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Stability and deactivation of spinel-type cobalt chromite catalysts for ortho-selective alkylation of phenol with methanol
Catalysis Communications 9 (2008) 2044–2047
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Stability and deactivation of spinel-type cobalt chromite catalysts for ortho-selective alkylation of phenol with methanol Yanli Wang a, Piaoping Yang a, Gang Liu b, Lei Xu b, Mingjun Jia b,*, Wenxiang Zhang b,*, Dazhen Jiang b a b
College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China College of Chemistry, Jilin University, Changchun 130023, PR China
a r t i c l e
i n f o
Article history: Received 16 January 2008 Received in revised form 12 March 2008 Accepted 28 March 2008 Available online 8 April 2008 Keywords: Cobalt chromite Phenol alkylation Catalyst deactivation Stability
a b s t r a c t A series of spinel-type cobalt chromite catalysts were investigated for the alkylation of phenol with methanol. The catalyst with Co/Cr = 0.8 shows the highest activity and good stability. The catalysts before and after reaction were characterized by the means of temperature programmed reduction (TPR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The experimental results indicate that reduction of cobalt and chromium ions to lower oxidation states is the main reason for the catalyst deactivation with time on stream. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Alkylation of phenol has been attracted much attention due to the importance of alkyl phenols as raw materials for various commercial applications, including manufacture of drugs, insecticides, antioxidants, dyes, specialty paints, plastics, and antiseptics. Currently, there is a significant interest in the ortho-selective alkylation of phenol with methanol, because o-cresol and 2,6-xylenol formed by the reaction are used as important intermediates for the production of herbicides, polyphenylene oxide (PPO), and special grade paints [1–5]. Many kinds of materials have been used in the reaction as catalysts, such as various zeolites, pure and mixed metal oxides, and hydrotalcites etc. [1–5]. Among these catalysts, oxide catalysts generally show better selectivity for ortho-alkylation phenols. It should especially be pointed out that spinel oxides mainly containing cations such as Co, Cr, Mn or Fe exhibited high activity and selectivity to ortho-alkylation for the phenol alkylation [6–11]. Spinels have a general formula AB2O4, where A- and B-cations occupy the tetrahedral and octahedral sites, respectively. Previously, the effect of surface acid–base properties of the spinel catalysts on catalytic performance has been investigated by several research groups [9–11]. It was proposed that the appropriate acidity on the surface of the spinel, which depends strongly on the nature of cations, the charge, and the distribution in tetrahedral/ octahedral sites, plays a dominating role for the phenol methylation. Spinel-type cobalt chromites as red–ox behavior have exhib* Corresponding authors. Tel.: +86 431 88499140; fax: +86 431 88499232. E-mail address: [email protected] (W. Zhang). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.03.049
ited special catalytic activity for various catalytic reactions, such as water gas shift reaction, combustion catalysis and the total oxidation of halogenated hydrocarbons [12–16]. Our earlier work has revealed that spinel-type cobalt chromite catalysts also show high activity for ortho-methylation of phenol [11]. Although it is important that a catalyst keeps its activity and selectivity for the longest time possible, the deactivation of the catalyst is still a problem. The catalyst deactivation for the alkylation of phenol and methanol has been reported by several papers [5– 7,17,18]. Coking is the main reason for zeolite catalyst deactivation [5,17]. It is well known that the mixed oxides are usually used to prevent coke formation to obtain better on-stream stability during many industrial processes. The reduction has usually been regarded as the main cause of the deactivation of these oxide catalysts [6,7,18]. To obtain catalysts with high activity and stability, our previous works have mainly focused on the correlation of the acidity with catalytic performance over spinel-type cobalt chromite catalysts [11]. Herein, the catalytic performance of the spinel-type cobalt chromite (Co/Cr = 0.8) for the ortho-alkylation of phenol with time on stream was investigated. The catalyst structure and the red-ox behavior change have been particularly studied by the methods of H2-TPR, XRD, and XPS. 2. Experimental 2.1. Catalyst preparation Spinel-type cobalt chromite were prepared by co-precipitation technique following a reference procedure [12]. Typically, a diluted
