Highly selective self-condensation of cyclic ketones using MOF-encapsulating phosphotungstic acid for renewable high-density fuel

Article abstract of DOI:10.1039/c5gc01287b

Transferring biomass-derived cyclic ketones such as cyclopentanone and cyclohexanone to a mono-condensed product through aldol self-condensation has great potential for the synthesis of a renewable high-density fuel. However, the selectivity is low for numerous catalysts due to the rapid formation of di-condensed by products. Herein, MIL-101-encapsulating phosphotungstic acid is synthesized to catalyze the self-condensation with selectivity of more than 95%. PTA clusters are uniformly dispersed in MOF cages and decrease the empty space (pore size), which provides both acidic sites and shape-selective capability. The optimal PTA amount decreases corresponding to the increase of reactant size. The shape-selectivity is also realized by changing the pore size of MOF such as from MIL-101 to MIL-100. Moreover, the catalyst is resistant to PTA leaching and performs stably after 5 runs. After hydrodeoxygenation of the mono-condensed product, high-density biofuels with densities of 0.867 g ml^-1 and 0.887 g ml^-1 were obtained from cyclopentanone and cyclohexanone, respectively. This study not only provides a promising route for the production of high-density biofuel but also suggests the advantage of MOF-based catalysts for shape-selective catalysis involving large molecular size.

Full text of DOI:10.1039/c5gc01287b

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Highly selective self-condensation of cyclic ketones using MOF
encapsulating phosphotungstic acid for renewable high-density fuel
Qiang Deng,a Genkuo Nie,a Lun Pan,a,b Ji-Jun Zou,*a,b Xiangwen Zhang,a,b and Li Wanga,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Transferring biomassꢀderived cyclic ketones like cyclopentanone and cyclohexanone to monoꢀcondensed
product by aldol selfꢀcondensation has great potential in synthesizing renewable highꢀdensity fuel. But
the selectivity is low for many catalysts due to the rapid formation of diꢀcondensed byproducts. Herein,
MILꢀ101 encapsulating phosphotungstic acid was synthesized to catalyze the selfꢀcondensation with
10 selectivity over 95%. PTA clusters are uniformly dispersed in MOF cages and decrease the empty space
(pore size), which provides both acidic sites and shapeꢀselective capability. The optimal PTA amount
decreases corresponding to the increase of reactant size. The shapeꢀselectivity is also realized by changing
the pore size of MOF, like from MILꢀ101 to MILꢀ100. Moreover, the catalyst is resistant to PTA leaching
and performs stably after 5 runs. After hydrodeoxygnation of monoꢀcondensed product, highꢀdensity
15 biofuels with density of 0.867 g/ml and 0.887 g/ml were obtained from cyclopentanone and
cyclohexanone, respectively. This work not only provides a promising way for highꢀdensity biofuel but
also suggests the advantage of MOFꢀbased catalysts for shapeꢀselective catalysis involving large reactant.
(including mineral acids, acidꢀtreated clays, ion exchange resins)
Introduction
and alkaline (including triethylamine, lithium perchlorate, sodium
50 hydroxide, aluminium oxide) catalysts have been used to catalyze
the selfꢀcondensation of cyclic ketones, but the selectivity of
monoꢀcondensed product is still low due to the quick formation
of diꢀcondensed side products.4,6(b),7,8,9 Microporous zeolites like
beta and ZSMꢀ5 have also been used to suppress the formation of
55 large side product by means of shapeꢀselectivity, but they are
nearly inactive because the pore size is too small.8(b)
The production of liquid transportation fuels from biomass is
a promising route toward decreasing the dependence on
20
petroleum and mitigating environmental issues.1 Synthetic jet fuel
produced by aldol condensation of biomassꢀderived furfural and
5ꢀhydroxymethylfurfural with other molecules followed by
subsequent hydrodeoxygenation has attracted many attentions.2
25 But the density of linear alkane produced from this route is
relative low (<0.770 g/ml). On the contrary, cyclic and polycyclic
hydrocarbons have high density and volumetric heating value,
such as conventional petroleumꢀbased highꢀdensity fuel JPꢀ10,
RJꢀ4, RJꢀ7 and RJꢀ5.3 To synthesize highꢀdensity fuel, cyclic CꢀC
