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Recent Progress in Synthesis and Application of Nanosized and Hierarchical Mordenite—A Short Review

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Recent Progress in Synthesis and Application of Nanosized and Hierarchical Mordenite—A Short Review catalysts Review Recent Progress in Synthesis and Application of Nanosized and Hierarchical Mordenite—A Short Review Yuri Kalvachev 1,* , Totka Todorova 1 and Cyril Popov 2,* 1 Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; [email protected] 2 Institute of Nanostructure Technologies and Analytics, University of Kassel, 34132 Kassel, Germany * Correspondence: [email protected] (Y.K.); [email protected] (C.P.); Tel.: +359-2-979-3989 (Y.K.); +49-561-804-4205 (C.P.) Abstract: Zeolites with their unique properties find applications in various fields, including medicine, agronomy, ecology, production of detergents and drying agents, and in a number of industrial pro- cesses. Among zeolites, mordenite is particularly widespread because of its high silica/alumina ratio, which allows it to resist exposure to high temperatures and to acidic gases and liquids. Mordenite is commercially available as a natural mineral and as a synthesized material. This zeolite is mostly used in its synthetic form as an acid catalyst in the petrochemical industry for the isomerization of alkanes and aromatics. In this review, we consider the scientific literature on the structure, synthesis, and two main types of modifications that solve the diffusion difficulties during catalytic processes. The first type of modifications is related to a reduction of the size of the mordenite crystals obtained to submicron or nanometric range, whereas the second ones aim to obtain hierarchical mordenite samples by appropriate post-synthetic treatments. Both types of modifications find many other applications besides solving diffusion constraints in catalytic processes. Attempts to fine-tune and control the particle size in the first type of modifications or the pore size in the second ones by adjusting various parameters during the synthesis are described. Citation: Kalvachev, Y.; Todorova, T.; Popov, C. Recent Progress in Keywords: zeolite; synthesis; nanosized mordenite; hierarchical material Synthesis and Application of Nanosized and Hierarchical Mordenite—A Short Review. Catalysts 2021, 11, 308. https:// 1. Introduction doi.org/10.3390/catal11030308 Zeolites are crystalline hydrated aluminosilicates. They possess right ordered structure Academic Editor: Roman Bulánek containing one-, two-, or three-dimensional cavities and channels with sizes up to 2 nm 4− through which a large number of molecules can pass. Their skeletons are built of [SiO4] 5− Received: 4 February 2021 and [AlO4] tetrahedra (T), linked by a shared oxygen atom. The presence of aluminum Accepted: 21 February 2021 atoms in zeolite structure determines the existence of negative skeleton charge, which Published: 26 February 2021 could be compensated by numerous cations of alkali and alkaline earth metals or by some organic cations. Typical cations involved in the zeolite structure are K+, Na+, Ca2+, Publisher’s Note: MDPI stays neutral Ba2+, and H+ [1–3]. The structural formula of a zeolite represents the smallest segment of with regard to jurisdictional claims in the structure: published maps and institutional affil- Mx/n[(AlO2)x(SiO2)y].wH2O iations. In the formula, n is the oxidation state of cation M, w the number of water molecule, and x and y are the numbers of aluminum and silicon atoms per unit cell. The ratio y/x varies for different zeolite structures [1,4]. The zeolite properties are determined by their chemical composition in combination with the specific channel system. Zeolites have been Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. extensively studied and applied in different fields like oil refining, petrochemical industry, This article is an open access article production of detergents and drying agents, separation of gas mixtures, in ecology, and distributed under the terms and also in many novel high-tech areas as electronics, optics, and sensor technology [5–9]. conditions of the Creative Commons Due to its unique textural properties and strong acidity, mordenite has been widely Attribution (CC BY) license (https:// employed as a heterogeneous catalyst in the petrochemical industry. Its superior acidic creativecommons.org/licenses/by/ and shape-selective properties are used in numerous processes, such as toluene dispro- 4.0/). portionation, alkylation, m-xylene transformation, hydroisomerization, carbonylation, etc. Catalysts 2021, 11, 308. https://doi.org/10.3390/catal11030308 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 16 Due to its unique textural properties and strong acidity, mordenite has been widely employed as a heterogeneous catalyst in the petrochemical industry. Its superior acidic