Organosilane-assisted synthesis of hierarchical porous ZSM-5 zeolite

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Organosilane-assisted synthesis of hierarchical porous ZSM-5 zeolite as a durable catalyst for light-olefins production from chloromethane Dafen Wen, Qing Liu, Zhaoyang Fei, Yanran Yang, Zhuxiu Zhang, Xian Chen, Jihai Tang, Mifen Cui, and Xu Qiao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02332 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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Industrial & Engineering Chemistry Research

Organosilane-assisted synthesis of hierarchical porous ZSM-5 zeolite as a durable catalyst for light-olefins production from chloromethane Dafen Wen,a,b Qing Liu,a Zhaoyang Fei,a, b* Yanran Yang,a,b Zhuxiu Zhang,a Xian Chen,a Jihai Tang,b,c Mifen Cui,b and Xu Qiaoa,b,c* a

College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China

b

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech

University, Nanjing 210009, PR China c

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),

Nanjing 210009, PR China

* Corresponding Author, E-mail: [email protected] (Z. Fei), [email protected] (X. Qiao); Tel: +86 025-83172298; Fax: +86 025-83587168.

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ABSTRACT: Hierarchical porous ZSM-5 (HP-ZSM-5) zeolites with both micro- and mesoporosity were synthesized by an organosilane-assisted method. Their catalytic activities were evaluated in the transformation of chloromethane into light-olefins (CMTO). The synthesized HP-ZSM-5 zeolites were characterized by XRD, Ar-sorption, SEM, NH3-TPD,

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Al NMR, Py-IR and uptake experiments of ethylene. The

organosilane treatment had a significant effect on the microstructure, pore chemistry and catalytic performance of HP-ZSM-5. Compared with the microporous ZSM-5, the abundant mesopore and appropriate acidity of HP-ZSM-5 samples are in favor of the inhibition of secondary reactions in CMTO process, such as hydrogen-transfer and aromatization. The selectivity of light-olefins reached 68.1% and the lifetime (the duration of chloromethane conversion > 98%) was up to 72 h, which is 4.5 times longer than that of the conventional ZSM-5.

KEYWORDS: ZSM-5, hierarchical pore, light-olefins, carbon deposit


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1 INTRODUCTION Light-olefins (e.g. C2H4 and C3H6) are important raw materials for modern chemical productions.1 The growing development of oil refinery and petrochemical industry presents an increasing demand for light-olefins2. Traditional petroleum route concerns the steam cracking of naphtha in the context of light-olefins production, but yields of light-olefins are lower than 10%.3 Additionally, petroleum resources become increasingly tense worldwide. Therefore, it is of strategic importance to develop non-petroleum routes, which are fit for the high-efficiency light-olefins production. The conversion of methane to light-olefins has been a topical research interest because methane is cheap and sufficient on the earth. There are direct and indirect pathways for this conversion. The direct conversion of methane to light-olefins is supposed to be the most efficient pathway, yet, it is not feasible for large-scale industrial application because of its harsh reactive conditions and the poor correlation between conversion and selectivity.4-6 A novel catalytic process of the direct conversion of methane to ethylene under anaerobic conditions was reported by Bao et al. in 2014,7 but this can only produce one type of light-olefins (ethylene), and requires a high reactive temperature (~1100 °C). Although methane direct conversion to light-olefins has been attracting more and more attention, methane conversion is often conducted by indirect methods. So far, there are two commercial techniques for the indirect conversion of methane to light-olefins. One is the conversion of methane to syngas which is subsequently converted to hydrocarbons by Fischer-Tropsch route.8 Another route refers to methanol to olefins (MTO) technology, in which firstly methane is converted into methanol via 3 ACS Paragon Plus Environment


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syngas, and then, methanol is converted to light-olefins through MTO process.9 However, the carbon-atom utilization efficiency of the process of syngas production from methane is below 50%. In addition, both the techniques can result in high production cost and enormous CO2 emission.10 Recently, a two-step route for the conversion of methane to light-olefins via methyl halides has been greatly impressed by considerable researchers owing to the simple process, mild conditions and the recyclable ability of hydrogen halide.11,12 Most importantly, the process does not require the syngas so as to reduce the energy consumption. This route includes two steps:

step 1: CH4 + HX + 1/2O2 step 2: CH3X

catalyst B

catalyst A

CH3X + H2O

1/nCnH2n + HX (X=Cl or Br)

