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Effects of Precursors on Preparation of Pd/r-alumina Catalyst for Synthesis of Dimethyl Oxalate Qian Lin,† Yang Ji,† Zhi-Dong Jiang,‡ and Wen-De Xiao*,† UNILAB, State Key Laboratory of Chemical Reaction Engineering, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China, and School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China
The influence of two palladium precursors, PdCl2 and Pd(NO3)2, on the preparation of catalyst applied in the CO coupling reaction to synthesize dimethyl oxalate was studied. It is revealed that PdCl2 is much more difficult to decompose than Pd(NO3)2 during calcination, and for catalyst prepared from PdCl2, temperatures as high as 773 K are necessary to obtain surface Pd0, while for catalyst prepared from Pd(NO3)2, the corresponding temperature is 473 K. Furthermore, two palladium distributions across the pellet supports are illustrated. The eggshell palladium distribution resulting from using Pd(NO3)2 as precursor was proven more suitable for the present coupling reaction than the uniform palladium distribution obtained by using PdCl2. The results suggest that Pd(NO3)2 is superior to PdCl2 in preparing catalysts for the CO coupling reaction, and the active components on the support outer face have a more efficient utilization than that on the support inner face, which makes moderate reduction of Pd loading feasible. 1. Introduction The CO coupling reaction to synthesize dimethyl oxalate (DMO)
2CO + 2CH3ONO f (COOCH3)2 + 2NO is considered the most valuable C1-chemistry process to be developed for the production of oxalic acid or ethylene glycol with moderate reaction conditions and low consumption of energy. Various catalysts were investigated for this reaction, where R-alumina was chosen as a suitable support. Although the exact reason for this option is still under discussion,1-3 the catalyst of palladium supported on R-alumina was really proven to be an effective one with high activity and good selectivity.4-8 However, in almost all relative research, the Pd catalysts were mostly made from the precursor palladium chloride, and the best Pd loading was confirmed to be 1.0 wt %. As the selection of the precursor is the first step in catalyst preparation, the succeeding steps such as drying, calcination, and reduction are decided accordingly, which together determine the final state of the active components on the support.9-13 For different metal precursors, the optimized preparation conditions may change and quite different catalysts are made. In this paper, the effects of the precursor on the preparation of Pd/R-alumina for the CO coupling reaction to synthesize dimethyl oxalate were studied. Two palladium precursors, palladium chloride and palladium nitrate, were investigated to clarify the differences in each individual catalyst preparation step, as well as the effect on the distribution and best loading amount of palladium on the R-alumina pellet support. 2. Experimental Section 2.1. Catalyst Preparation. R-Alumina supports from different sources were screened in the preliminary experiment, and * To whom correspondence should be addressed. Tel.: +86 21 64252814. Fax: +86 21 64252814. E-mail:
[email protected]. † East China University of Science and Technology. ‡ Shanghai Jiao Tong University.
Figure 1. Thermal decomposition temperature of palladium precursors (a) PdCl2 and (b) Pd(NO3)2.
the purest one (100%), which was from Jiangsu Jiangyan Chemical Additive with an average diameter of 4 mm and initial BET area of 16.25 m2/g, was chosen for the experiment. The support pellets were then impregnated by aqueous solutions of Pd precursors according to the incipient wetness technique for 15 min. Hydrochloric solution of PdCl2 and nitric solutions of Pd(NO3)2 were used. These solutions were prepared by solving certain amounts of solid PdCl2 and Pd(NO3)2 as needed in hydrochloric and nitric acids with pH 1.33, the value at which both solid PdCl2 and Pd(NO3)2 could be totally dissolved and was regulated by a digital pH meter (Suntex TS-1). After drying in an oven at 393 K for 12 h, all samples were calcined in air at prescribed temperatures for 4 h. Before reaction, the samples were in situ reduced at temperatures between 473 and 773 K in a flow of 25% H2/N2 at the rate of 100 mL STD/min for 4 h. 2.2. Catalyst Nomenclature. Two letters followed by numbers name all catalysts in this investigation. The first letter is “P”, which refers to the active metal palladium used to prepare all the catalysts. The second letter is either “C” or “N”, which reflects the precursor used (“C” for the chloride precursor and
10.1021/ie070640b CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7951 Table 1. Effect of Calcination Temperature on Dispersion of Palladiuma sampleb
calcn temp (K)
D(Pd)c (%)
d(Pd)d (nm)
A(Pd)e (m2/g)
PC1 PC2 PC3
473 573 773
8.4 7.6 3.9
8.8 14.7 28.4
57.0 34.0 17.6
a The reduction temperature of the catalysts is 473 K. b The palladium loading of the samples is 0.3 wt %. c D, metal dispersion. d d, active particle diameter. e A, metallic surface area, m2/g sample.
