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Improving low-temperature properties of lignin-derived jet-fuel-ranged hydrocarbons via hydroisomerization
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Improving low-temperature properties of lignin-derived jet-fuel-ranged hydrocarbons via hydroisomerization Genkuo Nie, Yiying Dai, Junjian Xie, Xiangwen Zhang, Lun Pan, Ji-Jun Zou* 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
A R T I C LE I N FO
A B S T R A C T
Keywords: Biofuel Hydroisomerization (Cyclopentylmethyl)cyclohexane Bi(cyclohexane) Cyclohexylphenol
Fuel blending generally exhibits better low-temperature flow properties (i.e. low viscosity, low freezing point) than pure component fuel. Here, a fuel bending containing bicyclohexane and (cyclopentylmethyl)cyclohexane was synthesized directly by hydroisomerization of cyclohexylphenol to reduce the freezing point. The selectivity of (cyclopentylmethyl)cyclohexane formed by isomerization is enhanced through inhibiting the quick hydrogenation of intermediates using mixing acid catalyst and metal catalyst. Zeolite with strong Brønsted acid site and big surface area, such as HZSM-5 is better for the isomerization. