The chemistry of interesterification processes. Albert J. Dijkstra

Transcription

1 The chemistry of interesterification processes Albert J. Dijkstra

2 What is interesterification? Interesterification of triglyceride oils can be defined as the exchange of fatty moieties between different triglyceride molecules (reshuffling) In the chemical industry, we call it ester interchange For instance: DMT + glycol monomer + methanol There are three types of interesterification: Randomization Directed interesterification 1,3-Specific interesterification There are two types of catalyst Alkaline catalysts Lipase enzymes 3 November 2015 World Congress on ils & Fats and 2

3 A bit of history Charles Friedel & James R. Crafts heat a mixture of ethyl benzoate and amyl acetate to 300 C and observe the formation of amyl benzoate and ethyl acetate Christiaan van Loon discloses several catalysts for the high-temperature interesterification process George B. Bradshaw & Walter C. Meuly (E.I. du Pont de Nemours) disclose low-temperature, alkali catalysed FAME production with glycerol co-product. The glycerol was needed for the War effort; the FAME were used in soap production Frank A. Norris & Karl F. Mattil demonstrate that interesterification causes the triglyceride composition to be random Josef Baltes proposes mechanism with glycerolate anion as catalytically active intermediate 3 November 2015 World Congress on ils & Fats and 31st ISF Lecture Series Rosario 3

4 A bit more history of interesterification chemistry Theodore J. Weiss et al. propose a mechanism based on β-keto ester formed from enolate that results from abstraction of an α-hydrogen from fatty acid moiety 1972, Acetone and DMS are found to accelerate the interesterification reaction early 1980s. Alisdair Macrae (Unilever) starts with lipase catalyzed interesterification for cocoa butter equivalents Linsen Liu demonstrates that acids without an α-hydrogen do not take part in the interesterification I propose the enolate mechanism Independent experimental support for the enolate mechanism 3 November 2015 World Congress on ils & Fats and 4

5 Reaction products When we randomize an equimolar A B mixture of A 3 and B 3, we get equimolar 4 A + 4 B mixture of all combinations A B A A 2 B + 3 AB 2 + B 3 A A A B A B B B A + A + B + A + B + A + B + B A B A A B A B B Now we assume A 3 to have a much higher melting point than B 3 and we cool the reaction mixture so that A 3 starts to crystallize: 3 A A A 2 B AB B 3 World Congress on ils & Fats and 3 November

6 Triglyceride composition after randomization The fatty acid composition of the fat blend determines the triglyceride composition of the reaction product [ABC] = mol fraction of triglyceride with fatty acids A, B and C [a], [b] and [c] mol fractions of fatty acids A, B and C in fat blend n = 1 if three fatty acids in triglyceride are the same, e.g. A 3 n = 3 if there are two different fatty acids in the triglyceride (A 2 B) n = 6 if the three fatty acids are different (ABC) The six permutations are: ABC. ACB, BAC, BCA, CAB, CBA 3 November 2015 World Congress on ils & Fats and 6

7 Triglyceride composition on directed interesterification First, the reaction mixture is randomized and made to crystallize by cooling to below the solubility of its highest melting component When this component then starts to crystallize, residual liquid is no longer random By maintaining random equilibrium, more of the highest melting component is formed A crystal slurry results with crystals of high melting components in a randomized liquid 3 November 2015 World Congress on ils & Fats and 7

8 The formerly accepted glycerolate mechanism starts with initiation The initiation step involves the nucleophilic attack of the methanolate anion on a triglyceride molecule to form the glycerolate anion and a fatty acid methyl ester C R C R 3 C C R C R C 3 C 3 triglyceride methoxide intermediate glycerolate + FAME 3 November 2015 World Congress on ils & Fats and 8

9 Then there is propagation leading to interesterification The glycerolate anion forms a complex with a triglyceride (nucleophilic attack) and then, the complex dissociates into a novel triglyceride while regenerating the glycerolate anion C R C R C R R triglyceride glycerolate intermediate glycerolate + novel triglyceride 3 November 2015 World Congress on ils & Fats and 9

10 Then the reaction is terminated by the addition of (acidulated) water Water is assumed to react with the sodium glycerolate to yield a partial glyceride and sodium hydroxide But the amount of soap (or FFA) is observed to be equimolar to the amount of sodium methanolate This observation can only be explained by assuming a quantitative reaction between the sodium hydroxide and triglyceride oil, which is most unlikely Soap boiling needs an excess lye and a long reaction time Using acidulated water prevents saponification The glycerolate mechanism cannot be the whole story 3 November 2015 World Congress on ils & Fats and 10

