Synthesis of bio-based surfactants from cashew nutshell liquid in water

Article abstract of DOI:10.1039/c8gc01686k

3-Pentadecylcyclohexylamines were synthesised via Pd/C-catalysed reductive amination of crude cashew nutshell liquid, an underused waste product, with hydrogen in water. The short, modular, and waste-minimised synthetic approach enables access to structurally diverse neutral, cationic, and zwitterionic tenside derivatives.

Full text of DOI:10.1039/c8gc01686k

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Synthesis of bio-based surfactants from cashew nutshell liquid in
water
Received 00th January 20xx,
Accepted 00th January 20xx
Valentina Bragoni, Raja K. Rit, Robin Kirchmann, A. Stefania Trita and Lukas J. Gooßen*
DOI: 10.1039/x0xx00000x
www.rsc.org/
3-Pentadecylcyclohexylamines were synthesised via Pd/C- detergents, cosmetics and personal care products, paints, and
catalysed reductive amination of crude cashew nutshell liquid, an agrochemicals is close to 20 megatons.27,28 Cationic (quats),
underused waste product, with hydrogen in water. The short, zwitterionic (betaines), and non-ionic surfactants (N-oxides),
modular, and waste-minimised synthetic approach enables all based on long-chain amines, are expensive and used for
accessing structurally diverse neutral, cationic, and zwitterionic high-end applications.29,30 They are characterised by low
tenside derivatives.
toxicity and skin irritation, good foam stability, and rapid
biodegradation, and are therefore used in detergents,
shampoos, cosmetics and personal care products.31–34
Cashew nutshell liquid (CNSL, Scheme 1) is an inedible waste
product of cashew nut production, obtained on a scale of 1
megaton per year.1 Cashew nutshells contain a mixture of
anacardic acids, substituted phenols, cardanol, and cardol,
each bearing a 15-carbon side chain with different degrees of
unsaturation. CNSL is obtained by pressing the nutshells at
elevated temperatures, causing decarboxylation of the acid
components. The resulting oil consists mainly of cardanols. The
use of cardanol as a chemical feedstock is highly advantageous
because it does not compete with land use for food
production.2–4 However, most current applications of this raw
material lead to low-value products. Cardanol has been used in
the synthesis of aromatic amines,5,6 modifiers of epoxy resins,7
rubber plasticisers,8 and adhesives.9
CNSL
anacardic acid
cardanol
cardol
R =
distillation 180 °C
- CO2
cardanols
70%
The long alkyl chain makes cardanol a promising source for
surfactants. Indeed, it has been converted to anionic
surfactants such as sulfonates, sulfates, carboxylates, as well
as non-ionic polyethoxylate surfactants.7,10–18 However, these
structures retain the aromatic ring, which is known to lead to
toxicity and bioaccumulation, and are particularly harmful for
the aquatic habitat.19–26 In this respect, they are inferior to
state-of-the-art surfactants, which are, however, mostly
prepared from petrochemical sources or fatty acids valuable
for food use.
HNR'R'' H2, Pd/C
N-oxides
betaines
quats
Scheme 1. Targeted conversion of CNSL into amine-based surfactants.
The global consumption of surfactants as softeners,
Exceptionally high value creation with maximal
sustainability would thus be achieved if a short and waste-
minimised synthesis of non-aromatic amine surfactants could
be devised starting from technical CNSL (Scheme 1). The
recent discovery of reductive amination processes of phenols
to cyclohexyl amines (Scheme 2) seemed to open up a window
of synthetic opportunity for the valorisation of cardanols.
Thus, Li35 and Taddei36 reported a Pd/C-catalysed reductive
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amination of simple phenols using aqueous HCO2Na as the
In non-polar and polar, aprotic and protic organic solvents,
reducing agent, leading aromatic or aliphatic amines, the yields were low (entries 1-6). This was not improved by
respectively. Beller37 and Fu38 used H2 as reducing agent, albeit addition of Lewis acids or alternative solid supports for Pd (see
in organic solvents.
the supporting information). However, the yields drastically
improved when switching to aqueous solvents (entry 7). To
our delight, the desired product 3a was obtained in almost
quantitative yield in water (entry 8). In this solvent, Pd/C was
superior to all other metals (rhodium, ruthenium, platinum,
Raney nickel) and solid supports (charcoal, alumina, barium
carbonate) tested (entries 9-13 and Table S1). A catalyst
loading of 2 mol%, a temperature of 100 °C, and a H2 pressure
of 5 bar are necessary to obtain 90% yield (entries 14-16).
Under ambient H2 pressure 3a could obtained in only 60%
yield (entry 17). Considered that the yields were calculated
based on the weight of technical CNSL, this is likely to be a
quantitative yield based on the cardanols. The reaction likely
proceeds via phenol hydrogenation followed by a classical
reductive amination (Scheme 2).37,39 In control experiments,
no product was formed when starting from the cyclohexanol,
whereas quantitative conversion was observed when starting
from the cyclohexanone (Scheme S1).
or
Li, 2015
addei, 2015
Beller, 2016
Fu, 2016
[H] = HCO2Na, toluene
