Znd. Eng. Chem. Res. 1992,31,1849-1856
1849
Characteristics of Carbon-Supported Palladium Catalysts for Liquid-Phase Hydrogenation of Nitroaromatics Dong Jin Suh and Tae-Jin Park Reaction Engineering Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea
Son-Ki Ihm* Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusung-Dong, Yusung-Gu, Taejeon 305-701, Korea
Effects of the properties of support and the preparation condition on the characteristics of carbon-supported palladium catalysts were investigated. The characteristics included dispersion and distribution of metal and the catalytic activity on the liquid-phase hydrogenation of dinitrotoluene. Oxidation of carbon support appeared to increase the number of surface oxygen groups and to enhance diapersion of palladium. However, the catalytic activity did not improve in proportion to the dispersion. In carbon-metal salt slurry, nuclei formation and growth by ion exchange and/or reduction was considered to be the most important step in determining the fmal state of the catalysb prepared by the alkali hydrolysis method. The catalytic activity was found to be strongly dependent on metal location throughout the mesopore structure especially in the case of catalysts supported on highly porous activated carbon.
Introduction Carbon-supportedpalladium catalysts are widely used in industrial chemical processes especially in liquid-phase hydrogenation of aromatic nitro compounds (Rylander, 1967; Dovell et al., 1979), for example, hydrogenation of dinitrotoluene for production of toluenediamine which is an intermediate of tolylene diisocyanate (TDI) (Rylander, 1983). Most often activated carbons are used as supports for noble metals because they have large surface area and low intrinsic chemical activity. Moreover, they are quite inexpensive. The primary role of a catalyst support is to provide a surface on which the active species can be dispersed to provide the reactants with greater surface area. Its role, however, is not limited only to that of a passive carrier since the surface properties-especially surface oxygen groups-and the pore structure of the carbon support may influence the characteristics of carbon-supported catalysts (Ehrburger et al., 1976; Linares-Solano et aL, 1980,Jenkins et al., 1980, Richard and Gallmt, 1987; Prado-Burguete et al., 1989). Due to the complicated characteristics of carbon-supported catalysts, little attention has been given to the influence of catalyst and support properties on the activity or selectivity of carbon-supported catalysts. Much still needs to be done to explore the chemical and physical properties of carbon supports in order to obtain optimum catalyst. In this work, thus, the effects of the support properties and preparation conditions of Pd/C catalysts on their catalytic properties have been investigated. Experimental Section Carbon supports used in this study were Strem activated carbon (SA), Darco G-60 activated carbon (DA), and Vulcan 3 carbon black (VB). For examination of the surface oxygen group effect, supports were treated with 70 w t % nitric acid at 80 "C for 15 h. Also chemical treatments with different oxidizing agents were intended to introduce oxygen functionalities to the Darco G-60 carbon surface. After the treatment, oxidized carbons were washed with distilled water until the filtrate became clear
* To whom correspondence should be addressed.
and neutral. The amount of surface oxygen groups was analyzed by temperature-programmeddesorption (TPD) and acid-base titrations. TPD of decomposition products (CO and C02) was obtained by flowing helium over the carbon support while raising the temperature at a rate of 10 "C/min. Identification of surface acid groups was performed, according to the method of Boehm et al. (1964), by neutralization with excess amount of various bases, followed by back titration with hydrochloric acid. Characterizations of the porous texture of the supports and catalysts were carried out by nitrogen adsorption-desorption at 77 K in a conventional volumetric apparatus. Total surface area was obtained via the BET equation, and mesopore size distribution was calculated from desorption isotherm assuming cylindrical pores. The micropore properties were analyzed by the t-plot method using the standard t-values given by Lecloux and Pirard (1979). Carbon-supportedpalladium catalysts were prepared by alkali hydrolysis of palladium chloride on various carbon supports followed by liquid-phase reduction of the hydrolyzed salt with saturated formaldehyde solution. The suspended catalyst was filtered off, washed with hot distilled water repeatedly, and dried in a vacuum oven. Palladium dispersion was determined by pulsewise