Selectivity Engineered FriedelâCrafts Acylation of Guaiacol with Vinyl

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Selectivity engineered Friedel-Crafts acylation of guaiacol with vinyl acetate to acetovanillone over cesium modified heteropolyacid supported on K-10 clay Ganapati D. Yadav, and Akhilesh R. Yadav Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401260a • Publication Date (Web): 05 Jul 2013 Downloaded from http://pubs.acs.org on July 11, 2013

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Industrial & Engineering Chemistry Research

Graphical abstract: Selectivity engineered Friedel-Crafts acylation of guaiacol with vinyl acetate to acetovanillone over cesium modified heteropolyacid supported on K-10 clay. Ganapati D. Yadav* and Akhilesh R. Yadav

OH O

OH

O O

CH3

Acid catalyst

+

H2C

O

+

CH3 H3C

2-methoxyphenol

vinyl acetate

O

CH3 H3C

H

O

1-(4-hydroxy-3-methoxyphenyl)ethanone

ACS Paragon Plus Environment

acetaldehyde


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Manuscript id: ie-2013-01260a – Revised and Clean

2

June 24, 2013

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Selectivity engineered Friedel-Crafts acylation of guaiacol with vinyl acetate

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to acetovanillone over cesium modified heteropolyacid supported on K-10

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clay

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Ganapati D. Yadav* and Akhilesh R. Yadav

8

Department of Chemical Engineering,

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Institute of Chemical Technology

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Nathalal Parekh Marg

11

Matunga, MUMBAI – 400 019,

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India

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Tel: +91-22-3361-1001/1002/1111

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Fax: +91-22-3361-1002/1020

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E-mail: [email protected], [email protected]

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* Author to whom correspondence should be addressed

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Abstract

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Acetovanillone or apocynin (4-hydroxy-3-methoxyacetophenone) is an effective natural drug. Its

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synthesis is typically done by using unsafe and expensive methods. The current paper deals with

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Friedel-Crafts acylation of guaiacol using several solid acids including modified heteropolyacids

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(HPA) and modified forms of sulfated zirconia, such as 20%w/w Cs2.5H0.5PW12O40/K-10, 20%

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w/w H3PW12O40/hexagonal mesoporous silica (HMS), K10 clay, UDCaT-5, UDCaT-6 and

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UDCaT-4. Amongst which 20% w/w cesium modified dodecatungstophosphoric acid supported

30

on K10 clay (Cs2.5H0.5PW12O40/K-10) was found to be the best. Vinyl acetate was the best

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acylating agent. The effect of various factors affecting rate of reaction were studied to establish

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kinetics and mechanism of reaction. A mathematical model is built to interpret the kinetic data

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and develop a mechanism. The reaction is intrinsic kinetically controlled and the apparent energy

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of activation was found to be 15.46 kcal/mol.

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Keywords: Friedel-Crafts acylation, Acetovanillone (apocynin), Heteropoly acid, heterogeneous

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catalysis, green chemistry, selectivity engineering.

37 38 39 40 41 42 43 44 45

2 ACS Paragon Plus Environment


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Introduction:

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Acetovanillone or also known commercially as apocynin is 4-hydroxy-3-methoxyacetophenone.

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It

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methoxybenzaldehyde). It has been isolated from a variety of plant sources and is being studied

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for its range of pharmacological properties. It is conventionally used as anti-arthritic, anti-

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asthmatic, atherosclerosis natural drug and also for treatment of bowel disease.1 The synthesis

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acetovanillone can be achieved by using Friedel Crafts acylation of guaiacol (2-

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methoxyphenol).2,3 Various papers have been published in order to explain the application of

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apocynin.4-7 The Friedel-Crafts acylation of aromatic compounds is a fundamental reaction in

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organic synthesis.8 The acylated products of aromatic compounds, ketones, are widely used in

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perfumery, metallurgy, pharmaceutics and as versatile intermediates in the synthesis of

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biologically potent compounds. Traditionally acylation of aromatic compounds has been carried

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out by using homogeneous Lewis acid catalysts such as AlCl3, FeCl3, ZnCl2, and Brønsted acids,

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like HF, H2SO4 and HCl extensively.8,9 Homogeneous acids cause a number of problems such

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as: need of more than stoichiometric quantities of catalyst, generation of large amount of toxic

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waste, problem of recovery and reusability, the corrosive nature of homogeneous acids leading

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to increase in costs, premature ageing of processing equipment, presence of deleterious

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impurities in the desired product, etc. Continuous search for suitable heterogeneous catalysts for

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Friedel-Crafts

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Ln(OTf)3-LiClO4,12 TiCl(OTf)3-TfOH,13 Re-Br(CO)5,14 LiClO4-acyl ahydride complex,15 cation

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exchange resins,16 acid treated clays17 and heteropolyacids supported on clays.18

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The synthesis of acetovanillone using a green process is highly relevant due to the following

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reasons. During oxidative or respiratory burst polymorphonuclear leukocytes consumes large

is

a

natural

organic

compound

structurally

related

to

vanillin

(4-hydroxy-3-

reactions has led to development of such catalysts as HZSM-5 zeolite,10,11


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amount of oxygen which is converted into reactive oxygen species (ROS) which an important

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role in host defense mechanism against invading microorganism, but it also gives rise to

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excessive tissue damage. Therefore, a compound which interferes with ROS production is useful

