Arrangement Models of Keggin-Al30 and Keggin-Al13 in the Interlayer

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Arrangement Models of Keggin-Al and Keggin-Al in Interlayer of Montmorillonite and the Impacts of Pillaring on Surface Acidity#A Comparative Study on Catalytic Oxidation of Toluene Ke Wen, Jianxi Zhu, Hanlin Chen, Lingya Ma, Hongmei Liu, Runliang Zhu, Yunfei Xi, and Hongping He Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03447 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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Arrangement Models of Keggin-Al30 and Keggin-Al13 in Interlayer of Montmorillonite and the Impacts of Pillaring on Surface Acidity：A Comparative Study on Catalytic Oxidation of Toluene Ke Wen a, b, #, Jianxi Zhu a, *, Hanlin Chen a, b, Lingya Ma a, Hongmei Liu a, Runliang Zhu a, Yunfei Xi c, *, Hongping He a, b a

CAS Key Laboratory of Mineralogy and Metallogeny, Guangdong Provincial Key Laboratory of

Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Guangzhou, 510640, China b

University of Chinese Academy of Sciences, Yuquan Road, Beijing 100049, China

c

School of Earth, Environmental and Biological Sciences, Queensland University of Technology

(QUT), Brisbane, Queensland, 4001, Australia Email addresses Ke Wen, [email protected] Jianxi Zhu, [email protected] Hanlin Chen, [email protected] Lingya Ma, [email protected] Hongmei Liu, [email protected] Runliang Zhu, [email protected] Yunfei Xi, [email protected] Hongping He, [email protected] Abstract Acid–base reactivity is a key factor for understanding the interfacial geochemistry of clay minerals. Numerous studies showed the significant role of surface acidity of clay minerals in the geological processes and environmentally related applications. In this work, montmorillonite (Mt) was pillared by polycations of Keggin-Al13 and Keggin-Al30, respectively. Arrangement models of Keggin-Al13 and Keggin-Al30 in the interlayer region of Mt were put forward based on the chemical composition analysis, structural formula calculation of Mt, and the results of powder X-

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ray diffraction. Ammonia temperature-programmed desorption and diffuse reflectance Fourier transform infrared method were applied to explore the impacts of pillaring by polycations (KegginAl13 and Keggin-Al30) on the surface acidic characteristics of Mt. Results demonstrated that one Keggin-Al30 polycation can affect an area of 9.5 unit cells (from two layers, with 4.7 – 4.8 unit cells in each layer) in Mt, while a Keggin-Al13 polycation controls an area of 7.1 unit cells (from two layers, with 3.5 – 3.6 unit cells in each layer). Pillaring by polycations could lead to a lot of surface acid sites (1.33 mmol NH3/g) on Mt with the main type of Bronsted acid sites. The increase of surface acid sites on both Keggin-Al13 pillared Mt (Al13-PILM) and Keggin-Al30 pillared Mt (Al30-PILM) are attributed to the high positive charge and high content of aluminum per unit of polycation, which affect the formation of Bronsted acid sites and structural changes of Mt layers. Catalytic oxidation of toluene provided evidences for the high catalytic activity of Al30-PILM under much lower temperature at 78°C compared with that of Al13-PILM and Mt at 207°C and 285°C, respectively. The basic finding in this study not only reveals the possible sources of abundant micropores and mesopores in the micro/mesoporous materials of Al13-PILM and Al30PILM, but also provides a reasonable mechanism for the formation of abundant Bronsted surface acid sites on these two types of pillared materials. The novel Al30-PILM with excellent micro/mesoporous structure and extremely high thermal stability also exhibits a potential ability in the application of heterogeneous acid catalysis. INTRODUCTION As the ubiquitous components in soils and sediments, clay minerals dominate numerous geochemical and environmental processes

1, 2.

They affect the fixation and migration of toxic

metals, organic matters, and trace elements in environments, due to their layered structures and excellent surface properties

3, 4, 5.

Recent studies have indicated that clay minerals also play

important roles in geological and biological events, such as the generation of petroleum 6, storage and transfer of natural organic matters 7, and the origin of life 8. Surface acidity of clay minerals is the key property in these processes, which has been attracting increasing research interests of both mineralogists and material researchers in the past decades 9, 10, 11, 12. Numerous studies have shown the important role of surface acidity of clay minerals in the geological processes and the applications as environmental-friendly heterogeneous acid catalysts 13, 14, 15, 16, 17, 18. Many factors can affect the surface properties of clay minerals, including chemical


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composition, nature of the surface atoms (mainly oxygen and hydrogen), extent and type of charge, and the type of exchangeable cations 19. Montmorillonite (Mt) is a kind of 2:1 clay mineral with the typical sandwiched (or called as layered) structure, which is composed with one octahedral layer and two tetrahedral layers (as shown in Figure 1). Layers of Mt are negatively charged due to isomorphic substitution, which are electrically balanced by equally exchangeable cations (such as Na+ and Ca2+) in the interlayer spaces 20, 21. The basal surface atoms of Mt are the oxygen atoms of the Si tetrahedron, which lead to the ability of Mt to donate protons (its surface acidity) 19. This ability causes the protonation of organic compounds and catalyzes many organic reactions 14, 22, 23, 24.

