Conversion of Methanol to Light Aromatics on Zn ... - ACS Publications

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Conversion of Methanol to Light Aromatics on Zn-Modified Nano-HZSM‑5 Zeolite Catalysts Gui Quan Zhang, Ting Bai, Teng Fei Chen, Wen Tao Fan, and Xin Zhang* School of Chemical Engineering, Northwest University, Xi’an 710069, China ABSTRACT: A series of Zn-modified nano-HZSM-5 (Zn/NZ) zeolite catalysts were prepared and used in the conversion of methanol to light aromatics (benzene, toluene, and xylene; BTX). The reaction conditions and loading of Zn exhibited a significant influence on the BTX yield. The interaction of Zn species with hydroxyl groups (OH) occurred on the HZSM-5 surface with the loading of Zn on a HZSM-5 zeolite. Such interaction distinctly changed the texture and acidity of the Zn/HZSM-5 catalyst. On the other hand, reduction of the Zn/NZ crystal size not only improved the interaction of the Zn species with HZSM-5 but also enhanced the resistance of coke deposition. The high BTX yield of 67.7% and good catalytic stability were obtained on a 0.5 wt % Zn/NZ catalyst, mainly because of its proper concentration and distribution of acid sites as well as the small crystal size with improved physical transport.

1. INTRODUCTION Light aromatics (benzene, toluene, and xylene, BTX) are the important fundamental chemicals in the industry of medicine, perfumery, dyestull, etc. Traditionally, BTX are mostly derived from naphtha reforming and oil thermal cracking.1 In recent years, production of BTX by the traditional route cannot reach the market demand for BTX because of the shortage of fuel oil and the fast growth rate of the demand for BTX. Thus, the other efficient route should be proposed to increase BTX. In recent years, the conversion of methanol to light aromatics (methanol aromatization, MTA) has attracted great attention because MTA is considered an effective route for using methanol and increasing BTX production,2 whereas it is necessary that a high-efficiency catalyst be developed for the industrial production of MTA. So far, few researches about the conversion of methanol to BTX have been reported.3−7 ZSM-5-zeolite-based catalysts, such as Ag-ZSM-5,3 Mo2C-ZSM-5,4 Zn-ZSM-5, H[Zn,Al]ZSM-5,5 Cu-ZSM-5,6 Zn−Sn/ZSM-5,7 etc., have been developed as catalysts for MTA reaction, and the catalytic performance of these catalysts and reaction conditions are listed in Table 1. Yoshihiro et al.3 reported that the Ag-modified ZSM-5 catalyst could afford a higher aromatic yield than Zn-ZSM-5, and the product distribution was significantly affected by the reaction conditions of temperature, partial pressure of methanol, and contact time. The highest aromatics yield of 80.3% was gained under the optimal reaction conditions of 477 °C, W/F = 9.0 g h mol−1, and PCH3OH = 20 kPa, but there were too much heavy aromatics and only 40% BTX was obtained. Ni et al.5 reported that the H[Zn,Al]ZSM-5 catalyst had a long lifetime of more than 160 h. However, the BTX yield was lower than 48%. The catalytic performance of the H[Zn,Al]ZSM-5 catalyst for the aromatization and cracking of C4+ paraffins was clearly enhanced with an increase of the reaction temperature. Xin et al.7 found that a bimetal-modified Zn−Sn/ZSM-5 catalyst greatly enhanced the BTX yield to 64.1%, which was much higher than that obtained on other reference catalysts, but the stability was poor. Despite the fact that some significant achievements have been gained, the catalytic reactivity and/or stability of the © 2014 American Chemical Society

Table 1. Catalytic Reactivity and Reaction Conditions of Topical Catalysts for MTA BTX yield (wt %)

catalyst

reaction conditions

ref

Ag-ZSM-5 (Si/Al2 = 42)a

40.4

477 °C, 0.1 MPa, WHSV = 0.7 h−1, PCH3OH = 20 kPa

3

Mo2C-ZSM-5 (Si/Al2 = 80)a

33.8


4

Zn-ZSM-5 (Si/Al2 = 50)a H[Zn,Al]ZSM-5 (Si/Al2 = 50)a Cu-ZSM-5 (Si/Al2 = 35)a

45.0

437 °C, 0.1 MPa, WHSV = 3.2 h−1

5

Zn−Sn/ZSM-5 Zn/NZ (Si/Al2 = 50)a a

−1

47.6

437 °C, 0.1 MPa, WHSV = 0.8 h

27.2


6

64.1 67.7

450 °C, 0.1 MPa, WHSV = 0.8 h−1 450 °C, 0.1 MPa, PCH3OH = 20 kPa, total flow rate = 25 mL min−1 (WHSV = 0.8 h−1)

