Clinoptilolite is one of the most common, widespread and abundant zeolites in nature. Its availability, low cost, and outstanding ion exchange properties make clinoptilolite an excellent candidate for both direct use and various modifications to create new low-cost functional materials for sustainable development. Specific applications in which clinoptilolite is already being used include water treatment and heavy metal ion removal, agricultural purposes, storage and conversion of unwanted gaseous emissions into the atmosphere, production of catalysts and photocatalysts, bioactive materials, and a number of others. Unlike some other zeolites, clinoptilolite is difficult to synthesize, which is why most publications refer to this zeolite in its natural form, either directly from the deposit or after applying various processes to this mineral to improve its properties. Among the modification methods used, ion exchange stands out. This review is devoted to the study of ion exchange processes in natural clinoptilolite with two goals: first, as its strategic property for use in processes in which cation exchange is fundamentally necessary; second, as a way to modify it to create composite materials with predetermined desired properties.

This article presents the results of a comprehensive study of copper-exchanged mordenite samples prepared from its ammonia and protonated forms (Si/Al = 10) using two different ion exchange methods: conventional and microwave (MW)-assisted. The protonated H-MOR-10 sample was obtained by calcination of commercial NH₄-MOR-10; in this case, a slight degradation of the mordenite framework was observed, but the resulting defects were partially restored after the first ion-exchange procedure of protons for copper ions. The level of copper exchange in the studied materials was found to be limited to 70%. Regardless of the exchange procedure, the replacement of ammonium or proton ions with copper led to a linear increase in the a/b ratio of cell parameters in accordance with an increase in the level of copper exchange, which means that all Cu²⁺ cations are ion-exchangeable and enter the main mordenite channel. Thermal analysis indicated a correlation between the replacement of various ammonium and hydroxyl groups by copper ions during the exchange treatment and their dehydroxylation energy during thermal decomposition. As a conclusion: MW-assisted treatment proved itself as an efficacious method for the synthesis of copper-exchanged mordenites, which not only significantly reduces preparation time but leads to a systematically higher copper exchange level.

Long-term changes in species of copper-silver bimetallic systems on natural clinoptilolite obtained by ion exchange of Cu²⁺ and Ag⁺ and then reduced at different temperatures were studied. Even after storage under ambient conditions, XRD and UV-Vis diffuse reflectance spectra indicate the presence of nanospecies and larger particles of reduced copper and silver. Scanning electron microscopy of aged bimetallic samples, reduced at the highest temperature (450 °C) and the pristine sample for their preparation, also aged, showed the presence of silver particles with a size of about 100 nm. They are formed in the initial ion-exchanged sample (without reduction) due to the degradation of Ag⁺ ions. The particles in the reduced sample are larger; in both samples they are evenly distributed over the surface. The presence of silver affects the stability and the mechanism of decomposition/oxidation of reduced copper species, and this stability is higher in bimetallic systems. The decomposition pattern of recently reduced species includes the formation of smaller nanoparticles and few-atomic clusters. This can occur, preceding the complete oxidation of Cu to ions. Quasicolloidal silver, which is present in fresh bimetallic samples reduced at lower temperatures, transforms after aging into Ag₈ clusters, which indicates the stability of these nanospecies on natural clinoptilolite.

Multimetallic systems, instead of monometallic systems, have been used to develop materials with diverse supported species to improve their catalytic, antimicrobial activity, etc., properties. The changes in the types of nanospecies obtained through the thermal reduction of ternary Ag⁺-Cu²⁺-Zn²⁺/mordenite systems in hydrogen, followed by their cooling in an air or hydrogen atmosphere, were studied. Such combinations of trimetallic systems with different metal content, variable ratios (between them), and alternating atmosphere types (during the cooling after reducing the samples in hydrogen at 350 °C) lead to diversity in the obtained copper and silver nanospecies. No reduction of Zn²⁺ was evidenced. A low silver content leads to the formation of reduced silver clusters, while the formation of nanoparticles of a bigger size takes place in the trimetallic samples with high silver content. The cooling of the reduced trimetallic samples in the air causes the oxidation of the obtained metallic clusters and silver and copper nanoparticles. In the case of copper, such conditions lead to the formation of mainly copper (II) oxide, while the silver nanospecies are converted mainly into clusters and nanoparticles. The zinc cations provoked changes in the mordenite matrix, which was associated with the formation of point oxygen defects in the mordenite structure and the formation of surface zinc oxide sub-nanoparticles in the samples cooled in the air.