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Influence of Lewis acidity of rhenium heptoxide supported on alumina catalyst on the catalytic performances in olefin metathesis
Applied Catalysis A: General 167 (1998) 237±245
In¯uence of Lewis acidity of rhenium heptoxide supported on alumina catalyst on the catalytic performances in ole®n metathesis F. Schekler-Nahama1,*, O. Clause, D. Commereuc, J. Saussey2 Kinetics and Catalysis Division, Institut Franc,ais du PeÂtrole, BP 311, 92506 Rueil-Malmaison Cedex , France Received 17 June 1997; received in revised form 14 October 1997; accepted 15 October 1997
Abstract Characterisation of surface acidity of the Re2O7/Al2O3 catalyst by infrared spectroscopy and ammonia thermodesorption reveals that the initiation and the good processing of the metathesis reaction is essentially governed by Lewis acidity. This strongly supports the role of Lewis acidity quanti®ed by pyridine and lutidine adsorption, which is exalted by the increase of the rhenium content and the calcination temperature and the concomitant increase of the metathesis activity. # 1998 Elsevier Science B.V. Keywords: Lewis acidity of Re2O7/Al2O3; Ole®n metathesis; IR spectroscopy
1. Introduction The metathesis reaction has drawn much interest because of its technological and fundamental importance [1]. It has been known since the pioneering work of researchers from British Petroleum [2] and from Phillips [3]. Concerning the nature of the catalytic active intermediates, surface carbene species, several authors conclude that partly oxidised transition metal ions seem to be necessary to bring about this reaction [4,5]. Most studies of the heterogeneously and homo*Corresponding author. Fax: 00 33 3 20 43 65 01; E-mail: [email protected] 1 Present address: Universite des Sciences et Technologies de Lille, Laboratoire de Catalyse HeÂteÂrogeÁne et HomogeÁne, URACNRS 402 BaÃt. C3, 59655 Villeneuve d'Ascq Cedex, France. 2 UMR.CNRS 6506-ISMRA, Laboratoire Catalyse et Spectrochimie, 6 boulevard du MareÂchal Juin, 14050 Caen Cedex, France. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00310-4
geneously catalysed metathesis have been carried out with molybdenum and tungsten based catalysts; rhenium based homogeneous catalysts are few and little active. However, the comparison is more interesting in the case of rhenium, since conditions of the catalysis are the same, especially the temperature. The literature data indicate that the environment of the active site does possess acid properties. However, the nature of the acid sites is still a matter of debate. It has been proposed that there is a relationship between the BroÈnsted acidity of the catalyst and the metathesis activity [6], the activity of the rhenium metathesis catalyst is not related to their Lewis acidity. In this respect, it is important to notice that Lewis acidity has been quanti®ed without considering the difference between weak and strong Lewis acidity. Consequently, Xiaoding's conclusions seem to be a little premature. On the other hand, Commereuc et al. proposed that the stability with time is governed at
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least in part, by the Lewis acidity of the active rhenium centres [7,10]. Hsu has claimed that both in¯uence the activity [8]. The purpose of the present work is to study thoroughly the surface acidity of the Re2O7/Al2O3 catalyst, and the nature of the acidity implicated in the metathesis reaction. Re2O7/Al2O3 catalyst is active even at subatmospheric pressures and at room temperature, and therefore particularly suited for an in situ infrared spectroscopic study. 2. Experimental 2.1. Preparation of the catalysts Catalysts with different Re contents were prepared by incipient wetness method. Our experimental conditions have been described in a previous paper (F. Schekler-Nahama, O. Clause, D. Commereuc, J. Saussey, to be published). The calcination temperature has been studied in the range 823±1023 K from 15 min up to 2 h. Other Re2O7/Al2O3 catalysts have been prepared by sol±gel method, using Al(OiPr)3 as an aluminium precursor, isopropanol as a solvent, HReO4 as a rhenium precursor, and water to hydrolyse. An aluminium triisopropylate±isopropanol (50 g± 35 ml) mixture was heated at 353 K under re¯ux. The solution was gently stirred with a magnetic stirrer. A hole in the cover allowed slow insertion of HReO4 solution, since hydrolysis is an exothermic reaction. The stirring has been maintained for 1 h to homogenise. After drying in an oven at 393 K to remove water and alcohol, the material was calcined at 823 K under air atmosphere for 2 h and then cooled in air. A WI catalyst indicates Re2O7/Al2O3 prepared by incipient wetness, and calcined for 2 h at 823 K under air. A WI 1023 (2 h) catalyst is calcined at 1023 K for 2 h, WI 1023 (15 min) is calcined at 1023 K for 15 min, and SG indicates a Re2O7/Al2O3 prepared by sol±gel method, as it has been described above. 2.2. FTIR spectroscopy 2.2.1. In situ static infrared cell In situ Fourier Transform Infrared measurements were carried out on a NICOLET MAGNA-750 Instru-
ment. The infrared spectra were recorded at beam temperature. The catalyst was activated at 823 K for 2 h under dry oxygen atmosphere. Regular replacements of oxygen were performed to remove desorbed water. The sample is then cooled for 45 min, and oxygen is evacuated. Dry pyridine or lutidine (3.102 Pa at equilibrium) was added at 298 K and physisorbed species evacuated under a dynamic vacuum (10ÿ3 Pa) at 50 K intervals up to 623 K. Subtraction of spectra after pyridine or lutidine desorption from those obtained before probe molecules adsorption evidences bands due to adsorbed species. Quanti®cation of Lewis and BroÈnsted acidity was realised by means of the literature molar extinction coef®cients [9]. 2.2.2. In situ dynamic infrared cell The high temperature infrared reactor cell has been installed on a NICOLET 5SX interferometer sample compartment. It is coupled on DELSI 200 chromatograph allowing automatic gas sampling. The complete system is characterised by a very low dead volume. The infrared reactor is made of stainless steel tubing with two CaF2 windows sealed in two te¯on holders with a Kalrez O-ring for each window. KBr plates are placed in the cell to minimise the dead space (1 mm). A thermocouple is ®xed on the upper side of the cell by a swagelock ®tting and is located in the sample holder. The sample was pressed into self-supporting disks 16 mm in diameter 0.2 mm thick, weighing 20 mg and held with a stainless steel sample holder. The reaction gases were introduced from one side of the sample holder. They were diverted onto both faces of the catalyst and evacuated on the other side. A detailed scheme of the apparatus is given in Fig. 1. Before spectra acquisition, the catalyst was activated under dry oxygen ¯ow at 673 K (with a temperature ramp of 4 K/min) for 2 h, and then cooled in 45 min to the reaction temperature (333 K) under a helium ¯ow to eliminate oxygen traces. The gas ¯ow rate (oxygen or helium) was 5 cm3 NTP/min. Dry propylene diluted with dry helium was admitted to the reactor at atmospheric pressure. The gas ¯ow rate was 1 cm3 NTP/min for each gas. The GC collection performed was every 15 min. The infrared spectra were recorded with the sample at 333 K.
