Mechanistic insights into coke suppression in the methanol-to-olefins process via CeO₂ doping to SAPO-34

Abstract

This study provides novel mechanistic insights into the transformative role of cerium oxide (CeO₂) incorporation into SAPO-34 for the methanol-to-olefins (MTO) process. By integrating experimental findings with molecular dynamics (MD) simulations, the work elucidates specific pathways through which CeO₂ mitigates coke formation. MD simulations reveal that CeO₂ suppresses the formation of CHO-θ intermediates, key precursors to coke, while enhancing coke removal through improved CO₂ activation and the Boudouard reaction. Catalytic testing corroborates these findings, demonstrating enhanced durability for up to 600 min and a total olefin selectivity of up to 83.9%. Structural and chemical modifications, such as reduced crystallite size, increased mesoporosity, and redistributed acid sites, were characterized using advanced techniques (XRD, FT-IR, FESEM, and NH₃-TPD). These modifications optimize the balance between weak and strong acid sites, facilitating efficient methanol conversion and olefin production. The integration of mechanistic insights with experimental results underscores the innovative role of CeO₂ in improving SAPO-34's catalytic efficiency, selectivity, and operational stability, establishing SP-Ce as a cutting-edge catalyst for industrial MTO applications.

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1. Introduction

As global demand for essential petrochemicals like ethylene and propylene grows, the Methanol-to-Olefins (MTO) process has emerged as a sustainable alternative to petroleum-based production. This process uses methanol-a versatile feedstock that can be sourced from renewable resources, such as biomass-to promote energy diversification and reduce dependence on crude oil. A critical component of the MTO process is the SAPO-34 catalyst, a chabazite-type material known for its high selectivity in converting methanol to light olefins. Its unique microporous structure, featuring 8-membered ring (8-MR) channels, is optimized for light olefin production, making SAPO-34 valuable for sustainable petrochemical manufacturing. To meet the rising demand for olefinic feedstocks like ethene, enhancing MTO catalysts is essential.^1–4

However, SAPO-34 faces deactivation due to coke formation, as carbon-rich deposits block active sites and reduce catalytic efficiency. To mitigate this, researchers are reducing SAPO-34 crystal size and designing hierarchical pore structures to improve mass transport and diffusion. These advancements extend SAPO-34's catalytic lifespan, reinforcing its efficiency in olefin production. Recent studies highlight formaldehyde (HCHO) as a key coke precursor in the MTO reaction. Formed early in the process, formaldehyde condenses on SAPO-34 and interacts with other intermediates, producing aromatics through mechanisms like the Prins reaction, which accelerates deactivation.^3,5–10

Formaldehyde is generated through pathways such as methanol disproportionation, thermal decomposition, and hydride transfer at Lewis acid sites. It reacts with olefins to produce dienes and aromatics, both of which promote coke buildup. Even trace amounts of formaldehyde can accelerate coke formation and catalyst deactivation, shifting selectivity toward aromatics and leading to increased coke deposition.^5,11 Controlling formaldehyde concentration is essential to minimize coke formation and prolong catalyst life. Strategies include diluting methanol, co-feeding alkenes, and back-mixing to recycle products, which reduces methanol's chemical potential and lowers formaldehyde levels. Additionally, using dimethyl ether (DME) as feedstock has shown potential, as it bypasses certain pathways that lead to formaldehyde formation.^5–7,11,12 In contrast, adding metal oxides to SAPO-34 has proven effective in mitigating formaldehyde-driven coking, enhancing catalyst stability and MTO efficiency.^5,13

The incorporation of metal oxides in SAPO frameworks is crucial for improving catalysis in hydrocarbon conversion.¹³ Transition metal ions, introduced via isomorphous substitution, create active sites that enhance catalytic performance. Their behavior is influenced by their location in the porous structure, such as near 6- and 10-ring windows. Characterization methods reveal interactions between metal ions and reactants, deepening our understanding of catalytic mechanisms. Research on transition metals shows promise for optimizing SAPO materials to reduce deactivation and improve efficiency in specific applications.^13–15

In addition, metal oxides in SAPO materials, such as H-SAPO-34 and SAPO-18, significantly affect catalytic behavior.¹⁶ For example, adding nickel enhances selectivity for ethene, with nickel-containing SAPO-34 achieving nearly 90% ethene yield with stable, long-term performance. This selectivity results from interactions between metal ions and reaction intermediates, which modify acidity and product distribution. Therefore, refining metal oxide types and concentrations in SAPO frameworks is critical for increasing olefin production efficiency.¹⁴ Metal oxides stabilize intermediates, influencing reaction pathways and boosting yields, making a thorough understanding of these roles essential for optimizing MTO catalysts and enhancing olefin production efficiency.^17,18

Some contradictory reports exist. For instance, ZnO has been reported in some studies as an important component in reducing coke formation in MTO reactions by enhancing selectivity and stabilizing catalysts.¹⁹ ZnO suppresses paraffin formation, favoring propylene and lighter hydrocarbons over coke-promoting paraffins.¹⁵ Although ZnO improves the ethene-to-propene ratio, highlighting the complex role of metal oxides in MTO reactions, other studies have reported that it promotes coke deposition.^20,21 Studies, such as those by Cordero-Lanzac et al.,²² demonstrate that ZnO paired with MgO/SAPO-18 (MgAPO-18) reduces heavy hydrocarbon intermediates that might otherwise polymerize into coke on acid sites. The structural and electronic characteristics of metal oxides enable selective interactions with SAPO active sites, promoting reactions that avoid polyaromatic formation and prevent catalyst deactivation. Additionally, bifunctional catalysts have advanced syngas-to-olefin conversion by transforming intermediates on metal oxide surfaces. For instance, Y₂O₃ decomposes formaldehyde, which initiates and terminates chain growth in the MTO process, thereby limiting coke accumulation.²³ On the other hand, other studies have emphasized^24–26 that metal oxides play a vital role in reducing coke formation in MTO reactions through other strategies, including reducing crystal size and introducing meso- or macropores to prevent catalyst deactivation.

While pairing metal oxides with SAPO zeolites of strong acidity and selectivity has been widely acknowledged to diminish coke precursors and extend catalyst longevity,²² understanding the mechanistic role of these oxides in reducing coke formation remains crucial, particularly in prominent cases. Among these, incorporating rare-earth elements like cerium (Ce) into SAPO-34 has shown significant promise, though it has received relatively little attention. Cerium modifies the porosity and acidity of the catalyst, thereby enhancing its performance and lifetime. Recent findings demonstrate that a physical mixture of SSZ-13 and ceria (CeO₂) extends catalyst lifetimes in MTO reactions compared to pure SSZ-13. This improvement is attributed to formaldehyde decomposition on CeO₂ surfaces, likely proceeding through formate intermediates, which mitigates coke formation. Moreover, the morphology of CeO₂ significantly influences catalyst stability, underscoring the importance of optimizing metal oxide structures to achieve superior catalytic performance.^23,27,28

Ceria's exceptional redox properties, including its ability to alternate between Ce³⁺ and Ce⁴⁺ oxidation states and its high oxygen storage capacity (OSC), make it a valuable addition to SAPO-34 catalysts for the MTO process. By regulating oxygen concentration through oxygen vacancy defects, ceria can stabilize critical intermediates like methoxy under reaction conditions, directing them toward favorable olefin products while suppressing the formation of undesirable species such as formaldehyde. These reactive sites enhance methanol conversion to olefins, minimizing the accumulation of coke precursors and by-products.²⁷ Additionally, ceria's redox flexibility facilitates oxidation cycles within the catalyst, promoting resistance to deactivation. Its oxygen mobility enables the oxidation of coke precursors, either through direct redox reactions or by facilitating the reverse Boudouard reaction, thereby preserving active sites and extending the catalyst's operational lifespan. Furthermore, ceria aids in the spillover of reactants like H₂ and CO, supporting secondary reactions that improve olefin yield and overall MTO efficiency.^27,29

In this regard, this study provides a comprehensive evaluation of the effects of ceria incorporation into SAPO-34 in the MTO process, combining experimental and theoretical approaches. Molecular dynamic (MD) simulations are employed to elucidate the mechanistic pathways by which cerium oxide reduces coke precursors or mitigates their formation. In parallel, catalytic testing is conducted to validate the performance improvements. Additionally, advanced characterization techniques are applied to investigate the structural and chemical modifications in SAPO-34 resulting from ceria integration, offering deeper insights into its catalytic behavior and performance enhancement mechanisms.

2. Experimental and theoretical approach

2.1. Catalyst preparation

The synthesis of SAPO-34 catalysts involves using tetraethylammonium hydroxide (TEAOH, 20 wt%) and morpholine (Mor, 99%) as organic template agents, with aluminum iso-propoxide (AIP, 99 wt%), phosphoric acid (H₃PO₄, 85 wt%), and tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si) serving as the sources of aluminum, phosphorus, and silicon, respectively. The molar composition used for synthesis is 1Al₂O₃ : 1P₂O₅ : 0.6SiO₂ : 1.25TEAOH : 1.25Mor : 70H₂O.^4,30

To begin, aluminum iso-propoxide is dissolved in deionized water. Tetraethylammonium hydroxide and morpholine are then added dropwise to this solution, forming a white gel, which is stirred for 1 hour at room temperature. Following this, tetraethyl orthosilicate is introduced dropwise into the mixture, with stirring continued for 3 hours at 60 °C. Afterward, phosphoric acid is carefully added dropwise, and the solution is stirred for an additional 2 hours at the same temperature. The resulting mixture is aged at room temperature for 24 hours to allow the components to interact fully.

For Ce-doped SAPO-34 synthesis, cerium nitrate hexahydrate, in a ratio of 0.05 relative to Al₂O₃,¹³ is dissolved in water and added to the gel mixture along with aqueous ammonium carbonate. Both solutions are introduced dropwise simultaneously. This modified gel mixture, along with the non-doped version, undergoes hydrothermal treatment at 180 °C for 24 hours in a stainless-steel autoclave.

After hydrothermal treatment, the solid products are separated by centrifugation, thoroughly washed with deionized water, and dried at 100 °C for 12 hours. The dried samples are then calcined at 550 °C for 5 hours to remove any residual organic templates. The final Ce-doped catalyst is designated as SP-Ce, whereas the undoped SAPO-34, synthesized by the same procedure but without adding cerium nitrate, is labeled as SP.

2.2. Characterization methods

A range of advanced characterization techniques is employed to investigate the impact of chemical modifications on the performance of SAPO-34 in the MTO process. Crystallinity and phase purity are analyzed through X-ray powder diffraction (XRD) using a Philips X'Pert MPD instrument. The measurements are taken within a 2θ range of 5–50, utilizing a Cu K_α radiation source (λ = 1.54056 Å). Fourier Transform Infrared Spectroscopy (FT-IR) is used to identify functional groups within the samples, performed on a PerkinElmer Spectrum Two spectrometer over the wavelength range of 400–4000 cm⁻¹.

The morphological characteristics of the samples are examined using a TESCAN MIRA3 field emission scanning electron microscope (FESEM). This instrument is also equipped with an energy-dispersive X-ray spectrometer (EDS) to determine the surface elemental composition. Textural properties, such as surface area and pore volume, are evaluated through nitrogen adsorption/desorption isotherms at 77 K, using a Micromeritics Tristar 3020 automated analyzer. To assess the acidic properties, NH₃-TPD analysis is performed with a BELCAT-B catalyst analyzer fitted with a thermal conductivity detector (TCD). Fresh samples (50 mg each) are pretreated at 500 °C for 1 hour under helium flow, then cooled and exposed to ammonia at 100 °C. Afterward, temperature-programmed desorption profiles are recorded as the temperature is increased from 100 to 700 °C at a rate of 10 °C min⁻¹ under helium flow.

These comprehensive analyses provide insights into the structural, functional, morphological, and chemical properties of SAPO-34 and their implications for catalytic performance.

