Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash

doi:10.21203/rs.3.rs-4846209/v1

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Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash

https://doi.org/10.21203/rs.3.rs-4846209/v1

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Solid waste fly ash has a great impact on the environment and pollutes water, air and soil to different degrees. Therefore, it is of great significance to realize the high-value utilization of fly ash. In this paper, high purity SiO₂ was successfully extracted from fly ash by high temperature calcination, alkali fusion activation and pickling, and then different morphologies of Silicalite-1 zeolites were synthesized by using SiO₂ extracted from fly ash as silicon source, TPAOH as templating agent and NaOH as alkali source. The influencing factors such as crystallization time, crystallization temperature, NaOH dosage, TPAOH dosage and different hydrosilica ratios were investigated separately. The results show that under the conditions of crystallization time of 12 h, crystallization temperature of 180 ℃ and molar ratio of water-silicon of 42, the prepared P-S (plate-like Silicalite-1) has good morphology, high relative crystallinity and adsorption capacity of CO₂ of 1.89 mmol/g. The CO₂ adsorption capacity of the S-S (spherical-like Silicalite-1) prepared by adjusting the amount of TPAOH is 1.34 mmol/g. The CO₂ adsorption capacity of the prepared C-S (cross-type Na-Silicalite-1) is only 1.06 mmol/g. The CO₂ adsorption capacities follow P-S > S-S > C-S. The results may provide a valuable reference for zeolite-based adsorbents in the adsorption removal or recovery of CO₂.

Fly ash

SiO2

Silicalite-1

Morphology

CO2 adsorption

Fly ash, as the main solid waste of thermal power plants^[1], can produce about 250 ~ 350 kg of fly ash for every 1 t of raw coal burned^[2]. The massive accumulation of fly ash not only encroaches on land resources, but also pollutes the environment^[3] and poses a threat to human health^[4], so it is of great significance for the high-value utilization of fly ash^[5]. The main oxide compositions of fly ash are SiO₂ and A1₂O₃, and the structure of both exists mainly in crystalline phase^[6]. Fly ash rich in Si and AI has the same chemical composition as zeolite^[7]. Zeolite molecular sieves are silicate inorganic minerals with skeletal elements consisting of silicon, aluminum and their coordinating atoms^[8]. It is characterized by the [SiO₄]⁻⁴ tetrahedron and [AlO₄]⁻⁵ tetrahedron constituting the primary structural unit. Various skeletal structures are formed by interconnecting shared oxygen atoms^[9]. This unique internal structure and physicochemical properties make zeolite molecular sieves have broad application prospects in the fields of wastewater treatment, exhaust gas adsorption and industrial catalysis ^[10]. With the increasing demand for zeolite molecular sieve, more and more attention has been paid to the preparation of low-cost molecular sieve using fly ash as raw material^[11].

As one of the global environmental problems, the “greenhouse effect” caused by the excessive use of fossil fuels has attracted the attention of various countries^[12]. As the source of the “greenhouse effect”, CO₂ capture and recovery has become the most promising method to solve the greenhouse effect at this stage^[13]. Fly ash has a high silica and aluminum content, and can be processed under certain conditions to obtain zeolite molecular sieves, which can be used to adsorb pollutants in water and capture CO₂ in the air^[14]. Silicalite-1 molecular sieve is an all-silicon molecular sieve without aluminum element, with good hydrophobicity and CO₂ adsorption and separation properties^[15]. Wang^[16] prepared microporous and mesoporous Silicalite-1 molecular sieves and investigated their adsorption and separation properties for CO₂/N₂/CH₄/C₂H_6, and the results demonstrated that microporous Silicalite − 1 has high adsorption capacity for CH₄, N₂, CO₂ and C₂H₆, while the selectivity of mesoporous Silicalite-1 is relatively high.Wang^[17] synthesized ZSM-5 and Silicalite-1 with rice husk ash as silicon source and studied their adsorption properties for CO₂ under humid conditions, the adsorption capacities are 81.69cm³/g and 69.96cm³/g. Razavian^[18] prepared Silicalite-1 by ultrasonic method and hydrothermal method respectively, and compared their CO₂/CH₄ adsorption properties. The results showed that the ultrasonic treated Silicalite-1 had high selectivity, while the hydrothermal method Silicalite-1 had high adsorption capacity. Therefore, fly ash can be used to prepare Silicalite-1 with high specific surface area and good adsorption performance for CO₂^[19].

