- Email: [email protected]
Alkaline-earth metal compounds as alcoholysis catalysts for ester oils synthesis
Applied Catalysis A: General 192 (2000) 23–28
Alkaline-earth metal compounds as alcoholysis catalysts for ester oils synthesis Stanislaw Gryglewicz Wroclaw University of Technology, Institute of Chemistry and Technology of Petroleum and Coal, 50-344 Wroclaw, ul. Gdanska 7/9, Poland Received 27 April 1999; received in revised form 26 July 1999; accepted 26 July 1999
Abstract Some esters of carboxylic acids and polyhydric alcohols are environmental friendly lubricants of superior properties. This work presents an attempt to use alkaline-earth metal compounds as catalysts for alcoholysis reaction in terms of synthesis di(2-ethylhexyl) adipate and an oligomeric ester of neopentyl glycol. Magnesium methoxide, calcium oxide, calcium alkoxides and barium hydroxide appear to be active catalysts for transesterification. The mechanism of catalytic activity of these compounds in alcoholysis reaction has been proposed. Synthesised esters possess suitable physicochemical properties as lubricant oils. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Transesterification; Ester oil; Alkaline catalyst
1. Introduction For proper work of most appliances it is necessary to use an efficient lubrication system. At present the lubricating media are mainly based on mineral oils obtained from petroleum. There are hundreds of thousands tons of used lubricants penetrating into the water and soil. They contribute to environment degradation. One of the recent policies aimed at changing this situation is an endeavour to replace petroleum derived lubricants by environment friendly compositions [1,2]. It was stated that for several technical applications esters of carboxylic acids and polyhydric alcohols are excellent substitutes for mineral oils [3,4,5]. Esters are related to natural oils as far as their chemical structure is concerned. Till now the limited application of esters as lubricants is affected by their high price. Therefore, it is a need to elaborate a new cheap method of their production.
Esters of polyhydric alcohols and carboxylic acids can be prepared by a direct reaction carried out in the presence of acids as catalyst [6]. However, this method has many drawbacks. Inorganic acids used as the catalysts, especially sulphuric acid, promote undesirable polymerisation reactions whereby the product is brown coloured and it has worse working properties. Moreover, it is very difficult to separate the ester from the catalyst and unreacted carboxylic acids. These impurities are responsible for the corrosivity of the product [7] and they can catalyse the hydrolysis reaction of the obtained esters [8]. The synthesis of esters by transesterification does not have the above mentioned drawbacks [9]. In this case the feedstocks are appropriately structured polyhydric alcohols and esters of carboxylic acids. Methyl esters are mainly used since the methoxide ion is a small group and thus, for steric reasons, a labile one. Acids, alkaline metal alkoxides, hydroxides and
0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 3 3 7 - 3
24
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
2.2. Solubility of catalysts in methanol and 2-ethylhexanol
Scheme 1.
carbonates, non-ionic bases as well as enzymes can serve as catalysts for this reaction [10]. In particular, a great interest is focused on immobilised guanidines, which can be used as heterogeneous catalyst, including in continuous processes. The mechanism of basic alcoholysis, catalysed by alkoxide ion [11] is presented in Scheme 1. The commonly used alkoxides of alkaline metals are inadvisable because of difficulties in separating them from highly viscous esters. Moreover, a high concentration of alkoxide ions may catalyse the undesirable Claisen condensation reaction. Taking into account the above factors, in this work an attempt to prepare the esters component of lubricating media by alcoholysis in the presence of alkaline earth metal compounds, was made. According to the fact that alkoxides of alkaline-earth metal are insoluble in organic solvents [12], they can be used as heterogeneous catalysts. For comparison the catalytic activity of metallic aluminium has been determined.
Approximately 1 g of a tested catalyst was added to 25 ml of alcohol and the mixture was maintained at the boiling point of alcohol for 3 h. Then after cooling down, the liquid was separated from the settling by means of a centrifuge. To hydrolyse the remaining compounds, 0.2 ml of water was added to 5 ml of the obtained extract and the whole was evaporated to dryness by raising the temperature gradually to 200◦ C. The solubility of catalyst in the alcohols expressed as hydroxide was determined. 2.3. Method of synthesising di(2-ethylhexyl) adipate 0.1 mole of dimethyl adipate (Aldrich), 0.2 mole of 2-ethylhexanol (Aldrich), 1.0% of a catalyst and 50 ml of isooctane (Merck) were placed in 500 ml capacity flask. The flask’s mouth was closed by Dean-Stark cap in order to carry out the azeotropic distillation (methanol–isooctane). The temperature has been raised to boiling point of reaction mixture. At that moment the formed methanol was continuously removed. The quantity of the methanol removed was exploited to measure the conversion of the reaction. When the process was over, the synthesised ester was separated from the catalyst by filtration. The unreacted substrates were separated by distillation at the temperature of 150◦ C under the pressure of 5 mm Hg. The reaction is illustrated in Scheme 2.
