J. Phys. Chem. 1993,97, 11979-1 1986
11979
Mechanisms of Methanol Adsorption on Silicalite and Silica: IR Spectra and ab-Initio Calculations A. G. Pelmenschikov,’*t**G. Morosi,f and A. Gambaf Dipartimento di Chimica Fisica ed Elettrochimica, Universita’ di Milano, Via Golgi 19, I-201 33 Milano, Italy, and Institute of Catalysis, Prosp. Lavrentieva 5, 630090 Novosibirsk, Russia
A. Zecchina,l S . Bordiga,l and E. A. Paukshtist: Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali. Universita’ di Torino, Via P.Giuria 7, I-1 0125 Torino. Italy, and Institute of Catalysis, Prosp. Lavrentieva 5, 630090 Novosibirsk, Russia Received: April 27, 1993; In Final Form: August 31, 1993’
Combined IR and ab-initio studies suggest the presence of two chemisorbed species of methanol on ZSMS-like silicalite, namely, free and hydrogen-bonded 9SiOCH3 groups, which are in thermodynamic equilibrium. Upon decreasing the temperature, the relative concentration of the free 9SiOH groups increases due to the > S O H transition from the hydrogen-bonded 3SiOCH3 groups into hydrogen-bonded 3 S i O H aggregates. Physically adsorbed methanol forms large hydrogen-bonded polymers, even at high concentration of unoccupied S i O H groups. The C H frequencies of the chemisorbed and physically adsorbed species on silicalite are equal to those on silica, indicating the identity of these forms on both solids. Owing to the extremely high local concentration of 3 S i O H groups in atomic-scale defects in silicalite, the effect of the reversible transformation of two chemisorbed forms to each other, on changing the temperature, is more pronounced for this solid than for silica.
Introduction The methanol adsorption on zeolites has been the subject of numerous experimental studies (see refs 1-10 and references therein), as it is the first step of several industrially important processes for methanol transformation to hydrocarbons.11.12 However, the role of non-alumina active sites in this adsorption has not been discussed in detail, as a discrimination between the methanol simultaneous interactions with alumina and nonalumina sites of activated zeolites is hardly possible. Recently designed Si02 materials with zeolite structure, silicalites,13-15give a unique opportunity for a separate study of the adsorption on non-alumina sites, as their aluminum content is negligible. Furthermore, the comparison of the well-explored methanol adsorptionon amorphous silica with that on silicalitecould reveal the effect of zeolitic structure on the properties of these sites. These considerations prompted us to a joint IR and quantum chemical study of the methanol adsorptionon highly pure ZSM5like silicalite.*5 Experimental Section The silicalite sample was prepared in a fully Na-free form following the method described in detail in ref 15. Before gas dosage at room temperature, the silicalite pellet was outgassed for 3 h at 773 K in vacuum. Methoxylated silicalite and silica (Aerosil, 180 mz g-1) were prepared by reacting the silicalite and silica pellets for 3 h at 623 K with 104 Pa of methanol, after which the cell was cooled and Pa. Before methoxylation the pellets were evacuated top = preheated to 973 K in vacuum. The IR spectra were recorded with a FTIR Bruker 66 spectrometer at room temperature. Thin pellets of compressed powder were inserted in an all-silica cell allowing in-situ thermal ~~~~~
* To whom correspondence should be addressed.
Present address: Dipartimento di Chimica Fisica ed Elettrochimica, UniversitB di Milano, Via Golgi 19, 1-20133 Milano, Italy. 1 Institute of Catalysis, Russia. 1 Universita’ di Milano, Italy. * Universita’ di Torino, Italy. *Abstract published in Aduance ACS Absrracts, October IS, 1993.
0022-365419312097-11979$04.00/0
treatments under high vacuum (p = The spectral resolution was 3 cm-l.
Pa) and gas dosages.
Theoretical Method and Models In a series of papers1619 we successfully used the molecular quantum chemical modeling of solids (refs 20-22 and references therein) to interpret IR data of water and methanol adsorptions on silica and zeolites. The vibrational frequencies of adsorption models were calculated with the 3-21G basis set, while the relative stabilitieswere estimated at the 6-3 lG* level. The same approach was applied in this investigation, and the calculations were performed using the Gaussian-88 package.23 Optimized geometries and total energies of the models utilized in this paper are collected in Tables I and 11,respectively. (See footnotesof Table I and refs 16-18 for more details.) Their relative relevance in interpreting the adsorption will be discussed in the following.
