https://vital.seals.ac.za/vital/access/manager/Index en-us 5 https://vital.seals.ac.za/vital/access/manager/Repository/vital:10980 Wed 12 May 2021 20:27:57 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10984 Wed 12 May 2021 20:12:53 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10958 Wed 12 May 2021 19:32:43 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10960 97% could be achieved where the resultant product was well within stipulated product specifications. In view of the results obtained, the following recommendations regarding the synthesis of atrazine in toluene as reaction solvent can be made: · Use a reagent addition sequence that staggers the addition of amine and NaOH in such a manner that amine is added first for a short while, followed by the simultaneous addition of amine and NaOH, and ending with NaOH. Use two reaction vessels in series, one for the IPA addition reaction and one for the MEA addition reaction. In this manner the reaction can be run on a continuous basis since no lag time between amine additions is required. Also, smaller reactors may be used whilst maintaining high production rates. Smaller reactors will improve both temperature control and mixing of reagents.]]> Wed 12 May 2021 15:57:46 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10978 Thu 13 May 2021 08:18:25 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10957 Thu 13 May 2021 07:22:50 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10950 Thu 13 May 2021 06:40:30 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10961 Thu 13 May 2021 06:08:05 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10959 Thu 13 May 2021 05:50:20 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10955 Thu 13 May 2021 04:43:14 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10951 Thu 13 May 2021 00:49:02 SAST ]]> https://vital.seals.ac.za/vital/access/manager/Repository/vital:10968 95%) and selectivities (>95%) may be obtained. However, the use of DMF, which poses a serious chronic health risk, is unacceptable in this process since the products are intended for use as food and flavouring chemicals. In view of the above the main objectives of this study were: · To find a suitable alternative solvent system, which could produce comparable results while still being economically viable; · To develop an appropriate experimental protocol in the laboratory based on the alternative solvent system; · To determine the important reaction variables by conducting statistically designed experiments; · To optimise the reaction to produce a reproducible and robust experimental protocol; and · To test the reaction thoroughly at bench-scale level and to obtain experimental data for scale-up to pilot plant The most promising alternative solvent system was a methanol/methyl acetate mixture, which produced satisfactory results in the preliminary assessment (conversion of 98.3% and selectivity of 92.0%). DMA and acetonitrile also produced promising results but were not considered for further investigation because of toxicity and cost issues. A set of statistically designed experiments was carried out on the methanol/methyl acetate solvent system where four variables were tested i.e., substrate concentration, temperature, catalyst loading, and methanol to methyl acetate volume ratio. The experimentally determined response surface model showed that the most important variable was catalyst loading (63.2%) for conversion. With respect to selectivity, the most important variables were catalyst loading (31.9%) and methanol to methyl acetate ratio (33.1%). The optimum reaction conditions were as follows: · Temperature: 120°C · Methanol:methyl acetate: 15:1 vol/vol · Catalyst loading: 8 mol % to substrate · Substrate concentration: 22 %m/m on solvent · Catalyst: Copper(I) bromide · Sodium Equivalents: 2.7 wrt substrate · Time: 3 hours The optimum conditions were tested for reproducibility in a 1 Labmax pressure reactor. Replicated reactions, two at a 10% and two at a 20% substrate concentration gave conversions and selectivities all greater than 90%. Although the reaction mixture was a slurry at these concentrations, the reactions were very fast and virtually complete within the first hour (~95% conversion). Initial scale-up studies were conducted in an 8 Parr reactor where five reactions were carried out using the optimum conditions described above. The conversion of substrate and vanillin selectivity was consistently high and compared favourably to the Labmax reactions. The average conversion was 97.3% (96.3 to 98.5%) at an average selectivity of 98.2% (97.4 to 99.1%). A study of the reaction kinetics confirmed that the reaction was first order with respect to the substrate as a plot of substrate concentration versus reaction rate gave a straight line. The rate constant was calculated as 1.1096 k(h-1). The reaction mechanism proposed for the copper assisted nucleophilic aromatic substitution involves the formation of an adduct between sodium methoxide, methyl acetate and copper(I) bromide. The formation of a transient intermediate with the substrate allows intramolecular delivery of the methoxide ion to the aryl moiety through a CuI – CuIII type cycle.]]> Thu 13 May 2021 00:19:35 SAST ]]>