7 this work

5

Molar ratio.

catalyst for MTA reaction must be further enhanced. Hence, it is necessary to deeply understand the relationship of the catalyst properties and its performance so as to develop an efficient catalyst. However, the properties−reactivity relationship of the catalyst for MTA has not yet been well understood. For example, Choudhary et al.8 and Song et al.9 considered that the strong acid sites of HZSM-5 zeolite reacted as active sites for the production of aromatics and coke in the conversion of light hydrocarbons. Thus, the reduction of strong acid sites was conducive to suppressing the coke but also led to the loss of aromatization activity. Song et al.10 also found that the coke formed on both weak and strong acid sites in butylene aromatization, and the stability of the catalyst was closely related to the acidic distribution. Aguayo et al.11 reported that the high Received: Revised: Accepted: Published: 14932

May 24, 2014 July 28, 2014 September 8, 2014 September 8, 2014 dx.doi.org/10.1021/ie5021156 | Ind. Eng. Chem. Res. 2014, 53, 14932−14940

Industrial & Engineering Chemistry Research

Article

Table 2. Reactivity of Zn-Modifed HZSM-5 Catalysts in the Conversion of Methanol yield (wt %) catalyst NZ 0.2 wt 0.5 wt 1.0 wt 2.0 wt HZ 0.5 wt

% % % %

Zn/NZ Zn/NZ Zn/NZ Zn/NZ

% Zn/HZ

conversion (wt %)

C1 and C2

C3

C4

C5+

B

T

X

C9+

yieldBTX (wt %)

100 100 100 100 100 100 100

1.7 1.8 1.5 2.5 3.2 2.6 2.7

13.1 10.3 7.2 9.2 9.7 14.3 11.9

29.5 25.1 22.2 20.2 19.2 30.5 25.5

4.0 4.8 5.3 4.4 4.5 5.7 4.8

2.9 2.0 2.5 1.5 1.4 2.4 1.9

15.5 15.3 19.5 16.9 15.3 16.3 17.4

23.1 29.4 29.7 31.6 31.2 24.5 30.6

10.2 11.3 12.1 13.7 15.5 3.7 5.2

41.5 46.7 51.7 50.0 47.9 43.2 49.9

Reaction conditions: 0.1 MPa, 400 °C, 0.5 g catalyst, PCH3OH = 20 kPa, total flow rate = 50 mL min−1, TOS = 3 h. B = benzene; T = toluene; X = xylene.

The 0.5 wt % Zn-modified HZ catalyst was prepared by the above method and remarked as 0.5 wt % Zn/HZ. 2.2. Catalytic Test. Methanol conversion was performed in a fixed-bed continuous-flow apparatus with a quartz tube reactor (650 mm length and 10 mm i.d.). Methanol (CH3OH, Tianjian Chem., A.R.) as the reagent was input into a vaporizer by a syringe pump (LabAlliance series III) and mixed with N2. The gaseous mixture passed through the 0.5 g catalyst bed (20−40 mesh). A gas chromatograph (Ruimin GC-2060) with a flame ionization detector (FID) and a thermal conductivity detector (TCD) was used to test the methanol and products online, respectively. FID with a SE-30 capillary column (30 m × 0.25 mm × 33 μm) detected C1−C4 and C5+ aliphatics and aromatics. A TCD with a GDX-104 packed column was used to analyze methanol. The methanol conversion and yield of the products were calculated from the number of C atoms with the following formula:

