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Selective Synthesis of Isoprene by Prins Condensation Using Molecular Sieves
J. Weitkamp, H.G. Karge, H. Pfeifer and W. Holderich (Eds.) Zeolifes and Relafed Microporow Maierials: Siafe of ihe Ari 1994 Studies in Surface Science and Catalysis, Vol. 84 0 1994 Elsevier Science B.V. All rights reserved.
1997
SELECTIVE SYNTHESIS OF ISOPRENE BY PRINS CONDENSATION USING MOLECULAR SIEVES E. Dumitriu, V. Hulea, C. Chelaru" and T. Hulea
Faculty of Industrial Chemistry, "Gh. Asachi" Technical University of Jassy, 71 Splai Bahlui, P.O.Box 483, Jassy-6600, Rcmania *Institute of Macromolecular Chemistry, 4l.A Aleea Gr.Ghica-Vcda, Jassy-6600 and S. Kaliaguine Genie Chimique - CERF'IC, Universite Laval, Cite Universitaire, Quebec, Canada, G1K 7P4 ABSTRACT
The formaldehyde condensation with isobutene was studied over mlecular sieves, such as HZSM-5 with various Si/Al ratios, HZSM-5 hpreqnated with H~FQL,SAPO-5 and AlFC-5, in vapour phase in the temperature range 175-400°C The dependence of the catalytic activity and the selectivity to isoprene on the acid strength distribution and the density of active sites was examined.
1. INTRODUCTION The formaldehyde condensation with isobutene is of a special interest since this reaction is involved in the isoprene manufacture, a raw material for various elastomers. The direct production of isoprene, the so-called "one stage process", is a very attractive industrial route due to its simplified prccedure, and y y attempts to find a proper catalyst have been done. Though various acidic oxides were tested and sane interesting features of this condensation have been outlined [l] , an efficient catalyst based on metal oxides has not been found till now. Therefore, it might be possible to obtain a m r e prcmising catalyst using molecular sieves, because these materials have a broad spectrum of acidities and pore structures. Unfortunately there are few reports on the Prins condensation catalyzed by zeolites [2-4] , and a detailed investigation of this process is necessary in order to design a proper catalyst for the selective synthesis of isoprene. With this idea in mind, the following two types of molecular sieves have been selected: ZSM-5, recognized as a shape selective and strong acid catalyst [5,6] , and SAPO-5 which is considered as mildly acidic [7-14 The regular three-dimensional pore structure of pentasil-type zeolites, their properties of shape selectivity, the possibility of modifying the surface properties by the Si/A1 ratio, oxide loading, ion exchange or by isomorphic substitution are all features of great interest in order to design a selective catalyst for the isoprene synthesis. The same reasons led us in the selection of SAPO-5. The objective of this study was to investigate scme factors influencing the activity and the selectivity to isoprene such the S102 /A1203 ratio in
.
1998
catalyst capsition and the oxide loading. A pulse type microreactor was used because it is m r e suitable at this stage of the investigations than a steady flow reactor, since certain information may be difficult to achieve with the latter reactor.
1. Catalyst Preparation. A series of ZSM-5 samples with SiO2/Al2O3 ratio ranging frcm 35 to 1000 were synthesized in the presence of tetrapropylamnonium b r d d e following the published patents (e.g. [12] ) . SAFG-5 samples and AlpO-5 were prepared according to the procedures presented in the literature [13-153 All samples were calcined prior to their use as catalysts in order to remove the organic template used in synthesis and to free the intracrystalline pore system. The catalyst powder was pressed binder free, ground, and used in a particle size between 0.25 and 0.5 mn. 2. Catalyst Modification. In order to obtain H+ forms the samples of zeolites were ion exchanged four times at 90°C in 0.1 M solution of NHLC~. After the ion exchange procedure the samples were washed with distilled water, dried and then calcined in air at 6OOOC for 6 hours. The impregnated forms of HZSM-5 zeolites were obtained following the [16] The resulting catalysts procedure described by Lercher et al. contained 2,3,4,6 and 8% H~FQLby weight. 3. Catalyst Characterization. The crystallinity of the samples was controlled by x-ray diffraction and by IR spectra in KBr, and found to be pure crystalline materials. The chemical compositions of the catalysts were determined by wet chemical methods, atcmic adsorption and flame photcnnetry. The main chemical characteristics together with acidity measurements are shown in Table 1.
.
.
Table 1 Main properties of the catalysts Catalyst Symbol HZSM-5
SiO2/Al2O3
2-35 2-75 2-222 2-400
2-600
2-1000
P/HZSM-5 P2 P3 P4 P6 P8 AlpD-5 SAPO-5
A
s1*
s2**
* si0.036 p0.529=O.L19 ** si0.225 '0.375 AIO.LOO
H~FQ,I,
Acidity , mnole/g.c. overall, 150-550°C
wt %
weak, 150-300°C
35 75 222 400 600 1000
-
0.175 0.087 0.032 0.043 0.028 0.037
0.504 0.322 0.095 0.051 0.034 0.047
35 35 35 35 35
2 3 4 6 8
0.174 0.173 0.167 0.161 0.158
0.482 0.461 0.423 0.388 0.313
0.066 0.108 0.228
0.070 0.162 0.308
1999
Acidic properties of the catalysts were determined by means of NH3-TPD. 4. Reagents. Tertiary butyl alcohol (TBA),(Fluka), and dimethoxymethane (M), (Aldrich), were used to generate in situ isobutene (iB) and formaldehyde (FA), respectively. Fresh aqueous solutions of formaldehyde were prepared by the thermal depolymerization of parafomldehyde. The concentration of formaldehyde (ca 30 % w t ) was determined by chemical methods. 5. Catalyst Evaluation. The condensation reactions were carried out in a pulse microreactor under the following conditions: weight of catalyst, 0.02- 0.04g; nitrogen flow rate, 30 ml/min; pressure, 250 kPa; pulse size, 0.5 p1. The reactor was directly connected to a 3.5 rn GC column filled with carbowax 20M on Chrmsorb M, or to a 1.2 m column packed with Porapak Q. A detailed description for the catalytic runs is given in earlier paper [4]. 3. RESULTS AND DISCUSSION
As previously reported [ 3J , the Prins condensation of fomldehyde and isobutene over medium pore zeolites is a selective reaction, because the molecular size constraint imposed by these zeolites inhibits the formation of bulkier cyclic condensation products, such as 4,4-dimethyl-l,3-dioxane. Thus, a selective production of the is0 rene via 2-methylbutenol can be provided. However, as shown earlier [45, over zeolites having a broad spectrum of acid sites the Prins condensation is accompanied by various side reactions of the olefins, such as oligcmerization, cracking into smaller olefins, disproportionation and aromatization. All these side reactions are also acid catalyzed, and they are undesirable due to their unfavourable carbon balance (for instance, at least two molecules of isobutene/isoprene are consumed for one armtic molecule). Therefore, according to the ideas described in the introductory part, modification of the acid properties of molecular sieves may enhance the catalytic performances. 3.1.Influence of Si02/A1203 As is well-known, the acidity of 234-5 can be controlled by varying the silica to alumina ratio. The yield of isoprene increases along with the increase of the Si02/A1203 ratio, showing a maximum at the Si02/A1203 ratio of %
E
300 O C
70
- 60 .-
0
x .> c
\
u 50-
-aJ %
Si02/AI203
,molar ratio
Figure 1. Effect of S1O2/Al2O3 ratio on the yield of isoprene; T = 30OOC.
LO
I
0.5
1
I
1.0
1.5
2.0
xi020 a.s/g
Figure 2. Effect of strong acid sites on the selectivity to isoprene.
2000
about 600 (Fig.1). Usually such an effect of the Si02 /A12 03 ratio is interpreted in terms of strength and amount of the acidic sites. As shown in Fig.2, the selectivity to isoprene increases as the amount of strong acid sites (measured by the m u n t of amnonia desorbed in the range of 300-6OO0C) decreases. It must be mentioned that the conversions were measured at the steadyactivity, i.e. when the cracking process has disappeared as a result of the deactivation of the strong acid sites by carbonaceous deposits [ 4 1 . This steady-activity is reached swner or later as the Si02/Al O3 ratio is increased or decreased; e.g. , over HZSM-5 with SiO2/AQO3 = 608 the cracking process disappears after the first pulse and a steady-activity is established (see Fig.1). On HZSM-5 the carbonaceous deposits are initially formed in the porous structure and each coke molecule neutralizes one active site [17] .The formation of coke is strongly dependent on the acid site density and the strength of acidic sites. 3.2. Influence of temperature The effect of the reaction temperature on the catalytic performances in the formaldehyde condensation with isobutene over HZSM-5 zeolites was examined over a large range of temperatures, 175-400OC. The typical dependence of conversion on temperature is depicted in Figure 3.
200
300
LOO
tempera ture,OC
Figure 3. The conversion of iB vs. 2-35 reaction temperature; (-1 and (---) 2-600; 0.02 g catalyst.
200
300 LOO t e m p e r o t u re,OC
Figure 4 . The selectivity dependence of reaction temperature over various molecular sieves; 0.02 g catalyst.
