J. Phys. Chem. 1983,87,683-689
683
Chemistry of Ru(NH3)2+ in Zeolites. 2. Interaction with Carbon Monoxide Jos J. Verdonck, Robert A. Schoonheydt, and Peter A. Jacobs' Centrum vwr Opperviaktescheikunde en Coiloldale Scheikunde, Kattwlieke universnen Leuven, de Croylaan 42, 8 3 0 3 0 Leuven (Heverlee), Belgium (Received: April 6, 1982: In Final Form: September 20, 1982)
Hexa"ineruthenium(II1) complexes in X- and Y-type zeolites were thermally activated in carbon monoxide and/or water atmospheres. The activation of the materials was followed by volumetric and gas chromatographic techniques and by in-situ IR and UV-visible spectroscopy. It turns out that, at temperatures below 400 K, a pentamminemonocarbonylruthenium(II)complex [Run(NH3)&O]is formed upon activation of the hexammine complex [RUII'(NH~)~] in carbon monoxide. At higher activation temperatures in CO, the major species is a triscarbonylruthenium(1) complex, while in HzO, a biscarbonylruthenium(1)complex dominates. In Y zeolites both species are always present simultaneously, while in zeolite X they can be better isolated. The results are in line with the previously reported low-temperature water-gas shift activity over these samples.
of Na-X and Na-Y zeolites with aqueous solutions of Introduction [Ru(NH3),]C13,has been given earlier,' as well as the Ruthenium-ammine complexes when exchanged in preparation procedure and the origin of the materials used. faujasite-type zeolites interact with several gases. HexThe number after the sample symbol represents the perammineruthenium(II1) in the presence of nitric oxide, cent exchange of Ru"' for Na'. oxygen, or water is transformed into several complexes, Pure CO and mixtures of CO in helium (1and 5% CO some of which have no homogeneous counterpart, since by volume, respectively) were from L'Air Liquide. The they are stable in the cages of the faujasite-type zeolite.' Penta"ine(dinitrogen)ruthenium(II) [ R U ~ ( N H ~ )in~ N ~ ] gases used were of high purity (99.999% by volume). In some experiments, the gases were passed through a therair oxidizes to Ru-red, [ (NH3)5RuORu(NH3)40Rumostatted water saturator, in order to make CO:HzO (NH3)5]6+,while in carbon monoxide a carbonyl complex mixtures. is formed.2 Volumetric Inuestigation. Isothermal adsorption meaThe formation of carbonyl complexes in zeolite Y has surements of CO and temperature-programmed experibeen studied extensively for RhS5 and Ir.6 Hexamminements in CO where done in a low-volume recirculation rhodium(II1) [Rh1''(NH3)6] and pentammine(chloro[rhoreactor, described earlier.'O The circulation circuit condium(II1) [Rh'xx(NH3)5Cl]exchanged in zeolite Y in the sisted of a high-speed pump, a quartz reactor, and a cooling presence of carbon monoxide are transformed into a ditrap in liquid nitrogen. The pressure changes in the system carbonylrhodium(I),M while penta"ine(ch1oro)iridiumwere measured with a pressure transducer which allowed (111) [Irm(NH3),C1]reacts to form an tricarbonyliridiumthe determination of the uptake or release of 0.08 wmol (I).6 The Rh' and Ir' carbonyls seem to be active in the quantities. Pressure measurements were usually made carbonylation reaction of methano1.a' every 180 s and the standard heating rate in temperature Hexammineruthenium(II1) in zeolite Y, activated in a programmed experiments was 83.3 mK s-l. This procedure carbon monoxide/water mixture, is transformed into an allows a determination of the degree of reduction of the active low-temperature water-gas shift (LTWGS) cataRuNa-zeolite as well as the average valency state of Ru. of the active site is a cationic ruthel y ~ t .The ~ ~nature ~ Gas Chromatographic Analysis. The volumetric menium-type carbonyl, the exact structure of which is not thod was found to have limitations mainly at high actientirely clear. vation temperatures, since noncondensable gases were It is therefore the aim of the present work to determine formed under these conditions. Therefore, sample actithe transformations which occur when hexamminevation was done in a continuous flow reactor, with a ruthenium(II1) exchanged in zeolites X and Y is thermally mixture of 5 vol % of CO in helium. On-line sampling of activated in the presence of carbon monoxide and/or the reactant and product gases was done with a six-port water. sampling valve (Valco). Analysis was performed at 310 K Experimental Section on a HP 5880 gas chromatograph, with a 1-m Carbosieve column, a nickel catalyst to transform CO and C02 into Samples and Materials. The chemical composition of CHI, and a FID detector. The analytical data allowed the the RuNa-X and RuNa-Y samples, prepared by exchange calculation of the rates of CO ad- or desorption and of COz formation. (1) Verdonck, J. J.; Schoonheydt, R.A,; Jacobs, P. A. J.Phys. Chem. IR and UV-Visible Spectroscopy. The techniques, in1981,85, 2393. strumental details, and conditions have been reported (2) Madhusudhan, C. P.;Patil, M. D.; Good, M. L. Inorg. Chem. 1979 18, 2384. earlier.' The IR spectra on the Perkin-Elmer 580 B in(3) Primet, M.;Vedrine, J. C.; Naccache, C. J . Mol. Catal. 1978,4,11. strument were taken at elevated sample temperature, while (4) Gelin, P.; Ben Taarit, Y.; Naccache, C. J. Catal. 1979, 59, 357. the UV-visible spectra always were scanned at ambiant (5) Okamoto, Y.;Ishida, N.; Imanaka, T.; Teranishi, S. J. Catal. 1979, 58,82. temperature. In the IR cell evacuation was done in vacuo (6)Gelin, P.; Ben Taarit, Y.; Naccache, C. J . Catal. 1979, 59, 357. and the CO adsorption in a static atmosphere. In the (7) Gelin, P.; Ben Taarit, Y.; Naccache, C. "Catalysis"; Seiyama, T.; quartz reflectance flow cell, evacuation was done in flowing Tanabe, K.; Ed.; Kodansha: Tokyo; Elsevier; Amsterdam, 1980; Part B, helium (at 1.3 cm3s-') and CO adsorption dynamically with p 898. (8) Verdonck, J. J.; Jacobs, P. A. Chem. Commun. 1979, 18.
