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Article

Enhanced Stability and Selectivity in Pt@MFI Catalysts for n-Butane Dehydrogenation: The Crucial Role of Sn Promoter

by
Nengfeng Gong
1,2,†,
Gaolei Qin
1,2,
Pengfei Li
2,†,
Xiangjie Zhang
1,2,
Yan Chen
1,2,
Yong Yang
1,* and
Peng He
1,*
1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
2
School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 101408, China
*
Authors to whom correspondence should be addressed.
†
These authors contribute equally to this work.
Catalysts 2024, 14(11), 760; https://doi.org/10.3390/catal14110760
Submission received: 29 September 2024 / Revised: 21 October 2024 / Accepted: 23 October 2024 / Published: 29 October 2024
(This article belongs to the Section Nanostructured Catalysts)
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Abstract

:
The dehydrogenation of n-butane to butenes is a crucial process for producing valuable petrochemical intermediates. This study explores the role of oxyphilic metal promoters (Sn, Zn, and Ga) in enhancing the performance and stability of Pt@MFI catalysts for n-butane dehydrogenation. The presence of Sn in the catalyst inhibits the agglomeration of Pt clusters, maintaining their subnanometric particle size. PtSn@MFI exhibits superior stability and selectivity for butenes while suppressing cracking reactions, with selectivity for C1–C3 products as low as 2.1% at 550 °C compared to over 30.5% for Pt@MFI. Using a combination of high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and Raman spectroscopy, we examined the structural and electronic properties of the catalysts. Our findings reveal that Zn tends to consume hydroxyl groups and substitute framework sites, and Ga induces more defective sites in the zeolite structure. In contrast, the interaction between SnOx and the zeolite framework does not depend on reactions with hydroxyl groups. The incorporation of Sn significantly prevents Pt particle agglomeration, maintaining smaller Pt particle sizes and reducing coke formation compared to Zn and Ga promoters. Theoretical calculations showed that Sn increases the positive charge on Pt clusters, enhancing their interaction with the zeolite framework and reducing sintering, albeit with a slight increase in the energy barrier for C-H activation. These results underscore the dual benefits of Sn as a promoter, offering enhanced structural stability and reduced coke formation, thus paving the way for the rational design of more effective and durable catalysts for alkane dehydrogenation and other high-value chemical processes.

1. Introduction

Light olefins, with their reactive carbon-carbon double bonds, serve as platform molecules that can be converted into a series of downstream chemicals with higher added value, driving increasing market demand [1,2,3]. Among them, butene stands out as the primary raw material for gasoline additives and a wide range of copolymer products with other olefins. Its unique chain length and structure are essential for producing high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), and polybutylene (PB) [4,5]. These copolymers exhibit distinct properties such as hardness, high heat distortion temperature, excellent resistance to creep and abrasion, and good overall durability [6]. Currently, butene is primarily produced by the cryogenic distillation of C4 fractions from naphtha cracking and the ethylene dimerization process, both of which heavily rely on petroleum resources [7,8]. To diversify the sources of butene, research has increasingly focused on converting n-butane, a byproduct of the Fischer–Tropsch process and the conversion of CO2 hydrogenation, into butene through dehydrogenation reactions [9,10,11,12]. This approach not only reduces dependence on petroleum but also provides a cost-effective and sustainable alternative for butene production, thereby enhancing the supply chain resilience for this critical industrial feedstock.
However, the dehydrogenation of n-butane to butene is thermodynamically constrained, requiring high temperatures exceeding 500 °C to achieve significant yields. Such extreme conditions promote undesirable side reactions, including C-C bond cleavage leading to C1–C3 by-products and excessive dehydrogenation resulting in coke formation on the catalyst surface [13,14,15]. This highlights the critical need for catalysts that maintain high activity and selectivity under these conditions. Pt-based catalysts are recognized for their high activity in light alkane dehydrogenation, though they suffer from deactivation over time, which reduces their selectivity for butene [4,5,9,14]. Extensive efforts have been directed towards enhancing the selectivity and stability of Pt catalysts. It has been shown that adding promoter metal species can modify the reaction pathway on Pt sites by altering their geometric and electronic structures, thereby optimizing catalytic properties for dehydrogenation [14]. Moreover, reducing Pt particle size can improve catalytic performance due to unique electronic structures and increased exposure of active sites, minimizing structure-sensitive side reactions such as C−C cracking, skeletal isomerization, and excessive dehydrogenation leading to coke formation [16]. Various bimetallic combinations, including PtSn, PtZn, PtGe, and PtGa, have shown promise in forming smaller, more effective Pt aggregates [17,18,19].
Nevertheless, stabilizing these small Pt aggregates under the harsh conditions of alkane dehydrogenation remains challenging. Encapsulating Pt within zeolites presents a promising strategy to address this issue. The rigid zeolite framework effectively constrains Pt atom migration and agglomeration, while the confined environment within the zeolite channels prevents the formation of large intermediates, thus reducing coke formation. Consequently, highly dispersed Pt particles exhibit high selectivity for alkenes and low activity towards carbon deposition. Exactly localizing metal species to tailor their reactivity within the zeolite framework remains a significant challenge. Recent advancements include the successful localization of subnanometric Pt clusters within the sinusoidal channels of purely siliceous MFI zeolite through an optimized one-pot synthesis method [20,21,22]. However, the limited affinity of reduced Pt clusters for the zeolite structure necessitates further improvements. Enhancing the interaction between Pt clusters and the zeolite framework by incorporating oxyphilic metals such as Sn, Zn, and Ga as promoters could address this issue. These metals may substitute for Si atoms within the framework or exist as extra-framework species. For instance, Zn atoms can react with Si-OH groups and occupy silanol nests [23,24]. Ga atoms may substitute for Si atoms within the framework during the crystallization of the zeolite structure [25,26,27] while incorporating Sn atoms into the zeolite framework during crystallization is more challenging due to the larger atomic radius of Sn, resulting in the formation of extra-framework Sn species [22,28,29]. These different positions and structures of oxyphilic metal species can significantly impact the chemical environment of Pt species. Therefore, it is crucial to explore how metal promoters affect Pt encapsulation and the overall catalytic efficiency in n-butane dehydrogenation.
In this study, we synthesized a series of Pt-modified MFI zeolites incorporating oxyphilic metals such as Sn, Zn, and Ga as promoters. These catalysts were employed in the dehydrogenation of n-butane to investigate the impact of these promoters on Pt encapsulation and catalytic efficacy. The encapsulation of Pt was examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and quantified by the selective hydrogenation of probe molecules with different diameters. Changes in the electron structure of the catalysts were analyzed using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). We compared the catalytic performance of these promoted catalysts in the dehydrogenation of n-butane, analyzing the dehydrogenation capacity of Pt from kinetic data. The progression of catalyst deactivation was studied through thermogravimetric analysis (TGA) curves and STEM images of the spent catalysts. Through these comprehensive investigations, our goal was to elucidate the influence of metal promoters on Pt encapsulation and to assess their impact on the performance of the catalysts in n-butane dehydrogenation, aiming for a deeper understanding of the mechanisms and potential improvements in catalytic design.

