Effect of Chlorine on the Properties of Ru/Al 2 O 3
4
KINETICS, CATALYSIS, AND REACTION ENGINEERING Effect of Chlorine on the Properties of Ru/Al 2 O 3 Vanina A. Mazzieri, ² Pablo C. L’Argentie ` re, ² Fernando Coloma-Pascual, ‡ and Nora S. Fı ´goli* ,² INCAPE (FIQ, UNL-CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina, and Servicios de Instrumentacio ´ n Cientı ´fica y Apoyo a la Investigacio ´ n, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Ru/Al 2 O 3 catalysts with different chlorine contents were prepared using RuCl 3 as the precursor. Different HCl concentrations in the impregnation solution or different treatments after impregnation were used. The catalysts were characterized by temperature-programmed reduc- tion, X-ray photoelectron spectroscopy, and hydrogen chemisorption. They were tested in the selective hydrogenation of benzene to cyclohexene in a stirred tank reactor at 373 K and 3 MPa hydrogen pressure. It was found that different electronic states of Ru appear according to the chlorine content on Ru/Al 2 O 3 . The electronic state of Ru influences the selectivity to cyclohexene. The results seem to support that the most electron-deficient Ru species may adsorb cyclohexene more weakly. When cyclohexene is more weakly adsorbed, it could be more easily desorbed by avoiding its further hydrogenation to cyclohexane, thus increasing selectivity. Introduction The selective hydrogenation of benzene to cyclohexene is an important reaction because cyclohexene is the raw material for the production of important products. Because, unlike cyclohexane, cyclohexene does not form an azeotropic mixture with benzene, an expensive separation step is avoided that is customary in the manufacture of cyclohexene by dehydrogenation of cyclohexane. Cyclohexane, a side product of the reaction, is returned to the inlet stream and dehydrogenated directly to benzene. 1 Various catalysts have been de- signed to improve the hydrogenation selectivity, among them Ru-based catalysts. 2 Ruthenium catalysts em- ployed in hydrogenation reactions in both the gas and liquid phases are most often supported on alumina, silica, titania, etc. It has been reported that the selectiv- ity to cyclohexene depends on the precursor used for the preparation of the samples. Catalysts obtained from RuCl 3 are more selective than the catalysts prepared by using Ru(acac) 3 or [Ru(NO)](NO 3 ) 3 . 3 Conventional impregnation 4 and the incipient wetness technique 5 are the methods reported most frequently for the prepara- tion of supported ruthenium catalysts. The activity and selectivity of the catalysts depend on the course and the experimental arrangement of the reduction and/or calcination of the catalyst precursor, bearing in mind that chlorides and other impurities may change the activity and selectivity of the final catalyst. 6 The objective of this paper is to analyze the effect of the chlorine content on the catalytic selectivity of Ru/ Al 2 O 3 during the benzene selective hydrogenation to cyclohexene. The chlorine content was varied by modi- fication of the HCl concentration of the impregnating solution and by different treatments after impregnation. Experimental Section Catalyst Preparation. The catalysts were prepared by wet impregnation of Al 2 O 3 CK 300 from Ketjen (S g ) 200 m 2 g -1 , pellets of 1.5 mm diameter and 4.0 mm length) using a solution of RuCl 3 (Strem Chemicals) of adequate concentration such as to obtain catalysts containing 4 wt % ruthenium. To achieve several chlorine concentrations in the samples, different pro- cedures were used. In one of them, the pH of the impregnating solution was changed by the addition of solutions having different HCl concentrations (2.4 and 3.8 M); in another case no HCl was added. The samples were then dried at 373 K for 24 h and reduced in a hydrogen stream at 673 K for 6 h. A part of the sample impregnated with a 3.8 M HCl solution was also calcined before reduction, and another part of the sample was treated with a 10 wt % NH 4 OH solution after calcination and before reduction. The procedures used for catalyst preparation and the nomenclature adopted for the different catalysts are summarized in Table 1. Catalyst Characterization. The ruthenium content was determined spectrophotometrically using a metha- nol solution saturated with N,N′-diphenylthiourea (DPTU), based on the Ru capacity to form a complex with DPTU, under certain conditions. 