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Alkylation of benzene with propylene catalyzed by fluorined alumina
Applied Catalysis A: General 169 (1998) 15±27
Alkylation of benzene with propylene catalyzed by ¯uorined alumina L.M. Rodrigueza, J. Alcaraza, M. Hernandeza, Y. Ben Taaritb, M. Vrinatb,* b
a Facultad de Quimica, UNAM, Ciudad Universitaria, 04510 Mexico DF, Mexico Institut de Recherches sur la Catalyse, 2 Av. A. Einstein, 69626 Villeurbanne Cedex, France
Received 3 April 1997; received in revised form 25 September 1997; accepted 14 October 1997
Abstract Fluorinated alumina catalysts have been prepared with different ¯uorine content and studied by combining textural and structural characterizations (BET, Porosity, DRX), surface composition (XPS), acidity (potentiometric titration) and catalytic properties in alkylation of benzene by propylene. Impregnation by NH4F solution modi®es the surface of the alumina phase, with the formation of ¯uoride surface species at low ¯uorine content and the formation of AlF3 and aluminium hydroxy¯uoride as the isolated crystallites at high ¯uorine content. Increase of the activity with the ¯uorine content up to 14.5 wt % also suggests the participation of these compounds in the catalytic sites. As far as the alkylation of benzene over these catalysts are concerned the reaction was found to be highly selective toward cumene, which avoid the formation of undesirable heavy products. # 1998 Elsevier Science B.V. Keywords: Catalysts; Alumina; Fluoride; Benzene; Alkylation
1. Introduction The 1990 Clean Air Act (CAA) Amendments target a substantial reduction of volatile organic compounds (VOC's) and toxics from vehicles and their fuels. The resultant gasoline reformulation is regarded as the most severe of all fuel speci®cations since the lead phaseout of the early 1970's. Such a legislation has many implications on the modern re®nery, including the role of catalytic reforming units, which are the most important sources of hydrogen of the re®ners. Among the toxic and pollutant compounds, benzene is considered as a powerful cancer-producing agent and its content in the reformulate gasoline must be less *Corresponding author. Tel.: +334 72 445 300; fax: +33 4 72 445 399. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00309-8
than 1.0 vol % As 70±80% of the benzene in the gasoline is coming from the catalytic reformate, attention has to be paid to that process or its resulting petroleum cut [1]. The reduction of benzene production could be obtained by minimizing its formation during the catalytic reforming process, or by elimination of the benzene-rich light reformate stream by subsequent benzene conversion or extraction. However, if avoiding the formation of benzene is the most logical option, economical and practical considerations limit the possibility to apply this strategy, a benzene net reduction below 1 vol % being impossible without a negative impact over the hydrogen production and in the octane number of the resulting gasoline. Light reformate processing therefore appears to be a more direct alternative to benzene reduction, and four basic
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potential options are available :saturation, saturation and isomerization, extraction or alkylation. The saturation of this aromatic compound consumes a lot of hydrogen and leads to a decrease of the octane number. Benzene saturation combined with isomerization results in a gain in octane number but the ®nal product has a slightly higher Reid Vapor Pressure and the net loss of hydrogen is greater. The extraction option is only considered by the re®ners also involved in the petrochemical market [2]. Taking into account the hydrogen consumption, benzene alkylation appears a real catalytic alternative to reduce net benzene. For benzene alkylation, the use of light ole®nes as ethylene, propylene or butene as alkylation agent could be considered since they are the by-products of catalytic cracking units, and, except butene, not always used in other gasoline production processes. Use of butene is limited by the boiling point (446± 456 K) of the butylbenzene obtained, close to the gasoline distillation end-point (443±473 K). Benzene alkylation by ethylene can be performed by zeolites and several technologies are available. However, in some re®neries propylene is more available than ethylene for alkylation. Moreover, alkylation of benzene catalyzed by a solid is commonly considered as processing via carbonium-ion type mechanism [3] and the proton af®nity of propylene is higher than that of ethylene (180.4 vs 160.6 kcal/mol) [4]. Therefore, in order to limit the severity of the experimental conditions and also the secondary reactions propylene was used in this work. Alkylation of benzene with propylene to produce cumene is a well-known reaction performed in the petrochemical industry using `solid phosphoric acid' as a catalyst. Other Friedel±Crafts catalysts, such as aluminium chloride have also been used [5]. However, application of such catalysts for processing a mixture of hydrocarbons in which benzene is only 15±20 vol % appears to be limited. Effort has been put in developing alternative catalytic systems for the oil industry. That was done using several zeolites [4,6±8], but only two commercial cumene processes catalyzed by zeolites have been developed [2]. Use of zeolites in such process was restricted by deactivation associated with pore plugging via formation of coke deposits [7]. On the contrary, only little work has been done with ¯uorined alumina though it is well known that such
a solid exhibits catalytic activity for acid catalyzed reactions as cracking, isomerization and polymerization [9]. This paper describes direct vapor-phase alkylation of benzene with propylene over a series of ¯uorinated alumina. Reaction was performed in a continuous ¯ow unit and the catalysts were characterized by various techniques. 2. Experimental 2.1. Catalysts preparation The starting alumina was a commercial g-Al2O3 with a surface area of 320 m2/g and a pore volume of 0.43 cm3/g. Except for the diffusion control study, all samples were issued from an Al2O3 powder with particle sizes between 100 and 200 mesh. The powder was calcined at 773 K for 18 h before impregnation. The aqueous incipient wetness impregnation was performed with aqueous solutions of NH4F prepared in order to get a ¯uorine loading between 0 and 20 wt%. After impregnation, the solids were left at room temperature for 2 h, dried at 353 K for 16 h and then calcined at 723 K for 4 h. Another powder alumina was impregnated with bidistillated water instead of NH4F solution. That sample was utilized as a reference for the solids characterizations. 2.2. Solids characterization The ¯uorine contents have been determined for samples calcined at 723 K by the ICP technique. Speci®c surface areas of the F/Al2O3 were obtained by nitrogen physisorption using an automated BET apparatus. The X-ray diffraction (XRD) patterns were collected using a Siemens D500, /2 diffractometer. The run source was a CuKa cell (1.5418 A8). The photoelectron spectra (XPS) have been obtained using a VG Scienti®c, Escalab 200R spectrometer using AlKa radiation. The reference utilized was the O1 s (531.2 eV) and the recorded peaks were Al2p and F1s. The total acidity (TA) of the solids was determined by n-butylamine potentiometric titration method as described in literature [10]. This procedure was used to measure the number of sites and the maximum acid strength (MAS8of the surface).
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Fig. 1. Schematic diagram of the reaction system. (1) Dryers, (2) saturator, (3) gas chromatograph and (4) reactor.
2.3. Catalytic activities The catalytic activity for alkylation of benzene with propylene was determined in a continuous ¯ow unit, schematized in Fig. 1. Nitrogen and propylene pressures were stabilized and a regulated inlet using a pressure control regulator. Their ¯ow was measured by rotameters. Benzene was introduced by bubbling N2 in a saturator±condensator device whose temperature was perfectly regulated. The reactor bed consisted of a 10 ml preheated zone containing quartz powder, the catalyst (0.15 to 2.0 g), and a post reaction zone containing 2 g of crushed quartz. Pressure in the reactor (2.1 atm) was maintained and measured using a pressure control regulator. All the lines were heated in order to avoid any condensation, and after the reactor the products are analyzed using a Gow-Mac 580 gas chromatograph (FID detector) equipped with a packed column (5% OV-1010N CHROM-P-AW-DMCS). Benzene, was supplied by Aldrich, (purity>99%) and propylene (purity>99%) was a product of Linde.
