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Article

Improving the Hydrothermal Stability of ZSM-5 Zeolites in 1-Octene Aromatization by Sequential Alkali Treatment and Phosphorus Modification

by
Jian Cao
1,2,3,
Mengjiao Xing
1,2,3,
Yuanlong Han
1,2,3,
Kun Hao
2,*,
Ling Zhang
2,
Fei Wang
1,
Wentao Zheng
1,*,
Zhichao Tao
2,
Xiaodong Wen
1,2,4,
Yong Yang
1,2 and
Yongwang Li
1,2
1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
2
National Energy Center for Coal to Liquids, Synfuels China Co., Ltd., Huairou District, Beijing 101400, China
3
University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China
4
Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing Information S & T University, Beijing 101400, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1629; https://doi.org/10.3390/catal12121629
Submission received: 18 November 2022 / Revised: 5 December 2022 / Accepted: 9 December 2022 / Published: 12 December 2022
(This article belongs to the Section Catalytic Materials)
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Versions Notes

Abstract

:
For the aromatization of olefins (in Fischer–Tropsch synthetic oil), especially for the fluidized bed reaction with steam as the fluidized medium, improving the catalytic and hydrothermal stabilities of ZSM-5 catalysts is a research focus because of both fundamental research interests and potential commercial applications. In this work, sequential alkali treatment and phosphorus modification were carried out for ZSM-5 samples. The results obtained from characterization and reaction evaluation show that the introduction of mesopores facilitates the dispersion of phosphorus species into the pores and improves the reaction stability for 1-octene aromatization. After the hydrothermal treatment, the P-0.5A-Z5-ST sample treated with an appropriate concentration of alkali retains the most acid sites and shows the highest aromatic selectivity (22.5%) at TOS = 660 min. Therefore, a moderate distribution of mesopores in a zeolite plays an important role in the diffusions of reactants and products, as well as in the distribution of phosphorus species.

1. Introduction

Long-chain olefins (C6–C10) are the main products of the coal-indirect-liquidation process. Generally, the content of α-olefins is often higher than 50%. One of the feasible methods of obtaining higher economic efficiency is to convert Fischer–Tropsch synthetic oil to high-octane-number gasoline through olefin aromatization [1,2,3,4]. Zeolites are known to convert olefins into more valuable (higher octane number) aromatic hydrocarbons due to their well-defined structures and tunable surface properties [5,6,7,8]. It is generally accepted that Brønsted acid sites of zeolites are the active centers for the conversion of olefins to aromatics. However, this type of catalyst suffers from rapid deactivation during the aromatization reaction [9].
The performances of zeolite catalysts can be improved by the proper tailoring of factors such as surface acidity and pore structure [4,10]. An effective approach to regulate the pore structure and the surface’s acid-base properties is known as post-synthesis treatment, which includes methods such as steam treatment, acid or alkali treatment [11,12,13]. For the acid or alkali treatment, dealumination or desilication from the zeolite framework happens and leads to the formation of structural defects in the crystal, followed by the formation of mesopores. Further-tunable mesoporosity could be achieved by changing the concentration of acid or alkali, as well as the time and temperature of the acid or alkali treatment. In addition, the introduction of phosphorus ions or their oxides could also change the acid properties of the resulted zeolite [14,15]. The combination of phosphorus modification with desilication or dealumination has also been used for the post-treatment synthesis of ZSM-5 [16,17,18]. Ding et al. [17] greatly improved on the activity and stability of hydrocarbon-cracking reaction through the combination of desilication and phosphorus modification and obtained more light olefin in the cracking reaction of 1-octene. Those phosphorus-containing species interact with the bridged hydroxyl groups, decreasing the zeolite’s acidity, modifying the pore structure, and then tuning the catalytic performance. More importantly, the introduction of phosphorus can significantly improve the hydrothermal stability of the ZSM-5 zeolite [19,20]. Our previous work found that a fluidized bed is more suitable for converting olefin-rich Fischer–Tropsch synthetic oil to high-quality gasoline, the steam of which is usually used as fluidizing medium [21,22].
While there is rich literature dealing with the performances of various zeolite catalysts for the C3–C4 short-chain olefin-aromatization reaction [23,24,25,26,27,28], the literatures on the aromatization of longer-chain olefins focus on olefin reduction and the upgrading of petroleum-based FCC gasoline, the olefin content of which is usually less than 50% [5,29,30,31,32]. A previous work from this laboratory established the relationship between the Al distribution of a properly prepared ZSM-5 solid and the catalytic performance of long-chain olefin aromatization [29]. By manipulation of the hydrothermal synthesis conditions to adjust the Al locations and proximities of the ZSM-5 catalyst, we demonstrated that the catalytic performance of the ZSM-5 catalyst for 1-octene aromatization is strongly related to the Al position and proximity, and that the balance between the isolated Al and pair-distributed Al sites was the key to the catalyst’s performance. These interesting findings have stimulated a systematic study on the modulation of Si or Al distribution and the surface acidities of zeolite catalysts with post-synthesis treatment for the 1-octene aromatization reaction.
In the present study, the post-synthesis treatment was adopted to adjust the ZSM-5 catalyst for the 1-octene aromatization reaction, and a protocol with sequential alkali treatment and phosphorus modification was established to enhance ZSM-5’s catalytic performance. The catalyst structure and surface acidity before and after the modification, or after serving the aromatization reaction sometimes, were characterized comprehensively with XRD, IR, NH3-TPD, NMR, and nitrogen-adsorption measurements. The responses of the catalyst’s pore structure, surface acidity, and catalytic performance to the change of post-treatment conditions for the modified ZSM-5 catalysts were presented for a clarification of the relationship between post-treatment conditions, surface acidity, and the catalytic performance for the aromatization reaction. The enhanced catalytic stability of the modified ZSM-5 catalyst in the 1-octene aromatization reaction implies that the combined modification of the ZSM-5 catalyst with the stepwise alkali treatment and phosphorus modification is a promising approach to enable the production of aromatics from long-chain olefins. The improved hydrothermal stability is also beneficial in converting Fischer–Tropsch synthetic oil into high-quality gasoline in the fluidized bed system.

