Selective syngas conversion to olefins over bifunctional Zn₂Al₃O₄/PLS-3 catalyst

Synthesis of PLS-3 zeolite

PLS-3 zeolites were synthesized using tetraethylammonium hydroxide solution (TEAOH) and H-kanemite as the structure-directing agent (SDA) and silica source, respectively, according to a previously reported method (Experimental details in Supplementary Materials)^[28]. Typically, H-kanemite, NaOH, and Al(NO₃)₃·9H₂O were added into the TEAOH solution, and then the solution was stirred for 30 min, resulting in the initial gels with molar compositions of 1.0 SiO₂ : x/2 Al₂O₃ : 0.04 NaOH : 0.2 TEAOH : 6.5 H₂O. The synthetic gel was crystallized at 443 K for 6 h, and then the products were collected by filtration, washing, and drying at 373 K for 12 h. The occluded organic species were removed by air calcination at 823 K for 6 h. The PLS-3 zeolite was ion-exchanged with NH₄Cl solution (1 M, the mass ratio of solid/liquid = 1:50) for 3 h at 353 K, and the process was repeated three times. Then, the sample was further calcined at 823 K for 6 h to obtain proton-form PLS-3 zeolites, denoted as PLS-3-x, where x is the Si/Al ratio in the synthetic gel.

Catalytic reactions

The syngas conversion reaction was tested using a high-pressure fixed-bed stainless steel reactor (inner diameter, 9 mm). For the preparation of the composite catalyst, the Zn₂Al₃O₄ and PLS-3-x zeolite powders were pressed, crushed, and sieved to 20-40 mesh granules separately. Then, the granules of the two components were physically mixed. Typically, 0.25 g composite catalyst with a metal oxide to zeolite mass ratio of 3:2 was used. The catalyst was pretreated using N₂ (30 mL·min^-1) at a temperature of 673 K for 2 h, cooled to 623 K, and then the N₂ gas was switched to syngas (volume ratio, 1/1; 6,000 mL·g^-1·h^-1) with the reaction pressure adjusted to 1 MPa. Subsequently, the reaction began after achieving the target temperature, and an online gas chromatograph, equipped with PLOT/Q capillary column (30 m × 0.32 mm × 20 μm) connected to a flame ionization detector (FID) detector and MS-13X column connected to a thermal conductivity detector (TCD) detector, was used to analyze the products. Notably, 4% Ar was used as an inner standard in the syngas flow. The CO conversion, along with CO₂ and hydrocarbon selectivity (C_nH_m), with CO₂ and DME excluded from the hydrocarbon selectivity calculation, was calculated.

The CO conversion [X(CO)] is given by

Where $$ A_{CO}^{out} $$ and $$ A_{Ar}^{out} $$ are peak areas of CO and Ar determined by the TCD detector at the outlet, $$ A_{CO}^{in} $$ and $$ A_{Ar}^{in} $$ are peak areas of CO and Ar determined by the TCD detector at the inlet.

The CO₂ selectivity [S(CO₂)] is given by

Where $$ A_{CO_2} $$ is the peak area of CO₂ determined by the TCD detector at the outlet, and $$ f_{CO_2} $$ and f_CO are the calibration factors of CO₂ and CO, respectively.

The hydrocarbon selectivity is given by

Where A_i is peak area of specific product i determined via the FID, and f_i is its calibration factor.