7 Ru was previ- ously extracted from the catalysts by heating the sample in the presence of a H 2 SO 4 solution until total dissolu- tion of the solid. Ruthenium dispersion was measured by H 2 chemi- sorption in a volumetric equipment at 373 K. Each point was measured after 5 min of stabilization. For calcula- tions, a H/Ru atomic ratio 1 was used. * To whom correspondence should be addressed. Fax: 54 342 4528062. E-mail: [email protected]. ² INCAPE (FIQ, UNL-CONICET). ‡ Universidad de Alicante, Spain. 2269 Ind. Eng. Chem. Res. 2003, 42, 2269-2272 10.1021/ie0209428 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/30/2003
Transcript of Effect of Chlorine on the Properties of Ru/Al 2 O 3
KINETICS, CATALYSIS, AND REACTION ENGINEERING
Effect of Chlorine on the Properties of Ru/Al2O3
Vanina A. Mazzieri,† Pablo C. L’Argentiere,† Fernando Coloma-Pascual,‡ andNora S. Fıgoli*,†
INCAPE (FIQ, UNL-CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina, and Servicios deInstrumentacion Cientıfica y Apoyo a la Investigacion, Universidad de Alicante, Apartado 99,E-03080 Alicante, Spain
Ru/Al2O3 catalysts with different chlorine contents were prepared using RuCl3 as the precursor.Different HCl concentrations in the impregnation solution or different treatments afterimpregnation were used. The catalysts were characterized by temperature-programmed reduc-tion, X-ray photoelectron spectroscopy, and hydrogen chemisorption. They were tested in theselective hydrogenation of benzene to cyclohexene in a stirred tank reactor at 373 K and 3 MPahydrogen pressure. It was found that different electronic states of Ru appear according to thechlorine content on Ru/Al2O3. The electronic state of Ru influences the selectivity to cyclohexene.The results seem to support that the most electron-deficient Ru species may adsorb cyclohexenemore weakly. When cyclohexene is more weakly adsorbed, it could be more easily desorbed byavoiding its further hydrogenation to cyclohexane, thus increasing selectivity.
IntroductionThe selective hydrogenation of benzene to cyclohexene
is an important reaction because cyclohexene is the rawmaterial for the production of important products.Because, unlike cyclohexane, cyclohexene does not forman azeotropic mixture with benzene, an expensiveseparation step is avoided that is customary in themanufacture of cyclohexene by dehydrogenation ofcyclohexane. Cyclohexane, a side product of the reaction,is returned to the inlet stream and dehydrogenateddirectly to benzene.1 Various catalysts have been de-signed to improve the hydrogenation selectivity, amongthem Ru-based catalysts.2 Ruthenium catalysts em-ployed in hydrogenation reactions in both the gas andliquid phases are most often supported on alumina,silica, titania, etc. It has been reported that the selectiv-ity to cyclohexene depends on the precursor used for thepreparation of the samples. Catalysts obtained fromRuCl3 are more selective than the catalysts preparedby using Ru(acac)3 or [Ru(NO)](NO3)3.3 Conventionalimpregnation4 and the incipient wetness technique5 arethe methods reported most frequently for the prepara-tion of supported ruthenium catalysts. The activity andselectivity of the catalysts depend on the course and theexperimental arrangement of the reduction and/orcalcination of the catalyst precursor, bearing in mindthat chlorides and other impurities may change theactivity and selectivity of the final catalyst.6
The objective of this paper is to analyze the effect ofthe chlorine content on the catalytic selectivity of Ru/Al2O3 during the benzene selective hydrogenation tocyclohexene. The chlorine content was varied by modi-
fication of the HCl concentration of the impregnatingsolution and by different treatments after impregnation.
