Modified Fischer–Tropsch Pathway for CO₂ Hydrogenation to Aromatics: Impact of Si/Al Ratio of H-ZSM-5 Zeolite on Light Aromatics Selectivity

Abstract

Despite significant advancements in designing tandem catalysts for CO₂ hydrogenation to aromatics, the role of zeolite acid property in regulating the selectivity of light aromatics (benzene, toluene, and xylene, abbreviated as BTX) remains unclear. Herein, we report H-ZSM-5 zeolite (denoted as HZ-X, where X represents the Si/Al ratio) integrated with a Na-promoted FeCo-based catalyst (NaFeCo) for CO₂ hydrogenation into aromatics via a modified Fischer–Tropsch synthesis pathway. This study systematically modulates the Si/Al ratio of acidic zeolite and examines its critical role in influencing the light aromatics selectivity. The optimized NaFeCo/HZ-50 catalyst achieves a CO₂ conversion of 43% with an aromatics selectivity of 41%, including a BTX fraction of 57% in total aromatics. Multiple characterization techniques (NH₃-TPD, Py/DTBPy-IR, ²⁷Al NMR, etc.) clarify that acidic zeolite HZ-50 exhibits appropriate acid density and lower external surface acid sites, which synergistically boost the efficient aromatics and BTX synthesis while suppressing the undesirable alkylation and isomerization reactions on the external acid sites. This work develops a highly efficient multifunctional catalyst for CO₂ hydrogenation to light aromatics, especially offering guidance for the rational design of acidic zeolite with unique shape-selective functions.

1. Introduction

In recent decades, the extensive utilization of fossil energy sources has substantially contributed to massive CO₂ emissions and unprecedented environmental challenges. The utilization of CO₂ as a raw material for valuable chemicals synthesis could provide a non-petroleum pathway for basic chemical supplies and alleviate environmental pressure [1,2,3,4]. Aromatics are an important chemical raw material with a wide range of applications. Benzene, toluene, and xylene (BTX), as the most valuable aromatic hydrocarbons, are important raw materials in the polymer industry [5,6]. Compared to the traditional fossil fuel-based aromatics production pathway, the direct transformation of CO₂ + H₂ into aromatics via thermal/catalytic strategies holds great potential to realize sustainable development [7,8,9].

Thermal/catalytic CO₂ hydrogenation to aromatics primarily involves a methanol-mediated route [10,11,12,13] and olefin-mediated route [14,15,16,17,18]. In the methanol-mediated pathway, CO₂ first reacts with hydrogen over a reduced metal oxide catalyst to form methanol. Then, the methanol molecules diffuse into the zeolite channels and are converted into aromatics with the aid of acid sites and the shape-selective catalysis function of zeolite. Even though high aromatics selectivity can be achieved from the methanol-mediated pathway, the thermodynamic mismatch between CO₂ hydrogenation to methanol and the methanol aromatization reaction results in low CO₂ conversion and high undesirable CO selectivity, hindering the high-yield synthesis of aromatics. In the olefin-mediated pathway, the reverse water–gas shift reaction first occurs to generate CO. Subsequently, CO undergoes Fischer–Tropsch synthesis (FTS) with H₂ to form olefins. At last, these olefins undergo aromatization in the channels of H-ZSM-5 zeolite to produce aromatics (Scheme 1). The olefin-mediated pathway (also known as the modified FTS pathway) delivers exceptional CO₂ conversion efficiency and low CO selectivity, offering promising potential for high-yield synthesis of aromatics [19,20]. From the viewpoint of industrial application, the olefin-mediated route, with the advantage of high-yield aromatics synthesis, could decrease the reactor size and investment cost.