Textural properties of PLS-3 zeolites

The as-made PLS-3 zeolite, synthesized with layered silicate H-kanemite as a silica source, showed the X-ray diffraction (XRD) patterns similar to a typical FER-type layered zeolite PREFER [Supplementary Figure 1]^[28]. The layer-related [200] diffraction peak of as-made PLS-3 zeolite shifted from 7.6° to 9.7° after calcination at temperature of 823 K for 6 h, due to the removal of occluded TEA organic species accompanied by the condensation of hydroxyl groups on the neighboring layers, leading to a topotactic transformation from 2D layer structure to 3D FER topology, as shown in Supplementary Figure 2. Figure 1A shows the XRD patterns of proton-type PLS-3 zeolites in calcined form with different Si/Al ratios by adjusting Al content in initial synthetic gels. The diffraction peaks corresponding to FER zeolite were well-resolved, and impurity phase was absent for all samples, indicating that highly crystalline PLS-3 zeolites were successfully synthesized with different Al amounts. The N₂ sorption isotherms were further measured to characterize the textural properties of PLS-3 zeolites [Figure 1B and Table 1]. The PLS-3 zeolites with different Si/Al ratios showed typical type-I isotherms, indicative of micropore materials. Meanwhile, the adsorbed N₂ amount dramatically raised at high relative pressure region of P/P₀ > 0.8, indicating that PLS-3 zeolite had abundant inter-crystal mesopores. The micropore surface area and volume were ~300 m²·g^-1 and ~0.13 cm³·g^-1 for all PLS-3 zeolites, respectively. The external surface area (~100 m²·g^-1) and mesopore volume (~0.50 cm³·g^-1) were higher than those of traditional FER zeolite with a bulk crystal morphology. Figure 2 showed that the rod-like crystal morphology with a short length of 50 nm was observed for all the PLS-3 zeolites, which is different from that of conventional FER with bulky crystal morphology [Supplementary Figure 3]. The crystal length of PLS-3 zeolite gradually reduced along with increasing Si/Al ratios.

Figure 1. The (A) XRD patterns and (B) N₂ sorption isotherms of PLS-3 zeolites with different Al contents. XRD: X-ray diffraction.

Figure 2. SEM images of (A) PLS-3-50, (B) PLS-3-100, (C) PLS-3-200 and (D) PLS-3-250. SEM: Scanning electron microscope.

Acidity of PLS-3 zeolites

The acidity of PLS-3 zeolites with different Si/Al ratios was analyzed via the ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR), hydroxyl infrared spectroscopy (OH-IR) and pyridine-adsorption infrared spectroscopy (Py-IR), and NH₃-temperature-programmed desorption (TPD) techniques. The ²⁷Al MAS NMR spectra of PLS-3-x samples are shown in Figure 3A. The resonance peaks at 53 and 0 ppm are related to the tetrahedrally coordinated framework Al (Al_F) and the extra-framework Al (EFAl), respectively^[29-32]. Nearly all the Al atoms were tetrahedrally coordinated in the framework, showing a strong peak centered at 53 ppm, and the intensity was significantly weakened with the decrease of Al content. In addition, the peak corresponding to Al_F gradually shifted to a higher field with chemical shift changing from 53.7 to 52.7 ppm with decreasing Al amount, indicating that the chemical environment of Al_F changed with the variation of Al content. There are four different tetrahedral (T) sites for the framework Al in FER topology. The T1/T2 sites are distributed in the intersection of 10R and 8R pores [Supplementary Scheme 1] while T3 sites are located in the 10R pores. T4 sites situated in 6R pockets can be accessed only through 8R pores^[26]. Hence, the peak at 53 ppm was further deconvoluted into four peaks at 60.5, 51, 53.5, and 56.6 ppm to identify the different chemical environment of Al in PLS-3 [Supplementary Figure 4 and Table 2], which are related to framework Al sited in T1, T2, T3 and T4, respectively^[26]. The Al_F species in T2 sites gradually increased from 40.5% to 68.5% with increasing Si/Al ratio. The proportion of Al_F in T4 sites rose from 15.1% to 22.5% with the Si/Al ratio of 50-200, and then slightly reduced to 18.7% when increasing the Si/Al ratio to 250. However, the percentage in T3 sites decreased from 38.2% to 11.5%. The results proved that the spatial distribution of Al_F sites can be tuned by the regulation of the Si/Al ratio in PLS-3 zeolite, and Al_F preferred to locate in T2 and T4 sites at relatively low Al content.

Figure 3. (A) ²⁷Al MAS NMR spectra and (B) IR spectra in the hydroxyl-stretching region of PLS-3 zeolites with different Al contents. MAS NMR: Magic angle spinning nuclear magnetic resonance; IR: infrared.