oxygen titration of adsorbed hydrogen at room temperature. Before pulse loading, all samples were dried in situ in a stream of helium at 100 "C and reduced in hydrogen (simultaneous adsorption) at room temperature. Then hydrogen was replaced with helium for the removal of p h y s i d y adsorbed/absorbed hydrogen on samples. Oxygen pulses of constant amount (0.017 mL) were injected through a six-portswitching valve into the helium stream in front of the catalyst bed. A thermal conductivity detector monitored the effluent composition and showed the amount of oxygen after each pulse. Catalytic activities of the prepared catalysts were evaluated by carrying out liquid-phase hydrogenation of 2,4-dinitrotoluene (2,4-DNT) in a 600-mL Parr reactor. The reaction rate was calculated from the consumption of hydrogen in a reservoir whose pressure was recorded. Standard reaction conditions were as follows: reaction temperature, 100 "C;hydrogen pressure, 162 psi; solvent, 250 mL of methanol; reactant, 18.2 g (0.1 mol) of 2,4-DNT;
0888-5885/92/2631-1849$03.00/00 1992 American Chemical Society
1850 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 Table I. Neutralization of Acidic Surface Oxides surf, consumption (mequiv/100 g) sumort area (m2/a) NaHCOn NazCOn NaOH SA 1140 19.6 38.8 SAON 766 113.5 192.4 254.5 SAON-1' 844 24.3 61.1 120.1 DA 742 5.0 7.5 17.8 DAON 618 158.4 222.0 338.8 DAOS 554 7.5 28.7 96.8 10.1 18.1 51.2 DAOH 742 DAOA 795 6.7 30.6 67.8 VB 73 2.5 15.5 VBON 85 27.3 37.6 96.6
~
"SA, activated carbon supplied by Strem chemicals; DA, Darco G-60 activated carbon; VB,Vulcan 3 carbon black; -ON, oxidized with nitric acid; -OS, oxidized with sodium hypochlorite, -OH, oxidized with hydrogen peroxide; -OA, oxidized by air burnoff at 350 OC. *SAON treated by flowing helium at 500 O C for 2 h.
0 0
100
3W 400 500 COa evolved,arbitrary unit
200
1
600
Figure 2. Relation between the amount of sodium carbonate neutralization and surface oxides evolved as COP Table 11. Effect of Surface Oxygen Groups on Catalytic Properties Pd Pd reaction rate loading dispersion (mol of HI/ support oxidant (wt %) (%) [min.(g of Pd)]) SA 4.33 17.0 4.67 SAON "03 3.79 62.4 6.32 DA 4.30 29.4 7.48 "08 3.96 68.4 4.02 DAON DAOS NaOCl 4.37 33.0 5.25 DAOH HzOt 4.40 28.7 7.44 VB 4.67 17.1 10.45 VBON HNOS 4.51 20.2 8.77
100
300
500 Temperature
700 (
900
'Cl
Figure 1. TPD profile of carbon oxides as decomposed products.
catalyst, 0.025 g of Pd/100 g of DNT. Results and Discussion Surface Oxygen Groups. Carbon supports were oxidized with various oxidants. The treatment resulted in a substantial increase in the amount of surface oxygen groups while not changing the textural properties noticeably. The content of acidic surface groups on the original and treated carbons and total surface areas are listed in the Table I. The differences in the quantities of the three bases consumed for neutralization are equivalent to the amounts of three different types of acidic groups: (I) carboxylic acid, (II)lactone, and (ID) phenol. For example, group I1 is neutralized by Na2C03,but not by NaHC03 (Boehm et al., 1964). Nitric acid treatment produced the highest amount of surface oxygen groups, especially the strongly acidic group I, but led to drastic reduction in surface area. Evidently, the attack by this strong acid was too severe and resulted in a collapse of the pore structure. Sodium hypochlorite and hydrogen peroxide as milder oxidizing agents introduced group I11 more than group I or group 11. Oxidation through partial gasificationresulted in substantial carbon loas and caused alterationsto surface area and pore size distribution (DAOA). It is well-known that the surface oxygen groups of a carbon decomposes to CO and C02 upon heating in an inert atmosphere. Figure 1,taken as a typical example,
shows the evolution of carbon oxides from thermal decomposition of the surface oxides on SA supports. As can be seen from Figure 1,the CO peak of the TPD curve for oxidized carbon support is shifted to lower temperature while the C02peak remains almost unchanged. This indicates the formation of another type of CO-evolving surface oxides by liquid-phase oxidation. The TPD c w e of CO desorbing from SAON-T, which was prepared by thermal treatment of SAON at 500 "C for 2 h under a helium environment, was almost identical with that of CO from SAON. The amounts of group I11 on these two carbon samples were also the same (cf. Table I). Therefore it is evident that the weaker acidic group, phenolic oxide, decomposesto CO while the more acidic group I and group I1 species decompose to C02. This relationship between the amount of Na2C03consumed for neutralization and the C02-evolvingsurface oxides is shown