30 rings have to be constructed via reaction or directly used as the
reactant. Actually, many biomassꢀderived intermediates have
such cyclic CꢀC rings, thus are ideal feedstock for highꢀdensity
jet fuel. Recently, hemicelluloseꢀderived cyclopentanone has
been converted to highꢀdensity fuel by aldol selfꢀcondensation
35 followed with hydrodeoxygenation.4 Similarly, cyclohexanone, a
derivative of lignin, can also be transferred to highꢀdensity fuel.5
Aldol selfꢀcondensation of aldehydes and crossꢀcondensation
of aldehydes with ketones can be easily catalyzed by acid and
base. But the selfꢀcondensation of ketones is limited to a few
40 examples because the steric hindrance of reactants makes the
reaction difficult to take place, especially for cyclic ketones.6
Selfꢀcondensation of cyclic ketones is often followed with inꢀsitu
dehydration that yields isomeric mixture of monoꢀcondensed
products. Unfortunately, especially for cyclohexanone, the monoꢀ
45 condensed products can further react with cyclic ketones to give
diꢀcondensed products and then dehydrate to polycyclic
aromatics that are not suitable for jet fuel. So far, many acidic
Metalꢀorganic framework (MOF) with high pore volume,
tailorable pore and structural versatility shows promising
perspectives in many aspects, such as gas adsorption, separation,
60 catalysis and biomedicine.10ꢀ13 In particular, MOFs with
supercage have been developed and their pore size can been
tuned by changing the organic linkers. For example, MILꢀ101
and MILꢀ100 have a 3D MTN zeolite topology. After the
removal of guest molecules, the cages have free internal diameter
65 of about 29 and 34 Å (MILꢀ101) and 25 and 29 Å (MILꢀ100) ,
which are accessible through microporous windows of 12 and 16
Å (MILꢀ101) and 5.5 and 8.6 Å (MILꢀ100), respectively.14 The
larger cage of MOF compared with zeolite allows many
modification chances like encapsulating large catalytic molecules
70 such as heteropolyacids, inorganic nanoꢀparticles and
metalloporphyrins.15 MILꢀ101 encapsulating heteropolyacids are
active in many reactions such as knoevenagel condensation,
esterification, dehydration of methanol, alkene epoxidation,
oxidative desulfurization, glucose and fructose dehydration to 5ꢀ
75 hydroxymethylfurfural, aldol condensation, baeyer condensation
and alcoholysis of styrene oxide,16 but in most cases these MOFꢀ
based heteropolyacid catalysts do not show obvious advantage
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over other catalysts from the aspects of activity and selectivity.
In this work, we report phosphotungstic acid (PTA)
encapsulated in the cage of MILꢀ101 as a highly selective catalyst
for selfꢀcondensation of cyclic ketones like cyclopenanone and
Morphology was obtained with a JEMꢀ2100F transmission
electron microscope. Porous structure was measured by N2
adsorption and desorption at ꢀ196 °C on a Micromeritics TriStar
3000 instrument. Specific surface area and the mesopore size
5
cyclohexanone. This catalyst shows selectivity of monoꢀ 60 distribution were calculated by BET method and DFT method
condensed product higher than 95%, attributed to the shapeꢀ
selectivity of pore size controlled by the amount of PTA
encapsulated. Moreover, PTA embedded in MOF is very stable
against leaching and shows unchanged performance after 5 runs.
respectively.18 Elemental analysis was performed by induced
coupled plasma atomic emission spectroscopy on an ICPꢀ9000
(N+M) spectrometer (TJA Co.). The amount of acidic protons in
PTA@MILꢀ101 was determined by acidꢀbase titration.
10 As a result, highꢀdensity biofuel with density of 0.867 g/ml and
0.887 g/ml are synthesized. The MOF catalysis represents a
promising route for producing renewable highꢀdensity fuel from
biomass derivatives.
65 Catalytic reaction
Aldol selfꢀcondensation of cyclic ketones (cyclopentanone,
cyclohexanone, cycloheptanone) was performed under magnetic
agitation in
a threeꢀnecked roundꢀbottom flask with N2
atmosphere. In a typical experiment, 30 ml ketone was heated to
70 130 °C in oil bath and then 1.5 g catalyst was added. The reaction
mixture was sampled 0.3 ml periodically and analyzed by a gas
chromatography (Agilentꢀ7890) equipped with an FID detector
and a capillary column HPꢀ1 capillary column (30 m × 0.53 mm).