acidic strength of the zeolite catalyst could lower the energy barrier and then increase the reaction rate. Therefore, no accordant acknowledge about the relationship of the catalyst properties and catalytic reactivity has been gained yet, and it is still a challenge to develop a catalyst with high catalytic reactivity and good stability. In this work, the effects of Zn loading and reactions on the catalytic performance of a Zn-modified nano-HZSM-5 catalyst in MTA were investigated. The properties of these catalysts were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 isothermal adsorption− desorption, Fourier transform infrared (FTIR) spectroscopy, pyridine-adsorbed infrared spectroscopy (Py-IR), and thermal gravity (TG). In addition, the effects of the texture and acidity of these catalysts on their catalytic performance were correlated.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The nano-HZSM-5 zeolite was prepared as follows. Calculated amounts of sodium aluminate (NaAlO2, Tianjin Chem., A.R.), tetraethoxysilane (TEOS, Tianjin Chem., A.R.), sodium hydroxide (NaOH, Shanghai Chem., A.R.), tetrapropylammonium hydroxide (TPAOH, Shanghai Chem., A.R.), cetyltrimethylammonium bromide (CTAB, Tianjin Chem., A.R.), and deionized water (H2O) were dropped in a beaker and stirred fast to generate gel. In this gel, Al2O3:SiO2: Na2O:TPAOH:CTAB:H2O = 2:100:3:25:5:1150 (molar ratio). After that, the mixture was crystallized at 110 °C for 24 h and then at 170 °C for 48 h in a stainless steel autoclave with Teflon lining. The resultant solid was washed with distilled water, dried at 110 °C for 12 h, and calcined at 550 °C for 4 h. Finally, the solid powder was ion-exchanged twice with a 1 mol L−1 ammonium nitrate (NH4NO3, Shanghai Chem., A.R.) aqueous solution at 90 °C for 2 h followed by filtering and washing. The ion-exchanged solid was dried at 110 °C for 12 h and then calcined at 500 °C for 4 h. The resultant solid was remarked as NZ. Zn/NZ catalysts were prepared by the wetness impregnation method. In a typical procedure, NZ powder was impregnated in an aqueous solution of zinc nitrate (Zn(NO3)2, Tianjin Chem., A.R.) with a concentrations of ca. 0.09, 0.23, 0.46, and 0.92 wt %. NZ powder was left in a zinc nitrate aqueous solution for 1 h to equilibrate at 50 °C, and then the H2O was removed in a rotary evaporator. The impregnated powder was dried at 110 °C for 12 h and then calcined at 500 °C for 4 h. The resultant solid was named as x wt % Zn/NZ, in which x wt % expressed the Zn loading. For comparison, normal HZSM-5 with Si/Al2 = 50 (molar ratio) was gained from Nankai University and named as HZ.

conversion wt % = (methanol feed − methanoloff‐gas) /methanol feed × 100% selectivityi wt % = Ai ni /(∑ Ai ni) × 100% yieldi wt % = conversion of methanol × selectivityi × 100%

where A represents the peak area and n represents the number of C atoms in the product i. 2.3. Catalyst Characterization. XRD was characterized by a Rogaku Rotflex D/Max-C powder X-ray diffractometer with Cu Kα radiation (λ = 0.15046 nm). SEM was carried out on a JEOL JEC-1600 instrument and a Hitachi S-4800 microscope meter at 200 kV. N2 isothermal adsorption−desorption was performed by a Micromeritics ASAP400 adsorption meter at 77 K. FTIR analysis of these samples was performed by a Bruker IF113 V spectrometer with a resolution of 4 cm−1. In the Py-IR experiment, a self-supported disk of a 30 mg catalyst with 6.5 mm radius was prepared and activated in an in situ cell under 1 × 10−3 Pa at 400 °C for 2 h first. After that, 20 mL min−1 helium saturated with pyridine was introduced into the IR cell for 2 h at 50 °C. Finally, the catalyst was heated to 50, 150, and 350 °C at a rate of 10 °C min−1 and collected as IR spectra. TG data were recorded on a Mettler TGA/SDTA851e thermal analyzer. TG analysis was carried out in 50 mL min−1 air flow and from 50 to 700 °C at a heating rate of 10 °C min−1.

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance. Table 2 displays the catalytic reactivity of Zn-modified HZSM-5 catalysts in the conversion 14933