Experimental Section
Catalyst Preparation. The catalysts were preparedby wet impregnation of Al2O3 CK 300 from Ketjen (Sg) 200 m2 g-1, pellets of 1.5 mm diameter and 4.0 mmlength) using a solution of RuCl3 (Strem Chemicals) ofadequate concentration such as to obtain catalystscontaining 4 wt % ruthenium. To achieve severalchlorine concentrations in the samples, different pro-cedures were used. In one of them, the pH of theimpregnating solution was changed by the addition ofsolutions having different HCl concentrations (2.4 and3.8 M); in another case no HCl was added. The sampleswere then dried at 373 K for 24 h and reduced in ahydrogen stream at 673 K for 6 h. A part of the sampleimpregnated with a 3.8 M HCl solution was alsocalcined before reduction, and another part of thesample was treated with a 10 wt % NH4OH solutionafter calcination and before reduction. The proceduresused for catalyst preparation and the nomenclatureadopted for the different catalysts are summarized inTable 1.
Catalyst Characterization. The ruthenium contentwas determined spectrophotometrically using a metha-nol solution saturated with N,N′-diphenylthiourea(DPTU), based on the Ru capacity to form a complexwith DPTU, under certain conditions.7 Ru was previ-ously extracted from the catalysts by heating the samplein the presence of a H2SO4 solution until total dissolu-tion of the solid.
Ruthenium dispersion was measured by H2 chemi-sorption in a volumetric equipment at 373 K. Each pointwas measured after 5 min of stabilization. For calcula-tions, a H/Ru atomic ratio 1 was used.
* To whom correspondence should be addressed. Fax: 54342 4528062. E-mail: [email protected].
† INCAPE (FIQ, UNL-CONICET).‡ Universidad de Alicante, Spain.
2269Ind. Eng. Chem. Res. 2003, 42, 2269-2272
10.1021/ie0209428 CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 04/30/2003
Ruthenium reducibility was determined by temper-ature-programmed reduction (TPR) using an Ohkura TP2002S instrument equipped with a thermal conductivitydetector. Nonreduced samples were heated to 975 K at10 K min-1 in a gas stream of 5% hydrogen in argon.
X-ray photoelectron spectroscopy (XPS) measure-ments were made using a VG-Microtech Multilab equip-ment, taken as reference the Al 2p signal at 74.5 eV.Determinations of the superficial atomic ratios weremade by comparing the areas under the peaks afterbackground subtraction and corrections due to differ-ences in escape depth and in photoionization crosssections.8 The samples were reduced in situ in thepretreatment chamber of the XPS equipment followingthe same operational conditions as those used duringthe preparation of the catalysts.
The benzene selective hydrogenation reaction wascarried out in a stainless steel stirred tank reactorequipped with a magnetically driven stirrer operatedat 125 rpm. The stirrer has a special design such as toobtain a good mixing; under these conditions, no diffu-sional external limitations were found. To investigatethe possibility of internal diffusional limitation, experi-ments were carried out by decreasing the catalystparticle size up to 1/4 of the original length. No modifica-tions in activity or selectivity were detected, thusneglecting the existence of internal diffusional limita-tions. The inner wall of the reactor was completelycoated with Teflon in order to neglect the catalytic actionof the steel of the reactor found by other authors.9 Thereaction was carried out at 373 K and 3 MPa constanthydrogen pressure using a volume of liquid of 200 mLand 4 g of catalyst. Reactant and products were ana-lyzed chromatographically, using a flame ionizationdetector and a capillary column CP-Wax 52 CB.