Table 2. Effect of Reduction Temperature on Valence State and Dispersion of Palladium (D)a
a
sampleb
redn temp (K)
BE(Pd 3d5/2)c (eV)
assignment
D(Pd) (%)
PC2 PC4 PC5 PN1
473 623 773 473
337.6 335.9 335.2 335.1
Pd0-PdCl2 Pd0-PdCl2 Pd0 Pd0
7.6 12.7 2.6 4.6
The calcination temperature of the catalysts is 573 K. b The palladium loading of the samples is 0.3 wt %. c BE, binding energies.
“N” for the nitrate precursor). The numbers directly following the letters are denote catalysts made under different conditions, and the decimal fraction number following the dash next to the letters represents the percentage of Pd loading in the sample. For example, “PC-0.1” corresponds to the palladium chloride made catalyst with palladium loading of 0.1 wt %. 2.3. Catalyst Characterization. ICP (inductively coupled plasma) was used to determine the loadings of Pd in the various prepared catalysts. This analysis was performed on an IRIS ADVANTAGE/1000 apparatus. The thermal decomposition temperature of each palladium precursor was detected by thermogravimetric analysis (TGA). The catalyst samples (about 20 mg) were heated in air from room temperature to 1473 K at a programmed ramp of 10 K/min. Metal dispersions were determined by carbon monoxide chemisorption with a Micromeritics ASAP-2010C apparatus. Catalyst samples (0.2-0.5 g) were placed in a flow-through cell for pretreatment (H2, at 673 K, 3 h) and then outgassed in a vacuum, and subsequent carbon monoxide chemisorption measurements were performed at room temperature. Temperature program reduction (TPR) was carried out on a Micromeritics AutoChem (II). The catalyst samples (about 0.3 g) were heated in 10% H2/Ar from room temperature to 1073 K at a programmed ramp of 10 K/min. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg KR radiation (hν ) 1253.6 eV). Binding energies were calibrated by using the containment carbon (C 1s ) 284.6 eV).14 Pictures of catalyst cross sections were taken by a Microscope XTZ-D (Shanghai Optical Instrument Factory No. 6) and were analyzed by the UTHSCSA Image Tool. The palladium distribution within the support pellet was measured by a scanning electron microprobe (EMP). The plastic mounted pellets were sectioned equatorially, and the cross sections were scanned by the EMP analyzer. 2.4. Measurement of Catalytic Activity. The catalytic activity was measured in a fixed-bed continuous flow reactor. A catalyst bed of 1.5 g was sandwiched with quartz sands in a quartz tube reactor with an inner diameter of 13 mm. After reduction by 25 vol % H2 diluted by nitrogen at 100 mL STD/ min, the catalyst bed was fed at 393 K with nitrogen diluted reactant gas (CH3ONO, 10 vol %; CO, 20 vol %; total flow rate, 100 mL STD/min). The feeding and outlet gases were sampled and analyzed in an on-line gas chromatograph (TECHCOMP 7890) equipped with a thermal conductivity detector.
Figure 2. TPR profiles of two kinds of Pd/R-Al2O3 made from different palladium precursors (a) catalyst PC and (b) catalyst PN. The calcination temperature of the catalysts is at 573 K, and the palladium loading of samples is 0.3 wt %.
3. Results and Discussion 3.1. Calcination Temperature. To find the optimal calcination temperature, the thermal decomposition of precursor salts Pd(NO3)2 and PdCl2 (though H2PdCl4 is formed during impregnation, after drying at 393 K the remaining palladium compound is thought to be PdCl2) was measured by TGA in air. The calculation of the curves in Figure 1 indicates that palladium nitrate totally decomposes into palladium oxide at 573 K, while for palladium chloride the decomposition begins above 773 K and proceeds directly to Pd. Though a high calcination temperature is required for the complete decomposition of PdCl2, calcination at too-high temperatures can lead to severe sintering. The Pd surface area measured by CO chemical adsorption (A in Table 1) reveals the extent of sintering with the increase of calcination temperature in preparation of catalyst PC. Due to the severe sintering, a sharp drop of palladium dispersion (D in Table 1) occurred when the calcination temperature was raised from 573 to 773 K. To avoid serious sintering, 573 K is adopted as the calcination temperature for both series of catalysts. It must be noted here that this temperature is high enough for Pd(NO3)2 to decompose completely in catalyst PN, but not for PdCl2 in catalyst PC. In the reduction of catalyst PC, the decomposition of PdCl2 will lead to the release of a highly erosive HCl and moisture mixture, which requires additional precautions, especially in industrial reactors.
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Figure 3. Microscope pictures of palladium distribution in catalyst PN with different palladium loadings: (a) 0.1, (b) 0.5, and (c) 1.0 wt %.
3.2. Reduction Temperature. It was found by TPR, as Figure 2 shows, that the catalyst PC and catalyst PN can be completely reduced at 623 and 473 K, respectively, temperatures that are a little higher than those reported by L’Argentiere et al.15 and Noronha.16 The PC catalyst showed a small peak at 373 K and a broad one between 500 and 700 K. On the other hand, the TPR profile of the PN catalyst was totally different. In this case, hydrogen consumption at room temperature was observed and complete reduction was at 473 K. In fact, the Pd oxychloride species formed during calcination in air, the existence of which was confirmed by Noronha16 and Rakai,17 are more strongly linked to the support than the PdO species, which explains the reduction at higher temperature. The negative peaks observed on both catalysts have been reported as the desorption of weakly adsorbed hydrogen from the palladium surface and the decomposition of the palladium hydride formed at room temperature.18-20 The extent of reduction and dispersion of palladium measured by CO chemisorption and XPS, respectively, are given in Table 2. The obtained Pd 3d5/2 BE values for Pd0 and PdCl2 were in good agreement with data measured by other authors14,21 and were used to identify the chemical states of Pd on catalyst samples. For Pd0 the peak position of Pd 3d5/2 is 335.1 eV, a value that is normally used as the probe for Pd0, and for PdCl2 the peak position of Pd 3d5/2 is 338.0 eV. For catalyst PC2, the Pd 3d5/2 peak position is at 337.6 eV; therefore, the reduction at 473 K is incomplete. For catalyst PC4, the Pd 3d5/2 peak position at 335.9 eV indicated the presence of PdCl2, which conflicts with the result of TPR. However, this disagreement is not surprising, since TPR gives the overall reduction state of the bulk phase, while XPS is more accurate but only detects the reduction state of the surface atoms. For catalyst PC5, reduction of PdCl2 is complete at 773 K, as indicated by its Pd 3d5/2 peak position of 335.1 eV. Different from calcination, the palladium dispersion values given in Table 2 indicate that reduction at moderate temperatures is in favor of palladium dispersion, but too-high temperature tends to induce sintering of metal particles. Hence, it suggests that the shift of the Pd 3d5/2 peak position of all the catalyst PC is due to the progressive reduction of Pd, which is almost complete at 773 K. Similar results were found by L’Argentiere et al.15 and Bozon-Verdiraz et al.22 As for catalyst PN1, the Pd 3d5/2 peak position indicates that Pd in PN1 is completely reduced at 473 K. However, it could be seen that, under the same preparation conditions, the dispersion of catalyst PC is higher than that of catalyst PN, which was also found by Panpranot et al.23 on silica-supported Pd catalyst. Although the dispersion of palladium in both catalyst PN and catalyst PC is relatively low, the magnitude is the same as that reported by Zhao et al.,8 by whom catalysts were made with palladium chloride as the precursor in the same condition.
