crystals EditorialEditorial NaturalNatural vs.vs. SyntheticSynthetic ZeolitesZeolites MagdalenaMagdalena Kr Królól FacultyFaculty of of Materials Materials Science Science and and Ceramic, Ceramic, AGH AGH University University of of Science Science and and Technology, Technology, 30 30 Mickiewicza Mickiewicza Av., Av., 30-05930-059 Krakow, Krakow, Poland; POLAND [email protected]; [email protected] Received: 14 July 2020; Accepted: 15 July 2020; Published: 17 July 2020 Received: 14 July 2020; Accepted: 15 July 2020; Published: 17 July 2020 Abstract: This brief review article describes the structure, properties and applications of natural and Abstract: This brief review article describes the structure, properties and applications of natural and synthetic zeolites, with particular emphasis on zeolites obtained from natural or waste materials. synthetic zeolites, with particular emphasis on zeolites obtained from natural or waste materials. Certainly, such short work does not exhaust the complexity of the problem, but it sheds light on Certainly, such short work does not exhaust the complexity of the problem, but it sheds light on some some outstanding issues on this subject. outstanding issues on this subject. Keywords: natural zeolite; zeolite synthesis; zeolite characterization; zeolite application Keywords: natural zeolite; zeolite synthesis; zeolite characterization; zeolite application 1. What Are Zeolites? 1. What Are Zeolites? Zeolites can be defined in two ways. They are hydrated tectoaluminosilicates with the general Zeolites can be defined in two ways. They are hydrated tectoaluminosilicates with the general formula [1,2]: formula [1,2]: MxM’MyNxM’z [TyNmzT’ [Tnm:::T’nO…O2(m+n)"–ε (OH)(OH)2z] (OH)br (aq)(aq)pp · qQqQ,, (1)(1) 2(m+n)- 2z br · wherewhere M, M, M’ M’ are are exchangeable exchangeable and and non-exchangeable non-exchangeable cations, cations respectively;, respectively N are; non-metallicN are non-metallic cations (generallycations (generally removable removable on heating); on heating) (aq) represents; (aq) represents chemically chemically bonded bonded water (orwater other (or stronglyother strongly held ligandsheld ligands of T-atoms); of T-atoms Q are); Q sorbate are sorbate molecules; molecules T,; T’ T, areT’ are Si andSi and Al, Al, but but also also Be, Be, B, B, Ga, Ga, Ge Ge andand P,P, amongamong others. others.This This formula formula is is particularly particularly useful useful when when describing describing natural natural zeolites zeolites [ 2[2],], but but also also those those synthesizedsynthesized from from natural natural or or waste waste materials, materials due, due to to their their complex complex chemical chemical composition. composition. OnOn the the other other hand, hand, zeolites zeolites can can be bemore more graphically, graphically, as as shown shown in in Figure Figure1 ,1 defined, defined as as crystalline crystalline inorganicinorganic polymers polymers consisting consisting of of [SiO [SiO4]4] andand [AlO [AlO44]] tetrahedra,tetrahedra, havinghaving thethe structurestructure filledfilled ions ions and and waterwater molecules, molecules, having having great great freedom freedom of of movement. movement. Figure 1. Scheme of zeolite structure. Figure 1. Scheme of zeolite structure. The specific structure of the zeolites gives them a number of unique properties. The most important The specific structure of the zeolites gives them a number of unique properties. The most regarding potential uses include [3,4]: important regarding potential uses include [3,4]: LowLow density density and and large large volume volume of of free free spaces; spaces; • TheThe existence existence of of channels channels and and chambers chambers of of strictly strictly defined defined dimensions dimensions (shape-selectivity); (shape-selectivity); • HighHigh degree degree of of hydration hydration and and the the presence presence of of so-called so-called "zeolite "zeolite water"; water"; • HighHigh degree degree of of crystallinity; crystallinity; • Possibility of sorption of molecules and ions; CrystalsCrystals2020 2020, 10, 10, 622;, x; doi: doi: FOR10.3390 PEER/cryst10070622 REVIEW www.mdpi.comwww.mdpi.com/journal//journal/crystalscrystals Crystals 2020, 10, 622 2 of 8 Possibility of sorption of molecules and ions; • CrystalsIon 2020 exchange, 10, x FOR capacity;PEER REVIEW 2 of 8 • Catalytic properties. • Ion exchange capacity; CatalyticThese properties, properties. caused by zeolites, arouse great interest among chemists, technologists and mineralogists.These properties Although, caused it may by seemzeolites that, arouse the years great of theinterest most among intensive chemists, research technologists on this group and of mineralogists.minerals have Although already