Article pubs.acs.org/IECR
Hydrogenation of Styrene Oxide to 2‑Phenyl Ethanol over Polyurea Microencapsulated Mono- and Bimetallic Nanocatalysts: Activity, Selectivity, and Kinetic Modeling Ganapati D. Yadav* and Yuvraj S. Lawate Department of Chemical Engineering, Institute of Chemical Technology+, Matunga, Mumbai-400 019, India ABSTRACT: Styrene oxide is an important precursor to prepare 2-phenyl ethanol (2-PEA) of high quality for its use as a fragrance chemical. Styrene oxide was selectively hydrogenated to 2-PEA with polyurea-microencapsulated (EnCat) Pd, Pd−Rh and Pd−Cu nanocatalysts with methanol as solvent and NaOH as promoter. The catalysts were fully characterized to understand their activity, selectivity, and stability. Pd−Cu EnCat was used for further studies. Complete conversion of styrene oxide with 92% selectivity to 2-PEA was obtained over Pd−Cu EnCat at 333 K and 607.8 kPa. Effects of various parameters were studied to understand the product profile. A bifunctional Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanistic model was proposed. The kinetics of reaction was established and tested against experimental data. The results are new and provide a green and cost-effective process for the synthesis of 2-PEA. the hydrogenation of styrene oxide to 2-phenyl-ethanol.10 However, a variety of byproducts are formed including phenylacetaldehyde, 2-cyclohexylethanol, styrene, ethylbenzene, 1-PEA, methoxymethylbenzene and cyclohexylethane.6−10 The product distribution is governed by the properties of solvent, nature of catalyst, concentration of styrene oxide, temperature, pressure, and type of reactor. The removal of the byproducts from 2-PEA becomes energy intensive, and the presence of even trace impurities is detrimental to the quality of 2-PEA required for perfumery applications. The application of heterogeneous catalysis for the production of fine chemicals and pharmaceuticals is important, particularly with increasing environmental awareness. Especially hydrogenation reactions using mono, bi-, and multimetal catalysts supported on inorganic and polymeric supports are valuable in pharmaceutical and specialty chemical industry.11−14 The nature, pore size distribution, and surface area of the support play a vital role in the activity and selectivity of heterogeneous catalysts. A variety of high-surface-area supports such as silica, alumina, calcium carbonate, barium sulfate, powdered KF with Al2O3 and poly(ethyleneimine) on alumina/silica have been used for supporting transition-metals. Supported transition metal (homogeneous) catalysts are produced by coordination of the metal to an immobilized ligand. Entrapment of homogeneous catalysts within a polymeric shell was developed by Kobayashi and co-workers.15,16 Microcapsules of a polymercoated catalyst are formed upon cooling a homogeneous solution of the catalyst and a polymer or copolymer. Various other methods and materials have been delineated in the published literature for entrapping homogeneous metal complexes and metal nanoclusters including sol−gel materials,
1. INTRODUCTION 2-Phenyl ethanol (2-PEA) is a valuable chemical having a wide range of applications. It is a colorless liquid with a faint but lasting odor of rose petals. Because of this property, 2-PEA is recognized as an important ingredient of perfumes, deodorants, soaps, and detergents.1,2 2-PEA possesses bacteriostatic and antifungicidal properties and hence is employed in the preparation of antiseptic creams and deodorants. It is extensively used in cosmetics, and also in hair shampoos and dyes to improve the texture and quality of hair.3−5 It is a precursor to manufacture some important chemicals such as phenylacetaldehyde, phenyl acetic acid, and benzoic acid.6 2-PEA is synthesized by various chemical routes. One of the most frequently used routes is the Friedel−Crafts alkylation of benzene using ethylene oxide and AlCl3 as catalyst.2 AlCl3 is used in far excess over stoichiometric amounts of ethylene oxide, adding significantly to the cost of raw materials. Major disadvantages of Friedel−Crafts homogeneous catalysts are that they are highly corrosive, polluting, nonreusable, and expensive. Thus, special corrosion-resistant equipment is needed. The cost of production is substantially increased due to the