N2O-Free single-pot conversion of cyclohexane to adipic acid catalysed by an iron(II) scorpionate complex

Article abstract of DOI:10.1039/c6gc03208g

The highly efficient eco-friendly synthesis of adipic acid (96% yield, 98% selectivity, TONs up to 1.8 × 10³) directly from cyclohexane is achieved by using ozone and [FeCl₂{κ³-HC(pz)₃}] (pz = pyrazol-1-yl) in a solvent-, heating-, radiation- and N₂O-free new protocol.

Full text of DOI:10.1039/c6gc03208g

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N2O-free single-pot conversion of cyclohexane to adipic acid
catalysed by an iron(II) scorpionate complex
Received 00th January 20xx,
Accepted 00th January 20xx
A.P.C. Ribeiro, a L.M.D.R.S. Martins*,a,b and A.J.L. Pombeiroa
DOI: 10.1039/x0xx00000x
www.rsc.org/
The highly efficient eco-friendly synthesis of adipic acid (96% yield, dicarboxylic acid, herein we report the one-pot oxidation of neat
98% selectivity, TONs up to 1.8 x 103) directly from cyclohexane is cyclohexane, by ozone, to adipic acid catalysed by the bio-inspired
achieved by using ozone and [FeCl2{κ3-HC(pz)3}] (pz=pyrazol-1-yl) C-scorpionate Fe(II) complex, [FeCl2{κ3-HC(pz)3}] (pz = pyrazol-1-yl)
in a solvent-, heating-, radiation- and N2O-free new protocol.
[11], as depicted in Scheme 1. This new catalytic system operates at
room temperature, in solvent- and radiation-free conditions. Adipic
acid precipitates from the reaction mixture, allowing a very simple
procedure (by filtration, see experimental, ESI) for its isolation in a
pure form (Figures S1 and S2).
Due to its versatility as building block for several processes in the
chemical, pharmaceutical and food industries, adipic acid is a
relevant commodity chemical produced worldwide (over 3.5
millions of metric tons/year, growing ca. 5%/year) [1,2]. Its primary
use is as a precursor for the synthesis of Nylon-6,6 polyamide
although also widely used to produce polyester and polyurethane
resins.
Adipic acid is industrially obtained [1,3] via a two-step process: i)
oxidation of cyclohexane to KA oil (a mixture of cyclohexanol and
cyclohexanone), and ii) oxidation of KA oil with an excess of nitric
acid leading to an extensive liberation of the greenhouse gas N2O
(300 kg of N2O per ton of adipic acid) [1,4]. Although alternative
substrates and synthetic routes have been explored towards a
future sustainable adipic acid production [4-10], the need for a
procedure that meets those requirements remains.
Scheme 1. One-pot oxidation of cyclohexane to adipic acid
catalysed by [FeCl2{κ3-HC(pz)3}].
As depicted in Figure 1 and Table S1 (ESI), a 6 h reaction time is
required for the quantitative formation of adipic acid. Shorter
reaction times (e.g., 3 h) led to the formation of KA oil instead of
this acid (entry 5, Table S1, ESI). The present reaction time is much
shorter than the reported [5] for the non-catalytic ozonolysis of
cyclohexane under UV irradiation (15 h).
A single-pot oxidation (from the alkane) would be especially useful
for the adipic acid production, in view of the adopted industrial
process. Moreover, it should also overcome the current undesirable
emission of N2O, thus avoiding the implementation of technologies
for recovering and reuse this pernicious gas. From a synthetic and
industrial viewpoint, such a single step procedure would also be
economically favourable.
The iron compound previously proved its catalytic activity in the
selective peroxidative (with H2O2) oxidation of cyclohexane to a
mixture of cyclohexanol and cyclohexanone [11-15], but neither
adipic acid was formed under those conditions, nor the complex
was tested with other oxidants.
With the aim of developing a more efficient and eco-friendly
process for the synthesis of the industrially most important
Besides cyclohexane-1,2-diol, obtained in very low amounts (<2.5%
Figure 2 and Table S1, ESI), no short-chain dicarboxylic acids or
other products were detected under the optimized conditions,
indicating a rather selective protocol for the adipic acid production
(with improved waste minimisation).
a.Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
b.Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa,
Instituto Politécnico de Lisboa, Rua Conselheiro Emídio Navarro, 1959-007 Lisboa,
Portugal. E-mail: lmartins@deq.isel.ipl.pt
The knowledge that acidic conditions (e.g., aq. HCl solution [5])
could promote the adipic acid formation, prompted us to use
pyrazine carboxylic acid (Hpca), a known promotor for the [FeCl2{κ3-
Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, analytical and catalytic data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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HC(pz)3}] catalysed cyclohexane oxidation to KA oil [14]. At Our catalytic system is also advantageous relative the reported
optimized reaction conditions Hpca notably improves the yield and photochemical one [5] affording higher yields of adipic acid (96%
selectivity of the reaction, leading to almost quantitative (>96%, and 75% [5], respectively).
Figure 1 and Table S1, ESI) formation of adipic acid with a turnover
number, TON (moles of oxygenated products per mol of catalyst) of Other advantages of this new catalytic protocol are i) very low
1.8 x 103. Moreover, it is obtained in a much shorter reaction time catalyst load (0.05 mol% vs. cyclohexane, Table S1, ESI), ii) mild and
(reduction from 15 [5] to 6 h) than when HCl is used.
environmental benign conditions, such as the solvent-, heating-,
radiation- and N2O-free protocol (no N2O abatement required), iii)
