Ind. Eng. Chem. Res. 2010, 49, 1027–1032
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Immobilized Palladium Nanoparticles Catalyzed Oxidative Carbonylation of Amines Mahesh R. Didgikar,† Debdut Roy,*,‡ Sunil P. Gupte,§ Sunil S. Joshi,§ and Raghunath V. Chaudhari*,‡ Geist Research PVt. Ltd., Yash Complex, Zadeshwar Road, Bharuch 392012, India, Department of Chemical and Petroleum Engineering, Center for EnVironmentally Beneficial Catalysis, The UniVersity of Kansas, Lawrence, Kansas 66047, and Catalysis, Reactors and Separation Unit (CReST), National Chemical Laboratory, Pune 411008, India
Catalytic application of immobilized palladium nanoparticles for synthesis of ureas by oxidative carbonylation of amines has been investigated. This is the first report on oxidative carbonylation of amines to ureas using immobilized palladium nanoparticles catalyst. The palladium nanoparticles were immobilized on a NaY zeolite support through 3-aminopropyl-trimethoxysilane (APTS) as anchoring agent. The [Pd]-APTS-Y catalyst along with NaI promoter showed high conversion and selectivity to the desired urea products over several amine compounds including an example of an aliphatic amine precursor even at 333 K. The immobilized catalyst was easily separated and recycled several times without any loss of activity. The role of different solvents, iodide promoters, iodide to Pd ratio, concentration of substrates, and temperature on the overall yield of the reaction was also investigated. 1. Introduction Catalysis has played an important role in producing new materials or developing new synthetic routes by which a chemical can be produced in a more economical as well as greener way.1,2 For example, the classical synthetic route for ureas from amines requires very corrosive and toxic reagents such as phosgene or isocyanates and, therefore, causes serious environmental problems.3 The urea derivatives have important applications in pharmaceuticals, agrochemicals, resins, polymers, and petrochemicals.4 Therefore, the demand of the class of compounds attracted researchers to develop a less hazardous synthetic route for urea. In recent years, oxidative carbonylation has emerged as an alternative pathway for urea synthesis wherein a simpler and cheaper raw material such as carbon monoxide has been used as a source of a carbonyl moiety.5 Second, the oxidative carbonylation is very attractive from atom economy point too as it utilizes amines, CO, and an oxidant as starting materials with high yields of desired products and no significant byproducts if appropriate reaction conditions are chosen.3 Stoichiometric reaction for oxidative carbonylation of amines with aniline as a model substrate is shown in Scheme 1. This route has received particular attention because of its atom efficiency and environmental friendliness. Though, CO oxidation to CO2 is a side reaction, under certain conditions (99.8% pure, Matheson Gas, USA) and oxygen (Indian Oxygen Ltd., India) were also used as received. Catalyst Synthesis and Characterization. Sources for the individual components for the synthesis of [Pd]-APTS-Y, the synthesis procedure, and characterization details are described in our previous publication.13 In a typical synthesis, 1 g of calcined NaY zeolite was treated with 5.73 mmol of APTS in anhydrous dichloromethane (DCM) at room temperature first to immobilize the amine functionality on the zeolite surface (APTS-Y). The solid was filtered, washed with DCM repeatedly to remove free APTS molecules, and dried under vacuum for further use. Colloidal Pd nanoparticles were synthesized sepaScheme 1. Oxidative Carbonylation of Aniline to Urea
* To whom correspondence should be addressed. E-mail: dsroy@ ku.edu (D.R.),
[email protected] (R.V.C.). Tel.: +1 785 864 1633. Fax: +1 785 864 6051. † Geist Research Pvt. Ltd. ‡ The University of Kansas. § National Chemical Laboratory. 10.1021/ie9007024 2010 American Chemical Society Published on Web 09/28/2009
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Scheme 2. Synthesis Procedure for [Pd]-APTS-Y
