Enhanced Light Olefin Production in Chloromethane Coupling over

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Kinetics, Catalysis, and Reaction Engineering

Enhanced Light Olefins Production in Chloromethane Coupling over Mg/Ca Modified Durable HZSM-5 Catalyst Jincan Huang, Wei Wang, Zhaoyang Fei, Qing Liu, Xian Chen, Zhuxiu Zhang, Jihai Tang, Mifen Cui, and Xu Qiao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05544 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Enhanced Light Olefins Production in Chloromethane Coupling over Mg/Ca Modified Durable HZSM-5 Catalyst Jincan Huanga,b, Wei Wanga,b, Zhaoyang Fei*a,b, Qing Liua, Xian Chena, Zhuxiu Zhanga, Jihai Tanga,c, Mifen Cui**a, Xu Qiao***a,b,c aCollege bState

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

Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech

University, Nanjing 210009, PR China cJiangsu

National Synergetic Innovation Center for Advanced Materials (SICAM),

Nanjing 210009, PR China

*Corresponding author. Tel.:+86 02583172298; fax: +86 02583172298, E-mail: [email protected] **Corresponding author. Tel.:+86 02583172298; fax: +86 02583172298, E-mail: [email protected] *****Corresponding author. Tel.:+86 02583172298; fax: +86 02583172298, E-mail: [email protected]

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Abstract A series of Mg/Ca modified HZSM-5 zeolite catalysts were prepared by the impregnation method for catalytic coupling of CH3Cl to light olefins. The catalysts were characterized using XRD, N2 adsorption-desorption, NH3-TPD and Pyridine-IR techniques. Results showed the Mg/Ca modification substantially reduced the strong Brønsted acid amount and greatly improved light olefins selectivity. The Mg-rich catalysts were more inclined to generate C3H6 and C4H8 due to less strong acid sites but shorten the lifetime due to smaller pore size. The 5Ca2Mg-HZ catalyst offered the best performance with 87.2% total olefins selectivity and 99% CH3Cl conversion over 58 h. Coke analysis revealed all the modified catalysts had similar coking behavior while the short-lived catalysts showed an early drop in surface area owing to the blocked channels. The deactivation-regeneration experiments showed the 5Ca2Mg-HZ catalyst also had good reproducibility.

Keywords: Chloromethane; Light olefins; ZSM-5; Metal modification

1. Introduction Light olefins, including ethylene (C2H4), propylene (C3H6) and butene (C4H8), are among the most important building blocks in modern chemical processes. 1-3 However, due to the

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increasing production demand and continuous consumption of petroleum resources, the traditional process based on cracking of naphtha to produce light olefins is now facing severe challenges. Thus it is very important to find new and commercially advantageous source of light olefins. The huge reserves of methane (CH4) have offered another possibility to produce light olefins and led to the discovery of several important processes.4-11 The most famous routes include the indirect processes of (a) CH4-syngas-methanol/dimethyl ether-olefins, (b) CH4-syngas-olefins and the direct processes of (c) CH4 oxidative coupling and (d) CH4 anaerobic aromatization. Considerable efforts have been devoted to these processes and remarkable progresses have been achieved. Objectively, each of these alternatives has its own limitations in the attempts with commercial scale. The development of novel routes for CH4 to light olefins, assisted by advanced computational methods and more detailed analytical tools, is still of high significance for industrial practice and fundamental research.12-14 In addition to the approaches from fossil resources mentioned above, hydrogenation of CO2 to olefins has also attracted much attention in recent years. 15, 16 However, the source of H2, usually ideally from the decomposition of water, has not achieved development in large-scale yet. Therefore, the development of this technology is multifaceted and needs sustained further study. In recent years, a two-step process based on methane oxyhalogenation and methyl halide coupling to produce light olefins has attracted widespread attentions.17-19 Notably, 3


