Realizing of ZSM-5 microspheres with enhanced catalytic properties prepared from iron ore tailings via solid-phase conversion method

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 prolongs catalytic lifetime from 18.7 to 36.0 h but also has higher selectivity for propylene by MP-ZSM-5 (43.7%) than that for HM-ZSM-5 (38.6%). 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.


Introduction
Methanol to propylene (MTP) conversion process is a promising technique for preparing propylene Xing et al. 2019;Zhang et al. 2018a, b), and the development of durable catalysts with higher selectivity for propylene is crucial (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 operation due to coke blocking intrinsic dominant micropores. Therefore, introducing mesopores into the zeolite crystal can be considered an effective method to improve the diffusion kinetics of the process. Moreover, such mesoporous zeolites exhibit relatively high acidity and hydrothermal stability, which can significantly increase their practical appeal for catalyzing the MTP reaction (Jia et al. 2018;Petushkov et al. 2011;Rafael et al. 2021;Yue et al. 2008;Azhati et al. 2016).
Various strategies have been proposed to generate mesopores in the zeolite crystals. Surfactant (Fang and Hu 2006) or polymer are used as soft templates, while carbon or resin-beads (Chen et al. 2011) are employed as hard templates. Desiliconization or dealumination is another strategy that can introduce mesopores in zeolites (Yang et al. 2017). However, Kortunov et al. (Kortunov et al. 2005) reported that the mesopores formed did not lead to the formation of an interconnected network according to the 3D-TEM and PFG-NMR analyses. Therefore, such process contributed little to alleviating the diffusion limitations. Incorporating mesopores into the zeolite crystals is generally considered destructive, whereby the zeolite framework may incur partial damage, resulting in decreased crystallinity and eventual collapse of the skeleton. Templating methods have also shown successful preparations of mesoporous zeolites.

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However, these methods typically involve complex multi-steps and the use of high-cost mesoporous templates, rendering them difficult to scale-up to the industrial level. Thus, alternative synthesis strategies are urgently needed to efficiently prepare mesoporous zeolites (Salavati-Niasari 2004).
The synthesis of zeolite nanocrystals has been an attractive route for producing mesoporous zeolites due to their nano-sized effects, which reduce diffusion path lengths and expose more acidic sites on the 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 leads to the production of polluted wastewater as a by-product due to the use of water as a solvent. Furthermore, silicon sources used in the synthesis of zeolites can be either (1) costly organic silicate such as silica sol, tetraethyl orthosilicate, or (2) inorganic silicate such as water glass and fumed silica. To overcome these challenges, Xiao et al. reported a facile solvent-free synthesis route that can produce ZSM-5, Beta, MOR, SOD, and FAU type zeolites without the presence of water solvent (Ren et al. 2012;Meng and Xiao 2013;Zhang et al. 2011). In addition, the solid-state conversion method has also been widely used in the manufacturing of perovskite devices (Wang et al. 2020;Xu et al. 2023;Cai et al. 2021;Mo et al. 2022). This approach has led to the development of a solid phase conversion method to prepare ZSM-5 aggregates consisting of zeolite nanocrystals. Additionally, iron ore tailing (IOTs), a by-product formed during 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. Several reports on the transformation of IOTs into zeolite materials, such as zeolite A (Zhang et al. 2018a, b), ZSM-5 (Zhang et al. 2019a, b), and MCM-41 (Yang et al. 2015), demonstrate the commercial value of this material. Overall, these developments highlight the potential for creating efficient and environmentally friendly synthesis routes to produce mesoporous zeolites using IOTs.
Here, we have successfully synthesized ZSM-5 microspheres using a solid-phase conversion method from IOTs as reported in our previous work . We have discussed in detail the influence of CTAB/SiO 2 molar ratio and the crystallization process and mechanism. Significantly, CTAB is not employed as templates to prepare mesoporous zeolites but act as a scaffold to confine 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.

Materials and chemicals
In this work, IOTs were retrieved from Kuancheng, Chengde (Northeastern China), Fig. S1 shows that the aluminosilicates present in the IOT are crystalline in nature, with quartz accounting for 36.79% of the total content. The chemicals used in the preparation of ZSM-5 zeolite were sodium aluminate (NaAlO 2 ), sodium hydroxide (NaOH), tetrapropylammonium bromide (TPABr), cetyltrimethylammonium bromide (CTAB), hydrochloric acid (HCl, 37 wt. % in water), and sodium sulfate decahydrate (Na 2 SO 4 ·10H 2 O), which were obtained from Sinopharm Chemical Reagent. Ammonium nitrate (NH 4 NO 3 , AR, 99%) was procured from Beijing Chemical Reagent Company. All chemicals were used without further purification.

