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 structurewere prepared and applied to selective hydrogenation of pyrolysis gasoline. The textural, structural, andmorphological properties of the catalyst supports were studied by SEM, XRD, and N2 adsorption-desorptiontechniques. The results showed that the hierarchically macro-/mesoporous structures of these materials wereformed by a spontaneous self-assembly mechanism, and the presence of surfactant molecules, which influencedthe mesopore size distribution, had almost no effects on the macroporous channels. Calcination treatmentsof the prepared materials revealed their high thermal stability. By comparison with a commercial catalystwithout 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 hierarchicalmesopores and macropores. Finally, the stability of the novel catalysts was tested. Both experimentalobservations and TGA analysis of the used catalysts indicated good stability of these catalysts.

1. Introduction

Pyrolysis gasoline (pygas) is a valuable byproduct of steamcracking of naphtha in an olefin unit. It contains a large quantityof aromatics, such as benzene, toluene, and xylene, which makesit a high potential feedstock for extraction of aromatics.1,2

Moreover, it can be used as a gasoline blending stock due toits generally high octane value.3 However, pygas is unstableand must be stabilized before the aforementioned utilizationbecause it contains substantial amounts of gum precursors, thatis, styrene and diolefins.3,4 Selective hydrogenation of pygasover heterogeneous catalysts has proved to be an effectiveprocess in the chemical industry, which aims at convertingstyrene and diolefins into ethylbenzene and monoolefins, re-spectively.5

The commercial catalysts for selective hydrogenation of pygasare usually Pd/Al2O3 with a pore size distribution between 4and 20 nm,6 which exhibits notable internal diffusion limitationsof species. Our recent work revealed that the influence of theinternal diffusion resistances was considerable, even if aneggshell catalyst was used for pygas hydrogenation.7 Takingthe 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 theresultant internal diffusion limitations will lead to two deficien-cies. First, some mesopores inside the catalyst will be blockedwith gums formed by polymerization of styrene and diolefinsafter a period of application, and consequently, some active sitesof the catalyst will be covered. Second, the intermediateproducts, monoolefins, will be further hydrogenated into satu-rated components, inevitably decreasing the hydrogenationselectivity of diolefins to monoolefins.

The strategy to overcome these deficiencies is to reduce theinternal diffusion to a great extent as well as to releasemonoolefins from the internal pores of the catalysts as soon aspossible. Previous investigations have showed that a bidispersepore structure can improve the catalytic performance for manyreaction systems, such as Fischer-Tropsch synthesis,8 total

oxidation of VOCs,9 CO oxidation,10 photocatalytic degradationof ethylene,11 autothermal reforming of methane,12 etc. Acatalyst with macro-/mesoporous structure can provide not onlylarge pores to reduce the pore diffusion resistance but also alarge specific surface area to increase the metal dispersion.

Recently, we prepared a Pd/TiO2 catalyst with a hierarchicallymacro-/mesoporous structure for styrene hydrogenation.13 Thisnovel catalyst showed high catalytic activity, but the selectivitybetween monoolefins and diolefins, a very important factor forselective hydrogenation of pygas, was not studied. Consideringthat alumina rather than titania is more generally used as acatalyst support for selective hydrogenation,14 herein, a seriesof hierarchically macro-/mesoporous Pd/Al2O3 catalysts forselective hydrogenation of pygas are prepared for the first time.The objective of this work is to highlight the predominant effectof the hierarchical pore structure on pygas hydrogenation,including the effect of preparation conditions on the catalysttexture, comparison of the catalytic performances of the novelcatalysts and the commercial catalyst, and the stability of thenovel catalysts.

2. Experimental Section

2.1. Alumina Support Preparation. Alumina samples witha hierarchically macro-/mesoporous structure were preparedunder 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 and15 mL of ethanol with slow stirring at room temperature. Thenthe 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 centrifuga-tion, washed by Soxhlet extraction for 30 h, and dried in airfor 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 inthe absence of the surfactant molecules, wherein the Soxhletextraction treatment of the samples was not required. The onlydifference in preparation of samples b and c was whether ethanolwas added. Corresponding to samples a, b, and c, the 800 °Ccalcined samples were labeled as a1, b1, and c1, respectively.The calcined samples were sieved, and those particles with a

* To whom correspondence should be addressed. Tel: +86-21-64252230. Fax: +86-21-64253528. E-mail: zmzhou@ecust.edu.cn.

