Study on the Acidic Modification of Mesoporous HZSM-5 Zeolite and Its Catalytic Cracking Performance

Abstract

Mesoporous HZSM-5 zeolites with nanocrystal stacking morphology were directly synthesized via hydrothermal methods without mesoporous templates. The synthesized mesoporous HZSM-5 was subjected to hydrothermal–citric acid washing treatment. The structural and acidic properties of the samples before and after modification were characterized using various techniques. The catalytic performance for butene conversion to propylene was investigated under atmospheric pressure, 500 °C, and a butene weight hourly space velocity (WHSV) of 10 h⁻¹ in a continuous-flow micro-fixed bed reactor. The results show that propylene selectivity increased significantly from 24.7% before modification to 44%, and propylene yield increased from 22% to 38%. After 2 h of hydrothermal–citric acid washing modification, the catalyst maintained a butene conversion rate of 76% and a selectivity of 47% at 525 °C and a WHSV of 10 h⁻¹ after 130 h of continuous reaction, with a propylene yield of 37%. The results indicate that moderate hydrothermal–citric acid washing modification leads to the removal of aluminum from the zeolite framework, reducing the amount and strength of acid but increasing the mesopore quantity. This helps control the reaction pathways and diffusion of intermediate products, suppresses some side reactions, and improves the selectivity and yield of the desired product, propylene, while significantly enhancing catalytic stability.

1. Introduction

Propylene, a crucial chemical raw material, is primarily used in the production of polypropylene, acrylonitrile, and propylene oxide. Its vast market size makes the propylene industry of significant importance to the national economy [1,2]. In recent years, driven by the increasing demand for downstream propylene derivatives, the demand for propylene has surged. The output of propylene from traditional ethylene co-production and catalytic cracking in refineries can no longer meet the market demand [3,4]. China’s propylene production capacity has been rapidly expanding, with an annual growth rate exceeding 10% in recent years [5]. Despite the growth in domestic propylene production capacity, it still falls short of meeting the increasing demand across various industries. By 2021, China’s propylene production was approximately 41.5 million tons, with a net import of 2.49 million tons and an apparent demand of around 43.9 million tons. Driven by downstream product needs, the apparent demand for propylene in China grew by 10.1% year on year [6].

Currently, industrial propylene production processes are primarily divided into two categories. The first is traditional by-product processes, including methods such as steam cracking [7] and catalytic cracking [8], which produce propylene as a by-product. The second is dedicated propylene production processes, which are specifically designed for propylene production and include propane dehydrogenation [9], methanol-to-propylene processes [10], olefin disproportionation [11], and catalytic cracking [12]. The share of dedicated propylene production facilities in the global propylene production landscape increased from 8% in 2009 to 26% in 2019, and is expected to reach 33% by 2029 [13]. This underscores the crucial role of dedicated propylene processes in balancing supply and demand. In addition to producing propylene and ethylene, traditional processes such as naphtha steam cracking [7], catalytic cracking [8], and methanol-to-olefin conversion also generate substantial amounts of C4 and higher hydrocarbons. With the rapid expansion of integrated refining and coal-to-olefins capacities in China, the availability of C4/C5 olefins has increased significantly. Utilizing olefin catalytic cracking technology to convert C4/C5 olefins into high-value propylene and ethylene represents a highly promising technological route [7,14].

Currently, several processes are being developed for converting low-value C4/C5 olefins into propylene and ethylene, including Mobil’s MOI process [15], Lurgi’s Propylur process [16], and Sinopec Shanghai Petrochemical’s OCC process [17]. Zeolites are widely used in catalytic cracking reactions due to their high hydrothermal stability [18], shape selectivity [19], and tunable acidity [20]. Among the various catalysts studied, ZSM-5 zeolites are predominant [21,22,23]. However, conventional ZSM-5 zeolites have small micropore sizes, which restrict the diffusion speed of product molecules through their pore channels. This limitation increases the likelihood of secondary reactions, reduces catalyst selectivity, and shortens the catalyst lifespan. Mesoporous zeolite catalysts, which have lower acid content and wider pore size distributions, are known to enhance the catalytic cracking of butenes to produce propylene. Therefore, the development of a high silica-to-alumina ratio in mesoporous zeolites is a key direction in optimizing butene catalytic cracking catalysts.

