760
Langmuir 1986,2. 760-765
conditions and for I > 150 dB (1> 145 dB), f 400 Hz, 1, > 0.9 mm, and lo < 0.33 mm. Hence eq 3.3 is more appropriate than (3.6).
Conclusion The experimental investigation that is developed in this paper shows that an increase in the global aerosol collection efficiency of a packed bed is obtained by the actions of an acoustic wave at appropriate intensities and frequencies. The most important acoustic effect on the submicronic aerosol particles is the acoustic turbulent diffusion. A significant increase in collection efficiency is obtained experimentally for particles 0.3 and 0.1 pm in diameter at high acoustic intensities ( I > 145 dB) and low frequencies ( f < 3 kHz). By associating this increase in efficiency to the acoustic turbulent diffusion for submicronic particles, a power law is obtained in term of the Peclet number, for the efficiency of a single spherical collector of the packed bed of spheres. At high intensities and a t low frequencies, good agreements are obtained between the experimental results and calculated values.
= G/(G
global efficiency of the packed bed, with and without acoustic wave frequency, s-l sedimentation parameter gravitational acceleration, m/s' acoustic intensity, W/m2 Boltzmann's constant, J / K Kudsen number, =lg/rp mean free path of the air molecules, =6.6 X lo-@m depth of the packed bed, m internal scale of the acoustic turbulence and characteristic length, m interception parameter Peclet numbers for Brownian and acoustic turbulent diffusions aerosol particle radius, m radius of spherical collectors of the packed bed, m Reynolds number relative to the spherical collectors Stokes number temperature, K mean frontal and interstitial flow velocities, U, = Uo/C m/s velocity amplitude of acoustic wave, m/s relative velocity of turbulent pulsation, m/s coefficients of attenuation, absorption, and scattering of the packed bed, for the acoustic wave, m-l porosity of the packed bed collection efficiency for single spherical collector
amplitude of acoustic oscillations of particle and of the acoustic wave, m
coefficient of dynamic viscosity, kg/m.s density of aerosol particle and of the air, kg/m3 relaxation time of the aerosol particle in the acoustic wave, s angular frequency of the acoustic wave, = 2 ~ frad/s ,
Appendix The efficiencies of different collection mechanisms for a single spherical collector of the packed bed, given in eq 1.3, are calculated by the following expressions where P, = 2RsUo/(DBc) and DB = c k T / ( G ~ ? ~ r , )
qB = 3.98P;213,
Tint
= PI/(PI+
721, PI = r p / R s
+ I), G = ~ ( 2 p , r , ~ g 4 / ( 9 ~ ~ U O ) vimp = st2/(st + 1/12, st = c(2pprp2Uo)/(gYgRs6) Tsed
Symbols A,, A ,
Millikan-Cunningham correction factor, =1 + Kn[1.25 + 0.4 exp(-l.l/Kn)] celerity of an acoustic wave in the air, =343.6 m/s at 20 "C Brownian and turbulent diffusion coefficients,m2/s
Fischer-Tropsch Reactions over Mono- and Bimetallic Solvated Metal Atom Dispersed Catalysts Hiroyoshi Kanai,? Beng Jit Tan, and Kenneth J. Klabunde* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received April 10, 1986. I n Final Form: August 6, 1986 Fischer-Tropsch (F-T) reactions were carried out over mono- and bimetallic solvated metal atom dispersed (SMAD) catalysts (which are prepared at low temperature in highly dispersed metallic form and require no preactivation/reduction step). The F-T reactions were studied at 503 K by using a H,/CO mole ratio of 2. Supported Co, Fe, and Ni monometallic SMAD systems were compared, and Co exhibited the highest activity and showed no deactivation over 4 h. The Ni SMAD catalyst, especially on Si02,was the least effective owing to loss of Ni as Ni(CO),. The Fe SMAD catalysts were very selective for terminal linear alkene formation. Bimetallic Co-Mn and Fe-Mn systems were less active than monometallic Co or Fe systems. It is proposed that the presence of Mn allows the formation of stronger Co-CO bonds leading to lower reaction rates. However, the presence of MnO as a support caused an increase in hydrogenation activity. All catalysts exhibited Schultz-Flory behavior except the Co-Cr system. Selective formation of linear, terminal alkenes may be due to the effect of carbonaceous species originally present on the SMAD catalysts. Overall, the Fe and Co SMAD catalysts were very active compared with conventional FT catalysts reported in the literature.
Introduction The development of new materials for catalytic applications deserves a great deal of attention. New synthetic
* Author to whom correspondence should be addressed. +Visiting Professor from Kyoto University. 0743-7463/86/2402-0760$01.50/0
methods are needed in order to obtain such materials. One novel approach recently described is that of solvated metal atom dispersion (SMAD).',' This new method of catalyst (1) (a) Matsuo, K.; Klabunde, K. J. J. Org. C h e n . 1982, 47, 843-848. (b) Matsuo, K.; Klabunde, K. J. J . Catal. 1982, 73, 216.
