J. Phys. Chem. 1994, 98, 6201-6211
6201
Lewis Acid Sites and Surface Aluminum in Aluminas and Zeolites: A High-Resolution NMR Study D. Coster, A. L. Biumenfeld, and J. J. Fripiat' Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201 Received: March 3, 1994; In Final Form: April 19, 1994'
The distribution of surface aluminum atoms among different coordination states is a key to the understanding of the acidity of aluminas and zeolites. Single-pulse 27Al MAS N M R spectra do not distinguish the surface from the bulk species. Proton-aluminum cross-polarization (CP-MAS) spectra, when hydroxyls are the source of protons, give a biased representation of the surface topology. Indeed, water chemisorption involves an unknown number of reconstructed layers because chemisorption is dissociative. On the other hand, when C P is operated from chemisorbed ammonia, the distribution of the surface A1 is observed, and it represents the state of the active catalytic surface. It was the main purpose of this work to study the nature of Lewis acidity in various catalysts (aluminas and zeolites) pretreated under controlled atmosphere at different temperatures. Isotopically enriched ammonia ISNHschemisorption was employed to probe the surface active sites and to allow for magnetization transfer to surface and close-to-surface A1 nuclei. A special deconvolution procedure was applied to obtain 27Al spectral parameters for different A1 sites in partially resolved spectra. The procedure accounted for the normal distribution of electric field gradient tensor components at a particular site. It is shown that there are two kinds of Lewis sites on aluminas: namely, a tetrahedral site with isotropic shift of about 58 ppm and quadrupolar coupling constant QCC 6 M H z and a pentagonal site with a isotropic shift of about 40 ppm and a slightly smaller QCC. In zeolites the same kinds of sites exist in the nonframework aluminum debris, and in addition, the Bronsted sites are associated with framework aluminum.
Introduction The acidity of aluminas and aluminosilicate catalysts depends on the coordination of aluminum and on the chemical nature of its neighbors. For instance, in zeolites it is generally accepted that the Brdnsted acid siteis the OH bridging a frameworksilicon to a framework aluminum. The dehydroxylation of alumina hydrates into transition aluminascreates coordinately unsaturated sites (CUS) which are at the origin of the Lewis acidity of these catalysts. Dehydroxylation,steaming, or dealumination of acid zeolites dislodges aluminum from the lattice into nonframework aluminum, present as tiny alumina debris.l.2 The structural similarity between these debris and transition aluminas rich in pentacoordinated aluminum has already been outlined.3 Of course, these debris contain Lewis sites as well. Thus, the zeolites which, as the ultrastable Y (USY), contain both Brdnsted and Lewis acid sites may combine their actions (synergy). Finally, the formation of CUS sites in zeolites, as well as in aluminas, must be accompanied by a redistribution of the charge on the aluminum and oxygen atoms. Some oxygen may be more electron donor than electron acceptor and work as a Lewis basic site.3 As a consequence of these general concepts, it is clear that the knowledge of the coordination state of aluminum on the surface of these catalysts is potentially very important, and 27Alhighresolution (magic angle spinning) nuclear magnetic resonance (MAS NMR) spectroscopy is the method of ~ h o i c e .Aluminum ~ being a quadrupolar nucleus, the coupling between its quadrupole moment (4)and the electric field gradient (EFG), that is, the quadrupole coupling constant (QCC), is strongly influenced by any modification of the symmetry of the oxygen ligands around the nucleus. The isotropic shift (6) on the other hand gives information on the electronic shielding of the nucleus. 27Aland 29Si MAS NMR have become routine characterization techniques in zeolites and other high surface area silicoaluminas. 29Si, in
* To whom correspondence should be addressed.
Abstract published in Aduance ACS Absfracrs, June 1, 1994.
particular, is extensively applied because the '12 spin resonance lines are easier to quantize than lines of quadrupolar nuclei. In solidswith large surfaceareas the ratio of "surface" (or subsurface) A1 to the total number of nuclei is large. In a Y zeolite, for instance, all the constituting nuclei can be considered as forming the surface. In a transition alumina with a surface area in the order of 200 m2/g, about 20% of the A1 nuclei are on the surface. The study of surface aluminum constitutes the goal of this contribution. The prerequisite question is about the possibility of observing surface Al. In a hydrated zeolite which contains tetrahedral framework A1 (A$') only, most of the A1 nuclei are observable in the signal corresponding to the Fourier transform of the free inductiondecay, the so-called one-pulse ( 1P) spectrum. However, dehydrating the zeolite by a mild thermal treatment broadens this AlF line so much that it may, at least partially, escape detection.5 Dehydration alters the symmetry of the sites and increases the QCC considerably. In fact, at the early stages of the NMR spectroscopy of quadrupolar nuclei, the possibility of studying surface Al, in alumina, was questioned because the absolute intensity of the 27Al 1P spectra decreases with increasing surface area.6 At the time this was interpreted by a too large QCC of 27Alin the surface layers. This interpretation was questioned more recently by Morris and Ellis,7who demonstrated the possibility of getting an 27Alsignal by cross-polarizing (CP) the surface or subsurface 27Althrough the proton of chemisorbed water (or surface OH group). In fact, Huggins and Ellis* showed by variabletemperature experiments that the intensity of the 1P 27Al signal is a function of frequency of the QCC fluctuations generated by proton hopping on surface oxygens. If this process can be treated as if the A1 nuclei were involved in an exchange process, there is a dynamic regime where an important loss of intensity is predictable. There are two possible solutionsfor overcoming this problem: namely, by recording 1P spectra at low temperature or by dehydrating the surface to an extent where the proton motion is inhibited and by using CP. Lowering the temperature in MAS experiments where a fast spinning is necessary to avoid super-
0022-3654/94/2098-6201%04.50/0 @ 1994 American Chemical Society
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6202 The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
TABLE 1: Some Physical Characteristics of the Zeolites’ A (m2/g) (Si/Al)cA (Si/AIr) AIt (XIOzl/g) AI? (X1O2I/g) 1.70 4.80 2.80 2.55 USY 750 VGz VG3 VGs
464 500 452
5.2 5.2 6.4
9.8 11 28
1.6 1.6 1.4
0.93 0.84 0.35
a A is the surface area obtained from the Langmuir isotherm of Nz physical adsorption. (Si/Al)cA and (Si/Alkv) are the ratios obtained from chemical analysis and 29SiMAS NMR, respectively. AIt and AI;’ are the absolute AI total contents and framework AI contents per gram of dehydrated zeolite (300 “C).
