Ind. Eng. Chem. Res. 1995,34, 2379-2382
2379
Characteristics of Catalyst for Carbon Monoxide Coupling Reaction Gen-hui Xu,* Xinbin Ma, Fei He, and Hong-Fong Chen Department of Chemical Engineering, Tianjin University, Tianjin, 300072, People's Republic of China
The carbon monoxide coupling reaction on supported metal catalysts Pd/Al203 and Pd-Fe/Al,Os was used to study the characteristics of the catalyst under different conditions. The results indicated that the catalytic activation temperature influenced the catalytic activity and that there was an optimum activation temperature. The characteristics of effective coupling catalysts were found to be smaller surface areas and bigger pores, for example the a-phase of aluminum oxide. X-ray photoelectron spectroscopic analysis results show that the catalytic reaction of CO coupling occurs via a redox mechanism.
Introduction The use of carbon monoxide to produce organic compounds is an important research area for the future. In particular the carbon monoxide coupling reaction takes place a t moderate reaction conditions, with low consumption of energy, t o produce oxalic acid and oxalate. The chemicals are feedstocks for ethylene glycol synthesis by hydrogenation of oxalate, as well as for products such as pesticides, pharmaceuticals, foods t d s , polymers, and fine chemicals. The present work was carried out in a flow reactor, involving the study of the following overall carbon monoxide coupling reaction:
2CO
+ 2CH,OH + (1/2)0, - (COOCH,), + H,O
(1)
The above reaction consists of two steps, the first reaction is the production of the oxalic diester from carbon monoxide and methyl nitrite under the effect of the coupling catalyst.
2CO
+ 2 C H 3 0 N 0- (COOCH,), + 2 N 0
(2)
In the second step, nitrogen oxide obtained from the first reaction participates in a regeneration reaction with oxygen and methanol, forming methyl nitrite. The methyl nitrite produced is then recycled as feed t o the coupling reaction.
NO
+ CH,OH + (1/4)0, - CH,ONO + (1/2)H,O
(3)
The technology of gaseous methyl nitrate promoted CO coupling oxidation is very efficient. In comparison with other technologies this process has favorable economics, without environmental pollution. In recent years there have been a number of studies of the CO coupling oxidation reaction, published in a series of papers (Shiomo et al., 1989; Bartley, 19871, but details of the catalytic nature of the CO coupling have been limited. In this paper, the characteristics of the metal coupling catalyst for the CO coupling reaction are examined by means of the XPS, SEM, and BET analysis methods in order to better understand the mechanism of catalytic activation and deactivation of the surface, metal valence state.
* Author to whom correspondence should be addressed.
Table 1. Relation between Activation Temperature and Catalytic Activity (Reaction Condition: Coupling Temperature 90 "C, CO/C&ONO = 2/1) P ( s ) not activated
0.91 1.14 1.37
no reaction no reaction no reaction
STY (ghL-cat)y at temperatureb ("C) 200 250 300 350 400 50.1 57.5 69.6 79.8 43.5 37.2 44.5 53.4 59.8 29.8 25.3 30.5 37.2 39.9 16.2
Space time yield. Activation temperature of catalyst. Contact time of reaction.
Reaction Experiments The experimental apparatus has been described in previous papers (Li et al., 1991; Chen et al., 1993). The experimental processing took place in a recycle flow reaction system. The catalyst, 1% Pd/Al203 or 1% Pd, 0.5%Fe/&03, 3 x 3 mm cylinders or spheres, was prepared by impregnation in 20% (wt %) aqueous solution of PdCl2 or 5% (wt %) FeCly6H20. After impregnation for 12 h the carrier was dried at 120 "C for 12 h, reduced in hydrogen a t 200-400 "C for 8 h, and finally left to cool in H2 a t room temperature. Two type catalysts were also used. The reactor was a quartz glass cylinder with an inner diameter of 25 mm and a length of 450 mm, the reaction zone was filled with 5.6 mL catalyst pellets. Before the catalytic reaction experiments great attention was paid to eliminate mass-transfer effects and the catalytic efficiencies (7) were complete (7 = 1). The experiments were carried out on PdlAlzO3 or Pd-Fe/AlzO, catalyst under the steady-state condition of activity of catalyst. The experimental conditions were reaction temperature 363-393 K, residence time of reaction (z) 2-6 s, concentration of carbon monoxide 20%(mol %), concentration of methyl nitrite 15%(mol %), concentration of methanol 10-15% (mol %), and the residual component was inert gas. The products were analyzed by an SP3700 gas chromatograph. Analytical measurements of the catalyst were made using a PHI-MOD-5400 for X-ray photoelectron spectroscopy (XPS)and an electron microscope. The experiments showed that the catalytic activation (pretreatment) temperature directly affected the catalytic activity. Without pretreatment of the catalyst, the reaction shows no catalytic activity. In order to compare the reaction results under different activation temperatures of the palladium catalyst, the experimental data are listed in Table 1. The experimental results are shown in Figure 1.