2. Experimental 2.1. Reagents Mg, Mg(OH)2 , MgO, Ca, Ca(OH)2 ,CaO, Ba(OH)2 , Al were of trade grade (POCH Gliwice, Poland). Mg(CH3 O)2 , Ca(CH3 O)2 , Ca(C8 H17 O)2 were synthesised by a direct reaction of appropriate metals with alcohols. In the case of the synthesis of Mg(CH3 O)2 , it was necessary to carry out the reaction in the presence of minute quantities of iodine as catalyst. The solubilization of metals in alcohols was carried out in glass flask under reflux condenser at the temperature about 68◦ C. The approximate rate of the solubilization of magnesium and calcium in 2-ethylhexanol and neopentyl glycol was also assessed in this experiment.
2.4. Method of synthesising ester of neopentyl glycol The synthesis was conducted similarly as for di(2-ethylhexyl) adipate. The substrates in the reaction were: 0.1 mole of neopentyl glycol and 0.2 mole of dimethyl adipate. The reaction is illustrated in Scheme 3.
Scheme 2.
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
25
Scheme 3.
Some experiments were done for the determination of catalytic activity of the following substances: Mg, Mg(OH)2 , MgO, Mg(CH3 O)2 , Ca, Ca(OH)2 , CaO, Ca(CH3 O)2 , Ca(C8 H17 O)2 , Ba(OH)2 . For comparison, a feasibility study of the use of metallic aluminium activated by a minute addition of mercuric chloride HgCl2 was undertaken. Molecular weight of synthesised esters was determined by Knauer Pressure Osmometer. Chloroform was used as a solvent.
3. Results Results of the trial syntheses of di(2-ethylhexyl) adipate and oligomeric ester of neopentyl glycol are illustrated in Figs. 1 and 2.
Fig. 2. Catalytic reactivity of alkaline-earth metals compounds in the synthesis of oligomeric ester of neopentyl glycol by alcoholysis.
The synthesis of both esters is catalysed effectively by Mg(CH3 O)2 , CaO, Ca(CH3 O)2 , Ca(C8 H17 O)2 , Ba(OH)2 . Moreover, the synthesis of di(2-ethylhexyl) adipate is catalysed by calcium and aluminium pulverised to grains below 0.2 mm in diameter. Their activity manifests itself after a relatively long time of inertia. Furthermore, metallic aluminium is active only after an activator addition (1 mg HgCl2 ). Metallic magnesium did not catalyse the investigated reactions. The physicochemical characteristic of prepared esters are given in Table 1. It was found that it is possible to obtain esters with low content of acidic compounds by alcoholysis in the presence of alkaline catalysts. Moreover, these esters possess low pour temperature and high viscosity indices. Table 1 Physicochemical properties of di(2-ethylhexyl) adipate and oligomeric ester of neopentyl glycol Properties
Fig. 1. Catalytic reactivity of calcium, aluminium and alkaline-earth metal compounds in the synthesis of di(2-ethylhexyl) adipate by alcoholysis.
Di(2-ethylhexyl) Ester of neopentyl adipate glycol
Viscosity [cSt], 40◦ C 8.43 2.35 Viscosity [cSt], 100◦ C Viscosity index, VI 90.05 Acid value [mg KOH/g] 0.1 Pour point [◦ C] −76 Molecular weight [u]a 370 Degree of oligomerisation – a
Experimental data.
46.1 7.6 129.0 0.1 −53 637 2.16
26
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
Table 2 Solubility of alkaline-earth metal compounds in methanol and 2-ethylhexanol Compound
Mg(OH)2 MgO Mg(CH3 O)2 Ca(OH)2 CaO Ca(CH3 O)2 Ca(C8 H17 O)2 Ba(OH)2 a
Solubility (%) Methanol
2-ethylhexanol
0.016 0.100 1.53 0.010 0.035 0.040 – 1.172
0.012 0.060 0.042 0.014 0.016 0.074 1.362a 0.016
Colloidal solution.
In fact, the prepared ester of neopentyl glycol is a mixture of components of different oligomerisation degree. This gives a great possibility to control physicochemical properties of prepared esters by change of substrates ratio. It will be a subject of a further study. The solubility of the catalysts in methanol and 2-ethylhexanol is given in Table 2. It has been found that anhydrous barium hydroxide and magnesium methoxide are relatively well soluble in methanol. Calcium 2-ethylhexanolate forms a suspension in 2-ethylhexanol with the concentration of about 1.36 wt.%. All other tested catalysts show low solubility in methanol as well as in 2-ethylhexanol. Test results for the solubility of metallic magnesium, calcium and aluminium specimens in methanol, 2-ethylhexanol and neopentyl glycol are given in Table 3. Magnesium, calcium and aluminium are diluted in methanol at a relatively fast rate. However, in the case of magnesium it is necessary to use a reaction activator in the form of minute quantities of iodine. Furthermore, magnesium does not react with 2-ethylhexanol in the experimental conditions, even in the presence of Table 3 Solubility of metallic magnesium, calcium and aluminium in alcohols Alcohol
Methanol 2-Ethylhexanol Neopentyl glycol a b
Time of dissolution (h) Mg
Ca
Al
2a
1 4 No
2b 4b No
No No
In the presence of I2 . In the presence of HgCl2 .
activator. Aluminium reacts with both methanol and 2-ethylhexanol but only in the presence of mercuric chloride(II). Magnesium, calcium and aluminium are not affected by the action of neopentyl glycol.
