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Porous aluminosilicic solids obtained by thermal-acid modification of a commercial kaolinite-type natural clay
Solid State Sciences 88 (2019) 29–35
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Porous aluminosilicic solids obtained by thermal-acid modification of a commercial kaolinite-type natural clay
T
J.A. Torres-Luna, J.G. Carriazo∗ Estado Sólido y Catálisis Ambiental (ESCA), Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Ciudad Universitaria-Carrera 30, # 4503, Bogotá, Colombia
A R T I C LE I N FO
A B S T R A C T
Keywords: Kaolinite Clay dealuminization Metakaolinite Porous material Catalytic support
Transformation of natural clay minerals is an interesting way to design new microporous solids with potential applications as adsorbents or heterogeneous catalysts. Particularly, kaolinite is a widely commercialized 1:1 clay mineral of promising modifiable structure. This paper reports the preparation of microporous solids with high surface areas and increased acidity by thermal-acid treatments of a natural kaolinite. Previous heating (700 °C and 900 °C) was used to activate the kaolinite (23 m2/g surface area) and produce metakaolinite. Then, aluminum extraction with 3 M HCl solution yielded solids with 320 m2/g and 526 m2/g micropore surface areas. The successful dissolution of Al3+ ions from the kaolinite octahedral sheets produces strong dealuminized solids with superior acidity (Brönsted and Lewis). The remarkable characteristics of these solids (high thermal stability, high porosity and enhanced acidity) allow considering them as important catalytic supports.
1. Introduction
interactions (hydrogen bonding) between layers, which make difficult the modification of this mineral in terms of exfoliation or delamination (layer separation from stacking) at molecular level. Several methods have been developed to enlarge the porosity, cationic exchange capacity, and the Brönsted acidity. Among the different methods to activate the kaolinite, it is worth to mention intercalation, mechanochemical activation, chemical activation, and thermal-chemical activation [8]. Some studies have been carried out on the chemical activation, in alkaline or acidic media, but only the acid treatments yield porous solids [8–10]. The acid-activation of natural kaolinite can be conducted by partially dissolving and extracting aluminum species using inorganic acids. Through acid-activation of kaolinite, the acid should hydrolyze Si-O-Al bonds and destroy the crystalline structure, removing octahedral Al3+ and yielding a porous structure. However, the layer sequence (stacking or structural order) of kaolinite is determined by abundant hydrogen bonds between the unit layers, which makes hard the acid attack. The loss of protons upon dehydroxylation may remove these bonds and destroys the ordering of layer sequence. Thus, pre-heating may be an effective way to facilitate the subsequent activation of kaolinite by inorganic acid attack. Several studies have shown the effect of acid concentrations, reaction time, and the use of some values of temperature, obtaining surface areas below 200 m2/g when 3 M (or lower) acid concentration and short time of reaction (lower to 2 h) were used
Clay minerals are particulate materials widely used in industrial applications [1–4]. At the last years, an enormous interest has been developed on the use of clay minerals for designing new adsorbents and heterogeneous catalysts [4,5]. Powders of pillared-clays from smectites, nanotubular halloysites, and modified kaolinites, among others, have been used as catalytic supports due to their porosity, controlled acidity, surface reactivity, and their availability in natural sources. Catalytic supports control the active phase stability and dispersion, molecular transport, and for some reactions the support directly participates in the catalytic process (bifunctional catalysts). Thus, the porosity and the chemical nature (composition, structure, and functional groups on the surface) of the catalytic supports are very important for designing heterogeneous catalysts. Natural kaolinite is an industrially available clay mineral (the commercial powder is named kaolin), which can be considered as a low cost green adsorbent. This clay mineral has tetrahedral sheets of silicates units (SiO4) covalently bonded to octahedral sheets of hydroxylated AlO6 entities [AlO6-x(OH)x] (Fig. 1), with the general composition of Al2Si2O5(OH)4 [6,7]. However, natural kaolinite has low adsorption capacity as compared to other synthetic adsorbents and catalytic supports. Furthermore, natural kaolinite has both low cation exchange capacity (1–15 meq/100 g) and large number of polar
∗
Corresponding author. Carrera 30 No. 45-03, Ciudad Universitaria, Edificio 451, oficina 109, Bogotá, Colombia. E-mail addresses: [email protected] (J.A. Torres-Luna), [email protected] (J.G. Carriazo).
https://doi.org/10.1016/j.solidstatesciences.2018.12.006 Received 15 June 2018; Received in revised form 1 November 2018; Accepted 8 December 2018 Available online 10 December 2018 1293-2558/ © 2018 Elsevier Masson SAS. All rights reserved.
Solid State Sciences 88 (2019) 29–35
J.A. Torres-Luna, J.G. Carriazo
0.01 °2θ, and step time of 10 s. A radiation power obtained with 40 kV/ 45 mA was used. In addition, infrared spectroscopy (FTIR) was used for complementing structural study. IR spectra (transmission mode) were taken in a NICOLET Thermo Scientific iS10 spectrometer, by mixing the sample with KBr (2 mg of sample in 200 mg of KBr) to form a compressed small tablet. Porosity and surface areas were measured from nitrogen adsorption isotherms. The isotherms were taken at 77 K using a Micromeritic ASAP 2020 adsorption analyzer in the P/P0 range of 1 × 10−5 to 0.99. The samples were previously outgassed at 200 °C for 8 h. Thermal behavior of samples was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA/DSC analyses were performed using a TA Instrument analyzer (TA SDT Q 600). The following conditions were used: N2 atmosphere, a gas flow of 40 mL/min, and 10 °C/min heating rate. The equipment was previously calibrated with high purity indium and sapphire. For each analysis, 8 mg of samples and alumina small crucibles were employed (the empty crucible as reference material). In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was used for estimating the acidity (Brönsted and Lewis) of synthesized solids [14,15], in order to compare them to the natural kaolinite. The assessment of acidity was carried out using NH3 as probe molecule, and an IR Tracer-100 (Shimadzu) FT-IR spectrometer with a Harrick Praying Mantis diffuse reflection spectroscopy accessory coupled to a high temperature reaction chamber. For each experiment, the sample was preheated at 400 °C in a flow of N2 (10 mL/min) for 1 h. Then, after the sample was cooled (30 °C), a stream (10 mL/min) of gas ammonia (5% in He) was passed through the sample for 15 min. After that, the sample was subjected to desorption for 30 min with a flow of N2 (10 mL/min) at different temperatures (room temperature, 100 °C, 200 °C, 300 °C, 400 °C). After desorption at each indicated temperature, the IR spectra were in situ recorded at 30 °C (150 scans with a resolution of 4 cm−1). Prior to ammonia adsorption, a background spectrum was acquired to be taken into account (subtracted) for the quantification of the intensities of IR bands corresponding to N-H vibrations when NH3 is linked (coordinated) to acid sites. Acidity was semiquantified by mathematical integration of the peaks at 1434 cm−1 (Brönsted acidity) and 1631 cm−1 (Lewis acidity).
Fig. 1. General structure of a kaolinite-type clay mineral.
[7,9–13]. In the present work, a systematic sequence of the temperatures (room temperature, 400 °C, 700 °C, and 900 °C) was used as pretreatment, to then activate the kaolinite with an acid solution of moderate concentration (3 M HCl) and a short time of reaction (2 h), obtaining higher surface area solids. 2. Experimental 2.1. Synthesis of porous solids A commercial natural kaolin (Caomin C08) was supplied by the company “Minerales Industriales” (Sabaneta-Antioquia, Colombia). The received material (pellets) was dried at 60 °C overnight, and then ground to be sieved through a mesh ASTM 100. This material (named k) was thermally treated for 2 h at 400 °C, 700 °C, and 900 °C, using a furnace with static air atmosphere. Both calcined (k-400, k-700, k-900) and not calcined (k) materials were separately refluxed with 3 M HCl (90 °C and magnetic stirring in a glass flask-condenser setup) during 2 h. Each sample was washed several times using deionized water, until chloride free. The samples were dried at 60 °C overnight, and then calcined at 400 °C for 2 h. The final solids were labeled as k-H-400 (fresh kaolinite activated with acid and then calcined at 400 °C), k-400H-400, k-700-H-400, and k-900-H-400, for the samples calcined at different temperatures, activated with acid, and then calcined at 400 °C. All the samples were finally ground and passed through a mesh ASTM 100.