Hydrothermal treatment is a simple and effective method to modify the acidity and structure of zeolites to improve propylene selectivity and catalyst stability [24,25,26]. This treatment can remove some framework aluminum, thereby increasing the silica-to-alumina ratio and reducing acidity while preserving the zeolite’s framework structure [18,27,28]. Further citric acid washing after hydrothermal treatment effectively removes non-framework aluminum and increases the pore volume, enhancing the zeolite’s carbon capacity. However, there is limited research on how hydrothermal treatment affects the acidity and catalytic performance of small-crystal mesoporous ZSM-5 zeolites in butene catalytic cracking reactions.

To improve the propylene yield and catalyst stability of ZSM-5 zeolites in butene catalytic cracking reactions, this study synthesized mesoporous ZSM-5 zeolites using a simple hydrothermal method without the addition of mesoporous templates. The resulting mesoporous ZSM-5 zeolites were then subjected to a combined hydrothermal–citric acid washing treatment to modify their acidity and pore structure. The catalysts were characterized using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption–desorption, X-ray fluorescence (XRF), and NH₃ temperature-programmed desorption (NH₃-TPD). The study investigated the effects of hydrothermal treatment time on the structure, acidity, and catalytic performance of mesoporous ZSM-5 zeolites in butene cracking reactions.

2. Results and Discussion

2.1. XRD

Figure 1 presents the X-ray diffraction (XRD) patterns of ZSM-5 zeolites before and after hydrothermal–citric acid treatment. From Figure 2, it can be observed that all modified samples exhibit two sets of characteristic diffraction peaks of MFI-type zeolite at 2θ angles of 7.6–8.9° and 22–25°. The diffraction peaks are sharp, indicating that the crystal-phase structure of HZSM-5 zeolite is not destroyed by the hydrothermal treatment. Additionally, as the hydrothermal treatment time increases, the intensity of the diffraction peaks initially increases and then decreases, suggesting that the crystallinity of the zeolite first improves and then deteriorates. This is because the mild hydrothermal–citric acid treatment helps remove non-framework aluminum from the zeolite’s pores and surface [28], enhancing the crystallinity. Hence, shorter hydrothermal treatment times increase the crystallinity of the zeolite. Although citric acid washing can remove some of the non-framework aluminum formed from aluminum removal, its effect on the ZSM-5 catalyst’s framework structure is minimal. However, with further extension of the hydrothermal treatment time, the continuous removal of framework aluminum leads to lattice defects, causing a reduction in diffraction intensity and a decrease in relative crystallinity [29]. The relative crystallinity of the five samples, calculated based on the sum of the intensities of the three strongest diffraction peaks in the 2θ range of 22.5°–25°, is greater than 90%, indicating that the synthesized mesoporous HZSM-5 zeolites possess good hydrothermal stability.

2.2. XRF

The X-ray fluorescence (XRF) characterization results of ZSM-5 zeolites before and after hydrothermal–citric acid treatment are presented in Table 1. From Table 1, it can be seen that the actual silicon-to-aluminum (Si/Al) ratio of the synthesized mesoporous ZSM-5 samples is lower than the initial Si/Al ratio. This is because during the synthesis process, the silicate solution did not fully react, leading to varying degrees of loss, which resulted in a lower solid yield for catalysts with high Si/Al ratios [30]. Additionally, the change in the framework Si/Al ratio before and after modification is noticeable. With the extension of the steam treatment time, the aluminum content gradually decreases, with the SiO₂/Al₂O₃ ratio increasing from 110 to 117. This significant increase may be due to the framework dealumination that occurs during hydrothermal treatment. Some of the non-framework aluminum removed during this process remains in the pores. Further acid washing eliminates this non-framework aluminum, while citric acid modification can help return some of the removed non-framework aluminum to the framework and rearrange it, thereby increasing the sample’s SiO₂/Al₂O₃ ratio [26].

2.3. SEM

Figure 2 displays the scanning electron microscopy (SEM) images of ZSM-5 zeolites before and after hydrothermal–citric acid treatment. As observed in Figure 3, the ZSM-5 zeolites are composed of spherical particles aggregated from primary nanoparticles with a diameter of approximately 40–50 nm, and the particle size reaches the micron scale. The results indicate that, under the condition of no mesopore template agent, a well-defined, uniform, and aggregated ZSM-5 zeolite with a regular morphology was obtained through a simple hydrothermal method.

