Oxidative Cleavage of Long-Chain Terminal Alkenes to Carboxylic Acids

Article abstract of DOI:10.1021/acs.oprd.8b00368

Oxidation of aliphatic terminal alkenes having more than 30 carbon atoms to carboxylic acids is presented. Such long carboxylic acids are useful as waxes, lubricants, plasticizers, etc. The oxidation was conducted under solvent-free conditions or with small amounts of diluents (heptane, methylcyclohexane, and tert-butylbenzene). Hydrogen peroxide was used as the oxidant, and tungstic acid was used as the catalyst. Halogen-free quaternary ammonium salts were used as phase-transfer catalysts, and the importance of the elimination of halide ions is discussed. Purification of the crude oxidized product and tungsten removal are also presented. The products have a maximum acid number of 75 mg of KOH/g, a saponification number of 100 mg of KOH/g, and tungsten content below 400 ppm.

Full text of DOI:10.1021/acs.oprd.8b00368

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Full Paper
Oxidative cleavage of long-chain terminal alkenes to carboxylic acids
Dariusz Pyszny, Tomasz Piotrowski, and Beata Orlinska
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00368 • Publication Date (Web): 21 Feb 2019
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Oxidative cleavage of long-chain terminal alkenes to
carboxylic acids
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Dariusz Pyszny, Tomasz Piotrowski, Beata Orlińska*
Department of Chemical Organic Technology and Petrochemistry, Silesian University of
Technology, Krzywoustego 4, 44-100 Gliwice, Poland
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ABSTRACT Oxidation of aliphatic terminal alkenes having more than 30 carbon atoms to
carboxylic acids is presented. Such long carboxylic acids are useful as waxes, lubricants,
plasticizers, etc. The oxidation was conducted in solvent-free conditions or with a small amount
of diluents (heptane, methylcyclohexane, and tert-butylbenzene). Hydrogen peroxide was used
as the oxidant, and tungstic acid was used as the catalyst. Halogen-free quaternary ammonium
salts were used as phase transfer catalysts, and the importance of the elimination of halide ions is
discussed. Purification of the crude oxidized product and tungsten removal is also presented. The
product has a maximum acid number of 75 mg KOH/g, a saponification number of 100 mg
KOH/g and tungsten content below 400 ppm.
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KEYWORDS Oxidative cleavage, α-olefins, hydrogen peroxide, phase transfer catalysis.
INTRODUCTION
The oxidative cleavage of alkenes into aldehydes, ketones and carboxylic acids is a reaction of
both commercial and synthetic interest 1.
There are many ways to cleave the olefinic C=C double bond to obtain carboxylic acids.
Different types of oxidants and catalysts have been used. Old-fashioned stoichiometric oxidants,
such as KMnO , Cr(VI) compounds, and OsO +NaIO (Lemieux–Johnson protocol), are toxic
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and/or generate many by-products and their industrial applications are limited. Methods that use,
Oxone 2, 3, PhI(OAc)2 4 have also been widely employed; however, these oxidants are quite
expensive. Tert-butylhydroperoxide has been successfully used as oxidant in the C=C bond
cleavage reaction of aromatic olefins together with suitable catalysts (Mn-MIL-100 5,
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.
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Neocuproine with Cu ions , FeCl 6H O , InCl , Ag/Au nanoparticles ). However tert-
3
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butylhydroperoxide generally does not react with aliphatic alkenes C8-C12 (only heptanoic acid
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was obtained from 1-octene in the presence of InCl ). Oxidation of higher aliphatic olefins
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using tert-butylhydroperoxide was not reported. Another well-known method is ozonolysis,
which has been used on the industrial scale for the production of azelaic acid from oleochemicals
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Although ozonolysis is still being developed , the method suffers from the toxicity of O and
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explosion risks. Currently, chemists are aware that the best choice for performing C=C bonds
cleavage is molecular oxygen or hydrogen peroxide because they are readily available and
environmentally friendly oxidants. Molecular oxygen is less expensive than H O and thus seems
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to be better for large-scale production. However, O works well only with aromatic olefins that
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have a benzene ring in the vinyl position . In the case of aliphatic olefins, the reaction requires
sophisticated and/or expensive catalysts to give good yields of the cleavage products for the
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reaction with molecular oxygen . The reactivity of aliphatic alkenes that have a terminal C=C
bond is even lower. Hydrogen peroxide is perhaps the most common choice for the oxidative
cleavage of aliphatic C=C bonds. It is used together with various catalysts that are based on
selenium , palladium , rhenium , ruthenium , iron and tungsten 19,22-33. Recently a lot of
attention is also paid to zeolites and MOF materials as catalysts. For example metal-organic
frameworks like MIL-101 were shown 20 to catalyse oxidative cleavage of terminal olefins to
carboxylic acids efficiently (e.g. 85% yield of undecanoic acid was achieved). However,
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according to literature , zeolites like Ti-Beta, TS1 or NaY catalyse oxidation of olefins with
H O that yields mostly epoxides and only small amounts of carboxylic acids.
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Iron is a ubiquitous metal in our civilization, and it would be great to use its compounds as
catalysts in the oxidative cleavage of olefins. However, a competitive reaction that leads to
unproductive H O decomposition (to H O and O ) may be a serious problem on a large scale.
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Tungsten compounds are known to be very efficient and versatile catalysts, and they work well
in the oxidation of almost all types of olefinic substrates. The oxidative cleavage of C=C double
bonds by H O begins with epoxidation catalysed by tungsten species. The epoxide undergoes
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hydrolysis and/or perhydrolysis, leading to the formation of a vicinal diol and/or
hydroperoxyalcohol. Theses intermediates undergo C-C bond scission and are also catalysed by
a tungsten species to form aldehydes. These aldehydes are further oxidized to give carboxylic
acids. (Figure 1). Nevertheless, other intermediate products are possible (hydroxyketones and
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lactones) . The generally accepted mechanism for the oxidative cleavage of C=C double bonds
involves activation of a W catalyst in a reaction with H O . This leads to the formation of O-O
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bridges in the structure of the catalyst (Figure 1), which then reacts with the double bonds of
olefins 22.
