Realizing of ZSM-5 Microspheres with Enhanced Catalytic Properties Prepared from Iron Ore Tailings via Solid-Phase Conversion Method

doi:10.21203/rs.3.rs-2693988/v1

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Research Article

Realizing of ZSM-5 Microspheres with Enhanced Catalytic Properties Prepared from Iron Ore Tailings via Solid-Phase Conversion Method

https://doi.org/10.21203/rs.3.rs-2693988/v1

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published 02 Jun, 2023

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The comprehensive utilization of iron ore tailings (IOTs) not only resolved environmental problems, but also brought huge economic benefits. In this study, the synthetic route presented herein provides a novel method for the synthesis of ZSM-5 microspheres from IOTs. The effects of Si/Al molar ratios and the pH of the precursor solution on the formation of zeolite was evaluated by various analytical methods. The catalytic performance of the catalyst prepared by the solid-phase conversion method (denoted as MP-ZSM-5) was evaluated by methanol-to-propylene (MTP) reaction. Compared with the zeolite catalyst that synthesized via the conventional hydrothermal method (denoted as HM-ZSM-5), MP-ZSM-5 not only can achieve a long catalytic lifetime and good selectivity for propylene, but it can also demonstrate superior ability towards coke toleration. In addition, Kissinger-Akahira-Sunose (KAS) model is applied to the TG result to study the template removal process kinetics. The average activation energy values required for the removal of CTAB and TPABr are 201.11±13.42 and 326.88±16.91 KJ∙mol^-1, respectively. Furthermore, this result is well coupled with the model-free kinetic algorithms to determine the conversion and isoconversion of the TPABr and CTAB decomposition in ZSM-5, which serves as important guidelines for the industrial production process.

ZSM-5 microspheres

Iron ore tailings

Model-free kinetics

MTP

Solid-phase conversion

Methanol to propylene (MTP) conversion process is regarded as a promising technique for preparing propylene (Chen et al., 2018; Xing et al., 2019; Zhang et al., 2018). However, to achieve an efficient MTP conversion process, the development of durable catalysts with higher selectivity for propylene is of great significance (Li et al., 2017). Among the various materials proposed as a catalyst for MTP process, high-silica three-dimensional zeolite, ZSM-5, is highly attractive due to its good thermal stability, high internal surface area, and high acidity (Shu et al., 2022; Kamimura et al., 2012; Wu et al., 2014). However, rapid deactivation of the catalyst may occur during the operation due to the blockage of the intrinsically dominant micropores by the coke. As a result of this rapid deactivation of the catalyst, the derivation of propylene from MTP process is considered to be impractical. To mitigate this issue, introducing mesopores into the zeolite crystal can be considered an effective method to improve the diffusion kinetics of the process. Furthermore, such mesoporous zeolites exhibit relatively high acidity and hydrothermal stability, which can significantly increase their practical appeal for catalyzing the MTP reaction (Li et al., 2021; Petushkov et al., 2011; Rafael et al., 2021; Yue et al., 2008; Azhati et al., 2016). To generate mesopores in the zeolite crystals, various strategies such as (i) the use of surfactants (Fang and Hu, 2016) or polymers (Zhou et al., 2006) as soft templates, (ii) the employment of carbon or resin-beads (Chen et al., 2011) as hard templates, and (iii) performing desiliconization or dealumination (Yang et al., 2017) to reconstruct the aluminum and silicon sites, have been proposed. Even though desiliconization or dealumination is an effective strategy to introduce mesopores in zeolites, it was reported by Kortunov et al. (Kortunov et al., 2005) that the mesopores formed did not lead to the formation of an interconnected network according to the 3D-TEM and PFG-NMR analyses, and therefore such process contributed little to minimal effects on alleviating the diffusion limitations. Furthermore, the incorporation of mesopores into the zeolite crystals is generally considered to be a destructive process, whereby the zeolite framework may incur partial damage. This can decrease the crystallinity of the zeolite, which may ultimately lead to the collapsing of the zeolite skeleton. On the other hand, templating methods, i.e., hard or soft templating, have also shown generally successful preparations of mesoporous zeolites. However, such templating methods typically involve complex multi-steps and the use of high-cost mesoporous templates, which render these routes highly difficult to scale-up to the industrial level. Thus, the available synthesis routes for preparing mesoporous zeolites are still considered to be ineffective, which in turn indicates the urgency of developing an alternative synthesis strategy to efficiently prepare the mesoporous zeolites.

