2632
J . Phys. Chem. 1992, 96,2632-2636
these results is that measurements of other physical or chemical properties of such systems will reflect the composite nature of the system.
Acknowledgment. We acknowledge the skilled technical assistance of A. Pittman and P. Hollins. We are especially grateful
for helpful comments from Professor R. R. Vold on the deuterium NMR spectroscopy. This N M R research was supported by a fellowship of the Sun Refining and Marketing Co. as well as by the Sponsors of the Center for Catalytic Science and Technology at the University of Delaware. C.D. acknowledges the support of the National Science Foundation under Grant CHEM-9013926.
13C NMR Studies of Ethylene Adsorbed on Ru-Y Zeolite
Y.S.Kye, S.X. Wu,and T . M. Apple* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 (Received: June 21, 1991)
The adsorption of ethylene on reduced co-cation-exchanged Ru-Y zeolites has been studied with "C solid-state NMR. Self-hydrogenation of ethylene to ethane and butane is observed at room temperature. 2-Butenes are intermediates in the reaction. Coadsorption of hydrogen and ethylene leads to an increase in the rate of hydrogenation, with the initial rate being roughly first order in hydrogen overpressure. The rate follows a co-cation dependence in the order Ru-H-Y > Ru-Ca-Y > Ru-Na-Y. At intermediate temperatures some butane isomerizes to isobutane in all zeolites studied, presumably through a carbocation intermediate formed at the acid sites either inherent in Ru-H-Y or created upon reduction of the metal in Ru-Ca-Y and Ru-Na-Y. At temperatures above 623 K, carbon-carbon bond cleavage is complete yielding only methane. Following evacuation of samples, signals from residual carbon are observed in the alkyl region, but are not well resolved.
Introduction The adsorption and subsequent reaction of ethylene on group VI11 metal surfaces has been investigated by a number of techniques including electron-energy loss low-energy electron d i f f r a c t i ~ n , ~infrared J~ spectroscopy,"-I3 and nuclear magnetic resonance. l4-I7 Ethylene is capable of forming several intermediates on surfaces, among them a wbonded form, a diu-bonded form, ethylidyne ( W C H 3 ) , ethylidene (=CHCH3), vinylidene (=C=CH2), and acetylide ( - C W H ) . The types and amounts of these intermediates depend on the adsorption temperature, the group VI11 metal, and the surface structure. The study of the reactions of ethylene on Ru surfaces by "C NMR has been reported by Gerstein and c o - ~ o r k e r s They .~~~~~ have shown that ethylene on silica-supported ruthenium is converted to ethane, n-butane, 2-butenes, and strongly adsorbed alkyl groups. Slichter et a1.16 using I3C-l3C and I3C-lH dipolar couplings concluded that, upon adsorption of ethylene on supported Pt, half ( I ) Backman, A. L.; Masel, R. I. J . Phys. Chem. 1990, 94, 5300. (2) Yagasaki, E.; Backman, A. L.; Masel, R. I. J . Phys. Chem. 1990, 94, 1066. (3) Yagasaki, E.; Masel, R. I. Surf.Sci. 1989, 222, 430. (4) Hatzikos, G . H.; Masel, R. I. Surf.Sci. 1987, 185, 479. (5) Zaera, F. J . Caral. 1990, 121, 318. (6) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W. H. J . Am. Chem. SOL.1986, 108, 3554. (7) Hills, M. M.; Parmeter, J. E.; Weinberg, W. H. J . Am. Chem. SOL. 1986, 108, 7215. (8) Baro, A. M.; Ibach, H. J . Chem. Phys. 1981, 74, 4194. ( 9 ) Dubois, L. H.; Castner, D. G.; Sormojai, G. A. J . Chem. Phys. 1980, 72, 5234. (IO) Kesmodel, L. L.; Dubois, L. H.; Sormojai, G. A. J . Chem. Phys. 1979, 70, 2180. ( 1 I ) Soma, Y. J . Caral. 1979, 59, 239. (12) Beebe. T.P., Jr.; Yates, J. T.,Jr. J . Phys. Chem. 1987, 91, 254. (13) Beebe,, T.P., Jr.; Yates, J. T.,Jr. J . Am. Chem. Soc. 1986, 108, 663. (14) Pruski, M.; Kelzenberg, J. C.; Gerstein, B. C.; King, T.S.J . Am. Chem. SOL.1990, 112, 4232. (15) Hwang, S.J.; King, T. S.; Gerstein, B. C., private communication. (16) Wang, P. K.; Slichter, C. P.; Sinfelt, J . H. J . Phys. Chem. 1985, 89, 3060. (17) Gay, I. D. J . Caral. 1987, 108, 1 5 .
of the carbons were nonprotonated and half were present in rotating methyl groups. Thus, they concluded that ethylene forms ethylidyne on supported Pt at room temperature. They reported carbon-carbon bond scission beginning at 450 K. Gay" studied the adsorption of ethylene on supported Pt by 13CNMR and found no evidence for nonprotonated carbon. His results were consistent with adsorption as ?r-bonded olefin; however, on A1203and those catalysts containing Cl-, reaction to di-a-adsorbed species retaining the double bond and to some surface alkyls was reported. We have used "C solid-state N M R to probe the adsorption of ethylene on Ru supported in Y zeolites. The influence of changing support characteristics by co-cation exchange, the effect of temperature on the reaction, and the incorporation of an overpressure of hydrogen on the reaction of ethylene have been investigated.
Experimental Section The Na-Y and H-Y zeolites used as starting materials for making Ru-exchanged Y zeolites were obtained from the Union Carbide Co. and the PQ Corp., respectively. Hexammineruthenium(II1) chloride was obtained from Strem Chemical Inc. Ca-Y zeolite was made from Na-Y zeolite by ion exchange.I8 A mixture of 25 g of Na-Y, 4.2 g of CaCl,, and 200 mL of pure water was stirred at 343 K for 24 h, filtered, washed with deionized HzO, and then dried in an oven at 373 K for about 20 h. The Ru-exchanged Y zeolites were made from Na-Y, Ca-Y, and H-Y zeolites by the same cation-exchanged method. During ion exchange at 343 K, the white color of the 15-g Na-Y (Ca-Y or H-Y) and 2.2-g R U ( N H ~ ) ~mixture C I ~ changed to pink and then purple due to the formation of "ruthenium red".Ig In the oven, the color became gray. The ruthenium weight percent was calculated by measuring the absorbance of the solution at 300 nm before and after cation exchange with a Shimadzu UV-visible 160U spectrometer. Ru-Na-Y, Ru-Ca-Y, and Ru-H-Y were 1.29, 1.28, and 0.95% Ru by weight, respectively. (18) Breck, D. W. Zeolite Molecular Sieve; John Wiley & Sons: New York, 1974; pp 529-588. (19) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemisrry; Interscience: New York, 1972; p 1011 .
0022~3654/92/2096-2632$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2633
13C NMR of Ethylene Adsorbed on Ru-Y Zeolite
a
PPY l
2M.M
,
l
2W.W
,
l
lM.M
,
l
LW.00
,
l
50.W
,
l
0.00
l
l
-50.00
1
1
L
-100.00
Figure 1. 13C single-pulse MAS N M R spectrum of ethylene adsorbed on dehydrated Na-Y zeolite at room temperature.
All Ru-exchanged Y zeolites were dehydrated in a glass vacuum line at 623 K and reduced under static hydrogen at the same temperature using a procedure similar to that of Uytterhoeven et a1.20*21Dispersions for all samples exceeded 0.9. For coadsorption studies of ethylene and hydrogen, ethylene was preadsorbed at 77 K and hydrogen was introduced immediately afterward. The double-labeled [Wlethylene (99%) used for adsorption was obtained from Icon Isotope Co. Instrument grade isobutane (Matheson) and methane (94.1'36, 13C;MSD Co.) were used to confirm chemical shifts of NMR peaks. All adsorptions were performed in a glass vacuum system. Helium gas was used to calibrate the volume of the glass vacuum line. The pressures were measured with an MKS capacitance manometer. For all experiments, ethylene was loaded at a concentration between 17 and 19 mg/g (approximately one ethylene per supercage), Zeolite samples were placed in 5 mm 0.d. tubes for solid-state NMR studies and in 4 mm 0.d. tubes for studies with a liquid-state spectrometer. The amount of zeolite packed into each NMR tube was 0.13 f 0.02 g. Glass wool was used to keep the zeolite in the NMR tube during evacuation. Samples were sealed such that all of the catalyst and any dead space above the catalysts was all contained in the NMR coil. In this way all reaction products leaving the zeolite cages could be quantified. Solid-state NMR data was acquired with a Chemagnetics M-100s solid-state NMR at a frequency of 25.04 MHz for I3C and 99.58 MHz for IH.Adamantane under magic angle spinning was used as a chemical shift reference, the most deshielded peak being 38.55 f 0.02 ppm from tetramethylsilane (TMS). A 9.5mm Kel-F rotor provided by Chemagnetics was used to spin samples sealed under vacuum at 3.0-3.3 kHz. All NMR experiments using the Chemagnetic M-100s were obtained using 4096 scans with a repetition rate of 1 s. Decoupling field strengths were 38 kHz. Checks were performed on several samples to ensure that the same intensities were obtained with larger repetition delays but fewer scans by using identically prepared samples. For experiments using the Varian VXR-200 NMR system, zeolite samples were prepared in 4 mm 0.d. NMR tubes which were inserted into 5 mm 0.d. NMR tubes filled with D 2 0 to provide a means to lock the magnet. A total of 2048 transients were collected for each sample. Broad-band decoupling was achieved with a WALTZ-16 sequence modulated at 9.9 kHz. Some spectra were acquired without using proton decoupling. A custom-built FT mass spectrometer interfaced with a Nicolet FTMS-2000 computer system was used to confirm hydrocarbon reaction products. Results and Discussion As shown in Figure 1, the NMR spectrum of ethylene adsorbed on Na-Y zeolite at room temperature is consistent with physisorbed ethylene. The spectrum remains unchanged with time,
I
200
O
I
100
I
0
I
I
-100
PPm
Figure 2. I3C single-pulse MAS N M R spectra of ethylene adsorbed on Ru-exchanged Na-Y zeolite at room temperature acquired (A) immediately, (B) 2 days, (C) 4 days, (D) 8 days, (E) 12 days, and (F) 14 days after sample preparation.