The design of a multifunctional catalyst that is capable of synergistically catalyzing RWGS, FTS, and aromatization reactions (tandem catalytic system) is the main challenge for the olefin-mediated route. Sun et al. developed a Na-modified ZnFeOx composite catalyst coupled with H-ZSM-5 zeolite for CO₂ hydrogenation to aromatics [21]. This study revealed that Zn acts as a structural accelerator to enhance the dispersion of Fe species, significantly facilitating the exposure of the active sites. In addition, the suitable amount of residual sodium (ca. 4.25 wt%), hierarchical pore structure, and the appropriate density of Brønsted acid sites in the ZnFeO_x-nNa/H-ZSM-5 composite catalyst significantly enhanced the aromatic selectivity and catalytic stability. Gascon et al. designed a multifunctional catalyst composed of Fe₂O₃@KO₂ and H-ZSM-5 [22]. The KO₂-doped Fe₂O₃ exhibited excellent olefin production activity, while H-ZSM-5 boosted the aromatization process. Under industrial conditions, the catalyst achieved a CO₂ conversion of 50% and an aromatic space-time yield (STY) of 9.2 mmol g⁻¹ h⁻¹. Although a tandem catalytic system based on modified FTS pathways for mixed aromatics (containing large amounts of heavy C₉₊ aromatics) has been extensively studied, the strategy for improving BTX selectivity needs to be further investigated.

The acidic property of zeolite (mainly H-ZSM-5 with MFI topology structure) is the key factor in optimizing the selectivity of BTX [23,24,25]. The charge imbalance caused by substituting Al³⁺ for Si⁴⁺ in the skeleton is the origin of the acidity of zeolite [26,27]. The substitution of Al for Si in the zeolite skeleton results in a negative charge that requires a cation to compensate, and when the cation is H⁺, a Brønsted acid site is formed [28]. These acid sites are crucial active sites for the aromatization process, promoting C–C bond formation and hydrogen transfer reaction through the proton transfer mechanism [29,30]. Furthermore, the molecular shape-selective effect of microporous in H-ZSM-5 permits only the molecules with kinetic diameters (e.g., BTX) matching the pore dimensions to diffuse through the channels, while bulkier C₉₊ aromatics are theoretically restricted within the channels. However, the formation of C₉₊ aromatics in practical application is mainly due to the alkylation and isomerization initiated by the acid sites at the pore mouth of channels and the external surface. These side reactions significantly reduce BTX selectivity. Therefore, by precisely modulating the spatial distribution of acid sites in H-ZSM-5 and rationally suppressing the side reactions on the external surface, the selectivity of BTX from olefin-mediated CO₂ hydrogenation can be enhanced effectively [31,32]. The current strategies for erasing external surface acid sites of zeolites are typically through epitaxial growth of Silicalite-1 or SiO₂ on the external surface, but these methods involve complicated procedures [13,33]. Recently, a novel H-ZSM-5 zeolite with unique Al distribution characteristics (Al-rich core and Al-deficient surface) has been discovered [34,35,36]. This special acid site distribution could eliminate the side reactions, offering a new approach for designing multifunctional catalysts in olefin-mediated CO₂ hydrogenation to BTX. Even though it has been shown that the spatial distribution characteristic of Al sites in this type of H-ZSM-5 zeolite could significantly enhance the BTX selectivity, its application in the modified FTS pathway for CO₂ hydrogenation into BTX has not been investigated. In particular, the detailed mechanism by which the Si/Al ratio (Si/Al) affects the product distribution through the modulation of acid strength, acid density, and shape-selective effect has not been systematically revealed.

In this work, we varied the Si/Al ratio of the special H-ZSM-5 zeolite with an Al-rich core and an Al-deficient surface (denoted as HZ-X, where X represents the Si/Al ratio, X = 25, 50, and 150) by adjusting the composition of the synthesized materials. We found a link between zeolite acidity and target product selectivity. NaFeCo/HZ-50 exhibited excellent BTX synthesis performance, which was attributed to its high Bronsted acidity and low external surface acidity. The multifunctional catalyst NaFeCo/HZ-50 achieved 43% CO₂ conversion and 41% aromatics selectivity, with BTX accounting for 57% of the total aromatics. At a high Si/Al ratio (HZ-150), the weak acid sites could not powerfully induce the aromatization of olefins toward aromatics. Conversely, under a low Si/Al ratio (HZ-25), the strong acidity at both internal and external surfaces induced the side reactions, resulting in a marked decline in BTX selectivity.

