Properties of a High Hydrolysis Stable Nitrogenous Benzo-heterocyclic Borates

Tian Ying; Lyu Ya

(East China Uniνersity of Science and Technology, Shanghai 200237)

Abstract: A benzotriazole-containing derivative was synthesized via the Mannich reaction. The structure was characterized by NMR spectroscopy. By utilizing computational chemistry and molecular simulation, the calculation and mapping of atomic charge and frontier molecular orbitals with complex structure of the borate were carried out. The acid number of BTBE (nitrogen-containing heterocyclic borate) was determined by the standard test method for acid number and the open observation method. The results showed that BTBE had a low initial acid number, because the acid number was changed only after 72 hours of hydrolysis, and it was negative after the hydrolysis reached 120 hours. The extreme-pressure friction resistance of BTBE and other four base oils was also determined. For 5 kinds of base oils, the friction experiment was carried out under a load of 30 kgf, and the BTBE showed a lowest wear scar diameter (0.365 mm) along with a highest maximum non-seizure load. The results showed that the EP friction resistance of BTBE was stronger than that of other four base oils under the same test conditions. The results of hydrolysis performance showed that the synthesized nitrogencontaining heterocyclic borate had a high resistance to hydrolysis, while demonstrating broad application prospects.

Key words: borate; Mannich reaction; acid number; hydrolysis; EP friction resistance.

1 Introduction

Borate is a kind of synthetic ester base oil with good application prospects. It is environmentally friendly, nonpolluting, and biodegradable and has excellent extreme pressure friction resistance[1]. Nevertheless, the ester base oils have special structures, since they contain ester groups. When they come into contact with water, they are more prone to hydrolysis than the mineral oils, causing raw material and economic losses and even harming the mechanical equipment.[2-3]An effective way to ameliorate this condition is to introduce various active elements into the borate, such as S, N, P, etc.[4]Since the N atom can form a coordination bond with the electron-deficient B atom through the lone pair of electrons[5], the structural stability is enhanced. The product is green, pollution-free,ashless, and phosphorus-free[6]. Therefore, it is selected to introduce a nitrogen-containing heterocyclic compound into the borate to enhance its hydrolysis resistance.Shen, et al.[7]found that the introduction of reactive nitrogen element in the borate molecule can signi ficantly improve its hydrolytic stability. At the same time, by means of molecular design, several borates containing N or O elements were synthesized, and their hydrolysis resistance was determined. Compared with the common borates, the conclusion is consistent with the design purpose. Li, et al.[8]introduced a benzotriazole group into a borate molecule to prepare a derivative called BTBM,which could be used as an additive for the rapeseed oil.The results veri fied that the BTBM has good EP friction resistance, and can simultaneously inhibit the corrosion of copper strips. Lei, et al.[9]used benzotriazole to react with boric acid, and introduced sulfhydryl groups into the synthesized esters. In this way, she increased the nitrogen content of borates and verified that the target materials have good hydrolytic stability and abrasion resistance.

It is well known that the properties of the materials have a de finite relationship with their composition and bulk state at the molecular level. For this reason, the atomic charge,the optimal configuration, and the frontier orbital map of a series of nitrogen-containing heterocyclic borates(BTBE) were obtained by using the Gaussian 09 software.The results are used to optimize the molecular structures and analyze the hydrolytic stability at the theoretical level. The Mannich reaction was used for synthesizing the BTBE. The introduction of a benzotriazole group and diethanolamine into the borate provides electrons for the boron atom and can simultaneously form a B-N internal coordinate bond, thus enhancing its hydrolytic stability.The Mannich reaction is environmentally friendly and non-polluting. The by-product is water, and its reaction condition is mild. The intermediates produced during the reaction have anti-wear, anti-oxidation, and anti-corrosion properties and are widely used as additives for various types of lubricating oils. Its hydrolytic stability test was adopted to investigate the hydrolysis stability of this borate, which was compared with the base oil widely used in the market to improve its potential application value.

