Organophilic Synthetic Stevensite-Zn: Synthesis and Characterization, an Alternative Simple Method

Abstract

Organophilic clays are used as components in petroleum fluids, lubricants, paints, adhesives, cosmetics, and others. This requires products that are increasingly pure with homogeneous properties, which are hardly found in natural clays. Synthetic clays can thus be a solution. For this, it is necessary to explore techniques for their production. The main focus of this study was to synthesize a smectite clay in a simple, fast, and economic way at a low temperature and pressure. To regulate the pH of the reaction and bring it closer to the ideal synthesis conditions, two different acids were used: nitric acid (the most widely used in the literature) and hydrochloric acid (more accessible and cheaper). Organostevensite was obtained by the direct incorporation of quarternary salt in synthetic stevensite using a semi-dry, fast, economic, and ecological method compared to that used in industry. The results showed characteristics of a uniform crystalline material with a porous structure similar to that of the smectite group. Small particle sizes of approximately 100 nm were observed by SEM. Stevensite clay was obtained by both synthesis methods using two different acids; the method using hydrochloric acid stood out. The greater practicality of its development provides a more viable option for large-scale industrial production.

1. Introduction

Smectite clays belong to the 2:1 phyllosilicate group and are important materials for a wide range of applications such as ceramics, rheological additives, binders, sorbents, ion exchangers, molecular sieve catalysts, and nanotechnology [1,2,3]. Their range of applications derives from their chemical and physical properties such as high cation exchange capacity (CEC) and high surface area [4]. Furthermore, the addition of organic modifiers such as quaternary salts makes them organophilic, which further expands their applications [5,6].

Natural clay minerals have been used for decades in many applications due to their good properties and low costs and for being easily accessible in nature; however, they are generally not homogeneous in composition or particle size, presenting impurities and variations that may limit their use. Furthermore, some clay minerals are not available in deposits in high enough amounts [3,7,8,9,10]. These negative factors are the greatest limitation of the application of natural clay minerals to some industrial and academic processes, increasing the interest in synthetic clay minerals produced under controlled conditions and with known compositions. Mostly due to their composition and structure, these clays can provide advanced functional materials for new applications or studies that require homogeneous and well-defined samples [3,11]. Some examples are the incomplete exfoliation of polypropene nanocomposites related to impurities [12], incomplete organophilization of clays [13], and formulation of pharmaceutical products [14].

Over the past 60 years, many smectite synthesis experiments have been carried out under different temperature, pressure, and moderate and extreme hydrothermal treatment conditions [15,16] such as montmorillonite [17]; nontronite [18]; saponite [19]; Mn–saponite, Mn–hectorite, and Mn–stevensite [20]; Zn-smectite [21]; and Mg, Ni, Co, Zn, Cu, and Fe smectites [22]. The main objective of the procedures is to obtain pure samples faster and at lower temperatures. These two parameters are important for recreating the formation of mineral clay on a laboratory scale and reducing the energy required for clay synthesis [23], as most conditions are high temperatures/pressures. Some trioctahedral smectitic clay minerals [4], such as saponite, hectorite, and stevensite, can be synthesized at low temperatures and pressures by the hydrolysis of urea using a silicon source and different oxides or hydroxides (magnesium oxide, aluminum oxide, zinc oxide, lithium hydroxide), as reported in [3]; however, they use more sophisticated methods with reactors, electrolytic buffers, and autotitrators [21,24], increasing the cost of manufacturing and hindering the industrial use of synthetic clays. Stevensite is a trioctahedral smectite, which contains an octahedral sheet rich in divalent cations (Zn${}^{2+}$, Co${}^{2+}$, Mg${}^{2+}$ among others) and a tetrahedral sheet rich in Si [25]. It has a basal space of around 13 to 15 Å [21,24]. Due to the ease of stevensite formation under less rigorous conditions and the existence of a gap in the studies on the organophilization of synthetic clays using simple and practical procedures, it would be of interest to obtain organophilic synthetic stevensite clay produced by a simple hydrothermal process and organically modified using a quaternary ammonium salt, also using a simple method. This work aims to present the process for obtaining a material with the potential for technological use with high purity, a known composition, a controlled particle size, and high reproducibility and compatibility with non-polar organic materials.