2. Results

2.1. Physicochemical Property of the H-ZSM-5 Zeolite

The preparation process of H-ZSM-5 zeolites (denoted as HZ-X, where X represents the Si/Al ratio) was illustrated in Figure 1a. The X-ray fluorescence (XRF) spectra verified the Si/Al ratio in HZ-X, as shown in Table S1. The morphology of the prepared HZ-X zeolites with different Si/Al ratios was investigated by scanning electron microscopy (SEM). As shown in Figure 1b–d, the HZ-X zeolite presented a hexagonal morphology. The average length of the zeolite was 20 μm.

The crystal phase structure and crystallinity of HZ-X zeolite with different Si/Al ratios were analyzed by X-ray diffractometer (XRD) patterns. The obtained HZ-X zeolites delivered a typical diffraction peak of the zeolite with MFI topology (PDF#47-0638) (Figure 2a), confirming the successful synthesis of H-ZSM-5 zeolite. As shown in Figure 2b and Table S2, with the increase in the Si/Al ratio, the relative crystallinity of zeolite first increased and then decreased. HZ-50 exhibited the highest relative crystallinity, indicating a more regular and stable crystal structure.

Nitrogen adsorption and desorption isotherms were employed to investigate the impact of the Si/Al ratio on the architecture property of the acidic zeolites. As demonstrated in Figure 2c, the nitrogen adsorption capacity of all zeolites rapidly increased at low relative pressures (P/P₀ = 0–0.2), which is a characteristic of type I isotherms and confirms the microporous-dominated structure. The detailed data are shown in Table S2. As the Si/Al ratio decreased, the specific surface area of the zeolite gradually increased from 378 m² g⁻¹ to 435 m² g⁻¹, and the micropore volume progressively rose from 0.161 cm³ g⁻¹ to 0.185 cm³ g⁻¹. The pore size distribution plots indicated that the pore size in zeolites all concentrated in the range of 0.5–0.6 nm, further confirming the microporous structure of the acidic zeolites with different Si/Al ratios (Figure 2d).