Comparing the morphology of the zeolites before and after hydrothermal treatment, there is no significant change in the appearance of the zeolites. This indicates that the hydrothermal treatment did not cause noticeable differences in the particle size of the catalyst. The zeolites maintained their intact nano-crystal aggregate structure, demonstrating good structural stability.

2.4. BET

Table 2 shows the results of specific surface area, pore volume, and pore size measurements of HZSM-5 zeolites before and after hydrothermal–citric acid treatment. From Table 2, it can be observed that as the hydrothermal treatment time increases, the specific surface area and pore volume of the HZSM-5 zeolite first increase and then decrease. This is primarily due to the removal of some framework aluminum during hydrothermal treatment, which creates defects in the zeolite framework, and some of the non-framework aluminum that is removed remains in the pores. After citric acid washing, the non-framework aluminum blocking the pores is removed, clearing the zeolite’s pore channels, thus increasing the specific surface area and pore volume.

The pore volume and pore size increase with longer hydrothermal treatment times. This is mainly because hydrothermal treatment results in the formation of larger secondary pores [24] and the creation of mesopore-like features. Further acid washing removes non-framework aluminum from the pore channels, increasing the pore volume and size. This increase is attributed to the framework aluminum removal during hydrothermal treatment, where the removed aluminum oxides cover the zeolite surface. Citric acid washing not only removes non-framework aluminum generated from framework aluminum removal, but also expands the existing mesopores formed by nanoparticle aggregation or creates new mesopores at the interfaces of nanoparticle aggregates, thus clearing the pores and increasing the pore volume.

As the hydrothermal treatment time is further extended, the removal of framework aluminum and the elimination of non-framework aluminum increase. This could cause the mesopore structures formed at the interfaces of nanoparticle aggregates to collapse due to the increased amount of aluminum removal or migration. This, in turn, strengthens the bonding between some nanoparticle grains, making the packing denser, and results in a decrease in the specific surface area and mesopore volume. Some of the framework aluminum in the zeolite undergoes hydrolysis to form Al(OH)₃, which also leads to silica replacement and the structural rearrangement of the zeolite. The removed aluminum oxides cover the zeolite surface, and the non-framework aluminum remaining in the zeolite pores blocks some of the micropores. Additionally, the shorter Si-O bond length (0.165 nm) compared to the Al-O bond length (0.175 nm) leads to a contraction of the zeolite framework, resulting in a reduction in the specific surface area and pore volume of the zeolite.

Figure 3 shows the N₂ adsorption–desorption isotherms of the ZSM-5 zeolites before and after hydrothermal–citric acid treatment. The low-temperature nitrogen adsorption curves exhibit a typical type IV adsorption–desorption isotherm with a clear hysteresis loop. The presence of a hysteresis loop in the zeolite after steam treatment indicates that the mesopores are still present. This suggests that the treated zeolite retains the pore structure characteristics of nanoparticle aggregates. Additionally, the hysteresis loops before and after modification do not show a distinct adsorption plateau, indicating that the zeolite primarily consists of irregular nanoparticle-packed pore structures. The smallest hysteresis loop corresponds to the zeolite treated with steam for 4 h, indicating a reduction in mesopore structure with increased hydrothermal time. The significant decrease in specific surface area and pore volume is mainly due to extensive framework aluminum removal and associated micropore destruction during prolonged hydrothermal treatment, with non-framework aluminum blocking the zeolite pores. This likely results from non-framework aluminum formed during steam treatment remaining in the zeolite pores, thus reducing the pore volume.

2.5. NH₃-TPD

Figure 4 presents the NH₃-TPD spectra of HZSM-5 zeolites before and after hydrothermal–citric acid treatment. The spectra reveal two distinct ammonia desorption peaks: one between 120 and 250 °C and another between 300 and 600 °C. These peaks correspond to the weak and strong acid sites of the zeolite, respectively [31,32]. The modification significantly impacts the acidity of the ZSM-5 zeolites. Compared to the unmodified HZSM-5, both the high-temperature and low-temperature desorption peak areas decrease with increasing steam treatment time, indicating a notable reduction in the catalyst’s acidity. At a steam treatment time of 4 h, the areas of both the strong and weak acid peaks are minimal. Furthermore, with prolonged steam treatment, the desorption peaks corresponding to strong and weak acids shift to lower temperatures, suggesting that the steam treatment also alters the strength of the acid sites, consistent with findings reported in the literature [18,26]. This reduction in acidity is due to the hydrothermal treatment process, where some of the tetrahedrally coordinated aluminum in the zeolite framework hydrolyzes to form Al(OH)₃, leading to the formation of aluminum compound fragments. This results in decreased acidity and acid strength [24]. The increase in acidity observed after citric acid treatment is attributed to the fact that steam treatment removes framework aluminum, thereby reducing the zeolite’s acidity. However, non-framework aluminum that forms during steam treatment can cover some of the catalyst’s active sites. Subsequent citric acid treatment removes this non-framework aluminum, exposing these active sites and increasing the zeolite’s acidity.