Figure 1 Schematic diagram of the hydrolysis process of triethoxy-boron

Table 1 Net charges of boron atoms (NBO)

2 Experimental

2.1 Molecular simulation of BTBE

2.1.1 Analysis of the hydrolysis amount

Figure 1 is a schematic diagram of the hydrolysis process of triethoxy-boron. The hydrolysis of triethoxy-boron is divided into three steps[15]: Firstly, after being attacked by water, triethyl borate is hydrolyzed to ethanol and diethyl borate (B02), then the diethyl borate continues to hydrolyze to ethanol and monoethyl borate (B03), and finally monoethyl borate is hydrolyzed to boric acid (B00).The DFT algorithm in Gaussian 09 software package was used to optimize the structure and frequency of each product in the hydrolysis process of triethoxy-boron at the molecular level of 6-31*G. The NBO net charges of each atom were calculated by the natural population analysis method[16]. The calculation results are shown in Table 1.Table 1 shows that the charges of the boron atom decrease gradually in the course of hydrolysis. Triethoxy-boron has a highest net charges of NBO equating to 1.605 eV,the number of charges carried by boric acid is the least which is equal to 1.321 eV. There are three ester groups in triethyl borate molecule, which are the most unstable and are prone to hydrolysis. In contrast, boric acid is the most stable hydrolytic product and will not continue to hydrolyze. It can be inferred that the hydrolysis stability of each kind of ester base oil has a speci fic relationship with the amount of the charges of boron atom in its structure: the more unstable the structure of boron ester is, the more charges the boron atom carries, and the closer the amount of the boron atom in the boric acid ester to the boron atom in the boric acid, the stronger the hydrolysis stability is.

It can occur mainly because the charges of boron atom are following the electron de ficiency theory. The less the positive charges of a boron atom, the easier it is to lose electrons, the stronger the stability it is, and it is not easy to be decomposed by water attack.

The structure of BTBE is imported from Gaussian view software for the same optimization calculation as mentioned in the above method (DFT/6-31G*). Table 2 shows that the band charges of a boron atom in BTBE are smaller than those of triethoxy-boron, which shows that the hydrolysis resistance of BTBE is stronger than that of triethoxy-boron in theory. Moreover, the charges of the boron atom in BTBE are very close to those in boric acid,so it can be inferred that the hydrolysis reaction cannot naturally occur. When the reactants rich in active nitrogen combine with boron atoms in boric acid to form BTBE,the nitrogen atoms can provide electrons to the empty p orbitals of boron atoms to form the coordination bonds between B-N, thus enhancing the stability of the structure of BTBE.

Table 2 Analysis of Boron atomic charges

2.1.2 Stereospeci fic blockade of BTBE

The hydrolysis stability of borate is also related to its little steric hindrance effect[17]. When the structure of borate contains a group of sizeable steric hindrance,it can prevent the water molecule from attacking the boron atom and makes it difficult to carry out hydrolysis.Figure 2 shows the molecular configuration of BTBE after molecular structure optimization. The formation of coordination bonds with B atom and N atom is not natural to destroy, so that it can enhance the stability of the structure.

Figure 2 Schematic diagram of the molecular structure after BTBE optimization

Table 3 shows the B-N atomic spacing of BTBE before and after molecular optimization. The distance between the B atom and the No. 1, 2, and 3 N atoms in the optimized molecular structure is reduced to some extent.The B atom and the N atom on the benzene ring are close to each other, which makes the B atom get electrons and not readily react.

Table 3 The distance between the boron atom and each nitrogen atom in BTBE

Figure 3 shows the 3D structure diagram of BTBE before and after optimization. The molecular structure of the optimized stable configuration is more compact than before, and the steric hindrance becomes more signi ficant than before. The long-chain distortion contains N and C makes the B and N atoms close to each other, and the spacing between B and N atoms is shortened. By increasing the steric hindrance effect of the molecule, the B atom is not easy to contact water during the hydrolysis reaction, which effectively hinders the occurrence of hydrolysis reaction.

Figure 3 Molecular structure of BTBE

2.1.3 Frontline track

Figure 4 HOMO orbital map of water molecules

The frontier orbital energy and electron cloud distribution of a series of substances in the hydrolysis of triethyl borate were calculated by DFT/6-31G* method. The first thing in the hydrolysis process of esters is nucleophilic attack[18].The frontier orbital theory suggests that the lower the LUMO energy level, the easier it is for borate molecules to accept electrons and make the reaction readily occur[19]. The HOMO orbitals of water molecules are shown in Figure 4. The LUMO orbitals of triethoxy-boron and BTBE are shown in Figure 4. The region covered by shadow indicates that the probability density of electrons is large. The color of shadow represents the positive or negative phase of the orbital wave function, and the red color indicates the positive phase, while the green color indicates the other.The area of the shaded part indicates that the delocalization space when electrons appear in the frontline orbit is the most active part of the hydrogen evolution reaction.When borate reacts with water, electrons flow out of electron-rich oxygen atoms in water molecules to attack sp2hybrid boron atoms with empty p orbitals and electron deficiency. Therefore, electrons flow out of the HOMO orbitals of water molecules into the LUMO orbitals of borates and combine with OH-to form a molecule of borate to complete the step-by-step reaction. In the HOMO diagram of water molecules, the wave functions positive and negative phase shadows to coincide, and the whole oxygen atom is wrapped in it, which indicates that the positive and negative phase electrons will appear in the HOMO orbital. Green shadows surround most of the LOMO diagrams of triethoxy-boron, and electrons with negative orbital wave function phase are more likely to appear in the LUMO orbitals of triethoxy-boron. It is shown that when they interact with foreign molecules,they are more likely to accept electrons with negative phase in HOMO orbitals and overlap to form bonds through molecular orbitals. Only the negative wave function of the LUMO orbitals of borates is more natural to overlap with the HOMO orbitals of water to achieve electron transfer. Nevertheless, in the LUMO orbital diagram of BTBE, the positive and negative phases of the wave function do not overlap, while the negative wave function region is obviously reduced, and the reaction activity is signi ficantly reduced.