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to prevent tissue destruction. In this context, apocynin or 4-hydroxy-3-methoxyacetophenone,

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isolated from Picrorhiza kurroa,19 is proven to be effective in the experimental treatment of

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several inflammatory diseases like arthritis, colitis, atherosclerosis and a potent inhibitor of the

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superoxide anion produced by NADPH oxidase of stimulated human neutrophils.20 Apocynin

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could be a prototype of a novel series of non-steroidal anti-inflammatory drugs (NSAID).21 The

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published literature so far provides an account of a variety of catalysts and reagents used for

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synthesis of apocynin. Coulthard et al. have tested ZnCl2 and AlCl3 for acylation of guaiacol

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using acetic acid as acylating agent.22 Selective demethylation of 3, 4-dimethoxy acetophenone

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was carried out by Dodge et al. using NaSEt.23 Recently polymer supported triphenylphosphine

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was reported as efficient catalyst for the total synthesis of apocynin.24 The processes reported so

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far are catalyzed by homogeneous catalysts which suffer from low conversion, costly reagents,

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harsh reaction conditions and use of environmentally hazardous components. Thus, there is

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scope to develop heterogeneous acid catalysts to synthesize apocynin. Solid catalysts are safe,

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benign and economical. Additionally the processes using them have advantages such as ease of

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separation, mild reaction conditions, better selectivity, waste minimization, less-expensive

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construction of material, etc.18,25 Acylation of activated aromatic compounds have been studied

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by using acetic acid, acetic anhydride and acetyl chloride as acylating agents in the presence of

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zeolites,25 acid treated metal oxides26 and heteropoly acids,27 amongst which heteropoly acids

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(HPAs) have been employed in a plethora of organic reactions.18,27 However, HPAs in

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unsupported form are beset with rapid deactivation, poor stability and low efficiency. Various



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supports like mesoporous silica, mesoporous aluminosilicate, alumina, carbon and zirconia have

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been used to enhance the stability and effectiveness of HPAs.18 Kimura et al.28 have

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demonstrated that acidic cesium salts of heteropoly acids are highly water tolerable catalyst,

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whereas our laboratory demonstrated a novel procedure to generate nano-catalyst

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Cs2.5H0.5PW12O40 in-situ within clay supports which can be reused repeatedly without loss of

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activity.29

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The use of solid superacids has grown over the years and our laboratory has developed novel

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solid super acid catalysts including sulfated zirconia,30 and its superior forms such as UDCaT-1

100

(stronger than sulfated zirconia)31, UDCaT-2 (shape selective and tunable),32 UDCaT-4

101

(synergism of persulfated alumina and zirconia into HMS)33, UDCaT-5 (stronger than UDCaT-

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1),34 UDCaT-6 (mesoporous)35 and modified heteropolyacids36 for Friedel-Crafts acylation and

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alkylation of commercially important molecules. The role of inorganic support, its synergetic

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effect on activity of various heteropoly acids and their application in a number of industrially

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fundamental reactions were well established by our group.

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The current work delineates process development and kinetic aspects of selective acylation of

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guaiacol with vinyl acetate as new acylating agent using cesium substituted heteropolyacid

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supported on K10 clay. There is no report on this system. Further some of the catalysts

109

developed by us merited attention to conduct this reaction. A comprehensive investigation of

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effects of various parameters was accomplished and a kinetic model developed to illustrate the

111

reaction pathway and validated against experimental results.

112 113

2. Experimental Section

114

2.1. Chemicals


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The following chemicals were procured from firms of repute and used without further

116

purification.

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dodecatungstophosphoric acid (DTP), chlorosulphonic acid, guaiacol and vinyl acetate were

118

obtained from s. d. Fine Chem. Ltd., Mumbai, India. Tetraethyl orthosilicate and K-10 clay were

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obtained from Fluka Germany. The catalysts were dried at 110 ºC for 4 h before use.

Zirconium

oxychloride,

aqueous

ammonia

solution,

cesium

chloride,

120 121

2.2. Preparation of catalysts

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UDCaT-433, UDCaT-534, UDCaT-635 and 20% w/w DTP/HMS37 were prepared by well

123

established procedures in our lab. 20% w/w Cs2.5H0.5PW12O40/K-10 was prepared by procedure

124

developed in our lab.29 Approximately 10 g of K-10 was dried in an oven to 120 ºC for 1h of

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which 8 g was weighed accurately. 0.2808 g (1.671×10−3 mol) of CsCl was weighed accurately

126

and dissolved in 10 cm3 of methanol. This volume of solvent used was approximately equal to

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the pore volume of the catalyst. The solution was added to the previously dried and accurately

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weighed 8 g of K-10 clay to form slurry. The slurry was stirred vigorously and air-dried. The

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resulted material was then dried in an oven at 120 ºC for 2 h. This was then further subjected to

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impregnation by an alcoholic solution of 2 g (6.688×10−4 mol) of DTP in 10 cm3 of methanol.