The development of efficient catalysts is of great importance for such route. ZSM-5 and SAPO-34 exhibit higher light-olefins yields in the conversion of CH3Cl than other zeolites such as H-beta and H-SAPO-11.1,13-20 The reason is that the narrow pores of SAPO-34 only facilitate the ingress and egress of small molecules (e.g., ethene, propene).21,22 Besides, the selectivity of propylene over SAPO-34 was relatively low, and in most cases, was lower than that of ethylene.16-20 Compared with SAPO-34, ZSM-5 presents higher catalytic activity and light-olefins selectivity in CMTO due to the special intrinsic porosity (5.6×5.3 Å channels crossed with 5.3×5.1 Å sinusoidal pores),23 large specific surface area, adjustable chemical composition, excellent hydrothermal stability and appropriate shape selectivity.1 Yin et al.23 reported an exceptional catalytic activity of a novel compound, HZSM-5@silicalite-1, in the 4 ACS Paragon Plus Environment

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synthesis of light-olefins (C2H4 and C3H6) from CH3Br. Xu et al.2 demonstrated that the acidity is a crucial factor to catalytic behaviors. The reduction of the acidity can improve the light-olefins selectivity in CMTO. The influence of acidity was also addressed by Gamero,24 in which the ZSM-5 sample (SiO2/Al2O3 = 80) with moderate acidity exhibited excellent kinetic behavior at 350 °C in the conversion of chloromethane into light-olefins. In addition, the catalyst can be re-activated by the coke combustion with air. Herein, it would be fair to assert that the catalyst with the highest Lewis acid concentration exhibited exceptional stability for the conversion of methyl bromide to light-olefins. Zhou et al.25-27 successfully prepared methylene-bridged mordenite zeolite with highly-conserved framework carbon by an improved DGC route, which can extend the application of traditional zeolites in the context of selective catalysis and adsorption. Despite many outstanding features of the ZSM-5 zeolite, a slow shift of produced light-olefins away from the catalytic site can facilitate their further reactions28 with deactivation as a consequence. The generation of hierarchical pores is a useful way to enhance the accessibility of the active sites.29-33 Various strategies have been developed to generate the secondary pores in zeolites, such as deliberately removing atoms,34,35 hard-template methods,36,37 dual-template with surfactants38,

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and silanization-based

methods.40,41 In case of silanization-based methods treatment, it is easy to graft organosilanes to silicate species and nanoparticles by reacting with the silanol groups. After calcination, hierarchical zeolites can be obtained in which the secondary porosity is directly related to the void occupied by the organosilane-containing group. The broad availability of organosilanes and their diversity on the molecular size, configuration and 5 ACS Paragon Plus Environment


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chemical properties enable the wide application of the silanization-based methods. For example, Serrano et al.42 introduced the secondary porosity into zeolite materials by perturbing the crystal growth, and the catalyst presented high catalytic activity for the conversion of molecules with steric hinderance. Wuamprakhon et al.43 synthesized hierarchical ferrierite nanosheet assemblies by using an organosilane surfactant. The prepared samples with a higher hierarchy factor showed an improvement of the catalytic activities in the benzylation of toluene by benzyl chloride. Zhou et al.44-45 synthesized a micro-meso-macroporous hierarchical Ti-containing hollownest-structured zeolite precursor (Ti-HSZ) by a one-step hydrothermal rotacrystallization method, and， the resulting Ti-HSZ presented an excellent catalytic activity for the epoxidation of alkenes. They also found that hierarchical MCM-22 with micro/mesopores is an efficient and robust catalyst for the isomerization of bpinene. Hence, the introduction of mesopores into zeolites is a feasible approach to enhance catalytic activities in catalytic reactions. In the current work, we use an organosilane-assisted method to prepare hierarchical porous ZSM-5 (HP-ZSM-5) by interfering the normal crystallization of the zeolite. The influence of adding different organosilanes on the crystallinity, porosity, morphology, framework and acidity has been investigated. In addition, we carried out the uptake experiments of ethylene to evaluate the differences in diffusion in ZSM-5 and HP-ZSM-5. Simultaneously, the influence of the diffusion and acidity on catalytic activity and coking was investigated in the chloromethane to the light-olefins reaction.