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the coupling of methyl halide can achieve high conversion and considerable olefin selectivity thus the process is expected to have a commercial outlet at sufficient scale. ZSM-5, ZSM-34, SAPO-34, and SAPO-11 etc. have proven to be reactive candidates for this process.18-21 Su et al. carried out a series of works using SAPO-34 as catalyst and made efforts to modify the catalyst for better performance.24-27 They also confirmed that coupling of CH3Cl on SAPO-34 followed the hydrocarbon pool mechanism, which proceeded via methylbenzenes as intermediates.28,29 Although SAPO-34 catalysts were studied intensively, there was an expansive growth of studies on HZSM-5 catalysts because the structure with medium micro pores was more prone to show higher selectivity and slower deactivation in various comparative works.30-35 Hence its further research is very attractive. Despite its advantages, the bare HZSM-5 zeolite is still limited for direct industrial application by its initial olefin selectivity and coke deposition problems.36-38 The inherent strong acid sites could reduce olefin selectivity and the coke deposits could block the tunnels and lead to deactivation. In general, adjusting the acidity39-41 and pore structure43-47 was often suggested to improve catalyst performance. The important work of Wang et al. introduced the fluorine doped HZSM-5 catalyst for the coupling of CH3Cl and CH3Br.32 In their work, the fluorine not only dramatically reduced the amount of strong acids, but also enlarged the pore size to better accommodate the precursors of high olefins. Thus the catalyst delivered higher C3H6 selectivity of 64% at 76% CH3Cl conversion and were stable at 400 °C for over 50 h during the reaction. In our previous work, a series of hierarchical HZSM-5 zeolites was synthesized using an organosilane-assisted method.48 The derived catalyst with improved mass transfer delivered much longer lifetime (over 72 h with 98% conversion) during the CH3Cl coupling reaction. Besides adjusting the pore structure, many efforts were made to regulate the surface acidity by introducing extra elements to essentially inhibit the 4



hydrogen transfer reaction and increase the olefin selectivity. Four alkali metals were adopted by Jaumain and Su for ion exchange of HZSM-5.30 Although the modification reduced the surface acidity and improved the olefin selectivity, it also brought disadvantages of lower conversion and easier deactivation. Wang et al. investigated the effect of rare earth on various zeolite catalysts and found that Ce modification on ZSM-35 could improve the selectivity of C3H6 while maintaining a high conversion.21 Transition metals of Fe, Co Mn and Zn were also considered for modification, and improved olefin selectivity in CH3Cl and CH3Br coupling.27,33,49,50 In summary, the modifications were mostly introduced to suppress side reactions by reducing the amount of strong acids, and each element introduced tended to have different effects on product distribution and durability. Therefore the above results indicate that the co-modification of HZSM-5 catalyst with multiple elements to display their respective advantages and even synergistic effect in olefins selectivity and stability is very promising for CH3Cl coupling reaction. In this work, we prepared a series of Mg and/or Ca modified HZSM-5 catalysts for the first time for CH3Cl coupling. The effect of Mg/Ca ratio on the structure, surface acidity and reactivity was investigated to study the interaction between Mg and Ca. And the coke analysis was subsequently conducted to reveal the reason of catalyst deactivation, which was further validated in a regeneration test.

2. Experimental section 2.1 Materials Precursors for preparing catalysts included HZSM-5 zeolite (SiO2/Al2O3=50, Nankai University Catalyst Co.), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and calcium nitrate hexahydrate (Ca(NO3)2·6H2O) (Sinopharm Chemical Reagent Co. Ltd). All materials were used without further treatment. Gases for reaction and characterization included N2 (99.999%), CH3Cl (99.5%), He (99.999%), and 4%/96% (v/v) NH3–He 5


