Next Article in Journal
Fusarium proliferatum PSA-3 Produces Xylanase-Aggregate to Degrade Complex Arabinoxylan
Previous Article in Journal
Immobilization and Catalytic Properties of Limonene-1,2-Epoxide Hydrolase
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 10
10.3390/catal15100987
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Investigating the Impact of Incorporating Alkali Metal Cations on the Properties of ZSM-5 Zeolites in the Methanol Conversion into Hydrocarbons

by
Senlin Dong
1,
Jie Yang
2,* and
Benoit Louis
1,*
1
Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES), UMR 7515, CNRS, University Strasbourg, F-67087 Strasbourg, France
2
Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(10), 987; https://doi.org/10.3390/catal15100987
Submission received: 9 September 2025 / Revised: 9 October 2025 / Accepted: 13 October 2025 / Published: 15 October 2025
(This article belongs to the Special Issue Catalytic Transformation of Biomass: From Waste to Fuels, Chemicals and High-Value Products)
Browse Figures
Versions Notes

Abstract

Alkali metal-modified M-ZSM-5 zeolites (M: Li+, Na+, K+) were synthesized by cationic exchange and characterized using ICP-MS, XRD, N2 adsorption–desorption, Py-IR and NH3-TPD techniques to evaluate their elemental composition, structure, textural and acidic properties. In addition, XPS and DFT calculations were employed to study the effects of metal ion doping on the electronic structure and catalytic behavior. The latter catalytic performance was assessed in the methanol-to-olefin (MTO) reaction. The results showed that alkali metal doping facilitated the enhancement of the zeolite structural stability, adjustment of acid density, and increase in the adsorption energy of light olefins onto the active sites. During the reaction, olefin products shifted from Brønsted acid sites to alkali metal sites, effectively minimizing hydrogen transfer reactions. This change in the active site nature promoted the olefin cycle, resulting in higher yields in propylene and butylenes, reduced coke deposition, and prolonged catalyst lifetime. Among all zeolites, Li-exchanged ZSM-5 exhibited the best and extending the catalyst lifetime by 5 h.

1. Introduction

Light olefins (ethylene, propylene and butylenes) are important building blocks of the modern chemical industry, being widely used in the production of polyolefins, synthetic rubber, fine chemicals and fuel additives [1,2,3]. Currently, industrial production of light olefins mainly relies on steam cracking and fluid catalytic cracking (FCC). However, these petroleum-based processes suffer from high energy consumption, large carbon emissions, and fluctuating feedstock supply, making them increasingly unsustainable toward stringent carbon footprint reduction policies [4,5]. In contrast, the MTO process uses coal, natural gas, or biomass gasification to produce methanol, providing a flexible and more sustainable alternative by reducing dependence on petroleum resources [6]. Since its discovery by accident in the 1970s, the MTO reaction has undergone decades of research and technological advancements, leading to successful industrial applications, particularly in China [7,8,9]. In line with the global carbon neutrality agenda, improving the selectivity toward light olefins and extending the catalyst lifetime have become key research priorities in the MTO chemistry [10,11].
The MTO reaction primarily occurs over zeolite-based acid catalysts and follows a dual-cycle mechanism, initially proposed by Dahl and Kolboe, consisting of an olefin-based cycle and an aromatic-based cycle [12,13]. The olefin cycle involves methylation, oligomerization, and cracking of higher olefins, favoring the formation of propylene and butylenes, whilst the aromatic cycle is dominated by polymethylbenzenes (polyMBs) as intermediates, accompanied by hydrogen transfer and condensation reactions, as shown in Figure S1 in the Supplementary Material. Hence, those consecutive reactions are leading to an increased ethylene production and coke formation [14]. Previous studies have demonstrated that the structure and acidity of the catalyst play a crucial role in regulating the reaction pathways, where fine-tuning the acidity can enhance the yield in light olefins while drastically reducing undesired aromatic formation and coke deposition [15].
ZSM-5 zeolite, due to its unique MFI microporous topology, strong acidity, and high framework stability, is widely employed in the conversion of methanol into hydrocarbons [16,17]. The strong Brønsted sites, bridging (Si–(OH)–Al) groups, in the ZSM-5 zeolite serve as primary active centers in acid-catalyzed paths involved in the MTO chemistry [18,19,20]. However, excessive Brønsted acid density promotes hydrogen transfer reactions, intensifying the aromatic cycle, thus accelerating catalyst deactivation, and reducing the selectivity toward light olefins [21,22]. Several modification strategies have been undertaken to regulate the acidity: including a fine-tuning of aluminum distribution, Si/Al ratio optimization, as well as metal ion incorporation to improve the catalytic performance [15,23,24]. Among those strategies, metal cations, due to their strong electron acceptability, can effectively regulate the overall acidity of the catalyst by substituting the protons of Brønsted acid sites or interacting with the framework aluminum, thereby influencing the MTO reaction pathway [25,26,27,28].
The catalytic behavior of ZSM-5 zeolites modified with alkali metal ions shows marked differences. Michael Dyballa et al. [29] systematically investigated the MTO reaction performance of H-ZSM-5 with a Si/Al ratio of 20 after partial ion exchange with Li, Na, and Cs. Their results revealed only a slight increase in propylene selectivity, while the catalyst lifetime was consistently shortened. They attributed this to the steric limitations of alkali metal ions, which reduce the accessible pore space of the zeolite and thus suppress reactivity. In contrast, Yajun Ji et al. [30] explored various alkali-metal-modified ZSM-5 catalysts (nSi/nAl = 50) for the supercritical cracking of n-dodecane. They found that the introduction of Na+ provided the most pronounced enhancement of catalytic activity, which was explained by the suppression of secondary reactions—such as light olefin consumption and coke formation—via a reduction in the number of strong acid sites. Similarly, Chanon Auepattana-aumrung et al. [31] demonstrated that Na+ ions selectively neutralized the strong Brønsted acid sites in ZSM-5, thereby inhibiting hydrogen transfer during 1-butene cracking, which in turn reduced coke deposition and improved selectivity. Among the samples they studied, Na-ZSM-5 with a Si/Al ratio of 20 yielded the highest target product selectivity.
These studies highlight that alkali metal ion modification can effectively tune both the strength and density of acid sites in ZSM-5. However, the outcome is highly dependent on the reaction system and the intrinsic composition of the zeolite, particularly its aluminum content. In addition, Naji et al. [32] reported significant differences in the ability of different alkali cations to neutralize the framework negative charge. Specifically, Li+, Na+, and K+ are considered “structure-stabilizing” cations because they effectively balance the framework charge and remain stably incorporated within the channels without compromising structural integrity. By contrast, Cs+ exhibits a “structure-disrupting” character due to its lower charge-compensation efficiency, which can induce local framework degradation or even collapse.
Nevertheless, most existing studies focus on a single Si/Al ratio, leaving the effect of alkali metal modification under varying aluminum contents insufficiently understood. Moreover, the MTO reaction imposes stringent demands on catalyst lifetime and light olefin selectivity, yet the beneficial versus detrimental effects of alkali modification on MTO remain ambiguous. This gap in knowledge hampers the rational design of optimized catalysts.
Against this background, the present work investigates Li+, Na+, and K+ modifications of H-ZSM-5 with different aluminum contents, systematically evaluating their effects on catalyst structure, acidity, adsorption behavior, and MTO performance. Our findings show that Li-modified ZSM-5 with high aluminum content dramatically extends catalyst lifetime while simultaneously enhancing the selectivity toward light olefins. Density functional theory (DFT) calculations were further employed to elucidate the underlying reasons for this improvement. Overall, this study not only broadens the application scope of alkali-metal-modified MFI zeolites in MTO but also offers a new strategy for designing efficient and durable catalysts for a variety of acid-catalyzed reactions.

