Zeolites are considered as promising adsorbents, ion-exchangers, and catalysts, and have been extensively applied in industrial processes [1-15]. Zeolite omega, an analogue to the natural mineral mazzite (MAZ), consists of gmelinite cages that are linked in columns parallel to the c-axis to form main pores with 12-membered rings [16]. Recently, increased attention has been paid on zeolite omega owing to its application in the alkylation of aromatics [17, 18], hydrocracking [19], oil cracking [20], and in the isomerization of paraffin and aromatics [19, 21, 22]. Coq et al. [19] reported that omega zeolite exhibits high selectivity for hydrocracking, while smaller omega particles permit the development of a nearly pure hydroisomerization catalyst. In turn, Perrotta et al. [20] showed that omega zeolites have increased α values in the cracking of n-hexane. Solinas et al. [22] reported that omega zeolite is highly active and selective in the isomerization of 1-methylnaphthalene. To-this-date, there are many reported examples of successful syntheses of zeolite omega. However, zeolite is associated with a relatively long crystallization time, and presents a major challenge for industrial applications owing to its costly synthesis. Coossens et al. [23] reported a zeolite omega synthesis following a 3-day aging process at room temperature, and a 10-d crystallization process at 100 ℃, in the presence of an organic template. Yang et al. [24] demonstrated zeolite omega synthesis in a Na₂O-SiO₂-Al₂O₃-H₂O-glycerol system at 125 ℃ for 45 d with extra silica as the silica source. Davis et al. [25] fabricated zeolite omega using dioxane as an organic template at 140 ℃ for 7 d, and Fajula et al. [26] obtained zeolite omega in a Na₂O-SiO₂-Al₂O₃-TMA₂O-H₂O system at 100 ℃ for 15 d. Additionally, Xu et al. [27] prepared zeolite omega using piperazine as an organic template at 150 ℃ in a 4-d period, and Cui et al. [28] synthesized zeolite omega from layered natural magadiite at 120 ℃ in a 10-day period.

Conversely, it is well known from the Arrhenius equation that the crystallization time of zeolites could be significantly reduced at increased crystallization temperatures [29, 30].

In this study, we systematically investigated the crystallization of zeolite omega at relatively low temperatures, and obtained the relationship between the crystallization time and crystallization temperature according to the Arrhenius equation. Subsequently, based on this relationship, we designed the fast crystallization of zeolite omega at higher temperatures. As expected, it was realized at 180 ℃ over a 5-h period. The zeolite omega product formed had good crystallinity and uniform nanocrystals. Most importantly, this zeolite supported Pt catalyst (0.5 wt% Pt/H-MAZ-180) and exhibited high selectivity for the isomers in the hydroisomerization of n-dodecane.

Sodium aluminate (NaAlO₂, 36.6% Na₂O and 43.3% Al₂O₃, Sinopharm Chemical Reagent Co., Ltd.), sodium hydroxide (NaOH, AR, 96%, Sinopharm Chemical Reagent Co., Ltd.), silica sol (LUDOX HS-40, 40% SiO₂ in H₂O, Sigma-Aldrich), tetramethylammonium hydroxide (TMAOH, 25% in water, Aladdin), and ammonium nitrate (NH₄NO₃, AR, 99%, Beijing Chemical Reagent Co., Ltd.), were employed without further purification.

In a typical run, 0.944 g of sodium aluminate was dissolved in 6.4 g of distilled water during stirring, followed by the addition of 0.532 g of sodium hydroxide. After stirring for 30 min, 6.0 g of silica sol and 0.64 g of tetramethylammonium hydroxide were added gradually. The reaction mixture was stirred for 2 h, and formed a gel with a molar ratio of 2.4 Na₂O/10 SiO₂/Al₂O₃/0.24 TMA₂O/1100 H₂O, followed by the crystallization of the compound in an autoclave at temperatures in the range of 100 to 180 ℃. After filtration, washing, drying, and calcination remove the organic template, the zeolite omega product was collected. The H-form of the sample was obtained following the ion-exchange with a solution of 1 mol/L NH₄HNO₃ at room temperature for 2 h, followed by its calcination at 500 ℃ for 4 h. The process was repeated three times, and the H-form of the product was finally obtained, which was designated as H-MAZ-x (x stands for the crystallization temperature).

