JACKH. LUNSFORD
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An Electron Paramagnetic Resonance Study of Y -Type Zeolites.
11. Nitric Oxide on Alkaline Earth Zeolites by Jack H. Lunsford Department of Chemistry, Texas A & M University, College Station, Texas 77843 (Received August RO, 1069)
The epr spectrum of the adsorbed nitric oxide molecule was used to study the alkaline earth Y-type zeolites. Results show that two types of sites existed on MgY, Cay, and SrY zeolites that had been degassed at 500°, but only one type of site was detected for the BaY zeolite. The spectrum on BaY was characterized by g1 = 1.994, gll = 1.89, and a i = 30 G for the N hyperfine splitting. A like spectrum was observed for the other zeolites; however, it was partially obscured by an overlapping spectrum, which showed no distinct hyperfine structure and a value of 911 = 1.95. This spectrum was similar to that previously observed on dehydroxylated HY zeolites. The spectrum of NO on CaY was observed as a function of dehydration temperature and extent of cation exchange. The HY-type site began to develop after dehydration at 300' and after 64% of the Na+ ions had been exchanged.
Introduction Since the early work of the LindelsZ group on the alkaline earth Y-type zeolites, there has been considerable interest in explaining the high activity and the systematic change in the catalytic behavior of these materials. It is now known that for a variety of reactions the activity increases as one moves up the series from Ba to Mg. At least three basic theories have been used to explain the experimental results. It was first proposed that strong electrostatic fields, emanating from the cations, polarize the reactants to the extent that they become pseudocarbonium ions.1,2 Next, it was suggested that the cations influence structural hydroxyl groups, thereby creating more acidic prot o n ~ .Finally, ~ ~ ~ it has been postulated that the diTialent cations are able to produce acidic sites through the hydrolysis of water which is held at the ~ a t i o n . ~ The' proton liberated in this process becomes attached to a lattice oxide ion where it forms the following type site 0 OH 0 a6
\ / Si / \
/
A1
/ \
0 0 0 0 which is identical with that proposed for decationated zeolites.' The ability to form these sites in cationic zeolites depends upon the hydrolyzing power of the cation. In an earlier paper from this laboratory on decationated (HY) zeolites, it was suggested that hydroxyl defect sites8 0 /O \ Si + A1
/ \
0
/ \
0 0
The Journal of Physical Chemistry
0
may form strong acids through their influence on adjacent SiOH group^.^ A similar mechanism may be operative for the cationic form, if indeed the same hydroxyl defect sites exist. Infrared data have shown that structural hydroxyl groups do occur on certain cationic Y zeolites, and there is growing infrared and X-ray evidence that lattice oxide ions are involved in the formation of water during the dehydroxylation p r o c e s ~ . ~ ,In ~,~ the ~'~~ dehydroxylation process; however, there are a number of alternate mechanisms that may occur, and it is very likely that several different reactions are proceeding to some extent. The present study has been carried out to determine the types of sites that exist on zeolites with divalent cations and to establish from epr data whether any Y-[OH] sites are formed upon dehydration. The electron paramagnetic resonance (epr) spectrum of the nitric oxide molecule was used to determine the crystal (1) P. E. Pickert, J. A. Rabo, J. A. Dempsey, and V. Schomaker, Proc. Int. Congr. Catat., 3rd, Amsterdam, 1964, 714 (1965). (2) J. A. Rabo, C. L. Angell, P. H. Kasai, and V. Schomaker, Discussions Faraday Soc., 41, 328 (1966). (3) A. E. Hirsohler, J . Catal., 2, 428 (1963). (4) J. T. Richardson, ibid., 9, 182 (1967). (5) J. W. Ward, J . Phys. Chem., 72, 4211 (1968); J . Catal., 10, 34 (1968). (6) L. G. Christner, B. V. Liengme, and W. K. Hall, Trans. Faraday SOC.,64, 1679 (1968). (7) J. B. Uytterhoeven, L. G. Christner, and W. K. Hall, J . Phys. Chem., 69, 2117 (1965). (8) For simplicity in nomenclature, the symbol [OH] will be used to denote a hydroxyl vacancy, If the vacancy is part of the Y zeolite lattice, the symbol Y-[OH] will be used; whereas, if the vacancy is on the cation M, the symbol M-[OH] will be used. A similar symbolism (X-[OH], AlgOs-[OH], etc.) could be extended to other systems. (9) J. H . Lunsford, J . Phys. Chem., 72, 4163 (1968). (10) J. B. Uytterhoeven, R. Schoonheydt, B. V. Liengme, and W. K. Hall, J. Catal., 13, 425 (1969). (11) D. H. Olson, J . Phys. Chem., 72, 1400 (1968).
