Zeolites have been extensively used as catalysts, adsorbents and ion-exchangers owing to their well-defined microporous structures, adjustable acidities and a large variety of available frameworks and chemical compositions [1-5]. Among the approximate reported 230 zeolite framework types, the vast majority of zeolites are synthesized in the presence of organic structure-directing agents (OSDAs), and more novel topologies are synthesized by the application of new OSDAs [6-8]. On the other hand, most common organic templates are quaternary ammonium or amines, which are usually unavailable or toxic or environmentally hazardous. Therefore, it is desirable to develop eco-friendly synthesis routes with the use of non-toxic and green OSDAs [9-11]. For example, polyquaternium-6, a typical component of shampoo, was successfully used as a template to synthesize EMT-rich faujasite, which offers the possibility of industrial applications of EMT zeolites [12]. However, it has been demonstrated that among a huge amount of organics, only a small minority of organics have the ability as structure-directing agents for zeolite synthesis. It is more difficult to find a non-toxic and eco-friendly OSDA with "fitting" size and shape for the targeted zeolite topology.

Herein we focus on the synthesis of Y zeolite with a 12-membered window size of 7.4 Åand a supercage cavity size of 12 Å, which has been vastly used as the main active component of fluidized catalytic cracking (FCC) catalysts so far [13-18]. It has been demonstrated that increasing the framework SiO₂/Al₂O₃ ratio of Y zeolite is helpful to enhance its acid strength, (hydro)thermal stability and catalytic activity [19, 20]. Whereas there has been a restricted upper limit for the SiO₂/Al₂O₃ ratio of Y zeolite prepared by traditional OSDA-free synthesis routes [21]. Nowadays, the most widely employed zeolite Y in industry is ultra-stable zeolite Y (or USY, SiO₂/Al₂O₃ ratio > 5) prepared by post-treatments to remove framework aluminium [22]. The post-synthesis treatments include hydrothermal dealumination and treatment with chemicals, which cause environmental pollution and high energy consumption, and compromise the structural integrity of zeolite, owing to reduplicative calcination and chemical treatments [23-26] Compared with these post-treatments, one-step direct synthesis methods in the presence of OSDAs are beneficial for the synthesis of high silica Y zeolite with higher crystallinity and avoid lots of complicated physicochemical treatments.

Delprato et al. [27] firstly developed a one-step direct synthesis method for zeolite Y with SiO₂/Al₂O₃ ratio of around 9 by employing crown ether (15-crown-5) as an OSDA in 1990. Inspired by crown ether, polyethylene oxides and tris(3, 6-dioxaheptyl)amine, which are acyclic molecules containing –OCH₂CH₂– groups similar to crown ether, were successfully used to synthesize zeolite Y (SiO₂/Al₂O₃ ratio = 6–7) [28]. In 2015, Xiao et al. [29] studied quaternary ammonium salts and synthesized high silica zeolite Y in the presence of N-methylpyridinium iodide. Our group used ethyl(or butyl)-3-methylimidazolium bromide and tetraethylammonium hydroxide as OSDAs for the synthesis of high silica zeolite Y [30, 31]. Nevertheless, the above OSDAs, especially crown ether, are expensive, toxic and environmentally unfriendly, which are unavailable for wide use in petrochemical processes. The development of an eco-friendly and "fitting" OSDA is difficult yet imperative to fill the gap in the synthesis of Y zeolite with the use of OSDAs.

Herein we focus on choline, a nontoxic, green and low-cost OSDA. Choline is a water-soluble vitamin-like essential nutrient that refers to quaternary ammonium salt containing N, N, N-trimethylethanolammonium cation. Choline was first isolated by Adolph Strecker from animal bile [32]. Nowadays, choline is industrially produced at a few thousand tons per year by 100% atom economy process [33]. Choline has been employed as a structure-directing agent to synthesize zeolites such as ZSM-4, SSZ-13 and SAPO-5 [34-36]. In the present work, the synthetic methodology is rationally designed to use choline as an eco-friendly and nontoxic OSDA in the synthesis of high silica zeolite Y for the first time. The sample ChCl-Y is synthesized with choline chloride and ChOH-Y is prepared with choline hydroxide. They both show good crystallinity and high framework SiO₂/Al₂O₃ ratio. By contrast, different properties of ChCl-Y and ChOH-Y reveal to us the separate roles and relationship of Na⁺ and OSDA⁺ in the synthesis of high silica Y zeolite. Both the products ChCl-Y and ChOH-Y show more outstanding high temperature thermal and hydrothermal stability, compared with conventional zeolite Y. Given the huge amount of zeolite Y used worldwide as adsorbents and FCC catalysts, the ability to enhance the SiO₂/Al₂O₃ ratio via the use of choline as an eco-friendly and economically viable OSDA would have a significant impact on the (petro)chemical industry and would be a great progress.

Firstly, 3.41 g choline chloride ([choline]Cl, Aladdin Chemical), 2.00 g sodium metaaluminate (NaAlO₂, Sinopharm Chemical) and 1.95 g sodium hydroxide (NaOH, Sinopharm Chemical) were dissolved in 26.65 g deionized water and stirred until a clear solution. Next, 24.60 g of silica sol (SiO₂ 29.8% aq. solution, Qingdao Ocean Chemical) was added in the above clear solution with strong agitation. The final gel was prepared with molar composition of 1.0NaAlO₂ : 5.0SiO₂ : 2.0NaOH : 1.0[choline]Cl : 100H₂O. The mixture was stirred for 12 h at ambient temperature and maintained in ovens at 110 ℃ for 10 d. Calcined sample ChCl-Y was obtained after calcination at 550 ℃ for 4 h.

Firstly, 6.60 g choline hydroxide ([choline]OH, 45% aq. solution, Acros Organics), 2.00 g NaAlO₂ and 0.98 g NaOH were dissolved in 23.00 g deionized water with stirring to form a clear solution. Next, 24.60 g silica sol was mixed with the above solution with strong agitation for 12 h at ambient temperature. ChOH-Y was synthesized in stainless steel autoclaves at the crystallization temperature of 110 ℃ for 12 d, with molar composition of 1.0NaAlO₂ : 5.0SiO₂ : 1.0NaOH : 1.0[choline]OH : 100H₂O. Calcined sample ChOH-Y was obtained after calcination at 550 ℃ for 4 h.

To investigate the role of choline in the synthesis of zeolite Y, the sample Na-Y was designed as a reference sample with a composition of 1.0NaAlO₂ : 5.0SiO₂ : 2.2NaOH : 100H₂O. Na-Y sample was synthesized at 100 ℃ with a reaction time of 14 d.

The X-ray diffraction (XRD) patterns were acquired using a PANalytical X'Pert PRO X-ray diffractometer using Cu-K_α radiation with λ = 1.54059 Å, operating at 40 kV and 40 mA. The samples were scanned at 12°/min between 2θ angles of 5°–65°. Scanning electron microscopy (SEM) images were collected with a Hitachi SU8020 scanning electron microscopy. N₂ adsorption-desorption isotherms of the samples were measured at –196 ℃ on a Micromeritics ASAP 2020 system. The solid state ²⁹Si-NMR and ²⁷Al-NMR experiments were conducted on a Bruker AvanceIII spectrometer equipped with a 14.1 T wide-bore magnet. TG-DSC measurement was conducted on a TA Q-600 analyzer with a heating rate of 10 ℃ /min from ambient temperature to 1100 ℃ in an air flow of 100 mL/min. The organic elemental composition of samples was measured by elemental analysis on an Elementar vario EL cube elemental analyzer. The chemical composition of the samples was calculated with a Philips Magix-601 X-ray fluorescence (XRF) spectrometer. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was conducted with a PerkinElmer 7300DV.

The XRD patterns of the as-synthesized products ChCl-Y and ChOH-Y are respectively shown in Fig. 1A and Fig. 1B. The X-ray diffraction peaks with a high intensity are in agreement with the typical topological features of theoretical FAU framework. It indicates that Y zeolites with good crystallinity are synthesized by employing [choline]Cl or [choline]OH as an OSDA. The crystal morphologies of ChCl-Y and ChOH-Y are entirely different in SEM micrographs (Figs. 1C, 1D and S1). ChOH-Y exhibits uniform cubic octahedral crystals, and ChCl-Y shows spherical agglomerates with superficial triangle cone. It is reported that the crystal morphologies are affected by some experimental factors, for examples, the species of structure-directing agents and the special silica source or aluminium source [37-39]. The relationship between the dramatically different morphologies and the crystallization curves of ChOH-Y and ChCl-Y has aroused our attention. The crystallization curves of ChCl-Y and ChOH-Y samples are presented in Fig. 1E and 1F wherein the relative crystallinity changes with various reaction time. The relative crystallinity is estimated by the reflection intensities of the X-ray diffraction peaks (1 1 1), (2 2 0) and (3 3 1) of the samples. Both the crystallization curves of ChCl-Y and ChOH-Y exhibit typical S-shaped curves, which indicate that the crystallization process of Y zeolite is comprised of induction period, continuous growth period and stable crystallization period. The induction period of ChOH-Y is 6 d, which is longer than that (4 d) of ChCl-Y. The crystallization of zeolite Y from ChOH-Y is completed within 12 d, which is also longer than that (9 d) of ChCl-Y. It is concluded that ChCl-Y has a faster generation rate and faster growth rate compared with ChOH-Y.

As the synthesis process and the composition of ChCl-Y and ChOH-Y are shown above, ChCl-Y and ChOH-Y are designed with the same feed molar ratio of [choline]⁺ and OH^–. However, the sample ChCl-Y is synthesized with Na⁺/SiO₂ feed ratio of 0.6, which is higher than that of ChOH-Y (0.4) in the starting gels. Therefore it means that the change of Na⁺ feed molar ratio has a major influence on zeolite morphologies and crystallization process. Na⁺ is proposed to coordinate water molecular with subsequent displacement by silicate and aluminate species to form aluminosilicate microorganization, or further nucleation centers in the synthesis of zeolites. Therefore, higher Na⁺ molar ratio of the starting gels contributes to shortening the crystallization time and improving the nucleation rate which trends to the agglomerates of small cubic octahedral crystal.

The N₂ adsorption-desorption isotherms of calcined and as-synthesized samples are respectively shown in Fig. 2A and Fig. 2B. Calcined samples ChCl-Y and ChOH-Y exhibit a typical type-I adsorption-desorption isotherm, indicating that they are mainly consisted of very uniform micropores. The BET specific surface area, t-plot micropore area and pore volume of the samples are listed in Table S1. The BET specific surface area of the calcined samples ChCl-Y and ChOH-Y are 735.2 and 730.7 m²/g, respectively, which are slightly larger than that (694.1 m²/g) of Na-Y. The t-plot micropore area of ChCl-Y and ChOH-Y are 679.0 and 675.9 m²/g, which indicates the presence of abundant micropores in the synthetic samples ChCl-Y and ChOH-Y. The large BET specific surface area and uniform micropore property demonstrated that the use of choline choride and choline hydroxide is helpful to the formation of micropore structure.