Results and Discussion During the first stages of methanol dosage on silicaliteseveral bands of chemisorbed species appear in the CH fundamental region (Figure 1, a and b, spectra 2 and 3). These bands were detected also in methanol adsorption on severely dehydroxylated (see also Figure 2) and assigned to chemisorbed species, as they do not disappear after prolonged evacuation (Figure 1, c and d, spectra 10 and 11). The 2997-, 2959-, and 2857-cm-1 bands are generally a~cepted2~.25~~~ to be the Ylm, v ’ ’ ~and , v, bands of the surface SSiOCHs group, designated as species I in the following. The pronounced shoulder at approximately2926 cm-* was attrib~ted24.2~ to an overtone or a combination of the 6(CH3) bending modes of this species, by analogy with the 2930-cm-1 band of methanol monomer in argon matrix isolation.30 According to Morrow’sz4theoretical analysis of the IR spectra of CH30H, CHzDOH, CHDzOH, and CDpOH adsorbed species on silica, the shoulder at 3010-3030 cm-1 (Figure 2) is certainly not a first overtone or a binary combination band of species I. We assign this band to another chemisorbed form whose nature will be discussed below. The first stages of the methanol dosage lead to a decrease of the complex skeletal band at 9 10-890 cm-1 (Figure 3), formed 0 1993 American Chemical Society
11980 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
Pelmenschikov et al.
TABLE I: Calculated Bond Lengths, AB (A), and Angles, ABC and ABCD (deg) molecule
basis set 6-31g* 6-31g’
3-21g 3-21g
3-21g
3-21g 3-218 3-21g 3-21g 3-21g 3-21g 3-21g 6-31g* 6-31g* 3-21g 3-21g 3-21g 3-21g
geometry Hl(1)0(2), 2.014; H1(1)0(2)C(2), 118.6 OHI, 0.950; CO, 1.394; CHz, 1.082; CH3, 1.089; COHI, 109.1; OCH2, 107.8; OCH3, 112.2; HIOCH3,61.1 OHl,O.946; CO, 1.405; CHz, 1.080; CH3, 1.086; COHI, 110.1; OCHz, 107.0; OCH3, 111.5; HlOCH3,61.2 Hl(1)0(2), 1.965; H1(2)0(3), 1.967; Hl(1)0(2)C(2), 119.5; Hl(2)0(3)C(3), 121.2 OH!, 0.952; CO, 1.392; CHz, 1.083; CH3, 1.090; COHI, 109.0; OCH2, 107.9; OCH3, 112.3; H I O C H ~61.1 , OH1,0.952; CO, 1.399; CHz, 1.081; CH3, 1.087; COHI, 109.9; OCHz, 107.6; OCH3, 111.7; H I O C H ~61.1 , OH1,0.947; CO, 1.407; CHz, 1.080; CH3, 1.085; COHI, 110.1; OCHz, 107.0; OCH3, 111.5; HlOCH,, 61.2 Hl(1)0(2), 1.785; Hl(1)0(2)C(2), 117.5 OHl,O.974; CO, 1.430; CHz, 1.081; CH3, 1.088; COHI, 110.3; OCHz, 107.3; OCH3, 112.7; HlOCH3,61.2 OH1, 0.964; CO, 1.447; CHZ,1.077; CH3, 1.083; COHI, 112.0; OCHZ, 105.9; OCH3, 111.5; H10CH3,61.3 Hl(1)0(2), 1.715; Hl(2)0(3), 1.716; Hl(1)0(2)C(2), 118.0; Hl(2)0(3)C(3), 119.3 OH!, 0.980; CO, 1.427; CHz, 1.081; CH3, 1.089; COHI, 110.1; OCH2, 107.6; OCH3, 112.8; HiOCH,, 61.2 OHI, 0.978; CO, 1.435; CHZ,1.079; CH3, 1.085; COHI, 112.3; OCHZ,107.0; OCH3, 111.8; HIOCH3,61.2 OHI, 0.964; CO, 1.449; CHz, 1.077; CH3, 1.082; COHI, 112.2; OCHz, 106.0; OCH3, 111.3; H I O C H ~61.4 , H1(1)0(2), 1.691; Hl(2)0(3), 1.623; Hl(3)0(4), 1.621; H1(4)0(5), 1.691; Hl(l)O(2)C(2), 117.6; Hl(2)0(3)C(3), 118.1; Hl(3)0(4)C(4), 119.0; Hl(4)0(5)C(5), 121.4 OH1, 0.983; CO, 1.426; CH2, 1.082; CH3, 1.089; COHI, 110.0; OCHz, 107.7; OCH,, 112.9; HIOCH3,61.2 OH1,0.990; CO, 1.431; CH2, 1.080; CH3, 1.086; COHI, 112.1; OCHz, 107.4; OCH3, 112.0; HlOCHp, 61.1 OHI, 0.990; CO, 1.433; CHz, 1.080; CHI, 1.086; COHI, 112.5; OCH2, 107.4; OCHp, 111.8; H1OCH3,61.1 OHI, 0.981; CO, 1.437; CHZ, 1.079; CH3, 1.084; COHI, 112.5; OCHZ, 107.1; OCH3, 111.6; HIOCH3,61.2 OHI, 0.964; CO, 1.450; CHz, 1.077; CH3, 1.082; COHI, 112.1; OCHz, 106.0; OCH3, 111.2; H I O C H ~61.3 , SiO, 1.642; OH, 0.957; S O H , 128.3 SiOI, 1.640; OlC, 1.429; CHI, 1.078; CHz, 1.084; SiOlC, 131.7; OICHI, 107.8; OICHZ,11 1.0; SiOlCH2,60.4 01H1, 0.973; HlOz, 1.730; 02Si2, 1.646; OzC, 