EPRSTUDYOF Y-TYPEZEOLITES
1519
and magnetic interactions at the surface. A particularly distinguishing feature in this spectrum is the line shape that results from the hyperfine interaction of NO with an 27Alnucleus. This interaction is available at the Y-[OH] site, but at all other adsorption sites the small aluminum ion is apparently hidden by large oxide ions.
Experimental Section The zeolites used have been described in the preceding paper.l2 They were degassed by heating the samples under vacuum for 1 hr at 100" intervals, usually up to 500". Following such pretreatment the zeolites were either (a) used in this form, (b) reduced for 2 hr in H2 at 500", or (e) oxidized in 02 for 2 hr at 500" and then reduced for 2 hr in H2 a t 500". The samples were evacuated at the high temperature for 30 min after exposure to Hz or Oz. After each pretreatment suitable epr spectra were obtained. Purified nitric oxide was admitted onto the samples at room temperature and usually at a pressure of 10 Torr. The samples were cooled by immersing the quartz sample tube slowly into liquid nitrogen. An alternate procedure which gave more uniform adsorption involved first cooling the sample tube in pentane a t its freezing point (- 130°), and then rapidly transferring the tube to liquid nitrogen. All spectra were recorded at - 196". Determination of the g values and spin concentrations were essentially the same as described in earlier work. Because of the relatively weak amplitude of the spectra, the error in the absolute concentration is estimated to be + 50%. 9r12
Results The spectra shown in Figure 1 were observed when NO was adsorbed on the respective Y-type zeolite that had been degassed at 500", treated with oxygen, and then reduced with hydrogen. Each of the spectra show the I4N hyperfine splitting from which aL was determined to be 30 A 1 G. It is clear, however, that as one moves up the series from BaY to MgY, the three line hyperfine structure shows less resolution. This loss in resolution may be ascribed to the presence of a second broad peak, such as that observed for NO on dehydroxylated HY zeolitesg and silica-alumina. l a In the latter cases the broad line was due to aluminum hyperfine interaction with the NO molecule. In addition to the loss in resolution, one may observe from Figure 1 that a high-field minimum is present for SrY, CaY-1, and MgY zeolites at g = 1.95, which is the same as the value of gll for NO on HY and silicaalumina. Only a very weak minimum a t g = 1.89 was observed for B a y , Still a third overlapping spectrum was observed a t g = 2.008 for SrY. This narrow line, like the major NO spectrum, can be removed by brief evacuation of the
g1 =1.994
100 G A U S S
Figure 1. Epr spectra of NO adsorbed on alkaline earth Y-type zeolites.