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aqueous ammonia (10 wt.%) solution was added to a mixed aqueous solution of chromium nitrate and cobalt nitrate with continuous stirring at room temperature until the pH value was 9.3. The precipitates were filtered, washed several times and dried in an air oven. Finally, the dried materials were calcined at 723 K for 5 h. The available powders were ready for the alkylation activity measurements after being crushed and sieved. 2.2. Characterization and catalyst test XRD patterns were recorded on a Lab XRD-6000 X-ray Diffractometer with nickel-filtered Cu Ka radiation operating at 40 kV and 30 mA in a 2h range of 10–70°. Temperature-programmed reduction (TPR) was carried out in a quartz reactor with 5% H2/Ar (molar ratio) reduction gas of flow rate of 30 ml/min. Catalyst (20 mg) was loaded in the quartz reactor. The temperature was raised from ambient temperature to 973 K at rate of 10 K/min. The hydrogen consumption was monitored by using an on-line thermal conductivity detector (TCD). X-ray photoelectron spectra (XPS) were recorded on a VG ESCA LAB MK-II X-ray electron spectrometer using Al Ka radiation (1486.6 eV, 10.1 kV). The spectra were referenced with respect to the C 1s line at 284.7 eV. The measurement error of the spectra was ±0.2 eV. The vapor phase alkylation of phenol with methanol was performed in a fixed bed continuous down-flow reactor at atmospheric pressure. Fresh catalyst (0.8 g) with the particle size from 40 to 60 mesh was charged each time. Before carrying out the reaction, the fresh catalyst was activated at 693 K for 1 h in nitrogen in a vertical glass-tube reactor. A pre-mixed phenol–methanol mixture was then fed from the top of the reactor using a SY-04 syringe pump along with N2 gas (3.2 ml/min). The reaction conditions are list as follows: phenol/methanol = 1/5 (mole ratio), reaction temperature = 693 K, weight hourly space velocity (WHSV) = 0.897 h1. The reaction products were analyzed by a gas chromatograph equipped with a HP–5 capillary column and identified with GC–MS. 3. Results and discussion
100
phenol conv.
90
o-cresol selec.
100
Conversion/Selectivity (%)
Conversion/Selectivity (%)
Catalytic performance of several representative cobalt chromite catalysts for the alkylation of phenol with methanol is presented in Fig. 1. It can be seen that all the catalysts show high activity and high selectivity to o-cresol and 2,6-xylenol as the major ortho
2,6-xylenol selec.
80
products. The catalyst with Co/Cr = 0.8 exhibits the highest catalytic conversion as 97.3%. Fig. 2 displays the catalytic results of the catalyst (Co/Cr = 0.8) with time on stream over a period of 35 h. As shown, the conversion of phenol is almost unchanged during the first 15 h. With further increase of the reaction time, the conversion of phenol decreases slightly, and reaches 92.5% at the reaction time of 35 h. Additionally, the selectivity to o-cresol is low at the beginning, and almost keeps unchanged form 3 to 10 h, then increases gradually after 10 h. On the contrary, the selectivity to 2,6-xylenol is high during the first 10 h, then decreases gradually after 10 h. The total ortho-selectivity is kept about 93%, which reveals that 2,6-xylenol is yielded at expense of o-cresol. On the whole, there is only a slight decrease in the catalytic activity. The results indicate spinel-type cobalt chromite catalysts exhibit good stability. The XRD patterns of various fresh cobalt chromite catalysts are displayed in Fig. 3a. All the fresh catalysts with different Co/Cr ratios show a characteristic diffraction assigned to the spinel phase. For the Cr rich catalysts (Co/Cr = 0.2, 0.5), the additional diffraction peaks attributed to Cr2O3 phase can also be observed. With the increase of Co contents (Co/Cr > 0.5), the peak intensity of spinel phase increases. This may be attributed to the formation of a solid 3þ solution of CoCr2O4 and Co3O4, which is like Co2þ Co3þ x Cr2x O4 solid solution [19]. For Co/Cr = 2.5 sample, the diffraction peaks shift to higher degree, which may be due to the cation distribution difference in the octahedral sites. In the case of normal spinel-type cobalt chromite sample, divalent Co2+ and trivalent Cr3+ cations occupy the tetrahedral and octahedral sites of the spinel lattice, respectively. Trivalent Co3+ and Cr3+ cations simultaneously occupy the octahedral sites of the spinel lattice when cobalt is excessive, and there are some difference in the ionic radius of Co3+ and Cr3+ [19]. For the spent samples (Fig. 3b), no significant change in the XRD pattern is observed for the spent Co/Cr = 0.2, 0.5, 0.8 catalysts compared to the fresh catalysts. However, for the spent Co/Cr = 2.5 catalyst, the CoO phase is obvious besides the spinel phase. The binding energy (BE), the line shapes, and the intensity of the satellite peaks of the XPS results have often been used for identification of the cobalt species. Though the BE value of CoO is close to that of Co3O4, the former is usually higher than the latter. In addition, Co(II) oxide has a strong shake-up satellite about 6 eV above oxide of Co 2p3/2 while Co(III) oxide usually does not [20–22]. The Co 2p core level photoelectron spectra of the fresh and spent Co/Cr = 0.8 cobalt chromite catalysts are depicted in Fig. 4a. Seen
70 60 50 40 30 20 10
90
phenol conv. o-cresol selec. 2,6-xylenol selec.
