Cotton Dyeing in Supercritical Carbon Dioxide

Carbon Dioxide

Carbon Dioxide

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J. T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op donderdag 1 september 2005 om 13:00 uur door

Maria Vanesa FERNÁNDEZ CID

Licenciada en Ciencias Químicas, Universidad de Santiago de Compostela geboren te Vigo, Spanje

Prof. dr. G.J. Witkamp

Samenstelling promotiecomissie:

Rector Magnificus Voorzitter

Prof. dr. G.J. Witkamp Technische Universiteit Delft, promotor Prof. dr. ir. H. van Bekkum Technische Universiteit Delft

Prof. dr. W. Buijs Technische Universiteit Delft Prof. Dr.-Ing. M. Wessling Universiteit Twente

Prof. dr. M. Poliakoff University of Nottingham Prof. dr. ir. M. J. Cocero Alonso University of Valladolid

Drs. J. van Spronsen FeyeCon Development & Implementation B.V.

This research was financially supported by the research program “Ecologie, Economie en Technologie” from the Ministry of Economic Affairs; Ministry of Housing, Spatial Planning and the Environment; and Ministry of Education, Culture and Science (project number EET K99101 Jump)

Cover design & layout by: Casper J. Koomen

ISBN: 90-9019765-6

Copyright © 2005 by M.V. Fernández Cid Printed by Febodruk B.V., Enschede

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilised in any form or by means, electronic or mechanical, including photocopy, recording or by any information storage and retrieval system, without written permission from the publisher.

por darme la mayor

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Table of contents

Chapter 1 Introduction

1

1.1 Conventional dyeing process for cotton

2

1.2 Supercritical dyeing: An alternative dyeing process

2

1.2.1 Advantages of supercritical CO₂ for textile dyeing

3

1.2.2 Drawbacks on dyeing cotton with supercritical CO₂

4

1.3 Aim of the thesis

6

1.4 Outline of this thesis

6

1.5

References

8

Chapter 2 Kinetics Study of a Dichlorotriazine Reactive Dye in

Supercritical Carbon Dioxide

11

2.1 Introduction

12

2.2

Materials and Methods

13

2.2.1 Materials

13

2.2.2 HPLC analysis

13

2.2.3 Set-up and Procedure

14

2.3 Results and Discussion

14

2.4 Conclusions

24

2.5 Acknowledgments

24

2.6 Reference

24

Chapter 3 Base promoted etherification reaction of a non-polar

reactive dye and methanol by zeolite KA

27

3.1 Introduction

28

3.2 Experimental

30

3.2.1 Materials

30

3.2.2 Analysis

30

Chapter 4 Acid-catalysed methanolysis reaction of non-polar

triazinyl reactive dyes in supercritical carbon

dioxide

47

4.1 Introduction

48

4.2 Materials and Methods

49

4.2.1 Materials

49

4.2.2 Dye synthesis

50

4.2.3 Experimental set-up and Procedure

51

4.2.4 Methanolysis reaction with addition of extra acid

51

4.2.5 HPLC analyses

51

4.3 Results and Discussion

52

4.3.1 Kinetics of RCl₂, RCl-OCH₃ and RCl-NHCH₃ in a neutral and

moderately acidic medium

52

4.3.2 Kinetics of monochlorotriazines in strongly acidic medium

59

61

64

4.4 Conclusions

68

4.5 Appendix.

69

4.6 References

71

Chapter 5 A Novel Process to Enhance the Dyeability of Cotton

in Supercritical Carbon Dioxide

73

5.1 Introduction

74

5.2 Materials & Methods

76

5.2.1 Materials

76

5.2.2 Pretreatment

77

5.2.3 Dyeing Equipment and Procedure

77

5.2.4 Extraction of Unfixed Dye

78

5.2.5 Colour Measurement

78

5.2.6 Determination of Dye Fixation

79

5.3 Results

79

5.4 Discussion

86

5.5 Conclusions

91

5.6 Acknowledgements

91

5.7 Appendix

92

5.8 References

94

Chapter 6 A significant approach to dye cotton in supercritical

carbon dioxide with fluorotriazine reactive dyes 97

6.1 Introduction

98

6.2 Materials & Methods

99

6.2.1 Materials

99

6.2.3 Experimental set-up

102

6.2.4 Experimental Procedure

102

6.2.5 Analysis

103

6.3 Results & Discussion

104

6.3.1 Kinetics of difluorotriazine reactive dye

104

6.3.2 Dyeing cotton with fluorotriazine reactive dyes

107

6.4 Conclusions

117

6.5 Acknowledgements

118

6.6 Appendix

119

6.7 References

121

Chapter 7 Excellent dye fixation on cotton dyed in supercritical

carbon dioxide using fluorotriazine reactive dyes

123

7.1 Introduction

124

7.2 Experimental

125

125

126

128

128

129

7.2.6 Determination of Colour Strength

130

7.2.7 Determination of Dye Fixation

130

7.3 Results and Discussion

130

7.3.1 Kinetic Study

130

7.3.2 Dyeing results and discussion

134

Chapter 8 Nucleophilic substitution of triazines with methanol:

a comparison of experimental and molecular

modelling data

151

8.1 Introduction

152

8.2 Experimental

153

8.2.1 Materials

153

8.2.2 Synthesis of 2,4-difluorotriazine dye (DFT)

153

8.2.3 Kinetic experiments

154

8.2.4 HPLC analyses

154

8.2.5 Molecular modelling

155

8.3 Results and Discussion

155

8.3.1 Experimental results & discussion

155

8.3.2 Modelling results & discussion

158

8.4 General Discussion

168

8.5 Conclusions

169

8.6 References

170

Chapter 9 Scale-up of the dyeing process of cotton

173

9.1 Introduction

174

9.2 Experimental

174

9.2.1 Scaled-up dyeing machine

174

9.2.2 Dyeing procedure

176

9.2.3 Colour characterization

176

9.3 Results and Discussion

176

9.3.1 Temperature of the dyeing vessel

176

9.3.2 Reaction time

178

9.3.3 Depth of colour

180

9.3.4 Influence of acids on the cotton colouration and dye fixation

181

9.3.5 Reproducibility

182

9.4 Conclusions

183

Summary

185

Samenvatting

189

Resumen

193

Acknowledgements

197

“All truths are easy to understand once they are discovered; the point is to discover them” - (Galileo Galilei, 1564-1642)

Chapter 1

Introduction

The current textile industry demands large amounts of water for dyeing cotton

with reactive dyes. At the end of the process, this water contains large quantities of

chemicals, salt and alkali, and becomes chemical waste, which is difficult to treat.

Therefore, there is a great need for a more ecological textile dyeing process. Supercritical

carbon dioxide has been proposed to replace water for textile dyeing; consequently the

water consumption and waste production can be potentially eliminated.