Fukui Kenichi’s frontier orbital theory[20]suggests that electrons can transfer from HOMO and LUMO orbitals and react only when there is a small frontier orbital energy difference between the molecules involved in the chemical reaction. Table 4 shows the LUMO orbitals of reactants and intermediate products calculated by quantum chemistry. The HOMO orbital energy levels of water molecules and the energy level differences(ΔE=ELUMO-EHOMO) are also shown in Table 4.

Figure 5 LUMO orbital map of triethoxy-boron and BTBE molecules

Table 4 The energy difference between LUMO orbital of each molecule and HOMO orbital of water

The smaller the energy difference between HOMO orbitals and LUMO orbitals, the stronger the interaction and the higher the degree of the system’s stability[21]. The frontier orbital energy difference between reactant molecules should be small enough to facilitate the occurrence of the reaction[22]. Table 4 shows the orbital energy difference.The massive front-line orbital energy difference between BTBE and water means that the reaction against water requires certain conditions, such as heating or adding some catalysts. In the same case, hydrolysis is difficult for BTBE. It is proved that the hydrolysis stability of BTBE is higher than that of triethoxy-boron.

2.2 Experimental procedure

The charge and molecular spacing of BTBE have been calculated by molecular simulation software, and it is theoretically inferred that BTBE has strong hydrolysis stability. In order to verify the correctness of the above conclusion, an experiment was designed to determine the hydrolysis resistance of BTBE. Because the carboxylic acid will be produced after hydrolysis of ester base oil, which will lead to the increase of acid number,the determination of acid number change after the coexistence of BTBE and water can clearly explain its ability to resist hydrolysis. Under the same conditions, the longer the hydrolysis time, the smaller the change of acid number would be, which indicates that the ester base oil has higher hydrolysis stability. The steps for determining the change of acid number in the hydrolysis process of BTBE are stated in the following section.

2.2.1 Experimental materials

Benzotriazole was supplied by the Shanghai Chemical Reagent Factory No. 3; Titan supplied the formaldehyde solution, absolute ethanol, and toluene; Sinopharm supplied the glacial acetic acid; Lingfeng supplied diethanolamine. These chemicals were pure analytical reagents. Moreover, the boric acid was an excellent-grade pure reagent, which was supplied by Feida.

2.2.2 Instruments

1H-NMR spectra were obtained using an AVANCE III 400 spectrometer operating at 400 MHz by the use of DMSO.Infrared spectra were recorded on a Nicolet 380 Fourier transform-infrared (FT-IR) spectrometer equipped with a Smart Diamond ATR head, having a resolution of 2 cm-1in the range of 400 cm-1to 4000 cm-1.

The ASTM D2619-09 method “Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method)” was used to detremine the acid number[10].The acid number was recorded on a LPH-802 pH-meter equipped with a composite electrode and a temperature electrode with a resolution of 0.1 mV and a measurement range from -1999 mV to 1999 mV.

The EP friction resistance of borate was recorded on an MS-800 four-ball friction tester equipped with a lever.The steel balls used in the experiments met the standard of ASTM D2266-01(2015), with a diameter of 12.7 mm,and a hardness of 61—65 HRC.

2.2.3 Synthetic method

6 g (0.05 mol) of benzotriazole, 5 mL of glacial acetic acid, and 100 mL of deionized water were successively added to an 100-mL round-bottom flask prior to slowly dropwise adding 10 mL of 36%—40% formaldehyde solution into the flask. The liquid mixture was stirred at room temperature for 0.5 h and was allowed to rest for 1.5 h. The filtered crude product was recrystallized with hot water. The intermediate Mannich base was obtained as a white solid powder with a yield of 92.48%. 7.45 g(0.05 mol) of intermediate Mannich base, 3.405 g of boric acid, 5.25 g of diethanolamine, and a proper amount of toluene were added to a 250-mL flask, and the mixture was re fluxed at 110 ℃ for 5 h and separated by water in a separator. The reaction terminated when the amount of water in the separator was consistent with the theoretical output. A yellowish viscous heavy oily liquid was obtained by vacuum distillation of reactants. The final product yield was 71.60%. Figure 6 shows the synthesis route.

