Selectivity Engineering in Synthesis of Thymol using Sulfated ZrO2-TiO2

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Selectivity Engineering in Synthesis of Thymol using Sulfated ZrO2-TiO2 Radhika Sadashiv Malkar, and Ganapati D. Yadav Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01454 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 8, 2017

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Industrial & Engineering Chemistry Research

Selectivity Engineering in Synthesis of Thymol using Sulfated ZrO2-TiO2

Radhika S. Malkar; Ganapati D. Yadav * Department of Chemical Engineering Institute of Chemical Technology Nathalal Parekh Marg, Matunga, Mumbai-400 019, India Email: [email protected], [email protected] Tel.: +91-22-3361-1001, Fax: +91-22-3361-1020

*Author to whom correspondence should be addressed

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Abstract: Selective synthesis of thymol (6-isopropyl-3-methylphenol), a perfumery chemical, is challenging and was studied in this work by a continuous vapor phase alkylation of m-cresol using isopropanol over different solid acids. A series of novel zirconia-titania mixed sulfated oxides were prepared by hydrothermal method and tested for isopropylation of m-cresol to prepare thymol. A sulfated catalyst with 2:1 mole ratio for zirconia: titaniashowed the best conversion and selectivity for thymol in continuous fixed bed vapor phase reactor. The catalyst is stable and active for a long period of time i.e up to 20 h (TOS). Isopropylation of m-cresol was carried out in continuous fixed bed vapor phase reactor under N2 as a carrier gas, 6.72 /h WHSV and 1:3 mole ratio of m-cresol to IPA at 220oC. Kinetic model was developed and thus reaction is kinetically controlled having activation energy 8.27 kcal/mol. Keywords: Sulfated zirconia-titania, thymol, isopropylation, m-cresol 1. Introduction A variety of fine chemicals are produced by using Friedel-Crafts alkylation, particularly using solid acids and different alkylating agents. For instance, products of alkylation of cresol are most effectively used in lubricating oils, stabilizers in jet fuels, as antioxidants & antibacterial, in petroleum products, cosmetics, rubber, pharmaceuticals etc.1,2 Isopropylation of m-cresol (1) is important because the product thymol (2) is a very valuable perfumery chemical. There are various isomers produced during the reaction such as 2 isopropyl 3-methylphenol (3), 4 isopropyl 3-methylphenol (4), 5 isopropyl 3-methylphenol (5) and thymol (6-isopropyl-3-methylphenol) along with isopropyl-3-methyl-benzylether, dialkylated and tri-alkylated products. Hence selectivity plays an important role in synthesizing 6isopropyl-3-methylphenol (Scheme 1).


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OH CH3

OH CH3

+ CH3

OH

OH

H3C

CH3 CH3

+

HO

CH3

CH3

CH3

CH3

+ H3C

2

1

3

CH3

4

CH3

+ H3C

Di & tri alkylated compounds + Diisopropyl ether

CH3 CH3

5

Scheme 1.Isopropylation of m-cresol Thymol shows antibacterial activity against food poisoning bacteria Escherichia 3

coli, fungicide,4 antimicrobial agent and as a preservative in food industries.5It preserves the quality of rape seed oil by preventing the oxidation of triglycerols in it.6Thymol on hydrogenation gives menthol7-9 which is having a peppermint like odor and is used for production of antiseptic, local anesthetic, flavors, fragrances and preservatives.10,11 Till date various catalyst have been developed and used in synthesis of thymol such as γAl2O3,7 activated alumina,12 H-ZSM-5,13,14 Mg-Al hydrotalcites,15 Zinc aluminate spinel,9 Al– MCM-41 molecular sieves8 were used in vapor phase using propylene as alkylating agent whereas UDCaT-5,16 Al (OC6H5)3, and ZnCl2-HCl10 used in liquid phase alkylation with isopropanol. Recently Liu et al.17have used SO3-H functionalized ionic liquids for alkylation of mcresol at 190 oC. Supercritical CO2 has been used in continuous fixed bed for selective synthesis of thymol over solid Lewis acid γ-Al2O3 and Bronsted acidNafion SAC-13 as catalysts.18 Industrially thymol is synthesized by liquid phase alkylation of m-cresol over activated alumina at 633 K and 50 atm pressure using isopropanol19 and in gas phase over supported metal sulfates20 or gamma alumina7using propylene as alkylating agent. In this process, use of high temperature and pressure are the two main drawbacks and thus it defines the scope for development of novel catalyst. These are the two important factors which add cost in the production of any product. Oligomerisation of propylene leads to coke formation and thus causes deactivation of catalyst.16Therefore there is a demand for highly selective catalyst that can perform better possessing high stability, activity and reusability for longer time at mild reaction conditions. 3 ACS Paragon Plus Environment


