Introduction

The methanol-to-olefins (MTO) process has emerged as a promising alternative to conventional olefin production routes that depend heavily on fossil resources such as naphtha or ethane steam cracking. In contrast, the MTO process utilizes methanol, which can be derived from renewable feedstocks including biomass, municipal solid waste, or CO2 hydrogenation, thus supporting a more sustainable and circular chemical economy1,2,3,4. Ethylene and propylene, the main products of the MTO process, are essential building blocks in the manufacturing of plastics, textiles, and other industrial chemicals.

The efficiency, selectivity, and durability of the MTO process are critically dependent on the catalyst used. Among various zeolite and zeotype catalysts, silicoaluminophosphate SAPO-34 has gained considerable attention due to its superior selectivity to light olefins and strong hydrothermal stability5,6. SAPO-34 crystallizes in the chabazite (CHA) structure, characterized by large internal cages (∼7.4 Å) interconnected by narrow 8-ring windows (∼3.8 Å). This structural topology offers unique advantages: it promotes the formation of light olefins while inhibiting the formation of heavier hydrocarbons due to size and shape selectivity, and it stabilizes the key reaction intermediates involved in the hydrocarbon pool mechanism.

However, despite its favorable properties, conventional SAPO-34 faces critical limitations in long-term operation, primarily due to diffusional constraints and catalyst deactivation by coke formation5,6,7,8. The small pore apertures limit the diffusion of reactants and products, which can lead to the accumulation of bulky reaction intermediates and coke precursors within the cages, ultimately blocking the active sites and reducing catalytic lifetime.

To mitigate these issues, the design and synthesis of hierarchical SAPO-34, materials that integrate both micropores and mesopores, have been explored extensively9,10,11. Introducing mesoporosity facilitates molecular transport, enhances mass transfer, and improves resistance to coke accumulation. Hierarchical structures also allow better accessibility to active sites and increase external surface area, which can enhance catalytic performance, particularly in reactions like MTO where rapid mass transfer is critica112.

Several strategies have been developed to synthesize hierarchical SAPO-34, including post-synthetic treatments (e.g., acid or base etching) and in situ templating methods. Among these, in situ dual-templating approaches have proven highly effective, as they allow for the simultaneous formation of microporous and mesoporous networks during crystallization5,12. In this method, organic structure-directing agents (OSDAs) such as tetraethylammonium hydroxide (TEAOH) are used to generate the CHA microporous framework, while hard templates (e.g., carbon particles, starch, or biopolymers) and soft templates (e.g., surfactants or cationic polymers) introduce mesoporosity5,6,13.

Traditional OSDAs and templating agents, however, pose economic and environmental concerns. They are often expensive and toxic. In response to these issues, recent research has shifted toward green synthesis approaches that utilize bio-derived, biodegradable, and low-toxicity alternatives. These methods aim to reduce the environmental footprint of SAPO-34 production while maintaining or improving catalytic performance5,6,13,14.

Natural materials such as polysaccharides, plant extracts, and biopolymers have shown promise as sustainable templating agents. For example, okra mucilage, a viscous plant extract rich in polysaccharides, can serve as a renewable hard template due to its ability to form stable gels and interact with silicoaluminophosphate precursors during hydrothermal synthesis. Similarly, brewed coffee, which contains nitrogenous compounds and polyphenols, can act as a soft template and nitrogen source, potentially enhancing surface properties, modulating acidity, and improving coke resistance in the resulting catalyst5,6,12,15.

In this context, the present study explores the synthesis of hierarchical SAPO-34 using a dual-template approach based on brewed coffee and okra mucilage as green templating agents. This strategy integrates the advantages of both meso- and microporosity while aligning with principles of green chemistry. Comprehensive physicochemical characterization techniques, including XRD, FT-IR, FESEM, nitrogen physisorption, EDS, and NH3-TPD, have been employed to understand the effects of these natural templates on textural properties, crystallinity, morphology, and acidity. Particular attention has been given to correlating mesoporosity, crystallite size, and acid site distribution with MTO catalytic performance in terms of light olefin selectivity, ethylene/propylene ratio, and catalyst stability.

The goal of this study is twofold:

  1. 1.

    To investigate whether the hierarchical porosity and moderated acidity induced by green templating improve MTO performance, especially by enhancing selectivity and reducing deactivation; and.

  2. 2.

    To demonstrate that eco-friendly templating using plant-based materials is a viable alternative to conventional synthesis, contributing to more sustainable catalyst production.

By integrating catalytic evaluation with detailed structural analysis, this work aims to advance the understanding of structure-performance relationships in SAPO-34-based MTO catalysts and establish a sustainable pathway for their synthesis.

Experimental and theoretical approach

Materials

SAPO-34 catalysts are synthesized using a hydrothermal crystallization method. Tetraethylammonium hydroxide (TEAOH, 20 wt%, Sigma-Aldrich) and morpholine (Mor, 99%, Merck) serve as template agents, while aluminum iso-propoxide (AIP, 99 wt%, Sigma-Aldrich), phosphoric acid (H3PO4, 85 wt%, Sigma-Aldrich), and tetra-ethyl-orthosilicate (TEOS, C8H20O4Si, Merck) act as sources of aluminum, phosphorus, and silicon, respectively. The synthesis follows a molar composition of 1Al2O3: 1P2O5: 0.6SiO2: 1.25TEAOH: 1.25Mor: 70H2O.