2. Materials and Methods

2.1. Materials

The reagents used as the silicon and zinc sources in the synthesis process were, respectively, sodium metasilicate pentahydrate (Na${}_2$SiO${}_3$·5H${}_2$O) and zinc nitrate hexahydrate (Zn(NO${}_3$)${}_2$·6H${}_2$O). Urea (CO(NH${}_2$)${}_2$) was used to complete the reaction mixture [24]. Two acids were also used to adjust pH: concentrated nitric acid (HNO${}_3$) and concentrated hydrochloric acid (HCl). For the organophilization of the clays, the cetyl trimethyl ammonium chloride (C${}_{19}$H${}_{42}$NCl) quaternary ammonium salt was used.

2.2. Methods

2.2.1. Clay Synthesis

The synthesis of the synthetic clay was performed with a simple and fast method using two different acids, nitric acid (more commonly used in the literature) and hydrochloric acid, obtaining two clays: NAS—nitric acid stevensite—and HAS—hydrochloric acid stevensite. The first solution was prepared by the dilution of 90 g of Na${}_2$SiO${}_3$·5H${}_2$O and 45 g CO(NH${}_2$)${}_2$ in 500 mL of water with a magnetic stirrer at an initial pH with a range of 1–2 adjusted with nitric acid [24] and hydrochloric acid. Then, 45 g of Zn(NO${}_3$)${}_2$·6H${}_2$O was added and the solutions were finally placed under mechanical stirring for 5 min (25 °C ambient temperature) leading to the formation of a rather thick gel. After that, the aliquot of the solution obtained was conditioned in a sealed glass reaction vessel and placed in an oven at 90 °C without stirring for 24 h. Finally, during the synthesis process, the samples were filtered, washed with water, and dried at 90 °C for 24 h. The samples were named according to the pH-regulating acid process. NAS—S: stenvensite; NA: nitric acid; HAS—S: stevensite; CA: cloridric acid; Organo: organophilic.

2.2.2. Organophilization

The organophilization of the clays was carried out using a non-aqueous method in which the salt was intercalated with small amounts of water and the washing step that occurs in the aqueous method was dismissed, making it an easier, more economic, and sustainable process. Therefore, the preparation of this process consisted of mixing 2 g of dry clay in a mortar with 1.28 mL of C${}_{19}$H${}_{42}$NCl and approximately 5 mL of distilled water to facilitate homogenization, forming a paste that was placed in an oven at 60 °C for 24 h and then disaggregated. The quaternary salt concentration used in the semi-dry method was calculated based on the CEC of the synthesized clays (NAS = 85.82 meq/100 g and HAS = 70.10 meq/100 g) [26,27].

2.3. Characterization

The samples of the synthetic clay were characterized by XRF, XRD, SEM, FTIR, BET, and TG-DTG. The chemical compositions of the clay samples were characterized by X-ray fluorescence (XRF) using a Zetium analyzer from Malvern Panalytical. The ignition (LOI) was carried out at 1020 °C for 2 h. XRD analyses were performed with PANanalytcal X Pert Pro Mpd equipment using CuK$\alpha$ radiation ($\lambda$ = 1.5406), 40 kV voltage, 30 mA current, scanning of 2$\theta$ from 5 to 70°, and a scanning speed of 2°/min. Infrared spectroscopy samples were analyzed using a Nicolet iS5 Fourier transform infrared spectrometer (FTIR) from Termo Fisher Scientific, scanning 4000 to 650 cm${}^{-1}$ [28]. The morphology and dimensions of the clay particles were evaluated by a scanning electron microscope (SEM) Inspect 50. The samples for the SEM analysis were coated with gold [29]. The thermal analyses of the clay were performed in a NETZSCH thermal analyzer, model STA 449F3 STA449F3A-0715-M, under nitrogen flow, between 20 and 1000 °C, with a heating rate of 20 °C per minute. The synthetic organophilic clay was evaluated by XRD and FTIR and the swelling capacity was evaluated through the Foster assay [30] in order to verify the affinity of the quaternary salt with the organic molecules of the solvents. The solvents used in this test consisted of water, ethyl alcohol, methyl, propyl alcohol, toluene, xylene and kerosene.

3. Results

3.1. Synthetic Clay

3.1.1. X-ray Fluorescence (XRF)

The chemical composition of synthetic clay (NAS and HAS) described in

Table 1. Chemical analysis data (wt.%) of stevensite NAS and HAS.

