Coupling Conversion of Methanol and C4 Hydrocarbon to Propylene

Sep 21, 2012 - and C4 hydrocarbon cracking to propylene, the coupling conversion remarkably ... effective utilization of C4 hydrocarbons and methanol...
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Coupling Conversion of Methanol and C4 Hydrocarbon to Propylene on La-Modified HZSM‑5 Zeolite Catalysts Ting Gong, Xin Zhang,* Ting Bai, Quiquan Zhang, Lin Tao, Mi Qi, Chao Duan, and Li Zhang School of Chemical Engineering, Northwest University, Taibai North Road 229, Xi’an, 710069, People’s Republic of China ABSTRACT: The coupling conversion of methanol and C4 hydrocarbon (62.2 wt % of n-butanes and 34.4 wt % of n-butylenes) to propylene was studied over La-modified HZSM-5 (La/HZSM-5) catalysts. Compared with the individual methanol to olefins and C4 hydrocarbon cracking to propylene, the coupling conversion remarkably improved propylene yield and the catalytic stability of La/HZSM-5. In the coupling conversion, propylene yield was strongly dependent on the reaction conditions and the loading of La. The interaction of La with hydroxyl groups (OH) adsorbed on HZSM-5 surface occurred. Such interaction modified the density and distribution of acid sites from the hydroxyl groups. Propylene yield was close relative to the density and distribution of acid sites. The comparatively high propylene yield of ca. 46.0 wt % was obtained on La/HZSM-5 catalyst with l.5 wt % load of La (1.5 wt % La/HZSM-5), possibly due to the moderate density and distribution of acid sites on the catalyst. understood. Mier et al.6,7 considered that a catalyst with considerable acid strength in its sites (≥120 kJ·mol NH3−1) was required for the conversion of n-butane in the joint transformation of n-butane and methanol. Li et al.9 found that a decrease in the acidic strength and/or the number of acids sites helped the coupling conversion of ethylene and methanol over modified HZSM-5. Without understanding the relationship of properties and catalytic reactivity, it is difficult to develop an effective catalyst by modifying the composition and properties of the catalyst. Although some significant achievement has been gained, it is still highly desirable to further explore properties−reactivity relationship and develop an effective catalyst. Recently, Wang et al.10 reported that the addition of La into HZSM-5 greatly enhanced the selectivity of propylene and the total selectivity of olefins in butanes cracking. Xue et al.11 found that the enhancement of propylene yield could be gained by the addition of La into P-modified HZSM-5 in butylenes cracking. These results encourage us to attempt La-modified HZSM-5 catalysts (La/HZSM-5) for the coupling conversion of methanol and C4 hydrocarbon to propylene. To the best of our knowledge, few research works concerning the coupling conversion over La/HZSM-5 have been reported so far. In this work, effects of reaction conditions and the load of La on the catalytic performance of La/HZSM-5 in the coupling conversion of methanol and C4 hydrocarbon (62.2 wt % of nbutanes and 34.4 wt % of n-butylenes) were investigated in detail. In addition, the properties of these catalysts were characterized by atomic absorption spectrophotometer (AAS), N2 isothermal adsorption−desorption, powder X-ray diffraction (XRD), infrared spectroscopy (IR), and pyridine-adsorbed infrared spectroscopy (Py-IR). Based on the obtained results, the relationship of the physiochemical properties with the

1. INTRODUCTION Propylene is one of the important raw chemicals for producing polypropylene, acrylonitrile, and propylene oxide, etc. Nowadays, propylene is mainly produced from steam cracking of naphtha and fluid catalytic cracking (FCC) units. In addition, methanol to olefins process (MTO) has been developed for the production of propylene by using nonpetroleum resources. However, in recent years, the production of propylene can not meet the huge market demand for propylene with its fast growth rate.1,2 Therefore, it is necessary to search for an alternative route to increase propylene production. Recently, the coupling conversion of methanol and C4 hydrocarbon to propylene has been proposed. Compared with individual MTO and C4 hydrocarbon cracking to propylene, the coupling conversion has some advantages, such as (1) thermoneutral and low energy consumption, (2) the improvement of butanes cracking, (3) the reduction of coke disposition, (4) the high yield of propylene, and (5) the simplification of reactor design.3−8 The coupling conversion can not only increase propylene yield, but also explore a new effective utilization of C4 hydrocarbons and methanol. Therefore, the coupling conversion of methanol and C4 hydrocarbon to propylene has attracted increasing attention. Up to now, few studies on the coupling conversion of methanol and C4 hydrocarbon to propylene have been published.3−8 Modified HZSM-5 catalysts, such as Fe-modified HZSM-5,3,4 Ga- modified HZSM-5,5 Ni- modified HZSM-5,6,7 and P- modified HZSM-5,8 etc., have been developed for the reaction. For example, Mier et al.6,7 reported that the methanol/n-butane molar ratio of 3 induced an energy-neutral integrated process and high yield of C2−C4 olefins (11.5% yield of propylene) was obtained on Ni-doped HZSM-5 catalyst in the joint transformation of methanol and n-butanes. The propylene yield of ca. 44.0% reached on P-doped HZSM-5 catalyst in the coupling conversion of methanol and 1-butylenes was much higher than that obtained in 1-butylene cracking or MTO.8 On the other hand, influences of acidity on the reactivity of catalyst in the coupling conversion have not yet been well © 2012 American Chemical Society

Received: Revised: Accepted: Published: 13589

February 27, 2012 July 28, 2012 September 21, 2012 September 21, 2012 dx.doi.org/10.1021/ie300515z | Ind. Eng. Chem. Res. 2012, 51, 13589−13598

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catalytic performance was correlated to show insight on the improvement of propylene yield by modifying the zeolite catalyst.

Scheme 1. Schematic Diagram of Catalytic Reaction System for the Coupling Conversion of Methanol and C4 Hydrocarbon

2. EXPERIMENTAL METHODS 2.1. Catalyst Preparation. Zeolite HZSM-5 (Si/Al2 = 50, molar ratio) was obtained from Nankai University. La/HZSM-5 was prepared by the wetness impregnation method. HZSM-5 powder impregnated the aqueous solution of lanthanum nitrate (La(NO3)3, Shanto Chem., A.R.) at 70−80 °C. The volume of lanthanum nitrate aqueous solution was about 1.5 times that of the volume of HZSM-5 powder. The lanthanum nitrate aqueous solution with the concentration of ca. 1, 2, 5, and 7 wt % was respectively used to prepare La/HZSM-5 with 0.5, 1.5, 5, and 7 wt % load of La. The impregnated powder was dried at 110 °C for 4 h and then calcined at 550 °C for 4 h in air to obtain n wt% La/HZSM-5, in which n wt% expresses the load of La. 2.2. Catalyst Characterization. The La-loading of these catalysts was measured by atomic absorption spectrophotometer (AAS TJA, Atomscan 16). XRD pattern of the catalyst was collected by Rogaku Rotflex D/Max-C powder X-ray diffractometer with Cu Kα radiation (λ = 0.15046 nm) operated at 40 kV and 30 mA to identify the structure of the catalyst. N2 isothermal adsorption−desorption characterization of the sample was performed at liquid nitrogen temperature by Micromeritics ASAP400 adsorption meter. The sample (ca. 240 mg) was degassed at 200 °C and 1.3 × 10−3 Pa for 4 h before the measurement of data. The specific surface area was calculated according to BET method. The volume of pores was evaluated by t-plot analysis of the adsorption isotherm. PyIR spectra were recorded by using a Bruker IF113 V FTIR spectrometer to study the acidity of the catalyst. The IR spectrometer was equipped with an in situ cell containing CaF2 windows. An 11.5-mg sample was pressed into a self-supported disk with radius 6.5 mm, which was introduced into the cell. In Py-IR experiment, the sample disk was activated under vacuum (1 × 10−3 Pa) and 20 mL/min He flow at 400 °C for 2 h and then cooled to room temperature. Subsequently, 20 mL/min He flow saturated with pyridine was introduced into the IR cell at room temperature for 2 h to ensure that all acid sites were covered. Afterward, the sample was respectively heated to 200 and 350 °C with the rate of 10 °C/min, and IR spectra were collected at the specific temperatures. Before measuring the spectra, the sample was purged by 20 mL/min He flow and evacuated at the temperature for 2 h to remove the desorbed pyridine. 2.3. Catalytic Test. The coupling conversion of methanol and C4 hydrocarbon was performed in a fixed-bed continuousflow catalytic reaction system (Scheme 1) with stainless steel reactor (650 mm length, 12 mm I.D.). The reaction temperature was controlled and measured by a temperature controller (YUDIAN AI-518/518P) and a Type K thermocouple. Weight hourly space velocity (WHSV) was referred to n-butylenes in feed. Before each test, the catalyst was first heated to 400 °C for 30 min and then to the reaction temperature in 20 mL/min N2 flow (Huayuan Gas, 99.95%). Methanol (Beijing Chem. Co., A.R.) was injected into the catalyst bed by a syringe pump (LabAlliance series II). C4 hydrocarbon was produced from FCC unit (Urumchi Shihua., SINOPEC), which was mainly composed of 62.2 wt % of nbutanes, 34.4 wt % of n-butylenes, and 3.3 wt % of propane. The off-gas was kept at ca. 120 °C by heating belt.

The feed and products were analyzed by gas chromatography (Fuli Anal. GC-9790) equipped with flame ionization detector (FID, 120 °C) and ATSE-30 capillary column (66 m × 0.25 mm × 1.0 μm, 60 °C). Mass balance more than 95% was used to calculate the conversion of reactants and the selectivity of products. Under the used reaction conditions, methanol almost completely converted on the investigated catalysts. The conversion of reactants and the selectivity of products were respectively calculated by the following formula. Conversion of n‐butanes wt% = (butanes in feed − butanes in off‐gas)/butanes in feed × 100% Conversion of n‐butylenes wt% = (butylenesin feed − butylenesin off‐gas)/butylenesin feed × 100% Conversion of methanol wt% = (methanol in feed − methanol in off‐gas)/methanol in feed × 100% Selectivity of product wt% = product/ ∑ product × 100%

Under the investigated reaction conditions, 100% conversion of methanol was obtained on these catalysts, but butanes did not convert. The conversion of butylenes was strongly dependent on the reaction conditions. Hence, the conversion of butylenes and methanol was very close relative to the yield of propylene and ethylene. The yield of product was calculated as the following formula. Yield of product wt% = conversion of n‐butylenes × selectivity of product × 100%

On the other hand, some important work and kinetic models on coke formation in MTO and the joint transformation of methanol and butanes have been reported previously.12−15 13590

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Concerning MTO, coke of the catalyst is highly dependent on the reaction conditions, such as, contact time, composition of feed, and reaction temperature. Similarly, coke composition and structure were close relative to the composition of the reaction medium and conditions in the joint transformation of methanol and butanes, whereby coke deposition increases as the concentrations of C2−C4 olefins and the C5−C11 fraction increase. In case of this work, the formation of coke was not so much and did not have great influence on the reactivity of the catalysts in the investigated reaction conditions and time. In addition, the instantaneous formation of coke was dependent on reaction time and difficult to estimate. Therefore, the conversion of C4 hydrocarbon and methanol to coke was neglected.

3. RESULTS AND DISCUSSION 3.1. Catalysts Characterization. The La-loading of these catalysts were measured by AAS. It can be seen from Table 1 Table 1. Specific Surface Area and Pore Volume of these Catalysts catalyst HZSM-5 0.5 wt % 1.5 wt % 5.0 wt % 7.0 wt %

La/HZSM-5 La/HZSM-5 La/HZSM-5 La/HZSM-5

La-loading (wt %)

SBET (m2/g)

Vpore (cm3/g)

0.35 1.42 4.72 6.55

276.5 275.2 275.1 239.5 212.1

0.23 0.23 0.23 0.20 0.18

that the final La-loading of these catalysts was lower than the La-loading calculated by the aqueous solution of lanthanum nitrate used in these catalysts’ preparation. Table 1 shows BET specific surface area (SBET) and pore volume (Vpore) of La/ HZSM-5 with the different loadings of La. Both 0.5 wt % LaHZSM-5 and 1.5 wt % La-HZSM-5 showed SBET and Vpore similar to the parent HZSM-5. These results suggest that the addition of La up to 1.5 wt % has no influence on either the BET surface area or the mesopore volume of the catalyst. When the La loading was more than 1.5 wt %, SBET of La/HZSM-5 decreased with the increase of La-loading; similarly, Vpore of these catalysts presented reduction tendency. Figure 1 shows XRD patterns of La/HZSM-5 and HZSM-5 as reference. Characteristic diffraction peaks of HZSM-5 (2θ = 7.8°, 8.7°, 23.0°, 23.8°, 24.2°) were found from these La/ HZSM-5 catalysts. On the other hand, the new peaks presented at 2θ = 23.2° and 23.6° were detected from La/HZSM-5 comparing with the parent HZSM-5. The peaks at 2θ = 23.2° slightly shifted to the high 2θ and the peaks at 2θ = 23.6° slightly shifted to low 2θ with the increase of La-loading. These phenomena are tentatively due to the interaction of La with HZSM-5 framework.10,11 On the other hand, no diffraction peaks belonging to La and La oxides were detected from these catalysts, indicating that La species might highly disperse on HZSM-5 surface. For La/ HZSM-5, the diffraction peaks intensity of HZSM-5 became weak with the increase of La-loading compared with the parent HZSM-5. The phenomenon was possibly due to the increase in the coverage of La species on HZSM-5 and/or the reduction in the crystallinity with the increase of La-loading. Figure 2 shows IR spectra of La/HZSM-5 with the different La-loading detected in the range of 400−1000 cm−1. Some useful information on the HZSM-5 framework structure can be gained from these IR spectra. These catalysts presented

Figure 1. XRD patterns of catalysts: (a) HZSM-5, (b) 0.5 wt % La/ HZSM-5, (c) 1.5 wt % La/HZSM-5, (d) 5.0 wt % La/HZSM-5, and (e) 7.0 wt % La/HZSM-5.

Figure 2. IR spectra of catalysts: (a) HZSM-5, (b) 0.5 wt % La/ HZSM-5, (c) 1.5 wt % La/HZSM-5, and (d) 7 wt % La/HZSM-5.

characteristic IR vibration of TO4 unit (ca. 799 cm−1) and the asymmetric and symmetric vibration of T-O binding/ double rings (ca. 624, 586, 544, and 451 cm−1) originated from HZSM-5.16−18 In case of La/HZSM-5, no IR band belonging to La−O species was clearly observed. In addition, IR bands of TO binding/double rings bands (ca. 624, 544, and 451 cm−1) slightly shifted toward the high wavenumber and their intensities were low relative to those for parent HZSM-5. 13591

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Moreover, the shift and intensity reduction of these IR bands were enhanced with the increase of La-loading. These results further confirm that the interaction of La with HZSM-5 occurred and became strong with the increase of La-loading. These results are in agreement with the results obtained from XRD. On the other hand, the change of the IR band intensity of OH group might roughly express the amount of La interacting with the OH group. With the increase of La-loading, the intensity of IR band belonging to OH group decreased. The result implies that the amount of La interacting with the OH group increased with the load of La. IR bands of these catalysts degassed at 200 °C were recorded in the range of wavenumber from 3500 to 3900 cm−1 for detecting surface hydroxyl (OH) groups. As seen from Figure 3, IR bands at ca. 3740, 3672, and 3610 cm−1 were observed

Figure 3. IR spectra of catalysts degassed at 200 °C: (a) HZSM-5, (b) 0.5 wt % La/HZSM-5, (c) 1.5 wt % La/HZSM-5, and (d) 7 wt % La/ HZSM-5.

Figure 4. Py-IR spectra of catalysts degassed at 200 and 350 °C: (a) HZSM-5, (b) 0.5 wt % LaHZSM-5, (c) 1.5 wt % LaHZSM-5, (d) 5.0 wt % LaHZSM-5, and (e) 7.0 wt % LaHZSM-5.

from these catalysts, attributed to the vibration of OH from Si(OH) (3740 cm−1) and the vibration of bridged OH in Si(OH)Al (3672 and 3610 cm−1).19−22 It is detected that IR bands belonging to OH in Si(OH) and Si(OH)Al for La/ HZSM-5 shifted toward the low wavenumber with respect to those for HZSM-5. Furthermore, the intensity of these IR bands distinctly reduced with the increase of La-loading. These results indicate that at least partial La species interacted with surface hydroxyl group and the interaction might become strong with the increase of La-loading. The introduction of La species into HZSM-5 resulted in the decrease in the density of OH. It is well-known that OH in Si(OH) and Si(OH)Al mainly provide Brønsted (B) acid sites for the catalyst. Thus, La species played important roles on the B acidity of the catalysts. It can be deduced that the density of B acid sites decreased with increasing La-loading. Py-IR spectroscopy was employed to comparatively investigate the acidity of HZSM-5 and La/HZSM-5. Figure 4 shows Py-IR spectra of these catalysts degassed at 200 and 350 °C. IR bands at ca. 1450, 1490, and 1540 cm−1 can be detected on these samples. IR band at 1450 cm−1 was attributed to the adsorption of pyridine coordinated on Lewis (L) acid sites.18,23 IR band near 1540 cm−1 was due to pyridinium ions formed by the transfer of protons from B acid sites on the samples to the organic base.18,23 In addition, IR band appearing at 1490 cm−1 was associated to the vibration of the pyridinic ring on B and L acid sites.18,23 These results confirm that both B and L acid sites

existed on these catalysts. B and L acid sites detected at 200 °C were attributed to weak/medium acid sites, and those observed at 350 °C were attributed to strong acid sites.18,23 IR bands belonging to L and B acid sites shifted to low wavenumber with the increase of La-loading. These results indicate that the strength of L and B acid sites become weak with the increase of La-loading. The concentration of acid sites (a.u./g) is identified as the amount of acid sites on unit mass catalyst. To estimate the concentration of acid sites, the areas of IR bands corresponding to B and L acid sites were respectively integrated. It can be seen in Table 2 that HZSM-5 has the higher total concentration of B acid sites and the lower total concentration of L acid sites than any other catalysts; moreover, it possessed the highest B/L ratio of ca. 3.42 among these catalysts. In view of La/HZSM-5, the concentration of B acid sites decreased with the increase of La-loading, whereas the concentration of L acid sites increased. It is noticed that the increase of La-loading clearly increased the concentration of weak and medium acid sites. B/L ratio decreased on La/HZSM-5 with the increase of La-loading. Hence, the introduction of La into HZSM-5 could not only modify the concentration of acid sites, but also affect the distribution of acid sites. Namely, La doping improved the concentration of L acid sites and reduced the concentration of B acid sites and the B/L ratio. 3.2. Catalytic Performance. Table 3 shows effect of Laloading on the reactivity of La/HZSM-5 in the coupling 13592

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Table 2. Concentration and Distribution of Brønsted and Lewis Acid Sites on Catalysts concn. of B (a.u./g) catalyst HZSM-5 0.5 wt % 1.5 wt % 5.0 wt % 7.0 wt %

La/HZSM-5 La/HZSM-5 La/HZSM-5 La/HZSM-5

concn. of L (a.u./g)

weak and med. (200 °C)

strong (350 °C)

total

weak and med. (200 °C)

strong (350 °C)

total

B/L

530 80 70 50 60

350 190 170 160 150

880 270 240 210 210

150 180 240 770 860

110 80 50 40 40

260 260 290 810 900

3.42 1.03 0.83 0.26 0.23

Table 3. Reactivity of La/HZSM-5 with Different La-Loading in the Coupling Conversion of Methanol and C4 Hydrocarbon yield (wt %) final La-loading (wt %)

conv. (wt %) methanol

conv. (wt %) n-butylenes

conv. (wt %) n-butanes

CH4

C2H4

C2H6

C3H6

C3H8

C5+

0.0 0.35 1.42 4.72 6.55

100.0 100.0 100.0 100.0 100.0

93.9 96.5 97.5 93.9 88.7

5.3 8.0 8.6 7.2 3.5

0.1 0.2 0.2 0.1 0.1

10.5 13.5 15.0 8.5 7.1

11.0 14.7 15.8 9.1 7.7

36.3 41.5 46.0 27.1 19.2

17.7 12.6 10.5 22.5 23.9

18.3 14.0 9.9 26.6 30.6

a

Reaction conditions: 450 °C, WHSV = 0.6 h−1, methanol/C4 hydrocarbon = 0.3 molar ratio, flow rate of methanol = 0.018 mL/min, TOS = 4 h.

conversion of methanol and C4 hydrocarbon. Under the used reaction conditions, distinct conversion of n-butylenes and little conversion of n-butanes were observed on these catalysts; in addition, products CH4, C2H4, C2H6, C3H6, and C3H8, as well as C5+ (liquid phase products, aliphatic and aromatics hydrocarbon), were detected. These results indicate that a series of reactions, such as, hydrocarbon cracking, hydrogen transfer, aromatization, dehydration reaction, and so on, might be involved in the coupling conversion of methanol and C4 hydrocarbon. Since the conversion of butanes was very low, these products were mainly produced from the conversion of butylenes and methanol. HZSM-5 exhibited 93.9% conversion of n-butylenes with 36.3% yield of propylene and 10.5% yield of ethylene. La/ HZSM-5 showed different catalytic reactivity from parent HZSM-5. The conversion of n-butylenes and n-butanes first increased and then gradually decreased with increasing the load of La. Similarly, both ethylene yield and propylene yield respectively increased to the maximal values of 46.0% and 15.0% on 1.5 wt % La/HZSM-5, and then began to drop with the increase of La-loading. It is found that loading La led to an increase in C5+ yield. Thus, La doping had great influences on the catalytic reactivity of La/HZSM-5. C4 hydrocarbons conversion and products distribution on La/HZSM-5 were strongly dependent on the load of La. La/HZSM-5 must possess the proper load of La for obtaining the high propylene yield. Figure 5 shows effect of methanol/C4 hydrocarbon molar ratio on the catalytic reactivity of 1.5 wt % La/HZSM-5 catalyst in the coupling conversion of methanol and C4 hydrocarbon. Comparing with only C4 hydrocarbon as feed, the cofeed of methanol and C4 hydrocarbon improved the conversion of nbutylenes and n-butanes. In addition, both n-butylenes conversion and n-butanes conversion initially increased to the maximal value and then decreased with the increase of methanol/C4 hydrocarbon ratio. Thus, it can be deduced that the coupling conversion of methanol and C4 hydrocarbon occurred on the catalyst. The proper methanol/C4 hydrocarbon ratio was required to obtain the high conversion of n-butylenes and n-butanes. Chang et al.24 reported that methanol enhanced the conversion of n-hexane on HZSM-5 during the coupling conversion of methanol and n-hexane. They thought that

