Porous frameworks from Ecuadorian clays
1 Universidad de Tecnología Experimental Yachay Tech, Escuela de Ciencias Químicas e Ingenieria, Grupo de Investigación Aplicada en Materiales y Procesos (GIAMP); email@example.com, firstname.lastname@example.org & email@example.com
* Correspondence: firstname.lastname@example.org; Tel.: +593995371712
Available from: http://dx.doi.org/10.21931/RB/2022.07.01.33
This research provides a literature review on several topics as a foundation to comprehend porous materials, their structure, and behavior to explore how they can be derived from clays and nanoclays. In this case, considering the several minerals present in some Ecuadorian clays, which are a potential starting material for the synthesis of porous frameworks, they constitute a solid source of metal atoms such as Silicon or Aluminum. This research presents the evaluation and characterization via XRD and AAS of clay samples collected in the southeast of Ecuador in the provinces of Azuay, Morona Santiago and Zamora Chinchipe, which present diversified soil mineralogy with many chemical and crystallographic features for suitable precursors in nanomaterials design.
Keywords. porous frameworks, clays, nanoclays, zeolites, X-ray powder diffraction, AAS.
The greenhouse effect, global warming, and climate change have come to a state where little to nothing can be done or approached in a traditional way to mitigate or tackle their effects. Scientists have a hard job developing innovative solutions for this pollution problem. Yet, since the early 2000s, there have been advances in gases capturing and removing them from the atmosphere through chemical processes. However, these efforts are not enough, and the approach has evolved to merge these chemical processes to physical ones using nanotechnology to achieve the adsorption of different kinds of gases. The presence of acid gases in the atmosphere has had a crucial impact on the quality of life of people everywhere. Nowadays, targeting the concentration of such gases in the atmosphere is one of the biggest goals of science. One of the recent techniques is the adsorption process of such gases through porous frameworks1.
Now, what can one understand how porous framework materials are? To answer that, one can see at these materials' nano or picoscopic scale that if the constituents are not densely stacked but form voids, the material is defined as porous material2. To exemplify this better, it is easy to picture a bee panel where there are blocks formed, leaving voids to be filled with honey. In materials science, those voids are to be filled with substances such as acid gases that are chemically and structurally compatible and trapped in the frameworks. Therefore, one can say that porous frameworks materials are becoming all those with voids and a structure with framework form.
Porous frameworks synthesis can imply a wide range of processes. Materials with permanently porous structures made either entirely from organic building blocks or a combination of organic ligands and inorganic nodes have been at the forefront of chemistry for two decades3.
Decades of painstaking observational and empirical synthetic advances have made it possible to predict, with a relatively high degree of confidence, which structures might result if certain building blocks are joined together4. Porous frameworks, whether they are organic or metal-organic frameworks or zeolites, have emerged as advanced materials with a wide range of applications such as chemical catalysis, gas adsorption, ion exchange, and advanced nanotechnology applications, as will be discussed later.
In the adsorption process, the molecules or ions are to be adhered to a surface rather than penetrate the framework. Particularly in nanostructured materials, the applications field for porous frameworks become a powerful tool since they present large surface areas, high stability, and small size5. For the adsorption process, the most remarkable feature is the large surface-volume ratio since it represents more binding sites.
In Chemistry, the applications of interest are generally removing undesired compounds, molecules, and impurities from different matrixes that may contain contaminants traces or subproducts. Therefore, the porous frameworks may act as sieves, binding layers, or regular adsorbents with an appropriate chemical reactivity, improving their efficiency when using nanostructured materials6.
Nowadays, nanomaterials constitute a wide range variety of materials. One of the emerging types is nanoclays, which are naturally occurring or synthetic clays treated and scaled to nanostructures7. Nanoclays, are of great interest since they represent an opportunity for industrial and technological applications. Nanostructures display enhanced functional features that are not found in larger dimensions materials8.
Consequently, the field also thrives because it has a relatively low barrier to entry: it does not generally require sophisticated apparatus or complicated synthetic techniques. This allows contributions from synthetic chemists and engineers, spectroscopists, and physicists3. All of whom are spurred by the increasing availability of these materials without necessarily relying on collaborators to supply samples.