2 EXPERIMENTAL SECTION 6 ACS Paragon Plus Environment

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2.1 Materials All chemicals were used without further purification, as follows: tetraethyl orthosilicate (TEOS, 28.5 wt%), sodium aluminate solid (NaAlO2, 41.0 wt% calculated from Al2O3), tetrapropylammonium hydroxide (TPAOH, 25 wt%) and four types of organosilanes as growth-inhibitors, including (3-aminopropyl) triethoxysilane (AMEO, 98 wt%), γ-chloropropyltriethoxysilane (CPTEO, 98 wt%), triethoxyvinylsilane (VTES, 98 wt%), and 3-triethoxysilypropylmercaptan (CPTES, 98 wt%). 2.2 Catalyst Preparation The HP-ZSM-5 samples were prepared with the following experimental procedure, which is proved an effective method to introduce mesopores into zeolites. In a typical synthesis, stoichiometric quantities of NaAlO2 was dissolved in distilled water under magnetic stirring, and then TPAOH and a certain amount of the organosilane were put into the above mixture stepwise. After the mixture had been stirred to clarify, TEOS was added slowly under vigorous stirring for 6 h. The molar ratio of the starting materials was as follows: 50 TEOS: 2 NaAlO2: 1.8 TPAOH: 0.5 organosilane: 1250 H2O. Finally, the mixture was transferred into an autoclave at 180 °C for 72 h. The sample was filtered, dried at 100 °C and calcined at 550 °C for 5 h to remove the template. The protonated sample was obtained from ion exchange with NH4Cl for four times, following by calcination at 550 °C for 4 h. The names of HP-ZSM-5 samples prepared by adding different organosilanes were listed in Table 1. For comparison, pure ZSM-5 was also synthesized under the similar procedures without organosilanes, and labelled as ZSM-5. (Table 1) 7 ACS Paragon Plus Environment


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As shown in Figure 1, the organosilane can hydrolyze in basicity solution, and the formed hydrolysis products can be grafted on the surfaces of zeolite precursors through the covalent Si-O-Si bond and hydrogen bond. The long chain organic groups connecting with the Si-species, can affect the growth of the zeolite crystals in the “bond block” manner. After crystallization and calcination, the hierarchical porous ZSM-5 samples with sufficient mesopores are formed.46 (Figure 1) 2.3 Catalyst Characterizations The crystallinity of the catalyst was determined by powder X-ray diffraction (PXRD) 5-40° on a SarmtLab powder diffractometer using Ni-filtered Cu Kα radiation (λ = 0.15406 nm) with a scan rate of 2 ° min-1 at 40 kV and 100 mA. The identification of the crystalline phases was achieved by comparing with JCPDS cards. The morphology and particle size of the samples were characterized by scanning electron microscope (SEM) on Hitachi S-4800 instrument at an acceleration voltage of 15 kV. Argon physisorption was performed on a BETSORP-II analyzer to gain information about the textural properties of the samples. The samples were pretreated at 250 °C under a vacuum of 10-3 Pa for 4 h prior to analysis. The Brunauere-Emmette-Teller (BET) theory was used to determine the total surface area in the p/p0 range between 0 and 0.15. Pore size analyses were obtained from the adsorption branches using the nonlocal density functional theory (NLDFT) model. To evaluate diffusion differences in the zeolites, the adsorptions of ethylene was 8 ACS Paragon Plus Environment

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investigated on an intelligent gravimetric analyzer apparatus (IGA-100, HIDEN). The samples were first dehydrated at 550 °C for 2 h and then exposed to the gaseous ethylene adsorbent at 30 °C, 4.7 kPa. 27

Al MAS-NMR were performed on a 600 MHz BrukerAvance III equipped with a 4

mm MAS probe. 27Al MAS NMR spectra were recorded using one pulse sequence with a spinning rate of 12 kHz. Chemical shifts were referenced to 1 M aqueous aluminum nitrate solution. The acidity was determined by NH3-temperature-programmed desorption (NH3-TPD) on a Micromeritics AutoChem 2920 II instrument. 50 mg of sample with a particle size of 40-60 mesh was degassed in a He stream (5 mL/min) at 500 °C for 1 h at a constant ramping rate of 10 °C/min and then cooled to 100 °C. NH3 adsorption was performed under a mixed gas flow of 10 vol % NH3/He (50 mL/min) for 1 h, and then the sample was exposed to flowing He (5 mL/min) for 30 min to remove weakly adsorbed NH3. Finally, a TPD profile was obtained when the NH3-TPD was promptly started at a heating rate of 10 °C/min from 100 to 600 °C under a He (50 mL/min) flow. The NH3-TPD analysis results were matched by using Origin software. Then, the matched profile was integrated to calculate the peak areas of different temperature ranges. Based on the peak areas, the density of strong acid and weak acid was calculated. Fourier transform infrared (FT-IR) spectroscopy measurements were performed on a Bruker TENSOR 27 infrared spectrometer manufactured by PE in the United States. The instrument was model Spectrum 2000 with a test wave number ranging from 1700-1400 cm-1 and a resolution of 4 cm-1. The sample (180 mg) was pressed into thin wafers and 9 ACS Paragon Plus Environment


