ALKYL IMIDAZOLINES AND THEIR ETHOXYLATED DERIVATIVES AS ANTIOXIDANTS FOR HYDROCARBON PRODUCTS

Alkyl imidazolines are prepared by reacting a polyethylenepolyamine structure (ethylenedi-amine, diethylenetriamine, triethylenetetraam-ine, and tetraethylenheptentamine) and a car-boxylic acid such as tall oil fatty acid or acid cuts [1-7].

In general, in addition to their applications in pharmaceutical industry as anti-bacterial agents, bitumen and asphalt as emulsifiers, detergents, and demulsifiers, alkyl imidazolines have also been reported to function as additives in oil pro-ducts with different applications.

Some of these functions include dispersing agents [8-10], metal inactivators [11], corrosion inhibitors [12-18], antirust agents [19], deter-gents [20-21], anti-gum formation agents [22], and antiabrasive agents [20]. Ethoxylated and propoxylated imidazolines have been used as multipurpose fuel additives [23]. These com-pounds prevent the decomposition of organic compounds accelerated in the presence of a metal. Antioxidants are one of fuel additives. An antioxidant is an organic compound used in rubbers, natural fats, food products, and fuels such as gasoline and diesel [24-25] to prevent and retard the oxidation and decomposition of molecules as well as gum formation. For example, an alkyl imidazoline has been used to prevent explosion. Furthermore, alkyl imidazo-lines have been applied in lubricants for corro-sion and oxidation inhibition [26].

The most pronounced outcome of not using an appropriate additive in petroleum products such as lubricants and different organic-based fuels is their conversion into peroxides and hydroper-oxides, which is also known as auto-oxidation. The acidification of the medium and corrosion and knocking in engines are some of the adverse effects of hydrocarbon degradation and conver-sion to peroxide, the mechanism of which is shown in Figure 1.

The oxidation of hydrocarbon substrates initi-ated by AIBN occurs according to a free radical mechanism shown in Figure 1.

Figure 1:Hydrocarbon degradation

where, r^● is a radical formed from the initiator; RH is a hydrocarbon; R^● and RO₂^● are free alkyl and peroxide radicals respectively; In stands for an inhibitor and In^● represents a radical resulted from the inhibitor [27].

A small amount of an antioxidant is added to prevent or retard the oxidation and formation of explosive peroxide intermediates. Depending on the effectiveness of the antioxidant, the amount of absorbed oxygen increases. Thus, antioxi-dants, which are soluble in hydrocarbon media are favorable.

Ethoxylated alkyl imidazolines have been syn-thesized and used as fuel antioxidants and their performance is evaluated in this work.

EXPERIMENTAL

Instrumentation and Chemical

Cumene was distilled over Na in a nitrogen atmosphere for 2 hours to obtain a fraction with a boiling point of 152 °C. Azobisisobutyronitrile (AIBN) was purchased from Fluka. Zinc dialky-dithiophosphoric acid (ZDDP) was prepared from Lubrizol Company. The imidazoline used herein was supplied by a local chemical factory.

Oxidation Rate Measurement

The apparatus used was previously described elsewhere [28]. In order to evaluate the oxidation rate, the following general method was applied (the proper amount of the chemi-cals needed for each experiment will be de-scribed later in each case). 25 ml of the testing sample was dissolved in purified cumene and 0.35 g of AIBN was added to a double-wall borosilicate reactor. Then the reactor was im-mediately purged with pure oxygen gas under 1 atmospheric pressure to empty out the any gases in the reactor. In this case, the oxidation reaction starts as soon as the temperature of the flask reaches 60 °C and since then the internal pressure of the reactor is measured as the indication of the amount of consumed oxygen in the reaction. The initial reactor pressure made as a result of the released heat of the reaction should be reduced by adjusting the mercuric manometer; another manometer filled with a chemical having a lower density such as dibutyl phthalate is applied. The measured pressures are corrected by the application of a conversion factor against the relative densities of dibutyl phthalate to mercury.

Synthesis

Ethoxylated alkyl imidazolines are prepared by reacting alkyl imidazolines and ethylene oxide using a base catalyst at a known temperature and pressure (Figure 2).

R=C₁₂-C₁₈

x=1-3

y=4, 8, 12, 18

Figure 2: Ethoxylated alkyl imidazolines

Evaluation

To evaluate the antioxidant performance, the amount of oxygen consumption and oxidation retardation time in isopropyl benzene (cumene) media in the presence of Azobisisobutyronitrile (AIBN) as a radical initiator are measured. Cumene is converted into cumene hydroper-oxide in this process (Figure 3). The influence of ZDDP and ethoxylated imidazolines on the oxidation retardation and peroxide formation is subsequently studied.

Figure 3: Isopropyl benzene(cumene) oxidizied to cumen hydroperoxide

A typical experiment of evaluation procedure is described as follows. The antioxidant agent and AIBN (0.35 g) were dissolved in pure cumene in a two-wall borosilicate reactor [28]. After purging the reactor with oxygen, the oxygen consumption rate was measured at atmospheric pressure at 60 °C by the difference of initial and final pressures of O₂. The effectiveness of ethoxylated imidazoline as antioxidant was expressed in terms of oxygen consumption as shown in Table 1 and Figures 4-9.

Table 1: Effectiveness of ethoxylated imidazoline as antioxidants in terms of oxygen consumption

As observed in Figures 4-9, oxygen is completely used at a retardation time of zero in the absence of any antioxidant and its consumption rate varies depending on antioxidant structure. For example, Lubrizol ZDDP, as a standard antioxi-dant, gives the same retardation time as alkyl imidazoline, while ethoxylated alkyl imidazolines of up to 12 EO increases this to 52 minutes, which is an appreciable enhancement.

Figure 4: Oxygen consumption as a function of time using 0.001 moles alkyl imidazoline in cumene oxidation (25 ml) in the presence of 0.35 g of AIBN at 60 °C

Figure 5: Oxygen consumption as a function of time using 0.001 moles Lubrizole ZDDP in cumene oxidation (25 ml) in the presence of 0.35 g of AIBN at 60 °C

Figure 6: Oxygen consumption as a function of time using 0.001 moles ethoxylated (4 ethylene oxide units) alkyl imidazoline in cumene oxidation (25 ml) in the presence of 0.35 g of AIBN at 60 °C

Figure 7: Oxygen consumption as a function of time using 0.001 moles ethoxylated (8 ethylene oxide units) alkyl imidazoline in cumene oxidation (25 ml) in the presence of 0.35 g of AIBN at 60 °C

Figure 8: Oxygen consumption as a function of time using 0.001 moles ethoxylated (12 ethylene oxide units) alkyl imidazoline in cumene oxidation (25 ml) in the presence of 0.35 g of AIBN at 60 °C

Figure 9: Oxygen consumption as a function of time using 0.001 moles ethoxylated (18 ethylene oxide units) alkyl imidazoline in cumene oxidation (25 ml) in the presence of 0.35 g of AIBN at 60 °C