As shown in Fig. 2A and Fig. 2B, BET specific surface area of the as-synthesized samples ChCl-Y and ChOH-Y is much smaller than that of calcined ChCl-Y and ChOH-Y accordingly. Clearly, as-synthesized ChCl-Y and ChOH-Y undergo minimal adsorption, suggesting that the microporous structure is occupied with an amount of organic template. As-synthesized ChCl-Y and ChOH-Y samples exhibit a similar ¹³C-NMR spectrum (Fig. 2C and Fig. 2D). There is a sharp peak appeared at 56.8 ppm in both ¹³C NMR spectra, which is specifically assigned to methyl groups nearby nitrogen atom in choline molecular. The peak shift at 68 ppm is attributed to methoxy groups of choline. It is demonstrated that [choline]⁺ acts as a structure-directing role in the synthesis of both ChCl-Y and ChOH-Y. The TG/DSC curves of as-synthesized ChCl-Y and ChOH-Y are shown in Fig. 2E and 2F. The TG/DSC results also prove that choline participates in the formulation of FAU framework as structure-directing agents. The endothermic weight loss at 100–300 ℃ is attributed to water desorption from samples. The water adsorption of ChCl-Y and ChOH-Y is respectively 16.93% and 16.90%. The weight loss accompanied by raised exothermic peaks at 300–650 ℃ in ChCl-Y and ChOH-Y is respectively 7.78% and 8.48%, which is attributed to the decomposition of organic templates. Carbon (C) and nitrogen (N) weight percentage of ChCl-Y and ChOH-Y is measured by elemental analysis (Table 1). The as-synthesized sample ChCl-Y contains 4.06 wt% carbon and 0.84 wt% nitrogen, which are obviously lower than 5.49 wt% carbon and 1.14 wt% nitrogen of ChOH-Y. The C/N molar ratios of both ChCl-Y and ChOH-Y are 5.6, close to that (5.0) of [choline]⁺ (chemical formulae of [choline]⁺ : C₅H₁₃OHN⁺). It is also a powerful evidence that [choline]⁺ acts as a structure-directing agent in the synthesis of high silica Y. The elemental analysis results of Na-Y are set out in Table 1, suggesting that the organic content of Na-Y is close to zero because Na-Y is synthesized without any organic template. In Table 1, the Na₂O/Al₂O₃ ratios of ChCl-Y and ChOH-Y are respectively 0.83 and 0.74, which are far smaller than that of Na-Y (0.98) by XRF data. The ICP calculation results exhibit that the Na₂O/Al₂O₃ ratios of ChCl-Y, ChOH-Y and Na-Y are respectively 0.83, 0.72 and 1.01, which are close to the XRF calculation. It is confirmed that [choline]⁺ acts as a structuring-directing agent partially replacing Na⁺ to occupy FAU supercages and compensate the negatively-charged Al framework sites. It is notable that carbon content of ChCl-Y is less than that of ChOH-Y and Na⁺ content of ChCl-Y is higher than that of ChOH-Y in the contrary.

The ²⁷Al MAS-NMR and ²⁹Si MAS-NMR spectrograms of ChCl-Y, ChOH-Y and Na-Y samples are presented in Fig. 3. They show almost the same ²⁷Al MAS-NMR spectrograms with only a sharp and symmetric peak centered at about 60 ppm, wich is indentified as tetrahedrally-coordinated framework aluminum, implying that the samples contain no octahedrally-coordinated aluminium of extra framework. The ²⁹Si MAS-NMR spectra of high silica ChCl-Y and ChOH-Y show that the Si(2Al) peak is less intense but the Si(1Al) and Si(0Al) signals are more intense, compared to the corresponding signals of low silica Na-Y, wherein Si(nAl) represents that one Si atom connects to n Al atoms and (4–n) Si atoms in the second coordination shell by a bridging oxygen. The results indicate that choline as an OSDA could decrease the number of framework aluminium atoms per unit cell and increase the framework SiO₂/Al₂O₃ ratio, which has been certified the calculation by XRF and ICP methods.

The SiO₂/Al₂O₃ ratio of the three products ChOH-Y, ChCl-Y and Na-Y presented in Table 1 are calculated by XRF, ²⁹Si-NMR and ICP. The XRF data indicate that ChCl-Y and ChOH-Y have higher SiO₂/Al₂O₃ ratios of 6.46 and 6.78, which are much higher than that (5.56) of Na-Y zeolites with the traditional synthesis method. It could be explained by the theory that relatively larger organic cations ([choline]⁺) with a lower charge density introduce a lower number of positive charges than small inorganic cations (Na⁺) within the zeolite cages, thus matching a lower number of framework Al atoms with negative charges, and increasing the framework SiO₂/Al₂O₃ ratio. ChCl-Y and ChOH-Y have SiO₂/Al₂O₃ ratios of 6.48 and 6.83 obtained by ICP analysis, which is in agreement with XRF calculation. The ²⁹Si NMR spectroscopy is also used as an accurate characterization measuring the framework SiO₂/Al₂O₃ ratio. The SiO₂/Al₂O₃ ratios of the samples are calculated with the signals of Si(nAl) species (n = 0–3) in Fig. 3. The framework SiO₂/Al₂O₃ ratios of ChCl-Y and ChOH-Y are respectively 6.34 and 6.54, whereas the SiO₂/Al₂O₃ ratio of Na-Y is only 5.4, far below the framework SiO₂/Al₂O₃ ratios of ChCl-Y and ChOH-Y. The ²⁹Si NMR results are slightly lower than those obtained by XRF and ICP, which could be explained by the presence of partial Al sites substituted by terminal OH species in the framework, decreasing the data calculated by the ²⁹Si-NMR spectrum. It is a great breakthrough that high silica zeolite Y could be synthesized with choline as a low-cost and eco-friendly organic template. Even more remarkable, ChOH-Y has a higher SiO₂/Al₂O₃ ratio than ChCl-Y.