An alternative synthesis route that can achieve an in-situ assembly of zeolite nanocrystals is developed to form the mesoporous zeolite. The preparation of nano-sized zeolite crystals is highly attractive due to the nano-sized effects, whereby diffusion path lengths are reduced, and more acidic sites are exposed due to the increase in its external surface area (Hayasaka et al., 2007). Generally, zeolite nanocrystals are synthesized via the hydrothermal treatment of amorphous aluminosilicate gels in the presence of high concentrations of organic additives (Mohamed et al., 2005). However, this strategy involves stringent experimental conditions, and, as a consequence of using water as a solvent, it also leads to the production of polluted wastewater as a by-product. Furthermore, silicon source used in the synthesis of zeolites can be either (1) the costly organic silicate, such as silica sol, tetraethyl orthosilicate, or (2) inorganic silicate, such as water glass and fumed silica. Thus, due to the use of costly silicon source during the synthesis of zeolite nanocrystals, its large-scale industrial production becomes less viable and appealing. Therefore, there is a strong desire to develop a convenient, economical, and environmentally friendly synthesis route to prepare the zeolite nanocrystals on a large-scale basis. Recently, Xiao et al. successfully synthesized ZSM-5, Beta, MOR, SOD, and FAU type zeolites (Ren et al., 2012; Meng and Xiao, 2013; Zhang et al., 2011) via a facile procedure that involved a solvent-free method to completely avoid the presence of water solvent. Building on this motivating result, the convenient and environmental advantages of the reported synthesis strategy have led to the development of a solid phase conversion in this work to prepare ZSM-5 aggregates that are comprised of zeolite nanocrystals. On the other hand, iron ore tailing (IOT), which is the by-product formed during the ore smelting process, is a cost-effective material that can be used in the preparation of ZSM-5 zeolite due to its high silica and alumina contents. To date, numerous reports on the transformation of IOT to zeolite materials, i.e., zeolite A (Zhang et al., 2018), ZSM-5 (Zhang et al., 2019), and MCM-41 (Yang et al., 2015), are available, and these works demonstrate a novel concept of enhancing the commercial value of IOT.

Here, we have successfully synthesized ZSM-5 microspheres based on a solid-phase conversion method from IOT as reported in our previous work (Zhang et al., 2020). The influence of CTAB/SiO₂ molar ratio and the crystallization process and mechanism are reported and discussed in detail. Significantly, CTAB are not employed as templates to prepare mesoporous zeolites, but act as a kind of scaffold to confined the growth space of zeolite crystals. In this study, we have further discussed the effects of Si/Al molar ratios and the pH of the precursor solution on the formation of zeolite. To evaluate the catalytic performances of the as-prepared zeolite catalysts, ZSM-5 microspheres and ZSM-5 zeolite (prepared by conventional hydrothermal method) are used as the catalysts for MTP reaction. In this work, the calculation of the kinetics of the template agent removal process is carried out, which provides significant guidelines for the actual production process.

2.1 Materials and Chemicals

In this work, IOTs were retrieved from Kuancheng, Chengde (Northeastern China). The chemicals used in the preparation of ZSM-5 zeolite were sodium aluminate (NaAlO₂), sodium hydroxide (NaOH), tetrapropylammonium bromide (TPABr), cetyltrimethylammonium bromide (CTAB), hydrochloric acid (HCl) and sodium sulfate decahydrate (Na₂SO₄·10H₂O), which were obtained from Sinopharm Chemical Reagent. Ammonium nitrate (NH₄NO₃) was procured from Beijing Chemical Reagent Company.

2.2 Preparation of ZSM-5 zeolite

The synthesis procedures and the amounts of ZSM-5 microspheres and conventional microporous ZSM-5 zeolites employed in the preparation of ZSM-5 zeolite were identical to those in our previous report [25]. The only exception was the use of different Si/Al molar ratios and the precursor solution with different pH values. To achieve the H-type ZSM-5, the samples were ion-exchanged in a 0.1 M NH₄NO₃ solution three times at 80°C for 7 h. After which, the preparation was washed and filtered before calcinating the dried powder at 450°C for 4 h. The as-prepared ZSM-5 microsphere is denoted as MP-ZSM-5, while the reference ZSM-5 that was synthesized via the hydrothermal method is denoted as HM-ZSM-5.