As can be seen, at l o w temperatures the conversion of isobutene to isoprene is the main reaction for both catalyst, but as the reaction temperature increases it passes through a maximum, around 3OO0C, then the conversion to isoprene decreases and this decreasing is more strongly for the catalyst with the higher aluminium content. A more pronounced difference between the two catalysts appears in relation to the dependence of total conversion on reaction temperature, when the shape of the curves indicates a strong influence of the SiO2/Al2O3 ratio on the product distribution at high temperatures. In relation to the selectivity to isoprene a more suggestive description is shown in Figure 4. At low temperatures all catalysts have a high selectivity, but as the reaction temperature increases the selectivity to
2001
isoprene decreases in strict dependence on the Si/Al ratio or the type of catalyst in the following order: SAPO-5 > 2-600 > P8 2-75 > 2-35. 3.3. Effect of the mole ratio of reagents Next, the change of conversion with varying molar ratio of the reagents has been studied over the 2-600 catalyst, at 300 OC. In Figure 5 the conversions are reported to the limiting reagent. Evidently, the conversion of formaldehvde increases as the TBA/FA is increased. On the other hand, the degree of conversion of isobutene decreases as the excess of formaldehyde increases. 501
0
501
1100
$ -LO.-O
aJ > c
0 U
302010 -
I
1
2
3
molar ratio
I
I
L
5
Figure 5. Dependence of conversion on the molar ratio of reagents; ( 0 ) TBA/FA; FA/TBA; ( + ) M/TBA. ( 0 ) TBA/M; 0.02 g of catalyst.
Figure 6. Effect of the degree of loading on the conversion ( x ) and the selectivity(0); T = 300OC; 0.02 g of catalyst.
This might be due to the differences among the adsorption strengths of water, isobutene and formaldehyde. If water is too strongly adsorbed, isobutene can hardly be adsorbed and the condensation reaction is diminished. As the GC analysis indicated, the dehydratation of TBA was ccmplete in all cases. when the methylal was introduced to generate in situ formaldehyde the conversions of limiting reagent decreased for both cases. We can conclude that the excess of formaldehyde has no favourable effect. 3.4. Influence of the modifier The activities and selectivities of the Prins condensation over various modified HZSM-5 zeolites are shown in Fig.6, where it is clear that HZSM-5 (Si02/A1203 = 35) modified by H~FQLgave a slight increase of the activity, but a high selectivity. This prmtion was attributed to the modification of the acid strength by H 3 POL. As shown by Lercher et al. [16], the bridging OH group, having significantly higher acid strength, is replaced by two "tenninal" hydroxyl groups and thus the acid strength is decreased. Also, it is worthwhile to note the loss of activity in cracking as the degree of loading increases (see the number of pulses used in order to suppress the process of cracking, Fig.6).
2002
Nature of catalytic sites The above presented data demonstrate that the acid-catalyzed condensation of formaldehyde and isobutene is governed by two factors: (a) the strength of the acidic sites, and (b) the density of active sites. Figure 7 illustrates the ccxnparison of the yield of isoprene of highly siliceous 2.94-5 with that of other solid acid catalysts. %e yield of 3.5.
ul
c
.-
C
3
x
L
0
L
.-
n
L
O
2 00
300 t e mpe ra t u re,OC
LOO
100
Figure 7. Dependence of the yield of isoprene on the nature of catalyst; 0.04 g of catalyst.
200
300 LOO 500 t e m p e ra, t u r e , o c
600
Figure 8. NH3-TPD spectra of tested catalysts; 180C/min heating rate.
isoprene decreases in the order:
H~FO,J+/HZSM-~(P~) > H Z . 9 4 - 5 ( 2 - 6 0 0 ) > > ~ - 5 ( S 2 ) > SAF+5(Sl) > AlFC-5(A)
This order may be correlated with the NH3-TpD spectra. It is clear that as the amount of strong acid sites decreases the selectivity to isoprene increases, but the yield of isoprene depends on the concentration of the acidic sites (for instance, see S1 and S2 catalysts).
200
300
t e m p e r a t ure,OC
LOO
Figure 9. Dependence of yp/y0 ratio on reaction tgnperature; ( 0 1 2-600; S2; 0.02 g of catalyst.
aJ
5
IIUU 100
80-
3
n
60 .-2 60-
‘
ix iX ‘X
c
0
‘X
c LO-
.-O ul
:
\X -It
20OoO
C
0 u
z
x-x-x-x-xo/oco-o-o-o-o-
,, O R :
I
Figure 10. Total conversion (x) and the conversion t o isoprene ( 0 ) vs. the a u n t of pyridine; T = 300OC.
