Ethylation of coking benzene over nanoscale HZSM-5 zeolites: Effects of hydrothermal treatment, calcination and La2O3 modification

Applied Catalysis A: General 355 (2009) 184–191

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Ethylation of coking benzene over nanoscale HZSM-5 zeolites: Effects of hydrothermal treatment, calcination and La2O3 modification Linping Sun, Xinwen Guo *, Min Liu, Xiangsheng Wang State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 July 2008 Received in revised form 5 December 2008 Accepted 10 December 2008 Available online 24 December 2008

The objective of this work is to study some post-treatment effects, including hydrothermal treatment, calcination and La2O3 modification, on the catalytic performance of a nanoscale HZSM-5 zeolite for ethylation of coking benzene. The nanoscale HZSM-5 zeolite was treated by hydrothermal treatment, calcination and La2O3 modification in series, and the prepared catalysts were evaluated in a fixed-bed down-flow reactor. The catalyst samples were also characterized by XRD, SEM, NH3-TPD, IR, N2 adsorption–desorption isotherms, cyclohexane adsorption isotherms and TG. The results showed that both the hydrothermal treatment and the calcination led to a drastic decrease in the total amount of acid sites, while the subsequent La2O3 modification resulted in only a slight increase in the number of acid sites. These post-treatments led to a decrease in the Bro¨nsted/Lewis ratio of the nanoscale HZSM-5 zeolites. The hydrothermal treatment followed by the La2O3 modification created new large micropores on the nanoscale HZSM-5 zeolites, which results in the coexistence of micropores and large micropores. The increase in the catalyst lifetime can be attributed to both suppression of carbon deposit formation and partial accommodation of the formed carbon deposit in La-C-HT-HZSM-5 catalyst with lower acidity, lower ratio of B/L and complicated pore structure. The prepared La-C-HT-HZSM-5 catalyst showed good catalytic stability within 1500 h of time on stream in the ethylation of coking benzene containing high sulfur content (375 ppmw) with ethylene under industrial reaction conditions. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Coking benzene Nanoscale HZSM-5 Hydrothermal treatment Calcination La2O3 modification

1. Introduction Ethylbenzene (EB) is a key intermediate in the manufacture of styrene which is the most important industrial monomer. The EB production in 2010 is estimated to be about 34 million metric tons [1]. The competing technologies for EB production based on zeolite catalysts mainly include Mobil-Badger, Mobil-Badger EBMax, Lummus-UOP and CDtech processes in the petrochemical industry [1–3]. In these processes, ethylene, ethanol and FCC off-gas [4] all can be used as the ethylating agents, but only in the alkylation of petroleum benzene. Ethylation of coking benzene, rather than petroleum benzene, without further purification, provides another source for EB production. It is a promising way to solve the supplement shortage of petroleum benzene. However, to our best knowledge, there has been no report about EB production from coking benzene. If conventional microscale HZSM-5 zeolites are used as the catalyst, it will be deactivated quickly. Nanoscale HZSM-5 zeolites [5] may be a good catalyst for the ethylation of coking benzene due to its larger external surface, shorter channels,

* Corresponding author. Tel.: +86 41139893990; fax: +86 41183689065. E-mail address: [email protected] (X. Guo). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.12.015

and more secondary pores in comparison with conventional microscale HZSM-5 zeolites. Hydrothermal treatment is often employed to adjust the acidity of zeolites [6] and to improve its stability in many reactions. The effect of hydrothermal treatment on the activity and catalytic stability can be attributed to the change in zeolites’ acidity. Song et al. found that hydrothermal treatment reduced both strong and weak acid sites of ZSM-5 zeolites, and thus reduced the carbon deposit during olefin aromatization [7]. Guo et al. also found hydrothermal treatment following HCl leaching improved the stability of HZSM-5 zeolites in the methylation of 4-methylbiphenyl with methanol, which was ascribed to dealumination after hydrothermal treatment [8]. On the other hand, many other researchers reported that dealumination of zeolites improved its activity in acid-catalyzed reactions due to the complex effect between Lewis acidity that resulted from the increased nonframework Al-oxide species and Bro¨nsted acidity after hydrothermal treatment [9–11]. Calcination is used as an effective method to reduce the acidity of zeolites, and therefore improve their lifetime in acid-catalyzed reactions. Campbell et al. found that HZSM-5 zeolites calcined at high temperature in N2 showed better catalytic stability than those without calcination in the conversion of methanol to gasoline.