dx.doi.org/10.1021/ie5021156 | Ind. Eng. Chem. Res. 2014, 53, 14932−14940


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of methanol to light aromatics. Methanol was nearly completely converted on all catalysts under the reaction conditions used. In addition, products of C1−C4 alkane and alkene, C5+ aliphatics (C5−C8 alkane and alkene), benzene, toluene, xylene, and C9+ aromatics were detected in the reaction. Thus, a series of complex reactions might occur on the catalysts, which include dehydration, oligomerization, cracking, hydrogen transfer, dehydrocyclization, aromatization, etc.5 NZ exhibited only 41.5% yield of BTX with more than 45% yield of C1−C5+ light hydrocarbons and 10.2% yield of C9+ aromatics in MTA reaction. Zn/NZ showed different catalytic reactivities relative to the parent NZ. With an increase of the Zn loading, the toluene yield rose to the maximal value of 19.5% on 0.5 wt % Zn/NZ and then began to drop. The yield of xylene slightly increased with an increase of the Zn loading, whereas the yield of benzene exhibited a slight decrease. It is observed that an increase of the Zn loading continuously increased the C1, C2, and C9+ aromatics yields but led to a decrease of the C4 yield. The yield of C3 initially decreased and then increased with the Zn loading. In general, 0.5 wt % Zn/NZ showed the highest BTX yield of 51.7% and a total aromatics yield of 63.8% versus any other samples. These results suggest that Zn doping distinctively modified the catalytic performance of Zn/NZ in MTA reaction. The product distribution on Zn/NZ was closely related with the Zn loading. In order to obtain the high BTX yield, Zn/NZ should have the proper Zn loading. On the other hand, although NZ exhibited a lower BTX yield than HZ, the Zn/NZ catalyst had higher reactivity in MTA reaction than Zn/HZ with the same Zn loading (0.5 wt %). Figure 1 shows the effects of the reaction temperature on the reactivity of 0.5 wt % Zn/NZ and 0.5 wt % Zn/HZ in the conversion of methanol. 100% methanol conversion was obtained on the investigated reaction temperature. It can be seen from Figure 1a,b that variation of the product distribution in methanol conversion with increasing reaction temperature on 0.5 wt % Zn/NZ was similar to that on 0.5 wt % Zn/HZ. For 0.5 wt % Zn/NZ, the yield of benzene gradually increased when the reaction temperature rose. The yield of toluene initially increased to 28% with an increase of the reaction temperature to 450 °C and subsequently decreased to 18.7% at 550 °C. It is possible that transformation of toluene might occur and was dominative at higher reaction temperature, which led to the reduction of toluene.4 In addition, with an increase of the reaction temperature, the yields of C1−C3 slightly rose and those of C4 and C5+ aliphatics gradually decreased. These results suggest that an increase in the reaction temperature favored oligomerization of light alkane/alkene and dehydrocyclization, whereas much higher reaction temperature suppressed dehydrocyclization and promoted the formation of coke from a secondary reaction.12 Thus, the reaction temperature significantly influenced the catalytic reactivity of 0.5 wt % Zn/NZ in MTA reaction. The highest BTX yield of 63.3% was obtained at 450 °C on 0.5 wt % Zn/NZ. As shown in Figure 1b, the highest BTX yield was also achieved at 450 °C on 0.5 wt % Zn/HZ. Figure 2 shows the effects of methanol pressure on the reactivity of 0.5 wt % Zn/NZ and 0.5 wt % Zn/HZ in the conversion of methanol at 450 °C. As shown in Figure 2a, the yield of toluene first increased and then decreased with an increase of the methanol pressure, whereas the benzene yield gradually decreased. The xylene yield hardly changed with the methanol pressure. The yield of BTX displayed a peak tendency with an increase of the methanol pressure, and the highest BTX yield was obtained at PCH3OH = 20 kPa. In addition, with an increase

Figure 1. Effects of the reaction temperature on the catalytic reactivity of (a) 0.5 wt % Zn/NZ and (b) 0.5 wt % Zn/HZ catalysts in methanol conversion to BTX. Reaction conditions: 0.1 MPa, 0.5 g catalyst, PCH3OH = 20 kPa, total flow rate = 50 mL min−1, and TOS = 3 h.

of the methanol pressure, the C1−C3 yields slightly decreased, while the C4, C5+ aliphatics, and C9+ yields gradually increased. Hence, the distribution of products in the reaction depended strongly on the methanol pressure. It is worth noting that variation of the product distribution with the methanol pressure on 0.5 wt % Zn/HZ (Figure 2b) was similar to that on 0.5 wt % Zn/NZ, but the yield of xylene sharply decreased at a methanol pressure above 40 kPa. Liu et al.13 reported that the reaction pressure changed the initial activation and reaction path and an increase of the reaction pressure favored hydrogen transfer, oligomerization, and aromatization reactions by chemical equilibrium. These might be the reasons why the BTX yield initially increased with an increase of the methanol pressure. Some research revealed that the formation of light aromatics could follow the “hydrocarbon pool” reaction mechanism with poly(methylbenzene) as the reaction intermediate in the methanol conversion.14,15 More poly(methylbenzene) was obtained with an increase of the methanol partial pressure, which made it difficult to transform into BTX by further transfer to the coke by oligomerization, hydrogen transfer, and so forth.16−18 Therefore, moderate methanol pressure should be used for achieving the high BTX yield in the reaction. Figure 3a shows the effect of the reaction contact time on methanol conversion over 0.5 wt % Zn/NZ. It can be seen that 14934



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Figure 2. Effects of the methanol pressure on the catalytic reactivity of (a) 0.5 wt % Zn/NZ and (b) 0.5 wt % Zn/HZ catalysts in methanol conversion to BTX. Reaction conditions: 0.1 MPa, 450 °C, 0.5 g catalyst, total flow rate = 50 mL min−1, and TOS = 3 h.

