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Phosphorus-modified b-axis oriented hierarchical ZSM-5 zeolites for enhancing catalytic performance in a methanol to propylene reaction
Applied Catalysis A, General 594 (2020) 117464
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Phosphorus-modified b-axis oriented hierarchical ZSM-5 zeolites for enhancing catalytic performance in a methanol to propylene reaction
T
Rui Fenga,*, Xinlong Yana, Xiaoyan Hua, Yixin Zhanga, Jianjun Wua, Zifeng Yanb a Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering, China University of Mining & Technology, Xuzhou, 221116, Jiangsu, China b State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZSM-5 zeolite B-axis orientation Hierarchical structure Phosphorus modification MTP reaction
In this work, a b-axis oriented hierarchical ZSM-5 zeolite was prepared with glucose as an additive and further modified by phosphorus to improve the propylene selectivity and prolong the lifetime of the catalyst in a methanol to propylene (MTP) reaction. The physiochemical properties were characterized by XRD, low-temperature N2 sorption, SEM, TEM, XPS, solid-state NMR, FTIR, and NH3-TPD analyses. The b-axis oriented ZSM-5 (Z5C) zeolite exhibited a microsized hexagonal lamellar structure with a thickness of approximately 220 nm. Analyzing the MTP reactions revealed that the newly prepared Z5-C catalyst exhibited a higher propylene selectivity of 53.01 % and a longer lifetime of 45 h compared to those of a conventional nanosized ZSM-5 catalyst (propylene selectivity of 43.97 % and lifetime of 10 h). In addition, the phosphorus modification remarkably increased the lifetime of the b-axis-oriented ZSM-5 catalyst to 79 h.
1. Introduction Methanol to hydrocarbon (MTH) conversion reactions have received wide attention due to the extensive sources of raw materials, such as coal and renewable biomass, as a replacement for conventional crude oil [1]. With the ever increasing demands of light olefins, including ethylene, propylene, and butene, the methanol to propylene (MTP) reaction is considered a promising alternative approach for propylene production to fill the demand gap from steam cracking naphtha [2]. ZSM-5-based catalysts, which possess a unique tri-dimensional pore structure and surface acidity, have been successfully applied in a commercial MTP process by Lurgi company [3]. However, improving the propylene selectivity and prolonging the catalytic lifetime of these ZSM-5 catalysts are the major goals for MTP technology at present [4–6]. To date, fabricating hierarchical structures, modifying crystal morphology and size, and adjusting the acid properties of ZSM-5 zeolites are the main strategies to improve the catalytic performance in MTP reactions [7–11]. It is recognized that microporous ZSM-5 with auxiliary mesopores and nanometer-sized crystals are beneficial for shortening the path length of molecular diffusion and enhancing accessibility to the internal surface acid sites. For instance, Li et al. [12] prepared an interconnected hierarchical ZSM-5 by a post dealumination-realumination method to improve the lifetime and propylene selectivity ⁎
of the MTP catalyst. For crystal sizes, it is not always a benefit toward improving the performance of the MTP reaction if the crystal sizes of ZSM-5 zeolite become too small [13–15]. Light olefins and coking precursors are more likely to adsorb on the acid sites of the highly exposed external surface of these nanosized crystals, although the catalytic lifetime increases in most cases [16–18]. In addition, small crystals decrease the shape-selective effect of micropores and lower propylene selectivity [19]. For example, Ryoo’s group synthesized ZSM-5 nanosheets with a thickness of 2 nm along the b-axis dimension, and the large number of acid sites on the external surface on these zeolites suppressed catalyst deactivation during a methanol to gasoline (MTG) conversion [20,21]. However, these acid sites on the external surface of nanosheets contributed little to improving the MTH conversion, as reported by Kim et al. [22]. Sugimoto et al. [13] found that the propylene selectivity over microsized ZSM-5 with large crystals of 3−4 μm was 8.1 % higher than that on nanosized ZSM-5 with crystal sizes less than 200 nm. A recent study by Wu et al. [14] also showed that ZSM-5 zeolite with an appropriate crystal size of approximately 150 nm promoted propylene selectivity and extended the catalytic lifetime compared with those of ZSM-5 zeolites with other crystal sizes. Recent studies have indicated that propylene is primarily produced in nonintersecting straight channels parallel to the b-axis of ZSM-5 zeolite, where an alkene-based cycle is active [23–25]. However, the
Corresponding author. E-mail address: [email protected] (R. Feng).
https://doi.org/10.1016/j.apcata.2020.117464 Received 10 December 2019; Received in revised form 24 January 2020; Accepted 10 February 2020 Available online 11 February 2020 0926-860X/ © 2020 Elsevier B.V. All rights reserved.
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2.3. Characterization
relatively long diffusion path of zig-zag channels parallel to the a-axis contributes more to deactivation [18,26]. Thus, an innovative solution is to design a microsized ZSM-5 zeolite that is thin along the b-axis direction; in this case, both shortening the diffusion path and strengthening the shape-selective effect of the straight channels are considered. For example, Xiao’s group reported a one-pot growth of TS1 crystals with controllable b-oriented lengths, which were synthesized with the aid of urea as an organic additive and a fluorinated surfactant [27]. Zhang et al. [28] synthesized a hexagonal lamellar ZSM-5 zeolite with a thin b-axis dimension of less than 450 nm under a neutral fluoride medium, and this catalyst performed with relatively high propylene selectivity (45.1 %) and a long lifetime in the MTP reaction. In this work, we demonstrate a facile hydrothermal crystallization method to synthesize a microsized b-axis oriented ZSM-5 zeolite (Z5-C) in the presence of environmentally friendly glucose. Z5-C exhibits superior catalytic performance in the MTP reaction with much higher propylene selectivity and a longer catalytic lifetime than conventional nanosized ZSM-5 (Z5-A). In addition, a phosphorus modification further prolongs the catalytic lifetime of the b-axis-oriented ZSM-5 zeolite (Z5CP2). The high selectivity for propylene and extended lifetime of Z5CP2 were related to its thinness along the b-axis direction, hierarchical structure, and appropriate surface acidity. Therefore, our work provides a new route for the facile synthesis of highly efficient MTP catalysts in the future.
Crystalline structure was measured by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (k = 0.15406 nm), operated at 40 kV and 30 mA and scanned from 5° to 75°. The relative crystallinity of ZSM-5 zeolites was calculated by comparing the sum of five characteristic peak areas at 2θ = 7°-25°. To investigate the zeolite morphologies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were measured on a Quanta 250 instrument and FEI Technai G2 F20 instrument, respectively. Low-temperature (−196 °C) nitrogen-sorption isotherms were obtained on a Micrometric Tristar 3000 analyzer to characterize the textural properties of the samples. Prior to measurement, the samples were degassed at 300 °C for 3 h. The Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method for the desorption branch of the isotherms were used to determine the specific surface area and mesopore size distribution, respectively. X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher 250Xi analyzer was used to measure the surface elemental composition of the zeolites. Solid-state 27Al and 29Si NMR analyses were performed on an Advance III HD 600 MHz instrument at a spectrometer frequency of 119.2 MHz, using a Bruker 4.0 mm double-resonance MAS detector to acquire 48 kHz MAS spectra. The framework silicon-to-alumina (SiO2/ Al2O3) ratio was derived from the resolved 29Si NMR spectra. To check the acid strength of the samples, temperature-programmed desorption of ammonia (NH3-TPD) measurements were performed at temperatures of 100−800 °C at a rate of 10 °C/min on a Quanta Chrome ChemStar™ instrument. Fourier transform infrared (FT-IR) analysis with a Nicolet iS5 (Thermo Co.) was used to characterize the acid sites using pyridine as the probe molecule, and the acid amounts were estimated using a semi-quantitative calculation proposed by Emeis et al. [29].
2. Experimental section 2.1. Raw materials In this study, raw materials, including tetraethyl orthosilicate (TEOS, AR), aluminum isopropoxide (AIP, AR), sodium hydroxide (NaOH, CP), tetrapropylammonium hydroxide aqueous solution (TPAOH, 25 wt.%), glucose (AR), and ammonium phosphate (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China, Shanghai) and used without any further purification.