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They contributed this difference to the fact that the calcined HZSM5 zeolites possessed lower densities of acid sites, which consequently decreased the rate of carbon deposit formation over HZSM-5 zeolites in this reaction [12]. Dong et al. also found that Mo/HZSM-5 catalyst calcined in N2 stream exhibited rather high stability in the methane dehydroaromatization due to the elimination of the strong Bro¨nsted acid sites, resulting in reduction of carbon deposit formation [13]. Rare-earth ions are often introduced into zeolites for improving their catalytic activity and catalytic stability by generating acidic sites via hydrolysis of hydrate cations and coverage of the active centers, respectively [14,15]. Biswas and Maxwell reported that introduction of rare-earth ions into Y zeolites was an effective method of improving the performance of FCC catalyst and of increasing the activity of the catalyst in many reactions due to the increase in the acid strength [16]. Zhao et al. found that HY zeolites modified with rare-earth lanthanum exhibited excellent stability in the alkylation of a-methylnaphthalene with long chain olefins, while the stability of the parent HY zeolites was poor. They attributed this to the differences in the coverage of acidic sites on the surface of HY zeolites [17]. All the above post-treatments of the acidic zeolites focused on the effects of hydrothermal treatment, calcination and La2O3 modification on the change in zeolitic acidity. However, there are no reports focused on the effects of these factors on the changes in pore structure of nanoscale HZSM-5 zeolites and on their applications in the ethylation of coking benzene. In the present study, nanoscale HZSM-5 zeolite was synthesized in an 8 m3 autoclave by using n-butylamine as the template. Using it as the precursor, we prepared the catalyst for the ethylation of coking benzene. In this paper, the effects of hydrothermal treatment, calcination and La2O3 modification on the physicochemical properties of the nanoscale HZSM-5 zeolites, and consequently on the ethylation of coking benzene was investigated. 2. Experimental 2.1. Materials Coking benzene comes from coal burning and is the main byproduct of coal oven gas. Benzene content in the coking benzene is larger than 99.5%. Both the appearance and the density of the coking benzene are the same as those of the petroleum type. On the other hand, the sulfur content in the coking benzene is about 100– 800 ppmw and dependent on its refining technologies. Thiophene and carbon disulfide are the sulfur species in the coking benzene. The other reactants used for the ethylation are pure ethylene (99.99%) and dehydrated ethanol (>99.7%). 2.2. Catalyst preparation NaZSM-5 powder (SiO2/Al2O3 = 24.7) was extruded by using alumina as a binder (30 wt%). After drying at 373 K overnight, the extrudate was calcined in air at 813 K for 4 h to remove the nbutylamine template. Then NaZSM-5/Al2O3 was exchanged with 0.4 mol/L NH4NO3 aqueous solution at room temperature for 2 h (solid/solution = 1 g/10 ml), then washed with deionized water. This procedure was repeated three times. Finally the sample was dried at 373 K overnight, and calcined in air at 813 K for 4 h. The obtained sample was denoted as HZSM-5 (P). Hydrothermal treatment was conducted in a fixed-bed reactor. In a typical process, 10 g of HZSM-5 (P) was packed in the middle of the reactor. Before treatment, the reactor was heated to 673 K at a rate of 10 K/min and held at this temperature for 1 h, and then 100% water vapor was fed at a rate of 2 g H2O/g sample/h through a piston pump for 3 h. The obtained sample was denoted as HT-HZSM-5.

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Calcination was conducted in a muffle oven. HT-HZSM-5 sample was calcined at 813 K for 4 h in air atmosphere. The obtained sample was denoted as C-HT-HZSM-5. La2O3 modification was accomplished by wet impregnation. The process is described as follows: HT-HZSM-5 and C-HT-HZSM-5 samples were impregnated with an aqueous solution of lanthanum nitrate for 4 h at room temperature. The concentration of the solution depended on the loading amount of La2O3. The ratio of the volume (ml) of aqueous solution to the weight (g) of sample was 6:1. The impregnated sample was dried at 373 K overnight and calcined at 813 K for 4 h. The obtained samples were denoted as LaHT-HZSM-5 and La-C-HT-HZSM-5, respectively. 2.3. Catalyst characterization XRD patterns were recorded using a Rigaku D/MAX-2400 diffractometer with Cu Ka1 radiation. SEM pictures were determined by using JEOL JSM-6700F field emission scanning electron microscope. N2 adsorption and desorption experiments were performed on an AUTOSORB-1 gas adsorption analyzer (Quantachrome, USA). Each sample was evacuated at 623 K for 4 h prior to adsorption. Pore size distributions were determined by applying the HK method to the adsorption branch of the isotherm. NH3-TPD experiments were carried out on a CHEMBET 3000 chemical absorber (Quantachrome, USA). Prior to each adsorption experiment, about 0.2 g of sample was pretreated at 813 K for 1 h in a quartz U-tube under a He flow. The temperature was raised from 373 K to 873 K at a heating rate of 10 K/min. IR spectra were recorded on an EQUINOX55 Fourier transform infrared spectrometer (Bruker Corp.) at 4 cm1 resolution. The sample used for IR was finely ground and pressed into a self-supporting wafer (8– 10 mg/cm2, diameter 15 mm). For FT-IR study, the sample was degassed at 723 K in vacuum, while for Py-IR, after adsorption of pyridine at room temperature, the sample was desorbed under vacuum at 423 K, 573 K and 723 K, respectively. Cyclohexane isotherm adsorption capacity was measured at 298 K according to a flow gravimetric method [18]. Before adsorption, the sample was dehydrated at 623 K for 50 min under a 35 ml/min N2 flow. The dehydrated sample was placed into the isothermal chamber (298 K). By using N2 as a carrier gas, the saturated cyclohexane vapor passed through the sample until the adsorption–desorption equilibrium was achieved. The equilibrium adsorption capacity was defined as the molar amount of the adsorbed cyclohexane per gram of sample. 2.4. Catalyst test Ethylation of coking benzene was carried out in a fixed-bed down-flow reactor. The reaction was conducted at a temperature of 673 K and a pressure of 1.4 MPa. When ethanol was used as an ethylating agent, the coking benzene was mixed with the ethanol, and then was fed together to the catalyst bed. When ethylene was used as an ethylating agent, the coking benzene and the ethylene were fed to the catalyst bed separately. Prior to being fed with the ethylene, the catalyst bed was fed with the coking benzene for 30 min to avoid the deactivation of the catalyst by the ethylene. In both cases, the molar ratio of the coking benzene to the ethylating agent was 6:1. The products were collected and analyzed using a GC (TECHCOMP 7890F) equipped with a SE-30 capillary column (50 m  0.32 mm  1.0 mm) and a FID detector. 2.5. Measurement of carbon deposit TG profiles were recorded on a TGA/SDTA851e instrument (METTLER TOLEDO). The spent catalyst powder was heated to 1123 K at a ramp rate of 10 K/min in an air flow with a flow rate of

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186

Fig. 1. SEM of nanoscale HZSM-5 zeolites.

60 ml/min. The weight loss between 573 K and 1123 K was attributed to carbon deposits on the spent catalyst and was calculated according to the following equation [7]: M1  M2 carbon deposit amount ðwt%Þ ¼  100 M1 Here M1 represents the weight percent of the spent catalyst after desorption of water and M2 represents the weight percent of the spent catalyst after burning-off the carbon deposit. 3. Results and discussion 3.1. Characterization of catalysts Fig. 1 displays the SEM image of the parent HZSM-5 powder. It is found that the crystal size of HZSM-5 is about 70 nm  100 nm. Unlike the hexagonal pattern of conventional microscale HZSM-5 zeolites, a single crystal of the nanoscale HZSM-5 zeolites presents an orthogonal pattern. Moreover, aggregation of these orthogonal particles occurs due to high surface Gibbs free energy of the HZSM5 nano-particles. Fig. 2 presents the XRD patterns of the samples before and after modification. The relative crystallinity is determined by comparing the integrated intensities of five characteristic XRD peaks in the 2u region of 5–458. It is shown that the characteristic diffraction peaks

Fig. 3. Pore distribution of the samples with and without the hydrothermal treatment followed by the La2O3 modification.

assigned to the nanoscale HZSM-5 zeolites are preserved, and that no La2O3 crystal phase is observed after 2.7 wt% La2O3 modification. This indicates that the structure of the nanoscale HZSM-5 zeolites is not destroyed by these post-treatments and that La2O3 is highly dispersed on the surface of the zeolite. The relative crystallinity values of HZSM-5 (P), HT-HZSM-5, La-HT-HZSM-5 and La-C-HT-HZSM-5 samples are 100%, 78.2%, 72.9% and 70.1%, respectively. The changes in pore distributions of the samples after the hydrothermal treatment followed by the La2O3 modification were determined by N2 adsorption–desorption isotherm technology and the profiles are presented in Fig. 3. A decrease in the micropore (diameter = 5.5 A˚) of La-HT-HZSM-5 sample is observed. This phenomenon indicates the blockage of the channels in the nanoscale HZSM-5 zeolites due to the deposition of nonframework Al species. On the other hand, the La-HT-HZSM-5 sample shows two newly created micropores (diameter = 7.2 A˚ and 9.5 A˚, respectively). As for the formation of the large micropores, the removal of framework aluminum from the zeolite lattice during the hydrothermal treatment plays a key role. Such results are consistent with the previous report by Bertea et al. [19]. The NH3-TPD data of HZSM-5 (P), HT-HZSM-5, La-HT-HZSM-5 and La-C-HT-HZSM-5 samples are summarized in Table 1. All the samples exhibit two NH3 desorption peaks characteristics of zeolites with MFI structure. One peak centers at about 540 K and the other at around 700 K, corresponding to the weak and the strong acidity, respectively. Usually, the specific peak area is proportional to the number of the acid sites of the sample and can be calculated based on the Gaussian curve-fitting method. The hydrothermal treatment decreases the amount of acid sites of HZSM-5 (P) by about 23.7%. Such reduction in the acid sites is due to dealumination during the hydrothermal treatment. However, Table 1 NH3-TPD and Py-IR data of the samples. Samples

HZSM-5 (P) HT-HZSM-5 La-HT-HZSM-5 La-C-HT-HZSM-5

Fig. 2. XRD patterns of the samples.

Acid amounta (mmol/g cat)

Tpeak (K) LT peak

HT peak

Total acidity

Weak acidity

Strong acidity

546 535 538 525

705 690 693 683

0.211 0.161 0.167 0.130

0.079 0.066 0.063 0.047

0.132 0.095 0.104 0.083

B/Lb

2.7 1.9 1.4 0.8

LT peak represents low temperature desorption peak. HT peak represents high temperature desorption peak. a Calculated with Gaussian function fit. b Calculated by the absorbance ratio AB/AL.

L. Sun et al. / Applied Catalysis A: General 355 (2009) 184–191

the subsequent La2O3 modification slightly increases the amount of acid sites of HT-HZSM-5 sample. This is against with the previous results [17,20]. It may be attributed to a co-contribution of newly formed Bro¨nsted acidity and Lewis acidity resulting from the hydration of lanthanum in the presence of trace amounts of water and from dehydration, respectively. Compared with La-HTHZSM-5 sample, La-C-HT-HZSM-5 sample possesses lower acidity. On the other hand, acid strengths for both strong and weak acidity are not influenced by the La2O3 modification because no change in desorption peak temperature is observed in the NH3-TPD data of HT-HZSM-5 and La-HT-HZSM-5. Such results are in agreement with the results reported by Sugi et al. [20]. But there is an obvious decrease in acid strength for HT-HZSM-5 and La-C-HT-HZSM-5 in comparison with HZSM-5 (P) and La-HT-HZSM-5, respectively. Fig. 4 displays the IR spectra of pyridine desorption at 573 K over HZSM-5 (P), HT-HZSM-5, La-HT-HZSM-5 and La-C-HTHZSM-5 samples in the 1400 cm1 and 1600 cm1 regions. The band at 1490 cm1 is attributed to the synergistic effect between Lewis acidity and Bro¨nsted acidity, while the bands at ca. 1545 cm1 and 1455 cm1 are assigned to protonation of the pyridine molecule by Bro¨nsted acidity and the pyridine coordinately bonded to Lewis acidity, respectively. As can be seen in Fig. 4, the hydrothermal treatment on HZSM-5 (P) results in an obvious decrease in both Bro¨nsted acidity and Lewis acidity. The subsequent La2O3 modification, however, leads to an increase in both Bro¨nsted acidity and Lewis acidity, especially noticeable for Lewis acidity. This is consistent with the above NH3-TPD results. In addition, the B/L ratio of the sample decreases in the modification process, being 2.7, 1.9, 1.4 and 0.8 for HZSM-5 (P), HT-HZSM-5, La-HT-HZSM-5 and La-C-HT-HZSM-5 samples, respectively, as listed in Table 1. It is due to the presence of Lewis acidity resulting from both the formation of extra lattice alumina during the hydrothermal treatment, the calcination and the La2O3 modification. This phenomenon was also observed by other researchers [21,22]. Fig. 5 compares the FT-IR spectra of the samples in the hydroxyl region after modification. The bands at 3612 cm1 and 3731 cm1 are ascribed to the acidic bridged OH groups (Bro¨nsted acidity) located in the internal cavities and the terminal silanol groups exposed at the external surface of HZSM-5 zeolites, respectively [23]. The bands at 3640–3680 cm1 are assigned to vibrations of OH groups attached to extraframework Al species [24]. As shown in Fig. 5, the band at 3612 cm1 decreases markedly due to dealumination after the hydrothermal treatment of HZSM-5 (P). In addition to a shift from 3612 cm1 to 3598 cm1, the band of the acidic bridged OH groups broadens obviously when more La2O3 was impregnated to HT-HZSM-5 sample. The terminal Si-OH decreases gradually with the modification processing. The other two bands associated with extraframework Al-OH undergo the

Fig. 4. Py-IR spectra of the samples. Evacuation desorption at 573 K.