2. Results and Discussion

2.1. Evaluation of the Z5 and xA-Z5 Catalysts for 1-Octene Aromatization

The 1-octene aromatization was used as a model reaction to evaluate the reactivity of the Z5 and xA-Z5 catalysts. Due to the fact that the 1-octene conversions were determined to be above 99.5% (Figures S1 and S2) in all test runs, Figure 1 presents the performances of the Z5 and xA-Z5 catalysts in the aromatization of 1-octene reaction by showing the time-courses of the aromatics and BTEX (BTEX: benzene, toluene, ethyl benzene, and xylene) selectivities. The aromatic selectivities and the light aromatic BTEX selectivities decreased more-or-less with the time on stream (TOS). The Z5 catalyst offered a sharply decreased aromatic selectivity (from 30.9% to 10.7%) and BTEX selectivity (from 26.9% to 5.0%) in the early stage (TOS < 200 min) of the reaction. According to a previous report, the deactivation of Z5 was mainly due to coke deposits [33]. The microporous structure of ZSM-5 limits the rapid diffusion of aromatic products, which undergo excessive reactions to form coke deposits.
Under identical reaction conditions, the aromatic selectivity and BTEX selectivity levels of the xA-Z5 catalysts were superior to those obtained from the non-treated Z5 catalyst (Figure 1A,B). Taking the 0.5A-Z5 sample as an example, the selectivities for total aromatics and light aromatics reached as high as 29.0% and 19.5% at TOS = 240 min, which were 4.5 and 7.2 times higher than those of the Z5 sample, respectively. It is deserving of note that the BTEX selectivity at TOS = 660 min increased from ca.9.0% with x = 0.25 to ca.14.3% with x = 0.5, but decreased to ca.10.9% when x was further increased up to x = 0.75, indicating that there is an optimal alkali treatment condition (i.e., x = 0.5) for offering the highest BTEX selectivity.
In addition, the zeolite will be irreversibly deactivated under hydrothermal conditions, which severely limits its long-term application [34]. Herein, the Z5 and xA-Z5 catalysts after hydrothermal treatment were also evaluated for 1-octene aromatization (see Section 3.3 for hydrothermal treatment conditions). As shown in Figure 1C,D, the steam-treated catalysts almost lost all of their aromatization selectivities after hydrothermal treatment (aromatic and BTEX selectivities are below 5%). This indicates a serious loss of catalytic activity of the zeolite. Therefore, improving the hydrothermal stability of ZSM-5 catalysts is another key problem of practical significance.

2.2. Structural Foundation for the Improved Catalytic Stability of the Alkali-Treated ZSM-5 Samples

The evaluation results in Section 2.1 show that alkali treatment can effectively improve the stability of the Z5 catalyst. To reveal the intrinsic mechanism, a series of characterization methods were carried out to explore the structural foundation.

2.2.1. Crystal Structure

The definite crystal structure of a zeolite provides the structural basis for pore structure and acid sites. The structural changes of the alkali-treated and the combined alkali-steam-treated samples with parent Z5 were studied by XRD techniques. The XRD patterns in Figure 2 show that all the samples exhibited the typical MFI topological structure. This result proves that the alkali treatment and hydrothermal treatment did not destroy or completely change the bulk-phase structure of ZSM-5. In addition, the relative crystallinities of the alkali-treated samples decreased gradually with the increase of the alkali treatment’s concentration. After hydrothermal treatment, the relative crystallinities of the samples were further reduced. The decreases in relative crystallinity after alkali treatment and hydrothermal treatment were caused by alkali desilication and hydrothermal dealumination, respectively.