If not mentioned, the reaction ended with 48 h. The total amount
75 of sample was 1.8 ml and did not affect the proceeding of
reaction.
Experimental section
15 Materials
Phosphotungstic acid, terephthalic acid, palladium dichloride
(PdCl2), ethanol (with 5% H2O) and cyclohexanone (99.5%) were
obtained from Tianjin Guangfu Fine Chemical Research Institute.
Chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O), Iron (III)
20 nitrate nonahydrate (Fe(NO3)3·9H2O), N,Nꢀdimethylformamide
(DMF), ammonium fluoride (NH4F), hydrofluoric acid (40 wt%
HF) and cyclopentanone (99.5%) were purchased from Tianjin
Jiangtian Chemical Technology Co., China. Chromium (VI)
oxide (CrO3), cycloheptanone (99%), trimesic acid and
25 amberlystꢀ15 were obtained from Shanghai Crystal Pure Reagent
Co., China. All the chemicals were used without further
purification. HZSMꢀ5 (SiO2/Al2O3=25), Hβ (SiO2/Al2O3=25) and
MCMꢀ41 purchased from Nankai Catalysts Co., China were
calcined in air at 550 °C for 6 h. AlꢀMCMꢀ41 (SiO2/Al2O3=26.8)
30 and PTA/MCMꢀ41 was synthesized according our previous
work.17 3wt%Pd/Hβ catalyst was prepared by the incipient
wetness impregnation method.
Leaching of PTA from PTA@MILꢀ101 was detected using
UV spectrometer method.19 0.1 g 42.7wt%PTA@MILꢀ101 was
stirred in 10 ml water or cyclopentanone for 4 h. Then mixture
3ꢀ
80 was filtered and the concentration of PW12O40 anions in the
supernatant was measured using a Uꢀ3010 UVꢀvis spectrometer
(Hitachi).
Hydrodeoxygenation of monoꢀcondensed product was carried
out based on literature.4 In a 250 mL batch autoclave reactor (Parr
85 Instrument Co.) equipped with stirring. The reactor was loaded
with monoꢀcondensed product (100.0 g) and Pd/Hβ (4.0 g),
sealed and purged with H2 for 4 times to exclude air. Then it was
heated to 250 °C under 6 MPa of H2 for 48 h. The final product
was recovered by filtering and used for fuel related property
90 measurement.
Synthesis of catalysts
MILꢀ101 encapsulating PTA (donated as PTA@MILꢀ101)
35 was synthesized using oneꢀpot hydrothermal method. A mixture
of Cr(NO3)3·9H2O (14 mmol), terephthalic acid (14 mmol),
HF(14 mmol), PTA (0~9.333 g) and water (70 mL) were mixed
under stirring, transferred into a Teflonꢀlined autoclave, sealed
and heated at 220 °C for 8 h. The formed light green solid was
Results and Discussion
Structural characteristics of PTA@MIL-101
Figure 1A shows that the XRD patterns of PTA@MILꢀ101
are almost identical to that of pure MILꢀ101. It confirms that the
40 recovered by filtration and washed using DMF, ethanol, NH4F 95 formation of MILꢀ101 metalꢀorganic framework in the
solution and water successively. The resultant solid was finally
dried and heated at 150 °C for 8 h. MILꢀ101 supporting PTA
(donated as PTA/MILꢀ101) was synthesized using wet
impregnation method.
synthesized materials, and PTA clusters should be encapsulated
in the cage of MOF and do not disturb the structure of MILꢀ101.
Moreover, there are no any patterns belonging to PTA crystals,
suggesting PTA is highly dispersed in the cage of MILꢀ101 in
45
To synthesize PTA@CrꢀMILꢀ100, a mixture of CrO3 (7 100 form of very small clusters. 31P MAS NMR spectra in Figure 1B
mmol), trimesic acid (7 mmol), HF (3.5 mmol), PTA (0, 3.60 g)
and water (40 mL) was used for hydrothermal treatment at 220
°C for 96 h. For PTA@FeꢀMILꢀ100, a mixture of Fe(NO3)3·9H2O
(5 mmol), trimesic acid (3.5 mmol), PTA (0, 2.73 g) and water
show that both PTA@MILꢀ101 and individual PTA present
identical peak at about ꢀ16 ppm, indicating PTA cluster in MOF
cage still retains good Keggin structure, in good agreement with
reported result.16
50 (25 ml) was hydrothermal treated at 160 °C for 12 h.
