Thermal analysis of montmorillonites modified with quaternary phosphonium and ammonium surfactants

The thermal stability of seven organically modified montmorillonites (‘organoclays’) has been investigated using differential thermal analysis, differential scanning calorimetry, and thermogravimetry in conjunction with X-ray diffractometry. Six organoclays were synthesised by replacing the interlayer inorganic cations, initially present, with quaternary phosphonium and ammonium surfactant cations. The samples modified with tetrabutylphosphonium (TBP), and butyltriphenylphosphonium (BTPP) ions have an appreciably higher thermal stability than the octadecyltrimethylammonium (ODTMA)-modified clays. Thus, in the case of TBP- and BTPP-modified montmorillonites, the onset temperature of decomposition is close to 300 °C. Samples modified with hexadecyltributylphosphonium (HDTBP) ions have a lower onset temperature of decomposition of 225 °C. In comparison, the onset temperature for ODTMA-montmorillonites (obtained at different concentrations of ODTMA-bromide) ranges from 158 to 222 °C, being highest where the concentration of intercalated surfactant is lowest. The onset temperature for a commercial alkylsilane-treated quaternary ammonium-modified organoclay (S-BEN N-400FP) is 207 °C. The basal spacing of the TBP- and BTPP-modified clays is 1.7–1.8 nm, indicating a monolayer arrangement of quaternary phosphonium ions in the interlayer space, while the value of 2.5 nm for HDTBP-montmorillonite indicates a more open structure. The ODTMA-modified samples have basal spacings ranging from 1.9 to 2.1 nm, indicative of a bilayer to pseudo-trilayer arrangement. The exceptionally high basal spacing of 3.4 nm for the S-BEN N-400FP organoclay might be due to interlayer penetration of organosilane hydrolysis products during synthesis. The thermal properties of organoclays are apparently related to the nature of the surfactants and their arrangement in the interlayer space of montmorillonite.     

Preparation of TiO2-pillared montmorillonite as photocatalyst Part I. Microwave calcination, characterisation, and adsorption of a textile azo dye

Two photocatalysts based on TiO₂-pillared intercalated montmorillonite have been prepared by microwave for 10 min at 700 W or by furnace heating at 673 K. Montmorillonite pillaring with TiO₂ increased the basal spacing to 14.7 Å (conventional heating) and 17.6 Å (microwave heating). XRD patterns of both materials showed the presence of 100% anatase with a slightly higher rate of crystallinity obtained through microwave calcination than by conventional heating at 673 K. The BET specific surface area of the microwave prepared photocatalyst (151 m² g^− 1) was 3 fold higher than those of the Degussa TiO₂ P25. At pH = 5.8, the maximum adsorption capacity of Solophenyl red 3BL (a textile azo dye) on the TiO₂-pillared montmorillonite calcined by microwave was 185 mg g^− 1, whereas it was 1.4 and 3 fold lower on the TiO₂-pillared montmorillonite calcined at 673 K, and on the Degussa TiO₂ P25 respectively. The influence of pH on the adsorption of the dye depended on the pH_ZPC of the pillared montmorillonites.     

Reversible collapse and Mg release of de- and rehydroxylated homoionic cis-vacant montmorillonites