3. Results and discussion Silicon and aluminum contents, and those of another minor elements contained in the natural clay, are typical of kaolinite [2,16,17]. The theoretical SiO2/Al2O3 ratio of 1:1 clay minerals is close to 1.2 [2], but in the present study the SiO2/Al2O3 ratio of 1.3 (Table 1) confirms a small additional quantity of other silicon source (silica impurities). Essentially, in raw clays the SiO2 content is increased due to the quartz contained in these natural materials [2], which was confirmed here by X-ray diffraction. This SiO2/Al2O3 ratio was maintained through thermal or thermal-acid treatments at room temperature and at 400 °C (samples k-400, k-H-400, k-400-H-400), indicating that no important dissolution of aluminum was evidenced as consequence of these activation conditions. Furthermore, thermal treatments at 700 °C and 900 °C (without acid attack) led to SiO2/Al2O3 ratios of 1.3 and 1.2, respectively (samples k-700 and k-900). Only the acid treatments after previous calcination of kaolinite at 700 °C and 900 °C yielded strongly dealuminized materials (samples k-700-H-400 and k-900-H-400), which is evident from the extremely higher SiO2/Al2O3 ratios (Table 1). Thus, the acid attack after calcining kaolinite at 700 or 900 °C facilitates the aluminum dissolution and its removal by washing. According to the literature [10,18], some important reactions involved in thermal and acid treatments of kaolinite can be described as:
2.2. Characterization techniques The elemental chemical analyses of the samples were performed by X-ray fluorescence (XRF), using a MagixPro PW-2440 Philips spectrometer with an Rh tube and a maximum power of 4 kW. Before analysis, dried samples were converted to standardized tablets from molten materials using a mixture of lithium tetraborate (Sigma Aldrich, 99.9%) in a relation 1:10. Scanning electron microscopy (SEM) was used to observe the morphological changes in the samples. Images were obtained in a microscope FEI Quanta 200, with 15 kV power and secondary electrons. The samples were previously metalized with a gold-palladium alloy using the sputtering technique. Several images were taken at different points on the surface of samples in order to observe representative morphologies. The structural variation was observed by X-ray diffraction (XRD). The analyses were carried out using an X Pert Pro MPD PANalytical equipment, with Bragg-Brentano configuration. The radiation source was a lamp with Cu anode (Cu-Kα radiation, wavelength of 1.54056 Å). Diffractograms were recorded at room temperature, with step size of
Al2O3.2SiO2.2H2 O (kaolinite)
400 − 500°C → Al2O3.2SiO2 + 2H2 O (metakaolinite)
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Table 1 Chemical analysis of the natural kaolinite and the modified solids. Solid
SiO2 (%)
Al2O3 (%)
SiO2/Al2O3
Fe2O3 (%)
TiO2 (%)
K2O (%)
MgO (%)
k (natural kaolinite) k-400 k-H-400 k-400-H-400 k-700 k-700-H-400 k-900 k-900-H-400
55.43 55.53 56.16 56.08 54.11 92.71 53.69 94.68
42.13 42.08 41.50 41.63 42.66 5.28 43.99 3.47
1.3 1.3 1.3 1.3 1.3 17.6 1.2 27.3
1.14 1.13 1.03 0.97 1.53 0.37 1.06 0.30
0.35 0.56 0.60 0.54 0.72 1.15 0.56 1.06
0.47 0.47 0.53 0.55 0.64 0.34 0.47 0.34
0.16 0.17 0.13 0.16 0.17 0.09 0.12 0.06
950°C 2 (Al2O3.2SiO2 ) → Si3Al 4 O3 O12 + SiO2 (amorphous) Al2O3.2SiO2 + 6HCl
reflux, 90°C → 2AlCl3 +2SiO2 +3H2 O
In this way, a previous heating of kaolinite at 700 °C or 900 °C was important to dehydroxylate its structure and eliminate the hydrogen bonds, destroying the stacking of aggregates and yielding an amorphous material (metakaolinite). The acid attacks disaggregated particles of metakaolinite and causes the dissolution of Al3+ ions from the octahedral sheets of kaolinite. Fig. 2 shows a short representative scheme of this {{}}process. Fig. 3 shows the SEM and EDX results for the natural kaolinite and some modified solids. Natural kaolinite has aggregates with the typical platelets morphology and relatively smooth surface [2,11]. The EDX analysis reveals high quantities of silicon and aluminum, with elevated intensities as response of the 1:1 ratio from silica/alumina sheets. This qualitative EDX result is in concordance with that (quantitatively) obtained by XRF. After modifications (including calcination and acid attack), all the yielded solids showed eroded surfaces, and those previously calcined at 700 °C or 900 °C revealed EDX spectra with small intensity of aluminum signal and high intensity of silicon signal. This EDX result is in agreement with XRF results, and confirms a partial dealuminization when thermal-acid treatments were carried out. X-ray diffraction (XRD) analysis confirms the kaolinite structure for the natural starting material (Fig. 4). All the identified signals were verified according to the literature [7,19,20]. The very intense peaks at 2θ = 12.5° (d001 = 7.07 Å) and 2θ = 25.09° (d002 = 3.55 Å) are characteristic XRD signals of kaolinite, which confirm that this clay mineral is the principal constituent of that clayed material. Two peaks of very
Fig. 3. Representative SEM micrographs and EDX analysis of the natural kaolinite and modified solids.
Fig. 4. X-ray diffraction patterns of the natural kaolinite and the modified solids by thermal or thermal-acid treatments (k = kaolinite, i = illite, q = quartz). Fig. 2. Schematized representation of thermal-acid activation of a natural kaolinite to give a dealuminized material. 31
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Fig. 5. FTIR spectra of natural kaolinite and the modified solids obtained by thermal or thermal-acid treatments. Fig. 6. TGA/DSC profiles for natural kaolinite and the solids obtained after thermal treatments at 400 °C, 700 °C and 900 °C.
low intensity at 2θ = 9.04° and 2θ = 17.91° indicate a minor contamination with illite mineral. Another contaminant in small quantity is quartz, whose peak of very low intensity is revealed at 2θ = 26.7°. The existence of these two minor contaminants in the clay clarifies the slight deviation of the SiO2/Al2O3 ratio (1.3) observed by XRF for this natural kaolinite, with respect to that reported for an ideal kaolinite (1.2). The XRD analyses for the modified kaolinite (Fig. 4) reveal an effective destruction of the structural organization (amorphization) of this mineral by thermal treatment at 700 °C or 900 °C, but not at 400 °C. In addition, the acid attack only, without previous thermal treatment, was unable to destroy the crystalline structure of kaolinite. This thermal behavior is well known for 1:1 clay minerals; thus when kaolinite is heated beyond the temperature of dehydroxylation, between 500 °C and 900 °C, metakaolinite (an amorphous phase) is the main product [21,22]. The IR analysis of kaolinite shows the typical vibrations of bonds in this clay mineral, such as Al-O-H, Si-O, and Si-O-Si (Fig. 5). The peaks at 3620 and 3700 cm−1 come from stretching vibrations of inner and outer (Al-O-H) hydroxyl groups bonded to aluminum ions, being 3620 cm−1 attributed to inner hydroxyl groups between the tetrahedral and octahedral sheets [23]. Small absorption bands at 3440 and 1610 cm−1 correspond to -OH stretching and bending, respectively, from small quantities of adsorbed water [24]. The peaks at 1020 and 1100 cm−1 are due to different stretching vibration modes from the silicon-oxygen (Si-O) bonds in the tetrahedral layer of the clay mineral, while the vibration at 912 cm−1 is attributed to Al-O-H deformation of inner hydroxyl groups [24]. Those absorption peaks between 600 and 800 cm−1 are originated from Si-O vibrations, and those at 535 and 470 cm−1 are due to Al–O–Si and Si–O–Si deformation, respectively [13,24]. Fig. 5 shows that alumina sheet hydroxyl (Al-O-H) stretching bands (3620 and 3700 cm−1) disappeared after calcination at 700 °C and 900 °C. This result indicates a strong dehydroxylation of the kaolinite structure. Furthermore, the peak at 535 cm−1, corresponding to Al–O–Si deformation, and that at 912 cm−1 (Al-O-H deformation of inner hydroxyl groups) also disappeared, which is consequence of dehydroxylation and destruction of Al–O–Si bonds. Bands between 600 and 800 cm−1 were converted to a unique band at 785 cm−1, and a wide band around 1100 cm−1 (with a shoulder at 1200 cm−1) displaced those two peaks at 1020 and 1100 cm−1, revealing important variations on the Si-O vibrations as result of possible symmetry changes during the formation of Si-O-Si linkages in metakaolinite [20,22]. The increase of intensity at 470 cm−1 (Si–O–Si deformation) confirms the predominant formation of silicate species. The IR spectra of the solids obtained from acid activation, after calcination at 700 °C and 900 °C, reveal a very broad band around 3400 cm−1, which is correlated with SiO-H bonds in silica-rich solids as result of dealuminization reaction [25]. This broad absorption band includes hydrogen-bonded OH groups
Fig. 7. TGA/DSC profiles for the solids obtained after acid treatment (k-H-400) and thermal-acid treatments (k-400-H-400, k-700-H-400 and k-900-H-400).
