H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
P R OP Y L E NE
ME T A T HE S IS
RE A C T I 0N
Y - ZE 0 L IT E S
M. tANIECKI Faculty of Chemistry, A. Mickiewicz University, 60-780 Poznafi Poland ABSTRACT The Mo(C0) loaded Y-zeolites differing in Si/Al ratio and surface acidity have been used to study the activity in propylene metathesis. Both sodium and hydrogen forms of Y-zeolites were applied as supports. Oecom7osition of adsorbed Mo(C0) , during heating,produces a variety of surface Mo species diffgrbng in $ k e degree of decarbonylation and oxidation number (from Mo to Mo 1. The catalytic activity decreased with increasing protonic acidity of the support and oxidation number of molybdenum. U V / V I S transmission spectroscopy was applied to study the influence of the support acidity on the formation of surface species during adsorption and decomposition of MO(CO)~. INTRODUCTION It is well known that one of the most difficult transition metals to ion exchange into the zeolites has been molybdenum, due to the absence of simple salts of this element which are stable under solution-ion-exchange conditions. F o r this reason, the different techniques of Mo implantation into the zeolite framework,including l ~ has been solid-state exchange with MoC15 (1) and M ~ ~ ( e n ) ~ C (2) applied. However, the less direct but m o r e efficient method of molybdenum loading into HY zeolites is based on the saturation of activated zeolite with volatile molybdenum carbonyl. Gallezot et a1.(3) and Ward and Lunsford (2), applying mainly IR spectroscopy, established that Mo(CO)~, initially adsorbed within the supercage of HY zeolite, become oxidized by zeolite protons during thermal activation. The IR and EPR studies by Abdo and Howe (4) cornplemented results reported earlier and indicated that i n the case of Mo(C0)6/NaY the formation o f supported zero-valent Mo is possible. Ward and Lunsford (2),studying normal and ultrastable HY zeolites, found that Mo ions are probably located at SII positions within the large cavities. Although the results presented by the groups of Howe (4), Luns-
ford(2) and Yashima(5-7) disclosed some chemistry of the Mo/Y-zeolites, there are few papers concerning the catalytic properties of these systems. Komatsu et al.(6),studying the propylene metathesis over Mo-loaded zeolites as a function of oxidation number, found that molybdenum species responsible for catalytic activity have an oxidation number lower than +4. They also stressed the importance of Mo dispersion on catalytic activity. It was established(7) that atomically dispersed ivlo with oxidation number close to zero is responsible for polymerization and hydrogenation of ethylene. Yong and Howe(8) reported recently the activity of molybdenum zeolites in the Fisher-Tropsch reaction. They established that application of Nay, KY and KL zeolites as supports leads to the formation of active centers in the Fisher-Tropsch process, namely zero-valent molybdenum. The propylene epoxidation reaction has also been studied with Mo-loaded zeolites ( 9 ) . The present paper seeks to contribute to an understanding of the surface chemistry and catalytic activity of molybdenum zeolites.The influence of zeolite composition and thermal pretreatment on the catalytic activity in propylene disproportionation reaction has been studied. EXPERIMENT NaY faujasites with Si/A1 ratio 2.0 and 2.7 were obtained according to the procedure described by Kacirek and Lechert(lO).The equivalent ammonium forms were obtained by triple exchange with 1M solution of NH4C1 at 345 K (for details s e e Table 1). The dehydration of sodium forms as well a s the dehydration and deammoniation of ammonium forms of Y-zeolites was carried out in a stream of purified helium inside the U-shaped quartz reactor at 675, 775 o r 875 K, for two hours. The amount of the support used was always adjusted to 0.25 g o f activated zeolite (for weight losses see Table l).After cooling and one-minute evacuation, the desired amount of doubly sublimed Mo(C016 was transferred at room temperature under vacuum onto the support and kept in a closed reactor for 1 2 hours. A similar procedure for alumina support has been described earlier(l1). The activation o f supported Mo(CO)~ was carried out in helium in 50 K intervals up to 675 K for 60 minutes. The evolved gases were trapped at liq. N2 temperature (for a H2 trap with molecular sieves 5A was used) and, after warming, analyzed with a thermal conductivity detector and 2.5 m
