Effect of Supercritical CO2 as Reaction Medium for Selective Hydrogenation of Acetophenone to 1-Phenylethanol.

1-Phenylethanol (PhE) is widely employed in the pharmaceutical industry as an anti-inflammatory and analgesic drug, as well as in chewing gums and yogurts as a food additive. In this work, we have investigated the selective synthesis of 1-phenylethanol (PhE) by hydrogenation of acetophenone using supercritical CO2 as a solvent. Supercritical carbon dioxide (scCO2) replaces organic solvent because it is inexpensive, nontoxic, nonflammable, inert, and environmentally benign. Polyurea-based encapsulated mono- and bimetallic catalysts were synthesized and characterized using different characterization techniques. The effects of various reaction parameters, such as co-solvent, catalyst loading, hydrogen pressure, total scCO2 pressure, and temperature, were studied to determine the reaction kinetics.

Introduction

In the past few decades, supercritical fluids (SCFs) have attracted considerable attention as reaction media. SCFs are some is of the new types or classes of fluids whose properties lie in between gases and liquids and which can be manipulated very easily by altering the pressure and temperature. This fluid medium has the capability to control the reaction environment with precision and to tune the selectivity. Among all SCFs, supercritical carbon dioxide (scCO2) has received special attention because organic solvents in the reaction can be easily replaced with scCO2 as it is nontoxic, nonflammable, inexpensive, and easy to dispose and recycle. It can also serve as a nonpolar solvent by manipulating its solvent strength over a wide range of polarities.[1−3] Reactions such as hydroformylation and hydrogenation, which are diffusion-limited in the normal solvent phase, can be boosted in the scCO2[4,5] because diffusivities in it are higher and viscosities are lower compared to conventional solvents.

The selective reduction of carbonyl group by means of heterogeneous catalysts is an essential step in industrial organic chemistry.[6,7] Acetophenone (AP) is one of the interesting compounds because its hydrogenation pathway gives a variety of products and byproducts due to the consecutive and competitive hydrogenation of the aromatic ring and the carbonyl function. Thus, it has become an ideal model compound for chemoselective hydrogenation reactions. 1-Phenylethanol (PhE) is one of the desired products used in pharmaceuticals as an intermediate compound for anti-inflammatory and analgesic drugs,[8] whereas in the food industry, it is used as additives in chewing gums and yogurts because of its typical strawberry scent.[9]

The main reason behind selecting this catalytic reduction is not only because aromatic keto compounds are essential in the synthesis of the pharmaceuticals, fine chemicals, and cosmetics industries, but also because the complex reaction pattern of this multistep reaction can be a useful tool to test the selectivity and activity properties of a catalytic system. Many heterogeneous catalysts employing Ru, Pd, Cu, Pt, Ni, and Ti metal catalysts are reported at a wide range of operating temperatures and pressures, with many of them involving “standard” supported noble metal catalysts in organic solvents.[10−14]

In many drug synthesis processes, hydrogenation is the final step, where contamination of the final product by catalyst leaching or traces of solvent is very challenging. The catalyst used for hydrogenation contains toxic metals, which are harmful to human health, and the very high grade of end purity required for pharmaceutical drugs would require an expensive and lengthy purification of the final product. To overcome the above difficulty of drug contamination by leached catalyst and/or solvents and to design a benign medium to synthesize drugs in compliance with the principles of green chemistry,[15] synthesis processes based on SCFs and ionic liquids as green solvents for reactions[16,17] and extractions[18] are emerging as a substitute for the replacement of standard volatile organic compounds.[19,20]