2.6. Catalytic Performance Evaluation

Figure 5 illustrates the variation in butene conversion rates over time for HZSM-5 catalysts with different hydrothermal treatment durations. It shows that as the hydrothermal treatment time increases, the butene conversion rate gradually decreases, though it remains between 80% and 85%. Ethylene yield decreases progressively, while propylene yield initially increases and then decreases. For the untreated catalyst, the propylene selectivity is 22% and the propylene yield is 18.7%. After 2 h of hydrothermal treatment, the propylene selectivity increases to 47% and the propylene yield to 40%. However, excessive hydrothermal treatment leads to a decrease in the catalyst’s conversion rate. The NH₃-TPD results after hydrothermal treatment indicate a gradual reduction in the areas of desorption peaks around 250 °C and 430 °C, with these peaks shifting to lower temperatures. This reflects a decrease in both the acidity and acid strength of the catalyst. The catalytic performance is closely related to the catalyst’s acidity and acid strength. Stronger acidity promotes butene cracking, but also increases the formation of aromatics and alkanes due to higher levels of aromatization and hydrogen transfer. Therefore, a moderate reduction in acidity and acid strength can help minimize secondary reactions like hydrogen transfer, thereby enhancing the selectivity and yield of the desired products.

The synthesized zeolites consist of irregular aggregates of nanocrystals. After hydrothermal treatment, the acidic sites can be covered or blocked by impurities in the pore channels. However, after treatment with citric acid, the specific surface area, pore volume, and pore size of the zeolites can be increased. This reduces the diffusion path of the products, thereby suppressing secondary reactions and enhancing propylene selectivity. The results indicate that the stability of the catalyst is not only related to its acidity and acid strength, but also closely tied to the structure of its pore channels. After hydrothermal and acid washing treatments, the non-framework aluminum blocking the pore channels is removed, improving the carbon capacity of the catalyst’s pore channels and thereby enhancing the catalyst’s stability.

The highest propylene yield is observed with 2 h of hydrothermal treatment. This is primarily due to the fact that the acidic sites of the ZSM-5 zeolite are the active centers for the cracking reaction, and both the acid strength and amount significantly affect the distribution of reaction products. As the hydrothermal treatment time increases, the severity of the treatment also increases, leading to a reduction in acid amount and strength. Catalytic cracking reactions occur when hydrocarbons interact with the acidic centers to form carbocations, which then undergo β-cleavage [22]. If the acidity of the zeolite is too weak, its catalytic cracking performance decreases. Additionally, during the hydrothermal treatment, framework aluminum is removed, and the resulting aluminum compounds can cover the zeolite surface and block the pore channels. This impedes the contact between reactants and the catalytic centers, thereby reducing the catalyst’s cracking performance and decreasing the propylene yield.

The more strong acid sites present on the surface of ZSM-5 zeolite, the poorer the catalyst stability, but having too few strong acid sites can also reduce the catalyst’s conversion rate. Therefore, the Z-AT2 catalyst, which was treated with 2 h of hydrothermal treatment followed by citric acid modification, was selected to investigate the stability of the butene catalytic cracking catalyst.

Figure 6 shows the changes in conversion rate, selectivity, and propylene yield over time for the catalyst that underwent 2 h of hydrothermal treatment followed by citric acid treatment. From Figure 7, it is evident that the catalyst treated with 2 h of steam followed by acid treatment has the highest butene conversion rate, approximately 85%. The propylene selectivity remains consistently around 47%. It is also apparent that the propylene yield and selectivity of the zeolite after steam and citric acid treatment are higher compared to the untreated zeolite. The zeolite that underwent both treatments exhibits higher and more stable propylene yield, indicating that the zeolite after two modifications is the desired catalyst. Over time, the catalyst gradually accumulates coke, eventually leading to catalyst deactivation.

