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Ind. Eng. Chem. Res. 2004, 43, 1192-1199
Carbamate Synthesis on Pd/C Catalysts: Gas-Solid versus Slurry Processes Pisanu Toochinda and Steven S. C. Chuang* Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325-3906
Carbamate synthesis has been studied over NaI-Pd/C in slurry, gas-solid, and tubular reactors at 373-438 K and 0.41-7.61 MPa. The gas-solid carbamate synthesis process in which the CO, O2, methanol, and aniline reactants are present in the gas phase and the catalyst is in solid form occurs at a significantly higher rate than the slurry-phase synthesis in which CO and O2 dissolve in the liquid methanol/aniline mixture. The high rate of the gas-solid carbamate synthesis compared with that of the slurry-phase synthesis can be attributed to (i) the intimate contact between the NaI promoter and the Pd on the carbon surface, (ii) the absence of solubility limitations in the gas-solid synthesis, and (iii) the slowing of the sintering of the Pd particles. Reaction pathway studies show that the direct oxidative carbonylation of aniline with methanol is the most effective pathway for carbamate synthesis. A low-cost, environmentally benign carbamate synthesis for the replacement of the isocyanate synthesis from phosgene/amine can be developed by coupling the high rate of the gas-solid synthesis with its intrinsic advantage of ease of catalyst recovery. 1. Introduction Catalysts play a key role in increasing the efficiency of chemical syntheses by lowering the reaction temperature and pressure, increasing the product yields, and reducing byproduct formation. The development of environmentally benign synthesis processes that bypass toxic feedstocks, combine process steps, and result in a net reduction of pollutants and energy use rests, to a great extent, on our capacity for innovation in the design of synthesis pathways and the associated catalysts.1,2 Carbonylation could provide new synthetic routes that minimize the use of hazardous and toxic reagents.3-7 Significant interest in carbonylation lies in its potential to replace the current isocyanate synthesis process,4 RNH2 + COCl2 f RNCO + 2HCl, which uses hazardous phosgene as a feedstock. Carbamate synthesis from the oxidative carbonylation of amine with methanol has been the most attractive process for its high selectivity toward carbamate, the stability of the resulting carbamate, and the highly selective conversion of carbamate (RNHCOOR′) to isocyanate (RNCO). The reactions can be written as follows4
Carbamate synthesis from the oxidative carbonylation of aniline with methanol RNH2 + R′OH + CO + 1/2O2 f RNHCOOR′ + H2O Isocyanate synthesis from carbamate RNHCOOR′ f RNCO + R′OH where R and R′ are alkyl or aryl groups. Both metal and carbon-supported metal catalysts in a slurry of solvent medium have been shown to exhibit high selectivities toward the desired carbamate products from carbonylation.8-10 However, these processes present four major problems: (a) difficulty in catalyst recovery, (b) the use of high pressure, (c) the rapid deactivation of the catalyst complexes by process excursion, and (d) * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 330-972-6993. Fax: 330-972-5856.
