The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
1174
P. A. Jacobs and H. K. Beyer
Evidence for the Nature of True Lewis Sites in Faujasite-Type Zeolites Peter A. Jacobs* Centrum voor Oppervlaktescheikunde en Collojdale Scheikunde, Katholieke Universiteit Leuven, De Croylaan 42, 8-3030 Leuven (Heverlee), Belgium
and Hermann K. Beyer Central Research Institute of Chemistry, Hungarian Academy of Sciences, 1 1 Pustaszeri-fit 57-69, Budapest, Hungary (Received September 5, 1978)
Experimental evidence was gathered for the nature of Lewis sites in copper-Y zeolites. The zeolite was subjected to several CO-O2 redox treatments. The kinetic behavior of the latter zeolite upon H2and CO treatment was compared to that of the original sample. From this the formation of A10’ and Cu2+-02--Cu2+was derived. Their existence was further confirmed using IR spectroscopy and treatment with 1802.It is concluded that Lewis sites are not trigonal A1 atoms in zeolites, but consist rather of A1-0 species leached from the zeolite framework.
Introduction Heat treatment of hydrogen zeolites results in the dehydroxylation of surface hydroxyl groups. This can be easily detected by infrared and gravimetric techniques. Based on these observations, Uytterhoeven, Christner, and Hall’ advanced the famous dehydroxylation scheme shown in eq 1. Trigonal aluminum atoms were thought to act
cu2’
0
2 A1 f
0 \
\ - 1
/
Si
/
I
\
2 Si
/
/
A1 \
\
0
A1
cu2
0
0 \
AI
\ I \ / \ /
’/,O,
+ cut
0
Si
Si’ A1
+
l \ l \ / \ l \
I \
-
\- / A T
/
Si
H 0
0
\
\ /
l A t \
I
\
\
\
I
Si -+ Si /
l
A1 Si t
2 A1
\ / \
Si I
\
\
A1 /
\
/
Si /
+ H,O
(1)
\
as true Lewis acid sites.l The species are easily visualized as adsorption centers for molecules with electron donor capabilities and explain the infrared vibration of adsorbed pyridine around 1455 cm-l. The Lewis centers were never reported to act as active sites in carbonium ion catalyzed reactions (for a recent review of the state of the art see ref 2-4). There seems to be an optimal concentration of Lewis sites, influencing the strength of the residual Brfinsted groups through induction via the zeolite l a t t i ~ e . ~ Kuh16 however using X-ray fluorescence techniques was unable to detect experimentally tricoordinated aluminum in dehydroxylated hydrogen-Y zeolite. It was found that 28-44% of the aluminum was hexacoordinated and the dehydroxylation scheme shown in eq 2 was advanced. This 0
H I
0
0
0
Si
0
A1 ( 3 )
Si
/ \ / \ / \ I \
Si
0
A1 t CO+CO,
Si
I \ / \ / \ /
0
I \ - I
\
I
A1
cut
0
+
t
0
H
0
0
\
0
0
+ cut
0
0
0
cut
I \ I \ - I \ I \ / \ - / Si Si’ A1 t A1 Si Si A1 ( 4 ) \ / \ I \ / \ I \ / \ / \ I \
\ - I \
A1 I
occur^.^ It is straightforward that the charge compensating cations due to their electron acceptor properties also are Lewis acid sites. The sites pictured in eq 1-4 will therefore be denoted as true Lewis sites (TLS). Upon oxygen treatment reactions 3 and 4 are found to be reversible, at least when the valence state of the transition metal ion is c o n ~ i d e r e d ,while ~ ~ ~ in a dehydroxylated H-Y with water a rehydroxylation is not obtained. We therefore report in this work physicochemical measurements in Cu(I1)-Y zeolite that allow us to characterize experimentally in more detail the nature of the true Lewis sites in zeolites. Experimental Section Materials. Starting from a Linde Na-Y sieve, a Cu(I1)-Y zeolite was prepared with the following anhydrous unit cell composition: Na17.5CU18,6(A102)54.7(Si02)137.5
situation is not found to be essentially different in Hr n ~ r d e n i t e .AS ~ shown in infrared studies with pyridine as a probe, identical Lewis sites can also be formed in Y zeolite exchanged with cupric ions. Upon thorough degassing of Cu(I1)-Y, autoreduction was observed,8 eq 3, while upon reduction with carbon monoxide reaction 4 0022-3654/79/2083-1174$0 1.OO/O
All the precautions taken to prepare a “stoichiometrically” exchanged sample were previously described in detail.1° Hydrogen, oxygen, and carbon monoxide were of ultra-high purity from Matheson Gas Products. Before use, they were catalytically treated to remove possible impurities. “So2 of 99% purity was from LCB (Belgium). Pyridine from Fluka was distilled before use. 0 1979 American Chemical Society