The acidity in PLS-3 zeolites was further investigated by hydroxyl-stretching IR spectra [Figure 3B]. Two main absorption bands were observed at 3,743 and 3,602 cm^-1, which are attributed to the terminal Si–OH and the bridging hydroxyl groups (Si–OH–Al), respectively. ^[25] The intensity of Si–OH–Al groups gradually decreased with increasing Si/Al ratio, in consistent with ²⁷Al MAS NMR spectra. Furthermore, NH₃-TPD curves give the acid amount in PLS-3 zeolites [Figure 4A and Table 3]. The NH₃-TPD curves show two desorption peaks at relatively low (about 423 K) and high temperatures (about 573 K), which are generally attributed to the desorption of NH₃ on weak acid sites (WAS) and strong acid sites (SAS), respectively. The amounts of WAS and SAS also dropped from 142 and 88 μmol·g^-1 to 20 and 28 μmol·g^-1, respectively, with the Si/Al ratio increased from 50 to 300. Moreover, the Py-IR spectra were used to analyze the content of Lewis acid site (LAS) and Brønsted acid site (BAS) after the desorption of physically adsorbed pyridine at 423 K [Figure 4B and Table 3]. The bands at 1,543 and 1,455 cm^-1 correspond to BAS and LAS, respectively^[33]. The band at ~1,490 cm^-1 is attributed to both BAS and LAS, and the band at 1,445 cm^-1 is due to hydrogen-bonded pyridine. The contents of BAS and LAS were dramatically reduced with increasing Si/Al ratio, consistent with NH₃-TPD results.

Figure 4. (A) NH₃-TPD curves and (B) Py-IR spectra of PLS-3 zeolites with different Si/Al ratios. Py-IR spectra were measured after desorption of pyridine at 423 K. TPD: Temperature-programmed desorption; Py-IR: pyridine-adsorption infrared spectroscopy.

Catalytic performance of syngas conversion

The PLS-3-x zeolites were mixed with Zn₂Al₃O₄ oxide to catalyze the syngas conversion reaction. The acidity of PLS-3 zeolites could be tuned by varying framework Al content, and the effect of acidity on the product selectivity was further studied by physically mixing PLS-3-x zeolites with Zn₂Al₃O₄ under the reaction condition at a space velocity of 6,000 mL·g^-1·min^-1, 623 K, and 1.0 MPa. As shown in Figure 5 and Table 4, the Si/Al ratio of PLS-3 zeolite had little effect on the CO conversion and the CO₂ selectivity, which remained almost unchanged. The selectivity of ethylene dramatically increased from 23.2% to 45.0%, but C₆⁺ and C_2-5⁰ selectivity decreased with increasing the Si/Al ratio from 50 to 250, thus leading to evidently increased E/P and O/P ratios from 3.6 to 7.0 and from 4.5 to 21.2, respectively. However, the catalytic performance became worse for sample PLS-3-300, showing the high CH₄ selectivity of 12.5% and DME selectivity of 3.1%, due to the difficulty in transforming intermediates over insufficient acid sites. Therefore, using PLS-3-250 as zeolite component combined with Zn₂Al₃O₄ oxide gave the optimal catalytic performance in the syngas conversion reaction, that is high selectivity of C_2-5⁼ (78.5%) and ethylene (45%) and high E/P (7.0) and O/P (21.2) ratios, which breaks the ASF distribution [Supplementary Figure 5]. Moreover, the bifunctional Zn₂Al₃O₄/PLS-3 catalysts for syngas conversion have a higher selectivity of ethylene, O/P and E/P ratio with comparison to the SAPO-type zeolite-based catalysts [Supplementary Table 1]. We speculated that the PLS-3 zeolite with fewer acid sites and more Al_F sited in the T2/T4 sites at a lower Al content may suppress the hydrogen transfer and aromatization reactions, thus leading to the high C_2-5⁼ and ethylene. These results indicated that the acidity of PLS-3 zeolite significantly affected the product selectivity of syngas conversion reaction. Furthermore, the CO conversion and product selectivity were not significantly changed within 100 h over Zn₂Al₃O₄/PLS-3-250 catalyst [Figure 6], indicating that this catalyst was durable. However, the catalytic performance gradually decreased after 100 h, accompanied by a significant increase in methane selectivity and a dramatic reduction in ethylene and C_2-5⁼ selectivity.

