I n d . Eng. Chem. Res. 1988,27, 2211-2213
2211
Vapor-Phase Carbonylation of Organic Compounds over Supported Transition-Metal Catalyst. 6. On the Character of Nickel/Active Carbon as Methanol Carbonylation Catalyst Kohji Omata,* Kaoru Fujimoto, Tsutomu Shikada, and Hiro-o Tominaga Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, J a p a n
The character of Ni/active carbon (A.C.) was investigated in comparison with Ni/A1,03 or Ni/Si02. The character of adsorbed CO corresponds to the catalytic activity: Ni/A.C., on which CO is adsorbed nondissociatively, is active for carbonylation but not active for CO hydrogenation, while Ni/A1203 and Ni/Si02, on which CO is adsorbed dissociatively, show poor activity for carbonylation and good activity on CO hydrogenation. Methyl iodide (MeI), which is essential for methanol carbonylation, was adsorbed on Ni a t 250 "C and was found to be a suitable probe to apprise the active site. Dispersion of Ni/A.C. calculated from the Me1 adsorption was 89.0% a t 1% nickel loading and 21.5% at 10% loading. Turnover frequency of the Ni/A.C. catalyst for methanol carbonylation was scarcely affected by the metal dispersion within the range mentioned above. The fact implies that the reaction is structure insensitive. It has been well-known that supported Ni is a highly selective catalyst for methanation of carbon monoxide. Recently, however, supported Ni clusters have been pointed out to have some strong interactions with support materials, and their catalytic properties are strongly influenced (Vannice, 1976). The authors have found that Ni supported on active carbon (A.C.) shows excellent activity for the carbonylations of methanol (MeOH), dimethyl ether (DME), and methyl acetate (AcOMe), yielding acetic acid (AcOH), methyl acetate, and acetic anhydride (Ac,~),respectively (Fujimotoet al., 1982). The most important feature of this catalyst is that the activity of carbonylation is outstanding only when Ni is supported on carbonaceous materials such as A.C. or carbon black. Thus, they are supposed to possess an unique interaction with Ni loading to the appearance of carbonylation activity. In this study, the characterization of supported Ni on a variety of supports was tried by methanol carbonylation, CO hydrogenation, adsorption measurements of H,,CO, and MeI, and also temperature-programmed reaction (TPR) of adsorbed CO.
Experimental Section Catalyst Preparation. Catalysts were prepared by an impregnation method from an aqueous solution of nickel acetate (Fujimoto et al., 1982). Carrier materials were active carbon (A.C.; Takeda Shirasagi C, charcoal base, 1200 m2/g), y-alumina (Tokaikonetu TKS99651, 160 m2/g), and silica gel (Fuji Davison ID, 270 m2/g), which are all commercially available. After the carriers were immersed in aqueous nickel acetate solution, they were placed in a vacuum desiccator for several hours and heated in a water bath t o remove water. Then the catalyst precursor was dried in an air oven at 120 "C for 1 2 h. It was reduced in flowing hydrogen at 400 "C for 2 h and then reduced again in situ at 400 OC for 1 h just before the reaction. The Ni loading was usually 2.5 wt 7'0 for each catalyst. Apparatus and Procedure. A continuous flow type reactor with a fixed catalyst bed was employed under pressurized conditions for the carbonylation of MeOH. Me1 was used as a promoter. The carbonylation reaction was performed under differential conditions as follows: temperature, 250 "C; total pressure, 10 atm; partial pressures of CO, MeOH, and MeI, 4.0, 2.0, and 0.6 atm, re0888-5885/88/2627-2211$01.50/0