OH
OH
[O]
H
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O
H O
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O
R
R
O
R
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Active form of catalyst :
+
CH2
OH
OH
O
H O
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3-
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O
O
O
O
O
O
H O
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O
W
W
O
O
OH
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OH
OH
O
O
W
W
O
O
O
-
H C=O
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R
R
+
+
O
O
P
O
H
O
O
OH
H O
2 2
R
O
O
O
O
R
H O
2
2
OH
O
O
H O
2
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O
O
O
O
OH
O
R
-
H C=O
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R
O
OH
Figure 1. Steps of C=C double bond oxidative cleavage by H O catalysed by a tungsten species
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19.
Much progress has been made since the discovery of tungsten catalysts. In particular,
tris(cetylpyridinium) 12-tungstophosphate has been used as a catalyst for the C=C bond
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oxidative cleavage with 35% H O in tert-butanol . The highest yields of carboxylic acids were
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achieved when cyclic olefins were used (up to 90%), and they were the lowest when 1-octene
was used (45%). Later, Noyori and co-workers found that [CH (n-C H ) N]HSO is an efficient
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acidic phase transfer catalyst (PTC) in the oxidative cleavage of cyclohexene to adipic acid (90%
yield) with 30% H O in the presence of Na WO 24. However, that acidic PTC is quite
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expensive. Venturello and co-workers 19 synthesized and used methyltrioctylammonium
tetrakis(oxodiperoxotungsto)phosphate to catalyse the oxidation of C=C bonds with 40% H O2
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in the absence of an organic solvent. They achieved corresponding carboxylic acids with yields
of approximately 80% from aromatic olefins (styrene), cyclic olefins (cyclohexene) and even
linear terminal olefins (1-hexadecene). Another example is the use of the Q {PO [WO(O ) ] }
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catalyst in the oxidation of cycloalkenes and unsaturated fatty acids to appropriate dicarboxylic
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+
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acids (yields of 60-86%) (where Q = [n-Bu N] or [C H N(n-C H )] or [Me(n-C H ) N] ).
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The Q {PO [WO(O ) ] } catalyst can be obtained from H PW O or H WO and H PO and an
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2 2 4
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12 40
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appropriate quaternary ammonium halide (QX) in the reaction with H O 25. Additionally, other
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peroxotungstate complexes with organic ligands (e.g., pyridinium) have been used in the
cleavage of functionalized cyclic olefins with 35% H O to produce bis-acids in high yields .
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The readily available phosphotungstic acid (H PW O ) was used without a PTC to obtain
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adipic acid (95% yield) and other polycarboxylic acids from cyclic olefins 27 in the reaction with
H O .
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Even simple H WO works well because in the presence of H PO it is converted in situ
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3-
(
during oxidation of olefin) to H PW O or to the peroxo-complex, {PO (WO(O ) ) } , in the
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presence of both H PO and hydrogen peroxide. Therefore, adipic acid can be produced on a
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large scale from cyclohexene using a recyclable catalyst system consisting of H WO , H SO
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and H PO without the need for any PTC or solvent .
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Recently, an article that describes the use of a tungstate species supported on zinc-modified tin
dioxide (W/Zn–SnO ) in the oxidative cleavage of C=C bonds with 30% H O was published.
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The so-called “release and catch” catalyst works well in the case of cyclic and aromatic olefins
but gave moderate yields of carboxylic acids in the case of aliphatic alkenes despite the use of a
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relatively large amount of W (10 mol%) .
Much attention has been paid to oxidative cleavage of unsaturated fatty acids. For example, the
double bond in oleic acid has been cleaved to azelaic acid (yield up to 91%) by the use of 60%
H O (amount up to 8 eq.) and H WO 30. The oxidative scission of oleic acid was quite
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efficiently performed (yields >80%) using an oxidant-catalyst system composed of 30% H O
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and [C H N(n-C H )] {PO [WO(O ) ] } (1/5/0.02 molar ratio) at 85°C within 5 h. Several
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other catalysts have also been prepared in situ from tungstophosphoric acid, H O and different
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quaternary ammonium salts (Q , Cl ) in the oxidation of oleic acid to azelaic acid . Recently, a
similar investigation on the oxidation of 1-octene, 1-decene and 1-dodecene via 30% H O in the
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presence of peroxotungsten compounds of the general formula of Q {PO [WO(O ) ] } was
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2 2 4
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reported .
Many different quaternary ammonium and phosphonium salts have been tested as PTCs by
Kadyrov and Hackenberger for the oxidation of aliphatic terminal olefins (C10, C12, C18) with
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H O in the presence of Na WO and H PO to yield the corresponding carboxylic acids . A
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PTC or solvent is required because of the poor solubility of olefins in water.
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Herein, we present results on the oxidation of long-chain aliphatic terminal alkenes (C30+) to
carboxylic acids with hydrogen peroxide in the presence of tungstic acid and a phase transfer
catalyst without solvent or with a small amount of diluents. Oxidative cleavage of such long
olefins has not yet been studied. The C30+ -olefin substrate can be obtained from ethylene in
the oligomerization process, and the long chain carboxylic acid products can be used for the
production of important chemicals, such as surface active agents, lubricants, greases, plasticizers,
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waxes, polymers, adhesives and household products (soaps, detergents, cosmetics) . The
importance of the elimination of halide ions in quaternary ammonium salts (phase transfer
catalysts) and the influence of H SO and H PO are discussed. Purification of the crude
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oxidized product and tungsten reuse are also presented.
EXPERIMENTAL SECTION
Materials
The following compounds were used as received: 1-dodecene (93-95%, Acros Organics),
Alpha Plus C30+ olefin (Chevron-Phillips According to Technical Data Sheet, it contains 71.4
wt% of n-α-olefins and 24.7 wt% of branched α-olefins), hydrogen peroxide (50 wt% in water,
stabilized, Acros Organics; 30 wt% in water POCH), tert-butylbenzene (Acros Organics),
methylcyclohexane – MCH (Alfa Aesar), heptane (Chempur), tungstic acid - H WO (Fluka),
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cetylpyridinium chloride monohydrate - CPC (Aldrich), cetyltrimethylammonium p-
toluenesulfonate - CTMAPTS (SIGMA), cetyldimethyl(2-hydroxyethyl)ammonium dihydrogen
phosphate - Luviquat mono CP AT1 (Aldrich 30% in H O), methyltrioctylammonium
2
hydrogensulfate - MTOAHS (Sigma-Aldrich), tetrabutylammonium hydrogensulfate - TBAHS
(Merck-Schuchardt).
