Abstract
Zeolites have extensively been applied in gas adsorption and separation, catalysis, and ion exchange thanks to their numerous interesting features. However, the synthesis of zeolites usually requires solvents, which often lead to water pollution, loss of silicon and aluminium components, and low synthesis efficiency. In this account, ZSM-5 zeolites were synthesized from natural clay palygorskite (PAL) by acid leaching, followed by thermal treatment under solvent-free conditions. The as-obtained ZSM-5 zeolites exhibited high crystallinity and mesoporous structure. The NMR spectra of ZSM-5 zeolites confirmed the presence of aluminium element derived from the acid-treated PAL (APAL) existed in zeolite framework. Thus, PAL was simultaneously employed as silica and aluminium sources for zeolite preparation. The Si/Al molar ratio of ZSM-5 zeolites could be adjusted by adding NaAlO2, but against growth of ZSM-5 zeolites, resulting in formation of analcime. On the other hand, long thermal treatment for 72 h caused the dissolution of ZSM-5 zeolite crystals and formation of analcime. Overall, calcination could enhance the activity of APAL and crystallinity of ZSM-5 zeolites, and solvent-free synthesis of zeolites from clay might efficiently reduce the cost of zeolites.
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1 Introduction
Zeolites have extensively been applied in gas adsorption and separation, catalysis and ion exchange thanks to their particular micropores, high hydrothermal stabilities, large surface areas and excellent adsorption performances [1,2,3]. The synthesis of zeolites usually requires solvents, which often leads to water pollution, loss of silicon and aluminium components and low synthetic efficiency. Alternatively, simpler and green methods have been developed for the synthesis of various types of zeolites under solvent-free conditions to reduce wastes and increase yield of zeolites [4,5,6,7]. Solvent-free synthesized zeolites also showed superior catalytic properties when compared to zeolites prepared in the presence of solvent [8,9]. In solvent-free synthesis of zeolites, raw materials like Na2SiO3·9H2O, fumed silica, solid silica gel and SiO2·3H2O have been explored as the silicon source to prepare zeolites [4,5,6,7]. However, the obtained materials are expensive when compared to natural minerals, resulting in the high cost of zeolites.
To reduce the cost of zeolites, various types of minerals containing silicon and aluminium have been employed in the preparation of zeolites after pre-treatment [10,11,12,13]. For instance, diatomite as mineral with high silica content (>90%) has been converted into sodalite zeolite after mixing with pseudoboehmite in the absence of solvent [14]. The amorphous silica in diatomite is available for zeolite synthesis to utilize diatomite, which could directly be converted to zeolites without purification or treatment. Clay are a class of important aluminosilicate minerals used as synthetic material of zeolites. Recently, metakaolin and fly ash were also attempted to synthesize zeolite A and zeolite NaP after calcination under solvent-free conditions [15,16]. ZSM-5 zeolites could also be obtained from illite clay after calcination with K2CO3 followed by leaching with H2SO4 under template- and solvent-free conditions [17]. However, with the significant differences in the structure and component of clay minerals, the solvent-free synthesis of zeolites from clay minerals is so far still a challenge to accomplish.
In this work, a green and economic synthesis of ZSM-5 zeolites was performed from natural clay palygorskite (PAL). ZSM-5 zeolites were successfully yielded from PAL as partial silica and aluminium source under the solvent-free condition. The synthetic parameters of clay-based ZSM-5 zeolites were investigated and the results were discussed. It was found that ZSM-5 zeolites with variable Si/Al molar ratios and mesoporous structure were obtained by controlling the synthesis conditions.