the limited solubility of CO and O2 in amines and alcohols. The limited solubility decreases the overall rate of the reaction process. The low rate of this process is evidenced by the fact that all of reported homogeneous catalytic carbonylation processes have required more than 2 h to achieve more than 50% carbamate yields in a batch reactor.10-15 In addition to the limited solubility, the low rate of the carbamate synthesis process can be attributed to the low catalyst activity. The problems associated with homogeneous processes and the use of phosgene as a reactant can be solved if an effective reaction pathway and heterogeneous catalytic process can be developed for oxidative carbonylation. The objectives of this study were (i) to identify an effective carbamate synthesis pathway on NaI-Pd/C catalyst and (ii) to explore the feasibility of carrying out the heterogeneous oxidative carbonylation of amine with methanol in a gas-solid mode where both reactants and products stay in the gas phase and the catalyst is in the solid form. NaI-Pd/C catalyst was selected for this study because Pd is the most active oxidative carbonylation catalyst in the slurry-phase carbamate synthesis.9,16 An examination of all of the possible methyl-N-phenyl carbamate synthesis pathways over NaI-Pd/C, presented in Figure 1, shows that CO contributes the carbonyl group, aniline provides C6H5NH, and methanol provides -OCH3 in the carbamate. Carbamate can be produced via three reaction pathways: (i) dimethyl carbonate synthesis followed by the reaction of dimethyl carbonate with aniline, (ii) indirect carbamate synthesis, and (iii) diphenyl urea synthesis followed by the reaction of diphenyl urea with methanol. Each step in Figure 1 has been demonstrated to occur at 373-453 K and 0.101-3.79 MPa on various forms of catalysts.6-8,17-19 Although the oxidative carbonylation of amine with methanol has been studied extensively, the specific reaction pathway leading to carbamate on NaI-Pd/C remains unclear. In this work, reaction pathway studies were carried out in the slurry phase in an autoclave (Figure 2a) to determine the carbamate yield of each pathway.
10.1021/ie0305768 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/03/2004
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1193
Figure 1. Oxidative carbonylation network.
Figure 2. Carbamate synthesis processes: (a) slurry phase, (b) gas-solid, and (c) gas-solid in a fixed-bed reactor.
The feasibility of the gas-solid carbonylation was studied in an autoclave (Figure 2b), where the solid NaI-Pd/C catalyst was exposed to CO/O2 and vapor phase methanol/aniline. Although NaI-Pd/C is in the solid form prior to the reaction, the Pd state might change during the reaction. Infrared (IR) spectroscopy and transmission electron microscopy (TEM) were used to monitor changes in the Pd states and catalyst morphology before and after the reaction. Observations of carbamate produced from the gas-solid reaction in the autoclave allowed us to take a step further in carrying out the reaction over NaI-Pd/C in a fixed-bed reactor in semibatch mode (Figure 2c). The results of reaction studies in the slurry, gas-solid, and fixed-bed configurations revealed the advantages of gas-solid and fixed-bed operations and provided the scientific and technical basis for the development of a large-scale carbonylation process. 2. Experimental Section 2.1. Catalyst Preparation. A 5 wt % Pd/C catalyst was prepared by the incipient wetness impregnation of Ambersorb-563 activated carbon (surface area ) 1050-