2.3 Material Characterization

An Ultima IV diffractometer was used to measure the X-ray diffraction (XRD) patterns of the samples. A Cu Kα radiation (λ = 0.15406 nm) in the 2θ range from 5° to 90° was used, and the operating voltage and current were 40 kV and 40 mA, respectively. A JSM-6701F scanning electron microscope (SEM) was used to observe the size and the morphology of the sample at an accelerating voltage of 5 kV. The composition of the IOT was analyzed with a Shimadzu XRF-1800 X-ray fluorescence analyzer (XRF). Magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) was performed using an AVANCEIIIHD500 spectrometer at room temperature. The ²⁷Al MAS NMR was conducted at a resonance frequency of 59.592 MHz, and a rotor spun at 4.0 kHz. Radio-frequency field was 37.0 kHz, the recycling time was 2 s, and the reversing angle was 60°. Micromeritics ASAP 2460 surface area and porosity analyzer was used to determine the specific surface area and pore volume based on the Brunauer–Emmett–Teller (BET) method at -196°C. Fourier Transform Infrared (FT-IR) spectroscopy was conducted to detect the existence of tetrahedral TO₄ (T = Si or Al) bonding and formation of samples by using a Nicolet IS10 instrument. The recordings of the FT-IR spectra of pyridine absorption (Py-FTIR) were carried out on a Thermo Fisher Nicolet iS50 instrument. Firstly, 20 mg catalyst was ground and pressed into an in-situ cell, then it was heated to 400°C for 3 hours with a ramping rate of 10°C∙min^− 1, while vacuum pretreatment was conducted concurrently. After the vacuum pretreatment, the temperature of the cell was cooled down to room temperature, and the pyridine was adsorbed for 30 min. Then the desorption of pyridine started from room temperature to 150 and 350°C with a ramping rate of 10°C∙min^− 1. TG curves were recorded with a simultaneous thermal analyzer (SDT Q600) at various heating rates in the presence of airflow.

2.4 Catalytic Performance Evaluation

The MTP reaction was conducted in a micro fixed-bed reactor with a stainless-steel tube (dimension of 8 mm×400 mm), as shown in Fig.S1. In a typical test, 1 g catalyst (with 20 ~ 40 mesh size) was weighed and placed into the middle portion of the reactor. Then, the upper and lower parts of the reactor were filled with quartz wool. After ensuring the tightness of the detection device, the reactor was connected with the heating furnace, and the catalytic performance of the catalyst was evaluated. During the heating process, the temperature of the fixed-bed was firstly raised to 450° C, and the N₂ was used for purging the furnace. After stabilizing for a period of time, N₂ flow was switched off, and methanol feeding began. The weight hourly space velocity (WHSV) was 9.8 h^− 1, and the product was collected every 40 min. The volumes of the gas, cooling gas, and liquid products of the ice-water mixture were calculated using a drainage gas gathering method. The methanol conversion and the product selectivity were analyzed using an off-line analysis method. The composition of the gas products was analyzed on-line with gas chromatography (Bruke GC-450), and the methanol in the liquid products was detected by Agilent 6820 chromatography. Methanol conversion (X) and selectivity of the product (S) are defined as follows:

$$\text{X}=\frac{{m}_{MeOH}^{i}-({m}_{MeOH}^{o}+\frac{32}{23}{m}_{DEM}^{o})}{{m}_{MeOH}^{i}}\times 100\%$$

$$\text{S}=\frac{n\times {m}_{product}^{o}}{{m}_{MeOH}^{i}-({m}_{MeOH}^{o}+\frac{32}{23}{m}_{DEM}^{o})}\times 100\text{\%}$$

Where m is the number of carbon atoms corresponding to C_mH_n, n is the number of carbon atoms, i and o indicate the components at the inlet and outlet of the reactor, respectively.

3.1 Effects of Si/Al Ratio on the formation of MP-ZSM-5

ZSM-5 zeolite is a type of zeolite with a high Si/Al ratio that can be tailored made. In general, the Si/Al ratio in ZSM-5 zeolite varies in a wide range of more than 10 (Liu et al., 2019). In this study, the effects of Si/Al ratio (10 ~ 100) on the crystallization and morphology of MP-ZSM-5 are investigated in detail. Figure 1 shows the XRD patterns of MP-ZSM-5 with different Si/Al ratios. Obvious characteristic diffraction peaks, i.e., 2θ = 7.99°, 9.07°, 23.08°, 23.88°, and 24.36°, that belong to ZSM-5 zeolite can be observed in the XRD spectra of MP-ZSM-5 with Si/Al molar ratio between 10 ~ 100. This result indicates that ZSM-5 zeolite can be synthesized with a Si/Al molar ratio that is in the range of 10 ~ 100. However, as the Si/Al ratio increases, the crystallinity of MP-ZSM-5 increases initially and then eventually remains unchanged. The lower crystallinity of ZSM-5 prepared from raw material with low Si/Al may be attributed to the following factor: A raw material with low Si/Al ratio possesses high aluminum content, which makes the formation of a stable five-membered ring structure during the depolymerization of the aluminum material highly challenging. As such, it becomes unfavorable to form ZSM-5 zeolite from raw material with low Si/Al, which in turn results in the poor crystallinity of the sample.