the short reaction time required, increasing atomic and energetic
efficiency, iv) the use of a non-toxic, biologic and cheap metal as
catalyst and v) a facile work-up; significant features towards the
development of a sustainable chemical process.
100
80
60
40
20
0
Control experiments under the same conditions but in the absence
of [FeCl2{κ3-HC(pz)3}] (leading to up to 15% cyclohexane conversion
with
a selectivity of 20, 60 and 20%, for cyclohexanol,
cyclohexanone and adipic acid, respectively, Table S1, ESI)
confirmed the crucial role of the scorpionate iron(II) complex to
efficiently catalyse the cyclohexane conversion to adipic acid.
Moreover, the use of Fe(II) or Fe(III) chlorides affords a maximum of
27 or 21% conversion of cyclohexane, respectively, but without the
formation of adipic acid (entries 14-17, Table S1, ESI).
0
1
2
3
4
5
6
Time / h
Although the detailed mechanistic pathway is still to be established,
the oxidation of cyclohexane by ozone is believed to proceed by a
mechanism related to that reported for the oxidation of
Figure 1. Effect of the reaction time on the relative amount (molar
%) of cyclohexane (*) and of products, cyclohexanol (ꢀ),
cyclohexanone (ꢀ), cyclohexane-1,2-diol (ꢁ) and adipic acid (ꢂ),
obtained by one-pot oxidation of cyclohexane with O3 catalysed by
[FeCl2{κ3-HC(pz)3}].
cyclohexane by dioxygen catalysed by
a mononuclear Fe(II)
porphyrin complex [16,17]. It is likely that the active catalytic
species is a formally Fe(IV)=O one, resulting from the oxidation of
the Fe(II) complex by O3. The mechanism consists of three major
The effect of Hpca is also clearly more pronounced than the oxidative steps (Scheme S1, ESI): i) cyclohexane oxidation to
observed with the inorganic nitric acid (Figure 2 and entries 8 and 9,
Table S1, ESI). The same behaviour was found for the peroxidative
(with H2O2) oxidation of cyclohexane to KA oil [14].
cyclohexane-1,2-diol (involving two consecutive hydroxylation
steps, each initiated by hydrogen-atom transfer from the
a
substrate to Fe(IV)=O); ii) cyclohexane-1,2-diol conversion to
hexane-1,6-dial (by intradiol C-C bond cleavage); and iii) hexane-
1,6-dial oxidation to adipic acid (via hydroxylation of the carbonyl
groups) [17]. The absence of cyclohexane-1,2-diol in our reaction
solution (Figure 1) would be in accord with the very rapid ring-
opening of the diol in the second main oxidative step of the
proposed mechanism [17].
100
80
60
40
20
0
Our new protocol is also advantageous relative to the so far
patented [7-9] alternative methods to produce adipic acid from
cyclohexane: i) by catalytic air oxidation of cyclohexane, requiring
high temperatures to yield a maximum of 70% of adipic acid and
thus purification operations [9]; ii) by oxidation of cyclohexane
dissolved in a halogenated hydrocarbon (e.g., CCl4) with an ozone-
containing gas while heating up to 65 ºC [7]; or iii) by using a
bimetallic catalyst supported on a transition metal oxide in an
autoclave at up to 300 ºC, leading to a cyclohexane conversion of
only 21% with an adipic acid selectivity of 34% [8].
none
Hpca
nitric acid
potassium
carbonate
Figure 2. Effect of additives on the yields of adipic acid obtained by
one-pot oxidation of cyclohexane with O3 catalysed by [FeCl2{κ3-
HC(pz)3}].
Thus, the reported [5] non-catalytic ozonolysis of neat cyclohexane
(affording 12 mol% of adipic acid) or of cyclohexane – 8 vol% aq. 0.5
M HCl (leading to 45 mol% of adipic acid) are now notably improved
by the action of catalytic amounts of [FeCl2{κ3-HC(pz)3}] and Hpca.
In summary, a new N2O-free simple method for the adipic acid
synthesis via the one-pot oxidation of cyclohexane with ozone has
2 | J. Name., 2016, 00, 1-3
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been developed. Green metrics (Table S1) such as atom economy
(molecular weight of desired product per combined molecular
weight of starting materials) or carbon efficiency (amount of carbon
in product per total carbon in reactants) highlight the need of a
careful synthetic evaluation, especially on the use of co-catalysts.
Further optimization studies will be attempted in view of the
interesting preliminary results achieved.
Funding from FCT (Fundação para a Ciência e a Tecnologia,
Portugal) (UID/QUI/00100/2013, PTDC/QEQ-ERQ/1648/2014 and
PTDC/QEQ-QIN/3967/2014 projects) are acknowledged. The
authors gratefully acknowledge Dr. Maria Cândida Vaz (IST) for the
direction of the elemental analysis service, the Portuguese NMR
Network (IST-UTL Centre) for providing access to the NMR facility.
Notes and references
1
Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.,
Wiley-VCH, Weinheim, Germany, 2002.
2
Adipic acid (ADPA): 2016 world market outlook and forecast
up to 2020, Merchant Research and Consulting, Birmingham,
UK, 2016.
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4
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2013, 3, 1465-1479.
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K.C. Hwang, A. Sagadevan, Science, 2014, 346, 1495-1498.
F. Cavani, S. Alini, in Sustainable Industrial Processes, F.
Cavani, G. Centi, S. Perathoner, F. Trifiró (eds.), Wiley-VCH,
Weinheim, 2009, Ch.7, pp. 367-425.
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Miyazaki Shiyouji, JPS565374 (B2), 1981.
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14 L.M.D.R.S. Martins, A.J.L. Pombeiro, Coord. Chem. Rev.,
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Fe
Iron catalysed efficient single-pot adipic acid synthesis by cyclohexane oxidation with ozone in a solvent-, heating-,
radiation- and N2O-free protocol.