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. 2025 Dec 26;148(1):1801–1811. doi: 10.1021/jacs.5c19117

Highly Selective Methylation of Ethylene to Propylene within the Confined Side Pockets of Mordenite Zeolite

Danyang Cheng †,‡, Yao Xiao ∥, Jiamin Yuan ∥, Dong Fan †, Nan Chen †, Jingfeng Han †, Shiping Liu †,*, Anmin Zheng §,*, Peng Tian †,*, Zhongmin Liu †
PMCID: PMC12814175  PMID: 41452701

Abstract

Ethylene methylation with C1 reagents provides a viable route for propylene production and enables flexible adjustment of olefin output ratios in industrial processes (e.g., naphtha steam cracking and methanol-to-olefins). However, this process faces the long-standing challenges of low propylene selectivity and rapid catalyst deactivation. Herein, a highly selective and stable catalyst for ethylene methylation to propylene was developed by precisely regulating the acid site distribution in mordenite (MOR) zeolite, where the acid sites in the 12-membered ring (12-MR) channels were passivated using pyridine, while those located in the confined 8-membered ring (8-MR) side pockets remained accessible. This spatial control of acid site distribution allowed the exclusive occurrence of ethylene methylation within the sterically confined pockets, effectively suppressing the side reactions requiring larger space (e.g., methanol-to-hydrocarbons reaction and olefin oligomerization). Remarkably, the optimized pyridine-modified MOR catalyst achieved an unprecedented propylene selectivity of 97% and exhibited exceptional stability with no sign of deactivation during a 70 h test. In situ Fourier transform infrared (FT-IR) spectroscopy, theoretical calculations, and isotope labeling experiments were utilized to elucidate the mechanism of ethylene methylation and establish the reaction network within the confined pockets. It is anticipated that the side pockets of the MOR zeolite, which can be considered as an angstrom-scale reactor, would provide more opportunities for the precise assembly of small organic molecules.

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1. Introduction

Propylene is one of the most essential organic chemical feedstocks globally, with extensive applications in the production of polypropylene, propylene oxide, acrylonitrile, acrylic acid, isopropanol, as well as various high-value chemicals. Currently, propylene is predominantly synthesized through several processes, including naphtha steam cracking, methanol-to-olefins, fluid catalytic cracking (FCC), and propane dehydrogenation. , Except for propane dehydrogenation, these industrial processes coproduce ethylene and propylene. , Development of the ethylene-to-propylene technology not only provides a facile propylene production route but also enables the flexible adjustment of olefin production ratios in industrial processes in response to market fluctuations. Various potential routes, for example, metathesis of ethylene and 2-butene to propylene, direct conversion of ethylene to propylene, and ethylene methylation, , have been reported to attain this objective. Among them, ethylene methylation is particularly appealing, as this route utilizes the relatively cheap C1 methylation agents, which can be produced cost-effectively from coal and natural gas. ,

The ethylene methylation over zeolite catalysts has been extensively investigated. − Various methylation agents, − such as methanol, dimethyl ether (DME), monosubstituted methyl halides (CH3 X), and mercaptan (CH3SH), have been explored for the reaction. Typically, DME and methanol have been demonstrated to be more active, ,, likely due to the easier activation of the C–O bond. Wu and Kaeding , cofed ethylene and methanol over H-ZSM-5, observing an increase in propylene selectivity compared to feeding methanol solely (27% → 42%). Svelle and Hill et al. − conducted kinetic studies of ethylene methylation over H-ZSM-5. Their results showed that reaction conditions significantly affected the product distribution. Under extremely short contact time, propylene became the dominant product (>80%), but the conversion was very low (<1%). Due to the shape selectivity of narrow 8-membered ring pores, SAPO-34 has been extensively investigated for ethylene methylation. Dahl and Kolboe , studied the coreaction of ethylene and methanol over SAPO-34 by using ethanol as an ethylene source. They found that propylene was mainly produced from methanol, and only a minor part of it was formed by ethylene methylation. As a result, the selectivity of propylene showed a slight increase. However, Wu and Anthony found that ethylene could react with methanol to form propylene on SAPO-34, but propylene was more active and could further couple with methanol to form butylenes. Li et al. , compared the catalytic performance of ZSM-5, SAPO-34, and ZSM-22. Under optimized conditions, they achieved a high propylene selectivity of 65% over that of SAPO-34.