1.440; CHz, 1.077; CH3, 1.081; HlOzC, HlOzSi, 113.7; OCHz, 107.0; OCH3, 110.3; Si202CH3, 60.4 01H1, 0.971; HlO2, 1.747; OzH2,0.958; OzSiz, 1.649; H I O Z H ~H102Si2, , 114.8 01H1, 0.979; H102, 1.685; 02H2,0.980; OzSiz, 1.641; H203, 1.664; 03H3,0.959; O&, 1.652; HIOZHZ,HIOZSiZ, Hz03H3, Hz03Si3, 114.8 SiOl, 1.625; 01H1, 0.977; H102, 1.663; OZC,1.438; CHz, 1.078; CH3, 1.084; SiOlHI, 131.3; HlOzC, 122.3; OzCHz, 106.7; OzCH3, 110.7; H102CH3, 119.3 SiOl, 1.650; A101, 1.861; 01H1, 1.035; H102, 1.426; OzC, 1.439; CH2, 1.078; CH3, 1.082; SiOlHI, 119.7; HlOzC, 121.5; OzCHz, 106.5; 0zCH3, 110.2; HlOzCH3, 119.1 Sios, 1.621; o$, 1.407; CHI, 1.081; CHI, 1.083; CH3, 1.085; OnSi01, 115.5; 03si02, 106.0; SO&, 126.5; O I S ~ O ~72.6; C , O ~ C H I107.9; , SiO3CH1, 172.3; O3CH2, 111.1; SiO3CH2, 52.8; O3CH3, 110.7; SiO3CH3, -68.6 SiO3,1.622; O3C, 1.397; CHI, 1.081; CHz, 1.086; CH3, 1.087; O~Si01,114.6; O,SiOz, 109.0; SiOsC, 129.1; O3CH1, 108.2; SiO3CH1, 175.3; 03CH2, 111.7; SiO3CH2, 55.9; O3CH3, 111.5; SiOsCH3, -65.8 SiOl, 1.625; 01H1,0.976; H102, 1.671; OzH2,0.964; OzC, 1.447; CH3, 1.077; CH4, 1.082; SiOlHI, 131.3; H102H2, 126.1; H ~ O Z C121.1; , 0zCH3, 105.8; OzcH4, 111.3; H~OzCH4~61.3 SiOl, 1.657; 01H1,0.961; Hz01, 1.835; OzH2,0.972; OzC, 1.431; CH3, 1.080; CH4, 1.088; SiOlH1, 124.8; OzCH3, 107.3; OzCH4, 112.8; HzOzCH4,61.3 SiOl, 1.635; Si03, 1.668; OIHI,0.975; H102, 1.785; OZHZ,0.974; OZC, 1.436; CH3, 1.079; CHI, 1.084; SiOlHI, 122.3; OIHIOZ,141.8; H I O ~ C150.0; , 0zCH3, 107.0; 0zCH4, 111.8; COZHZ,113.9; H20zCH4, 61.1 SiOl, 1.612; Si04, 1.676; 01H1,0.990; HIOZ,1.581; 02‘2, 1.431; CH5, 1.079; CH6, 1.085; 02Hz,O.986; 04H3, 1.696; O,Hp, 0.984; 03C, 1.434; CH7, 1.079; CHs, 1.085; SiOlHI, 137.8; 01H102, 167.1; HIOZC, 138.1; OzCHs, 107.4; OzCH6, 111.7; HzOzCH6, 61.0; HIOZHZ,115.1; Si04H3, 120.5; 04H303, 174.8; H303C, 113.6; O3CH7, 107.4; O Z C H ~111.7; , H303CHs, 61.1 ~~
Indication of the geometry parameters corresponds to Figures 5 , 6, 7, 9, and 10. Numbers in parentheses correspond to the CHJOH subunit numbers in Figure 5 . 0-H bond lengths and TOH angles (T = Si, Al) of the terminal OH groups not involved in the modeled interactions (H atoms (ROH= 0.951 A and LAlOH = 140° 16-17). are not numbered in the figures) are fixed and equal to those in Si(OH)4 (model 2a) and HIO.A~(OH)~
during the activation procedure. The same effect was observed on dehydroxylated silica.31-32 According to experimental studies for silica32-34 and ab-initio calculations,3s this band belongs to thevery active edge-shared tetrahedral defect produced by water elimination from adjacent isolated silanols a t T 1 800 K.32-36
1
Like in structure 1, the hydroxyl group in structure 3 should not be hydrogen bonded. However, the growth of a complex broad band at 3400-3550 cm-*, occurring during the first stages of methanol dosage (Figure la, spectra 2 and 3), indicates the formation of hydrogen bonds of moderate strength. We ascribe this band to hydrogen-bonded 3SiOCH3 groups produced by chemisorption on the other active sites, the strained siloxane bridges2733
2
Being geometrically too far apart for hydrogen bonding,33.36these silanols adsorbat 3743 cm-’ (cf. the 3747-cm-’ band of theisolated M O H group36). Structure 1 was also found on ZSM5-like silicalite in a recent study by Zecchina et al.37 The adsorption on the edge-shared defects should result in species I formation. ,CH3 OH
2
0
3
4
5
which are not associated with definite skeletal bands in the IR spectra. These active sites are formed by dehydroxylation reaction between hydrogen-bonded M i O H gro~ps,?~,~**36 where silica atoms are not connected through a siloxane bridge, like in structure 1.