sample at room temperature. It is much less prominent a t a lower surface concentration of nitric oxide. The spin concentration, from the spectra of Figure 1, was about 5 X 10'' spins/g. This concentration, which constitutes only a small fraction of the total NO concentration on the zeolite, was not increased upon increasing the pressure above 10 Torr. A comparison in Figure 2 of the spectra for 16N0and 14N0 on CaY-1 shows that the nitrogen hyperfine splitting changes as expected for the two isotopes. The I4N isotope has a spin of 1, whereas the '5N isotope has a spin of '/2, For '5NO the presence of the broad line can be more clearly seen at a low magnetic field where there is less interference from a nitrogen hyperfine line. The steep slope of the hyperfine line is also convincing evidence that the loss in resolution is not due to broadening of the nitrogen hyperfine lines, but rather to an interfering spectrum. The similarity in the spectra of NO on CaY and HY zeolites may be seen by comparing the curves in Figures 2b and 2c. The spectrum of NO on silica-alumina shows somewhat less structure than that in Figure 2c. Though not shown here, the spectrum of 15N0 on HY zeolites is likewise compatible with the spectrum of Figure k g It was of interest t o study changes in the spectra as the dehydration temperature was increased. In this process the sample was heated under vacuum but no O2 or HZwas employed. After the dehydration of CaY-1 at 100" and addition of 10 Torr NO the spectrum shown in Figure 3a was observed. Under these conditions only the nitrogen hyperfine lines along with a weak minimum at g = 1.92 were detected. It is interesting to note that a shoulder also appeared at this same g (12) K. M. Wang and J. H. Lunsford, J. Phys. Chem., 74, 1512 (1970). (13) J. H.Lunsford, J . Catal., 14, 379 (1969). Volume 7.4, Number 7 April 2,lQrO
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JACK H.LUNXFORD g Il.994 11
g,, = I . 95
I
Figure 2. Spectra of: a, I5NO adsorbed on CaY-1; b, adsorbed on Cay-1;c, I4NO adsorbed on a HY zeolite.
1 4 ~ 0
value in the spectrum of XgY as shown in Figure 1. The dashed line in Figure 3 denotes a low-field impurity signal which could be removed by reduction with Hz a t elevated temperatures. Following a maximum dehydration temperature of 300", the weak and poorly resolved spectrum in Figure 3b was detected. Here it appears that the broad spectrum was beginning to develop. The presence of the high-field minimum at g = 1.95 was also observed. The sharp line at q = 2.003 is an additional part of the background spectrum. It is probably the result of carbonaceous material on the zeolite since it may be removed by the oxygen treatment. In addition, a very weak line at g = 1.96, which is sometimes observed, is part of the NOzspectrum.14 When the same sample was dehydrated at a maximum temperature of 500", the spectrum shown in Figure 3c was recorded. One should note that the amplitude was about twice that observed for the spectrum in Figure 3b. Apart from the background, the spectrum in Figure 3c is the same as was shown for NO on CaY-1 in Figures l b and 2c. Dehydration at higher temperatures did not significantly alter the shape or the amplitude of the spectrum. In an attempt to explore the reversibility of this dehydration procedure 2 X lozomolecules of water per gram of zeolite were added back to the previous sample at 23". This sample had been degassed to remove residual oxides of nitrogen. After addition of water, the sample was heated under vacuum for 1 hr at loo", and the NO spectrum was recorded under the usual conditions. The spectrum was quite similar to that shown in Figure 3b and did not revert back to the spectrum of Figure 3a. In a separate experiment the sample was degassed at 23" after rehydration and the subsequent KO spectrum was similar to that in Figure 3a. Following dehydration at 500" the spectrum was essentially the same as that shown in Figure 3c. The Journal of Physical Chemistry
Figure 3. Spectra of NO on CaY-1 following dehydration at: a, 100'; b, 300'; e, 500'.
g , z1.994 I-
\
g,,: 1.95
Figure 4. Spectra of NO on: a, CaY-3 (43% exchanged); b, CaY-2 (65%) exchanged); c, CaP-1 (92% exchanged).
Since the catalytic activity and adsorption properties of the divalent zeolites are dependent upon the degree of exchange,iti5 a study was made to determine the effect of this parameter upon the NO spectrum. Samples of CaY-1, CaY-2, and CaY-3 were dehydrated a t a maximum temperature of 500" and reduced in Hz. The spectrum of NO on CaY-3 (43% exchanged) is shown in Figure 4a. This curve has much in common with the spectrum of NO on NaY zeolite.0 The poorly resolved 14Nhyperfine structure is thought in this case to result from a weak magnetic interaction of the N O molecule with a 23Nanucleus. A value of gl I = 1.88 f 0.01 may be compared with a value of gi I = 1.86 f 0.01 for NO on NaY. The amplitude of the curve in (14) J. H. Lunsford, J. Colloid Interfac. Sci., 26, 356 (1968). (15) S.E.Tung and E. McInninch, J . Catal., 10, 166 (1968).