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The solution was added to the previously treated K-10 clay with CsCl again to form slurry. The

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slurry was stirred vigorously and air-dried. The preformed catalyst was dried in an oven at 120

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ºC for 2 h and then calcined at 300 ºC for 3 h.29

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2.3. Experimental set up

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All experiments were carried out in 100 cm3 stainless steel autoclave (Amar Equipments,

136

Mumbai). A four bladed –pitched turbine impeller was used for agitation. The temperature was 6 ACS Paragon Plus Environment


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maintained at ±1°C of the desired value. Known quantities of reactants and catalyst were charged

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into autoclave, the temperature raised to the desired value and agitation started. Then, an initial

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sample was withdrawn. Further samples were withdrawn at periodic intervals upto 2 h. A

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standard experiment consists of 0.005 mol of guaiacol, 0.03 mol of vinyl acetate and a catalyst

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loading of 0.009 g/cm3 with respect to total volume of the liquid. The temperature was

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maintained at 170 °C and the speed of agitation at 1000 rpm. The total volume of liquid phase

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was 33.5 cm3.

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2.4. Method of analysis of reaction mixture

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Clear liquid samples were withdrawn periodically and analysis of the reaction mixture was

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performed on GC (Chemito, model 1000) by using FID and a capillary wax column BP-20 (0.25

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mm diameter and 30 m length). The product was confirmed by GC–MS (Perkin Elmer

148

instrument, Clarus 500) with BP-1 capillary column (0.25 mm i.d., 30 m length) and EI mode of

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MS. The injector and detector temperatures were maintained at 280 °C. The oven temperature

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was programmed from 90 °C to 270 °C, with a ramp rate of 15 °C/min. Nitrogen was used as the

151

carrier gas at a flow rate of 0.5 cm3/s. The conversions were based on the disappearance of

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guaiacol, which was the limiting reactant.

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Isolation of product was done as follows. 1,4-Dioxane, unreacted vinyl acetate and the co-

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product acetaldehyde, in trace quanity, were removed by distillation under reduced pressure.

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Indeed, acetaldehyde, being low boiling (b.p. 20.4 oC), used to escape from the reaction mass.

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Ice-cold water was added to the reaction mass, stirred for ~ 10-15 min and the white product

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isolated through filtration. The co-product acetaldehyde being volatile did not cause any


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problems. The product obtained was white solid with melting point 115 °C. The product was

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confirmed by gas chromatography-mass spectroscopy (GC-MS) and its physical constants.

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2.5. Reaction scheme OH O

OH

O O

CH3

Acid catalyst

+

H2C

O

+

CH3 H3C

161

2-methoxyphenol

vinyl acetate

O

CH3 H3C

H

O

1-(4-hydroxy-3-methoxyphenyl)ethanone

acetaldehyde

Scheme 1: Acylation of 2-methoxyphenol

162 163 164

3. Results and Discussion

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3.1. Catalysts characterization

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The catalyst was thoroughly characterized and recently reported by us.36-39 So only few rominent

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features are discussed here. X-ray diffractometer confirmed the crystalline and textural nature of

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20%w/w CsDTP/K10 clay. DTP was found to be crystalline whereas K10 retained its amorphous

169

characteristic. The diffractogram obtained suggested that although the CsDTP salt had lost some

170

of its crystallinity during the process of supporting it on K10, the Keggin structure of DTP

171

remained uninterrupted (Figure 1). The FTIR analysis confirmed the preservation of the Keggin

172

structure of DTP in the catalyst. The direct interaction between Keggin polyanion and Cs+ was

173

confirmed by the characteristic split in W=O band of CsDTP (Figure 2). The scanning electron

174

micrographs identified that both K-10 and 20%w/w CsDTP samples possess rough and rugged

175

surface, whereas 20%w/w CsDTP/K10 clay shows a smoother surface because of a layer Cs-salt

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of DTP over the external surface of K-10 (Figure 3). The Brunauer-Emmett-Teller surface area,

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pore volume and pore diameter analysis of 20% w/w CsDTP/K10, K-10, and 20% DTP/K-10 are 8 ACS Paragon Plus Environment


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given in Table 1. The decrease in surface area of catalyst is due to incorporation of CsDTP salt in

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pores of K-10. As given in Table 2 on ammonia TPD analysis of K-10 and DTP based catalysts,

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an increase in acidity of catalyst was observed when CsDTP is loaded on K-10. The confirmation

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of mesoporous nature was done by adsorption desorption isotherm of 20%w/w CsDTP/K-10

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which exhibits type IV isotherm with hysteresis loop of type H3.36-39

183 184

3.2. Efficacy of Various Catalysts

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Different solid acid catalysts were used to demonstrate their efficacy in the liquid phase acylation

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of guaiacol (Figure 4). A 0.009 g/cm3 loading of catalyst based on total volume of the reaction

187

mixture was employed at 170 °C. The mole ratio of guaiacol to vinyl acetate was kept 1:6 with

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speed

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(Cs2.5H0.5PW12O40)/K10 was found to be very active and highly efficient catalyst. 20% w/w DTP

190

(H3PW12O40) supported on hexagonal mesoporous silica (HMS) was found to be active. Sulfated

191

zirconia based catalysts, namely, UDCaT-5 (superacidic modified sulfated zirconia), UDCaT-4

192

(persulfated alumina and zirconia on HMS) and UDCaT-6 (modified sulfated zirconia

193

nanoparticles on HMS) were also used for batch reaction. It was observed that these Lewis acid

194

catalysts give much lower conversion as compared to Bronsted acid catalysts. Although the

195

activity of 20% w/w DTP/HMS was similar to 20% w/w CsDTP/K10, the reusability of Cs

196

doped DTP on K10 was much superior to previous one. The catalyst was filtered, dried and

197

reused with

198

during first reuse because of leaching of DTP from the pores of HMS, whereas the activity of

199

20% w/w CsDTP/K10 remained practically the same (Table 3). Hence 20% w/w CsDTP/K-10

200

was selected for further exploration.

of

agitation

1000

rpm.