4. Discussion 4.1. Basic character of alkaline-earth metal compounds The basicity of alkaline-earth metal compounds, described by general formula Me(OH)2 increases in the order Mg(OH)2 < Ca(OH)2 < Ba(OH)2 , following the radii of metal cations which increase in this direction and their decreasing electronegativity [13]. According to the Lewis theory, alkaline-earth metal oxides with formula MeO are also bases, which exist, in an non-aqueous medium. Two free electron pairs associated with oxygen atoms result in a higher alkalinity of the oxides than that of the corresponding hydroxides. Thus the alkaline-earth metal oxides and hydroxides can be ordered according to their basicity as follows: BaO > CaO > MgO and MeO > Me(OH)2 . Undoubtedly, alkaline-earth metal alkoxides, which are stable in a non-aqueous medium, are strong bases and nucleophilic reagents. Magnesium, calcium and aluminium are not bases but reducers. They can form bases, i.e. alkoxides with alcohols. Taking into account these facts, the observed catalytic reactivity of alkaline-earth metal compounds in alcoholysis can be related to their basicity (Table 4). The alkalinity of a given compound is a chief factor, which determines its catalytic activity in alcoholysis reaction but not the only one. Alkaline-earth metal compounds are heterogeneous catalysts and the degree of their dispersion in the reaction system has a considerable bearing on the level of their catalytic activity thus limiting the diffusion aspects of the reaction. Table 4 Catalytic reactivity of alkaline-earth metal compounds in alcoholysis relating to their basicity Increase of basicity ↓
→Increase of basicity Mg(OH)2 a MgOa
a b
Not reactive. Reactive.
Ca(OH)2 a CaOb
Ba(OH)2 b –
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
27
An alkoxide can be formed also through the exchange of alcohol from another alkoxide which, as a matter of fact, is an alcoholysis reaction as well [11]. MeR1 O + R2 OH ↔ MeR2 O + R1 OH
The above observations are general in a character. In many cases, the course of alcoholysis reaction is determined by factors difficult to foresee. The catalytic activity of particular catalysts is described in the next paragraph using as an example of the reaction of dimethyl adipate with 2-ethylhexanol leading to the formation of di(2-ethylhexyl) adipate. Similar description can be made for the synthesis of an oligomeric ester of neopentyl glycol.
Scheme 4.
4.2. Mechanism of alkaline-earth metal compounds catalytic activity The classic mechanism of alcoholysis assumes an attack of an alkoxide anion on the trigonal hybridised carbon atom in the ester group converting it to a tetrahedral intermediate. However, the considered alkaline-earth metal alkoxides are very slightly soluble in alcohols, except magnesium methoxide in methanol. Therefore, it seems, that alcoholysis can be catalysed not only by free alkoxylate ions but also by solid alkoxylate which can be regarded as adducts of a strong alkoxylate ion base and a metal cation acid. The proposed mechanism of alcoholysis catalysed by undissociated alkoxylates is showed in Scheme 4. It can be postulated that appropriate alkaline-earth metal alkoxides are the real catalysts in the investigated alcoholysis reactions. An alkoxide can be introduced directly into the reaction system. Actually, alkoxides have been found to be highly catalytically active in such cases. An appropriate alkoxide can be formed from a relevant metal and alcohol if the reaction proceeds at a possibly fast rate. It is well known that hydroxides of strong alkaline metals coexist in equilibrium with alkoxides. At present, such reactions are regarded as a standard method of alkoxides production [12]. Alkoxides may also be formed in a metal oxide–alcohol reaction. Such a reaction for calcium oxide was discovered at the beginning of this century [14,15]. It proceeds in the condition when very fine and freshly roasted calcium oxide is used. CaO + CH3 OH ↔ Ca(OH)CH3 O
(2)
(1)
4.3. Catalytic activity of tested catalysts 4.3.1. Magnesium Magnesium does not catalyse directly the alcoholysis reaction. Test has shown that magnesium is not reactive towards 2-ethylhexanol, therefore, this reaction could not have been a source of catalytically active alkoxide. 4.3.2. Calcium Pulverised calcium is an active catalyst of alcoholysis. There is a characteristic delay in catalytic activity because an appropriate concentration of calcium 2-ethylhexanolate must be reached. The latter can be formed in two different ways according to the reactions given below. Ca + 2C8 H17 OH → Ca(C8 H17 O)2 + H2 ↑
(3)
Ca + 2CH3 OH → Ca(CH3 O)2 + H2 ↑
(4)
Ca(CH3 O)2 + C8 H17 OH ↔ Ca(C8 H17 O)CH3 O + CH3 OH
(5)
The process is of autocatalytic features, which underscores the role of calcium methoxide in the process. Calcium reacts slowly with 2-ethylhexanol. This reaction is a source of increasing quantities of methanol in the reactor. Methanol reacts intensively with calcium producing calcium methoxide from which calcium 2-ethylhexanolate is formed. The increasing quantities of calcium 2-ethylhexanolate accelerate dramatically, after a time, the rate of alcoholysis leading to the synthesis of di(2-ethylhexyl) adipiate.
