4991
J . Phys. Chem. 1992,96,4991-4997
Sllicallte Characterization. 2. I R Spectroscopy of the Interaction of CO with Internal and External Hydroxyl Groups A. Zecchina,* S. Bordiga, G.Spoto, L.Marcbese, Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universith degli Studi di Torino, via P . Giuria 7, 10125 Turin, Italy
G.Petrid, G.Leofanti, and M.Padovan ENICHEM ANIC Centro Ricerche di Bollate, via S. Pietro 50, 20021 Bollate, Italy (Received: August 26, 1991; In Final Form: January 22, 1992)
In paper 1 we have shown that the structural properties of silicalites depend upon the preparation procedure. In particular Na- and Al-free sample (S) prepared following a specifically designed method is characterized by the presence of internal c and computer graphic simulations, atomic-dimension defects (nanodefects and microcavities). On the basis of s p e c t r ~ p idata it is shown that these microcavities derive from one or more missing [SO4] units and can contain up to four OH groups per missing tetrahedron. These OH groups form chains of hydrogen-bonded species which, upon water elimination at high temperature, give Si-0-Si distorted bridges characterized by peculiar vibrational features. It is also shown that internal and external OH groups have distinguishable IR manifestations. Terminal and isolated hydroxyl groups located in internal positions give 1:l hydrogen-bonded adducts with CO at 77 K, which are associated with red and blue shifts of the OH and CO stretching frequencies, respectively. External OH groups can be perturbed only by dosing CO at more elevated pressures.
Introduction In paper 1 we have concluded that Na- and Al-free silicalite S (synthesized following a properly designed method) is characterized by (i) full crystallinity and orthorhombic structure, (ii) prismatic microcrystalscharacterized by very regular contour and an external surface area of 22 mz g-',l and (iii) presence of internal atomielike defects (nanodefects or microcavities). These defects are the cause of the anomalous abundance of OH groups as evidenced by the IR spectroscopy (in respect to "normal" SNa samples prepared using the Flanigen patente2 To go deeper in the characterization of this material, in this paper the evolution of the IR spectra of hydroxyls with the outgassing temperatures is studied in detail. The concentrations of internal and external silanols were also determined by means of temperature-programmed H20desorption. The structure of exposed faces and examples of internal defects are described using computer graphic simulations. Due to the high sensitivity of the stretching frequency of CO to the interaction with the hydroxyl groups (Brijnsted sites) and Lewis site^,^-^ an IR study of CO adsorption at low temperature was carried out in order to study (i) the local fields present in channels, microcavities, and external surfaces; (ii) the acidity of silanols in the microcavities; and (iii) the possible presence of Lewis centers after dehydroxylation at high temperature. Experimental Section Materiels The synthesis of the samples was described in paper 1. Metbods. The IR spectra were obtained on a Bruker IFS 48 FTIR spectrometer equipped with a MTC cryodetector (resolution 2 cm-I). Thin pellets of compressed powder were inserted in an all-silica IR cell permanently connected to a high vacuum (p = lop5Torr; 1 Torr = 1.33 mbar) manifold allowing in situ outgassing procedures (up to 1123 K) and gas dosages. In CO adsorption experiments, the temperature of the pellet was decreased to =lo0 K by cooling the sample holder with liquid Nz. Lower temperatures were difficult to achieve, because of the heating effect of the IR beam and of the low conductivity of the sample. Because of the difference in temperature, spectroscopic and volumetric isotherms (discussed in paper 1) cannot be fully compared. Computer graphic simulations of the external faces and of the internal defects were performed with the CHEM-x program distributed by Chemical Design Ltd., Oxford.