column of activated carbon (Mcrck, 25-35 mesh). The propylenc metathesis reaction was studied by a pulse method. The propylone pulses were injected from the gas-sampling vale (1.21 cm3) into the carrier gas (He o r H2) flowing at a rate of 3 0 cm3- n1in-l and carried through the catalyst bed at 335 K . Reaction products were collected in a trap and, after each injection, analyzed with gas chromatography. The average oxidation number ( O . N . ) of Mo was calculated on the basis of oxygen titrations performed at 575 I( with a well calibrated sampling valve. The U V / V I S experiments were carried out in the high vacuum system. The details of the cell, high-vacuum and gas-dosing system have been described elsewhere (12). The self-supporting wafers of zeolite with a thickness of 2 - 3 m g + c m - 2 were employed. The U V / V I S spectra were recorded at room temperature in transmission mode with a Perkin-Elmer U V / V I S spectrometer (Model 556) equipped with baseline corrector. Similarily t o the catalytic experiments, in the spectroscopic studies the zeolite wafers were dehydroxylated under vacuum in the U V / V I S cell at elevated temperatures prior to the Mo(CO)~ admission. In the spectroscopic studies the amount of supported Mo(CO)~ was calculated from the pressure drop of carbonyl vapours, whereas in the catalytic experiments the Mo content was analyzed with atomic absorption spectrometry. TABLE 1 Characterization of zeolites used. Unit cell
Pretr. Weight temp. loss*
IR absorbance**(a.u.) OH vibrations
(NH4)35Na14A152Si1420384 6 7 5 Si/A1=2.72 775 875
1.00 0.13 0.03
0.30 1.46 1.58
*after 2 hours at indicated temperature in N **after 2 hours activation under vacuum follo8ed by interaction with pyridine at 475 K (2h adsorption, lh desorption). 8Py and LPy represent the density o f the Brunsted and Lewis centers, respectively.
AND DISCUSSION The application of tIY zeolites with different S i / A 1 ratio shows that the initial composition (and consequently also the initial acidity) influences the decomposition of supported Mo(CO)(,. Figure 1 shows the amount of CI1 and H2 cvolvcd upon heating the Mo(CO16 loaded zaolites, pretreated at 6 7 5 K. In the case of zeolite with higher A 1 content the decomposition of Mo(CO)~ proceeds at lower temperatures than with the sample where Si/A1=2.7. The complete decarbonylation for these samples was achieved at 520 and 6 5 0 K , respectively. RESULTS
F i g . 1 . CO and H evolution 1
2 P rn
during decomposftion of supported Mo(CO) HY-1 and HY-2 refiresent supports with Si/A1=2.11 and 2.72 respectively. Pretreatment temperature of the support - 6 7 5 K.
LOO 500 600 TEMPERATURE K 1
The quantities of hydrogen released in the reaction of Mo(C0l6 with surface hydroxyl groups are smaller than those found previously ( 2 ) . These amounts, however, can serve a s well as titrations for the estimation of an average oxidation nurnber(0.N.) of molybdenum, because for the samples activated at 575 K a good agreement has been found. The evaluation o f the O . N . was based on the assumption that during oxygen titrations all Mo species are oxidized to Mo'~. It was found for the applied HY zeolites pretreated at 675 K that in every step o f thermal decomposition the 0.11. is always lower f o r the zeolite with lower Si/A1 ratio. F o r example, samples activated at 675 K show the O.N. close to + 4 f o r those with Si/Al=2.1, whereas +6 is shown f o r those with Si/A1=2.7. Similar behaviour has been observed f o r HY supports when pretreated at 775 K. The values of O.N. ( s e e Table 2) for the Mo s p e cies were very much lowered in comparison with pretreatment at 675 K.The decarbonylation process had been also completed at lower temperatures in both cases. As previously, the hydrogen evolution was observed, but only f o r the sample with lower A1 content.
When HY zeolite with Si/Al 2.1 was pretreated at 875 K only carbon monoxide was found in the decomposition products. Comparing the influence o f the initial acidity, expressed a s the average density of Ertlnsted and Lewis centers (Table 11, with the average oxidation number (Table 2 ) one can conclude that the surface hydroxyl groups are essentialy responsible for the oxidation process of supported molybdenum carbonyl. Abdo and Howe (4), applying IR spectroscopy, found that adsorbed Mo(C016 interacts with surface hydroxyls even at r o o m temperature and thermal activation causes the complete disappearance of IR bands originating from OH vibrations. In o u r case, while the support is practically dehydroxylated, the molybdenum carbonyl decomposes instantly, releasing only carbon monoxide with parallel formation of Mo species with O.N.close to zero. Such a situation exists when the HY zeolite is pretreated at high temperatures o r when the parent NaY zeolites are applied as supports. I n the absence of an oxidizing agent such a s a hydrogen from OH groups, as well as in the presence of the basic sites present in NaY zeolites, the decomposition of Mo(C0l6 started immediately after deposition onto the support. After one hour of the contact a t room temperature,the species with the average formula M o ( C O ) ~ were formed. Only the activation at elevated temperatures caused the complete decarbonylation of supported Mo(C016. F o r the sample with Si/A1=2.11 the complete decarbonylation was observed above 455 I ( , and above 400 I( for the sample with Si/A1=2.72. The oxygen titrations indicated that application of NaY zeolites a s supports, pretreated at 6 7 5 I ( , leads to the fornation o € zerovalent 140 particles. TABLE 2 Average oxidation number of Support pretreatment temperature ( I( )
675 775 075
supported on hydrogen Y-zeolites.
Average oxidation number of 110 after activation at indicated temperature ( K ) Si/Al=2.11 Si/A1=2.72 415 575 675 475 515 615 +0.0 -
+1.1 + 3 . 3
Samples differing i n Mo oxidation state and containing two Mo atoms per supercage of Y zeolite were tested i n propylene metathesis reaction. Catalyst activities were determined by passing pulses of propylene over the catalyst kept at 335 K in a stream of helium. Assuming pure disproportionation and equilibrium between the 2-butene isomers, the equilibrium conversion i s 42.8% ( 1 3 ) . Pure disproportionation will yield only 2-butenes and ethylene in equimolar amounts. Analysis up through the hexenes revealed only tiny amounts of 1-butene as an additional product f o r any of the studied samples, showing a selectivity higher than 99.5%. The ratio of ethylene/Z-butenes was usually higher than 1.4, suggesting higher adsorption of 2-butenes on the catalyst. It was also dependent on the pulse number. The ratio of trans2-butene to cis-2-butene was close to the equilibrium value of 2.8. -
10 15 PULSE NUMBER
Fig.2. Variation of activity with the pulse number for catalysts activated at 575 K . Support pretreatment temperature - 675 K . H Y - 1 , Si/Al=2.11 H Y - 2 , Si/A1=2.72 Nay-1, Si/A1=2.00 Nay-2, Si/A1=2.70 Open triangles represent activity obtained after 6 pulses o f ethylene followed by propylene pulses over HY-1.
The results o f catalytic activity measurements,expressed as propylene conversion, for catalysts pretreated at 675 K are shown in Fig.2. F o r both hydrogen forms of Y-zeolites a low activity after first pulses of propylene was observed. The effect is the reverse of that with the alumina as a support ( 1 4 ) . The maximum activity usually was reachod with 15 p u l s s s , and n z x t a sln1.i ;!::cr>az:? l i a s n:?:;-svzC. ?rli,iy:-.nc conv~rsiari ov?c th-: c a t a l y s t : ; ,.:it:i:ii:1:i::r : < i n t s , i t as in all studied c3s05 always higher than f o r those tdhcro Si/A1=2.7. For iblo-loadcd ;!aY zeolites the activity bias cJnstant f r o m t h c first pulse, however, very much lowered in comparison with HY supports. Very similar results have been obtained €or the catalysts where t l i o sup7orts uere pretreated at 775 o r 875 I<. The initial activity after the first pulses of propylane was at 775 K twice as high and
- 475 K - 575 K
Fig.3. Activity versus average oxidation number (O.N.
OXIDATION NUMBER at 875 I( over three times as high as for those pretreated at 6 7 5 K. The increase of pretreatment temperature for NaY zeolites above 675 K did not influence the changes in the catalytic activity. In order to establish the influence of the reaction products on the catalytic activity, pulses of ethylene and trans-2-butene were passed over the fresh catalyst before propylene injections. Preadsorption of butene did not change the activity very much, but significant changes were observed for the samples originating from the HY supports pretreated with ethylene. I n a l l cases when ethylene was applied before propylene admission the initial activity increased at least six times that expected for fresh catalyst ( s e e Fig.2.). The explanation for such behaviour can be based on the assumption that carbene-Mo complexes are formed much faster with ethylene than with propylene and butene. This would also e x plain thc presence of an induction period during the first pulses of propylene. It i s known that carbene complexes, which themselves can undergo disproportionation, can be isolated from homogencous olefin metathesis systems(l5,ld). According to the mechanism proposed by Casey and Rurkhardt (15), a carbene complex is initially required in order ts catalyze disproportionation. Figure 3 shows the influence of the value of oxidation number on the catalytic activity, measured after 15 pulses. Similar to the results presented by Komatsu et a1.(6), the activity decreases
with increasing oxidation number for HY supports. The low activity o f catalysts originating from NaY zeolites i s due probably to the low dispersion of Moo particles ( 7 ) . In the present paper U V / V I S transmission spectroscopy has been applied to study the adsorption and decomposition o f supported M o ( C 0 l 6 . Figure 4 shows the U V / V I S spectra of molybdenum carbonyl supported over HY zeolite, pretreated at 6 7 3 K . The band positions at 230, 285 and 3 2 5 n m , characteristic for Mo(C016 (171, indicate that at room temperature only physisorption occurs. The changes in V."
Fig.4. U V / V I S spectra of Mo(C016 supported at r o o m temperature over hydrogen form o f Y-zeolite recorded after : a - 5 min.; b - 20 min.; c - 1 hr; d - 2 h r ; e - 4 hr. The amount of Y O ( C O ) ~ introduced 0.0127 mmo1-g- .
the absorbance after M o ( C O ) ~ admission suggest a very slow migration of carb0.2 onyl particles inside the zeolite cavities. The decrease of the BrMnsted site 0.0 population (Table 1 ) with increased 200 300 400 pretreatment temperature to 7 7 5 :< WAVELENGTH Inm 1 causes the reaction of introduced W O ( C O ) ~ with the zeolite framework already at room temperature. This was observed a s changes in the main band intensities as well as the ayp:'arance o f th:: ahou1dL.r b a n d s 3 t 7 5 0 a n d 2 7 3 nn. I n a C L i l i o n , t.volutioii
~ i n s ~ : :;i i z
cnszd Zrbnstzrl acidity was cven
pronounczd 1,ihen the zeolite 'Fig.5.
300 wx) WAVELENGTH Inm 1
supported at room temperYturc over Nay zeolites. a - after mission of 0.0091 mmo1.g and recorded after 1 min. b - recordod after 30 min. c - after adnissior another 0.0091 mno1.g- , recorded after 1 min. d - recorded after 1 hr.
:40( 30) I
;,PI'Jct o f cir3cr-
was almost completely dehydroxylated at 875 K. The band at 230 nm was present only at the initial stage of adsorption, whereas the bands at 250, 270 and 340 nm increased their intensity.App1ication of the support with Si/Al=2.11 produced the same tendency as in the previous case; however, the decomposition of Mo(C016 started already at room temperature, even if the support was pretreated at 675 K . The instant decomposition of molybdenum carbonyl at room temperature was observed for NaY zeolites. Figure 5 shows spectra after adsorption of Mo(C0)6 over these supports. The prolonged contact of molybdenum carbonyl with NaY caused the appearance of new bands at 260, 320 and 390 nm. Concomitantly,a strong evolution
-300 K - _ _ _ 375 K 175 K
Fig.6. U V / V I S spectra of Mo(C0) adsorbed at room temperature ovgr HY-zeolite and activated at different temperatures. Support pretreatment temp. -675 K Si/Al = 2.72 Amount-pf Mo(CO)~ adsorbed - 0.0138 mmol-g .
of CO was observed. The amount of released CO, both under static and 200 300 Loo 500 dynamic (He flow) conditions suggests WAVELENGTH Inm 1 the formation of Mo(CO)~ ads.species. On the other hand, the bands at 320 and 3 9 0 nm can originate from dimolybdenum-like species (18). The appearance of the same bands has been also noticed for HY supports however, only after activation up to 425 K ( s e e fig.6). Activation at higher temperatures was always associated with the decrease of band quantity and intensity. The results presented suggest that mainly basic sites are involved in the decomposition of supported MO(CO)~. In both cases, of NaY and HY zeolites, the oxygen anions act as basic sites. However, f o r NaY the oxygen ion carries a much higher negative charge than the corresponding oxygen in the H-form of the zeolite. Additional U V / V I S experiments with olefins and Mo-loaded zeolite s indicated the interaction of propylene and butenes with the acidic supports (both protonic and non-protonic) with fast formation o f r - a l l y 1 carbocations (19). Contrary to that, the interaction of ethylene and the zeolite acidic centers was much slower, -1
and formation of r - a l l y l i c structures was very much delayed. Such behaviour can explain the increased activity in propylene metathesis after ethylene pulses, as well as the existence of an induction period at the initial stage of the reaction. In the case of propylene the competition reaction of j-r-ally1 complexe formation proceeds first, and next the carbene-Mo complexes are formed. With ethylene the reverse sequence is probably involved, and 7 - a l l y 1 carbocations can be developed after the formation of carbene complexe. CONCLUSIONS 1. The decomposition of Mo(C016 over NaY and HY zeolites occurs on the basic sites. Surface hydroxyl groups of HY zeolites are responsible for the oxidation of supported molybdenum hexacarbony1 at elevated temperatures. 2. In the absence of hydroxyl groups in NaY zeolites the supported metallic Mo can be formed. 3 . The catalytic activity in propylene metathesis is strongly influenced by the value of the oxidation number of supported Mo species. The lower the O.N. of Mo, the higher i s the catalytic activity. Application o f NaY supports leads to the formation of a catalyst with low activity due to the poor dispersion of Moo particl es. 4. Carbene-metal complexes are probably involved in the mechanism of propylene disproportionation over these catalyts. The mechanism via metallocyclobutane intermediate has to be questioned because of its validity on a thermodynamic basis. Besides, no cyclopropane has been observed from propylene, as would be the case if a metallocyclobutane were a n intermediate. ACKNOWLEDGEMENT The author is indebted to Or. H.G.Karge f o r the critical revision o f the manuscript and the provision of the UV/VIS facility which made this work possible. REFERENCES 1 P.E.Dai, J.H.Lunsford, J.Cata1. 6 4 (1980) 173-184 2 M.B.Ward, J.H.Lunsford, Proceed.6th 1nter.Zeolite Conf. July 1983, Reno (D.Olson,A.Bisio Eds.), Butterworth 1984, pp. 405 3 P.Galezot, G.Courdier, M.Primet, B.Imelik, ACS Symposium Ser. 40 (1977) 144-155
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