EPRSTUDYOF Y-TYPEZEOLITES
1521 CaO
,"\
- 2+
Ca(OH,) 0
0 - 0 Si
/
CaOH' 0 - 0
A1
Si
\ / \/
0
A1
Si
B
0 - 0
\ / \ / \ /O\
/O
/"\
/O\-/"\ S i A1
Si
Si
+
/7
A1
/O\
_c
Ca2+
\ / \/ \
(a)
+
+
+
+
Ca -0- Ca
Ca -0- Ca
0 - 0
0 - 0
0
4.
0
Figure 5. Proposed mechanisms for dehydroxylation of a CaY zeolite.
Figure 4a was less than the amplitude of the NO spectrum on pure Nay. For Cay-2 (65% exchanged) the intensity of the spectrum decreased by almost a factor of 3, and the apparent value of y l ~shifted to 1.95 as shown in Figure 4b. This suggests that some Y-[OH] sites were formed. The typical spectrum of Cay-1 (92% exchanged), shown in Figure 4c, reveals a large increase in the number of Y-[OH] sites.
Discussion The role of the cation in hydrolyzing water to form the lattice hydroxyl is clear from infrared dah6Z6 The OH stretching band at a nominal 3640 cm-' has been generally assigned to this particular group. Recent work by Ward6 has shown that this band was quite prominent in the ir spectra of MgY, Cay, and SrY, but not in BaY zeolites. The hydrolysis reaction and subsequent dehydration as proposed by Ward are shown in Figures 5a, 5b, and 5c, using CaY as an example. According to this mechanism, dehydroxylation results in the Y-[OH] defect. Ward5 concluded that the electrostatic field associated with the barium cation was insufficient to bring about the dissociation of the adsorbed water to any appreciable extent. The BaY situation is somewhat ambiguous, however, since Hall and coworkersaJoobserved a weak band at 3650 cm-1 following dehydration at 250". All infrared work on the cationic zeolites shows that the 3640-cm-I band, when observed, rapidly decreases in amplitude upon heating the sample above 400".
This means that lattice protons are involved in the formation of water during dehydration, but it does not necessarily mean that lattice oxide ions are involved. An example of the latter case is shown in Figure 5d. Recently, Uytterhoeven, Schoonheydt, Liengme, and Halllo pointed out that the infrared band associated with M(OH) + ions is more sensitive to dehydroxylation than the -Si-OH groups. They suggested that the early stages of dehydroxylation lead to a product similar to that shown in Figure 5e. If this is true, during the latter stages of dehydroxylation (Figure 5f) lattice oxide ions would be removed and the Y-[OH] defect would be formed. One may reason from the epr results that upon dehydration of RlgY, Cay, and SrY zeolites a lattice oxygen is removed as molecular water is formed. This oxide vacancy uncovers a trigonal aluminum which is capable of adsorbing nitric oxide. The magnetic interaction between the aluminum and a nitric oxide molecule leads to the broad line which is depicted in Figure 2 . The spectrum of NO on BaY indicates that few, if any, Y-[OH] defects were formed upon dehydration. This result would be expected from the ir data of Ward since the -Si-OH group is a precursor of the Y-[OH] defect. It is clear from the data presented here and in a previous paper" that more than one type of adsorption site is involved, but only one type of adsorption site is characterized by the aluminum hyperfine interaction. The one-dimensional zeolite in Figure 5g illustrates Volume 74, Number 7 April 8 , 1970
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the type of interaction which may be operating in these zeolites to produce a very acidic proton. The electron induction may be even stronger here than in partially dehydroxylated BY since a cation is compensating for part of the negative charge which is formally placed on the aluminum. It should be emphasized that, acccording to this model, the active center for Bronsted acidity includes both the Y-[OH] defect and an adjacent hydroxyl group. Inclusion of the hydroxyl group as part of the active site is consistent with the loss in catalytic activity for certain reactions upon excessive dehydration.lB It was interesting to note that the Y-[OH] defect could be reproduced following