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10.1021/ie1003043 2010 American Chemical SocietyPublished on Web 04/26/2010

diameter range of 50-75 µm were used as catalyst support. Asfor the commercial catalyst, it was also ground and sieved to50-75 µm for a catalytic test.

2.2. Catalyst Preparation. The palladium-supported cata-lysts with 0.3 wt % metal loading (measured by inductivelycoupled plasma optical emission spectroscopy) were preparedby incipient-wetness impregnation of the aforementioned 800°C calcined samples with palladium chlorite aqueous solution.The impregnated powders were placed at room temperature for12 h and then at 120 °C for 6 h. Finally, these powders werecalcined in air from room temperature to 400 °C with a rampof 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 iso-therms and the corresponding pore-size distributions wereacquired at -196 °C on a Micromeritics ASAP 2010 instrument.All the samples were degassed at 190 °C and 1 mmHg for 6 hprior to nitrogen adsorption measurements. The pore diameterand the pore size distribution were determined by the BJHmethod. The morphology and the macroporous array of theAl2O3 powders were examined with a JEOL JSM 6360 LVscanning electron microscope (SEM). High-resolution transmis-sion electron microscopy (HRTEM) investigation was performedusing a JEOL JEM-2010 transmission electron microscope. Thesamples prepared for HRTEM investigation were first dispersedin ethanol under ultrasound, and then a drop of the sample-ethanol solution was transferred onto a carbon-coated coppergrid. X-ray diffraction (XRD) patterns of the prepared sampleswere obtained on a Rigaku D/Max 2550 VB/PC diffractiometerwith Cu KR radiation scanning 2θ angles ranging from 10° to80°. 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 tohelium (100 mL/min) at 190 °C for 20 min to remove adsorbedhydrogen. After the catalysts were cooled to 35 °C in a heliumflow, carbon monoxide pulses were injected into the quartzreactor, and the net volume of CO was monitored with a thermalconductivity detector (TCD). A chemisorption stoichiometry ofone CO molecule per surface palladium atom was assumed.Thermal analysis was performed using a TA SDTQ600 ther-mogravimetric analyzer under an air flow of 100 mL/min. Theused catalysts were heated from room temperature to 1000 °Cwith a ramp of 10 °C/min. The metal loading of each catalystwas measured by inductively coupled plasma optical emissionspectroscopy (ICP) using a Thermo Elemental IRIS 1000instrument.

2.4. Selective Hydrogenation of Pygas. Hydrogenation ofa model pygas composed of styrene, cyclopentadiene, 1-hexene,and n-heptane (solvent) was carried out in a stirred autoclaveat 40 °C and 2.0 MPa. Detailed information on the experimentalprocedure was reported elsewhere.5 To compare the hydrogena-tion selectivity of different catalysts, two parameters are definedaccording to the reaction system in this work; that is,

It is evident that higher values of S1 and S2 signify poorerhydrogenation selectivity of the catalyst.

3. Results and Discussion

3.1. Physicochemical Characterization. As shown in Figure1, the as-prepared three samples display similar macroporouschannels, and these channels are parallel to each other andperpendicular to the tangent of the outer surface. The texturalproperties of these samples are listed in Table 1. Note that thediameter of the macroporous channel varies with the preparationconditions. The similar macropore sizes of samples a and bindicate that the addition of surfactant molecules seemingly hasno effect on the formation of the macroporous channels, whereasthe complete different macropore sizes of samples b and c implythat the water concentration can greatly affect the formation ofthe macrochannels.

Table 1. Preparation Conditions and Textural Properties of Samplesa

no. water (mL) ethanol (mL) CTAB (g) pH SBET (m2/g) pore volume (cm3/g) macropore sizeb (µm) mesopore sizec (nm)

a 35 15 0.4 12 514.1 0.81 0.45 5.9b 35 15 0 12 403.5 0.39 0.60 3.5c 50 0 0 12 320.8 0.31 1.75 3.5

a Preparation conditions: room temperature, 350 rpm, 1 h. b The average macropore diameter obtained from analysis of the image (the number ofcounts 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 aluminumoxide samples and (B) the 800 °C calcined samples.