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supplied by Sanle Gas (Nanjing, China). 2.2 Catalyst Preparation The Mg and/or Ca modified HZSM-5 catalysts were prepared using the impregnation method: a certain amount of Mg(NO3)2·6H2O and Ca(NO3)2·6H2O were dissolved in 30 ml of deionized water to form a nitrate aqueous solution. Then, 10 g of HZSM-5 zeolite powders was dispersed in the solution. After sonicated at 30 °C for 2 h, the precursor was further stirred at room temperature for 24 h before dried at 110 °C for 24 h. The catalyst was finally obtained after calcined at 550 °C for 3 h. A series of Mg and Ca modified zeolite catalysts were prepared based on 7 wt% of total loading, including 7Ca-HZ, 5Ca2Mg-HZ, 3.5Ca3.5Mg-HZ, 2Ca5Mg-HZ, 7Mg-HZ. 2.3 Catalyst Characterization X-ray diffraction (XRD) was carried out on a Smart Lab diffractometer of Rigaku Corporation of Japan. CuKα was used as the x-ray source (λ=0.15406 nm). The tube current was 100 mA, the voltage was 40 kV, the scanning rate was 0.05 s/step, and the scanning range was 5°-40° with a scanning step of 0.02 °/min. The N2 adsorption-desorption test was performed on a Japanese BELSORP II type adsorption apparatus, and the sample was vacuum-pretreated at 230 °C for 3 h before tested. The specific surface area of the sample was determined by BET equation. The pore size distribution and pore size of the sample were determined by HK equation. NH3 temperature programmed desorption (NH3-TPD) was measured on Auto Chem II 2920, Micromeritics, USA. FTIR and Pyridine-IR data were collected on a Bruker TENSOR 27 infrared spectrometer. The wave number ranged from 1400 to 1700 cm-1 and 3000 to 4000 cm-1 with a resolution of 4 cm-1. Thermogravimetric analysis was carried out on PerkinElmer TG/DTA 6300. The sample was first heated to 250 °C under nitrogen flow to remove the adsorbed water before heated to 800 °C at a rate of 10 °C/min. And the Raman spectra of used catalysts were gathered on JY Horiba LabRam HR800 using 6



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514-nm radiation at a power of 2 mW. 2.3 Catalytic Evaluation The reaction of CH3Cl coupling to prepare low-carbon olefins (C2H4, C3H6 and C4H8) was carried out in a fixed-bed glass reactor with an equivalent diameter and length of 24 mm and 0.5 m, respectively. 3 g catalyst and 30 g of quartz sand (16 – 40 mesh) were uniformly mixed and filled in the middle of the glass reaction tube. The temperature of the reactor oven was controlled by a thermocouple. The gas flows (CH3Cl and N2) were controlled using digital mass flow meters. The reactor was firstly heated to 450 °C in N2 atmosphere to remove the substances adsorbed on the catalysts and the quartz tube. Then, 90 ml·min-1 feed gas of the stoichiometric mixture (CH3Cl: N2=1:8) was continuously flowed through the reactor. After the reaction was stable for 2 h, the gas sample was taken with a collecting bag, and was analyzed by a FULI-9750 gas chromatograph for calculation. The CH3Cl conversion (X) and the selectivity (Si) was calculated according to follow equations X = (1 - FCH3Clin / FCH3Clout) * 100%

(1)

Si = Fi / (FCH3Clin - FCH3Clout) * 100%

(2)

where FCH3Clin and FCH3Clout were the molar flow rate of the CH3Cl in the feed and the reactor outlet stream, and Fi was the molar flow rate of the i product with C equivalent unit in the reactor outlet stream.

3. Results and discussion 3.1 Characterizations Figure 1 presented the XRD curves of nCamMg-HZ catalysts. All the catalysts displayed obvious characteristic peaks group at 8°, 8.8°, 23.5°, 23.8°, 24.3°, and 29.8° fitted to the MFI structure of ZSM-5 zeolite, indicating that the HZSM-5 zeolite base was preserved. Although the Mg/Ca ratio varied, no diffraction peaks of related crystal phase of CaO or MgO appeared, showing that the impregnation of Mg/Ca composite was 7


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uniformly dispersed on the surface of the zeolites. Furthermore, the intensity of the diffraction peaks of nCamMg-HZ catalysts was decreased in comparison with HZSM-5.

