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Synthesis of Bio-Lubricant Trimethylolpropane Trioleate and Its Lubricant Base Oil Properties Sen Qiao, Yonggang Shi, Xiaojuan Wang, Zhenxing Lin, and Yunxuan Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00876 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017
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Energy & Fuels
Synthesis of Bio-Lubricant Trimethylolpropane
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Trioleate and Its Lubricant Base Oil Properties
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Sen Qiao 1, Yonggang Shi 1＊, Xiaojuan Wang 2, Zhenxing Lin 2 and Yunxuan Jiang 1
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1 Administrater Center for Instruments &Equipments, Logistical Engineering University,
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Chongqing 401311, PRC.
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2 technic Center, Ningbo Entry-Exit Inspection and Quarentine Bureau, Ningbo 315012, PRC.
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KEYWORDS: trimethylolpropane trioleate, benzene sulfonic acid, oxidation stability, infrared
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spectrometry and PDSC
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ABSTRACT: Trimethylolpropane trioleate (TMPTO) could significantly reduce the
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environmental pollution compared with mineral oils. In this study, oleic acid and
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trimethylolpropane (TMP) were used to synthesize TMPTO, and the synthesis process was
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optimized by single-factor experiments to obtain the highest yield. Firstly, two kinds of operating
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conditions (vacuum distillation and atmospheric reflux) were compared, and then four kinds of
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catalysts were investigated to select the most efficient one in esterification. Afterwards, five
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reaction parameters were investigated, and the optimal conditions obtained were as follows:
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catalyst/TMP as 2% w/w, oleic acid/TMP as 3.2/1(molar ratio), temperature as 180°C, reaction
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time as 3h and vacuum degree as 0.095MPa. Under the optimized conditions, TMP conversion
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rate reached up to 92.8%. Furthermore, TMPTO was confirmed by Fourier transform infrared
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spectroscopy (FTIR). Finally, the typical properties of the product were evaluated respectively,
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including the kinematic viscosity at 40°C and 100°C, viscosity index, solidifying point and
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thermal oxidation stability of TMPTO.
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1. INTRODUCTION
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With the rapid development of modern industry, lubricant consumption increased year by year.
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Most of the traditional lubricants are based on mineral oil, which has poor biodegradability in the
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environment. Recently, the impact of mineral lubricants oils on the environment has become an
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increasingly important issue.1 Worldwide concern about the environmental issues and energy
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savings are encouraging the study of advanced lubricants as a substitute for mineral oil.
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Therefore, this requires us to utilize renewable resources for accelerating the research on
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environmentally friendly lubrications.2, 3 Due to renewable, higher lubricity, viscosity indices,
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shear stability, lower volatility and biodegradable, bio-lubricant has broad development
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prospects.4-7
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Vegetable oils are investigated as a potential source of bio-lubricants because of these
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properties. However, vegetable oil has some drawbacks, such as poor oxidation stability, poor
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hydrolytic instability and poor low-temperature properties. Therefore, bio-lubricants derived
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from branched neopentyl polyols, such as trimethylolpropane (TMP), neopentyl glycol (NPG) or
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pentaerythritol, have received wider acceptance due to its improved performance. Because of the
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saturation level and the presence of branching, the bio-lubricants are modified base oil with good
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oxidative stability and good low temperature properties. In general, the higher the molecule
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degree of branching is, the better its pour point and hydrolytic stability are, but the lower its
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oxidative stability is. Conversely, the higher the saturation of this molecule is, the better its
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oxidation stability is, and the worse its pour point is.8 So, Polyol esters have a good resistance to
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attacks by water molecules.9 Among the three polyols, TMP have a moderate price level,
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relatively low melting point. Hence it is being used extensively to produce environmentally
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friendly lubricants.
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All vegetable oils are triglycerides with a distribution of saturated, monounsaturated, and
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polyunsaturated acids. The components of fatty acids in vegetable oils could affect the stability
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of vegetable oils. Previous study has documented that the oxidative stability potential of
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vegetable oils could be improved with the increase of oleic acid component and the decrease of
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both the linolenic and the linoleic acid components. Besides, oleic acid is economical and easier
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to store than linoleic acid and linolenic acid, so oleic acid has better development value.10
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Trimethylolpropane trioleate (TMPTO) is a chemically modified product of triglyceride.
