2022.07.01.33

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Porous frameworks from Ecuadorian clays

María Calle Luzuriaga¹, Edward E. Ávila¹, Dario Alfredo Viloria¹

¹ Universidad de Tecnología Experimental Yachay Tech, Escuela de Ciencias Químicas e Ingenieria, Grupo de Investigación Aplicada en Materiales y Procesos (GIAMP); [email protected], [email protected] & [email protected]

* Correspondence: [email protected]; Tel.: +593995371712

Available from: http://dx.doi.org/10.21931/RB/2022.07.01.33

ABSTRACT

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.

INTRODUCTION

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.

Theoretical Background

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, etc^10,11. 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.