105
TEM images in Figure 2 shows MILꢀ101 has perfect
octahedral structure. PTA/MILꢀ101 synthesized by wet
impregnation exhibits many PTA aggregates on the surface of
host, indicating most of the PTA is located on the outer surface,
and MILꢀ101 just serves as a supporting material, like many other
Catalyst characterization
Crystal structure was recorded on a D/MAXꢀ2500 Xꢀray
diffractometer with (CuꢀKα) radiation. Coordination of
phosphorus species was determined via Solidꢀstate 31P MAS
55 NMR spectra on an Infinity plus 300WB spectrometer.
110 PTAꢀsupported catalysts. There are no aggregates on the surface
of PTA@MILꢀ101, but EDS analysis on randomly selected area
2
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DOI: 10.1039/C5GC01287B
detects both W and P elements, which demonstrates the 15 maximum number of PTA molecules encapsulated in the large
homogeneous dispersion of Keggin clusters.
and small cage of MILꢀ101 is 5 and 3, respectively. The
theoretical maximum encapsulation capability is 3.7 PTA
molecules in each cage, very close to the case of
42.7wt%PTA@MILꢀ101 (3.2 PTA per cage).
A
B
42.7wt
%
PTA@MIL-101
PTA@MIL-101
33.5wt
%
42.7wt%PTA@MIL-101
20
Figure 3A shows the N2 adsorptionꢀdesorption isotherms of
MILꢀ101 and PTA@MILꢀ101. The isotherms have secondary
uptakes at about P/P0=0.1 and P/P0=0.2, indicating the presence
of the two nanoporous windows of MOF structure.14(b),15,20 The
BET surface area of MILꢀ101 is 2306 m2/g and decreases with
21.2wt
%
PTA@MIL-101
PTA@MIL-101
13.4wt
%
9.8wt%PTA@MIL-101
PTA
20
MIL-101
3
6
9
12
0
-20
2θ
25 the encapsulation of PTA (Table 1). Figure 3B shows the pore
size distribution centers around 12.9, 14.7, 19.3 and 25.3 Å for
MILꢀ101, corresponding to two types of window and cage. With
the inclusion of PTA, the pore size (cage void) is substantially
decreased. As shown in Table 1 and Figure S1, the pore size and
30 volume of small and large cage decrease equally with the increase
of PTA amount, indicating PTA molecules are evenly
encapsulated in two type cages.
δ
(ppm)
Figure 1 (A) Xꢀray diffraction patterns and (B) 31P MAS NMR of
PTA@MILꢀ101.
5
0.16
0.12
0.08
0.04
0.00
1200
1000
800
a
b
c
d
e
f
a
b
c
d
e
f
B
A
600
400
200
0.0
0.2
0.4
0.6
0.8
1.0
10
15
20
25
30
Pore diameter (Å)
P/P
0
Figure 3 (A) N2 adsorption isotherms and (B) pore size distribution of (a)
35 MILꢀ101, (b) 9.8wt%PTA@MILꢀ101, (c) 13.4wt%PTA@MILꢀ101, (d)
21.2wt%PTA@MILꢀ101,
42.7wt%PTA@MILꢀ101.
(e)
33.5wt%PTA@MILꢀ101,
(f)
Since the acidity of PTA greatly depends on the protonation
state, the amount of acidic proton in PTA@MILꢀ101 was
40 detected by acidꢀbase titration.15(a),16(h) As shown in Table 1, the
acidic proton increases with the amount of PTA encapsulated and
is generally in agreement with the theoretic value calculated
according to three acidic protons per Keggin anion. This indicates
the acidity of PTA is not affected when being encapsulated in
45 MOF cage.
Figure
2 TEM micrographs and EDS of (A,B) MILꢀ101, (C,D)
44.4wt%PTA/MILꢀ101, (E,F) 42.7wt%PTA@MILꢀ101.