It arouses our interest that not only ChCl-Y and ChOH-Y exhibit entirely different crystal morphologies, but also the choice of choline chloride and choline hydroxide has a great influence on the framework SiO₂/Al₂O₃ ratio. This gives us a brand new idea how to increase SiO₂/Al₂O₃ ratio of Y zeolite, and impels us to further investigate the roles and relationship of [choline]⁺ and Na⁺ in the synthesis of zeolite Y with OSDAs. As discussed in the previous part, the physicochemical differences of ChCl-Y and ChOH-Y are caused by the various feed molar ratio of Na⁺ in the starting gels. In general, alkali-metal cations (Na⁺) play two roles in the synthesis of zeolite, acting as (ⅰ) a source of alkalinity (NaOH is commonly used in Y zeolite synthesis) and (ⅱ) a limited structure-directing agent [2]. [choline]⁺ has been confirmed to be a structure-directing agent in the synthesis process of both ChCl-Y and ChOH-Y, but only choline hydroxide can be used as the source of alkalinity. Therefore choline hydroxide could provide sufficient alkalinity in the synthesis of ChOH-Y, use only half the amount of raw material NaOH during the synthesis of ChCl-Y, and sharply decrease the feedstock amount of Na⁺ (nearly one third). Based on the asNa+sumption that the differences of ChCl-Y and ChOH-Y are caused by the variation of Na⁺ feed ratio, a typical sample ChCl/ChOH:0.5/0.5-Y is designed with the Na⁺/SiO₂ feed molar ratio (0.5) between that of ChCl-Y (0.6) and ChOH-Y (0.4). The synthetic methodology is rationally designed with the same [choline]⁺ and OH^– concentration as ChCl-Y and ChOH-Y by changing ChOH/NaOH ratio. The corresponding XRD pattern shows that ChCl/ChOH:0.5/0.5-Y has FAU topological structure with good crystallization (Fig. 4A). The SEM image (Fig. 4B) shows uniform irregular polyhedron, which is different with the morphology of both ChCl-Y and ChOH-Y. Therefore, the change of Na⁺ feed molar ratio has a decisive influence on the crystal morphology. Comparing the SiO₂/Al₂O₃ ratio, Na₂O/Al₂O₃ ratio and OSDA content in Table 1, it is obviously shown a regular variation of the final products: as the Na⁺/SiO₂ feed molar fraction (0.6, 0.5 and 0.4 corresponding to ChCl-Y, ChCl/ChOH:0.5/0.5-Y and ChOH-Y) in their starting gels reduces, the SiO₂/Al₂O₃ ratio increased (6.48, 6.67 and 6.83 by ICP), the Na₂O/Al₂O₃ ratio decreased (0.72, 0.77 and 0.83 by ICP), and OSDA content raised accordingly (C %: 4.06%, 5.11% and 5.49% by elemental analysis). In the final liquid-solid mixture, a part of [choline]⁺ and Na⁺ dissociate in the solution, and the other part of them fill in the micropores. There is inevitably a competition between Na⁺ and [choline]⁺ acting as structure-directing agents and skeletal fillers in the synthesis of Y zeolite. The results demonstrate that the molar ratio of Na⁺ to [choline]⁺ is a crucial factor for their competition and further influences the framework SiO₂/Al₂O₃ ratio. The use of OSDAs in a OH^– form is a direct and efficient method to enhance the framework SiO₂/Al₂O₃ ratio, which reduces the feedstock amount of Na⁺ to weaken the competiveness of Na⁺. However, when the feed of Na⁺ is reduced below 0.3, it is difficult to form uniform zeolite Y; the result accompanies lots of amorphous aluminosilicate (samples #1 and #2 in Table S2, Fig. S2 and Fig. S3). A certain amount of Na⁺ is an essential part of the synthesis of Y zeolite.

