Using metrics and sustainability considerations to evaluate the use of bio-based and non-renewable Brønsted acidic ionic liquids to catalyse Fischer esterification reactions
© Clark et al.; licensee Chemistry Central Ltd. 2013
Received: 22 September 2013
Accepted: 19 November 2013
Published: 22 November 2013
Ionic liquids have found uses in many applications, one of which is the joint solvation and catalysis of chemical transformations. Suitable Brønsted acidic ionic liquids can be formed by combining lactams with sulphonic acids. This work weighs up the relative benefits and disadvantages of applying these Brønsted acidic ionic liquid catalysts in esterifications through a series of comparisons using green chemistry metrics.
A new bio-based ionic liquid was synthesised from N-methyl pyrrolidinone and p-cymenesulphonic acid, and tested as a catalyst in three Fischer esterifications under different conditions. An evaluation of the performance of this Brønsted acidic ionic liquid was made through the comparison to other ionic liquid catalysts as well as conventional homogeneous Brønsted acids.
Extending the argument to feedstock security as well as mass utilisation, ultimately in most instances traditional mineral acids appear to be the most sensible option for Brønsted acid esterification catalysts. Ester yields obtained from Brønsted acidic ionic liquid catalysed procedures were modest. This calls into question the diversity of research exploring esterification catalysis and the role of ionic liquids in esterifications.
KeywordsBio-based products Brønsted acidic ionic liquid p-Cymenesulphonic acid Esterification Green chemistry Ionic liquids Metrics
Esters are useful products for a variety of applications, including plastics, fragrances and pharmaceuticals. The least complicated and most useful of several esterification methodologies is the Fischer esterification, which requires an acid catalyst, and in the case of solid reactants often a solvent as well. Fischer esterification combines a carboxylic acid and an alcohol to produce the desired ester, and water as the only by-product. Popular catalysts are the familiar Brønsted acidic mineral acids and Lewis acidic transition metal salts, but supported catalysts (including enzymes) and recyclable heterogeneous solid acids are becoming increasing popular because of the influence of green chemistry [1, 2]. Other methods of esterification may employ nucleophilic catalysts or stoichiometric activators, and as such are harder to justify except for the most unreactive substrates. Applying the philosophy of green chemistry to esterification, the use of equimolar quantities of each reactant to give 100% conversion, whilst minimising unnecessary VOC emissions, is desirable. Recycling of the catalyst and any solvent would then result in minimal waste. Nevertheless all auxiliary species should be sustainable, present low toxicity and health concerns, as well as effective in assisting the transformation.
A summary of mass based green chemistry metrics
Mass of desired product/Theoretical maximum mass of product
Relative molecular mass (RMM) of desired product/RMM reactants
Mass of desired product/Total mass of reactants used
Total mass waste/Mass of desired product
Total mass required/Mass of desired product = E-Factor + 1
Brønsted acidic ionic liquids in esterifications
Ionic liquid cation
Ionic liquid anion
Yield (2–4 hours)
Results and discussion
As already identified in previous studies, esters produced by Fischer esterification in certain Brønsted acidic ionic liquids will form a unique phase, offering a means of easy removal from the reaction system . The continuous separation of the product will adjust the equilibrium position in the reactive ionic liquid phase, promoting further esterification. This is the same principle used in more typical esterification strategies, which include distillation and the use of an excess of one reactant . The advantage of the ionic liquid phase is that it is low energy (often operating at room temperature) and reusable whilst also being compatible with the reaction of two solids. After stirring overnight at the ambient temperature without an auxiliary solvent, the synthesis of benzyl acetate assisted by 25 mol% of [H-NMP][HSO4] did indeed result in a biphasic system. The ester containing organic phase could be retrieved without additional workup.
Ethyl levulinate synthesis protocols
The most typical of catalysts, sulphuric acid arguably offers the best performance as a function of catalyst loading, whilst maintaining low environmental impact, if in fact ultimately unsustainable. Thus it is important to ask why so much effort has been invested into the development of more elaborate catalysts, often requiring sulphuric acid as a reagent anyway. Sulphuric acid is known to be corrosive , and also able to dehydrate alcohols . It is also not recyclable, which becomes a concern in high volume processes. Designing and implementing heterogeneous acid catalysts that can be reused, or employing sulphonic acids or their salts which are considered to be less corrosive than sulphuric acid , has been a major topic within green chemistry, and work will continue in this area for as long as the productivity of sulphuric acid catalysed reactions continues to provide the best product returns.
