Characterization and Catalytic Performance of Deactivated and

Article pubs.acs.org/IECR

Characterization and Catalytic Performance of Deactivated and Regenerated TS‑1 Extrudates in a Pilot Plant of Propene Epoxidation Yi Zuo, Mengli Wang, Wancang Song, Xiangsheng Wang, and Xinwen Guo* State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, P. R. China S Supporting Information *

ABSTRACT: TS-1 extrudates that were used for propene epoxidation in a pilot plant for about 1700 h deactivated partly. To study the reason for the deactivation, the deactivated and regenerated catalysts were investigated with X-ray diffraction analysis, Fourier-transform infrared, Ultraviolet−visible diffuse reflectance, thermogravimetry−differential thermogravimetry, N2 sorption, n-hexane adsorption, cyclohexane adsorption, and elemental analysis. The catalytic performance of the deactivated and regenerated TS-1 extrudates in the epoxidation of propene was evaluated in a fixed-bed reactor and in a batch reactor. The activity of the deactivated catalyst from the inlet of the pilot-plant reactor was higher than that from the outlet of the reactor in which more propene oxide oligomers were generated than in the pores of catalyst taken from the inlet of the reactor. External and in situ regeneration could reinstate the activity of the deactivated catalysts, while the in situ regeneration was preferred.

1. INTRODUCTION Propene oxide (PO) is an important industrial chemical feedstock, especially for the manufacture of polyether polyols and propene glycols. At present, PO is commercially produced by the chlorohydrin route or the coproduction (Halcon) route.1 The chlorohydrin process, which is the oldest process, uses chlorine and propene to produce propene chlorohydrins as intermediates. It suffers from environmental pollution, while the Halcon route uses ethyl benzene or tert-butyl hydroperoxide to oxidize propene. The Halcon process coproduces styrene or tert-butyl alcohol, respectively, so its economics is linked to that of the coproducts.2 Therefore, the development of a new process, freeing propene oxide from its coproducts, is of great interest. The invention of titanium silicate (TS-1) by Taramasso et al. in 19833 opened an alternative route for selective oxidation, such as alkene epoxidation,4,5 alkane oxidation, 6,7 aromatic hydroxylation, 8 and oxidation of ammonia to hydroxylamine.9 High conversion of hydrogen peroxide and high selectivity of PO can be obtained in the epoxidation of propene over TS-1 catalyst under relatively mild conditions. Furthermore, it is an environmental friendly route, with only chlorine-free water as the byproduct. Therefore, the epoxidation of propene catalyzed by TS-1 has attracted much attention.10−12 Nevertheless, there are still two obstacles for the use of TS-1 in industry. One obstacle for industrial application of TS-1 was its high cost. To reduce the cost, tetrapropylammonium bromide (TPABr) instead of tetrapropylammonium hydroxide (TPAOH) was adopted as the template to synthesize TS-1,13−15 and seeds were introduced to shorten the synthesis period.16 The other obstacle was the shaping method. There are mainly two methods for shaping the powdered TS-1. One is spraying or in situ synthesizing TS-1 on inert supports, of which the active component loading is low, so that the output of PO is very low. The other shaping method is extruding the powdered TS-1 with supports, producing TS-1 extrudates. The disadvantage of © XXXX American Chemical Society