Figure 5. (A) Effect of Si/Al ratio of PLS-3 on the catalytic performance of Zn₂Al₃O₄/PLS-3-x for syngas conversion; (B) Dependence of E/P and O/P ratios on the Si/Al ratio of PLS-3. Reaction conditions: catalyst, 0.25 g; metal oxide/zeolite mass ratio, 3:2; temperature, 623 K; pressure, 1 MPa; the WHSV of syngas (H₂/CO volume ratio = 1:1), 6,000 mL·g^-1·h^-1; TOS, 10 h. E/P: Ethylene to propylene; O/P: olefin to paraffin; WHSV: weight hourly space velocity; TOS: time on stream.

Figure 6. CO conversion and product selectivity with TOS over the bifunctional composite catalyst of Zn₂Al₃O₄/PLS-3-250. Reaction conditions: see Figure 5. TOS: Time on stream.

As shown in Supplementary Figures 6-8 and Supplementary Table 2, we investigated the effects of reaction conditions of weight hourly space velocity (WHSV), temperature, and proximity between two components on the syngas conversion performance using the bifunctional catalysts composed of Zn₂Al₃O₄ and PLS-3-250. Increasing the WHSV of syngas from 3,000 to 7,200 mL·g^-1·h^-1 [Supplementary Figure 6 and Supplementary Table 2], the CO conversion was decreased (from 13.3% to ~11%) and the selectivity of ethylene (from 29% to >40%) dramatically increased, while the selectivity of C_2-5⁰ and C₆⁺ was reduced. Among the hydrocarbon products, the E/P (from 5.4 to 7.1) and O/P (from 16.1 to 23.6) ratios also increased with WHSV. However, the selectivity of long-chain olefins (such as the butene and pentene) was reduced with increasing the WHSV [Supplementary Figure 6A], and they were produced from the ethylene oligomerization reaction. These results indicated that increasing the WHSV of syngas promoted the diffusion of olefin products, which depressed the further hydrogen transfer and ethylene oligomerization to form the alkanes and long-chain hydrocarbons. The reaction temperature showed a more significant effect on the catalytic performance of Zn₂Al₃O₄/PLS-3-250 in the syngas conversion reaction [Supplementary Figure 7 and Supplementary Table 2]. The CO conversion and CO₂ selectivity increased with temperature from 603 to 663 K. Furthermore, the higher methane (20.6%) and DME (1.2%) selectivity were observed at a lower temperature of 603 K. The result indicated that the lower temperature is unfavorable for the conversion of methanol intermediate. Meanwhile, the selectivity of C₆⁺ exhibited a volcanic trend. At a lower temperature (603 K), the difficulty in converting methanol intermediates led to high CH₄ selectivity but low C₆⁺ selectivity, while at higher temperatures (663 K), the suppression of chain growth reactions resulted in low C₆⁺ selectivity. In addition, when the temperature was increased from 623 to 663 K, the CH₄ and C_2-5⁰ were promoted, but C_2-5⁼ and ethylene selectivity were reduced as a result of the enhancement of hydrogen transfer reactions at higher temperatures^[3,5], resulting in the highest E/P (7.0) and O/P (21.2) ratios at 623 K. With comprehensive consideration of selectivity and conversion, the optimal reduction temperature was 623 K.

In addition, the proximity between two components of zeolite and metal oxide was reported to affect the product distribution in previous studies^[5,25]. We thus compared the catalytic performance among three integration manners, including dual bed, granule mixing and mortar mixing [Supplementary Figure 8 and Supplementary Table 2]. Over a dual-bed configuration with PLS-3-250 zeolite downstream from Zn₂Al₃O₄ oxide, a lower C_2-5⁼ (33.3%) and ethylene (11%) selectivity but a higher CO₂ (~44%) and CH₄ (~34%) selectivity were observed at low CO conversion (6.7%), and the E/P and O/P ratios were only 2.0 and 2.2, respectively. For the granule stacking mode, a higher C_2-5⁼ selectivity (78.5%), a higher ethylene selectivity (45%) in olefins, and a lower selectivity of CO₂ (24%) were achieved at CO conversion of 11%. The powder of metal oxide and zeolite was mixed and then compressed into pellets to achieve the increased contact distance. The CH₄ and C_2-5⁰ selectivity significantly increased, but the C_2-5⁼ selectivity decreased to 68%; thus, the O/P ratio dramatically dropped from 21.2 to 7.2. Meanwhile, the ethylene selectivity was slight increased (46%) and that of long-chain C₆⁺ was decreased. These results indicated that too far spatial distance of the two components was unfavorable for the syngas conversion due to the difficulty in transferring intermediate, while the too close spatial distance may cause partial shielding of acid sites in PLS-3 zeolite^[34]. Hence, the granule mixing of two components provided the most suitable spatial proximity to obtain high selectivity of C_2-5⁼ and ethylene.