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Melting points were determined on EZ-Melt Automated Melting Point Apparatus Digital
Image Processing Technology, Stanford Research System. The amount of tungsten in oxidized
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C30+ product was determined using AAS or ICP-AES methods. The H NMR spectra were
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recorded on an Agilent 400-NMR working at 400 MHz. Samples were prepared in CCl or
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CDCl3.
Oxidation of 1-dodecene with H O
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-dodecene (30 mmol; 6.7 ml; 5.05 g), a catalyst containing H WO (3 mol%; 0.9 mmol; 0.22
2 4
g), cetylpyridinium chloride (3 mol%; 0.9 mmol; 0.32 g), phosphoric acid (0.5 ml; 0.5 M),
hydrogen peroxide (50 wt.%; 120 mmol; 6.8 ml; 8.16 g) and diphenyl (0.42 g) as the internal GC
standard were introduced to a 25-ml round bottom flask equipped with a magnetic stirrer and
condenser. The reaction mixture was stirred (1400 rpm) at 70°C for 800 min. During the
reaction, samples were taken, and their compositions were determined by GC.
Oxidation of C30+ olefins
C30+ α-olefin (40-60 g), a catalyst containing H WO (1.2-7.2 mmol), PTC (2.0-7.2 mmol), 1
2
4
M H PO (0-2 mmol) or 1 M H SO (0-4 mmol) and (optional) a hydrocarbon diluent (32 g)
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were introduced to a 250-ml round bottom reactor equipped with a mechanical stirrer and a
condenser. The mixture was heated to 75°C; then, hydrogen peroxide (5-8 equiv. 50 wt.% or 30
wt.%) was pumped into the reactor (ca. 0.5 g/min), and the mixture was heated to the desired
temperature. The reaction was performed at 80-100°C for 10-16 h at 300-600 rpm.
During oxidation, reaction samples of the crude product were taken and dried at 60°C, and
their acid numbers (AN) were determined.
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When oxidation was finished, the crude products were washed with 500 ml of distilled water
and dried at 60°C, and their acid numbers (AN) and saponification numbers (SN) were
determined.
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Crude product purification by sedimentation, decantation and crystallization
The purification of the chosen oxidation product (AN 64 mgKOH/g) from tungsten was
performed via sedimentation followed by decantation and crystallization in a selected solvent. To
a round-bottomed flask (250 ml), we introduced 15.0 g of dried oxidation product and 30-100 ml
of the solvent. The flask was immersed in an oil bath at a temperature of 90°C. After melting and
dissolving the oxidation product (approximately 15 min), the contents were mixed for a while
and left at 90°C for sedimentation for 135 min. Then, we poured (decanted) the supernatant into
a crystallizer and allowed it to crystallize overnight. The resulting crystals were filtered and dried
at 20-40°C, and the tungsten content was determined.
To purify the product by adsorption of tungsten compounds on Al O , the hot supernatant was
2
3
directly poured on a heated (90°C) adsorption column (diameter of 15 mm and length of 160
mm) filled with active Al O (particles < 0.1 mm; previously calcined at 600°C for 3 h). The
2
3
purified solution was allowed to crystallize overnight. The resulting crystals were filtered and
dried at 20-40°C, and the tungsten content was determined.
Crude product purification by extraction
To ca. 72 g of the oxidation products obtained in reactions No. 26 and 27 (Table 1), 25 g of
H O was added, and the mixture was extracted twice with methylcyclohexane (MCH) (2 x 70 or
2
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5 g of MCH). The MCH was evaporated from the organic phase. A white solid product was
obtained, and the tungsten content was determined.
Recovery and reuse of tungsten compounds
1
1
1
1
1
1
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1
1
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According to the above-described procedure (sedimentation, decantation, and crystallization),
the precipitate obtained by purification of the product of reaction No. 21 (Table 1) was combined
with the aqueous filtrate from washing of the crude product. The solvents were evaporated, and
the recovered catalyst was reused in reaction No. 22 (Table 1).
Oxidation of C30+ olefins in a reaction calorimeter
Oxidation of C30+ olefins was conducted in a Mettler Toledo reaction calorimeter (RC1e)
equipped with an RTCal heat flow sensor at a volume of 500 ml. First, the C30+ α-olefin (80 or
9
0 g), the catalyst containing H WO (4.8 or 5.4 mmol), CTMAPTS (4.4 or 4.9 mmol), 1 M
2 4
H PO (1.4 or 1.5 mmol) and tert-butylbezene (64 or 36 g) as the diluent were introduced. The
3
4
mixture was heated to 80°C; then, hydrogen peroxide (33 or 38 g, 50 wt.%) was pumped into the
reactor (0.4 g/min). The reaction was conducted at 80°C for 6 h at 300 rpm. The temperature of
the reaction mixture was automatically controlled. The heat of the process was measured.
Calculation of bromine number and maximum acid number
The double bond content of C30+ α-olefins was calculated using 1H NMR spectroscopy with
naphthalene as an internal standard (Figure S1, Suppl.). The NMR tube was charged with a
solution prepared by dissolving 0.50062 g of α-olefins and 0.05572 g of naphthalene in carbon
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tetrachloride. The ratio of double bonds count to number of naphthalene molecules was
calculated by determination of the surface area of peaks corresponding to appropriate molecule
fragments. The calculated double bond count per unit mass of α-olefins (0.002035 mol of C=C
0
1
2
3
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0
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per 1 g of olefin) corresponds to theoretical bromine number 32.52 g Br /100 g. Based on the
2
content of double bonds the average molecular mas of C30+ olefins is calculated as 491 g/mol
which is very close to molecular mas of C H . We assumed that all the C=C bonds are terminal
35 70
(
CH =CH-R) and CH group is oxidized to formic acid which is removed during washing the
2 2
product. Oxidation of C H yields carboxylic acid C H COOH, so the calculated maximum
3
5
70
33 67
theoretical value of AN was 110 mg KOH/g. Detailed description of above calculations is given
in Supplementary Information.