80 70 60 50 40 30 20 10
0.5
1.0
1.5
2.0
2.5
Co/Cr (mol ratio) Fig. 1. Catalytic performance of cobalt chromite catalysts with different Co/Cr ratios. (Reaction conditions: phenol/methanol = 1/5, WHSV = 0.897 h1, T = 693 K, N2 flow rate = 3.2 ml/min, reaction time = 4 h).
0
5
10
15
20
25
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35
Time on stream (h) Fig. 2. Stability of Co/Cr = 0.8 cobalt chromite catalyst for ortho-selective alkylation of phenol with methanol. (Reaction conditions: phenol/methanol = 1/5, WHSV = 0.897 h1, T = 693 K, N2 flow rate = 3.2 ml/min).
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Co 2p3/2
a
a Co 2p1/2
0.5 0.8
sat.
Intensity (a. u.)
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765 770 775 780 785 790 795 800 805 810 815 20
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Binding energy (eV)
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b Cr 2p1/2
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2 theta (degree)
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570
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Fig. 3. Powder XRD patterns for fresh (a) and spent (b) cobalt chromite catalysts with different Co/Cr ratios calcined at 723 K, Characteristic diffraction peaks of spinel phase (.), CoO (j) and Cr2O3 (d) are indicated.
Fig. 4. Co 2p (a) and Cr 2p (b) core level photoelectron spectra of fresh and spent Co/Cr = 0.8 cobalt chromite catalysts.
from the figure, the Co 2p3/2 peaks of fresh and spent Co/Cr = 0.8 catalysts appear at 781.4 eV and 782.2 eV, respectively. The Co 2p3/2 peak of the spent catalyst shifts to higher bonding energy in comparison with that of the fresh catalyst, and the intensity of the satellite for the spent catalyst is much stronger than that of the fresh catalyst, which can be attributed to an increase of Co2+ [23,24]. The results show that some of Co3+ ions have been converted to Co2+ ions during the reaction. Cr 2P photoelectron spectra (Fig. 4b) show the Cr 2p3/2 peak of the fresh cobalt chromite catalyst is resolved into a peak about 577.4 eV and a shoulder peak about 580.3 eV. The peaks are assigned to Cr3+ and Cr6+, respectively [25]. The BE of the Cr 2p3/2 peak for the spent catalyst doesn’t show any change compared to the fresh catalyst. However, the shoulder perk disappears in the spent catalyst. The result reveals that Cr6+ ions are turned into Cr3+ ions in the course of the reaction. Moreover, the peak intensity for the spent catalyst is lower than that for the fresh one, which may be due to the formation of polyalkylated compounds or deposited coke on the surface of the catalyst during the phenol methylation. H2-TPR profiles of fresh cobalt chromite catalysts and Co3O4 are shown in Fig 5a. For the Co/Cr = 0.2 catalyst, the peaks at about 513 K and 638 K can be attributed to the reduction of Cr6+ to Cr3+ and the chemisorption of hydrogen, respectively [14,16,26]. The intensity of the peak at 638 K decreases with the increase of Co/
Cr ratio. The peak at around 923 K can be assigned to the reduction of Co2+ of CoCr2O4, and shifts to lower temperature with the increase of Co contents. In the case of Co3O4, the two peaks at 615 K and 669 K correspond to the reduction of Co3+ and Co2+. For the Co/Cr = 2.5 catalyst, the peak at about 644 K corresponds to the reduction of Co3+, and the peak at about 818 K can be attributed to the overlap of the two reduction peaks of Co2+ of CoCr2O4 and Co3O4. Fig. 5b shows TPR profiles of spent catalysts. The peak around 923 K is ascribed to the reduction of Co2+ of CoCr2O4. The XRD indicates that the CoO phase is present for the Co/Cr = 2.5 catalyst. So the other peak can be the reduction of Co2+ of CoO. The methanol to phenol ratio applied over various spinel-type cobalt chromite catalysts is 5, which is far exceeding the stoichiometric value of 2. As a result, the superfluous methanol may decompose to CO and H2 [6,7]. Hence the reduction gases are definitely capable of reducing the fresh catalysts. The reduction processes including the complete reduction of Cr6+ to Cr3+, Co3+ to Co2+ and partial reduction of Co2+ may occur on the surface of catalysts, which has been identified by the XRD, TPR and XPS results. Reduction may firstly proceed as complete reduction of Cr6+ to Cr3+, because the reduction peak of Cr6+ is not observed in the TPR spectra of the spent catalysts, and Cr6+ is more unstable than Co3+, which could result in the prompt increase of the selectivity to o-cresol and prompt decrease of the selectivity to 2,6-xylenol
Y. Wang et al. / Catalysis Communications 9 (2008) 2044–2047
Intensity (a. u.)
