A Preliminary Study on the Role of the Internal and External Surfaces

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Kinetics, Catalysis, and Reaction Engineering

A preliminary study on the role of internal and external surface of Nano-ZSM-5 Zeolite in the alkylation of benzene with methanol Ji Qian, Guang Xiong, Jiaxu Liu, Chunyan Liu, and Hongchen Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00291 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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A preliminary study on the role of internal and external surface of Nano-ZSM-5 Zeolite in the alkylation of benzene with methanol Ji Qian, Guang Xiong, Jiaxu Liu, Chunyan Liu, Hongchen Guo* Department of Catalytical Chemistry and Engineering, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, PR China * Corresponding author. Tel & Fax: +86-411-84986120 Mobile: +8618698614478 E-mail: [email protected]

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Abstract The catalytic behaviors of the internal and external Brönsted acid sites on HZSM-5 zeolite were investigated in the alkylation of benzene with methanol (MeOH). The external surface passivated HZSM-5 zeolites were prepared by the silanization of tetraethyl orthosilicate, while the internal surface passivated HZSM-5 zeolites were obtained by sodium cation exchange followed by tetrapropylammonium cation re-exchange. Ammonia temperature-programmed desorption (NH3-TPD) and pyridine/ 2,6-Di-tert-butylpyridine adsorption FT-IR spectroscopy were used to characterize the acidity of the modified zeolites. Results indicate that the internal surface of the HZSM-5 zeolite is not an ideal place for the alkylation of benzene with methanol since the formation of the by-product (ethylbenzene) is favored. In contrast, the external Brönsted acid sites on the internal surface passivated HZSM-5 zeolites exhibit not only significantly better xylene selectivity but also good alkylation activity, anti-coking deactivation ability and regeneration ability through coke burning.

Keywords: internal surface; external surface; ethylbenzene selectivity; alkylation of benzene with methanol; HZSM-5 zeolite

1.

Introduction Toluene and xylene are important chemical intermediates for the production of chemicals.1-4 The

majority of toluene is used to produce benzene and xylenes. Toluene can also be used as solvent, or be blended into unleaded gasoline due to its high octane number.5, 6 Xylenes are used for the production of phthalic anhydride, isophthalic acid, terephthalic acid and dimethyl terephthalate. Terephthalic acid and dimethyl terephthalate are used to produce polyethylene terephthalate fibers, polyester resins and films. 6-9 In 2016, the world demand for paraxylene (PX) was approximately 39 million metric tons. The conventional source of toluene and xylene is naphtha, which is expensive and limited as fossil energy resources. Toluene and xylene can also be obtained by the alkylation of benzene with MeOH. On one hand, MeOH is a renewable resource, which can be produced from biomass, coal and natural gas via syngas. The technologies for the production of MeOH from coal and natural gas are very mature currently. On the other hand, the steam cracking process produces large amounts of benzene. With the progress of the gasoline standard in China, the benzene content in gasoline pool will 2



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be reduced from 1.0 to 0.8 vol.%, which will release a large amount of benzene from the gasoline pool. Therefore, the alkylation of benzene and MeOH displays an important industrial significance. In 1970, Yashima et al.10 firstly reported that PX can be produced by the alkylation of toluene and MeOH on Y zeolite. Their results indicated that the selectivity of PX reached 50%, which is much higher than that of thermodynamic equilibrium. Later, the shape selective alkylation of toluene with MeOH was extensively studied over high-silica ZSM-5 zeolite. High selectivity of para-xylene can also be obtained with the modified HZSM-5 zeolite catalysts.11-13 It is well known that the alkylation of toluene with MeOH is catalyzed by the Brönsted acid sites of the zeolite catalysts10, 14. Recently, a multiple fixed-beds process of the toluene alkylation with MeOH to produce PX (MTX) has been announced15. The Over the rare earth and alkaline earth elements modified ZSM-5 catalyst, the MTX process demonstrated 27% toluene conversion and 94% PX selectivity. The fluidized-bed toluene/MeOH alkylation and MeOH to olefins (MTO) reactions integration process was also claimed16. This integrated process used the modified ZSM-5 zeolite as catalyst, high selectivity to PX was obtained (10% toluene conversion and 97% PX selectivity) together with considerable amount of the ethylene and propylene co-products (0.2-0.3 tons of ethylene and propylene will be generated for each ton of PX produced). According to the literatures, both the alkylation of toluene with MeOH and alkylation of benzene with MeOH are catalyzed by the Brönsted acid sites17. There are several experimental and theoretical studies on the mechanism of benzene alkylation with MeOH.18-22 Generally, the literatures showed two distinctly different routes for the benzene alkylation with MeOH: one called stepwise (consecutive) mechanism which is characterized by the adsorption of MeOH over Brönsted acid sites to form methoxy group, and the reaction of methoxy group with benzene to form toluene; The other called direct (concerted) mechanism which is featured by the co-adsorption of MeOH and benzene over Brönsted acid sites to directly form toluene. So far, many zeolites including X, Y, MOR, ZSM-5, ZSM11, ZSM-12, MCM-22 and SAPO-34/11 have been employed as the catalysts for the alkylation of benzene and MeOH10, 23-32. ZSM-5 seems to be better than the other zeolites. There are several studies concerned the comparison of the catalytic performances of the different HZSM-5 zeolites in alkylation of benzene with MeOH.33 Results showed that the small ZSM-5 crystal displayed high benzene conversion and anti-deactivation ability. The advantages of ZSM-5 zeolite include low price, good 3