0 1986 American Chemical Society
Fischer-Tropsch Reactions ouer
Langmuir, Vol. 2, No. 6, 1986 761
SMAD Catalysts
synthesis employs the use of free metal atoms (vapor) which are solvated a t low temperature. This solution is then allowed to permiate catalyst supports and metal atoms/clusters are deposited. Since SMAD materials are formed under mild conditions in a reduced and dispersed state, they serve as effective catalysts without pretreatment.1-4 They have shown promise in high-pressure Fischer-Tropsch (F-T) reactions: and, recently, bimetallic SMAD catalysts, which were prepared by simultaneous vaporization and solvation of two metals, have exhibited greatly enhanced catalytic activity for hydrogenation, isomerization, and hydrogenolysis of alkenes and cyclop r ~ p a n e .Cobalt-manganese ~ catalysts were of particular interest in that catalytic activity of Co/SiO, was greatly enhanced by manganese a d d i t i ~ n . This ~ effect was explained in terms of an electronic rather than an ensemble e f f e ~ t . ~It was also noted that residual carbonaceous species in the SMAD catalysts may play a role. In order to further explore SMAD catalysts in important reactions, we have carried out F-T reactions6 over mono- and bimetallic SMAD systems, which are reported herein. Recently, several catalysts which have been prepared by decomposition of adsorbed metal carbonyls or mixed bimetallic carbonyls were shown to exhibit unusual behavior in F-T reactions.' For example, Co-Mn systems showed higher activities and sometimes more stability than Co catalysts alone.8 Likewise, Fe-Ru systems prepared from bimetallic Fe-Ru carbonyl compounds exhibited different properties than Fe-Ru catalysts prepared by simply mixing Fe and Ru carbonyls (where intimate mixing of Fe and Ru apparently did not O C C U ~ ) . ~However, fragmentation of Fe-Os carbonyl clusters on SiOz by heat treatment caused the disappearance of bimetallic effects.1° Hydrogen-deficient carbon species which are formed by dissociation of carbon monoxide are believed to be key intermediates for F-T r e a ~ t i o n s . l ' - ~There ~ are several kinds of carbon species which are responsible for the initiation and the retardation of F-T r e a ~ t i 0 n s . l ~As mentioned earlier, SMAD catalyst particles contain carbonaceous residues and are actually pseudoorganometallic in nature because (1)carbon fragments form during catalyst preparation, these fragments remain bound to the metal particle surfaces, and (2) these carbonaceous groups apparently help stabilize the metallic particles toward sin(2) Klabunde, K. J.; Tanaka, Y. J. Mol. Catal. 1983, 21, 57-79. (3) (a) Klabunde, K. J.; Imizu, Y. J. Am. Chem. SOC.1984, 106, 2721-2722. (b) Imizu, Y.; Klabunde, K. J. In Catalysis of Organic Reactions; Augustine, R. L., Ed.; Marcel Dekker: New York, 1985; pp 225-250. (4) Meier, P. F.; Pennella, F.; Klabunde, K. J.; Imizu, Y. J . Catal., in press describes high-pressure F-T reactions over SMAD catalysts. (5) (a) Sachtler, W. M. H.; Santen, R. A. Adu. Catal. 1977,26,69. (b) Ponec, V. Ibid. 1983, 32, 149. (c) Sinfelt, J. C. Bimetallic Catalysts; Wiley: New York, 1983. (6) (a) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: New York, 1984. (b) Henrici-Olive, G.; Olive, S. The Chemistry of the Catalyzed Hydrogenation of Carbon Monoxide; Spinger-Verlag: Berlin, 1984; pp 143-196. (7) (a) Zwart, J.; Snel, R. J. Mol. Catal. 1985, 30, 305. (b) Ichikawa, M. Tailored Metal Catalysts; Iwasawa, Ed.; D. Reidel: Dordrecht, 1985; D 183. (8) Vanhove, D.; MaKambo, L.; Blanchard, M. J. Chem. Res., Miniprint 1980, 4121-4132. (9) Kaminsky, M.; Yoon, K. J.; Geoffroy, G. L.; Vannice, M. A. J. &tal. 1985, 91, 338-351. (10)Chopin, A,; Leconte, M.; Bassett, J. M.; Shore, S. G.; Hsu, W.-L. J. Mol. Catal. 1983, 21, 389. (11)Fischer, F.; Tropsch, H. Brennst.-Chem. 1926, 7, 97. (12) Ponec, V. In Catalysis; Bond, G. C . ; Webb, G., Eds.; The Royal Society of Chemistry: London, 1982; Vol. 5, pp 48-79. (13) Ekerdt, J. G.; Bell, A. T. J . Catal. 1980, 62, 19. (14) (a) Hofer, L. J. E.; Peebles, W. C. J. Am. Chem. SOC.1947,69,893. (b) McCarty, J. G.; Wise, H. J . Catal. 1979, 57, 406. (c) Tanaka, K.; Okuhara, T.; Miyahara, K.; Aomura, K. J. Chem. SOC. Jpn. 1980, 945.