The SEM images of the synthesized samples are presented in Fig. 2, whereby it can be seen that Si/Al ratio exert a significant influence on the morphology of MP-ZSM-5. At a Si/Al ratio of 10, the products are mainly amorphous substances, with a small amount of ZSM-5 zeolite crystals. At a higher Si/Al ratio of 20, the as-obtained MP-ZSM-5 reveals prismatic morphology, whereby crystals are formed due to the stacking effect. As the Si/Al ratio increases to more than 30, MP-ZSM-5 starts to transforms into microspheres, and the morphology of the microspheres becomes more uniform with the increase in Si/Al ratio. This may be due to the different nucleation rates of zeolite with different Si/Al ratios during the crystallization process, because the nucleation starts from the stirring and aging process. The nucleation rate can be calculated according to the Von Weimarn equation (Weiser et al., 2002), as shown in Eq. (3).

V = D/S×O×(Q-L) = D/S×O×P (3)

Where S is the thickness of the adherent film, D is the diffusion coefficient, Q is the total concentration of the substance to be precipitated, O is the extent of surface, P = Q-L is the absolute supersaturation., and L is the solubility.

Therefore, the quality of Si in the raw materials is a constant value, and the absolute value and concentration of Al absolute supersaturation are variables. According to Eq. (3), with the increase in the Si/Al ratio, the supersaturation of the solution decreases, which leads to the reduction in the size of the crystal particle. Furthermore, the diameter of MP-ZSM-5 is about 3–6 um observed by TEM image (Fig. 2G), which are composed of many nanoparticles in size of about 50–100 nm (Fig. 2H), and the HRTEM image (Fig. 2I) of the selected region shows the distinct lattice fringes throughout MP-ZSM-5, proving that the MP-ZSM-5 crystals grow continuously along the certain directions in the conversion of solid material.

3.2 Effect of precursor solution pH on the formation of MP-ZSM-5

During the preparation of ZSM-5, the presence of OH^- can improve the rate of zeolite crystal growth, and therefore this may lead to a change in its morphology (Nada et al., 2017). However, OH^- may inhibit the nucleation process, or lead to the formation of other phases. Thus, the pH of the precursor solution may affect the formation of ZSM-5. As such, the effects of precursor solution pH on the formation of ZSM-5 are investigated between a pH range of 7 to 12 in order to determine the optimal crystallization of ZSM-5.

Figure 3 shows the XRD spectra and ²⁷Al MAS NMR spectra of MP-ZSM-5 prepared with precursor solutions in different pH values. All samples exhibit obvious typical diffraction of MFI structure at pH > 7, which indicates that MP-ZSM-5 is well crystallized, and no hetero-crystal is formed. It is noteworthy that when the pH of the solution is 12, the gel cannot be formed as the silicon and aluminum species are dissolved into the solution, which ultimately leads to the inability to form ZSM-5. As shown in Fig. 3 (A), at pH = 7, no characteristic peak that belongs to ZSM-5 can be observed. Such a result is due to the neutrality of the solution that does not promote the dissolution of silicon and aluminum in the raw material, which results in no ZSM-5 being formed. It is also noticed the ²⁷Al MAS NMR of samples (pH = 7) reveals a major peak δ = 80 ppm, which is due to the tetrahedral-coordinated Al (Zhang et al., 2018), which corresponds to the added aluminum source NaAlO₂. As shown in Fig. 3 (B), as pH > 7, one peak centered at 54 ppm is observed in the samples, which can be ascribed to the tetrahedral Al in the ZSM-5 zeolite framework. The Al distributions in MP-ZSM-5 are observed using the energy-dispersive X-ray spectroscopy (EDS) result (Fig. 4). According to the EDS mappings, it can be observed that the Al element is evenly distributed across the crystal, which further indicates that Al is embedded in the ZSM-5 framework.