2. Results and Discussion

2.1. The Effect of Aluminum Content on the Catalytic Performance of Alkali Metal-Doped ZSM-5 Zeolite

In order to study the effect of Al content on the properties of alkali metal ion modified ZSM-5 catalyst, the MTO catalytic properties of different parent Na-ZSM5 zeolites and M-ZSM-5 (M = Li, Na, K) zeolites prepared by cationic exchange were investigated.

2.1.1. MTO Reaction Properties of Na-ZSM-5 Zeolites with Different Si/Al Ratios

The catalytic performance of Na-ZSM-5 zeolites was evaluated under MTO reaction conditions at a weight hourly space velocity (WHSV) of 1.35 h−1 and a reaction temperature of 500 °C.
Experimental results show that hydrothermally synthesized Na-ZSM-5 zeolites with a Si/Al ratio greater than 30 produce almost no light olefins in the MTO reaction, whereas samples with a Si/Al ratio below 30 exhibit significant catalytic activity. As illustrated in Figure 1, Na-ZSM-5 with Si/Al ratios below 30 maintains a light olefin (ethylene, propylene, and butylene) selectivity of 44–61% (left axis, bar chart), with a catalytic lifetime of 2–4 h (right axis, green spheres).

2.1.2. MTO Reaction Properties of Alkali Metal Ion-Loaded ZSM-5 Zeolite

Figure 2 provides a clear comparison of the MTO catalytic behavior of M-ZSM-5 zeolites (M = Li, Na, K) prepared by introducing alkali metal ions into H-ZSM-5 frameworks with different Si/Al ratios. For the low-aluminum H-ZSM-5 samples (CBV280 and ZP90), modification with any of the three alkali metals resulted exclusively in the formation of dimethyl ether (DME), with no detectable peaks corresponding to the target light olefins. In contrast, the high-aluminum samples (CBV30 and H16) modified with Li+ or Na+ exhibited noticeable catalytic activity, producing light olefins together with alkanes and certain heavier hydrocarbons. However, none of the K+-modified zeolites showed MTO activity. These results demonstrate that the effect of alkali metal modification is strongly dependent on the aluminum content of the parent zeolite. This correlation can be attributed to the direct relationship between aluminum content and the number of Brønsted acid sites, while alkali metal ions suppress these sites to varying extents depending on their nature, ultimately governing the catalytic performance [17].
On the basis of the analysis in Section 2.1 regarding the influence of aluminum content, it can be concluded that the catalytic behavior of both hydrothermally synthesized Na-ZSM-5 and ion-exchanged alkali-metal ZSM-5 is significantly controlled by aluminum content, with MTO activity only observable in high-aluminum zeolites. Accordingly, subsequent studies will employ H-ZSM-5 with a Si/Al ratio of 15 (CBV3024E, Zeolyst) as the parent zeolite, and different alkali metal ion loadings will be introduced to optimize its physicochemical properties, with the goal of enhancing both the selectivity toward light olefins and the catalytic lifetime in the MTO reaction.

2.2. The Effect of Different Alkali Metal Ions on the Physicochemical Properties of ZSM-5 Zeolite

Modification of H-CBV30 zeolite of equal mass was carried out using solutions of lithium, sodium, and potassium of equal concentration to investigate the effects of different alkali metal cations on the physicochemical properties of zeolite.

2.2.1. Effect on Zeolite Structure and Crystallinity

XRD patterns confirmed that after incorporating Li+, Na+, or K+ into H-ZSM-5 zeolite (Figure 3), the samples maintained their characteristic diffraction peaks from the MFI topology. The diffraction peaks observed at 7.92°, 8.80°, 14.78°, 23.10°, 23.90° and 24.40° [33] further verified that alkali metal loading did not drastically impact the MFI topology. The relative crystallinity [34] was evaluated by XRD analysis by calculating the integrated peak area between 23° and 25°and presented in Table 1. The introduction of Li+ and K+ increased the crystallinity of ZSM-5 by approximately 22% and 57%, whereas Na+ exerted only a minor negative effect, reducing the crystallinity by approximately 3%. Previous studies have demonstrated that relative crystallinity serves as an indicator of zeolite thermal stability [35], and the intervention of alkali metal ions neutralizes the negative charge of the zeolite framework and enhances the stability of the framework. Accordingly, the impact of different alkali metal ions on the structural stability of ZSM-5 follows the trend: K+ > Li+ > H+ > Na+.

2.2.2. Effect on Pore Size and Surface Area

As shown in Figure 4a, the N2 adsorption–desorption isotherms of ZSM-5 zeolites modified with Li+, Na+, and K+ show that the adsorption and desorption curves of all samples are highly consistent. In the low-pressure region (P/P0 < 0.1), the isotherms exhibit monolayer adsorption behavior. In the intermediate pressure range (0.1 < P/P0 < 0.8), the adsorption approaches saturation, while at high pressures (P/P0 approaching 1), there is a sharp increase in adsorption. The overall trend conforms to the characteristics of an IUPAC Type I isotherm, suggesting that the samples primarily possess a microporous structure. Notably, all modified zeolites exhibit distinct hysteresis loops in the medium-to-high pressure range (P/P0 > 0.4) [36], suggesting the presence of disordered mesopores. Compared with the parent H-CBV30, the total adsorption capacity decreases slightly for Li+ and Na+-modified samples, while the reduction is more pronounced for K-CBV30, implying that alkali-metal incorporation partially alters the pore structure.
Notably, the desorption branches of Na-CBV30 and K-CBV30 do not overlap with their respective adsorption branches, a phenomenon typically associated with pore blocking or cavitation effects [37,38]. This behavior suggests that Na+ and K+ introduction may lead to partial micropore obstruction or mesopore network rearrangement, resulting in irreversible adsorption and incomplete desorption of nitrogen. By contrast, Li-CBV30 exhibits a nearly reversible adsorption–desorption curve, indicating that its pore structure remains relatively intact after modification. These observations highlight that the degree of pore structure disturbance strongly depends on the type of alkali metal cation, with K+ exerting the most significant influence.
Pore size distribution analysis further reveals the structural evolution of the zeolite particles, as shown in Figure 4b. The parent H-CBV30 mainly features pores in the 4–10 nm range, indicative of intercrystalline mesopores [39]. Li+ and Na+ modification preserves this distribution, whereas K+ modification leads to the disappearance of mesopores. Combined with XRD results, which show no obvious framework collapse, it can be inferred that K+ incorporation suppresses the formation of intercrystalline mesopores.
In addition, as shown in Table 1, the specific surface area of all modified zeolites was slightly reduced compared to the pristine H-zeolite. This trend, consistent with previous literature reports [40], further confirms the effect of alkali metal ion loadings on the relative surface area, mainly due to the fact that alkali metal cations possess a larger ionic radius than H+ and they occupy a portion of the zeolite skeleton pores, thereby reducing the available surface area. The order of influence degree is K+ > Na+ > Li+-ZSM-5, suggesting that K+ has the strongest perturbation on the porous framework of MFI-type zeolites.

