Optimization of the oxirane ring opening reaction in biolubricant base oil production

Jumat Salimon; Bashar Mudhaffar Abdullah; Nadia Salih

doi:10.1016/j.arabjc.2011.11.002

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Original article

9 (

2_suppl

); S1053-S1058

doi:

10.1016/j.arabjc.2011.11.002

Optimization of the oxirane ring opening reaction in biolubricant base oil production

Jumat Salimon^⁎, Bashar Mudhaffar Abdullah, Nadia Salih

School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

⁎Corresponding author. Tel.: +60 3 8921 5412, fax: +60 3 8921 5410. jumat@ukm.my (Jumat Salimon)

Received: 2011-3-16, Accepted: 2011-11-22, Published: 2011-11-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

This study has successfully optimized the conversion of monoepoxide linoleic acid (MEOA) into biolubricant via oxirane ring opening reaction using oleic acid (OA) with p-toluene sulfonic acid (PTSA) as a catalyst. The four main factors were studied according to a D-optimal design at three levels. These factors were OA/MEOA ratio, PTSA/MEOA ratio, temperature and reaction time. This analysis evidenced the best operating conditions of the oxirane ring opening reaction performed at the following condition; OA/MEOA ratio of 0.30:1 (w/w), PTSA/MEOA ratio of 0.50:1 (w/w), reaction temperature at 110 °C and reaction time at 4.5 h, an optimum yield of 84.61% and OOC of 0.05%. This model results showed a good agreement with the predict value, demonstrating that this methodology may be useful for industrial process optimization.

Keywords

Ring opening

D-optimal design

Optimization

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1 Introduction

The development of new efficient lubricants and environmentally benign pathways, which can lead to new value added products, has still a high potential (Baumann et al., 1988). Furthermore, vegetable oil based products have become more cost competitive with their petroleum derived counterparts as crude petroleum oil prices have increased dramatically in recent years due to a number of geopolitical factors (Moser et al., 2007).

Due to the high reactivity of the oxirane ring, the epoxidation of the double bonds opens up a wide range of feasible reactions that can be carried out under moderate reaction conditions. A variety of chemical modifications of epoxidized vegetable oils and fatty acids are possible through epoxy moiety, and one of the most commonly used is the ring opening reaction (Lozada et al., 2009).

The oxirane ring opening takes place through cleavage of one of the carbon–oxygen bonds. It can be initiated by either electrophiles or nucleophiles, or catalyzed by either acids or bases. The acid catalyzed of an epoxide is a useful procedure for preparing hydroxy ester compounds (Von, 2002). The nucleophilic addition of a carboxyl group to the epoxide center can easily be promoted by protonation using solid acid catalysts (Fig. 1). Nonetheless, the rate of the oxirane ring opening of epoxidized fatty acids strongly depends on the nature and structure of the carboxylic acid (Fig. 2) (Schuster et al., 2008).

Nucleophilic addition of a carboxyl group to the epoxide promoted by protonation using acid catalysts.

Nature and structure of the carboxylic acid toward the oxirane ring opening.

Many nucleophilic reagents are known to add to an oxirane ring, resulting in ring opening (Salimon et al., 2011). These ring-opening reactions could result in branching at the oxirane ring opening (earlier sites of unsaturation in LA). The appropriate branching groups would interfere with the formation of macro-crystalline structures during low-temperature applications and would provide enhanced fluidity to plant oils. TAGs that are hydrogenated to eliminate polyunsaturation will solidify at room temperature as a result of alignment and stacking of adjacent molecules (Hwang and Erhan, 2001).

The aim of the present work is the determination of the feasibility, reliability and best operating conditions for the oxirane ring opening reaction process using D-optimal design on the optimization of the process variables. Fig. 3 demonstrates the scheme for the oxirane ring opening reaction of MEOA.

Oxirane ring opening reaction of MEOA. Notes: 9-10-monoepoxy 12-octadecanoic acid (1a); 12-13-monoepoxy 9-octadecanoic acid (1b); 9-Hydroxy-10-oleioxy-12-octadecanoic acid (1b); 12-hydroxy-13-oleioxy-9-octadecanoic acid (2b); oleic acid (R).