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imposition of the center -1/2 l / 2 transitions with spinning sidebands (SSB) is not easy. On the other hand, CP experiments are time-consuming and the surface must be protected against rehydration. The only possibility is to use sealed vials in which the catalyst is transferred and kept under vacuum after the desired pretreatment. This is the technology that has been successfully developed for this work. As anticipated, and as demonstrated in the following, water is dissociatively chemisorbed, and thus, water chemisorption provokes an extensive surface reconstruction. Thus, CP from surface OH gives a wrong representation of the catalyst surface. Since the main goal of this work is to study acid sites, the problem can be solved in adsorbing a protonated base. This idea was exploited by Ellis et a1.,7 who used pyridine (Py) containing eventually lsN. As well-documented from infrared spectroscopy (IR) investigations: Py is adsorbed on Lewis sites (L) or on Bronsted sites forming either the Py:L or the PyH+ adduct. It was shown that surface 27Al can be cross-polarizedfrom Py:L on alumina, which was treated with a small amount (overall surface coverage) of liquid Py. Although these experimentswerecarried out carefully to avoid surface rehydration, it is not possible to exclude this possibility completely. The use of dry ammonia (lSNH3 or 14NH3),made possible by the sealed vial technique, gives morecertainty about theexclusion of water from thesystem. In addition, if chemisorbed NH3 is used in CP experiments, the bond with the surface being through nitrogen, the average distance between Z7Aland 1H in an Al:NH3 configuration is shorter than in an A1:Py configuration. Short distance increases the CP efficiency and this is a welcome benefit, because even with large surface areas the number of sites on which NH3 can be chemisorbed is small and a very large number of accumulations will be necessary to obtain CP spectra with acceptable signalto-noise ratio (S/N). In summary, the goal of this workis to explorethecoordination state of surface aluminum in selected catalysts through magnetization transfer from the protons of chemisorbed ammonia. Simultaneous to the 27Al CP experiments, lSN CP spectra will be deciphered. Finally, a few words should be said about the materials chosen for this study. The zeolites are the well-known ultrastable Y and three mordenites with different degrees of dealumination. They all contain nonframeworkaluminum (NFAl). It has been shown previously that the nonframework alumina has deep structural similarities with aluminas rich in pentacoordinated A1 (Alv) and that it contains strong Lewis acid ~ites.~JO Besides the zeolites, two aluminas rich in AIVand a “classical” y-alumina containing 4-fold (A1IV)and 6-fold (AlVI)alumina will constitute references for the interpretation of the data collected for the zeolites. The activity of these catalysts for o-xylene and n-pentane isomerizations will be discussed elsewhere. Also, in another paper, the FTIR study of CO chemisorptionby the very same solids will be reported and connected with the NMR data.
Experimental Section Materials. Some physical characteristics of the studiedzeolites are shown in Table 1. The USY zeolite was used as received. It
was calcined at 500 OC under vacuum before being exposed to NH3 (or 15NH3). The Si/Albv ratio obtained from the *9Si spectrum and the chemical Si/Al ratio are shown in Table 1. The VG2 mordenite was obtained by exchanging four times Na+ by 1 M NH4NO3, washed and dried at 120 OC, heated to 250 OC at 5 OC/min, and kept for 2 h at 250 OC for removing hydration water. The temperature was then increased to 500 OC at 5 OC/min and kept for 2 h. The sample was recalcined under vacuum at 480 OC before exposure to NH3. VG3 is obtained by steaming VG2 for 1 h at 500 OC. VGSis obtained by steaming VG2 for 1 h at 600 OC and by treating the solid with 1 M HCl for 1 h at room temperature. VG3 and VGS were outgassed at 480 OC before exposure to NH3, as was VG2. The y-alumina was a commercial sample with a surface area of 268 m2/g. The alumina rich in AIVwas obtained from the limited hydrolysis of aluminum tri-sec-butoxide,as described by Coster et al.11 Two samples with different contents in A P (two different hydrolysis ratios), namely 1 and 2,have been prepared. Their specific surface areas were 360 f 2 m2/g. The aluminas were treated at 600 ‘C under vacuum before being brought into contact with NH3 at lower temperature. In all cases, the calcination under vacuum was preceded by calcination in the presence of 30 Torr of 0 2 at 400 OC in order to clean the surface from carbonaceous contaminants. The anhydrous ammonia used for this study was obtained with a purity of 99.9% from Bentley. It was carefully dried in three steps, namely, (a) condensation on dry KOH, (b) transfer through a NaA molecular sieve, and (c) distillation under vacuum. 15NH3 ammonia (98% atomic enriched) was obtained from Aldrich. I S N Hand ~ 14NH3ammonias were probably of the same dryness because no spectral feature which would raise suspicion about the presence of water was observed in the A1 CP spectra obtained from either one. 27Al1P and CPspectra of thevery same samples but rehydrated by contact during 24 h with the atmosphere will be reported in order to illustrate the difference between reconstructed (H20) and (NH3) nonreconstructed surface features. Ammonia chemisorption was carried out at either 115 or 190 OC for 1 h, followed by outgassing for 10 min at the same temperature. NMR Technique. All 27Alexperiments were performed in a static field of 11.7 T under MAS conditions, the spinning rate being 12 f 0.05 kHz. For the 1P spectra, the length of the pulse was 0.5 ps (selective excitation of the center line), the delay between pulses was 50 ms, and the number of acquisitions was on the order of a few thousand. For the 27AlCP MAS spectra, the proton pulse width was 6 ps while the delay between pulses was 1 s. The radio-frequency field for the proton (500 MHz) was Hy = 50 kHz and kept constant for all CP experiments.The field for aluminum H’: was adjusted in each case for the maximum signal. It corresponded to the general relationship shown by VegaQ13 3w,, = oH
+ n21rv~
where is the MAS frequency. The highest intensity was consistently found for n = -2. The positive values of n were not explored because the power applied to the probe would have been at the highest acceptable limit. The contact time for the CP experiments from chemisorbed NH3 or from OH (for the rehydrated samples) was between 0.6 and 1 ms. Decoupling during acquisition had no effect. While about 10 000 accumulations were generally enough to achieve an acceptable S / N ratio in the 27AlCP experiments from OH, it was necessary to accumulate between 60 000 and 90 000 spectra for the CP experiments from NH,. It was carefully checked that a sample dehydrated, as described in the previous section, and kept in the sealed vial in a back-pressure of about 10-5 Torr did not produce any detectable z7Alsignal upon CP and
Surface Aluminum in Aluminas and Zeolites between 60 000 and 90 000 accumulations when not exposed to NH3. Some examples will be shown. The procedure to fill and seal the vial is described elsewhere. Behind this short sentence lays the success of this work and about 6 months of technical trials and fai1~res.I~ Now about 80% of the preparations are standing the g's experienced by the -3mm- 0.d. vial fitting in the 5-mm high-speed Doty probe spinning at 12 kHz. The NMR parameters used to record the 15N CP spectra were as follows: proton pulse width 15 ps and the contact time 1 ms. The delay between pulses was 1.5 s. The reference used for measuring the lSN chemical shift was 15NH4Cl(solid). With this reference the line of liquid ammonia is at - 4 4 ppm,l5 and that of 15NH4+in solution is at -14 ppm. As usual, a 0.01 M Al(N03)s solution was used as reference for 27Al. Deconvolution Procedure. When simulating experimental NMR spectra (either l5N or Z7Al),one should consider various sources of line broadening that contribute to an overall line shape. For 27Al these may be (i) second-order quadrupolar broadening that is not averaged out under MAS conditions, (ii) residual hetero- and homonuclear dipolar interactions, and (iii) disorder effects coming from random distributions of sites with different geometries. The first reason mentioned does not apply to 15N spectra (spin I/2) and should be replaced by the dipolar coupling of spin 1/2 nuclei with quadrupolar nuclei.I6 If the magnitude of any of the above-mentioned interactions is not much less than the line width of the experimentalspectrum, it should be accounted for in the simulation procedure. The simulation of the 27Al spectra was performed with a modified standard software for calculation of quadrupolar line shapes in powder under MAS conditions.'' The program allows one to introduce an individual line broadening over each line of a spectrum. Thus, it takes into account both quadrupolar effects (by setting quadrupolar constants QCC and asymmetry parameters r] for each line) and dipolar broadening (by setting widths of Gaussian and Lorentzian convoluted with an ideal powder averaged quadrupolar line shape). However, disorder effects at a particular site may manifest itself as a distribution of either chemical shifts or quadrupolar parameters of a corresponding line. While the former is easily accounted for by the introduction of additional line broadening, the latter calls for a different approach. Different structural