Poly(trimethylolpropane)trimethacrylate-bound Rh-phosphine complexes as catalysts in continuous gas-phase hydroformylation of propene

Journal of Molecular Catalysis, 81 (1993) 333-347

Elsevier Science Publishers B.V., Amsterdam 333

Ml22

Poly (trimethylolpropane)trimethacrylate-bound Rh-phosphine complexes as catalysts in continuous gas-phase hydroformylation of propene

Bernd Heinrich and Jes Hjortkjaer* Dept. of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800,

Lyngby (Denmark); tel. (+ 45.42)883288, fax. (-I- 45.42)882258

Antonios Nikitidis and Carlaxel Andersson* Dept. of Inorganic Chem. 1, Chemical Center, University of Lund, P.O. Box 124, S 22100 Lund

(Sweden)

(Received November 2,1992; accepted February 9,1993)

Abstract

Polymer supports composed of rigid, small sized porous particles of poly- (trimethylolpropane) trimethacrylate (poly-TRIM) and a grafted linear acrylate polymer bearing phosphines; e.g. N-methyl-2- (diphenylphosphino)benzylamide, N-methyl-4- (diphenylphosphino)benzylamide and (2-dipheny1phosphino)benzylmethacrylat.e as pendant side groups were prepared. Reaction of these polymer particles with [ Rh (acac) (CO),] affordedpoly-TRIM-bound rhodium catalysts which were studied in continuous gas-phase hydroformylation of propene. At 333K and Ptit 600 kPa, these catalysts were found to be highly active with total hydroformylation rates between 3 and 110 x 1Om6 mol butanal s-i (g Rhh’) and also highly stable showing no loss in activity after 215 h on stream. The influence on selectivity (rJr&) and total activity (rtot) for different phosphine to rhodium ratios in the catalyst, as well as for the substituent pattern of the phosphines and particle type, were studied.

Key words: bound rhodium catalyst; hydroformylation; phosphine-containing graft copolymer; poly(trimethylolpropane)trimethacrylate

Introduction

Research in the field of heterogenized metal complex catalysts is moti- vated mainly by the need to handle such catalysts more easily, i.e. easier separation and recovery of the catalyst in batch operations or easier process design in continuous flow operation, as compared to metal complexes in solution. Several reviews and monographs covering the field of heterogenized metal complex catalysts have been published [l-4]. Hydroformylation of olefins is

*Corresponding authors.

0304-5102/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All,rights reserved.

334 B. Heinrich et al./J. Mol. Catal. 81 (1993) 333-347

a catalytic reaction where homogeneous catalysis has reached large industrial volumes [ 51. It is also a field where considerable research efforts have been devoted to the development of heterogenized versions [6-171 of the homogeneous catalyst HRhCO(PR3)3. These efforts, however, have met limited suc- cess, the main problems being related to the stability of the catalysts under long-term continuous operation [ 18,191. Metal particle formation [ 20-221, metal leakage [23], phosphine degradation via oxidation [24] or quarterni- zation [ 251 and metal-induced cleavage of carbon-phosphorus bonds [ 26-301 are observed side reactions leading to a limited lifetime for the polymer-bound catalysts.

In the cycle describing the hydroformylation reaction with rhodium phosphine complexes as catalysts, several rhodium-phosphine dissociation-asso- ciation equilibria operate. Under CO pressure, successive substitution of phosphine ligands with carbonyl ligands can therefore lead to the formation of carbonyl complexes not linked to the support [ 121. Thus, a depletion of the metal from the support can occur. A high CO partial pressure, a low reaction temperature and a low concentration of free (uncomplexed) phosphine are factors that favour a shift from phosphine to carbonyl complexes. CO pressure and temperature are process variables that can be selected to minimize hydri- docarbonyl complex formation. The concentration of free phosphine can, to some extent, be controlled in the synthesis of the catalyst; but the restricted mobility and accessibility inherent to ligands bound to crosslinked polymers can lead to a true free phosphine concentration substantially lower than the nominal free phosphine concentration.

Regarding the true free ligand concentration, a closer resemblance to the situation in solution can be achieved by applying ligands which are bound to linear non-crosslinked polymers [31-351. In this case, however, the ease of separation is lost to some extent.