from the solid surface and possibly adsorbed water. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses show important changes in the solids after calcination and acid treatment (Figs. 6 and 7). Initially, a typical TGA/DSC profile was observed for the natural kaolinite (Fig. 6). A small loss of weight (4%) corresponding to adsorbed water, associated to a small endothermic peak about 45 °C, and a second loss of weight (13.5%) with an endothermic peak at 500 °C, corresponding to dehydroxylation of the 1:1 clay mineral structure, were observed for the natural kaolinite. Similar profile was obtained for the solid k-400 (with 1% of weight loss about 45 °C, and 13% of weight loss at 500 °C), which confirms that no significant structural transformation was carried out when the kaolinite was treated at 400 °C (Fig. 6). Thermal analyses of these two samples clearly show that strong dehydroxylation and the formation of metakaolinite occurred at 500 °C [10,13,20]. According to Cheng et al. [26], the theoretical value of mass loss in this dehydroxylation process, calculated from the ideal kaolinite formula, is 13.96%. On the other hand, the samples k-700 and k-900 show profoundly different TGA/DSC profiles regarding original kaolinite. Changes in the structure have been reported in literature, indicating that strong dehydroxylation at these temperatures leads to the formation of amorphous metakaolinite [10,13,20]. For these metakaolinite samples (k-700 and k-900), TGA analyses reveal a very small loss of mass (1.5% and 1% respectively) up to 500 °C or higher temperatures, which confirms that structure was previously dehydroxylated. The continuous drop of the DSC base-line in 32
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both profiles of k-700 and k-900 is a consequence of the amorphous character of the samples, which is typical of siliceous materials, and perhaps may be related to a small continuous re-accommodation of these amorphous solids during thermal increase, in addition to some deviation of base-line at high temperature. Fig. 7 shows the TGA/DSC profiles of acid-treated samples. The solids k-H-400 and k-400-H-400 showed similar profiles, with small loss of weight (3% and 1.5%, respectively) at 45 °C. A second important change was registered at 500 °C (loss of mass about 11.5% and 12.5% respectively), accompanied with a strong endothermic peak. Thermal behavior of these solids (k-H-400 and k-400-H-400) is similar to that observed for natural kaolinite, which indicates that probably no relevant change was occurred at structural level when these treatments were carried out. These results are coherent with those obtained by XRD and IR analyses. However, the samples k-700-H-400 and k-900-H400 show an important endothermic peak at 75 °C and 70 °C, respectively, with higher loss of weight (9% for k-700-H-400, and 12.5% for k900-H-400). In both cases, a continuous loss of mass (13.5% and 16.5%, respectively) was observed up to the final temperature of analysis. The weight loss below 200 °C is correlated to releasing water bonded to the silanol groups of the silica-rich solid surface, and the continuous weight loss above 200 °C is associated with gradual removal of silanol groups from the surface by a condensation reaction in which siloxane bonds are formed [27]. It is important to take into account that through the acid treatment (reflux operation) silanol groups are formed by aluminum extraction, and from rehydration of {{}}the material [28]. The nitrogen adsorption isotherms (Fig. 8) for the natural kaolinite and the samples thermally treated (k-400, k-700 and k-900) have
Table 2 Textural properties (surface area and pore volume) of the natural kaolinite and the modified solids. Solid
k (natural kaolinite) k-400 k-700 k-900 k-H-400 k-400-H-400 k-700-H-400 k-900-H-400 a b c
Surface area (m2 g−1)
Pore volume (cm3 g−1)
BET
Micropore (tplot)a
Mesoporeb
Microporea
Totalc
23
6
17
0.0020
0.1088
26 23 17 36 40 _ _
8 8 3 5 5 320 526
18 15 14 31 35 31 68
0.0028 0.0028 0.0012 0.0017 0.0018 0.1132 0.1864
0.1249 0.1178 0.1505 0.1343 0.1352 0.1761 0.4357
The Harkins-Jura equation was used for statistical thickness. De Boer's method was employed. According to Gurvitsch's method.
similar characteristics (type II and IVa), according to the new IUPAC classification [29]. These curves reveal solids with small surface areas and textures composed of macro and mesopores. The surface areas and pore volumes of all the solids appear in Table 2. The BET surface areas of these solids are very low and comparable each other (about 23 m2/ g), with slight decrease for k-900. In these solids, the contribution of micropores to the surface areas is negligible (Table 2). In the same way, the solids k-H-400 and k-400-H-400 have comparable isotherms, but with slightly higher adsorption levels (about 40 m2/g) than that of the natural kaolinite. These results clearly indicate that the formation of metakaolinite solely does not contribute to increase the porosity, and the acid attack to the kaolinite without previous metakaolinization only can slightly modify the mesopore surface area (Table 2). However, the acid attack after calcination at 700 °C or 900 °C gives solids with high surface areas and prominent nitrogen uptake (Fig. 8). In these cases, the high values of micropore surface areas (320 m2/g and 526 m2/g for k700-H-400 and k-900-H-400, respectively) and the shape of isotherms (a combination of types Ib and IVa [29]) confirm the formation of highly microporous solids, with contribution of mesopores (31 and 68 m2/g, respectively). It is important to take into account that when materials have high level of microporosity, the BET model is not representative. For metakaolinite formation, the coordination number of Al changes from 6 to 5 and 4, and metakaolinite with the highest content of five-coordinated Al is more reactive with acids [22]. This aspect is coherent with our results, and explain the formation of highly porous materials from kaolinite after preheated at high temperatures and then attacked with acid. The materials k-700-H-400 and k-900-H-400 have high microporosity and some important mesoporosity, as shown in Table 2 and Fig. 9. The BJH pore size distributions (Fig. 9) reveal important population of mesoropores, with average radii around 18 Å, for these two solids. Evidently, the remarkable increase of mesoporosity is a consequence of the (700 °C and 900 °C) thermal-acid treatment on the parent kaolinite. On the other hand, taking into account the outstanding textural characteristics of the solids k-700-H-400 and k-900-H400, it is obvious to propose these materials to be used as interesting catalytic supports. Some interesting studies on this subject have shown materials with lower surface areas than these achieved in this work, e.g. Belver et al. [13] reported thermal-acid treated metakaolinites with 219 m2/g, Rodríguez-Castillo et al. [12] got similar materials of 200 m2/g, and Lenarda et al. [10] obtained an acid-activated metakaolinite of 288 m2/g. Panda et al. [7] reported solids of 143 m2/g, Gao et al. [18] achieved an acid-activated metakaolinite of 465 m2/g, and Gao et al. [9] reported materials of 258 m2/g. Thus, it is very important to continue improving the textural characteristics of this type of materials and attempting to understand the chemical {{}}process.
Fig. 8. Nitrogen adsorption isotherms (at 77 K) of the natural kaolinite and the solids obtained after thermal or acid-thermal modifications. 33
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Brönsted acidity
Intensity (a. u.)
k-900-H-400 k-700-H-400 k-700 k-900 kaolinite (k)
0
100
200
300
400
Temperature (°C)
Intensity (a. u.)