2 Experimental
2.1 Synthesis of ZSM-5
The acid treatment of PAL was carried out in 3 mol l−1 HCl solution at 80°C for 48 h [11,12]. The contents of SiO2 and Al2O3 in acid-treated PAL (APAL) were set to 80.66 and 2.17%, respectively [12]. In a typical synthesis, 1.315 g Na2SiO3·9H2O, 0.36 g APAL, 0.24 g tetrapropylammonium bromide (TPABr) and 0.4 g NH4Cl were initially mixed together and then milled for 20 min. Next, the homogenized mixture was transferred into a sealed stainless steel autoclave with Teflon liner. After heating in an oven at 180°C for 48 h, the as-obtained solid powder was washed with deionized water and then dried overnight at 100°C. The synthesized product was finally calcined at 550°C for 3 h to remove the template. H-form ZSM-5 zeolites were obtained by ion exchange of the synthesized ZSM-5 zeolites with the NH4NO3 solution (0.2 mol l−1) at 80°C for three times and subsequent calcination at 550°C for 3 h.
2.2 Characterization
Scanning electron microscopy (SEM) images were collected on an S-3000N scanning electron microscope (Hitachi) operating at 20 kV. The silicon and aluminium element analyses of the synthesized ZSM-5 zeolite were determined using inductively coupled plasma-optical emission spectroscopy (Optima 7000DV, PerkinElmer). The samples were dissolved in hydrofluoric acid and then diluted by the deionized water. Transmission electron microscopy (TEM) images of the calcined ZSM-5 specimens were viewed on a JEM 2100 microscope operating at 200 kV. Powder X-ray diffraction (XRD, D8-Discover diffractometer, Bruker) was used for crystal structure identification using CuKα radiation (at 40 kV and 40 mA) at scanning velocity of 2° min−1 and from 5 to 40°. Nitrogen adsorption–desorption experiments at 77 K were measured on a Micromeritics TriStar II 3020. The specimens were first degassed at 300°C for 5 h under nitrogen atmosphere. 27Al MAS NMR spectra of the synthesized ZSM-5 zeolite were recorded on a Bruker AVANCE III 400 MHz NMR spectrometer, and infrared spectra of the samples were performed on a Nicolet 5700 spectrometer with a resolution of 4 cm−1 using KBr method. Temperature-programmed desorption of ammonia (NH3-TPD) of H-form ZSM-5 zeolites was carried out on a AMI-300 chemisorption analyzer equipped with a thermal conductivity detection (TCD). The samples were degassed at 300°C for 3 h before the measurements.
3 Results and discussion
3.1 Effect of SiO2/Al2O3 molar ratio
Figure 1 shows the XRD patterns of PAL and APAL. After acid treatment, the characteristic peak of PAL at 2θ of 8.3º completely vanished. However, the quartz peaks at 2θ of 20.7 and 26.6º significantly increased due to its high stability in hydrochloric acid. This indicated that destruction of the crystal structure of PAL to form amorphous silica [11]. Hence, APAL might be used to replace fumed silica in solvent-free synthesis of zeolite. The SiO2/Al2O3 molar ratio of synthesis mixture without exogenous aluminium source was 124.
To investigate the effects of aluminium source, SiO2/Al2O3 molar ratios of the raw materials were adjusted to 60, 24 and 17 by adding 0.02, 0.08 and 0.12 g NaAlO2, respectively. The XRD patterns of products synthesized using sodium silicate and APAL as silica source under solvent-free conditions are presented in figure 1. Obviously, all products displayed well-resolved peaks at 8–23°, consistent with data of MFI zeolites [6]. Zeolites could not be formed from APAL without NH4Cl, in which the product still exhibited amorphous structure in absence of NH4Cl (data not shown). On the contrary, a mixture composed of APAL and Na2SiO3·9H2O could be transformed into zeolite by mineralization of NH4Cl under solvent-free conditions. Small amounts of water and alkaline substances derived from sodium silicate might dissolve APAL and facilitate the hydrolysis and condensation of Si−O−Si bonds [4]. Finally, mineralization of NH4Cl and template action of TPA+ ions gradually transformed silica–alumina gel into MFI zeolites during synthesis. In addition, the intensities of the characteristic peaks of as-synthesized ZSM-5 zeolites reduced with NaAlO2 amount, and new zeolite phase (analcime) appeared at SiO2/Al2O3 molar ratio of 24 [18]. The characteristic peaks of analcime looked very obvious at SiO2/Al2O3 molar ratio of 17. Therefore, excess aluminium could affect the crystallinity of ZSM-5 zeolites [19,20], resulting in the formation of impurity phase analcime [21]. The SiO2/Al2O3 molar ratios of the synthesized ZSM-5 zeolites were reduced with the increase in the NaAlO2 amount and were 78, 44, 24, 12, respectively. The difference between the SiO2/Al2O3 molar ratios of the synthesized ZSM-5 zeolites and raw materials was caused by incomplete utilization of silicon component during the formation of zeolite.