Microencapsulation is the method of entrapping a metal or material into a polymeric coating or shell.[21,22] Recently, the use of microencapsulation in catalyst has been exploited.[23,24] Generally, in a transition-metal-supported catalyst, the metal is coordinated to a ligand, which is covalently bound to a polymeric backbone or sometimes adsorbed on an inert surface, such as carbon or silica. These catalyst systems are very often referred to as “supported” or “polymer-anchored” catalysts. These polymer-anchored catalysts are produced by copolymerization method, which is very tedious, lengthy, and expensive. The use of interfacial microencapsulation to immobilize homogeneous catalysts was reported by Ley and co-workers,[25] suggesting that it might solve the challenging limitations of previous approaches. In this method, microcapsules are prepared by an in situ interfacial polymerization method, which involves a dispersion of organic phase containing polyfunctional monomers/oligomers (along with the material to be encapsulated) into an aqueous phase, which contains a mixture of emulsifiers and protective colloid stabilizers. This gives oil-in-water emulsion, which undergoes in situ interfacial polymerization, with the monomers/oligomers reacting spontaneously at the phase interface to form microcapsule walls. One can easily tune the coordinating properties of the polymer material and the permeability and size of these microcapsules by varying other reagents, the nature of monomers/oligomers, and the encapsulation conditions used in the procedure. For efficient entrapment of transition-metal-based catalysts, we need to design systems having ligating functionality to retain the metal species.[26] These systems should be chemically inert and physically robust to reaction environment although also being economical. The presence of a backbone of urea functionality in polyurea microcapsules was found to be a suitable chemical structure to ligate and retain metal species (e.g., Pd(OAc)2).[21] The polyurea-encapsulated catalyst was used in reductive Heck coupling,[21] Suzuki reactions,[21,27] hydrogenation,[28] transfer hydrogenation,[29,30] ring-opening epoxidation, and nitroreduction.[31] Polyurea-encapsulated catalysts have good leaching resistance in reaction media due to their ability to ligate to the catalyst metal.[21,27,31]

This work deals with the synthesis and characterization of polyurea-encapsulated mono- and bimetallic catalysts. This work is also delved with a detailed hydrogenation of acetophenone in the presence of an encapsulated catalyst using scCO2 as the reaction medium. Thus, the idea of using scCO2 as solvent came forward, keeping enhancement of conversion and selectivity in mind. Experimental results were acquired by varying the operational conditions (catalyst, temperature, H2 pressure, CO2 pressure) and using three different co-solvents (methanol, ethanol, and 2-propanol). Results were inferred by kinetic modeling.

Experimental Section

Chemicals

Acetophenone, methanol, ethanol, 2-propanol, toluene, hexane, ruthenium chloride hydrate, copper acetate, toluene diisocyanate, sodium dodecasulfonate (SDS), ethylene diamine, diethylene triamine, and poly(ethylene glycol) (PEG) 400 were obtained from SD Fine Chemicals Ltd., Mumbai, India. Pd(OAc)2 was procured from Monarch Catalyst Pvt. Ltd., Mumbai, India. All chemicals used in this study were analytical-grade reagents and used without further purification. Hydrogen and liquid CO2 cylinder were obtained from Industrial Oxygen Co., Ltd., Mumbai, India.

Catalyst Synthesis

The catalysts were synthesized by the interfacial polymerization method[31,32] and further modified in our laboratory for polyurea-based encapsulated catalysts, namely, Pd(PdEnCat), Pd-Ru(Pd-RuEnCat), and Pd-Cu(Pd-CuEnCat).[33,34]

The synthesis of polyuria-based encapsulated catalyst involves two steps: making organic-phase and aqueous-phase solutions. Organic-phase solution was prepared by dissolving palladium acetate (0.1 g) in toluene (2 mL) at 70 °C under stirring, followed by addition of surfactant Aliquat 336. Once dissolution of palladium acetate was complete, toluene diisocyanate (3.66 gm) was added and the solution stirred to make it homogeneous. The aqueous-phase solution was prepared by dissolving 72 mg of sodium dodecasulfonate (SDS), 50 mg of PEG 400, and 0.15 g of copper acetate monohydrate in deionized water (20 mL). Further, diethylene triamine (0.72 g) and ethylene diamine (0.68 g) were added in 1:1 molar ratio to this solution and stirred until homogenized. Amines are used to increase the cross-linking and functionalize the ligation of the metal. This increase in cross-linking due to higher amines gives mechanical properties to the polymeric beads.[31] The organic-phase solution of palladium was added dropwise over the aqueous-phase solution of copper at a temperature below 5 °C to prevent polymerization reaction and to form stable oil-in-water microemulsion of proper size. Once microemulsion was formed, the temperature was increased to 45 °C and the emulsion was kept under vigorous stirring for 16 h. The encapsulated catalyst was formed, which was then filtered and subsequently washed with water, ethanol, and hexane three times. Catalyst reduction was done in the reactor at 15 bar hydrogen pressure and 100 °C for 4 h with ethanol as solvent. The yield of the catalyst was 98% based on metal encapsulated in the polymeric bead.