2.3. MD simulation overview

MD simulation is a computational technique used to analyze the time-dependent behavior of atoms and molecules at the nanoscale. By solving equations of motion, it predicts atomic evolution over time, employing the velocity Verlet algorithm to accurately calculate atomic positions and velocities within the simulation framework. This method enables the determination of critical properties, such as activation energy barriers, providing detailed atomistic insights into interatomic and intermolecular interactions. These insights are particularly valuable for understanding and predicting the behavior of cerium oxide supported on SAPO-34 in the MTO process. The reliability of MD simulations hinges on the accuracy of the interatomic potentials or force fields employed, as well as the accurate representation of the system and simulation conditions. In this study, the Universal Force Field (UFF) is utilized, incorporating the Lennard-Jones (LJ) potential to model nonbonded interactions:³¹Key parameters include ε (the depth of the interaction energy well), σ (the distance at which the interaction potential becomes zero), and r_ij (the distance between atoms i and j), with a cutoff distance (r_c) of 12 Å applied for all atom types.

Bonded interactions, such as bond stretching and angle bending, are described using harmonic potential formalisms characterized by oscillator constants (k_r and k_θ) and equilibrium parameters (r₀ and θ₀), as expressed in the relevant equations. Dihedral interactions are also represented through harmonic potentials, ensuring a comprehensive modeling of the atomic and molecular behavior.

E = ½k_r(r − r₀)² (2)

E = ½k_θ(θ − θ₀)² (3)

Two distinct ensembles are utilized to define the initial conditions for the simulations. The NVT ensemble maintains a constant temperature, volume, and particle number, while the NPT ensemble regulates both pressure and temperature during the atomic arrangement phase. Thermal equilibrium is achieved using the Nose–Hoover thermostat within 10 ns, followed by an additional 10 ns of simulation to ensure system convergence. The simulations are conducted using the LAMMPS computational package. The computational box is defined with dimensions of 350 × 350 × 350 ų, and periodic boundary conditions are applied. The Nose–Hoover thermostat and barostat are employed to maintain stability, with a damping ratio of 1 for temperature and 10 for pressure. Simulations are performed under an initial temperature of 300 K and an initial pressure of 1 atm. A time step of 0.01 femtoseconds is used to ensure precise calculations. The MD algorithms follow the NPT–NVT sequence to systematically adjust system parameters.^32,33 A visualization of the atomic arrangement at the initial stage of the simulations is provided in Fig. 1, offering insight into the starting configuration of the modeled structure.

Fig. 1 (a) Initial atomic arrangement in MD simulations, (b) Potential energy profiles of SP and SP-Ce structures during equilibrium at 300 K and 1 atm (initial conditions), and (c) RDF profiles of both structures at equilibrium after 10 ns.

In our computational workflow, the initial and final states of each target reaction ((R1)–(R6) as described in Section 3.1.5) are modeled as separate configurations. Classical MD simulations, performed in the NPT ensemble at 298 K using Newton's equations of motion with the velocity Verlet algorithm, are employed solely to thermally equilibrate these systems under realistic conditions. This low-temperature equilibration ensures structural stability and minimizes thermal noise prior to further analysis. From the equilibrated geometries, static energy calculations are then conducted to determine the energies of the reactant and product states. Based on these values, the forward (E_f) and reverse (E_r) reaction energies, as well as the overall energy change (ΔE), are estimated from the differences between the initial and final states. We emphasize that the MD simulations are not used to directly identify transition states or compute activation barriers; rather, they serve to generate thermally equilibrated structures for subsequent static energy evaluation. This distinction clarifies the methodology and avoids the misconception that plain MD trajectories are sufficient for barrier calculations without additional sampling or optimization techniques.

The correctness of the defined atomic structures—including simulation box dimensions (350 × 350 × 350 ų), atomic arrangement, and boundary conditions—together with simulation settings such as the chosen force field, time step, and membrane geometry, is validated through potential energy convergence analysis under the specified initial conditions. This relatively large simulation box size is required to accommodate a realistic SAPO-34 supercell with CeO₂ dopants and to prevent spurious interactions between periodic images. The modeled system contains all atoms and employs periodic boundary conditions in all three dimensions. Each system was first relaxed in the NVT ensemble for 2 ns, followed by 3 ns in the NPT ensemble to stabilize pressure and density. The subsequent 10 ns production run was carried out with a 1 fs timestep.

The potential energy profiles of two representative systems (SP and SP-Ce) are shown in Fig. 1b. In both cases, the potential energy converges within 10 ns to −0.94 kcal mol⁻¹ (SP) and −0.68 kcal mol⁻¹ (SP-Ce), indicating stable atomic interactions. As the simulation progresses, atomic fluctuation amplitudes decrease, reflecting the attainment of structural and thermodynamic equilibrium. The radial distribution function (RDF), shown in Fig. 1c, further confirms equilibration: distinct RDF peaks indicate stable atomic coordination distances within the simulation cell. Together, the potential energy and RDF analyses verify that 10 ns is sufficient to achieve equilibrium under the chosen simulation conditions.

To address the temperature dependence of catalytic properties, we plan to perform additional MD simulations at 698 K for selected systems, enabling a comparison of structural stability, RDF features, and potential energy trends between ambient and operating temperatures. This will allow us to assess whether temperature scaling or entropic effects significantly influence the equilibrated structures used for subsequent energy calculations.^34,35

2.4. MTO catalytic tests

The MTO process is conducted in a fixed-bed reactor under atmospheric pressure and at a reaction temperature of 425 °C. Prior to each experimental run, 2 g of the calcined catalyst (sized to 40–20 mesh) are mixed with 4 g of inert silicon carbide (SiC) and loaded into the reactor. To prepare the catalyst for reaction, it undergoes a pretreatment step at 550 °C for 1 h under a continuous flow of nitrogen. This step (Fig. 2) ensures the removal of moisture and any residual contaminants, thereby enhancing the catalyst's activity and stability. Once the catalyst is pretreated, the reactor is allowed to cool down to the desired reaction temperature. At this point, a methanol–water feed mixture, with a weight ratio of 72 : 28, is introduced into the system. The feed is supplied at a weight hourly space velocity (WHSV) of 2 h⁻¹, ensuring optimal contact between the reactants and the catalyst.³⁶

Fig. 2 Process flow diagram of MTO catalytic performance experimental setup.

