Important Synthesis Parameters Affecting Crystallization of Zeolite T

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 institutional 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 purity and crystallinity of zeolite T were synthesized in 100 °C for 168 h Additionally, the synthesis method of zeolite T has evolved with the emergence of [11,12]. other methods fused with hydrothermal treatment, such as (i) microwave-assisted [9–12], Additionally, the synthesis method of zeolite T has evolved with the emergence of (ii) sonochemical-assisted, and (iii) secondary growth techniques, for the development of other methods fused with hydrothermal treatment, such as (i) microwave-assisted [9–12], zeolite T membrane. (ii) sonochemical-assisted, and (iii) secondary growth techniques, for the development of zeolite1.2. Microwave-Assisted T membrane. Method The microwave-assisted method has been widely applied in zeolite synthesis for 1.2. Microwave-Assisted Method a decade due to its capability to rapidly produce high purity products with less time consumptionThe microwave-assisted [9]. With microwave method assistance, has been the wi zeolitedely applied T membrane’s in zeolite preparation synthesis for time a wasdecade shortened due to its by capability 12 times, to to only rapidly 9 h [produce3,11]. high purity products with less time con- sumptionThe effect[9]. With of microwavemicrowave assistance, irradiation the gives zeolite impetus T membrane’s to the annihilation preparation of time normal was shortenedhydrogen (H)by 12 bonding times, into water,only 9 causing h [3,11]. ion oscillation and dipole rotation, which produces isolated,The so-calledeffect of microwave active water irradiation molecules, gives which impetus respond to tothe the annihilation high rate of of dissolving normal hy- of drogen (H) bonding in water, causing ion oscillation and dipole rotation, which produces

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Materials 2021, 14, 2890 3 of 13 isolated, so-called active water molecules, which respond to the high rate of dissolving of isolated, so-called active water molecules, which respond to the high rate of dissolving of the composition gel. Coupling the behavior of the microwave with the hydration of shell thethe composition composition gel. gel. Coupling Coupling the the behavior behavior of of the the microwave microwave with with the the hydration of shell water molecules is efficient and favors the coordination change of the alumina octahedral water molecules is efficient efﬁcient and favors the coordination change of the alumina octahedral layer to the tetrahedral layer, which is more reactive and essential in growing zeolite T layerlayer to to the the tetrahedral tetrahedral layer, layer, which which is is more more reactive reactive and and essential essential in ingrowing growing zeolite zeolite T materials. Hence, it results in thinner membranes that affect zeolite T’s permeability prop- materials.T materials. Hence, Hence, it results it results in thinner in thinner membranes membranes that affect that affect zeolite zeolite T’s permeability T’s permeability properties [10,11]. ertiesproperties [10,11]. [10 ,11]. Figure 3 shows the morphology of thin, compact zeolite T membranes. Growing the FigureFigure 33 showsshows thethe morphologymorphology ofof thin,thin, compactcompact zeolitezeolite TT membranes.membranes. GrowingGrowing thethe rod-like zeolite T upon the alumina support tube required a secondary growth technique rod-likerod-like zeolite zeolite T T upon upon the the alumina alumina support support tube tube required required a secondary growth technique and several heating steps. The heating process affects the distribution of zeolite T on the andand several several heating heating steps. steps. The The heating heating process process affects affects the the distribution of of zeolite T on the membrane, and the distribution affects the membrane selectivity performance. It was membrane,membrane, and and the distribution affects the membranemembrane selectivityselectivity performance.performance. It was found that the low power of microwave heating (4.8 kW/m33) led to insufficient growth of foundfound that that the the low power of microwave heatingheating (4.8(4.8 kW/mkW/m3)) led led to to insufficient insufﬁcient growth growth of zeolite T (Figure 3a), while as the power density increased to 6.0 kW/m3 3, the membrane zeolitezeolite T (Figure 33a),a), whilewhile asas thethe power power density density increased increased to to 6.0 6.0 kW/m kW/m3,, the membrane evolved with more defects (Figure 3b). Therefore, by controlling the heating power den- evolvedevolved with more defectsdefects (Figure(Figure3 b).3b). Therefore, Therefore, by by controlling controlling the the heating heating power power density density of the microwave, high-performance zeolite T membranes can be synthesized [11]. sityof the of microwave,the microwave, high-performance high-performance zeolite zeolite T membranes T membranes can becan synthesized be synthesized [11]. [11].