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hydrothermal stability, suitable channel, sufficient acidity and excellent anti-coking deactivation ability. However, HZSM-5 catalyst suffers one major drawback in the alkylation of benzene with MeOH. That is the formation of considerable amount of ethylbenzene.34-38 Ethylbenzene is an isomer of C8 aromatics. The co-existence of ethylbenzene with xylenes will remarkably decrease the efficiency of xylene isomerization in PX production. Therefore, the inhibition of ethylbenzene formation is an important task, which must be accomplished in order to develop an industrially viable benzene and MeOH alkylation catalytic technology. Recent publications1, 37 indicated that, the loading of the precious metal Pt on ZSM-5 can prominently suppress the formation of ethylbenzene under H2 atmosphere in the alkylation of benzene with MeOH. The effect of the Pt modification lies in the hydrogenation of ethylene to ethane, which restrained the alkylation of benzene with ethylene to form ethylbenzene. However, when 0.05 wt.% Pt/ZSM-5 was used as the catalyst, the utilization efficiency of MeOH decreased significantly from 72.7% to 52.7%. Besides, zinc modification of the hierarchical porous ZSM-5 was also reported to suppress the formation of ethylbenzene under hydrogen atmosphere.38 The effect of zinc was interpreted as decreasing Brönsted acidity and therefore suppressing MeOH to olefins. In addition to the Pt and Zn modifications, the effects of other metal and non-metal modification, including Na, Co, Cu, P, B and N, on the catalytic performance of HZSM-5 in the alkylation of benzene with MeOH were also investigated39-43. General speaking, the decrease of Brönsted acidity and ethylene formation are the essential methods to suppress ethylbenzene in the alkylation of benzene with MeOH. In this paper, a nano-ZSM-5 with SiO2/Al2O3 ratio of 30 was employed as the catalyst for the alkylation of benzene with MeOH. This study focused on the effect of the internal and external surface acid sites of ZSM-5 zeolite on the formation of ethylbenzene. The external surface passivation of ZSM5 catalyst was achieved by the silanization of HZSM-5 with tetraethoxysilane. The internal surface passivation of ZSM-5 catalyst was obtained by the ion exchange of NaZSM-5 with tetrapropylammonium bromide solution. The acidity characterization was measured by the pyridine (Py)/2,6-Di-tert-butylpyridine (DTBPy) adsorption FT-IR spectroscopy and the ammonia temperatureprogrammed desorption (NH3-TPD) method. Surprisely, the results show that ethylbenzene is mainly formed inside the micropores of ZSM-5. This phenomenon has not been observed in the literatures.

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2.