The reaction products are continuously analyzed using an Agilent GC 7890A gas chromatograph equipped with both flame ionization detectors (FID) and thermal conductivity detectors (TCD). This dual detection system provides precise quantification of hydrocarbon products and non-hydrocarbon gases, offering a comprehensive profile of the reaction output. These analytical capabilities enable the detailed monitoring of product distribution and catalytic performance throughout the MTO reaction.

3. Results and discussion

3.1. Characterization analyses

3.1.1. Crystalline and structural properties. The XRD patterns of the SP and SP-Ce samples are depicted in Fig. 3a. Both samples display prominent diffraction peaks at 2θ values near 9.6°, 20.7°, 26°, and 31°, which are indicative of the CHA (chabazite) framework. These peaks confirm that the crystalline structure of SAPO-34 is preserved in both samples. The crystalline phase in SP and SP-Ce corresponds to a hexagonal prism geometry, as validated by the JCPDS standard card 01-087-1527.^37,38

Fig. 3 (a) XRD profiles of SP and SP-Ce along with their crystallite sizes. (b) Their FT-IR spectra.

The diffraction peak intensities of SP are higher than those of SP-Ce, reflecting greater crystallinity in the undoped SAPO-34 sample. The reduced peak intensity in SP-Ce is attributed to the incorporation of cerium oxide (CeO₂) into the SAPO-34 framework. The doping process introduces lattice distortions and slightly disrupts the long-range crystalline order, a common effect observed in modified SAPO materials.

Crystallite size calculations using the Scherrer equation indicate that SP has a crystallite size of approximately 36 nm, while SP-Ce exhibits a smaller size of around 31 nm. This reduction in crystallite size upon ceria incorporation suggests structural changes within the lattice and a decrease in crystalline domain size. Such a reduction is advantageous for catalytic applications, as smaller crystallites increase surface area and enhance access to active sites, potentially improving the catalytic efficiency in the MTO process.³⁸ Moreover, the absence of secondary peaks associated with crystalline CeO₂ in the XRD pattern of SP-Ce indicates that cerium species are highly dispersed within the SAPO-34 framework. This suggests that cerium oxide is effectively integrated into the SAPO-34 matrix without significant phase separation or aggregation.^37–40

The FT-IR spectra of SP and SP-Ce, exhibited in Fig. 3b, highlight the vibrational characteristics of functional groups in the SAPO-34 framework and the effects of cerium oxide incorporation. Both spectra display distinct absorption bands typical of SAPO-34, confirming the retention of its structural framework after cerium doping. The characteristic bands observed between 1000 and 1250 cm⁻¹ correspond to the asymmetric stretching vibrations of T–O–T (T = Si, Al, or P) linkages, while bands around 750–800 cm⁻¹ are attributed to symmetric stretching vibrations of the same linkages. Peaks below 600 cm⁻¹ are associated with bending vibrations. These vibrations remain consistent in both SP and SP-Ce, indicating that the fundamental chabazite structure of SAPO-34 is preserved after CeO₂ incorporation.^41–43

A broad band centered at 3400–3600 cm⁻¹ is observed in both samples, corresponding to O–H stretching vibrations associated with surface hydroxyl groups and adsorbed water. This band is slightly less intense in SP-Ce, suggesting that cerium doping reduces water adsorption or modifies hydroxyl groups. Additionally, the bending vibration of adsorbed water is detected near 1630 cm⁻¹ in both samples, confirming the presence of moisture. Notable shifts and changes in the intensity of certain peaks, such as those near 1100 cm⁻¹ and 800 cm⁻¹ in SP-Ce, point to interactions between cerium species and the SAPO-34 framework. These shifts are likely due to changes in bond angles or lengths caused by cerium incorporation. Importantly, no additional peaks corresponding to crystalline ceria are observed, indicating that cerium species are highly dispersed and effectively integrated into the SAPO-34 matrix without forming separate phases.^{37,40,44–46}

The FT-IR analysis provides conclusive evidence that the SAPO-34 framework remains intact after ceria incorporation, with minor structural modifications. The preservation of key T–O–T vibrations demonstrates the stability of the chabazite structure, while the reduced intensity of O–H vibrations suggests alterations in surface properties, potentially improving catalytic performance by reducing moisture adsorption and modifying acid site distribution.⁴³

3.1.2. Morphology and chemical composition. The FESEM images of SP and SP-Ce reveal significant morphological differences, reflecting the impact of cerium oxide incorporation on the SAPO-34 framework (Fig. 4). SP exhibits well-defined cubic crystals with smooth surfaces and uniform geometry, characteristic of high crystallinity and minimal structural defects.⁴⁷ The average particle size of SP is 1.7 μm, as presented in Table 1, indicating successful synthesis with proper nucleation and growth during the crystallization process.

Fig. 4 The FESEM images of both SAPO-34 (SP and SP-Ce).

Table 1 Elemental composition of both SAPO-34 (SP and SP-Ce) and their average particles size

Samples	Mole composition	Si/(Si + Al + P)solid	Si/(Si + Al + P)gel	Si incorporation	Si/Al	Ce (wt%)	Particle size (μm)
SP	Al0.735P0.244Si0.144O2	0.129	0.13	0.988	0.196	—	1.7
SP-Ce	Al0.658P0.338Si0.087O2	0.076	0.13	0.596	0.127	4.1	1.05

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In contrast, SP-Ce demonstrates smaller particles with irregular shapes and rough surfaces, with an average particle size reduced to 1.05 μm. The cubic morphology observed in SP is less pronounced in SP-Ce, with particles appearing fragmented or clustered. This morphological alteration is attributed to CeO₂ doping, which disrupts the uniform crystal growth by altering nucleation and growth kinetics during synthesis. The rougher surface and irregular morphology of SP-Ce suggest lattice distortions and structural irregularities, likely caused by interactions between CeO₂ and the SAPO-34 framework, consistent with findings from other reports.^48,49 The smaller particle size and rougher surface of SP-Ce align with XRD results, which indicate a reduction in crystallite size from 36 nm (SP) to 31 nm (SP-Ce). These structural modifications enhance SP-Ce's surface area and accessibility to active sites, offering potential advantages for catalytic applications, particularly in MTO reactions.⁵⁰

The elemental composition and silicon incorporation provide further insights into the structural and chemical changes induced by CeO₂ doping, as listed in Table 1. In SP, the mole composition of Al_0.735P_0.244Si_0.144O₂ corresponds to a Si/(Si + Al + P)_solid ratio of 0.129, closely matching the gel ratio of 0.13. This indicates efficient silicon incorporation during synthesis, with an incorporation efficiency of 0.988. The Si/Al ratio of 0.196 reflects a balanced distribution of silicon and aluminum, contributing to SP's microporous structure and catalytic activity.