2.2.4 Acid number method

Firstly, 25 g of BTBE and 75 g of deionized water were placed together in a clear 150-mL glass bottle at room temperature, then were kept in contact with a weighed and polished smooth copper strip, while the bottle was sealed with an inertia cork. The whole system was placed in a furnace preheated at a temperature of 93 ℃, making the glass bottle rotate at 5 r/min, while keeping the glass bottle head and tail upside down. The glass bottle was taken out after a hydrolysis reaction time of 48 h, 96 h,144 h, and 192 h, respectively. The product was poured into a liquid separation funnel, while the oil phase and the copper strip were washed repeatedly with distilled water, and the washing solution was poured into the water phase. After dehydration, the standard test method for determining the acid number was performed as described previously[11]. The quality change of the copper strip was measured after washing and drying. All washing fluids were collected and mixed with the aqueous phase to determine the acidity of the aqueous phase at the same time.

Figure 6 Route for synthesis of BTBE

2.2.5 Open observation method

The alkalinity of amino groups in BTBE structure after hydrolysis could affect the end point for determining the acid number. The specific time of boric acid produced by the hydrolysis of BTBE was verified by an open observation method[12]to determine its stability accurately.

The experimental principle is based on the idea showing that the boric acid produced by hydrolysis of borate is insoluble in the paraffin oil and the solid crystal can be precipitated. The end point of the experiment can be determined by observing the precipitation of boric acid.24.8 g of liquid paraffin oil and 0.2 g of deionized water were added into two 100-mL glass bottles, respectively,while 0.25 g of ethyl borate were added dropwise into them at the same time. The bottle with BTBE was shaken properly to disperse the system. The glass bottle was placed in a 75℃ thermostatic box to accelerate the process, and the time was recorded when the liquid in the cup began to become turbid.

2.2.6 Extreme pressure wear resistance experiment

Two kinds of base oils with a viscosity of 34 mm/s2and 92 mm/s2, respectively, trimethylolpropane trioleate, and pentaerythritol oleate[13]with excellent friction resistance were selected as the control of BTBE. The EP friction resistance of five base oils was evaluated by a MS-800 four-ball friction tester. The experiment was carried out at room temperature according to ASTM D2266-01(2015)[14]. The shaft speed of the instrument was 1 200 r/min,while the friction resistance test was conducted for 30 min, and the test time of last non-seizure load (PB)was 30 s. At the end of the experiment, the wear scar diameters (WSD) of the three test balls were measured by a reading microscope with an accuracy of 0.01 mm,and the average value was recorded. A total of these groups of experiments were carried out to record the average value twice and ensure that the diameter error was within 5%.

3 Results and Discussions

3.1 Characterization of BTBE

Figure 7 shows the FTIR spectra of BTBE. The -CH2-groups were observed at a wavenumber of 2876.1 cm-1.The -N=N- group as a particular functional group of benzotriazole was observed at 1 652.7 cm-1. The -NH-group was observed at 1 316.1 cm-1. The -C-O-groups of BTBE were observed at 1 076.2 cm-1. The absorption peak at 749.2 cm-1in the fingerprint region indicates that there was an o-disubstituted benzene group of the structure.

Figure 7 Infrared spectrum of BTBE

Figure 8 shows the1H-NMR spectra of BTBE. The characteristic peak value is as follows:1H NMR(400 MHz, DMSO-d6, 298 K,δ): 8.06 - 7.91 (m, 1H, Ar),7.46 (d,J= 3.1 Hz, 1H, Ar), 7.26 (t,J= 7.5 Hz, 1H, Ar),7.16 (dd,J= 14.8, 7.3 Hz, 1H, Ar), 6.04 (s, 1H, -CH2-NH-CH2-), 5.66 (s, 2H, N-CH2-O), 3.56 - 3.44 (m, 4H,O-CH2-CH2-), and 2.61 (d,J= 79.5 Hz, 4H, N-CH2-CH2).

Figure 8 400 MHz nuclear magnetic resonance spectrum of BTBE

Through the above characterization results, it can be determined that the synthetic material is in good agreement with the expected structure of the target.

