8734
J . Phys. Chem. 1994, 98, 8734-8741
Proton Transfer between H-Zeolite and Adsorbed Acetone or Acetonitrile: Quantum Chemical and FTIR Study Jan FloriPn’~+~~~~ and Ludmila KubelkovPt J . Heyrovsk$ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejfkova 3, 182 23 Prague 8, Czech Republic; Institute of Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic: and Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901 Received: September 24, 1993; In Final Form: May 12, 1994@
The proton-transfer energetics, heats of adsorption, and vibrational spectra of acetone and acetonitrile complexes with the H3Si-OH-AlH2-OH zeolite model were studied by ab initio HF/3-21G, HF/6-31G* and MP2/63 1+G** methods. The results were compared with FTIR examination of acetone, acetone-&, and acetonitriled3 adsorbed a t room temperatureon the skeletal hydroxyls of HZSM-5, H Y , and H X zeolites. The experimental results indicate that both acetonitrile and acetone preferably form the H-bonded “neutral” complex with bridging hydroxyls of the zeolites. Yet, for acetone/HZSM-5 complex, the formation of the ion-pair type of hydrogen bond cannot be entirely ruled out. These findings are supported by the trend in proton transfer energies calculated for these systems. The higher ability of less basic water to be protonated a t the zeolite surface is attributed to two point adsorption that enables coupled movement of protons in two adjacent hydrogen bonds. For acetone (acetonitrile), the interaction of the methyl group hydrogens with the skeletal oxygens can ease the proton transfer between the bridging hydroxyl and the adsorbate carbonyl (nitrile) in a similar but less effective way.
Introduction The stimulation of catalytic reactions by acid crystalline aluminosilicates (zeolites) promoted many debates on cationic surface complexes as active reaction intermediates. One possible route for the formation of these complexes is proton transfer between the strongly acidic zeolite bridging hydroxyl and the adsorbed molecule, but the direct experimental evidence is rather rare and ambiguous. Special attention has been paid to the activation of methano1,lJdimethyl ether,3 ~ a t e r ,and ~ , acetone6,’ ~ whose protonated forms in HZSM-5 were deduced from the infrared (IR) and the !H and 13C magic angle spinning nuclear magnetic resonance (MAS NMR)* spectra of adsorbed species. Recently, the mentioned IR spectroscopy evidence for the formation of ion-pair surface complexes was doubted and a new interpretation of the IR spectra of complexes of HZSM-5 with water, methanol and acetonitrile was proposed by Pelmenschikov and co-workers.9-ll This reinterpretation was based on the resonant theory that explains the spectral features observed in the IR spectra of medium and strongly hydrogen-bonded (AH-.B) as pseudobands in terms of Fermi resonance of the stretching (VAH)and bending vibrations AH) with the in-plane and out-of-plane overtones (2bAH, 2yAH).l2-I6 In addition, an extensive anharmonic coupling of the V A H and Y A...B vibrations makes the A, B, C, D pseudobands, usually found around -2900, -2500, 1800, and -1000 cm-1, respectively, very broad and strongly dependent on the strength of the hydrogen bond. Unfortunately, this theory does not provide direct means for distingushing the neutral and zwitterionic types of hydrogen bondsboth providing essentially the same AB spectral pattern.14 Therefore,joint experimental and ab initio computational studies, involving calculations of the proton transfer energies and the spectral changes caused by hydrogen bonding and the ion formation, are necessary to elucidate tha possible pathways for
-
* Corresponding address: Institute of Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic. t Academy of Sciences of the Czech Republic. t Charles University. 8 Southern Illinois University. e Abstract published in Advance ACS Abstracts, July 15, 1994. 0022-3654/94/2098-8734$04.50/0
the surface reactions catalyzed by zeolites. In the presented paper this approach is applied to the analysis of the FTIR spectra of the zeolite complexes with acetonitrile and acetone. From the point ofview of the calculated potential energy surface (PES), the hydrogen-bonded complexes Rl-A-H*-B-R2 can, in principle, be divided into two groups with one or two distinct PES minima as a function of the position of the bridging proton.17 For most hydrogen-bonded complexes formed by neutral molecules only one minimum corresponding to the R l-A-H-B-R2 “neutral” arrangement is obtained because of the strong electrostatic attraction felt by the proton in the ion-pair Rl-A-.-HB-R2+ system. The zwitterionic structure is stabilized in polar solution due to dipole-dipole interactions and in the systems forming multiple hydrogen bonds. The probability PBof finding the zwitterionic structure is given by the Boltzmann law as In PB = -(EB - EA)/kT, where EA, E B are the total energies of the neutral and zwitterion structures, respectively, k is the Boltzmann constant, and T is the temperature in kelvin. The exponential dependence makes the accuracy requirements for the quantumchemical calculations very high. So, we focused our attention to the comparison of acetonitrile and acetone results and to the relative trends calculated for their vibrational spectra. An additional comparison, which enabled us to stress the importance of the two-point adsorption for the proton transfer, was made with the previous ab initio calculations’s-20 of the zeolite complexes with water, methanol, and ammonia. Also, the effects of the size of the zeolite model on the calculated proton transfer energies are discussed.