a
2047
4. Conclusions
2.5
In summary, the spinel-type cobalt chromite catalysts exhibit good catalytic performance for the gas phase catalytic ortho-alkylation of phenol with methanol. By combination with the XRD, TPR, XPS measurements and the reaction results, it is proposed that a slight decrease in the catalytic activity of cobalt chromite catalysts could be mainly ascribed to complete reduction of Cr6+ to Cr3+, Co3+ to Co2+ and partial reduction of Co2+ during the reaction process.
0.8
Acknowledgment
Co3O4
0.5 Financial support from the foundation of Harbin Engineering University (Grant number HEUFT07053) is greatly acknowledged.
0.2
373
473
573
673
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873
973
Temperature (K)
Intensity (a. u.)
b
2.5 0.8 0.5 0.2 373
473
573
673
773
873
973
Temperature (K) Fig. 5. H2-TPR spectra of fresh (a) and spent (b) cobalt chromite catalysts with different Co/Cr ratios.
at the beginning of reaction. With further increase of reaction time, reduction may mainly happened as the complete reduction of Co3+ to Co2+ and partial reduction of Co2+, because the reduction peak of Co3+ is not found in the TPR spectra of the spent catalysts, and the reduction peak of Co2+ is over the reaction temperature (693 K), which may lead to gradual increase of the selectivity to o-crosel and gradual decrease of the selectivity to 2,6-xylenol.
References [1] K.R. Reddy, K. Ramesh, K.K. Seela, V.V. Rao, K.V.R. Chary, Catal. Commun. 4 (2003) 112. [2] S. Velu, C.S. Swamy, Appl. Catal. A 162 (1997) 81. [3] S. Sato, K. Koizumi, F. Nozaki, J. Catal. 178 (1998) 264. [4] A.R. Gandhe, J.B. Fernandes, Catal. Commun. 5 (2004) 89. [5] M. Bregolato, V. Bolis, C. Busco, P. Ugliengo, S. Bordiga, F. Cavani, N. Ballarini, L. Maselli, S. Passeri, I. Rossetti, L. Forni, J. Catal. 245 (2007) 285. [6] A.S. Reddy, C.S. Gopinath, S. Chilukuri, J. Catal. 243 (2006) 278. [7] T. Mathew, N.R. Shiju, K. Sreekumar, B.S. Rao, C.S. Gopinath, J. Catal. 210 (2002) 405. [8] K. Sreekumar, S. Sugunan, Appl. Catal. A 230 (2002) 245. [9] K. Sreekumar, S. Sugunan, J. Mol. Catal. A 185 (2002) 259. [10] T. Mathew, B.B. Tope, N.R. Shiju, S.G. Hegde, B.S. Rao, C.S. Gopinath, Phys. Chem. Chem. Phys. 4 (2002) 4260. [11] Y.L. Wang, Z. Zhou, M.J. Jia, X.M. Zhu, W.X. Zhang, D.Z. Jiang, Catal. Lett. 104 (2005) 67. [12] G.J. Hutchings, R.G. Copperthwaite, F.M. Gottschalk, R. Hunter, J. Mellor, S.W. Orchard, T. Sangiorgio, J. Catal. 137 (1992) 408. [13] J.R. Mellor, R.G. Copperthwaite, N.J. Coville, Appl. Catal. A 164 (1997) 69. [14] J. Słoczyn´ski, J. Janas, T. Machej, J. Rynkowski, J. Stoch, Appl. Catal. B 24 (2000) 45. [15] M.I. Vass, V. Georgescu, Catal. Today 29 (1996) 463. [16] D.C. Kim, S.K. Ihm, Environ. Sci. Technol. 35 (2001) 222. [17] P.D. Chantal, S. Kaliaguine, J.L. Grandmaison, Appl. Catal. 18 (1985) 133. [18] H. Grabowska, R. Klimkiewicz, W. Tylus, P.J. Godowski, Appl. Catal. A 240 (2003) 111. [19] G.L. Castiglioni, G. Minelli, P. Porta, A. Vaccari, J. Solid State Chem. 152 (2000) 526. [20] F.B. Noronha, M. Schmal, B. Moraweck, P. Delichère, M. Brun, F. Villain, R. Fréty, J. Phys. Chem. B 104 (2000) 5478. [21] D.C. Frost, C.A. McDowell, I.S. Woolsey, Mol. Phys. 27 (1974) 1473. [22] K. Omata, T. Takada, S. Kasahara, M. Yamada, Appl. Catal. A 146 (1996) 255. [23] S. Velu, K. Suzuki, C.S. Gopinath, J. Phys. Chem. B 106 (2002) 12737. [24] J.G. Kim, D.L. Pugmire, D. Battaglia, M.A. Langell, Appl. Surf. Sci. 165 (2000) 70. [25] J.M. Rao, A. Sivaprasad, P.S. Rao, B. Narsaiah, S.N. Reddy, V. Vijayakumar, S.V. Manorama, K.L. Krishna, K. Srinivas, K.R. Krishnan, S. Badrinarayanan, J. Catal. 184 (1999) 105. [26] M.I. Zaki, M.A. Hasan, N.E. Fouad, Appl. Catal. A 171 (1998) 315.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Stability and deactivation of spinel-type cobalt chromite catalysts for ortho-selective alkylation of phenol with methanol Yanli Wang a, Piaoping Yang a, Gang Liu b, Lei Xu b, Mingjun Jia b,*, Wenxiang Zhang b,*, Dazhen Jiang b a b