Mt is characterized with solid acidity in its structure and simultaneously includes both Bronsted

and Lewis acid sites (shown as Figure 2) 25. It has been established that the exchangeable cations in the interlayer space of Mt intensively affect the ability of Mt to donate protons to the adsorbed molecules 19, 22, 26, 27. Since 1970s, a new class of porous catalysts, named as the intercalated pillared clays (PILCs), have received a great deal of attention for the catalytic reactions, which are of primary interests in petroleum processing 28. The intercalation of large polycations and the subsequent calcination of the intercalated solids give rise to stable structures with constant interlayer space up to high temperatures. The obtained solids with the adequate porosity and high thermal stability are favorable in catalysis applications

29.

Polycations of Keggin-Al13 (AlO4Al12(OH)24(H2O)127+)

formed by the base hydrolysis of Al3+ aqueous solutions, was used as intercalants to prepare the PILCs 30. From then on, researchers have tried different polycations as intercalants to get PILCs with enhanced acid acidity and catalyst reactivity 31, 32, 33, 34, 35. Bradley et al. 36 compared the acidity and reactivity of Ga13, Al13, GaAl12, and Chromium pillared Mt, and found that the Bronsted acidic characters for PILCs are dependent on the pillars and the calcination temperature. Binitha and Sugunan 37 synthesized a Ti-PILC and evaluated its catalytic activity with the ethylation reaction of benzene to ethylbenzene. Based on the PILC with pure polynuclear species of Ti and Zr, Bahranowski et al.

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reported the [Ti, Zr]-PILC with a

significant population of Lewis acid sites which are much stronger than Ti-PILC and Zr-PILC. González-Rodríguez et al. 39 found that doping with Fe and Cu in the structure of polycations of Ti could increase the acidity of PILCs, while in the Cu-doped solid it rapidly decreases with the increasing of temperature. Liang et al.

40

studied the performance of Ti-PILC supported Fe

catalysts for toluene oxidation, and found that the introduction of Fe3+ on the surface of Ti-PILC


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didn’t obviously change its delaminated structure and porosity but enhanced the catalytic activity with the increase of Fe content (below 3.75%). In our previous work, a novel kind of pillared clay (Keggin-Al30 pillared Mt, Al30-PILM) was prepared by intercalating the Keggin-Al30 polycations (Al2O8Al28(OH)56(H2O)2418+) into the interlayer spaces of Mt 41. This Al30-PILM is established with high thermal stability even after calcination at 800˚C 42. In this work, NH3 temperature-programmed desorption (NH3-TPD) and NH3-adsorbed diffuse reflectance Fourier transform infrared (DR-FTIR) were applied to explore the effects of pillaring with polycations of Keggin-Al13 and Keggin-Al30 onto the surface acidity of Mt. Mechanisms were discussed together with the results of chemical components analysis and chemical reactions under high temperature. Based on the chemical composition analysis, calculation of structural formula of Mt, and together with the results of powder X-ray diffraction, possible arrangement models of Keggin-Al13 and Keggin-Al30 in the interlayer of Mt were revealed. Results of NH3-TPD analysis and NH3-adsorbed DR-FTIR measurement have provided evidences for the increasing of surface acid sites on pillared products. Deep oxidation of toluene experiment was conducted to evaluate the catalytic activities of Mt with/without pillaring by polycations (Keggin-Al13 and Keggin-Al30). The aim of this study is to gain insights into the relationship between the polycations of aluminum in interlayer spaces of Mt and its surface acidic properties. EXPERIMENTAL Materials Calcium-montmorillonite (Mt) was obtained from Inner Mongolia, China. This sample has a very high purity above 95%, only a small amount of quartz was identified as the impurity by the powder X-ray diffraction (XRD) measurement. The main chemical composition (wt%) of Mt was investigated by X-ray fluorescence (XRF), the results are listed in Table 1. Cation exchange capacity (CEC) of this Mt was determined by the adsorption quantity of [Co(NH3)6]3+ 43, with a value at 110.5 cmol/100kg 44. Raw Mt was used directly without further purification, because the extreme small amount of impurity (quartz) has little impacts on the results. Aluminum chloride (AlCl3·6H2O), sodium hydroxide and silver nitrate (AgNO3) were purchased from Guangzhou Chemical Reagent Factory. All chemicals are of analytical grade and used as received. Distilled water (18.2 MΩ) from an Ultra-Pure Water Purifier was used for preparing solutions and washing samples.


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Preparation of the pillared products Keggin-Al13 intercalating solution (Al13-INTS): Similar to a procedure reported in our previous work

