Article pubs.acs.org/IECR
Chemoselective Hydrogenation of Cinnamaldehyde over a Pt-Lewis Acid Collaborative Catalyst under Ambient Conditions Hongli Liu, Zhong Li, and Yingwei Li* School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China S Supporting Information *
ABSTRACT: A novel Pt-Lewis acid collaborative catalyst system for selective hydrogenation of cinnamaldehyde is developed. The Pt/MIL-101 catalyst is able to efficiently catalyze the selective hydrogenation of the CC group in cinnamaldehyde to hydrocinnamaldehyde at atmospheric pressure and room temperature with >99.9% selectivity at conversions >99.9%. The remarkably enhanced catalytic activity and selectivity of Pt/MIL-101 can be attributed to the synergism effect between highly dispersed Pt and Lewis acid sites. In situ ATR-IR spectroscopic studies and reaction results demonstrated that the Lewis acid sites on MIL-101 suppressed the reactivity of the CO bond in cinnamaldehyde while enhancing the hydrogenation activity of the conjugated CC bond through a strong interaction with the CO bond, which subsequently inhibited the consecutive hydrogenation of the produced hydrocinnamaldehyde. The kinetic parameters of cinnamaldehyde hydrogenation over the Pt/ MIL-101 catalyst were investigated, and a kinetic model was established based on the reaction mechanism and compared with experimental observations.
1. INTRODUCTION The ever increasing demand for natural resources has triggered an enormous amount of interest in green sustainable chemistry.1 In this sense, the production and processing of chemicals especially require more efficient processes working under mild conditions (even ambient conditions) employing easily recoverable catalysts.1,2 Particularly, it is noticeable that achieving 100% selectivity is often referred to as one of the key goals for present and future research in the heterogeneous catalysis field.3 In this context, selective hydrogenation of α,βunsaturated aldehydes to their corresponding semihydrogenated products has attracted considerable attention in both fundamental research and industrial scales, due to its wide application in the syntheses of many important chemicals and intermediates.4,5 Cinnamaldehyde, a typical α,β-unsaturated aldehyde, has two distinct competing sites of hydrogenation: a CO bond and a conjugated CC group, which generally leads to a complex reaction network involving a parallel and consecutive reduction at different functional groups (Figure 1). For instance, cinnamyl alcohol from the CO bond hydrogenation and hydro-
cinnamaldehyde from the conjugated CC bond hydrogenation are both widely used in pharmaceuticals, perfumes, and flavors. Despite their importance, however, achieving 100% selectivity toward a desirable hydrogenated product without any side-product formation, even at a thermodynamically more favorable CC moiety, is still quite difficult. Thus, there is a strong incentive to develop highly active and chemoselective catalysts for the sole hydrogenation of the CC (or CO) bond in order to avoid any loss of energy efficiency and waste post-treatment. In view of these settings, there have been many attempts to find alternative and more environmentally sound catalytic hydrogenation systems. For the selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde, a number of effective heterogeneous catalysts have been explored, e.g., Pd/ γ-Al2O3,6 Au/CNTs,7 Pt/SBA-15,8 Pd/C,9 Pt/MTNTs,10 ̂ u/SiO2,11 Pt/CeO2−ZrO2,12 Rh/HAP,13 PdAu/MSN,14 PtA and Cu−Au/SiO2.15 Although these catalytic systems are efficient for the hydrogenation, the attainment of high hydrocinnamaldehyde selectivity at (almost) quantitative conversion still remains a challenge, due to the complexity of the hydrogenation system involving a rather complicated reaction network. It is well-known that specific sites are able to promote different molecular reactions taking place on the surface of a catalyst.3 Thus, the selectivity could be manipulated efficiently based on understanding of the molecular interrelations. Because a Lewis acid can coordinate with the Lewis basic CO group of cinnamaldehyde,16,17 such an acid−base interaction affects the electron distribution of the conjugated CC group, which Received: Revised: Accepted: Published:
Figure 1. Reaction network of cinnamaldehyde hydrogenation. © 2015 American Chemical Society
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November 4, 2014 January 19, 2015 January 22, 2015 January 22, 2015 DOI: 10.1021/ie504357r Ind. Eng. Chem. Res. 2015, 54, 1487−1497
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Industrial & Engineering Chemistry Research would make the CC bond more reactive for hydrogenation. Therefore, the CC bond could be hydrogenated preferentially to the competing CO group. At the same time, the Lewis acid−base interaction between the Lewis acid and the CO group of the produced hydrocinnamaldehyde may inhibit the further hydrogenation. On the other hand, the type of metal active sites for hydrogenation has been demonstrated to influence the reaction selectivity significantly, and platinum is among the most selective metals for the CO group hydrogenation.18−20 To prove that Lewis acids may inhibit the parallel and consecutive hydrogenation of the CO bond, Pt was chosen as the metal active site in this study. Herein, we report a highly efficient and chemoselective heterogeneous Pt catalyst, which was deposited on a Lewis acidic metal−organic framework (MOF) such as chromium terephthalate MIL-101 {Cr3(F,OH)O[(O2C)C6H4(CO2)]3}.21 The Pt-Lewis acid catalyst system showed excellent synergy in selective hydrogenation of the conjugated CC group in cinnamaldehyde, leaving the CO bond untouched. The reaction could be implemented effectively at atmospheric pressure and room temperature with >99.9% selectivity to hydrocinnamaldehyde at cinnamaldehyde conversions >99.9%.