surroundings for a given A1 site result in the distribution of the EFG tensor parameters at this site. This in turn gives rise to a set of parameters (QCC and r ] ) of a quadrupolar interaction experienced by A1 atoms with a given coordination number. Similar reasonings were introduced recently by Meinhold, Slade, and Norman.18 In our work we followed their computational strategy with some modifications. For the details of the procedure readers are referred to the Appendix. We have modified the standard simulation software in such a way that a new dimensionless parameter g, that is, the width of normal distribution imposed over principal values of EFG tensor, is supplemented to the default set of adjustable parameters (QCCi, vi, biipo,fi, LBi). As far as l5N is concerned, it should be pointed out that dipolar coupling of 15Nnuclei with quadrupolar moment of neighboring 27Al must not be considered. An estimation of the magnitude of this interactionI6shows that even with the largest possible values of QCC and 15N-27Aldipolar interaction (QCC zz 7 MHz, %-AI = 2 A) it does not exceed 1 ppm, which is much less than the observed spectral line width. Thus, a distribution of 15Nchemical shifts is the major source of spectral broadening, since dipolar interactions of 15Nnuclei are effectively averaged out by MAS and proton decoupling. Simulation Strategy for 27AlNMR. Two peculiar features of Z7AlNMR spectra immediately catch the eye (see Figures 1-4). First, in most cases, individual lines when resolved are strongly asymmetrical with extensive high-fieldwings, and second, a very broad base is observed in the spectra (both CP and 1P) of almost
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6203
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Figure 1. "A1 MAS spectra of y-alumina. Three top spectra and from the top: the one-pulse (IP)spectrum of the sample exposed to the atmosphere (rehyd), the sample having chemisorbed NH3 at 115 OC after dehydration at 600 OC (NH3), and the dehydrated sample (vac).
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The three bottom spectra are the corresponding 1H *7AlMAS CP spectra. Number of accumulations: typically SO00 for the one-pulse experimentsand 40 OOOor 80 OOO for the CP(0H) or (NHa)experiments, respectively. All traces have been rescaled to have the same maximum amplitude except the bottom one.
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PPm Figure 2. Z7Al MAS spectra of Super S(2). Legend as for Figure 1. The MAS CP experiment of the dehydrated sample being also featurelessas in Figure 1 is not shown. The MAS CP (OH) spectrumof therehydrated
sample was obtained after 5000 accumulations. all compounds. As shown in ref 18 and in this work (see Appendix), the line asymmetry is easily simulated with an appropriate value of 5. Simulating the base does not yield a defined solution. We restricted ourselves within reasonableranges
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Figure 3. 27A1MAS spcctra of Super 5(1). Legend as for Figure 2. Ammonia has been adsorbed at either 115 or 190 OC.
of adjustable parameters, that is, QCC I7 MHz, l < 0.45, total LB I2.5 kHz. It was found out that any change in asymmetry parameter 7 hasvery littleeffect on the total line shape, especially with distributions being imposed over individual lines. This is not particularly surprising, since the poor resolution smoothes out all possible singularities resulting from the asymmetry of the EFG tensors. As a consequence and also to reduce a number of adjustable parameters, we assumed the asymmetry parameter to be zero in all cases. Some examples of spectral simulations are given in Figure 5 , and the obtained parameters are listed in Tables 2 and 3. IJNNMR Spectral Simulation. This was performed with the standard software Peakfit. It allows simulation of experimental spectra with an arbitrary number of lines with preassigned shape. We used Gaussian functions in all cases and set limits on the number of lines (three at the most) and their widths ( 1 8 ppm).
Experimental Results The experimentalresults will be shown by displayingthe NMR spectra and tables containing the numerical results of the deconvolution procedure. It should be outlined that there has not been any attempt to reach the absolute intensity of the 27Al 1P or CP signals for several reasons. It is practically impossible to know the amount of material introduced into thevial; the large distribution of QCC suggests that there may be some "NMR silent" *7Al in the 1P spectra.4 As far as the 27Al CP MAS experiment are concerned, the difficulty of obtaining absolute intensity measurements is well documented.l9820 Note that the silicon nitride spinner used in this work contain -0.5% AlN showing up in a signal at -110 ppm. In the 1P spectra the amplitude of the signalcorrected by the number of accumulations can provide some rough intensity information. It would be easier to obtain quantitative information on the 15N contents (spin l/z), but the measurement of the chemisorbed amount of NH3 has to be carried out independently and not in the sample actuallystudied. At this point, it is worthwhile to remind the reader that there are - 5 AIV1and -3 AIIVpresent per nm2 on the surface of a rhombooctahedron (5-30-nm diameter) constructed on the low index planes of a spinel structure such as that of aluminas.21The
Coster et al. average of many measurements of surface OH density reveals 13 OH per nm2 at 100 "C and -2 OH at 600 "C. On the Alv-rich alumina studied here, after outgassing NH3 at 80 "C the ratio (NH3/Al) on the surface was less than 2 while the differential heats of adsorption weredistributed between 135and 73 kJ/mol. These values obtained by A. AurouxzZare in the range of those reported by Dumesic et al.23for yalumina. On an H-mordenite at 150 "C, these authors observed a monolayer coverage corresponding to about 10-3 mol/g and differential heats of adsorption of about 150 kJ mol-'. Assuming that there are 15 X 10-3 g of alumina in the sealed vial and that the surface area of the alumina is 300 m2/g, there are -3 X 1019z7Alnuclei and 4 X 1018 15N (in l5NH3) potentially detectable by CP MAS. Thus, the experimental results reported here are at the limit of the sensitivity of the NMR instrument. For each sample two very different aspects of the surface compositions will be explored. The 27AlCP MAS spectrum obtained from the polarization transfer from the (chemisorbed)ammonia protons to Z7Alshould give information on the state of the surface after calcination at high temperature, that is, the catalytically active surface. The ammonia being outgassed in all cases below 200 OC, ammonialysis is very unlikely. On the opposite, the 27AlCP MAS spectrum obtained from the polarization transfer from chemisorbed water will show extensive surface reconstructionbecause the water chemisorption is dissociative. The material used to obtain this spectrum is that outgassed at high temperature and exposed for 24 hat atmospheric moisture. The two kinds of z7Al CP MAS spectra, labeled 27Al CP MAS (NH3) and 27AlCP MAS (OH), are compared to the corresponding *7AI 1P spectra, recorded immediately before or after the CP MAS spectra. It must be, of course, outlined that the chemisorption of ammonia on a CUS aluminum is likely to shift its resonance line upfield, as does oxygen or OH. The extent of this shift is, however, not known, but it may be assumed that it will be about the same as that observed for oxygen ligands or OH.24 As far as we are aware, it is the first time the 1H 27Al CP from chemisorbed ammonia is experimentally observed. This fact alone raises an interesting question about the efficiency of the magnetization transfer. As it results from the above remarks and from a rough estimate of the ratio OH/Al on the surface of an alumina outgassed at -600 "C (namely, -2 OH/8 Al), the efficiency of the magnetization transfer from OH to A1 is not enough to produce an observable signal after 8 X 104 accumulations. By contrast, when this ratio is in the order of 1 OH/1 Al, an intense 27AlCP MAS (OH) spectrum is observed after about 8 X lo3 accumulations. In the 27Al MAS CP (NH3) spectra decent S / N ratios are achieved after 8 X 104 accumulations for an ammonia/Al ratio less than 1/4. Thus, the magnetization transfer from chemisorbed ammonia is quite efficient. This observation suggests that a fraction of the surface aluminum must be coordinatively bound to NH3. In order words, the AI-H distances must be relatively short, since the cross-polarization rate is a direct function of the heteronuclear second m ~ m e n t . l ~From - ~ ~ the number of accumulations reported above, the S / N ratio and the rough approximate for the surface concentration, it turns out that the number of surface aluminum evidenced by CP cross-polarization is less than 10% of the surface aluminum, in alumina. Figures 1-3 display the z7Al CP MAS and 1P MAS obtained for the aluminas while Table 2 shows the parameters used to deconvolute these spectra as described in the Experimental Section. Four lines are necessary to accommodate the 27Al 1P spectra calcined at "high temperature" (see figure captions) whether or not NH3 is present. There are two lines with isotropic shifts at 74 f 2 and 59.5 f 2.5 ppm, which have to be assigned to A P . The line with isotropic shift at 38.5 f 2.5 ppm is most
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The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6205
Surface Aluminum in Aluminas and Zeolites
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1P Reh.