2.2. The Catalytic Performance of Aromatics Synthesis from CO₂ Hydrogenation

The catalytic performance of NaFeCo/HZ-X catalysts was evaluated in a fixed-bed reactor at 320 °C and 3 MPa for the hydrogenation of CO₂ to aromatics. NaFeCo and HZ-X zeolites were pelletized into 20–40 mesh particles, respectively, after which they were physically mixed and loaded into the reactor. The detailed results are shown in Figure 3, with specific data in Tables S3 and S4. Increasing the Si/Al ratio of the zeolitic catalyst led to a continuous reduction in CO₂ conversion, dropping from 44.3% to 35.5%. The variation in CO₂ conversion can be attributed to the different driving forces stemming from the HZ-X zeolites with different Si/Al ratios and acid strengths. For the product distribution, as the Si/Al ratio increased, the selectivity of CH₄ and light olefins was improved, while the selectivity of aromatics exhibited a decreasing trend. These results demonstrated that a lower Si/Al ratio is favorable to enhance the efficiency of olefin transformation and aromatization synthesis. To further compare the aromatization capability of different zeolites, the product distribution of C₅₊ hydrocarbon compounds is presented in Figure 3c. With the Si/Al ratio increased, the selectivity of olefins and iso-paraffins showed a slight enhancement, while the selectivity of n-paraffins experienced a mild decline. We then analyzed the selectivity of the most important products, light aromatics, BTX. Figure 3b demonstrates the BTX selectivity and space-time yield (STY) obtained from the NaFeCo&HZ-X catalysts. It was not difficult to find that when NaFeCo&HZ-50 had the highest selectivity of BTX and STY, NaFeCo&HZ-25 had the second highest performance, and NaFeCo&HZ-150 had the lowest performance. The above results may be because the excessively strong acidity in HZ-25 resulted in the conversion of the produced light aromatics to C9+ heavy aromatics. In contrast, the excessively weak acidity in HZ-150 was not sufficient to support the aromatization reaction. The NaFeCo&HZ-50 catalyst showed the optimal BTX synthesis performance with a BTX selectivity of 23.7% and a BTX STY of 59.8 mg_BTX·g_FeCo⁻¹·h⁻¹. The study of the distribution of aromatics fraction (Figure 3d and Table S4) revealed that toluene (T), xylene (X), and C₉₊ heavy aromatics were dominant in the products. When the Si/Al ratio increased from 25 to 150, the proportion of toluene in total aromatics gradually decreased from 22% to 12.8%, and the proportion of ethylbenzene also declined from 5.3% to 4.2%. In contrast, the proportion of C₉₊ heavy aromatics first decreased and then increased, first from 40.7% to 37.2% and then to 47.1%. The above-discussed product distribution characteristics indicated that adjusting the Si/Al ratio can effectively optimize the product distribution. The NaFeCo&HZ-50 catalyst achieved high selectivity of aromatics and BTX, with aromatics selectivity of 41.1% and BTX fraction of 57.7% in total aromatics. Furthermore, benefiting from the higher CO₂ conversion, NaFeCo&HZ-50 also delivered an excellent STY of light aromatics.

2.3. Acidic Property of HZ-X Zeolites

In the CO₂ hydrogenation reaction, the synthesis of aromatics, especially BTX, is closely related to the acidic property of zeolites. Optimizing the density of Brønsted acid sites plays a critical role in enhancing aromatics selectivity. Figure 4a presents the ammonia temperature-programmed desorption (NH₃-TPD) profiles of different zeolites, where two distinct desorption peaks appeared at 130 °C and 400 °C, respectively. The peak at 130 °C was associated with the weak acid sites, while the peak at 400 °C corresponded to the medium and strong acid sites. As the Si/Al ratio decreased, the desorption peaks shifted to higher temperatures, indicating an increase in acid strength. The specific density of the acid sites is provided in Table S5. HZ-25 exhibited the highest acid density, reaching a total of 144 μmol g⁻¹, while the acid density of HZ-50 decreased to 54 μmol g⁻¹. To further investigate the acidic property of the zeolites, we performed pyridine infrared (Py-IR) spectroscopy analysis (Figure 4b). The three typical peaks at 1445, 1490, and 1545 cm⁻¹ can be assigned to the Lewis acid sites, both Lewis and Brønsted acid sites, and Brønsted acid sites, respectively. In H-ZSM-5 zeolite, the Brønsted acid is associated with the inserted Al³⁺ in a tetrahedral framework as Si–OH–Al in H-ZSM-5 zeolite, and the Lewis acid is generally derived from the Al³⁺ located in the extra-framework [37]. As the Si/Al ratio decreased, the amount of Brønsted acid sites increased, which is associated with the increased Al species content. HZ-25 exhibited the highest Brønsted acid concentration of 0.72 mmol g⁻¹, while HZ-50 exhibited a slightly lower value of 0.52 mmol g⁻¹ (Table S6). The variation trends in acid density observed from the Py-IR spectra aligned well with the NH₃-TPD results. It should be noted that the difference in the amount of acid sites determined by NH₃-TPD and Py-IR was due to the different accessibilities of probe molecules with different molecular sizes (pyridine and NH₃) to the acid sites [38]. With a lower Si/Al ratio in zeolite, its acidity gradually enhances, leading to increased CO₂ conversion and aromatics selectivity, whereas BTX selectivity first improved and then decreased.