2 Methodology

2.1

2.1 Experimental procedure

The oxirane ring opening reaction was carried out using oleic acid (OA) and p-toluene sulfonic acid (PTSA) as catalyst to prepare 9,12-hydroxy-10,13-oleioxy-12-octadecanoic acid (HYOOA) (Salimon et al., 2011). Table 1 shows the different OA/MEOA ratio, different PTSA/MEOA ratio, different reaction temperature and different reaction time using D-Optimal design. Factors (variables) such as ratio OA/MEOA (w/w, X₁), PTSA/MEOA (w/w, X₂), reaction temperature (°C, X₃) and reaction time (h, X₄) were performed under the same experimental conditions. MEOA (1.55 g, 0.0052 mol) and ratio PTSA/MEOA (0.2:1–0.5:1 w/w) were dissolved in toluene (10 mL) in a 250-mL three-neck flask equipped with a cooler, dropping funnel and thermometer. The mixture was kept at 50 °C. OA/MEOA ratio (0.30:1–0.60:1 w/w) was added during 1.5 h in order to keep the reaction mixture temperature under 70–80 °C. The reaction mixture was subsequently heated to different temperatures 90–110 °C and refluxed at different times 3–6 h at this temperature range. After reaction termination, the heating was stopped and the mixture was left to stand overnight at ambient temperature. The mixture was washed with water and the organic layer was dried over anhydrous sodium sulfate and the solvent was removed using the vacuum evaporator. The oxirane ring content (OOC%), yield % and iodine value (IV mg/g) were measured.

Table 1 Independent variables and their levels for D-optimal design of the oxirane ring opening reaction.

Independent variables		Variable levels
		−1	0	+1
1. OA/MEOA (w/w)	X₁	0.30	0.45	0.60
2. PTSA/MEOA (w/w)	X₂	0.2	0.35	0.5
3. Temperature (°C)	X₃	90	100	110
4. Time (h)	X₄	3	4.5	6

2.2

2.2 Experimental design and statistical analysis

A quadratic polynomial equation by central composite design was developed to predict the response as a function of independent variables and their interaction. In general, the response for the quadratic polynomials is described below:

(1)

Y = β_{0} + \sum β_{i} x_{i} + \sum β_{ii} x_{i}^{2} + \sum \sum β_{ij} X_{i} x_{j}

where β₀; β_i; β_ii and β_ij are constant, linear, square and interaction regression coefficient terms, respectively, and x_i and x_j are independent variables (Razali et al., 2010). Analysis of variance (ANOVA) was applied to estimate the effects of main variables and their potential interaction effects on the OOC% of the biolubricant.

3 Results and discussion

The nucleophilic attack by fatty acid molecules on the oxirane ring of MEOA in the presence of PTSA resulted in the ring opened products 9,12-hydroxy-10,13-oleioxy-12-octadecanoic acid (HYOOA), as shown in Fig. 4. In the first pathway mechanism, the acid reacts with the epoxide to produce a protonated epoxide and finally alcohol compound form by nucleophilic substitution reaction. OA acts like nucleophiles during the acid-catalyzed epoxy ring opening reaction. This modification proposes that for one oxirane ring present in the MEOA, one ester and one hydroxyl functional groups will generate in the molecule.

Mechanism for the MEOA ring opening, 10 or 13-hydroxy (a); 9 or 12-hydroxy (b).

Optimization study of the oxirane ring opening using D-optimal design took place in the presence of OA by using PTSA as a catalyst. The design is used to obtain 25 design points within the whole range of four factors for experiments. The designs and the response OOC% (Y) are given in Table 2 by measuring the yield % and IV mg/g for the lowest OOC% product. To see the impact on the oxirane ring opening by OA reaction, different ratio of OA/MEOA (w/w, X₁), different ratio of PTSA/MEOA (w/w, X₂), different reaction temperature (°C, X₃) and different reaction time (h, X₄) were evaluated Table 2.

Table 2 D-optimal design arrangement and OOC% response of HYOOA.