187

Fig. 5. FT-IR spectra in the range 3200–4000 cm1 for the samples.

same changes as do the bridged OH groups during the modification. These results are in agreement with the results from NH3-TPD and Py-IR. In addition, the changes in both the terminal Si-OH and the bridged OH groups indicate that the loaded La2O3 is located in both the external and the internal surface of HT-HZSM-5. From the above results of the modified nanoscale HZSM-5 zeolites, the following changes can be summarized: (1) newly formed large micropores at 7.2 A˚ and 9.5 A˚ were observed in LaHT-HZSM-5 sample, (2) both the acidity and the B/L ratio decrease after the hydrothermal treatment, the calcination and the La2O3 modification over the nanoscale HZSM-5 zeolites. As shown in the following study, these changes in the properties of the samples cause some differences in their catalytic performance. 3.2. Ethylation of coking benzene over HZSM-5 (P), HT-HZSM-5, LaHT-HZSM-5 and La-C-HT-HZSM-5 samples Ethylation of coking benzene over HZSM-5 (P), HT-HZSM-5, LaHT-HZSM-5 and La-C-HT-HZSM-5 samples was conducted. The conversion and the total EB selectivity as a function of time on stream (TOS) over the first three samples are shown in Fig. 6. Conversion of coking benzene is calculated as a ratio of the converted coking benzene amount to the coking benzene amount

Fig. 6. Variation of coking benzene conversion with TOS over HZSM-5(P), HT-HZSM5 and La-HT-HZSM-5 samples: 673 K, 1.4 MPa, coking benzene/ethanol = 6:1 (molar ratio), sulfur content = 120 ppmw, WHSV = 10 h1. Solid symbol: coking benzene conversion; open symbol: total EB selectivity; square: HZSM-5 (P); triangle: HT-HZSM-5; circle: La-HT-HZSM-5.

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in the feed. Due to transalkylation of diethylbenzene (DEB) to EB, the total EB selectivity is defined as EB selectivity plus DEB selectivity. Besides, the deactivation rate is employed as a standard of anti-deactivation of the sample and is calculated by using a ratio of the decrease in the coking benzene conversion to TOS. As shown in Fig. 6, La-HT-HZSM-5 undergoes rather slow deactivation compared to the other two samples. The deactivation rate over HZSM-5 (P) and HT-HZSM-5 is 0.17%/h and 0.13%/h, respectively. But for La-HT-HZSM-5, the deactivation rate is only 0.06%/h. For HZSM-5 (P) sample, the conversion of coking benzene is 16.9% at 2 h and 5.4% at 66 h of TOS. For HT-HZSM-5 sample, the coking benzene conversion is 16.2% and 7.4% at 2 h and 67 h of TOS, respectively. For La-HT-HZSM-5 sample, however, the coking benzene conversion is 16.2% at 2 h of TOS and 11.8% at 69 h of TOS. In addition, the change in the total EB selectivity as the reaction proceeds shows again that La-HT-HZSM-5 sample exhibits more stability in this reaction. The total EB selectivity over HZSM-5 (P) sample decreases from 97.3% to 95.3%, decreasing by 2% after the reaction. For HT-HZSM-5 sample, it decreases from 97.6% to 96.1%, decreasing by 1.5%. However, for La-HT-HZSM-5 sample it decreases from 97.2% to 96.9%, decreasing by only 0.3%. In order to further improve the catalyst lifetime of La-HTHZSM-5 sample, the calcination was conducted after the hydrothermal treatment and before the La2O3 modification. Fig. 7 displays the conversion of coking benzene and the total EB selectivity as a function of TOS over La-HT-HZSM-5 and La-C-HTHZSM-5 samples. It is found that La-C-HT-HZSM-5 sample exhibits higher catalytic stability than La-HT-HZSM-5 sample. When TOS is 240 h, the conversion of coking benzene on La-CHT-HZSM-5 sample is about 15.1%, and the total EB selectivity is about 96.7%; while on La-HT-HZSM-5 sample, the conversion of coking benzene and the total EB selectivity at 240 h of TOS is 7.4% and 94.7%, respectively. Even at 386 h of TOS, the conversion of coking benzene over La-C-HT-HZSM-5 sample is 9.6%, and it is still higher than that value over La-HT-HZSM-5 sample at 240 h of TOS. Fig. 8 shows the test results of catalyst lifetime of La-C-HTHZSM-5 sample under industrial reaction conditions. Unlike the above reaction, ethylene, instead of ethanol, was used as ethylating agent in this reaction because ethylene is widely employed in commercial units for EB production. It is found that after 1500 h of TOS, the conversion of coking benzene and the total EB selectivity over the catalyst are around 14.5% and 98.8%, respectively.

Fig. 7. Ethylation of coking benzene over La-HT-HZSM-5 samples with and without calcinations: 673 K, 1.4 MPa, coking benzene/ethanol = 6:1 (molar ratio), sulfur content = 170 ppmw, WHSV = 3 h1. Solid symbol: coking benzene conversion; open symbol: total EB selectivity .

Fig. 8. Catalytic stability of La-C-HT-HZSM-5 sample under industrial reaction conditions: 673 K, 1.4 MPa, coking benzene/ethylene = 6:1 (molar ratio), sulfur content = 375 ppmw, WHSV = 2.7 h1. Solid symbol: coking benzene conversion, open symbol: total EB selectivity.

