- Email: [email protected]
FTIR study of the effects of water pretreatment on the acid sites and the dispersion of metal particles in Y zeolites and mordenites
Jmmnal of Mokular
Catalysis,
233
71 (1992) 233-244
M2737
FTIR study of the effects of water pretreatment on the acid sites and the dispersion of metal particles in Y zeolites and mordenites Leonid M. Kustov* and Wolfgang M. H. Sachtler V. N. Ipati@ Laboratarg, Center for Catalysis University, Evanston, IL 60208 (USA)
and Su?$ace Science,
Northwestern
(Received April 11, 1991; accepted June 12, 1991)
Abstract FUR results on zeolite-supported Pt and Pd show that the presence of water during metal reduction modifies both the acid and the metal functions of these catalysts. The water treatment eliminates Lewis acid sites and lowers the concentration of Bronsted acid sites by partial deahrmination. At low temperature CO is adsorbed on Bronsted sites; the position of the corresponding FTIR band indicates that for Pt/HMOR, (MOR = mordenite) unlike Pt/HY, the intrinsic acid strength of these sites is increased by the wet reduction procedure. FTIR spectra after CO adsorption at room temperature show that wet reduction markedly improves the dispersion of Pt in HMOR; this effect is weaker for HY and absent for the Na forms of the zeolites. Bands of ga-Pt(CO),, which are indicative of very small, possibly electron-deficient Pt clusters, are detected in Pt/HMOR.
Introduction
The catalytic behaviour of bifunctional catalysts is determined by the presence of acidic centers (Brgnsted and/or Lewis) and metal particles, as well as by interaction between the acid and metal functions. Hydrothermal treatment of zeolites is known to affect their acidic properties via partial dealumination and alteration of aluminum ordering in the framework [ 1, 2 ]. Therefore, this kind of pretreatment is widely used to modify catalytic activity and selectivity of acidic forms of zeolites. For instance, partial dealumination of high-silica zeolites strongly enhances their activity in paraffin conversions [ 3-5 1. Little information is, however, available on how the presence of water during reduction affects the acidity and properties of the metal such as its reducibility, dispersion or electronic. state. For Ni, Pd and Pt on Y zeolites, it was shown that reduction with moist hydrogen results in a lower metal dispersion [6, 71. The present research is devoted to the study of these effects for Pt and Pd particles supported on the decationized and the sodium forms of zeolite *On leave from N. D. Zelinsky Institute of Organic Chemistry, Academy of Sciences, Moscow, Russia.
0304-5102/92/$5.00
0 1992 - Elsevier Sequoia. All rights reserved
234
Y and mordenite. This work was initiated by AKZO Chemical Division Research Centre, Amsterdam, The Netherlands [8, 91. IR spectroscopy is one of the most informative methods for the investigation of acid sites in zeolites [lo]. The use of adsorbed probe-molecules, e.g. CO, provides additional information about the state and dispersion of reduced metal particles in zeolite cavities [ll, 121. At low temperature, the same molecular probe (CO) can also be used for the investigation of Lewis and Brensted acid sites which are able to form complexes with such weak bases [ 13 ]. This approach has been used in the present work to study the effect of water treatment on the properties of mordenites and Y-type zeolites with supported Pt and Pd.
Experimental
Pt/HMOR (AKZO No. 90-BK-444-101; Si/Al=8.8; 0.56 wt.% Pt) and Pt/ HY (AKZO No. 90-BK-444-102; Si/Al=4.75; 0.53 wt.% Pt) were provided by AKZO, Chemical Division, Research Centre Amsterdam, The Netherlands. Pt/NaMOR (Si/!= 9.3; 1 wt.% Pt), Pt/NaY (SW = 2.4; 5 wt.% Pt) and Pd/ NaY (Si/Al= 2.4; 4.7 wt.% Pd) were prepared by ion exchange of the H or Na form with dilute aqueous solutions of [Pt(NH&](NO& and [Pd(NH,),](NO,), at room temperature. The I-IYzeolite used for the preparation of Pt/HY was partially deahnninated. Thin self-supported wafers of the samples, without binders, with a diameter of 8 mm and a mass per cross-sectional area of - 7-13 mg cmW2 were pressed and placed into the sample holder of the IR cell. The low temperature IR cell used for the study of Pt and Pd-containing zeolites was described in [4]. The spectra were measured at 80 and 300 K using a Nicolet 60SX Fourier-transform spectrometer with a resolution of 1 cm- ‘. Calcination, water treatment, and reduction, were carried out in situ. Before the spectroscopic measurements, the samples were calcined from 300 to 770 K at 0.5 K min- ’ and then holding the temperature at 770 K for 2 h in a flow of oxygen (120 ml min- I). After calcination the system was purged with He at 770 K for an additional 20 min and cooled to 300 K in He. Following this purge, the samples were ready for reduction. The He flow was replaced by a hydrogen flow (60 ml min-‘). Metal reduction was initiated by ramping the temperature from 300 to 620 K at 8 K ruin-’ and holding the temperature at 620 K for 20 min. The samples were subsequently purged at 620 K with He for 20 min and cooled to room temperature. These samples will be denoted as ‘dry’. Prior to the reduction, if water treatment was desired, the samples were saturated by a He flow containing 15 torr of water vapor for 30 min. These samples will be denoted as ‘wet’. After the reduction, the background spectra of the reduced samples were measured. The samples were then cooled to 80-90 K and a He/CO mixture with a volume ratio of 30:5 and a total flow rate of 70-80 ml mir- ’ was passed through the cell for 10 min. The cell was subsequently purged with
235
He for 10-20 min to remove gaseous CO. The same procedure was repeated at 300 K.
Results
and discussion
Acid sites in zeolites Hydroxgl groups In all figures, spectra of the ‘wet’ samples will be denoted by the symbol (1) and those of the ‘dry’ samples by (2). The IR spectra of IWHY and Pd/ NaY are shown in Fig. 1 with (a) denoting Pt/HY and (b) Pd/NaY. The narrow bands at 3650-3615 cm-’ and the broad band at 3550 cm-’ are conventionally ascribed to bridging acid hydroxyls (I):
According to literature data [lo], the bands at 3650-3615 and 3550 cm- ’ are attributed to bridging groups located in supercages and hexagonal prisms, 3635
3560
O.lA
3285 I
I 8 f B 8 z
2b
J?ig. 1. IR spectra of OH-groups in FVHY (a) and Pd/NaY (b) reduced in the presence (1) or absence (2) of water.
F’ig. 2. IR spectra of ‘wet’ (1) and ‘dry’ (2) Pt/HY (a) and Pd/NaY (b) samples measured after adsorption of CO at 80-90 K. The corresponding spectra of the samples before CO admission are subtracted.
