Hydrogenation
iekei/Silica Catalysts
Robert A. Ross,* George D. Martln, and Welter 6. Cook Department of Chemistry, Lakehesd llniversity, Thunder Bay, Ontario, Canada P7B X I
The hydrogenation of benzene, toluene, and 1,3-butadiene has been studied in a flow system at atmospheric pressure over a range of nickel-on-silica catalysts which varied in nickel content from 4 to 79 % . The method of preparation was designed to avoid the effects of thermal sintering of the catalysts. Total catalyst and nickel metal surface areas were determined by low-temperature nitrogen adsorption and hydrogen chemisorption, respectively. Information regarding the form and dispersion of the nickel particles was obtained from electron micrographs. The rates of hydrogenation have been calculated with reference to total catalyst mass for each reaction which was studied from 35 to 8OoC, 70 to 120°C, and 85 to 175OC for benzene, toluene, and 1,3-butadiene, respectively. Reaction rates increase steadily with increase in the catalyst nickel content for all hydrogenations, while the hydrogen chemisorption results show that these increases coincide with a decrease in metal areas from 125 to 60 m2 g-’ and an increase in the average nickel crystallite size from 34 to 72 A. Selectivity ratios in the hydrogenation of 1,3=butadienehave been calculated and the results show that the selectivity of the catalysts for the formation of 1-butene increases slightly and linearly with increase in nickel crystallite size.
Introduction I t is well established that variations in the methods of preparation of supported metal catalysts can have significant effects on their activities in given reactions. Activity may be related to the degree of dispersion of the metal on the carrier and linked with metal particle size although the effect of the latter is subject to various interpretations (Boudart, 1969). Since, however, supported metal catalysts are of substantial technical significance it 1s important in developing methods of preparation to investigate how catalytic activity may vary with key physical properties of the catalysts. The method of preparation of catalyst samples has been found to have considerable effects on the activity of catalysts (Dzis’ko et al., 1972) and many methods have been used to achieve dispersion of the metal on the support, ranging from coprecipitation of’ hydroxides to thermal sintering. Studies of the activity of a series of preparations within which the metal particle size is varied are of particular interest and significance. There have been a number of investigations where particle size was found to have no effect on reaction rate. Yates et al. (1964) noted that there was no dependence of the dctivity of nickelhilica catalysts on particle size for ethane hydrogenolysis and Borisova et ai. (1971) found that the activity of nickel on various supports remained constant as tested by the benzene hydrogenation reaction. Similarly, Taylor and Staffin (19ti7) found that nickel crystallite size had no influence on the course of the benzene hydrogenation reaction and Aben et al. (1970) found that the activity per exposed metal atom is independent of the metal crystallite size or of the support used in the hydrogenation o i benzene on supported platinum, palladiunl, and nickel a t 100°C and 5.6 atm hydrogen pressure. On the other hand, Carter et al. (1966) reported that the activity of nickel/silica -alumina catalysts for ethane hydrogenolysis appeared to decrease by a factor of 20 as the relative particle size increased by a factor of 4 and work on various nickel and palladium catalysts supported on Aerosil revealed that a measurable amount of the infrared-active form of adsorbed nitrogen occurred only on metal crystallites with diameters in the range between 15 and approximately 70 8, (van Hardeveld and Hartog, 1972). I t has been proposed that the term “facile” be used to describe reactions in which activity does not depend on
particle size, and the term “demanding” for cases where there is such dependence (Boudart et al., 1966). In order to study further the effect of metal particle size on catalytic activity a t atmospheric pressure, the present work examines the hydrogenation of benzene, toluene, and 1,3-butadiene on a series of silica-supported nickel samples where the nickel content was varied during precipitation so that all catalysts would subsequently receive the same heat treatment, thereby limiting surface effects which might be caused by different degrees of thermal sintering.