determined quantitatively by atomic absorption spectroscopy (AAS) on a HITACHI Z-2300 instrument. The morphologies of the samples were characterized by a FEI Tecnai G2 fitted with a CCD camera for ease and speed of use. The powders were dispersed in ethanol and dropped on a copper grid for TEM analysis. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AxisUltra DLD system with a base pressure of 10−9 Torr. The surface acidity was measured in a dynamic mode by means of a pulse chromatographic technique of gas-phase adsorption of pyridine (PY, sum of Brönsted and Lewis acid sites) and 2,6-dimethylpyridine (DMPY, Brönsted sites) as probe molecules. The basic probe was introduced into a stream of dehydrated and deoxygenated inert carrier gas flowing through the chromatographic column at 100 °C. Then the chromatographic oven temperature was increased with a heating rate of 10 °C/min. When the temperature was reached to 200 °C, the sample was equilibrated for 1 h at this temperature to desorb the reversibly adsorbed base. Subsequently, the oven temperature was decreased to 100 °C, and the above procedure was repeated. The amount of base adsorbed irreversibly (q) at 200 °C can be calculated by the following equation
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. H2PtCl6·6H2O (A.R.), Cr(NO3)3·9H2O (A.R.), HF (40% aqueous solution), ethanol (A.R.), hexane (A.R.), toluene (A.R.), THF (A.R.), ethyl acetate (A.R.), and isopropyl alcohol (A.R.) were purchased from the Sinopharm Chemical reagent Co., Ltd. 1,4Benzenedicarboxylic acid (98%), polyvinylpyrrolidone (PVP, M.W. = 30000), NaBH4 (98%), and cinnamaldehyde (98%) were purchased from Alfa Aesar. Active carbon was purchased from Sigma-Aldrich. All chemicals were used without further purification. MIL-101 was synthesized according to the reported procedures.21 MIL-101 supported platinum catalysts were prepared by using a simple colloidal deposition method.22 In a typical preparation, the required amount of PVP (PVP monomer/Pt = 10:1, molar ratio) was added to an appropriate volume of H2PtCl6 methanol solution (1 × 10−3 M). The mixture was stirred for 1 h at 0 °C. Then, a freshly prepared methanol solution of NaBH4 (0.1 M, NaBH4/Pt = 5:1, molar ratio) was rapidly added to the mixture under vigorous stirring. After sol formation in a few minutes, the activated MIL-101 was immediately added, and the solution was further stirred for 8 h. Afterward, the sample was washed thoroughly with methanol and dried under vacuum at 100 °C for 2 h to obtain the Pt/ MIL-101. In order to investigate the effects of different supports, an activated carbon supported Pt sample was also prepared by using the same recipe as described above for the synthesis of Pt/MIL-101. 2.2. Catalyst Characterization. Powder X-ray diffraction patterns of the samples were obtained on a Rigaku diffractometer (D/MAX-IIIA, 3 kW) employing Cu Kα radiation (λ = 0.1543 nm) at 40 kV, 40 mA at room temperature. 2θ scans were performed from 2° to 60° at 3°/ min. The low-temperature nitrogen adsorption−desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 instrument. The sample was evacuated at 150 °C for 12 h before the analysis. The metal contents of the samples were
q=
x ΔA · w A
where x is the amount of injected basic probe, μmol; w is the weight of catalyst, g; ΔA is the difference between areas of the two consecutive chromatograms; and A is the area of the second chromatogram. 2.3. Liquid-Phase Hydrogenation of Cinnamaldehyde. The liquid-phase hydrogenation of cinnamaldehyde was carried out in a Teflon-lined stainless steel autoclave equipped with a pressure gauge and a magnetic stirrer. Typically, a mixture of 0.5 mmol of cinnamaldehyde, 24 mg of Pt/MIL-101 catalyst, and 5 mL of isopropyl alcohol was introduced into the reactor at room temperature. Air in the autoclave was purged several times with H2. Then, the reaction began by starting the agitation (600 r/min) when hydrogen was regulated to the desired pressure after the reaction temperature was reached. After reaction for the desired time, the solid was isolated from the solution by centrifugation. The products in the solution were quantified and identified by a gas chromatograph (Thermo Focus) equipped with a HP-innowax capillary column (30 m × 0.25 mm) and a flame ionization detector (FID) using n-tetradecane as an internal standard. The typical gas chromatograph analysis program was as follows: initial column temperature 120 °C, hold 2 min, to 240 °C at 10 °C/min, and hold for 6 min. For the recyclability test, the reactions were performed maintaining the same reaction conditions as described above, except using the recovered catalyst. At the end of the catalytic reaction, the mixture was centrifuged, and the solid was recovered, which was thoroughly washed with isopropyl alcohol, and then reused in the next run. To determine whether there was any leaching of Pt during the reaction or not, hydrogenation of cinnamaldehyde was performed at 25 °C, and the solid catalyst was removed from the reaction solution by filtration after 20 min. The reaction was continued with the filtrate in the absence of catalyst for an additional 40 min to check if the solution in the absence of solid exhibited further reactivity. 1488
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Industrial & Engineering Chemistry Research 2.4. In Situ ATR-IR Spectroscopy. ATR-IR spectra were recorded on a Thermo Fisher iS10 equipped with a liquid nitrogen cooled MCT detector. The spectra were obtained by averaging 32 scans at a resolution of 1 cm−1. The thin film of support or catalyst powder deposited on the ZnSe element for ATR-IR spectroscopic study was prepared as follows. A suspension of ca. 50 mg of support or catalyst powder in 10 mL of isopropyl alcohol was stirred overnight to eliminate any serious agglomeration in solution. Then, the slurry was dropped onto a ZnSe internal reflection element (IRE) and dried out at room temperature, which was subsequently dried in a vacuum oven for a complete evaporation of isopropyl alcohol. The asprepared solid film adhered to the ZnSe IRE and exhibited excellent stability under the investigated conditions. For the ATR-IR monitoring of the hydrogenation reaction, typically, the H2-saturated isopropyl alcohol solution of cinnamaldehyde (0.1 M) was first prepared through vigorously stirring the mixture for 1 h in H2. The H2-saturated mixture was quickly added to the above coated ZnSe IRE mounted in the liquid cell under H2 atmosphere. Subsequently, the cell was quickly sealed, and the in situ ATR-IR spectra of the solid− liquid interface started to collect. It should be mentioned that the cell was connected to a hydrogen cylinder to maintain 1 atm H2 during the experiment. The spectra of adsorbed cinnamaldehyde on MIL-101 or Pt/MIL-101 were measured using the similar procedures as described above, except under Ar atmosphere instead of H2.