Figure 5. Effect of methanol/C4 hydrocarbon molar ratio on the reactivity of 1.5 wt % La/HZSM-5 in the coupling conversion of methanol and C4 hydrocarbon. Reaction conditions: 450 °C, WHSV = 0.6 h−1, TOS = 4 h.

methanol was adsorbed on acid sites prior to n-hexane and immediately transformed into surface methoxy groups.24 These methoxy groups acted as the active sites for the conversion of nhexane and improved the initial activity of n-hexane by bimolecular hydride transfer.12 On the other hand, both ethylene yield and propylene yield in the coupling conversion were higher than those in the 13593

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of methanol and C4 hydrocarbon. Methanol was almost completely converted in the investigated reaction temperature range. It is noticed that the conversion of butylenes was almost the same and reached ca. 97%. In addition, the conversion of butanes slightly increased with the increase of reaction temperature. The yield of ethylene and ethane constantly increased when the reaction temperature rose. C5+ yield and propane yield showed decreased tendency with the increase of reaction temperature. It is well-known that the high temperature favored the cracking of hydrocarbon and the transformation of methanol to produce ethylene and ethane. Therefore, the yield of ethylene and ethane increased with increasing temperature. On the other hand, propylene yield initially increased with the reaction temperature, reached a maximum value of ca. 46 wt % at 450 °C, and subsequently decreased to 39 wt % at 510 °C. The result implies that propylene could further transfer to secondary reaction products at the high reaction temperature. The joint transformation of methanol and C4 hydrocarbon to olefins is the coupling of exothermic reaction and endothermic reaction, resulting in the complicated effect of reaction temperature on the reactivity of La-HZSM-5. The effect of reaction temperature on the joint transformation of methanol and C4 hydrocarbon might be different from that on the dependent Propylur process and MTO. Reaction temperature played an important role on the catalytic reactivity and stability of La-HZSM-5 in the coupling reaction. At the high reaction temperature, the produced propylene might further transfer by oligomerization, aromatization, and coke. Hence, the proper reaction temperature is required for gaining the high yield of propylene. Figure 7 shows effects of weight hourly space velocity (WHSV) on the reactivity of 1.5 wt % La/HZSM-5 in the coupling conversion of methanol and C4 hydrocarbon at 450 °C. The conversion of n-butanes and n-butylenes gradually decreased with the increase of WHSV, possibly because the high WHSV blocked the contact of reactants with active sites. Ethylene yield and propylene yield first increased and then decreased with the increase of WHSV; propane yield and C5+ yield gradually increased whereas ethane yield decreased. Thus, the product distribution in the coupling conversion was strongly dependent on WHSV. The previous revealed that propylene and ethylene were primary products during the cracking of C4 hydrocarbon and MTO and they could further transfer to secondary products by hydrogen transfer, oligomerization, and aromatization reaction, and so forth.25−31 Thus, in order to achieve the high yield of propylene and ethylene, the moderate WHSV should be used in the coupling conversion. On the other hand, it is worth noting that ethylene yield and propylene yield showed the similar tendency as a variety of WHSV. Figure 8 shows time course of the coupling conversion of methanol and C4 hydrocarbon, C4 hydrocarbon cracking, and MTO on 1.5 wt % La/HZM-5. With prolonging time on stream (TOS), the yield of ethylene and propylene constantly decreased in MTO and C4 hydrocarbon cracking. These results suggest that 1.5 wt % La/HZM-5 did not show perfect catalytic stability in MTO and C4 hydrocarbon cracking. On the other hand, it is noticed that propylene yield and ethylene yield almost kept constant in the coupling conversion of methanol and C4 hydrocarbon with the increase of TOS. Thus, the coupling conversion significantly improved the catalytic stability

individual cracking of C4 hydrocarbon and MTO. These results further indicate that the coupling conversion of methanol and C4 hydrocarbon occurred and favored the formation of propylene and ethylene. For the coupling conversion, propylene yield and ethylene yield initially increased and then decreased with increasing methanol/C4 hydrocarbon ratio. The highest yields of propylene and ethylene were obtained at methanol/C4 hydrocarbon ratio of ca. 0.3. Hence, methanol/C4 hydrocarbon ratio greatly affected the products distribution of the coupling conversion. The proper methanol/C4 hydrocarbon ratio was necessary to gain the high propylene yield and ethylene yield. Mier et al.6,7 reported that the coupling conversion of methanol and n-butane with the proper methanol/n-butane ratio favored the reaction heat neutral integrated process and generated the active intermediate species to enhance cracking of hydrocarbon and methanol to olefins. In addition, the methanol and/or methoxy adsorbed on acid sites avoided the secondary reaction of propylene.24 As a result, the coupling conversion with the proper methanol/C4 hydrocarbon ratio improved the yield of propylene and ethylene compared with individual MTO and C4 hydrocarbon cracking. Figure 6 shows effect of reaction temperature on the reactivity of 1.5 wt % La/HZSM-5 in the coupling conversion

Figure 6. Effect of reaction temperature on the reactivity of 1.5 wt % La/HZSM-5 catalyst in the coupling conversion of methanol and C4 hydrocarbon. Reaction conditions: WHSV = 0.6 h−1, methanol/C4 hydrocarbon = 0.3 molar ratio, flow rate of methanol = 0.018 mL/min, TOS = 4 h. 13594

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Figure 7. Effect of WHSV on the reactivity of 1.5 wt % La/HZSM-5 in the coupling conversion of methanol and C4 hydrocarbon. Reaction conditions: 450 °C, methanol/C4 hydrocarbon = 0.3 molar ratio, flow rate of methanol = 0.018 mL/min, TOS = 4 h.