Catalyst Characterization

The catalyst was characterized using various techniques, such as X-ray diffraction (XRD) for the determination of crystalline nature of the polymer, Fourier transform infrared (FTIR) spectroscopy for the determination of adsorbed species, scanning electron microscopy (SEM) for textural determination, and physical appearance of catalyst surface and energy-dispersive X-ray spectroscopy (EDXS) for surface elemental analysis.

Infrared spectra of the samples were recorded on a PerkinElmer instrument with reference to blank potassium bromide pellet in each sample. These spectra were collected at 2 cm–1 resolution and between 400 and 4000 cm–1 wavelengths.

XRD patterns of the catalyst recorded on a Bruker instrument (λ = 0.1541 nm) at room temperature were used to interpret the crystalline nature of the catalyst. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy data for catalyst were obtained on a JEOL JSM-6380LA microscope operated at 10 kV, 1 nm resolution, and 2.6–8 mm working distance.

Catalyst pore size distribution, pore volume, and surface area were determined by multipoint Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods on Micromeritics (ASAP-2010) instrument by nitrogen adsorption–desorption. The catalyst sample was degassed at 150 °C, and analysis was performed in liquid nitrogen at a temperature of −196 °C.

Reaction Scheme

Reaction network for acetophenone hydrogenation is presented in Scheme .

Scheme 1

Reaction Network for Acetophenone Hydrogenation. Acetophenone (AP); 1-Phenylethanol (PhE); Cyclohexylmethylketone (CHMK); Ethylbenzene (EB); 1-Cyclohexylethanol (CHE)

Experimental Setup

Reactions under scCO2 condition were performed in a specially designed high-pressure reactor (Figure A) of 100 cm3 capacity from Thar Technologies, Inc. The reactor was provided with a magnetic stirrer assembly with an impeller and a speed controller. The process control package included a process controller, electrical cartridge heaters, a pressure transducer, a thermocouple, and an electrical enclosure box mounted on the reactor stand assembly. It also had an inbuilt video camera to monitor the phase change. Video overlay kit display was provided with recordings of date, time, sample number, pressure, set temperature, and actual temperature. Figure B,C shows the images taken by the inbuilt camera, while observing the phase change of compound under scCO2 in the phase-monitor (B) and reaction proceedings in the supercritical reactor (C).

Figure 1

(A) Supercritical reactor for hydrogenation under scCO2. (B, C) Reaction monitoring by the inbuilt camera.

Reaction Procedure

Desired quantities of the catalyst and reactants were loaded into the reactor system, which was then closed, flushed three times with carbon dioxide, and heated to reaction temperature. Then, hydrogen gas was introduced into the reactor at the desired pressure and liquid carbon dioxide was pumped into the reactor using a high-pressure liquid pump at the desired reaction pressure. The reaction mixture was stirred for the desired time, and the reaction samples were collected at a particular interval. The reactor was cooled down to room temperature. Gases were vented carefully, and the liquid reaction mixture was filtered to separate the catalyst.

Analytical Method

The reaction samples were analyzed using a Chemito GC 1000 gas chromatography instrument with 30 m capillary (BPX-50) columns using a flame ionization detector and nitrogen as carrier gas. Constant pressure mode of the carrier gas was used. The detector and injector were kept at 250 °C, and the oven temperature ramp was programmed from 100 °C/1 min to 10 °C/min, 120 °C/4 min, 15 °C/min, and 240 °C/1 min. The conversion was calculated based on the disappearance of the starting material acetophenone using the internal standard method. The products were identified by GC mass spectroscopy.

Results and Discussion

Effect of Various Catalysts

Various encapsulated catalysts were studied for the hydrogenation of acetophenone reaction, including monometallic PdEnCat, bimetallic Pd-RuEnCat, and Pd-CuEnCat. The catalytic activities for the conversion of acetophenone and the selectivity of 1-phenylethanol were compared for all of the catalysts under similar reaction conditions. During hydrogenation of acetophenone, various products (Scheme ), such as 1-phenylethanol (PhE), cyclohexylmethylketone (CHMK), ethylbenzene (EB), and 1-cyclohexyl-ethanol (CHE), were produced, whose conversion and selectivity profiles are shown in Figure . Among all catalysts, Pd-RuEnCat was found to be more active, with 97% conversion, but less selective, 71% toward the PhE. On the other hand, Pd-CuEnCat showed the highest selectivity of 95% toward PhE with a reasonable conversion of 85% and PdEnCat shows the least 70% conversion with 80% PhE selectivity. Ruthenium metal shows a high capability to reduce the aromatic ring in acetophenone and hence less selectivity toward synthesizing phenylethanol.[35,36] Pd-RuEnCat consisted of precious bimetals, i.e., palladium and ruthenium, in equal proportion, which makes this catalyst relatively costly. On the other hand, Pd-CuEncat shows enhanced selectivity to 1-phenylethanol compared to PdEnCat and Pd-RuEnCat catalysts because copper metal is the top candidate for selectively making PhE.[37] Therefore, Pd-CuEnCat was selected for further study.

Figure 2

Effect of various catalysts on conversion and PhE selectivity. Acetophenone: 0.05 mol; catalyst loading: 0.015 g/cm3; temperature: 80 °C; H2 pressure: 20 bar; total CO2 system pressure: 80 bar; reaction time: 4 h; co-solvent (methanol): 25 mL; speed of agitation: 800 rpm.

Details of the catalyst characterization are discussed in the Supporting Information.

Effect of the Speed of Agitation

To eliminate the external mass-transfer resistance, the effect of speed of agitation on the rate of reaction was studied. The agitation speed was studied in the range of 600–1000 rpm under otherwise similar reaction conditions over Pd-CuEnCat, maintaining the temperature at 80 °C. As can be seen from Figure , no substantial change in the overall acetophenone conversion was observed, indicating that mass-transfer limitation was absent for the diffusion of hydrogen from the the bulk fluid phase through fluid−solid film and subsequently to the external surface of the catalyst. It was quite expected that hydrogen gas is completely miscible with scCO2. Thus, it was ensured that there is no influence of external mass-transfer effect under the current experimental conditions.[38] All further experiments were carried out keeping the agitation speed at 800 rpm.

Figure 3

Effect of the speed of agitation. Acetophenone: 0.05 mol; catalyst: Pd-CuEnCat; catalyst loading: 0.015 g/cm3; temperature: 80 °C; H2 pressure: 20 bar; total CO2 system pressure: 80 bar; co-solvent (methanol): 25 mL.

Effect of Catalyst Loading

It is well known that the rate of reaction is directly proportional to catalyst loading under no influence of external mass-transfer resistance and internal diffusion limitation. The effect of catalyst loading was studied in the range of 0.005–0.02 g/cm3 based on the total volume of the reaction solution. Figure indicates the effect of catalyst loading on acetophenone conversion. The conversion of acetophenone increases with increasing catalyst loading due to the proportional increase in the number of active sites. At very high catalyst loading, the reaction will be too violent to control.[39] Therefore, all further reactions were carried out at optimum catalyst loading (0.015 g/cm3).

Figure 4

Effect of catalyst loading on conversion of acetophenone. Acetophenone: 0.05 mol; catalyst: Pd-CuEnCat; temperature: 80 °C; H2 pressure: 20 bar; total CO2 system pressure: 80 bar; co-solvent (methanol): 25 mL; speed of agitation: 800 rpm.

Effect of the Partial Pressure of Hydrogen

The effect of hydrogen pressure on acetophenone hydrogenation over Pd-CuEnCat was studied. The catalytic experiments were conducted at hydrogen pressures of 5, 10, 15, and 20 bar. The obtained conversion vs time plot is presented in Figure . It is observed that the reaction rate increases with initial hydrogen pressure. At higher pressure, conversion as good as 85% was obtained. At low hydrogen pressure, the conversion was low because of the limited hydrogen present inside the reactor. The total volume of the supercritical reactor used in this work is 100 cm3, and the volume of the feed charge was 30 cm3; therefore, free volume for hydrogen gas was 70 cm3. Under the reaction conditions of 80 °C and 20 bar pressure, only 0.0483 gmol of hydrogen gas could be accessed in 70 cm3 for the reaction, as shown by the calculations below. Moreover, for complete hydrogenation of 0.05 mol (5.84 mL) of acetophenone, 0.05 mol of hydrogen is required. Thus, the reaction may inhibit by insufficient hydrogen at lower pressure, so maximum hydrogen pressure, i.e., 20 bar was used for the reaction study.P = 20 bar, V = 70 mL, T = 353 K, and R = 0.082 (bar L/mol K). Hence, n = 0.0483 mol.

Figure 5

Effect of partial pressure of hydrogen on conversion of acetophenone. Acetophenone: 0.05 mol; catalyst: Pd-CuEnCat; catalyst loading: 0.015 g/cm3; temperature: 80 °C; total CO2 system pressure: 80 bar; co-solvent (methanol): 25 mL; speed of agitation: 800 rpm.

Effect of Supercritical CO2

The effect of scCO2 on the conversion of acetophenone was studied. Here, we selected three different conditions: (1) without CO2; (2) below supercritical region; and (3) under supercritical condition. The conversion profile, as shown in Figure , clearly indicates the effect of CO2 on conversion. The conversion is very low without CO2 and increases with CO2 pressure from subcritical condition to supercritical condition.

Figure 6

Effect of CO2 pressure on conversion of acetophenone. Acetophenone: 0.05 mol; catalyst: Pd-CuEnCat; catalyst loading: 0.015 g/cm3; temperature: 80 °C; H2 pressure: 20 bar; co-solvent (methanol): 25 mL; speed of agitation: 800 rpm.

At low CO2 pressure, the reaction medium was still in liquid phase, but the solubility of H2 in the liquid phase was enhanced and it was more than that without CO2. Therefore, it gives higher conversion rate than without CO2. Under the supercritical condition, hydrogen solubility was enhanced further and it was completely soluble in solution. The effective concentration of hydrogen in a supercritical CO2 solution can be almost an order of magnitude higher than that in a conventional solvent.[40] As a consequence, hydrogen concentration at the surface of the catalyst can be significantly increased, which leads to very high reaction rates compared to the standard liquid-phase operation. Thus, further reactions were carried out under the supercritical condition at 80 bar pressure.

Effect of Co-Solvent

The effect of co-solvent on catalyst activity or conversion and the selectivity for hydrogenation of acetophenone were studied using isopropanol, ethanol, and methanol. The effect of the above-mentioned solvents on the evolution of acetophenone conversion as a function of time is shown in Figure . It was observed that the catalyst activity follows the sequence: MeOH > EtOH > IPA. On the other hand, catalyst selectivity does not have any effect on solvent and shows higher than 99% selectivity toward 1-phenylethanol during the complete catalytic runs. The results from Figure notably indicate that activity of the catalyst for acetophenone hydrogenation depends on the solvent used in the reaction. The variation of the catalyst activity with solvent is commonly observed in liquid-phase-catalyzed reactions, but it is barely explained in terms of simple reaction parameters.

Figure 7

Effect of co-solvent on conversion of acetophenone. Acetophenone: 0.05 mol; catalyst: Pd-CuEnCat; catalyst loading: 0.015 g/cm3; temperature: 80 °C; H2 pressure: 20 bar; total CO2 system pressure: 80 bar; speed of agitation: 800 rpm.

In the solid-catalyzed reaction, the role of the solvent is a very complex phenomenon due to the fact that the solvent effect may be related to catalyst–solvent, product–solvent, and reactant–solvent, interactions, which were well explained in previous works.[41,42] The acetophenone conversion rate on the catalyst is clearly higher in MeOH. Here, we correlate the conversion rate with dielectric constant (ε), dipole moment (μ), and hydrogen donor capability (α) of the solvents.[43]Table indicates that MeOH has higher dielectric constant and dipole moment than EtOH and IPA. Because of these reasons, MeOH shows higher conversion rate than EtOH and IPA. Another reason was hydrogen donor capability, as the solvent with lower H-bond capability can interact very weakly with acetophenone (AP). MeOH has higher hydrogen donor capability than EtOH and IPA, so it interacts strongly and gives higher reaction rate. Because of this reason, methanol had a higher reaction rate than ethanol and isopropanol and hence we selected methanol as the co-solvent to conduct further study.

Table 1

Polarity Parameters of the Solvents[44]

#	solvent	ε	μ	α
1	MeOH	32.7	1.70	0.98
2	EtOH	24.6	1.69	0.86
3	IPA	19.9	1.66	0.76

Effect of Temperature

Experiments were conducted in the temperature range of 60–80 °C to study the effect of temperature on the rate of reaction. Figure shows the effect of temperature on conversion of acetophenone. It was expected that the reaction rate will improve with an increase in temperature, which could be due to enhancement in the surface reaction rate constant at a higher temperature. Also, higher temperature reduced the viscosity of supercritical carbon dioxide and improved the diffusivity, which is responsible for the enhancement of the reaction rate.

Figure 8

Effect of temperature on conversion of acetophenone. Acetophenone: 0.05 mol; catalyst: Pd-CuEnCat; catalyst loading: 0.015 g/cm3; H2 pressure: 20 bar; total CO2 system pressure: 80 bar; co-solvent (methanol): 25 mL; speed of agitation: 800 rpm.

Reaction Kinetics and Modeling

Here, we had assumed bifunctional catalytic sites S1 and S2. On S1 sites, substrate is chemisorbed, and on S2 sites, hydrogen is adsorbed.

Adsorption of acetophenone:

Chemisorption of acetophenone on site S1A = acetophenone

S1 = type 1 catalyst active site

AS1 = occupied catalyst active site by acetophenone.

Chemisorption rate is given bywhere KA is adsorption equilibrium constant = ka/ka′

Cv is the vacant sites S1 concentration.

Hydrogen adsorption:

Assuming that hydrogen is dissociatively adsorbed on type S2 sitewhere KH is the adsorption equilibrium constant (kh/kh′)

C is the vacant sites S2 concentration.

Surface reactionB = product

BS1 = catalyst active site occupied by product

kS = reaction rate constant.

If the surface reaction is rate controlling, thenDesorptionwhere KB′ is the desorption equilibrium constant (kd/kd′)

Assuming that the rate-controlling step is a surface reaction,

rAD and rHD become 0,

so from eq From eq , the following is obtainedFrom eq , the following is obtainedBy replacing desorption constant KB′ with adsorption constant KBThe total site balance for sites S1 and S2 can be written as followsSubstituting CAS, CHS, CBS, Cv, and Cv from eqs –14, 17, and 18 into eq , we getAssuming negligible adsorption of B, the above equation reduces towhere w = catalyst loading (g/L)The above equation is simplified as follows:

Case 1: If KACA ≪ 1, thenInverting the above equation, we getThus, a plot of left-hand side (wCA/rS) for a fixed concentration of A, fixed w, and initial rate versus was made to evaluate the slope of and the intercept of Figure . The values of k2 and KH were calculated as 1.04 × 10–3 mol/gcat/s and 0.2319 bar–1, respectively.

Figure 9

Plot of wCA/rSi vs 1/(P)−0.5.

Case 2:

If KACA ≪ 1 and KHPH ≫ 1, thenwhere k2 = kSKA

The above equation indicates a typical pseudo-first-order reaction. The rates of reaction would not be dependent on the hydrogen partial pressure at high pressures. Integration of the above equation givesThe plot of −ln(1–XA) vs time is shown in Figure . The initial rates were obtained and utilized to make the Arrhenius plot as shown in Figure . From Figure , apparent activation energy was calculated to be 14.8 kcal/mol.

Figure 10

Pseudo-first-order plot [−ln(1–XA) vs time].

Figure 11

Arrhenius plot (ln kSR vs 1/T).

Conclusions

Hydrogenation of acetophenone was investigated over polyurea-microencapsulated mono- and bimetallic catalysts, such as PdEnCat, Pd-RuEnCat, and Pd-CuEnCat, using supercritical CO2 as solvent. Of these catalysts, Pd-RuEnCat was found to be more active but less selective toward PhE. On the other hand, Pd-CuEnCat was found to be superior with 95% selectivity for the desired product PhE. The effects of various reaction parameters on the reaction rates using Pd-CuEnCat were studied thoroughly, and a kinetic model based on bifunctional catalyst sites was derived, which showed a very good agreement with the experimental data. The reaction was found to follow pseudo-first-order mechanism. The activation energy was found to be 14.8 kcal/mol.