Green synthesis of ester base oil with high viscosity — Part II: Reaction kinetics study

Highlights

• Equal reactivity of functional groups applies to reaction kinetics of complex ester.

• Mutation of reaction order is easily caused by adsorption and high viscosity.

• Activation energy increases with the increase in steric hindrance of complex ester.

• Sn/Zr mixed oxides can reduce the activation energy of esterification reaction.

Abstract

As an eco-friendly lubricant base oil, ester base oil is receiving increasing attention. However, studies on synthetic reaction kinetics of high viscosity complex ester have been rarely reported. In this work, based on the principle of equal reactivity of all functional groups and simplified kinetics models, synthetic reaction kinetics for high viscosity complex ester was investigated in two steps. As for the esterification of trimethylolpropane with glutaric acid, the activation energies of the first and second stage were 55.3 and 73.5 kJ/mol, respectively. As for the esterification of the first step products with 2-ethylhexanoic acid, the activation energies of the first and second stage were 60.6 and 98.2 kJ/mol, respectively. As for the esterification of the first step products with n-heptanoic acid, its activation energy was 68.9 kJ/mol. A mutation phenomenon on reaction order from zero to second order was discovered, which could be explained by the strong adsorption of organic acid on catalyst surface. The conversion rate of carboxyl of synthetic reaction for mixed acid ester was estimated by simulation, and average relative error was less than 3.0%. The synthetic process of high viscosity complex ester was considered to consist of a series of parallel–consecutive reactions with addition–elimination mechanism.

Introduction

In addition to lubrication, lubricating oil has functions of cooling, cleaning, sealing, buffering, corrosion prevention, power transmission, and electrical insulation as well, which can make mechanical devices more efficient, meanwhile, can significantly reduce their friction, wear, and energy consumption (Zulkifli et al., 2013; Pinheiro et al., 2017; Panchal et al., 2017). Therefore, it is widely applied in almost all industrial fields, especially in transportation, manufacturing, and power generation (Zhou and Qu, 2017). High viscosity lubricating oil is usually used in heavy-duty diesel engine, vehicle transmission, large-scale industrial installation, and vehicles which run in the tropics or seriously worn. Commonly, base oil which accounts for 80–99 wt.% of lubricating oil plays a critical role in the basic performance of lubricating oil, therefore, high quality base oil is the key to endowing lubricating oil with excellent performance (Soni and Agarwal, 2014; Gao et al., 2015). With the quality criterion of base oil and environmental regulations becoming strict increasingly, ester base oil has been considered as an excellent substitute for mineral base oil, owing to its flexible structural design, outstanding lubricity, wonderful viscosity–temperature performance, favorable low-temperature fluidity, high thermal and oxidation stability, satisfactory biodegradability, low volatility, good miscibility, and nontoxicity (Seyidov and Mansoori, 2008; Geethanjali et al., 2016; Raghunanan and Narine, 2016). In general, the viscosity of ester base oil cannot be high enough only by using the esterification of polyol with monocarboxylic acid, unless uncommon chemicals or special functional groups are added. Nowicki et al. (2016) synthesized pentaerythritol isostearate and trimethylolpropane dimer isostearate as high viscosity ester base oil, however, the high price of isostearic acid limits the large-scale production of ester base oil. Walter et al. (2018) synthesized a kind of complex ester by introducing dicarboxylic acid into reacting system. By the oligomerization of dicarboxylic acid with polyol, the number of side chain increases with the increase in main chain length, which can enhance the viscosity and viscosity index of ester base oil markedly. In addition, the viscosity of complex ester can be flexibly changed by adjusting the molar ratio of polyol to dicarboxylic acid. Hence, the complex ester has the potential to be a high viscosity ester base oil.