Figure 3. Effects of the contact time on the catalytic reactivity of (a) 0.5 wt % Zn/NZ and (b) 0.5 wt % Zn/HZ in methanol conversion to BTX. Reaction conditions: 0.1 MPa, 450 °C, 0.5 g catalyst, PCH3OH = 20 kPa, and TOS = 3 h.

the contact time remarkably affected the product distribution on the catalyst. All of the yields of C1−C4 and C5+ aliphatics slightly decreased with prolonged reaction contact time, while the yields of benzene and toluene initially increased and then remained constant. The xylene yield had a slight fluctuation with an increase of the contact time. The C9+ aromatics yield initially dropped and then increased slowly with prolonged contact time. As a result, an induction period might exist in the conversion of methanol to BTX on 0.5 wt % Zn/NZ. The long contact time favored oligomerization, cyclization, hydrogen transfer, and aromatization. The yield of toluene increased along with a significant decrease in C9+ aromatics, suggesting that competition existed in the production of toluene and C9+ aromatics. Figure 3b shows the effect of the reaction contact time on the conversion of methanol over 0.5 wt % Zn/HZ. The yields of benzene, toluene, and xylene initially increased with prolonged reaction contact time, while the yields of C1−C4 and C5+ aliphatics gradually decreased. The maximum BTX yield was obtained at 2.0 × 10−2 g mL−1 min−1. The C9+ yield continuously rose with an increase of the reaction contact time. These phenomena imply that the formation of BTX and C9+ aromatics might be produced through a common intermediate, such as poly(methylbenzene),14,15 and the parallel pathways on these catalysts. Figure 4 shows the time course of methanol conversion on 0.5 wt % Zn/NZ and 0.5 wt % Zn/HZ catalysts under the

optimal reaction conditions of 0.1 MPa, 450 °C, PCH3OH = 20 kPa, and total flow rate = 25 mL min−1. Both catalysts of 0.5 wt % Zn/NZ and 0.5 wt % Zn/HZ exhibited almost 100% methanol conversion at the first time on stream. After 16 h on stream, methanol conversion over 0.5 wt % Zn/HZ sharply decreased, while 0.5 wt % Zn/NZ still showed 100% methanol conversion after 40 h on stream. As shown in Figure 5, the product distribution varied as a function of the time on stream over 0.5 wt % Zn/NZ and 0.5 wt % Zn/HZ catalysts. It can be seen from Figure 5a,b that a variety of product distributions as time on stream on 0.5 wt % Zn/NZ exhibited similar tendency with those on 0.5 wt % Zn/HZ. In the case of 0.5 wt % Zn/HZ (Figure 5b), the yields of benzene, toluene, and xylene continuously decreased with prolonged time on stream, especially for toluene. The BTX yield initially increased to 62.9% at a time on stream of 1 h and then momentously dropped to 37% after a reaction time of up to 20 h. The yields of C1, C2, and C4 hydrocarbons continuously increased with time on stream, while the yields of C3, C5+, and C9+ aromatics fluctuated with time on stream. These results suggest that the catalyst did not display perfect catalytic stability in MTA reaction. The oligomerization of light alkane/ alkene on the catalyst might be suppressed with deactivation of the coke deposition. In contrast, 0.5 wt % Zn/NZ showed no 14935



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Figure 4. Time course of methanol conversion on 0.5 wt % Zn/NZ and 0.5 wt % Zn/HZ catalysts. Reaction conditions: 0.1 MPa, 450 °C, 0.5 g catalyst, PCH3OH = 20 kPa, and total flow rate = 25 mL min−1.

Figure 6. XRD patterns of catalysts: (a) NZ; (b) 0.2 wt % Zn/NZ; (c) 0.5 wt % Zn/NZ; (d) 1.0 wt % Zn/NZ; (e) 2.0 wt % Zn/NZ; (f) HZ; (g) 0.5 wt % Zn/HZ.

peak, corresponding to the reduction of crystallinity. It is noted that no diffraction peaks assigned to Zn species were observed from Zn-modified HZSM-5 samples, suggesting that Zn species might existed on the surface of HZSM-5 with high dispersion.17 With an increase of the Zn loading, the diffraction peak intensity of Zn/NZ became weak, possibly attributed to an increase of the coverage by Zn species. In addition, the diffraction peak at 2θ = 23.1° of Zn/NZ slightly shifted to low 2θ with an increase of the Zn loading. Sami et al.18 reported that Zn species dispersed on the external surface of HZSM-5 and stabilizied at the cation-exchange sites, which could cause lattice distortion of HZSM-5. Hence, Zn species partly interacted with the HZSM-5 framework and influenced the lattice structure of HZSM-5. According to the XRD peaks of HZSM-5, the Si/Al2 ratio of these catalysts was calculated by the empirical formula of Al2O3 % = 16.5−30.8d, where d represents the difference between the two peaks around 2θ = 45°. As shown in Table 3, the Si/Al2 Table 3. Textural Properties of These Catalysts catalyst

