Chapter 3 LDHs as Solid Base Catalyst for Epoxidation of Styrene with Molecular Oxygen

Transcription

1 Chapter - 3

2 Selective and efficient epoxidation of olefins have been extensively studied because epoxides are acting as versatile intermediates in organic syntheses ( ). Styrene oxide is a commercially important intermediate used in the synthesis of fine chemicals and pharmaceuticals. Styrene oxide is traditionally produced by the epoxidation of styrene using stoichiometric amounts of peracids as an oxidizing agent (342). However, peracids are expensive, hazardous to handle, non-selective for the epoxide formation leading to formation of undesirable products, creating waste (343). A number of studies have been reported on the epoxidation of styrene and other olefins over solid base catalysts including LDH such as Ti/SiO 2 ( ), TS-1 ( ), Ti-MCM-41 ( ), Fe or V/SiO 2 (346), TBS-2 and TS-2 (350) using TBHP (tertiary butyl hydrogen peroxide) (345), H 2 O 2 (167, 346, ) or urea-h 2 O 2 adduct (165). Styrene conversion is very high when H 2 O 2 is used as an oxidizing agent but the selectivity for styrene oxide is poor. On the other hand, use of TBHP (345) and urea H 2 O 2 (345) as oxidizing agents yield high styrene oxide selectivity (>%) with low styrene conversion (9.8 and 17.7%) respectively. The Mg-Al LDH has been studied extensively as an efficient base catalyst for the epoxidation of various olefins such as styrene, limonene and N-oxidation of pyridines using hydrogen peroxide (165, , ). Modified Mg-Al LDH has also been reported as efficient catalyst for epoxidation such as novel chiral sulphonato-salenmanganese (III)-pillared Mg-Al hydrotalcite Catalysts for the asymmetric epoxidation of styrene and cyclic alkenes (167). Different bivalent system like Zn-Al, Ni-Al other than Mg- Al LDH have also been studied as solid base catalyst for the epoxidation of olefins like bicycloalkenes using hydrogen peroxide as oxidant (165). However, there is a need to develop catalyst which employ oxidants safer than H 2 O 2 and produce little waste (167). The heterogeneous oxidation of olefins by air/oxygen is one of the 156

3 most challenging and promising subject (344, ). The catalytic oxidation of terminal olefins, including styrene, by O 2 to the corresponding 2-ketones and 2-alcohols using a cobalt (II) complex has been reported ( ). It has been recently reported that the selective epoxidation of styrene can be carried out using molecular oxygen and Co exchanged zeolites, Co-ZSM-5 coordinated with ligands, CoOx and CoOx/SiO 2 (358-3). LDH has also reported as efficient catalyst for oxidation of thiols using air as oxidant (166). The present study is focused on the synthesis of binary and ternary LDHs and their application as catalyst for the epoxidation of styrene to styrene oxide using molecular oxygen as oxidant. A series of binary Mg-Al, Co-Al and ternary Co-Mg-Al LDHs sample with varied cation molar ratio and carbonate as intercalated anion were synthesized under optimized synthesis conditions as described in chapter 2. The synthesized LDHs samples were studied for epoxidation reaction. Effect of cation molar ratio, temperature, time, calcination, DMF volume, catalyst concentration, reuse of catalyst, reconstruction after calcination and effect of water content on catalytic activity of catalyst has been studied as described in following parts I and II. 157

4 Part - I

5 3.1.1 Materials Magnesium nitrate was procured from s.d. fine chem. Ltd India, Aluminium nitrate and cobalt nitrate were from LOBA Chemie Pvt. Ltd. India, Sodium carbonate was from National chemicals India. Sodium hydroxide was procured from Ranbaxy Lab. Ltd India. Styrene and tridecane were from Sigma Aldrich and N-N dimethylformamide was from Qualigens fine chemicals Ltd. India. Oxygen (99.9% purity) was from Inox Air Product Ltd., Vadodara Synthesis of Co-Al and Mg-Al LDHs with varied cation molar ratios A series of Mg-Al and Co-Al LDH samples with varied cation molar ratio and carbonate as intercalated anion were synthesized by conventional co-precipitation method under optimized conditions. Solution A was prepared for binary cation combination by dissolving bivalent (M 2+ = Mg/Co) and trivalent (M 3+ = Al) nitrate salts with different molar concentration such as M 2+ = 0.63M, M 3+ = 0.37M for M 2+ : M 3+ = 1.7:1, M 2+ = 0.66M, M 3+ = 0.34M for M 2+ : M 3+ = 2:1, M 2+ = 0.75M, M 3+ = 0.25M for M 2+ : M 3+ = 3:1 and M 2+ = 0.M, M 3+ = 0.20 M for M 2+ :M 3+ = 4:1. For different molar concentration ratio of M 2+ and M 3+, x (x= M 3+ / M 3+ + M 2+ ) value is 0.37, 0.34, 0.25 and 0.20 for 1.7:1, 2:1, 3:1 and 4:1 respectively. Solution B having sodium carbonate (Na 2 CO 3 : M 2+ (NO 3 ) 2 = 1:1molar ratio dissolved in 100 ml NaOH (2.2 M) was prepared separately. Both solution A and solution B were added into a container simultaneously at a flow rate of 8-10 ml min -1 at room temperature followed by agitation at 65 C for 18h. The resultant precipitate was dried at room temperature over CaCl 2 for ~24h. The yield of LDH samples was calculated as below and given in Table 1. Yield (wt %) = (Obtained weight of product / Theoretical weight of product) x

6 Table 1. Synthesis of Co-Al and Mg-Al LDH samples with varied cation molar ratios Sample Cation Combination Cation Molar Ratio Yield (%) Mg-Al-1.7 Mg-Al 1.7:1 69 Mg-Al 2,, 2:1 68 Mg-Al 3,, 3:1 72 Mg-Al 4,, 4:1 65 Co-Al 1.7 Co-Al 1.7:1 64 Co-Al 2,, 2:1 58 Co-Al 3,, 3:1 69 Co-Al 4,, 4: Characterization of LDHs Powder X-Ray Diffraction Studies (PXRD): The d-spacing and crystallinity of LDH samples was measured by X-ray powder diffractometer (Philips X pert) using Cu Kα radiation (λ=1.5405ǻ). The samples were scanned in 2θ range 2 to 70 at a scanning rate of 0.4 deg sec -1. Crystallite size, i.e., disc diameter was calculated from 003 reflections (2θ = 11.6) using the Scherrer formula (107) Crystallite size = Kλ/ W cos θ Where W = Wb - Ws Wb = broadened profile width of experimental sample and Ws = standard profile width of reference silicon sample FT-IR-Spectroscopic studies: The Fourier transform infrared (FT-IR) spectra of LDH samples in the range of cm - 1 with a resolution of 4 cm -1 as KBr pellets were measured using Perkin-Elmer GX- Spectrometer. 1