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was degassed in a vacuum cell for 2 h at 400 °C and 10-2 Pa. Then, the sample was cooled down to room temperature, and a background spectrum was recorded before the pyridine adsorption step. Next, consecutive doses of pyridine with saturated vapor pressure of 4 kPa were added to the sample until saturation. Finally, the sample was heated to 320 °C under vacuum environment for 2 h and the spectra were then recorded. The coke amounts of the catalysts in the initial reaction period of 30 h were measured on a thermal gravimetric analyzer (TG, PerkinElmer TG/DTA 6300) in an air flow. For this purpose, the temperature of the sample was increased to 800 °C at a heating rate of 10 °C/min. Firstly, approximately 10 mg of catalyst was heated up to 250 °C under N2 flow (20 mL/min) until no weight loss occurred, then heated (2 °C/min) in an air flow (20 mL/min) to 800 °C while starting the test. The weight loss between 250-600 °C was considered as the total carbonaceous deposit content. 2.3 Catalyst Evaluation The catalytic reaction was carried out in a fixed-bed reactor. The equivalent diameter and length of the reactor are 24 mm and 0.5 m, respectively. The catalyst (3 g, 16-40 mesh) mixed with 30 g quartz sand (20-40 mesh) was loaded into the reactor. The catalyst was pre-treated with high-purity N2 gas flow at 500 °C for 1 h in order to remove guest molecules residing on the catalysts. Then, the feed gas of mixed N2 and chloromethane (V N 2：VCH 3Cl = 8 : 1 ) passed through the catalyst in the reactor at a flow rate of 90 mL/min. To quantify the catalytic behaviour of the samples, the reaction indices were shown as followed: 10 ACS Paragon Plus Environment

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Conversion of chloromethane (X): X =

( FCH3Cl )in − ( FCH3Cl ) out ( FCH3Cl ) in

× 100%

Where ( FCH 3Cl )in and ( FCH 3Cl ) out are the molar flow rate of the chloromethane in the feed and the reactor outlet stream, respectively. Selectivity of i product (Si):

Si =

( FCH3Cl )in

Fi − ( FCH3Cl ) out

Where Fi is the molar flow rate of the i product in the reactor outlet stream, expressed in C equivalent units.

3 RESULTS AND DISCUSSION 3.1 Catalyst Properties 3.1.1. XRD analysis

(Figure 2) Figure 2 shows the XRD patterns of the as-prepared ZSM-5 and HP-ZSM-5 samples synthesized with different organosilanes. Diffraction peaks at 2θ of 7.86 °, 8.78 °, 14.78 °, 23.18 °, 23.90 ° and 24.40 °, which are in good agreement with those peaks of ZSM-5 zeolite (JCPDS no. 43-0321), were detected for all samples. This result suggests that all samples show the MFI-type framework.47 However, with the involvement of organosilanes, the intensity of the characteristic diffraction peaks attributed to ZSM-5 decreased to varying degrees. It is because that the organosilanes were grafted on the crystal seed and limited the growth of zeolite fusion, which may lead to a slight decrease in crystallinity. It is deduced that the crystallinity of ZSM-5 can be retained, but the



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crystallinity decreased by adding different organosilanes as the growth inhibitors. 3.1.2. SEM analysis

(Figure 3) SEM images of ZSM-5 and HP-ZSM-5 samples are depicted in Figure 3. The organosilanes remarkably influence the morphology and particle size of HP-ZSM-5. The ZSM-5 sample exhibits the MFI-typical hexagonal morphology and shows smooth and angular surface, while the HP-ZSM-5 samples present spherical aggregates of crystals with roughen surfaces and relatively small particle sizes (about 1~2 µm in diameter), consistent with the decreasing intensity of the corresponding XRD patterns. This result is mainly contributed to the factor that the organosilanes were grafted on the surface of crystal seed through the covalent Si-O-Si bond and hydrogen bonds,42 which hindered the growth of ZSM-5 during crystallization process and eventually affected the structure and morphology. Furthermore, the small particle sizes of the HP-ZSM-5 reduce the diffusion rate, which is conducive to mass transfer.48-50 3.1.3. Ar adsorption/desorption analysis

(Figure 4) (Table 2) Compared with N2 physisorption analysis, Ar physisorption analysis is a quite more accurate characterization of the porosity obtained by using the NLDFT model. Figure 4 illustrates the Ar sorption isotherms and pore size distributions of ZSM-5 and HP-ZSM-5, and their textural properties are listed in Table 2. The ZSM-5 shows typical type-I isotherm, while all HP-ZSM-5 samples show the characteristic of type IV 12 ACS Paragon Plus Environment