In this introductory chapter a description of the conventional dyeing process for

cotton, and a comparison with the supercritical dyeing process are provided. The

advantages and drawbacks of cotton dyeing in scCO

are also described.

The aim and outline of this thesis are given at the end of the Chapter.

1.1

Conventional dyeing process for cotton

Nowadays, in the textile industry over 50 % of the dyeing of cotton in water is carried out with reactive dyes [1]. The excellent wet fastness, due to the fixation of the dye via covalent bonding with the substrate, and the brilliance and variety of hues, are responsible for the extended used of reactive dyes to dye cotton [2].

However, reactive dyeing of cotton in water suffers from severe economical en enviromental drawbacks. The process consumes nearly 1000 million tons of water per year and demands considerably large quantities of salt and alkali. Electrolyte is required for overcoming the anionic dye-fibre repulsion, so adsorption can take place. Alkali is needed for the fixation because cellulosic anions are generated, which easily react with the reactive dye. During the dyeing process up to 40% of the dye is hydrolysed. This hydrolysed dye is highly substantive for the cotton, which needs to be removed via a wash-off step in order to achieve the characteristc wet fastness of reactive dyes. The wash-off step is a laborious operation, which is often longer than the dyeing step itself and requires large volumes of water.

At the end of the dyeing process high levels of polluted water are produced. The effluent contains up to 0.1 million tons of hydrolysed dye (when a dye concentration of 2 % on weight of the fiber is applied), 16 million tons of salt and 5 million tons of alkali per year [2, 3]. The treatment of the dyeing effluents is complicated and constitutes a major economic and environmental issue. As much as 50% of the total cost of the process is attributed to the washing-off stages and waste-water treatment of the effluent [4, 5].

The water-based process of dyeing cotton with reactive dyes is an expensive, time-consuming process and a commination for the environment. The development of a more ecological dyeing process for cotton without compromising the desirable attributes of the reactive dyes is greatly demanded.

1.2

Supercritical dyeing: An alternative dyeing process

alternative process to eliminate the water usage and water pollution [6]. From an environmental and safety point of view, supercritical carbon dioxide is the best solvent to replace water in textile dyeing. It is inexpensive, non-toxic, non-flammable, environmentally friendly and is chemically inert under many conditions. Simple flow schemes of the alternative superctitical dyeing and the current process are shown in Figure 1.1.

���������������������������� �������������������� ������ ���� ������� ����� ����� ���������� ��������� ���� ������ �������������� ���� ���� ������ �������������� ���� ������� ��� ��������� �� ������ ��� �� � ������

Figure 1.1 Flow schemes of dyeing steps involved in the water dyeing

process and in supercritical dyeing process of cotton.

1.2.1 Advantages of supercritical CO₂ for textile dyeing

Textile industry can greatly benefit from the use of supercritical CO₂ as dye solvent. The high diffusion rates and low mass transfer resistance observed in scCO₂ compared to water; facilitate the dye penetration into the fibres,

by simply lowering the pressure. Carbon dioxide and dye can be then reused, making the process economically feasible and environmentally attractive. A comparison of the conventional water dyeing and the supercritical dyeing process is given in Table 1.1 [7, 8].

Table 1.1 Comparison of textile dyeing processes

Conventional water textile dyeing

Alternative textile dyeing with scCO2

• Large usage of water • Elimination of water usage • High levels of salt and alkali • No additives

• Hydrolysis of dye molecules • No hydrolysis of dye molecules

• Costly water purification • No production of polluted water

• Drying step of textile • No drying step (energy saving) • Shorter process time due to

high diffusion coefficients and low mass transfer resistance • Easy separation of the dye from

scCO₂ with dye recovery • Carbon dioxide can be reused

1.2.2 Drawbacks on dyeing cotton with supercritical CO₂

Supercritical carbon dioxide for dyeing synthetic fibres has been shown excellent results on a laboratorium scale [9- 11]. Nevertheless, very poor colour depth and colour fastness on dyed natural fibres, especially cotton, were observed when using scCO₂. Considering that 35% of the world market share is represented by cotton [12], the development of a method for dyeing cotton is essential. The possibility of dyeing all kinds of textiles in scCO₂ will make the process economicably feasible. To understand the limitations of the dyeing process with cotton, it is necessary to examine the structure of cotton, the properties of CO₂ as a dyeing medium and the dye itself.

1.2.2.1 Cotton structure

The cotton consists of an assembly of cellulose chains connected via inter-chain hydrogen bonding. The chemical structure of the cellulose can be described as a condensation polymer of β-D-glucopyranose with 1,4-glycosidic bonds. The intermediate units possess one primary and two secondary alcohols groups each, which are capable for reaction. However, it is generally accepted that the primary alcohol group is more reactive than the secondary groups. Figure 1.2 illustrates the cellulose structure.

� �� � � � � �� � �� � � � �� � �� � � � �� � � �� � � �� ��� Figure 1.2 Cellulose structure

The dyeing properties of the cotton are determined by its structure, which contains crystalline as well as amorphous domains. The dye can only access the reactive site of the cotton in the amorphous region, which represents only some 30% of the whole cotton structure. The dyeability of the cotton can be improved, however, by swelling the fibres. The cotton structure is, then, partially disrupted, increasing the accessibility of the reactive sites of the cotton [5].

1.2.2.2 Carbon dioxide as dyeing medium for cotton

The major limitation of carbon dioxide as dye medium is its inability to swell the cotton [13]. Moreover, scCO₂ can extract the natural moisture present in the cotton fibers. Hence, the cotton loses its flexibility and its glass transition temperature (T_g) increases up to 220°C [14], which is a prohibited operating temperature because it causes cotton damage.

polar disperse dyes are soluble in scCO₂, but they have very low affinity for cotton.

To overcome these limitations disperse dyes were modified by attaching a reactive group, which can react with cotton [15]. Reactive disperse dyes has been developed, up to now, only for investigation purposes. An example of a reactive dye is given in Figure 1.3.

� � �� � � � �� �� Figure 1.3 Structure of a reactive disperse dye

1.3

Aim of the thesis

The aim of this thesis was to develop a method for dyeing cotton in supercritical carbon dioxide with non-polar reactive dyes. This included an extended study of reaction mechanisms and process optimisation.