7 Thermal Gravimetric Analysis: TG/DTA analysis of LDH samples was performed using TGA (Mettler Toledo Star e System) in the temperature range of 50 C to 750 C with a heating rate of 10 C min -1 under N 2 flow (40 cm 3 min -1 ) Surface Area Analysis: N 2 adsorption and desorption isotherms on various LDH samples were measured at 77.4 K using ASAP 2010 Micromeritics, USA. Prior to measurement, the samples were activated in situ by heating at 150 C under vacuum (1x 10-3 mmhg) for 2 h. Surface area was determined from N 2 adsorption data using BET equation (113) Chemical Analysis: The elemental analyses of Mg and Al in the synthesized LDH samples was carried out using inductivity coupled plasma (ICP) emission spectroscopy with Perkin-Elmer optima 2000 DV Optical Emission Spectrometer. The solution used for the analysis was prepared by dissolving 4 ppm (for magnesium) and 10 ppm (for aluminium) of LDH samples in 5% HNO 3. The total water content of LDH sample has been analyzed by Karl Fischer titrator (Titra Master 85) and also calculated on the basis of weight loss observed during TG analysis. The water content was found in the range of moles per mole of LDH by Karl Fischer titrator however theoretically calculated values are in the range of moles per mole of LDH. The values obtained by Karl Fischer titrator included in the molecular formula of LDH sample shown in Table Diffuse reflectance spectroscopy (DRS) Diffuse reflectance spectroscopy (DRS) studies were performed with a Shimadzu UV- 3101PC equipped with an integrating sphere. BaSO 4 was used as the reference material. The spectra were recorded at room temperature in the wavelength range of nm. 161

8 Temperature-programmed desorption (TPD) The temperature-programmed desorption of CO 2 was studied using a TPD Micromeritics Pulse Chemisorb 2720 with a TCD detector. Before the TPD measurements, the LDH sample (50 ± 0.5 mg) was activated at 150 C with heating rate 10 C/ min under He flow (35 ml/min) for 2h. After activation, the sample was treated with CO 2 (20 ml/min) at C for 1h. After that, desorption process was carried out from to 900 C at 10 C/min Catalytic activity for epoxidation of styrene with O 2 O O Styrene Hydrotalcite/O 2 DMF Styrene Oxide + Benzaldehyde H The binary LDH samples dried at room temperature were used for the catalytic studies. The catalytic epoxidation reactions were carried out in liquid phase as a batch reaction at 100 C. Typically, a 50-ml round bottomed flask equipped with an efficient water condenser is kept in a constant temperature oil bath with the temperature maintained at 100 C±2. Then styrene (1g) along with N,N-dimethylformamide (DMF) (10 ml) and catalyst (100 mg) were added to the flask. Tridecane was used as an internal standard. The reaction mixture was magnetically stirred at 0 rpm. The reaction was started by bubbling O 2 at atmospheric pressure into the reaction mixture at the rate of 6 8 ml min 1. After 4 h of reaction, the liquid organic products were quantified using a gas chromatograph (Hewlett-Packard model 6890) equipped with a flame ionization detector and an HP-5 capillary column (30 m long and 0.32 mm in diameter, packed with silica-based, supel cosil), a programmed oven (temperature 162

9 range C 220 C), and N 2 as the carrier gas (2.5ml/min). Gas chromatograph mass spectrometer (GC MS), analysis using a Shimadzu GCMS-QP-2010, was done with the GC oven programmed at the temperature range of C 220 C and helium as the carrier gas and MS in the EI mode with a 70-eV ion source. The reaction kinetics were monitored by carefully withdrawing small amounts of the reaction liquid with a micro syringe from the reaction flask, avoiding solid catalyst, at 1h intervals and analyzing its composition by GC. The conversion was calculated on the basis of molar percent of styrene; the initial molar percent of styrene was divided by initial area percent (styrene peak area from GC) to get the response factor. The un-reacted moles of styrene remaining in the reaction mixture were calculated by multiplying the response factor by the area percentage of the GC peak for styrene obtained after the reaction. The conversion of styrene and selectivity for styrene oxide were calculated as follows: Styrene conversion (mol%) = 100 x [(Initial mol% -final mol%) / Initial mol%] Styrene oxide selectivity = 100 x [GC peak area of styrene oxide/gc peak area of all products] Results and Discussion Characterization of LDHs Powder X-Ray Diffraction Studies The Mg-Al and Co-Al LDH samples prepared with different cation molar ratios showed structure typical of hydrotalcite (JCPDS ) displaying the characteristic reflections: (i) sharp and intense basal reflections (00l) of 003, 006 planes in the low angle region (2θ 25 ), (ii) broad 0kl reflections of 012, 015, 018 planes in the mid angle region (2θ =30-50 ) and (iii) sharp hk0 and hkl reflections of 110, 113 and 116 planes in the high angle region (2θ =55-65 ) but with variation in the crystallinity and d spacing (Table 2). We have focused on d spacing of 003 plane and crystallinity in terms of peak intensity, counts/sec of 003 planes 163

