The influence of different monodentate P-ligand mixtures on Rh-catalyzed 1-butene hydroformylation☆

WeiliJiang,JinxinChu,JieYang,PengyuZang,LijieGao,GuanglinZhou,HongjunZhou,*,HuiboWei

1Institute of New Energy,Beijing Key Laboratory of Biogas Upgrading Utilization,China University of Petroleum,Beijing 102249,China

2Department of Chemistry,University of California,Riverside,CA 92521,USA

3Beijing National Laboratory for Molecular Sciences,State Key Laboratory of Rare Earth Materials Chemistry and Applications,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China

Keywords:Homogeneous catalysis Hydroformylation Syngas 1-Butene Rhodium catalyst

A B S T R A C T Four monodentate P-ligands and their mixtures (six groups of double-ligand systems, four groups of triple-ligand systems and one group of tetra-ligand system)were used with Rh(acac)(CO)2(acac=acetylacetonate)or Rh(acac)CO(PPh3)as the catalyst in the hydroformylation reaction of 1-butene.It was found that different Rh catalysts showed little difference in the catalysis performance. The general order of catalysis performance is doublelig and system＞single-ligand system＞triple-ligand system＞tetra-ligand system.Some synergistic effect in the double-ligand system was detected which needs a further investigation.

1.Introduction

Because of more complex separation of products and feedstocks and even catalysts,homogeneous catalysis is not so commonly applied in industry as heterogeneous catalysis.However,owing to its higher reaction activity and selectivity,as well as better understanding of the mechanisms,homogeneous catalysis is still a focus in the fields of pharmaceutical intermediates and fine chemicals[1–3].Driven by limited petroleum resources,more attention is directed to the conversion of light olefins into high-value products, such as aldehyde [4–6]. The most important industrial homogeneous catalysis is the hydroformylation reaction for aldehyde,which was discovered by Otto Roelen of Ruhr Company in the 1930s. This reaction is believed to be atomically economic, highly efficient and environmentally friendly[7–9].The common catalyst candidates for this reaction are rhodium and cobalt complexes.Due to different activities,the former is usually applied in the hydroformylation of light olefins such as propylene and butene,while the latter is generally used for higher olefins[1,10–12].The higher price of rhodium,however,drives chemists to improve the turnover frequency(TOF)of this type of catalyst.Ligands containing P atomsuch as phosphines and phosphites are recommended in hydroformylation to control the geometry of the complex,which play a key role in the catalysis performance through electronic effect and steric effect[13–17].For example,Rh complexes with bulky aryl monophosphites have been reported to show excellent chemo-and regioselectivities when catalyzing disubstituted and internal olefins[18–20].Diphosphine ligands with large bite angles are found to result in better selectivities toward liner aldehyde,especially in the of internal alkenes [21–24]. As the research progresses, the structure of ligand becomes more and more complex, from monodentate ligands to bidentate ligands and then to multidentate ligands,and the synthesis process becomes more and more complicated[25–28].However,the industry prefers the opposite—ligands with simple structures and can be easily prepared.Monodentate P-ligands usually have such advantages but their activities(especially the product n/i ratio)may be slightly lower than multidentate ligands.To our best knowledge,the Rh complexes with monodentate P-ligands used in hydroformylation are usually homocombinations,with a structure of[MLaLa],meaning the ligands coordinating to one Rh atom are the same.Recently,a group of catalysts with hybrid bidentate P-ligands were prepared to combine the high performances displayed by two different donors,and show better activities in Rh-catalyzed asymmetric hydroformylation[29–32].Since it is difficult to get a pure heterocombination structure like[MLaLb]in solution due to slow ligand exchange[33,34],very little was known about the performance of[MLaLb]catalysts until year 2003,during which Reetz et al.put forward a new concept of using mixtures of chiral P-ligands in Rhcatalyzed asymmetric hydrogenation[35].A further study combining two free monodentate ligands with Rh precursor in hydroformylation was carried out later,which showed interesting and diverse results and confirmed the viability of this method[36].This concept of ligand mixtures not only offers a guide for synthesizing new active Rh complexes,but also exhibits an easier way to conduct effective reactions without preparing new complexes.Because it is difficult to measure the accurate content of the three spices([MLaLa],[MLaLb]and[MLbLb]),the prediction of their performance is so difficult that such an approach calls for extensive accumulation of samples. Few rules can be followed as only a few examples have been published within the field of hydroformylation and they are all focusing on the two-ligand mixtures.No system containing mixtures of three or more ligands has been reported as far as we know,leaving a blank on this topic[34,37].