2.4. Catalyst evaluation in the MTP reaction The MTP reactions were performed in a fixed-bed reactor with an inner diameter of 6 mm and operating at 470 °C and atmospheric pressure. For each test, 0.10 g of H-form ZSM-5 catalyst with a particle size of 40–60 mesh was loaded in the flat-temperature zone of the reactor. The methanol was pumped into the reactor using 50 mL/min of argon as the carrier gas, and the weight hourly space velocity (WHSV) was set at 4.0 h−1. The products were analyzed by an online gas chromatograph (GC 2014C, Shimadzu) with a flame ionization detector (FID) and a capillary column (TG-BONG Q). The amount of coke on the spent catalysts was measured by thermogravimetric (TG) analysis on a Netzch STA449F5 instrument. Typically, the TG curve was recorded as the catalyst (10 mg) was heated from 30 °C to 850 °C in air with a heating rate of 10 °C/min. The methanol conversion and selectivity to the i product were defined as follows:
2.2. Synthesis of the b-axis oriented ZSM-5 zeolite The b-axis oriented ZSM-5 zeolite and its counterpart ZSM-5 sample were synthesized using the following chemicals as raw materials. A typical synthesis of b-oriented ZSM-5 zeolite using glucose as an additive was performed as follows: TEOS, AIP, NaOH, TPAOH, deionized water, and glucose were mixed together at room temperature and magnetically stirred at a speed of 1000 rpm for 12 h to give a final mole ratio of 1.0 Al2O3:50 SiO2:2 Na2O:8 TPAOH:3000 H2O:x glucose (x = 0, 6, 12). The mixture was then transferred into a PTFE-lined stainlesssteel autoclave and precrystallized at 110 °C for 24 h and crystallized at 170 °C for 48 h. After crystallization, the obtained product was filtered, washed with deionized water, and dried overnight at 110 °C. By varying the amount of glucose, three samples were obtained and denoted as Z5A, Z5-B, and Z5-C, corresponding to samples with x = 0, 6, and 12, respectively. To obtain H-form zeolites, the as-synthesized samples in this work were calcined at 550 °C for 4 h with a heating rate of 3 °C/min to remove the organic template and then ion-exchanged twice in a solution of 1.0 mol/L NH4Cl at 80 °C for 4 h, followed by heating at 550 °C for 4 h. A phosphorus modification of b-oriented ZSM-5 zeolite (Z5-C) was carried out using a wet impregnation of ammonium phosphate. Typically, 1.0 g of Z5-C was impregnated with a (NH4)2HPO4 solution of 3.0 mL and then left standing at room temperature for 2 h. Next, the sample was dried at 110 °C overnight and calcined at 550 °C for 4 h with a heating rate of 3 °C/min. The phosphorus-modified samples Z5CPx (x = 1, 2, 3) were finally obtained with phosphorus increments of 1.5 wt.%, 2 wt.%, and 3 wt.%, respectively.
XMeOH = Si =
(mMeOH (in) − 1.39mDME (out ) − mMeOH (out ) ) mMeOH (in)
mi Σmi
where mi is the mass weight of compound i and Σ mi is the total mass weight of all products. The dimethyl ether (DME) product was regarded as two molecules of unconverted methanol. The mass balance was carefully controlled with an average error within 5%. 3. Results and discussion 3.1. Physiochemical properties of the as-synthesized ZSM-5 zeolites The XRD patterns of the as-synthesized samples in Fig. 1 show that 2
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Fig. 1. XRD patterns of the as-synthesized ZSM-5 zeolites and phosphorus-modified ZSM-5 zeolites. Table 1 Physiochemical properties of the as prepared ZSM-5 samples. Item
Z5-A Z5-B Z5-C Z5-CP1 Z5-CP2 Z5-CP3 a b c d
Relative crystallinity (%)
98.2 93.7 100.0 90.9 95.1 97.6
SiO2/Al2O3 molar ratio
85c 114c 136c 117d 121d 118d
Specific surface area (m2·g−1)
Pore volume (cm3·g−1)
SBETa
Smeso
Smicrob
Vpb
Vmeso
Vmicrob
414.9 336.8 320.4 302.5 287.3 277.2
127.0 147.0 184.2 140.0 122.9 121.5
287.9 189.8 136.2 162.5 164.4 155.7
0.236 0.204 0.299 0.271 0.257 0.233
0.120 0.125 0.239 0.211 0.204 0.181
0.116 0.079 0.060 0.060 0.053 0.052
Derived from the multipoint Brunauer-Emmett-Teller (BET) method. Derived from the t-plot method. Derived from the 29Si NMR spectra. Detected by ICP-OES method.
demonstrates the feature of the short b-axis for the Z5-C sample. As for glucose, various chemical reactions such as dehydration, condensation, or polymerization and aromatization can take place under the hydrothermal conditions, especially in the alkaline solution, leading to a complex mixture of organic compounds [35–37]. During the hydrothermal crystallization process, nucleation and subsequent crystal growth of ZSM-5 zeolites happen [38,39]. Simultaneously, the polyhydroxy saccharide species would preferentially adsorb on the (010) crystal surface by hydrogen-bond, van der Waals, or electrostatic interaction [39,40]. Thus, the further attachment of silica or growth units to crystal surface of (010) was reduced, thereby changing the growth rates along the three-dimensional directions and resulting in the b-axis oriented ZSM-5 zeolite. The XPS spectra in Fig. 3 display the surface elemental compositions of zeolites. Fig. 3b shows that compared to the conventional Z5 sample, the Al content on the surface of the b-axis oriented Z5-B shows little change. In contrast, the Al content on the surface of the b-axis-oriented Z5-C obviously decreases. The FTIR spectra in Fig. S1 also show that the concentrations of extraframework and framework Al species on the external surface of the Z5-C sample are much lower than those of the Z5-A sample. In contrast, there is little change in the intensity of the Si2p bands for the three samples; however, the band position of the Si atom on Z5-C shifts to a high binding energy, as shown in Fig. 3c. This is probably attributed to its low concentration of Al atoms on the surface, which have a smaller electronegativity (χ = 1.61) than that of Si atoms (χ = 1.90). The coordinate states of Al and Si atoms in ZSM-5 zeolites are revealed by the 27Al and 29 Si NMR spectra and shown in Fig. 4 and Fig. S2. In Fig. 4a, the peaks of the 27Al NMR spectra at 52.9 ppm and -2.7
all samples exhibit the characteristic peaks of ZSM-5 zeolite. These peaks appear at 2θ of 8.0°, 8.9°, 23.2°, 24.0°, and 24.5°, which correspond to (101), (020), (501), (151) and (303), respectively (JCPSD no. 44-0003) [30–32]. The partial increase of peaks at 8.0° and 8.9° shows that with the increase of glucose, the two peaks of Z5-C shift to higher 2θ positions, which is indicative of a higher framework silicon-to-alumina (SiO2/Al2O3) ratio compared to that of the Z5-A sample [33]. In contrast, the peaks shift to lower 2θ positions with increasing amounts of ammonium phosphate, indicating that the phosphorus atoms were introduced into the framework of the Z5-C sample and resulted in an increase in crystalline size [34]. The relative crystallinity (R.C.) results in Table 1 show that the relative crystallinities of phosphorus-modified Z5-C samples decreased slightly compared to that of the parent Z5-C. The morphologies of the as-synthesized ZSM-5 zeolites were detected by SEM and TEM analysis and are displayed in Fig. 2. In Fig. 2a, uniform structure of bulky zeolite particles with sizes centered at approximately 550 nm is observed for the conventional Z5-A sample. These bulky particles are wrapped with uniformly and closely packed small particles with sizes of approximately 50 ± 10 nm (see Figs. 2b and c). In contrast, the Z5-B sample in Fig. 2d–f possess a hexagonal lamellar structure with a particle size of approximately 3.5 μm × 1.4 μm × 420 nm, exhibiting thinness along the b-axis direction. In contrast, the Z5-C sample exhibits a similar b-axis oriented structure but a large particle size of approximately 5 μm × 2.1 μm × 220 nm (see Fig. 2g and h). It is noted that the thickness along the b-axis direction of the Z5-C sample is comparatively smaller than that of the Z5-B sample, indicating that the glucose additive decreases the crystal growth along the b-axis direction in the process of hydrothermal crystallization. In addition, the electron permeability in the TEM image (see Fig. 2i) also 3
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Fig. 2. Morphologies of the as-synthesized ZSM-5 zeolites: SEM images (a) & (b) and TEM image (c) of Z5-A; SEM images (d) & (e) and TEM image (f) of Z5-B; and SEM images (g) & (h) and TEM image of Z5-C. Note, the inset in Fig. 2a is the particle size distribution chart of the Z5-A sample.