more clearly in Figure 2. Similar results were obtained for all other carbon supports. Furthermore, it was found that the amounts of decomposing surface oxides on oxidized carbon-supported palladium catalysts were slightly lese than those on the oxidized carbon itself while their TPD curves were almost identical. This observation implies that the catalyst preparation step will not appreciably change the surface functionality of the carbon. Carbons oxidized in acidic medium evolved more C 0 2in comparison to CO on heating than did the sample treated with oxygen. This phenomenon was also confirmed by acid-base titration as shown in Table I. These surface oxygen groups can affect the carbon-metal interaction and the catalytic activity of the carbon-supported catalyst. The effects of surface oxygen groups on palladium dispersion of carbon-supported catalysts and their catalytic activities are presented in Table 11. The
Ind. Eng. Cham. Res., Vol. 31, No.8,1992 1851 Pd dispersion, measured by pulsed oxygen titration, increased with increasing amounts of surface groups, but was
not significantly improved by milder oxidation of the carbon support, as indicated by samples DAOS and DAOH in Table 11. The Pd dispersion increased by about 3 times upon nitric acid treatment of the carbon support SA while a substantial reduction of total surface area was unavoidable under such a severe oxidation condition. In fact, the possible influence of alterations to the textural properties on metal dispersion m o t be ignored. In order to examine such a possibility, two carbons having almost identical surface areas were compared: DAOS, with a large amount of surface groups; and DAON, with much larger amount of surface groups. Furthermore, the difference between C0,-evolving surface groups in both carbon samples was more significant than that between CO-evolving groups. Thus it is clear that the increase in metal dispersion is mainly due to the surface oxygen groups, especially COP-evolvinggroups (groups I and 11). The promotional effect of oxygen functional group on a metal dispersion can be explained by the interaction between the metal precursor and the carbon support in the catalyst preparation stages. The metal salt precursor (HQdClJ was added to.the carbon slurry before the alkali. This procedure allows the formation of nuclei by ion exchange and reduction that can be grown to the desired crystallite sue by the careful control of preparation condition. The surface oxygen groups may provide preferred sites for the formation of nuclei; thus they may promote metal dispersion of the prepand catalyat by increasing the number of nuclei. The characteristica of the metal precursor and solvent-carbon interactions may play an important role in the metal precursor distribution state. In fact, since carbon is a poor adsorbent for anions (Mattaon and Mark, 1971), it is likely that the anionic metal precursor, [PdCIJz, from an aqueous solution is adsorbed nonuniformly on the support. Moreover, the hydrophobic nature of the original carbon support may not be favorable for the deep penetration of the aqueous metal salt solution into the small pores. The change in hydrophobicity of the carbonsurface upon oxidative treatment may be effective for more homogeneous distribution of the metal precursor because acidic groups are introduced to the carbon. These surface oxygen groups will then increase the affhity of the carbon for the aqueous solution and finally lead to more uniform distribution of the precursor throughout the internal pore structure. This explanation is closely related to the results described by Machek et al. (1983), who investigated the role of the solvent on the dispersivity of supported platinum using H,PtCI, as a precursor. They found that, for activated carbons, the use of nonpolar solvents such as ketones led to a higher dispersion than that of polar solvents. In case of a hydrophilic support like A1208,the opposite situation was found to occur. Prado-Burguete et al. (1989) suggested another possible explanation for the promotional effect of surface oxygen groups on metal dispersion. They proposed that an increase in the metal-carbon interaction would hinder the sintering of the metal particle during decomposition and reduction step. However, in this study, the hydrolyzed metal precursor which had been distributed over the carbon support was reduced to metal in the liquid phase by a saturated formaldehyde solution below 100 OC. This liquid-phase reduction is not as fraught with sintering problems as gas-phase reduction, but appreciable loss of metal area can occur due to the redistribution and/or agglomeration of the metal particles. In other words,
..
a-
.