Si/Al2 (M)

crystal size (nm)

SBET (m2 g−1)

Vpore (cm3 g−1)

NZ 0.2 wt % Zn/NZ 0.5 wt % Zn/NZ 1.0 wt % Zn/NZ 2.0 wt % Zn/NZ HZ 0.5 wt % Zn/HZ

52.1 53.6 53.3 52.2 54.5 48.9 50.5

24 23 22 23 23 153 149

392 394 389 373 365 263 257

0.40 0.38 0.37 0.33 0.31 0.22 0.20

ratio of NZ was somewhat lower than that of the synthesis mixture because of the low crystallinity and without an obvious change after Zn loading. On the other hand, the crystal sizes of the catalysts were calculated by the Scherrer equation and are listed in Table 3. HZ shows the bigger crystal size of 153 nm than that of NZ (24 nm). Figure 7 shows SEM images of HZ and NZ zeolites. It can be seen that the two catalysts present different particle size and morphology. HZ appeared as irregular hexagonal crystal morphology with a mean particle size of more than 2 μm. NZ contained uniform cylinder aggregation particles (∼200 nm) with nanocrystals ranging from 20 to 30 nm. These results indicate that further smaller nanosized HZSM-5 was prepared

Figure 5. Time course of product distribution on (a) 0.5 wt % Zn/NZ and (b) 0.5 wt % Zn/HZ catalysts. Reaction conditions: 0.1 MPa, 450 °C, 0.5 g catalyst, PCH3OH = 20 kPa, and total flow rate = 25 mL min−1.

distinct decreases in the yield of benzene, toluene, and xylene until a reaction time of up to 20 h, and 48.9% BTX yield was still retained after 40 h of time on stream. Thus, the 0.5 wt % Zn/NZ catalyst showed good catalytic stability. 3.2. Catalyst Characterization. Figure 6 shows XRD patterns of catalysts. As shown in Figure 6, XRD peaks of HZSM-5 (2θ = 7.8, 8.7, 23.1, 23.8, and 24.3°) were detected from all of the catalysts. In contrast to HZ, NZ displayed a low diffraction 14936



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bridged Si(OH)Al of zeolite mostly belonged to the Brønsted (B) acid sites. Thus, the introduction of Zn species into NZ significantly affected the B acid sites. The intensity of B acid sites decreased with an increase of the Zn loading. Figure 9 shows the Py-IR spectra of Zn/NZ catalysts. Three characteristic peaks at 1453, 1540, and 1490 cm−1 attributed to Lewis (L) acid sites, B acid sites, and both L and B acid sites appeared in all of the samples.23 According to Emeis24 and Zhang et al.,25 the density and concentration of the acid sites on Zn/NZ catalysts were calculated, as shown in Table 4. It can be seen that the total concentration of the catalyst hardly exhibited an obvious change with the introduction of Zn. However, the concentration of B acid sites sharply decreased, and the concentration of L acid sites (especially the weak L acid sites) significantly increased with an increase of the Zn loading. Thus, the B/L ratio reduced with increasing Zn loading, suggesting that partial B acid sites were translated to L acid sites due to the interaction of Zn and surface OH groups. It is noted that the weak acid sites slightly increased while the strong acid sites reduced as the Zn loading increased. These results indicate that the introduction of Zn into NZ not only modified the concentration of L and B acid sites but also influenced the acidic strength distribution. After 0.5 wt % Zn/HZ and 0.5 wt % Zn/NZ catalysts were on the stream of methanol/N2 for 20 and 80 h, respectively, under the optimal reaction conditions of 0.1 MPa, 450 °C, PCH3OH = 20 kPa, and total flow rate = 25 mL min−1, TG experiments of these used catalysts were performed to study the coke on the catalysts. Figure 10 depicts the TG curves of the catalysts used. A distinctive weight loss (ca. 400−700 °C) was observed, which tentatively corresponds to the coke amount. 0.5 wt % Zn/NZ showed a higher coke amount (ca. 14.4%) than 0.5 wt % Zn/HZ (ca. 7.3%). On the other hand, the weight loss per hour is identified as the average rate of coke formation on the catalysts, and 0.5 wt % Zn/NZ presented a lower average rate of coke formation (ca. 0.18%) than 0.5 wt % Zn/HZ (ca. 0.37%). These results indicate that 0.5 wt % Zn/NZ has a better carbon capability and resistance to coke deposition. Although a high BTX yield of 64.1% was obtained on the Zn−Sn/ZSM-5 catalyst, it also showed a much higher average rate of coke formation (ca. 0.5%) than that of 0.5 wt % Zn/ HZ.7 Ni et al.5 reported that H[Zn,Al]ZSM-5 showed a lower average rate of coke formation, approximately 0.2% h−1, and a longer lifetime than the other reference catalysts;3−7 however, the BTX yield was lower than 50%. In the case of this work, high BTX yield (67.7%) and low average rate of coke formation (ca. 0.18%) was achieved on 0.5 wt % Zn/NZ, indicating the best catalytic performance and resistance to coke deposition. 3.3. Effects of the Catalyst Properties on the Reactivity. From the results obtained in this work, it is found that the interaction between Zn and the NZ framework occurred with the addition of Zn into the NZ zeolite. The properties, such as texture and acidic distribution of Zn/NZ, were significantly modified by the interaction, which, in turn, influenced its catalytic performance and stability in MTA reaction. In order to further understand the effect of acidity on the catalytic reactivity of Zn/NZ, we correlated the relationships of the concentration of acid sites and the B/L ratio with the specific yield of BTX, as shown in Figure 11. The specific yield of BTX is defined as the production of BTX (g) per hour (h) and unit catalyst (g). It can be seen from Figure 11 that, with an increase of the specific yield of BTX, the concentration of B acid sites initially decreased and then increased, while the