Results and Discussion IR Spectra of Hydroxyl Groups at Increasing Outgassing Temperatures. The evolution of the IR spectrum of the silicalite
S recorded in transmission mode in the 3800-3000-cm-' range as a function of the outgassing temperature is illustrated in Figure 1. The spectra are shown in absorbance scale. For comparison, the spectrum of a nonporous SiOz sample (curve 5 ) , treated at 973 K in vacuo, is shown. In the inset, the parallel thermogravimetric curve of the weigh loss at increasing temperature is also illustrated. The absorption bands can be roughly divided into two main groups: those falling in the 3800-3650-cm-' interval and those falling at lower frequency (3400-3500 cm-I). (a) 380&3650-cm-' Range. The 3800-3650-cm-' absorption show well distinguishable tailed components having intensity varying with the outgassing temperature. Three main bands (A, B, C) can be singled out from the 3800-3650-cm-' envelope. Band A, observed at 3700 cni' on the low-temperaturetreated sample, decreases strongly in intensity in the 573-773 K interval (curves 2-3) and is practically absent on the 973 K (curve 4) treated sample. At the same time, the frequency of the maximum undergoes a gradual upward shift (Av = 16 cm-I). Band B, initially observed at 3725 cm-l on the 300 K treated sample (curve l), decreases gradually with the increase of the outgassing temperature. Unlike the previous case, it is still present on the sample treated at 973 K. The B peak undergoes a gradual upward shift from 3725 to 3733 cm-' (AB = +8 cm-'). Band C, observable as an ill-defined shoulder on the low-temperature-treated sample, becomes a prominent peak on the sample outgassed at high temperature (curve 4); this behavior is the typical one for isolated species. Comparison with the spectrum of OH groups on nonporous silica shows that, even on the samples outgassed at 973 K,the half-width of the C peak = 50 cn-') is definitely larger than that of the corresponding one of silica. Also the frequency of the maximum (3742 instead of 3747 an-') is not the same. ( b ) 3600-3200-cm-' Range. The very broad 3600-3200-cm-* absorption is clearly composite and two broad components (A', B') can be singled out. The first band (A', shoulder) is centered at =3400 cm-' while the second one ( A v ~ ,=~ 250 cm-I) is in the 3460-3500-cm-' interval (a moderate upward shift AB = 40 cm-l is observed by increasing the outgassing temperature). The A' band disappears first, while the B' is more resistant and is still observed on the samples treated at 973 K (curve 4). This behavior
0022-3654/92/2096-499 1%03.00/0 0 1992 American Chemical Society
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4992 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
z 11
Zecchina et ai.
38.
la
Because at this distance hydrogen bonding is extremely weak: or negligible, we can conclude that on these faces the hydroxyl groups are substantially unperturbed (isolated). (100)Faces. Figure 2B represents the hydroxyl groups distribution on (100)faces seen along the [loo] direction. Also in this case the OH density is 3.1 OH/nm2 and the minimum O H - O H distance is 0.36nm; the hydroxyl groups concentration is the same as in the surface structures shown in la. Also in this case the silanols are isolated. (001) Faces. Figure 2C represents the hydroxyl groups distribution on (0011faces seen along the [001] direction. On these faces the structures
10.1
terminal
O,rH
\
H-0 0-li /H
if
I isolated
",'terminal
SI /I'
/ \
/I\
lb
4
3530
1
3738
3650
3503 3603 3300 WfiVtVdMBtS C M - I
3239
3!80
Fwe 1. IR spectra of the silicalite S outgassed under vacuo at (1) 300 K, (2) 573 K, (3) 773 K, (4) 973 K. Curve 5 corresponds to S O l (Aerosil) outgassed at 973 K. In the inset is reported the thermogravimetric curve of S. TABLE I 3800-3650 cm-' Stretching modes of OH groups not involved in
hydrogen bonding. These groups include (a) isolated OH (either external and internal) and (b) terminal OH (either internal and external) 3650-3200 cm-' Stretching modes of hydrogen-bonded OH groups
(internal and external) is parallel to that found for the A and B species (and this is the reason for the similar label). On the basis of the immense literature concerning the hydroxyl bands on silica and siliceous materials (refs 6-16 and references therein), the two groups of bands observed on S silicalite can be broadly assigned as illustrated in Table I. Before trying a more detailed assignment, we must previously answer the following question: Are the observed bands all associated with the OH groups of the external surfaces (as usually assumed), or do we need to invoke internal species as well? In order to answer this question we have examined the structure of the most commonly exposed faces by means of computer graphic simulation. On the basis of the high-resolution electron microscopy results (showing that the exposed faces are regular and flat) the simulations can be considered as realistic. structure of the Externrl Faces rind the OH C-tion. The structure of the most commonly exposed faces is shown schematically in Figure 2,parts A-D. As shown in Figure 4 of paper 1, the faces are generated by cutting the crystal along the planes where the structure shows the smallest density of bonds. The dangling bonds centered on silicon and oxygen atoms formed during this procedure are then saturated with O H and H groups, respectively, to generate S i 4 H groups and to ensure the stoichiometry. (010)Faces. Figure 2A represents the hydroxyl groups distribution on (010)faces seen along the [ O l O ] direction. The OH density is 3.1 OH/nm2 and the minimum OH-OH distance (for instance, 0 1 - 0 2 in the figure) is 0.36 nm in the structure shown as follows.
