Introduction
Synthetic porous aluminosilicates are promising elements for a whole range of relevant interdisciplinary problems. For a long time, the use of zeolites has been reduced mainly to animal feed or water purification. This is unjustified because, as materials, they have several unique properties. These include, first of all, their ability for ionic exchange, high cation-exchange capacity, micro-and nanoporous structure, and availability of surface-active centers of various natures. Since these properties were discovered, they have evolved from an object of laboratory research into the most crucial components. They have found wide applications in many branches of the chemical industry. It is believed that the tremendous success in the field of the introduction of zeolites is connected with oil refining. There is also an opinion that they can act as sorbents of toxic substances in liquid and gas environments.
Since zeolites are a prevalent subject for analysis, there are many studies in the literature related to the structure and basic properties of elements, but they are often voluminous and superfluous. The work aims to analyze the selected sources with the subsequent systematization of information and establish basic structural and functional features of zeolites that distinguish them from other substances. Moreover, the issue of the possibility of using zeolite for methane storage is considered debatable. In recent years, many chemical studies have been carried out; it is vital to evaluate their results and answer whether it is reasonable to use zeolite and whether it can compete with other materials.
Literature Review
The study will address the structure and definition of zeolites (Rashed & Palanisamy, 2018) and (Tuel, 2016). However, there will be an explanation of the need to use zeolite and its principles (Bhatia, 2020) and (Wen, et al., 2018). The article will focus on zeolite ion exchange and the definition of sodium zeolite as the most traditional ion-exchange method (Wang, 2017). The interaction of zeolite with ions and the charge phase of ion exchanges will also be described (Wang, 2019). In addition, attention will be paid to the actual exchange of zeolite cations (Price, et al., 2017). Factors influencing the balance and kinetics of ion substitution will also be analyzed (Krol, 2016). Also, the paper will identify areas where the use of zeolite is possible (Eroglu, et al. 2017). An analysis of experiments involving zeolite will be performed to determine its ability to store methane (Denning, et al., 2021) and (Li, et al., 2016). A comparison of different types of zeolite will be made and based on this the most suitable one for gas will be determined (Xiao, 2016) and (Do, et al., 2020).
Zeolites and their Application
Zeolites are crystalline inorganic microporous solids formed by TO4 tetrahedrons, where T is an atom in the tetrahedral position. Common bridging oxygens connect neighboring tetrahedrons, so the resulting framework has a total O/T ratio of 2. Thus, zeolites and zeolite-like materials belong to the tectosilicates group and may differ from thicker phases by framework density (FD), defined as the number of T atoms per 1000 A˚3 (Rashed & Palanisamy, 2018). The minimum FD for dense structures such as quartz or feldspars ranges from 20 to 22, whereas zeolites are characterized by FD values below 21, usually even below 19. From a strict structural point of view, the bound zeolite framework is formed by an angle separating aluminate [AlO4] and silicate [SiO4] tetrahedrons. Pure silicate frameworks (SiO2) are neutral, but the substitution of Si4þ for Al3þ in the positions creates a negative charge, which charge-balancing cations must compensate in the extra-framework positions in the cells or pores of the structure (Tuel, 2016). The original cations present in the synthesized sample can be replaced with other, more convenient ones that give the zeolite its cation-exchange capacity and other specific properties.
When rapidly heated, the substances release water and seem to boil. This behavior inspired the Swedish mineralogist A. F. Cronstedt, who discovered these minerals in 1756, to call them ‘zeolites,’ which comes from the Greek words Zeo and Lithos and can be translated as ‘stones that boil’ (Tuel, 2016). Today, 235 different zeolite frameworks have been identified, and the International Zeolite Association has assigned a three-letter code to each.
The ion-exchange capacity of zeolites is one of the main parameters characterizing their sorption and technological properties. The maximum ion-exchange capacity resembles the complete replacement of one ion by another in all crystal positions, corresponding to the maximum sorption capacity. Zeolites are present in daily life, widely used as sorbents, ion exchangers in detergents, or industrial catalysts for upgrading and producing liquid fuels and intermediates for the petrochemical, chemical, or pharmaceutical industries (Bhatia, 2020). In addition to their outstanding physicochemical properties and high functionality, zeolites have the added advantage of being environmentally friendly. They are safe and sustainable, which is the key driving force behind their growing use as ‘green’ alternatives to, for example, chlorine in pools, polyphosphates in detergents, or mineral acids such as hydrochloric, in many industrial processes.