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Compare with vegetable oil, it has better oxidative stability, better low temperature properties
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and could be used as a green lubricant.11 TMPTO has excellent lubricity and high viscosity
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index. In the ecological environment, the biodegradation rate of TMPTO can exceed 90%.8, 12-14
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TMPTO can be used as environmentally friendly hydraulic oil, chain oil and water yacht engine
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oil. TMPTO is also widely used in cold rolling of steel plate, steel drawing oil and other
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metalworking fluids.15
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Nowadays, TMPTO is synthesized by direct esterification or transesterification. The use of
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catalysts is essential in the TMPTO’s synthesis.16-18 The types of catalysts used in
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esterification/esterification are homogeneous catalysts, heterogeneous catalysts, and enzyme
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catalysts. However, there are some disadvantages of the catalysts, such as long reaction time, low
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reaction rate, large dosage of catalyst. Besides, the separation process is quite troublesome.
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Although heterogeneous catalysts can be recycled easily, they have other shortcomings such as
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deactivation from coking, high costs and the tendency to cause saponification. For enzyme
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catalysts, Enzyme catalysts are an environmentally friendly catalyst, but enzymes are susceptible
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to poisoning and lose activity in esterification which leads to low catalytic efficiency. From the
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above literature work, it is worthwhile to develop a low dosage and high efficiency catalyst for
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bio-lubricant’s synthesis. In this work, attempts were made to have a better understanding of the
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use of a new homogeneous catalyst, which has the advantages of high catalytic efficiency, short
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reaction time, low usage and no deactivation. Also, systematic work were undertaken toward the
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synthesis, characterization, and performance evaluation (mainly physical chemistry index and
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oxidation stability) of TMPTO, bio-lubricant oil. To far as we know, similar work has not been
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done yet.
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2.EXPERIMENTAL SECTION
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2.1 Raw materials and instruments
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Oleic Acid(AR), TMP(AR), Benzene sulfonic acid(CP), Sodium bicarbonate(AR),
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Methylbenzene (CP), Isopropyl Alcohol(AR) were obtained from Tianjin Zhiyuan Chemical
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Reagent Co. Ltd. The main component of oleic acid(AR) is cis-9-octadecenoic acid. Solid Super-
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acid was obtained from Nanjing Jiena Si New Material Co. Ltd. HZSM-5 was obtained from
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Nankai Catalyst Factory. Strongly acidic styrene type cation exchange resin (001×7) was
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obtained from Chengdu Kelong Chemical Reagent Factory.
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Electronic analytical balance (FA2204B) was from Shanghai Tian Mei Balance Instrument
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Co., Ltd. Electric heating magnetic agitator (MS7-H550-S) was from Czech Sino American
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company. Circulating water device multipurpose vacuum pump (SHB-3A) was from Zhengzhou
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Du Fu Instrument Factory. Metrohm potentiometric titrator (877) was from Metrohm China Co.,
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Ltd. Fourier transform infrared spectrometer was Nicolet-6700. Pressure differential scanning
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calorimeter (DSC Q2000) was from TA Instruments. High-speed refrigerated centrifuge (CR
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22GIII) was from Hitachi (China) Co., Ltd.
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2.2 Experimental Design
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The esterification is a reversible reaction. Reaction equation was shown in Fig.2. In the
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presence of catalysts, TMPTO was synthesized through dehydration condensation from oleic
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acid and TMP. Two kind of synthesis methods were utilized. One is vacuum distillation method,
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where the water could be removed by distillation and the reaction progress was monitored by the
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acid value of the reactants. Another is atmospheric refluxing methods, where a water-carrying
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agent, such as benzene or toluene, could form an azeotrope with water to carry out the water.
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Two types of experimental devices are shown in Figure 1, Fig 1(1) is the Atmospheric reflux
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device, Fig 1(2) is the vacuum distillation device.