Experimental and Computational Methods The infrared (IR) spectra of zeolite plates with a thickness of 7 mg cm-I were measured in situ using a Nicolet MX-1E Fourier transform infrared spectrometer with a resolution of 2 cm-1. Adsorption of acetone or acetonitrile was carried out on the acid form of samples, which were pretreated in vacuo at lo4 Pa and 400 OC. Liquid adsorbates (Fluka, p.a. purity) were stored over dried KA zeolite and degassed by repeating freezing and thawing before admitting their vapors into the zeolite. Acetone-d6 (99.5% of D) and CD3CN (99.95% of D) were supplied by the Institute 0 1994 American Chemical Society
Proton Transfers in H-Zeolite Complexes
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 8735
i
ii
A? H1
H17
iii
iv Figure 1. Structure and numbering of zeolite models. of Nuclear Research in Poland and ISOCOMMERZ (Rosendorf, Germany), respectively. Zeolites were synthesized in the Research Institute for Oil and Hydrocarbon Gases, Bratislava. Ab initio HF gradient optimizations of the geometry of zeolite complexes were carried out using the 3-21G and 6-31G* basis sets by the Gaussian 92 program.*' The C, symmetry of the complexes and of the model (iv) was assumed. The contributions of polarization functions on hydrogens and of electron correlation to the total energies of the zeolite complexes were evaluated by thesingle point HF/6-31+G** and MP2/6-31+GS* calculations in the HF/6-3 l G * geometries of the complexes. The interaction energies were corrected for the basis set superposition error (BSSE). The standard Boys-Bernardi counterpoise (CP) correction scheme was slightly modified to take into account the geometry reorganization when going from the isolated subsystems to the complex.22 Namely, C P correction for each monomer was determined as the difference between the energy of the monomer in the complex geometry with the basis set of the whole complex and that of the same monomer without ghost orbitals. The search for the ion-pair forms of the complexes was done in several steps. First, the partial geometry optimization with the N-H (0-H) bond of protonated acetonitrile (acetone) fixed at 1.09 (1 -03) A was carried out. Several values for the length of the fixed bonds were tested. For the resulting structure the full gradient optimization was atempted. The harmonic vibrational frequencies, normal modes, and IR intensities were evaluated from the
Figure 2. Geometries of the H-bonded complexes of acetone and acetonitrile with the (iii) fragment. The ion-pair complexes are formed by the transfer of the proton H3 from zeolite to acetone or acetonitrile. HF/3-21G analytic force constants and atomic polar tensors of the isolated subsystems and hydrogen-bonded complexes. The resulting vibrational frequencies were uniformly scaled by the factor of 0.93, which was determined by comparing experimental (3611 cm-1 for HZSM-5) and calculated (3862 cm-1) uOH frequency of the bridging hydroxyl. This is equivalent to the multiplying all elements of the force constant matrix by the factor 0.865. The 3-21G and 6-31G* basis sets were chosen for their previous wide use in the field of zeolite chemistry.23 In addition, split-valence basis sets are considered to represent the method of choice for calculations of vibrational spectra of medium sized systems.24 The zeolite skeleton was modeled by several fragments (Figure 1). For the calculations of the vibrational spectra and energetics of the proton transfer in the acetone/zeolite and acetonitrile/zeolite hydrogen-bonded complexes (Figure 2) we used mainly model (iii) because it yields resonable deprotonation energies and enables evaluation of the two-point interaction pattern. Results Interaction Complexes and Their Reactivity: IVIR Experiments. The I R spectra of the interaction complexes of acetone and acetonitrile with HZSM-5, HY, and HmNa4& zeolites are given in Figures 3-5. The acid strengths of these adsorbents decrease in the sequence of decreasing Si/Al ratio, which is equal to 13.5 (19.0), 2.90, and 1.33 for HZSM-5, HY, and HX, respectively. In other words, the HZSM-5 zeolite is the most acidic in the chemical sence. Bridging hydroxyls in HZSM-5 zeolite exhibit single IR band at 3611 cm-1 (Figures 3 and 4),
Florihn and Kubelkovh
8736 The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 3640 3611
1
1 t
2661
1620
a
t t
10.3
2272 2435
t
..... ...2
2995
I
I
2000
3000
(cm-l)
' \ 10.1 \ \ \
800
\
900 (cm-1) 1000
Figure 3. FTIR spectra of adsorbed acetone (0.51 mmol/g) on (-) HZSM-5 and (- - -) HY. (a) Spectra of zeolite hydroxyls before adsorption. (b) Difference spectra of zeolite with the adsorbate and the original zeolite. (c) Spectra in the transmission windowsof zeolites without (1) and with (2) adsorbates.
t
3611
! I
I
800
V I
_.
1500
2000
2600
(cm-1)
1
3400
Figure 4. FTIR spectra of HZSM-5 zeolite with acetone-&: (-) 0.82 mmol/g, (- - -) 2.10 mmol/g, number of bridging hydroxyls 0.71 mmol/ g. (a) Spectra of zeolite hydroxyls before adsorption. (b) Difference spectra of zeolite with the adsorbate and the original zeolite.
while couple of bands, reflecting O H groups in large cavities and small cages, is seen in the spectra of faujasites (Figures 3-5): 3640, 3550 cm-l for H Y and 3657, 3572 cm-I for HX. The low-frequency band of H X is negligible in comparison with the high-frequency band. The adsorption conditions avoid the interaction of faujasites hydroxyls in small cavities with both acetonitrile and acetone. All the zeolites contain silanol groups (3745 cm-I) that arenot involvedin theinteraction with adsorbates (Figures 3-5).
