Synthesis of fatty monoester lubricant base oil catalyzed by Fe-Zn double-metal cyanide complex

Article abstract of DOI:10.1007/s12039-014-0669-x

Fatty monoester lubricant base oils as high as 96.7 mol% were prepared by reacting methyl oleate with long-chain alcohols viz., 2-ethyl-1-hexanol (C ₈-OH), 1-decanol (C₁₀OH) and 1-dodecanol (C ₁₂OH) in the presence of a solid Fe-Zn double-metal cyanide (DMC) complex catalyst. Unlike many other acid catalysts, DMC doesn't produce undesired ether side products. The catalyst was reusable in four recycling experiments with little loss in catalytic activity and ester yield. The long-chain esters prepared in the study have the desired physical properties for their application as lubricant base oils.

Full text of DOI:10.1007/s12039-014-0669-x

J. Chem. Sci. Vol. 126, No. 4, July 2014, pp. 997–1003. ꢀc Indian Academy of Sciences.
Synthesis of fatty monoester lubricant base oil catalyzed by Fe-Zn
double-metal cyanide complex
∗
RAVINDRA K RAUT, MEHEJABEEN SHAIKH and SRINIVAS DARBHA
Catalysis Division, CSIR-National Chemical Laboratory, Pune 411 008, India
e-mail: d.srinivas@ncl.res.in
MS received 15 February 2014; revised 7 April 2014; accepted 8 April 2014
Abstract. Fatty monoester lubricant base oils as high as 96.7 mol% were prepared by reacting methyl oleate
with long-chain alcohols viz., 2-ethyl-1-hexanol (C8−OH), 1-decanol (C10OH) and 1-dodecanol (C12OH) in
the presence of a solid Fe-Zn double-metal cyanide (DMC) complex catalyst. Unlike many other acid catalysts,
DMC doesn’t produce undesired ether side products. The catalyst was reusable in four recycling experiments
with little loss in catalytic activity and ester yield. The long-chain esters prepared in the study have the desired
physical properties for their application as lubricant base oils.
Keywords. Fatty monoester; lubricant base oil; transesterification; Fe-Zn double-metal cyanide (DMC)
catalyst.
bio-based lubricants.3,7 They are produced by esteri-
fication of a fatty acid or transesterification of veg-
1. Introduction
High performance, long product life and better eco-
logical compatibility are the characteristic features
that make fatty monoesters important as high qual-
ity biodegradable lubricant base oils.1 Unlike other
synthetic oils ca. poly alpha-olefins (PAO) and poly
alkylene glycols (PAG), the ester oils are miscible
with a large number of additives to formulate effective
biodegradable lubricants. Currently, more than 10,000
lubricant formulations are needed to meet the world
demand of 12 billion gallons per year.3 Through a
proper choice of fatty acid and long-chain alcohol, a
wide variety of lubricants with custom design properties
can be synthesized. Biodegradable lubricants reduce
environmental pollution which the conventional min-
eral oil-based lubricants cause after their usage.
etable oil with a long-chain alcohol in presence of a
8–10
homogeneous acid/base catalyst
or by using an ion-
11,12
exchange resin.
Their synthesis from the reaction
,2
of fatty acid methyl esters (FAME-biodiesel) and long-
chain alcohol is attractive, as a part of biodiesel pool, can
be converted to more valuable lube product. This diver-
sion possibly makes biodiesel production economical
even in the absence of incentives from the government.
Cost effective production of synthetic esters is of
utmost importance for their wider use. The homoge-
neous mineral acid or base catalysts currently used
are non-reusable. Additional process steps are needed
for catalyst neutralization and separation. Significant
amount of salt and hard water is generated as waste
by-product. Ion exchange resins have limited thermal
stability confining their application to reactions with
short chain alcohols only. Use of solid catalysts in place
of homogeneous catalysts make the process attractive
and cost-effective. Solid catalysts can be recyclable
and processes involving them can be green and zero-
waste generating. Silica-sulphuric acid, Amberlyst-15,
–5
Note: about 53% of mineral oil lubricants are col-
lected as waste endangering the planet.6
Monoesters are categorized as ‘non-water pollutant’
lubes which reduce the expenses of oil spillage or dis-
posal. It is realized that more than 90% of all the
current lubricants could be formulated to be rapidly
6
biodegradable. The ester oils are normally for high-
ꢀ
immobilized lipase (Novozym 435), sulfated zirco-
end usage due to their higher price. Biolubricants
are priced twice as high as conventional petroleum
lubricants. As a consequence, only a few percentage
of the market for slippery fluids is commanded by
nia, calcium methoxide, titanosilicates and zirconium
phenyl phosphonate phosphite are a few solid cata-
13–18
lysts reported for biolubricants’ preparation.
Ear-
lier, we reported the application of Fe-Zn double-
metal cyanide (DMC) complex as an efficient cata-
lyst for producing biolubrciants by transesterification
of vegetable oils with linear chain monohydric C8
∗
For correspondence
9
97