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Experimental

2.1. Catalyst preparation Two series of catalysts were prepared to study the roles of internal and external acid sites of ZSM5 zeolite in the alkylation of benzene with MeOH. The extruded strip HZSM-5 catalyst (zeolite: Al2O3= 80: 20) manufactured with nano-ZSM-5 zeolite (nSiO2/nAl2O3 = 30, crystal size: 20~50 nm) was used as parent catalyst. The internal surface passivated ZSM-5 catalysts were prepared as follows: First, the NaZSM-5 catalyst was prepared by sodium cation exchange. The sodium cation exchange was carried out at 80 ℃ with 1.0 mol/L NaNO3 solution (liquid/solid ratio= 5: 1). The process was repeated three times to guarantee a complete sodium cation exchange of hydrogen protons. The NaZSM-5 catalyst was dried at 120 ℃ for 12 h, and then calcined at 540 ℃ for 6 h. Then, the NaZSM-5 catalyst was subjected to a tetrapropylammonium cation re-exchange with 1.0 mol/L tetrapropylammonium bromide solution (25℃, liquid/solid ratio= 5: 1), which was followed by drying at 120 ℃ for 12 h, and calcination at 540 ℃ for 6 h. Finally, the internal surface completely passivated catalyst was obtained and named as In-ZSM-5-2.3% (2.3% is the weight percent of sodium in the catalyst, which was measured by ICP-AES). Another internal surface partly passivated catalyst In-ZSM-5-1.9% was prepared by the same method except that the sodium ion exchange of HZSM-5 was repeated twice. The external surface passivated ZSM-5 catalyst was prepared by the silanization of HZSM-5 with tetraethyl orthosilicate in the presence of cyclohexane solvent44. 5.0 gram of HZSM-5 was impregnated in 4.5 ml solution (1.4 mol tetraethyl orthosilicate /L cyclohexane). The impregnation was carried out at room temperature for at least 3 h. Then, cyclohexane solvent was removed by distillation. The sample was dried at 110 ℃ for 60 min and calcined at 540 ℃ for 6 h. The calcined sample was an external surface partly passivated catalyst which was named as Ex-ZSM-5-5% (5% is the weight percent of SiO2 provided by tetraethyl orthosilicate based on the dry basis weight of HZSM-5). The external surface completely passivated catalyst Ex-ZSM-5-15% was prepared by the same method except that the impregnation operation was repeated three times after drying. 2.2. Catalyst characterization 2.2.1 Acidity The acidity characterization was done by pyridine (Py)/2,6-Di-tert-butylpyridine (DTBPy) probe 5


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molecule adsorption FT-IR spectroscopy and ammonia temperature-programmed desorption (NH3TPD) method. The IR spectra were recorded with a Nicolet is 10 FT-IR spectrometer in the range from 4000 to 400 cm-1 with an optical resolution of 4 cm-1. The catalysts were pressed into self-supporting thin wafers (approximately 15 mg) and decontaminated at 400 ℃ under vacuum (10-3 Pa) for 4 h in a quartz IR cell equipped with CaF2 windows. After the pretreatment, the cell was cooled down to room temperature to record the background spectrum. The spectra of the adsorbed probe molecule were obtained as follows: first, probe molecule adsorption was carried out at 35℃; second, the evacuation treatment (10-3 pa) was conducted for 30 min at 150 ℃; Then the cell was cooled down to room temperature for the measurement; finally, the spectra were obtained by subtracting the background spectrum from the measured spectra. The total amount of Brönsted acid sites was calculated from the absorption band at 1540 cm-1, by using the integrated molar extinction coefficient (IMEC) of the band at 1540 cm-1 (2.22 cm/μmol). The calculation equation of the Brönsted acid sites is as follows: C (pyridine on Brönsted sites) = πR2·IA/(IMEC(Brönsted)·W); R = radius of catalyst disk (cm); W = weight of disk (mg). The amount of the external Brönsted acid sites was determined by the absorption band at 1540 cm-1. Since the IMEC of the band is unavailable, a conversion factor between Pyridine (Py) and 2,6Di-tert-butylpyridine (DTBPy) was calculated by the absorption bands of Py and DTBPy adsorption on the internal surface completely passivated catalyst In-ZSM-5-2.3%, supposing that the Brönsted acid sites on the external surface of catalyst In-ZSM-5-2.3% could be equally approached and adsorbed by Py and DTBPy. Supposing that the interaction of Brönsted acid site and probe molecule is 1: 1 for both Py and DTBPy.45, 46 The NH3-TPD was carried on a Quantachrome Chembet 3000 chemisorb instrument. First, 0.14 gram of catalyst (20-40 mesh) was loaded into a quartz U reactor and decontaminated at ammonia temperature-programmed desorption (NH3-TPD) method under helium for 1.0 h; Second, the temperature was cooled down to 150 ℃ for NH3 adsorption which was lasted for 30 minutes. Then, physically adsorbed NH3 was removed by continuous helium purging. Finally, the desorption pattern was recorded from 150 ℃ to 600 ℃ in 20 ml/min helium carrier gas. The temperature was increased at a rate of 15 ℃·min-1. 6