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239
Fig. 1. Infrared reactor cell.
2.2.3. Ammonia pulse injections 50 ml portions of dry ammonia were successively introduced to a helium ¯ow in the reactor cell at 333 K. IR spectra were recorded after each ammonia injection. 2.3. Metathesis reaction The deactivation experiments were carried out in a conventional ®xed-bed micro-catalytic down ¯ow reactor. The reactor was a quartz tube (10 mm in diameter), that could be placed in a vertical oven. In a typical experiment, a catalyst sample of 2 g (200±700 mm sifted) was placed in the reactor and calcined for 2 h in an oxygen stream (13.5 l/(h.g)) at 823 K (heating rate 5 K/min). The reactor was subsequently taken out of the oven, and cooled to the reaction temperature in 15 min. The reaction temperature is thermostatically controlled at 323 2 K. Standard reaction conditions were a LHSV11 hÿ1 (2 g of catalyst and a propylene ¯ow of 44 l/h), and an atmospheric pressure. Oxygen and propylene were dried on 4A molecular sieves, and propylene was deoxygenated by means of a deoxo cartridge (CHROMPACK). The reaction products were analysed on line by a gas phase chromatograph, and an alumina-PLOT capillary column (CHROMPACK), 50 m in length
and 0.53 mm in diameter. The GC signals were processed by a Hewlett Packard integrator. 2.4. Activity measurements The experimental results have been interpreted on the basis of a Langmuir±Hinshelwood model including a second order equilibrated reaction. The rate constants derived from this model were used as a tool for the estimation of activity of the catalyst. They were calculated according to the following relations. For laboratory-scale experiments, perfectly-stirred reactor, reactant propylene : kt f2
N3e ÿ N3s with 2mf2 4 1 ÿ x2 ÿ x2 =Keq
2Q= b3 N3e RT 2 1 ÿ x B2 B4 x2
where kt: rate constant at time t, m: mass of catalyst, N3e: molar ¯ow rate of propylene in, N3s: molar ¯ow rate of propylene out, Keq equilibrium constant, Q: total volume ¯ow supposed to be constant, R: perfect gas constant (R8.314 m3 Pa/(K mol), T: reaction temperature, B2b2/b3, B4b4/b3 (bi: adsorption coef®cient of the ole®n i). In our calculations, we have adopted the following correlation previously checked by Commereuc et.al. [10]
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Keq 2:833 10ÿ4 T K ÿ 273 0:0833 between 293 and 333 K: Calculation of the rate constant needs the knowledge of the ratios of the adsorption coef®cients bi/b3 relative to propylene for all components i of the medium, and the knowledge of the adsorption coef®cient b3. These values have been extracted from Kapteijn's thesis [11]. The deactivation of the catalyst was interpreted as if the amount of the active catalyst were decreasing with time. This process has been shown to follow a ®rstorder rate law, according to the relation : loge kt loge ko ÿ kd t in which ko: the initial metathesis rate constant, and kd: the ®rst order deactivation rate constant are the two parameters used to characterise the performances of the catalysts. 3. Results and discussion 3.1. Metathesis activity The activity of the rhenium on alumina catalysts increases quasi exponentially with their rhenium con-
tent (Figs. 2±4). Consequently, rhenium is better used at high content and the environment of the active site has to possess an optimal rhenium content. On the other hand, the metathesis reaction does not work at very low rhenium contents (1 wt% Re or less). Another point remarked is that beyond a given content (10 wt% Re), the activity indeed continues to increase, but an oligomerisation reaction of the ole®n present superimposes on the metathesis reaction. This is ascertained by a brown colouring of the catalyst contacting with propylene, due to the presence of byproducts. This phenomenon was also accompanied by a faster rate of catalyst deactivation (more than 3 orders of magnitude). Comparing the preparation methods, sol±gel catalysts are far less active (5 orders of magnitude) than their homologous wet impregnated Re2O7/Al2O3 catalysts. Furthermore faster deactivation is observed in the case of the sol±gel catalysts. Regarding the effect of the calcination temperature, increasing temperature (from 823 to 1023 K) and calcination time enhances metathesis activity. In order to explain the increase of the activity with Re content, we suspected the role of acidity in the metathesis mechanism. Furthermore, oligomerisation side reaction could have a cationic origin resulting
Fig. 2. Infrared spectra of Re2O7/Al2O3 with varying Re loadings after pyridine adsorption thermodesorption in vacuum (10ÿ3 Pa) at 423 K. (a) 0.0 wt % Re; (b) 1.8; (c) 8.0; (d) 13.7.
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Fig. 3. Infrared spectra of Re2O7/Al2O3 and Al2O3 after lutidine adsorption thermodesorption in vacuum (10ÿ3 Pa) at 423 K. (a) 0.0 wt % Re; (b) 8.0.
Fig. 4. Propylene metathesis T323 K: effects of the rhenium content (support g-Al2O3), the preparation method, and the calcination temperature on the metathesis activity.
from the presence of BroÈnsted acidity at high rhenium content. Acidity measurements were performed by means of FTIR spectroscopy using pyridine and lutidine adsorp-
tion. Pyridine interacts, after desorption at 423 K, give rise to protonated species at 1539 cmÿ1, and coordinated species at 1452 and 1618 cmÿ1 for weak Lewis acidity, at 1456 and 1624 cmÿ1 for strong Lewis
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Table 1 Correlation between the number of Lewis acid sites and BroÈnsted acid sites on Re2O7/Al2O3 catalysts and the metathesis activity wt% Re
nL(1508C) (mmol/mg)
nB(1508C) (mmol/mg)
ko (mol(h.gcata))
0.0 1.8 8.0 13.7
0.16 0.19 0.21 0.26
0.00 0.00 0.00 0.09
0.0 0.4 11.1 74.7
(50% strong) (75% strong) (100% strong) (100% strong)
Table 2 Correlation between the number of Lewis acid sites and BroÈnsted acid sites as a function of the preparation methods of Re2O7/Al2O3 7.0 wt% Re catalysts (incipient wetness "WI" impregnation and sol gel "SG") and the metathesis activity Reference sample
nL(423 K) (mmol/m2)
nL(623 K) (mmol/m2)
nB(423 K) (mmol/m2)
ko (mol/(h.gcata))
WI 7.0 SG 7.0
0.93 0.57
0.15 0.05
0.00 0.00
7.9 1.0
acidity (Fig. 2). The evolution of the metathesis activity as a function of the rhenium content is correlated with the acid properties of alumina or nearest rhenium oxide sites. Table 1 summarises our main results. Increasing rhenium content leads to the simultaneous increase of the strong Lewis acidity of Re2O7/ Al2O3 materials at the expense of the weak Lewis acidity of alumina, the appearance of BroÈnsted acidity, and the increase of the metathesis activity. Therefore, there is a distinct correlation between the increase of Lewis acidity and metathesis activity at such a rhenium loading that there is no BroÈnsted acidity detected yet. Below 10 wt% Re, increasing the rhenium content is almost proportional with increasing Lewis acidity, whereas the increase of activity is quasi exponential. In fact, we should consider both the numerical and strength increase of the Lewis acid sites to establish a distinct correlation with the non proportional increase of activity. Furthermore, the role of BroÈnsted acidity cannot be ruled out since activity of the Re2O7/Al2O3 13.7 wt% Re catalyst corresponds to a drastic increase of the metathesis activity. However, the presence of BroÈnsted acidity on the Re2O7/Al2O3 13.7 wt% Re catalyst enhances oligomerisation or polymerisation side-reactions which have a cationic origin. Resulting by-products behave as diffusional barriers and block the active sites. This phenomenon was also accompanied by a faster rate of catalyst deactivation. At least, let us notice that BroÈnsted acidity on the Re2O7/ Al2O3 13.7 wt% Re catalyst is weak, since the pyridinium species band at 1539 cmÿ1 entirely disappears
after thermodesorption at 523 K. The result is the same using lutidine as a probe molecule. Lutidine interaction give rise to protonated species at 1641 and 1654 cmÿ1 and coordinated species at 1468 and 1621 cmÿ1 (Fig. 3). Nevertheless, BroÈnsted acidity is detected from 8 wt% Re, which is not surprising since lutidine is more basic than pyridine. The role of the preparation mode was also investigated, as Table 2 indicates. Given speci®c area of sol±gel catalysts is two times larger than speci®c area of wet impregnated catalysts, we prefer to express the number of acid sites in mmol/m2, rather than in mmol/ mg. Sol±gel catalysts are far less active (eight times), and far less Lewis acid than their wet impregnated homologue. The local acidity around an active rhenium site for a wet impregnated catalyst is stronger. Infrared spectroscopy also reveals that one part of the rhenium initially introduced is occluded in the mass of the sol±gel materials. Consequently, it is not accessible to propylene, and the performances in propylene metathesis are smaller for a sol±gel catalyst. As for the role of the calcination temperature, Table 3 here after gathers our results. The increase of both the metathesis activity and calcination temperature is concomitant with the increase of Lewis acidity for a Re2O7/Al2O3 8 wt% Re catalyst. Consequently, this is another indication of the favourable role of Lewis acidity on the Re2O7/Al2O3 catalysts performances in ole®n metathesis.
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Table 3 Correlation between the number of Lewis acid sites and BroÈnsted acid sites as a function of the activation temperature of Re2O7/Al2O3 8.0 wt% Re catalysts, and the metathesis activity Tcalcination (K)
nL(423 K) (mmol/mg)
nL(623 K) (mmol/mg)
nB(423 K) (mmol/mg)
ko (mol/(h.gcata))
823 1023
0.21 0.28
0.05 0.18
0.00 0.00
11.1 24.3
As a conclusion, there is a clear correlation between the Lewis acidity of a fresh Re2O7/Al2O3 catalyst and the metathesis activity. This result is explained by: the increase of both numerous and strong Lewis acid sites and activity, the appearance of weak BroÈnsted acidity on a Re2O7/Al2O3 13.7 wt% Re catalyst detected by means of pyridine adsorption and the quasi exponential increase of activity for this rhenium content.
characteristic of the ethylene product. However, after thermodesorption at 523 K (spectrum "c"), the Re2O7/ Al2O3 8 wt% Re catalyst is active again. Accordingly, Lewis acid sites appear to be necessary for the catalyst to start the metathesis reaction, whereas BroÈnsted acid sites are not able to initialise the metathesis reaction.
3.2. Acidity measurements: in situ infrared spectroscopy in the static mode
In order to con®rm or to infer the important role of Lewis acidity which has been evidenced previously, dynamic mode infrared spectroscopy measurements were performed by poisoning the Re2O7/Al2O3 8 wt% Re catalyst with ammonia. The evolution with time on stream of the infrared spectra in the 1700±1100 cmÿ1 region is reported in Fig. 6. The infrared spectrum "a" of the Re2O7/Al2O3 8 wt% Re catalyst under propylene ¯ow at 333 K before ammonia injection exhibits the bands of adsorbed ethylene, propylene and cis butene-2. The broad band around 3600 cmÿ1 corresponds to the OH groups of the Re2O7/Al2O3 catalyst affected by propylene. The two poorly de®ned bands around 3000 cmÿ1 characterise the ethylenic and aliphatic stretching vibrations of the CH groups. At least, the band near 1650 cmÿ1 is assigned to the stretching vibration of the double bond, and the band around 1450 cmÿ1 corresponds to the deformation vibrations of the ethylenic and aliphatic CH groups. Spectrum "b" exhibits ole®n adsorbed on the partly deactivated catalyst after successive introductions of NH3 pulses. Subtracted spectrum "b±a" shows the detail of the characteristic bands of chemisorbed ammonia under propylene ¯ow at 333 K. Ammonia adsorption gives rise to coordinated species (bands at 1618, 1326 and 1276 cmÿ1), but not to the protonated species which
In order to investigate the role of Lewis acidity in the reaction mechanism, metathesis reaction was performed after ammonia thermodesorption at various temperatures, followed by FTIR spectroscopy. The IR spectra of thermodesorbed ammonia have been given previously in (F. Schekler-Nahama, O. Clause, D. Commereuc, and J. Saussey, to be published). It can be noticed that after ammonia thermodesorption at 423 K, all the BroÈnsted acid sites, characterised by the 1680 and 1456 cmÿ1 bands, are restored whereas Lewis acid sites, characterised by the 1624, 1333 and 1276 cmÿ1 bands, are still coordinated. Nevertheless, after ammonia thermodesorption at 523 K, all the Lewis acid sites are almost regenerated. Fig. 5 illustrates the gaseous spectra in propylene metathesis on Re2O7/Al2O3 catalyst after 1 h of reaction time at 353 K. The ! (CH2) band at 950 cmÿ1 characterises gaseous ethylene, and the band at 912 cmÿ1 characterises gaseous propylene. Spectrum "a" is a blank experiment. Without ammonia, the Re2O7/Al2O3 8 wt% Re catalyst is active for propylene metathesis as the 950 cmÿ1 band characteristic of the ethylene product indicates. When ammonia is thermodesorbed at 423 K (spectrum "b"), the Re2O7/Al2O3 8 wt% Re catalyst is not active, since there is no 950 cmÿ1 band
3.3. Acidity measurements: in situ infrared spectroscopy in the dynamic mode
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Fig. 5. Gaseous spectra in propylene metathesis on Re2O7/Al2O3 8wt% Re catalyst, after 1 h of reaction time at 353 K. (a) Without NH3; (b) after NH3 thermodesorption under vacuum at 423 K (10ÿ3 Pa); after NH3 thermodesorption under vacuum at 523 K (10ÿ3 Pa).
would have been characterised by bands at 1680 and 1456 cmÿ1. Numerous studies are devoted to the relationship between the BroÈnsted acidity of the Re2O7/ Al2O3 catalyst and the metathesis activity [6,12]. Spronk revealed that the ReOÿ 4 that reacted with strong BroÈnsted acid surface hydroxyls resulted in the formation of the active centres for metathesis. In this work, BroÈnsted acidity doesn't seem to be indispensable in the metathesis mechanism, whereas Lewis acidity is necessary for the good processing of the reaction. Given our latest data (F. Schekler-Nahama, O. Clause, D. Commereuc, and J. Saussey, to be published), Lewis acidity of aluminium perrhenate, which is supposed to stand at the Re2O7/Al2O3 catalyst surface, would be responsible for the metathesis activity. We suggest that the active species in the metathesis reaction are located on the aluminium
perrhenate compound, probably near the Al3 ions initially engaged in Al(ReO4)3. 4. Conclusion Characterisation of the surface acidity of the Re2O7/ Al2O3 catalyst by infrared spectroscopy and ammonia thermodesorption reveals that the initiation of the metathesis reaction is essentially governed by Lewis acidity. This result strongly supports the role of Lewis acidity exalted by increasing the rhenium content and the calcination temperature. In this respect, acidity measurements clearly indicate that increasing the rhenium content leads to emphasise strong Lewis acidity of alumina at the expense of weak Lewis acidity. Besides, weak BroÈnsted acidity which appears above 14 wt% Re with pyridine adsorption seems to be inef®cient for the good processing of the reaction. Furthermore, acidity measurements by means of the in
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245
Fig. 6. IR spectra of adsorbed alkenes under flow. (a) Without NH3, C11.1% ; (b) after 250 ml NH3, C5.0%.
situ infrared spectroscopy in the dynamic mode clearly indicates that Lewis acidity plays an important role for the good processing of the reaction. Acknowledgements We express our thanks to Mrs. J. Boussard, N. Dos Santos, E. Leplat, V. Poitrineau and Mr. B. Leze for X-ray ¯uorescence and inductively coupled plasma emission spectroscopy, and to Mr. Chauvin for fruitful discussions. References [1] K.J. Ivin, Olefin Metathesis, Academic Press, London, 1983, p 32.
[2] U.S. Patent, 3 641 189 to British Petroleum (1964). [3] U.S. Patent, 3 676 520 to Phillips Petroleum (1964). [4] R. Nakamura, E. Echigoya, Bull. Japan. Petrol. Inst. 14 (1972) 187. [5] R.F. Howe, I.R. Leith, J. Chem. Soc. Faraday I 69 (1973) 1967. [6] X. Xiaoding, J.C. Mol, C. Boelhouwer, J. Chem. Soc., Faraday Trans. 1(82) (1986) 2707. [7] D. Commereuc, Y. Chauvin , J. Chem. Soc. Chem. Commun., 462 (1992). [8] C.C. Hsu, Ph. D. Thesis, Okahoma University, USA, 1980. [9] J. Take, T. Yamaguchi, K. Miyamoto, H. Ohyama, M. Misono, in Y. Murakami (Ed.), Proceedings of the 7th International Zeolite Conference, Tokyo, 1986, Kodansha± Elsevier, Tokyo±Amsterdam, 495 (1986). [10] D. Commereuc, P. Amigues, Y. Chauvin, J. Mol. Catal. 65 (1991) 39. [11] F. Kapteijn, Ph.D. Thesis, Amsterdam, 1980. [12] R. Spronk, Ph.D. Thesis, Amsterdam University, 1991.
In¯uence of Lewis acidity of rhenium heptoxide supported on alumina catalyst on the catalytic performances in ole®n metathesis F. Schekler-Nahama1,*, O. Clause, D. Commereuc, J. Saussey2 Kinetics and Catalysis Division, Institut Franc,ais du PeÂtrole, BP 311, 92506 Rueil-Malmaison Cedex , France Received 17 June 1997; received in revised form 14 October 1997; accepted 15 October 1997
Abstract Characterisation of surface acidity of the Re2O7/Al2O3 catalyst by infrared spectroscopy and ammonia thermodesorption reveals that the initiation and the good processing of the metathesis reaction is essentially governed by Lewis acidity. This strongly supports the role of Lewis acidity quanti®ed by pyridine and lutidine adsorption, which is exalted by the increase of the rhenium content and the calcination temperature and the concomitant increase of the metathesis activity. # 1998 Elsevier Science B.V. Keywords: Lewis acidity of Re2O7/Al2O3; Ole®n metathesis; IR spectroscopy
1. Introduction The metathesis reaction has drawn much interest because of its technological and fundamental importance [1]. It has been known since the pioneering work of researchers from British Petroleum [2] and from Phillips [3]. Concerning the nature of the catalytic active intermediates, surface carbene species, several authors conclude that partly oxidised transition metal ions seem to be necessary to bring about this reaction [4,5]. Most studies of the heterogeneously and homo*Corresponding author. Fax: 00 33 3 20 43 65 01; E-mail: [email protected] 1 Present address: Universite des Sciences et Technologies de Lille, Laboratoire de Catalyse HeÂteÂrogeÁne et HomogeÁne, URACNRS 402 BaÃt. C3, 59655 Villeneuve d'Ascq Cedex, France. 2 UMR.CNRS 6506-ISMRA, Laboratoire Catalyse et Spectrochimie, 6 boulevard du MareÂchal Juin, 14050 Caen Cedex, France. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00310-4
geneously catalysed metathesis have been carried out with molybdenum and tungsten based catalysts; rhenium based homogeneous catalysts are few and little active. However, the comparison is more interesting in the case of rhenium, since conditions of the catalysis are the same, especially the temperature. The literature data indicate that the environment of the active site does possess acid properties. However, the nature of the acid sites is still a matter of debate. It has been proposed that there is a relationship between the BroÈnsted acidity of the catalyst and the metathesis activity [6], the activity of the rhenium metathesis catalyst is not related to their Lewis acidity. In this respect, it is important to notice that Lewis acidity has been quanti®ed without considering the difference between weak and strong Lewis acidity. Consequently, Xiaoding's conclusions seem to be a little premature. On the other hand, Commereuc et al. proposed that the stability with time is governed at
238
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least in part, by the Lewis acidity of the active rhenium centres [7,10]. Hsu has claimed that both in¯uence the activity [8]. The purpose of the present work is to study thoroughly the surface acidity of the Re2O7/Al2O3 catalyst, and the nature of the acidity implicated in the metathesis reaction. Re2O7/Al2O3 catalyst is active even at subatmospheric pressures and at room temperature, and therefore particularly suited for an in situ infrared spectroscopic study. 2. Experimental 2.1. Preparation of the catalysts Catalysts with different Re contents were prepared by incipient wetness method. Our experimental conditions have been described in a previous paper (F. Schekler-Nahama, O. Clause, D. Commereuc, J. Saussey, to be published). The calcination temperature has been studied in the range 823±1023 K from 15 min up to 2 h. Other Re2O7/Al2O3 catalysts have been prepared by sol±gel method, using Al(OiPr)3 as an aluminium precursor, isopropanol as a solvent, HReO4 as a rhenium precursor, and water to hydrolyse. An aluminium triisopropylate±isopropanol (50 g± 35 ml) mixture was heated at 353 K under re¯ux. The solution was gently stirred with a magnetic stirrer. A hole in the cover allowed slow insertion of HReO4 solution, since hydrolysis is an exothermic reaction. The stirring has been maintained for 1 h to homogenise. After drying in an oven at 393 K to remove water and alcohol, the material was calcined at 823 K under air atmosphere for 2 h and then cooled in air. A WI catalyst indicates Re2O7/Al2O3 prepared by incipient wetness, and calcined for 2 h at 823 K under air. A WI 1023 (2 h) catalyst is calcined at 1023 K for 2 h, WI 1023 (15 min) is calcined at 1023 K for 15 min, and SG indicates a Re2O7/Al2O3 prepared by sol±gel method, as it has been described above. 2.2. FTIR spectroscopy 2.2.1. In situ static infrared cell In situ Fourier Transform Infrared measurements were carried out on a NICOLET MAGNA-750 Instru-
ment. The infrared spectra were recorded at beam temperature. The catalyst was activated at 823 K for 2 h under dry oxygen atmosphere. Regular replacements of oxygen were performed to remove desorbed water. The sample is then cooled for 45 min, and oxygen is evacuated. Dry pyridine or lutidine (3.102 Pa at equilibrium) was added at 298 K and physisorbed species evacuated under a dynamic vacuum (10ÿ3 Pa) at 50 K intervals up to 623 K. Subtraction of spectra after pyridine or lutidine desorption from those obtained before probe molecules adsorption evidences bands due to adsorbed species. Quanti®cation of Lewis and BroÈnsted acidity was realised by means of the literature molar extinction coef®cients [9]. 2.2.2. In situ dynamic infrared cell The high temperature infrared reactor cell has been installed on a NICOLET 5SX interferometer sample compartment. It is coupled on DELSI 200 chromatograph allowing automatic gas sampling. The complete system is characterised by a very low dead volume. The infrared reactor is made of stainless steel tubing with two CaF2 windows sealed in two te¯on holders with a Kalrez O-ring for each window. KBr plates are placed in the cell to minimise the dead space (1 mm). A thermocouple is ®xed on the upper side of the cell by a swagelock ®tting and is located in the sample holder. The sample was pressed into self-supporting disks 16 mm in diameter 0.2 mm thick, weighing 20 mg and held with a stainless steel sample holder. The reaction gases were introduced from one side of the sample holder. They were diverted onto both faces of the catalyst and evacuated on the other side. A detailed scheme of the apparatus is given in Fig. 1. Before spectra acquisition, the catalyst was activated under dry oxygen ¯ow at 673 K (with a temperature ramp of 4 K/min) for 2 h, and then cooled in 45 min to the reaction temperature (333 K) under a helium ¯ow to eliminate oxygen traces. The gas ¯ow rate (oxygen or helium) was 5 cm3 NTP/min. Dry propylene diluted with dry helium was admitted to the reactor at atmospheric pressure. The gas ¯ow rate was 1 cm3 NTP/min for each gas. The GC collection performed was every 15 min. The infrared spectra were recorded with the sample at 333 K.
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Fig. 1. Infrared reactor cell.
2.2.3. Ammonia pulse injections 50 ml portions of dry ammonia were successively introduced to a helium ¯ow in the reactor cell at 333 K. IR spectra were recorded after each ammonia injection. 2.3. Metathesis reaction The deactivation experiments were carried out in a conventional ®xed-bed micro-catalytic down ¯ow reactor. The reactor was a quartz tube (10 mm in diameter), that could be placed in a vertical oven. In a typical experiment, a catalyst sample of 2 g (200±700 mm sifted) was placed in the reactor and calcined for 2 h in an oxygen stream (13.5 l/(h.g)) at 823 K (heating rate 5 K/min). The reactor was subsequently taken out of the oven, and cooled to the reaction temperature in 15 min. The reaction temperature is thermostatically controlled at 323 2 K. Standard reaction conditions were a LHSV11 hÿ1 (2 g of catalyst and a propylene ¯ow of 44 l/h), and an atmospheric pressure. Oxygen and propylene were dried on 4A molecular sieves, and propylene was deoxygenated by means of a deoxo cartridge (CHROMPACK). The reaction products were analysed on line by a gas phase chromatograph, and an alumina-PLOT capillary column (CHROMPACK), 50 m in length
and 0.53 mm in diameter. The GC signals were processed by a Hewlett Packard integrator. 2.4. Activity measurements The experimental results have been interpreted on the basis of a Langmuir±Hinshelwood model including a second order equilibrated reaction. The rate constants derived from this model were used as a tool for the estimation of activity of the catalyst. They were calculated according to the following relations. For laboratory-scale experiments, perfectly-stirred reactor, reactant propylene : kt f2
N3e ÿ N3s with 2mf2 4 1 ÿ x2 ÿ x2 =Keq
2Q= b3 N3e RT 2 1 ÿ x B2 B4 x2
where kt: rate constant at time t, m: mass of catalyst, N3e: molar ¯ow rate of propylene in, N3s: molar ¯ow rate of propylene out, Keq equilibrium constant, Q: total volume ¯ow supposed to be constant, R: perfect gas constant (R8.314 m3 Pa/(K mol), T: reaction temperature, B2b2/b3, B4b4/b3 (bi: adsorption coef®cient of the ole®n i). In our calculations, we have adopted the following correlation previously checked by Commereuc et.al. [10]
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Keq 2:833 10ÿ4 T K ÿ 273 0:0833 between 293 and 333 K: Calculation of the rate constant needs the knowledge of the ratios of the adsorption coef®cients bi/b3 relative to propylene for all components i of the medium, and the knowledge of the adsorption coef®cient b3. These values have been extracted from Kapteijn's thesis [11]. The deactivation of the catalyst was interpreted as if the amount of the active catalyst were decreasing with time. This process has been shown to follow a ®rstorder rate law, according to the relation : loge kt loge ko ÿ kd t in which ko: the initial metathesis rate constant, and kd: the ®rst order deactivation rate constant are the two parameters used to characterise the performances of the catalysts. 3. Results and discussion 3.1. Metathesis activity The activity of the rhenium on alumina catalysts increases quasi exponentially with their rhenium con-
tent (Figs. 2±4). Consequently, rhenium is better used at high content and the environment of the active site has to possess an optimal rhenium content. On the other hand, the metathesis reaction does not work at very low rhenium contents (1 wt% Re or less). Another point remarked is that beyond a given content (10 wt% Re), the activity indeed continues to increase, but an oligomerisation reaction of the ole®n present superimposes on the metathesis reaction. This is ascertained by a brown colouring of the catalyst contacting with propylene, due to the presence of byproducts. This phenomenon was also accompanied by a faster rate of catalyst deactivation (more than 3 orders of magnitude). Comparing the preparation methods, sol±gel catalysts are far less active (5 orders of magnitude) than their homologous wet impregnated Re2O7/Al2O3 catalysts. Furthermore faster deactivation is observed in the case of the sol±gel catalysts. Regarding the effect of the calcination temperature, increasing temperature (from 823 to 1023 K) and calcination time enhances metathesis activity. In order to explain the increase of the activity with Re content, we suspected the role of acidity in the metathesis mechanism. Furthermore, oligomerisation side reaction could have a cationic origin resulting
Fig. 2. Infrared spectra of Re2O7/Al2O3 with varying Re loadings after pyridine adsorption thermodesorption in vacuum (10ÿ3 Pa) at 423 K. (a) 0.0 wt % Re; (b) 1.8; (c) 8.0; (d) 13.7.
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Fig. 3. Infrared spectra of Re2O7/Al2O3 and Al2O3 after lutidine adsorption thermodesorption in vacuum (10ÿ3 Pa) at 423 K. (a) 0.0 wt % Re; (b) 8.0.
Fig. 4. Propylene metathesis T323 K: effects of the rhenium content (support g-Al2O3), the preparation method, and the calcination temperature on the metathesis activity.
from the presence of BroÈnsted acidity at high rhenium content. Acidity measurements were performed by means of FTIR spectroscopy using pyridine and lutidine adsorp-
tion. Pyridine interacts, after desorption at 423 K, give rise to protonated species at 1539 cmÿ1, and coordinated species at 1452 and 1618 cmÿ1 for weak Lewis acidity, at 1456 and 1624 cmÿ1 for strong Lewis
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Table 1 Correlation between the number of Lewis acid sites and BroÈnsted acid sites on Re2O7/Al2O3 catalysts and the metathesis activity wt% Re
nL(1508C) (mmol/mg)
nB(1508C) (mmol/mg)
ko (mol(h.gcata))
0.0 1.8 8.0 13.7
0.16 0.19 0.21 0.26
0.00 0.00 0.00 0.09
0.0 0.4 11.1 74.7
(50% strong) (75% strong) (100% strong) (100% strong)
Table 2 Correlation between the number of Lewis acid sites and BroÈnsted acid sites as a function of the preparation methods of Re2O7/Al2O3 7.0 wt% Re catalysts (incipient wetness "WI" impregnation and sol gel "SG") and the metathesis activity Reference sample
nL(423 K) (mmol/m2)
nL(623 K) (mmol/m2)
nB(423 K) (mmol/m2)
ko (mol/(h.gcata))
WI 7.0 SG 7.0
0.93 0.57
0.15 0.05
0.00 0.00
7.9 1.0
acidity (Fig. 2). The evolution of the metathesis activity as a function of the rhenium content is correlated with the acid properties of alumina or nearest rhenium oxide sites. Table 1 summarises our main results. Increasing rhenium content leads to the simultaneous increase of the strong Lewis acidity of Re2O7/ Al2O3 materials at the expense of the weak Lewis acidity of alumina, the appearance of BroÈnsted acidity, and the increase of the metathesis activity. Therefore, there is a distinct correlation between the increase of Lewis acidity and metathesis activity at such a rhenium loading that there is no BroÈnsted acidity detected yet. Below 10 wt% Re, increasing the rhenium content is almost proportional with increasing Lewis acidity, whereas the increase of activity is quasi exponential. In fact, we should consider both the numerical and strength increase of the Lewis acid sites to establish a distinct correlation with the non proportional increase of activity. Furthermore, the role of BroÈnsted acidity cannot be ruled out since activity of the Re2O7/Al2O3 13.7 wt% Re catalyst corresponds to a drastic increase of the metathesis activity. However, the presence of BroÈnsted acidity on the Re2O7/Al2O3 13.7 wt% Re catalyst enhances oligomerisation or polymerisation side-reactions which have a cationic origin. Resulting by-products behave as diffusional barriers and block the active sites. This phenomenon was also accompanied by a faster rate of catalyst deactivation. At least, let us notice that BroÈnsted acidity on the Re2O7/ Al2O3 13.7 wt% Re catalyst is weak, since the pyridinium species band at 1539 cmÿ1 entirely disappears
after thermodesorption at 523 K. The result is the same using lutidine as a probe molecule. Lutidine interaction give rise to protonated species at 1641 and 1654 cmÿ1 and coordinated species at 1468 and 1621 cmÿ1 (Fig. 3). Nevertheless, BroÈnsted acidity is detected from 8 wt% Re, which is not surprising since lutidine is more basic than pyridine. The role of the preparation mode was also investigated, as Table 2 indicates. Given speci®c area of sol±gel catalysts is two times larger than speci®c area of wet impregnated catalysts, we prefer to express the number of acid sites in mmol/m2, rather than in mmol/ mg. Sol±gel catalysts are far less active (eight times), and far less Lewis acid than their wet impregnated homologue. The local acidity around an active rhenium site for a wet impregnated catalyst is stronger. Infrared spectroscopy also reveals that one part of the rhenium initially introduced is occluded in the mass of the sol±gel materials. Consequently, it is not accessible to propylene, and the performances in propylene metathesis are smaller for a sol±gel catalyst. As for the role of the calcination temperature, Table 3 here after gathers our results. The increase of both the metathesis activity and calcination temperature is concomitant with the increase of Lewis acidity for a Re2O7/Al2O3 8 wt% Re catalyst. Consequently, this is another indication of the favourable role of Lewis acidity on the Re2O7/Al2O3 catalysts performances in ole®n metathesis.
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Table 3 Correlation between the number of Lewis acid sites and BroÈnsted acid sites as a function of the activation temperature of Re2O7/Al2O3 8.0 wt% Re catalysts, and the metathesis activity Tcalcination (K)
nL(423 K) (mmol/mg)
nL(623 K) (mmol/mg)
nB(423 K) (mmol/mg)
ko (mol/(h.gcata))
823 1023
0.21 0.28
0.05 0.18
0.00 0.00
11.1 24.3
As a conclusion, there is a clear correlation between the Lewis acidity of a fresh Re2O7/Al2O3 catalyst and the metathesis activity. This result is explained by: the increase of both numerous and strong Lewis acid sites and activity, the appearance of weak BroÈnsted acidity on a Re2O7/Al2O3 13.7 wt% Re catalyst detected by means of pyridine adsorption and the quasi exponential increase of activity for this rhenium content.
characteristic of the ethylene product. However, after thermodesorption at 523 K (spectrum "c"), the Re2O7/ Al2O3 8 wt% Re catalyst is active again. Accordingly, Lewis acid sites appear to be necessary for the catalyst to start the metathesis reaction, whereas BroÈnsted acid sites are not able to initialise the metathesis reaction.
3.2. Acidity measurements: in situ infrared spectroscopy in the static mode
In order to con®rm or to infer the important role of Lewis acidity which has been evidenced previously, dynamic mode infrared spectroscopy measurements were performed by poisoning the Re2O7/Al2O3 8 wt% Re catalyst with ammonia. The evolution with time on stream of the infrared spectra in the 1700±1100 cmÿ1 region is reported in Fig. 6. The infrared spectrum "a" of the Re2O7/Al2O3 8 wt% Re catalyst under propylene ¯ow at 333 K before ammonia injection exhibits the bands of adsorbed ethylene, propylene and cis butene-2. The broad band around 3600 cmÿ1 corresponds to the OH groups of the Re2O7/Al2O3 catalyst affected by propylene. The two poorly de®ned bands around 3000 cmÿ1 characterise the ethylenic and aliphatic stretching vibrations of the CH groups. At least, the band near 1650 cmÿ1 is assigned to the stretching vibration of the double bond, and the band around 1450 cmÿ1 corresponds to the deformation vibrations of the ethylenic and aliphatic CH groups. Spectrum "b" exhibits ole®n adsorbed on the partly deactivated catalyst after successive introductions of NH3 pulses. Subtracted spectrum "b±a" shows the detail of the characteristic bands of chemisorbed ammonia under propylene ¯ow at 333 K. Ammonia adsorption gives rise to coordinated species (bands at 1618, 1326 and 1276 cmÿ1), but not to the protonated species which
In order to investigate the role of Lewis acidity in the reaction mechanism, metathesis reaction was performed after ammonia thermodesorption at various temperatures, followed by FTIR spectroscopy. The IR spectra of thermodesorbed ammonia have been given previously in (F. Schekler-Nahama, O. Clause, D. Commereuc, and J. Saussey, to be published). It can be noticed that after ammonia thermodesorption at 423 K, all the BroÈnsted acid sites, characterised by the 1680 and 1456 cmÿ1 bands, are restored whereas Lewis acid sites, characterised by the 1624, 1333 and 1276 cmÿ1 bands, are still coordinated. Nevertheless, after ammonia thermodesorption at 523 K, all the Lewis acid sites are almost regenerated. Fig. 5 illustrates the gaseous spectra in propylene metathesis on Re2O7/Al2O3 catalyst after 1 h of reaction time at 353 K. The ! (CH2) band at 950 cmÿ1 characterises gaseous ethylene, and the band at 912 cmÿ1 characterises gaseous propylene. Spectrum "a" is a blank experiment. Without ammonia, the Re2O7/Al2O3 8 wt% Re catalyst is active for propylene metathesis as the 950 cmÿ1 band characteristic of the ethylene product indicates. When ammonia is thermodesorbed at 423 K (spectrum "b"), the Re2O7/Al2O3 8 wt% Re catalyst is not active, since there is no 950 cmÿ1 band
3.3. Acidity measurements: in situ infrared spectroscopy in the dynamic mode
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Fig. 5. Gaseous spectra in propylene metathesis on Re2O7/Al2O3 8wt% Re catalyst, after 1 h of reaction time at 353 K. (a) Without NH3; (b) after NH3 thermodesorption under vacuum at 423 K (10ÿ3 Pa); after NH3 thermodesorption under vacuum at 523 K (10ÿ3 Pa).
would have been characterised by bands at 1680 and 1456 cmÿ1. Numerous studies are devoted to the relationship between the BroÈnsted acidity of the Re2O7/ Al2O3 catalyst and the metathesis activity [6,12]. Spronk revealed that the ReOÿ 4 that reacted with strong BroÈnsted acid surface hydroxyls resulted in the formation of the active centres for metathesis. In this work, BroÈnsted acidity doesn't seem to be indispensable in the metathesis mechanism, whereas Lewis acidity is necessary for the good processing of the reaction. Given our latest data (F. Schekler-Nahama, O. Clause, D. Commereuc, and J. Saussey, to be published), Lewis acidity of aluminium perrhenate, which is supposed to stand at the Re2O7/Al2O3 catalyst surface, would be responsible for the metathesis activity. We suggest that the active species in the metathesis reaction are located on the aluminium
perrhenate compound, probably near the Al3 ions initially engaged in Al(ReO4)3. 4. Conclusion Characterisation of the surface acidity of the Re2O7/ Al2O3 catalyst by infrared spectroscopy and ammonia thermodesorption reveals that the initiation of the metathesis reaction is essentially governed by Lewis acidity. This result strongly supports the role of Lewis acidity exalted by increasing the rhenium content and the calcination temperature. In this respect, acidity measurements clearly indicate that increasing the rhenium content leads to emphasise strong Lewis acidity of alumina at the expense of weak Lewis acidity. Besides, weak BroÈnsted acidity which appears above 14 wt% Re with pyridine adsorption seems to be inef®cient for the good processing of the reaction. Furthermore, acidity measurements by means of the in
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Fig. 6. IR spectra of adsorbed alkenes under flow. (a) Without NH3, C11.1% ; (b) after 250 ml NH3, C5.0%.
situ infrared spectroscopy in the dynamic mode clearly indicates that Lewis acidity plays an important role for the good processing of the reaction. Acknowledgements We express our thanks to Mrs. J. Boussard, N. Dos Santos, E. Leplat, V. Poitrineau and Mr. B. Leze for X-ray ¯uorescence and inductively coupled plasma emission spectroscopy, and to Mr. Chauvin for fruitful discussions. References [1] K.J. Ivin, Olefin Metathesis, Academic Press, London, 1983, p 32.
[2] U.S. Patent, 3 641 189 to British Petroleum (1964). [3] U.S. Patent, 3 676 520 to Phillips Petroleum (1964). [4] R. Nakamura, E. Echigoya, Bull. Japan. Petrol. Inst. 14 (1972) 187. [5] R.F. Howe, I.R. Leith, J. Chem. Soc. Faraday I 69 (1973) 1967. [6] X. Xiaoding, J.C. Mol, C. Boelhouwer, J. Chem. Soc., Faraday Trans. 1(82) (1986) 2707. [7] D. Commereuc, Y. Chauvin , J. Chem. Soc. Chem. Commun., 462 (1992). [8] C.C. Hsu, Ph. D. Thesis, Okahoma University, USA, 1980. [9] J. Take, T. Yamaguchi, K. Miyamoto, H. Ohyama, M. Misono, in Y. Murakami (Ed.), Proceedings of the 7th International Zeolite Conference, Tokyo, 1986, Kodansha± Elsevier, Tokyo±Amsterdam, 495 (1986). [10] D. Commereuc, P. Amigues, Y. Chauvin, J. Mol. Catal. 65 (1991) 39. [11] F. Kapteijn, Ph.D. Thesis, Amsterdam, 1980. [12] R. Spronk, Ph.D. Thesis, Amsterdam University, 1991.