It is widely believed that the accumulation of carbon on HZSM-5 catalysts, which covers the acidic sites, is a primary cause of catalyst deactivation [4,33,34]. The coking process on the catalyst is mainly influenced by two factors: acidity and pore structure [35,36]. Unmodified catalysts have a high number of acidic centers and strong acid sites. While these catalysts generate the target products, they are also more prone to hydrogen transfer and aromatization reactions. The products of these reactions serve as precursors for coke formation, which covers the catalytic active sites and poisons the catalyst, leading to decreased catalytic stability.

After hydrothermal modification, the amount of strong acid sites and the acid strength of the ZSM-5 zeolite are reduced. This helps suppress the formation of by-product aromatics during the reaction, thereby inhibiting the formation of coke from aromatic condensation products and reducing the coke formation rate. Additionally, the hydrothermal modification increases the mesopore volume and mesopore surface area of the catalyst. This enhances the diffusion rate of reactant and product molecules within the catalyst’s pores, thereby reducing the likelihood of further reactions that lead to coke formation. Furthermore, mesopores can accommodate coke precursors or accumulated coke on active sites without affecting the diffusion rate of reactants or products, thus inhibiting coke formation during the reaction. As a result, the stability of the catalyst is significantly improved after hydrothermal treatment.

We conducted a long-term stability test on the catalyst modified by hydrothermal treatment combined with citric acid washing. After 130 h of operation, we observed the following: During the first 50 h of the reaction, the initial propylene yield was around 40%. Over the subsequent 80 h, as shown in the figure, the propylene yield was maintained by adjusting the temperature. After 110 h of reaction, the propylene yield slightly decreased but remained at 38%. The ethylene yield was stable at around 5% for an extended period. The butene conversion rate was about 90% during the first 50 h and remained relatively stable at approximately 85% over the next 80 h. After 110 h, it started to decline slowly. These results indicate that the ZSM-5 zeolite catalyst, after modification, exhibits good catalytic activity and stability.

3. Experimental Materials and Methods

3.1. Materials

Tetrapropylammonium bromide (TPABr): 99% purity, purchased from Shanghai Aladdin Biochemical Technology Co. (Shanghai, China); sodium hydroxide (NaOH): analytical, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Water glass (sodium silicate, Na₂O₃Si): industrial grade, purchased from Qingdao Ocean Chemical Co. (Qingdao, China).

Aluminum sulfate octadecahydrate (Al₂(SO₄)₃·18H₂O): analytical grade, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; sulfuric acid (concentrated, 98%): purchased from Saen Chemical Technology Co. (Shanghai, China); citric acid (HOC(COOH)(CH₂COOH)₂): analytical grade, purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China); ammonium chloride (NH₄Cl): analytical grade, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; butene: 99% purity, purchased from Beijing Millennium Jingcheng Gas Co., Ltd. (Beijing, China); deionized water.

3.2. Catalyst Preparation

At 40 °C under constant stirring, a gel was prepared with the following molar ratios: SiO₂:Al₂O₃:TPABr:Na₂O:H₂O = 1:0.006:0.1–0.3:0.01–0.05:30. Aluminum sulfate solution and Tetrapropylammonium bromide (TPABr) solution were slowly and uniformly added to a water glass (sodium silicate) solution. After dynamic aging at low temperature for 24 h, the gel was transferred to a PTFE (Polytetrafluoroethylene)-coated crystallization vessel and crystallized at 150 °C for 96 h. Post-crystallization processing: After crystallization, the product was filtered, washed with deionized water until neutral, dried at 110 °C, and calcined at 550 °C in air to remove the organic template. The calcined material was then exchanged with a 1 mol/L ammonium chloride solution at a solid-to-liquid ratio of 1:10 in a 90 °C water bath for 4 h, repeated twice. The solid product was filtered, washed until neutral, dried at 110 °C, and calcined at 550 °C to obtain the HZSM-5 zeolite, named Z1. Preparation of mesoporous ZSM-5 catalyst hydrothermal treatment: 5 g of Z1 was placed in a reaction tube. Under a nitrogen atmosphere, the temperature was increased to 600 °C at a rate of 10 °C/min and held at this temperature. At atmospheric pressure, 100% steam was passed through the HZSM-5 zeolite (with a steam hourly space velocity of 3.75 mL·g⁻¹·h⁻¹) for hydrothermal treatment. Acid treatment: After hydrothermal treatment, the zeolite was treated with a 1 mol/L citric acid solution at a solid-to-liquid ratio of 1:20 in a 90 °C water bath for 1 h. The product was filtered, washed until neutral, dried at 110 °C, and calcined at 550 °C to obtain the hydrothermally modified ZSM-5 catalyst. Based on the treatment duration, the products were sequentially named AT-1, AT-2, AT-3, and AT-4.