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isotherm with a hysteresis loop at a p/p0 range of 0.3~1.0 (Figure 4a). It is reckoned that with the organosilanes treatment, HP-ZSM-5 samples possess open porous structures with slit-shaped pores.51-54 Pore diameter distributions (Figure 4b) reveal that the ZSM-5 without any treatment has no mesopores. After organosilanes treatment, except for typical micropores (~0.54 nm), the HP-ZSM-5 samples also possess mesopores 2~4 nm in size. It is clear that the addition of different organosilanes caused significant changes to the mesopore structures. HP-ZSM-5N and HP-ZSM-5C own smaller sizes of the mesopores, about 2.4 and 2.8 nm, respectively. HP-ZSM-5Cl and HP-ZSM-5S hold relatively larger mesopores, with pore sizes of ~4.0 nm. And there is also a big difference among them in terms of the pore volumes. As listed in Table 2, the BET surface area and total pore volume of ZSM-5 are 367 m2/g and 0.1528 cm3/g, respectively. Nevertheless, the HP-ZSM-5 samples possess larger surface area (maximum 422 m2/g) and more pore volume (maximum 0.2189 cm3/g), mainly attributed to the presence of nanocrystals that increases the external surface area and intergranular accumulation of mesopores.55 3.1.4. Diffusion analysis

(Figure 5) To evaluate the influence of the additional mesopores on the diffusion in the ZSM-5 and HP-ZSM-5, the adsorption of ethylene was investigated in Figure 5. The uptake rate of ethylene is only 22.5 µg/g/s for ZSM-5, while that are much faster over HP-ZSM-5 samples (27.9~40.8 µg/g/s), in the order of HP-ZSM-5S> HP-ZSM-5Cl> HP-ZSM-5C> HP-ZSM-5N> ZSM-5. In other words, when achieving the same uptake amount of 13 ACS Paragon Plus Environment


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ethylene, the HP-ZSM-5 samples are much quicker than ZSM-5, so the HP-ZSM-5 samples show the enhanced mass transfer rate. These sequences are consistent with the pore volume. It indicates the presence of mesopores and the shortened diffusion length of the HP-ZSM-5 samples that accelerate the mass transfer.56 3.1.5. 27Al MAS NMR analysis

(Figure 6) MAS NMR was used to study the environment of Al of ZSM-5 and HP-ZSM-5. As shown in Figure 6, there are two peaks presented in the spectra of all samples and no change in chemical shift of the samples under study. The strong resonance peak at ~54 ppm corresponds to AlO4 tetrahedra. The aluminum spectra also exhibited another peak at ~0 ppm which should be ascribed to the presence of octahedral Al outside the framework.57 Furthermore, the peaks at ~0 ppm for all four HP-ZSM-5 samples with high intensity indicates large amounts of octahedral Al present in the ZSM-5 framework.58 On the other hand, the intensity of the peak at ~54 ppm slightly decreased. These indicate that an increasing amount of tetrahedral Al atoms from the framework of HP-ZSM-5 is converted into octahedral Al atom from outside the framework with the organosilanes treatment. One can deduce that the acidity of HP-ZSM-5 samples is significantly different from that of ZSM-5 sample, which is further confirmed by the following NH3-TPD and Py-IR characterization. 3.1.6. NH3-TPD analysis

(Figure 7) (Table 3) 14 ACS Paragon Plus Environment

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The acidities of ZSM-5 and HP-ZSM-5 were characterized by the NH3-TPD (Figure 7 and Table 3). There are two distinct NH3 desorption peaks at ~200 and ~400 °C for all samples, which could be caused by the NH3 molecules adsorbed on the acid sites with weak and moderate acidities respectively. Although the SiO2/Al2O3 ratios of the samples are close to each other (Table 3), there is a difference in the acidity of ZSM-5 and HP-ZSM-5 synthesized using different types of organosilanes. Compared with ZSM-5, the acid density of HP-ZSM-5 (385~425 µmol/g) declined significantly and the peaks shifted to the lower temperature slightly, especially the mid-strong acid sites. In other words, the addition of different types of organosilanes leads to a decrease in acidity. 3.1.7. Py-IR analysis

(Figure 8) To obtain further information about the acid sites, FT-IR studies on adsorbed pyridine from ZSM-5 and HP-ZSM-5 were exhibited in Figure 8. The IR bands at 1450 cm–1 could be attributed to Lewis acid sites, mainly derived from the acidic hydroxyl groups produced by non-skeletal aluminum. The IR bands at 1540 cm–1 could be ascribed to the Brönsted acid sites, derived from the acidic hydroxyl produced by the skeleton aluminum. The IR band at 1487 cm–1 was originated from the vibration of the pyridine ring on both Brönsted and Lewis acid sites.59,60 For HP-ZSM-5, the intensity of the IR band belonging to Lewis acid sites increased slightly whereas the intensity of Brönsted acid sites amount reduced compared with those of ZSM-5.58,61 These differences in acidity are mainly due to the addition of organosilanes. According to Al MAS NMR analysis, the skeleton structure of HP-ZSM-5 samples is partially destroyed and partial 15 ACS Paragon Plus Environment


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Al atoms of the Brönsted acid active center are divorced from the framework of zeolite. The newly generated AlO+ ions form the new Lewis acid center，resulting in the increasing intensity of Lewis acid.62 This demonstrates that the organosilanes treatment has different effects on the Brönsted and Lewis acid sites.