The present work aims to develop an effective and convenient hydroformylation method to synthesize pentanal from 1-butene,which is a useful intermediate in both pharmaceutical and chemical industries[7,38–40].Four monodentate P-ligands were selected to form a series of mixtures(six groups of double-ligand systems,four groups of triple-ligand systems and one group of tetra-ligand system).Two types of Rh complexes,Rh(acac)(CO)2and Rh(acac)CO(PPh3),were used as catalyst precursors to mix with the ligand system to test their catalysis performance in the hydroformylation of 1-butene.Our results show that the double-lig and systems are generally more active than the other systems,and the tetra-ligand system is the worst.

2.Experimental

2.1.Materials

1-Butene(＞99.95 v%)and syng as(VH2:VCO=1:1)were purchased from Beijing Beiwen Qiti Company.Rh(acac)(CO)2(＞99%)and Rh(acac)CO(PPh3)(＞97%)were purchased from Beijing Persisted Technology Company Limited.Triphenylphosphine(TPP,＞99%)was purchased from Aisinaladdin-e.com.Diphenyl-2-pyridyl Phosphine(＞93%)and Diphenyphosphino-2-benzoic acid(＞98%)were purchased from Beijing Haoersi Company.Triphenyl phosphite(＞98%)was purchased from Tianjin Guangfu Fine Chemical Research Institute.n-Pentanal(＞97%)was purchased from Alfa Aesar Company.

2.2.Solubility of ligand C

About 1.0 g of ligand C was dispersed in 50 ml of n-pentanal.The mixture was heated to 100°C with a condenser under stirring for 2.5 h.Then the mixture was transferred to a centrifuge tube to separate the insoluble solid and the liquid at a stirring rate of 4000 r·min-1for 10 min.Finally,the solid was dried at 80°C under vacuum overnight.The solubility of C can be calculated through the following equation:

In the equation,S(g·L-1)is the solubility of C;Mostands for the initial mass of C,and M′stands for the final mass of the insoluble C.

2.3.The alcoholysis of ligand D

In a round-bottom flask,0.968 g of ligand D and 50 ml of ethyl alcohol were mixed together to form a uniform solution.Then the solution was heated to reflux under stirring for about 2 h.The UV–vis absorption spectra(Shimadzu UV-3100 spectrometer)of the mixture were measured at 0 h,0.5 h,1.0 h and 2 h during heating.

2.4.Theoretical calculation

Quantum chemistry calculation on the ligands was carried out using B3LYP density functional theory(DFT)method with6-31gbasis set.Molecular orbital distribution,energy level and Mulliken charge were obtained from the calculated results based on optimized singlet ground state geometry.The calculation was performed through Gaussian 09 program.

2.5.Experimental method

In a typical reaction,a certain amount of P-ligand,Rh catalyst,and 125 ml of n-pentanal(as the solvent)were rapidly put into a 500 ml stainless steel autoclave with mechanical stirring.The concentration of Rh catalyst was 1.25 mmol·L-1and the total molar ratio of P-ligand to Rh catalyst(P/Rh)was50.N2was then flowed into the reactor continually for about 10 min before a syngas flow was introduced to replace N2totally.Subsequently,the autoclave was filled with syngas to a pressure of 0.5 MPa,and 10 ml of 1-butene was pumped into the reactor.The reactor was then heated to 100°C under a stirring rate of 200 r·min-1.Additional syngas was flowed into the autoclave to raise the pressure to2.5 MPa,when the reaction started.After2.5 h,the reaction was stopped and the system was cooled down to room temperature for analysis.All products were analyzed by a GC2000-II gas chromatograph with a flame ionization detector(FID),equipped with a DB-1 column(60 m×0.25 mm×0.25 μm).The evaluation indicators including the conversion of 1-butene(X),the ratio of n-pentanal to ipentanal(n/i)and the turnover frequency(TOF)of the catalyst were calculated using the following equations:

where Nnstands for the moles of n-pentanal;Niis the moles of ipentanal;NBrepresents the moles of 1-butene;NCis the moles of Rh catalyst and t is the reaction time.