1.4

Outline of this thesis

The processes of dyeing cotton in supercritical carbon dioxide and in water are basically similar, as they involve the following steps: dissolution, transport, adsorption, diffusion and reaction of the dye with the cotton. Yet, the dyeing mechamisms of both processes will be radically different, because the chemical and physical properties of water and CO₂ are radically different. Consequently, this requires an innovative approach towards the development of a dyeing process of cotton in scCO₂. This thesis is about the progress made and the breakthroughs, which have been accomplished, in developing said process.

occurr in scCO₂. In Chapter 2 the focus is on the effect of supercritical carbon dioxide on the reactivity of a reactive disperse dye with dichlorotriazine as reactive group. The reaction of the dye and the cotton is quite complex due to the heterogenity of the dyeing medium. Therefore, methanol was used as model for cotton, which enables the study of the reaction isolated from other dyeing factors. From the kinetic study a positive influence of scCO₂ on the reactivity of the dye was observed and more suitable dye structures were indicated.

The chemical reaction of the dye and methanol was extremely accelerated when the methanol was dried over zeolite KA. In Chapter 3, it is shown that besides its excellent drying properties zeolite KA, can also induce base promoted organic reactions, especially when protic solvents or reactants are involved.

Chapter 4 extends the kinetic studies to reactive disperse dyes with a monochlorotriazine reactive group. The relation between dye structure and reaction medium was determined, which was essential for establishing the reaction mechanism of triazinyl compounds with methanol in scCO₂.

Though reaction is possible, the cotton was poorly dyed. Dyeing steps as dye transport and diffusion were examinated and improved in Chapter 5. A novel process to enhance the dyeability of cotton was developed, in which chemical modification of the cotton is not required. Presoaking in methanol and addition of extra cosolvent facilitated the transport of the dye and its diffusion, as reflected in the greatly improved on the coloration of the cotton dyed in scCO₂.

An important breakthrough in cotton dyeing in scCO₂ was achieved and is described in Chapter 6. A series of newly developed fluorotriazine reactive dyes were synthetised and they were successfully applied to dye cotton. The excellent reactivity of fluorotriazine dyes in combination with our own pretreatment method improved the cotton coloration and the dye fixation, approaching these values required for commercialization.

water-free dyeing process for textiles.

Chapter 8 deals with the mechanistic description of the nucleophilic reaction of dichlorotriazine and difluorotriazine dyes with methanol using molecular modelling. The combination of experimental data and molecular modelling serves as a powerful tool to elucidate reaction mechanisms and unravel solvent effects. The results obtained are very useful for the further optimisation of the dyeing process of cotton in scCO₂.

For a future industrial process of cotton dyeing in scCO₂, it is very important that the process is gradually scaled-up. In Chapter 9 the scale-up of the dyeing process to a medium scale is presented.

1.5

References

[1] D.A.S. Phillips, Environmentally friendly, productive and reliable: priori-ties for cotton dyes and dyeing processes J. Soc. Dyers Colour., 112 (1996) 183.

[2] A. Hunter, M. Renfrew, Reactive dyes for textile fibres, Soc. Dyers Col-our., 1999, p.168.

[3] R.S. Blackburn and S.M. Burkinshaw, A greener approach to cotton dye-ings with excellent wash fastness, Green Chem., 4 (2002) 47.

[4] C. Allegre, M. Maisseu, F. Charbit and P. Moulin, Coagulation-floccula-tion-decantation of dye house effluents: concentrated effluents, J. Haz-ard. Mater., B116 (2004) 57.

[5] J. Shore, Cellulosic Dyeing, Soc. Dyers Colour., 1995, p. 191.

[6] D. Knittel and E. Schollmeyer, Environmentally friendly dyeing of syn-thetic fibres and textile accessories, Int. J. Clothing Sci. Technol., 7 (1995) 36.

[7] W.A. Hendrix, Progress in supercritical CO₂ Dyeing, J. Ind. Text., 31 (2001) 43.

[8] E. Bach, E. Cleve and E. Schollmeyer. Past, present and future of super-critical dyeing technology - an overview, Rev. Prog. Color, 32 (2002) 88.

[10] E. Bach, E. Cleve and E. Schollmeyer, Dyeing of poly(ethylene terephtha-late) fibers in supercritical carbon dioxide, in: High Pressure Chemical Engineering, Eds.: Ph. Rudolf von Rohr and Ch. Trepp. Elsevier Science, 1996. p.581.

[11] M. van der Kraan, M.V. Fernandez Cid, W.J.T. Veugelers, G.F. Woerlee and G.J. Witkamp, Dyeing natural and synthetic textiles in supercritical carbon dioxide with reactive dyes, Proceedings of the 4th International Symposium on High Pressure Technology and Chemical Engineering, Venice, Vol. 1 (2002) 199.

[12] T. Johnson, Chem. Fibers Intern., 49 (1999) 455.

[13] Saus, W., Hoger, S., Knittel, D., and Schollmeyer, E., Fäber aus über-kritischem Kohlendioxid: Dispersionsfarbstoffe und Baumwollgewebe, Textilveredlung, 28 (1993) 38-40.

[14] Beltrame, P.L., Castelli, A., Selli, E., Mossa, A., Testa, G., Bonfatti, A.M., and Seves, A., Dyeing of Cotton in Supercritical Carbon Dioxide. Dyes Pigm., 39 (1998) 335.

[15] A. Schmidt, E. Bach and E. Schollmeyer, The dyeing of natural fibres with reactive disperse dyes in supercritical carbon dioxide. Dyes Pigm., 56 (2003) 27.

Chapter 2

Kinetics Study of a Dichlorotriazine

Reactive Dye in Supercritical Carbon

Dioxide

The kinetics of the reaction between a dichlorotriazine reactive dye and methanol

was studied in supercritical carbon dioxide and in a solution of pure methanol. The

experiments were carried out in a batch reactor at different temperatures between

333 and 393 K and at 300 bar when supercritical carbon dioxide was the solvent

medium. The rate constants and the parameters of the Arrhenius equation were

determined and compared. A significant influence of the supercritical carbon dioxide

on the rate constants was found. The formation of methoxy-dye was faster over the

whole temperature range in pure methanol than in supercritical carbon dioxide. But

the consecutive reaction, the formation of dimethoxy-dye, was considerably faster,

up to 20 times, in supercritical carbon dioxide than in pure methanol. Moreover,

the lowest value for the activation energy, 50±13 kJmol

_{, was found for the}

consecutive reaction in supercritical carbon dioxide. In this paper new kinetic data

for the methanolysis reaction of a dichlorotriazine reactive dye in supercritical carbon

dioxide are shown.

2.1

Introduction

The conventional aqueous (batchwise) textile dyeing process uses a large amount of water that has to be treated before discharge. The wastewater treatment increases the cost of the dyeing process and it is expected to rise in the near future because of more stringent legislation. In order to reduce or even eliminate this disadvantage, research has been carried out to develop a dyeing process where supercritical carbon dioxide is proposed as dye solvent instead of water [1-3].