2. Results and Discussion

2.1. Characterization of Pt@MFI with Various Promoters

A series of Pt-modified MFI zeolites were synthesized using a one-pot synthesis method and then subjected to comprehensive spectroscopic and microscopic analyses to confirm their composition and structural integrity. By varying the concentration of the metal precursors, we successfully produced samples with distinct compositions. The concentrations of metals in the Pt@MFI, PtSn@MFI, PtZn@MFI, and PtGa@MFI samples were quantified using inductively coupled plasma-optical emission spectrometry (ICP-OES), as presented in Table S1. The Pt content was consistently found to be approximately 0.45 wt%. The molar ratios of the promoter metals to Pt were maintained at about 4. Building on insights from our previous research, which demonstrated a significant improvement in Pt dispersion with the addition of K species, each sample was doped with K at a concentration of approximately 0.4 wt%. The acidity of catalysts plays a crucial role in cracking reactions, as alkanes are prone to catalytic cracking at acidic sites [30]. Therefore, pyridine adsorption infrared spectroscopy (Py-IR) was employed to characterize the acidity of the catalysts. The Py-IR spectrum at 473 K and 623 K represented the weak and strong acids of catalysts, respectively. The results, presented in Table S1, indicate that all catalysts exhibited low acidity. This low acidity benefits the dehydrogenation of n-butane, as it minimizes undesired cracking reactions and aligns with our catalyst design objectives.
X-ray diffraction (XRD) analysis revealed that each sample displayed the characteristic peaks associated with the MFI zeolite structure, as shown in Figure S1. Scanning electron microscopy (SEM) images, also depicted in Figures S2–S5, revealed a uniform particle size across the samples, with an average diameter of about 300 nm. This uniformity suggests that integrating metal species into the MFI framework does not significantly impact the topological structure of zeolites, a conclusion supported by the XRD results. Notably, this consistency in particle size was maintained even after the samples underwent calcination at 550 °C and subsequent reduction in a hydrogen atmosphere at 600 °C. The ability of the MFI zeolite structure to retain its particle size and structural integrity after such treatments underscores its durability, which is essential for its use in high-temperature catalytic applications, such as co-aromatization reactions, etc. [31,32,33]. This robustness of the MFI framework, combined with the strategic incorporation of metal species, positions these materials as promising catalysts for the dehydrogenation of n-butane and potentially other high-value chemical processes. The nitrogen adsorption-desorption isotherms and the pore size distribution obtained from the adsorption isotherms of all the samples, as shown in Figures S6 and S7, indicate isotherms characteristic of microporous structures in the catalysts. The analysis results of surface areas and pore volumes are presented in Table S2. It is observed that there is no significant difference in the total surface area and pore volume among the catalysts with different promoters. This consistency indicates that the catalyst samples have similar textural properties, which align with the expectations of our catalyst design.
HAADF-STEM was employed to evaluate the dispersion of Pt species within the MFI zeolite samples. The HAADF-STEM images of the as-synthesized samples, presented in Figure 1, suggest their subnanometric size or high dispersion. After undergoing air calcination and hydrogen reduction, the Pt nanoparticles in the Pt@MFI samples remained exceptionally small, with an average diameter of 0.94 nm (Figure 1a). This indicates that the Pt was effectively encapsulated within the MFI framework, maintaining a high dispersion. The addition of Sn as a promoter appears to preserve the high dispersion of Pt, as evidenced by the PtSn@MFI samples, which exhibited Pt particles with an average size of 0.86 nm (Figure 1b).
Conversely, incorporating Zn and Ga as promoters resulted in slightly larger Pt particle sizes, measuring 1.13 nm and 1.18 nm for PtZn@MFI and PtGa@MFI, respectively (Figure 1c,d). This suggests that the addition of Zn and Ga may interfere with the stabilization of Pt within the zeolite, potentially facilitating the migration and aggregation of Pt particles under the conditions of high-temperature pretreatment. These metals could alter the dynamics of Pt diffusion and nucleation during the synthesis process, leading to a broader distribution of Pt particle sizes and a reduction in uniformity. These observations underscore the critical role that promoter metals play in influencing the physicochemical characteristics of the catalytic sites, particularly during the formation of Pt sites. The effects of Zn and Ga on Pt dispersion highlight the nuanced interactions between promoter metals and the zeolite support, which can significantly impact the catalytic performance by modifying the size and, potentially, the spatial distribution of active metal sites.
To overcome the limitations of two-dimensional STEM imaging in accurately depicting the three-dimensional distribution of Pt within the MFI zeolite, we adopted a hydrogenation reaction-based method using 1-hexene (kinetic diameter = 0.49 nm) and cyclooctene (kinetic diameter = 0.79 nm) as probes. These molecules were selected based on their size difference relative to the zeolite pore size (~0.5 nm). This allows us to distinguish between Pt clusters inside the zeolite channels and those on the external surface. The 1-hexene can access Pt sites both inside the channels and on the external surface, participating in hydrogenation reactions at both sites. In contrast, the larger cyclooctene is restricted to reacting with Pt sites on the external surface of the zeolite. By comparing the hydrogenation rates of these two molecules, we can effectively quantify the distribution of Pt clusters, thereby gaining insights into the degree of Pt encapsulation within the zeolite. This approach provides a clearer understanding of Pt spatial distribution and its impact on catalytic performance, offering a more nuanced view than STEM imaging alone.
The reaction rate ratios between 1-hexene and cyclooctene (Xsample = r1-hexene/rcyclooctene) across various catalysts are tabulated in Table 1, providing insight into the Pt distribution on the MFI’s internal versus external surfaces. Given the inherent differences in the intrinsic reaction rates of 1-hexene and cyclooctene, which could be further influenced by the presence of promoters, it was crucial to isolate the effect of these variables. To achieve this, we conducted hydrogenation reactions of 1-hexene and cyclooctene using Pt/SiO2 and its promoter variants (PtSn/SiO2, PtZn/SiO2, and PtGa/SiO2), synthesized through a standard impregnation method, under the same conditions to determine the reaction rate ratios (XPt-M/SiO2). To adjust for the inherent reactivity difference between the two reactants, we calculated the ratio between Xsample and XPt/SiO2 (Ysample = Xsample/XPt/SiO2). A Ysample value nearing 1 indicates a predominance of Pt on the external surface of MFI zeolite. Meanwhile, a higher Ysample value suggests a significant internal encapsulation of Pt within the MFI channels [34]. According to Table 1, the Y values for Pt@MFI, PtSn@MFI, PtZn@MFI, and PtGa@MFI are 28.9, 32.1, 25.6, and 23.9, respectively. These findings suggest that in each case, a majority of the Pt species are encapsulated within the zeolite framework, highlighting the effectiveness of the encapsulation strategy.