To combine the mechanical and separational advantages of crosslinked polymers with the ligand mobility advantages of non-crosslinked polymers, we have developed a new type of phosphinated polymer [ 36-371, called phosphinated poly-TRIM. These consist of crosslinked porous particles of poly (methylolpropane) trimethacrylate and a linear acrylamide or acrylester polymer bearing phosphines as pendant functional groups grafted in the pore system of the particles. In a previous study [38] the feasibility of the poly- TRIM concept was demonstrated by the continuous gas-phase hydroformylation of propene. The present studies are aimed at evaluating the long-term catalyst stability, together with the influence of the phosphine to rhodium ratio, the type of phosphine and the type of polymer particle on catalyst activity, selectivity and stability.

Experimental

Materials and methods Two different poly-TRIM particles, prepared as described [39,40] were

used: PM1 particles, were kindly supplied by Casco Nobel, Sundsvall, Sweden

B. Heinrich et al./J. Mol. Catal. 81 (1993) 333-347 335

and PM2 particles were supplied by the Dept. of Chemical Technology, Uni- versity of Lund. Table 1 shows the characteristics of the particles used. Re- agent grade solvents and chemicals from commercial sources were dried before use by standard procedures. Propene (Alfax 99% ) was purified by passing it through a reactor containing highly dispersed copper (I) (BASF R3-11) at 473-493 K and a reactor containing activated molecular sieves (5A). Hydro- gen (Dansk Ilt, > 99% as determined by GLC ) and carbon monoxide (Linde 99.97% ) were used as received, except in testing the long-term stability of the catalysts. In this case, hydrogen and carbon monoxide were passed through a reactor containing a palladium catalyst (Engehardt, Deoxo D and Deoxo DS, respectively) and a reactor containing activated molecular sieves (5A). The reactant gases were premixed in a steel cylinder to the desired composition. The exact gas composition was determined by GC.

The TRIM particles used to prepare catalysts 6 and 7 were phosphinated as described [ 371 by polymerizing 2- (diphenylphosphino )benzylmethacrylate (I, Fig. 1) in the pores of PM1 particles. TRIM-particles used to prepare catalysts l-5 were phosphinated by polymerizing acryloylchloride in the pores of PM2 particles and subsequently reacting the acid-chloride functionalized particles with amino-phosphines as described [ 361. The phosphines; N-methyl- 2- (diphenylphosphino ) benzylamine (II, Fig. 1) , N-methyl-4- (diphenylphosphino)benzylamine (III, Fig. 1) and 2- (diphenylphosphino) benzylmethacrylate were prepared as described earlier [ 411. The phosphorus content in the phosphinated particles was determined by Mikrokemi AB, Uppsala Sweden, or by Malissa and Reuter Microanalytisches Laboratorium, Gummersbach, FRG. [ Rh (acac) (CO),] was purchased from Johnson Matthey Chemicals. Determination of particle size, porosity, surface area and weight-swelling of the TRIM-particles were carried out as described [ 361.

TABLE 1

Physical characteristics of the poly-TRIM particles

Initial particle Particle size (pm) Pore volume (cm3/g) 15-60 A >60A Total

Swelling (g/g) Specific area (m’/g)

Initial particle After grafting acryloyl chloride After phosphination

Phosphorus content wt.-%

PM1 PM2

4.1

0.27 0.20-0.25 0.49 1.0-1.8 0.76 1.7-2.0 3.9 1.5-1.8

486 450 227 165 64 15

4.2 2.7

5-150

B. Heinrich et al.fJ. Mol. Catal. 81 (1993) 333-347

0 CHs \\ I

HzC ,0-c-c+

‘%

Method I

t\\

H,C/ WHdH

t

+I

(11)

W’

N(CH3H

6Ph2 (III)

Method 2

t\\ + CH?= CH- C(O)CI

- {+‘t ] iii ii

0 n

Fig. 1. The phosphines and the methods used in the phosphination of the TRIM-particles. \u denotes residual double bonds in the pores of the TRIM-particle. (i) AIBN/toluene/water suspension; 60°C. (ii) AIBN/toluene; 60°C. (iii) (II) or (III); toluene/NEt3; 60°C.

Preparation of the catalysts The following general procedure was used in the preparation of the cata-

lysts. A weighed amount of the phosphinated particles was transferred to a Schlenk tube connected to a vacuum line. The tube was evacuated and filled with argon repeatedly before addition of CH,Cl, (about 150 ml/g poly-TRIM). The particles were allowed to swell for 30 min under constant magnetic stirring before the calculated amount of [ Rh (acac) (CO),] dissolved in CH,Cl, was added in one portion under a constant stream of argon. The reaction between the phosphine groups in the particles and the rhodium precursor is conve- niently monitored by an instant disappearance of the green-red color of the solution and evolution of CO gas. After gas evolution had ceased, the Schlenk tube was purged with argon and thereafter closed and left magnetically stirred for 24 h. The yellow polymer particles were then collected by filtration, washed with 5x20 ml CH2C12, 2x20 ml ethanol and 2 x20 ml diethyl ether before drying in air for 30 min and under high vacuum for 4 h.

Preparation of HRh(CO)(PPhJ), from [Rhacac(CO)d A heavy-walled Schlenk tube with a Teflon valve was charged under argon

with triphenylphosphine (1.0 g, 3.8 mmol) dissolved in ethanol (35 ml) and


[ Rhacac (CO) 2] (100 mg, 0.39 mmol) . The solution turned yellow as the solid dissolved. The magnetically stirred tube was filled with Hz (1 atm) by repeated vacuum/H, cycles, sealed and heated to 50’ C in an oil bath. The clear solution turned turbid after a short while at the reaction temperature. The yellow pre- cipitate formed after continued reaction for 20 min at 50’ C and at room temperature for 30 min was collected by filtration, washed with 3 x 15 ml ethanol and air dried. Yield 260 mg (72% ) of a light yellow powder. This was identified as the title compound by comparing the IR spectra (V Rh-H 2038 cm-‘, Y C- O 1920 cm-’ ) with that of an authentic sample.

Hydroformylation procedures

The continuous gas-phase hydroformylation of propene was carried out in a tubular stainless steel reactor in which the catalyst formed a fixed bed. Details of the reactor system and the analytical procedures have been described previously [ 421. The premixed reactant gases were passed through the reactor containing 0.25 g catalyst at 600 kPa and 333 K. Flow rates between 0.3-5 cm”/s were controlled by a fine metering valve and measured at the reactor outlet by a soap film meter. The exit gas from the reactor was periodically analyzed by GLC via a sample loop. To ensure differential conditions, the conversion was kept below 3% of the limiting reactant unless otherwise stated. This allows rates to be calculated as: r= [aldehyde] x F/ W, where F is the flow rate (cm3/s) from the reactor, [aldehyde] is the concentration of n- or iso-butanal (mol/cm3) in the exit gas and W,, is the weight of rhodium in the reactor. There was no detectable formation of propane.

The reproducibility of the measured rate was very good. In a previous paper [38] we reported an experimental rate law for the propene hydroformylation with poly-TRIM catalyst containing 2.7% P and having a P/Rh mo- lar ratio of 2.6 (catalyst 4, Table 2). The total hydroformulation rate at 333K was 2.6 x 10-5Xp~~g XJ&, Xpco -“.75 mol/s gRh, where the reactant partial pres- sures are in atmospheres [ 381, For the independent experiments reported here, this equation predicts a rate of 48 x 10m6 mol/s g Rh, the measured rate being 47 x lop6 mol/s g Rh. The reproducibility is then clearly demonstrated. Fur- thermore, for two independent experiments with another poly-TRIM catalyst (2.7% P, P/Rh = 9) under identical conditions (except for the amount of catalyst and total flow rate), the total hydroformylation rate could be reproduced within 2%.

Results and discussion

Preparation and activation of the catalysts Figure 1 gives a schematic presentation of the method used to prepare the

catalysts supports. This has been thoroughly described elsewhere [ 36,371. The


TABLE 2

Rate data for the hydroformylation of propene for various polymer-bound catalysts. T=333 K, total pressure 600 kPa; C,:CO:H,=2.22: 1.18: 1.00; 0.25 g catalyst

Catalyst Particle” Phosphine’ P-content’ Rh:P rtotX 1OF R ((w/w)%) (mol mol-‘) (mol s-’ gRh_‘)

1 PM1 III 4.2 I:5 18 6.0 2 PM2 III 2.7 1:5 39 2.9 3 PM2 III 2.7 1:20 3.7 3.8

4 PM2 III 2.7 1: 2.6 47 2.9 5 PM1 II 4.3 1:3 16 2.1

6 PM1 I 2.2 1:2 47 2.0 7 PM1 I 2.2 1:lO 38 2.0

“See Table 1. bSee Fig. 1. “Content before reaction with Rh.

basic idea behind this concept is to obtain a polymer matrix with highly mobile and flexible phosphine groups.

The complex HRhCO ( PRB ) 2 is a very active hydroformylation catalyst [ 43-451. In preparing a polymer-bound rhodium hydroformylation catalyst, it is therefore highly desirable to fix this type of complex on the polymer. This is most easily done using the parent complex HRhCO ( PR3) 3 in ligand exchange with polymer-bound phosphines, a method successfully used in several studies [ 11,14-161. An exchange of all the phosphine groups on the parent complex, however, is often not possible to achieve. Thus, a catalyst is obtained containing phosphines linked to the polymer as well as phosphines not linked to the polymer, which is a serious drawback to this method. Redistribution of the metal over the ligands present, which can occur under operating conditions, can lead to metal depletion via formation of phosphine complexes with no attachment to the support. To avoid metal leakage and catalyst deactivation in this way, a different synthetic approach was applied in the present study, assuring that all the phosphine ligands are provided by the support. Reaction of the precursor complex [ Rh (acac) (CO )J with the poly-TRIM bound phosphines leads to carbon monoxide-phosphine exchange and to the formation of a complex [ Rh (acac) (CO )P-poly 1, (P-poly denoting poly-TRIM bound phosphine) on the polymer, as evident from the CO stretching vibration at 1977 cm-l [ 461. For catalyst 4, Table 2, the symmetric CO stretching band at 1580 cm-’ and the antisymmetric C=C stretching band at 1520 cm-’ of coordinated acac- are visible. These bands are obscured by overlapping bands from the poly-TRIM matrix for the catalysts with a lower P/Rh ratio.

Although not explicitly stated in any of the previous studies [8,12,46,47] using [Rh(acac) (CO)PR,] type complexes as a catalyst in the hydroformylation reaction, it is assumed that this complex undergoes oxidative addition


of Hz followed by reductive elimination of Hacac. To confirm that this reaction actually occurs, [Rh(acac) (CO),] in ethanol solution was treated with triphenylphosphine (10 eq) under 1 atm of hydrogen (see the Experimental sec- tion). Within minutes at 50°C a nearly quantitative yield of ]HRh(CO) W’h,),l was obtained. Thus, under Hz/CO pressure and in the presence of free phosphine, the precursor complex [ Rh (acac ) (CO)P-poly ] in the polymer is converted to complexes with the composition [H Rh (CO) 2 (P- poly)sl orH Rh(CO) U’-poly)sl.

A typical reaction profile for the catalysts investigated in the present study is given in Fig. 2. The steady increase in the reaction rate with time up to around 80 h on stream, where steady state conditions are reached, indicates a very long induction period for catalyst activation. The regio-selectivity, r,JriSo, of the reaction is given in Fig. 3. During the first hours there is a steady increase in the regioselectivity and a maximum is reached after about 4 h. Thereafter a slow decline is observed and, finally, a steady state level is reached after about 20 h. This indicates that the chemical transformations of the precursor complex contribute to the activation process, at least in the initial stage. The increase in activity over the whole period of activation, however, can also be related to changes in the polymer structure, causing readier access to the active sites in the polymer, as observed for polystyrene-bound catalysts [ 151. Chem- ical transformations of the type discussed above should in principle be possible to monitor, either by the presence of Hacac in the reactor outlet gas or by changes in the IR spectrum of the catalyst in the CO stretching region. Hacac however, has not been traced in the product stream during the activation. Whether this is an effect of the high dilution of Hacac in the product stream or due to the condensation of released Hacac in the polymer is not known.

Fig. 2. Catalytic activity of a poly-TRIM bound Rh catalysts (2.7% P, 3.5% Rh, P/Rh=2.6 mol mol-‘) as a function of time on stream. T= 333 K, Ptit = 600 kPa; [ C3 : HZ: CO : N2 = 5 : 3.4 : 1: 4.61; 0.22 g catalyst. Conversion of CO<7%. Between the measurements at 50 h, 123 h and 215 h; temperature and reactant composition were changed several times. q = rate of n-butanal formation; 0 = rate of isobutanal formation.


7 R ( regioselectivity )

,I Hp

P T/h 1 I I

2.

5 15 25

Fig. 3. Selectivity, R, as function of time on stream in the activation phase. Conditions as in Fig.

2100 2ooo 1950 cm-' I I I

Fig. 4. IR spectrum in the region of CO stretching vibrations of catalyst 4 after 30 h on stream.