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2.2.1 Atmospheric refluxing method
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A quantity of oleic acid was added to the three-necked flask and heated to 60°C, then TMP
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was added to the flask at a certain ratio, stirred. After TMP was melted, the acid value at the
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starting point was determined. And then catalyst and water agent (methylbenzene) were added.
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The acid value was measured periodically, and the degree of reaction was calculated by the acid
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value. After a certain period, the heating was stopped, and the mixture was cooled to room
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temperature. The experimental installation is shown in Fig. 1(1).
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2.2.2 Vacuum distillation method
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A quantity of oleic acid (55.2g, 0.196mol) was added to the three-necked flask and heated to
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60°C, then TMP (8.2g, 0.61mol) was added to a three-necked flask, stirred. After TMP melted,
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the acid value at the starting point was determined. Then catalyst was added, the degree of
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vacuum was controlled above 0.09MPa, the temperature was at 180°C, the acid value was
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measured periodically, and the degree of reaction was calculated by the acid value. After a
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certain period, the heating was stopped, and the mixture was cooled to room temperature under
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vacuum. The experimental installation is shown in Fig. 1(2).
(1)
(2)
Fig.1 Two types of experimental devices
Fig.2 Reaction equation of oleic acid and Trimethylolpropane 113
The reaction mixture was washed with hot water at 80°C for 3-4 times in a separating funnel,
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and the aqueous layer was separated and the catalyst was removed from the reaction mixture.
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Then the unreacted oleic acid of the reaction mixture was neutralized with 10% sodium
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carbonate solution. The soaps were removed by a high-speed centrifuge to obtain the
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supernatant. The product was washed once again with hot water, and the aqueous layer was
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removed. The final product was dried with anhydrous calcium chloride and filtered.
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2.2.3 Single-Factor Experiments
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To investigate the synthesis yield of TMPTO from TMP and oleic acid, single-factor tests were
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conducted to find the optimal conditions in the tested ranges, such as catalyst amount (0.5%-
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4.0%), acid-to-alcohol ratio (2.5:1-3.4:1), reaction temperature (120-200°C), reaction time (1-
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7h).
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2.2.3 Statistical Analysis
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All the studies were conducted in triplicate and the results were recorded in the form of mean ±
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SD (standard deviation). Statistical analysis was carried out by Excel 2016 (Microsoft,
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Redmond, WA, USA) and OriginPro 8 (Northampton, MA 01060, USA).
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2.3 Properties of the Product
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2.3.1 Acid value
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The initial acid value and the acid value of the mixture during the reaction are determined
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according to GB/T7304-2014. Potentiometric titration was used to determine the acid value of
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petroleum products. The acid value is calculated as follows:
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Acid value = (V × C × 56.1)/M
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In the formula: V - consumed sodium hydroxide volume (mL); C - concentration of potassium
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hydroxide solution (mol/L); M - mass of sample (g)
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2.3.2 Esterification rate
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The esterification degree was evaluated by the ratio of TMP esterification, which was
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calculated as follows:
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TMP esterification rate% = (1-acid value/initial acid value)×acid-to-alcohol ratio/3×100%
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2.3.3 Fourier transform infrared spectroscopy
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The products were analyzed by the Nicolet-6700 Fourier transform infrared spectroscopy
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(FTIR). The spectrum was obtained on the 400–4000cm-1 for 16 repeated scans at 4cm-1 spectral
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resolution with the liquid film. All the spectra were recorded and processed using the OMNIC
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software.
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2.3.4 Lubricant base oil properties of TMPTO
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The capillary viscometer was used to determine the kinematic viscosity according to GB/T265-
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1988. At a strictly controlled temperature, the time of a given volume of liquid flowing through
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the capillary viscometer under gravity was determined. Viscosity index was determined by
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kinematic viscosity of the specific oil at 40°C and 100°C according to GB/T1995-1998.
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The pour point was measured by GB/T3535-2006. The sample was put in the specified tube
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and cooled to the expected temperature then the test tube was tilted to 45° and hold for one
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minutes. If the liquid surface is not flowing, this temperature is the pour point.