(b) F i r e S. Electron micrographs of the Pd particlea on activated carbon support: (a) on nitric acid treated support (SAON); (b) on the original support (SA).
deposition of the metal precursor by nucleation and alkali hydrolysis may be a dominant step in deciding the final state of metal dispersion of the prepared catalyst. The more acidic groups (CO,-evolving groups, especially group I) are expected to have a major influence on the metal dispersion by providing nucleation sites and enhancing the hydrophilicity of the carbon surface. Figure 3 gives further evidence for the explanation given above; transmission electron microscopic (TEM) views of Pd supported on SA and SAON show more uniform distribution of the smaller Pd particles on an oxidized carhon support in comparison to that on the original support. In Table II, the activities of the catalysta prepared from the oxidized carbon supporta are compared with those of the catalysts using untreated carbons. Although the s u p port pretreatment led to a substantial increase in Pd dispersion, catalytic activities of those catalysts for DNT hydrogenation did not improve as well. Presumably, the contradictory result is due to the fact that the catalytic activity is dependent not only on metal dispersion but also on the distribution of the metal particles and/or surface polarity. In other words, even if metal particles were well dispersed, those located in micropores would not easily contact large reactant molecules, especially in liquid-phase reactions. A t the same time, oxidized carbons with hydrophilic surfam would not be w i l y wetted with nonpolar reactant. At this point, it cannot be stated whether the oxidative treatments of carbon supporta may advanta-
1852 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 Table 111. Effect of Alkali Hydrolysis Procedure Pd Pd reaction rate loading dispersion (mol H2/ support procedure' (wt %) (%) [min.(g of Pd)]) 4.67 SA I 4.33 17.0 4.87 13.9 7.86 I1 7.48 DA I 4.30 29.4 11.40 I1 4.34 25.3 10.45 VB I 4.67 17.1 10.43 4.69 17.2 I1 6.32 3.79 62.4 SAON I 5.31 3.85 61.1 I1 4.02 DAON I 3.96 68.4 7.45 3.91 81.1 I1 a I, adding alkali precipitant into metal salt-carbon slurry; 11, adding metal salt solution into alkali-carbon slurry.
geously alter catalyst activity. As can be seen in Table 11, the nitric acid treatment of two different activated carbon supports had an opposite effect on catalytic activity, which is probably due to the fact that the relative importance of the different effects stated above would be dependent on the structure and surface nature of the parent carbon itself. The results for the oxidized carbon black support show the importance of the surface polarity effect on catalytic activity because carbon blacks consist of nonporous spherical particles frequently aggregated into chains and thus eliminate the possibility of pore structure effects. It was reported that hydrogen spillover onto active carbon was observed only above 250 "C by a temperature programmed reduction study (Chen et al., 1983). In this study, however, hydrogen spillover from highly dispersed palladium onto nitric acid treated activated carbon support was observed at 100 "C. Hydrogen titration at 100 "C to measure Pd dispersion could not be employed for oxidized activated carbon-supported catalysts since the hydrogen uptake greatly exceeded the possible quantity absorbed by palladium alone (H/Pd = 1). In the absence of palladium, the oxidized carbon support absorbed a negligible amount of hydrogen. In view of the present observation, the oxidative treatment of the activated carbon support is believed to facilitate the hydrogen spillover at such a low temperature. Effects of Catalyst Preparation Conditions. Nearly all preparations of the catalysts by the alkali hydrolysis technique consisted of slurrying the carbon support with metal salt solution, heating to desired temperature, and then adding an alkali continuously in order to hydrolyze the salt onto the carbon surface (procedure I). In another variation, the catalytic metal salt solution was added to the alkali-carbon support slurry at constant rate (procedure 11). The effects of alkali hydrolysis procedure are presented in Table 111. According to procedure I, the carbon support is allowed to come into contact with the H2PdC14solution. Once the metal salt solution has filled the pore structure, it reacts with the surface oxygen groups, deposits part of the metal by ion exchange and/or reduction, and forms nuclei on which further growth can take place during the later steps. On the other hand, in procedure 11, the hydrolysis products are formed in the bulk solution, and diffusion to the carbon surface will favor the growth of particle located on the surface of the carbon particle (or in the wider pores). Thus the catalysts prepared by procedure I have a more uniform distribution of small Pd particles throughout the pore structure than those prepared by procedure 11. It is now evident that the difference in the observed catalytic properties by these two methods of catalyst preparation is related to the metal distribution throughout the pore structure. Hence, for the catalyst design to maximize utilization of the active com-