2.2.3. Effect on Zeolite Acidity

Temperature-programmed desorption of ammonia (NH3-TPD) was used to characterize the acidity of ZSM-5 zeolites, which is an effective method to determine the acid site concentration and acid strength, as shown in Figure 5a. In general, the NH3-TPD curve can be deconvoluted with Gaussian fitting into two main peaks, the area of the peak estimates the acid content, and the position of the peak reflects the strength of the acid site [15]. Low-temperature peaks are associated with weakly adsorbed NH3, such as adsorption by H-bonding, silanol groups (Si-OH), and aluminum sites outside the skeleton [40]. In contrast, the high-temperature peaks reflect the need for higher temperatures for NH3 desorption, for instance Si–OH–Al sites, indicating that these sites are more acidic [19].
As shown in Table 2, the results of NH3-TPD show that the acid centers decrease after alkali metal loading, indicating that the alkali metal ions partially replace the H+ in the Si-OH-Al group of zeolite. Specifically, the Li+ and Na+ modified zeolites retain some strong acid sites, while the K+ modified zeolites completely eliminate the strong acid sites. This suggests that under the same loading conditions, the ability of alkali metal ions to replace strong acid sites follows the order of K+ > Na+ > Li+. According to the literature [30], this trend is related to the hydration radius of alkali metal cations, and ions with smaller hydration radius are more likely to enter within the zeolite channels and effectively replace H+, thus reducing the density of strong acid sites, the order of hydration radius of alkali metal cations is known to be K+ < Na+ < Li+.
In addition, the NH3 desorption temperature decreased significantly after loading with alkali metals, indicating that the strength of both strong and weak acid sites was weakened. This effect may be due to the electrostatic properties of the alkali metal ions, which favor the adsorption and desorption of NH3 while reducing the acid site strength. This change may enhance the occurrence of the olefin cycle during the MTO reaction.
To further investigate the mechanism of acid site regulation, pyridine-adsorbed Fourier transform infrared spectroscopy (Py-IR) was employed to quantitatively analyze the distribution of Brønsted and Lewis acid concentrations of the zeolites under different pyridine desorption temperatures. The spectrum obtained at 150 °C is shown in Figure 5b, while those recorded at 250 °C and 350 °C are provided in the Supplementary Material (Figure S2). The characteristic peaks at 1540 cm−1 and 1450 cm−1 correspond to the adsorption of pyridine on Brønsted acid and Lewis acid sites, respectively [41], while the peak at 1490 cm−1 represents the co-adsorption of pyridine on the two types of acid sites. Considering that the MTO reaction mainly depends on the Brønsted acid center, Table 2 summarizes the Brønsted and Lewis acid contents of each zeolite at 150 °C. The results showed that the Brønsted acid concentration was significantly reduced after alkali metal loading compared to the original H-ZSM-5 zeolite, which is consistent with the results of NH3-TPD, further supporting the inference that H+ in the Si−OH−Al structure was partially replaced by alkali metal cations. In addition, the Lewis acid content of the alkali metal-modified samples also decreased, which may be related to the alkali metal-framework interaction or the change in the coordination environment of Al3+ in part of the framework [42].
Studies have shown that changes in the concentration of zeolite acid sites have a significant impact on the MTO (methanol to olefins) reaction pathway. Especially when the Brønsted acid concentration is low, the contribution of the olefin cycle mechanism is enhanced, which is conducive to the formation of light olefins and prolongs the catalyst life. On the contrary, when the acid site density is high, it is easier to stimulate the aromatic cycle mechanism, promote side reactions and carbon deposition, and ultimately lead to rapid catalyst deactivation [16].

2.2.4. Speciation of Alkali Metal Ions in Zeolite Channels

X-ray photoelectron spectroscopy (XPS) was employed to investigate the oxidation states and distribution of alkali metal ions in ZSM-5 zeolites. Figure 6a presents the XPS measurements of ZSM-5 samples modified with different alkali metals. It is noteworthy that the presence of O, Si, Al, C and their respective alkali metal elements could be assessed. Notably, the characteristic peaks corresponding to Li, Na and K were observed in the modified ZSM-5 zeolites, as shown in Figure 5b, Figure 5c, and Figure 5d respectively, indicating that these metal ions were successfully introduced and evenly distributed in the channels.
Further analysis of the O 1s spectrum (Figure 6e) shows that after the introduction of alkali metals, the O 1s peak splits towards high binding energy and low binding energy. This indicates that the chemical environment of oxygen has changed, which may be related to the formation of a new M–O–Si/Al bond structure after some acidic sites are replaced by alkali metal ions. In addition, this change may also originate from the presence of different forms of oxygen, such as framework oxygen (Si–O–Al), non-bridging oxygen (Si-O-M+), and surface adsorbed hydroxyl or moisture [43,44]. High-resolution XPS spectra of different alkali metals further confirmed the coexistence of alkali metal ions and possible oxide or hydroxyl species in modified ZSM-5 (Figure 6b–d). This finding shows that the introduction of alkali metals not only changes the local electronic structure of zeolite, but also plays a crucial role in tuning the acidity and hence the catalytic properties of the zeolite.