spectively. Nitrogen was employed as a diluent. Under these conditions, the products were mostly AcOMe and DME. The rate of carbonylation, rM,was calculated as xM1
1
m=--100 2 W / F
(1)
where XM is the conversion of MeOH to AcOMe (%), F is feed rate of MeOH (mmol/h), and W is catalyst weight (g). The rate was measured under the conditions where XMis less than 10%. The products were analyzed by gas chromatography. The hydrogenation of CO was also performed in a fixed bed reactor. The reaction conditions were temperature, 400 "C; W / F ,0.3 (gh)/mol where F is the total feed rate of N2,H,, and CO; N2/H2/C0,30/4/1 molar ratio; and total pressure, 4 atm. The adsorption measurement was conducted by using a flow cell connected to a conventional volumetric vaccum line which could be evacuated up to lo* Torr (1.3 X atm). The amount of adsorbed gas was measured by a volumetric technique. The H2 used for adsorption or reduction of the sample was purified over a Deoxo unit (Engelhard Co.) and in a trap cooled at liquid nitrogen temperature. CO was purified by passing it through a column packed with Cr/Si02 granules and the cold trap. Me1 used for adsorption measurement was purified by a repeated freeze-pumpthaw cycle. The catalyst sample was reduced in flowing hydrogen at 450 "C for 2 h after evacuation at 150 "C for 0.5 h and then was evacuated at 480 "C for 1h. Adsorption isotherms of hydrogen were measured at 0 "C or at room temperature. The intercept of an adsorption isotherm was adopted as irreversible adsorption of hydrogen. After the sample was evacuated at 300 "C for 1 h, CO adsorption was measured. The second isotherm of CO adsorption was measured after the sample was evacuated for 0.5 h. The difference of the two isotherms at 80 Torr was adopted as the irreversible adsorption. The adsorption of Me1 was measured at 250 "C after the sample was reduced at 360 "C for 2 h and evacuated for 0.5 h at the same temperature. The irreversible adsorption was determined in the same way as that of the CO. The temperature-programmed reaction (TPR) was performed according to a similar procedure as that in the previous study (Fujimoto et al., 1980). The sample on which CO had been already adsorbed was heated in flowing hydrogen at a constant rate (400 "C/h), and the desorbed products such as CH,, CO, 0 1988 American Chemical Society
2212 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 Table 1. Activity of Ni Catalyst and catalyst Ni/A.C. carbonylation activitp AcOMe, (7c 53.1 rnethanat,ion activity* CHI, c7c 3.8 COB, 90 0.4 chemisorption, mmol/g 0.0067 H2uptake CO uptake 0.68C 1.59 CO/Ni CO/H 50.5
CO/HZ Adsorption Ni/A1,03
Ni/SiOp
0.9
0.1
43.0 4.3
78.3 18.3
0.0072 0.24 0.56 16.5
0.037 0.21 0.48 2.8
I
n
N1
IS102
'MeOH conversion to AcOH a t 250 "C, 11 atm, W / F = 5 ( g h)/mol, CO/MeOH/MeI = 100/19/1. *CO conversion at 400 "C, 4 atm. W / F = 0.3 (gh)/mol, N 2 / H 2 / C 0 = 30/4/1/. "0 uptake a t 80 Torr (0.11 atm). ~.
.~~
/ S I
2
100
200
300
400
Temperature ( ' C i
Figure 2. TPR spectra of adsorbed CO on Ni catalyst. Programmed rate 400 "C/h; 2.5 wt % Ni on carrier.
Pressure
Torr 1
Figure 1. CO uptake on Ni/A.C. and Ni/SiOz. (0) ( 0 )Ni/A.C., (A) (A)Ni/SiOz. ( 0 )(A)First isotherm, (0) (A)second isotherm; 2.5 wt 7'0 Ni on carrier.
and C 0 2 were determined with a gas chromatograph.