H2
CO
CO-HZ
Vac u um pump
Circulation DUmD
Manometer
Ther
Reactor v
Trap
1
Outlet for sampling
II
I
Figure 1. Circulation reactor for F-T catalysis studies. tering.lv2J5 It is possible that these carbonaceous species could play a role in F-T reactions.
Experimental Section Materials. Metals were obtained from Matheson, Coleman, and Bell (Cr, Co, Fe, Mn), Cerac, Inc. (Co), and Fisher (Ni). Potassium carbonate and Mn(N03)2.6H20were obtained from Fisher and Fluka Chemical Corporations, respectively. The catalyst support materials were Davison S i 0 2 (300 m2/g), Si02-A1203 (100 m2/g), and American Cyanamide y-Alz03 (200 m,/g). These supports were calcined a t 773 K for 3 h in flowing dry air (420mL/min) and cooled in flowing Nz (500 mL/min). Commercially available low surface area MnO (Alpha Products) and MnOz (Fisher) were dried in vacuo. The KZCO3/Al2O3and MnO/AlZO3 were prepared by impregnating the A1203 with aqueous solutions of K2C03 or Mn(N03)2.6Hz0 followed by calcination at 773 K for 3 h in flowing air and Nz, respectively. Solvents were purified according to methods described earlier.2 A high purity 2:l mixture of H 2 / C 0 was obtained from the Linde co. Catalyst Preparation. The metal vapor reactor and catalyst preparation methods have been described in previous publications and references F-T Reactions. Catalytic reactions were carried out in a closed circulation reactor (total volume 287 mL) equipped with a liquid nitrogen t r a p for product collection (Figure 1). A standard procedure for reaction startup was used for each catalyst. I t consisted of evacuating the 100-mg sample a t room temperature for 1h, admitting the H 2 / C 0 mixture (20kPa), raising the temperature to 503 K, and holding the temperature for 15 min. The entire glass system was then evacuated and a new H 2 / C 0 sample (66.5 kPa, 500 torr) introduced onto the 503 K catalyst. The gas mixture was circulated by means of a magnetically driven glass piston p u m p which pumped 280 mL of gas/min. Reaction products other than methane were trapped in the liquid nitrogen trap as they were formed and in this way circulated away from the catalyst. Reaction rates were followed by change in CO concentration. Products were analyzed by GLPC using a Poropak N column (FID, TCD, 6 ft, room temperature) for COz, CzH4,and C2Hs, a bis(2-methoxyethy1)adipate column (25% on Chromosorb-PAW, FID, 24 ft, room temperature) for C , C 7 hydrocarbons, and an active carbon column (TCD, 10 ft, room temperature) for CO and CH4. Traces of > C7 hydrocarbons were observed as small (15) (a) Davis, S. C.; Klabunde, K. J. J. Am. Chem. SOC.1978, 100, 5973-5974. (b) Davis, S. C.; Severson, S. J.; Klabunde, K. J. J.Am. Chem. SOC. 1981,103, 3024-3029.
762 Langmuir, Vol. 2, No. 6, 1986
4.25% 4.64% 3.48% 4.16% 4.16% 3.67% 5.38% 3.64% 5.43%
Kanai et al.