Experimental Section Catalyst Preparation. The various catalysts were prepared by dripping nickel nitrate a t concentrations varying from 0.1 to 2.6 M , with mixing, into a sodium hydroxide solution at pH 12 to precipitate nickel hydroxide which was then mixed with Aerosil and subsequently washed and dried a t 125OC. The dried solid was then crushed and particle sizes lying between 440 and 600 p were used for all experiments after activation of the product in a 100 ml min-l stream of hydrogen for 14 hr a t 45OOC (Ross and McBride, 1960). Nickel concentrations were determined gravimetrically with dimethylglyoxime and the results confirmed in volumetric titrations using EDTA. The nickel contents of the reduced samples are given in Table I. X-Ray diffraction showed that detectable amounts of crystalline material were absent in all samples before and after catalysis as judged by the absence of lines on powder photographs. Apparatus. The flow system, catalytic reactor, and preliminary control tests have been described (Ross and Walsh, 1961). In the benzene and toluene experiments the liquids were contained in a thermostated gas saturator through which a hydrogen flow of 100 ml min-l was passed. Benzene and toluene partial pressures were 119 and 110 Torr, respectively. For the hydrogenation of 1,3-butadiene the gases were passed through separate flow meters and subsequently mixed before passing through the reactor. The flow rates for hydrogen and 1,3-butadiene, (190 Torr), were 75 and 25 ml min-l, respectively. About 0.1 g of catalyst was used and diluted to 1 ml with catalytically inactive quartz sand of particle size range 400 to 600 p. The effluent from the benzene and toluene reactions was collected in a liquid nitrogen trap and injected into a Beckman GC-5 gas chromatograph using an analytical column Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 3, 1975
151
Table I. Properties of Catalysts Amount of nickel
Catalyst (DZ)
(D5) (D15) (D30) (D50) (D75)
in reduced sample, $&
Nickel metal area, mz g-'
4.32 9.36 24.16 38.82 51.15 19.43
125 120 120 90 80
60
Average nickel crystallite size, A 34 36 36 48 54 72
of 20% silicone oil on Chromosorb W, 8O/lOO mesh. In the 1,3-hutadiene study the effluent was passed directly into the gas chromatograph via a I-ml sample loop. A stainless steel analytical column, 12 ft X 0.125 in. o.d., packed with 30% dimethylsulfolane on Chromosorb W, 60/80 mesh, was used a t 0% to separate the components of the gas stream. Electron micrographs were taken with a Philips EM300 instrument operated at 100 kV. Crystallite size ranges were examined by direct transmission electron microscopy. Specimens were crushed in an agate mortar and suspended in n-butyl alcohol. Drops of the suspension were then placed on the carbon support film on the microscope grids and the alcohol was subsequently removed by evaporation at Torr. BET surface areas were determined on a conventional apparatus (McCaffrey, 1972) with nitrogen as the adsorbate and nickel metal areas were ohtained by hydrogen chemisorption a t 2 2 T (Yates e t al., 1964). Materials. Ni(NOdy6HzO was Baker Analyzed reagent; the silica was Degussa Aerosil380 (>99.8% pure: given surface area 380 m2 g-'). Benzene and toluene were spectrophotometric grade (Aldricb Chemical Co.). Benzene was further purified by refluxing over NaRb-alloy and then distilled. Toluene was purified by a preparative gas chromatographic technique. 1,3-Butadiene (99.9%) and carbon monoxide (99.5%) were research grade materials (Matheson of Canada Ltd.). Hydrogen (Canadian Liquid Air Ltd.) was purified before use by passage through a "deoxo" unit. Results Catalyst Features. Tahle I shows nickel areas and the average nickel crystallite size as determined by hydrogen chemisorption for each catalyst (Coenen et al., 1973). Size distribution data were obtained from electron micrographs which showed that the crystallite size range was narrow and increased with increase in catalyst nickel content in agreement with the hydrogen chemisorption results. Figure 1 shows that in the dark-field micrographs the nickel particles appeared as distinct bright spots and hence were easily measured whereas in the light-field micrographs the particles appeared as ill-defined dark spots. Dark-field micrographs were also useful for an examination of the extent of dispersion of the nickel crystallites on the support. Tilt photographs were taken a t a 6 O angle to the electron heam on all samples hut these gave no indication of the presence of major lattice discontinuities such as twin boundaries in any of the observed nickel particles as deduced by the absence of light-shading effects (van Hardeveld and Hartog, 1972). Benzene and Toluene Hydrogenations. For both henzene and toluene hydrogenations the rate of reaction was obtained in terms of moles of product min-' g-' of catalyst and Figures 2a and 2h present the rate results as plots of lag rate vs. 103/T (OK) for benzene and toluene, respectively. 152
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 3, 1975
Figure 1. Transmission electron micrographs (microscope magnification 126,000, total magnification 357,000) for (a) (D15)and (b) (D50). The corresponding dark field micrographs are shown in (e) and (d) respectively.