S1 ) conversion of 1-hexeneconversion of cyclopentadiene

× 100% (1)

S2 ) conversion of 1-hexeneconversion of styrene

× 100% (2)

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The nitrogen adsorptionsdesorption isotherms of the alumi-num oxide samples and the corresponding pore-size distributioncurves are presented in Figures 2A and 3A, respectively.Different from the inappreciable effect of surfactants on themacropore size, the addition of the surfactant molecules has asignificant effect on the mesopore size of the prepared samples.The average mesopore sizes of samples b and c prepared in theabsence of surfactants are almost the same; however, they aresmaller than that of sample a prepared in the presence ofsurfactants.

On the basis of the experimental results in this work togetherwith those reported in the literature,15-23 a spontaneous self-assembly formation mechanism is proposed for the hierarchicallymacro-/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 frontthen proceeds inwardly, and the hydrolysis and condensationreactions occur in concert to produce the solid phase (aluminumoxide nanoparticles) and the liquid phase (water and alcohol),which consequently results in the microphase-separated regionsthat generate the macroporous channels (Figure 4c). Figure 5clearly demonstrates the formation mechanism of this uniquestructure. In the first 10 min of reaction, the macroporouschannels have already been formed in the outer zone of thedroplet, but in the inner zone, no macrochannels are observed(Figure 5A). In marked contrast, after 1 h’s reaction, themacrochannels completely pass through the whole particle(Figure 5B).

For the samples without surfactant addition (Figure 4d), theself-assembly and aggregation of aluminum oxide particlesproduce the mesopores; while for the CTAB-assisted samples(Figure 4e), the CTAB molecules can adsorb on the surfacesof these particles to form a bilayer structure,18,23 which isfavorable for the widening of the mesopore size distribution.This is probably the reason that the pore width of surfactant-assisted samples is larger than that of nonsurfactant-assistedones. 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 themacroporous channels is determined by the hydrolysis andcondensation rates of the alkoxide precursors. Considering thata small quantity of CTAB is added into the solution (samplea), which can hardly influence the hydrolysis rate of TBOA,the similar values of the macropore sizes of samples a and bare explainable. However, compared with sample c, a largequantity of ethanol is added into the solution during preparationof 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.

Figure 3. Pore-size distribution curves (determined from the adsorptionbranch) of (A) the prepared aluminum oxide samples and (B) the 800 °Ccalcined samples.

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some extent and, consequently, decreases the hydrolysis rateof TBOA. As a result, the macropore size of sample b is smallerthan that of sample c. In another independent experiment, whosepreparation conditions are very similar to those of sample a,except that 40 mL water and 10 mL ethanol replace 35 mLwater and 15 mL ethanol, it is found that the averagemacroporous diameter of this sample is 0.70 µm, which is largerthan that of sample a (0.45 µm). Therefore, both cases indicatethat the water concentration can greatly affect the macrochannels.

The SEM images of the 800 °C calcined samples arepresented in Figure 6. It is obvious that the macroporouschannels of the calcined alumina are well preserved, indicatingtheir high thermal stability. The nitrogen adsorptionsdesorptionisotherms of the calcined samples and their pore-size distributioncurves are presented in Figures 2B and 3B, respectively. Table2 summaries their textural properties. Compared with thesamples without calcination, the 800 °C calcined aluminashowed a little bit smaller macropores due to expansion of thewalls after thermal treatment, which was consistent with theincrease in the mesopore size (as illustrated in Figure 3). Samplecc listed in Table 2 is a commercial Pd/δ-Al2O3 catalyst forpygas selective hydrogenation,6 and it has no macroporouschannels, which is evidenced by the SEM images (see Figure7).

Figure 8 presents the X-ray diffraction patterns of the as-prepared and the corresponding calcined alumina. Sample aexhibits diffraction peaks assigned to the boehmite phase(JCPDS 21-1307), whereas samples b and c show a mixture ofthe boehmite phase and the bayerite phase (JCPDS 20-0011).

The difference may be caused by the effect of the surfactantmolecules 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 chemi-

sorption 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 presentedin Figure 9. According to energy-dispersive X-ray analysis, themetallic particles inside the catalysts (indicated by arrow) arepalladium metal. In addition, it can be clearly seen from theseimages that the palladium particles deposit on the mesopores.