1150 m2/g; Rohm and Haas Co.) with an aqueous solution of PdCl2‚2H2O/HCl (Fisher Scientific). The ratio of the volume of the salt solution to the weight of support was 1 cm3:1 g. The impregnated sample was dried in air at 298 K for 24 h and calcined in a 30 cm3/ min air flow at 573 K for 10 h. A 2.5 wt % NaI-5 wt % Pd/C catalyst was prepared by the incipient wetness impregnation of 5 wt % Pd/C with a NaI/CH3OH (molar ratio of NaI to CH3OH ) 1:19) solution. The impregnated sample was dried in air at 298 K for 24 h. The molar ratio of NaI to Pd obtained was 0.35:1. 2.2. Reaction Pathways. Table 1 lists the reactant compositions and reaction conditions used for the investigation of the reaction pathways on NaI-Pd/C. The reactions in Table 1 were studied in the slurry mode, with gaseous CO and O2 dissolved in the liquid reactants (methanol or/and aniline) containing 0.5 g of solid NaI-Pd/C catalyst particles. Because of the dual use of methanol as a solvent and as a reactant, the number of moles of methanol used in the reaction was significantly higher than that in the reaction stoichiometry. The specific experimental procedures consisted of (i) loading the catalyst and methanol/aniline mixture into the autoclave, (ii) sealing the autoclave and flushing the
1194 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 Table 1. Reaction Conditions and Product Yields of the Oxidative Carbonylation Network over NaI-Pd/C producta yield (%) reaction
reactant molar ratio
CH3OH + CO + 1/2O2 f (CH3O)2CO + H2O (CH3O)2CO + C6H5NH2 f C6H5NHCOOCH3 + CH3OH C6H5NH2 + CH3OH + CO + 1/2O2 f C6H5NHCOOCH3 + H2O 2C6H5NH2 + CO + 1/2O2 f (C6H5NH)2CO + H2O (C6H5NH)2CO + CH3OH f C6H5NHCOOCH3 + C6H5NH2
T(K)
P(MPa)
time (min)
(i) DMC Synthesis and the Reaction of DMC with Aniline CH3OH/CO/O2 403 1.38 480 ) 2221:10:1 aniline/DMC ) 1:1 453 2.42 480
MPC
MA
MB
DMC
DPU
0.0
0.0
0.0
0.0
0.0
0.5
32.4
38.6
0.0
0.0
12.8
5.3
0.0
0.0
0.0
0.0
0.0
37.3
0.5
0.8
0.0
0.0
(ii) Direct Carbamate Synthesis CH3OH/aniline/CO/O2 438 2.15 600 70.0 ) 2926:68:10:1 (iii) DPU Synthesis and the Reaction of DPU with Methanol CH3OH/aniline/CO/O2 373 2.15 480 2.1 ) 2926:68:10:1 438 2.15 120 82.0
a DMC ) dimethyl carbonate, DPU ) diphenyl urea, MPC ) Methyl-N-phenyl carbamate, MA ) monomethylaniline, and MB ) methyl benzoate.
Table 2. Comparison of Slurry and Gas-Solid Carbamate Syntheses at 438 K liquid-phase CH3OH/ concentration aniline aniline/ 3 yield (%) CO/O2 P time (mmol/cm ) conversion a (%) MPC MA MB molar ratio (MPa) (min) CO O2a 78:1.8:10:1 78:1.8:10:1 2926:68:10:1 2926:68:10:1
7.61 7.61 2.15 2.15
120 480 120 600
2926:68:10:1 1.93 618:277:10:1 0.51 156:207:10:1 0.41
120 120 120
a
Slurry 0.79 0.11 0.79 0.11 0.75 0.01 0.75 0.01 Gas-Solid N/A N/A N/A N/A N/A N/A
71.6 84.5 8.7 88.2
67.0 82.0 5.5 70.0
4.0 0.4 3.2 12.8
0.7 1.8 0.0 5.3
80.2 25.3 97.1
74.8 5.4 0.0 14.8 9.7 0.0 1.42 89.8 5.9
Determined using the Peng-Robinson equation of state.
headspace with gaseous reactants (CO/O2 ) 10:1) at a flow rate of 30 cm3/min, and (iii) raising the CO and O2 pressure and temperature to the desired conditions. 2.3. Gas-Solid Oxidative Carbonylation Reaction Studies in the Batch System. The heterogeneous gas-solid oxidative carbonylation reaction was carried out in an autoclave with 0.5 g of NaI-Pd/C placed in a pouch made of fiberglass fabric (Fisher Scientific) and suspended above 120 cm3 of a methanol and aniline mixture, as shown in Figure 2b. The molar ratios of reactants and reaction conditions are listed in Table 2. 2.4. Gas-Solid Oxidative Carbonylation Reaction Studies in a Semibatch Fixed-Bed Reactor. Seven hundred milligrams of NaI-Pd/C was packed with glass wool at both ends in a fixed-bed reactor (i.d. ) 0.95 mm), as shown in Figure 2c. The mixture of methanol and aniline vapor was brought into the reactor by flowing CO/O2 (10:1 ratio) at a rate of 20 cm3/min through a saturator containing methanol/aniline (molar ratio of 2.25:1) at 313 K. The resulting mixture in the reactor contained the reactants CO, O2, methanol, and aniline in the molar ratio of 75:7.5:67:1. The reactor was heated to 