are simultaneously present. The minimum OH-OH distances (01-02 and 0 1 - 0 3 in the figure) are 0.25 nm (second structure of scheme 1 b). This distance is sufficiently low to ensure hydrogen bonding. Following ref 6,such distance could well justify the peak in the 3300-3600-cm-' interval. The hydroxyl density on this face is estimated to be about 5 OH/nm2. {loll Faces. Figure 2D represents the hydroxyl groups distribution on (101)faces seen along the [loll direction. In this case the situation is similar to that found on (001)faces: the minimum OH.-OH distance is 0.25 nm and the O H density is 3.8 OH/nm2. The surface concentration of OH groups (per gram of zeolite) estimated on the basis of the particles dimensions and of the results derived from Figure 2,parts A-D is =6 X 1019 OH/g. A high proportion of these groups is isolated (=90%) while a minor fraction (=lo%) is hydrogen bonded. This figure, obtained by combined use of electron microscopy and computer graphic simulation, can be considered as reasonably correct, at least as far as the order of magnitude is concerned. However, it totally disagrees with the OH groups concentration resulting from the weight lass measurement illustrated in the inset of Figure 1, (which is 1 order of magnitude larger, -9 X lo2' OH/&. This undoubtedly means that (i) the external OH groups represent only a small fraction of the total, (Le., 6-7%) and (ii) that the vast majority of the OH groups is located in the inner parts of the microcrystals. Detailed M i t of the Hydroxyl Bands. The previous conclusion is reinforced by purely spectroscopic considerations. In fact, from parts A-D of Figure 2 it can be seen that, although on these faces both isolated and hydrogen-bonded hydroxyl groups are simultaneously present, the relative concentration of these species (Le., isolated vs hydrogen bonded) does not correspond to the experimental spectra (characterized by an exceedingly high absolute and relative intensities of the hydrogen-bonded species). Moreover, even on samples outgassed at high T (973 K) the characteristic narrow band at 3750 cm-l of the isolated, external OH groups cannot be seen, because it is overshadowed by a broader and tailed adsorption at 3742 cm-'(Cband). This purely spectroscopic result fully confirms the hypothesis that silanols at internal positions are abundantly present on silicalite samples and contrasts with the model of perfect zeolitic samples, where the hydroxyl groups are considered to be located on external faces only. This in turn implies that the OH excess is associated with internal defects. The substantial absence of extended defects shown by the HRTEM investigation, indicates that these defects have atomic scale dimension (vide infra), i.e., involve defect extended over a limited number of [SO4] building units. We have hypothesized that the imperfections derive from one or more missing [Si04]unit in the framework and in Figure 3 possible
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4993
Silicalite Characterization. 2
I
i
Figure 2. Computer graphic modeling of the OH distribution on four different faces. (A) (010) face; dol,, 0.36 nm. (C) (001) face; dol,, and do143 = 0.25 nm. (D) (101) face; dola, and do143 = 0.25 nm.
