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Synthesis of jet fuel rang cycloalkane from isophorone with glycerol as a renewable hydrogen source
Accepted Manuscript Title: Synthesis of jet fuel rang cycloalkane from isophorone with glycerol as a renewable hydrogen source Authors: Hao Tang, Ning Li, Shanshan Li, Fang Chen, Guangyi Li, Aiqin Wang, Yu Cong, Xiaodong Wang, Tao Zhang PII: DOI: Reference:
S0920-5861(17)30492-3 http://dx.doi.org/doi:10.1016/j.cattod.2017.07.009 CATTOD 10923
To appear in:
Catalysis Today
Received date: Accepted date:
21-10-2016 5-7-2017
Please cite this article as: Hao Tang, Ning Li, Shanshan Li, Fang Chen, Guangyi Li, Aiqin Wang, Yu Cong, Xiaodong Wang, Tao Zhang, Synthesis of jet fuel rang cycloalkane from isophorone with glycerol as a renewable hydrogen source, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of jet fuel rang cycloalkane from isophorone with glycerol as a renewable hydrogen source
Hao Tanga,b, Ning Lia,c,*, Shanshan Lia,b, Fang Chena,b, Guangyi Lia,c, Aiqin Wanga,c, Yu Conga, Xiaodong Wanga, Tao Zhanga,c,*
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, No. 457 Zhongshan Road, Dalian 116023, China. b
University of Chinese Academy of Sciences, Beijing 10049, China.
c
iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Corresponding author: Prof. Tao Zhang, Tel: +86-411-94379015; Fax: +86-411-84691570; E-mail: [email protected] Prof.
Ning
Li,
Tel:
+86-411-84379738;
[email protected]
1
Fax:
+86-411-84685940;
E-mail:
Graphical abstract
Highlights Jet fuel range C9 cycloalkane was first synthesized by the HDO of isophorone. Glycerol was first used as a renewable hydrogen resource for the HDO process. Pt/Al2O3 was highly effective for the coupling of APR and HDO reactions. Other lignocellulosic polylols can also be used as the hydrogen resources. C9 cycloalkane as obtained can be used as an additive to conventional bio-jet fuel.
Abstract For the first time, 1,1,3-trimethyl-cyclohexane (a jet fuel range cycloalkane) was synthesized by coupling the aqueous phase reforming (APR) of glycerol and the 2
hydrodeoxygenation (HDO) of isophorone which can be obtained from lignocellulose. Among the investigated catalysts, Pt/Al2O3 was found to be the most active for the production of 1,1,3-trimethyl-cyclohexane with the hydrogen which was in-situ generated by the APR of glycerol. Over it, high carbon yield (67.0%) of 1,1,3trimethyl-cyclohexane can be achieved at 533 K in the absence of external hydrogen. The excellent performance of Pt/Al2O3 catalyst can be explained because Pt is highly active for both APR and HDO reactions. Besides glycerol, many other lignocellulose derived polylols (such as ethylene glycol, xylitol and sorbitol) and methanol can also be used as renewable hydrogen source for the HDO of isophorone. As a potential application, the 1,1,3-trimethyl-cyclohexane as obtained can be blended into the conventional jet fuels to improve their volumetric heat values.
Keywords: Aqueous phase reforming; Hydrodeoxygenation; Lignocellulose; Jet fuel; Cycloalkane.
3
1. Introduction With the increase of social concern about energy and environmental problems, the catalytic conversion of renewable and CO2-neutral biomass to fuels [1-5] and chemicals [6-16] has become a very hot research topic. Lignocellulose is the major component of agricultural and forestry wastes. Compared with other forms of biomass, lignocellulose is much cheaper and more abundant. Jet fuel is one of the most important transportation fuels with increasing demand. In the past years, many routes has been developed for the synthesis of renewable jet fuel range hydrocarbons by the C-C coupling reactions of lignocellulose-derived platform compounds followed by the hydrodeoxygenation (HDO) [17-27]. However, most of the reported work about the lignocellulosic bio-jet fuel was concentrated on the synthesis of jet fuel range chain alkanes. Compared with conventional jet fuel which is a mixture of chain alkanes and cyclic hydrocarbons, these chain alkanes have lower densities (or volumetric heat values). In real application, they need to be blended with conventional jet fuel to meet the specification of aviation fuel [3]. As a solution to this problem, it is imperative to develop new routes for the synthesis of jet fuel range cyclic hydrocarbons with lignocellulosic platform compounds [28-32]. Another bottleneck for the industrialization of lignocellulosic bio-jet fuel is hydrogen which is greatly needed in the HDO processes. As we know, most of the hydrogen we are using today is derived from fossil energy. To fulfil the need of sustainable development, it is also necessary to develop new technologies for the production of renewable bio-jet fuel without using external hydrogen [33, 34]. Aqueous phase reforming (APR) is an important method for the production of renewable 4