Among

clay

based

catalyst

20%

w/w

CsDTP

make up quantity. There was 30% decrease in conversion with 20% DTP/HMS


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201

Okuhara et al.38 have disclosed that alkali exchange with proton of HPA results in enhanced

202

activity, better stability and higher surface area of heteropoly acids. Yadav and Asthana

203

reported that the incorporation of Cs+ ion into dodecatungstphosphoric acid supported on

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montmorillonite K10 which results in enhancement of catalytic activity and also change in

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surface structural properties such as pore volume, pore diameter and surface area (Table 1). Salts

206

of small cations like (group A) are highly soluble in water while salts of large cations (like Cs of

207

group B) are completely insoluble in aqueous medium and any organic solvents, making them

208

useful for heterogeneous reactions.

209

The lower loadings we have studied earlier by us 39,40 and found that 20% w/w HPA/K-10 is the

210

best which on substation with substation with Cs renders higher surface area.29 The function of

211

support is to enhance the homogeneous distribution of catalyst on pore walls of K-10 and also to

212

add to the overall acid strength of the catalyst. However, with increase in loading of Cs-DTP on

213

K-10, the surface area was decreased due to clogging of fine pores of support by Cs-DTP

214

particles but increase in acidity was observed.29 Therefore 20% CsDTP/K-10 was used as the

215

catalyst.

216

3.3. Effect of acylating agents

217

The effect of acylating agents on the acylation of guaiacol with acetic anhydride, acetic acid,

218

vinyl acetate, methyl acetate, ethyl acetate and phenyl acetate were studied using

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Cs2.5H0.5PW12O40/K-10 solid acid catalyst. The results are given in Table 4. It was observed that

220

vinyl acetate was highly efficient acylating agent as compared to others. No reaction was

221

observed with ethyl acetate and acetic acid whereas phenyl acetate was found to be pretty

222

impressive. Unfortunately the use of acetic acid and acetic anhydride is associated with lot of

223

environmental and corrosion problems. The high activity and selectivity of vinyl acetate toward

29



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acylation was due to stability of co-product acetaldehyde which escapes the reaction mass since

225

its boiling point is 20.3 oC and the reaction becomes irreversible. Therefore it was selected for

226

further exploration.

227

3.4. Effect of Speed of Agitation

228

To assess the effect of external mass transfer resistance on the rate of reaction, the speed of

229

agitation was shuffled from 600 to 1200 rpm at 180 °C at the catalyst loading of 0.009 g/cm3

230

(Figure 5). From 600 to 1200 rpm, the conversion and product distribution were practically

231

similar in all the cases. Thus, the external resistance to mass transfer was absent beyond 1000

232

rpm. However, to be on safer side and to avoid abrasion of the catalyst, further experiments were

233

conducted at 1000 rpm.

234 235

3.4.1. Proof of absence of external mass transfer resistance

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This is a typical solid-liquid slurry reaction involving the transfer of guaiacol, the limiting

237

reactant (A) and vinyl acetate (B) from the bulk liquid phase to the catalyst wherein transfer of

238

the reactant to the outer surface of the catalyst particle takes place, which is followed by intra-

239

particle diffusion, adsorption, surface reaction and desorption.29 The influence of external solid-

240

liquid mass transfer resistance must be ascertained before true kinetic model could be developed.

241

This has been explained in some of our publications.

242 243

The reaction of guaiacol (A) with vinyl acetate (B) produces 4-hydroxy-3-methoxyacetophenone

244

(C) and acetaldehyde (D).

245

A+ B → C + D

(1)


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246

At steady state, the rate of mass transfer per unit volume of the liquid phase (mol/cm3s) is given

247

by:

248

R A = k SL

249

(Rate of transfer of A from bulk liquid phase to external surface of the catalyst particle)

250

R A = k SL

251

(Rate of transfer of B from bulk liquid phase to external surface of the catalyst particle)

252

RA = robs

253

(Observed rate of reaction within the catalyst particle).

254

Here the subscripts “o” and “s” denote the concentrations in bulk liquid phase and external

255

surface of catalyst, respectively. Depending on the relative magnitudes of the external resistance

256

to mass transfer and reaction rates, different controlling mechanisms have been put forward.

257

When the external mass transfer resistance is negligible, then the following inequality holds:

258

1 1 >> robs kSL − A a p[ A o ]

(5)

259

1 1 >> robs kSL − Bap[ Bo ]

(6)

260

The observed rate robs could be given by three types of models wherein the contribution of intra-

261

particle diffusion resistance could be accounted for by incorporating the effectiveness factor η.

262

These models are as follows:

263

(a) The power law model if there is weak adsorption of reactant species.

264

(b) Langmuir-Hinshelwood-Hougen-Watson model.

265

(c) Eley-Rideal mechanism.