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Metal oxides have been used as supports and catalysts in a number of reactions, amongst which zirconia and sulfated zirconia (SZ) are particularly attractive.21 SZ has been employed in many catalytic reactions such as alkylation, isomerization, etherification, cracking, cyclization, esterification, transesterification, acylation, etc.22-27 Recently, a novel magnetically separable SZ was prepared by our group and evaluated for synthesis of ethyl levulinate from furfuryl alcohol.23However, SZ undergoes deactivation at high temperature and can produce SOx and H2S under reduced atmosphere.28 Besides SZ is not suitable for liquid phase reactions where water is generated in situ. Therefore many ways are reported to improve its thermal stability and catalytic activity. One of the methods is to promote SZ with another metal oxide to prepare binary or tertiary mixed oxide system. The use of mixed oxides in catalysis is an attractive strategy as surface characteristics of one oxide can be changed by incorporation of second oxide. Mixed oxides show enhanced acidity and stability than single metal oxide.29-31 SiO2 based binary oxides have been used as solid acid catalysts such as SiO2-Al2O3, SiO2-MgO, SiO2-ZrO2 etc.32 Co and Mo promoted TiO2-ZrO2 was investigated as the best catalyst for hydrodesulfurization reaction33 whereas effect of boron in Co-Mo-B2O3-Al2O3 on vacuum gas oil hydrotreatment was studied.34 Recently, Ti-M (M=Si, Al, Zr) mixed oxide catalyst was used for esterification of succinic acid and butyric acid to notice that performance of mixed oxide is better than single oxide.35 Generally mixed oxides have high surface area and therefore are potential candidates to serve as support in catalytic reactions; for instance, CO oxidation over Au supported Ce-Zr-O mixed oxideprepared by a deposition-precipitation method36 and Pd-ZnII-CrIIIbifunctional catalyst for synthesis of methyl isobutyl ketone (MIBK).37 Along with pure SZ, sulfated mixed oxides have also been examined for industrially useful reactions. Barthos et al studied the effect of S-ZrO2-TiO2 and S-ZrO2-SiO2 on isomerization of cyclopropane and n-hexane.38In this work Bronsted acidity of surface was enhanced by using sulfate group from H2SO4 and found that it was sufficient to carry out the reaction at low temperature. Role of different promoters like sulfate, molybdate and tungstate on the acidity of Al2O3-ZrO2 was evaluated by Reddy et al.39 Among all promoters SO42-/Al2O3ZrO2 was found to be giving good product yield under mild reaction conditions. Azambre et al.40 used acidic CeO2-ZrO2for selective catalytic reduction of NOx by methane and reported that it was stable and active up to 700oC beyond which S-species started decomposing to SO2. It is very


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well documented in literature that precursor of sulfate ion and calcination temperature affects catalytic activity and selectivity.41 In present work, we are reporting for the first time use of chlorosulfonic acid for the sulfonation of binary metal oxide to get sulfated mixed oxide (S-ZrO2-TiO2) for selective synthesis of thymol. It has been reported that ZrO2-doped titania showed increased photo catalytic activity. The reason behind is decreased surface area of TiO2 because of doped ZrO2 into it.42 Composite of ZrO2-TiO2 possesses mechanical strength, shows acid-base properties and better surface area.43 Surface characteristics of this catalyst is highly dependent on molar ratio of Zr and Ti, method of preparation, precursor of sulfate ion etc. Das et al.44 previously have studied isopropylation of benzene over sulfated ZrO2-TiO2. Here they have used sodium dodecyl sulfate to modify binary mixed oxide with SO4 group. They have reported a catalyst with better activity because of improved textural properties and surface acidity. Due to high pore volume they obtained the best selectivity for isopropylbenzenes. Here we have also tried to improve textural properties of binary mixed oxide by synthesizing it with hydrothermal method and using chlorosulfonic acid as SO4-2 group for modification of surface. 2. Experimental: 2.1. Chemicals Isopropanol, m-cresol (98%), ethylene dichloride (AR) and nitric acid (70%) were procured from Thomas Baker, Mumbai, India. Titanium tetra-isopropoxide, zirconium (IV) propoxide and chlorosulfonic acid were obtained from Spectrochem Pvt. Ltd., Mumbai. All chemicals were used as received. 2.2. Catalyst Preparation Fuel lean sulfated zirconia (FLSZ), fuel rich sulfated zirconia (FRSZ) and UDCaT-5 were prepared by well known methods reported by us.45,46 A series of modified zirconia-titania mixed oxides with varying mol ratio of zirconium (IV) propoxide and titanium tetra-isopropoxide [ZrTi(1:1), Zr-Ti(1:2), Zr-Ti(2:1)] along with single oxides

ZrO2 and TiO2were prepared by

47

hydrothermal method. The synthetic procedure consisted of first dilution of Zrpropoxide and Ti isopropoxide in IPA to get alkoxide solution. A mixture of water and nitric acid was added dropwise to the above mixture under vigorous stirring at ~273 K to promote hydrolysis. Here, mole ratio of ROH/alkoxide, H2O/alkoxide and HNO3/alkoxide were maintained at 65, 20 and 5 ACS Paragon Plus Environment