2.2.2. Textural Properties

Figure 3A presents the N2 adsorption and desorption curves for the Z5 and xA-Z5 samples. The Z5 sample showed a typical type-I isotherm, indicating a typical microporous structure. The xA-Z5 samples, however, exhibited type-IV isotherms and suggested the generation of mesopores. Correspondingly, from the pore-size distributions (Figure 3B), Z5 showed no obvious mesoporous distribution. However, more mesopores formed over a wide range of pore widths with increasing alkali concentrations. These results indicate the formation of mesopores in the process of alkali treatment.
From the measured nitrogen physical adsorption–desorption isotherms, the BET surface areas, external surface areas, micropore areas, micropore volumes, and mesopore volumes are shown in Table 1. However, the BET surface areas were similar for the Z5 and the alkali-treated samples (approximately 370 m2/g). The external surface area increased significantly from 32 to 116 m2/g with an increasing alkali concentration. However, the micropore area decreased from 342 to 255 m2/g. The retention rate for the micropores in this process gradually decreased from 100% to 72.3%. These results prove the partial microporous pore structure’s transformation into a mesoporous pore structure in the process of alkali treatment.
The pore-structure data obtained for the catalysts after the reaction evaluations are shown in Table S1 and Figure S3. The micropore area and volume of the Z5 catalyst disappeared after undergoing the reaction for 660 min. Therefore, the reason for the deactivation of the Z5 catalyst is that the acid sites located in micropores were blocked by coke deposits. However, the alkali-treated samples retained more micropore areas and volumes, proving that the formation of mesopores can enhance the diffusion of products and suppress coke deposition in the micropores to some extent.
Similar to alkali-treated samples, a small hysteresis loop in the low-pressure region (P/P0 = 0.4–0.8) and the corresponding 2.5 nm mesopores were observed for the xA-Z5-ST samples (Figure 3C,D). This can be attributed to the structural damage of micropores by the hydrothermal dealumination. As shown in Table 1, the corresponding micropore-specific surfaces decreased by approximately 53% (from 342 to 160 m2/g for the Z5 and Z5-ST samples). At the same time, the external surface areas increased from 32 to 125 m2/g, which can also be attributed to the partial microporous pore structure being transformed into a mesoporous pore structure. It should be noted that the alkali treatment suppressed the partial pore structure’s transformation to some extent (Figure 3D and Table 1). However, considering the extremely low selectivities for total aromatics and light aromatics of all of the samples after hydrothermal treatment, the partial loss of the microporous structure should not be the key reason. The loss of acid sites caused by the removal of the framework aluminum after hydrothermal treatment may be the reason for the decline of their reaction performances.

2.2.3. Acidic Properties

For the 1-octene aromatization, the acid sites, especially the Bronsted acid in the micropore, are the key active sites [29]. In this work, the acidic characteristics of the different catalysts were investigated using NH3-TPD and pyridine-probed FTIR spectroscopy (Py-IR). As shown in Figure 4, all ZSM-5 zeolites showed two typical NH3 desorption peaks (peak centers at approximately 210 °C and 380 °C), which can be attributed to weak and strong acid sites, respectively. Compared with the Z5 sample, the intensities of both peaks decreased to some extent for the alkali-treated samples. In addition, it should be noted that the peak intensities for both peaks decreased significantly for the hydrothermally treated samples (Figure 4). Especially for the characteristic peak of the strong acid at approximately 380 °C, the peaks almost disappeared for all samples, which is an important reason for their deactivation in 1-octene aromatization.
To further determine the types and concentrations of the acid sites in the zeolite, Py-IR experiments were also performed. Pyridine was used as a probe to distinguish Brønsted acid sites (BASs) and Lewis acid sites (LASs) based on differences in the vibration peaks. Typically, the peaks observed at 1544 and 1455 cm−1 are assigned to Brønsted and Lewis acid sites, respectively. The peak at 1490 cm−1 is characteristic of both Brønsted and Lewis acid sites. After Gaussian function fitting and the integration of the spectra (Figure S4), the contents of LASs and BASs for these samples are shown in Table 2. The total acid content for the Z5 sample was 493.4 and 397.7 μmol/g at 200 and 350 °C, respectively. Compared with Lewis acid sites, Brønsted acid sites were the dominant acid-site type for ZSM-5. For the alkali-treated samples, the amount of BASs was decreased to approximately 300 and 200 μmol/g at 200 and 350 °C, respectively. The amount of LASs increased significantly with the alkali concentration. This led to an obvious increase in the L/B ratio (e.g., from 0.05 to 0.34 at 200 °C). The 27Al NMR aluminum spectrum (Figure S5) shows that the characteristic peak for framework aluminum (55 ppm) significantly decreased after the alkali treatment. At the same time, the characteristic peak for the extra-framework aluminum (0 ppm) increased correspondingly. According to a previous report [35], alkali-treatment-induced desilication can lead to the breaking of some Si-O-Al bonds, which converts the framework aluminum to extra-framework aluminum. At the same time, this also leads to a decrease and increase in the number of BASs and LASs, respectively.
For the hydrothermally treated samples, it should be noted that the number of acid sites decreased significantly. This result coincides with the NH3-TPD results. Combined with the extremely low aromatic selectivities of the samples after hydrothermal treatment, it is vital to preserve the acid sites for the olefin aromatization reaction.