Figure 3. Cross-section of zeolite T membrane heated at 4.8 kW/m3 (a) and 6.0 kW/m3 (b); the circle Figure 3. Cross-section of zeolite T membrane heated at 4.8 kW/m33 (a) and 6.0 kW/m33 (b); the circle Figureindicates 3. Cross-section the area defects of zeolite[11] (Reproduced T membrane with heated permission at 4.8 kW/m from Zhou(a) and et 6.0al., kW/m Microwave-assisted(b); the circle indicates the area defects [11] (Reproduced with permission from Zhou et al., Microwave-assisted indicateshydrothermal the area synthesis defects of [11 a&b-] (Reproducedoriented zeolite with T permission membranes from and Zhoutheir etpervaporation al., Microwave-assisted properties; hydrothermal synthesis of a&b-oriented zeolite T membranes and their pervaporation properties; hydrothermalpublished by Elsevier, synthesis 2009 of a&b-oriented). zeolite T membranes and their pervaporation properties; published by Elsevier, 2009). published by Elsevier, 2009). 1.3. Sonochemical-Assisted Method 1.3.1.3. Sonochemical-Assisted Sonochemical-Assisted Method Method The sonochemical-assisted method uses powerful ultrasound radiation (20 kHz–10 The sonochemical-assisted method uses powerful ultrasound radiation (20 kHz–10 MHz)The to sonochemical-assisted treat the homogenous method mixture uses before powerful hydrothermal ultrasound treatment. radiation (20 Nucleation kHz–10 MHz) and MHz) to treat the homogenous mixture before hydrothermal treatment. Nucleation and toformation treat the homogenousof zeolite T have mixture been before superior hydrothermal due to sonochemical treatment. Nucleation effects from and acoustic formation fre- formation of zeolite T have been superior due to sonochemical effects from acoustic fre- ofquency zeolite usage, T have and been this superior method due has to shortened sonochemical the synthesis effects from period acoustic from frequency168 h to 24usage, h [13]. quency usage, and this method has shortened the synthesis period from 168 h to 24 h [13]. andFigure this 4 method shows the has morphology shortened the of synthesis zeolite T periodsynthesized from 168with h and to 24 without h [13]. Figuresonication4 shows pre- Figure 4 shows the morphology of zeolite T synthesized with and without sonication pre- thetreatment. morphology of zeolite T synthesized with and without sonication pre-treatment. treatment.

Figure 4. Morphology of zeolite T synthesized in 168 h without sonication (a), and zeolite T form FigureFigure 4. 4. MorphologyMorphology of of zeolite zeolite T synthesize synthesizedd in 168 h without sonication (a), and zeolitezeolite TT formform in in 24 h assisted with sonication pre-treatment (b) [12] (Reproduced with permission from Jusoh et in24 24 h assistedh assisted with with sonication sonication pre-treatment pre-treatment (b ()[b)12 [12]] (Reproduced (Reproduced with with permission permission from from Jusoh Jusoh et et al. Rapid-synthesis of Zeolite T via sonochemical-assisted hydrothermal growth method; published by Elsevier, 2017).

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al. Rapid-synthesis of Zeolite T via sonochemical-assisted hydrothermal growth method; pub- Materials 2021, 14, 2890 lished by Elsevier, 2017). 4 of 13

The sonochemical-assisted method incorporates a similar concept to the microwave- assistedThe method, sonochemical-assisted where the effect method of vibrat incorporatesions influences a similar the physicochemical concept to the microwave- phenom- enaassisted that method,impute the where nucleation the effect and of vibrationscrystallization inﬂuences of zeolite the physicochemical T. In contrast to phenomena the microwave-assistedthat impute the method, nucleation however, and crystallization the sonochemical-assisted of zeolite T. Inme contrastthod stimulates to the microwave- a chemical reactionassisted method,between however, solid and the liquid sonochemical-assisted in the zeolite T methodmixture stimulates by producing a chemical high reactionenergy acousticbetween cavitation, solid and liquidwhich inwill the lead zeolite to the T mixtureproduction by producingof extremely high high energy temperature acoustic ◦ (>5000cavitation, °C), pressure which will (>20 lead MPa), to the and production very high ofcooling extremely rates high(>107 temperature K s−1) [14]. (>5000 C), pressure (>20 MPa), and very high cooling rates (>107 K s−1)[14]. 1.4. Secondary Growth/Dip-Coating 1.4. Secondary Growth/Dip-Coating The secondary method is a combination of primary method, where zeolite T is syn- The secondary method is a combination of primary method, where zeolite T is synthesized by following the optimum formulation. It is repeated twice using a supporter thesized by following the optimum formulation. It is repeated twice using a supporter membrane (organic or inorganic), allowing more zeolite crystals to grow within the first membrane (organic or inorganic), allowing more zeolite crystals to grow within the ﬁrst system in two or more different composition gel formulations. Secondary growth is al- system in two or more different composition gel formulations. Secondary growth is always ways related with seeding zeolite crystal seed on the supports, in direct hydrothermal in related with seeding zeolite crystal seed on the supports, in direct hydrothermal in situ situcrystallization, crystallization, either either in ain clear a clear solution solution or or in in an an aqueous aqueous gel gel inside inside an an autoclaveautoclave atat a different temperature, temperature, depending depending on on the the types types of of zeolite; zeolite; 90–100 90–100 °C◦ forC for faujasite faujasite (FAU) (FAU) or Lindeor Linde type-A type-A (LTA) (LTA) while while 180 180 °C ◦forC forMFI MFI type type membranes membranes [15]. [15 The]. The standard standard method method for secondaryfor secondary growth growth is dip-coating, is dip-coating, where where the su thepport support base baseis dipped is dipped into the into solution the solution mix- turemixture containing containing the seed the seed or crystal or crystal of zeolite of zeolite T. T. Figure 55 shows shows the the illustrated illustrated dip-coating dip-coating techniques’ techniques’ execution. execution. Using Using this techniquethis tech- niquegives angives advantage an advantage in controlling in controllin the desiredg the desired thickness thickness of a membrane. of a membrane.

