Evaluation of Lewis acid catalysed and other nitration strategies for the selective nitration of cresols
- Van Niekerk, Rudolf Jacob Francois
- Authors: Van Niekerk, Rudolf Jacob Francois
- Date: 2001
- Subjects: Nitration
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10957 , http://hdl.handle.net/10948/60 , Nitration
- Description: The nitration of m- and p-cresol was investigated under mild reaction conditions in a number of solvents; the effects of certain nitration catalysts were also evaluated. These different reaction parameters were evaluated in terms of their effect on a number of important reaction responses. Other important factors that were investigated were the manipulation of the isomer ratios by changing reaction parameters, as well as important process chemistry information, such as product distribution, isolation and purification, identification of side products formed, and evaluation of the heat of reaction. Use was made of an experimental design system to evaluate the effect of reaction parameters on the chosen design responses. It was found that the ratio of para to ortho nitrocresol products could be influenced slightly by using low concentrations of nitric acid and low reaction temperatures. A different mechanism for the formation of 2-methyl-1,4-benzoquinone (from mcresol nitration) was proposed (compared with that previously reported), which could explain a “one mole nitric acid” pathway and the fact that only the pbenzoquinone isomer was observed. Reaction side products were identified and found to consist of dimers of cresol and nitrocresol, which were probably the result of oxidation of the cresol, subsequent formation of a quinomethide intermediate and reaction with either the product or the substrate. The heat of nitration was determined for various reaction systems and found to be governed by two factors, namely the actual nitration process and also an oxidation process, which is responsible for the formation of side products.
- Full Text:
- Date Issued: 2001
- Authors: Van Niekerk, Rudolf Jacob Francois
- Date: 2001
- Subjects: Nitration
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10957 , http://hdl.handle.net/10948/60 , Nitration
- Description: The nitration of m- and p-cresol was investigated under mild reaction conditions in a number of solvents; the effects of certain nitration catalysts were also evaluated. These different reaction parameters were evaluated in terms of their effect on a number of important reaction responses. Other important factors that were investigated were the manipulation of the isomer ratios by changing reaction parameters, as well as important process chemistry information, such as product distribution, isolation and purification, identification of side products formed, and evaluation of the heat of reaction. Use was made of an experimental design system to evaluate the effect of reaction parameters on the chosen design responses. It was found that the ratio of para to ortho nitrocresol products could be influenced slightly by using low concentrations of nitric acid and low reaction temperatures. A different mechanism for the formation of 2-methyl-1,4-benzoquinone (from mcresol nitration) was proposed (compared with that previously reported), which could explain a “one mole nitric acid” pathway and the fact that only the pbenzoquinone isomer was observed. Reaction side products were identified and found to consist of dimers of cresol and nitrocresol, which were probably the result of oxidation of the cresol, subsequent formation of a quinomethide intermediate and reaction with either the product or the substrate. The heat of nitration was determined for various reaction systems and found to be governed by two factors, namely the actual nitration process and also an oxidation process, which is responsible for the formation of side products.
- Full Text:
- Date Issued: 2001
Strategies for the improvement of the industrial oxidation of cymene
- Authors: Harmse, Nigel
- Date: 2001
- Subjects: Oxidation , Cymene
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10958 , http://hdl.handle.net/10948/73 , Oxidation , Cymene
- Description: The oxidation of cymene with dioxygen has been investigated in some detail with the view of establishing the feasibility of improving the efficiency of the oxidation process. Of particular interest were the rate of cymene oxidation and the selectivity of the oxidation process for the tertiary cymene hydroperoxide, especially at conversions above 15%. In order to be able to evaluate the selectivity of oxidation processes, a reliable method for analysis of the individual hydroperoxides had to be established. Two methods were investigated, namely reduction of the hydroperoxides to alcohols using ferrous sulphate and reduction using triphenylphosphine, and analysing the reduction products by gas chromatography. Of these two methods, the triphenylphosphine method proved to be superior to the ferrous sulphate method and was used as the method of choice for this investigation. A number of oxidation systems were evaluated in an initial screening experiment for the oxidation of p-cymene. The results of this screening experiment showed that three-phase oxidation systems, i.e. systems containing an organic phase, an aqueous phase and gas, gave significantly lower activities than two-phase oxidation systems. In addition, the use of a base in the aqueous layer does not improve the overall selectivity of the oxidation process, but improves the selectivity towards the tertiary hydroperoxide to some extent due to the decomposition and extraction of primary hydroperoxide into the basic aqueous phase. Oxidation systems using a non-autoxidation catalyst, i.e. a catalyst that does not catalyse the conventional autoxidation of organic compounds, gave by far the most promising results. These systems gave both a high selectivity as well as high reaction rate. From the initial screening experiment, and using multi-factorial statistical techniques, two catalyst systems were selected for investigation, namely vanadium phosphate and boron phosphate. The results of these investigations showed that these two catalysts are remarkably active and selective for the oxidation of p-cymene, giving the cymene tertiary hydroperoxide in selectivities exceeding 85% and at substrate conversions as high as 25%. These results are a considerable improvement over currently known oxidation systems and may offer opportunities for further commercial exploitation.
