1219
Langmuir 1988, 4 , 1219-1221
surrounding the two types of phosphorus species. This is then observed as what might be a fortuitous binary mechanism for ensemble formation, wherein the Lewis base, by its decomposition, eliminates the Ni sites that would otherwise compete with the ensembles for adsorption of C0.20 In searching for proof of a Lewis base induced surface ensemble, one must first determine precisely how this type of adsorption would manifest itself in an unambiguous manner. Model systems that presume the formation of such ensembles indicate that changes in the relative surface ratio of two species coadsorbed on a surface are an ineffective probe of the process4 and might be construed as ambiguously differentiating localized electronic interactions from steric (local) and global electronic effects. A change in the surface-adsorbate bond strength is in and of itself ambiguous with respect to similar electronic considerations. In particular, if the change is observed to be a gradual change in bond strength, then one might rightly conjecture that the new surface-adsorbate interaction is based upon global electronic effects. What would ideally be observed is the simultaneous existence of both the new adsorbate state and the unperturbed adsorbate state. These can be seen in Figure 2, with resonances at 2023 and 2057 cm-', respectively. A further observation that can be targeted is the relative rate of reformation of the two states in the presence of a surface with a preadsorbed base. In the case of a positive ensemble, wherein the effect of the base is to strengthen the adsorbate-surface bonding, it might be possible to observe the adsorption of the new state prior to the original state (Figure 3). For a negative ensemble, special conditions might be necessary to force adsorption in the new sites.14 Since the purpose of this research is to lead toward preconfiguration of catalytic utility, the positive ensemble was targeted. The next step in this process should be the determination of a donor species sufficiently robust so as to be stable under conditions of catalytic utility. Registry No. CO, 630-08-0;P(OCH3)3, 121-45-9; Ni, 7440-02-0. (20) This donor/acceptor interaction has interesting implications for the formation of (two-dimensional)surface agglomerations (islands). If, for example, an electron-donating adsorbate (H2S,P!OCH,)j, etc.) is a precursor to an electron-withdrawing and relatively immobile product species (S, P), then the precursor/prcduct interaction should tend to form islands of product species. The size of these agglomerations should be related to the stability and surface mobility of the adsorbed precursor. Conversely, a stable adsorbate should result in a chaotic and dispersed surface state.
Determining Template Removal from a Crystalline P e n t a d Zeolite by Xenon NMR Spectroscopy Chihji Tsiao,' Cecil D bowski,**' Darrell Walker,' Vincent Durante, and David R. Corbins
7
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, Sun Refining and Marketing Co., Applied Research and Development, Marcus Hook, Pennsylvania 19061, and Central Research and Development Department, E . I . du Pont de Nemours and Co., Wilmington, Delaware 19898 Received May 17, 1988. In Final Form: July 6, 1988
In the production of highly crystalline silicoaluminates such as the pentasil zeolites, one often uses a template
r\
/
145
140
135
I
130
125
120
119
CHEMICAL SHIFT (PPM)
Figure 1. lzeXe NMR spectra of xenon adsorbed in silicalite samples treated in a flow of air at 480 "C. The numbers at the left of each spectrum are the treatment times. T h e shift at 12 h is not meaningful.
molecule to promote the growth of structure. After the synthesis is complete, this molecule must be removed by high-temperature combustion to produce the pure silicoaluminate. The removal of a template is a protracted process, and, typically, the loss of template is monitored by DTA or some spectroscopic method.' In this paper we describe a method for following the kinetics of this process based on the NMR spectroscopy of xenon gas sorbed in the material. The NMR spectral parameters of xenon-129 are quite sensitive to the details of collisions in which it is involved.24 When it is sorbed in microporous materials such as zeolites, the dominant collisional process involves collisions with the structure. Because a collision with the template molecule makes the spectroscopy of xenon in cavities containing them different from the spectroscopy of xenon in cavities without the template, one may infer the percentage of xenon in these environments directly
* Author to whom correspondence should be addressed. University of Delaware. *Sun Refining and Marketing Co. 8 E. I. du Pont de Nemours and Co. Contribution no. 4762. (1)Song, T.; Xu, R.; Li, L.; Ye, Z. R o c . Int. Cor$ Zeolite, 7th (Tokyo) 1986, 201-206. (2) Ito, T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (3) Ripmeester, A.; Davison, D. J . Mol. Struct. 1981, 75, 67. (4) Ito, T.; deMenoval, L.; Guerrier, E.; Fraissard J . Chem. Phys. Lett. 1984, I l l , 271.