The morphology of MFI zeolite is closely related to their structure, micropore size and crystal size, which have a significant role in adsorption performance^[20]. The difference in the preferred orientation of pore channels will affect the morphology and diffusion efficiency^{[21, 22]}. Chen et al.^[23] investigated the adsorption properties of toluene on Silicalite-1 zeolites with different morphologies and found that morphologies have significant and different effects on toluene adsorption under dry or wet conditions. Under dry conditions (303 K), the dynamic adsorption capacity followed S-S (spherical Silicalite-1) > P-S (plate-like Silicalite-1) > B-S (brick-like Silicalite-1). Under humid conditions (RH = 50%, 303 K), the dynamic adsorption capacity was in the order of P-S > S-S > B-S.Liu et al^[24] found that the particle size of zeolite molecular sieves could be varied by the length of the b-axis to adjust the distribution of aromatic hydrocarbons. Wang et al.^[25] compared the catalytic performance of nano-scale and micron-scale ZSM-5 in different reactions (disproportionation of toluene, alkylation of toluene with methanol and trimethylbenzene cracking) and found that nano-scale ZSM-5 exhibited excellent reactivity due to increased pore openings and high accessibility of acid sites. Wang et al.^[26] synthesized straight channel-covered hemispherical bicrystalline zeolites and obtained p-xylene with greater than 99% selectivity by optimizing shape-selective catalysis of toluene in sinusoidal channels. Based on the advantages of molecular sieve morphology, grain size and adsorbability, the effect of MFI zeolite morphology on CO₂ adsorption removal was proposed, which has been rarely reported so far.

In order to promote the high-value utilization of fly ash and reduce the production cost of molecular sieves, in this work, high purity SiO₂ was successfully extracted from fly ash by high-temperature calcination, alkali fusion activation and acid leaching, and then Silicalite-1 zeolites with three different morphologies (plate-like, spherical-like and cross-type), were synthesized using SiO₂ as the silica source, and then used for the adsorption of CO₂. Finally, the purity, morphology, pore size and CO₂ adsorption performance of the Silicalite-1 zeolites with different morphologies were determined by characterization means such as XRD, SEM, FT-IR and BET.

2.1 Materials and reagents

Fly ash, purchased from Xinjiang Changji thermal power plant. Hydrochloric acid (HCl, 38%), sodium carbonate (Na₂CO₃), tetrapropyl ammonium hydroxide (TPAOH, 25%) and sodium hydroxide (NaOH) were all analytically pure, and were acquired from Sinopharm Group Chemical reagent Co, LTD. Deionized water for homemade.

2.2 Sample preparation

2.2.1 Extraction of SiO₂ from fly ash

The fly ash was placed in a box-type resistance furnace and roasted at 600 ℃ for 2 h to remove the unburned carbon. After cooling to room temperature, the fly ash was activated with Na₂CO₃ as the activator. Fly ash and Na₂CO₃ were mixed according to the mass ratio of 1:1 and calcined at 850 ℃ for 100 min in the box resistance furnace, and the calcined clinker was obtained by thorough grinding after cooling. The calcined clinker was stirred with 5 mol/L HCl according to the solid-liquid ratio of 1 g :10 mL on a magnetic stirrer for 4 h, and then filtered, washed with distilled water until neutral and dry, and SiO₂ is obtained.

2.2.2 Preparation of P-S (plate-like Silicalite-1)

P-S were prepared by using SiO₂ extracted from fly ash as the silica source, TPAOH as the template agent and distilled water as the solvent. Weighing a certain mass of deionized water, TPAOH and the prepared SiO₂ at room temperature according to a certain molar ratio on a magnetic stirrer after stirring vigorously for 4 h, and then the mixture was transferred to a 100 mL stainless steel reactor PTFE liner, and then placed into the oven has been preheated, crystallization at a certain temperature for a certain period of time. After crystallization is completed, take out of the reactor, cool the product, filter, wash to neutral, dry and grind. Finally, the product was roasted in a muffle furnace at 550 ℃ for 6 h, so as to remove the template agent and obtain the P-S (plate-like Silicalite-1).

