Photoinduced cycloaddition of biomass derivatives to obtain high-performance spiro-fuel

Article abstract of DOI:10.1039/c9gc02790d

The photoinduced conversion of biomass-derived chemicals to high value chemicals and advanced fuels is of great significance but still challenging. Herein, a green and efficient self-sensitized [2 + 2] cycloaddition process is developed to convert biomass-derived β-pinene and isophorone to spirocyclic molecules, which cannot be achieved by thermal catalytic conversion. The photoreaction can take place with isophorone as the self-sensitizer, with high selectivity and a yield of up to 91.1%. A triplet sensitization mechanism is disclosed by a combination of triplet quenching, phosphorescence quenching, Stern-Volmer kinetic analysis, DFT calculations and photochemical kinetic studies. When combined with hydrodeoxygenation, spiro-fuel is obtained with an overall yield of 85.0% showing a high density of 0.911 g mL^-1 which is 16.8% higher than that of conventional aviation kerosene (ca. 0.78 g mL^-1), along with excellent cryogenic properties. Notably the self-sensitized cycloaddition strategy can be extended to a wide range of biomass derived α,β-unsaturated ketones and alkenes. Thus, this work provides a promising ring-increasing route to upgrade low-density bio-derived feedstocks to high-density hydrocarbons.

Full text of DOI:10.1039/c9gc02790d

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Paul T. Anastas et al.
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DOI: 10.1039/C9GC02790D
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Photoinduced cycloaddition of biomass derivatives to high-
performance spiro-fuel
Junjian Xie1, Lun Pan1, Genkuo Nie, Jiawei Xie, Yakun Liu, Chi Ma, Xiangwen Zhang, Ji-Jun Zou*
Received 00th January 20xx,
Accepted 00th January 20xx
Photoinduced conversion of biomass-derived chemicals to high value chemicals and advanced fuels is of great significance
but still challenging. Herein, a green and efficient self-sensitized [2+2] cycloaddition process is developed to convert
biomass-derived β-pinene and isophorone to spirocyclic molecules, which can not be achieved by thermal catalytic
conversion. The photoreaction can take place in high selectivity with isophorone as the self-sensitizer, with yield up to
91.1%. Triplet sensitization mechanism is disclosed by combination of triplet quenching, phosphorescence quenching, Stern-
Volmer kinetic, DFT calculation and photochemical kinetic studies. Combined with hydrodeoxygenation the spiro-fuel with
an overall yield of 85.0% is obtained, which shows a high density of 0.911 g mL-1 that is 16.8% higher than conventional
aviation kerosene (ca. 0.78 g mL-1), along with excellent cryogenic properties. Notably the self-sensitized cycloaddition
strategy can be extended to a wide scope of biomass derived α, β-unsaturated ketones and alkenes. Thus, this work provides
a promising ring-increasing route to upgrade low-density bio-derived feedstocks to high-density hydrocarbons.
DOI: 10.1039/x0xx00000x
www.rsc.org/
furanic aldehydes15 and aromatic oxygenates with furfural
alcohol,16 Robinson annulation of furfural with 2,4-
pentanedione,17 Diels-Alder addition of diacetone alcohol,18
Introduction
High-density aviation fuels are critically important for
aerospace vehicles because they can provide more propulsion
energy to extend the flight range and increase the payload,
attributed to their higher density and volumetric heat value
than conventional distilled fuel.1 Considering the low
temperature environment encountered by aerospace vehicles,
the fuels must have satisfied cryogenic properties such as low
freezing point and low viscosity. Traditional high-density liquid
fuels are mainly synthesized using petroleum-based feedstocks,
which are consisted of multi-cyclic hydrocarbons to afford high
density.2 For example, JP-10, RJ-4, RJ-7 and HDF-T1 are typic
polycyclic or bridged-cyclic hydrocarbons.1,2 Recently
alternative routes for production of fuels from renewable
resources like biomass is receiving considerable attention
because of the increasing demand on energy and the need for
sustainable development.3,4 Many C-C coupling strategies have
been adopted to upgrade the biomass-derived compounds to
high value fuels,5 like aldol condensation of ketones (such as
cyclic ketones, benzaldehydes and aliphatic ketones)6,7, furanic
aldehydes8 and isophorone9, oligomerization of olefins10,11
(chain olefins, cyclic olefins and terpene), Michael addition of
methyl ketones12 or 1,3-dicarbonyl compounds with enones,13
hydroxyalkylation/alkylation of 2-methylfuran with ketones14 or
reductive coupling/rearrangement of cyclic ketones19 and so on.