Supercritical carbon dioxide is the most attractive alternative fluid for environmental, economical and chemical reasons. Its high diffusion rates and low mass transfer resistance facilitates the penetration of the dye into the fibre. Besides, the rates of the reactions increase over those observed in an aqueous medium [4]. A major advantage of dyeing with supercritical carbon dioxide is the absence of water, which competes with the textile to react with the dye. Therefore, it might be expected that the use of supercritical carbon dioxide for dyeing textiles might provide a faster process than the conventional dyeing under aqueous conditions.

Dyeing of synthetic fibres with supercritical carbon dioxide has been successfully achieved at laboratory scale; however this is currently not possible for cotton with reactive dyes [5].

A non-polar reactive dye was used in this work, because it can be dissolved in supercritical carbon dioxide, and can react with the reactive sites of the textiles forming a covalent bond.

Moreover, reactive dyes with two reactive sites, as our non-polar reactive dye, tend to exhibit high colour yields, whilst offering high wash-fastness properties. This was observed for water dyeing. However, Schmidt et al. [5] observed low wash-fastness on cotton dyed in supercritical carbon dioxide using a non-polar dye with the same reactive group as in our studies.

The reaction kinetics of the reactive dyes has been studied by different authors [6-9] as an essential step for the development of the dyeing process of natural fibres, especially cotton. In order to study the kinetics of the fixation process separately from the influence of other dyeing factors, as affinity or diffusivity. In our studies the heterogeneous system dye-textile was

that methanol is a satisfactory model for cotton [11]. The kinetics of the methanolysis reaction of a reactive dye in supercritical carbon dioxide as a solvent medium has not been extensively studied yet [12, 13].

In this work, to estimate the potential for fixing reactive dyes to cotton in supercritical carbon dioxide, the kinetics of the homogeneous reaction between a dichlorotriazine reactive dye and methanol was studied in a solution of pure methanol and in supercritical carbon dioxide.

2.2

Materials and Methods

2.2.1 Materials

A non-polar dichlorotriazine reactive dye, with the chromophore and reactive group shown in Figure 2.1, was synthesized by a well-known manufacturer of reactive dyes. The carbon dioxide from Hoek Loos exhibited a purity of 99.97%. The methanol and acetonitrile used were HPLC grade from Acros and Rathburn respectively. � �_�� � ���_� � �_� ����������� � � � �� �� ����������� �������������� Figure 2.1 Structure of the chromophore and reactive group of the

non-polar reactive dye used in this work. The two components are connected by a NH group resulting from the NH₂ group of the chromo-phore.

acetonitrile and 15% water as mobile phase at a flow rate of 1 ml/min. Samples of 50 µl were injected with a Marathon autosampler and the chromatographic column was maintained at 295 K. The dye samples were detected at their maximum absorption wavelength (513 nm) with a Varian ProStart 310UV/VIS Detector.

2.2.3 Set-up and Procedure

A high-pressure batch reactor was designed to carry out experiments in supercritical conditions. The reactor consists of a 150 ml pressure vessel connected to a pressure manometer and a needle valve, and it is capable of operating up to 350 bar.

A suspension of 0.001 g of the dye in 50 ml of CH₃OH was made up and placed in an ultrasonic bath for 5 min to complete the dissolution of the dye. For reactions in supercritical carbon dioxide the procedure was as follows. 1 ml of the dye solution was poured into the reactor, then, a known amount of liquid carbon dioxide at 60 bar and 295 K was added to the reactor via the valve. The reactor was weighed before and after being filled to assure the right amount of CO₂ for each temperature to reach 300 bar. The molar ratio CO₂: CH₃OH: Dye is 0.98: 0.01: 3.02 x 10-8_{. For the methanolysis reaction}

in pure methanol, 25 ml of the dye solution described above were added to the reactor. In both reaction procedures the reactor was placed then in a thermostatic oil bath set at temperatures of 333, 353, 373 and 393 K. After a sufficient reaction time, the reaction was stopped by cooling and a sample was very carefully taken from the reaction mixture by opening the valve and was immediately analysed by HPLC.

2.3

Results and Discussion

Dichlorotriazine reactive dyes react with nucleophiles, like methanol, by a nucleophilic substitution mechanism [14]. A consecutive reaction may occur where the second reactive site is substituted.

When the solvent is one of the reactants and is present in large excess, the reaction may be considered as a pseudo-first order reaction [14]. In our

chosen to conform to such reaction kinetics.

At the pressure and temperature conditions of our experiments a homogeneous phase for the system CO₂/MeOH is present [15]. Unfortunately, no solubility data for our non-polar reactive dye are available. However, it has been found [16] for non-polar dyes with similar structure that the solubility in CO₂ in mol fraction is 10-5_{. The dye concentration in our experiments is 10}-8 _{in mol}

fraction, therefore a complete dissolution in carbon dioxide is assumed. Furthermore, no precipitation or crystallization of the dye in the reactor has been found during our experiments.

The reaction, assumed irreversible, can be schematically written as follows:

� � � �� �� ����� � � � ���_� �� ����� � � �����

�

�

where, chrom= 4-NO₂C₆H₄N₂C₆H₃N(C₂H₅)₂

NH-Scheme 2.1 Model reaction of dichlorotriazine dye with methanol.

Where k₁ and k₂ are the pseudo-first order rate constants for the formation of methoxy-dye [RCl-OCH₃] and dimethoxy-dye [R-(OCH₃)₂], respectively.

An example of a HPLC chromatogram is shown in Figure 2.2 for 1h reaction of the dye with methanol in supercritical carbon dioxide at 393K and 300bar.

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Figure 2.2 HPLC Chromatogram of the non-polar dichlorotriazine

reac-tive dye after 1h reaction with methanol in supercritical carbon dioxide at 393K and 300bar.

The decrease in the concentration of dye can be expressed by the equation:

� � � �

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(1)

The peak areas of the components calculated from the chromatograms are linearly related to their amounts by the Lambert-Beer law. The dye samples were measured at their wavelength of maximum absorption; therefore, the concentration of the dye and its products can be replaced by their relative peak areas in equation (1). The relative peak area is defined such that the total peak area in a given chromatogram equals one. The differential equation (1) is solved and equation (2) is obtained.

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When ln([RCl₂]₀/[RCl₂]_t) was plotted against t, a straight line was obtained

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Figure 2.3 Plot of ln([RCl₂]₀/[RCl₂]_t) against time at different temperatures in pure methanol.

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In order to calculate the rate constants k₁ and k₂, a model to predict the concentrations of the three forms of the dye was used. The model is represented by equation (2) and the two following equations (3) and (4) [17].

�� � � ���� � � � ����� � � � ��� � � � � � ���� � ��� ��� � � ��� ��� � � � � � � � � � � � � � � � � � � � � � � (3) � � � � � � � � �� � ���� � ���� ��� � ���� � ��� � � � � � � � �

₍₄₎

In equation (3) the term, [RCl-OCH₃]₀ exp(-k₂t), can be omitted since no reaction

has taken place at time 0 minutes.