Figure 4. Normalized radial EMP palladium profiles in 4 mm diameter catalyst PN pellets with different palladium loadings: (a) 0.1 and (b) 1.0 wt %.
3.3. Palladium Distribution across the Pellet. In industry, catalyst is normally formed as a pellet to reduce the pressure drop across the bed. The performance of catalytic reactions can be greatly influenced by the distribution of active components within the pellet, especially for fast reactions. For supported catalysts, the optimized distribution of the active phase will raise the utilization efficiency of metal and reduce its loadings. The distribution of palladium in series of catalyst PN and catalyst PC was studied by microscopy and scanning electron microprobe as shown by Figures 3-6. All catalysts experienced pretreatment of calcinations under 573 K and reduction under 473 K. In catalyst PN, palladium is generally eggshell distributed, and the depth increases with the palladium loadings as indicated
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Figure 5. Microscope pictures of palladium distribution in catalyst PC with different palladium loadings: (a) 0.1, (b) 0.5, and (c) 1.0 wt %. Table 3. Palladium Concentration in Ex-Nitrate Pd/r-Alumina Catalystsa metal content (g/g R-alumina)
a
sample
solution introduced
measured by ICP
eggshell thickness (mm)
metal concn in eggshell (wt %)
PN-0.1 PN-0.3 PN-0.5 PN-0.7 PN-1.0
0.10 0.30 0.50 0.70 1.00
0.10 0.27 0.48 0.72 0.95
0.50 0.93 1.17 1.32 1.34
0.17 0.32 0.52 0.75 0.98
The calcination and reduction temperatures of the catalysts are 573 and 473 K, respectively.
Figure 6. Normalized radial EMP palladium profiles in 4 mm diameter catalyst PC pellets with different palladium loadings: (a) 0.1 and (b) 1.0 wt %.
in EMP pictures (Figure 4). The thickness of palladium shell measured by a special staff gauge is around 0.50 and 1.34 mm for PN-0.1 and PN-1.0, respectively, in Figure 3a and Figure 3c. The Pd concentration inside the shell layer was calculated and is listed in Table 3; the value is higher than the average
concentration measured by ICP, and the thinner the shell is, the larger the difference. As for catalyst PC, a tree-ring-type distribution can be seen in Figures 5 and 6. It seems that the tree ring tends to be thicker and to shift to center with the increase of Pd loading. The EMP pictures (Figure 6) show that the surface palladium increased to a high level in catalyst PC at 1 wt % palladium loading. The difference of Pd distribution in these two catalyst series can be explained by the interaction of the precursor with the support. The isoelectric point (IEP) of R-alumina is 7.0-9.0; in the acidic impregnation solution (pH 1.33), the support tends to adsorb compensating anions such as PdCl42-, which is formed as PdCl2 solved in chloric acid, while the excessive chloride anions compete with the palladium ion for adsorption sites, in which way a uniform catalyst is made. Work done by Maatman24 also showed that a uniform profile could be obtained for platinum deposition from hexachloroplatinic acid on an alumina support by adding simple inorganic acids such as HCl and HNO3. On the contrary, it is not beneficial for the adsorption of cations like Pd(H2O)42+, which is formed as Pd(NO3)2 is solved in diluted nitric acid. Because the positively charged support surface dispels Pd(H2O)42+ species and palladium only precipitates on the outer surface of support during drying, the eggshell palladium distribution is formed. Besides, the tree-ring distribution reflects the tight-loose alternating layers formed during the support pelletizing. 3.4. The Best Palladium Loadings. Almost all research concerning this coupling reaction used palladium as catalyst, and took palladium chloride as precursor.1-8 The optimal palladium loading researchers confirmed is 1.0 wt %, which is in accordance with our catalyst PC in Figure. 7. The new finding in our research is for catalyst PN, which has taken palladium nitrate as the precursor; the optimal Pd loading is 0.1 wt %. Therefore, it reduced the Pd loading by about 10 times. This benefit comes from the eggshell distribution of Pd in catalyst PN, which suggests the CO coupling reaction is a fast reaction. In this case, the eggshell catalyst distribution has shortened the diffusion path of the molecules and raised the utilization efficiency of the catalyst phase on the support.