9
98
Ravindra K Raut et al.
alcohol19 and esterification of oleic acid with polyhy- atmospheric pressure under a flow of nitrogen. Nitro-
20
dric alcohols. We now extend the study reporting their gen drives out the by-product methanol formed in the
use for transesterfication of methyl oleate (a component reaction and pushes the equilibrium towards the prod-
of biodiesel) with 2-ethyl-1-hexanol (C OH), 1-decanol uct. Then, the temperature of the reaction mixture was
8
◦
(
C OH) and 1-dodecanol (C OH) producing the cor- brought down to room temperature (25 C) and the cat-
10
12
responding long-chain alcohol monoesters. DMC com- alyst was separated by centrifugation (RCF = 7000 g
plexes are hydrophobic and Lewis acid catalysts.
for 5 min using a Remi Laboratory Instrument, Model
The lubricant properties of the produced monoesters are No. R-24)/filtraton. Excess, unreacted alcohol in the
19,20
reported.
product mixture was removed by steam-distillation. The
product monoester was analyzed by a Perkin-Elmer
(
Series 200) high performance liquid chromatography
2
. Experimental
fitted with an ELSD detector (Gilson) and reverse-phase
Brownlee column (C-18, Spheri-5, 250 × 4.6 mm). For
kinetics studies, reactions were conducted at five dif-
ferent temperatures (433, 443, 453, 463 and 473 K) for
a period of 4h. Rate constants were determined from
the concentration verses time plots using a pseudo-
first order rate equation. Arrhenius equation enabled
determination of thermodynamic parameters (activation
energy and Gibbs free energy). The yield of oleic fatty
2.1 Catalyst preparation
DMC was prepared as reported by us earlier19 using
K Fe(CN) ·3H O as a source of Fe, ZnCl as a source
4
6
2
2
of Zn, tert.-butanol as complexing agent and PEG-4000
as co-complexing agent. Prior to use in reactions, the
DMC catalyst was activated at 180 C for 2 h.
◦
1
monoester was confirmed also by H NMR (Bruker
2.2 Characterization techniques
Avance 200 MHz) and gas chromatography (GC, Var-
ian CP 3800; column - Varian CP 9079, 15 m long
X-ray powder diffraction (XRD) patterns of fresh and
spent DMC catalysts were recorded on a Phillips X’pert
Pro diffractometer equipped with a Ni-filtered Cu-Kα
radiation (λ = 0.15418 nm, 40 kV and 30 mA) and
a proportional counter detector. The diffraction pat-
×
0.32 mm ID × 0.1 um film thickness) techniques.
It should be noted that methyl palmitate and stearates
present in MO as impurity (30%) have also been
converted to the corresponding lube esters with equal
efficiency as that of methyl oleate.
0
terns were recorded in the 2θ range of 10–80 with
◦
◦
a scan rate of 4 /min and with a step size of 0.02 .
Fourier transform infrared (FTIR) spectra of the sam- 3. Results and discussion
ples as KBr pellets (1 wt %) were measured in the
−1
3.1 Catalyst characterization
wavenumber range of 400–4000 cm with a spectral
−1
resolution of 4 cm on a Shimadzu 8300 spectropho-
A detailed characterization of Fe-Zn DMC catalyst was
tometer. All the other characterization studies were
1
9,20
done as reported elsewhere.1
9,20
reported by us earlier.
istic parameters of the catalyst used in this study. DMC
molecular formula: K Zn [Fe(CN) ] .6H O.2(tert.-
Table 1 lists some character-
Physical properties of
fatty acid monoesters were determined at Chem-Tech
Laboratories Pvt. Ltd., Pune, India (Supplementary
Information).
(
4
4
6 3
2
Table 1. Physicochemical characteristics of Fe-Zn DMC
catalyst.
2.3 Reaction procedure
Physical property
Molecular formula
Value
Methyl oleate (MO, Sigma-Aldrich Co., technical
grade 70%; balance 30% being methyl palmitate and
stearate) was transesterified with C –C alcohols [2-
K4Zn4[Fe(CN)6]3.6H2O.2
(
tert.-BuOH)
8
12
Crystal lattice
Unit cell parameter (a)
Average crystallite size
Cubic
0.904 nm
36 nm
ethyl-1-hexanol (Loba Chemie, 99%), 1-decanol and
-dodecanol (SD Fine Chem.]. In a typical reac-
1
tion, 6.76 mmol of MO and three equivalent excess Average particle size
0.8 μm
Particle morphology
Specific surface area (SBET )
Acidity
Cubic/spherical
of alcohol and 0.06 g of DMC catalyst (3 wt% of
MO) were taken in a glass, round-bottom flask placed
in a temperature-controlled oil bath and connected
with water-cooled condenser. Temperature was raised
2
52 m /g
2
1.96 m /g
IR bands
2925, 2096, 1260 – 1450
and 1095 cm
−1
◦
to 180 C and the reaction was conducted for 8h at