The hydrogenation of benzene was achieved between 35 and SO°C, and that of toluene hetween 70 and 120°C. The only product of the benzene reaction was cyclohexane and the only product of the toluene reaction was methylcyclohexane. In all determinations with each hydrocarbon the readily reproducible values for the degree of hydrogenation were limited to less than 10%. The linear plots in Figures 2a and 2b indicate that the rate of hydrogenation increases with increase in the amount of nickel in the catalysts. The temperature coefficients of the reaction rates were 13.5 -+ 0.5 and 14.5 f 0.5 kcal mol-' on all catalysts for benzene and toluene hydrogenations, respectively. 1,a-Butadiene Hydrogenation. The hydrogenation of 1,3-butadiene was carried out between 85 and 175% Total conversion to the three major products, 1-butene, tram-2butene, and cis-%butene, was kept t o less than 10% of the starting material. n-Butane could not he detected in the products of catalysis and while some longer chain or cyclic condensation products were present, the total amount of these was still less than 5% of the total conversion even a t the highest temperature studied. Figures 3a-c show separate plots of log rate against 103/T (OK) for 1,3-butadiene hydrogenation to I-butene and trans-2-butene and also for the total conversion. The amount of cis-2-butene formed a t any one temperature was small and its rate of formation was a factor of 100 times less than that for 1-butene or trans-2-butene. The hydrogenation results for Wbutadiene were easily reproduced in duplicate runs with the same catalyst samples and with fresh samples of the same catalyst. The products of catalysis were stable under the conditions of the reaction and no homogeneous or reactor-wall effects were observed in blank runs with no catalysts. The selectivity for the formation of 1-hutene was calculated as S = rJ(r1 + r2) where rl and r2 represent the re-
200,
1
0
29
30
31
32
33
240
250
260
270
280
2M
240
250
260
270
280
T
-
I
28
230
pl
34
t
e
01
050
t P
000-
240
'
250
260
IITEMPERATURE
270
280
290
w
IOOC
300
(OK) xi03
Figure 2. The dependence of the reaction rate on temperature for (a) benzene and (b) toluene hydrogenation: 0, (D2); 0,(D5); A, (D15);0 , (D30); (D50);A, (D75).
spective rates of formation of 1-butene and trans- %butene in mol min-' m-2 Ni. Figure 4 shows these results for the range 85 to 165OC. For each catalyst the selectivity increases linearly with temperature which indicates that hydrogenation to 1-butene becomes more favored with increase in temperature, although not appreciably so. A more significant observation is that the selectivity increases with increase in crystallite size, ranging from a value of about 0.34 for (D5) to 0.45 for (D50) a t 100OC. The selectivities of (D2), (D5), and (D15) are quite similar throughout the temperature range and may be considered to be nearly identical.
230
240
250
IITEMPERATURE
260
270
280
.,
( O K ) X 103
Figure 3. The dependence of reaction rate on temperature for
1,3-butadiene hydrogenation to (a) 1-butene; (b) trans-2-butene, and (c) total products: 0 , (D2); 0, (D5); A, (D15); 0 , (D30); (D50). E
P t
05-
fL
P 2.
