Langmuir 1989,5, 319-325
319
IR Investigation of Ni(CO)*Interaction with Silicalite, ZSM-5, Zeolite-Y, and yAl,O, K. Mohana Rao,? G. Spoto, and A. Zecchina" Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitci d i Torino, c.so M.D'azeglio 48, 10125 Torino, Italy Received June 1, 1988. I n Final Form: October 25, 1988 The interaction of Ni(C0I4 with Lewis and Bronsted acid centers at the surface of ZSM-5, H-Y, and 7-Al,03 gives 0-bonded adducts with approximate CBvsymmetry. The (Al)' stretching mode essentially localized on carbonyls 0-bonded to Lewis acid centers is lowered by about 100-150 cm-' with respect to the unperturbed carbonyl molecule. The downward shift of the (Al)' mode is roughly equal to the sum of the upward shifts of the remaining and E modes of the surface adducts. In Ni(C0)40-bonded (or hydrogen bonded) to Bronsted sites (H-Y zeolite), the perturbed carbonyl frequency is 40 cm-' lower than that of the unperturbed carbonyl, while the other modes are practically unaltered. On all samples, upon lowering the pressure, a decomposition process is favored with formation of subcarbonylic species and small neutral particles.
Introduction Anchoring of metal carbonyls on various supports for the preparation of catalysts has been the subject of many investigations.' The anchoring process follows different routes depending upon the basicity, acidity, and hydroxylation state of the support. On surfaces dominated by the chemical properties of the Lewis acid sites, the coordination of the metal cluster carbonyls occurs via 0-bonding2 of the bridging CO. Similarly, well defined mononuclear adducts 0-bonded to coordinatively unsaturated A P Lewis centers are observed upon the interaction of group VI mononuclear metal carbonyls with fully dehydroxylated y-A1203.394Nearly identical results have been found for the interaction of Fe(C0)5on fully dehydrated ZSM-5, H-Y, and 7-AI2Op5 It is most noticeable that the IR spectra of surface 0-bonded adducts are nearly identical with those of the homogeneous analogues, prepared by the interaction of metal carbonyls with soluble Lewis acids (i.e., AlCl, and/or A1BqJe6 T h e interaction of metal carbonyls with protons (Bronsted acids) has been found to occur under homogeneous conditions.'p8 Moreover, there are numerous reports which confirm such an interaction on surfaces. For example, Bein e t a1.8 have observed that the absorption of Fe(CO)6 in H-Y zeolite causes a distinct perturbation of the acidic OH groups (Bronsted acids) present in the supercages: this observation has been interpreted in terms of hydrogen bonding between the oxygen of the carbonyl group and the proton. In a recent study some of us have confirmed this interpretation and have demonstrated that a parallel perturbation (although smaller in magnitude) is induced on the stretching mode of the hydrogen-bonded carbonyl. On this basis we have suggested5 that 0-bonding is always the primary step during the interaction of metal carbonyls (both mononuclear and polynuclear) with solid surfaces having acidic (Lewis and Bronsted) character. In order to prove the general validity of this hypothesis, we report and discuss the IR spectra of Ni(C0)4adsorbed on silicalite (highly dehydrated, no Lewis acidity), ZSM-5 (highly dehydrated, Lewis acidity only), H-Y (low sodium NH4+,dehydrated a t 573 K, Bronsted acidity only), H-Y (nearly totally dehydrated a t 1073 K, Lewis acidity only), and 7-A1203(totally dehydrated a t 973 K, Lewis acidity only). *Author to whom correspondence should be addressed. 'From Chemistry Department, Indian Institute of Technology, Bombay, India.