hydrogen with biomass derived oxygenates [35-37]. To the best of our knowledge, there is no report for the synthesis of jet fuel range hydrocarbons by the coupling of APR and HDO reactions. Isophorone is the trimerization product of acetone [38] which is the by-product in the manufacture of bio-butanol and bio-ethanol from the Acetone-Butanol-Ethanol (ABE) fermentation of lignocellulose [39]. Considering its cyclic chemical structure, isophorone can be used as a precursor for jet fuel range hydrocarbons. Glycerol is a by-product in in the manufacture of biodiesel. Due to its low price and high availability, glycerol can be utilized as a promising hydrogen resource for the HDO process [37]. In this work, it was reported for the first time that 1,1,3-trimethyl-cyclohexane (a jet fuel range cycloalkane) can be obtained at high carbon yield by coupling the APR of glycerol and the HDO of isophorone. At the same time, we also explored the feasibility for the utilization of other lignocellulose derived polylols as renewable hydrogen sources for the HDO of isophorone. 2. Materials and methods 2.1. Preparation of catalysts The Pt/Al2O3, Pd/Al2O3, Rh/Al2O3, Ru/Al2O3 and Ir/Al2O3 catalysts were prepared by the incipient wetness impregnation of commercial Al2O3 with aqueous solutions of H2PtCl6·6H2O, PdCl2, RhCl3·6H2O, RuCl3·3H2O, and H2IrCl6·6H2O, respectively. The theoretical contents of noble metals in these catalysts were controlled as 3% by weight (denoted as 3wt.%). Subsequently, the products were dried at 383 K for 8 h and reduced by hydrogen flow at 533 K for 6 h. After being cooled down to room temperature in 5
hydrogen flow, the catalysts were passivated with 1% O2 in N2. For comparison, we also prepared the Pt/SiO2-Al2O3, Pt/AC, Pt/TiO2, Pt/Fe2O3 and Pt/TiO2 catalysts by the same method using SiO2-Al2O3 (SiO2/Al2O3 molar ratio = 25), activated carbon (NORIT Company), TiO2 P25 (Degussa), Fe2O3 and ZrO2 as supports. The theoretical Pt contents in these catalysts were fixed as 3wt.%. 2.2. Hydrodeoxygenation (HDO) of isophorone The HDO of isophorone was conducted at 533 K in a stainless steel fixed-bed tubular reactor which has been described in literature [35, 36]. For each test, 1.80 g catalyst was used. The isophorone and 20wt.% glycerol (or ethylene glycol, xylitol and sorbitol) aqueous solution were fed into the reactor from bottom by two HPLC pumps at a rate of 0.02 mL min-1 and 0.1 mL min-1, respectively. The products from the outlet of the tubular reactor became two phases in a gas-liquid separator. The gas products (carried by nitrogen flow at a rate of 120 mL min-1) passed through a back-pressure regulator which was used to maintain the system pressure at 6.0 MPa and were analyzed on-line by an Agilent 6890N GC. The liquid products were drained periodically from the separator and analyzed by another Agilent 7890 GC. 3. Results and discussion 3.1. Effect of noble metal At the beginning of this work, we compared the activities of several Al2O3 loaded noble metal catalysts for the HDO of isophorone with the hydrogen which was in-situ generated
by
the
APR
of
glycerol. 6
On
the
basis
of
our
analysis,
3,3,5-trimethylcyclohexanone,
3,3,5-trimethylcyclohexanol
and
1,1,3-trimethylcyclohexane, (i.e. the 1A, 1B and 1C in Scheme 1) were identified as three major products from this process. The reaction pathways for the generation of these compounds were proposed in Scheme 1. It is interesting that no 3,5,5-trimethylcyclohex-2-enol (i.e. the 1D in Scheme 1) was detected in the product. This result can be comprehended because the hydrogenation of C=C bond in isophorone molecule is relatively easier than the hydrogenation of C=O bond (due to thermodynamic
reason
[40]).