3.2 Catalytic Performance 3.2.1. Stability test

(Figure 9) (Table 4) Chloromethane conversion to light-olefins as a typical acid catalyzed reaction was performed to evaluate the catalytic performance of all samples. We first investigated the stability of ZSM-5 and HP-ZSM-5. The chloromethane conversion versus time over all samples is shown in Figure 9. It is apparent that ZSM-5 was more prone to deactivation than HP-ZSM-5, showing a rapid reduction in catalytic conversion after 20 h. Its conversion even dropped to 45% after 45 h. Conversely, after the introduction of mesopores, the four HP-ZSM-5 samples performed much slower deactivation than ZSM-5. Here, we also employed the time at which the conversion rate of chloromethane decreased to 98% (t98) to illustrate the stability of the catalysts, as this is more applicable for commercial practice.63 The corresponding data are displayed in Table 4. In Table 4, HP-ZSM-5S with the highest stability can maintain a conversion over 98% for 72 h. Even in the worst case among all HP-ZSM-5 samples, HP-ZSM-5N also displayed an enhanced catalytic activity compared with ZSM-5. Their stability follows the order of HP-ZSM-5S> HP-ZSM-5Cl> HP-ZSM-5C> HP-ZSM-5N> ZSM-5. 16 ACS Paragon Plus Environment

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(Figure 10) There is a positive correlation between the catalytic lifetime and the pore volume shown in Figure 10. It is deduced that the abundant mesoporosity and short diffusion length of HP-ZSM-5 samples enhance the mass transfer and the tolerance capability54 for coke formation, and further improve their stability. Besides, the presence of mid-strong acid sites is also the major cause of inactivation.55 According to NH3-TPD analysis, it can be seen that the acidity of HP-ZSM-5 samples, especially the acidity of mid-strong acid sites, depleted severely which is not feasible for the conversion of light-olefins to compounds with larger molecular weight64 on HP-ZSM-5, thereby prolonging the lifetime of HP-ZSM-5 samples. 3.2.2. Products selectivity

(Figure 11) In addition to stability, the light-olefins selectivity is also important to the evaluation of the catalytic activity. Figure 11 shows the selectivity of lower olefins on ZSM-5 and HP-ZSM-5 when the chloromethane conversion rate reaches 98%, and the detail of the product contribution are listed in Table 4. Compared with ZSM-5, the HP-ZSM-5 samples exhibit higher selectivity of light-olefins (60.8%~68.1%), of which the reasons are as follows. On the one hand, light-olefins can be further converted into oligomerization and/or aromatization products in the pore channels of zeolites, during which the acidity of catalysts played an important role in the second reactions. After organosilanes treatment, both strength and density of mid-strong acid sites decreased to a great degree, which may suppress the further conversion of light-olefins in the pore 17 ACS Paragon Plus Environment


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channels of the catalysts. On the other hand, the shorter diffusion path of the HP-ZSM-5 sample makes light-olefins more susceptible to diffuse out to avoid deep reaction. In a word, the improved selectivity of light-olefins over HP-ZSM-5 samples is attributed to the reduced amount of mid-strong acidity and the shortened length of pore channels.

3.3 Coke Analysis (Figure 12) Coke amounts

of

all

spent

ZSM-5

and

HP-ZSM-5

were

analyzed

by

thermogravimetric analyses in Figure 12. Since the adsorbed water has been removed during the pretreatment, there is only one weight loss from 400-600 °C due to the decomposition of the carbon on the spent samples.2 It can be seen from Figure 11 that the carbon deposition of all discharged HP-ZSM-5 (3.6%~4.8%) samples is less than that of the discharged ZSM-5 (6.6%), and the order of the weight loss is ZSM-5 >> HP-ZSM-5C > HP-ZSM-5Cl > HP-ZSM-5S > HP-ZSM-5N, in consistent with their acidities sequence. According to the literature,65 the carbon are in the form of mainly polymethyl benzenes derived from the side reactions of light-olefins of which the acidity of the samples plays an important role. With the organosilanes treatment, the less amount of mid-strong acid sites in the HP-ZSM-5 samples hindered the side reactions so as to decrease the coking rate. Apart from this, the introduction of mesopores in HP-ZSM-5 increases the pore size and pore volume, which is conducive to the rapid diffusion of light-olefins, thus reducing the chance of side reactions.66

4. CONCLUSIONS


Page 19 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In this research, we successfully prepared a series of HP-ZSM-5 samples by the organosilane-assisted method. All as-synthesized HP-ZSM-5 samples exhibited enhanced stability and increased selectivity of light-olefins for CMTO reaction. Simultaneously, we verified that the pore volume and acidity are critical for the catalytic performance. The introduction of a number of mesopores in HP-ZSM-5 can facilitate mass transfer inside the catalysts and deactivate at a lower rate in the CMTO reaction. Meanwhile, the modification of the acidity retains abundant amount of the weak acid sites and weakens the mid-strong acid sites in HP-ZSM-5 samples, which promote the increased light-olefins selectivity through inhibiting the hydrogen-transfer and aromatization reactions.