Esterification of oxidized product with MeOH
1
0 g of dry oxidized product, 25 ml of MeOH and 0.1 ml of conc. H SO was placed in the
2 4
stainless steel reactor. The esterification was conducted at 90°C during 20 h under the pressure
of N (7 bar). Then the esterified product was washed with MeOH (3 x 20 ml) and dried.
2
Gas chromatography analyses
GC analyses were performed on a Hewlett-Packard 5890 Series II gas chromatograph with a
flame ionization detector (FID) using a (ZB-5HT) capillary column (30 m ˣ 0.32 mm ˣ 0.10 μm
film). Carrier gas: helium. The following conditions were used: column head pressure of 60 kPa,
injection port temperature of 280°C, and an oven program of 80°C for 1 min that was then
ramped to 280°C at 20°C/min with a final time of 2 min at 280°C. The detector temperature was
2
80°C. The qualitative analyses were performed using a GC (Agilent Technologies 7890C)
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connected with to an MS (Agilent Technologies 5975C) detector. The GC-MS was equipped
with an HP 5MS column (30 m × 0.25 mm × 0.25 μm film, ionization with EI at 70 eV).
Determination of H O concentration
2
2
1
1
1
1
1
1
1
1
1
1
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2
2
2
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2
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A sample from the aqueous phase taken after the oxidation process was dissolved in pure
acetic acid and iodometrically analysed (using 0.1 M solution of Na S O for titration).
2
2
3
Acid number AN
Samples (ca 0.3 g) taken from reactor were dried at 60°C and then introduced to a conical flask
equipped with condenser. Then 25 ml of xylene with 2-methyl-2,4-pentanediol (2:1 v/v) was
added. Mixture was heated up to get clear solution. Hot mixture was titrated with 0.05 M KOH
in ethanol. Phenolphthalein was used as an indicator. Acid number (AN) is the amount of KOH
(mg) required to neutralize carboxylic groups in 1 g of sample.
Saponification number SN
Samples (ca 1 g) of product were dried at 60°C and then introduced to a conical flask. Then 25
ml of xylene with 1-propanol (4:1 v/v) and 25 ml of 0.1 M KOH in ethanol was added. Mixture
was heated under reflux for 30 minutes. Hot mixture was titrated with 0.1 M HCl.
Phenolphthalein was used as an indicator. In blank test the same mixture without the sample
from reaction was used. Saponification number (SN) is the amount of KOH (mg) required to
neutralize free carboxylic groups and esters in 1 g of sample.
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RESULTS AND DISCUSSION
A mixture of long-chain terminal olefins with more than 30 carbon atoms (C30+ α-olefins)
was used as the starting material in this study (containing 71.4 wt.% of n-α-olefins and 24.7
wt.% of branched α-olefins). The double bond content of the C30+ α-olefins was calculated
0
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2
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2
3
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0
1
using H NMR spectroscopy with naphthalene as the internal standard (Figure S1, Suppl.), which
gave the theoretical bromine number of 32.52 g Br /100 g. The oxidation reactions of C30+
2
olefins with H O (30-50 wt.% ) that were catalysed by H WO and PTC were conducted in the
2
2
aq
2
4
presence or absence of a mineral acid and a diluent (heptane, methylcyclohexane or tert-
butylbenzene). Commercially available halogen-free quaternary ammonium compounds,
including cetyltrimethylammonium p-toluenesulfonate (CTMAPTS), cetyldimethyl(2-
hydroxyethyl)ammonium dihydrogen phosphate (Luviquat), methyltrioctylammonium
hydrogensulfate (MTOAHS), and tetrabutylammonium hydrogensulfate (TBAHS), were used as
the PTCs. For comparison, cetylpyridinium chloride monohydrate (CPC) was used. The course
of the oxidation was monitored by measuring the acid numbers (AN) and saponification numbers
(
SN), which represent the total amount of carboxylic acids and total amount of esters plus
carboxylic acids, respectively. The effect of the amount of H WO , PTC, H O , H SO , and
2
4
2
2
2
4
H PO , the mixing rate and temperature were studied. The acid numbers of the crude products
3
4
and final products obtained after washing with water are presented in Table 1. The acid numbers
of the washed products are discussed in this section.
1
Based on the H NMR spectroscopy of oxidized products, it has been established that the
conversion of C30+ olefins for these reactions is 100% (characteristic peaks for the protons in
the vicinity of C=C bonds were not detected; Figure S6, Suppl.). The maximum theoretical value
of AN is 110 mg KOH/g. It was calculated based on the amount of C=C bonds in the 1 g of
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substrate, which was determined using 1H NMR with naphthalene as internal standard. We
assumed that all the C=C bonds are terminal (CH =CH-R) and formic acid formed by cleavage
2
of C=C bonds is removed during washing the product. In fact the substrate contains some
internal olefins (RCH=CHR’) where R’ may be a methyl, ethyl, propyl or other alkyl
substituents. These alkyl substituents with short hydrocarbon chains are converted to carboxylic
acids soluble in water (eg. acetic, propionic) which are mostly removed during washing the
products (proved by GC-MS analysis of water phase). Therefore, yield of long chain carboxylic
acids based on AN or SN values can be only estimated, eg. AN=80 mg KOH/g corresponds to
the yield ca. 72%. The dependence of maximum yield of carboxylic acids as a function of acid
number (AN) or saponification number (SN) in the oxidative cleavage of C30+ olefins are given
in Supplementary Information (Figure S5, Suppl.).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1. Oxidation of C30+ olefins with hydrogen peroxide a.