236
respectively. The hydroxyls characterized by the band at 3550 cm- ’ are usually considered to be hydrogen atoms bonded to oxygen atoms of the framework and inaccessible to adsorbed molecules. In addition to acidic OH-groups, low-intensity bands at 3670 and 3720 cm-’ were found; they are usually assigned to extra-framework AlOH species [ 1, 2, 4, lo]. Terminal silanols responsible for the band at 3740 cm- ’ are also present in the samples. Water treatment followed by reduction at 620 K results in a -20% decrease in the concentration of the bridging OH-groups of different types in Pt/HY, as evidenced by a decrease in the intensities of the corresponding bands (3635,3615 and 3550 cm-‘). Unlike bridging hydroxyls, the intensities of the OH-groups attributed to extra-framework al~inium (3670 and 3720 cm- ‘) increase after the water treatment. This shows that partial dealumination of Pt/HY takes place, presumably in the course of the ‘wet’ reduction. In the spectra of ‘wet’ and ‘dry’ Pd/NaY reduced at 620 K, the narrow band at 3645 cm-’ predominates due to the bridging OH-groups located in the supercages; these protons are a co-product of metal reduction. The presence of water during reduction has no significant effect on the intensity of these bands. No additional bands appear in the ‘wet’ sample of Pd/NaY as compared with its ‘dry’ counterpart. Similar spectra were observed for ‘wet’ and ‘dry’ Pt/NaY. However, in this case the intensity of the band at 3650 cm-’ which appeared after reduction was lower than for Pd/NaY, because of the lower molar concentration of Pt. As in the case of Pd/NaY, the presence of water during reduction had practically no effect on the spectral pattern. Low temperature adsorption of carbon monoxide probes for the acid strength of hydroxyl groups in zeolites [ 131. Absorption of CO at liquid nitrogen temperature leads to formation of weak complexes between CO and acidic hydroxyls; they are characterized by broad bands at V< 3500-3200 cm-‘. The bands are attributed to the stretching vibrations of O-H bonds in hydroxyl groups perturbed by CO. The extent of the shift towards lower wavenumbers of this broad band, with respect to isolated hydroxyls, is a direct measure of the acid strength of Bronsted sites. In Pig. 2 difference spectra for ‘wet’ and ‘dry’ Pt/HY and Pd/NaY are presented; they were obtained by subtraction of the spectrum before CO admission from the spectrum after CO admission. Bands with negative intensities, thus, correspond to those OH-groups which disappear after CO adsorption due to formation of complexes. Broad bands with positive intensities are attributed to hydroxyls perturbed via H-bonding to CO molecules. The most intense band at 3285-3295 cm-’ for Pt/HY and at 3410 cm-’ for PdMaY could be assigned to CO complexes with bridging OH-groups. The shoulder at 3465 cm-’ which is more intense for the ‘wet’ PtJHY, and a band at 3580 cm-’ appear to be due to interaction of CO with less acidic OH-groups, i.e. OH-groups connected with extra-framework aluminium (v=3670 and 3720 cm-‘) and silanol groups (~“3740 cm-‘) respectively.
237
be seen in Fig. 2 for both ‘wet’ and ‘dry’ samples of Pt/I-IY, the maxima of the broad bands are close to each other. For Pd/NaY or Pt/NaY reduced in the presence or absence of water vapor the spectra are almost identical. We, therefore, conclude that the acid strength of the bridging OHgroups in Pt/HY, Pd/NaY and Pt/NaY is not changed significantly by the water treatment. The IR spectra of the reduced RA-IMOR and Pt/NaMOR prepared by ‘wet’ and ‘dry’ reduction procedures are shown in Fig. 3. The band at 3610 cm-‘, which is the most intense band in the spectrum of Pt/HMOR, is ascribed to bridging OH-groups [ 11. Silanol groups (3740 cm-‘) are also present in both samples. Also an unresolved band near 3680-3720 cm-’ appears in the spectrum of ‘wet’ Pt/HMOR; it is attributed to OH-groups connected with extra-framework aluminum [ 1, 2, lo]. The concentration of the bridging OH-groups is approximately 40-50% lower in ‘wet’ Pt/HMOR than in the ‘dry’ sample. There is also a broad band at YN 3300-3400 cm-’ in the spectrum of the ‘wet’ Pt/I-IMOR which is absent in the spectrum of the ‘dry’ sample. It may be attributed to water coordinated to Lewis acid sites. This hypothesis is confirmed by the presence of the band of the bending vibrations of water at 1640 cm- 1 in the spectrum of ‘wet’ zeolite (not shown in Fig. 3). In contrast to Pt/HMOR, the presence of water vapor does not influence the Bronsted acidity of Pt/NaMOR; the spectra of both ‘wet’ and ‘dry’ Pt/ NaMOR are identical. As
can
3270
3605
n 0.05A
1
Fig. 3. IR spectra of Pt/HMOR (a) and PtMah$R (2) of water.
@) reduced in the presence (1) or absence
Fig. 4. lR spectra of ‘wet’ (1) and ‘dry’ (2) FVHMOR (a) and PtMaMOR (b) measured after adsorption of CO at 80-90 K. The corresponding spectra of the samples before CO admission are subtracted.
238
Figure 4 shows the difference spectra measured after adsorption of CO at 80-90 K on Pt/HMOR and IWNaMOR zeolites. Negative peaks show the disappearance of the bands of isolated bridging OH-groups (3610 cm-‘) which form complexes with adsorbed CO. These complexes are characterized by broad bands at 3270-3215 cm-’ for Pt/HMOR and at 3375 cm-’ for Pt/NaMOR. The shifts toward lower frequencies for PtJHMOR, relative to the corresponding bands for Pt/HY zeolites (~‘3285-3295 cm-‘), clearly indicate a stronger acidity of hydroxyl groups in IW-IMOR than in Pt/HY zeolites. This is consistent with the literature data on the acidity of Hmordenites and HY zeolites [ 1, lo]. The band due to the complexes of OH-groups with CO is shifted in the ‘wet’ Pt/HMOR to 3215 cm-‘, whereas the corresponding band observed for the ‘dry’ sample peaks is at 3270 cm-‘. This fact evidences an enhancement of the acid strength of the OH-groups in Pt/HMOR due to the water treatment before the reduction, as well as a - 40-50% decrease in their concentration. Unlike PVHMOR, no band shift is observed for Pt/NaMOR; i.e. the bands corresponding to the complexes of CO and bridging hydroxyls exhibit the same maxima. It follows that for Pt/NaMOR the acid strength of the OH groups remains unaffected by the water treatment.