393 K for diphenyl urea synthesis. A 5-cm3 sample of gaseous products was collected every 5 min during the diphenyl urea synthesis. After 1 h of the synthesis reaction, 25 mg of the solid catalyst and product were removed from the reactor for analysis. Then, the reactor temperature was brought from room temperature to 438 K, and 20 cm3 of CO/O2 (10:1) was admitted into the reactor at 438 K for 1 h of carbamate synthesis. The product produced was purged from the reactor by flowing CO2 and collected in a 5-cm3 sample vial. 2.5. CO Adsorption Study. CO adsorption on NaIPd/C was carried out in a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell. The catalyst sample was first exposed to 0.2 cm3 of a
methanol/aniline mixture with a molar ratio of 1:42.7 and then to 30 cm3 of CO/O2 (10:1) at 298 K. 2.6. Reactant, Product, and Catalyst Analysis. The compositions of the gaseous and liquid products were determined by a Nicolet MAGNA 550 Series II transmission infrared (IR) spectrometer, a HewlettPackard 5890A gas chromatograph (GC) with a 6-ft × 1/ -in. 12% SE-30 packed column (Supelco Inc.), and a 8 Gemini 300 13C nuclear magnetic resonance (NMR) spectrometer. The mixture of liquid reactants and products was further analyzed by Bucksci 200A atomic absorption (AA) to determine the potential of the dissolution of Pd into the liquid reactant mixture during reaction. The solid diphenyl urea product and the solid sample were first diluted with potassium bromide (KBr) at a ratio of 1:10 for the DRIFTS analysis. Following the DRIFTS analysis, the product accumulated on the solid catalyst was removed from the catalyst surface by dissolving in methanol for quantitative GC analysis. The catalyst samples before and after the reaction were examined by an FEI-TACNAI 12 transmission microscope (TEM) with a 120-keV source to determine the effect of the reaction on the size of the Pd particles on the carbon surface. The compositions of the catalysts before and after reaction were determined by Galbraith Laboratory (Knoxville, TN), using inductively coupled plasma optic emission spectroscopy for Pd and Na analyses and the ion-selective electrode technique for iodide analysis. 3. Results and Discussion 3.1. Reaction Pathway. The objective of the reaction pathway study was to identify the effective pathways and conditions for carbamate synthesis on the NaIPd/C catalyst. The reaction temperature was selected on the basis of the optimum temperature for each reaction reported in the literature.6-8,17-19 Table 1 lists the yields of products produced during the reaction pathway studies on NaI-Pd/C. The carbamate yield is defined as the ratio of the number of moles of carbamate produced to the initial number of moles of aniline; the dimethyl carbonate yield is defined as the ratio of the number of moles of dimethyl carbonate produced to the initial number of moles of methanol. In pathway i, the NaI-Pd/C catalyst shows no activity for the synthesis of dimethyl carbonate at 403 K. This is in contrast to CuCl2 and Cu-Y zeolite catalysts, which are highly active for dimethyl carbonate synthesis from the oxidative carbonylation of methanol at 393 K.17 The NaI-Pd catalyst also exhibited little activity for the synthesis of carbamate from the reaction of dimethyl carbonate with aniline. The major products from this reaction are monomethylaniline (MA) from the methylation of aniline and
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1195