The Journal of Physical Chemistty, Vol. 83, No. 9, 1979
True Lewis Sites in Faujasite-Type Zeolites
cu'
cu'
AL SI AL !jl SI AL !SI AL-AL
/ \ /\ / \ /\ /\
0 I
\
/\
/\
ooooooooodoo;oo 0 I I I 1 / I 1 1 I1 I1 I1 I
Cu'
(ALO)+
cu'
51 SI SI AL SI AL / \ / \ I\ /\ I\ / \ 1 1 0 00 000000000 0 0 I
1175
E-----
I 1 l 1 \ 1 l l I I l I I 1
"0
303
7
1200
930
603 T I M E / S
CU' H 0 0 0 0
0 0 0 0
/\-/\/ d \ l \ / \ / \ l \ - P \
AL SI AL I:!
SI AL 9 AL / \ / \ I \ / \ / \ / \ /\ / \ 0 0 0 00 00 00 co 0000 0 I
I I l l
1 1
I1 1 1 I l l 1 I
(V)
p-0
@LO)+
0 0 0 0 0 0-0 \ / \ / \ / \ AL SI 51 SI Si AL SI AL / \ / \ \ \ I\ /\ I\ I\ / \ 0 00000000000 00 00 0
I \ - / \ / \ / \/ \
I
I I l l \ l I l l I I I
I l l 1 I
(IV)
Figure 2. Reduction of Cu(I1)-Y zeolite with carbon monoxide at 673 K after different pretreatments: a, fresh sample dehydrated at 673 K; b, treated with oxygen at 673 K (0)and after one redox cycle (treatment CO followed by 0,)(0);c, treated with CO for 1 h at 723 K and reoxidized at 673 K; d, reduced with CO for 1 h at 823 K and reoxidized at 673 K. The dashed line corresponds to uptake of 1 CO per 2 CU'+.
Figure 1. Schematic representation of several redox reactions occurring in Cu(I1)-Y zeolites.
Methods. Kinetic measurements of hydrogen reduction are done in a recirculation reactor. The precautions taken to avoid overheating were described earlierelo The reduction with CO and treatment with 1802 is done in the same reactor, attached to a Balzers quadrupole mass spectrometer, using a sampling valve with a very low internal volume. Infrared measurements were done on a Beckman IR12 spectrometer in the double beam/absorbance mode. The resolution at 3700 crn-l was 5 cm-l and at 2000 cm-' it was 2 cm-l.
Results and Discussion The ideas a t the basis of this work are schematically represented in Figure 1 for Cu(I1)-Y zeolite. Upon treatment of the Cu(I1)-Y zeolite (I) with carbon monoxide, lattice oxygen is removed as carbon dioxide and true Lewis sites have to be f ~ r m e d .The ~ TLS sites can be represented using either the formalism proposed by Uytterhoeven et al.' (species 11) or by Kuh16 (species 111). If I11 is the final product, I1 may well be a metastable intermediate. Upon hydrogen treatment of Cu(I1)-Y under mild conditions, structure V containing Cu+ ions and surface OH groups is formed.lOJ When CO reduction and reoxidation precede the mild hydrogen treatment, different species will be present in the final material. Depending on whether structure I1 or I11 represent the nature of the Lewis sites, the final zeolite will contain V or VI, respectively. In the following paragraphs experimental evidence will be advanced for the nature of the Lewis sites. This will be derived from the probabilities at which the different reaction pathways of Figure 1 occur. Euidence for [A10]+and [CU-O-CU]~+ Species in Cu-Y Zeolites. Reduction with Carbon Monoxide. The first suspicion that successive treatments of Cu(I1)-Y with CO and O2 does not restore the initial zeolite (species I) can be derived from the data of Figure 2. In every case 1COz is formed per CO consumed indicating that no carbon is left on the surface. Reduction with CO at 673 K of Cu(I1)-Y which has undergone different pretreatments is given. A freshly dehydrated sample which has never been exposed to oxygen a t elevated temperature is reduced according to curve a. Reoxidizing this sample at the same temperature and reducing it again gives curve b. The final degree of reduction remains unchanged, but the rate at which this is reached has increased. This clearly shows that a conversion from species I to I1 and vice versa has not occurred. Curves c and d (Figure 2) represent a
0
3
9
6 T IM E
12
s x 103
Flgure 3. Reduction of Cu(I1)-Y zeolite with hydrogen at 473 K after different pretreatments: a, outgassed at 673 K (0),followed by one H,/O, cycle at 473 K and dehydration at 673 K (0); b, treated with CO at 773 K for 1 h and reoxidized at 673 K; c, treated with CO at 823 K and reoxidized at 673 K.