(9) Verdonck, J. J.; Schoonheydt, R. A.; Jacobs, P. A.; 'Catalysis"; Seiyama, T.; Tanabe, K.; Ed.; Kodansha: Tokyo; Elsevier; Amsterdam, 1980; Part B, p 911. 0022-3654l83l2087-0683$0 1.5010
(10) Jacobs, P.A.; Tielen, M.; Linart, J.-P.; Uytterhoeven, J. B.; Beyer, H.K. J. Chem. SOC.,Faraday Trans. 1 1976, 12, 2793.
0 1983 American Chemical Society
684
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983
Verdonck et al.
TABLE I : Volumetric Results (mmol g-') of Interaction of CO with [ R U ( N H ~ ) ~in] RuNa-Y-40 ~+
co
treatment TP ( C 0 ) - 3 7 3 Ka evacuation-373 K TP ( C 0 ) - 4 2 3 Ka evacuation-423 K TP ( C 0 ) - 4 2 3 Ka evacuation-423 K CO adsorption-293 K TP ( C 0 ) - 4 2 3 K' evacuation-423 K CO adsorption-293 K evacuation-293 K TPD-423 K b
co
consumed (1)
COZ formed (11)
noncondensable gas released (I11 (N,~:H,:co )
0.75
0.21
0.00
0.54
0.97
0.38
1.04
0.86
0.00
0.1F
0.32
1.57
0.00
0.69
0.75 0.09:0.65:0.01)
0.00
0.72 0.00
0.00 0.39
0.00 0.67 0.07:0.40:0.20)
0.72
0.76
0.00
0.00
0.76
1.38
0.00
0.04
0.63
0.00
1.06
d
ads orbed (IV) = (I)-( 11)
CO/RU
CO, formed/ Ru
ads
1.24 1.30
0.00
0.71
0.06
Temperature programmed experiment in CO up the final temperature indicated. Temperature programmed desorption From a up t o the final temperature indicated. Corrected for desorbed ammonia as determined by a blank experiment. mass-spectrometric analysis. a
a mixture of CO in helium (1%by volume). This lower CO content was selected to avoid overheating in the flow cell, since in the present case larger amounts of sample ( 3 g) are used.
C
-
Results Interaction of CO with [Ru=(NH~)~] in RuNa-Y-40, As Derived from Volumetric Measurements. The temperature-dependent interaction between CO and [Rum(NH&] in faujasite zeolites was studied by heating a RuNa-Y-40 sample in CO at different temperatures in the recirculation reactor. The amount of gases consumed, formed, and/or adsorbed are given in Table I. Initially and in the temperature region 293-373 K, part of the CO is transformed into COzand another part is sorbed. The latter part is very close to 1 CO molecule per Ru atom. Subsequently, in the temperature region from 373 to 423 K, an additional amount of CO is taken up and at the same time NH3 is released from the sample. During a second activation in CO up to 423 K, no CO is adsorbed but C 0 2 is formed, as well as N2 and H2. The sample is then subjected repeatedly to the following treatments: (i) sorption of CO at ambiant temperature, followed by (ii) heating in CO to 423 K, and (iii) evacuation at the same temperature. As far as the consumption and sorption of CO is concerned, the behavior of the sample is reproducible during each of the three treatments. Since the amount of C 0 2 formed during two subsequent cycles decreases, it seems that at least part of the C 0 2 is a reaction product of the water-gas shift reaction? CO HzO + C02 + H, (1) Indeed, the amount of physisorbed water is also continuously decreasing during the subsequent thermal cycles and so is the extent to which reaction 1 occurs. The rather small amounts of N2 formed are most probably the result of a catalytic decomposition of NH3 ligand. The amount of CO adsorbed at room temperature and per Ru atom is higher than 1 (actual values are between 1.30 and 1.38). Part of this quantity is very weakly held since it can be desorbed at the temperature of adsorption (Table I, last line). Another part can only be desorbed upon thermal activation of the sample. The latter quantity is equal to the amount of Ru present (Ru:CO ratio is 1:l). Reduction of Rum and Adsorption of CO in RuNa-Y-40. A product of the reduction of [Ru"'(NH,),] in hydrated Na-Y zeolite by CO possibly is COz. However, from previous paragraphs it follows that C 0 2 is also produced via the water-gas shift reaction. So that a more quanti-
r
8 ,*0
.-C
E
'T m
-
EE
-
I
3-
I
I
E
I
P
+
1
I
I
100
200
300
time / min. Flgure 1. Survey of the activation of [Ru"'(NH~)~] in RuNa-Y-40 with CO as derived from the gas chromatographic experiments: A, timetemperature curves for activation of a fresh sample; B, rate of CO, formation (rWJ during the activation procedure (curve c is for a sample prereduced with hydrogen at 773 K); C, rate of CO adsorption and desorption.
tative picture of the reduction of the Ru"' complex could be obtained, the interaction of CO with R u ( N H ~ ) was ~~+ followed under dynamic conditions with a continuous flow reactor on-line with a gas chromatograph. The RuNa-Y-40 sample was subjected to a linear temperature increase while interacting with carbon monoxide. During such run, the rate of CO uptake shows two maxima (Figure 1, A and C) located at 373 and 453 K,
Chemistty of Ru(NH,):+
in Zeolites
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983 685
TABLE 11: Cumulative Amounts of CO Adsorbed and CO, Formed upon Activation of RuNa-Y-40 in COa temp/K
COiRu
CO,/Ru
313 1.11 0.4Tb 423 2.80 0,8gb 6 53 1.19 1.43 As derived from the integrated peak maxima of Figure CO, resulting from reduction of cationic 1, B,a and C. ruthenium.
t
.-5
.-InIn E In
C
i?
CI
frequency / cm-1
Flgure 2. Infrared investigation of the interaction of CO with [Ru"'(NH,),] in RuNa-Y-40: a, sample evacuation at 293 K followed by CO adsorption at the following temperatures: b, 373 K; c, 423 K; d, 458 K; and e, 496 K. The spectra are taken at the treatment temperature.
respectively. At 653 K, CO is desorbed again at a maximum rate. The rate of formation of COz shows three maxima at approximately the same temperatures (Figure 1, A,a and B,a). However, the rather broad maximum around 473 K hardly shows a resolved shoulder at the low-temperature side. The phenomenon associated with this shoulder can be better isolated when a different heating procedure is applied during activation (Figure 1, A,b and B,b). Therefore, the shoulder in the rate of COz formation represents most probably the reduction process. In this way a site can be formed which is very active for COzformation via a catalytic watel-gas shift reaction. The maximum at 653 K (Figure 1, B,a), on the other hand, shows a better resolved shoulder on the high-temperature side. This shoulder is also observed when a RuNa-Y-40 sample, prereduced at high temperatures (773 K) and containing only highly dispersed Ruol' and no residual hydration water is contacted with CO (Figure 1, B,c). It has therefore most probably its origin as a reaction product of the Boudouard reaction on RuO: 2co c cop (2) A corresponding maximum in the rate of CO adsorption does not occur, since in this temperature region part of the previously adsorbed CO is desorbed. The cumulative amounts of CO consumed and C o p formed from a reduction of cationic ruthenium can be obtained by integration of the peak maxima in Figure 1, B(a,b) and C. These data are given in Table 11. It should be noted that the CO/Ru ratios for the first two maxima are, within experimental error, equal to 1 and 3, respectively. Interaction of CO with [ R u ~ ( N ~ &in) RuNa-Y-40 ~] by in Situ ZR and UV-Visible Spectroscopy. The parent sample has bands at 1360 and 1640 cm-' (Figure 2a) which can be attributed12 to vz, G(H-N-H) and v4, G(H-N-H) of 4
+
(11) Verdonck, J. J.; Jacobs, P. A. J. Chem. SOC.,Faraday Trans. 1 1980, 76, 403.
t
c
.e
E
s
n
400 w a v ~ l o n & ~ nm
Figure 3. Interaction of CO with [Ru"'(NH,),] in RuNa-Y-40 by in-situ reflectance spectroscopy: a, evacuation at 293 K followed by CO adsorption at b, 373 K, and c, 423 K, and d, subsequent evacuation at 423 K and readsorption of CO at e, 423 K.