and shape-selective properties are used in numerous processes, such as toluene dispro- Catalysts 2021, 11, 308 portionation, alkylation, m-xylene transformation, hydroisomerization, carbonylation,2 of 16 etc. Soon after its discovery, mordenite became one of the most useful industrial ze- olites. The reason is the high thermal and acidity stability in combination with a large channel system. Soon after its discovery, mordenite became one of the most useful industrial zeolites. The Despite the relatively large pores of mordenite, their size is often a drawback when reason is the high thermal and acidity stability in combination with a large channel system. the dimensions of the molecules subjected to transformations on the zeolites or that of the Despite the relatively large pores of mordenite, their size is often a drawback when resulting products are large. There are two approaches to overcome this disadvantage and the dimensions of the molecules subjected to transformations on the zeolites or that of the to provide improved diffusion of molecules—one is the synthesis of nanosized zeolite, the resulting products are large. There are two approaches to overcome this disadvantage and other the preparation of hierarchical material. to provide improved diffusion of molecules—one is the synthesis of nanosized zeolite, the The goal of the present review is to summarize and systematize the data on the other the preparation of hierarchical material. structure,The modifications, goal of the present characterizations, review is to summarize and applications and systematize of mordenite the datawith ona fo- the cusstructure, on various modifications, approaches characterizations, for modifications and that applications solve the diffusion of mordenite problems. with a Synthe- focus on sisvarious methods approaches for nanosized for modifications and hierarchical that mordenite solve the diffusion are discussed problems. and their Synthesis applications methods infor the nanosized field of catalysis and hierarchical are presented. mordenite are discussed and their applications in the field of catalysis are presented. 2. Structure of Mordenite 2. StructureThe mordenite of Mordenite type of zeolite was discovered and named by Professor How in 1864. The name mordenite was introduced type of zeolite for a was minera discoveredl found andin region named Morden, by Professor Nova HowScotia in in1864. Canada The [10,11]. name wasThis introduced new-for-the-time for a mineral mineral found has high in region silicon Morden, content Nova structure Scotia within Canada approximate[10,11]. Thischemical new-for-the-time composition mineral (Na2,K has2,Ca) high4(Al silicon8Si40O content96)∙28H structure2O. The zeolite with ap- mordeniteproximate belongs chemical to composition pentasil family, (Na2,K typical2,Ca)4 (Alfor8 Siwhich40O96 )are·28H five2O. Themembered zeolite mordeniterings as secondarybelongs to building pentasil family,units [12–14]. typical for The which mord areenite five structure membered is rings composed as secondary of connected building 5-memberedunits [12–14 ].rings The forming mordenite a composite structure is buildin composedg unit of connected“mor”. These 5-membered building ringsunits form-are connecteding a composite by 4-membered building rings unit “mor”. which determ These buildingine the formation units are of connected 12-membered by 4-membered channels withrings a size which of determine0.7 × 0.65 nm the and formation strongly of 12-memberedcompressed 8-membered channels with channels a size of with0.7 × a 0.65size nmof 0.57and × strongly0.26 nm along compressed the crystallographic 8-membered channelsdirection with[001] a(Figure size of 10.57 a). ×Only0.26 8-membered nm along the channelscrystallographic (0.34 × 0.48 direction nm), [001]referred (Figure as 1sidea). Only pockets, 8-membered are lined channels up in parallel(0.34 × to0.48 [010] nm) , directionreferred as(Figure side pockets, 1 b). The are reason lined up is in that parallel the channels, to [010] direction running (Figure staggered1b). The along reason the is “b”-axis,that the channels,intersect runningthe other staggered two types along of channels the “b”-axis, arranged intersect alongside the other the two “c”-axis types of [11,15,16].channels arranged alongside the “c”-axis [11,15,16]. FigureFigure 1. 1.SkeletalSkeletal structure structure of of mordenite projectedprojected ( a()a perpendicular) perpendicular to to axis axis “c” “c” and and (b) to(b) axis to “b”. axis “b”. The mordenite structure possesses an orthorhombic unit cell of topological space- groupThe ofmordenite symmetry structureCmc21. possesses This space-group an orthorhombic is very close unit to cell space-group of topologicalCmcm space-group(a = 18.2560 of Å, symmetry b = 20.5340 Cmc2 Å,1 c. This = 7.5420 space-group Å); the differenceis very close is dueto
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