Fig.1.Molecular structures of four monodentate P-ligands.

3.Results and Discussion

3.1.The electronic properties of the four ligands

As the most commonly used ligand,triphenylphosphine(TPP,A)was chosen as a reference and the other three ligandswith similar structures are diphenyl-2-pyridyl phosphine (B), diphenyphosphino-2-benzoic acid(C)and triphenyl phosphite(D).Their molecular structures are shown in Fig.1.

Fig.2 shows the UV–vis absorption spectra of the four ligands in toluene solvent. Ligands A–C all demonstrated a single absorption peak between 300 and 330 nm,assigned to spin-allowed ligand-centered transitions.Ligand D showed no obvious absorption in the measuring range,and no absorption above400nm was detected for all the ligands.After the ligands were mixed with Rh(acac)(CO)2respectively(Fig.3),all the solutions of A–C exhibited longer wavelength absorptions above 400 nm,and obvious shoulder absorptions between 350 nm and 450 nm were also observed for A and B solutions,attributed to metal-to-ligand charge-transfer or ligand-to-ligand charge-transfer[41].Ligand D,however,showed an intense absorption band around 300 nm.From Figs.1 and 2 it can be confirmed that all the ligands would coordinate with Rh atom in the solution.

Fig.2.UV–vis absorption spectra of the four ligands in toluene.

Fig.3.UV–vis absorption spectra of the four ligands with Rh(acac)(CO)2in toluene.

TD-DFT analysis was carried out to explore the electron configurations of the ligands,in order to compare their coordination abilities with metalatoms(Table 1).For all the four ligands,the Mulliken charge that distributed on the phosphorus atom is positive,with a value order of A＜B＜C＜D.In addition,the electron cloud density on P atom on HOMO of D is smaller than those of the other ligands.Therefore,it can be deduced that the coordination ability of D with Rh is weaker than A–C.Besides,the calculated energy gap(Eg)values match very well their adsorption bands in Fig. 1. The reason that D showed no adsorption band might be attributed to the vibration of its flexible P--O--chains.

Table 1Calculated Mulliken charge on the P atom,and HOMO,LUMO,energy gap(Eg)values as well as electron cloud distribution of HOMOs of the ligands

3.2.Rh(acac)(CO)2as the catalyst

3.2.1.Single-ligand–Rh(acac)(CO)2system

First,each ligand was tested together with Rh(acac)(CO)2in the hydroformylation of 1-butene.The results are shown in Table 2.

Table 2Catalysis performances of single-ligand–Rh(acac)(CO)2systems

The catalysis performance of ligand A was found to be slightly higher than the others in all the evaluation indicators.Ligand B just showed a slightly lower activity than A,which was in agreement with the result of tri(2-pyridyl)phosphine in Wilkinson's work[42].Although the electron withdrawing group of pyridine in ligand B is considered to be capable of increasing the reaction rate by reducing the bond between Rh atom and CO,the ortho-N atom in bis(2-pyridyl)phenylphosphine with coordination property showed a much lower TOF[18].The calculated negative Mulliken charge on N atom(-0.433)also confirmed its coordination possibility.

Ligand D is usually reported to be more reactive than ligand A due to its strong electron withdrawing property(the χ value of D is 30.20,while that of A is 13.25)[43].The P--O bonds make D a strong πacceptor which will facilitate CO dissociating from Rh center.However,it was not the case in our experiment,where D exhibited a slightly lower activity than A.A probable reason is that the water or alcohols existing in the solvent(97%n-pentanal)lead to the hydrolysis or alcoholysis of ligand D[44],which was also confirmed by the UV–vis absorption spectra in Fig.4.When D was refluxed with ethanol,the absorption of the solution was increasing rapidly with time going on,indicating that some new compound was generated.On the other hand,the smaller cone angle of D(θ-value is 128 for D,and 145 for A)[43]causes its smaller n/i ratio.