indicating that ethylene is unreactive. This is in agreement with the results of Denney et a1.22 NMR spectra of ethylene adsorbed on Ru-exchanged Y zeolites at room temperature are shown in Figure 2. Clearly several moieties are produced. These spectra are similar in several respects to those observed by Pruski et al.I4 on Ru-Si02. Resonance assignments may be aided by comparison of chemical shifts of weakly adsorbed species with those of liquids.23 Peak a (1 28 f 2 ppm from TMS) corresponds to carbon-carbon double bonds of unsaturated hydrocarbons. The intensity of this peak continuously decreases with time, while the intensities of peaks b, d, and e increase monotonically. Peak e (2 f 1 ppm) corresponds to ethane formed by hydrogenation of ethylene. This peak is very narrow due to the high mobility of ethane in the zeolite. The chemical shifts of peaks b (27 f 2) and d (16 f 3 ppm) along with the fact that their intensities rise proportionately suggest their assignment to n-butane. Peak b corresponds to secondary carbons (CH,) of butane, while peak d is the terminal carbons. Peak d is quite broad and is never fully resolved. This is most evident in (E) and (F), which are no longer complicated by the presence of peak c (20 f 2 ppm). The NMR intensity in Figure 2F in the region between 10 and 40 ppm may be fit well by two equally intense resonances, a broad one at 20 ppm and a narrower one at 27 ppm. During the first several days after sample preparation, the intensity of peak c increases very rapidly and then decreases. Therefore peak c derives from intermediates in the reaction. Although ethylene contributes to peak a initially, it very rapidly ~~~
(20) Nijs, H. H.; Jacobs, P. A.; Uytterhwen, J. B. J . Chem. Soc., Chem. Commun. 1979, 180. (21) Verdonck, J. J.; Jacobs, P. A.; Uytterhoeven, J. B. J . Chem. Soc., Chem. Commun. 1979, 181.
I
~
~~
~
~
(22) Denney, D.; Mastikhin, V. M.; Namba, S.;Terkevich. J. J . Phys. Chem. 1978,82, 1752. (23) Engelhardt, G.; Michel, D. High Resolurion Solid-Stare N M R of Silicates and Zeolifes;John Wiley & Sons: New York, 1987; pp 368-478.
2634
Kye et al.