3.3. Catalyst Characterization

The X-ray diffraction (XRD) technique was used to identify the crystal structure and crystallinity of the catalysts. XRD patterns of the samples were collected in the 2θ range of 5 to 50° with 0.01° step size by using a Rigaku D/max-2500 X-ray diffractometer manufactured by Rigaku Corporation (Akishima, Japan) using Ni-filtered CuKα radiation. A scanning electron microscope (SEM) was chosen to determine the morphology and size of the crystals. The Si/Al ratios of the samples were determined by X-ray fluorescence spectrometry (XRF) using a Magix 601 (Philips, Amsterdam, The Netherlands). SEM images were investigated using a Hitachi Field Emission Scanning Electron Microscope (Tokyo, Japan, S-4700). The N₂ physicsorption technique was used to determine the surface area and pore volume of the sample. The sample was outgassed under N₂ for 4 h, and then the treated sample underwent N₂ physicsorption at −196 °C. The Brunauer–Emmett–Teller (BET) method was used to determine the specific surface area of the sample, while the Barrett–Joyner–Halenda (BJH) method was applied to calculate the pore volume of the sample. All samples were characterized on a Micrometric Chemisorbs 2750 model ASAP 2000 automated system (Norcross, GA, USA). The temperature-programmed desorption of ammonia (NH₃-TPD) method was used to investigate the total acidity and acid strength of catalysts with an Autosorb-1C-TCD-MS automated system. First, 0.1 g of sample was packed in a quartz U-tube reactor, and then it was preheated with a He gas stream (flow rate = 25 mL/min) at 550 °C for 1 h. Subsequently, the preheated sample was cooled down to 40 °C, and then the catalyst was adsorbed with 15% of NH₃/He mixed gas stream (flow rate = 25 mL/min) for 30 min. Afterward, the sample was purged with a He gas stream (flow rate = 25 mL/min) to remove the physisorbed NH₃. Finally, the temperature was linearly raised to 600 °C with a heating rate of 10 °C/min and the desorbed ammonia was detected by a TCD detector.

3.4. Reaction Performance Evaluation

First, the catalyst sample was ground and then shaped using a small pellet press, with a pressure of 20 MPa applied and maintained for 10 min. After shaping, the pellet was removed, crushed, and catalyst particles with a size of 20–40 mesh were selected for use.

The butene catalytic cracking evaluation system uses a custom-made mini fixed-bed reactor, as shown in Figure 8. The reaction tube is 550 mm long with an outer diameter of 20 mm and an inner diameter of 10 mm. Here, 0.5 g of catalyst is placed in the constant temperature section in the middle of the reaction tube, with quartz sand of a 20–40-mesh particle size packed at both ends of the tube. The reactor is placed in a two-zone temperature-controlled furnace, with the reaction pressure at atmospheric pressure. First, the catalyst is heated to 400 °C. Then, the catalyst is activated using nitrogen at a flow rate of 100 mL/min. When the catalyst temperature reaches 500 °C, butene feed gas (diluted with nitrogen at a mass ratio of 1:1) is introduced into the reactor with a weight hourly space velocity (WHSV) of 10 h⁻¹. Gas-phase products are analyzed every hour using a gas chromatograph (Agilent Technologies, Santa Clara, CA, USA, Model GC979, with a capillary column specification of 66 m × 0.25 mm × 1.0 µm, and a hydrogen flame ionization detector), with the workstation being an HVJ-2000. The chromatographic analysis conditions are as follows: the column temperature is set to 60 °C, the detector temperature is set to 120 °C, and the injector temperature is set to 110 °C. The butene conversion rate (X, %), propylene selectivity (S, %), and propylene yield (Y, %) are calculated using Equations (1)–(3).