The best values for the rate constants, k₁ and k₂, were obtained using the least square method where the differences between the experimental and model data were minimised.

The Figures 2.5 and 2.6 show that the experimental data are well fitted by the model for the methanolysis reaction in pure methanol at 393 K and in supercritical carbon dioxide at 393 K and 300 bar. These examples are representative for all experiments.

� ��� ��� ��� ��� � � �� ��� ��� ��� ��� ��� ���������� ������������������� ���� �������� ��������� �����

� ��� ��� ��� ��� � � �� �� �� �� ��� ��� ��� ���������� ������������������ ���� �������� ��������� �����

Figure 2.6 Comparison of the experimental results with the model in

supercritical carbon dioxide at 393 K and 300 bar.

The fitted values for the rate constants k₁ and k₂ are listed in Table 2.1.

Table 2.1 Values of the rate constants k₁ and k₂ for the consumption of dye at different temperatures in pure methanol (pure MeOH) and supercritical carbon dioxide (scCO₂).

105_{x k} 1 (s -1) 105x k2 (s -1) T(K) pure MeOH scCO2 pure MeOH scCO2 333 1.8 0.3 0.02 0.4 353 11 1.9 0.1 0.4 373 31 3.3 0.6 2.0

As expected, both rate constants increase with the temperature independently of the solvent medium.

Using Arrhenius’ law follows,

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Figure 2.7 Arrhenius plot for the rate constant k₁ for the reaction of the dye and methanol in pure methanol and supercritical carbon dioxide.

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Figure 2.8 Arrhenius plot for the rate constant k2 in pure methanol

and in supercritical carbon dioxide.

The activation energies E and the pre-exponential factors k₀ were calculated. Their values are given in Table 2.2 for the reactions in pure methanol and in supercritical carbon dioxide.

Table 2.2 Calculated activation energy (E) and pre-exponential

fac-tor (k0) for k1 and k2 in pure methanol (pure MeOH) and supercritical

carbon dioxide (scCO2), over the range of temperature (333K – 393K)

together with the standard deviations.

pure MeOH Sc-CO₂

E (kJ mol-1_{) k}

0 (s-1) E (kJ mol-1) k0 (s-1) k₁ 60 ± 10 532x102 _{± 27 61 ± 7 163x10}2 _{± 1600}

s-1 _{for the rate constant of a monochlorotriazine dye at 353 K and at a pH of}

approximately 11. A larger value for the rate constant of our dichlorotriazine dye was expected because of the two reactive sites of the dye. However a value of 11x10-5 _s-1 _{in pure methanol was obtained at the same temperature of}

353 K. This decrease in the rate constant is probably related to the higher pH value, 11, in the Klan_{čnik’s research. At this high pH the nucleophilic attack} is favoured because the methanol is partly ionized to form the methoxide ion, which reacts more rapidly with the dye than the unionized methanol at the lower pH of our experiments in pure methanol

The value of k₁ in supercritical carbon dioxide was 1.9x10-5 _s-1 _{at 353 K. It was}

a factor 50 lower than that from Klan_{čnik’s studies. In supercritical carbon} dioxide it is difficult to refer to pH. The only reference of pH measurements is discussed by Toews et al. [18], they measured the pH of water saturated with CO₂, and the pH was approximately 3 due to the formation of carbonic acid. Even if our experimental conditions are different from those of Toews, it is assumed that formation of methoxide ion does not occur, because the pH is closer to the values of our reactions in pure methanol than pH 11 from Klan_{čnik’s studies. Therefore, the lower k1} values can be also explained by unfavoured nucleophilic attack, due to the absence of methoxide ion.

The reaction mechanism is, consequently different from that at higher pH. In our study the methanolysis reaction occurs via protonation of the triazine ring of the reactive group. In the course of the reaction HCl is formed, which provides protons to protonate the triazine ring. Protonation in acid catalysed reactions in supercritical carbon dioxide has been previously reported by Gray et al. [19]. In both solvents pure methanol and supercritical carbon dioxide, HCl is formed. However, the values of the rate constant are clearly different depending of the solvent medium. This indicates the important role of the solvent in the reaction of the dye with methanol.

Independently of the solvent medium, k₁ was always larger than k₂, except at 333K where in supercritical carbon dioxide, k₁ and k₂ were almost equal. When the same rate constant is compared considering the solvent medium, pure methanol or supercritical carbon dioxide, a different effect is observed. The rate constant k₁ is approximately up to a factor 10 larger in pure methanol than in supercritical carbon dioxide. However, there is no significance

pure methanol the activation energy for k₁ has a value of 60±10 kJmol-1 _and

in supercritical carbon dioxide 61±7 kJmol-1_.

The larger k₁ values in pure methanol are related to the solvent phase, where the reaction took place, and to the density of the liquid phase. In supercritical carbon dioxide, the reaction occurs in a homogeneous phase, which is less dense than the liquid phase.

In the case of the rate constant k₂ a remarkable behaviour is observed. Contrary to k₁, the values of k₂ were always larger, by a factor of 3 to 20, when the reaction was carried out in supercritical carbon dioxide. This effect was observed for the whole temperature range. In pure methanol the activation energy for k₂ was 74±11 kJmol-1 _{while in supercritical carbon dioxide a value}

of 50±13 kJmol-1 _{was found. This low value of the activation energy of k}

2 in

supercritical carbon dioxide indicates that supercritical carbon dioxide as a solvent medium favoured the attack of the methanol on the first reaction product RCl-OCH₃ to form the second reaction product R-(OCH₃)_2.

These large k₂ values in supercritical carbon dioxide are explained by protonation of the triazine ring of the reactive group of the dye. This is related to the solvation of the product molecule HCl. In both solvents the HCl is solvated by molecules of methanol. In pure methanol, however, the number of coordinating molecules around HCl is higher than in supercritical carbon dioxide making the protonation more difficult. In supercritical carbon dioxide, methanol is diluted in carbon dioxide so fewer molecules of methanol are present in the reaction medium. Also, Pfund et al [20] observed relatively strong interactions between carbon dioxide and methanol molecules. Consequently, HCl is not solvated to the same extent as in pure methanol, promoting the protonation of the triazine ring of the dye.

The large k₂ value observed in supercritical carbon dioxide compared with pure methanol, can provide important information about which could be the most suitable reactive dyes for supercritical dyeing. From these data it seems that reactive dyes with a structure similar to the first reaction product (RCl-OCH₃) might react faster and more easily with cotton in supercritical carbon dioxide.