The elemental yield of Si and Al in MP-ZSM-5 prepared from precursors with different pH values are shown in Table 1. It is observed that the yield of Si and Al decreases with the increase in the pH value of the precursor. Such a trend may be a result of the increase in the dissolution rate of Si and Al in raw materials with the increase in the alkalinity of the precursor, which prevents the participation of these dissolved Si and Al in the crystallization reaction, and therefore these elements remain in the solvent. This reason can also provide a good explanation for the higher yield of silicon and aluminum in the solvent-free method than that in the hydrothermal method (Zhang et al., 2020).

Table 1

Yield of Si and Al at different pH values
pH values	7	8	9	10	11
Average yield of Si and Al, %	99.2	93.3	89.6	90.2	86.1

The SEM results of MP-ZSM-5 synthesized under different pH values (Fig. 4) suggest that the morphology of the product can be significantly influenced by the pH of the precursor. There is no ZSM-5 crystal in the product at pH = 7, which is consistent with the XRD results. As the pH of the precursor increases from 8 to above 9, the morphology of the MP-ZSM-5 changes from a coffin-like structure to a microsphere. It was reported by Ban et al (Ban et al., 2005). that the dissolution of the silica gel and sequential deposition of Si and Al in the solution under different pH values could lead to a difference in the crystallization process. At a low pH, the synthesized zeolite possesses a long plate-like morphology, which grows into a long rod shape as the pH value increases. The main reason for this difference is that the supersaturation of silicon and aluminum components is different in various pH values, which can affect the growth rate along the b-axis of the ZSM-5. As such, this can result in a morphological change in the zeolite. At pH = 8, the typical coffin-like morphology of the MP-ZSM-5 indicates low solubility and nucleation rate of aluminosilicate under weak alkali conditions, which leads to the preferential formation of large grains (usually single crystals). Between pH = 9 ~ 11, the morphology of MP-ZSM-5 gradually transforms from coffin-like crystals to microspheres. Figure 4D1 shows that the formation of ZSM-5 is due to the accumulation of flaky grains with obvious gaps between the small grains. This result indicates that using a precursor with an appropriate pH value not only can effectively reduce the grain size of the zeolite, but it can also promote the formation of mesoporous structure (mesopores are formed due to the voids generated by the accumulation of small grains). The textural properties result of MP-ZSM-5 further prove that ZSM-5 microspheres contain mesopores. It can be seen from Fig. 5A that the samples displayed type-IV isotherms that contain a hysteresis loop when pH ≥ 9, which was the characteristic of capillary condensation in mesoporous channels. Moreover, the broken microsphere, as presented in Fig. 4D2, reveals the existence of macropores, as marked by the rectangle box. The BJH adsorption pore distributions of MP-ZSM-5 (Fig. 5B) also proof that ZSM-5 microspheres contained a certain amount of macropores (> 50 nm). Our previous studies [25] have shown that macropores are formed by calcination of CTAB. Thus, the mesopores or macropores that are generated based on the accumulation of flaky grains and the removal of CTAB can improve the diffusion efficiency of propylene in the zeolite. Based on Table S1, MP-ZSM-5 contains a certain amount of Fe, and according to the EDS mapping of Fe, it can be clearly observed that there is a uniform distribution of Fe across the surface of zeolite (Fig. 4H). FTIR spectra of the samples (Fig. S2) indicates that a new absorption peak appears at 3670 cm^-1 as the synthesis time exceeds 12 hours, which belongs to the vibration of (FeOH) that is loaded on the zeolite. Thus, all results show that the residual iron in the raw material is present in the zeolite, which could promote the catalytic performance of zeolite (Meliancabrera et al., 2006).

3.3 Analysis of the template removal kinetics

Before using MP-ZSM-5 as a catalyst in MTP reaction, it is necessary to first remove the organic template, i.e., CTAB and TPABr. One of the most common methods in the removal of the template is through a high-temperature calcination process (Vyazovkin et al., 1999), whereby the determination of the optimal temperature and heating duration is essential in fully eliminating the surfactant from the product.