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2.2.2 Dynamic adsorption/diffusivity characterization To study the diffusivity the dynamic adsorption of benzene and MeOH over the samples was measured on Simultaneous Thermal Analysis (Netzsch STA449F3) apparatus. For this purpose, a selfassembled accessory was used. First, 0.15 gram of catalyst (40-60 mesh) was loaded into a crucible and decontaminated at 600 ℃ under N2 for 3.0 h; Second, the temperature was cooled down to 35℃ for probe molecule adsorption and adsorption pattern recording. The diffusion coefficient of benzene and MeOH in the catalysts was calculated according to Frick's law.47, 48 ∞

mt − m0 6 1 n2 π2 Dt = 1 − 2 ∑ 2 exp⁡(− ) m∞ − m0 π n R2 n=1

D— diffusion coefficient, m2/s; t — absorption time, s; mt — absorption capacity of catalyst, when the time at t s, mg/g; m∞— equilibrium absorption capacity, mg/g; R— Particle radius, m; 2.2.3 Other characterizations Powder X-ray diffraction (XRD) patterns were recorded with a RigakuD/max-2004 diffractometer using Cu Kα radiation (40 kV, 100 mA) at a scanning rate of 0.02° min-1 (2θ). Field Emission Scanning Electron Microscopy (FE-SEM) images were collected on a Hitachi S-4800 microscope. Argon adsorption/desorption isotherms of samples was recorded at 77 K on a Micromeritics ASAP 2020 instrument according to reported procedures49. Simultaneous Thermal Analysis (STA) of the samples was conducted using a Netzsch STA449F3 apparatus. The experiments were carried out in the temperature range of 313-1073 K, at a heating rate of 10 K·min-1 in flowing air. The chemical compositions of the samples were determined by a Shimadzu ICPS-8000E inductively coupled plasma atomic emission spectrometer (ICP). The ratio of C and H in coke was measured by Perkin-Elmer 2400CHN elemental analysis apparatus.

2.3. Catalytic reaction The alkylation performances of the catalysts were evaluated on a fixed-bed reactor. Catalyst (2 g, 7


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20-40 mesh) was loaded into the thermostatic region of the reactor and filled with inert ceramic balls at both ends. The alkylation was carried out under the following conditions: 400 ℃, 1.0 MPa, weight hourly space velocity (WHSV) of benzene 1.66 h-1, and benzene/MeOH molar ratio 2/1. Before the mixture of benzene and MeOH was pumped into the reactor, the catalyst was activated at 400 ℃ in a dried N2 flow for 1 h. Gas products were sampled every 12 h and analyzed with a gas chromatograph (domestic GC-7890F equipped with FID detector (matched with PLOT Al2O3 capillary column) and TCD detector (matched with 60-80 mesh-TDX-01 packed column)). Liquid products were collected every 24 h. Oil phase was analyzed by a domestic GC SP-6800A (PEG-20M capillary column, FID detector), water phase was analyzed by an Agilent 6890 offline gas chromatograph (GC) equipped, respectively. The conversion of benzene and MeOH, the selectivity of ethylbenzene, the methylation selectivity of aromatics, and the percentage of ethylbenzene in C8 (EB/C8) were defined as follows (wt.): Conversion of benzene = (benzene in feed-benzene in product)/ benzene in feed×100% Conversion of MeOH = (MeOH in feed-MeOH in product)/ MeOH in feed×100% Selectivity of ethylbenzene = ethylbenzene/ alkylaromatics×100% Selectivity of ethylene = ethylene/ (olefins + alkanes)×100% EB/C8 = ethylbenzene/ (ethylbenzene + xylene)×100%

3.

Results and discussion

3.1. Basic physicochemical properties characterization

Figure 1. XRD pattern (left) and SEM image (right) of powder ZSM-5 zeolite parent

Figure 1 shows XRD pattern and SEM image of the parent ZSM-5 zeolite. The parent ZSM-5 8



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zeolite is a nano-sized ZSM-5 zeolite with silica to alumina ratio is 30. The primary crystal size is in the range of 20-100 nm50. Figure 2 shows that both the parent HZSM-5 zeolite extrudate and the modified samples show similar argon adsorption/ desorption isotherms, which belong to typical IVtype isotherm. However, the internal surface passivated samples (In-ZSM-5-1.9% and In-ZSM-5-2.3%) show smaller pore size (approximately 0.51 nm) than that of the parent HZSM-5 zeolite extrudate. The external surface passivated samples (Ex-ZSM-5-5% and Ex-ZSM-5-15%) exhibit the similar pore size (approximately 0.54 nm) to that of the parent HZSM-5 zeolite extrudate. This indicates that the Na+ partly occupied the micropores of the internal surface passivated HZSM-5 zeolites, which narrows the micropores. However, the silica deposition on the external surface passivated HZSM-5 zeolite cannot effect the micropores of the zeolite although quite high amount of silica was deposited. Table 1 shows that both the internal and the external surface passivated samples lose their surface area significantly. This implies that silanization can block the micropores. Nevertheless, the process of the internal surface passivation can lead to the sintering of the aggregated nanometer zeolite (see Figure 1) and the decrease in the external surface area, which is due to the repeated calcination in the tetrapropylammonium cation exchange. The explanation for this will be the task of our future study. However, the following reaction evaluation can illustrate the influence of the external surface by repeated tetrapropylammonium cation exchange of external Na+.

