**1. Introduction**

From the discovery of zeolites until today, the interest in scientific research and use of this amazing, interesting, and useful material has continuously grown.

Natural zeolites are hydrated aluminosilicate materials. Primary building units (PBUs) are tetrahedra of Si and Al oxides which are interconnected by oxygen ions into two-dimensional and three-dimensional secondary units (secondary building units [SBUs]). The zeolite network consists of channels and cavities filled with hydrated alkali and alkaline earth metal ions. With the presence of these metal ions in the structure of zeolites, a balance between positive electric charge of metal ions and negative charge of zeolite network is achieved.

The hydrating and dehydrating properties of zeolites were crucial in discovering their physical and chemical properties and their wider use, and that is why this mineral was named "boiling stone" (ζέω (zéō) meaning "to boil" and λίθος (líthos) meaning "stone") [1].

Unlike natural zeolites which are formed as crystals in cavities of basalt rocks or as volcanic tuff in different geological environments at relatively low temperatures, synthetic zeolites are created by hydrothermal synthesis in laboratory conditions. During hydrothermal synthesis in the presence of certain chemical compounds ("template" or structure directing agent (SDA)), which act on the organization of SBUs and consequential formation of tertiary or composite building units (TBUs), a final crystal zeolite is created in the form of a polyhedron which contains Si-O-Al bonds. Control of kinetic processes during zeolite synthesis is especially important since most of the zeolites created via this process are metastable phases [2].

### **2. Zeolite's history**

Zeolite materials were discovered in the eighteenth century when a Swedish chemist, founder of modern mineralogy, Baron Axel Fredrik Cronstedt, discovered that during the heating of mineral "stilbite," moisture appears on its surface [3]. His discovery began the research of chemical, physical, and mineralogical characteristics of natural zeolites.

In spite of the limited capabilities of structural research of zeolites at the time of their discovery (unlike today's modern methods), researchers set the foundations for today's application of natural and synthetic zeolites (**Figure 1**). Based on Cronstedt's observations and research, in the middle of the nineteenth century according to the available literature, the first synthetic zeolite-levinite [5] was synthesized, while in the first half of the twentieth century (after the first structural analysis

#### **Figure 1.**

*The mineral stilbite (left) and the crystal structure (framework) of the stilbite group (right) [4].*

#### **Figure 2.**

*Early discoveries of zeolite materials and their characteristics, the beginning of the modern zeolite science, and their industrial applications [7, 8].*

of zeolite materials), a synthetic analog zeolite-mordenite [6] was synthesized. Intensive research on the synthesis as well as physical and chemical properties of zeolites (absorption, ion exchange, dehydration) enabled the industrial application of zeolites. Significant discoveries and research of zeolites, as well as the beginning of modern zeolite science (parallel with the development of new instrumental techniques), are shown in **Figure 2**.