Synthesis of fatty monoester lubricant base oil
999
Scheme 1. Possible structure of DMC and the uses of fatty monoesters.
BuOH) and molecular structure as shown in scheme 1) mesoporosity of less ordered nature in its structure.21
depicted XRD pattern (figure 1a) which could be Diffuse reflectance Fourier transform infrared (DRIFT)
indexed to a cubic lattice with a unit cell parameter spectra of adsorbed pyridine and temperature-
of 0.904 nm. FTIR spectrum of DMC (figure 1b) programmed desorption of ammonia (NH
-TPD) poin-
Tetrahedrally coor-
bridged cyanide group and other bands at 2925, dinated Zn are the sites of Lewis acidity which were
3
−1
19,22
showed a characteristic band at 2096 cm due to ted out that it is Lewis acidic.
2+
−
1
23
1
260–1450 and 1095 cm corresponding to C–H and active in transesterification reactions. Water adsorp-
C–O stretching vibrations of the coordinated tert.- tion studies revealed that this catalyst is hydrophobic
19
butanol. Nitrogen-physisorption and high resolution like silicalite-1. Hydrophobicity and Lewis acidity are
transmission electron microscopy (HRTEM) studies the two unique characteristic features that differentiate
revealed that DMC is mostly microporous but has some DMC from several other solid acid catalysts.
(a)
1
1
600
200
DMC-Spent
(b)
8
00
00
0
4
1
1
600
200
DMC-Fresh
8
00
00
0
DMC-Spent
DMC-Fresh
-
1
4
2096 cm
4000 3500 3000 2500 2000 1500 1000 500
1
0
20
30
40
50
-
1
Wavenumber
(
cm
)
2θ (degree)
Figure 1. (a) XRD and (b) FTIR spectra of fresh and spent DMC catalysts.

1
000
Ravindra K Raut et al.
7
7
6
6
5
5
4
4
3
3
5
0
5
0
5
0
5
0
5
0
80
(
a)
(b)
70
60
50
40
30
20
10
0
0
1
2
3
4
5
0
2
4
6
8
10
12
14
16
Catalyst Weight %
2-Ethyl-1-hexanol / MO (molar ratio)
Figure 2. Effects of (a) catalyst amount and (b) 2-ethyl-1-hexanol/MO molar ratio on the
yield of 2-ethylhexyl oleate. Reaction conditions: For (a): methyl oleate (MO) = 2 g, 2-ethyl-
◦
1
-hexanol = 2.64 g, MO : 2-ethyl-1-hexanol (molar ratio) = 1:3, temperature = 180 C, reac-
tion time = 8 h, pressure = 1 atm, under a positive flow of nitrogen. For (b): MO = 2 g,
◦
DMC = 3 wt% of MO, temperature = 180 C, reaction time = 8 h, pressure = 1 atm, under
a positive flow of nitrogen.
3.2 Catalytic activity
(figure 2a). In presence of DMC catalyst, the yield was
higher. It increased with increasing amount of the cat-
Transesterification of methyl oleate (MO) with long- alyst, reaching a maximum of 70 mol% at a catalyst
chain monohydric alcohols (2-ethyl-1-hexanol, 1- loading of 3 wt% of MO and became stable there-
decanol and 1-dodecanol) yields fatty acid monoesters after (reaction conditions: methyl oleate (MO) = 2 g,
◦
that find application not only in synthetic lubricants 2-ethyl-1-hexanol = 2.64 g, temperature = 180 C and
but in paint additives, plasticizers, pharmaceuticals and reaction time = 8 h). External mass transfer is the pos-
cosmetics (scheme 1). Methanol is a by-product. For- sible cause for this behavior at a catalyst loading above
mation of ethers via condensation of long-chain alco- 3 wt%. 2-Ethyl-1-hexanol/MO molar ratio had also a
hols or methanol was not detected at our experimen- marked effect on the product yield (figure 2b). The yield
1
tal conditions over DMC catalyst (HPLC, GC and H of 2-ethylhexyl oleate increased, reached a maximum
◦
NMR). Control experiments at 180 C for 8 h with 2- of 70 mol% at a 2-ethyl-1-hexanol/MO molar ratio of
◦
ethyl-1-hexanol/MO molar ratio = 3, revealed that this 3 and decreased thereafter (reaction conditions: 180 C,
reaction occurs even in the absence of a catalyst. But reaction time = 8h and catalyst = 3 wt% of MO). Tran-
the yield of 2-ethylhexyl oleate was 37 mol% only sesterification is an equilibrium reaction. Higher
1
00
o
1
1
1
1
2
60 C
o
70
60
2-Ethyl-1-hexanol
1-Decanol
1-dodecanol
70 C
o
80
90
00
C
C
C
8
6
4
2
0
0
0
0
o
o
5
4
0
0
30
2
1
0
0
(
a)
(b)
1
.0
1.5
2.0
2.5
3.0
3.5
4.0
0
1
2
3
4
5
6
7
8
9
Reaction time (h)
Reaction time (h)
Figure 3. Effect of (a) reaction temperature and time and (b) type of long-chain alcohol
on the yield of fatty monoester. Reaction conditions: For (a): methyl oleate (MO) = 2 g,
2
-ethyl-1-hexanol = 2.64 g, MO: 2-ethyl-1-hexanol (molar ratio) = 1 : 3, DMC = 3 wt% of
MO, pressure = 1 atm, under positive flow of nitrogen. For (b): MO = 2 g, MO: long-chain
◦
alcohol (molar ratio) = 1:3, DMC = 3 wt% of MO, reaction temperature = 180 C, reaction
time = 8 h, pressure = 1 atm, under a positive flow of nitrogen.