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0.15, respectively. The alcogel obtained after alkoxide hydrolysis was transferred to a bomb reactor and the temperature was raised to 240 oC for 24 h under autogenic pressure. Thereafter the sample was oven dried at 100 oC to evaporate isopropanol and finally calcined at 650 oC in air to obtain binary oxide mixture of ZrO2-TiO2. It was then immersed in 1M solution of chlorosulfonic acid in ethylene dichloride and dried for 24 h at 120 oC.46 Dried sample was calcined at 650 oC for 3 h to obtain crystalline S-ZrO2-TiO2 catalyst. 2.3. Reaction set up and analysis All the experiments ofvapour phase alkylation of m-cresol were performed in a down flow fixed bed reactor having 120 mm diameter and 4100 mm height. It is having an upstream vaporizer and a downstream condenser (Chemito, India). Liquid feed of m-cresol and isopropanol in the optimum mole ratio was fed by a pump (HPLZ grade, Knauer, USA) to the pre heater, then into the reactor. N2 gas was used as a carrier at specific weight hourly space velocity (WHSV). The reaction mass was analyzed by GC with OV-17 column and a FID detector. Catalyst bed was packed between th glass beads and quartz wool. All the experiments were carried out at optimum reaction conditions to study the effect of various parameters such as mole ratio, WHSV, temperature, and W/FAo (weight of catalyst per unit molar flow rate of reactant), feed flow rate and carrier gas flow rate was studied for all the reactions.

3. Results and discussion 3.1. Catalyst Characterization Catalysts were characterized by various techniques using XRD, ammonia TPD, FTIR, nitrogen BET surface are and SEM. All the details about characterization method have been provided in Supplementary information (ESI). 3.1.1. X-ray Diffraction XRD pattern of Zr-Ti (2:1) support, fresh S-Zr-Ti (2:1) and reused S-Zr-Ti (2:1) are presented here (Figure 1).Graph shows 3 strongest peaks at 30.38, 50.66 of pure tetragonal phase of zirconia and 25.27 of anatase phase of titania. 2θ values at 31.7, 35.77 and 42.74 corresponding to (1 1 1), (1 1 1) and (1 1 2) planes respectively shows monoclinic phase of ZrO2. Characteristic peaks of anatase phase of TiO2 appear at 37.32, 48.01 and 63.32 corresponding to (1 1 2), (2 0 0) and (2 0 4) diffraction pattern. However, binary mixture of titania and zirconia results into 6 ACS Paragon Plus Environment

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formation of ZrTiO4 phase. It is difficult to differentiate ZrTiO4 phase from tetragonal phase of ZrO2 because both the phases have strong peak at 2θ = 30o.48The crystallite size of Zr-Ti (2:1), fresh and reused S-Zr-Ti(2:1) catalystswere calculated by Scherrer formula as 11.2, 9.6 and 9.49 nm respectively. Sulfonation treatment causes decrease in crystallite size of catalyst. XRD pattern of S-ZrO2-TiO2does not contain sharp reflections like ZrO2-TiO2, which indicates that crystallinity of ZrO2-TiO2 decreases with loading of sulfur. All the peaks were successfully retained after using the catalyst two times.

Figure 1. XRD spectra of Zr-Ti (2:1) (A), fresh S-Zr-Ti (2:1) (B) and Reused S-Zr-Ti (2:1) (C) 3.1.2. Ammonia- temperature programmed desorption Temperature desorption using ammonia as a probe has been used to measure acidic strength of the catalyst. Sulfate group generates Lewis acidic cites on the surface of mixed oxide. The total acidity of S-Zr-Ti (2:1) is 0.48 mmol/g and of Zr-Ti (2:1) is 0.18mmol/g (Table 1). Reused S-Zr-Ti (2:1) catalyst shows minor reduction in acidity content. Presence of strong Lewis and Bronsted acidic sites is due to sulfate group. S-Zr-Ti (1:1) ratio shows 0.26mmol/g of total acidity whereas S-Zr-Ti (1:2) shows 0.21mmol/g of acidity. Acidity of sulfated zirconia catalyst decreases in the order of FLSZ>UDCaT-5>FRSZ. Among all S-TiO2 shows very less acidity because of its poor surface area. 7 ACS Paragon Plus Environment


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Table 1.Ammonia TPD Analysis of catalysts Catalyst

Total acidity in mmol/g

S-Zr-Ti (1:1)

0.26

S-Zr-Ti (1:2)

0.21

Zr-Ti (2:1)

0.186

S- Zr-Ti (2:1)

0.48

Reused S- Zr-Ti (2:1)

0.475

FLSZ

0.60

FRSZ

0.50

UDCaT-5

0.58

S-TiO2

0.19

S-ZrO2

0.20

3.1.3. Fourier transform infrared spectroscopy (FTIR) FTIR spectroscopy is used to study the nature of sulfur retained on surface and nature of acidic sites generated. FTIR spectra of Zr-Ti (2:1) supportand virgin S-Zr-Ti (2:1) with reused catalyst are presented in Figure 2 & 3 respectively. Band at 1631 cm-1 from all three spectra is attributed to the δ-OH bending frequency of water molecules associated with sulfate group. The set of peaks from Zr-Ti (2:1) sample, in the range of 810-500 cm-1 is showing Zr-O, Ti-O and ZrO-Ti groups vibrations which confirmed the ZrO2-TiO2 phase.49 IR specta of virgin and reused sulfated catalysts has distinguishing peaks from the region 1000-1400 cm-1 which are characteristic of bidentate sulfate ion.The peak around 1400 cm-1shows νS=O vibration in free sulfate ions.28


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Figure 2. FTIR spectra of binary oxide support Zr-Ti (2:1)

Figure 3. FTIR spectra of fresh S-Zr-Ti (2:1) (A) and reused S-Zr-Ti (2:1) (B) 3.1.4. Surface area analysis Textural characteristics of various catalysts were calculated by nitrogen adsorption method at liquid nitrogen temperature (-195.8oC). S-ZrO2-TiO2 shows the Type IV isotherm 9 ACS Paragon Plus Environment


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which gives information on the mesopore structure through its hysteresis loop (Figure 4). Specific surface area of mixed oxide varies as the mole ratio of zirconia and titania varies. They follow increasing order as Zr-Ti (1:2) UDCaT-5.