10 because this plane representing the total thickness of layer including d spacing as well as thickness of cationic layer. The variation in d spacing and crystallinity is as discussed below: For Mg-Al LDH it was observed that as cationic molar ratio increases the d 003 increases. The d 003 value for cation molar ratios 1.7, 2, 3, 4 was 0.7 nm, 0.7 nm, nm, nm respectively. But the disc diameter showed opposite trend, i.e., with increase in cation molar ratio disc diameter decreases (1.7= 51 nm, 2 = 40 nm, 3 = 30 nm and for 4 = 28 nm). As cation molar ratio increases (1.7 to 3) crystallinity were observed to decrease (1500 to 922 counts/sec). However, for cation molar ratio of 4, the crystallinity was found to be 1174 counts/sec (Table 2 & Figure 1a). The XRD of calcined Mg-Al LDH sample showed peaks for magnesium oxide at 2θ = 43 and 62 with d and respectively (Figure 2a). The d 003 of Co-Al LDH was found in the range of nm and no proper trend was observed for d 003 with respect to cation molar ratios (Table 2). The crystallinity of LDH samples (Figure 1b) prepared with varied Co-Al molar ratio gradually decreases from 713 to 292 counts/sec with increase in cation molar ratio (1.7 to 4). The disc diameter decreased from 138 to 54 nm with an increase in Co-Al molar ratio from 1.7 to 3, however, further increased to 68 nm with an increase in cation molar ratio to 4. Our results in contrast to the results reported in literature (334) that addition of cobalt increases the crystal perfection due to increasing crystallite size. The XRD of calcined Co-Al LDH sample showed peaks for cobalt oxide at 2θ = 37, and 65 with d , 0.15 and 0.42 (Figure 2b) Lattice parameters The Mg-Al and Co-Al LDH samples with varied cation molar ratios was found to have a = nm and c = nm wherein, a corresponds to the average metal-metal distance within the layer and c corresponds to layer-to-layer distance (129) with 164

11 rhombohedral structure and resembles with Hydrotalcite, syn (JCPDS ) reported in JCPDS data. As d 003 of the Mg-Al-LDH and Co-Al-LDH samples with carbonate anion = nm, the samples have six times layer-to-layer distance showing 6R (42) stacking of the layers. Structural disorders are known to occur in LDHs, among which stacking disorders are common (40-42). Table 2. Structural and textural characterization of LDH samples Sample d 003 (nm) Crysta - llinity (counts/s) 003Disc Diameter (nm) Lattice Strain (%) Surface Area (m 2 /g) Formula Mg-Al Mg 0.63 Al 0.37 (OH) 2 (CO 3 ) H 2 O Mg-Al Mg 0.67 Al 0.33 (OH) 2 (CO 3 ) H 2 O Mg-Al Mg 0.75 Al 0.25 (OH) 2 (CO 3 ) H 2 O Mg-Al Mg 0. Al 0.20 (OH) 2 (CO 3 ) H 2 O Co-Al Co 0.63 Al 0.37 (OH) 2 (CO 3 ) H 2 O Co-Al Co 0.67 Al 0.33 (OH) 2 (CO 3 ) H 2 O Co-Al Co 0.75 Al 0.25 (OH) 2 (CO 3 ) H 2 O Co-Al Co 0. Al 0.20 (OH) 2 (CO 3 ) H 2 O Mg-Al-3R a Mg 0.75 Al 0.25 (OH) 2 (CO 3 ) H 2 O Mg-Al- 3RWW b ,, Mg-Al 3R c ,, a =Reformed LDH [calcined LDH (1g) + water (50ml)], b Uncalcined Mg-Al-3 LDH after reaction in presence of water (1 ml), c = Mg-Al 3 after calcination constructed during reaction with 1 ml water 165

12 Figure 1a. XRD pattern of Mg-Al LDH with varied cation molar ratio b Figure 1b. XRD pattern of Co-Al LDH with varied cation molar ratio 166

13 Figure 2a. XRD pattern of Mg-Al-3 LDH sample after calcination Figure 2b. XRD pattern of Co-Al-3LDH sample after calcination 167

14 Figure 3a. XRD pattern of LDH Mg-Al R a samples Figure 3b. XRD pattern of LDH Mg-Al R b samples Figure 3c. XRD pattern of LDH Mg-Al R c samples 168

15 FT-IR Spectroscopic studies FTIR spectra of Mg-Al and Co-Al LDH samples prepared with varied cation molar ratio with carbonate anion are similar in nature (Figure 4 & Table 3). The spectra showed all bands related to hydrotalcite structure for example an intense and broad band at ~3449 cm -1 that is attributed to stretching frequency of OH groups of interlayer water as well as to OH groups bound to metal cation (Mg-Al or Co/Al) with a shoulder at ~3070 cm -1 indicating the hydrogen bonding between interlayer water and CO 2-3 species. A broad peak at ~1636 cm -1 is 2- attributed to the bending frequency of interlayer water molecules. The incorporated CO 3 anions in the interlayer of LDH samples showed a sharp and intense peak for anti-symmetric stretching frequency at ~1369 cm -1 (٧ 3 ), shoulder for out of plane bending at 871 cm -1 (٧ 2 ) and a intense and broad peak for in- plane bending at 656 cm -1 (٧ 4 ). A broad peak at 784 cm - 1 and a sharp peak at 555 cm -1 is associated with Al-OH translation, whereas, a shoulder at 949 cm -1 for Al-OH deformation. An intense peak at 448 cm -1 could be assigned to M 2+ -OH (M 2+ = Mg/Co) Table 3. FT-IR peak assignments for Mg-Al and Co-Al LDHs Peak Position cm Assignment 3437 ν- OH, interlayer water 3070 H-bonding of CO 2-3 with interlayer water 1636 δ OH interlayer water 1369 ν 3 CO 3-2 antisymmetric stretching 871 ν 2 CO 3-2 out of plane bending 656 ν 4 CO 3-2 in-plane bending 949 δ Al-OH deformation 784 Al-OH translation 554 Al-OH translation 448 Mg-OH/Co-OH translation 418 Al-OH translation

16 %T cm Figure 4. FT-IR Spectrum of Co-Al and Mg-Al LDH Thermal Analysis The Mg-Al and Co-Al LDH samples prepared with cation molar ratio under optimized conditions showed the total weight loss in the range of 33-46%. The thermal stability of the samples with varied cation molar ratios is as discussed below: The Mg-Al LDH samples prepared with varied cation molar ratio showed the total weight loss in the range of 42-46%. The cation molar ratio was found to have significant effect on thermal stability of LDH samples. With an increase in cation molar ratio, thermal stability of LDH sample was decreased. The DTA curve showed two endothermic peaks, first peak was due to dehydration of interlayer water and the second peak due to decarbonation (Figure 5a-d). The first peak related to dehydration (weight loss 13-17%) was shifted towards lower temperature from 210ºC to 140ºC with an increase in cation molar ratio from 1.7 to 4. Similar observation was found for the second peak related to decarbonation (weight loss 26-30%) which shifted from 390ºC to 3ºC with increased Mg-Al molar ratio 170