It has been proven that the acidic characteristic of zeolite possesses a significant correlation with the spatial distribution of Al sites in the skeletal structure [38]. The zeolite framework contains silanol (Si–OH) and silanol–aluminum (Si–OH–Al) groups [39,40]. Si–OH corresponds to weak acid sites, while Si–OH–Al typically corresponds to stronger Brønsted acid sites associated with tetrahedral coordinated Al³⁺ in the framework. Therefore, as the amount of tetrahedral Al increases, the acidity strengthens [41]. To characterize the coordination of Al³⁺ in the zeolite framework and elucidate the association of Al³⁺ with acidic properties, we conducted an analysis of the coordination states using ²⁷Al solid-state magic angle spinning nuclear magnetic resonance (²⁷Al MAS NMR) spectroscopy (Figure 4c). The spectrum exhibited a prominent peak at 55 ppm, which can be diagnosed as frame Al species. In contrast, the peak at 0 ppm corresponded to the non-framework Al species. All zeolite samples exhibited typical tetrahedral coordination framework Al structure characteristics. It was worth noting that the HZ-25 sample exhibited a weak signal of the non-framework Al species due to the high content of Al³⁺.

The acid sites on the external surface of the zeolites tend to induce the formation of C₉₊ heavy aromatics compounds through the alkylation and isomerization of light aromatics [25,42,43]. To investigate the variation in external surface acidity of zeolites with different Si/Al ratios, the 2,6-di-tert-butylpyridine infrared (DTBPy-IR) spectra were obtained. The bulky DTBPy could primarily only interact with the acid sites on the external surface of the zeolite, thereby avoiding interference from acid sites within the pore interiors. Thus, it is an effective probing molecule for studying the external surface acidity of zeolites. As shown in Figure 4d, it had large peaks at 1600, 1630, 1445, 1490, and 1545 cm⁻¹. The large peak at 1600 cm⁻¹ is related to the bending vibration of C=C in the pyridine ring, while the large peak at 1630 cm⁻¹ is related to the hydrogen bonding in the pyridine molecule. Since DTBPy also has a pyridine ring structure, three desorption peaks at 1445, 1490, and 1545 cm⁻¹ also appeared in the spectra. The peak between 1600 and 1700 cm⁻¹ reflected the acidity of the external surface of zeolites. It can be observed that the external surface acidity of the zeolites initially decreased and then increased as the Si/Al ratio increased, with HZ-25 and HZ-150 displaying stronger external surface acidity. In conclusion, moderate Si/Al ratio zeolites featured an Al-rich core and Al-deficient surface, generating spatially distinct acid site distribution (high internal but low surface acid density), which enhances BTX formation efficiency.

Through comprehensive analysis, it was found that the Si/Al ratio of zeolite was significantly and positively correlated with the Brønsted acid strength and framework aluminum density (Figure 4). HZ-50 exhibited moderate Al content, with Al exclusively participating in framework formation, which promotes the synthesis of total aromatics. Meanwhile, its lower external surface acid density facilitated the efficient synthesis of light aromatics BTX by avoiding the undesirable side reactions.

2.4. Relationship Between Zeolite Acidic Property and Light Aromatics Synthesis Performance

The relationship between the acidic property of zeolites with different Si/Al ratios and CO₂ hydrogenation performance is shown in Scheme 2. In the modified FTS reaction system, olefin intermediates were first aromatized on acid sites in the pore of the zeolite to form light aromatics. During their diffusion to the external surface, these light aromatics underwent secondary reactions (such as alkylation and isomerization) with the aid of acid sites on the external surface, generating undesirable by-products, such as heavy aromatics or iso-paraffins. The experimental results showed that when the Si/Al ratio was too high, the acid strength was insufficient for the aromatization process. When the Si/Al ratio was too low, a part of the Al might be located on the external surface of the zeolite, which leads to the undesirable side reactions that lower the BTX selectivity. Under the optimal Si/Al ratio, all Al species in the zeolite were involved in the skeleton formation process. It’s appropriate acid site density and distribution optimized the aromatization process and minimized the side reactions, therefore achieving the superior BTX synthesis performance. An increase in the Si/Al ratio of zeolites led to a gradual decrease in acid site density, resulting in slower conversion of intermediates and a decreased CO₂ conversion due to hampered driving force in the tandem catalytic system.