However, most of the synthesized biofuels are linear
hydrocarbons or branched monocycloalkanes with density <
0.85 g mL-1, which cannot meet the requirement of high-density
fuels (Table S1 in ESI). Although several multi-cyclic
hydrocarbons like tri(cyclopentane),20 HCWO and cedrane21
with high densities (> 0.91 g mL-1) have been synthesized, their
cryogenic properties are not good. Actually, most of
abovementioned biofuels are synthesized by direct C-C coupling
of cyclic reactants without constructing new rings, so these
syntheses are highly dependent on cyclic feedstocks and not
able to increase the density significantly. Thus, the key to
synthesize high performance biofuel is to develop routes for
constructing new ring structure to endorse high density.
Specifically, spirocycloalkanes are reported to have both high
density and good cryogenic properties, which represent a new
type of very promising high-density fuels.19
Recently, light-driven reaction has attracted increasing
interests as a new route for conversion of biomass.22-24 Many
visible-light-driven reactions have been developed to convert
biomass and its derivates to valuable chemicals,25 such as
photocatalytic oxidative cycloaddition of phenols and alkenes
to dihydrobenzo furan derivates,26 photocatalytic upgrading of
furan aldehyde to valued bio-products and H2,27 photodegrade
Key Laboratory for Green Chemical Technology of the Ministry of Education, School
of Chemical Engineering and Technology, Tianjin University, Collaborative
Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,
China. E-mail: jj_zou@tju.edu.cn
1These authors contributed equally (J. Xie and L. Pan)
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9GC02790D
Scheme 1. Synthesis route of high-performance spiro-fuel through self-photosensitized cycloaddition and hydrodeoxygenation.
lignin into functionalized aromatics.28 Compared with thermal Materials
reactions, photoreactions occur under relatively mild conditions
Isophorone (99%), β-pinene (99%), α-pinene (98%), 3-
methylcyclopent-2-enone (98%), 2-cyclopentenone (98%),
cinnamaldehyde (98%), coumarin (99%), carvone (99%), n-
hexene (99%) and camphene (95%) were provided by Shanghai
Aladdin Biochemical Technology Co., China. Cyclohexane (99%),
CH2Cl2 (99%), cinene (98%), 2-methoxy-4-vinylphenol (98%), 2-
methylfuran (99%), 2,5-dimethylfuran (99%) and chloroplatinic
acid (99%) were supplied by Tianjin Yuanli Chemical Co., China.
All chemicals were directly used as received without further
purification. HY (SiO2/Al2O3=5.4) was purchased from Nankai
Catalysts Company and calcined in air at 580 oC for 3 h. 3% Pt/HY
was prepared by the incipient wetness impregnation method.
and, in many cases, show a high selectivity. Although biomass
conversion to various platform compounds have been
industrialized and hydrodeoxygenation of biomass derivatives
to fuels or fine chemicals have been studied intensely, to the
best of our knowledge, photoinduced cycloaddition for biomass
derivatives to synthesize high-performance biofuels has not
been reported.