17 of 1.7 to 4. The shape of both endothermic peaks was also observed. The first peak becomes sharper in the sample having Mg-Al molar ration 1.7 to 3, however, the sample having Mg- Al molar ration of 4 showed broader and less intense peak. The second peak is also broad with a shoulder for lower cation molar ratio (1.7:1) and become sharper as cation molar ratio increases % W t. L o s s Temperature ( 0 C) a /M in % W t L o s s Temperature( 0 C) b /M in % W t L o s s /M in % W t L o s s /M in Temperature( 0 C) Temperature( 0 C) c d Figure 5. TG-DT Analysis Mg-Al LDH with varied cation molar ratio a; 1.7:1, b; 2:1, c; 3:1, d; 4:1 The Co-Al-LDH samples prepared with varied cation molar ratios showed the total weight loss in the range of 33-36% with two-step weight loss (Figure 6a-d). In the first step weight loss was % as shown by a sharp peak at 200 C. However, the temperature for this 171

18 weight loss gradually decreased to 155 C with increasing Co-Al molar ratio from 1.7 to 4. In the second step the weight loss was % at C for the LDH sample having lower cation molar ratios of 1.7:1 and 2:1 and, however, it decreased to 220 C for higher Co-Al cation molar ratio of 4: % W t L o s s /M in % W t L o s s /M in Temperature( 0 C) Temperature( 0 C) a b % W t L o s s /M in % W t L o s s /M in Temperature( 0 C) c Temperature( 0 C) d Figure 6. TG-DT Analysis Co-Al LDH with varied cation molar ratio a; 1.7:1, b; 2:1, c; 3:1, d; 4: Surface Area Analysis BET surface area values for the various Mg-Al and Co-Al LDH was found in the range of m 2 /g (Table 2) and the variations in surface area are as discussed below: 172

19 It was found that the cation molar ratio have significant effect over surface area, for example Mg-Al LDH samples prepared with varied cation molar ratio showed gradual increase in surface area from 65 to 539 m 2 /g with increase in cation molar ratio 1.7 to 4 (Table 2). Our results of surface area were found contradictory with results observed by Frances Medina et al. (167) who found decreased surface area (157 to 46 m 2 /g) with increase in Mg-Al cation molar ratio (from 2:1 to 4:1). For Co-Al LDH samples with varied cation molar ratio the surface area increased from m 2 /g with increase in Co-Al cation molar ratio from 1.7 to 4 as shown in Table 2. Higher surface area values of LDH materials are reported in the literature but having different cations or anions. For example, Mg-Fe-CO 3 (Mg:Fe=2.45:1) having 354 m 2 /g and Mg-Al-Fe(CN) 6 (Mg:Al=3.3:1) having 419 m 2 /g surface area value are reported (38) Diffuse reflectance spectroscopy (DRS) 110 Reflectance(%) Co-Al 1.7:1 Co-Al 2:1 Co-Al 3:1 Co-Al 3.5:1 Co:Al 4: Wavelenth(nm) Figure 7. DRS Spectrum of Co-Al LDH with varied cation molar ratio 173

20 The Co-Al LDH catalysts having cation molar ratio 1.7 and 2 showed spectral band at around 240 nm in the UV region along with a band at 530 nm in the visible region (Figure 7), which were assigned to the transitions of the octahedral Co(II) ions located in the cationic layer of LDH. The intensity of the band at 530 nm was reduced in the catalysts having cation molar ratio of 3 and completely disappeared in the catalysts having cation molar ratio of 4. An additional broad band appeared at around 620 nm in the catalysts having cation molar ratio of 2, which increased in the catalysts having cation molar ratio of 3. The DRS spectra of Mg-Al LDH catalysts exhibited only one band at around 240 nm Temperature-programmed desorption (TPD) The temperature ( C) at which desorption of CO 2 takes place indicates the strength of the basic sites, whereas the amount of CO 2 desorbed (mmol/g) is the measure of number of basic sites present in the samples (167). For Co-Al LDH, CO 2 desorption occurred towards lower temperature (from 299 to 257 C) indicating the decrease in basic strength with increasing Co-Al cation molar ratio from 1.7 to 4 (Figure 8a). However, number of basic sites was found maximum in the sample having Co- Al molar ratio of 3:1. The trend of number of basic sites with cation molar ratio was 3:1>2:1>1.7:1>4:1 (Table 4 and Figure 8b). Mg-Al LDH showed CO 2 desorption at higher temperature ( C) as well as amount of CO 2 desorption (11-13 mmol/g) as compared to Co-Al LDH (Table 4). These results clearly showed that Mg-Al-LDHs have higher basicity in terms of strength and number of basic sites than Co-Al LDH samples. 174

21 Epoxidation of styrene to styrene oxide The Mg-Al and Co-Al LDH samples studied for epoxidation of styrene with oxygen showed formation of styrene oxide and benzaldehyde as two major products confirmed by GC-mass. The mass data showed standard fragmentation patterns corresponding to styrene epoxide (m/z = 120, 91, 65, 51) and benzaldehyde (m/z = 106, 77, 51) Temperature ( C) Co-Al molar ratio Figure 8a. Effect of cation molar ratio on temperature for main CO 2 desorption peak during TPD 10 Carbon dioxide desorbed (mmol/g) Co-Al molar ratio Figure 8b. Effect of cation molar ratio on amount of carbondioxide desorption during TPD 175

22 Effect of cation molar ratios The cation molar ratios were found to show significant effect on the styrene conversion and styrene oxide selectivity (Table 4). For example, in case of Mg-Al LDH, both styrene conversion and epoxide selectivity increased significantly with an increase in Mg:Al molar ratio from 1.7 to 3. However, further increasing the Mg:Al molar ratio to 4, both conversion and selectivity were found to decrease. The Mg-Al LDH having Mg:Al molar ratio of 3 showed the highest conversion of 45% with 77% epoxide selectivity (Figure 8c). In case of Co-Al LDH, results were different (Table 4). Styrene conversion decreased from 42 to 13% with an increase in cobalt content from 1.7 to 4 and selectivity increased from 67 to 71% with an increase in cobalt content from 1.7 to 3. With further increase in cobalt content to 4, selectivity decreased to %. The highest selectivity for styrene oxide of 71% with 26% conversion was observed with Co-Al LDH having cation molar ratio 3:1 (Figure 8d). As discussed above in section temperature-programmed desorption (TPD) (Table 4, Figure 8a and 8b) that the change in cation molar ratio influenced the basicity of the LDH samples, which affect the activity of the studied catalysts. For example, data given in Table 4 showed that sample Co-Al-1.7 with highest basic strength showed highest styrene conversion and Co-Al 4 with lowest basic strength showed lowest styrene conversion, however, Co-Al 3 having maximum number of basic sites resulted in maximum styrene oxide selectivity. These results clearly showed the co-relation between strength and number of basic sites of Co-Al LDH with their activity and selectivity for desired product leading to an important finding that the conversion of styrene was observed to depend on the strength of basic sites and the selectivity for styrene oxide on number of basic sites available on the catalyst. 176