Synthesis of fatty monoester lubricant base oil
amount of 2-ethyl-1-hexanol drives the equilibrium
1001
toward right, forming fatty monoester lubricant.
At much higher concentrations of 2-ethyl-1-hexanol
(
molar ratio of 2-ethyl-1-hexanol/MO ≥ 5), accessibil-
ity of MO to the active sites was low and hence, lower
yields of 2-ethylhexyl oleate were obtained. Under
similar experimental conditions, the product yield
increased with increasing reaction time and attained
equilibrium at 8 h. Also with increasing temperature,
◦
the yield of 2-ethylhexyl oleate increased. At 180 C
and in 4h, monoester yield as high as 96.7 mol% was
obtained (figure 3a). At high temperature, most of the
known acid catalysts form ethers. The unique feature
◦
of DMC is that even at 180 C, formation of such unde-
sired ether products was not detected. The selectivity
for the monoester and methanol were 100%. DMC was
reused in four recycling experiments. At the end of
each run, the catalyst was separated from the reaction
◦
mixture; it was washed with methanol, dried at 100 C
for 12h and used in another recycle. The yield of 2-
ethylhexyl oleate was nearly the same in each recycle.
XRD and FTIR spectra of the fresh and spent cata-
lyst (at the end of 4th recycle) were almost the same
(figure 1) confirming the structural integrity of DMC
in transesterification runs. No leaching of metal ions
was detected as revealed by atomic absorption spec-
troscopy (AAS). Conversion verses time plots at five
different temperatures enabled rate constant and energy
of activation (E from the Arrhenius plot). The E value
a
a
was determined to be 75.9 kJ/mol. Using Eyring equa-
#
tion enthalpy of activation (ꢀH ) and entropy of acti-
#
vation (ꢀS ) were determined to be 72.3 kJ/mol and
283.3 J/mol, respectively. As this reaction proceeds to
some extent even in the absence of a catalyst, the acti-
vation energy has contributions from both thermal and
catalytic reactions.
DMC is effective for transesterification of MO with
different long-chain alcohols (figure 3b). A marginal
decrease in equilibrium conversion of MO from 70 to
62 mol% was observed when 1-dodecanol instead of 2-
ethyl-1-hexanol was used. The monoester product after
removal of methanol and unreacted long-chain alcohol
was pale yellow in color. Its physical properties (table 2)
match well with those of hydraulic oil lubricants
confirming that these long-chain alcohol fatty esters are
suitable lubricant base oil. Kinematic viscosity of long-
chain alcohol oleates were higher than that of the start-
ing ester (MO). As expected, 2-ethylhexyl oleate has
pour point lower than the other esters and MO.
A tentative mechanism for this reaction over Lewis
2+
acidic Zn active sites in DMC catalyst is shown in
2+
figure 4. Activation of MO at Zn site; formation
of carbocation; nucleophilic attack by the long-chain

1
002
Ravindra K Raut et al.
Figure 4. Tentative mechanism for the reaction of methyl oleate with 2-ethyl-1-hexanol.
alcohol (2-ethyl-1-hexanol, for example) on the carbo Acknowledgements
cation, followed by removal of methanol and long-chain
alcohol oleate ester formation are the steps involved in
the reaction.
M K acknowledges the Council of Scientific and Indus-
trial Research (CSIR), New Delhi for the award of
Senior Research Fellowship. This work forms a part of
the Network Project ‘Catalysts for Sustainable Energy
4. Conclusions
(ECat, CSC 0117)’ sponsored by CSIR.
Catalytic activity of DMC for forming fatty monoester
lubricants through transesterification of methyl oleate References
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◦
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2
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