Figure 5. SchematicSchematic diagram diagram of of dip-coating dip-coating method method in in preparing preparing zeolite zeolite T/carbon T/carbon composite composite [16] [16 ] ((ReproducedReproduced withwith permissionpermission fromfrom Yin Yin et et al., al., Thin Thin zeolite zeolite T/carbon T/carbon composite composite membranes membranes supported sup- portedon the porouson the porous alumina alumina tubes for tubes CO 2forseparation; CO2 separation published; published by Elsevier, by Elsevier, 2013). 2013).

Consequently, FigureFigure6 6shows shows the the morphology morphology of of the the zeolite zeolite T membraneT membrane prepared prepared in indifferent different coating coating layers. layers. Its thicknessIts thickness controls controls the selectivitythe selectivity and permeabilityand permeability properties proper- of tiesthe membrane.of the membrane. As the coatingAs the cyclecoating increases, cycle increases, the membrane the membrane terrace’s thickness terrace’s and thickness hence, andthe gas’hence, ideal the selectivity gas’ ideal is selectivity improved, is while improved, the gas’ while permeance the gas’ is reducedpermeance [9,16 is]. reduced [16,9].Despite the ﬂexibility of the seeding method, several factors need close attention to maintain the quality and purity of the zeolite T crystal that grows on the support. Defects may occur during the process of growing the zeolite T particles, and the combination of this technique with microwave treatment helps to form a uniform oriented zeolite T membrane [11]. Moreover, (i) the surface roughness of the support, (ii) suspension concentration, (iii) seed size, and (iv) coating temperature are essential factors that will affect the coverage, thickness, and defect-free property of the dip-coating technique [15]. As the zeolite T seeds grow on the support surface, the seeds’ distribution is controlled by the suspension concentration, which is attributed to the layer thickness. Eventually, the thick membrane

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the quality of the membranes [9,17,18].

Figure 6. Morphology of zeolite T membrane undergone one coating cycle (a) and four coating Figure 6. Morphology of zeolite T membrane undergone one coating cycle (a) and four coating cycle cycle (b) [16] (Reproduced with permission from Yin et al., Thin zeolite T/carbon composite mem- (b)[16] (Reproduced with permission from Yin et al., Thin zeolite T/carbon composite membranes branes supported on the porous alumina tubes for CO2 separation; published by Elsevier, 2013). supported on the porous alumina tubes for CO2 separation; published by Elsevier, 2013). Despite the flexibility of the seeding method, several factors need close attention to Additionally, the temperature for drying and curing the membrane also plays a vital rolemaintain in the the prepared quality membrane’sand purity of quality. the zeolite Figure T crystal7 shows that the grows morphology on the support. of the zeoliteDefects Tmay membrane occur during using the a SiCprocess polished of growing tubular the mullite zeolite support T particles, (outer and diameter the combination = 12 mm, of thicknessthis technique = 1.5 with mm, microwave and average treatment pore of 1.0 helpµm)s to that form has a undergone uniform oriented the drying zeolite and T curing mem- processbrane [11]. at temperatures ~100 ◦C and ~150 ◦C, respectively [5]. As the drying and curing temperatureMoreover, increased (i) the tosurface 150 ◦C, roughness the top layer’s of the intensity support, is (ii) more suspension robust than concentration, the membrane (iii) preparedseed size, at and 100 (iv)◦C. coating temperature are essential factors that will affect the coverage, thickness, and defect-free property of the dip-coating technique [15]. As the zeolite T seeds 2.grow Factors on the Affecting support Crystallization surface, the seeds’ of Zeolite distribution T is controlled by the suspension concentration,Zeolite which T can is be attributed prepared to using the layer various thickness. ways,either Eventually, using the independent thick membrane chemicals will orlead aided to the with formation seeding of support. microcracks The onlyduring matter drying in synthesizingand curing, and zeolite ultimately T is during affectthe the crystallizationquality of the membranes phase, which [9,17,18]. determines the effectiveness of the preparation requirements. Meanwhile,Additionally, crystal the formation temperature of zeolite for drying T (either and curing by hydrothermal the membrane synthesis also plays or aideda vital withrole in microwave-, the prepared or membrane’s sonochemical- quality. irradiation) Figure 7 is shows affected the bymorphology several primary of the zeolite factors. T Frommembrane the past using and a recent SiC polished literature, tubular several mullite cases support involve crystals’(outer diameter behavior = 12 upon mm, several thick- factors,ness = 1.5 namely mm, and SiO 2average/Al2O3 poreratio, of alkalinity 1.0 µm) that (n (OH)–)/n has undergone (SiO2)), the water drying ratio, and synthe- curing sisprocess temperature at temperatures and crystallization ~100 °C and time, ~150 and °C, therespectively addition of[5]. structure-directing As the drying and agents curing (SDA)temperature [1,3,4,8 ,increased16,19–22] ,to while 150 pressure°C, the top is notlayer’s directly intensity involved is more in zeolite robust T than synthesis the mem- [23]. Thisbrane review prepared summarizes at 100 °C. the factors that affect the growth of zeolite T.