To establish an environmentally friendly approach to catalyst fabrication, the process starts with creating a natural organic template from okra plants. Freshly harvested okras from the TMU Research Centre farm are dried at 40 °C for 48 h before being ground into a fine powder to improve mucilage extraction16. Five grams of this powdered okra is combined with 50 ml of water and stirred at 50 °C for one hour17,18. The resulting mixture undergoes centrifugation to separate the mucilage from solid residues, producing around 30 ml of mucilage. Next, to prepare the brewed coffee, coffee powder sourced from Indonesian Arabica coffee beans, specifically the roasted Aceh Gayo type, is brewed for 30 min, and its liquid is separated as a soft template.

In this study, okra mucilage is classified as a hard template due to its gel-like, polysaccharide-rich structure, which provides a semi-rigid framework during crystallization, promoting mesopore formation. Brewed coffee, rich in small organic molecules like caffeine and polyphenols, is used as a soft template, guiding mesopore development through chemical interactions and self-assembly without forming a physical scaffold. This classification follows conventional definitions based on the physical versus chemical templating roles of the agents.

Catalyst fabrication

To start, deionized water is poured into a beaker, and AIP is dissolved by simultaneously adding TEAOH and morpholine. Okra mucilage (10% by volume) is then gradually introduced into the solution. Once the aluminum source and organic templates are fully incorporated, the mixture is stirred at 600 rpm for 1 h at room temperature. Subsequently, TEOS is added dropwise and stirred for 3 h at 60 °C, followed by the slow addition of phosphoric acid. The mixture continues stirring at 600 rpm for another 2 h at 60 °C. The resulting gel is then left to age at room temperature for 24 h.

The aged gel undergoes hydrothermal treatment inside a stainless-steel autoclave, where controlled crystallization occurs. The first stage of the process is conducted at 180 °C for 18 h, facilitating the stepwise formation of SAPO-34 particles through nucleation, crystallization, and nanoparticle aggregation19. Prior to the autoclave step, brewed coffee (10% by volume) is added to the aged gel. Both brewed coffee and okra mucilage act as structure-directing agents, contributing to the morphological control of the final product. During the nucleation phase, small precursor clusters form in the supersaturated gel, initiating the framework of SAPO-34. This is followed by the crystallization stage, where the nuclei grow into well-defined crystalline structures as the system maintains a high temperature inside the autoclave. As the reaction progresses, nanoparticles aggregate, forming larger SAPO-34 particles, which ultimately lead to the development of a well-structured microporous framework. It should be noted that brewed coffee mainly contains C, O (> 60 wt%), N (2.6 wt%), and K (1 wt%), along with minor Ca, Mg, P, Fe, and Mn20. Okra mucilage consists of 50 wt% C, 40 wt% O, and trace N (0.5 wt%), Ca, Mg, and K21.

After hydrothermal treatment, the synthesized materials are separated via centrifugation, washed four times with deionized water, and dried at 100 °C for 12 h. Finally, the dried samples are calcined at 550 °C for 5 h to eliminate organic templates and residual organic matter. The final product, excluding brewed coffee and okra mucilage, is labeled as SAPO-34-P (SP). The fabrication steps for the eco-friendly SAPO-34 catalysts, synthesized via a dual-template strategy using brewed coffee as a soft template and okra mucilage as a hard template (SPG), and using only okra mucilage (SPG1), are illustrated in Fig. 1.

Fig. 1
figure 1

Step-by-step process of SPG fabrication.

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Characterization methods

A range of analytical techniques is utilized to examine the influence of the dual green template on SAPO-34 synthesis and its effectiveness in the MTO process. The crystallinity and phase purity of the samples are determined using X-ray powder diffraction (XRD) with a Philips X’Pert MPD system, scanning within the 2θ range of 5–50° and employing a Cu Kα radiation source (λ = 1.54056 Å). The crystallite size is calculated using the Scherrer equation, and the relative crystallinity is determined by calculating the ratio of the area under the crystalline peaks to the total area under the XRD pattern, expressed as a percentage. The presence of functional groups in the samples is identified through Fourier Transform Infrared Spectroscopy (FT-IR) using a PerkinElmer Spectrum Two spectrometer, covering wavelengths between 400 and 4000 cm− 1. The morphological characteristics of the samples are assessed using a TESCAN MIRA3 field emission scanning electron microscope (FESEM), while elemental composition is simultaneously analyzed using an energy-dispersive X-ray spectrometer (EDS) attached to the FESEM22.

The textural properties, including surface area and pore volume, are determined via nitrogen adsorption/desorption at 77 K, performed with a Micromeritics Tristar 3020 automated physical adsorption analyzer. The acidic properties of the catalysts are investigated through temperature-programmed ammonia desorption (NH3-TPD) using a BELCAT-B catalyst analyzer from MicrotracBEL, which is equipped with a thermal conductivity detector (TCD).

MTO catalytic tests

The MTO process takes place in a fixed-bed reactor at atmospheric pressure and a temperature of 425 °C. Before each experiment, 2 g of calcined catalyst (40–20 mesh) and 4 g of inert silicon carbide (SiC) are introduced into the reactor. To eliminate moisture and potential contaminants, the catalyst undergoes a pretreatment at 550 °C for 1 h under nitrogen flow. Once the reactor stabilizes at the reaction temperature, a methanol-water mixture (72:28 by weight) is fed into the system at a weight hourly space velocity (WHSV) of 2 h− 1.

The gaseous products formed during the reaction are analyzed using an Agilent GC 7890 A gas chromatograph, equipped with flame ionization (FID) and thermal conductivity (TCD) detectors for precise product characterization. This analytical setup enables a comprehensive evaluation of reaction performance and product distribution, as depicted in Fig. 2.

Fig. 2
figure 2

MTO Process flow diagram experimental setup.

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Fig. 3
figure 3

(a) XRD patterns with average crystallite sizes and relative crystallinity; (b) FT-IR spectra of all SAPO-34, and (c) FT-IR spectra of both natural templates (brewed coffee and okra mucilage powder).

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Results and discussion

Characterization analyses

Crystalline and structural properties

The XRD patterns of all samples, depicted in Fig. 3a, exhibit distinct diffraction peaks at 2θ values of 9.6°, 20.7°, 26°, and 31°, which are characteristic of the CHA framework. These peaks confirm the successful synthesis of SAPO-34 with a crystalline structure consistent with the standard CHA topology. Both SPG and SPG1 retain structural integrity, as validated by the reference JCPDS card 01-087-1527, confirming the presence of the crystalline phase associated with a hexagonal prism geometry23,24.

Despite maintaining the fundamental CHA framework, notable differences arise among SP, SPG and SPG1 due to their distinct synthesis routes. The SP sample, synthesized through conventional methods, exhibits higher peak intensities, indicating a higher degree of crystallinity. Conversely, the SPG and SPG1 catalyst demonstrate slightly lower peak intensities and reduced crystallite sizes. The average crystallite sizes, derived from XRD data, reveal that SP possesses larger crystals (46 nm) compared to SPG1 (39 nm) and SPG (34 nm). In addition, the relative crystallinity of all samples is evaluated, showing the following order: SP (82%) > SPG1 (74%) > SPG (67%). This reduction, particularly, in crystallite size suggests that okra mucilage, acting as a hard template, influences the nucleation and growth processes, and that the dual-template synthesis method further intensifies this effect, resulting in finer crystal morphology and enhanced textural properties.

The shift in peak intensities and differences in crystallite sizes indicate that the green templating method significantly alters the physicochemical properties of SAPO-34. The use of natural templates, such as brewed coffee and okra mucilage, in SPG synthesis results in a reduced crystallite size compared to the conventionally synthesized SP sample. This reduction may enhance diffusion properties and catalytic performance in the MTO process by facilitating molecular transport within the hierarchical pore network24. However, the lowest crystallinity of SPG could potentially affect its structural stability and catalytic lifespan. Nevertheless, these modifications have significant implications for its catalytic behavior, as the hierarchical porosity and eco-friendly synthesis approach may enhance mass transport and reduce coke formation.

The FT-IR spectra of all catalysts, as shown in Fig. 3b, provide valuable insights into their structural and functional characteristics. The spectra display characteristic absorption bands associated with the SAPO-34 framework, confirming the successful synthesis of both modified catalysts while highlighting the impact of green templating on their physicochemical properties. All samples exhibit absorption bands typical of SAPO-34, including those corresponding to the fundamental vibrations of aluminosilicate and phosphate species. The broad absorption band observed around 3400 cm− 1 is attributed to the O–H stretching vibrations of hydroxyl groups, indicating the presence of adsorbed water and surface hydroxyl groups. The intensity of this band is slightly higher in SP25,26,27,28.

The strong absorption bands in the range of 1200–900 cm− 1 are characteristic of asymmetric stretching vibrations of the TO4 tetrahedral units (T = Si, Al, P) that constitute the SAPO-34 framework15,29,30. All Sapo-34 samples exhibit these bands, confirming the integrity of the zeolitic structure. However, slight shifts in peak positions and variations in intensity between these samples suggest subtle differences in framework composition and bonding environments. These variations may be attributed to the influence of natural templating agents, which can impact the crystallization process and the incorporation of silicon into the framework.

Notably, SPG shows slight deviations in transmission intensity compared to SP, which may be linked to the hierarchical porosity introduced by the dual-template synthesis approach. The use of brewed coffee as a soft template and okra mucilage as a hard template likely influences the porosity and defect sites, resulting in minor spectral variations. Overall, the FT-IR analysis confirms that both SPG1 and SPG retain the characteristic SAPO-34 framework while highlighting the impact of sustainable green templating on structural and surface properties. The observed spectral differences suggest modifications in hydroxylation, framework composition, and porosity, which could influence the catalytic behavior of SPG.

Additionally, FT-IR spectroscopy is employed to characterize the brewed coffee and okra mucilage powder, used as natural templates in the synthesis of SAPO-34. The spectra, as shown in Fig. 3c, reveal a range of characteristic absorption bands corresponding to various organic and inorganic functional groups inherent in both materials.

A broad and intense absorption band centered around 3366 cm− 1 is attributed to the O–H stretching vibrations, which arise due to inter- and intra-molecular hydrogen bonding. This feature confirms the presence of hydroxyl groups, which are abundant in both natural materials due to the presence of water, polysaccharides, and phenolic compounds. The broad nature of this band suggests a complex hydrogen bonding network, indicative of the amorphous organic structure in both coffee and okra mucilage. The peak observed at approximately 2925 cm− 1 corresponds to the C–H stretching vibrations, including both symmetric and asymmetric modes, typically associated with aliphatic –CH and –CH2 groups. This feature confirms the presence of organic acids, lipids, and other aliphatic constituents in both natural templates.

The fingerprint region of the FT-IR spectra (1700 –900 cm− 1) provides deeper insights into the complex molecular structures. Notably, between 1500 and 1700 cm− 1, significant absorption bands are detected, which are indicative of amide functional groups, including C = O (amide I) and C–N (amide II) stretching vibrations. These peaks suggest the presence of proteins or amino-sugar derivatives within the organic matrices.