R' = Ar or aliphatic
[H] = HCO2Na, water
[H] = H2, p-xylene
[H] = H2, hexane
T
[H]
[Pd]
[H]
[Pd]
H2NR' or HNR'R''
H2O
or
Scheme 2. Reductive amination of phenols to cyclohexyl amine derivatives.
The use of hydrogen in combination with a Pd/C catalyst
and an ecologically benign solvent would be optimal for large-
scale applications. We thus began investigating the coupling of
technical CNSL (1) with di-n-butyl amine (2a) as a model
We next investigated the generality of this reductive
amination (Table 2). Cyclic, mono- and dialkylamines were all
converted in good yields. Functionalities including methoxy,
acetal, or dialkylamino groups were well tolerated. This
confirms that this reaction step is suitable for designing
structurally diverse tensides for various applications.
reaction using 2 mol% Pd/C at 10 bar H2 pressure at 100 °C
(Table 1). Compared to structurally simple and highly purified
phenols, dark-brown, oily CNSL proved to be a challenging
substrate.
Table 1. Optimisation of the reductive amination conditions. a
The scalability of the reaction was demonstrated by
synthesizing N,N-dibutyl-3-pentadecyl cyclohexylamine
66% yield) and N,N-dimethyl-3-pentadecyl cyclohexylamine
3b, 64% yield) on 15-20 g scale.
(3a,
catalyst, H2
HN(n-Bu)2
solvent, T, 15 h
(
1
2a
Catalyst
3a
Table 2. Substrate scope for the reductive amination of cardanol.a
Entry
Solvent
Yield of 3a (%)
2 mol% Pd/C
5 bar H2
1
2
Pd/C
EtOH
iPrOH
15
11
"
H2O, 100 °C, 15 h
3
"
toluene
17
4
"
cyclohexane
15
5
"
mesitylene
11
1
2a-l
3a-l
6
"
diglyme
H2O/iPrOH
5
7
"
51
8
"
Rh/C
Ru/C
Al/Ni
Pd/Al2O3
Pd/BaSO4
Pd/C
"
H2O
90
9
"
"
"
"
"
"
"
"
"
49
10
11
12
13
14b
15c
16d
17e
n.d.
n.d.
trace
n.d.
18
40
"
90
"
60
a Reaction conditions: 0.5 mmol 1, 0.55 mmol 2a, 2 mol% catalyst, 10 bar H2, 2 mL
solvent, 100 °C, 15 h. Yields were determined by GC using n-hexadecane as
internal standard and calculated based on CNSL weight. b 1 mol% catalyst. c 80 °C.
d 5 bar H2. . e 1 bar H2. n.d. = not detected.
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Benzy l chloride
3a 86%b
3b 85%b
3c 70%
5b, 62%
66%c
64%d
3b
Bromoacetic acid
3d 74%
3e 74%b
3f 80%
84%b
6b, 67%
Scheme 4. Synthesis of quaternary ammonium salt and betaine.
The surface tension and aggregation behaviour of 4b, 5b,
and 6b were explored using drop shape analysis. Their critical
micellar concentration (CMC) was found to be 28 M, 0.5 M,
and 10 M respectively (see SI for the data). These values
3g 77%b
3h 61%
3i 85%
µ
µ
µ
compare favourably with those of the state-of-the-art
commercial fatty acid-derived tensides lauryldimethylamine
oxide (1.7 mM),44
stearyldimethylbenzylammonium chloride (0.31 mM),45 and
lauryl betaine (2.86 x 10-3 mM),46 proving their exceptional
potential as surfactants. These results form an excellent basis
for the targeted design of cardanol-derived amine-based
tensides for various applications.
3j 67%
3k 85%
3l 80%
a Reaction conditions: 1 mmol 1, 1.1 mmol 2, 2 mol% Pd/C, 5 bar H2, 3 mL H2O,
b
c
100 °C, 15 h. Isolated yields after purification. 10 bar H2. 50 mmol scale.
d
68 mmol scale.
The oxidation of amines 3a and 3b to the corresponding N-
oxides proceeded in quantitative yields using hydrogen
peroxide (Scheme 3). Moreover, both reaction steps were
successfully combined into one pot. The resulting process
involves reductive amination of cardanol and dimethylamine in
the presence of Pd/C at 10 bar H2 and 100 °C in water. After 15
h, the intermediate amine was oxidized with H2O2 at room
temperature. Water and pentane were added after 30
minutes, and the aqueous phase was concentrated, yielding 4b
in 71% yield. Unreacted amine and 3-pentadecylphenol side-
product remained in the organic layer, and the catalyst was
recovered by filtration. This way, an E-factor of 2 (total kg of
waste per kg product, see SI) was reached which for large-scale
manufacture can certainly be improved further.40,41
Conclusions
A straightforward, eco-friendly and waste-minimised concept
for the synthesis of amine-based surfactants from CNSL, an
inedible waste product of the cashew nut industry, has been
devised. The N-oxide, betaine, and quaternary ammonium salt
tensides thus obtained show comparable or better properties
than state-of-the-art commercial surfactants. The key step, a
catalytic reductive amination, is carried out in water using
molecular H2 as reductant. In the case of the particularly
valuable N-oxide surfactant, the entire reaction sequence can
be performed in one pot with water as the sole solvent,
resulting in a remarkably low E-factor.
Acknowledgements
HNR'2
Pd/C, H2
H2O2
We thank Marvin Meinecke for assistance with analyses, the
DFG (Cluster of Excellence EXC1069 “RESOLV”) for financial
support, and Umicore and Cardolite Corporation for donation
of chemicals.
r.t., 30 min
H2O, 100 °C, 15 h
R'= n-Bu, 4a, 93%
R' = Me, 4b, 94% (71%)a
1
3
Scheme 3. Synthesis of N-oxides from cardanols. a yield of one pot procedure.
Conflicts of interest
The authors declare no conflict of interest.
Amine 3b was also successfully converted to the benzyl-
quat
N-benzyl-N,N-dimethyl-3-pentadecylcyclohexan-1-
5b and to the betaine 2-(dimethyl(3-
aminium
(
)
Notes and references
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