2.1. Effects of SiO2/Al2O3 Ratio

Zeolite T is classiﬁed as intermediate silica zeolites (SiO2/Al2O3 composition ratio from 2 to 5 [22–24], hence the ratio of SiO2/Al2O3 will affect the crystallization behavior of zeolite T. Percentage yield of zeolite T can be maximized by increasing the molar ratio of SiO2/Al2O3 in the mixture. However, the optimum ratio for zeolite T is in the range of 3 to 4 [1–6,9–11,13,15,20–22,24]. However, the ratio of SiO2/Al2O3 depends on the alkalinity of the composition. Since zeolite T favors high alkalinity crystallization, controlling the ratio of SiO2/Al2O3 is crucial to prevent it from exceeding the optimal range (3 to 4). Otherwise, the composition mixture can promote the crystallization of competitive phases, such as zeolite L and/or W [2,20]. The SiO2/Al2O3 ratio was found to affect the crystallization behavior, such as nucleation rate, crystal purity and intensity, and application performance [1,2,4,5]. The amount of silica used to synthesize zeolite T, being higher than the amount of alumina, contributes to the stability characteristics in both thermal and acid, while remain- ing hydrophilic [11,25]. Hence, the ratio of SiO2/Al2O3 not only affects the crystallization behavior but also the application performances of the synthesized zeolite T.

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FigureFigure 7. 7.SEMSEM for for zeolite zeolite T membrane T membrane at at373 373 K (100 K (100 °C),◦C), 35 h 35 for h for(a) and (a,b );(b 423); 423 K (150K (150◦C) °C) for for 20 20 h h for for(c ,(dc)[) and5] (Reproduced (d) [5] (Reproduced with permission with permissi fromon Zhou from et Zhou al., Synthesis et al., Synthesis of oriented of oriented zeolite Tzeolite membranes T membranesfrom clear solutionsfrom clear and solutions their pervaporation and their pervaporation properties; properties published; bypublished Elsevier, by 2013). Elsevier, 2013).

2.2.2. Factors Effects Affecting of Alkalinity Crystallization of Zeolite T ZeoliteZeolite T Tcan is be synthesized prepared using under various activation ways, of either the alkaline using independent medium, which chemicals is usually or aidedmade with up ofseeding mineralizer support. components The only ofmatter Na2O in and synthesizing K2O. The alkalinityzeolite T is strongly during inﬂuencesthe crys- tallizationthe level ofphase, crystallinity which determines and stability the of effe thectiveness zeolites structures, of the preparation thus affecting requirements. the perfor- Meanwhile,mance of zeolite crystal Tformation in its applications. of zeolite T Generally,(either by hydrothermal when the medium synthesis is in or high aided relative with microwave-,alkalinity at or about sonochemical- 0.71 [2], it willirradiation) accelerate is theaffected nucleation by several of zeolite primary T crystals factors. and From shorten the pastthe and crystallization recent literature, time, because several at cases a high invo pH,lve Na crystals’2O and Kbehavior2O tend upon to loosen several and factors, dissolve namelyby releasing SiO2/Al their2O3 ratio, OH–, alkalinity and this accelerates(n (OH)–)/n the (SiO reaction2)), water between ratio, synthesis aluminate temperature and silicate andions, crystallization while the degree time, of silicate and anionsthe addition polymerization of structure-directing decreases [13,26 ]. agents (SDA) [1,3,4,8,16,19–22],However, at while higher pressure relative alkalinityis not directly beyond involved 0.71 [2 ],in synthesized zeolite T synthesis crystals of[23]. T-zeolite This reviewtend to summarizes redissolve, resultingthe factors in that a low-density affect the growth product, of affecting zeolite T. the performance properties of zeolite T [13]. This phenomenon is due to the characteristic of aluminosilicate layers, ≥ 2.1.which Effects tend of toSiO aggregate,2/Al2O3 Ratio forming another phase in alkalinity 0.82 [27]. This study proved that the relative alkalinity has a substantial impact on zeolite T crystallization [2]. Different Zeolite T is classified as intermediate silica zeolites (SiO2/Al2O3 composition ratio relative alkalinity yields different purity of the product; as the value increases, the zeolite from 2 to 5 [22–24], hence the ratio of SiO2/Al2O3 will affect the crystallization behavior of T crystals’ growth was affected. Table1 and Figure8 elucidate the product formed from zeolite T. Percentage yield of zeolite T can be maximized by increasing the molar ratio of different zeolite species due to different relative alkalinity values. Meanwhile, the evolution SiOphase2/Al2 ofO3 zeolite in the Tmixture. crystals However, and the morphologies the optimum for rati eacho for sample zeolite areT is shown in the inrange Figure of 38 a,b.to 4 [1–6,9–11,13,15,20–22,24].Thus, the synthesis of zeolite T must be in between a relative alkalinity of 0.71 [1,2] heldHowever, only at optimum the ratio crystallizationof SiO2/Al2O3 depends time (24–168 on the h) alkalinity to allow theof the growth composition. inclination. Since As zeolitethe relative T favors alkalinity high alkalinity increases, crystallization, it will trigger controlling the evolution the ofratio species of SiO zeolite2/Al2O L3 andis crucial W. to prevent it from exceeding the optimal range (3 to 4). Otherwise, the composition mix- ture2.3. can Effects promote of pH andthe Watercrystallization Ratio of competitive phases, such as zeolite L and/or W [2,20]. TheThe waterSiO2/Al component2O3 ratio was added found into to affect the zeolite the crystallization T system is calculatedbehavior, such and presentedas nucle- ationexplicitly rate, crystal in an adequate purity and amount, intensity, as theand medium’s application alkalinity performance depends [1,2,4,5]. on the amount of waterThe added. amount Zeolite of silica formation used to synthesize is greatly affectedzeolite T, by being thepH higher values than range the amount (0.71–0.82), of alumina,which corresponds contributes to to the the stability values ofcharacteristics water precursors in both of thermal the solutions and acid, [3]. while Thus, remain- the pH ingof hydrophilic the alkaline [11,25]. medium Hence, can be the varied ratio byof alteringSiO2/Al2O the3 not amount only affects of water the in crystallization moles of 8–16 behavior but also the application performances of the synthesized zeolite T.