3.3. Effects of various modification methods Generally, acidity is the origin of the catalytic activity of zeolites. The coverage of acid sites and/or the channel blockage caused by carbon deposit, resulting in the inaccessible inner acid sites, is responsible for catalyst deactivation. So the variation of acidity after the modification and the carbon deposit over the spent samples are the main aspects to be discussed in this section. 3.3.1. Effect of the hydrothermal treatment Fig. 6 shows that HT-HZSM-5 gives higher catalytic stability than HZSM-5 (P). It may be attributed to the decrease in the acidity and the B/L ratio after the hydrothermal treatment. Table 1 shows that the peak belonging to strong acidity shifts from 705 K to 690 K, while the peak assigned to weak acidity shifts from 546 K to 535 K, indicating that the strengths of both acidities decrease after the hydrothermal treatment. In addition, the density of acid sites decreases after the hydrothermal treatment. The number of strong acidity decreases from 0.132 mmol/g cat to 0.095 mmol/g cat, and that of weak acidity decreases from 0.079 mmol/g cat to 0.066 mmol/g cat. In order to investigate the specific changes of Bro¨nsted acidity and Lewis acidity during the hydrothermal treatment, we also employed FT-IR and Py-IR technologies in this paper. In the FT-IR spectra, we take the ratio of peak intensity at 3612 cm1 to that at 3650 cm1, I3612 cm1 =I3650 cm1 , as a standard for comparing the relative abundance of Bro¨nsted acidity and Lewis acidity over the nanoscale HZSM-5 zeolites. FT-IR spectra show that I3612 cm1 =I3650 cm1 for HT-HZSM-5 is lower than the value for HZSM-5 (P), indicating that the hydrothermal treatment preferentially decreases Bro¨nsted acidity. Again, Py-IR spectra display that the B/L ratio decreases after the hydrothermal treatment. We ascribe the decrease in the acidity and the B/L ratio to the dealumination and the subsequent transformation of the hydroxylated Al-containing intermediate species to the stable Aloxide species with Lewis acidity [25]. This phenomenon is preferential over nanoscale HZSM-5 because it is more easily dealuminated by the hydrothermal treatment than microscale HZSM-5 zeolites [21]. Usually, the amount of carbon deposit over acidic zeolites is proportional to its acidity. The decrease in the acidity, especial the Bro¨nsted acidity, suppresses the carbon deposit formation. This is in agreement with the TG results shown in Table 2. The measured carbon deposit amount over the spent HT-HZSM-5 is 3.8 wt%, while it is 4.8 wt% on the spent HZSM-5 (P).

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Table 2 Carbon deposit on the spent samples. Samples

HZSM-5 (P)a

HT-HZSM-5a

La-HT-HZSM-5a

La-HT-HZSM-5b

La-C-HT-HZSM-5b

TOS (h) Carbon deposit amount (wt%)

66 4.8

67 3.8

69 4.3

240 7.9

390 7.8

a b

673 K, 1.4 MPa, coking benzene/ethanol = 6:1 (molar ratio), sulfur content = 120 ppmw, WHSV = 10 h1. 673 K, 1.4 MPa, coking benzene/ethanol = 6:1 (molar ratio), sulfur content = 170 ppmw, WHSV = 3 h1.

3.3.2. Effect of the La2O3 modification Fig. 6 also shows that the deactivation rate over La-HT-HZSM-5 is 0.06%/h, almost half of that over HT-HZSM-5. The newly created micropores and this further decrease in the B/L ratio may be the reasons for improvement of catalytic stability. Compared with the FT-IR spectrum of HT-HZSM-5 sample, the increase in the intensities of both Bro¨nsted acidity and Lewis acidity bands for La-HT-HZSM-5 sample is observed. On the other hand, I3612 cm1 =I3650 cm1 further decreases after the La2O3 modification. This suggests that the La2O3 modification increases Lewis acidity preferentially. Py-IR spectra also show the decrease in the B/L ratio after the La2O3 modification, which is consistent with the FT-IR results. On the other hand, the La2O3 modification also changes the pore distribution of the catalyst. Two types of the newly created micropores (7.2 A˚ and 9.5 A˚) are also observed in La-HT-HZSM-5 sample. The decrease in the B/L ratio is responsible for the improvement of catalytic stability as mentioned above. In the following discussion, we focus on the role of the newly created large micropores in the improvement in catalytic stability. The coking benzene conversion over La-HT-HZSM-5 sample at 69 h of TOS is much higher than those on both HZSM-5 (P) sample at 66 h of TOS and HT-HZSM-5 sample at 67 h of TOS, as shown in Fig. 6. This result indicates that there are severe carbon deposits on both spent HZSM-5 (P) and spent HT-HZSM-5 samples during the reaction, which blocks the catalyst channel. However, the hydrothermal treatment followed by the La2O3 modification is likely to suppress this blockage. The channel blockage by carbon deposit on both HZSM-5 (P) and HT-HZSM-5 samples restricts the access of reactants/intermediates to the internal active sites. Due to the weakness of channel blockage of spent La-HT-HZSM-5 samples, most active sites are still accessible to the reactants and intermediates. Consequently, the coking benzene conversion over spent La-HT-HZSM-5 sample is much higher than those over spent HZSM-5 (P) and HT-HZSM-5 samples. In order to clarify this suppression of channel blockage of La-HT-HZSM-5 sample, we employed cyclohexane isotherm adsorption experiments with a flow gravimetric method. The results are listed in Table 3. On fresh HZSM-5 (P) sample, the adsorption capacity of cyclohexane is 714.0 mmol/g cat. After the reaction, it decreases down to 538.3 mmol/g cat, about 24.6% decrease due to the carbon deposit. As for HT-HZSM-5 sample, the adsorption capacities of fresh and spent samples are 573.8 mmol/g cat and 440.6 mmol/g cat, respectively, indicating a decrease of 23.2% after the reaction. However, the cyclohexane adsorption capacities of fresh and spent La-HT-HZSM-5 samples are 607.2 mmol/g cat and 499.4 mmol/g