A distinct peak appearing in the range of 1420–1450 cm− 1 can be assigned to the bending vibrations of aliphatic CH and CH2 groups, as well as the stretching vibration of the carbonyl (C = O) group. This supports the presence of both aliphatic structures and carboxylic acid functionalities, potentially derived from fatty acids or oxidized carbohydrate residues. In the region of 1160–1240 cm− 1, characteristic peaks corresponding to C–O–C linkages in polysaccharides and C–N amide stretching are observed. These features reflect the polysaccharide-rich nature of okra mucilage and the complex carbohydrate content in coffee residues, both of which are key to templating behavior in SAPO synthesis. Additionally, strong absorption in the 950–1150 cm− 1 range is assigned to C–O stretching vibrations of carbohydrates, as well as contributions from C = C, C = N, and C–H stretching in heterocyclic or aromatic ring systems. These bands are commonly associated with polyphenolic and lignin-like compounds, particularly prevalent in brewed coffee powder. Importantly, absorption bands between 500 and 600 cm− 1 are attributed to metal-oxygen (M–O) vibrations, confirming the presence of metal ions or oxides, especially in the coffee-derived template. The presence of these inorganic components is consistent with literature reports20,21 on the elemental composition of coffee and okra, which may influence the crystallization process and framework formation during SAPO-34 synthesis.

The FT-IR analysis clearly demonstrates that both brewed coffee and okra mucilage powders are rich in functional groups conducive to acting as natural templates in SAPO-34 synthesis. The abundance of hydroxyl, carbonyl, amide, and ether linkages, along with carbohydrate and protein-based structures, suggests a high potential for directing the assembly of silicoaluminophosphate frameworks via hydrogen bonding and other intermolecular interactions. Furthermore, the presence of metal–oxygen species in coffee may enhance the nucleation process or influence the acidity and porosity of the resulting SAPO-34 structure. Overall, the complex but informative FT-IR spectra support the feasibility of utilizing these sustainable, biodegradable materials as environmentally friendly alternatives to conventional synthetic templates in molecular sieve synthesis.

Fig. 4
figure 4

FESEM images of all SAPO-34 samples, illustrating their particle size distribution.

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Table 1 Elemental analysis of all SAPO-34 samples obtained through FESEM-EDS.
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Morphological characteristics and elemental analysis

The FESEM images of all samples, which offer critical insights into their morphological differences, are presented in Fig. 4. The micrographs clearly show variations in particle size, shape, and aggregation behavior, demonstrating the impact of the green templating method on SAPO-34 crystal formation. The SP sample exhibits well-defined, cubic-shaped crystals with smooth surfaces and a relatively uniform particle size distribution31. The average particle size for SP is approximately 1.7 μm, as indicated in the images. The sharp edges and uniformity suggest a controlled crystallization process, typical of conventional SAPO-34 synthesis methods using standard SDAs. The relatively large and well-formed crystals indicate a high degree of crystallinity, which correlates with the higher XRD peak intensities observed in previous analyses.

In contrast, both modified samples synthesized using natural green templates exhibit noticeable differences in morphology. Their particles appear more aggregated and slightly irregular compared to the well-defined cubic crystals of SP. The average particle size of SPG1 is smaller than that of SP, approximately 1.4 μm, while SPG shows the smallest particle size of 1.1 μm, suggesting that the presence of brewed coffee and okra mucilage during synthesis influenced the nucleation and growth processes. The reduced particle size in SPG is likely attributed to the dual-template approach, where the hard template (okra mucilage) and the soft template (brewed coffee) create hierarchical porosity and alter crystallization dynamics. The increased surface roughness and particle aggregation observed in SPG further indicate the formation of meso- and macropores, which could enhance molecular diffusion and catalytic performance.

The particle size distribution analysis further supports these findings. The SP sample exhibits a broader particle size distribution, with most particles ranging between 1.3 and 2.0 μm, whereas the SPG sample demonstrates a more concentrated size distribution around 0.9–1.3 μm. This reduction in particle size, combined with increased porosity, may contribute to improved mass transport properties, making SPG potentially more efficient for catalytic applications of MTO process.

Overall, the FESEM analysis confirms that the introduction of bio-derived templates in SPG synthesis significantly alters the morphology of SAPO-34. The smaller, more aggregated crystals with enhanced porosity suggest potential advantages in catalytic performance, particularly in diffusion-limited reactions. While the lower crystallinity observed in SPG could influence stability, the improved textural properties and eco-friendly synthesis approach make it a promising alternative to conventional SAPO-34.

On the other hand, the EDS analysis of SP and SPG provides critical insights into the elemental composition and silicon incorporation behavior of these catalysts, as presented in Table 1. The mole compositions of both samples confirm the presence of aluminum (Al), phosphorus (P), and silicon (Si) as key framework components, with oxygen completing the SAPO-34 structure. A key difference between the two catalysts is the Si/(Si + Al + P)solid ratio, which represents the extent of silicon incorporation into the final catalyst structure. The SP sample exhibits a higher Si content (0.129) compared to SPG1 (0.1) and SPG (0.086), indicating that the green templating approach affects silicon distribution within the framework. Despite both samples having the same Si/(Si + Al + P)gel ratio (0.13), the reduced Si incorporation in SPG1 and SPG suggests that the bio-derived templates influence silicon mobility and interaction during crystallization, potentially modifying acidity and catalytic behavior.

The Si/Al ratio further reflects this trend, showing a higher value in SP (0.988) compared to SPG1 (0.769) and SPG (0.663). This reduction in SPG suggests a lower framework acidity, which could impact methanol conversion and reaction selectivity. However, a balanced acidity profile can enhance catalyst stability and reduce coke formation, making SPG potentially advantageous for prolonged MTO operation32.