According
to
literature
[41,
42],
the
1,1,3-trimethylcyclohexane as obtained has a density of 0.79 g mL-1 and a freezing point of 207.3 K. As a potential application, it can be blended into the conventional bio-jet fuels to improve their volumetric heat values. The 3,3,5-trimethylcyclohexanone (i.e. 1A) as obtained can be used as a solvent for vinyl resins, lacquers, varnishes, paints and other coatings. The 3,3,5-trimethylcyclohexanol (i.e. 1B) as obtained can be used as an intermediate in the production of cyclandelate. As another option, the mixture of 3,3,5-trimethylcyclohexanone and 3,3,5-trimethylcyclohexanol as obtained can also be recycled and used for the HDO process to increase the carbon yield of 1,1,3-trimethylcyclohexane. As we can see from Fig. 1, Pt/Al2O3 exhibited the highest activity for the HDO of isophorone among the investigated catalysts. Over it, the isophorone was completely converted. High carbon yield (67.0%) of 1,1,3-trimethylcyclohexane was achieved at 533 K. The activities of noble metal catalysts decreased in the order of Pt/Al2O3 > Pd/Al2O3 > Rh/Al2O3 > Ru/Al2O3 > Ir/Al2O3. In the previous work of Dumesic et al. 7
[43], it was found that the activities of noble metal catalysts for the APR of ethylene glycol decreased in the order of Pt > Ru > Rh ~ Pd > Ir, while the hydrogen selectivity in the APR product decreased in the order of Pd > Pt > Ru > Rh. In another work of the same group [44], it was suggested that Pt is highly active for the aqueous HDO of biomass derived oxygenates. In the recent work of Suppes et al. [45], it was also found that the hydogenolysis activities of noble metal catalysts decreases in the order of Ru > Pt > Pd. As we know, hydogenolysis is an important pathway for the HDO of biomass derived oxygenates [46]. Based on these reports, we can attribute the excellent performance of Pt/Al2O3 catalyst to the high activities of Pt for both the production of hydrogen from APR of glycerol and the HDO of isophorone. 3.2. Effect of support The influence of support on the HDO of isophorone over Pt catalyst was also investigated. Compared with the other Pt catalysts which were used in this work, the Pt/Al2O3 catalyst exhibited evidently higher HDO activity (see Fig. 2). The carbon yields of 1,1,3-trimethylcyclohexane over the Pt catalysts decreased in the order of Pt/Al2O3 > Pt/SiO2-Al2O3 > Pt/AC > Pt/TiO2 > Pt/Fe2O3 > Pt/ZrO2. This sequence is similar as the one which has been reported by Tian et al. in their previous work about the APR of glycerol (Pt/Al2O3 > Pt/MgO > Pt/SiO2 > Pt/HUSY > Pt/AC > Pt/SAPO-11) [37]. The higher activity of Pt/Al2O3 catalyst can be explained by two reasons: 1) The higher activity of Pt/Al2O3 for the generation of hydrogen from the APR of glycerol. 2) The proper acidity of Al2O3 support. As we know, the acidity of support is very important for the HDO process because acid-catalyzed dehydration follow metal 8
catalyzed hydrogenation has been suggested as a major pathway for the HDO of the biomass derived oxygenates [46]. However, too high acidity of support will also lead to the dehydration of glycerol (or the polyols and alcohols generated from the APR of glycerol) and the generation of unexpected C1-C3 alkanes by self-HDO reaction of glycerol [44] which is competitive reaction for the HDO of isophorone. 3.3. Effect of Pt content The effect of metal content on the HDO performance of the Pt/Al2O3 catalyst was explored. As we can see from Fig. 3, the isophorone conversion and the carbon yield of 1,1,3-trimethylcyclohexane increased with the increment of Pt content in the Pt/Al2O3 catalyst, reached the maximum when Pt content was about 3wt.%, then leveled off with the further increase of Pt content. The carbon yield of 3,3,5-trimethylcyclohexanone decreased with the increment of Pt content from 1wt.% to 3wt.%, then stabilized with the
further
increase
of
Pt
content.
In
contrast,
the
carbon
yield
of
3,3,5-trimethylcyclohexanol slightly increased with the increment of Pt content in the Pt/Al2O3 catalyst from 1wt.% to 5wt.%, then stabilized. These results can be rationalized because 3,3,5-trimethylcyclohexanone is the intermediate during the HDO of isophorone to 1,1,3-trimethylcyclohexane. The conversion of isophorone to 3,3,5-trimethylcyclohexanone is very fast. As the result, high carbon yield of 3,3,5-trimethylcyclohexanone was achieved even at low Pt content. Because the hydrogenation
is
a
reversible
reaction,
there
is
an
equilibrium
between
3,3,5-trimethylcyclohexanone and 3,3,5-trimethylcyclohexanol, which may be the reason
why
the
carbon
yields
of 9
3,3,5-trimethylcyclohexanone
and
3,3,5-trimethylcyclohexanol
level
off
at
high
Pt
content.
Considering the economic reason, we fixed the Pt content in the Pt/Al2O3 catalyst in the following work. 3.4. Effect of polyol The feasibility for the utilization of other lignocellulose derived polyols as hydrogen sources for the HDO of isophorone was also explored. Among them, ethylene glycol can be obtained from the hydrogenolysis of cellulose [47, 48], xylitol and sorbitol can be produced from the hydrolysis-hydrogenation of hemicellulose [49] and cellulose [50-53]. From Fig. 4, we can see that all of these polyols can be used as the hydrogen sources for the HDO of isophorone. Compared with glycol, lower carbon yields of 1,1,3-trimethylcyclohexane were obtained when ethylene glycol, xylitol and sorbitol were used as renewable hydrogen resource. Taking into consideration of lower price of glycerol than those of ethylene glycol, xylitol and sorbitol, we believe that glycerol is a promising hydrogen resource for the hydrogenation of isophorone. Besides polyols, short chain alcohol (such as methanol) can also be used as hydrogen resource for the HDO of isophorone, which means that we can use the mixture of glycerol and unreacted methanol from the production of bio-diesel by transesterification. This is advantageous in real application because it can decrease the cost and energy consumption during the separation of glycerol and unreacted methanol. In the future research, it is also possible to use cheaper biomass derived oxygenates (hydrogenated bio-oil [54] or hydrogenated hemicellulose extract which is the by-product in wood-processing industries such as 10
biomass boilers or pulp mills [55]) as potential hydrogen sources for the HDO of isophorone. 3.5. Stability of catalyst Finally, we also checked the stability of the Pt/Al2O3 catalyst in the HDO of isophorone. According to Fig. 5, it was noticed that the carbon yield of 1,1,3-trimethylcyclohexane decreased with the increment of time on stream. In contrast, the carbon yields of 3,3,5-trimethylcyclohexanone and 3,3,5-trimethylcyclohexanol increased with the time on stream. This result indicates that the activity of Pt/Al2O3 catalyst decreases during the HDO of isophorone, which may be explained by the low hydrothermal stability of Al2O3 support. In the future work, this problem should be solved by improving the hydrothermal stability of Al2O3 support or using more water-tolerant materials as the supports for Pt catalysts. 4. Conclusions In this work, we developed a new a route for the synthesis of jet fuel range cycloalkane (i.e. 1,1,3-trimethylcyclohexane) by the hydrodeoxygenation (HDO) of bio-derived isophorone with the renewable hydrogen which was produced from the aqueous phase reforming (APR) of glycerol. Due to its high activities for both APR and HDO reactions, Pt catalyst was found to be most active for the conversion of isophorone to 1,1,3-trimethylcyclohexane. Al2O3 is better than the other investigated supports for the HDO of isophorone over Pt catalysts, which can be rationalized by the proper acidity of Al2O3. Besides glycerol, a series of lignocellulose derived polyols (such as ethylene glycol, xylitol and sorbitol) and methanol can also be used as the hydrogen 11
resources for the HDO of isophorone. With the increasing of time on stream, the activity of the Pt/Al2O3 for the HDO of isophorone decreased, which can be explained by the low hydrothermal stability of the Al2O3 support. In the future research, this problem should be overcome by improving the hydrothermal stability of Al2O3 support or using more water-tolerant supports. Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21277140; 21476229; 21506213), Dalian Science Foundation for Distinguished Young Scholars (no. 2015R005), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100), Department of Science and Technology of Liaoning Province (under contract of 2015020086-101) and 100-talent project of Dalian Institute of Chemical Physics (DICP).
12
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Figure Captions Fig. 1. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over Al2O3 loaded noble metal catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 2. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over different Pt catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 3. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the Pt/Al2O3 catalysts as the function of Pt content. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 4. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 3wt.% Pt/Al2O3 catalyst using different hydrogen sources. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 5. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 17
3wt.% Pt/Al2O3 catalyst as the function of reaction time. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
18
Conversion or yield (%)
100
Conversion of isophorone Carbon yield of 1A Carbon yield of 1B Carbon yield of 1C
80
60
40
20
0 l O3 /A 2 t P
l 2O 3
/A Pd
l 2O 3
/A Rh
l 2O 3
/A Ru
l 2O 3 Ir/A
Fig. 1. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over Al2O3 loaded noble metal catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
19
Conversion of isophorone Carbon yield of 1B
Conversion or yield (%)
100
Carbon yield of 1A Carbon yield of 1C
80 60 40 20 0 /A Pt
l 2O 3 /S Pt
-A
iO 2
l 2O 3
Pt/
AC
/T Pt
iO 2
O3 /Fe 2 t P
/Z Pt
rO 2
Fig. 2. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over different Pt catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
20
Conversion or yield (%)
100 Conversion of isophorone Carbon yield of 1A Carbon yield of 1B Carbon yield of 1C
80
60
40
20
0 1
5 3 Pt content (wt.%)
10
Fig. 3. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the Pt/Al2O3 catalysts as the function of Pt content. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
21
Conversion of isophorone Carbon yield of 1B
Carbon yield of 1A Carbon yield of 1C
Conversion or yield (%)
100 80 60 40 20 0 n th a
ol
Me
ol
e l en
c gly
Gl
y
l ro e c
X
tol i l y
So
l ito b r
hy
Et
Fig. 4. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 3wt.% Pt/Al2O3 catalyst using different hydrogen sources. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
22
Conversion or yield (%)
100 Conversion of isophorone Carbon yield of 1A Carbon yield of 1B Carbon yield of 1C
80
60
40
20
0 3
5
7
9 11 13 15 17 19 21 23 25 Time on stream (h)
Fig. 5. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 3wt.% Pt/Al2O3 catalyst as the function of reaction time. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
23
Scheme 1. Reaction pathways for the generation of different products during the HDO of isophorone.