3. RESULTS AND DISCUSSION 3.1. Catalysts Characterization. The powder XRD patterns of the MIL-101 and Pt/MIL-101 samples are shown in Figure S1. For the pristine MIL-101, the characteristic diffraction peaks were in excellent agreement with the already published XRD patterns.21 When Pt nanoparticles (NPs) were incorporated into MIL-101, the X-ray diffractograms showed no significant changes relative to those of the support, implying that the MIL-101 support was very stable during the loading of the metal. Moreover, the characteristic diffraction peaks corresponding to Pt were not observed, due to the low doping amount of Pt in the samples. The actual Pt loading in all samples was about 1 wt %, as measured by AAS analysis. The specific surface areas, pore volumes, and average pore diameters of the synthesized samples were determined by N2 physisorption, and the results are given in Figure 2 and Table 1. As can be seen from Figure 2a, the N2 adsorption isotherms of MIL-101 before and after loading Pt both showed a mixture of type I and IV curves. The BET surface area and total pore volume of MIL-101 were calculated to be 3249 m2 g−1 and 1.68 cm3 g−1, respectively. As compared to MIL-101, the BET surface area and pore volume of 1 wt % Pt/MIL-101 decreased to 2731 m2 g−1 and 1.37 cm3 g−1, respectively, mainly owing to the cavity blockage of MIL-101 by the highly dispersed metal NPs. Moreover, the possible residual protecting agent PVP in Pt/MIL-101 could also have some influence on the gas adsorption capacities. As indicated in the pore size distribution curves (Figure 2b), the incorporation of Pt NPs led to a slight decrease of the pore diameters of MIL-101. TEM images of the Pt/MIL-101 material were obtained to visually confirm the distribution and size of Pt NPs. The micrographs showed that small Pt NPs were uniformly distributed on the MIL-101, and no significant formation of aggregate was observed (Figure 3a and 3b). The size distribution histogram of Pt NPs displayed in Figure 3c was
Figure 2. Nitrogen adsorption isotherms (a) and pore size distribution curves (b) at 77 K for the pristine MIL-101 (●), Pt/MIL-101 (◆), and Pt/MIL-101 after a catalytic reaction (▲).
Table 1. Physicochemical Properties of the Pt/MIL-101 Catalyst surface acidity (μmol g−1) sample MIL-101 Pt/MIL-101 Pt/C Pt/MIL-101 reused-5 times
N2 physisorption
PY
DMPY
Lewis acidity
SBET (m2 g−1)
SLangmuir (m2 g−1)
Vpore (cm3 g−1)
2023 1939 163 1845
42 28 71 31
1981 1911 92 1814
3249 2731 1105 2557
4542 3449 1381 3268
1.68 1.37 0.75 1.33
obtained by counting more than 200 particles, and the mean diameter of these Pt NPs was 2.3 ± 0.55 nm. The high dispersion is believed to be related to the high porosity and surface area of the support. The high-resolution image in Figure 3a showed that the interplanar spacing of the particle lattice was 0.197 nm, which agreed well with the (200) lattice pacing of fcc Pt. The acidity of the materials was also investigated using a pulse chromatographic methodology, and the results are summarized in Table 1. The total amount of Lewis acidic sites on parent MIL-101 was 1.98 mmol g−1, which root from potentially unsaturated chromium sites upon removal of the terminal water molecules in the MOF structure. After incorporation of Pt into MIL-101, the quantity of Lewis acidic sites showed a slight decrease as compared to the pristine MIL101. 1489
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Figure 3. TEM images of Pt/MIL-101 (a, b) and corresponding size distribution of Pt NPs (c); and the Pt/MIL-101 after a catalytic reaction (d).
The surface compositions and the chemical states of Pt were analyzed by XPS. As displayed in Figure 4a, the Pt 4f, C 1s, O 1s, and Cr 2p were observed from the complete spectral survey of the Pt/MIL-101 sample. The binding energies of the Pt 4f7/ 2 and Pt 4f5/2 peaks were observed at 71.8 and 75.2 eV,
respectively. According to previous reports, the binding energies of Pt 4f 7/2 at 71.1−72.0 eV and Pt 4f 5/2 at 74.2−75.6 eV could be attributed to the characteristics of the metallic Pt.23−25 The results of XPS characterization indicated that the Pt species was mostly in a metallic state on the Pt/ MIL-101. 3.2. Selective Hydrogenation of Cinnamaldehyde. The hydrogenation of cinnamaldehyde was carried out at 25 °C and atmospheric H2 pressure. In order to determine the baseline rate, the hydrogenation reaction was initially performed without catalyst or with MIL-101 in the presence of H2, or using Pt/ MIL-101 as catalyst in the absence of H2. Essentially no conversion was found in these blank reactions (entries 1−3, Table 2), confirming as expected the need of both H2 and a metal to perform the hydrogenation of cinnamaldehyde. Subsequently, some commonly used solvents, such as isopropyl alcohol, hexane, toluene, and ethanol, were employed in the reaction. Results of the cinnamaldehyde hydrogenation indicated that isopropyl alcohol was the best solvent (entries 4−9, Table 2). Although isopropyl alcohol is known as a hydrogen transfer reagent,26 these results would suggest that the effect of transfer hydrogenation was negligible under the investigated conditions. It is noteworthy that a relatively low selectivity to hydrocinnamaldehyde was observed in ethanol due to the formation of the aldol condensation side products (entry 7, Table 2). When isopropyl alcohol was used as solvent, the reaction proceeded up to complete conversion with a >99.9% selectivity to hydrocinnamaldehyde via sole CC bond hydrogenation (entry 4, Table 2). Besides cinnamaldehyde, the Pt/MIL-101 system was also found to be highly efficient in the selective hydrogenation of other α,β-unsaturated aldehydes (Table S1). Notably, it has been reported that Pt-based catalysts generally exhibit activity for both CO and CC bonds hydrogenation for α,β-unsaturated aldehydes.12,27−31 However, only hydrocinnamaldehyde was produced over the present catalyst system, without the formation of either of the CO hydrogenation products, i.e., cinnamyl alcohol and hydrocinnamyl alcohol. To account for this phenomenon, we
Figure 4. XPS spectra of Pt/MIL-101: (a) survey spectra of Pt/MIL101 and (b) Pt 4f. 1490
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Industrial & Engineering Chemistry Research Table 2. Catalytic Results of the Hydrogenation of Cinnamaldehyde (CAL)a sel. (%) entry
catalyst
1 2
MIL-101
3b
Pt/MIL101 Pt/MIL101 Pt/MIL101 Pt/MIL101 Pt/MIL101 Pt/MIL101 Pt/MIL101 Pt/MIL101 Pt/C
4 5 6 7 8 9 10 11
solvent
t (h)
conv. (%)
HCAL
1
>99.9
>99.9
isopropyl alcohol isopropyl alcohol isopropyl alcohol isopropyl alcohol hexane
3
1
63.2
>99.9
toluene
1
72.2
>99.9
ethanol
1
>99.9
97.3
THF
1
48.6
87.6
ethyl acetate
1
60.0
>99.9
10
>99.9
96.9
3
3.5
70.5
isopropyl alcohol isopropyl alcohol
COL
HCOL
3 3
8.2
Figure 5. Reuses of the Pt/MIL-101 catalyst in the hydrogenation of cinnamaldehyde. Reaction conditions: cinnamaldehyde (0.5 mmol), catalyst (Pt 0.25 mol %), isopropyl alcohol (5 mL), 25 °C, 1 atm H2, 1 h.
4.2
essentially no efficiency loss was observed in the cinnamaldehyde hydrogenation for up to five runs. For the heterogeneity test, the solid catalyst was filtered from the reaction solution after 20 min, and the resultant solution was continued for an additional 40 min without observing any further reactivity under the conditions (Figure S3). Meanwhile, AAS analysis of the solution indicated that no metal had leached into the liquid phase during reaction. These findings were in accordance with the results from TEM, N2 physisorption, and XRD characterization of the reused catalyst. The XRD patterns of the catalyst after the fifth run showed no appreciable changes as compared to the fresh Pt/MIL-101 (Figure S1). The BET surface area and pore volume of the reused catalyst displayed a slight decrease, which was possibly related to a little residual reaction product remaining in the pores of the catalyst. Furthermore, TEM images of the recycled catalyst showed no significant aggregation of Pt particles (Figure 3d). These results demonstrated the high stability and reusability of the Pt/ MIL-101 catalyst under the investigated conditions. 3.4. Kinetic Parameters in the Chemoselective Hydrogenation of Cinnamaldehyde. 3.4.1. Effects of Stirring Speed. In a heterogeneous catalytic reaction, external mass transfer could play an important role in the conversion and product formation, because the catalyst is usually in a different phase from the reactant. Enhancing stirring speed is an efficient approach to eliminate external mass transfer limitations. To obtain intrinsic kinetic data independent of external diffusion effects, the hydrogenation reaction over the Pt/MIL-101 catalyst was conducted under the same conditions (cinnamaldehyde/metal molar ratio = 400:1, 1 atm, 25 °C, 1 h), except using different stirring speeds. As shown in Figure 6, the conversion of cinnamaldehyde was obviously enhanced with an increase of stirring speed from 0 to 400 r/min, indicating that the reaction rate was limited by external mass transfer under low agitation. With a further increase of the stirring speed, there was no obvious improvement of reaction rate, suggesting that the stirring rate was sufficiently high for negligible gas−liquid mass-transfer effect. Therefore, the stirring speed was set at 600 r/min for all the reaction tests in this study. 3.4.2. Effects of Pt Concentration. The dependence of the reaction rate of cinnamaldehyde hydrogenation on the Pt concentration employed was investigated for the range from 0.125 to 1.0 mM. As it can be seen in Figure 7a, the reaction
3.1 10.8
18.7
a
Reaction conditions: cinnamaldehyde (0.5 mmol), catalyst (Pt 0.25 mol %), solvent (5 mL), 25 °C, 1 atm H2. bIn the absence of H2.
performed the hydrogenation of cinnamaldehyde at a longer reaction time to investigate the further hydrogenation of the hydrocinnamaldehyde product. Interestingly, only 3.1% hydrocinnamaldehyde was further hydrogenated at the CO bond to hydrocinnamyl alcohol (HCOL) after an additional time of 9 h (entry 10, Table 2). These results clearly demonstrated that the CC bond hydrogenation was relatively faster than the CO bond, and the CO hydrogenation was effectively suppressed over the Pt/MIL-101 catalyst. For comparison, a Pt/C catalyst prepared by using the same recipe as Pt/MIL-101 preparation was also tested in cinnamaldehyde hydrogenation. The use of activated carbon as support led to inferior activity and selectivity in the hydrogenation, giving only 3.5% conversion of cinnamaldehyde even though the reaction time was prolonged to 3 h (entry 11, Table 2). Interestingly, TEM micrographs (Figure S2) showed that the Pt NPs were highly dispersed on activated carbon and exhibited similar particle size and morphology as Pt/MIL-101. Moreover, the high-resolution images of Pt/C indicated that the interplanar distance between adjacent lattice planes was ascribed to the (200) plane of fcc Pt, which was also identical to the Pt/MIL-101. The results suggested that the remarkably different catalytic performances of the two catalysts could not be related to the size and morphology effect of Pt NPs. It was therefore suggested that a new catalytic site was present in Pt/ MIL-101 as compared to Pt/C, a site which could enhance the activity and selectivity of the CC bond hydrogenation. 3.3. Stability and Reusability of Pt/MIL-101. One important issue when involving heterogeneous systems is to verify the stability and reusability of catalyst during the course of reaction. To address this point we performed a recycling experiment and heterogeneity test, respectively. For the recyclability of the Pt/MIL-101 catalyst, the hydrogenation of cinnamaldehyde was carried out at 25 °C and 1 atm H2 pressure. The results included in Figure 5 indicated that 1491
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between the concentration of Pt in the reaction and the hydrogenation rate. 3.4.3. Effects of Reaction Temperature. It is known that the reaction temperature and H2 pressure may affect the chemoselectivity in hydrogenation of α,β-unsaturated aldehydes.32,33 Thus, the effects of reaction temperature on the cinnamaldehyde hydrogenation were first investigated at various temperatures in the range of 10−30 °C under 1 atm H2. The results illustrated in Figure 8a indicated the exceptional catalytic
Figure 6. Effects of stirring speed on the hydrogenation of cinnamaldehyde. Reaction conditions: cinnamaldehyde (0.5 mmol), catalyst (Pt 0.25 mol %), isopropyl alcohol (5 mL), 25 °C, 1 atm H2.
Figure 8. Cinnamaldehyde (CAL) concentration as a function of time for different reaction temperatures (a) and the Arrhenius plot (b). Reaction conditions: cinnamaldehyde (0.5 mmol), catalyst (Pt 0.25 mol %), isopropyl alcohol (5 mL), 1 atm H2.
activity of Pt/MIL-101 even at 10 °C. As expected, increasing the reaction temperature from 10 to 30 °C led to continuous shortening of the reaction time for achieving complete conversion of cinnamaldehyde. Importantly, the selectivity of the desired hydrocinnamaldehyde product was kept at >99.9%, unchanged with the reaction temperature. Notably, the concentration of cinnamaldehyde (CAL) was observed to decrease linearly with the reaction time up to reaction completion, indicating a typical characteristic of zero-order dependence on cinnamaldehyde concentration. By plotting ln k versus the inverse of reaction temperature, a highly linear Arrhenius plot was obtained from the kinetic parameters (Figure 8b). From the plot, the apparent activation energy (Ea) and pre-exponential factor (A) were determined from the slope and intercept of the straight line for the five temperatures covered, which was ca. 42.8 kJ/mol and 5.48 × 104 min−1 MPa−1, respectively. The value of activation energy
Figure 7. Cinnamaldehyde (CAL) concentration as a function of time for different Pt concentrations (a) and plot of ln r0 vs ln[Pt] (b). Reaction conditions: cinnamaldehyde (0.5 mmol), isopropyl alcohol (5 mL), 25 °C, 1 atm H2.
rate of cinnamaldehyde (CAL) hydrogenation significantly increased with an increase in Pt concentration. For instance, the time to achieve complete conversion of cinnamaldehyde was about 1 h with 0.25 mM Pt but only 18 min in the presence of 1.0 mM Pt. It has to be pointed out that the only product detected in all cases was hydrocinnamaldehyde. Plotting of the initial reaction rate versus Pt concentration in the system, both in logarithmic scales (Figure 7b), follows a linear relationship with a slope of 0.92, indicating a first-order dependence 1492
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saturated isopropyl alcohol at different catalyst surfaces was investigated. The difference spectra of adsorbed cinnamaldehyde on MIL-101 are displayed in Figure 10a. In this plot, the
for the hydrogenation of cinnamaldehyde over the Pt/MIL-101 is relatively lower than that for PdAu0.2/MSN (57 kJ mol−1),14 Pt0.05Au/SiO2 (55 kJ mol−1),11 and Pd/C (65 kJ mol−1) reported in the literature.34 3.4.4. Effects of H2 Pressure. In another set of experiments, the effect of H2 pressure on the hydrogenation performance was investigated at H2 pressures ranging from 0.1 to 0.8 MPa. As shown in Figure 9a, the cinnamaldehyde (CAL) conversion
Figure 10. ATR-IR difference spectra of CAL in Ar-saturated isopropyl alcohol solutions on MIL-101 (spectrum of MIL-101 film was subtracted) (a) and Pt/MIL-101 (spectrum of Pt/MIL-101 film was subtracted) (b).
spectrum of the pristine MIL-101 (Figure S4) was subtracted. Bands at 1021 and 1507 cm−1 can be correlated to phenyl C C stretching vibration.35 The band at 1626 cm−1 relates to the CC bond conjugated with the carbonyl group in cinnamaldehyde, which showed no band shifts. A CO stretching vibration of cinnamaldehyde in the presence of MIL101 could be visualized at 1663 cm−1. Comparably, the aldehydic CO band of pure cinnamaldehyde can be observed at 1676 cm−1 (Figure S5). The observed remarkable red shift (by 13 cm−1) of the carbonyl band upon adsorption may be attributed to coordination to the Cr Lewis acid sites of MIL101 through one of the oxygen lone pairs.35−37 Interestingly, the band of cinnamaldehyde v(CO) adsorbed on the MIL101 support (1663 cm−1) quickly reached a maximum intensity after 1 min, implying a strong interaction with the support. In contrast, no appreciable change in band intensity and position of the aldehydic CO was observed for the Pt/C and C support (Figures S7 and S8), indicating a weak adsorption of cinnamaldehyde on the carbon. Considering the significant difference in surface acidity between the MIL-101 and carbon supports, these results suggested that Lewis acidic sites in MIL101 played an important role in cinnamaldehyde adsorption. Similarly to MIL-101, the band intensity and position of the CO stretching vibration of cinnamaldehyde were observed to
Figure 9. Cinnamaldehyde (CAL) concentration as a function of time at different H2 pressures (a) and plot of lg r0 vs lg PH2 (b). Reaction conditions: cinnamaldehyde (0.5 mmol), catalyst (Pt 0.25 mol %), isopropyl alcohol (5 mL), 25 °C.
was enhanced obviously with an increase in H2 pressure, while the selectivity of hydrocinnamaldehyde was still maintained at >99.9%. When H2 pressure was increased from 0.1 to 0.8 MPa, the reaction rate over Pt/MIL-101 increased 6-fold. It is plausible that a high pressure could improve the solubility of hydrogen, rendering more hydrogen molecules accessible in the reaction solution. The logarithm of r0 plotted against logarithm of PH2 with the initial reaction rates (r0) determined from the time evolution of cinnamaldehyde concentration for each run is presented in Figure 9b. The fitting data followed a good linear relationship with a slope of ca. 0.90, indicating a first-order dependence between the H2 pressure and reaction rate. 3.5. In Situ ATR-IR Spectroscopic Study of SingleComponent Adsorption. In situ ATR-IR spectroscopy was employed to gain some molecular insight into the adsorption and surface species present on the catalysts under the reaction conditions. Initially, cinnamaldehyde (CAL) adsorption in Ar1493
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vibration of hydrocinnamaldehyde was observed, and its intensity was increased with reaction time. At the same time, the intensity of the characteristic absorption band at 1664 cm−1 for the CO stretching vibration of cinnamaldehyde gradually declined with time. The above observation suggested that hydrocinnamaldehyde was obtained via selective hydrogenation of the CC bond in cinnamaldehyde, and hydrocinnamaldehyde was competitively adsorbed on the catalyst with cinnamaldehyde. The strong adsorption of hydrocinnamaldehyde was suggested to be responsible for the high selectivity because such an interaction could inhibit further hydrogenation of hydrocinnamaldehyde. This speculation was also confirmed from the reaction results obtained under similar conditions (entries 4 and 10, Table 2). It is worth mentioning that the hydrogenation rates in ATR-IR experiments were obviously slower than that obtained in a batch reactor, likely due to the slower mass-transfer without stirring. In addition, the band intensities at 1020 (phenyl CC stretching vibration), 1430 (ν(COO−) groups within the MIL101 framework), 1500 (phenyl CC stretching vibration), and 1547 cm−1 (ν(COO−) groups of MIL-101) were also gradually increased with the growing of the band at 1715 cm−1. As can be clearly seen from Figure 11b, the asymmetric stretching vibration of the dicarboxylate COO− groups within MIL-101 framework at 1547 cm−1 consistently became stronger and broader with the production of hydrocinnamaldehyde. This is possibly due to the CC bond of cinnamaldehyde being adsorbed on the Pt surface, while the conjugated CO bond was adsorbed on the unsaturated Cr sites of MIL-101. However, with more and more hydrocinnamaldehyde produced via hydrogenation of cinnamaldehyde, less and less CC bonds were adsorbed on Pt. At the same time, the CO bond of hydrocinnamaldehyde gradually replaced that of cinnamaldehyde to adsorb on the Lewis Cr sites of MIL-101. Therefore, the electron densities of both the CO bond and its adsorbed Cr atom could undergo a redistribution, which would consequently affect the asymmetric stretching vibration of the COO− groups coordinated with Cr within the MIL-101 framework.38 3.7. Reaction Mechanism for the Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde over Pt/MIL101. On the basis of the catalyst characterization and ATR-IR study, a plausible reaction mechanism for the hydrogenation reaction is proposed (Figure 12). It is well-known that platinum NPs are capable of dissociating activation of H2. In view of the very small Pt particles (2.3 ± 0.55 nm) in Pt/MIL-101, the steric constraint generated by the phenyl group of the cinnamaldehyde for its adsorption on the surface of Pt particles could be eliminated.10,39 On the other hand, low coordination sites (such as corners, kinks, adatoms, and defective sites) in Pt NPs favor interactions with the CC bond, which increase accordingly as the size of Pt NPs decreases. Thus, the CC bond may adsorb on the Pt NPs with a small size.10,39 It is known that Lewis acids may coordinate to the CO group via one of the oxygen lone pairs.35,40 Moreover, the ATR-IR spectra also demostrated the coordination of the CO group to the Lewis acidic Cr site of the MIL-101 support (Figure 10). Therefore, an interaction between the CO group with the Lewis acidic Cr site should be involved in the mechanism. Such a Lewis acid−base interaction might affect the electron distribution of the CC group, which gave rise to relatively high electron density on the carbon atom of the CC close to the CO bond. As a consequence, the CC bond should
be almost identical for Pt/MIL-101 (Figure 10b). This result indicated that the presence of Pt particles did not increase the adsorption capacity with respect to the CO bond and also further supported a strong interaction between the carbonyl bond of cinnamaldehyde and MIL-101. The phenyl ν(CC) band at 1502 cm−1 became weaker and slightly red-shifted (by ca. 5 cm−1) as compared to MIL-101-adsorbed cinnamaldehyde. In addition, the bands at 1547 and 1429 cm−1, which are attributed to the stretching vibration of the dicarboxylate −(O−C−O)− groups within the MIL-101 framework, also changed in intensity and symmetry. It should be mentioned that the two bands for MIL-101 were still observed in the difference spectra, possibly due to the enhancement in intensity after cinnamaldehyde adsorption. Unfortunately, the conjugated CC bond with the CO at 1625 cm−1 in the presence of Pt/MIL-101 revealed no clear changes as compared to MIL101 because of the low ratio of Pt to cinnamaldehyde (1:400) in the system. From the above observations, a strong adsorption of cinnamaldehyde via its CO bond (possibly on the Cr Lewis acid sites of MIL-101) was apparently present in the systems together with a plausible adsorption of the conjugated CC group on the Pt sites, both responsible for the observed significant variations in ATR-IR spectra. 3.6. ATR-IR Monitoring of the Hydrogenation Reaction. To gain more in-depth information about the hydrogenation reaction, the reaction process was also followed by in situ ATR-IR. As seen in Figure 11, a new characteristic absorption band at 1715 cm−1 related to the CO stretching
Figure 11. ATR-IR spectra of CAL in H2-saturated isopropyl alcohol solutions on Pt/MIL-101 at 25 °C in the ranges of 2000−1000 cm−1 (a) and 1750−1400 cm−1 (b). 1494
DOI: 10.1021/ie504357r Ind. Eng. Chem. Res. 2015, 54, 1487−1497
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Industrial & Engineering Chemistry Research
Figure 12. Plausible mechanism for the hydrogenation of cinnamaldehyde over the Pt-Lewis acid collaborative Pt/MIL-101 catalyst.
r = − d[H 2]/dt = ka,1[H 2]θ 2 S1 = kd,1θ 2 H−S1
undergo an electrophilic addition more easily to form hydrocinnamaldehyde. Meanwhile, the hydrogenation of C O in cinnamaldehyde was inhibited due to its strong adsorption interaction with the Lewis Cr sites. As a result, no cinnamyl alcohol was produced in the hydrogenation reaction over the Pt/MIL-101 catalyst (Table 2). According to the literature reports,6,8 the product hydrocinnamaldehyde could be further hydrogenated to hydrocinnamyl alcohol (Figure 1). However, in this study, prolonging the reaction time did not enhance the yield of hydrocinnamyl alcohol (HCOL) significantly for our catalytic system (entry 10, Table 2). These results indicated that the hydrogenation of the CO bond in hydrocinnamaldehyde was effectively suppressed over Pt/MIL-101, which was suggested to be related to the Lewis acid−base interaction, as demonstrated in the ATR-IR study. Such an interaction made the CO bond of hydrocinnamaldehyde to adsorb onto the Lewis acidic Cr site instead of Pt, thereby inhibiting the CO hydrogenation to form hydrocinnamyl alcohol. 3.8. Kinetic Model of Cinnamaldehyde Hydrogenation over Pt/MIL-101. Based on the experimental results and reaction mechanism, the four elementary steps over Pt/MIL101 for hydrogenation of cinnamaldehyde, including reactant activation by adsorption of H2 (1) and cinnamaldehyde (2), surface reaction between the adsorbed reactants (3) and desorption of product (4), can be written as follows
where θS1 and θH−S1 refer to the number of free adsorption sites and the surface coverage of hydrogen on site S1, respectively. According to the conventional Langmuir−Hinshelwood mechanism, the assumption of quasi-equilibrated H2 adsorption on S1 site obtains the following expression for surface coverage of hydrogen θH − S1 =
C H2 = PH2/H
where H is a constant. Surface coverage of hydrogen can be obtained by substituting CH2 with the above equation, which is expressed as follows: θH − S1 =
ka,2
θH − S1 =
(2)
k d,3
k4
RH−S2 + H−S1 → RH 2 + S1 + S2
KHPH2
KHPH2 =
1/2 KH PH2
Combining the preceding equations gives the following rate expression
ka,3
R−S2 + H−S1 ⇀ RH−S2 + S1 ↽⎯⎯⎯
1+
It is well-known that the adsorption of H2 on Group VIII metals is weakly dissociative adsorption.41,42 As a consequence, (K1PH2)1/2 ≪1. So the equation for the surface coverage of hydrogen can be simplified to
R + S2 ⎯⇀ R−S2 k d,2
KHPH2
KH = α /H
(1)
↽⎯⎯⎯
αC H2
where CH2 refers to the liquid-phase concentration of H2. According to Henry’s law
ka,1 k d,1
1+
α = ka,1/kd,1
H 2 + 2S1 ⇀ 2H−S1 ↽⎯⎯⎯
αC H2
(3)
1/2 2 r = kd,1θ 2 H − S = kd,1( KH PH2 ) = kd,1KHPH2
= ka,1PH2/H = kPH2
(4)
where S1 represents the surface Pt site on the catalyst, S2 represents the unsaturated Cr site in the MIL-101 support, and R refers to cinnamaldehyde. On the basis of the ATR-IR study, cinnamaldehyde could be rapidly adsorbed on the S2 site. Furthermore, the halfhydrogenated intermediate RH was never detected in the reaction mixture using the Pt/MIL-101 catalyst. It could be speculated that the RH species was very active and thus underwent further hydrogenation (step 4) very quickly. Therefore, steps 2 and 4 could be viewed as a state of rapid equilibrium and would not be considered as a candidate for the rate-determining step. Assuming step 1 to be the rate-determining step, the reaction rate can be described by
with k = ka,1/H
In another case, we assume step 3 as the rate-determining step. The saturated adsorption of cinnamaldehyde on the surface of catalyst is very fast, so [RH−S2] can be considered as a constant. Analogously, the corresponding expression of the reaction rate can be described as 1/2 1/2 r = ka,3[RH − S2]θH − S = ka,3[R H − S2] KH PH2 = βPH2
with
β = ka,3[RH − S2] KH 1495
DOI: 10.1021/ie504357r Ind. Eng. Chem. Res. 2015, 54, 1487−1497
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(4) Saudan, L. A. Hydrogenation Processes in the Synthesis of Perfumery Ingredients. Acc. Chem. Res. 2007, 40, 1309. (5) Dupau, P. In Organo-metallics as Catalysts in the Fine Chemical Industry; Beller, M., Blaser, H.-U., Eds.; Springer: Heidelberg, 2012. (6) Galletti, A. M. R.; Antonetti, C.; Venezia, A. M.; Giambastiani, G. An easy microwave-assisted process for the synthesis of nanostructured palladium catalysts and their use in the selective hydrogenation of cinnamaldehyde. Appl. Catal., A 2010, 386, 124. (7) Zhang, X.; Guo, Y. C.; Zhang, Z. C.; Gao, J. S.; Xu, C. M. High performance of carbon nanotubes confining gold nanoparticles for selective hydrogenation of 1,3-butadiene and cinnamaldehyde. J. Catal. 2012, 292, 213. (8) Handjani, S.; Marceau, E.; Blanchard, J.; Krafft, J.; Che, M.; MäkiArvela, P.; Kumar, N.; Wärnå, J.; Murzin, D. Y. Influence of the support composition and acidity on the catalytic properties of mesoporous SBA-15, Al-SBA-15, and Al2O3-supported Pt catalysts for cinnamaldehyde hydrogenation. J. Catal. 2011, 282, 228. (9) Zhao, F.; Ikushima, Y.; Chatterjee, M.; Shiraia, M.; Araib, M. An effective and recyclable catalyst for hydrogenation of α,β-unsaturated aldehydes into saturated aldehydes in supercritical carbon dioxide. Green Chem. 2003, 5, 76. (10) Hsu, C. Y.; Chiu, T. C.; Shih, M. H.; Tsai, W. J.; Chen, W. Y.; Lin, C. H. Effect of Electron Density of Pt Catalysts Supported on Alkali Titanate Nanotubes in Cinnamaldehyde Hydrogenation. J. Phys. Chem. C 2010, 114, 4502. (11) Hong, Y. C.; Sun, K. Q.; Zhang, G. R.; Zhong, R. Y.; Xu, B. Q. Fully dispersed Pt entities on nano-Au dramatically enhance the activity of gold for chemoselective hydrogenation catalysis. Chem. Commun. 2011, 47, 1300. (12) Bhogeswararao, S.; Srinivas, D. Intramolecular selective hydrogenation of cinnamaldehyde over CeO2-ZrO2-supported Pt catalysts. J. Catal. 2012, 285, 31. (13) Huang, L.; Luo, P.; Xing, W.; Huang, J.; Pei, W.; Liu, X.; Wang, Y.; Wang, J. Selective Hydrogenation of Nitroarenes and Olefins over Rhodium Nanoparticles on Hydroxyapatite. Adv. Synth. Catal. 2012, 354, 2689. (14) Yang, X.; Chen, D.; Liao, S.; Song, H.; Li, Y.; Fua, Z.; Su, Y. High-performance Pd-Au bimetallic catalyst with mesoporous silica nanoparticles as support and its catalysis of cinnamaldehyde hydrogenation. J. Catal. 2012, 291, 36. (15) Yuan, X.; Zheng, J. W.; Zhang, Q.; Li, S. R.; Yang, Y. H.; Gong, J. L. Liquid-Phase Hydrogenation of Cinnamaldehyde over Cu-Au/ SiO2 Catalysts. AIChE J. 2014, 60, 3300. (16) Cook, D. Infrared Spectra of Xanthone: Lewis Acid Complexes. Can. J. Chem. 1963, 41, 522. (17) Liu, H. L.; Li, Y. W.; Jiang, H. F.; Vargas, C.; Luque, R. Significant promoting effects of Lewis acidity on Au-Pd systems in the selective oxidation of aromatic hydrocarbons. Chem. Commun. 2012, 48, 8431. (18) da Silva, A. B.; Jordão, E.; Mendes, M. J.; Fouilloux, P. Effect of metal-support interaction during selective hydrogenation of cinnamaldehyde to cinnamyl alcohol on platinum based bimetallic catalysts. Appl. Catal., A 1999, 14, 253. (19) Kahsar, K. R.; Schwartz, D. K.; Medlin, J. W. Control of Metal Catalyst Selectivity through Specific Noncovalent Molecular Interactions. J. Am. Chem. Soc. 2014, 1, 520. (20) Guo, Z.; Xiao, C.; Maligal-Ganesh, R. V.; Zhou, L.; Goh, T. W.; Li, X.; Tesfagaber, D.; Thiel, A.; Huang, W. Pt Nanoclusters Confined within Metal-Organic Framework Cavities for Chemoselective Cinnamaldehyde Hydrogenation. ACS Catal. 2014, 4, 1340. (21) Férey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surblé, S.; Margiolaki, I. A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area. Science 2005, 309, 2040. (22) Liu, H.; Liu, Y.; Li, Y.; Tang, Z.; Jiang, H. Metal-Organic Framework Supported Gold Nanoparticles as a Highly Active Heterogeneous Catalyst for Aerobic Oxidation of Alcohols. J. Phys. Chem. C 2010, 114, 13362.
According to the preceding kinetic results over the Pt/MIL101, the reaction order with respect to H2 and cinnamaldehyde was 1 and 0, respectively. Therefore, the assumption of the reaction rate of step 1 as the rate-determining step was reasonable.
4. CONCLUSIONS To summarize, we have demonstrated a highly efficient and chemoselective heterogeneous catalyst based on Pt nanoparticles deposited on an acidic MOF for the hydrogenation of α,β-unsaturated aldehydes. The catalyst is able to effectively catalyze cinnamaldehyde to the corresponding hydrocinnamaldehyde via the CC bond reduction with >99.9% selectivity at >99.9% conversion under atmospheric H2 pressure and room temperature. The synergistic effect between Pt and Lewis acid is considered to be the critical reason for the excellent catalytic performance. Lewis acid coordination with the CO group of cinnamaldehyde made the CC bond more reactive, while Pt activated H2, and hydrocinnamaldehyde was formed quickly. Subsequently, the formed hydrocinnamaldehyde adsorbed onto the MIL-101 support via the CO group coordination to the Lewis acidic sites, which inhibited further hydrogenation of hydrocinnamaldehyde. The kinetic study indicated a zero-order dependence on cinnamaldehyde while a first-order dependence on H2 pressure. A kinetic model has been proposed based on the Langmuir−Hinshelwood kinetics as well as the reaction results. The striking improvement in activity and selectivity by Pt-Lewis acid collaboration provides an alternative and environmentally benign pathway for controlling the reaction selectivity of α,β-unsaturated aldehydes.
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ASSOCIATED CONTENT
S Supporting Information *
Powder X-ray diffraction patterns, activity profile for the hydrogenation of cinnamaldehyde, and ATR-IR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NSF of China (21322606, 21436005, and 21406075), the Doctoral Fund of Ministry of Education of China (20120172110012), Guangdong NSF (S2011020002397, 2013B090500027, and 10351064101000000), the Fundamental Research Funds for the Central Universities (D214244w), and the China Postdoctoral Science Foundation (2014M550437).
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