rately by reduction of 100 mL 10-4 M aqueous palladium nitrate (Pd(NO3)2 · 3H2O) solution using 0.01 g NaBH4 dissolved in water at room temperature. The colloidal Pd suspension was then treated with 0.02 g of APTS-Y and stirred for 12 h to immobilize the Pd nanoparticles on the zeolite surface ([Pd]APTS-Y). The solid was allowed to settle, then filtered, and washed with copious amounts of water. The gray solid was dried at 323 K overnight under vacuum and used for characterization and catalysis studies. A schematic of the synthesis procedure is shown in Scheme 2. The [Pd]-APTS-Y nanocomposite was characterized by spectroscopic techniques such as inductively coupled plasmaoptical emission spectroscopy (ICP-OES), UV-visible, Fourier transform infrared (FTIR), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and by thermogravimetric analysis (TGA). The details of characterization were described in our previous publication,13 and the important results are summarized as follows: (i) Pd metal loading in the [Pd]-APTS-Y was 3.6% wt/wt as estimated by ICP-OES analysis. (ii) TEM results showed that the Pd nanoparticles were narrowly distributed in the range of 4-5 nm. (iii) The nanocomposite showed predominantly Pd(0) species from the XPS studies. (iv) XRD results showed that the zeolite structure remained intact during catalyst synthesis and the Pd nanoparticles were bound to the outer surface of NaY selectively. (v) FTIR and TGA results indicated that the metal nanoparticles were attached to the zeolite surface through APTS molecules. NaY was selected as the support matrix for this work to selectively immobilize the Pd nanoparticles outside of the zeolite surface and not inside the channels. This strategy was considered to eliminate pore diffusion limitations of reactant species during catalytic applications. Zeolite NaY has a 3-dimensional pore structure with a pore (channel) diameter of 7.4 Å since the aperture is defined by a 12 member oxygen ring and leads into a larger cavity (super cage) of diameter 12 Å. Each super cage
is connected to four other tetrahedrally arranged super cages to form a continuous three-dimensional framework of interconnecting super cages.14 If we assume that the APTS molecule can bind to the silanol groups inside the channels, the tethered moiety ((-O)3SiCH2CH2CH2NH2) will have a length of 6.065 Å (calculated using Mercury 1.1.2 software, Cambridge Crystallographic Data Centre). But the bare Pd nanoparticles possess an average particle size in the range of 4-5 nm (particle size of Pd nanoparticles were measured in a separate experiment just after NaBH4 reduction of palladium nitrate), which is far bigger than the channels of the zeolite Y. Therefore, we assume that the Pd nanoparticles are selectively immobilized on the outer surface of the NaY zeolite. Experimental Procedure. The oxidative carbonylation reactions were carried out in a magnetically stirred round bottomed flask with 8:1 CO to O2 supplied from a bladder. To prepare the 8:1 CO:O2 gas mixture, CO and O2 from two separate reservoirs were filled into a third reservoir maintaining an 8:1 proportion in total pressure. The reservoir with a CO + O2 gas mixture was used further to fill the bladder. In a typical experiment, known amounts of the amine compound and iodide promoter were dissolved in organic solvent and charged into a round bottomed flask along with a known amount of [Pd]-APTSY catalyst. A bladder containing an 8:1 ratio of CO to O2 was used as a source of CO/O2 for the reaction. The flask was purged 2-3 times with nitrogen and twice with CO-O2 mixture at room temperature under mild agitation conditions. The flask was heated to a desired temperature, the initial liquid sample withdrawn, and the CO-O2 gas mixture was introduced into the reactor. The reaction was started by setting the agitation to a specific value. The catalyst performance studies with [Pd]APTS-Y for different amine compounds (presented in Table 1) were continued for 8 h to achieve high conversion. The reactions for parametric studies were carried out for 2 h, after which the content was cooled and the sample was collected for analysis. For the catalyst recycle studies, the liquid phase from the previous run was pipetted out, the catalyst at the bottom was washed several times with methanol, followed by diethyl ether, and vacuum-dried. The fresh substrate and promoter in a solvent
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a
Table 1. Results on Oxidative Carbonylation of Different Amines Using [Pd]-APTS-Y Catalyst
a
Reaction conditions: amine 0.80 kmol/m3; [Pd]-APTS-Y 1.854 kg/m3; NaI 1.24 ×10-3 kmol/m3; temp 333 K; P(CO:O2) 50 psig; solvent DMF, time
8 h.
