5247
J . Phys. Chem. 1987, 91, 5247-5251 predissociation of the ground-state complex ion, leading to the fragmentation into ChHsOan HN(CH3)3+. Most of the excess energy after the dissociation may be converted into vibrational energies of the phenoxyl radical in the ground state. Summary and Conclusions (1) Characterization of the neutral complex C6HsOH-N(CH3)3 was done by fluorescence spectroscopy. Dissociation energy of the neutral complex was found to be 4500 A 1000 and 5320 A 1000 cm-' for the So and SI states, respectively. Those states are characteristic of the hydrogen-bonded heterodimer of C6HsOH with N(CH3),. (2) Two-color photoionization provides the yield spectrum for the complex cation generation via the S, state of the neutral complex. The threshold energy of the yield spectrum shows an upper limit of adiabatic ionization potential of the complex (ca. 56 400 cm-') which is extremely reduced by the complex formation from that of free phenol. The dissociation energy was found to be about 16700 cm-' (2.07 eV) in the ground-state complex cation. (3) The protonated fragment ion HN(CH3)3+is efficiently generated from the photoionized complex cation whose excited state is found to be responsible for the protonated fragmentation.
(4) Correlation between energy levels of the complex cation and the dissociation limit suggests the following photodissociation route: C,H jOH-N(CH,),
hui
C,HSOH*-N(CH3)3
hY2 ---*
dissocn
C6HsO-HN(CH3)3+-!% C6HsO*-HN(CH3)3+ C6HSO + HN(CH3)3+ (5) The above process occurs at hu2 photon energy of about 25400 cm-I, which corresponds to the local excitation of the phenoxyl radical in the complex ion. (6) In conclusion, it is has been demonstrated that a combination of the mass-selected photoionization and the fluorescence spectroscopies is quite useful for investigation of the dissociation process of the complex ion. Acknowledgment. We are grateful to Prof. Mitsuo Ito for valuable discussions and encouragement during this work. We also thank Y . Sugahara for assistance in the early stage of this work. Registry No. HN(CH3)3+,16962-53-1;C6H,0H, 108-95-2;N(CH3),, 75-50-3.
Adsorption of Carbon Monoxide on ZSM-5 Zeolites. Infrared Spectroscopic Study and Quantum-Chemical Calculations Leonid M. Kustov, Vladimir B. Kazansky, The N . D . Zelinsky Institute of Organic Chemistry. Academy of Sciences of USSR, Leninsky Prospekt 47, I 1 791 3 Moscow, USSR
Stanislav Beran, Ludmila Kubelkovi,* and Pave1 Jiru The J . Heyrovskj Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, M6chova 7 , 121 38 Prague 2, Czechoslovakia (Received: August 25, 1986)
Low-temperature adsorption of CO was studied on H-ZSM-5 zeolites modified by dehydroxylation, ionic exchange with A13+,and impregnation with A1203and on Na-ZSM-5 and CaH-ZSM-5 zeolites. It was found that interaction of CO with framework OH groups results in the formation of a hydrogen-bonded CO complex whose OH bond frequency is decreased by 310-320 cm-l compared with that of free hydroxyls. For the less acidic framework hydroxyls in large cavities of H70Na30:Y zeolite the observed shift is 275 cm-I. With ZSM-5 zeolites, at least six types of electron-acceptingsites are observed originating from nonframework A1 species (bands of CO in the interaction complex: 2132, 2222,2202, 2195, and 2198 cm-I) and the A1203microcrystalline phase (CO band at 2153 cm-I). The CO bond orders calculated by the CNDO/2 method for the CO interaction complexes with models of surface sites increase in the following order: >0-CO < >OH-CO r +AI-CO Na-CO < alumina-CO z Ca-CO < >Si-CO < Al(cationic)-CO. A correlation between the calculated bond orders of CO and the observed vibrational frequencies of CO-forming interaction complexes is drawn.
Zeolites have found broad application as acid-type catalysts, whose proton-donor and electron-acceptor acidic sites of different nature are commonly regarded as the active centers. Proton-donor sites can be investigated directly by infrared spectroscopy (IRS) in the region of the OH bond stretching vibrations, where the band shifts resulting from the formation of hydrogen-bonded complexes of hydroxyls with adsorbed weak bases characterize their proton-donor ability (acid strength). However, IR spectroscopic identification and characterization of electron-accepting acid sites can be performed only indirectly using the adsorption of test molecules.' Carbon monoxide has often been employed to study aprotic sites of various adsorbents and catalysts.'-'' For zeolites, however, (1) Ward, J . W. Zeolite Chemistry and Catalysis; ACS Monograph 171; Rabo, J., Ed., American Chemical Society: Washington, DC, 1976; p 118.
0022-3654/87/2091-5247$01 .50/0
the field of its application has long been limited to cationic forms predominantly of Y zeolite^.^-'^ As for the Z S M family, only (2) Angell, C. L.; Schaffer, P. C. J . Phys. Chem. 1966, 70, 1413. (3) Fenelon, P. J.; Rubalcava, H. E. J . Chem. Phys. 1969, 51, 961. (4) Rabo, J. A.; Angell, C. L.; Schomaker, V. Proc. Int. Congr. Catal., 4th 1971, 2, 96. (5) Huang, Y . Y , J . A m . Chem. SOC.1973, 95, 6636. (6) Bregadze, T. A.; Seleznev, V. A.; Kadushin, A. A,; Krylov, 0. V. Izu. Akad. Nauk SSSR, Ser. Khim. 1973, 2701. (7) Egerton, T. A,; Stone, F. S . J . Chem. Soc., Faraday Trans. 1 1973, 69, 22. (8) Huang, Y . Y. J . Catal. 1974, j2, 482. (9) Kasai, P. H.; Bishop, Jr., R. J.; McLeod, Jr., D. J . Phys. Chem. 1978, 82. 279. (10) Dombrowskii, D.; Dyakonov, S. S.;Kiselev, A. V.; Lygin, V . I . Kinet. Katal. 1978, 19, 1067. ( 1 1 ) Ballivet-Tkatchenko, D.; Courdusier, G. Inorg. Chem. 1979, 18, 558. (12) Lokhov, Yu. A,; Davydov, A. A. Kinet. Katal. 1980, 21, 1515, 1523.
Q 1987 American Chemical Society
5248 The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
Kustov et al.
TABLE I: Composition and Sorption Capacity of Zeolites"
no.
zeolite
Si/AI (lattice)
1 2 3 4
Na-ZSM-5 H-ZSM-S(a) AI j H-ZSM-5 AlllH-ZSM-5 H-ZSM-5.AI203 CaH-ZSM-5 H-ZSM-5 (b) H-ZSM-S(c)
19 19 19 19 13.6 19 24 30
5 6 7 8
100 Na+/Alr 77 5 5 3 3 2 5 5
SC(Ar), mmol/g
'4120, per unit cell
100 AI3+/Alr
100 Ca2+/Alr 7
5.3
5.5
1 0.2 0.3
4 8
5.6 5.5 5.1 7.5 n.d. n.d.
5.5
n.d. 44
"n.d., not determined; AIf, number of AI atoms in the framework; SC(Ar), sorption capacity measured with Ar.
recently has brief information appeared concerning H-ZSM- 1 1 ze01ite.I~ The present paper deals with the IRS study of the adsorption of C O at 77 K and low pressures on H-ZSM-5 zeolites modified by dehydroxylation, ion exchange with AI3+, and impregnation with A1203as well as on Na-ZSM-5 and CaH-ZSM-5 zeolites. Quantum-chemical calculations of the interaction complexes of C O with cluster models of different types of sites then assist not only in the interpretation of the observed C O bands but also in a more detailed discussion of the origin and properties of the active acidic sites.