Normally, zeolite Y is synthesized at 100 ℃ with high alkalinity as the sample Na-Y. ChCl-Y and ChOH-Y are designed with low alkalinity at 110 ℃ because the low alkalinity and high crystallization temperature during the synthesis are helpful for enhancing the SiO₂/Al₂O₃ ratio of the final zeolites. However, when the temperature is increased to 120 ℃, the same aluminosilicate gel produces FAU and MAZ zeolite (samples #3 and #4 in Table S2, Fig. S4 and Fig. S5). When the feed molar fraction of choline cation is increased to 2.0, it is plainly a tendency that the final products transform from the FAU topology to MAZ structure (sample #5 in Table S2 and Fig. S6). Choline is also used as an OSDA for synthesizing zeolite ZSM-4 with MAZ framework [34].

It is well known that the (hydro)thermal stability of zeolite at high temperature has a strong relationship with the framework SiO₂/Al₂O₃ ratio, which is a crucial factor for industrial applications. The exothermic peaks without weight variation above 800 ℃ in the TG/DSC curves are attributed to the structural collapse of zeolite Y, which is used as a main evaluation of the thermal stability of zeolites at high temperature. Na-Y starts to collapse at 970 ℃ (Fig. S7), but the skeleton of ChCl-Y and ChOH-Y remains stable until 1024 and 1030 ℃ (Fig. 2E and 2F). The results are also additional evidence that our materials have unusually ultrahigh SiO₂/Al₂O₃ ratio. The crystallinity change before and after steam treatment at high temperature is a measure of the hydrothermal stability of zeolites. The XRD patterns of HChCl-Y, HChOH-Y and HNa-Y (the correspond H-form samples after ammonium ion exchange and calcination) before and after 100% steam treatment at 750 ℃ for 2 h are shown in Fig. 5. This indicates that the crystallinity of HNa-Y descents more steeply than that of HChCl-Y and HChOH-Y after steam treatment. The crystallinity of HChCl-Y and HChOH-Y retains 54.2% and 56.6%, higher than 38.5% of HNa-Y; BET specific surface area and t-plot micropore surface area of HNa-Y are obviously lower than that of HChCl-Y and HChOH-Y after steam treatment (Table 2). The results indicate that ChCl-Y and ChOH-Y exhibit a more superior hydrothermal and thermal stability owing to the higher framework SiO₂/Al₂O₃ ratio. It is known that the bond energy of the silicon–oxygen bond (Si–O–Si) is higher than that of the aluminium–oxygen bond (Al–O–Si), so the Si–O–Si bond is more stable than the Al–O–Si bond. ChCl-Y and ChOH-Y have higher Si–O–Si/Al–O–Si ratio than Na-Y, which has been interpreted with the ²⁹Si-NMR spectrum (Fig. 4). Collectively, these studies confirm the practical advantages of increasing Y zeolite SiO₂/Al₂O₃ ratio to improve the (hydro)thermal stability, which enhances the adaptation of Y zeolite for catalytic applications.

A FAU zeolite with good crystallinity has been synthesized with the use of choline as an eco-friendly and biodegradable OSDA for the first time. The synthetic samples ChCl-Y and ChOH-Y have SiO₂/Al₂O₃ ratios of 6.48 and 6.83, which are much higher than that of common Y zeolite prepared by OSDA-free routes. Both ChCl-Y and ChOH-Y exhibit outstanding thermal and hydrothermal stability owing to higher SiO₂/Al₂O₃ ratio. Based on a thoughtful experiment design, it is proved that Na⁺ show a seemingly stronger competitiveness than [choline]⁺ in the cage of FAU zeolite and the feed fraction of Na⁺ has a crucial influence on the FAU framework SiO₂/Al₂O₃ ratio. The use of OSDAs with OH^– anion enhancing the zeolite SiO₂/Al₂O₃ ratio is a valued inspiration for the synthesis of high silica zeolite. These unique findings are of potential importance for the future industrial application of high silica FAU zeolite as an adsorbent and a catalyst.