Homoionic Na⁺, Ca²⁺, Sr²⁺, Li⁺, Cu²⁺ and Zn²⁺ samples of the <2 μm fraction of a cis-vacant montmorillonite from Linden (Bavaria) were steam treated at 200°C (≈1.5 MPa), 240°C (≈3.3 MPa) and 300°C (≈8.0 MPa) after dehydroxylation at temperatures up to 630°C. Cation exchange capacity (CEC) measurements, determination of exchangeable cations and X-ray diffraction (XRD), supplemented by thermoanalytical investigations of the evolved water in a thermobalance linked to a mass spectrometer, infrared (IR) and electron spin resonance (ESR) spectroscopy were employed to obtain information about the state of expandability and structural changes of swellable montmorillonite and the sites of interlayer and octahedral cations after heating and rehydroxylation.The XRD pattern of the initial samples showed a well-defined (001) reflection according to the interlayer cation and its hydration state under laboratory atmosphere. After dehydroxylation the pattern exhibited (001) reflections between 9.6 and 9.8 Å, corresponding to a collapsed structure for all samples. The Na⁺-, Ca²⁺- and Sr²⁺-rich montmorillonites regained partial expandability after rehydroxylation at 200°C and full expandability after rehydroxylation at 300°C if the dehydroxylation temperature was less than 630°C. Rehydroxylation at 300°C of the Cu²⁺- and Zn²⁺-rich montmorillonites did not cause reexpansion, whereas the Li⁺-rich samples recovered a partial swellability after rehydroxylation at 240°C and nearly the full swellability after rehydroxylation at 300°C.The Li⁺-, Cu²⁺- and Zn²⁺-rich samples underwent a strong CEC reduction due to migration of the interlayer cations into the 2:1 layer before dehydroxylation started. After rehydroxylation under water steam Cu²⁺- and Zn²⁺-rich samples released 16–30 meq/100 g of Mg²⁺ from the structure, increasing with the steam temperature. Mg²⁺ release was not observed for the Li⁺-rich montmorillonite.     

Study on ibuprofen/montmorillonite intercalation composites as drug release system

The present paper focused on the intercalation of ibuprofen (IBU) into montmorillonite as a sustained release drug carrier. The intercalation compounds were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FT-IR), and thermogravimetric analysis (TGA). The basal spacing of montmorillonite increased from 1.25 nm to 1.57 nm. The decomposition temperature of intercalated IBU was increased to 471 °C. The in vitro release experiments revealed that IBU was released from MMT steadily and pH dependent.     

Adsorption of tannic acid on chitosan-montmorillonite as a function of pH and surface charge properties

Chitosan, a natural biopolymeric cation, is a candidate to modify montmorillonite for the adsorption of anions. As an anionic organic pollutant the adsorption of tannic acid was studied. Because of protonation/deprotonation reactions of both chitosan-montmorillonite and tannic acid, the adsorption process is strongly pH-dependent. The objective of this work is to characterize the pH dependency of adsorption in combination with surface charge determinations.Montmorillonite was modified with different amounts of chitosan, corresponding to 20–1000% of the cation exchange capacity (CEC). The deacetylation degree of chitosan was determined by polyelectrolyte titration and was found to be 74%. The uptake of chitosan was determined by the C-content. The interlayer expansion was investigated by X-ray powder diffraction. The adsorption capacity for tannic acid was investigated with the batch technique at pH 3, 4, 5 and 8. As a measure for the adsorption properties, the electrokinetic surface charge was determined with a particle charge detector.The uptake of chitosan by montmorillonite is up to 152% (1.69 mol_c kg^− 1) of the CEC. The resulting anion exchange capacity of chitosan-montmorillonite calculated from C-content is 0.43 mol_c kg^− 1. At low loadings with chitosan (24.7 and 49.5% uptake), a monolayer is formed in montmorillonite. At an uptake of 96.8%, a bilayer structure is observed, which becomes more dominant at higher loadings. On the external surface, a monolayer of chitosan was formed. From pH 4 to 8, the surface charge of all modified montmorillonites is with − 9 to 8 mmol_c kg^− 1 close to the point of zero charge. The maximal adsorption capacity for tannic acid is found with 240 g kg^− 1 (0.14 mol_c kg^− 1) at pH 4. The adsorption process fits in well with the Freundlich isotherm. At lower as well as higher pH values the adsorption capacity decreases up to about 25%. Most probably the exchange sites in the interlayer do not contribute to the adsorption of tannic acid. The observed surface charge is lower than the adsorbed amount of tannin. It is thought that tannin is adsorbed also by van der Waals forces besides ionic forces.     