24
S0920-5861(17)30492-3 http://dx.doi.org/doi:10.1016/j.cattod.2017.07.009 CATTOD 10923
To appear in:
Catalysis Today
Received date: Accepted date:
21-10-2016 5-7-2017
Please cite this article as: Hao Tang, Ning Li, Shanshan Li, Fang Chen, Guangyi Li, Aiqin Wang, Yu Cong, Xiaodong Wang, Tao Zhang, Synthesis of jet fuel rang cycloalkane from isophorone with glycerol as a renewable hydrogen source, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of jet fuel rang cycloalkane from isophorone with glycerol as a renewable hydrogen source
Hao Tanga,b, Ning Lia,c,*, Shanshan Lia,b, Fang Chena,b, Guangyi Lia,c, Aiqin Wanga,c, Yu Conga, Xiaodong Wanga, Tao Zhanga,c,*
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, No. 457 Zhongshan Road, Dalian 116023, China. b
University of Chinese Academy of Sciences, Beijing 10049, China.
c
iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Corresponding author: Prof. Tao Zhang, Tel: +86-411-94379015; Fax: +86-411-84691570; E-mail: [email protected] Prof.
Ning
Li,
Tel:
+86-411-84379738;
[email protected]
1
Fax:
+86-411-84685940;
E-mail:
Graphical abstract
Highlights Jet fuel range C9 cycloalkane was first synthesized by the HDO of isophorone. Glycerol was first used as a renewable hydrogen resource for the HDO process. Pt/Al2O3 was highly effective for the coupling of APR and HDO reactions. Other lignocellulosic polylols can also be used as the hydrogen resources. C9 cycloalkane as obtained can be used as an additive to conventional bio-jet fuel.
Abstract For the first time, 1,1,3-trimethyl-cyclohexane (a jet fuel range cycloalkane) was synthesized by coupling the aqueous phase reforming (APR) of glycerol and the 2
hydrodeoxygenation (HDO) of isophorone which can be obtained from lignocellulose. Among the investigated catalysts, Pt/Al2O3 was found to be the most active for the production of 1,1,3-trimethyl-cyclohexane with the hydrogen which was in-situ generated by the APR of glycerol. Over it, high carbon yield (67.0%) of 1,1,3trimethyl-cyclohexane can be achieved at 533 K in the absence of external hydrogen. The excellent performance of Pt/Al2O3 catalyst can be explained because Pt is highly active for both APR and HDO reactions. Besides glycerol, many other lignocellulose derived polylols (such as ethylene glycol, xylitol and sorbitol) and methanol can also be used as renewable hydrogen source for the HDO of isophorone. As a potential application, the 1,1,3-trimethyl-cyclohexane as obtained can be blended into the conventional jet fuels to improve their volumetric heat values.
Keywords: Aqueous phase reforming; Hydrodeoxygenation; Lignocellulose; Jet fuel; Cycloalkane.
3
1. Introduction With the increase of social concern about energy and environmental problems, the catalytic conversion of renewable and CO2-neutral biomass to fuels [1-5] and chemicals [6-16] has become a very hot research topic. Lignocellulose is the major component of agricultural and forestry wastes. Compared with other forms of biomass, lignocellulose is much cheaper and more abundant. Jet fuel is one of the most important transportation fuels with increasing demand. In the past years, many routes has been developed for the synthesis of renewable jet fuel range hydrocarbons by the C-C coupling reactions of lignocellulose-derived platform compounds followed by the hydrodeoxygenation (HDO) [17-27]. However, most of the reported work about the lignocellulosic bio-jet fuel was concentrated on the synthesis of jet fuel range chain alkanes. Compared with conventional jet fuel which is a mixture of chain alkanes and cyclic hydrocarbons, these chain alkanes have lower densities (or volumetric heat values). In real application, they need to be blended with conventional jet fuel to meet the specification of aviation fuel [3]. As a solution to this problem, it is imperative to develop new routes for the synthesis of jet fuel range cyclic hydrocarbons with lignocellulosic platform compounds [28-32]. Another bottleneck for the industrialization of lignocellulosic bio-jet fuel is hydrogen which is greatly needed in the HDO processes. As we know, most of the hydrogen we are using today is derived from fossil energy. To fulfil the need of sustainable development, it is also necessary to develop new technologies for the production of renewable bio-jet fuel without using external hydrogen [33, 34]. Aqueous phase reforming (APR) is an important method for the production of renewable 4