41,

an Al13 intercalating solution was prepared by dropwise addition of a 0.6 M

NaOH solution into a 1.0 M AlCl3 solution at a rate of 1 mL/min, under vigorous stirring using a water bath at 60°C to get a final [OH-]/[Al3+] molar ratio of 2.4. The concentration of Al in the obtained solution is 0.2 M. Subsequently, the mixture was continuously stirred for another 12 hours, and “aged” for 24 hours at 60°C. Keggin-Al30 intercalating solution (Al30-INTS): Under constant stirring, a 0.6 M of NaOH solution was slowly added into a 1.0 M AlCl3 solution at a rate of 1 mL/min using an oil bath at 95°C to obtain a final molar ratio [OH-]/[Al3+] = 2.4. The resulting solution was stirred at 95°C for another 12 hours, then a further “aging” at 95°C was applied for a day. The intercalation of Mt by Keggin-Al13 and Keggin-Al30 polycations was carried out through ion exchange reaction with an Al/clay mole ratio of 4.0 mmol/g. Under vigorous stirring, Mt powder was dispersed into Al13-INTS at 60°C and Al30-INTS at 95°C, respectively. Two suspensions were stirred continuously for 24 hours and then aged for 24 hours at 60°C and 95°C, respectively. After cooled down to the room temperature, the products were separated by filtration and washed with distilled water until the supernatant solution was free of chloride as indicated by AgNO3 solution. The solid products were freeze-dried for 48 hours and denoted as Al13-INTM and Al30-INTM, respectively. Keggin-Al13 pillared interlayered Mt (Al13-PILM) and Keggin-Al30 pillared interlayered Mt (Al30-PILM) samples were obtained after calcination of the intercalated products at 300°C for 2 hours. Characterization methods The main chemical composition (wt%, shown as elemental oxides) of Mt and the pillared products were analyzed using a Rigaku RIX 2000 X-ray fluorescence spectrometer (XRF) on fused glass beads. Calibration lines used in quantification were produced by bivariate regression of data from 36 reference materials encompassing a wide range of silicate composition, and analytical uncertainties are mostly between 1 and 5%. Acidic characteristics of these samples were studied by the ammonia temperatureprogrammed desorption (NH3-TPD) method. It was carried out in a quartz tube reactor, using a


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chemisorption analyzer equipped with a TCD detector (mass spectrometry, Hiden QIC-20). About 50 mg sample was loaded in a reactor and pretreated at 300 °C in Ar flow for 1 h. The sample was cooled to 50 °C under Ar flow, and then pure NH3 adsorption was performed until equilibrium. Subsequently, the sample was introduced to Ar flow for removing physically adsorbed ammonia. Then the TPD analysis was carried out from 50°C to 900°C at 10 °C/min under Ar. The quantitative analysis was corrected by predefining the peak area of known volume NH3. NH3 adsorption was performed at room temperature. The mixture of about 0.9 mg of solid sample and 80 mg KBr was pressed into a thin pellet and placed on the bottom of a ceramic crucible and evacuated for 1 h. NH3 was then slowly introduced and kept airtight for 12 h. After that, samples were heated at 120°C for 1 h under vacuum to remove physically adsorbed NH3. The resulting NH3 adsorbed samples were denoted as Mt/NH3, Al13-PILM/NH3, and Al30-PILM/NH3. Diffuse reflectance Fourier transform infrared spectroscopy (DR-FTIR) were obtained using the KBr pressed pellet method with a Bruker VERTEX 70 Fourier transform infrared spectrometer. The KBr pellets were prepared by pressing mixtures of 0.9 mg of sample powder and 80 mg of KBr. All spectra were collected at room temperature using 64 scans in the range of 4000–400 cm−1 with a resolution of 4 cm−1. The measurement lasted for 2 min (from the sample loading to the spectrum recording) and the ambient relative humidity was ~ 30%. Baseline correction was performed by using the OPUSTM 6.5 software (Bruker Optik, Ettingen, Germany). The band component analysis was conducted using a Gauss–Lorentz cross-product function applied by PEAKFIT software package. The minimum number of component bands was obtained with squared correlations ≥0.995. Complete oxidation experiments of toluene were performed in a conventional fixed-bed reactor in the temperature range of 100 – 700 °C under atmospheric pressure. 200.0 mg of catalyst was loaded in a quartz tube reactor (i.d., 6 mm) supported by a porous quartz plate. Gaseous toluene was generated by flowing N2 into liquid toluene. For catalytic activity evaluation, the inlet gas composes of 1000 ppm toluene and 20 vol% oxygen balanced by N2. The total flow rate was 100 mL min-1, corresponding to gas hourly space velocity (GHSV) of 30 000 cm3 g-1 h-1. To investigate the resistance to moisture, water was introduced into the feed gas with a syringe pump. The oxidation of toluene can be expressed by the generation of CO2 and calculated as (C/C0) × 100%, where C0 is the theoretical concentration of CO2 when toluene in the inlet gas was completely oxidized. Concentration of CO2 in outlet gas was analyzed in the steady state at certain


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temperature by a non-dispersive infrared CO2 analyzer (Beijing Huayun GXH-3010E). Before the detection of CO2, the water in outlet gas was removed by concentrated sulfuric acid. RESULTS AND DISCUSSION Analysis of the main chemical composition Based on the results of chemical composition of Mt in Table 1, the complete chemical structural formula of Mt can be calculated as (process of calculation is shown in Table 2): (Ca0.381, Na0.185, K0.017, Mg0.036)[Si7.960, Al0.040]IV(Al2.714, Mg0.996, Fe0.290)VIO20(OH)4. The charge per formula unit, X, is the net negative charge per layer, expressed as a positive number 45. And the negative layer charge resulting from the isomorphism substitution of Al by Mg and Fe in the octahedral sheets, can be compensated by the positively charged cations in the interlayer. This represents an important factor affecting both hydration and cation speciation in the interlayer space of Mt. On the basis of the obtained chemical structural formula, the net charge of Mt is 1.036 per unit cell, which is in accordance with the value reported in literature 45. In other words, the amount of charge per unit area is 2.254 per square nm, since the structural parameter is a = 0.517 nm, b = 0.889 nm, and c = 10.3 nm with a C2/m layered symmetry 46. Noticing the contents of Al and Si in Mt are 15.93% and 54.28% (after deduction of quartz impurity content), thus, the ratio of Al/Si in the raw Mt is 0.29. Accordingly, calculations of the amounts of Al inserted into Mt were done and the results are shown in Table 3. Calculations on the number of unit cells occupied and charges needed per polycation were undertaken based on 1000 g of Mt (Table 4). Molar mass of the unit cell in Mt was calculated according to the chemical structural formula above. As reported in our previous work, the d001 values of Al13-PILM and Al30PILM are 1.65 nm and 1.86 nm respectively 41. Therefore, models for the structures of Al13-PILM and Al30-PILM could be speculated in Figure 3, with considering the particle size of polycations (approximate values: Keggin-Al13, 0.9×0.9×0.9 nm3; Keggin-Al30, 0.9×1.2×2.0 nm3). Obviously, 9.5 unit cells (from two layers, with 4.7 – 4.8 unit cells in each layer) of Mt can be affected by one Keggin-Al30 polycation (Figure 3B and D), while 7.1 unit cells (from two layers, with 3.5 – 3.6 unit cells in each layer) hold one Keggin-Al13 in the interlayer of Mt (Figure 3A and C). This arrangements of polycations in the interlayers of Mt cause the differences on the amounts of micropores and mesopores, and the surface area between Al13-PILM and Al30-PILM. In other words, the arrangement models of Keggin-Al30 in the interlayer of Mt is the source of large