Results and Discussion Characterization of Adsorbed CO. The activities for MeOH carbonylation and CO hydrogenation were investigated on Ni catalysts supported on three kinds of supports, active carbon (A.C.), A1203,and SiOz. The two reactions con+,rastwith respect to the cleavage of the C-0 bond in the reaction step. The results are shown in Table I. Ni shows excellent activity for the carbonylation when it is supported on A.C. The order of the activity is as follows: Ni/A.C. >> Ni/Alz03> Ni/Si02. Apparently, A.C. exerts a special influence on the character of Ni. In contrast, Ni is active for CO hydrogenation on Alz03and Si02 The results of H2 adsorption and CO adsorption are also summarized in the table. The amount of chemisorbed H2 was less than that of CO for every sample, and thus the ratio of chemisorbed CO to that of H was higher than 1.0. Similar phenomena were reported by Kikuchi et al. (1982) on a Fe/A.C. catalyst and by Bartholomew et al. (1981) on a Ni/A1203. The latter group indicated that the small uptake of hydrogen on well-dispersed Ni/AlZO3is due to the strong metal-support interaction. Since the phenomena was most remarkable with the Ni/A.C. in this study, it is suggested that the metal-support interaction is most predominant on A.C. It should be also noted that the molar amount of chemisorbed CO is larger than that of Ni loaded on A.C. above 10 Torr (0.013 atm), which indicates that more than one molecule of CO is adsorbed on the surface Ni atom while at lower pressure the ratio is less than one. Figure 1 shows the adsorption isotherms of CO on Ni supported on A.C. and Si02. It is apparent that the amount of chemisorbed CO on Ni/A.C. depends strongly on CO pressure, while it is not the case for the other supports. This phenomenon suggests that the adsorption of CO on Ni/A.C. is multiple and that the CO-Ni bond is rather weak.
TPR of adsorbed CO has been known to be useful for characterizing the chemisorbed CO on supported metal catalysts (Fujimoto et al., 1980). The formation of CO2 and CH4 could be interpreted by co-+c+o (2) 0 + co -..+ coz (3) C + 4H --* CH4 (4) 0 + 2H HzO (5) Figure 2 shows the results of the TPR of CO which is adsorbed on Ni on A1203, Si02,and A.C., respectively. The reactivity of CO on Ni is markedly different for each support as indicated in the Table I. In the case of Ni/ A1203,CO is desorbed at temperatures lower than 100 "C and then C02 is formed and finally CH4 is formed. The dissociation of the C-0 bond should proceed easily on Ni/A1203 (Zagli, 1979). In the case of Ni/SiOp, the main product is CHI and the peak temperature of CHI desorption is lower than that on Ni/A1203,while the reverse is correct for CO desorption. In the case of Ni/A.C., however, almost all of the adsorbed CO was desorbed at temperatures below 120 "C, whereas little CH, was formed at any temperature. This means that the G O bond dissociation proceeds most easily on Ni/Si02 and then on Ni/A1203. On Ni/A.C., the adsorbed CO is scarcely dissociated during TPR but is desorbed as CO. Methane formation at high temperature is probably due to the hydrogenolysis of the A.C. used as the support (Egashira et al., 1982), which means the C in reaction 4 comes from A.C. but not from CO. This characteristic feature of supported nickel catalysts is supported by the fact that the catalytic activity of methanation is the descending order, Ni/Si02 > Ni/A120, > Ni/A.C., where the C-0 dissociation step is essential. It is concluded that active carbon exerts special effects on nickel, which cause the suppression of hydrogen adsorption, the promotion of multiple CO adsorptions, and the inhibition of dissociation of adsorbed CO. The stable associative adsorption of CO should be essential for the appearance of catalytic activity of methanol carbonylation. Determination of Surface Nickel. It is clear from the above discussion that neither H2 nor CO adsorption is suitable as a probe for determining surface area, or active site, of Ni supported on A.C. As we have already reported (Fujimoto et al., 1982), the reaction order of AcOMe formation with respect to the partial pressure of MeI, which is essential for the reaction, is close to 0 on Ni/A.C. This suggests that Me1 is adsorbed strongly on Ni under reac+
Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 2213
, I
I
-p l I t -I
1
1st
#
0 0
100
200
300
Pressure o f Me1 (Torr)
Figure 3. Me1 uptake on 2.5 w t % Ni/A.C. at 250 O C . 1
While dispersion decreases with Ni loading, the turnover number is almost constant. This may indicate that the carbonylation of MeOH over Ni/A.C. is a structure-insensitive reaction. Two kinds of speculation of the mechanism of the phenomenon are possible. The first one is that the interaction between Ni and carbon remains unchanged even if particle size of Ni increases. The electron transfer from A.C. to Ni is supposed to be essential to facilitate the oxidative addition of Me1 on Ni/A.C. Provided the high activity of Ni/A.C. is due to the above-mentionedelectron donor-acceptor function of A.C., the capacity of electron acceptance by carbon support should be large enough irrespective of the amount of supported Ni. Another possible explanation is that there exist two kinds of Ni on A.C. One keeps intimate contact with the support and is active for carbonylation, on which Me1 is adsorbed at 250 "C. Another Ni species has nothing to do with the catalytic activity. When the loading of Ni increases, only the latter species increases. According to this model, a large part of Ni supported on A.C. is of no use at high loading (>5 wt
5%). 0
' 0
8 N1 Loading ( W t X ) 4
12
Figure 4. Effect of Ni loading on A.C. Reaction conditions: tem0.6 2 atm; , perature, 250 "C; Pba, 10 atm; PCO,4 atm; P M ~ H atm.