28
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
4.3.3. Aluminium It is necessary to introduce minute quantities of mercuric chloride(II) into reactor which initiates the reaction of aluminium with 2-ethylhexanolate and methanol. Without a suitable activator aluminium does not react with alcohol because of the passivating action of the oxide film on the surface of this metal. 4.3.4. Magnesium, calcium and barium hydroxides Only barium hydroxide is an active catalyst for alcoholysis reaction. In comparison with magnesium and calcium hydroxides, the Ba–(OH)2 bonds have most ionic character, being strongly polarised. Barium alkoxides can be formed in the reactions: Ba(OH)2 +C8 H17 OH↔Ba(OH)C8 H17 O+H2 O
(6)
Ba(OH)2 + CH3 OH ↔ Ba(OH)CH3 O + H2 O
(7)
Ba(OH)CH3 O + C8 H17 OH ↔ Ba(OH)C8 H17 O + CH3 OH
(8)
The good solubility and the high degree of dissociation of barium hydroxide in methanol are beneficial to the last two reactions. Unfortunately, barium hydroxide is a poisonous substance which limits its practical application. 4.3.5. Magnesium and calcium oxides Magnesium oxide, in which the covalent bonds have a high contribution, does not react with 2-ethylhexanol towards the formation of magnesium 2-ethylhexanolate. It is quite different in the case of more basic calcium oxide, according to the following reaction: CaO + C8 H17 OH ↔ Ca(OH)C8 H17 O
(9)
4.3.6. Calcium 2-ethylhexanolate, magnesium and calcium methoxylates Calcium 2-ethylhexanolate introduced into the reaction mixture catalyses directly the process of alcoholysis. Both magnesium and calcium methoxylates when reacting with 2-ethylhexanolate are a source of active catalysts, i.e. magnesium 2-ethylhexanolate and calcium 2-ethylhexanolate. The higher activity of magnesium methoxylate in comparison with that of calcium methoxylate is
probably due to the formation of high-concentration colloidal solutions in 2-ethylhexanol by magnesium methoxylate. As a result, a large active surface is developed in the reaction. 5. Conclusions This study showed that some alkaline-earth metal compounds, i.e. Mg(CH3 O)2 , CaO, Ca(CH3 O)2 , Ba(OH)2 appear to be active heterogeneous catalysts for alcoholysis reactions. The alkalinity of a given compound is a chief factor which determines its catalytic activity in alcoholysis. Moreover, the synthesis of di(2-ethylhexyl) adipate is catalysed by metallic calcium and aluminium. These metals reacts with 2-ethylhexanol forming alkoxides which are real catalysts for alcoholysis. Using pulverised CaO as catalyst, di(2-ethylhexyl) adipate and oligomeric ester of neopentyl glycol were synthesised. These esters possess excellent physicochemical properties as lubricant oils. References [1] T. Mathiesen, Proc. 10th Int. Colloquium, Tribology, Solving, Friction and Wear Problems,Technische Akademie Esslingen, 1996. [2] S.C. Harold, Proc. 9th Int. Colloquium, Ecological and Economical Aspect of Tribology, Technische Akademie Esslingen, 1994. [3] R.L. Shubkin, Synthetic Lubricants and High-Performance Functional Fluids, Marcel Dekker, New York, 1993. [4] G. Fiscaro, S. Fattori, J. Synth. Lubr. 10 (1993) 237. [5] S.J. Randles, M. Wright, J. Synth. Lubr. 9 (1992) 145. [6] J. Koskikallio, Alcoholysis, acidolysis and redistribution of esters, in: Saul Patai (Ed.), The Chemistry of Carboxylic Acids and Esters, Wiley, New York, 1969. [7] S. Gryglewicz, E. Beran, R. Janik, M. Steininger, J. Synth. Lubr. 13 (1996) 337. [8] S. Boyde, Proc. 11th Int. Colloquium, Industrial and Automotive Lubrication, Technische Akademie Esslingen, 1998. [9] J. Otera, Chem. Rev. 93 (1993) 1449. [10] U. Schuchardt, R. Sercheli, R.M. Vargas, J. Braz. Chem. Soc. 9 (1998) 199. [11] R.T. Morrison, R.N. Boyd, Organic Chemistry, Allyn and Bacon, Boston, 1973. [12] D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Metal Alkoxides, Academic Press, London, 1978. [13] L. Pauling, P. Pauling, Chemistry, Freeman, San Francisco, 1975. [14] C. Weise, B. Neuberg, Biochem. Z. 9 (1908) 537. [15] B. Berner, Ber. 71 (1938) 2015.
