materials ReviewReview ImportantImportant SynthesisSynthesis ParametersParameters AffectingAffecting CrystallizationCrystallization of of ZeoliteZeolite T:T: AA Review Review SitiSiti Z. Z. Patuwan Patuwan and and Sazmal Sazmal E. E. Arshad Arshad * * FacultyFaculty of of Science Science and and Natural Natural Resources, Resources, Universiti Universiti Malaysia Malaysia Sabah, Sabah, Kota 88450 Kinabalu Kota Kinabalu, 88450, Sabah, Sabah, Malaysia; Malaysia; [email protected]@gmail.com ** Correspondence:Correspondence: [email protected] [email protected] Abstract:Abstract:Synthesis Synthesis of of zeolite zeolite T T with with a varietya variety of of desired desired characteristics characteristics necessitates necessitates extensive extensive work work in formulationin formulation and and practical practical experiments, experiments, either either by conventional by conventional hydrothermal hydrothermal methods methods or aided or aided with differentwith different approaches approaches and synthesis and synthesis techniques, techniques such as, such secondary as secondary growth growth or microwave or microwave irradiation. irra- Thediation. objectives The objectives of this review of this are review to adduce are to the adduce potential thework potential in zeolite work T in (Offretite-Erionite) zeolite T (Offretite-Erionite) synthesis, evaluatingsynthesis, determiningevaluating determining factors affecting factors the affectin synthesisg the and synthesis quality ofand zeolite quality T crystals. of zeolite Attention T crystals. is givenAttention to extensive is given studiesto extensive that interconnect studies that withinterconnect other significant with othe findings.r significant findings. Keywords:Keywords:zeolite zeolite T;T; offretite;offretite; erionite; erionite; hydrothermal; hydrothermal; microwave; microwave; secondary secondary growth growth 1.1. IntroductionIntroduction Zeolites are well-known porous crystal systems that exist in various sizes (nano-, Zeolites are well-known porous crystal systems that exist in various sizes (nano-, mi- micro-,cro-, etc.) etc.) and and have have been been widel widelyy incorporated incorporated in different in different applications. applications. They They consist consist of a Citation: Patuwan, S.Z.; Arshad, S.E. ofuniform a uniform size sizeof pore of poredistributions distributions [1] which [1] which are also are known also known as voids. as voids.Voids and Voids cavities and Important Synthesis Parameters cavities within the zeolite system are the factors that contribute to the unique properties within the zeolite system are the factors that contribute to the unique properties of the AffectingCitation: Crystallization Patuwan, S.Z.;of Arshad, Zeolite S.E. T: of the zeolite. These voids and cavities are controlled by the type of crystals that grow zeolite. These voids and cavities are controlled by the type of crystals that grow and in- AImportant Review. Materials Synthesis2021 Parameters, 14, 2890. and intergrowth within the zeolite membrane. Introduced for the first time by Bennet and tergrowth within the zeolite membrane. Introduced for the first time by Bennet and Grad https://doi.org/10.3390/Affecting Crystallization of Zeolite Grad [2], nanoporous zeolite T (the rod-like shape shown in Figure1) is identified to have T: A Review. Materials 2021, 14, x. [2], nanoporous zeolite T (the rod-like shape shown in Figure 1) is identified to have ma14112890 within it an intergrowth framework that makes zeolite type T an exceptional species in the https://doi.org/10.3390/xxxxx within it an intergrowth framework that makes zeolite type T an exceptional species in zeolite family. Academic Editors: Alain Moissette the zeolite family. andAcademic Jordi Sort Editor: Alain Moissette and Jordi Sort Received: 28 January 2021 Accepted:Received:19 28 April January 2021 2021 Published:Accepted: 2819 MayApril 2021 2021 Published: 29 April 2021 Publisher’s Note: MDPI stays neutral withPublisher’s regard to Note: jurisdictional MDPI stays claims neu- in publishedtral with maps regard and institutionalto jurisdictional affil- iations.claims in published maps and institu- tional affiliations. Copyright:Copyright: ©© 2021 2021 by by the the authors. authors. FigureFigure 1. 