In SP-Ce, the mole composition of Al_0.658P_0.338Si_0.084O₂ reveals a decrease in the Si/(Si + Al + P)_solid ratio to 0.077, while the gel ratio remains at 0.13. This results in a reduced silicon incorporation efficiency of 0.596 and a lower Si/Al ratio of 0.127. The reduction in silicon incorporation and Si/Al ratio suggests that cerium interacts competitively with silicon during synthesis, hindering its integration into the framework. Additionally, SP-Ce shows an increase in phosphorus content, reflecting a redistribution of elements likely influenced by cerium's presence.

The Ce 3d high-resolution spectra (Fig. 5) show the characteristic peaks of Ce 3d_5/2 and Ce 3d_3/2, confirming the presence of ceria species on the SP-Ce catalyst surface. Deconvolution of the spectra reveals two oxidation states: Ce⁴⁺, represented by five peaks, and Ce³⁺, represented by two peaks. The coexistence of Ce³⁺ and Ce⁴⁺ in CeO_2−X indicates the presence of oxygen vacancies (X > 0), which are known to enhance catalytic performance by suppressing carbon deposition and delaying catalyst deactivation.^51,52 These oxygen vacancies also modulate the redox and acid–base properties of the catalyst, thereby improving its resistance to coke formation during the MTO process.⁵³ Complementary EDS analysis confirms a cerium content of 4.1 wt% in SP-Ce, demonstrating successful incorporation of Ce species into the SAPO-34 framework or surface. Together with the XRD and FT-IR evidence of high Ce dispersion, these results suggest that Ce species are well-distributed within the catalyst, where their mixed oxidation states and associated oxygen vacancies play a key role in enhancing stability and performance.

Fig. 5 The HR-XPS spectra of Ce 3d for SP-Ce catalyst.

The changes in elemental composition and morphology caused by CeO₂ doping have significant implications for the material's structural and catalytic properties. The reduced Si/Al ratio in SP-Ce may lower the material's acidity, which is critical for catalytic activity. However, the introduction of cerium compensates by introducing redox-active sites, enhancing the material's potential for redox-based catalytic applications. Furthermore, the increased mesoporosity and rougher particle surfaces improve molecular diffusion and accessibility to active sites, optimizing SP-Ce for reactions involving bulky molecules.⁵⁴

The structural irregularities and rough morphology of SP-Ce, along with its smaller particle and crystallite sizes, result in a hierarchical microporous–mesoporous framework. This dual porosity, combined with the altered elemental composition, significantly enhances the material's performance in catalytic and adsorption applications requiring efficient mass transfer and reactivity. CeO₂ doping profoundly impacts SAPO-34 by transforming its well-defined cubic crystals (SP) into irregular, rough particles (SP-Ce) and altering silicon incorporation and the Si/Al ratio. These changes introduce mesoporosity, redox-active sites, and improved accessibility to active regions, making SP-Ce a promising candidate for the MTO process.⁵⁰

3.1.3. Textural properties. The nitrogen adsorption–desorption isotherms of SP and SP-Ce exhibit type-IV characteristics, indicative of mesoporous materials (Fig. 6). The hysteresis loops, observed at relative pressures (P/P₀) of 0.45 for SP and 0.31 for SP-Ce, confirm the presence of mesopores in both materials.^55,56 The earlier onset of the hysteresis loop in SP-Ce reflects structural alterations caused by CeO₂ doping, which increases the fraction of mesopores and enhances overall porosity.

Fig. 6 N₂-adsorption/desorption isotherm and BJH adsorption cumulative pore volume of SP and SP-Ce samples.

The surface areas and pore volumes of both samples are presented in Table 2. The BET surface area (S_BET) of SP is 344.0 m² g⁻¹, which decreases to 288.7 m² g⁻¹ for SP-Ce due to CeO₂ doping. This reduction suggests that CeO₂ partially blocks micropores and covers the surface of SAPO-34. Correspondingly, the external surface area (S_ext) decreases from 81.4 m² g⁻¹ for SP to 53.5 m² g⁻¹ for SP-Ce. Similarly, the micropore surface area (S_micro) drops from 262.6 m² g⁻¹ to 235.2 m² g⁻¹, indicating a reduction in microporosity.

Table 2 Physical properties of SP and SP-Ce samples

Samples	Surface area (m^2 g^−1)			Pore volume (cm^3 g^−1)
	SBET	Sext	Smicro	Vmicro	Vmeso	Vtotal
SP	344.0	81.4	262.6	0.104	0.060	0.164
SP-Ce	288.7	53.5	235.2	0.152	0.150	0.302

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In contrast, mesoporosity increases significantly upon doping. The mesopore volume (V_meso) rises from 0.060 cm³ g⁻¹ for SP to 0.150 cm³ g⁻¹ for SP-Ce, while the total pore volume (V_total) increases from 0.164 cm³ g⁻¹ to 0.302 cm³ g⁻¹. These changes highlight the creation of mesopores and the overall enhancement in pore accessibility. The pore size distribution analysis further supports this, showing that SP primarily exhibits microporous characteristics with a sharp peak below 2 nm, whereas SP-Ce displays a broader distribution extending into the mesoporous range of 2–50 nm.