tion conditions, and thus it is expected to be suitable as a probe molecule. Chemisorption of Me1 was measured at 250 "C to estimate the number of active site of the Ni catalyst under reaction temperature. The adsorption isotherms are demonstrated in Figure 3. The amount of chemisorbed Me1 is determined as the difference between the first and the second isotherms, while the latter may show the reversible adsorption on active carbon. As mentioned, the adsorption of Me1 is shown to be so strong even at 250 "C that the amount of chemisorbed Me1 depends only slightly on pressure. Dispersion of Ni on A.C. was calculated from the uptake of chemisorbed Me1 at 250 "C by DNi = MeI,/Ni,d (6) where DNiis the dispersion of Ni, MeI, is the amount of Me1 uptake (mmol/g), and Nihd is the amount of loaded Ni (mmol/g). The dispersion level is illustrated in Figure 4 as a function of the Ni loading. Apparently the dispersion decreases with Ni loading from 89.0% at 1% Ni to 21.5% at 10% Ni, and thus the particle size of Ni increases. The rate of AcOMe formation indicated in the figure is rather complicated. It has a maximum at about 5% Ni loading, but the higher loading causes the depression of the activity. The catalytic activity in terms of turnover number (NM in h-l) is defined by the ratio of AcOMe formation rate to the number of sites of Me1 adsorption (eq 7 ) and is presented in Figure 4. NM = rM/MeI, (7)
Conclusions The characterization of the active site of supported Ni was pursued by adsorption of H2, CO, and Me1 and also by TPR of chemisorbed CO, giving the following conclusions: 1. The character of CO adsorbed on Ni corresponds to the reaction activity. Ni/A.C., on which CO is adsorbed nondissociatively and multiply, shows excellent activity for carbonylation and poor activity for CO hydrogenation. In contrast, Ni/Si02 and Ni/A1203,on which CO is adsorbed dissociatively, shows poor activity for carbonylation and good activity on CO hydrogenation. 2. The activity in terms of turnover number for MeOH carbonylation over the Ni/A.C. was found to be almost unchanged with the dispersion of the Ni metal. The mechanism was ascribed to either the electronic character of active carbon or bimodal dispersion of Ni on it. 3. Me1 was adsorbed strongly on Ni at 250 "C and was found to be a suitable probe molecule for determining the surface Ni. Registry No. Ni, 7440-02-0; C, 7440-44-0; MeOH, 67-56-1; MeI, 74-88-4; CO, 630-08-0.
Literature Cited Bartholomew, C. H.; Pannel, R. B.; Butler J. L.; Mustard D. G . Znd. Eng. Chem. Prod. Res. Dev. 1981,20, 296. Egashira, M.; Honda, M.; Kawasumi, S. Nippon Kagaku Kaishi 1982,323. Fujimoto, K.; Kameyama, M.; Kunugi, T. J. Catal. 1980, 61,7. Fujimoto, K.; Shikada, T.; Omata, K.; Tominaga, H. Znd. Eng. Chem. Prod. Res. Dev. 1982, 21, 429. Kikuchi, E.; Inoue, S.; Morita, Y. Nippon Kagaku Kaishi 1982,185. Vannice, M. A. J. Catal. 1976,44, 152. Zagli, A. E. J. Catal. 1979, 56, 453.
Received for review April 5 , 1988 Accepted July 29, 1988