The hydrothermal synthesis method of porous frameworks has become of consistent and urgent interest for material scientists due to its easy access when one refers to equipment and reagents used in the laboratory as vital factors for the manufacture of the monomers producing the framework afterward. Performing a hydrothermal method for the synthesis, there is a higher chance of successfully modifying the framework. It is well known that clays display many exciting components to produce multiple porous frameworks3 since they contain silicon, iron, and aluminum minerals.
Porous materials and frameworks
Porous material can be defined as every solid or primarily solid material that presents pores in its structure, giving certain features relative to the system's porosity, pore size, and the fraction of pore volume concerning the total volume of the material9. Applications of porous materials occupy a varied assortment; they are commonly used as insulators, transistors, and conductors in the electronic industry as well as sieves for the water filtration system, chemical catalysis, etc10,11. Yet research in this area continues to take innovative and necessary paths.
The efforts of researchers have been reflected in the synthesis and production of materials that are being applied in processes for the elimination of polluting substances, employing the adsorption process of compounds whether they are in the liquid or gaseous phase. These materials are known as adsorbents, and these have high demand, but like any kind of material, their applications may be limited because of the precursors, specifically to the use of contaminating substances as templates, as well as due to the synthesis methods. So, the zeolites are examples of excellence since their synthesis is a greener approach.
Zeolites, silica gel, intercalated layered materials, etc., are common minerals used in such applications due to their pore dimensions9. These kinds of materials can be used in complex conditions given their ilk. The particularity of zeolites is that their structure is what gives such behaviors that can be utilized in several industrial applications like catalysis, gas adsorption, water purification, and treatment12.
Figure 1. Formation of a zeolite, from the primary TO4 to secondary building units and further assemble to form extended zeolite13.
There is particular importance in the synthesis of the porous materials because of the specifications that they must present to be adequate and suitable for specific applications; therefore, the pore size, the porosity, the surface area, etc., must be controlled. Even in a more intricate way when the material is of nanometric scale. Engineering the design of porous material in clays is intended to change porosity, surface area14, surface content of solids, as well as thermal stability15.
Although the engineering process for the obtention of zeolites from layered silica clays has reached a high interest in the material sciences scenario, the research on three-dimensional structures (3D) from these compounds has been limited. This may be due to the limited application of pillared clay, so the effort to increase the use of coated material through pore engineering begins to transform the coating into a zeolite structure16. Now, data shows over two hundred types of zeolites based on a silica-alumina ratio17.
Clays are an extended collection of minerals, yet in chemistry, they are known as hydrous silicates18. They are found in nature from different natural sources and can be produced in their synthetic form. Their natural origin is after geological processes; whether they were physical or chemical processes, weather-depending processes, decompositions, etc., it will determine both the composition and properties of the clays. Even though they can travel because of natural phenomena, they are generally found near their origin site. Clay minerals may be divided into four major groups, mainly in terms of the layered structure variation, as shown in Table 1. These include the kaolinite group, the smectite group, the illite group, and the chlorite group19.
Table 1. Clay minerals classification19.
The arrangement in these crystals consists of silicate layers that coordinate two tetrahedral atoms combined to edge-shared octahedral sheets20. Thus, generally, clay structures are proposed in layers, and these layers are seen in sheets both tetrahedral and octahedral. This indicates that the tetrahedral layer is composed of silica-oxygen molecules sharing the corners to other tetrahedrons of the same type. Instead, the octahedral layer is structured by aluminum or magnesium in sixfold coordination, for instance, halloysite as seen in Figure 118,21, that displays this conformation in nature. The layers are then arranged by interactions such as van der Waals forces, hydrogen bonds, cationic or static forces, etc. Clay's ability for surface modification is what allows the dispersion of layered silicates into separate sheets20.
Figure 2. Chemical structure of halloysite21.
Clay particles can absorb or lose water in response to simple humidity content changes in the surrounding environment; when water is absorbed, it fills the spaces between the stacked silicate layers22.
With regards to the soil mineralogy of Ecuador, there is barely any research done. Despite the large variety and existence of clays, there is still a significant lack of knowledge or information about their conformation, structure, and chemical formula.
Clays in Ecuador may have several compositions, yet they all show approximate levels of silica 60%, alumina 15%, low alkali and carbonates, and high levels of iron23. However, the lack of information opens the door to new research being done. There is a crescent interest in using clays for nanotechnology purposes, and most Ecuadorian lands provide them. As mentioned before, the clays may be found to be of various types, as shown in Table 1, even more than the few ones already discovered. And therefore, it can be used for many applications, including the development of nanomaterials and the improvement of nano-polymers.