− A

− B

a p{ [ A o ] − [ A s ]}

(2)

a p{ [ B o ] − [ B s ]}

(3)

(4)



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According to Equations (5) and (6), it is necessary to calculate the rates of mass transfer of

267

guaiacol (A) and vinyl acetate (B) and to compare them with the overall rate of reaction. For a

268

typical spherical particle, the particle surface area per unit liquid volume is given by

269

ap =

270

Where w is the catalyst loading (g/cm3) of liquid phase, ρp the density of particle (g/cm3) and dp

271

is the particle diameter (cm). For this reaction, for the maximum catalyst loading used (0.0089

272

g/cm3) with the particle size (dp) of 0.005 cm, in the current studies, ap= 12.59 cm2/cm3 liquid

273

phase. The liquid phase diffusivity values of the reactant A denoted by DAB was calculated by

274

using Wilke-Chang equation41 at 170 °C as 1.15×10-4 cm2/s. The corresponding value for

275

reactant B was calculated as 1.44×10-4 cm2/s at 170 °C. The solid-liquid mass transfer

276

coefficients for A were calculated from the limiting value of the Sherwood number (e.g. ShA=kSL-

277

Adp/DAB)

278

agitated systems but for conservative estimations a value of 2 is taken. The solid-liquid mass

279

transfer coefficients k SL − A and k SL − B

280

respectively. The initial rate of reaction was calculated from the conversion profile at 170 °C. A

281

typical initial rate of reaction was calculated as 1.62 × 10-6 mol/cm3s. Therefore, putting the

282

appropriate values in Eqs. (5) And (6), i.e.

283

1 1 >> robs k SL − A a p [ A0 ]

284

i. e. 6.17 × 107 >> 3.43 × 102 and

285

1 1 >> robs k SL − B a p [B 0 ]

286

i. e. 6.17× 107 >> 0.45 × 102

6w = 12.59 cm −1 ρpdp

(7)

of 2. The actual Sherwood numbers are typically higher by order of magnitude in well-

were obtained as 4.63×10-2 cm/s and 5.77×10-2 cm/s,

(8)

(9)


Page 15 of 44

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287

The above inequality demonstrates that there is an absence of resistance due to the solid-liquid

288

external mass transfer for both the species A and B and the rate may be either surface reaction

289

controlled or intra-particle diffusion controlled. Therefore, the effects of catalyst loading at a

290

fixed particle size and temperature were studied to evaluate the influence of intra-particle

291

resistance.

292 293

3.5. Effect of Catalyst loading

294

In the absence of mass transfer and intra-particle diffusion resistances, the rate of reaction is

295

directly proportional to the catalyst loading based on the total reaction volume. The catalyst

296

loading was varied from 0.009 to 0.012 g/cm3 keeping mole ratio 1:6 (Figure 6). It was

297

concluded that both rate of reaction as well as conversion had increased up to 0.012 g/cm3 with

298

increase in catalyst loading due to proportional increase in active sites available for the reaction.

299

The conversion was almost the same for catalyst loading of 0.009 g/cm3 and 0.012 g/cm3 which

300

was 96.6% in both the cases. This may be because of the fact that beyond a certain loading, the

301

number of catalyst sites was greater than that actually required by the reactant molecules.

302 303

3.5.1. Proof of absence of intraparticle diffusion resistance:

304

The average particle diameter of the Cs2.5H0.5PW12O40/K-10 was 50 µm and thus theoretical

305

calculation was done by using the Wiesz-Prater criterion to assess the influence of intraparticle

306

diffusion resistance. According to the Weisz-Prater criterion, the value of − robs ρ p R p2 De − A [C AS ]

307

has to be much less than unity for the reaction to be intrinsically kinetically controlled; which

308

represents the ratio of the intrinsic reaction rate to the intraparticle diffusion rate.42 According to

309

Weisz-Prater criterion, Cwp can be evaluated from the observed rate of reaction, particle radius



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Page 16 of 44

310

(Rp), effective diffusibility of the limiting reactant (De) and concentration of reactant on the

311

surface of the particle. − robs ρ p R p2

312

(i) If C wp =

313

(ii) If Cwp

314

The effective diffusivity of guaiacol ( De − A ) inside the pores of the catalyst was obtained from the

315

ε  bulk diffusivity ( DAB ), porosity (ε) and tortuosity (τ) as 1.54 ×10−5 cm2/s where De − A = DAB   τ 

316

. In the present case, the value of Cwp was calculated as 1.12×10−6 for the initial observed rate

317

which is much less than 1 and therefore the reaction is intrinsically kinetically controlled. A

318

further proof of the absence of the intra-particle diffusion resistance was obtained through the

319

study of the effect of temperature and it will be discussed later.

De − A [C AS ]

1 , then the reaction is limited by severe internal diffusional resistance.

1 , then the reaction is intrinsically kinetically controlled.

320 321

3.6. Effect of mole ratio

322

The mole ratio of guaiacol to vinyl acetate was varied from 1:2 to 1:8 with a catalyst loading of

323

0.009 g/cm3 at 170 °C (Figure 7). It was observed that the initial rate of the reaction increased

324

with increase in mole ratio. It can be concluded from the graph that there is not much difference

325

in the conversions between mole ratios of 1:6 to 1:8. Hence further all reactions were carried out

326

at mole ratio of 1:6 of guaiacol with vinyl acetate.

