TiO2 Catalyst in CO2

ARTICLE pubs.acs.org/JPCC

Hydrogenation of Nitrostyrene with a Pt/TiO2 Catalyst in CO2Dissolved Expanded Polar and Nonpolar Organic Liquids: Their Macroscopic and Microscopic Features Hiroshi Yoshida,† Katsumasa Kato,† Jinyao Wang,§ Xiangchun Meng,§ Satomi Narisawa,† Shin-ichiro Fujita,† Zhijian Wu,|| Fengyu Zhao,§ and Masahiko Arai*,† †

Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China

)

§

bS Supporting Information ABSTRACT: Hydrogenation of nitrostyrene (NS) was studied with a Pt/TiO2 catalyst in CO2-dissolved ethanol and toluene phases at 323 K. The effects of CO2 pressure and organic solvents on the total conversion and the product distribution were examined. The conversion simply decreases with CO2 pressure in the both expanded solvents. The pressurization with CO2 varies the product selectivity in the nonpolar solvent but not in the polar solvent. The interactions among the substrate, solvent, and CO2 molecules were measured by in situ high-pressure FTIR methods in transmittance and attenuated total reflection modes. The reactivity of the nitro group of NS is lowered by interactions with CO2, which is responsible for the change in the product distribution. The local structures of an NS molecule in the expanded solvents are discussed from the results of FTIR and molecular dynamics simulations. The change in the product distribution is explained by the change of the local composition around the substrate molecule depending on CO2 pressure. The local composition is likely to change in the toluene but not in the ethanol, in accordance with the changes in the product selectivity in these two solvents. The substrate-CO2 interactions are important in toluene while those between the substrate and the solvent are predominant in ethanol even at high CO2 pressures.

1. INTRODUCTION Carbon dioxide dissolved expanded liquids (CXLs) are a new class of interesting reaction media. A CXL can occur when an organic solvent or substrate is pressurized by CO2 and a large quantity of CO2 is dissolved into the liquid phase. The properties of the organic liquid phase can widely and easily be varied with CO2 pressure. The dissolution of CO2 can promote the dissolution of gaseous reactants such as H2, O2, and CO. The CXLs are effectively applicable for organic synthetic reactions including reduction, oxidation, hydroformylation, and others. When the rate of reaction is enhanced by using CXLs, the volume of organic solvents can significantly be reduced. Those attractive features of CXLs as promising reaction media are demonstrated in recent review articles.1-5 For example, Subramaniam and his co-workers showed the effectiveness of CXLs for accelerating oxidation reactions with homogeneous and heterogeneous catalysts by oxygen.6-8 The CXLs would receive further increasing attention from industry and academia because of their interesting features. Compared to homogeneous systems in which all reacting species r 2011 American Chemical Society

are dissolved in the CO2 gas phase, for CXLs, we have no concern about the solubility of reacting species in the gas phase, may use the catalysts active in organic liquid and aqueous phases without modifications of increasing their solubility into the gas phase, and can process larger amounts of substrates. The pressure of CO2 is crucial in determining the reaction rate and the product distribution in chemical reactions in CXLs. The reactions in CXLs are complicated multiphase reactions and the pressurization with CO2 should have different physical and/or chemical impacts, positive or negative, on the reactions. The authors investigated the hydrogenation reactions of carbonyl9-13 and nitro14-16 compounds in CXLs with homogeneous and heterogeneous catalysts. In addition to the roles of promoting the dissolution of H2 and diluting the reacting species in the CXLs, the pressurization with CO2 has another important role of Received: November 3, 2010 Revised: December 13, 2010 Published: January 5, 2011 2257

dx.doi.org/10.1021/jp1105339 | J. Phys. Chem. C 2011, 115, 2257–2267

The Journal of Physical Chemistry C Scheme 1. Reaction Pathways in Hydrogenation of Nitrostyrene (NS)a

a

VA: vinylaniline. ENB: ethylnitrobenzene. EA: ethylaniline. The broken arrow from VA to EA means that this hydrogenation pathway is more difficult compared to the others under the conditions used in the present work.

modifying the reactivity of substrates and intermediates. The dissolved CO2 molecules may interact with certain functional groups of the reacting species and change their reactivity. The presence of such molecular interactions of CO2 is proved by in situ high-pressure FTIR measurements.17 For example, in hydrogenation of nitrobenzene using conventional supported Ni catalysts, the interactions of CO2 with the nitro group of the substrate decrease its reactivity while those with nitrosobenzene and N-phenylhydroxylamine increase their reactivity. As a result, for the hydrogenation reactions of nitrobenzene and chloronitrobenzene, 100% selectivity to the desired final aniline products can be achieved at any conversion level up to 100%.14-16 Note that CXLs are also interesting media for organic reactions that include no gaseous reactants such as Heck and Sonogashira coupling reactions.18,19 Although there are a number of previous works on chemical reactions in CXLs, further study should be required to clarify in more detail the roles of dissolved CO2 molecules and the features of chemical reactions in CXLs. The present work has been undertaken to shed light on the roles of CO2 in chemical reactions in CXLs, using hydrogenation of nitrostyrene (NS) as a test reaction (Scheme 1). We selected NS having two functional groups to be hydrogenated to examine the effects of CO2 on both reaction rate and product selectivity. The hydrogenation of NS was conducted with a conventional Pt/ TiO2 catalyst in toluene and ethanol in the presence of compressed CO2. The rate of hydrogenation of NS in either toluene or ethanol has been observed to merely decrease on CO2 pressurization. It has also been shown that the presence of compressed CO2 changes the product distribution in toluene but not in ethanol. In addition to macroscopic characterization (phase behavior observation) of the present gas-liquid-solid multiphase reaction systems, microscopic characterization was also made using in situ high-pressure FTIR with transmittance and attenuated total reflection (ATR) modes and molecular dynamics simulation. These were used to inspect molecular interactions among substrate, solvent, and CO2 molecules and to examine the microscopic local structure of a substrate molecule surrounded by solvent (toluene or ethanol) and CO2 cosolvent molecules in the CXLs. On the basis of those reaction and