Lewis acidity
Fig. 9. BJH pore size distributions for the natural kaolinite and the solids obtained after thermal or acid-thermal modifications.
k-900-H-400 k-700-H-400 k-700 k-900 kaolinite (k)
0
100
200
300
400
Temperature (°C)
Important changes in both Brönsted and Lewis acidity of the solids, when the parent kaolinite was preheated or thermal-acid modified, is inferred from Fig. 10. Increased levels of both types of acidity (Brönsted and Lewis) were observed when the natural kaolinite was calcined at 700 °C and 900 °C, being the k-700 acidity higher than that of k-900. The lower level of acidity for k-900 perhaps is a consequence of strong collapse of structure and lower accessibility of NH3 molecules. In general, the increase of acidity for the metakaolinite was expected due to the disaggregation and destruction of the kaolinite stacking, which exposes internal sites of the material to be accessed by NH3 molecules. Furthermore, as above mentioned the high contents of Al(IV) and Al(V) yield a more reactive material (metakaolinite). Previous literature has reported the low coordination aluminum and the existence of –OH groups in the metakaolinite [20,22]. On the other hand, among all the synthesized solids the highest Brönsted and Lewis acidity levels (higher number of acid sites) were obtained for the solids k-700-H-400 and k900-H-400 (Fig. 10). These results clearly reveal that dissolution of Al3+ ions from the octahedral sheets of kaolinite, by acid attack after calcination at these temperatures (700 °C and 900 °C), yields solids with very superior acidity to that of natural kaolinite. A high number of acid sites (DRIFT spectra band intensities as described in experimental section), for Brönsted and Lewis acidity, was semiquantified at different temperatures, providing information on the nature and strength of the acid sites in the solids. According to different acidic strengths, the adsorbed species (NH3) are released at different temperatures while the intrinsic species of solids are restored [15]. Using the comparative scale of acid strength employed for Rodríguez-Castillo et al. [12] and previously reported by Yadav and Nair [30], in which the sites releasing the adsorbed ammonia below 200 °C are considered weak acid sites, and those that release ammonia above 200 °C are considered strong
Fig. 10. Brönsted and Lewis acidity variation in the solids obtained after thermal or acid-thermal modifications of the natural kaolinite.
acid sites, it can be concluded that the solids k-700-H-400 and k-900-H400 have important level of strong acid sites (Brönsted and Lewis). For a better comparison, the slopes of curves have to be taken into account. A dramatic drop of the curve slope is observed for the solid k-700-H400 in the Lewis acidity (Fig. 10), revealing that a higher quantity of weak Lewis sites are contained in this solid, and smaller quantities of strong sites are preserved after 200 °C. A less dramatic drop of the Lewis acidity curve was observed for k-900-H-400, which indicates that a superior number of strong acid sites is present. However, a minor variation in the slopes of Brösnted acidity curves for these solids indicates the levels of Brönsted acidity are moderately preserved through heating, which shows that a higher population of strong Brönsted sites are present. Finally, the estimated acidity levels for the solids k-700-H400 and k-900-H-400 reveal the high potentiality of these solids to be used as either catalytic supports or catalysts for acid-catalyzed reactions. 4. Conclusions Previous thermal treatment at temperatures higher than 500 °C, e.g. 700 °C and 900 °C, was required to make the amorphization of a natural kaolinite, yielding a more reactive material (metakaolinite) whose structure could be effectively attacked with HCl of moderate concentration (3 M). Thermal activation can be interpreted as a dehydroxylation that leads to disaggregation, hydrogen bond disruption, 34
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and destruction of stacking. After metakaolinization of the natural clay mineral, a 3 M HCl solution allowed the partial aluminum extraction (strong dealuminization) to produce aluminosilicic minerals with high surface area values, micro and mesoporosity, high thermal stability, and acidity (Brönsted and Lewis) superior to that of the natural kaolinite. The high values of micropore (320 and 536 m2/g) and mesopore (31 and 68 m2/g) surface areas for the solids, their combined acidity concerning both the nature and strength (weak and strong acid sites), and the high thermal stability, make of these materials interesting supports to design new catalysts.
org/10.1016/j.jcis.2007.03.015. [11] C. Zhang, Z. Zhang, Y. Tan, M. Zhong, The effect of citric acid on the kaolin activation and mullite formation, Ceram. Int. 43 (2017) 1466–1471, https://doi.org/ 10.1016/j.ceramint.2016.10.115. [12] L. Rodríguez-Castillo, A. Castillo-Mares, R. García-Alamilla, R. Silva-Rodrigo, G. Sandoval-Robles, S. Robles-Andrade, Preliminary study of the acid properties of a kaolin treated with ultrasonic energy and inorganic acid solutions, Rev. Mex. Ing. Quim. 5 (2006) 329–334. [13] C. Belver, M. Bañares-Muñoz, M. Vicente, Chemical activation of a kaolinite under acid and alkaline conditions, Chem. Mater. 14 (2002) 2033–2043, https://doi.org/ 10.1021/cm0111736. [14] J.C. Cortés, M. Muñoz, L. Macías, R. Molina, S. Moreno, Incorporation of Ni and Mo on delaminated clay by auto-combustion and impregnation for obtaining decane hydroconversion catalysts, Catal. Today 296 (2017) 205–213, https://doi.org/10. 1016/j.cattod.2017.08.028. [15] L. Wang, W. Li, S. Schmieg, D. Weng, Role of Brönsted acidity in NH3 selective catalytic reduction reaction on Cu/SAPO-34 catalysts, J. Catal. 324 (2015) 98–106, https://doi.org/10.1016/j.jcat.2015.01.011. [16] S. Bhattacharyya, P.S. Behera, Synthesis and characterization of nano-sized α-alumina powder from kaolin by acid leaching process, Appl. Clay Sci. 146 (2017) 286–290, https://doi.org/10.1016/j.clay.2017.06.017. [17] A.R. Mermut, A.F. Cano, Baseline studies of the clay minerals society source clays: chemical analyses of major elements, Clay Clay Miner. 49 (2001) 381–386. [18] Z. Gao, X. Li, H. Wu, S. Zhao, W. Deligeer, S. Asuha, Magnetic modification of acidactivated kaolin: synthesis, characterization, and adsorptive properties, Microporous Mesoporous Mater. 202 (2015) 1–7, https://doi.org/10.1016/j. micromeso.2014.09.029. [19] S. Chipera, D. Bish, Baseline studies of the clay minerals society source clays: powder X-ray diffraction analyses, Clay Clay Miner. 49 (2001) 398–409. [20] A.K. Chakraborty, Phase Transformation of Kaolinite Clay, Springer, Kolkata (India), 2014. [21] D.O. Obada, D. Dodoo-Arhin, D. Dauda, F.O. Anafi, A.S. Ahmed, O.A. Ajayi, The impact of kaolin dehydroxylation on the porosity and mechanical integrity of kaolin based ceramics using different pore formers, Results Phys 7 (2017) 2718–2727, https://doi.org/10.1016/j.rinp.2017.07.048. [22] L. Heller-Kallai, Thermally modified clay minerals, second ed., in: F. Bergaya, G. Lagaly (Eds.), Handbook of Clay Science, 5A Elsevier, Oxford (UK), 2013, pp. 411–433. [23] J. Madejová, FTIR techniques in clay mineral studies, Vib. Spectrosc. 31 (2003) 1–10, https://doi.org/10.1016/S0924-2031(02)00065-6. [24] J. Madejová, P. Komadel, Baseline studies of the clay minerals society source clays: infrared methods, Clay Clay Miner. 49 (2001) 410–432. [25] E. Abdullayev, A. Joshi, W. Wei, Y. Zhao, Y. Lvov, Enlargement of halloysite clay nanotube lumen by selective etching of aluminum oxide, ACS Nano 6 (2012) 7216–7226, https://doi.org/10.1021/nn302328x. [26] H. Cheng, Q. Liu, J. Yang, S. Ma, R.L. Frost, The thermal behavior of kaolinite intercalation complexes-A review, Thermochim. Acta 545 (2012) 1–13, https://doi. org/10.1016/j.tca.2012.04.005. [27] K. Tettey, D. Lee, Effect of thermal treatment and moisture content on the charge of silica particles in non-polar media, Soft Matter 9 (2013) 7242–7250, https://doi. org/10.1039/C3SM51121A. [28] A. Christy, The nature of silanol groups on the surfaces of silica, modified silica and some silica based materials, Adv. Mater. Res. 998–999 (2014) 3–10, https://doi. org/10.4028/www.scientific.net/AMR.998-999.3. [29] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC technical report), Pure Appl. Chem. 87 (2015) 1051–1069, https://doi.org/10.1515/pac-2014-1117. [30] G. Yadav, J. Nair, Sulfated zirconia and its modified versions as promising catalysts for industrial processes, Microporous Mesoporous Mater. 33 (1999) 1–48, https:// doi.org/10.1016/S1387-1811(99)00147-X.