900
(cm-') iboo
Figure 5. (a) FTIR difference spectra of acetonitrile-d3 on HX, (1) 25 OC, 0.68 mmol/g adsorbed, (2) 125 OC, 3.44 mmol/g added to the reactionvolume. (b) FTIRdifferencespectraof acetonitrile-d3onHZSM5, (1) 25 OC,0.54 mmol/g adsorbed, (2) 125 OC, 3.00 mmol/g added to the reaction volume. (c,d) FTIR spectra of H X (c) and HZSM-5 (d) at 25 "C in the transmission window between 800 and 1000 cm-I, before (1') and after (2') the adsorption of acetonitrile-d3.
After the complex formation, the intensity of stretching vibration band of the free bridging hydroxyls at 3657-361 1 cm-I decreases, and two very broad bands at 2995-2770 and 24352370 cm-I arise (Figures 3-5). They can be attributed to OH stretching vibrations in the interaction complex. These features are considered to be typical for strong hydrogen bonding.13J6.25 Specific differences in the spectra, which point to variations in thecharacter and properties of the interaction complexes, become clearly apparent when the acid strength of bridging OH is varied and methyl groups are labelled with deuterium. This labeling allows us to monitor the mobility of methyl deuterium resulting in exchange of zeolite O H for OD. It also enables to recognize
Proton Transfers in H-Zeolite Complexes the in-plane deformation modes of the perturbed hydroxyls between 1500 and 1300 cm-I that are overlapped by the CH bending bands. The corresponding CD bending vibrations can be found below 1200 cm-I. The spectra of the same number of aCetOne-h6 complexes with bridging hydroxyls of HZSM-5 and H Y are given in Figure 3. It can be seen that the decrease in acid strength of bridging hydroxyls favors the high-frequency component of the -2800 and 2400 cm-I broad-band couple. For HZSM-5, the integral intensity ratio & ~ / A 2 mis equal to 0.58 while for the less acidic H Y it amounts to 0.88. It is also evident that the complexation significantly affects thespectra below 1800 cm-I. For HZSM-5, the maximum of the broad band of the C - 0 stretching vibration is found at 1655 cm-I, Le., 84 cm-1 below that of the gaseous molecules. Appreciable absorption extends further over the lower wavenumber region wherein intensities of 1420 and 1380 cm-I bands of CH deformation vibrations are unusually high (Figure 3). However, the spectra of adsorbeddeuterated acetone (Figure 4, curve 1) reveal that there is another strong band a t 1380 cm-I. The adsorption of both acetone-h6 and acetone-d6 results in appearance of a new strong band a t 884 cm-I in the transmission window between 1000 and 800 cm-l (Figures 3 and 4). The exchange of deuterium from acetone CD3 group for zeolitic hydrogen (Figure 4, curve 2) causes the suppression of bands a t 2820, 2370, 1380, and 884 cm-I so that these bands can be attributed to the vibrations of OH groups. For HY, a weaker interaction of aCetOne-h6 C=O group with bridging hydroxyls results in a smaller shift of the C 4 stretching band from 1739 (gas phase) to 1676 cm-I, Le., by 63 cm-I. Simultaneously, an absorption between 1600 and 1300cm-1 (Figure 3b) andintensity of a new band a t 840 cm-I in the transmission window (Figure 3c) are notably lower than it was observed in the spectrum of the HZSM-5 complex. Under the laboratory-temperature conditions, the chemical reaction of acetone was observed, but only at high filling of cavities, i.e., for the higher acetone concentrations than one molecule per bridging hydroxyl. These molecules were only slightly perturbed (see for the C=O stretching band of aCetOne-da a t 1709 cm-I in Figure 3, curve 2). In this reaction, the aldolization to mesityl oxide, indicated by bands at 1585and 1526 cm-I, was accompanied by the exchange of deuterium from methyl groups for zeolitic hydrogen (vide supra). The pair of broad bands at 3000-2300 cm-1 in the spectra of acetonitrile-dj complexes (Figure 5) is more sensitive to the change in the hydroxyl acid strength than it was observed for the acetone complexes. In the HZSM-S/acetonitrile-d3 spectrum the high-wavenumber component shifts about 200 cm-I downward and the intensity ratio of the high to low-wavenumber component decreases more than 10-times, compared to the HX/acetonitriled3 complex. In addition, no enhanced absorption is found between 1800 and 1300 cm-I for HX, while a broad band of low intensity appears at 1700 cm-I for HZSM-5. In the transmission window between 1000 and 800 cm-I, a new weak band at 833 cm-I is seen in the spectrum of HX, while a much stronger band a t 870 and the weak band a t 840 cm-I are found in the spectrum of HZSM-5 (Figure 5c). The C N stretching vibration is shifted to 2296 cm-* for HZSM-5/acetonitrile-d:, and to 2272 cm-I for HX/acetonitrile-d3 compared to liquid (2263 cm-l)*6a or gaseous (2273 cm-1)26b acetonitrile. In contrast to acetone, the exchange of deuterium from methyl group of acetonitrile for zeolite hydrogen of HZSM-5 does not occur at laboratory temperature. It slowly proceeds above 120 O C as it can be inferred from the appearance of bands of free bridging O D a t 2661 cm-I (stretching vibration, Figure 5b, curve 2) and 890 cm-1 (bending vibration, not shown in Figure 5). Simultaneously, new surface species, suggested by the band at 1620 cm-I (Figure 5b, curve 2), are formed. H X zeolite binds acetonitrile complexes more weakly than HZSM-5