7Mg-HZ

2Ca5Mg-HZ

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


3.5Ca3.5Mg-HZ

5Ca2Mg-HZ 7Ca-HZ

HZSM-5 5

10

15

20

25

2 (degree)

30

35

40

Figure 1. XRD patterns of HZSM-5 and nCamMg-HZ catalysts. The N2 adsorption-desorption isotherms of HZSM-5 and nCamMg-HZ catalysts were displayed in Figure 2. The HZSM-5 and all nCamMg-HZ catalysts exhibited type I isotherms, which indicated that the nCamMg-HZ catalysts were still typical microporous materials. The detailed structural data were listed in Table 1. Compared to HZSM-5, the introduction Mg/Ca composite led to the decreases of both surface areas (from 408.1 to 306.3 - 329.9 m2·g-1) and pore volumes (from 0.222 to 0.167 - 0.181 cm3·g-1). Meanwhile, the reduction degrees were affected by Mg/Ca ratio, and the 5Ca2Mg-HZ catalyst showed the maximum surface area and pore volume among the nCamMg-HZ catalysts. Additionally, the pore diameters of all nCamMg-HZ catalysts, lower than that of HZSM-5 (0.66 nm) though, increased monotonously with the Ca content (0.49 - 0.53 nm). These results revealed that the Mg/Ca composites were introduced into the pores of HZSM-5 by ion exchange or surface loading.

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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

Intensity (a.u)


                                      

0.0

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HZSM-5                     7Mg-HZ                                  2Ca5Mg-HZ                    3.5Ca3.5Mg-HZ                     5Ca2Mg-HZ                        7Ca-HZ                       

0.2

0.4

p/p0

0.6

0.8

1.0

Figure 2. N2 adsorption-desorption isotherms of HZSM-5 and nCamMg-HZ catalysts.

Table 1. BET results of HZSM-5 and nCamMg-HZ catalysts Isotherm

Sbeta

Vpb

Dp

Type

/(m2·g-1)

/(cm3·g-1)

/(nm)

HZSM-5

I

408.1

0.222

0.60

7Mg-HZ

I

306.3

0.167

0.49

2Ca5Mg-HZ

I

316.5

0.175

0.52

3.5Ca3.5Mg-HZ

I

319.4

0.173

0.53

5Ca2Mg-HZ

I

329.9

0.181

0.53

7Ca-HZ

I

321.3

0.178

0.54

Sample

aBET

surface areas were evaluated in the P/P0 range of 0.05–0.25. bPore volumes were

evaluated at P/P0 = 0.95. The acidity of the derived catalysts was further studied by NH3-TPD techniques. 9


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Figure 3 showed the NH3-TPD curves of conventional HZSM-5 and nCamMg-HZ catalysts and Table 2 displayed the detailed desorption temperature and calculated quantities of acid. The results showed that both HZSM-5 and nCamMg-HZ catalysts had two desorption peaks. For HZSM-5, the peak at 200 °C was corresponded to the weak acid while the peak around 415 °C was corresponded to the medium strong acid. The quantity of weak and strong acids was 0.78 and 0.57 mmol·g-1, respectively. Compared with the conventional HZSM-5, the weak acid peaks of nCamMg-HZ catalysts shifted to lower temperature (173 – 183 °C) and the weak acid amount were slightly decreased (0.66 - 0.72 mmol·g-1). However, the peaks of strong acid significantly reduced with calculated acid amount declined to 0.01 - 0.14 mmol·g-1, which intuitively indicated that the Mg/Ca modification mostly covered the strong acid sites. In addition, with increasing of Ca content of nCamMg-HZ catalysts, the desorption peaks of both weak and medium strong acid had little difference in the location, while the amount of weak acid gradually decreased and the strong acid increased. The increase of strong acid sites with increasing Ca content was due to the larger molecular weight of CaO. Since the modification weight was the same (7 wt% in this work), the Ca-rich catalyst left more uncovered acidic sites. Noticeably, the surface acidity of the modified catalysts was still much less than that of the unmodified HZSM-5, although it varied with the Mg/Ca composition. The results above suggested that modification had a significant effect on the surface acidity of zeolite and was related to the Mg/Ca ratio.