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amounts of micropores and big surface area of Al30-PILM, when compared with Al13-PILM. This arrangement model responds very well to the results of N2 adsorption-desorption in our previous study 41, 42. Acidic characteristics TPD of adsorbed ammonia is a reliable method to feature the strength but not the nature (Bronsted or Lewis) of surface acid sites in solid acid catalysts 47. Figure 4 shows the NH3-TPD spectra of the initial Mt (A), Al13-PILM (B), and Al30-PILM (C). Four types of desorption peaks were shown in the temperature range of 50 – 900°C, which could be assigned to the weak acid sites (150 – 250°C), moderate acid sites (250 – 350°C), strong acid sites (350 – 450°C), and very strong acid sites (> 450°C), respectively 40. Different temperatures for the desorption of NH3 reflect different relative strengths of acid sites. The amount of desorbed ammonia (Table 5) can be determined from the area under the TPD curve, corresponding to the adsorbed ammonia on Lewis and Bronsted acid sites 48. Differences in acid sites concentration and relative strength between Mt and the pillared products can be seen from Figure 4 and Table 5. After pillaring with polycations of Keggin-Al13 or Keggin-Al30, total amounts of acid sites on Mt were greatly increased, especially for the weak acid sites (NH3 per gram increased from 56 μmol in Mt to 173 μmol in Al13-PILM and 956 μmol in Al30-PILM) and very strong acid sites (NH3 per gram increased from 66 μmol in Mt to 213 μmol in Al13-PILM and 378 μmol in Al30-PILM). For Mt, it can be speculated that ammonia could be “locked” in the space of collapsed layers when temperature is high enough. This “locked” ammonia is the reason for the occurring of very strong acid peak at 686°C for the raw Mt. Therefore, enhancement of the structure thermal stability give rise to the decrease of temperature for ammonia desorption in the region of >450°C. Amounts of the weak acid sites were significantly increased, which is caused by the increase of aluminum quantities. Aluminum ions in the interlayer of Mt could engender weak and moderate acid sites in the form of the OH groups bonded to the pillars Al ions (Al-OH)

49.

According to the calculation results of amounts of aluminum inserted into interlayers of Mt, the larger amounts of aluminum were intercalated into the interlayers of Mt for Al30-PILM. And that is the reason why the most abundant weak acid sites were detected in the sample of Al30-PILM (956 μmol NH3 per gram). All in all, pillaring with Keggin-Al13 and Keggin-Al30 into interlayers


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of Mt results different ratio of Al/Si, which significantly affects the surface acidic characteristics of Mt. Inserting Keggin-Al30 polycations into interlayers of Mt can lead to more acid sites and more strong acid on the surface of Mt. Differences in the amount of acid sites on Mt, Al13-PILM, and Al30-PILM can also be tracked from the NH3-adsorption DR-FTIR results (Figure 5, Table 6, Figure 6 and Table 7). Intensities of OH stretching (3416 cm−1, Figure 5A) and bending (1638 cm−1, Figure 5A) vibrational bands of adsorbed water in Al13-PILM/NH3 and Al30-PILM/NH3 are much weaker than those in Mt/NH3. It indicated the adsorbed water in (the surface, interlayer space or edge sites of) Al13-PILM and Al30-PILM was desorbed by NH3 adsorption. Noticeable, the structural Si-O-Si stretching bands (Figure 5A) in Al13-PILM/NH3 and Al30-PILM/NH3 slightly shifted to the left (with a higher wavenumber), which was caused by the pillaring with polycations of Keggin-Al13 and Keggin-Al30. It is because of the presence of polycations (Keggin-Al13 and/or Keggin-Al30) with the high positive charge and large particle size, which can impose a very strong electrostatic attracting force to the surficial O atoms and lengthen the bond distance of Si-O. Si-O-Si stretching bands shift to a higher wavenumber since the bond length increases, can result in a lower vibrational stretching frequency. The adsorbed NH3 molecules can not only replace molecules (in the interlayer space or on the surfaces), but also react with the water molecules to form NH4+ ions or react with the structural Lewis and Bronsted acid sites. Ammonia molecules on Lewis acid sites, NH4+ ions, and the hydrogen-bound ammonia can coexist in the interlayer spaces of Mt, Al13-PILM, and Al30-PILM 50, 51, 52, 53.