Methanol Adsorption on Silicalite and Silica
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11981 OH
I
Si
6
7
As found by Zecchina et al.,37 there are two geometricallydifferent types of structure 6 in ZSM5-like silicalite: one of them adsorbs at 372513500 cm-1 and theother at 3700/3400 cm-l, the highest and lowest wavenumberscorrespondingto the free and hydrogenbonded OH groups of structure 6, respectively. Morrow et al.34 reported two similar types of structure 6 for silica, adsorbing at 372013520 and 3700 cm-l. Therefore, we assign the two broad OH bands at 3520 and 3410 cm-l, which can be easily distinguishedafter the first methanoldosage (Figure la, spectrum 2), to the hydrogen-bonded OH groups of two geometrically different types of structure 5, analogous to those of structure 6,343 In a recent ab-initio study, Pelmenschikov et al.17J9showed that the formation of a hydrogen bond to the oxygen atom of the 3SiOCH3 group should shift the das, u ” ~ and ~ , us frequencies to higher wavenumbers. They explained this effect on the basis of Gutmann’s second rule:38 an intramolecular electron density redistribution decreasing the C+-H- polarity of the C-H bonds should increase their strengths. Therefore, the 301&3030-~m-~ shoulder can be ascribed to the u’,~frequency of structure 5 (hereafter speciesII), shifted to higher wavenumberswith respect H H &H OH*** 0 ‘
I
I
A\ 0.’j\ 5
to the d , = 2997 cm-1 of species I by electron density withdrawal from the CH3 group. The broadening of this band in comparison with the CH bands of species I can be explained by the abovementioned inhomogeneity of structure 5. By increasingthe coverage (Figure 1b, spectra 4-6), the maxima of the 2997- and 2959-cm-I bands shift gradually to 2994 and 2950 cm-l, and a new band appears at 2845 cm-l very close to the 2857-cm-l band. This is due to the formation of a new species, whose 2994-, 2950-, and 2845-cm-1 frequencies are shifted by 3, 9, and 12 cm-1 to lower wavenumbers in comparison with the corresponding CH frequencies of species I. The 2950- and 2845-cm-* bands were also detected by Borello et al.27.28in the adsorption on silica and ascribed to the physically adsorbed methanol species on the 3SiOH groups. By increasing the concentration of physically adsorbed methanol, a band at 3632 cm-l grows (Figure la, spectra 4-6); this band was observed in the physical adsorption on silica27.28 and tentatively assigned to the OH stretching frequency of terminal methanols in hydrogenbonded polymeric aggregates (see below). The degassing at room temperature completely removes the physically absorbed methanol from the samples (Figure 1, c and d, spectra 10 and 1l), leaving only the chemisorbed species. The 2997-, 2959-, and 2857-cm-l bands of chemisorbed species I are at least 3-4 times more intense after the degassing than after the initial stages of the chemisorption, when this species is formed through reaction 2 with the edge-shared defects (Figure 1, a and b, spectra 2 and 3), as stated above. This increase of species I concentration could occur either through the reaction of esterification
4\
+ CH30H
-
/CH3
0
I
+ HO,
’
(5)
or through local structural reconstructionsaccompanyingreaction 3 of structure 5 formation
5
in regions of high 3SiOH concentration: both the reactions are slower than reaction 2 on the very active edge-shared defects. Reaction 5 can be excluded,as the 1630-cm-l OH bending characteristic of physically adsorbed water, was not found in the spectra. Further arguments in favor of reaction 6 come from the IR spectra, and the results of calculations and will be discussed below. As to the CH bending region, the first dosages of adsorbed methanol produce a complicated 1475-1455-cm-1 band (Figure 4a,b), which was to the b(CH3) vibrations of the chemisorbed methanol, by analogy with the 1474-, 1466-, and 1450-cm-l b(CH3) modes of the methanol molecu1e.M Upon increasing the coverage, a parallel growth of two other bands occurs in the same spectral region: a sharp band at 1450 cm-1 and a broad one whose maximum moves gradually from 1380 to 1400 cm-l (Figure 4a). These bands belong to the physically adsorbed methanol, as they are removed by degassing at room