Despite these advancements, the propylene selectivity remained insufficiently competitive. The challenges in enhancing propylene selectivity stem from the inherent complexity of the reaction network. − Both ethylene and methanol/DME are highly reactive compounds that are prone to undergo oligomerization and conversion into hydrocarbons, respectively. Additionally, the produced propylene is prone to successive methylation. These reactions occur concurrently or sequentially, resulting in an intricate reaction network that makes the propylene formation pathways difficult to regulate. Furthermore, the methanol-to-hydrocarbons (MTH) reactions mediated by the hydrocarbon pool mechanism not only lower the propylene selectivity but also cause the formation of bulky aromatic coke species that deactivate the catalyst.

Understanding the reaction mechanisms is pivotal for catalyst design. The mechanistic investigation of ethylene methylation over zeolites has been driven by the interest in elucidating the mechanism of MTH reactions, as ethylene methylation is involved in the complex reaction network. ,,,,,, According to spectroscopy studies and theoretical calculations, ,− two distinct mechanisms, namely the concerted mechanism and the stepwise mechanism, have been proposed for ethylene methylation. In the concerted mechanism, ,, both methanol/DME and ethylene molecules are assumed to adsorb on a Brønsted acid site (BAS) before undergoing the methylation reaction, while in the stepwise mechanism, ,− ,,,,− methanol/DME first adsorbs on a BAS to form surface methoxy, which then reacts with the ethylene molecule via nucleophilic attack. Apparently, the former mechanism requires a slightly larger reaction space than the latter. The two mechanisms are currently under debate.

By comparing ethylene methylation with its side reactions, it can be found that the main side reactions require a relatively large space. In principle, these side reactions can be suppressed within a confined space, allowing the ethylene methylation reaction to proceed exclusively. Herein, we report mordenite (MOR) zeolite with confinement of acid sites within the small 8-MR side pockets as a highly selective catalyst for ethylene methylation to propylene. The designed pyridine-modified MOR catalyst exhibits propylene selectivity as high as 97% and long catalytic stability (>70 h). It is revealed that pyridine modification, by passivating the acid sites in 12-MR channels, not only effectively suppresses the side reactions but also preserves pore accessibility to allow the diffusion of reactants and products, thereby contributing to the outstanding catalytic performance of the catalyst. In situ Fourier transform infrared spectroscopy, density functional theory (DFT) calculations, as well as isotope labeling experiments, were combined to investigate the reaction mechanism, and a detailed reaction network within the confined space was proposed based on the investigation.

2. Results and Discussion

2.1. Catalytic Performance over Different Zeolite Catalysts

A variety of acidic zeolites, including H–Y (FAU), H-Beta (*BEA), H-ZSM-5 (MFI), H-SAPO-34 (CHA), and H-EU-7 (BIK), were first investigated to illustrate the effect of zeolite topology on catalytic performance for ethylene coupling with DME. The SEM images and XRD patterns of the samples used in this study are displayed in Figures S1 and S2, respectively, demonstrating their good crystallinity and phase purity. The framework type, channel dimensionality, and chemical composition of the samples are summarized in Table S1, and the reaction results are shown in Figures a and S3. It can be seen that the channel architecture exerts great influence on the product distribution. Large-pore 12-MR zeolites (H–Y and H-Beta) exhibit high selectivity toward C5+ hydrocarbons, while medium-pore 10-MR zeolite (H-ZSM-5) predominantly produces C4 hydrocarbons. Only for small-pore 8-MR zeolites (H-SAPO-34 and H-EU-7), a high propylene selectivity of more than 58% is attained, which should be attributed to the shape selectivity of the 8-MR apertures, effectively restraining the generation of C4 and C5+ hydrocarbons. Additionally, the soluble carbonaceous species deposited on the spent catalysts were extracted and analyzed by gas chromatography–mass spectrometry (GC-MS). From Figure b, a multitude of aromatic compounds are detected on H–Y, H-Beta, H-ZSM-5, and H-SAPO-34. The phenomena imply the great contribution of the aromatic-based hydrocarbon pool pathway to the reaction. Conversely, only trace amounts of polymethylbenzenes and coke are detected on spent H-EU-7. It suggests that the narrow, straight 8-MR channels (cage-free) of H-EU-7 restrict the formation of aromatic species, which is distinct from the behavior of small-pore SAPO-34 with large cages. Given the low coke amount (Figure S4) and rapid deactivation behavior (Figure S3), it is speculated that coke species deposit on the external surface of H-EU-7. This speculation is corroborated by the matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance (MALDI FT-ICR) mass spectra (Figure S5), ,, which show the existence of coke deposits on the external surface of the spent H-EU-7. These coke species are proposed to block the entrance of the 8-MR pores and thus cause the rapid deactivation of H-EU-7.