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fused in. When the amount of water inside the system decreases, it initiates the mineralizer from Na2O and K2O to release OH-, causing the pH and alkalinity to increase [3]. As the synthetic medium is high in alkalinity, zeolite T particles’ size decreases because zeolite T’s aggregation size decreases as the water component decreases [27]. Moreover, the Materials 2021, 14, x FOR PEER REVIEW 8 of 14 mixture’s water content will reduce the degree of OH– concentration, leading to decreasing monotonic zeolite growth and crystallinity [28].

(a)

(b)

FigureFigure 8. 8. (a() aXRD) XRD spectrum spectrum for sample for sample S1-S8 [2] S1–S8 (b) SEM [2]( morphologiesb) SEM morphologies for sample S1–S8 for [2] sample (Re- S1–S8 [2] produced with permission from Rad et al., Development of T type zeolite for separation of CO2 (Reproduced with permission from Rad et al., Development of T type zeolite for separation of CO2 from CH4 in adsorption processes; published by Elsevier, 2012). from CH4 in adsorption processes; published by Elsevier, 2012). Thus, the synthesis of zeolite T must be in between a relative alkalinity of 0.71 [1,2] held only at optimum crystallization time (24–168 h) to allow the growth inclination. As the relative alkalinity increases, it will trigger the evolution of species zeolite L and W.

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Table 1. Effect of relative alkalinity toward product phases.

Sample ηRM α Silica Sources Relative Crystallinity Product Phase S1 20 0.82 Silica acid 0 L + W S2 20 0.71 Silica acid 69 T + L S3 25 0.71 Colloidal silica 80 T + L S4 25 0.71 Colloidal silica 100 T S5 25 0.82 Colloidal silica 54 T + L S6 20 0.82 Colloidal silica 26 T + L S7 20 0.71 Colloidal silica 82 T S8 25 0.82 Silica acid 35 T + L

ηRM: SiO2/Al2O3 ratio, α: relative alkalinity. Source: [2] (Reproduced with permission from Rad et al., Develop- ment of T type zeolite for separation of CO2 from CH4 in adsorption processes; published by Elsevier, 2012).

Unfortunately, crystallization of zeolite T will be stunted if the relative amount of water is lower than 14 H2O, as the synthetic mixture’s reagent’s chemical reaction is incomplete. This is due to the function of water in the system as an ion transporter, also known as cavities. However, an excessive amount of water needed in the system will diminish the zeolite T crystals into an amorphous system [19]. Thus, an excessive amount of water inside the synthetic solution will cause poor selectivity of the zeolite T crystals intergrowth layer. Meanwhile, Figure9 shows the XRD analysis for samples synthesized at water, mole 25, with no zeolite crystal form and indicates the amorphous phase evolved. Therefore, only an adequate mount of water, mole 14–16 H2O, is needed in performing the optimum crystallization of zeolite T, as the water controls the pH ranges [2,3].

2.4. Effects of Synthesis Temperature and Crystallization Time Generally, temperature and time have a crucial impact upon T-type zeolites during crystallization. Although zeolite T synthesis is aided with different nucleation agents, the crystals phase may develop defects that are observable in other zeolite phases and species if the composition gel is exposed to insufficient or excessive temperature, or inappropriate crystallization time. Meanwhile, the authors [1] studied the effect of crystallization temperature time in synthesizing zeolite T and parsed Ostwald’s theory, where under the same synthesizing condition, zeolite T can coexist with other species, which develops either depending on time or temperature variations. Table2 has a summary of the findings of their study. Zeolite T can be developed at 100 ◦C only if it is conducted at 168 to 216 h, disappearing if the temperature increases to ≥120 ◦C[29]. Samples synthesized at 120 ◦C in a time of 144 h had no crystalline phase, while as the temperature increased to 150 ◦C, zeolite T peaks subdued and increased steadily as the temperature of 180 ◦C was applied. However, at 180 ◦C, crystallization of zeolite W was also observed (2T = 19.8 and 24.3) (Figure 10). At 130 ◦C, zeolite T with high purity was synthesized in 144 h, while restricting the formation of other species like zeolite W. Table3 shows the effect of different synthesis methods as well as the temperature and time towards the crystallization of zeolite T. Different methods were employed, namely (i) conventional refluxing heating (CR), (ii) conventional hydrothermal heating (CH), (iii) microwave refluxing heating (MR) and (iv) microwave hydrothermal heating (MH) [23]. As the synthesis temperature remains constant at 100 ◦C, the crystallization behavior varies in all synthesis techniques. The result shows that the lowest time of 48 h did not provide sufficient conditions to impute the formation and growth of zeolite T nuclei. However, as the zeolite T crystals form at 120 h, the crystallization remained, even though synthesis time was prolonged to 168 h. Materials 2021, 14, x FOR PEER REVIEW 9 of 14