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 ). 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 NH 4 NO 3 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.

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 27 Al MAS NMR was conducted at a resonance frequency of 59.592 MHz, and a rotor spun at 4.0 kHz. Radiofrequency 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 TO 4 (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 h 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. The structure and properties of samples were characterized by ultraviolet Raman spectroscopy (UV-Raman) spectroscopy using a LabRAM HR Evolution with 325 nm laser wavelength.

Catalytic performance evaluation
The MTP reaction was conducted in a micro fixedbed reactor with a stainless-steel tube (dimension of 8 mm × 400 mm), as depicted in Fig. S2. In a typical test, 1 g catalyst (with 20 ~ 40 mesh size) was weighed and placed into the middle portion of the reactor. The upper and lower sections of the reactor are then filled with quartz wool to ensure proper operation of the detection device. Once the reactor was connected to the heating furnace, its catalytic performance was evaluated during the heating process. Initially, the temperature of the fixed-bed was raised to 450 °C, and N 2 gas was used for purge the furnace. After stabilizing for a period of time, N 2 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. To calculate the volumes of the gas, cooling gas, and liquid products of the ice-water mixture, a drainage gas gathering method is employed. The methanol conversion and the product selectivity were analyzed using an offline analysis method. Online gas product composition analysis is conducted using gas chromatography (Bruke GC-450), while Agilent 6820 chromatography is utilized to detect methanol in the liquid products. Methanol conversion (X) and selectivity of the product (S) are defined as follows: where m is the number of carbon atoms corresponding to C m H n , n is the number of carbon atoms, i and o indicate the components at the inlet and outlet of the reactor, respectively.

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 . 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 1A shows the XRD patterns of MP-ZSM-5 with different Si/Al ratios. Obvious characteristic diffraction peaks associated with the typically well-crystallized ZSM-5 lattice structure (PDF code: 00-039-0225) can be observed in the XRD spectra of MP-ZSM-5 with Si/Al molar ratio between 10 and 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 structure of MP-ZSM-5 with various Si/Al ratios was studied using UV-Raman spectrum excited at 325 nm. Figure 1B shows that the UV-Raman spectrum of all samples exhibited characteristic bands of the MFI structure at 375 cm −1 , which is assigned to the characteristic bending mode of 5-membered rings in ZSM-5 zeolite. This suggests the formation of ZSM-5 crystals in all samples (Yu et al. 2001). The characteristic bands belonging to ZSM-5 have low strength when Si/Al < 30, which is consistent with those of XRD analyses.
The SEM images of the synthesized samples are presented in Fig. 2, whereby it can be seen that Si/Al ratio exert 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 transform 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, as shown in Eq. (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 µm observed by TEM image (Fig. 2G), which are composed of many nanoparticles in size of about 50-100 nm (Fig. 2H).

Effect of precursor solution pH on the formation of MP-ZSM-5
During ZSM-5 preparation, OH − can enhance zeolite crystal growth rate and change its morphology (Nada and Larsen 2017). However, OH − may also inhibit nucleation or lead to other phase formation. Therefore, the pH of the precursor solution can affect ZSM-5 formation. The effects of precursor solution pH on the formation of ZSM-5 are investigated within a pH range of 7 to 12 to determine the optimal crystallization conditions for ZSM-5. Figure 3 shows the XRD spectra and 27 Al MAS NMR spectra of MP-ZSM-5 prepared with precursor solutions at different pH values. All samples exhibit typical diffraction of MFI structure at pH > 7, indicating 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, resulting in no ZSM-5 formation. At pH = 7, no characteristic peak belonging to ZSM-5 can be observed due to neutrality of solution not promoting dissolution of silicon and aluminum in the raw material, which results in no ZSM-5 being formed. The 27 Al MAS NMR of samples (pH = 7) reveals a major peak δ = 80 ppm, which is due to the tetrahedral-coordinated Al (Zhang et al. 2018a, b), corresponding to added aluminum source NaAlO 2 . As pH > 7, one peak centered at 54 ppm is observed in samples ascribed to tetrahedral Al in 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, Al element is evenly distributed across the crystal further indicating that Al is embedded in 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 ).
The SEM images presented in Fig. 4 indicate that the morphology of Mes-ZSM-5 product is significantly influenced by the pH value of the precursor synthesis solution. When the pH of the solution is 7, there is no ZSM-5 crystal in the product. As the precursor pH increases from 8 to above 9, the morphology of Mes-ZSM-5 changes from coffin-shaped to microspheres, which is similar to previous research results (Lermer et al. 1985). Ban et al. (Ban et al. 2005) found that when the pH value of the solution was different, there would be differences in the dissolution of silica gel and the sequential deposition of silicon and aluminum in the crystallization process. When the alkalinity was low, the synthesized zeolite was in a long plate shape, and when the alkalinity was high, it was in a long rod shape. The main reason for this difference is that due to different alkalinity, the supersaturation degree of silicon and aluminum components is different, which affects the growth rate of ZSM-5 zeolite along b-axis and then changes its morphology. When pH = 8, Mes-ZSM-5 has a typical coffin-shaped ZSM-5 crystal morphology, indicating that under weak alkaline conditions, the solubility and nucleation rate of silicoaluminates  are low, so it is easier to form large crystals (usually single crystals). When pH = 9, Mes-ZSM-5 has a diameter of about 3 ~ 4 µm microspheres. By marking out a rectangular area in Fig. 4, it can be seen that microspheres are formed by stacking nanosheet-like small crystals with obvious gaps between them. This indicates that an appropriate pH value of the solution can not only effectively reduce the particle size of zeolite but also promote the generation of mesoporous structure (the gaps formed by stacking small crystals) to improve diffusion efficiency. 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. 4, 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 ) 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. S3) indicates that a new absorption peak appears at 3670 cm −1 as the synthesis time exceeds 12 h, 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).