3.3 Hydrolysis stability

The hydrolysis degree of BTBE was judged by measuring the change of the acid number after hydrolysis. Figure 9 shows the change of the acid number of BTBE over 120 h. The initial acid number of BTBE determined in the laboratory was 0.230 mgKOH/g. After 48 hours, the change of acid number did not fluctuate much, indicating that BTBE had an specific ability to resist hydrolysis.Until the hydrolysis time reached 72 h, the acid number of BTBE began to decrease. After the hydrolysis time reached 120 h, the acid number decreased to less than 0,because the structure of BTBE contains amino groups,which could make it alkaline after hydrolysis.

Figure 9 Trends in acid number of BTBE during hydrolysis

Table 5 shows the average of three open observation experiments. As a control, the hydrolysis stability time of BTBE was much longer than that of ethyl borate, and it could remain stabile at high temperature for more than 24 hours. It was mainly because B atom in BTBE and N atom in the benzotriazole group could form a kind of internal coordination bond, which could lead to stronger hydrolysis resistance.

Table 5 Experimental results of the open observation

3.4 EP friction resistance

Figure 10 shows the WSD of five different base oils under the same test pressure (294 N). Although the smallest WSD in the four kinds was 0.543 mm of base oil 92, still the WSD of BTBE was equal to 0.365 mm. The results show that BTBE had the best friction resistance among the five kinds of base oils tested thereby.

Figure 11 shows the different last non-seizure loads(PB). For the five base oils under the same conditions,the higher thePBvalue, the better the extreme pressure would be. ThePBof the two base oils (with a viscosity of 34 mm2/s and 92 mm2/s, respectively,) was relatively low because physical methods could extract them, and the original hydrocarbons did not contain active molecules to improve the extreme pressure performance. Also, the polarity of the two ester base oils was quite significant,and the molecules would adhere to the metal surface so that the lubricating property and the extreme pressure abrasion resistance could all be improved[23]. It can be seen from Figure 11 that thePBof BTBE is much higher than the other four base oils. As an ester oil, BTBE has more than a strong polarity and also contains the active elements such as B and N in the molecule. In the process of friction, the Fe-B or Fe-B-C reaction film is formed onto the friction surface so that the direct friction between the metal is prevented. In addition to the B atom, the N atom radius of the active element in the BTBE has a high electronegativity and is very prone to forming hydrogen bonds with the B atoms in the molecule to improve the lateral gravity, so that the abrasion resistance of BTBE is enhanced.

Figure 10 Different wear scar diameter of base oil under the same pressure (294 N)

Figure 11 Last non-seizure load for different base oil

4 Conclusions

Overall, the studies have establish three important discoveries as shown below:

(1) The structure and frequency analysis of BTBE were carried out using the DFT/6-31*G method of the Gaussian 09 software. The calculation results of the net charge of boron atom and the distance between B and N atoms showed that the difficulty of hydrolysis of borate ester could be re flected by the change of charges of boron atom in its structure in a way. The greater the amount of charge is, the more unstable the molecule is, and the closer the quantity of charge is in relation to that of the boron atom in boric acid, and the better the hydrolysis stability of the borate is. The steric hindrance also affects the hydrolysis to a certain extent. The more compact the molecular structure and the smaller the distance between atoms, the more difficult it is for water molecules to contact B atom, so that it is hard to hydrolysis. The most stable conformational feature of BTBE is simulated by calculation. The data showed that the distance between B and N atoms is greatly reduced compared with that before optimization, which confirms the above point of view.With the help of the frontier molecular orbital theory, the reaction is beneficial only when there are small frontier orbital energy gaps between the molecules involved in the chemical reaction. The energy difference between the LUMO orbitals of BTBE and the HOMO orbitals of water is obviously larger than that of triethyl borate,which indicates that BTBE has better hydrolysis stability than triethyl borate in theory.

(2) In practice, the experimental results showed that BTBE has excellent hydrolysis stability. The stabilization time in paraffin oil determined by the open observation method exceeded 24 h at a specific temperature, which was much larger than that of ethyl borate without active nitrogen in the structure. The initial acid number of BTBE was low and changed slowly with an increase in hydrolysis time. Because the nitrogen elements in BTBE make it alkaline after being dissociated, the acid number will be negative after hydrolysis for a speci fied period.

(3) BTBE has good EP friction resistance. Its structure contains active nitrogen which can bind with B atom to forms a dense oxide film onto the metal surface. The WSD of BTBE is smaller than other base oils, and the last non-seizure load is higher than that of the other four kinds of widely used base oils in the market.