structures (a-c) of these defects consisting of some adjacent missing tetrahedra located in different position of the unit cell are represented. In order to keep the stoichiometry, in defects derived by one missing [Si04] unit, four hydroxyl groups must be present and the minimum OH--OH distance is 0.25 nm. As this distance is suitable for hydrogen bonding, the abundant presence of hydrogen-bonded species absorbing at 3600-3200 cm-l is explained (Figure 1). Similar situations are expected when two or more adjacent tetrahedral units are simultaneously missing (forming a microcavity). In such cases we can have also geminal hydroxyl groups. In all cases a common feature is observed: chains of hydrogen-bonded hydroxyl groups, whose length depends upon the number of missing [SO4] units, populate the surface of the microcauity. It is interesting to notice (Figure 3) that some of the holes are totally internal (a) (and consequently inaccessible to gases) and some others are connected with one channel only (b) while the remaining one connects two straight channels (c). In reality, it is better to speak about a distribution of ultramicropores instead of a few types of structures, even if we must stress that these defects always involve a very limited number of tetrahedra and so escape the electron microscopy observation. The presence of these microcavitiesis in agreement with the volumetric results (data shown in paper 1). The model of the defective sites is similar to that proposed by van Santen For a more quantitative determination, 29Si NMR et al." spectroscopy will be used in the near future.'* Having positively concluded about the presence of internal hydroxyl groups, we have to answer the final question: How big is the spectroscopiccontribution of the hydroxyls located on ex-
= 0.36 nm. (B) (100) face; dola, =
F'igure 3. Models of possible nanodefects in the zeolite structure derived from a few missing [SiO,] units.
ternal surfaces and where is the involved stretching band? The answer is not straightforward. However, we can reasonably hypothesize that on samples outgassed at 973 K, the external hydroxyl groups density (OH per nm2) is the same found on nonporous silica after a similar treatment. This means that the contribution expected from external groups should be similar to that of a nonporous silica with the same specific surface area of S sample (22 m2 g-') outgassed at 973 K. As the Aerosil sample
4994 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Zecchina et al.
SCHEME I
3ted internal
,terminal internal ( in accessible nanopores)
used here for the sake of comparison has a specific surface area of 180 m2 g-l, we have reduced the observed peak intensity by a factor -10. This leads to the small narrow peak shown in the bottom of Figure 1 (curve 5). In conclusion, the IR manifestations in the OH stretching region observed on S sample are essentially determined by the internal hydroxyl groups while the contribution of the external groups is minor. We shall fully demonstrate the correctness of this conclusion in the paragraph concerning the interaction of CO at low temperature. This conclusion is of c o w a fortiori valid also for the SNa silicalite. The next step of the discussion is the assignment of the A-C and A'-B' bands described before, which have been firmly established to belong to terminal and hydrogen-bonded species located at internal position. Bands A, B, and C (3800-365O-cm-' Range). Their behavior upon outgassing suggests that they are associated with substantially unperturbed terminal (A, B) and isolated (C) species, respectively (because they are influenced by the thermal treatments in an opposite way, Le., A and B decrease while C increases). Bands A'and B' (3600-3200-~m-~ Range). On the basis of their thermal behavior, frequency, and half-width, they are assigned to hydrogen-bonded hydroxyl groups present in the same (complex) structures (A, B). AU these considerations are consistent with the following picture: hydroxyl groups present at internal defects are prevalently grouped together to form chains which can be schematized as follows:
'I
isolated Internal
( 7)
,hydrogen b d e d internal
1( in accessible nanopoms J
Olated external
4 3800
i
3700
3600
3500
3400
3300
WAVENUMBER E M - I
Figure 4. Schematic representation of different surface structure contributing to the OH stretching bands of an S sample outgassed at 973 K. The narrow band at 3750 cm-' corresponds to the stretching mode of isolated OH groups located on the external surface of nonporous silica (Aerosil, 180 m2 g-l) outgassed at the same temperature.
2
These chains can have variable length and shape (for instance, they could be in form of rings), depending upon the structure of the defect (the participation of geminal hydroxyl groups is not excluded). This explains why the involved bands are broad and the intensity of the components is slightly changing from one sample to the other and with dehydroxylation. Hydroxyl groups in terminal position are responsible for A and B peaks, while the other are hydrogen bonded (A'-B' peaks). The OH-OH distance in A' and B' hydrogen-bonded species (as derived from computer graphic modeling, Figure 3) is totally consistent with the observed frequency and half-width of the peaks? This disordered situation explains why N2 and CO adsorption isotherms without or with ill-defined steps have been found. Silanol condensation and water elimination induced by thermal treatments in vacuo lead to a decrease of the hydrogen-bonded chains length and to an increase of the isolated species (Scheme I). This explains the gradual frequency change of the OH-0 groups bands with dehydroxylation. As a final point, we have to explain why the C species (isolated OH groups at internal defects) are characterized by a frequency lower than that observed on nonporous silica treated at the same temperature and by a broader and more asymmetric character. Isolated silanols located in a channel or pore of very small radius are not really isolated because they are more or less sensing the cage effect of the pore and/or channel walls. This is physically similar to the 'solvent" effect and can cause a small downward shift of the stretching frequencies together with an increase of the bands half-width? This effect clearly depends upon the dimension of the micropores and state of dehydroxylation, and it is difficult to quantify. In agreement with this view, it is worth recalling that the IR spectra (in the OH region) of nonporous and microporous silica (as obtained from the literature) arc always slightly different; in particular, on microporous samples, the OH stretching bands are invariably observed at slightly lower frequencies and have broader c h a r a ~ t e r . ' ~ J ~
4000
3500
3000
2500
2000
1500
1000
YRVENUHBERS C M - I
Figure 5. IR spectra in the 4000-800-cm-' range of S outgassed at 300 K (1) and 773 K (2). In the inset an exploded view of the 1000-8OO-cm-' range is illustrated.