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 e
1
a A
a A
‘1
I Ill
i
Figure 3. Plot of concentrations of species versus time: (0) ethylene, ( A ) butane, ( 0 ) butene, (0) ethane. TABLE I: Rate of Ethylene Hydrogenation initial slope initial [HJ, (X10-5), [I3C2H4], mg/g mg/g mg/gs-’ 5.781 1 18.955 0.000 0.117 9.614 2 18.705 14.850 3 17.187 0.222
\ \
initial rate (X10-5), mol/(Ru atoms) 1.52 2.43 4.45
converts to 2-butenes which also give rise to peak c. The resolution obtained in these experiments does not allow separation of the cis and trans isomers. Both 2-butene and n-butane were released from the catalyst upon evacuation as determined by I T mass spectrometry. After 14 days, virtually all of the ethylene and butene has been hydrogenated. In Figure 3 the concentrations of ethylene, butenes, butane, and ethane are given as a function of time as determined from the N M R data. Ethylene persists on the Ru-Y zeolite system for a longer time than observed by Pruski et al.I4 on Ru-Si02. On Ru-Si02 all ethylene is converted to other species within 1 day. If the contribution of 2-butenes to peak a in Figure 2 is equated with the intensity of peak c, the remaining intensity of peak a must be ascribed to ethylene. It is clear that on Ru-Y zeolite ethylene is still present in Figure 2A and B. By 4 days (Figure 2C) virtually all ethylene has disappeared. Ethylene’s longer lifetime on Ru-Y zeolite may be related to the electronic behavior of the very small metal particles formed in zeolites. It has been postulated that small particles may behave as if “electron d e f i ~ i e n t ” . ~Thus, ~ less back-donation into the antibonding orbital of ethylene is expected upon interaction of ethylene with Ru. Figure 4 shows spectra obtained at 50 MHz of ethylene a d s o r b e d with 0.222 mg/g H2 on reduced Ru-exchanged Na-Y zeolite at room temperature. Although the spectra are similar to those in Figure 2, the reaction is more rapid and the proportion of ethane produced is remarkably higher. No further changes are observed in the NMR spectrum after 7 days. In another experiment the H2 dose was reduced by half. The rate and the amount of ethane produced are diminished. The amount of ethylene remaining versus time for the catalysts as a function of hydrogen dose is shown in Figure 5. The initial rate for ethylene reaction on Ru-Na-Y zeolite is dependent upon hydrogen concentration to approximately first order. This agrees with previous I lists the rate destudies of ethylene h ~ d r o g e n a t i o n . ~Table ~ (24) R a b , J.; Shomaker, V.; Pickert, P. Proceedings, Third International
Congress on Catalysis; North Holland Publishing: Amsterdam, 1965; Vol.
2, p 1264. (25) Thomson, S. J.; Webb, G. Heterogeneous Catalysis; John Wiley & Sons: New York, 1968; p 1 1 8 .
F 200
100
0
,
’
Ppm Figure 4. ’)C single-pulse NMR spectra of ethylene coadsorbed with 0.222 mg/g H2on Ru-exchanged Na-Y zeolite at room temperature acquired (A) immediately, (B) 2 days, (C) 3 days, (D) 4 days, (E) 6 days, and (F) 7 days after sample preparation.
h
eR
‘ 0
6
2 3 6
.I
3
C U04
do
; 0
u
02
00
Time (day) Figure 5. Concentration of ethylene versus time for differing initial loadings of hydrogen: (m) 0.00, (A) 0.1 17, and (0)0.222 mg/g.
pendence of loss of ethylene on the initial concentration of hydrogen. Figure 6 presents the reactant and product concentrations of coadsorbed ethylene and hydrogen as a function of time. N M R results for the adsorption of ethylene on reduced Ruexchanged Ca-Y zeolite are shown in Figure 7. The reaction of ethylene to butane and ethane appears to be complete in 5 days. It is interesting that the line width of peak d is reduced compared to spectra obtained from Ru-Na-Y zeolite. This increases the resolution of peaks b and d on Ru-Ca-Y. The decrease in line width may be due to the fewer number of ions contained in these zeolites following substitution of the divalent Ca2+for the singly
"C NMR of Ethylene Adsorbed on Ru-Y Zeolite
0
.3
4 (d
L
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2635
4
+J
F: 0.4
Q,
F:
"L
0
f
A
0
u02 Figure 8. I3CNMR spectra of ethylene adsorbed on Ru-exchanged H-Y zeolite at room temperature acquired (A) immediately and (B) after adsorption and then evacuated at room temperature for 30 min.
0.0 0
2
6
4
Time (day) Figure 6. Plot of concentrations of s ecies versus time for ethylene coadsorbed with H2:( 0 )ethylene, butane, (A) butene and f@ ethane.
&
a
WH
I z5o.00
I
I z0o.00
,
I i50.00
,
J 100.00
l
l 50.00
,
l n.00
,
/ -50.00
,
I
/
-100.m
Figure 9. 13Csingle-pulse MAS NMR spectra of ethylene adsorbed on RuCa-Y zeolite: (A) sample heated at 373 K; (B) same sample without proton decoupling.
