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Hydroformylation of butene-1 and butene-2 over rhodium-slp catalysts, as compared with the hydroformylation of ethene and propene
Journal of Molecular
Catalysis, 31 (1986)
371
3’71 - 383
HYDROFORMYLATION OF BUTENE-1 AND BUTENE-2 OVER RHODIUM-SLP CATALYSTS, AS COMPARED WITH THE HYDROFORMYLATION OF ETHENE AND PROPENE PART III. THE INFLUENCE OF THE DEGREE OF PORE-FILLING ON THE PERFORMANCE H. L. PELT, N. A. DE MUNCK*, J. J. F. SCHOLTENt Department of Chemical Technology, 2628 BL Delft (The Netherlands) (Received July 12,1984;
R. P. J. VERBURG, Delft
University
J. J. J. J. BROCKHUS** of Technology,
Julianalaan
and 136,
accepted November 26,1984)
Summary The catalytic hydroformylation of butene-1 and butene-2 (cis and &ens) is compared with the hydroformylation of ethene and propene, using supported liquid phase rhodium catalysts (Rh-SLP catalysts). Attention is paid to the reaction rates, the linear-to-branched alkanal ratios (l/b), and the catalyst stabilities found with the various alkenes. It is shown that the degree of pore-filling of SLP catalysts not only determines the catalytic activity per unit weight of rhodium, but the linearto-branched alkanal ratio as well. This is due to the fact that the alkene conversion must be attributed to two classes of rhodium complexes, viz. rhodium complexes at the gas-liquid interface and those dissolved in the liquid phase. The relative contribution of these two classes of complexes to the catalytic performance changes as a function of the degree of pore-filling. In studying SLP catalysts with the macroreticular resin XAD-2 as a support, it appeared that on decreasing the degree of pore-filling, the linearto-branched alkanal ratio decreased with a simultaneous increase in catalytic activity per unit weight of rhodium. Catalysts with a very low degree of pore-filling are attractive with respect to their high activity per rhodium complex. It is shown that the rhodium complexes at the gas-liquid interface exhibit a higher activity and a lower l/b ratio than the complexes in solution. Butene-2, which exhibits an extremely low hydroformylation activity due to steric hindrance, could only be hydroformylated over Rh-SLP catalysts having a very low degree of pore-filling (monolayer catalysts).
*Present address: Essochem. Holland Inc., Rozenburg, The Netherlands. **Present address: Unichema, Gouda, The Netherlands. *Author to whom correspondence should be addressed; also affiliated with the Dept. of Catalysis, Central Laboratories, DSM, Geleen, The Netherlands. 0304-5102/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
372
Introduction Several authors have described the gas-phase hydroformylation of alkenes and of substituted alkenes over rhodium-SLP catalysts, applying triphenylphosphine as solvent-ligand [ 1 - 71, and have demonstrated the high stability and high linear-to-branched alkanal ratios which may be realized with such catalysts. The influence of type of support on the performance of a SLP has been demonstrated by us in two previous articles [8,9]. It was shown that many supports induce adsorptive complex withdrawal, which often gives rise to elimination of the complexes as catalytic centres. Hence, supports negatively influence the rhodium utilization in SLP catalysts, but an exception must be made for the styrene-divinylbenzene resin (XAD-2), which exhibits almost no complex adsorption. Therefore this support is an excellent choice with respect to maximum rhodium utilization. In our previous work it was also pointed out that during hydroformylation the alkanals produced dissolve into the solvent-ligand, and attain a steady state concentration which depends on experimental conditions such as temperature and feed composition [f&9]. The dissolved alkanal may influence the position of the equilibria between the various rhodium complexes in solution, and also the extent of rhodium complex adsorption. Therefore the amount of dissolved alkanal co-determines the catalytic performance. Furthermore, dissolution of the alkanal limits the maximum allowable alkene conversion, because too large a conversion to alkanal leads to soaking of the catalyst. Moreover, we concluded that the reaction takes place at the gas-liquid interface as well as in the bulk solution. In the present article, the performance of some rhodium--SLP catalysts in the hydroformylation of butene-1 and butene-2 (cis and tmns) is reported, and compared with the results of hydroformylation experiments with ethene and propene. It is generally known that internal alkenes such as butene-2 need fairly active catalysts. As explained above, XAD-2 is the best choice in such a case. Moreover, from exploratory research by de Munck [lo], we know that XAD-2 as a support, combined with a very low degree of porefilling, leads to catalysts with a very high activity per rhodium complex. Such catalysts were coined monolayer catuZysts by us, and in connection with their high specific activity they were tested in the hydroformylation of butene-2. The fact that these catalysts give rise to low linear-to-branched alkanal ratios is no disadvantage in this case, since the introduction of the formyl group on carbon atom 2 or 3 in butene-2 leads to the same alkanal.
Experimental Materials RhH(CO)(PPh,), was prepared by the method of Ahmad et al. [ll]. Triphenylphosphine (Merck, F.R.G., 99%) was used as received. Benzene and toluene (Merck. F.R.G.. 99%) were dried over molecular sieve 3A, from
373
Union Carbide, U.S.A. Butene-1 (99.5%), cis- and trans-butene-2 (99%), nitrogen (99.9%) and hydrogen (99.99%) were obtained from Hoek Loos NV, The Netherlands. Carbon monoxide (99.5%), helium (99.98%), propene (99.5%) and ethene (99.9%) were obtained from Air Products, U.S.A. The gases were freed from traces of oxygen, water and carbon dioxide over BASF catalyst R3-11, molecular sieve 3A and sodium hydroxide on asbestos (‘Ascarit’) respectively. Hydrogen was freed from traces of oxygen and water over Pd-on-alumina catalyst (BASF, F.R.G.) and over molecular sieve 3A. The support material Silica-H is a non-commercial product from DSM, The Netherlands (SBET= 46 m2 g-l, VP = 0.44 cm3 g-l, sodium content 10 ppm). The macroreticular resin Amberlite XAD-2, (Serva, F.R.G.) has a BET surface area of 300 m2 g-’ and a pore volume of 0.69 cm3 g-l. Catalyst preparation The rhodium-SLP catalysts were prepared by impregnation of the supports with a solution of RhH(CO)(PPh,), and of PPh, in benzene to incipient wetness, followed by evaporation of the benzene. This method is described extensively by Gerritsen et al. [ 11, Some catalysts on XAD-2 (monolayer catalysts) were prepared by wet impregnation. Dry XAD-2 (10 g) was stirred with a solution containing the desired amount of the rhodium complex (0 JO - 0.35 g) and 3 mol PPhJ per mol rhodium in 80 cm3 toluene, saturated with hydrogen. The solution was slowly heated to 338 K during 1 h, and kept under hydrogen at 338 K for 2 h. The complex-loaded resin was then separated from the solution, and carefully washed three times with 100 cm3 toluene and twice with 100 cm3 diethyl ether in order to remove excess rhodium complex and triphenylphosphine. Additional triphenylphosphine was added to the surface by contacting the catalyst with a solution of PPh3 in diethyl ether for 0.5 h. Finally the catalyst was dried under hydrogen. Catalyst characterization The phosphorus content of the catalysts was determined by digestion of the samples, followed by calorimetric phosphorus titration. Digestion of the samples was carried out with a solution of sodium sulfate/mercury oxide in 95 - 98% sulfuric acid, at 743 K for 24 h. In a Technicon Autoanalyser, a blue colour is formed on addition of orthophosphate, molybdate and antimony ions, followed by reduction with ascorbic acid at pH = 7, see [12]. The phosphomolybdenum concentration was measured at X = 660 nm. The rhodium content of the catalysts was determined by neutron activation analysis, based on detection of the isotope ‘@‘Rh. The single comparator method was used, with Ti as a reference material [ 131. The textures of supports and catalysts were determined by physical adsorption and capillary condensation of nitrogen at 77 K using a Carlo Erba Sorptomatic apparatus, type 1800.