Additionally, carbon (C) and nitrogen (N) contents exhibit noticeable variations among all samples. SPG contains higher amounts of carbon (51.1 atomic%) and nitrogen (8.8 atomic%), with SPG1 showing similar values, compared to SP, which has 46.4 atomic% carbon and 5.7 atomic% nitrogen. Although all three samples are calcined to remove organic templates and residual matter, elemental analysis (Table 1) reveals trace amounts of carbon and nitrogen that are not fully eliminated by thermal treatment, particularly when incorporated into microporous domains or chemically bonded to the surface, as previously reported by Wang et al.33. These residual elements may also influence the surface polarity or functionality of the materials, as evidenced by FT-IR characterization. The nitrogen incorporation is likely derived from organic compounds present in the natural templates (brewed coffee), which remain embedded in the structure and alter the electronic environment of the catalyst. The presence of nitrogen could enhance the catalyst’s resistance to framework degradation and improve coke suppression by modifying adsorption energy and reaction pathways34,35,36.

Therefore, the EDS analysis confirms that green templating significantly alters the composition of SAPO-34 by reducing Si incorporation while increasing C and N content. These modifications suggest that SPG may exhibit different acidity characteristics and catalytic stability compared to conventional SAPO-34. The influence of nitrogen in SPG may play a critical role in enhancing methanol adsorption, improving coke resistance, and optimizing catalytic longevity, making it a promising candidate for industrial MTO applications37.

The incorporation of bio-derived templates significantly influences the crystal growth behavior and framework composition of SAPO-34. Okra mucilage, acting as a hard template, introduces a gel-like medium rich in polysaccharides that physically impedes crystal overgrowth, promoting nucleation over growth and resulting in smaller crystal sizes. This semi-rigid matrix also modulates the diffusion of precursor species, leading to more uniform and confined crystal formation.

Brewed coffee, serving as a soft template, contains low-molecular-weight nitrogenous compounds such as caffeine and polyphenols, which chemically interact with silicon and aluminum species during synthesis. These interactions can competitively coordinate with silicon sources, thereby limiting excessive silicon incorporation and favoring a more balanced Si/Al distribution. Furthermore, the nitrogen-containing organics can act as conventional SDAs, guiding the framework assembly while influencing silicon incorporation kinetics.

Together, these bio-derived templates not only reduce crystal size by altering the nucleation-growth balance but also contribute to a lower silicon content in the final crystal structure. This is corroborated by XRD and SEM/EDS results, which show a consistent trend of smaller crystal morphology and reduced Si content in SPG and SPG1 compared to SP.

Fig. 5
figure 5

N2-adsorption/desorption Isotherm and BJH Adsorption Cumulative Pore Volume of all SAPO-34 samples.

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Table 2 Physical properties of all SAPO-34 samples.
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Surface area and porosity

The nitrogen adsorption/desorption isotherm and BJH adsorption cumulative pore volume analyses, as shown in Fig. 5, provide crucial insights into the textural properties of SAPO-34 synthesized using different templating strategies. The isotherm profiles reveal distinct adsorption behaviors for SP, SPG1 and SPG, highlighting the impact of hierarchical porosity on gas adsorption performance.

The nitrogen adsorption-desorption isotherms of all samples exhibit characteristic type-IV curves with hysteresis loops, confirming their mesoporous nature38,39. However, SP displays a notably narrower hysteresis loop and a sharp nitrogen uptake at low relative pressures (Fig. 5), indicating a structure more dominated by micropores. This is quantitatively supported by BET surface area measurements (Table 2), where SPG shows the highest total surface area (518.4 m2/g), followed by SPG1 (437.1 m2/g) and SP (386.0 m²/g). Although the micropore surface area (Smicro) follows the same trend—SPG (449.0 m2/g) > SPG1 (378.9 m2/g) > SP (336.7 m2/g)—the Smicro/SBET ratio reveals that SP has the highest proportion of micropore surface area (0.872). This highlights the dominant contribution of micropores to its overall surface area, likely due to its single-template synthesis method, which promotes microporosity over mesoporosity.

In contrast, SPG demonstrates the highest nitrogen uptake across the full relative pressure range and the largest BET surface area (518.4 m2/g), reflecting a more open mesoporous architecture. The decreasing Smicro/SBET ratios from SP to SPG1 and SPG suggest an increasing mesoporous contribution in the latter samples. This is further evidenced by the Sext/Smicro ratio, with SPG exhibiting the highest external surface area contribution (0.155) and SP the lowest (0.146). These findings underscore the tunability of pore structure through different templating strategies, enabling tailored microporous or mesoporous characteristics depending on the synthesis route.

Pore volume analysis further differentiates the structural characteristics of the three samples, particularly emphasizing the enhanced mesoporosity of SPG. The total pore volume (Vtotal) of SPG reaches 0.325 cm3/g, nearly double that of SP (0.160 cm3/g) and significantly higher than SPG1 (0.230 cm3/g), indicating a considerable increase in available pore space. SPG also exhibits the highest micropore volume (Vmicro = 0.169 cm3/g), outperforming SPG1 (0.145 cm3/g) and SP (0.104 cm3/g), demonstrating the effectiveness of the dual-template approach in generating well-developed micropores.

More notably, SPG possesses a mesopore volume (Vmeso) of 0.156 cm3/g, approximately three times that of SP (0.056 cm3/g) and about 1.5 times that of SPG1 (0.102 cm3/g), further confirming the successful incorporation of mesoporous domains. This is supported by the Vmicro/Vtotal ratios, which follow the order SP (0.650) > SPG (0.557) > SPG1 (0.520), indicating a greater microporous contribution in SP, while the Vmeso/Vmicro ratios, SPG (0.923) > SPG1 (0.797) > SP (0.538), highlight the enhanced mesoporosity in SPG. These results underscore the significant role of brewed coffee as a soft template in promoting mesopore formation. Such a hierarchical pore structure is particularly beneficial for MTO catalytic applications, as it facilitates improved molecular accessibility and mass transport within the SAPO-34 framework.