To summarize the results the position and half-width of all components involving internal and external silanols can be schematically represented as in Figure 4. This figure will be of great help for the interpretation of the gross spectral features observed on silicalite, and we shall refer to it in the following. Effect of Dehydroxyhtion (w Zeolites Skeletal Modes. Outgassing at increasing temperature is not only affecting the IR spectrum in the OH region (as expected because of the hydroxyl group condensation) but also modifies the spectrum of the zeolite itself (Figure 5; outgassing temperature 7 7 3 K). In fact upon dehydroxylation, a strong and complex peak centered at 890 cm-' shows up in the region between the asymmetric and symmetric stretching modes of the [SiOo] units (1200-1100 and 800
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4995
Silicalite Characterization. 2 1
a
b
h
C
W aJ 0 c m
W
u c m
-", 0
z
m
vr
n
f n
m
3500
31 0
3600 3A00 3200 WRVENdMBtR CM-1
2300
2200 2100 YRVENUMBER CM-1
a0
21 0 YRvENdMafR C M - I
Figure 6. IR spectra of CO adsorbed at 77 K (increasing doses) on S outgassed at 773 K. (a) OH stretching region: the evolution of the bands is indicated by the arrows. (b) CO stretching region. (c) As (b) (third most intense spectrum) after CO gas contribution subtraction.
cm-1).19-22This peak is very similar to that observed on silica outgassed at very high temperature and assigned to a stretching mode of a greatly distorted siloxane bridge formed upon dehydroxylation6v10(Scheme I). We consider this assignment as totally valid also for the present case. However, the high intensity of the peak (at least 1 order of magnitude more intense than on silica) and the lower formation temperature strongly suggest that the distorted bridges are mainly formed inside the particles, for instance, at defective sites. The higher concentration (with respect to nonporous silica) of distorted siloxane bridges which can be tolerated by the zeolitic framework is likely a consequence of the higher flexibility of its open framework. The observed frequency, intermediate between the asymmetric and symmetric modes of the [Si041 building units, closely corresponds to that of an hypothetic mechanically uncoupled S i 4 bond (with identical force constant K ) as shown below:
1 uncouplad oscillator
/\
9si r ? i
I
I
TZ
A1
4 ccupled oscillatm (tatrahadral unit)
3
This observation seems to suggest that the Si-0-Si bridges formed upon dehydroxylation are highly distorted as shown below:
4
In fact, in this structure one of the two bonds is elongated (and hence the associated IR mode is occurring at much lower v), while the other is approaching the situation of an uncoupled Si-0 oscillator (which could be well responsible for the band at 890 cm-I formed during the dehydroxylation). A fully open structure containing a tetrahedral unit in local C,, symmetry (with a terminal + S O - dangling bond) and a Lewis acid +Si+ site in adjacent position Oe
is also compatible with the 890-cm-' band. However, as will be shown in the next paragraph, no a-adduct with Lewis acids centers are formed upon CO dosage at low T; consequently, the fully open structure must be discarded. CO Adsorptioa at Low Temperatures.As is well-known, carbon monoxide is a good surface probe, because in the adsorbed state its stretching frequency is highly sensitive to local fields (at positively charged sites), to hydrogen bonding, and to u-bond formation on sites with strong Lewis acid In our case, CO can be used to probe the local fields present in the channels, microcavities, and external surfaces in order to (a) detect the presence of Lewis centers (if any) on S and SNa samples dehydroxylated at high temperature (through the possible formation of weakly bonded 1:l adducts); (b) ascertain the polarity (acidity) of silanols, either isolated and terminal (through the formation of hydrogen-bonded species); (c) distinguish between external and internal silanol groups; (d) probe the local field in the hydroxyl-free part of the internal surface. The IR spectra of CO adsorbed at low T on S samples outgassed at 773 K are shown in Figure 6a-c. Part a of the figure illustrates the perturbation induced by CO adsorption on the hydroxyl group while, in part b) the spectrum of adsorbed CO is reported. The following comments can be made: (a) Hydroxyl groups (both isolated and terminal) located in the microcavities are selectively perturbed by CO. The original bands are progressively eroded and two new bands are generated (an h b e s t i c point is clearly seen, definitely indicating 1:l adducts formation following Scheme 11. The hydroxyl groups in terminal position at 3704 cm-l are consumed first, giving a peak at 3582 cm-l (downward shift AE = 120 cm-'). The isolated hydroxyl groups at 3742 cm-' are consumed at higher CO pressure giving a peak at 3637 cm-' (Av = 100 cm-l). As the downward shift gives a qualitative measure of the acidity of the silanols, it is inferred that terminal species