No
H
2
WO
4
PTC
PTC
H
2
SO
4
H
3
PO
4
H
2
O
2
temp. mixing ANcr AN
w
SN
63
w
EN
w
2 2
H O left
wt.% b
1.5%
3.0%
1.5%
1.5%
1.5%
1.5%
1.5%
1.5%
1.5%
1.5%
1.5%
wt.% b mol/g b
equiv. b °C
rpm
500
500
500
500
500
450
400
600
500
500
500
mgKOH/g
wt.% c
5.4%
1
2
3
4
5
6d
7
8
9
Luviquat CP 2.5%
Luviquat CP 2.5%
Luviquat CP 5.0%
Luviquat CP 5.0%
0
0
0
0
17
17
17
34
0
5
5
5
5
5
8
5
5
5
5
5
90
90
90
90
90
90
90
90
90
90
90
49
74
26
47
67
63
63
49
68
74
75
39
76
13
33
59
50
64
48
68
76
73
24
102 26
1.0%
22
55
75
71
80
63
83
88
90
9
12.8%
12.2%
2.5%
22
16
21
16
15
15
12
18
Luviquat CP 5.0% 17
Luviquat CP 2.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
0
0
0
0
0
0
17
0
13.1%
- k
0
- k
0
- k
1
1
0
1
17
34
1.2%
1.1%
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
2
1.5%
1.5%
1.5%
1.5%
CTMAPTS 2.5%
0
34
0
8
5
5
5
8
7
5
5
5
5
5
5
6
6
6
8
6
6
5
5
5
90
90
90
90
90
500
500
500
500
500
77
62
68
62
70
67
70
24
78
59
50
75
48
48
70
76
54
63
65
3
73
62
66
59
63
64
67
25
76
58
47
74
64
63
65
62
58
60
63
5
96
82
87
85
95
94
86
37
87
87
71
98
78
81
81
79
75
75
87
13
31
23
19
21
26
32
31
19
12
10
29
24
24
15
18
17
17
17
15
24
8
13.1%
0.8%
1.5%
1.3%
11.3%
1.2%
2.2%
17.4%
1.2%
1.1%
0.8%
1.2%
7.9%
8.7%
1.7%
0.8%
5.7%
9.7%
<0.1%
32.0%
<0.1%
13
CTMAPTS 2.5% 34
CTMAPTS 2.5% 34
CTMAPTS 2.5% 68
CTMAPTS 2.5% 34
CTMAPTS 2.5% 34
1
1
1
1
4
5
17
17
17
17
17
17
17
34
17
17
17
17
17
17
17
17
17
17
17
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6d 1.5%
7
1.5%
1.5%
0.5%
1.5%
3.0%
100 500
18
CTMAPTS 1.5%
CTMAPTS 2.5%
CTMAPTS 3.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
CTMAPTS 5.0%
CTMAPTS 5.0%
CTMAPTS 2.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
CTMAPTS 2.5%
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
90
90
90
90
90
90
80
80
80
80
80
80
90
90
90
500
500
500
500
500
500
400
300
300
300
300
300
500
500
500
1
2
2
2
9
0
1
2e 0%
23f 1.5%
2
2
2
4g 1.5%
5g 1.5%
6gj 3.0%
27dgj 3.0%
28h 1.5%
2
3
3
9i 1.5%
0
1
1.5%
1.5%
1.5%
MTOAHS
TBAHS
CPC
2.5%
2.5%
2.5%
32
13
13
19
AN = acid number of the crude product; AN , SN , EN are the acid number, saponification
cr
w
w
w
number and ester number of the washed product, respectively (theoretical maximum of AN and
SN is ca. 110 mg KOH/g).
a) Conditions: 40-60 g of C30+ -olefin, hydrogen peroxide (50 wt.%) of 0.5 g/min., 10 h. b)
Relative to C30+ -olefin. c) In aqueous phase. d) Reaction was conducted for 8 h with 4 equiv.
of H O ; then, the next 4 equiv. of H O were added, and the reaction was continued for another
2
2
2
2
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h (total of 16 h). e) Catalyst from reaction No. 21 was reused. f) 30 wt.% H O was used. g)
2
2
2 g of MCH was used as the diluent. h) 32 g of heptane was used as the diluent. i) 32 g of tert-
butylbenzene was used as the diluent. j) Extraction with MCH was performed instead of
washing. k) not determined.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
We faced many problems when working with C30+ olefins that are not present in the case of
short chain olefins. First, C30+ olefins are solids at room temperature and have melting points
near 70°C. Molten C30+ olefins form a homogenous mixture with tert-butanol, but the addition
of water (or H O2 aq) results in the formation of two phases. Thus, tert-butanol, which is widely
2
applied in the oxidation of short-chain olefins, is not a suitable solvent in the case of long chains.
Perhaps other solvents that are miscible with water will also tend to create a separate phase
(containing water and/or H O ), and the situation would be similar as that when a very low
2 2
concentration of H O was used; therefore, oxidation would be negligible. Furthermore, during
2
2
the oxidation reaction of C30+ olefins, products with higher melting points are formed. The
solvent-free processes suffer from high viscosities (because of the required PTC presence); thus,
to achieve vigorous mixing, a mechanical stirrer is required (at 500 rpm for mixing a solvent-free
mixture and 300 rpm for the process conducted with a diluent).
Effect of amount and type of acid
The effect of the addition of H PO , H SO or their combination on C30+ olefin oxidation
3
4
2
4
reactions that are catalysed by H WO and CTMAPTS or Luviquat was studied. When a more
2
4
efficient PTC catalyst, CTMAPTS, was used, only the positive effect of adding a small amount
of H PO was observed (Table 1, No. 9, 10, and 13). The effect of H SO is negligible. In
3
4
2
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3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
contrast, when Luviquat was used, a much higher AN and SN were achieved when H SO was
2
4
applied instead of H PO (Table 1, No. 3 and 5). We observed that a considerable amount of heat
3
4
was produced during the first 1-2 h of the processes conducted with H SO . This suggests that
2
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
one of the steps in the reaction pathway under these conditions is very fast and/or highly
exothermic (probably hydrolysis of the epoxide).
According to the mechanism and pathway of oxidative cleavage of C=C bonds, intermediate
epoxides and diols are formed. Thus, esterification reactions between carboxylic acids and
intermediate diols can occur. The amount of esters obtained was higher in the presence of H SO
2
4
than H PO . Despite the presence of significant amount of water, hydrolysis of these esters did
3
4
not proceed efficiently under applied conditions. Even when we used more H O (and
2
2
simultaneously H O) and performed the reaction for longer (16 h instead of 10 h) (Table 1, No.
