Important synthesis parameters that affecting crystallization of zeolite T: A review

Synthesis of zeolite T with a variety of desired characteristics necessitate extensive work in the formulation and practical experiments either by conventional hydrothermal methods or aided with different approaches and synthesis techniques such as secondary growth or microwave irradiation. The objectives of this review are to adduce the potential work in zeolite T (Offretite-Erionite) synthesis evaluating determining factors affecting synthesis and quality of the zeolite T crystals. Attention given to the extensive studies that interconnect with other significant findings.


Introduction
Zeolites are well known porous crystals systems existed in various size (nano-, micro-, etc) that had been widely used in different field of applications. It consists a uniform size of pores distribution [1] which also known as voids. Voids and cavities within the zeolite system that gives the unique properties to the zeolite. And this voids and cavities are controlled by the types of crystals that grow and intergrowth within the zeolite membrane. Introduced for the very first time by Bennet and Grad [2], nanoporous of zeolite T (rod-like shape as shown in figure 1) are identify to have an intergrowth frameworks lies within it which makes zeolite type T a very special species in zeolites family. The intergrowth systems are made up from chabazite group of hexagonal crystals of offretite (OFF) and erionite (ERI) (Figure 2). Interframing of these two entities had cause alteration in crystals arrangement of zeolite T which are AABAACAABAAC.. instead of AABAA-BAAB.. [3]. Despite of stacking within the zeolite system, these OFF-ERI had given characters to zeolite T in terms of thermal stability, mechanical and chemical stability [1] acid resistance, highly ion selectivity [4] which had made zeolite T as promising candidate in catalysis, molecular sieves, ion exchanger and other applications [5].

Figure 1
Rod-like shape of zeolite T synthesized by [14]. Unlike others zeolite members, zeolite T possesses different voids size due to the intergrowth group of OFF and ERI. Offretite has 12-rings of channels that parallel to the c-axis and additional 8-rings of sub channels that normal to c-axis. The pore size for the 12-rings of channels are 0.67 nm x 0.68 nm while 0.36 nm x 0.49 nm are pore size for 8-ring sub channels. Alternately, erionite has only 8-ring channels which also parallel to c-axis with pore diameter of 0.36 nm x 0.51 nm [1]. Generally, the framework of OFF and ERI may be differ but they were closely related (Bennet & Grad, 1967 [3,7]) stated that ionic behavior of pure OFF and ERI are similar to the synthetic intergrowth OFF-ERI stacked in zeolite T and thus, it can be considered that intergrowth phases of zeolite T are homogenous mixtures of two pure components phases of OFF and ERI [3].
pressure in presence of water that acts as solvent, transferring pressure, mediator in reaction, changing physical and chemical properties of reagent or products and potential to be an accelerator in chemical reaction [8]. Hydrothermal synthesis technique usually conducted on mixture in sealed vessel where heat treatment are applied. This procedure were also labelled as an in-situ reaction which are strongly depending on temperature and time. Temperature needed to initiate the crystallization and time taken to complete the reaction are the most crucial parameters in conduction zeolization of T-type zeolite under hydrothermal technique [1]. Extensive reseaarch has been conducted in various ways to investigate various parameters upon hydrothermal treatment and most findings are considered successful as nano-sized crystals of zeolite T are manages to be synthesized. Hence, this method is easily to control during the synthesis phase and recently, thru hydrothermal, high purity and crystallinity of zeolite T with particles size of 150-200 nm were successfully synthesized by [9]. Yin et al (2013) proclaimed about 300 nm of zeolite T successfully synthesized based on additional parameters such as structure directing agent (SDA) and precipitate silica [10], while study by Sazmal et al., 2018 had successfully obtained nanosized zeolite T crystals via SDA additions [11][12].

Microwave-assisted method
Synthesis of zeolite T using microwave-assisted is one of an alternative ways that can be used to be in-cooperate with hydrothermal technique. It has been widely applied in zeolite synthesis for a decade due to its capability in producing high purity product with less time consuming [13]. Yin et al (2015) able to developed zeolite T membrane at only 9 h crystallization time [14]. The modification of conventional method is to investigate the relation between synthesis product and time taken thru comparison of synthesis temperatures. Concept of microwave is based on frequency power density where hydrothermal treatment of the composite gel mixture in sealed vessels used when carried out prior to microwave irradiation.
Theoretical microwave irradiation mentioned by Park et al [15][16] stated that normal hydrogen (H) bonding of water were annihilated due to microwave irradiation causing ion oscillation and dipole rotation, and this produces isolated so-called active water molecules which responsible to the high rate of dissolving of the composition gel. Meanwhile, Carmona et al [15][16] proposed that coupling behavior of microwave with hydration of shell water molecules is efficient and yet favoring the coordination change of alumina octahedral layer to tetrahedral layer which is more reactive and essential in growing of zeolite materials. as impurities when microwave technique used for zeolization of T. Hence, microwaveassisted method surely enhance the crystallization and maximized the formation of crystalized zeolite T with economical time consumed [15][16].