the ammonia ligand, respectively. It is understood that the adsorption at 1640 cm-' is partly due to v2, 6 of water physisorbed on the zeolite. When the sample is heated in CO at 373 K, an intense CO band at 1951 cm-' appears (Figure 2,b). At the same time, the band at 1360 cm-' shifts to 1325 cm-l, while its intensity decreases and a new band appears at 1450 cm-l, which can be assigned to v4, 6 p + ~ )of the NH4+ions. This change in the IR spectrum of the N-H vibrations is typical for reduction of Ru"' 1,12 in Na-Y sieves. Besides all this, two new bands of small intensity are found at 2220 and 1270 cm-'. Upon interaction with CO, the reflectance spectrum of [Ru'"(NH,),] shows three bands at 540,360, and 265 nm (Figure 3, a and b). The band at 540 nm is due to Rured.lJ4J5 The band at 265 nm, also found on the parent (12) Nakamoto, K.'Infrared and Raman Spectra of Inorganic and Coordination Compounds";Wiley: New York, 1978; 3rd ed., p 199.
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TABLE 111: Influence of the Aluminum Content of the Ru-Zeolite of the Faujasite-Type on vco of the Most Intense Peaks V C O , em-' treatment RuNa-X-30 RuNa-Y-40 CO,373K CO. 423 K CO: 493 K
Y
X
473
-
323
v
v)
I
1
I
I
t
I
I
2
4
6
time
/
I
h
Figure 4. Variation of the band intensities of CO chemisorbed on RuNa-Y-40 A, heating in CO atmosphere; B, degassing in vacuo; C, heating in the presence of water vapor: and D, degassing. The bands are at a, 1950, b, 2090, c, 2025, d, 2055, e, and f, 1960 cm-'.
sample, increases in intensity when the sample is heated in CO at 373 K (Figure 3b). The position of the 360-nm absorption is close to a band of Ru-red, but, compared to the intensity of the band at 540 nm, its intensity is too high to be attributable only to Ru-red.14J5As derived from band intensities, it is clear that the formation of Ru-red is suppressed in the presence of CO as well as it was found to be in the presence of O2 and NO.' Indeed, when the sample is vacuum degassed at the same temperature, a higher amount of the Ru-red is formed (compare with ref 1,Figure 2, spectrum 3). After activation in CO at 423 K, the UV-visible spectrum shows bands a t 460, 295, and 240 nm (Figure 3 4 . Upon evacuation at the same temperature only the 240-nm band does not decrease in intensity (Figure 3d). The absorption maximum centered around 295 nm is now seen to be composed of several components. However, spectrum c is regenerated again when CO is readsorbed at 423 K (Figure 3e). From spectrum f, it can be derived that upon activation at 473 K in CO a new band around 380 nm must have been formed. This absorption disappears again after low temperature degassing (Figure 3g). Upon activation of the RuNa-Y-40 zeolite in CO at 423 K, the IR bands at 2025 and 2090 cm-' increase in intensity at the expense of the 1951-cm-l band (Figure 212). A t higher activation temperatures, the former pair of bands starts to decrease also (Figure 2d) and new bands grow at 1960, 2005, and 2055 cm-' (Figure 2e). A plot of the (13)Laing, K.R.;Leubner, R. L.; Lunsford, J. H. Znorg. Chem. 1975, 14, 1400.
(14)Earley, J. E.;Fealey, T.J.Chem. SOC.,Chem. Commun. 1971,331. (15) Stanko, J. A.; Stanishak, T. W. Inorg. Chem. 1969,8,2156.
1939 2010. 2080 1937: 1988, 2040
1951 2025. 2090 19601 2005, 2055
spectral changes of the important bands against the activation temperature (Figure 4A) clearly shows that in a CO atmosphere subsequently three different species are formed, characterized by a single band, a pair, and a triplet of bands, respectively. During the heat treatment also a band at 2220 cm-' disappears at the expense of a new one at 2255 cm-'. The latter band finally is removed completely. Actiuation of [Ru"'(NH&] in CO and H@. The activation described earlier is always done in the presence of a small but not negligible amount of zeolite hydration water. So that the effect of water on the position and intensities of CO vibrations could be determined, RuNaY-40 activated in CO at 493 K was degassed, water was added, and the heating was continued to 513 K (Figure 4B-D). The figure shows that during this process the bands at 2025 and 2090 cm-' increase at the expense of the bands at 1960, 2005, and 2055 cm-'. Upon readdition of CO, the initial spectrum reappears again, which is indicative of reversible changes. The same reversible behavior is also observed with the UV-visible spectra of RuNa-Y-40 pretreated in equimolar amounts of CO and H 2 0 at 493 K (see ref 9, Figure 5). During CO activation bands are found at 390,295, and 240 nm. Upon subsequent exposure to water, bands are found at 460, 295, and 240 nm. The first set of bands reappears when the sample is contacted again with carbon monoxide. Influence of the Aluminum Content of the Zeolite upon the Spectral Behauior of [Ru"'(NH&] in Its Interaction with CO andlor HzO. The major bands in the CO stretching region after different treatments are given in Table I11 for RuNa-X and -Y zeolites. During activation of RuNa-X-30 in CO essentially the same bands are present as in zeolite Y, although they are systematically found at lower wavenumbers (10-20 cm-') for zeolite X. Representative UV-visible reflectance spectra of RuNa-X-15 after typical activation treatments are given in Figure 5. The original spectrum of [ R u ~ ( N H ~at) ~ 275 ] nm (Figure 5a) upon activation in a H20/C0 mixture at 492 K is transformed into an absorption curve with maxima around 460,390,300, and 240 nm (Figure 5b). This spectrum is close to that of RuNa-Y after a similar treatment, although in the former case the wavelength of maxima generally is found at higher values, which is in line with the IR differences. In a CO atmosphere the 390-nm band grows at the expense of the 460-nm band and shifts to 405 nm (Figure 5c). In a water vapor atmosphere, the 390-405-nm band is eliminated and intense bands remain at 460, 295, and 240 nm, even after thorough degassing (Figure 5d). It is demonstrated that on RuNa-X as well as on RuNa-Y the same spectral changes occur when a CO atmosphere is alternated with a water vapor phase. Moreover, analogous changes are revealed both by IR and UV-visible spectroscopy. However, careful examination of the UVvisible spectra for both zeolites (compare Figure 3f with Figure 5, b,c) reveals that, at comparable intensities for the other bands, the 405-nm band is more intense in zeolite X. This is confirmed by the IR results. Two spectra taken
Chemistry of Ru(NH,),~+ in Zeolites
The Journal of Fhysical Chemistry, Vol. 87, No. 4, 1983 687
of carbon dioxide formed. Indeed, the rate of this reaction at the temperature mentioned (its maximum value is 2.3 X lo4 mmol g-' s-')~) is much lower than the actual rate of COz formation during activation of the material, as derived from the gas chromatographic experiments (2.7 X mmol g-' s-'). Under the same conditions, the IR spectra show a single carbonyl stretching vibration in the region 1951-1939 cm-'. A single CO stretching is found for [ R U ( N H ~ ) ~ ( C Oat )JC~~ 1918 cm-'.15 The slight shift to higher wavenumbers can be accounted for by the presence of the zeolitic environment. The formation of NH4' ions, as established by IR, confirms that NH, under these conditions is removed from the coordination sphere of the complex. This can also directly be derived from the intensity decrease of the ligand ammonia band. Bands at 360 and 265 nm in the UV-visible spectrum can be assigned to the presence of the penta"inemonocarbonylruthenium(I1) c0mp1ex.l~ The 360-nm band represents the 'Alg 'T1 transition in a pseudo-octahedral d6environment (i.e., Ru%). The band at 265 nm is a charge transfer band, corresponding to the transition dn(Ru) r*(CO), but may also contain the lAl, 'T2 transition. All tkese data can be rationalized by the following overall equation:
-
t s
-
-
-
+
~ [ R U ( N H , ) ~+] ~3CO + HzO 2[Ru(NH3)5C0I2++ C02
u)
n
+ 2NH4'
(3)
In the presence of hydration water, the hexammine complex is first hydr01yzedl~~J~ into the pentamminehydroxyruthenium(I1) complex: [Ru(NH3)613'
+ HzO
-
[Ru(NH3)5(OH)I2' + NH4+ (4)
This is also the first step in the formation of other hydrolyzed species, such as Ru-red.'J4J5 The present UVvisible results indicate that, in the presence of excess CO, the formation of Ru-red is largely suppressed and that a monocarbonyl is formed, which may be visualized as follows:
-
2[Ru(NH3)5(0H)J2' + 3CO 2[Ru(NH3)5C0lz' 200 Figure 5. Reflectance spectra of Rub-X-15 with Na-X as reference after a, evacuation at 293 K; b, activation at 492 K in equimolar amounts of HO , and CO; c, degassing following by CO adsorption at 373 K; d, repeating step b followed by evacuation at 523 K.
under comparable conditions of excess CO on both zeolite X and Y are distinctly different. On zeolite Y, five different bands, representing the two sets of bands, are present at 2090,2025 and 2055,2005, and 1960 cm-l. In zeolite X the set of bands at 2040,1988, and 1937 cm-' are dominant, while those at 2080 and 2010 cm-' are hardly visible.