addition of water to a dehydrated sample. A similar reversible behavior has been noted in the infrared and catalytic data for cationic ~eolites.6~~7J8 Likewise, there is favorable agreement between the formation of the Y-[OH] center and the onset of catalytic activity,lIl6 beginning when roughly 60% of the sodium ions are exchanged and increasing until more than 90% of the sodium ions are exchanged with calcium ions. The calcium ions in the dehydrated zeolite strongly prefer site I and will tend to fill all of those sites (57y0exchanged) before the site I1 positions begin to fill. Apparently the Y-[OH] defect is promoted by the site I1 divalent cations or by a cooperative effect of the site 11’ and site I’ cations such as that shown in Figure 5f. The 3640-cm-l band has been detected in divalent Y zeolites for all exchange levels tested, including 42 and 64% exchanged CaY. The observation of this band at low Ca2+levels is difficult to reconcile with the epr data unless one assumes that at these low exchange levels dehydration does not involve removal of oxide ions from the lattice. If one assumes that the spin concentration is linearly proportional to the site concentration, then it follows that the number of Y-[OH] sites on the divalent zeolites is about 1/60 the number found for dehydroxylated HY.g This is in agreement with the reported concentration of hydroxyl groups on these materials.& The spin concentration is several orders of magnitude less than the active site concentration as determined by catalytic poisoning experiments,lg but it is also several orders of magnitude greater than the site concentration as calculated from transition state t h e ~ r y . ~ While a study of the reaction of NO to form NzO and
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JACKH.LUNSFORD NO2 has not been the purpose of this paper, it is perhaps worthwhile to state that this reaction has been observed to occur on the zeolites reported here. Addison and BarrerZ0were the first to report that the reaction occurred on sodium and calcium faujasites as well as on other zeolites. Infrared evidence indicated the formation of XtO, J S 0 2 , and NOS- groups on cationic X zeolites when NO was adsorbed.21 There may, in fact, be competition for certain sites between NO and NzO or N02. This competition for sites by oxides of nitrogen could explain why the YO spin concentration was much less than the maximum 02-spin concentration on the same divalent zeolites. It should be recalled that the 02- species was largely on sites other than the Y-[OH] defect.’.’ These sites (sites I1 cations, for example) may stabilize the 02- ion but cause the NO molecule to react. On the HY zeolites the NO spin concentrations exceeds the 0 2 - spin concentration, and both species are primarily at the Y-[OH] defect.22 This may mean that the Y-[OH] site adsorbs NO in the molecular form and does not promote further reaction. Conclusions The work presented here strongly suggests that lattice oxide ions are removed upon extensive dehydration of MgY, Cay, and SrY zeolites. The resulting defect sites, which adsorb NO, are quite similar to those found on dehydroxylated HY zeolites. Other investigators have reached a similar conclusion bmed mainly upon infrared data. A second type of adsorption site which does not involve aluminum ions is present on MgY, Cay, SrY, and BaY zeolites. Acknowledgment. The author wishes to acknowledge the support of this work by The Robert A. Welch Foundation under Grant No. A-257. (16) P. D. Hopkins, J . Catal., 12, 325 (1968). (17) J. W. Ward, J . Phus. C h e w , 72, 2689 (1968). (18) C. S. Plank, Proc. Int. Congr. Catal., Srd, Amsterdam, 1964, 727 (1965). (19) J , Turkevich, F. Nozaki, and D. Stamires, (bid., 516 (1965). (20) W. E. Addison and R. M. Barrer, J . Chem. Doc., 757 (1955). (21) A. V. Alekseyev, V. N. Filimonov, and A. N. Terenin, Dolcl. Akad. Nauk SSSR, 147, 1392 (1962). (22) K. M. Wang and J. H. Lunsford, J . Phys. Chem., 73, 2069 (1969).