Fig.4.UV–vis absorption spectra of ligand D with ethanol at different time after reflux.

The rarely low activity of ligand C is most likely caused by its low solubility,which was measured to be only 4.8 g·L-1in pentanal at 100 °C.That means a large amount of C was insoluble in the hydroformylation experiment.Besides,the coordinating ability of carboxyl group in ligand C made it possible to form insoluble dinuclear or polynuclear complexes between C and Rh(acac)(CO)2[18].

3.2.2.Double-ligand–Rh(acac)(CO)2system

The four ligands were mixed with a molar ratio of 1:1 to form six groups of double-ligand systems which were then used in the experiment with Rh(acac)(CO)2(Table 3).

Table 3Catalysis performances of double-ligand–Rh(acac)(CO)2systems

It was found that both AB and AD demonstrated similar activities to their corresponding single-ligand systems.The results of AC and BC were much worse than A or B,but better than C.Considering the equal molar amounts of A and C in the solution(A:C:Rh=25:25:1),together with the better coordinating ability of C than A,there should be only a small amount of HRh(CO)(PPh3)3generated in the solution.On the other hand, the coordinating ability of B was better than A, resulting in a higher conversion in system BC than AC.It was surprising that the performance of BD was lower than either B or D.The high electronegativity of D was possibly helpful to increase the binding of Rh atom with N atom in B,resulting in the formation of more inert complexes.Most interestingly,the activity for mixture CD was detected to be higher than either C or D,indicating that there must be a synergistic effect between the two ligands.

In order to study the mixing effect,two systems,AD and CD,with X＞80%and n/i＞6 were chosen for further experiment on the mixing ratios.The results of AD mixtures with different ratios of A:D were shown in Table 4.The conversion reached its highest value of 87.4%at the ratio of 1.25:1,while the n/i got to its top point of 7.0 at the ratio of 1:1.Nevertheless,all these results showed no obvious advantage compared to single A or D system in Table 2, implying there was no significant synergistic effect in the AD-mixture system.

Table 4Catalysis performances of A–D–Rh(acac)(CO)2systems with different ligand ratios

The results of CD mixtures with different ratios were shown in Table 5.As the amount of C increased,both the conversion and n/i arrived at the top point at the ratio of 0.75:1(C:D).Further addition of C resulted in worse performance.It could be certain that a notable synergistic effect took place when C and D were mixed in the system,and the effect could be either between the two kinds of ligands or through their bondings with Rh atom.

Table 5Catalysis performances of C–D–Rh(acac)(CO)2systems with different ligand ratios

3.2.3.Multiple-ligand–Rh(acac)(CO)2system

No study on hydroformylation reaction catalyzed by a triple-ligand mixture system or more was reported before.Every three in our four ligands were mixed with a molar ratio of 1:1:1 to form four groups of triple-ligand systems.Then each group together with Rh(acac)(CO)2was employed in hydroformylation reaction.The results are shown in Table 6.

Table 6Catalysis performances of multiple-ligand–Rh(acac)(CO)2systems

Unexpectedly,the catalysis performance of triple-ligand mixtures did not exhibit comparative differences like the former groups.All the results were similar but with lower conversions and n/i ratios.In particular,better results were obtained whenever single ligands A, B and D, or mixtures AB and AD were used,whereas mixture ABD showed a lower performance,just higher than BD.On the contrary,the mixtures AC and BC both showed worse catalysis activity,but the mixture ABC performed much better than either of them,due to the reduced C amount.Another trend was that the conversion,n/i and TOF were all similar for mixtures ABC,ABD and ACD.The activity of BCD was the worst in the above-mentioned four groups.Surprisingly,in the tetra-ligand system(molar ratio 1:1:1:1),a much lower catalysis performance was detected.

Taken all the results together,a general tendency can be summarized on the catalysis performance:double-ligand system＞singlelig and system＞triple-ligand system＞tetra-ligand system.It can be inferred that some cooperative effect inevitably happened when mixed ligands were used in the reaction.A positive influence has been observed in the double-lig and system,especially for CD mixture.However,a negative impact was found for BD mixture and multi-ligand systems.