pretreatment in CO at 723 and 823 K, respectively. After oxidation under the same conditions, the sample treated in CO at the higher temperature becomes reducible to a greater degree. If upon CO reduction of Cu(I1)-Y, structure I11 is obtained rather than I1 (Figure l),the difference in reduction rate in the first and further redox cycles is explained if it is assumed that the (CU-O-CU)~+species loose oxygen faster than the zeolite lattice. All this constitutes a first indication that the nature of the TLS is of the type proposed by Kuhl.6 The reversibility of the system after the first redox cycle during successive CO/02 treatments is easily explained by the interconversion of species I11 and IV. Reduction with Hydrogen. It is known that under mild conditions cupric ions in Y zeolite can be reduced selectively into cuprous i0ns.l' It may be expected that cupric ions and (CU-O-CU)~+species will be reduced at a different rate. Therefore, in Figure 3 is a comparison of the reducibility of these two species with hydrogen under mild conditions. The concentration of possible Cu-0-Cu species is gradually increased using more severe CO treatments followed by reoxidation. It is clearly seen that when the CO treatment becomes more severe and, as required by Figure 1, the contribution of species IV increases, the degree of reduction with hydrogen at 473 K increases gradually. This confirms the previous statement about the reducibility of Cu2+ ions in the lattice and (CU-O-CU)~+species in the zeolite. Figure 3 also shows that structures I and V can be interconverted under mild conditions (curve a). Detailed evidence for this was advanced earlier.ll
The Jmrnal of Physical Chemistry, Vol. 83,
No. 9, 1979
P. A. Jacobs and H. K. Beyer
16
15
1 c nl.1 x 10.2
I
I
17
16
1
c nl-l x 1 6 2
Figure 4. Infrared spectra at room temperature of Cu(I1)-Y treated with hydrogen at 473 K for 2 h after the following treatments: a, dehydration at 673 K; b, dehydration at 723 K, CO treatment at 723 K for 1 h and O2treatment at 673 K (1 h); c, same as b, but treated at 773 K and d, at 823 K. Thickness of the different films = 5 f 0.3 mg cm-2.