-
04-
* 0
m 03
Discussion The low-temperature nitrogen adsorption results, with the exception of that for (D50), showed that the method of preparation yielded catalysts of very similar surface area values, 240 m2 g-' f 7%. This feature is explained by the avoidance of a thermal sintering process in the method employed. The steady variation in nickel metal areas and nickel crystallite size may be related simply and respectively to the increase in the amount of nickel in the catalysts and the well-known effects of solution concentrations on the particle size of precipitates. Further development of the present method could lead to catalyst samples with an even smaller distribution of crystallite sizes. The rate results for the hydrogenation of benzene are in general agreement with previous work on nickel surfaces (Ross and Walsh, 1961; Madden and Kemball, 1961) when the reaction rate dependence on benzene concentration was found to be zero order. The present temperature coefficients of 13.5 f 0.5 and 14.5 f 0.5 kcal mol-' for benzene and toluene, respectively, indicate low activation energies for both hydrogenation reactions. A value of 14 f 1 kcal mol-' has been determined for the apparent activation energy for benzene hydrogenation on supported nickel (Aben et al., 1970). These values are all consistent with high catalyst activity as evidenced here by the necessity to dilute the
90
100
110
120
IM
140
150
160
170
180
TEMPERATURE ("C)
Figure 4. The relationship between the catalyst selectivities and temperature for 1-butene in the 1,3-butadienehydrogenation reaction: 0, (D2); 0,(D5);A, (D15),0 , (D30); W, (D50).
catalyst with quartz sand in order to maintain low conversions. Figures 2 and 3 show that the hydrogenation rates increased with increase in nickel content which might be anticipated since nickel is regarded as the only active component of this type of catalyst (Hill and Selwood, 1949). Further, no catalysis was detected in hydrogenation runs with Aerosil alone and so synergetic effects are probably absent. Figure 2 and Table I show that in the hydrogenation of toluene at 90°C, the rate, X104, increases from 4.0 mol min-' g-l in moving from (D2) to (D75). This represents an approximate eightfold change in activity which is similar to that observed for benzene at 6OoC. The rate change for total hydrogenation of 1,3-butadiene between (D2) and (D50) is about tenfold. Thus for all of the hydrogenations, there is a pronounced trend for the reaction rates, based on catalyst mass, to increase progressively with decrease in the nickel metal area, which is in step with an increase in the average nickel crystallite size. Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 3,1975
153
In terms of the fraction of metal atoms exposed to the reactants, Aben and coworkers found an approximate twofold increase in activity for benzene hydrogenation on nickelhilica when this fraction was raised from 0.1 to 0.2. However, a direct comparison with these results is obscured since the earlier workers used a much greater hydrogen pressure, individual catalysts with a wide spectrum of crystallite sizes ranging from 10 to 200 %, and an impregnation procedure for catalyst preparation which would be expected to yield a catalyst series with a lower range of nickel content than those used in the present work. The hydrogenation of the diolefin 1,3-butadiene produced as its main products the monoolefins 1-butene, trans-2-butene, and cis-2-butene with no evidence of the occurrence of complete hydrogenation to n- butane. These findings are in agreement with earlier work (Phillipson et al., 1969; Oliver et al., 1973) when results were not specifically related to metal crystallite size. There is a general trend for the selectivity toward l-butene formation to increase with both increase in temperature and nickel content. Previously, a similar effect of temperature was detected and explained by a decrease in the availability of adsorbed hydrogen which would tend to favour the formation of 1-butene (Bond et al., 1965). The amount of this isomer produced would also be expected to increase with nickel content if both Ni(I1) and Ni(II1) oxidation states occur in the nickel precipitate prior to reduction since it has been concluded from mechanistic studies that this type of surface is most likely to coordinate the intermediate 1-methyl-r-allyl ligand to a metal atom (Oliver e t al., 1973).