2.2. Effect of Promoter on the Structure of Pt-M@MFI

The varied sizes of Pt clusters indicate that the interaction between Pt clusters and the zeolite framework is significantly influenced by oxyphilic metal promoters. These promoters can either substitute for Si atoms within the framework or exist as extra-framework species. For example, Ga atoms may replace Si atoms during the crystallization of the zeolite structure. Meanwhile, incorporating Sn atoms is more challenging due to their larger atomic radius, resulting in the formation of extra-framework Sn species. To elucidate the impact of promoters on the catalyst structure, we utilized FTIR spectroscopy. As depicted in Figure 2, the peaks at 3524 cm−1 and 3732 cm−1 are attributed to silanol (Si-OH) nests and silanol groups on the zeolite surface, respectively [35,36,37,38]. These peaks confirm the presence of Si-OH groups within the zeolite structure. When compared to Pt@MFI, the IR spectra of catalysts containing Zn, Ga, and Sn exhibit notable differences.
In the presence of Zn, the signal for OH groups is significantly reduced, suggesting the consumption of these hydroxyl groups. Zhao et al. [23] investigated the reaction between ZnO and defective OH groups in MFI zeolite under reductive or oxidative conditions at 550 °C. They found that ZnO reacts with H2 to form Zn0 species, which then react with Si-OH groups. This leads to Zn atoms substituting Si sites in the MFI framework and forming unsaturated ZnOx species. Given that the PtZn@MFI sample in this study was reduced by H2 at 600 °C, it is likely that Zn atoms similarly substitute Si sites within the MFI framework.
Conversely, the OH signal in PtGa@MFI is significantly enhanced compared to Pt@MFI, indicating the formation of additional OH sites. The peaks associated with Si-OH and silanol nests at 3524 cm−1 and 3732 cm−1 are more pronounced than those in the spectra of Pt@MFI, suggesting that Ga induces more defective sites during the crystallization of the MFI zeolite structure. Additionally, the broad peak at 3665 cm−1 is assigned to Ga-OH, indicating the presence of Ga(OH)x species.
In contrast to both PtZn@MFI and PtGa@MFI, the IR spectrum of PtSn@MFI closely resembles that of Pt@MFI, suggesting that Sn atoms do not significantly alter the OH groups in the structure. Previous studies on the PtSn@MFI structure have shown that Sn exists as an extra-framework SnOx species rather than substituting Si sites within the MFI framework. Therefore, a plausible explanation for the similar FTIR spectra of Pt@MFI and PtSn@MFI is that Sn does not affect the crystallization of the MFI zeolite structure, and SnOx species do not need to react with Si-OH groups to be stabilized within the zeolite tunnels. A more uniform distribution of SnOx within the MFI zeolite framework could enhance the distribution of Pt clusters, as the metal species would be more evenly dispersed rather than concentrated at specific sites within the zeolite structure [39].
The influence of oxyphilic metal promoters on the zeolite structure significantly affects the chemical environment of Pt clusters, which are critical for the dehydrogenation reaction. XPS was employed to investigate the electronic state of Pt in the catalysts. Typical XPS measurements primarily probe Pt atoms on the external surface, while those encapsulated within the zeolite structure remain covered. To expose the Pt clusters inside the zeolite, Ar ion etching was performed. As shown in Figure 3a, the Pt 4f7/2 XPS signal for Pt@MFI appears at 71.5 eV. This signal shifts to 71.6 eV and 72.0 eV in the presence of Sn and Ga, respectively, indicating a decreased electron density around Pt atoms. Conversely, the 4f7/2 XPS signal for PtZn@MFI shifts to a lower binding energy of 71.1 eV, suggesting an increased electron density around Pt due to the addition of Zn.
Given that XPS mainly provides information about surface atoms, we employed IR spectroscopy to investigate the chemical environment of Pt species within the zeolite tunnels. Carbon monoxide (CO) was used as a probe molecule to indirectly determine the distribution and electronic properties of Pt atoms by measuring the CO stretching vibration frequency.
After introducing CO over Pt for adsorption and subsequently purging with the flow of He, FTIR spectra showing the C-O stretching mode of CO adsorbed on Pt clusters were collected (Figure 3b). CO signals at 2046 cm−1 could be ascribed to CO molecules linearly adsorbed on Pt clusters [40]. The redshift of the CO vibration wavenumber from the gas phase value (2143 cm−1) is due to the donation of Pt d-electrons to the π* antibonding orbital of CO [40]. The degree of this redshift signifies the electron density of the Pt atoms adsorbing the CO molecule.
The introduction of promoter metal species alters the IR signal in distinct ways. For PtGa@MFI, the IR peak blue-shifts to 2061 cm−1, indicating that Ga species reduce the electron density of Pt clusters by modifying their electronic structure. In contrast, the CO IR signal for PtZn@MFI red-shifts to 2039 cm−1, indicating an increased electron density of Pt clusters due to Zn. The effect of Sn addition on the CO signal in PtSn@MFI is less pronounced compared to the Ga and Zn counterparts, suggesting a more moderate impact on the electronic structure of Pt clusters [41], which is consistent with the XPS observations.