temperature (Figure 4b). The 1450-cm-1 band can be ascribed to a b(CH3) vibration, while the broad band is very similar to the OH bending band of methanol polymers,30 which also moves from 1380 to 1407 cm-l on transition from dimer to multimer. Moreover, the 2994-, 2950-, and 2845-cm-1 CH frequencies of the physically adsorbed methanol (cf. above) are very close to the 2996-, 2949-, and 2840-cm-l C H frequencies of methanol polymers in argon.30 This indicates a tendency of the physically adsorbed methanol to form polymeric aggregates, even at quite high concentration of unoccupied 3SiOH groups in the samples. (See the 3743-3747-cm-1 band intensity of the free 3SiOH groups in Figure la, spectra 4 and 5 . ) In Table I11 the calculated CH frequency shifts of methanol dimer and trimer ( l a and lb, Figure 5) with respect to free methanol are presented. The experimental shifts of the dimer compared to those of the monomer in argon matrix isolation, reported in Table 111,were evaluatedon the basis of our assignment of CH frequencies detected in ref 30 (Table IV). According to the second Gutmann rule and the C+-H- polarity of the CH bonds (see refs 17 and 19 for more details), the electron density perturbation, caused by the hydrogen bonding in the dimer and the trimer (Figure 5), shifts the CH stretching frequencies of the electron-acceptor (la( 1) and lb(1)) and electron-donor (la(2) and lb(3)) terminal methanols to lower and higher wavenumbers, respectively. As the 3-21G basis set overestimatesthe hydrogenbonding effect,M these 3-21G frequency shifts are about 1.5 times larger than the corresponding 6-3 1G* frequency shifts. The CH
Pelmenschikov et al.
11982 The Journal of Physical Chemistry, Vol; 97, No. 46, 1993
bl
29%
d Wavenumbers cm-‘
Figure 2. IR spectra of methoxylated silicalite (-) and silica (- -) at
room temperature.
3600
3200
2800 3000 Wavenumbers cm-‘
2800
Figure 1. IR spectra of CH3OH on silicalite. (a) Spectrum 1: silicalite outgassed at 773 K. Spectra 2-6: increasing number of doses up to CH30H pressure (2) 50, (3) 102, (4) 5 X 102, (5) IO3, and (6) 2 X IO3
Pa, (b) Extended view of the CH stretching region of (a) (after background and hydrogen-bondingband tail subtraction). (c) Effect of outgassing. Spectrum 6 corresponds to spectrum 6 of (a) Spectra 7-1 1: effect of decreasing CH3OH pressure to (7) 5 X lo2,(8) lo2, (9) 50, (10) 20, and (1 1) 5 Pa. (d) Extended view of the CH stretching region of (c) (after background and hydrogen-bonding band tail subtraction).
TABLE II: Total Energies (hartrees) basis set molecule 3-21g la
lb IC
2a 2b 2c 24l
2e 3a 3b 41 4b Sa 5b 5c
sd
-228.8124 -343.23 10 -572.0722 -587.7192 -626.5301 -1214.2697 -1 175.4581 -1763.2028 -740.9532 -1206.8768
6-31g* -230.0789 -345.1241
-629.9 127 -629.9063 -702.1388 -702.1 304 -702.1467 -8 16.5771
frequencies of the internal methanol (lb(2)) in the trimer are slightly shifted, as the electron density changes of the CH3 group of this molecule, produced by the two hydrogen bonds, compensate each other. Like for the electron-donormethanols in the dimer and trimer, the 3-21G CH frequencies (Table V) of the electron-donor 9SiOCH3 group of species II (model ZC, Figure 6) are shifted by about 20-30 cm-1 to higher wavenumberswith respect to those of the free SSiOCH, group, species I (model 2b, Figure 6). This strengthens our suggestion about the nature of the 3010-3030-~m-~ shoulder. To demonstrate further this effect in adsorption
950
I
I
900
850
Wavenumbers cm-’ Figure 3. Effect of CH3OH dosage on the 910-890-cm-* band of the edge-shared tetrahedral defects. Spectra 1-6 corresponds tospectra 1 - 6 of Figure 1.
interactions, the calculated and experimental C H frequency” shifts of dimethyl ether on the terminal SSiOH and bridging SSi-(OH).-Alf group of zeolites (models 3a and 3b, Figure 7) are presented in Table VI. As dimethyl ether can act toward these OH groups as an electron donor only, the CH frequencies should undergo upward shifts in both the interactions. The shifts for structure 9 is larger than for structure 8 (Table VI) as the electron density withdrawal from the CH3 groups of dimethyl ether on the strongly acid bridging OH group is larger than on the 9SiOH.