ponent, procedure I1 will be efficient. The above explanation can be confirmed from the fact that the catalytic properties of nonporous carbon black (VB) supported catalysts do not depend on the different procedures in catalyst preparations. However, in the case of oxidized activated carbon support, this behavior was not observed. As discussed from the effect of surface oxygen groups, this is probably because the driving force for more uniform distribution of the metal precursor even into the micropore greatly increases for oxidized carbon supports. This explanation will be confirmed by the results of pore structural effect discussed later. Hydrolysis temperature appears to be the most important factor when carbon-supported Pd catalyst is prepared by the alkali hydrolysis method and procedure I, presumably because the temperature is closely related to the rate of nuclei formation and growth. In Figure 4a, there exists an optimum temperature, about 40 "C, when activated carbon with a considerable amount of surface oxygen groups is used as support. This is presumably because the rate of formation of nuclei is not sufficiently fast at lower temperatures while that of particle growth is too fast at higher temperatures. On the other hand, with nonporous carbon black support which has very few surface oxygen groups, metal dispersion improves as the hydrolysis temperature decreases, as seen in Figure 4b. In procedure 11, the growth rate of particles by diffusion to the carbon surface may be more important than that of nuclei formation and growth. Hence, Figure 4c shows the existence of the optimum temperature, but the dependencies of metal dispersion and catalytic activity on the hydrolysis temperature are very low. A similar dependency on the hydrolysis temperature for other activated carbon supports gives further evidence for the explanation stated above. However, the optimum hydrolysis temperature changes slightly due to different pore structure and surface functionality. For any type of carbon support, a common feature in the influence of the hydrolysis temperature is distinctive growth rate of metal particles at higher temperature. It has been shown that the formation of nuclei is the important step in the preparation of activated carbonsupported Pd catalysts. Ion exchange and reduction may be possible reactions in this step. In general, the ion-exchangeable species on the original carbons are insufficient in number to provide high concentration of active metal. In addition, anionic metal salt used in this study is not effective for exchange of a hydrogen ion for a metal ion. In practical ion-exchange method, the metal precursor is generally present as a cation which reacts with the surface oxygen groups of the support (Anderson, 1975). Another reaction which leads to the formation of nuclei may be reduction of the hydrolyzed metal salt to active metal. It is known that salts of some metals undergo partial or complete reduction to metals when adsorbed on activated carbons (Hassler, 1963; Morikawa et al., 1969). Although the origin of the reducing power of activated carbons is not still well understood, it may have a close relationship with the surface oxygen groups. In the present work, nucleation (or soaking) without further reduction produced a significant amount of palladium metal particles detected by X-ray diffraction as shown in Figure 5. An electron micrograph of this sample showing the uniform distribution of palladium metal particles is given in Figure 5b. It is also noted that size distribution of Pd is considerably broad, covering the range from 10 A to larger than 200 A. This is probably because nucleation and growth are carried out simultaneouslyby redistribution and/or agglomeration.
Ind. Fng. Chem. Res., Vol. 31,No.8, 1992 1853 25
61
25
20
15
-
x
s ._ t 10
..
.F D
(b) 5
0
m
10
so
80
i
0
Hydmlyrir tapenturs Itl
pieUre 6. Morphdcgy and state of Pd particles on activated carbon aupport prepared by nucleation alone: (a) XRD pattern; (b) eleebon micrograph. Tabls IV. Effect of Nnclsstion Time reaction rata nucleation temp loading dispersion (mol of H,/ support time (h) ("C) (wt 46) (46) [min&ofPd)I) SA 2 16.75 2 16.75
VB 2 17.0
0
m
u)
60
80
tm
Hydrolysis b q s r a t u r s 1 tl
Figure 4. FSect of hyclmlysia t8mperak. (a,top) SA aupport; (b, middle) VB support; (c, bottom)SA support and procedure 11.