Run no.	Coded independent variable levels				Response
Run no.	OA^a/MEOA^b (w/w, X₁)	PTSA^c/MEOA (w/w, X₂)	Temperature (°C, X₃)	Time (h, X_h)	OOC^d (%, Y)
1	0.60	0.20	110	4.5	2.60
2	0.60	0.50	110	6	0.90
3	0.30	0.20	90	4.5	3.80
4	0.30	0.50	90	6	3.20
5	0.30	0.20	110	3	3.40
6	0.30	0.50	90	3	3.10
7	0.45	0.50	100	4.5	2.70
8	0.60	0.50	90	4.5	2.40
9	0.30	0.35	100	4.5	2.10
10	0.30	0.50	100	3	1.80
11	0.30	0.20	100	6	1.50
12	0.30	0.50	90	4.5	2.90
13	0.45	0.20	100	4.5	3.05
14	0.30	0.20	90	3	3.50
15	0.30	0.35	110	6	0.60
16	0.60	0.50	110	3	0.80
17	0.45	0.20	110	6	0.40
18	0.60	0.50	110	3	0.30
19	0.60	0.20	90	3	2.95
20	0.45	0.35	100	3	1.40
21	0.45	0.35	100	5.25	0.20
22	0.60	0.20	100	6	0.90
23	0.60	0.20	90	6	0.70
24	0.30	0.50	110	4.5	0.05
25	0.60	0.35	100	4.5	0.60

Notes:

a Oleic acid.

b 9(12)-10(13)-Monoepoxy 12(9)-octadecanoic acid

c p-Toluene sulfonic acid.

d Oxirane oxygen content.

Table 2 illustrates the OOC% effect related with catalyst, temperature and reaction time. As expected, at high temperature, 110 °C, the OOC% shows a great reduction 0.05% compared with the temperatures of 90 and 100 °C. This abrupt reduction on OOC% percent by high temperature (110 °C) shows a high increment on yield of 84.61%, and iodine value of 134.82 mg/g compared with initial iodine value (IV_°) of 66.65 mg/g. At lower temperatures, fairly low oxirane ring reduction (3.80%) was observed at 90 °C for 4.5 h of reaction compared with 110 °C for 4.5 h. At 100 °C, the OOC% shows a smooth reduction (Table 2) during the reaction.

The quadratic regression coefficient obtained by employing a least squares method technique to predict quadratic polynomial models for the OOC% (Y) of HYOOA are given in Table 3. The OOC% of HYOOA (Y), the linear term of PTSA catalyst amount (X₂) and quadratic terms of PTSA catalyst amount (X₂₂) were significant (p < 0.05). The intercept of the reaction was highly significant (p < 0.01).

Table 3 Regression coefficients of the predicted quadratic polynomial model for response variables of the OOC% of HYOOA.

Variables	Coefficients (β), OOC% (Y)	T	P	Notability
Intercept	1.63	5.19	0.0064	^⁎⁎⁎

Linear
X₁	−0.31	1.76	0.2143
X₂	−0.55	5.73	0.0378	^⁎⁎⁎
X₃	−0.11	0.23	0.6395
X₄	−0.064	7.548E-003	0.9325

Quadratic
X₁₁	0.16	0.19	0.6699
X₂₂	1.18	8.1	0.0172	^⁎⁎
X₃₃	−0.078	0.65	0.4377
X₄₄	−1.01	1.47	0.2537

Interaction
X₁₂	0.13	0.51	0.4912
X₁₃	0.14	2.05	0.1830
X₁₄	−0.22	0.40	0.5423
X₂₃	−0.23	5.99	0.0344
X₂₄	1.02	8.22	0.0168
X₃₄	−0.017	8.374E-003	0.9289
R²	0.87

Notes: X₁ = OA/MEOA ratio; X₂ = PTSA/MEOA ratio; X₃ = reaction temperature; X₄ = reaction time, See Table 2 for a description of the abbreviations.

⁎⁎ P < 0.05.

⁎⁎⁎ P < 0.01. T:F test value.