College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China College of Chemistry, Jilin University, Changchun 130023, PR China
a r t i c l e
i n f o
Article history: Received 16 January 2008 Received in revised form 12 March 2008 Accepted 28 March 2008 Available online 8 April 2008 Keywords: Cobalt chromite Phenol alkylation Catalyst deactivation Stability
a b s t r a c t A series of spinel-type cobalt chromite catalysts were investigated for the alkylation of phenol with methanol. The catalyst with Co/Cr = 0.8 shows the highest activity and good stability. The catalysts before and after reaction were characterized by the means of temperature programmed reduction (TPR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The experimental results indicate that reduction of cobalt and chromium ions to lower oxidation states is the main reason for the catalyst deactivation with time on stream. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Alkylation of phenol has been attracted much attention due to the importance of alkyl phenols as raw materials for various commercial applications, including manufacture of drugs, insecticides, antioxidants, dyes, specialty paints, plastics, and antiseptics. Currently, there is a significant interest in the ortho-selective alkylation of phenol with methanol, because o-cresol and 2,6-xylenol formed by the reaction are used as important intermediates for the production of herbicides, polyphenylene oxide (PPO), and special grade paints [1–5]. Many kinds of materials have been used in the reaction as catalysts, such as various zeolites, pure and mixed metal oxides, and hydrotalcites etc. [1–5]. Among these catalysts, oxide catalysts generally show better selectivity for ortho-alkylation phenols. It should especially be pointed out that spinel oxides mainly containing cations such as Co, Cr, Mn or Fe exhibited high activity and selectivity to ortho-alkylation for the phenol alkylation [6–11]. Spinels have a general formula AB2O4, where A- and B-cations occupy the tetrahedral and octahedral sites, respectively. Previously, the effect of surface acid–base properties of the spinel catalysts on catalytic performance has been investigated by several research groups [9–11]. It was proposed that the appropriate acidity on the surface of the spinel, which depends strongly on the nature of cations, the charge, and the distribution in tetrahedral/ octahedral sites, plays a dominating role for the phenol methylation. Spinel-type cobalt chromites as red–ox behavior have exhib* Corresponding authors. Tel.: +86 431 88499140; fax: +86 431 88499232. E-mail address: [email protected] (W. Zhang). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.03.049