2.2.3 Preparation of S-S (spherical-like Silicalite-1)

S-S were prepared by adjusting the dosage of TPAOH with SiO₂ extracted from fly ash as the silica source, TPAOH as the templating agent and distilled water as the solvent, keeping the crystallization time, crystallization temperature and water-to-silica ratio certain. The specific steps were the same as 2.2.2.

2.2.4 Preparation of C-S (cross-type Na-Silicalite-1)

C-S was prepared with SiO₂ extracted from fly ash as silicon source, TPAOH as template agent, NaOH as base source and distilled water as solvent. The specific steps were the same as 2.2.2.

2.3 Characterization

X-ray powder diffraction test (XRD): The D/max-2400 X-ray diffractometer of Rigaku Company in Japan, the radiation source is Cu target Kα radiation, the scanning speed is 10°/min, the scanning range is 5°-80°. X-ray fluorescence analyzer (XRF): SRF3400 X-ray fluorescence spectrometer from Bruker, Germany; Scanning electron microscope (SEM): Beijing KYKY-2800B. FT-IR: IFS88 from Bruker, Germany, with a range of 700–4000 cm^− 1. Specific surface area test (BET): Micrometrics Tristar II instrument was used to analyze the specific surface area, pore structure, and nitrogen adsorption and desorption curves of the catalysts. The samples were degassed under vacuum at 200°C for 8 h prior to the test, and the N₂ adsorption and desorption tests were performed on the samples under 77 K liquid nitrogen. The samples were purged at 300 ℃ for 12 h and then tested for CO₂ adsorption at 25 ℃.

3.1 Investigation of the synthesis conditions of P-S

3.1.1 Effect of different template agents

The type of template agent also affects the synthesized Silicalite-1 molecular sieve, and TPABr and TPAOH are the most commonly used template agents. Under the same experimental conditions, TPABr and TPAOH were used as templates to investigate the effects of different TPA⁺ on the preparation of Silicalite-1 molecular sieve. As can be seen from Fig. 1, the sample with TPAOH as the template agent has an obvious characteristic peak of Silicalite-1 molecular sieve. However, when TPABr is used as the template agent, only amorphous SiO₂ diffraction peaks appear, indicating that Silicalite-1 molecular sieve cannot be successfully synthesized when only TPABr is used as the template agent. At this time, Br^- in the system can only balance charge and cannot promote silicate rearrangement to form Silicalite-1 molecular sieve crystals. When TPAOH is the template agent, the reaction process contains the appearance of OH^-, which provides the base source for the synthesis environment of the sample and balances the negative charge at the same time, playing a structure-oriented role, so the Silicalite-1 molecular sieve can be successfully synthesized.

Figure 2.is the SEM image of Silicalite-1 zeolite synthesized under different template agents. It can also be seen from the figure that when the template agent is TPAOH, the crystalline phase structure of Silicalite-1 zeolite with smooth surface, regular morphology and uniform size appears, and the grain size is between 6–8 µm. When TPABr was used as template, only amorphous phase structure appeared. Therefore, TPAOH was selected as the template agent to synthesize Silicalite-1 zeolite.

