Abstract
The influence of thermal activation, decationation, and dealumination on adsorption properties of rock from Transcarpathian deposit (Lypcha village, Ukraine) containing mordenite and clinoptilolite at the ratio 1:1 was studied in this research. Taking into account the need of preliminary thermal activation of adsorbent, the research of thermal stability of zeolite component of rock was conducted. Parameters of microporous structure of decationated and dealuminated samples were calculated. Obtained results show that decationation of mordenite–clinoptilolite rock leads to insignificant increase and dealumination leads to significant decrease of adsorption capacity for water. Maximal adsorption ability for methanol is about 50% dealumination degree of the sample.
Introduction
Today the role of synthetic zeolites applied as adsorbents and catalytic agents can hardly be overestimated. Natural zeolites often do not rank in technological properties below the synthetic analogs. However, the cost of natural zeolites preparation is much lower than the cost of production of synthetic zeolites.
Although the deposits of zeolites have the broad geography and possess its large volumes, the scope of its application is now limited mainly with the fields of agriculture and construction. However, researches of these materials for increasing of their scope of application are permanently carried out. Due to their high cation-exchange ability natural zeolites can be used as adsorbents in water treatment (Wang and Peng, 2010) including drinking water (Li et al., 2011), effluents from industrial enterprises (Yousefa et al., 2011), agricultural productions (Montegut et al., 2016; Smedt et al., 2015), and landfills (Tuncan et al., 2003). The molecular sieve properties allow application of initial and modified forms of natural zeolites for separation and purification of gas mixtures (Ackley et al., 2003; Aguilar-Armenta and Romero-Perez, 2009; Cakicioglu-Ozkan and Ulku, 2003; Shafie et al., 2012). Also the works for application of these materials in pharmacy are carried out (Cerria et al., 2016; Gennaro et al., 2016).
Possibilities of natural zeolites application in energy recovery technologies are now expanding (Nizamia et al., 2016). In the process of biofuels production these materials can be used as recyclable adsorbents/catalysts for biogas upgrading (Cabrera-Codony et al., 2017) and bioethanol absolutization (Karimi et al., 2016). As a result of chemical and structural modification of natural raw materials, catalysts and catalyst supports for such processes as conversion of methanol to dimethyl ether (Kustovska and Kosenko, 2014; Royaee et al., 2008) and dimethyl ether to olefins (Nassera et al., 2016) can be obtained. These processes allow obtaining synthetic fuels with improved ecological characteristics.
The main reasons of narrow use of mineral raw materials are highly variable mineral content and nonhomogeneity of phase composition of zeolite rocks. That is why most of processes mentioned above require preliminary modification of minerals. Chemical modification allows the regulation of сhemical and phase compositions of natural materials, and therefore, adsorption properties that determine main exploitation characteristics of adsorbents and catalysts. Since in the deposits different zeolite structures often crystalize jointly, research of regularities of modification of rocks containing the mixture of zeolites, namely mordenite–clinoptilolite, is the subject of special interest. The relevant rocks can be found in Transcarpathian deposits, which are unique in terms of amount and quality of raw materials (Survey of natural zeolites market in the CIS, 2010).
Therefore, the aim of the work was to study the adsorption of methanol and water vapor on modified rocks containing a genetic mixture of zeolites for the purpose of their further use as adsorbents and catalysts for the methanol conversion toward dimethyl ether or hydrocarbons.
Materials and methods
As the main object of research sample of zeolite rock from Transcarpathian deposit (Lypcha village, Ukraine) containing mordenite and clinoptilolite at the ratio 1:1 with little amount of feldspar was applied. Total content of zeolites in rock was more than 90% mass. Chemical content of rock (mas%): SiO2—–64.95; TiO2—0.20; А12O3—12.23; Fe2O3—1.06; FeO—0.22; MgO—1.21; CaO—3.22; Na2O—0.70; K2O—2.35; S—0.03; P2O5—0.11; CO2—1.03; H2O—12.33.
Thermal activation of samples was conducted in the temperature of 300–1000℃ during 6 h. Powder X-ray diffraction spectra were measured in an X-ray diffractometer DRON-3M using Cu Kα radiation. The crystallinity of mordenite and clinoptilolite components of the thermally processed rock was estimated by relative intensity of nonintersecting analytical diffraction maxima of mordenite and clinoptilolite (d = 0.453 nm and d = 0.298 nm, respectively). As a reference object a sample of the original rock was used (Figure 1).