Oxides (%)	${\mathbf{Na}}_2{O}$	MgO	${\mathbf{Al}}_2{{O}}_3$	${\mathbf{SiO}}_2$	${\mathbf{SO}}_3$	Cl	${{K}}_2{O}$	CaO	${\mathbf{Fe}}_2{{O}}_3$	ZnO	LOI *
NAS	1.2	-	0.1	59.1	-	-	-	-	-	27.8	11.9
HAS	1.3	-	0.1	61.1	-	0.8	-	-	-	25.6	11.1

* LOI: weight loss on ignition at 1020 °C.

was obtained from the X-ray fluorescence analysis. The main constituent of the clays was silicon oxide (NAS: 59.1% and HAS: 61.1%). Aluminum oxide was present (0.1%), probably due to the material in the sample holder; in addition, zinc oxide (25.6%–27.8%) was identified in the composition of a trioctahedral zinc smectite corresponding to stevensite. A small percentage of chlorine was observed for the HAS clay (0.8%), indicating the presence of hydrochloric acid in the synthesis. The presence of chlorine may have influenced the decrease in the percentage of ZnO in the HAS clay. The LOI value corresponds to the loss in mass resulting from the heat treatment of the sample at 1020 °C. The LOI value was observed to be about 12% due to the loss of adsorbed water, dehydroxylation, and decomposition of unreacted matter. These results agree with those of the infrared analysis [31].

3.1.2. X-ray Diffraction (XRD)

Figure 1 presents the XRD results of the NAS and HAS clays in which a peak was observed corresponding to the basal distance $d_{\left(060\right)}$ of 1.53 Å (2$\theta$ = 60.6${}^{\circ }$) that is typical of trioctahedral smectite clay. The clays exhibited behavior and characteristics similar to those of a synthetic stevensite clay, suggested by the peaks at 2$\theta$ = 5.8${}^{\circ }$(14–15 Å), 19.8${}^{\circ }$ (4.5 Å), and 34.6${}^{\circ }$ (2.6 Å), as reported in the literature [24,29,32]. The values found were also similar to those of natural stevensite clay, as observed in [33,34,35], where it had a maximum base spacing of 16.8 Å. Variations in the reflections of the peak $d_{001}$ were expected due to the way the clay layers were stacked during the synthesis period. In addition, the XRD results revealed the presence of ZnO corresponding to a peak at 34.6${}^{\circ }$ (2.61 Å) according to the standard crystallography database (JCPDS 36-1451), indicating the formation of synthetic zinc-rich stevensite, and were in agreement with the XRF and FTIR analyses. The 2$\theta$ peaks at 29.3${}^{\circ }$ for the NAS clay and at 31.4 and 45.6${}^{\circ }$ for the HAS clay, respectively, correspond to sodium nitrate $\left({\mathrm{NaNO}}_3\right)$ and sodium chloride (NaCl) and were attributed to an incomplete clay-washing process.

3.1.3. Infrared Spectroscopy (FTIR)

In the NAS and HAS infrared spectra shown in Figure 2, the bands located at around 3650–3690 and 3350–3370 cm${}^{-1}$ were attributed to the OH stretching of the clay, which is typical of trioctahedral smectites. The band at 1624 cm${}^{-1}$ corresponds to the stretching vibrations of the OH group, which were related to the adsorbed water present in the smectite. The band at around 1000–1060 cm${}^{-1}$ corresponds to the elongation vibrations of the Si-O bonds [36] and the band at 660 cm${}^{-1}$ corresponds to the vibration of the Si-O-Zn bond in the tetrahedral sheets [21]. It was noted that there were no significant changes in the specific bands of clay minerals in either clay spectrum; however, the appearance of a band at 2360 cm${}^{-1}$ in the NAS clay can be attributed to the presence of adsorbed water molecules [4].

3.1.4. Scanning Electron Microscopy (SEM)

Figure 3 and Figure 4 show the SEM images of the clays. The micrographs allowed us to observe that the clays had similar behavior. The material appeared quite uniform, with regular faces (characteristic of crystalline materials), a high-porosity structure, and some [16] aggregates. It was observed that the samples presented small particles with dimensions of the order of nanometers and diameters of approximately 200 nm, which tended to aggregate and curl up on the edges of the thinnest plates, a typical structure of clays of the smectite group [36,37]. Other authors observed similar behavior [24,29,38]. These results are important, as natural smectitic clays generally have diameters greater than 1 micrometer. Note that there were no appreciable changes in the morphology of the clay particles produced with nitric acid and hydrochloric acid, indicating that the use of hydrochloric acid in the synthesis did not change the morphology of the clay, corroborating the results from the XRD analysis.