Lack of fit F-value for all the responses showed that the lack of fit is not significant (p > 0.05) relative to the pure error. This indicates that all the models predicted for the responses were adequate. Regression models for data on responses Y were significant (p < 0.05) with satisfactory R². However R² for Y was (0.87), although the model was significant. Table 4 summarizes the analysis of variance (ANOVA) of all the responses of this study.

Table 4 Analysis of variance, showing the effect of the variables as linear, square and interactions on the response OOC% of HYOOA (Y) of the D-optimal design.

Source	Df	Sum of squares	Mean square	F value	P
Mean	1	85.75	85.75
Linear	4	19.89	4.97	5.76	0.0030
2FI	6	8.48	1.41	2.25	0.0987
Quadratic	4	4.28	1.07	2.38	0.1214
Lack-of-fit	10	4.50	0.45
Pure error	25	122.90	4.92

From the experimental design in Table 1, experimental results in Table 2 and Eq. (2) developed a second order polynomial equation (in coded units) that could relate biodiesel yield to the parameters study. The following quadratic model was explained in Eq. (2).

(2)

Y = + 1.63 - 0.31 X_{1} - 0.55 X_{2} - 0.11 X_{3} - 0.064 X_{4} + 0.16 X_{1}^{2} + 1.18 X_{2}^{2} - 0.078 X_{3}^{2} - 1.01 X_{4}^{2} + 0.13 X_{1} X_{2} + 0.14 X_{1} X_{3} - 0.22 X_{1} X_{4} - 0.23 X_{2} X_{3} + 1.02 X_{2} X_{4} - 0.017 X_{3} X_{4}

RSM is one of the best ways of evaluating the relationships between responses, variables and interactions that exist. Significant interaction variables in the fitted models Table 3 were chosen as the axes (OA/MEOA ratio; X₁, PTSA/MEOA ratio; X₂, reaction temperature; X₃ and reaction time; X₄) for the response surface plots. In a contour plot, curves of equal response values are drawn on a plane whose coordinates represent the levels of the independent factors. Therefore, different surface height values enable one to focus attention on the levels of the factors at which changes in the surface height occur (Wu et al., 2008).

Canonical analysis was performed on the predicted quadratic polynomial models to examine the overall shape of the response surface curves and used to characterize the nature of the stationary points. Canonical analysis is a mathematical approach used to locate the stationary point of the response surface and to determine whether it represents a maximum, minimum or saddle point (Wu et al., 2008).

Fig. 5 is the Design-Expert plots for the response (Y). In the oxirane ring opening reaction of HYOOA, performing the technique using high ratio of PTSA/MEOA would give the desired OOC% of HYOOA. The relationships between the parameters and oxirane ring opening of HYOOA were linear or almost linear. Lowest OOC% could be obtained by using high ratio of PTSA/MEOA at high reaction temperature, otherwise; other literatures have used lower PTSA such as (Salimon et al., 2011). Experimental variables should be carefully controlled in order to reduce the OOC% of interest with reasonable yield.

Response surface (a) and contour plots (b) for the effect of the OA/MEOA ratio (X1, w/w) and PTSA. MEOA ratio (X2, w/w) on the OOC% of HYOOA.

Optimum conditions of the experiment to obtain high yield% of HYOOA and lowest OOC% were predicted at ratio of OA/MEOA of 0.30:1 (w/w), ratio of PTSA/MEOA of 0.50:1 (w/w), reaction temperature 110 °C and 4.5 h of reaction time. At this condition, the OOC of HYOOA was 0.05%, yield was 84.61% and IV was 134.82 mg/g. The observed value was reasonably close to the predicted value as shown in Fig. 6.

Predicated vs. actual plot of Y (HYOOA).

4 Conclusion

The process describes a systematic approach to modify chemically the MEOA to yield biolubricant base oil. Preparation was based on the opening of the formed oxirane ring in an appropriate medium using OA. The optimization using D-optimal design of the catalytic nucleophilic addition of OA to the MEOA using different variables was studied. Based on the results obtained, the nucleophilic addition of OA resulted in hydroxy ester.

Acknowledgements

We thank UKM and the Ministry of Higher Education for research grant UKM-GUP-NBT-08-27-113 and UKM-OUP-NBT-29-150/2011.

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