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(a) Comparison of the catalytic performance of ethylene methylation with DME over different zeolite catalysts (TOS = 2 h). (b) GC-MS chromatograms of the organic species extracted from the spent catalysts after 24 h of reaction. Reaction conditions: T = 563 K, P total = 101.3 kPa, P(DME) = 3.3 kPa, P(C2H4) = 33 kPa, GHSV = 1200 mL/gcat/h, N2 as the balance gas. The peak at the retention time of 9.65 min in panel b corresponds to the internal standard (hexachloroethane).

The above results demonstrate that the reaction pathways in zeolites are dominated by their structural features. The fast deactivation observed for small-pore SAPO-34 and H-EU-7 (Figure S3) implies that, although the 8-MR channels facilitate propylene selectivity, the formation of coke deposition can aggravate the diffusion limitation and cause catalyst deactivation. An ideal catalyst for ethylene methylation to propylene should combine 8-MR pores (providing spatial confinement for the target reaction) with interconnected, large acid-free channels that facilitate efficient diffusion of reactants/products while suppressing undesired side reactions.

2.2. Design and Construction of Highly Selective Catalysts with Confined Space

MOR zeolite, featuring intersecting 12-MR channels and 8-MR side pockets, represents a promising catalyst candidate for ethylene methylation to propylene. Previous studies have demonstrated that acid sites within the 12-MR channels can be selectively passivated using bulky organic amines, − while those located in the 8-MR side pockets remain accessible. To evaluate the suitability of these 8-MR side pockets for selective ethylene methylation, we performed molecular dynamics (MD) simulations to investigate olefin transport between the side pockets and 12-MR channels. MD simulation snapshots (Figure a–c) reveal that the occupancy of the 8-MR pockets decreases with increasing molecular size. This trend is corroborated by the density profiles of the olefins within the MOR framework (Figure d–f). The color gradient (blue → yellow → red) clearly indicates that smaller molecules (ethylene and propylene) readily access the 8-MR side pockets, whereas larger molecules (butylene and pentene) predominantly reside within the 12-MR channels. To further examine the molecular exchange dynamics between the 8-MR pockets and 12-MR channels, we tracked the real-time trajectories of guest molecules (Figure S6). Ethylene exhibits rapid and frequent exchange between the pore systems, facilitating its participation in subsequent reaction steps, such as propylene formation. In contrast, propylene shows significantly reduced exchange frequency, indicating a preferential residence within the larger 12-MR channels. This spatial confinement likely suppresses the further transformation of propylene into bulkier species within the side pockets, enhancing propylene selectivity. Diffusion of butylene molecules into the side pockets is substantially hindered, although molecules initially positioned within the pockets can migrate into the 12-MR channels within the simulated time frame (2 ns, Figure S7). Pentene diffusion into the side pockets is virtually negligible. Collectively, these findings demonstrate the potential for the 8-MR side pockets to provide a constrained environment conducive to the selective methylation of ethylene to propylene while simultaneously suppressing the formation of C4+ hydrocarbons, particularly C5+ species.

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Representative MD simulation snapshots and corresponding density distributions of (a, d) ethylene, (b, e) propylene, and (c, f) butylene within the MOR zeolite framework throughout the simulations.