posttreatment processes such as destruction of the catalyst from reaction mass by neutralization and further purification of the crude 2-PEA by distillation. In another industrial route, 2-PEA is prepared by Grignard reaction of chlorobenzene with ethylene oxide, in which chlorobenzene, is converted to phenylmagnesium chloride, which reacts with ethylene oxide to give phenylethoxy magnesium chloride followed by its decomposition in sulfuric acid. The drawbacks of the Grignard process are the use of hazardous and explosive phenylethoxy magnesium chloride, high level of pollution, effluent treatment cost, and poor quality of 2-PEA.1 In the context of Green Chemistry, styrene oxide is viewed as a very attractive starting material for synthesizing 2-PEA. Various homogeneous6 and heterogeneous catalysts7−9 have been used for the hydrogenation of styrene oxide to 2-PEA. Recently, a copper catalyst prepared using a silica zirconia support gave up to 80% yield in © 2013 American Chemical Society
Received: Revised: Accepted: Published: 4027
September 23, 2012 January 26, 2013 February 20, 2013 February 20, 2013 dx.doi.org/10.1021/ie302587j | Ind. Eng. Chem. Res. 2013, 52, 4027−4039
Industrial & Engineering Chemistry Research
Article
was reduced in hydrogen atmosphere using ethanol as a solvent for 4 h at 1.515 MPa pressure and 373 K. Reduction of the catalyst was visible by the color change from pale yellow or brown to black beads of polymers. It was stored in methanol. The catalyst was dried at 388 K after reduction under inert atmosphere before use. With reference to the material balance on metal trapped in the polymer, the yield is 98%. Catalyst Characterization. The catalyst characterization studies included the following: determination of crystalline nature of polymer by X-ray diffraction (XRD), determination of adsorbed species by Fourier transform infrared spectroscopy (FTIR), textural determination like physical appearance of encapsulated species by scanning electron microscopy (SEM), and surface elemental analysis by energy dispersive X- ray spectroscopy (EDXS). 2.2.1. Surface Area Analysis. The sample was prepared under high vacuum at 423 K for 3 h, and then the surface area measurement was done by liquid nitrogen adsorption on a Micromeritics ASAP 2010 instrument at 77 K. 2.2.2. X-ray Diffraction (XRD). The crystalline nature of the catalyst was analyzed with X-ray diffraction (XRD) technique using a Bruker AXA-D8 Advance instrument with Cu Kα radiation (λ = 1.540562). 2.2.3. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS). Surface morphology and elemental composition of encapsulated catalysts were taken on JEOL-JSM 6380 LA instrument. Prior to SEM analysis, the catalyst samples were put on a carbon film and platinum coating was done (sputtering) so as to make the sample surface anodic or conductive for electron. The samples were scanned at various magnifications. 2.2.4. Fourier Transform Infrared Spectroscopy (FTIR). Infrared spectra of the samples pressed in KBr pellets were obtained at a resolution of 2 cm−1 between 4000 and 400 cm−1. These spectra were collected with a Perkin-Elmer instrument and in each case the sample was referenced against a blank KBr pellet. 2.3. Hydrogenation Experiments. Hydrogenation of styrene oxide was carried out in a 4 cm i.d. 100 mL autoclave manufactured by Amar Autoclaves, Mumbai. The autoclave was equipped with a standard four-bladed pitched-turbine impeller for agitation. The autoclave was charged with the reaction mixture containing styrene oxide, methanol, n-decane as internal standard (I.S.), and the catalyst. The reactor was flushed first with nitrogen and then with hydrogen. The temperature was raised and maintained at ±1 °C of the set value with the help of an in-built proportional-integralderivative (PID) controller. Once the temperature had reached the desired value, the autoclave was pressurized with hydrogen to the desired pressure level and agitation was started. A constant pressure was maintained throughout the reaction by means of the mass flow controller (MFC). The analysis was done by using GC (model 8510, Chemito Instruments, Mumbai). A 4 m × 3.25 mm internal diameter stainless steel column packed with 10% OV-17 on chromosorb WHP was used with a flame ionization detector (FID).
dendrimers, and polyoxyalkylene resins. Microencapsulated metal catalysts prepared by interfacial polymerization have a potential to be one of the most useful techniques for immobilizing homogeneous catalysts. Consequently, the preparation of polyurea-microencapsulated metal catalysts appears interesting for redox reactions.15−18 Styrene oxide was thus thought to be an excellent precursor to synthesize 2-PEA using polyurea-microencapsulated metal catalysts. The current work is thus concerned with selective production of 2-PEA by hydrogenation of styrene oxide using novel polyurea-encapsulated Pd, Rh, and Cu nanocatalysts. These catalysts have been fully characterized. A bifunctional Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanistic model is proposed. The kinetics of reaction is established and tested against experimental data. The results are new and provide a green and cost-effective process for the synthesis of 2PEA.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Styrene oxide was obtained from Cinch Chemicals Private Limited, Mumbai, India. Methanol, ethylene diamine (EDA), diethylene triamine (DETA), toluene, toluene diisocynate (TDI), sodium dodeca-sulfonate, polyethylene glycol (PEG 400), ruthenium trichoride and copper acetate were obtained from M/s s.d. Fine Chemical Ltd., Mumbai. Palladium acetate was obtained from M/s. Loba Chemie Pvt. Ltd., Mumbai. All chemicals were of analytical grade and used without further purification. Hydrogen cylinder was obtained from M/s Industrial Oxygen Co. Ltd., Mumbai. 2.2. Catalyst Preparation. The catalysts were synthesized by the interfacial polymerization method17,18 which was further developed in our laboratory for polyurea encapsulated catalysts, namely, Pd (Pd EnCat), Pd−Ru (Pd−Ru EnCat) and Pd−Cu (Pd−Cu EnCat).19,20 Pd−Cu EnCat was synthesized as follows: Palladium acetate (Pd(OAc)2) (0.1g) was dissolved in toluene (2 mL) and stirred for 45 min at 343 K. Aliquat 336 (tricaprylmethylammonium chloride or methyltrioctylammonium chloride, C25H54ClN) was added as a surfactant. After palladium acetate was completely dissolved, toluene diisocynate (TDI) (3.66 g) was added and the solution was stirred for 2 h to make it homogeneous. Aqueous solution of 20 mL deionized water containing copper acetate monohydrate (0.15 g) was prepared in which sodium dodecasulfonate (SDS) (72 mg) and PEG 400 (50 mg) were added. To this solution ethylene diamine (0.68 g) and diethylene triamine (0.72 g) (1:1 molar ratio) were added. Amines are used to increase cross-linking and subsequently functional ligation of the metal. Increasing the amine number (i.e., diamine, triamine, pentaamine, etc.) increases the crosslinking which in turn increases the mechanical strength of the polymer beads.17 The aqueous solution was homogenized by stirring for 4 h. The above solution containing palladium acetate in toluene was added into the aqueous solution containing copper acetate drop by drop for 1 h while maintaining the temperature below 278 K. This ensured that no polymerization occurred before the stable oil-in-water microemulsion was formed. The mixture was stirred for 10 min to get a stable microemulsion of oil-in-water of a proper drop size. The temperature was then increased and maintained at 318 K and the resulting oil-in-water emulsion was stirred vigorously for 16 h. It formed the encapsulated catalyst which was filtered and washed thrice with 100 mL solvent using the following sequence: water, ethanol, and hexane. The catalyst
3. RESULTS AND DISCUSSION 3.1. Efficacy of Various Catalysts. Scheme 1 lists four possible reaction products. Styrene oxide upon hydrogenation by reactions a, b, and c leads to 2-PEA, 1-PEA, and methoxy methyl benzene, respectively. Isomerization of styrene oxide (reaction d) gives phenyl acetaldehyde which upon subsequent 4028
dx.doi.org/10.1021/ie302587j | Ind. Eng. Chem. Res. 2013, 52, 4027−4039
Industrial & Engineering Chemistry Research
Article
Scheme 1. Possible Products of Hydrogenation of Styrene Oxide
Figure 1. Effect of various catalysts on selectivity and conversion in methanol as a solvent. Reaction conditions: temperature, 333 K; H2 pressure, 607.8 kPa; reaction volume, 67 mL; catalyst loading, 0.45 kg/ m3; concentration of styrene oxide, 0.65 kmol/m3; speed of agitation, 16.67 rps. Selectivity (%), blue; conversion (%), red.
Table 1. EDXS Surface Analysis of Polyurea Encapsulated Catalysts catalyst
C
N
O
Pd
Ru
Cu
PdEnCat Pd-RuEnCat Pd-CuEnCat
68.13 61.24 65.39
18.82 16.89 17.40
3.61 10.11 5.21
3.98 2.82 9.74
0 8.94 0
0 0 2.26
proposed for the hydrogenation of the styrene oxide to 2-PEA.4 However, Rode et al.6 further studied hydrogenation of styrene oxide to 2-PEA over charcoal-supported 5% w/w Pd and 5% w/ w Pt in different solvents, where the order of activities was heptane < supercritical CO2 < methanol (H2 pressure, 3 MPa; CO2 pressure, 10 MPa; temperature, 323 K). The conversion of styrene oxide was 73.7% and selectivity to 2-PEA was 66% in supercritical CO2 in 15 min. The formation of dehydroxylated byproducts was suppressed in supercritical carbon dioxide. Negative carbon dioxide pressure effect was observed over the 5% w/w Pd/C. The superior activity of Pd−Cu EnCat to Pd EnCat can be explained. It is well-known that bimetallic catalysts often show different and superior properties from the corresponding monometallic catalysts due to several reasons such as smaller particle sizes of bimetallic catalysts, modifications of the electronic structure through a ligand effect, different geometry of the bimetallic structure, and better ability of the bimetallic surface to bind with adsorbates.21 In the current case, both Pt− Cu and Pt−Ru particles were smaller than the corresponding monometals, thereby increasing the particle surface area and active sites. All catalysts were nanometric in size, loaded on a porous polymer matrix. These were reduced under hydrogen atmosphere. Pd−Ru EnCat was found to be more active in comparison with Pd−Cu EnCat. However, it consisted of two precious metals Pd and Ru, making it quite expensive. On the other hand Pd−Cu Encat was cheaper and offered better selectivity to 2-PEA as compared to the other two catalysts. Therefore, Pd−Cu EnCat was selected for further study. To understand the activity of Pd−Cu EnCat, it was further characterized vis-à-vis others and only some salient studies are discussed here. 3.2. Catalyst Characterization. 3.2.1. X-ray Diffraction (XRD). The crystallographic structure of polyurea encapsulated catalysts was determined by XRD. One diffraction peak in the
hydrogenation (reaction e) produces 2-PEA. The product profile is a strong function of the following variables: nature of catalyst; pore size distribution; distribution of acid, base, and metal sites on pore surface; type of support; type of solvent; concentration of styrene oxide; pressure; temperature; type of promoter and its concentration. On the basis of our previous studies,19,20 it was decided to use polyurea-encapsulated monometallic and bimetallic catalysts, since several authors have used Pd/C and Pd/MgO type catalysts and their findings could be used for comparison. Three polyurea encapsulated catalysts were synthesized such as Pd EnCat, Pd−Ru EnCat and Pd−Cu EnCat and evaluated for their performance in the hydrogenation of styrene oxide. The conversion and selectivity of 2-PEA were compared under identical conditions: styrene oxide concentration, 0.65 kmol/ m3; catalyst loading, 0.45 kg/m3; speed of agitation, 16.67 rps; solvent, methanol; temperature, 333 K; H2 pressure, 607.8 kPa (Figure 1). All three catalysts were found to give 100% conversion of styrene oxide in 1 h and showed good selectivity to 2-PEA; the selectivity was 51, 53, and 65% with Pd EnCat, Pd−Ru EnCat, and Pd−Cu Encat, respectively (Table 1). The selective hydrogenation of styrene oxide to 2-PEA has been investigated using different catalysts and supports by Rode et al.5 who reported that 1% w/w Pd/C was the best catalyst, under milder temperature (313 K) and pressure (2.048 MPa). 2-PEA was selectively formed when alkali was used as a promoter. A plausible mechanistic pathway has also been 4029
dx.doi.org/10.1021/ie302587j | Ind. Eng. Chem. Res. 2013, 52, 4027−4039
Industrial & Engineering Chemistry Research
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Figure 2. XRD of polyurea encapsulated palladium copper catalyst, Pd−Cu EnCat.
Figure 3. FTIR of polyurea encapsulated palladium catalyst, Pd EnCat.
low angle region (2θ = 14−30°) for Pd−Cu EnCat is visible indicating that a long chain carbon was formed (Figure 2). These peaks also indicate that polyurea has long-range ordering. The XRD pattern shows that polyurea has a semicrystalline structure. It is postulated that when polymer molecules precipitate out at high rates, they do not get sufficient time to arrange themselves in an ordered lattice.22,23 So the polyurea formed at a higher rate of reaction shows less crystalline structure than that formed at a lower rate of reaction. However, it is possible to alter the structure of these polymer films by varying the conditions. Pd−Ru EnCat shows amorphous structure. When Ru is infused in the polyurea matrix, it distorts the matrix and thus does not lead to the ordered structure of the polymer. The average size of metal crystallites could be estimated by XRD. However, the determination of crystallite size for catalysts containing less than 0.5% metal or for very small metal particles (
