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In this study, we communicate our initial findings regarding the development of novel inorganic composite clinoptilolite membranes. The membranes were formed from natural clinoptilolite micropartic...

In this study, we report on the effects of Ag+ cations exchange within clinoptilolite zeolite in relation to both the physicochemical and the gas adsorption characteristics. Clinoptilolites cation-exchanged with AgNO3 to varying degrees were investigated and the influence of naturally occurring cations (Mg2+, Ca2+, K+) distributed within the material is discussed. The morphology and elemental composition of the untreated clinoptilolite and the Ag+ cation exchanged variants were characterized by XRD, EDS, FTIR, and N2 sorption experiments performed at 77 K. From the pore property analysis, we show that a slight reduction in the micropore distribution occurs for the highly exchanged Ag+ clinoptilolite compared to the untreated clinoptilolite, indicating a likely decrease in the average effective micropore size. Equilibrium adsorption isotherms of CH4 and N2 were measured at 303 K. The adsorption uptake rate of both gases was also studied and it slowed by the addition of Ag+ cations. The modified samples showed decreasing adsorption capacity for the larger CH4 gas molecule, whereas adsorption of the smaller N2 gas was less affected. This effect was most pronounced for the highly exchanged Ag+ clinoptilolite samples (AgNO3 exchange solution above 0.2M) whereby more N2 was adsorbed than CH4. At 1 atm and 303 K, the N2/CH4 equilibrium selectivity values of highly exchanged Ag+ clinoptilolite ranged from 2.8 to 7.6, which are higher than many clinoptilolite types.

This study details an experimental analysis of the thermodynamics of adsorption equilibrium behaviour of pure CO2, CH4, and N2 gases on raw, Fe3+, Ca2+, and Cs+ cation exchanged clinoptilolite. Equilibrium adsorption isotherms were experimentally determined for CO2, CH4, and N2 at three different temperatures ranging from 303 K up to 363 K, at pressures up to 8.0atm using the microgravimetric technique. Kinetic adsorption uptake data was measured at different temperatures for CH4 and N2 gases, and the results were compared for raw and Ca2+ clinoptilolite. Heat of adsorption values were calculated for all three gases on the studied clinoptilolite samples using the chromatographic method. These results were compared to the isosteric adsorption enthalpies determined from the adsorption isotherms at different temperatures. The isosteric heat of adsorption was the highest for CO2 followed by CH4, and N2. Temperature dependent Sips, Toth, and Dual-Site Langmuir isotherm models were fitted with experimental data and selectivity factor values were calculated for CO2/CH4 and CH4/N2 gas separations using the regressed isotherm models. Ca2+ clinoptilolite shows the best potential for CO2/CH4 separations due to its high selectivity. However, higher temperatures are needed in order to compensate for the slow kinetic behaviour. For CH4/N2 separations, Fe3+ clinoptilolite selectivity displays both high values and less variability compared to Cs+ clinoptilolite over a broader range of temperatures and pressures.

In this study, natural clinoptilolite was modified by cation exchange using alkali metal cations (Li+, Na+, K+, Rb+, Cs+), alkaline earth metal cations (Be2+, Mg2+, Ca2+, Sr2+, Ba2+), transition metal cations (Fe3+, Ni2+, Cu2+, Zn2+, Ce3+), and acid treatment (H+). The composition and structural properties of the modified samples were assessed using EDS and XRD analysis, and compared to the original clinoptilolite sample. Adsorption isotherms for CO2, N2, and CH4 gases were measured using a microgravimetric adsorption analyser. For all of the clinoptilolite samples, the adsorption equilibria and the diffusion rates were measured at 303 K over a pressure range from 0 to 8atm. The results of this screening analysis show that the cation exchanged clinoptilolites exhibit a wide range of adsorption characteristics, which make them suitable for a variety of gas separation applications by pressure swing adsorption. The high adsorption selectivity towards CO2 and CH4 found with the Cs+ exchanged clinoptilolite makes it advantageous for possible CH4/N2 and CO2/N2 equilibrium separations. Reduced CH4 equilibrium capacity resulting from pore blocking in the Ca2+ exchanged clinoptilolite leads to high selectivity for CO2/CH4 and N2/CH4 selective adsorbent. However, the potential of this material is limited due to increased microporous diffusion resistance. While both Li+ and Ni2+ clinoptilolites exhibit low CH4/N2 ideal selectivity, they present high N2/CH4 kinetic selectivity, thereby making them suitable for possible N2/CH4 kinetic separations.

The characterization of adsorption properties of binary gas mixtures is an important factor in the design of cyclic periodic adsorption processes. Presently, there are a few methods for determining the binary adsorption behaviour. However, most of these methods are very time demanding (static approaches). One dynamic approach for determining the binary adsorption behaviour is by employing the concentration pulse method. Although this experimental method has been shown to reliably reproduce the data obtained with static approaches, the application has been limited to systems where the adsorption capacities are similar. In our previous work, a novel method (HT-CPM) was shown to be capable of handling systems where the adsorption capacity of the heavy component is much greater than the light component.