ARTICLE

characterization results, the roles of dense phase CO2 have been discussed at molecular level. In addition to spectroscopic measurements, molecular dynamics simulations were employed to investigate the microscopic structures of CXLs.1,20-28 For example, Eckert et al. studied the local compositions in CO2-dissolved methanol and acetone phases by solvatochromic experiments using a dye of Coumarin 153 and molecular dynamic simulations.20 Maroncelli et al. discussed the solvation dynamics in CO2-dissolved acetonitrile phase by timeresolved fluorescence of trans-4-(dimethylamino)-40 -cyanostilbene and molecular dynamics computer simulations.25 The combination of experiments and simulations is synergistic and complementary for the elucidation of local molecular structures in CXLs and their potential features in chemical reactions therein.1 In the present work, local structures of a NS molecule in CO2-dissolved expanded ethanol and toluene phases have also been examined by molecular dynamics simulation.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A 0.5 wt % Pt/TiO2 was prepared by impregnation using TiO2 (anatase/rutile = 7/3, BET surface area 49 m2/g, Catalysis Society of Japan, JRC-TIO4(2)) and H2PtCl6 (Wako). The Pt-loaded TiO2 sample was dried at 373 K for 5 h and reduced under 4% H2 (N2 balance) based on N2 at 473 K for 3 h. 2.2. Hydrogenation of Nitrostyrene. The hydrogenation reaction runs were carried out in a 50 cm3 stainless steel autoclave. In a typical run, 3.4 mmol of substrate and 10 cm3 of solvent were charged together with 20 mg of Pt/TiO2 catalyst and the reactor was flushed with CO2 three times to remove the air. After the reactor was heated to 323 K, H2 (4 MPa) was introduced, and then liquid CO2 was introduced into the reactor with a high-pressure liquid pump (JASCO SCF-Get) to the desired pressure. The reaction was conducted while the reaction mixture was being stirred with a magnetic stirrer. After the reaction, the reactor was cooled with an ice-water bath and then depressurized carefully, and the composition of reaction mixture was analyzed by a gas chromatograph (GL Science GC390B) using a capillary column (GL science TC-1701) and a flame ionization detector. In several runs, the nitrostyrene conversion was controlled to be different values by changing the catalyst amount and the reaction time. The total conversion of nitrostyrene was determined from the final amount of nitrostyrene unreacted divided by the initial amount of nitrostyrene loaded. In this reaction, vinylaniline (VA), ethylnitrobenzene (ENB), and ethylaniline (EA) were formed and no other products were observed (Scheme 1). The selectivity to each product was determined from the amount of each product formed divided by the total amount of products formed. 2.3. High-Pressure FTIR. Molecular interactions between NS and CO2 molecules were examined by in situ high-pressure FTIR measurements using a JASCO FTIR-620 spectrometer with a triglycine sulfate detector at a wavenumber resolution of 2 cm-1. The FTIR spectra were collected at 323 K (reaction temperature) for NS in the compressed CO2 gas phase by transmittance mode17 and for the CO2-dissolved expanded liquid NS phase by ATR mode. For the former measurement, a 1.5 cm3 cell was used; it was loaded with a small amount of NS, purged with CO2 three times, and loaded with CO2 to the desired pressure. The sample temperature was controlled by circulation of preheated oil around the cell (JULABO F25-HP). The ATR 2258

dx.doi.org/10.1021/jp1105339 |J. Phys. Chem. C 2011, 115, 2257–2267

The Journal of Physical Chemistry C measurement was made with a home-designed cell of about 1 cm3 using procedures similar to those mentioned above. The temperature was adjusted to 323 K by resistivity heating with coils embedded in the cell. The ATR measurement was also made to examine the interactions among NS, solvent, and CO2 molecules in the CO2-dissolved expanded liquid phases. In addition, the formation and adsorption of CO on the Pt/TiO2 catalyst in the presence of compressed H2 and CO2 was measured by in situ transmittance FTIR using the same high-pressure cell and procedures as used previously.30 The adsorption of CO was reported to occur and cause the deactivation of supported metal catalysts in hydrogenation reactions.30-32 2.4. Volume Expansion of Liquid Phases. The volume expansion ratios of ethanol and toluene used as solvents in the presence of compressed CO2 were measured with a 10 cm3 highpressure view cell equipped with two sapphire windows. The cell was loaded with 2 cm3 solvent, it was purged with CO2 three times, and then it was heated to a temperature of 323 K while stirring. After the desired temperature was obtained, 4 MPa H2 was introduced, compressed CO2 was fed into the cell to the desired pressure, and then stirring was stopped after 2 min. The depth of the liquid was measured to determine the volume of the liquid phase.