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Figure 7. Influence of palladium loadings on catalytic behavior in catalyst PC and catalyst PN.
Combining the above catalyst characterization with the catalytic activity measurement, it seems that only the outer layer palladium is effective for the CO coupling reaction. Therefore, it is very interesting to make a thinner and more concentrated eggshell catalyst and thus to even further reduce the palladium loading to less than 0.1 wt %. 4. Conclusions Based on the results reported in this paper, it is concluded that the preparation conditions and the catalytic behavior of the supported Pd catalysts for the CO oxidative coupling to dimethyl oxalate is strongly affected by the selection of palladium precursors. The advantages of taking Pd(NO3)2 over PdCl2 as the Pd precursor are obvious. First of all, the temperature of calcination and reduction is much lower, which means severe sintering of Pd particles can be avoided. Furthermore, the reduction of catalyst in the reactor is practical, and the release of HCl is avoided. Last, but not least, an optimized eggshell distribution of Pd in support can be realized, which can greatly reduce the Pd loading, from 1 to 0.1 wt %. These conclusions are very important for the development of industrial catalysts. Acknowledgment The authors are indebted to X. G. Zhao for supporting the research. The analytical contributions of Dr. W. L. Dai, W. J. Qian, and Dr. W. Wu are gratefully acknowledged. Alumina supports were kindly supplied by Jiangsu Jiangyan Chemical Additive, Inc., People’s Republic of China. Literature Cited (1) Song, R. J. Effect of support pore structure on Pd-catalyst activity and selectivity. J. Nat. Gas Chem. 1992, 2, 146-152. (2) Jiang, X. Z.; Su, Y. H.; Lee, B. J.; Chien, Sh. H. A study on the synthesis of diethyl oxalate over Pd/R-Al2O3 catalysts. Appl. Catal., A: Gen. 2001, 211, 47-51. (3) Zhao, X. G.; Lv, X. L.; Zhao, H. G.; Xiao, W. D. Study on Pd/RAl2O3 catalyst for vapor-phase coupling reaction of CO with CH3ONO to (CH3OOC)2. Chin. J. Catal. 2004, 25, 125-128.