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HZSM-5 7Ca-HZ

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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5Ca2Mg-HZ 3.5Ca3.5Mg-HZ 2Ca5Mg-HZ 7Mg-HZ

100

200

300

400

Temperature (℃ )

500

600

Figure 3. NH3-TPD profiles of HZSM-5 and nCamMg-HZ catalysts. Table 2. Acid data of HZSM-5 and nCamMg-HZ catalysts Sample

aAcid

Peak temperature/(°C)

Acid amounta/(mmol·g-1)

Weak

Strong

Weak

Strong

HZSM-5

200

415

0.78

0.57

7Mg-HZ

176

485

0.72

0.01

2Ca5Mg-HZ

176

483

0.71

0.03

3.5Ca3.5Mg-HZ

182

483

0.70

0.06

5Ca2Mg-HZ

176

483

0.70

0.10

7Ca-HZ

182

483

0.66

0.14

amounts were measured by NH3-TPD.

While the TPD technique showed apparent changes in weak and medium strong acids, the measurement of Pyridine-IR could provide further information to reveal the types of acids particularly. FTIR was then conducted to further study the effect of the Mg/Ca modification on the surface acidity of the catalysts. Figure 4 showed the FTIR spectra of HZSM-5 and nCamMg-HZ catalysts around 3000-4000 cm-1. It could be found 11


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that HZSM-5 contained a characteristic peak around 3600 cm-1, corresponding to the bridge Al-OH hydroxyl groups, which were generally considered as B acid sites. The bulging peaks around 3640 cm-1 corresponded to the abundant terminal Si-OH hydroxyl groups31, 32. After Mg/Ca modification, all the characteristic peaks around 3600 cm-1 were absent on the modified catalysts, which indicated the Al-OH hydroxyl groups had disappeared. It could be inferred that the modifications preferentially replaced the hydrogen of the Al-OH hydroxyl groups, and formed the structure of Al-O-Ca (Mg), thus eliminating the previous Al-OH hydroxyl groups31. Combined with TPD results, the strong acid sites of the modified catalysts decreased significantly, while the weak acid sites were less affected. This indicated that the Mg/Ca modification reduced the B-acid sites on the catalyst surface by covering the acidic bridge Al-OH hydroxyl groups. Figure 5 showed the Pyridine-IR spectra of HZSM-5 and nCamMg-HZ catalysts. It was clear that HZSM-5 had three bands at 1445, 1489 and 1542 cm-1. The bands at 1445 cm-1 and 1545 cm-1 were corresponded to Lewis acid and Brønsted acid, respectively. And the band at 1489 cm-1 was assigned to the pyridine on both Lewis and Brønsted acid sites.39 For all nCamMg-HZ catalysts, it could be observed that as the Mg/Ca modification was induced, the Lewis acid bands were almost maintained but the bands of Brønsted acid at 1542 cm-1 nearly disappeared contrarily. Combining the NH3-TPD results from Figure 3, one could deduce that the reduction of the strong acid peaks was corresponding to the absence of Brønsted acid.

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HZSM-5 7Mg-HZ 5Mg2Ca-HZ 3.5Mg3.5Ca-HZ 2Mg5Ca-HZ 7Ca-HZ

3800

3700

3600

3500

3400

Wavenumber (cm-1)

Figure 4. FTIR spectra of HZSM-5 and nCamMg-HZ catalysts.

(b) B-acid

B-+L-acid L-acid HZSM-5

Absorbance (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

Absorbance (a.u)


7Ca-HZ 5Ca2Mg-HZ 3.5Ca3.5Mg-HZ 2Ca5Mg-HZ 7Mg-HZ

1560

1530

1500

1470

1440

-1

Wavenumbers (cm )

1410

1380

Figure 5. Pyridine-IR spectra of HZSM-5 and nCamMg-HZ catalysts. 3.2 Catalytic Performance The prepared catalysts were tested in the catalytic coupling of CH3Cl to produce light olefins. The stability of ZSM-5 and nCamMg-HZ catalysts was first investigated under the temperature of 450 °C and the mass space velocity of 0.45 h-1 (VN2. VCH3Cl=8:1). The CH3Cl conversion versus time over all catalysts was shown in Figure 6. The initial conversion of all tested catalysts was close to 100% under the reaction 13