2.4

Conclusions

Our non-polar dichlorotriazine reactive dye reacts with methanol under pseudo-first order kinetics in pure methanol and in supercritical carbon dioxide.

As expected, the rate constants increase with the temperature independently of the solvent medium. The activation energies in pure methanol for the first and second nucleophilic substitution of chloride for methoxy were 60±10 kJmol-1 _{and 74±11 kJmol}-1 _{respectively. In supercritical carbon dioxide these}

values were 61±7 kJmol-1 _{and 50±13 kJmol}-1_.

Supercritical carbon dioxide as a solvent medium had a significant influence on the reactivity of the methanolysis reaction of the dichlorotriazine reactive dye. The formation of the methoxy-dye (the first reaction product) was over the whole temperature range 5 to 10 times faster in pure methanol than in supercritical carbon dioxide. However, the consecutive reaction, the formation of dimethoxy-dye (the second reaction product) was faster, 3 to 20 times, in supercritical carbon dioxide than in pure methanol. Solvation of HCl and protonation mechanism can explain these large k₂ values.

These results indicate the most suitable reactive group for non-polar reactive dyes for reacting with methanol, and probably cotton, in supercritical carbon dioxide.

2.5

Acknowledgments

Jill Djodikromo and Laura Nájar are acknowledged for their contribution to the experimental work of this chapter.

2.6

Reference

[1] W. Saus, D. Knittel and E. Schollmeyer, Dyeing of textiles in supercritical carbon dioxide, Text. Res. J., 63 (1993) 135.

39 (2000) 4806.

[3] E. Bach, E. Cleve and E. Schollmeyer, Past, present and future of super-critical dyeing technology - an overview, Rev. Prog. Color, 32 (2002) 88. [4] P.G. Jessop, W. Leitner, Chemical Synthesis Using Supercritical Fluids,

Willey-VCH, 1999.

[5] A. Schimdt, E. Bach and E. Schollmeyer, The dyeing of natural fibres with reactive disperse dyes in supercritical carbon dioxide, Dyes Pigm., 56 (2003) 27.

[6] L. Xiao-Tu, Z. Zheng-Hua and C. Kong-Chang, The kinetics of the hydrol-ysis and alcoholhydrol-ysis of some model monofluorotriazinyl reactive dyes, Dyes Pigm., 11 (1989) 123.

[7] A.H.M. Renfrey and J.A. Taylor, Reactive dyes for cellulose. Concurrent methoxide-hydroxide reactions of triazinyl reactive systems: a model system for assessment of potential fixation efficiency, JSDC, 105 (1989) 441.

[8] M. Klan_{čnik, The influence of temperature on the kinetics of concurrent} hydrolysis and methanolysis reactions of a monochlorotriazine reactive dye, Dyes Pigm., 46 (2000) 9.

[9] M. Klančnik and M. Gorenšek, Kinetics of hydrolysis of monofunctional and bifunctional monochloro-s-triazine reactive dye, Dyes Pigm., 33 (1997) 337.

[10] Z. Zheng-Hua, C. Kongchang and Y.Ronggeng, Study of competitive alcoholysis and hydrolysis of vinylsulfonyl reactive dyes, Dyes Pigm., 14 (1990) 129.

[11] T.W. Bentley, J. Ratcliff, A.H.M. Renfrew and J.A. Taylor, Homogeneous models for the chemical selectivity of reactive dyes on cotton-develop-ment of procedures and choice of model, JSDC, 11 (1995) 288. [12] M.V. Fernandez Cid, M. van der Kraan, W.J.T. Veugelers, G.F. Woerlee

and G.J. Witkamp, Kinetics study of reactive dyes in supercritical carbon dioxide, Proceedings of the 4th _{International Symposium on High}

Pres-(2003) 1167.

[14] A. Johnson, The Theory of Coloration of Textiles, Society of Dyers and Colourists, 1995.

[15] J. Liu; Z. Qin, G. Wang, X. Hou and J. Wang, Critical properties of binary and ternary mixtures of hexane + methanol, hexane + carbon dioxide, methanol + carbon Dioxide, and hexane + carbon Dioxide + methanol, J. Chem. Eng. Data, 48 (2003) 1610.

[16] T. Shinoda and K. Tamura, Solubilities of C.I. disperse red 1 and C.I. disperse red 13 in supercritical carbon dioxide, Fluid Phase Equilib., 213 (2003) 115.

[17] J.E. House, Principles of Chemical Kinetics, WCB, 1997.

[18] K.L. Toews, R.M. Shroll, C.M. Wai, N.G. Smart, pH-defining equilibrium between water and supercritical CO₂. Influence on SFE of organics and metal chelates, Anal. Chem., 67 (1995) 4040.

[19] W.K. Gray, F.R. Smail, M.G. Hitzler, S.K. Ross, M. Poliakoff, The continu-ous acid-catalyzed dehydration of alcohols in supercritical fluids: a new approach to the cleaner synthesis of acetals, ketals, and ethers with high selectivity, J. Am. Chem. Soc., 121 (1999) 10711.

[20] D.M. Pfund, J.L. Fulton, R.D. Smith, Aggregation of methanol in super-critical carbon dioxide: A molecular dynamics simulation in: E. Kiran, J.F. Brennecke (Eds.), Supercritical Fluid Engineering Science: Fundamentals and Applications, American Chemical Society, 1993, p. 158

Chapter 3

Base promoted etherification

reac-tion of a non-polar reactive dye and

methanol by zeolite KA

An unexpected base promoted etherification reaction of a non-polar

4,6-dichloro-1,3,5-triazine type reactive dye and methanol was observed when methanol was

dried over zeolite KA. A dye conversion of 100% was measured after 3 min reaction

time at room temperature while, when methanol was not pre-contacted with zeolite

KA, the dye conversion was only 31% after 24 h. The rate constants of the two-step

consecutive reaction between dye and methanol were calculated in order to determine

the accelerating effect. At 293 K, k

increased a factor of 3 000 and k

a factor

of 26 000 compared to these values observed with undried methanol. It has been

found that the ion-exchange properties of the zeolite KA are responsible for the base

promoted reaction. Elemental analyses of the zeolite KA, pellets and powder, and

methanol extracts showed an exchange of cations between the two phases. In the case

of pellets the binder material contributes to the cation content of the methanol. The

potential of zeolite KA as a base promoter is discussed.

3.1

Introduction

Non-polar reactive dyes have recently been developed for application in a water free dyeing process where supercritical carbon dioxide is the dye solvent [1]. The major advantages of this new process are the elimination of waste water and of a drying step. Supercritical carbon dioxide is the most attractive alternative fluid for environmental, economical and chemical reasons; moreover a faster dyeing process may be achieved due to the high diffusion rates in carbon dioxide and its low viscosity [2].