Alkaline-earth metal compounds as alcoholysis catalysts for ester oils synthesis Stanislaw Gryglewicz Wroclaw University of Technology, Institute of Chemistry and Technology of Petroleum and Coal, 50-344 Wroclaw, ul. Gdanska 7/9, Poland Received 27 April 1999; received in revised form 26 July 1999; accepted 26 July 1999
Abstract Some esters of carboxylic acids and polyhydric alcohols are environmental friendly lubricants of superior properties. This work presents an attempt to use alkaline-earth metal compounds as catalysts for alcoholysis reaction in terms of synthesis di(2-ethylhexyl) adipate and an oligomeric ester of neopentyl glycol. Magnesium methoxide, calcium oxide, calcium alkoxides and barium hydroxide appear to be active catalysts for transesterification. The mechanism of catalytic activity of these compounds in alcoholysis reaction has been proposed. Synthesised esters possess suitable physicochemical properties as lubricant oils. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Transesterification; Ester oil; Alkaline catalyst
1. Introduction For proper work of most appliances it is necessary to use an efficient lubrication system. At present the lubricating media are mainly based on mineral oils obtained from petroleum. There are hundreds of thousands tons of used lubricants penetrating into the water and soil. They contribute to environment degradation. One of the recent policies aimed at changing this situation is an endeavour to replace petroleum derived lubricants by environment friendly compositions [1,2]. It was stated that for several technical applications esters of carboxylic acids and polyhydric alcohols are excellent substitutes for mineral oils [3,4,5]. Esters are related to natural oils as far as their chemical structure is concerned. Till now the limited application of esters as lubricants is affected by their high price. Therefore, it is a need to elaborate a new cheap method of their production.
Esters of polyhydric alcohols and carboxylic acids can be prepared by a direct reaction carried out in the presence of acids as catalyst [6]. However, this method has many drawbacks. Inorganic acids used as the catalysts, especially sulphuric acid, promote undesirable polymerisation reactions whereby the product is brown coloured and it has worse working properties. Moreover, it is very difficult to separate the ester from the catalyst and unreacted carboxylic acids. These impurities are responsible for the corrosivity of the product [7] and they can catalyse the hydrolysis reaction of the obtained esters [8]. The synthesis of esters by transesterification does not have the above mentioned drawbacks [9]. In this case the feedstocks are appropriately structured polyhydric alcohols and esters of carboxylic acids. Methyl esters are mainly used since the methoxide ion is a small group and thus, for steric reasons, a labile one. Acids, alkaline metal alkoxides, hydroxides and
0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 3 3 7 - 3
24
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
2.2. Solubility of catalysts in methanol and 2-ethylhexanol
Scheme 1.
carbonates, non-ionic bases as well as enzymes can serve as catalysts for this reaction [10]. In particular, a great interest is focused on immobilised guanidines, which can be used as heterogeneous catalyst, including in continuous processes. The mechanism of basic alcoholysis, catalysed by alkoxide ion [11] is presented in Scheme 1. The commonly used alkoxides of alkaline metals are inadvisable because of difficulties in separating them from highly viscous esters. Moreover, a high concentration of alkoxide ions may catalyse the undesirable Claisen condensation reaction. Taking into account the above factors, in this work an attempt to prepare the esters component of lubricating media by alcoholysis in the presence of alkaline earth metal compounds, was made. According to the fact that alkoxides of alkaline-earth metal are insoluble in organic solvents [12], they can be used as heterogeneous catalysts. For comparison the catalytic activity of metallic aluminium has been determined.
Approximately 1 g of a tested catalyst was added to 25 ml of alcohol and the mixture was maintained at the boiling point of alcohol for 3 h. Then after cooling down, the liquid was separated from the settling by means of a centrifuge. To hydrolyse the remaining compounds, 0.2 ml of water was added to 5 ml of the obtained extract and the whole was evaporated to dryness by raising the temperature gradually to 200◦ C. The solubility of catalyst in the alcohols expressed as hydroxide was determined. 2.3. Method of synthesising di(2-ethylhexyl) adipate 0.1 mole of dimethyl adipate (Aldrich), 0.2 mole of 2-ethylhexanol (Aldrich), 1.0% of a catalyst and 50 ml of isooctane (Merck) were placed in 500 ml capacity flask. The flask’s mouth was closed by Dean-Stark cap in order to carry out the azeotropic distillation (methanol–isooctane). The temperature has been raised to boiling point of reaction mixture. At that moment the formed methanol was continuously removed. The quantity of the methanol removed was exploited to measure the conversion of the reaction. When the process was over, the synthesised ester was separated from the catalyst by filtration. The unreacted substrates were separated by distillation at the temperature of 150◦ C under the pressure of 5 mm Hg. The reaction is illustrated in Scheme 2.