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conditions. For HZSM-5, The CH3Cl conversion was stable at ~100% in the first 22 h and decreased rapidly to 21% in the next 26 h. Compared to HZSM-5, the 7Mg-HZ catalyst was much easier to deactivate with a stable time of only 6 h while the 7Ca-HZ catalyst delivered slightly longer lifetime of 26 h. Previous work had shown that the difference in deactivation over modified ZSM-5 zeolites was caused by the coking formation which was influenced by the metal variety introduced into the catalysts.30 In this work, the exceedingly distinct performance in reaction stability of 7Mg-HZ and 7Ca-HZ catalysts was most probably caused by different coking orientation, which would be later explored in detail. For three catalysts modified with both Mg and Ca (2Ca5Mg-HZ, 3.5Ca3.5Mg-HZ and 5Ca2Mg-HZ), the lifetime was effected by Mg/Ca ratio. With the increase of the Ca content, the lifetime of the catalyst became longer, and the 5Ca2Mg-HZ catalyst showed the longest lifetime of 58 h with the CH3Cl conversion above 99.0%. These results further proved that the Ca modification was more preferred than Mg for improving catalyst durability, which was consistent with the conclusion from the comparison between 7Ca-HZ and 7Mg-HZ catalysts. Moreover, it was also observed that the 5Ca2Mg-HZ catalyst offered even longer lifetime than the Mg-free 7Ca-HZ catalyst, which implied the existence of the synergistic reaction between Ca and Mg. Another effect of Mg/Ca modification was reflected in the olefin selectivity presented in Table 3 and Figure 7. For bare HZSM-5, the total olefin selectivity was only 52.0% with quite a number of alkanes and C5+ hydrocarbons. The excessive products mainly came from the secondary reactions of hydrogen transfer and aromatization occurring on the abundant strong Brønsted acid sites according to numerous reports concerning ZSM-5 based catalysts.31,32 Notably, the modified nCamMg-HZ catalysts all showed much higher selectivity of total olefins (from 85.3% to 90.3%). The vast improvement in the selectivity of total olefins was believed to be caused by the remarkable reduction in the number of strong Brønsted acid, which had been 14



confirmed by TPD and Pyridine-IR data. In particular, different Mg/Ca ratio also led to variety in C2-C4 olefins distribution. As listed in Table 3, all the nCamMg-HZ catalysts exhibited lower C2H4 selectivity and higher selectivity of C3H6 and C4H8 than the original HZSM-5 catalyst. Moreover, with the different Mg/Ca ratio, the changes in selectivity showed different tendencies in Figure 7. One could see as the Mg modification increased, the C2H4 selectivity gradually decreased from 20.7% to 12.2%. On the contrary, the C3H6 selectivity increased with Mg addition. Taking 7Ca-HZ catalyst as an example, the C3H6 selectivity was enhanced from 23.9% to 50.2% compared with HZSM-5, and kept growing gradually with Mg modification to 60.3%. The C4H8 selectivity raised with the Mg content from 14.6% to 17.8%, also showing a trend of growth. Besides, the initial increase of Mg from 0% to 2% had a great improvement on the C3H6 selectivity (from 50.2% to 57.4%). The increasing effect of further Mg addition had leveled off (from 58.7% to 60.3%). These results indicated that moderate Mg modification was more conducive to improving selectivity of C3H6 and C4H8. The distribution of olefins on nCamMg-HZ catalysts was further related to the strong acid amount. In previous works, the acidity of bare HZSM-5 catalyst was responsible for the low C3H6 selectivity and high C2H4 selectivity for the latter product was thought to be generated from the cracking of high-carbon products on these acid sites.32 In this work, as the Mg modification increased, the strong acid amount was continuously reduced from 0.14 mmol·g-1 to 0.01 mmol·g-1. The lack of strong acid inhibited the cracking reaction and further increased the fraction of C3H6 and C4H8. Moreover, these acid sites were also considered to be where the hydrogen transfer reaction took place to produce alkanes. So it could be seen from Table 3 that the selectivity of C1-C4 alkanes all decreased with the increase of Mg.