Experimental Section ZSM-5 zeolites 1-6 in Table I were synthesized according to the procedure described in ref 18. Na-ZSM-5 was then obtained by calcination at 870 K for 6 h in an oxygen stream. NH4-ZSM-5 was prepared from Na-ZSM-5 by using repeated ion exchange with a 0.5 M solution of NH,NO3. AI ions were introduced into this zeolite by ion exchange in a 0.1 M A1(N03)3solution. An increase of their concentration was attained by repetition (twice) of this procedure. Preparation of CaNH,-ZSM-5 was carried out in the same way with a 0.3 M solution of Ca(NO&. HZSM-5Al2O3 zeolite was obtained by impregnation of NH4ZSM-S(a) with A1(N03)3, followed by calcination in an oxygen stream at 800 K for 6 h. r-A1203and silica gel (Cabosil) (150 and 200 m2/g, respectively) were also studied for the sake of comparison. The zeolites are characterized in Table I by the silicon-toaluminum ratio of the original sample before modification, which is denoted as (Si/AI),,,,ic,. Amounts of Na and Ca ions as well as AI added by ion exchange are related to the amount of lattice AI and expressed in the atomic ratios. For the impregnated sample, the number of added A1203molecules is calculated per unit cell of H-ZSM-5. The chemical composition of the samples was determined by using wet chemical analysis. The sorption capacities measured with Ar after evacuation of the zeolites at 670 K are given in the last column of Table I. They show that neither collapse nor appreciable plugging of pores occurred during the modification. Thin pellets of the samples (4-12 mg/cm2) were pressed and placed in a glass IR cell fitted with KBr windows. This is suitable both for thermovacuum pretreatment of samples up to 1070 K in its upper part and for IR measurements at 77 or 300 K in the bottom of the cell that can be cooled with liquid nitrogen. The samples were preheated at 670 K for 16 h in vacuo ( Pa). The rate of the temperature increase was 10 K/min. The zeolites were then treated in steps (2 h) at 870 and 1070 K in vacuo. At (13) Ione, K. G. Paukshtis, E. A,; Mastikhin, V. M. Stepanov, V . G.; Nefedov, B. K.; Yurchenko, E. N. Izu. Akad. Nauk SSSR, Ser. Khim. 1981, 1717. (14) Novlkovd, J.; Kubelkovl, L.; Wichterlovd, B.; JuSka, T.; DolejSek, 2. Zeolites 1982, 2, 17. (15) BoslEek, V.; Brechlerovl, D.; Kiivlnek, M. In Adsorpfion o f H p drocarbons in Microporous Adsorbents; Schirmer, W.; Stach. H., Eds.; Academy of Sciences of GDR: Berlin, 1982: p 26. (16) Paukshtis, E. A.; Yurchenko, E. N. Usp. Khim. 1983, 52, 426. (17) Tsyganenko, A. A.; Denisenko, L. A.; Zverev, S. M.; Filimonov, V. N. J . Catal. 1985, 94, 10 and references herein. (18) Argauer, R. J.; Landolt, G. R. U.S. Patent 3 702886, Nov 14, 1972. (19) Kubelkovl, L.; Novlkovl, J.; Beran, S., submitted for publication in J . Mol. Catal.
TABLE 11: Shifts of Bands of OH Vibrations Caused by Formation of Hydrogen-Bonded Complexes with CO (at 77 K ) and C6H6 (at 300
K) AuOH, cm-'
uOHIcm-l
zeolite ZSM-5 ZSM-5 ZSM-5 ZSM-5 SiO, H,,Na,*-Y
300 K 3612-3615 3665 3725 3745 3747 3647
77 K 3615-3620 3670 3730 3750 3750 3652
C O (77 K) C6H6 (300 K ) 310-320 260 240
350 280
90 275
120 326"
"See refer 22.
L 1 A , 1700 3500 1100 Yltm.'1 3100
Figure 1. IR spectra of OH groups in the H-ZSM-S(b) zeolite before (1,2) and after the adsorption of CO (3). Temperature of measurement was 300 K (1) and 77 K (2, 3).
each stage in the calcination, the IR spectra of the samples were measured before and after C O adsorption at 77 K and equilibrium pressure until 65 Pa. The amount of adsorbed carbon monoxide was determined by using a Pirany manometer to measure the pressure change in the volume of the IR cell. IR spectra in the 4000-16QO-cm-' range were recorded with a Nicolet MX-1 E Fourier-transform infrared spectrometer. Carbon monoxide was purified by using traps filled with K-A zeolite and Cu chips. Results and Discussion The Interaction of CO with Hydroxyl Groups. The IR spectra of studied zeolites revealed several types of OH groups with the characteristic bands listed in Table 11. The most important ones are the bridged framework hydroxyls of the Si-OH-A1 type, which are responsible for the high acid strength of the H-ZSM-5 zeolites and whose intense band at 3612-3616 cm-' was found in the spectra of all our samples pretreated at 670 K and measured at room temperature, except Na-ZSM-5; in Na-ZSM-5 no framework-bridged hydroxyls were detected. In addition, OH groups analogous to SiOH on silica were present in all the zeolites. Their band at 3745 cm-I, however, was always of considerably lower intensity than the band of the framework HO hydroxyls. Two more weak bands at 3725 and 3665 cm-' were observed in
The Journal of Physical Chemistry, Vol. 91. No. 20, 1987 5249
Adsorption of C O on ZSM-5 Zeolites 2'95J
TABLE III: Heights of Bands of Adsorbed CO in the Spectral Region of 2235-2220 and 2205-2185 cm-'
2172
absorbance, cm2/g, at
Si/A1
2175
J.