-
-
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 8737
TABLE 1: Calculated and Measured Wavenumbers (cm-1) and IR Band Intensities of Zeolite Hydroxyls and Stretching Skeletal Vibrations measured calculated' HZSM-5 HY HX (iv) fragment interpretation 3611 1O5Oc
364Y 3548d 1015cf 105SCf 42W
3657c 3572d 103oC 990'
3592(295)
vOH
1152 (283) 1123(155) 600 (163) 863 (152) 521 (82) 506 (1 19)
vSi03-vO8A1, 6OH 60H yOH vSi02 vSi08 + &EA1 vA102
a Values of integral molar absorption coefficients (km/mol) are given in parentheses. 6 and y denote the in-plane and out-of-plane bending
vibrations, respectively, Y denotes stretching modes. In-phase (out-ofphase) combinationsof the vibrations are denoted by the + (-) sign. For atom numbering see Figure 1. Directed to big cavities. Directed to small cavities. e Determined from near-infrared spectra.32f One band at 1060 cm-1 was found by neutron inelastic~cattering.~~ 8 Determined using inelastic neutron scattering." zeolite, so that they are almost completely desorbed at 120 OC and no evidence of D exchange is found in the spectrum (Figure sa). Calculations of Vibrational Spectra. The HF/3-21G vibrational frequencies and I R intensities of characteristic vibrations of acidic bridging OH groups and of the stretching lattice (skeletal) modes of zeolites are compared with experimental data in Table 1. The H3-Si-OH-Al(OH)2-OSM3 cluster (iv), which includes all types of covalent bonds present in aluminosilicates, was used for this calculation. Because of the lack of the closed ring structure is this fragment its use for calculations of bending skeletal modes of zeolites, lying below 500cm-I, can be considered insufficient. For stretching and OH bending modes, fair agreement with observed spectral features was obtained. The stretching vibrations of the A l - O H S i bridges are less mutually coupled than the AI-OSi for which the picture of in-phase (521 cm-1) and out-of-phase (v,(AlOSi, 1152cm-I) coupled stretching vibrational modes is more appropriate. In addition, significant coupling ofthe v,(AlOSi) stretching with thein-planeOH bending mode of the bridging hydroxyl (1 123 cm-1) was calculated. The use of the fragment (iv) for the purpose of comparison with the spectra of the (iii) complexes might seem inappropriate. However, wavenumbers calculated for the free (iii) fragment cannot be directly compared with the wavenumbers from its complexes because an intramolecular hydrogen bond is formed in this fragment. As a result, the "free" OH stretching frequency in (iii) lies as low as at 3162 cm-'-that is, at lower wavenumber than for itscomplexes. Obviously, the artificial H3-08 attraction (Figure l), which originates in part from the large BSSE of the 3-21G basis set, is screened by adsorbed molecules. This interaction is negligible in the larger (iv) fragment. The comparison of significant vibrational modes of isolated aCetOne-d6, protonated acetone-d6, acetonitrile-ds, and protonated acetonitrile-d3 with the calculated spectra of their neutral hydrogen-bonded complexes with the zeolite fragment (iii) is presented in Tables 2 and 3. The larger red-shift and intensity of the OH stretching vibrations of bridging hydroxyls in the acetone-(iii) complex compared to those calculated for the acetonitrile-(iii) complex clearly indicate the presence of the larger interaction energy and stronger hydrogen bonds in the former complex. This finding was confirmed by our calculations of binding enthalpies (Table 4). The S C F (HF/6-31G*) interaction enthalpies do not include the dispersion attraction so that the resulting BSSE corrected heats of adsorption (-34 and -25 kJ/mol for acetone and acetonitrile, respectively) are underestimated. Indeed, the single point MP2 calculations a t the HF/6-3 l G * geometries result in a large increase of absolute values of interaction enthalpies. However, the calculated
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994
8738
TABLE 2: Vibrational Spectra of Acetone-d&Protonated Acetone-de and Acetone-d6 Complexes with HZSM-5 and with the (iii) Zeolite Model IR [cm-'1' HZSM-5/ (CD3)zCO 2572c 2820w, vb 2370w, vb 2367vw 2257w 2216w 2142mw 2112mw 2089mw 1664s 1600m, vb 1380m, b 882m
-
H F / 3-2 1G [cm-'1 [(CD3)2COH]+
(iii) ... OC(CD&
assignment
3498 (324)
2986 (2500)
2302 (4.8) 2277 (2.7) 2226 (11.3) 2224 (1 1.9) 2135 (13.3) 2127 (24.9) 1548 (169)
2284 (2.3) 2256 (39) 2224 (2.8) 2229 (1.7) 2144 (0.3) 2120 (33) 1745 (464)
1342 (256) 812 (122)
1410 (258) IO32 (77)
vOH A band B band vCD uCD uCD vCD uCD uCD vC0 C band 60H yOH
(CD3)z-
co
2279 (4.0) 2276 (2.7) 2241 (9.2) 2234 (0.0) 2142 (0.8) 2137 (0.1) 1792 (147)
IR intensities denoted as strong (s), medium (m), weak (w), broad (b), very (v). Only modes falling in the zeolite transmission windows are presented. Calculated integral molar absorption coefficientsare given in parentheses [km/mol]. The frequency of the vOH mode estimated empiricallyz9assuming Fermi resonance origin of the A and B bands (see the text).