- Full Text:
- Date Issued: 2001
- Authors: Harmse, Nigel
- Date: 2001
- Subjects: Oxidation , Cymene
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10958 , http://hdl.handle.net/10948/73 , Oxidation , Cymene
- Description: The oxidation of cymene with dioxygen has been investigated in some detail with the view of establishing the feasibility of improving the efficiency of the oxidation process. Of particular interest were the rate of cymene oxidation and the selectivity of the oxidation process for the tertiary cymene hydroperoxide, especially at conversions above 15%. In order to be able to evaluate the selectivity of oxidation processes, a reliable method for analysis of the individual hydroperoxides had to be established. Two methods were investigated, namely reduction of the hydroperoxides to alcohols using ferrous sulphate and reduction using triphenylphosphine, and analysing the reduction products by gas chromatography. Of these two methods, the triphenylphosphine method proved to be superior to the ferrous sulphate method and was used as the method of choice for this investigation. A number of oxidation systems were evaluated in an initial screening experiment for the oxidation of p-cymene. The results of this screening experiment showed that three-phase oxidation systems, i.e. systems containing an organic phase, an aqueous phase and gas, gave significantly lower activities than two-phase oxidation systems. In addition, the use of a base in the aqueous layer does not improve the overall selectivity of the oxidation process, but improves the selectivity towards the tertiary hydroperoxide to some extent due to the decomposition and extraction of primary hydroperoxide into the basic aqueous phase. Oxidation systems using a non-autoxidation catalyst, i.e. a catalyst that does not catalyse the conventional autoxidation of organic compounds, gave by far the most promising results. These systems gave both a high selectivity as well as high reaction rate. From the initial screening experiment, and using multi-factorial statistical techniques, two catalyst systems were selected for investigation, namely vanadium phosphate and boron phosphate. The results of these investigations showed that these two catalysts are remarkably active and selective for the oxidation of p-cymene, giving the cymene tertiary hydroperoxide in selectivities exceeding 85% and at substrate conversions as high as 25%. These results are a considerable improvement over currently known oxidation systems and may offer opportunities for further commercial exploitation.
- Full Text:
- Date Issued: 2001
The development and evaluation of procedures for the synthesis of phenolic ethers by Baeyer-Villiger oxidation
- Authors: Hoffmann, Petra
- Date: 2001
- Subjects: Ethers , Phenols
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10955 , http://hdl.handle.net/10948/51 , Ethers , Phenols
- Description: A novel preparative method has been discovered in our laboratories which produces hydroxy-substituted phenolic ethers in a simple one-pot process. Initially it was found that 4-hydroxyacetophenone, when oxidized in the presence of ammonium peroxydisulfate and concentrated sulfuric acid in methanol, resulted in 4- methoxyphenol as product in high yield. Being novel, no information about the method was available in the literature, and hence it was our task to study the reaction more extensively. The scope of the reaction was investigated in that different reagents and substrates, as well as alternative oxidants, catalysts and alcohols (or reagents which may be envisaged to behave similarly to alcohols) were used. The studies showed that 4- hydroxy-substituted benzaldehyde and acetophenone, vanillin, fuchsones and stilbenequinones as substrates all successfully afforded the phenolic ether. Other oxidants such as hydrogen peroxide, sodium perborate etc. also afforded the phenolic ether but were less effective. The use of other acids than sulfuric acid resulted in comparable or lower yields of the desired phenolic ether. When the use of alcohols or similar reagents was varied, it was found that methanol, ethanol, benzyl alcohol, allyl alcohol, but-3-en-1-ol, hex-5-en-1-ol and undec-10-en-1-ol all afforded the desired ethers in significant yields. In an investigation to determine how this method may be useful for industrial applications, it was observed that compounds such as 3,5-di-tert-butyl-4-hydroxysubstituted alkenyloxy phenols, having potential application as antioxidant precursors in the polymer industry, may be prepared. Results showed that the reaction became less efficient when the alkenyl chain