3.1.5. Thermal and Differential Analysis (TG/ DTG)

The TG/DTG curves of synthetic clays are shown in Figure 5. The thermal behavior of the clays was observed as showing the same tendencies and characteristics as those of stevensite [39]. An intense peak was identified in the range of 80∼96 °C, indicating the dehydration of the stevensite clay, followed by a second peak at a temperature of approximately 210 °C, which was attributed to the loss of water coordinated to interlamellar cations. This loss of water occurs at a higher temperature because coordinated water molecules are more strongly bound, requiring more energy in the form of heat for this bond to be broken [40].

3.1.6. Nitrogen Adsorption Analysis—BET

Figure 6 shows the nitrogen adsorption and desorption isotherms for the NAS and HAS samples. The isotherms correspond to type IV isotherms with a hysteresis loop compatible with type H3 according to the IUPAC classification. The hysteresis loop H3 indicates the presence of micropores associated with mesopores and macropores [41]. This type of loop is normally observed in the presence of nonrigid aggregates of particles in the form of plates, giving rise to slit-like pores characteristic of lamellar materials, which is a similar behavior to natural stevensite clay [42].

Table 2. BET total specific area values, pore volume, external surface area, and average pore and nanopore diameter of the samples.

Data	NAS	HAS
BET area (m${}^2$/g)	163.16	197.41
Pore volume (cm${}^3$/g)	0.93	1.09
External surface area (m${}^2$/g)	189.60	213.29
Average pore diameter (Å)	196.82	205.48
Mean diameter of nanopores (Å)	367.73	303.93

shows the results extracted from the BET tests related to the specific surface area, pore volume, and pore diameter of the materials produced. The analysis shows that the area values found for the synthesized clays were similar to those verified by other authors [24,33,42] and were characterized by smaller particle sizes (thus supporting the SEM analysis) and higher surface areas. In addition, it was observed that the HAS clay sample had a higher BET surface area (197.41 m${}^2$/g) (sum of the external surface area and micropore area) and higher absorption of ${\mathrm{N}}_2$ (708.78 cm${}^3$/g) than the NAS sample (163.16 m${}^2$/g; 607.13 cm${}^3$/g), suggesting that the HAS clay had narrower and deeper pores favoring nitrogen absorption. These differences may have been the result of the change in the chemical composition of the clay, as seen in the XRF analysis, and the evaporation and condensation that occurred during the synthesis [24].

Furthermore, it was verified that the values obtained for the specific area of the synthesized stevensite clays were even higher than those of the natural bentonite clays (specific area = 30∼80 m${}^2$/g), which are the most commonly used in industry. This high surface area of stevensite revealed a high porosity that may be responsible for the high capacity of this material to fix some cations and non-ionic [37] compounds, a very important factor for their use in delivery systems.

3.2. Organophilic Synthetic Clay

3.2.1. Evaluation of Organophilization by X-ray Diffraction—XRD

From the XRD analysis (Figure 7) of organophilic stevensite, it was possible to identify that some stevensite reflections were not modified compared to Zn-stevensite; however, displacements to smaller angles and, consequently, small increases in the basal distance from the peak $d_{001}$ of the samples for NAS (14 Å) and HAS (15 Å), respectively, were observed of 15.6 Å and 16 Å after the organophilization process. According to the literature, this small increase is probably related to the interlamellar composition and the degree of isomorphic substitutions [16,24]. Additionally, there was a widening of the peaks, possibly related to the lack of long-range ordering and the low amount of stacking, which was also observed in [16]. Therefore, it can be suggested that the quaternary ammonium salt caused the modification of the clay structure and that the semi-dry process is a good alternative for the modification of clay minerals since it is an easier and more economic process and reduces the amount of excess water used in the washing step.

