Next Article in Journal
Particle Conditioning for Improving Blockage Resistance of Denitration Catalysts in Sintering Flue Gas
Previous Article in Journal
Self-Assembled PDI-COOH/PDINH Supramolecular Composite Photocatalysts for Highly Efficient Photodegradation of Organic Pollutants
Previous Article in Special Issue
Catalysis on Zeolites and Zeolite-like Materials II
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 14
Issue 10
10.3390/catal14100697
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Seeds Combining Pyrrolidine Control the Framework Al Distribution of FER Zeolite to Enhance Its Performance in the Skeletal Isomerization of n-Butene

by
Jinlong Fan
1,2,
Xuedong Zhu
1,*,
Fan Yang
1,
Yarong Xu
2 and
Lantian Chen
2
1
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
2
Research Institute of Urumqi Petrochemical Company, Petrochina Co., Ltd., Urumqi 830019, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(10), 697; https://doi.org/10.3390/catal14100697
Submission received: 31 August 2024 / Revised: 29 September 2024 / Accepted: 3 October 2024 / Published: 7 October 2024
(This article belongs to the Special Issue Catalysis on Zeolites and Zeolite-Like Materials, 3rd Edition)
Browse Figures
Versions Notes

Abstract

:
FER zeolites have a unique framework structure and acid distribution, and are widely studied as a catalyst for reactions such as n-butene skeletal isomerization and dimethyl ether carbonylation. The Brönsted acid site (BAS) located in the 10-member ring (10-MR) of FER zeolites serves as the active site for the isomerization reaction of skeletal n-butene to produce isobutene. This study prepared five types of FER zeolites using different methods: using pyrrolidine (PY) alone as a template; using Na-form FER as seeds (SN) or H-form FER as seeds without organic structure directing agents (OSDAs); and combining the seeds of SN or SH with PY as OSDAs. The differences in the structure and acid distribution of the five zeolites were investigated, as well as their catalytic performance for the skeletal isomerization of n-butene. Experiments and characterization results showed that under hydrothermal synthesis conditions, the FER-PY+SH zeolites synthesized by using both H-form zeolites seeds and pyridine exhibited the highest aluminum concentrations at T1 and T3 sites, along with the greatest BAS located in the 10-MR. This unique composition contributed to the highest selectivity of isobutene. The FER-PY+SH catalyst was continuously used for 720 h at 350 °C, 0.1 MPa, and an n-butene mass space velocity of 2.0 h−1 for three cycles of 2160 h. During this period, the conversion of n-butene was over 39%, while the selectivity of isobutene exceeded 95%. The FER-PY+SH catalyst exhibited excellent stability and activity.

1. Introduction

Isobutene is an important basic organic chemical raw material, which can be used to produce many high value-added products, including butyl rubber, polyisobutylene, methyl tert-butyl ether (MTBE), and methyl methacrylate. One of the primary industrial methods of production involves the skeletal isomerization of n-butene to generate isobutene [1,2,3]. In the past few decades, extensive research has focused on the production of isobutene through the skeleton isomerization of n-butene, utilizing various catalysts such as phosphoric acid, metal halides, aluminum oxide, phosphated and halogenated aluminum oxide, and zeolite molecular sieves [4,5]. The pore structure of FER zeolite includes two vertically intersecting two-dimensional pore systems: an 8-member ring (8-MR) pore (0.35 × 0.48 nm) along the [010] direction, a 10-member ring (10-MR) pore (0.43 × 0.55 nm) along the [001] direction, and a ferrierite cage with a diameter of approximately 0.6–0.7 nm formed by the intersection of a 6-member ring (6-MR) and 8-MR [6,7]. FER zeolite is currently the highest-performing n-butene skeleton isomerization catalyst and was successfully applied in industrial applications in the 1990s [8]. In the following decades, extensive research was conducted on the reaction mechanism of the skeletal isomerization of n-butene and on the synthesis and post-treatment modification of high-performance FER zeolites [9,10,11].
Numerous studies have demonstrated that the Brönsted acid site (BAS) located on the 10-MR in FER zeolite acts as a selective active site for catalyzing the skeleton isomerization of n-butene to produce isobutene. In contrast, the Lewis acid site (LAS) and BAS located on the 8-MR can exacerbate the occurrence of side reactions such as butene polymerization and cracking [12,13,14,15,16]. As is well known, zeolites are crystal compounds composed of TO4 (T atoms are generally Si or Al) tetrahedra according to certain rules. Their acidity mainly arises from the introduction of protons, which balance the negative charge of aluminum atoms in the framework of zeolites. It can be seen that the catalytic performance of zeolites is closely linked to the strength and position of acid sites. The strength and position of these acid sites are directly influenced by the distribution of Al atoms within the zeolite framework [17,18,19,20]. According to relevant information from the International Zeolite Association (IZA), the framework T atom of FER zeolites exhibits four distinct positions, namely the T1/T3 position, which can be connected to a 10-MR, and the T2/T4 position, which is solely connected through an 8-MR (note: labels of T atoms have been unified according to the FER framework in the IZA). Xiong et al. [21,22,23] used characterization methods such as MAS NMR and X-ray emission spectroscopy to study the distribution of framework aluminum atoms in FER zeolites. The results showed that FER zeolites induced by different organic structural directing agents (OSDAs) had different distributions of framework aluminum. Moreover, they combined MAS NMR spectroscopy and density functional theory to calculate the chemical shifts of aluminum located at positions T1/T2/T3/T4 in FER zeolites, which were 61, 51, 53, and 58 ppm, respectively.
In the synthesis process of zeolites, OSDAs play a crucial role in balancing the negative charge introduced by the aluminum atoms in the framework. Consequently, these OSDAs significantly influence the distribution of aluminum in zeolites. Pinar et al. [14,24,25] synthesized FER zeolites using a fluorinated system without the use of inorganic cations, using a combination of 1-benzoyl-1-methylpyrrolidinium, trimethylamine (TMA), pyridine, pyrrolidine (PY), and several organic template agents. The investigations indicate that FER zeolites induced by different organic structure directing agents have different skeletal aluminum distributions and exhibit different performances in n-butene skeletal isomerization reactions. Due to the different sizes of template molecules and their interactions with skeleton atoms, FER zeolites with different framework aluminum distributions can be obtained. When larger size OSDAs are used, a longer crystallization time is required to increase the distribution probability of aluminum atoms in the 10-MR, enhance the BAS of the 10-MR, and improve the control of the activity and selectivity of n-butene skeleton isomerization. Conversely, this tends to lead to side reactions such as butene polymerization and cracking, and accelerate catalyst deactivation. In addition, the studies of Leshkov [26] and Chu [27] also indicate that when different sizes of cyclic or chain amines are used as OSDAs, FER zeolites with different 10-MR and 8-MR aluminum distributions can also be obtained. Numerous studies have shown that the distribution of aluminum in the framework of zeolites is not only influenced by OSDAs, but also by the synthesis conditions of zeolites [28] and the selection of silicon and aluminum sources [29,30], solvents [31], inorganic cations [32,33,34], and post-treatment [35,36,37]. During the seed-induced synthesis of zeolites, the seed can play a role in reducing or avoiding the use of OSDAs, shortening crystallization time, avoiding impurities, and regulating the crystal morphology of target zeolites [38,39,40]. Xiao et al. [41] believe that due to the multiple roles of the seed in inducing the synthesis of zeolites, the seed can be regarded as the third type of structure directing agents, in addition to inorganic cations and OSDAs. Zhang et al. [42] synthesized FER zeolite with good crystallinity, large surface area, uniform micropores, and abundant acidic sites using RUB-37 as the crystal seed without OSDAs. Studies by Ham et al. [43,44,45] showed a change in the distribution of framework aluminum atoms in seed-induced synthesized FER zeolites as compared to FER zeolites as the seed. The higher BAS content of the 8-MR than the 10-MR resulted in better dimethyl ether carbonylation activity and stability of the seed-induced FER zeolites than the seed. In addition, Nishitoba [30] found that when using TMA as an OSDA to synthesize CHA zeolites, the addition of FAU zeolites as crystal seeds and aluminum sources could regulate the distribution of aluminum atoms in the framework of CHA zeolites. These studies indicate that the distribution of aluminum in the zeolite framework can be modulated using the seed induction methods.
In this work, the synthesis of FER zeolites under different conditions was investigated as follows: using only pyrrolidine in OSDA synthesis; using only Na-form seed (SN)-induced synthesis; using only H-form seed (SH)-induced synthesis; and combining SN or SH + PY. The distribution of aluminum atoms in the framework of FER zeolites synthesized by different methods and their catalytic performance for n-butene skeleton isomerization reaction were investigated. The obtained catalyst of FER zeolites by inducing H-form seeds + PY as OSDAs has the highest distribution of 10-MR framework aluminum, which enhanced the acidic strength in the 10-MR and significantly improved the catalytic activity and selectivity of FER zeolites for the n-butene skeleton isomerization reaction. A new method for regulating the distribution of the framework of aluminum and acid in the FER zeolites was proposed.