length was increased. The use of different acids and oxidants, the change of the oxidant:substrate ratio and the addition of a potential lanthanide catalyst did not improve the yield of the desired ether. A mechanism for the reaction was proposed which involved an initial Baeyer-Villiger oxygen insertion step to afford an ester which was hydrolysed in situ to hydroquinone. The remaining oxidant was then involved in oxidizing some of the soformed hydroquinone to benzoquinone. It was then proposed that these two compounds, i.e., hydroquinone and benzoquinone, interacted with each other in solution, forming a charge-transfer complex. According to our proposal, the carbonyl group of the benzoquinone, when complexed with hydroquinone in solution, was then attacked by nucleophilic reagents such as alcohols. Etherification hence resulted as a consequence of this attack. A statistical experimental evaluation and optimization study was then performed which led to a system that produced the phenolic ether, 4-methoxyphenol, in an 86% yield. An analysis of the effects of the reaction parameters on the reaction showed that the reaction temperature and the oxidant:substrate ratio played a major role in deciding the effectiveness of the reaction, while the amount of substrate and acidic catalyst had a much smaller influence on the ether yields.
- Full Text:
- Date Issued: 2001
- Authors: Hoffmann, Petra
- Date: 2001
- Subjects: Ethers , Phenols
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10955 , http://hdl.handle.net/10948/51 , Ethers , Phenols
- Description: A novel preparative method has been discovered in our laboratories which produces hydroxy-substituted phenolic ethers in a simple one-pot process. Initially it was found that 4-hydroxyacetophenone, when oxidized in the presence of ammonium peroxydisulfate and concentrated sulfuric acid in methanol, resulted in 4- methoxyphenol as product in high yield. Being novel, no information about the method was available in the literature, and hence it was our task to study the reaction more extensively. The scope of the reaction was investigated in that different reagents and substrates, as well as alternative oxidants, catalysts and alcohols (or reagents which may be envisaged to behave similarly to alcohols) were used. The studies showed that 4- hydroxy-substituted benzaldehyde and acetophenone, vanillin, fuchsones and stilbenequinones as substrates all successfully afforded the phenolic ether. Other oxidants such as hydrogen peroxide, sodium perborate etc. also afforded the phenolic ether but were less effective. The use of other acids than sulfuric acid resulted in comparable or lower yields of the desired phenolic ether. When the use of alcohols or similar reagents was varied, it was found that methanol, ethanol, benzyl alcohol, allyl alcohol, but-3-en-1-ol, hex-5-en-1-ol and undec-10-en-1-ol all afforded the desired ethers in significant yields. In an investigation to determine how this method may be useful for industrial applications, it was observed that compounds such as 3,5-di-tert-butyl-4-hydroxysubstituted alkenyloxy phenols, having potential application as antioxidant precursors in the polymer industry, may be prepared. Results showed that the reaction became less efficient when the alkenyl chain length was increased. The use of different acids and oxidants, the change of the oxidant:substrate ratio and the addition of a potential lanthanide catalyst did not improve the yield of the desired ether. A mechanism for the reaction was proposed which involved an initial Baeyer-Villiger oxygen insertion step to afford an ester which was hydrolysed in situ to hydroquinone. The remaining oxidant was then involved in oxidizing some of the soformed hydroquinone to benzoquinone. It was then proposed that these two compounds, i.e., hydroquinone and benzoquinone, interacted with each other in solution, forming a charge-transfer complex. According to our proposal, the carbonyl group of the benzoquinone, when complexed with hydroquinone in solution, was then attacked by nucleophilic reagents such as alcohols. Etherification hence resulted as a consequence of this attack. A statistical experimental evaluation and optimization study was then performed which led to a system that produced the phenolic ether, 4-methoxyphenol, in an 86% yield. An analysis of the effects of the reaction parameters on the reaction showed that the reaction temperature and the oxidant:substrate ratio played a major role in deciding the effectiveness of the reaction, while the amount of substrate and acidic catalyst had a much smaller influence on the ether yields.