0888-5885/95/2634-2379$09.00/0 1995 American Chemical Society
2380 Ind. Eng. Chem. Res., Vol. 34, No. 7,1995
eo 70
60
50
Em 30
20
10
0 0
50
100
150
150
100 Te~nporolwro (C)
Figure 1. Relation between activation temperature and STY.
The catalytic activation temperature's influence on the activity of catalyst can be seen to be quite sensitive, and shows a maximum catalytic activity at an activation temDerature of 350 "C. The results of online XPS anaiysis are shown in Figure 2A. The curve shown is the electron energy spectra of the metal element on the coupling catalyst before and after reaction. It was discovered that the palladium electron binding energy was 76.5 eV (i.e., zero valence) before the reaction and 77.0 eV after the reaction (corresponding to a 2+ valence), therefore the valence state of the palladium is altered from PdO t o Pd2+ in the catalytic process. This provides confirmation of a redox mechanism for the CO coupling reaction with methyl nitrite. We also studied bimetallic Pd-Fe/Al203 catalysts. The Pd spectra from XPS analysis of the bimetallic catalyst are similar t o the spectra of the PdAl203 catalyst. For the iron element the XPS results are shown in Figure 2B. Figure 2B shows that the electron binding energy of the iron element is changed from 336.0 eV before the reaction is carried out t o 338.8 eV after the reaction. In the catalytic coupling reaction, the iron is regarded as an electronic promoter where the iron valence changes from Fe2+ to Fe4+. Fe2+ ions can easily contribute electrons as a donor. Therefore it appears to effect valency state regulation of the active species. In addition, the experiments show that the activated catalyst produces the reduced state of the palladium. As the activation temperature is increased, the amount of reduced state increases and catalytic activity therefore increases. When the activation temperature reaches a sufficiently high temperature (the experiments show this to be about 350 "C), a continued increase in temperature decreases the catalytic activity due t o sintering of the metal particles on the surface of the catalyst. The result showed that the dispersity of the active species at 400 "C is poor; the dispersity at 300 "C in Figure 3A is better than the dispersity at 400 "C in Figure 3B.
Analysis of Catalyst Physical Structure The catalyst physical structure was examined by BET analysis, and the phase state of the catalyst support was
Table 2. Relations between Catalyst Physical Structure and Activity" phase state Y a
s, r
132.4 60
45
174.0
4.11 200
13.6 180
38.5 64.5 85.3
40.6 52.3 89.7
70.2 182.2 254.1
77.8 183.4 267.5
STY at t ("C) 90 100 120
S,, specific surface areas (M2/g);r, average aperture radius
(A); t, reaction temperature (TI;STY,catalytic activity (spacetime yield [g/(h.L-cat.)l).
measured by X-ray diffraction (XRD) spectral analysis. The analyses found that catalytic activity was related to the catalyst physical structure, such as specific surface area, pore radius, and the phase state of the support. The relations between catalyst physical structure and activity are shown in Table 2. The results indicated that the smaller surface area and larger pores of the a-phase aluminum oxide showed increased activity. It also demonstrated that the CO coupling reaction does not require a large surface area of catalyst, but requires larger pores in order to supply sufficient reactant molecules. In general the CO adsorptions were divided into line adsorption and bridged absorption; the former favorably benefits the carboncarbon bond formation for the CO coupling reaction, shown by in-situ IR spectra (Vannice et al., 1981).
Discussion of Reaction Mechanisms and Conclusion Studies of the reaction mechanism of the gas phase CO coupling reaction are limited; most previous studies of the mechanism for this reaction are for liquid phase homogeneous catalysis. An explanation of the reaction mechanism of gaseous CO coupling in the presence of methyl nitrite over palladium catalysts, by using the research of modern organometallic chemistry was to identify existence of carbomethoxypalladium complex (Rivetti and Romano,
Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995 2381 ESCAMILTXPLEX lU19/30 EL411 REG 1 IYYXE=l5deg KOTM=1.67nin FILL: T w 1 2 AI203/Pd 8 Fe before reactlon SC#E FIICTOR, OFFSET: 0.689, 2.668 k c/s PnSS ENERGY= 35.758 eV Ng 250 H
ESEll MllTIPLEX 12/19/90 ELspdI REG 3 M E = 45 deg KO TIK=1.67 nin FILE: TWL4 A120Md 8 Fa after reaction SCNE FIYTOR, OFFSET= 0.252, 11.840 k CISP&S ENERGY: 35.750 eV Ng 2513 II
B
f
a a
4‘
d ‘
10 9
a 7
1
6
SI
5
d
4
3 2
1
351.2
349.1
347.1
345.0
343.0 340.9 338.9 IIWING ENERGY, eV
336.0
334.8
332.7
330.6
Figure 2. Energy spectra analysis results by XPS: (A) Pd electron energy spectra on Pd/Al203 catalyst; (B)Fe electron energy spectra on Pd-Fe/AlzOs catalyst.