Figure 8. Time course of the coupling conversion of methanol and C4 hydrocarbon, C4 hydrocarbon cracking, and MTO on 1.5 wt % La/ HZSM-5 catalyst. Reaction conditions: (1) the coupling conversion: 450 °C, methanol/C4 hydrocarbon = 0.3 molar ratio, flow rate of methanol = 0.018 mL/min, WHSV = 6 h−1; (2) C4 hydrocarbon cracking: 450 °C, WHSV = 6 h−1; (3) MTO: 450 °C, total flow of gasphase feed = 80 mL/min, Pmethanol = 25 kPa, 1 g catalyst.

of 1.5 wt % La/HZM-5 compared with the individual C4 hydrocarbon cracking and MTO. It is known that the deactivation of HZSM-5 based catalyst in MTO and C4 hydrocarbon cracking was mainly due to the deposition of coke on the catalyst surface. The previous works reveal that the coupling conversion favored neutralizing the endothermal C4 hydrocarbon cracking and exothermal methanol transformation to block the formation of coke.3−7 On the other hand, the coupling conversion of methanol and C4 hydrocarbon might attenuate the formation of coke by methanol fast adsorbing on acid sites prior to C4 hydrocarbon.24 Therefore, the catalytic stability of 1.5 wt % La/HZSM-5 greatly improved in the coupling conversion. 3.3. Coupling Conversion of Methanol and C 4 Hydrocarbon to Propylene. The catalytic test reveals that the coupling conversion of methanol and C4 hydrocarbon to propylene occurred over La/HZSM-5. The coupling conversion significantly improved the yield of propylene and catalytic stability of 1.5 wt % La/HZSM-5 compared with the individual C4 hydrocarbon cracking and MTO process. For achieving the high yield of propylene, it was necessary that the coupling reaction was performed on La/HZSM-5 with the proper La-loading and under the optimal reaction conditions. The maximal propylene yield of ca. 46 wt % with ethylene yield of 16 wt % was gained on 1.5 wt % La/HZSM-5 at 450 °C, WHSV = 0.6 h−1, methanol/C4 hydrocarbon = 0.3 molar ratio,

flow rate of methanol = 0.018 mL/min, and TOS = 4 h. Under the similar reaction conditions, 1.5 wt % La/HZSM-5 showed good catalytic reactivity relative to these previously reported catalysts.3−8 To further evaluate the improvement in the coupling conversion, an improvement coefficient (α) is calculated by the following formula proposed by Mier et al.:6,7 α = YC3H6,coupling − (nYC3H6,MTO + (1 − n)YC3H6,C4 cracking)

where YC3H6, coupling is propylene yield obtained from the coupling conversion of methanol and C4 hydrocarbon; YC3H6, MTO and YC3H6,C4 cracking are propylene yield calculated in independent MTO and C4 hydrocarbon cracking process under the same space velocity; and n is methanol/(methanol + C4 hydrocarbon) molar ratio. In case of this work, the improvement coefficient α is ca. 29.5% on 1.5 wt % La/HZSM-5 under the reaction conditions of 450 °C, WHSV = 0.6 h−1, methanol/C4 hydrocarbon = 0.3 molar ratio, flow rate of methanol = 0.018 mL/min and TOS = 4 h. The result further confirms that the coupling conversion of methanol and C4 hydrocarbon to propylene took place on 1.5 wt % La/HZSM-5 and the coupling reaction enhanced the 13595

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formation of propylene with respect to the independent MTO and C4 hydrocarbon cracking. 3.4. Influences of La on Structure and Acidity. On the basis of the above results, it is found that the introduction of La into HZSM-5 caused the interaction of La with HZSM-5 framework. Such interaction greatly modified the structure and acidity of the La/HZSM-5. Thus, La doping played important roles on the physicochemical properties of these catalysts. Up to the present, different descriptions of effects of introducing La to HZSM-5 on the structure and properties of La-doped HZSM-5 catalyst have been reported. For example, Wang et al.10 reported that La not only modified the concentration and distribution of acid site but also altered the basic properties of La-modified HZSM-5 catalyst. Xue et al.11 and Tynjala et al.20 observed that the addition of La into HZSM-5 decreased the total amount of B acid sites and generated new acid sites by splitting of water with La3+. Wakui et al.32 found that the acidity of the La-modified HZSM-5 catalysts hardly changed and the basic sites newly generated on the surface by the loading of La. These different results about the roles of La might be mainly due to the use of different catalyst and characterization methods. Clearly, a convicting model should be proposed to account for the influences of La additive on the structure and acidity of La/HZSM-5 for the coupling conversion of methanol and C4 hydrocarbon to propylene. From the obtained results in this work, we can get insight into the relationship of La species with the structure and acidity of La/HZSM-5. La species highly dispersed on HZSM-5 surface; in addition, the interaction of La species with HZSM-5 framework and surface OH group occurred. Such interaction became stronger and stronger with the increase of La-loading. The introduction of La eliminated some OH groups from Si(OH) and Al(OH)Si. Generally speaking, OH groups from Si(OH) are weak and medium B acid sites, while OH groups from Si(OH)Al provide strong B acid sites. Therefore, the increase of La-loading decreased the total concentration of B acid sites and modified the strength distribution of B acid sites. On the other hand, new L acid sites La(OH)2+ could be generated by the interaction of La species with bridging OH group and splitting coordinated water molecular with La3+ ions.11,20 As a consequence, the concentration of L acid sites increased on La/HZSM-5 with the increase of La-loading. The addition of La modified the type distribution of acid sites; that is, B/L ratio gradually decreased on La/HZSM-5 with the increase of La-loading. 3.5. Relationship of Acidity with Catalytic Reactivity. It is noticed that the catalytic reactivity and acidity of La/ HZSM-5 varied as a function of the La-loading. For revealing the influence of the acidity on the catalytic reactivity of La/ HZSM-5, the relationships of the density of acid sites and B/L ratio with specific yield of propylene were correlated (Figure 9). The density of acid sites (a.u./m2) is identified as the amount of acid sites on unit-surface-area catalyst. The specific yield of propylene (g/(m2·h)) is identified as the formation of propylene in an hour and per surface area catalyst. It can be seen from Figure 9, the acidity had complex influences on the catalytic reactivity of La/HZSM-5. The moderate density of acid sites and B/L ratio on La/HZSM-5 favored the formation of propylene in the coupling conversion of methanol and C4 hydrocarbon. It is widely accepted that C4 hydrocarbon cracking follows the bimolecular reaction mechanism which is involved in