2
1
6) or at a higher temperature (100°C) (Table 1, No. 17), we did not achieve a lower EN. This is
most likely due to the very high esterification equilibrium constant. It is known that for a longer
hydrocarbon chain in the carboxylic acid and/or alcohol, there will be a higher equilibrium
constant 34, 35, and the presence of acids (sulfuric, phosphoric) increases both the rate of
hydrolysis and the rate of ester formation (equilibrium is achieved faster).
To obtain better insight into the oxidation process a 1-dodecene was used as the model
compound, because its oxidation course can be monitored via GC. The oxidation of 1-dodecene
with 50 wt.% H O gave undecanoic acid as the main cleavage product. GC analysis also
2
2
revealed other products (1,2-epoxydodecene, 1,2-dodecanediol, and undecanal), which are
intermediates in this process according to the aforementioned pathway. The results show (Figure
2
) that full conversion of olefin is easily achieved, but the intermediates do not react as rapidly.
Some studies (e.g., ref. 36,37) that describe the oxidative cleavage of olefins with H O have
2
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shown a high concentration of epoxide even after a long reaction time (24 h). Our investigation
on the course of oxidation in the presence of catalytic amounts of a strong acid (p-
toluenesulfonic acid p-TSA) showed that the concentration of epoxide is very low during the
entire reaction time. However, in the presence of weaker acids (e.g., H PO ), the epoxide
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
4
accumulates at the beginning of the reaction, reaches a maximum and then decreases. This
clearly means that hydrolysis is much faster in the presence of strong acids. This is in accordance
3
8
with literature data reporting that epoxides are relatively unstable below a pH of 2-3 . Because
strong acids (p-TSA and probably H SO ) increase the rate of epoxide hydrolysis, the diol is
2
4
quickly produced; however, oxidation of the diol may be generally slower (requires phase
transfer of an oxidizing agent) than esterification with carboxylic acid when the concentration of
the latter increases (esterification occurs in the same phase). Therefore, huge amounts of water
positively influence the equilibrium constant; however, this is undesirable because of the low
hydrogen peroxide concentration. Hence, the oxidation rate is very slow. Based on the
investigation with 1-dodecene we assumed that it was possible to stop reaction on the stage of
epoxides. To achieve this goal we oxidized 1-dodecene (30 mmol) using lower amount of
hydrogen peroxide (substrate : H O 1 : 2 mol/mol) in the presence of tungstic acid (3 mol%) and
2
2
PTC (3 mol% CPC) without sulfuric acid or phosphoric acid. The product contained 70% of 1,2-
epoxydodecane after 2 h (after 10 h content of epoxide decreased to 64% due to slow hydrolysis
to 1,2-dodecanodiol).
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
4
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5
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5
6
100%
90%
80%
a)
1
-dodecene
undecanal
1,2-epoxydodecane
undecanoic acid
7
6
5
4
3
2
1
0%
0%
0%
0%
0%
0%
0%
1
,2-dodecanediol
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
%
0
200
400
600
800
Time, min
1
00%
b)
9
8
7
6
5
4
3
2
1
0%
0%
0%
0%
0%
0%
0%
0%
0%
1-dodecene
undecanal
1
,2-epoxydodecane
undecanoic acid
1,2-dodecanediol
0
%
0
500
Time, min
Figure 2. Conversion of 1-dodecene and yields of products obtained in the oxidation reaction
with H O catalysed with H WO and CPC in the presence of a) H PO or b) p-TSA.
2
2
2
4
3
4
In contrast to the literature 28 in which H SO was found to be important in oxidative cleavage
2
4
of cyclohexene, our results show that H SO is not required or is even unfavourable for C30+
2
4
oxidation in the presence of CTMAPTS. Nonetheless, H PO is important.
3
4
Effect of amount and concentration of H O
2
2
In our studies, a slight excess of H O was used, i.e., 5 equivalents, because it has been
2
2
observed that a stoichiometric amount of H O (4 equiv.) leads to a lower AN. In experiments
2
2
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No. 6, 16 and 27 (Table 1), first reactions were conducted for 8 h in the presence of 4 equivalents
of H O . Then, another 4 equivalents of H O were added to these mixtures, and reactions were
2
2
2
2
continued for another 8 h. The ANs increased after the addition of the second amounts of H O
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
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5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
Figure 3), indicating that 4 equivalents are not enough to complete oxidation. The AN and SN
achieved after 16 h for the reactions were similar (or even slightly lower) to those found after 10
h of reaction using 5 equivalents of H O .
2
2
The investigation of the reaction course revealed (Figure 3) that the AN was almost constant 2
h before the reaction was terminated, although a considerable amount of H O was detected in
2
2
the water phase (13.1%) (Table 1, No. 6).
8
7
6
5
0
0
0
0
40
30
2 2
+H O
No.6
No.10
No.16
2
1
0
0
0
0
200
400
Time, min
600
800
1000
Figure 3. Course of oxidation of C30+ olefin monitored by measuring the AN of the crude
product. First, 4 equivalents of H O were introduced to the reactor; then, after 8 h, another 4
2
2
equivalents of H O were added (No. 6 and 16). Only 5 equivalents of H O were added to the
2
2
2
2
reactor at the beginning of reaction No. 10. The numbers in the legend refer to Table 1.
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2
2
2
2
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6
We established that 50 wt.% H O can be replaced by 30 wt.% H O , which is less expensive
2
2
2
2
and safer. Similar results were obtained in both cases (Table 1, No. 20 and 23). However, the
yield from the reactor volume unit will be lower when 30 wt.% H O is used.
2
2
0
1
2
3
4
5
6
7
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9
0
1
2
3
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1
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0
1
2
3
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9
0
1
2
3
4
5
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8
9
0
Effect of amount of H WO
2
4
The investigation of the effect of the amount of H WO (0.5-3 wt.%) showed that it is
2
4
beneficial to perform this oxidation in the presence of 1.5% catalyst in combination with
CTMAPTS. Using a lower or higher amount of H WO (0.5 or 3 wt.%) led to a lower AN, SN,
2
4
and EN (Table 1, No. 19 and 21).