2. Results and Discussion

2.1. Structural and Textural Properties

As shown in Figure 1, XRD patterns of FER-PY, FER-SN, FER-SH, FER-PY+SN, and FER-PY+SH exhibit typical diffraction peaks at 9.3°, 22.3°, 23.5°, 24.3°, 25.2°, 25.7°, and 28.5° out of the (200), (321), (330), (112), (040), (202), and (312) crystal planes in the FER topology (PDF #44-0104), respectively, indicating the successful formation of the FER zeolite without detectable impurities [22,23]. The relative crystallinity (RC) was calculated by integrating the areas of diffraction peaks within 9.3-28.5°, and the sum of the areas of FER-PY was set as 100% for reference. As listed in Table 1, the calculated RC values are comparable, measuring 101%, 99%, 100%, and 101% for FER-SN, FER-SH, FER-PY+SN, and FER-PY+SH, respectively. The above results indicate that several templates, such as PY, SN, SH, PY+SN, and PY+SH, can induce the synthesis of well-crystallized FER zeolites. The composition information is listed in Table 1, where the SiO2 to Al2O3 ratios (SARs) of the five samples are within 24.6–25.6. Additionally, the Na2O contents are all below 43 mg/g, suggesting that all FER-X samples have similar compositions and acidic densities. The calculated texture parameters are shown in Table 1; in line with the XRD observations, the specific surfaces and pore volumes remain relatively uniform, for instance, the values of SBET average around 430 m2/g, while Vtotal and Vmicro are all about 0.27 cm3/g and 0.14 cm3/g, respectively.
The SEM images (Figure 2) present the morphologies of the obtained FER-X zeolites. All samples are composed of hexagonal flake-like crystals with smooth surfaces; the average size of each flake is ~1 μm in length, ~500 nm in width, and 20~50 nm in thickness. Combined XRD and SEM analyses indicate that their morphology and structure are identical.

2.2. Acidity Characterization

The acidic properties were characterized using NH3-TPD and Py-IR. As illustrated in Figure 3, two NH3 desorption peaks are observed at 195 °C and 450 °C (Figure 3a), corresponding to the desorption of NH3 molecules that interact with the weak acids and strong acids, respectively. The positions of these peaks provide insight into acid strength, indicating that the weak and strong acid sites in FER-X samples exhibit similar acidic densities. Figure 3b–f shows the Py-IR spectrum was extracted at 150 °C, 250 °C, and 350 °C. The IR adsorption peak at 1450 cm−1 corresponds to the C-N stretching of pyridine adsorbed on LAS, while the peak at 1490 cm−1 is attributed to the C-H vibration of pyridine on both BAS and LAS. The peak at 1540 cm−1 is due to the N-H stretching of pyridinium cations generated by protonation of pyridine on BAS. As shown in Figure 3b–f, with the increase in desorption temperature, both BAS and LAS in the sample decrease. When the desorption temperature reaches 350 °C, LAS decreases to a very low level, indicating a scarcity of strong LAS in the FER-X samples. In contrast, the decrease in BAS is considerably smaller, indicating a stronger acidity of BAS present in the FER-X samples.
The quantitative analysis of NH3-TPD and Py-IR experiments is presented in Table 2. Notably, NH3-TPD provides an overall assessment of the acidity of the samples, as the kinetic diameter of NH3 (3.7 Å) is smaller than the 8-MR and 10-MR apertures in the FER structure. In contrast, Py-IR could only detect the acid sites located in 10-MR due to its larger kinetic diameter (5.7 Å for pyridine vs. 4.8 Å for 8-MR). Consequently, NH3-TPD can be used to characterize the total acidity of FER-X zeolites, and Py-IR can be used to characterize the acidity of the 10-MR of FERX-zeolites (see the caption below Table 2 for more details). From the NH3-TPD profiles, all samples have a similar acid distribution and acid content, with acid densities around approximately 1.3 mmol/g for the five samples, consistent with their structural and compositional properties. However, more BAS can be identified over Py-IR for the FER-X samples (0.29 mmol/g for FER-PY and 0.28–0.34 mmol/g for others), especially for FER-PY+SH, whose value is 0.34 mmol/g. In addition, the BAS of desorption temperature as measured from Py-IR (B10-350) is also shown in Table 2; this value is 0.20 mmol/g for FER-PY and increases monotonically to 0.32 mmol/g for FER-PY+SH. The differing acid distributions may arise from the distinct nucleation and crystal growth processes of zeolites synthesized using seed crystals, or a combination of seed crystals and organic templates, compared to those formed solely with organic templates. The different B10-350 provides direct evidence that utilizing seeds or employing a combination of seeds and organic templates will alter the acid distribution in FER zeolite.