In this work, we report for the first time the green and
efficient self-photosensitized [2+2] cycloaddition to synthesize
high-performance spiro-fuel from biomass-derived β-pinene
and isophorone, as illustrated in Scheme 1. It should be pointed
out that [2+2] cycloaddition is thermally forbidden but is
kinetically accessible by photochemical methods.29 Several
sunlight-driven cycloaddition reactions have been carried out,
such as the Diels-Alder cycloaddition of maleic anhydride and
anthracene,30 retro Diels-Alder conversion of dicyclopentadiene
to cyclopentadiene31 and intramolecular [2+2] cycloaddition of
norbornadiene,32-34 Paterno-Buchi reaction of carbonyl
Synthesis of spiro-fuel
The photoreaction was carried out in a 25 mL jacket reactor
irradiated by a high-pressure mercury lamp with adjustable light
intensity. The temperature was controlled by using constant
temperature circulation tank. Typically, the reaction solution
(25.5 mmol isophorone and 102 mmol β-pinene) was bubbled
with nitrogen gas for 0.5 h, and then irradiated for 9 h under
stirring. Subsequently, the product was separated by vacuum
distillation, and subjected to hydrodeoxygenation (HDO) in a
100 mL autoclave (Easy Chem E100) with a mechanical
agitation. 10 g of substrate, 5 g of 3% Pt/HY and 50 mL of
cyclohexane were loaded, flushed with N2 gas for three times,
and then hydrodeoxygenated under H2 pressure of 6 MPa at
temperature of 150 oC for 10 h. The target spiro-fuel was finally
obtained through filtering and vacuum distillation. The reaction
product was diluted with CH2Cl2 and quantitative analyzed by
using an Agilent 7820A GC system equipped with FID detector
and HP-1 capillary column (30 m × 0.53 mm), and qualitatively
measured using an Agilent 6890/5975 GC-MS equipped with
HP-5 capillary column (30 m × 0.5 mm). The products were also
compound to an olefin.35 Notably, β-pinene,
a major
component of turpentine with a productivity of around 330,000
tons per year, is a versatile natural product and extensively used
as solvents, resin precursors, pharmaceutical intermediates and
commodities,36 and have been used to synthesize high-density
fuels but their cryogenic properties are not good.37,38
Meanwhile isophorone, a typical biomass platform molecule,
can be produced from lignocellulose in industrial scale via
Acetone-Butanol-Ethanol fermentation followed with ease
trimerization of acetone.9,39 For instance, isophorone can be
industrially obtained in 83% yield by catalytic acetone
condensation in continuous process.40 And it has been used as
feedstock
to
produce
biofuels
through
direct
hydrodeoxygenation or selective hydrogenation and then self-
aldol condensation or aldol condensation of isophorone with
furanic aldehydes.9,39,41 Herein, the [2+2] cycloaddition of β-
pinene and isophorone is facilitated under light irradiation to
construct a spiro skeleton. More importantly, the obtained
spirocycloalkane possesses high density, high volumetric neat
1
identified by 13C and H NMR spectra collected using Bruker
Avance 500 MHz Spectrometer.
Calculations and measurements
heat of combustion (NHOC) and excellent cryogenic properties, The computations on photoreaction pathway were conducted
which represents a new type of renewable high-density fuel with the Gaussian 09 program package at the B3LYP/6-31g(d)
with great application potential. Also this self-sensitized density functional level.42 Geometries of the reactant, product
cycloaddition strategy can be extended to a wide scope of and transition state were optimized using Berny’s algorithms.43
biomass derived α, β-unsaturated ketones and alkenes.
The computed harmonic frequencies of fully optimized minima
were real, whereas transition structures had a single imaginary
frequency. Intrinsic reaction coordinate calculations were
Experimental section
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conducted to ensure that the transition states connect to the
correct reactants and products.
Isophorone/-pinene mVixietwurAerticle Online
Isophorone
a
DOI: 10.1039/C9GC02790D
The density of fuel was measured with a Mettler Toledo
DE40 density meter according to ASTM D4052. Freezing point
was measured as outlined in ASTM D2386. Kinematic viscosity
was determined using capillary viscometer according to ASTM
D445. The net heat of combustion (NHOC) was measured using
the IKA-C6000 isoperibol Package 2/10 Calorimeter according to
ASTM D240-02.
-pinene
Results and discussion
Photoinduced self-sensitized [2+2] cycloaddition
200
C
300
400
500
Wavelength/nm
o
First the thermal reaction (150 C, 24 h) without light were
S
S
C
self-adduct
100
isophorone
co-adduct
-pinene
30
20
10
0
carried out as blank reaction, and as expected β-pinene and
isophorone do not undertake any reaction under thermal
activation. Pure β-pinene cannot undergo any cycloaddition or
coupling reactions under irradiation, except small amount of
oxidation products (Scheme S1 and Fig. S1, ESI), suggesting β-
pinene cannot be excited by light for photoreaction. Differently,
under irradiation pure isophorone undergoes [2+2]
cycloaddition with conversion of 42.9%, along with small
amount of isomerized product (Fig. S2-S4, ESI), indicating
isophorone molecules can be photoexcited for intermolecular
[2+2] cycloaddition. For the mixture of β-pinene and
isophorone, as shown by the time-dependent product
distribution in Fig. 1, there are two reactions, i.e. [2+2] co-
cycloaddition between β-pinene and isophorone and [2+2] self-
cycloaddition of isophorone. But the former one is dominant,
with a selectivity of 95.8% for co-adduct (Scheme 1 and Fig. S5-
S7, ESI) and just 1.5% for self-adduct.
b
80
60
40
20
0
p
m
m
m
n
m
n
n
n
m
a
0
6
3
5
6
0
2
l
4
5
2
y
r
3
4
=


=
u
=
c
r
e



M
Figure 2. (a) UV-vis absorption spectra of reactants. (b) Effect of wavelength on
photoinduced cycloaddition of isophorone and β-pinene. Reaction conditions: 10 oC, 9 h,
25.5 mmol isophorone and 102 mmol β-pinene, light intensity of 27 mW/cm2.