TABLE 3: Vibrational Spectra of Acetonitrile-& Protonated Acetonitrile-d3, and Acetonitrile-d3 Complexes with HZSM-5 and Zeolite Model (iii) IR [cm-I] NCCD3/ HZSM-5
HF/3-21G [cm-Ila (CD3)CN
(iii). .. NCCD?
[(CD3)CNHl+ ~~
3538 (876) 2552b 2296s C
2413 (5.2) 2263 (2.2)
2386 (27) 2260 (17)
2148 (1.3)
2129 (24)
931 (0.9)
953 (47)
832 (9.8)
780 (0.1) 781 (392)
C
-
2116w 1700w, b 1320m 870s 840w
assignment
~~
2770m, b 2405s, b 3232 (1686) 2414 (50) 2285 (4.0) 2271 (0.6) 2153 (2.6) 1327 (258) 947 (21) 820 (237) 939 (18)
A-band B-band vNH vOH vCN vCD vCD VCD C-band 6OH GCCD, 6CCN rOH vCC, GCCD 6NH
Only modes falling in the zeolite transmission windows are presented. Calculated integral molar absorption coefficients aregiven in parentheses [km/mol]. The frequency of the vOH mode estimated empiri~ally2~ assuming Fermi resonance origin of the A and B bands (see the text). c Overlapped. (I
magnitude of this increase should be taken with great caution since the geometry relaxation contributions were not treated properly at the MP2 level.
Florih and Kubelkovii For distinguishing the bands originating from the neutral and ion-pair complexes, thedirect comparison of the calculated spectra of the both these structures would be helpful. However, the ionpair complexes do not represent stationary points on the potential energy surface, at our level of approximation (for details see below). Therefore, we limited ourselves to calculations of the effects of protonation on isolated bases. Certain extrapolation of these effects to the hypothetical ion-pair complex can be done by considering an increase in NH and O H bending frequencies and a decrease in their stretching frequencies caused by hydrogen bonding. The O H vibrational modes of protonated acetone are found at the same wavenumber regions as O H modes of aliphatic alcohols. Considering the influence of H-bonding, the O H stretching frequencies of protonated acetone in the ion-pair complex might be very close to those of zeolitic hydroxyls in the "neutral" complex. Also the OH in-plane bending frequencies and intensities of zwitterionic and neutral complexes are very similar, regardless the proton being covalently bonded to different molecules. These findings preclude the use of the O H stretching and in-plane bending bands as structural markers. The O H outof-plane vibrations of the protonated acetone exhibit somewhat higher specifity. Unfortunately, these bands fall into the region overlapped by the skeletal vibrations of zeolites. Possibly more spectroscopic evidence could be obtained by using deuterated zeolites the OD out-of-plane bending vibrations of which fall into the 800-900 cm-1 transmission window.2' The most pronounced difference between the spectra of the ion-pair and neutral structures of the acetone-&-zeolite complex involve the frequency of the CO stretching vibration. Our calculations show that the adsorption of acetone results in the -50 cm-' red-shift of the CO stretching band. Even so, the calculated 1745 cm-I frequency of this band is too high (Table 2). Such a frequency overestimation is a common feature for HF calculations with small basis sets. It can be corrected by the differential scaling, that is by introducing individual scale factors for force constants or characteristic frequencies corresponding to different internal ~oordinates.2~The avail of scaling concepts stems from the good transferability of scale factors between chemically related molecules. To estimate the position of the C O stretching band in the spectra of ion-pair complex we will use the 0.95 frequency scale factor obtained by comparing calculated (1745 cm-I) and observed (1664 cm-I) frequencies of the C O stretching vibration in the neutral type of the complex. In addition the calculated effect of protonation of free acetone, and the effect of ionic hydrogen bonding upon the CO stretching frequency must be considered. The protonation of free acetone results in the red-shift of the 1745 cm-1 band by -250 cm-1 (Table 2). On adsorption of hypothetical free protonated acetone its O H bond is weakened. It is well-known that this weakening results in a strengthening of the neighboring CO bond so that an -50 cm-1
TABLE 4 Energetics of the Proton Transfer in the Hydrogen-Bonded Complexes of Acetone and Acetonitrile with the Zeolite Model (iii) acetone-(iii) method (energy//geometry) HF/3-21G//HF/3-21G HF/6-31G*//HF/6-31G* HF/6-31+G**//HF/6-31G* MP2/6-3 1+G**//HF/6-3 l G *
total energy [au] -871.974 -876.684 -876.728 -877.888
690 729 162 529
acetonitrile...(iii)
G",
AEL [kJ/mol]
[kJ/mol]
78(62)b 84(91)' 78 64
-87 -50 -45 -58
[kJ/mol]
total energy [au]
[kJ/mol]
AGnt [kJ/mol]
APH', [kJ/mol]
-43 -37 -35 -40
-812.266499 -816.646 115 -816.682713 -817.667688
86 102(113)' 98 68
-54 -39 -34 -49
-1 7 -25 -24 -33
a Em values were calculated as the differences between the total energies of the complexes with the ion-pair and neutral type of the hydrogen bond. The energies of the ion-pair complexes were calculated at the partially optimized geometries. In the (iii)--[H-O(CH3)2]+ complex, the 0 - H bond of the protonated acetone was fixed at 1.03 A. In the (iii)--[H-NCCH3]+ complex, the N-H bond was fixed at 1.09 A. The proton-transfer energies calculated with other constraints are given in parentheses. 0-H+ bond fixed at 1.08 A. 0-H+ bond fixed at 0.99 A. N-H+ bond fixed at 1.04 A. The interaction energy AEinl was calculated as a difference between the total energy of the complex (presented in this table) and the sum of total energies of its isolated constituents. /The basis set superposition error (BSSE) corrected interaction enthalpy (heat of adsorption) at 0 K: AHo = Mint - BSSE- AZPE, where zero-point energies were obtained at the HF/3-21G level (AZPE = 6 kJ/mol and 7 kJ/mol for acetonitrile and acetone complex, respectively.