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c
1P NH3
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~~
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Figure 4. (A) One-pulse (1P) *’A1 MAS of WSY,VG5, vG3, and VGz from top to bottom. All samples were rehydrated prior to recording. Typically 10 000 accumulations. (B) ‘H Z7AlCP MAS (OH) spectra of the same samples. Number of accumulations: 40 000, 50 OOO, 45 000, and 5000 (from top to bottom). (C) One-pulse 27AlMAS spectra of the same samples dehydrated under vacuum at 500 “C after ammonia chemisorption at 1 15 OC. Typically: 5-10 000 accumulations. (D) 1H 27A1MAS CP(NH3) of the same sample. Number of accumulations: 80 OOO, 80 000, 80 0o0, 50 000. All traces have been rescaled to have the same maximum amplitude.
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likely attributable to AIV,and finally, the h e with isotropic shift at 11 f 2 ppm is that of AIV’. QCC of the four lines are in the sequence of decreasing chemical shift, 6,5.5, 5.7, and 3.8 MHz (f6%). The CP spectra of the dehydratd alumina may be simulated with three lines because only one AIIV resonance can be evidenced, in most cases. When the surface is rehydrated, three lines are necessary to simulate the 27A1 1P and CP MAS spectra, namely, those with isotropic shifts at 74 f 2, 38.5 0.5, and 11 f 2 ppm. These shifts are practically identical to those observed for the three corresponding lines in the dehydrated materials and so are the
*
corresponding QCC. The differentiation between samples results mainly from the relative contributions of each line to the total intensity and, as shown by a selected example of deconvolution in Figure Sa, by the width of the normal distribution of QCC. The sum of the Gaussian and Lorentzian broadenings does not change much. The only noticeable exception is the AP’ line, which is generally narrower. The z7Al spectra obtained for the four studied zeolites are shown in Figure 4A-D whereas the parameters used to simulate the spectra are in Table 3. For the rehydrated as well as for the dehydrated samples, the deconvolution of the spectra with
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The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
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n
for AllVand Alv and 3.2 MHz for A1VI. As evident from the examination of Figure 4C, the 1P MAS (NH3) spectra obtained for the zeolites do not show enough characteristic features to permit a reasonable deconvolution. It should also be pointed out that the residual acidic O H bridging framework silicon and aluminum, Si-O(H)-AIF, in the dehydrated sample protonates NH3.25 In HY zeolite at room temperature the acidic proton is not mobile at the NMR time scale whereas NH4+ may have mobility in this time scale at 100 "C as long as the NH4Y has not been calcined at a temperature higher than 300 0C.26*27 However, it has been reported that the 15N resonance associated with 15NH4+ (where H+ is the acidic proton in an 89% Na-exchanged HY) disappearsin the static NMR spectrumwhile it is observed if an excess ammonia is present.4 Cross-polarization requires a strong reduction of translational molecular mobility. As a consequence, the proton in the acidic form or the NH4+ could participate in the transfer of polarization toward 27Alin 27Al MAS CP (NH3) experiments. Theother contribution comes from NH3 adsorbed on Lewis sites, as that allowing the observation of 27Al in dehydrated aluminas with chemisorbed NH3. As said before, the l5N MAS CP resonance of chemisorbed l5NH3 has been studied in five catalysts with the results shown in Figure 6. Note that the singularities caused by the coupling with the quadrupolar 27A1 nuclei should be observableonly if the line width was less than 1 ppm. The deconvolution parameters shown in Table 4 are obtained assuming superimposition of Gaussianlines with theindicatedfull width at half-height (fwhh). In the aluminas two lines are generallyobserved: the most intense one is at -23.8 f 1 ppm, the fwhh being 10.4 f 2 ppm and the average contribution being 77 f 11%. The weaker contribution is at -33 f 4 ppm. In the Super 5 (1) aluminas, there is probably a weak contribution around -20 ppm. In the two studied zeolites, three lines are required to simulate the l5N MAS CP spectra. The first one is at -24 f 1 ppm, that is, near the position observed in aluminas, but it is narrower and the relative population is smaller, namely, 45 f 5%. The second line is at -30 f 2 ppm, again like the range of the second line observed in the aluminas, but it is also narrower and the population is markedly higher. Finally, there is a third contribution at about -36 f 2 ppm which was not observed in the aluminas. The unexpected result is that in these CP MAS spectra there is no contribution near -14 ppm which should have been assigned to l5NH4+. Indeed, the shift of NH4+ in solution is upfield by this amount with respect to solid 15NH4Cl. The expected result is that there is no resonance near -44 ppm, since physisorbed NH3 has not survived outgassing carried out at 115 or 190 OC (for 10 min) The absence of a line characteristic of 15NH4+in acid zeolites such as VG3 and USY is in contradiction with the observation of the characteristic 3lP resonance of the proton adduct of trimethylphosphine (TMPH+) in the same catalysts.28 TMP being a weaker base than NH3, ammonia should be adsorbed more strongly.
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.
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Figure 5. Simulations of selected spectra from Figures 1-4. All parameters used for the simulations are summarized in Tables 2 and 3.