Z5-CP3 indicated more octahedral Al due to the dealumination from zeolite caused by phosphorus modification [42]. The 31P NMR spectra in Fig. S2d shows that with increasing the phosphorus modification amount, the peak intensity of phosphorus species and the extraframework octahedral Al species gradually increased, emphasizing the fact that phosphorus modification has led to the dealumination of framework Al species. The 29Si NMR spectra in Fig. S2 of the three samples and their resolutions of overlapped peaks. All the spectra show similar features and can be resolved into eight resonances at approximately
ppm are attributed to framework tetracoordinated Al species and extraframework hexacoordinated Al species, respectively [41]. In addition, a small weak shoulder at 28.5 ppm is observed for the Z5-C sample, which is attributed to pentacoordinated Al species. It is obvious that the relative intensity of tetracoordinated Al follows the order of Z5C > Z5-A > Z5-B, which is consistent with the variation of relative crystallinity in Table 1. The weaker peak of octahedral Al shifted from -2.7 to −14 ppm after phosphorus modification, indicating the octahedral Al attached to phosphorus and the higher signal at −14 ppm of
Fig. 3. XPS spectra of the as-synthesized ZSM-5 zeolites: (a) full spectrum of Z5-A sample using C 1s at 284.8 eV for a calibration reference; (b) Al2p spectra; and (c) Si2p spectra. 4
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Fig. 4.
27
Al NMR spectra of the as-synthesized ZSM-5 zeolites and phosphorus modified Z5-C samples.
−98, −102, −106, −109, −114, −116, −119 and −122 ppm. Among them, the peak at approximately −98 corresponds to Q2(2Al), and the peak at approximately −102 ppm corresponds to Q3(0Al) on the external surface of the zeolites [41]. The peaks between −106 and −109 ppm are attributed to Q4(1Al), while other intense peaks from −114 to −122 ppm are attributed to Q4(0Al) [43]. Here, the resolved resonances are used to estimate the framework SiO2/Al2O3 ratios of the ZSM-5 zeolites using previously reported methods [31,43]. The calculated results gave framework SiO2/Al2O3 ratios of 85, 114, and 136 for Z5-A, Z5-B, and Z5-C, respectively; the trend of these values is similar to the XRD results. N2 sorption measurements (Fig. 5 and Table 1) show that Z5-A and Z5-B samples have a type-I isotherm with a nearly invisible hysteresis loop; in contrast, Z5-C possesses type IV sorption curves typical of mesoporous materials, according to the IUPAC classification. The hysteresis loop of Z5-C gradually decreases with increasing phosphorus content after modification, which is indicative of the decrease in mesopores. The pore size distributions in Figs. 5b and c show the coexistence of the intrinsic micropores of ZSM-5 and its mesopores, and the parent and phosphorus-modified Z5-C have a clear mesoporous structure in the range of 5−50 nm. The textural parameters in Table 1 show that with increasing amounts of glucose, the mesoporous surface area and pore volume for Z5-C increase to approximately 184.2 m2 g−1 and 0.239 cm3 g−1, respectively, which is higher than that of Z5-A (approximately 127.0 m2 g−1 and 0.120 cm3 g−1, respectively). However, the microporous surface area and pore volume of Z5-C decrease compared to that of Z5-A, probably due to a remarkable decrease in nanocrystals. The microporous surface areas of the phosphorus-modified Z5-C samples are larger than those of the parent Z5-C, probably due to the channel cleaning action of weakly alkaline ammonium phosphate. In addition, a micropore feature in the range from 0.50 nm to 0.75 nm
is observed in Fig. 5, corresponding to the microporous structure of an MFI zeolite, which possesses 2D sinusoidal channels (0.51 × 0.55 nm, a direction) crossed with 1D straight channels (0.53 × 0.56 nm, b direction) [23,44]. Pyridine adsorbed IR analysis and NH3-TPD analysis were conducted to measure the acid properties of ZSM-5 zeolites, and the calculated results are shown in Fig. 6 and Table 2. In Fig. 6a, the assignment of bands are similar to that of what we previously reported [45–47]. That is, the bands at 1448 and 1545 cm−1 are attributed to pyridine-adsorbed Lewis acid sites (LAS) and Brönsted acid sites (BAS), respectively. The acid amount results in Table 2 show that with increasing amounts of glucose, the amounts of Lewis-, Brönsted-, and total acid sites gradually decrease, and the phosphorus modification further decreases the acid amounts of the Z5-C samples. In comparison, the NH3-TPD curves in Fig. 6b display the existence of three types of acid sites: weak acid sites at ammonia desorption temperatures lower than 300 °C, moderately strong acid sites at approximately 300∼500 °C, and strong acid sites at temperatures higher than 500 °C. Obviously, the amount of total acid sites and strong acid sites decrease with increasing amounts of glucose, as shown in Table 2. Compared to the parent Z5-C sample, the phosphorus-modified ZSM-5 samples show a decrease in total acid sites and strong acid sites, which is similar to those reported by others [34,48,49]. Among the as-prepared samples, the Z5-CP2 sample with an appropriate phosphorus doping possesses the lowest amounts of strong acid sites and total acid sites, up to 126.3 μmol g-1 and 494.9 μmol g−1, respectively. 3.2. Catalytic performance of the MTP reaction A methanol to propylene (MTP) reaction was used to evaluate the catalytic performance of the as-synthesized ZSM-5 catalysts. Fig. 7 and
Fig. 5. N2 sorption curves (a) and pore width distributions (b) & (c) of the as-synthesized ZSM-5 samples. 5
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Fig. 6. Pyridine-adsorbed FT-IR spectra (a) and NH3 TPD curves (b) of the as-synthesized ZSM-5 zeolites.
propylene selectivity and light olefin selectivity of Z5-CPx catalysts gradually decreased with increasing amounts of phosphorus. Notably, compared to Z5-A, the selectivity of light alkanes (C2°-C4°) obviously decreased for the parent Z5-C and phosphorus-modified Z5-CPx catalysts. This result was probably attributed to the low surface acid density of the b-axis-oriented ZSM-5 catalysts (see Fig. 6 and Table 2), which decreased the hydrogen transfer activity of the light olefins to form light alkanes [51,52]. Similarly, the ethylene selectivities of phosphorus-modified Z5-CPx catalysts were much lower than those of Z5-A and Z5-C catalysts, resulting in a remarkable increase of propylene to ethylene (P/E) ratios, as shown in Fig. 7b. To the best of our knowledge, the Z5-CP3 catalyst possesses the highest observed P/E ratio of 14.3, which has never been documented before. As is known, olefin-based and aromatic-based cycles are the two cycles simultaneously running in the methanol to hydrocarbon (MTH) reaction over the ZSM-5 catalysts. The former produces ethylene, and the latter produces propylene and C3+ olefins [53,54]. On the one hand, shortening the distance of the straight channels along the b-axis direction and introducing an additional mesoporous structure reduced the residence time of polymethylbenzene in the micropores, which increased the ratio of olefin-based cycle reactions to aromatic-based cycle reactions and resulted in high propylene selectivity and low ethylene selectivity [19,51,52,55–57]. On the other hand, phosphorus doping brought about a tiny but not negligible reduction of the microporous channels and further increased the shape-selective effect of the straight channels [34]. In addition, reducing the acid density of the ZSM-5 catalysts prolonged the lifetimes of the various catalysts by reducing coke formation; however, the excessive phosphorus doping of Z5-CP3 had the opposite effect for prolonging the catalyst lifetime due to pore blockage and rising amounts of moderately and strong acid sites (see
Table 2 Surface acid properties of the as-synthesized ZSM-5 zeolites. Item
Z5-A Z5-B Z5-C Z5-CP1 Z5-CP2 Z5-CP3 a b
Surface acid amounta (μmol·g−1)
Surface acid amount (μmol·g−1)b
Total
BAS
LAS
Total
Weak
Moderate
Strong
230.5 218.5 186.4 191.4 165.2 168.7
156.0 148.5 130.3 136.8 126.2 133.2
74.5 70.0 56.1 54.6 39.0 35.5
658.9 588.1 572.9 539.5 494.9 540.7
184.6 188.0 180.2 175.1 188.4 196.7
224.1 211.4 209.8 191.6 180.2 188.3
250.2 188.7 182.9 172.8 126.3 155.7
Derived from pyridine-adsorbed FT-IR spectra in Fig. 6a. Derived from the NH3-TPD curve in Fig. 6b.
Table S1 show the average selectivities of the products and the lifetimes of the as-synthesized ZSM-5 catalysts in a continuous flow fixed-bed reactor at 470 °C. Here, the lifetime of the catalyst was defined as the reaction time until the methanol conversion was approximately 85 %. In Fig. 7a, it could be found that the microsized b-axis oriented ZSM-5 catalysts (Z5-B and Z5-C) presented higher selectivity to propylene and selectivity to light olefins (C2=-C4=) compared to the conventional nanosized ZSM-5 catalyst (Z5-A). However, it did not mean this microsized ZSM-5 had better performance in the MTP reaction than the smaller nanosized ZSM-5 with particle sizes less than 20 nm. For instance, Corma et al. reported that nanocrystalline ZSM-5 zeolite with particle sizes of approximately 15 nm exhibited a much larger catalyst lifetime and higher selectivity to propylene than nanosized ZSM-5 with particle sizes of approximately 200 nm in the MTP reaction, due to the fast mass transfer caused by short path lengths [50]. In contrast, the
Fig. 7. Average product selectivity (a) and catalyst lifetime (b) for MTP reactions over the ZSM-5 catalysts. The average values on the basis of mass weight were calculated before methanol conversions were less than 85 %. The lifetime of the catalyst was until the reaction decreased to ∼85 % methanol conversion. Reaction conditions: WHSV of methanol =4.0 h−1, 470 °C, 0.1 MPa, catalyst =0.10 g.