0743-7463/89/2405-0319$01.50/0
Experimental Section The dehydroxylated ZSM-5 sample was obtained by heating a thin pellet (20 mg/cm2)of ZSM-5 (Montedipe,%/AI = 28) under Torr) for 3 h at 1073 K. After this thermal vacuum treatment, the intensity of the hydroxyl stretching modes (in the 3750-3500-cm-' range) is negligible. This ensures that the Bronsted acidity is almost nill. A similar thermal treatment was performed on the silicalite pellet. In this case the OH stretching modes were barely detectable. The H-Y zeolite was obtained from NH4+-exchangedY-zeolite (Linde, low sodium LZ-Y82),and the typical composition was as follows: SiOz 72.2 w t % Na20 0.2 wt %
(NHM
22.8 wt % 4.4 wt %
Na20/A1203
molar ratio molar ratio
5.38 0.01
Si02/A1203
A1203
The protonated form (H-Y) was obtained according to the procedure described by Ward.s H-Y samples outgassed at 573 K contain Bronsted acidity pred~minantly.~J~ Complete dehydroxylation and the maximum Lewis acidity state were achieved by outgassing the samples at 1073 K under vacuum for 3 h. Even though the sample treated at this high temperature underwent some skeletal degradation, this had no effect on the present study. Fully dehydrated r-A1203 (Degussa,aluminium oxide C) was obtained by degassing the sample at 973 K for 1 h. This sample contains maximum surface Lewis acidity. The morphology of this sample as observed by Reller and Cocke" was reported to be microparticles of perfect single crystals of hexagonal shape (diameter 10-20 nm and thickness < 5 nm). The Ni(C0)4(STREM)was dosed from the gas phase following condensation at 77 K and double distillation. The IR spectra were obtained by using a Perkin Elmer 580 B instrument equipped with a data station, and the spectra reported (1)Jacobs, P. A. In Studies in Surface Science and Catalysis; Gates, B. C., Guczi, L., Knozinger, H., Eds.; Elsevier: Amsterdam, 1986;Vol. 21,p 357. (2)Tossier-Youngs, C.; Correa, F.; Ploch, D.; Burwell, R. L., Jr.; Shriver, F. Organometallics 1983,2, 898. (3) Bilhou, J. L.; Theolier, A.; Smith, A. K.; Basset, J. M. J.Mol. Catal. 1977178,3,245. (4)Zecchina, A.; Platero, E. E.; Arean, C. 0. Inorg. Chem. 1988,27, 102. ( 5 ) Rao, K. M.; Spoto, G.; Guglielminotti, E.; Zecchina, A. Inorg. Chem., in press. ( 6 ) Horwitz, C. P.; Shriver, D. F. Adu. Organomet. Chem. 1984,23,219. Fink, P.; Kohler, A. Z. Chem. 1977,17, 41. (7)Winde, H.; Faraday Trans.I 1984,80, (8)Bein, T.; Jacobs, P. A. J. Chem. SOC., 1391. (9)Ward, J. W. J. Catal. 1967,9, 225. (10)Hughes, T. R.; White, H. M. J. Phys. Chem. 1967, 71, 2192. (11)Reller, A.; CocKe, D. L., private communication.
0 1989 American Chemical Society
Rao et al.
320 Langmuir, Vol. 5 , No. 2, 1989 in the figures were corrected for background absorption.
Results and Discussion Adsorbing Sites on Silicalite, ZSM-5, H-Y Zeolites, and 7-A1203. 1. Dehytfrated Silicalite and ZSM-5. Both silicalite and ZSM-5 belong to the same pentasil family12 and have identical structure and similar pore diameter (-5.3 A). Silicalite does not show significant Bronsted or Lewis acidity due to the absence of aluminium ions. The external and internal surface is very covalent and is highly hydrophobic. Consequently, the adsorption on the fully dehydrated surface of silicalite is thought to be dominated by van der Waals forces, which are predominantly of a physical nature. ZSM-5, on the contrary, contains highly acidic protons, and following complete dehydration, strong Lewis acid centers are generated (the process is accompanied by some decomposition of the structure). The Lewis centers are generally considered to consist of coordinatively unsaturated aluminum ions exposed on the external and internal surfaces.