2.3. Effect of Alkali Metal Loading on the Physicochemical Properties of Zeolites

2.3.1. MTO Catalytic Performance After Optimizing Alkali Metal Loading

To control alkali metal loading, the concentration of metal salt solutions was varied while keeping other conditions constant. Using the H-CBV30 zeolite as the parent material, ion exchange was carried out with Li+, Na+, and K+ solutions at concentrations of 0.25, 0.5, and 1.0 mol/L, under a solid-to-liquid ratio of 5.5 g/L. The modified samples were then tested for MTO performance at 500 °C and WHSV = 1.35 h−1.
The MTO catalytic results indicate that K+, regardless of loading amount, completely suppresses the catalytic activity of the zeolite, suggesting that it strongly neutralizes the Brønsted acid sites. For Li+- and Na+-modified zeolites, no catalytic activity was observed at a loading of 1.0 mol/L, while lower loadings helped to preserve part of the catalytic function. Figure 7 presents the methanol conversion and product selectivity of the samples that exhibited MTO activity.
As shown in Figure 7a, the parent zeolite H-CBV30 achieved full methanol conversion at the beginning of the reaction. However, after approximately 13 h, the conversion dropped sharply from 95% to around 40%. Taking 70% conversion as the criterion for deactivation, the catalytic lifetime of the parent zeolite was defined as 13 h. In contrast, the Na+-modified zeolites at both loadings fell below 70% conversion within 5–6 h. For Li+ modification, the catalyst with a loading of 0.5 mol/L showed a lifetime comparable to the parent zeolite (about 13 h), while the 0.25 mol/L sample exhibited a prolonged lifetime of around 18 h, extending the stability by nearly 5 h. Moreover, the decline in conversion for the Li+-modified sample was more gradual, indicating improved stability.
Figure 7b illustrates the product selectivity during the MTO reaction. The focus of this study is on light olefins. The parent H-CBV30 zeolite delivered a light olefin selectivity of 62%. Na+ modification did not improve this parameter, as the selectivity remained at or below 62%. In contrast, Li+ modification significantly enhanced light olefin formation: the 0.25 mol/L sample reached a selectivity of 71%, representing an increase of about 9% compared with the parent zeolite; the 0.5 mol/L sample showed a slightly lower value of 68%, suggesting that higher Li+ loading could suppress light olefin production.
In summary, the influence of alkali metal cations on the MTO performance of H-ZSM-5 varies significantly with the loading amount. K+ completely deactivates the catalyst; Na+ shows no improvement; while Li+ distinctly enhances catalytic behavior. At the optimal loading (Li-CBV30-0.25 mol/L), the catalyst lifetime was extended by ~5 h, and the selectivity toward light olefins improved by ~9%.
The C4 hydrogen transfer index (C4-HTI) and the (propylene–ethylene)/ethylene ratio (PE/E) are commonly employed to evaluate the relative contributions of the olefin and aromatic cycles. The C4-HTI is calculated as C4°/(C4° + C4=), where C4° and C4= represent the selectivities of butanes and butenes, respectively. A higher C4-HTI indicates a greater tendency toward the aromatic cycle. In contrast, PE/E is defined as (propylene − ethylene)/ethylene, and a higher value reflects a stronger preference for the olefin cycle [15].
As shown in Figure 8, the C4-HTI values derived from product selectivities follow the order: H-CBV30 > Na-CBV30 > Li-CBV30. The PE/E ratios, however, display the opposite trend: Li-CBV30 > Na-CBV30 > H-CBV30. These results suggest that, during the MTO reaction, H-ZSM-5 favors the aromatic cycle, Na-ZSM-5 exhibits an intermediate behavior, and Li-ZSM-5 promotes the olefin cycle, with the latter thereby generating more propylene and butenes, which contributes to extending the catalyst lifetime [14].

2.3.2. Properties of Li-ZSM-5 Zeolites with Different Loading Amounts

Due to the excellent catalytic performance of Li-ZSM-5 zeolites in the MTO reaction, a series of Li-modified zeolites with different loadings were systematically characterized, as summarized in Table 3. It is worth noting that with increasing Li loading, the Brønsted acid sites, which serve as the primary active centers in the MTO reaction, decrease significantly. A difference in Li loading of 0.24 wt% leads to nearly a 90 μmol/g variation in acid content, indicating a strong ability of Li to diminish Brønsted acid sites, thereby directly affecting the catalytic activity. In addition, Li loading also affected the textural and structural properties of the zeolite, including specific surface area, relative crystallinity, Lewis acid sites, and the distribution of strong and weak acid sites detected by NH3-TPD. Among these, the specific surface area and relative crystallinity showed a clear negative correlation with increasing Li loading, indirectly impacting the catalytic performance, while the distribution of Lewis acid sites and NH3-TPD-derived acid strengths did not exhibit a clear trend and are therefore not further discussed. In summary, Li modification of ZSM-5 zeolites can effectively regulate their acidity and structural characteristics, but precise control of Li loading is essential.
In summary, introducing a moderate amount of Li+ into high-aluminum H-ZSM-5 can significantly enhance its performance in MTO reactions, especially in terms of olefin selectivity and catalyst durability. Na+ and K+ modifications, however, failed to improve and in some cases worsened performance. These findings offer practical insights for tuning ZSM-5 catalysts in MTO applications.

2.4. DFT Study on the Mechanism of Li+-Enhanced MTO

According to the dual-cycle mechanism, hydrogen transfer serves as the key bridge between the aromatic and olefin cycles, and the introduction of Li+ appears to inhibit this process. To verify this hypothesis, density functional theory (DFT) calculations were performed to construct an H-ZSM-5 zeolite model with a Si/Al ratio of 15. Li+ ions were introduced in the MFI structure at concentrations consistent with experimental conditions to develop the Li-ZSM-5 model. Ethylene (C2H4) and propylene (C3H6) molecules were then incorporated into the straight channels of these models, and their adsorption energies were calculated, as summarized in Table 4.
The results indicate that replacing H+ by the more metallic Li+ in H-ZSM-5 increases the adsorption energy of ethylene and propylene. This effect is likely due to the electronegativity of the C=C double bond in olefins [45], which interacts via long-range electrostatic forces with either H+ or Li+ within the zeolite framework. Further analysis suggests that Li doping at the bridging oxygen site causes olefinic products to migrate away from the O-H site and toward the Li site, thereby hindering further hydrogenation reactions, the general process is shown in Figure 9.

3. Experimental

3.1. Catalyst Preparation

3.1.1. Hydrothermal Synthesis of Na-ZSM-5 Zeolite

The inorganic precursors solution was prepared by mixing silicon sources (TEOS 99%, Sigma-Aldrich, St. Louis, MO, USA) and aluminum sources (NaAlO2, Riedel de Haën, Seelze, Germany), with defined molar ratios, with a template agent (TPAOH, 20 wt% aqueous solution, Sigma-Aldrich, St. Louis, MO, USA), according to our former study [46]. After stirring for 3 h, the mixture was transferred into an autoclave and subjected to hydrothermal crystallization at 170 °C for 3 days. The recovered crystalline solids were cleaned with deionized water until the pH was neutral, and then calcined in a Muffle furnace at 550 °C for 12 h. The resulting samples were designated as NaX, where X represents the molar ratio of the silicon and aluminum sources.

3.1.2. Cationic Exchange Method for Alkali Metal Ion-Loaded ZSM-5 Zeolite

Four ZSM-5 zeolite samples with different aluminum contents were selected as parent materials, including HCZP90 (Si/Al = 42, Clariant, Muttenz, Switzerland), hydrothermally synthesized H-ZSM-5 (home-made, Si/Al = 16), and two NH4-ZSM-5 zeolites (Zeolyst, Kansas City, KS, USA), named CBV3024E (Si/Al = 15) and CBV28014 (Si/Al = 140). Prior to use, NH4-ZSM-5 samples were calcined at 500 °C for 5 h to remove NH3 and yield acidic H-form.
Alkali metal ion solutions of varying molar concentrations were prepared using analytically pure MNO3 (M = Li, Na, K). The pristine H-ZSM-5 zeolite powders were dispersed in MNO3 solutions (Merck, Darmstadt, Germany) at a solid-to-liquid ratio of 5.5 g/L and magnetically stirred at 80 °C for 3 h at a speed of 1000 rpm. The powders were then dried and calcined at 550 °C for 5 h. The resulting samples were designated by the alkali metal ion, its concentration, and the parent zeolite name. For example, Na-CBV30-0.25 refers to the CBV3024E zeolite ion-exchanged with a 0.25 mol/L NaNO3 solution.