TS-1 extrudates is that the diffusion path is too long for substrates and reaction heat, so that the PO may react with solvent and H2O2 may decompose, lowering the PO selectivity and utilization of H2O2. BASF and Dow Chemical established a hydrogen peroxide to propene oxide (HPPO) process in Antwerp, Belgium,17 which has a PO capacity of 300 000 tons per year. Evonik and Uhde jointly developed a HPPO process, which has a PO capacity of 100 000 tons per year and uses 0.77 kg of propene and 0.75 kg of hydrogen peroxide (100%) for 1 kg of PO. Although TS-1 exhibits a high activity for selective oxidation, the deactivation cannot be ignored. Even though the process of Evonik-Uhde is commercial, the reaction temperature has to be increased with the time on stream to maintain the catalytic activity. Thiele and Roland18 found that deactivation occurred by blocking of the zeolite micropores with PO oligomers, which could be eliminated by refluxing with dilute hydrogen peroxide. Yan et al.19 studied the deactivation by a simulation method. The results showed that deactivation occurred by blocking the pores with di- and trioligomers of ether with PO. Our previous work showed that TS-1 extrudates exhibit high catalytic activity, high selectivity of PO, and a long lifetime in the epoxidation of propene.19,20 Therefore, a pilot plant was built to determine the catalytic performance and stability of TS1 extrudates. The experiment ran for about 1700 h and the average conversion of H2O2 and selectivity of PO were both above 95%, while the utilization of H2O2 was about 85%. However, the activity of the catalyst declined with the reaction time, so that the reaction temperature had to be increased to maintain the activity. Therefore, it is important to study the reason for the deactivation of the catalyst, so that the catalyst Received: March 3, 2012 Revised: July 20, 2012 Accepted: July 26, 2012

A

dx.doi.org/10.1021/ie300581z | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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μm). PO was the main product and propene glycol (PG) and its monomethyl ethers (MME) were byproducts. The conversion of H2O2, selectivity of PO, and utilization of H2O2 were calculated as follows:

would have a longer lifetime for industrial application. In this work, we characterized the used TS-1 extrudates to find the reason for the deactivation of the catalyst. We also regenerated the deactivated catalyst by different methods to study if the catalyst could be recycled.

2. EXPERIMENTAL SECTION 2.1. Preparation of the Catalyst. TS-1 was synthesized as described in ref 21, using colloidal silica and TiCl4 as silicon and titanium sources, respectively. TPABr was used as the template, and aqueous ethyl ammonium as the base. The molar composition of the gel was n(SiO2)/n(TiO2)/n(TPABr)/ n(C2H5NH2)/n(H2O) = 1:0.02:0.15:1.0:16. The gel was crystallized at 170 °C for 48 h, then washed and calcined at 540 °C for 6 h. The as-synthesized TS-1 was modified by adding an aqueous TPAOH solution and TS-1 to a Teflon lined autoclave, and heating at 170 °C for 48 h.22 The TS-1 extrudates were prepared by mixing the TS-1 powder with colloidal silica (TS-1/SiO2 = 4/1, w/w), and extruding the mixture. The resulting product was dried at room temperature for 48 h and calcined at 540 °C for 6 h.23 2.2. Regeneration of the Catalyst. The used TS-1 extrudate catalysts were regenerated with an external and with an in situ regeneration method. The external regeneration was performed by calcining the used catalyst in air at 300 or 540 °C for 2 or 6 h. In the in situ regeneration, the used catalyst was first washed with methanol for 1 h (WHSV 1 h−1) and then washed with dilute 1.5 or 5 wt % H2O2 for 12 or 24 h at a WHSV of 5 h−1. 2.3. Characterization of the Catalyst. The appearance of the TS-1 extrudates was determined by scanning electron microscopy (SEM) on a Hitachi S-4800 instrument. X-ray diffraction (XRD) was performed on a Rigaku Corporation D/ MAX-2400 instrument using Cu Kα radiation. Fouriertransform infrared (FT-IR) spectra were recorded on a Bruker EQUINOX55 spectrometer from 4000 to 400 cm−1, and the KBr pellet technique was adopted. Ultraviolet−visible diffuse reflectance (UV−vis) spectra were obtained on a Jasco UV-550 spectrometer from 190 to 500 nm, and pure BaSO4 was used as a reference. Nitrogen sorption measurements were performed at liquid nitrogen temperature on a Quantachrome AUTOSORB-1 physical sorption apparatus. Total surface area and pore volume were calculated according to the BET method and HK model. Thermogravimetry (TG) and differential thermogravimetry (DTG) were performed on a Mettler-Toledo TGA/ SDT851e instrument with a nitrogen flow rate of 40 mL/min. The sample was heated from ambient temperature to 800 °C with a heating rate of 10 °C/min. N-Hexane sorption and cyclohexane sorption were carried out on a homemade physical sorption apparatus. A Bruker SRS3400 X-ray fluorescence spectrometer (XRF) and Perkin-Elmer OPTIMA 2000DV ICP optical emission spectrometer provided elemental analysis of TS-1. 2.4. Test of the Catalyst. 2.4.1. Reactions in a Fixed-Bed Reactor. The epoxidation of propene was carried out in a fixedbed reactor. The amount of catalyst used in the reaction was 7.0 g and typical reaction conditions were as follows: solvent, acetone/methanol 3/1(vol); temperature, 45 °C; concentration of H2O2, 1.10 mol/L; ratio of H2O2/C3H6, 1/3(mol); pressure, 3.2 MPa; WHSV of propene, 0.5 h−1. The residual H2O2 was determined by iodometric titration. The products were analyzed on a Tianmei 7890F gas chromatograph with a FID and a PEG-20 M capillary column (30 m × 0.25 mm × 0.4

X(H 2O2 ) = (n0(H 2O2 ) − n(H 2O2 ))/n0(H 2O2 )

(1)

S(PO) = n(PO)/(n(PO) + n(MME) + n(PG))

(2)

U (H 2O2 ) = (n(PO) + n(MME) + n(PG)) /(n0(H 2O2 ) × X(H 2O2 ))

(3)

The n0(H2O2) and n(H2O2) stand for the initial and final molar content of H2O2, respectively. The n(PO), n(MME), and n(PG) represent the molar content of PO, MME, and PG, respectively. 2.4.2. Reactions in a Batch Reactor. The propene epoxidation was also carried out in a stainless steel batch reactor. At first, 0.4 g of the powdered catalyst, 24 mL of acetone, 8 mL of methanol, and 30 wt % of hydrogen peroxide were added to the reactor. The concentration of hydrogen peroxide was 0.6 mol/L. Propene was then charged to the reactor to reach 0.4 MPa. The reaction was finished after the mixture was heated at 60 °C for 1 h. The residual H2O2 and the products were analyzed in the same way as the products of the reaction in the fixed-bed reactor.

3. RESULTS AND DISCUSSION 3.1. Deactivation of TS-1 Extrudates. 3.1.1. Characterization of Deactivated TS-1 Extrudates. The TS-1 extrudates that we analyzed had been used for 1700 h in a pilot plant, which had a catalyst loading of 100 kg and an output of 100 tons per year. SEM images of the fresh TS-1 extrudates show that the size of the crystals is about 2 μm and that the crystals are coated by amorphous silica, which is introduced in the extrusion step (Figure 1). The deactivated catalyst that was taken from the pilot plant was divided into four parts from the inlet (the bottom of the reactor) to the outlet (the top of the reactor) of the pilot plant reactor, which were denoted as sample 1 to sample 4, respectively. These parts were separately tested in a fixed-bed reactor and in a batch reactor. In addition, we tested fresh TS-1 extrudates (denoted as sample 5). The explanation of the samples was shown in Table 1. The XRD patterns showed that all samples had MFI topology, with characteristic peaks at 7.8, 8.8, 23.0, 23.9, and 24.4° (Figure 2); in other words, the framework of the zeolite was not destroyed after 1700 h reaction. Liu et al.24 found that the relative crystallinity decreased to 65% of the fresh catalyst after 1000 h reaction on the lamina TS-1 (the loading of lamina TS-1 is 5 kg), which was ascribed to the covering of pores by reactants or products. However, the relative crystallinity, which was calculated by comparing the total intensity of the characteristic peaks mentioned above with those of the standard TS-1, increased sharply after long reaction time. This may be due to the loss of amorphous SiO2 (introduced in the extrusion with silica sol), which was washed out by hydrogen peroxide during the long run, as proved by the elemental analysis (Table 2). The amorphous SiO2 did not appear in the lamina TS-1, thus the crystallinity will not be affected by it. The FT-IR spectra of the samples are shown in Figure 3a. The bands at 550 and 1230 cm−1 are characteristic for the MFI B



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Figure 2. The XRD patterns of the TS-1 extrudates: (1−4) the deactivated TS-1 extrudates from the bottom to the top of the pilot plant fixed-bed reactor; (5) the fresh TS-1 extrudates.

