Preparation of Flame-Resistant Liquids Based on Mixed Tri(phenyl, p-tert-butylphenyl) Phosphates by Transesterification of Triphenyl Phosphate with p-tert-Butylphenol

Article abstract of DOI:10.1134/S1070427220080170

Abstract: The possibility of controlling the composition of a mixture of triphenyl phosphate, p-tert-butylphenyl diphenyl phosphate, di(p-tert-butylphenyl)phenyl phosphate, and tri(p-tert-butylphenyl) phosphate, formed by transesterification of triphenyl phosphate with p-tert-butylphenol, was demonstrated. The amount of p-tert-butylphenol necessary for transesterification of triphenyl phosphate to yield a mixture of phosphates of required composition was determined. If necessary, the composition of the phosphates can be adjusted by selective distillation of triphenyl phosphate in a vacuum.

Full text of DOI:10.1134/S1070427220080170

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2020, Vol. 93, No. 8, pp. 1237–1243. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Prikladnoi Khimii, 2020, Vol. 93, No. 8, pp. 1181–1187.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Preparation of Flame-Resistant Liquids Based on Mixed
Tri(phenyl, p-tert-butylphenyl) Phosphates by Transesterification
of Triphenyl Phosphate with p-tert-Butylphenol
Yu. G. Noskova, T. E. Krona, O. G. Karchevskayaa, and G. A. Korneevaa,*
a Joint Research and Development Center, Moscow, 119333 Russia
*e-mail: KorneevaGA@rn-rdc.ru
Received November 13, 2019; revised April 5, 2020; accepted April 17, 2020
Abstract—The possibility of controlling the composition of a mixture of triphenyl phosphate, p-tert-butylphenyl
diphenyl phosphate, di(p-tert-butylphenyl)phenyl phosphate, and tri(p-tert-butylphenyl) phosphate, formed
by transesterification of triphenyl phosphate with p-tert-butylphenol, was demonstrated. The amount of p-tert-
butylphenol necessary for transesterification of triphenyl phosphate to yield a mixture of phosphates of required
composition was determined. If necessary, the composition of the phosphates can be adjusted by selective distillation
of triphenyl phosphate in a vacuum.
Keywords: transesterification, triphenyl phosphate, p-tert-butylphenol, flame-resistant liquids
DOI: 10.1134/S1070427220080170
Low-toxic biodegradable flame-resistant oil based
on mixed p-tert-butylphenyl phenyl phosphates is wide-
ly used as an additive to polymers and lubricants; it is
the base of the majority of liquids for hydraulic systems
and working lubricating liquids, ensuring accident-free
operation of turbines with high parameters of the work-
ing medium, including power facilities of nuclear power
plants [1–3].
Synthetic flame-resistant butylated oil is prepared
using a mixture of phenol and p-tert-butylphenol (BP).
The reaction yields an equilibrium mixture of esters 1–4
(Scheme 1) with statistical distribution of components,
usually containing 15–30% 1 (in some cases, up to 50%)
[1–3, 10].
The quantitative composition of a mixture of esters
1–4 strongly influences the physicochemical and opera-
tion properties of the mixture (viscosity–temperature
characteristics, pour point, lubricating properties, resist-
ance to hydrolysis and oxidation, deaeration, etc.) and
its toxicological characteristics. The composition is usu-
ally optimized to impart to the oil the properties meeting
the specific requirements.
Wide range of functionalized phosphates, phospho-
nates, and phosphinates can be prepared by catalytic es-
terification of compounds containing P(O)–OH groups
with diaryliodonium triflates [4], with phenols in the
presence of silicon chlorides as catalysts [5], and when
using as phosphorylating agents white phosphorus [6],
phosphorus pentoxide [7], and phosphorylpyridine de-
rivatives [8]. Singh and Nolan [9] reported preparation
of phosphates by transesterification in the presence of
N-heterocyclic carbenes. However, the process that is
widely used for commercial synthesis of triaryl phos-
phates is based on the reaction of phenols with phospho-
rus oxychloride:
The major components of the oil used for turbine
systems are unsymmetrical esters 2 and 3 at minimal
content of symmetrical phosphates 1 and 4 [1–3, 10].
This is due, in particular, to the fact that triphenyl
phosphate exhibits low resistance to hydrolysis. Its
presence accelerates the degradation of phosphate
esters under the action of moisture [1–3] and causes
undesirable increase in the deaeration time of the
O=PCl3 + 3ArOH → (ArO)3P=O + 3HCl.
(1)
1237

1238
NOSKOV et al.
Scheme 1. Components of butylated oil.