4996 The Journal of Physical Chemistry, Vol. 96, No. 12, I992
are slightly more acidic than the isolated ones. (b) Under the adopted pressure and temperature conditions, the hydroxyl groups located on the external faces are not consumed a t all upon CO dosage (because their interaction with CO does not benefits of the "condensation" effect of the walls of the micromres). Due to this fact. the unmrturbed narrow Deak of undompiexed isolated and external silanols shows up giadually upon CO dosage, because it is no more obscured by the stronger peaks of the hydroxyl species in the cavities (which have moved to lower frequencies because of the formation of complexes). It is a matter of fact that the intensity and half-width of this peak is as expected on the basis of the electron microscopy and of the computer graphic simulations. Half-width and frequency are also very similar to those observed on Aerosil, where the isolated hydroxyls are all located on external faces. We consider this result as fully conclusive. (c) The band of the hydrogen-bonded OH groups is not substantially eroded when CO is dosed and the hydrogen-bonded adducts on terminal OH groups are formed. Only a small continuous downward shift to low frequency is observed. The explanation of this effect is as follows. The formation of hydrogen-bonded adducts at the terminal OH groups of the chains is associated with OH bond polarization. This induces a slight reinforcement of the negative polarity of the oxygen atom and of its proton-accepting character. As a consequence, a small reinforcement of the hydrogen bonding is induced in the chain with subsequent downward shift of the stretching frequency. (d) The frequency of hydrogen-bonded CO is observed at 2 156 cm-l (i.e., at a frequency definitely higher than that of gaseous CO). This upward shift is typically associated with the formation of 1:l adducts with isolated and terminal groups (the two species are not differentiated as far as the CO stretching is concerned). (e) The very strong peak at 2138 cm-' is due to CO physically adsorbed on channels walls, because it reaches the saturation under higher pressure conditions and its frequency is nearly identical to that of liquid C0.23324 An IR spectrum of the physically adsorbed species (after subtraction of the CO gas and elimination of the contribution of the OH-.CO (1:l) complex (broken curve) is reported in Figure 6c. The main peak a t 2138 cm-' (with a well-defined component at 21 36 cm-l) closely corresponds to the spectrum of liquid and/or solid C0.23-27 However, the strong tails on the high- and low-frequency sides of the main peak are indicative of the presence of a residual rotational envelope, suggesting that at least a fraction of molecules behaves as partially hindered rotators.23 This means that, under the condition of the spectroscopic experiment, a fraction of the CO molecules adsorbed in the channels have high mobility. This conclusion seems to contradict the picture coming from the volumetric isotherm which were interpreted in terms of extensive C O C O interactions similar to those present in the liquid or solid phase. However, this contradiction is only apparent; in fact the two experiments are done at different temperatures ( T = 77 K for the volumetric experiment and T 1 100 K for the IR experiment) and it is quite possible that at 100 K a non negligible fraction of adsorbed CO molecules can keep some rotational freedom. ( f ) The IR spectrum of CO adsorbed a t low temperature on SNa (not illustrated for sake of brevity) shows similar features. Two main differences are, however, observed: (i) the peak of 1:l OH-CO adducts has negligible intensity; (ii) a weak but welldefined band associated with Na+-CO adduct is observed at 2174 cm-I. This species is similar to that observed on Na-ZSMS and will be thoroughly discussed elsewhere. We only remark that Na+ impurities present on SNa can be easily detected by IR spectr oscopy . (8) Finally, no trace of C O u-adducts formation with Lewis centers is found on the silicalite S outgassed at high temperature (Le., when the formation of the 890-cm-I band is observed). This means that this band is associated with a distorted bridge without Lewis unsaturation a t one of the silicon atoms. This result definitely favors the structure depicted in 4. In addition, CO does not perturb the band at 890 cm-l.