Thermal degradation of organic matter in the interlayer clay–organic complex: A TG-FTIR study on a montmorillonite/12-aminolauric acid system

The storage of natural organic matter within the interlayer space of layered silicate is an important type of clay–organic association in sediment. However, the role of the interlayer space of clay minerals in the thermal degradation of organics and the generation of hydrocarbons has not been well understood. In this study, an interlayer clay–organic complex was synthesized using montmorillonite (Mt) and 12-aminolauric acid (ALA). An Mt–ALA complex in which Mt and ALA were simply mixed was also prepared for comparison. Thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR) was applied to monitor the thermal events and the corresponding products during the thermal degradation of the Mt–ALA complexes. In the absence of Mt, ALA decomposed at 467 °C via the cleavage of CC bonds, producing aliphatic hydrocarbon, N-containing compounds, and carboxylic acid. The decomposition temperatures of organic matter in the mixed Mt–ALA complex and the interlayer Mt–ALA complex decreased to 402 and 342 °C, respectively. The most characteristic products of the interlayer Mt–ALA complex were NH₃ and saturated hydrocarbons. The Brønsted acid sites in the interlayer space of Mt, arising from the dissociated interlayer water, initiated the deamination of ALA via the Hoffmann elimination pathway and significantly promoted the cracking of hydrocarbons via a carbonation mechanism. Lewis acid sites had little effect on the thermal degradation of ALA. This work indicated that the interlayer space of clay minerals provided the storage space for organic matter. Moreover, the active sites within the interlayer space strongly promoted the thermal degradation of organics.     

Role of the interlayer space of montmorillonite in hydrocarbon generation: An experimental study based on high temperature–pressure pyrolysis

The interlayer clay–organic complex is an important clay–organic association in sedimentary environments. However, the effects of organic matter storage in the interlayer space of clay minerals on the thermal degradation of organics and the generation of hydrocarbons have not been investigated. In this study, montmorillonite (Mt) and 12-amoniolauric acid (ALA) were used to prepare an interlayer Mt–ALA complex and a Mt–ALA complex in which Mt and ALA were simply mixed. Pyrolysis experiments on the ALA and Mt–ALA complexes were conducted in a confined gold capsule system at a fixed temperature and pressure of 350 °C and 36 MPa, respectively. X-ray diffraction, elemental analysis and Fourier transform infrared spectroscopy were used to characterize the Mt–ALA complexes, and automatically controlled gas chromatography along with a pyrolysis furnace was used to detect the volatile components released during pyrolysis. In the absence of Mt, the pyrolysis of ALA yielded only a small amount of C_1–5 hydrocarbons and CO₂. The amounts of C_1–5 hydrocarbons released from the pyrolysis of the interlayer Mt–ALA complex and the mixed Mt–ALA complex are approximately 43 and 5 times greater than the amounts released from ALA alone, respectively. The Brønsted acid sites in the interlayer space of Mt, which arise from the interlayer dissociated water, significantly promote the cracking of hydrocarbons through a carbocation mechanism, the isomerization of normal hydrocarbons and alkene–alkane conversion through hydrogenation, resulting in high i-alkane/n-alkane and alkene/alkane ratios in the pyrolysis products. The Lewis acid sites of Mt are primarily involved in the decarboxylation of ALA during pyrolysis and are responsible for CO₂ generation.     