The solid lines correspond to each individual contribution and the sum. The markers represent the experimentaldata. (A) 27AlMAS spectra of Super 5(1). From top to bottom, the one-pulse experiment of the sample exposed to NH3 at 190 OC after dehydrationat 600 OC,the CP spectrum of the sample exposed to the atmosphere, and the CP spectra of the sample exposed to NH3 at 190 OC after dehydrationat 600OC. (B) 27A1 MAS spectra of USY. From top to bottom, the 1P spectrum of the sample exposed to the atmosphere,the cross-polarizationexperiment of the same sample, and the cross-polarizationof the sample exposed to
NH3 at 1 I5 OC after dehydration at 500 OC.
relatively well-defined features can be accounted for by three lines with isotropic shifts at 60 f 1 ppm ( A P ) and 35 f 5 ppm (AlV). The isotropic shift of the third line (AlVI)is noticeably lower than that observed in the aluminas. The QCC are generally smaller than those observed in aluminas being about 4.2 MHz
Discussion
First, we center the discussion on the 27Al spectra obtained for the aluminas. The 27Al MAS 1P of dehydrated aluminas with or without NH3 are composed of four lines correspondingto four kinds of aluminum in the bulk and in the surface. The rehydrated aluminas have 1P or CP (OH) MAS spectra made from three lines. The line with a isotropic shift near 59.5 f 2 ppm in the dehydrated alumina has disappeared. The difficulty linked to intensity measurements of 27Al signals obtained from cross-polarization, even for getting the relative distribution of 27Al within three coordination states, has already been emphasized. An aluminum which cross-polarizes slowly must be given the time to equilibrate with the spin-locked proton reservoir which decays in time and the cross-polarization
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Surface Aluminum in Aluminas and Zeolites
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TABLE 2 Deconvolution of *‘A1 Spectra Obtained for the Aluminas in Three of Four Lines (See Deconvolution Procedures for Explanation) broadening sample pulse/state bi, (ppm) QCC (MHz) € G (Hz) L (Hz) distribution (76) Gamma AI 1P rehyd 1 2 3
Super5 (1)
1P dehyd 1 2 3 4 CP OH 1 2 3 C P + N H , a t 115OC 1 2 3 1P rehyd 1 2 3 1P dehyd 1 2 3 4 1P dehyd + NH3 at 115 OC 1 2 3 4 CP OH 1 L
3 1P dehyd + NH3 at 190 OC 1 2 3 4 CP+NH3at11S0C 1 2 3 CP NH3 at 190 OC 1 2 3 1P rehyd 1 2 3 1P dehyd 1 2 3 4 1P dehyd NH3 at 115 OC 1 2 3 4 CP OH 1
+
Super5 (2)
+
2
3 C P + N H g a t 115OC 1 2 3 4
71.5 44.0 10.0
5.1 5.1 3.55
0.19 0 0.31
1000 900 600
1000 800 600
32 2 66
72.5 58.0 43.0 11.5
5.1 4.9 5.1 3.5
0.23 0.09 0.27 0.29
1000 1000 1000 900
1000 800 lo00 800
27 24 6 43
72.5 38.5 10.0
5.1 3.5 3.5
0.14 0 0.29
lo00 900 800
1000 900 900
11 2 87
71.5 35.0 8.5
5.5 5.5 3.5
0.37 0.34 0.21
1100 1100 800
1100 1100 900
33 23 44
73.0 39.5 11.5
6.75 5.45 4.5
0.21 0.35 0.25
1100 1100 1000
1000 800 800
44 34 22
72.0 57.5 39.0 12.5
6.1 6.1 5.7 3.8
0.21 0.3 1 0.27 0.26
1000 1100 1000 1100
900 lo00 800 1100
30 30 21 19
72.2 58.0 40.0 13.5
5.9 5.9 5.5 3.8
0.19 0.19 0.3 1 0.21
1100 1000 1000 1100
700 800 800 1100
30 21 28 21
72.5 39.0 9.5
6.75 4.9 3.8
0.25 0.21 0.21
lo00 1000 1100
800 800 700
48 35 17
73.0 58.0 39.5 11.5
5.9 5.9 5.5 3.8
0.19 0.28 0.39 0.27
1100 1000 1000 1100
700 800 800 1100
28 21 29 22
66.5 42.5 8.5
6.8 5.5 3.8
0.27 0.21 0.19
1400 1100 1000
1900 1000 500
54 34 12
67.0 40.5 9.5
6.8 5.5 3.8
0.29 0.21 0.21
1400 1100 lo00
1300 lo00 500
49 38 13
77.5 38.5 13.0
6.75 5.45 4.45
0.15 0.19 0.21
1000 1100 700
800 1100 700
38 17 45
76.0 57.0 33.0 12.5
6.45 4.95 6.5 4.45
0.21 0.21 0.23 0.27
1300 1100 1300 700
800 800 1100 700
34 21 12 33
76.5 57.0 37.0 13.5
6.2 4.45 5.8 4.45
0.19 0.175 0.19 0.21
1100 1100 lo00 700
1000 lo00 800 lo00
34 16 10 40
74.5 38.0 10.0
6.75
1100
4.45
0.19 0.29 0.21
700
800 700 700
35 26 39
73.0 58.0 41.0 10.5
4.8 5.0 5.5 3.8
0.23 0.19 0.13 0.27
1400 1100 1000 1000
1300 1100 lo00 500
27 13 32 28
eP.”Tp
time must be shorter than the shorest We checked that this was the case for Z7Al CP from OH where was 2 4 ms while T A ~was H I 1 ms in a Super 5 alumina and a steamed H Y
5.45
1000
zeolite. Similar results were obtained for lH-15N CP dynamics N 11 ms and TH-NN 0.5 ms. in the USY zeolite. We found The contact time for maximum intensity was between 0.5 and 1
Coster et al.