For photoreaction a photosensitizer is necessary to absorb
light energy and trigger the reaction through energy or electron
transfer.25 Ketone compounds including benzophenone,
thioxanthone, acetophenone, acetone etc. are typical
sensitizers with its carbonyl groups as chromophore.44 The
present experimental results suggest isophorone works as not
only a reactant but also a sensitizer, attributed to the presence
of carbonyl group. Therefore, this reaction should be a self-
sensitized cycloaddition. Light absorption ability of the
photosensitizer, especially the light wavelength, is the key
factor in photoreaction. UV-vis absorption spectra in Fig. 2a
depicts that β-pinene cannot absorb wavelength longer than
275 nm due to the absence of suitable chromophore, whereas
isophorone shows a strong absorption at ca. 300-375 nm
attributed to the carbonyl group. The photoreaction was then
carried out under different wavelength with the same intensity
as exhibited in Fig. 2b. Under 365 nm, the photoreaction shows
the highest activity, while 420 nm cannot trigger the
photoreaction at all because none of the reactants can absorb
visible light. When the full wavelength or incident wavelength
larger than 360 nm is applied, the photoreaction is much slower
than that at 365 nm, because most of the light are in the range
beyond 400 nm that cannot be absorbed by isophorone.
Notably, under 254 nm, the reaction activity is much lower than
that under 365 nm. The above results further confirm that
isophorone serves as the photosensitizer in the reaction, and
especially the absorption in the range of 300-375 nm is the most
effective to trigger the photoaddition.
100
C
80
60
40
20
0
isophorone
C
S
-pinene
co-adduct
S
self-adduct
Quenching experiments including triplet quenching and
phosphorescence quenching were further conducted to
understand the self-sensitized photoreaction. It is well
established that cis and trans isomerization of alkenes can be
effective sensitized by α, β-unsaturated cyclic ketones through
triplet energy transfer.29 As shown in Fig. 3a, direct exciting
isophorone by 350 nm light at 77 K generates a moderate
1
2
3
4
5
6
7
8
9
Time (h)
Figure 1. Time-dependent photoinduced cycloaddition of isophorone and β-pinene.
Reaction conditions: 10 oC, 25.5 mmol isophorone and 102 mmol β-pinene, light intensity
of 230 mW/cm2.
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phosphorescence emission with λmax at 476 nm due to the phosphorescence intensity of isophorone is gradually
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transition from triplet state to ground state, which corresponds decreased, which infers that triplet isophDoOroI:n1e0.1t0ra3n9/sCfe9GrsCe0n27e9r0gDy
to a triplet energy of 251.4 kJ mol-1 for isophorone. Thus trans- to β-pinene for further reaction. However, phosphorescence
stilbene with a lower triplet energy of 209.2 kJ mol-1 is used as emission of β-pinene in Fig. 4b shows a λmax at 356 nm (the
the triplet quencher to confirm the existence of triplet emission less than 300 nm is attributed to fluorescence),
isophorone.45 It can be seen from Fig. 3b that the trans-stilbene equivalent to a triplet energy of 336.1 kJ mol-1 and much higher
greatly suppresses the cycloaddition, with the conversion being than that of isophorone (251.4 kJ mol-1). Thus, it is more likely
reduced from 94.1% to 25.5% for isophorone (from 36.6% to that triplet isophorone collides with ground state β-pinene to
9.6% for β-pinene) and the selectivity of co-adduct from 95.8% generate a complex (instead of triplet β-pinene) and further
to 63.6%. Correspondingly, large amount of cis-stilbene is induce the cycloaddition. Specifically, a perfect linear Stern-
detected. It should be noted that trans-stilbene cannot be Volmer plot of phosphorescence intensity attenuation vs β-
excited by UV light to produce the cis-isomer,45 and cis-stilbene pinene concentration is obtained in Fig. 4c, with a slope of 0.77
must be generated through energy transfer from triplet L mol-1. As depicted in Eq. (1), the slope of this plot equals to
isophorone. These observations strongly suggest that triplet kqτ, where kq is the rate constant for quenching and τ is the
isophorone is the key to triggering the cycloaddition. Then lifetime of triplet isophorone in the absence of β-pinene. F0 and
phosphorescence quenching experiments with trans-stilbene as F represent the phosphorescence intensity without and with β-
the quencher were carried out to further prove the existence of pinene, respectively. Given that the lifetime of triplet
triplet isophorone. During the quenching, triplet isophorone will isophorone is around 60 ns,46 the quenching rate constant kq in
cross intersystem to singlet through energy transfer to trans- cycloaddition of isophorone and β-pinene is in the order of 1.28
stilbene other than emitting phosphorescence. As exhibited in
x
107, which is comparable to the rate constant of
Fig. 3a, as the concentration of trans-stilbene increases, the cyclohexenone quenched with alkenes.46 It is noteworthy that
phosphorescence intensity of isophorone is greatly reduced triplet isophorone self-quenching is considerable in pure
because of the enhanced triplet energy transfer.