ited special catalytic activity for various catalytic reactions, such as water gas shift reaction, combustion catalysis and the total oxidation of halogenated hydrocarbons [12–16]. Our earlier work has revealed that spinel-type cobalt chromite catalysts also show high activity for ortho-methylation of phenol [11]. Although it is important that a catalyst keeps its activity and selectivity for the longest time possible, the deactivation of the catalyst is still a problem. The catalyst deactivation for the alkylation of phenol and methanol has been reported by several papers [5– 7,17,18]. Coking is the main reason for zeolite catalyst deactivation [5,17]. It is well known that the mixed oxides are usually used to prevent coke formation to obtain better on-stream stability during many industrial processes. The reduction has usually been regarded as the main cause of the deactivation of these oxide catalysts [6,7,18]. To obtain catalysts with high activity and stability, our previous works have mainly focused on the correlation of the acidity with catalytic performance over spinel-type cobalt chromite catalysts [11]. Herein, the catalytic performance of the spinel-type cobalt chromite (Co/Cr = 0.8) for the ortho-alkylation of phenol with time on stream was investigated. The catalyst structure and the red-ox behavior change have been particularly studied by the methods of H2-TPR, XRD, and XPS. 2. Experimental 2.1. Catalyst preparation Spinel-type cobalt chromite were prepared by co-precipitation technique following a reference procedure [12]. Typically, a diluted
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Y. Wang et al. / Catalysis Communications 9 (2008) 2044–2047
aqueous ammonia (10 wt.%) solution was added to a mixed aqueous solution of chromium nitrate and cobalt nitrate with continuous stirring at room temperature until the pH value was 9.3. The precipitates were filtered, washed several times and dried in an air oven. Finally, the dried materials were calcined at 723 K for 5 h. The available powders were ready for the alkylation activity measurements after being crushed and sieved. 2.2. Characterization and catalyst test XRD patterns were recorded on a Lab XRD-6000 X-ray Diffractometer with nickel-filtered Cu Ka radiation operating at 40 kV and 30 mA in a 2h range of 10–70°. Temperature-programmed reduction (TPR) was carried out in a quartz reactor with 5% H2/Ar (molar ratio) reduction gas of flow rate of 30 ml/min. Catalyst (20 mg) was loaded in the quartz reactor. The temperature was raised from ambient temperature to 973 K at rate of 10 K/min. The hydrogen consumption was monitored by using an on-line thermal conductivity detector (TCD). X-ray photoelectron spectra (XPS) were recorded on a VG ESCA LAB MK-II X-ray electron spectrometer using Al Ka radiation (1486.6 eV, 10.1 kV). The spectra were referenced with respect to the C 1s line at 284.7 eV. The measurement error of the spectra was ±0.2 eV. The vapor phase alkylation of phenol with methanol was performed in a fixed bed continuous down-flow reactor at atmospheric pressure. Fresh catalyst (0.8 g) with the particle size from 40 to 60 mesh was charged each time. Before carrying out the reaction, the fresh catalyst was activated at 693 K for 1 h in nitrogen in a vertical glass-tube reactor. A pre-mixed phenol–methanol mixture was then fed from the top of the reactor using a SY-04 syringe pump along with N2 gas (3.2 ml/min). The reaction conditions are list as follows: phenol/methanol = 1/5 (mole ratio), reaction temperature = 693 K, weight hourly space velocity (WHSV) = 0.897 h1. The reaction products were analyzed by a gas chromatograph equipped with a HP–5 capillary column and identified with GC–MS. 3. Results and discussion
100
phenol conv.
90
o-cresol selec.
100
Conversion/Selectivity (%)
Conversion/Selectivity (%)
Catalytic performance of several representative cobalt chromite catalysts for the alkylation of phenol with methanol is presented in Fig. 1. It can be seen that all the catalysts show high activity and high selectivity to o-cresol and 2,6-xylenol as the major ortho
2,6-xylenol selec.