L 11 I!
sample H-ZSM-5(a)
---I -2
A11H-ZSM-5 AIIIH-ZSM-5 H-ZSM-5sA1203 H-ZSM- 5(b) H-ZSM-S(c)
pretreatment 2222 2195 (lattice) in vacuo 1, K cm-' cm-' 19 670 0.34' 1.05 870 6.13 2.71 9.14 2.71 1070 0.4Y 0.30 19 670 12.8 0.45 1070 1.04" 0.64 19 670 1070 11.6 0.92 framework OH of HNa-Y > O H of H-ZSM-5 vibrating at 3670 cm-' > OH of H-ZSM-5 vibrating at 3720 cm-' > > OH groups of S O z , in agreement with the present knowledge on the acidity of zeolites. Observed values of the shifts of OH bands caused by the adsorption of CO at 77 K are similar to those found after adsorption of benzene at room temperature (see Table 11).
Figure 2 depicts the IR spectrum of H-ZSM-5 zeolite pretreated at 670 K, measured in the region of stretching vibrations of carbon monoxide. It consists of an intense line at 2175 cm-', a shoulder at 2170-2160 cm-', and a band with a maximum at 2138 cm-'. The latter is observed in the IR spectra of all the samples under study at any pretreatment temperature. Its position is very close to the vibration frequency of CO in the gas (2143 (20) Peri, J. B. In Cutalysis; Hightover, J. W., Ed.; Elsevier: Amsterdam, 1973; Vol. 1, p 329. (21) Scherzer, J. In Catalytic Materials: Relationship between Structure and Reactiuiry: Whyte, T. E., Dalla Betta, R. A., Derouane, E. G., Baker, R.T.K., Eds.; American Chemical Society: Washington, DC, 1984, ACS Symp. Ser. No 248, p 157.
cm-I). This carbon monoxide is weakly adsorbed as it can be removed by evacuation at 17 K. Therefore, the band at 2138 cm-' should be assigned to C O molecules physically adsorbed in zeolite channels. The band near 2170 cm-' can be attributed to the complex of CO with the remaining Na+ cations as the IR spectrum of carbon monoxide adsorbed on Na-ZSM-5 zeolite contains a single intense band at 2172 cm-l (Figure 2). In connection with the results obtained with Na-ZSM-5, it should be mentioned here that adsorption of CO on the CaH-ZSM-5 zeolite was accompanied by the appearance of a strong band at 2195 cm-I. This is depicted in Figure 2 for the sample dehydroxylated at 1070 K. Similar C O band positions were published in ref 1-3 for Na and Ca faujasites. The most intensive band of H-ZSM-5 zeolites with a maximum at 2175 cm-' corresponds to C O complexes with framework hydroxyls, as after dehydroxylation of the samples at 870 and 1070 K the intensity of this band sharply decreases. The frequency of C O vibration in this complex should reflect the energy of interaction with the OH group, so that its decrease is expected with the decrease of acid strength of the O H group. Indeed, for silanol groups of silica gel the adsorbed C O was found to vibrate at 2157 cm-'. Interaction of CO with A1 or Si Electron-Accepting (Lewis) Sites. It is known that nonframework AI species can be introduced into zeolites by ion exchange and that they can act as a strong electron-accepting sites even after pretreatment at 670 K because of the thermal stability of their hydroxyls which is lower than that of the framework hydroxyls. According to ref 13, 15, and 16, CO interaction complexes with nonframework A1 Lewis sites are characterized by bands near 2230 and 2190 cm-I. It can be seen from Table 111 that both H-ZSM-5 and Al,H-ZSM-5 zeolites pretreated at 670 K exhibited very weak CO bands in these regions. Only a small increase of the broad band at 2230 cm-' was observed when the amount of added A1 increased, Le., with AlIIH-ZSM-5 zeolite. As H-ZSM-5 was impregnated with Alz03,no such bands were found, but a new specific band appeared at 2153 cm-I. this band was also present in the spectrum of the AlIIH-ZSM-5 zeolite (Table 111). A substantially more pronounced effect on C O adsorption was found after vacuum treatment carried out at 870 and 1070 K. From the IR spectra of CO complexes depicted in Figure 3 it can be seen that there are up to five bands, 2232, 2222, 2205, 2195, and 2108 cm-l, in the spectral region 2235-2185 cm-', which can be divided into two sets in the 2235-2220- and 2205-2185-cm-' ranges. Their intensities are represented in terms of the heights of the bands at 2223 and 2195 cm-I, respectively, for the individual samples in Table 111. The table also contains data on the band at 2153 cm-'. The above-mentioned bands were preferentially formed after adsorption of a small amount of CO. Similarly, after adsorption at higher pressures (65 Pa), followed by evacuation for 15-60 min at 77 K, the bands at v C 2185 cm-I disappeared almost completely
5250
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
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