1.Rod-like Rod-like shape shape of of zeolite zeolite T T synthesized synthesized by by [3 [3]] (Reproduced (Reproduced with with permission permission from from Yin Yin et al.,et LicenseeLicensee MDPI,MDPI, Basel, Basel, Switzerland. Switzerland. al., Hydrothermal synthesis of hierarchical zeolite T aggregates using tetramethylammonium hy- Hydrothermal synthesis of hierarchical zeolite T aggregates using tetramethylammonium hydroxide ThisThis article article isis an open open access access article article droxide as single template; published by Elsevier, 2015). as single template; published by Elsevier, 2015). distributeddistributed under the the terms terms and con- and conditionsditions of the of theCreative Creative Commons Commons At- This intergrowth system is made up of a cancrinite group of hexagonal crystals of Attribution (CC BY) license (https:// This intergrowth system is made up of a cancrinite group of hexagonal crystals of tribution (CC BY) license (http://cre- offretite (OFF) and erionite (ERI) (Figure 2). The interframe of these two entities causes an creativecommons.org/licenses/by/ offretite (OFF) and erionite (ERI) (Figure2). The interframe of these two entities causes an ativecommons.org/licenses/by/4.0/). alteration in the crystal arrangement of zeolite T, which is AABAACAABAAC, instead of 4.0/). alteration in the crystal arrangement of zeolite T, which is AABAACAABAAC, instead of Materials 2021, 14, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/materials Materials 2021, 14, 2890. https://doi.org/10.3390/ma14112890 https://www.mdpi.com/journal/materials Materials 2021,, 1414,, 2890x FOR PEER REVIEW 22 of of 1314 AABAABAAB [[4].4]. DespiteDespite stackingstacking within within the the zeolite zeolite system, system, OFF-ERI OFF-ERI gives gives characters characters to tozeolite zeolite T in T termsin terms of thermal of thermal stability, stability, mechanical mechanical and chemical and chemical stability stability [1], acid [1], resistance, acid re- and high ion selectivity [5], which makes zeolite T a promising candidate in catalysis, sistance, and high ion selectivity [5], which makes zeolite T a promising candidate in ca- molecular sieves, ion exchange, and other applications [6]. talysis, molecular sieves, ion exchange, and other applications [6]. Figure 2. Framework of offretite (OFF) [001] (a) and erionite (ERI) [001][001] ((b)) (IZA).(IZA). Unlike other zeolite members, zeolite T possesses different void sizes due to offretite and erionite intergrowth group. Offretite has 12-ring channels parallel to the c-axis and additional 8-ring subchannels that are normal to the c-axis. The pore size for the 12-ring12-ring channels is 0.67 nm ×× 0.680.68 nm, nm, while 0.36 nm × 0.490.49 nm nm is is the the pore size for the 8-ring8-ring subchannels. Alternatively, Alternatively, erionite only has 8-ring8-ring channels parallel to the c-axis, with a pore diameter of 0.36 nmnm ×× 0.510.51 nm nm [1]. [1]. Generally, Generally, the framework of OFF and ERI may differ, but they are closelyclosely related.related. Notably, the ionic behavior of pure OFF and ERI is similar to the synthetic intergrowth, offretite-erionite, in zeolite T. Thus, zeolite Ts can be considered asas intergrowth intergrowth phases phases and and are homogenousare homogenous mixtures mixtures of two of component two component phases phasesof offretite of offretite and erionite and erionite [4,7]. [4,7]. 1.1. Hydrothermal Conventional Method Hydrothermal treatment is the most conventional and widely applied method for synthesizing zeolitezeolite TT and and is is considered considered to to be be the the earliest earliest synthesis synthesis method, method, leading leading to high to percentagehigh percentage (88–90%) (88–90%) product product yields. yields. It is aIt treatment is a treatment that involvesthat involves high high temperature temperature and pressureand pressure in the in presence the presence of water of water that acts that not acts only not asonly a solvent, as a solvent, but also but aids also in aids transferring in trans- pressure.ferring pressure. It is a reaction It is a reaction mediator, mediator, capable of capable altering of the altering physical the and physical chemical and properties chemical propertiesof reagents of or reagents products, or andproducts, it possesses and it thepossesses potential the topotential be an accelerator to be an accelerator in chemical in reactions [8–10]. The hydrothermal synthesis technique is usually conducted on a mixture chemical reactions [8-10]. The hydrothermal synthesis technique is usually conducted on in a sealed vessel, where heat treatment is applied. This procedure is also labeled as an a mixture in a sealed vessel, where heat treatment is applied. This procedure is also la- in situ reaction that strongly depends on temperature and time [1]. This method is easy beled as an in situ reaction that strongly depends on temperature and time [1]. This to control during the synthesis phase, and recently, via hydrothermal treatment, the high method is easy to control during the synthesis phase, and recently, via hydrothermal treat- purity and crystallinity of zeolite T were synthesized in 100 ◦C for 168 h [11,12]. ment, the high