Proton Transfers in H-Zeolite Complexes increase of the CO stretching frequency compared to free protonated acetone can be expected. The additivity of these two effects and subsequent frequency scaling implies the presence of a new medium band near 1470 cm-I in the IR spectrum of the hypothetical ion-pair structure. However, a strong coupling of this mode with the COH in-plane bending vibration may result in a broad envelope band in the 1500-1300 cm-1 region rather than in a clearly distinguishable pair of C-0 stretching and dOH+ bending peaks (Figures 3 and 4). As for the CD stretching bands, the weak interaction of the methyl group of acetone with the basic zeolitic hydrogen was calculated to induce 2-fold increase in the IR intensities of stretching vibrations of those C D bonds that form the closest contacts with the zeolite. Protonated acetonitrile does not possess the NH bending vibration near the zeolitic OH-bending mode in “neutral” H-bonded complex, Le., between 1200 and 1400 cm-I. As the other possible indication of the presence of the ion-pair acetonitrile-zeolite complex in the sample could serve the lack of the OH out-of-plane bending vibration in the 800-900 cm-I transmission window. Finally, the N-protonation shifts the C N stretching vibration downward by 30 cm-1, whereas the “neutral” H-bond almost does not change its frequency. The -30 cm-I frequency shift is determined by the mass increase caused by protonation rather than by the change in the C N stretching force constant. A significant increase in I R intensity of tliis mode is connected with protonation as well as with hydrogen bonding. Due to the weaker contacts of the methyl group of acetonitrile with the zeolite oxygen the intensities of acetonitrile-d3 CD stretching modes are only little influenced by the formation of the complex. Surprisingly, their frequencies increase in the complex, too. For this perturbation, the O-H.-N interaction plays probably a decisive role. Hydrogen-Bonded Complexes and Proton Transfer: Ab Initio Calculations. The calculated stable surface species correspond to the hydrogen-bonded complexes with protons forming covalent bonds to the bridging oxygen of the zeolite. The HF/6-31G* geometries of these complexes are presented in Figure 2 and Table 5 . These data show that acetone as well as acetonitrile are stabilized on the surface by the hydrogen bond with bridging hydroxyl 02-H3 and by the interaction of the methyl group with oxygen 08.The HF/6-3 lG* lengths of the hydrogen bonds are 2.75 and 2.92 A for acetone (0-0) and acetonitrile (N-0), respectively. The distance between the methyl group and the (weakly basic) skeletal oxygen is also shorter for acetone than for acetonitrile. These findings agree well with the higher interaction energy of the former complex indicated by the calculated frequency shifts and interaction energies (vide supra). The interaction of the acetonitrile methyl group with the 0 8 oxygen of the model (iii) forces the adsorbate to depart slightly from the optimal, linear, C-C-N--H geometry. In the real zeolite structures, more favorably localized skeletal oxygens can be found on the opposite side of the cavity. They may be capable of forming the C-H-.O hydrogen bonds without generating structural tensions. To elucidatedifferencesbetween the 2-point and 1-point interaction patterns we studied acetonitrile complex with the (ii) model by the HF/3-21G method. Despite the 34 kJ/mol smaller HF/3-21G deprotonation energy of the (ii) fragment than that of the (iii) model, the proton transfer energy in the (ii)-.acetonitrile complex was calculated to be larger by 13kJ/mol compared to that for the (iii).-acetonitrile complex. Taking into account also that the basicity of the 0 8 oxygen is somewhat overestimated by the (iii) model, the stabilization of the “ion-pair” hydrogen bonds by C-H-0 contacts may be estimated to be in the 20-40 kJ/mol range. The calculated energies required for transferring the proton in the hydrogen bond from the zeolite to the adsorbed acetone or acetonitrile are collected in Table 4. The calculated potential
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 8739
TABLE 5: HF/6-31G* Optimized Geometries of the Complexes (1 angst” = 0.1 nm, degree) of Acetone and Acetonitrile with the Zeolite Model (iii) bond length or angle (iii)-.(CH~)zC0 A1-02 A1-08 02-H3 0 2 4 02-A1-08 A1-02-H Si-02-A1
c-0
C 12-C14 C12-C 15 0-C-Cl4 0-C-Cl5 C-H 19 C-H20 C-H16 C-H17 C-C-H 16 C-C-H 17 C-C-H 19 C-C-H2O 0-C-C-H2 1 0-C-C-H 17 08-H16 013-H3 08-H 16-C A1-08-H 16 02-H3-0 H3-0-C
(iii)-CH3CN 1.957 1.744 0.970 1.694 95.0 113.7 130.1 1.202 1.506 1.509 122.3 120.3 1.08 1 1.086 1.080 1.087 110.6 109.5 110.2 110.0 121.1 121.5 2.319 1.796 169.2 135.1 162.9 136.9
Al-02 A148 02-H3 0 2 4 02-AI-08 AI-02-H Si-02-A1 N-C C-C C-H16 C-H 15 N-C-C A1-C 1-C2-H 17 C-C-H 15 C-C-H 16
1.963 1.750 0.964 1.698 96.1 112.8 131.3 1.136 1.465 1.082 1.080 178.9 119.7 107.5 109.8