The dye applied in the present work was N-(4,6-dichloro-1,3,5-triazin-2-yl)-N-{5-(diethylamino)-2-[(Z)-(4-nitrophenyl)diazenyl]phenyl}amine, (C₁₉H₁₈N₈O₂Cl₂), with a dichlorotriazinyl ring as the reactive group. The chromophore and the reactive group of the dye are shown in Figure 3.1.

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Figure 3.1 Chromophore and reactive group of the dye used in this

study, connection via amino function.

Non-polar reactive dyes of this type can be dissolved in carbon dioxide and they react with the reactive sites of the textiles: the OH groups of cotton, the NH₂ and OH groups of wool, forming a covalent bond. Consequently, they are expected to exhibit high colour yields, whilst offering high wash-fastness properties.

A detailed study of the fixation reaction of the dye with the textile is an essential step to develop a carbon dioxide based dyeing process of natural fibres, especially cotton [3, 4]. Methanol is used as the simplest model for cotton [5], providing a homogeneous reaction medium, which facilitates the study of the fixation. In our previous work [6] the reactivity of the dye with methanol in supercritical carbon dioxide has been studied.

65% and 293 K, will contain some 7.5 wt% absorbed water [7]. When treated with supercritical carbon dioxide cotton will lose part of its water; drying occurs [8]. Both the residual cotton-absorbed water and the water dissolved in the carbon dioxide can react with the dye in a non-productive hydrolysis reaction.

In view of the role of water in the dyeing process, it was decided to apply in the reaction of the dye with methanol, non-dried methanol as well as zeolite KA-dried methanol. To our surprise, a dramatic increase in the reactivity of the dye/methanol reaction was observed upon treating the methanol with zeolite KA.

Drying of liquids and gases constitutes one of the early and well-established uses of zeolites. Especially the hydrophilic zeolite A (NaA, KA) is used for this purpose. Alternatively, the more acid-stable small-pore zeolite chabazite can be used. At low water pressures or concentrations zeolites are superior to conventional sorbents like silica. The high affinity for water is coupled with a fairly high saturation capacity (25 wt%).

When drying organic liquids, size and nature of the organic compound have of course a bearing on the drying efficiency; the more hydrophobic the compound, the deeper the drying [9, 10]. For drying of small molecules, zeolite KA is the desiccant of choice. KA is also applied industrially for drying of bioethanol [11].

Though the drying properties of zeolite A seem well established there are several reports in literature where the effects of adding zeolite NaA or KA are not understood. This is especially the case when alcohols are applied as solvent or reactant [12-14]. Therefore it was decided to study the accelerating effect of “drying” methanol with zeolite KA in our model reaction in more detail.

3.2

Experimental

3.2.1 Materials

The non polar reactive dye, used in this work (Figure 3.1), was supplied by a well-known dye manufacturer. The methanol and acetonitrile used were HPLC grade from Acros and Rathburn, respectively. Zeolite KA pellets, and powder, as well as the potassium methoxide (95%) were supplied by Sigma-Aldrich.

3.2.2 Analysis

3.2.2.1 HPLC analysis

A Chrompack liquid chromatograph was used for the chromatographic analyses. The column was an Inertsil 5 ODS-2 (250×4.6 mm) and the mobile phase consisted of a 85:15 (v/v) mixture of acetonitrile and ultra-pure water applied at a flow of 1 ml/min. Samples of 50 μl of the reaction mixture were injected with a Marathon autosampler while the chromatographic column was maintained at 295 K. A Varian ProStart 310 UV/VIS detector was used to detect the dye and its methoxy derivatives at their wavelength of maximum absorbance (513 nm).

3.2.2.2 pH analysis

Methanol samples were diluted 1:1 (v/v) with ultra-pure water to measure the pH with a combined glass electrode-reference.

3.2.2.3 ICP-HRMS / ICP-AES analysis

Inductive Coupled Plasma, High Resolution Mass Spectrometry (ICP-HRMS), type Element 1, from Finnigan, was used in the first screenings for detecting trace elements (range of ng/kg and pg/kg) in the methanol extracts. Inductive Coupled Plasma, Atomic Emission Spectroscopy (ICP-AES), type Spectroflame, from Spectro, was used to analyze sub-traces elements with a detection limit

For both techniques, the sample preparation was as follows. A solution of 10 wt% of the methanol extracts in 3% HNO₃ was made. As internal standards, 10 μg/kg of In was added for ICP-HRMS analyses and 10 mg/kg of Sc for ICP-AES analyses. Standards from untreated methanol were prepared as the methanol extracts.

3.2.2.4 XRF-analysis

A Philips X-ray spectrometer, type PW2400 with UniQuant-5 software was used for determining the elemental composition of the zeolite KA in pellet and powder form.

The zeolite pellets were ground till the particle size was lower than 40 μm. Then, a grounded zeolite sample of 0.5 g was mixed with 5 g of Li₂B₄O₇ and melted at 1473 K to obtain a glass pearl, which was analyzed in the X-ray spectrometer.

3.2.3 Methods

3.2.3.1 Etherification reaction procedure: zeolite KA as base promoter

The experiments were carried out in an Erlenmeyer flask at room temperature. Methanol was applied as received or treated with zeolite KA. In the latter case, 50 ml methanol was contacted with 15 wt% of zeolite KA pellets or powder and kept at room temperature overnight. The pH of the methanol solution treated with zeolite was 11, while the pH of methanol as received was 6. ICP-HRMS analyses were done to determine the elemental composition of the treated methanol. Before carrying out the etherification reaction, the methanol was separated from the zeolite KA by filtration.

A solution of 3x10-2 _{wt% of the dye was prepared in fresh methanol. The}

solution was kept at 280 K to avoid reaction between dye and methanol before being mixed with the methanol, treated with zeolite KA.

3.2.3.2 Reactions with binderless zeolite KA and potassium methoxide

To determine the base promoted mechanism of the methanolysis reaction of the dye, experiments with binderless zeolite KA (powder form) and potassium methoxide were performed. The experimental procedure for zeolite KA powder was the same as the one described above for zeolite KA pellets.

For the experiment with potassium methoxide, 0.035 g of it was dissolved in 50 ml methanol. The concentration of K+ _{present in this experiment was}

comparable to that measured by ICP-HRMS in a methanol extract of zeolite KA.

The etherification reaction, in both experiments, was done as described in section 2.3.1.

3.2.3.3 Reaction under acidic conditions

Zeolite KA pellets in a concentration of 15 wt% were suspended in an Erlenmeyer with methanol. H₂SO₄ was added to the methanol in the presence of zeolites till pH 2. After 48 h the pH was measured again and a value of 4 was obtained. This methanol solution was used to carry out the reaction with the dye in a solution, prepared as previously described, in a ratio of (4:1) (v/v), respectively. HPLC analyses of the reaction mixture were performed at suitable times to follow the reaction. It may be noted that sulphuric acid treatment might lead to some dissolution of the zeolite elements.