Figure 2 shows the 27Al MAS-NMR spectra of zeolites synthesized from APAL without the addition of NaAlO2. The high-intensity signal at 55.39 ppm suggested the presence of Al atoms in the internal framework of zeolite [12]. A weak signal was observed around −0.28 ppm associated with extra-framework octahedrally coordinated Al species [22]. Thus, some Al atoms derived from PAL succeeded to enter into the synthesized zeolite framework. Hence, ZSM-5 zeolites can be prepared by APAL as silica and aluminium sources using simple solvent-free method. From ICP (table 1), the corresponding Si/Al molar ratios of as-synthesized ZSM-5 zeolites were determined as 18.5, 9.2, 7.3 and 5.8 at SiO2/Al2O3 molar ratios of the synthesis mixture were 124, 60, 24 and 17, respectively. Therefore, Si/Al molar ratio of ZSM-5 zeolites could be adjusted by adding the extrinsic NaAlO2.
Figure 3 illustrates SEM images of PAL, APAL and ZSM-5 zeolites synthesized from APAL under solvent-free conditions. For PAL and APAL, particles with irregular morphologies were visible at low resolution. Subsequently, round-shaped crystals with average particle sizes of 8–9 µm were obtained at SiO2/Al2O3 molar ratio of 124. The crystals exhibited typical ZSM-5 zeolite morphology [20], indicating high crystallinity of the as-synthesized ZSM-5 zeolites. The fragmented amorphous materials increased as SiO2/Al2O3 molar ratio declined in the synthesis mixture. These findings further confirmed that excess aluminium element was not conducive for synthesis of ZSM-5 zeolites under solvent-free conditions.
The N2 sorption isotherms and pore-size distributions (PSD) of ZSM-5 zeolites synthesized from APAL at different SiO2/Al2O3 molar ratios are illustrated in figure 4. All the ZSM-5 zeolite samples exhibit typical type-IV N2 adsorption isotherms with a hysteresis loop corresponding to capillary condensation at relative pressure from 0.4 to 1.0, suggesting large intracrystal mesoporosity [6]. Those hysteresis loops varied according to SiO2/Al2O3 molar ratio of raw materials, indicating that the synthesized ZSM-5 zeolites have different mesoporous or macroporous structures. For the ZSM-5 zeolites obtained at SiO2/Al2O3 molar ratio range of 24–124, the hysteresis loop has the general type B form, indicating that zeolites have slit-shaped pores and the distribution of pore size and shape is not well-defined [23]. However, the hysteresis loop became smaller and the general type A form and moved to the higher relative pressure when SiO2/Al2O3 molar ratio was 17, indicating the existence of larger mesopores and the materials consist well-defined cylindrical-like pore channels [23]. The existence of mesopores was further confirmed by BJH PSD curves obtained from desorption branch of the synthesized ZSM-5 zeolites (figure 4B). An obvious peak of mesopores (3–5 nm) was noticed in those ZSM-5 zeolites. In addition, the weak and broad peak for mesopores (7–40 nm) was also existed in the ZSM-5 zeolites and the peak became obvious at SiO2/Al2O3 molar ratio of 17, which was well agreed with the result of N2 sorption isotherms shown in figure 4A. The meso- and macro-pores existed in the aggregated nanosized APAL crystals might result in the formation of mesoporous structure in the ZSM-5 zeolites [7]. Table 1 lists the texture properties of APAL and the above ZSM-5 zeolites (S1). The BET surface area, pore volume, micropore volume and mesopore volume of ZSM-5 zeolites were determined as 250 m2 g−1, 0.136, 0.092 and 0.044 cm3 g−1, respectively. Compared to APAL, BET surface area and micropore volume of ZSM-5 zeolites were obviously increased, but pore and mesopore volumes decreased. The latter suggested the existence of zeolite phase with micropore structure in the synthesized product. As shown in table 1, the BET surface area and micropore volume of ZSM-5 zeolites (S1–S4) declined with added amount of NaAlO2. By contrast, the mesopore volume of ZSM-5 zeolites displayed a different changing trend, confirming that excess aluminium was not conducive for the formation of ZSM-5 zeolites from PAL under solvent-free conditions.