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2.3.5 Thermal oxidation stability analysis
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Pressure differential scanning calorimetry (PDSC) was used to evaluate the thermal oxidation
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stability of lubricating oil by two methods: isothermal method and programmed temperature
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method, respectively. The oxidation stability of the lubricating oil is measured by the oxidation
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onset temperature (OOT).19-21
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The oxidation onset temperature was measured by ASTM E2009 Standard Test Method for
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Oxidation Onset Temperature of Hydrocarbons by Differential Scanning Calorimetry.
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Temperature calibration of the instrument with standard materials indium and tin was performed
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by ASTM D969. The heating rate was 10°C/min. The oxygen pressure is 1.24MPa and the
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temperature is 270°C. OOT was determined by ASTM E1858. The intersection of the baseline
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extrapolated line and the maximum tangent of the oxidation peak rate is defined as OOT.
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3.RESULTS AND DISCUSSION
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3.1 Effect of the catalytic type
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The effects of different catalysts on the esterification rate of the production at two devices
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were investigated under acid-to-alcohol ratio 3.2:1, reaction temperature 140°C and reaction time
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5h. As shown in Figure 3, the catalytic efficiency increased significantly from 18% to 94% as the
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reaction time progressed from 1h to 5h. Apparently, the catalytic efficiency of the vacuum
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distillation reaction apparatus was higher than that of the reflux reaction apparatus. With the
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vacuum distillation reaction apparatus, the reaction temperature could reach 140°C. In the
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refluxing apparatus, the boiling point of water and benzene was 110°C. It was found that the
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higher reaction temperature was helpful for improving the catalytic efficiency because of higher
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molecules effective collisions. Resin catalyst(Atmospheric reflux) Resin catalyst(Vacuum distillation) Solid superacid(Atmospheric reflux) Solid superacid(Vacuum distillation) HZSM-5 Zeolite Catalyst(Atmospheric reflux) HZSM-5Zeolite Catalyst(Vacuum distillation) Benzene sulfonic acid (Atmospheric reflux) Benzene sulfonic acid (Vacuum distillation)
100 90 80
TMP Esterification Rate%
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175 176
1
2
3
4
5
Reaction time/h
Figure 3. Catalytic effects of catalyst types
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The amount of Resin catalyst, HZSM-5 zeolite catalyst and solid super acid was 10% of the
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total mass of oleic acid and TMP. The catalytic efficiency of each catalyst was clearly seen in
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Fig.3. The results showed that the solid super acid and HZSM-5 molecular sieve were better, but
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the TMP esterification rate was lower than 90%. The catalytic efficiency of Resin catalyst 732
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H-type Resin was the lowest due to catalyst 732 H-type maximum temperature of 100°C. After 3
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hours, Resin catalyst 732 H-type became black and failure, the catalytic efficiency decreased
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significantly. Solid superacid and HZSM-5 are both heterogeneous catalyst. For HZSM-5, oleic
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acid could not enter the molecular sieve of HZSM-5 due to its large molecular weight, which led
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to inability of reducing the activation energy of esterification. For solid super acid, during the
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esterification process, the water or steam would contact the surface accelerators (such as SO42-)
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of solid super acid and cause a loss of surface SO42-, resulting in a reduction of acid center
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number on surface and acid strength, which eventually led to a decrease of catalyst activity.
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Benzene sulfonic acid, which is a strong acid, is commonly used to catalyze esterification and
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organic synthesis. The amount of benzene sulfonic acid was 2% of the mass of TMP, the
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catalytic effect was obviously improved than that of the other three catalysts. The esterification
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rate could reach 90% at about 3 hours, and the esterification rate was not obvious after 3h. So,
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the benzene sulfonic acid was chosen as the catalyst.
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3.2 Effect of catalyst amount
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The effect of the amount of catalyst on the esterification reaction was investigated at acid-to-
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alcohol ratio of 3.2: 1, reaction temperature of 140 °C and reaction time of 3h. As shown in
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Fig.4, the TMP esterification rate increased significantly from 46.32% to 96.31% as the catalyst
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amount increased from 0.50% to 4.00% of TMP mass. When the amount of the catalyst reaches
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2.00% of the TMP mass, the esterification rate reached to 91.70%. Although the esterification
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rate kept increasing slowly as the catalyst amount increasing 2.00% to 4.00%, the color of the
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product began to darken by Tab. 1 which indicated the polymerization of oleic acid and other
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reactions. Therefore, the catalyst amount of 2.00% was selected for the subsequent studies.