(1)$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}={\displaystyle \frac{{{(W}_{c_4^{=}})}_0-{{(W}_{c_4^{=}})}_t}{{{(W}_{c_4^{=}})}_0}}\times 100\%$

(2)$\mathrm{S}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{{{(W}_{c_n})}_t}{{{(W}_{c_4^{=}})}_0-{{(W}_{c_4^{=}})}_t}}\times 100\%$

(3)$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}=\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\times \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}$

In the equations:

${{(W}_{c_4^{=}})}_0$ is the mass fraction of butene in the feed;

${{(W}_{c_4^{=}})}_t$ is the mass fraction of butene in the products;

${{(W}_{c_n})}_t$ is the mass fraction of the desired product in the products.

4. Conclusions

• (1). In the absence of mesopore templates, HZSM-5 zeolite with a nanocluster mesopore structure was synthesized using a simple hydrothermal method. The zeolite particles have a diameter of approximately 40–50 nm. The ZSM-5 zeolite has a silicon-to-aluminum ratio of 110 and a mesopore volume of 0.188 cm³/g. It is free of other impurities and exhibits a relatively high crystallinity.

• (2). Hydrothermal modification reduced the surface acid strength and acid amount of the zeolite catalyst. The combined hydrothermal–citric acid treatment removed non-framework aluminum from the pore channels, created new mesopores, increased the catalyst’s specific surface area and pore volume, and decreased the external specific surface area of the catalyst. The reduced strong acid sites and increased mesopore volume suppressed hydrogen transfer reactions during butene catalytic cracking, enhancing propylene selectivity and yield while extending the catalyst’s lifespan. After 2 h of hydrothermal treatment and citric acid washing, the propylene selectivity increased from 24.7% to 44%, and the propylene yield increased from 22% to 38%.

• (3). The long-term evaluation of the catalyst showed an initial propylene yield of 40%. After 130 h of reaction, the butene conversion rate was 76%, with selectivity still reaching 47% and the propylene yield remaining at 37%. These results indicate that the synthesized ZSM-5 zeolite catalyst possesses good catalytic activity and stability, providing valuable insights for its industrial application.

Author Contributions

Conceptualization, Y.T.; methodology, Y.T.; validation, Y.T.; investigation, Y.T.; resources, M.K.; writing—original draft preparation, Y.T. writing—review and editing, M.K.; visualization, Y.T.; supervision, M.K. project administration, Y.T. and M.K.; funding acquisition, M.K. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors are indebted to the State Key Laboratory of Heavy Oil Processing (China University of Petroleum) for the SEM, BET, XRF, NH₃-TPD, and XRD measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. XRD pattern of ZSM-5 molecular sieve before and after modification.

Figure 2. SEM photo of modified molecular sieve. (a): Z1; (b): ST-2; (c): AT-2.

Figure 3. Nitrogen adsorption–desorption isotherms and pore distribution of Z1 zeolite treated with citric acid after hydrothermal treatment.

Figure 4. NH3-TPD curve of ZSM (150) catalyst treated with citric acid after hydrothermal treatment.

Figure 5. Effect of citric acid treatment on conversion, selectivity, and yield after different hydrothermal treatment times.

Figure 6. Conversion selectivity and yield of molecular sieve after 2 h hydro-heat-treatment pickling.

Figure 7. Catalyst long-period operation test results.

Figure 8. Butene catalytic cracking evaluation device. 1–3, pressure reducing valve; 4–9, valve; 10–12, flow controller; 13–15, T-pipe; 16, gas-mixing tanks; 17, three-stage heating furnace; 18, product buffer tank; 19, on-line gas chromatograph; 20, liquid product collection tanks.

Table 1

XRF data of citric acid treatment after different heat treatment times.

Sample	SiO2 (wt%)	Al2O3 (wt%)	SiO2/Al2O3 (mol/mol)
Z1	97.20	1.50	110
AT-1	97.32	1.47	112
AT-2	97.40	1.45	114
AT-3	97.50	1.43	116
AT-4	97.51	1.42	117

Table 2

Structural properties of molecular sieve treated with acid after water vapor treatment.

Sample	Surface Area (m2/g)			Pore Volume (cm3/g)			AverageDiameter (nm)
	Stotal	Smicro	Sext	Vtotal	Vmicro	Vmeso
Z1	399	179	220	0.293	0.105	0.188	4.14
AT-1	398	183	215	0.292	0.102	0.189	4.31
AT-2	400	221	179	0.296	0.105	0.191	4.42
AT-3	402	215	187	0.303	0.110	0.193	4.39
AT-4	395	195	200	0.290	0.112	0.178	4.23

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