327 328

3.7. Effect of Temperature

329

The temperature was varied from 150 °C to 180 °C in order to investigate its effect on the rate of

330

reaction (Figure 8). It was found that with an increase in temperature the rate of reaction and


Page 17 of 44

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331

conversion increased substantially, which confirms that reaction was intrinsically kinetically

332

controlled. The Arrhenius plot of lnk vs. 1/T gives the activation energy for acylation of guaiacol

333

with vinyl acetate. The apparent energy of activation was found to be 15.46 kcal/mol

334

3.8. Mechanism of reaction

335 336

Scheme 2: Plausible reaction mechanism on catalytic surface

337

338

3.9. Development of mechanistic model and kinetics of the reaction



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Page 18 of 44

339

From the calculated values of mass transfer rates of A and B, initial observed rate, it is evident

340

that the rate is independent of the external mass transfer effects. Activation energy also indicates

341

that intra-particle diffusion resistance is absent.

342

The reaction is assumed to occur by Langmuir-Hinshelwood-Hougen-Watson (LHHW) type of

343

mechanism with a weak adsorption of the reactants in the absence of any diffusion resistance.

344

Let A= Guaiacol, B= Vinyl acetate, ES= Apocynin, WS= Acetaldehyde, S= vacant sites.

345

Adsorption:-

346

Adsorption of guaiacol (A) on a vacant site S is given by:

347 348 349

KA  → AS A + S ←

(10)

Adsorption of vinyl acetate (B) on a vacant site S is given by: KB  → BS B + S ←

(11)

350

Surface reaction:-

351

Surface reaction of AS and BS in the vicinity of an active site, leading to the formation of ES and

352

WS on the active

353

 → ES + WS AS + BS ←

354

Desorption:-

355

Desorption of apocynin (ES) and acetaldehyde (WS) is given by:

356

 →E + S ES ← 

(13)

357

 →W + S WS ← 

(14)

358

The total concentration of the sites, Ct , expressed as,

359

Ct = CS + C AS + CBS + CES + CWS

360

or

K2

(12)

1/ K E

1/ KW

(15)


Page 19 of 44

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361

C t = C S + K A C A C S + K B C B C S + K S C E C S + K W CW C S

362

When the adsorption and desorption steps are assumed to be in equilibrium,

363

The concentration of vacant sites can be written as,

364

CS =

365

Ct (1 + + K ACA + K BCB + KS CE + KW CW )

(16)

(17)

If the surface reaction controls the rate of reaction, then the rate of reaction of A is given by

K E KW CE CW 2 }Ct dC A K2 − = (1 + + K AC A + K B CB + K S CE + KW CW ) 2 dt k2 {K A K B C ACB −

366

367

When the reaction is far away from equilibrium

368

−

369

−

(18)

dC A k 2Ct2 K A K B C AC B = dt (1 + ∑ K i Ci ) 2

(19)

dC A kR2 wK B CACB = dt (1 + ∑ Ki Ci ) 2

(20)

370

Where kR2 w = k2Ct2 K A K B where, w is catalyst loading. If the adsorption constants are small,

371

then the above equation reduces to:

372

−

dCA dX A = CA0 = kR2 wCA0 (1 − X A )CB 0 dt dt

(21)

373 374

Because vinyl acetate was taken in molar excess over guaiacol ( CB0

375

first order equation which can be integrated as follows:

376 377

− ln(1 − X A ) = k1wt

C A0 ), it becomes a pseudo

(22)

Where, k1 is the pseudo-first order constant.



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

Page 20 of 44

378

A plot of –ln (1-XA) versus time was made as different temperatures (Figure 9). It is seen that

379

the data fits well and hence validates the model. Arrhenius plot was made (Figure 10) to get the

380

energy activation as 15.46 kcal/mol which shows that the reaction is intrinsically kinetically

381

controlled.

382 383

3.10. Reusability of Catalyst

384

The reusability of Cs2.5H0.5PW12O40/K-10 was tested by conducting 3 runs. After each run the

385

catalyst was filtered and refluxed with 50 cm3 of 1,4-dioxane in order to remove any adsorbed

386

material from the catalyst surface, then it was dried in an oven at 120 ºC for 2 h. In a typical

387

batch reaction, there was an inevitable loss of particles during filtration due to attrition. The

388

actual volume of catalyst used in next batch was almost 5% less than the previous batch. The

389

volume of reaction mixture was adjusted to make the catalyst loading of 0.009 g/cm3. It was

390

observed that there was only a marginal decrease in the conversion (Figure 11). Thus, the

391

catalyst was active and reusable.

392 393

4. Conclusion:

394

The Friedel-Crafts acylation of guaiacol with vinyl acetate was studied over Lewis as well as

395

Bronsted acid catalysts. Among various catalysts like Cs2.5H0.5PW12O40/K-10, 20%DTP/HMS,

396

K10, UDCaT-5, UDCaT-6 and UDCaT-4, Cs2.5H0.5PW12O40/K-10 was most active, stable and

397

reusable catalysts for acylation reaction. Screening of acylating agents demonstrates vinyl acetate

398

as highly effective acylating agent. The effects of various parameters like speed of agitation,

399

catalyst loading, mole ratio and influence of temperature on the rate of reaction were discussed in

400

detail. A kinetic model for the reaction was successfully developed and the energy of activation


Page 21 of 44

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401

was found to be 15.46 kcal/mol. The reaction was found to follow Langmuir-Hinshelwood-

402

Hougen-Watson type of mechanism with a weak adsorption of the reactants in the absence of

403

any diffusion resistance. In conclusion we have developed a simple, efficient, convenient and

404

reusable method for the preparation of acetovanillone or 4-hydroxy-3-methoxyacetophenone.