As such, MP-ZSM-5 are subjected to the calcination process at different heating rates, and the XRD patterns of these samples are presented in Fig. 6A. According to Fig. 6A, the crystallinity of the product remains relatively unchanged when the calcination process is conducted at a heating rate of 5 ℃/min to 550 ℃ for 4 hours. However, as the heating rate increases, it can be observed that there is a gradual decrease in the crystallinity of MP-ZSM-5. In particular, at a high heating rate of 20 ℃/min, the crystallinity of the as-prepared MP-ZSM-5 is less than half of that of the original sample, which clearly suggests that the zeolite framework structure may collapse when subjected to a high heating rate. Furthermore, the difference in the MP-ZSM-5 calcinated at various heating rates can also be observed visually by their color change. As shown in Fig. 6B, obvious color changes across the samples can be observed in their respective digital photographs. The color of MP-ZSM-5 remains unaltered when being calcinated with a heating rate of 5 ℃/min. On the other hand, when the heating rate is 10 ℃/min, the color of MP-ZSM-5 turns gray initially, and then it converts back to the original white powder. At the heating rate of 20℃/min, the color of MP-ZSM-5 rapidly turns black and remains unchanged. The reason for this phenomenon may be due to the rapid decomposition of a large number of organic guest molecules at a very high heating rate, i.e., exceeds 10 ℃/min, which results in the accumulation of a large quantity of gas product molecules in the zeolite cage. This leads to the sudden increase in the pressure, whereby once the pressure reaches a certain level, the framework of zeolite will be destroyed. To explore the template removal process kinetics, Kissinger-Akahira-Sunose (KAS) model is applied to the TG data. The analysis is based on an isoconversional method, which can calculate the kinetic parameters without understanding the reaction mechanism. In the isoconversional method, reaction rate is only a function of temperature at a constant conversion, while reaction kinetic does not exhibit any relationship with heating rate.

The removal of the template from MP-ZSM-5 calcinated at heating rates of 5, 10, and 20 ℃/min is also investigated using thermogravimetry, and the results are shown in Fig. 6C. Four typical mass losses can be observed in all three TG curves: (ⅰ) The first mass loss below 200°C can be attributed to the thermodesorption of physically adsorbed water. (ⅱ) The second mass loss below 200 ~ 400°C can be ascribed to the decomposition of CTAB. (ⅲ) The last mass loss at 400 ~ 550°C can be ascribed to the decomposition of TPABr. (ⅳ) The residual template decomposition may occur at 550 ~ 700°C. The collapse of the zeolite skeleton is mainly caused by the decomposition of CTAB and TPABr. Therefore, only these two stages, i.e., 200 ~ 400°C and 400 ~ 550°C, are investigated in this study.

Figure 6D and E presents the conversion curve as a function of the temperature during the decomposition of CTAB and TPABr under various heating rates. The conversion percentage can be calculated based on Eq. (4).

$${\alpha }=\frac{{m}_{\theta }-{m}_{t}}{{m}_{\theta }-{m}_{f}}$$

where m_θ is the initial mass of the sample, m_t is the mass of the sample that varies with time or temperature, and m_f is the final mass of the sample at the end of the study.

A series of conversion percentages, i.e., α = 10, 20, 30, 40, 50, 60, 70, 80, and 90%, are chosen to determine the activation energy (E_k) of the template removal process, as described in the supporting information. By using Eq. (S9), E_k can be determined based on the slope of the $ln\frac{{\beta }}{{T}^{2}}$ versus $\frac{1}{T}$ plot. As shown in Fig. 6F and H, it can be observed that the obtained experimental data can be fitted perfectly (R > 0.98) for the CTAB and TPABr decomposition stages. Based on this result, the activation energy of the removal of templates can be estimated, and the results are presented in Fig. 6G. The average activation energies needed to remove CTAB and TPABr based on a 0 ~ 90% conversion as suggested by the KAS model are 201.11 ± 13.42 and 326.88 ± 16.91 KJ∙mol^− 1, respectively.

The obtained activation energy curve of the CTAB and TPABr removal process can be used in conjugation with the model-free kinetic algorithms to determine the conversion and iso-conversion parameters at different stages of the template decomposition. Table 2 shows the various parameters of the CTAB and TPABr removal process as a function of the conversion and time. Such results can serve as important guidelines for the industrial production process.

Table 2

Temperature to removal of CTAB and TPABr of MP-ZSM-5 at different events
Time/min	α/%
Time/min	10	30	50	70	90
Temperature range: 180 ~ 280 ℃
10	214.5	230.8	240.0	249.1	268.5
20	208.7	224.9	234.2	243.2	263.3
30	205.3	221.6	230.8	239.9	260.4
40	202.9	219.2	228.5	237.5	258.3
50	201.1	217.4	226.7	235.7	256.7
60	199.6	215.9	225.2	234.3	255.4
Temperature range: 380 ~ 500 ℃
10	-	382.3	392.5	403.6	437.1
20	-	-	381.3	389.1	418.4
30	-	-	-	380.2	407.7
40	-	-	-	-	400.3
50	-	-	-	--	394.7
60	-	-	-		390.2