11 We need another/additional catalytic intermediate than just glycerolate This intermediate must form a free fatty acid when reacting with water We observe that the amount of FAME formed is equivalent to the amount of Na-methoxide Even when some of the methoxide has been inactivated by water or FFA to give free methanol This intermediate must form fatty acid methyl esters (FAME) by reacting with free methanol The enolate anion meets these requirements 3 November 2015 World Congress on ils & Fats and 11

12 What is an enolate anion? The enolate anion is formed by abstracting an α- hydrogen atom from a fatty acid moiety The methoxide anion can abstract the hydrogen The enolate anion is stabilized by mesomerism fatty acid that is part of glyceride C R C 3 methoxide anion 3 November 2015 World Congress on ils & Fats and enolate anion R R C 3 keto-form enol-form methanol 12

13 The enolate anion reacts with water enolate anion R C C water R C C FFA glycerolate anion The glycerolate anion will react further with water and form a partial glyceride and caustic soda The caustic will react with the FFA and form soap If the water had been acidified, the acid would neutralize the caustic and leave the FFA as such 3 November 2015 World Congress on ils & Fats and 13

14 Methanol also has a hydroxyl group The enolate anion reacts with hydroxyl groups enolate anion R C C methanol C 3 R C C FAME C 3 glycerolate anion With free methanol, it reacts to give a FAME and a glycerolate anion The enolate anion mops up all free methanol FAME formation equivalent to sodium methanolate 3 November 2015 World Congress on ils & Fats and 14

15 Partial glyceride + enolate anion interesterification enolate anion R C C partial glyceride glycerolate anion R C C novel triglyceride Enolate anion abstracts hydrogen from hydroxyl group in partial glyceride Bond is formed between α-carbon and partial glyceride Glycerolate anion is split off 3 November 2015 World Congress on ils & Fats and 15

16 Regeneration of enolate anion C R R R glycerolate anion enolate anion partial glyceride There is an equilibrium between enolate anion and glycerolate anion igh concentration of fatty acid moieties compared to free hydroxyl groups favours enolate anion Enthalpy difference may also favour the enolate anion 3 November 2015 World Congress on ils & Fats and 16

17 The role of acetone In 1972, Muller & Kock (Unilever) show that acetone accelerates the interesterification process Could acetone act as proton transfer agent? enolate anion R C C D D D acetone CD 3 Proof was provided in 2005 when fully deuterated acetone was added to an interesterification reaction mixture and the deuterium was found to be incorporated in the α-position of fatty acid moieties R C C + + D D D CD 3 keto D D enol CD 3 3 November 2015 World Congress on ils & Fats and 17

18 What about the β-keto ester? C 2 C R 1 C C R 1 R R 1 1 C C C C C C C C 2 C C 2 C R 2 C 2 R 2 R 2 R 2 Its presence has been spectrophotometrically demonstrated It has been claimed as a catalytic intermediate Now its formation is considered to be the cause of the loss of catalytic activity at elevated temperature Reaction temperature should be kept below 100 C 3 November 2015 World Congress on ils & Fats and 18

19 Transesterification or methanolysis (Biodiesel production) When producing biodiesel (FAME), methanol is made to react with triglycerides with sodium methanolate or potassium hydroxide as catalyst. The process starts with two phases il at the bottom and methanol floating on top There is a period with a single phase igh concentration of partial glycerides Finally, a glycerol phase settles at the bottom The alkalinity is concentrated in the glycerol phase Because polarity of glycerol favors presence of ions? 3 November 2015 World Congress on ils & Fats and 19

20 Which catalytic intermediate when? In the beginning, the excess of methanol favors the methoxy anion but energy arguments favour the enolate anion With FAME formation, the methanol concentration decreases and hydroxyl groups in partial glycerides start playing a role This favours the glycerolate anion At the end, the alkalinity in the glycerol phase is present as glycerolate anion In the organic phase, the high concentration of fatty acid moieties causes the alkalinity to stem mainly from enolate 3 November 2015 World Congress on ils & Fats and 20

21 Conclusions to be drawn 1. There are several catalytic anionic intermediates methoxy, glycerolate, enolate, hydroxy, carboxy 2. They are in equilibrium with each other, for instance glycerolate + FA in TAG partial glyceride + enolate 3. The equilibrium position is determined by free energy difference of the intermediates relative concentrations 4. Interesterification reaction favours the enolate anion As demonstrated by the formation of FFA on catalyst inactivation by the addition of water 3 November 2015 World Congress on ils & Fats and 21