3.3.Rh(acac)CO(PPh3)as the catalyst

3.3.1.Single-ligand–Rh(acac)CO(PPh3)system

Table 7 shows the catalysis performances of single-ligand systems with Rh(acac)CO(PPh3)as the catalyst precursor.It was found that the performances of A and B were nearly the same,and they showed the best conversion among the four ligands—the same as the results in Table 2.In the C–Rh(acac)CO(PPh3)system,1-butene conversion was higher than that in the C–Rh(acac)(CO)2system,but the n/i ratio was slightly lower.The result in system D,however,exhibited a slightly lower conversion and a higher n/i ratio.In conclusion,the amount of PPh3ligand introduced by Rh(acac)CO(PPh3)was so small that the performance of the single-ligand system was not affected significantly.

Table 7Catalysis performances of single-ligand–Rh(acac)CO(PPh3)systems

3.3.2.Double-ligand–Rh(acac)CO(PPh3)system

The results of double-ligand (1:1) systems were shown in Table 8. Like the results in Table 3,AD still showed a much higher n/i than either AorD.The 1-butene conversion of AC or BC was much lower than A or B,but higher than C.A synergetic promotion effect in the group of CD could also be concluded for its excellent activities in both 1-butene conversion and n/i,better than either one of its components.Besides,the AC group exhibited a much higher 1-butene conversion in Table 8 than in Table 2.

Table 8Catalysis performances of double-ligand–Rh(acac)CO(PPh3)systems

The best two systems,AD and CD,with conversion＞80%and n/i＞7 were chosen for further study of the mixing effect.The results in Table 9 demonstrate that as the content of A increased,1-butene conversion also rose,so did the n/i ratio.When the ratio of A:D reached 1:1,the n/i ratio reached the highest point of 8.6;then at the ratio of 1.5:1(A:D),1-butene conversion arrived its maximum value of 94.2%,with a highest TOF of 360.5 h-1.At the same time,the n/i ratio went down slightly.A further increase of the amount of A would reduce the catalysis performance. By comparing Table 4with Table 9,we can see the two different catalysts led to slightly different catalysis performances.Generally,the A–D–Rh(acac)CO(PPh3)system shows a slightly better performance in both 1-butene and n/i ratio than the A–D–Rh(acac)(CO)2system.

Table 9Catalysis performances of A–D–Rh(acac)CO(PPh3)systems with different ligand ratios

The results of CD mixtures with different ratios were shown in Table 10.As the amount of C increased,1-butene conversion,n/i and TOF were all enhanced at first and then declined.A highest conversion of 88.6%was obtained at C:D=0.75:1,and the highest n/i was 7.4 at C:D=1:1.Within the range of C:D=0.5:1–1:1,the conversion was always better than that of single C or D system,which was similar to the results with the Rh(acac)(CO)2catalyst.

Table 10Catalysis performances of C–D–Rh(acac)CO(PPh3)systems with different ligand ratios

3.3.3.Multiple-ligand–Rh(acac)CO(PPh3)system

The results of triple-ligand–Rh(acac)CO(PPh3)systems and tetralig and system are shown in Table 11.Similar to Table 6,every system showed a lower conversion and n/i ratio together with a lower TOF compared with the double-ligand systems.It can be further concluded that the effects of ligands were averaged when three ligands were mixed.The performance of the tetra-ligand system was even worse than the triple-ligand systems.

Table 11Catalysis performances of multiple-ligand–Rh(acac)CO(PPh3)systems

4.Conclusions

For all the catalyst systems studied,the catalysis performance follows the order of double-ligand system＞single-ligand system＞triple-ligand system＞tetra-ligand system.A certain cooperative effect inevitably happened when different ligands were mixed in the hydroformylation reaction.A positive influence has been observed in the double-ligand system,especially for mixture of CD.On the other hand,negative impacts were found for BD mixture and multi-ligand systems.Furthermore,there is very little difference between the results with Rh(acac)(CO)2and Rh(acac)CO(PPh3)as the catalyst precursors.Further effort needs to be made on the characterization of the real composition of the solution and the measurement of the performance of each component.