Figure 4.N2 adsorption desorption isotherm of S-Zr-Ti (2:1)

Figure 5. Pore size distribution in S-Zr-Ti (2:1)


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Table 2.Textural properties of catalysts Catalyst

BET surface area (m2/g) 244 153 306 260 259 53.3 21.6 83 22 12.8 19.6 10.6

S-Zr-Ti (1:1) S-Zr-Tiv(1:2) Zr-Ti (2:1) S-Zr-Ti (2:1) Reused S-Zr-Ti (2:1) FLSZ FRSZ UDCaT-5 ZrO2 TiO2 S-ZrO2 S-TiO2

Avg pore diameter (nm) 2.3 5.2 14.7 11.71 11.65 9.8 9.1 4 0.52 0.053 0.51 0.051

Pore volume (cm3/g) 1.12 0.81 1.24 0.98 0.96 0.13 0.06 0.21 0.12 0.016 0.17 0.014

3.1.5. Scanning electron microscopy (SEM) Typical SEM micrographs of Zr-Ti (2:1) and S-Zr-Ti (2:1) catalyst are shown in Fig. 6-8. It can be seen that particles are agglomerated with irregular shape and size. Surface morphology did not change even after sulfonation which can be seen in higher resolution images (Figure 8). EDX elemental mapping was performed and it showed even distribution of sulfur on the surface of Zr-Ti (2:1) support with 3.2 mass %. (Figure 9). Further quantitative analysis of sulfur from SZr-Ti (2:1) was carried out by x-ray fluorescence (XRF). The sample was analyzed on Shimadzu Energy Dispersive X-Ray fluorescence Spectrometer, Mumbai, India. Complete analysis showed 3.27% of sulfur content in the catalyst. As there is absence of chloride species, it concludes that chlorosulfonic acid has not been decomposed to sulfuryl chloride, pyrosulfuryl dichloride and chlorine. Table 3. Elemental analysis of S-Zr-Ti (2:1) by XRF Element

Oxygen

Sulfur

Zirconia

Titania

Total

Mass (%)

40.02

3.27

38.26

18.45

100



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Figure 6.SEM image of Zr-Ti (2:1)

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Figure 7. SEM image of S-Zr-Ti (2:1)

Figure 8.Higher resolution SEM images (A) Zr-Ti (2:1) (B) S-Zr-Ti (2:1)

Figure 9. Elemental mapping image of sulfur over S-Zr-Ti (2:1)

3.2. Catalyst Screening The efficacy of different catalysts namely FLSZ, FRSZ, UDCaT-5 and hydrothermally synthesized sulfated ZrO2-TiO2 catalyst with varying mole ratio of zirconia and titania was tested 12 ACS Paragon Plus Environment

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in terms of conversion and selectivity to select the best catalyst (Figure 10). Mole ratio was changed to compare the effect of proportion of zirconia and titaniain catalyst mixture on the conversion and selectivity.

90 80 70 % Percentage

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60 50 40 30 20 10 0 FLSZ

FRSZ

UDCaT-5

S-Zr-Ti (1:1)

S-Zr-Ti (1:2)

S-Zr-Ti (2:1)

S-TiO2

S-ZrO2

Catalyst % Conversion of m-cresol

% Selectivity of Thymol

Figure 10. Efficacy of various catalysts: 1:10 mol ratio of m-cresol: IPA, 1.5 g of catalyst, 200oC, 50 ml/min of N2 flow rate, 0.2 ml/min of feed flow rate and 6.54 h-1 of WHSV, 4h Combustion synthesized sulfated zirconia catalysts i.e. FLSZ and FRSZ gave highest conversionofm-cresol because of its high acidity whereas its less surface area leads to decreased selectivity for thymol. Acidic nature of catalysts decreases in the order of FLSZ > UDCaT-5 > FRSZ > S-Zr-Ti (2:1) > S-Zr-Ti (1:1) > S-Zr-Ti (1:2). Superacidic UDCaT-5, which was already reported in liquid phase synthesis of thymol, gives 61% conversion with 55% selectivity towards thymoland 39% selectivity for di-alkylated product.16 Because of its high acidity, formation of isomers of thymol, di and tri-alkylated products was also observed. Hydrothermally synthesized catalysts, among which S-Zr-Ti (1:1) having the same moles of zirconia and titania and S-Zr-Ti (1:2), show decreased conversion as compared to S-Zr-Ti (2:1). High selectivity of 77% was observed for this mole ratio because of high surface area and pore diameter as compared to other catalysts. Surface area of catalyst decreases as S-Zr-Ti (2:1) > S-Zr-Ti (1:1)> S-Zr-Ti



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(1:2)>UDCaT-5 > FLSZ > FRSZ. Therefore, S-Zr-Ti (2:1) was selected as choice of catalyst for further reactions. Mixed oxide Zr-Ti (2:1) had much higher surface area (260 m2/g), pore volume (0.98 cm3/g) and pore size (11.71 nm) than single oxide, ZrO2(22 m2/g, 0.12 cm3/g, 0.52 nm)and TiO2 (12.8 m2/g, 0.016 cm3/g, 0.053 nm) (Table 2). On sulfation of oxides the acidity, surface area, pore volume and pore size were as follows, S-Zr-Ti (2:1) (0.48 mmol/g, 260 m2/g, 0.98 cm3/g, 11.71 nm), S-ZrO2 (0.20 mmol/g, 19.6 m2/g, 0.17 cm3/g, 0.51 nm) and S-TiO2 (0.19 mmol/g, 10.6 m2/g, 0.014 cm3/g, 0.051 nm)(Table 2) which clearly indicates that the mixed oxide has better catalytic properties with superior acidity. And thus conversion showed by S-Zr-Ti (2:1), SZrO2 and S-TiO2 were 55.54, 15.33 and 10.23 %respectively whereas selectivity were as follows 77.69, 31. 23 and 21.14 %, respectively. It clearly indicates that because of better textural properties of S-Zr-Ti (2:1) it is better catalyst than single oxide. According to literature, use of high temperature is dominant in all studies on vapor phase alkylation of m-cresol with isopropanol (Table 4). Velu et al.15 carried out alkylation of m-cresol over Mg-Al calcined hydrotalcite to get a mixture of O- and C- alkylated products. It was observed that at low temperature formation of O-alkylated product, isopropyl-3-methylphenyl ether was dominant whereas at high temperature (400 oC) selectivity for thymol was more. Zincaluminate spinel which was prepared by hydrothermal method gave 78.2% conversion of mcresol and 88.4% selectivity for thymol at 255oC.9 Iron catalyst containing mixed oxides of Cr, Si and K showed lowest conversion for m-cresol at very high temperature.52 Mesoporous Al-MCM418 molecular sieves with different Si/Al ratios (59, 103 and 202) were synthesized and tested. Catalyst with Si/Al ratio of 59 gave 100% selectivity for thymol at 300 oC at 34.2% conversion of m-cresol. The major drawback of this catalyst was in its rapid deactivation after 3 h due to which there was a sudden drop in conversion up to 14%. The catalyst with lower Si/AL ratio possesses high number of acid sites but leads to faster deactivation and thus reduction in catalytic activity. Selvaraj and co-workers54 carried out this reaction with Zn-Al-MCM-41 and obtained 100% selectivity for thymol. However, when TOS reached >3 h the yield and selectivity started decreasing because of formation of dialkylated product. It showed that the catalyst was not stable thereafter and could not be used for long period. Comparatively, S-Zr:Ti (2:1) catalyst in the


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current work gives good selectivity and conversion at lower temperature over long period of time. Table 4: Literature study of m-cresol alkylation over vapor phase using iso-propnol m-cresol Catalyst

Mg-Al

Temp (K)

WHSV (h-1)

conversion (%)

Thymol selectivity (%)

Reference

673

1

40

80

[15]

528

0.5

78.2

88.4

[9]

693

0.5

17

60

[52]

Al–MCM-41

573

1.4

34.2

100

[8]

γ-Al2O3

523

0.5

74.4

66.7

[53]

Zn–Al–MCM-41

623

1.45

90.7

100

[54]

S-Zr-Ti (2:1)

493

6.72

73.12

75.31

This work

hydrotalcites ZnAl2O4 Iron catalyst containing Cr, Si, & K oxides

3.3. Effect of Temperature Temperature effect was studied for this reaction to understand the variation in conversion and selectivity profile (Figure 11). It was observed that selectivity for 6-isopropyl 3 methylphenol increases with increase in temperature but the conversion of m-cresol was found to be decreased. At higher temperature dehydration of isopropanol was found to be increased and it led to the formation of water and propylene. Oligomerisation of propylene causes coke formation and it blocks many active sites of catalyst therefore deactivate the catalyst. So, further conversion got reduced. Synthesis of diisopropyl ether was also observed with increase in temperature. At 220 oC optimum conversion of m-cresol (59%) and selectivity for thymol (52%) were obtained whereas yield for thymol was found to be 30.33%. Chemisorption of m-cresol occurs on the acid sites of the catalysts surface through its –OH group while o-carbon favors to react with isopropyl 15 ACS Paragon Plus Environment


cation for a reaction. Isopropyl cation attacks on the sixth position of carbon for higher yield of thymol and attack at second carbon is hindered because of the presence of bulky methyl group. Generally thymol i.e. 6 isopropyl 3 methylphenol, isomers of thymol, di and tri-alkylated products were formed over S-Zr-Ti (2:1). Total selectivity for di and tri-alkylated compound was observed to be up to 7%. 80 70 60 Percentage (%)