2.3. Effects of pH and Water Ratio The water component added into the zeolite T system is calculated and presented explicitly in an adequate amount, as the medium’s alkalinity depends on the amount of water added. Zeolite formation is greatly affected by the pH values range (0.71–0.82), which corresponds to the values of water precursors of the solutions [3]. Thus, the pH of the alkaline medium can be varied by altering the amount of water in moles of 8–16 fused in. When the amount of water inside the system decreases, it initiates the mineralizer from Na2O and K2O to release OH-, causing the pH and alkalinity to increase [3]. As the synthetic medium is high in alkalinity, zeolite T particles’ size decreases because zeolite T’s aggregation size decreases as the water component decreases [27]. Moreover, the mixture’s water content will reduce the degree of OH– concentration, leading to decreasing monotonic zeolite growth and crystallinity [28]. Unfortunately, crystallization of zeolite T will be stunted if the relative amount of Materials 2021, 14, x FOR PEER REVIEWwater is lower than 14 H2O, as the synthetic mixture’s reagent’s chemical reaction 10is ofin- 14 complete. This is due to the function of water in the system as an ion transporter, also known as cavities. However, an excessive amount of water needed in the system will diminish the zeolite T crystals into an amorphous system [19]. Thus, an excessive amount same synthesizing condition, zeolite T can coexist with other species, which develops ei- of water inside the synthetic solution will cause poor selectivity of the zeolite T crystals ther depending on time or temperature variations. Table 2 has a summary of the findings intergrowth layer. Meanwhile, Figure 9 shows the XRD analysis for samples synthesized of their study. at water, mole 25, with no zeolite crystal form and indicates the amorphous phase evolved.Table 2. Summary Therefore, of onlyevolution an adequa phases tein mountsynthesis of zeolite water, T mole hydrothermally. 14–16 H2O, is needed in per- Materials 2021, 14, 2890 forming the optimum crystallization of zeolite T, as the water controls the pH ranges9 [2,3]. of 13 Crystallization Parameters Sample Obtained Product (s), Zeolite Type Temperature (°C) Time (h) 1 100 120 Majority: W 2 100 168 Majority: T 3 100 216 Majority: T, minor: W 4 120 120 Majority: T 5 120 168 Majority: T 6 120 216 Majority: W, minor: L 7 140 120 Majority: W, minor: L 8 140 168 Majority: L, minor: T, W 9 140 216 Majority: L Source: [1] (Reproduced with permission from Mirfendereski et al., Investigation of hydrothermal synthesis parameters on characteristics of T type zeolite crystal structure; published by Elsevier, 2011).

Zeolite T can be developed at 100 °C only if it is conducted at 168 to 216 h, disappearing if the temperature increases to ≥ 120 °C [29]. Samples synthesized at 120 °C in a time of 144 h had no crystalline phase, while as the temperature increased to 150 °C, zeolite T peaks subdued and increased steadily as the temperature of 180 °C was applied. However, FigureFigureat 180 9.9. °C,XRD XRD crystallization spectrumspectrum oror zeolitezeolite of zeolite T’s samples W was synthesized also observed at at different different (2T = 19.8moles moles and of of water water24.3) [3](Figure [3 ](Repro- (Repro- 10). ducedducedAt 130 withwith °C, permissionpermission zeolite T fromwithfrom YinYinhigh et et al., purityal., Hydrothermal Hydrothermal was synthesized synthesis synthesis in of of 144 hierarchical hier h,archical while zeolite zeoliterestricting T T aggregates aggregates the for- usingusingmation tetramethylammonium tetramethylammonium of other species like hydroxide hydroxide zeolite W. as as single single template; template; published published by by Elsevier, Elsevier, 2015). 2015).

FigureFigure 10. 10.XRD XRD analysis analysis for for zeolite zeolite T T samples samples synthesizedsynthesized atat differentdifferent temperaturetemperature for 144 h with formulation of 1 SiO2: 0.025 Al2O3: 0.15 Na2O: 0.15 K2O: 14 H2O: 0.06 TMAOH, where T and W formulation of 1 SiO : 0.025 Al O : 0.15 Na O: 0.15 K O: 14 H O: 0.06 TMAOH, where T and W indicate peaks of zeolite2 T and2 W,3 respectively2 [29] (Reproduced2 2 with permission from Jiang et al., indicate peaks of zeolite T and W, respectively [29] (Reproduced with permission from Jiang et al., Synthesis of T-type zeolite nanoparticles for the separation of CO2/N2 and CO2/CH4 by adsorption Synthesisprocess; ofpublished T-type zeolite by Elsevier, nanoparticles 2013). for the separation of CO2/N2 and CO2/CH4 by adsorption process; published by Elsevier, 2013). Table 3 shows the effect of different synthesis methods as well as the temperature Therefore, at a favorable temperature of 120–130 ◦C, increasing and prolonging the and time towards the crystallization of zeolite T. Different methods were employed, crystallization time does not signiﬁcantly affect the crystallinity of zeolite T [23,29]. This suggests that 120 h is the minimum time needed to develop and grow zeolite T, via the hydrothermal method.