Table I. F-T Reactions rate, lo+ mol (g of Co)-' 5-l CZ 44.3 13.6 (58) 31.6 10.3 (69) 25.7 15.4 (56) 17.8 13.4 (62) 16.2 13.1 (62) 13.5 11.2 (57) 9.88 10.9 (44) 9.66 9.3 (54) 4.05 11.7 (71)
catalvst Co/A1203 Co-Mn0,/A1,03d C0-3.80% Cr/AlzO3 Co-4.24% Mn/AlZO3 co-1.33% Mn/A1,03 Co/SiO, Co/MnO Co/SiOz-AlzO, Co/MnO,
over Co S M A D Catalysts products," % C3
c4
20.9 (92) 21.8 (92) 23.2 (91) 21.3 (94) 20.1 (93) 19.4 (90) 15.9 (63) 20.9 (86) 20.2 (81)
15.3 (90) 20.2 (90) 17.1 (89) 15.4 (93) 15.0 (92) 16.3 (87) 10.7 (71) 15.0 (87) 15.1 (82)
alkene,b %
C5-C7C 14.9 (69) 27.9 (75) 16.9 (69) 16.6 (79) 17.4 (79) 18.9 (71) 16.8 (70) 15.5 (76) 21.1 (74)
Co2 Cl-C, (CZ-C,)
C1
34.1 19.2 26.2 31.2 32.6 32.6 38.2 37.7 20.8
51.0 65.7 57.6 55.4 53.4 51.0 33.3 46.4 57.6
1.1
0.5 1.2 2.1 1.7 1.6 7.4 1.6 11.1
(80.8) (84.6) (80.3) (84.5) (83.7) (80.8) (61.4) (79.3) (78.3)
"Product percentages are expressed in terms of carbon efficiency. The number in parentheses is the fraction of alkene. bFraction of alkenes among Cl-Cj hydrocarbons. The number in parentheses is the fraction of alkenes among C2-C, hydrocarbons. cProducts >C7 were observed in small amounts but not analyzed. dMnO, was prepared by thermal decomposition of 0.73% Mn(N03)2/A1z03at 773 K.
4.67% 4.58% 5.68% 4.93% 3.76% 4.80%
Table 11. F-T rate, lo4 mol (g of Fe)-'s-l 7.84 6.54 6.35 5.47 3.98 2.67
catalyst Fe-5.0% K,C03/A1203 Fe/Al,O, Fe/SiOz Fe-1.40% Mn/A1203 Fe-0.72% Mn/AlZO3 Fe/MnO
Reactions over Fe S M A D Catalysts products," % CZ c3 c 4 cj-c,c 12.4 (95) 16.8 (96) 12.5 (96) 17.0 (82) 15.3 (92) 19.2 (97) 14.0 (96) 16.6 (77) 11.3 (98) 22.3 (100) 20.8 (99) 35.8 (84) 17.1 (95) 20.0 (97) 13.3 (96) 17.3 (80) 20.0 (97) 17.6 (80) 16.2 (93) 13.5 (96) 19.0 (94) 22.4 (96) 15.4 (96) 17.4 (80)
alkene,b % C,-c, 30.3 77.0 15.7 69.3 1.2 85.5 16.5 74.0 11.7 68.4 7.8 73.5
c1
co,
10.9 19.2 8.6 15.8 21.0 18.0
(C*-C,) (93.7) (92.5) (96.0) (94.0) (93.3) (93.3)
"**Thesame as those in Table I. cProducts >C7 were observed in small amounts but were not analyzed. Table 111. F-T Reactions over Ni S M A D Catalysts products: % rate, lo4 mol (g of Ni)-I C2 c3 c4 C5-C7C 2.00 16.9 (21) 17.0 (65) 8.1 (73) 6.0 (58) 1.45 22.8 (73) 16.5 (91) 4.4 (60) 7.0 (91)
catalyst 7.31% Ni/MnO 5.10% Ni/A1,03
The same as those in Table I.
COP 4.4 1.1
C,-Cs (Cz-C,) 24.1 (49.4) 40.9 (81.1)
Fe catalysts
Product distribution 10
C1
47.7 48.2
Products >Ci were observed in small amounts but were not analyzed.
Co c a t a l y s t s
0
alkene,b %
Product distribution
%
0
2 0 3 0 4 0 5 0 6 0 7 0 80 9 0 1 0 0
10
20 30 4 0
%
5 0 60 7 0 8 0 9 0 100
4.67%Fe-5.0%K2C03/A1203
4 . 2 5 % C o / A 1 203
4.58%FelA 1203 4.6 4 % C o -0.7 3 %M n 0
IA1
O3
I
/
\\
5.66%Fe/Si02
3 4 8 % C o - 3 . 8 0 % C r l A l 203
4.16%Co-4.24%Mn/AI2O3
4.93%Fe-1.40%MnlA l2 O 3
4.1 6 % C 0 - 1 . 3 3 % M n / A 1 203
3.7 6 % F e - 0 . 7 2 % M n lS i 0 2
3 . 6 7 %C o I S I 0
4.80%FelMn0 cp
5.3 8% Co / MnO
c3
c4
cs-c7
c1 co2
B a s e d on c a r b o n e f f i c i e n c y
m: : :k:;
3.64%CoISiO2-A I 2 O 3
F i g u r e 3. Product distributions of Fe SMAD catalysts. 5.43%CoIMnO2 c2
c3
c4
c -c7
q
cop
Based on carbon e f f i c i e n c y ,Alkene \Alkane
F i g u r e 2. Product distributions for Co SMAD catalysts. and broad peaks and were not included in Tables 1-111. Reactions of Ethene a n d Propene with Fe SMAD C a t a lysts. One catalyst, 5.78% Fe-1.0% K2CO3/Al2O3(440 mg) was
evacuated at room temperature for 1 h. Ethene (13.3 kPa) was added and the reaction vessel was heated to 373 K for 1.F, h. Gases were analyzed by GLPC. Propene (13.3 P a ) was allowed to react similarly with another sample of the same cataly,t (420 mg).