To quantify the crystallinity of the samples, the zeolite omega synthesized at 180 ℃ for 5 h was referred to as a 100% crystallinity sample.

X-ray diffraction (XRD) patterns were measured with a Rigaku Ultimate Ⅵ X-ray diffractometer (40 kV, 40 mA) using Cu Kα₁ radiation (λ = 1.5406 Å). Solid-state NMR spectra were measured with a Varian Infinity Plus 400 spectrometer. Scanning electron microscopy (SEM) images were acquired with a Hitachi SU-1510 electron microscope. Transmission electron micrograph (TEM) experiments were performed with a Hitachi HT-7700 transmission electron microscope. Additionally, N₂ sorption experiments at the temperature of liquid nitrogen were measured on a Micromeritics TriStar Ⅱ apparatus. Thermogravimetry-differential thermal analysis (TG-DTA) curves were acquired on a Perkin-Elmer TGA 7 unit at a heating rate of 10 ℃/min in air. The sample composition was analyzed by Perkin-Elmer inductively coupled plasma (ICP). The acidity of the catalysts was measured according to the temperature-programmed-desorption of ammonia (NH₃-TPD). The catalyst was prepared at 550 ℃ in a He flow for 1 h, followed by the adsorption of NH₃ at 100 ℃ for 1 h. After saturation, the catalyst was purged by a He flow for 3 h to remove the physically adsorbed ammonia on the sample. Subsequently, desorption of NH₃ was carried out from 100 to 600 ℃ with a heating rate of 2.5 ℃/min. The amount of NH₃ desorbed from the sample was detected using a thermal conductivity detector.

H-MAZ-100 and H-MAZ-180 supported Pt catalysts which were prepared using the wet impregnation method using an appropriate amount of Pt(NH₃)₄Cl₂. The samples were crushed to mesh sizes in the range of 20–40 by a hand-operated press after they were dried at 100 ℃ overnight, and after their calcination at 450 ℃ for 3 h. These were named as Pt/H-MAZ-100 and Pt/H-MAZ-180, respectively.

Hydroisomerization of n-C₁₂ was carried out in a fixed-bed reactor. First, catalysts were reduced in flowing hydrogen at 400 ℃ for 2 h. Subsequently, n-C₁₂ was pumped into the reactor [31]. The reaction conditions were as follows: the H₂/n-dodecane (V/V) ratio was equal to 400, the pressure was 1.0 MPa, the weight hourly space velocity (WHSV) was set to 1.5 h^–1, and the temperature was in the range of 300 to 380 ℃. The products were analyzed online using gas chromatography.

Figs. S1 and S2 show XRD patterns and SEM images of fully crystallized omega crystals from 100 to 150 ℃ in the aluminosilicate gels in the presence of organic TMAOH, which are well consistent with those of zeolite omega reported previously [32, 33]. Fig. 1 shows the crystallization curves of the zeolite omega in the temperature range from 100 to 150 ℃. Interestingly, the crystallization time was dependent on the crystallization temperature in the synthesis of zeolite omega. For example, the full crystallization of zeolite omega at 100 ℃ required 240 h, while the full crystallization at 150 ℃ required 17 h.