3.2. Characterization

The powder XRD patterns were recorded using a Bruker AXS D8 Advance diffractometer (Bruker AXS GmbH, Karlsruhe, Germany), and the Cu Kα radiation was between 5° and 65° 2θ range. X-ray photoelectron spectroscopy (XPS, Scienta Omicron, Uppsala, Sweden) was used to determine the oxidation state and the distribution of the elements, using an ultra-high vacuum system (10−9 mbar) with VSWClass WA hemispherical electron Energy Analyzer (radius: 150 mm) and monochromatic Al Kα X-ray source (hν = 1486.6 eV). The specific surface areas and pore volumes of the samples were obtained by N2 adsorption–desorption isotherms at 77 K, using ASAP 2020 Micromeritics equipment (Micromeritics Instrument Corporation, Norcross, GA, USA). Temperature programmed desorption of NH3 (NH3-TPD) was performed on a Micromeritics AutoChem II instrument (Micromeritics Instrument Corporation, Norcross, GA, USA).
Pyridine adsorption infrared spectroscopy (Py-IR) measurements were performed using an in situ infrared spectrometer (Bruker Tensor 27, Vertex 80v, Karlsruhe, Germany). The Li content was analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using Agilent 7850 ICP-MS(Agilent Technologies Inc, Santa Clara, CA, USA). The determination of elemental composition on zeolite surfaces was also performed using an Epsilon3XL Energy dispersive X-ray fluorescence (XRF, Malvern Panalytical, Worcestershire, United Kingdom) spectrometer, equipped with silver tubes.

3.3. Methanol-to-Olefin (MTO)

The MTO reaction was carried out at 500 °C in a continuous-flow fixed-bed quartz tubular reactor (length: 28 cm, inner diameter: 1 cm, Merck, Darmstadt, Germany) under atmospheric pressure. The prepared ZSM-5 zeolite samples were pelletized, crushed and sieved to a particle size of 14–22 mesh before being loaded in the middle part of a vertical quartz tube, with quartz wool packed on both sides of the catalyst bed.
During the reaction, methanol (99.9%, HPLC pure grade, Fisher Chemical, Pittsburgh, PA, USA) was fed using argon as carrier gas. The weight hourly space velocity (WHSV) was controlled by adjusting the carrier gas flow rate and catalyst mass, with additional corrections based on the temperature of the methanol saturator. The reaction products were analyzed using a gas chromatograph (HP5890 Series I, Agilent Technologies, Santa Clara, CA, USA) equipped with a 50 m capillary column (PONA) and a flame ionization detector (FID). Methanol conversion and product selectivity were evaluated after 1 h of operation. The degree of methanol conversion of 70% was arbitrarily defined as the catalyst deactivation threshold.

3.4. DFT Calculations

The ZSM-5 model with MFI configuration was obtained from Xing’s work [47] with the lattice parameter of a = 20.373 Å, b = 20.077 Å and c = 13.506 Å, respectively. For the Al-substituted ZSM-5 (Al-ZSM-5), 6 out of 8 equivalent Si on the T7 site were replaced by Al along with the addition of H on the corresponding T7-O-T11 site to achieve the most stable structure of 15:1 Si/Al ratio. Al-O-Al bonds were avoided during model construction in order to obey the Löwenstein rule. The Vienna Ab initio Simulation Package (VASP) was used to conduct the DFT calculations [48]. The Projected Augmented Waves (PAWs) basis and Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional was employed [49]. Grimme’s DFT-D3 method [50] was used to account for the long-range van der Waals interaction. The 400 eV energy cut-off was used during the optimization. Gamma-only k-point mesh was employed for the calculations. All the structures were optimized until the residual force less than 0.03 eV/Å.
The adsorption energy (Eads.) was calculated according to Equation (1) below:
Eads. = Ecomp. − Esub. − Emol
Ecomp., Esub., and Emol. stand for the energy of the adsorption complex, the substrates and the small molecules, respectively.

4. Conclusions

This study systematically investigates the effects of alkali metal cations (Li+, Na+, K+) on the physicochemical properties of ZSM-5 zeolites and their performance in the methanol-to-olefin (MTO) reaction. The results show that alkali metal incorporation enhances the structural stability of the zeolite, modifies the acid site density, and influences both pore structure and surface area. However, the introduction of K+ completely deactivates the catalyst in the MTO reaction. Li+ and Na+ only exhibit catalytic activity when introduced into high-aluminum-content ZSM-5. While Na+ incorporation does not improve the catalytic performance, an appropriate amount of Li+ significantly enhances MTO activity, particularly in terms of olefin selectivity and catalyst durability, under optimal conditions, the selectivity toward light olefins reaches 71% (an increase of ~9%), and the catalyst lifetime is extended by approximately 5 h. Computational analysis further reveals that Li+ incorporation increases the adsorption energy of light olefins at the active sites. During the reaction, olefin products tend to migrate from Brønsted acid sites to Li+ sites, effectively suppressing hydrogen transfer and other side reactions. This migration promotes the olefin cycle, thereby improving propylene and butylene yields, reducing coke formation, and prolonging catalyst stability. These findings provide valuable insights for the rational design and optimization of ZSM-5 catalysts in MTO applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15100987/s1, Figure S1: Schematic Diagram of the double cycle mechanism in the stable stage of MTO reaction; Figure S2: PY-IR spectra of pyridine adsorption at 25 °C, and 350 °C; Figure S3: Reproducibility results of the MTO reaction over Li-CBV30-0.25 mol zeolite; Table S1: PY-IR characterization results.