3.4 Catalytic Performance of the Catalysts

Py-FTIR is used to study the type of acidic sites in MP-ZSM-5 and in HM-ZSM-5 (synthesized by hydrothermal method), and the result is shown in Fig. 7. According to the result, two bands located at around 1455 and 1540 cm^− 1 can be observed for both samples, which can be ascribed to Lewis acid (L acid) and Brønsted acid (B acid), respectively (Lou et al., 2016). Such a result suggests that L acid and B acid are both present on the surfaces of MP-ZSM-5 and HM-ZSM-5. The other peak located at 1490 cm^− 1 is also observed due to the mixed vibration of pyridine molecules that are adsorbed on these two types of acid sites (Zhang et al., 2019). Interestingly, when compared to HM-ZSM-5, MP-ZSM-5 possesses a higher concentration of B acid sites, which could be attributed to the following two factors: (ⅰ) As indicated in Fig. S3, MP-ZSM-5 possesses more Al (86%) in its framework at the channel intersection than HM-ZSM-5 (80%) according to the ²⁷Al MAS NMR result, despite using a raw material with Si/Al ratio of 50 in both methods. (ⅱ) The microporous channels of MP-ZSM-5 are connected through the mesopores, as shown in Fig. S4 and Table S2, which can effectively enhance the absorbability of the zeolite catalyst towards the pyridine molecules. As a result, MP-ZSM-5 contains a higher amount of B acid sites than HM-ZSM-5, which suggests that MP-ZSM-5 should exhibit better catalytic performance in MTP reaction as compared to HM-ZSM-5.

The MTP catalytic performances of MP-ZSM-5 and HM-ZSM-5 prepared from a raw material with Si/Al ratio of 50 are investigated at 450°C under atmospheric pressure, with a WHSV of 9.8 h^− 1. Figure 8 shows the methanol conversion percentage as a function of time on stream of MP-ZSM-5 and HM-ZSM-5. At the initial 7 h, a 100% methanol conversion percentage is observed for both catalysts, however, the catalytic lifetime for which methanol conversion percentage is above 90% varies with the time on stream. It is noteworthy that during the MTP reaction, deactivation of zeolite catalyst is caused by the deposition of coke (Wodarz et al., 2020). Since the size of the ZSM-5 zeolite is in the range of 0.51 ~ 0.56 nm (Peng et al., 2018), diffusion of macromolecular material can be greatly limited. As the formation of carbonaceous precursors such as polycyclic aromatic hydrocarbons in the pore channel is difficult, the deposition of carbon occurs mainly on the outer surface of the zeolite catalyst. Thus, as shown in Fig. 8, the HM-ZSM-5 prepared via the conventional hydrothermal method shows a shorter catalytic lifetime of 18.7 h, with methanol conversion percentage dropping to below 90%. Interestingly, for the as-prepared MP-ZSM-5, a longer catalytic lifetime of 36.0 h can be obtained under similar conditions. This extended catalytic lifetime of MP-ZSM-5 can be attributed to the mesoporous structure that is generated by the accumulation of flaky grains crystals (Fig. 4). It is commonly accepted that a larger mesopore volume translates to better diffusion properties. Thus, light olefin can escape from the zeolite channels more easily, which inhibits the secondary reactions. Furthermore, according to the TG curves of the zeolite catalysts after the operation (Fig. S5), a greater mass loss due to the combustion of retained coke species is observed for MP-ZSM-5 (15.6%) as compared to that for HM-ZSM-5 (9.4%), which indicates that MP-ZSM-5 has superior tolerance to coke, and there it is able to exhibit a longer catalytic lifetime.