15


Page 16 of 29

Page 17 of 29

100

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 60 40 HZSM-5 7Ca-HZ 5Ca2Mg-HZ 3.5Ca3.5Mg-HZ 2Ca5Mg-HZ 7Mg-HZ

20 0

0

10

20

30

40

50

60

Time on stream (h)

70

80

90

Figure 6. CH3Cl conversion vs time over HZSM-5 and nCamMg-HZ catalysts. Table 3. Selectivity of HZSM-5 and nCamMg-HZ catalysts. aSelectivity

Samples

/%

C1

C2

C2=

C3

C3=

C4

C4=

C5+

Total olefins

H-ZSM-5

6.7

1.1

23.7

17.5

23.9

13.6

3.4

10.1

52.0

7Ca-HZ

2.4

0.1

20.7

3.6

50.2

4.2

14.6

2.0

85.5

5Ca2Mg-HZ

1.8

0.1

15.2

3.2

57.4

3.8

14.6

2.2

87.2

3.5Ca3.5Mg-HZ 1.5

0.1

13.7

2.2

58.7

2.4

17.5

2.9

89.9

2Ca5Mg-HZ

1.7

0.1

13.1

0.8

59.5

1.1

17.5

4.2

90.1

7Mg-HZ

1.1

0.1

12.2

0.1

60.3

0.1

17.8

4.4

90.3

aSelectivity

was calculated when CH3Cl conversion was above 99.0%.

16



80

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

Page 18 of 29

60

C2= C3= C4=

40

20

0

7Ca-HZ

5Ca2Mg-HZ3.5Ca3.5Mg-HZ2Ca5Mg-HZ

7Mg-HZ

Figure 7. Selectivity of C2H4, C3H6 and C4H8 over nCamMg-HZ catalysts. To get more insights into the deactivation process, the thermogravimetric analysis was introduced to study the coking behavior on three zeolite catalysts (5Ca2Mg-HZ, 7Mg-HZ, and 7Ca-HZ). Figure 8(a) showed the changes in the coke amount of three zeolite catalysts. In the first 6 h of reaction, the carbon deposition accumulated in three catalysts was almost the same and all reached ~4 wt%. Subsequently, the coke on 7Mg-HZ catalyst gradually reached 6 wt% and gained no further increase. According to the preceding reaction data, the catalyst had been deactivated so there was no more coke accumulation generated from the reaction. The coking process on 7Ca-HZ catalyst lasted for 36 h before it became sluggish at the amount of ~10 wt% which echoed the sharp decline of its CH3Cl conversion in the corresponding time. The coke deposited on 5Ca2Mg-HZ catalyst during 56 h was continuously increasing. Similar cases had been reported in other literatures. Zeolite catalysts (such as SAPO-34 and HZSM-5) containing moderate coke deposition were, on the contrary, considered to be the most efficient and durable ones.36-38 It could be speculated that the difference in lifetime of various zeolite catalysts was due to the discrepancy in the orientation of carbon deposition. Figure 8(b) subsequently showed another expression of the coke effect on surface area to expand the 17


Page 19 of 29 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


inquiry. The specific surface area of the three catalysts were significantly reduced with the initial 2 h, which was attributed to the existence of the induction period of reaction to generate a large number of polymethylbenzene. The specific surface area of 7Mg-HZ decreased sharply from 241 m2/g-1 to 57 m2/g-1 when the coke content increased from 4% to 6%, which indicated the coke had blocked the channels, created dead volume and led to the rapid deactivation of 7Mg-HZ referring to Figure 6. The specific surface area of 7Ca-HZ underwent a slower decline from 2 h to 26 h, which was corresponded to the moderate coking process that tended to disperse on the channel surface uniformly. The gentle process continued for the longest time of 56 h on 5Ca2Mg-HZ catalyst. Combining Figure 8(a) and (b), it was suggested that the nCamMg-HZ catalysts had similar coking behaviors with uniformly distribution on the channel surface. And the over loading of the coke to obstruct the channels was the main reason of deactivation. As mentioned above, the modification weight was the same for all modified catalysts. The catalysts with more Mg content had smaller pore size according to the BET data in Table 1, which made coke deposition easier to block the channels. As shown in Scheme 1, for short-lived catalysts (such as HZSM-5 and 7Mg-HZ), the coke deposition had a greater tendency to block the channels while for others (such as 7Ca-HZ) the moderate coking could last longer.