NOTES The authors declare no competing financial interest.

ACKNOWLEDGEMENT Financial supports from National Natural Science Foundation of China (Grant No. 21306089, 21606130), Science and Technology Department of Jiangsu (Grant No. BY2015005-02) and State Key Laboratory of Materials-Oriented Chemical Engineering (Grant No. ZK201610) are greatly appreciated. We are also grateful to Prof. W. -J. Ji and Dr. Y. Yao for their help in the 27Al NMR test.



Page 20 of 47

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Table Captions Table 1. The Organosilanes used herein and the name of corresponding samples.

Table 2. Physical and diffusion parameters of ZSM-5 and HP-ZSM-5 samples. Table 3. Acidic parameters of ZSM-5 and HP-ZSM-5 samples.

Table 4. Lifetime and product contribution of CMTO reaction at 98% chloromethane conversion over ZSM-5 and HP-ZSM-5 samples.



Page 30 of 47

Table 1. The Organosilanes used herein and the name of corresponding samples. Organosilanes AMEO VTES

Structure (H5C2O)3

Si

(H5C2O)3

Name of samples NH2

HP-ZSM-5N HP-ZSM-5C

Si

CPTEO

(H5C2 O)3

Si

Cl

HP-ZSM-5Cl

CPTES

(H5C2O)3

Si

SH

HP-ZSM-5S


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Table 2. Physical and diffusion parameters of ZSM-5 and HP-ZSM-5 samples. Vp[a]/(cm3/g)

S/ Sample

Dp[a]/

Ethylene uptake

(m2/g)

Micropore

Mesopore

Total

(nm)

rate[b]/(µg/g/s)

ZSM-5

367

0.1465

0.0063

0.1528

-

22.5

HP-ZSM-5N

385

0.1451

0.0355

0.1806

2.4

27.9

HP-ZSM-5C

409

0.1475

0.0505

0.1980

2.8

31.9

HP-ZSM-5Cl

418

0.1462

0.0632

0.2094

3.9

35.9

HP-ZSM-5S

422

0.1474

0.0715

0.2189

4.0

40.8

[a] Obtained by applying the NLDFT model; [b] Determined by diffusion analysis.



Page 32 of 47

Table 3. Acidic parameters of ZSM-5 and HP-ZSM-5 samples. Sample

Acid amount/(µmol/g)

SiO2/Al2O3[a]

Total

Weak[b]

Strong[c]

ZSM-5

25.0

444

229

215

HP-ZSM-5N

25.5

385

202

183

HP-ZSM-5C

26.5

425

220

205

HP-ZSM-5Cl

26.0

420

220

200

HP-ZSM-5S

26.5

419

226

193

[a] Determined by XRF analysis; [b] Acid sites at ~200 °C; [c] Acid sites at ~400 °C.


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Table 4. Lifetime and product contribution of CMTO reaction at 98% chloromethane conversion over ZSM-5 and HP-ZSM-5 samples. t98[a]/

Selectivity / %

samples h

C1

C2

C2=

C3

C3=

C4

C4=

C5+

Total olefins

ZSM-5

16

8.0

2.7

21.6

31.3

19.0

11

3.5

2.9

44.1

HP-ZSM-5N

26

8.3

1.5

25.0

16.6

26.9

8.3

8.9

4.5

60.8

HP-ZSM-5C

37

8.5

1.4

26.6

16.0

28.3

6.4

7.3

5.5

62.2

HP-ZSM-5Cl

51

8.1

1.2

25.2

13.7

29.9

6.9

8.9

6.1

64.0

HP-ZSM-5S

72

8.0

1.6

28.7

10.7

31.6

6.4

7.8

5.2

68.1

[a] Lifetime is referred to the time to reach 98% of chloromethane conversion.



Page 34 of 47

Figure Captions Figure 1. The schematic representation of the formation process of hierarchical ZSM-5 zeolites.

Figure 2. XRD patterns of ZSM-5 and HP-ZSM-5 samples. Figure 3. SEM images of ZSM-5 and HP-ZSM-5 samples synthesized with different organosilanes.