3.1.2 Effect of crystallization time

Figure 3.shows the XRD (a) and crystallinity (b) diagrams of P-S synthesized with different crystallization times, as shown in the figure: when the crystallization time is 2 h, it can be clearly seen that the characteristic peaks belonging to MFI-type molecular sieves appeared in the 2θ of 7.94 °, 8.87 °, 23.08 °, 23.31 ° and 23.96 °, but with lower peak heights and weaker peak strengths. The initial induction period of ZSM-5 zeolite molecular sieve, which is also of MFI type, is about 5 h without the addition of crystal seed, and Silicalite-1 has entered the crystal growth stage at 2 h, which is a shorter induction period than that of ZSM-5. This may be due to the fact that there is no involvement of aluminate in the crystallization process of all-silica molecular sieves, thus shortening the crystallization time. With the prolongation of the crystallization time, the intensity of the characteristic diffraction peaks was gradually enhanced, indicating that more and more amorphous silicate sols have begun to transform to Silicalite-1 zeolite molecular sieve crystals, and the intensity of the characteristic diffraction peaks of Silicalite-1 zeolite gradually showed a downward trend when the crystallization time was greater than 12 h, which may be attributed to the fact that the crystallization time is too long, the crystals reacted with solution, making the crystals grow excessively. This may be due to the long crystallization time, the reaction between the crystals and the solution, which makes the crystals grow excessively and thus transcrystallization phenomenon occurs, which reduces its crystallinity ^[27]. As can be seen from Fig. (b), the crystallinity of the samples reached the highest when the crystallization time was 12 h, and the crystallinity of the samples gradually decreased when the time was extended.

Figure 4.is the SEM diagram of P-S synthesized under different crystallization times. As can be seen from the diagram, when the crystallization time is 2 h, there is no obvious regular crystal phase structure, which is due to the short crystallization time so that the basic molecular sieve skeleton cannot be formed. When the crystallization time is 7 h, a very small amount of P-S crystal phase structure appears, and the surface is smooth and flat, the grain size is between 4–6 µm, but still dominated by large amorphous particles.When the crystallization time was extended to 12 h, regular and dispersed plate crystals appeared with good morphology and uniform size, and the grain size was between 6–8 µm. However, with the continuous extension of crystallization time, the molecular sieve appeared obvious agglomeration and fracture phenomenon. When the crystallization time is 24 h, almost no single crystal structure can be found, and all are large agglomerated crystals. When the crystallization time is 36 h, P-S crystals not only agglomerate, but also fracture, with a large number of broken crystals attached to the surface of the molecular sieve. The results indicated that the long crystallization time was not conducive to the synthesis of P-S, but would promote the target product to continue to react with the solution, resulting in crystallization transformation.Therefore, based on the above analysis, 12 h is selected as the best crystallization time.

3.1.3 Effect of crystallization temperature

A higher crystallization temperature may shorten the induction period and promote the growth of the molecular sieve crystals, because the higher temperature helps rearrangement between atoms of the substance and promotes the formation of nuclei and the growth of crystals. However, when the crystallization temperature is too high, it may lead to other side reactions of the formed crystals, such as crystal transformation or crystal dissolution. When the crystallization temperature is low, it is not conducive to the atomic activity and rearrangement between the reactive substances, resulting in a decrease in the reaction rate^[28]. Figure 5.shows the XRD (a) and crystallinity (b) plots of P-S synthesized at different temperatures. From the figure, it can be seen that the products at five crystallization temperatures all showed the characteristic peaks of MFI molecular sieves between 2θ of 7.94 °, 8.87 °, 23.08 °, 23.31 ° and 23.96 °. When the crystallization temperature was 150 ℃, the characteristic diffraction peak strength of Silicalite-1 zeolite was weaker, which indicated that only the basic skeleton structure could be formed at 150 ℃. With the continuous increase of the crystallization temperature, the peak area of the characteristic peaks of Silicalite-1 gradually becomes higher, and when the temperature reaches 180 ℃, the characteristic diffraction peak intensity of Silicalite-1 reaches the highest. As the crystallization temperature continues to increase, the characteristic diffraction peak intensity decreases, which may be due to the fact that the crystallization temperature is too high, which makes the crystal transcrystallization or rupture phenomenon, thus leading to the decrease in the intensity of the characteristic peaks and the degree of crystallinity.

Figure 6.shows the SEM images of P-S synthesis at different crystallization temperatures. It can be seen from the figure that when the crystallization temperature is 150 ℃, although a small amount of plate crystalline phase structure appears, it still contains a large amount of bulk amorphous material. When the crystallization temperature is 160 ℃, the crystallization is relatively complete and more uniform plate structure appears, but a large number of small particles are still gathered on the crystal surface, which may be due to the low temperature and the adhesion of SiO₂ which is not involved in crystallization to the crystal surface. When the crystallization temperature is 170 ℃, the small particles gathered on the surface of Silicalite-1 crystals decrease, indicating that the unreacted amorphous SiO₂ has begun to transform into Silicalite-1 crystals. When the crystallization temperature is 180 ℃, the P-S crystal structure with uniform size and smooth surface appears. When the crystallization temperature was further extended to 190 ℃, cracks appeared on the surface of some samples and a large number of samples were agglomerated. According to the above analysis, either too high or too low temperature is not conducive to the synthesis of P-S, so 180℃ is chosen as the best synthesis temperature.