- Full Text:
- Date Issued: 2001
The evaluation of autoxidation procedures for the selective oxidation of aliphatic alcohols
- Authors: Bacela, Siyabulela Mawande
- Date: 2001
- Subjects: Oxidation , Aliphatic compounds
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10950 , http://hdl.handle.net/10948/59 , Oxidation , Aliphatic compounds
- Description: The homogeneously catalyzed oxidation of 1-propanol by dioxygen in glacial acetic acid using cobalt(II)acetate and sodium bromide as the catalyst system has been investigated with the view of determining the significance of various experimental variables during the oxidation. The results of this investigation show unequivocally that a number of reaction variables have a direct influence upon catalytic activity and hence the reaction products. It is quite evident that the major product of this autoxidation reaction is propionic acid with the respective esters as side-products. This is an indication that the autoxidation mechanism occurs via a two-stage pathway, namely, the oxidation of 1-propanol to propionaldehyde as the primary product and, subsequently, the further oxidation of the propionaldehyde to propionic acid as the major product. Thus the esterification process of the propionic acid with the substrate 1-propanol could be termed as a side-reaction because its not facilitated by the catalyst system and it consumes the formed product. The catalyst activity has been demonstrated to depend on a number of factors, including the bromide concentration, the cobalt(II)acetate concentration, the water concentration, reaction temperature, and the presence of metal acetates as co-catalysts. There is an observed decrease in catalytic activity at high bromide concentration, which may be explained in terms of cobalt bromide complexes that form at these high concentrations. Subsequently, the same trend of catalyst activity reduction at high cobalt(II)acetate concentration may be ascribed to the “inactive” metal complexes that are susceptible to form at high metal ion concentrations. The catalytic activity increases with increase in total concentration and rapidly decreases at very high concentrations. This can be explained in terms of the observations made during the investigation of the effect of cobalt(II)acetate and bromide concentrations. The high increase in catalytic activity with increasing temperature is ascribed to the Arrhenius law, which relates the rate constant for a particular reaction to temperature. However, there is an observed loss of catalyst selectivity at high temperatures which maybe due to two possible factors. The first is simply related to an increased loss of volatile material from the reactor in the oxygen gas stream as the temperature is increased. The second relates to the increasing activity of the catalyst system for the selective decarboxylation of the carboxylic acid product. The addition of water to the reaction system rapidly reduces the catalyst activity. This detrimental effect is an indication that there is an effective competition by water with bromide for coordination sites on cobalt(II), thereby preventing the formation of the active catalyst species. The introduction of metal acetates as co-catalyst reduces the catalyst activity quite dramatically. This inhibition effect is suggested to relate to the redox potential of the respective metal ions. The results of statistical analysis of the experimentally derived response surface during the oxidation of 1-propanol, show no significant lack of fit, and the residuals obtained by applying the response surface to the design settings show that the data is normally distributed. The response surface is therefore reliable, but keeping in mind that the central composite design used is not rotatable so that its predictive power, especially outside the experimental domain investigated, is quite limited. However, several interesting observations were still possible The oxidative dehydrogenation of ethanol over supported noble-metal catalysts has been investigated with the view of identifying the most active supported noble-metal and also to compare this oxidation procedure with the autoxidation procedure. Secondly, the effect of an acidic resin as a co-catalyst was also investigated during the said oxidation. On the basis of results presented in this study during oxidative dehydrogenation of ethanol, catalysts no.2 (10% Pd/C), 8 (2% Pd/Al – Pb-promoted) and 9 (2% Pt/8% Pd/C) appear to be the most active in terms of relative rates, while catalysts 6 (10% Pd/C- Pbpromoted), 7 (5% Pd/C-shell reduced-Pb -promoted) and 10 (5% Pt 5% Pd on C) are more active based on the comparison of average rates. Two other observations are of interest. Firstly, the promotion of the Pd catalysts with lead appears to improve catalyst activity to some extent as shown by the comparisons between catalysts 1 and 5, 4 and 8, 2 and 6 and 3 and 7. Secondly, the introduction of Pt up to equal amounts with palladium seems to produce the most active catalysts. On its own, platinum appears to be a better catalyst than Pd when supported on activated carbon (catalysts 1 and 12). In comparison with the homogeneous, cobalt-bromide catalyzed oxidation of 1- propanol in the liquid-phase, oxidations over noble-metal catalysts in the liquid-phase appear to be significantly less active. The presence of the resin promoted the formation of ethyl acetate to some extent, the improvements are not as dramatic as expected.