Funding This work was supported by the National University of Colombia, Bogotá-D.C. [grant number 36038, official announcement 2016–2018]. Acknowledgments The authors gratefully acknowledge the service of Lab-DRES (Laboratorio de Diseño y Reactividad de Estructuras Sólidas) at the National University of Colombia-Bogotá. References [1] M. Masheane, L. Nthunya, M. Mubiayi, T. Thamae, S. Mhlanga, Physico-chemical characteristics of some Lesotho's clays and their assessment for suitability in ceramics production, Part. Sci. Technol. 36 (2018) 117–122, https://doi.org/10.1080/ 02726351.2016.1226226. [2] M.D. Awad, A. López-Galindo, M. Setti, M.M. El-Rahmany, C. Viseras-Ibarra, Kaolinite in pharmaceutics and biomedicine, Int. J. Pharm. 533 (2017) 34–48, https://doi.org/10.1016/j.ijpharm.2017.09.056. [3] R.A. Schoonheydt, Reflections on the material science of clay minerals, Appl. Clay Sci. 131 (2016) 107–112, https://doi.org/10.1016/j.clay.2015.12.005. [4] C.C. Harvey, G. Lagaly, Industrial applications, second ed., in: F. Bergaya, G. Lagaly (Eds.), Handbook of Clay Science, 5B Elsevier, Oxford (UK), 2013, pp. 451–490. [5] L. Deng, P. Yuan, D. Liu, F. Annabi-Bergaya, J. Zhou, F. Chen, Z. Liu, Effects of microstructure of clay minerals, montmorillonite, kaolinite and halloysite, on their benzene adsorption behaviors, Appl. Clay Sci. 143 (2017) 184–191, https://doi. org/10.1016/j.clay.2017.03.035. [6] S.-H. Liu, C. Xue, H. Yang, X.-Q. Huang, Y. Zou, Y.-N. Ding, L. Li, X.-M. Ren, Intercalated hybrid of kaolinite with KH2PO4 showing high ionic conductivity (∼10−4 S cm−1) at room temperature, Solid State Sci. 74 (2017) 95–100, https:// doi.org/10.1016/j.solidstatesciences.2017.10.007. [7] A.K. Panda, B. Mishra, D. Mishra, R. Singh, Effect of sulphuric acid treatment on the physico-chemical characteristics of kaolin clay, Colloids Surf., A 363 (2010) 98–104, https://doi.org/10.1016/j.colsurfa.2010.04.022. [8] A. Magdy, Y. Fouad, M. Abdel-Aziz, A. Konsowa, Synthesis and characterization of Fe3O4/kaolin magnetic nanocomposite and its application in wastewater treatment, J. Ind. Eng. Chem. 56 (2017) 299–311, https://doi.org/10.1016/j.jiec.2017.07.023. [9] W. Gao, S. Zhao, H. Wu, W. Deligeer, S. Asuha, Direct acid activation of kaolinite and its effects on the adsorption of methylene blue, Appl. Clay Sci. 126 (2016) 98–106, https://doi.org/10.1016/j.clay.2016.03.006. [10] M. Lenarda, L. Storaro, A. Talon, E. Moretti, P. Riello, Solid acid catalysts from clays: preparation of mesoporous catalysts by chemical activation of metakaolin under acid conditions, J. Colloid Interface Sci. 311 (2007) 537–543, https://doi.
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Porous aluminosilicic solids obtained by thermal-acid modification of a commercial kaolinite-type natural clay
T
J.A. Torres-Luna, J.G. Carriazo∗ Estado Sólido y Catálisis Ambiental (ESCA), Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Ciudad Universitaria-Carrera 30, # 4503, Bogotá, Colombia
A R T I C LE I N FO
A B S T R A C T
Keywords: Kaolinite Clay dealuminization Metakaolinite Porous material Catalytic support
Transformation of natural clay minerals is an interesting way to design new microporous solids with potential applications as adsorbents or heterogeneous catalysts. Particularly, kaolinite is a widely commercialized 1:1 clay mineral of promising modifiable structure. This paper reports the preparation of microporous solids with high surface areas and increased acidity by thermal-acid treatments of a natural kaolinite. Previous heating (700 °C and 900 °C) was used to activate the kaolinite (23 m2/g surface area) and produce metakaolinite. Then, aluminum extraction with 3 M HCl solution yielded solids with 320 m2/g and 526 m2/g micropore surface areas. The successful dissolution of Al3+ ions from the kaolinite octahedral sheets produces strong dealuminized solids with superior acidity (Brönsted and Lewis). The remarkable characteristics of these solids (high thermal stability, high porosity and enhanced acidity) allow considering them as important catalytic supports.
1. Introduction
interactions (hydrogen bonding) between layers, which make difficult the modification of this mineral in terms of exfoliation or delamination (layer separation from stacking) at molecular level. Several methods have been developed to enlarge the porosity, cationic exchange capacity, and the Brönsted acidity. Among the different methods to activate the kaolinite, it is worth to mention intercalation, mechanochemical activation, chemical activation, and thermal-chemical activation [8]. Some studies have been carried out on the chemical activation, in alkaline or acidic media, but only the acid treatments yield porous solids [8–10]. The acid-activation of natural kaolinite can be conducted by partially dissolving and extracting aluminum species using inorganic acids. Through acid-activation of kaolinite, the acid should hydrolyze Si-O-Al bonds and destroy the crystalline structure, removing octahedral Al3+ and yielding a porous structure. However, the layer sequence (stacking or structural order) of kaolinite is determined by abundant hydrogen bonds between the unit layers, which makes hard the acid attack. The loss of protons upon dehydroxylation may remove these bonds and destroys the ordering of layer sequence. Thus, pre-heating may be an effective way to facilitate the subsequent activation of kaolinite by inorganic acid attack. Several studies have shown the effect of acid concentrations, reaction time, and the use of some values of temperature, obtaining surface areas below 200 m2/g when 3 M (or lower) acid concentration and short time of reaction (lower to 2 h) were used
Clay minerals are particulate materials widely used in industrial applications [1–4]. At the last years, an enormous interest has been developed on the use of clay minerals for designing new adsorbents and heterogeneous catalysts [4,5]. Powders of pillared-clays from smectites, nanotubular halloysites, and modified kaolinites, among others, have been used as catalytic supports due to their porosity, controlled acidity, surface reactivity, and their availability in natural sources. Catalytic supports control the active phase stability and dispersion, molecular transport, and for some reactions the support directly participates in the catalytic process (bifunctional catalysts). Thus, the porosity and the chemical nature (composition, structure, and functional groups on the surface) of the catalytic supports are very important for designing heterogeneous catalysts. Natural kaolinite is an industrially available clay mineral (the commercial powder is named kaolin), which can be considered as a low cost green adsorbent. This clay mineral has tetrahedral sheets of silicates units (SiO4) covalently bonded to octahedral sheets of hydroxylated AlO6 entities [AlO6-x(OH)x] (Fig. 1), with the general composition of Al2Si2O5(OH)4 [6,7]. However, natural kaolinite has low adsorption capacity as compared to other synthetic adsorbents and catalytic supports. Furthermore, natural kaolinite has both low cation exchange capacity (1–15 meq/100 g) and large number of polar
∗
Corresponding author. Carrera 30 No. 45-03, Ciudad Universitaria, Edificio 451, oficina 109, Bogotá, Colombia. E-mail addresses: [email protected] (J.A. Torres-Luna), [email protected] (J.G. Carriazo).
https://doi.org/10.1016/j.solidstatesciences.2018.12.006 Received 15 June 2018; Received in revised form 1 November 2018; Accepted 8 December 2018 Available online 10 December 2018 1293-2558/ © 2018 Elsevier Masson SAS. All rights reserved.