AI 3+
'I' 2. H-Y and Fully Dehydrated H-Y. Y-zeolite belongs to the fujasite family. The diameter of the pore is larger (-7.8 A)13 with respect to the ZSM-5, and consequently, the penetration of larger molecules (such as metal carbonyls) is easier. At outgassing temperatures lower than 600 K, Bronsted acidity is dominant?JO Outgassing at 1073 K is known to cause nearly total dehydroxylation of the sample and the appearance of the maximum Lewis acidity. Also, in this case, the zeolite structure is partly modified. The Lewis centers are thought to be coordinatively unsaturated aluminium ions. 3. y-A120p yA1203 has a defective spinel-type structure with the A13+ions of the bulk occupying both tetrahedral and octahedral holes (A13+tet/A13+oct = 8/13 (1/3)).14 Consequently, both A13+(tet(cus) and A13+oct(cus) ions (Lewis acid centers) will emerge on the surface, although not necessarily in the same ratio as in the bulk (cus means coordinatively unsaturated). An important difference between dehydroxylated zeolites (ZSM-5 and H-Y) and -pAl,03 is that on the zeolites the Lewis centers are isolated (particularly on ZSM-5, because the Al/Si ratio is very small) whereas on -pAl,O, they are not. For this reason the surface Lewis acid properties of r-Al,O, cannot be considered simply as the mere sum of the properties of individual coordinatively unsaturated A13+t,t and A13+octsurface sites. Indeed, surface models have been constructed which show that the surface coordinative unsaturation can be shared by both tetrahedral and octahedral A13+ ions.15 Consequently, it is more appropriate to speak in terms of Lewis sites whose acidity is predominantly determined by A13+,,, and A13+octions, (12) Jacobs, P. A.; Beyer, H. K.; Valyon, J. Zeolites 1981, 1, 161. (13) Zeolite Chemistry and Catalysis; Rabo, J. A,, Ed.; ACS Monograph 171, American Chemical Society; Washington, D.C., 1976. (14) Lippens, B.C.; Steggerda,J. J. In Physical and Chemical Aspects of Adsorbents and Catalysts; Lingen, B. G., Ed.; Academic Press: New York, 1970; Chapter 4. (15) Knozinger, H.; Ratnaswamy, P. Catal. Reu.-Sci. Eng. 1978, 17, 31.
respectively. The second difference between dehydrated H-Y, ZSM-5, and 7-A1203lies in the fact that nonisolated Lewis acid sites communicate electronically via inductive e f f e ~ t s .Consequently, ~ the adsorption on one site necessarily modifies the adsorptive properties of adjacent sites (through the solid chemical effect). On 7-A1203,besides Lewis acid sites we have to consider the reactivity of 02-cus ions (which represent the other component of the surface: OH- groups are not considered here, because our data concern only the fully dehydrated samples). The nucleophilic activity of 02 ,, toward simple mononuclear carbonyls is well documented for the Ni(CO),/MgO, Fe(CO),/MgO, and Me(CO),/MgO systems16J7 and leads to the formation of carbenoid-type complexes, following the reaction MeOKO),
+
Mg2+02-cus
-
0. M g 2 +I, C C -
Meo C O ) , -
I
'-
These complexes are characterized by a pair of characteristic bands at 1480-1525 and 1050-1063 cm-l associated with the stretching modes of C 7 . 0 groups. A similar reaction has been hypothesized for Mo(CO), on 7-A1203,although without having evidence of the formation of the carboxylate-type peaks mentioned above.18 IR Spectrum of Ni(C0)4 and Its Lewis and Bronsted 0-Bonded Adducts. Ni(C0I4 is a tetrahedral molecule with Td symmetry in the gas phase. The CO stretching representation is rc0= A, + T2 and only the T2 (triply degenerate) mode is IR active (actually one IR band at 2058 cm-' (T2)is found).Ig In solution, Ni(C0)4behaves very similarly, but the only small difference is represented by the shift of the T2mode to 2045 cm-'. The Raman active (A,) mode is found in the range 2132 (gas)-2125 cm-l (~olution).'~Due to the similarity between the state of Ni(C0)4in the condensed phase and physically adsorbed on covalent surfaces (the van der Waals forces predominate in both the cases), the spectrum of Ni(C0)4in solution will be used as a reference for the assignment of the IR spectrum of Ni(C0)4physically adsorbed on the surfaces. The formation of adducts with Lewis acid surface sites via 0-bonding leads to surface species with approximate CSusymmetry.