3.1.4 Effect of water amount

Water is not only a solvent in the synthesis of molecular sieve, but also has an important impact on the composition and structure of silicate gels as well as the formation and growth of crystal nuclei by changing the concentration of silicate ions in the solution and adjusting the alkalinity^[29]. Figure 7.shows the XRD pattern and crystallinity of P-S synthesized under different water amounts. As can be seen from the figure, pure phase Silicalite-1 zeolite can be synthesized within the molar ratio of H₂O/SiO₂ of 21 ~ 49, and no other crystalline heterogeneous peaks appeared. When H₂O/SiO₂ = 21, the characteristic peak-to-peak strength of Silicalite-1 molecular sieve is low, which may be due to the large silicate concentration, incomplete crystallization and low crystallinity caused by too low water content. The crystallinity gradually increased with the increase of water volume, and the relative crystallinity reached the optimum when the molar ratio of H₂O/SiO₂ was 42. While the water-silicon ratio continues to increase to 49, the relative crystallinity begins to decrease, which may be due to too much water, low concentration of silicate, and the existence of a large number of incomplete crystallized silica, resulting in a decrease in crystallinity.

Figure 8.is the SEM photo of P-S synthesized with different water-silicon ratios. It can be seen from the SEM image that when the molar ratio of H₂O/SiO₂ is 21, although there is a small amount of Silicalite-1 crystals, there are still a large number of amorphous substances attached to the crystal surface. With the increase of the water-silicon ratio, the amorphous material gradually decreases. When the molar ratio of H₂O/SiO₂ is 42, the crystal with smooth surface and uniform particle size is between 6–8 µm. Continue to increase the water-silicon ratio to 49, at this time, a large number of incomplete crystallization substances appear on the crystal surface again, indicating that too much water will lead to a relatively low concentration of silicate, making a large amount of SiO₂ is not fully crystallized. Based on the above analysis, the water-silicon ratio 42 is selected as the best water-silicon ratio.

3.2 Investigation of the synthesis conditions of S-S

3.2.1 Influence of TPAOH dosage

During the crystallization process of molecular sieves, the template agent will affect the interaction of silicaluminate, which have an impact on the gelation and nucleation process. Organic template agents are expensive, and their dosage also determines the cost of molecular sieve production. Figure 9 shows the XRD (a) and relative crystallization (b) of Silicalite-1 zeolites synthesized under different TPA⁺/SiO₂ molar ratio. It can be seen from the figure: When the molar ratio is 0.01, there is no characteristic peak belonging to MFI zeolite molecular sieve, indicating that the guiding effect is weak when the dosage of template agent is small, and it cannot provide structural guiding effect for amorphous SiO2 in the system^[30]. With the increase of the amount of template agent, within a certain range, the characteristic diffraction peaks and peaks of MFI zeolites are strong, and the relative crystallinity of Silicalite-1 is also high, indicating that TPAOH as the template agent can be synthesized within a certain range of Silicalite-1 with high crystallinity and single crystalline phase. When the molar ratio of TPAOH/SiO₂ is 0.28, the relative crystallinity reaches the maximum. The relative crystallinity decreased slightly with increasing the amount of TPAOH. It may be because the concentration of the template agent is too high, which will lead to excessive growth or aggregation of the crystal, thus affecting the relative crystallinity.