H
E
9
The Journal of Physical Chemistry, Vol. 97,No. 46, 1993 11983
Methanol Adsorption on Silicalite and Silica
TABLE I V CH Stretching Frequencies (cm-l) of Methanol in Argon assgnt" J,
J',
VI
species
obsdb
dimer methanol (2) monomer dimer methanol (1) dimer methanol (2) monomer dimer methanol (1) dimer methanol (2) monomer dimer methanol (1)
3019 3005 2992 2979 2962 2949 2848 2833
Figure 4. IR spectra of the CH bending region. (a) Increasing number of CH3OH doses. Spectra 2-6 correspond to spectra 2-6 of Figure 1. (b) Effect of outgassing. Spectra 6-10 correspond to spectra 6-10 of Figure 1.
a This reassignment for methanol dimer, with respect to a tentative assignment made in ref 30, follows from the observation based on the 6-31G' and 3-21G calculations (Table 111) that the Ju, V"u, and v, frequencies of the electron-acceptor and electron-donormethanols of the dimer, dimer methanol (l), and dimer methanol (2)undergo downward and upward shifts of approximately equal absolute values. b Reference 30.
TABLE III: CH Frequency Shifts (cm-l)'
TABLE V: CH Frequency Shifts (cm-l)
1500
1400
Wavenumbers cm-'
zc
la(2) lb(1) lb(2) lb(3) obsdb calcd obsdb calcd calcd calcd -21 (-31) -13 17(22) 14 -27(-42) -2(-3) 16(24) 5(0) 32(45) -22(-41) -13 25 (36) 17 -32(-38) -22 (-43) 3 (-1) 21 (29) -16(-28) -15 17(23)
141)
assgnt
v', J', V, 0
calcd
Calculations with 6-31G' (3-21G)basis set. Reference 30.
JU
J'u VS
calcd'
obsdb
22 35 20
10-30
The 3-21G basis set. On the basis of the 3010-3030-cm-' band assignment to the JU of species II. J', and uU bands of species II are supposed to be indistinguishable in the spectra (see text).
(2)
la
assgnt
Ib 2a
2b
2d 10
Figure 5. Methanol dimer, trimer, and pentamer. Arrows indicate directions of electron charge transfer.
20
2e
Figure 6. Molecular models.
The energies of the reactions
2a + 2a 2a + 2d 2a
+ 2b
-
+ 52 kJ/mol
(7)
2e + 67 kJ/mol
(8)
+ 53 kJ/mol
(9)
2d
2c
evaluated from the 3-21G total energies (Table 11) illustrate the well-known cooperative effect (refs 42-44 and references therein) for the + S O H groups (cf. energies of reactions 7 and 8). Like for OH-containing molecules in solution, this effect causes the H I O H groups of silica and silicalite to form large hydrogenbonded associates.36~3~ This effect explains also why reaction 6, proposed on the basis of the IR spectra, should become energetically favorable when the local ?&OH concentration is high enough for the association to occur (cf. energies of reactions 8 and 9). Apparently, the predominant 3SiOH location in atomic-scale cavities in silicalite, as stated by Zecchina et al.,37 should make the +SiOH local concentrationin silicalitemaximal for the known Si02 materials. Therefore,during the methoxylation,the channel
3a 3b Figure 7. Molecular models of dimethyl ether adsorbed on terminal and bridging OH groups of zeolites.
of species I formation through reaction 6 might become considerably more important for silicalite than for silica. The IR spectra of methoxylated silicalite and silica, prepared using the usual procedure of silica methoxylation24 (Figure 2), reinforce this supposition. At the same CH band intensities of species I on both samples, the OH band of + S O H associates at 3420 cm-1,37 appearing during the methoxylation, is considerably more intense for silicalitethan for silica, as speciesI formation through reaction 6 is accompanied by the growth of these associates.
11984 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
Pelmenschikov et al.