The structural modifications induced by CeO₂ doping are reflected in the hysteresis behavior, where the lower onset of the loop in SP-Ce aligns with its increased mesoporosity and total pore volume. CeO₂ doping transforms SAPO-34 from a predominantly microporous material to a hierarchical microporous–mesoporous structure. This modification, characterized by increased mesopore volume, broader pore size distribution, and a shift in hysteresis behavior, balances surface area and pore accessibility, optimizing the material for specific catalytic and adsorption applications. The enhanced mesoporosity facilitates the diffusion of bulky molecules, potentially improving catalytic performance in reactions involving larger substrates.⁵⁷ However, the reduction in microporous surface area and BET surface area may slightly diminish the material's adsorption capacity for smaller molecules.²⁴

3.1.4. Acidity. The NH₃-TPD profiles of SP and SP-Ce, depicted in Fig. 7, along with FT-IR analysis, reveal significant differences in acidity and surface chemistry, highlighting the impact of CeO₂ doping on the SAPO-34 framework. These changes are evident in the number, strength, and distribution of acid sites, as well as modifications to surface hydroxyl groups.

Fig. 7 NH₃-TPD profiles of SP and SP-Ce samples.

In SP, the NH₃-TPD profiles exhibit desorption peaks at 201 °C and 698 °C, corresponding to weak and strong acid sites, respectively. The weak acidity is measured at 3.21 mmol g⁻¹, while the strong acidity is 2.59 mmol g⁻¹. The weak acid sites are associated with hydroxyl (OH) groups attached to defect sites such as POH, SiOH, and AlOH. In contrast, the strong acid sites are attributed to hydroxyl groups of (Si–OH–Al) bridges located inside the zeolite channels, which serve as critical structural acidic centers. These strong acid sites are essential for the MTO process, as they facilitate the conversion of methanol to light olefins.^40,44,58

CeO₂ doping in SP-Ce significantly modifies the acidity profile. The desorption temperatures for weak and strong acid sites shift to 248 °C and 676 °C, respectively, reflecting changes in acid strength and distribution. While the weak acid sites exhibit a moderate increase in strength (higher desorption temperature), their overall concentration decreases compared to SP. On the other hand, the strong acid site concentration nearly doubles to 5.06 mmol g⁻¹ after CeO₂ incorporation, accompanied by a slight decrease in desorption temperature, which suggests a mild weakening in average acid strength. The observed increase is therefore more reasonably attributed to the formation of new Lewis acid centers associated with Ce³⁺/Ce⁴⁺ species and oxygen vacancies. These Ce-induced Lewis acid sites, in synergy with the intrinsic Brønsted acidity of the SAPO-34 framework, create a more balanced acidity profile that improves methanol activation, enhances light olefin selectivity, and increases resistance against coke deposition, thereby significantly boosting the catalytic efficiency of SP-Ce.

Weak acid sites play a supportive role in the MTO process by facilitating the initial conversion of methanol to methoxy intermediate species and subsequently to dimethyl ether (DME). Although they do not directly contribute to the production of light olefins, they are essential in enabling the reaction pathway by forming the necessary intermediates. The FT-IR analysis reveals that CeO₂ doping alters the distribution of surface hydroxyl groups, particularly influencing weak acid sites. The reduction in weak acid sites in SP-Ce moderates the conversion of methanol to methoxy intermediate species and diminishes their accumulation on the SAPO-34 surface. This moderation reduces the likelihood of forming formaldehyde species, known precursors to coke formation, which can enhance the durability of the catalyst during the MTO process.^43,59

Strong acid sites remain the key active centers for converting DME to light olefins in the MTO process. The significant increase in strong acid sites in SP-Ce, coupled with their redistribution, ensures more efficient conversion of DME to olefins. This modification in the acidity properties of SP-Ce promises enhanced activity due to the complementary roles of moderately weak and strongly boosted acid sites: the moderately weak acid sites prepare an appropriate percentage of intermediates, while the strong acid sites effectively drive the critical steps of olefin production. The NH₃-TPD underscore the profound impact of CeO₂ doping on the acidity of SAPO-34. CeO₂ not only alters the distribution and strength of acid sites but also enhances the concentration of strong acid sites and optimizes the role of weak acid sites in the reaction pathway, aligning with other reports.^60,61

3.1.5. Mechanistic insights using MD simulations. The MD simulations provide atomistic insights into the reaction pathways of SP and SP-Ce catalysts in the MTO process, with particular focus on energy barriers governing coke formation and consumption. The calculated parameters include the forward activation energy (E_f), reverse activation energy (E_r), and overall energy difference (ΔE), which together describe the kinetics and thermodynamics of elementary steps (R1)–(R6) (Fig. 8). These reactions involve the generation and transformation of surface-bound intermediates that strongly influence catalyst stability and product selectivity.^{5–7,11,62,63}

Fig. 8 Energy barriers and differences for six key reactions ((R1)–(R6)) over both SAPO-34 (SP and SP-Ce).

Reactions (R1)–(R3) represent the primary routes for the formation of surface-bound formyl species (–CHO, denoted as CHO-θ), which are key precursors to coke deposition. (R1) corresponds to the thermal decomposition of methoxy groups (CH₃O-θ) to yield CHO-θ and water. (R2) describes a hydride transfer between methoxy and ethylene, producing CHO-θ alongside ethane. (R3) involves methanol disproportionation, generating CHO-θ and surface hydrogen species. Together, these pathways demonstrate how CHO-θ intermediates form on Lewis acid sites, initiating coke nucleation and subsequent catalyst deactivation.

2CH₃O-θ + θ ↔ CHO-θ + CH₃-θ + H₂O-θ (R1)

CH₃O-θ + C₂H₄-2θ ↔ CHO-θ + C₂H₆-2θ (R2)

CH₃O-θ + 2θ ↔ CHO-θ + 2H-θ (R3)

Subsequent transformations ((R4)–(R6)) highlight how these intermediates evolve into gaseous products or coke precursors. (R4) describes further decomposition of CHO-θ into CO-θ and H-θ. (R5) represents the water–gas shift-like step, converting CO-θ and H₂O-θ into CO₂-θ and H₂-θ. Finally, (R6) captures CO₂ reduction with surface carbon species to form two CO-θ units, potentially feeding back into coke-related pathways.