Discussion Formation of Pentamminemorwcarbonyl ruthenium(Il) in CO at Low Activation Temperature. Upon activation around 373 K of hexammineruthenium(II1) in faujasitetype zeolites, on the average 1 CO per Ru atom present is adsorbed as can be derived from the volumetric measurements. The same experiments have established that part of the CO sorbed is transformed into C02. The chromatographic measurements show that this C02/Ru ratio is very close to 0.5, which is expected for the complete reduction of Ru"' to Ru". It can be excluded that the water-gas shift reaction would contribute to this amount
+ HzO + COZ
(5)
The small IR bands at 2220 and 2255 cm-l, which disappear above 258 K, represent side reactions of CO with traces of (nitrosy1)ruthenium complexes to form isocyanateruthenium species. Indeed, -NCO absorbs around 2260 cm-' l6 and -NCO- around 2220 cm"." Our earlier work' also shows that traces of (nitrosy1)ruthenium complexes can be formed from hexammineruthenium(II1) in the presence of zeolite hydration water even in a reducing atmosphere. This may occur as follows: Ru-NO + 2CO Ru-NCO C02 (6)
-
+
Formation of Triscarbonylruthenium(I)in CO at Elevated Temperatures. Upon interaction of CO at 423 K or above with the previously formed pentamminemonocarbonylruthenium(I1) complex, the amount of CO adsorbed corresponds to 2.8 CO molecules per Ru atom. This ratio is sufficiently accurate as to indicate that a mixture of bis- and triscarbonylruthenium complexes is formed. In addition to this a supplementary amount of CO is converted into COz,corresponding to 0.42 carbon dioxide molecules per Ru atom (volumetric and chromatographic results). This indicates that Run should be reduced to Ru' (16) Earley, J. E.; Fealey, T. Znorg. Chem. 1973,12, 323. (17) Unland, M.L.J.Cutul. 1973,31,459.Solymosi, F.; h k o , J. Ibid. i977,49,240.
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The Journal of Physical Chemistty, Vol. 87,No. 4, 1983
Verdonck et al.
almost quantitatively according to the following stoichiometric equation: 2[Ru-C0I2+ + CO
+ H2O
NH3
--*
+ H+
2[Ru-CO]+
-
+ 2H+ + C02 (7)
NH4+
(8)
The presence of G,(H-N-H) of ligand NH3 under these conditions confirms that ammonia is still present in the coordination sphere of Ru, and therefore that eq 8 is likely to occur. In the IR spectra, the CO stretching frequencies of the penta"inemonocarbonylruthenium(I1) complex (1951 cm-') decreases and two other bands appear. These two bands are found at 2025 and 2090 cm-'. The antisymmetric and symmetric CO stretching of biscarbonylruthenium(1) in Na-Y is at 2113-2100 and 2048-2023 cm-l, re~pectively.~~'The corresponding vibrations of biscarbonylruthenium(1)on silica are at 2130 and 2070 cm-l,18 and on alumina two different mononuclear low-valency biscarbonylruthenium species have been found, one adsorbing at 2070 and 2002 cm-' and the other at 2050 and 1970 cm-l.19 At higher activation temperatures in CO the doublet at 2025 and 2090 cm-' is further transformed into a triplet of bands at 2055, 2005, and 1960 cm-'. If we take into account that (i) the dominant ruthenium species present under these conditions is Ru' as is derived from the amount of C02 formed, (ii) there exists a mixture of bisand triscarbonylruthenium as is derived from the amount of CO adsorbed, and (iii) upon increasing activation temperature the single CO vibration of the pentammine monocarbonyl is replaced subsequently by a doublet and by a triplet of bands, (iv) increasing temperature means lower NH3 and residual water contents, and therefore higher CO concentration in the ligand sphere, it is logical to assign the IR bands as follows. The bands at 2025 and 2090 cm-' represent the antisymmetric and symmetric CO stretching vibration in biscarbonylruthenium(1) while the triplet of bands at 2055, 2005, and 1960 cm-l are to be associated with the existence of triscarbonylruthenium(1). Given the unknown symmetry of this particular complex when encaged in zeolites, it is not possible to advance a more detailed assignment of the three particular vibrations. Anyway, there is ample evidence of triscarbonylruthenium complexes characterized by three CO stretching vibrat i o n ~ .The ~ ~parallel ~ changes in intensity of the three bands and the volumetric results make an assignment of the lower frequency band to a distinct species of the type monocarbonylruthenium(1) highly improbable. The formation of a triscarbonylruthenium(1) may be visualized as follows: 2[Ru(C0)l2++ 5CO
+ H20
-
2[Ru(C0)3]+ + 2H++ C02 (9)
A further confirmation of the mononuclear nature of these (18) Davydow, A. A,; Bell, A. T.J. Catal. 1977,49, 322. Zhao, Y.; Tesche, B.; Barth, R.; Epstein, R.; Gates, (19) KnBzinger, H.; B. C.; Scott, J. P. Faraday Discuss. 1981, 72, paper 4. (20) Cullen, W. R.;Harborne, D. A. Inorg. Chem. 1970, 9, 1839. (21) Trevati, A.;Ozaneo, A.; Ugugliati, P.; Zingales, F. Inorg. Chem. 1970, 9,671. (22) Calderazzo, F.;L'Eplattenier, F. Inorg. Chem. 1967, 6,1220. (23) Madden, D.P.;Carty, A. J.; Buchall, T. Inorg. Chem. 1972, 11, 1453. (24) Benedetti, E.;Braca, G.; Sbroma, G.; Salvetti, F.; Grassi, B. J. Orgonometall. Chem. 1972, 37, 361. (25) Kuznetsov, V. L.;Bell, A. T. J. Catal. 1980, 65, 374.