Figure 2. Argon adsorption/desorption isotherms (left) and pore size distributions (right) of parent HZSM5 zeolite extrudate and its modified samples (internal surface passivation: In-ZSM-5-1.9% and In-ZSM-52.3%; external surface passivation: Ex-ZSM-5-5% and Ex-ZSM-5-15%)

Table 1 Textural data of parent HZSM-5 zeolite extrudate and its modified samples obtained from argon adsorption 9


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/desorption isotherms SBET

Sexter

Vtotal

Vmicro

m2g-1

m2g-1

cm3g-1

cm3g-1

HZSM-5

325

100

0.290

0.128

NaZSM-5

216

56

0.229

0.090

Ex-ZSM-5-5%

255

87

0.243

0.095

Ex-ZSM-5-15%

217

73

0.195

0.082

In-ZSM-5-1.9%

250

74

0.263

0.099

In-ZSM-5-2.3%

230

62

0.247

0.095

Sample

3.2. Acidity characterization Figure 3 and table 2 show the ammonia temperature-programmed desorption (NH3-TPD) profiles. Both the silanization modifications with tetraethyl orthosilicate and the sodium cation exchange followed by tetrapropylammonium cation re-exchange significantly alter the acidity of HZSM-5 zeolite. However, the two modifications exhibit the striking difference. For the silanization modification, the decrease in the amount of the strong acid sites (corresponding to high temperature desorption peak) is limited. Considerable amount of the strong acid sites is reserved on the catalyst Ex-ZSM-5-15% even though very large amount of tetraethyl orthosilicate is applied (equivalent to 15 % SiO2 based on the dry basis weight of HZSM-5 zeolite). In the case of sodium cation exchange treatment, however, the complete loss of the strong acid sites is observed on NaZSM-5 zeolite after three times repeated sodium cation exchange. The small amount of the strong acid sites is recovered after tetrapropyl-ammonium cation re-exchange of the NaZSM-5 (see In-ZSM-5 2.3%). This is because that the silanization passivates the external surface of HZSM-5 zeolite while the sodium cation exchange-tetrapropylammonium cation re-exchange passivates the internal surface of HZSM-5 zeolite. Compared with the slight recovery of the strong acid sites by the sodium cation exchange-tetrapropylammonium cation re-exchange, the silanization results in a more obvious decrease in the amount of the strong acid sites. Theoretically, both changes are related to the amount of external surface acid sites. Most of the strong acid sites disappeared during the silanization process should be ascribed to the inaccessibleness of the internal surface strong acid sites when the pore mouth was blocked by the 10



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silanization. As mentioned above in the argon adsorption experiment in table 1.

Figure 3 NH3-TPD profiles of HZSM-5 and its modified counterparts obtained by internal surface passivation (InZSM-5-1.9% and In-ZSM-5-2.3% in left) and external surface passivation (Ex-ZSM-5-5% and Ex-ZSM-5-15% in right)

Table 2 Acidic properties of HZSM-5 and its modified counterparts obtained by internal surface passivation (InZSM-5-1.9% and In-ZSM-5-2.3%) and external surface passivation (Ex-ZSM-5-5% and Ex-ZSM-5-15%) Acidic by strength a (mmol/g) Samples

a

Strong

Medium

Weak

Total

HZSM-5

0.289

0.097

0.168

0.553

Ex-ZSM-5-5%

0.195

0.086

0.107

0.388

Ex-ZSM-5-15%

0.136

0.057

0.056

0.249

In-ZSM-5-1.9%

0.114

0.146

0.109

0.369

In-ZSM-5-2.3%

0.075

0.169

0.158

0.402

Na-ZSM-5

---

0.274

0.136

0.410

Density of the acid sites, assorted according to the acidic strength, determined by NH3-TPD. Strong, NH3 desorbed

at 350-550 ℃; medium, NH3 desorbed at 250-350 ℃; weak, NH3 desorbed at 150-250 ℃.