If the proposed schemes for interconversion between I and V and between I11 and IV are true, other differences have to exist. (i) Upon hydrogen treatment of IV, protons are not fixed in the zeolite lattice but are removed as water. The difference in reaction stoichiometry upon hydrogen treatment of I and IV can easily be seen using infrared spectroscopy. Several samples of Cu(I1)-Y were subjected to different carbon monoxide-oxygen treatments and then reduced with hydrogen a t 473 K. As already known, species I is transformed into V, protons are fixed in the lattice, and no water is formed. This is confirmed again (Figure 4a). The treatment with CO at increasing temperatures according to our previous data (Figures 2 and 3) results in an increase of the number of 111which upon oxidation are converted into IV. Figure 4b-d shows that upon hydrogen treatment fewer hydroxyl groups are present (bands at 3640 and 3550 cm-l) and more water is physically held in the lattice (aHzOaround 1640 cm-'). This again is in favor of the existence of true Lewis sites as proposed by KuhP since it provides evidence for the interconversion of I11 and IV. (ii) Further discrimination between structures I1 and 111can be made using infrared spectroscopy with pyridine as probe. Upon oxygen treatment of 11, the lattice vacancies are supposed to be filled and the TLS concentration should decrease. On the other hand, if IV is formed upon oxidation of 111, the concentration of (A10)+species should remain unchanged. That the latter is true is shown in Figure 5. Initially, pyridine is shown to interact only with Cu2+ ions (1450 cm-', spectrum a). When lattice oxygen is removed, a new band appears at 1455 cm-' (b) indicative of TLS, while the Cu+ ions formed also seem to adsorb pyridine (shoulder a t 1447 cm-'). Upon reoxidation (spectrum c), the concentration of TLS is hardly changed. This is expected if a structure similar to I11 (Figure 1)represents the nature of the TLS. The existence of a structure such as IV is further proved in spectrum d (Figure 5). Upon hydrogen treatment of structure IV, the (A10)' concentration remains unchanged. Since there are still Cu2+ions present as such, it is logical that the same treatment also gives supplementary Cu+ ions and H+ ions, the latter upon pyridine adsorption form pyridinium ions (1550 cm-l). Spectra f and g explain an earlier ob~ervation.~ In the presence of TLS and HzO, pyridine adsorbed on Lewis
Figure 5. Infrared spectra of pyridine adsorbed and evacuated at 423 K on Cu(I1)-Y after different treatments: a, degassed at 673 K; b, treated with CO at 723 K; c, reoxidized at 623 K; d, hydrogen treated at 473 K and degassed at 673 K; e, reoxidized at 673 K and CO treated at 773 K; f, 1 mmol/g of water adsorbed at 423 K.
sites is transformed into the corresponding ions. This may be explained according to (-410)'
+ HzO + Al(OH)Z+
A1(OH)z++ pyridine + pyridine H+
+ AlOOH
(5) (6)
A1(OH)z+seems to be a very weak acid, since a strong base is required for its dissociation. Kuhl' showed that A10+ can hydrolyze according to A10+ + H20
4
AlOOH
+ H+
(7)
if dehydroxylated mordenite is treated with 0.1 N NaOH. It is therefore possible that in some reactions A1(OH)2+can function as a proton donor. This was suggested by Breck et al."J3 for dehydroxylated Y zeolites. In the presence of high water partial pressures (A10)' may hydrolyze and polymerize. Species may be formed as proposed for deep-bed calcined and ultrastable NH4-Y and mordenite.7J4J5Under these conditions, the Cu-0-Cu species may undergo similar transformations. Evidence for the Nature of Cu-0-Cu Species. From the data already presented, it is evident that upon oxidation of Cu(1)-Y zeolite derived from Cu(I1)-Y, Cu(0)Cu species are formed. Using labeled oxygen molecules, it should be possible to gather more information about their nature. In earlier work,16 it was found that oxygen was chemisorbed on Cu(I1)-Y. The nature of the species was proposed as 0 Cuz+-0-0
'i
and Cu2+ \
0
This type of chemisorbed oxygen could be desorbed around 573 and 693 K, respectively. Figure 6B shows that oxygen adsorbed below 523 K is indeed adsorbed in a nondissociated form. The concentration of this species increases when more Cu+ are preformed using a CO treatment. Most probably the following reaction occurs: cu+ + 0 2 (CU2+ 02-) (8)
-
This is more probable than the sites proposed by Iwamoto et a1.16 The formation of 0, under similar conditions was proposed several time~.l'-'~ It should be noted that when protons are in the solid, Cu+ ions are reoxidized completely at 523 K." The mechanism for this reaction is proposed to occur over a
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
True Lewis Sites in Faujasite-Type Zeolites
1177
6A). This indicates clearly that under these conditions species similar to Cu2+-Oz--Cu2+are formed. Figure 7 shows that only this form of dissociatively adsorbed oxygen is capable of interacting with carbon monoxide. The oxygen molecules chemisorbed as such do not interact with CO but are simply desorbed.