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0.01 °2θ, and step time of 10 s. A radiation power obtained with 40 kV/ 45 mA was used. In addition, infrared spectroscopy (FTIR) was used for complementing structural study. IR spectra (transmission mode) were taken in a NICOLET Thermo Scientific iS10 spectrometer, by mixing the sample with KBr (2 mg of sample in 200 mg of KBr) to form a compressed small tablet. Porosity and surface areas were measured from nitrogen adsorption isotherms. The isotherms were taken at 77 K using a Micromeritic ASAP 2020 adsorption analyzer in the P/P0 range of 1 × 10−5 to 0.99. The samples were previously outgassed at 200 °C for 8 h. Thermal behavior of samples was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA/DSC analyses were performed using a TA Instrument analyzer (TA SDT Q 600). The following conditions were used: N2 atmosphere, a gas flow of 40 mL/min, and 10 °C/min heating rate. The equipment was previously calibrated with high purity indium and sapphire. For each analysis, 8 mg of samples and alumina small crucibles were employed (the empty crucible as reference material). In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was used for estimating the acidity (Brönsted and Lewis) of synthesized solids [14,15], in order to compare them to the natural kaolinite. The assessment of acidity was carried out using NH3 as probe molecule, and an IR Tracer-100 (Shimadzu) FT-IR spectrometer with a Harrick Praying Mantis diffuse reflection spectroscopy accessory coupled to a high temperature reaction chamber. For each experiment, the sample was preheated at 400 °C in a flow of N2 (10 mL/min) for 1 h. Then, after the sample was cooled (30 °C), a stream (10 mL/min) of gas ammonia (5% in He) was passed through the sample for 15 min. After that, the sample was subjected to desorption for 30 min with a flow of N2 (10 mL/min) at different temperatures (room temperature, 100 °C, 200 °C, 300 °C, 400 °C). After desorption at each indicated temperature, the IR spectra were in situ recorded at 30 °C (150 scans with a resolution of 4 cm−1). Prior to ammonia adsorption, a background spectrum was acquired to be taken into account (subtracted) for the quantification of the intensities of IR bands corresponding to N-H vibrations when NH3 is linked (coordinated) to acid sites. Acidity was semiquantified by mathematical integration of the peaks at 1434 cm−1 (Brönsted acidity) and 1631 cm−1 (Lewis acidity).
Fig. 1. General structure of a kaolinite-type clay mineral.
[7,9–13]. In the present work, a systematic sequence of the temperatures (room temperature, 400 °C, 700 °C, and 900 °C) was used as pretreatment, to then activate the kaolinite with an acid solution of moderate concentration (3 M HCl) and a short time of reaction (2 h), obtaining higher surface area solids. 2. Experimental 2.1. Synthesis of porous solids A commercial natural kaolin (Caomin C08) was supplied by the company “Minerales Industriales” (Sabaneta-Antioquia, Colombia). The received material (pellets) was dried at 60 °C overnight, and then ground to be sieved through a mesh ASTM 100. This material (named k) was thermally treated for 2 h at 400 °C, 700 °C, and 900 °C, using a furnace with static air atmosphere. Both calcined (k-400, k-700, k-900) and not calcined (k) materials were separately refluxed with 3 M HCl (90 °C and magnetic stirring in a glass flask-condenser setup) during 2 h. Each sample was washed several times using deionized water, until chloride free. The samples were dried at 60 °C overnight, and then calcined at 400 °C for 2 h. The final solids were labeled as k-H-400 (fresh kaolinite activated with acid and then calcined at 400 °C), k-400H-400, k-700-H-400, and k-900-H-400, for the samples calcined at different temperatures, activated with acid, and then calcined at 400 °C. All the samples were finally ground and passed through a mesh ASTM 100.
3. Results and discussion Silicon and aluminum contents, and those of another minor elements contained in the natural clay, are typical of kaolinite [2,16,17]. The theoretical SiO2/Al2O3 ratio of 1:1 clay minerals is close to 1.2 [2], but in the present study the SiO2/Al2O3 ratio of 1.3 (Table 1) confirms a small additional quantity of other silicon source (silica impurities). Essentially, in raw clays the SiO2 content is increased due to the quartz contained in these natural materials [2], which was confirmed here by X-ray diffraction. This SiO2/Al2O3 ratio was maintained through thermal or thermal-acid treatments at room temperature and at 400 °C (samples k-400, k-H-400, k-400-H-400), indicating that no important dissolution of aluminum was evidenced as consequence of these activation conditions. Furthermore, thermal treatments at 700 °C and 900 °C (without acid attack) led to SiO2/Al2O3 ratios of 1.3 and 1.2, respectively (samples k-700 and k-900). Only the acid treatments after previous calcination of kaolinite at 700 °C and 900 °C yielded strongly dealuminized materials (samples k-700-H-400 and k-900-H-400), which is evident from the extremely higher SiO2/Al2O3 ratios (Table 1). Thus, the acid attack after calcining kaolinite at 700 or 900 °C facilitates the aluminum dissolution and its removal by washing. According to the literature [10,18], some important reactions involved in thermal and acid treatments of kaolinite can be described as:
2.2. Characterization techniques The elemental chemical analyses of the samples were performed by X-ray fluorescence (XRF), using a MagixPro PW-2440 Philips spectrometer with an Rh tube and a maximum power of 4 kW. Before analysis, dried samples were converted to standardized tablets from molten materials using a mixture of lithium tetraborate (Sigma Aldrich, 99.9%) in a relation 1:10. Scanning electron microscopy (SEM) was used to observe the morphological changes in the samples. Images were obtained in a microscope FEI Quanta 200, with 15 kV power and secondary electrons. The samples were previously metalized with a gold-palladium alloy using the sputtering technique. Several images were taken at different points on the surface of samples in order to observe representative morphologies. The structural variation was observed by X-ray diffraction (XRD). The analyses were carried out using an X Pert Pro MPD PANalytical equipment, with Bragg-Brentano configuration. The radiation source was a lamp with Cu anode (Cu-Kα radiation, wavelength of 1.54056 Å). Diffractograms were recorded at room temperature, with step size of
Al2O3.2SiO2.2H2 O (kaolinite)
400 − 500°C → Al2O3.2SiO2 + 2H2 O (metakaolinite)
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Table 1 Chemical analysis of the natural kaolinite and the modified solids. Solid
SiO2 (%)
Al2O3 (%)
SiO2/Al2O3
Fe2O3 (%)
TiO2 (%)
K2O (%)
MgO (%)
k (natural kaolinite) k-400 k-H-400 k-400-H-400 k-700 k-700-H-400 k-900 k-900-H-400
55.43 55.53 56.16 56.08 54.11 92.71 53.69 94.68
42.13 42.08 41.50 41.63 42.66 5.28 43.99 3.47
1.3 1.3 1.3 1.3 1.3 17.6 1.2 27.3
1.14 1.13 1.03 0.97 1.53 0.37 1.06 0.30
0.35 0.56 0.60 0.54 0.72 1.15 0.56 1.06
0.47 0.47 0.53 0.55 0.64 0.34 0.47 0.34
0.16 0.17 0.13 0.16 0.17 0.09 0.12 0.06
950°C 2 (Al2O3.2SiO2 ) → Si3Al 4 O3 O12 + SiO2 (amorphous) Al2O3.2SiO2 + 6HCl
reflux, 90°C → 2AlCl3 +2SiO2 +3H2 O
In this way, a previous heating of kaolinite at 700 °C or 900 °C was important to dehydroxylate its structure and eliminate the hydrogen bonds, destroying the stacking of aggregates and yielding an amorphous material (metakaolinite). The acid attacks disaggregated particles of metakaolinite and causes the dissolution of Al3+ ions from the octahedral sheets of kaolinite. Fig. 2 shows a short representative scheme of this {{}}process. Fig. 3 shows the SEM and EDX results for the natural kaolinite and some modified solids. Natural kaolinite has aggregates with the typical platelets morphology and relatively smooth surface [2,11]. The EDX analysis reveals high quantities of silicon and aluminum, with elevated intensities as response of the 1:1 ratio from silica/alumina sheets. This qualitative EDX result is in concordance with that (quantitatively) obtained by XRF. After modifications (including calcination and acid attack), all the yielded solids showed eroded surfaces, and those previously calcined at 700 °C or 900 °C revealed EDX spectra with small intensity of aluminum signal and high intensity of silicon signal. This EDX result is in agreement with XRF results, and confirms a partial dealuminization when thermal-acid treatments were carried out. X-ray diffraction (XRD) analysis confirms the kaolinite structure for the natural starting material (Fig. 4). All the identified signals were verified according to the literature [7,19,20]. The very intense peaks at 2θ = 12.5° (d001 = 7.07 Å) and 2θ = 25.09° (d002 = 3.55 Å) are characteristic XRD signals of kaolinite, which confirm that this clay mineral is the principal constituent of that clayed material. Two peaks of very
Fig. 3. Representative SEM micrographs and EDX analysis of the natural kaolinite and modified solids.