23 Table 4. Epoxidation of styrene with molecular O 2 over LDH catalysts having varied cation molar ratio Cation molar ratio Temperature ( C) for main CO 2 desorption peak b CO 2 desorbed (mmol/g) c Styrene conversion (%) Styrene Oxide Selectivity (%) Benzaldehyde Mg-Al Co-Al Mg-Al Co-Al Mg-Al Co-Al Mg-Al Co-Al Mg-Al Co-Al 1.7: : : : Mg-Al-3R a a Reformed LDH, b & c are TPD results, Reaction conditions: Temperature 100 C, Time 4 hr, Styrene:LDH wt.ratio 10:1, DMF 10 ml Furthermore, Mg-Al-3 showed maximum conversion and selectivity for styrene oxide. As Mg-Al LDH showed higher basicity as compared to Co-Al LDH, so these samples showed slightly higher conversion of styrene (45%) and selectivity for styrene oxide (77%) as compared to Co-Al LDH (42% conversion and 71% selectivity). However, due to lack of basicity data for all Mg-Al LDH samples, the co-relation between strength and number of basic sites with activity and selectivity could not be fined for Mg-Al LDH like Co-Al LDH samples. There have been reports ( ) showing the role of Co 2+ ions for oxygen activation and also for epoxidation of styrene with oxygen or air. Similar mechanism could be operative with Co-Al LDH samples. In case of Mg-Al LDH samples, to the best of our understanding, there are following ways by which observed oxidation of styrene to styrene epoxide could be explained: (i) The strong Bronsted basic sites present in Mg-Al LDH could activate oxygen to give perhydroxyl anion species which are responsible for catalytic activity (ii) Autooxidation of styrene under studied reaction conditions where in the role of LDH could be in stabilization of radicals formed through interaction with its basic sites. 177

24 Styrene Oxide Selectivity % Benzaldehyde Mg-Al Molar ratio Figure 8c. Effect of cation molar ratio on % selectivity over Mg-Al LDH Styrene Oxide Selectivity % Benzaldehyde Co-Al Molar ratio Figure 8d. Effect of cation molar ratio on % selectivity over Co-Al LDH Effect of reaction temperature The epoxidation reaction of styrene with molecular O 2 carried out at different temperatures in the range of 90 to 120 C (Table 5). In case of Mg-Al 3 LDH, both styrene conversion (2 %) and oxide selectivity (55 %) were lower at 90 C, which sharply increased to 45 % and 77 % respectively with increasing the temperature to 100 C. However, further increase in temperature to 120 C resulted in decrease of conversion and selectivity both. 178

25 The Co-Al 3 LDH showed gradual increase in styrene conversion (26 to 46%) as reaction temperature increased from 90 to 120 C but selectivity was similar (71-72%) at C. For further studies, 100 C temperature have been chosen as optimized temperature for both Mg-Al and Co-Al LDH samples. According to data it is clear that 100 C is sufficient temperature to activate molecular oxygen and further increase in temperature leads to fast activation of molecular oxygen which leads to increase in the conversion but higher temperature also promote the formation of other side products (benzaldehyde) and cause loss of selectivity for styrene oxide. Therefore, 100 C temperatures have been chosen as optimized temperature for further studies. Table 5. Epoxidation of styrene with molecular O 2 over LDH catalysts at different temperatures Temperature (ºC) Selectivity (%) Conversion (%) Styrene Oxide Benzaldehyde Mg-Al-3 Co-Al-3 Mg-Al-3 Co-Al-3 Mg-Al-3 Co-Al Reaction conditions: Time 4h, Styrene:LDH wt. ratio 10:1, DMF 10 ml, Kinetic studies For Mg-Al and Co-Al LDHs the kinetic studies were carried out for 8h with cation molar ratio 3:1 (Table 6). For Mg-Al LDH sample conversion was gradually increased with time (11 to 50%) (Figure 9), however, maximum selectivity for styrene oxide (72%) was achieved in 4h and then decreased with time. For Co-Al LDH sample conversion was increased with time (0.19 to 43%) (Figure 9) and maximum selectivity for styrene oxide (69%) was observed in 2h and maintained upto 6h and then decreased with time upto 8h. The reaction 179

26 was slower in Co-Al LDH as compared to Mg-Al LDH as conversion was 11% in 0.5h for Mg-Al LDH, however, for Co-Al LDH only 0.19%. On the basis of kinetic study the optimized time decided was 4h. In temperature study for Co-Al LDH sample the conversion as well as selectivity increased with increase in temperature. The maximum conversion (46%) and selectivity (72%) observed at temperature 120 C. So that kinetic study was also carried out at 120 C. The results showed that styrene conversion increased (2 to 64%) with time and higher as compared to 100 C (0.19 to 43%). However, the time to achieved maximum selectivity for styrene oxide was similar as for 100 C i.e. 2h and maintained upto 6h and then decreased with time upto 8h (Table 6). So it can be conclude that the optimized time for maximum activity was not much affected by reaction temperature Table 6. Kinetic studies with respect to time for epoxidation of styrene with molecular oxygen over LDH catalysts Time (h) Conversion (%) Styrene Oxide Selectivity (%) Benzaldehyde Mg- Co-Al Co-Al* Mg- Co-Al Co-Al* Mg- Co-Al Al 3:1 3:1 3:1 Al 3:1 3:1 3:1 Al 3:1 3: Co-Al 3:1* Reaction conditions=temperature 100 C, Time 8h, Styrene:LDH wt. ratio 10:1, DMF 10 ml, catalyst with cation molar ratio=3:1, * at reaction temperature 120 C 1