Because the zeolite crystallization is a kinetic process, the growth of omega crystals could be expressed by the Arrhenius equation:

(1)

where R₁ denotes the crystallization rate, E_a is the activation energy of crystallization, and T is the crystallization temperature. Because the reciprocal of the crystallization time linearly increases with the crystallization rate, Eq. (1) is simplified into Eq. (2) as follows:

(2)

where the t is the crystallization time. If the values of ln(1/t) are plotted as a function of 1/T in the temperature range from 100 to 150 ℃, a linear relationship is obtained, as shown in Fig. 2. From the slope of this relationship, the activation energy for the omega crystallization can be estimated to equal to 70.0 kJ/mol. Therefore, we can predict any crystallization time for any temperature in the case of the synthesis of zeolite omega based on Eq. (2). Interestingly, if the crystallization temperature is selected to be 160 ℃, the crystallization time would be reduced to 8.5 h. When the crystallization temperature is increased to 180 ℃, the crystallization time should be shortened to 3.6 h. Even though we can theoretically predict the shortening of the crystallization time for zeolite omega using Eq. (2), this deviates for experimental syntheses of zeolite omega at temperatures higher than 150 ℃. This may be related to a relatively difficult crystallization process at such high temperatures.

Table S1 presents the outcomes of a systematic investigation on the crystallization of zeolite omega at 180 ℃ from aluminosilicate gels in the presence of organic TMAOH. Interestingly, when the value of ratio of Na₂O/SiO₂ in the gels is adjusted from 0.205 to 0.437, products with amorphous phases are obtained, including a mixture of an amorphous phase with the omega (MAZ) structure, pure MAZ, a mixture of MAZ with the analcime (ANA) structure, and a pure ANA structure (Fig. S3). The Na₂O/SiO₂ ratio for the crystallization of a pure MAZ structure was optimized and ranged from 0.305 to 0.338. In addition, the TMAOH/SiO₂ ratio also influences the crystallization of zeolite omega. The TMAOH/SiO₂ ratio is below 0.038, and the product is normally a mixture of an amorphous phase with MAZ. When this ratio reaches 0.046, a pure phase of the MAZ structure is elicited (Fig. S4). Furthermore, it is found that the SiO₂/Al₂O₃ ratio is associated with the crystallization of zeolite omega. When the SiO₂/Al₂O₃ ratio is lower than 10, a mixture of MAZ with ANA is always elicited. When this ratio is distributed in the range of 10–20, it usually forms a pure phase in reference to the MAZ structure (Figs. S5 and S6). After the systematic synthesis, an appropriate form of aluminosilicate gel is obtained for the crystallization of zeolite omega at 180 ℃ with a molar ratio of 0.305 Na₂O/SiO₂/0.046 TMAOH/0.1 Al₂O₃ /110 H₂O.

Fig. 3 shows the crystallization curves of omega zeolites from aluminosilicate gels with the optimized molar ratios at 160 and 180 ℃. It is worth noting that the crystallization times at 160 and 180 ℃ is ca. 9 and 5 h, respectively, which are consistent with those elicited based on theoretical data shown in Fig. 2.

Fig. 4 displays the XRD pattern, SEM image, TEM image, nitrogen sorption isotherms, ²⁷Al NMR spectra, and the TG-DTA curve of the zeolite omega synthesized at 180 ℃ (MAZ-180) for 5 h. The XRD pattern of the as-synthesized MAZ-180 contains a series of characteristic peaks (Fig. 4(A)) associated with those reported previously [32, 33], thus confirming the MAZ structure. After calcination, these peaks remain, thus indicating their good thermal stability (Fig. 4(A)). Fig. 4(B) shows a SEM image of the as-synthesized MAZ-180 with the assembled nanorods. These nanorods are uniform with widths of ca. 200 nm and lengths in the range of ca. 2–3 μm.

By contrast, conventional synthesis of zeolite omega at 100 ℃ (MAZ-100) yields larger crystals (widths of ca. 400 nm and lengths of ca. 5–6 μm, Fig. S2). This phenomenon may be associated with the fact that higher temperatures have faster nucleation rates in the synthesis of zeolite omega. Notably, after ultrasonic treatment for 1 h, the sample morphology is still retained (Fig. S7), thus suggesting that the assembled nanorods are stable.