3.2. Selective Hydrogenation of Pygas. Figures 10 and 11separately show concentration variations and conversion varia-tions of various species over the four different catalysts. RCPD,RSTY, and RHEX (Y-axis of Figure 11) represent reactionconversions of cyclopentadiene, styrene, and 1-hexene, respec-tively. It can be seen that the needed time for almost completeconversion of cyclopentadiene is about 44 min for the Pd/cccatalyst, 24 min for Pd/c1, and 16 min for Pd/a1 and Pd/b1. Asfar as styrene is concerned, the needed time for 99% conversionor 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.

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 126.4 0.51 0.45 14.3b1 108.0 0.38 0.50 12.7c1 87.6 0.24 1.25 11.9cca 98.1 0.37 14.9

a The commercial Pd/δ-Al2O3 catalyst.

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

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Pd/b1. As for the Pd/cc catalyst, the conversion of styrene isstill lower than 99% after 48 min of reaction. Therefore,compared with the commercial catalyst Pd/cc, the three novelcatalysts (Pd/a1, Pd/b1, and Pd/c1) with the hierarchically porousstructure exhibit much higher reaction activities.

It is interesting to note that although Pd/c1 has a smaller porevolume and specific surface area than Pd/cc, the former showsmuch higher activities than the latter. Apparently, the uniquelyhierarchically macro-/mesoporous structure of the novel catalystsplays a significant role in improving the activity. First, theinternal diffusion limitations of reactants can be greatly reducedin the macroporous channels;13,23,25 second, the narrow walls(0.4-1.5 µm, Figure 6) with the accessible mesopores canshorten 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 similaractivities, and both of them display higher activities than Pd/c1. This is reasonable, considering that Pd/a1 and Pd/b1 possesssimilar textural properties and have a larger pore volume andspecific surface area than Pd/c1.

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

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

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

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

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

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The macroporous channels and the narrow walls can shortenthe residence time of monoolefins (intermediate products) in-side the catalyst and prevent further hydrogenation of monoole-fins to saturated hydrocarbons, which is favorable for the highselectivity of diolefins to monoolefins.

When it comes to the three novel catalysts, they exhibitsimilar hydrogenation selectivity in terms of S1, that is, on thebasis of cyclopentadiene. However, from the viewpoint of S2

(i.e., on the basis of styrene), the Pd/b1 catalyst manifests thehighest hydrogenation selectivity among the novel catalysts. Bycomprehensive consideration of the activity and the selectivityof the four catalysts, the Pd/b1 catalyst presents the best catalyticperformance for selective hydrogenation of pygas.

3.3. Stability of the Catalysts. The stability of the catalystsis 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 cyclopentadieneduring each run. It can be seen that all three concentration curvesfor each catalyst almost completely overlap, and no catalystdeactivation is observed from the experimental results, indicatinggood stability of the novel catalysts. Of course, a long-term testof the stability of the novel catalysts is necessary, and this workwill 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

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

4. Conclusions

A series of alumina with a hierarchically macro-/mesoporousstructure were successfully prepared in the presence and absenceof surfactant molecules. A spontaneous self-assembly formationmechanism was proposed for the uniquely macro-/mesoporousstructure. The use of surfactant was found to influence themesopore size distribution of the alumina materials, but not todirect the formation of the macroporous channels. Characteriza-tion of the 800 °C calcined alumina by SEM and N2

adsorption-desorption analysis revealed the high thermal stabil-ity of the prepared alumina.

Three novel palladium catalysts were prepared by incipient-wetness impregnation of the 800 °C calcined alumina andapplied to selective hydrogenation of pyrolysis gasoline. Theresults showed that the hierarchically macro-/mesoporousstructure of the novel catalysts played an important role inimproving the catalytic activity and the hydrogenation selectivityin comparison with a commercial catalyst without the monolithicmacrochannels. Reuse of the novel catalysts displayed goodstability, and no coke was formed on the used catalysts. It canbe expected that these novel catalysts will be among the mostideal catalysts for selective hydrogenation of pyrolysis gasoline.

Acknowledgment

The authors express their gratitude to the National NaturalScience Foundation of China (No. 20706018), the National HighTechnology Research and Development Program of China (No.2008AA05Z405), and the Program for Changjiang Scholars andInnovative Research Team in University (No. IRT0721) forfinancial support of this work.

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

Accepted April 16, 2010

IE1003043

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