Ferrierite (FER) zeolite is a medium-pore zeolite with two perpendicular intersecting channels, namely an 8-membered ring (MR) channel (0.35 nm × 0.48 nm) along the [010] direction and a 10-MR channel (0.42 nm × 0.54 nm) along the [001] direction [1]. FER zeolite is an industrialized zeolite that has been successfully applied as a catalyst to the skeletal isomerization of 1-butene [2-6]. FER zeolite has also been used in other reactions, including nitrous oxide decomposition and reduction [7, 8], dimethyl ether carbonylation [9-11], methanol-to-olefin (MTO) conversion [12], alkane hydroisomerization [13], and n-paraffin cracking [14, 15].

FER zeolite is usually synthesized with the assistance of organic structure-directing agents (OSDAs), which are generally organic amine compounds, such as ethylenediamine [16, 17], pyrrolidine [6, 18, 19], pyridine [20, 21], and cyclohexylamine [22]. Furthermore, oxygen-containing compounds, such as tetrahydrofuran, can be used to direct the synthesis of FER zeolite [23, 24]. FER zeolite can also be synthesized using two organic molecules of different sizes as co-OSDAs, with the resultant materials showing unique physical, chemical or catalytic properties. Hierarchical FER zeolite was synthesized using piperidine (PI) and tetramethylammonium hydroxide as co-OSDAs, leading to more than three-fold external surface area and better catalytic performance in catalytic low-density polyethylene cracking compared to the bulk zeolite [25]. Hierarchical FER zeolite nanosheet assemblies with ball-shaped morphologies have been prepared using an organosilane surfactant, 3-(trimethoxysilyl)propyl octadecyl dimethyl ammonium chloride, and pyrrolidine as co-OSDAs. Compared with conventional FER zeolite, these assemblies showed greatly improved catalytic activity in the benzylation of toluene with benzyl chloride [26]. Recently, FER zeolite was synthesized using a novel combination of OSDAs, namely 1, 6-bis(N-methylpyrrolidinium)hexane and tetramethylammonium [27].

Previously, Jongkind et al. [28] found that FER zeolite was synthesized using PI as an OSDA during the preparation of mordenite (MOR) zeolite using saturated cyclic amines as OSDAs. However, these products were always contaminated with the MOR phase. After that, FER zeolite was synthesized by solid-state recrystallization of aluminum-containing magadiite [29] and kanemite [30]. Generally, it is necessary to prepare the raw materials, magadiite or kanemite, before recrystallization, which makes this synthetic approach more complex. Furthermore, the products are always contaminated with magadiite or quartz. Wu et al. [31] found that FER zeolite was obtained from the crystallization of MCM-22 (MWW type) zeolite when the silica/aluminum molar ratio (SAR, SiO₂/Al₂O₃) was lower than 25. However, synthesis of FER zeolite using PI as an OSDA in a simple and workable way is still needed to be resolved.

In this work, we focused on optimizing the synthetic conditions of FER zeolite using PI as an OSDA. The entire crystallization process at different temperatures was monitored by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influence of initial gel composition (alkalinity, water content, and SAR) on the crystal phase and morphology of the products was investigated in detail. The optimal synthetic conditions for FER zeolite using PI as an OSDA are summarized. Furthermore, the obtained samples were evaluated as catalysts in the skeletal isomerization of 1-butene.

FER zeolite was synthesized using PI as an OSDA under traditional hydrothermal conditions. In a typically procedure, sodium aluminate solution (16.8 wt% Al₂O₃, 24.2 wt% Na₂O, homemade), deionized water, sodium hydroxide (96.0%, Sinopharm Chemical Reagent Co., Ltd.), PI (99.0%, Sinopharm Chemical Reagent Co., Ltd.), and silica sol (29.7 wt% SiO₂, Qingdao Haiyang Chemical Co., Ltd.) were added into a Teflon-lined stainless-steel autoclave. The batch composition obtained in the initial gel mixture was 2.1Na₂O:0.55Al₂O₃:20SiO₂:3.2PI:280H₂O. After stirring for 30 min, the mixture was heated under rotation (60 rpm) at a specified reaction temperature for a given time. The reaction was quenched with tap water and the product was separated by repeated centrifugation several times until the solution was neutral. The solid was dispersed in water again and dried at 120 ℃ overnight.

After removing the OSDA by calcination at 540 ℃ for 3 h, the sodium-form zeolite was exchanged to the ammonium form by treatment with NH₄NO₃ solution (1.0 mol/L) at 80 ℃ for 2 h with stirring. This procedure was repeated twice for each sample. The sample was then washed with deionized water three times, dried, and calcined at 520 ℃ for 3 h to afford the proton-form zeolite.