methyl benzoate (MB) possibly from CO insertion, oxidation, and esterification reactions.18 The lack of NaI-Pd/C activity for catalyzing the dimethyl carbonate synthesis and carbamate synthesis from the dimethyl carbonate/aniline reaction ruled out pathway i for the synthesis of carbamate from CO, O2, methanol, and aniline. The direct oxidative carbonylation, pathway ii, produced carbamate with a high yield (70%) at 438 K. Pathway iii, which consists of diphenyl urea synthesis at 373 K and the reaction of diphenyl urea with methanol at 438 K, resulted in a higher carbamate yield (82%) than pathway ii. The high overall carbamate yield and the decrease in the formation of MA and MB in pathway iii might be due to the depletion of aniline during diphenyl urea synthesis at 373 K. The low concentration of aniline might lead to the low rate of the reaction of methanol with aniline and CO at 438 K, resulting in the low overall yields of MA and MB for pathway iii. Analysis of the reactant and product mixture after diphenyl urea synthesis at 373 K showed that the aniline conversion was 74.8% and the diphenyl urea yield was 37.3% after 480 min. Diphenyl urea synthesis dominated at temperatures below 438 K regardless of the presence or absence of methanol. At temperatures above 438 K, diphenyl urea, which was formed from aniline, CO, and O2, reacted rapidly with methanol to produce carbamate. Although pathway iii gives the highest carbamate yield, the use of a two-stage operation at different temperatures could result in a significant increase in the overall cost of carbamate production. 3.2. Slurry versus Gas-Solid Carbamate Synthesis from the Direct Oxidative Carbonylation. Table 2 compares the product yields of the slurry (Figure 2a) and gas-solid (Figure 2b) carbamate syntheses from the direct oxidative carbonylation of aniline with methanol over NaI-Pd/C at 438 K in a batch autoclave. The slurry reaction produced carbamate as the major product, as well as monomethylaniline (MA) and methyl benzoate (MB) as byproducts. Product yield in Table 2 is defined as the ratio of the number of moles of product formed to the initial number of moles of aniline. In the gas-solid synthesis, the number of moles of products used in calculating the yield include those retained on the solid catalyst in the pouch and those in solution. Increasing the reaction time increased carbamate (MPC) yields; raising the CO/O2 pressure at a constant ratio further enhanced the carbamate yield in the slurry carbamate synthesis. The latter can be attributed to a significant increase in the solubility of O2 in the liquid methanol/aniline mixture. Figure 3 shows the infrared spectra of the NaI-Pd/C catalyst and liquid reactants and products after 120 min of the gas-solid carbamate synthesis in the autoclave. A comparison of the carbamate IR intensity on the solid catalyst and in the liquid samples after the reaction shows that the majority of the products, including carbamate, were in the liquid reactant/product mixture. The results suggest that carbamate produced from the gas-solid synthesis on the catalyst in the pouch was dissolved in methanol and dripped down with the liquid reactant/product mixture during cooling of the reactor. A comparison of reaction conditions and carbamate yields in Table 2 shows that the carbamate yield (MPC) increased from 5.5 in the slurry synthesis at 2.15 MPa to 74.8 in the gas-solid synthesis at 1.93 MPa. Changing from the liquid to the gaseous medium exposed the
Figure 3. IR spectra of fresh and used NaI-Pd/C and liquid product obtained from gas-solid carbamate synthesis in a batch reactor.