Results F-T Reactions (General). Rates and product distributions for F-T reactions over Co, Fe, and Ni SMAD catalysts are shown in Tables 1-111 and Figures 2-4. The rates are expressed on a gram of metal basis which gives
Langmuir, Vol. 2, No. 6, 1986 763
Fischer-Tropsch Reactions over SMAD Catalysts Ni catalysts
Product distribution
0
Table IV. Probability f o r a Chain Growth S t e p a n d t h e Fraction of Alkenes alkene selectivity, CY0 wt % * catalyst 4.25% Co/A1203 0.45 48.9 4.64% Co-MnO,/AI2O3 0.57 64.6 3.48% Co-3.80% Cr/A1203 0.50 (C,-C4) 54.5 0.33 (C&) 4.16% co-4.22% Mn/A1203 0.47 54.2 4.16% co-1.33% Mn/A1203 0.47 52.5 3.67% Co/Si02 0.47 49.5 5.38% Co/MnO 0.45 34.1 3.64 % Co/ Si02-A1203 0.45 45.8 5.43% Co/Mn02 0.53 57.0 4.67% Fe-5.0% KzCO3/Al2O3 0.58 75.3 4.58% Fe/A1203 0.52 67.6 0.61 83.8 5.68% SiOz 4.93% Fe-1.40% Mn/A1203 0.56 72.5 3.76% Fe-0.72% Mn/Si02 0.52 67.2 4.80% Fe/MnO 0.54 71.7 7.31% Ni/MnO 0.33 22.9 5.10% Ni/A1203 0.33 38.4
%
2 0 3 0 40 5 0 6 0 7 0 8 0 9 0 1 0 0
10
7.31%Ni/MnO
5.1%Ni/A1203
cp
c3 c,' c>-c7
c1
CO2
B a s e d on c a r b o n e f f i c i e n c y
,Alkene -Alkane
Figure 4. Product distributions for N i SMAD catalysts.
I
I
nProbability for a chain growth step. *Alkene fraction in hydrocarbons. Table V. Composition of n -Butane a n d 1-Butene i n C4 Hydrocarbons
-3
catalyst 4.25% Co/A1203 4.64% Co-Mn0,/A1203 3.48% Co-3.80% Cr/A1203 4.16% Co-4.24% Mn/A1203 4.16% Co-1.33% Mn/Al2O3 3.67% Co/Si02 5.38% Co/MnO 3.64% Co/Si02-A1203 5.43% Co/Mn02 4.67% Fe-5.0% K2C03/A1203 4.58% Fe/A1203 5.68% Fe/Si02 4.93% Fe-1.40% Mn/A1203 3.76% Fe-0.72% Mn/Si02 4.80% Fe/MnO 7.31% Ni/MnO 5.1% Ni/A1203
t 1
2
3 4 5 6 Carbon number
7
Figure 5. Schultz-Flory plots (mp = weight fraction; p = carbon number) for F-T reactions over 4.16% C0-1.33% Mn/A1203 (O), 4.93% Fe-1.40% Mn/A1203 (A), a n d 5.1% Ni/A1203 (0).
a good representation of the metal utilization. Product distributions (selectivities) are expressed in terms of carbon efficiency.16J7 Alkene selectivities are expressed as the fraction of alkenes among C145 and C245 hydrocarbons.16 Product distributions in each case appear to obey the Schultz-Flory equation (Figure 5),18 except in the case of Co-Cr. The probability for a chain growth step, a, is shown in Table IV. The C4 fraction is divided into nbutane, 1-butene, trans-Zbutene, and cis-2-butene. Isobutane and isobutene were not observed. The ratios of n-butane/C4 hydrocarbons and 1-butene/butenes are taken as indicators of the ability of each catalyst for hydrogenation and isomerization of alkenes, respectively (Table V). Co, Co-Mn, and Co-Cr SMAD Catalysts. Monometallic Co SMAD catalysts showed the highest activities and lowest CO, formation in F-T reactions. The addition of Mn caused a lowering of reaction rate and Mn/Co ratios of 0.34 and 1.1yielded similar activities and product dis(16) Commereuc, D.; Chauvin, Y.; Hugues, F.; Basset, J. M.; Olivier,
D.J . Chem. SOC.,Chem. Commun. 1980, 154.