- Full Text:
- Date Issued: 2001
- Authors: Bacela, Siyabulela Mawande
- Date: 2001
- Subjects: Oxidation , Aliphatic compounds
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10950 , http://hdl.handle.net/10948/59 , Oxidation , Aliphatic compounds
- Description: The homogeneously catalyzed oxidation of 1-propanol by dioxygen in glacial acetic acid using cobalt(II)acetate and sodium bromide as the catalyst system has been investigated with the view of determining the significance of various experimental variables during the oxidation. The results of this investigation show unequivocally that a number of reaction variables have a direct influence upon catalytic activity and hence the reaction products. It is quite evident that the major product of this autoxidation reaction is propionic acid with the respective esters as side-products. This is an indication that the autoxidation mechanism occurs via a two-stage pathway, namely, the oxidation of 1-propanol to propionaldehyde as the primary product and, subsequently, the further oxidation of the propionaldehyde to propionic acid as the major product. Thus the esterification process of the propionic acid with the substrate 1-propanol could be termed as a side-reaction because its not facilitated by the catalyst system and it consumes the formed product. The catalyst activity has been demonstrated to depend on a number of factors, including the bromide concentration, the cobalt(II)acetate concentration, the water concentration, reaction temperature, and the presence of metal acetates as co-catalysts. There is an observed decrease in catalytic activity at high bromide concentration, which may be explained in terms of cobalt bromide complexes that form at these high concentrations. Subsequently, the same trend of catalyst activity reduction at high cobalt(II)acetate concentration may be ascribed to the “inactive” metal complexes that are susceptible to form at high metal ion concentrations. The catalytic activity increases with increase in total concentration and rapidly decreases at very high concentrations. This can be explained in terms of the observations made during the investigation of the effect of cobalt(II)acetate and bromide concentrations. The high increase in catalytic activity with increasing temperature is ascribed to the Arrhenius law, which relates the rate constant for a particular reaction to temperature. However, there is an observed loss of catalyst selectivity at high temperatures which maybe due to two possible factors. The first is simply related to an increased loss of volatile material from the reactor in the oxygen gas stream as the temperature is increased. The second relates to the increasing activity of the catalyst system for the selective decarboxylation of the carboxylic acid product. The addition of water to the reaction system rapidly reduces the catalyst activity. This detrimental effect is an indication that there is an effective competition by water with bromide for coordination sites on cobalt(II), thereby preventing the formation of the active catalyst species. The introduction of metal acetates as co-catalyst reduces the catalyst activity quite dramatically. This inhibition effect is suggested to relate to the redox potential of the respective metal ions. The results of statistical analysis of the experimentally derived response surface during the oxidation of 1-propanol, show no significant lack of fit, and the residuals obtained by applying the response surface to the design settings show that the data is normally distributed. The response surface is therefore reliable, but keeping in mind that the central composite design used is not rotatable so that its predictive power, especially outside the experimental domain investigated, is quite limited. However, several interesting observations were still possible The oxidative dehydrogenation of ethanol over supported noble-metal catalysts has been investigated with the view of identifying the most active supported noble-metal and also to compare this oxidation procedure with the autoxidation procedure. Secondly, the effect of an acidic resin as a co-catalyst was also investigated during the said oxidation. On the basis of results presented in this study during oxidative dehydrogenation of ethanol, catalysts no.2 (10% Pd/C), 8 (2% Pd/Al – Pb-promoted) and 9 (2% Pt/8% Pd/C) appear to be the most active in terms of relative rates, while catalysts 6 (10% Pd/C- Pbpromoted), 7 (5% Pd/C-shell reduced-Pb -promoted) and 10 (5% Pt 5% Pd on C) are more active based on the comparison of average rates. Two other observations are of interest. Firstly, the promotion of the Pd catalysts with lead appears to improve catalyst activity to some extent as shown by the comparisons between catalysts 1 and 5, 4 and 8, 2 and 6 and 3 and 7. Secondly, the introduction of Pt up to equal amounts with palladium seems to produce the most active catalysts. On its own, platinum appears to be a better catalyst than Pd when supported on activated carbon (catalysts 1 and 12). In comparison with the homogeneous, cobalt-bromide catalyzed oxidation of 1- propanol in the liquid-phase, oxidations over noble-metal catalysts in the liquid-phase appear to be significantly less active. The presence of the resin promoted the formation of ethyl acetate to some extent, the improvements are not as dramatic as expected.
- Full Text:
- Date Issued: 2001
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