0743-7463/88/2404-1219$01.50/0 0 1988 American Chemical Society
Notes
1220 Langmuir, Vol. 4, No. 5, 1988 from xenon NMR spectra. Thus, it is in principle possible to monitor the change in relative amounts of cavities with and without templates in a simple stopped-flow system. In Figure 1 are shown the spectra of xenon taken at a fixed pressure of xenon sorbed in a sample of the aluminum-free pentasil, silicalite, that was made in the laboratory a t Sun Refining and Marketing Company and had been previously treated in oxygen. All analyses of the material, including X-ray crystal structure and silicon NMR spectroscopy, indicate that it is purely siliceous and has a pentasil structure. The times indicate the amount of exposure of the sample to a flow of air at 480 "C. One can see that the spectra are sums of two components. Using a Simplex algorithm, we decomposed each spectrum into the sum of a Gaussian and a Lorentzian subspectrum, corresponding to regions with template present and regions in which there is no template. The combination of two Gaussian or two Lorentzian subspectra gave significantly worse fits to the data. The solid lines are the best fits to this model, and the dashed lines show the subspectra for the two phases. From the areas of the calculated subspectra, one can estimate the fraction of xenon atoms contributing to the Lorentzian resonance: where A iis the area of the Lorentzian or the Gaussian subspectrum, L or G, respectively. Assuming that we detect all the xenon atoms in these two areas, this fraction is also the fraction of xenon atoms in the particular Lorentzian environment for this pressure of xenon: The sorption isotherms for xenon in the two environments may be different. In that case, at a given pressure the site occupancies, dXe,Land dXe,G, for use regions may not be the same, where we use of the nomenclature dxe,i = Ni/ Wi, Wi being the dry weight of the material for phase i. The parameter of interest in this case is the fraction of the weight of the sample that gives the Lorentzian environment, FL:
FL = WL(WL + WJ1
(3) We can deduce this parameter from the xenon-NMR-derived quantity, f L , by the formula
FL = X f L / ( l
- (1 - X ) f L )
(4)
where x = dxe,G/dXe,L for the condition of the xenon NMR measurement. For example, if these densities happen to be identical, then FL = f L . However, this may generally not be the case, and one may need to determine it from a complete pressure dependence of the composite line. The details of the experiments are as follows: A sample containing template in some areas is subjected to a 120 mL/min flow of air at 480 "C for a time, after which it is placed in a resealable NMR test tube having a coaxial top cock.^ It is outgassed to Torr at ambient temperature and slowly heated under vacuum to 400 "C over 6 h, after which it is maintained at this temperature for 12 h to drive off any adsorbates that may have been picked up in the transfer. We have shown by previous experiments that this heating does not change the composite line. After it is brought to room temperature, xenon gas is sorbed to a specific pressure, and the NMR spectrum is recorded. The sample is treated in air for a subsequent period and reexamined in a similar manner. From the spectra of Figure 1, we obtain values of fL. From pressure (5) Mesaros, D.; Dybowski, C. Appl. Spectrosc. 1987, 41, 610.
Table I. Fraction of Silicalite Sample That Gives Rise to a Phase with a Lorentzian for Sorbed Xenon
i
88
treatment time, h
FL
0 4 8
0.716 f 0.011 0.823 f 0.017 0.891 f 0.010
79
70
62
53
44
CHEMICAL SHIFT
35
26
18
9
0
(PPMI
Figure 2. Carbon-13 NMR spectrum of the original material obtained with cross-polarizationand magic-angle spinning. t,, = 8 ms. profiles and the previously known adsorption isotherms of the untreated sample and a template-free sample, we calculate FL for each sample, as given in Table I. Note that treatment in air at 480 OC for 12 h produces a situation in which this procedure cannot determine the presence of template. The original sample was examined with cross-polarization magic-angle-spinning carbon-13 NMR spectroscopy by using a Chemagnetics mlOOs spectrometer. The spectrum, shown in Figure 2, indicates that this sample contains organic material, which can be identified as tetrapropylammonium A similar spectrum of the material after treatment in air at 480 O C for 12 h shows no trace of carbon-containing material. Thus we infer that the two-phase behavior exhibited in the xenon spectrum is an indicator of xenon atoms in regions with and without templates. To understand the xenon data one assumes a mechanism for the elimination of template (whose presence gives rise to the Gaussian component). We assume that the combustion is first order in the template, all other parameters held constant. Then the rate of appearance of template-free material is given by where k is the first-order rate constant for the process. k , of course, will depend on temperature, oxygen concentration, and other parameters held fixed in this experiment. The integrated form of this equation is FL(t) = 1 - (1 - FL(0))e-kt
(6)
The data of Table I, when plotted according to this equation, are shown in Figure 3. From the analysis of the data, we conclude that the value of k a t 480 OC in air is 0.120 f 0.01 h-l. (6) Boxhoorn, G.; van Santen, R. A.; van Erp, W. A.; Hays, G. R.; Huis, R. J . Chem. Soc., Chem. Commun. 1982, 264. (7) Boxhoorn, G.; van Santen, R. A.; van Erp, W. A.; Hays, G . R.; Alma, N. C. M.; Huis, R.; Clague, A. D. H. Proc. Int. Conf. Zeolite, 6th 1983, 694-703.