08-H 15 N-H3 08-H 15-C A1-08-H 15 02-H3-N H3-N-C
2.572 1.955 129.9 156.9 175.6 113.5
For numbering see Figure 2. Optimizations were carried out under the Cs symmetry constraint. The geometry of the SiH and AlH bonds, which are not present in the real zeolite structures, is omitted. energy surface (PES) for the proton transfer exhibit only one minimum corresponding to the H-bonded “neutral” complex of acetone or acetonitrile. For this reason the “ion pair” energies and A E ~differences T could be obtained only by forcing the proton to be adjoined to the adsorbed acetone or acetonitrile. Therefore, the A E ~ Tenergies presented in Table 4 can be considered as estimates of the steepness of the given single-minimum well rather than as real proton-transfer energies. The HF/6-31GS A E ~ T values calculated for complexes with the (iii) fragment equal 84 and 102 kJ/mol for acetone and acetonitrile, respectively, and correlate with the proton affinity order of the isolated adsorbate molecules.Z* These numbers vary by 20% when the lengths of the fixed OH or N H distances are changed within reasonable limits. Similarly, our tentative HF/3-21G calculations showed that enlargement of the model surface fragment from (i) and (ii) to (iii) was reflected in a 20% variation in AEp7. By including the MP2 contribution of electron correlation to the total energy the energies of ion-pair structures were lowered by 20 kJ/mol (acetone) and 34 kJ/mol (acetonitrile). Similar MP2 stabilization of the ion-pair structures was obtained by Sauer et a1.18 for water-(i) complex. The relatively computationally cheap HF/ 3-21G method provided proton-transfer energies for (iii) complexes with acetone and acetonitrile as well as for the water...(i) complex in good agreement with the MP2 results.
Discussion For acetonitrile-d3 interaction with bridging hydroxyls, our FTIR data confirm the formation of H-bonded “neutral” complex. This conclusion is supported by the following observations: (i) According to the model calculations (see Table 3), the protonation should decrease thevibrational frequency of C N group toward lower wavenumbers. In contrast, the increasing acid strength of bridging hydroxyls shifts the C N frequency upwards, which is typical for H-bonded “neutral” complex.
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(ii) A trio of bands at 2770,2405, 1700 cm-I found with CD3CN/HZSM-5, and 2995, 2435 cm-1 bands in CDsCN/HX conform well with the ABC band pattern of strong H-bonded complexes and with AB structure of weaker H-complexes in solutions.~~J4 It is well-known that the center of gravity of the AB components is shifted to the lower wavenumbers with increasing strength of the hydrogen bond. Under the assumption of the ”neutral” type of hydrogen bond in the CD3CN/HX and CD3CN/HZSM-5 complexes, the higher acidity of the HZSM-5 zeolite should result in stronger hydrogen bonding. Indeed, the component position in the band couple is much closer, the center is shifted downward and the intensity ratios are changed in favor of the low-wavenumber component in the CD3CN/HZSM-5 system. Roughly, the position of the Evans window between A and B bands was measured at the frequency corresponding to the 2*60H overtone (Figure 5b, Table 3), for both HZSM-5 and H X complexes. To provide more quantitative data we calculated the 6 0 H and vOH fundamental frequencies by the equations published by O~erend.2~ The spectral parameters of the AB Fermi resonance couple in the IR spectra of CD3CN/HZSM-5 and CD3/HX complexes served as input data for this calculation. The uOH = 2552 cm-I, 60H = 1310 cm-l, vOH = 2942 cm-’, and 6 0 H = 1243 cm-1 frequencies were calculated for HXSM-5 and H X complexes with acetonitrile, respectively. The 60H fundamental frequencies calculated in this way are in a reasonable agreement with the results of ab initio calculations (Table 3). The too high ab initio frequency of the vOH mode (3232 cm-I) indicates that the zeolite acidities are underestimated by our computational model, Le., by the (iii) model and the 3-21G basis set. (iii) The presence of the C band near 1700 cm-1 in the spectrum of CD3CN/HZSM-5 complex and its lack for the weaker CD3CN/HX complex also confirms ”neutral” type of hydrogen bonding. Here, the Fermi resonance of the y O H overtone with a broad v(0H) f k v ( 0 - N ) combination band plays a role. The yOH fundamental was observed at 87Ocm-I for HZSM-5 complex (figure 5d), in agreement with its position calculated for the “neutral” zeolite/acetonitrile complex (Table 3). Due to the weaker acid strength of H X the y O H vibration of its complex might occur below 810 cm-I, Le., in the opaque region. The assignment of the weak 833 cm-1 candidate for the r O H fundamental is not clear because it was found also for the HZSM-5 complex. (iv) Catalytic transformation of surface species as well as exchange