Results and Discussion
The Ru content of all catalysts was 4 wt %.The TPR profiles of the catalysts are presented in
Figure 1. The profile of A shows a peak with a maximumat about 415 K, being similar to the reduction temper-ature of unsupported RuCl3.10,11 Another peak, smallerthan the previous one, with a maximum at 468 K, isalso observed and can be attributed to a rutheniumoxide because it was found that RuCl3 can be oxidizedat the surface by air exposition at room temperature.11
Moreover, we have observed color changes (from darkred to black) in the RuCl3-impregnated catalyst uponair exposition, suggesting the transformation of thechloride into an oxide. Bossi et al.12 also observed asimilar modification. It could also be considered theformation of the oxide during drying at 373 K for 24 h.The profile of B is similar to the one of A, but the firstpeak is smaller, probably because of the lower chlorineconcentration used during impregnation; this is evenmore evident in the case of C, which was prepared
without HCl. The profile of D shows a peak with amaximum at about 415 K previously assigned to RuCl3reduction, thus indicating that chlorine was not com-pletely eliminated after calcination at 773 K. A shoulderat 425 K is also observed which may correspond,according to the literature,12 to ruthenium oxychloride.Another peak appears at about 470 K, which is relatedto a ruthenium oxide reduction; Koopman et al.11 andBetancourt et al.10 assigned a peak between 450 and478 K to the reduction of RuO2. The profile of E presentsa peak with a maximum at 470 K and a shoulder atabout 427 K that can be attributed to the reduction ofruthenium oxide and of ruthenium oxychloride, respec-tively. The presence of RuCl3 and of the oxychlorideindicates that chlorine is not completely eliminated aftercalcination and washing with a diluted NH4OH solutionand that, according to the preparation conditions, dif-ferent Ru species are present before reduction, whichmay lead to different species when the reduction stepis performed.
The presence of different Ru species was also foundby XPS. Table 2 presents the binding energy (BE)corresponding to Ru species and the Ru0/Ruδ+ and Cl/Al atomic ratios calculated from the XPS analysis,
Table 1. Preparation Conditions for the DifferentCatalysts
catalystHCl solution concn (M)used for impregnation
treatment afterimpregnation
A 3.8 reductionB 2.4 reductionC 0 reductionD 3.8 calcination + reductionE 3.8 calcination + washing with
NH4OH - reduction
Figure 1. TPR profiles of catalysts prepared using different HClconcentrations and different treatments after impregnation.
Table 2. Ru Oxidation State, Ru0/Ruδ+, and Cl/Al AtomicRatios, Selectivity to Cyclohexene (SCHE, %) at 5 min ofOperation and Dispersion (D)
catalystRu 3d5/2
(eV)Ru0/Ruδ+
(at./at.)Cl/Al
(at./at.)SCHE at 5 min
(%)D
(%)
A 280.0 1.50 0.10 32.6 19281.5
D 279.6 1.60 0.07 25.6 5280.9
E 279.7 4.10 0.04 15.9 5281.1
B 279.6 4.30 0.03 11.3 15281.1
C n.m. n.m. n.m. 7.7 11
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where Ruδ+ means electron-deficient Ru species. The Rudispersion (D, %) and selectivity to cyclohexene (SCHE,%) values are also shown. To compare more easily thevalues displayed in Table 2, the catalysts have beenordered following increasing Ru0/Ruδ+ atomic ratiovalues. In every case, the Ru 3d XPS peak was carefullydeconvoluted. The referencing of the BE scale is difficultin the case of ruthenium because the Ru 3d3/2 peakappears superposed to the C 1s line. For this reason,the Al 2p signal at 74.5 eV was taken as the reference.There are also discrepancies in the BE reported in theliterature for ruthenium compounds. Sample A showsa signal at 280.0 eV, attributed to Ru0, but not all Ruwas reduced, because a signal at 281.5 eV (Ru inRuCl3
13) also appears. The position of the Ru 3d peakon sample D shows the presence of Ru0 species at 279.6eV, although another peak at 280.9 eV was also de-tected, which may be ascribed to ruthenium oxychloridespecies.12 The position of the Ru 3d peak for catalyst Esuggests mainly the presence of Ru0 (279.7 eV) andruthenium oxychloride (281.1 eV). For catalyst B,mainly the presence of Ru0 (279.6 eV) and rutheniumoxychloride (281.1 eV) may be suggested. Obviously,decreasing the Cl/Al ratio increases Ru0/Ruδ+. Van derSteen and Scholten14 and Milone et al.3 affirmed thatchlorine modifies the electronic state of Ru, favoring theformation of electron-deficient Ru species. Figure 2shows, as an example, the Ru 3d spectra of catalyst B.