3. MOLECULAR DYNAMIC SIMULATIONS Molecular interactions of an NS molecule with solvent (toluene or ethanol) and CO2 cosolvent molecules in the CO2dissolved expanded solvent phases were also studied by molecular dynamic simulations. The simulation results were compared with the above-mentioned experimental ones and the significance of the molecular interactions for the product selectivity in the hydrogenation of NS in those expanded solvents was discussed. The details of the simulations are described in a previous work.33 For the calculation concerning CO2, the “EMP2” potential parameters were used, which was constructed to reproduce the gas-liquid coexistence curve of the real CO2 fluid.34 The united-atom version of the transferable potential for phase equilibrium (TraPPE-UA) force field developed for alcohols was used to model the ethanol molecule. In the TraPPE-UA, the methyl and ethylene groups were replaced by pseudoatoms at their carbon sites, and the oxygen and hydrogen atoms of hydroxyl were modeled explicitly. Similar to the case for ethanol, the TraPPE-UA force field for alkylbenzenes was used to model toluene. For the NS molecule, the OPLS-AA force field was used.35 Coulombic interactions were handled using the Ewald summation method. The simulations reported here were performed in the NPT ensemble. The temperature coupling of Nose-Hoover was used to maintain a constant temperature of 323.15 K in all cases, while the pressure coupling of ParrinelloRahman was used to maintain constant pressure. The equations of motion were integrated using a leapfrog scheme with a 2 fs time step. All bond lengths were kept constant using the SHAKE algorithm.36 The simulation data were obtained in production run of 3 ns after an equilibration period of 1 ns. 4. RESULTS 4.1. Hydrogenation. The hydrogenation of 3-nitrostyrene (NS) was examined in toluene and ethanol with a Pt/TiO2 catalyst at 323 K. Figure 1 gives the changes of total conversion with CO2 pressure in hydrogenation reactions. The conversion

ARTICLE

Figure 1. Influence of CO2 pressure on total conversion of hydrogenation of nitrostyrene in ethanol (a) and in toluene (b) using 0.5% Pt/ TiO2 at 323 K and at a H2 pressure of 4 MPa.

obtained in ethanol was larger than that in toluene but it decreased with CO2 pressure in the both solvents. Hence, the pressurization with CO2 had a negative impact on the total conversion of NS hydrogenation reactions in the polar and nonpolar solvents. The relationship between total conversion and product selectivity was obtained by using different reaction times and amounts of catalyst. Under the conditions used, vinylaniline (VA), ethylnitrobenzene (ENB), and ethylaniline (EA) were observed to form as products (Scheme 1). Figure 2 shows that, in ethanol, the selectivity to the three products was in the order VA > ENB > EA at a low conversion of around 20%. The selectivity to VA did not change with total conversion so much while the selectivity to ENB decreased but that to EA increased. The product distribution at an atmospheric CO2 pressure was the same as that at a high pressure of 10 MPa. These results indicate that NS is hydrogenated to either VA or ENB at comparable reaction rates and the latter is further hydrogenated to the final product EA in the latter course of reaction but not the former. Very similar results were also obtained in toluene at 0.1 MPa CO2, as shown in Figure 3. When the CO2 pressure was raised to 2 MPa, the selectivity to ENB increased while that to VA decreased. These effects of CO2 pressurization were more significant at 10 MPa, at which the selectivity to ENB was much larger (>90%) than those to VA and EA. That is, the pressurization with CO2 changed the product distribution in a nonpolar solvent toluene but not in a polar solvent ethanol. Recently, the authors revealed the presence of significant interactions of dense phase CO2 molecules with the nitro groups of nitrobenzene and chloronitrobenzene by in situ high-pressure FTIR, and the interactions decreased their reactivity.14-16 The present result of the decreased selectivity to VA, which is produced via hydrogenation of nitro group of NS, in the presence of dense phase CO2 is in accordance with those previous results. Further, the hydrogenation of a semihydrogenated product VA was also examined in toluene and ethanol. Table 1 presents the results obtained with VA and, for comparison, styrene and a substituted styrene of 3-methylstyrene. The conversion of VA obtained in either toluene or ethanol was significantly smaller than those of styrene and methylstyrene, which gave similar conversion levels. That is, the hydrogenation of VA is even slower than the ones of styrene and methylstyrene. The rates of hydrogenation of these three substrates were larger in ethanol than in toluene and the differences in the rate of hydrogenation between VA and the others seems to be larger in ethanol than in toluene. It can be assumed that the hydrogenation of VA should 2259


The Journal of Physical Chemistry C

ARTICLE

Figure 2. Selectivity against conversion in hydrogenation of NS in the CO2-dissolved expanded ethanol at CO2 pressures of 0.1 MPa (a) and 10 MPa (b). Abbreviations: VA, vinylaniline; ENB, ethylnitrobenzene; EA, ethylaniline.

Figure 3. Selectivity against conversion in hydrogenation of NS in the CO2-dissolved expanded toluene at CO2 pressures of 0.1 MPa (a), 2 MPa (b), and 10 MPa (c). Abbreviations: VA, vinylaniline; ENB, ethylnitrobenzene; EA, ethylaniline.

Table 1. Results of Hydrogenation Reactions of 3-Vinylaniline, Styrene, and 3-Methylstyrene in Toluene and Ethanola

conversion (%) entry 1 2 3 a

substrate 3-vinylaniline

in toluene

in ethanol

18.0 (1)b

20.3 (1)b b

styrene

65.8 (3.2)b

44.8 (2.5)

b

3-methylstyrene

64.7 (3.2)b

45.7 (2.5) 3

3

Reaction conditions: solvent 10 cm , substrate 0.5 cm (3.4 mmol), Pt/ TiO2 catalyst 5 mg, temperature 323 K, time 20 min. b Relative conversion with respect to that of vinylaniline.