22 Industrial interesterification (current practice) Interesterification is carried out as a batch process in a vessel with heating coils, agitator and vacuum connection The oil or oil blend to be interesterified must be neutral but need not be bleached Its acidity is measured and enough caustic soda is added to neutralize this acidity The batch is heated to some C Vacuum is applied to dry the batch When the piping connecting the vessel to vacuum is no longer warm, the batch is dry 3 November 2015 World Congress on ils & Fats and 22

23 Industrial interesterification (former practice) A continuous process has been operated using a sodium glycerolate catalyst Dissolve sodium hydroxide in a mixture of glycerol and water. Add the solution to preheated (150 C) oil Spray dry mixture in vacuo Pump dry mixture through coil with few minutes residence time to refining stage Suitable for large production volumes Avoids yield loss resulting from FAME 3 November 2015 World Congress on ils & Fats and 23

24 Industrial interesterification (catalyst and yield loss) Sodium methanolate (methoxide) is commonly used as interesterification catalyst A neutral and dry batch does not need more than 0.05 wt% catalyst (0.5 kg/tonne of oil) but most people use more A low catalyst dosage leads to a low concentration of catalytic intermediate and reduces by-product formation, especially in second order reactions (ketone formation?) Using more catalyst is costly since yield loss (FAME and FFA) is proportional to catalyst loading Yield loss can be the most important cost factor 3 November 2015 World Congress on ils & Fats and 24

25 Ways to reduce yield loss Use sodium (or potassium) glycerolate instead of sodium methanolate as catalyst Avoids the formation of FAME Anhydrous catalyst inactivation avoids the formation of FFA Glacial acetic acid has been reported in literature But what about: Fatty acids that will be removed during deodorization Concentrated phosphoric acid? Acid activated bleaching earth? 3 November 2015 World Congress on ils & Fats and 25

26 Experimental support Interesterification with large excess of catalyst Take 50 g soya bean oil; measure FFA Dry under vacuum and add 0.5 g sodium methanolate (1%) eat under vacuum to 100 C Inactivate with 5 g glacial acetic acid Wash with water until neutral Measure FFA in interesterified oil Amount of FFA formed: = 0.70 % Equivalent amount of 1% NaC : 54 = 5.22 % FFA Far less FFA formation by anhydrous inactivation 3 November 2015 World Congress on ils & Fats and 26

27 There are other catalysts besides the bases mentioned earlier Water can be used at high temperatures Causes hydrolysis and subsequent esterification when pressure is released Metal salts have been mentioned (Van Loon) Are also used during manufacture of polyester polymer from DMT and ethylene glycol Mechanism not well-understood Then there are enzymes Especially those produced by genetically modified micro-organisms 3 November 2015 World Congress on ils & Fats and 27

28 Enzymatic interesterification using 1,3-specific lipase enzymes The process was developed to produce cocoa butter equivalents, fats with a high content of symmetric monounsaturated triglycerides Therefore, the starting material is an oil/fat with a high oleic acid content at the 2-position igh oleic acid sunflower seed oil Shea butter olein And exchanging the fatty acid at the 1,3-position with a saturated fatty acid like stearic acid 3 November 2015 World Congress on ils & Fats and 28

29 Fundamental problems of (stereospecific) enzymatic interesterification The first step in enzymatic interesterification is the hydrolysis of a triglyceride Too much water causes extensive hydrolysis Too little water diminishes catalytic activity The 1,3-specific hydrolysis produces a 1,2- or 2,3-diglyceride These diglycerides isomerize to the more stable 1,3-diglycerides Loss of stereospecificity 3 November 2015 World Congress on ils & Fats and 29

30 Mechanisms of loss of specificity (former proposal) S - - S + S S S - S S S + S Enthalpy difference between α and β positions is some 4 kj/mol Mechanism requires 8 consecutive steps All steps are reversible S S + S S S 3 November 2015 World Congress on ils & Fats and 30

31 Mechanism of loss of specificity (current proposal) + S S - S S S S S + S Involves fewer steps than previous mechanism Therefore more likely Both mechanisms operate probably simultaneously 3 November 2015 World Congress on ils & Fats and 31