These three types of corresponding bands are always overlapped with each other around

1635 cm-1 and 1400 cm-1 26, 54, 55. After pillaring by polycations of Keggin-Al13 and Keggin-Al30, the amount of adsorbed ammonia increased (Figure 5B), particularly by Keggin-Al30. In order to quantificationally figure out each component of adsorbed ammonia on three samples, PEAKFIT 4.12 software was used to do the deconvolution and simulation on DR-FTIR spectra of NH3-adsorbed samples of Mt, Al13-PILM and Al30-PILM, and the results were shown in Figure 6 and Table 7. According to previous studies 24, 25, 56, three vibrational bands at the range 1620 – 1710 cm-1 are attributed to the vibrational mode of L/NH3 (~1630 cm-1), hydrogen-bound NH3 (~1650 cm-1), and NH4+ ion species (~1700 cm-1). Band at around 1400 cm-1 is the characteristic of NH4+ species. Bands at wavenumber higher and lower than 1400 cm-1 at the 1350 – 1550 cm-1 region are corresponding to the CO32- stretching mode of calcite and the vibration of


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NH3 coordinated to the of polycations through surficial O atoms. Since Keggin-Al30 possesses much more positive charge (18) than Keggin-Al13 (7), energy needed for NH3 coordinating to the surface of Keggin-Al13 to compensate the repulsive force is much lower than that of Keggin-Al30. Thus, the vibration of NH3 coordinated to Keggin-Al30 is relatively weaker than that of KegginAl13, showing a band at a higher wavenumber of 1478 cm-1 (Figure 6C) than 1426 cm-1 of KegginAl13 (Figure 6B). In comparison with the area of each component in NH3-adsorbed DR-FTIR spectra of Mt and its pillared products (Table 7), Al13-PILM and Al30-PILM adsorbed more NH3 molecules under the same treatment conditions. These adsorbed ammonia molecules performed as NH4+ species in the interlayer space (~1700 cm-1), on the surface of polycations or Mt layers, and edge sites of Mt. Some of them can also exist in the interlayer space through hydrogen bonding. The relative areas of NH4+ and L/NH3 bands (denoted as 𝐴𝑁𝐻4+ and 𝐴𝐿/𝑁𝐻3) represented the relative amounts of Bronsted and Lewis acid sites, respectively. Then the amount of Bronsted acid sites (QB) and Lewis acid sites (QL) in each sample can be obtained by equations (1) and (2), based on the semiquantitative calculation using the DR-FTIR bands (Figure 6 and Table 7). Results were shown in Figure 7 and Table 8. Obviously, after pillaring with polycations of Keggin-Al13 and Keggin-Al30, the amount of total acid sites of Mt increased dramatically, especially the amount of Bronsted acid sites. Two reasons contributed to this change on the amount of acid sites. One is the extremely high positive charge on polycations of Keggin-Al13 and Keggin-Al30, which caused the polarization of water molecules and consequently donated protons to ammonia easily. The other reason is because, polycations of Keggin-Al13 and Keggin-Al30 contain plenty of AlO6 octahedral units on the outsides. Polarized water molecules (H3O+ ions) associated with these negatively charged AlO6 octahedral also represents Bronsted acid sites and can still donate protons even after the removal of H2O molecules 57. Besides, pillaring by Keggin-Al30 polycations impose a more significant impact than that of Keggin-Al13 on the amount of acid sites, particularly for Bronsted acid sites. This can be attributed to the high positive charge (18) and Al components (30 atoms) in one unit of Keggin-Al30, higher than those of Keggin-Al13, with 7 positive charge and 13 aluminum atoms in one unit of Keggin-Al13. 𝑄𝐵 = 𝑄𝑇 × 𝐴 𝑄𝐿 = 𝑄𝑇 × 𝐴