Powder XRD data were collected on a PANalytical X'Pert PRO diffractometer using Cu K_α monochromatized radiation operating at 40 kV and 40 mA. The samples were scanned in the 2θ range of 5°-50°with a step size of 0.02°. The relative crystallinity (RC) of samples was calculated from the characteristic peak heights (2θ = 9.3° ± 0.1°, 22.3° ± 0.1°, 23.5° ± 0.1°, 24.3° ± 0.1°, 25.2° ± 0.1°, 25.7° ± 0.1°, and 28.5° ± 0.1°). The sample (S15) with an initial gel composition of 3.0Na₂O: 0.50Al₂O₃: 20SiO₂:3.2PI:280H₂O synthesized at 160 ℃ for 48 h was chosen as the reference sample. Chemical compositions were analyzed using a Philips Magix 601 X-ray fluorescence (XRF) spectrometer. Crystal morphology and particle size were observed by SEM using FEI Quanta-200F and Hitachi SU1510 microscopes. Samples were coated with gold before testing. N₂ adsorption/desorption experiments were carried out at -196 ℃ on a Micromeritics ASAP-2020 HD88 instrument. Prior to analysis, samples were degassed at 350 ℃ for 10 h.

The 1-butene skeletal isomerization reactions were carried out at atmospheric pressure on a continuous-flow fixed bed microreactor with an inner diameter of 6 mm. In a typical procedure, catalyst (0.50 g) was activated under flowing N₂ (20 mL/min) at 500 ℃ for 2 h and then cooled to 400 ℃. The mixture of 1-butene and N₂ was then fed into the fixed bed. The 1-butene/N₂ molar ratio was 1/1 and the weight hourly space velocity (WHSV) of 1-butene was 8 h^-1. The products were analyzed using an online gas chromatograph (Agilent 7890B) equipped with a flame ionization detector (FID) and an Al₂O₃ capillary column.

Crystalline phases of zeolites are sensitive to crystallization temperature and time due to the metastable nature of zeolites [32]. Initial gels were crystallized at different temperatures for a certain time, and the XRD patterns of the corresponding samples are shown in Fig. 1. Only FER zeolite appeared in the crystallization process when crystallized at 150 ℃ (Fig. 1(a)). At 160 ℃ and 170 ℃, the crystallization processes were significantly different from that at 150 ℃. MWW and FER composite zeolites were obtained when initial gels were heated at 160 ℃ for 32 h. When the crystallization time was extended to 36 h, both the MWW and FER zeolite contents increased, as shown in Fig. 1(b). As the crystallization time was further increased, FER zeolite gradually began to dominate the product and MWW zeolite completely disappeared, with pure FER zeolite obtained after 40 h. As PI could direct the synthesis of MWW zeolite [31, 33] and MWW zeolite could transform into FER zeolite [34-36], it was reasonable that MWW zeolite was present in the crystallization process of FER zeolite as a metastable phase that finally transformed into FER zeolite. Furthermore, high temperatures favored the crystallization of MWW zeolite in this system. This was further illustrated by the crystallization process at 170 ℃, which was similar to that at 160 ℃, while the crystallization rate was significantly increased and the FER zeolite was well crystallized after 34 h. With the crystallization time further extended to 48 h, the product was contaminated with cristobalite as an impurity phase, indicated by the peak at 21.7° in the corresponding XRD pattern.

Crystallization curves of FER zeolite at different temperatures are shown in Fig. 2. The induction period was approximately 34 h when the initial gel mixture was crystallized at 150 ℃. Well-crystallized FER zeolite was obtained when the crystallization time was extended to 45 h. Further extending the crystallization time had little effect on the RC of the FER zeolite. When the crystallization temperature was increased to 160 ℃, a crystallization curve similar to that at 150 ℃ was obtained, but with the induction period shortened to 26 h. Well-crystallized FER zeolite was obtained after 40 h. When the temperature was further increased to 170 ℃, the induction period was just 18 h and FER zeolite was well crystallized after 34 h. However, with further increasing crystallization time, the RC of the FER zeolite declined rapidly. As mentioned previously, a peak not attributed to FER zeolite was found in the XRD pattern of the sample crystallized for 48 h (Fig. 1(c)), indicating that the decrease in RC was due to FER zeolite transforming into cristobalite. This was similar to the results reported by Pál-Borbély et al [30], who found that FER zeolite transformed into quartz over time when the crystallization temperature was above 147 ℃ using a solid-state recrystallization method.

SEM images were taken after crystallization at 160 ℃ (Fig. 3). An amorphous phase was obtained after crystallization for 24 h. Extending the crystallization time to 32 h afforded aggregates consisting of numerous platelets, which is the typical morphology of MWW zeolite. When the crystallization time was 36 h, block FER zeolite was clearly observed on the surface of the MWW aggregates. After 38 h, these MWW aggregates had largely disappeared, and the product was mainly composed of block FER zeolite. Pure FER zeolite was obtained after crystallization for 40 h. These observations were in accordance with the XRD results.