T E M P IK
Flgure 6. Desorption of isotopic oxygen molecules from Cu(I1)-Y: a, outgassed at 773 K; b, reduced with CO for 2 h at 773 K. A, 180180 adsorbed at 623 K and B, ,at 523 K: (-) 1sO'80;(- - -) 1s0180; (- - -)
-
180160
Conclusions The model for true Lewis sites in zeolite Y proposed by Kuh16 on the basis of X-ray fluorescence data is strongly confirmed in the present work on Cu(I1) exchanged zeolites: upon reduction of Cu(I1)-Y with CO, Cu(1) and (A10)+ are formed; upon reoxidation the concentration of (AlO)+ remains unchanged, while Cu(1) ions are transformed into the Cu2+-02--Cu2+species. The different reactivity and stoichiometry of reaction of the latter species with CO and H2is demonstrated. As a general conclusion, it is clear now, that the so-called true Lewis sites are best represented as A1-0 species external to the lattice. No evidence is found for the existence of trigonal aluminum atoms. Acknowledgment. P. A. Jacobs acknowledges a permanent research position as "Bevoegdverklaard Navorser" from N.F.W.O., Belgium. Financial support from the Belgian Government (Diensten voor Programmatie van het Wetenschapsbeleid) is also acknowledged. References and Notes
T E M P I K
Flgure 7. Reduction with CO of Cu(I1)-Y after a, degassing at 773 K and b, treating with CO at 773 K for 1 h in each case, followed by at 623 K, cooled in 180'80 at 473 K and degassed: treatment with rs0180 (-) 180'80; (---) C'801y).
few Cu+ ions that are able to dissociate oxygen. The formation of Cu2+-02--Cu2+ is proposed to be rate determining. Figure 6B shows that dissociative adsorption at 523 K has occurred to a limited extent, since around 740 K ls0l8O is desorbed. When oxygen is adsorbed on species I11 (Figure 1)at 623 K, only ls0l8O species are desorbable around 740 K (Figure
(1) J. B. Uytterhoeven, L. G. Christner, and W. K. Hall, J . Phys. Chem., 69,2117 (1965). (2) J. A. Rabo, ACS Monog., No. 171 (1976). (3) P. A. Jacobs, "Carboniogenic Activity of Zeolites", Elsevier, Amsterdam, 1977. (4) R. Rudham and A. Stockwell, "Catalysis l", The Chemical Society, London, 1977,Chapter 3. (5) J. H. Lunsford, J . Phys. Chsm., 72,4163 (1968). (6) G.H. Kuhl in "Molecular Sieves", J. B. Uytterhoeven, Ed., Leuven University Press, 1973,p 227. (7) G. H. Kuhl, ACS Symp. Ser , No. 40,96 (1977). (8) P. A. Jacobs, W. De Wilde, R. Schoonheydt, J. 6. Uytterhoeven, and H. Beyer, J. Chem. Soc., Faraday Trans. 7, 72, 1221 (1976). (9) C. M. Naccache and Y. Ben Taarit, J . Catal., 22, 171 (1971). (10) P. A. Jacobs, M. Tielen, J. P. Linart, J. B. Uytterhoeven, and H. Beyer, J . Chem. Soc., Faraday Trans. 7, 72,2793 (1978). (11)R. G. Herman, J. H. Lunsford, H. Beyer, P. A. Jacobs, and J. B. Uytterhoeven, J . Phys. Chem., 79,2388 (1975). (12) D.W. Breck and G. W. Skeels, Proc. Inf. Congr. Catal. 6fh, 2,645 (1976). (13) D. W. Breck and G. W. Skeels, ACS Symp. Ser., No. 40,271 (1977). (14) P. A. Jacobs and J. 8. Uytterhoeven, J. Chem. Soc., Faraday Trans. I, 69,373 (1973). (15)J. Scherzer and J. L. Bass, ,J. Catal., 46, 100 (1977). (16) M. Iwamoto, K. Maruyama, N. Yamazoe, and T. Seiyama, J . Phys. Chem., 81,622 (1977). (17) T. Imai and H. W. Habgood, J. Phys. Chem., 77, 925 (1973). (18) Y. Ono, K. Suzuki, and T. Keii, J. Phys. Chem., 78,218 (1974). (19) S.Krzyzanowski, J. Chem. Soc., Faraday Trans. 7,72,1573 (1976).