isophorone because the self-cycloaddition of pure isophorone
In order to find out how the energetic triplet isophorone can also take place smoothly in the blank test. However, co-
works in the photoreaction, phosphorescence emission of cycloaddition completely overwhelms the self-cycloaddition in
isophorone with varied β-pinene concentration was carried out
in Fig. 4a. With the increase of β-pinene concentration, the
a
0 mol/L
0.20 mol/L
0.73 mol/L
2.18 mol/L
3.40 mol/L
a
0 mol/L
0.03 mol/L
0.09 mol/L
0.27 mol/L
0.81 mol/L
450
500
550
600
650
Wavelength (nm)
476
450
500
550
600
650
Wavelength (nm)
4
3
2
1
c
100
80
60
40
20
0
C
F0/F= 0.77 -pinene] + 1.35
R2= 0.998
b
isophorone
C
-pinene
S
co-adduct
b
356
300
400
500
600
Wavelength (nm)
0
1
2
3
-pinene] (mol/L)
None
Trans-stilbene
Figure 4. (a) Phosphorescence spectrum of isophorone as a function of β-pinene
concentration with excitation at 350 nm under 77 K. (b) Phosphorescence emission of β-
pinene with excitation at 260 nm under 77 K. (c) Stern-Volmer phosphorescence
intensity quenching studies of isophorone and β-pinene.
Figure 3. (a) Phosphorescence spectrum of isophorone as a function of trans-stilbene
concentration with excitation at 350 nm under 77 K. (b) Triplet quenching effect on the
cycloaddition of isophorone and β-pinene with 0.5 mol/L trans-stilbene.
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isophorone/β-pinene mixture. This indicates the reaction of from 96.6% to 86.3%, and the selectivity of self-adduct reaches
View Article Online
DOI: 10.1039/C9GC02790D
isophorone and β-pinene induced by triplet sensitization is 12.5% at the ratio of 4:1. Therefore, excess β-pinene is used to
highly efficient.
inhibit the formation of self-adduct, because excess β-pinene
F0 / F = 1+ kqτ [β-pinene]
(1)
can restrain the collision of triplet isophorone with the ground
Thus, we deduce that the photoreaction should be a self- state isophorone to generate the self-adduct, and ensure high
sensitized triplet energy transfer process as shown in Scheme 2. selectivity of the co-adduct. Therefore, preferred
a
Under irradiation, isophorone molecule is excited to triplet isophorone/β-pinene ratio of 1:4 is adopted, with the
state by absorbing photons. Then the triplet isophorone collides conversion of isophorone and β-pinene being 95.1% and 36.6%,
with ground state β-pinene to form an exciplex, which evolves respectively, while a co-adduct selectivity of 95.8% and yield of
into the energy-rich triplet 1 through an energetic triplet 91.1%.