2. Experimental 2.1. Reagents Mg, Mg(OH)2 , MgO, Ca, Ca(OH)2 ,CaO, Ba(OH)2 , Al were of trade grade (POCH Gliwice, Poland). Mg(CH3 O)2 , Ca(CH3 O)2 , Ca(C8 H17 O)2 were synthesised by a direct reaction of appropriate metals with alcohols. In the case of the synthesis of Mg(CH3 O)2 , it was necessary to carry out the reaction in the presence of minute quantities of iodine as catalyst. The solubilization of metals in alcohols was carried out in glass flask under reflux condenser at the temperature about 68◦ C. The approximate rate of the solubilization of magnesium and calcium in 2-ethylhexanol and neopentyl glycol was also assessed in this experiment.
2.4. Method of synthesising ester of neopentyl glycol The synthesis was conducted similarly as for di(2-ethylhexyl) adipate. The substrates in the reaction were: 0.1 mole of neopentyl glycol and 0.2 mole of dimethyl adipate. The reaction is illustrated in Scheme 3.
Scheme 2.
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
25
Scheme 3.
Some experiments were done for the determination of catalytic activity of the following substances: Mg, Mg(OH)2 , MgO, Mg(CH3 O)2 , Ca, Ca(OH)2 , CaO, Ca(CH3 O)2 , Ca(C8 H17 O)2 , Ba(OH)2 . For comparison, a feasibility study of the use of metallic aluminium activated by a minute addition of mercuric chloride HgCl2 was undertaken. Molecular weight of synthesised esters was determined by Knauer Pressure Osmometer. Chloroform was used as a solvent.
3. Results Results of the trial syntheses of di(2-ethylhexyl) adipate and oligomeric ester of neopentyl glycol are illustrated in Figs. 1 and 2.
Fig. 2. Catalytic reactivity of alkaline-earth metals compounds in the synthesis of oligomeric ester of neopentyl glycol by alcoholysis.
The synthesis of both esters is catalysed effectively by Mg(CH3 O)2 , CaO, Ca(CH3 O)2 , Ca(C8 H17 O)2 , Ba(OH)2 . Moreover, the synthesis of di(2-ethylhexyl) adipate is catalysed by calcium and aluminium pulverised to grains below 0.2 mm in diameter. Their activity manifests itself after a relatively long time of inertia. Furthermore, metallic aluminium is active only after an activator addition (1 mg HgCl2 ). Metallic magnesium did not catalyse the investigated reactions. The physicochemical characteristic of prepared esters are given in Table 1. It was found that it is possible to obtain esters with low content of acidic compounds by alcoholysis in the presence of alkaline catalysts. Moreover, these esters possess low pour temperature and high viscosity indices. Table 1 Physicochemical properties of di(2-ethylhexyl) adipate and oligomeric ester of neopentyl glycol Properties
Fig. 1. Catalytic reactivity of calcium, aluminium and alkaline-earth metal compounds in the synthesis of di(2-ethylhexyl) adipate by alcoholysis.
Di(2-ethylhexyl) Ester of neopentyl adipate glycol
Viscosity [cSt], 40◦ C 8.43 2.35 Viscosity [cSt], 100◦ C Viscosity index, VI 90.05 Acid value [mg KOH/g] 0.1 Pour point [◦ C] −76 Molecular weight [u]a 370 Degree of oligomerisation – a
Experimental data.
46.1 7.6 129.0 0.1 −53 637 2.16
26
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
Table 2 Solubility of alkaline-earth metal compounds in methanol and 2-ethylhexanol Compound
Mg(OH)2 MgO Mg(CH3 O)2 Ca(OH)2 CaO Ca(CH3 O)2 Ca(C8 H17 O)2 Ba(OH)2 a
Solubility (%) Methanol
2-ethylhexanol
0.016 0.100 1.53 0.010 0.035 0.040 – 1.172
0.012 0.060 0.042 0.014 0.016 0.074 1.362a 0.016
Colloidal solution.
In fact, the prepared ester of neopentyl glycol is a mixture of components of different oligomerisation degree. This gives a great possibility to control physicochemical properties of prepared esters by change of substrates ratio. It will be a subject of a further study. The solubility of the catalysts in methanol and 2-ethylhexanol is given in Table 2. It has been found that anhydrous barium hydroxide and magnesium methoxide are relatively well soluble in methanol. Calcium 2-ethylhexanolate forms a suspension in 2-ethylhexanol with the concentration of about 1.36 wt.%. All other tested catalysts show low solubility in methanol as well as in 2-ethylhexanol. Test results for the solubility of metallic magnesium, calcium and aluminium specimens in methanol, 2-ethylhexanol and neopentyl glycol are given in Table 3. Magnesium, calcium and aluminium are diluted in methanol at a relatively fast rate. However, in the case of magnesium it is necessary to use a reaction activator in the form of minute quantities of iodine. Furthermore, magnesium does not react with 2-ethylhexanol in the experimental conditions, even in the presence of Table 3 Solubility of metallic magnesium, calcium and aluminium in alcohols Alcohol
Methanol 2-Ethylhexanol Neopentyl glycol a b
Time of dissolution (h) Mg
Ca
Al
2a
1 4 No
2b 4b No
No No
In the presence of I2 . In the presence of HgCl2 .
activator. Aluminium reacts with both methanol and 2-ethylhexanol but only in the presence of mercuric chloride(II). Magnesium, calcium and aluminium are not affected by the action of neopentyl glycol.