Zecchina et al. In part 3 of this work will be shown how this band can be Perturbed by stronger bases than c o , like €320, NH3, and CHSOH,showing that the reactivity of these distorted bridges depends upon the strength of the interacting base.
Conclusions Internal defects (nanocavities as already suggested in part 1 of this paper) must be invoked to explain the IR manifestation of the OH groups and of the skeletal modes in S samples. These defects are less abundant on Na- and Al-containing samples prepared in the conventional way. The defects can be represented by computer graphic design as one or more missing [SO4] units in the zeolitic framework. In order to preserve the stoichiometry, the microcavities so obtained are saturated with OH groups; these groups are close enough to generate hydroxyls chains interacting through hydrogen bonding. The IR features of these OH groups are different from those of the OH groups located on external faces. Water elimination at high temperature from these hydroxyls leads to formation of distorted Si-0-Si bridges characterized by well observable local stretching modes. Adsorption of CO a t low temperature occurs primarily in the channels and in the microcavities forming hydrogen bonded OH-CO adducts and (at higher pressure) a liquidlike state. Under the adopted temperature and pressure conditions, OH groups located on the external faces are not perturbed at all. The hydrogen-bond formation is well documented by the red and blue shift of the OH and CO stretching modes, respectively. CO adsorption at low temperature does not reveal any kind of acid-base interaction between CO and the siliceous structure; consequently, it is inferred that the distorted siloxane bridges do not have any Lewis acid character. Acknowledgment. This research has been supported by the Consiglio Nazionale delle Ricerche, Progetto Chimica Fine 11. We thank CSI Piemonte for allowing us the use of the CHEM-x program. R-hy
NO. CO, 630-08-0.
References and Notes (1) Leofanti, G.; Genoni, F.,; Padovan, M.; Petrini, G.; Trezza, G.;Zecchina, A. In Characterization of Porous Solids Ik Rodriquez-Reinoso, F., Roquerol, J., Sing, K. S . W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1991; p 62. (2) Grose, R. W.; Flanigen, E. M.; U S . Pat. 4061724, 1977. (3) Escalona Platero, E.; Scarano, D.; Spoto, G.; Zecchina, A. Faraday Discuss. Chem. SOC.1985, 80, 183. (4) Hush, N. S.; Williams, L. J . Mol. Spectrosc. 1974, 50, 349. (5) Zaki, M. I.; Knozinger, H. J . Catal. 1989, 119, 31 1. (6) b e n t e l , G. C.; McClellan, A. L. The hydrogen bond, W. H. Freeman: London, 1960. (7) Borello, E.; Zecchina, A.; Morterra, C. J. Phys. Chem. 1967, 71,2938. (8) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; John Wiley and Sons: New York, 1975. (9) Morrow, B. A.; Cody, I. A.; Lee, L. S. M. J . Phys. Chem. 1976, 80, 761. (10) (1 1) (12) (13) (14)
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A Study on Hydrogen Encapsulation in Cs,.,-Zeolite
A
Jong-Ho Yoon* Department of Industrial Chemistry, Kyungpook Sanup University, 55, Hyomokdong, Donggu, Taegu, 701- 702 Korea
and Nam Ho Heo Department of Industrial Chemistry, Kyungpook National University, 1370, Sankyukdong, Pukgu, Taegu, 702-701 Korea (Received: October I, 1991; In Final Form: December 30, 1991)
The numbers of hydrogen molecules encapsulated in the cavities of Csz,5-zeoliteA were calculated with respect to pressure as well as temperature, and the results were compared with known experimental data. The calculations were performed by the combined use of the statistical theory of the radial distribution function and the theory of the perfect 3-D lattice gas. The pressures and the temperatures used for the calculations ranged from 10 to 129 atm and from 373 to 623 K,respectively. The results indicated that these two theories were enough to reproduce the pressuredependent experimental data which increase with increasing pressure at the pressures studied.