3. Materials and Methods

3.1. Materials and Chemicals

Ferric nitrate nonahydrate [Fe(NO₃)₃·9H₂O] (Macklin Biochemical Co., Ltd., Shanghai, China), cobalt nitrate hexahydrate [Co(NO₃)₂·6H₂O] (Macklin Biochemical Co., Ltd., Shanghai, China), sodium meta-aluminate (NaAlO₂) (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China), tetrapropylammonium bromide (C₁₂H₂₈BrN) (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China), n-butylamine (C₄H₁₁N) (Macklin Biochemical Co., Ltd., Shanghai, China), Urea (CH₄N₂O) (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China), anhydrous sodium carbonate (Na₂CO₃) (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China), Colloidal silicon oxide LUDOX^® HS-40 (SiO₂) (Sigma-Aldrich Life Science & Tech Co., Ltd., Shanghai, China) were of analytical grade and utilized without further purification. The deionized water used for catalyst preparation had a resistivity of 18.2 megohms and was obtained through an ultrapure water system.

3.2. Catalyst Preparation

3.2.1. Synthesis of NaFeCo Catalyst

The FeCo catalyst was synthesized by the urea precipitation method. Typically, Fe(NO₃)₃·9H₂O (12.12 g), Co(NO₃)₂·6H₂O (2.9103 g), and urea (33.033 g) were dissolved in deionized water (330 mL) to form a homogeneous solution. The solution was then placed in an oil bath, heated to 85 °C, and stirred for 2 h. The solution was then kept at 85 °C for 12 h for aging. The product was collected by centrifugation, washed four times with deionized water, and then dried at 70 °C overnight in an oven. The catalyst was then calcined at 350 °C for 4 h under an air atmosphere at a ramp rate of 5 °C min⁻¹ to obtain the FeCo catalyst. Finally, the FeCo catalyst was impregnated with sodium carbonate solution for 3% sodium impregnation, followed by drying in an oven at 70 °C overnight to obtain the final catalyst NaFeCo.

3.2.2. Synthesis of H-ZSM-5 Zeolites

The H-ZSM-5 zeolites with different Si/Al ratios (denoted as HZ-X, where X represents the Si/Al ratio) were synthesized by the hydrothermal method. Typically, deionized water of 91.5 g was added to a 200 mL PTFE bottle. Under the stirring condition, NaAlO₂ of 0.08–0.48 g, TPABr of 1.83 g, and C₄H₁₁N of 3.92 g were added into the above solution and magnetically stirred for 15 min to make them dissolve thoroughly. Then, SiO₂ of 22.53 g was slowly added into the above solution drop by drop, and stirring was continued for 30 min. The molar ratio of all the reagents was SiO₂:Al₂O₃:H₂O:TPABr:NBA = 300:1–6:10,160:13.78:107.2. The obtained solution was poured into a stainless steel reactor with PTFE lining and tightly sealed to ensure that the liquid did not leak. The reactor was transferred to a rotary drying oven and fixed on a rotating holder. The hydrothermal temperature was maintained at 180 °C for 48 h, and the rotation speed was 2.0 r min⁻¹. The obtained emulsion was centrifugally washed three times with deionized water, followed by drying overnight at 70 °C to acquire the HZ-X precursor. The precursor was calcinated at 550 °C for 6 h in a muffle furnace with a heating rate of 5 °C min⁻¹ to remove the organic template in HZ-X.