The alkylation of benzene with MeOH is mainly catalyzed by Brönsted acid sites. Therefore both the internal surface and external surface passivated HZSM-5 zeolites were further subjected to Py and DTBPy adsorption FT-IR spectroscopy characterization44,

51-55

. The infrared absorption band of

pyridine adsorption at 1540 cm-1 corresponds to Brönsted acid sites located on both internal and 11


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external surface, while the absorption of 2,6-Di-tert-butylpyridine adsorption at 1530 cm-1 is related with Brönsted acid sites on the external surface45, 46, 52. As it is expected, Figure 4 confirms that, there are no Brönsted acid site on the external surface of the passivated HZSM-5 zeolites (Ex-ZSM-5-5% and Ex-ZSM-5-15%). However, there are still a lot of Brönsted acid sites on the internal surface of the zeolites. The internal surface passivated HZSM-5 zeolites (In-ZSM-5-1.9% and In-ZSM-5-2.3%), all have considerable amount of Brönsted acid sites on their external surface. In order to quantitatively compare the effect of the internal and external Brönsted acid sites on the catalytic performance, the percentage of the external Brönsted was estimated by comparing the absorption peak areas of 2,6-Ditert-butylpyridine adsorption and pyridine adsorption on the internal surface completely passivated catalyst In-ZSM-5-2.3% according to literatures45, 46. Finally, the amount of external Brönsted acid sites was obtained for all catalysts in table 3. A

B

B′ A′

Figure 4. FT-IR spectra of pyridine absorption (A and A′) and 2,6-Di-tert-butylpyridine absorption (B and B′) on HZSM-5 and its modified counterparts (Ex-ZSM-5-5% and Ex-ZSM-5-15% were external surface passivated HZSM-5 zeolites, In-ZSM-5-1.9% and In-ZSM-5-2.3% were internal surface passivated HZSM-5 zeolites )

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Table 3 Brönsted acid site (BAS) amount of external surface and internal surface of HZSM-5 zeolite and its modified counterparts Total BAS

External BAS

Internal

Percentage

(Py)

(DTBPy)

BAS

of internal

(μmol.g-1)

(μmol.g-1)

(μmol.g-1)

BAS (%)

HZSM-5

81

46

35

43

Ex-ZSM-5-5%

62

7

55

89

Ex-ZSM-5-15%

15

1

14

93

In-ZSM-5-1.9%

50

46

4

8

In-ZSM-5-2.3%

45

45

0

0

NaZSM-5

0

-

-

-

Sample

3.3 Alkylation of benzene with MeOH over the external and internal surface passivated HZSM-5 zeolites Figure 5 indicates that, under same reaction conditions, both the external and internal surface passivated HZSM-5 zeolites show similar conversions for MeOH and benzene. However, it is amazing that the external surface passivated catalysts (Ex-ZSM-5-5% and Ex-ZSM-5-15%) remarkably enhance the formation of ethylbenzene by-product. On the contrary, the internal surface passivated catalysts (In-ZSM-5-1.9% and In-ZSM-5-2.3%) significantly suppress the formation of ethylbenzene. Table 4 shows that the passivation of the internal surface simultaneously promotes the formation of xylenes. As a result, the ratio of ethylbenzene to total C8 aromatics (EB/C8) decreases notably for the internal surface passivated catalysts. Undoubtedly, the inhibition of the ethylbenzene formation and meanwhile promotion of the production of xylenes (i.e. increase of EB/C8 ratio) is a distinct merit of the internal surface passivated HZSM5 zeolites. In order to evaluate the practical significance of the internal surface passivated zeolites in the alkylation of benzene with MeOH, the In-ZSM-5-1.9% catalyst was selected for durability investigation. Table 3 and Figure 6 (A) indicate that the parent HZSM-5 zeolite has 81 μmol.g-1 total Brönsted acid sites. In addition, the In-ZSM-5-1.9% catalyst has 50 μmol.g-1 total Brönsted acid sites, which has more than 90% of the active sites located on external surface. But the In13


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ZSM-5-1.9% and parent catalysts have similar activity and stability. Figure 6 (B) exhibits that, the internal surface passivated catalyst also shows a good coke-burning regeneration performance. No obvious change in the activity and selectivity is observed after 3 cycles of regeneration-reaction. Besides, if a more careful comparison can be made between the parent HZSM-5 zeolite and the modified counterpart in Figure 6, the more pronounced drops of ethylbenzene selectivity and EB/C8 ratio with time on stream can be seen for the internal surface passivated catalyst, which is also desired. Figure 7 and table 5 indicate that, the structure and composition of the catalysts has not been obviously changed during the regeneration process. Moreover, compared with HZSM-5C, In-ZSM-5-1.9%-C has a small amount of coke, which has a lower molar ratio of C and H. There are two possible reasons. On the one hand, the MTO reaction weakens due to a small amount of Brönsted acid sites on In-ZSM-5-1.9%. On the other hand, the Brönsted acid sites of In-ZSM-51.9% are concentrated on the external surface, which is the main place of the reaction. Therefore, it is conducive to the diffusion of products and slow down the carbon deposition rate.