The reaction was performed at temperatures between 593 K and 693 K. The results were expressed by : the benzene conversion, the propylene conversion, the cumene selectivity referred to benzene defined by the ratio : mole number of cumene obtained/ mole number of benzene transformed, the cumene selectivity referred to propylene defined by the ratio : mole number of cumene obtained/mole number of propylene transformed. 3. Results and discussion. 3.1. Solids characterization 3.1.1. Chemical analysis and surface area The solids prepared are listed in Table 1 with the ¯uorine content given in wt%. From this table it appears that there is a difference between the ¯uorine expected and the ¯uorine obtained. This indicates that an important fraction of ¯uorine is lost during the preparation, the halogen losses being greater when the
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Table 1 Chemical analysis and surface area of the catalysts Sample number
% F content (% wt) Expected
LRF-00 LRF-02 LRF-03 LRF-04 LRF-05 LRF-06 LRF-10 LRF-15 LRF-20 a
0 2 3 4 5 6 10 15 20
Obtained
a a
0 1.9 3.19
a
6.5 13.1 14.5
Surface area (m2/g) 231 201 196 185 199 172 160 133 187
NANot available.
solutions have higher concentrations of NH4F. This behavior could be explained through the more and more dif®cult reaction of ¯uorine with the alumina as soon as the super®cial and then the second layer of aluminium atoms are attacked. At high ¯uorine content unreacted NH4F is therefore maintained on the catalyst during the drying and sublimated during calcination. The evolution of the surface area as a function of the ¯uorine content is shown in Table 1. The solids lose surface area when the halogen content rises. This progressive decrease is observed until the wt% of ¯uorine reaches about 13. However, when the ¯uorine content get a value of 14 wt%, a sharp increase occurs. Fig. 2 shows that when the ¯uorine content increases the pore size distribution is larger than in the original alumina. The adsorption±desorption isotherms of the solid without ¯uorine and LRF-20 (14.5 w%) are presented in Fig. 3 and it shows that the desorption step is slightly displaced to a higher P/Po value for the support with more ¯uorine content. The modi®cation in the pores size distribution and the change of desorption P/Po value indicate an increase of the pores diameters induced by the ¯uorination. 3.1.2. XRD. Three samples were analyzed by XRD :LRF-10, LRF-15 and LRF-20. Their diffractograms are given in Fig. 4. From the patterns obtained we observe : the three solids give rise to the typical diffraction peaks of the alumina support (2678, 468 and 37.58);
Fig. 2. Variation, with respect to fluorine content, of pore distribution of some F/Al2O3 samples.
Fig. 3. Adsorption±desorption isotherms of N2 at 77 K for the LRF-00 and LRF20 samples.
the patterns obtained for the LRF-15 and LRF-20 give additional peaks at 214.88, 25.18, 25.88, 29.88 and 51.18, some of which are due to the
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
Fig. 4. DRX diffractograms of some F/Al2O3 catalysts. (ÐÐÐ) g-AlF3 and AlF3.3H2O; (Ð-) AlF3-X(OH)X.
presence of g-AlF3*nH2O; peaks at 215.68, 308, 31.58 and 52.88 are also evidenced and these peaks are attributed to the presence of AlF1.65(OH)1.35*nH2O; in the case of the LRF-15, small additional peaks at 217.58, 19.98 and 62.18 are identical with the typical pattern of (NH4)3AlF6; the intensities of the g-AlF3*nH2O diffraction lines are greater in the sample LRF-20 than in the LRF-15; the peaks at 2168, 238, 26.88, 29.18, 51.38 and 52.98 suggest that AlF3*3H2O was present on the surface of the LRF-15 and LRF-20 catalysts. Evidence of the formation of (NH4)3AlF6 over ¯uorinated alumina was reported by Kowalak [11,12] who suggested this compound to be formed by the following reaction : Al3 6NH4 F ) NH4 3 AlF6 3NH3 3H : The transformation of (NH4)3AlF6 into (NH4)AlF4, g-AlF3 and ®nally a-AlF3 has been reported to occur respectively at 573, 773 and 873 K [13]. However, Kerkhof et al. [14] proposed the formation of AlF3 to be the result of consecutive reactions between Al(OH)3 and HF, leading to Al(OH)2F, Al(OH)F2 and other intermediates as AlF1.65(OH)1.35*nH2 O, or in a general formula AlF3-X(OH)X.
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Fig. 5. Comparison between DRX diffractograms of LRF-20 and AlF33H2O not calcined.
The presence of (NH4)3AlF6 in the LRF-15 sample and of AlF3-X(OH)X in both LRF-15 and LRF-20 catalysts suggest that AlF3 could be formed according to both mechanisms in agreement with Bulgakov and Antipina [15]. However, the proportion of these two compounds would be related to the Al2O3/NH4F ratio, which means to the concentration of the NH4F solution used for impregnation. For catalysts with a ¯uorine content lower than 13 wt%, the XRD technique does not allow to get any information. A commercial AlF3*3H2O compound (surface area of 48 m2/g) was also studied and its diffraction diagram given in Fig. 5 presents peaks at 2168, 238, 26.88, 29.18, 51.38 and 52.98, as also observed for the LRF-15 and LRF-20 samples. 3.1.3. XPS. The XPS spectra have been recorded on samples after calcination and the scale of binding energy has been obtained by taking a value of 531.20 eV for O1s. This value is in satisfactory agreement with the literature data. Fig. 6 gives the detailed Al2p peak of the four samples. For LRF-04 catalyst, the Al2p peak is symmetric and the binding energy at 74.3 eV in agreement with that of Al in Al2O3. For the other three samples, the asymmetric peak suggests the occurrence of aluminium involved in other environ-
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Fig. 6. Effect of the fluorine content on the Al2p XPS signal.
ment. Indeed, the deconvolution of the Al2p spectra, as given in Fig. 7 for the LRF-15 sample, indicates that this peak is the sum of a peak at 74.3 eV (Al belonging to Al2O3) and of a peak at about 76.5± 77.0 eV, which correspond to Al in ¯uorine environment. The results of these deconvolutions are reported in Table 2. For the F1s peak, pro®les reported in Fig. 8 clearly evidence an evolution of the binding energies with the ¯uorine content of the catalyst. Such asymmetric peaks also indicate the presence of different ¯uorine species on the surface of the samples. Deconvolution of the peaks was therefore attempted assuming two ¯uorine species. The ®rst peak with a binding energy of 685.5 eV corresponds to super®cial ¯uoride ions at the surface of alumina and would be the result of the following reactions : Al ÿ OH NH4 F ! Al ÿ F H2 O NH3 Al ÿ O ÿ Al NH4 F ! Al ÿ OH F ÿ Al NH3
Fig. 7. Deconvolution of the Al2p and F1s XPS spectra for the LRF-15 sample.
Such entities are dominant at low ¯uorine contents. The second peak at about 688 eV could be attributed to aluminium hydroxy¯uorides and (or) to aluminium ¯uoride crystallites. Such a peak, which increase with the ¯uorine content, is rather important for the catalysts with more than 6% of ¯uorine. Such a value was founded in several reports to correspond to the amount of ¯uorine needed to form a monolayer on the surface of Al2O3 [16±18]. These XPS results also allow to understand the surface area evolution of the LRF-20 sample. In that sample, the percentage of ¯uorine involved in the formation of super®cial ¯uorine atoms is higher than over the LRF-15 sample, which could
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Table 2 XPS results for some F/Al2O3 catalysts LRF-04
LRF-10
LRF-15
LRF-20
wt% F Al2p (eV)
3.19 74.3 (100%)
F1s (eV)
685.4 (95%) 688.0 (5%) 0.17 0.65
6.5 74.4 (91.5%) 76.5 (8.5%) 685.5 (60%) 687.8 (40%) 0.34 0.74
13.1 74.35 77.30 685.4 688.2 0.46 0.78
14.5 74.4 (92.0%) 76.3 (8.0%) 685.5 (52%) 688.1 (48%) 0.50 0.70
F1s/Al2p Al2p/O1s
(85.0%) (15.0%) (35%) (65%)
intensity, the cross section as given by Sco®eld [20] and Ec the kinetic energy. The results given in Table 2 show that for the low ¯uorine content, the F1s/Al2p ratio increases linearly with the ¯uorine content, which is the proof of a high and regular dispersion of the ¯uorine on the alumina surface. For higher ¯uorine content, the F1s/Al2p ratio tends to a plateau suggesting the formation of bulk ¯uorinated compounds. For the Al2p/O1s atomic ratio, the calculated value for the LRF-04 catalyst is in good agreement with the theoretical value (0.66). For higher ¯uorine content, the increase of the Al2p/ O1 s ratio re¯ects the replacement of OH species by ¯uoride ones. 3.1.4. Acidity The effect of ¯uorination on the total number of acid sites of alumina and over the maximum acid strength is illustrated in Figs. 9 and 10. The potentiometric method, used in this work, allows the determination of all acid sites irrespective of their Lewis or BroÈnsted nature. 3.2. Catalytic activities Fig. 8. Effect of the fluorine content on the F1s XPS signal.
be explained by the higher surface area of the sample. Finally, estimation of the F1s/Al2p and Al2p/O1s atomic ratios was made using the equation [19] : p m1 I1 2 Ec2 p m2 I2 1 Ec1 where m is the number of atoms analyzed, I is the peak
3.2.1. Preliminary studies: control of the kinetic regime In heterogeneous catalysis, the measured rate not only depends on the surface reaction rate but could be modi®ed by reactor wall effects and mass transfer rates due to the gradients of concentration. In order to control such phenomena related to experimental conditions, a set of experiments has been performed. A blank run using quartz powder was made at 673 K in order to verify that the stainless steel reactor does not contribute to the reaction. Effect of the granulometry
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L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
Fig. 9. Evolution of the total acidity (TA) of F/Al2O3 as a function of the fluorine content.
of the catalyst was controlled using three samples prepared with 5% of ¯uorine over Al2O3 powder crushed and screened in particle fractions between 80 and 100 mesh, 100 and 200 mesh and larger than 200 mesh. At 673 K and for the same reactant ¯ow rate, the same conversions have been obtained. External diffusion was controlled at 623 K using various amounts of catalyst LRF-20, keeping the space velocity constant (W/F0.0175 g mn cmÿ3) and varying the ¯ow rate in a large extent. The results given in Fig. 11 allow to de®ne the region outside the control of external diffusion limitation. Therefore, it appears that experiments performed with 0.5 g of catalyst prepared over Al2O3 with a granulometry between 100 and 200 mesh and with a total ¯ow rate of 1.510ÿ3 mol minÿ1, could be considered as belonging to the chemical kinetic regime. Attention has also been given to thermal effect. Indeed, though alkylation is a very exothermic reaction, no variation of the temperature of the catalytic bed has been observed. That could be explained by the presence of N2 used as a carrier gas for benzene (dilution effect) and to the low conversions which were obtained under our experimental conditions.
Fig. 10. Evolution of the maximum acid strength (MAS) of F/Al2O3 as a function of the fluorine content.
Fig. 11. Control of external diffusion for the LRF-20: activity as a function of linear velocity at constant space velocity (reaction temperature 623 K, molar ratio C 3 H 6 /C 6 H 6 0.6, W/F 0.0175 g mn cmÿ3).
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
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Table 3 Activities of the F/Al2O3 catalysts in the alkylation reactiona at 673 K Catalyst Number
C6H6 Conversion (%)
Cumene Selectivity (%)
C3H6 Conversion (%)
Cumene Selectivity (%)
LRF-00 LRF-01 LRF-02 LRF-03 LRF-04 LRF-05 LRF-10 LRF-15 LRF-20
0 0 0.9 5.0 7.2 9.5 11.5 12.0 11.8
± ± ± 99.9 99.7 99.5 99.3 99.2 99.5
0 0 0.24 18.7 27 30.2 38.4 54.8 66
± ± ± 9.9 11.5 10.3 6.11 5.9 3.7
a
Catalyst 0.5 g; pressure 2.13 atm; C3H6/C6H65.8 mol/mol.
3.2.2. Comparison of catalysts at 673 K :effect of the fluorine content The various catalysts were tested at 673 K, under a C3H6/C6H6 molar ratio equal to 5.8 and results are summarized in Table 3. Conversions and selectivities were calculated after 120 min time on stream when the catalyst reached a steady state regime, which one is maintained during all the experiment (about 7 h). For a free ¯uorine catalyst no conversion occurs. When the ¯uorine content rises, both benzene and propene conversions increase, but whatever the catalyst considered, the selectivity of transformation of benzene to cumene is very close to 100% since no dialkylbenzene was evidenced. That is in agreement with thermodynamic calculations which indicate the formation of dialkylbenzene to be very low at 673 K. For propene a high conversion is associated with the formation of methylcyclopentane as the major product; cyclohexane was only observed as traces. As a complementary experiment, the transformation of propene alone was performed at 623 and 673 K over the LRF20 catalyst. Only the methylcyclopentane and the cyclohexane were formed. For catalysts with a ¯uorine content higher than 10 wt% the benzene conversion appears to be limited to about 12%. Such a limitation of the benzene conversion while in the meantime propylene is still transformed may suggest some thermodynamical limitation for the benzene alkylation reaction. In order to verify such an assumption, the alkylation thermodynamic equilibrium was calculated at different temperatures for a C3H6/C6H6 ratio of 5.8, taking into account that a signi®cant amount of propylene was
involved in a dimerization secondary reaction. The results are presented in Fig. 12. It appears that at 673 K, if 60% of the propylene is consumed by the secondary reaction, the equilibrium conversion is close to 15%. High conversions of C3H6, observed and reported in Table 3 for the LRF-15 and LRF-20 catalysts, are in agreement with such an interpretation. Study of these catalysts at a lower reaction tempera-
Fig. 12. Benzene conversion at thermodynamic equilibrium as a result of the competition between alkylation and secondary reactions. (Propylene consumed in % mol. by secondary reaction is indicated on the right).
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ture was therefore undertaken to try to differentiate the various samples. 3.2.3. Effect of the temperature over the conversion Limitation of the conversion by thermodynamic equilibrium could be evidenced by changes in the reaction temperature. That was done for the LRF-15 sample, the C3H6/C6H6 ratio and the contact time being maintained constant. The catalytic behavior for the benzene alkylation as a function of the temperature is given in Fig. 13. Such a curve is
Fig. 13. Activity of LRF-15 as a function of the reaction temperature. (Catalyst 0.5 g, molar ratio C3H6/C6H65.8).
typical for a reaction that reaches the thermodynamic equilibrium. For a reaction temperature higher than 663 K the conversion is controlled by the thermodynamic equilibrium. However, under our reaction conditions a large amount of propylene was consumed by the dimerization reaction. That could be related to the high C3H6/C6H6 ratio used. Therefore a new set of experiments will be performed under a lower C3H6 partial pressure and at a lower reaction temperature. 3.2.4. Comparison of the catalysts at 623 K Catalytic activities and selectivities obtained for the various catalysts at 623 K and under a C3H6/C6H6 molar ratio of 0.6 are given in Table 4. It is observed that the conversion increased with the ¯uorine content in the catalyst. Assuming 30% of propylene to be consumed by the secondary reaction, the calculated conversion of benzene at thermodynamic equilibrium is 25%. Therefore, under these new experimental conditions our catalysts could be compared without thermodynamical limitations. Selectivity of benzene toward cumene is high and the propylene involved in the polymerization reaction is deeply reduced as compared to the previous experiments. That last point could be probably explained by the lower C3H6/C6H6 molar ratio in the feed. Finally it could be concluded that under such conditions the reaction is not thermodynamically limited and that higher conversions could be obtained by increasing the contact time. Attempt could therefore be made to correlate the catalytic activity patterns to some physico±chemical characterization.
Table 4 Activities of the F/Al2O3 catalysts in the alkylation reactiona at 623 K Catalyst number
C6H6 Conversion (%)
Cumene Selectivity (%)
C3H6 Conversion (%)
Cumene Selectivity (%)
LRF-02 LRF-04 LRF-06 LRF-10 LRF-15 LRF-20 AlF3NC ALF3650
0.05 0.31 1.73 2.42 4.90 6.90 3.3 0.0
100 99.9 100 97.9 100 99.8 99.8 0.0
0.15 1.5 8.2 10.3 15.8 28.3 16.7 0.0
± 35.3 34.4 39.0 37.0 32.7 34.3 0.0
a
Catalyst 0.5 g; pressure 2.13 atm; C3H6/C6H60.6 mol/mol.
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3.3. Relationship between characterizations and catalytic activity The textural characterization of the various catalysts indicates that the chemical attack of the alumina phase by NH4F provokes a decrease of the surface area of the starting material. At low ¯uorine content the size distribution of the mesopores does not change very much, but at higher ¯uorine contents a slight increase of the average pore radius is noticed. From XRD data, it appears that when alumina is impregnated with a large amount of NH4F (®nal ¯uorine content higher than 6%), compounds as (NH4)3AlF6, AlF3-x(OH)x and AlF3nH2O are formed. As indicated in Table 4, a commercial AlF3, 3H2O has a noticeable activity. On the contrary, after calcination during 6 h at 923 K (weight variation corresponding to the loss of 2.9 mol of water), the sample did not
25
evidence any more activity toward alkylation and such a loss of activity could not be solely related to the decrease of the surface area. These results would suggest hydrated AlF3 to participate to the catalytic activity of these solids. However, question still arises about the effect of these various ¯uorine species over the surface acidity and consequently over the activity. The starting alumina, as all pure aluminium oxides is devoid of BroÈnsted sites, although a sizable fraction of its surface hydroxyl groups are able to retain pyridine, via strong hydrogen bonding, up to as high temperature as 473 K, yet no pyridium ion is formed. On the other hand, active alumina is known to possess strong Lewis acid sites that withhold pyridine beyond 673 K. However, none of this Lewis acid sites of pure alumina and none of its surface hydroxyl groups is able to catalyze the isopropylation of benzene with propene as is
Fig. 14. Schematic representation of the formation of BroÈnsted acid-sites (a) medium acid sites; (b) strong acid sites; (c) strongest acid sites.
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L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
apparent from Table 3, showing that no conversion of benzene was achieved over the starting alumina. Even alumina with almost 2% of ¯uoride is quasi inactive (0.05% of conversion at 623 K) as seen from Table 4 as well. It is remarkable that a threshold ¯uoridation is necessary to affect alkylation of benzene. Indeed although the increase of the total number of acid sites as measured by the potentiometric method is important in the early stages of ¯uoridation, the activity increment up to 3% ¯uorine is only modest. The early stage of ¯uoridation correspond mainly to the substitution of hydroxide ions by ¯uoride ions which probably increases the mobility of the protons of close OH groups to such an extent that they are only able to be transferred to a strong base as n-butylamine but not to a milder one such as propene. Such BroÈnsted type acid sites may be formed according to the simpli®ed picture given in Fig. 14(a). The next ¯uorine fraction might convert some Lewis acid sites into strong BroÈnsted acid sites, strong enough to generate propenium ions with long enough lifetime to react with the aromatic molecule to achieve alkylation (Fig. 14(b)). Thus, even a slight increment of acid sites will result in a signi®cant increase of the conversion as may be seen while comparing the increase of the number of acid sites in Fig. 9 with the ®vefold increase of the conversion XPS results strongly suggested the formation of aluminium hydroxy¯uorides and even that of aluminium tri¯uoride. Such species are very likely to possess very mobile hydrogen due to the strong inductive effect of the gem-¯uoride ions. In addition a strong interaction between surface BroÈnsted sites as those represented in Fig. 14(b) and adjacent aluminium tri¯uoride may result in even stronger BroÈnsted sites which are schematized in Fig. 14(c). Formation of stronger acid sites during the ¯uoridation was suggested by Hirscher [21] and Covini et al. [22]. Notice that the LRF-15 sample characterized by the highest « AlF3 » fraction (65%) is proportionally much more active than samples having a lower ¯uorine content when the corresponding number of acid sites or the corresponding ¯uorine content are considered. Appropriate IR studies, in association with pyridine adsorption are planed which will help to discriminate the various nature of acid sites as well as varying strength at various steps of the ¯uoridation.
4. Conclusion The alkylation of benzene with propylene is catalyzed by ¯uorinated alumina. As commonly considered, this alkylation reaction proceeds via a protonation of propylene by the acid sites to form the active species. The latter can react with benzene, producing cumene, or it can react with another propylene molecule producing a C6 species. In our case, both reaction are observed, but the transformation of benzene is almost 100% selective toward benzene, and di and trialkylbenzenes are not observed. Such a point is important for the gasoline reformulation in which the formation of heavy products has to be avoided. Our results also indicate that consumption of propylene by oligomerization was important enough, but that could be probably reduced by optimization of the propylene/ benzene ratio. According to the effect of ¯uorine over the alumina, the chemical attack of the alumina results in an increase of the average diameter of the mesopores and a decrease of the surface area, except at high ¯uorine content. The presence of AlF3 and aluminium hydroxy¯uoride was clearly evidenced by XRD diffraction over the samples with a high ¯uorine content. However, the XPS experiments indicate that different types of ¯uoride species are present over the surface of the catalysts as soon as the ¯uorine content was higher than about 3%. Therefore, it appears that the surface is a mixture of residual alumina and of aluminium tri¯uoride and hydroxyaluminium ¯uoride particles. Comparison of the catalytic activities, determined in the absence of thermodynamical limitation, with the surface characterization, would suggest that AlF3 and AlF3-xOHx structures are also involved in the catalytic sites. Complementary characterizations of the acid sites are in progress for a better understanding of these samples. Acknowledgements This work was performed in the framework of a P.C.P (Programme de CoopeÂration Post-GradueÂ) France-Mexique with ®nancial support from Consejo Nacional.de Ciencia y TecnologõÂa (CONACyT), MinisteÁre des Affaires EtrangeÁres (France) and Petroleos Mexicanos. We are indebted to M. Brun and P. Deli-
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
chere for their help in XPS experiments and results analysis. References [1] C.C. Johnson III, H.F. Moore, J.M. Kersey, M.M. Mitchell, ACS Symposium Series, 37 (1993). [2] A.R. Goelzer, A. Hernandez Robinson, S. Ram, A.A. Chin, M.N. Harandi, Oil and Gas J. 13 (1993) 63. [3] R.T. Morrison, R.N. Boyd, QuõÂmica Organica (SustitucioÂn aromaÂtica electrofõÂlica), SITESA, MeÂxico (1985). [4] G. Bellusi, G. Pazzuconi, C. Perego, G. Girotti, G. Terzoni, J. Catal. 157 (1995) 227. [5] W.W. Kaeding, R.E. Holland, J. Catal. 109 (1988) 212. [6] E.M. Kennedy, F. Lonyi, T.H. Ballinger, M.P. Rosynek, J.H. Lunsford, Energy and Fuels 8 (1994) 846. [7] G.S. Lee, J.J. Maj, S.C. Rocke, J. M GarceÂs, J. Catal. Lett. 2 (1989) 243. [8] E. Ruckenstein, Ind. Eng. Chem. Res. 34(5) (1995) 1517.
27
[9] V.M. Allenger, D.D. McLean, M. Ternan, J. Catal. 131 (1991) 305. [10] R. Cid, G. Pecchi, Appl. Catal. 14 (1985) 15. [11] S. Kowalak, Acta. Chim. Acad. Sci. Hung. 107 (1981) 27. [12] S. Kowalak, Acta. Chim. Acad. Sci. Hung. 107 (1981) 19. [13] P.O. Scokard, S.A. Selim, J.P. Damon, P.G. Rouxhet, J. Colloid Interface Sci. 70 (1979) 209. [14] F.P. Kerkhof, J.C. Oudejans, J.A. Moulin, E.R.A. Natulewiez, J. Colloid Interface Sci. 77 (1980) 120. [15] O.V. Bulgakov, T.V. Antipina, Russ. J. Phys. Chem. 41 (1980) 120. [16] H.R. Gerberich, F.E. Lutinski, W. Keith Hall, J. Catal. 6 (1966) 209. [17] H.J. Reitsma, C. Boelhouwer, J. Catal. 33 (1974) 39. [18] S.M. Davis, G.D. Meitzner, D.A. Fischer, J. Gland, J. Catal. 142 (1993) 368. [19] V.I. Nefedov, N.P. Sergushin, I.M. Mbandan, M.B. Trhaskoskaya, J. Electron Spectros. 2 (1973) 283. [20] J.M. Scofield, J. Electron Spectros. 8 (1976) 129. [21] A.E. Hischer, J. Catal. 2 (1963) 428. [22] R. Covini, V. Fattore, N. Giordano, J. Catal. 7 (1967) 126.
Alkylation of benzene with propylene catalyzed by ¯uorined alumina L.M. Rodrigueza, J. Alcaraza, M. Hernandeza, Y. Ben Taaritb, M. Vrinatb,* b
a Facultad de Quimica, UNAM, Ciudad Universitaria, 04510 Mexico DF, Mexico Institut de Recherches sur la Catalyse, 2 Av. A. Einstein, 69626 Villeurbanne Cedex, France
Received 3 April 1997; received in revised form 25 September 1997; accepted 14 October 1997
Abstract Fluorinated alumina catalysts have been prepared with different ¯uorine content and studied by combining textural and structural characterizations (BET, Porosity, DRX), surface composition (XPS), acidity (potentiometric titration) and catalytic properties in alkylation of benzene by propylene. Impregnation by NH4F solution modi®es the surface of the alumina phase, with the formation of ¯uoride surface species at low ¯uorine content and the formation of AlF3 and aluminium hydroxy¯uoride as the isolated crystallites at high ¯uorine content. Increase of the activity with the ¯uorine content up to 14.5 wt % also suggests the participation of these compounds in the catalytic sites. As far as the alkylation of benzene over these catalysts are concerned the reaction was found to be highly selective toward cumene, which avoid the formation of undesirable heavy products. # 1998 Elsevier Science B.V. Keywords: Catalysts; Alumina; Fluoride; Benzene; Alkylation
1. Introduction The 1990 Clean Air Act (CAA) Amendments target a substantial reduction of volatile organic compounds (VOC's) and toxics from vehicles and their fuels. The resultant gasoline reformulation is regarded as the most severe of all fuel speci®cations since the lead phaseout of the early 1970's. Such a legislation has many implications on the modern re®nery, including the role of catalytic reforming units, which are the most important sources of hydrogen of the re®ners. Among the toxic and pollutant compounds, benzene is considered as a powerful cancer-producing agent and its content in the reformulate gasoline must be less *Corresponding author. Tel.: +334 72 445 300; fax: +33 4 72 445 399. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00309-8
than 1.0 vol % As 70±80% of the benzene in the gasoline is coming from the catalytic reformate, attention has to be paid to that process or its resulting petroleum cut [1]. The reduction of benzene production could be obtained by minimizing its formation during the catalytic reforming process, or by elimination of the benzene-rich light reformate stream by subsequent benzene conversion or extraction. However, if avoiding the formation of benzene is the most logical option, economical and practical considerations limit the possibility to apply this strategy, a benzene net reduction below 1 vol % being impossible without a negative impact over the hydrogen production and in the octane number of the resulting gasoline. Light reformate processing therefore appears to be a more direct alternative to benzene reduction, and four basic
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L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
potential options are available :saturation, saturation and isomerization, extraction or alkylation. The saturation of this aromatic compound consumes a lot of hydrogen and leads to a decrease of the octane number. Benzene saturation combined with isomerization results in a gain in octane number but the ®nal product has a slightly higher Reid Vapor Pressure and the net loss of hydrogen is greater. The extraction option is only considered by the re®ners also involved in the petrochemical market [2]. Taking into account the hydrogen consumption, benzene alkylation appears a real catalytic alternative to reduce net benzene. For benzene alkylation, the use of light ole®nes as ethylene, propylene or butene as alkylation agent could be considered since they are the by-products of catalytic cracking units, and, except butene, not always used in other gasoline production processes. Use of butene is limited by the boiling point (446± 456 K) of the butylbenzene obtained, close to the gasoline distillation end-point (443±473 K). Benzene alkylation by ethylene can be performed by zeolites and several technologies are available. However, in some re®neries propylene is more available than ethylene for alkylation. Moreover, alkylation of benzene catalyzed by a solid is commonly considered as processing via carbonium-ion type mechanism [3] and the proton af®nity of propylene is higher than that of ethylene (180.4 vs 160.6 kcal/mol) [4]. Therefore, in order to limit the severity of the experimental conditions and also the secondary reactions propylene was used in this work. Alkylation of benzene with propylene to produce cumene is a well-known reaction performed in the petrochemical industry using `solid phosphoric acid' as a catalyst. Other Friedel±Crafts catalysts, such as aluminium chloride have also been used [5]. However, application of such catalysts for processing a mixture of hydrocarbons in which benzene is only 15±20 vol % appears to be limited. Effort has been put in developing alternative catalytic systems for the oil industry. That was done using several zeolites [4,6±8], but only two commercial cumene processes catalyzed by zeolites have been developed [2]. Use of zeolites in such process was restricted by deactivation associated with pore plugging via formation of coke deposits [7]. On the contrary, only little work has been done with ¯uorined alumina though it is well known that such
a solid exhibits catalytic activity for acid catalyzed reactions as cracking, isomerization and polymerization [9]. This paper describes direct vapor-phase alkylation of benzene with propylene over a series of ¯uorinated alumina. Reaction was performed in a continuous ¯ow unit and the catalysts were characterized by various techniques. 2. Experimental 2.1. Catalysts preparation The starting alumina was a commercial g-Al2O3 with a surface area of 320 m2/g and a pore volume of 0.43 cm3/g. Except for the diffusion control study, all samples were issued from an Al2O3 powder with particle sizes between 100 and 200 mesh. The powder was calcined at 773 K for 18 h before impregnation. The aqueous incipient wetness impregnation was performed with aqueous solutions of NH4F prepared in order to get a ¯uorine loading between 0 and 20 wt%. After impregnation, the solids were left at room temperature for 2 h, dried at 353 K for 16 h and then calcined at 723 K for 4 h. Another powder alumina was impregnated with bidistillated water instead of NH4F solution. That sample was utilized as a reference for the solids characterizations. 2.2. Solids characterization The ¯uorine contents have been determined for samples calcined at 723 K by the ICP technique. Speci®c surface areas of the F/Al2O3 were obtained by nitrogen physisorption using an automated BET apparatus. The X-ray diffraction (XRD) patterns were collected using a Siemens D500, /2 diffractometer. The run source was a CuKa cell (1.5418 A8). The photoelectron spectra (XPS) have been obtained using a VG Scienti®c, Escalab 200R spectrometer using AlKa radiation. The reference utilized was the O1 s (531.2 eV) and the recorded peaks were Al2p and F1s. The total acidity (TA) of the solids was determined by n-butylamine potentiometric titration method as described in literature [10]. This procedure was used to measure the number of sites and the maximum acid strength (MAS8of the surface).
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
17
Fig. 1. Schematic diagram of the reaction system. (1) Dryers, (2) saturator, (3) gas chromatograph and (4) reactor.
2.3. Catalytic activities The catalytic activity for alkylation of benzene with propylene was determined in a continuous ¯ow unit, schematized in Fig. 1. Nitrogen and propylene pressures were stabilized and a regulated inlet using a pressure control regulator. Their ¯ow was measured by rotameters. Benzene was introduced by bubbling N2 in a saturator±condensator device whose temperature was perfectly regulated. The reactor bed consisted of a 10 ml preheated zone containing quartz powder, the catalyst (0.15 to 2.0 g), and a post reaction zone containing 2 g of crushed quartz. Pressure in the reactor (2.1 atm) was maintained and measured using a pressure control regulator. All the lines were heated in order to avoid any condensation, and after the reactor the products are analyzed using a Gow-Mac 580 gas chromatograph (FID detector) equipped with a packed column (5% OV-1010N CHROM-P-AW-DMCS). Benzene, was supplied by Aldrich, (purity>99%) and propylene (purity>99%) was a product of Linde.
The reaction was performed at temperatures between 593 K and 693 K. The results were expressed by : the benzene conversion, the propylene conversion, the cumene selectivity referred to benzene defined by the ratio : mole number of cumene obtained/ mole number of benzene transformed, the cumene selectivity referred to propylene defined by the ratio : mole number of cumene obtained/mole number of propylene transformed. 3. Results and discussion. 3.1. Solids characterization 3.1.1. Chemical analysis and surface area The solids prepared are listed in Table 1 with the ¯uorine content given in wt%. From this table it appears that there is a difference between the ¯uorine expected and the ¯uorine obtained. This indicates that an important fraction of ¯uorine is lost during the preparation, the halogen losses being greater when the
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Table 1 Chemical analysis and surface area of the catalysts Sample number
% F content (% wt) Expected
LRF-00 LRF-02 LRF-03 LRF-04 LRF-05 LRF-06 LRF-10 LRF-15 LRF-20 a
0 2 3 4 5 6 10 15 20
Obtained
a a
0 1.9 3.19
a
6.5 13.1 14.5
Surface area (m2/g) 231 201 196 185 199 172 160 133 187
NANot available.
solutions have higher concentrations of NH4F. This behavior could be explained through the more and more dif®cult reaction of ¯uorine with the alumina as soon as the super®cial and then the second layer of aluminium atoms are attacked. At high ¯uorine content unreacted NH4F is therefore maintained on the catalyst during the drying and sublimated during calcination. The evolution of the surface area as a function of the ¯uorine content is shown in Table 1. The solids lose surface area when the halogen content rises. This progressive decrease is observed until the wt% of ¯uorine reaches about 13. However, when the ¯uorine content get a value of 14 wt%, a sharp increase occurs. Fig. 2 shows that when the ¯uorine content increases the pore size distribution is larger than in the original alumina. The adsorption±desorption isotherms of the solid without ¯uorine and LRF-20 (14.5 w%) are presented in Fig. 3 and it shows that the desorption step is slightly displaced to a higher P/Po value for the support with more ¯uorine content. The modi®cation in the pores size distribution and the change of desorption P/Po value indicate an increase of the pores diameters induced by the ¯uorination. 3.1.2. XRD. Three samples were analyzed by XRD :LRF-10, LRF-15 and LRF-20. Their diffractograms are given in Fig. 4. From the patterns obtained we observe : the three solids give rise to the typical diffraction peaks of the alumina support (2678, 468 and 37.58);
Fig. 2. Variation, with respect to fluorine content, of pore distribution of some F/Al2O3 samples.
Fig. 3. Adsorption±desorption isotherms of N2 at 77 K for the LRF-00 and LRF20 samples.
the patterns obtained for the LRF-15 and LRF-20 give additional peaks at 214.88, 25.18, 25.88, 29.88 and 51.18, some of which are due to the
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
Fig. 4. DRX diffractograms of some F/Al2O3 catalysts. (ÐÐÐ) g-AlF3 and AlF3.3H2O; (Ð-) AlF3-X(OH)X.
presence of g-AlF3*nH2O; peaks at 215.68, 308, 31.58 and 52.88 are also evidenced and these peaks are attributed to the presence of AlF1.65(OH)1.35*nH2O; in the case of the LRF-15, small additional peaks at 217.58, 19.98 and 62.18 are identical with the typical pattern of (NH4)3AlF6; the intensities of the g-AlF3*nH2O diffraction lines are greater in the sample LRF-20 than in the LRF-15; the peaks at 2168, 238, 26.88, 29.18, 51.38 and 52.98 suggest that AlF3*3H2O was present on the surface of the LRF-15 and LRF-20 catalysts. Evidence of the formation of (NH4)3AlF6 over ¯uorinated alumina was reported by Kowalak [11,12] who suggested this compound to be formed by the following reaction : Al3 6NH4 F ) NH4 3 AlF6 3NH3 3H : The transformation of (NH4)3AlF6 into (NH4)AlF4, g-AlF3 and ®nally a-AlF3 has been reported to occur respectively at 573, 773 and 873 K [13]. However, Kerkhof et al. [14] proposed the formation of AlF3 to be the result of consecutive reactions between Al(OH)3 and HF, leading to Al(OH)2F, Al(OH)F2 and other intermediates as AlF1.65(OH)1.35*nH2 O, or in a general formula AlF3-X(OH)X.
19
Fig. 5. Comparison between DRX diffractograms of LRF-20 and AlF33H2O not calcined.
The presence of (NH4)3AlF6 in the LRF-15 sample and of AlF3-X(OH)X in both LRF-15 and LRF-20 catalysts suggest that AlF3 could be formed according to both mechanisms in agreement with Bulgakov and Antipina [15]. However, the proportion of these two compounds would be related to the Al2O3/NH4F ratio, which means to the concentration of the NH4F solution used for impregnation. For catalysts with a ¯uorine content lower than 13 wt%, the XRD technique does not allow to get any information. A commercial AlF3*3H2O compound (surface area of 48 m2/g) was also studied and its diffraction diagram given in Fig. 5 presents peaks at 2168, 238, 26.88, 29.18, 51.38 and 52.98, as also observed for the LRF-15 and LRF-20 samples. 3.1.3. XPS. The XPS spectra have been recorded on samples after calcination and the scale of binding energy has been obtained by taking a value of 531.20 eV for O1s. This value is in satisfactory agreement with the literature data. Fig. 6 gives the detailed Al2p peak of the four samples. For LRF-04 catalyst, the Al2p peak is symmetric and the binding energy at 74.3 eV in agreement with that of Al in Al2O3. For the other three samples, the asymmetric peak suggests the occurrence of aluminium involved in other environ-
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L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
Fig. 6. Effect of the fluorine content on the Al2p XPS signal.
ment. Indeed, the deconvolution of the Al2p spectra, as given in Fig. 7 for the LRF-15 sample, indicates that this peak is the sum of a peak at 74.3 eV (Al belonging to Al2O3) and of a peak at about 76.5± 77.0 eV, which correspond to Al in ¯uorine environment. The results of these deconvolutions are reported in Table 2. For the F1s peak, pro®les reported in Fig. 8 clearly evidence an evolution of the binding energies with the ¯uorine content of the catalyst. Such asymmetric peaks also indicate the presence of different ¯uorine species on the surface of the samples. Deconvolution of the peaks was therefore attempted assuming two ¯uorine species. The ®rst peak with a binding energy of 685.5 eV corresponds to super®cial ¯uoride ions at the surface of alumina and would be the result of the following reactions : Al ÿ OH NH4 F ! Al ÿ F H2 O NH3 Al ÿ O ÿ Al NH4 F ! Al ÿ OH F ÿ Al NH3
Fig. 7. Deconvolution of the Al2p and F1s XPS spectra for the LRF-15 sample.
Such entities are dominant at low ¯uorine contents. The second peak at about 688 eV could be attributed to aluminium hydroxy¯uorides and (or) to aluminium ¯uoride crystallites. Such a peak, which increase with the ¯uorine content, is rather important for the catalysts with more than 6% of ¯uorine. Such a value was founded in several reports to correspond to the amount of ¯uorine needed to form a monolayer on the surface of Al2O3 [16±18]. These XPS results also allow to understand the surface area evolution of the LRF-20 sample. In that sample, the percentage of ¯uorine involved in the formation of super®cial ¯uorine atoms is higher than over the LRF-15 sample, which could
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
21
Table 2 XPS results for some F/Al2O3 catalysts LRF-04
LRF-10
LRF-15
LRF-20
wt% F Al2p (eV)
3.19 74.3 (100%)
F1s (eV)
685.4 (95%) 688.0 (5%) 0.17 0.65
6.5 74.4 (91.5%) 76.5 (8.5%) 685.5 (60%) 687.8 (40%) 0.34 0.74
13.1 74.35 77.30 685.4 688.2 0.46 0.78
14.5 74.4 (92.0%) 76.3 (8.0%) 685.5 (52%) 688.1 (48%) 0.50 0.70
F1s/Al2p Al2p/O1s
(85.0%) (15.0%) (35%) (65%)
intensity, the cross section as given by Sco®eld [20] and Ec the kinetic energy. The results given in Table 2 show that for the low ¯uorine content, the F1s/Al2p ratio increases linearly with the ¯uorine content, which is the proof of a high and regular dispersion of the ¯uorine on the alumina surface. For higher ¯uorine content, the F1s/Al2p ratio tends to a plateau suggesting the formation of bulk ¯uorinated compounds. For the Al2p/O1s atomic ratio, the calculated value for the LRF-04 catalyst is in good agreement with the theoretical value (0.66). For higher ¯uorine content, the increase of the Al2p/ O1 s ratio re¯ects the replacement of OH species by ¯uoride ones. 3.1.4. Acidity The effect of ¯uorination on the total number of acid sites of alumina and over the maximum acid strength is illustrated in Figs. 9 and 10. The potentiometric method, used in this work, allows the determination of all acid sites irrespective of their Lewis or BroÈnsted nature. 3.2. Catalytic activities Fig. 8. Effect of the fluorine content on the F1s XPS signal.
be explained by the higher surface area of the sample. Finally, estimation of the F1s/Al2p and Al2p/O1s atomic ratios was made using the equation [19] : p m1 I1 2 Ec2 p m2 I2 1 Ec1 where m is the number of atoms analyzed, I is the peak
3.2.1. Preliminary studies: control of the kinetic regime In heterogeneous catalysis, the measured rate not only depends on the surface reaction rate but could be modi®ed by reactor wall effects and mass transfer rates due to the gradients of concentration. In order to control such phenomena related to experimental conditions, a set of experiments has been performed. A blank run using quartz powder was made at 673 K in order to verify that the stainless steel reactor does not contribute to the reaction. Effect of the granulometry
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L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
Fig. 9. Evolution of the total acidity (TA) of F/Al2O3 as a function of the fluorine content.
of the catalyst was controlled using three samples prepared with 5% of ¯uorine over Al2O3 powder crushed and screened in particle fractions between 80 and 100 mesh, 100 and 200 mesh and larger than 200 mesh. At 673 K and for the same reactant ¯ow rate, the same conversions have been obtained. External diffusion was controlled at 623 K using various amounts of catalyst LRF-20, keeping the space velocity constant (W/F0.0175 g mn cmÿ3) and varying the ¯ow rate in a large extent. The results given in Fig. 11 allow to de®ne the region outside the control of external diffusion limitation. Therefore, it appears that experiments performed with 0.5 g of catalyst prepared over Al2O3 with a granulometry between 100 and 200 mesh and with a total ¯ow rate of 1.510ÿ3 mol minÿ1, could be considered as belonging to the chemical kinetic regime. Attention has also been given to thermal effect. Indeed, though alkylation is a very exothermic reaction, no variation of the temperature of the catalytic bed has been observed. That could be explained by the presence of N2 used as a carrier gas for benzene (dilution effect) and to the low conversions which were obtained under our experimental conditions.
Fig. 10. Evolution of the maximum acid strength (MAS) of F/Al2O3 as a function of the fluorine content.
Fig. 11. Control of external diffusion for the LRF-20: activity as a function of linear velocity at constant space velocity (reaction temperature 623 K, molar ratio C 3 H 6 /C 6 H 6 0.6, W/F 0.0175 g mn cmÿ3).
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
23
Table 3 Activities of the F/Al2O3 catalysts in the alkylation reactiona at 673 K Catalyst Number
C6H6 Conversion (%)
Cumene Selectivity (%)
C3H6 Conversion (%)
Cumene Selectivity (%)
LRF-00 LRF-01 LRF-02 LRF-03 LRF-04 LRF-05 LRF-10 LRF-15 LRF-20
0 0 0.9 5.0 7.2 9.5 11.5 12.0 11.8
± ± ± 99.9 99.7 99.5 99.3 99.2 99.5
0 0 0.24 18.7 27 30.2 38.4 54.8 66
± ± ± 9.9 11.5 10.3 6.11 5.9 3.7
a
Catalyst 0.5 g; pressure 2.13 atm; C3H6/C6H65.8 mol/mol.
3.2.2. Comparison of catalysts at 673 K :effect of the fluorine content The various catalysts were tested at 673 K, under a C3H6/C6H6 molar ratio equal to 5.8 and results are summarized in Table 3. Conversions and selectivities were calculated after 120 min time on stream when the catalyst reached a steady state regime, which one is maintained during all the experiment (about 7 h). For a free ¯uorine catalyst no conversion occurs. When the ¯uorine content rises, both benzene and propene conversions increase, but whatever the catalyst considered, the selectivity of transformation of benzene to cumene is very close to 100% since no dialkylbenzene was evidenced. That is in agreement with thermodynamic calculations which indicate the formation of dialkylbenzene to be very low at 673 K. For propene a high conversion is associated with the formation of methylcyclopentane as the major product; cyclohexane was only observed as traces. As a complementary experiment, the transformation of propene alone was performed at 623 and 673 K over the LRF20 catalyst. Only the methylcyclopentane and the cyclohexane were formed. For catalysts with a ¯uorine content higher than 10 wt% the benzene conversion appears to be limited to about 12%. Such a limitation of the benzene conversion while in the meantime propylene is still transformed may suggest some thermodynamical limitation for the benzene alkylation reaction. In order to verify such an assumption, the alkylation thermodynamic equilibrium was calculated at different temperatures for a C3H6/C6H6 ratio of 5.8, taking into account that a signi®cant amount of propylene was
involved in a dimerization secondary reaction. The results are presented in Fig. 12. It appears that at 673 K, if 60% of the propylene is consumed by the secondary reaction, the equilibrium conversion is close to 15%. High conversions of C3H6, observed and reported in Table 3 for the LRF-15 and LRF-20 catalysts, are in agreement with such an interpretation. Study of these catalysts at a lower reaction tempera-
Fig. 12. Benzene conversion at thermodynamic equilibrium as a result of the competition between alkylation and secondary reactions. (Propylene consumed in % mol. by secondary reaction is indicated on the right).
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L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
ture was therefore undertaken to try to differentiate the various samples. 3.2.3. Effect of the temperature over the conversion Limitation of the conversion by thermodynamic equilibrium could be evidenced by changes in the reaction temperature. That was done for the LRF-15 sample, the C3H6/C6H6 ratio and the contact time being maintained constant. The catalytic behavior for the benzene alkylation as a function of the temperature is given in Fig. 13. Such a curve is
Fig. 13. Activity of LRF-15 as a function of the reaction temperature. (Catalyst 0.5 g, molar ratio C3H6/C6H65.8).
typical for a reaction that reaches the thermodynamic equilibrium. For a reaction temperature higher than 663 K the conversion is controlled by the thermodynamic equilibrium. However, under our reaction conditions a large amount of propylene was consumed by the dimerization reaction. That could be related to the high C3H6/C6H6 ratio used. Therefore a new set of experiments will be performed under a lower C3H6 partial pressure and at a lower reaction temperature. 3.2.4. Comparison of the catalysts at 623 K Catalytic activities and selectivities obtained for the various catalysts at 623 K and under a C3H6/C6H6 molar ratio of 0.6 are given in Table 4. It is observed that the conversion increased with the ¯uorine content in the catalyst. Assuming 30% of propylene to be consumed by the secondary reaction, the calculated conversion of benzene at thermodynamic equilibrium is 25%. Therefore, under these new experimental conditions our catalysts could be compared without thermodynamical limitations. Selectivity of benzene toward cumene is high and the propylene involved in the polymerization reaction is deeply reduced as compared to the previous experiments. That last point could be probably explained by the lower C3H6/C6H6 molar ratio in the feed. Finally it could be concluded that under such conditions the reaction is not thermodynamically limited and that higher conversions could be obtained by increasing the contact time. Attempt could therefore be made to correlate the catalytic activity patterns to some physico±chemical characterization.
Table 4 Activities of the F/Al2O3 catalysts in the alkylation reactiona at 623 K Catalyst number
C6H6 Conversion (%)
Cumene Selectivity (%)
C3H6 Conversion (%)
Cumene Selectivity (%)
LRF-02 LRF-04 LRF-06 LRF-10 LRF-15 LRF-20 AlF3NC ALF3650
0.05 0.31 1.73 2.42 4.90 6.90 3.3 0.0
100 99.9 100 97.9 100 99.8 99.8 0.0
0.15 1.5 8.2 10.3 15.8 28.3 16.7 0.0
± 35.3 34.4 39.0 37.0 32.7 34.3 0.0
a
Catalyst 0.5 g; pressure 2.13 atm; C3H6/C6H60.6 mol/mol.
L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
3.3. Relationship between characterizations and catalytic activity The textural characterization of the various catalysts indicates that the chemical attack of the alumina phase by NH4F provokes a decrease of the surface area of the starting material. At low ¯uorine content the size distribution of the mesopores does not change very much, but at higher ¯uorine contents a slight increase of the average pore radius is noticed. From XRD data, it appears that when alumina is impregnated with a large amount of NH4F (®nal ¯uorine content higher than 6%), compounds as (NH4)3AlF6, AlF3-x(OH)x and AlF3nH2O are formed. As indicated in Table 4, a commercial AlF3, 3H2O has a noticeable activity. On the contrary, after calcination during 6 h at 923 K (weight variation corresponding to the loss of 2.9 mol of water), the sample did not
25
evidence any more activity toward alkylation and such a loss of activity could not be solely related to the decrease of the surface area. These results would suggest hydrated AlF3 to participate to the catalytic activity of these solids. However, question still arises about the effect of these various ¯uorine species over the surface acidity and consequently over the activity. The starting alumina, as all pure aluminium oxides is devoid of BroÈnsted sites, although a sizable fraction of its surface hydroxyl groups are able to retain pyridine, via strong hydrogen bonding, up to as high temperature as 473 K, yet no pyridium ion is formed. On the other hand, active alumina is known to possess strong Lewis acid sites that withhold pyridine beyond 673 K. However, none of this Lewis acid sites of pure alumina and none of its surface hydroxyl groups is able to catalyze the isopropylation of benzene with propene as is
Fig. 14. Schematic representation of the formation of BroÈnsted acid-sites (a) medium acid sites; (b) strong acid sites; (c) strongest acid sites.
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L.M. Rodriguez et al. / Applied Catalysis A: General 169 (1998) 15±27
apparent from Table 3, showing that no conversion of benzene was achieved over the starting alumina. Even alumina with almost 2% of ¯uoride is quasi inactive (0.05% of conversion at 623 K) as seen from Table 4 as well. It is remarkable that a threshold ¯uoridation is necessary to affect alkylation of benzene. Indeed although the increase of the total number of acid sites as measured by the potentiometric method is important in the early stages of ¯uoridation, the activity increment up to 3% ¯uorine is only modest. The early stage of ¯uoridation correspond mainly to the substitution of hydroxide ions by ¯uoride ions which probably increases the mobility of the protons of close OH groups to such an extent that they are only able to be transferred to a strong base as n-butylamine but not to a milder one such as propene. Such BroÈnsted type acid sites may be formed according to the simpli®ed picture given in Fig. 14(a). The next ¯uorine fraction might convert some Lewis acid sites into strong BroÈnsted acid sites, strong enough to generate propenium ions with long enough lifetime to react with the aromatic molecule to achieve alkylation (Fig. 14(b)). Thus, even a slight increment of acid sites will result in a signi®cant increase of the conversion as may be seen while comparing the increase of the number of acid sites in Fig. 9 with the ®vefold increase of the conversion XPS results strongly suggested the formation of aluminium hydroxy¯uorides and even that of aluminium tri¯uoride. Such species are very likely to possess very mobile hydrogen due to the strong inductive effect of the gem-¯uoride ions. In addition a strong interaction between surface BroÈnsted sites as those represented in Fig. 14(b) and adjacent aluminium tri¯uoride may result in even stronger BroÈnsted sites which are schematized in Fig. 14(c). Formation of stronger acid sites during the ¯uoridation was suggested by Hirscher [21] and Covini et al. [22]. Notice that the LRF-15 sample characterized by the highest « AlF3 » fraction (65%) is proportionally much more active than samples having a lower ¯uorine content when the corresponding number of acid sites or the corresponding ¯uorine content are considered. Appropriate IR studies, in association with pyridine adsorption are planed which will help to discriminate the various nature of acid sites as well as varying strength at various steps of the ¯uoridation.
4. Conclusion The alkylation of benzene with propylene is catalyzed by ¯uorinated alumina. As commonly considered, this alkylation reaction proceeds via a protonation of propylene by the acid sites to form the active species. The latter can react with benzene, producing cumene, or it can react with another propylene molecule producing a C6 species. In our case, both reaction are observed, but the transformation of benzene is almost 100% selective toward benzene, and di and trialkylbenzenes are not observed. Such a point is important for the gasoline reformulation in which the formation of heavy products has to be avoided. Our results also indicate that consumption of propylene by oligomerization was important enough, but that could be probably reduced by optimization of the propylene/ benzene ratio. According to the effect of ¯uorine over the alumina, the chemical attack of the alumina results in an increase of the average diameter of the mesopores and a decrease of the surface area, except at high ¯uorine content. The presence of AlF3 and aluminium hydroxy¯uoride was clearly evidenced by XRD diffraction over the samples with a high ¯uorine content. However, the XPS experiments indicate that different types of ¯uoride species are present over the surface of the catalysts as soon as the ¯uorine content was higher than about 3%. Therefore, it appears that the surface is a mixture of residual alumina and of aluminium tri¯uoride and hydroxyaluminium ¯uoride particles. Comparison of the catalytic activities, determined in the absence of thermodynamical limitation, with the surface characterization, would suggest that AlF3 and AlF3-xOHx structures are also involved in the catalytic sites. Complementary characterizations of the acid sites are in progress for a better understanding of these samples. Acknowledgements This work was performed in the framework of a P.C.P (Programme de CoopeÂration Post-GradueÂ) France-Mexique with ®nancial support from Consejo Nacional.de Ciencia y TecnologõÂa (CONACyT), MinisteÁre des Affaires EtrangeÁres (France) and Petroleos Mexicanos. We are indebted to M. Brun and P. Deli-
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