6
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Fig. 8. Conversion percentage of methanol and selectivity to light olefins for the MTP reactions vs. time on stream over the ZSM-5 catalysts: Z5-A (a); Z5-B (b); Z5-C (c); and Z5-CP2 (d). Reaction conditions: WHSV of methanol =4.0 h−1, 470 °C, 0.1 MPa, and catalyst =0.10 g.
time, the acid sites of the ZSM-5 catalysts were gradually covered by coking species, resulting in a decrease in the cracking activity of the C5+ products and their related propylene selectivity, as shown in Fig. S4. The thermogravimetry (TG) curves in Fig. 10 show that coke deposition should be responsible for the deactivation of the MTP catalyst and that the coke contents on the deactivated Z5-A, Z5-C, and Z5-CP2 catalysts were 12.68 wt.%, 7.18 wt.%, and 2.78 wt.%, respectively. In view of the lifetimes for the three catalysts in Fig. 8, the coking rate of the Z5-CP2 catalyst was even lower than that of the Z5-A catalyst. As shown in Fig. S5, it could be attributed to the synergistic effect of the fast mass transfer benefiting from the thinness along the b-axis direction and hierarchical structure, the relatively high cracking activity of the C5+ products catalyzed by acid sites of appropriate strength, and the low coking rate owing to the low acid site density.
Table 1 and Table 2). Among the as-prepared samples, the Z5-CP2 catalyst had better catalytic performance with a catalyst lifetime of 79 h, while it performed with a relatively high P/E ratio of 8.6 and a high propylene selectivity of 49.94 %. Fig. 8 and Figs. S3 and S4 comparatively show the methanol conversion and selectivities of light olefins as a function of time on stream for the MTP reaction over the ZSM-5 catalysts. All catalysts exhibited a similarly high conversion of methanol (> 99 %) in the initial stage of the reaction that continued for some time. This was then followed by an attenuation with increasing reaction time. Fig. 8 shows that propylene is the primary product of the light olefins over the Z5-A, Z5-B, Z5-C, and Z5-CP3 catalysts, which is consistent with the dominance of the olefinbased cycle reactions over the ZSM-5 catalysts. Thus, the variation trend of selectivities to light olefins was similar to that of propylene selectivity; in contrast, the selectivities to ethylene and butene varied slightly with time on stream. It was interesting and significant that the ethylene selectivity over Z5-CP3 was less than 5 wt.% during the catalytic lifetime cycle when propylene was regarded as a target product (see Figs. 8d and S4). Fig. 9 shows that the methane selectivity and C5+ product selectivity over the four catalysts gradually increased with time on stream. Compared to that of the conventional nanosized Z5-A, the baxis oriented catalysts had a much lower increasing rate of methane selectivity as a function of time on stream. In contrast, the increasing rate of the C5+ product selectivity on all catalysts showed little change despite the differences in the C5+ product selectivity. In addition, the results in Fig. S4c show that the selectivities of the C5+ products over the Z5-C and Z5-CPx catalysts at the same reaction time followed the order of Z5-CP3 > Z5-CP2 > Z5-CP1≈ Z5-C. In view of the gradual decrease of surface acid density with a phosphorus modification, the cracking rate of C5+ products into light olefins decreased, resulting in a low propylene selectivity (Fig. 7a). Similarly, with an extended reaction
4. Conclusion Microsized b-axis oriented hierarchical ZSM-5 zeolites with high crystallinity, hexagonal lamellar structures with a thickness of 220 nm along the b-axis direction, and low concentrations of acid sites were synthesized through hydrothermal crystallization in the presence of glucose. In addition, a phosphorus modification could further decrease the acid amounts and acid strength of the b-axis oriented ZSM-5 zeolite. The catalytic performance of the parent- and phosphorus-modified baxis-oriented ZSM-5 was evaluated by the methanol to propylene (MTP) reaction and compared with that of the conventional nanosized ZSM-5 (Z5-A). Significant improvements of enhanced propylene selectivity, prolonged catalytic lifetime, and superior coke toleration suggested baxis oriented ZSM-5 to be a candidate catalyst for the MTP process due to its thinness along the b-axis direction, hierarchical structure, and appropriate surface acidity. In particular, the sample of the b-axis 7
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Fig. 9. Methane selectivity (a) and C5+ selectivity (b) for the MTP reactions vs. time on stream over the ZSM-5 catalysts: Z5-A (a); Z5-B (b); Z5-C (c); and Z5-CP2 (d). Reaction conditions: WHSV of methanol =4.0 h−1, 470 °C, 0.1 MPa, and catalyst =0.10 g.
of Jiangsu Province (No.BK20190625), the China Postdoctoral Science Foundation (No.2018M642363), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2020.117464. References [1] P. Tian, Y. Wei, M. Ye, Z. Liu, ACS Catal. 5 (2015) 1922–1938. [2] M. Khanmohammadi, S. Amani, A.B. Garmarudi, A. Niaei, Chin. J. Catal. 37 (2016) 325–339. [3] H. Koempel, W. Liebner, Lurgi’s Methanol To Propylene (MTP®) Report on a successful commercialisation, F. Bellot Noronha, M. Schmal, E. Falabella Sousa-Aguiar (Eds.) Stud. Surf. Sci. Catal. Elsevier, 2007, pp. 261–267. [4] I. Yarulina, K. De Wispelaere, S. Bailleul, J. Goetze, M. Radersma, E. Abou-Hamad, I. Vollmer, M. Goesten, B. Mezari, E.J.M. Hensen, J.S. Martínez-Espín, M. Morten, S. Mitchell, J. Perez-Ramirez, U. Olsbye, B.M. Weckhuysen, V. Van Speybroeck, F. Kapteijn, J. Gascon, Nat. Chem. 10 (2018) 804–812. [5] T. Zhao, Y. Wang, C. Sun, A. Zhao, C. Wang, X. Zhang, J. Zhao, Z. Wang, J. Lu, S. Wu, W. Liu, Microporous Mesoporous Mater. 292 (2020) 109731. [6] J. Ding, Y. Jia, P. Chen, G. Zhao, Y. Liu, Y. Lu, Chem. Eng. J. 361 (2019) 588–598. [7] R.T. Carr, M. Neurock, J. Catal. 278 (2011) 78–93. [8] Y. Ono, T. Mori, J. Chem. Soc., Faraday Trans. 1 (77) (1981) 2209–2221. [9] A.G. Gayubo, A. Alonso, B. Valle, A.T. Aguayo, M. Olazar, J. Bilbao, Chem. Eng. J. 167 (2011) 262–277. [10] S. Müller, Y. Liu, M. Vishnuvarthan, X. Sun, A.C. van Veen, G.L. Haller, M. SanchezSanchez, J.A. Lercher, J. Catal. 325 (2015) 48–59. [11] J. Zhang, H. Zhang, X. Yang, Z. Huang, W. Cao, J. Nat. Gas Chem. 20 (2011) 266–270. [12] J. Li, M. Liu, X. Guo, S. Xu, Y. Wei, Z. Liu, C. Song, ACS Appl. Mater. Interf. 9 (2017) 26096–26106. [13] M. Sugimoto, H. Katsuno, K. Takatsu, N. Kawata, Zeolites 7 (1987) 503–507. [14] Z. Wu, K. Zhao, Y. Zhang, T. Pan, S. Ge, Y. Ju, T. Li, T. Dou, Ind. Eng. Chem. Res. 58 (2019) 10737–10749. [15] R. Feng, X. Wang, J. Lin, Z. Li, K. Hou, X. Yan, X. Hu, Z. Yan, M.J. Rood, Microporous Mesoporous Mater. 270 (2018) 57–66. [16] J. Li, Y. Wei, G. Liu, Y. Qi, P. Tian, B. Li, Y. He, Z. Liu, Catal. Today 171 (2011) 221–228. [17] K. Barbera, F. Bonino, S. Bordiga, T.V.W. Janssens, P. Beato, J. Catal. 280 (2011) 196–205. [18] D. Cai, N. Wang, X. Chen, Y. Ma, Y. Hou, X. Li, C. Zhang, Z. Chen, W. Song, M.T. Arslan, Y. Li, Y. Wang, W. Qian, F. Wei, Nanoscale 11 (2019) 8096–8101. [19] R. Khare, D. Millar, A. Bhan, J. Catal. 321 (2015) 23–31. [20] M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, R. Ryoo, Nature 461 (2009) 246. [21] K. Na, M. Choi, W. Park, Y. Sakamoto, O. Terasaki, R. Ryoo, J. Am. Chem. Soc. 132 (2010) 4169–4177. [22] W. Kim, R. Ryoo, Catal. Lett. 144 (2014) 1164–1169. [23] T. Liang, J. Chen, Z. Qin, J. Li, P. Wang, S. Wang, G. Wang, M. Dong, W. Fan, J. Wang, ACS Catal. 6 (2016) 7311–7325. [24] C. Wang, Q. Wang, J. Xu, G. Qi, P. Gao, W. Wang, Y. Zou, N. Feng, X. Liu, F. Deng, Angew. Chem. Int. Ed. 55 (2016) 2507–2511. [25] C. Li, A. Vidal-Moya, P.J. Miguel, J. Dedecek, M. Boronat, A. Corma, ACS Catal. 8 (2018) 7688–7697. [26] N. Wang, W. Sun, Y. Hou, B. Ge, L. Hu, J. Nie, W. Qian, F. Wei, J. Catal. 360 (2018) 89–96. [27] Z. Shan, H. Wang, X. Meng, S. Liu, L. Wang, C. Wang, F. Li, J.P. Lewis, F.-S. Xiao,
Fig. 10. TG curves of the three spent ZSM-5 catalysts after the MTP reactions at 470 °C.