biradical intermediate. Meanwhile the triplet 1 will convert to
Then the effect of light intensity on photoreaction was
the target co-adduct through intersystem crossing. To obtain studied. As illuminated in Fig. 6b, the increase of light intensity
further mechanistic evidence, DFT calculation was employed to can greatly enhance the conversion of isophorone and β-
explore the key steps of cycloaddition. As presented in Fig. 5, pinene, while the selectivity of co-adduct always remains at a
the pathway to get the co-adduct encounters a highest energy high level. It is reasonable because more isophorone molecules
barrier of 340.7 kJ/mol, which can thus be inferred that the can be excited under stronger light intensity. Different with the
cycloaddition of isophorone and β-pinene is almost impossible great influence of light intensity, as presented in Fig. 6c, when
to occur under thermal catalytic conditions. In addition, the the temperature goes from -20 oC to 20 oC, the photoreaction is
energy barrier is very close to the energy of 365 nm light (327.9 almost unchanged. It can be inferred that reaction temperature
kJ/mol), which indicates a good agreement with the high has little effect on the photoreaction, which is highly aligned
activity under 365 nm irradiation in Fig. 2b. It can also be seen with prior findings47 and can also be supported by the high
from Fig. 5 that the formation of biradical intermediate has the energy barrier of cycloaddition in Fig. 5.
activation energy of 188.2 kJ/mol. Considering that the lowest
Under the optimized reaction conditions, quantum yield (Φ)
triplet energy of isophorone is 252.0 kJ/mol, thence the of the photocycloaddition was further evaluated under single
excitation of isophorone to its triplet state is the key step and wavelength of 365 nm using the Eq. (2).
the collision between triplet isophorone and ground state β-
pinene to generate biradical intermediate should be an
exothermic process. The biradical intermediate closing the ring
to form triplet state of product has an energy barrier of 152.5
kJ/mol, along with the endothermicity of 120.1 kJ/mol. And the
configuration of transition state (TS) indicates
a good
agreement with the biradical in Scheme 2. The entire
cycloaddition is a non-spontaneous process with ΔG = 17.5
kJ/mol.
Optimization of reaction condition
To optimize the photoreaction parameters, we then conducted
the reaction under various conditions. As shown in Fig. 6a, with
the molar ratio of isophorone/β-pinene increasing from 1:6 to
4:1, the conversion of isophorone is suppressed and that of β-
pinene is enhanced, while the selectivity of co-adduct declines
Figure 5. DFT calculated energy profile for cycloaddition of isophorone and β-pinene.
C
C
S
co-adduct
C
C
S
co-adduct
C
S
co-adduct
100
80
60
40
20
0
a
100
80
60
40
20
0
C
-pinene
isophorone
-pinene
isophorone
-pinene
c
isophorone
b
100
80
60
40
20
0
1:6
1:1
4:1
1:4
1:2
2:1
130mW/cm2
170mW/cm2
210mW/cm2
Isophorone/-pinene molar ratio
-20 oC
-10 oC
10 oC
20 oC
0 oC
Figure 6. Effect of (a) isophorone/β-pinene molar ratio, (b) light intensity and (c) temperature on the photoinduced cycloaddition. Reaction conditions: (a) 10 oC, 9 h, isophorone and
β-pinene with a total volume of 20 mL, light intensity of 230 mW/cm2. (b) 10 oC, 9 h, 25.5 mmol isophorone and 102 mmol β-pinene. (c) 9 h, 25.5 mmol isophorone and 102 mmol
β-pinene, light intensity of 230 mW/cm2.
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DOI: 10.1039/C9GC02790D
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O
O
O
O
O
O
3
3
3
3
*
*
*
*
hv
co-adduct
triplet
exciplex
biradical
triplet 1
Scheme 2. Proposed mechanism for self-sensitized cycloaddition.
푁
where Q is the light intensity (W/cm2); S and n represent the
illuminated area (cm2) and the molar amounts of reactants
(mol), separately; A is preexponential factor. As described in Fig.
7c, the minus natural log of the reaction kinetic rate constant (-
lnk) was plotted as a function of inverse light intensity to obtain
the photochemical activation energy (Ea, W/mol). The
calculated Ea for the photoreaction is 10.25 W/mol.
훷 = 퐼푡(1 ― 푇)
(2)
where N means the molar amounts of co-adduct generated
during the irradiation time t; I and T are the flux of incident light
(mol/h) and transmittance of the reaction solution to incident
light, respectively. The cycloaddition shows a quantum yield of
0.22, indicating 0.22 mol cycloadducts can be produced by
absorbing 1 mole photons.
As discussed above, light intensity instead of reaction
temperature has a decisive influence on the photoreaction.