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Table 2. Summary of evolution phases in synthesis zeolite T hydrothermally.

Crystallization Parameters Sample Obtained Product(s), Zeolite Type Temperature (◦C) Time (h) 1 100 120 Majority: W 2 100 168 Majority: T 3 100 216 Majority: T, minor: W 4 120 120 Majority: T 5 120 168 Majority: T 6 120 216 Majority: W, minor: L 7 140 120 Majority: W, minor: L 8 140 168 Majority: L, minor: T, W 9 140 216 Majority: L Source: [1] (Reproduced with permission from Mirfendereski et al., Investigation of hydrothermal synthesis parameters on characteristics of T type zeolite crystal structure; published by Elsevier, 2011).

Table 3. Different synthesis methods and parameters affecting the crystallization behavior of zeolite T.

Experiment Technique Time (h) Crystallization Behavior (XRD) 1 48 Amorphous 2 72 Amorphous 3CR, T = 100 ◦C, P = 1 atm 96 T + amorphous 4 120 T 5 168 T 6 5 Amorphous 7 10 Amorphous 8MR, T = 100 ◦C, P = 1 atm 20 Amorphous + T 9 30 T 10 48 T 11 48 Amorphous 12 72 Amorphous + T 13CH, T = 100 ◦C, P = 1 atm 96 T 14 120 T 15 168 T 16 5 Amorphous 17 10 Amorphous 18MH, T = 100 ◦C, P = 1 atm 15 Amorphous + PHI 19 20 PHI 20 30 PHI Source: [23] (Reproduced with permission from Zhou et al., Synthesis of zeolite T by microwave and conventional heating; published by Elsevier, 2013).

2.5. Effects of the Addition of SDA Structure directing agent (SDA) is the substance implanted into the composition gel when synthesizing any desired zeolite. It acts as a template base and will be dismissed right after the crystallization phase. There are different SDA types: tetramethylammonium hydroxide (TMAOH), TMPBr2, tetraethylammonium hydroxide (TEAOH), to name a few. The presence of SDA in the synthesis system carries a signiﬁcant role, as it determines product crystals’ impeccability. Moreover, SDA can accelerate the nucleation and crystallization phase. As an example, for TMAOH, the hydroxide anion within it will provide the necessary alkalinity to facilitate the formation of anionic silicates, thus expediting the rate of dissolution of silicate and aluminate [3,7,20,21,28,30]. Additionally, the authors in [7] reported that providing an abundant amount of SDA will create high supersaturation conditions and steric stabilization of proto-nuclei, which are the critical factors in forming zeolite nanoparticles without aggregation [30]. Determination of the exact amount of SDA needed in synthesizing zeolite, especially ERI-type zeolite, is crucial because erionite (ERI) Materials 2021, 14, 2890 11 of 13