Figure 5 shows the NH3-TPD profiles of H-form ZSM-5 zeolites prepared from APAL at different SiO2/Al2O3 molar ratios. A obvious desorption peak in the temperature region of 50–300°C, ascribed to weak acid cites [24], was observed in all H-form ZSM-5 zeolites. The weak acid cites are related to the non-framework Al atoms and zeolite defects [25]. On the other hand, the weak desorption peak at 300–500°C was attributed to strong acid cites [24]. The NH3 desorption peak of ZSM-5 zeolites shifted to high temperature region with a decrease of SiO2/Al2O3 molar ratio, indicating that the ZSM-5 zeolites possessed more strong acid cites. The strong acid cites are associated to the lattice Al atoms in zeolite framework, which means that more aluminium atoms entered into ZSM-5 zeolite framework with a decrease in SiO2/Al2O3 molar ratio [25]. However, the NH3 desorption peak at 300–500°C was weakened when the SiO2/Al2O3 molar ratio of ZSM-5 zeolites was 17, indicating that the strong acid cites were reduced. The phenomenon was caused by the formation of analcime despite higher aluminium content in zeolites.
Figure 6 exhibits TEM image of the ZSM-5 zeolites prepared from APAL without added NaAlO2. Obviously, the synthesized ZSM-5 zeolite crystals looked large and discernible with intracrystal mesopores of 20–40 nm, consistent with the N2 adsorption–desorption data (figure 4).
3.2 Effect of crystallization time
ZSM-5 zeolites were synthesized by varying the crystallization time from 1 to 72 h without adding NaAlO2 under the solvent-free conditions. Figure 7 presents the XRD patterns of corresponding ZSM-5 zeolites. After 1 h crystallization, the product always looked amorphous. Weak characteristic peaks of ZSM-5 zeolites appeared in the XRD profiles of products obtained after 1.5 h. The intensity of peak, ascribed to MFI zeolite, rose with crystallization time from 1.5 to 48 h. However, the characteristic peaks of ZSM-5 zeolites reduced at prolonged crystallization time of 72 h. On the other hand, peaks of analcime appeared in XRD spectra of the obtained products. Both silica and aluminium in metastable ZSM-5 zeolite crystals might re-dissolve to form more stable analcime when crystallization time was prolonged [26]. Overall, the optimal crystallization time was determined as 48 h.
The texture properties of ZSM-5 zeolite obtained at crystallization times of 12 and 72 h are compiled in table 1. The BET surface area and total volume of ZSM-5 zeolites increased with crystallization time. However, the micropore volume of ZSM-5 zeolites firstly enhanced and then declined with crystallization time, but the corresponding mesopore volume reduced first and then increased. The latter might be caused by the presence of analcime with low surface area and re-dissolution of ZSM-5 zeolites as crystallization time prolonged to 72 h. The formation of analcime reduced BET surface area, but partial elimination of ZSM-5 zeolites phase also increased mesopore volume.
FTIR spectra of ZSM-5 zeolites prepared in the absence of NaAlO2 and solvent at 180°C for 1–72 h are displayed in figure 8. The adsorption bands near 550 and 1224 cm−1 were assigned to the double-ring tetrahedral vibration and asymmetric stretching of Si and AlO4 tetrahedra in zeolite framework, respectively [12,22]. These peaks enhanced with crystallization time from 1.5 to 48 h. However, the peak intensity reduced at crystallization time of 72 h, further confirming the formation of ZSM-5 zeolites from APAL under solvent-free conditions. The infrared spectroscopy results of ZSM-5 zeolites corroborated well with those from XRD and SEM.