Although many compositions have been obtained, the ability to synthesize customized zeolites is still a challenge concerning their porous structure and the distribution and arrangement of their active sites. The hydrothermal stability of large- and extra-large-pore zeolites is also a key factor, especially to their potential commercial applications in areas such as catalytic cracking or hydrocracking. The area with the most significant expansion and the greatest environmental and economic impact is the use of zeolites as heterogeneous catalysts. In this sense, all zeolite structures can offer exciting possibilities. For example, zeolites with tiny pores, in addition to their potential applications for the separation of CO2, N2, and CH4, are promising materials for the reduction of NOx in automotive emissions (Wen, et al., 2018). Medium pore zeolites are helpful in many catalytic processes in petroleum refining, petrochemicals, chemistry, and fine chemistry. However, they must be sufficiently stable and low enough in cost to be competitive. The properties of zeolites are prevalence, availability, cheapness, and possibility of repeated application, which makes them indispensable and essential substances for study.
Zeolite Ion Exchange
Ion exchangers transfer one ion for another, temporarily hold it, and then deliver it into the regeneration suspension. In the ion exchange system, undesired ions in the water supply are replaced by more acceptable ions. For example, calcium and magnesium ions that form scale are replaced by sodium ions in the sodium zeolite softener. ‘Softening of sodium zeolite is the most traditional method of ion exchange’ (Wang, 2017). When the zeolite is softened, liquid, including system-forming ions such as calcium and magnesium, crosses through a layer of resin in the form of sodium. Hardness ions are exchanged with sodium in the resin, and sodium spreads into a bulk aqueous solution.
Zeolites are widely used in ion exchange applications, where they interact with cations from their structure to dissolve. Ion exchange is one of the methods applied to remove several toxic substances, including heavy metals, from industrial and municipal wastewater. Clinoptilolite is a type of zeolite and is found in extensive quantities in many deposits around the world. It belongs to the group of heulandites with a three-dimensional silicon frame and aluminum tetrahedra having the typical chemical formula Na6 [(Al2O3) (SiO2) 30] · 24H2O (Wang, 2019). The most meaningful feature of zeolites is the expressive ion exchangeability, the ability of the material to absorb cations.
It is known that ion exchange is the replacement of ions between the solid and liquid stages. The solid phase is charged, and this is there balanced by ions of opposite charge, announced counterions. The diffusion process occurs when the ion exchanger containing ions A is immersed inside the liquid phase containing ions B. Therefore, it is established that ions A and B distribute into a strong structure (Price, et al., 2017). This dispersion is the result of a significant variation in concentrations between phases. If the ions do not carry electric charges, these concentration differences are compensated by diffusion. However, such a process would disrupt electroneutrality because ions are charged.
There are no separate ionic groups in zeolites: in fact, certain ionic combinations are missing. In zeolites, the lattice consists of tetrahedra SiO4 and AlO4, which have ordinary oxygen atoms. This charge is balanced by alkaline and alkaline earth cations, which do not occupy fixed positions but can quickly move channels of a lattice framework. Moreover, these ions act as counterions and can be replaced by other cations. Thus, the negative charge of zeolites is not localized but evenly distributed within (Krol, 2016). In zeolites, the frame charge is balanced by free cations, and in this case, these cations are counterions.
It is recognized that zeolites are rare in nature in their pure form. Zeolite tuffs containing several impurities that may be active or inactive for exchange (TEC) should be considered. However, in pure zeolites, the TEC corresponds to the actual number of complementary cations. In order to distinguish the case of TPPs of pure zeolites, the term ‘ideal metabolic capacity (IEC) is introduced. Real exchange rate (REC) refers to the sum of the actual exchange rate of zeolite cations. It is a characteristic constant of the heat exchanger ion regardless of the experimental conditions. In essence, REC is the upper limit of the actual capacity of the zeolite. In zeolite ores (tuffs), TEC is always higher than REC; this difference is because some of the zeolite cations cannot be removed from the zeolite structure (Price, et al., 2017). They have solid binding forces in the structure material and are not exchanged.
Partial exchange in zeolites is mainly the result of the phenomenon of ion sifting into the microporous structure. Hence, the nature (shape, size, and charge) of the input cation is critical. Moreover, this distinction’s inability explains that some zeolite cations cannot be removed under certain experimental conditions through low mobility and strong adhesion forces in the material structure. That is why zeolites are often pre-treated and converted into homoiconic forms, mainly in Na-. For example, it is known that Na + ions, the weakest bound ions of clinoptilolite, are most easily exchanged by cations from solvents. Finally, temperature is a critical factor influencing the balance and kinetics of ion replacement. For example, in clinoptilolite systems, the level of exchange of Ca2 + zeolite ions for 1 M solutions of Cs + and Na + was 59% and 57% at 5 ◦C and 91% and 100% at 90 ◦C, sequentially (Eroglu, et al. 2017). The maximum exchange level increases in these instances, but the REC is in an active and independent position in each case.