1978). Therefore we imagine that adsorbable CO molecule inserts in Pd(I1) alkoxyl compound t o form the carbomethoxypalladium complex (DCPC):
+
Pd2+(OMe), Pd(I1) alkoxyl compound 2co* -Pd2+(COOMe)2 (4) CO line adsorption DCPC where * is the active site. When palladium atom is in the line-adsorption state, the n-backbonding in the Pd atom is an obvious weakness as compared with the bridge-adsorption state. Consequently the carboncarbon bond formation is very favorable. So long as the space field of reaction was sufficient (without space resistance), the reduction-elimination reaction was
produced and formed the dimethyl oxalate: Pd2+(COOMe),-
DCPC Pdo -k (COOMe), (5) Pd atom dimethyl oxalate (product) Usually methyl nitrite is added as promoter into the coupling reaction to accelerate the reoxidation reaction for palladium catalysis function: Pdo Pd atom
+
-
2MeONO methyl nitrite Pd2+(OMe), 2 N 0 (6) Pd(I1) alkoxyl compound Formula 6 was the redox reaction carried out between
+
2382 Ind. Eng. Chem. Res., Vol. 34, No. 7,1995
Figure 3. SEM photographs of samples: (A) activation temperature at 300 “C, PdAl203; (B) activation temperature at 400 “C, PdAl203.
palladium and methyl nitrite. Methyl nitrite played a carrier of oxygen function in the redox reaction. Considering the commercial use of the coupling process above, reaction 6, production byproduct NO must be recycled to regenerate with oxygen and methanol again to produce methyl nitrite (see eq 3). From eqs 1-3 was shown that the catalytic characteristics of the CO coupling reaction are the simultaneous procedure of both coupling oxidation and regeneration reaction. The reaction mechanisms were carried out according to the matrix of “second order common catalysis recycle”:
3. The physical nature of coupling catalyst, especially specific surface, aperture, and phase state, were important factors to influence catalytic activity.
Acknowledgment This research was supported by the National Natural Science Foundation of the People’s Republic of China. This paper was paid great attention by Prof. Richard G. Mallinson and his leadership research groups in the Sarkeys Energy Center of the University of Oklahoma, the authors are very grateful. Literature Cited
Our experimental study results show the following: 1. Carbon monoxide coupling reaction over Pd/Al203 or Pd-Fe/AlzOs catalysts in the presence of methyl nitrite (as promoter) was an redox process. In the process of valence of palladium was varied from PdO to Pd2+and iron valence from Fe2+to Fe4+. Methyl nitrite as promoter plays a carrier of oxygen function. 2. The catalytic activation temperature directly affected catalytic activity; the optimum activation temperature was 350 “C. The activation temperature influence on the catalytic activity was more sensitive.
Bartley, William J.; et al. U.S.Patent, 4,677,234,1987. Chen Jing-Wen; Xu Gen-Hui; Li Zhen-hua; Chen Hong-fang. Kinetics of Regeneration Reaction of CO Coupling. J. Chem. Eng. (China) 1993,44,66-72. Li Zhen-Hua; Xu Gen-Hui; Chen Hong-fang. Preparation of Dimethyl Oxalate from CO by Vapor Phase Catalytic Coupling Reaction. Chem. Ind.Eng. (China) 1991,8,15-20. Rivetti, Franco; Romano, Ugo. Alkoxy Carbonyl Complexes of Palladium and their role in Alcohol Carbonylation. J. Orga“ e t . Chem. 1978,154,323-326. Shiomi, Yasu hui; Matsuzaki, Tokuo; Masunaga, Katsuro. U.S. Patent 4,874,888,1989. Vannice, Albert, M.; Wang, S.-Y.; Moon, S. H. The Effect of SMSI (Strong Metal-Support Interaction) Behavior on CO Adsorption and Hydrogenation on Pd catalysts. 1. IR Spectra of Adsorbed CO prior to and during Reaction Conditions. J . Catul. 1981, 71,152-166.
Received for review March 23, 1994 Revised manuscript received February 10,1995 Accepted March 21, 1995@ IE9401824 @Abstractpublished in Advance ACS Abstracts, May 1, 1995.