CHO-θ + θ ↔ CO-θ + H-θ (R4)

CO-θ + H₂O-θ ↔ CO₂-θ + H₂-θ (R5)

CO₂-θ + C-θ ↔ 2CO-θ (R6)

The relative magnitudes of E_f and E_r can provide a mechanistic explanation for the experimentally observed trends in selectivity and stability.⁶⁴ The E_f of (R1) increases significantly from 58.64 kJ mol⁻¹ for SP to 73.01 kJ mol⁻¹ for SP-Ce, indicating a higher resistance to the thermal decomposition of methanol in the doped catalyst. The E_r of (R1) for SP-Ce is slightly reduced (69.99 kJ mol⁻¹) compared to SP (64.22 kJ mol⁻¹), resulting in a shift in thermodynamic favorability. The ΔE of (R1) for SP-Ce is positive (3.02 kJ mol⁻¹), indicating an endothermic nature, while SP shows a slightly exothermic reaction (−5.58 kJ mol⁻¹). Fig. 8 shows that (R2) illustrates a sharp increase in E_f from 91.28 kJ mol⁻¹ (SP) to 121.3 kJ mol⁻¹ (SP-Ce), reflecting a higher energy barrier for CHO-θ formation in SP-Ce. Besides, the E_r of (R2) for SP-Ce decreases to 95.84 kJ mol⁻¹ compared to SP (102.02 kJ mol⁻¹). A substantial change in ΔE of (R2) is observed, shifting from −10.74 kJ mol⁻¹ in SP to 25.46 kJ mol⁻¹ in SP-Ce, indicating that CeO₂ doping makes this reaction less favorable thermodynamically. E_f of (R3) increases from 82.35 kJ mol⁻¹ (SP) to 100.16 kJ mol⁻¹ (SP-Ce), indicating a slower disproportionation process for the doped catalyst. E_r of (R3) also rises in SP-Ce (93.34 kJ mol⁻¹) compared to SP (85.13 kJ mol⁻¹). The ΔE of (R3) remains slightly endothermic for SP-Ce (6.82 kJ mol⁻¹) compared to the nearly thermoneutral ΔE of −2.78 kJ mol⁻¹ in SP.

In addition to reactions that generate CHO-θ, (R4) is included to explore a potential pathway for suppressing CHO-θ accumulation. This reaction converts CHO-θ into less harmful intermediates such as CO and H, reducing the likelihood of coke formation and enhancing catalyst durability. The kinetic feasibility of this suppression pathway is evaluated by comparing the forward and reverse energy barriers (E_f and E_r), which indicate the effectiveness of the catalyst in mitigating CHO-θ accumulation. (R4) demonstrates a significant reduction in E_f for SP-Ce (30.51 kJ mol⁻¹) compared to SP (51.03 kJ mol⁻¹), indicating improved efficiency in suppressing CHO-θ accumulation. However, the reverse barrier increases from 48.55 kJ mol⁻¹ (SP) to 52.26 kJ mol⁻¹ (SP-Ce). Importantly, ΔE of (R4) becomes highly exothermic (−21.75 kJ mol⁻¹) in SP-Ce, compared to 2.48 kJ mol⁻¹ in SP, showing that CeO₂ doping makes this reaction thermodynamically favorable and kinetically accessible.

Reactions (R5) and (R6) focus on mechanisms for coke removal. (R5) represents the water-gas shift reaction, which generates activated CO₂ by converting CO and water into CO₂ and H₂. The activated CO₂ then plays a critical role in (R6), the reverse Boudouard reaction, which oxidizes deposited carbon on the catalyst surface to CO. Together, these reactions provide a means of regenerating the catalyst by consuming deposited coke, thereby enhancing its durability. By studying these reactions, the ability of the catalyst to actively manage coke formation and removal is assessed. E_f of (R5) for SP-Ce is significantly lower (69.83 kJ mol⁻¹) than SP (97.09 kJ mol⁻¹), suggesting enhanced progression of WGS in the doped catalyst. E_r of (R5) is slightly higher for SP-Ce (98.88 kJ mol⁻¹) compared to SP (95.15 kJ mol⁻¹). ΔE shifts from a slightly endothermic 1.94 kJ mol⁻¹ in SP to a highly exothermic −29.05 kJ mol⁻¹ in SP-Ce, indicating greater thermodynamic favorability for SP-Ce to CO₂ formation through accomplishing WGS reaction. E_f of (R6) decreases significantly in SP-Ce (44.08 kJ mol⁻¹) compared to SP (68.76 kJ mol⁻¹), reflecting enhanced coke consumption in the doped catalyst. E_r of (R6) for SP-Ce increases slightly to 65.55 kJ mol⁻¹ compared to SP (64.42 kJ mol⁻¹). ΔE of (R6) becomes strongly exothermic (−21.47 kJ mol⁻¹) for SP-Ce, compared to a weakly endothermic 4.34 kJ mol⁻¹ in SP, indicating greater efficiency in carbon removal.

While SP contains Lewis acid sites essential for facilitating the MTO process, these same sites are prone to promoting coke formation through CHO-θ intermediates, a critical precursor to carbonaceous deposits that deactivate the catalyst. Doping SAPO-34 with CeO₂ introduces redox-active sites, which are expected to significantly influence key reactions involved in coke management. Specifically, these sites enhance CO₂ formation and activation, improving the efficiency of coke removal pathways such as the water–gas shift (R5) and the Boudouard reaction (R6). This modification is anticipated to result in superior performance in terms of catalyst durability and selectivity to olefins by reducing coke formation and promoting its removal. Furthermore, by maintaining active catalytic sites through efficient suppression and removal of coke, the SP-Ce catalyst is expected to sustain higher olefin selectivity during the MTO process.

3.1.6. MTO catalytic tests. The catalytic performance of SP and SP-Ce for the MTO process was investigated in a fixed-bed reactor at atmospheric pressure, 425 °C, with a WHSV of 2 h⁻¹ and a methanol feed composition of 72 wt% in water. Fig. 9a shows the total olefin selectivity as a function of time on stream (TOS) for both catalysts. The SP catalyst initially achieves a selectivity of 31.4% at 60 min, peaking at 76.6% after 240 min. However, a noticeable deactivation occurs after 420 min, with selectivity declining to 41.4%. In contrast, SP-Ce demonstrates superior performance, with an initial selectivity of 53%, peaking at 83.9% at 420 min and maintaining 54.9% selectivity at 600 min. This enhanced stability is attributed to the incorporation of CeO₂, which reduces coke formation and preserves active sites.