200
0
100
-100
PPM
Figure 7. I3C single-pulse MAS NMR spectra of ethylene adsorbed on Ru-exchanged Ca-Y at room temperature acquired (A) immediately, (B) 1 day, (C) 2 days, (D)3 days, (E) 4 days, and (F)5 days after sample preparation.
valent Na+ ions. This then provides more free volume for the included butane. Figure 8A shows the 13CN M R spectrum of ethylene coadsorbed with hydrogen on reduced Ru-exchanged H-Y zeolite. Data acquisition was complete within 1 h of adsorption. Ethylene had completely reacted to form butane and ethane. With time no further changes are observed with 13C NMR. Figure 8B shows the N M R spectrum following evacuation of this sample at room temperature for 30 min. Ethane, shown as
a narrow peak in spectrum A, disappears upon evacuation in B. The remaining peaks originate from butane and more strongly adsorbed species, which still remain after evacuation. Butane diffusion out of the zeolite pores is clearly slower than that of ethane. The intermediates of ethylene's reaction on metal surfaces depend upon the temperature.s Figure 9A is a spectrum of a sample of Ru-Ca-Y which was exposed to ethylene at room temperature and then the temperature was raised to 373 K for 3 days. While ethane and n-butane remain in the sample tube after heating, two new peaks appear at 23 i 1 and -8 f 2 ppm. The peak a t 23 ppm could arise from either cyclobutane or isobutane. To confirm the assignment, the spectrum was acquired without proton decoupling. This spectrum is shown in Figure 9B. The splitting indicates that this peak is due to isobutane. For confirmation, the spectrum of isobutane adsorbed on this same sample is shown in Figure 10A. The chemical shift agrees with that observed in the sample of Figure 9. The resolution does not allow separation of the tertiary and terminal carbons of isobutane. The formation of isobutane from olefins in Y zeolites has previously been r e p ~ r t e d . ~ ~The ? ~ isobutane ' probably arises from conversion of n-butane by acid sites in the zeolite via a carbonium ion intermediate. The peak observed at -8 ppm corresponds to methane produced by the cleavage of the carbon-carbon bond. (26) White, J. L.; Lazo, N. D.; Richardson, B. R.; Haw, J. F. J . Catal. 1990, 125, 260.
(27) Weitkamp, J. In Catalysis by Zeolites, Studies in Surjace Science and Cafalysis;Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; Vol. 5 , p 65.
Kye et al.
2636 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
w YO
C, H '
*
RuCa.Y
5 Days
C, H 4 + F7uH.Y immedlalely
.............................. 7 Days with 1 6 X 10 5 mol pz l O D a y s w i t h O 8 X l O 5molHl
-I
a.a.@i
,
I
mum
,
I
IUDD
,
1L
**-tn
I
rmm
,
I
5am
,
I
,
0.00
I
.
I
,
iso. C,
-%am
Figure 10. 13C single-pulse MAS NMR spectra of (A) isobutane and (B) methane adsorbed on Ru-Ca-Y zeolite at room temperature.
Figure 12. Scheme of the reactions of ethylene adsorbed on Ru-exchanged Y zeolites.
adsorbed on Ru-Na-Y zeolite is immediately converted to ethane and some methane. At 623 K, only methane is observed. Thus, the carbon-carbon bond is completely cleaved in the temperature region between 573 and 623 K. A summary of the reactions of ethylene observed by 13CNMR on Ru-exchanged Y zeolites is shown in Figure 12.
Conclusions m.m
zs~m
I L ) . ~
im.00
w.m
-w.m
D.M
-iw.m
CY
I
25D.00
m.0)
isam
rw.m
5 ~ w
O.DO
.w m
A comparison of the reactions of ethylene on three kinds of Ru-Y zeolites shows that the self-hydrogenation rate has the order Ru-H-Y > Ru-Ca-Y > Ru-Na-Y. These results suggest that the self-hydrogenation of ethylene is more rapid with increasing acidity of the co-cation. On the other hand, the hydrogenation rate depends on the amount of coadsorbed hydrogen13to roughly first order. The presence of polymerization products beyond butane is not observed. The reactions of ethylene are affected by reaction temperature with some rearrangement of butane to isobutane at intermediate temperatures. Carbon-carbon bond cleavage to methane predominates above 573 K.
-IM DO
Figure 11. 13Csingle-pulse MAS N M R spectra of ethylene adsorbed on Ru-Na-Y zeolite: (A) 573 K; (B) 623 K.
Enriched methane adsorbed on a similar zeolite sample is also found at -8 ppm as seen in Figure 10B. Figure 11 shows I3C NMR spectra of ethylene adsorbed on Ru-Na-Y zeolites at elevated temperatures. At 573 K, ethylene
Acknowledgment. This work was supported by the National Science Foundation under Grant CHE8718850. We thank Prof. Rich Shoemaker for helpful discussions. Registry No. C,H,,74-85-1; Ca, 7440-70-2; Na, 7440-23-5; Ru, 7440-18-8.