All reactions were performed under air without any effort made to exclude water. p-Cymene-2-sulphonic acid was prepared by the method previously described in the literature . All other chemicals were used as received. Characterisation of the ester products was consistent with authentic samples.
N-Methyl pyrrolidinonium p-cymene-2-sulphonate
To p-CSA, crystallised as the dihydrate (2.50 g, 0.01 mol), dissolved in DCM (20 mL) was added NMP (0.96 mL, 0.01 mol) dropwise. The resultant reaction mixture was stirred overnight to give N-methyl pyrrolidinonium p-cymene-2-sulphonate as a pale yellow liquid (3.05 g, 97% of the theoretical yield). NMR: δH (400 MHz, DMSO-d6) 0.98 (6H, d, 3 J = 6.9 Hz, CHCH3), 1.70 (2H, q, CH2-4), 2.00 (2H, t, CH2-5), 2.28 (3H, s, NCH3), 2.65 (1H, m, CHCH3), 3.11 (2H, t, CH2-3), 6.88 (2H, m, ar), 7.43 (1H, d, ar)/ppm; NMR: δC (100 MHz, DMSO-d6) 17.32, 19.69, 24.03, 29.18, 20.30, 33.09, 48.70, 124.46, 126.97, 131.02, 132.96, 144.97, 145.25, 174.13 /ppm. IR: ν (neat) 2962, 1659, 1489, 1456, 1404, 1229, 1156, 1084, 1015, 964, 907, 828, 720, 687, 619/cm-1.
In the absence of an auxiliary solvent acetic acid (12 mmol) and benzyl alcohol (12 mmol) were added to the chosen catalyst (3 mmol) and stirred for 18 hours at the ambient temperature at 300 rpm. If [H-NMP][HSO4] was used as the catalyst the product could be decanted from the reaction. Otherwise diethyl ether (10 mL) and water (10 mL) was added, the organic phase separated and washed with water (2 × 10 mL), dried with magnesium sulphate, filtered and concentrated in vacuo to give benzyl acetate as a yellow liquid.
The kinetics of benzyl acetate synthesis were monitored by taking 1H-NMR spectra at suitable intervals and integrating the benzylic CH2 signal of the product and comparing it to the corresponding benzyl alcohol signal. The procedure mirrors that utilised by Wells et al. previously . Typically acetic acid (0.32 mL, 5.50 mmol) and benzyl alcohol (0.52 mL, 5.00 mmol) were added to the desired quantity of catalyst as a preheated solution in p-cymene (5 mL) at 323 K.
Acetic acid (0.46 mL, 8.00 mmol) and 1-butanol (0.73 mL, 8.00 mmol) were added to [H-NMP][HSO4] (0.394 g, 25 mol%) and stirred at 300 rpm at the desired temperature for 18 hours. After this time the product was decanted from the reaction mixture and characterised.
Levulinic acid (0.464 g, 12 mmol) and the chosen catalyst (1 mmol) were added to ethanol (0.70 mL, 12 mmol), and the resultant mixture stirred for 18 hours at the ambient temperature at 300 rpm. At this point diethyl ether (10 mL) and potassium carbonate were added, and the mixture filtered before concentrated in vacuo to give ethyl levulinate as a colourless oil.
The synthesis of Brønsted acidic ionic liquids for implementation in esterification chemistries has become popular in the last decade, but the state of the art lacked a greater context with respect to what benefits these catalysts actually provide in the widest sense. To address this, a bio-based ionic liquid, [H-NMP][OCym], was synthesised for the first time and compared to its non-renewable analogues and more typical Brønsted acid catalysts. Although generally the shift from unsustainable chemicals to renewable bio-waste derived alternatives cannot be viewed as anything but agreeable, adequate yields whilst minimising waste could only realistically be achieved with sulphuric acid in the synthesis of ethyl levulinate. The results of other authors are consistent with this . The other case studies presented in this work could only be concluded in a similar way, provoking a rethink towards the design, synthesis, implementation, and inevitable waste disposal issues of Brønsted acidic ionic liquids. This is particularly poignant for esterifications, which are in fact suited to low polarity solvents , and not ionic liquids .
General reaction mass efficiency
Proton nuclear magentic resonance
Nuclear magnetic resonance
Process mass intensity
Reaction mass efficiency
Relative molecular mass
Revolutions per minute
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