3.3. Catalyst Characterization

X-ray diffraction (XRD) analysis of the obtained catalysts was conducted using Cu Kα radiation and a DX-2700BH X-ray diffractometer (Dandonghaoyuan Instrument Co., Ltd., Dandong, China). The relative crystallinity of the zeolites was obtained by comparing the crystal peak areas in the XRD patterns of the zeolites under the same conditions. The morphology of the catalysts was observed via scanning electron microscopy (SEM) using a Regulus 8100 instrument (Hitachi High-Tech Co., Shanghai, China). Further, the specific surface area and pore size distribution of the samples were determined by the JW-BK200 instrument (JingweiGaoBo Co., Ltd., Beijing, China). The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. The Horvath–Kawazoe (HK) method was employed to analyze the pore size distribution of microporous. NH₃ temperature-programmed desorption (NH₃-TPD) experiments were carried out on a DCA-1200 instrument (Builder Electronic Technology Co., Ltd., Beijing, China). Prior to the experiment, the catalyst was pretreated at 150 °C for 1 h to eliminate the H₂O present in the microporous zeolite. Subsequently, the sample was saturated with NH₃ at 50 °C, and the physisorbed NH₃ was removed using He flow. Further, NH₃-TPD profiles were recorded under a He flow with a heating rate of 10 °C min⁻¹. The Si/Al ratio of the HZ-X catalyst was evaluated using a PANalytical AXIOS-Petro X-ray fluorescence spectrometer (PANalytical B.V. Co., Netherlands, Beijing, China). Pyridine infrared (Py-IR) and 2,6-di-tert-butylpyridine infrared (DTBPy-IR) spectra were obtained using a Bruker iS50 FTIR spectrometer (Thermo Fisher Scientific Instrument Co., Ltd., Waltham, MA, USA) to distinguish the acid type in HZ-X. Prior to the Py-IR or DTBPy-IR test, the zeolite was pressed into a self-supported wafer and degassed at 400 °C and 10⁻⁴ Pa for 1 h. After that, inert gas was saturated with pyridine or 2,6-di-tert-butylpyridine and was introduced into the IR cell for 1 h at room temperature. Finally, the catalyst was heated to 150 °C with a heating rate of 10 °C min⁻¹, then desorbed under a helium atmosphere for 30 min, and finally, Py-IR or DTBPy-IR spectra were collected. The distribution of Al in HZ-X zeolites was analyzed using an AVANCE3-400M (Bruker Co., Beijing, China) wide-cavity solid-state Nuclear Magnetic Resonance (NMR) spectrometer. The acidity data for zeolite was calculated using the following equation:

(1)

NH₃-TPD:

$$acidity={\displaystyle \frac{{\mathrm{V}}_0\times w\times A}{m\times {\mathrm{A}}_0\times V_m}}$$

where V₀ is the volume of the quantized ring, w is the volume fraction of NH₃, A₀ and A are the peak areas of the TCD signal peaks measured for the quantized ring and the catalyst, respectively, m is the mass of the catalyst, and V_m is the molar volume at standard conditions.

(2)

Py-IR:

$$Brønstedacidity=1.88\times A_B\times R^2/m$$

$$Lewisacidity=1.42\times A_L\times R^2/m$$

The quantitative determination of Bronsted and Lewis acids of the catalysts was based on previous studies [44]. where 1.88 and 1.42 are the extinction coefficients of Brønsted acid and Lewis acid, respectively. The peak areas of the characteristic peaks of Brønsted acid and Lewis acid are labeled as A_B/A_L. The radius of the zeolite compression tablet is labeled as R. The mass of the compression tablet is labeled as m.