ethers often show bands below 185 nm. Acetaldehyde, which was generated in the pilot plant experiment, shows a band at 290 nm, but the small amount of acetaldehyde may be covered by the band of nonframework Ti and anatase TiO2. Figure 4 shows the TG and the DTG curves of the deactivated and in situ regenerated TS-1. All deactivated samples show two major DTG peaks, indicating that the main weight loss is at 117 and 278 °C. The peak at around 117 °C is due to propene glycol monomethylether (MME), which is the main byproduct generated by etherification of PO with methanol. The other peak is attributed to an oligomer of PO with ethers or with itself, for example, dipropene glycol monomethylether.18 It is obvious that the intensity of the peak centered at 278 °C increased from sample 1 to sample 4, indicating that the amount of PO oligomers increased from the inlet to the outlet of the pilot-plant reactor. The same trend can be seen from the amount of weight loss at 200−500 °C (cf. Table 3). On the other hand, the weight loss at lower temperature (25−200 °C) decreased from 5.25% to 3.45%, corresponding to the reactor from the inlet to the outlet. In other words, PO and MME generated in the beginning at the bottom of the pilot-plant reactor, flowed upward along with the solvent methanol and water, and reacted with them or with themselves to oligomers, such as di- or trioligomers, in the upper part of the reactor. This would deactivate the TS-1 extrudates by blocking the pores and covering the active centers. The increasing concentrations of PO and MME along

Figure 1. The SEM images of the fresh TS-1 extrudates.

topology,25 while the band at 960 cm−1 is due to the stretching vibration of [SiO4] units strongly influenced by titanium ions in neighboring coordination positions,26,27 which is a proof of the introduction of Ti into the framework. The intensity of this band was weaker for samples 1−4 than for sample 5, because of the partially washed away framework of Ti (cf. elemental analysis in Table 2). UV−vis spectroscopy is a sensitive detection method for the coordination state of Ti. There are three major bands in the UV−vis spectra (Figure 3b). The characteristic band at around 210 nm is assigned to tetrahedrally coordinated Ti, which is also called framework Ti.28,29 The band at ∼270 nm was assigned to a charge-transfer process in isolated [TiO4] or [HOTiO3] units, and is called nonframework Ti, while the band at ∼310 nm is due to anatase TiO2. There is no clear difference between the used and fresh TS-1, although we expect that some deposition would show a band in the UV−vis spectrum. It may be due to the fact that alkenes, alcohols, and Table 1. The Explanations of the Denotation of Samples samples 1−4 5 6−9 10−13 14−17 18−21 22 23 24

explanations deactivated TS-1 extrudates from the inlet to the outlet of the pilot-plant fresh TS-1 extrudate externally regenerated TS-1 extrudates, corresponding to samples 1−4 by externally regenerated TS-1 extrudates, corresponding to samples 1−4 by externally regenerated TS-1 extrudates, corresponding to samples 1−4 by externally regenerated TS-1 extrudates, corresponding to samples 1−4 by in-situ regenerated sample 4 by 5 wt % H2O2 at 80 °C for 12 h in-situ regenerated sample 4 by 1.5 wt % H2O2 at 80 °C for 24 h fresh TS-1 without extrusion C

reactor calcination calcination calcination calcination

at at at at

540 300 300 540

°C °C °C °C

for for for for

6 6 2 2

h h h h



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Table 2. Elemental Analysis of the Externally Regenerated and Fresh TS-1

a

cat.

SiO2 (wt %)

TiO2 (wt %)

Al2O3 (wt %)

Na2O (wt %)

Ca (wt %)

Fe2O3 (wt %)

P2O5 (wt %)

n(Si/Ti)

5 6 7 8 9

97.00 90.89 90.84 92.67 93.78

2.170 2.143 2.131 2.134 2.145

0.251 0.299 0.105 0.113 0.108

0.119 0.085 0.019 0.021 0.020

0.97 1.04 1.09 1.12

0.149 0.123 0.101 0.077

0.62 0.57 0.51 0.34

59.60 56.55 56.84 57.90 58.29

C stands for the content of carbon element in the samples.

Figure 3. The FT-IR (a) and UV−vis (b) spectra of the TS-1 extrudates. For sample notation, see caption to Figure 2.

Figure 4. The TG (a) and DTG (b) curves of the deactivated and in situ regenerated TS-1 extrudates. (1−4) the deactivated TS-1 extrudates from the bottom to the top of the pilot plant fixed-bed reactor; (22) the in situ regenerated TS-1 extrudates of sample 4 by 5 wt % H2O2 at 80 °C for 12 h; (23) the in situ regenerated TS-1 extrudates of sample 4 by 1.5 wt % H2O2 at 80 °C for 24 h.

Table 4), with fresh TS-1 (sample 5). The N2 sorption isotherms of both samples have a hysteresis loop, indicating the existence of mesopores, which were generated during the modification by TPAOH.22 The pore diameter distributions of the deactivated TS-1 (sample 4) and the fresh TS-1 (sample 5) are clearly different. The pore volume at around 0.5 and 1.5 nm diameter decreased after 1700 h reaction by the blocking of pores by ethers and oligomers. The total and micropores surface area and pore volume of the samples exhibit the same trend as the pore distribution curves. To further study the relative deactivation of the four deactivated catalysts taken from different parts of the pilotplant reactor, we examined the amount of n-hexane and cyclohexane adsorption for samples 1−9 (cf. Figures 7 and 8, respectively). N-hexane can access the pores of TS-1, while cyclohexane can only adsorb on the external surface and orifices. Therefore, the two adsorbates characterize different parts of the catalysts. The saturation adsorption amount of n-

Table 3. The Weight Loss of the Deactivated and the in Situ Regenerated TS-1 Extrudates weight loss (%) cat.

25−200 °C

200−500 °C

500−800 °C

total

1 2 3 4 22 23

5.25 4.69 4.53 3.45 0.74 0.26

2.94 3.74 4.18 4.77 0.47 0.56

0.37 0.39 0.32 0.34 0.11 0.26

8.56 8.82 9.03 8.56 1.32 1.08

the axial direction make the side reactions easier, thus the catalysts deactivated more seriously near the outlet of the pilotplant reactor. The nitrogen sorption isotherms and pore diameter distribution are shown in Figures 5 and 6, respectively. We first compare sample 4, which deactivated most seriously (see D



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Figure 6. The pore diameter distribution of the catalysts. For sample notation, see caption to Figure 5.

Figure 5. The nitrogen sorption isotherms of the catalysts: (4) the deactivated TS-1 extrudates; (5) the fresh TS-1 extrudates; (9) the externally regenerated TS-1 extrudates of sample 4; (22) the in situ regenerated TS-1 extrudates of sample 4 by 5 wt % H2O2 at 80 °C for 12 h; (23) the in situ regenerated TS-1 extrudates of sample 4 by 1.5 wt % H2O2 at 80 °C for 24 h.

and then deposited in the channels or on the external surface and these block the pores of TS-1. This is the reason that the catalyst near the outlet of the pilot-plant reactor deactivated more strongly. The selectivity of PO increased slightly after 1700 h reaction, which is due to the covering of the acid sites which may catalyze the side reactions, by oligomers. The utilization of H2O2 increased somewhat after long time reaction, because H2O2 can decompose in the channels when the exothermic heat of reaction cannot be released in time. The blocking of pores restrained the spread of H2O2 into the channels, so that the decomposition of H2O2 decreased. The TS-1 extrudates were powdered to about 60 mesh for the batch reaction, but the silica used for extrusion was not removed. This might block the pores of TS-1 and influence the catalytic activity. The catalytic performance of fresh TS-1 without extrusion (sample 24) was excellent. However, the conversion of H2O2 over fresh TS-1 extrudate was very low and similar to that of sample 1 (Table 4). The same trend was obtained for the epoxidation of propene on the deactivated TS1 extrudates in the batch and fixed-bed reactor. From sample 1 to sample 4, the catalytic activity decreased gradually, which is due to serious blocking of the pores, as follows from the adsorption characterization. Therefore, the reaction distribution in the reactor should be controlled strictly by adjusting the temperature or other impacts, otherwise the catalyst near the outlet of the bed will deactivate more easily. The selectivity of PO and the utilization of H2O2 obtained on the fresh TS-1 were almost the same as those on the deactivated TS-1.

hexane of the deactivated TS-1 extrudates decreased from sample 1 to sample 4 (from 760 to 610 μmol/g), and all values were much less than that of the fresh TS-1 (sample 5, 1160 μmol/g). The same trend was obtained for the adsorption of cyclohexane. This indicates that both the external surface (characterized by cyclohexane) and the pore channels (characterized by n-hexane) are partly covered by reactants or products (mainly MME and oligomers, as proved by the DTG analysis), and that the extent of the covering of surface and the blocking of pores by oligomers is more serious near the outlet of the pilot-plant reactor. 3.1.2. Catalytic Performance of the Deactivated TS-1 Extrudates. The catalytic performance of the deactivated TS-1 extrudates in the propene epoxidation in a fixed-bed reactor and a batch reactor are shown in Table 4. The results obtained in the fixed-bed reactor are the average data for 100 h reaction after the catalytic activity became stable. The conversion of H2O2 over the used TS-1 decreased to varying extents. From the inlet of the pilot-plant reactor to the outlet of the reactor, the conversion of H2O2 decreased sharply, and was always much lower than that of the fresh catalyst. The concentration of PO in the pilot-plant reactor increased from the inlet to the outlet, so that side reactions, such as etherification, might occur more easily at the outlet. Thus, more byproducts are generated E



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Table 4. Catalytic Performances of the Deactivated, Regenerated, and Fresh TS-1 fixed-bed reactionsa cat. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

batch reactionsb

X(H2O2) (%)

S(PO) (%)

U(H2O2) (%)

X(H2O2) (%)

S(PO) (%)

U(H2O2) (%)

82.6 67.2 56.9 52.5 95.3 98.0 96.3 95.8 94.2 88.1 87.4 84.2 83.1 87.1 86.3 83.7 80.9 91.0 89.5 84.4 81.8 97.7 98.3

98.1 99.0 99.2 99.3 96.2 95.3 94.4 94.4 95.3 95.7 95.7 95.7 95.9 96.0 96.2 95.8 95.6 96.1 96.3 96.2 96.4 96.8 96.5

87.4 87.8 85.4 86.8 83.2 83.5 82.9 83.1 83.3 86.5 86.2 80.5 78.3 81.8 82.3 80.8 79.0 87.4 87.4 87.3 87.5 87.6 91.0

52.8 36.7 26.0 17.0 52.2 98.9 98.9 98.8 97.1 97.4 96.4 96.2 94.6 96.7 96.7 96.2 95.8 98.7 98.7 98.5 98.5 92.6 95.6 95.2

99.2 98.7 98.3 98.3 98.7 95.0 95.2 94.8 94.0 91.1 89.5 91.4 94.4 89.2 89.5 89.6 89.6 93.8 92.1 92.7 93.9 94.9 94.6 96.0

68.5 68.2 68.2 67.9 68.9 86.4 85.2 83.4 80.2 77.8 78.8 80.7 80.4 79.4 80.7 81.4 82.6 83.4 82.8 81.6 82.3 86.5 88.5 84.6

Figure 8. The cyclohexane adsorption curves of the catalysts. For sample notation, see caption to Figure 7.

Table 2 shows the elemental composition of samples 5−9. The Si/Ti ratio decreased appreciably when the amount of silicon decreased. Calcination cannot remove silicon or titanium, so the change must have occurred during the pilot plant run. Combining the results of the crystallinity measurement and of the elemental analysis, we believe that the silicon was not lost from the framework, which was difficult to remove, but from the amorphous silica introduced by extrusion. The silica can block the pores, and removing it will improve the activity of the catalyst. The activity of the catalysts decreased with increasing Si/Ti mole ratio (section 3.2.2), because less hydrogen peroxide was left in the substrate and thus less silica was removed from the catalyst in the upper part of the pilotplant reactor. Petrini et al.30 found that Ti was removed in the reaction, which was an irreversible deactivation. A small amount of Ti was lost in our experiment, but the activities obtained with the externally regenerated catalysts were not affected by the loss of titanium. The TG and DTG curves of samples 10 to 21 were shown in Figure 9 columns a and b, respectively. The weight loss of all the samples was less than 3%, and the primary weight loss occurred below the temperature of 100 °C, which was assigned to the physically adsorbed water. The TG curve showed a weak weight loss at about 400 °C for the samples calcined at 300 °C, but a nearly straight line for that at 540 °C, indicating that the external regeneration was not complete at 300 °C, and the temperature of 540 °C was preferred. The adsorption curves of n-hexane and cyclohexane of the samples 5−9 are shown in Figures 7 and 8, respectively. The external surface and orifices occupied by amorphous silica were washed out by hydrogen peroxide after a long time reaction, according to the former characterization analysis. Therefore, more cyclohexane can adsorb on the surface and orifices after regeneration than on those of the fresh TS-1, which were covered by the amorphous silica before the pilot-plant reaction. The amount of n-hexane adsorption on the regenerated TS-1 was still less than on the fresh sample and may be because the channels were not cleared thoroughly by the regeneration or because small parts of the framework may have sintered due to the high temperature of calcination, so that the pore volume decreased slightly. 3.2.2. Catalytic Performance of the Externally Regenerated TS-1 Extrudates. The catalytic performances of the externally regenerated TS-1 in the fixed-bed reactor and batch

Conditions: 45 °C; 3.2 MPa; solvent, V(acetone)/V(methanol) = 3; H2O2, 1.10 mol/L; n(propene)/n(H2O2) = 3; WHSV of propene, 0.6 h−1. bConditions: catalyst, 0.4 g; 60 °C; 1 h; solvent, V(acetone)/ V(methanol) = 3; H2O2, 0.6 mol/L; pressure of propene, 0.4 MPa. a

Figure 7. The n-hexane adsorption curves of the catalysts: (1−4) the deactivated TS-1 extrudates from the bottom to the top of the pilot plant fixed-bed reactor; (5) the fresh TS-1 extrudates; (6−9) the externally regenerated TS-1 extrudates from the bottom to the top of the pilot plant fixed-bed reactor.

3.2. External Regeneration of TS-1 Extrudates. 3.2.1. Characterization of the Regenerated TS-1 Extrudates. The four parts of the deactivated TS-1 extrudates taken from the pilot-plant reactor were regenerated by calcination for 2 or 6 h in air at 300 or 540 °C. The sign of the regenerated catalysts in different conditions of external regeneration were shown in Table 1. F



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Figure 9. The TG (a) and DTG (b) curves of the externally regenerated TS-1 extrudates: (10−13) 300 °C, 6 h; (14−17) 300 °C, 2 h; (18−21) 540 °C, 2 h.

reactor are shown in Table 4. The results obtained in the fixedbed reactor are the average data for 100 h of reaction after the catalytic activity had become stable. The conversion of H2O2 over samples 8 and 9 recovered in the fixed-bed reactor. The conversion over samples 6 and 7 was even higher than that of fresh TS-1, because the washing out of amorphous silica by H2O2 and methanol in the pilot plant unblocks the channels, so that active centers can more easily be achieved by the reactants. Moreover, the conversion was not affected by the leaching of a small amount of titanium. However, the loss of titanium and sodium ions (see Table 2) would expose some acid sites, such as Si−OH, which could catalyze the side reaction of PO with solvent or others, so that the selectivity of PO cannot recover to the fresh level, although it was still above 94%. The utilization of H2O2 was almost the same on the externally regenerated TS1 as on the fresh TS-1. The activity of samples 10−21 did not recover completely, due to the incomplete clearance of channels, according to the TG analysis. Therefore, the condition of calcining at 540 °C for 6 h was the best one for the TS-1 regeneration. A drastic increase in the conversion of H2O2 was obtained for the externally regenerated TS-1 in the batch reactor (Table 4). In the epoxidation of propene in the batch reactor, reactants react with each other both on the external surface and in the channels, while in the fixed-bed reactor more reactions happen on the external surface and the channels close to the surface. Therefore, the clearance of the pores of the catalyst by washing