6208 The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
TABLE 3: Deconvolution of 27AI Spectra Obtained for the Zeolites (See Deconvolution Procedures for Explanation) broadenine " sample pulse/state 4M ( P P d QCC (MHz) 5 G (Hz) L (Hz) distribution (9%) USY 1P rehyd 1 2 3 CP OH 1 2 3 CP NH3 at 115 OC 1 2 3 1P rehyd 1
+
VGz
3 CP OH 1 2 3 CP NH3 at 115 OC 1
+
L
VG3
VG5
3 1P rehyd 1 2 3 CP OH 1 2 3 C H + N H s a t 115'C 1 2 3 1P rehyd 1 2 3 CP OH 1 2 3 CP NH3 at 115 1 2 3
+
60.0 34.5 4.0
2.8 4.1 2.9
0.23 0.31 0.35
1000 1200 700
1000 1200 900
58 21 21
59.5 31.5 4.5
3.9 3.2 3.1
0.34 0.34 0.29
1100 1000 700
500 1000 700
40 15 45
60.0 38.0 10.5
5.9 5.5 3.9
0.36 0.09 0
1400 1300 800
700 500 500
81 15 4
57.5 40.0 7.5
2.0 5.8 4.0
0.31 0.23 0.23
100 900 1000
500 1100 1000
63 30 7
59.5 34.0 4.8
4.6 4.8 3.0
0.14 0.1 1 0.19
1000 1100 1000
900 1100 1000
42 31 27
57.0 39.0 13.5
4.0 5.5 5.0
0.29 0.18 0
700 1300 700
700 1200 700
65 31 4
57.5 38.5 6.5
2.0 5.1 3.8
0.31 0.23 0.23
400 1000 lo00
700 1100 1100
54 36 11
61.0 34.5 5.0
4.0 4.0 3.0
0.14 0.11 0.26
1000 1000 1000
900 900 900
37 27 36
57.0 38.5 12.5
4.0 6.0 5.5
0.27 0.29 0
800 1300 1000
800 1200 1000
51 41 8
57.0 41.5 8.5
2.0 5.1 5.5
0.23 0.11 0.19
800 900 700
1000 900 500
42 28 30
64.0 36.5 6.0
4.8 4.4 3.8
0.12 0.14 0.21
1000 lo00 lo00
400 400 400
23 18 59
60.0 36.5 6.0
4.4 4.8 3.0
0.14 0.14 0
1600 1600 400
200 900 400
40 51 9
ms under the conditions specified in the Experimental Section. In the case of Z7Al CP from NH3, because of the large number of accumulations, it was practically impossible to perform the dozen of intensity measurements with respect to the contact time required to observe the maximum. However, for USY and Super 5(1) it was found that the maximum intensity was also between 0.5 and 1 ms. Therefore, the relative distribution of the surface Z7Al within the different coordinations is represented by the intensity distribution observed in the CP spectra, in first approximation. We have tried hard to figure out possible contributions of trigonal aluminum but without success. Even in assuming an isotropicchemical shift a t about 95-100 ppm, a QCC of 10 MHz, and 1) equal to one, we were not able to assign any of the features observed in the 1P dehydrated (no NH3) spectrum to the hypothetical AlIII. Unless this species is in vanishing concentration, with respect to surface A P , AIV,and Alvl (that is much less than 10% of the observable surface sites), we are obliged to conclude that the theoretical calculations29on the configurational energy of A1 clusters in which trigonal A1 is the working horse are not experimentally founded. As shown in Table 2, which reports the results obtained for the three aluminas, there is an extremely clear-cut separation be-
tween the distribution of the A1 species with and without surface reconstruction. The comparison of the distribution of the aluminum atoms in the 1P or the C P spectra has to take into account that, in the CP spectra, only those A1 in close proximity with the protons are observable. For instance, when the surface is reconstructed by chemisorbing water fewer than 10 000 accumulations yield a spectrum with a S / N ratio better than that obtained with 10 times more accumulations on the nonreconstructed surface having chemisorbed ammonia. As mentioned earlier, there are more protons on the rehydrated surface (and most probably more than one layer is affected) than on the dehydrated surface with NH3. Theoretically, two different cross-polarization processes are possible. The first and more efficient process is the direct transfer of polarization from the protons of an ammonia or OH to an aluminum, to which this ammonia or hydroxyl is coordinated. The second and less efficient one is the polarization transfer to a near-neighbor aluminum which is "oxo-linked" to that enjoying direct transfer. This second mechanism will be called proximity transfer. In the absence of information on the surface topology, it is impossible to assess the respective contributions of "direct" and "proximity" transfer. It is conceivable that on an activated
Surface Aluminum in Aluminas and Zeolites
75
'
=I!
Super5 (2)
0
m
t
'Bc20
0
-20
-40
-60
PPm
Figure 6. 15N MAS CP spectra of chemisorbed NH3 on selected catalysts. One example of deconvolution is shown for "NH3 chemisorbed on USY
zeolite.
TABLE 4 AI Distribution in the NF Debris in Zeolites (%) without reconstruction (NH3) with reconstruction(rehydrated) NFAtV NFAIV NFAIVt NFA1IV NFAIV NFAIV1
USY VG2 VG3 VG5
52.6 16.0 -0
19.1
37.4 74.3 85.4 68.8
9.9 9.8 17.4 12.0
39.5 42.2 31.2 22.9
i
8
15.2 31.1 26.7 17.5
44.9 26.1 36.1 59.6
surface the CP spectra show AIVcoming from AI1" having one more NH3 ligand in the coordination shell or AIV1produced from AIV. AllVcan not be observed except by proximity transfer in CP spectra, since trigonal A1 has not been detected. Of course, AIV and API may also be observed in CP(NH3) from proximity transfer. The following considerationsillustrate the difficulties encountered for comparing results. As shown in Figure 7, there is apparently a linear relationshipbetweentheAIV1relativecontents obtained from the 1P (NH3) or 1P (OH) spectra and the correspondingvalues obtained from the CP (NH,) or CP (OH) spectra in aluminas. The nature of the proton reservoir (OH or NH3) does not seem to matter. Thus, the surface API (CP experiment) is not favored in the cross-polarizationtransfer with respect to surface AllVor AIV(1P spectra). In spite of the fact that the overall *7A1CP MAS spectrum is at least 10times weaker than a 1P spectrum, the relative contribution of AlvI is practically the same in the 1P as in the CP spectrum. However, the sum of the relative AItVand AIVcontributions fluctuates. In aluminas the contribution of AIVis always higher in CP (NH3) and CP (OH) spectra than in the 1P spectra. Of course, theoppositesituation is observed for the A1lVcontribution. It is likely that the AllVline which appears upon dehydration (line 2 in 1P spectra, see Tables 2 and 3) provides adsorption sites for NH3, transforming them into AIV, with 4 0, and 1 NH3 ligand. These AIV (originally A P ) would, thus, be observable through direct cross-polarization,while the remaining AllVwould be excited through proximity cross-polarization. Meanwhile, A P could be observable in the CP spectra because some of the structural surface AIV, a part of which is observable in the 1P
0 0
0
25
50
75
100
Y7AlW1 P
Figure 7. Relative AIVtcontent obtained from the CP experiments using either NH3 ( 0 )or OH (m) as a source of protons us relative AIV' content obtained from the 1P experiments.