The bands present at 1580 cm-l and at 1520 cm-’ in the IR spectrum of fresh catalyst 4, which are characteristic of coordinated acac, are no longer observed after 30 h on stream. This supports the idea that the Hacac is elimi- nated in the activation process,

An IR-spectrum of a catalyst sample after 30 h on stream, the period in which changes in selectivity occurs, is shown in Fig. 4. The dominant feature is the rather broad and strong peak at 1977 cm-‘. Compared to the spectra of the fresh catalyst, which also show a strong peak at 1977 cm-l, the changes are not pronounced. The main differences are additional weak bands at 2038

B. Heinrich et cd/J. Mol. Catal. 81 (1993) 333-347 341

and 2067 cm-‘, and weak shoulders at 1940 and 1998 cm-‘. From the gross similarities in the spectrum of the fresh and the activated catalyst (Fig. 4) it can be inferred that only a small fraction of the precursor complex has been chemically transformed, however, it may be totally responsible for the new ;/ n ratio. An alternative explanation of the very strong 1977 cm-’ peak in the spectrum, which we favour, may also be valid. The reported IR spectra [48] for the complex HRh (CO ) 2 ( PPhB ) 2, which is the homogeneous counterpart to one of the complexes most likely formed by the activation process, shows three peaks: viz. v Rh-H at 2040 (w) and v CO at 1977 (s) and 1943 (sh) cm-‘. Thus the two peaks at 2038 and 1977 cm-’ and the shoulder at 1940 cm-’ (Fig. 4 ) may indicate that a complex, HRh (CO ) 2 (P-poly ) 2, is indeed formed in the polymer during activation. No definite conclusions can be drawn, because the main IR band of the precursor complex and the complex HRh (CO) 2 (P-poly ), coincide. The interpretation of the observed IR spectra is further complicated by the possibility that any HRh (CO )2 (P-poly ), formed in under reaction conditions, because of the extreme air-sensitivity of that complex [ 481, is likely to be converted to the dimer [Rh( CO), (P-poly),], once the catalyst is re- moved from the reactant gas mixture and transferred to the spectrometer. Moreover, any formation of a dimer [Rh(CO),(P-poly), I2 in the polymer is also very difficult to prove because the strong indicators for a complex of this type - i.e. strong IR bands at 1985 and 1765 cm-l [ 48]- almost coincide with the main band of the precursor [Rh(acac) (CO)P-poly] and with the CO stretching band of the poly-TRIM backbone.

Based on IR-spectroscopic evidence, the formation of different Rh-car- bony1 clusters has been suggested to occur in the hydroformylation of 1-hexene catalyzed by Rh(C0) (acac) (PPhB) [47]. The small additional peaks and shoulders at 1998,2067 and 2079 cm-l observed in the present study (Fig. 4) show that new, unidentified carbonyl complexes are formed in the activation process. The absence of any strong peaks in the region 1900-1800 cm-‘, which are indicative of rhodium clusters with bridging carbonyls [49,50], suggest that these new complexes in the polymer are not clusters such as Rh, (CO) i2 or Rh,(CO),,, which were claimed in the case of the homogeneous catalyst.

Activity and selectivity of the catalysts The equilibria (Eqn. 1) are of fundamental importance in the hydrofor-

mylation reaction. Two different catalytic cycles have been suggested to ac- count for the observed regioselectivity of the reaction [ 441.

HRh(C0)2PRB +PR,=HRh(CO), (PR,),=HRh(CO) (PR,), +CO (1)

(B) (A) (C)

Complex B is the active catalyst in the less regioselective dissociative cycle and complex A is the active intermediate in the regioselective, associative cycle. Considerable debate has been focused on the validity of the traditional associative cycle because this invokes a 20-electron intermediate. Recent kinetic


analysis [51,52] and spectroscopic studies [53] indicate that the 16-electron complex C is the actual olefin capturing complex in the associative cycle.

The distribution between complexes B and C at constant CO pressure is affected by the concentration of free phosphine in the system. A high phosphine concentration favours the formation of complex C. In the present study parameters such as particle type, P/Rh ratio in the catalyst and the type of phosphine have been varied. These are all parameters which might influence the concentration of free phosphine seen by the metal and thereby of importance for the activity and the selectivity of the catalysts.