80
products. The catalyst with Co/Cr = 0.8 exhibits the highest catalytic conversion as 97.3%. Fig. 2 displays the catalytic results of the catalyst (Co/Cr = 0.8) with time on stream over a period of 35 h. As shown, the conversion of phenol is almost unchanged during the first 15 h. With further increase of the reaction time, the conversion of phenol decreases slightly, and reaches 92.5% at the reaction time of 35 h. Additionally, the selectivity to o-cresol is low at the beginning, and almost keeps unchanged form 3 to 10 h, then increases gradually after 10 h. On the contrary, the selectivity to 2,6-xylenol is high during the first 10 h, then decreases gradually after 10 h. The total ortho-selectivity is kept about 93%, which reveals that 2,6-xylenol is yielded at expense of o-cresol. On the whole, there is only a slight decrease in the catalytic activity. The results indicate spinel-type cobalt chromite catalysts exhibit good stability. The XRD patterns of various fresh cobalt chromite catalysts are displayed in Fig. 3a. All the fresh catalysts with different Co/Cr ratios show a characteristic diffraction assigned to the spinel phase. For the Cr rich catalysts (Co/Cr = 0.2, 0.5), the additional diffraction peaks attributed to Cr2O3 phase can also be observed. With the increase of Co contents (Co/Cr > 0.5), the peak intensity of spinel phase increases. This may be attributed to the formation of a solid 3þ solution of CoCr2O4 and Co3O4, which is like Co2þ Co3þ x Cr2x O4 solid solution [19]. For Co/Cr = 2.5 sample, the diffraction peaks shift to higher degree, which may be due to the cation distribution difference in the octahedral sites. In the case of normal spinel-type cobalt chromite sample, divalent Co2+ and trivalent Cr3+ cations occupy the tetrahedral and octahedral sites of the spinel lattice, respectively. Trivalent Co3+ and Cr3+ cations simultaneously occupy the octahedral sites of the spinel lattice when cobalt is excessive, and there are some difference in the ionic radius of Co3+ and Cr3+ [19]. For the spent samples (Fig. 3b), no significant change in the XRD pattern is observed for the spent Co/Cr = 0.2, 0.5, 0.8 catalysts compared to the fresh catalysts. However, for the spent Co/Cr = 2.5 catalyst, the CoO phase is obvious besides the spinel phase. The binding energy (BE), the line shapes, and the intensity of the satellite peaks of the XPS results have often been used for identification of the cobalt species. Though the BE value of CoO is close to that of Co3O4, the former is usually higher than the latter. In addition, Co(II) oxide has a strong shake-up satellite about 6 eV above oxide of Co 2p3/2 while Co(III) oxide usually does not [20–22]. The Co 2p core level photoelectron spectra of the fresh and spent Co/Cr = 0.8 cobalt chromite catalysts are depicted in Fig. 4a. Seen
70 60 50 40 30 20 10
90
phenol conv. o-cresol selec. 2,6-xylenol selec.
80 70 60 50 40 30 20 10
0.5
1.0
1.5
2.0
2.5
Co/Cr (mol ratio) Fig. 1. Catalytic performance of cobalt chromite catalysts with different Co/Cr ratios. (Reaction conditions: phenol/methanol = 1/5, WHSV = 0.897 h1, T = 693 K, N2 flow rate = 3.2 ml/min, reaction time = 4 h).
0
5
10
15
20
25
30
35
Time on stream (h) Fig. 2. Stability of Co/Cr = 0.8 cobalt chromite catalyst for ortho-selective alkylation of phenol with methanol. (Reaction conditions: phenol/methanol = 1/5, WHSV = 0.897 h1, T = 693 K, N2 flow rate = 3.2 ml/min).
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Co 2p3/2
a
a Co 2p1/2
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Intensity (a. u.)
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spent
2.5
765 770 775 780 785 790 795 800 805 810 815 20
30
40
50
60
70
Binding energy (eV)
2 theta (degree)
Cr 2p3/2
0.5 0.8
Intensity (a. u.)
Intensity (a. u.)
0.2
b Cr 2p1/2
580.3
b
fresh
spent 2.5
20
30
40
50
60
70
2 theta (degree)
565
570
575
580
585
590
595
600
605
Binding energy (eV)
Fig. 3. Powder XRD patterns for fresh (a) and spent (b) cobalt chromite catalysts with different Co/Cr ratios calcined at 723 K, Characteristic diffraction peaks of spinel phase (.), CoO (j) and Cr2O3 (d) are indicated.