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 and Wight 1999), whereby the determination of the optimal temperature and heating duration is essential in fully eliminating the surfactant from the product. MP-ZSM-5 samples were subjected to the calcination at different heating rates, and their XRD patterns are presented in Fig. 6A. The crystallinity of the product remains relatively unchanged when the calcination process is conducted at a heating rate of 5 ℃·min −1 to 550 ℃ for 4 h. However, as the heating rate increases, there is a gradual decrease in the crystallinity of MP-ZSM-5. In particular, at a high heating rate of 20 ℃·min −1 , the crystallinity of the as-prepared MP-ZSM-5 is less than half of that of the original sample, which suggests that the zeolite framework structure may collapse when subjected to a high heating rate. The difference in MP-ZSM-5 calcinated at various heating rates can also be observed visually by their color change (Fig. 6B). Obvious color changes across the samples can also 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 −1 . On the other hand, when the heating rate is 10 ℃ min −1 , the color of MP-ZSM-5 turns gray initially and then converts back to the original white powder. At a heating rate of 20 ℃·min −1 , the color of MP-ZSM-5 rapidly turns black and remains unchanged. 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 −1 , 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 it once 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 −1 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 present the conversion curve as a function of the temperature during the decomposition of CTAB 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 β T 2 versus 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.

Catalytic performance of the catalysts
Py-FTIR was used to study the type of acidic sites in MP-ZSM-5 and HM-ZSM-5, and the result is shown in Fig. 7. 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). This 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. 2019a, b). Interestingly, when compared to HM-ZSM-5, MP-ZSM-5 possesses a higher concentration of B acid sites. This could be attributed to the following two factors: (i) As indicated in Fig. S4, MP-ZSM-5 possesses more Al (86%) in its framework at the channel intersection than HM-ZSM-5 (80%) according to the 27 Al 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. S5 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%  (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. S6), 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 9A 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 C 5 + species increases with time on stream. (iii) The selectivity of C 1 ~ C 4 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 C 5 + 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.

Conclusion and prospect
ZSM-5 microspheres were successfully synthesized using a solid-phase conversion method by deploying iron ore tailings (IOTs) as the silicon source. The following conclusion can be drawn from the results: (a) The optimal conditions for catalyst formation are Si/Al > 30 and pH = 9 ~ 11. Additionally, the Al content in the MP-ZSM-5 framework is higher than that of HM-ZSM-5. This suggests that more Al from the raw material can be synthesized into the zeolite framework using the solid-phase conversion method compared to 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; (c) MP-ZSM-5 has been shown to have significantly higher catalytic activity than HM-ZSM-5 during MTP reaction. It has a long catalytic lifetime and high propylene selectivity. Additionally, it has superior ability to tolerate coke.
This reports the successful preparation of ZSM-5 microspheres using IOTs as raw materials under laboratory conditions, achieving the high-value utilization of iron tailings. However, whether this process can "bring ideals into reality" and achieve industrialization of results still needs further verification through pilot testing and industrial production.