n-C,H,rJ/ (n-C4HIo+ C4H8) 0.10 0.10 0.11 0.07 0.08 0.13 0.29 0.13 0.18 0.04 0.04 0.01 0.04 0.04 0.04 0.27 0.09
1-C4&/ (l-C4H8+ 2-CdH8) 0.67 0.69 0.68 0.66 0.63 0.58 0.97 0.87 0.89 0.96 0.86 0.96 0.94 0.88 0.93 0.63 0.59
tributions. However, addition of Cr caused a higher percentage of C2 and C3 hydrocarbons to be formed and a break in the Schultz-Flory plot was observed a t C4 hydrocarbon. Also, the Co-Cr SMAD catalyst showed an increased a, the chain growth probability (for C1 C2 and C3). Further work on this particular catalyst is planned. Addition of M I I ( N O ~ )to~ the Co/A1203 system, after thermal decomposition, allowed a C0-Mn0,/A1203 catalyst to be prepared. (It should be noted that ignition of Mn(NO,), on y-A1203is reported to form Mn(III).)lg This catalyst exhibited a slightly decreased reaction rate but increased chain growth and alkene content. Support effects were pronounced for Co SMAD systems. Activities decreased in the order A1203> Si02> MnO > Si02-A1203> MnO,. Product distributions for A1203and SiOz were similar. However, large amounts of hydrogenated products were formed by using the MnO support. Also, terminal alkenes were highly favored over this support (Table V). Similar changes in selectivities were observed for Mn02-supported Co catalysts. However, the M n 0 2 system had one complication in that M n 0 2 was
-
(17) van den Berg, F. G. A.; Glezer, J. H. E.; Sachtler, W. M. H. J .
Cotal. 1985, 93,340. (18) Henrici-Olive, G.; Olive, S. Angew. Chem., Int. Ed. Engl. 1976, 15, 136.
~~
(19) Selwood, P. W.; Moore, T. E.; Ellis, M.; Wethington, K. J. Am. Chem. SOC.1949, 71, 693.
764 Langmuir, Vol. 2, No. 6, 1986 partially reduced under reaction con2itions by Hz/CO and large yields of COz resulted. Fe and Fe-Mn SMAD Catalysts. The characteristic feature of the Fe SMAD catalysts is their selectivity for production of alkenes, especially terminal alkenes, compared to conventional impregnated catalystsz0(Tables I1 and V). The probabilities for chain growth and alkene selectivity are highest for these systems. The addition of Mn as a second component slightly lowered the reaction rate and only slight changes in selectivities were noted. However, a potassium ion promoted catalyst was more active than the nonpromoted, a finding also true for conventional Fe F-T catalysts.21 In our system the presence of potassium ion also facilitated chain growth and alkene formation, but about 1/3 of the CO converted ended up as
cop
Support effects were less pronounced on rates for the Fe SMAD systems (as compared with Co). The same general order of activity was observed; A1,0, > Si02 > MnO > Si02-A1,03 > MnO,. In this system, MnO as a support caused a preferential formation of lower alkenes (C2-C4), while Si02 caused a preferential formation of higher alkenes (C5-C7). Ni SMAD Catalysts. These systems performed poorly in F-T reactions, compared with both Co and Fe SMAD materials but also with conventional Ni catalysts.22 In the case of Ni/Si02 SMAD, the Ni was readily lost from the support surface due to Ni(C0)4formation and evaporation (as confirmed by mass analysis). The Ni SMAD systems yielded low chain growth and alkene selectivities. Methane was produced a t 50% carbon efficiency. Interestingly, MnO was the best support for Ni, based on F-T catalysis activity. However, alkene selectivity was low. Hydrogen, Ethene, and Propene Reactions. When a potassium ion promoted Fe/A1203 SMAD catalyst was treated with hydrogen a t 488 K, the product evolved was mainly CH4. Ethene treatment of another sample of this catalyst at 373 K yielded small amounts of propene, nbutane, and butenes. When propene was used as the reactant, the products were ethane, ethene, 1-butene, isobutene, cis-2-butene, hexenes, and CO,. The ratio of C2H,/C2H6/C02was 73:16:11.