The pore size distribution analysis, presented in the inset, further supports these findings. SPG exhibits a broader pore distribution with a significant fraction of mesopores, whereas SP remains largely microporous. The formation of mesopores in SPG is attributed to the self-assembly of nitrogen-containing organic molecules from the brewed coffee template, which interact with silica or aluminosilicate precursors during crystallization40,41. This mechanism ensures a more uniform and interconnected micro-mesoporous framework, aligning with the dual-template synthesis principles.

The superior textural properties of SPG can be attributed to the dual-template and secondary synthesis approaches, wherein the initial microporous framework is established using okra mucilage (hard template), followed by mesopore formation guided by brewed coffee (soft template). The integration of these natural green templates enhances hierarchical porosity while maintaining structural integrity. In contrast, SP, synthesized via a conventional templating approach, lacks the extensive mesopore network observed in SPG.

Fig. 6
figure 6

NH3-TPD profiles of all SAPO-34 samples.

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Acidity

The NH3-TPD analysis (Fig. 6) provides crucial insights into the acidity profiles of SP, SPG1, and SPG, revealing significant variations in acid strength and distribution that directly influence their catalytic performance in MTO reactions. The desorption profiles indicate three distinct peaks corresponding to weak, strong, and very strong acid sites, highlighting the hierarchical acidity of these materials and its impact on catalytic efficiency and stability25,26,42.

Both SP and SPG exhibit an initial desorption peak at approximately 199 °C, corresponding to weak acid sites primarily associated with hydroxyl groups (POH, SiOH, and AlOH) at defect sites. In comparison, SPG1 displays a slightly higher peak at 204 °C. These weak acid sites primarily facilitate the adsorption and desorption of reactant species rather than directly participating in methanol conversion25,26,42. The slightly higher concentration of weak acid sites in SPG (4.7 mmol/g) compared to SP (4.4 mmol/g) suggests a marginally increased accessibility of such sites, which may enhance initial interactions with methanol, though they have a limited influence on the overall conversion process. In contrast, SPG1 exhibits the lowest concentration of weak acid sites, at 4.1 mmol/g.

The strong acid sites, attributed to the structural hydroxyl groups bridging Si-OH-Al within the SAPO-34 framework, show a notable distinction between the two samples43,44,45. SP displays a pronounced desorption peak at 388 °C, with a significantly higher acidity concentration of 10.6 mmol/g, while SPG exhibits a peak at a slightly lower temperature (369 °C) with a much lower acidity concentration of 3.1 mmol/g. In contrast, SPG1 exhibits a moderate desorption peak at 387 °C, accompanied by a higher acidity concentration (5.6 mmol/g) than SPG, but lower than that of SP. This suggests that the conventional SP sample possesses a denser network of active acidic centers, making it more effective in methanol and dimethyl ether (DME) conversion to olefins15,46. However, excessive strong acidity in SP negatively affects olefin selectivity and catalyst stability by promoting undesired side reactions, such as over-cracking of light olefins into paraffins and aromatics, thereby reducing the yield of ethylene and propylene. Additionally, the increased density of strong acid sites in SP accelerates coke formation by facilitating consecutive polymerization reactions, leading to micropore blockage, reduced accessibility of active sites, and rapid catalyst deactivation. The strong acidity also enhances secondary reactions such as hydrogen transfer and cyclization, further diminishing the selectivity toward light olefins.

In contrast, the reduction in strong acidity in SPG and SPG1, which align with its lower Si/Al ratio (0.663 and 0.769 compared to 0.988 for SP), leads to a more balanced catalytic environment. A lower concentration of strong acid sites minimizes excessive olefin cracking, preserving ethylene and propylene selectivity while reducing secondary side reactions47. Furthermore, the superior controlled acidity in SPG suppresses excessive hydrogen transfer and cyclization, ensuring a more stable product distribution. By decreasing the rate of coke formation, SPG maintains its catalytic activity for a longer duration, ultimately extending its effective lifetime in MTO applications.

The third desorption peak observed between 600 and 800 °C, initially ascribed to very strong acid sites, is more plausibly linked to structural phenomena such as framework dehydroxylation, Al–O or Si–O bond cleavage, and phase transitions. These unusually high temperatures exceed the typical range for NH3 desorption from Brønsted or Lewis acid sites, suggesting that the peak reflects both residual ammonia release from deep defect sites and the onset of structural degradation. Although not directly catalytic, these transformations may influence long-term catalyst stability and coke tolerance. Notably, the presence of such deep or defect-related acidic environments may also contribute to sustained methanol conversion at elevated temperatures, while simultaneously increasing the risk of olefin degradation and coke deposition, highlighting a dual catalytic effect48,49,50. SP exhibits a high-intensity peak at 742 °C, with an acidity concentration of 1.8 mmol/g, whereas SPG shows a peak at a lower temperature (676 °C) with a reduced acidity of 0.9 mmol/g. Like SPG, SPG1 exhibits a low concentration of very strong acid sites (0.9 mmol/g), but its desorption peak occurs at a higher temperature of 707 °C. The lower concentration of very strong acid sites in SPG, along with the reduced peak temperature, suggests that SPG may offer improved catalyst stability over extended reaction times by mitigating secondary reactions and minimizing coke accumulation. Additionally, the shift of the very strong acid peak to a lower temperature in SPG implies that the catalyst’s strongest acid sites are more readily available for reaction at moderate temperatures, potentially improving olefin selectivity.