spectra of the dehydrated aluminas, are adsorption sites for NH3. This would mean that some of the observed surface A P have 5 0 and 1 NH3 ligand. However, the linearity shown in Figure 7 suggeststhat most of the Alvl are structural and observed through proximity cross-polarization. Thus, the Lewis acid sites, available to NH3, on the surface of aluminas are tetrahedral sites with isotropic shift of -59 ppm and QCC -5.5 f 0.5 MHz and pentagonal sites with isotropic shift near 38 ppm and QCC near 6 MHz. The ratios of these surface A P to AIVLewis sites should be close to the ratios of the AIVto the AP1 line intensities in the CP (NH3) spectra, namely, 0.54,2.8,and 1.2in y-AlzO3,Super 5(1),andSuper 5(2) aluminas, respectively. Chemisorption of water occurs on the same sites as those chemisorbing NH3, but the water chemisorption is dissociative and the profound surface reconstruction is clearly observed in comparing CP (OH) and CP (NH3) spectra in aluminas and zeolites. As will be shown elsewhere, there seems to be a relationship between both these Lewis acidic sites and those observed by FTIR of CO TPD. The interpretation of the 27Alspectra obtained for the zeolites (Figure 4a-D) is relatively simple if the following facts are taken into account. 1. All the zeolites studied here contain nonframeworkAI: the 1P (OH) spectra show lines attributable to nonframework (NF) AIV and AP1 while the N F A P is overlapped by the F A P contribution. The absolute amount of NFAl ranks as follows: VG2 < VG3 < USY < VGs (Table 1) in agreement with the NFAIV and API lines intensity in Figure 4A. 2. The CP (OH) spectra give the distribution of the NFAl among the three coordination states in a reconstructed surface structure of the AI N F debris. Indeed, the F A P resonance is not observable in the CP (OH) spectrum of Y zeolites.30 It is also probably not observable in the mordenites. The CP(0H) spectra are similar to the CP (OH) spectra observed in the aluminas. The smaller is the absolute amount of NFAI, the larger is the relative content. 3. All the 1P spectra of the dehydrated samples contain an A1IVpeak with its center of gravity between 52 and 55 ppm on top of a broad featureless component. Therelativeintensityofthe APlineinCPMAS (NH3) spectra should be a function of the relative AlF content obtained from 29SiNMR (Table 1). Figure 8 shows the increasing contribution of A$' in the A P CP (NH3) signal. The composition of the NFAl debris normalized to the NFAl content is shown in Table 4 where it is compared to the composition after reconstruction (CP OH). The relative AIV contents are higher than those observed in the Super 5 alumina. As observed for the aluminas, reconstruction by H2O chemisorption favors AIV' at the expense of both APV and AIV.
Coster et al.
6210 The Journal of Physical Chemistry, Vol. 98, No. 24, I994 100
15
1
I
I
,,.d
TABLE 5 I5N Resonance Line of Chemisorbed NH3 Chemical Shift, fwhh, and Corresponding Relative Contribution (Weak Line and Uncertain Deconvolution Results in Parentheses) sample shift (ppm) fwhh (ppm) distribution (%) 7-Al203 -23.9 11.3 88 Super 5(1) (115 "C)
Super 5(1) (190 "C) Super 5(2) 0
0
25
50
75
100
VG3
%AllVframework
Figure 8. Relativeintensity of the AIIVMAS CP (NH3) signal observed for the zeolites (Table 3) us the relative content in AIkv/AI is from Table
USY
1.
Considering the similarity between the C P MAS (NH3) spectra of Super 5 alumina and those of the NFAl debris in zeolites, it is not too far-fetched to suggest that the Lewis sites are of similar nature. Of course, the zeolites contain, in addition, the Brbnsted sites Si - O(H) - Al: which becomes Si - O(NH4) - A l F after exposure to NH3. Other differences exist between aluminas and nonframework A1 debris in zeolites. First, as shown in Table 3, the QCCs of the various coordinations are generally smaller than in aluminas. The highest QCC is observed for AP. The isotropic chemical shifts of the AIiF and A$ lines are lower than in alumina. The important question that is not answered so far is about the nature of the synergy between Brbnsted and Lewis acid sites. Indeed, this is merely a function of the distance between both kinds of sites. The last step of this work is to check the assignments which have been suggested for the Lewis sites by an independent technique. We pointed out theexistenceof a relationshipbetween the nature of the two Lewis sites and the vibrational modes of chemisorbed CO, as will be shown elsewhere. A more direct approach was desirable, and of course, it was expected that the study of the l5N resonance of the chemisorbed NH3 used for polarizing the surface aluminum would shed light on the Lewis (and Brbnsted) sites. The crucial point in the interpretation of I5N spectra resides in the validity of one-to-one spectrostructural assignments of separate lines like in the case of 27Alspectra. From various previous studies of ammonia ad~orption,~l+32 it is well-known that ammonia on the catalyst surface is far from making up a 'rigid" structure. The dynamic nature may result from a variety of processes, such as proton exchange between NH4+ and NH3 or NH3 and residual OH groups. Being of the utmost significance for 15N shifts and line shapes, these dynamic effects may be of secondary importance for Z7Al NMR, since (i) they do not markedly affect the immediate surrounding of active sites and (ii) the frequency range of 27Al spectra (75 ppm, Le., 104 Hz) is 10 times larger than that of 15N (20 ppm, Le., lo3 Hz). This means that processes that are fast on the 15N time scale might be slow or intermediate on the z7Altime scale. Furthermore, one must not rule out the possibility of ammonia adsorption over two or three sites a t a time. This can occur due to the A1 cluster formation on alumina surfaces, as in thequantum-chemical model developed by Kawakami and Yoshida.29 From the above line of reasoning it is not surprising that the interpretation of these l5N spectra has not been straightforward in spite of the fact that two major contributions are observed for NH3 chemisorbed on aluminas, while there are three for zeolites (see Figure 6b and Table 5).
-34.6 -12.6 -23.3 -32.0 -19.1 -24.2 -31.1 -23.9 -32.0 -25.0 -31.9 -38.5 -23.1 -28.3 -34.2
6.8 9.2 9.3 10.2 4.2 5.8 9.4 13.2 9.5 5.9 6.8 5.8 8.6 4.0 8.3
12 (9) 62 29 12 45 43 81 19 48 39 13 41 21 32
Upon examination of the 15N chemical shift scale for ammonia and ammonium salts,15it becomes immediately apparent that unfortunately there are no distinctive shift ranges for 15NH3and 15NH4+. Indeed, *5NH4+shifts spread from 0 ppm (ISNH4C1, solid) to-20 ppm (15NhN03,solution), while l5NH3shifts appear between -60 ppm (15NH3, gaseous) and -23 ppm (ISNH3, phy~isorbedl~). If any exchange process occurs a t the surface, it should be fast on the experimental time scale and gives rise to averaged signals at intermediate positions between actual chemical shifts. This might explain why ISN spectra for alumina and zeolites are so similar in appearance, in spite of large differences in the surface acidity, such as the absence of Brbnsted site in aluminas. We first examined the aluminas and assumed that the line near -23.8 f 0.5 ppm was due to NH3 adsorbed on the surface A P , corresponding to the chemical shift near 58 ppm. Thus, the line near -32 ppm has to be assigned to NH3 adsorbed on the pentagonal AI. The contributions of these two lines in the 1P (no NH3) MAS spectrum of the dehydrated aluminas were renormalizedandcompared with therelativecontributionsof the-23.8 and -32.4 ppm lines in the l5N spectra. The rankings of the alumina samples according to both these criteria are the same. A difficulty arises from the zeolites. The Brbnsted sites surviving dealumination and calcination are obviously associated the A1IVresonance observed in the C P (NH3) spectra (Figure 8). Thus, a line attributable to NH4+ should be observed, and this line should be between -1 4 and -20 ppm where shifts are observed for 15NH4+solutions. This line has not been observed. Yet, the most intense contribution has a chemical shift very similar to that associated to NH3 on the surface AllVin alumina, that is, at -23 f 1 ppm. On USY a signal appears at about -28 ppm. On VG3 two lines are a t about the same shift as in the aluminas, the third one being a t a higher field. Weconclude that the interpretation of 15N spectra at a higher level will have to wait until additional experimental data (e.g., low-temperature measurements) become available. Conclusions The step forward described in this work is in the qualitative characterization of two kinds of Lewis acid sites in aluminas and in the alumina debris in dealuminated zeolites. This progress has been achieved by observing the 27Alcrosspolarization spectra obtained from the protons of chemisorbed ammonia. The degree of efficiency of the magnetization transfer being unknown, the information obtained from spectral deconvolutioncannot indicatemorethan general trends in the variations of the surface composition.