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50 40 30 20 10 0 200

220

240 Temp

% Conversion of m-cresol

260

OC


Figure 11. Effect of temperature: 1:10 mol ratio of m-cresol:IPA, 1.5 g of catalyst weight, 200260oC, 50 ml/min of N2 flow rate, 0.2 ml/min of feed flow rate, 6.54 h-1 of WHSV, and 4h 3.4. Effect of feed mole ratio The effect of feed mole ratio was studied over the range of 1:7, 1:5, 1:3 and 1:1 of mcresol:IPA (Figure 12) . With increasing moles of IPA, conversion of m-cresol increased but the selectivity for 6-isopropyl-3 methylphenol got decreased because of formation of other side products. Therefore at 1:7 mole ratio, we have observed the formation of isomers of thymol and di as well as tri-alkylated compounds resulting in to decreased selectivity for 6-isopropyl 3 methyl phenol. Hence, higher content of IPA improves the conversion but not selectivity and yield of desired product. At 1:3 mole ratio, optimum selectivity obtained as 73.76%. For 1:1, the concentration of isopropanol in reaction mixture was low and thus rate of reaction was also low. Therefore it did not lead to higher conversion of m-cresol. At the same mole ratio, selectivity for one of isomer of thymol was found to be equal as of thymol. So this mole ratio was unfavorable for optimization. Hence, 1:3 mole ratio was chosen for further reactions.


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80 70

% Percentage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30 20 10 0 1:7

1:5

1:3

1:1

Mole ratio % Conversion of m-cresol


Figure 12. Effect of feed mole ratio: 1.5 g of catalyst weight, 220oC, 50 ml/min of N2 flow rate, 0.2 ml/min of feed flow rate, and 4h 3.5. Effect of catalyst loading The catalyst loading was varied over the range of 0.5-2 g. With the increase in amount of catalyst loading, the catalytic activity can also be increased due to availability of higher number of active sites on the surface. Hence, the effect of catalyst loading on the conversion of m-cresol was evaluated by charging the fixed bed reactor with 0.5, 1.0, 1.5 and 2.0 g of S-Zr-Ti (2:1) catalyst with WHSV value 20.16, 10.08, 6.72 and 5.04 h-1, respectively. Figure 13 illustrates, as the loading of catalyst increases, conversion of m-cresol increases but selectivity for thymol decreases. At higher catalyst loading i.e. 2.0 g, formation of isomers of thymol and di-alkylated product with 51.5% and 17% selectivity respectively were observed. It was due to more availability of active sites on catalyst bed for higher chemisorptions of m-cresol. Because of these side products, selectivity for thymol got reduced. Highest selectivity for thymol was obtained at 0.5 g catalyst loading but the conversion of m-cresol was very low. Optimum selectivity and conversion was found at 1.5 g catalyst loading. Total selectivity for other isomers was 25% and for di and tri alkylated product was found to be 10%. Hence, it was selected for further reactions.


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Percentage (%)


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100 90 80 70 60 50 40 30 20 10 0 0.5

1 1.5 Catalyst loading (g)


2


Figure 13. Effect of Catalyst loading: Feed mole ratio 1:3, 220oC, 50 ml/min of N2 flow rate, 0.2 ml/min of feed flow rate, and 4h 3.6. Effect of feed flow rate Experiments were performed by using 0.1, 0.2 and 0.4 ml/min of reactant flow rate at 3.36, 6.72 and 13.44 h-1 WHSV respectively. It was observed that with increase in flow rate conversion of m-cresol got decreased but selectivity for thymol increased (Figure 14). It can be elucidated that with increase in feed flow rate, the residence time of reacting molecule minimized. The decrease in catalytic activity was observed at a flow rate of0.4 ml/min.Formation of isomers of thymol along with multi-alkylated products was observed at 0.1 ml/min because of greater contact time of reacting molecules with the active sites of catalyst. Highest conversion and selectivity was obtained at 0.2 ml/min and thus selected as optimum.


Page 19 of 33

90 80 Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 10 0 0.1

0.2 Feed flow rate (ml/min)


0.4


Figure 14. Effect of Feed flow rate: Feed mole ratio 1:3, 220oC temp, 50 ml/min of N2 flow rate, 1.5g catalyst loading and 4h Influence of WHSV was examined at constant temperature. With increase in WHSV conversion of m-cresol was decreased but selectivity for thymolwas increased. Plot of conversion and selectivity against WHSV was made. As the WHSV value increases the residence time of reacting molecules on the surface of catalyst decreases. Therefore it resultsin to decrease in conversion. At highest WHSV 13.4 h-1, conversion of m-cresol is 55% while selectivity for thymol and di-alkyalted compound is 81 and 17% respectively. The trend of selectivity for dialkylated compound is similar with conversion. The optimum selectivity and conversion was obtained at 6.72 h-1 and therefore used for further experiments.