Figure 7. SEM images of catalysts.

during the hydrothermal synthesis than the conventional method, which was in line with that of XRD. During the hydrothermal synthesis of NZ, cationic polymer CTAB was used as a protecting agent, which prevented the crystal nucleus from selfaggregation and intergrowth. Negatively charged inorganic silica and aluminum species generated by hydrolysis of TEOS and NaAlO2, which interact with the cationic polymer CTAB by electrostatic self-assembly. Then, the isolated nanosized precursor transferred into NZ by in situ crystallization. Yin et al.19 synthesized nanosized MCM-22 zeolite by a similar process. The specific surface area (SBET) and pore volume (Vpore) of Zn/HZSM-5 catalysts were characterized by N2 isothermal adsorption−desorption, and the results are listed in Table 3. NZ exhibited higher SBET and Vpore than those of HZ, possibly because of its small particle size.20 With an increase of the Zn loading, SBET and Vpore of Zn/NZ gradually decreased, suggesting that Zn species were mainly located on the external surface and/or the pore mouth of zeolite. FTIR bands were collected to detect the surface OH groups of Zn/NZ catalysts. As shown in Figure 8, IR bands assigned

Figure 8. IR spectra of catalysts: (a) NZ; (b) 0.2 wt % Zn/NZ; (c) 0.5 wt % Zn/NZ; (d) 1.0 wt % Zn/NZ; (e) 2.0 wt % Zn/NZ.

to the lattice terminal Si(OH) (ca. 3723 cm−1) and bridged Si(OH)Al (ca. 3680 and 3600 cm−1) groups appeared in all of the catalysts.21 With an increase of the Zn loading, the peaks assigned to the Si(OH) (3723 cm−1) and Si(OH)Al (3600 cm−1) groups obviously shifted toward high wavenumber. In addition, the peak intensity of Si(OH) and Si(OH)Al of Zn/NZ clearly decreased relative to that of NZ-5, and this tendency became more pronounced as the Zn loading increased. These phenomena indicate that the interaction between Zn species and hydroxyl groups occurred on the surface of NZ zeolite, which became more pronounced with increasing Zn loading. Vedrine et al.22 reported that the OH groups in terminal Si(OH) and 14937



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Figure 9. Py-IR spectra of catalysts evacuated at specific temperature: (a) NZ; (b) 0.2 wt % Zn/NZ; (c) 0.5 wt % Zn/NZ; (d) 1.0 wt % Zn/NZ; (e) 2.0 wt % Zn/NZ.

Table 4. Distribution of L and B Acid Sites on Zn/NZ Catalysts concentration of B acid sites (μmol g−1)

concentration of L acid sites (μmol g−1)

Zn loading (wt %)

weak (50−150 °C)

medium (150−350 °C)

strong (>350 °C)

total

weak (50−150 °C)

medium (150−350 °C)

strong (>350 °C)

total

B/L

0.0 0.2 0.5 1.0 2.0

99.5 57.3 21.2 8.8 5.1

41.0 33.6 25.1 15.1 13.9

64.2 40.9 27.5 15.2 12.5

204.7 131.8 73.8 39.1 31.5

87.5 147.3 180.7 194.1 201.4

21.3 24.4 36.6 51.0 50.0

22.5 27.8 40.6 45.7 51.7

131.3 199.5 257.9 290.8 303.1

1.56 0.66 0.29 0.13 0.10

Figure 10. TG curves of the catalysts used. Figure 11. Relationship of the acidity and specific yield of BTX in MTA reaction over Zn/NZ catalysts. Reaction conditions: 0.1 MPa, 450 °C, 0.5 g catalyst, PCH3OH = 20 kPa, total flow rate = 25 mL min−1, and TOS = 3 h.