Langmuir 1988,4, 1221-1222
--
u
O'O
to determine the rate constant for loss of template by a silicalite sample treated in flowing air at 480 OC. It should be possible to study many other similar problems of loss of material from silicoaluminate pores by this means.'O The limitation of this technique seems to arise from the ability of xenon to move among pores with different environments on the NMR time scale. When about 3% of the pores (or less) contains template, the method fails to indicate the presence of template, as in the spectrum of the sample treated for 12 h (based on the kinetic constant), because of rapid averaging. An investigation of the spectrum at lower temperature may enable one to make the measurement more precisely because of the decreased mobility of the xenon atom and the longer residence time in a single region. This technique could form the basis of a simple, quick test for the removal of template in such systems.
1
-1.0
-3.0
1221
1
0.0
4.0
TIME
8.0
12.0
(HOURS)
Figure 3. Plot of In [l- FL,(t)]as a function of time for the sample of Figure 1.
The sensitivity of xenon NMR spectroscopic parameters to the local environment provides a convenient means to differentiate between two very similar environment^.^^^ Since one can easily measure the areas corresponding to the resonances of xenon in the various regions, one can quickly quantify the amount of each region present in the particular sample by using a routine liquid-state NMR spectrometer. Quantification in this manner permits one (8) Fraissard, J.; Ito, T.; Springuel-Huet, M.; Demarquay, J. Stud. Surf. Sci Catal. 1986,28, 393. (9) Springuel-Huet, M.; Ito, T.; Fraissard, J. Structure and Reactiuity of Modified Zeolites;Elsevier: Amsterdam, 1984; pp 13-21.
Acknowledgment. This work was supported in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society, the sponsors of the Center for Catalytic Science and Technology, and the Sun Refining and Marketing Co. (with a grant to C. R.D.). Registry No. lt9Xe, 13965-99-6; silica, 7631-86-9; tetrapropylammonium, 13010-31-6. (10) Ryoo, R.;Liu, S.-B.; de Menorval, L. C.; Takegoshi, K.; Chmelka, B.;Trecoske, M.; Pines, A. J . Phys. Chem. 1987, 91, 6675.
Comments On the Comparison of Evaporation Rates The comparisons of evaporation rates through monolayer-covered and monolayer-free water surfaces reported by Archer and La Mer' are invalid because the value they used for the apparent evaporation coefficient was not relevant to their experimental conditions. In a recent article2reference was made to the calculation of Archer and La Mer' that purports to show that spread monolayers of certain alkanoic acids reduce the rate of water evaporation by a factor of approximately lo4 relative to a monolayer-free water surface. This calculation is incorrect, and the actual reduction achieved is not nearly so spectacular. It must be emphasized that Archer and La Mer were not comparing the evaporation rates through monolayer-covered surfaces with their experimental rates through monolayer-free surfaces but with theoretical rates calculated from the Hertz-Knudsen equation. Their reasons for choosing this procedure are sound and arise from the practical details of evaporation rate measurements. The observed evaporation rate depends on the transport resistance of the entire evaporation pathway, with the interface contributing only a portion of the total resistance. Thus relative rates depend on the experimental arrangement. The argument is given in detail in a review article: (1) Archer, R. J.; La Mer, V. K. J . Phys. Chem. 1966,59, 200. (2) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034.
but the analogy with electric current is informative: comparing evaporation rates is like comparing the electric currents through two resistors when the measuring circuit contains a large resistance in series with the resistance of interest-the ratio of the two currents depends on the magnitude of the large resistance. The ratio of the two resistances is more useful, as it is independent of the large resistance, and this is what Archer and La Mer attempted to calculate for evaporation. The determination of the monolayer resistance follows what is now a standard procedure1and involves measuring the total resistances of the transport pathways with and without monolayer and taking the difference. However, there is no simple method for measuring the resistance of the monolayer-free surface, so Archer and La Mer attempted to calculate it. The origins of their calculation and the logical fallacy employed are discussed in detail el~ewhere.~ I will here outline the argument. The evaporation rates of liquids are governed by the Hertz-Knudsen equation:
J = ~ ( R T / ( ~ X M ) ) ' /-~C)( C ~ ~
(1)
where J is the net evaporative flux, a is the evaporation coefficient, R is the gas constant, T is temperature, M is the molar mass of water, and c q and c are respectively the equilibrium and actual water vapor concentrations at the (3) Barnes, G. T. Adu. Colloid Interface Sci. 1986, 25, 89.
0743-7463/88/2404-1221$01.50/00 1988 American Chemical Society