Lewis acid sites in mm-de&es metals
and Y-zeolites with supported
Lewis sites, unlike OH groups, do not exhibit characteristic bands in IR spectra; their study, therefore, requires adsorption of suitable molecular probes. Low temperature adsorption of carbon monoxide has been used for the identification of Lewis acid sites in zeolites [ 131. Due to the presence of an unoccupied p-orbital, Lewis acid sites are able to interact with CO molecules. This leads to a shift towards higher frequencies of the stretching vibrations of C -0 bonds in comparison to gaseous CO (2143 cm- ‘). The bands attributed to such complexes are usually located near 2240-2 180 cm-‘, the value of the high frequency shift being a measure of the acid strength of the Lewis acid sites. In Pigs. 5-9 the spectra of ‘wet’ and ‘dry’ samples of PVHY, PVHMOR, Pt/NaY, Pt/NaMOR and Pd/NaY, respectively, are presented in the region of stretching vibrations of adsorbed CO. The bands below 2 143 cm-’ are attributed to metal carbonyls and will be discussed in the next paragraph. No bands above 2180 cm-’ which might be ascribed to complexes of carbon monoxide with Lewis acid sites were found in the spectra of the sodium forms, i.e. for Pt/NaMOR, Pt/NaY and Pd/NaY. However, for the H forms of these zeolites, i.e. Pt/HY and PtkIMOR, a characteristic band at 2228 cm-’ due to complexes of CO with strong Lewis acid sites is present for the ‘dry’ preparation. Its intensity is 4-5 times higher for the ‘dry’ mordenite than for ‘dry’ Pt/HY. This band is not observed in the spectra of ‘wet’ samples. Thus, the presence of water before or during reduction removes strong Lewis acid sites in both Pt/HMOR and PWHY. Presumably water is strongly adsorbed on these sites, and it is not desorbed
239
2087
1
\ Fig. 5. IR spectra of ‘wet’ vibrations.
(1) and ‘dry’ (2)
Pt/HY zeolites
Fig. 6. IR spectra of ‘wet’ (1) and ‘dry’ (2) PtMMOR
in the region
I
8
5 B t?
1986
!
0.02A 2066, P,l
:
Fig.
7.
IR
spectra of CO adsorbed
Fig. 8. IR spectra of CO adsorbed
2
of CO stretching
in the region of CO stretching vibrations.
024
0.2A
‘\ ‘._-’
on ‘wet’ (1) and ‘dry’ (2) F’t/NaY. on ‘wet’ (1) and ‘dry’ (2) PtMaMOR.
2114 I? 0.05Aj
1900
Fig. 9. IR spectra of CO adsorbed on ‘wet’ (1) and ‘dry’ (2) Pd/NaY.
under the conditions of reduction (T= 620 K, t = 20 ruin) and further with He. Water molecules coordinated to Lewis sites probably create new Brgnsted acid sites: H\ H’
0
---3
L
(II)
In the spectrum of CO adsorbed on ‘wet’ Pt/HMOR, two additional bands at 2197 and 2182 cm- ’ appear. They are absent in the spectrum of the ‘dry’ zeolite and may be assigned to CO complexes with weaker Lewis acid sites created via partial deahunination of zeolite during water treatment and further reduction. State and dispersion of supported metals CO was also used to study the state and dispersion of Pt and Pd in zeolites. When CO is adsorbed on reduced metal particles, it forms carbonyl moieties which may be either linear or bridging [ 11, 121. For different PtContaining catalysts, mainly bands at 2050-2120 cm-’ due to single linear complexes were reported in the literature. Accordingly, the only band at 2090-2092 cm-’ observed in the spectra of CO adsorbed on ‘dry’ samples of Pt/HMOR (Fig. S), PtDIY (Fig. 5) and Pt/NaY (Fig. 7) could be assigned to single linear P&CO located on large Pt particles. Only a low-intensity shoulder at * 2125 cm-’ is found in the spectrum of Pt/HMOR. In ‘dry’ Pt/HY the dominant feature is the band at 2092 cm-‘, which is attributed to linear CO. After water treatment of Pt/I-IY, the intensity of this band increases and its maximum is shifted from 2092 to 2087 cm-‘. Also two bands at 2035 and 1996 cm-’ appear, which are ascribed to the antisymmetric and symmetric stretching vibrations of two CO molecules in linear ‘twin’ complexes of Pt:
241
c=o
Pt /
’
c=o
(III)
The presence of these ‘twin’ complexes is an indication of very high coordinative unsaturation of platinum, which suggests that water treatment leads to an enhanced metal dispersion. In addition, bands > 2100 cm-’ are observed. They are attributed to electron-deficient linear complexes, possibly CO on monoatomic or very small Pt particles: pt+--C=O
The band at 2072 cm-’ seems to be characteristic of neutral I% particles which are likely to be smaller than those responsible for the band at 2092 cm-’ observed for ‘dry’ Pt/HY. The effect of water treatment on the state and dispersion of Pt is most pronounced in the case of FWIMOR (Fig. 6). The intensity of the band due to linear complexes is higher for the ‘wet’ sample; the band maximum is shifted from 2090 cm-’ toward 2070 cm-‘. This frequency shift is much larger (20 cm-‘) than for Pt/I-IY (5 cm-‘). An additional intense band at 2124 cm- ’ in the spectra of the ‘wet’ sample is assigned to linear CO with electron-deficient, probably very small pt particles. The most drastic changes occur in the region of CO stretching vibrations of the twin F’t(CO)a complexes. They are absent in the ‘dry’ sample, but in the spectrum of ‘wet’ PtkIMOR two bands at 2033 and 1996 cm- ’ are visible, which may be ascribed to antisymmetric and symmetric stretching vibrations in twin complexes (III). This assignment is confirmed by the simultaneous disappearance of these bands upon purging with He. Table 1 summarizes the changes in intensities of different types of carbonyls which occur after preliminary water treatment of Pt/HY and Pt/ HMOR zeolites. It includes the intensities of the bands at 2128-2110 cm- I, those at 2092-2070 cm- ’ and the intensities of the symmetric band of twin complexes (1996 cm- ‘). TABLE
1
Intensities
of the IR bands of different
Pkarbonyls cm-’
Zeolite
Reduction conditions
2110-2130
IWHMOR Pt/HMOR PtiHY WHY
dry ivet dry wet
<0.005 -0.030
A A
-0.010
A
2090-2070
0.064 0.080 0.071 0.096
A A A A
cm-’
1996
cm-’
0.020 0.004 0.017
A A A
242
The data presented in Figs. 5 and 6 and Table 1 show that water treatment enhances the dispersion of platinum in the H-form of both zeolites and thereby creates new Pt species. These may include atomic Pt carrying a partial positive charge due to interaction with acidic protons, and small particles exposing strongly coordinatively unsaturated Pt atoms. This effect is much more characteristic for Pt/HMOR than for IWHY zeolite. Unlike Pt/HMOR or Pt/HY zeolites, the intensities of the bands corresponding to carbonyls of platinum are significantly decreased after water treatment preceding reduction in the case of Pt supported on NaY zeolite (Fig. 7). Simultaneously, the band at 2090-2000 cm-’ broadens, probably consisting of several unresolved bands. It should be noted that reduction of Pt/NaY in the absence of water vapour leads to the IR spectrum which exhibits the only narrow band at 2092 cm-‘, as in the case of Pt/HY or IWHMOR zeolites. This evidences that the dispersion of Pt decreases in Pt/ NaY reduced in a moist atmosphere, and that distribution of Pt particles is not as uniform compared to ‘dry’ Pt/NaY. For Pt/NaMOR (Fig. 8) and Pd/NaY (Fig. 9), the intensities of the carbonyl bands are only slightly lower in the spectra of the ‘wet’ zeolites than in those of the ‘dry’ zeolites. No new bands appear after reduction in moist atmosphere, and the intensity ratio of the bands remains essentially unchanged. The effect of water vapor during reduction is thus much smaller for the Na-forms of zeolites supporting Pt or Pd than for the H-forms of these zeolites. This pronounced difference in the effect of water treatment on the dispersion of reduced metals apparently reflects the different nature of the sites capable of strongly adsorbing water molecules. Only the H-forms contain Bronsted and Lewis acid sites in appreciable concentrations. Taking into account that the concentration of Lewis acid sites is much higher for Pt/ HMOR than for Pt/HY and that the effect of water treatment on the dispersion and properties of supported platinum is most pronounced for H-mordenite, we conclude that the presence of Lewis acid sites plays a key role in the reduction process in the presence of water vapor. The formation of very small particles of the reduced metal in the channels of H-mordenite in addition to rather large particles is consistent with the following three arguments. (1) The mordenite channels contain ‘side-pockets’ formed by eightmembered rings; these side-pockets are likely sites accommodating small (even atomic) metal particles. (2) As bridging hydroxyls in mordenites are stronger acids than those in Y-type zeolites, the anchoring and stabilizing effect of protons on the metal particles will be stronger for Pt/HMOR than for PVHY. (3) Due to the higher Si/Al ratio in mordenites, the distance between Pt ions prior to reduction may be larger in mordenites than in faqjasites. This might favor formation of larger Pt particles in Pt/HY. According to our previous data [14-171, reduced Pt particles exhibit weak mobility in zeolites. Their mobility may further be hindered by water
243
molecules coordinated to Lewis acid sites and by extra-framework species created by partial dealumination of zeolite.
aluminium
Conclusions The spectroscopic data clearly show that water treatment of Pt/HMOR and Pt/HY prior to their reduction affects both the acidic sites and the state and dispersion of the supported metal. This contrasts with the sodium forms of these zeolites: Reduction of Pt/NaY, Pt/NaMOR or Pd/NaY in a moist atmosphere does not lead to changes in metal state or a higher metal dispersion than reduction with dry Hz. The effects on the H-forms of the metal/zeolites of reduction in the presence of water are: 1) transformation of Lewis sites into Brensted acid sites via coordination of water molecules to low-coordinated ions; 2) decrease in the concentration of Brensted acid sites; 3) partial deahunination of the zeolite framework; 4) increase in metal dispersion and a change in the morphology or electronic structure of platinum, as evidenced by the appearance of linear or twin carbonyl complexes which are indicative of the presence of electrondeficient particles and coordinatively unsaturated Pt atoms. The modification of acid and metal functions is much greater for Pt/ HMOR than for Pt/HY zeolite. At present we are studying relations between structural effects, as described in this paper, and the catalytic performance in C5/C6 hydroisomerization. The results will be described in a subsequent paper.
Acknowledgement Financial support for this work by AKZO Corporate is gratefully acknowledged.
Research America
References 1 J. Schener, in T. E. Wlyte, R. Dalla Betta, E. G. Derouane and R. T. Baker (eds.), Catalytic Mat&k: Relationship between S&u&m-e and Rem&&~, ACS Symp. Ser. No. 248, Am. Chem. Sot., Washington D. C., 1984, p. 158. 2 J. Schener, CataL Rev. Sci. Eng., 31 (1989-1990) 215. 3 R. M. Lago, W. 0. Haag, R. J. Mikovsky, D. H. Olson, S. H. Hellring, K. D. Schmidt and G. T. Kerr, in Y. Murakami, A. Iijima and J. Ward (eds.), Proc. 7th Iti. Coqf Zeoliites, Tokyo, 1986, Elsevier-Kodansha, Tokyo, Amsterdam, 1986, p. 677. 4 V. L. Zholobenko, L. M. Kustov, V. B. Kazansky, E. Loeffler, U. L&se, C. Peuker and G. Oehlmann, Zeolites, 10 (1990) 304. 5 E. Brunner, H. Ernst, D. Freude, M. Hunger, C. B. Krause, D. Praeger, W. Reschetilowski, W. Schwieger and K.-H. Bergk, Zeolties, 9 (1989) 282. 6 P. Gallezot, Catal. Rev. Sci. Eng., 20 (1979) 121.
244 7 U. S. Pat. 4 139433 (1979) to J. W. Ward. 8 F. R. Mas Cabre, B. Klaver and R. G. Oudhaarlem,Abstr.
9 10 11 12 13 14 15 16
17
11th N. American
Meeting
CataL Sot., Dearborn, MI, May 7-l 1, 1989, C35. F. R. Mas Cabre, B. Klaver and R. G. Oudhaarlem, Catal. Symp. Dutch Cha. Sot., Temeuzen, The Netherlands, May 18-19, 1989. J. W. Ward, in J. Rabo (ed.), Zeoltie Chemistry and Catalysis, ACS Monograph 171, ACS, Washington DC, 1976, p. 118. N. Sheppard and T. T. Nguyen, in R. J. H. Clark and R. E. Hester (eds.), Advances in Iqfrared and Raman Spectroscopy, Vol. 6, Heyden, London, 1978. L. H. Little, Iqfrared Spectm of Adsorbed Molecules, Academic Press, New York, 1966, p. 428. L. M. Kustov, V. B. Kaaansky, S. Beran, L. Kubelkova and P. Jir&, J. Phys. Chem., 91 (1987) 5247. H. J. Jiang, M. S. Tzou and W. M. H. Sachtler, Appl. Cataf., 39 (1988) 256. M. S. Tzou, H. J. Jiang and W. M. H. Sachtler, React. Kin.&. Ca.taL I.&t., 35 (1987) 207. M. S. Taou and W. M. H. Sachtler in J. W. Ward (ed.), Froc. 10th North Am. Meeting Catalysis Society, San Diego, 1987,Vol. 38, Studies in [email protected] Science and Catalysis, Elsevier, Amsterdam, 1987, p. 233-241. M. S. Tzou, H. J. Jiang and W. M. H. Sachtler, Solid State Zonics, 26 (1988) 71.
Catalysis,
233
71 (1992) 233-244
M2737
FTIR study of the effects of water pretreatment on the acid sites and the dispersion of metal particles in Y zeolites and mordenites Leonid M. Kustov* and Wolfgang M. H. Sachtler V. N. Ipati@ Laboratarg, Center for Catalysis University, Evanston, IL 60208 (USA)
and Su?$ace Science,
Northwestern
(Received April 11, 1991; accepted June 12, 1991)
Abstract FUR results on zeolite-supported Pt and Pd show that the presence of water during metal reduction modifies both the acid and the metal functions of these catalysts. The water treatment eliminates Lewis acid sites and lowers the concentration of Bronsted acid sites by partial deahrmination. At low temperature CO is adsorbed on Bronsted sites; the position of the corresponding FTIR band indicates that for Pt/HMOR, (MOR = mordenite) unlike Pt/HY, the intrinsic acid strength of these sites is increased by the wet reduction procedure. FTIR spectra after CO adsorption at room temperature show that wet reduction markedly improves the dispersion of Pt in HMOR; this effect is weaker for HY and absent for the Na forms of the zeolites. Bands of ga-Pt(CO),, which are indicative of very small, possibly electron-deficient Pt clusters, are detected in Pt/HMOR.
Introduction
The catalytic behaviour of bifunctional catalysts is determined by the presence of acidic centers (Brgnsted and/or Lewis) and metal particles, as well as by interaction between the acid and metal functions. Hydrothermal treatment of zeolites is known to affect their acidic properties via partial dealumination and alteration of aluminum ordering in the framework [ 1, 2 ]. Therefore, this kind of pretreatment is widely used to modify catalytic activity and selectivity of acidic forms of zeolites. For instance, partial dealumination of high-silica zeolites strongly enhances their activity in paraffin conversions [ 3-5 1. Little information is, however, available on how the presence of water during reduction affects the acidity and properties of the metal such as its reducibility, dispersion or electronic. state. For Ni, Pd and Pt on Y zeolites, it was shown that reduction with moist hydrogen results in a lower metal dispersion [6, 71. The present research is devoted to the study of these effects for Pt and Pd particles supported on the decationized and the sodium forms of zeolite *On leave from N. D. Zelinsky Institute of Organic Chemistry, Academy of Sciences, Moscow, Russia.
0304-5102/92/$5.00
0 1992 - Elsevier Sequoia. All rights reserved
234
Y and mordenite. This work was initiated by AKZO Chemical Division Research Centre, Amsterdam, The Netherlands [8, 91. IR spectroscopy is one of the most informative methods for the investigation of acid sites in zeolites [lo]. The use of adsorbed probe-molecules, e.g. CO, provides additional information about the state and dispersion of reduced metal particles in zeolite cavities [ll, 121. At low temperature, the same molecular probe (CO) can also be used for the investigation of Lewis and Brensted acid sites which are able to form complexes with such weak bases [ 13 ]. This approach has been used in the present work to study the effect of water treatment on the properties of mordenites and Y-type zeolites with supported Pt and Pd.
Experimental
Pt/HMOR (AKZO No. 90-BK-444-101; Si/Al=8.8; 0.56 wt.% Pt) and Pt/ HY (AKZO No. 90-BK-444-102; Si/Al=4.75; 0.53 wt.% Pt) were provided by AKZO, Chemical Division, Research Centre Amsterdam, The Netherlands. Pt/NaMOR (Si/!= 9.3; 1 wt.% Pt), Pt/NaY (SW = 2.4; 5 wt.% Pt) and Pd/ NaY (Si/Al= 2.4; 4.7 wt.% Pd) were prepared by ion exchange of the H or Na form with dilute aqueous solutions of [Pt(NH&](NO& and [Pd(NH,),](NO,), at room temperature. The I-IYzeolite used for the preparation of Pt/HY was partially deahnninated. Thin self-supported wafers of the samples, without binders, with a diameter of 8 mm and a mass per cross-sectional area of - 7-13 mg cmW2 were pressed and placed into the sample holder of the IR cell. The low temperature IR cell used for the study of Pt and Pd-containing zeolites was described in [4]. The spectra were measured at 80 and 300 K using a Nicolet 60SX Fourier-transform spectrometer with a resolution of 1 cm- ‘. Calcination, water treatment, and reduction, were carried out in situ. Before the spectroscopic measurements, the samples were calcined from 300 to 770 K at 0.5 K min- ’ and then holding the temperature at 770 K for 2 h in a flow of oxygen (120 ml min- I). After calcination the system was purged with He at 770 K for an additional 20 min and cooled to 300 K in He. Following this purge, the samples were ready for reduction. The He flow was replaced by a hydrogen flow (60 ml min-‘). Metal reduction was initiated by ramping the temperature from 300 to 620 K at 8 K ruin-’ and holding the temperature at 620 K for 20 min. The samples were subsequently purged at 620 K with He for 20 min and cooled to room temperature. These samples will be denoted as ‘dry’. Prior to the reduction, if water treatment was desired, the samples were saturated by a He flow containing 15 torr of water vapor for 30 min. These samples will be denoted as ‘wet’. After the reduction, the background spectra of the reduced samples were measured. The samples were then cooled to 80-90 K and a He/CO mixture with a volume ratio of 30:5 and a total flow rate of 70-80 ml mir- ’ was passed through the cell for 10 min. The cell was subsequently purged with
235
He for 10-20 min to remove gaseous CO. The same procedure was repeated at 300 K.
Results
and discussion
Acid sites in zeolites Hydroxgl groups In all figures, spectra of the ‘wet’ samples will be denoted by the symbol (1) and those of the ‘dry’ samples by (2). The IR spectra of IWHY and Pd/ NaY are shown in Fig. 1 with (a) denoting Pt/HY and (b) Pd/NaY. The narrow bands at 3650-3615 cm-’ and the broad band at 3550 cm-’ are conventionally ascribed to bridging acid hydroxyls (I):
According to literature data [lo], the bands at 3650-3615 and 3550 cm- ’ are attributed to bridging groups located in supercages and hexagonal prisms, 3635
3560
O.lA
3285 I
I 8 f B 8 z
2b
J?ig. 1. IR spectra of OH-groups in FVHY (a) and Pd/NaY (b) reduced in the presence (1) or absence (2) of water.
F’ig. 2. IR spectra of ‘wet’ (1) and ‘dry’ (2) Pt/HY (a) and Pd/NaY (b) samples measured after adsorption of CO at 80-90 K. The corresponding spectra of the samples before CO admission are subtracted.
236
respectively. The hydroxyls characterized by the band at 3550 cm- ’ are usually considered to be hydrogen atoms bonded to oxygen atoms of the framework and inaccessible to adsorbed molecules. In addition to acidic OH-groups, low-intensity bands at 3670 and 3720 cm-’ were found; they are usually assigned to extra-framework AlOH species [ 1, 2, 4, lo]. Terminal silanols responsible for the band at 3740 cm- ’ are also present in the samples. Water treatment followed by reduction at 620 K results in a -20% decrease in the concentration of the bridging OH-groups of different types in Pt/HY, as evidenced by a decrease in the intensities of the corresponding bands (3635,3615 and 3550 cm-‘). Unlike bridging hydroxyls, the intensities of the OH-groups attributed to extra-framework al~inium (3670 and 3720 cm- ‘) increase after the water treatment. This shows that partial dealumination of Pt/HY takes place, presumably in the course of the ‘wet’ reduction. In the spectra of ‘wet’ and ‘dry’ Pd/NaY reduced at 620 K, the narrow band at 3645 cm-’ predominates due to the bridging OH-groups located in the supercages; these protons are a co-product of metal reduction. The presence of water during reduction has no significant effect on the intensity of these bands. No additional bands appear in the ‘wet’ sample of Pd/NaY as compared with its ‘dry’ counterpart. Similar spectra were observed for ‘wet’ and ‘dry’ Pt/NaY. However, in this case the intensity of the band at 3650 cm-’ which appeared after reduction was lower than for Pd/NaY, because of the lower molar concentration of Pt. As in the case of Pd/NaY, the presence of water during reduction had practically no effect on the spectral pattern. Low temperature adsorption of carbon monoxide probes for the acid strength of hydroxyl groups in zeolites [ 131. Absorption of CO at liquid nitrogen temperature leads to formation of weak complexes between CO and acidic hydroxyls; they are characterized by broad bands at V< 3500-3200 cm-‘. The bands are attributed to the stretching vibrations of O-H bonds in hydroxyl groups perturbed by CO. The extent of the shift towards lower wavenumbers of this broad band, with respect to isolated hydroxyls, is a direct measure of the acid strength of Bronsted sites. In Pig. 2 difference spectra for ‘wet’ and ‘dry’ Pt/HY and Pd/NaY are presented; they were obtained by subtraction of the spectrum before CO admission from the spectrum after CO admission. Bands with negative intensities, thus, correspond to those OH-groups which disappear after CO adsorption due to formation of complexes. Broad bands with positive intensities are attributed to hydroxyls perturbed via H-bonding to CO molecules. The most intense band at 3285-3295 cm-’ for Pt/HY and at 3410 cm-’ for PdMaY could be assigned to CO complexes with bridging OH-groups. The shoulder at 3465 cm-’ which is more intense for the ‘wet’ PtJHY, and a band at 3580 cm-’ appear to be due to interaction of CO with less acidic OH-groups, i.e. OH-groups connected with extra-framework aluminium (v=3670 and 3720 cm-‘) and silanol groups (~“3740 cm-‘) respectively.
237
be seen in Fig. 2 for both ‘wet’ and ‘dry’ samples of Pt/I-IY, the maxima of the broad bands are close to each other. For Pd/NaY or Pt/NaY reduced in the presence or absence of water vapor the spectra are almost identical. We, therefore, conclude that the acid strength of the bridging OHgroups in Pt/HY, Pd/NaY and Pt/NaY is not changed significantly by the water treatment. The IR spectra of the reduced RA-IMOR and Pt/NaMOR prepared by ‘wet’ and ‘dry’ reduction procedures are shown in Fig. 3. The band at 3610 cm-‘, which is the most intense band in the spectrum of Pt/HMOR, is ascribed to bridging OH-groups [ 11. Silanol groups (3740 cm-‘) are also present in both samples. Also an unresolved band near 3680-3720 cm-’ appears in the spectrum of ‘wet’ Pt/HMOR; it is attributed to OH-groups connected with extra-framework aluminum [ 1, 2, lo]. The concentration of the bridging OH-groups is approximately 40-50% lower in ‘wet’ Pt/HMOR than in the ‘dry’ sample. There is also a broad band at YN 3300-3400 cm-’ in the spectrum of the ‘wet’ Pt/I-IMOR which is absent in the spectrum of the ‘dry’ sample. It may be attributed to water coordinated to Lewis acid sites. This hypothesis is confirmed by the presence of the band of the bending vibrations of water at 1640 cm- 1 in the spectrum of ‘wet’ zeolite (not shown in Fig. 3). In contrast to Pt/HMOR, the presence of water vapor does not influence the Bronsted acidity of Pt/NaMOR; the spectra of both ‘wet’ and ‘dry’ Pt/ NaMOR are identical. As
can
3270
3605
n 0.05A
1
Fig. 3. IR spectra of Pt/HMOR (a) and PtMah$R (2) of water.
@) reduced in the presence (1) or absence
Fig. 4. lR spectra of ‘wet’ (1) and ‘dry’ (2) FVHMOR (a) and PtMaMOR (b) measured after adsorption of CO at 80-90 K. The corresponding spectra of the samples before CO admission are subtracted.
238
Figure 4 shows the difference spectra measured after adsorption of CO at 80-90 K on Pt/HMOR and IWNaMOR zeolites. Negative peaks show the disappearance of the bands of isolated bridging OH-groups (3610 cm-‘) which form complexes with adsorbed CO. These complexes are characterized by broad bands at 3270-3215 cm-’ for Pt/HMOR and at 3375 cm-’ for Pt/NaMOR. The shifts toward lower frequencies for PtJHMOR, relative to the corresponding bands for Pt/HY zeolites (~‘3285-3295 cm-‘), clearly indicate a stronger acidity of hydroxyl groups in IW-IMOR than in Pt/HY zeolites. This is consistent with the literature data on the acidity of Hmordenites and HY zeolites [ 1, lo]. The band due to the complexes of OH-groups with CO is shifted in the ‘wet’ Pt/HMOR to 3215 cm-‘, whereas the corresponding band observed for the ‘dry’ sample peaks is at 3270 cm-‘. This fact evidences an enhancement of the acid strength of the OH-groups in Pt/HMOR due to the water treatment before the reduction, as well as a - 40-50% decrease in their concentration. Unlike PVHMOR, no band shift is observed for Pt/NaMOR; i.e. the bands corresponding to the complexes of CO and bridging hydroxyls exhibit the same maxima. It follows that for Pt/NaMOR the acid strength of the OH groups remains unaffected by the water treatment.
Lewis acid sites in mm-de&es metals
and Y-zeolites with supported
Lewis sites, unlike OH groups, do not exhibit characteristic bands in IR spectra; their study, therefore, requires adsorption of suitable molecular probes. Low temperature adsorption of carbon monoxide has been used for the identification of Lewis acid sites in zeolites [ 131. Due to the presence of an unoccupied p-orbital, Lewis acid sites are able to interact with CO molecules. This leads to a shift towards higher frequencies of the stretching vibrations of C -0 bonds in comparison to gaseous CO (2143 cm- ‘). The bands attributed to such complexes are usually located near 2240-2 180 cm-‘, the value of the high frequency shift being a measure of the acid strength of the Lewis acid sites. In Pigs. 5-9 the spectra of ‘wet’ and ‘dry’ samples of PVHY, PVHMOR, Pt/NaY, Pt/NaMOR and Pd/NaY, respectively, are presented in the region of stretching vibrations of adsorbed CO. The bands below 2 143 cm-’ are attributed to metal carbonyls and will be discussed in the next paragraph. No bands above 2180 cm-’ which might be ascribed to complexes of carbon monoxide with Lewis acid sites were found in the spectra of the sodium forms, i.e. for Pt/NaMOR, Pt/NaY and Pd/NaY. However, for the H forms of these zeolites, i.e. Pt/HY and PtkIMOR, a characteristic band at 2228 cm-’ due to complexes of CO with strong Lewis acid sites is present for the ‘dry’ preparation. Its intensity is 4-5 times higher for the ‘dry’ mordenite than for ‘dry’ Pt/HY. This band is not observed in the spectra of ‘wet’ samples. Thus, the presence of water before or during reduction removes strong Lewis acid sites in both Pt/HMOR and PWHY. Presumably water is strongly adsorbed on these sites, and it is not desorbed
239
2087
1
\ Fig. 5. IR spectra of ‘wet’ vibrations.
(1) and ‘dry’ (2)
Pt/HY zeolites
Fig. 6. IR spectra of ‘wet’ (1) and ‘dry’ (2) PtMMOR
in the region
I
8
5 B t?
1986
!
0.02A 2066, P,l
:
Fig.
7.
IR
spectra of CO adsorbed
Fig. 8. IR spectra of CO adsorbed
2
of CO stretching
in the region of CO stretching vibrations.
024
0.2A
‘\ ‘._-’
on ‘wet’ (1) and ‘dry’ (2) F’t/NaY. on ‘wet’ (1) and ‘dry’ (2) PtMaMOR.
2114 I? 0.05Aj
1900
Fig. 9. IR spectra of CO adsorbed on ‘wet’ (1) and ‘dry’ (2) Pd/NaY.
under the conditions of reduction (T= 620 K, t = 20 ruin) and further with He. Water molecules coordinated to Lewis sites probably create new Brgnsted acid sites: H\ H’
0
---3
L
(II)
In the spectrum of CO adsorbed on ‘wet’ Pt/HMOR, two additional bands at 2197 and 2182 cm- ’ appear. They are absent in the spectrum of the ‘dry’ zeolite and may be assigned to CO complexes with weaker Lewis acid sites created via partial deahunination of zeolite during water treatment and further reduction. State and dispersion of supported metals CO was also used to study the state and dispersion of Pt and Pd in zeolites. When CO is adsorbed on reduced metal particles, it forms carbonyl moieties which may be either linear or bridging [ 11, 121. For different PtContaining catalysts, mainly bands at 2050-2120 cm-’ due to single linear complexes were reported in the literature. Accordingly, the only band at 2090-2092 cm-’ observed in the spectra of CO adsorbed on ‘dry’ samples of Pt/HMOR (Fig. S), PtDIY (Fig. 5) and Pt/NaY (Fig. 7) could be assigned to single linear P&CO located on large Pt particles. Only a low-intensity shoulder at * 2125 cm-’ is found in the spectrum of Pt/HMOR. In ‘dry’ Pt/HY the dominant feature is the band at 2092 cm-‘, which is attributed to linear CO. After water treatment of Pt/I-IY, the intensity of this band increases and its maximum is shifted from 2092 to 2087 cm-‘. Also two bands at 2035 and 1996 cm-’ appear, which are ascribed to the antisymmetric and symmetric stretching vibrations of two CO molecules in linear ‘twin’ complexes of Pt:
241
c=o
Pt /
’
c=o
(III)
The presence of these ‘twin’ complexes is an indication of very high coordinative unsaturation of platinum, which suggests that water treatment leads to an enhanced metal dispersion. In addition, bands > 2100 cm-’ are observed. They are attributed to electron-deficient linear complexes, possibly CO on monoatomic or very small Pt particles: pt+--C=O
The band at 2072 cm-’ seems to be characteristic of neutral I% particles which are likely to be smaller than those responsible for the band at 2092 cm-’ observed for ‘dry’ Pt/HY. The effect of water treatment on the state and dispersion of Pt is most pronounced in the case of FWIMOR (Fig. 6). The intensity of the band due to linear complexes is higher for the ‘wet’ sample; the band maximum is shifted from 2090 cm-’ toward 2070 cm-‘. This frequency shift is much larger (20 cm-‘) than for Pt/I-IY (5 cm-‘). An additional intense band at 2124 cm- ’ in the spectra of the ‘wet’ sample is assigned to linear CO with electron-deficient, probably very small pt particles. The most drastic changes occur in the region of CO stretching vibrations of the twin F’t(CO)a complexes. They are absent in the ‘dry’ sample, but in the spectrum of ‘wet’ PtkIMOR two bands at 2033 and 1996 cm- ’ are visible, which may be ascribed to antisymmetric and symmetric stretching vibrations in twin complexes (III). This assignment is confirmed by the simultaneous disappearance of these bands upon purging with He. Table 1 summarizes the changes in intensities of different types of carbonyls which occur after preliminary water treatment of Pt/HY and Pt/ HMOR zeolites. It includes the intensities of the bands at 2128-2110 cm- I, those at 2092-2070 cm- ’ and the intensities of the symmetric band of twin complexes (1996 cm- ‘). TABLE
1
Intensities
of the IR bands of different
Pkarbonyls cm-’
Zeolite
Reduction conditions
2110-2130
IWHMOR Pt/HMOR PtiHY WHY
dry ivet dry wet
<0.005 -0.030
A A
-0.010
A
2090-2070
0.064 0.080 0.071 0.096
A A A A
cm-’
1996
cm-’
0.020 0.004 0.017
A A A
242
The data presented in Figs. 5 and 6 and Table 1 show that water treatment enhances the dispersion of platinum in the H-form of both zeolites and thereby creates new Pt species. These may include atomic Pt carrying a partial positive charge due to interaction with acidic protons, and small particles exposing strongly coordinatively unsaturated Pt atoms. This effect is much more characteristic for Pt/HMOR than for IWHY zeolite. Unlike Pt/HMOR or Pt/HY zeolites, the intensities of the bands corresponding to carbonyls of platinum are significantly decreased after water treatment preceding reduction in the case of Pt supported on NaY zeolite (Fig. 7). Simultaneously, the band at 2090-2000 cm-’ broadens, probably consisting of several unresolved bands. It should be noted that reduction of Pt/NaY in the absence of water vapour leads to the IR spectrum which exhibits the only narrow band at 2092 cm-‘, as in the case of Pt/HY or IWHMOR zeolites. This evidences that the dispersion of Pt decreases in Pt/ NaY reduced in a moist atmosphere, and that distribution of Pt particles is not as uniform compared to ‘dry’ Pt/NaY. For Pt/NaMOR (Fig. 8) and Pd/NaY (Fig. 9), the intensities of the carbonyl bands are only slightly lower in the spectra of the ‘wet’ zeolites than in those of the ‘dry’ zeolites. No new bands appear after reduction in moist atmosphere, and the intensity ratio of the bands remains essentially unchanged. The effect of water vapor during reduction is thus much smaller for the Na-forms of zeolites supporting Pt or Pd than for the H-forms of these zeolites. This pronounced difference in the effect of water treatment on the dispersion of reduced metals apparently reflects the different nature of the sites capable of strongly adsorbing water molecules. Only the H-forms contain Bronsted and Lewis acid sites in appreciable concentrations. Taking into account that the concentration of Lewis acid sites is much higher for Pt/ HMOR than for Pt/HY and that the effect of water treatment on the dispersion and properties of supported platinum is most pronounced for H-mordenite, we conclude that the presence of Lewis acid sites plays a key role in the reduction process in the presence of water vapor. The formation of very small particles of the reduced metal in the channels of H-mordenite in addition to rather large particles is consistent with the following three arguments. (1) The mordenite channels contain ‘side-pockets’ formed by eightmembered rings; these side-pockets are likely sites accommodating small (even atomic) metal particles. (2) As bridging hydroxyls in mordenites are stronger acids than those in Y-type zeolites, the anchoring and stabilizing effect of protons on the metal particles will be stronger for Pt/HMOR than for PVHY. (3) Due to the higher Si/Al ratio in mordenites, the distance between Pt ions prior to reduction may be larger in mordenites than in faqjasites. This might favor formation of larger Pt particles in Pt/HY. According to our previous data [14-171, reduced Pt particles exhibit weak mobility in zeolites. Their mobility may further be hindered by water
243
molecules coordinated to Lewis acid sites and by extra-framework species created by partial dealumination of zeolite.
aluminium
Conclusions The spectroscopic data clearly show that water treatment of Pt/HMOR and Pt/HY prior to their reduction affects both the acidic sites and the state and dispersion of the supported metal. This contrasts with the sodium forms of these zeolites: Reduction of Pt/NaY, Pt/NaMOR or Pd/NaY in a moist atmosphere does not lead to changes in metal state or a higher metal dispersion than reduction with dry Hz. The effects on the H-forms of the metal/zeolites of reduction in the presence of water are: 1) transformation of Lewis sites into Brensted acid sites via coordination of water molecules to low-coordinated ions; 2) decrease in the concentration of Brensted acid sites; 3) partial deahunination of the zeolite framework; 4) increase in metal dispersion and a change in the morphology or electronic structure of platinum, as evidenced by the appearance of linear or twin carbonyl complexes which are indicative of the presence of electrondeficient particles and coordinatively unsaturated Pt atoms. The modification of acid and metal functions is much greater for Pt/ HMOR than for Pt/HY zeolite. At present we are studying relations between structural effects, as described in this paper, and the catalytic performance in C5/C6 hydroisomerization. The results will be described in a subsequent paper.
Acknowledgement Financial support for this work by AKZO Corporate is gratefully acknowledged.
Research America
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244 7 U. S. Pat. 4 139433 (1979) to J. W. Ward. 8 F. R. Mas Cabre, B. Klaver and R. G. Oudhaarlem,Abstr.
9 10 11 12 13 14 15 16
17
11th N. American
Meeting
CataL Sot., Dearborn, MI, May 7-l 1, 1989, C35. F. R. Mas Cabre, B. Klaver and R. G. Oudhaarlem, Catal. Symp. Dutch Cha. Sot., Temeuzen, The Netherlands, May 18-19, 1989. J. W. Ward, in J. Rabo (ed.), Zeoltie Chemistry and Catalysis, ACS Monograph 171, ACS, Washington DC, 1976, p. 118. N. Sheppard and T. T. Nguyen, in R. J. H. Clark and R. E. Hester (eds.), Advances in Iqfrared and Raman Spectroscopy, Vol. 6, Heyden, London, 1978. L. H. Little, Iqfrared Spectm of Adsorbed Molecules, Academic Press, New York, 1966, p. 428. L. M. Kustov, V. B. Kaaansky, S. Beran, L. Kubelkova and P. Jir&, J. Phys. Chem., 91 (1987) 5247. H. J. Jiang, M. S. Tzou and W. M. H. Sachtler, Appl. Cataf., 39 (1988) 256. M. S. Tzou, H. J. Jiang and W. M. H. Sachtler, React. Kin.&. Ca.taL I.&t., 35 (1987) 207. M. S. Taou and W. M. H. Sachtler in J. W. Ward (ed.), Froc. 10th North Am. Meeting Catalysis Society, San Diego, 1987,Vol. 38, Studies in [email protected] Science and Catalysis, Elsevier, Amsterdam, 1987, p. 233-241. M. S. Tzou, H. J. Jiang and W. M. H. Sachtler, Solid State Zonics, 26 (1988) 71.