concentration of L acid sites showed the opposite tendency. The highest BTX yield was obtained with moderate concentrations of B and L acid sites. Hence, the catalytic reactivity of Zn/NZ in MTA reaction was closely related to its acidic distribution, and the moderate B/L ratio was favored to the production of BTX. It is well accepted that MTA is a typical acid catalytic reaction, and the synergetic effect occurred between B and L acid sites. Liu et al.26 found that a small of fraction of B acid sites was sufficient for transformation of the intermediates in aromatization of methane and propane, while the aromatic carbonaceous deposits increased with the superfluous strong B acid sites. As reported by Yu et al.,27 the cracking of C7−C9+ polymers was suppressed, which is due to a decrease of the B acid sites. In addition, the dehydrogenation reaction of C7−C9+ intermediates was enhanced with an increase of the L acid sites. Thus, the Zn/NZ catalyst mainly increased the yields of toluene, xylene, and C9+ aromatics with a decrease of C3 and

C4 hydrocarbons when Zn loading was increased to 0.5 wt %. With a further decrease of the B/L ratio, hydrogen-transfer reaction might be restrained, which suppressed transformation of the poly(methylbenzene) intermediate to light aromatics (benzene and toluene). On the other hand, carbon and/or coke produced on excess of L or B acid sites by dehydrogenation, hydrogen transfer, polymerization, etc.9,24 Hence, the acidic distribution of catalyst played an important role in the catalytic reactivity. The catalyst should have a moderate B/L ratio so as to obtain the high BTX yield, which favored the formation of BTX and also suppressed generation of carbon and coke. Coke deposition was known to be the major reason for catalyst deactivation in MTA reaction.28 It is reported that shortening the diffusion path and residence time of bulky products in 14938



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aromatics yield increased. Excessive Zn loading led to a decrease of the BTX yield, indicating that much lower B acid sites of Zn/HZSM-5 prevented dealkylation of poly(methylbenzene).

the crystal pore was a good way to improve the catalyst lifetime.29,30 Rownaghi and Hedlund31 reported that nano-ZSM-5 zeolite displayed more improved BTX yield and catalytic stability than the conventional one, mainly because of reduction of the micropore diffusion path length and an increase of the external surface area by decreasing the zeolite crystal size. Therefore, the decrease of the crystal size favored reduction of the diffusion pathway of products in zeolite catalyst, which suppressed the formation of coke. As pointed out by Ni et al.,5 the nanostructure was beneficial for the dispersion of metal species in the zeolite, which might enhance the interaction of metal species and surface active sites. The 0.5 wt % Zn/NZ catalyst showed nanostructure and high SBET as well as high Vpore, which significantly enhanced the interaction of Zn with surface OH groups and improved the physical transport and carbon capacity. Thus, 0.5 wt % Zn/NZ exhibited higher BTX yield and better catalytic stability than 0.5 wt % Zn/HZ, mainly because of its small particle size. On the basis of the results obtained and the literature,15,16 we deduced possible transformation pathways of methanol over the Zn/HZSM-5 catalyst, as shown in Scheme 1. Dimethyl

4. CONCLUSION Zn-modified nano-HZSM-5 catalyst exhibited improved catalytic performance in MTA reaction. The catalytic reactivity of Zn/NZ was closely related to the Zn loading and reaction conditions. Under the optimal conditions of 0.1 MPa, 450 °C, PCH3OH = 20 kPa, and total flow rate = 25 mL min−1, the comparatively high BTX yield of 67.7% was gained on the 0.5 wt % Zn/NZ catalyst. The addition of Zn into NZ led to the interaction between Zn species and HZSM-5, which reduced the concentration of B acid sites and generated new L acid sites. In addition, reduction of the HZSM-5 crystal size enhanced the interaction of Zn with surface OH groups and improved physical transport, which improved the catalytic reactivity and resistance to coke deposition. The combination of small crystal size and moderate distribution of acid sites makes 0.5 wt % Zn/NZ a potentially interesting catalyst in MTA reaction with high BTX yield and good stability.

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Scheme 1. Diagram of the Transformation Pathways of Methanol over the Zn/HZSM-5 Catalyst

AUTHOR INFORMATION

Corresponding Author

*Tel: (+86)-29-88302853. Fax: (+86)-29-88302883. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Ministry of Education (Grant NCET-10-878,), Shaanxi “13115” Innovation Project (2009ZDKJ-70), and Shaanxi Key Innovation Project (2011ZKC4-08).

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REFERENCES

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