Discussion Our earlier work with SMAD catalysts has shown that active metals such as Co and Ni are very highly dispersed.,r3 For example, a 2.3% c o on SiOz system was 50% dispersed according to Hz chemisorption s t ~ d i e s .The ~ addition of a small amount of Mn actually aided further dispersion of Co so that essentially 100% Co dispersion was achieved even with higher Co loadings; i.e., all atoms of Co were exposed and on the surface. Furthermore, our earlier studies of alkene hydrogenation and isomerization and cyclopropane hydrogenolysis demonstrated that Mn addition had little influence on selectivities but a dramatic influence on activities. These results suggested an electronic effect of added Mn rather than an ensemble effect. In the present study SMAD catalysts once again show unique behavior, albeit quite different and perhaps unexpected. Let us briefly enumerate these findings: (1) Co and Fe SMAD catalysts are highly active in F-T reactions compared with conventional systems. Rates ex(20) Amelse, J. A.; Schwartz, L. H.; Butt, J. B. J. Catal. 1981, 72,95. (21) (a) Dry, M. E. In Catalysis-Science and Technology, Anderon, J. R., Boudsrd, M., Eds.; Springer-Verlag: Berlin, 1981; pp 191-194. (b) McVicker, G. B.; Vannice, M. A. J. Catal. 1980, 63, 25. (22) (a) Vannice, M. A. J. Catal. 1975, 37, 449. (b) Jensen, K. B.; Massoth, F. E. Ibid. 1985, 92, 109. (c) Vannice, M. A,; Garten, R. L. J . Catal. 1979, 56. 236.
Kanai et al. pressed in mole of CO per gram of metal per second have been estimated from extrapolation of Arrhenius plots.21b*22 The rates shown in Tables I and I1 are very high when compared with those of similar F-T catalysts prepared by conventional means or by metal carbonyl deposition/deThus, literature values of 2.5-3.4 ymol (g of metal-' s-l zlb compared with 2.6 up to 44 for our SMAD systems. If we assume 100% d i s p e r s i ~ nturnover , ~ ~ ~ numbers for these Co SMAD catalysts are in the range 2.61 X to 2.38 X s-l and those of Fe SMAD catalysts are in the range 4.37 X to 1.49 X s-l a t 503 K. It should also be noted that the Co SMAD system exhibited no deactivation during the F-T reaction over 4 h, whereas the Fe system was slightly deactivated in this reaction time. (2) Bimetallic Co-Mn and Fe-Mn SMAD catalysts exhibit lower activities than Co and Fe SMAD monometallic systems. Product distributions were effected only marginally. Since selectivities are only slightly affected by addition of varying amounts of Mn, ensemble effects do not appear to be important, and any electronic effects present work to the slight detriment of the reaction. It may be worth speculating on the reason for this lack of activation by Mn. In another F-T catalysis study the presence of Mn(I1) with Rh caused significant activation, although high pressures of CO were used in this In the present work much of the Mn may be in partially oxidized states since the presence of C 0 2 and H20 (as products) would probably cause oxidation of Mn On the other hand, if some Mn survived as Mn metal, it might, through electronic effects, cause Co to interact more strongly with CO, thereby reducing the reaction rate slightly, as has been found for Cu additions to transition metals in F-T studieseZ4 This idea is supported by the fact that the presence of potassium ions increases the reaction rate (this work), suggesting that is not Mn ions that are important here. Also note that Mn ions (MnO from Mn(N0J2 decomposition) had less of a deactivating effect than Mn metal and a more significant effect on selectivity. Whatever Mn is doing, it is interesting that its effect on Co is very different in hydrogenation/ hydrogenolysis reactions3 than in low-pressure F-T reactions. (3) Support effects also appear to have some importance. Earlier work with Co/A1203, Co/SiO,, Ni/A1203, and Ni/Si02 showed that metal dispersion was not significantly influenced by changing from A1203to Si02(although more highly acidic supports did influence Ni dispersion).z In the present work, however, Co/Alz03was more active than Co/Si02. The strength of interaction with A120, may be important here; EXAFS studies revealed a stronger interaction of Ru or Ni with AlZO3than Si02.25J6A similar effect might be expected with the Co and Ni SMAD catalysts. Our work certainly supports this idea a t least with Ni since Ni was readily lost as Ni(C0I4 on S i 0 2 but not on A1203. A particularly interesting case is the Fe/SiOz SMAD catalyst. No significant rate effect for Fe/A1203 vs. Fe/ Si02was observed, but the SiOz supported system greatly enhanced higher hydrocarbon formation (C5-C7,see Figure (23) Wilson, T. P.; Kasai, P. H.; Ellgen, P. C. J. Catal. 1981, 69, 193-201 and references therein. (24) (a) Araki, M.; Ponec, V. J. Catal. 1976,44,439. (b) Bond, G. C.; Turnham, B. D. Ibid. 1976,45,128. (c) Shah, Y. T.; Perotta, A. H. Ind. Eng. Chem., Prod. Res. Deu. 1976, 15, 123. (25) Asakura, K.; Yamada, M.; Iwasawa, Y.; Kuroda, H. Chem. Lett. 1985, 511. (26) Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J . Catal. 1980, 65, 335.