405

406

Acknowledgement

407

G.D.Y. received support from R.T. Mody Distinguished Professor Endowment and as J.C. Bose

408

National Fellow from Department of Science and Technology, Government of India. A.R.Y.

409

thanks UGC for providing JRF under its BSR in Green Technology.

410

411

412

413

414

415

416

417 418

Nomenclature

419

A

reactant species A, gauaicol



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

420

B

reactant species B, vinyl acetate

421

AS

chemisorbed gauaicol

422

BS

chemisorbed vinyl acetate

423

C

product species C, apocynin

424

D

product species D, acetaldehyde

425

S

vacant site

426

CA

concentration of A in (mol/cm3)

427

CAo

initial concentration of A in bulk liquid phase, (mol/cm3)

428

CAS

concentration of A at catalyst surface, (mol/cm3)

429

CB

concentration of B, (mol/cm3)

430

CBo

initial concentration of B in bulk liquid phase, (mol/cm3)

431

CBS

concentration of B at solid surface, (mol/cm3)

432

CC

concentration of C, (mol/cm3)

433

Ccs

concentration of C at solid surface, (mol/cm3)

434

Cs

concentration of vacant sites, (mol/cm3)

435

CT

total concentration of sites, (mol/cm3)

436

kSR

reaction rate constant, cm6gmol-1s-1

437

Ki

adsorption equilibrium constant for species i, cm3/mol

438

w

catalyst loading g/cm3 of the liquid volume

439

XA

fractional conversion of A

440

-rA

441

k1

pseudo-first order surface reaction constants

442

kR2

second order surface reaction constants

Page 22 of 44

rate of surface reaction, g mol cm-3 s-1


Page 23 of 44

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443

KA

Equilibrium constant for adsorption of A on catalyst surface (mol-1 min-1)

444

KB

Equilibrium constant for adsorption of B on catalyst surface (mol-1 min-1)

445

DAB

Diffusion coefficient of A in B (cm2/s)

446

DBA

Diffusion coefficient of B in A (cm2/s)

447

De-A

Effective diffusivity (cm2/s)

448

kSLA, kSLB

solid liquid mass transfer coefficient(cm/s)

449

robs

Observed rate of reaction (mol/g cat/s)

450

S

Vacant site

451

Sh

Sherwood number

452

t

Time (min)

453

w

Catalyst loading (g/cm3 of liquid phase)

454

XA

Fractional conversion of A

455

Cwp

Weisz-Prater constant

456

ap

solid liquid interfacial area (cm2/cm3)

457

Rp

radius of catalyst particle (cm)

458 459

Greek letters

460

ε Porosity

461

ρp Density of catalyst particle (g/cm3)

462

τ Tortuosity (-)

463

µ Viscosity of reaction mixture (kg/m.s)

464

Abbreviation for Catalysts

465

UDCaT-4

University Department of Chemical Technology catalyst-4



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

466

UDCaT-5

University Department of Chemical Technology catalyst -5

467

UDCaT-6

University Department of Chemical Technology catalyst -6

468

CsDTP/K10

Cs2.5H0.5PW12O40 supported on montmorillonite acid treated clay K-10

469

DTP/HMS

H3PW12O40 supported on HMS

470

K10

Montmorillonite acid treated clay K-10

Page 24 of 44

471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487

488


Page 25 of 44

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489

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490

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491

activity of the newly developed neutrophil oxidative burst antagonist apocynin. Free

492

Radicals in Biology and Medicine, 1990, 9, 127–131.

493

(2) Martin R. (Ed), Handbook of Hydroxyacetophenones: Preparation and Physical Properties, Volume 1. Springer, 2005.

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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

Page 30 of 44

Figure Captions and Figures

601 602 603

Figure 1.

XRD of 20% CsDTP/K10

Figure 2.

FTIR spectra of catalysts

Figure 3.

SEM 20% CsDTP/K10

Figure 4.

Effect of Catalyst. Guaiacol-0.005 mole, Vinyl acetate-0.03 mole, Catalyst loading

604 605 606 607 608

0.009 g/cm3, Speed of agitation-1000 rpm, Temperature -170 °C, solvent-1,4 Dioxane

Figure 5.

Effect of Speed of Agitation. Guaiacol-0.005 mole, Vinyl acetate-0.03 mole, Catalyst loading- 0.009g/ cm3, Temperature-170 °C, solvent-1,4 Dioxane

Figure 6.

Effect of Catalyst Concentration. Guaiacol-0.005 mole, Vinyl acetate-0.03 mole, Speed of agitation-1000 rpm, Temperature-170 °C, solvent-1,4 Dioxane

Figure 7.

Effect of mole ratio. Guaiacol: Vinyl acetate, Catalyst loading- 0.009g/ cm3, SOA1000 rpm, Temperature-170 °C, solvent-1,4-Dioxane

Figure 8.

Effect of Temperature. Guaiacol-0.005 mole, Vinyl acetate-0.03 mole, Catalyst loading 0.009g/ cm3, Speed of agitation-1000 rpm, solvent-1,4-Dioxane

Figure 9.

Kinetic plot of –ln (1-Xa) Vs time. Guaiacol-0.005 mole, Vinyl acetate-0.03 mole, Catalyst loading-0.009g/ cm3, Speed of agitation-1000 rpm, Temperature 150 °C180 °C, solvent-1,4 Dioxane

Figure 10.