Adsorption separation of carbon dioxide from nitrogen (N2, which represents air) was investigated by using H-ZSM-5 as the adsorbent with SiO2/Al2O3 ratios of 30 and 280. Pure and binary isotherms at 40°C have been determined experimentally using concentration pulse chromatography. Presently available methods for determining binary isotherms from concentration pulse chromatography data have been reviewed and shown incapable of interpreting this particular binary system. A novel solution method is presented and shown to be superior to the other methods. The binary isotherms for the CO2-N2 system show an increase in CO2 selectivity with increase in surface heterogeneity. The optimal separation factor for H-ZSM-5-30 was found to be 215 at 0.7% CO2 in the feed, whereas, H-ZSM-5-280 has an optimal separation factor of 48 at 2.7% CO2 in the feed.

Adsorption of pure methane, ethane and ethylene on molecular sieve zeolites was examined via the gas chromatographic method to determine the potential for the separation of ethylene from light hydrocarbons. The molecular sieves chosen for the study were H-mordenite and 13X, CaX, 4A and 5A zeolites. Henry's law constants were determined over a variety of temperature ranges between 233 and 473 K. van't Hoff plots are presented for all three gases on 4A and 5A zeolites and for methane and ethylene on the CaX and 13X zeolites and H-mordenite. Equilibrium separation factors for the ethylene/methane system are provided for all zeolites (except clinoptilolite) over various temperature ranges. Separation is most promising with CaX zeolite, which yielded separation factors ranging from 1100 at 100 °C to 100 at 200 °C. Separation seems possible in 5A and CaX zeolites at very high temperatures due to the strong affinity of their divalent cations for the ethylene π-bond.

Adsorption of N 2 , CO, CO 2 and NO has been studied on various molecular sieves using the gas chromatographic method to determine the potential for separation of these common atmospheric contaminants from air. The molecular sieves studied include H-Mordenite, 4A and 5A zeolite, a natural clinoptilolite and an activated carbon. Henry's law constants have been determined over a variety of temperature ranges from 243 to 473 K. Van't Hoff plots are presented for CO on all materials and for NO on all but 4A zeolite. Adsorption of CO 2 on the clinoptilolite was too strong to produce an interpretable response peak. Results of CO adsorption on 4A and 5A zeolites have been compared to and are supported by data available in the literature. Heats of adsorption for CO, NO and N 2 were determined. For CO the heats of adsorption decrease in the order of clinoptilolite > 5A zeolite > 4A zeolite > H-Mordenite > activated carbon. For adsorption of NO the heats of adsorption decrease in the order of clinoptilolite > 5A zeolite > activated carbon. Separation factors are presented for the CO/N 2 and NO/N 2 systems. The natural clinoptilolite shows most promise for the separation of CO and NO from N 2 at the temperature range 273–398 K. Diffusion coefficients for CO and N 2 on clinoptilolite between 348 and 423 K were also determined. Micropore diffusion proved to be the dominant mass transfer mechanism for both CO and N 2 in clinoptilolite under the conditions examined.

A study was made of the interaction of H2S, CO, and SO2 over activated clinoptilolite at 400°C using the electron spin resonance (e.s.r.) technique. It was observed that the CO- paramagnetic ions are formed over fresh activated clinoptilolite when treated with CO at ∼ 400°C. It was also observed that two types of SO2− ions, i.e., A-type SO2− with hyperfine structure (h.f.s.) and B-type SO2− without h.f.s. are formed over clinoptilolite depending on the experimental conditions. The B-type SO2− ion takes part in the reduction of SO2 by CO or H2S. The presence of CO over fresh activated clinoptilolite poisons the sites that are responsible for the formation of B-type SO2− ions. At room temperature, adsorbed H2S on the fresh activated clinoptilolite is responsible for the formation of paramagnetic Al3+ ions in the structure of clinoptilolite. Clinoptilolite has both electron-accepting and -donating sites. A plausible mechanism for the interaction of H2S, CO, and SO2 over activated clinoptilolite is discussed.

A chromatographic study of the adsorption kinetics and equilibrium of CH 4 , CO and N 2 gases on molecular sieves zeolite 4A, zeolite 5A, H-mordenite and chabazite has been made. The objective of this study was to determine the adsorption characteristics of these zeolites and look at the viability of separation of the gases mentioned. Adsorption equilibrium constants, selectivities and nitrogen zeolitic diffusivities have been determined in the Henry law region. The results indicate that the synthetic zeolites (H-mordenite, zeolites 4A and 5A) have a higher capacity for the adsorption of all three gases compared with naturally occurring chabazite. Also, adsorption equilibrium constants are higher for all the gases with the larger-pore zeolites H-mordenite and zeolite 5A. Among the gas-zeolite systems studied, CO-CH 4 and CO-N 2 were the easiest pairs to separate on the basis of adsorption equilibrium selectivities. Intra-crystalline diffusional resistance was the major contributing factor to mass transfer for nitrogen in both H-mordenite and zeolite 4A.

Publisher Summary This chapter describes adsorption of nitrogen and methane on natural clinoptilolite. The volumetric method was used to study the adsorption of pure N 2 and CH 4 gases on natural clinoptilolite from Bigadic, Turkey. The Langmuir equation and the Flory-Huggins form of the Vacancy Solution theory were used to fit the pure gas adsorption isotherms obtained for N 2 and CH4, and the binary adsorption isotherms, as well as the binary adsorption phase diagrams of these gases were predicted by the Extended Langmuir model, the Flory-Huggins form of the Vacancy Solution theory, and the Ideal Adsorbed Solution theory. The influence of the structural, physical and chemical characteristics of these adsorbates on clinoptilolite is discussed to explain the behavior predicted by each model.