oriented ZSM-5 (Z5-C) catalyst had a high propylene selectivity of 53.01 % and a long lifetime of 45 h compared with that of the conventional nanosized ZSM-5 (Z5-A) catalyst (propylene selectivity of 43.97 % and a lifetime of 10 h). The phosphorus-modified Z5-CP2 catalyst revealed the longest catalytic lifetime (79 h) along with a considerably high propylene selectivity (49.94 %) and large P/E ratio (8.6). Author contributions Rui Feng contributed to the conception of the study. Xinlong Yan and Xiaoyan Hu contributed significantly to analysis and manuscript preparation; Rui Feng and Yixin Zhang performed the data analyses and wrote the manuscript; Jianjun Wu and Zifeng Yan helped perform the analysis with constructive discussions. Acknowledgements This work was supported by the National Natural Science Foundation of China (No.21908240), the State Key Laboratory of Heavy Oil Processing (No. SKLOP201902001), the Natural Science Foundation 8
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Phosphorus-modified b-axis oriented hierarchical ZSM-5 zeolites for enhancing catalytic performance in a methanol to propylene reaction
T
Rui Fenga,*, Xinlong Yana, Xiaoyan Hua, Yixin Zhanga, Jianjun Wua, Zifeng Yanb a Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering, China University of Mining & Technology, Xuzhou, 221116, Jiangsu, China b State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZSM-5 zeolite B-axis orientation Hierarchical structure Phosphorus modification MTP reaction
In this work, a b-axis oriented hierarchical ZSM-5 zeolite was prepared with glucose as an additive and further modified by phosphorus to improve the propylene selectivity and prolong the lifetime of the catalyst in a methanol to propylene (MTP) reaction. The physiochemical properties were characterized by XRD, low-temperature N2 sorption, SEM, TEM, XPS, solid-state NMR, FTIR, and NH3-TPD analyses. The b-axis oriented ZSM-5 (Z5C) zeolite exhibited a microsized hexagonal lamellar structure with a thickness of approximately 220 nm. Analyzing the MTP reactions revealed that the newly prepared Z5-C catalyst exhibited a higher propylene selectivity of 53.01 % and a longer lifetime of 45 h compared to those of a conventional nanosized ZSM-5 catalyst (propylene selectivity of 43.97 % and lifetime of 10 h). In addition, the phosphorus modification remarkably increased the lifetime of the b-axis-oriented ZSM-5 catalyst to 79 h.
1. Introduction Methanol to hydrocarbon (MTH) conversion reactions have received wide attention due to the extensive sources of raw materials, such as coal and renewable biomass, as a replacement for conventional crude oil [1]. With the ever increasing demands of light olefins, including ethylene, propylene, and butene, the methanol to propylene (MTP) reaction is considered a promising alternative approach for propylene production to fill the demand gap from steam cracking naphtha [2]. ZSM-5-based catalysts, which possess a unique tri-dimensional pore structure and surface acidity, have been successfully applied in a commercial MTP process by Lurgi company [3]. However, improving the propylene selectivity and prolonging the catalytic lifetime of these ZSM-5 catalysts are the major goals for MTP technology at present [4–6]. To date, fabricating hierarchical structures, modifying crystal morphology and size, and adjusting the acid properties of ZSM-5 zeolites are the main strategies to improve the catalytic performance in MTP reactions [7–11]. It is recognized that microporous ZSM-5 with auxiliary mesopores and nanometer-sized crystals are beneficial for shortening the path length of molecular diffusion and enhancing accessibility to the internal surface acid sites. For instance, Li et al. [12] prepared an interconnected hierarchical ZSM-5 by a post dealumination-realumination method to improve the lifetime and propylene selectivity ⁎
of the MTP catalyst. For crystal sizes, it is not always a benefit toward improving the performance of the MTP reaction if the crystal sizes of ZSM-5 zeolite become too small [13–15]. Light olefins and coking precursors are more likely to adsorb on the acid sites of the highly exposed external surface of these nanosized crystals, although the catalytic lifetime increases in most cases [16–18]. In addition, small crystals decrease the shape-selective effect of micropores and lower propylene selectivity [19]. For example, Ryoo’s group synthesized ZSM-5 nanosheets with a thickness of 2 nm along the b-axis dimension, and the large number of acid sites on the external surface on these zeolites suppressed catalyst deactivation during a methanol to gasoline (MTG) conversion [20,21]. However, these acid sites on the external surface of nanosheets contributed little to improving the MTH conversion, as reported by Kim et al. [22]. Sugimoto et al. [13] found that the propylene selectivity over microsized ZSM-5 with large crystals of 3−4 μm was 8.1 % higher than that on nanosized ZSM-5 with crystal sizes less than 200 nm. A recent study by Wu et al. [14] also showed that ZSM-5 zeolite with an appropriate crystal size of approximately 150 nm promoted propylene selectivity and extended the catalytic lifetime compared with those of ZSM-5 zeolites with other crystal sizes. Recent studies have indicated that propylene is primarily produced in nonintersecting straight channels parallel to the b-axis of ZSM-5 zeolite, where an alkene-based cycle is active [23–25]. However, the
Corresponding author. E-mail address: [email protected] (R. Feng).
https://doi.org/10.1016/j.apcata.2020.117464 Received 10 December 2019; Received in revised form 24 January 2020; Accepted 10 February 2020 Available online 11 February 2020 0926-860X/ © 2020 Elsevier B.V. All rights reserved.
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2.3. Characterization
relatively long diffusion path of zig-zag channels parallel to the a-axis contributes more to deactivation [18,26]. Thus, an innovative solution is to design a microsized ZSM-5 zeolite that is thin along the b-axis direction; in this case, both shortening the diffusion path and strengthening the shape-selective effect of the straight channels are considered. For example, Xiao’s group reported a one-pot growth of TS1 crystals with controllable b-oriented lengths, which were synthesized with the aid of urea as an organic additive and a fluorinated surfactant [27]. Zhang et al. [28] synthesized a hexagonal lamellar ZSM-5 zeolite with a thin b-axis dimension of less than 450 nm under a neutral fluoride medium, and this catalyst performed with relatively high propylene selectivity (45.1 %) and a long lifetime in the MTP reaction. In this work, we demonstrate a facile hydrothermal crystallization method to synthesize a microsized b-axis oriented ZSM-5 zeolite (Z5-C) in the presence of environmentally friendly glucose. Z5-C exhibits superior catalytic performance in the MTP reaction with much higher propylene selectivity and a longer catalytic lifetime than conventional nanosized ZSM-5 (Z5-A). In addition, a phosphorus modification further prolongs the catalytic lifetime of the b-axis-oriented ZSM-5 zeolite (Z5CP2). The high selectivity for propylene and extended lifetime of Z5CP2 were related to its thinness along the b-axis direction, hierarchical structure, and appropriate surface acidity. Therefore, our work provides a new route for the facile synthesis of highly efficient MTP catalysts in the future.