Accordingly, a series of experiments were conducted under
different light intensity to determine the pseudo reaction
kinetics of the photoaddition. Firstly, photoreaction with
different initial molar ratios of isophorone and β-pinene under
light intensity of 170 mW/cm2 is carried out to check the
reaction order (Fig. 7a). Subsequently, the reaction rate
equation is calculated and shown as Eq. (3). In order to obtain
the photochemical apparent activation energy, the
photoreaction is conducted under different light intensity in Fig.
7b. A formula Eq. (4) similar to Arrhenius function is proposed
to disclose the relationship between light intensity (Q) and the
reaction kinetic rate constant (k).
Self-sensitized cycloaddition of broader biomass derivatives
With the optimized conditions and proposed mechanism
established, we applied this self-sensitized cycloaddition
strategy to a broader range of biomass derived α, β-unsaturated
ketones and alkenes. As shown in Table 1, a variety of alkene,
cyclic olefin and aryl substituted olefin can afford the
corresponding [2+2] cycloadducts with high selectivity when
isophorone is used as the self-sensitizer (entry 1-5, GC-MS
spectra S8-S18 in ESI). Among them, the cycloaddition between
isophorone and 2-methoxy-4-vinylphenol affords a relatively
low 68.5% selectivity of co-adduct because large amount of self-
adduct of 2-methoxy-4-vinylphenol is detected owing to the
triplet energy transfer from triplet isophorone to 2-methoxy-4-
vinylphenol (entry 5).48 In addition, various biomass-derived
cyclic enones and aromatic enone compounds can also work as
푑퐶isophorone
0.04
―
= 0.1774퐶
퐶0.29
(3)
(4)
β ― pinene isophorone
dt
퐸푎
―ln푘 = 푄푆/푛 ―ln퐴
3.20
1.9
1.20
1.15
1.10
a
c
3.16
b
3.12
3.08
1.8
1.7
n
n
n
/n
=1:1 R2= 0.994
=1:2 R2= 0.999
=1:4 R2= 0.997
isophorone -pinene
3.04
2.04
2.00
1.96
-lnk=10.25n/QS+0.78
R2=0.986
1.6
/n
isophorone -pinene
130mW/cm2 R2=0.9925
170mW/cm2 R2=0.9973
210mW/cm2 R2=0.9978
1.20
1.18
1.16
1.14
1.12
1.10
1.5
1.4
/n
isophorone -pinene
7.0x10-5
8.0x10-5
9.0x10-5
1.0x10-4
1.1x10-4
0.1
0.2
0.3
Time (h)
0.4
0.1
0.2
0.3
0.4
n/QS
Time (h)
Figure 7. Fitting results of photoinduced cycloaddition of isophorone and β-pinene (scatter: experimental data; line: fitting result) (a) versus irradiating time at different initial
concentration, (b) versus irradiating time at different total light intensity. (c) The lnk-1/Q curve for the photocycloaddition of isophorone and β-pinene. Reaction conditions: (a) 10
oC, 170 mW/cm2, the initial molar ratios of isophorone and β-pinene are 1:1, 1:2 and 1:4, respectively, with a total volume of 20 mL; (b) 10 oC, the initial molar ratio of isophorone
and β-pinene is 1:4.
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Green Chemistry
View Article Online
DOI: 10.1039/C9GC02790D
Green Chemistry
ARTICLE
Table 1 Self-sensitized cycloaddition of series biomass derivatives.a
Entry
1
Substrate
Co-adduct
Conv. [%]b
Sel. [%]c
Enone
Olefin
O
O
99.0%
68.9%
95.5%
85.1%
O
O
2
O
O
O
3
4
92.5%
97.0%
96.5%
90.4%
O
O
+
OH
HO
H3CO
O
5
O
55.4%
68.5%
OCH3
O
O
6
7
O
O
96.0%
89.4%
90.1%
94.3%
8
9
82.0%
35.5%
92.5%
81.5%
O
O
O
O
O
O
O
10
11
12
O
40.5%
41.1%
35.8%
78.5%
92.1%
80.5%
O
O
O
O
O
O
O
O
a Reaction conditions: 9 h, 10 oC, 25.5 mmol enone and 102 mmol olefin, light intensity of 230 mW/cm2.
b Conversion of enone.
c Selectivity of co-adduct.
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Journal Name
Table 2. Properties of fuels synthesized using biomass derivatives.