X-ray diffraction pattern of the mordenite–clinoptilolite rock. M: mordenite component; C: clinoptilolite component. Analytical diffraction maxima are marked with asterisks.
Chemical treatment of samples was carried out in aqua solutions of NH4Cl and HCl at the ratio of liquid and solid phases 1:10. After processing, the samples were washed with distilled water until no Cl− ions were present in the filtrate (qualitative reaction to AgNO3) and dried in air at room temperature. Decationated (hydrogen) forms were obtained by processing of the sample with a solution of NH4Cl with further calcination in air for 2 h at 600℃. Dealumination was conducted with HCl solution. Depth of decationation and dealumination was regulated through the concentration of solutions and the duration of modification. Control of modification processes was performed through chemical analysis of the spent solutions. Degree of decationation and dealumination of zeolites were determined as the number of cations and aluminum removed from the sample during processing, as percentage of their content in zeolite phase of starting sample.
An adsorption study was carried out at room temperature using a laboratory setup based on the McBain–Bakr quartz spiral balance. The sensitivity of quartz springs was about 2.5 mg/mm. The Dubinin–Radushkevich equation for volume filling of micropores was used for a theoretical analysis of adsorption isotherms
Results and discussion
Taking into account the need of preliminary thermal activation of adsorbents and catalysts, the research of thermal stability of zeolite component of sample (Figure 2) was conducted. With increasing calcination temperature, the crystal structure of zeolites is gradually destroyed. Moreover, resistance to temperature for mordenite is substantially higher than for clinoptilolite. After heat treatment at 400℃ destruction of mordenite is less than 10%, whereas clinoptilolite is destroyed for more than 50%. In 500℃ more than 80% of mordenite structure is retained, while reflexes of clinoptilolite on the diagram cannot be found. Even during calcination at 800℃ mordenite undergoes less than 50% degradation. In the same time the higher thermal stability of natural mordenite comparing to synthetic can be observed. It can be explained by the stabilization of the structure of the natural mordenite with alkaline earth metal cations.
Dependence of crystallinity degree on temperature of preliminary processing for zeolites investigated. Curve 1: clinoptilolite component of natural sample; Curve 2: mordenite component of natural sample; Curve 3: synthetic mordenite.
Adsorption isotherms of initial rock as well as thermally processed samples are typical for microporous sorbents. Figure 3 shows the dependence of adsorption capacity of mordenite–clinoptilolite rock on temperature of preliminary processing and reveals that above-mentioned destruction of the crystal structure of zeolite components is accompanied by a decrease in the adsorption capacity of the sample. For initial sample (T = 50℃), full sorption volumes (P/Ps = 1) for water and methanol are the same, although the nature of the isotherms is different. In the case of adsorption of water about 75% of volume accounts for micropores (P/Ps = 0.4), whereas only 60% of methanol is adsorbed in micropores and the rest accounts for macropores that may be explained by the inaccessibility of polycationic form of clinoptilolite for methanol molecules. For samples heated at 500 and 600℃ adsorbed volume of water and methanol are significantly different, and at 700℃ they become close again. This is the result of sintering (seal) of the amorphous phase, created during the destruction of clinoptilolite. After calcination at 500 and 600℃, this phase is partially accessible for water molecules, and after heating to 700℃ it is sealed so much that the adsorption of water is minimized. According to X-ray analysis (Figure 2), heat treatment of the sample at 700℃ leads to the complete destruction of clinoptilolite, while mordenite is destroyed for only 23%, and remains the only active sorbent component. So, the total content of mordenite in the sample is 38%. The total volume of micropores of such sample should theoretically be 3.0 ± 0.3 mmol of H2O/g, which coincides with the experimentally determined at P/Ps = 0.4 adsorption magnitude 3.1 mmol of H2O/g. This shows that results are quite correct. So, for maximum preservation of the microporous structure of such zeolite rocks the preliminary thermal activation at temperatures above 400–450℃ is undesirable.
Dependence of adsorption capacity of mordenite–clinoptilolite rock on temperature of preliminary processing. Curve 1: for water as adsorbent at P/Ps=1; Curve 2: for methanol as adsorbent at P/Ps = 1; Curve 3: for water as adsorbent at P/Ps = 0.4; Curve 4: for methanol as adsorbent at P/Ps = 0.4.