Fig. 4. Co 2p (a) and Cr 2p (b) core level photoelectron spectra of fresh and spent Co/Cr = 0.8 cobalt chromite catalysts.
from the figure, the Co 2p3/2 peaks of fresh and spent Co/Cr = 0.8 catalysts appear at 781.4 eV and 782.2 eV, respectively. The Co 2p3/2 peak of the spent catalyst shifts to higher bonding energy in comparison with that of the fresh catalyst, and the intensity of the satellite for the spent catalyst is much stronger than that of the fresh catalyst, which can be attributed to an increase of Co2+ [23,24]. The results show that some of Co3+ ions have been converted to Co2+ ions during the reaction. Cr 2P photoelectron spectra (Fig. 4b) show the Cr 2p3/2 peak of the fresh cobalt chromite catalyst is resolved into a peak about 577.4 eV and a shoulder peak about 580.3 eV. The peaks are assigned to Cr3+ and Cr6+, respectively [25]. The BE of the Cr 2p3/2 peak for the spent catalyst doesn’t show any change compared to the fresh catalyst. However, the shoulder perk disappears in the spent catalyst. The result reveals that Cr6+ ions are turned into Cr3+ ions in the course of the reaction. Moreover, the peak intensity for the spent catalyst is lower than that for the fresh one, which may be due to the formation of polyalkylated compounds or deposited coke on the surface of the catalyst during the phenol methylation. H2-TPR profiles of fresh cobalt chromite catalysts and Co3O4 are shown in Fig 5a. For the Co/Cr = 0.2 catalyst, the peaks at about 513 K and 638 K can be attributed to the reduction of Cr6+ to Cr3+ and the chemisorption of hydrogen, respectively [14,16,26]. The intensity of the peak at 638 K decreases with the increase of Co/
Cr ratio. The peak at around 923 K can be assigned to the reduction of Co2+ of CoCr2O4, and shifts to lower temperature with the increase of Co contents. In the case of Co3O4, the two peaks at 615 K and 669 K correspond to the reduction of Co3+ and Co2+. For the Co/Cr = 2.5 catalyst, the peak at about 644 K corresponds to the reduction of Co3+, and the peak at about 818 K can be attributed to the overlap of the two reduction peaks of Co2+ of CoCr2O4 and Co3O4. Fig. 5b shows TPR profiles of spent catalysts. The peak around 923 K is ascribed to the reduction of Co2+ of CoCr2O4. The XRD indicates that the CoO phase is present for the Co/Cr = 2.5 catalyst. So the other peak can be the reduction of Co2+ of CoO. The methanol to phenol ratio applied over various spinel-type cobalt chromite catalysts is 5, which is far exceeding the stoichiometric value of 2. As a result, the superfluous methanol may decompose to CO and H2 [6,7]. Hence the reduction gases are definitely capable of reducing the fresh catalysts. The reduction processes including the complete reduction of Cr6+ to Cr3+, Co3+ to Co2+ and partial reduction of Co2+ may occur on the surface of catalysts, which has been identified by the XRD, TPR and XPS results. Reduction may firstly proceed as complete reduction of Cr6+ to Cr3+, because the reduction peak of Cr6+ is not observed in the TPR spectra of the spent catalysts, and Cr6+ is more unstable than Co3+, which could result in the prompt increase of the selectivity to o-cresol and prompt decrease of the selectivity to 2,6-xylenol
Y. Wang et al. / Catalysis Communications 9 (2008) 2044–2047
Intensity (a. u.)
a
2047
4. Conclusions
2.5
In summary, the spinel-type cobalt chromite catalysts exhibit good catalytic performance for the gas phase catalytic ortho-alkylation of phenol with methanol. By combination with the XRD, TPR, XPS measurements and the reaction results, it is proposed that a slight decrease in the catalytic activity of cobalt chromite catalysts could be mainly ascribed to complete reduction of Cr6+ to Cr3+, Co3+ to Co2+ and partial reduction of Co2+ during the reaction process.
0.8
Acknowledgment
Co3O4
0.5 Financial support from the foundation of Harbin Engineering University (Grant number HEUFT07053) is greatly acknowledged.
0.2
373
473
573
673
773
873
973
Temperature (K)
Intensity (a. u.)
b
2.5 0.8 0.5 0.2 373
473
573
673
773
873
973
Temperature (K) Fig. 5. H2-TPR spectra of fresh (a) and spent (b) cobalt chromite catalysts with different Co/Cr ratios.
at the beginning of reaction. With further increase of reaction time, reduction may mainly happened as the complete reduction of Co3+ to Co2+ and partial reduction of Co2+, because the reduction peak of Co3+ is not found in the TPR spectra of the spent catalysts, and the reduction peak of Co2+ is over the reaction temperature (693 K), which may lead to gradual increase of the selectivity to o-crosel and gradual decrease of the selectivity to 2,6-xylenol.
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