In the subsequent investigation, pyridine modification − was adopted to passivate the acid sites in the 12-MR channels of MOR zeolite, and the resulting catalyst was labeled as Py-HMOR. For comparison, two reference catalysts were prepared: protonated MOR (H-MOR) and the MOR without acid sites in the side pockets (denoted 8Na-12H-MOR). Figure a presents the FT-IR spectra of the hydroxyl vibration region of H-MOR (Si/Al = 16) before and after pyridine modification. An asymmetrical band in the acidic hydroxyl stretching region is observed for H-MOR, which can be deconvoluted into two bands at 3604 and 3580 cm–1, attributed to the BAS in 12-MR channels and side pockets, respectively. , According to the deconvolution results, the proportion of BAS in the side pockets is about 50%. After pyridine modification, the high-frequency band disappears, and only the band at 3580 cm–1 remains, indicating that the BAS in the 12-MR channels of Py-HMOR are titrated by pyridine molecules. The 1H MAS NMR spectra of the samples are displayed in Figure b. For H-MOR, a strong signal at 4.0 ppm and two small signals at 2.6 and 1.7 ppm are observed, corresponding to the BAS, extraframework aluminum hydroxyls (AlOH), and silanol groups (SiOH), respectively. Pyridine modification causes an obvious decrease of the signal at 4.0 ppm, which should result from the consumption of 12-MR acid sites by pyridine, as evidenced by the FT-IR spectra. The reaction of pyridine with protons also leads to the appearance of new signals at 15.5–7.0 ppm, which are owing to the formation of pyridine–H+ complexes and the H atoms of pyridine. Based on the integrated area of the peak at 4.0 ppm, the amounts of BAS are estimated to be 0.604 mmol/g for H-MOR and 0.272 mmol/g for Py-HMOR. Moreover, the acid site distribution in 8Na-12H-MOR is also measured by FT-IR spectroscopy (Figure S8), verifying the BAS exclusively in the 12-MR channels. The ethylene methylation performance of MOR zeolites with a controlled acid site distribution is presented in Figure c. Remarkably, Py-HMOR shows an impressive propylene selectivity of 97% with DME conversion of 21% under the investigated conditions. To our knowledge, this represents the highest propylene selectivity reported for ethylene methylation to date. In contrast, H-MOR and 8Na-12H-MOR give low propylene selectivity (<5%), with C5+ hydrocarbons as the main products. These results demonstrate that the 8-MR pockets can enable selective ethylene methylation through the spatial confinement effect, while the coverage of 12-MR acid sites is essential for suppressing side reactions. This interpretation is further supported by GC-MS and thermal analysis of the postreaction catalysts (Figures S9 and S10), showing that Py-HMOR has trivial coke deposition, whereas H-MOR and 8Na-12H-MOR accumulate abundant polycyclic aromatics.

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(a, b) FT-IR spectra and 1H MAS NMR spectra of H-MOR (Si/Al = 16) before and after pyridine modification. (c, d) The catalytic performance of ethylene methylation with DME over MOR (Si/Al = 16) with different acid site distributions and over Py-HMOR with different Si/Al ratios, respectively. Reaction conditions: T = 563 K, P total = 101.3 kPa, P(DME) = 3.3 kPa, P(C2H4) = 33 kPa, GHSV = 1200 mL/gcat/h, and N2 as the balance gas.

Considering that the Si/Al ratio (SAR) is a key parameter affecting the acid properties of zeolites, the effect of the SAR on ethylene methylation is further explored. The FT-IR spectra and NH3-TPD of the samples with SAR values of 9, 16, and 20 (Figures S11 and S12) show that both the total acid amount and the acid amount in 8-MR side pockets decline with the increase of SAR. Correspondingly, the pyridine loading amount decreases as the acid site density diminishes (Figure S13). The ethylene methylation performance of the samples after pyridine adsorption is shown in Figures d. The sample with Si/Al = 9 exhibits the highest DME conversion but relatively low propylene selectivity. This should be attributed to the higher acid density of the sample, which is unfavorable for the complete coverage of the 12-MR acid sites by pyridine molecules, causing ethylene dimerization into butylenes and other side reactions. The high-silica sample (Si/Al = 20) exhibits inferior catalytic activity, likely due to its low acid density. Overall, Py-HMOR with Si/Al = 16 demonstrates the best catalytic performance among the samples, with high propylene selectivity and activity. Furthermore, the notably smaller crystal size of this catalyst is believed to facilitate mass transport, contributing to its superior catalytic performance.