3.2.2. Infrared Spectroscopy (FTIR) of Synthetic Clays Organophilized Using the Semi-Dry Method

The curves of the organophilic clays, as seen in Figure 8, showed the specific bands of the clay mineral since the organophilization did not change the structure of the stevensite; however, bands related to the carbonic chain of the salt were observed, with bands at 2920 and 2840 cm${}^{-1}$ corresponding to the asymmetric and symmetric stretching of the ${\mathrm{CH}}_2$ group, respectively. A band at 1640 cm${}^{-1}$ corresponds to the H–O–H bond of the adsorbed water. The vibrations at around 1480 and 1384 cm${}^{-1}$ are attributed to the bending of the C–H bonds of the ${\mathrm{CH}}_3$ and ${\mathrm{CH}}_2$ groups. The appearance of the absorption band at a frequency of 719 cm${}^{-1}$ is attributed to the asymmetric angular deformation of the ${\mathrm{CH}}_2$ group [43]. The FTIR indicated that there was a possible insertion of the carbonic chains of the quaternary salt in the clay. The FTIR results are complementary to the XRD diffractometry studies, suggesting the presence of salt and that the low increase in the basal spacing of organophilic clays observed in the XRD did not characterize the non-incorporation of quarternary salt and that possibly, the process carried out for organophilization may have caused interactions of the quaternary ammonium salt molecule in different positions between the clay layers. Therefore, the semi-dry process of organophilization, is a good alternative for obtaining organophilic stevensite.

3.3. Swellability in Water and Organic Liquids of Synthetic and Organophilized Clays

Table 3. Swelling values of organophilic clays after stirring.

Samples	Swelling (mL/g)
	Water	Ethyl Alcohol	Methyl Alcohol	Isopropyl Alcohol	Toluene	Xylol	Kerosene
NAS Organo	2.0	2.0	4.0	4.0	2.0	2.0	4.5
HAS Organo	2.0	8.0	4.0	4.0	2,0	2.0	4.0

shows the results of the swelling test of the organophilic clays in water and organic solvents by volume. The clays did not swell in water (2 mL/g) as expected. The NAS sample did not swell in ethanol, toluene, or xylene (2 mL/g) but swelled in methanol, isopropanol, and kerosene, with swelling values of 4.0 and 4.5 mL/g, respectively. The HAS sample did not swell in toluene or xylene (2.0 mL/g) but swelled in methanol, isopropanol, kerosene (4 mL/g), and ethanol (8 mL/g). These swelling values show the organophilic character of the materials obtained. Note that depending on the acid used in the synthesis, materials that did and did not swell in ethanol could be obtained, and both samples swelled in polar solvents such as methylene and non-polar solvents such as kerosene.

To verify if there was any change in the crystalline structure of the organophilic clays dispersed in ethyl alcohol and kerosene (they showed greater swelling), they were analyzed using XRD, as shown in Figure 9. Through the diffractograms, it was verified that the peak $d_{001}$ disappeared for the clays in ethyl alcohol and kerosene, indicating a possible destructuring of the clay layers and that organophilization may have facilitated the interaction with the organic liquids, supporting what has been reported in the literature [30,44]. There seems to be no direct relationship between the cation exchange capacity and expansion volume. The effect of the intercalation of other types of quaternary ammonium salts and the physicochemical and surface changes between the HAS and NAS samples will be the subjects of subsequent studies.

4. Conclusions

The synthesis of stevensite clay using a hydrothermal process and the preparation of organophilic clays from synthetic stevensite in a semi-dry process were successfully carried out. The results showed that it was possible to synthesize Zn-stevensite smectite clay at low temperatures and pressures using both methods. The static process, which uses hydrochloric acid to regulate pH, stood out because it is a simple and economic method (without sophisticated equipment) and reduces the zinc stevensite time of the synthesis process by 50% compared to the methods established in the literature. The use of hydrochloric acid in the development of the synthetics was essential since it is an acid of easier commercialization for large-scale production compared with nitric acid. The X-ray patterns suggested that the clays produced are expandable and that their morphology is composed of small, regular, and uniform particles in the order of 100 nanometers, unlike the natural form that usually presents particles in the order of micrometers. It was evidenced that the semi-dry organophilization method is efficient; the clays presented similar properties compared to the properties of the clays prepared using conventional methodology. In addition, it was shown to be a faster process that was easier to execute, with less water consumption, lower environmental impact, and more industrial advantages. The techniques and characteristics presented in this study are important for future applications of the new stevensite synthetics and organoclays.