𝐴𝑁𝐻 + 4

𝑁𝐻4+

+ 𝐴𝐿/𝑁𝐻3

𝐴𝐿/𝑁𝐻3 𝑁𝐻4+

+ 𝐴𝐿/𝑁𝐻3

(1), (2),


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QT represents the total amount of acid sites obtained from the NH3-TPD measurement results. Catalytic performance for toluene oxidation Catalytic properties of Mt and its pillared products are evaluated by the oxidation of toluene. To check whether toluene is decomposed spontaneously, a blank test is carried out without any catalyst, where no generation of CO2 is observed. Toluene conversion % to CO2 for Mt, Al13-PILM and Al30-PILM in the temperature range of 100 – 700°C are shown in Figure 8. Temperatures of 10% (T10), 50% (T50), and 90% (T90) toluene conversion (CO2 generation) are summarized in Table 9, together with the temperatures for detectable CO2. Both pillaring by Keggin-Al13 and Keggin-Al30 could lower the temperature of toluene catalytic oxidation (Figure 8B), with the temperature for detectable CO2 at 78°C (Al30-PILM) and 207°C (Al13-PILM) respectively, significantly lower than that of Mt at 285°C (Table 9). It is consistent with the NH3-TPD results (Figure 4 and Table 5). Intercalation of Keggin-Al30 in interlayer of Mt could induce a higher aluminum percentage, which increases the weak and moderate acid sites. These weak acid sites formed in Al30-PILM make the catalytic oxidation of toluene much easier under a relative low temperature (78°C) when compared with Al13-PILM and Mt. The catalytic activity enhances with pillaring by Keggin-Al13 and Keggin-Al30, because of the increase of acid sites (type: very strong acid sites) in Al13-PILM and Al30-PILM. These increased acid sites cause the increase of toluene adsorption and oxidation. This is evidence that the more toluene was converted to CO2 at about 700°C (Figure 8A). Thus, the more amount of Al intercalated into the interlayer of Mt, the more very strong acid sites are induced, causing the enhance of catalytic activity at high temperature. SUMMARY AND CONCLUSIONS In this work, pillared products (Al13-PILM and Al30-PILM) were synthesized by using polycations of Keggin-Al13 and Keggin-Al30 to intercalate the Mt layers. Calculation of chemical structural formula of Mt was operated based on the main chemical composition analysis. Amounts of inserted aluminum into interlayers of Mt were obtained from the normalized calculation. Arrangement models of Keggin-Al13 and Keggin-Al30 in interlayers of Mt were put forward quantificationally for the first time, according to the chemical components in pillared samples, results of powder X-ray diffraction, and structural parameters of Mt. One Keggin-Al30 polycation can affect 9.5 unit cells (from two layers, with 4.7 – 4.8 unit cells in each layer), while a Keggin-


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Al13 controls an area of 7.1 unit cells (from two layers, with 3.5 – 3.6 unit cells in each layer). These arrangement models reveal the key source of micropores and mesopores in structures of Al13-PILM and Al30-PILM. Pillaring with Keggin-Al13 and Keggin-Al30 into interlayers results in more acid sites (particularly Bronsted acid sites) on Mt, including both weak and very strong acid sites. The ratio of Al/Si affects the amounts of acid sites and the acidic strength dramatically. Large amounts of aluminum in interlayers of Mt could enhance the surface acidic characteristics. Pillaring by polycations of Keggin-Al30 into the interlayer of Mt could lead to more Bronsted acid sites and strong acid sites, due to the large amounts of aluminum atoms and the extremely high positive charge in one unit of Keggin-Al30. Deep catalytic oxidation experiments of toluene demonstrated that pillaring of polycations (especially the Keggin-Al30) can lower the temperature for catalytic oxidation of toluene. More toluene could be converted to CO2 at high temperature by Al13-PILM and Al30-PILM, due to the more very strong acid site on their surfaces. These findings in this work demonstrate evidences for the source of porous structure in pillared interlayered Mt, and the pillaring impacts on the surface acidity of Mt. The key of this work is that it develops the knowledge of ratio of Al/Si affecting the surface acidity significantly. This fundamental work also expands the potential possibility of Al30-PILM in the application as heterogeneous acid catalysts with a promising future, especially under high temperature. AUTHOR INFORMATION * Corresponding authors J.X. Zhu (Email: [email protected]) Y.F. Xi (Email: [email protected]) #

Present address

Department of Ecosystem Science and Management, University of Wyoming, Laramie, Wyoming 82071, United States ORCID Ke Wen: 0000-0002-2080-6541 Jianxi Zhu: 0000-0002-9002-4457 Yunfei Xi: 0000-0003-2924-9494


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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the Collaborative Innovation Platform and Environment Construction of Special Funds of Guangdong Province, China (Grant No. 2017A050501048), National Natural Science Foundation of China (Grant No. 41502031), Youth Innovation Promotion Association CAS (Grant No. 2018387), Science and Technology Program of Guangzhou, China (Grant No. 201607010280). This is a contribution from GIGCAS. REFERENCES 1.

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a combined IR and Raman study. Applied Surface Science 2004, 227 (1-4), 30-39. TABLE CAPTIONS Table 1 The main chemical composition of Mt and the pillared products (wt%, as elemental oxides). Table 2 Calculation of the structural chemical formula of Mt. Table 3 Analysis of the inserted amounts of Al (as Al2O3) into the interlayer of Mt. Table 4 Calculations about the number of unit cells occupied and charges needed per polycation. Table 5 NH3-TPD results of Mt, Al13-PILM, and Al30-PILM. Table 6 Positions and assignments of the DR-FTIR vibrational bands in NH3-adsorbed samples of Mt, Al13-PILM and Al30-PILM. Table 7 Results of deconvolution in the 1350 – 1750 cm-1 region of NH3-adsorbed DR-FTIR spectra of Mt, Al13-PILM and Al30-PILM. Table 8 The amount of Bronsted acid sites (QB) and Lewis acid sites (QL) of Mt, Al13-PILM and Al30-PILM. Table 9 Catalytic activity of Mt, Al13-PILM and Al30-PILM characterized by the temperature (°C) at detectable, 10%, 50% and 90% of CO2 generation. FIGURE CAPTIONS Figure 1. Schematic structure of montmorillonite, which is composed with two octahedral layers and one tetrahedral layer. Figure 2. Schematic representation of the possible sources of solid acid sites of Mt. Figure 3. Arrangement models of polycations in montmorillonite. (A) top view of Keggin-Al13 in Mt, (B) top view of Keggin-Al30 in Mt, (C) three-dimensional view of Keggin-Al13 in Mt, (D) three-dimensional view of Keggin-Al30 in Mt. Cations and/or anions are not shown here. Figure 4. NH3-TPD spectra of (A) Mt, (B) Al13-PILM, and (C) Al30-PILM. Figure 5. DR-FTIR spectra of NH3-adsorbed Mt, Al13-PILM and Al30-PILM. Figure 6. DR-FTIR spectra of NH3-adsorbed (A) Mt, (B) Al13-PILM and (C) Al30-PILM in the