3.4. Catalytic Activity Evaluation

The catalytic performance of the catalysts was measured in a fixed-bed reactor (ZXBLUE Co., Ltd., Beijing, China) with an internal diameter of 12 mm. As shown in Figure 5, 0.2 g of NaFeCo and 0.6 g of HZ-X were made into 20–40 mesh granules and mixed homogeneously. The mixture was mixed with an equal volume of quartz sand and loaded into the reactor. Prior to the reaction, the catalyst was reduced by pure H₂ at 400 °C for 4 h. After cooling down to room temperature, the reactant gas (23.75% CO₂, 71.25% H₂, and 5% Ar) was fed into the reactor until the pressure reached 3 MPa. At the same time, the temperature of the reactor was increased to 320 °C. The exhaust gas was analyzed by an online gas chromatograph (Fuli 9790II, Fuli Analytical Instruments Co., Ltd., Wenling, China) equipped with two detectors (thermal conductivity detector (TCD) and flame ionization detector (FID). The TCD is connected to the TDX-01 (60–80 mesh, 3 mm × 2 m, RuideJingke Instrument Co., Ltd., Zaozhuang, China); Porapak-Q (60–80 mesh, 3 mm × 1 m, Agilent Technologies Co., Ltd., Santa Clara, CA, USA) packed columns for the analysis of Ar, CO, CH₄, and CO₂; and the FID is connected to the HP-PLOT/Q capillary column (0.53 mm × 30 m, Agilent Technologies Co., Ltd., Santa Clara, CA, USA) to detect gas-phase hydrocarbons. An off-line gas chromatograph (Fuli 9790II, Fuli Analytical Instruments Co., Ltd., Wenling, China) equipped with an FID and a capillary column InerCap-5 (30 m × 0.25 mm, GL Sciences, Tokyo, Japan) was employed to analyze the liquid products.

The specific catalytic performance evaluation method is as follows:

(1)

CO₂ conversion was calculated according to

$${CO}_{2}Conversion={\displaystyle \frac{{CO}_{2inlet}-{CO}_{2outlet}}{{CO}_{2inlet}}}\times 100\%$$

in which the mole of CO₂ in the inlet gas is labeled as the CO₂ inlet and that in the outlet gas is labeled as the CO₂ outlet.

(2)

Product selectivity was calculated as the percentage of CO₂ converted into a given product and according to

$${Sel}_i={\displaystyle \frac{N_i\times n_i}{{\sum }_1^i(N_i\times n_i)}}\times 100\%$$

where N_i and n_i represent the mole percentage and carbon number of product i, respectively.

(3)

CO selectivity for CO₂ hydrogenation was calculated according to

$$COselectivity={\displaystyle \frac{{CO}_{outlet}}{{CO}_{2inlet}-{CO}_{2outlet}}}\times 100\%$$

in which the mole of CO in the outlet gas is labeled as the CO outlet. The carbon balances in all catalytic runs were calculated and are all higher than 90%. Typically, the experimental data after the reaction of 24 h were used for discussion.

4. Conclusions

In summary, we synthesized H-ZSM-5 (denoted as HZ-X, where X represents the Si/Al ratio) zeolites with varying Si/Al ratios and evaluated their catalytic performance in CO₂ hydrogenation to aromatics after combining with the NaFeCo catalytic component. The acidic property of the zeolites, particularly the acid density and distribution, plays a crucial role in light aromatics synthesis performance. The structural characteristics of zeolites govern their acid site distribution, and these acid sites serve as the active centers for catalyzing the conversion of CO₂ to aromatics. Therefore, tuning the active sites of zeolites can promote the selective synthesis of the target product. The zeolite with a Si/Al ratio of 50 possessed Al-rich core and Al-deficient surface characteristics. This acidic property synergistically boosted the efficient light aromatics synthesis by suppressing the undesirable alkylation and isomerization reactions on the external acid sites. In contrast, the zeolite with a high Si/Al ratio exhibited insufficient acid site density for the aromatization reaction, while the zeolite with a low Si/Al ratio delivered high heavy aromatics selectivity due to the undesirable side reactions, such as alkylation and isomerization. The optimized NaFeCo/HZ-50 catalyst achieved a CO₂ conversion of 43% with an aromatics selectivity of 41%, including a BTX fraction of 57% in total aromatics. This work demonstrates that the rational design of zeolite acidic property is critical for efficient CO₂ hydrogenation into BTX, offering design guidance for bifunctional catalysts in a targeted synthesis system.