3.2.3.4 Ion-exchange capacity of zeolite KA

To study the service time of the zeolite KA as base promoter a continuous experiment was carried out.

For this experiment, 7 g of zeolite KA were placed in a glass column. Then, methanol was pumped through the zeolite bed using a HPLC pump at a flow rate of 0.5 ml/min and collected in a vessel. The first sample of methanol was taken immediately after passing the zeolite KA in the column. This sample was coded time 0. Subsequently, samples of 10 ml were taken every 2 h directly from the methanol stream. The 10 ml methanol samples were reacted with the dye solution in acetonitrile (3x10-2 _{wt% of the dye) in a ratio (4:1)}

determine the rate constants. All methanol samples were analysed by ICP-AES to relate the cation concentrations with the rate constant values from the methanolysis reaction. Moreover, pH measurements were done to monitor the change in pH of the methanol extracts.

3.3

Results and Discussion

The non-polar reactive dye reacts with methanol via a two-step nucleophilic aromatic substitution (Figure 3.2).

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NH-Figure 3.2 Reaction between the reactive dichlorotriazinyl group of the

dye and methanol.

When the non-polar reactive dye [R-Cl₂] reacted with dried methanol over zeolite KA a dramatic increase of the dye conversion was observed compared to the reaction in undried methanol. At room temperature a conversion of 100% was measured after 3 min with dried methanol, whereas with undried methanol even after 24 h a conversion of only 31% was observed. The dye conversion for undried and dried methanol is shown in Figure 3.3 (a) and (b), respectively.

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���

���

Figure 3.3 Dye conversion vs. time for the methanolysis at room

tem-perature with undried methanol (a) and methanol dried (b) over zeolite KA (■ pellets and

▲

In the experiments the solvent is one of the reactants and its concentration is very large compared to that of the dye. Therefore, a pseudo-first order reaction will occur [15] for the methanolysis of the dye. This kinetics was confirmed in our previous work [6].

The rate constants were experimentally estimated and then optimized by fitting a model to the experimental data. The model for first order consecutive reactions is represented by the following equations [16].

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The calculated rate constants, k₁ and k₂, are shown in Table 3.1. They are also compared with data from previous work [6] on the methanolysis reaction of the dye with undried methanol at 393 K.

Table 3.1 Rate constant values, k1 and k2, for the two step

methanoly-sis reaction of the dye with undried methanol and with methanol dried over zeolite KA pellets and powder.

Dried MeOH over

Zeolite KA Undried MeOH

Pellets room temp

Powder

room temp room Temp T=393 K [6]

k₁(s-1_{) >2.5 x 10}-2 _{2.5 x 10}-2 _{8.4 x 10}-6 _{4.7 x 10}-4

k₂(s-1_{) 3.4 x 10}-3 _{1.9 x 10}-4 _{1.3 x 10}-8 _{9.3 x 10}-6

The accelerating effect of zeolite KA on the methanolysis reaction is reflected by the rate constants. The determination of k₁ when zeolite KA pellets were used was not possible due to the fast reaction. Within 3 min the dye has fully reacted to the methoxy-derivatives. However, for zeolite KA powder, due to its slightly lower reactivity, a k₁ value of 2.5x10-2 _s-1 _{(see Table 3.1) was}

Since in these experiments the dye had never been in direct contact with zeolite KA, the base promoted mechanism of the methanolysis reaction must be related to the methanol. Besides the excellent drying properties of the zeolites KA and NaA, lately their potential capacity of exchanging cations with the environment has been reported [13, 14, 17]. In our model reaction, the cations present in the structure of zeolite KA can be exchanged with the protons of the methanol forming a cation-methoxide. This species is very reactive, and therefore, the nucleophilic substitution with the dye is extremely accelerated. Although this ion-exchange mechanism seems to be the most likely to occur, no information about the type of cations that are involved in the process, and to which extent they are exchanged, has been found. Therefore, in this study, elemental analyses were carried out using different techniques. The type and concentration of cations present in the structure of the zeolite KA pellets and powder were determined by XRF, and by ICP-HRMS and ICP-AES in the methanol extracts. The results of these elemental analyses are listed in Table 3.2.

Table 3.2 Elemental analysis of zeolites KA in pellet and powder form

by XRF (*) and of methanol extracts by ICP-HRMS (**).

Zeolite KA*

MeOH** _{dried over}

Zeolite KA MeOH**

Element

(mol/kg) Pellets Powder Pellets Powder undried

K 2.4 2.8 1.9×10-3 _0.3×10-3 ₀ Na 3.8 3.7 4.7×10-3 _0.5×10-3 ₀ Al 10.3 10.3 0.1×10-3 _0.7×10-3 ₀ Si 15.1 11.5 0 0 0 Element (mg/kg) Mg 25 800 0 11.0 0 0 Ca 9 500 1 180 80.0 0.19 0 Fe 8 550 253 13.6 0 0 Mn 182 0 0.3 0 0 Ti 1 090 0 0.5 0 0

and Na, were found in the zeolite KA pellets and powder. In the case of zeolite KA pellets additional cations were found to be present. This will be due to the presence of inorganic binder material in the pellets.

Si, Al, and K constitute the structure of the zeolite KA. This type of zeolite is made by partial ion exchange of zeolite NaA, which explains the presence of Na in the elemental analysis.

From these analyses, it is possible to calculate the molar concentration of methoxide. Assuming that K and Na are most likely to exchange with the protons of methanol, the sum of both was taken into account for the calculations. As a result, in our experiments the ratio mol dye per mol methoxide was (1:37) (mol/mol).

Consider the elemental analyses of the undried and dried methanol over zeolites KA in pellet and powder form (see Table 3.2). It was confirmed that none of these cations were present in the methanol before being in contact with the zeolites, so a migration of cations from the zeolite to the methanol must have taken place.

However, it is possible that the binder material used to produce the zeolite pellets could be co-responsible of the base promoted effect on the methanolysis reaction of the dye. To exclude the binder effect, therefore, in a separate experiment a binderless zeolite KA in powder form was used to dry the methanol, which was reacted afterwards with the dye. Moreover, the reaction with the dye was performed using directly the ionic form of the methanol, potassium methoxide as a solution in methanol.

Though Fig. 3.3 (b) shows a small difference between methanol dried with zeolite KA in pellets or powder form, accordingly the rate constants of the reaction carried out with methanol treated with zeolite pellets were higher (Table 3.1). These results indicated that the acceleration of the methanolysis reaction of the dye is partly a consequence of the binder material of the zeolite KA pellets, but it is mostly due to the zeolite itself.