Table 1 shows the elemental composition of PTA@MILꢀ101.
10 The content of W increases with the amount PTA added in the
synthesis. The amount of PTA in MILꢀ101 varies from 9.8 to
42.7 wt%, corresponding to 0.5ꢀ3.2 PTA molecules in each MILꢀ
101 cage, calculated according to the MILꢀ101 topology
structure.14(b),20 Based on the size of cage and keggin anion, the
Table 1 Composition and physicochemical properties of PTA@MILꢀ101. [a] detected by titration, in brackets are theoretic values. [b] the volume of small
cage. [c] the volume of large cage.
The mass of PTA
W
(wt%)
ꢀ
Cr
(wt%)
ꢀ
PTA
(wt%)
ꢀ
PTA/Cage
Acidic protons[a]
SBET
(m2/g)
2306
Vtotal
(cm3/g)
1.681
V[b]
(cm3/g)
V[c]
(cm3/g)
added (g)
0
(mmol/g)
ꢀ
ꢀ
0.447
0.348
0.583
1.167
2.333
7.49
10.30
15.98
13.14
12.76
10.95
9.8
0.5
0.7
1.3
0.110 (0.102)
0.147 (0.139)
0.251 (0.220)
2165
2011
1761
1.404
1.271
1.081
0.385
0.349
0.305
0.288
0.248
0.213
13.4
21.2
5.176
9.333
25.71
32.7
9.05
8.58
33.5
42.7
2.4
3.2
0.318 (0.348)
0.402 (0.444)
1611
1429
0.942
0.875
0.235
0.186
0.156
0.145
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Highly selective self-condensation of cyclopentanone
35 shows a low selectivity because the channel is too large for shape
selection. This indicates that inhibiting the side reactions is the
key to selectively synthesize monoꢀcondensed product that can be
used as precursor for highꢀdensity biofuel.
As aforementioned, apart from the formation of monoꢀ
condensed product (2ꢀcyclopentylidenecyclopentanone, 7.0×4.5
Å),
large
diꢀcondensed
byproduct
(2,5ꢀ
100
5
dicyclopentylidenecyclopentanone, 11.5×4.5 Å) can also be
formed and then dehydrated to trindane in acidic condition
(Figure 4A). Figure 4B shows the timeꢀdependent product
distribution over PTA and PTA@MILꢀ101. The conversion of
cyclopentanone is very quick over PTA (78% at 24 h) but the
10 concentration of monoꢀcondensed product is very low, and
trindane is the dominant product due to the easilyꢀoccurring side
reactions. Fortunately, over PTA@MILꢀ101, the trindane and diꢀ
condensed byꢀproducts are negligible, and the selectivity of
monoꢀcondensed product remains above 98%. This suggests that
15 encapsulating PTA in MOF cage can completely suppress the
side reaction.
cyclopentanone
2-cyclopentylidenecyclopentanone
A
80
60
40
20
0
O
O
A
Acid/Base
-
2
H2O
100
2ꢀcyclopentylidenecyclopentanone
cyclopentanone
2-cyclopentylidenecyclopentanone
B
O
O
O
80
60
40
20
0
Acid/Base
-
+
H2O
2,5ꢀdicyclopentylidenecyclopentanone
O
Acid
-
H2O
trindane
100
cyclopetanone:
2-cyclopentylidenecyclopentanone:
2,5-dicyclopentylidenecyclopentanone:
trindane:
PTA;
PTA;
PTA;
PTA;
PTA@MIL-101
PTA@MIL-101
PTA@MIL-101
PTA@MIL-101
B
80
60
40
20
0
40
Figure 5 Activity and selectivity of (A) often used catalysts and (B)
PTA@MILꢀ101 with different PTA amount in cyclopentanone selfꢀ
condensation. Except PTA@MILꢀ101, the reaction time of these catalysts
was optimized to give the highest yield of monoꢀcondensed product: PTA
45 and NaOH, 0.5 h; amberlystꢀ15, 1.5 h; AlꢀMCMꢀ41, 48 h; HZSMꢀ5,
PTA/MILꢀ101 and PTA/MCMꢀ41, 48 h.