Langmuir 1986, 2, 765-770 2). Subtle effects seem to be a t work here, and perhaps further work will elucidate these. When MnO and Mn0, were used as supports, unusual effects were also noted. Since these materials are much lower in surface area than the A1203or SiOz materials, reaction rates cannot be rationally compared. However, product distributions were changed significantly. The greater hydrogenation activities may be due to electron withdrawal of MnO or M n 0 2 from the Fe, Co, and Ni particles, thereby weakening the metal-C0 interaction and enhancing metal-H2 interactions (as expressed earlier by Sachtler and co-workers)." (4)Perhaps the most interesting behavior of Co and Fe SMAD catalysts is their selective formation of linear, terminal alkenes. There may be some connection between the facts that (a) adsorbed C, CH, and/or CH2 are likely intermediates in the F-T mechanism15aand (b) such species are already present on SMAD catalyst parti~1es.l~The presence of these species where methylene species are statistically a b ~ n d a n t ~has ~ . been ' ~ ~ demonstrated by earlier work3J5 and supported by the present work in that hydrogen, ethene, or propene exposures to the catalysts under mild conditions yielded CH, hydrogenation (CH,), CH2 oligomerization, and dimerization, as well as metathesis products.27
CHz=CHR
Fe
765
+ Fe=CH2 + CH2=C-CH3 I R
/CH2\
E Fe
\
/CH2\
/CHR CHZ
/
+
Fe\CHR/CH2
Fe=CHR Fe
+
+
CHz=CH2
or
CH,=CHCH,R
Sites favorable for CH2 formation may be formed during SMAD catalyst preparation and perhaps such sites facilitate further CH2 formation and/or stzbilization during F-T catalysts. That insertion of such species into C=C bonds and rapid release of oligomerized product before isomerization take place would explain such selective linear, terminal alkene formation. A model for a SMAD catalyst particle must be presented a t a later date, after surface spectroscopic studies have been completed.
Acknowledgment. The generous support of the National Science Foundation is acknowledged with gratitude. Registry No. Fe, 7439-89-6; Co, 7440-48-4; Ni, 7440-02-0; Mn, 7439-96-5; Cr, 7440-47-3; CO, 630-08-0; MnO, 1344-43-0. (27) Hugues, F.;Besson, B.; Basset, J. M. J . Chem. Soc., Chem. Commun. 1980,719.
Effect of Corona Discharge Treatment of Poly(ethy1ene terephthalate) on the Adsorption Characteristics of the Fluorosurfactant Zonyl FSC As Studied via ESCA and Surface Energy Measurements L. J. Gerenser,* J. M. Pochan, J. F. Elman, and M. G. Mason Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received May 2, 1986. I n Final Form: July 21, 1986 ESCA and surface energy measurements have been used to study the effect of corona discharge treatment (CDT) of poly(ethy1ene terephthalate) (PET)on the adsorption characteristics of the cationic fluorosurfactant Zonyl FSC. Oxidation of the surface via CDT causes the surfactant to adsorb uniformly and continuously over the surface, whereas on non-CDT PET the surfactant coverage is incomplete. Within a monolayer coverage regime, the surfactant molecules reorient as the coverage is increased. The ESCA data suggest that the surfactant molecule is chemisorbed to the CDT PET surface due to an ion-exchange process. Models and correlations are derived to relate dispersion energy measurements to surface coverage as determined by ESCA.
Introduction ESCA is an important analytical tool for ascertaining the surface composition of thin films.'P2 Surface energies obtained via contact angle measurements are used to describe the wettability and other surface characteristics of materialsS3p4 While contact angle measurements are (1)Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergmark, T.; Karlsson, S.; Lindgren I.; Lindberg, B. ESCA Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy; Almquest and Wiksells: Uppsala, 1967. (2) Clark, D.T.In Advances in Polymer Science; Springer-Verlag: New York, 1977; pp 126-187. (3) Wu, S. Polymer Interface and Adhesion; Marcel1 Dekker: New York, 1982. (4) Osipow, L. I. Surface Chemistry; Reinhold: New York, 1962.
0143-1463/S6/2402-0165$01.50/0
thought to be influenced by the first chemical layer, ESCA can detect the first 10-50 A of a substrate surface. We have been actively involved in understanding the role of surfactant and substrate structure on the adsorption characteristics of surfactant molecules. Recently, we presented a study of the adsorption characteristics of the fluorosurfactant Zonyl FSC on clean SiO, and clean poly(ethy1eneterephthalate) (PET).5 In that study ESCA was used to probe the structural characteristics of an adsorbed monolayer of FSC on each of the surfaces studied. On SiOz it was shown that FSC coverage is uniform and continuous a t all coating thicknesses and that the sur(5) Gerenser, L. J.; Pochan, J. M.; Mason M. G.; Elman, J. F. Langmuir 1985, 1 , 305.
0 1986 American Chemical Society