Figure 9. Relationship of acidity and propylene yield in the coupling of methanol and C4 hydrocarbon over La-modified HZSM-5 catalysts with the different La-loading. Reaction conditions: 450 °C, WHSV = 0.6 h−1, methanol/C4 hydrocarbon = 0.3 molar ratio, flow rate of methanol = 0.018 mL/min, TOS = 4 h.

hydride transfer, isomerization, alkylation, and β scission of the carbenium ion.25,33 In this mechanism, propylene produces from the cracking of n-butylenes dimers and it can be further transferred to secondary products by hydrogen transfer, oligomerization, and aromatization reaction. In case of MTO reaction on acidic zeolite catalyst, “Hydrocarbon Pool” mechanism was proposed to be dominant routes for the first C−C bond formation in MTO reaction and the first C−C bond is closely correlated with the existence of surface methoxy groups as the most important active species.26−31 Ethylene, propylene, and butylenes produce from a pool of adsorbed hydrocarbon as intermediate, such as pentabenzene and hexemethylbenzene, in the conversion of methanol.26−31 The reaction mechanism concerning the coupling conversion of methanol and C4 hydrocarbon has remained unclear. Few works have been done to understand the reaction mechanism of the coupling reaction. It is difficult to probe the coupling reaction mechanism because the two different reactants result in the complexity of the coupling system. Li et al.9 considered that the methylation of ethylene by methanol was responsible for the enhancement of propylene yield in the coreaction of ethylene and methanol. The further methylation of propylene by methanol and the cracking of higher olefins were also operative under the used reaction conditions.9 Chang et al.24 thought that during the coupling of methanol and n-hexane methoxy species produced from methanol were the active sites for the conversion of n-hexane and improved the initial activity of n-hexane by bimolecular hydride transfer. In view of above points, we can deduce the possible reaction pathways of the coupling conversion of methanol and C4 hydrocarbon on La/HZSM-5. Methanol was activated to methoxy species and C4 hydrocarbon transferred to carbenium ion following bimolecular mechanism on B acid sites. Methoxy species reacted with carbenium ion to produce olefins such as ethylene and propylene. Nevertheless, propylene and ethylene could further convert to secondary reaction products on L and B acid sites by hydrogen transfer, isomerization, and aromatization reaction, etc. Thus, the density of acid sites and B/L ratio on the catalyst had important roles in its catalytic reactivity in the coupling reaction. To obtain the high propylene yield, the catalyst must possess the moderate density and type distribution (B/L ratio) of acid sites which can not only enhance the formation of propylene but also hold back the 13596

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(6) Mier, D.; Aguayo, A. T.; Gayubo, A. G.; Olazar, M.; Bilbao, J. Catalyst discrimination for olefin production by coupled methanol/nbutane cracking. Appl. Catal., A 2010, 383, 202. (7) Mier, D.; Aguayo, A. T.; Gayubo, A. G.; Olazar, M.; Bilbao, J. Synergies in the production of olefins by combined cracking of nbutane and methanol on a HZSM-5 zeolite catalyst. Chem. Eng. J. 2010, 160, 760. (8) Wang, Z.; Jiang, G.; Zhao, Z.; Feng, X.; Duan, A.; Liu, J.; Xu, C.; Gao, J. Highly Efficient P-Modified HZSM-5 Catalyst for the Coupling Transformation of Methanol and 1-Butene to Propene. Energy Fuels 2010, 24, 758. (9) Li, J.; Qi, Y.; Xu, L.; Liu, G.; Meng, S.; Li, B.; Li, M.; Liu, Z. Coreaction of ethene and methanol over modified H-ZSM-5. Catal. Commun. 2008, 9, 2515. (10) Wang, X.; Zhao, Z.; Zhang, L.; Jiang, G.; Xu, C.; Duan, A. Effects of fight Rare earth on Acidity and Catalytic Performance of HZSM-5 Zeolite for Catalytic Clacking of Butane to Light Olefins. J. Rare Earth 2007, 25, 321. (11) Xue, N.; Liu, N.; Nie, L.; Yu, Y.; Gu, M.; Peng, L.; Guo, X.; Ding, W. 1-Butene cracking to propene over P/HZSM-5: Effect of lanthanum. J. Mol. Catal. A 2010, 327, 12. (12) Aguayo, A. T.; Castaño, P.; Mier, D.; Gayubo, A. G.; Olazar, M.; Bilbao, J. Effect of co-feeding butane with methanol on the deactivation by coke of a HZSM-5 zeolite catalyst. Ind. Eng. Chem. Res. 2011, 50, 9980. (13) Kaarsholm, M.; Joensen, F.; Nerlov, J.; Cenni, R.; Chaouki, J.; Patience, G. S. Phosphorous modified ZSM-5: Deactivation and product distribution for MTO. Chem. Eng. Sci. 2007, 62, 5527. (14) Mier, D.; Gayubo, A. G.; Aguayo, A. T.; Olazar, M.; Bilbao, J. Olefin production by co-feeding methanol and n-butane. Kinetic modeling considering the deactivation of HZSM-5 zeolite. AIChE J. 2011, 57, 2841. (15) Schulz, H. “Coking” of zeolites during methanol conversion: Basic reactions of the MTO, MTP and MTG processes. Catal. Today 2010, 154, 183. (16) Perego, G.; Bellussi, G.; Corus, C.; Toramasso, M.; Buonomo, F.; Esposito, A. In New Development in Zeolite Science and Technology; Murakami, Y., Lijima, A., Ward, J. W., Eds.; Kodansha/Elsevier: Tokyo/Amsterdam, 1986; p 129. (17) Yoo, J.; Lee, C.; Chang, J.; Park, S.; Ko, J. Characterization and catalytic properties of Ti-ZSM-5 prepared by chemical vapor deposition. Catal. Lett. 2000, 66, 169. (18) Zhang, X.; Zhong, J.; Wang, J.; Zhang, L.; Gao, J.; Liu, A. Catalytic performance and characterization of Ni-doped HZSM-5 catalysts for selective trimerization of n-butene. Fuel Process. Technol. 2009, 90, 863. (19) Kazansky, V.; Borovkov, V.; Serikha, A.; van Santen, R.; Anderson, B. Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation. Catal. Lett. 2000, 66, 39. (20) Tynjala, P.; Pakkanen, T. Acidic properties of ZSM-5 zeolite modified with Ba2+, Al3+ and La3+ ion-exchange. J. Mol. Catal. A 1996, 110, 153. (21) Meusinger, J.; Corma, A. Influence of Zeolite Composition and Structure on Hydrogen Transfer Reactions from Hydrocarbons and from Hydrogen. J. Catal. 1996, 159, 353. (22) Jaumain, D.; Su, B. Monitoring the Brønsted acidity of zeolites by means of in situ FT-IR and catalytic testing using chloromethane as probe molecule. Catal. Today 2002, 73, 187. (23) Zhang, X.; Wang, J.; Zhong, J.; Liu, A.; Gao, J. Characterization and catalytic performance of SAPO-11/Hβ composite molecular sieve compared with the mechanical mixture. Microporous Mesoporous Mater. 2008, 108, 13. (24) Chang, F.; Wei, Y.; Liu, X.; Zhao, Y.; Xu, L.; Sun, Y.; Zhang, D.; He, Y.; Liu, Z. A mechanistic investigation of the coupled reaction of n-hexane and methanol over HZSM-5. Appl. Catal., A 2007, 328, 163. (25) Li, L.; Gao, J.; Xu, C.; Meng, X. Reaction behaviors and mechanisms of catalytic pyrolysis of C4 hydrocarbons. Chem. Eng. J. 2006, 116, 155.

secondary reactions of propylene. The addition of La into HZSM-5 remarkably modified the acidity of the catalyst, which in turn affected the catalytic reactivity of the catalyst in the coupling reaction. The 1.5 wt % La/HZSM-5 catalysts exhibited the high propylene yield, mainly owing to its moderate density and type distribution of acid sites.

4. CONCLUSION The 1.5 wt % La/HZSM-5 was an effective catalyst for the coupling conversion of methanol and C4 hydrocarbon. The reactivity of La/HZSM-5 was strongly dependent on the reaction conditions and La-loading. The comparatively high propylene yield of ca. 46 wt % was obtained on 1.5 wt % La/ HZSM-5 catalyst under the optimal reaction conditions of 450 °C, WHSV = 0.6 h−1, methanol/C4 hydrocarbon = 0.3 molar ratio, flow rate of methanol = 0.018 mL/min, and TOS = 4 h. Under the optimal coupling reaction conditions, the propylene yield obtained from the La-modified HZSM-5 catalyst was higher than that from other HZSM-5 based catalysts, such as Pmodified HZSM-5, Ga-modified HZSM-5, and Ni-modified HZSM-5.3−8 The La introduced into HZSM-5 caused the interaction of La species with HZSM-5 framework. Such interaction reduced the density of OH groups and generated new L acid sites by La interacting with surface OH groups and splitting adsorbed water molecular, which modified the density and distribution of acid sites on La/HZSM-5. The acidity of La/HZSM-5 had important roles on its catalytic reactivity in the coupling reaction. The 1.5 wt % La/HZSM-5 exhibited the high propylene yield, mainly owing to its moderate density and distribution of acid sites.

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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

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REFERENCES

This work was financially supported by National Ministry of Education (20096101120018, NCET-10-878, SRFRCS-200937th), Shaanxi Project (2011zkc04-8, 2009ZDKJ-70, 09JK793), Northwest University (PR09005, 10YSY08), State Key Lab for Physical Chemistry of Solid Surfaces (2009), and State Key Lab of Chemical Resource Engineering (CRE-2011-C-304).

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