2.3. 29Si and 27Al MAS NMR

The 29Si MAS NMR spectra show closely similar features (Figure 4), exhibiting four sub-peaks from the curve at approximately −100, −107, −112, and −116 ppm, respectively. The peaks at −100 and −107 ppm correspond to the Si atoms bonded to two Al atoms (Si(2Al)) and one Al atom (Si(1Al)), respectively, while the peaks at −112 and −116 ppm can be attributed to the Si atoms that are not adjacent any Al atoms (Si(0Al)), and silicon connecting to four Si tetrahedrons in the FER framework [21,47]. The weaker signal at −100 ppm of all samples indicates a lower prevalence of Al–O–Si–O–Al species on FER-X zeolites.
The 27Al MAS NMR spectra of various FER-X zeolites are shown in Figure 5. As shown in Figure 5a, all of the FER-X samples exhibit a prominent resonance peak at 55 ppm corresponding to tetrahedrally coordinated Al species, while no signal peak appears at 0 ppm. This indicates that the aluminum atoms in zeolites samples are located on the framework of FER-X zeolites, and there is no non-framework aluminum present [42,43,48]. Previous studies have shown that aluminum atoms exhibit different NMR chemical shifts depending on their positions of T1, T2, T3, and T4 within the FER framework [21,23,47], and the 27Al MAS NMR spectrum at 55 ppm is deconvolved into four peaks located at 61, 51, 53, and 58 ppm, respectively, as illustrated in Figure 5b–f. The Al population at T1–T4 sites in the FER-X samples was quantified from the corresponding 27Al NMR spectra (Figure 5b–f) and Table 3. The FER-PY sample exhibited a minimal fraction of Al atoms on the T1 + T3 sites (30%). In comparison, the distribution proportion of aluminum on the T1 + T3 positions in FER-SN, FER-SH, and FER-PY+SN gradually increased to 41%, 44%, and 50% respectively, while the distribution proportion of aluminum at T1 + T3 positions on FER-PY+SN reached the highest value of 60%. Notably, each of the five samples contained a small amount of Al on the T1 site (0~12%).
In summary, using PY as an OSDA to induce the synthesis of FER-PY zeolites as a reference, all samples possessed similar compositional and textural properties, are total acid densities were closely aligned. Nevertheless, the combined use of pyrrolidine and H-form FER seeds renders a distinctive acid distribution, with a greater number of strong acid sites located in the 10-MR. To elucidate the relationship between the aluminum distribution and the acid distributions of FER-X zeolites, Figure 6 correlates the aluminum distribution data at T1 + T3 sites of different samples with B10-350. A linear relationship can be observed between T1 + T3 and B10-350, implying the acid distributions positively correlate with the quality of the aluminum distribution on T1 + T3. Several factors may contribute to this result. Firstly, the use of seed crystals to facilitate FER zeolite synthesis accelerates the nucleation and crystallization processes, potentially leading to a higher distribution of aluminum within the 10-MR. Secondly, the lower sodium content in H-form FER seeds allows for larger species to balance the framework charge, thereby increasing the likelihood of aluminum being positioned at T1 + T3 sites in the 10-MR. Moreover, this effect is more pronounced in the presence of PY as OSDAs alongside H-form FER seeds.

2.4. Catalytic Performance

The catalytic performances of all FER-X samples in n-butene skeletal isomerization are shown in Figure 7; the C4 hydrocarbons produced by fluidized catalytic cracking in refineries react with methanol to produce MTBE after removing isobutene (post-MTBE C4) and are used as feed.
Figure 7a illustrates the relationship between the n-butene conversion and time on stream for FER-X catalyzed n-butene skeletal isomerization. As can be seen from the figure, FER-X catalysts exhibit the same trend, that is, with the extension of time on stream, the conversion initially decreases rapidly and then gradually stabilizes. Both FER-PY and FER-SN showed the best initial activities, achieving n-butene conversions exceeding 65%, while the initial conversions of the other samples fall within the range of 60% to 63%. This may be mainly due to the presence of more framework aluminum at the T2 + T4 sites in FER-PY and FER-SN, which resulted in more side reactions such as polymerization and cracking, leading to a higher conversion of n-butene. As the reaction proceeded, all conversions dropped rapidly during the first 12 h and then stabilized, by approximately 45% after running for 48 h.
In contrast, the selectivity of isobutene showed a completely reversed trend from that in Figure 7b. In the initial stage, the selectivity over FER-PY, FER-SN, and FER-SH was below 20%, while FER-PY+SN and FER-PY+SH recorded 25% and 40%, respectively. With the extension of time on stream, the selectivity over FER-PY gradually increased to ~55% after 48 h. FER-SN and FER-SH both presented a sharp increase in the first 12 h and then kept steady, with the selectivity reaching ~63% at 48 h. The selectivity over FER-PY+SN and FER-PY+SH also presented a sharp increase in the first 12 h, leveling off at about 78% and 90% after 48 h, respectively.
The experimental results in Figure 7 are mainly due to the different acid distributions of the FER-X catalysts. It is generally believed that the skeleton isomerization of n-butene to produce isobutene is carried out through the joint action of the monomolecular mechanism and the bimolecular mechanism. The BAS located in the 10-MR of FER zeolites is the active site for the selective produce of isobutene from n-butene [48,49,50]. In the monomolecular mechanism, n-butene is first adsorbed and protonated into a carbenium cation on the acid site, after which it undergoes skeleton isomerization and proton feedback to afford isobutene. The bimolecular mechanism mainly involves polymerization and isomerization to generate various long-chain carbocations, which subsequently crack to produce isobutene and by-products such as propylene. In the initial stage of this reaction, the strong acid sites of FER zeolite were well retained, resulting in the predominance of the bimolecular mechanism, which was followed by cracking and the generation of by-products. After the initial stage, carbon deposition began to cover the non-selective acid sites, leading to a decrease in catalyst activity while suppressing undesired side reactions. Previous investigations have proved that the BAS in 10-MR of FER zeolites is accountable for the skeleton isomerization of n-butene, while BAS in the 8-MR and LAS can exacerbate side reactions such as polymerization and cracking, despite their higher activity [51,52]. Therefore, FER-PY+SH demonstrates improved selectivity and isobutene yields owing to its higher B10-350 concentration. As shown in Figure 8, the yield of isobutene increases with the increase in B10-350 and Al distribution on the T1 + T3.

2.5. Catalyst Stability

Figure 9 displays the long-term stability tests using FER-PY+SH as candidate catalysts. The catalytic results were obtained with three cycles of regeneration (2160 h). The one-way life of the catalyst reaches 720 h, the conversion of n-butene (xn) is greater than 39%, the selectivity of isobutene (si) is greater than 95%, and the yield of isobutene is greater than 37.5%. After 720 h of continuous reaction, the conversion of n-butene was below 39%, and catalyst regeneration was carried out at this time. After stopping the reaction, all deactivated catalysts were placed in a muffle furnace and the temperature was raised at a rate of 1 °C/min from 30 °C to 500 °C, and calcined at 500 °C for 12 h to complete the regeneration of the deactivated catalysts. Interestingly, after 720 h of one-way operation, the activity of FER-PY+SH in skeletal isomerization of n-butene was fully recovered after regeneration. This finding substantiates that FER-PY+SH can be effectively reused, thereby extending its lifespan beyond most reported catalysts in the skeletal isomerization of n-butene. The catalyst of FER-PY+SH exhibits superior catalytic activity due to its unique distribution of framework aluminum and enhanced acidic strength in the 10-MR.
In conclusion, the preparation of FER-PY+SH zeolites using a combination of H-form seeds and pyrrolidine enriches the number of BASs in the 10-MR, while improving the selectivity of isobutene and effectively suppressing the side reactions such as polymerization and cracking. The FER-PY+SH samples thus demonstrate excellent isobutene selectivity and yields in the skeletal isomerization of n-butene.

3. Experimental

3.1. Materials

The reagents used for the synthesis of FER zeolite were silica sol (SS3015, 30% SiO2, 0.02%NaO2, Shandong Baite New Material Co., Ltd. Linyi, Shandong, China), sodium meta-aluminate (NaAlO2, 99.0%, Tianjin Kermel Chemical Reagent Co., Ltd. Tianjin, China), sodium hydroxide (NaOH, 96.0%, Tianjin Kermel Chemical Reagent Co., Ltd. Tianjin, China), pyrrolidine (PY) (C4H9N, 99%, Tianjin Kermel Chemical Reagent Co., Ltd. Tianjin, China), ammonium chloride (NH4Cl, 99%, Tianjin Kermel Chemical Reagent Co., Ltd. Tianjin, China), and deionized water.

3.2. Preparation of FER-X Zeolites

3.2.1. Preparation of Seeds

Typically, deionized water and NaOH aqueous solution were slowly added to silica sol (SiO2, 30 wt%), and stirred vigorously for 1 h. Then, NaAlO2 aqueous solution and PY were added, followed by stirring for 2 h to form a homogeneous suspension with a molar composition of 0.1 Na2O:0.033 Al2O3:1 SiO2:25 H2O:0.22 PY. The suspension was transferred into a Teflon-lined stainless-steel autoclave and crystallized at 160 °C under a rotation of 200 rpm for 72 h. After the hydrothermal synthesis, the product was washed with distilled water until it reached a neutral pH. It was then dried at 120 °C overnight and calcinated at 550 °C in air for 6 h to obtain Na-form FER. The Na-form FER zeolites were subsequently converted into NH4-form via ion exchange with 1 mol/L NH4Cl solution at 80 °C for 2 h; this process was repeated twice. Then, the resulting white slurry was filtered and dried at 120 °C for 6 h. Finally, H-form FER zeolite catalysts were obtained by calcination of NH4-form FER zeolites in air at 500 °C for 4 h. These Na-form FER and H-form FER were used as the seeds, and the seeds’ abbreviations are SN and SH respectively.

3.2.2. Preparation of Seed-Derived FER-X Zeolites

The seed-derived FER zeolites were prepared as fellows: organic structure directing agents were not used, while the remaining synthetic processes were kept identical to the procedures described above. Finally, the SN and SH were added to form a homogeneous suspension with a molar composition of 0.1 Na2O:0.033 Al2O3:1 SiO2:25 H2O: 15% wt seeds. The prepared mother gel was transferred into a Teflon-lined stainless-steel autoclave and crystallized at 160 °C under a rotation of 200 rpm for 72 h, followed by centrifuging, rinsing, and drying overnight at 120 °C to afford Na-form FER.
The method of synthesizing FER zeolites using both PY and SN or SH as seeds simultaneously is similar to the synthesis methods of the two aforementioned FER zeolites. Finally, the homogeneous suspension with a molar composition of 0.1 Na2O:0.033 Al2O3:1 SiO2:25 H2O: 0.12 PY:15% wt SN or SH was transferred into a Teflon-lined stainless-steel reactor, and then hydrothermal synthesis was conducted for 72 h at 160 °C with 200 rpm. The white gel was obtained after several washing steps and dried at 120 °C for 6 h, and further calcined at 550 °C in air for 6 h to afford Na-form FER.

3.2.3. Preparation of H-form FER Zeolites

All of the Na-form FER zeolites were transformed into NH4-form via ion exchange with 1 mol/L NH4Cl solution at 80 °C for 2 h. This process was repeated twice. Then, the resulting white slurry was filtered and dried at 120 °C for 6 h. Finally, H-form FER zeolite catalysts were obtained by calcining of NH4-form FER zeolites in air at 500 °C for 4 h. The H-form FER zeolite prepared using PY was labeled as FER-PY, the H-form FER zeolite prepared by Na-form FER zeolites as seeds induction was denoted as FER-SN, the H-form FER zeolite prepared by H-form FER zeolites as seeds induction was denoted as FER-SH, and the H-form FER zeolite prepared with PY and Na-form FER zeolites or H-form FER zeolites as seeds simultaneously was denoted as FER-PY+SN and FER-PY+SH, respectively. Finally, the H-form FER zeolite was compressed and formed to obtain a catalyst for the skeletal isomerization of n-butene.

3.3. Characterization of Catalysts

X-ray diffraction (XRD) was used to measure the crystallization and phase of the FER catalysts performed with a Rigaku D/max 2550 VB/PC diffractometer apparatus with Cu Ka radiation (λ = 0.15406 nm) at a voltage and current of 40 kV and 40 mA, respectively. The 2θ scanning range was from 5° to 40° with a scan rate of 4°/min and the step size of 0.02°. The R.C.s of the catalysts were obtained by calculating the sum of the peak intensity at 2θ = 9.3 ± 0.1°, 22.3 ± 0.1°, 23.5 ± 0.1°, 24.3 ± 0.1°, 25.2 ± 0.1°, 25.7 ± 0.1°, and 28.5 ± 0.1°, and the R.C.s of FER-PY was set as 100%.
The scanning electron microscopy (SEM) images of the samples were analyzed using a Nava Nano SEM 450 microscope to confirm the crystal morphology and size. The sodium content and SiO2/Al2O3 molar ratio of the samples were determined by X-ray fluorescence (XRF) analysis using a ZSX Primus Ⅱ instrument (Rigaku, Tokyo, Japan) operated at 60 kV and 150 mA. The textural characterization of the FER catalysts was performed by N2 adsorption–desorption at −196 °C with a US Micromeritics ASAP 2020 V3.00 H nitrogen adsorption analyzer (Atlanta, GA, USA). The FER catalysts were outgassed at 350 °C for 5 h under the vacuum of 10−3 Pa to remove moisture and volatile impurities before the adsorption measurement. The specific surface areas were obtained by the Brunauer–Emmett–Teller (BET) method and the pore volume was calculated by N2 adsorption at a relative pressure of 0.99.
The temperature-programmed desorption of ammonium (NH3-TPD) with a Micromeritics chemisorb 2720 (Atlanta, GA, USA) instrument equipped with a thermal conductivity detector was used to study the acid properties of the catalysts. Typically, 150 mg of the sample was first preheated under a He stream of 30 mL∙min−1 at 550 °C for 1 h, then purged in a 5% NH3-He flow of 30 mL/min at 100 °C for 40 min. Subsequently, the sample was treated under a He flow of 30 mL/min at 100 °C for 60 min to remove the physically adsorbed NH3. Finally, the samples were heated from 100 °C to 550 °C using a temperature ramp of 10 °C/min, and the quantity of NH3 desorbed was measured by a TCD detector.
Pyridine adsorption Fourier transform infrared (Py-IR) was used to determine B acid and L acid concentrations using a Bruker Tensor 27 infrared spectrometer. Approximately, the sample powder (30 mg) was formed into a self-supporting wafer (dimeter 20mm) and then placed inside the reaction tank. The sample was degassed under vacuum at 500 °C for 2 h under high vacuum conditions of 10−3 Pa, and then the sample was saturated with the probe molecule pyridine at 40 °C for 30 min, and subsequently, the physisorbed molecules were removed at 150 °C for 30 min. IR spectra were recorded at 350 °C. The adsorption peaks located at 1540 cm−1 and 1450 cm−1 were assigned to BAS and LAS, respectively. The concentration of acid sites was calculated from the IR results using the extinction coefficient and integrated intensity of the desorption peak and the sample weight reported by Emeis [46].
Magic-angle-spinning nuclear-magnetic-resonance (MAS NMR) spectra of 27Al and 29Si were collected on a Bruker Avance III 600 MHz Wide Bore spectrometer (Bruker, Rheinstetten, Germany). The single-pulse sequence was adopted with a 10° pulse and a delay time of 0.3 s. The chemical shifts for 27Al and 29Si were calibrated by referring to AlCl3 and tetramethylsilane, respectively. The 27Al MAS NMR spectra were deconvoluted using the mixed Gaussian–Lorentzian equation.