of deuterium from acetonitrile methyl for hydrogen of bridging hydroxyl need relatively high temperatures. As for acetone, the HF/3-21G calculations reveal a similarity of the IR spectra of neutral and ion-pair zeolite complexes with acetone (Table 2). We suggested in the previous papers6%’that H-bonded “neutral” species on bridging hydroxyls of HZSM-5 are in equilibrium with protonated “ion-pair” forms at room temperature. Coexistence of two forms of acetone interaction complexes was deduced from the presence of C=O band at 1655 cm-1, characterizing “neutral” complex, and two strong O H bands at 1380 and 884 cm-I, which were attributed to vibrations of OH groups in the ion-pair complex. In a previous paper, we tentatively considered the individual components, A and B, of the highfrequency O H band to be characteristic of “neutral” and ”ionpair” structures, respectively. As pointedout by Pelmenschikovet aL9-I1theuseoftheresonant theory of the O H band profiles may be more appropriate for the interpretation of the ABC band structure. However, because the H-bonding affects OH vibrations of both complexes, Fermi resonance of O H vibrations can result in a similar AB structures of the O H band in a high-frequency region. Such an intuitive resoning was confirmed by the IR spectra of ion-pair Rl-A--HB+-R2 complexes of pyridinium salts in solutions.~4 The study of OdinokovI4 also revealed that, in contrast to the neutral type
Floridn and Kubelkovd of H bond, interionic H-bonding energies increase with increasing basicity of anion A-. As a result, the intensities in the AB duo of bands redistribute into the lower frequency component as the basicity of the molecule A- increases. Because bridging hydroxyls of zeolites are weaker acids in H Y than in HZSM-5, negatively charged lattice oxygens (conjugated bases) are stronger bases in H Y than in HZSM-5. Therefore, if the hydrogen bond formed between the zeolite and acetone was of the ion-pair nature the center of gravity of the AB bands would be found at lower and the 6 0 H band a t higher wavenumbers for H Y than for the HZSM-5 complex. Instead, the vOH and 60H frequencies calculated29 from the AB Fermi resonance components (Figure 4) show the reverse order that is typical for zeolitic hydroxyls in H-bonded ”neutral” complexes: vOH(HY) = 2572 cm-I, vOH(HZSM-5) = 2517 cm-I, 6OH(HY) = 1314cm-’, GOH(HZSM5 ) = 1336 cm-1. This trend is confirmed by bands at 840 and 884 cm-1 found with acetone/HY and acetone/HZSM-5, respectively, which can be attributed to r O H vibrations. The formation of neutral H-bonded complex is also evidenced by the C=O band, which shows a weaker perturbation of carbonyl by H bonding for H Y than for HZSM-5. The positions of minima in Evans windows resonably correspond to the doubled frequencies of the 6 0 H vibrations. The effect of acid strength thus allows us to conclude, that “neutral” H-bonded complexes are formed both in H Y and HZSM-5 zeolite. For HZSM-5, the strong “neutral” hydrogen bonding, which is close to the symmetrical O--H-O type of hydrogen bond, is the most probable when the number of adsorbed acetone does not exceed appreciably the number of bridging hydroxyls. Yet, there remains a possibility for acetone to form a slightly asymmetric ion-pair hydrogen bond with HZSM-5. This concerns mainly the strong 6 0 H band at 1380 cm-1 and the broad band observed below 1600 cm-I. To exclude formation of such an “ion-pair” structure in HZSM-5 needs, in our opinion, further experimental work, namely, with labeled compounds. The reactivity of acetone significantly increases when the acetone concentration in cavities exceeds one acetone molecule per Bronsted acid site. Also, the recent I3C N M R study3O revealed the sharp transition from a static adsorbent molecular complex at low coverages to rapid adsorbent reorientations at high coverages of HZSM-5 surface by acetone. We believe that it is the relationship between the proton transfer from zeolite to the adsorbed acetone and the 0-.H-C interactions which may, due to the exponential dependence in the Boltzman law, result in such a triggering mechanism of acetone rotation dynamics and reactivity. The full cavity models would be necessary for a detailed insight into this phenomenon. The calculated energetics of the proton transfer in the complexes of acetonitrile and acetone with zeolite models agree with the observed findings in that the neutral hydrogen-bonded complexes are energetically more stable. On the other hand, the previous MP2/DZP study of the adsorption of water and methanol on the zeolite surface1*showed the formation of oxonium and methoxonium ions in spite of their smaller proton affinities as compared to acetonitrile and acetone. Such a difference can be explained by the number and nature of the hydrogen bonds in the surface complexes. Whereas there is only one hydrogen bond acceptor atom in acetone (*O=C