Zeolite for Methane Storage
The most prevalent greenhouse gas on Earth is carbon dioxide. There are many, albeit not cheap, ways to capture it from the atmosphere. However, methane’s situation is more complicated because this gas interacts poorly with other substances. Increasing attention to the synthesis of suitable materials for physical methane storage has been the primary goal of many researchers in recent years. Many materials have been studied, such as metal-organic frameworks (MOFs), which have favorable characteristics, such as exceptional specific surface area and easily tunable pore sizes. These elements exhibit high methane storage capacity, but their structure usually degrades after a long period of sorption cycles. Although, a group of scientists once created many porous polymer networks (~18,000 structures) for CH4 adsorption (Denning, et al., 2021). However, only three structures achieved CH4 adsorption of 180 cm3 /cm3. From this result, it follows that finding superior materials for methane storage is a great challenge.
Employees of the U.S. national laboratory found that the natural mineral zeolite, widely used as a filter in various technological processes, can also be adapted to absorb and store methane. Among other adsorption elements with more excellent structural stability, zeolites are considered the most promising options. Like MOFs, covalent organic frameworks (COFs) have a crystalline structure with controlled pore sizes. However, COFs have the definite advantage of containing only non-metallic elements such as C, Si, B, O, and H. These elements are bound together by a considerable number of covalent bonds, forming the structures of COFs. A great abundance of studies has been done on these porous materials. Scientists have analyzed the characteristics of nearly 100,000 different zeolites and selected several samples that can be technologically used for methane capture. For example, some scientists reported a high methane storage capacity of about 8.2 wt% at 3 MPa and 298 K for the CaX zeolite (Li, et al., 2016). In comparison, others obtained a methane capacity of about 6.8 wt% at 3 MPa and ambient temperature for the 13X zeolite.
Regarding other theoretical studies, scientists developed two materials with appropriate functional groups to improve the methane storage properties of some COF materials. These were COF-103-Eth-trans and COF-102-Ant with methane adsorption capacity exceeding the target set by the U.S. Department of Energy (Do, et al., 2020). Another group of scientists modified COF-102 by double halogen substitution. Their simulation result showed that the methane adsorption capacity of COF-102-14-2I is 181 B (STP)/B. The latter is a bikitaite zeolite (BIK), which has the chemical composition of the unit cell Li2(Al2Si4O12)-2H2O (Xiao, 2016). It is a zeolite with tiny pores (diameter 0.28-0.37 nm) studied for various applications and, in particular, has shown the best performance in gas separation and storage.
The ability of bikitaite zeolite to accumulate hydrogen is commendable, and the separation efficiency of H2 from H2-CO2 and H2-N2 mixture for DDR membrane and SAPO 34 zeolite is significantly higher compared to the data presented (Xiao, 2016). However, it should be recorded that there are still many unresolved issues before the industrial and commercial use of these developed separation and storage materials. In natural separation systems, the gas mixtures and operating conditions are more complicated, and many factors must be considered for successful industrial operation. There is no doubt that zeolite materials represent an area of great excitement and potential importance in all fields. It would be an outstanding achievement if such materials were put into practice.
A literature review suggests that zeolites are a class of natural and synthetic substances whose properties make them suitable catalysts. Their molecular sieve effect plays a unique role: each of hundreds of zeolite species has a unique porous structure, specific geometry, and mutual arrangement of many channels inside the crystals. It is essential to use zeolites as catalysts; they contain a specific type of branded solid acid centers. The combination of these properties makes them valuable materials for various chemical processes. In the late 1980s, it was discovered that if metal cations were chemically introduced into zeolites, the resulting system acquired new properties. Several recent scientific papers have demonstrated the possibility of the chemical activation of methane and its transformation into other valuable products like methanol, carboxylic acids, or aromatic hydrocarbons (Do, et al., 2020). This area of research has continued to evolve and is now an important area because methane is one of the most abundant and affordable hydrocarbons.
Consequently, among all porous materials, especially in the family of microporous materials, zeolites are the first and newest ones. They are attracting increasing interest due to their unique physical and chemical properties. These include high surface area, chemical resistance, and unusual mechanical and catalytic properties due to specific surface chemistry. These peculiar and surprising characteristics have highlighted the potential of this material in various applications, especially in gas separation and storage. The development of carbon porous materials with predetermined finely tuned nanoscale reaction chambers and channels (which can have application-specific surface properties) is of paramount importance because of their low weight, high bulk density, and controllable composition. Such materials have great potential to achieve, among other things, high and reversible methane adsorption.
Conclusion
Zeolites are a large group of minerals with similar properties and compositions. Their molecular structure is a dense network of AlO4 and SiO4, which forms cavities where water and other polar molecules or ions are inserted or replaced. One of the essential characteristics of zeolites is their ability to ionic exchange: they selectively release and re-absorb various kinds of reagents as well as exchange cations. Due to their exceptional and individual properties, they are used for various medical, industrial, and environmental purposes, mainly to absorb toxic pollutants from industrial effluents and waste. They also have a unique crystalline structure and are excellent materials for gas storage, mainly methane. After the analysis of numerous scientific types of research, it can be concluded that zeolites have a high surface area and are composed only of non-metallic elements. They have unique physical and chemical characteristics that provide the material with a high potential for methane storage compared with other elements.
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