Figure 4. (a) Ar adsorption/desorption isotherms and (b) pore size distributions of ZSM-5 and HP-ZSM-5 samples.

Figure 5. Ethylene uptake experiments at 30 °C over ZSM-5 and HP-ZSM-5 samples. Figure 6. 27Al MAS NMR spectra of all samples. Figure 7. NH3-TPD profiles of ZSM-5 and HP-ZSM-5 samples. Figure 8. FT-IR spectra of the pyridine adsorption on ZSM-5 and HP-ZSM-5 samples. Figure 9. Chloromethane conversion vs. time of CMTO reaction over ZSM-5 and HP-ZSM-5 samples.

Figure 10. The relationship between catalytic lifetime (t98) and the total pore volume. Figure 11. Product contribution over ZSM-5 and HP-ZSM-5 samples at conversion rate of 98%.

Figure 12. TG profiles of spent ZSM-5 and HP-ZSM-5 samples after 30 h reaction.


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Figure 1. The schematic representation of the formation process of hierarchical ZSM-5 zeolites.



Relative crystallinity

1

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 47

5

10

ZSM-5

0.85

HP-ZSM-5N

0.53

HP-ZSM-5C

0.67

HP-ZSM-5Cl

0.58

HP-ZSM-5S

15

20

25

2θθ (degree)

30

35

40

Figure 2. XRD patterns of ZSM-5 and HP-ZSM-5 samples.


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Figure 3. SEM images of ZSM-5 and HP-ZSM-5 samples synthesized with different organosilanes.



V (cm3/ g)

(a)

ZSM-5 HP-ZSM-5N HP-ZSM-5C HP-ZSM-5Cl HP-ZSM-5S

0.0

0.2

0.4

0.6

0.8

Relative pressure (p/p0)

1.0

(b) ZSM-5

dVp/dW (cm3/nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 47

HP-ZSM-5N

HP-ZSM-5C

HP-ZSM-5Cl

HP-ZSM-5S

0

1

2

3

4

Pore diameter (nm)

45

50

Figure 4. (a) Ar adsorption/desorption isotherms and (b) pore size distributions of ZSM-5 and HP-ZSM-5 samples.


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8

Adsorbed amount (mgg-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


·

7 6 5 4 3


2 1 0 0

50

100

150

200

250

300

350

Time (s)

Figure 5. Ethylene uptake experiments at 30 °C over ZSM-5 and HP-ZSM-5 samples.




Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100

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80

60 27

40

20

0

-20

-40

Al MAS NMR (ppm)

Figure 6. 27Al MAS NMR spectra of all samples.


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ZSM-5 HP-ZSM-5N

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HP-ZSM-5C HP-ZSM-5Cl HP-ZSM-5S

100

200

300

400

500

600

Temperature (°C)

Figure 7. NH3-TPD profiles of ZSM-5 and HP-ZSM-5 samples.




Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1400 1420 1440 1460 1480 1500 1520 1540 1560 -1

Wavenumber (cm )

Figure 8. FT-IR spectra of the pyridine adsorption on ZSM-5 and HP-ZSM-5 samples.


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100


90

CH3Cl conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 60 50 40 30 20

0

20

40

60

80

100

120

140

Time (h)

Figure 9. Chloromethane conversion vs. time of CMTO reaction over ZSM-5 and HP-ZSM-5 samples.



80 HP-ZSM-5S

70 60

Lifetime (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 HP-ZSM-5Cl

40 HP-ZSM-5C

30 HP-ZSM-5N

20 ZSM-5 10 0.15

0.16

0.17

0.18

0.19

0.20

0.21

0.22

3

Total pore volume (cm /g) Figure 10. The relationship between catalytic lifetime (t98) and the total pore volume.


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70

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

C2= C3=

50

C4= Total olefins

40 30 20 10 0 ZSM-5

HP-ZSM-5N HP-ZSM-5C HP-ZSM-5Cl HP-ZSM-5S

Samples Figure 11. Product contribution over ZSM-5 and HP-ZSM-5 samples at conversion rate of 98%.



3.9 3.6

6.6

4.8

98

4.5

100

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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96 ZSM-5 HP-ZSM-5N

94

HP-ZSM-5C HP-ZSM-5Cl HP-ZSM-5S

92 300

400

500

600

700

800

Temperature (°C)

Figure 12. TG profiles of spent ZSM-5 and HP-ZSM-5 samples after 30 h reaction.


Page 47 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Hierarchical ZSM-5 catalysts synthesized by a growth-inhibition strategy with abundant mesoporosity and appropriate acidity exhibited enhanced stability (26~72 h) and light-olefins selectivity (60.8% ~ 68.1%) for in the transformation of CH3Cl into light-olefins.


Organosilane-assisted synthesis of hierarchical porous ZSM-5 zeolite

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