When H WO was 3 wt.% in a reaction with Luviquat, very high AN and SN values (Table 1,
2
4
No. 2) were achieved; however, the reaction mixture reached a temperature above 100°C for a
while at the beginning of this exothermic process, indicating very fast reactions after activation
of the catalyst, what is unwanted for safety reasons.
Effect of temperature
Due to solidification of the reaction mixture at approximately 80°C, most of the reactions were
performed at 90°C. A further increase in the reaction temperature to 100°C resulted in a high SN
and EN but a relatively low AN (Table 1, No.17). Additionally, we observed that a considerable
amount of heat was released at the beginning of the process, which can be dangerous in a scale-
up process.
Effect of diluent addition
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The solidification and high viscosities of the reaction mixture in the studied solvent-free
reactions encouraged us to employ aliphatic and aromatic hydrocarbons (methylcyclohexane,
heptane and tert-butylbenzene) as diluents. MCH and heptane were used because of their
relatively low boiling points (101°C and 98°C, respectively); thus, they can be easily distilled off
at the end of the reaction. The application of a small amount of diluent (diluent/olefin mass ratio
is only 4/5) allowed the reaction to proceed smoothly at 80°C even when a relatively high
amount of CTMAPTS (5 wt.%, Table 1, No. 24 and 25) or tungstic acid (3 wt.%, Table 1, No. 26
and 27) was used. Due to the presence of the diluent, the possibility of overheating was
significantly reduced. The AN and SN achieved in the presence of MCH were approximately 60
and 80 mgKOH/g, respectively; thus, they were slightly lower than in the reactions at 90°C
without diluent. Again, 8 equiv. of H O and 16 h of reaction (Table 1, No.27) gave similar
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
results as the use of 5 equiv. of H O and 10 h of reaction. Heptane and tert-butylbenzene also
2
2
worked well as diluents (Table 1, No.28 and 29).
Effects of the amount and type of PTC
In the investigated reaction, commercially available alkylammonium salts were used as the
PTC catalysts. As we expected, lipophilic salts are more effective in this reaction. This was
confirmed by comparing the products obtained in the reactions performed in the presence of
tetrabutylammonium hydrogensulfate (TBAHS) (Table 1, No. 31) and methyltrioctylammonium
hydrogensulfate (MTOAHS) (Table 1, No. 30). The reaction mixture easily separated into 2
phases when TBAHS was used (Table 1, No. 31), and negligible oxidation occurred. Therefore,
salts having cations with at least one alkyl chain with ≥ 8 carbon atoms, such as CTMAPTS,
MTOAHS, Luviquat, and CPC (Table 1, No. 10, 30, 3, and 32), were used in further studies.
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1
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1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
4
4
4
4
4
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5
5
5
5
5
5
5
6
The highest AN was achieved in the presence of CTMAPTS (Table 1, No. 10). Investigation of
the influence of the CTMAPTS amount in the range of 1.5-3.5 wt.% (Table 1, No. 18, 10, 20)
revealed that it is beneficial to use 2.5 wt.% CTMAPTS. Use of a smaller amount (1.5 wt.%) of
this PTC resulted in a lower AN (Table 1, No. 18). Surprisingly, the reaction was very
exothermic at the beginning (the temperature increased to 100°C for a short time). When a larger
amount (3.5 wt.%) of CTMAPTS was used, AN and SN did not significantly change. A further
increase in the CTMAPTS amount is rather undesirable due to the increase in viscosity of the
reaction mixture and problems with mixing.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-
When CPC containing Cl ions (Table 1, No.32) was used, very low AN and SN values were
3
3
achieved, and most of the H O unproductively decomposed. Kadyrov and Hackenberger
2
2
tested a wide range of ammonium and phosphonium salts as PTCs in the oxidative cleavage of
-
olefins and found that salts with Br anions catalyse the decomposition of H O ; however, they
2
2
did not provide any reason for this phenomenon. In the literature, little information is given
about H O selectivity and its decomposition to water and O in the presence of a PTC.
2
2
2
Generally, the decomposition of H O is unavoidable even if H O is stabilized. However, it
2
2
2
2
may be limited if halogen ions are eliminated from the catalytic system. We confirmed this
experimentally by heating the mixtures of H O (50 wt.% aq), tert-butylbenzene or MCH,
2
2
H WO and different PTCs without the olefinic substrate, which resulted in the fast evolution of
2
4
O and full decomposition of H O in the case of CPC and a very slow evolution in other cases
2
2
2
(Table 2). GC analyses of the applied solvents revealed that they are resistant to oxidation at the
applied conditions.
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The obtained results can be explained by considering the redox potentials of the
halides/dihalogens compared with that of H O , which may react as an oxidant or reductant
2
2
(Figure 4). The redox data show clearly that Cl and Br ions can catalyse the decomposition of
H O (in an acidic environment) because the redox potentials of chlorine and bromine are above
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
0
.695 V and below 1.766 V (values for H O oxidation and reduction, respectively). On a small
2 2
scale, the influence of Cl ions may be weak because Cl can easily escape from the reaction
2
mixture (breaking the catalytic cycle) since the surface-to-volume ratio is often high and the Cl2
is volatile (more than Br ). Perhaps because of this, Kadyrov and Hackenberger did not find a
2
-
negative influence due to Cl ions.
Our studies demonstrated that any PTC with halogen ions should be avoided. This can be done
by synthesizing the tungstic catalyst in the form of [R R R R N] [P W O ] prior to the main
1
2
3
4
3
x
y
z
2
5
oxidation reaction . However, this approach requires an additional synthetic step. Another
possibility is to perform the in situ synthesis of the active form of the catalyst from H WO ,
2
4
H PO , H O and a halogen-free PTC.
3
4
2
2
a
Table 2. Decomposition of H O in the presence of chosen PTC.