Figure 10.is the SEM diagram of Silicalite-1 molecular sieve with different dosage of template agent. According to the diagram, when the dosage of template agent is too low, it cannot fully polymerize with silicate to form gel, resulting in the failure to form crystal nucleus. With the increase of the concentration of template agent, TPA⁺ can bind to silicate ions and promote the formation of silicate gel. Silica tetrahedrons tend to form pores or cages on the gel surface, increasing the number of crystal nuclei^[31]. When the molar ratio of TPAOH/SiO₂ increases from 0.07 to 0.28, the particle size becomes smaller and smaller. When the molar ratio is 0.28, spherical-like crystals with smooth surface and uniform size appear.When the molar ratio of TPAOH/SiO₂ is greater than 0.28, the dosage of template agent exceeds a certain threshold, and the contribution to molecular sieve synthesis is reduced. This is due to the large size of TPA⁺, which may prevent silicon species from entering the molecular sieve skeleton^[32], and affect the formation of molecular sieve skeleton to a certain extent, resulting in larger molecular sieve particle size. Based on the above analysis, a molar ratio of TPAOH/SiO₂ of 0.28 is the best choice for the synthesis of S-S.

3.3 Investigation of the synthesis conditions of C-S

3.3.1 Influence of NaOH dosage

The amount of NaOH has a great influence on the synthesis of Silicalite-1 zeolite, which can adjust the pH value of the reaction medium, thus affecting the reaction rate and the crystal structure of the product. Alkaline environment can promote the reaction to a certain extent, but too high alkalinity may cause side reactions or affect the crystal morphology of the product^[33]. Figure 11.shows the XRD (a) and relative crystallinity (b) of Silicalite-1 molecular sieve with different amounts of NaOH. It can be seen from the figure that the synthesized zeolites at different proportions all show the standard MFI structure characteristic diffraction peaks at 2θ of 7.94 °, 8.87 °, 23.08 °, 23.31 ° and 23.96 °, and the crystallinity of the five products does not change significantly, indicating that Na-Silicalite-1 zeolite with high crystallinity can be synthesized in a certain range when SiO₂ extracted from fly ash is used as the silicon source.

Figure 12.shows the SEM images of Silicalite-1 zeolites at different NaOH dosages, from which it can be seen that the grain size decreases gradually with the increase of NaOH dosage. When the molar ratio of NaOH/SiO₂ is 0.011, there are plate-like structures of different sizes in the system, when the molar ratio of NaOH/SiO₂ is 0.055, at this time, there are not only plate-like structures in the system, but also smaller spherical-like structures, and when the molar ratio of NaOH/SiO₂ is 0.11, the crystalline structure of the system at this time becomes cross-type. When the molar ratio of NaOH/SiO₂ is 0.165, at this time it is a spherical-like crystal structure with uniform size. Continue to increase the dosage of NaOH to the molar ratio of NaOH/SiO₂ is 0.22, at this time, the solution is more alkaline, the surface of the molecular sieve is etched, and it becomes similar to the amorphous material morphology.

3.4 Characterization of different morphologies of Silicalite-1 zeolites and their CO₂ adsorption properties

3.4.1 Nitrogen adsorption-desorption

Figure 13.shows the nitrogen adsorption-desorption curves and pore size distribution of Silicalite-1 zeolites with different morphologies. As can be seen from the figure, both the P-S and S-S have no obvious hysteresis loops and are type I isotherms. The main performance is: the adsorption capacity rises rapidly under low relative pressure, and the adsorption saturation occurs after reaching a certain relative pressure. This is because in the narrow micropores, the adsorbent-adsorbent interaction is enhanced, which leads to the micropores being filled rapidly at very low relative pressure, but when the saturation pressure is reached, the adsorbent condenses, resulting in the curve beginning to flatline.However, C-S has an insignificant hysteresis ring, and the adsorption curve is a composite isotherm with type I as the main and type IV as the auxiliary. According to the analysis of pore size distribution by BJH theory, it can be seen that the pore size distribution of Silicalite-1 zeolites with three kinds of morphology are mainly concentrated in the range of 1 nm ~ 4 nm, which mainly exists in the form of micropores and has relatively narrow mesoporous pores.

Table 1 shows the pore structure parameters of Silicalite-1 zeolites with different morphologies, from the table, it can be seen that the specific surface area of S-S is 460.24 m²/g, of which the specific surface area of the micropores is 275.25 cm²/g, and the total pore volume is 0.365 cm³/g, of which the microporous pore volume is 0.141 cm³/g, which indicates that the prepared P-S have high specific surface area and pore volume. Not only that, the P-S have larger specific surface area, total pore volume and microporous pore volume than the other two morphologies of Silicalite-1. The specific surface area of S-S is much lower than that of P-S and C-S, except for the specific surface area, the rest of the parameters of S-S and C-S are not much different.

Table 1

Pore structure parameters of Silicalite-1 zeolites with different morphologies: (a) P-S; (b) S-S; and (c) C-S.
	S_BET(m²/g)	S_micro(m²/g)	V_total(cm³/g)	V_micro(cm³/g)	Average pore size/nm
a	460.24	275.25	0.365	0.141	3.16
b	413.83	304.77	0.183	0.120	1.76
c	454.58	298.95	0.201	0.119	1.771

3.4.2 FT-IR analysis

Figure 14.shows the FT-IR spectra of Silicalite-1 zeolites with different morphologies. It can be seen from the figure that all the three morphologies of Silicalite-1 have skeleton vibration peaks belonging to MFI-type molecular sieves at 1230, 1080 and 800 cm^− 1, among which the peak at 1230 cm^− 1 is attributed to the tensile vibration of the five-membered ring unique to MFI, the peak at 1080 cm^− 1 is attributed to the asymmetric telescopic vibration of the Si-O-Si external connection, and the peak at 800 cm^− 1 is attributed to the telescopic vibration of Si-OH. and the 800 cm^− 1 peak is attributed to the stretching vibration of Si-OH. However, the FT-IR spectra of C-S did not show any obvious Na-O peaks, probably due to less Na in the skeleton, and thus the absorption vibration peaks were not obvious.

3.4.3 CO₂ adsorption isotherm

Figure 15.shows the CO₂ adsorption isotherms of Silicalite-1 zeolites with different morphologies. It can be seen from the figure that both P-S and S-S overlapped the adsorption curve and desorption curve of CO₂, indicating that their adsorption properties for CO₂ are reversible and mainly dominated by physical adsorption. In the low pressure region, the adsorption capacity rises rapidly, indicating that there is strong adsorption in the low pressure region, and this adsorption process is related to the large number of micropores in the sample.In the medium pressure section, the adsorption curve rose slowly, indicating that the mesoporous content of the sample was low. When the adsorption pressure is increased to 101.325KPa, the CO₂ adsorption capacity of the prepared P-S is 1.89mmol /g, indicating that it can be used as an excellent CO₂ adsorption material. The adsorption capacity of S-S for CO₂ is 1.34mmol /g, which is much lower than that of P-S.This may be due to the fact that P-S has a larger specific surface area, total pore volume and microporous pore volume than the remaining two morphologies of Silicalite-1.

In addition, it can be seen from the figure that the adsorption curve and desorption curve of C-S for CO₂ do not completely coincide, indicating that the adsorption performance of C-S for CO₂ is partially irreversible. Although it is mainly physical adsorption, there is still a certain amount of chemical adsorption due to the introduction of alkaline metals. Under the test condition of 25 ℃, when the adsorption pressure was increased to 101.325 KPa, the adsorption capacity of C-S for CO₂ was 1.06 mmol / g. Although a small amount of chemical adsorption phenomenon appeared in C-S, its adsorption performance was much lower than that of P-S and S-S,which could be attributed to the decrease of CO₂ adsorption capacity due to the reduction of the specific surface area and the micropore of molecular sieves by adding inorganic alkali. Among the three morphologies of Silicalite-1 zeolites, the adsorption effect of P-S was better due to the slightly larger specific surface area compared with the others, so it can be hypothesized that the CO₂ adsorption performance is mainly related to the specific surface area of the molecular sieves.