In the same way, there is a need to discuss strategies and their implementation in nanoscale technology to take advantage of the naturally occurring resources, such as the Ecuadorian clays that can bring new and emerging materials, tackling ecological issues also industrial and economic concerns, etc. The lack of information on Ecuadorian clays is an opportunity to research them and discover and develop many other applications.
This review considered the data from the repositories of Ecuadorian Clays by the INEDITA project at Yachay Tech University, Ecuador. Under the INEDITA project, the clays were collected from Azuay, Zamora Chinchipe, and Morona Santiago provinces, located in the southeast of Ecuador, also known as the austral region of Ecuador. The total area of these provinces covers about 33000 km2, yet the samples were extracted from specific points, as shown below in Figure 3.
Figure 3. ap of the Ecuadorian provinces and sites where clays were collected. This sampling was performed by G.I.A.M.P (Research Group Applied in Materials and Processes).
They are labeled and enlisted in Table 2, considering the numbers used for classifying and identifying the clay series of the INEDITA-GIAMP project. The code corresponds to the origin site of the clay and the number of the sample in the set. Bouyoucos' method was used to collect the clays. Clay minerals were characterized via X-Ray Powder Diffraction. This technique is a non-destructive method that permits a collection of data about the composition of the crystallographic structure and, therefore, the physical behavior of materials24. The mentioned clays were previously used in nanocomposite research.
The physical appearance of each clay sample is shown below in Figures 4 and 5, which allow seeing how they vary in color. It could be related to the content of iron oxide or iron salts, not being this the only reason for the color variance in each of the samples, as can be observed. In Table 3 as well, it is indicated the iron concentration present in them. Still, the color of the samples will derive from other factors as the different mineralogical composition, environmental factors, organic compounds, etc. It must be said that the texture of the clays was very alike between all the samples. Between clays 301, 302, and 303 there is a notorious difference in color, while there is a similarity between 303 and 306 which were collected in the same province. It could be said that 302 and 308 also look alike, although 302 is more pinkish while 308 tends to a yellowish tone.
Figure 4. Samples corresponding to clays CAN-301, CZN-302 and CZP-303
Figure 5. Samples corresponding to clays CZQ-306, COL-307 and COL-308
Over 2950 phases matching the XRD data were obtained and analyzed with the software SmartLab Studio II (Rigaku Corporation, Japan), on average for all the samples. It was necessary to see what compounds were present in the analyzed samples, from which it was observed that Quartz and other forms of silicon oxides appeared in all the samples in different compositions. As it is noticeable, the spectra show similarities for data sets that correspond to the exact location of provenience, and they also are alike in composition.
The most remarkable features obtained of the analyzed samples are shown in Table 2:
Table 2. Data corresponding to known mineral phases of each sample analyzed by XRD and treated via QUALX2.0, the values in parenthesis correspond to relative abundance (%).
Besides their mineralogical composition, the samples are also composed mainly of inorganic complexes, oxides, and long carbon chains. There is a high relative abundance of Quartz in all samples except for the CAN-306 sample, composed mainly of Kaolinite.
From the characterization of clays through Atomic Absorption Spectroscopy' AAS', during the INEDITA project activities, the iron concentration data (in parts per million) was obtained for each sample. This is shown in Table 3:
Table 3. Data corresponding to Iron (Fe) concentration and percentage present in each sample25.
The similar iron percentage in the analyzed clay samples indicates that almost all the clays share a similar element concentration. Moreover, sample CZY-305 shows a doubled concentration concerning the other clays, see Figure 6, which suggests that this sample is the most suitable precursor for porous framework synthesis designed for H2S capture.
Figure 6. Percentage of iron concentration in clay samples obtained by Atomic Absorption Spectroscopy
Nanomaterials, specifically nanoclays, permit and present various chemical formulas because of their modifiable structure7. For example, Kaolinite or Montmorillonite which chemical formula varies depending on the environment since its structure adapts to both water and soil18. Besides, materials like clays present characteristics such as the Cation Exchange Capacity (CEC), which allows the core of the material to create a charge imbalance that leads to a change in chemical composition. Additionally, it indicates the level of potential substitution in the core of the material, depending on which cations are used to fill the voids somewhat; thus, resulting in a promising new electronic or polymeric application. Thus, the exact theoretical formula is rarely presented in such a way in nature; rather, the material will have a certain number of waters, etc.