2
2
time
[h]
No hydrocarbon PTC
1b t-BuPh
CPC
O [mmol]
2
4
30.9
11.0
4.1
2
3
4
5
t-BuPh
t-BuPh
t-BuPh
MCH
MTOAHS 14
Luviquat 14
CTMAPTS 14
CTMAPTS 14
5.2
7.4
a
Conditions: 62 mmol H O (50 wt.% ), 0.15 g PTC, 0.09 g H WO , 6.0 g tert-butylbenzene
2
2
aq
2
4
b
(t-BuPh) or methylcyclohexane (MCH) with 0.1 mmol H PO , 1000 rpm, 80°C, 70°C.
3
4
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1
1
1
1
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2
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2
2
2
2
2
2
3
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Cl2
H O
2
2
H O
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
-
+
H O + 2H
2Cl
O + 2H
2
2
2
-
I2 + 2e
0,5355V
0,695V
1,066V
1,3582V
1,766V
2,153V
2I
+
H O
O + 2H + 2e
2
2
2
-
Br + 2e
2Br
2
-
Cl + 2e
2
2Cl
+
H O + 2H + 2e
2 H2O
2
2
-
F2 + 2e
2F
Figure 4. Decomposition of hydrogen peroxide catalysed by halides and explained using
standard electrode potentials.
Crude product purification
After the reaction was finished, the mixture was washed with water to remove phosphoric,
sulfuric, formic and other water-soluble carboxylic acids (Figure S2, Suppl.). Therefore, the AN
of the crude product was generally higher than the AN of the washed product (Table 1).
In the studied process, tungstic acid is present in form of oxo(diperoxotungsto)phosphates
(formed in the reaction with hydrogen peroxide and H PO ) until H O is present in the reaction
3
4
2
2
mixture. When H O is consumed, the tungstic catalyst is decomposed and partially precipitates.
2
2
Unfortunately, the waxy nature of the oxidation product makes it difficult to separate this
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catalyst. Therefore, the C30+ oxidation product was purified from tungsten compounds in a
multistep process that included sedimentation, decantation, crystallization and a filtration
procedure. For this purpose, an organic solvent with a boiling point above the melting point of
the product (>90°C) was employed. Using polar and non-polar solvents, different degrees of
separation of tungsten from the post-reaction mixture were achieved (Table 3, No. 1, 2, 3, 6).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 3. The influence of solvent type on the amount of tungsten separated from the oxidation
product a.
No. Solvent [bp.]
Amount of Recovered Remaining Crystallization
b
solvent [ml] W [wt.%]
W [wt.%] yield
o
1
2
3
4
5
6
7
Toluene [110 C]
100
100
100
30
93.1%
90.9%
85.1%
93.2%
93.6%
93.5%
93.0%
0.051%
0.067%
0.11%
70.3%
76.3%
83.3%
79.2%
78.4%
73.0%
o
n-Butyl Acetate [126 C]
o
1-Propanol [97 C]
c
o
Heptane [98 C]
0.050%
0.047%
0.048%
0.052%
c
o
Heptane [98 C]
60
c
o
Heptane [98 C]
100
150
c
o
Heptane [98 C]
75.1%
a
b
1
5 g of oxidation product (obtained in the reaction using 1.8 wt.% of H WO ); content of
2
4
c
tungsten in the untreated sample: 0.74%; mixture of isomers (fraction from petroleum).
Solvent addition reduced the viscosity of the obtained mixture of the higher aliphatic
carboxylic acids and facilitated aggregation of the catalyst particles. Solvents that allowed the
greatest degree of purification of the product from tungsten are aliphatic (e.g., heptane) or
aromatic (e.g., toluene) hydrocarbons.
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Although toluene produced a similar tungsten separation degree as heptane, the toluene filtrate
was yellowish, indicating contamination with tungsten(VI) oxide. The heptane filtrate was
colourless and was analysed by means of GC, and no detectable amounts of other organic
substances were found. We assume that heptane could be added to the next purification step
without any problems. Polar solvents with similar boiling points as 1-propanol will increase the
crystallization yield; however, the recovery of tungsten is significantly smaller. Therefore, a
solvent with a low viscosity and a high lipophilicity is preferred.
0
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0
Based on these results, heptane was chosen for following experiments (Table 3, No. 4, 5, 6, 7)
with different heptane/oxidation product ratios. It was not necessary to use huge amounts of this
hydrocarbon. The recovered tungsten amount was very similar and even decreased with an
increase in the amount of heptane. Smaller amounts of heptane also increased the yield of
crystallization; however, its minimal dosage was limited by the viscosity of the solution after
crystallization, thus should be larger than 30 ml per 15 g of oxidation product. The purification
using heptane also had no influence on the AN in contrast to 1-propanol (which decreased AN).
An increased purification of the product from tungsten compounds can be performed using
adsorption on activated alumina (Al O ). The obtained purified product contained 0.037% W
2
3
(
95.0% separated W). This result shows that the tungsten catalyst can be adsorbed on Al O ;
2 3
however, the remaining tungsten amount was insignificantly lower compared that in the other
results (Table 3, No. 5, 6).
To simplify the process of oxidation product purification, the use of extraction with MCH was
studied. Products of reactions No. 26 and No. 27 (Table 1; obtained using 3 wt.% of H WO )
2
4
were extracted with MCH at 75°C instead of washing with water (with 2 portions of MCH);
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then, the extracts were evaporated to obtain white solid wax products (Figure S4, Suppl.). The
extraction method is simpler than purification via the sedimentation-crystallization described
above. However, the amount of remaining tungsten was relatively high (0.19% of W determined
in the solid product obtained from the first extract from reaction No. 26; 91.4% of W was
removed) compared with the values given in Table 3.
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GC-MS of the substrate and product
The C30+ oxidation product and the C30+ olefinic substrate were qualitatively analysed using
GC-MS (Figure 5). Due to the low vapour pressure of carboxylic acids, the oxidation product
was esterified with MeOH prior to analysis. Chromatograms show that the C30+ olefin contains
hydrocarbon chains with an even number of carbon atoms (primarily C28 to C42), whereas the
product contains hydrocarbon chains that have mostly an odd number of carbon atoms (primarily
C27 to C39). Thus, the results clearly show that hydrocarbon chains were shortened by one
carbon atom (i.e., the =CH group) during the oxidation of olefin and the formation of carboxylic
2
acid. There is only a small amount of short chain carboxylic acids, which are most likely
obtained from internal double bond cleavage.
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