18



16

7Mg-HZ 7Ca-HZ 5Ca2Mg-HZ

14

(a)

Coke (wt%)

12 10 8 6 4 2 0 0

Surface area (m2·g-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

Page 20 of 29

10

20

30

40

Time on stream (h)

(b)

50

60

7Mg-HZ 7Ca-HZ 5Ca2Mg-HZ

300

200

100

0 0

10

20

30

40

Time on stream (h)

50

60

Figure 8. Evolution of (a) coke content and (b) surface area vs. time over 7M-HZ, 7Ca-HZ and 5Ca2Mg-HZ catalysts.

19


Page 21 of 29

Scheme 1. Schematic illustration of coking behaviors and deactivation on zeolite catalysts with Mg/Ca modification. 7Mg-2h 7Mg-10h 7Ca-2h 7Ca-10h 5Ca2Mg-2h 5Ca2Mg-10h

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


600

800

1000

1200

1400

Wavenumber (cm-1)

1600

1800

Figure 9. Raman spectra of 7M-HZ, 7Ca-HZ and 5Ca2Mg-HZ catalysts with different reaction time. In addition, the types of coke deposits on the surface of used catalysts were also studied by Raman spectroscopy. As shown in Figure 9, two peaks appeared around 20



1100-1400 cm-1 and 1580 cm-1 in the Raman spectra of 7M-HZ, 7Ca-HZ and 5Ca2Mg-HZ catalysts after 2 h and 10 h of reaction. According to the literature, the peak group around 1100-1400 cm-1 was attributed to polyaromatics, and the second single peak was attributed to the coke with graphite structure51. It could be easily found that the ratios of the two peaks were very close regardless of the catalyst and reaction time, and the graphite peaks were much smaller than those of polyaromatics. This indicated that the coke deposits of the three catalysts in the first 10 h were mainly polyaromatic compounds.

100

Convention (%)

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

60

40

20

Fresh catalyst

0

30

60

1st regeneration

90

2nd regeneration

120 150 180 210 240 270

Time on stream (h)

Figure 10. Regeneration of 5Ca2Mg-HZ catalyst in the coupling reaction. Finally, the repeatability of 5Ca2Mg-HZ catalyst was investigated with 550 °C calcination in air as regeneration method. As shown in Figure 10, the fresh catalyst could run stably for 50 h with ~100% CH3Cl conversion. After the first and second regeneration, the CH3Cl conversion remained unchanged compared to the fresh catalyst and the reaction continued 54 h and 56 h stably. These results showed that the 5Ca2Mg-HZ catalyst had remarkable reproducibility. The high activity and excellent 21


Page 22 of 29

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stability of the catalyst could be maintained by burning off coke. It was also confirmed that the deactivation was not attributed to the irreversible structural damage of active sites, but due to the coke deposition to block the channels, which led to the separation of the active sites from the reactants.

4. Conclusions In this work, a series of Mg/Ca modified HZSM-5 zeolites were first prepared by for CH3Cl coupling to produce light olefins. Compared to bare HZSM-5, the nCamMg-HZ catalysts all had higher conversion of ~100% and total olefin selectivity of ~90%. The promoted olefins selectivity was attributed to their decreased amount of strong acid sites which inhibited the secondary reactions of hydrogen transfer and aromatization. The Ca-rich catalysts tended to have longer lifetime during the reaction. The Mg/Ca ratio also affected the selectivity of C2H4, C3H6 and C4H8. The Mg modification was more conducive to improving selectivity of C3H6 and C4H8 than Ca, which was further related to its ability to lower the strong acid amount. Coke analysis demonstrated that the nCamMg-HZ catalysts had similar coking behaviors before getting deactivated, and the over loading of coke to block the channels was responsible for catalyst deactivation. The 5Ca2Mg-HZ catalyst had the best performance and could still kept for 58 h with the C3H6 selectivity of 57% at ~100% CH3Cl conversion after 2 regenerative cycles. The regeneration test also showed that the coke deposit on the catalyst could be removed easily through calcination in air.

Acknowledgments Financial supports from 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.

22



Page 24 of 29

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Enhanced Light Olefin Production in Chloromethane Coupling over

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