Influence of particle type The porosity and the surface area of TRIM particles can be varied by the

choice of solvent and porogenic agent in the initial preparation [ 391. Previous studies have shown that the amount of functional polymer that can be post- copolymerized in the pore system of the particle roughly follows the surface area and the swelling capacity of the particle. Physical characteristics, such as porosity, surface area and swelling capacity for the PM1 and PM2 particles used in the present study are given in Table 1. The main difference between these particles is a substantially higher swelling of the unfunctionalized PM1 particle, leading to a higher phosphine group content in the final particle. The surface area of both phosphinated particles, however, are very similar.

As is evident if one compares the activity and selectivity data of catalysts 1 and 2, given in Table 2, the difference in phosphine content of the particles has a great impact. At the same P/Rh ratio and under otherwise identical reaction conditions, the particle with the higher phosphine content gives twice as high a selectivity, accompanied by a decreased activity to about half of that of catalyst 2. Thus, a catalyst prepared from particles with a higher phosphine content per gram of polymer favours n-butyraldehyde formation at the ex- pense of overall reaction rate.

Since the surface areas of both particles are very similar after the phosphination step there is no reason to ascribe the observed difference to surface phenomena, chemical differences relating to equilibrium (1) is a more likely cause.

In essence, catalyst 2 can be regarded as a more diluted version of catalyst 1. An equilibrium between complexes B and C (Eqn. 1) involves three species which are contained within the volume of the polymer particle; B and free phosphine on the left-hand side and C on the right-hand side. Thus, the position of this equilibrium is affected by dilution. The overall effect of a decrease in the total phosphine content of the polymer particle is therefore a shift to the more active but less selective monophosphine complex B. Similar concentration-dependent effects have been observed in SLP-catalyst systems [54] i.e. holding the P/Rh ratio constant while diluting the liquid phosphine phase with inert solvents gave an increase in activity and a decrease in selectivity.

From an economic standpoint a high selectivity is often more important


than high activity. The catalyst prepared from PM1 is therefore the more interesting of the two types tested, since this provides a higher phosphine content.

Influence of the phosphinerrhodium ratio Within the same type of support particle, the equilibrium (Eqn. 1) can be

shifted more straightforwardly by changes in the free (uncomplexed) phosphine concentration. This can be achieved by varying the phosphine-rhodium ratio applied in the syntheses of the catalysts, as was done in the synthesis of catalyst 2,3 and 4 using the PM2 particle. The activity and selectivity data for these three catalysts are displayed in Table 2. The differences in activity and selectivity in this series are not correlated with changes in the P/Rh ratio in a simple proportional manner; changing the P/Rh ratio from 2.6 to 5 results in a slight decrease in the activity from 47 to 39 x lob6 mol butanal s-l gRh-‘, while the selectivity is unchanged. Large changes in both the activity and the selectivity are seen first at the fairly high P/Rh ratio of 20. This can be caused by a somewhat restricted mobility of the phosphine ligands in the polymer matrix, which results in the concentration of free ligand sensed by the metal center to be lower than the nominal concentration of free ligand.

The selectivity data for the poly-TRIM catalysts 2, 3 and 4, which range from 2.9-3.8, compare favourably with the selectivity reported previously [ 151 for rhodium phosphine complexes bound to crosslinked polystyrene particles. In this latter case a P/Rh ratio of 26.4 gave a selectivity of only 0.9 corresponding to 47% n-butanal. That the poly-TRIM and polystyrene systems respond so differently to changes in the P/Rh ratio, as reflected in the regioselectivity, is striking. Part of this difference can be explained by the low concentration (0.26 wt.-% ) of phosphine groups in the polystyrene polymer used; cf. the dilution effects discussed above. Additionally the differences in ligand mobility of the two systems may also be of importance.

Because of restricted mobility of the ligands in polymer-bound systems the true concentration of free ligand, which determines the position of the equilibrium (Eqn. 1) and thereby the regioselectivity, is lower than the nominal concentration of free ligand, the more so the less mobile the ligands are. The ligands in the polystyrene system are integrated parts of the highly crosslinked polymeric network, implying low ligand mobility. In the TRIM system, on the other hand, the ligands are attached to a linear flexible polymer chain enabling a high degree of ligand mobility. Thus, the regioselectivity of the poly- TRIM catalysts are more sensitive to changes in the P/Rh ratio than are catalysts bound to highly cross-linked polymer systems. The importance of ligand mobility in determining the selectivity has also been noted recently [ 171; the selectivity to n-butanal was found to increase with decreasing crosslink density in the polystyrene polymers used in that study.