3.4. Catalyst Evaluation

The skeletal isomerization of n-butene was conducted under atmospheric pressure in a fixed-bed stainless-steel reactor unit, with the tubular reactor having 12 mm inner diameter and 800 mm length. Firstly, 3 g of the FER-X catalyst (20–40 mesh) was placed in the middle of reactor, the catalyst was pretreated under a N2 flow at a heating rate of 10 °C/min, and the system was purged for 1 h. The C4 hydrocarbons produced by fluidized catalytic cracking in refineries react with methanol to produce MTBE after removing isobutene (post-MTBE C4) and were used as reaction materials. The feed stream of post-MTBE C4 was fed into the reactor by a mass flow controller to start the skeletal isomerization; when heated to 350 °C, the weight hourly space velocity (WHSV) of n-butene was 2.0 h−1. The properties of the post-MTBE C4 are listed in Table 4.
The products were analyzed using an Agilent 7890B gas chromatograph (Santa Clara, CA, USA) equipped with a flame ionization detector and an Agilent-GS-Alumina capillary column (50 m × 0.532 mm). Since the equilibrium of 1-butene, trans-2-butene, and cis-2-butene was established rapidly through double-bond isomerization at the temperature of the reaction, and all the straight-chain butene isomers were transformed to isobutylene, all of the straight-chain butene was considered as the n-butene reactant.
The conversion of n-butene (xn) was defined as:
x n = m 0 − m 1 m 0 × 100 %
The selectivity of isobutene (si) was defined as:
s i = m 2 − m 3 m 0 − m 1 × 100 % s i = m 2 − m 3 m 0 − m 1 × 100 %
where m0, m1, m2, and m3 are the weights of n-butene in the feed, the weights of n-butene in the products, the weights of isobutene in the reaction products, and the weights of isobutene in the feed, respectively.
The production rate of isobutene was defined as the average moles of isobutene produced at the reaction duration of 24–48 h [mmol g−1 h−1].

4. Conclusions

FER-type zeolites have been successfully prepared in pyrrolidine as an organic template or without pyrrolidine systems utilizing the Na-form or H-form as the seed. The seed-derived FER samples own similar topological and morphological properties compared to the pyrrolidine-derived sample, as well as similar total acid densities. The choice of the H-form seed and pyrrolidine induces more BAS in the 10-MR of the FER structure. As the BAS in 8-MR and LAS is responsible for the bimolecular mechanism that tends to cause more carbon depositions and side-products in n-butene skeleton isomerization, the FER-PY+SH samples exhibit better isobutene selectivity and higher stability than the counterparts synthesized by Na-form as the normal seed. By optimizing the synthetic parameters, FER-PY+SH could be continuously operated for 720 h at 350 °C and 0.1 MPa for three cycles, and a weight hourly space velocity of n-butene of 2.0 h−1, during which the n-butene conversion keeps higher than 39%, the selectivity of isobutene is higher 95%, and the isobutene yield is higher than 37.5%.

Author Contributions

J.F.: investigation, data curation, writing—original draft. X.Z.: methodology, conceptualization, supervision, writing—review and editing. F.Y.: methodology, writing—original draft. Y.X.: methodology formal analysis, project administration. L.C.: validation, formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This research received the funding of the project by CNPC (2018B-1907).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Author Jinlong Fan, Yarong Xu and Lantian Chen were employed by the company of Research Institute of Urumqi Petrochemical Company, Petrochina Company Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

ContentAbbreviations
Brönsted acid sitesBAS
10-member ring10-MR
pyrrolidinePY
Na-form FER as seedsSN
H-form FER as seedsSH
organic structure directing agentsOSDAs
8-member ring8-MR
6-member ring6-MR
International Zeolite AssociationIZA
methyl tert-butyl etherMTBE
trimethylamineTMA
relative crystallinityRC
SiO2 to Al2O3 ratiosSARs
BAS of desorption temperature as measured from Py-IRB10-350
C4 hydrocarbons produced by fluidized catalytic cracking in refineries react with methanol to produce MTBE after removing isobutenepost-MTBE C4
weight hourly space velocityWHSV