2.3. Effect of Reaction Conditions and Stability Test

Figure a displays the effect of reaction temperature on the ethylene methylation performance of Py-HMOR. Low DME conversion and propylene selectivity are observed at 473 K. The main byproducts are ethane and methoxyethane, which likely arise from ethylene hydrogen transfer and ethylene etherification, respectively. With the temperature increasing from 473 to 563 K, both the DME conversion and propylene selectivity rise simultaneously. However, the catalyst stability deteriorates at the reaction temperature of 593 K (Figure S14) due to slow pyridine desorption (Figure S15), which causes the recovery of acid sites in 12-MR channels. The impact of the reaction pressure is depicted in Figure b. Increasing the pressure (0.1 → 3.0 MPa) can enhance both DME conversion and the propylene synthesis rate, but it synchronously promotes the side reactions. The main byproducts are C4 hydrocarbons, implying aggravated ethylene dimerization under high reaction pressures. Figure c illustrates the influence of reactant partial pressures. With the increase of DME partial pressure from 1 to 4 kPa, the propylene synthesis rate remains constant at 1.33 mol/(mol H8MR)/h. Conversely, the increase of ethylene partial pressure leads to an almost linear increase in the propylene synthesis rate. This observation suggests that the methylation rate has a zero-order and first-order dependence on the partial pressure of DME and ethylene, respectively. These findings are in good agreement with the previous kinetic studies by Hill et al. − In addition, the catalytic performance is investigated as a function of gas hourly space velocity (GHSV). As illustrated in Figure S16, increasing the GHSV from 1200 to 8400 mL/gcat/h leads to a progressive decrease in DME conversion due to the shortened contact time. However, there is no obvious change in the propylene synthesis rate and product distribution.

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(a–c) Ethylene methylation over Py-HMOR (Si/Al = 16) under different reaction conditions and (d) the stability test of the catalyst. Reaction conditions for (a): P total = 101.3 kPa, P(DME) = 3.3 kPa, P(C2H4) = 33 kPa, and GHSV = 1200 mL/gcat/h; for (b): T = 563 K, P(DME)/P(C2H4)/P(N2) = 1/10/19.7, GHSV = 1200 mL/gcat/h; for (c): T = 563 K, P total = 101.3 kPa, P(DME) = 1–4 kPa, P(C2H4) = 33 kPa, or P(DME) = 3.3 kPa, P(C2H4) = 0–33 kPa, GHSV = 1200 mL/gcat/h; for (d): T = 563 K, P total = 101.3 kPa, P(DME) = 3.3 kPa, P(C2H4) = 33 kPa, GHSV = 1200 mL/gcat/h. N2 as the balance gas.

Based on the above results, the long-term stability of the Py-HMOR catalyst was evaluated under optimal conditions at 563 K, atmospheric pressure, and a GHSV of 1200 mL/gcat/h. From Figure d, the DME conversion is maintained at 20 ± 1.0%, with propylene selectivity exceeding 97% during the 70 h test, and no obvious deactivation is observed under an overall carbon balance of approximately 99% (Figure S17), demonstrating the excellent stability of the developed catalyst.

2.4. Mechanistic Insights into Ethylene Methylation with DME

To elucidate the catalytic mechanism of ethylene methylation in the confined 8-MR side pockets of the Py-HMOR catalyst, in situ FT-IR spectroscopy and online mass spectrometry were combined to simultaneously monitor the evolution of surface species and the effluent products. From Figure a, upon introducing DME into the IR cell, a multitude of absorption bands appear in the C–H vibration region (2800–3300 cm–1), which should arise from both the gaseous and surface-adsorbed DME. Meanwhile, a negative peak at 3575 cm–1 attributed to the acidic hydroxyls in the 8-MR side pockets − can be observed, suggesting the consumption of BAS by DME. The corresponding MS signals of the effluent products (Figure b) show the generation of methanol (m/z = 32) and water (m/z = 18), which confirms the dissociation of DME at the BAS. Following He purging, the bands at 2970 cm–1 and 2860 cm–1, assigned to the asymmetric and symmetric C–H stretching vibrations of methyl groups, respectively, remain unaffected, verifying the formation of surface methoxy species. ,,

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In situ FT-IR spectra of Py-HMOR (Si/Al = 16) and the corresponding mass spectra of the effluent gas from the IR cell following the sequential introduction of DME (a, b) and ethylene (c, d). Experimental conditions: T = 523 K, P total = 101.3 kPa, DME (1% in He, 5 mL/min), C2H4 (2% in He, 5 mL/min), and He purging (20 mL/min) for 15 min.