Ni I C I
m
y f
*
= coordinatively unsaturated Lewis center
Hereafter, the adducts with sites whose acidity is mainly due to A13+,, and A13+,t will be named as type a and type b adducts, respectively. For reasons briefly outlined above, type a adducts should also be present on 7-A1203surfaces together with adducts involving octahedral A13+oct(cua) ions (type b). (16) Guglielminotti, E.; Zecchina, A. J. Mol. Catal. 1984, 24,331. (17) Zecchina, A.;Garrone, E.; Guglielminotti, E. Catalysis (London) 1983, 6,90. (18) Laniecki, M.; Burwell, R. L., Jr. J . Colloidal Interface Sci. 1980, 75,95. (19) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London, 1975; pp 186-188 and references therein.
Ni(CO)4Interaction with Zeolites
Langmuir, Vol. 5, No. 2, 1989 321 Table I
physically adsorbed
Al
adduct a
T2 (Alh E
silicalite (1073 K), cm-'
ZSM-5 (1073 K), cm-'
2120 2053-2042
2120 2040 2145 2095 1903
('4112
adduct c
(573 K), cm-'
H-Y
(1073 K), cm-'
H-Y
T-Al,O, (973 K), cm-'
2130-2135 2060-2050 2145 2100 1900 2130-2135 2065-2050 2005
2130 2060-2050 2145 2100-2090 1900
2130 2055 2150-2140 2100-2080 1950-1900
(Alh
E
2080 (?)
2065 1985 1835
The I'co of the 0-bonded adducts a and b (local symmetry C3J is rc0= 2A1 + E (E = doubly degenerated)
1860 (?)
1850 (?)
NI CCO& Td
By analogy with the known homogeneous 0-bonded analogues6 and with the heterogeneous ones formed by interaction of metal VI carbonyls on 7-A12034)and interaction of Fe(C0)5 on ZSM-5, H-Y and 7-A1203,50bonding has the following specific effects: (1) The stretching band of the 0-bonded CO (A1)2is expected to be red shifted (A3 = -150 cm-' for the a-type complex and AP = -100 cm-' for the b type). The red shift of the mode is expected to be smaller than that of the hexacarbonyl and pentacarbonyl, because the Lewis basicity of the terminal CO is in the order6 Me(CO)6> Me(C0)5 > Me(C0)4. (2) The stretching modes of the remaining Ni(CO)3 moiety (a total of two IR bands: A, E) are expected to be blue shifted with respect to the modes of the unperturbed carbonyl. On the basis of the results shown in ref 4 and 5, the sum of the upward shifts of the (A,), modes and of the degenerate E mode should be equal to the downward shift of the (AJ2 mode: A3(Al), + 2ApE AF(A,)~i= 0
+
+
The expected approximate IR pattern for a-type and btype adducts is schematized in Figure 1. In this figure the IR peaks of the various types of 0-bonded adducts are located at frequencies that are assumed to be independent of the surface coverage changes. However, at least for 7-A1203,this is a crude approximation, because, due to the chemical effects (through solid), the Lewis acidity of the non-independent sites is expected to change with 8 (vide infra). On hydroxylated samples the formation of (CO)3NiC0-H-0 structures (adduct c) must also be considered because of many reports regarding hydrogen bonding, both under homogeneous6p20r21and heterogeneous conditions.6b,22.23
ck
t?
co
\J;/ I
c I
0
Y
0 (20)Edgell, W. F.;Hegde, S.; Barbetta, A. J. Am. Chem. Soc. 1978, 100,1406. (21)Wilkinson, J. R.;Tood, L. J. J. Organomet. Chem. 1976,118,199. (22)Ballivet, D.;Tatchenko, T.; Courdurier, G. Inorg. Chem. 1979,18, 558.
c2v
a' I
I
AI'+
I
Figure 1. Schematic representation of the vibrational modes of Ni(C0)4adducts formed upon interaction with different types of Lewis sites.
The spectroscopic effect of hydrogen bonding is to cause a blue shift of one of the carbonyl frequencies6 and of the stretching band of the acidic OH groups. The shift of the CO stretching mode should be