These acidity variations highlight the significant impact of the dual-template synthesis method employed in SPG, where the incorporation of natural soft and hard templates results in a more hierarchical pore structure with a finely tuned acidity profile. The lower overall acidity in SPG may slightly reduce its methanol conversion rate compared to SP; however, this is counterbalanced by improved long-term stability, reduced coke formation, and enhanced selectivity toward desirable light olefins. The optimized balance between strong and very strong acidity plays a pivotal role in maximizing olefin production while mitigating deactivation, positioning SPG as a promising and sustainable alternative to conventional SAPO-34 catalysts in MTO applications.

Fig. 7
figure 7

(a) Selectivity of light olefins and (b) propylene to ethylene ratio (P/E) versus time on stream (min). (c) Product Distribution (C2=–C4=) for all SAPO-34 Samples at their maximum selectivity.

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Catalytic performance in the MTO process

The MTO catalytic performance of all samples was evaluated based on total olefin selectivity, propylene-to-ethylene (P/E) ratio, and selectivity toward ethylene, propylene, and butene, as presented in Fig. 7.

The total olefin selectivity results indicate that SPG outperforms both SPG1 and SP in producing light olefins over the course of the reaction. Initially, all catalysts exhibit similar performance, with selectivity values ranging from 31 to 34% at 60 min. However, as the reaction progresses, SPG1, and particularly SPG, show a substantial increase in selectivity, reaching peak values at 240 min.: 89.8% for SPG and 78.1% for SPG1, while SP achieves a maximum of 76.6% at the same time point. This suggests that the green templating method, especially in the case of SPG, enhances catalytic efficiency, likely by modifying the material’s textural and acidic properties. After reaching their peak performance, all catalysts show a decline in selectivity, with SP dropping sharply to 41.4% at 420 min, whereas SPG maintains a higher level of selectivity (74.0%) at the same time, further reinforcing its superior catalytic stability. At this time, SPG1 exhibits a lower selectivity of 55% compared to SPG. In addition to differences in acidity and porosity, the prolonged catalytic lifetime of SPG can also be partially attributed to its lower silicon content (Si/(Si + Al + P)solid = 0.086), compared to SPG1 (0.1) and SP (0.129). A reduced Si content can moderate Brønsted acid site density, thereby limiting excessive secondary reactions that accelerate coke deposition and catalyst deactivation. This more balanced acidity profile, associated with lower Si incorporation, likely contributes to the extended stability observed for SPG during MTO operation.

Regarding the P/E ratio, SP generally exhibits higher values than SPG and SPG1 throughout the reaction, with the highest ratio observed at 120 min (2.6), followed by a gradual decrease over time. In contrast, SPG and SPG1 maintain a consistently lower P/E ratio, fluctuating between 0.6 and 1.1. This indicates that SP favors the formation of propylene over ethylene to a greater extent compared to SPG. Furthermore, at their maximum total olefin selectivity points, the selectivity toward ethylene, propylene, and butene further highlights the differences in catalytic behavior. SPG and SPG1 show a significantly higher ethylene selectivity (52.5%) and (41.9%) than SP (35.5%), whereas SP demonstrates a slightly better propylene selectivity (38.3%) compared to SPG (35.5%) and SPG1 (34.9%). The selectivity for butene is generally low for both catalysts, but SP exhibits a slightly higher value (2.8%) than SPG (1.8%) and SPG1 (1.2%).

The MTO catalytic performance of SP, SPG1, and SPG demonstrates significant differences in olefin selectivity, catalytic stability, and overall lifetime, which are directly correlated with their distinct physicochemical properties. The experimental results reveal that the higher acidity of SP enhances propylene selectivity, while the lower acidity of SPG1 and SPG favor ethylene production. Furthermore, the integration of a micro-mesoporous structure in SPG through the dual-template approach significantly extends catalyst lifetime and enhances total olefin selectivity. Moreover, lower Si content in SPG likely contributes to this balanced acidity, offering fewer but better-distributed acid sites that are sufficient for methanol activation but less prone to causing coke-inducing secondary reactions. This structural feature, derived from the altered silicon incorporation pathway in the green templating method, further supports the longer catalytic life of SPG.

The NH3-TPD analysis confirms that SP possesses a higher concentration of strong and very strong acid sites compared to SPG and SPG1. This higher acidity contributes to an increased rate of methanol conversion but also leads to undesired secondary reactions, including the excessive cracking of light olefins, hydrogen transfer, and cyclization reactions, which ultimately favor the formation of heavier hydrocarbons and coke deposits. Consequently, the P/E ratio is notably higher for SP than for SPG and SPG1, particularly in the early stages of the reaction. The strong acidity of SP enhances the activation of methanol and DME, promoting the formation of propylene; however, this also accelerates coke deposition, resulting in a shorter catalyst lifetime. In contrast, SPG and SPG1, with their moderated acidity, exhibit a lower P/E ratio, preserving ethylene selectivity while reducing side reactions that promote coke formation51.

A critical advantage of SPG over SP and SPG1 lies in its enhanced mesoporosity, which plays a pivotal role in prolonging catalyst lifetime and improving overall olefin selectivity. The nitrogen adsorption/desorption isotherm and BJH pore volume analysis confirm that SPG exhibits significantly higher mesoporosity than SP and SPG1, attributed to the incorporation of a soft template (brewed coffee) and a hard template (okra mucilage) during synthesis. The mesoporous network in SPG enhances mass transfer, facilitating the diffusion of reactant and product molecules, thereby reducing diffusion limitations and minimizing coke formation. This improvement in transport properties translates to a longer catalytic lifetime, as evidenced by the prolonged stability of SPG, which maintains high olefin selectivity even after extended reaction times52.