Figure 9 shows SEM images of the above ZSM-5 zeolites. At crystallization time of 1 h, particles’ morphology remained irregular. As crystallization time was prolonged to 1.5 h, small round grains with particle size of 200 nm appeared, showing formation of ZSM-5 zeolite crystals. The round grains continued to grow to yield twin crystals when crystallization time reached 2 h. Further increase in crystallization time enlarged the particle size and morphology of ZSM-5 zeolite crystals. However, small grains started to form as crystallization time reached 72 h, meaning dissolution of ZSM-5 zeolite crystals and formation of new zeolite phase. These results agreed well with the XRD analyses.
3.3 Effect of ageing time
The ageing stage is essential to the hydrothermal synthesis of ZSM-5 zeolites. ZSM-5 zeolites were prepared from APAL without NaAlO2 after ageing at 12–72 h, followed by crystallization time of 48 h at 180°C. Figure 10 shows the XRD patterns of ZSM-5 zeolites prepared from APAL at different ageing times. The intensity of characteristic peaks of ZSM-5 zeolites reduced gradually with the increase in ageing time and then, those characteristic peaks disappeared when the ageing times was 48 h. The results indicate that the growth of ZSM-5 zeolites was restrained by ageing time. Figure 10 also illustrates that ZSM-5 zeolite crystal nuclei were not formed when the ageing time of zeolite synthesis gel was extended to 72 h. The results were caused by the evaporation of water in zeolite synthesis gel at room temperature during the ageing stage. The loss of water in zeolite synthesis gel will restrict the formation of ZSM-5 zeolites. Hence, the synthesis gel could directly be used to synthesize ZSM-5 zeolites under the solvent-free condition without ageing stage.
3.4 Effects of TPABr and activation method
The effects of template on solvent-free synthesis of ZSM-5 zeolites from APAL were investigated by increasing the amounts of TPABr. To improve the quality of ZSM-5 zeolites, PAL was calcined after acid-treatment and then used to prepare ZSM-5 zeolites under solvent-free conditions and the results are gathered in figures 11, 12 and table 1. No significant changes in the intensity and texture properties of ZSM-5 zeolites were observed despite presence of more template ions (figure 11 and table 1). More aggregated crystals appeared as TPABr content rose (figure 12). The calcination of APAL could also improve the utilization of silica and aluminium components, which would efficiently enhance the intensity and BET surface area of the obtained ZSM-5 zeolites (table 1). Aggregated spherical particles of zeolite crystals could be obtained with larger particle size than that of ZSM-5 zeolites synthesized after acid treatment. Hence, calcination could improve the utilization of silica and aluminium components in PAL.
4 Conclusions
ZSM-5 zeolites were successfully prepared from natural clay PAL via solvent-free method. The acid-treated PAL was employed as partial silica and aluminium source during synthesis of ZSM-5 zeolites, where Si/Al molar ratio of ZSM-5 zeolites could be adjusted by adding NaAlO2. The synthesized ZSM-5 zeolites also exhibited mesoporous structures. The calcination of acid-treated PAL might enhance the utilization of silica and aluminium components in PAL to yield high quality zeolites. Overall, the proposed solvent-free synthetic strategy of zeolites from natural clay looks promising as low-cost and green-synthesis route of zeolites.
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Acknowledgements
We thank the National Natural Science Foundation of China (nos. 51574130, 51908240 and 21606098), Six Talent Peaks Project in Jiangsu Province (2018-JNHB-009), Natural Science Foundation of Jiangsu Province (BK20181064), Natural Science Key Project of the Jiangsu Higher Education Institutions (18KJA430006 and 19KJA430015) and College Students’ Innovation and Entrepreneurship Training Program of Jiangsu Province (201911049012Y), for financial support.
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Wu, M., Jiang, W., Jiang, J. et al. Synthesis of ZSM-5 zeolites using palygorskite as raw material under solvent-free conditions. Bull Mater Sci 43, 289 (2020). https://doi.org/10.1007/s12034-020-02263-8
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DOI: https://doi.org/10.1007/s12034-020-02263-8