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range of 1350 – 1750 cm-1 (dashed: experimental, solid: simulation, marked: deconvolution). Three vibrational bands at the range 1620 – 1710 cm-1 are attributed to the vibrational mode of L/NH3 (~1630 cm-1), hydrogen-bound NH3 (~1650 cm-1), and NH4+ ion species (~1700 cm-1). Band at around 1400 cm-1 is the characteristic of NH4+ species. Bands at wavenumber higher and lower than 1400 cm-1 at the 1350 – 1550 cm-1 region are corresponding to the CO32- stretching mode of calcite and the vibration of NH3 coordinated to the of polycations through surficial O atoms. Figure 7. The amount of Bronsted acid sites (QB) and Lewis acid sites (QL) of Mt, Al13-PILM and Al30-PILM. Figure 8. The conversion curves of CO2 generation over Mt, Al13-PILM and Al30-PILM catalysts in the temperature range of 100 – 700°C (A, 300 – 700°C and B, 100 – 300°C). Reaction conditions: [toluene] = 1000 ppm, [O2] = 20%, catalyst mass = 200.0 mg, total flow rate = 100 mL min-1 and GHSV = 30 000 cm3 g-1 h-1. Pillaring by Keggin-Al13 and Keggin-Al30 could lower the temperature of toluene catalytic oxidation, with the temperature for detectable CO2 at 78°C (Al30-PILM) and 207°C (Al13-PILM) respectively, significantly lower than that of Mt at 285°C.


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TABLES Table 1 The main chemical composition of Mt and the pillared products (wt%, as elemental oxides). Sample

Al2O3

CaO

Fe2O3

K2O

MgO

Na2O

P2O5

SiO2

TiO2

LOI

Mt

15.93

2.42

5.25

0.09

4.72

0.65

0.31

59.28

0.27

9.63

Al13-PILM

24.13

0.03

4.37

0.08

3.80

0.01

0.01

52.37

0.23

13.71

Al30-PILM

28.35

0.02

4.06

0.04

3.31

0.06

0.20

47.54

0.21

15.11

Table 2 Calculation of the structural chemical formula of Mt. aOxides

Weight (wt%)

Atomic weight (g)

cg

eq cations

dg

eq cations in total

eAtoms

per unit cell

SiO2

b54.28

60.09

3.613

31.844

7.960

Al2O3

15.93

101.96

0.937

8.261

2.754

CaO

2.42

56.08

0.086

0.761

0.381

Fe2O3

5.25

159.70

0.099

0.869

0.290

K2O

0.09

94.20

0.002

0.017

0.017

MgO

4.72

40.31

0.234

2.064

1.032

Na2O

0.65

61.98

0.021

0.185

0.185

a

Total analysis of the Mt expressed as oxides and based on a full-cell chemical formula unit;

b

Weight of Si in Mt is obtained by deducting the contents of impurity (quartz) from the total Si contents;

c

(Weight %/atomic weight) × (valence of cation) (numbers of atoms of the cation), e.g., for SiO2 (54.28/60.06);

d

g eq cations × 44 (number of positive charges per unit cell) / (normalization factor). The normalization factor is Σ g

eq cations, which for this example is 4.995; e

g eq cations in total/valence of cation.


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Table 3 Analysis of the inserted amounts of Al (as Al2O3) into the interlayer of Mt.

a

Sample

Al2O3 (wt%)

SiO2 (wt%)

Mt

15.93

Al13-PILM Al30-PILM

a Coe.

of

b Normalized

c Inserted

Al2O3 (wt%)

Norm

Al2O3 (wt%)

54.28

1.00

15.93

/

24.13

47.37

1.15

27.65

11.72

28.35

42.54

1.28

36.17

20.24

Coe. of Norm represents the coefficient of normalization, it is obtained by using the contents of SiO2 in Mt as

standard, and equals to (contents of SiO2 in Mt) / (contents of SiO2 in pillared products); b

Normalized Al2O3 is the contents of Al2O3 when assuming that these samples have the same contents of SiO2 in their

structures. In this way, the differences within their contents of Al2O3 could be compared; c Inserted Al2O3 contents represent the amounts of Keggin-Al13 and Keggin-Al30 into interlayers of Mt.

Table 4 Calculations about the number of unit cells occupied and charges needed per polycation. Items

Amounts

Molar numbers of unit cells (molar mass: 792.80 g)

1.26 mol

Weight of Al2O3 inserted into interlayers for Al13-PILM

117.20 g

Weight of Al2O3 inserted into interlayers for Al30-PILM

202.49 g

Molar numbers of Keggin-Al13 inserted

0.18 mol

Molar numbers of Keggin-Al30 inserted

0.13 mol

Number of unit cells of Mt occupied by per Keggin-Al13 polycation

7.14

Number of unit cells of Mt occupied by per Keggin-Al30 polycation

9.53

Number of charges needed per Keggin-Al13 polycation

0.39 (positive)

Number of charges needed per Keggin-Al30 polycation

8.13 (negative)


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Table 5 NH3-TPD results of Mt, Al13-PILM, and Al30-PILM. Desorption temperature (°C)