TMP Esterification Rate 100 90 80
TMP Esterification Rate(%)
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0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Catalyst Amount(%)
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Figure 4. The relationship between the amount of catalyst and TMP esterification rate
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Table 1. The relationship between the amount of catalyst and the color of product catalyst/ %
TMP esterification rate /%
Color
1 2
65.54 91.70
Pale yellow Pale yellow
3
93.66
Reddish brown
4
96.31
Brown black
206 207
Esterification of OA with TMP was a stepwise reaction. Firstly, OA and TMP were esterified
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into monoester (ME). Afterwards, OA and ME were esterified into diester (DE). Finally, OA and
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DE were esterified into TMPTO.22 The reaction was reversible at each step. As seen in Fig 4, the
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TMP esterification rate increased significantly as the catalyst amount increased from 0.50% to
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2.00% of TMP mass. The reason may be that the number of acid sites in the reaction system was
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increasing as the amount of catalyst increasing, leading to an increased in TMP esterification
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rate. When catalyst amount was increased from 2.00% to 4.00%, the catalytic efficiency was
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reduced. There may be two reasons for the reduction. On one hand, excessive amount of catalyst
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might accelerate the rate of reverse reaction. In other words, the rates of reaction from TMPTO
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to DE and from DE to ME were accelerated, which slowed down the increase of TMP
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esterification rate. Secondly, as can be observed in Tab.1, when the amount of catalyst increased
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from 1% to 4%, the color of the product changed from pale yellow to brown black. This
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indicated that excessive acidity could induce side reactions, because OA(C18:1) contains active
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double bond C=C, which was prone to oxidation, polymerization and other side reactions,
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leading to the production of some side reaction products such as dipolymer and tripolymer. 22
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3.3 Effect of acid-to-alcohol ratio
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The effect of molar ratio of oleic acid/alcohol on the esterification rate was investigated under
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conditions as follow: catalyst amount was 2% of TMP, the vacuum degree was 0.095MPa,
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reaction temperature was 140°C and time was 3 h. The experimental results are shown in Fig. 5. TMP esterification rate
100 90 80
TMP esterification rate %
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70 60 50 40 30 20 10 0 2.5:1
226 227
2.8:1
3.0:1
3.2:1
3.4:1
Oleic acid/alcohol Ratio
Figure 5. Relationship between oleic acid/alcohol molar ratio and TMP esterification rate
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As can be seen from Fig.5, the TMP esterification rate increased significantly from 82.50% to
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91.73% as the oleic acid/alcohol increased from 2.5:1 to 3.4:1. When the molar ratio of alcohol
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to acid was 2.5:1, TMP was excessive, the esterification rate of TMP was 82.50%. With the
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increase of the molar mass of oleic acid, the esterification rate of TMP increased gradually.
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When the molar ratio of alcohol to acid was 3.2:1, the esterification rate of TMP is the highest,
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and the amount of oleic acid is increased. When the amount of oleic acid was further increased,
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the esterification rate of TMP began to decrease. Therefore, the molar ratio of alcohol to acid of
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3.2:1 was selected in subsequent studies.
236
From the reaction formula of TMPTO, the theoretical molar ratio of oleic acid to TMP was
237
3:1. When the reaction raw material TMP was excessive, the reaction rate of TMPTO was not
238
high. Because in the reaction of TMP and oleic acid, the production of monoester or disester was
239
increased, and the production of TMPTO was reduced. At first, the TMP esterification rate
240
increased rapidly with the increase of reactant molar ratio. The reason might be that the excess
241
reactant increased the collision of the nucleophilic particles, which facilitated the reaction to the
242
direction of TMPTO formation. However, with the molar ratio further increasing, the TMP
243
esterification rate decreased. The reason may be that when OA content in the reaction system
244
was too high, the concentration of TMP in the system was diluted, and the collision probability
245
of the nucleophilic particle was decreased, thus TMP esterification rate was reduced.