were charged into the round bottomed flask containing the used catalyst. The amines in the liquid phase sample were analyzed by GC using an HP-5 column and flame ionization detector (FID; see the Supporting Information for details). The separation and quantification of 1,3-diphenylurea (DPU) was done on an HPLC, using a Waters RP-8 column, UV detector, and acetonitrile-water as a mobile phase (see the Supporting Information for details). All the products reported in this work were isolated by column chromatography and characterized by NMR (13C and 1H in Bruker-AV-200 and 1H in Bruker-MSL300 machines), FTIR (Biorad FTS 175C machine in transmission mode), and elemental analysis (CHNS-O EA 1108, Elemental analyzer of Carlo-Erba Instruments, Italy). The material balance for all the reactions agreed >95% as per the stoichiometry of the reaction scheme shown in Scheme 1. 3. Results and Discussions Catalytic activity of the [Pd]-APTS-Y catalyst was tested for conversion of several amines to the corresponding ureas in the presence of NaI as a promoter and N,N-dimethylformamide (DMF) as a solvent at 333 K. The results on conversion, selectivity, yield, and turnover frequencies (TOFs) for different amines are presented in Table 1. The products were separated and characterized by NMR, FTIR, and elemental analysis (see the Supporting Information). Therefore, the yields and selectivities reported in Table 1 are based on the isolated yields of the reactions, which are always expected to be lower than the actual due to handling loss of the reaction mixture (it may drop from 97% true yield to 91% when isolated15). In that case, the selectivities to the ureas are expected to be >95% with all the amines reported in Table 1. As the amines transformed almost quantitatively to the corresponding ureas, isolation and characterization of the trace amount of byproduct were not attempted. However, azobenzenes are expected to be the side products with aromatic amines, which formed without a catalyst
as reported earlier by Orito et al.16 The formation of azobenzene may be predominant at higher temperature (393 K). It is evident from the table that the immobilized Pd nanocatalyst showed good conversions (69-98%) and selectivities (>90%) to the urea derivatives for all the amines. This indicates the usefulness of this route for oxidative carbonylation of a wide range of amines with or without other functional groups in it under mild reaction conditions. The influence of different p-substituents on conversion is shown in entries 3-5 in Table 1. The conversion level improved from electron withdrawing (deactivating) groups such as chloride (entry 3; 68.7%) to methoxy (better activating group; entry 4; 78.3%) to methyl (good activating group; entry 5; 95.9%) as the p-substituents. Very often it is difficult to compare catalyst activities (conversion, selectivity, and turnover frequency (TOF)) with reported literature due to difference in reaction conditions such as temperature, catalyst loading, pressure of the gas phase components, substrate concentrations, and reaction time while all of these can have influence on the efficiency of a catalyst. We have compared the activity of our catalyst with literature reports with respect to TOF rather than conversion so that effect of catalyst loading and reaction time can be avoided while comparing our catalyst with reported ones. We have not considered selectivity as a criterion to compare Pd catalysts for this reaction as oxidative carbonylation of amines to ureas or carbamates are highly selective reaction with any kind of Pd catalysts depending on temperature range. We compared catalytic performance of the [Pd]-APTS-Y catalyst with literature reports with Pd(0) catalysts for oxidative carbonylation of aniline to urea. Shi et al.12 synthesized polymer immobilized Pd nanoparticles (particle size 5% Pt/C (36 h-1) > IrCl3 (17 h-1). These results indicate that the [Pd]-APTS-Y has higher activity (TOF of 157 h -1) even at lower temperature and pressure compared to the catalysts reported previously. To study the effects of reaction parameters, aniline was chosen as a model substrate and the effects of different solvents, aniline concentration, iodide promoters, and ratio of iodide promoter to Pd were studied. Solvent Screening. The solvent effect on oxidative carbonylation of aniline in the presence of [Pd]-APTS-Y catalyst and NaI promoter was studied, and the results are shown in Figure 1. The selectivity for DPU was almost constant (∼98%) for all the solvents whereas conversion (and consequently the yield also) varied depending upon the solvent used. A decrease in the conversion was directly correlated to the polarity of the solvents used (polarity indexes of the solvents from DMF to THF as shown in Figure 1 are 6.4, 5.1, 5.2, 4.7, and 4.0, respectively19). Therefore, DMF was used as a solvent for further studies. It is important to note here that the selectivity to DPU remained unchanged and phenyl carbamate was not identified as a product with solvents like methanol or ethanol. This is attributed to the lower temperature (333 K) in our present study. Kanagasabapathy et al.20 has studied the effect of temperature on oxidative carbonylation of n-butyl amine using alcohols as
Figure 2. Effect of iodide promoters on oxidative carbonylation of aniline. Reaction conditions: aniline 0.82 kmol/m3; [Pd]-APTS-Y 1.854 kg/m3; iodide 1.24 × 10-3 kmol/m3; P(CO:O2) 50 psig; temp 333 K; solvent DMF; time 2 h.