catalyst directly to high concentrations of gaseous CO and O2, thereby enhancing the carbamate (i.e., MPC) yields. Decreasing the total reaction pressure for the gas-solid carbamate synthesis caused a decrease in the carbamate yield. Decreasing the methanol concentration further decreased the carbamate yield and shifted the product toward MA. 3.3. Gas-Solid Diphenyl Urea and Carbamate Synthesis in a Fixed-Bed Reactor. Fixed-bed reactors exhibit a number of advantages: simplicity, separation of the catalysts from the reactants and products, large-scale operation, and flexibility of adding/withdrawing reactants/products. To determine the feasibility of a low-pressure fixed-bed carbamate synthesis process, a two-step carbamate synthesis via DPU, pathway iii, was carried out over NaI-Pd/C in a fixed-bed reactor (Figure 2c). Figure 4a shows the concentration of CO2 versus time during the batch diphenyl urea synthesis in the fixed-bed reactor at 373 K and 0.505 MPa. The results show that CO2 formation stopped due to depletion of O2 after 20 min of reaction. IR analysis of the solid sample after 1 h of the reaction, presented in Figure 4b, shows that diphenyl urea in the solid form is the major product at 373 K; decreasing the reaction pressure with a constant CO/O2 ratio resulted in a significant decrease in the diphenyl urea concentration on the solid catalyst. Figure 5 shows the IR spectra of the (a) liquid and (b) solid products of the carbamate synthesis at 438 K in the fixed-bed reactor. Carbamate was obtained by the reaction of diphenyl urea produced at 373 K with methanol, CO, and O2 at 438 K for 120 min. The carbonyl peaks of carbamate at 1710 and 1734 cm-1 were observed for both the liquid product and solid catalyst. The liquid product collected from the reactor exit gave a carbamate concentration of 0.086 mmol/mL and an MA concentration of 0.032 mmol/mL, corresponding to a carbamate yield of 15.3% and an MA yield of 5.6%. Table 3 summarizes the carbamate yields produced from the carbamate synthesis over Pd- and Cu-based cata-
1196 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004
Figure 4. Urea syntheses from the oxidative carbonylation of aniline over NaI-Pd/C at 373 K in a fixed-bed reactor: (a) concentration of CO2 versus time in the fixed-bed reactor and (b) IR spectra of fresh and used NaI-Pd/C at 0.202 and 0.505 MPa. Table 3. Carbamate Yields from the Gas-Solid Oxidative Carbonylation
catalyst 5.0 wt % CuCl2/ HZSM-5 4.7 wt % CuCl24.4 wt % PdCl2/ HZSM-5 4.4 wt % PdCl2/ HZSM-5 5.0 wt % NaIPd/C a
Figure 5. IR spectra of liquid products and solid catalyst obtained from carbamate synthesis over NaI-Pd/C in a fixed-bed reactor at 438 K.
lysts in the fixed-bed reactor. NaI-Pd/C exhibited the highest carbamate yield among the catalysts tested. Although PdCl2 and CuCl2 exhibited good activities for
reaction conditions
aniline time conversion carbamate (min) (%) yield (%)
1.156 MPa, 438 K
60
43.5a
2.0a
1.020 MPa, 438 K
60
9.7a
4.4a
1.170 MPa, 438 K
60
6.3a
1.8a
0.505 MPa, 438 K
120
27.2
15.3
From ref 26.
carbamate synthesis in the liquid phase, these catalysts exhibited significantly lower activities than NaI-Pd/ C. Figure 6 shows the TEM micrographs of NaI-Pd/C before and after reactions in the slurry, gas-solid, and fixed-bed modes. The Pd particle size on the fresh catalyst surface falls in the 5-10-nm range, with an average particle size (∑nidi/∑ni, where ni is the number of particles of type i and di is the diameter of particles of type i) of 6 nm from 30 counts, as shown in Figure 6a. Carbamate synthesis in the slurry phase caused agglomeration of the Pd particles, bringing the size of the Pd particles from 5-10 to 60-200 nm (average ) 108 nm), as shown in Figure 6b. Sintering of the Pd particles led to a decrease in the number of Pd active sites, resulting in a low reaction rate. Sintering of the Pd particles is dramatically reduced for the syntheses in the gas-solid and fixed-bed mode. Parts c and d of Figure 6 show that the size of the Pd particles after the gas-solid synthesis lies in the 20-40-nm range, with
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1197
Figure 6. TEM micrographs of NaI-Pd/C before and after oxidative carbonylation reaction in different modes: (a) before reaction, (b) after slurry reaction, (c) after gas-solid reaction, and (d) after reaction in a fixed-bed reactor.