References

  1. Díaz Velázquez, H.; Likhanova, N.; Aljammal, N.; Verpoort, F.; Martínez-Palou, R. New Insights into the Progress on the Isobutane/Butene Alkylation Reaction and Related Processes for High-Quality Fuel Production. A Critical Review. Energy Fuels 2020, 34, 15525–15556. [Google Scholar] [CrossRef]
  2. Xu, Q.; Yang, W.; Chen, Z.; Ye, Y.; Luo, Y.; Street, J.; Zhou, H.; Xu, C. Formation and regeneration of shape-selective ZSM-35 catalysts for n-Butene skeletal isomerization to isobutylene. ACS Omega 2018, 3, 8202–8211. [Google Scholar] [CrossRef] [PubMed]
  3. Corma, A. Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. Rev. 1995, 95, 559–614. [Google Scholar] [CrossRef]
  4. Houzvicka, J.; Ponec, V. Skeletal isomerization of n -Butenes. Catal. Reviews. Sci. Eng. 1997, 39, 319–344. [Google Scholar] [CrossRef]
  5. Paul, M.; Naccache, C. Skeletal isomerisation of n-butenes catalyzed by medium-pore zeolites and aluminophosphates. Adv. Catal. 2010, 31, 505–543. [Google Scholar]
  6. Vaughan, P.A. The crystal structure of the zeolite ferrierite. Acta Crystallogr. 1966, 21, 983–990. [Google Scholar] [CrossRef]
  7. Xu, H.; Zhu, J.; Zhu, L.; Zhou, E.; Shen, C. Advances in the synthesis of ferrierite zeolite. Molecules 2020, 25, 3722. [Google Scholar] [CrossRef]
  8. Mooiweer, H.H.; de Jong, K.P.; Kraushaar-Czarnetzki, B.; Stork WH, J.; Krutzen BC, H. Skeletal isomerisation of olefins with the zeolite ferrierite as catalyst. Stud. Surf. Sci. Catal. 1994, 84, 2327–2334. [Google Scholar]
  9. Guo, G.Q.; Long, Y.C. Advances in the catalysis of skeletal isomerization of n-butene into iso-butene on FER zeolite. Chem. Ind. Eng. Prog. 2006, 6, 16–19. [Google Scholar]
  10. Liu, W.; Hu, H.; Liu, Y.; Zhang, L.; Xia, C.; Wang, Q.; Ke, M. Distribution of effective ferrierite active sites for skeletal isomerization of n-butene to isobutene. ChemistrySelect 2019, 4, 7851–7857. [Google Scholar] [CrossRef]
  11. Wang, Y.; Gao, Y.; Chu, W.; Zhao, D.; Chen, F.; Zhu, X.; Li, X.; Liu, S.; Xie, S.; Xu, L. Synthesis and catalytic application of FER zeolites with controllable size. J. Mater. Chem. A 2019, 7, 7573–7580. [Google Scholar] [CrossRef]
  12. Wichterlová, B.; Žilkova, N.; Uvarova, E.; Čejka, J.; Sarv, P.; Paganini, C.; Lercher, J.A. Effect of Broensted and Lewis sites in ferrierites on skeletal isomerization of n-Butenes. Appl. Catal. A Gen. 1999, 182, 297–308. [Google Scholar] [CrossRef]
  13. Domokos, L.; Lefferts, L.; Seshan, K.; Lercher, J. The importance of acid site locations for n-Butene skeletal isomerization on ferrierite. J. Mol. Catal. A Chem. 2000, 162, 147–157. [Google Scholar] [CrossRef]
  14. Pinar, A.; Márquez-Álvarez, C.; Grande-Casas, M.; Pérez-Pariente, J. Template-controlled acidity and catalytic activity of ferrierite crystals. J. Catal. 2009, 263, 258–265. [Google Scholar] [CrossRef]
  15. Seo, G.; Jeong, H.S.; Hong, S.B.; Uh, Y.S. Skeletal isomerization of 1-Butene over ferrierite and ZSM-5 zeolites: Influence of zeolite acidity. Catal. Lett. 1996, 36, 249–253. [Google Scholar] [CrossRef]
  16. Van Donk, S.; Bus, E.; Broersma, A.; Bitter, J.H.; de Jong, K.P. Probing the accessible sites for n-Butene skeletal isomerization over aged and selective H-Ferrierite with d3-acetonitrile. J. Catal. 2002, 212, 86–93. [Google Scholar] [CrossRef]
  17. Corma, A. From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis. Chem. Rev. 1997, 97, 2373–2420. [Google Scholar] [CrossRef]
  18. Li, J.; Gao, M.; Yan, W.; Yu, J. Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties. Chem. Sci. 2023, 14, 1935–1959. [Google Scholar] [CrossRef]
  19. Mercedes, B.; Corma, A. What Is Measured When Measuring Acidity in Zeolites with Probe Molecules? ACS Catal. 2019, 9, 1539–1548. [Google Scholar]
  20. Palčić, A.; Valtchev, V. Analysis and control of acid sites in zeolites. Appl. Catal. A Gen. Int. J. Devoted Catal. Sci. Its Appl. 2020, 606, 117795. [Google Scholar] [CrossRef]
  21. Xiong, Z.; Qi, G.; Bai, L.; Zhan, E.; Chu, Y.; Xu, J.; Ta, N.; Hao, A.; Deng, F.; Shen, W. Preferential population of Al atoms at the T4 site of ZSM-35 for the carbonylation of dimethylether. Catal. Sci. Technol. 2022, 12, 4993–4997. [Google Scholar] [CrossRef]
  22. Bohinc, R.; Hoszowska, J.; Dousse, J.-C.; Błachucki, W.; Zeeshan, F.; Kayser, Y.; Nachtegaal, M.; Pinar, A.B.; van Bokhoven, J.A. Distribution of aluminum over different T-sites in ferrierite zeolites studied with aluminum valence to core X-ray emission spectroscopy. Phys. Chem. Chem. Phys. 2017, 19, 29271–29277. [Google Scholar] [CrossRef] [PubMed]
  23. Pinar, A.B.; Verel, R.; Pérez-Pariente, J.; van Bokhoven, J.A. Direct evidence of the effect of synthesis conditions on aluminum sitting in zeolite ferrierite: A 27Al MQ MAS NMR study. Microporous Mesoporous Mater. 2014, 193, 111–114. [Google Scholar] [CrossRef]
  24. Pinar, A.B.; Gómez-Hortigüela, L.; McCusker, L.B.; Pérez-Pariente, J. Controlling the Aluminum Distribution in the Zeolite Ferrierite via the Organic Structure Directing Agent. Chem. Mater. 2013, 25, 3654–3661. [Google Scholar] [CrossRef]
  25. Márquez-Alvarez, C.; Pinar, A.B.; García, R.; Grande-Casas, M.; Pérez-Pariente, J. Influence of Al distribution and defects concentration of ferrierite catalysts synthesised from na-free gels in the skeletal isomerisation of n-Butene. Top. Catal. 2009, 52, 1281–1291. [Google Scholar] [CrossRef]
  26. Román-Leshkov, Y.; Moliner, M.; Davis, M.E. Impact of Controlling the Site Distribution of Al Atoms on Catalytic Properties in Ferrierite-Type Zeolites. J. Phys. Chem. C 2011, 115, 1096–1102. [Google Scholar] [CrossRef]
  27. Chu, W.; Liu, X.; Yang, Z.; Nakata, H.; Tan, X.; Liu, X.; Xu, L.; Guo, P.; Li, X.; Zhu, X. Constrained Al sites in FER-type zeolites. Chin. J. Catal. 2021, 42, 2078–2087. [Google Scholar] [CrossRef]
  28. Kim, S.; Park, G.; Woo, M.H.; Kwak, G.; Kim, S.K. Control of hierarchical structure and framework-Al distribution of ZSM-5 via adjusting crystallization temperature and their effects on methanol conversion. ACS Catal. 2019, 9, 2880–2892. [Google Scholar] [CrossRef]