Secondary growth / Dip-coating
Extensive work has been carried in order to improve the synthesis method, performance of synthesis method and the performance of desired product [1,4,10,[17][18][19][20]. Recent technique that had been widely applied are dip-coating as a secondary method. Secondary method is a combination of primary method in which synthesizing zeolite T following the optimum formulation and yet repeated twice by using supporter membrane (organic or inorganic) or allowing more zeolite crystals to growth within the first system in two or more different formulation of composition gel. Secondary growth always related with seeding zeolite crystals seed on the supports in the direct hydrothermal in situ-crys- and vacuum-coating [23].

Figure 4
Schematic diagram of dip-coating method in preparing zeolite T/carbon composite [10].
Despite the frequent usage of porous tube in secondary growth, zeolite T also can be prepared on α-alumina disc compact as reported by Yeo et al, (2014). Hybrid membrane of zeolite T with ZIF-8 dispersed uniformly at α-alumina support involving hydrothermal with ultrasonic bath treatment [22]. The purpose of ultrasonic bath is to remove the loose particles from polished disc support. Milky viscous mixture of zeolite T was prepared in composition gel ratio of  In fact, the alteration in zeolite T synthesis using secondary growth had boost and enhance the properties and performance of free-standing of zeolite T. CO2/CH4 selectivity increase from 68 to 182 [22], good stability for dehydration of a propanol solution [23] and good hydrothermal stability [6].

Factors affecting crystallization of zeolite T
Zeolite T can be prepare by using various ways either using independent chemicals,  [25]. This review summarized the factors that affecting zeolite T's growth.

Effects of SiO2/Al2O3 ratio
Silicate and aluminate sources are the crucial components in zeolite mixtures system.
It is proven that value ratio of SiO2/Al2O3 will affect the crystallization behavior of zeolite ratio with presence of silica sources in 0.71 alkalinity [2]. Different literatures has different optimum condition regarding the SiO2/Al2O3 ratio due to the presence of others reagents such as silica sources, structure directing agents and also types of alumino-silicate that were used.
Usually, the effects of SiO2/Al2O3 ratio were determined on the crystallization behavior that has been applied by [1][2], while some other literatures used properties behavior of zeolite T to determine the effectiveness of SiO2/Al2O3 ratio. [26] figured that SiO2/Al2O3 ratio strongly influenced the behavior of synthesized zeolite T in terms of its performance Hence, ratio of SiO2/Al2O3 not only affect the crystallization behavior but also the application performances of the synthesized zeolite T.

Effects of alkalinity
Zeolite T are typically and mostly synthesized under activation of basic medium which usually made up from mineralizer components of Na2O and K2O. The alkalinity will strongly influence the level of crystallinity and stability of the zeolites structures. It is also effecting the performance of zeolite T in its applications. Generally, when the medium in high alkalinity at about 0.71 [2], it will accelerate the nucleation of zeolite T crystals and shorten the crystallization time because, at high pH, Na2O and K2O tend loose and dissolved by releasing their OH-, and this will accelerate the reaction between aluminate and silicate ions while the degree of silicate anions polymerization decrease [17,27].
However, at higher alkalinity beyond 0.71 [2], synthesized crystals of T-zeolite tends to re-dissolve again and also create low density of product which will affect the performance properties of zeolite T [17]. This is due to the characteristic of aluminosilicate layers tend to aggregates and forming another phase because of high alkalinity (≥0.82) surroundings [28]. The theory are proven when literature reported by [2] that relative alkalinity had strong impact in crystallization of zeolite T. Resulting from their analyzed samples, relative crystallinity of zeolite T are reduced as the relative alkalinity increase because under high alkalinity, the condition has given favorable gel for the formation of competitive phase of zeolite L. Therefore, synthesis of T-type zeolite must in low optimum alkalinity (0.71) conditions though the theory of higher alkalinity will enhance the solubility of Si and Al and speed up crystallization of zeolites. This is because at higher alkalinity, crystallization of zeolite increase but only at time range of 24 h to 168 h before the desired product re-dissolved and undergo evolution phase to forms impurities phases. In table 1 listed the formulation used in study conducted by [2] while the evolution phase of zeolite T crystals along with the morphologies for each sample shown in figure 7 (a) and (b). Table 1 Formulations of zeolite T samples conducted by [2].