Author Contributions

S.D.: Writing—original draft, Writing—Review & Editing, Formal Analysis, Validation, Visualization and Investigation; J.Y.: Writing—review & editing, Visualization; B.L.: Validation, Writing—review & editing, Visualization, Supervision, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC, grant number 52300160). SD acknowledges the China Scholarship Council (CSC) for supporting her PhD studies at the University of Strasbourg.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors would like to thank the Solid Waste High-Value Resource Utilization Innovation Team at Kunming University of Science and Technology for their valuable support and technical guidance during the experimental work. The authors also appreciate the analytical assistance provided by the University of Strasbourg.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Tabibian, S.S.; Sharifzadeh, M. Statistical and analytical investigation of methanol applications, production technologies, value-chain and economy with a special focus on renewable methanol. Renew. Sustain. Energy Rev. 2023, 179, 113281. [Google Scholar] [CrossRef]
  2. Almuqati, N.S.; Aldawsari, A.M.; Alharbi, K.N.; González-Cortés, S.; Alotibi, M.F.; Alzaidi, F.; Dilworth, J.R.; Edwards, P.P. Catalytic production of light olefins: Perspective and prospective. Fuel 2024, 366, 131270. [Google Scholar] [CrossRef]
  3. Witoon, T.; Lapkeatseree, V.; Numpilai, T.; Cheng, C.K.; Limtrakul, J. CO2 hydrogenation to light olefins over mixed Fe–Co–K–Al oxides catalysts prepared via precipitation and reduction methods. Chem. Eng. J. 2022, 428, 131389. [Google Scholar] [CrossRef]
  4. Alabdullah, M.A.; Gomez, A.R.; Vittenet, J.; Bendjeriou-Sedjerari, A.; Xu, W.; Abba, I.A.; Gascon, J. A viewpoint on the refinery of the future: Catalyst and process challenges. ACS Catal. 2020, 10, 8131–8140. [Google Scholar] [CrossRef]
  5. Zhao, S.; Li, H.; Wang, B.; Yang, X.; Peng, Y.; Du, H.; Zhang, Y.; Han, D.; Li, Z. Recent advances on syngas conversion targeting light olefins. Fuel 2022, 321, 124124. [Google Scholar] [CrossRef]
  6. Tian, P.; Wei, Y.; Ye, M.; Liu, Z. Methanol to olefins (MTO): From fundamentals to commercialization. ACS Catal. 2015, 5, 1922–1938. [Google Scholar] [CrossRef]
  7. Van Bekkum, H.; Flanigen, E.M.; Jansen, J.C. Introduction to Zeolite Science and Practice, 2nd ed.; Elsevier Science: Amsterdam, The Netherlands, 2001; pp. 248–249. [Google Scholar]
  8. Sun, H.; Zhang, Z.; Sun, H.; Yao, B.; Lou, C. Numerical Investigation of Exergy Loss of Ammonia Addition in Hydrocarbon Diffusion Flames. Entropy 2022, 24, 922. [Google Scholar] [CrossRef] [PubMed]
  9. Xu, S.; Zhi, Y.; Han, J.; Zhang, W.; Wu, X.; Sun, T.; Wei, Y.; Liu, Z. Advances in Catalysis for Methanol-to-Olefins Conversion. Adv. Catal. 2017, 61, 37–122. [Google Scholar] [CrossRef]
  10. Boltz, M.; Losch, P.; Louis, B. A General Overview on the Methanol to Olefins Reaction: Recent Catalyst Developments. Adv. Chem. Lett. 2014, 1, 247–256. [Google Scholar] [CrossRef]
  11. Olsbye, U.; Svelle, S.; Bjørgen, M.; Beato, P.; Janssens, T.V.W.; Joensen, F.; Bordiga, S.; Lillerud, K.P. Conversion of methanol to hydrocarbons: How zeolite cavity and pore size controls product selectivity. Angew. Chem. Int. Ed. 2012, 51, 5810–5831. [Google Scholar] [CrossRef]
  12. Dahl, I.M.; Kolboe, S. On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34: I. Isotopic labeling studies of the co-reaction of ethene and methanol. J. Catal. 1994, 149, 458–464. [Google Scholar] [CrossRef]
  13. Ilias, S.; Bhan, A. Mechanism of the catalytic conversion of methanol to hydrocarbons. ACS Catal. 2013, 3, 18–31. [Google Scholar] [CrossRef]
  14. Valecillos, J.; Epelde, E.; Albo, J.; Aguayo, A.T.; Bilbao, J.; Castaño, P. Slowing down the deactivation of H-ZSM-5 zeolite catalyst in the methanol-to-olefin (MTO) reaction by P or Zn modifications. Catal. Today 2020, 348, 243–256. [Google Scholar] [CrossRef]
  15. Xue, Y.; Li, J.; Wang, P.; Cui, X.; Zheng, H.; Niu, Y.; Dong, M.; Qin, Z.; Wang, J.; Fan, W. Regulating Al distribution of ZSM-5 by Sn incorporation for improving catalytic properties in methanol to olefins. Appl. Catal. B 2021, 280, 119391. [Google Scholar] [CrossRef]
  16. Yang, M.; Fan, D.; Wei, Y.; Tian, P.; Liu, Z. Recent progress in methanol-to-olefins (MTO) catalysts. Adv. Mater. 2019, 31, 1902181. [Google Scholar] [CrossRef]
  17. Chizallet, C.; Bouchy, C.; Larmier, K.; Pirngruber, G. Molecular views on mechanisms of Brønsted acid-catalyzed reactions in zeolites. Chem. Rev. 2023, 123, 6107–6196. [Google Scholar] [CrossRef] [PubMed]
  18. Haag, W.O.; Lago, R.M.; Weisz, P.B. The active site of acidic aluminosilicate catalysts. Nature 1984, 309, 589–591. [Google Scholar] [CrossRef]
  19. Mortier, W.J.; Sauer, J.; Lercher, J.A.; Noller, H. Bridging and terminal hydroxyls. A structural chemical and quantum chemical discussion. J. Phys. Chem. 1984, 88, 905–912. [Google Scholar] [CrossRef]
  20. Xia, W.; Wang, J.; Wang, L.; Chen, Q.; Chen, K. Ethylene and propylene production from ethanol over Sr/ZSM-5 catalysts: A combined experimental and computational study. Appl. Catal. B 2021, 294, 120242. [Google Scholar] [CrossRef]
  21. Guisnet, M.; Costa, L.; Ribeiro, F.R. Prevention of zeolite deactivation by coking. J. Mol. Catal. A Chem. 2009, 305, 69–83. [Google Scholar] [CrossRef]
  22. Müller, S.; Liu, Y.; Kirchberger, F.M.; Tonigold, M.; Sanchez-Sanchez, M.; Lercher, J.A. Hydrogen transfer pathways during zeolite catalyzed methanol conversion to hydrocarbons. J. Am. Chem. Soc. 2016, 138, 15994–16003. [Google Scholar] [CrossRef]
  23. Setiabudi, H.D.; Jalil, A.A.; Triwahyono, S.; Kamarudin, N.H.N.; Jusoh, R. Ir/Pt-HZSM5 for n-pentane isomerization: Effect of Si/Al ratio and reaction optimization by response surface methodology. Chem. Eng. J. 2013, 217, 300–309. [Google Scholar] [CrossRef]
  24. Smit, B.; Maesen, T.L.M. Towards a molecular understanding of shape selectivity. Nature 2008, 451, 671–678. [Google Scholar] [CrossRef]
  25. Yaripour, F.; Shariatinia, Z.; Sahebdelfar, S.; Irandoukht, A. Effect of boron incorporation on the structure, products selectivities and lifetime of H-ZSM-5 nanocatalyst designed for application in methanol-to-olefins (MTO) reaction. Microporous Mesoporous Mater. 2015, 203, 41–53. [Google Scholar] [CrossRef]