Reaction mechanism of syngas conversion catalyzed by Zn₂Al₃O₄/PLS-3

The investigation of coke precursors helps to understand the reaction mechanism in zeolite-catalyzed syngas conversion^[25]. The carbonaceous species deposited on the spent PLS-3 catalysts were investigated by thermogravimetry (TG) [Figure 7A]. The percentage of coke deposition gradually decreased from 4.2% to 2.2% with the decrease of Al content in PLS-3 zeolite, suggesting that the weakened acidity of PLS-3 zeolite can suppress the generation of carbon deposition for the syngas conversion reaction. The soluble coke species confined in the pore channels of used PLS-3 zeolite were further analyzed by the GC-MS technique [Figure 7B]. Polymethylbenzene (PMB) species, including trimethylbenzene, tetramethylbenzene, pentamethylbenzene, hexamethylbenzene, naphthalenes, and phenanthrenes, were detected in spent catalysts, potentially serving as coke precursors that contribute to catalyst deactivation. These coke precursors were dramatically reduced over the PLS-3 zeolite with a higher Si/Al ratio, consistent with the TG result. Moreover, the oxygenate species, such as dimethylbenzaldehyde, were detected, probably related to the intermediate species of syngas conversion in PLS-3 zeolite^[35], and their amounts were also decreased with increasing Si/Al ratios.

Figure 7. (A) TG curves and (B) GC-MS analysis of the hydrocarbons on the spent catalysts. ^*The internal standard. TG: Thermogravimetry; GC-MS: gas chromatography-mass spectrometry.

For the bifunctional catalyst of metal oxide and zeolite used in syngas conversion, the zeolite component is responsible for the transformation of intermediate into desired hydrocarbons. It is currently recognized in research that methanol is the main intermediate for the bifunctional catalytic system of ZnAlO_x-zeolite^[5,25]. Hence, the reaction mechanism of syngas conversion over Zn₂Al₃O₄/PLS-3-250 was further studied by the probe reaction of methanol conversion using the in-situ IR spectra as shown in Figure 8. Methanol was first introduced into the wafer of PLS-3-250 zeolite under the He atmosphere at 623 K. The stretching vibration of C–H of surface methoxy species (2,980, 2,966 and 1,455 cm^-1) and C–H bending vibration of -CH₃ (2,952 and 2,923 cm^-1) were observed, while the band at 2,891 cm^-1 was related to the DME molecules hydrogen-boned to Si–OH^[36]. Meanwhile, the symmetrical stretching vibration of =CH₂ at 1,463 and 1,417 cm^-1 and the C–H vibration of olefin species at 2,876, 2,855 and 2,817 cm^-1 were observed^[25,37]. The negative bands at 3,500-3,800 cm^-1 were related to the interaction of hydrocarbon species with hydroxyl-stretching (-OH) groups^[25]. We found that the intensity of signals corresponding to olefin species at 2,876, 2,855 and 2,817 cm^-1 slightly increased from 0 to 10 min. In the same time range, those bands corresponding to acetate species (1,544, 1,550 and 1,574 cm^-1) and carbonate species (1,516 and 1,508 cm^-1) were observed over the PLS-3 zeolite for the methanol conversion under the He [Figure 8B]^[16]. The bands at 1,716 and 1,768 cm^-1 related to the C=O asymmetric vibrations in methyl acetate (MA)^[38,39], and the C=O vibrations of AA at 1,744 and 1,730 cm^-1 and acetyl at 1,704 cm^-1 also were observed^[38,40]. The intensity of the bands related to acetate, carbonate, MA, AA, and acetyl species kept increasing over time until a steady state was reached after 10 min of methanol conversion, but their intensity is relatively low under the He flow. These species were considered as key intermediates supporting the main carbonylation mechanism in methanol conversion reactions over the PLS-3 zeolite, but the hydrocarbon pool mechanism of MTO may also exist in this process^[41]. Once CO/H₂ was introduced into the system to replace He after 10 min, the bands related the vibrations of gaseous CO were observed at 2,181 and 2,115 cm^-1 over PLS-3-250 zeolite at 623 K^[2,25]. Importantly, the intensity of bands corresponding to acetate, carbonate, MA, AA, and acetyl species significantly increased with the introduction of syngas, which is due to CO insertion into the methoxy group for promoting formation of these intermediates^[36]. Furthermore, these key intermediates could be transformed to ethanol by hydrogenation reaction, and then ethanol further undergoes dehydration to obtain the ethylene over the PLS-3 zeolite^[16].