Silicalite-1 zeolite molecular sieves with different morphologies were prepared using SiO₂ extracted from fly ash as silicon source. The results show that under the conditions of crystallization time of 12 h, crystallization temperature of 180 ℃ and molar ratio of H₂O/SiO₂ of 42, the P-S prepared has good morphology, uniform size and high relative crystallinity. The specific surface area is 460.24 m²/g, the specific surface area of the micropores is 275.25 cm²/g, and the total pore volume is 0.365 cm³/g, among which the micropore volume is 0.141 cm³/g, indicating that the prepared P-S has a high specific surface area and pore volume. S-S was prepared by adjusting the amount of TPAOH. When TPAOH/SiO₂ = 0.28, the prepared S-S had better morphology and higher relative crystallinity. The specific surface area is 413.83 m²/g, the specific surface area of the micropores is 304.77 cm²/g, the total pore volume is 0.183 cm³/g, and the micropore volume is 0.120 cm³/g. When preparing Na-Silicalite-1 molecular sieve, it is found that when NaOH/SiO₂ = 0.11, C-S can be synthesized. The specific surface area is 454.58 m²/g, the specific surface area of the micropores is 298.95 cm²/g, the total pore volume is 0.201 cm³/g, and the micropore volume is 0.119 cm³/g.

Under the test conditions of 298 K and 1bar, the adsorption capacity of P-S for CO₂ is 1.89 mmol/g, and that of S-S for CO₂ is 1.34 mmol/g, while the adsorption capacity of C-S for CO₂ is only 1.06 mmol/g, and the adsorption capacity of CO₂ is P-S > S-S > C-S.

Author Contribution

Wu Xianglian wrote the main manuscript text Aisha Nulahong was responsible for overseeing the experimentChangmin Tuo,Tiezhen Ren,Abulikemu Abulizi ,Jian Li and Fei Xu

Data Availability

Data is provided within the manuscript .

Fund Project: Supported by the 2022 Xinjiang Uygur Autonomous Region Natural Science Foundation Project (Joint Fund) (2002D01C378)

Declaration of Interest Statement：There are no conflicts of interest to declare.All authors disclosed no relevant relationships.

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You are reading this latest preprint version

Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experiment Materials and Methods

2.1 Materials and reagents

2.2 Sample preparation

2.2.1 Extraction of SiO₂ from fly ash

2.2.2 Preparation of P-S (plate-like Silicalite-1)

2.2.3 Preparation of S-S (spherical-like Silicalite-1)

2.2.4 Preparation of C-S (cross-type Na-Silicalite-1)

2.3 Characterization

3. Results and discussion

3.1 Investigation of the synthesis conditions of P-S

3.1.1 Effect of different template agents

3.1.2 Effect of crystallization time

3.1.3 Effect of crystallization temperature

3.1.4 Effect of water amount

3.2 Investigation of the synthesis conditions of S-S

3.2.1 Influence of TPAOH dosage

3.3 Investigation of the synthesis conditions of C-S

3.3.1 Influence of NaOH dosage

3.4 Characterization of different morphologies of Silicalite-1 zeolites and their CO₂ adsorption properties

3.4.1 Nitrogen adsorption-desorption

3.4.2 FT-IR analysis

3.4.3 CO₂ adsorption isotherm

4. conclusion

Declarations

Author Contribution

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1

Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experiment Materials and Methods

2.1 Materials and reagents

2.2 Sample preparation

2.2.1 Extraction of SiO2 from fly ash

2.2.2 Preparation of P-S (plate-like Silicalite-1)

2.2.3 Preparation of S-S (spherical-like Silicalite-1)

2.2.4 Preparation of C-S (cross-type Na-Silicalite-1)

2.3 Characterization

3. Results and discussion

3.1 Investigation of the synthesis conditions of P-S

3.1.1 Effect of different template agents

3.1.2 Effect of crystallization time

3.1.3 Effect of crystallization temperature

3.1.4 Effect of water amount

3.2 Investigation of the synthesis conditions of S-S

3.2.1 Influence of TPAOH dosage

3.3 Investigation of the synthesis conditions of C-S

3.3.1 Influence of NaOH dosage

3.4 Characterization of different morphologies of Silicalite-1 zeolites and their CO2 adsorption properties

3.4.1 Nitrogen adsorption-desorption

3.4.2 FT-IR analysis

3.4.3 CO2 adsorption isotherm

4. conclusion

Declarations

Author Contribution

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1

2.2.1 Extraction of SiO₂ from fly ash

3.4 Characterization of different morphologies of Silicalite-1 zeolites and their CO₂ adsorption properties

3.4.3 CO₂ adsorption isotherm