Figure 9 (A) and (B) reveals the change in the product selectivity of MP-ZSM-5 and HM-ZSM-5 over time on stream, respectively. It can be observed that both catalysts exhibit similar product selectivity trends: (i) The selectivity for ethylene and propylene decrease with time on stream. (ii) The selectivity of C₅⁺ species increases with time on stream. (iii) The selectivity of C₁ ~ C₄ alkanes shows negligible changes with time on stream. As the MTP reaction proceeds, the amount of carbon deposition increases gradually, and this will cause the selectivity of propylene, which is the key product of the MTP process, decreases for both catalysts. On the other hand, the cracking reaction of high-carbon olefins is limited by the contents of the coke deposit, which results in an increase in the selectivity of C₅⁺ species. Furthermore, the propylene/ethylene ratio of the catalyst, i.e., P/E, is an important index in the MTP process. It is commonly accepted that a higher propylene yield can be achieved in the recirculation process at a higher P/E ratio (Ding et al., 2017). Table S3 shows that the P/E ratio of MP-ZSM-5 (4.2) is considerably higher than HM-ZSM-5 (2.8), which leads to a higher selectivity for propylene by MP-ZSM-5 (43.7%) than that for HM-ZSM-5 (38.6%). This is because the incorporation of mesopores can improve the diffusion performance of the catalyst. This allows propylene, as the main product, to achieve an efficient diffusion process due to a shortened diffusion path and enhancing its removal. This can help the proceeding of the reaction in the direction that is conducive to the formation of propylene to a certain extent. Thus, according to the collective results, the ZSM-5 microsphere prepared by the solid-phase conversion method is a promising catalyst for MTP conversion.

ZSM-5 microsphere is successfully prepared based on a solid-phase conversion method by deploying IOTs as the silicon source. After which, the as-prepared ZSM-5 is then investigated as a potential catalyst in the MTP reaction. The following conclusion can be drawn from the results:

(a) Si/Al ratio and pH values of the precursor solution exert significant influence on the morphology of the catalyst. Based on the results, the optimum parameters for the formation of catalyst are Si/Al > 30 and at pH = 9 ~ 11. Moreover, the Al content in the MP-ZSM-5 framework is higher than that of HM-ZSM-5, which suggests that more Al from the raw material can be synthesized into the zeolite framework by the solid-phase conversion method as compared to that by the conventional hydrothermal method.

(b) The kinetics study of the template removal process reveals that the activation energies needed for the removal of CTAB and TPABr are 201.11 ± 13.42 and 326.88 ± 16.91 KJ∙mol^− 1, respectively. When coupled with the model-free algorithms, the conversion and iso-conversion parameters at different stages of CTAB and TPABr decomposition are estimated, which presents important guidelines for the industrial production process.

(c) When compared with the ZSM-5 zeolite synthesized via the conventional hydrothermal method (HM-ZSM-5), MP-ZSM-5 shows a significantly enhanced catalytic activity during MTP reaction, whereby it not only demonstrates a long catalytic lifetime and high propylene selectivity, but it also exhibits superior ability towards coke toleration. This phenomenon may be due to its intrinsic micro-porosity and the unique hierarchical porous structure that arises from the accumulation of flaky grains and the removal of CTAB.

Ethical Approval: This work was obtained from the Academic Committee of School of Materials Science and Engineering.

Competing interests: The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Consent to Participate: All authors (Peng Zhang, Xingyue Chen, Yang Wang, Wei Peng, Zhifeng Rena, Yihong Li, Yibo He and Baoshuai Chu) are consented to participate.

Consent to Publish: Consent to Participate: All authors (Peng Zhang, Xingyue Chen, Yang Wang, Wei Peng, Zhifeng Rena, Yihong Li, Yibo He and Baoshuai Chu) are consented to publish.

Authors' contributions: Peng Zhang and Yihong Li wrote the main manuscript text and Xingyue Chen, Zhifeng Ren, Yibo He, Baoshuai Chu, Yang Wang and Wei Peng prepared all figures. All authors reviewed the manuscript.

Funding: This work was supported by Fundamental Research Program of Shanxi Province (No. 202103021223277), Taiyuan University of Science and Technology Doctoral Research Fund (No. 20212025), National Natural Science Foundation of China (No. 51904278) and Shanxi Excellent Doctor Award Fund Project (No. 20222080). The authors would like to thank Qinya Li from Shiyanjia lab for support in the ²⁷Al MAS NMR analysis (www.shiyanjia.com).

Availability of data and materials: All data generated or analysed during this study are included in this published article and its supplementary information files.

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Realizing of ZSM-5 Microspheres with Enhanced Catalytic Properties Prepared from Iron Ore Tailings via Solid-Phase Conversion Method

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Abstract

Figures

1 Introduction

2 Experimental Section

2.1 Materials and Chemicals

2.2 Preparation of ZSM-5 zeolite

2.3 Material Characterization

2.4 Catalytic Performance Evaluation

3 Results And Discussion

3.1 Effects of Si/Al Ratio on the formation of MP-ZSM-5

3.2 Effect of precursor solution pH on the formation of MP-ZSM-5

3.3 Analysis of the template removal kinetics

3.4 Catalytic Performance of the Catalysts

4 Conclusion

Declarations

References

Supplementary Files

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