Surface Aluminum in Aluminas and Zeolites However, these approximations are good enough to show the restructuring role of chemisorbed water and to rule out the use of *’A1 CP from surface hydroxyls for representing the catalytic active surface. More work is needed to elucidate the significance of the 15N resonance spectra of chemisorbed 15NH3.
Acknowledgment. The financial support of DOE Grant DEFG02-90 ER 1430is gratefully acknowledged. The data in Table I were obtained by Dr. V. Gruver and Mr. Yong Hong. The chemical analysis were performed in the Advanced Analytical Facility of our University.
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6211 be. Now we define a new principal axes system so that the largest perturbed EFG is a new z-axis and the smallest one is a new y-axis. New parameters QCCi and are then calculated and become input values for a standard line shape simulation. The 343 line shape functionscalculated in this way are then multiplied by their weights wtjk (see eq A2) and added up to give a total line shape. In order to be convinced in the uniformity of our, in fact discrete, distribution, we checked the procedure with increased sampling (13 numbers) and over broader range (f3f). Even for largevalues of QCC (- 8 MHz) and f (-0.45) we did not observe any significant improvement in the line shape smoothness. It should be noted that one distribution parameter f has the same meaning as that from ref 18 but differs by the factor of 4 6 .
Appendix The Z7Al simulation procedure used in this work with some modifications followed the basic pattern given in ref 18. We describe this strategy here in more detail. If a given 27Al site is characterized by a quadrupolar interaction with parameters QCCO and 70, one can specify the principal component of EFG tensor as follows:
“a, = QCC,
In so doing, we exploit the tracelessnds of EFG tensors and skip out all constant factors, so that the EEG’s are expressed now in frequency units. In order to impose a normal (Gaussian) distribution on the EFG’s, we introduce a new dimensionless parameter f (0 < f I 0.4) and construct a set of distributed parameters QCC and 9 in the following way. Here is where a modification of the original procedure18 occurs. First, we defirre seven numbers, ai, that are evenly distributed between -2f and +2f. Say, for f = 0.15, these numbers are -0.3, -0.2, -0.1, 0, 0.1,0.2, and 0.3. Then, we construct 7 3 = 343 triplets (ai,aj,a&), where i, j , k = 0, 1, ..., 7, and assign a Weighting factor W#jk for each of them:
wilk = (1/5d(27F))3exp(-u,2/2t2) x e x p ( - a j W > exp(-a,2/2t2) (A21 For each triplet (a~,a,,ak)we now assign the EFG perturbations (following the notation in ref 18) V. = ai(QCC),Vy= a,(QCC), V, = ak(QCC) and define the perturbation EFG‘s:
v‘, = “a,+
2vx-
vy- v,
Pyy= PYY+ 2vy - v,- v, = “a,,+
2vz-
v,- vy
One can easily see that the new EFG set is traceless, as it should
References and Notes (1) Pellet, R. J.; Scott-Blackwell, C.; Rabo, J. A. J. Catal. 1988, 114, 71. (2) Sam, J.; Fornes, V.; Corma, A. J. Chem., Soc., Faraday Trans. I 1988, 84, 3113.
(3) Chen, F. R.; Davis, J. G.; Fripiat, J. J. J . Caral. 1992, 133, 263. (4) Engelhardt, G.; Michel, D. High Resolution Solid State NMR of Silicates and Zeolites; John Wiley and Sons: New York, 1987. ( 5 ) Vega, A. J.; Luz, Z. J . Phys. Chem. 1987, 91, 365,376. (6) OReilly, D. E. Symposium on Instrumental Techniques in Study of Catalysis Mechanism; Divisionof Petroleum Chemistry, American Chemical Society: Washington, DC,1959; Vol. 4, #2-C, p C157. (7) Morris, H. D.; Ellis, P. D. J. Am. Chem. Soc. 1989, 111, 6045. (8) Huggins, B. A.; Ellis, P. D. J . Am. Chem. SOC.1992, 114, 2098. ( 9 ) Parry, E. P. J . Catal. 1963, 2, 371. (IO) Chen, F. R.; Fripiat, J. J. J . Phys. Chem. 1992, 96, 819. (11) Coster, D.; Fripiat, J. J. Chem. Mater. 1993, 5, 1204. (12) Vega, A. J. J. Magn. Reson. 1992, 96, 50. (13) Vega, A. J. Solid Stare Nucl. Magn. Reson. 1992, I , 17. (14) Ponton, R. J. 38th Annual Symposium of the American Scientific Glassblowers Society; June 1993. Fusion, in press. (15) Witanowski, M.; Stefaniak, L.; Webb, G. A. In Annual Report on NMR Spectroscopy; Webb, G. A., Ed.; Academic Press: New York, 1993; Vol. 25, p 86. (16) Olivieri, A. C. J . Magn. Reson. 1989, 81, 201. (17) This software was kindly provided by Dr. Sethi, the Analytical and Services Department, AMOCO Corp., Naperville, IL. (18) Meinhold, R. H.; Slade, R. C. T.; Newman, R. H. Appl. Magn. Reson. 1993, 4, 921. (19) Mehring, M. Principles of High Resolution NMR in Solids, 2nd 4.; Springer-Verlag: New York, 1983. (20) Walter, T. H.; Turner, G. L.; Oldfield, E. J. Magn. Reson. 1988,76, 106. (21) Knozinger, H.; Ratnasamy, P. Caral. Reo.-Sci. Eng. 1978,17,31. (22) Auroux, A. Institut de Recherche sur le Catalyse; Lyon, France. Personal communication. (23) Spiewak, B. E.; Handy, B. E.; Sharma, S.B.; Dumesic, J. A. Caral. Lett. 1994, 23, 207. (24) Stein, A.; Wehrle, B.;Jansen, M. Zeolites 1993, 13, 291. (25) Uytterhoeven, J. B.; Christner, L. G.; Hall, W. K. J. Phys. Chem. 1965.69. 21.. 17.. .- -, - . , (26) Mestdagh, M. M.; Stone, W. E. E.; Fripiat, J. J. J. Chem. SOC., Faraday Trans. I 1976, 72, 154. (27) Mestdagh, M. M.; Stone, W. E. E.; Fripiat, J. J. J. Catal. 1975,38, 357. (28) Coster, D. J.; Bendada, A,; Chen, F. R.; Fripiat, J. J. J . Catal. 1993, 140, 497. (29) Kawakami, H.; Yoshida, S.J . Chem.SOC.,Faraday Trans. 1986,82, 1385. More examples of the abuse of trigonal AI can be easily reported. (30) Rocha, J.; Carr, S. W.; Klinowski, J. Chem. Phys. L r t . 1991, 187, 401. (31) Michel, D.; Germanus, A.; Pfeifer, H. J . Chem.Soc., Faraday Trans. 1 1982. 78. 237. (32) Earl, W. L.; Fritz, P. 0.; Gibson, A. A. V.;Lunsdorf, J. H. J. Phys. Chem. 1987,91, 2091.