27 50 Mg-Al % Converstion Co-Al Time(h) Figure 9. Effect of time on % conversion of styrene over Mg-Al LDH and Co-Al LDH Effect of catalyst concentration The effect of catalyst concentration was carried out by varying catalyst amount from mg for Mg-Al 3 and Co-Al 3 (Table 7). The data showed that for both Mg-Al and Co-Al LDH conversion increase with increase in catalyst concentration to 200 mg. However, on further increase catalyst concentration leads to almost constant styrene conversion for Co-Al LDH however, decreased for Mg-Al LDH. For both Mg-Al and Co-Al LDH samples the selectivity of styrene oxide were maximum for 10:1 substrate to catalyst ration (100 mg catalyst) and further increase leads to gradual decrease in selectivity (Figure 10a-b). 181

28 Table 7. Epoxidation of styrene with molecular O 2 over LDH catalysts at varied catalyst concentration Catalyst Amount (mg) With Substrate: Catalyst ration Conversion (%) Selectivity (%) Styrene Oxide Benzaldehyde Mg-Al Co-Al Mg-Al Co-Al Mg-Al 3:1 3:1 3:1 3:1 3:1 50 (20:1) Reaction conditions=temperature 100 C, Time 4h, DMF 10 ml, catalyst with cation molar ratio=3:1 Co-Al 3:1 100 (10:1) (5:1) (3.4:1) (2.5:1) Styrene Oxide Selectivity % Benzaldehyde Catalyst concentration(mg) Figure 10a. Effect of catalyst concentration on % selectivity over Mg-Al-3 LDH Styrene Oxide Selectivity % Benzaldehyde Catalyst concentration(mg) Figure 10b. Effect of catalyst concentration on % selectivity over Co-Al-3 LDH 182

29 Effect of calcination The layered structure of LDH samples changes to metal oxides after calcination which may have catalytic activity different from LDH. To study the effect of calcination on catalytic activity of LDH, the Mg-Al 3 and Co-Al 3 LDH samples were calcined for 4h at 500 C under inert atmosphere and the resultant oxides formed studied for epoxidation of styrene (Table 8). As LDH on calcination at 500 C, for 4h gives metal oxide (Figure 2a & b) the reactivity of metal oxides as compared to hydrotalcite for styrene conversion to styrene oxide may be different for both Co-Al and Mg-Al LDH. In the case of calcined Mg-Al-3 LDH, the mixed MgAlOx or MgO like phase formed after calcination showed conversion in the similar range (41%) as the uncalcined Mg-Al-3 LDH (45%), however, with slight lower selectivity (67%) for styrene oxide. On the other hand, the calcined Co-Al LDH resulted in the increase in conversion (36%) as compared to uncalcined one (26%) with selectivity of 71%. Table 8. Epoxidation of styrene with molecular O 2 over calcined LDH catalysts Catalyst Conversion (%) Selectivity (%) Without Calcination After Calcination Reaction conditions=temperature 100 C, Time 4h, Styrene:LDH wt. ratio 10:1, DMF 10 ml, catalyst with cation molar ratio=3: Effect of reconstruction and water content Styrene Oxide Without Calcination After Calcination Benzaldehyde Without Calcination After Calcination Mg-Al Co-Al The LDHs are known to reform or reconstruct their layered structure in presence of water or air. The calcined LDH sample was reformed with water in presence of air and then studied for its catalytic activity under the similar experimental conditions. The reformed LDH showed lower styrene conversion (21%) and selectivity (69%) as compared to fresh LDH 183

30 catalyst (Table 9). The water adsorbed on the surface of LDH during reforming along with the lower crystallinity and disc diameter (Mg-Al-3R a, Table 2 & Figure 3a) as compared to fresh Mg-Al 3 LDH resulted in lower catalytic activity. The calcined and uncalcined Mg-Al 3 LDH was also studied for epoxidation of styrene in presence of water (1 ml) in the reaction mixture to study the effect of the water molecules on the oxidation reaction. However, no conversion was observed in presence of water molecules with both calcined and uncalcined Mg-Al 3 LDH (Table 9). It showed that water molecules have adverse affect on the activity of the catalyst and blocked the active sites in both calcined as well as uncalcined LDH. The structural properties like d 003, crystallinity and disc diameter of uncalcined (Mg-Al-3R b ) and calcined (Mg-Al-3R c ) LDH catalyst after reaction in presence of 1 ml water showed that after reaction the calcined LDH has reconstructed and the uncalcined LDH sample has increased d 003 as well as crystallinity (Table 2 and Figure 3b and 3c). Table 9. Effect of activation and water content on epoxidation of styrene with molecular O 2 over Mg-Al -3 LDH catalysts Catalyst Conversion (%) Selectivity (%) Styrene Oxide Benzaldehyd e Without calcination Without calcination+1ml water After calcination After calcination+1ml water After calcination +Ex-situ reconstructed Reaction conditions=temperature 100 C, Time 4h, Styrene:LDH wt. ratio 10:1, DMF 10 ml Effect of DMF volume DMF has been reported as a good solvent for epoxidation of styrene using molecular O 2 over solid catalysts such as zeolites, Co-Y-ZrO 2 and Co-hydroxypatite catalysts (358, ). 184

31 The present results also indicated that DMF plays a significant role in the epoxidation of styrene with O 2 as both Mg-Al-3 and Co-Al-3 LDH showed enhanced styrene conversion as well as styrene oxide selectivity by increasing the concentration of DMF from 5 to 20 ml (Table 10 & Figure 11). Mg-Al-3 LDH showed highest styrene conversion of 85% with % selectivity for oxide, whereas Co-Al-3 LDH exhibited 97% conversion with 83% styrene oxide selectivity in presence of higher concentration (20 ml) of DMF. DMF is reported to associate with catalyst through metal and activates the O 2 molecule to form an intermediate complex, which reacts with styrene to give styrene oxide and benzaldehyde (358). Baiker et al. (362) observed N-formyl-N-methylformamide (FMF) resulting from the autoxidation of DMF identified by GC-mass spectroscopy (MS) analysis (m/z = 87, 73, 59, 42, 30, 15). We did not find any DMF oxidized products by GC-mass spectroscopy except standard fragmentation patterns corresponding to styrene epoxide (m/z = 120, 91, 65, 51) and benzaldehyde (m/z = 106, 77, 51). GC-mass data confirmed that DMF remained unchanged at the end of reaction and act as a solvent and not as a reagent. However, DMF plays important role in epoxidation of styrene as conversion was found to increase with increase in DMF volume. Increased styrene conversion with higher volume of DMF was due to better dispersion of catalyst in the solvent leading to enhanced mass transfer between active sites and styrene molecules and also due to increased quantity of dissolved oxygen in DMF solvent. As we found maximum conversion for Co-Al-3 LDH with 20 ml DMF with catalyst concentration 100 mg in 4h at 100 C. So, the same reaction was carried out at 120 C reaction temperature without changing other parameters (Table 10). It has been observed that conversion and selectivity both influenced by reaction temperature. Like conversion decreased 97 to 90%, and selectivity for styrene oxide was 83 to 67% with increased reaction 185