Fig. 4. X-ray diffraction patterns of the natural kaolinite and the modified solids by thermal or thermal-acid treatments (k = kaolinite, i = illite, q = quartz). Fig. 2. Schematized representation of thermal-acid activation of a natural kaolinite to give a dealuminized material. 31
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Fig. 5. FTIR spectra of natural kaolinite and the modified solids obtained by thermal or thermal-acid treatments. Fig. 6. TGA/DSC profiles for natural kaolinite and the solids obtained after thermal treatments at 400 °C, 700 °C and 900 °C.
low intensity at 2θ = 9.04° and 2θ = 17.91° indicate a minor contamination with illite mineral. Another contaminant in small quantity is quartz, whose peak of very low intensity is revealed at 2θ = 26.7°. The existence of these two minor contaminants in the clay clarifies the slight deviation of the SiO2/Al2O3 ratio (1.3) observed by XRF for this natural kaolinite, with respect to that reported for an ideal kaolinite (1.2). The XRD analyses for the modified kaolinite (Fig. 4) reveal an effective destruction of the structural organization (amorphization) of this mineral by thermal treatment at 700 °C or 900 °C, but not at 400 °C. In addition, the acid attack only, without previous thermal treatment, was unable to destroy the crystalline structure of kaolinite. This thermal behavior is well known for 1:1 clay minerals; thus when kaolinite is heated beyond the temperature of dehydroxylation, between 500 °C and 900 °C, metakaolinite (an amorphous phase) is the main product [21,22]. The IR analysis of kaolinite shows the typical vibrations of bonds in this clay mineral, such as Al-O-H, Si-O, and Si-O-Si (Fig. 5). The peaks at 3620 and 3700 cm−1 come from stretching vibrations of inner and outer (Al-O-H) hydroxyl groups bonded to aluminum ions, being 3620 cm−1 attributed to inner hydroxyl groups between the tetrahedral and octahedral sheets [23]. Small absorption bands at 3440 and 1610 cm−1 correspond to -OH stretching and bending, respectively, from small quantities of adsorbed water [24]. The peaks at 1020 and 1100 cm−1 are due to different stretching vibration modes from the silicon-oxygen (Si-O) bonds in the tetrahedral layer of the clay mineral, while the vibration at 912 cm−1 is attributed to Al-O-H deformation of inner hydroxyl groups [24]. Those absorption peaks between 600 and 800 cm−1 are originated from Si-O vibrations, and those at 535 and 470 cm−1 are due to Al–O–Si and Si–O–Si deformation, respectively [13,24]. Fig. 5 shows that alumina sheet hydroxyl (Al-O-H) stretching bands (3620 and 3700 cm−1) disappeared after calcination at 700 °C and 900 °C. This result indicates a strong dehydroxylation of the kaolinite structure. Furthermore, the peak at 535 cm−1, corresponding to Al–O–Si deformation, and that at 912 cm−1 (Al-O-H deformation of inner hydroxyl groups) also disappeared, which is consequence of dehydroxylation and destruction of Al–O–Si bonds. Bands between 600 and 800 cm−1 were converted to a unique band at 785 cm−1, and a wide band around 1100 cm−1 (with a shoulder at 1200 cm−1) displaced those two peaks at 1020 and 1100 cm−1, revealing important variations on the Si-O vibrations as result of possible symmetry changes during the formation of Si-O-Si linkages in metakaolinite [20,22]. The increase of intensity at 470 cm−1 (Si–O–Si deformation) confirms the predominant formation of silicate species. The IR spectra of the solids obtained from acid activation, after calcination at 700 °C and 900 °C, reveal a very broad band around 3400 cm−1, which is correlated with SiO-H bonds in silica-rich solids as result of dealuminization reaction [25]. This broad absorption band includes hydrogen-bonded OH groups
Fig. 7. TGA/DSC profiles for the solids obtained after acid treatment (k-H-400) and thermal-acid treatments (k-400-H-400, k-700-H-400 and k-900-H-400).
from the solid surface and possibly adsorbed water. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses show important changes in the solids after calcination and acid treatment (Figs. 6 and 7). Initially, a typical TGA/DSC profile was observed for the natural kaolinite (Fig. 6). A small loss of weight (4%) corresponding to adsorbed water, associated to a small endothermic peak about 45 °C, and a second loss of weight (13.5%) with an endothermic peak at 500 °C, corresponding to dehydroxylation of the 1:1 clay mineral structure, were observed for the natural kaolinite. Similar profile was obtained for the solid k-400 (with 1% of weight loss about 45 °C, and 13% of weight loss at 500 °C), which confirms that no significant structural transformation was carried out when the kaolinite was treated at 400 °C (Fig. 6). Thermal analyses of these two samples clearly show that strong dehydroxylation and the formation of metakaolinite occurred at 500 °C [10,13,20]. According to Cheng et al. [26], the theoretical value of mass loss in this dehydroxylation process, calculated from the ideal kaolinite formula, is 13.96%. On the other hand, the samples k-700 and k-900 show profoundly different TGA/DSC profiles regarding original kaolinite. Changes in the structure have been reported in literature, indicating that strong dehydroxylation at these temperatures leads to the formation of amorphous metakaolinite [10,13,20]. For these metakaolinite samples (k-700 and k-900), TGA analyses reveal a very small loss of mass (1.5% and 1% respectively) up to 500 °C or higher temperatures, which confirms that structure was previously dehydroxylated. The continuous drop of the DSC base-line in 32
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both profiles of k-700 and k-900 is a consequence of the amorphous character of the samples, which is typical of siliceous materials, and perhaps may be related to a small continuous re-accommodation of these amorphous solids during thermal increase, in addition to some deviation of base-line at high temperature. Fig. 7 shows the TGA/DSC profiles of acid-treated samples. The solids k-H-400 and k-400-H-400 showed similar profiles, with small loss of weight (3% and 1.5%, respectively) at 45 °C. A second important change was registered at 500 °C (loss of mass about 11.5% and 12.5% respectively), accompanied with a strong endothermic peak. Thermal behavior of these solids (k-H-400 and k-400-H-400) is similar to that observed for natural kaolinite, which indicates that probably no relevant change was occurred at structural level when these treatments were carried out. These results are coherent with those obtained by XRD and IR analyses. However, the samples k-700-H-400 and k-900-H400 show an important endothermic peak at 75 °C and 70 °C, respectively, with higher loss of weight (9% for k-700-H-400, and 12.5% for k900-H-400). In both cases, a continuous loss of mass (13.5% and 16.5%, respectively) was observed up to the final temperature of analysis. The weight loss below 200 °C is correlated to releasing water bonded to the silanol groups of the silica-rich solid surface, and the continuous weight loss above 200 °C is associated with gradual removal of silanol groups from the surface by a condensation reaction in which siloxane bonds are formed [27]. It is important to take into account that through the acid treatment (reflux operation) silanol groups are formed by aluminum extraction, and from rehydration of {{}}the material [28]. The nitrogen adsorption isotherms (Fig. 8) for the natural kaolinite and the samples thermally treated (k-400, k-700 and k-900) have
Table 2 Textural properties (surface area and pore volume) of the natural kaolinite and the modified solids. Solid
k (natural kaolinite) k-400 k-700 k-900 k-H-400 k-400-H-400 k-700-H-400 k-900-H-400 a b c
Surface area (m2 g−1)
Pore volume (cm3 g−1)
BET
Micropore (tplot)a
Mesoporeb
Microporea
Totalc
23
6
17
0.0020
0.1088
26 23 17 36 40 _ _
8 8 3 5 5 320 526
18 15 14 31 35 31 68
0.0028 0.0028 0.0012 0.0017 0.0018 0.1132 0.1864
0.1249 0.1178 0.1505 0.1343 0.1352 0.1761 0.4357
The Harkins-Jura equation was used for statistical thickness. De Boer's method was employed. According to Gurvitsch's method.