TABLE VI: CH Freauencr Shifts (cm-9 ~
3b
3a
assnnt ~~
calcd“
obsdb
calcd‘
obsdb
18 38 23
9 21 11
28 62 33
23 51 24
~~~
V’ll
V”u VI
a The 3-21G basis set. b On the basis of IR data from ref 25 and of the CH frequencies of gaseous dimethyl ether.41
I2858
4a 4b Figure 9. Geometry of the Si(OH)(01H)(02H)fragment equal to that in Si(OH)4 optimized with the 6-31g* basis set and S4 symmetry ( 4 4 The 03CHlHzH3 fragment is fully optimized. (4b) re~triction.~~
The CH3 group is rotated around the S i 4 3 bond from the equilibrium position in model 4a (torsional angle OlSiO3C is equal to -90°, while it is 73O in 4 4 . All other geometrical parameters of the OICH~H~H, fragment are optimized.
bution of the 2959-cm-1 band to the v”, mode of species however, this is mainly based on IR spectra recorded at temperatures considerably higher than 97 K,25*26J9usually at room temperature, when the 2959-cm-l band is the only one one could associate with the Y”,, as the 2974-cm-I band becomes its hardly visible shoulder (see spectrum b in Figure 8). There are two possiblereasons for the observed change of species I spectrum (Figure 8) by increasing the temperature from 93 to 193 K: either the appearance of new optical transitions in this species or a surface reconstruction,causing species I to transform to another species (or to be perturbed). A new optical transition in the unperturbed 3SiOCH3 group could result only from the appearance of a new rotational degree of freedom in this group. It could be the rotation either of the hydrogens around the C-O bond or of the CH3 group around the Si-O bond. Quantitative e~timates2~ exclude the possibility of free rotation of the hydrogens around the C-0 bond even a t room temperature. The experimental energy barrier (4.5 kJ/mol) of this rotation in gaseous methanol24 gives a probability for this rotation a t 193 K, exp(4S/RT) = 0.06, a very small value. The 6-31G* energy difference between two conformations of (H0)3SiOCHp molecule (models 4a and 4b, Figure 9), corresponding to two different positions of the CH3 group around the S i 4 bond, is equal to 17 kJ/mol. The calculated frequency of the CH3 torsional vibration with respect to the S i 4 bond for this molecule is 71 cm-1. On this basis the energy barrier for the free CH3 group rotation around the Si-0 bond (216 kJ/mol) is too high for this rotation to occur. Moreover, the suggestion of a rotational degree of freedom for this structure at 193 K is in contradiction with the absence of a vibrational-rotational subband on the right side of thevery sharp 2858-cm-1 band in the spectrum. Such a subband of the 2858-cm-1 band appears only a t about 800 K (the spectrum is not reported), when a considerable fraction of the +SiOCH3 structures should be in a state with the CH3 group hydrogens rotating around the C-O bond, as exp(d.S/RT) 0.5 a t this temperature. As seen from the spectra in Figure 8, a decrease of species I 2997, 2974, 2957, 2924, and 2857 cm-I band intensities is accompanied by an increaseof the 3010-3030-~m-~ bandintensity. According to our hypothesis, this shows the transformation of species I to species 11, whose J,, J’,, and v8 bands are shifted by about 10-30 cm-1 to higher frequencies with respect to the corresponding v’, = 2997, Y“, = 2974, and vl = 2857-cm-1 bands of species I and broadened due to the structural inhomogeneity of species 11. Two other broad CH bands of species II, namely, the Y“, and vs bands, expected to be a t approximately 2990 and 2880 cm-1, cannot be distinguished among other sharp bands in the complex 3000-2860-cm-1 spectral region. One can also suppose that, owing to the extremely high local M i O H concentration in silicalite, the transformation of species I1 to species I by decreasing the temperature should be facilitated in silicalite in comparison with other Si02 materials, being promoted by the stronger cooperative effect (see above). This
k2c26929
m V
m
?
s
9
I
I
3000 2800 Wavenumbers cm-’ Figure 8. IR spectra of methoxylated silicalite at (a) 93 and (b) 193 K.