2.3. Evaluation of the ZSM-5 Catalysts with P Modification in 1-Octene Aromatization

Figure 1C,D show that the Z5 and alkali-treated ZSM-5 samples almost lose their catalytic activities for the aromatization reaction after hydrothermal treatment. To improve the hydrothermal stabilities of the samples, the P-xA-Z5 samples were prepared by sequential alkali treatment and phosphorus modification. As shown in Figure S7, the initial aromatics selectivities of the phosphorus-modified samples all decreased slightly, with the highest decrease being in P-0.75A-Z5, from 35.2% to 28.5%. As is consistent with the results before modification, sample P-0.5A-Z5 retained the highest aromatics selectivity at TOS = 660 min. After hydrothermal treatment, the corresponding samples maintained much higher selectivities for aromatics (Figure 5A). Taking P-Z5-ST as an example, the initial aromatic selectivity was 20.3%. In addition, compared with P-Z5-ST, the P-xA-Z5-ST samples with alkali treatment showed better catalytic stability, which were similar to the differences observed for the Z5 and xA-Z5 samples (Figure 1A,B). The intrinsic reason for this can be attributed to the enhanced catalytic stability due to the introduction of a mesoporous structure, which will not be further studied in this section. The detailed product-distribution of the 1-octene reaction at TOS = 660 min is shown in Figure 5B. It can be easily seen that with the increase of the alkali treatment’s concentration, the aromatics selectivity first increases and then decreases, and the aromatics selectivity of P-0.5A-Z5-ST is the highest. However, the selectivity of cracking products C3=-C4= showed an opposite trend. This is mainly because in the process of the 1- octene reaction, light olefins were generated by cracking at first, and then through dimerization, cyclization, and hydrogen transfer reactions formed aromatics. The catalyst modified by proper desilication and phosphorus retained more acidic center sites after hydrothermal treatment, thus showing a more-excellent aromatization performance.

2.4. Structural Characterizations of the ZSM-5 Catalysts with P Modifications

The ZSM-5 catalysts with P modifications exhibited significantly improved aromatic selectivities after hydrothermal treatment. A series of characterization methods were carried out to explore the structural foundation.

2.4.1. Crystal Structure

The powder XRD patterns (Figure S8) show that all the P-modified samples exhibited a typical MFI topological structure before and after hydrothermal treatment [36].

2.4.2. Textural Properties

For the Z5 and P-Z5 samples (Table 1 and Table 3), after phosphorus modification, the micropore volumes decreased from 0.166 to 0.128 m2/g (the retention of the micropore volume decreased by 22.9%). This indicates that the introduction of phosphorus species blocked part of the micropores. In contrast, the micropore volume of the alkali-treated samples showed little change. Especially for samples 0.5A-Z5 and P-0.5A-Z5, the retained micropore volume decreased by 12.6%. The introduction of mesopores in ZSM-5 weakened the effect of phosphorus modification on the micropore structure.
It should be noted that the micropore area and volume even increased after hydrothermal treatment. On the one hand, this proves the high hydrothermal stability of the P-modified samples. On the other hand, the partial recovery of micropores can be attributed to the migration of phosphorus species during the streaming treatment. In addition, the hydrothermal-treatment-induced hysteresis loop in the low-pressure region and the corresponding 2.5 nm mesopores were not observed (Figure S9C,D), further proving the improvement in the hydrothermal stability against structural damage.

2.4.3. Acidic Properties

For the phosphorus-modified samples, the NH3 desorption peaks also decreased in intensity after the hydrothermal treatment (Figure 6). They retained significantly more acidic sites than the unmodified samples (Figure 4B). In addition, P-0.5A-Z5-ST retained more acidic sites than the P-0.25A-Z5-ST, P-0.75A-Z5-ST, and P-Z5-ST samples.
The Py-IR experiments showed a similar rule to NH3-TPD (Figure S6 and Table 4). Compared with the samples without phosphorus modification, the phosphorus-modified samples retained more acid sites, especially B-acid sites, after the hydrothermal treatment. In addition, P-0.5A-Z5-ST contained the largest number of total acid sites (e.g., 69.5 μmol Py/g at 200 °C) after the hydrothermal treatment. This result shows that the alkali treatment is conducive to the improvement of hydrothermal stability. By comparing the catalytic activity and acidity characterization, the most important effect of phosphorus modification is that more acid sites are preserved in hydrothermal treatment.
The chemical compositions of the phosphorus-modified samples are shown in Table 5. After phosphorus modification under the same conditions, the phosphorus content increased with the alkali concentration. This proves that the introduction of mesopores in a zeolite improves its capacity for accommodating phosphorus species. At the same time, sample P-0.5A-Z5 had the closest bulk- and surface-phosphorus–aluminum ratio, indicating that the phosphorus species were more uniformly distributed in sample P-0.5A-Z5. For P-0.75A-Z5, the bulk phosphorous-to-aluminum ratio was greater than that on the surface, which can be attributed to the accumulation of phosphorus species in the large mesopores. Therefore, the formation of suitable mesopores is conducive to the distribution of phosphorus species.
To further understand the interaction between phosphorus and aluminum, 31P MAS NMR was used to characterize the state of the phosphorus species (Figure 7). Several resonance peaks appeared at 0, −6, −13, −15, −26, −32, −38, and −44 ppm, and their ascriptions are described in Table 6.
For the P-Z5 sample, the peaks at 0 and −44 ppm are prominent. They can be attributed to ortho- and polyphosphates on the external surface, which could hardly interact with the framework aluminum [37]. However, for the alkali-treated samples, a larger variety of phosphorus species appeared. The P-0.5A-Z5 sample had the highest content of phosphorus species that interacted with framework aluminum (around −26 ppm). This phosphorus species was more attached to the tetrahedral framework aluminum structure, which could play a better stabilizing role.
Figure 7B shows the phosphorus spectra measured for the samples after hydrothermal treatment. The peaks mainly concentrated between 10 and −50 ppm, indicating that the steam treatment destroyed the structure of zeolites and changed the phosphorus distribution [38]. The peak fitting results for the phosphorus spectrum are shown in Table S2. For sample P-Z5-ST, more phosphorus species interacted with the zeolite, which is also related to the effect of steam on the zeolite structure (Figure 3D). However, the alkali-treated samples contained fewer phosphorus species that interacted with the zeolite, and more condensed polyphosphates were formed after hydrothermal treatment.
Table
Table 6. Integral peak-fitting results of 31P MAS NMR for the P-xA-Z5 samples.
Table 6. Integral peak-fitting results of 31P MAS NMR for the P-xA-Z5 samples.
Chemical Shift/ppmDescriptionP-0.25A-Z5P-0.5A-Z5P-0.75A-Z5
0Monomeric phosphates [39]2.7%0.0%5.3%
−4–−6Terminal phosphates [20,33]12.1%15.3%3.8%
−11Middle group phosphates [17]5.2%17.0%11.9%
−15–−17Physical interaction with framework aluminum, nonbonded [34]22.6%6.1%32.3%
−26Interaction between phosphorus and framework aluminum (similar to AlPO4 structure) [40,41]20.9%32.5%18.6%
−32–−38Extra-frameworkaluminum phosphate [42]12.6%16.5%6.1%
−42–−56Condensed polyphosphates [43]23.9%12.6%22.0%

3. Materials and Methods

3.1. Alkaline Treatment of ZSM-5 Zeolite

The parent HZSM-5 zeolite (nSi/nAl = 15) was purchased from Nankai Catalyst Factory and denoted as Z5 for simplicity. The alkaline treatment of ZSM-5 was carried out at 80 °C in a series of sodium hydroxide (NaOH) solutions with different concentrations. In a typical process, 20 g of ZSM-5 was added into the desired aqueous NaOH solution (200 mL) and heated to 80 °C under mechanical stirring (200 rpm) for 4 h. Then, the resultant suspensions were separated by filtration and the solid was washed thoroughly with deionized water and dried overnight at 120 °C. The obtained alkaline-treated Na-form ZSM-5 was then used to prepare the H-ZSM-5 by ion exchange with ammonium chloride solution (1 M) three times before being dried overnight at 120 °C and finally calcined at 550 °C for 6 h to give the alkaline-treated H-form ZSM-5 samples (designated as xA-Z5, x = 0.25, 0.5, and 0.75 M, respectively, which represent the concentrations of the NaOH solution).

3.2. Phosphorus Modification

Phosphorus was introduced into the zeolite with the hydrothermal dispersion method [19,20]. In a typical procedure, (NH4)2HPO4 was first dissolved in deionized water to prepare a 2 M solution. A total of 5 g of the sample of xA-Z5 was added to 20 mL of the aqueous (NH4)2HPO4 (2 M) and mixed thoroughly by magnetic stirring. After that, the mixture was transferred into a Teflon-lined autoclave and heated under hydrothermal conditions at 140 °C for 2 h with rotation. The slurry was then centrifuged, and the resulting solid was washed four times with deionized water (solid-liquid ratio 1:90) to remove excess phosphorus species and was then dried overnight at 120 °C. Finally, calcination was performed at 550 °C for 6 h. The sample was named P-xA-Z5.

3.3. Hydrothermal Treatment

Hydrothermal treatment was carried out at 700 °C in 100% steam (H2O) for 17 h. Specifically, the samples were heated to 700 °C in a tube furnace, and then 100% steam was injected through a steam generator for 17 h. The sample was named Y-ST (Y represents the sample before the hydrothermal treatment).

3.4. Evaluation of the Catalytic Performance

The catalytic reaction was conducted in a fixed-bed microreactor under atmospheric pressure at 380 °C. In a typical experiment, 200 mg of the catalyst (40~60 mesh) was sandwiched in the middle section of the stainless-steel tube reactor with quartz sand to preheat and completely vaporize the liquid feed. The 1-octene was introduced into the reactor with a 5 h−1 of WHSV and 13.9 mL/min of nitrogen as the reaction carrier gas. The effluents were analyzed by online gas chromatography (Agilent 7980A) equipped with two flame-ionization detectors. Heavy- and light-hydrocarbon products were separated through PONA and Al2O3 columns, respectively.

3.5. Characterization

Powder X-ray diffraction (XRD) patterns for the samples were recorded using a D8 Advance diffractometer (Bruker, Bremen, Germany) with Cu Kα radiation at 40 kV and 40 mA.
The BET surface areas, pore volumes, and average pore diameter results were derived from N2 adsorption–desorption isotherms at −196 °C measured on an ASAP 2020 instrument (Micromeritics, Norcross, GA, USA). Prior to the adsorption–desorption test, all samples were degassed under 5 × 10−3 Torr at 350 °C for 8 h. The micropore surface areas and volumes were determined using the τ-plot method. The total surface areas were calculated by the Brunauer–Emmett–Teller (BET) method in the P/P0 range of 0.05–0.30.
The bulk Si/Al ratios for the various samples were determined by X-ray fluorescence (XRF) using a ZSX Primus II spectrometer (Rigaku, Tokyo, Japan). The surface Si/Al ratios for the various samples were determined by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250Xi spectrometer (Thermo Scientific, Waltham, MA, USA) with Al Ka radiation and a multichannel detector. Accurate binding energies were calibrated with respect to the C 1s peak at 284.8 eV.
Solid-state nuclear magnetic resonance (NMR) was performed using an Advance 600 NMR spectrometer (Bruker, Bremen, Germany). The 27Al MAS NMR spectra were recorded using a 4 mm ZrO2 rotor operated at a spinning frequency of 12 kHz with a pulse angle of π/12 and a recycle delay of 1 s. The chemical shifts were determined relative to a 1 M aqueous solution of aluminum nitrate. The 31P MAS NMR spectra were recorded with a recycle delay of 10 s. An aqueous phosphoric acid solution (1 M) was used as the chemical-shift reference.
The surface acidic properties were evaluated by the temperature-programmed desorption of ammonia (NH3-TPD) using an Autochem II 2920 instrument (Micromeritics, Norcross, GA, USA) equipped with a well-calibrated mass spectrometer as the detector. First, the samples were pretreated at 550 °C in flowing He (50 mL/min) for 1 h and then saturated with NH3 at 100 °C for 10 min. Subsequently, the physically adsorbed ammonia was removed by flowing He (50 mL/min) for 60 min. Finally, NH3 desorption was performed by heating the samples to 600 °C at a ramping rate of 10 °C/min under a flow of pure helium (50 mL/min). The signals were recorded simultaneously by a mass spectrometer.
Pyridine-adsorbed Fourier-transform infrared (Py-FTIR) spectroscopy was collected using a VERTEX 70 spectrometer (Bruker, Bremen, Germany). Typically, a self-supporting wafer of the sample was placed in an in-situ cell equipped with a vacuum system, pretreated at 350 °C for 1.0 h under evacuation (pressure < 10−2 Pa), and then absorbed pyridine until an equilibrium was reached at room temperature. The spectra were recorded after desorbing pyridine at 200 and 350 °C, respectively.

4. Conclusions

In this work, sequential alkali treatment and phosphorus modification were carried out for ZSM-5 samples. The characterization results show that the phosphorus species can interact with the framework aluminum better after alkali treatment, and the phosphorus dispersion is improved after alkali treatment at a concentration of 0.5M. After the hydrothermal treatment, a P-0.5A-Z5-ST sample treated with an appropriate concentration of alkali retained the most acid sites and showed the highest aromatic selectivity (22.5%) at TOS = 660 min. Therefore, a moderate distribution of mesopores in a zeolite plays an important role in the diffusion of reactants and products, as well as in the distribution of phosphorus species.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121629/s1, Figure S1. Performance of (A) xA-Z5 and (B) xA-Z5-ST catalysts in the aromatization reaction of 1-octene by conversion time courses; Figure S2. Performances of (A) P-xA-Z5 and (B) P-xA-Z5-ST catalysts in the aromatization reaction of 1-octene by conversion time courses; Figure S3. Nitrogen physical adsorption–desorption isotherms (A) and pore-structure distributions (B) of the xA-Z5 samples after reaction evaluation; Figure S4. FT-IR spectra of pyridine adsorptions on different samples at 200 °C (A,B) and 350 °C (C,D); Figure S5. (A) 27Al MAS NMR and (B) proportion of extra-framework and framework Al obtained by curve-fitting of the 27Al MAS NMR spectra of fresh ZSM-5 and Alkali-treated zeolites; Figure S6. FT-IR spectra of pyridine adsorptions on different samples at 200 °C (A,B) and 350 °C (C,D); Figure S7. Performance ((A): total aromatics selectivity, (B): BTEX selectivity) of P-xA-Z5 catalysts in the aromatization reaction of 1-octene by product-selectivity time courses; Figure S8. XRD patterns of the P-xA-Z5 and P-xA-Z5-ST samples; Figure S9. Nitrogen physical adsorption–desorption isotherms (A,C) and pore size distributions (B,D) of the P-xA-Z5 and P-xA-Z5-ST samples; Table S1. Textural properties of the xA-Z5 samples after reaction; Table S2. Integral peak-fitting results of 31P MAS NMR for the P-xA-Z5-ST samples.

Author Contributions

Investigation, Formal analysis, Validation, Data curation, Writing—original draft, J.C.; Formal analysis, M.X. and Y.H.; Conceptualization, Formal analysis, Methodology, K.H.; Formal analysis, Writing—Review & Editing, L.Z.; Formal analysis, Visualization, Writing—Review & Editing, F.W. and W.Z.; Formal analysis, Conceptualization, Z.T.; Resources, Funding acquisition, X.W. and Y.Y.; Supervision, Resources, Funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for financial support from the Ministry of Science and Technology of China (Grant No. 2022YFA1604104), the National Natural Science Foundation of China (No. 21473229, No. 91545121, No. 21902172), the Ordos Science and Technology Cooperation Key Special Project 2021EEDSCXQDFZ008 and funding support from Synfuels China, Co. Ltd. Wentao Zheng appreciates the support from NPL, CAEP (No. 2019BB08).