Figure 5 Reaction performance of benzene alkylation with MeOH over external surface passivated (left) and internal surface passivated (right) HZSM-5 zeolites (T= 400 ℃, P = 1.0 Mpa, WHSV= 2 h-1, nBenzene/ nMeOH= 2)

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Table 4 Product distributions of benzene alkylation with MeOH over external surface passivated and internal surface passivated HZSM-5 zeolites Sample

HZSM-5 Ex-ZSM-5-5%

Ex-ZSM-5-15%

In-ZSM-5-1.9% In-ZSM-5-2.3%

CH4

1.58

1.48

0.56

1.31

1.67

C2H6

0.29

0.36

0.21

0.26

0.21

C2H4

0.62

0.22

0.07

0.54

5.98

C3H8

2.96

7.56

5.43

2.57

1.34

C3H6

1.14

0.16

0.16

0.93

1.01

C4H10

1.49

1.47

2.04

2.10

1.37

C4H8

0.11

-

-

0.09

0.04

Toluene

52.70

50.04

50.38

53.67

53.22

Xylene

15.77

14.37

12.85

14.17

15.14

Ethylbenzene

11.67

11.78

14.83

6.40

3.03

Trimethylbenzene

2.36

2.09

1.41

2.25

2.30

Propylbenzene

1.72

2.47

3.36

6.94

7.03

Ethyltoluene

1.47

1.37

1.65

2.37

1.27

Tetramethylbenzene

0.83

0.77

0.55

1.00

1.03

C10+

5.29

5.86

6.50

5.40

5.36

Note: reaction was carried out at T = 400 ℃, P= 1.0 Mpa, WHSV =1.66 h-1, nBenzene/nMeOH=2, TOS =24 h.

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Figure 6 A comparison on the durability of internal surface passivated catalyst In-ZSM-5-1.9% with parent HZSM-5 (A) and coke-burning regeneration performance of catalyst In-ZSM-5-1.9% (B). The alkylation of benzene with MeOH was conducted at T= 400 ℃, P = 1.0 Mpa, WHSV= 2 h-1, and nBenzene/nMeOH= 2.

Table 5 Textural data and composition of regeneration HZSM-5 and In-ZSM-5-1.9% catalysts SBET

Sexter

Vtotal

Vmicro

Na+

n(C)/n(H) ratio

m2g-1

m2g-1

cm3g-1

cm3g-1

%

of Coke

HZSM-5-Re

323

100

0.291

0.128

0.03

1.216

In-ZSM-5-1.9%-Re

240

71

0.261

0.098

1.9

0.462

Sample

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B

A

Figure 7 Weight loss of HZSM-5-C and In-ZSM-5-1.9%-C under air by STA (A). And the XRD patterns of catalysts (B). HZSM-5-C and In-ZSM-5-1.9%-C are the used catalysts, after reaction 48 h. HZSM-5-Re and In-ZSM-5-1.9%-Re are regenerated by coke burning.

4.

Disscusion It is fascinating that more ethylbenzene by-product is formed inside the micropores of HZSM-5

zeolite while less ethylbenzene by-product is formed at the external surface (Schematic 1). However, the clarification of the phenomenon is difficult due to the possible involvements of a lot of factors including zeolitic pore structure, active site acidity, diffusivity, competitive adsorptions and reactions, etc. The difference in the acid strength of the internal and external Brönsted acid sites is one of the main related factors. It is generally accepted that the bridging hydroxyls were formed when the tetrahedral framework aluminum atoms of ZSM-5 zeolite are located at the intersections of micropores. However, the terminal hydroxyls were formed when they are located at the external surface. We have carried out some other experiments in this regards. As shown in Schematic 1, to understand the difference of external and internal surface for the alkylation of benzene and MeOH, the MeOH to olefin (MTO) reaction was conducted in a pulse microreactor over the internal and external passivated HZSM-5 zeolites. It is found that both the external surface passivated HZSM-5 zeolites along and the parent HZSM-5 facilitate the MTO reactions as indicated by the rather high ethylene selectivity (Figure 8). In contrast, the MTO reaction over the internal surface passivated HZSM-5 zeolites is not favorable. Many publications1, 37, 38, 41, 42, 56, 57 indicated that, the formation of ethylbenzene is directly related to the ethylene, which is produced by MeOH through MTO in the alkylation of benzene with MeOH.