In order to obtain additional information about the effect of nucleation, a series of carbon-supported Pd catalysts were prepared varying nucleation t i e only. As shown in Table IV, the longer the activated carbon is allowed to come into contact with the H,PdCI, solution, the higher is the metal dispersion of the prepared catalyst. However, during this period, the increase in catalytic ac-
45 45 45 60
60 60 45 45 45
4.72 4.72 4.69 4.78 4.68 4.85 4.24 4.51 4.39
13.4 16.9 23.8 8.2 13.7 23.6 17.9 18.4 17.4
4.22 4.49 6.07 2.25 2.96 5.08 11.09 11.11 11.14
tivity with contact time is much smaller than that in metal dispersion since the solution is allowed to penetrate into the smaller pores. Table IV also indicates that this metal diatribution effect is found to be more signiiicant at higher hydrolysis and nucleation temperatures. As expected, for carbon black support, the difference in contact time gives little effect on catalytic properties. Consequently, the interaction between metal precurmr and carbon surfam in solvent cannot be simply explained. An optimum catalyst preparation rep& careful attention to the main factors affecting catalytic properties stated above and full understanding of the metal precursor carbon support-solvent system. Pore Structure Effect. As discussed above, the pore structure is considered to be an important factor which influences the activities of carbon-supported metal catalysts. The pore structure governs transport proceaaes of reactants and producta to and from the surface and determines the effective surfam area of the active metal. At the same time the pore structure influences the metal dispersion. Activated carbon, which is widely used BS a
1854 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
b
.3 f
1i
a e
SA support
+-L"
9
SA-66 (Dispersion :17.02)
-2-
*
SA-64 (Dispersion : 3.5X)
..
D E c
0 D
0
n 0
.?
-
4
*1
..
3
a 0
2
4
6
8
1
0
1
2
0 -
i 4
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1855
t
14t
i =I
2
Ill
12
m
'E 10
4.*
> I
i L/
Y
.4 e
4
0 0
10
20
30
40
Pd dispersion ( X I
Figure 8. Relationship between catalytic activity and Pd dispersion for various carbon-supported catalysts: VB,supported on Vulcan 3 carbon black; JB,supported on carbon black supplied by Korea Steel Chemical Co.; SA-I, supported on SA (prepared by procedure I); SA-II, supported on SA (prepared by procedure 11); DA-I, supported on DA (prepared by procedure I); DA-11, supported on DA (prepared by procedure 11).
micropore diameter and increase in the micropore area by gasification indicates the production of new micropores instead of the growth of some micropores to larger ones. Thus oxidation of the carbon support by gasification does not improve the pore characteristics as a catalyst support because the support is expected to have wider pores rather than micropores. Figure 7 illustrates the typical mesopore size distribution in activated carbon-supportedPd catalysts and the original support. As expected from the results in Table V, considerable loss in mesopore volume of activated carbon support was observed even at low metal loadings. Thus the apparent reaction rate over nonoxidized carbon-supported Pd catalyst in this work is predominantly dependent on the metal distribution in the mesopores rather than in the micropores. In principle, the activity per unit mass of palladium is supposed to be proportional to the palladium surface area (or palladium dispersion) since DNT hydrogenation is a structure-insensitivereaction. In many case%,however, this proportionality is not apparent because part of the palladium surface area is not accessible for the reactant and/or product molecules. To understand this pore structure effect on catalyst activity, the activity data have been plotted against palladium dispersion in Figure 8 for a series of carbon-supported palladium catalysts prepared by the alkali hydrolysis method. It was found that the catalysts could be divided into three groups: group I, proportional; group 11, almost proportional but slightly lower activity; group 111, not proportional. The supports used in the group I catalysts are nonporous carbon black, so that palladium crystallites are located on the outside of the carbon particles and are fully available for the reaction. O n the other hand, for activated carbon supports, the group I1 and I11 catalysts were prepared by procedures I1 and I, respectively. Different behavior of both groups in Figure 8 may be attributed to the difference in palladium dispersion throughout the mesopore structure. In group 11, the catalyst activity is slightly depressed due to inevitable pore diffusion problem, which is not significant. In group 111,the extent of the pore structural effect may be changed considerably according to different catalyst parameters such as preparation conditions. This pore
structure effect on catalytic activity is much more significant for higher dispersion, possibly because more homogeneous distribution of metal may result in higher dispersion.