Figure 8. In-situ FTIR spectra when gas-phase methanol was introduced to PLS-3-250 zeolite under the He or syngas flow at temperature of 623 K. Methanol was continuously brought into in-situ chamber under He flow for 10 min, and then under syngas flow for another 10 min. (A) Enlarge view of the region at 3,800-2,000 cm^-1; (B) Enlarge view of the region at 1,800-1,400 cm^-1. FTIR: Fourier transform infrared spectroscopy.

Based on the above results, we propose a possible formation mechanism of key intermediates in the syngas conversion over the bifunctional Zn₂Al₃O₄/PLS-3 catalyst. As shown in Scheme 1, methanol carbonylation is the dominant route to form olefins (especially ethylene) for the PLS-3 zeolite with FER topology. The syngas is activated to generate the methanol/methoxy intermediate over Zn₂Al₃O₄ oxide^[5]. Subsequently, the acetyl group is converted into a methoxy group by insertion of CO over the acidic Si–OH–Al groups of PLS-3 zeolite. The formed MA and AA species are further converted into ethylene, and then longer-chain olefins (such as propylene, butene, and pentene) are formed by the oligomerization of ethylene. The previous study has indicated that the T2/T4 sites in FER are beneficial to the insertion of CO into the surface methoxy group to produce acetyl species for promoting the carbonylation reaction^[26]. In this work, the proportion of Al in T2/T4 sites gradually raised with Si/Al ratios of PLS-3 zeolite as evidenced by ²⁷Al MAS NMR investigation [Supplementary Figure 3 and Table 2]. Figure 9A showed that the ethylene selectivity and E/P ratio gradually increased with the proportion of T2 and T4 acid sites over the PLS-3 zeolite. Hence, we speculated that the T2 and T4 sites are the main active centers, contributing to the carbonylation route in Zn₂Al₃O₄/PLS-3-catalyzed syngas conversion reaction. Moreover, the olefin selectivity and O/P ratio also gradually increased with the decreasing acid amount in PLS-3 as measured by Py-IR spectra and NH₃-TPD curves [Figure 9B]. The result indicated that weakened acidity favors the formation of olefins by suppressing the hydrogen transfer reactions to form the alkanes. Hence, the high E/P and O/P ratios are presumably attributed to two main reasons: the suitable acid site distribution and acid density of PLS-3 zeolite, which are achieved by tuning the Si/Al ratio.

Figure 9. (A) Dependence of ethylene selectivity and E/P ratio on the proportion of (T2 + T4) sites; and (B) relationship between the olefin selectivity or O/P ratio and the acid amount. The (T2 + T4) site proportion was obtained from the ²⁷Al MAS NMR spectra. The acid amount was measured by Py-IR and NH₃-TPD techniques. E/P: Ethylene to propylene; O/P: olefin to paraffin; MAS NMR: magic angle spinning nuclear magnetic resonance; Py-IR: pyridine-adsorption infrared spectroscopy; TPD: temperature-programmed desorption.

Scheme 1. Possible formation routes of key intermediates in the syngas conversion reaction over Zn₂Al₃O₄/PLS-3 catalyst.