The total light olefin selectivity data further demonstrate the superior catalytic efficiency of SPG. While all catalysts exhibit an initial increase in selectivity, SPG consistently outperforms SP and SPG1, reaching a maximum total olefin selectivity of 89.8% at 240 min on stream compared to 76.6% for SP and 78.1% for SPG1. Additionally, SPG retains a higher selectivity over longer reaction times, maintaining 62.4% olefin selectivity at 480 min, whereas SP and SPG1 deactivate more rapidly, dropping to 41.4% and 55.0% by 420 min. This extended activity is directly linked to the hierarchical porosity of SPG, which effectively reduces coke deposition and preserves active sites for methanol conversion.

Another significant finding from the catalyst characterization is the higher nitrogen incorporation in SPG, attributed to the use of brewed coffee as a bio-derived soft template. The elemental composition analysis confirms increased nitrogen content in SPG compared to SP and SPG1, suggesting that nitrogen-containing organic species interact with the silica-alumina framework during crystallization. This incorporation may contribute to modified acidity characteristics, enhancing methanol adsorption while simultaneously suppressing excessive Brønsted acidity, which in turn helps regulate the balance between olefin formation and coke resistance. The presence of nitrogen in SPG could also play a role in stabilizing the catalyst structure, further improving its longevity in MTO applications53,54,55.

The influence of acidity on product distribution is particularly evident in the selectivity data at the point of maximum total olefin selectivity. SP favors propylene formation, reaching a propylene selectivity of 38.3%, whereas SPG exhibits higher ethylene selectivity at 52.5%. This shift is directly linked to the lower acidity of SPG, which prevents excessive secondary reactions that lead to olefin cracking and hydrogen transfer. In contrast, the higher acidity of SP promotes more extensive hydrocarbon transformations, increasing the yield of propylene and butene but also accelerating catalyst deactivation. The lower butene selectivity in SPG further supports the conclusion that reduced acidity helps suppress secondary cracking, preserving ethylene and propylene selectivity56.

The structural characterization of all catalysts also highlights key differences in crystallite size and morphology, which influence their catalytic behavior. XRD analysis reveals that SP and SPG1 possess larger crystallites than SPG, suggesting that the dual-template synthesis method alters nucleation and growth processes, leading to smaller and more uniform crystals in SPG. This reduction in crystallite size enhances diffusion properties, improving molecular transport within the hierarchical pore network and contributing to the prolonged catalytic activity observed in SPG.

Moreover, the eco-friendly and cost-effective nature of the templating strategy used for SPG and SPG1 synthesis presents a significant advantage over conventional methods. The use of natural, biodegradable templates such as brewed coffee and okra mucilage reduces reliance on expensive and toxic organic SDAs, making SPG, in particular, a more sustainable alternative. This approach aligns with the growing emphasis on green chemistry and sustainable catalyst development, offering a scalable and environmentally benign route for SAPO-34 production.

Overall, the MTO catalytic results clearly demonstrate that SPG, synthesized via a dual-template strategy, exhibits superior performance in terms of higher total olefin selectivity, prolonged catalytic lifetime, enhanced ethylene selectivity, and improved mass transfer properties due to its micro-mesoporous structure. The lower acidity of SPG helps mitigate undesired side reactions, reducing coke deposition while preserving ethylene and propylene selectivity. The reduced silicon content in SPG also plays a supportive role in achieving a more stable acidity profile, which contributes to extended catalytic life. Additionally, the incorporation of nitrogen from brewed coffee enhances catalytic stability, further optimizing performance. The integration of sustainable, bio-derived templates not only enhances catalytic efficiency but also presents a cost-effective and eco-friendly alternative for SAPO-34 synthesis, making SPG a highly promising catalyst for industrial MTO applications.

Conclusion

This study evaluated the catalytic performance of three SAPO-34 catalysts in the MTO process: conventional SAPO-34 (SP), green SAPO-34 synthesized using okra mucilage (SPG1), and a dual-template SAPO-34 synthesized using brewed coffee and okra mucilage (SPG). The findings demonstrated that SPG exhibited superior catalytic properties, highlighting its promise as a sustainable and efficient alternative to conventional SAPO-34.

The incorporation of natural templates introduced hierarchical micro-mesoporosity in SPG, which enhanced molecular diffusion, reduced coke deposition, and extended catalyst life. Compared to SP and SPG1, SPG achieved higher total olefin selectivity and significantly improved ethylene selectivity, confirming its potential for applications requiring high light olefin yields. The improved porosity and accessibility of active sites contributed to these enhancements.

NH3-TPD analysis revealed that SPG possessed a lower concentration of strong acid sites, which helped suppress undesirable secondary reactions such as hydrogen transfer and cyclization. This moderated acidity profile supported longer catalyst lifespans and more stable performance over time. The increased nitrogen content in SPG, likely originating from brewed coffee, may have contributed to the stabilization of the SAPO-34 framework and increased coke resistance.

The dual-template strategy employed in SPG synthesis leveraged the functional diversity of both templates. Brewed coffee, rich in nitrogen-containing organics such as caffeine and polyphenols, played a role in moderating acidity and enhancing structural integrity. Okra mucilage, a polysaccharide-rich material with gel-like properties, helped guide the formation of mesopores and promoted a more stable hierarchical structure. These bio-derived materials offered an eco-friendly, low-cost, and biodegradable alternative to synthetic structure-directing agents. Beyond catalytic improvements, this synthesis strategy aligned with green chemistry principles by reducing toxic waste and lowering synthesis costs. The use of renewable materials not only enhanced the environmental profile of the catalyst but also demonstrated the viability of natural templating approaches for scalable, industrial catalyst production.

In summary, SPG outperformed its counterparts in terms of stability, selectivity, and sustainability. Its eco-friendly synthesis, optimized porosity, and balanced acidity position it as a promising candidate for industrial MTO applications, offering a pathway toward more efficient and sustainable olefin production.