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Moreover, there have been many in-depth studies58-63 on the MTO/MTP reaction of MeOH. Most studies indicate that MeOH is easy to produce light olefin hydrocarbon through the mechanism of hydrocarbon pool on the Brönsted acid sites of zeolite. Besides, if a more careful comparison can be made between the parent HZSM-5 zeolite and the modified counterpart in Figure 5, 8 and table 3. It seems that the selectivity of ethylene in MTO reaction decreases with the decrease in the percentage of internal BAS. Accordingly, the ethylbenzene selectivity in the alkylation of benzene with methanol decreases with decreasing the selectivity of ethylene in MTO. The other interesting experiment is the characterization of the competitive adsorption of benzene and MeOH over the modified catalysts. As shown in Figure 9 and table 6, both the passivations on the external and internal surface have impact on the adsorptions of benzene and MeOH on HZSM-5 zeolite. It seems that the diffusion coefficient of MeOH is more sensitive to the modifications than that of benzene, because of the diffusion coefficient of methanol over the different catalysts changed greatly (HZSM-5>Ex-ZSM-5>In-ZSM5), while the diffusion coefficient of benzene barely changed (HZSM-5≈Ex-ZSM-5≈In-ZSM-5). However, no relevant interrelation is found for the ethylbenzene selectivity in the alkylation of benzene and MeOH so far. Therefore, the percentage of internal BAS is one of the factors that contribute to the formation of ethylbenzene, because of it can be able to regulate the selectivity of ethylene in MTO reaction. However, more effort will be put into finding out the other cause of the ethylbenzene formation on HZSM-5 zeolite during the alkylation of benzene with MeOH.

Schematic 1. Difference of external and internal surface for the alkylation of benzene and MeOH

18



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Figure 8 Conversion of MeOH and Selectivity of ethylene of MeOH to olefin reaction (MTO) over the external and internal surface passivated HZSM-5 zeolites evaluated with a pulse microreactor (conditions: catalyst loading: 0.2 g, 20-40 mesh; reaction temperature: 400 ℃; carrier gas: N2, 20 mL s-1; input dosage: 1 μL)

A

B

B′

A′

Figure 9 Dynamic adsorption of benzene and MeOH over external surface (A and B) and internal surface (A’ and B’) passivated HZSM-5 zeolites Table 6 The adsorption and diffusion properties of the internal and external surface passivated HZSM-5 zeolites

19


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obtained from benzene and MeOH dynamic adsorptions. Diffusion Maximum

Diffusion Maximum adsorption

coefficient of Sample

adsorption capacity

coefficient of capacity of benzene

MeOH (DMeOH) of MeOH (g.g-1), %

DMeOH/DBen benzene (DBen)

(g.g-1), % ×108

(m2.S-1)

×108 (m2.S-1)

ZSM-5

9.77

7.17

7.94

2.95

2.43

Ex-ZSM-5-5%

8.34

1.43

6.08

2.86

0.50

Ex-ZSM-5-15%

7.04

1.34

4.87

2.37

0.57

In-ZSM-5-1.9%

7.69

0.85

6.18

2.45

0.35

In-ZSM-5-2.3%

7.23

0.98

5.58

2.45

0.40

Note: the diffusion coefficient was calculated based on the first two-minute dynamic adsorption of benzene and MeOH.

5. Conclusions This study shows that the internal surface of HZSM-5 zeolite provided by micropores is not an ideal place for the alkylation of benzene with MeOH because it facilitates the formation of ethylbenzene by-product. Sodium cation exchange followed by tetrapropylammonium cation reexchange is an effective method to passivate the Brönsted acid sites inside the pores and catalyze the alkylation on external Brönsted acid sites alone. The internal surface passivated HZSM-5 zeolites exhibit not only significantly better xylene selectivity but also good alkylation activity, anti-coking deactivation ability and regeneration ability through coke burning. The percentage of internal BAS is one of the factors that contribute to the formation of ethylbenzene. However, other primary causes of ethylbenzene formation on HZSM-5 zeolite during the alkylation of benzene with MeOH will be investigated in the future.

6. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (No. 21603023) for financial support.

20



Page 22 of 27

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TOC

26


A Preliminary Study on the Role of the Internal and External Surfaces

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