4. Discussion 4.1. Basic character of alkaline-earth metal compounds The basicity of alkaline-earth metal compounds, described by general formula Me(OH)2 increases in the order Mg(OH)2 < Ca(OH)2 < Ba(OH)2 , following the radii of metal cations which increase in this direction and their decreasing electronegativity [13]. According to the Lewis theory, alkaline-earth metal oxides with formula MeO are also bases, which exist, in an non-aqueous medium. Two free electron pairs associated with oxygen atoms result in a higher alkalinity of the oxides than that of the corresponding hydroxides. Thus the alkaline-earth metal oxides and hydroxides can be ordered according to their basicity as follows: BaO > CaO > MgO and MeO > Me(OH)2 . Undoubtedly, alkaline-earth metal alkoxides, which are stable in a non-aqueous medium, are strong bases and nucleophilic reagents. Magnesium, calcium and aluminium are not bases but reducers. They can form bases, i.e. alkoxides with alcohols. Taking into account these facts, the observed catalytic reactivity of alkaline-earth metal compounds in alcoholysis can be related to their basicity (Table 4). The alkalinity of a given compound is a chief factor, which determines its catalytic activity in alcoholysis reaction but not the only one. Alkaline-earth metal compounds are heterogeneous catalysts and the degree of their dispersion in the reaction system has a considerable bearing on the level of their catalytic activity thus limiting the diffusion aspects of the reaction. Table 4 Catalytic reactivity of alkaline-earth metal compounds in alcoholysis relating to their basicity Increase of basicity ↓
→Increase of basicity Mg(OH)2 a MgOa
a b
Not reactive. Reactive.
Ca(OH)2 a CaOb
Ba(OH)2 b –
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
27
An alkoxide can be formed also through the exchange of alcohol from another alkoxide which, as a matter of fact, is an alcoholysis reaction as well [11]. MeR1 O + R2 OH ↔ MeR2 O + R1 OH
The above observations are general in a character. In many cases, the course of alcoholysis reaction is determined by factors difficult to foresee. The catalytic activity of particular catalysts is described in the next paragraph using as an example of the reaction of dimethyl adipate with 2-ethylhexanol leading to the formation of di(2-ethylhexyl) adipate. Similar description can be made for the synthesis of an oligomeric ester of neopentyl glycol.
Scheme 4.
4.2. Mechanism of alkaline-earth metal compounds catalytic activity The classic mechanism of alcoholysis assumes an attack of an alkoxide anion on the trigonal hybridised carbon atom in the ester group converting it to a tetrahedral intermediate. However, the considered alkaline-earth metal alkoxides are very slightly soluble in alcohols, except magnesium methoxide in methanol. Therefore, it seems, that alcoholysis can be catalysed not only by free alkoxylate ions but also by solid alkoxylate which can be regarded as adducts of a strong alkoxylate ion base and a metal cation acid. The proposed mechanism of alcoholysis catalysed by undissociated alkoxylates is showed in Scheme 4. It can be postulated that appropriate alkaline-earth metal alkoxides are the real catalysts in the investigated alcoholysis reactions. An alkoxide can be introduced directly into the reaction system. Actually, alkoxides have been found to be highly catalytically active in such cases. An appropriate alkoxide can be formed from a relevant metal and alcohol if the reaction proceeds at a possibly fast rate. It is well known that hydroxides of strong alkaline metals coexist in equilibrium with alkoxides. At present, such reactions are regarded as a standard method of alkoxides production [12]. Alkoxides may also be formed in a metal oxide–alcohol reaction. Such a reaction for calcium oxide was discovered at the beginning of this century [14,15]. It proceeds in the condition when very fine and freshly roasted calcium oxide is used. CaO + CH3 OH ↔ Ca(OH)CH3 O
(2)
(1)
4.3. Catalytic activity of tested catalysts 4.3.1. Magnesium Magnesium does not catalyse directly the alcoholysis reaction. Test has shown that magnesium is not reactive towards 2-ethylhexanol, therefore, this reaction could not have been a source of catalytically active alkoxide. 4.3.2. Calcium Pulverised calcium is an active catalyst of alcoholysis. There is a characteristic delay in catalytic activity because an appropriate concentration of calcium 2-ethylhexanolate must be reached. The latter can be formed in two different ways according to the reactions given below. Ca + 2C8 H17 OH → Ca(C8 H17 O)2 + H2 ↑
(3)
Ca + 2CH3 OH → Ca(CH3 O)2 + H2 ↑
(4)
Ca(CH3 O)2 + C8 H17 OH ↔ Ca(C8 H17 O)CH3 O + CH3 OH
(5)
The process is of autocatalytic features, which underscores the role of calcium methoxide in the process. Calcium reacts slowly with 2-ethylhexanol. This reaction is a source of increasing quantities of methanol in the reactor. Methanol reacts intensively with calcium producing calcium methoxide from which calcium 2-ethylhexanolate is formed. The increasing quantities of calcium 2-ethylhexanolate accelerate dramatically, after a time, the rate of alcoholysis leading to the synthesis of di(2-ethylhexyl) adipiate.