Introduction The storage (encapsulation) and release (decapsulation) of hydrogen molecules by the use of molecular sieves, such as dehydrated M,-zeolite A (M,Na12-mSi12A112048;M can be any monopositive cation) has been the subject of interest to chemists as well as physicists during the past two Since the use of hydrogen as a multipurpose fuel has been thought to be useful, the importance of the en(de)capsulation of hydrogen has been emphasized. In general, the encapsulation of hydrogen in zeolite A is possible by forcing hydrogen molecules into the cavities developed in zeolite A through the apertures of the cavities under a high-temperature (400-600 K) and high-pressure (more than 10 atm) condition followed by the quenching of the zeolite apertures to the ambient condition? The principles involved in this process are the thermal activation of the vibrational motion of the apertures and the molecular diffusion which allows molecules to be distributed in the cavities. That is, around rmm temperatures, the vibrational motion is not activated (closed) enough to allow the entrance (diffusion) of hydrogen molecules into the cavities. However, if the temperature is sufficiently raised, the aperture’s vibrational motion will be fully activated to allow the entrance of hydrogen molecules into the cavities. Since this process is reversible, the decapsulation of hydrogen molecules from the cavities becomes possible upon heating the quenched zeolite A. From the work of Fraenkel and Shabtai* in 1977, it has been known empirically that the number of hydrogen molecules encapsulated in C ~ ~ , ~ - z e o lAi t increases e almost linearly with increasing the encapsulation pressure in the log-log scale. However, detailed consideration to account for this linear relationship has not yet emerged. Most of the studies done on such a system are limited to the investigation of the state of the encapsulated hydrogen molecules in a zeolite A cavity at fairly low temperatures (10-20 K).4*S From these studies, it has been shown that the encapsulated hydrogen molecules are adsorbed on the molecules constituting the zeolite A cavity wall.e6 However, since hydrogen encapsulations in zeolite A are possible only at a high-temperature and high-pressure condition, the low-temperature results may not be directly applicable in studying high-temperature hydrogen encapsulations.
Bearing this in mind, we applied the statistical theory of the radial distribution function (rdf)’ and the theory of perfect 3-D lattice gasEto calculate the number of the encapsulated hydrogen molecules in Csz,S-zeolite A for the purpose of more detailed understanding on this phenomenon. That is, by comparing the theoretically calculated hydrogen encapsulations with known experimental data: we examined the effect of pressure as well as temperature on the hydrogen encapsulation in Csz,S-zeolite A.
Theory The two most important thermodynamic factors making hydrogen encapsulation in Cs,,,-zeolite A cavities possible are temperature and pressure. In principle, by raising or lowering temperature one may open or close the apertures of zeolite A cavities to allow or to prohibit the entrance of hydrogen molecules, respectively. On the other hand, pressure plays a role by forcing hydrogen molecules into the cavities. Hence, the number of hydrogen molecules encapsulated in Csz,S-zeolite A cavities (hydrogen encapsulation) should be a function of both pressure and temperature. According to the theory of statistical mechanics, the maximum number of molecules contimed in a finite volume can be calculated, by making use of rdf.’ That is d
N =p
g(r)47rP dr
+1
where d is the radius of the zeolite cavity, r the intermolecular separation, N the maximum number of molecules encapsulated in a cavity, p the number density, and g(r) the radial distribution fimction (rdf). In eq 1, the fmt term counts the maximum number of neighboring molecules distributed around a probe hydrogen molecule in a cavity. The second term, 1, comes from the contribution of the probe molecule itself. For gaseous hydrogen, rdf may be written asI3
Here, U(r) is the intermolecular potential, k the Boltzmann constant, and T the temperature in absolute scale. As can be seen in eq 2, rdf is independent of pressure. For the potential function
0022-3654/92/2096-4997%03.00/0 0 1992 American Chemical Society