The other important conclusion following from reaction 6 is that chemisorbed species I and I1 should be in thermodynamic equilibrium: the temperature decrease should shift the thermodynamic equilibrium in reaction 6 to the left (cf. energies of reactions 8 and 9); Le., it should increase the relative concentration of species I. Such reversible increase of species I band intensities upon decreasing the temperature of methoxylated silica, with approximately constant overall band area in the 3030-2840-cm-l range, was first observed by Morrow.24 He concluded in favor of a thermodynamic equilibrium between species I and someother chemisorbed form of methanol adsorbing in the same complex spectral range, as the experiments were carried out in the absence of both physically adsorbed and gas-phase methanol. This progressive transformation of some chemisorbed form to species I on decreasing the temperature was observed on methoxylated silica24 within the temperature range from 673 to 101 K. The IR spectra of methoxylated silicalite samples at 193 and 93 K, obtained after removal of physically adsorbed and gasphase methanol, are shown in Figure 8. According to our interpretation of the adsorption, the complete absence of the 3010-3030-cm-l shoulder at 93 K means that only species I is present. The nature of the bands at 2974 and 2957 cm-l in this spectrum needs a special discussion. One of these two bands should be associated with the v”, mode, while the other with an overtone or a combination of the 6(CH3) modes of species 1.25829330 We assume that the 2974-cm-I band corresponds to the vrrasand the 2957-cm-I band to the highest S(CH3) overtone of species I, which is expected to be approximately equal to 2 X 1475 = 2950 cm-l (see above). This interpretation is analogous to the one accepted in the literature for the 2961- and 2956-cm-l bands of methanol monomer in arg0n,~~J5+30 assigned respectively to the v’lglland the highest S(CH3) overtone (2 X 1474 = 2948 cm-1). Our assignment is in contradiction with the agreed-upon attri-
Methanol Adsorption on Silicalite and Silica
The Journal of Physical Chemistry, Vol. 97, No. 46, I993
TABLE MI: Adsorption Energies (kJ/mol) and CH Frequency Shifb (cm-I) CalCd“
adsenergy J, J’, VI
Sa
Sb
35
57
77
81
81
-25 -47 -32
26
-1 11 8
-8 6
-11 7 3
43 27
Sc
Sd(1)
1
Sd(2)
lc(3)
obsdb
-14
-11 -11 -2
-6
-8
The 3-21Gbasis set. Shifts with respect to the methanol monomer in argon matrix isolation.30 a
11985
11 (Table VII), due to the extra hydrogen bonds of methanols in structures 12 and 13 in comparison with structures 10 and 11. The specific adsorption energy increases also from structure 12 to 13, in agreement with the tendency of methanol to polymerization even at high concentration of unoccupied SSiOH groups (see above). This effect can be explained by two factors, namely, by a decrease of the hydrogen-bonded ring strains17943 and an increase of the cooperativity effect in going from structure 12 to structure 13. The best agreement between calculated and observed shifts (Table VII) is found for the internal methanol in the pentamer (lc(3), Figure 5), supporting the conclusion that the physically adsorbed methanol is mainly in the form of large hydrogen-bonded polymers. Conclusion
I
p”g H
Sb
Sa
5c 5d Figure 10. Molecular models of physically adsorbed methanol. Arrows
show directions of electron charge transfer.
explainswhy in Morrow’s study24of this reaction on methoxylated silica a high concentration of species II remained on the surface even at 101 K (see Figure 2 in ref 5), unlike their complete transformation to species I at approximatelythe same temperature in silicalite (see Figure 8). In our opinion for a considerable fraction of methoxylated silica surface, prepared following the usual procedure,24the local SSiOH concentration is not high enough for the cooperative effect to occur. Like for the electron-acceptor, electron-donor, and internal methanols in thedimer and trimer (Table 111), 3-21G calculations show (Table VII) that the CH frequencies of methanols in structures 10,11,12and 13 (models5a, 5b,5,and 5d, respectively,
10
11
I
12
I 13
Figure 10) should be shifted to higher wavenumbers (structure lo), to lower wavenumbers (structure l l ) , and slightly shifted (structures 12 and 13) in comparison to free methanol. These structures correspond to different mechanisms of physical adsorption at very low coverages, when the formation of large polymeric associates of methanol could be neglected. The calculated specific energies of methanol adsorption are significantly larger for structures 12 and 13 than for structures 10 and
In conclusion, chemisorbed and physically adsorbed methanol species on silica and silicalite are chemically identical. Two chemisorbed forms are suggested, taking place through the reaction with the edge-shared tetrahedral defects and with the strained siloxanebridges. The first one, SSiOCH3 group (species I), manifests itself by vJaS = 2997, = 2974, and Y, = 2857cm-l stretching frequencies. The previous assignments of the 2959-cm-1 band to the Pas of this group are suggested to be erroneous. The second chemisorbed form, hydrogen-bonded SSiOCH3 group (species 11), adsorbs at v’, = 3020, v”- N 2990, and vu = 2880 cm-l, with the IR band broadened by the structural inhomogeneity of this form. These two forms are in thermodynamicequilibrium,and upon decreasingthe temperature the relative concentration of the free SSiOCH3 groups increases. This is due to the 3SiOH transformation from species II into hydrogen-bonded SSiOH aggregates, caused by the cooperative effect. We suppose this effect to be similar in nature to the wellknown phenomenon of phase separation on decreasing the temperature in two-component liquid mixtures. Due to the extremely high 3SiOH concentration in atomic-scale defects in silicalite, this effect is more pronounced for silicalite than for silica, being promoted by the stronger cooperative effect. Owing to the cooperative effect, the physically adsorbed methanol is present on the surface mainly in hydrogen-bonded polymeric forms. It adsorbs at v’, = 2994, V”as = 2950, and v, = 2845 cm-l, very close to the corresponding frequencies (2996, 2949, and 2840 cm-l) of the polymeric methanol in argon matrix isolation.
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