hydrogen with biomass derived oxygenates [35-37]. To the best of our knowledge, there is no report for the synthesis of jet fuel range hydrocarbons by the coupling of APR and HDO reactions. Isophorone is the trimerization product of acetone [38] which is the by-product in the manufacture of bio-butanol and bio-ethanol from the Acetone-Butanol-Ethanol (ABE) fermentation of lignocellulose [39]. Considering its cyclic chemical structure, isophorone can be used as a precursor for jet fuel range hydrocarbons. Glycerol is a by-product in in the manufacture of biodiesel. Due to its low price and high availability, glycerol can be utilized as a promising hydrogen resource for the HDO process [37]. In this work, it was reported for the first time that 1,1,3-trimethyl-cyclohexane (a jet fuel range cycloalkane) can be obtained at high carbon yield by coupling the APR of glycerol and the HDO of isophorone. At the same time, we also explored the feasibility for the utilization of other lignocellulose derived polylols as renewable hydrogen sources for the HDO of isophorone. 2. Materials and methods 2.1. Preparation of catalysts The Pt/Al2O3, Pd/Al2O3, Rh/Al2O3, Ru/Al2O3 and Ir/Al2O3 catalysts were prepared by the incipient wetness impregnation of commercial Al2O3 with aqueous solutions of H2PtCl6·6H2O, PdCl2, RhCl3·6H2O, RuCl3·3H2O, and H2IrCl6·6H2O, respectively. The theoretical contents of noble metals in these catalysts were controlled as 3% by weight (denoted as 3wt.%). Subsequently, the products were dried at 383 K for 8 h and reduced by hydrogen flow at 533 K for 6 h. After being cooled down to room temperature in 5
hydrogen flow, the catalysts were passivated with 1% O2 in N2. For comparison, we also prepared the Pt/SiO2-Al2O3, Pt/AC, Pt/TiO2, Pt/Fe2O3 and Pt/TiO2 catalysts by the same method using SiO2-Al2O3 (SiO2/Al2O3 molar ratio = 25), activated carbon (NORIT Company), TiO2 P25 (Degussa), Fe2O3 and ZrO2 as supports. The theoretical Pt contents in these catalysts were fixed as 3wt.%. 2.2. Hydrodeoxygenation (HDO) of isophorone The HDO of isophorone was conducted at 533 K in a stainless steel fixed-bed tubular reactor which has been described in literature [35, 36]. For each test, 1.80 g catalyst was used. The isophorone and 20wt.% glycerol (or ethylene glycol, xylitol and sorbitol) aqueous solution were fed into the reactor from bottom by two HPLC pumps at a rate of 0.02 mL min-1 and 0.1 mL min-1, respectively. The products from the outlet of the tubular reactor became two phases in a gas-liquid separator. The gas products (carried by nitrogen flow at a rate of 120 mL min-1) passed through a back-pressure regulator which was used to maintain the system pressure at 6.0 MPa and were analyzed on-line by an Agilent 6890N GC. The liquid products were drained periodically from the separator and analyzed by another Agilent 7890 GC. 3. Results and discussion 3.1. Effect of noble metal At the beginning of this work, we compared the activities of several Al2O3 loaded noble metal catalysts for the HDO of isophorone with the hydrogen which was in-situ generated
by
the
APR
of
glycerol. 6
On
the
basis
of
our
analysis,
3,3,5-trimethylcyclohexanone,
3,3,5-trimethylcyclohexanol
and
1,1,3-trimethylcyclohexane, (i.e. the 1A, 1B and 1C in Scheme 1) were identified as three major products from this process. The reaction pathways for the generation of these compounds were proposed in Scheme 1. It is interesting that no 3,5,5-trimethylcyclohex-2-enol (i.e. the 1D in Scheme 1) was detected in the product. This result can be comprehended because the hydrogenation of C=C bond in isophorone molecule is relatively easier than the hydrogenation of C=O bond (due to thermodynamic
reason
[40]).
According
to
literature
[41,
42],
the
1,1,3-trimethylcyclohexane as obtained has a density of 0.79 g mL-1 and a freezing point of 207.3 K. As a potential application, it can be blended into the conventional bio-jet fuels to improve their volumetric heat values. The 3,3,5-trimethylcyclohexanone (i.e. 1A) as obtained can be used as a solvent for vinyl resins, lacquers, varnishes, paints and other coatings. The 3,3,5-trimethylcyclohexanol (i.e. 1B) as obtained can be used as an intermediate in the production of cyclandelate. As another option, the mixture of 3,3,5-trimethylcyclohexanone and 3,3,5-trimethylcyclohexanol as obtained can also be recycled and used for the HDO process to increase the carbon yield of 1,1,3-trimethylcyclohexane. As we can see from Fig. 1, Pt/Al2O3 exhibited the highest activity for the HDO of isophorone among the investigated catalysts. Over it, the isophorone was completely converted. High carbon yield (67.0%) of 1,1,3-trimethylcyclohexane was achieved at 533 K. The activities of noble metal catalysts decreased in the order of Pt/Al2O3 > Pd/Al2O3 > Rh/Al2O3 > Ru/Al2O3 > Ir/Al2O3. In the previous work of Dumesic et al. 7
[43], it was found that the activities of noble metal catalysts for the APR of ethylene glycol decreased in the order of Pt > Ru > Rh ~ Pd > Ir, while the hydrogen selectivity in the APR product decreased in the order of Pd > Pt > Ru > Rh. In another work of the same group [44], it was suggested that Pt is highly active for the aqueous HDO of biomass derived oxygenates. In the recent work of Suppes et al. [45], it was also found that the hydogenolysis activities of noble metal catalysts decreases in the order of Ru > Pt > Pd. As we know, hydogenolysis is an important pathway for the HDO of biomass derived oxygenates [46]. Based on these reports, we can attribute the excellent performance of Pt/Al2O3 catalyst to the high activities of Pt for both the production of hydrogen from APR of glycerol and the HDO of isophorone. 3.2. Effect of support The influence of support on the HDO of isophorone over Pt catalyst was also investigated. Compared with the other Pt catalysts which were used in this work, the Pt/Al2O3 catalyst exhibited evidently higher HDO activity (see Fig. 2). The carbon yields of 1,1,3-trimethylcyclohexane over the Pt catalysts decreased in the order of Pt/Al2O3 > Pt/SiO2-Al2O3 > Pt/AC > Pt/TiO2 > Pt/Fe2O3 > Pt/ZrO2. This sequence is similar as the one which has been reported by Tian et al. in their previous work about the APR of glycerol (Pt/Al2O3 > Pt/MgO > Pt/SiO2 > Pt/HUSY > Pt/AC > Pt/SAPO-11) [37]. The higher activity of Pt/Al2O3 catalyst can be explained by two reasons: 1) The higher activity of Pt/Al2O3 for the generation of hydrogen from the APR of glycerol. 2) The proper acidity of Al2O3 support. As we know, the acidity of support is very important for the HDO process because acid-catalyzed dehydration follow metal 8
catalyzed hydrogenation has been suggested as a major pathway for the HDO of the biomass derived oxygenates [46]. However, too high acidity of support will also lead to the dehydration of glycerol (or the polyols and alcohols generated from the APR of glycerol) and the generation of unexpected C1-C3 alkanes by self-HDO reaction of glycerol [44] which is competitive reaction for the HDO of isophorone. 3.3. Effect of Pt content The effect of metal content on the HDO performance of the Pt/Al2O3 catalyst was explored. As we can see from Fig. 3, the isophorone conversion and the carbon yield of 1,1,3-trimethylcyclohexane increased with the increment of Pt content in the Pt/Al2O3 catalyst, reached the maximum when Pt content was about 3wt.%, then leveled off with the further increase of Pt content. The carbon yield of 3,3,5-trimethylcyclohexanone decreased with the increment of Pt content from 1wt.% to 3wt.%, then stabilized with the
further
increase
of
Pt
content.
In
contrast,
the
carbon
yield
of
3,3,5-trimethylcyclohexanol slightly increased with the increment of Pt content in the Pt/Al2O3 catalyst from 1wt.% to 5wt.%, then stabilized. These results can be rationalized because 3,3,5-trimethylcyclohexanone is the intermediate during the HDO of isophorone to 1,1,3-trimethylcyclohexane. The conversion of isophorone to 3,3,5-trimethylcyclohexanone is very fast. As the result, high carbon yield of 3,3,5-trimethylcyclohexanone was achieved even at low Pt content. Because the hydrogenation
is
a
reversible
reaction,
there
is
an
equilibrium
between
3,3,5-trimethylcyclohexanone and 3,3,5-trimethylcyclohexanol, which may be the reason
why
the
carbon
yields
of 9
3,3,5-trimethylcyclohexanone
and
3,3,5-trimethylcyclohexanol
level
off
at
high
Pt
content.