0
10
20
30
40
50
Reaction time (h)
Different from the catalysts mentioned above, PTA@MILꢀ
101 shows considerably high selectivity of monoꢀcondensed
product, see Figure 5B. Since the PTA clusters are
50 homogeneously located in the cage, the catalytic reaction has to
occur inside the empty space of cage, which provides the
possibility for shapeꢀselective catalysis. With the increase of PTA
encapsulated, both the activity and selectivity of selfꢀ
condensation increase simultaneously. The pore window of MILꢀ
Figure 4 (A) Reaction scheme of cyclopentanone selfꢀcondensation and
20 (B) product distribution of cyclopentanone selfꢀcondensation using PTA
and 42.7wt%PTA@MILꢀ101.
Figure 5A further compares the conversion and selectivity of
some often used catalysts. It should be noted that the reaction
time of these catalysts, except PTA@MILꢀ101, is optimized to
25 get the highest yield of monoꢀcondensed. Acidic amberlyst and
basic NaOH show high activity but low selectivity. Microporous 55 101 (12 Å, 16 Å) can allow the free diffusion of product and byꢀ
HZSMꢀ5 has a very low activity because the pore diameter
(5.1×5.5 Å) is even smaller than the size of monoꢀcondensed
product. Mesoporous AlꢀMCMꢀ41 (with pore diameter 37 Å)
30 shows higher conversion than HZSMꢀ5 but the selectivity is very
products, so the selectivity is controlled by the pore size that is
adjusted by PTA amount encapsulated. First, the increase of PTA
means more active sites that are accessible for the reaction.
Second, the inclusion of PTA can reduce the pore size and thus
low due to the very large pore size. When PTA is supported on 60 suppress the formation of lager diꢀcondensed byꢀproducts. For
MILꢀ101 surface, the selectivity is increased but still less than
50% because many acid sites are dispersed on the surface and
thus nonꢀselective. PTA dispersed in MCMꢀ41 channel still
42.7wt%PTA@MILꢀ101, although the amount of PTA almost
reaches the theoretical maximum value, that is 5 (3) PTA
molecules in each large (small) cage, it still provides enough
4
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DOI: 10.1039/C5GC01287B
O
O
empty space for monoꢀcondensation but effectively restrict the diꢀ
condensation (Scheme 1). As a result, it shows the highest
conversion of 46.4% and selectivity of 98.1%.
A
Acid/Base
-
2
H2O
2ꢀ(1ꢀcyclohexenꢀlꢀyl)cyclohexanone
O
O
O
Acid/Base
-
+
H
2
O
2,6ꢀdi(1ꢀcyclohexenyl)cyclohexanone
O
Acid
-
H2O
dodecahydrotriphenylene
100
80
60
40
20
0
cyclohexanone
2-(1-cyclohexen-l-yl)cyclohexanone and isomer
B
5
Scheme
1
Highly
selective
synthesis
of
2ꢀ
cyclopentylidenecyclopentanone via shapeꢀselectivity in MOF cage.
In order to elaborate the shape selectivity of MOF cage and
exclude the effect of metal ion in MOF, Crꢀ and FeꢀMILꢀ100 with
smaller topology space was further studied. The maximum
10 number of PTA molecules in large and small cage of MILꢀ100 is
3 and 1, respectively. 21.2wt%PTA@CrꢀMILꢀ100 with averaged
1 PTA molecule in each cage and 23.2wt%PTA@FeꢀMILꢀ100
with averaged 1.1 PTA molecules in each cage were synthesized.
Figure S2, S3 and Table S1 shows that the crystal and porous
15 structures are in agree with the reported results.21 As shown in
Figure S4, compared with MILꢀ101 with very close PTA loading
(21.2wt%PTA@MILꢀ101), a higher selectivity is obtained for
PTA@MILꢀ100, confirming that smaller cage size is more
efficient in suppressing the diꢀcondensation. It is also noted that
20 pure MILꢀ100 and PTA@MILꢀ100 synthesized using Fe and Cr
ions show very similar conversion and selectivity, which
indicating the catalytic performance is determined by the cage
size of MOF host and acidity of PTA encapsulated, having
nothing to do with the type of metal ion in MOF framework.