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3.4 Effect of reaction temperature
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Effects of temperature on the reaction were investigated. The experiment was under the
248
following conditions molar ratio of oleic acid / alcohol of 3.2:1, benzene sulfonic acid used as
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the catalyst, the amount of catalyst of 2% of TMP mass, the reaction time of 3h, the vacuum
250
degree of 0.095MPa. The experimental results are shown in Fig.6.
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It can be seen from Fig. 6 that the TMP esterification rate ranged from 87.71% to 92.80% as
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the reaction temperature increased from 120°C to 180°C. When the reaction temperature was
253
120°C, the TMP esterification rate was 87.71%. With the reaction temperature increasing, the
254
TMP esterification rate was increased. The TMP esterification rate was the highest when the
255
temperature was 180°C. When the temperature was increased to 200°C, not only the color of the
256
TMPTO was deepened, which indicated that side reactions such as oleic acid polymerization
257
were likely to occur leading to a decreased esterification rate and an energy waste, but also the
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difficulty of post-processing was increased. Therefore, the reaction temperature of 180°C was
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selected in subsequent studies.
TMP esterification rate% 100 90 80
TMP esterification rate%
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70 60 50 40 30 20 10 0 120
262
160
180
200
temperature/℃
260
261
140
Figure 6. Effect of reaction temperature on TMP esterification rate
3.5 Effect of reaction time
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The effect of reaction time on the esterification rate was investigated under conditions as
264
follow: The molar ratio of oleic acid/alcohol was 3.2:1, the catalyst was benzene sulfonic acid,
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the vacuum degree was 0.095MPa. The acid value of the reactants was measured in each hour to
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calculate the esterification rate. The results are shown in Fig.7.
TMP esterification rate 100 90 80
TMP esterification rate%
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70 60 50 40 30 20 10 0 0
1
2
3
4
5
6
Reaction time/h
267
268
Figure 7. Effect of reaction time on TMP esterification rate
269
It can be seen from Fig.7 that the TMP esterification rate increased significantly from 56.73%
270
to 93.56% as the reaction time increased from 1h to 5h, After 5h, the TMP esterification rate
271
began to decrease, the TMP esterification rate was 90.01% at 6h. The TMP esterification rate
272
increased obviously as the reaction time progressed from 1h to 3h. With the reaction time
273
increasing, TMP esterification rate is not obvious. When the reaction time to 5h later,
274
trimethylolpropane oleate color significantly deepened, side reaction of oxidation occurs. The
275
results could be due to the fact that oleic acid underwent the polymerization and some unknown
276
reactions. From the view of energy saving and optimal yield, the optimal reaction time was fixed
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at 3h.
278
3.6 Structure characterization
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The IR spectrum of the product is shown in Fig 8.
100 90 80
3467.3
70
Transmittance%
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60 50 40 30
3004.5
20 1164.7
10 2931.2
0 4000
3500
3000
2854.1
2500
1743.3
2000
1500
1000
500
-1
280
281
wave number/cm
Figure 8. FT-IR spectrum of trimethylolpropane tiroleate
282
As shown in figure 8, the product could be identified as TMPTO, judged by the following
283
peaks: peak at 3004.5cm-1 is the characteristic peak of -C=C-H CIS stretching vibration; peaks at
284
2931.2 cm-1 and 2854.1 cm-1 are -CH3 and -CH2- stretching vibration absorption peaks; peak at
285
1743.3cm-1 is a typical absorption peak of -C=O stretching vibration; peak at 1164.7 cm-1 is the
286
C-O-C single bond stretching vibration; The characteristic absorption peak of -OH at 3467.3cm-1
287
is very small, which indicates that the product was TMPTO.
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3.7 Physical and chemical properties of lubricant base oil
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When the base oil is not added with any functional additives, the higher the viscosity is, the
290
higher the oil film strength is and the worse the flowability is. As can be seen from Table 2,
291
compared with two commercial oils, the viscosity of TMPTO at 40°C is lower than that of two
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commercial oils. This indicates that TMPTO is suitable for working under low load conditions.