Conclusions It has been shown that the chemical nature of the carbon support, catalyst preparation method, and the pore structure play an important role in the properties of the carbon-supported palladium catalysts. The influence of these factors on the catalyst activity in DNT hydrogenation could be explained by metal dispersion,metal location throughout the pore structure, and surface nature. 1. The amount of surface oxygen groups of carbon support and metal dispersion of carbon-supported palladium catalyst prepared thereof can be increased by oxidative treatment of the support. However, the catalytic activity cannot be improved accordingly due to more uniform distribution of metal into the smaller pores. The increase in hydrophilicity of the carbon surfaces upon oxidative treatment reduces the affinity with nonpolar reactant and leads to lower catalytic activity. 2. To prepare highly active carbon-supportedpalladium catalysts by the alkali hydrolysis method, careful attention should be given to the control of nuclei formation/growth rate during the metal precursor-carbon contact period. Control of metal distribution over the catalyst support can be achieved by some variation in the hydrolysis procedure. The hydrolysis temperature, an important preparation parameter, should be kept as low as possible when carbon black is used as a support. On the other hand, there exists an optimum temperature for activated carbon support which has a considerable amount of oxygen groups on the surface. 3. The alkali hydrolysis method yields catalysts with metal distributed mainly in the wider pores or on the outside of carbon particles in case of nonoxidized carbon supports. The apparent reaction rate over nonoxidized activated carbon-supported palladium catalyst prepared by the alkali hydrolysis method is mainly dependent on the metal distribution in the mesopore range rather than that in the micropore range. Hence catalytic activities of nonporous carbon black-supported palladium catalysts are quite linearly proportional to metal dispersion,while those of porous activated carbon-supported ones have poor proportionality, especially at a higher dispersion range. Registry No. Pd, 7440-05-3;C,7440-44-0. Literature Cited Anderson, J. R. Dispersed Metal Catalysts. In Structure of Metallic Catalyst; Academic Press: New York, 1975;Chapter 4. Boehm, H. P.; Diehl, E.; Heck, W.; Sappok, R. Surface Oxides of Carbon. Angew. Chem. 1964, 76, 742. Chen, G.; Chou, W.-T.; Yeh, C.-T. The Sorption of Hydrogen on Palladium in a Flow System. Appl. Catal. 1983,8, 389. Dovell, F. S.; Ferguson, W. E.; Greenfield, H. Kinetic Study of Dinitrotoluene and m-Dinitrobenzene. Znd. Eng. Chem. Prod. Res. Dev. 1970, 9, 224. Ehrburger, P.; Mahajan, 0. P.; Walker, P. L., Jr. Carbon as a Support for Catalysts, I. Effect of Surface Heterogeneity of Carbon on Dispersion of Platinum. J. Catal. 1976, 43, 61. Hassler, J. W. Nature of Activated Carbon. In Activated Carbon; Chemical Publ. Co.: New York, 1963;Chapter 9. Jenkins, R. G.; Walker, P. L., Jr.; Linares-Solano, A.; RodriguezReinoso, F.; Salinas-Martinez de Lecea, C. Platinum Catalysts Supported on Graphitized Carbon Black-11, Characterization of the Platinum by Small Angle X-ray Scattering and Transmission Electron Microscopy. Carbon 1980, 20, 185. Lecloux, A.; Pirard, J. P. The Importance of Standard Isotherm in the Analysis of Adsorption Isotherms for Determining the Porous
1856
Ind. Eng. Chem. Res. 1992,31, 1856-1864
Texture of Solids. J. Colloid Interface Sci. 1979, 70,66. Linares-Solano, A.; Rodriguez-Reinoso,F.; Salinas-Martinez de Lecea, C.; Mahajan, 0. P.; Walker, P. L., Jr. Platinum Catalysts Supported on Graphitized Carbon Black-I, Characterization of the Platinum by Titrations and Differential Calorimetry. Carbon 1980,20, 177. Machek, V.; Ruzicka, V.; Sourkova, M.; Kunz, J.; Janacek, L. P r e p aration of PtJActivated Carbon and PtJAlumina Catalysts by Impregnation with Platinum Complexes. Collect. Czech. Chem. Commun. 1983,48,517. Mattson, J. S.; Mark, H. B., Jr. Activated Carbon: Surface Chemistry and Adsorption from Solution; Dekker: New York, 1971. Morikawa, K.;Shirasaky, J.; Okada, M. Correlation Among Methods of Preparation of Solid Catalysts, Their Structures, and Catalytic Activities. Adv. Catal. 1969,20,97.
Pradc-Burguete, C.; Linares-Solano, A.; Rudriguez-Reinoso,F.;Salinas-Martinez de Lecea,c. The Effect of Oxygen Surface Groups of the Support on Platinum Catalysts. J. Catal. 1989, 115, 98. Richard, D.; Gallezot, P. Preparation of Highly Dispersed, Carbon Supported, Platinum Catalysts. In Preparation of Catalysts ZV; Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Academic Press: Amsterdam, 1987;pp 71-81. Rylander, P. N. Nitro Compounds. In Catalytic Hydrogenution over Platinum Metals; Academic Press: New York, 1967; Chapter 11. Rylander, P. N. Catalytic Processes in Organic Conversions. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1983;Vol. 4,Chapter 1.