Figure 3. Effect of iodide promoter to catalyst ratio on oxidative carbonylation of aniline. Reaction conditions: aniline 0.82 kmol/m3; [Pd]APTS-Y 1.854 kg/m3; promoter NaI; P(CO:O2) 50 psig; temp 333 K; time 2 h.
solvents and reported that the noncatalytic reaction between the urea and alcohols to carbamates is significant only at or above 378 K, with urea as the favored product at lower temperature. Iodide Promoters. In our preliminary studies, it was observed that the oxidative carbonylation of aniline to urea did not occur without a halide promoter. Similar observation was made earlier by Gupte et al.21 Fukuoka et al.18 reported that the catalytic activity of Pd catalysts with halide promoters for oxidative carbonylation reaction decreases in the order I- > Br- > Cl-; however, the role of halide promoters is not well-understood in the reported literature. On the basis of this literature information, iodide promoters were considered for further studies. The effect of different iodide promoters was investigated on conversion of aniline to 1,3-diphenyl urea in the presence of [Pd]-APTS-Y catalyst at 333 K, and the results are presented in Figure 2. The nano-Pd catalyst along with NaI, TEAI (tetraethyl ammonium iodide), and I2 promoters gave very high conversion (26, 24, and 27%, respectively) and selectivity (∼98%) in 2 h. The efficiency of the promoters was found to be linearly proportional to their ability to release iodides in the reaction medium. NaI was selected as a promoter for further studies. Figure 3 shows the overall productivity of 1,3-diphenyl urea (DPU) as a function of the ratio of iodide to Pd concentrations. The results indicate that an optimum ratio of the iodide promoter to catalyst (2 for the present catalyst) exists for the best performance of the catalytic system for oxidative carbonylation of aniline to DPU. Gupte et al.21 has reported the optimum ratio
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Table 2. Effect of Aniline Concentration on Conversion, Selectivity, and Yielda aniline conc, kmol/m3
conversion, %
selectivity, % DPU
yield, % DPU
0.41 0.82 1.21 1.60 2.05
15.04 26.13 36.58 50.99 69.86
96.41 97.25 97.66 94.51 96.15
14.5 25.58 35.72 48.19 67.17
a Reaction conditions: [Pd]-APTS-Y 1.854 kg/m3; NaI 1.24 × 10-3 kmol/m3; P(CO:O2) 50 psig; temp 333 K; solvent DMF; time 2 h.
Figure 5. Arrhenius plot.
Figure 4. Temperature effect on oxidative carbonylation of aniline with [Pd]-APTS-Y catalyst. Reaction conditions: aniline 0.824 kmol/m3; [Pd]APTS-Y 1.854 kg/m3; NaI 1.24 × 10-3 kmol/m3; P(CO:O2) 50 psig; solvent DMF; time 2 h.
as 1 for Pd metal catalyst and 3.6 for 5% Pd/C catalyst. The difference was attributed to the differences in the adsorption of NaI on the support material. At higher I-/Pd ratio, the activity of the catalyst decreased due to blocking of the possible active Pd metal sites for CO, O2, and aniline adsorption followed by the surface reaction. Effect of Aniline Concentration. The effect of aniline concentration on conversion of aniline, selectivity, and yield of DPU was studied in the range of 0.41-2.05 kmol/m3 of aniline concentration, and the results are presented in Table 2. The conversion of aniline and yield of DPU increased with increase in aniline concentration with selectivity to DPU remaining constant. From the initial rate of reaction vs aniline concentration data, it was calculated that the reaction order is approximately 1.6 with respect to aniline concentration. A similar rate dependence for oxidative carbonylation of aniline to DPU in the presence of Pd/C catalyst and NaI promoter was reported earlier by Gupte et al.21 Effect of Temperature. The conversion of aniline and formation of DPU were found to increase with temperature, and the experimental results are shown in Figure 4. The selectivity to DPU remained in the range of 96-99% in the studied temperature range. Reactions were carried out at different agitation speeds (16-20 Hz), and it was confirmed that the reactions were carried out under mass transfer free conditions. The influence of gas-liquid, liquid-solid, and intraparticle mass transfer resistances were also evaluated using the criteria discussed earlier by Ramachandran and Chaudhari to ensure the kinetically controlled conditions for the experiments (see the Supporting Information).22 The activation energy was calculated from the Arrhenius plot (shown in Figure 5) as 29.7 KJ/mol for oxidative carbonylation of aniline with [Pd]-APTS-Y catalyst.