Conclusions In the hydrogenation of benzene, toluene, and 1,3-butadiene, nickel-on-silica catalysts, prepared without thermal sintering, exhibit a pronounced trend for reactions rates based on catalyst mass to increase progressively with (i) an
Lanthanum Titanate Catalyst-Sulfur
increase in catalyst nickel content, (ii) an increase in the average nickel crystallite size, and (iii) a decrease in nickel metal area. The selectivity of the catalyst series toward 1-butene formation increases with increase in the reaction temperature from 85 to 175OC and with an increase in the catalyst nickel content from 4.32 to 79.43%.
Acknowledgement We thank Michael Fraser for experimental assistance in the preparation of the catalyst samples. Literature Cited Aben, P. C., Platteeuw, J. C., Stouthamer, B., Roc. Int. Congr. Catal., 4th, (1970). Bond, G. C., Webb, G., Wells, P. B., Winterbottom, J. M., J. Chem. SOC., 3218 (1965). Borisova, M. S.,Dzis'ko, V. A., Bulgakova, Yu. O., Kinet. Catab, 12, 344 (1971). Boudart, M., Adv. Catal., 20, 153 (1969). Boudart, M., Aldag, A,, Benson, J. E. Dougharty, N. A,, Harkins, C. G.. J. Cafal., 8, 92 (1966). Carter, J. L., Cusumano, J. A,. Sinfelt, H. H., J. fhys. Chem., 70, 2257 (1966). Coenen. J. W. E., van Meerten, R. 2. C., Rijnten, H. T.. Roc. Int. Congr. Cafal., 5th. (973). Dzis'ko, V. A., Noskova, S. P.,Karachiev, L. G.. Borisove, M. S., Bolgova, V. G., Tyulikova, 1.Ya., Kinet. Catab, 13, 366 (1972). Hill, F. N., Selwood, P. W., J. Amer. Chem. SOC.,71, 2522 (1949). Madden, W. F., Kernball, C., J. Chem. SOC.,302 (1961). McCaffrey, E., Ph.D. Thesis, The Queen's University of Belfast, N. Ireland, 1972. Oliver, R . G., Wells, P. B., Grant, J., Proc. h t . Congr. Catab, 5th. (1973). Phillipson, J. J., Wells, P.B., Wilson, G. R., J. Chem. SOC..1351 (1969). Ross, R. A,, McBride. G. B., Chem. lnd, 1504 (1960). Ross, R. A,. Walsh, B. G., J. Appl. Chem., 11, 469 (1961). Schoiten, J. J. F., van Montfoort. A,, J. CataL, 1, 85 (1962). Taylor, W. F., Staffin, H. K., Trans. Faraday SOC.,63, 2309 (1967). van Hardeveld, R., Hartog, F., Adv. CataL, 22, 75 (1972). Yates, D. J. C., Taylor, W. F.. Sinfelt, J. H., J. Amer. Chem. Soc., 86, 2996 (1964).
Received for review November 14,1974 Accepted April 7,1975
Dioxide Reduction
John Happel,' Miguel A. Hnatow, Laimonis Bajars, and Mlchael Kundrath Columbia University, New York, New York,
Preliminary studies on dry gases indicate that lanthanum titanate is an active catalyst for the reduction of sulfur dioxide to elemental sulfur by means of carbon monoxide. This material is characterized by a low rate of carbonyl sulfide (COS) formation by further reduction of the sulfur initially produced. Catalysts previously studied do not possess this ability. Minimizing COS formation is important in stack gas purification systems because this substance is extremely toxic.
Introduction
No substantial commercial experience exists for catalysts employing CO for the reduction of SOZ. Laboratory studies have been published by Khalafalla et al. (1971, 1972) using an iron oxide catalyst supported on alumina and by Ryason and Harkins (1967), Quirido and Short (1973), Quinlan et al. (1973a) and Kasaoka et al. (1973), all using a copper 154
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14. No. 3, 1975
supported catalyst. Further studies by Quirilan et al. (1973b) and Goetz et al. (1974) included screening of a number of catalysts for simu~taneousreduction of sozand NO with co. all the studies that so far have been reported it was found that in addition to the desired reaction
2co + so2
-
2c02
+ 'hS2
(1)
a second reaction occurs between carbon monoxide and ele-