32 temperature 100 to 120 C. The results showed that 100 C is sufficient to activate the molecular oxygen and further increase in temperature will promote the formation of other products (benzaldehyde) rather than epoxide. Table 10. Epoxidation of styrene with molecular O 2 over LDH catalysts at varied DMF volume Catalyst Conversion (%) Selectivity (%) Styrene Oxide Benzaldehyde DMF (ml) DMF (ml) DMF (ml) Mg-Al Co-Al Co-Al-3* Reaction conditions: Temperature100 ºC, Time 4h, Styrene:LDH wt. ratio 10:1, catalyst with cation molar ratio=3:1, * at reaction temperature 120 C 100 % Converstion Mg-Al Co-Al DMF Volume (ml) Figure 11. Effect of DMF volume on % conversion of styrene over Mg-Al-3 LDH and Co-Al-3 LDH Reusability of binary LDH The catalyst was recovered from the reaction mixture by filtration, washed with acetone, dried at room temperature and reused for the oxidation reaction under the similar reaction 186

33 conditions (Table 11 & Figure 12). The reused Mg-Al-3 LDH showed significant decrease in the conversion from 45 to 15 % along with decrease in selectivity after first cycle. The basic sites of reused Mg-Al-3 LDH seem to be deactivated even after first cycle may be due to blockage of active sites with products which could not be removed by washing with acetone or room temperature drying. However, Co-Al-3 LDH can be reused upto 4-5 cycles with similar conversion as well as selectivity as fresh catalyst (Table 11 & Figure 12). Table 11. Epoxidation of styrene with molecular O 2 over reused LDH catalysts No. of cycle Conversion (%) Styrene Oxide Selectivity (%) Benzaldehyde Mg-Al 3:1 Co-Al 3:1 Mg-Al 3:1 Co-Al 3:1 Mg-Al 3:1 Co-Al 3: Reaction conditions=temperature 100 C, Time 4h, Styrene:LDH wt. ratio 10:1, DMF 10 ml, catalyst with cation molar ratio=3:1 % Conversion Co-Al Mg-Al No. of Cycles Figure 12 Reusability study in terms of no. of cycle on % conversion of styrene over Mg- Al-3 LDH and Co-Al-3 LDH 187

34 3.1.6 Conclusions Mg-Al and Co-Al binary LDH were found as efficient solid base catalysts for the epoxidation of styrene to styrene oxide with molecular O 2 in presence of DMF. The cation molar ratio of LDH was observed to significantly influence the basicity and hence the catalytic activity. The concentration of DMF solvent played a significant role in the epoxidation of styrene. The highest conversion and oxide selectivity was found for cation (Mg/Co:Al) molar ratio of 3:1, wherein Mg-Al LDH showed 85% styrene conversion with % epoxide selectivity and Co- Al LDH sample exhibited 97% conversion with 83% styrene oxide selectivity in presence of DMF (20 ml) at 100 C after 4 h. Effect of water study showed water has adverse affect on catalytic activity. Calcination leads to increase conversion for Co-Al LDH however, Mg-Al LDH showed slightly lower conversion and selectivity both. Reused Mg-Al LDH showed deactivation of the basic sites only after the first cycle, however, Co-Al LDH could be reused till 4 th reaction cycle without affecting the styrene conversion and epoxide selectivity. 188

35 Part - II

36 Subsequent to the study reported in part I, the ternary Co-Mg-Al LDH with varied combinations of cation molar ratio were prepared, characterized and studied for epoxidation of styrene with molecular O 2 are discussed in this Part Synthesis of Co-Mg-Al LDHs with varied cation combinations A series of Co-Mg-Al LDH samples with varied cation molar ratio and carbonate as intercalated anion was synthesized by conventional co-precipitation method under optimized condition. Solution A was prepared by dissolving Co and Mg nitrate salt with varied molar concentration with Al nitrate salt (assuring that x value for all cation molar ratio is 0.25, where x = Al/Al+Mg, i.e., M 2+ : M 3+ molar ratio should be 3:1). Five combinations of varied molar ratio for Co 2+ :Mg 2+ :Al 3+ were prepared as follows: (1) 2.5:0.5:1 (Co 2+ = 0.625M, Mg 2+ = 0.125M), (2) 2:1:1 (Co 2+ = 0.5M, Mg 2+ = 0.25M), (3) 1.5:1.5:1 (Co 2+ = 0.375M, Mg 2+ = 0.375M), (4) 1:2:1 (Co 2+ = 0.25M, Mg 2+ = 0.5M) and (5) 0.5:2.5:1 (Co 2+ = 0.125M, Mg 2+ = 0.625). Solution B was prepared by dissolving sodium carbonate (Na 2 CO 3 : Mg(NO 3 ) 2 +Co(NO 3 ) 2 = 1:1 molar ratio) in 100 ml NaOH (2.2 M). Both solution A and solution B were added simultaneously into a container at a flow rate of 8-10 ml min -1 at room temperature. The solution was agitated at 65 C for 18 h. The resultant precipitate was dried at room temperature over CaCl 2 for ~24 h. The samples were designated as per cations molar ratio, e.g., Co-Mg-Al LDH with cation molar ratio of 2.5:0.5:1 was represented as Co-Mg-Al 2.5:0.5:1. The yield for all LDH samples was calculated as below and given in Table 12 Yield (wt %) = (Obtained weight of product / Theoretical weight of product) x