XRD patterns of samples synthesized at different alkalinities are shown in Fig. 4, and detailed synthetic parameters are listed in Table 1. FER zeolites without any impurity phase were obtained using Na₂O/SiO₂ molar ratios of 0.090-0.140. When the Na₂O/SiO₂ ratio was lower (S1), no FER phase was found, with the products mainly consisting of MWW and MTN phases. In contrast, FER zeolite was obtained at a higher alkalinity (Na₂O/SiO₂ = 0.150), but was contaminated with MOR phase. Furthermore, samples were crystallized at different alkalinities when the SAR was 30.3 and 40.0 (Table 1). Pure FER zeolite was synthesized using Na₂O/SiO₂ molar ratios of 0.105-0.150 (initial gel SAR = 40.0), which were slightly higher than those used to prepare FER zeolite with an initial gel SAR of 36.4. When the alkalinity was increased, a mixture (S16) of FER and MOR was obtained, which was also observed for initial gels with low SARs (S6 and S10). However, FER zeolite doped with MTN phase (S11) was obtained at low alkalinity, which was different from the case at low alkalinity when the initial gels had an SAR of 36.4. The alkalinity used to synthesize FER zeolite was lower (Na₂O/SiO₂ ≤ 0.125) when the initial gels had an SAR of 30.3.

Product morphology was closely related to alkalinity. SEM images of FER zeolites synthesized at different alkalinities are shown in Fig. 5. Well-dispersed lamellar samples were obtained at low alkalinity (S2, Na₂O/SiO₂ = 0.090), and the crystal size ranged from 0.5 μm to 1.0 μm. The morphology of the sample (S5) synthesized at high alkalinity was very different to that of S2, showing smaller primary particles (0.4-0.5 μm) and closer stacking. The SARs of the two products were also different, decreasing from 29.0 to 26.5 when the Na₂O/SiO₂ ratio was increased from 0.090 to 0.140 (Table 1).

N₂ adsorption/desorption isotherms of FER zeolites synthesized at different alkalinities are shown in Fig. 6, all of which showed type-I N₂ adsorption isotherms. Detailed data of the textural properties are summarized in Table 2. The micropore volumes of the three samples were large and similar to reported values [17], indicating that all three samples were well crystallized. The external surface area of S2 was larger than that of S5 (38 m²/g vs. 27 m²/g), which seemed abnormal because the crystal size of S2 (Fig. 5(a) and (b)) was larger than that of S5 (Fig. 5(c) and (d)). Notably, the crystal particles of S2 were loosely stacked, which resulted in each particle making a large contribution to the external surface area and mesopore volume. Therefore, the external surface area and mesopore volume of S2 was slightly larger than that of S5. Furthermore, the total volume of S2 was also larger than that of S5.

Water content is important in FER zeolite crystallization [37]. Well-crystallized FER zeolite without any impurity phase was obtained when the H₂O/SiO₂ ratio was 14.0 (Fig. 7(2)). However, FER zeolite contaminated with impurity phase, as indicated by the peaks at 25.9° and 27.9°, was obtained when silica/aluminum species were concentrated in the initial gel mixture (H₂O/SiO₂ = 8.5). In contrast, a low concentration of silica/aluminum species (H₂O/SiO₂ = 25.0) diminished FER zeolite crystallization, affording amorphous phase with several weak diffraction peaks (S18, Fig. 7(3)). When H₂O/SiO₂ was further increased to 35.0, no peaks were observed in the XRD pattern. Nevertheless, well-crystallized FER zeolite was obtained by adding 5 wt% FER seeds to the high-water-content system (S19, Fig. 7(4)).

In summary, well-crystallized FER zeolites were successfully synthesized at 150-170 ℃ with the following initial gel compositions: (1.8-3.0)Na₂O:(0.50-0.66)Al₂O₃:20SiO₂:3.2PI: 280H₂O. The particle size was in the range of 0.4-1.0 μm and the Brunauer-Emmett-Teller (BET) surface area was around 395 m²/g.

FER zeolite is a good catalyst candidate in the skeletal isomerization of 1-butene [2, 5]. Herein, HS2 and HS5, obtained at different alkalinities, were chosen and evaluated as catalysts in the 1-butene skeletal isomerization reaction (Fig. 8). These two samples showed similar catalytic changing trends, as reported previously [6, 38]. 1-Butene conversion gradually decreased with time on stream, while the product selectivity for isobutene increased with reaction time. Therefore, the isobutene yield increased with time on stream. As shown in Fig. 8(c), the isobutene yield over HS5 was slightly higher than that over HS2.

The crystallization behavior and optimal synthetic conditions of FER zeolites directed by PI were investigated in detail. XRD patterns and SEM images showed that product evolution was closely related to the crystallization temperature. Only the FER phase was observed during crystallization at 150 ℃. However, MWW phase appeared as the interphase when crystallization occurred at 160-170 ℃. Alkalinity proved to be the key factor in obtaining FER zeolites with controllable chemical compositions, morphologies, and textural properties. FER zeolite with a large external surface area and loosely stacked large particles was obtained at low alkalinity, while high alkalinity led to the formation of FER zeolite with closely stacked small particles. MOR and MTN were the main impurity phases produced using this synthesis system. Based on the above experimental results, the optimal initial gel compositions for FER synthesis was (1.8-3.0)Na₂O:(0.50-0.66)Al₂O₃:20SiO₂:3.2PI: 280H₂O. The obtained FER zeolites exhibited good catalytic performance in the 1-butene skeletal isomerization reaction.