374
Hydroformylation equipment The fixed bed continuous flow equipment and the method of experimentation is described in [ 11. Gas chromatographic analysis of the product stream was performed in two apparatus. The first, self-constructed apparatus analysed the isomerisation and hydrogenation products of the alkene, and was equipped with a flame ionisation detector and a Hewlett Packard 5701A electrometer. The column used was filled with Spherosil XOB-75 (length 3 m, i.d. 0.0021 m). The selectivity for hydrogenation and isomerisation products was very low, and never exceeded 2% of the alkene conversion described in this paper. The second apparatus was a Hewlett Packard gas chromatograph 5710A, with which the hydroformylation and the aldol condensation products were analysed. Use was made of a column filled with 25% OV 101 on Chromosorb WHP (length 4 m, i.d. 0.0021 m). Peak areas were integrated with a microprocessor LCI 11/03 from Digital Equipment Co., U.S.A.
Results Comparison of the hydroformylation rates of ethene, propene and butene-1 over a Rh-SLPC In the experiments reported in this paper, the reactor was operated differentially (conversion per pass <5%), and hence the reaction rate (r) can be expressed by:
r = tlP/V
(1)
where W is the weight of rhodium in the reactor in grams, F is the alkene flow expressed in cm3 alkene per second (at 0.1 MPa, 293 K), t is the alkene conversion, and r is the reaction rate consequently expressed in cm3 alkene converted (at 0.1 MPa, 293 K) per gram rhodium per second. The reaction rates in Table 1 are the rates measured after a stable activity level was reached, which took less than 2 h. Stable activity is observed for at least 1000 h, and after this period there were no indications of deactivation. The SLPC was supported on Silica-H lumps, mean particle size 0.5 mm, with a degree of pore-filling (6) of 0.5, and a P/Rh ratio of 100 mol phosphorus per mol rhodium. In the seventh column of Table 1, standardized reaction rates are presented, fixing the reaction rate with ethene at 100 arbitrary units. The standardized reaction rates were calculated at the same alkene concentration in PPh3 and at the same space velocity used in the ethene experiment. We took into account the facts that the reaction rate is first order in the alkene pressure and zero order in hydrogen and carbon monoxide. These orders were reported by Gerritsen et al. [3] and by Herman [ 51 ‘for the case of propene, and were found in our own experiments for,the case of butene-1. Furthermore, allowance is made for differences in alkene solubilities [14,15].
0.40
1.20
0.40
0.40 0.17 0.40 0.17 0.40
0.40 0.17 0.40 0.40 0.40
1.20 0.50 1.20 1.20 1.10
Palkene
Alkene pressure
WV
CO pressure PC0 (MPa)
WW
PT
Total pressure
aNot applicable. bUnknown.
ethene propene butene-1 butene-1 butene-2 (cb) butene-2 (Pa=?)
alkene
Type of
b
b
0.024 0.029 0.056 0.024
hkene)
Alkene fraction in PPh,
0.0
145 1.2 11.5 4.9 0.0
Reaction rate, r (cm3 alkene gRhl s-r)
0
2 3 3 0
100
Standardized reaction rate, r5 (arbitrary units)
a
18 21 21 a
a
Linear-to-branched ratio, l/b
Reaction rates and linear-to-branched alkanal ratios (I/b) for various alkenes over a silica-supported rhodium SLPC at 363 K
TABLE 1
376
The linear-to-branched ratios (I/b) The influence of the various reactant partial pressures on the linear-tobranched ratio (l/b) is shown in Fig. 1. On lowering the carbon monoxide partial pressure, l/b strongly increases, whereas the hydrogen and butene-1 partial pressures hardly influence the l/b ratio. This is in agreement with the results found by Gerritsen et al. [ 31 in the hydroformylation of propene over a SLPC. The l/b ratios for the butene-1 hydroformylation, in comparison with propene hydroformylation, are on an average 60% higher over the entire range of carbon monoxide partial pressures investigated. In studying the effect of the degree of pore-filling (6) on the linear-tobranched ratio, a very interesting result was obtained. In order to avoid any influence of adsorptive complex withdrawal, XAD-2 was chosen as a support. The results are presented in Fig. 2, in which l/b, together with the butene-1 hydroformylation rate, is plotted as a function of 6. By way of comparison, the ethene hydroformylation reaction rate as a function of 8, published earlier by Gerritsen et al. [2], is reproduced in the same Figure. The course of curve (a) is explained in [9] : the lower the degree of porefilling, the higher the percentage of rhodium complexes at the gas-liquid interface. These complexes are in direct contact with the gas-phase carbon monoxide, and therefore their mean coordination number with
50
LO
30
20
10
01 0
01
02
03
P, IMPal
04
0.5
0.6
0
02
OL
06
08
10
-6
Fig. 1. Linear-to-branched ratio (l/b) for the hydroformylation of butene-1 as a function of the partial pressure of the reactants (Pi). Supports: Silica-H, 6 = 0.5, P/Rh = 100,T = 363 K, total pressure = 1.2 MPa, W/F = 0.98m&h s cmV3. Curve a: (X) the influence of the partial pressure of butene-1: curve a: (0) the influence of the partial pressure of hydrogen; curve b: (a) the influence of the partial pressure of carbon monoxide; (- - -): Gerritsen’s result for the hydroformylation of propene and the influence of the partial pressure of carbon monoxide on it [ 3 1. Fig. 2. Reaction rate of pore-filling, in the MPa, P/Rh = 100, T: Gerritsen’s result of
r21.
(curve a) and the linear-to-branched ratio (curve b) versus the degree hydroformylation of butene-1. Support: XAD-2, total pressure 1.2 363 K, Hz:CO:butene-1 = l:l:l, W/F = 0.57 mgnn s crnb3; curve c: the reaction rate of the ethene hydroformylation as function of 6
triphenylphosphine ligands will be lower than the coordination number of rhodium complexes in the bulk liquid, and hence they exhibit a higher activity. By comparing curve (a) with curve (c), the ratio of the hydroformylation rates of ethene and butene-1 can be calculated. We obtain the following results: at 6 = 0.1, the rate ratio is 7.3; at 6 = 0.2, the rate ratio is 11.3; at 6 = 0.5, the rate ratio is 29; at 6 = 0.8, the rate ratio is 43. Hence it appears that the lower the ratio of the rates of ethene and butene-1 hydroformylation, the lower the PPhs coordination around the rhodium centre. Curve (b) in Fig. 2 represents the change in the linear-to-branched ratio in butene-l hydroformylation, as influenced by 6. The higher the percentage of interface complexes (lower 6), the lower the l/b ratio. This is in agreement with our previous conclusion that the gas-liquid interface complexes have a lower average PPh, coordination. Catalytic behaviour of monolayer catalysts In the foregoing section the high activity per rhodium complex of SLP catalysts with very low degrees of pore-filling was demonstrated. Lowering the degree of pore-filling leads to catalysts covered with a monolayer of PPh3, in which the complexes are dissolved, or even to sub-monolayer catalysts. In this article, all these catalysts are termed ‘monolayer catalysts’. A number of such catalysts were prepared and tested in the hydroformylation of propene, butene-1 and cis- and trans-butene-2. The rate of propene hydroformylation as a function of time, for a series of XAD-2-supported monolayer catalysts with various coverages, is shown in Fig. 3. Experimental conditions are indicated in the legend to Fig. 3, and the characteristic catalyst parameters are given in Table 2. These catalysts were
24
2.0
b
0 0
LO
80
120
160
200
240
Fig. 3. The hydroformylation rate of propene as a function of time using monolayer catalysts with varying PPh3 coverage (for catalyst parameters see Table 2); T = 363 K, total pressure 0.1 MPa, Hp:CO:propene = 1:l:l; for comparison the behaviour of SLPC k (S = 0.66) is given.
378 TABLE 2 Final activities and linear-to-branched ratios in propene hydroformylation over rhodium complex monolayer catalysts at 363 K and 0.1 MPa total pressure, Hz:CO:propene, 1:l:l Catalyst
ePPh,
r
6PPh,
(cm3 propene
l/b
P/Rh
WIF (mgRh
s cm-‘)
k!Rh 5)--l)
0.112 0.16 0.28 0.31 0.32 0.32 0.35 0.43 0.48
0.019 0.027 0.047 0.053 0.054 0.054 0.059 0.073 0.081
2.29
1.32 0.89 0.82 0.73 0.77 0.72 0.26 0.24
prepared by wet impregnation coverage with PPh3, containing 6 PPh,
mol P (g support)-’ =
S(BET)
7.7 9.6 12.6 14.5 14.4 14.2 15.0 15.5 16.5
25.6 55.0 75.3 140.2 66.6 97.2 153.4 29.1 49.1
10.22 6.9 3.2 5.4 8.2 6.3 5.7 32.5 27.0
as described in the Experimental section. dissolved complexes, is defined as:
The
NAVS(PPhs) (2)
where S(PPh,) is the cross-sectional area of a PPh, molecule (1.2 X 1O-18 m’), NAV is Avogadro’s number (6.02 X 10z3 molecules mol-‘), and S(BET) is the BET surface area of the support (300 m2 g-i). Using XAD-2 as a support, a degree of pore-filling of 0.15 corresponds with monolayer coverage. By comparison, catalyst k shows the activity of a SLPC with 6 = 0.66, It is seen from the Figure that the catalytic activity of monolayer catalysts; per unit weight of rhodium, is appreciably higher than found for SLP catalysts with a higher degree of pore-filling. In the case of catalyst a (0 is a 0.112 only), the activity is even a factor of 23 higher than that of k (0 is 4.4) (Fig. 3). The explanation for this effect is the same as that presented above for the behaviour of SLP catalysts. Just as found for this last type of catalyst, an increase in the activity is accompanied by a decrease in the linear-tobranched ratio (Fig. 4). Monolayer catalysts are stable only at reaction temperatures below 373 K. At higher degrees of pore-filling, the catalyst remains stable up to at least 393 K. Hydroformylation Alkenes with more slowly than butene-1. Alkenes slowly. In studying strates, Brown and
of butene-1 and butene-2, and mixtures thereof the double bond in the /I-position, like butene-2, react alkenes with the double bond in the o-position, like with the general formula R1R2C=CH2 react even more the hydroformylation of a number of unsaturated subWilkinson [ 161 observed an activity difference of a factor
379 -6 002 004 . I r, cm3propene
30.
gRh
006 I
008 II
010 3 rlcrn butew) gRh'S
I/b
'
t_
t
20
20 -
I/b
f
20-t
20
.
A--XX.",'"
0
X'
10 -
,dX
10
cl
10 -
10
0 7-x" 0
;_;_
0 0 -9
0 02
OL
06O
/-;i 0 __t
I/ 50 %
0 100 buterwlm butene
Fig. 4.. The rate of propene hydroformylation and the l/h ratio as a function of the PPha coverage using monolayer catalysts. For catalysts and experimental conditions see legend to Fig. 3. Fig. 5. The reaction rate of butene hydroformylation and the linear-to-branched ratio over a rhodium-SLPC as a function of the percentage butene-1 in the butene feed. 0: butene-l/helium mixture, X: butene-l/butene-2 mixture. Support is silica-H, Hz:CO: butene = 1:l:l; for catalyst parameters and experimental conditions see legend to Fig. 1.
of about 25 between alkenes-1 and alkenes-2, using RhH(CO)(PPhs)s dissolved in benzene as a catalyst at 293 K and 0.1 MPa total pressure. We investigated to what extent our catalytic system shows the same trend. In Fig. 5 the rate of butene hydroformylation per unit weight of rhodium is plotted as a function of the percentage of butene-l in the butene feed. -The type of catalyst used and the experimental conditions are indicated in the legend of the Figure. In some cases part of the butene-l was replaced by helium, in others by cis-butene-2. From the Figure it is shown that dilution of butene-1 by butene-2 has an effect on the reaction rate similar to that of dilution by helium, in accordance with the fact that the rate of reaction is 30 times lower for butene-2 compared to butene-1. It is also shown in Fig. 5 that the linear-to-branched ratio (l/b) is perfectly constant, independent of the percentage of butene-1. Hence in the hydroformylation of a mixture of butene-1 and butene-2, butene-2 behaves practically as an inert gas. During hydroformylation, butene-l is separated from a mixture of butenes. Butene-2 can be hydroformylated, provided extremely active catalysts are used. Such catalysts are the monolayer catalysts described above. In Table 3 the results of the hydroformylation experiments are given. Under identical experimental conditions, the butene-l conversion per unit weight of rhodium is six times higher over the monolayer catalyst than over the rhodium SLPC, and the l/b ratio is lower, as explained earlier. Over the monolayer catalyst, both cis- and trans-butene-2 can be hydroformylated with measurable rates, which are about 30 times slower than that found for butene-1. Using the monolayer catalyst we observe a behaviour in the
380 TABLE 3 The rate of hydroformylation of butene-1, cis-butene-2 and trans-butene-2 using a rhodium monolayer catalyst (catalyst B); for comparison, the rate of hydroformylation of butene-1 over a Rh-SLPC is added (catalyst A) Type of alkene
Catalyst
Rate of hydroformylation to linear/branched alkanal (cmbutene3 gnhq s-9
Linear-tobranched ratio
butene-1 butene-1
Rh-SLPC ( A)a monolayer catalyst ( B)b monolayer catalyst ( B)b monolayer catalyst ( B)b
17.3 86.0
17.3 3.7
cis-butene-2 trans-butene-2
1.0 23.0
c
3.3
c
c
3.6
c
Experimental conditions: T = 363 K, PT = 1.2 MPa, Ha:CO:butene = 1:l:l. Wris catalyst is described in the legend to Fig. 2. bSupport is XAD-2,8 = 0.14, P/Rh = 84. CNot applicable.