Figure 6 (A) Reaction scheme for cyclohexanone selfꢀcondensation and
50 (B) activity and selectivity of often used catalysts and PTA@MILꢀ101.
Except PTA@MILꢀ101, the reaction time was optimized to give the
highest yield of monoꢀcondensed product: PTA, amberlystꢀ15 and NaOH,
5.5 h; AlꢀMCMꢀ41, 24 h; HZSMꢀ5, PTA/MILꢀ101 and PTA/MCMꢀ41, 24
h.
55 Self-condensation of cycloheptanone
25 Highly selective self-condensation of cyclohexanone
To further illustrate the relationship between cage size and
shapeꢀselectivity, cycloheptanone, a larger cyclic ketone, was
also used in the reaction, although it is not a biomass derivative.
Different from cyclopenanone and cyclohexanone, the selfꢀ
60 condensation of cycloheptanone is highly selective because the
monoꢀcondensed product (2ꢀcycloheptylidenecycloheptanone,
8.6×5.1 Å) quickly isomerizes to 2ꢀmethylꢀ2,3ꢀcyclohexanoꢀ4,5ꢀ
cycloheptenoꢀ(ꢁ5)ꢀ2,3,4,5ꢀtetrahydrofuran that cannot takes part
in subsequent diꢀcondensed reaction (Figure 7A).22 As shown in
65 Figure 7B and Figure S6, the selectivity of monoꢀcondensed
product stays at 70%. But the conversion begins to decrease after
the PTA amount exceeds 13.4wt%, because the pore size
decreases to such a degree that the formation of monoꢀcondensed
product is restricted. The decrease of optimal PTA amount
70 (corresponding to the increase of pore size) with the increase of
monoꢀcondensed product size clearly supports the shapeꢀselective
catalysis in MOF cage.
Similar to cyclopentanone, cyclohexanone firstly generates
monoꢀcondensed 2ꢀ(1ꢀcyclohexenꢀlꢀyl)cyclohexanone (8.6×4.3
Å) and then form diꢀcondensed product 2,6ꢀdi(1ꢀ
cyclohexenyl)cyclohexanone (12.0×8.0 Å) or its isomers and
30 dodecahydrotriphenylene (Figure 6A). Similarly, Figure S5
shows that pure PTA produces considerable diꢀcondensed
product and dodecahydrotriphenylene, whereas PTA@MILꢀ101
gives a monoꢀcondensed selectivity above 95% because of the
shape selection of cage. Figure 6B also compares the activity of
35 different catalysts. Amberlystꢀ15, PTA/MCMꢀ41, PTA/MILꢀ101,
NaOH, AlꢀMCMꢀ41 have low selectivity whereas HZSMꢀ5 has
low conversion, indicating they are not suitable for this reaction.
All the PTA@MILꢀ101 catalysts show very high selectivity, and
the diꢀcondensed byꢀproduct is almost completely suppressed. It
40 is noted that, the conversion first increases and then decreases
with the amount of PTA encapsulated, with the highest value
appears with 21.2wt%PTA@MILꢀ101. Since the molecular size
of monoꢀcondensed product is larger than the case of
cyclopentanone,
a bigger pore size, namely less PTA
45 encapsulation, is needed.
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DOI: 10.1039/C5GC01287B
O
O
to
a
mixture of bicyclohexane (84.9%), its isomer
A
cyclopentylmethylꢀcyclohexane (6.4%) and some oxygenates.
35 After vacuum distillation, two biofuels containing bicyclopentane
(99.5%), bicyclohexane (93.3%) and cyclopentylmethylꢀ
cyclohexane (6.4%) were obtained, respectively.
Acid/Base
-
2
H2O
2ꢀcycloheptylidenecycloheptanone
Acid
O
100
cyclopentanone
2-cyclopentylidenecyclopentanone
O
80
2ꢀmethylꢀ2, 3ꢀcyclohexanoꢀ4,5ꢀ
cycloheptenoꢀ(ꢁ5)ꢀ2,3,4,5ꢀtetrahydrofuran
80
60
40
20
0
cycloheptanone
2-cycloheptylidenecycloheptanone and isomer
60
40
20
0
B
1
2
3
4
5
Recycle times
40 Figure
8
Cycling performance of 42.7wt%PTA@MILꢀ101 in
cyclopentanone selfꢀcondensation.