Data Availability Statement

Not applicable.

Acknowledgments

We gratefully acknowledge Zhengang Lv, Guoyan Zhao, and Caixia Hu at Synfuels China Co., Ltd., for assistance in performing XPS, in situ FTIR, and NMR, respectively.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Catalysts 12 01629 g001 550
Figure 1. Performance ((A,C) total aromatics selectivity, (B,D) BTEX selectivity) of xA-Z5 and xA-Z5-ST catalysts in the aromatization reaction of 1-octene by product-selectivity time courses.
Figure 1. Performance ((A,C) total aromatics selectivity, (B,D) BTEX selectivity) of xA-Z5 and xA-Z5-ST catalysts in the aromatization reaction of 1-octene by product-selectivity time courses.
Catalysts 12 01629 g001
Catalysts 12 01629 g002 550
Figure 2. XRD patterns for the (A) xA-Z5 and (B) xA-Z5-ST samples. The relative crystallinities were calculated from the ratio of the integrated intensities of the x-ray peaks in the 22.5–25° region, assuming that the crystallinity of the parent zeolite Z5 was 100%.
Figure 2. XRD patterns for the (A) xA-Z5 and (B) xA-Z5-ST samples. The relative crystallinities were calculated from the ratio of the integrated intensities of the x-ray peaks in the 22.5–25° region, assuming that the crystallinity of the parent zeolite Z5 was 100%.
Catalysts 12 01629 g002
Catalysts 12 01629 g003 550
Figure 3. Nitrogen physical adsorption–desorption isotherms (A,C) and pore–size distributions (B,D) of xA-Z5 and xA-Z5-ST samples.
Figure 3. Nitrogen physical adsorption–desorption isotherms (A,C) and pore–size distributions (B,D) of xA-Z5 and xA-Z5-ST samples.
Catalysts 12 01629 g003
Catalysts 12 01629 g004 550
Figure 4. NH3-TPD profiles of (A) xA-Z5 and (B) xA-Z5-ST samples.
Figure 4. NH3-TPD profiles of (A) xA-Z5 and (B) xA-Z5-ST samples.
Catalysts 12 01629 g004
Catalysts 12 01629 g005 550
Figure 5. (A) Performances of P-xA-Z5-ST catalysts in the aromatization reaction of 1-octene by aromatics selectivity time courses. (B) The detailed product distributions over the P-xA-Z5-ST catalysts for 1-octene aromatization at time-on-stream = 660 min. (Ci means non-aromatic hydrocarbons with i carbon atoms; Ci= and Ci0 mean the alkene and alkane hydrocarbons with i carbon atoms, respectively; CiA means aromatics hydrocarbons with i carbon atoms).
Figure 5. (A) Performances of P-xA-Z5-ST catalysts in the aromatization reaction of 1-octene by aromatics selectivity time courses. (B) The detailed product distributions over the P-xA-Z5-ST catalysts for 1-octene aromatization at time-on-stream = 660 min. (Ci means non-aromatic hydrocarbons with i carbon atoms; Ci= and Ci0 mean the alkene and alkane hydrocarbons with i carbon atoms, respectively; CiA means aromatics hydrocarbons with i carbon atoms).
Catalysts 12 01629 g005
Catalysts 12 01629 g006 550
Figure 6. NH3-TPD profiles of (A) P-xA-Z5 and (B) P-xA-Z5-ST samples.
Figure 6. NH3-TPD profiles of (A) P-xA-Z5 and (B) P-xA-Z5-ST samples.
Catalysts 12 01629 g006
Catalysts 12 01629 g007 550
Figure 7. The31P MAS NMR of the (A) P-xA-Z5 and (B) P-xA-Z5-ST samples.
Figure 7. The31P MAS NMR of the (A) P-xA-Z5 and (B) P-xA-Z5-ST samples.
Catalysts 12 01629 g007
Table
Table 1. Textural properties of the xA-Z5 and xA-Z5-ST samples.
Table 1. Textural properties of the xA-Z5 and xA-Z5-ST samples.
SampleBET
Surface Area
[m2/g]		t-Plot
External
Surface
Area
[m2/g]		t-Plot
Micropore Area
[m2/g]		t-Plot
Micropore
Volume
[cm3/g]		Mesopore Volume
[cm3/g]		Micropore
Retention
Rate [%]
Z5374323420.1660.079100.0
0.25A-Z5364603040.1490.13089.7
0.5A-Z53801052750.1350.24881.3
0.75A-Z53711162550.1200.36472.3
Z5-ST2851251600.0820.15449.4
0.25A-Z5-ST2941131810.0910.19554.8
0.5A-Z5-ST3021051970.0970.27358.4
0.75A-Z5-ST2721111610.0790.34747.6
Micropore retention rate = the micropore volumes of different samples/the micropore volume of the Z5 sample.
Table
Table 2. Acidity and its acidic-site distributions of the xA-Z5 and xA-Z5-ST samples according to Py-IR measurement.
Table 2. Acidity and its acidic-site distributions of the xA-Z5 and xA-Z5-ST samples according to Py-IR measurement.
Sample200 °C (μmol Py/g) 350 °C (μmol Py/g)
BASLASTotalL/BBASLASTotalL/B
Z5467.825.6493.40.05378.019.7397.70.05
0.25A-Z5285.251.4336.60.18213.845.1258.90.21
0.5A-Z5324.085.2409.20.26218.275.6293.80.35
0.75A-Z5308.0103.5411.50.34205.4102.3307.70.50
Z5-ST16.34.921.20.307.32.29.50.30
0.25A-Z5-ST19.59.529.00.492.76.79.42.48
0.5A-Z5-ST23.124.147.21.046.015.821.82.63
0.75A-Z5-ST25.828.354.11.105.923.229.13.93
The infrared spectrum of absorbed pyridine on samples is shown in Figure S4.
Table
Table 3. Textural properties of the samples.
Table 3. Textural properties of the samples.
SampleBET Surface Area
[m2/g]		t-Plot
Micropore Area
[m2/g]		t-Plot
External Surface Area
[m2/g]		t-Plot
Micropore Volume
[cm3/g]		Mesopore Volume
[cm3/g]		Micropore Retention Rate [%]
P-Z5283259240.1280.07977.1
P-0.25A-Z5269233360.1150.11269.3
P-0.5A-Z5285232530.1140.19768.7
P-0.75A-Z5239171680.0850.25151.2
P-Z5-ST296280160.1380.09483.1
P-0.25A-Z5-ST303276270.1350.12681.3
P-0.5A-Z5-ST293244490.1220.20973.5
P-0.75A-Z5-ST242184580.0900.25554.2
Micropore retention rate = the micropore volumes of different samples/the micropore volume of the Z5 sample.
Table
Table 4. Acidity and its acidic-site distributions of the P-xA-Z5 and P-xA-Z5-ST samples according to Py-IR measurement.
Table 4. Acidity and its acidic-site distributions of the P-xA-Z5 and P-xA-Z5-ST samples according to Py-IR measurement.
Sample200 °C (μmol Py/g) 350 °C (μmol Py/g)
BASLASTotalL/BBASLASTotalL/B
P-Z5167.712.4180.10.07121.610.1131.70.08
P-0.25A-Z5134.113.3147.40.1091.711.2102.90.12
P-0.5A-Z5121.516.3137.80.1397.914.9112.80.15
P-0.75A-Z589.210.599.70.1264.410.274.60.16
P-Z5-ST35.78.744.40.2416.93.620.50.21
P-0.25A-Z5-ST52.011.863.80.2332.011.543.50.36
P-0.5A-Z5-ST53.016.569.50.3129.912.942.80.43
P-0.75A-Z5-ST53.26.359.50.1218.04.422.40.24
The infrared spectrum of absorbed pyridine on samples is shown in Figure S6.
Table
Table 5. Chemical compositions of the catalysts.
Table 5. Chemical compositions of the catalysts.
CatalystSi/Al aSi/Al bP/% aBulk
P/Al a		Surface
P/Al b
P-Z515151.30.40.8
P-0.25A-Z514131.70.51.0
P-0.5A-Z511102.20.60.8
P-0.75A-Z51095.81.51.1
a Measured by XRF. b Measured by XPS.
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Cao, J.; Xing, M.; Han, Y.; Hao, K.; Zhang, L.; Wang, F.; Zheng, W.; Tao, Z.; Wen, X.; Yang, Y.; et al. Improving the Hydrothermal Stability of ZSM-5 Zeolites in 1-Octene Aromatization by Sequential Alkali Treatment and Phosphorus Modification. Catalysts 2022, 12, 1629. https://doi.org/10.3390/catal12121629

AMA Style

Cao J, Xing M, Han Y, Hao K, Zhang L, Wang F, Zheng W, Tao Z, Wen X, Yang Y, et al. Improving the Hydrothermal Stability of ZSM-5 Zeolites in 1-Octene Aromatization by Sequential Alkali Treatment and Phosphorus Modification. Catalysts. 2022; 12(12):1629. https://doi.org/10.3390/catal12121629

Chicago/Turabian Style

Cao, Jian, Mengjiao Xing, Yuanlong Han, Kun Hao, Ling Zhang, Fei Wang, Wentao Zheng, Zhichao Tao, Xiaodong Wen, Yong Yang, and et al. 2022. "Improving the Hydrothermal Stability of ZSM-5 Zeolites in 1-Octene Aromatization by Sequential Alkali Treatment and Phosphorus Modification" Catalysts 12, no. 12: 1629. https://doi.org/10.3390/catal12121629

APA Style

Cao, J., Xing, M., Han, Y., Hao, K., Zhang, L., Wang, F., Zheng, W., Tao, Z., Wen, X., Yang, Y., & Li, Y. (2022). Improving the Hydrothermal Stability of ZSM-5 Zeolites in 1-Octene Aromatization by Sequential Alkali Treatment and Phosphorus Modification. Catalysts, 12(12), 1629. https://doi.org/10.3390/catal12121629

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