hydroformylation of alkene-1 and the corresponding alkene-2 similar to that found by Brown and Wilkinson for benzene-dissolved catalysts. Obviously the degree of PPhs coordination in both types of catalysts is the same. SLP catalysts, on the other hand, show a behaviour corresponding with a higher degree of PPh, coordination. It is noteworthy that the difference in hydroformylation rates between cis- and truns-butene-2 is very small. Obviously the difference in steric hindrance during formation of the n-bonded complex is small. During hydroformylation of butene-2, no isomerisation to butene-1 was observed, and hence no n-pentanal was observed in the product gas. Discussion The results summarized in Table 1 are typical of SLP catalysts with a relatively high degree of pore-filling. In such catalysts the ratio of the number of active rhodium complexes in solution to the number of active rhodium complexes at the gas-liquid interface is very high, and therefore their catalytic behaviour is mainly governed by the complexes in the bulk of the liquid. This liquid is characterized by a high triphenylphosphine concentration, whereas the carbon monoxide concentration is very low [14]. It is very likely that the rhodium centre is provided with at least two triphenylphosphine ligands, and this finds expression in a high l/b ratio (see the last column in Table 1). In the homologous series of alkenes, an activity decline by a factor of about 50 is found, proceeding from ethene to propene (seventh column of Table l), but the activity found with butene-1 as a substrate does not deviate significantly from that of propene. Experiments with gaseous octene-1 in the feed (not reported here) showed that the activity of this higher alkene with
381
the double bond in the o-position did not deviate substantially from that of propene. However, higher alkenes gave rise to higher l/b ratios. In the hydroformylation mechanism, the choice between the formation of a linear or a branched alkyl ligand is made during the interligand reaction between the n-bonded alkene and the hydrogen ligand. It is likely that, for steric reasons, the formation of a linear alkyl ligand is increasingly favoured the longer the alkyl chain in the reacting alkene. By comparison with more highly diluted solutions of the Wilkinson complex, the activity of the rhodium complexes in our SLP catalysts is relatively low, in particular the activity to give the branched product. This is probably why the activity for the hydroformylation of cis- and tmns-butene2 is virtually zero (see last two rows of Table 1). The most important observation reported in this paper is that the hydroformylation activity of SLP catalysts, calculated per rhodium complex, increases considerably with decreasing degree of pore-filling. This effect is accompanied by a decrease in the linear-to-branched ratio (see Fig. 2). The explanation is as follows. On decreasing the degree of pore-filling, the surface area of the gas-liquid interface greatly increases (see, for instance, Fig. 9 in [9]). Hence the percentage of complexes at the gas-liquid interface markedly increases, and these complexes have a lower average number of triphenylphosphine ligands around the rhodium centre. This follows from the observation that the l/b ratio is much lower than that found for rhodium complexes in the bulk of the solvent-ligand. The lower average coordination with triphenylphosphine ligands at the gas-liquid interface we attribute to the direct contact these complexes have with the gas-phase carbon monoxide molecules. The concentration of carbon monoxide in the gas-phase (at 0.4 MPa partial carbon monoxide pressure, 1.2 MPa total pressure, and at 363 K) is a factor of about 15 higher than the concentration of carbon monoxide dissolved in the solvent-ligand triphenylphosphine. This follows from the solubility of CO in PPh, experimentally determined by Herman et al. [ 141, and it means that the PPh, ligand replacement by CO ligands is higher at the gas-liquid interface than in the bulk of the solvent-ligand. A secondary factor may be that the coordination of the rhodium complexes at the gas-liquid interface with free non-liganded PPh, molecules is half the value in the bulk liquid, which means the thermodynamic activity of PPhs is lower. The advantages of catalysts with a high gas-liquid interface-to-bulk liquid ratio manifest themselves most clearly in catalysts with a degree of pore-filling so low that a monolayer, or less than a mqnolayer, of PPhs, in which the rhodium complexes are dissolved, is present. From Table 3 we calculate that cis- or truns-butene-2 reacts more slowly by a factor of about 30 than butene-1 over these monolayer catalysts. This factor is about equal to that found by Brown and Wilkinson [ 161. In comparing the reaction rates of pentene-l and pent.ene-2 (cis and trans) and heptene-1 and heptene-2 (cis and trans), these investigators arrived at a factor of 25 and 30 respectively. Just as in our experiments with cilr-and trans-butene-2, Brown and Wilkinson
[16] did not find a significant difference between the rates with cis- or trunsheptene-2 as a substrate. Obviously the catalytic performance of the complexes at the gas-liquid interface in our catalysts is comparable with that of the complexes used by Wilkinson in his experiments with homogeneous dilute systems. Contrary to what was found with SLP catalysts with a high degree of pore-filling, monolayer catalysts are able to hydroformylate alkenes with the double bond in the P-position, such as butene-2 (see Table 3). The activity found for butene-2 (cis and truns) is a factor of about 30 lower than that for butene-1. This may be due both to steric hindrance and to the electrondonating strength of the two CH, groups in butene-2. Acknowledgements We thank Mr. P. Bode of the Interuniversity Reactor Institute, Delft, The Netherlands, for carrying out the neutron activation analyses. The investigations were supported (in part) by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). List of symbols F l/b N AV pi
PT
P/Rh r r17 rb
S(BET) S(PPh3) T WRh xi
6 PPh,
e PPh, t
flow of alkene at 0.1 MPa and 293 K ratio linear-to-branched alkanal Avogadro’s number partial pressure of component i total pressure molar ratio phosphorus to rhodium reaction rate rates of the formation of linear and branched alkanals respectively surface area cross-sectional surface area of PPh3 temperature weight of rhodium metal in the reactor mole fraction of component i in the liquid phase degree of pore-filling of the support with PPh3 surface coverage with PPh, conversion
cm3 s-l molecules mol-’ MPa MPa mol mol-’ cm3 alkene ga,-’ cm3 alkene g,,-’
s-1’ s-
m2 g-’ m2 K g -
References 1 L. A. Gerritsen, A. van Meerkerk, M. H. Vreugdenhil and J. J. F. Scholten, J. Mol. Catal., 9 (1980) 139. 2 L. A. Gerritsen, J. M. Herman and J. J. F. Scholten, d. Mol. Catal., 9 (1980) 241.
333 3 L. A. Gerritsen, W. Klut, M. H. Vreugdenhil and J. J. F. Scholten, J. Mol. Catal., 9 (1980) 265. 4 N. A. de Munck, J. P. A. Notenhoom, J. E. de Leur and J. J. F. Scholten, J. Mol. Catal., 11 (1981) 233. 5 J. M. Herman, Ph.D. Thesis, Delft, The Netherlands, 1983. 6 J. Hjortkjaer, M. S. ScurreiI and P. Simonsen, J. Mol. Catal., 6 (1979) 405. 7 J. Hjortkjaer, M. S. Scurrell, P. Simonsen and H. Svendsen, J. Mol. Catal., 12 (1981) 179. 8 H. L. Pelt, G. van der Lee and J. J. F. Scholten, J. Mol. Catal., 29 (1985) 319. 9 H. L. Pelt, J. J. J. J. Brockhus, R. P. J. Verhurg and J. J. F. Scholten, J. Mol. Catal., 31 (1985) 107. 10 N. A. de Munck, Ph.D. Thesis, Delft, The Netherlands, 1980. 11 N. Ahmad, S. D. Robinson and M. F. Uttley, J. Chem. Sot., Dalton Trans., (1972) 343. 12 Industrial Method No. 329-74 W/B Technicon Industrial System, U.S.A. 13 P. J. M. Korthoven and M. de Bruin, J. Radioanal. Chem., 35 (1977) 127. 14 J. M. Herman, L. A. Gerritsen and Th. W. de Loos, J. Chem. Eng. Data, 26 (1981) 185. 15 H. L. Pelt, J. J. J. J. Brockhus, R. P. J. Verhurg and Th. W. de Loos, to be published. 16 C. K. Brown and G. Wilkinson, J. Chem. Sot. A., (1970) 2753.
Catalysis, 31 (1986)
371
3’71 - 383
HYDROFORMYLATION OF BUTENE-1 AND BUTENE-2 OVER RHODIUM-SLP CATALYSTS, AS COMPARED WITH THE HYDROFORMYLATION OF ETHENE AND PROPENE PART III. THE INFLUENCE OF THE DEGREE OF PORE-FILLING ON THE PERFORMANCE H. L. PELT, N. A. DE MUNCK*, J. J. F. SCHOLTENt Department of Chemical Technology, 2628 BL Delft (The Netherlands) (Received July 12,1984;
R. P. J. VERBURG, Delft
University
J. J. J. J. BROCKHUS** of Technology,
Julianalaan
and 136,
accepted November 26,1984)
Summary The catalytic hydroformylation of butene-1 and butene-2 (cis and &ens) is compared with the hydroformylation of ethene and propene, using supported liquid phase rhodium catalysts (Rh-SLP catalysts). Attention is paid to the reaction rates, the linear-to-branched alkanal ratios (l/b), and the catalyst stabilities found with the various alkenes. It is shown that the degree of pore-filling of SLP catalysts not only determines the catalytic activity per unit weight of rhodium, but the linearto-branched alkanal ratio as well. This is due to the fact that the alkene conversion must be attributed to two classes of rhodium complexes, viz. rhodium complexes at the gas-liquid interface and those dissolved in the liquid phase. The relative contribution of these two classes of complexes to the catalytic performance changes as a function of the degree of pore-filling. In studying SLP catalysts with the macroreticular resin XAD-2 as a support, it appeared that on decreasing the degree of pore-filling, the linearto-branched alkanal ratio decreased with a simultaneous increase in catalytic activity per unit weight of rhodium. Catalysts with a very low degree of pore-filling are attractive with respect to their high activity per rhodium complex. It is shown that the rhodium complexes at the gas-liquid interface exhibit a higher activity and a lower l/b ratio than the complexes in solution. Butene-2, which exhibits an extremely low hydroformylation activity due to steric hindrance, could only be hydroformylated over Rh-SLP catalysts having a very low degree of pore-filling (monolayer catalysts).
*Present address: Essochem. Holland Inc., Rozenburg, The Netherlands. **Present address: Unichema, Gouda, The Netherlands. *Author to whom correspondence should be addressed; also affiliated with the Dept. of Catalysis, Central Laboratories, DSM, Geleen, The Netherlands. 0304-5102/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
372
Introduction Several authors have described the gas-phase hydroformylation of alkenes and of substituted alkenes over rhodium-SLP catalysts, applying triphenylphosphine as solvent-ligand [ 1 - 71, and have demonstrated the high stability and high linear-to-branched alkanal ratios which may be realized with such catalysts. The influence of type of support on the performance of a SLP has been demonstrated by us in two previous articles [8,9]. It was shown that many supports induce adsorptive complex withdrawal, which often gives rise to elimination of the complexes as catalytic centres. Hence, supports negatively influence the rhodium utilization in SLP catalysts, but an exception must be made for the styrene-divinylbenzene resin (XAD-2), which exhibits almost no complex adsorption. Therefore this support is an excellent choice with respect to maximum rhodium utilization. In our previous work it was also pointed out that during hydroformylation the alkanals produced dissolve into the solvent-ligand, and attain a steady state concentration which depends on experimental conditions such as temperature and feed composition [f&9]. The dissolved alkanal may influence the position of the equilibria between the various rhodium complexes in solution, and also the extent of rhodium complex adsorption. Therefore the amount of dissolved alkanal co-determines the catalytic performance. Furthermore, dissolution of the alkanal limits the maximum allowable alkene conversion, because too large a conversion to alkanal leads to soaking of the catalyst. Moreover, we concluded that the reaction takes place at the gas-liquid interface as well as in the bulk solution. In the present article, the performance of some rhodium--SLP catalysts in the hydroformylation of butene-1 and butene-2 (cis and tmns) is reported, and compared with the results of hydroformylation experiments with ethene and propene. It is generally known that internal alkenes such as butene-2 need fairly active catalysts. As explained above, XAD-2 is the best choice in such a case. Moreover, from exploratory research by de Munck [lo], we know that XAD-2 as a support, combined with a very low degree of porefilling, leads to catalysts with a very high activity per rhodium complex. Such catalysts were coined monolayer catuZysts by us, and in connection with their high specific activity they were tested in the hydroformylation of butene-2. The fact that these catalysts give rise to low linear-to-branched alkanal ratios is no disadvantage in this case, since the introduction of the formyl group on carbon atom 2 or 3 in butene-2 leads to the same alkanal.
Experimental Materials RhH(CO)(PPh,), was prepared by the method of Ahmad et al. [ll]. Triphenylphosphine (Merck, F.R.G., 99%) was used as received. Benzene and toluene (Merck. F.R.G.. 99%) were dried over molecular sieve 3A, from
373
Union Carbide, U.S.A. Butene-1 (99.5%), cis- and trans-butene-2 (99%), nitrogen (99.9%) and hydrogen (99.99%) were obtained from Hoek Loos NV, The Netherlands. Carbon monoxide (99.5%), helium (99.98%), propene (99.5%) and ethene (99.9%) were obtained from Air Products, U.S.A. The gases were freed from traces of oxygen, water and carbon dioxide over BASF catalyst R3-11, molecular sieve 3A and sodium hydroxide on asbestos (‘Ascarit’) respectively. Hydrogen was freed from traces of oxygen and water over Pd-on-alumina catalyst (BASF, F.R.G.) and over molecular sieve 3A. The support material Silica-H is a non-commercial product from DSM, The Netherlands (SBET= 46 m2 g-l, VP = 0.44 cm3 g-l, sodium content 10 ppm). The macroreticular resin Amberlite XAD-2, (Serva, F.R.G.) has a BET surface area of 300 m2 g-’ and a pore volume of 0.69 cm3 g-l. Catalyst preparation The rhodium-SLP catalysts were prepared by impregnation of the supports with a solution of RhH(CO)(PPh,), and of PPh, in benzene to incipient wetness, followed by evaporation of the benzene. This method is described extensively by Gerritsen et al. [ 11, Some catalysts on XAD-2 (monolayer catalysts) were prepared by wet impregnation. Dry XAD-2 (10 g) was stirred with a solution containing the desired amount of the rhodium complex (0 JO - 0.35 g) and 3 mol PPhJ per mol rhodium in 80 cm3 toluene, saturated with hydrogen. The solution was slowly heated to 338 K during 1 h, and kept under hydrogen at 338 K for 2 h. The complex-loaded resin was then separated from the solution, and carefully washed three times with 100 cm3 toluene and twice with 100 cm3 diethyl ether in order to remove excess rhodium complex and triphenylphosphine. Additional triphenylphosphine was added to the surface by contacting the catalyst with a solution of PPh3 in diethyl ether for 0.5 h. Finally the catalyst was dried under hydrogen. Catalyst characterization The phosphorus content of the catalysts was determined by digestion of the samples, followed by calorimetric phosphorus titration. Digestion of the samples was carried out with a solution of sodium sulfate/mercury oxide in 95 - 98% sulfuric acid, at 743 K for 24 h. In a Technicon Autoanalyser, a blue colour is formed on addition of orthophosphate, molybdate and antimony ions, followed by reduction with ascorbic acid at pH = 7, see [12]. The phosphomolybdenum concentration was measured at X = 660 nm. The rhodium content of the catalysts was determined by neutron activation analysis, based on detection of the isotope ‘@‘Rh. The single comparator method was used, with Ti as a reference material [ 131. The textures of supports and catalysts were determined by physical adsorption and capillary condensation of nitrogen at 77 K using a Carlo Erba Sorptomatic apparatus, type 1800.
374
Hydroformylation equipment The fixed bed continuous flow equipment and the method of experimentation is described in [ 11. Gas chromatographic analysis of the product stream was performed in two apparatus. The first, self-constructed apparatus analysed the isomerisation and hydrogenation products of the alkene, and was equipped with a flame ionisation detector and a Hewlett Packard 5701A electrometer. The column used was filled with Spherosil XOB-75 (length 3 m, i.d. 0.0021 m). The selectivity for hydrogenation and isomerisation products was very low, and never exceeded 2% of the alkene conversion described in this paper. The second apparatus was a Hewlett Packard gas chromatograph 5710A, with which the hydroformylation and the aldol condensation products were analysed. Use was made of a column filled with 25% OV 101 on Chromosorb WHP (length 4 m, i.d. 0.0021 m). Peak areas were integrated with a microprocessor LCI 11/03 from Digital Equipment Co., U.S.A.
Results Comparison of the hydroformylation rates of ethene, propene and butene-1 over a Rh-SLPC In the experiments reported in this paper, the reactor was operated differentially (conversion per pass <5%), and hence the reaction rate (r) can be expressed by:
r = tlP/V
(1)
where W is the weight of rhodium in the reactor in grams, F is the alkene flow expressed in cm3 alkene per second (at 0.1 MPa, 293 K), t is the alkene conversion, and r is the reaction rate consequently expressed in cm3 alkene converted (at 0.1 MPa, 293 K) per gram rhodium per second. The reaction rates in Table 1 are the rates measured after a stable activity level was reached, which took less than 2 h. Stable activity is observed for at least 1000 h, and after this period there were no indications of deactivation. The SLPC was supported on Silica-H lumps, mean particle size 0.5 mm, with a degree of pore-filling (6) of 0.5, and a P/Rh ratio of 100 mol phosphorus per mol rhodium. In the seventh column of Table 1, standardized reaction rates are presented, fixing the reaction rate with ethene at 100 arbitrary units. The standardized reaction rates were calculated at the same alkene concentration in PPh3 and at the same space velocity used in the ethene experiment. We took into account the facts that the reaction rate is first order in the alkene pressure and zero order in hydrogen and carbon monoxide. These orders were reported by Gerritsen et al. [3] and by Herman [ 51 ‘for the case of propene, and were found in our own experiments for,the case of butene-1. Furthermore, allowance is made for differences in alkene solubilities [14,15].
0.40
1.20
0.40
0.40 0.17 0.40 0.17 0.40
0.40 0.17 0.40 0.40 0.40
1.20 0.50 1.20 1.20 1.10
Palkene
Alkene pressure
WV
CO pressure PC0 (MPa)
WW
PT
Total pressure
aNot applicable. bUnknown.
ethene propene butene-1 butene-1 butene-2 (cb) butene-2 (Pa=?)
alkene
Type of
b
b
0.024 0.029 0.056 0.024
hkene)
Alkene fraction in PPh,
0.0
145 1.2 11.5 4.9 0.0
Reaction rate, r (cm3 alkene gRhl s-r)
0
2 3 3 0
100
Standardized reaction rate, r5 (arbitrary units)
a
18 21 21 a
a
Linear-to-branched ratio, l/b
Reaction rates and linear-to-branched alkanal ratios (I/b) for various alkenes over a silica-supported rhodium SLPC at 363 K
TABLE 1
376
The linear-to-branched ratios (I/b) The influence of the various reactant partial pressures on the linear-tobranched ratio (l/b) is shown in Fig. 1. On lowering the carbon monoxide partial pressure, l/b strongly increases, whereas the hydrogen and butene-1 partial pressures hardly influence the l/b ratio. This is in agreement with the results found by Gerritsen et al. [ 31 in the hydroformylation of propene over a SLPC. The l/b ratios for the butene-1 hydroformylation, in comparison with propene hydroformylation, are on an average 60% higher over the entire range of carbon monoxide partial pressures investigated. In studying the effect of the degree of pore-filling (6) on the linear-tobranched ratio, a very interesting result was obtained. In order to avoid any influence of adsorptive complex withdrawal, XAD-2 was chosen as a support. The results are presented in Fig. 2, in which l/b, together with the butene-1 hydroformylation rate, is plotted as a function of 6. By way of comparison, the ethene hydroformylation reaction rate as a function of 8, published earlier by Gerritsen et al. [2], is reproduced in the same Figure. The course of curve (a) is explained in [9] : the lower the degree of porefilling, the higher the percentage of rhodium complexes at the gas-liquid interface. These complexes are in direct contact with the gas-phase carbon monoxide, and therefore their mean coordination number with
50
LO
30
20
10
01 0
01
02
03
P, IMPal
04
0.5
0.6
0
02
OL
06
08
10
-6
Fig. 1. Linear-to-branched ratio (l/b) for the hydroformylation of butene-1 as a function of the partial pressure of the reactants (Pi). Supports: Silica-H, 6 = 0.5, P/Rh = 100,T = 363 K, total pressure = 1.2 MPa, W/F = 0.98m&h s cmV3. Curve a: (X) the influence of the partial pressure of butene-1: curve a: (0) the influence of the partial pressure of hydrogen; curve b: (a) the influence of the partial pressure of carbon monoxide; (- - -): Gerritsen’s result for the hydroformylation of propene and the influence of the partial pressure of carbon monoxide on it [ 3 1. Fig. 2. Reaction rate of pore-filling, in the MPa, P/Rh = 100, T: Gerritsen’s result of
r21.
(curve a) and the linear-to-branched ratio (curve b) versus the degree hydroformylation of butene-1. Support: XAD-2, total pressure 1.2 363 K, Hz:CO:butene-1 = l:l:l, W/F = 0.57 mgnn s crnb3; curve c: the reaction rate of the ethene hydroformylation as function of 6
triphenylphosphine ligands will be lower than the coordination number of rhodium complexes in the bulk liquid, and hence they exhibit a higher activity. By comparing curve (a) with curve (c), the ratio of the hydroformylation rates of ethene and butene-1 can be calculated. We obtain the following results: at 6 = 0.1, the rate ratio is 7.3; at 6 = 0.2, the rate ratio is 11.3; at 6 = 0.5, the rate ratio is 29; at 6 = 0.8, the rate ratio is 43. Hence it appears that the lower the ratio of the rates of ethene and butene-1 hydroformylation, the lower the PPhs coordination around the rhodium centre. Curve (b) in Fig. 2 represents the change in the linear-to-branched ratio in butene-l hydroformylation, as influenced by 6. The higher the percentage of interface complexes (lower 6), the lower the l/b ratio. This is in agreement with our previous conclusion that the gas-liquid interface complexes have a lower average PPh, coordination. Catalytic behaviour of monolayer catalysts In the foregoing section the high activity per rhodium complex of SLP catalysts with very low degrees of pore-filling was demonstrated. Lowering the degree of pore-filling leads to catalysts covered with a monolayer of PPh3, in which the complexes are dissolved, or even to sub-monolayer catalysts. In this article, all these catalysts are termed ‘monolayer catalysts’. A number of such catalysts were prepared and tested in the hydroformylation of propene, butene-1 and cis- and trans-butene-2. The rate of propene hydroformylation as a function of time, for a series of XAD-2-supported monolayer catalysts with various coverages, is shown in Fig. 3. Experimental conditions are indicated in the legend to Fig. 3, and the characteristic catalyst parameters are given in Table 2. These catalysts were
24
2.0
b
0 0
LO
80
120
160
200
240
Fig. 3. The hydroformylation rate of propene as a function of time using monolayer catalysts with varying PPh3 coverage (for catalyst parameters see Table 2); T = 363 K, total pressure 0.1 MPa, Hp:CO:propene = 1:l:l; for comparison the behaviour of SLPC k (S = 0.66) is given.
378 TABLE 2 Final activities and linear-to-branched ratios in propene hydroformylation over rhodium complex monolayer catalysts at 363 K and 0.1 MPa total pressure, Hz:CO:propene, 1:l:l Catalyst
ePPh,
r
6PPh,
(cm3 propene
l/b
P/Rh
WIF (mgRh
s cm-‘)
k!Rh 5)--l)
0.112 0.16 0.28 0.31 0.32 0.32 0.35 0.43 0.48
0.019 0.027 0.047 0.053 0.054 0.054 0.059 0.073 0.081
2.29
1.32 0.89 0.82 0.73 0.77 0.72 0.26 0.24
prepared by wet impregnation coverage with PPh3, containing 6 PPh,
mol P (g support)-’ =
S(BET)
7.7 9.6 12.6 14.5 14.4 14.2 15.0 15.5 16.5
25.6 55.0 75.3 140.2 66.6 97.2 153.4 29.1 49.1
10.22 6.9 3.2 5.4 8.2 6.3 5.7 32.5 27.0
as described in the Experimental section. dissolved complexes, is defined as:
The
NAVS(PPhs) (2)
where S(PPh,) is the cross-sectional area of a PPh, molecule (1.2 X 1O-18 m’), NAV is Avogadro’s number (6.02 X 10z3 molecules mol-‘), and S(BET) is the BET surface area of the support (300 m2 g-i). Using XAD-2 as a support, a degree of pore-filling of 0.15 corresponds with monolayer coverage. By comparison, catalyst k shows the activity of a SLPC with 6 = 0.66, It is seen from the Figure that the catalytic activity of monolayer catalysts; per unit weight of rhodium, is appreciably higher than found for SLP catalysts with a higher degree of pore-filling. In the case of catalyst a (0 is a 0.112 only), the activity is even a factor of 23 higher than that of k (0 is 4.4) (Fig. 3). The explanation for this effect is the same as that presented above for the behaviour of SLP catalysts. Just as found for this last type of catalyst, an increase in the activity is accompanied by a decrease in the linear-tobranched ratio (Fig. 4). Monolayer catalysts are stable only at reaction temperatures below 373 K. At higher degrees of pore-filling, the catalyst remains stable up to at least 393 K. Hydroformylation Alkenes with more slowly than butene-1. Alkenes slowly. In studying strates, Brown and
of butene-1 and butene-2, and mixtures thereof the double bond in the /I-position, like butene-2, react alkenes with the double bond in the o-position, like with the general formula R1R2C=CH2 react even more the hydroformylation of a number of unsaturated subWilkinson [ 161 observed an activity difference of a factor
379 -6 002 004 . I r, cm3propene
30.
gRh
006 I
008 II
010 3 rlcrn butew) gRh'S
I/b
'
t_
t
20
20 -
I/b
f
20-t
20
.
A--XX.",'"
0
X'
10 -
,dX
10
cl
10 -
10
0 7-x" 0
;_;_
0 0 -9
0 02
OL
06O
/-;i 0 __t
I/ 50 %
0 100 buterwlm butene
Fig. 4.. The rate of propene hydroformylation and the l/h ratio as a function of the PPha coverage using monolayer catalysts. For catalysts and experimental conditions see legend to Fig. 3. Fig. 5. The reaction rate of butene hydroformylation and the linear-to-branched ratio over a rhodium-SLPC as a function of the percentage butene-1 in the butene feed. 0: butene-l/helium mixture, X: butene-l/butene-2 mixture. Support is silica-H, Hz:CO: butene = 1:l:l; for catalyst parameters and experimental conditions see legend to Fig. 1.
of about 25 between alkenes-1 and alkenes-2, using RhH(CO)(PPhs)s dissolved in benzene as a catalyst at 293 K and 0.1 MPa total pressure. We investigated to what extent our catalytic system shows the same trend. In Fig. 5 the rate of butene hydroformylation per unit weight of rhodium is plotted as a function of the percentage of butene-l in the butene feed. -The type of catalyst used and the experimental conditions are indicated in the legend of the Figure. In some cases part of the butene-l was replaced by helium, in others by cis-butene-2. From the Figure it is shown that dilution of butene-1 by butene-2 has an effect on the reaction rate similar to that of dilution by helium, in accordance with the fact that the rate of reaction is 30 times lower for butene-2 compared to butene-1. It is also shown in Fig. 5 that the linear-to-branched ratio (l/b) is perfectly constant, independent of the percentage of butene-1. Hence in the hydroformylation of a mixture of butene-1 and butene-2, butene-2 behaves practically as an inert gas. During hydroformylation, butene-l is separated from a mixture of butenes. Butene-2 can be hydroformylated, provided extremely active catalysts are used. Such catalysts are the monolayer catalysts described above. In Table 3 the results of the hydroformylation experiments are given. Under identical experimental conditions, the butene-l conversion per unit weight of rhodium is six times higher over the monolayer catalyst than over the rhodium SLPC, and the l/b ratio is lower, as explained earlier. Over the monolayer catalyst, both cis- and trans-butene-2 can be hydroformylated with measurable rates, which are about 30 times slower than that found for butene-1. Using the monolayer catalyst we observe a behaviour in the
380 TABLE 3 The rate of hydroformylation of butene-1, cis-butene-2 and trans-butene-2 using a rhodium monolayer catalyst (catalyst B); for comparison, the rate of hydroformylation of butene-1 over a Rh-SLPC is added (catalyst A) Type of alkene
Catalyst
Rate of hydroformylation to linear/branched alkanal (cmbutene3 gnhq s-9
Linear-tobranched ratio
butene-1 butene-1
Rh-SLPC ( A)a monolayer catalyst ( B)b monolayer catalyst ( B)b monolayer catalyst ( B)b
17.3 86.0
17.3 3.7
cis-butene-2 trans-butene-2
1.0 23.0
c
3.3
c
c
3.6
c
Experimental conditions: T = 363 K, PT = 1.2 MPa, Ha:CO:butene = 1:l:l. Wris catalyst is described in the legend to Fig. 2. bSupport is XAD-2,8 = 0.14, P/Rh = 84. CNot applicable.
hydroformylation of alkene-1 and the corresponding alkene-2 similar to that found by Brown and Wilkinson for benzene-dissolved catalysts. Obviously the degree of PPhs coordination in both types of catalysts is the same. SLP catalysts, on the other hand, show a behaviour corresponding with a higher degree of PPh, coordination. It is noteworthy that the difference in hydroformylation rates between cis- and truns-butene-2 is very small. Obviously the difference in steric hindrance during formation of the n-bonded complex is small. During hydroformylation of butene-2, no isomerisation to butene-1 was observed, and hence no n-pentanal was observed in the product gas. Discussion The results summarized in Table 1 are typical of SLP catalysts with a relatively high degree of pore-filling. In such catalysts the ratio of the number of active rhodium complexes in solution to the number of active rhodium complexes at the gas-liquid interface is very high, and therefore their catalytic behaviour is mainly governed by the complexes in the bulk of the liquid. This liquid is characterized by a high triphenylphosphine concentration, whereas the carbon monoxide concentration is very low [14]. It is very likely that the rhodium centre is provided with at least two triphenylphosphine ligands, and this finds expression in a high l/b ratio (see the last column in Table 1). In the homologous series of alkenes, an activity decline by a factor of about 50 is found, proceeding from ethene to propene (seventh column of Table l), but the activity found with butene-1 as a substrate does not deviate significantly from that of propene. Experiments with gaseous octene-1 in the feed (not reported here) showed that the activity of this higher alkene with
381
the double bond in the o-position did not deviate substantially from that of propene. However, higher alkenes gave rise to higher l/b ratios. In the hydroformylation mechanism, the choice between the formation of a linear or a branched alkyl ligand is made during the interligand reaction between the n-bonded alkene and the hydrogen ligand. It is likely that, for steric reasons, the formation of a linear alkyl ligand is increasingly favoured the longer the alkyl chain in the reacting alkene. By comparison with more highly diluted solutions of the Wilkinson complex, the activity of the rhodium complexes in our SLP catalysts is relatively low, in particular the activity to give the branched product. This is probably why the activity for the hydroformylation of cis- and tmns-butene2 is virtually zero (see last two rows of Table 1). The most important observation reported in this paper is that the hydroformylation activity of SLP catalysts, calculated per rhodium complex, increases considerably with decreasing degree of pore-filling. This effect is accompanied by a decrease in the linear-to-branched ratio (see Fig. 2). The explanation is as follows. On decreasing the degree of pore-filling, the surface area of the gas-liquid interface greatly increases (see, for instance, Fig. 9 in [9]). Hence the percentage of complexes at the gas-liquid interface markedly increases, and these complexes have a lower average number of triphenylphosphine ligands around the rhodium centre. This follows from the observation that the l/b ratio is much lower than that found for rhodium complexes in the bulk of the solvent-ligand. The lower average coordination with triphenylphosphine ligands at the gas-liquid interface we attribute to the direct contact these complexes have with the gas-phase carbon monoxide molecules. The concentration of carbon monoxide in the gas-phase (at 0.4 MPa partial carbon monoxide pressure, 1.2 MPa total pressure, and at 363 K) is a factor of about 15 higher than the concentration of carbon monoxide dissolved in the solvent-ligand triphenylphosphine. This follows from the solubility of CO in PPh, experimentally determined by Herman et al. [ 141, and it means that the PPh, ligand replacement by CO ligands is higher at the gas-liquid interface than in the bulk of the solvent-ligand. A secondary factor may be that the coordination of the rhodium complexes at the gas-liquid interface with free non-liganded PPh, molecules is half the value in the bulk liquid, which means the thermodynamic activity of PPhs is lower. The advantages of catalysts with a high gas-liquid interface-to-bulk liquid ratio manifest themselves most clearly in catalysts with a degree of pore-filling so low that a monolayer, or less than a mqnolayer, of PPhs, in which the rhodium complexes are dissolved, is present. From Table 3 we calculate that cis- or truns-butene-2 reacts more slowly by a factor of about 30 than butene-1 over these monolayer catalysts. This factor is about equal to that found by Brown and Wilkinson [ 161. In comparing the reaction rates of pentene-l and pent.ene-2 (cis and trans) and heptene-1 and heptene-2 (cis and trans), these investigators arrived at a factor of 25 and 30 respectively. Just as in our experiments with cilr-and trans-butene-2, Brown and Wilkinson
[16] did not find a significant difference between the rates with cis- or trunsheptene-2 as a substrate. Obviously the catalytic performance of the complexes at the gas-liquid interface in our catalysts is comparable with that of the complexes used by Wilkinson in his experiments with homogeneous dilute systems. Contrary to what was found with SLP catalysts with a high degree of pore-filling, monolayer catalysts are able to hydroformylate alkenes with the double bond in the P-position, such as butene-2 (see Table 3). The activity found for butene-2 (cis and truns) is a factor of about 30 lower than that for butene-1. This may be due both to steric hindrance and to the electrondonating strength of the two CH, groups in butene-2. Acknowledgements We thank Mr. P. Bode of the Interuniversity Reactor Institute, Delft, The Netherlands, for carrying out the neutron activation analyses. The investigations were supported (in part) by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). List of symbols F l/b N AV pi
PT
P/Rh r r17 rb
S(BET) S(PPh3) T WRh xi
6 PPh,
e PPh, t
flow of alkene at 0.1 MPa and 293 K ratio linear-to-branched alkanal Avogadro’s number partial pressure of component i total pressure molar ratio phosphorus to rhodium reaction rate rates of the formation of linear and branched alkanals respectively surface area cross-sectional surface area of PPh3 temperature weight of rhodium metal in the reactor mole fraction of component i in the liquid phase degree of pore-filling of the support with PPh3 surface coverage with PPh, conversion
cm3 s-l molecules mol-’ MPa MPa mol mol-’ cm3 alkene ga,-’ cm3 alkene g,,-’
s-1’ s-
m2 g-’ m2 K g -
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