Table 2 shows the fuel synthesized from cyclopentanone has
a density of 0.867 g/ml and a freezing point of ꢀ38.0 °C, C/H ratio
of 6.71 and heating value of 42.42 kJ/g. The density is 11.1%
45 higher than petroleumꢀbased jet fuel with density of ~0.78 g/ml.
The kinematic viscosity increases very slightly from 1.28 mm2/s
to 4.68 mm2/s when the temperature goes down from 45 °C to ꢀ35
°C, indicative of very good fluid property. The fuel derived from
cyclohexanone has a higher density of 0.887 g/ml, although its
50 freezing point of 1.2 °C is a little high as liquid fuel. However, it
can be mixed with other fuels as a density promote. For example,
the 1/1 blend of two biofuels produced in this work has a density
of 0.877 g/ml and freezing point of ꢀ34.5 °C.
Figure 7 (A) Reaction scheme for cycloheptanone selfꢀcondensation and
(B) activity and selectivity of PTA@MILꢀ101 with different PTA
amount. The reaction was conducted for 48 h.
5
Catalyst stability and recycling
Fast hot catalyst filtration test was performed and no further
substrate conversion in the filtrate was found after the removal of
PTA@MILꢀ101 (Figure S7), which proves the true
10 heterogeneous catalysis nature of selfꢀcondensation and suggests
PTA leached from the catalyst is negligible. Figure S8 shows that
pure PTA is easily dissolved in water and cyclopentanone. But
the amount of PTA dissolved from PTA@MILꢀ101 is only 7.4
ꢂg/ml, corresponding to 7.4×10ꢀ4 g PTA leaching per gram
15 PTA@MILꢀ101. This confirms the leaching of PTA from MOF
cage is negligible, which guarantees a stable performance in
reaction.
The cycling of PTA@MILꢀ101 was further evaluated in the
selfꢀcondensation of cyclopentanone. The catalyst was
20 centrifuged after the reaction and used directly for the next run.
As shown in Figure 8, after 5 runs, the selectivity still keeps
above 98%, and the conversion is only slightly decreased
compared the first run. The used catalyst remain green (Figure
S9), and the crystallinity is well preserved as evidenced by XRD
25 (Figure S10), indicating the framework and PTA encapsulated are
fairly stable under the present reaction conditions.
Table 2 Properties of highꢀdensity biofuels.
Property
cyclopentanoneꢀ cyclohexanoneꢀ 1/1 blend of
derived fuel
0.867
derived fuel
0.887
two fuels
0.877
density at 20 °C (g/ml)
freezing point (°C)
ꢀ38.0
1.2
ꢀ34.5
kinematic viscosity
(mm2/s)
1.62 (25 °C)
4.68 (ꢀ35 °C)
6.71
3.72 (25 °C)
6.33 (5 °C )
6.56
2.27 (25 °C)
9.78 (ꢀ30 °C)
ꢀ
C/H ratio
heating value (kJ/g)
42.42
42.97
ꢀ
55 Conclusions
We developed a highly selective route to synthesize highꢀ
density biofuel using PTA@MOF shapeꢀselective selfꢀ
condensation
of biomassꢀderived
cyclopentanone
and
Properties of mono-condensed biofuels
cyclohexanone. PTA clusters are encapsulated in MOF cage,
60 which not only provides active sites but also controls the pore
size. Different from many catalysts like PTA, Amberlystꢀ15,
PTA/MCMꢀ41, PTA/MILꢀ101, NaOH and AlꢀMCMꢀ41 which
show either low selectivity or low conversion, PTA@MOF shows
selectivity of monoꢀcondensed product higher than 95% with
After the condensation reaction, the monoꢀcondensed
products are recovered by vacuum distillation. Then they were
30 hydrodeoxygnated using Pd/Hβ catalyst. Monoꢀcondensed of
cyclopentanone was converted to bicyclopentane (95.8%) along
with some oxygenates, and that of cyclohexanone was transferred
6
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DOI: 10.1039/C5GC01287B
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5
65
70
selectivity
42.7wt%PTA@MILꢀ101,
for
different
reactants.
21.2wt%PTA@MILꢀ101
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Graphical Abstract