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Also, the viscosity of TMPTO can be increased by tackifier to improve its carrying capacity. The
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viscosity index of TMPTO is obviously better than that of two commercial oils, which shows
295
that TMPTO has better viscosity-temperature characteristics. TMPTO can keep good viscosity
296
under the condition that the temperature changes obviously. Pour points of TMPTO and two
297
commercial oils are basically the same, indicating that the minimum temperature of TMPTO
298
could satisfy the requirements of lubrication oil. In general, synthesized TMPTO could be used
299
as lubricant base oil.
300
Table 2. Physical and chemical properties of lubricant base oil
Properties
TMPTO OA
TULUX T600
Mobil CF-4
CJ-4 15W-40
15W-40
Test Method
0.80
210
0.52
0.55
GB/T73042014
Kinematic viscosity at 49.2 40 ◦C (mm2 /s)
23.4
110
106
GB/T2651988.
Kinematic viscosity at 10.5 100 ◦C (mm2/ s)
3.5
14.8
14.2
GB/T2651988.
Viscosity index
210
--
139
136
GB/T19951998(2004)
Pour point (◦C)
-27
8
-34
-27
GB/T35352006
Total acid vaule (mg KOH/g)
301 302
3.8 Thermal oxidative stability
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The oxidation onset temperature of OA and TMPTO was determined by the program
304
temperature method in an oxygen atmosphere at a heating rate of 10°C/min and an initial furnace
305
pressure of 1.24MPa.
306
The oxidation of the base oil was an exothermic reaction. The base oil was heated in the
307
presence of oxygen, and an exothermic peak appeared on the PDSC curve if the base oil began to
308
oxidize. The lower the initial oxidation temperature of the base oil was, the worse the thermal
309
oxidation stability of the base oil was. It was seen from Fig.9 that the thermal oxidation stability
310
of TMPTO was significantly better than that of oleic acid, since the oxidation onset temperature
311
of oleic acid and TMPTO were 125°C and 142°C, respectively. TMPTO is a polyol ester, and
312
there is no hydrogen on the beta carbon of TMP, so TMPTO has good oxidation stability. oleic
313
acid is a monounsaturated fatty acid, the oxidation initiation temperature of oleic acid is lower
314
than that of TMPTO, therefore the thermal stability of the ester group in TMPTO might be better
315
than that of the carboxyl group in oleic acid.
OA TMPTO
80 70 60
DSC/(W/mg)
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50 40 30 20 10 0
40
316
317
142℃
125℃
-10 60
80
100
120
140
160
180
200
220
240
260
Temperature/℃
Figure 9. OOT curves of OA and TMPTO using program temperature method
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4 CONCLUSIONS
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In this paper, the optimum conditions to synthesize TMPTO using TMP and oleic acid were
320
studied. We compared the two methods of atmospheric reflux and vacuum distillation. We
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selected benzene sulfonic acid as catalyst, and studied the influences of several optimum
322
conditions. The physical and chemical properties and thermal oxidation stability of the
323
synthesized products (TMPTO) were analyzed. Based on the above results and discussions, it
324
was found that benzene sulfonic acid was the best catalyst and the optimal conditions were as
325
follows: oleic acid/TMP: 3.2/1, catalyst: 2% of the TMP mass, temperature at 180°C, reaction
326
time of 3h and vacuum degree at 0.095MPa. The product was identified as TMPTO by FTIR.
327
Under the optimal conditions, the TMP esterification rate was more than 92%. The thermal
328
oxidation stability of TMPTO was better than that of oleic acid by pressure differential scanning
329
calorimetry.
330
The TMPTO synthesized could been used as lubricant base oil. The synthesis method
331
overcome the disadvantages of the traditional method such as overlong reaction time, separation
332
process, high costs, and catalyst deactivation from coking and the production ability could
333
increase more than 90%.
334
AUTHOR INFORMATION
335
Corresponding Author
336
* Tel: +86 023 86730260; E-mail:
[email protected] (Yonggang Shi)
337
Notes
338
Any additional relevant notes should be placed here.
339
ACKNOWLEDGMENT
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The authors thanks CIQ R&D program (grant No. 2017IK171) for its financial support. Thanks
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are also given to my classmates and the staff of Administrater Center for Instruments &
342
Equipments at logistical engineering university for their selflessness help.
343
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