(1) Triphenyl phosphate, (2) p-tert-butylphenyl diphenyl phosphate, (3) di(p-tert-butylphenyl) phenyl phosphate, and
(4) tri(p-tert-butylphenyl) phosphate.
where Ph is phenyl; Pn, phenylene; and t-Bu, tert-
butyl.
resulting oil [10]. On the other hand, increased
concentrations of 3 and 4 lead to an increase in the
viscosity of the mixture [11].
EXPERIMENTAL
The content of triphenyl phosphate in a quaternary
mixture of 1–4 is decreased to a residual concentration
of ~2% by distillation using a film evaporator; in the
process, 5–30 wt % of the initial mixture is lost [12].
Triphenyl phosphate (>99%) was prepared at the
Rosneft Research and Development Center according
to [13]. p-tert-Butylphenol (99.5%, Novokuibyshevsk
Petrochemical Company, Russia) was used without
additionalpurification.Potassiumcarbonate(99%,Acros
Organics) was preliminarily calcined to constant weight
at 200°С. Hydrochloric acid (35–38%, NevaReaktiv,
Russia) was used for neutralizing reaction mixtures.
The quantitative composition of triaryl phosphate
mixtures was determined by gas chromatography with
a Thermo Finnigan Focus GC device equipped with
a flame ionization detector and a ZB 5HT Inferno
capillary column (30 m × 0.32 mm × 0.25 μm) under
the following conditions: vaporizer temperature 350°C;
carrier gas helium, column inlet pressure 130 kPa, flow
rate before splitting 80 mL–1, detector temperature
350°C; flow rates through detector, mL min–1: air 350,
In addition, the concentration of 1 can be reduced
to <5% by using as a phosphorylating agent diphenyl
phosphorochloridate and/or phenyl phosphorodichlori-
date [10]. However, implementation of this procedure is
complicated by difficult availability of the phosphoro-
chloridates and their easy hydrolysis.
In this study, we examined the possibility of control-
ling the content of the components in a mixture of triaryl
phosphates formed by transesterification:
(2)
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PREPARATION OF FLAME-RESISTANT LIQUIDS
1239
hydrogen 40, helium 30; column temperature schedule:
initial temperature 60°С, 6 min; heating to 340°С at a
rate of 15 deg min–1; 340°С, 10 min.
mV
2
The quantitative composition of the mixtures was
calculated by the equation
Si Ki
n
(3)
cwi

100,
3
(Si Ki )
i1
where cwi is the weight fraction of ith component in
the mixture; n, number of components; Si, peak area
of ith component; Ki, relative response factor of ith
component.
Response factors Ki: phenol 0.868, p-tert-butylphenol
0.831, 1 1.000, 2 0.960, 3 0.930, and 4 0.910. A typical
chromatogram of the triaryl phosphate mixture is shown
in Fig. 1.
1
4
min
Fig. 1. Typical chromatogram of a mixture of aryl phosphate
esters. (1) Triphenyl phosphate, (2) p-tert-butylphenyl diphe-
nyl phosphate, (3) di(p-tert-butylphenyl) phenyl phosphate,
and (4) tri(p-tert-butylphenyl) phosphate.
The transesterification of triphenyl phosphate was
performed on a laboratory installation consisting of a
three-necked flask as a still pot and a heated packed
column d 40 × 360 mm (spiral packing d 3.5 × 3 mm
made of wire d 0.5 mm; steel 12Cr18Ni10Ti). The still
pot was heated with a CMUA2000/CE Electrothermal
flask heater–stirrer. The temperatures of the still pot,
column, and distilled phenol vapor were monitored with
thermoelectric sensors. The vacuum was made with a
Vacuubrand pump (Germany).
mixture was stirred for 4 h. The KCl precipitate was
filtered off, and the reaction mixture composition was
determined by gas chromatography.
To isolate the flame-resistant oil, the neutralized
reaction mixture was subjected to fractional distilla-
tion using the heated packed column. As a result, we
obtained 894.60 g of the triphenyl phosphate fraction
with Тb 235–244°С/10 mmHg (97 wt %) and 737.66 g
of the target fraction of the flame-resistant oil with Тb
255–287°С/10 mmHg of the composition (wt %): 1 0.45,
2 81.48, 3 17.01, and 4 1.06; Тcong = –18°С; kinematic vis-
cosity at 50°С 26 mm2 s–1, p-tert-butylphenol conversion
99.5%. The isolated triphenyl phosphate fraction can be
reused in the next cycle of flame-resistant oil preparation.
The pour point was determined by method B of GOST
(State Standard) 20287, Petroleum Products. Methods
for Determining the Flow and Pour Points, and the kin-
ematic viscosity, by the ASTM D445 method, Standard
Test Method for Kinematic Viscosity of Transparent and
Opaque Liquids (and Calculation of Dynamic Viscosity).