Fig. 4(C) shows a TEM image of the as-synthesized MAZ-180, exhibiting clear micropores in the zeolite. Obviously, the corresponding SAED pattern of the MAZ-180 shows the electron pattern associated with the single crystal of the MAZ nanorods (insert in Fig. 4(C)). Fig. 4(C) shows the solid-state ²⁷Al NMR spectrum of the as-synthesized and calcined MAZ-180 samples comprising two obvious signals at ~60 and ~55 ppm, which can be assigned to the two T-sites in the MAZ framework [34]. In addition, we cannot observe a signal at 0 ppm associated with the extra framework of the Al species in the zeolite framework [35]. Figs. 4(E) and S8 show the N₂ sorption isotherms of calcined MAZ-180 and calcined MAZ-100 indicating steep increases in the adsorbed volumes at a relative pressure less than 0.01, which is owing to the filling of micropores. In addition, the hysteresis loop appeared to be at the relative pressure range of 0.5–0.95. This is indicative of mesoporosity [36] that may be attributed to the spaces among the nanorods. The BET surface area and micropore volume of MAZ-180 are 187 m²/g and 0.05 cm³/g. These values are similar to those of zeolite omega in the literature [28], as listed in Table S2. Fig. 4(F) displays the TG-DTA curves of the as-synthesized MAZ-180 in the temperature range from 25 to 900 ℃. The weight loss associated with the adsorbed water appears to be below 250 ℃, and the weight loss assigned to the combustion of organic templates shows that the temperature ranged from 300 to 650 ℃. After 700 ℃, no obvious weight loss was observed. The DTA curve exhibits an endothermic peak within the temperature range of 100–250 ℃, while the main exothermic peaks occur at 400–600 ℃. These results suggest that the weight loss in the temperature ranges of 100–250 and 400–600 ℃ correspond to adsorbed water and to the organic template, respectively, and are accompanied by weight losses which are approximately equal to 11.76% and 5.65%, respectively. Furthermore, the Si/Al ratio measured by the ICP technique is equal to 2.5 for MAZ-180. A brief summary of these characterizations confirms that MAZ-180 has a good crystallinity and contains uniform nanocrystals. The very short crystallization time could be important for the low-cost production of zeolite omega in industrial processes.

The acidity values of the H-MAZ-100 and H-MAZ-180 were measured with the NH₃-TPD technique. The results showed that both samples had similar acidity values and strengths—a finding attributed to their similar Si/Al ratios in the zeolite framework.

Given that long-chain paraffin hydroisomerization is one of the industrially important reactions, it has received increased attention owing to the requirement of fuel upgrading [37-42]. Normally, one-dimensional zeolites with 10-membered and 12-membered pore openings (e.g., SAPO-11, ZSM-48, and ZSM-22) are employed in the preparation of the hydroisomerization catalyst [43-49]. The fast crystallization of zeolite omega with one-dimensional, 12-membered pore openings in this work may offer a new opportunity for the preparation of highly efficient hydroisomerization catalysts. After loading similar Pt nanoparticles (0.5 wt%), the Pt/H-MAZ-100 and Pt/H-MAZ-180 catalysts have similar Pt size distributions (Fig. S9) but they exhibit a significant difference in their catalytic performances in the hydroisomerization of n-dodecane. At any temperature, Pt/H-MAZ-180 always has a higher isomeric selectivity and a lower cracking selectivity than Pt/H-MAZ-100 catalysts, which may be related to their differences in nanocrystal sizes. Smaller and shorter one-dimensional zeolite crystals are favorable for the formation of isomers in the n-dodecane hydroisomerization [50-52].

In summary, we have designed a fast crystallization process for nanosized zeolite omega crystals based on the relationship between crystallization time and temperature in accordance with the Arrhenius equation. This design led to a crystallization time of 5 h at 180 ℃ (MAZ-180) which was much shorter than those reported previously. More importantly, the product of zeolite omega had good crystallinity and uniform nanocrystals. After the loading of Pt nanoparticles, Pt/H-MAZ-180 exhibited good isomer selectivity in the hydroisomerization of n-dodecane. The combination of fast crystallization and good catalytic performance presents good opportunities for practical applications of zeolite omega in the future.