32 There are also kinetic problems This is the reaction that we would like: S 4 8 S 4 8 S This is the reaction we get S S 4 8 S 4 S 4 S S nly one out of four triglycerides is SS 3 November 2015 World Congress on ils & Fats and 32

33 Solutions to the problems The kinetic problem can be alleviated by shifting the equilibria by using a large excess of stearic acid (or its methyl ester) This necessitates substantive purification The hydrolytic problem and the loss of stereospecificity can be alleviated by using a large amount of rather dry catalyst This is costly when the enzyme loses its activity Sadly enough, in Europe, enzymatic CBE is not allowed in chocolate 3 November 2015 World Congress on ils & Fats and 33

34 Positioning of modification processes There are several modification processes that change the physical properties of oils and fats Blending; simple, cheap and limited in scope ydrogenation converts liquid oil into solid fat Makes oil more stable; expensive; forms trans isomers Interesterification changes physical properties Product properties depend only on fatty acid composition Fractionation generates two fractions Desirable fraction may have useless byproduct Byproduct may be used in interesterification blend 3 November 2015 World Congress on ils & Fats and 34

35 Modification processes Neutral oil Fractionation Bleaching Chemical interesterification ydrogenation Bleaching Deodorisation Fully refined modified oil Bleaching Deodorisation Deacidification Enzymatic interesterification 3 November

36 Industrial enzymatic interesterification The process is continuous Four reactors with immobilized enzyme in series The first reactor acts as guard to protect the others Changing from one type to another leads to the production of unspecified material Can be reused based on its fatty acid composition The raw material should not inactivate the enzyme Therefore the raw material has to be neutralized, bleached, deodorized and deacidified Using RBD palm oil is ideal since its neutralisation by physical refining comprises bleaching and deodorisation 3 November 2015 World Congress on ils & Fats and 36

37 Cost comparison chemical / enzymatic (Kellens and Calliauw, il modification processes, in Edible il Processing, amm, amilton, Calliauw (eds), Wiley-Blackwell, 2013, Table 6.9, pp ) Cost element Chemical Enzymatic Usage/ tonne Cost ($/tonne) Usage/ tonne Cost ($/tonne) Labour Steam 150 kg kg 0.30 Electricity 15 kwh kwh 0.60 Catalyst/Enzyme 1 kg kg Citric acid 0.5 kg 3.56 Bleaching earth 5 kg 3.30 il losses 18 kg kg 0.51 Total cost, incl. Capital cost and maintenance 3 November 2015 World Congress on ils & Fats and

38 Assumptions underlying the cost comparison Chemical Enzymatic Plant capacity (tonnes/day Plant capacity (tonnes/year) Capital investment (US$) Equipment and engineering Structural works Installation Total investment Annual maintenance (US$) November 2015 World Congress on ils & Fats and 38

39 Comments on cost comparison The authors assume 1 kg catalyst per tonne of oil instead of the perfectly viable 0.5 kg/tonne This would reduce catalyst cost by $ 1.25/tonne and the oil loss by $ 4.80/tonne The authors include the post-treatment in the cost of the chemical process but do not include any pre- or post-treatment in the enzymatic process costs This makes a major difference because the treatment preceding the enzymatic process is extensive The published cost comparison is biased in favour of the enzymatic process 3 November 2015 World Congress on ils & Fats and 39

40 Towards a more impartial comparison of current interesterification processes The enzymatic process is continuous and product changes are awkward Large amount of oil involved in rinsing the reactors Really only suitable for single-product installations The chemical process can be batch or continuous Batch process is mandatory in multi-product situation In continuous process, no cross-contamination on change-over The chemical process is cheaper than the enzymatic process Especially when raw materials are chemically refined 3 November 2015 World Congress on ils & Fats and 40

41 Look at the future Based on insight into the chemistry of the chemical process, improvements are possible Yield loss is largest cost element Avoiding FAME formation improves yield Use catalyst made from glycerol and Na/K Also reduces sensitivity to water Avoiding FFA formation improves yield Anhydrous catalyst inactivation Strong acid, cationic ion exchange resin? 3 November 2015 World Congress on ils & Fats and 41

42 Take home messages (the same as this morning) Understanding the chemistry of our processes can lead to unexpected process improvements This also holds for established processes Understanding originates from questioning established truths They can be myths that people got used to Biased reporting will eventually be exposed Then it may seriously backfire 3 November 2015 World Congress on ils & Fats and 42

43 DIXI (I have spoken) 3 November 2015 World Congress on ils & Fats and 31st ISF Lecture Series, Rosario 43