In a typical experiment, the still pot was charged
with 1532.70 g (4.67 mol) of triphenyl phosphate,
328.45 g (2.186 mol) of p-tert-butylphenol, and 15.18 g
(0.110 mol) of anhydrous potassium carbonate as a
catalyst (1 wt % based on triphenyl phosphate). The
transesterification was performed at a residual pressure
of 10 mmHg with continuous distillation of phenol
through the packed column. The still pot temperature
was increased from 130 to 220°С at such a rate that the
column temperature did not exceed the phenol boiling
point at a residual pressure of 10 mmHg (about 90°С).
After separation of the major fraction of phenol, the
column temperature was elevated to 250°С by electric
heating for more complete removal of residual phenol
from the packing, and the still pot was kept at 220°C
for 20 min to stabilize the equilibrium between esters
1–4. After the completion of the phenol distillation, the
reaction mixture was cooled to 40–50°С, hydrochloric
acid was added for neutralization (2 mol per mole of
potassium carbonate taken into the reaction), and the
RESULTS AND DISCUSSION
Requirements to flame-resistant turbine oils concern
a number of parameters. According to [1–3], to ensure
the hydrolytic stability and required deaeration level,
the content of phosphate 1 in the oil should not exceed
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NOSKOV et al.
Figure 2 shows how the pour point and kinematic
viscosity depend on the oil composition (2/3 weight
ratio). As this ratio is changed from 1.2 to 4.0, the pour
point decreases from –11 to –17°С, and the kinematic
viscosity measured at 50°С decreases from 37 to
25 mm2 s–1. As can be seen, the 2/3 weight ratio of
4.0–4.5 in the butylated oil formula ensures the required
(prescribed by regulations) levels of kinematic viscosity
and pour point of the oil. Thus, ester 2 should be the
major component of the oil (about 70–80 wt %).
The base-catalyzed reaction of 1 with p-tert-
butylphenol (BP) allows preparation of products with
different degrees of transesterification [Eq. (2)]. The
fractions and proportions of products 2–4 depend on the
ratio of 1 and BPin the loaded reactant mixture.After the
completion of phenol distillation (phenol is distilled off
in the equimolar amount relative to the loaded BP), an
equilibrium mixture of triaryl phosphates 1–4 is formed
(Scheme 1). The equilibrium between components 1–4
is considered as attained if the component concentrations
do not further vary in time. The following equations
of equilibrium chemical reactions correspond to the
attained equilibrium in the mixture of esters 1–4:
2/3 weight ratio
Fig. 2. Pour point and kinematic viscosity (50°C) of butylated
oil consisting of esters 1–4 as functions of the 2/3 weight
ratio. (1) Triphenyl phosphate, (2) p-tert-butylphenyl diphenyl
phosphate, (3) di(p-tert-butylphenyl) phenyl phosphate, and
(4) tri(p-tert-butylphenyl) phosphate. Total content of 1 + 4
3–10 wt %.
4–5 wt %. The kinematic viscosity should be no lower
than 23 mm2 s–1 at 50°С, and the pour point should not
exceed–17°С.1 Ithasbeenfoundthatthesecharacteristics
also directly correlate with the quantitative composition
of the butylated oil.
(4)
(5)
(6)
given in mole fractions, and BP0 and I0 are the loaded
amounts (moles) of p-tert-butylphenol and triphenyl
phosphate, respectively:
where K1 and K2 are the equilibrium constants of
reactions (4) and (5), respectively.
Equation (6) is linearly dependent [it can be obtained
by summation of Eqs. (4) and (5)]; therefore, it is
excluded from further consideration.
Then, the equilibrium is described by the system of
Eqs. (7)–(10), where the equilibrium concentrations are
–––––––––––––––
1 Document RD EO 1.1.2.05.04442016: Requirements to Operation
and to Organization and Procedure of Tests of Transformer and
Turbine Oils at Nuclear Power Plants.
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PREPARATION OF FLAME-RESISTANT LIQUIDS
1241
Equation (9) reflects the molar ratio of tert-
butylphenyl fragments (substituents) to the sum of
the esters, equal to the molar ratio of BP0 to I0 in the
reactant mixture loaded for transesterification. Equation
(10) is the balance with respect to phosphate esters.
Knowing the equilibrium constants K1 and K2, we can
calculate the composition of the equilibrium mixture
and determine the BP0/I0 ratio ensuring the required
2/3 ratio in the transesterification product mixture.
To determine the equilibrium constants K1 and K2
under the transesterification conditions, we modeled
experimentally the equilibrium mixtures with different
ratios of the phosphate esters (Table 1).
60
40
20
3
4
2
1
BP0/I0 molar ratio
Fig. 3. Composition of the equilibrium mixture of transes-
terification products as a function of the BP0/I0 molar ratio.
Points: experiment; lines: calculation by Eqs. (7)–(10). (BP0)
p-tert-Butylphenol, (I0) triphenyl phosphate, (2) p-tert-butyl-
phenyl diphenyl phosphate, (3) di(p-tert-butylphenyl)phenyl
phosphate, and (4) tri(p-tert-butylphenyl) phosphate.
From the mean values of the constants, K1 = 2.8639
and K2 = 3.0201, by numerical solution of Eqs. (7)–
(10) we calculated the dependence of the esterification
mixture composition on the BP0/I0 molar ratio. The
calculation results are plotted in Fig. 3. The calculated
dependence of the 2/3 weight ratio in the equilibrium
mixture of transesterification products on BP0/I0 is
plotted in Fig. 4.
2–4 to the preset residual content by vacuum
distillation. For example, at the preset residual content
of I of 3 wt %, the oil composition is characterized by
the following component ratio (wt %): I, 3; 2, 79; 3,
17; and 4, 1. Such oil had the pour point of –18°С and
the kinematic viscosity at 50°С of 26.3 mm2 s–1 [15].
With reasonable agreement between the calculated
and experimental values (Fig. 3), at a constant
amount of triphenyl phosphate loaded, an increase
in the amount of the BP loaded leads to a monotonic
decrease in the equilibrium concentration of 1 and to
an increase in the concentrations of 3 and 4, whereas
the concentration of 2 passes through a maximum at
BP0/I0 molar ratios in the interval 0.8–1.0. The 2/3
weight ratio of 4.0–4.4, required for obtaining the
oil that meets the regulation requirements to flame-
resistant turbine oils with respect to viscosity and
temperature characteristics, is reached at the BP0/I0
molar ratio in the starting mixture of ~0.5 (Fig. 4).
In this case, the concentration of 4 in the equilibrium
mixture is satisfactorily low (~1 wt %), whereas the
concentration of triphenyl phosphate is inadmissibly
high (about 53 wt %). Decreasing the concentration
of 1 by increasing the BP0/I0 ratio leads to undesirable
distribution of components 2–4 (Table 1). Indeed, as
shown in [14], at the molar ratio BP0/I0 = 3 the fraction
of unchanged 1 is 11 wt %, but a large amount of 4
(19 wt %). is formed. The excess triphenyl phosphate
after deactivating the transesterification catalyst can
be selectively separated from a mixture of phosphates
Thus, the equilibrium constants found and
the mathematical description allow calculation
of the optimum amounts of reactants loaded for
transesterification to obtain the reaction mixture of
BP0/I0 molar ratio
Fig. 4. Calculated dependence of the 2/3 weight ratio in the
equilibrium mixture of transesterification products on the
BP0/I0 molar ratio. (BP0) p-tert-Butylphenol, (I0) triphenyl
phosphate, (2) p-tert-butylphenyl diphenyl phosphate, and
(3) di(p-tert-butylphenyl) phenyl phosphate.
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1242
NOSKOV et al.
Table 1. Equilibrium composition of the phosphate mixture as a function of the molar ratio of p-tert-butylphenol to triphenyl phosphate and
calculated equilibrium constants of reactions (4) and (5)
Calculated ester mixture composition, wt %
BP0/I0 molar
2/3, g g–1
K1
K2
ratio
1
2
3
4
1.060
0.818
0.864
0.689
0.613
0.498
0.497
0.474
0.464
0.468
0.478
24.43
35.05
30.99
41.16
45.65
53.46
53.52
55.36
56.00
55.72
55.01
41.36
41.69
45.23
42.7
27.12
20.10
21.25
14.83
12.13
8.57
8.57
7.96
7.61
7.71
8.01
7.09
3.17
2.54
1.74
1.20
0.65
0.65
0.58
0.52
0.54
0.58
1.53
2.07
2.13
2.85
3.38
4.36
4.35
4.54
4.71
4.67
4.54
2.5268
2.4144
3.0390
2.8640
2.9735
2.9747
2.9636
2.8943
2.9549
2.9570
2.9407
2.4674
3.0094
3.8739
2.9446
2.9517
2.9543
2.9924
2.9583
3.0492
3.0295
2.906
41.02
37.32
37.27
36.10
35.87
36.03
36.40
preset composition (preset ratio of phosphate esters
2, 3, and 4 in the flame-resistant oil). For example,
to obtain flame-resistant butylated oil similar to
commercial butylated oil Fyrquel® EHC Plus for
electrohydraulic control in the turbine lubrication
system (component content, wt %: 1, 0–4; 2, 32–78;
3, 10–30; 4, 0–10),2 the calculated molar ratio of the
reactants taken for transesterification, BP0/I0, should
be in the interval 0.35–0.65 (Fig. 4). The required
residual content of triphenyl phosphate is reached by
removing its excess by vacuum distillation after the
catalyst neutralization.
p-tert-butylphenol necessary for transesterification of
triphenyl phosphate to yield the required composition of
the product mixture.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest
requiring disclosure in this article.
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