View Article Online
DOI: 10.1039/C9GC02790D
Property
Molecular formula
Density at 20 oC (g·mL-1)
Freezing point (oC)
C19H32
0.911
C18H32
0.892
-40
C10H18
0.870
-76
2 (25 oC)
3 (0 oC)
20 (-60 oC)
42.72
C20H34
0.938
-23 ± 3
-51
3 (20 oC)
15 (0 oC)
176 (-20 oC)
42.45
29 (20 oC)
103 (0 oC)
1315 (-20 oC)
42.74
38.58
This work
Kinematic viscosity (mm2·s-1)
19,000-32,000
(-15 oC)
NHOC (MJ·kg-1)
Volumetric NHOC (MJ·L-1)
Reference
42.11
39.5
[37]
38.67
This work
37.17
[19]
self-sensitizers for the [2+2] cycloaddition with β-pinene (entry
6-10, GC-MS spectra S19-S30 in ESI). Moreover, cellulose
derived furans can undertake [4+2] cycloaddition with
isophorone (entry 11-12, GC-MS spectra S31-S34 in ESI).
Therefore, the self-sensitized cycloaddition provides a universal
and green ring-increasing route of biomass derivatives.
Fuel properties
After the photoreaction of isophorone and β-pinene, the co-
adduct was achieved with > 95% purity by vacuum distillation.
Then, the co-adduct was hydrodeoxygenated to saturated
hydrocarbon by bifunctional catalyst 3% Pt/HY, because acid
sites and platinum can show a synergistic effect for HDO,
namely acidic support HY can catalyse the dehydration of
oxygenates and metallic Pt can catalyse the hydrogenation.49 As
shown in Fig. 8, the XRD patterns of Pt/HY shows sharp and peak
of fcc Pt. TEM images illustrate that Pt nanoparticles with a size
of ca. 5.5 nm are uniformly dispersed on HY zeolite. The HDO
achieves 93.2% yield along with some ring cracking side reaction
under 150 oC and 6 MPa H2 for 10 h. Finally, the target spiro-fuel
with 98.5% purity is obtained through vacuum distillation (Fig.
S35-S37, ESI).
Figure 8. (a) XRD patterns, (b) EELS-mapping and (c) (d) TEM images of Pt/HY.
[2+2] cycloaddition endows not only higher density but also
better cryogenic properties. (Table S1 in ESI)
Conclusions
The properties of the spiro-fuel are tested and compared
with some fuels from similar feedstocks or with similar
structures. As shown in Table 2, the spirocycloalkane has a high
density (0.911 g mL-1), a low freezing point (-51 oC) and a NHOC
of 42.45 MJ·kg−1. Compared with the reported spiro[4,5]decane
(density of 0.870 g mL-1), the spiro-fuel obtained in this work
shows much higher density due to the presence of multi- and
bridged-cyclic structure. In addition, compared with the fuel
synthesized from self-cycloaddition of isophorone (Fig. S38-S39,
ESI), the spiro-fuel derived from isophorone/β-pinene mixture
has a higher density (0.911 g mL-1 vs. 0.892 g mL-1) and an
evidently better cryogenic properties including freezing point (-
We report a self-sensitized photocycloaddition ([2+2] and [4+2])
approach to produce high-performance fuels using a wide scope
of biomass derivatives. Especially, the cycloaddition of
isophorone and β-pinene can take place smoothly with a
quantum yield of 0.22 under 365 nm without additional
photosensitizer because isophorone acts as an excellent
photosensitizer. Triplet isophorone is involved in the reaction
and it will collide with β-pinene to generate a complex and
trigger the photoaddition. The absorption in the wavelength
range of 300-375 nm is the most effective to induce the
photoreaction. Under optimal conditions, the finally spiro-fuel
is achieved with an overall yield of 85.0%. Having a high density
of 0.911 g·mL-1, a freezing point of -51 oC and a volumetric NHOC
of 38.67 MJ L-1, the obtained spirocycloalkane is very
competitive as renewable high-density liquid fuels. Thus, this
work provides a promising route to produce high-performance
spiro-fuel via self-photosensitized biomass conversion.
o
51 oC vs -40 C) and low-temperature kinematic viscosity (176
mm2·s-1 vs 1315 mm2·s-1 at -20 oC). Also, the cryogenic viscosity
of spiro-fuel is two orders of magnitude lower than that of
pinene dimer. Moreover, compared with the reported linear,
branched monocyclic and multi-cyclic bio-alkanes, the spiro
skeleton and high tension four-membered ring constructed by
8 | J. Name., 2012, 00, 1-3
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Acknowledgements
The authors appreciate the supports from the National Natural
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