Received for review January 24, 1992 Accepted May 19, 1992
Diffuse Reflectance Infrared and Transient Studies of Oxidative Coupling of Methane over Li/MgO Catalyst Soujanya C. Bhumkar and Lance L. Lobban* School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019
In situ studies of surface species at steady and unsteady state were conducted using Fourier transform infrared spectrometry and transient analysis to study the oxidative coupling of methane over a Li/MgO catalyst. The diffuse reflectance Fourier transform infrared spectroscopic (DRIFTS) technique was employed to monitor the surface features of the catalyst and ita interaction with adsorbates under reaction conditions. The effect of C02on the catalyst activity and selectivity was also studied, and a model based on evidence of the heterogeneous steps and the intermediates involved in the reaction was proposed. Introduction There is much interest in the conversion of natural gas to higher value products such as ethane and ethylene (C2), and a large number of catalysts have been shown to be active for the oxidative coupling of CHI. However, in addition to the desired Cz products, the oxidative coupling process also yields undesirable products such as carbon oxides ((20,). Keller and Bhasin (1982) first proposed a mechanism for the oxidative coupling of methane over a metal oxide catalyst. Other researchers have investigated the kinetics of the reaction over various catalysts, and several mechanisms have been proposed. Lunsford and co-workers (Ito et al., 1985; Driscoll and Lunsford, 1985; Wang and Lunsford, 1986; Driscoll et al., 1985; Aika and Lunsford, 1977; Lunsford, 1984)have characterized the active centers for the formation of methyl radicals over Li/MgO catalyst. Korf et al. (1987) have studied the influence of COPon the reaction, emphasizing the effects of products on the activity and selectivity of the catalyst. The oxidative coupling of CHI over Li/MgO was studied by Tung and Lobban (1992). They used steady-state measurements to discriminate between simple reaction mechanisms. However, these mechanisms did not include the influence of products, nor was any direct information on the surface species' or intermediates' concentrations available. Many other studies have been undertaken (Amenomiya et ai., 1990; Dubois and Cameron, 1990; Garibyan and Margolis, 1990, Lee and Oyama, 1988),but, to date, there have been few direct measurements of the surface intermediates under reaction conditions. It is extremely valuable to observe surface speciea under reaction conditions to determine the changes that occur
* To whom correspondence should be addressed.
in response to changes in the operating conditions. In order to understand important catalyst characteristics and to develop a more complete reaction mechanism, therefore, in situ investigations of surface species at steady and unsteady state were carried out using Fourier transform infrared spectrometry (FTIRS). In particular, the diffuse reflectance Fourier transform infrared spectroscopic (DRIFTS)technique was employed to monitor the surface features of a Li/MgO catalyst and its interaction with adsorbates under reaction conditions. This technique has been similarly employed in other in situ studies of surface species (Ferraro and Basile, 1982; Fuller and Griffiths, 1978; Hamadeh et al., 1984; Schraml-Marth et al., 1990; Prairie et al., 1991). Transient studies were also conducted in a tubular plug flow reactor (PFR) in which differential conversion conditions were maintained. The PFR allows for a high catalyst volume to void volume ratio, which is desirable in transient and dynamic experiments. Reaction paths were distinguiehed qualitatively by the transient responses following changes in reactant partial pressure. Experimental Section FTIR experiments were carried out using a Digilab FTS-40 FTIR spectrometer equipped with a 3200 Data Station computer. Spectra were obtained at various resolutions (8, 4, and 1 cm-') and are presented in the absorbance format. The absorbance format has been shown to be more appropriate for neat samples (i.e. samplea which are not diluted in any matrix) with highly absorbing species than is the Kubelka-Munk format commonly used for diffuse reflectance measurements (Smryl et al., 1983). A Harrick Scientific evacuable diffuse reflectance accessory (DRA) with 'praying mantis" design [Model HVC-DRB] was employed as a reactor (Figure 1). K type thermocouples were used for temperature measurements, and the
0888-5885f 92f 2631-1856$Q3.QQ f 0 0 1992 American Chemical Society