2.3. Catalytic Performance in n-Butane Dehydrogenation

To assess the influence of promoters on the catalytic performance in n-butane dehydrogenation, the synthesized samples were employed as catalysts at a reaction temperature from 400 to 550 °C. As depicted in Figure 4, Figures S8–S11 and Tables S3–S7, significant differences in n-butane conversion and product selectivity are witnessed across the catalysts with different oxyphilic metal promoters. For Pt@MFI, n-butane conversion increased from 18.5% at 400 °C to 73.8% at 550 °C due to enhanced C-H activation at elevated temperatures. However, butene selectivity decreased to 55.7% at 550 °C, with C1–C3 molecules, indicative of substantial carbon chain cracking, comprising 30.5% of the products (Figure 4a and Figure S8 and Tables S3 and S7).
The introduction of Sn, Zn, and Ga as promoters significantly reduced the formation of these cracking products. When PtGa@MFI and PtZn@MFI were used as catalysts, n-butane conversion remained similar to that of Pt@MFI. However, the selectivity for C1–C3 products decreased to 22.4% and 14.3% for PtGa@MFI and PtZn@MFI, respectively (Figure 4c,d, Figures S10 and S11 and Tables S5–S7). Notably, the Sn-promoted PtSn@MFI catalyst drastically reduced the selectivity towards C1–C3 products to just 2.1% at 550 °C (Figure 4b and Figure S9 and Tables S4 and S7), while increasing the selectivity for linear butene isomers from 55.7% with Pt@MFI to 86.1% with PtSn@MFI. The beneficial effect of Sn in reducing n-butane cracking was consistently observed across various temperatures. At reaction temperatures of 400 °C, 450 °C, and 500 °C, PtSn@MFI exhibited lower selectivity for C1–C3 products and higher selectivity for linear butene isomers compared to Pt@MFI, PtGa@MFI, and PtZn@MFI. This comparative analysis underscores the particularly strong impact of Sn in both suppressing unwanted cracking reactions and enhancing butene selectivity, surpassing the effects seen with Zn and Ga promoters. In addition, we compared the performance of the PtSn@MFI catalyst with other catalyst systems for butane dehydrogenation reported in the literature, particularly those based on zeolites and alumina (Table S8). Our catalyst demonstrated significantly superior performance, showing lower yields of C1–C3 cracking products, which is closely related to the catalyst stability compared to the reported systems.
Meanwhile, to evaluate the contribution of parent purely siliceous MFI zeolite and promoter, the catalytic performance of these reference catalysts (MFI, Sn@MFI, Zn@MFI, and Ga@MFI) was evaluated under the same conditions, respectively. As depicted in Figures S12–S15 and Table S9, both the Sn@MFI and Zn@MFI catalysts exhibited a n-butane conversion of approximately 1.6%, comparable to that of purely siliceous MFI zeolite. In contrast, the Ga@MFI catalyst demonstrated a significantly higher n-butane dehydrogenation conversion of approximately 2.8%, attributed to its enhanced acidity [42,43,44]. These results suggest that the MFI support and promoters themselves are inert to this dehydrogenation reaction, and the performance variations in Pt-based catalysts modified with different promoters are primarily due to the specific interactions between Pt and the promoters.
Furthermore, the intrinsic activities of Pt@MFI, PtSn@MFI, PtZn@MFI, and PtGa@MFI catalysts were evaluated through kinetic tests, as illustrated in Figure S16 and detailed in Table S10. The apparent activation energies were calculated according to Arrhenius plots. The activation energies for n-butane catalyzed by Pt@MFI, PtSn@MFI, PtZn@MFI, and PtGa@MFI were determined to be 87.7, 95.1, 88.1, and 83.9 kJ·mol−1, respectively. These results serve as a reliable indicator of the activity.
As the reaction proceeded at 550 °C, n-butane conversion gradually declined with Pt@MFI, PtZn@MFI, and PtGa@MFI catalysts (Figure 5a, Figures S17, S19, and S20). In contrast, adding Sn as a promoter allowed the conversion to remain stable at around 60% (Figure S18), demonstrating remarkable stability. Considering different initial conversion rates, different hydrogen partial pressures will be generated on Pt active sites, which will affect the comparison of stability; thus, the weight hourly space velocity (WHSV) was adjusted to compare catalytic performance at similar conversion levels. As illustrated in Figure 5b, n-butane conversion over PtSn@MFI slightly decreased from 70.3% to 67.5% (Figure S21), whereas conversion over Pt@MFI significantly dropped from 73.8% to 49.9% (Figure S17). Similar reduced conversion was observed over PtZn@MFI and PtGa@MFI (Figures S19 and S22). This indicates that including Sn in Pt@MFI effectively mitigates catalyst deactivation, a benefit not observed with Zn and Ga.