  29. Gábová, V.; Dědeček, J.; Čejka, J. Control of Al distribution in ZSM-5 by conditions of zeolite synthesis. J. Chem. Commun. 2003, 10, 1196–1197. [Google Scholar] [CrossRef]
  30. Nishitoba, T.; Yoshida, N.; Kondo, J.N.; Yokoi, T. Control of Al distribution in the CHA-type aluminosilicate zeolites and its impact on the hydrothermal stability and catalytic properties. Ind. Eng. Chem. Res 2018, 57, 3914–3922. [Google Scholar] [CrossRef]
  31. Heard, C.J.; Grajciar, L.; Nachtigall, P. The effect of water on the validity of Lwenstein’s rule. Chem. Sci. 2019, 10, 5705–5711. [Google Scholar] [CrossRef] [PubMed]
  32. Di Iorio, J.R.; Gounder, R. Controlling the isolation and pairing of aluminum in Chabazite zeolites using mixtures of organic and inorganic structure-directing agents. Chem. Mater. 2016, 28, 2236–2247. [Google Scholar] [CrossRef]
  33. Di Iorio, J.R.; Li, S.; Jones, C.B.; Nimlos, C.T.; Wang, Y.; Kunkes, E.; Vattipalli, V.; Prasad, S.; Moini, A.; Schneider, W.F.; et al. Cooperative and competitive occlusion of organic and inorganic structure directing agents within Chabazite zeolites influences their aluminum arrangement. J. Am. Chem. Soc. 2020, 142, 4807–4819. [Google Scholar] [CrossRef]
  34. Fletcher, R.E.; Ling, S.; Slater, B. Violations of Löwensteins rule in zeolites. Chem Sci 2017, 8, 7483–7491. [Google Scholar] [CrossRef]
  35. Inagaki, S.; Yamada, N.; Nishii, M.; Nishi, Y.; Kubota, Y. Control of framework Al distribution in ZSM-5 zeolite via post-synthetic TiCl4 treatment. Microporous Mesoporous Mater. 2020, 302, 1102231–1102239. [Google Scholar] [CrossRef]
  36. Vjunov, A.; Fulton, J.L.; Huthwelker, T.; Pin, S.; Mei, D.; Schenter, G.K.; Govind, N.; Camaioni, D.M.; Hu, J.Z.; Lercher, J.A. Quantitatively Probing the Al Distribution in Zeolites. Am. Chem. Soc. 2014, 136, 8296–8306. [Google Scholar] [CrossRef]
  37. Holzinger, J.; Beato, P.; Lundegaard, L.F.; Skibsted, J. Distribution of Aluminum over the tetrahedral sites in ZSM-5 zeolites and their evolution after steam treatment. J. Phys.Chem. C Nanomater. Interfaces 2018, 122, 15595–15613. [Google Scholar] [CrossRef]
  38. Meng, X.J.; Xiao, F.S. Green routes for synthesis of zeolites. Chem. Reviews. 2014, 114, 1521–1543. [Google Scholar] [CrossRef]
  39. Ye, Z.; Zhang, H.; Zhang, Y.; Tang, Y. Seed-induced synthesis of functional MFI zeolite materials: Method development, crystallization mechanisms, and catalytic properties. Front. Chem. Sci. Eng. 2020, 14, 143–158. [Google Scholar] [CrossRef]
  40. Zhang, H.; Zhao, Y.; Zhang, H.; Wang, P.; Shi, Z.; Mao, J.; Zhang, Y.; Tang, Y. Tailoring zeolite ZSM-5 crystal morphology porosity through flexible utilization of Silicalite-1 seeds as templates: Unusual crystallization pathways in a heterogeneous system. Chem. A Eur. J. 2016, 22, 7141–7151. [Google Scholar] [CrossRef]
  41. Ji, Y.; Wang, Y.; Xie, B.; Xiao, F.S. Zeolite Seeds: Third Type of Structure Directing Agents in the Synthesis of Zeolites. Comments Inorg. Chem. 2016, 36, 1–16. [Google Scholar] [CrossRef]
  42. Zhang, H.; Guo, Q.; Ren, L.; Yang, C.; Zhu, L.; Meng, X.; Li, C.; Xiao, F.-S. Organotemplate-free synthesis of high-silica ferrierite zeolite induced by CDO-structure zeolite building units. J. Mater. Chem. 2011, 21, 9494–9497. [Google Scholar] [CrossRef]
  43. Ham, H.; Jung, H.S.; Kim, H.S.; Kim, J.; Cho, S.J.; Lee, W.B.; Park, M.J.; Bae, J.W. Gas-phase carbonylation of dimethyl ether on the stable seed derived Ferrierite. ACS Catal 2020, 10, 5135–5146. [Google Scholar] [CrossRef]
  44. Kim, J.; Ham, H.; Jung, H.S.; Wang, Y.; He, Y.; Tsubaki, N.; Cho, S.J.; Han, G.Y.; Bae, J.W. Dimethyl ether carbonylation to methyl acetate over highly crystalline zeolite-seed derived Ferrierite. Catal. Sci. Technol. 2018, 8, 3060–3072. [Google Scholar] [CrossRef]
  45. Kwak, S.J.; Kim, H.S.; Park, N.; Park, M.J.; Lee, W.B. Recent progress on Al distribution over zeolite frameworks: Linking theories and experiments. Korean J. Chem. Eng. 2021, 38, 1117–1128. [Google Scholar] [CrossRef]
  46. Emeis, C.A. Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts. J. Catal. 1993, 141, 347–354. [Google Scholar] [CrossRef]
  47. Guo, Y.; Wang, S.; Geng, R.; Wang, P.; Li, S.; Dong, M.; Qin, Z.; Wang, J.; Fan, W. Enhancement of the dimethyl ether carbonylation activation via regulating acid sites distribution in FER zeolite framework. iScience 2023, 26, 107748. [Google Scholar] [CrossRef]
  48. Xue, T.; Li, S.S.; Wu, H.H. Surfactant-promoted synthesis of hierarchical zeolite ferrierite nano-sheets. Microporous Mesoporous Mater. 2021, 312, 1107481–1107488. [Google Scholar] [CrossRef]
  49. Xie, S.; Peng, J.; Xu, L.; Wu, Z.; Wang, Q. Synthesis of ZSM-35 zeolite using cyclohexylamine as organic template and its catalytic performance. Chin. J. Catal. 2003, 24, 531–534. [Google Scholar]
  50. Houzvicka, J.; Nienhuis, J.G.; Ponec, V. The role of the acid strength of the catalysts in the skeletal isomerisation of n-Butene. Appl. Catal. A Gen. 1998, 174, 207–212. [Google Scholar] [CrossRef]
  51. Xu, W.Q.; Yin, Y.G.; Suib, S.L.; Edwards, J.C.; O’Young, C.L. Modification of non-template synthesized ferrierite/ZSM-35 for n-Butene skeletal isomerization to isobutylene. J. Catal. 1996, 163, 232–244. [Google Scholar] [CrossRef]
  52. Comelli, R.A. Skeletal Isomerization of Linear Butenes on Boron Promoted Ferrierite: Effect of the Catalyst Preparation Technique. Catal. Lett. 2008, 122, 302–309. [Google Scholar] [CrossRef]
Catalysts 14 00697 g001
Figure 1. XRD patterns of various FER-X zeolites.
Figure 1. XRD patterns of various FER-X zeolites.
Catalysts 14 00697 g001
Catalysts 14 00697 g002
Figure 2. SEM images of various FER-X zeolites. (a) FER-PY, (b) FER-SN, (c) FER-SH, (d) FER-PY+SN, (e) FER-PY+SH.
Figure 2. SEM images of various FER-X zeolites. (a) FER-PY, (b) FER-SN, (c) FER-SH, (d) FER-PY+SN, (e) FER-PY+SH.
Catalysts 14 00697 g002
Catalysts 14 00697 g003aCatalysts 14 00697 g003b
Figure 3. NH3-TPD and Py-IR profiles of various FER-X zeolites. (a) NH3-TPD profiles of FER-X zeolites, (b) Py-IR profiles of FER-PY, (c) Py-IR profiles of FER-SN, (d) Py-IR profiles of FER-SH, (e) Py-IR profiles of FER-PY+SN, (f) Py-IR profiles of FER-PY+SH.
Figure 3. NH3-TPD and Py-IR profiles of various FER-X zeolites. (a) NH3-TPD profiles of FER-X zeolites, (b) Py-IR profiles of FER-PY, (c) Py-IR profiles of FER-SN, (d) Py-IR profiles of FER-SH, (e) Py-IR profiles of FER-PY+SN, (f) Py-IR profiles of FER-PY+SH.
Catalysts 14 00697 g003aCatalysts 14 00697 g003b
Catalysts 14 00697 g004
Figure 4. 29Si MAS NMR spectra of various FER-X zeolites.
Figure 4. 29Si MAS NMR spectra of various FER-X zeolites.
Catalysts 14 00697 g004
Catalysts 14 00697 g005
Figure 5. 27Al MAS NMR spectra of various FER-X zeolites and Al distribution obtained from the curve deconvolved of FER-X zeolites. (a) 27Al MAS NMR spectra of FER-X zeolites, (b) deconvolved of FER-PY, (c) deconvolved of FER-SN, (d) deconvolved of FER-SH, (e) deconvolved of FER-PY+SN, (f) deconvolved of FER-PY+SH.
Figure 5. 27Al MAS NMR spectra of various FER-X zeolites and Al distribution obtained from the curve deconvolved of FER-X zeolites. (a) 27Al MAS NMR spectra of FER-X zeolites, (b) deconvolved of FER-PY, (c) deconvolved of FER-SN, (d) deconvolved of FER-SH, (e) deconvolved of FER-PY+SN, (f) deconvolved of FER-PY+SH.
Catalysts 14 00697 g005
Catalysts 14 00697 g006
Figure 6. The influence of the Al populations on the T1 + T3 sites on the B10-350 of various FER-X zeolites.
Figure 6. The influence of the Al populations on the T1 + T3 sites on the B10-350 of various FER-X zeolites.
Catalysts 14 00697 g006
Catalysts 14 00697 g007
Figure 7. Catalytic performance of various FER-X zeolites for skeletal isomerization of n-butene with post-MTBE C4 as feed. (a) n-butene conversion of FER-X zeolites within time on stream, (b) iso-butene selectivity of FER-X zeolites within time on stream. Reaction conditions: T = 350 °C; p = 0.1 MPa; WHSVn-butene = 2.0 h−1.
Figure 7. Catalytic performance of various FER-X zeolites for skeletal isomerization of n-butene with post-MTBE C4 as feed. (a) n-butene conversion of FER-X zeolites within time on stream, (b) iso-butene selectivity of FER-X zeolites within time on stream. Reaction conditions: T = 350 °C; p = 0.1 MPa; WHSVn-butene = 2.0 h−1.
Catalysts 14 00697 g007
Catalysts 14 00697 g008
Figure 8. Relationship between yield of isobutene and Al populations on the T1 + T3 sites.
Figure 8. Relationship between yield of isobutene and Al populations on the T1 + T3 sites.
Catalysts 14 00697 g008
Catalysts 14 00697 g009
Figure 9. Stability of FER-PY+SH for skeletal isomerization of n-butene. Reaction conditions: T = 350 °C; p = 0.1 MPa; WHSVn-butene = 2.0 h−1.
Figure 9. Stability of FER-PY+SH for skeletal isomerization of n-butene. Reaction conditions: T = 350 °C; p = 0.1 MPa; WHSVn-butene = 2.0 h−1.
Catalysts 14 00697 g009
Table 1. Textural properties of various FER-X zeolites.
Table 1. Textural properties of various FER-X zeolites.
SampleNa2O
(mg/g)		SARsRC (%)Surface Area (m2/g)Pore Volume (cm3/g)
TotalMicroTotalMicro
FER-PY4025.61004313880.270.14
FER-SN3925.31014283860.270.14
FER-SH3624.6994223800.260.13
FER-PY+SN4225.31004303780.280.14
FER-PY+SH4124.81014323900.280.15
Table 2. Amounts of acid in the various FER-X samples.
Table 2. Amounts of acid in the various FER-X samples.
SampleAcidity by NH3-TPD a
(mmol/g)		Acidity by Py-IR b
(mmol/g)
WeakStrongTotalBrønstedLewisTotalB10-350 c
FER-PY0.790.561.350.290.030.320.20
FER-SN0.780.551.330.280.030.310.24
FER-SH0.750.531.280.280.030.310.25
FER-PY+SN0.760.521.280.300.030.330.28
FER-PY+SH0.810.561.370.340.020.350.32
a Acid sites on FER-X and their amounts were measured by NH3-TPD analysis, and the quantities of weak and strong acid sites were calculated by using an integrated area at the maximum desorption temperatures of NH3 at 195 °C and 450 °C, respectively. b Amounts of BAS and LAS were measured by Py-IR spectroscopy at a desorption temperature of 150 °C and 350 °C, and the number of acid sites was calculated using the well-known formula of BAS = 1.88ABR2/W, LAS = 1.42ALR2/W, where AB and AL are the integrated absorbance peak areas of BAS and LAS located at 1550 and 1450 cm−1, respectively, R is the radius of a catalyst pellet of FER-X, cm, and W is the weight of catalyst, g [46]. c The number of BAS located in 10-MR at 350 °C was calculated by conducting Py-IR measurements at a desorption temperature of 350 °C.
Table 3. Aluminum distribution obtained from the curve fitting of 27Al MAS NMR spectra of various FER-X zeolites.
Table 3. Aluminum distribution obtained from the curve fitting of 27Al MAS NMR spectra of various FER-X zeolites.
SampleAluminum Distribution (%)
T1T2T3T4T1 + T3
FER-PY030303930
FER-SN819334041
FER-SH1025343144
FER-PY+SN1221382950
FER-PY+SH615542560
Table 4. Composition of post-MTBE C4 from refinery.
Table 4. Composition of post-MTBE C4 from refinery.
Compositionwt/%
propane0.12
propylene0.15
isobutane52.86
butane10.94
trans-2-butene13.76
1-butene12.00
isobutene0.26
cis-2-butene9.91
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Fan, J.; Zhu, X.; Yang, F.; Xu, Y.; Chen, L. Seeds Combining Pyrrolidine Control the Framework Al Distribution of FER Zeolite to Enhance Its Performance in the Skeletal Isomerization of n-Butene. Catalysts 2024, 14, 697. https://doi.org/10.3390/catal14100697

AMA Style

Fan J, Zhu X, Yang F, Xu Y, Chen L. Seeds Combining Pyrrolidine Control the Framework Al Distribution of FER Zeolite to Enhance Its Performance in the Skeletal Isomerization of n-Butene. Catalysts. 2024; 14(10):697. https://doi.org/10.3390/catal14100697

Chicago/Turabian Style

Fan, Jinlong, Xuedong Zhu, Fan Yang, Yarong Xu, and Lantian Chen. 2024. "Seeds Combining Pyrrolidine Control the Framework Al Distribution of FER Zeolite to Enhance Its Performance in the Skeletal Isomerization of n-Butene" Catalysts 14, no. 10: 697. https://doi.org/10.3390/catal14100697

APA Style

Fan, J., Zhu, X., Yang, F., Xu, Y., & Chen, L. (2024). Seeds Combining Pyrrolidine Control the Framework Al Distribution of FER Zeolite to Enhance Its Performance in the Skeletal Isomerization of n-Butene. Catalysts, 14(10), 697. https://doi.org/10.3390/catal14100697

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop