Preparation of a Catalyst for Selective Hydrogenation of Pyrolysis

Res. , 2010, 49 (21), pp 11112–11118. DOI: 10.1021/ie1003043. Publication Date (Web): April 26, 2010. Copyright © 2010 American Chemical Society. *...
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Ind. Eng. Chem. Res. 2010, 49, 11112–11118

Preparation of a Catalyst for Selective Hydrogenation of Pyrolysis Gasoline Zhiming Zhou,* Tianying Zeng, Zhenmin Cheng, and Weikang Yuan State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China

Novel palladium catalysts supported on alumina supports with the hierarchically macro-/mesoporous structure were prepared and applied to selective hydrogenation of pyrolysis gasoline. The textural, structural, and morphological properties of the catalyst supports were studied by SEM, XRD, and N2 adsorption-desorption techniques. The results showed that the hierarchically macro-/mesoporous structures of these materials were formed by a spontaneous self-assembly mechanism, and the presence of surfactant molecules, which influenced the mesopore size distribution, had almost no effects on the macroporous channels. Calcination treatments of the prepared materials revealed their high thermal stability. By comparison with a commercial catalyst without the hierarchically porous structure, these novel catalysts exhibited much better catalytic performance, that is, higher activity and selectivity, which was ascribed mainly to their unique structures of hierarchical mesopores and macropores. Finally, the stability of the novel catalysts was tested. Both experimental observations and TGA analysis of the used catalysts indicated good stability of these catalysts. 1. Introduction Pyrolysis gasoline (pygas) is a valuable byproduct of steam cracking of naphtha in an olefin unit. It contains a large quantity of aromatics, such as benzene, toluene, and xylene, which makes it a high potential feedstock for extraction of aromatics.1,2 Moreover, it can be used as a gasoline blending stock due to its generally high octane value.3 However, pygas is unstable and must be stabilized before the aforementioned utilization because it contains substantial amounts of gum precursors, that is, styrene and diolefins.3,4 Selective hydrogenation of pygas over heterogeneous catalysts has proved to be an effective process in the chemical industry, which aims at converting styrene and diolefins into ethylbenzene and monoolefins, respectively.5 The commercial catalysts for selective hydrogenation of pygas are usually Pd/Al2O3 with a pore size distribution between 4 and 20 nm,6 which exhibits notable internal diffusion limitations of species. Our recent work revealed that the influence of the internal diffusion resistances was considerable, even if an eggshell catalyst was used for pygas hydrogenation.7 Taking the reaction occurring at 323 K and 3.0 MPa as an example, the effectiveness factors for styrene, cyclopentadiene, 1-hexene, and hydrogen were 0.182, 0.135, 0.332, and 0.138, respectively.7 The conventional pore size distribution of the catalyst and the resultant internal diffusion limitations will lead to two deficiencies. First, some mesopores inside the catalyst will be blocked with gums formed by polymerization of styrene and diolefins after a period of application, and consequently, some active sites of the catalyst will be covered. Second, the intermediate products, monoolefins, will be further hydrogenated into saturated components, inevitably decreasing the hydrogenation selectivity of diolefins to monoolefins. The strategy to overcome these deficiencies is to reduce the internal diffusion to a great extent as well as to release monoolefins from the internal pores of the catalysts as soon as possible. Previous investigations have showed that a bidisperse pore structure can improve the catalytic performance for many reaction systems, such as Fischer-Tropsch synthesis,8 total * To whom correspondence should be addressed. Tel: +86-2164252230. Fax: +86-21-64253528. E-mail: [email protected].

oxidation of VOCs,9 CO oxidation,10 photocatalytic degradation of ethylene,11 autothermal reforming of methane,12 etc. A catalyst with macro-/mesoporous structure can provide not only large pores to reduce the pore diffusion resistance but also a large specific surface area to increase the metal dispersion. Recently, we prepared a Pd/TiO2 catalyst with a hierarchically macro-/mesoporous structure for styrene hydrogenation.13 This novel catalyst showed high catalytic activity, but the selectivity between monoolefins and diolefins, a very important factor for selective hydrogenation of pygas, was not studied. Considering that alumina rather than titania is more generally used as a catalyst support for selective hydrogenation,14 herein, a series of hierarchically macro-/mesoporous Pd/Al2O3 catalysts for selective hydrogenation of pygas are prepared for the first time. The objective of this work is to highlight the predominant effect of the hierarchical pore structure on pygas hydrogenation, including the effect of preparation conditions on the catalyst texture, comparison of the catalytic performances of the novel catalysts and the commercial catalyst, and the stability of the novel catalysts. 2. Experimental Section 2.1. Alumina Support Preparation. Alumina samples with a hierarchically macro-/mesoporous structure were prepared under various conditions (see Table 1). In a typical synthesis (sample a), 0.4 g of cetyltrimethylammonium bromide (CTAB) was added into a mixture of 35 mL of twice-distilled water and 15 mL of ethanol with slow stirring at room temperature. Then the pH of the solution was adjusted to 12.0 by NH3 · H2O. Finally, 2 g of aluminum tri-sec-butoxide (TBOA) was added. After 1 h, the precipitates formed were separated by centrifugation, washed by Soxhlet extraction for 30 h, and dried in air for 24 h. The as-prepared samples were finally calcined at 800 °C for 5 h. Different from sample a, samples b and c were prepared in the absence of the surfactant molecules, wherein the Soxhlet extraction treatment of the samples was not required. The only difference in preparation of samples b and c was whether ethanol was added. Corresponding to samples a, b, and c, the 800 °C calcined samples were labeled as a1, b1, and c1, respectively. The calcined samples were sieved, and those particles with a

10.1021/ie1003043  2010 American Chemical Society Published on Web 04/26/2010

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a

Table 1. Preparation Conditions and Textural Properties of Samples no.

water (mL)

ethanol (mL)

CTAB (g)

pH

SBET (m2/g)

pore volume (cm3/g)

macropore sizeb (µm)

mesopore sizec (nm)

a b c

35 35 50

15 15 0

0.4 0 0

12 12 12

514.1 403.5 320.8

0.81 0.39 0.31

0.45 0.60 1.75

5.9 3.5 3.5

a Preparation conditions: room temperature, 350 rpm, 1 h. b The average macropore diameter obtained from analysis of the image (the number of counts is 50). c BJH pore diameter determined from the adsorption branch.

Figure 1. SEM images of the synthesized aluminum oxide samples. Scale bar: 10 µm.

Figure 2. N2 adsorptionsdesorption isotherms of (A) the prepared aluminum oxide samples and (B) the 800 °C calcined samples.

diameter range of 50-75 µm were used as catalyst support. As for the commercial catalyst, it was also ground and sieved to 50-75 µm for a catalytic test. 2.2. Catalyst Preparation. The palladium-supported catalysts with 0.3 wt % metal loading (measured by inductively coupled plasma optical emission spectroscopy) were prepared by incipient-wetness impregnation of the aforementioned 800 °C calcined samples with palladium chlorite aqueous solution. The impregnated powders were placed at room temperature for 12 h and then at 120 °C for 6 h. Finally, these powders were calcined in air from room temperature to 400 °C with a ramp of 1 °C/min and kept at 400 °C for 5 h. Before activity tests, the catalysts were reduced by hydrogen at 150 °C for 8 h. 2.3. Characterization. Nitrogen adsorption-desorption isotherms and the corresponding pore-size distributions were acquired at -196 °C on a Micromeritics ASAP 2010 instrument. All the samples were degassed at 190 °C and 1 mmHg for 6 h prior to nitrogen adsorption measurements. The pore diameter and the pore size distribution were determined by the BJH method. The morphology and the macroporous array of the Al2O3 powders were examined with a JEOL JSM 6360 LV scanning electron microscope (SEM). High-resolution transmission electron microscopy (HRTEM) investigation was performed using a JEOL JEM-2010 transmission electron microscope. The samples prepared for HRTEM investigation were first dispersed in ethanol under ultrasound, and then a drop of the sampleethanol solution was transferred onto a carbon-coated copper grid. X-ray diffraction (XRD) patterns of the prepared samples were obtained on a Rigaku D/Max 2550 VB/PC diffractiometer with Cu KR radiation scanning 2θ angles ranging from 10° to 80°. Metal dispersions were measured by using CO pulse

chemisorption on a Micromeritics AutoChem 2920 apparatus. The weighed catalysts were reduced in a mixture of 10% H2/ Ar (100 mL/min) at 150 °C for 2 h, followed by a switch to helium (100 mL/min) at 190 °C for 20 min to remove adsorbed hydrogen. After the catalysts were cooled to 35 °C in a helium flow, carbon monoxide pulses were injected into the quartz reactor, and the net volume of CO was monitored with a thermal conductivity detector (TCD). A chemisorption stoichiometry of one CO molecule per surface palladium atom was assumed. Thermal analysis was performed using a TA SDTQ600 thermogravimetric analyzer under an air flow of 100 mL/min. The used catalysts were heated from room temperature to 1000 °C with a ramp of 10 °C/min. The metal loading of each catalyst was measured by inductively coupled plasma optical emission spectroscopy (ICP) using a Thermo Elemental IRIS 1000 instrument. 2.4. Selective Hydrogenation of Pygas. Hydrogenation of a model pygas composed of styrene, cyclopentadiene, 1-hexene, and n-heptane (solvent) was carried out in a stirred autoclave at 40 °C and 2.0 MPa. Detailed information on the experimental procedure was reported elsewhere.5 To compare the hydrogenation selectivity of different catalysts, two parameters are defined according to the reaction system in this work; that is, S1 )

conversion of 1-hexene × 100% conversion of cyclopentadiene

(1)

conversion of 1-hexene × 100% conversion of styrene

(2)

S2 )

It is evident that higher values of S1 and S2 signify poorer hydrogenation selectivity of the catalyst. 3. Results and Discussion 3.1. Physicochemical Characterization. As shown in Figure 1, the as-prepared three samples display similar macroporous channels, and these channels are parallel to each other and perpendicular to the tangent of the outer surface. The textural properties of these samples are listed in Table 1. Note that the diameter of the macroporous channel varies with the preparation conditions. The similar macropore sizes of samples a and b indicate that the addition of surfactant molecules seemingly has no effect on the formation of the macroporous channels, whereas the complete different macropore sizes of samples b and c imply that the water concentration can greatly affect the formation of the macrochannels.

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Figure 3. Pore-size distribution curves (determined from the adsorption branch) of (A) the prepared aluminum oxide samples and (B) the 800 °C calcined samples.

The nitrogen adsorptionsdesorption isotherms of the aluminum oxide samples and the corresponding pore-size distribution curves are presented in Figures 2A and 3A, respectively. Different from the inappreciable effect of surfactants on the macropore size, the addition of the surfactant molecules has a significant effect on the mesopore size of the prepared samples. The average mesopore sizes of samples b and c prepared in the absence of surfactants are almost the same; however, they are smaller than that of sample a prepared in the presence of surfactants. On the basis of the experimental results in this work together with those reported in the literature,15-23 a spontaneous selfassembly formation mechanism is proposed for the hierarchically macro-/mesoporous structure, which is illustrated in Figure 4. When a TBOA droplet is introduced into the water solution (Figure 4a), a thin semipermeable shell is simultaneously formed

at the outer surface of the droplet (Figure 4b). The reaction front then proceeds inwardly, and the hydrolysis and condensation reactions occur in concert to produce the solid phase (aluminum oxide nanoparticles) and the liquid phase (water and alcohol), which consequently results in the microphase-separated regions that generate the macroporous channels (Figure 4c). Figure 5 clearly demonstrates the formation mechanism of this unique structure. In the first 10 min of reaction, the macroporous channels have already been formed in the outer zone of the droplet, but in the inner zone, no macrochannels are observed (Figure 5A). In marked contrast, after 1 h’s reaction, the macrochannels completely pass through the whole particle (Figure 5B). For the samples without surfactant addition (Figure 4d), the self-assembly and aggregation of aluminum oxide particles produce the mesopores; while for the CTAB-assisted samples (Figure 4e), the CTAB molecules can adsorb on the surfaces of these particles to form a bilayer structure,18,23 which is favorable for the widening of the mesopore size distribution. This is probably the reason that the pore width of surfactantassisted samples is larger than that of nonsurfactant-assisted ones. Similar results were also reported by Yuan et al.19,23 The formation mechanism of the hierarchically macro-/ mesoporous structure demonstrates that the formation of the macroporous channels is determined by the hydrolysis and condensation rates of the alkoxide precursors. Considering that a small quantity of CTAB is added into the solution (sample a), which can hardly influence the hydrolysis rate of TBOA, the similar values of the macropore sizes of samples a and b are explainable. However, compared with sample c, a large quantity of ethanol is added into the solution during preparation of sample b, which in turn dilutes the water concentration to

Figure 4. Proposed formation mechanism of the hierarchically macro-/mesoporous aluminum oxide.

Figure 5. SEM images of the hierarchically macro-/mesoporous aluminum oxide at different reaction times: (A) 10 min; (B) 1 h. Scale bar: (A) 20 µm; (B) 50 µm.

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Figure 6. SEM images of the 800 °C calcined alumina (corresponding to Figure 1). Scale bar: 5 µm. Table 2. Textural Properties of the 800 °C Calcined Al2O3 no.

SBET (m2/g)

pore volume (cm3/g)

macropore size (µm)

mesopore size (nm)

a1 b1 c1 cca

126.4 108.0 87.6 98.1

0.51 0.38 0.24 0.37

0.45 0.50 1.25

14.3 12.7 11.9 14.9

a

The commercial Pd/δ-Al2O3 catalyst.

some extent and, consequently, decreases the hydrolysis rate of TBOA. As a result, the macropore size of sample b is smaller than that of sample c. In another independent experiment, whose preparation conditions are very similar to those of sample a, except that 40 mL water and 10 mL ethanol replace 35 mL water and 15 mL ethanol, it is found that the average macroporous diameter of this sample is 0.70 µm, which is larger than that of sample a (0.45 µm). Therefore, both cases indicate that the water concentration can greatly affect the macrochannels. The SEM images of the 800 °C calcined samples are presented in Figure 6. It is obvious that the macroporous channels of the calcined alumina are well preserved, indicating their high thermal stability. The nitrogen adsorptionsdesorption isotherms of the calcined samples and their pore-size distribution curves are presented in Figures 2B and 3B, respectively. Table 2 summaries their textural properties. Compared with the samples without calcination, the 800 °C calcined alumina showed a little bit smaller macropores due to expansion of the walls after thermal treatment, which was consistent with the increase in the mesopore size (as illustrated in Figure 3). Sample cc listed in Table 2 is a commercial Pd/δ-Al2O3 catalyst for pygas selective hydrogenation,6 and it has no macroporous channels, which is evidenced by the SEM images (see Figure 7). Figure 8 presents the X-ray diffraction patterns of the asprepared and the corresponding calcined alumina. Sample a exhibits diffraction peaks assigned to the boehmite phase (JCPDS 21-1307), whereas samples b and c show a mixture of the boehmite phase and the bayerite phase (JCPDS 20-0011).

Figure 8. X-ray diffraction patterns of (A) the as-prepared alumina and (B) the 800 °C calcined alumina.

The difference may be caused by the effect of the surfactant molecules during preparation.24 After calcination at 800 °C, sample a1 exhibits δ-Al2O3 (JCPDS 16-0394), and samples b1 and c1 show θ-Al2O3 (JCPDS 11-0517). Values of palladium dispersion obtained from CO chemisorption are similar for the four catalysts: Pd/a1, Pd/b1, Pd/c1, and Pd/cc, being 32.5%, 26.0%, 37.2% and 29.6%, respectively. The HRTEM images for the three novel catalysts are presented in Figure 9. According to energy-dispersive X-ray analysis, the metallic particles inside the catalysts (indicated by arrow) are palladium metal. In addition, it can be clearly seen from these images that the palladium particles deposit on the mesopores. 3.2. Selective Hydrogenation of Pygas. Figures 10 and 11 separately show concentration variations and conversion variations of various species over the four different catalysts. RCPD, RSTY, and RHEX (Y-axis of Figure 11) represent reaction conversions of cyclopentadiene, styrene, and 1-hexene, respectively. It can be seen that the needed time for almost complete conversion of cyclopentadiene is about 44 min for the Pd/cc catalyst, 24 min for Pd/c1, and 16 min for Pd/a1 and Pd/b1. As far as styrene is concerned, the needed time for 99% conversion or higher is around 28 min for Pd/c1 and 20 min for Pd/a1 and

Figure 7. SEM images of the commercial catalyst. Scale bar: (A) 10 µm; (B) 5 µm.

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Figure 9. HRTEM images of the novel catalysts: (A) Pd/a1; (B) Pd/b1; (C) Pd/c1. Scale bar: 20 nm.

Figure 11. Comparison of catalytic activities of different catalysts: (A) cyclopentadiene, (B) styrene, and (C) 1-hexene.

Figure 10. Concentration variations of various species with time over different catalysts: (A) Pd/cc, (B) Pd/a1, (C) Pd/b1, and (D) Pd/c1.

Pd/b1. As for the Pd/cc catalyst, the conversion of styrene is still lower than 99% after 48 min of reaction. Therefore, compared with the commercial catalyst Pd/cc, the three novel catalysts (Pd/a1, Pd/b1, and Pd/c1) with the hierarchically porous structure exhibit much higher reaction activities. It is interesting to note that although Pd/c1 has a smaller pore volume and specific surface area than Pd/cc, the former shows much higher activities than the latter. Apparently, the uniquely hierarchically macro-/mesoporous structure of the novel catalysts plays a significant role in improving the activity. First, the internal diffusion limitations of reactants can be greatly reduced in the macroporous channels;13,23,25 second, the narrow walls (0.4-1.5 µm, Figure 6) with the accessible mesopores can shorten the diffusion distance of reactants to the active sites, which is helpful for increasing the reaction rate. As shown in Figures 10 and 11, Pd/a1 and Pd/b1 have similar activities, and both of them display higher activities than Pd/ c1. This is reasonable, considering that Pd/a1 and Pd/b1 possess similar textural properties and have a larger pore volume and specific surface area than Pd/c1.

Figure 12. Hydrogenation selectivity of different catalysts based on cyclopentadiene (A) and styrene (B).

Comparison of the hydrogenation selectivity of diolefins to monoolefins among the four catalysts is presented in Figure 12. It is clear that the three novel catalysts possess a higher hydrogenation selectivity than the commercial Pd/cc catalyst. The high hydrogenation selectivity of the novel catalysts is mostly ascribed to the uniquely macro-/mesoporous structure.

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to analysis. There are two distinct weight loss steps. The first step up to around 180 °C is assigned to the removal of physisorbed water and some organic species. The second step up to around 500 °C is associated with the combustion of the organic groups. Almost no weight loss is detected above 500 °C, which is evidence that no coke is formed on the surface of the catalyst during the three runs. This result is in accordance with that obtained from Figure 13. 4. Conclusions

Figure 13. Concentration variations of cyclopentadiene during each run: (A) Pd/a1, (B) Pd/b1, and (C) Pd/c1.

A series of alumina with a hierarchically macro-/mesoporous structure were successfully prepared in the presence and absence of surfactant molecules. A spontaneous self-assembly formation mechanism was proposed for the uniquely macro-/mesoporous structure. The use of surfactant was found to influence the mesopore size distribution of the alumina materials, but not to direct the formation of the macroporous channels. Characterization of the 800 °C calcined alumina by SEM and N2 adsorption-desorption analysis revealed the high thermal stability of the prepared alumina. Three novel palladium catalysts were prepared by incipientwetness impregnation of the 800 °C calcined alumina and applied to selective hydrogenation of pyrolysis gasoline. The results showed that the hierarchically macro-/mesoporous structure of the novel catalysts played an important role in improving the catalytic activity and the hydrogenation selectivity in comparison with a commercial catalyst without the monolithic macrochannels. Reuse of the novel catalysts displayed good stability, and no coke was formed on the used catalysts. It can be expected that these novel catalysts will be among the most ideal catalysts for selective hydrogenation of pyrolysis gasoline. Acknowledgment

Figure 14. TGA profiles of the three novel catalysts (after three uses) in air at a heating rate of 10 °C/min.

The macroporous channels and the narrow walls can shorten the residence time of monoolefins (intermediate products) inside the catalyst and prevent further hydrogenation of monoolefins to saturated hydrocarbons, which is favorable for the high selectivity of diolefins to monoolefins. When it comes to the three novel catalysts, they exhibit similar hydrogenation selectivity in terms of S1, that is, on the basis of cyclopentadiene. However, from the viewpoint of S2 (i.e., on the basis of styrene), the Pd/b1 catalyst manifests the highest hydrogenation selectivity among the novel catalysts. By comprehensive consideration of the activity and the selectivity of the four catalysts, the Pd/b1 catalyst presents the best catalytic performance for selective hydrogenation of pygas. 3.3. Stability of the Catalysts. The stability of the catalysts is always important. To check the stability of the novel catalysts, we repeated three runs of reaction for each kind of catalyst. Figure 13 shows the concentration variations of cyclopentadiene during each run. It can be seen that all three concentration curves for each catalyst almost completely overlap, and no catalyst deactivation is observed from the experimental results, indicating good stability of the novel catalysts. Of course, a long-term test of the stability of the novel catalysts is necessary, and this work will be done in the near future in our group. Figure 14 shows the TGA profiles of the three used catalysts. These catalysts were dried at room temperature for 2 days prior

The authors express their gratitude to the National Natural Science Foundation of China (No. 20706018), the National High Technology Research and Development Program of China (No. 2008AA05Z405), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0721) for financial support of this work. Literature Cited (1) Zhou, Z. M.; Cheng, Z. M.; Yang, D.; Zhou, X.; Yuan, W. K. Solubility of hydrogen in pyrolysis gasoline. J. Chem. Eng. Data 2006, 51, 972–976. (2) Mostoufi, N.; Sotudeh-Gharebagh, R.; Ahmadpour, M.; Eyvani, J. Simulation of an industrial pyrolysis gasoline hydrogenation unit. Chem. Eng. Technol. 2005, 28, 174–181. (3) Kaminsky, M. P. Pyrolysis gasoline stabilization. United States Patent 6,949,686, 2005. (4) Nijhuis, T. A.; Dautzenberg, F. M.; Moulijn, J. A. Modeling of monolithic and trickle-bed reactors for the hydrogenation of styrene. Chem. Eng. Sci. 2003, 58, 1113–1124. (5) Zhou, Z. M.; Cheng, Z. M.; Cao, Y. N.; Zhang, J. C.; Yang, D.; Yuan, W. K. Kinetics of the selective hydrogenation of pyrolysis gasoline. Chem. Eng. Technol. 2007, 30, 105–111. (6) Li, S. Q.; Men, X. T.; Liu, G. S.; Liang, S. Q.; Zhang, X. G. Selective hydrogenation catalyst for pyrolysis gasoline. United States Patent 6,576,586, 2003. (7) Zhou, Z. M.; Zeng, T. Y.; Cheng, Z. M.; Yuan, W. K. Kinetics of selective hydrogenation of pyrolysis gasoline over an eggshell catalyst. Chem. Eng. Sci. 2010, 65, 1832–1839. (8) Zhang, Y.; Koike, M.; Tsubaki, N. Preparation of alumina-silica bimodal pore catalysts for Fischer-Tropsch synthesis. Catal. Lett. 2005, 91, 193–198. (9) Tidahy, H. L.; Hosseni, M.; Siffert, S.; Cousin, R.; Lamonier, J.-F.; Aboukaı¨s, A.; Su, B. L.; Giraudon, J.-M.; Leclercq, G. Nanostructured

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ReceiVed for reView February 7, 2010 ReVised manuscript receiVed April 15, 2010 Accepted April 16, 2010 IE1003043