intergrowth crystals will not form effectively if the system contains a large amount of cation [7,13,20,21]. In order to determine the effect of SDA upon crystallization behavior of T-type zeolite, a different amount of TMAOH was added into the mixture system of 1 SiO2: 0.025 Al2O3: Materials 2021, 14, x FOR PEER REVIEW 12 of 14 0.15 Na2O: 0.15 K2O: x TMAOH: 14 H2O, (x = 0, 0.01, 0.02, 0.04, 0.06, 0.1), where it was determined that under zero SDA, no crystalline phases are formed up to 192 h. Aided by XRD spectrum analysis, the addition of SDA at a temperature of 120 ◦C demanded 0.06 ofamount TMAOH of toSDA synthesis needed zeolite in synthe T, withsizing a particle zeolite, size especially of 500–700 ERI- nm,type where zeolite, it showedis crucial the be- maximumcause erionite intensity (ERI) peak intergrowth (2 theta =crystals 7.7◦) and will gradually not form effectively decreased asif the more system TMAOH contains was a added.large amount of cation [7,13,20,21]. WhenIn order a different to determine ratio of the TMAOH effect of was SDA added upon into crystallization the composition beha gelvior of of 1 SiOT-type2: 0.055 zeo- Allite,2O 3a: 0.23different Na2O: amount 0.008 Kof2 O:TMAOHx TMAOH: was 14added H2O, into where the mixturex = 0, 0.05, system 0.10, of 0.15 1 SiO and2: 0.22,0.025 itAl resulted2O3: 0.15 in Na different2O: 0.15 crystallization K2O: x TMAOH: behavior 14 H2O, [3, 21(x ,24= 0,]. 0.01, Synthesized 0.02, 0.04, zeolite 0.06, T,0.1), without where a it TMAOHwas determined template, that took under 192 zero h to developSDA, no 5crystallineµm length phases in 100 are◦C, formed while up at 0.05to 192 TMAOH, h. Aided pureby XRD zeolite spectrum T started analysis, to develop the addition as early of as SDA 48 h. at Up a temperature to 0.15, 1–2 µofm, 120 rod-like °C demanded crystals 0.06 of zeoliteof TMAOH T were to obtained synthesis as zeolite pictured T, with in Figure a part 11icle. However, size of 500–700 at 0.22 nm, TMAOH, where it the showed rod-like the shapemaximum crystals intensity deformed peak into (2 smalltheta irregular= 7.7°) and particles, gradually which decreased conﬁrmed as more that zeolite TMAOH T with was nano-sizedadded. crystals could be prepared in 0.22 TMAOH. Therefore, the varied additions of TMAOHWhen amount a different into theratio synthesis of TMAOH gel was can prepareadded into zeolite the composition T aggregates gel of of a 1 size SiO range2: 0.055 fromAl2O 23:µ 0.23m to Na 2002O: nm. 0.008 Though K2O: thex TMAOH: addition of14 SDAH2O, is where proven x to= 0, enhance 0.05, 0.10, the 0.15 crystallization, and 0.22, it itresulted is crucial in that different alkaline crystallization cation content behavior in the system [3,21,24]. is maintained Synthesized at a lowzeolite level T, to without prevent a theTMAOH aggregation template, of the took negatively 192 h to charged develop aluminosilicate 5 µm length in subcolloidal 100 °C, while particles at 0.05 [ 22TMAOH,,30]. pureThe zeolite concept T started of the to participation develop as early of SDA as 48 in h. zeolite Up to 0.15, T crystallization 1–2 µm, rod-like is due crystals to the of offretite/erionite-typezeolite T were obtained zeolite’s as pictured favorable in Figu condition,re 11. However, which is at typically 0.22 TMAOH, developed the rod-like in the presenceshape crystals of tetramethylammonium deformed into smallion irregular or NCOTs, particles, nitrogen-containing which confirmed organic that zeolite templates, T with namelynano-sized benzyltrimethylammonium crystals could be prepared (BTMA) in 0.22 cations,TMAOH. and Therefore, 1,4-diazabicyclo-2,2,2-octane the varied additions of (DABCO)TMAOH cationsamount [ 7into]. Even the synthesis though the gel synthesis can prepare of offretite/erionitezeolite T aggregates tends of toa size develop range infrom the presence2 µm to 200 of nm. proper Though NCOT, the NCOTsaddition are of SDA costly is andproven consume to enhance high the temperatures crystallization, in templates-removal,it is crucial that alkaline which cation generally content affect in the the system structure is maintained of synthesized at a low zeolite level to crystals. prevent Hence, NCOTs are avoided in the crystallization of zeolite crystals [7]. the aggregation of the negatively charged aluminosilicate subcolloidal particles [22,30].

Figure 11. SEM images of zeolite T at 373 K (100 °C) at different amounts of TMAOH: (a) 0.05, (b) Figure 11. SEM images of zeolite T at 373 K (100 ◦C) at different amounts of TMAOH: (a) 0.05, (b) 0.10, (c) 0.15 and (d) 0.22 [3] (Reproduced with permission from Yin et al., Hydrothermal synthesis 0.10, (c) 0.15 and (d) 0.22 [3] (Reproduced with permission from Yin et al., Hydrothermal synthesis of hierarchical zeolite T aggregates using tetramethylammonium hydroxide as single template; ofpublished hierarchical by Elsevier, zeolite T 2015 aggregates). using tetramethylammonium hydroxide as single template; published by Elsevier, 2015). The concept of the participation of SDA in zeolite T crystallization is due to the offretite/erionite-type zeolite’s favorable condition, which is typically developed in the presence of tetramethylammonium ion or NCOTs, nitrogen-containing organic templates, namely benzyltrimethylammonium (BTMA) cations, and 1,4-diazabicyclo-2,2,2-octane (DABCO) cations [7]. Even though the synthesis of offretite/erionite tends to develop in

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Therefore, for the past decade, TMAOH has been the only favored SDA parame- ter in developing offretite/erionite zeolites—zeolite T. TMAOH is actively involved in the templating of the crystallization of zeolite T, while potential alternatives to SDAs remain unfound.

3. Conclusions In synthesizing the zeolite T, several aspects are taken into account to achieve the high purity of zeolite crystals. By using the conventional method of hydrothermal treatment, it is proven to produce zeolite T, with respect to SiO2/Al2O ratios, temperature, time, alkalinity, and pH, with or without SDA. However, if the synthesis is aided with assistance methods, such as microwave irradiation or sonication, the time taken to produce a similar quality of zeolite T is effectively less. Modiﬁcation of zeolite T into a membrane has been possible via secondary growth and has improved the potential for further performances towards gas selectivity and permeability. Meanwhile, under certain conditions, zeolite T can develop and coexist with other zeolite phases (treated as impurities), as this phenomenon does affect the performance of zeolite T. Finally, we hope that this review will contribute to the emergence of a new exploitation of zeolite T, both in synthesis and applications, which will provide extensive information about the interrelation on the evolution phase of zeolite T, L, and W, and more.

Author Contributions: All authors have read and agree to the published version of the manuscript. Funding: The APC was funded by Universiti Malaysia Sabah. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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