an average at 30 nm; after the synthesis in the fixed bed, the size lies in the 15-30-nm range, with an average of 16 nm. Elemental analysis of NaI-Pd/C catalysts before and after the reaction showed that there was no loss of Pd from the catalyst during the slurry synthesis in the autoclave and gas-solid synthesis in the fixed-bed reactor. AA analysis of the liquid reactant/product mixture confirmed that the Pd on the carbon surface did not dissolve into the liquid medium. The Na and I compositions also remained the same before and after the gas-solid synthesis. In contrast, the composition of iodide on the catalyst dropped from 7 to 0.02% after slurry carbamate synthesis. The loss of NaI is a result of its high solubility in methanol. Because NaI is an essential promoter, the absence of intimate contact between Pd and NaI will significantly decrease the catalyst activity for carbamate synthesis. 3.4. Infrared Study of CO Adsorption. CO adsorption was used to probe the nature of the Pd surface in the presence of the reactant mixture. Knowledge of the nature of Pd site in the reactant environment can assist in elucidating the gas-solid carbamate synthesis mechanism. The CO adsorption study was conducted by exposing the catalyst to a methanol/aniline mixture at a molar ratio of 42.7 in the presence of 0.101 MPa CO/
O2 (10:1), which corresponds to the reactant molar ratio used for carbamate synthesis at 298 K. Figure 7a shows the IR spectrum of adsorbed CO, methanol, and aniline. The aniline gave an intense N-H bending at 1600 cm-1 and an aromatic carbon-carbon stretching at 1500 cm-1; methanol showed O-H bending at 1450 cm-1. These intense bands are due to liquid methanol and aniline and overlapped with the characteristic bands of adsorbed methanol and aniline. Figure 7b enlarges the region showing the IR spectra of adsorbed CO on NaIPd/C during CO adsorption in the presence of methanol, aniline, and O2. CO2 emerged along with linear CO on Pd0 at 2048 cm-1.20 The formation of linear CO on Pd0 revealed that the catalyst surface was reduced to Pd0 by converting CO to CO2. The absence of any significant variation in the wavenumber of the linear CO band with its intensity is due to the lack of dipole-dipole coupling between neighboring adsorbed CO molecules.21 These results suggest that the Pd0 sites might be in isolated form, separated by NaI promoters. 3.5. Reaction Mechanism. Although the mechanism of carbamate synthesis is still not well understood, it is instructive to use the proposed mechanism to shed light on the possible reaction pathways and the nature of the sites for the reaction on the catalyst surface. The mechanism of the slurry carbamate synthesis proposed
1198 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004
Figure 7. CO adsorption over NaI-Pd/C with O2, methanol, and aniline: (a) full-range IR spectrum and (b) IR spectrum of adsorbed CO at 348 K with respect to time.
Figure 8. Proposed mechanism of carbamate synthesis from oxidative carbonylation over NaI-Pd/C.
by Fukuoka et al. and Chaudhari’s group8,9 consists of (i) the formation of the active species Pd+I-, (ii) the dehydrogenation of ArNH2 (Ar ) aromatic group), (iii) the insertion of CO into Pd-ArNH to form a carbamoyl species, (iv) the reaction of the carbamoyl species with methanol, and (v) the reoxidation of the Pd sites to regenerate the initial Pd+I- species, as shown in Figure 8. Some of these proposed steps for the slurry carbamate synthesis can be integrated with the related reaction steps on the surfaces and our experimental observations to gain a better understanding of the reaction pathway and the nature of the active sites for carbamate synthesis on the NaI-Pd/C catalyst surface. Figure 8 shows the proposed mechanism for carbamate synthesis on the surface of NaI-Pd/C. The Pd sites depicted in Figure 8 consist of the surface of Pd crystallites that are decorated with Pd+I- and Na+. The proposed catalyst structure is based on the TEM observations of Pd crystallites and the presence of high concentrations of NaI on the catalyst determined by elemental analysis. These Na+ and I- species appear to be uniformly distributed on the Pd surface, giving isolated Pd0 sites that chemisorb CO as linear CO at 2048 cm-1. The conjecture of the presence of the isolated Pd0 sites is based on the lack of dipole-dipole interac-