Chemical and thermal properties of organoclays derived from highly stable bentonite in sulfuric acid

Generally, acid activation modified the physico-chemical properties of the raw clay minerals. The extent of these modifications depended on the type, origin of the clay minerals and the conditions of the acid activation. In this study, a bentonite exhibited a strong stability toward the acid treatment at 90 °C and at higher acid/clay mineral ratios, with slight depletion of Mg^2 +, Fe^3 + and Al^3 + cations (about 5%). The resulting organo-acid activated clays prepared after a reaction with cetyltrimethylammonium (C16TMA) hydroxide solution, exhibited uptaken amounts of surfactants between 0.80 mmol and 0.7 mmol/g with interlayer spacings of 2.20 nm and 1.80 nm, independently of the initial concentrations of the organic molecules. These organoclays were stable in acidic and basic solutions. However, after heating at 200 °C, the interlayer spacing shrunk due to the degradation of the organic surfactants as indicated by thermogravimetric analysis. The rehydration of the calcined organoclays at temperatures below 200 °C, did not lead to the increase of the basal spacings, due to change in configuration of the C16TMA cations.     

Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

The aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Na_x(Al_(2 − x)Mg_x)Si₄O₁₀(OH)₂), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA).Montmorillonite phase was only produced at 120 and 200 MPa.At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite.At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00.A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming.     

The Effects of Thermal Treatment on Associations between Fatty Acids and Montmorillonite

The sorption from CCl₄ solutions of acetic, lauric and stearic acids by montmorillonites saturated with H, Li, Na, K, Cs, Ca, Mg, Cu, Al and Fe, was studied by IR and X-ray methods combined with various thermal treatments. Two distinct species were detected in the clay phase, the carboxylic acid (RCOOH) and the carboxylate anion (RCOO⁻). The ratios of the two sorbed species were found to be dependent both on the nature of the exchangeable cation and on the chain length of the acid. In Al⁻, Fe⁻ and Cu—montmorillonites, the ionic species predominates whereas in Cs—montmorillonite the acidic species predominates. Stearic acid shows the highest tendency to dissociate. The following associations were identified as occurring in the interlayer space: (I) and (II) — linkages between COOH group and oxygen sheet of silicate layer or with hydrophobic structured water; (III) — linkage between a COOH group and an exchangeable cation through a “water bridge”; (IV) — direct linkage between a COOH group and an exchangeable cation; (V) — linkage between a COO⁻ group and an exchangeable cation through a “water bridge” and (VI) — direct linkage between a COO⁻ group and an exchangeable cation. Although several types of associations are associated with every homoionic montmorillonite, some are more predominant. Thus, with Cs, association (I) predominates. With Li, Ca and Mg, association (III) predominates and is transformed to (IV) after thermal dehydration; whereas with Na and K association (IV) appears in freshly prepared sample. Al, Fe and Cu—montmorillonites mainly produce association (V). This structure is transformed into (VI) in the case of Al—montmorillonite by stearic and lauric acids, only. In the case of Cu—montmorillonite, all the three acids are transformed into (VI), but at 200°C decarboxylation of lauric and stearic acids occur.     

Mercury adsorption by montmorillonite and vermiculite: a combined XRD, TG-MS, and EXAFS study

Synchrotron-based extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray diffraction at room and high temperature, thermal analyses combined with mass spectrometry of evolved gas, and chemical analyses contributed to assess the influence of mercury on montmorillonite and vermiculite layers.The adsorbed Hg amount was higher for montmorillonite (Hg=37.7 meq/100 g) than for vermiculite (Hg=28.0 meq/100 g). The basal spacing for the Hg treated samples was 15.2 (montmorillonite) and 14.5 Å (vermiculite). Thermal and evolved gas spectrometry analyses suggest that Hg was released at T230 °C and at 600 °C for montmorillonite, but at 550, 800 and 860 °C for vermiculite.The effect of temperature on Hg release is also apparent when the basal spacing at 230 °C for montmorillonite (d₀₀₁=10.3 Å) is compared to that for vermiculite (d₀₀₁=11.8 Å).EXAFS analyses provide qualitative evidence that oxygen atoms occupy the first coordination shell of Hg in both clay minerals. The best fit between observed and calculated spectra is obtained when montroydite is assumed as a reference model compound.     