Crystalline structure was measured by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (k = 0.15406 nm), operated at 40 kV and 30 mA and scanned from 5° to 75°. The relative crystallinity of ZSM-5 zeolites was calculated by comparing the sum of five characteristic peak areas at 2θ = 7°-25°. To investigate the zeolite morphologies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were measured on a Quanta 250 instrument and FEI Technai G2 F20 instrument, respectively. Low-temperature (−196 °C) nitrogen-sorption isotherms were obtained on a Micrometric Tristar 3000 analyzer to characterize the textural properties of the samples. Prior to measurement, the samples were degassed at 300 °C for 3 h. The Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method for the desorption branch of the isotherms were used to determine the specific surface area and mesopore size distribution, respectively. X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher 250Xi analyzer was used to measure the surface elemental composition of the zeolites. Solid-state 27Al and 29Si NMR analyses were performed on an Advance III HD 600 MHz instrument at a spectrometer frequency of 119.2 MHz, using a Bruker 4.0 mm double-resonance MAS detector to acquire 48 kHz MAS spectra. The framework silicon-to-alumina (SiO2/ Al2O3) ratio was derived from the resolved 29Si NMR spectra. To check the acid strength of the samples, temperature-programmed desorption of ammonia (NH3-TPD) measurements were performed at temperatures of 100−800 °C at a rate of 10 °C/min on a Quanta Chrome ChemStar™ instrument. Fourier transform infrared (FT-IR) analysis with a Nicolet iS5 (Thermo Co.) was used to characterize the acid sites using pyridine as the probe molecule, and the acid amounts were estimated using a semi-quantitative calculation proposed by Emeis et al. [29].
2. Experimental section 2.1. Raw materials In this study, raw materials, including tetraethyl orthosilicate (TEOS, AR), aluminum isopropoxide (AIP, AR), sodium hydroxide (NaOH, CP), tetrapropylammonium hydroxide aqueous solution (TPAOH, 25 wt.%), glucose (AR), and ammonium phosphate (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China, Shanghai) and used without any further purification.
2.4. Catalyst evaluation in the MTP reaction The MTP reactions were performed in a fixed-bed reactor with an inner diameter of 6 mm and operating at 470 °C and atmospheric pressure. For each test, 0.10 g of H-form ZSM-5 catalyst with a particle size of 40–60 mesh was loaded in the flat-temperature zone of the reactor. The methanol was pumped into the reactor using 50 mL/min of argon as the carrier gas, and the weight hourly space velocity (WHSV) was set at 4.0 h−1. The products were analyzed by an online gas chromatograph (GC 2014C, Shimadzu) with a flame ionization detector (FID) and a capillary column (TG-BONG Q). The amount of coke on the spent catalysts was measured by thermogravimetric (TG) analysis on a Netzch STA449F5 instrument. Typically, the TG curve was recorded as the catalyst (10 mg) was heated from 30 °C to 850 °C in air with a heating rate of 10 °C/min. The methanol conversion and selectivity to the i product were defined as follows:
2.2. Synthesis of the b-axis oriented ZSM-5 zeolite The b-axis oriented ZSM-5 zeolite and its counterpart ZSM-5 sample were synthesized using the following chemicals as raw materials. A typical synthesis of b-oriented ZSM-5 zeolite using glucose as an additive was performed as follows: TEOS, AIP, NaOH, TPAOH, deionized water, and glucose were mixed together at room temperature and magnetically stirred at a speed of 1000 rpm for 12 h to give a final mole ratio of 1.0 Al2O3:50 SiO2:2 Na2O:8 TPAOH:3000 H2O:x glucose (x = 0, 6, 12). The mixture was then transferred into a PTFE-lined stainlesssteel autoclave and precrystallized at 110 °C for 24 h and crystallized at 170 °C for 48 h. After crystallization, the obtained product was filtered, washed with deionized water, and dried overnight at 110 °C. By varying the amount of glucose, three samples were obtained and denoted as Z5A, Z5-B, and Z5-C, corresponding to samples with x = 0, 6, and 12, respectively. To obtain H-form zeolites, the as-synthesized samples in this work were calcined at 550 °C for 4 h with a heating rate of 3 °C/min to remove the organic template and then ion-exchanged twice in a solution of 1.0 mol/L NH4Cl at 80 °C for 4 h, followed by heating at 550 °C for 4 h. A phosphorus modification of b-oriented ZSM-5 zeolite (Z5-C) was carried out using a wet impregnation of ammonium phosphate. Typically, 1.0 g of Z5-C was impregnated with a (NH4)2HPO4 solution of 3.0 mL and then left standing at room temperature for 2 h. Next, the sample was dried at 110 °C overnight and calcined at 550 °C for 4 h with a heating rate of 3 °C/min. The phosphorus-modified samples Z5CPx (x = 1, 2, 3) were finally obtained with phosphorus increments of 1.5 wt.%, 2 wt.%, and 3 wt.%, respectively.
XMeOH = Si =
(mMeOH (in) − 1.39mDME (out ) − mMeOH (out ) ) mMeOH (in)
mi Σmi
where mi is the mass weight of compound i and Σ mi is the total mass weight of all products. The dimethyl ether (DME) product was regarded as two molecules of unconverted methanol. The mass balance was carefully controlled with an average error within 5%. 3. Results and discussion 3.1. Physiochemical properties of the as-synthesized ZSM-5 zeolites The XRD patterns of the as-synthesized samples in Fig. 1 show that 2
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Fig. 1. XRD patterns of the as-synthesized ZSM-5 zeolites and phosphorus-modified ZSM-5 zeolites. Table 1 Physiochemical properties of the as prepared ZSM-5 samples. Item
Z5-A Z5-B Z5-C Z5-CP1 Z5-CP2 Z5-CP3 a b c d
Relative crystallinity (%)
98.2 93.7 100.0 90.9 95.1 97.6
SiO2/Al2O3 molar ratio
85c 114c 136c 117d 121d 118d
Specific surface area (m2·g−1)
Pore volume (cm3·g−1)
SBETa
Smeso
Smicrob
Vpb
Vmeso
Vmicrob
414.9 336.8 320.4 302.5 287.3 277.2
127.0 147.0 184.2 140.0 122.9 121.5
287.9 189.8 136.2 162.5 164.4 155.7
0.236 0.204 0.299 0.271 0.257 0.233
0.120 0.125 0.239 0.211 0.204 0.181
0.116 0.079 0.060 0.060 0.053 0.052
Derived from the multipoint Brunauer-Emmett-Teller (BET) method. Derived from the t-plot method. Derived from the 29Si NMR spectra. Detected by ICP-OES method.
demonstrates the feature of the short b-axis for the Z5-C sample. As for glucose, various chemical reactions such as dehydration, condensation, or polymerization and aromatization can take place under the hydrothermal conditions, especially in the alkaline solution, leading to a complex mixture of organic compounds [35–37]. During the hydrothermal crystallization process, nucleation and subsequent crystal growth of ZSM-5 zeolites happen [38,39]. Simultaneously, the polyhydroxy saccharide species would preferentially adsorb on the (010) crystal surface by hydrogen-bond, van der Waals, or electrostatic interaction [39,40]. Thus, the further attachment of silica or growth units to crystal surface of (010) was reduced, thereby changing the growth rates along the three-dimensional directions and resulting in the b-axis oriented ZSM-5 zeolite. The XPS spectra in Fig. 3 display the surface elemental compositions of zeolites. Fig. 3b shows that compared to the conventional Z5 sample, the Al content on the surface of the b-axis oriented Z5-B shows little change. In contrast, the Al content on the surface of the b-axis-oriented Z5-C obviously decreases. The FTIR spectra in Fig. S1 also show that the concentrations of extraframework and framework Al species on the external surface of the Z5-C sample are much lower than those of the Z5-A sample. In contrast, there is little change in the intensity of the Si2p bands for the three samples; however, the band position of the Si atom on Z5-C shifts to a high binding energy, as shown in Fig. 3c. This is probably attributed to its low concentration of Al atoms on the surface, which have a smaller electronegativity (χ = 1.61) than that of Si atoms (χ = 1.90). The coordinate states of Al and Si atoms in ZSM-5 zeolites are revealed by the 27Al and 29 Si NMR spectra and shown in Fig. 4 and Fig. S2. In Fig. 4a, the peaks of the 27Al NMR spectra at 52.9 ppm and -2.7
all samples exhibit the characteristic peaks of ZSM-5 zeolite. These peaks appear at 2θ of 8.0°, 8.9°, 23.2°, 24.0°, and 24.5°, which correspond to (101), (020), (501), (151) and (303), respectively (JCPSD no. 44-0003) [30–32]. The partial increase of peaks at 8.0° and 8.9° shows that with the increase of glucose, the two peaks of Z5-C shift to higher 2θ positions, which is indicative of a higher framework silicon-to-alumina (SiO2/Al2O3) ratio compared to that of the Z5-A sample [33]. In contrast, the peaks shift to lower 2θ positions with increasing amounts of ammonium phosphate, indicating that the phosphorus atoms were introduced into the framework of the Z5-C sample and resulted in an increase in crystalline size [34]. The relative crystallinity (R.C.) results in Table 1 show that the relative crystallinities of phosphorus-modified Z5-C samples decreased slightly compared to that of the parent Z5-C. The morphologies of the as-synthesized ZSM-5 zeolites were detected by SEM and TEM analysis and are displayed in Fig. 2. In Fig. 2a, uniform structure of bulky zeolite particles with sizes centered at approximately 550 nm is observed for the conventional Z5-A sample. These bulky particles are wrapped with uniformly and closely packed small particles with sizes of approximately 50 ± 10 nm (see Figs. 2b and c). In contrast, the Z5-B sample in Fig. 2d–f possess a hexagonal lamellar structure with a particle size of approximately 3.5 μm × 1.4 μm × 420 nm, exhibiting thinness along the b-axis direction. In contrast, the Z5-C sample exhibits a similar b-axis oriented structure but a large particle size of approximately 5 μm × 2.1 μm × 220 nm (see Fig. 2g and h). It is noted that the thickness along the b-axis direction of the Z5-C sample is comparatively smaller than that of the Z5-B sample, indicating that the glucose additive decreases the crystal growth along the b-axis direction in the process of hydrothermal crystallization. In addition, the electron permeability in the TEM image (see Fig. 2i) also 3
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Fig. 2. Morphologies of the as-synthesized ZSM-5 zeolites: SEM images (a) & (b) and TEM image (c) of Z5-A; SEM images (d) & (e) and TEM image (f) of Z5-B; and SEM images (g) & (h) and TEM image of Z5-C. Note, the inset in Fig. 2a is the particle size distribution chart of the Z5-A sample.