Subsequently, ethylene was introduced into the IR cell (Figure c). The bands associated with surface methoxy groups show a progressive decrease following the introduction of ethylene. The MS signals reveal the formation of propylene (m/z = 42) (Figure d), which increases sharply during the initial period and then decreases gradually. The phenomena suggest that propylene is yielded from the reaction between ethylene and surface methoxy species. Afterward, the catalyst was flushed with He to remove the weakly adsorbed species, as evidenced by the disappearance of the bands at 3060 and 3030 cm–1, attributed to the C–H stretching vibrations of olefins. By subtracting the spectrum before introducing ethylene from the spectrum after He purging, it is evident that the acidic hydroxyl band is partially recovered, while the bands of methoxy groups decrease, demonstrating the consumption of surface methoxy groups by reaction with ethylene. These findings imply that ethylene methylation with DME in the 8-MR side pockets follows a stepwise mechanism.

In addition to experimental studies, theoretical calculations were performed to elucidate the molecular-level reaction mechanism within a confined environment. The FT-IR spectroscopy study has revealed that the reaction follows a stepwise mechanism. Moreover, the concerted mechanism was evaluated by theoretical calculations but found to be kinetically unfavorable, with a significantly higher activation barrier of 191.9 kJ/mol (Figure S18). Therefore, the concerted pathway is excluded from further discussion. Figure presents the Gibbs free energy surface for ethylene methylation in the side pockets at 563 K, with the corresponding optimized structures provided in Figure S19. The reaction initiates via DME adsorption on the BAS through hydrogen bonding (adsorption energy: 9.2 kJ/mol). The adsorbed DME undergoes protonation followed by dissociation, forming a surface methoxy species while releasing methanol. , The energy barrier for this methoxylation step is 59.6 kJ/mol, which is significantly lower than values reported for SAPO-34 and ZSM-5, , attributable to the confinement effect of the side pockets. , This facile formation of surface methoxy species further indicates that DME is a more favorable methylating agent than methanol. Subsequently, ethylene adsorbs within the pocket. Optimal fitting of ethylene in this confined space stabilizes the adsorption. The following methylation step, where ethylene nucleophilically attacks the methoxy group, requires a barrier of 153.8 kJ/mol. Intrinsic bond orbitals (IBO) analysis of this transition state (Figure S20) reveals orbital overlap between ethylene’s π-orbital and the methyl carbocation’s σ*-orbital, confirming the nucleophilic attack mechanism. The methylation yields protonated cyclopropane, a nonclassical carbocation that can be effectively stabilized by the confinement of the 8-MR side pockets. − The protonated cyclopropane is a highly reactive intermediate that readily isomerizes to the 2-propyl cation via an intramolecular hydride shift (barrier: 45.9 kJ/mol). The 2-propyl cation then rapidly deprotonates (barrier: 31.3 kJ/mol) to form propylene and regenerate the BAS. Among these elementary steps, ethylene methylation exhibits the highest activation energy (153.8 kJ/mol), identifying it as the rate-determining step. This conclusion aligns with experimental kinetics showing first-order dependence on the ethylene partial pressure.

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Gibbs free energy surface (inside) for the proposed ethylene methylation reaction mechanism (outside) within the 8-MR side pockets of the MOR zeolite.

Isotope labeling experiments were conducted to monitor the transformation of reactants on the Py-HMOR catalyst. 13C-labeled ethylene (13CH213CH2) and DME were cofed into the reactor, and the effluents were analyzed by GC-MS. The mass spectra of the products resulting from unlabeled ethylene and DME were also detected for comparison. As illustrated in Figure a, the main fragments of propylene derived from 13C-labeled ethylene and DME were at m/e = 41, 43, and 44, while those of propylene from ordinary ethylene and DME appeared at m/e = 39, 41, and 42, indicating that two carbons of propylene come from the ethylene molecule. This confirms that propylene is generated through the coupling of ethylene with DME.

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Isotopic distribution of products from reactions of ethylene with DME over Py-HMOR (Si/Al = 16). In all panels: top: 13C-ethylene feed; bottom: 12C-ethylene feed.

The mass spectra of the minor products of methoxyethane and butylenes are presented in Figure b–f. The observation of two 13C atoms in the ethyl group of methoxyethane (Figure b, top panel) confirms the formation of methoxyethane via the ethylene etherification reaction. This implies the formation of a surface ethoxy group and its reaction with DME, despite the relatively weak adsorption of ethylene on BAS, as revealed by FT-IR spectra (Figure c). Moreover, the activation of ethylene also contributes to the formation of cis-2-butylene and trans-2-butylene via dimerization, as evidenced by the mass spectra in Figure c and d. The formation of iso-butylene is somewhat complex. In Figure e and f, two distinct ion mass distributions are observed for iso-butylene, corresponding to iso-butylenes with two 13C atoms and four 13C atoms. This observation indicates that two different pathways contribute to iso-butylene formation: ethylene dimerization and propylene methylation. The former leads to the formation of iso-butylene (accounting for 59%) with four 13C atoms, while the latter gives rise to iso-butylene (constituting 41%) with two 13C atoms.

Based on the above findings, the reaction network within the confined 8-MR side pockets of Py-HMOR is proposed, as depicted in Figure . The adsorption of DME on BAS leads to the formation of surface methoxy groups, which undergo methylation with ethylene to form propylene. A minor fraction of propylene may further convert to iso-butylene by successive methylation. On the other hand, the surface ethoxy, resulting from the adsorption of ethylene on BAS, can react with ethylene and DME, delivering butylenes and methoxyethane, respectively. Owing to the spatial constraint of the side pockets and the poisoning of the acid sites in the 12-MR channels, the aromatic-based cycles of MTH reactions are greatly suppressed, thereby simplifying the reaction network and enabling significant enhancement of the propylene selectivity and long-term stability of the catalyst.

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Reaction network of ethylene methylation with DME in the confined 8-MR side pockets of the Py-HMOR catalyst.

3. Conclusion

A highly selective and stable catalyst for ethylene methylation to propylene has been rationally developed through selectively passivating the acid sites in the 12-MR main channels of the MOR zeolite. The side reactions, which require a larger space (e.g., MTH reaction and olefin oligomerization), can be effectively suppressed in the acid-free 12-MR channels, thereby allowing the exclusive occurrence of ethylene methylation within the confined side pockets. The optimized Py-HMOR catalyst shows a high propylene selectivity of 97% and excellent catalytic stability. By a combination of experimental and theoretical analyses, the ethylene methylation to propylene reaction is revealed to follow a stepwise mechanism in the confined space, and the reaction network involving the formation of byproducts is well established. This work demonstrates that the unique structure of modified MOR zeolite with acid-free 12-MR channels makes the material a promising catalyst candidate for the selective transformation of small organic molecules. To further advance this catalytic system, future efforts could focus on: (1) enhancing the population of Al sites within the 8-MR side pockets to increase the density of active centers and (2) developing more robust methods for selectively and permanently deactivating BAS in the 12-MR channels to extend the catalyst’s operational temperature window and applicability.

Supplementary Material

ja5c19117_si_001.pdf (5.7MB, pdf)

Acknowledgments

This work is supported by the National Key Research and Development Program of China (2024YFE0207000), the National Natural Science Foundation of China (No. 22288101, 22372170, 22272713, 22125304, and 22032005), DICP funding (DICP1202420), the Natural Science Foundation of Hubei Province of China (2025AFA008), and funding from the Sino-French International Research Network (IRN Zeolites).

Glossary

Abbreviations

MOR

mordenite

12-MR

12-membered ring

8-MR

8-membered ring

FT-IR

Fourier transform infrared

FCC

fluid catalytic cracking

DME

dimethyl ether

MTH

methanol-to-hydrocarbons

BAS

Brønsted acid site

DFT

density functional theory

GC-MS

gas chromatography–mass spectrometry

MD

molecular dynamics

SAR

Si/Al ratio

GHSV

gas hourly space velocity

IBO

intrinsic bond orbitals

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.5c19117.

  • Additional details of experimental procedures, sample preparation, and catalyst characterization; supplementary results of reaction performance, MD simulation, and DFT calculation (PDF)

#.

D.C. and Y.X. contributed equally.

The authors declare no competing financial interest.

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