Effects of pH and water ratio
Water component added into the zeolite T system are calculated and specifically present in an adequate amount as the alkalinity of the medium are depending on the amount of water added. Zeolite formation greatly affected by the pH values range (0.71-0.82) which correspond to the values of water precursors of the solutions [14]. Thus, pH of the alkaline medium can be vary by altering the amount of water in mole of 8 -16 [14] fused in. When amount of water inside the system is decrease, it initiate the mineralizer from Na2O and K2O to release OH-and this will cause the pH increase and also the alkalinity [14]. As the synthetic medium is high alkalinity, the size of zeolite T particles are decrease because the size aggregation of zeolite T can be obtained in smaller size as water component is decrease [29]. Moreover, water content inside the mixture will decrease the degree of OH-concentration and this will lead to decreasing of monotonic of zeolite growth and crystallinity [30].
Unfortunately, crystallization of zeolite T will stunt if the relative amount of water low at less than 14 H2O as the chemical reaction of the synthetic mixture's reagent are incomplete. This is due to the function of water in the system as ion transporter which also known as cavities. However, the addition of excessive amount of water that as needed in the system will diminished the zeolite T crystals into an amorphous system Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 29 January 2021 doi:10.20944/preprints202101.0617.v1 [14]. Kuperman et al (1993) reported that water is an important factor in synthesis of zeolite T as it required small amount of water and utterly controlled [7]. Thus, excessive amount of water inside the synthetic solution will cause poorest selectivity of zeolite T crystals intergrowth layer. Meanwhile, in figure 8 shows the XRD analysis and for samples synthesized at water mole 25 with no zeolite crystals form and indicates the amorphous phase evolved [14]. Therefore, only an adequate mole amount of water, 14 -16 H2O, are needed in performing the optimum crystallization of zeolite T as the water control the pH ranges (Veerapur et al, 2008 [2,14]).

Effects of synthesis temperature and crystallization time
Generally, temperature and time have high crucial impact upon T-type zeolites during crystallization. Though synthesizing zeolite T aided with different nucleation agents, the crystals phase are capable to develop defects of other zeolite phases and species if the composition gel exposed in insufficient or excessive temperature or omitted inappropriate crystallization time. Meanwhile, [1] studied the effect of crystallization temperature time in synthesizing zeolite T and has parsing the Ostwald's theory that under same synthesizing condition, zeolite T can coexist with other species which literally developed either depending on time and temperature variations. Table 2 has the summary of the findings of their study. where T and W indicates peaks of zeolite T and W respectively [9]. it will provide the necessary alkalinity and facilitates the formation anionic silicates, thus, it expedited the rate of dissolution of silicate and aluminate [9,14,28]. Additionally [9] reported that by providing abundant amount of SDA will create high supersaturation condition and steric stabilization of proto-nuclei, which are the critical factors in formation of zeolite nanoparticles without aggregation [28]. Determination of the exact amount of SDA needed in synthesizing zeolite especially ERI-type zeolite is crucial because erionite (ERI) intergrowth crystals will not be form effectively if the system contains large amount of cation [9,[11][12] In order to determine the effect of SDA upon crystallization behavior of T-type zeolite, different amount of TMAOH has been added into the mixture system of addition of SDA is proven to enhance the crystallization but it is crucial that alkaline cation content in the system at low level in order to prevent the aggregation of the negatively charged of aluminosilicate subcolloidal particles [28].  [14].
The concept of participation of SDA in zeolite T crystallization are due to offretite/erionite type zeolite favor's condition, which are normally developed in presence of tetramethylammonium ion or NCOTs, nitrogen-containing organic templates namely benzyltrimethylammonium (BTMA) cations, 1,4-diazabicyclo-2,2,2-octane (DABCO) cations (Sand, 1978; [7]. Despite the synthesis of offretite/erionite tend to developed under proper NCOT present, NCOTs are specifically high cost and consumed high temperature to remove the templates which normally will affect the structure of synthesized zeolite crystals. Hence, NCOTs had been avoided in crystallization of zeolite crystals [7].
Therefore, for the past decade, TMAOH is the only favor SDA parameters in developing offretite/erionite zeolites -zeolite T. TMAOH had been actively involved in templating the crystallization of zeolite T while potential of other available SDAs remain unfold.

Conclusions
The synthesis parameters revealing the strongest factors influenced the zeolite T formation. Demands in synthesizing of zeolite T rose effectively with some precaution consideration toward the parameters and formulations. As T-type zeolite can be synthesized in various Si/Al sources such as using amorphous metakaolin or mulite from kaolinite or using independent chemicals or aided with nano or micro sized tube as a supporter. Via hydrothermal synthesis, zeolite T can be obtained at temperature and time as low as 120 °C in 168 h without any alteration in silica sources, meanwhile the time factor needed that can be altered by providing more source of nucleation (silica sources). However, with aided of microwave irradiation, crystallization only consumed time at lower than hydrothermal conventional method. It is noted that, zeolite T can only be formed at optimum formulation depending on the technique and alkalinity. Zeolite T does not favor high alkaline medium but developed excellently in presence of SDA. Meanwhile, under certain condition, zeolite T can developed and coexist together with other zeolite phases as impurities and this system do affect the performance of zeolite T. Therefore, some consideration must be taken extensively prior to synthesis of zeolite T. .References