28
S. Gryglewicz / Applied Catalysis A: General 192 (2000) 23–28
4.3.3. Aluminium It is necessary to introduce minute quantities of mercuric chloride(II) into reactor which initiates the reaction of aluminium with 2-ethylhexanolate and methanol. Without a suitable activator aluminium does not react with alcohol because of the passivating action of the oxide film on the surface of this metal. 4.3.4. Magnesium, calcium and barium hydroxides Only barium hydroxide is an active catalyst for alcoholysis reaction. In comparison with magnesium and calcium hydroxides, the Ba–(OH)2 bonds have most ionic character, being strongly polarised. Barium alkoxides can be formed in the reactions: Ba(OH)2 +C8 H17 OH↔Ba(OH)C8 H17 O+H2 O
(6)
Ba(OH)2 + CH3 OH ↔ Ba(OH)CH3 O + H2 O
(7)
Ba(OH)CH3 O + C8 H17 OH ↔ Ba(OH)C8 H17 O + CH3 OH
(8)
The good solubility and the high degree of dissociation of barium hydroxide in methanol are beneficial to the last two reactions. Unfortunately, barium hydroxide is a poisonous substance which limits its practical application. 4.3.5. Magnesium and calcium oxides Magnesium oxide, in which the covalent bonds have a high contribution, does not react with 2-ethylhexanol towards the formation of magnesium 2-ethylhexanolate. It is quite different in the case of more basic calcium oxide, according to the following reaction: CaO + C8 H17 OH ↔ Ca(OH)C8 H17 O
(9)
4.3.6. Calcium 2-ethylhexanolate, magnesium and calcium methoxylates Calcium 2-ethylhexanolate introduced into the reaction mixture catalyses directly the process of alcoholysis. Both magnesium and calcium methoxylates when reacting with 2-ethylhexanolate are a source of active catalysts, i.e. magnesium 2-ethylhexanolate and calcium 2-ethylhexanolate. The higher activity of magnesium methoxylate in comparison with that of calcium methoxylate is
probably due to the formation of high-concentration colloidal solutions in 2-ethylhexanol by magnesium methoxylate. As a result, a large active surface is developed in the reaction. 5. Conclusions This study showed that some alkaline-earth metal compounds, i.e. Mg(CH3 O)2 , CaO, Ca(CH3 O)2 , Ba(OH)2 appear to be active heterogeneous catalysts for alcoholysis reactions. The alkalinity of a given compound is a chief factor which determines its catalytic activity in alcoholysis. Moreover, the synthesis of di(2-ethylhexyl) adipate is catalysed by metallic calcium and aluminium. These metals reacts with 2-ethylhexanol forming alkoxides which are real catalysts for alcoholysis. Using pulverised CaO as catalyst, di(2-ethylhexyl) adipate and oligomeric ester of neopentyl glycol were synthesised. These esters possess excellent physicochemical properties as lubricant oils. References [1] T. Mathiesen, Proc. 10th Int. Colloquium, Tribology, Solving, Friction and Wear Problems,Technische Akademie Esslingen, 1996. [2] S.C. Harold, Proc. 9th Int. Colloquium, Ecological and Economical Aspect of Tribology, Technische Akademie Esslingen, 1994. [3] R.L. Shubkin, Synthetic Lubricants and High-Performance Functional Fluids, Marcel Dekker, New York, 1993. [4] G. Fiscaro, S. Fattori, J. Synth. Lubr. 10 (1993) 237. [5] S.J. Randles, M. Wright, J. Synth. Lubr. 9 (1992) 145. [6] J. Koskikallio, Alcoholysis, acidolysis and redistribution of esters, in: Saul Patai (Ed.), The Chemistry of Carboxylic Acids and Esters, Wiley, New York, 1969. [7] S. Gryglewicz, E. Beran, R. Janik, M. Steininger, J. Synth. Lubr. 13 (1996) 337. [8] S. Boyde, Proc. 11th Int. Colloquium, Industrial and Automotive Lubrication, Technische Akademie Esslingen, 1998. [9] J. Otera, Chem. Rev. 93 (1993) 1449. [10] U. Schuchardt, R. Sercheli, R.M. Vargas, J. Braz. Chem. Soc. 9 (1998) 199. [11] R.T. Morrison, R.N. Boyd, Organic Chemistry, Allyn and Bacon, Boston, 1973. [12] D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Metal Alkoxides, Academic Press, London, 1978. [13] L. Pauling, P. Pauling, Chemistry, Freeman, San Francisco, 1975. [14] C. Weise, B. Neuberg, Biochem. Z. 9 (1908) 537. [15] B. Berner, Ber. 71 (1938) 2015.