cat, respectively, and the adsorption capacity decreases only 17.8%. Usually, the more carbon deposit on the spent zeolites leads to the larger decrease in its cyclohexane adsorption capacity. The carbon deposit on spent HT-ZSM-5 is 3.8 wt%, while it is 4.3 wt% over spent La-HT-HZSM-5. It seems that the extent of decrease in cyclohexane adsorption capacity over spent La-HT-HZSM-5 sample should be larger than that over spent HT-HZSM-5 sample. But the measurement of cyclohexane adsorption capacity shows an opposite trend. This apparent contradiction can be explained by assuming that the newly created large micropores accommodate the partial carbon deposit and therefore the channel blockage is relieved. In addition, DTG profiles are employed to further investigate the location of the carbon deposit on the spent samples, as displayed in Fig. 9. Two peaks centering at 831 K and 699 K appear in the DTG profile of spent HZSM-5 (P) sample. These represent the hardcombustion carbon and easy-combustion carbon, respectively. Both the peak position and its area change with the modification going on. Compared with the DTG profile of spent HZSM-5 (P), the peak at higher temperature transfers to lower temperature (831 K versus 820 K) over spent La-HT-HZSM-5. Besides, the 820 K peak area decreases and the 699 K peak area increases. These results indicate that the combustion of carbon deposit on spent La-HTHZSM-5 is easier than that on spent HZSM-5 (P). It is reasonable to conclude that a part of the carbon deposits on the newly created large micropores. Therefore, channel blockage by carbon deposit is somewhat improved by the hydrothermal treatment followed by the La2O3 modification. 3.3.3. Effect of the calcination Fig. 7 shows that La-C-HT-HZSM-5 sample exhibits higher stability than La-HT-HZSM-5 sample in the ethylation of coking benzene with ethanol. The deactivation rate of La-HT-HZSM-5 sample is 0.03%/h, about three times larger than that of La-C-HTHZSM-5 sample. This increase is due to the further decrease in the acidity and the B/L ratio after HZSM-5 (P) has been treated by the

Table 3 Adsorption capacity for cylcohexane on the fresh and spent samples. Samples

HZSM-5 (P)

HT-HZSM-5

La-HT-HZSM-5

Fresh (mmol/g cat) spent (mmol/g cat) D (%)

714.0 538.3 24.6

573.8 440.6 23.2

607.2 499.4 17.8

D represents the decrease in the adsorption capacity 673 K, 1.4 MPa, coking benzene/ethanol = 6:1 (molar ratio), sulfur content = 120 ppmw, WHSV = 10 h1.

Fig. 9. DTG profiles of the spent samples.