complexes is derived from previous work? Indeed, these complexes function as active sites in the low-temperature w a t e q a s shift reaction and the activity is maximum when on the average one Ru species per supercage is present. Formation o f Biscarbonylruthenium(0 in Excess Water. In a CO-lean and water-rich atmosphere the IR bands at 2025 and 2090 cm-' are dominant. In Na-Y the triscarbonyl is transformed into the biscarbonyl through interaction with H 2 0 (Figure 4B). The presence of weak G,(H-N-H) vibrations in the 1350-1270-cm-' region indicates that coordinated ammonia remains at high temperatures in CO and/or H20 atmosphere. These changes can be represented by the following equation:
CO + [RU(NH~)~(H~O),+~(CO)~I+ (10) I1
with x + y < 3. The reflectance UV-visible spectra confirm the changes observed in IR. In a CO-rich atmosphere the 405-nmband is found at its maximum intensity. In a water-rich atmosphere the 460-nm band has grown to its maximum intensity at the expense of the 405-nm band. Unfortunately the nature of the two bands is unknown. By analogy with the IR results, the 460- and 405-nm bands should be typical for bis- and triscarbonylrutheniu(I), respectively. The shift from 460 to 405 nm can be explained if the bands represent predominantly d-d transitions. In such a case, replacement of a weak by a stronger ligand would cause the shift. An alternative explanation is the change in symmetry of these complexes. Influence of the Faujasite-Type Zeolite upon the Nature of the Carbonylruthenium Complexes. For the same absolute yield of Ru in X and Y zeolites, the number of triscarbonylruthenium(1) species formed in a CO atmosphere is higher in zeolite X. In a water atmosphere, also the relative concentration of the dominant biscarbonylruthenium(1) species is higher in zeolite X. This directly is in line with the higher water-gas shift activity of this zeolite. Indeed, RuNa-X zeolites are also more active low-temperature water-gas shift catalysts? Although the reason for this difference is not clear, it suggests that, at least in Y zeolites, the total amount of Ru is not entirely present as Ru' in the supercages. Since no interaction of CO with Ru inside the sodalite cage is possible, this would be a possible site for the catalytically inactive Ru. High-Temperature Interaction with CO. Around 650 K, the cationic complexes are destroyed. From the amount of C02formed, it results that Ru' is further reduced. From the amount of CO adsorbed, it is quite clear that one CO molecule per Ruo remains chemisorbed. This kind of carbon monoxide easily reacts according to the Boudouard disproportionation reaction at higher temperatures. Conclusions
The present work shows that, in hexammineruthenium(111) complexes exchanged in faujasite-type zeolites, the ammonia ligands are gradually substituted by carbon monoxide when the zeolite is thermally activated in the presence of CO. In this way, mono-, bis-, and triscarbonyl complexes can be formed. At the same time a stepwise reduction of Ru"' to Ru' occurs. Steam affects the relative concentration of the bis- and triscarbonyl species in such a way that it competes with CO for a ligand position. The aluminum content of the zeolite definitely influences the concentration of bis- and triscarbonyl-
J. phys. Chem. 1083, 87, 689-695
ruthenium(1) complexes. It remains to be investigated why this is so. Acknowledgment. P. A. Jacobs and R. A. Schoonheydt acknowledge a research position as Senior Research Associate from the Belgian National Fund of Scientific Re-
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search. This research was supported by the Belgian Government (Geconcerteerde Onderzoehakties, Ministerie voor Wetenschapsbeleid). Registry No. CO, 630-08-0; HzO, 7732-18-5; Ru(NH3):+, 18943-33-4; R ~ ( N H ~ ) ~ ( c31418-66-3; ~ ) ~ + , RU(NH,)~(OH)~+, 38331-41-8.
structure of Rotator Phases in n-Alkanes Goran Ungar RMBr Boskovic Institute, 41001 Zagreb, Yugoslavia (Received: June 8, 1982; In Final Form: September 28, 1982)
Polymorphic behavior of crystalline odd-numbered n-alkanes from CllHa to C&52, as well as of binary mixtures, was studied by X-ray diffraction,differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. With increasing temperature alkanes up to C21H44 undergo one first-order transition into an orthorhombic plastic (“rotator”)phase with a face-centered unit cell, space group Fmmm (phase denoted FCO). Longer n-alkanes (C23H48 and C25H52) undergo a further weak first-order transition into a rhombohedral (trigonal)modification 3-5 K below melting point. Its space group is R3m and it has a hexagonal subcell, while the unit cell extends through three molecular layers. In the shortest paraffin, CllH24, the ratio of lateral unit-cell parameters ao/bo in the FCO phase is 1.45-1.47 and is lower than in the ordered orthorhombic form (1.49). However, in the FCO phase of longer alkanes this ratio increases steeply with temperature, the increase being accelerated as ao/boapproaches the hexagonal lattice value of 3lI2. Another C-face-centered plastic phase is present in C23H48 between 39.5 and 41 “C. It is also observed in CzsHszon cooling. The more highly ordered modification, which appears in C26H52 1-2 K below the transition into the plastic state, and which is also found in longer alkanes, is shown by X-ray diffraction to be the modification B previously described in C33H68.
