrum is the average of 254 scans. Sample morphology was characterized by a JEOL field emission scanning electron microscope (FE-SEM, JSM-6500F) at an acceleration voltage of 10 kV. Nitrogen sorption isotherms were measured at −196 °C on a MicrotracBEL BELSORP-max. Prior to analysis, a powder sample (∼40 mg) in a preweighed analysis tube was heated under dynamic vacuum at 350 °C. After 2 h of heating, the analysis tubes were cooled to ambient temperatures, filled with inert gas, and capped to prevent the intrusion of air and moisture during transfer. The analysis tubes containing activated samples were transferred to an electrical balance and weighed to determine the mass of the samples. The tubes were transferred back to the analysis port of the instrument and sorption isotherms were collected using N2 gas (99.999% purity, Hokkaido Air Water) with an initial dosing amount of 0.1 cm3 (STP) g–1. Specific surface areas were calculated using the BET model,[31] selecting initial low-pressure points in the relative pressure range below 0.3 and assuming the cross-sectional area of a N2 molecule to be 0.162 nm2. Pore size distributions were determined by applying data in the adsorption branch to the Dollimore–Heal method.[32] CO2 adsorption isotherms were collected on a MicrotracBEL BELSORP-max at 25 °C. Samples were pretreated under dynamic vacuum at 350 °C for 2 h prior to adsorption experiments. To analyze the thermal activation steps of MgAl–LDH samples, thermogravimetric (TG) experiments were performed on a NETZSCH STA 2500 Regulus thermogravimetric analyzer. Approximately 10 mg of a sample was heated to 1200 °C in N2 or air flowing at a rate of 20 mL min–1. Analysis of thermal activation steps was also conducted using a tubular reactor. Approximately 50 mg of a sample was held on a plug of quartz wool within a quartz tubular reactor (10 mm I.D.) placed in a resistively heated furnace (Asahi Rika, ARF-30K). Samples were heated to 1000 °C at a ramp rate of 20 °C min–1 in 10% H2 in helium balance, air, or helium flowing at a flow rate of 30 mL min–1, while the effluent gas was analyzed by a MicrotracBEL BELMass quadrupole mass spectrometer.