Effect of different phosphines The type of ligand bound to the TRIM support particle can be varied

widely. The only restriction is that an NHR or OH group is required on the


ligand to enable attachment via reaction with acryloylchloride/metacryloyl- chloride or poly (acryloylchloride) /poly (methacryloylchloride ). Some varia- tions in side chain substituents and the substituent pattern of the bound phosphine ligand have been included in the present study, i.e. the catalytic activity and selectivity have been studied using the substituted phosphines in Fig. 1 bound to poly-TRIM. The ortho isomers I and II, Fig. 1, were considered interesting because, after reaction with the acid chloride, these will have an ester or amide group close to the phosphine functionality. These two ligands are therefore potential chelating agents via complexation with the phosphine and the amide/ester group [55] which could affect the catalytic performance of catalysts prepared from them. Catalysts 6 and 7 (Table l), prepared from phosphine I (Fig. 1) , are both highly active catalysts. As can be seen from the activity and selectivity data of these catalysts, the effect of an increased P/Rh ratio using support particles derived from phosphine I, however, is rather limited. The catalytic activity decreases only marginally and the effect on the selectivity is negligible.

In this context it is interesting to compare how the catalysts based on phosphines I and III differ in their response to changes in the P/Rh ratio. Going from catalyst 4 (P/Rh = 2.6) to catalyst 2 (P/Rh = 5 ) the rate decreases from 47 x lop6 to 39 x lop6 with no measurable change in the selectivity while between catalyst 6 (P/Rh= 2.0) and 7 (P/Rh= lo), the rate decreases from 47 x 10W6 to 38x 10V6, also in this case with no measurable change in the selectivity. Thus, despite a much larger increase in the P/Rh ratio in the case of phosphine I the changes in activity and selectivity are very similar. The influence of an increased P/Rh ratio is so small, in the case of the phosphine I, that it can most likely be ascribed to effects of the side chain substituent on this phosphine. As noted in studies with cationic rhodium phosphine complexes [56] there is a fairly strong complexation of the ester group in a monophosphine complex. This coordination of the ester carbonyl group competes with the coordination of a second phosphine molecule, thus making the formation of the more selective but less active diphosphine complex more difficult.

Catalyst 5, which is based on phosphine II, is the least active of all catalysts included in Table 1 and its selectivity is also rather low. In accord with the observations in a study of hydrogenation using a rhodium complex based on this particular phosphine [56], the very low activity of catalyst 5 indicates that the coordination of the amide side chain is substantially stronger than the coordination of the ester side chain in the case of phosphine I. This strong coordination creates a poisoning effect which reduces the catalytic activity but which does not enhance selectivity.

Comparison with other polymer bound systems The hydroformylation experiments in the present study were carried out

under differential conditions to enable the evaluation of rate constants and a proper rate law [ 381. Most of the previous studies which have been published

B. Heinrich et al.fJ. Mol. Catal. 81 (1993) 333-347 345

in the field use batch-wise reactions or continuous flow processes at high con- versions. Rate constants for the hydroformylation of propene using polystyrene-bound rhodium phosphine complexes as catalysts in continuous flow reactor under differential conditions have been published, however [ 151. These rate constants permit a comparison of the polystyrene system with the poly- TRIM system presented above. The high activity of the poly-TRIM system, at reaction temperatures above 333 K, lead to a conversion outside the range of differential conditions. The determined energy of activation and the derived rate law [ 381 is therefore strictly only applicable in the temperature interval 298-333 K. For a rough comparison one can, however, extend the rate expression and the energy of activation to hold also at 363 K. Inserting the conditions of Scholten et al. [ 151 into the rate expression for the TRIM catalyst viz.

r,,, = r, exp ( - E,I~~) (ppropene >“.““PH, (PCO ) -o0.75

with r,= 3.4 x log, E,=90 kJ/mol and p in atm. rtot= 1.5 x lop4 mol butanal S -' (g Rh)-’ is obtained which compares favourably with the value, r tot= 1.5 X lop5 for the PS-30 catalyst in the study cited. It is also worth noting that the selectivity in the case of the TRIM-catalysts is in no case below 2.1, while the reported selectivity of catalyst PS-30 is 0.9. Concerning the stability of the two systems there are no great differences.

Acknowledgements

Financial support from TFR (Swedish Research Council for Engineering Sciences) and from NUTEK (Swedish National Board for Industrial and Technical Development) is gratefully acknowledged.

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