90 80 Percentage %

70


60 50

% Selectivity for thymol

40 30 20

% selectivity for di-alkylated product

10 0 0

5

10 WHSV

15

h-1

Figure 15. Effect of WHSV 3.7. Effect of N2 flow rate Effect of carrier gas (N2) flow rate was studied by varying in the range of 25, 50, 100 and 150 ml/min. It was observed that with increasing flow rate, conversion of m-cresol decreased while selectivity towards thymol increased (Figure 16). It is because of reduction in residence time of reactants on the surface of catalyst at higher N2 flow rate. Therefore, optimum value was chosen as 50 ml/min which was giving 71% m-cresol conversion and 73.76% selectivity for thymol. 80 70 Percentage (%)

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60 50 40 30 20 10 0 25

50 100 N2 Flow rate (ml/min)


150


Fig. 16. Effect of N2 flow rate: Feed mole ratio 1:3, 220oC temp, 0.2 ml/min of feed flow rate, 1.5g catalyst loading and 4h 20 ACS Paragon Plus Environment

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3.8. Time on stream study (TOS) The stability and activity of S-Zr-Ti (2:1) catalyst was studied by time on stream (TOS) data up to 50 h. A good stability and activity of catalyst was maintained up to 20 h (Figure 17).After that it started showing gradual deactivation. Because as the time passes coke formed in situ starts depositing over the surface and blocks the active sites and thus leads to loss in its activity.18 80 70 60 Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30 20 10 0 5

10

15

20

25

30

35

40

45

50

Time (h) % Conversion of m-cresol


Figure 17. Time on stream data: 1:3mol ratio of m-cresol: IPA, 1.5 g of catalyst weight, 220oC, 50 ml/ min of N2 flow rate, 0.2 ml/min of feed flow rate (m-cresol and isopropanol solution), 6.72 h-1, and 50h 3.9. Regeneration and reusability of S-Zr-Ti (2:1) For regeneration of catalyst, first the spent catalyst was flushed with N2 gas at 100 ml/min for 2 h at 220 oC. After that dry catalyst was taken out and washed with isopropanol to get rid off the adsorbed material from the catalyst. Then, it was subjected to calcination at 650 oC for 4 h. We have made up the loss of catalyst during the regeneration step by adding fresh catalyst to get the same catalyst loading and the catalyst shows almost the same activity which confirms the



reusability. Figure 18 elucidates that there was no considerable decrease in the activity of catalyst even after third reuse. Hence, S-Zr-Ti (2:1)was a robust and reusable catalyst. 100 80 Percentage (%)

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60 40 20 0 Fresh

1st reuse


2nd reuse

3rd resuse

% selectivity of Thymol

Figure 18. Catalyst Reusability study: 1:3mol ratio of m-cresol: IPA, 1.5 g of catalyst weight, 220oC, 50 ml/ min of N2 flow rate, 0.2 ml/min of feed flow rate (m-cresol and isopropanol solution), 6.72 h-1, and 4h 3.10. Kinetic Model The following observations were made from the experimental data, 1) The rate of reaction is proportional to partial pressure of m-cresol. At a given m-cresol feed rate and temperature selectivity of thymol remains constant. 2) The yield of thymol isomer, di-alkylated product and ether are proportional to the partial pressure of m-cresol in the feed. 3) The initial rate of reaction of m-cresol and hence conversion of m-cresol and yield of thymol increase with increase in temperature up to a certain limit and beyond that it gets decreased, at fixed molar ratio of reactants. Based on above observations, the overall reaction can be considered from the reactants, m-cresol (A), isopropanol (B) and products formed are thymol (C), di-alkylated product (DI), di isopropylether (E), isomer of thymol (TI) and water (W)


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A + B  → C + W + DI + E + TI

(Overall reaction)

(1)

As, the selectivity for ether and isomer of thymol is very less; it can be neglected. The chemisorption of m-cresol on a vacant site (S) should be the first step followed by other reactions. K

A A + S ← → AS

(2)

Isopropanol is adsorbed on the adjacent sites: K

B B + S ← → BS

(3)

In above equations KA and KB are adsorption coefficients for m-cresol and isopropanol respectively. The chemisorbed species undergoes reaction to produce thymol, di-alkyalated and ether products. KS AS + BS → CS + WS

(4)

The concentration CS of the vacant sites (S) is calculated from the total site (Ct), adsorption equilibrium constants (Ki) and concentrations of relevant species (Ci)

Ct = CS + CAS + CBS + CES + CWS + CDIS + CTI

CS =

(5)

Ct

(1 + ∑ KiCi )

(6)

The overall rate of reaction of m-cresol (A) (mol/s.g-cat), assuming equation (4) as rate determining step, is given as follows,

− rA = k S C AS C BS

(7)

−rA = kS K A KBCACBCS 2

(8)

 Ct − rA = k S K A K B C AC B   1 + ∑ K i Ci 

(

)

   

2

(9) 23

ACS Paragon Plus Environment


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Page 24 of 33

When all species are weakly adsorbed and if,

∑ KiCi >CA0 hence the overall reaction becomes pseudo first order and can be written as,

− rA = −

dC A w = kC A dt FA0

(11)

Where, the overall pseudo constant for reaction of A (K) is given by

k = k S K A K B C B Ct 2

(12)

Now, Equation (11) is integrated to get the following for a fixed bed vapor phase catalytic reactor:

− ln (1 − X A ) = kC A0

w FA0

(13)

Thus, a plot of –ln(1-XA) vs.w/FA0 was made as a function of temperatures (Figure 19) to get an excellent fit.This is pseudo first order reaction.