Figure 7. Adsorption of positively charged molecules on the negatively charged surface of a nanoclay26.
Research on nanoclays has made significant progress in the last years because of the urgent and rising interest in the polymer market7. These nanomaterials show many advantages as additives, coatings, etc. In clays, it is essential to notice that they present isomorphous substitution, which is why they tend to have a charge resulting from the exchange of Si4+ with Al3+ leaving a negative charge to be balanced later. This gives the matrix the space to accept positive charges, which allow the clay sheet structure to undergo modification, providing them the possibility of exciting applications as precursor material since it allows the obtention, synthesis, or manufacturing of additives for polymeric chains27. Also, nanoclays are layered mineral silicate with layered nanoparticles that form a crystalline structure after stacking. After, those stacks can be dispersed in a polymeric matrix to achieve a specific new material or feature of a polymer19.
Nanoclays heavily affect the performance and behavior of nanocomposites, although research is still to be done. An extensive set of characteristics may determine the best and most optimum usage of a specific type of nanoclays but is still little and unknown. The level of intercalation and exfoliation are some of the prior mentioned features, and they could severely change the output of the nanomaterials. Nowadays, research has turned its eyes into attaining unique and functional combinations of polymer matrix and nanoclays. Some of these so desired features go from an immiscible or intercalated structure to an exfoliated structure, or even all of them together19.
Most of the research has been directing and investing its resources in flame retardancy and thermal modification, and the remaining efforts go to organophilic clays for mechanical or barrier properties18. Nonetheless, as mentioned before, extending the field of research on nanoclays features will lead to the synthesis of new materials and could even expand technology as one knows it. For example, suppose one considers the decomposition rate of certain polymers (already of commercial use) when added a more organophilic and high exfoliating nanoclay to its matrix or as a coating. In that case, it could disintegrate much more easily48 resulting in a greener material or a vector that adsorbs toxins from the environment, etc.
Still, there is an urgency to consider the environmental impact and concerns, and the proper production of new nanoclays, as every emerging material, must be examined against its ecological cost. In this case, the extraction of clays and the subsequent nanoclays for both nature as a whole and humans are two main factors.
Although it can be a completely clean process, the extraction represents a depletion of resources and must be carefully considered, primarily when they represent a non-renewable good, which in the long term will leave a mark on its primary source.
On the other hand, it must be considered the size of the particle and the chemical composition and its effect on both living and inert beings. Will these nanoclays be safe? For example, for living beings, if the nanomaterial has some aspects in its structure, could it enter the blood circulation system and harm it? Or could they accumulate and cause disease? And so on, many questions must be considered.
Toxicity and ecological challenges
The studies and analysis of nanoparticle toxicology focus on the capacity of nanoparticles to damage or change the function of cells, genetic code, and living environments caused by shaping factors such as the very high surface area of these molecules. It aims to determine in what mechanism the damage is caused, and more importantly, it attempts to find a proper response or a bridge to change the hazardousness into a non-destructive feature.
The different modifiers and their incorporation or combination in the layered silicates can result in nanoclays very miscible in polymers but could also represent a highly dangerous when inhaled or a general direct exposure28. In the same way, the factors mentioned above can lead to toxicological profiles after random scenarios as thermal degradation and chemical composition affect the nanoclays properties and their size and shape, which provokes a variation in toxicity that can be somewhat hazardous.
It was previously discussed that the modification generally occurs via ion exchange, and this process will directly impact the resulting nanocomposite28. Despite the obtention of a high-quality polymer, the usage or consumption of them will certainly open the possibility of degradation and, therefore, the release of the nanoclays from the matrix29, which can turn into a pool of free nanocomposites and depending on the medium or vicinity it can increase the risk of toxicity.
The obtention of naturally sourced clays for the posterior manufacture of nanoclays composites represents another ecological challenge. It is not a threatening challenge, yet it is essential to consider these factors since clays are obtained from limited resources that constitute not only the ecosystem for living beings but also the inorganic portion of a wholesome environment, and like everything in nature, it provides a balance that must be taken care of. In the obtention of clays, it is fundamental to consider the alterations in the surroundings rather than in the clays themselves.
In recent years, functionalization has been the main objective of research in nanomaterials, since it seems to be, for now, the mechanism which has shown higher effectiveness for targeting many problems, such as drug delivery, surface activity for medical and mechanical purpos