Polyimide/organo‐montmorillonite nanocomposites: A comparative study of the organoclays modified with aromatic diamines

Six organophilic clays have been obtained through cation‐exchange between sodium montmorillonite (Na^+‐Mt) and the hydrochloride salts of aromatic diamines (DA1–6). The results obtained by thermogravimetric analysis (TGA) showed that the organophilic clays start to decomposition within 150–340°C, which shows that they are thermally stable compared with conventional montmorillonite modified with aliphatic long‐chain quaternary alkyl ammonium salts. The highest thermal stability and interlayer basal spacing were observed for the organoclay obtained from 3,3′‐sulfonyl dianiline (DA2), and therefore it was chosen for preparing clay/polymer nanocomposite materials (CPN). Polyimide/clay nanocomposite materials consisting of benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride (BTDA) and 2‐(5‐(3,5‐diaminophenyl)‐1,3,4‐oxadiazole‐2‐yl)pyridine (POBD) were also obtained by an in situ polymerization reaction through a thermal imidization. DA2‐Mt was used as filler at different concentrations. Both the thermal stability and the glass transition temperature (T_g) are increased with respect to pure polyimide (PI) at low clay loadings. At high clay concentrations, the organoclay particles make aggregate and as results of this phenomena T_g and thermal stability are decreased. POLYM. COMPOS., 36:613–622, 2015. © 2014 Society of Plastics Engineers     

Changes in potassium exchangeability in heated lepidomelane

Changes in the K behavior of lepidomelane were induced by heating samples of the mica. These changes were related to concurrent alterations in the heated mineral to obtain information about the role of mineral properties in the exchangeability of interlayer K. The masking effects of small particles and fixable cations in the extractant were minimized by using 5–10 μm samples and NaCl---NaBPh₄ extracting solutions, respectively. The mica samples were heated in a muffle furnace for different periods at several temperatures between 300° and 950°C before they were characterized in terms of their K release to the NaCl---NaBPh₄ solutions. The effects of these thermal treatments on the Fe²⁺ content, d-spacing and weight of the mica samples were also recorded.The thermal treatments reduced the rate and degree of interlayer K exchange in the lepidomelane. Heat treatments at 300°C had relatively little effect, but the extraction of interlayer K required progressively longer periods when the temperature was increased incrementally to 950°C or the heating period at these higher temperatures was increased. Using the amounts of K extracted in 3 days as an index of the average rate of K release, a close relationship between the K release rate and Fe²⁺ oxidation in the heated mica was shown to exist. At the same time, however, reductions in the sample weight and basal spacing of the mineral occurred and were attributed to dehydroxylation in the heated mica. These changes in basal spacing probably had some effect on the rate with which all the K was released but had a more pronounced effect on the last 30% of interlayer K and finally made this part of the K nonexchangeable. The reduction in maximum extractable K indicates that a portion of the mica layers have a higher charge density. The processes of dehydroxylation and Fe²⁺ oxidation occurred independently in the heated mica and are reflected in the combined effects of basal spacing, layer charge and OH status of the mineral on interlayer K exchange.     

Changes in the morphology of organoclays with HDTMA surfactant loading

The detailed understanding of the interlayer structure of organoclays is of importance in the design of organoclay based materials and their industrial applications. In this study, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to provide new insights into the interlayer structure and morphology of HDTMA⁺/montmorillonite organoclays. XRD patterns show that thermal treatment has an important effect on the stability of organoclays, reflected by significant changes in the basal spacing. TEM and SEM micrographs demonstrate that the organoclays with lower surfactant packing density are mainly composed of irregular layer stacking with a number of curved organoclay layers, while those with higher surfactant packing density are mainly composed of regularly intercalated and flat layers. Variations of the interlayer distances exist in all organoclays and are more pronounced in the organoclays with lower surfactant packing density. This study demonstrates that not only the arrangement model of surfactant but also the morphology of organoclay strongly depend on the surfactant packing density within the montmorillonite interlayer space.     