Page 25 of 33

1.4

y = 6E-06x + 1.0425 R² = 0.7913

1.2 1 -ln(1-Xa)

y = 5E-06x + 0.5574 R² = 0.9675 y = 8E-06x + 0.2039 R² = 0.9713

0.8 0.6 0.4

200 210 220

y = 4E-06x + 0.2966 R² = 0.9865

0.2

240

0 0

10000

20000

30000

40000

w/FAo(s) Figure 19. Pseudo first order plot: –ln(1-XA) versus w/FA0 for synthesis of Thymol -13.9 -14 -14.1 -14.2 ln k

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-14.3 -14.4 -14.5

y = -4162.8x - 5.8211 R² = 0.9932

-14.6 -14.7 0.0019

0.00195

0.002 0.00205 1/T (K-1)

0.0021

0.00215

Figure20. Arrhenius Plot ofln k v/s 1/T for synthesis of Thymol The rate constant (k) values were calculated at different temperature. Arrhenius plot (Figure 20) was made to calculate the energy of activation. The value of apparent energy of activation was calculated to be 8.27 kcal/mol. Thus, it can be concluded that reaction was intrinsically kinetically controlled. 4. Conclusion Synthesis of thymol was carried out over sulfated ZrO2-TiO2 catalyst and vapor phase reactor. Catalyst was completely characterized and showed very good catalytic activity and 25 ACS Paragon Plus Environment


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Page 26 of 33

stability for long time. Excellent selectivity for thymol is a characteristic of textural properties of S-Zr-Ti (2:1) and better conversion is dependent on acidity of catalyst. Therefore at reduced temperature it is giving best selectivity for thymol. A kinetic model was developed suggesting that the reaction is kinetically controlled. The activation energy calculated is 8.27 kcal/mol for thymol synthesis. Time on stream study was performed up to 50 h and there was no decrease in conversion of m- cresol and selectivity for thymol up to 20 h which means catalyst is robust and can be used for prolonged time. The maximum conversion of m-cresol, selectivity and yield of thymolare 73.12, 75.31% and 55.07% respectively at 1:3 feed mole ratio of m-cresol:IPA, 1.5g catalyst loading and at 220 oC. This is relatively very low temperature in comparison with other published reports for vapor phase reaction.

Acknowledgment R. S. Malkar acknowledges University Grants Commission (UGC), India for the award of fellowship in M Tech Green Technology. G. D. Yadav acknowledges support from R. T. Mody Distinguished Professor Endowment and J. C. Bose National Fellowship of Department of Science and Technology, Government of India. Conflict of Interest Statement The author declares no conflict of interest. Abbreviations: A

reactant species A, m-cresol

B

reactant species B, isopropanol

C

thymol

DI

di-alkylated product

E

isopropyl-3-methyl-benzylether

W

water

TI

isomer of thymol

AS

chemisorbed A

BS

chemisorbed B 26 ACS Paragon Plus Environment

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CS

chemisorbed C

WS

chemisorbed W

KA

adsorption coefficients for m-cresol

KB

adsorption coefficients for isopropanol

KS

rate constant for surface reaction

CA

concentration of A, mol/g

CB

concentration of B, mol/g

CS

concentration of vacant sites, mol/g

CAS concentration of adsorbed A, mol/g CBS concentration of adsorbed B, mol/g CES concentration of adsorbed E, mol/g CWS concentration of adsorbed W, mol/g CDIS concentration of adsorbed DI, mol/g CTI

concentration of adsorbed TI, mol/g

Ki

adsorption equilibrium constant

Ci

concentrations of relevant species, mol/g

-rA

rate of surface reaction of A, molg-1 s-1

K

reaction rate constant

W

weight of the catalyst, g

FAo molar flow rate of reactants, mol/h EDX Energy dispersive X-ray spectroscopy References: [1] Kondamudi, K.; Elavarasan, P.; Dyson, P. J.; Upadhyayula, S. Alkylation of p-Cresol with Tert-Butyl Alcohol Using Benign Bronsted Acidic Ionic Liquid Catalyst. J. Mol. Catal. A Chem.2010,321, 34–41. [2] Ashaduzzaman, M,; Chowdhury, A. M. S.; Saha, M. Synthesis Of Cyclhexylcresols by Alkylation of Cresols with Cyclohexanol in the Presence of Perchloric Acid. J. Sci.2012,60, 195–198. [3] Xu, J.; Zhou, F.; Ji, B. P.; Pei, R. S.; Xu, N. The Antibacterial Mechanism of Carvacrol and Thymol Against Escherichia coli.Lett. Appl. Microbiol.2008,47, 174–179. [4] Ahmad, A.; Khan, A.; Akhtar, F.; Yousuf, S.; Xess, I.; Khan, L. A.; Manzoor, N. Fungicidal 27 ACS Paragon Plus Environment


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Thymol synthesis 100x63mm (600 x 600 DPI)

ACS Paragon Plus Environment

Selectivity Engineering in Synthesis of Thymol using Sulfated ZrO2-TiO2

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