  26. Jiang, X.; Su, X.; Bai, X.; Li, Y.; Yang, L.; Zhang, K.; Zhang, Y.; Liu, Y.; Wu, W. Conversion of methanol to light olefins over nanosized [Fe,Al]ZSM-5 zeolites: Influence of Fe incorporated into the framework on the acidity and catalytic performance. Microporous Mesoporous Mater. 2018, 263, 243–250. [Google Scholar] [CrossRef]
  27. Chen, X.; Jiang, R.; Hou, H.; Zhou, Z.; Wang, X. Facile synthesis of an Mg-incorporated ZSM-5 zeolite from dual silicon sources and its application for conversion of methanol to olefins. ChemistrySelect 2021, 6, 7056–7061. [Google Scholar] [CrossRef]
  28. Zhou, Y.; Santos, S.; Shamzhy, M.; Marinova, M.; Blanchenet, A.M.; Kolyagin, Y.G.; Simon, P.; Trentesaux, M.; Sharna, S.; Ersen, O.; et al. Liquid metals for boosting stability of zeolite catalysts in the conversion of methanol to hydrocarbons. Nat. Commun. 2024, 15, 2228. [Google Scholar] [CrossRef] [PubMed]
  29. Dyballa, M.; Obenaus, U.; Blum, M.; Dai, W. Alkali metal ion exchanged ZSM-5 catalysts: On acidity and methanol-to-olefin performance. Catal. Sci. Technol. 2018, 8, 4440–4449. [Google Scholar] [CrossRef]
  30. Ji, Y.; Yang, H.; Yan, W. Effect of alkali metal cations modification on the acid/basic properties and catalytic activity of ZSM-5 in cracking of supercritical n-dodecane. Fuel 2019, 243, 155–161. [Google Scholar] [CrossRef]
  31. Auepattana-aumrung, C.; Suriye, K.; Jongsomjit, B.; Panpranot, J.; Praserthdam, P. Inhibition effect of Na+ form in ZSM-5 zeolite on hydrogen transfer reaction via 1-butene cracking. Catal. Today 2020, 358, 237–245. [Google Scholar] [CrossRef]
  32. Nagy, J.B.; Bodart, P.; Collette, H.; El Hage-Al Asswad, J.; Gabelica, Z.; Aiello, R.; Nastro, A.; Pellegrino, C. Aluminium distribution and cation location in various M-ZSM-5-type zeolites (M = Li, Na, K, Rb, Cs, NH4). Zeolites 1988, 8, 209–220. [Google Scholar] [CrossRef]
  33. Yuanyuan, S.; Li, Z.; Zhou, X.; Li, G.; Tan, M.; Ao, S.; Sun, W.; Chen, H. Mesoporous zeolite ZSM-5 confined Cu nanoclusters for efficient selective catalytic reduction of NOx by NH3. Appl. Catal. B 2024, 346, 123747. [Google Scholar] [CrossRef]
  34. ASTM D5758-01(2021); Test Method for Determination of Relative Crystallinity of Zeolite ZSM-5 by X-Ray Diffraction. The World Trade Organization Technical Barriers to Trade (TBT) Committee: Geneva, Switzerland, 2021. [CrossRef]
  35. Taherizadeh, A.; Harpf, A.; Simon, A.; Choi, J.; Richter, H.; Voigt, I.; Stelter, M. Thermochemical study of the structural stability of low-silicate CHA zeolite crystals. Results Chem. 2022, 4, 100466. [Google Scholar] [CrossRef]
  36. Qi, R.; Fu, T.; Wan, W.; Li, Z. Pore fabrication of nano-ZSM-5 zeolite by internal desilication and its influence on the methanol to hydrocarbon reaction. Fuel Process. Technol. 2017, 155, 191–199. [Google Scholar] [CrossRef]
  37. Matadamas, J.; Alférez, R.; López, R.; Román, G.; Kornhauser, I.; Rojas, F. Advanced and delayed filling or emptying of pore entities by vapor sorption or liquid intrusion in simulated porous networks. Colloids Surf. A Physicochem. Eng. Asp. 2016, 496, 39–51. [Google Scholar] [CrossRef]
  38. Kutarov, V.V.; Robens, E.; Tarasevich, Y.I.; Aksenenko, E.V. Adsorption hysteresis at low relative pressures. Theor. Exp. Chem. 2011, 47, 163–168. [Google Scholar] [CrossRef]
  39. Fu, T.; Chang, J.; Shao, J.; Li, Z. Fabrication of a nano-sized ZSM-5 zeolite with intercrystalline mesopores for conversion of methanol to gasoline. J. Energy Chem. 2017, 26, 139–146. [Google Scholar] [CrossRef]
  40. Miyamoto, T.; Katada, N.; Kim, J.H.; Niwa, M. Acidic property of MFI-type gallosilicate determined by temperature-programmed desorption of ammonia. J. Phys. Chem. B 1998, 102, 6738–6745. [Google Scholar] [CrossRef]
  41. Chen, K.; Wu, X.; Zhao, J.; Zhao, H.; Li, A.; Zhang, Q.; Xia, T.; Liu, P.; Meng, B.; Song, W.; et al. Organic-free modulation of the framework Al distribution in ZSM-5 zeolite by magnesium participated synthesis and its impact on the catalytic cracking reaction of alkanes. J. Catal. 2022, 413, 735–750. [Google Scholar] [CrossRef]
  42. Yarulina, I.; De Wispelaere, K.; Bailleul, S.; Goetze, J.; Radersma, M.; Abou-Hamad, E.; Vollmer, I.; Goesten, M.; Mezari, B.; Hensen, E.J.M.; et al. Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process. Nat. Chem. 2018, 10, 804–812. [Google Scholar] [CrossRef]
  43. Liu, C.; Luo, J.; Dong, H.; Zhao, Z.; Xu, C.; Abuelgasim, S.; Abdalazeez, A.; Wang, W.; Chen, D.; Tang, Q. Hydrogen-rich syngas production from biomass char by chemical looping gasification with Fe/Ca-based oxygen carrier. Sep. Purif. Technol. 2022, 300, 121912. [Google Scholar] [CrossRef]
  44. Du, Y.; Shi, T.; Guo, S.; Li, H.; Qin, Y.; Wang, Y.; He, C.; Wei, Y. Unraveling the intrinsic mechanism behind the retention of arsenic in the co-gasification of coal and sewage sludge: Focus on the role of Ca and Fe compounds. J. Hazard. Mater. 2024, 470, 134211. [Google Scholar] [CrossRef] [PubMed]
  45. Domingo, L.R.; Pérez, P.; Contreras, R. Reactivity of the carbon–carbon double bond towards nucleophilic additions: A DFT analysis. Tetrahedron 2004, 60, 6585–6591. [Google Scholar] [CrossRef]
  46. Zhang, Y.; Louis, B. Tailoring structure and acidity of ZSM-5 zeolite by algae carbon modification: Promoting effect in the MTO reaction. Microporous Mesoporous Mater. 2023, 350, 112431. [Google Scholar] [CrossRef]
  47. Xing, B.; Ma, J.; Li, R.; Jiao, H. Location, distribution and acidity of Al substitution in ZSM-5 with different Si/Al ratios—A periodic DFT computation. Catal. Sci. Technol. 2017, 7, 5694–5708. [Google Scholar] [CrossRef]
  48. Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15–50. [Google Scholar] [CrossRef]
  49. Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. [Google Scholar] [CrossRef]
  50. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H–Pu. J. Chem. Phys. 2010, 132, 154104. [Google Scholar] [CrossRef]
Catalysts 15 00987 g001
Figure 1. Selectivity and catalytic lifetime of Na-ZSM-5 zeolites in the MTO reaction.
Figure 1. Selectivity and catalytic lifetime of Na-ZSM-5 zeolites in the MTO reaction.
Catalysts 15 00987 g001
Catalysts 15 00987 g002
Figure 2. Gas chromatograms of the different products obtained during the MTO reaction over M-ZSM-5 zeolites with different Al contents loaded with various alkali metal ions (GC-SC: Gas chromatography-Standard time peak diagram).
Figure 2. Gas chromatograms of the different products obtained during the MTO reaction over M-ZSM-5 zeolites with different Al contents loaded with various alkali metal ions (GC-SC: Gas chromatography-Standard time peak diagram).