Z5-CP3 indicated more octahedral Al due to the dealumination from zeolite caused by phosphorus modification [42]. The 31P NMR spectra in Fig. S2d shows that with increasing the phosphorus modification amount, the peak intensity of phosphorus species and the extraframework octahedral Al species gradually increased, emphasizing the fact that phosphorus modification has led to the dealumination of framework Al species. The 29Si NMR spectra in Fig. S2 of the three samples and their resolutions of overlapped peaks. All the spectra show similar features and can be resolved into eight resonances at approximately
ppm are attributed to framework tetracoordinated Al species and extraframework hexacoordinated Al species, respectively [41]. In addition, a small weak shoulder at 28.5 ppm is observed for the Z5-C sample, which is attributed to pentacoordinated Al species. It is obvious that the relative intensity of tetracoordinated Al follows the order of Z5C > Z5-A > Z5-B, which is consistent with the variation of relative crystallinity in Table 1. The weaker peak of octahedral Al shifted from -2.7 to −14 ppm after phosphorus modification, indicating the octahedral Al attached to phosphorus and the higher signal at −14 ppm of
Fig. 3. XPS spectra of the as-synthesized ZSM-5 zeolites: (a) full spectrum of Z5-A sample using C 1s at 284.8 eV for a calibration reference; (b) Al2p spectra; and (c) Si2p spectra. 4
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Fig. 4.
27
Al NMR spectra of the as-synthesized ZSM-5 zeolites and phosphorus modified Z5-C samples.
−98, −102, −106, −109, −114, −116, −119 and −122 ppm. Among them, the peak at approximately −98 corresponds to Q2(2Al), and the peak at approximately −102 ppm corresponds to Q3(0Al) on the external surface of the zeolites [41]. The peaks between −106 and −109 ppm are attributed to Q4(1Al), while other intense peaks from −114 to −122 ppm are attributed to Q4(0Al) [43]. Here, the resolved resonances are used to estimate the framework SiO2/Al2O3 ratios of the ZSM-5 zeolites using previously reported methods [31,43]. The calculated results gave framework SiO2/Al2O3 ratios of 85, 114, and 136 for Z5-A, Z5-B, and Z5-C, respectively; the trend of these values is similar to the XRD results. N2 sorption measurements (Fig. 5 and Table 1) show that Z5-A and Z5-B samples have a type-I isotherm with a nearly invisible hysteresis loop; in contrast, Z5-C possesses type IV sorption curves typical of mesoporous materials, according to the IUPAC classification. The hysteresis loop of Z5-C gradually decreases with increasing phosphorus content after modification, which is indicative of the decrease in mesopores. The pore size distributions in Figs. 5b and c show the coexistence of the intrinsic micropores of ZSM-5 and its mesopores, and the parent and phosphorus-modified Z5-C have a clear mesoporous structure in the range of 5−50 nm. The textural parameters in Table 1 show that with increasing amounts of glucose, the mesoporous surface area and pore volume for Z5-C increase to approximately 184.2 m2 g−1 and 0.239 cm3 g−1, respectively, which is higher than that of Z5-A (approximately 127.0 m2 g−1 and 0.120 cm3 g−1, respectively). However, the microporous surface area and pore volume of Z5-C decrease compared to that of Z5-A, probably due to a remarkable decrease in nanocrystals. The microporous surface areas of the phosphorus-modified Z5-C samples are larger than those of the parent Z5-C, probably due to the channel cleaning action of weakly alkaline ammonium phosphate. In addition, a micropore feature in the range from 0.50 nm to 0.75 nm
is observed in Fig. 5, corresponding to the microporous structure of an MFI zeolite, which possesses 2D sinusoidal channels (0.51 × 0.55 nm, a direction) crossed with 1D straight channels (0.53 × 0.56 nm, b direction) [23,44]. Pyridine adsorbed IR analysis and NH3-TPD analysis were conducted to measure the acid properties of ZSM-5 zeolites, and the calculated results are shown in Fig. 6 and Table 2. In Fig. 6a, the assignment of bands are similar to that of what we previously reported [45–47]. That is, the bands at 1448 and 1545 cm−1 are attributed to pyridine-adsorbed Lewis acid sites (LAS) and Brönsted acid sites (BAS), respectively. The acid amount results in Table 2 show that with increasing amounts of glucose, the amounts of Lewis-, Brönsted-, and total acid sites gradually decrease, and the phosphorus modification further decreases the acid amounts of the Z5-C samples. In comparison, the NH3-TPD curves in Fig. 6b display the existence of three types of acid sites: weak acid sites at ammonia desorption temperatures lower than 300 °C, moderately strong acid sites at approximately 300∼500 °C, and strong acid sites at temperatures higher than 500 °C. Obviously, the amount of total acid sites and strong acid sites decrease with increasing amounts of glucose, as shown in Table 2. Compared to the parent Z5-C sample, the phosphorus-modified ZSM-5 samples show a decrease in total acid sites and strong acid sites, which is similar to those reported by others [34,48,49]. Among the as-prepared samples, the Z5-CP2 sample with an appropriate phosphorus doping possesses the lowest amounts of strong acid sites and total acid sites, up to 126.3 μmol g-1 and 494.9 μmol g−1, respectively. 3.2. Catalytic performance of the MTP reaction A methanol to propylene (MTP) reaction was used to evaluate the catalytic performance of the as-synthesized ZSM-5 catalysts. Fig. 7 and
Fig. 5. N2 sorption curves (a) and pore width distributions (b) & (c) of the as-synthesized ZSM-5 samples. 5
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Fig. 6. Pyridine-adsorbed FT-IR spectra (a) and NH3 TPD curves (b) of the as-synthesized ZSM-5 zeolites.
propylene selectivity and light olefin selectivity of Z5-CPx catalysts gradually decreased with increasing amounts of phosphorus. Notably, compared to Z5-A, the selectivity of light alkanes (C2°-C4°) obviously decreased for the parent Z5-C and phosphorus-modified Z5-CPx catalysts. This result was probably attributed to the low surface acid density of the b-axis-oriented ZSM-5 catalysts (see Fig. 6 and Table 2), which decreased the hydrogen transfer activity of the light olefins to form light alkanes [51,52]. Similarly, the ethylene selectivities of phosphorus-modified Z5-CPx catalysts were much lower than those of Z5-A and Z5-C catalysts, resulting in a remarkable increase of propylene to ethylene (P/E) ratios, as shown in Fig. 7b. To the best of our knowledge, the Z5-CP3 catalyst possesses the highest observed P/E ratio of 14.3, which has never been documented before. As is known, olefin-based and aromatic-based cycles are the two cycles simultaneously running in the methanol to hydrocarbon (MTH) reaction over the ZSM-5 catalysts. The former produces ethylene, and the latter produces propylene and C3+ olefins [53,54]. On the one hand, shortening the distance of the straight channels along the b-axis direction and introducing an additional mesoporous structure reduced the residence time of polymethylbenzene in the micropores, which increased the ratio of olefin-based cycle reactions to aromatic-based cycle reactions and resulted in high propylene selectivity and low ethylene selectivity [19,51,52,55–57]. On the other hand, phosphorus doping brought about a tiny but not negligible reduction of the microporous channels and further increased the shape-selective effect of the straight channels [34]. In addition, reducing the acid density of the ZSM-5 catalysts prolonged the lifetimes of the various catalysts by reducing coke formation; however, the excessive phosphorus doping of Z5-CP3 had the opposite effect for prolonging the catalyst lifetime due to pore blockage and rising amounts of moderately and strong acid sites (see
Table 2 Surface acid properties of the as-synthesized ZSM-5 zeolites. Item
Z5-A Z5-B Z5-C Z5-CP1 Z5-CP2 Z5-CP3 a b
Surface acid amounta (μmol·g−1)
Surface acid amount (μmol·g−1)b
Total
BAS
LAS
Total
Weak
Moderate
Strong
230.5 218.5 186.4 191.4 165.2 168.7
156.0 148.5 130.3 136.8 126.2 133.2
74.5 70.0 56.1 54.6 39.0 35.5
658.9 588.1 572.9 539.5 494.9 540.7
184.6 188.0 180.2 175.1 188.4 196.7
224.1 211.4 209.8 191.6 180.2 188.3
250.2 188.7 182.9 172.8 126.3 155.7
Derived from pyridine-adsorbed FT-IR spectra in Fig. 6a. Derived from the NH3-TPD curve in Fig. 6b.
Table S1 show the average selectivities of the products and the lifetimes of the as-synthesized ZSM-5 catalysts in a continuous flow fixed-bed reactor at 470 °C. Here, the lifetime of the catalyst was defined as the reaction time until the methanol conversion was approximately 85 %. In Fig. 7a, it could be found that the microsized b-axis oriented ZSM-5 catalysts (Z5-B and Z5-C) presented higher selectivity to propylene and selectivity to light olefins (C2=-C4=) compared to the conventional nanosized ZSM-5 catalyst (Z5-A). However, it did not mean this microsized ZSM-5 had better performance in the MTP reaction than the smaller nanosized ZSM-5 with particle sizes less than 20 nm. For instance, Corma et al. reported that nanocrystalline ZSM-5 zeolite with particle sizes of approximately 15 nm exhibited a much larger catalyst lifetime and higher selectivity to propylene than nanosized ZSM-5 with particle sizes of approximately 200 nm in the MTP reaction, due to the fast mass transfer caused by short path lengths [50]. In contrast, the
Fig. 7. Average product selectivity (a) and catalyst lifetime (b) for MTP reactions over the ZSM-5 catalysts. The average values on the basis of mass weight were calculated before methanol conversions were less than 85 %. The lifetime of the catalyst was until the reaction decreased to ∼85 % methanol conversion. Reaction conditions: WHSV of methanol =4.0 h−1, 470 °C, 0.1 MPa, catalyst =0.10 g.
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Fig. 8. Conversion percentage of methanol and selectivity to light olefins for the MTP reactions vs. time on stream over the ZSM-5 catalysts: Z5-A (a); Z5-B (b); Z5-C (c); and Z5-CP2 (d). Reaction conditions: WHSV of methanol =4.0 h−1, 470 °C, 0.1 MPa, and catalyst =0.10 g.
time, the acid sites of the ZSM-5 catalysts were gradually covered by coking species, resulting in a decrease in the cracking activity of the C5+ products and their related propylene selectivity, as shown in Fig. S4. The thermogravimetry (TG) curves in Fig. 10 show that coke deposition should be responsible for the deactivation of the MTP catalyst and that the coke contents on the deactivated Z5-A, Z5-C, and Z5-CP2 catalysts were 12.68 wt.%, 7.18 wt.%, and 2.78 wt.%, respectively. In view of the lifetimes for the three catalysts in Fig. 8, the coking rate of the Z5-CP2 catalyst was even lower than that of the Z5-A catalyst. As shown in Fig. S5, it could be attributed to the synergistic effect of the fast mass transfer benefiting from the thinness along the b-axis direction and hierarchical structure, the relatively high cracking activity of the C5+ products catalyzed by acid sites of appropriate strength, and the low coking rate owing to the low acid site density.
Table 1 and Table 2). Among the as-prepared samples, the Z5-CP2 catalyst had better catalytic performance with a catalyst lifetime of 79 h, while it performed with a relatively high P/E ratio of 8.6 and a high propylene selectivity of 49.94 %. Fig. 8 and Figs. S3 and S4 comparatively show the methanol conversion and selectivities of light olefins as a function of time on stream for the MTP reaction over the ZSM-5 catalysts. All catalysts exhibited a similarly high conversion of methanol (> 99 %) in the initial stage of the reaction that continued for some time. This was then followed by an attenuation with increasing reaction time. Fig. 8 shows that propylene is the primary product of the light olefins over the Z5-A, Z5-B, Z5-C, and Z5-CP3 catalysts, which is consistent with the dominance of the olefinbased cycle reactions over the ZSM-5 catalysts. Thus, the variation trend of selectivities to light olefins was similar to that of propylene selectivity; in contrast, the selectivities to ethylene and butene varied slightly with time on stream. It was interesting and significant that the ethylene selectivity over Z5-CP3 was less than 5 wt.% during the catalytic lifetime cycle when propylene was regarded as a target product (see Figs. 8d and S4). Fig. 9 shows that the methane selectivity and C5+ product selectivity over the four catalysts gradually increased with time on stream. Compared to that of the conventional nanosized Z5-A, the baxis oriented catalysts had a much lower increasing rate of methane selectivity as a function of time on stream. In contrast, the increasing rate of the C5+ product selectivity on all catalysts showed little change despite the differences in the C5+ product selectivity. In addition, the results in Fig. S4c show that the selectivities of the C5+ products over the Z5-C and Z5-CPx catalysts at the same reaction time followed the order of Z5-CP3 > Z5-CP2 > Z5-CP1≈ Z5-C. In view of the gradual decrease of surface acid density with a phosphorus modification, the cracking rate of C5+ products into light olefins decreased, resulting in a low propylene selectivity (Fig. 7a). Similarly, with an extended reaction
4. Conclusion Microsized b-axis oriented hierarchical ZSM-5 zeolites with high crystallinity, hexagonal lamellar structures with a thickness of 220 nm along the b-axis direction, and low concentrations of acid sites were synthesized through hydrothermal crystallization in the presence of glucose. In addition, a phosphorus modification could further decrease the acid amounts and acid strength of the b-axis oriented ZSM-5 zeolite. The catalytic performance of the parent- and phosphorus-modified baxis-oriented ZSM-5 was evaluated by the methanol to propylene (MTP) reaction and compared with that of the conventional nanosized ZSM-5 (Z5-A). Significant improvements of enhanced propylene selectivity, prolonged catalytic lifetime, and superior coke toleration suggested baxis oriented ZSM-5 to be a candidate catalyst for the MTP process due to its thinness along the b-axis direction, hierarchical structure, and appropriate surface acidity. In particular, the sample of the b-axis 7
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Fig. 9. Methane selectivity (a) and C5+ selectivity (b) for the MTP reactions vs. time on stream over the ZSM-5 catalysts: Z5-A (a); Z5-B (b); Z5-C (c); and Z5-CP2 (d). Reaction conditions: WHSV of methanol =4.0 h−1, 470 °C, 0.1 MPa, and catalyst =0.10 g.
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Fig. 10. TG curves of the three spent ZSM-5 catalysts after the MTP reactions at 470 °C.
oriented ZSM-5 (Z5-C) catalyst had a high propylene selectivity of 53.01 % and a long lifetime of 45 h compared with that of the conventional nanosized ZSM-5 (Z5-A) catalyst (propylene selectivity of 43.97 % and a lifetime of 10 h). The phosphorus-modified Z5-CP2 catalyst revealed the longest catalytic lifetime (79 h) along with a considerably high propylene selectivity (49.94 %) and large P/E ratio (8.6). Author contributions Rui Feng contributed to the conception of the study. Xinlong Yan and Xiaoyan Hu contributed significantly to analysis and manuscript preparation; Rui Feng and Yixin Zhang performed the data analyses and wrote the manuscript; Jianjun Wu and Zifeng Yan helped perform the analysis with constructive discussions. Acknowledgements This work was supported by the National Natural Science Foundation of China (No.21908240), the State Key Laboratory of Heavy Oil Processing (No. SKLOP201902001), the Natural Science Foundation 8
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