37 Table 12. Synthesis of Co-Mg-Al LDH samples with varied cation molar ratio Sample Cation Molar Ratio Yield (%) Co-Mg-2.5:0.5:1 2.5:0.5:1 64 Co-Mg-2:1:1 2:1:1 62 Co-Mg-1.5:1.5:1 1.5:1.5:1 59 Co-Mg-1:2:1 1:2:1 62 Co-Mg-0.5:2.5:1 0.5:2.5: Characterization of LDHs All samples prepared has been characterized by XRD, thermal analysis, FT-IR, surface area, diffuse reflectance spectroscopy (DRS), temperature programmed desorption (TPD) as described in part I Results and Discussion Characterization of LDHs Powder X-Ray Diffraction Studies The Co-Mg-Al LDH samples prepared with different cation molar ratios showed structure typical of hydrotalcite (JCPDS ) displaying the characteristic reflections: (i) sharp and intense basal reflections (00l) of 003, 006 planes in the low angle region (2θ 25 ), (ii) broad 0kl reflections of 012, 015, 018 planes in the mid angle region (2θ =30-50 ) and (iii) sharp hk0 and hkl reflections of 110, 113 and 116 planes in the high angle region (2θ =55-65 ) but with variation in the crystallinity and d spacing (Table 13). We have focused on d spacing of 003 plane and crystallinity in terms of peak intensity, counts/sec of 003 planes because this plane representing the total thickness of layer including d spacing as well as thickness of cationic layer. The variation in d spacing and crystallinity is as discussed below: 191

38 For Co-Mg-Al-LDH the gradual increase in cobalt concentration (0.5 to 2.5) leads to decrease crystallinity from 632 to 406 counts/sec and d 003 from to nm. However, disc diameter initially decreased from 36 to 31 nm with increase in cobalt content from 0.5 to 1.5 and then increased from 31 to 39 nm with further increase in cobalt concentration from 1.5 to 2.5. It was found minimum for molar ratio Co-Mg =1.5:1.5, i.e., 31 nm (Table 13 & Figure 13a). The results of disc diameter are in good agreement with the result reported in literature (334) that addition of cobalt increases the crystal perfection due to increasing crystallite size. Table 13. Structural and textural characterization of Co-Mg-Al ternary LDH samples Sample d 003 (nm) Crysta -llinity (counts /s) 003 Disc Dia - meter (nm) Lattice Strain (%) Surface Area (m 2 /g) Formula Co-Mg-2.5:0.5: Co Mg Al 0.25 (OH) 2 (CO 3 ) H 2 O Co-Mg-2:1: Co 0.50 Mg 0.25 Al 0.25 (OH) 2 (CO 3 ) H 2 O Co-Mg-1.5:1.5: Co Mg Al 0.25 (OH) 2 (CO 3 ) H 2 O Co-Mg-1:2: Co 0.25 Mg 0.50 Al 0.25 (OH) 2 (CO 3 ) H 2 O Co-Mg-0.5:2.5: Co Mg 0625 Al 0.25 (OH) 2 (CO 3 ) H 2 O a =Reformed LDH [calcined LDH (1g) + water (50ml)], b Uncalcined Mg-Al-3 LDH after reaction in presence of water (1 ml), c = Mg-Al 3 after calcination constructed during reaction with 1 ml water The XRD of calcined Co-Mg-Al LDH sample showed the peaks of CoO cobalt oxide (2θ = 42 & 62) and MgO (2θ = 43 & 62). As explain in part I section powder x-ray diffraction studies that Co-Al LDH gives peaks for CoO (2θ = 37, & 65) and Mg-Al LDH for MgO (2θ = 43 & 63) after calcinations. For Co-Mg-Al LDH the peaks are shifted and broad with increased peak width indicating the formation of Co-Mg mixed oxide (Figure 13b). 192

39 Figure 13a. XRD pattern of Co-Mg-Al LDH with varied cation molar ratio Figure 13b. XRD pattern of Co-Mg-Al LDH (1.5:1.5:1) LDH sample after calcination Lattice parameters The Co-Mg-Al LDH samples with varied cation molar ratios was found to have a = nm and c = nm wherein, a corresponds to the average metal-metal distance within the layer and c corresponds to layer-to-layer distance (129) with rhombohedral structure and 193

40 resembles with Hydrotalcite, syn (JCPDS ) reported in JCPDS data. As d 003 of Co- Mg-Al LDH samples with carbonate anion = nm, the samples have six times layer-to-layer distance showing 6R staking of layers. Structural disorders are known to occur in LDHs, among which stacking disorders are common (40-42) FT-IR-Spectroscopic studies FT-IR spectra of ternary Co-Mg-Al LDH samples prepared with varied cation molar ratio were similar in nature as for binary Mg-Al and Co-Al LDH (part I section FT-IR- Spectroscopic studies) Thermal Analysis The Co-Mg-Al LDH samples prepared with varied cation molar ratio under optimized conditions showed the total weight loss in the range of %. For Co-Mg-Al LDH sample the first sharp peak was for dehydroxylation (12-17%) and second broad step for decarboxylation and decarbonation (22-30%). The concentration of magnesium and cobalt in Co-Mg-Al LDH sample affect the thermal stability significantly. The increase in magnesium content leads to increase thermal stability of LDH sample. The first peak related to dehydroxylation shifted gradually towards higher temperature from 170 to 190ºC with increase in magnesium content from 0.5 to 2.5. Similarly the second step also shifted towards higher temperature from 2-310ºC to 3ºC with increase in magnesium content from 0.5 to 2.5. The second peak was broad for lower magnesium content of 0.5 to 2 (Figure 14a-d), however, the peak is sharper for higher magnesium content of 2.5 (Figure 14e). As discussed earlier (part I section Thermal analysis) that the thermal stability was decreased with increase in Co or Mg content for Co-Al LDH and also for Mg-Al LDH. But when we compare the thermal stability of Co-Al LDH and Mg-Al LDH, we observed that 194

41 the thermal stability of Mg-Al LDH was higher. In the similar way, increase in Mg content in Co-Mg-Al LDH increase its thermal stability % W t L o s s /M in % W t L o s s % W t L o s s 1 /M in /M in Temperature( 0 C) Temperature( 0 C) Temperature( 0 C) a b c % W t L o s s /M in % W t L o s s /M in Temperature( 0 C) Temperature( 0 C) d e Figure 14. TG-DT Analysis Co-Mg-Al LDH with varied cation molar ratio of Co-Mg-Al a:; 2.5:0.5:1, b; 2:1:1, c; 1.5:1.5:1, d; 1:2:1, e; 0.5:2.5: Surface Area Analysis Co-Mg-Al LDH samples prepared with varied cation molar ratios were observed to have the BET surface area in the range of m 2 /g (Table 13). The LDH samples with higher cobalt content (Co=2.5) having higher surface area (396 m 2 /g) and the surface area decreased to 357 m 2 /g with decreased cobalt content to 1.5. However, further decrease in cobalt content 195