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hydrothermal treatment, the calcination and the La2O3 modification in series in comparison with HZSM-5 (P) treated with the hydrothermal treatment followed by the La2O3 modification. Both NH3-TPD results and Py-IR spectra show that the calcination reduces the acidity of La-HT-HZSM-5 sample. The B/L ratio decreases about 43% after calcination. The B/L ratio is 1.4 over La-HT-HZSM-5 sample, while it is 0.8 for La-C-HT-HZSM-5 sample. Usually, the tetra-coordinated Al in the HZSM-5 zeolite framework is the origin of its acidity. The hydroxyl group connected with this framework Al acts as Bro¨nsted acid site, while the OH group linked with nonframework Al acts as Lewis acid site. The transformation of framework Al to nonframework Al leads to the decrease in B/L ratio. The reason for the decrease in acidity and B/L is that the calcination results in dealumination of the nanoscale HZSM-5 zeolite lattice and the transformation of framework Al to nonframework Al, respectively. The lower acidity and the lower B/L ratio enable La-C-HT-HZSM-5 sample to possess the excellent property of anti-carbon deposit. The carbon deposit on spent LaHT-HZSM-5 sample is 7.9 wt% at 240 h of TOS, while it reaches the same level over spent La-C-HT-HZSM-5 sample only at 386 h of TOS. A similar result was also found by Campbell et al. [12]. In brief, the decrease in both the acidity and the B/L ratio together with the newly created large micropores lead to the improvement of catalytic stability during the ethylation of coking benzene. The channel blockage by carbon deposits and the carbon deposit amount are the key factors for deactivation of the nanoscale HZSM-5 zeolites in this reaction. The channel blockage prevents the access of the reactants/intermediates to the inner active sites uncovered by the carbon deposits. Thus the conversion of coking benzene decreases rapidly as the reaction goes on. However, the newly created large micropores in La-HT-HZSM-5 sample accommodate a part of carbon deposit, consequently suppressing the formation of carbon deposit in its inherent micropores to some extent. Meanwhile, the newly formed large micropores reduce the transport constraints of the reactants/the intermediates/the products, thus increasing the TOF (turnover frequency) of each active site. In addition, the decrease in the acidity and the B/L ratio suppresses the carbon deposit formation, therefore relieving the coverage of active sites and channel blockage. 3.3.4. Effect of feed composition When one uses ethylene as ethylating agent, La-C-HT-HZSM-5 catalyst exhibits higher EB selectivity and much longer catalyst lifetime compared with the values obtained using ethanol as ethylating agent. In spite of the same benzene/ethylating agent ratio in the feed, the total EB selectivity over La-C-HT-HZSM-5 catalyst reaches about 99% in the case of using ethylene as ethylating agent, while it is about 96% in the case of using ethanol as ethylating agent. Due to the hydrophilicity of HZSM-5 zeolites from aluminum in both zeolite framework and nonframework groups and external surface silanol groups [26], it adsorps ethanol preferentially when benzene coexists with ethanol. Under the condition of using ethanol as ethylating agent, the actual benzene/ ethanol ratio around the active sites on surface of La-C-HT-HZSM-5 catalyst becomes smaller than that in the feed, consequently leading to the decrease in the total EB selectivity. But under the condition of using ethylene as ethylating agent, the case is contrary to the above-mentioned. The difference may be ascribed to the preferential adsorption of benzene on HZSM-5 zeolites when benzene coexists with ethylene. This behavior is due to the adsorption energy of benzene being much lower than that of ethylene, as reported in the literature [27]. On the other hand, LaC-HT-HZSM-5 catalyst shows much higher stability in the reaction in the case of using ethylene as ethylating agent than that in the case of using ethanol as ethylating agent. In the case of ethylene,

the coking benzene conversion is 14.5% and remains almost unchanged during 1500 h of TOS. However, in the case of ethanol, the coking benzene conversion at the initial stage is 15.0% and decreases down to 9.6% within only 386 h of TOS. Usually, ethylene poisons the active sites on the surface of HZSM-5 zeolites by the formation of carbon deposit precursor such as oligomer and polysubstituted alkylaromatics. In the case of ethanol, the hydrophilicity of HZSM-5 zeolites leads to the enrichment of ethanol around the active sites on the surface of La-C-HT-HZSM-5 catalyst. Then ethanol is easily converted to ethylene under this reaction condition. It means that the enrichment of poison occurs around the active sites on the surface of La-C-HT-HZSM-5 catalyst. Therefore, its catalyst lifetime is shortened. But when ethylene is used as ethylating agent, the preferential adsorption of benzene over La-C-HT-HZSM-5 catalyst leads to the dilution of ethylene. Thus its catalyst lifetime is prolonged. 4. Conclusions Combination of the hydrothermal treatment, the calcination and the La2O3 modification in series is an effective method to adjust the acidity and the pore structure of the nanoscale HZSM-5 zeolites. Modification by these post-treatments reduces both the total number of acid sites and the B/L ratio, and changes pore structure from the single micropores to the coexistence of micropores and large micropores over the nanoscale HZSM-5 zeolites. A new catalyst, La-C-HT-HZSM-5, was prepared and exhibited higher stability in the ethylation of coking benzene under industrial reaction conditions with the coking benzene conversion of 14.5% and the total EB selectivity of 98.8%. The catalytic activity and selectivity remained almost unchanged in a test with 1500 h of TOS. The improvement in the stability can be attributed to both a low rate of carbon deposit and the slowing of channel blockage, which would be caused by carbon deposits in the catalyst channel. When ethylene is used as ethylating agent, both the total EB selectivity and the catalyst lifetime of La-C-HT-HZSM-5 catalyst are better than those in the case of using ethanol as ethylating agent. Acknowledgements The project was supported by the program for New Century Excellent Talent in University (NECT-04-0268) and by the 111 project. The authors thank Dr. Guang Xiong and Dr. Xiaoliang Ma for their help in revising the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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