As the catalysts widely used in the industrial production of ester base oil, H₂SO₄ and p-toluenesulfonic acid usually cause excessive by-products, high sulfur content, difficult separation, deepening of oil chromaticity, and poor stability of product (Zhang et al., 2015; Zaccheria et al., 2016). Moreover, the follow-up problems including product emulsification, complex refining processes, and sewage discharge are inevitable in alkali cleaning process (Zhang et al., 2015; Singh et al., 2016). Recently, mixed oxide catalysts have been extensively applied into various esterification reactions, due to fewer by-products, lighter chromaticity, less pollution, easier separation, and more convenient refining (Park et al., 2010; Zaccheria et al., 2016; Singh et al., 2016). In our previous work (Hu et al., 2020), a Sn/Zr mixed oxide catalyst, i.e., PMO-Sn₈Zr-9.0-160-18, was prepared for the synthesis of high viscosity complex ester without adding any templates, surfactants, and sulfur element, which can reduce pollution and avoid introducing sulfur element into ester base oil. The conversion rate of carboxyl, kinematic viscosity, viscosity index, pour point, and chromaticity of the crude complex ester were 97.2%, 156.41 mm²/s (40 °C), 18.57 mm²/s (100 °C), 134, −37 °C, and 0–0.5 (ISO standard), respectively.

To deeply understand reaction process and to guide industrial production of ester base oil with high viscosity, it is meaningful to investigate the synthetic reaction kinetics of complex ester. Up to now, there have been numerous synthetic reaction kinetics studies on the ester with low viscosity and molecule weight. Kopyshev et al. (2015) studied the esterification kinetics of pentaerythritol with different monocarboxylic acids, and calculated the formation rates of mono-, di-, and tri-ester. However, they could not give activation energy data because of increasing steric hindrance. Wang et al. (2008) investigated the esterification kinetics of low-concentration naphthenic acids with methanol in mineral base oil (100 °C, 5.4 mm²/s) using SnO as a catalyst, and found the experiment data following the rule of second-order reaction with an activation energy of 104.2 kJ/mol. Hoo and Abdullah (2015) focused on the esterification kinetics of glycerol and lauric acid in a molar ratio of 1:1, and proposed a composite catalytic mechanism which combined nucleophilic substitution mechanism with Langmuir–Hinshelwood mechanism. Nevertheless, very few synthetic reaction kinetics studies have been reported on the complex ester with high viscosity, because the qualitative and quantitative analysis of complex ester with high viscosity are difficult problems for researchers, and are also still absent in the published references. Firstly, the intermediates and final products of complex ester with high viscosity are extremely complex and numerous, which are difficult to be identified in detail; secondly, complex ester with high viscosity is a high boiling mixture whose boiling range exceeds the highest detection temperature of available analytical instruments, such as GC, GC–MS, etc.; thirdly, the intermediates which are rich in hydroxyl are thermosensitive substances. Furthermore, as reactions proceed, the applicability of kinetics model will be gradually reduced with the increase of steric hindrance of products. Therefore, traditional kinetics methods which need to detect the concentrations of reactants and intermediates are not suitable for this complex reacting system. Instead, determining concentration of hydroxyl or carboxyl by neutralization titration is the most common method to acquire the reaction extent of complex esterification reaction, especially for the synthetic reaction of complex ester with high viscosity. Hence, it is necessary to develop a convenient and simple investigation method for the synthetic reaction kinetics of complex ester with high viscosity.

Fortunately, by investigating a large amount of kinetics of linear polyesterification reaction catalyzed by p-toluenesulfonic acid, the Nobelist Flory (1939, 1953) concluded that the reaction rate of polyester synthesis is not significantly affected by either increase in molecular weight or the concurrent increase in viscosity, and he proposed the principle of equal reactivity of all functional groups. Therefore, it is the concentration of carboxyl that should be focused and measured for complex esterification reaction, rather than the concentrations of reactants and intermediates. In this work, complex ester as a high viscosity ester base oil was synthesized in two steps. The first step is a heterogeneous three-dimensional (3D) oligomeric reaction, and the second step is a heterogeneous non-polymerization reaction, which are different from the homogeneous linear polyesterification reaction. Hence, the principle of equal reactivity of all functional groups should be flexibly applied, and be combined with simplified kinetics models reasonably.