NMR evaluation of montmorillonite's d-spacings on the formation of phosphocarbonaceous species in intumescent systems

To further investigate the influence of the d-spacing of montmorillonite types derived from Cloisite Na on the process of the intumescent layer formation, composites of poly(ethylene-co-butylacrylate), EBA-30, with an intumescent formulation consisting of ammonium polyphosphate and pentaerythritol were processed and heated at different temperatures. The residues were analyzed by ¹³C, ³¹P, and ²⁷Al solid-state nuclear magnetic resonance (NMR). ¹³C NMR peaks in the aromatic range of 100–150 ppm for samples heated at 350 °C suggest that the montmorillonites with smaller d-spacing (13.5 and 21.8 Å) accounted for the greater amounts of condensed aromatic structures, leading to the char. For d-spacing of 35 Å, the condensation of the polyaromatic structures occurs more slowly, retarding the char formation. The ²⁷Al NMR peak around −15 ppm for samples heated at 280 °C indicates the destruction of montmorillonite structure, producing aluminophosphate species, meaning that the d-spacing influences the reactions under this temperature. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48053.     

High-pressure adsorption of methane on montmorillonite,kaolinite and illite

Methane (CH₄) adsorption of Ca^2 +-montmorillonite (Mt), kaolinite (Kaol) and illite (Il) at 60 °C and pressures up to 18.0 MPa was investigated, during which the adsorption capacity was evaluated by the Langmuir adsorption model. The influences of adsorbed water and the interlayer distance of the clay minerals on CH₄ adsorption were explored by using heated Mt products with different interlayer distances as the adsorbent. Mt, Kaol and Il showed high CH₄ adsorption capacities, and their maximum Langmuir adsorption capacities were Mt, 6.01 cm³/g; Kaol, 3.88 cm³/g; and Il, 2.22 cm³/g, respectively. CH₄ was adsorbed only on the external surface of Kaol and Il; however, adsorption also occurred in the interlayer space of Mt, which had a larger interlayer distance than the size of a CH₄ molecule (0.38 nm). CH₄ adsorption in the interlayer space of Mt was supported by the lower CH₄ adsorption capacity of heated Mt products (with the interlayer distance < 0.38 nm) than that of Mt at high pressures despite the higher external surface areas of the heated Mt samples. The entrance of CH₄ into the interlayer space of Mt occurred at low pressures, and more CH₄ molecules entered the interlayer space at high pressures. Moreover, the adsorbed water occupied the adsorption sites of the clay minerals and decreased the CH₄ adsorption capacity. These results indicate that clay minerals play a significant role in CH₄ adsorption of shale and indicate that the structure and surface properties of clay minerals are the important parameters for estimating the gas storage capacity of shale.     

Removal of methyl orange from aqueous solutions using a bentonite modified with a new gemini surfactant

Glycol bis-N-cetylnicotinate dibromide (designated as GN16-1-16), a new cationic gemini surfactant, was prepared and used to modify bentonite. Bentonite modified with commercial cetyltrimethylammonium bromide (CTMAB) was also prepared for comparison purposes. FTIR and XRD revealed that both surfactants successfully intercalated into the bentonite layers. The basal spacing was 2.65 nm for GN-Bt (the bentonite modified by GN16-1-16) and 2.14 nm for C-Bt (the bentonite modified by CTMAB), indicating that GN16-1-16 was more efficient than CTMAB in expanding the interlayer space of montmorillonite. The optimum reaction time and temperature in the modification were 1 h and 30 °C for GN16-1-16 and 3 h and 70 °C for CTMAB. The GN16-1-16 reacted with bentonite faster than CTMAB. The decoloration rate and the COD removal of methyl orange (MO) solution were 99.02% and 90.62% for GN-Bt and 80.12% and 75.49% for C-BT. Therefore, GN-Bt was more effective than C-Bt to remove MO from aqueous solution. However, the efficiency of GN-Bt decreased rapidly at pH > 6, which might be due to the hydrolysis of the ester groups in GN16-1-16 molecule under alkaline environment.     