tions between neighboring adsorbed CO molecules, as shown in Figure 7b. Although no studies on the oxidative dehydrogenation activity of Pd0 for methanol and aniline have been reported, the surfaces of Pd0, Au0, Ag0, and Cu0 have been shown to exhibit Bronsted basicity to oxidatively dehydrogenate formic acid and formaldehyde;22,23 Au, Ag, and Cu surfaces covered with adsorbed oxygen have been found to exhibit activity for converting methanol to methoxy; and Cu and Ag with adsorbed oxygen have also been shown to activate N-H bonding.24 The results of these studies suggest that Pd0 sites might play a role in assisting the dehydrogenation of aniline and methanol to produce ArNH and CH3O species, as shown in steps vi and vii in Figure 8. The presence of Na+ could also further enhance the basic environment to enhance dehydrogenation on the Pd0 sites. The Pd0-OCH3 and Pd0-NHAr species produced might further interact with I- to produce Pd+-OCH3 and Pd+-NHAr, respectively. The insertion of CO into Pd-ArNH might occur on Pd+ sites (step iii), rather than on Pd0 sites, because oxidized metal sites are known to be more active for catalyzing CO insertion than reduced metal sites.25 The presence of I- could further stabilize the Pd+ sites to enhance CO insertion. Although Pd+ species might
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catalyze the carbamate synthesis as in the homogeneous or slurry-phase syntheses,6-8 the synergistic interaction between the potential dehydrogenation role of the Pd0 sites and the CO insertion activity of Pd+I- might lead to a significant enhancement in the carbamate synthesis activity on NaI-Pd/C during the gas-solid oxidative carbonylation. 4. Conclusion This study demonstrated that the direct oxidative carbonylation of aniline with methanol is an effective pathway for the synthesis of carbamate from CO, O2, methanol, and aniline. The gas-solid carbamate synthesis process slowed the sintering of Pd particles on the carbon support; retained NaI on the Pd/C surface; allowed all of the reactants to directly access the catalyst surface; and eliminated the limitation of the low solubilities of O2 and CO in the liquid methanol/aniline reactant mixture, resulting in a significant increase in the carbamate yield as compared to the slurry-phase synthesis. Combining these advantages with the intrinsic merit of ease of catalyst recovery, oxidative carbonylation over NaI-Pd/C in a gas-solid fixed-bed reactor could provide an effective pathway for carbamate and isocyanate syntheses. Acknowledgment Support for this work was provided by NSF Grant CTS 9816954 and Ohio Board of Regents Grant R5538. Literature Cited (1) Green Chemistry: Frontiers in Benign Chemical Synthesis and Processes; Anastas, P. T., Williamson T. C., Eds.; Oxford University Press: Oxford, U.K., 1998. (2) Chuang, S. C.; Toochinda, P.; Konduru, M. Low-Pressure Oxidative Carbonylation of Aniline with Pd/C and Ag-Pd/C. In Green Engineering; Anastas, P. T., Heine, L. G., Williamson T. C., Eds.; ACS Symposium Series 766; American Chemical Society: Washington, DC, 2001; p 136. (3) Weissermel, K.; Arpe, H. Industrial Organic Chemistry, 2nd ed.: VCH Publishers: Weinheim, Germany, 1993. (4) Cenini, S.; Pizzotti, M.; Crotti, C. In Aspect of Homogeneous Catalysis: A Series of Advances; Ugo, R., Ed.; D. Reidel Publishing: Dordrecht, The Netherlands, 1998; Vol. 6, pp 97-198. (5) Parshall, G. W.; Ittel, S. D. Homogeneous CatalysissThe Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.; John Wiley and Sons Publishers: New York, 1992. (6) Lin, I. J. B.; Chang, C. S. Palladium-catalyzed formatenitrobenzene-carbon monoxide reaction: Formation of carbamate ester. J. Mol. Catal. 1992, 73, 167. (7) Chen, B.; Chuang, S. C. CuCl2-PdCl2 catalyst for oxidative carbonylation of aniline with methanol. J. Mol. Catal. 2003, 195 (122), 37.
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Received for review July 9, 2003 Revised manuscript received October 27, 2003 Accepted November 5, 2003 IE0305768