also be slower compared to styrene and methylstyrene when the reaction mixture is pressurized by CO2. This is because interactions of CO2 molecules with the vinyl group are so weak, as will be shown later, that its reactivity does not change with the presence of dense phase CO2. A similar chemical impact of CO2 on chemical reactions was previously reported by Eckert et al.29 These authors studied the

hydrogenation of nitriles with NiCl2/NaBH4 in CO2-dissolved expanded ethanol and observed an impact of CO2 on the product selectivity. The primary amines can be protected by CO2, resulting in an increase in the selectivity to the primary amines but effectively suppressing the formation of the secondary amines. For hydrogenation reactions using H2 with supported transition metal catalysts in the presence of compressed CO2, the catalysts were observed to lose their hydrogenation activity.30-32 An important cause of the catalyst deactivation is the formation and adsorption of CO on the catalysts, which is produced via reverse water gas shift reaction. In the present reaction of NS with Pt/TiO2, however, such a catalyst deactivation did not occur, as confirmed from conversion-time profiles (not shown), and the adsorption of CO on the catalyst was not detected by in situ highpressure FTIR measurements. 4.2. Phase Behavior. The volume expansion of toluene and ethanol on CO2 pressurization was examined at 323 K and at different pressures in the presence of 4 MPa H2 (those visual observations of the phase behavior are given in Supporting Information). The extent of volume expansion is plotted against CO2 pressure in Figure 4. The liquid to reactor volume ratio used was the same as used in the hydrogenation runs. An extent of volume expansion of 5 means that the gas-liquid biphasic mixture changed to the single gas phase state under the conditions used. The toluene phase expanded slightly at CO2 pressures 12 MPa. A similar volume expansion was also observed 2260



Figure 4. Extent of volume expansion of ethanol (a) and toluene (b) on pressurization with CO2 at 323 K in the presence of 4 MPa H2.

with ethanol and the liquid phase disappeared at around 13 MPa. A macroscopic feature of the volume expansion with CO2 pressurization was not so different between these two polar and nonpolar solvents. 4.3. High-Pressure FTIR. In situ high-pressure FTIR measurements were made for NS dissolved in compressed CO2 gas phase and in the expanded organic solvent phases at a temperature of 323 K and at different pressures to examine the molecular interactions among NS, solvent, and CO2 molecules. Figure 5a presents the FTIR spectra obtained with transmittance mode in the range 1250-1600 cm-1 for NS dissolved in the CO2 gas phase. Two absorption bands appeared at around 1530 and 1350 cm-1, which can be assigned to asymmetric and symmetric stretching vibrations of NO2, respectively.37 The peak positions of the two absorption bands were blue-shifted as the CO2 pressure was raised to 6 MPa, after which the peak positions were red-shifted but only marginally (Figure 5c). Thus, the strength of NO bonds of the NS molecule is stronger in dense phase CO2 than in the atmospheric gas phase. These results are similar to those obtained previously with the nitro groups of nitrobenzene and chloronitrobenzene.15,16 Thus, the dense phase CO2 should decrease the reactivity of the NO bond of NS. In contrast, it did not influence the peak position of an absorption band assigned to ν(CdC) located at 1636 cm-1,37 as shown in Figure 5b. No changes were also observed for the peak positions of the other absorption bands due to overtone of δ(C-H) and combination. Namely, the reactivity of the vinyl group of NS was unlikely to be varied by dense phase CO2 molecules. The results with liquid NS in the presence of compressed CO2 measured by ATR are given in Figure 6. The peak positions of νs(NO2) and νas(NO2) were blue-shifted with CO2 pressure although the extent of the blue shift was smaller than that for NS in the CO2 gas phase (Figure 5). The peak position of νas(NO2) changed from 1525 cm-1 at ambient pressure through 1530 cm-1 at 14 MPa. No changes were observed for ν(CdC) and δ(C; H) for liquid NS compressed by CO2 (Figure 6b). The results of Figures 5 and 6 indicate that CO2 molecules have similar interactions with the nitro group of NS in the gas and liquid phases. Figure 7 gives the FTIR spectra for NS in the CO2-dissolved ethanol phase. When CO2 pressure was raised, the absorption band of the nitro group appeared at a wavenumber higher than that of the vinyl group at the same position (not shown). The peak position of νas(NO2) was 1536 cm-1 at 0.1 MPa and increased to 1538 cm-1 at 8 MPa. This blue shift of 2 cm-1 was

ARTICLE

smaller compared to 4 cm-1 for the CO2-dissolved liquid NS phase at the same pressure range (Figure 6). In addition to the nitro group of the NS solute, the dissolved CO2 affected the absorption of the OH bond of the ethanol solvent (Figure 7b). The peak position of ν(OH) was shifted from 3337 to 3359 cm-1 at pressures up to 8 MPa. When compressed Ar was used instead of CO2, no changes were observed for the absorption bands of nitro and vinyl groups of NS, indicating that high pressure was not responsible but the chemical nature of CO2 was important. A high pressure is needed for the dissolution of CO2 into the organic liquid phases. Figure 8 presents the results for NS in CO2-dissolved toluene phase at different pressures. The peak positions of νs(NO2) and νas(NO2) were also blue-shifted with CO2 pressure, similar to those with ethanol; note, however, that the extent of blue shift of νas(NO2) was larger, the shift (ca. 4 cm-1) at 0.1-8 MPa being comparable to the maximum shift (ca. 4 cm-1) expected from the results with the CO2-dissolved liquid NS phase (Figure 6). It is believed from the above-mentioned FTIR results that the local structure around an NS molecule is different in ethanol and in toluene in the presence of compressed CO2. For a nonpolar solvent of toluene, the dissolved CO2 molecules should interact with an NS molecule with its nitro group even at low CO2 pressures (at low CO2 concentrations in toluene). This is because no significant interactions are expected to occur between NS and toluene molecules. Thus, the blue shift of ν(NO2) in CO2dissolved toluene is comparable to that in CO2-dissolved liquid NS (Figures 6 and 7). In ethanol, in contrast, the nitro group of NS molecules may interact with the dissolved CO2 molecules and with the hydroxyl group of the ethanol solvent. The nearest neighbor sites of the nitro group of an NS molecule should be occupied by ethanol molecules more predominantly than by CO2. This may explain that the peak positions of ν(NO2) absorption bands are not blue-shifted so much, compared with the changes in the CO2-dissolved toluene and liquid NS phases. The CO2 pressurization causes the peak positions of ν(NO2) to be blueshifted slightly for the CO2-dissolved ethanol. This could be explained by either dissolved CO2 molecules replacing some ethanol ones and having direct interactions with the nitro group or CO2 molecules interacting with an ethanol molecule through its hydroxyl group, changing the nitro-hydroxyl interactions indirectly. Those local structures of an NS molecule in the CO2-dissolved ethanol and toluene phases and their implications in hydrogenation reactions of NS will further be discussed in the following. As referred to in section 4.1, Eckert et al. pointed out the importance of interactions with CO2 cosolvent molecules in selective hydrogenation of nitriles to primary amines, interacting with the desired products (primary amines) and suppressing their further hydrogenation to secondary amines.29 The present authors also revealed the significance of interactions of CO2 with carbonyl9-13 and nitro14-16 groups in selective hydrogenation of R,β-unsaturated aldehydes and aromatic nitro compounds. One can say that, in addition to such solvent properties as polarity, interactions of solvent molecules with reacting species would be an interesting factor for manipulating the product distribution. For example, Anderson et al. have recently reported the Pd-catalyzed hydrogenation of several substituted phenyls in water and cyclohexane.38 These authors assume that interactions of external functional groups (-CHO, -COOH, -COCH3, -CONH2, and others) with water molecules protect them from interactions with the surface of supported Pd particles, resulting in selective hydrogenation of the aromatic ring. 2261



ARTICLE

Figure 5. High-pressure FTIR spectra of NS in dense phase CO2 at 323 K and at different pressures given in the ranges of stretching vibrations of N-O (a) and C-C (b) bonds. (c) displays the changes of the peak positions of ν(NO) absorption bands with CO2 pressure.

Figure 6. High-pressure FTIR spectra of the CO2-dissolved expanded liquid NS phase at 323 K and at different pressures.

5. DISCUSSION The present reaction results show that the CO2 pressurization decreases the total conversion of hydrogenation of NS in both toluene and ethanol and it affects the product distribution in the former but not in the latter. In the following, our discussion will be given to the effects of CO2 pressurization on the hydrogenation (total conversion and product selectivity), the interactions of CO2 with the other molecules (substrate and solvent), and the local molecular structures around the substrate. 5.1. Total Conversion. The conversion of NS hydrogenation decreases with CO2 pressure when either toluene or ethanol is used as a solvent (Figure 1). The reactivity of the nitro group of

NS is likely to decrease through interactions with CO2 molecules dissolved in the solvents but not the vinyl group, according to the high-pressure FTIR results. The hydrogenation rates of nitro and vinyl groups of NS are not so different, and then the decrease in the reactivity of the nitro group only is not responsible for the decreased total conversion observed. The dilution of NS, hydrogen, and Pt/TiO2 in the expanded organic liquid phases occurs by dissolution of CO2 and H2 gases at high pressures. We assume that this simple dilution is mainly responsible for the decrease of the total conversion on CO2 pressurization. Another possible factor is the influence of dense phase CO2 on the properties of supported Pt particles. Such a possibility was 2262



ARTICLE

Figure 7. High-pressure FTIR spectra of NS in the CO2-dissolved ethanol phase at 323 K and at different pressures. At higher pressures the gas-liquid sample mixture changed into a single phase and this resulted in a significant decrease in the strength of absorption of nitro and hydroxyl groups.

Figure 8. High-pressure FTIR spectra of NS in the CO2-dissolved toluene phase at 323 K and at different pressures. At higher pressures the gas-liquid sample mixture changed into a single phase and this resulted in a significant decrease in the strength of absorption of nitro group.

assumed from the results of optical absorption experiments with a model catalyst of gold particles deposited on a quartz plate by vacuum evaporation.39 The UV/vis spectra were collected for this sample in dense phase CO2 at different pressures. The peak position of the plasmon absorption band was blue-shifted (from 555 nm through 575 nm at pressures up to 20 MPa) and its full width at half-maximum increased (from 102 nm through 125 nm) with CO2 pressure, suggesting the presence of interactions between gold particles and CO2 molecules at high pressures. Similar in situ high-pressure UV/vis measurements are difficult for powdered catalyst samples like the present one owing to the limitation of our current setup. It will be improved in the future to collect the UV/vis spectra with granular catalyst samples, which are helpful in discussing the impact of dense phase CO2 on the nature and catalytic activity of small metal particles. The total rate of NS hydrogenation is larger in ethanol than in toluene (Figure 1). Similarly, the hydrogenation reactions of VA, styrene, and 3-methylstyrene are faster in the former than in the latter (Table 1). Intrinsic natures of the polar solvent may be more beneficial for the hydrogenation of vinyl and nitro groups of those substrates compared with the nature of the nonpolar one. The state of dispersion of solid Pt/TiO2 catalyst in the organic solvents could also be important; so, we had a look at the dispersion of the catalyst in those solvents compressed by CO2. The catalyst granules were seen to be well dispersed in the both CO2dissolved ethanol and toluene phases (not shown).

5.2. Product Selectivity. A very similar product distribution is seen for the hydrogenation reactions of NS in ethanol in the absence and presence of compressed CO2 and in toluene in the absence of compressed CO2. Under these conditions, the selectivity to VA is larger than those to ENB and EA and the final product EA is produced from ENB (Figure 2, Table 1). In toluene in the presence of dense phase CO2, however, the selectivity to ENB is even larger compared with those for VA and EA. That is, the effect of CO2 pressurization appears for toluene but not for ethanol. The high selectivity to ENB is ascribable to molecular interactions of CO2 with nitro groups of NS and ENB, which increase the strength of NO bonds and then decrease their reactivity. One may say that such molecular interactions of CO2 are likely to occur in toluene but not in ethanol even at a high CO2 pressure of 10 MPa. The volume expansion on CO2 pressurization is not so different between these two organic solvents (Figure 3), which means that the quantities of CO2 molecules dissolved in the solvents are similar. According to the previous solubility data reported in the literature,40-43 the mole fraction of CO2 in ethanol was 0.61 at 9.95 MPa and at 333.4 K40 and 0.52 at 9.19 MPa and at 333.8 K.42 In toluene a similar mole fraction of CO2 of 0.59 was reported at 9.56 MPa and at 352.6 K.43 Then, we should consider the microscopic structure of a substrate molecule surrounded by solvent and CO2 molecules in the expanded organic liquids.1 The above-mentioned hydrogenation and FTIR results let us assume that, in ethanol, an NS substrate molecule is surrounded by the solvent molecules even at high CO2 pressures. This is because the interactions of NS with ethanol molecules are more significant compared to those with CO2 molecules; the interactions should occur between the O atom of the nitro group of the substrate and the H atom of the OH group of the solvent and between the N atom of the substrate and the O atom of the solvent. Thus, the local composition of an NS substrate changes little with CO2 pressure, resulting in no change in the product selectivity (Figure 2). In toluene, however, such interactions are absent between the substrate and the solvent and the interactions of the substrate with CO2 molecules should become more significant at increasing CO2 pressure. The interactions may occur between the O atom of NS and the C atom of CO2 and between the N atom of NS and the O atom of CO2. The nearest neighbor sites of an NS molecule are occupied by toluene molecules when the concentration of CO2 in the solvent phase is small (at low pressures). At higher CO2 pressure, however, more CO2 molecules can come to the nearest neighbor sites and interact with the nitro group of NS substrate. The 2263



Figure 9. Radial distribution functions g(r) for (a) ethanol and (b) toluene phases. (a) Solid line: g(r) of the H atom of ethanol around the O atom of the NO2 group of NS in pure ethanol. Dotted line: g(r) of the C atom of CO2 around the O atom of NO2 in the expanded ethanol at 10 MPa. Broken line: g(r) of the H atom of ethanol around the O atom of NO2 in the expanded ethanol at 10 MPa. (b) Solid line: g(r) of the COM of toluene around the O atom of the NO2 group of NS in pure toluene. Dotted line: g(r) of the C atom of CO2 around the O atom of NO2 in the expanded toluene at 10 MPa. Broken line: g(r) of the COM of toluene around the O atom of NO2 in the expanded toluene at 10 MPa.

interactions decrease the reactivity of the nitro group and, hence, the selectivity to its hydrogenated product VA decreases with CO2 pressure but that to ENB, the product of hydrogenation of the vinyl group does not change (Figure 3). As shown in Figure 7 for ethanol, the peak position of ν(NO2) absorption band is slightly blue-shifted. How is this blue shift related to the local structure of an NS substrate in the CO2dissolved ethanol? There are two possible explanations, as mentioned above: one is the direct interactions of the nitro group with CO2 molecules that replace some ethanol molecules and the other is the indirect effects of CO2 molecules, which interact with ethanol molecules and then change the interactions between the ethanol and the nitro group. After considering the results of molecular dynamics simulation as given below, the latter may be more likely. That is, the authors suppose that the molecule in nearest contact with an NS molecule is ethanol even at high CO2 pressures (at high CO2 concentrations in the solvent phase) but

ARTICLE

Figure 10. Radial distribution functions g(r) for (a) ethanol and (b) toluene phases. (a) Solid line: g(r) of the O atom of ethanol around the COM of the vinyl group of NS in pure ethanol. Dotted line: g(r) of the C atom of CO2 around the COM of vinyl in the expanded ethanol at 10 MPa. Broken line: g(r) of the O atom of ethanol around the COM of vinyl in the expanded ethanol at 10 MPa. (b) Solid line: g(r) of the COM of toluene around the COM of vinyl in pure toluene. Dotted line: g(r) of the C atom of CO2 around the COM of vinyl in the expanded toluene at 10 MPa. Broken line: g(r) of the COM of toluene around the COM of vinyl in the expanded toluene at 10 MPa.

the interactions of NS with the ethanol molecules may be changed by the interactions of the solvent with CO2 molecules that exist in the second nearest sites of the substrate. Furthermore, the local structures of an NS molecule in the CO2-dissolved ethanol and toluene phases will be examined by molecular dynamics simulations. 5.3. Molecular Dynamics Simulation. The simulation was based on an equilibrated cubic, periodic box containing a single substrate molecule and 1013 molecules of organic solvent and/or CO2 cosolvent. The calculation was made for the two-component systems in the absence of CO2 and for the three-component ones in the presence of 10 MPa CO2, for which the solvent/ cosolvent ratio was 6/4. This ratio corresponds to a mole fraction of CO2 in the liquid phase of 0.4, which is slightly smaller than the solubility expected from the literature.40-43 Figure 9 shows the radial distribution function (g(r)) curves around the O atom of NO2 group for the mixtures of NS in CO2 (10 MPa) dissolved expanded ethanol and toluene phases at 323 K. Figure 9a indicates that the C atom of CO2 has the first 2264



ARTICLE

Figure 11. Solvent and cosolvent (CO2) distribution maps around an NS molecule in the neat (a) and CO2-dissolved expanded ethanol and toluene phases at 10 MPa (b, c). (a) Ethanol and toluene around NS. (b) Ethanol and toluene around NS. (c) CO2 around NS.

peak at 0.34 nm while the H atom of ethanol is at a shorter distance of 0.20 nm. This radial distribution curve of the H atom is similar to that calculated for the mixture in the absence of compressed CO2. These results imply that the presence of CO2 does not affect the interactions of NS with the ethanol so much, which should be stronger than those with the CO2. That is, the features of the cycotactic region of NS in the CO2 dissolved expanded ethanol do not differ from those of NS in pure ethanol although a large amount of CO2 molecules is dissolved in the liquid phase. This may explain that the product selectivity obtained in ethanol does not change on pressurization with CO2 (Figure 2). However, different results were obtained with toluene (Figure 9b). The center of mass (COM) of toluene presents the first peak at 0.48 nm around the O atom of the NO2 group but the C atom of CO2 at a shorter distance of 0.30 nm. In contrast to ethanol, the interactions of toluene with NS should be affected by CO2 pressurization and dissolution, which are weaker than those

with the dissolved CO2 molecules. This is responsible for the significant change in the product distribution in hydrogenation of NS in toluene by CO2 pressurization (Figure 2). The molecular simulation was also performed with respect to the vinyl group of NS (Figure 10). Figure 10a shows that the g(r) for the O atom of ethanol around the COM of the vinyl group of NS in pure ethanol is flat at 0.40 nm or longer distance; in the presence of 10 MPa CO2, the C atom of CO2 has the first peak at 0.47 nm and the g(r) for the O atom of ethanol is slightly reduced at 0.40-0.65 nm. This implies that the interactions of the vinyl group with ethanol are slightly affected by CO2 molecules. In toluene, the g(r) for the COM of toluene around the COM of the vinyl group of NS changes little with the presence of compressed CO2, having the first peak at 0.55 nm. The first peak of g(r) for the C atom of CO2 around the COM of the vinyl group appears at a shorter distance of 0.45 nm. The influence of 10 MPa CO2 on the g(r) for the C atom of CO2 around the vinyl group of NS is 2265


The Journal of Physical Chemistry C less significant compared to that around the nitro group in toluene (Figure 9b). In addition, Figure 11 illustrates the solvent (cosolvent) distribution maps around an NS substrate molecule in pure and CO2dissolved solvent phases. The comparison of (a) and (b) implies that the local composition of an NS molecule in toluene is significantly changed by CO2 pressurization at 10 MPa. In contrast, a slight change is apparent for the expanded ethanol phase at the same high CO2 pressure. The results of (c) show a large difference in the distribution of CO2 molecules around an NS molecule; more CO2 molecules can be in contact with the substrate molecule in toluene, compared with ethanol. Those molecular dynamics simulation results support the above-mentioned models of the local structures of an NS molecule in the CO2-dissolved ethanol and toluene phases, as envisaged from the results of hydrogenation runs and in situ highpressure FTIR measurements.

6. CONCLUSIONS The pressurization with CO2 decreases the rate of hydrogenation of NS in both ethanol and toluene and changes the product distribution in toluene but not in ethanol. In toluene, the selectivity to VA, the product of hydrogenation of the nitro group, decreases with CO2 pressure while that to ENB, the one of hydrogenation of the vinyl group, increases. This results from a decrease in the reactivity of the nitro group of NS by interactions with the dissolved CO2 molecules in toluene. Such a change is absent in the case of ethanol because the interactions of the nitro group with the ethanol through its hydroxyl group are more significant. The local composition around an NS molecule changes little for the CO2-dissolved ethanol phase even at high CO2 pressures but does change for the CO2-dissolved toluene phase. In the latter phase, more CO2 molecules occupy the nearest neighbor sites around an NS substrate at higher pressure. This induces the significant interactions of its nitro group with CO2 molecules, which changes the product distribution depending on CO2 pressure. Generally, the CO2-dissolved expanded liquids are interesting media for organic synthetic reactions, in which the product distribution can be tuned by CO2 pressure and the suitable selection of solvent to be used. ’ ASSOCIATED CONTENT

bS

Supporting Information. The phase behavior of the solvents of toluene and ethanol in the presence of pressurized H2 (4 MPa) and CO2 at different CO2 pressures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Japan Society for the Promotion of Science with a Grant-in-Aid for Scientific Research (B) 22360327 and with a Bilateral Program for Joint Research Project with the Chinese Academy of Sciences PG25100001 and by the Ministry of Education, Culture, Sports, Science and

ARTICLE

Technology, Japan, with the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science).

’ REFERENCES (1) Hallett, J. P.; Kitchens, C. L.; Hernandez, R.; Liotta, C. L.; Eckert, C. A. Acc. Chem. Res. 2006, 39, 531. (2) Jessop, P. G.; Subramaniam, B. Chem. Rev. 2007, 107, 2666. (3) Kruse, A.; Vogel, H. Chem. Eng. Technol. 2008, 31, 23. (4) Akien, G. R.; Poliakoff, M. Green Chem. 2009, 11, 1083. (5) Arai, M.; Fujita, S.; Shirai, M. J. Supercrit. Fluids 2009, 47, 351. (6) Wei, M.; Musie, G. T.; Busch, D. H.; Subramaniam, B. J. Am. Chem. Soc. 2002, 124, 2513. (7) Wei, M.; Musie, G. T.; Busch, D. H.; Subramaniam, B. Green Chem. 2004, 6, 387. (8) Kerler, B.; Robinson, R. E.; Borovik, A. S.; Subramaniam, B. Appl. Catal. B Environ. 2004, 49, 91. (9) Liu, R.; Cheng, H.; Wang, Q.; Wu, C.; Ming, J.; Xi, C.; Yu, Y.; Cai, S.; Zhao, F.; Arai, M. Green Chem. 2008, 10, 1082. (10) Liu, R.; Zhao, F.; Fujita, S.; Arai, M. Appl. Catal. A Gen. 2007, 316, 127. (11) Fujita, S.; Akihara, S.; Arai, M. J. Mol. Catal. A Chem. 2006, 249, 223. (12) Fujita, S.; Akihara, S.; Zhao, F.; Hasegawa, M.; Arai, M. J. Catal. 2005, 236, 101. (13) Zhao, F.; Fujita, S.; Akihara, S.; Arai, M. J. Phys. Chem. A 2005, 109, 4419. (14) Zhao, F.; Zhang, R.; Chatterjee, M.; Ikushima, Y.; Arai, M. Adv. Synth. Catal. 2004, 346, 661. (15) Meng, X.; Cheng, H.; Akiyama, Y.; Hao, Y.; Qiao, W.; Yu, Y.; Zhao, F.; Fujita, S.; Arai, M. J. Catal. 2009, 264, 1. (16) Meng, X.; Cheng, H.; Fujita, S.; Hao, Y.; Shang, Y.; Yu, Y.; Cai, S.; Zhao, F.; Arai, M. J. Catal. 2010, 269, 131. (17) Akiyama, Y.; Fujita, S.; Senboku, H.; Rayner, C. M.; Brough, S. A.; Arai, M. J. Supercrit. Fluids 2008, 46, 197. (18) Fujita, S.; Tanaka, T.; Akiyama, Y.; Asai, K.; Hao, J.; Zhao, F.; Arai, M. Adv. Synth. Catal. 2008, 350, 1615. (19) Akiyama, Y.; Meng, X.; Fujita, S.; Chen, Y.; Lu, N.; Cheng, H.; Zhao, F.; Arai, M. J. Supercrit. Fluids 2009, 51, 209. (20) Gohres, J. L.; Kitchens, C. L.; Hallett, J. P.; Popov, A. V.; Hernandez, R.; Liotta, C. L.; Eckert, C. A. J. Phys. Chem. B 2008, 112, 4666. (21) Shukla, C. L.; Hallett, J. P.; Popov, A. V.; Hernandez, R.; Liotta, C. L.; Eckert, C. A. J. Phys. Chem. B 2006, 110, 24101. (22) Skarmoutsos, I.; Dellis, D.; Samios, J. J. Chem. Phys. 2007, 126, 224503. (23) Skarmoutsos, I.; Dellis, D.; Samios, J. J. Chem. Phys. 2007, 126, 044503. (24) Galand, N.; Wipff, G. New J. Chem. 2003, 27, 1319. (25) Swalina, C.; Arzhantsev, S.; Li, H.; Maroncelli, M. J. Phys. Chem. B 2008, 112, 14959. (26) Li, H.; Arzhantsev, S.; Maroncelli, M. J. Phys. Chem. B 2007, 111, 3208. (27) Li, H.; Maroncelli, M. J. Phys. Chem. B 2006, 110, 21189. (28) Song, W.; Biswas, R.; Maroncelli, M. J. Phys. Chem. A 2000, 104, 6939. (29) Xie, X.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem. Res. 2004, 43, 7907. (30) Fujita, S.; Yamada, T.; Akiyama, Y.; Cheng, H.; Zhao, F.; Arai, M. J. Supercrit. Fluids 2010, 54, 190. (31) Minder, B.; Mallat, T.; Pickel, K. H.; Steiner, K.; Baiker, A. Catal. Lett. 1995, 1, 1. (32) Ichikawa, S.; Tada, M.; Iwasawa, Y.; Ikariya, T. Chem. Commun. 2005, 924. (33) Wang, J.; Zhao, F.; Wu, Z. Chem. Phys. Lett. 2010, 492, 49. (34) Harris, J. G.; Yung, K. H. J. Phys. Chem. 1995, 99, 12021. 2266



ARTICLE

(35) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179. (36) Berendsen, H. J. C.; Postma, J. P. M.; Van Gunsteren, W. F.; DiNola, A.; Haak, J. R. J. Chem. Phys. 1984, 81, 3684. (37) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden-Day: San Francisco, CA, US, 1977. (38) Anderson, J. A.; Athawale, A.; Imrie, F. E.; McKenna, F.-M.; McCue, A.; Molyneux, D.; Power, K.; Shand, M.; Wells, R. P. K. J. Catal. 2010, 270, 9. (39) Arai, M.; Nishiyama, Y.; Ikushima, Y. J. Supercrit. Fluids 1998, 13, 149. (40) Suzuki, K.; Sue, H.; Itou, M.; Smith, R. L.; Inomata, H.; Arai, K.; Saito, S. J. Chem. Eng. Data 1990, 35, 63. (41) Jennings, D. W.; Lee, R.; Teja, A. S. J. Chem. Eng. Data 1991, 36, 303. (42) Galicia-Luna, L. A.; Ortega-Rodriguez, A. J. Chem. Eng. Data 2000, 45, 265. (43) Ng, H.; Robinson, D. B. J. Chem. Eng. Data 1978, 23, 325.

2267


TiO2 Catalyst in CO2

Recommend Documents