away of silica has little effect in the fixed-bed reactor, but gives a significant improvement in the batch reactor. 3.3. In-Situ Regeneration of TS-1 Extrudates. 3.3.1. Characterization of in Situ Regenerated TS-1 Extrudates. In an external regeneration one must remove the catalyst from the reactor, which is laborious and interrupts the industrial production for a considerable period of time. Therefore, the development of an alternative in situ regeneration may be helpful for the use of TS-1 in industry. At present, there are two main methods of in situ regeneration. The first method is extracting the organic compounds with different solvents, such as methanol and tetrahydrofuran.31,32 The second method is oxidizing the compounds by dilute H2O2.33,34 In this work, we combined the two methods to regenerate the deactivated TS-1 extrudates. As we have seen in section 3.1, sample 4, which is taken from the nearest part of the outlet of pilot-plant reactor, had deactivated most strongly. Therefore, we chose this sample to examine the conditions of in situ regeneration with washing with methanol for 1 h followed by washing with dilute H2O2. Two conditions were tested. One was washing with 5 wt % H2O2 at 80 °C for 12 h, giving sample 22, the other was washing with 1.5 wt % H2O2 at 80 °C for 24 h, giving sample 23. The TG curves of samples 22 and 23 are shown in Figure 4a. The comparison of the amount of weight loss at different regions of temperature is shown in Table 3. The total weight loss decreased sharply after regeneration, which is due to the G



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Notes

oxidation of the oligomers that blocked the pores by the diluted H2O2. Only a little weight loss at about 100 °C of the regenerated catalysts was assigned to physically adsorbed water. The nitrogen sorption isotherms and pore diameter distribution of samples 9, 22, and 23 are shown in Figures 5 and 6, respectively; those of samples 4 and 5 are shown for contrast. The pore diameter distribution of sample 9 is similar to that of the fresh sample 5, and the pore volume recovered after external regeneration by calcination at 540 °C for 6 h. Insitu regeneration by washing with dilute H2O2 can also regenerate the deactivated TS-1. The pores, with a diameter centered at about 0.5 nm, are even larger than those of fresh TS-1. The BET surface area and pore volume of sample 23 are the largest of all the three regenerated samples, so the catalytic activity is expected to be the highest of all. 3.3.2. Catalytic Performance of the in Situ Regenerated TS-1 Extrudates. The catalytic performance of the in situ regenerated TS-1 in the fixed-bed reactor and batch reactor are shown in Table 4. The results obtained in the fixed-bed reactor are the average data for 100 h reaction after the catalytic activity becomes stable. The conversion of H2O2 increased to a value higher than 97% after regeneration, which was higher than that of the fresh TS-1 and equaled that of sample 1 in the fixed-bed reactor. The selectivity of PO and utilization of H2O2 also recovered after regeneration. The conversion of H2O2 over sample 23 was higher than that over sample 22, which indicates that washing with dilute H2O2 for a longer time is more effective than using a higher concentration of H2O2. The selectivity of PO decreased, because of the removal of amorphous silica of extrusion and exposure of the acid sites. The utilization of H2O2 increased after regeneration, because the removed amorphous silica had lead to the decomposition of H2O2.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the program for New Century Excellent Talent in University (Grant NCET-04-0268) and the Plan 111 Project of the Ministry of Education of China.

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4. CONCLUSIONS The TS-1 extrudate catalyst deactivated partly after 1700 h pilot-plant reaction. The activity decreased from the inlet to the outlet of the pilot-plant reactor. The main reason for the deactivation in the pilot plant is similar to that for the deactivation in the laboratory, which is the blocking of pores and covering of active centers by ethers or oligomers. The more oligomers that are generated, the more seriously the catalyst deactivates. The loss of a small amount of framework titanium had little influence on the catalytic activity. The deactivated catalysts could be externally regenerated by calcination at 540 °C for 6 h and in situ regenerated by washing with dilute H2O2. The in situ regeneration, rather than the external regeneration, can be adopted in industry. When using in situ regeneration with dilute H2O2, a longer time is more effective than a higher concentration of H2O2.

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ASSOCIATED CONTENT

S Supporting Information *

Characterizations of the externally regenerated and in situ regenerated TS-1 extrudates; additional figures and tables. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Characterization and Catalytic Performance of Deactivated and

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