Adsorption of water vapor on initial and decationated forms at 25℃.
Correspondence of adsorption capacity (for water and methanol) and degree of decationation is provided in Figure 4. Comparison of adsorption amount of different adsorbates with decationated samples has shown that almost full replacement of cations leads to relatively low increasing of sorption capacity for water, whereas methanol adsorption significantly increases and becomes similar to water adsorption. Along with this, the most significant changes of adsorption capacity take place after removal of nearly half of exchange cations from sample. It may be related to the fact that the size of entries to intracrystalline cavities of zeolites in initial sample doesn’t limit water adsorption, so the influence of deactivation is low. Adsorption of bigger molecules of CH3OH in initial sample is complicated, but even 45% decationation ensures its free diffusion. Thus, in order to increase the efficiency of methanol adsorption it is necessary to conduct decationization of rocks at a minimum of 45%.
Dependence of adsorption capacity of mordenite–clinoptilolite rock on decationation degree. Curve 1: for water as adsorbent at P/Ps = 1; Curve 2: for methanol as adsorbent at P/Ps = 1; Curve 3: for water as adsorbent at P/Ps = 0.4; Curve 4: for methanol as adsorbent at P/Ps = 0.4.
Adsorption curves of water and methanol vapors depending on the degree of sample’s dealumination are presented in Figure 5. With the extent of dealumination the form of isotherms is gradually changing. While the increasing of dealumination depth, adsorption amount of water is decreasing in case of low values of P/Ps, and increasing in case of high values of this ratio. It demonstrates the change in the character of porosity—microporous structure of clinoptilolite breaks and wide-pore amorphous phase appears. It should be noted that acid modification affects on changes of adsorption capacity of water and methanol samples not equally. At almost the same total amount of adsorbed water and methanol (P/Ps = 1) micropore capacity for various adsorbates is significantly different. With increasing of dealumination activity degree, adsorption for water vapor decreases, whereas for methanol extremum dependence is observed. This is probably a result of change in both pore structure and chemical nature of zeolite surface in the process of acid treatment. Thus, adsorption of methanol occurs mainly in large channels of mordenite, because the pores of clinoptilolite are not available for this adsorbate. Dealumination, which is accompanied by widening of zeolite channels until their destruction, leads first to increase of adsorption of relatively large methanol molecules (at dealumination degree up to 50%), and then to its reduce as a result of dealumination, the concentration of hydrophilic adsorption centers is decreased and the crystalline structure is destroyed. Furthermore, in the process of dealumination the formation of X-ray amorphous microporous phase that is able to sorb large molecules is possible. However, the contribution of this phase in the sorption capacity of the sample is obviously insignificant. The adsorption capacity of micropores by water at dealumination monotonically decreases as small water molecules do not experience steric constraints upon adsorption on initial form, whereas during removing of aluminum from framework, concentration of active adsorption centers is reduced. It leads to reduction of micropores capacity of this adsorbate.
Dependence of adsorption capacity of mordenite–clinoptilolite rock on dealumination degree. Curve 1: for water as adsorbent at P/Ps = 1; Curve 2: for methanol as adsorbent at P/Ps = 1; Curve 3: for water as adsorbent at P/Ps = 0.4; Curve 4: for methanol as adsorbent at P/Ps = 0.4.
Adsorption of water vapor on initial and dealuminated forms at 25℃.
Conclusions
Obtained results show that decationation of mordenite–clinoptilolite rock leads to insignificant increase and dealumination leads to significant decrease of adsorption capacity for water. Therefore, for preparation of adsorbents for dehydration of gases and organic liquids on the basis of zeolite-containing rocks, it is possible to carry out only fractionation and washing of raw materials. It will lead to significant cheapening of process. In case of adsorption of larger molecules, like methanol, widening of zeolite channels is necessary. It can be achieved through acid treatment, which is necessary for preparation of catalysts or catalyst carriers for methanol conversion processes.
Footnotes
Acknowledgment
First presented at the 15th Ukrainian–Polish Symposium on Theoretical and Experimental Studies of Interfacial Phenomena and their Technological Applications, Lviv, Ukraine, 12–15 September 2016.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.