Catalysts 15 00987 g002
Catalysts 15 00987 g003
Figure 3. XRD patterns of ZSM-5 zeolites prior and after alkali metal loading (alkali metal solution concentration: 0.5 mol/L).
Figure 3. XRD patterns of ZSM-5 zeolites prior and after alkali metal loading (alkali metal solution concentration: 0.5 mol/L).
Catalysts 15 00987 g003
Catalysts 15 00987 g004
Figure 4. Adsorption–desorption isotherms (a) and pore size distributions (b) of different alkali metal-modified ZSM-5 zeolites (alkali metal solution concentration: 0.5 mol/L).
Figure 4. Adsorption–desorption isotherms (a) and pore size distributions (b) of different alkali metal-modified ZSM-5 zeolites (alkali metal solution concentration: 0.5 mol/L).
Catalysts 15 00987 g004
Catalysts 15 00987 g005
Figure 5. NH3−TPD profiles (a) of ZSM-5 zeolites modified with different alkali metals and FT−IR spectra of pyridine adsorption at 150 °C (b) (alkali metal solution concentration: 0.5 mol/L).
Figure 5. NH3−TPD profiles (a) of ZSM-5 zeolites modified with different alkali metals and FT−IR spectra of pyridine adsorption at 150 °C (b) (alkali metal solution concentration: 0.5 mol/L).
Catalysts 15 00987 g005
Catalysts 15 00987 g006
Figure 6. XPS results of ZSM-5 zeolites modified with different alkali metals: (a) survey spectrum; (b) Li 1s; (c) Na 1s; (d) K 2p; (e) O 1s (alkali metal solution concentration: 0.5 mol/L).
Figure 6. XPS results of ZSM-5 zeolites modified with different alkali metals: (a) survey spectrum; (b) Li 1s; (c) Na 1s; (d) K 2p; (e) O 1s (alkali metal solution concentration: 0.5 mol/L).
Catalysts 15 00987 g006
Catalysts 15 00987 g007
Figure 7. Methanol conversion (a) and product selectivities (b) obtained over alkali cation-modified ZSM-5 zeolites.
Figure 7. Methanol conversion (a) and product selectivities (b) obtained over alkali cation-modified ZSM-5 zeolites.
Catalysts 15 00987 g007
Catalysts 15 00987 g008
Figure 8. Comparison of C4-HTI and PE/E indices prior and after zeolite modification.
Figure 8. Comparison of C4-HTI and PE/E indices prior and after zeolite modification.
Catalysts 15 00987 g008
Catalysts 15 00987 g009
Figure 9. Schematic diagram of model adsorption of ethylene and propylene by different cationic ZSM-5 zeolite clusters.
Figure 9. Schematic diagram of model adsorption of ethylene and propylene by different cationic ZSM-5 zeolite clusters.
Catalysts 15 00987 g009
Table 1. Characterization data of different alkali metal modified ZSM-5 zeolites.
Table 1. Characterization data of different alkali metal modified ZSM-5 zeolites.
SampleLoaded Metal Cation Solution Concentration (mol/L) aTarget Cation Loading (wt%) bBET Surface Area (m2/g) cRelative Crystallinity d
H-CBV30//359100%
Li-CBV300.50.43341122%
Na-CBV300.51.4832597%
K-CBV300.53.89306157%
a. Preparation of target concentration solutions by precise weighing of solid solutes. b. Determination by XRF and ICP-MS (for Li). c. Total surface area obtained using adsorption data by BET method. d. Relative crystallinity calculated from the sum of peak intensities between 23 and 25°. H-CBV30 zeolite was chosen as the reference sample and set to 100% crystallinity.
Table 2. Number of acid sites present on the catalysts obtained by NH3-TPD and Py-IR adsorption (alkali metal solution concentration: 0.5 mol/L).
Table 2. Number of acid sites present on the catalysts obtained by NH3-TPD and Py-IR adsorption (alkali metal solution concentration: 0.5 mol/L).
SampleNH3-TPD (μmol/g) aPy-IR (μmol/g) b
Weak Acid ContentTemperature (°C)Strong Acid ContentTemperature (°C)Amount of B AcidAmount of L Acid
H-CBV3026030643578431360
Li-CBV3013729035518146346
Na-CBV30662712942446260
K-CBV303032820/23314
a. The amount of weak and strong acid sites is determined by the amount of ammonia (NH3) desorbed at 200–350 °C and 350–600 °C, respectively. b The amount of Brønsted and Lewis acid sites is determined by Py-IR and the amount of pyridine (Py) desorbed at 150 °C.
Table 3. Summary table of characterisation indicators for ZSM5 zeolite with different Li loadings.
Table 3. Summary table of characterisation indicators for ZSM5 zeolite with different Li loadings.
SampleTarget Cation Loading (wt%) aBET Surface Area (m2/g) bRelative Crystallinity cNH3-TPD (μmol/g) dPy-IR (μmol/g) e
Weak Acid ContentTemperature (°C)Strong Acid ContentTemperature (°C)Amount of B AcidAmount of L Acid
Li-CBV30-0.25 mol/L0.4345123%18729633523146346
Li-CBV30-0.5 mol/L0.43341122%13728928518116434
Li-CBV30-1 mol/L0.64313120%1673004053459379
a. Determination by ICP-MS. b. Total surface area obtained using adsorption data by BET method. c. Relative crystallinity calculated from the sum of peak intensities between 23 and 25°. H-CBV30 zeolite was chosen as the reference sample and set to 100% crystallinity. d. The amount of weak and strong acid sites is determined by the amount of ammonia (NH3) desorbed at 200–350 °C and 350–600 °C, respectively. e. The amount of Brønsted and Lewis acid sites is determined by Py-IR and the amount of pyridine (Py) desorbed at 150 °C.
Table 4. Adsorption energies of different cationic ZSM-5 zeolite clusters modelled for the adsorption of ethylene and propylene.
Table 4. Adsorption energies of different cationic ZSM-5 zeolite clusters modelled for the adsorption of ethylene and propylene.
ModelE-C2H4 (eV)E-C3H6 (eV)
H-ZSM-5−0.68−0.86
Li-ZSM-5−0.87−1.02
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Dong, S.; Yang, J.; Louis, B. Investigating the Impact of Incorporating Alkali Metal Cations on the Properties of ZSM-5 Zeolites in the Methanol Conversion into Hydrocarbons. Catalysts 2025, 15, 987. https://doi.org/10.3390/catal15100987

AMA Style

Dong S, Yang J, Louis B. Investigating the Impact of Incorporating Alkali Metal Cations on the Properties of ZSM-5 Zeolites in the Methanol Conversion into Hydrocarbons. Catalysts. 2025; 15(10):987. https://doi.org/10.3390/catal15100987

Chicago/Turabian Style

Dong, Senlin, Jie Yang, and Benoit Louis. 2025. "Investigating the Impact of Incorporating Alkali Metal Cations on the Properties of ZSM-5 Zeolites in the Methanol Conversion into Hydrocarbons" Catalysts 15, no. 10: 987. https://doi.org/10.3390/catal15100987

APA Style

Dong, S., Yang, J., & Louis, B. (2025). Investigating the Impact of Incorporating Alkali Metal Cations on the Properties of ZSM-5 Zeolites in the Methanol Conversion into Hydrocarbons. Catalysts, 15(10), 987. https://doi.org/10.3390/catal15100987

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop