CHAPTER 6. SOLVENT-FREE SELECTIVE OXIDATION OF -PINENE OVER Co-SBA-15 CATALYST

Transcription

1 135 CHAPTER 6 SOLVENT-FREE SELECTIVE OXIDATION OF -PINENE OVER Co-SBA-15 CATALYST 6.1 INTRODUCTION -Pinene is a terpenoid family of organic compound which is inexpensive, readily available and renewable starting material for the production of a variety of valuable products such as flavors, fragrances, medicines and agrochemicals (Erman et al 1985, Bauer et al 1997). The oxidation products of -pinene such as verbenol, verbenone and -pinene oxide are important intermediates for the production of fine and specialty chemicals (Lewis and Hedrick 1965, Wender and Mucciaro 1992). The oxidative functionalisation of olefins is an important unit operation in the fine chemical synthesis. However, olefins can be oxidized by different ways such as allylic C-H bond, epoxidation and oxidative cleavage of carbon-carbon double bond. The metalperoxo species favor epoxidation of olefins and free radical species favor allylic oxidation of olefins. It is found that epoxidation and allylic oxidation of olefins are often competitive reactions which normally yield mixture of products (Murphy et al 2000). However, allylic oxidation of olefins through hydrogen abstraction is the dominant reaction. Traditionally allylic oxidation of olefins is carried out using toxic and expensive metallic oxidants. The development of a reaction process using clean oxidant such as hydrogen peroxide is an environmentally acceptable green process.

2 136 Allylic oxidation of -pinene was carried out using both homogeneous and heterogeneous catalyst such as cobalt based compounds (Lajunen and Koskinen 1994, Lajunen et al 2000, Lajunen et al 2001, Joseph et al 2002, Lajunen et al 2003, Chakrabarty and Das 2004, Guo et al 2005, Maksimchuk et al 2007), copper salts (Allal et al 2003), titano-silicates (Morn et al 2000), Cr-AlPO-5 (Lempers et al1996), Cr-pillered clay (Maksimchuka et al 2005), Fe and Cr-MIL-101 (Timofeeva et al 2012), and Uranyl-MCM-41 (Selvam et al 2011). Although these catalytic systems used drastic reaction condition and toxic oxidants, the conversion and selectivity of the product were low. The development of a heterogeneous catalyst for the selective oxidation of -pinene using a green oxidant is the major demand in chemical industry. Heterogeneous catalyst with eco-friendly oxidant was used in chemical reactions which led to efficient process. Hence, heterogeneous catalyst with high surface area, ordered pore arrangement and tunable pore size are good choice in the field of catalysis. Mesoporous SBA-15 possesses large pore diameter and thicker wall compared to MCM-41 and MCM-48. However, siliceous SBA-15 do not find application as catalyst due to lack of active sites. Hence incorporation of transition metal ions into SBA-15 is a challenge under strongly acidic condition due to hydrolysis of M-O-Si network. There are many reports on the direct incorporation of heteroatoms such as Al, Ti, V, Co, Cr, Mn and Fe into SBA-15 framework by direct method under suitable ph condition (Vinu et al 2005, Selvaraj and Lee 2006, Chandrasekar et al 2007, Sathish Kumar et al 2007, Berube et al 2010, Selvaraj et al 2010). Similarly, a few reports are available on the allylic oxidation of -pinene using mesoporous supported catalyst (Margolese et al 2000, Selvam et al 2011). However, cobalt incorporated SBA-15 material has not been attempted in the allylic oxidation of -pinene.

3 137 Hydrothermal synthesis of Co-SBA-15 with appropriate ph adjustment and its catalytic performance in the liquid phase oxidation of -pinene using H 2 O 2 as the oxidant under solvent-free condition are presented in this chapter. The reaction parameters such as molar ratio of -pinene/h 2 O 2 and effect of reaction time are also studied in order to improve the conversion and selectivity of the product. The plausible reaction mechanism is proposed for the selective oxidation of -pinene. The stability, recyclability and heterogeneity of the catalyst are also established in this study. 6.2 CHARACTERIZATION OF Co-SBA X-ray Diffraction (XRD) The small-angle X-ray diffraction patterns of SBA-15 and Co-SBA-15 (Figure 6.1) exhibited three well resolved peaks corresponding to (100), (110) and (200) reflections of ordered hexagonal mesopores with space group of p6mm (Lou et al 2008). The diffraction patterns of Co-SBA-15 samples are similar to that of pure SBA-15. It is interesting to note that the intensity of peaks increased with increase of metal content due to increased ordering of mesoporous nature. However, the diffraction patterns of Co-SBA-15 materials are slightly shifted to lower angle, which is attributed to expansion of mesopores while increasing the cobalt content. The unit cell parameters (calculated using the equation a o = 2d 100 3) increased with increase of cobalt content in SBA-15 framework and the results are presented in Table 6.1. The increase of unit cell parameter (a o ) from to nm is due to dilation of mesoporous SBA-15 framework. The wall thickness of Co-SBA-15 materials increased slightly more than that of parent SBA-15. These results suggest the incorporation of cobalt species in the silica framework.

4 138 (100) Intensity (a.u) (110) (200) (d) (c) (b) (a) (degree) Figure 6.1 Small angle XRD patterns of (a) SBA-15, (b) Co-SBA-15(25), (c) Co-SBA-15(50) and (d) Co-SBA-15(100) Table 6.1 Textural properties of Co-SBA-15 Sample Unit cell parameter (a o ) Surface area (m 2 /g) Pore volume (cm 3 /g) Pore diameter (nm) Pore wall thickness (nm) SBA Co-SBA-15 (25) Co-SBA-15 (50) Co-SBA-15 (100) a o calculated from =2d 100 3, Pore wall thickness= a o - D p Nitrogen Sorption Studies The nitrogen adsorption-desorption isotherms of SBA-15 and Co-SBA-15 are shown in Figure 6.2. The N 2 sorption isotherm of SBA-15 exhibited type IV isotherm with H1 hysteresis loop, which is characteristic of

5 139 well ordered hexagonal mesoporous material. The capillary condensation showed a sharp inflection in the range of relative pressure, which revealed uniform mesopores in SBA-15. The cobalt incorporated SBA-15 materials showed isotherms similar to that of SBA-15. The capillary condensation step slightly shifted to lower range with increase of cobalt content which clearly indicated the expansion of pores. The textural parameters viz., surface area decreased from 649 to 562 m 2 /g and pore volume decreased from to cm 3 /g while pore diameter increased slightly with increase of cobalt content. These results clearly evidenced the incorporation of cobalt into SBA-15 framework with a slight effect on the micropores. The pore size distribution of SBA-15 and Co-SBA-15 are depicted in Figure 6.3. SBA-15 exhibits a very narrow pore size distribution while cobalt incorporated SBA-15 materials showed a broad distribution, which clearly indicated well dispersion of cobalt species into SBA-15 framework. Volume of N 2 adsorped (cm 3 /g, STP) (d) (c) (b) (a) Relative pressure (p/p o ) Figure 6.2 N 2 sorption isotherms of (a) SBA-15, (b) Co-SBA-15(100), (c) Co-SBA-15(50) and (d) Co-SBA-15(25)

6 140 Figure 6.3 Pore size distribution of (a) SBA-15, (b) Co-SBA-15(25), (c) Co-SBA-15(50) and (d) Co-SBA-15(100) Diffuse Reflectance Ultraviolet-Visible (DRSUV-Vis) Spectroscopy The chemical state and coordination environment of cobalt incorporated SBA-15 were investigated by DRSUV-Vis spectra as shown in Figure 6.4. The DRSUV-Vis spectra of all Co-SBA-15 materials showed four distinct absorption peaks at 250, 526, 581 and 665 nm. The peak centered at 250 nm is assigned to O-Co 2+ charge transfer transition. The other three peaks at 526, 581 and 665 nm are the characteristic absorption for 4 A 2 4 T 1 (P) transition of Co 2+ ion incorporated in tetrahedral coordination into SBA-15 framework (Sexton et al 1986). The intensity of absorption band increased with increase of metal content thus confirming the increased cobalt content in SBA-15 framework. This result is yet another strong evidence for the incorporation of cobalt in tetrahedral coordination into SBA-15.

7 Absorbance (a.u.) (a) (b) Absorbance (a.u.) Wave length(nm) (c) Wave length (nm) Figure 6.4 UV-DRS spectra of (a) Co-SBA-15(25), (b) Co-SBA-15(50) and (c) Co-SBA-15(100) Fourier Transform- Infra Red (FT-IR) Spectroscopy Figure 6.5 exhibits the FT-IR spectra of SBA-15 and Co-SBA-15 with different Si/Co ratios. The pure SBA-15 exhibited a band around 1057 cm 1 which is assigned to asymmetric stretching vibration of Si Si, and the bands close to 813 cm 1 is assigned to symmetric stretching and deformation modes of Si Si framework. Co-SBA-15 materials showed additional bands at 970 cm -1 which is attributed to stretching vibration of Si Co bond. In addition, the intensity of Si-O-Si asymmetric stretching vibration and deformation modes decreased with increase of cobalt content. This result also confirmed the incorporation of cobalt into silica framework.

8 142 Transmittance (a.u.) (d) (c) (b) (a) Si-O-Co Si-O-Si Wave number (cm -1 ) Figure 6.5 FT-IR spectra of (a) SBA-50, (b) Co-SBA-15(25), (c) Co-SBA-15(50) and (d) Co-SBA-15(100) Temperature Programmed Reduction (TPR) Profile The H 2 -TPR profile of Co-SBA-15 (Figure 6.6) was recorded to understand the reducibile property and the nature of cobalt species in SBA-15 framework. The H 2 -TPR profile of Co-SBA-15 exhibited only one broad reduction peak centered at 825 C. This reduction peak is ascribed to Co 2+ species that are effectively incorporated in the silica framework, which is reduced to Co 0. These results clearly revealed that cobalt species are present in the silica framework without the presence of cobalt oxide. Furthermore, the intensity of reduction peak area increased with increase of cobalt content due to increased number of cobalt species in SBA-15. These results confirmed the strong interaction of cobalt species with SBA-15 framework (Martinez et al 2003).

9 143 Co-SBA-15 (25) Co-SBA-15 (50) Co-SBA-15 (100) TCD Signal (a.u.) Temperature ( o C) Figure 6.6 TPR profile of ( ) Co-SBA-15(25), ( ) Co-SBA-15(50) and ) Co-SBA-15(100) SEM and TEM The morphology of SBA-15 and Co-SBA-15 with Si/Co ratios of 25, 50 and 100 was studied by SEM images as shown in Figure 6.7. The SEM image of SBA-15 showed a rod like morphology which are bundled together, and the SEM images of Co-SBA-15 with different Si/Co ratios exhibited similar morphology (Zhao et al 1998). The hexagonal array of mesoporosity and homogeneous distribution of cobalt species are confirmed by TEM images as shown in Figure 6.8. The TEM image of Co-SBA-15 (25) exhibited well-organized hexagonal mesoporous structure with uniform channels but did not show any distinct metal oxide particles.

10 144 Figure 6.7 SEM images of (a) SBA-15, (b) Co-SBA-15(25), (c) Co-SBA-15(50) and (d) Co-SBA-15(100) Figure 6.8 TEM images of (a) SBA-15 and (b) Co-SBA-15 (25)

11 CATALYTIC ACTIVITY The catalytic activity of Co-SBA-15 was evaluated in the oxidation of -pinene using H 2 O 2 as the oxidant under solvent-free condition (Scheme 6.1). The influence of reaction parameters such as Si/Co ratios, different molar ratio of pinene/oxidant and reaction time was also attempted to optimize the reaction condition so as to obtain high conversion and high selectivity of the product. The influence of cobalt content in SBA-15, absence of catalyst, absence of H 2 O 2 and different amount of catalyst on the oxidation of -pinene is presented in Table 6.2. The entries 1 to 3 in Table 6.2 results revealed the influence of Si/Co ratios, and the results cocluded that the conversion and selectivity of the product increased with increase of cobalt content. This may be attributed to high density of redox sites that led to high conversion and high selectivity of the product. Additionally, the same reaction carried out in the absence of catalyst showed not only low conversion but also resulted isomerization of -pinene (others) which is due to insufficient reactive oxygen species. The same reaction conducted in the absence of oxidant (H 2 O 2 ) resulted only 23% conversion with low selectivity of allylic oxidized product. The entries 6 to 8 in Table 6.2 showed the influence of catalyst amount on the conversion of -pinene, which increased with increase of catalyst amount from 50 to 100 mg and further increase of catalyst amount did not change the conversion. This result revealed that 100 mg of catalyst is found to be sufficient for the oxidation of -pinene. O Co-SBA Others H C H 3 2 O 2 OH O H 3 C Scheme 6.1 Allylic oxidation of -pinene over Co-SBA-15

12 Table 6.2 Allylic oxidation of -pinene under different reaction conditions a a S. No Catalyst Selectivity (%) Conversion (%) b -Pinene Verbenol Verbenone epoxide b Campholenic aldehyde 1 Co-SBA-15(100) Co-SBA-15(50) Co-SBA-15(25) Others Co-SBA-15(25) c Co-SBA-15(25) d Co-SBA-15(25) e Co-SBA-15(25) f Reaction conditions: Catalyst (100 mg), -pinene (10 mmol), H 2 O 2 (30 mmol), Time 14 h, Room temperature, b Determined by gas chromatograph, c Without H 2 O 2, d Catalyst (50 mg), e Catalyst (75 mg), f Catalyst (125 mg). 146

13 147 The plausible mechanistic pathway for the selective oxidation of -pinene is shown in Scheme 6.2. The first step is the decomposition of H 2 O 2 over Co 2+ -SBA-15 generating active oxidant species of hydroxyl and hydroperoxyl radicals. The hydroxyl radical abstracts a proton from the allylic position of -pinene to form a radical intermediate (I). This radical intermediate (I) reacts with (i) hydroxyl radical to form verbenol (major product 1) and (ii) hydroperxyl radical to form verbenyl hydroperoxide intermediate(ii). The verbenyl hydroperoxide further reacts with Co 2+ active sites in three different pathways such as (a) distant oxygen coordinates with Co 2+ sites forming a complex, which reacts with another molecule of -pinene to form -pinene oxide (minor product 2), (b) homogeneous decomposition of verbenyl hydroperoxide gives hydroxyl radical and intermediate (III). This intermediate then reacts with another molecule of -pinene by abstracting a proton from the allylic position to form verbenol (major) and (c) subsequent oxidation of verbenyl hydroperoxide into verbenone (minor product 3).

14 148 Decomposition of H 2 O 2 Co 2+ + H 2 O 2 Co 3+ + HO + HO - Co 3+ + H 2 O 2 Co 2+ + O OH+H + Mechanism + HO OH CH HO OH (Intermediate I) (1) H O H3C H 3C CH3 CH3 O OH O O OH (Intermediate III) (Intermediate II) Co3+ + HO (b) Co 2+ (3) O (c) HO O (a) Co 2+ Co 2+ + O H 3 C (2) Scheme 6.2 Plausible mechanism for the allylic oxidation of -pinene

15 Effect of Molar Ratio Figure 6.9 depicts the influence of molar ratio of -pinene/h 2 O 2 in the oxidation over Co-SBA-15(25) catalyst. The conversion of -pinene increased from 34 to 92 % with increase of molar ratio of -pinene/ H 2 O 2 from 2:1 to 1:3, and further increase of molar ratio to 1:4 did not enhance the conversion of -pinene. The selectivity of the product also increased with increase of -pinene/ H 2 O 2 ratio from 2:1 to 1:3. This is attributed to the increased decomposition of H 2 O 2 with increase of H 2 O 2 concentration. The results revealed that high concentration of H 2 O 2 showed rapid decomposition. It is concluded that -pinene/h 2 O 2 ratio of 1:3 is found to be the optimal reaction condition for selective oxidation of -pinene with high conversion and selectivity of the product. 100 Conversion Verbenol Verbenone Conversion/ Selectivity (%) :1 1:1 1:2 1:3 1:4 olar ratio of -pinene/h 2 O 2 Figure 6.9 Effect of Si/Co ratios on the oxidation of -pinene

16 Effect of Reaction Time The influence of reaction time was studied over Co-SBA-15 (25) in the oxidation of -pinene, keeping other reaction parameters the same and the results are depicted in Figure The conversion of -pinene increased with increase of reaction time up to 14 h. The selectivity of the product (verbenol) remained the same whereas other products such as verbenone increased slightly with increase of reaction time. This is attributed to increased decomposition of verbenyl hydroperoxide with increase of reaction time. The epoxide selectivity decreased with increase of reaction time. This is attributed to the acidic property of Co-SBA-15 which may involve in the isomerisation of epoxide yielding campholenic aldehyde. It is concluded that the efficiency of H 2 O 2 increased with increase of reaction time up to 14 h. Conversion/ Selectivity (%) Conversion Verbenol Verbenone Pineneoxide Campholenic aldehyde Others Time (h) Figure 6.10 Effect of reaction time on the oxidation of -pinene

17 Reusability and Heterogeneity Studies The recyclability of Co-SBA-15(25) catalyst was examined in the oxidation of -pinene using H 2 O 2 as the oxidant at room temperature for 14 h under solvent-free condition. The Co-SBA-15(25) catalyst was recovered after the reaction from the reaction mixture by filtration. The recovered catalyst was thoroughly washed with acetonitrile to remove of the organic substrate, dried at 100 C for 5 h and activated at 300 C. The activated catalyst was used for the next catalytic cycle and the same procedure was repeated for 5 cycles. The results revealed that Co-SBA-15 (25) catalyst showed almost the same efficiency without significant loss of its activity up to 5 cycles (Figure 6.11). The heterogeneous nature of the catalyst was also studied and results are presented in Figure In order to verify the heterogeneous nature of Co-SBA-15 (25) catalyst in the oxidation of -pinene the following study was performed. The catalyst was removed after 6 h of reaction and the -pinene conversion was found to be only 33%. The reaction was further continued with filtrate for another 8 h. The reaction product was analyzed and the results indicated that there was no conversion of -pinene after the removal of catalyst. This suggested that the reaction is heterogeneous in nature. It is also concluded that there is no leaching of cobalt from SBA-15 framework.

18 Conversion (%) No. of cycles Figure 6.11 Recyclability studies 100 With catalyst Catalyst removed after 6 h 80 Conversion (%) Catalyst filtration Time (h) Figure 6.12 Heterogeneity studies

19 CONCLUSION The cobalt incorporated hexagonal ordered SBA-15 was prepared by hydrothermal method with lower acidic condition (ph=3). The ordering of mesoporous nature increased with increase of cobalt content as confirmed by XRD. The presence of isolated Co 2+ site was confirmed by DRSUV-Vis and TPR profile. The catalytic activity of Co-SBA-15 in the oxidation of -pinene with H 2 O 2 as the oxidant under solvent-free condition at room temperature achieved high selectivity of the product (verbenol). The Co 2+ species played crucial role in the selective allylic oxidation of -pinene due to homogeneous decomposition of H 2 O 2. The selective product formation of verbenol indicated the involvement of hydroxyl radical in this reaction. The recyclability study revealed that cobalt species are highly dispersed in the silica framework without leaching. The heterogeneity test performed by hot filtration method showed that Co-SBA-15 present truly in heterogeneous nature in the oxidation of -pinene.