Figure 6. Recycle studies of [Pd]-APTS-Y catalyst for oxidative carbonylation of aniline. Reaction conditions: aniline 0.82 kmol/m3; [Pd]-APTS-Y 1.854 kg/m3; NaI 1.24 × 10-3 kmol/m3; P(CO:O2) 50 psig; temp 333 K; solvent DMF; time 4 h.
Recycle Studies. In order to check the stability of the immobilized Pd nanoparticles under the oxidative carbonylation reaction conditions, the catalyst was recycled five times and it was observed that conversion of aniline and selectivity of DPU remained unchanged during all the runs. The results of recycle runs are shown in Figure 6, which show excellent stability of the [Pd]-APTS-Y catalyst during recycles. The leaching of Pd was checked in the liquid samples after each experiments. The reaction mixture was allowed to settle and then the liquid pipetted out carefully was filtered through Whatman filter paper, and the filtrate was analyzed for dissolved Pd by ICP-OES. The ICP-OES analysis of the liquid phase samples after each recycle runs showed that there was no traceable Pd metal present in the supernatant liquid, which indicates that the amount of Pd leached in the liquid phase if at all is below the detectable limit of ICP. This proves the stability and robustness of [Pd]-APTSY as a catalyst. 4. Conclusions Pd nanoparticles immobilized on NaY zeolite through APTS as an anchoring agent have shown excellent catalytic activity for oxidative carbonylation of aliphatic and aromatic amine compounds (with diverse substituents) to their corresponding symmetrical ureas in the presence of iodide promoters. The strategically designed catalyst showed improved activity for oxidative carbonylation with respect to other reported Pd(0) catalysts possibly due to the absence of pore diffusional limitations and very good site isolation achieved by the synthesis. The [Pd]-APTS-Y catalyst showed very consistent activity and excellent stability over repeated recycle experiments.
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The reaction parameters such as solvent, iodide promoter, iodide promoter to catalyst ratio, aniline concentration, and temperature on [Pd]-APTS-Y catalyzed oxidative carbonylation of aniline were also addressed. Acknowledgment M.R.D. wishes to thank the Council of Scientific and Industrial Research (CSIR), Government of India, for financial support as a senior research fellow. D.R. acknowledges The University of Kansas for financial support as a postdoctoral researcher. Supporting Information Available: Further experimental details, Table S1, Figures S1-S6, and analysis of mass transfer resistences. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Sheldon, R. A. Catalysis: The key to waste minimization. J. Chem. Tech. Biotechnol. 1997, 68, 381–388. (2) Sheldon, R. A. The E factor: Fifteen years on. Green Chem. 2007, 9, 1273–1283. (3) Dı´az, D. J.; Darko, A. K.; McElwee-White, L. Transition metalcatalyzed oxidative carbonylation of amines to ureas. Eur. J. Org. Chem. 2007, 4453–4465. (4) Hegarty, A. F.; Drennan, L. J. ComprehensiVe Organic Functional Group Transformations; Katritzky, A. R., MethoCohn, O., Rees, C. W., Eds.; Pergamon: Oxford, 1995; Vol. 6, pp 499-526. (5) Bigi, F.; Maggi, R.; Sartori, G. Selected syntheses of ureas through phosgene substitutes. Green Chem. 2000, 2, 140–148. (6) Kelkar, A. A.; Kolhe, D. S.; Kanagasabapathy, S.; Chaudhari, R. V. Selectivity behavior in catalytic oxidative carbonylation of alkylamines. Ind. Eng. Chem. Res. 1992, 31, 172–176. (7) Dombek, B. D.; Angelici, R. J. A mechanistic investigation of the decacarbonyldimanganese-catalyzed carbonylation of amines. J. Organomet. Chem. 1977, 134 (2), 203–217. (8) Sonoda, N. Selenium-assisted carbonylation with carbon monoxide. Pure Appl. Chem. 1993, 65, 699–706. (9) Venkatesh, P. K.; Chaudhari, R. V. Activity and selectivity of supported Rh catalysts for oxidative carbonylation of aniline. J. Catal. 1994, 145 (1), 204–15.
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ReceiVed for reView April 30, 2009 ReVised manuscript receiVed September 9, 2009 Accepted September 14, 2009 IE9007024