Catalyst B presents the Cl 2p3/2 peak at 199.0 eV asshown in Figure 3. The Cl 2p XPS spectra of the othersamples are very similar, although the Cl/Al atomicratios change, as shown in Table 2. For A and B, theCl/Al atomic ratio is related to the HCl concentrationin the impregnating solution. For A, D, and E, preparedusing the same HCl concentrations, the Cl/Al atomicratio is related to the treatments after impregnation.It can be noticed that calcination and reduction producea catalyst with a lower chlorine content than when onlyreduction was performed. Washing with a NH4OHsolution before reduction reduces the chlorine contenteven more. It must be taken into account that, underthese treatments, chlorine was not completely elimi-nated. Mieth and Schwarz15 also found that it is very
difficult to eliminate chlorine from catalysts preparedfrom RuCl3.
The only reaction products detected were cyclohexeneand cyclohexane. Selectivity to cyclohexene is definedas the weight percentage of benzene transformed intocyclohexene. The selectivity to cyclohexene values after5 min of operation (time at which the maximumselectivity values were observed for all catalysts),presented in Table 2, indicates a good correlationbetween the Ru0/Ruδ+ atomic ratio and selectivity tocyclohexene: the selectivity increases when the Ru0/Ruδ+ ratio decreases. The total conversion values after5 min of operation for all catalysts were within the0.27-0.29 range.
Catalysts A-C have been prepared in order to obtaindifferent Cl contents following the same pretreatmentprocedure. To achieve this, several chlorine concentra-tions in the precursor solutions were used. These threecatalysts have the same surface species but increasingRu0/Ruδ+ ratios. For these catalysts, dispersion de-creases with the chlorine content, as was also observedby other authors.11 A similar trend can be observed forselectivity, as was also found by Milone et al.3 Compar-ing catalysts B and E, having different pretreatmentsbut similar Cl/Al ratios, we see that calcination de-creases dispersion.
Figure 2. Ru 3d XPS spectrum of catalyst B. Deconvolutions of the Ru 3d3/2 and Ru 3d5/2 peaks are presented by solid lines.
Figure 3. Cl 2p XPS spectrum of catalyst B.
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Catalysts A and D were prepared using the sameimpregnating solution (which means the same chlorineconcentration) but following different pretreatmrents.Catalyst D was calcined and catalyst A was not. It canbe observed that calcination decreases the chlorinecontent.
Catalysts D and E were prepared using the sameimpregnating solution, and they were both calcined.Catalyst E was also treated with a NH4OH solution todecrease its chlorine content; this last treatment did notaffect the dispersion.
Conclusions
It can be concluded that different electronic states ofRu can be obtained by modifying the chlorine contentof Ru/Al2O3 (prepared using RuCl3 as a Ru precursor)using different HCl concentrations in the impregnationsolution or different treatments after impregnation. Theelectronic state of the ruthenium present at the surfaceinfluences the selectivity to cyclohexene. The increasein selectivity to cyclohexene when decreasing the Ru0/Ruδ+ ratio suggests that the most electron-deficient Ruspecies may adsorb cyclohexene more weakly. Whencyclohexene is more weakly adsorbed, it could be moreeasily desorbed by avoiding its hydrogenation to cyclo-hexane, thus increasing selectivity.
Acknowledgment
The experimental assistance of C. Mazzaro and M.Gonzalez and the financial assistance of CAI+D (UNL)and ANPCyT are greatly acknowledged.
Literature Cited
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(4) Narita, T.; Miura, H.; Ohira, M.; Hondou, H.; Sugiyama,K.; Matsuda, T.; Gonzalez, R. The effect of reduction temperatureon the chemisorptive properties of Ru/Al2O3: Effect of chlorine.Appl. Catal. 1987, 32, 185.
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Received for review November 25, 2002Revised manuscript received March 20, 2003
Accepted March 28, 2003
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