Arrhenius plot (–lnk Vs. 1/T) 29 ACS Paragon Plus Environment

Page 31 of 44

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

Figure 11.


Reusability of Catalyst. Guaiacol-0.005 mole, Vinyl acetate-0.03 mole, Catalyst loading 0.009g/ cm3, Speed of agitation-1000 rpm, Temperature 170 °C, solvent1,4 Dioxane

Table Captions Table 1. Surface area, pore volume and pore diameter analysis

Table 2. Ammonia TPD analysis of DTP based catalyst

Table 3. Reusability of various DTP based catalyst: Reaction conditions: Guaiacol0.005 mole, Vinyl acetate-0.03 mole, Catalyst loading

0.009g/ cm3, Speed of

agitation-1000 rpm, Temperature 170 °C, solvent-1,4 Dioxane

Table 4. Effect of various acylating agents: Reaction conditions: Guaiacol-0.005 mole, Vinyl acetate-0.03 mole, Catalyst loading 0.009g/ cm3, Speed of agitation1000 rpm, Temperature 170 °C, solvent-1,4 Dioxane



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

Page 32 of 44

609

610 611

Figure 1

612

613

614


Page 33 of 44

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


615

616

617

618 619

Figure 2

620

621

622

623



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

Page 34 of 44

624

625 626

Figure 3

627

628

629

630

631

632

633

634


Page 35 of 44

100 90 80 70 % Conversion

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

Catalyst 635 636

Figure 4

637

638

639

640

641

642



100 90 80 70 60 % Conversion

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

Page 36 of 44

50 40 30 20 10 0 0

20

40 600rpm

60 80 Time,mins 800rpm 1000rpm

100

120

140

1200rpm

643 644

Figure 5

645

646

647

648

649

650

651


Page 37 of 44

652

100 90 80 70

% Conversion

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 0

20

40

60

80

100

120

Time, mins 0.003g/cc

0.006g/cc

0.009g/cc

0.012g/cc

653 654

Figure 6

655

656

657

658

659

660 36 ACS Paragon Plus Environment


661


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

Page 38 of 44

60 50 40 30 20 10 0 0

20

40

60

80

100

120

Time, mins 1:02

1:04

1:06

1:08

662 663

Figure 7

664

665

666

667

668


Page 39 of 44

669

100 90 80 70

% conversion

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 0

20 150°C

670 671

40

60 80 Time, mins 160°C

170°C

100

120

180°C

Figure 8

672

673

674

675

676



678

0.7 y = 0.00036x R² = 0.99905 0.6 y = 0.00028x R² = 0.99971 0.5

0.4 -ln(1-Xa)

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

Page 40 of 44

y = 0.00017x R² = 0.99827 0.3 y = 0.00011x R² = 0.99931

0.2

0.1

0 0

200

400

600 150°C

800

1000 1200 Time, sec 170°C 160°C

1400

1600

1800

2000

180°C

679 680

Figure 9

681

682

683

684

685


Page 41 of 44

687

-7.8

-8

-8.2

-8.4

y = -7785.3x + 9.31 R² = 0.9877

lnk

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


-8.6

-8.8

-9

-9.2 0.00218

0.00221

0.00227

0.0023

0.00233

0.00236

1/T

688 689

0.00224

Figure 10

690

691

692

693

694




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

Page 42 of 44

60 50 40 30 20 10 0 0

20

40

60

80

100

120

140

Time, mins Fresh

1st reuse

2nd reuse

3rd reuse

695 696

Figure 11

697

698

699

700

701

702


Page 43 of 44

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

703 704


Table 1. Surface area, pore volume and pore diameter analysis

Catalyst

Surface area

Pore

volume Pore

(m2/g)

(cm3/g)

nm

K-10

230

0.36

6.4

20% DTP/K-10

107

0.32

7.1

20% Cs2.5H0.5PW12O40/K-10

207

0.29

5.8

20% DTP/HMS

909

0.54

2.6

diameter

705 706 707 708

Table 2. Ammonia TPD analysis of DTP based catalyst Sr.No.

Catalyst

Total acidity (mmol/g)

1

K-10

0.139

2

20% DTP/K-10

0.328

3

20% Cs2.5H0.5PW12O40/K-10

0.405

4

20% DTP/HMS

0.245

709 710 711 712 713 714 715 716 717 718 719 42 ACS Paragon Plus Environment


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

Page 44 of 44

Table 3. Reusability of various DTP based catalyst

720 721

Catalyst

Fresh catalyst

Reused catalyst

(% Conversion)

(% Conversion)

K-10

65

50

20% DTP/K-10

80.5

61.4

20% Cs2.5H0.5PW12O40/K-10

96.6

93.6

20% DTP/HMS

95

65

722 723

Table 4. Effect of various acylating agents

724 725

#

726 727 728

Acylating agent

Conversion, %

Vinyl acetate

96.6

Phenyl acetate

37.5

Methyl acetate

15.3

Ethyl acetate

0

Acetic anhydride

7.4

Acetic acid

0

N.B. Guaiacol-0.005 mol, Vinyl acetate-0.03 mol, Catalyst loading 0.009g/ cm3, Speed of agitation-1000 rpm, Temperature 170 °C, solvent-1,4 Dioxane


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