similar characteristics (type II and IVa), according to the new IUPAC classification [29]. These curves reveal solids with small surface areas and textures composed of macro and mesopores. The surface areas and pore volumes of all the solids appear in Table 2. The BET surface areas of these solids are very low and comparable each other (about 23 m2/ g), with slight decrease for k-900. In these solids, the contribution of micropores to the surface areas is negligible (Table 2). In the same way, the solids k-H-400 and k-400-H-400 have comparable isotherms, but with slightly higher adsorption levels (about 40 m2/g) than that of the natural kaolinite. These results clearly indicate that the formation of metakaolinite solely does not contribute to increase the porosity, and the acid attack to the kaolinite without previous metakaolinization only can slightly modify the mesopore surface area (Table 2). However, the acid attack after calcination at 700 °C or 900 °C gives solids with high surface areas and prominent nitrogen uptake (Fig. 8). In these cases, the high values of micropore surface areas (320 m2/g and 526 m2/g for k700-H-400 and k-900-H-400, respectively) and the shape of isotherms (a combination of types Ib and IVa [29]) confirm the formation of highly microporous solids, with contribution of mesopores (31 and 68 m2/g, respectively). It is important to take into account that when materials have high level of microporosity, the BET model is not representative. For metakaolinite formation, the coordination number of Al changes from 6 to 5 and 4, and metakaolinite with the highest content of five-coordinated Al is more reactive with acids [22]. This aspect is coherent with our results, and explain the formation of highly porous materials from kaolinite after preheated at high temperatures and then attacked with acid. The materials k-700-H-400 and k-900-H-400 have high microporosity and some important mesoporosity, as shown in Table 2 and Fig. 9. The BJH pore size distributions (Fig. 9) reveal important population of mesoropores, with average radii around 18 Å, for these two solids. Evidently, the remarkable increase of mesoporosity is a consequence of the (700 °C and 900 °C) thermal-acid treatment on the parent kaolinite. On the other hand, taking into account the outstanding textural characteristics of the solids k-700-H-400 and k-900-H400, it is obvious to propose these materials to be used as interesting catalytic supports. Some interesting studies on this subject have shown materials with lower surface areas than these achieved in this work, e.g. Belver et al. [13] reported thermal-acid treated metakaolinites with 219 m2/g, Rodríguez-Castillo et al. [12] got similar materials of 200 m2/g, and Lenarda et al. [10] obtained an acid-activated metakaolinite of 288 m2/g. Panda et al. [7] reported solids of 143 m2/g, Gao et al. [18] achieved an acid-activated metakaolinite of 465 m2/g, and Gao et al. [9] reported materials of 258 m2/g. Thus, it is very important to continue improving the textural characteristics of this type of materials and attempting to understand the chemical {{}}process.
Fig. 8. Nitrogen adsorption isotherms (at 77 K) of the natural kaolinite and the solids obtained after thermal or acid-thermal modifications. 33
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Brönsted acidity
Intensity (a. u.)
k-900-H-400 k-700-H-400 k-700 k-900 kaolinite (k)
0
100
200
300
400
Temperature (°C)
Intensity (a. u.)
Lewis acidity
Fig. 9. BJH pore size distributions for the natural kaolinite and the solids obtained after thermal or acid-thermal modifications.
k-900-H-400 k-700-H-400 k-700 k-900 kaolinite (k)
0
100
200
300
400
Temperature (°C)
Important changes in both Brönsted and Lewis acidity of the solids, when the parent kaolinite was preheated or thermal-acid modified, is inferred from Fig. 10. Increased levels of both types of acidity (Brönsted and Lewis) were observed when the natural kaolinite was calcined at 700 °C and 900 °C, being the k-700 acidity higher than that of k-900. The lower level of acidity for k-900 perhaps is a consequence of strong collapse of structure and lower accessibility of NH3 molecules. In general, the increase of acidity for the metakaolinite was expected due to the disaggregation and destruction of the kaolinite stacking, which exposes internal sites of the material to be accessed by NH3 molecules. Furthermore, as above mentioned the high contents of Al(IV) and Al(V) yield a more reactive material (metakaolinite). Previous literature has reported the low coordination aluminum and the existence of –OH groups in the metakaolinite [20,22]. On the other hand, among all the synthesized solids the highest Brönsted and Lewis acidity levels (higher number of acid sites) were obtained for the solids k-700-H-400 and k900-H-400 (Fig. 10). These results clearly reveal that dissolution of Al3+ ions from the octahedral sheets of kaolinite, by acid attack after calcination at these temperatures (700 °C and 900 °C), yields solids with very superior acidity to that of natural kaolinite. A high number of acid sites (DRIFT spectra band intensities as described in experimental section), for Brönsted and Lewis acidity, was semiquantified at different temperatures, providing information on the nature and strength of the acid sites in the solids. According to different acidic strengths, the adsorbed species (NH3) are released at different temperatures while the intrinsic species of solids are restored [15]. Using the comparative scale of acid strength employed for Rodríguez-Castillo et al. [12] and previously reported by Yadav and Nair [30], in which the sites releasing the adsorbed ammonia below 200 °C are considered weak acid sites, and those that release ammonia above 200 °C are considered strong
Fig. 10. Brönsted and Lewis acidity variation in the solids obtained after thermal or acid-thermal modifications of the natural kaolinite.
acid sites, it can be concluded that the solids k-700-H-400 and k-900-H400 have important level of strong acid sites (Brönsted and Lewis). For a better comparison, the slopes of curves have to be taken into account. A dramatic drop of the curve slope is observed for the solid k-700-H400 in the Lewis acidity (Fig. 10), revealing that a higher quantity of weak Lewis sites are contained in this solid, and smaller quantities of strong sites are preserved after 200 °C. A less dramatic drop of the Lewis acidity curve was observed for k-900-H-400, which indicates that a superior number of strong acid sites is present. However, a minor variation in the slopes of Brösnted acidity curves for these solids indicates the levels of Brönsted acidity are moderately preserved through heating, which shows that a higher population of strong Brönsted sites are present. Finally, the estimated acidity levels for the solids k-700-H400 and k-900-H-400 reveal the high potentiality of these solids to be used as either catalytic supports or catalysts for acid-catalyzed reactions. 4. Conclusions Previous thermal treatment at temperatures higher than 500 °C, e.g. 700 °C and 900 °C, was required to make the amorphization of a natural kaolinite, yielding a more reactive material (metakaolinite) whose structure could be effectively attacked with HCl of moderate concentration (3 M). Thermal activation can be interpreted as a dehydroxylation that leads to disaggregation, hydrogen bond disruption, 34
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and destruction of stacking. After metakaolinization of the natural clay mineral, a 3 M HCl solution allowed the partial aluminum extraction (strong dealuminization) to produce aluminosilicic minerals with high surface area values, micro and mesoporosity, high thermal stability, and acidity (Brönsted and Lewis) superior to that of the natural kaolinite. The high values of micropore (320 and 536 m2/g) and mesopore (31 and 68 m2/g) surface areas for the solids, their combined acidity concerning both the nature and strength (weak and strong acid sites), and the high thermal stability, make of these materials interesting supports to design new catalysts.
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