(4) Tahara, S.; Fujii, K.; Nishihira, K.; Matsuda, M.; Mizutare, K. Process for continuously preparing a diester of oxalic acid. EP 46,598 A1, 1982. (5) Uchiumi, S.; Ataka, K.; Matsuzaki, T. Oxidative reactions by a palladium-alkyl nitrite system. J. Organomet. Chem. 1999, 576, 279289. (6) Chen, G. S.; Chen, Y. D. Synthesis of oxalic acid via catalytic reaction of carbon monoxide with nitrite esters. CN 85,101,616 A, 1986. (7) Meng, F. D.; Xu, G. H.; Guo, Q. R. Kinetics of the catalytic coupling reaction of carbon monoxide to diethyl oxalate over Pd-Fe/R-Al2O3 catalyst. J. Mol. Catal. A: Chem. 2003, 201, 283-288. (8) Zhao, X. G.; Lin, Q.; Xiao, W. D. Characterization of Pd-CeO2/ R-alumina catalyst for synthesis of dimethyl oxalate. Appl. Catal., A: Gen. 2005, 284, 253-257. (9) Scire`, S.; Minico`, S.; Crisafulli, C. Selective hydrogenation of phenol to cyclohexanone over supported Pd and Pd-Ca catalysts: an investigation on the influence of different supports and Pd precursors. Appl. Catal., A: Gen. 2002, 235, 21-31. (10) Toebes, M. L.; van Dillen, J. A.; de Jong, K. P. Synthesis of supported palladium catalysts. J. Mol. Catal. A: Chem. 2001, 173, 75-98. (11) Gurrath, M.; Kuretzky, T.; Boehm, H. P.; Okhlopkova, L. B.; Lisitsyn, A. S.; Likholobov, V. A. Palladium catalysts on activated carbon supports: Influence of reduction temperature, origin of the support and pretreatments of the carbon surface. Carbon 2000, 38, 1241-1255. (12) Sakauchi, J.; Sakagami, H.; Takahashi, N.; Matsuda, T.; Imizu, Y. Comparison of dinitrodiamminepalladium with palladium nitrate as a precursor for Pd/SiO2 with respect to catalytic behavior for ethane hydroformylation and carbon monoxide hydrogenation. Catal. Lett. 2005, 99, 257-261. (13) Basova, Y. V.; Edie, D. D.; Badheka, P. Y.; Bellam, H. C. The effect of precursor chemistry and preparation conditions on the formation of pore structure in metal-containing carbon fibers. Carbon 2005, 43, 15331545. (14) Wagner, C. D.; Riggs, W. N.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Muileuberg, G. E., Ed.; PerkinElmer: Waltham, MA, 1978. (15) L’Argentiere, P. C.; Figoli, N. S. Effect of reduction temperature and support on the surface electronic state of supported Pd catalysts. React. Kinet. Catal. Lett. 1991, 43, 413-417. (16) Noronha, F. B.; Baldanza, M. A. S.; Schmal, M. CO and NO adsorption on alumina-Pd-Mo catalysts: effect of the precursor salts. J. Catal. 1999, 188, 270-280. (17) Rakai, A.; Tessier, D.; Bozon-Verduraz, F. Palladium-alumina catalysts: a diffuse reflectance study. New J. Chem. 1992, 16, 869-875. (18) Noronha, F. B.; Primet, M.; Frety, R.; Schmal, M. Characterization of palladium-copper bimetallic catalysts supported on silica and niobia. Appl. Catal., A: Gen. 1991, 78, 125-139. (19) Chen, G.; Chou, W. T.; Yeh, C. T. The sorption of hydrogen on palladium in a flow system. Appl. Catal., A: Gen. 1983, 8, 389-397. (20) Chang, T. C.; Chen, J. J.; Yeh, C. T. Temperature-programmed reduction and temperature-resolved sorption studies of strong metal-support interaction in supported palladium catalysts. J. Catal. 1985, 96, 51-57. (21) He, F.; Gao, Z. H.; Song, Y.; Xu, G. H. XPS study of Pd-Fe/Al2O3 catalyst for CO coupling reaction with hydrogen. Chin. J. Catal. 2002, 23, 223-226. (22) Bozon-Verduras, F.; Omar, A.; Escard, J.; Pontvienne, B. Chemical state and reactivity of supported palladium: I. Characterization by XPS and uv-visible spectroscopy. J. Catal. 1978, 53, 126-134. (23) Panpranot, J.; Tangjitwattakorn, O.; Praserthdam, P.; Goodwin, J. G., Jr. Effects of Pd precursors on the catalytic activity and deactivation of silica-supported Pd catalysts in liquid phase hydrogenation. Appl. Catal., A: Gen. 2005, 292, 322-327. (24) Maatman, R. W. How to make a more effective platinum-alumina catalyst. Ind. Eng. Chem. 1959, 51, 913-914.
ReceiVed for reView May 7, 2007 ReVised manuscript receiVed August 8, 2007 Accepted August 29, 2007 IE070640B