Adsorption of gaseous SO2 and structural changes of montmorillonite

Several montmorillonite samples after adsorption of gaseous SO₂ were analyzed to evaluate structural and textural changes. The equilibrium adsorption of the SO₂ gas was measured at 25 °C and 0.1 MPa. The samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), swelling index (SI), pH measurements, and N₂ adsorption–desorption isotherms. SO₂ adsorption increased with the specific surface area of montmorillonite. SO₂ retention decreased pH of the dispersed samples from 6 to 1 and released interlayer and octahedral cations from the structure, which increased the specific BET surface area and specific micropore surface similar to that of acid-activated montmorillonite.     

Chitosan intercalated montmorillonite: Preparation, characterization and cationic dye adsorption

Chitosan intercalated montmorillonite (Chi-MMT) was prepared by dispersing sodium montmorillonite (Na⁺-MMT) into chitosan solution at 60 °C for 24 h. The Chi-MMT was characterized by XRD, XRF and FT-IR. The intercalation was accomplished via the ion-exchange of Na⁺ ions with –NH₃⁺ of chitosan, resulting in the expansion of d₀₀₁ from 1.42 nm of Na⁺-MMT to 2.21 nm of Chi-MMT. The chitosan content in the Chi-MMT measured by TGA was about 17 mass%. The adsorption capacity of Chi-MMT was investigated in comparison with the starting Na⁺-MMT and chitosan using three different cationic dyes, i.e. basic blue 9 (BB9), basic blue 66 (BB66) and basic yellow 1 (BY1). The Chi-MMT showed the highest adsorption capacity in the range of 46–49 mg/g when the initial dye concentration was 500 mg/L, being equivalent to 92–99 wt.% of dye removal. The adsorption capacities of Chi-MMT for all basic dyes increased with an increase of initial dye concentration. An increase of adsorption capability of Chi-MMT was attributed to the existence of intercalate-chitosan. It could enlarge the pore structure of Chi-MMT, facilitating the penetration of macromolecular dyes, and also electrostatically interact with the applied dyes. These results indicated the competency of Chi-MMT adsorbent for basic dye adsorption.     

Quantitative characterization of the solid acidity of montmorillonite using combined FTIR and TPD based on the NH3 adsorption system

The complete parameters of montmorillonite solid acidity, namely amount, strength, and types of acidity, were determined and the properties of the acid sites after heating were proposed by combining the temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR) based on the NH₃ adsorption system. The total amount of montmorillonite acid sites was 1.15 mmol/g, which was higher than the value obtained by the Hammett indicator method because of the detection of solid acid sites in the montmorillonite interlayer space. These acid sites were composed of 1.00 mmol/g Brønsted and 0.15 mmol/g Lewis acid sites. The acidity of montmorillonite was primarily derived from the interlayer polarized water, Si–OH, H₃O⁺ adsorbed by negatively charged tetrahedral AlO₄, and unsaturated Al^3 + ions, all of which were attributed to the Brønsted acid sites with the exception of the unsaturated Al^3 + ions (Lewis acid sites). Heating led to an increase in the acid strength and the acid amount and altered the type of the partial acid sites. The interlayer polarized water provided more protons after heating at 120 °C and exhibited higher acid strength than that of raw montmorillonite. After heating at 400 °C, the interlayer polarized water acted as very strong acid sites. The H₃O⁺ adsorbed by tetrahedral AlO₄ was attributed to weak-strength acid sites and transformed into Si–O(H)–Al after dehydration, while displaying strong-strength acidity. The unsaturated Al^3 + ions showed medium-strength Lewis acidity, although a portion of these ions adsorbed water molecules and exhibited weak Brønsted acidity. After dehydroxylation at 600 °C, an abundance of unsaturated Al^3 + ions appeared and the amount of Lewis acid sites increased.