2003
The influence of the strength of the acidic sites can be understood better by performing condensation runs over pyridine poisoned catalysts. First, the catalyst sample (the most selective catalysts, 2-600 and 52, were chosen) was saturated with pyridine at 200OC, then it was purged for 20 min, and the condensation reactions were performed at higher temperatures than the saturation temperature. The ratio between the yield of isoprene on poisoned catalyst (yp)and the yield of isoprene on unpoisoned catalyst as a function of the reaction temperature is shown in Figure 9. It can be observed fran the experimental data, and especially fran the value of the temperature for which yp = yo, that in the case of the SAPO-5 catalyst the acidic sites involved in the Prins condensation are mainly weak, whereas for HZSM-5 with Si02/A1203 = 600 they are of rather moderate strength. These results are in agreement with those shown in Fig.8, and they may explain the different activities of the two catalysts (Fig.7), when the strength of the acidic sites is more important than their number (Table 1). The second kind of experiments consisted of a consecutive poisoning of the catalyst by pyridine at 300 OC, i.e. pulses of 1 p l pyridine were alternated by pulses of reagent mixture. For these experiments 2-35, a catalyst with a broad spectrum of acidities, was chosen and the dependence of the total conversion of isobutene (iB) and the conversion to isoprene on the m u n t of poison is shown in Figure 10. It is worthwhile to note that the conversion to isoprene increases and the side reactions are diminished when the strong acid sites are progressively poisoned. 4.
mcLus1oN
At this stage of our investigations the following conclusions m y be drawn. The synthetic molecular sieves can catalyze the vapour phase condensation of formaldehyde and isobutene, but the catalytic activity and selectivity are strongly dependent on the strength and density of the acid sites. The experimental results indicated that in the Prins condensation the most favourable acid sites are those of slight to moderate strength. The strong acidic sites favoured the side reactions such as the oligcmerization, arcmatization and cracking of isobutene and especially of the formed isoprene F r m the pentasil-type zeolites, HZSM-5 with high SiO*/Al2 03ratio (about 600) and H3FO&npreynated HZSM-5 show high catalytic performances. type are very selective, but their activity is The catalysts of -5 dependent on the Si/Al ratio. The optimum temperature for the acid-catalyzed condensation of formaldehyde and isobutene over molecular sieves in vapour phase was found to be around 300OC. REFERENCES 1. M. Ai, J. Catal, 1106 (1987). 2. P.V. Venuto and P.S. Laandis, Adv. Catal., 18 (1968) 259.
3. C.D. Chany, W.H.Lang and W.K. B e l l , Catalysis of Organic Reactions, W.R. Moser ( e d s ) , Marcel Lkkker, Inc., New York, 1981, p.73. 4. E. Dumitriu, D. Gongescu and V. Hulea, Heterogeneous Catalysis and Fine Chemicals 111, M. Guisnet, J. Earbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier (eds), Elsevier Sci.Publ., Amsterdam, 1993, p.669.
2001
5. J.Dotka and Piwowarska, Zeolites, 1 (1988) 30. 6. S.G. Hedge, P. Ratnasamy, L.M. Kustov and V.B. Kazansky, Zeolites, 8(3), (1988) 137. 7. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T. R. Cannan and E. M. Flanigan, J. Amer. Chem. Soc., 106 (1984) 6092. 8. E.M. Flanigan, B.M. Lok, R.L. Patton and S.T. Wilson, Stud. Surf. Sci. Catal., 28 (1986) 103. 9. X. Quinhua, Y. Aizhen, B. Shulin and X. Kaijun, Stud. Surf. Sci. C a t a l . , 28 (1986) 835. 10. R.J. Pellet, G.N. Long and J.A. Rabo, Stud. Surf. Sci. Catal., 28 (1986) 843. 11. C. Halik and J.A. Lercher, J. Chem. Soc., Faraday Trans. I, 84 (1988) 4457. 12. R.J. Argauer and G.R. Landolt, U.S. Patent 3,702,886 (1972). 13. H. Weyda and H. Lechert, Zeolites, 10 (1990) 251. 14. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E. M. Flanigan, U.S.Patent 4,440,871 (1984). 15. S.T. Wilson, B.M. Lok and E.M. Flanigan, U.S.Patent 4,310,440 (1982). 16. J.A. Lercher, G. Runplrrrayer and H. Noller, Proc. Int. Symp. Zeol. C a t a l . Siofok (Hungary), May 13-16, 1985, p.71. 17. M. Guisnet, P. Magnoux and C.Canaff, New Developnents in Zeolite Science Technology, Y. M u r a k a m i , A. I i j h and J.W. Ward ( e d s ) , Kcdansha Ltd, Tokyo, 1986, p.201.
1997
SELECTIVE SYNTHESIS OF ISOPRENE BY PRINS CONDENSATION USING MOLECULAR SIEVES E. Dumitriu, V. Hulea, C. Chelaru" and T. Hulea
Faculty of Industrial Chemistry, "Gh. Asachi" Technical University of Jassy, 71 Splai Bahlui, P.O.Box 483, Jassy-6600, Rcmania *Institute of Macromolecular Chemistry, 4l.A Aleea Gr.Ghica-Vcda, Jassy-6600 and S. Kaliaguine Genie Chimique - CERF'IC, Universite Laval, Cite Universitaire, Quebec, Canada, G1K 7P4 ABSTRACT
The formaldehyde condensation with isobutene was studied over mlecular sieves, such as HZSM-5 with various Si/Al ratios, HZSM-5 hpreqnated with H~FQL,SAPO-5 and AlFC-5, in vapour phase in the temperature range 175-400°C The dependence of the catalytic activity and the selectivity to isoprene on the acid strength distribution and the density of active sites was examined.
1. INTRODUCTION The formaldehyde condensation with isobutene is of a special interest since this reaction is involved in the isoprene manufacture, a raw material for various elastomers. The direct production of isoprene, the so-called "one stage process", is a very attractive industrial route due to its simplified prccedure, and y y attempts to find a proper catalyst have been done. Though various acidic oxides were tested and sane interesting features of this condensation have been outlined [l] , an efficient catalyst based on metal oxides has not been found till now. Therefore, it might be possible to obtain a m r e prcmising catalyst using molecular sieves, because these materials have a broad spectrum of acidities and pore structures. Unfortunately there are few reports on the Prins condensation catalyzed by zeolites [2-4] , and a detailed investigation of this process is necessary in order to design a proper catalyst for the selective synthesis of isoprene. With this idea in mind, the following two types of molecular sieves have been selected: ZSM-5, recognized as a shape selective and strong acid catalyst [5,6] , and SAPO-5 which is considered as mildly acidic [7-14 The regular three-dimensional pore structure of pentasil-type zeolites, their properties of shape selectivity, the possibility of modifying the surface properties by the Si/A1 ratio, oxide loading, ion exchange or by isomorphic substitution are all features of great interest in order to design a selective catalyst for the isoprene synthesis. The same reasons led us in the selection of SAPO-5. The objective of this study was to investigate scme factors influencing the activity and the selectivity to isoprene such the S102 /A1203 ratio in
.
1998
catalyst capsition and the oxide loading. A pulse type microreactor was used because it is m r e suitable at this stage of the investigations than a steady flow reactor, since certain information may be difficult to achieve with the latter reactor.
1. Catalyst Preparation. A series of ZSM-5 samples with SiO2/Al2O3 ratio ranging frcm 35 to 1000 were synthesized in the presence of tetrapropylamnonium b r d d e following the published patents (e.g. [12] ) . SAFG-5 samples and AlpO-5 were prepared according to the procedures presented in the literature [13-153 All samples were calcined prior to their use as catalysts in order to remove the organic template used in synthesis and to free the intracrystalline pore system. The catalyst powder was pressed binder free, ground, and used in a particle size between 0.25 and 0.5 mn. 2. Catalyst Modification. In order to obtain H+ forms the samples of zeolites were ion exchanged four times at 90°C in 0.1 M solution of NHLC~. After the ion exchange procedure the samples were washed with distilled water, dried and then calcined in air at 6OOOC for 6 hours. The impregnated forms of HZSM-5 zeolites were obtained following the [16] The resulting catalysts procedure described by Lercher et al. contained 2,3,4,6 and 8% H~FQLby weight. 3. Catalyst Characterization. The crystallinity of the samples was controlled by x-ray diffraction and by IR spectra in KBr, and found to be pure crystalline materials. The chemical compositions of the catalysts were determined by wet chemical methods, atcmic adsorption and flame photcnnetry. The main chemical characteristics together with acidity measurements are shown in Table 1.
.
.
Table 1 Main properties of the catalysts Catalyst Symbol HZSM-5
SiO2/Al2O3
2-35 2-75 2-222 2-400
2-600
2-1000
P/HZSM-5 P2 P3 P4 P6 P8 AlpD-5 SAPO-5
A
s1*
s2**
* si0.036 p0.529=O.L19 ** si0.225 '0.375 AIO.LOO
H~FQ,I,
Acidity , mnole/g.c. overall, 150-550°C
wt %
weak, 150-300°C
35 75 222 400 600 1000
-
0.175 0.087 0.032 0.043 0.028 0.037
0.504 0.322 0.095 0.051 0.034 0.047
35 35 35 35 35
2 3 4 6 8
0.174 0.173 0.167 0.161 0.158
0.482 0.461 0.423 0.388 0.313
0.066 0.108 0.228
0.070 0.162 0.308
1999
Acidic properties of the catalysts were determined by means of NH3-TPD. 4. Reagents. Tertiary butyl alcohol (TBA),(Fluka), and dimethoxymethane (M), (Aldrich), were used to generate in situ isobutene (iB) and formaldehyde (FA), respectively. Fresh aqueous solutions of formaldehyde were prepared by the thermal depolymerization of parafomldehyde. The concentration of formaldehyde (ca 30 % w t ) was determined by chemical methods. 5. Catalyst Evaluation. The condensation reactions were carried out in a pulse microreactor under the following conditions: weight of catalyst, 0.02- 0.04g; nitrogen flow rate, 30 ml/min; pressure, 250 kPa; pulse size, 0.5 p1. The reactor was directly connected to a 3.5 rn GC column filled with carbowax 20M on Chrmsorb M, or to a 1.2 m column packed with Porapak Q. A detailed description for the catalytic runs is given in earlier paper [4]. 3. RESULTS AND DISCUSSION
As previously reported [ 3J , the Prins condensation of fomldehyde and isobutene over medium pore zeolites is a selective reaction, because the molecular size constraint imposed by these zeolites inhibits the formation of bulkier cyclic condensation products, such as 4,4-dimethyl-l,3-dioxane. Thus, a selective production of the is0 rene via 2-methylbutenol can be provided. However, as shown earlier [45, over zeolites having a broad spectrum of acid sites the Prins condensation is accompanied by various side reactions of the olefins, such as oligcmerization, cracking into smaller olefins, disproportionation and aromatization. All these side reactions are also acid catalyzed, and they are undesirable due to their unfavourable carbon balance (for instance, at least two molecules of isobutene/isoprene are consumed for one armtic molecule). Therefore, according to the ideas described in the introductory part, modification of the acid properties of molecular sieves may enhance the catalytic performances. 3.1.Influence of Si02/A1203 As is well-known, the acidity of 234-5 can be controlled by varying the silica to alumina ratio. The yield of isoprene increases along with the increase of the Si02/A1203 ratio, showing a maximum at the Si02/A1203 ratio of %
E
300 O C
70
- 60 .-
0
x .> c
\
u 50-
-aJ %
Si02/AI203
,molar ratio
Figure 1. Effect of S1O2/Al2O3 ratio on the yield of isoprene; T = 30OOC.
LO
I
0.5
1
I
1.0
1.5
2.0
xi020 a.s/g
Figure 2. Effect of strong acid sites on the selectivity to isoprene.
2000
about 600 (Fig.1). Usually such an effect of the Si02 /A12 03 ratio is interpreted in terms of strength and amount of the acidic sites. As shown in Fig.2, the selectivity to isoprene increases as the amount of strong acid sites (measured by the m u n t of amnonia desorbed in the range of 300-6OO0C) decreases. It must be mentioned that the conversions were measured at the steadyactivity, i.e. when the cracking process has disappeared as a result of the deactivation of the strong acid sites by carbonaceous deposits [ 4 1 . This steady-activity is reached swner or later as the Si02/Al O3 ratio is increased or decreased; e.g. , over HZSM-5 with SiO2/AQO3 = 608 the cracking process disappears after the first pulse and a steady-activity is established (see Fig.1). On HZSM-5 the carbonaceous deposits are initially formed in the porous structure and each coke molecule neutralizes one active site [17] .The formation of coke is strongly dependent on the acid site density and the strength of acidic sites. 3.2. Influence of temperature The effect of the reaction temperature on the catalytic performances in the formaldehyde condensation with isobutene over HZSM-5 zeolites was examined over a large range of temperatures, 175-400OC. The typical dependence of conversion on temperature is depicted in Figure 3.
200
300
LOO
tempera ture,OC
Figure 3. The conversion of iB vs. 2-35 reaction temperature; (-1 and (---) 2-600; 0.02 g catalyst.
200
300 LOO t e m p e r o t u re,OC
Figure 4 . The selectivity dependence of reaction temperature over various molecular sieves; 0.02 g catalyst.
As can be seen, at l o w temperatures the conversion of isobutene to isoprene is the main reaction for both catalyst, but as the reaction temperature increases it passes through a maximum, around 3OO0C, then the conversion to isoprene decreases and this decreasing is more strongly for the catalyst with the higher aluminium content. A more pronounced difference between the two catalysts appears in relation to the dependence of total conversion on reaction temperature, when the shape of the curves indicates a strong influence of the SiO2/Al2O3 ratio on the product distribution at high temperatures. In relation to the selectivity to isoprene a more suggestive description is shown in Figure 4. At low temperatures all catalysts have a high selectivity, but as the reaction temperature increases the selectivity to
2001
isoprene decreases in strict dependence on the Si/Al ratio or the type of catalyst in the following order: SAPO-5 > 2-600 > P8 2-75 > 2-35. 3.3. Effect of the mole ratio of reagents Next, the change of conversion with varying molar ratio of the reagents has been studied over the 2-600 catalyst, at 300 OC. In Figure 5 the conversions are reported to the limiting reagent. Evidently, the conversion of formaldehvde increases as the TBA/FA is increased. On the other hand, the degree of conversion of isobutene decreases as the excess of formaldehyde increases. 501
0
501
1100
$ -LO.-O
aJ > c
0 U
302010 -
I
1
2
3
molar ratio
I
I
L
5
Figure 5. Dependence of conversion on the molar ratio of reagents; ( 0 ) TBA/FA; FA/TBA; ( + ) M/TBA. ( 0 ) TBA/M; 0.02 g of catalyst.
Figure 6. Effect of the degree of loading on the conversion ( x ) and the selectivity(0); T = 300OC; 0.02 g of catalyst.
This might be due to the differences among the adsorption strengths of water, isobutene and formaldehyde. If water is too strongly adsorbed, isobutene can hardly be adsorbed and the condensation reaction is diminished. As the GC analysis indicated, the dehydratation of TBA was ccmplete in all cases. when the methylal was introduced to generate in situ formaldehyde the conversions of limiting reagent decreased for both cases. We can conclude that the excess of formaldehyde has no favourable effect. 3.4. Influence of the modifier The activities and selectivities of the Prins condensation over various modified HZSM-5 zeolites are shown in Fig.6, where it is clear that HZSM-5 (Si02/A1203 = 35) modified by H~FQLgave a slight increase of the activity, but a high selectivity. This prmtion was attributed to the modification of the acid strength by H 3 POL. As shown by Lercher et al. [16], the bridging OH group, having significantly higher acid strength, is replaced by two "tenninal" hydroxyl groups and thus the acid strength is decreased. Also, it is worthwhile to note the loss of activity in cracking as the degree of loading increases (see the number of pulses used in order to suppress the process of cracking, Fig.6).
2002
Nature of catalytic sites The above presented data demonstrate that the acid-catalyzed condensation of formaldehyde and isobutene is governed by two factors: (a) the strength of the acidic sites, and (b) the density of active sites. Figure 7 illustrates the ccxnparison of the yield of isoprene of highly siliceous 2.94-5 with that of other solid acid catalysts. %e yield of 3.5.
ul
c
.-
C
3
x
L
0
L
.-
n
L
O
2 00
300 t e mpe ra t u re,OC
LOO
100
Figure 7. Dependence of the yield of isoprene on the nature of catalyst; 0.04 g of catalyst.
200
300 LOO 500 t e m p e ra, t u r e , o c
600
Figure 8. NH3-TPD spectra of tested catalysts; 180C/min heating rate.
isoprene decreases in the order:
H~FO,J+/HZSM-~(P~) > H Z . 9 4 - 5 ( 2 - 6 0 0 ) > > ~ - 5 ( S 2 ) > SAF+5(Sl) > AlFC-5(A)
This order may be correlated with the NH3-TpD spectra. It is clear that as the amount of strong acid sites decreases the selectivity to isoprene increases, but the yield of isoprene depends on the concentration of the acidic sites (for instance, see S1 and S2 catalysts).
200
300
t e m p e r a t ure,OC
LOO
Figure 9. Dependence of yp/y0 ratio on reaction tgnperature; ( 0 1 2-600; S2; 0.02 g of catalyst.
aJ
5
IIUU 100
80-
3
n
60 .-2 60-
‘
ix iX ‘X
c
0
‘X
c LO-
.-O ul
:
\X -It
20OoO
C
0 u
z
x-x-x-x-xo/oco-o-o-o-o-
,, O R :
I
Figure 10. Total conversion (x) and the conversion t o isoprene ( 0 ) vs. the a u n t of pyridine; T = 300OC.
2003
The influence of the strength of the acidic sites can be understood better by performing condensation runs over pyridine poisoned catalysts. First, the catalyst sample (the most selective catalysts, 2-600 and 52, were chosen) was saturated with pyridine at 200OC, then it was purged for 20 min, and the condensation reactions were performed at higher temperatures than the saturation temperature. The ratio between the yield of isoprene on poisoned catalyst (yp)and the yield of isoprene on unpoisoned catalyst as a function of the reaction temperature is shown in Figure 9. It can be observed fran the experimental data, and especially fran the value of the temperature for which yp = yo, that in the case of the SAPO-5 catalyst the acidic sites involved in the Prins condensation are mainly weak, whereas for HZSM-5 with Si02/A1203 = 600 they are of rather moderate strength. These results are in agreement with those shown in Fig.8, and they may explain the different activities of the two catalysts (Fig.7), when the strength of the acidic sites is more important than their number (Table 1). The second kind of experiments consisted of a consecutive poisoning of the catalyst by pyridine at 300 OC, i.e. pulses of 1 p l pyridine were alternated by pulses of reagent mixture. For these experiments 2-35, a catalyst with a broad spectrum of acidities, was chosen and the dependence of the total conversion of isobutene (iB) and the conversion to isoprene on the m u n t of poison is shown in Figure 10. It is worthwhile to note that the conversion to isoprene increases and the side reactions are diminished when the strong acid sites are progressively poisoned. 4.
mcLus1oN
At this stage of our investigations the following conclusions m y be drawn. The synthetic molecular sieves can catalyze the vapour phase condensation of formaldehyde and isobutene, but the catalytic activity and selectivity are strongly dependent on the strength and density of the acid sites. The experimental results indicated that in the Prins condensation the most favourable acid sites are those of slight to moderate strength. The strong acidic sites favoured the side reactions such as the oligcmerization, arcmatization and cracking of isobutene and especially of the formed isoprene F r m the pentasil-type zeolites, HZSM-5 with high SiO*/Al2 03ratio (about 600) and H3FO&npreynated HZSM-5 show high catalytic performances. type are very selective, but their activity is The catalysts of -5 dependent on the Si/Al ratio. The optimum temperature for the acid-catalyzed condensation of formaldehyde and isobutene over molecular sieves in vapour phase was found to be around 300OC. REFERENCES 1. M. Ai, J. Catal, 1106 (1987). 2. P.V. Venuto and P.S. Laandis, Adv. Catal., 18 (1968) 259.
3. C.D. Chany, W.H.Lang and W.K. B e l l , Catalysis of Organic Reactions, W.R. Moser ( e d s ) , Marcel Lkkker, Inc., New York, 1981, p.73. 4. E. Dumitriu, D. Gongescu and V. Hulea, Heterogeneous Catalysis and Fine Chemicals 111, M. Guisnet, J. Earbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier (eds), Elsevier Sci.Publ., Amsterdam, 1993, p.669.
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5. J.Dotka and Piwowarska, Zeolites, 1 (1988) 30. 6. S.G. Hedge, P. Ratnasamy, L.M. Kustov and V.B. Kazansky, Zeolites, 8(3), (1988) 137. 7. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T. R. Cannan and E. M. Flanigan, J. Amer. Chem. Soc., 106 (1984) 6092. 8. E.M. Flanigan, B.M. Lok, R.L. Patton and S.T. Wilson, Stud. Surf. Sci. Catal., 28 (1986) 103. 9. X. Quinhua, Y. Aizhen, B. Shulin and X. Kaijun, Stud. Surf. Sci. C a t a l . , 28 (1986) 835. 10. R.J. Pellet, G.N. Long and J.A. Rabo, Stud. Surf. Sci. Catal., 28 (1986) 843. 11. C. Halik and J.A. Lercher, J. Chem. Soc., Faraday Trans. I, 84 (1988) 4457. 12. R.J. Argauer and G.R. Landolt, U.S. Patent 3,702,886 (1972). 13. H. Weyda and H. Lechert, Zeolites, 10 (1990) 251. 14. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E. M. Flanigan, U.S.Patent 4,440,871 (1984). 15. S.T. Wilson, B.M. Lok and E.M. Flanigan, U.S.Patent 4,310,440 (1982). 16. J.A. Lercher, G. Runplrrrayer and H. Noller, Proc. Int. Symp. Zeol. C a t a l . Siofok (Hungary), May 13-16, 1985, p.71. 17. M. Guisnet, P. Magnoux and C.Canaff, New Developnents in Zeolite Science Technology, Y. M u r a k a m i , A. I i j h and J.W. Ward ( e d s ) , Kcdansha Ltd, Tokyo, 1986, p.201.