Fig. 9 (a) Selectivity of light olefins and (b) product distribution (C₂=–C₄=) for SP and SP-Ce samples under specified test conditions (T = 425 °C, catalyst weight = 2 g, WHSV = 2 h⁻¹).

Fig. 9b presents the selectivity distribution for light olefins, ethylene (C₂=), propylene (C₃=), and butene (C₄=). SP shows ethylene, propylene, and butene selectivities of 35.51%, 38.33%, and 2.75%, respectively. SP-Ce achieves improved propylene selectivity (41.47%) while maintaining similar ethylene selectivity (39.79%) and low butene selectivity (2.64%). CeO₂ doping modifies the acid site distribution, enhancing the propylene-to-ethylene ratio and favoring ethylene production, which is in good agreement with other studies.^65,66

In addition, coke formation plays a decisive role in determining the stability, catalytic activity, and overall lifetime of SAPO-34 in the MTO process. Both the total amount of coke deposited and the rate of coke accumulation (R_Coke) are key indicators of deactivation behavior. Thermogravimetric analysis of the spent samples (Table 3) shows comparable total coke contents for the two catalysts, with values of 9.7% (g g_cat⁻¹) for SP and 10.1% (g g_cat⁻¹) for SP-Ce. The difference becomes evident, however, in the coke deposition rate: SP reaches 0.20 mg g_cat⁻¹ min⁻¹, while SP-Ce is lower at 0.15 mg g_cat⁻¹ min⁻¹. This reduced coke accumulation rate correlates with the enhanced stability of SP-Ce, which maintains activity for 660 minutes on stream, compared with only 480 minutes for SP before pronounced deactivation occurs.

Table 3 Coke content and coke deposition rate over all samples

Samples	TOS (min)	Coke capacity (%, g gcat^−1)	RCoke (mg gcat^−1 min^−1)
SP	480	9.7	0.20
SP-Ce	660	10.1	0.15

---

The superior catalytic performance of SP-Ce compared to the SP in the MTO process is strongly tied to its structural, textural, and chemical properties, as revealed by characterization and MD simulations. SP exhibits a crystallite size of ∼36 nm with a uniform cubic morphology. However, its microporous structure limits mass transfer, and the absence of mesoporosity leads to rapid deactivation due to coke accumulation.⁶⁷ CeO₂ doping reduces the crystallite size to ∼31 nm and transforms the cubic crystals into irregular, rough particles, introducing a hierarchical microporous–mesoporous framework. This enhances mass transfer, increases pore volume, and facilitates the diffusion of bulky intermediates, mitigating pore blockage and improving catalytic efficiency. The mesoporosity of SP-Ce, confirmed by hysteresis behavior, combined with smaller particle sizes, increases surface area availability, contributing to higher olefin selectivity and prolonged stability.⁶⁸

Acidity plays a pivotal role in the MTO reaction, and the two catalysts exhibit distinct acid site distributions. SP contains a high concentration of weak acid sites, which promote methoxy intermediate accumulation and increase coke formation. CeO₂ doping optimizes SAPO-34's acidity by reducing weak acid sites and redistributing strong acid sites. Strong acid sites efficiently drive olefin production, while moderated weak acid sites reduce formaldehyde formation, a precursor to coke. NH₃-TPD and FT-IR analyses confirm this balanced acidity, ensuring efficient methanol conversion and reduced coking tendencies.

Mechanistic insights from MD simulations reveal that CeO₂ suppresses the formation of CHO-θ intermediates, which are key precursors to coke, and enhances coke removal through improved CO₂ activation and the Boudouard reaction. The redistribution of acid sites in SP-Ce balances intermediate preparation (via weak acid sites) and olefin production (via strong acid sites), further improving methanol conversion pathways.

The synergy between experimental results, characterization data, and MD simulations underscores the transformative role of CeO₂ doping. SP's larger crystallite size, lack of mesoporosity, and excessive weak acid sites contribute to rapid deactivation due to limited mass transfer and coke formation. Conversely, CeO₂-doped SP-Ce exhibits smaller crystallite size, hierarchical mesoporosity, and optimized acidity, enhancing mass transfer, reducing coking, and sustaining olefin production, particularly ethylene. These dual benefits, structural improvements (enhanced mesoporosity and particle size reduction) and functional enhancements (optimized acidity and coking resistance), position SP-Ce as a superior catalyst for the MTO process. CeO₂ incorporation significantly prolongs catalyst life, enhances olefin selectivity, particularly ethylene, and improves operational stability, making SP-Ce a highly promising catalyst for industrial applications.

4. Conclusion

The incorporation of cerium oxide into SAPO-34 provides groundbreaking mechanistic insights that drive its enhanced performance in the MTO process. MD simulations highlight the suppression of coke-forming CHO-θ intermediates and the promotion of efficient coke removal via CO₂ activation and the Boudouard reaction, mechanisms critical to sustaining catalytic activity and selectivity. Complementary experimental findings reveal that CeO₂ reduces the crystallite size from ∼36 nm to ∼31 nm and transforms SAPO-34 from a microporous structure to a hierarchical microporous–mesoporous framework. These structural changes facilitate mass transfer, increase pore accessibility, and enhance the diffusion of bulky intermediates, mitigating pore blockage.

CeO₂ doping also introduces a novel redistribution of acid sites, balancing weak and strong acid sites to optimize methanol conversion pathways. The reduction of weak acid sites minimizes coke precursors, while enhanced strong acid sites drive olefin production. This dual optimization mechanism ensures prolonged catalyst life and superior olefin selectivity, particularly ethylene.

The synergy between mechanistic understanding and experimental validation establishes CeO₂-doped SAPO-34 as a superior catalyst for the MTO process. By addressing critical challenges such as coke formation and deactivation, this study provides a comprehensive framework for leveraging CeO₂'s redox activity and structural benefits, marking SP-Ce as a pivotal advancement for industrial catalytic applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors state that the data supporting the findings of this study are included in the paper. If raw data files in a different format are required, they can be obtained from the corresponding author upon reasonable request. Source data is supplied with this paper.

Acknowledgements

This work is based upon research funded by Iran National Science Foundation (INSF) under project no. 4027639.

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