Considering the economic reason, we fixed the Pt content in the Pt/Al2O3 catalyst in the following work. 3.4. Effect of polyol The feasibility for the utilization of other lignocellulose derived polyols as hydrogen sources for the HDO of isophorone was also explored. Among them, ethylene glycol can be obtained from the hydrogenolysis of cellulose [47, 48], xylitol and sorbitol can be produced from the hydrolysis-hydrogenation of hemicellulose [49] and cellulose [50-53]. From Fig. 4, we can see that all of these polyols can be used as the hydrogen sources for the HDO of isophorone. Compared with glycol, lower carbon yields of 1,1,3-trimethylcyclohexane were obtained when ethylene glycol, xylitol and sorbitol were used as renewable hydrogen resource. Taking into consideration of lower price of glycerol than those of ethylene glycol, xylitol and sorbitol, we believe that glycerol is a promising hydrogen resource for the hydrogenation of isophorone. Besides polyols, short chain alcohol (such as methanol) can also be used as hydrogen resource for the HDO of isophorone, which means that we can use the mixture of glycerol and unreacted methanol from the production of bio-diesel by transesterification. This is advantageous in real application because it can decrease the cost and energy consumption during the separation of glycerol and unreacted methanol. In the future research, it is also possible to use cheaper biomass derived oxygenates (hydrogenated bio-oil [54] or hydrogenated hemicellulose extract which is the by-product in wood-processing industries such as 10
biomass boilers or pulp mills [55]) as potential hydrogen sources for the HDO of isophorone. 3.5. Stability of catalyst Finally, we also checked the stability of the Pt/Al2O3 catalyst in the HDO of isophorone. According to Fig. 5, it was noticed that the carbon yield of 1,1,3-trimethylcyclohexane decreased with the increment of time on stream. In contrast, the carbon yields of 3,3,5-trimethylcyclohexanone and 3,3,5-trimethylcyclohexanol increased with the time on stream. This result indicates that the activity of Pt/Al2O3 catalyst decreases during the HDO of isophorone, which may be explained by the low hydrothermal stability of Al2O3 support. In the future work, this problem should be solved by improving the hydrothermal stability of Al2O3 support or using more water-tolerant materials as the supports for Pt catalysts. 4. Conclusions In this work, we developed a new a route for the synthesis of jet fuel range cycloalkane (i.e. 1,1,3-trimethylcyclohexane) by the hydrodeoxygenation (HDO) of bio-derived isophorone with the renewable hydrogen which was produced from the aqueous phase reforming (APR) of glycerol. Due to its high activities for both APR and HDO reactions, Pt catalyst was found to be most active for the conversion of isophorone to 1,1,3-trimethylcyclohexane. Al2O3 is better than the other investigated supports for the HDO of isophorone over Pt catalysts, which can be rationalized by the proper acidity of Al2O3. Besides glycerol, a series of lignocellulose derived polyols (such as ethylene glycol, xylitol and sorbitol) and methanol can also be used as the hydrogen 11
resources for the HDO of isophorone. With the increasing of time on stream, the activity of the Pt/Al2O3 for the HDO of isophorone decreased, which can be explained by the low hydrothermal stability of the Al2O3 support. In the future research, this problem should be overcome by improving the hydrothermal stability of Al2O3 support or using more water-tolerant supports. Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21277140; 21476229; 21506213), Dalian Science Foundation for Distinguished Young Scholars (no. 2015R005), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100), Department of Science and Technology of Liaoning Province (under contract of 2015020086-101) and 100-talent project of Dalian Institute of Chemical Physics (DICP).
12
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Figure Captions Fig. 1. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over Al2O3 loaded noble metal catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 2. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over different Pt catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 3. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the Pt/Al2O3 catalysts as the function of Pt content. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 4. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 3wt.% Pt/Al2O3 catalyst using different hydrogen sources. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1. Fig. 5. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 17
3wt.% Pt/Al2O3 catalyst as the function of reaction time. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
18
Conversion or yield (%)
100
Conversion of isophorone Carbon yield of 1A Carbon yield of 1B Carbon yield of 1C
80
60
40
20
0 l O3 /A 2 t P
l 2O 3
/A Pd
l 2O 3
/A Rh
l 2O 3
/A Ru
l 2O 3 Ir/A
Fig. 1. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over Al2O3 loaded noble metal catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
19
Conversion of isophorone Carbon yield of 1B
Conversion or yield (%)
100
Carbon yield of 1A Carbon yield of 1C
80 60 40 20 0 /A Pt
l 2O 3 /S Pt
-A
iO 2
l 2O 3
Pt/
AC
/T Pt
iO 2
O3 /Fe 2 t P
/Z Pt
rO 2
Fig. 2. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over different Pt catalysts. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
20
Conversion or yield (%)
100 Conversion of isophorone Carbon yield of 1A Carbon yield of 1B Carbon yield of 1C
80
60
40
20
0 1
5 3 Pt content (wt.%)
10
Fig. 3. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the Pt/Al2O3 catalysts as the function of Pt content. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% glycerol solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
21
Conversion of isophorone Carbon yield of 1B
Carbon yield of 1A Carbon yield of 1C
Conversion or yield (%)
100 80 60 40 20 0 n th a
ol
Me
ol
e l en
c gly
Gl
y
l ro e c
X
tol i l y
So
l ito b r
hy
Et
Fig. 4. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 3wt.% Pt/Al2O3 catalyst using different hydrogen sources. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
22
Conversion or yield (%)
100 Conversion of isophorone Carbon yield of 1A Carbon yield of 1B Carbon yield of 1C
80
60
40
20
0 3
5
7
9 11 13 15 17 19 21 23 25 Time on stream (h)
Fig. 5. Conversions of isophorone and carbon yields of 3,3,5-trimethylcyclohexanone (1A), 3,3,5-trimethylcyclohexanol (1B), 1,1,3-trimethylcyclohexane (1C) over the 3wt.% Pt/Al2O3 catalyst as the function of reaction time. Reaction conditions: 533 K, 6 MPa; 1.8 g catalyst, 20wt.% polylol (or methanol) solution flow rate 0.1 mL min-1; isophorone flow rate: 0.02 mL min-1.
23
Scheme 1. Reaction pathways for the generation of different products during the HDO of isophorone.
24