Sample

Weak

Moderate Strong

Acidity distribution (μmol NH3/g)

Very strong

Weak

Moderate

Strong Very strong

Total

Mt

218

0

372

686

56

0

525

66

647

Al13-PILM

233

0

379

525

173

0

516

213

902

Al30-PILM

195,236

0

0

473

956

0

0

378

1334

Table 6 Positions and assignments of the DR-FTIR vibrational bands in NH3-adsorbed samples of Mt, Al13-PILM and Al30-PILM. Position (cm-1)

Assignment

3625

Structural OH stretching

3416

OH stretching of water

1638

OH bending of water

1402

Bending mode of NH4+

1020

Si―O―Si stretching

917

Al―OH bending


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Table 7 Results of deconvolution in the 1350 – 1750 cm-1 region of NH3-adsorbed DR-FTIR spectra of Mt, Al13PILM and Al30-PILM. Sample

Mt/NH3

Al13-PILM/NH3

Al30-PILM/NH3

Central wavenumber (cm-1)

Assignment

Relative Area (%)

1707

NH4

1.76

1651

Hydrogen-bound NH3

66.59

1633

L/NH3

48.81

1405

NH4

+

2.80

1707

NH4+

4.44

1646

Hydrogen-bound NH3

111.41

1626

L/NH3

25.48

1478

NH4+, coordinated to Keggin-Al13

1.99

1401

NH4+

4.99

1364

2-

CO3 stretching mode

1.78

1703

NH4+

5.71

1644

Hydrogen-bound NH3

109.08

1626

L/NH3

21.13

1426


10.37

1411


2.31

1400

NH4

5.12

+

+

Table 8 The amount of Bronsted acid sites (QB) and Lewis acid sites (QL) of Mt, Al13-PILM, and Al30-PILM. Sample

QB (μmol NH3/g)

QL (μmol NH3/g)

QT* (μmol NH3/g)

Mt

384

263

647

Al13-PILM

747

155

902

Al30-PILM

1151

183

1334

*QT represents the total amount of acid sites in each sample.

Table 9 Catalytic activity of Mt, Al13-PILM and Al30-PILM characterized by the temperature (°C) at detectable, 10%, 50% and 90% of CO2 generation. Sample

Start

*T10

*T50

*T90

Ca-Mt

285

395

530

Not observed

Al13-PILM

205

549

666

Not observed

Al30-PILM

78

495

582

632

*T10, T50 and T90 are ascribed to the temperature of 10%, 50%, and 90% CO2 generation, respectively.


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TABLE OF CONTENT


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Figure 1. Schematic structure of montmorillonite, which is composed with two octahedral layers and one tetrahedral layer. 154x68mm (300 x 300 DPI)


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Figure 2. Schematic representation of the possible sources of solid acid sites of Mt. 146x99mm (300 x 300 DPI)


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Figure 3. Arrangement models of polycations in montmorillonite. (A) top view of Keggin-Al13 in Mt, (B) top view of Keggin-Al30 in Mt, (C) three-dimensional view of Keggin Al13 in Mt, (D) three-dimensional view of Keggin-Al30 in Mt. Cations and/or anions are not shown here. 145x114mm (300 x 300 DPI)


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Figure 4. NH3-TPD spectra of (A) Mt, (B) Al13-PILM, and (C) Al30-PILM. 85x181mm (600 x 600 DPI)


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Figure 5. DR-FTIR spectra of NH3-adsorbed Mt, Al13-PILM and Al30-PILM. 86x144mm (600 x 600 DPI)


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Figure 6. DR-FTIR spectra of NH3-adsorbed (A) Mt, (B) Al13-PILM and (C) Al30-PILM in the range of 1350 –

1750 cm-1 (dashed: experimental, solid: simulation, marked: deconvolution). Three vibrational bands at the range 1620 – 1710 cm-1 are attributed to the vibrational mode of L/NH3 (~1630 cm-1), hydrogen-bound

NH3 (~1650 cm-1), and NH4+ ion species (~1700 cm-1). Band at around 1400 cm-1 is the characteristic of NH4+ species. Bands at wavenumber higher and lower than 1400 cm-1 at the 1350 – 1550 cm-1 region are corresponding to the CO32- stretching mode of calcite and the vibration of NH3 coordinated to the of polycations through surficial O atoms. 88x192mm (600 x 600 DPI)


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Figure 7. The amount of Bronsted acid sites (QB) and Lewis acid sites (QL) of Mt, Al13-PILM and Al30-PILM. 85x66mm (600 x 600 DPI)


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Figure 8. The conversion curves of CO2 generation over Mt, Al13-PILM and Al30-PILM catalysts in the temperature range of 100 – 700°C (A, 300 – 700°C and B, 100 – 300°C). Reaction conditions: [toluene] = 1000 ppm, [O2] = 20%, catalyst mass = 200.0 mg, total flow rate = 100 mL min-1 and GHSV = 30 000 cm3 g-1 h-1. Pillaring by Keggin-Al13 and Keggin-Al30 could lower the temperature of toluene catalytic oxidation, with the temperature for detectable CO2 at 78°C (Al30-PILM) and 207°C (Al13-PILM) respectively, significantly lower than that of Mt at 285°C. 83x135mm (600 x 600 DPI)


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Graphic Abstract 82x44mm (300 x 300 DPI)


Arrangement Models of Keggin-Al30 and Keggin-Al13 in the Interlayer

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