Introduction Odd-numbered n-alkanes with 9 to approximately 4 3 carbon atoms, as well as even ones with 22 to approximately 4 0 C atoms, exist in an orientationally disordered (plastic) crystalline state at temperatures several degrees below their melting points.’ Muller2 first observed that orthorhombic paraffin crystals tend toward, and in some cases reach, hexagonal symmetry on approaching melting temperature. He also proposed that molecules in the hexagonal phase rotate as rigid rods around their long axes and hence the name “rotator” phase for the disordered crystal form. The disordered phase has ever since received continuous attention and has been studied by a number of experimental techniques, as well as theoretically. One of the outstanding studies on polymorphic transitions in n-alkanes is that of Strobl and co-workers (see ref 3 for a review) performed on n-C33H68,where four crystal modifications were found and studied in detail. The highest-temperature modification can be classified as the rotator phase (the term “rotator” phase as used here does not imply existence of free rotation of molecules), but it is not hexagonal and the paraffin chains are tilted with respect to the molecular layer normal. Also, it was found to contain a considerable proportion of nonplanar molecules with one or two gauche bonds. As regards shorter paraffins, much less reliable information exists about the rotator phase. A number of theoretical studies however were performed in the past.44 Recently two primarily vibrational studies were reported, one by Zerbi and co-workers on C1gH4o7and the other by (1)M. G. Broadhurst, J. Res. Natl. Bur. Stand., Sect. A, 66, 241 (1962). (2)A. Miiller, h o c . R. SOC.London, Ser. A , 138,514 (1932). (3)B. Ewen, G.R. Strobl, and D. Richter, Faraday Discuss. Chem. SOC., 69,19 (1980). (4)J. D. Hoffman, J. Chem. Phys., 20, 541 (1952). (5)D. W.McClure, J. Chem. Phys., 49,1830 (1968). (6)D. H.Bonsor and D. Bloor, J. Mater. Sci., 12,1552 (1977). 0022-3654f 8312087-0689$01.50/0
Snyder and co-workers on odd alkanes C17H36 through C29HW8 Whereas previous papers, with the exception of that by Pecholde and the above-mentioned paper of Strobl and c o - ~ o r k e r sviewed ,~ the molecules as rigid all-trans forms displaying rotational and translational motion,lOJ1 the newest studies reveal a considerable fraction of molecules with some gauche bonds both near the chain ends7y8 and, in longer alkanes, also in the chain interior.8 Concerning the structural aspects, already in 1948 Mazee12 observed that the rotator phase in nJ&HG and nC23H48has a base-centered orthorhombic unit cell. Subsequently, however, Larsson13 reported that C19H4,possesses a hexagonal subcell above the rotator transition but that the overall unit cell, which extends through two molecular layers, has orthorhombic symmetry due to the mode in which neighboring layers are stacked. While the present work was in progress, Doucet and co-workers14reported the rotator phase in C17H36, C1gH40, and C21H44 to have a base-centered orthorhombic unit cell, space group Ccmm, which in CsHu and C25H525transforms into a hexagonal modification of undetermined structure a couple of degrees below the melting point. In the present study all odd-numbered n-alkanes from CllHU to C25H52, as well as selected binary mixtures, were investigated by X-ray diffraction, differential scanning calorimetry (DSC), and infrared (IR) and Raman spec(7)G. Zerbi, R.Magni, M. Gwoni, K. Holland Moritz, A. Bigotto, and S. Dirlikov, J . Chem. Phys., 76, 3175 (1981). (8)R.G. Snyder, M. Maroncelli, S. P. Qi, and H. L.Strauss, Science, 214,188(1981);M. Maroncelli, S.P. Qi, H. L. Strauss, and R. G. Snyder, J. Am. Chem. SOC.,104,6237(1982). (9)S. Blasenbrey and W. Pechold, Rheol. Acta, 6,174 (1967). (10)J. D. Barnes and B. M. Fanconi, J . Chem. Phys., 56,5190(1972). (11)J. D. Barnes, J. Chem. Phys., 58, 5193 (1973). (12)W.M. Mazee, R e d . Trau. Chim. Pays-Bas, 67,197 (1948). (13)K.Larsson, Nature (London),13,383 (1967). (14)J. Doucet, I. Denicolo, and A. Craievich, J. Chem. Phys., 75,1523 (1981). (15)J. Doucet, I. Denicolo, A. Craievich, and A. Collet, J. Chem. Phys., 75, 5125 (1981).
0 1983 American Chemical Society