passed,it may seem interest that in the them years is notof the decreasing. most intensive Constantly research conducted on this group research of mineralson the specific have already properties passed, of zeolites interest show in them comprehensive is not decreasing. possibilities Constantly of using conducted this type research of mineral, on theas shownspecific in properties Figure2. Of of course, zeolites all show applications comprehensive of zeolites possibilities cannot be mentionedof using this (only type examples of mineral will, beas shownpresented in Figure in this 2. paper), Of course, but evenall applications those presented, of zeolites for example, cannot be give mentioned a picture (only of the examples great importance will be presentedof zeolites in in this various paper), applications. but even those In addition, presented other, for example properties, give and a possibilitiespicture of the of great practical importance use are ofbeing zeolites discovered. in various applications. In addition, other properties and possibilities of practical use are being discovered. FigureFigure 2. 2. ZZeoliteeolite application applications.s. 2. Natural Zeolites 2. Natural Zeolites Natural zeolites are hydrothermal and of mainly volcanic origin. They can occur both in Natural zeolites are hydrothermal and of mainly volcanic origin. They can occur both in crystallized forms found in igneous and metamorphic rocks, as well as in grains of smaller diameters crystallized forms found in igneous and metamorphic rocks, as well as in grains of smaller diameters accumulated in sedimentary rocks [5]. Ocean bottom sediments are relatively huge and rich in zeolites, accumulated in sedimentary rocks [5]. Ocean bottom sediments are relatively huge and rich in but these deposits are so far inaccessible to humans. However, these minerals may also constitute zeolites, but these deposits are so far inaccessible to humans. However, these minerals may also important components of tuffs or clay. Such surface retention of zeolite sediments, and therefore constitute important components of tuffs or clay. Such surface retention of zeolite sediments, and relatively simple mining using the opencast method, creates perfect conditions for their wider use. therefore relatively simple mining using the opencast method, creates perfect conditions for their It should be mentioned here that the zeolites naturally occurring in nature, possessing operational wider use. It should be mentioned here that the zeolites naturally occurring in nature, possessing significance, are: clinoptilolite; mordenite; chabazite. operational significance, are: clinoptilolite; mordenite; chabazite. Application Examples 2.1. Application Examples Natural zeolites have a high selectivity for heavy metal ions [6–8] and ammonium ions [9]. Thus, zeolitesNatural have zeolites found important have a high uses selectivity in environmental for heavy protection metal ions and [6–8 agriculture.] and ammonium Wastewater ions [ treatment9]. Thus, zeolitesfrom heavy have metal found ions important [10,11] or uses radioactive in environmental isotopes [12 ]protection can take place and inagriculture. sorption columns Wastewater filled treatmentwith zeolite. from Ammonium heavy metal ions ions contained [10,11] inor municipal,radioactiveindustrial isotopes [12] and agriculturalcan take place wastes in so canrption also columns filled with zeolite. Ammonium ions contained in municipal, industrial and agricultural wastes can also be removed in a similar way [13]. In agriculture, zeolites can be used as carriers of agrochemical compounds, in the treatment of soil and fish ponds and as a feed additive [2,14]. Attempts to modify the structure to give them catalytic [15] or antibacterial [16] properties are also made. After all, they are also widely used in many households as pet litter. Crystals 2020, 10, 622 3 of 8 be removed in a similar way [13]. In agriculture, zeolites can be used as carriers of agrochemical compounds, in the treatment of soil and fish ponds and as a feed additive [2,14]. Attempts to modify the structure to give them catalytic [15] or antibacterial [16] properties are also made. After all, they are also widely used in many households as pet litter. On the other hand, natural zeolites have limited applications in industry because, as already mentioned, their properties are strictly dependent on their crystal structure. The main disadvantage is that the channel diameters are too small (in the case of clinoptilolite, which is the most common in nature, it is 0.30–4 nm [17]), which do not allow for the adsorption of larger gas molecules and organic compounds. In addition, zeolite deposits are a non-renewable resource. The need for the synthesis of molecular sieves and