A comprehensive analysis of the spent catalysts was conducted to uncover the reasons behind their deactivation and identify factors contributing to the enhanced stability of Pt@MFI catalysts. HAADF-STEM imaging was used to examine Pt particle agglomeration during the reaction (Figure 1 and Figure 6). The analysis revealed that Pt particle size in the spent Pt@MFI catalyst increased from 0.94 nm in the fresh catalyst to 1.68 nm after use. In contrast, with Sn incorporation, Pt particles in PtSn@MFI remained notably smaller, averaging 0.96 nm in diameter post-reaction. The spent PtZn@MFI and PtGa@MFI catalysts showed larger Pt particles, measuring 1.92 nm and 1.89 nm, respectively. This indicates that the Sn promoter uniquely prevents Pt migration and agglomeration, thereby enhancing Pt dispersion during the reaction, an effect not achieved with Zn and Ga.
In addition to Pt species agglomeration, coke deposition is another critical factor influencing catalyst stability in dehydrogenation reactions. To evaluate coke deposits in the spent catalysts, energy-dispersive X-ray (EDX) spectroscopy was employed during STEM image collection. Increased carbon signals were observed in all catalyst samples (Figures S23–S26), indicating coke formation on the catalyst particles. To quantify coke deposition, TGA of the spent catalysts was conducted, attributing the weight loss between 300 °C and 800 °C to coke combustion (Figure 7a). The coke content on Pt@MFI was found to be 1.7%. In comparison, PtSn@MFI, PtZn@MFI, and PtGa@MFI showed significantly reduced coke contents of 0.6%, 1.2%, and 1.4%, respectively.
Additionally, Raman spectroscopy was employed to distinguish the chemical nature of the coke deposited on the dehydrogenation catalysts. The Raman spectra of the deactivated catalysts are shown in Figure 7b. Several Raman bands appear at around 1380 cm−1 and around 1600 cm−1, which are assigned to the D and G peaks due to sp² carbon species, respectively [45,46]. The G peak is due to the bond stretching of all pairs of sp² atoms in both rings and chains, while the D peak is due to the breathing modes of sp² atoms in rings. These two peaks are common in various forms of disordered, noncrystalline, and amorphous carbons in Raman spectra [47]. A higher frequency for the G peak is possibly related to conjugated olefinic species or polycyclic aromatic hydrocarbons (PAHs) [48]. Thus, a lower band on the spectrum of PtSn@MFI at 1600 cm−1 suggests that the formation of PAHs is suppressed. Different PAHs also exhibit UV Raman peaks in the spectral region where the D peak is located [49]. The strong D peak observed in the Raman spectrum of Pt@MFI indicates that the coke on the spent Pt@MFI catalysts consists of deeply dehydrogenated carbonaceous compounds, including conjugated olefinic species and PAHs.
To better compare the coke species in the spent catalysts, the intensity ratio of the D peak to the G peak (I(D)/I(G)) was calculated and is listed in Table S11 [50,51]. The ratios of I(D)/I(G) for Pt@MFI, PtGa@MFI, and PtZn@MFI are 0.44, 0.66, and 0.77, respectively. A higher ratio of I(D)/I(G) for PtSn@MFI (0.87) indicates that the degree of graphitization or amorphous nature of the coke deposited on PtSn@MFI is inhibited. These observations align with the finding that PtSn@MFI had the lowest selectivity for cracking products, suggesting superior resistance to coking.
When comparing the catalytic performance of Pt@MFI catalysts with various promoters, the differences in product selectivity, Pt sintering behavior, and coke formation underscore the critical role of Sn in preventing carbon chain cracking, Pt migration and agglomeration, and coke precursor development. The PtSn@MFI catalyst exhibits improved selectivity for butenes and suppresses carbon chain cracking during the dehydrogenation process. The significantly reduced selectivity for cracking products is beneficial in inhibiting the formation of coke precursors and reducing coke deposits. Additionally, the Sn promoter mitigates the tendency for Pt sintering. These effects collectively suggest that Sn not only stabilizes Pt clusters but also plays a pivotal role in inhibiting the coking process, thus enhancing the catalytic performance.

2.4. Theoretical Study on the Effect of Promoter

To better understand the inhibition of Pt agglomeration by Sn as a promoter and scrutinize the influence of promoters on the electronic structure of Pt, a theoretical study was conducted by directly analyzing the electron density. Based on the experimental observations, it is inferred that the reaction primarily takes place in the straight channels of the zeolite. Therefore, our theoretical analysis mainly focused on Pt clusters within the context of promoter introduction. Recent studies on the local structure of Pt clusters within MFI channels suggest that Pt clusters with 4–6 atoms are more stable than smaller ones [28]. Additionally, ICP-OES analysis of the catalyst composition revealed an atomic ratio of 1:4 between Pt and the promoters. Consequently, clusters with five atoms, i.e., Pt5, Pt1Ga4, Pt1Zn4, and Pt1Sn4, were employed in the theoretical modeling (Figure 8).
According to our calculation of the Bader charge, the Pt5 cluster is positively charged at 0.97e (Figure 8a), and the value of the positive Bader charge increased in the presence of promoter metal atoms (Figure 8b–d) to 1.54e, 1.49e, and 1.44e, respectively. Among them, Pt1Sn4 is the most positively charged (1.54e), suggesting that the formed Pt cluster with Sn atoms introduced results in more electrons transferred from clusters to the nearby oxygen atoms in the zeolite framework. This effect may enhance the interaction between the metal cluster and the zeolite framework, as the stability of positively charged metal particles is reinforced by electrostatic pinning to the negatively charged surface, thereby increasing resistance to sintering [52].
Considering the critical role of Pt in C-H activation, a key step in dehydrogenation, we infer that the presence of promoter metal atoms significantly influences the catalytic behavior of Pt species in our catalyst system. To verify our speculation, the C-H activation pathway over the Pt1M4 clusters was studied using CH4 as a probe molecule. The free energy diagram is depicted in Figure 9, and their corresponding geometric structures and energy references are summarized in Figure S27 and Table S12, respectively. The cleavage of the C-H bond is most kinetically favorable over the Pt5 cluster as only 0.27 eV of the energy barrier is required to overcome, and Pt1Ga4@MFI and Pt1Zn4@MFI exhibit 0.57 and 0.47 eV for energy barriers, respectively, while the highest energy barrier (1.56 eV) has been detected on Pt1Sn4@MFI. Their reaction energy (−0.13, 0.16, and 0.19 eV vs. 0.73 eV) for the C-H dissociation follows a similar pattern compared with the corresponding energy barriers. The order of our calculated energy barriers (Pt1Sn4 > Pt1Ga4 > Pt1Zn4 > Pt5) is in good accordance with the results of different n-butane conversions obtained over catalysts with different promoters in experiments (Figure 4 and Figure 5).
In addition, the calculation of projected density of states (pDOS) further demonstrated the active site Pt with present Sn is relatively inactive compared to other metal promoters and pure Pt clusters (Figure S28, −2.29 vs. −2.11, −1.65 and −1.59, respectively). Comprehensively, the slightly reduced catalytic activity of PtSn@MFI compared to other catalysts can be attributed to the reduced electron density of Pt clusters in the presence of Sn atoms, resulting in an increased C-H activation energy barrier. This understanding underscores the importance of precisely adjusting the electron density at catalytic sites to balance catalytic activity and stability against metal sintering. It highlights a crucial aspect for future catalyst design and optimization.

3. Conclusions

In this study, we systematically investigated the influence of various oxyphilic metal promoters (Sn, Zn, and Ga) on the structural and catalytic properties of Pt@MFI catalysts for n-butane dehydrogenation. Our findings reveal several critical insights into the role of these promoters in enhancing catalytic performance and stability. These interactions affect the stability and performance of the catalysts differently; however, neither Zn nor Ga matches the dual benefits provided by Sn in preventing Pt agglomeration and reducing coke formation. High-resolution STEM analysis demonstrated that Sn as a promoter uniquely prevents Pt particle agglomeration, maintaining a significantly smaller Pt particle size compared to Pt@MFI, PtZn@MFI, and PtGa@MFI catalysts, contributing to the improved catalytic stability of PtSn@MFI. The spectroscopic study on the catalyst structure reveals that Zn tends to react with hydroxyl groups and substitute framework sites, while Ga induces more defective sites in the zeolite structure. The stabilization of SnOx species does not require the reaction with hydroxyl groups. TGA and Raman spectroscopy analyses indicated that Sn significantly reduces coke formation on the catalyst surface. PtSn@MFI exhibited the lowest coke yield and a lower degree of graphitization or amorphous nature of the coke deposits, thereby mitigating one of the primary causes of catalyst deactivation. Additionally, PtSn@MFI not only exhibits higher stability over a prolonged period of dehydrogenation reaction but also maintains high selectivity for butenes while suppressing unwanted cracking reactions. The selectivity for C1–C3 products over PtSn@MFI is as low as 2.1% at 550 °C, compared to over 30.5% for Pt@MFI, highlighting the reduced cracking product selectivity. Theoretical calculations and XPS studies revealed that incorporating Sn increases the positive charge on Pt clusters, enhancing their interaction with the zeolite framework and reducing sintering, albeit with a slight increase in the energy barrier for C-H activation. In summary, our comprehensive study highlights the pivotal role of Sn as a promoter in Pt@MFI catalysts, providing enhanced structural stability, reduced coke formation, and lower selectivity for cracking products. These findings emphasize the importance of tuning electronic properties to optimize catalytic performance, paving the way for the rational design of more effective and durable catalysts for alkane dehydrogenation and other high-value chemical processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14110760/s1, Section S1 Experimental; Section S2 Supplementary Tables; Section S3 Supplementary Figures; Section S4 Reference; Refs. [53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69] are cited in the Supplementary Materials.

Author Contributions

Methodology, N.G. and G.Q.; Software, P.L.; Validation, P.L.; Formal analysis, X.Z. and P.H.; Investigation, Y.C.; Data curation, N.G., P.L., X.Z. and P.H.; Writing—original draft, N.G.; Writing—review & editing, N.G.; Visualization, X.Z. and Y.C.; Supervision, Y.Y. and P.H.; Project administration, P.H.; Funding acquisition, Y.Y. and P.H. All authors have read and agreed to the published version of the manuscript.

Funding

We acknowledge support from the National Key R&D Program of China (No. 2023YFB4103102), the Beijing Natural Science Foundation (No. L245019) and Shanxi Provincial Science and Technology Department (No. YDZJSX2022A074), the National Natural Science Foundation of China (No. 22172186, 22302221, and 22202229), the Major Science and Technology Project of Ordos (No. 2022EEDSKJZDZX001), and the Inner Mongolia Key Research and Development Program (No. 2023YFHH0009).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We appreciate the support of characterization equipment and experimental platform from Synfuels China Technology Co., Ltd.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. HAADF-STEM images of (a) Pt@MFI, (b) PtSn@MFI, (c) PtZn@MFI, and (d) PtGa@MFI upon calcination in air and reduction by H2.
Figure 1. HAADF-STEM images of (a) Pt@MFI, (b) PtSn@MFI, (c) PtZn@MFI, and (d) PtGa@MFI upon calcination in air and reduction by H2.
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Figure 2. FTIR spectra in the -OH stretching region of the catalysts after pretreatment under vacuum at 400 °C for 1 h.
Figure 2. FTIR spectra in the -OH stretching region of the catalysts after pretreatment under vacuum at 400 °C for 1 h.
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Catalysts 14 00760 g003
Figure 3. (a) XPS spectra in the Pt 4f region of the catalysts (the surface of catalysts was etched with Ar ions) reduced at 600 °C for 1 h. (b) IR spectra of the catalysts upon CO adsorption at 40 °C.
Figure 3. (a) XPS spectra in the Pt 4f region of the catalysts (the surface of catalysts was etched with Ar ions) reduced at 600 °C for 1 h. (b) IR spectra of the catalysts upon CO adsorption at 40 °C.
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Figure 4. The conversion of n-butane and the dehydrogenation product selectivity obtained over (a) Pt@MFI, (b) PtSn@MFI, (c) PtZn@MFI, and (d) PtGa@MFI. (The bar chart in the figure corresponds to the product selectivity on the left vertical axis. The red curve corresponds to the n-butane conversion on the right vertical axis).
Figure 4. The conversion of n-butane and the dehydrogenation product selectivity obtained over (a) Pt@MFI, (b) PtSn@MFI, (c) PtZn@MFI, and (d) PtGa@MFI. (The bar chart in the figure corresponds to the product selectivity on the left vertical axis. The red curve corresponds to the n-butane conversion on the right vertical axis).
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Figure 5. The conversion of n-butane during the dehydrogenation reaction over Pt@MFI, PtSn@MFI, PtZn@MFI, and PtGa@MFI. The reactions were conducted at identical (a) WHSV (5.7 h−1) and (b) conversions, respectively.
Figure 5. The conversion of n-butane during the dehydrogenation reaction over Pt@MFI, PtSn@MFI, PtZn@MFI, and PtGa@MFI. The reactions were conducted at identical (a) WHSV (5.7 h−1) and (b) conversions, respectively.
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Figure 6. HAADF-STEM images and particle size distribution of spent catalysts, including (a) Pt@MFI, (b) PtSn@MFI, (c) PtZn@MFI, and (d) PtGa@MFI.
Figure 6. HAADF-STEM images and particle size distribution of spent catalysts, including (a) Pt@MFI, (b) PtSn@MFI, (c) PtZn@MFI, and (d) PtGa@MFI.
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Figure 7. (a) Thermogravimetric analysis (TGA) curves and (b) Raman spectra of catalysts after n-butane dehydrogenation reaction for 23 h.
Figure 7. (a) Thermogravimetric analysis (TGA) curves and (b) Raman spectra of catalysts after n-butane dehydrogenation reaction for 23 h.
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Figure 8. The optimized structures with the charge of Pt1M4 clusters in the straight channels of (a) Pt5@MFI, (b) Pt1Sn4@MFI, (c) Pt1Zn4@MFI, and (d) Pt1Ga4@MFI (O: red, Si: yellow, Pt: blue, Ga: sage, Sn: pink, and Zn: green).
Figure 8. The optimized structures with the charge of Pt1M4 clusters in the straight channels of (a) Pt5@MFI, (b) Pt1Sn4@MFI, (c) Pt1Zn4@MFI, and (d) Pt1Ga4@MFI (O: red, Si: yellow, Pt: blue, Ga: sage, Sn: pink, and Zn: green).
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Figure 9. Free energy diagram of CH4 dissociative adsorption on Pt5@MFI, Pt1Sn4@MFI, Pt1Zn4@MFI, and Pt1Ga4@MFI. (“*” indicates the state of adsorption on the catalyst surface).
Figure 9. Free energy diagram of CH4 dissociative adsorption on Pt5@MFI, Pt1Sn4@MFI, Pt1Zn4@MFI, and Pt1Ga4@MFI. (“*” indicates the state of adsorption on the catalyst surface).
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Table 1. Comparison of the encapsulation degree of Pt encapsulated in MFI zeolite and the measurement parameters.
Table 1. Comparison of the encapsulation degree of Pt encapsulated in MFI zeolite and the measurement parameters.
Samplesr1-hexenercycloocteneX aY b
Pt/SiO2145,000605424.0-
PtSn/SiO2131,957354337.2-
PtZn/SiO2105,028402326.1-
PtGa/SiO296,011506619.0-
Pt@MFI61,55889691.728.9
PtSn@MFI37,073311195.932.1
PtZn@MFI32,10548668.925.6
PtGa@MFI28,95464452.423.9
a Xsample = r1-hexene/rcyclooctene. b Ysample = Xsample/X Pt-M/SiO2.
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MDPI and ACS Style

Gong, N.; Qin, G.; Li, P.; Zhang, X.; Chen, Y.; Yang, Y.; He, P. Enhanced Stability and Selectivity in Pt@MFI Catalysts for n-Butane Dehydrogenation: The Crucial Role of Sn Promoter. Catalysts 2024, 14, 760. https://doi.org/10.3390/catal14110760

AMA Style

Gong N, Qin G, Li P, Zhang X, Chen Y, Yang Y, He P. Enhanced Stability and Selectivity in Pt@MFI Catalysts for n-Butane Dehydrogenation: The Crucial Role of Sn Promoter. Catalysts. 2024; 14(11):760. https://doi.org/10.3390/catal14110760

Chicago/Turabian Style

Gong, Nengfeng, Gaolei Qin, Pengfei Li, Xiangjie Zhang, Yan Chen, Yong Yang, and Peng He. 2024. "Enhanced Stability and Selectivity in Pt@MFI Catalysts for n-Butane Dehydrogenation: The Crucial Role of Sn Promoter" Catalysts 14, no. 11: 760. https://doi.org/10.3390/catal14110760

APA Style

Gong, N., Qin, G., Li, P., Zhang, X., Chen, Y., Yang, Y., & He, P. (2024). Enhanced Stability and Selectivity in Pt@MFI Catalysts for n-Butane Dehydrogenation: The Crucial Role of Sn Promoter. Catalysts, 14(11), 760. https://doi.org/10.3390/catal14110760

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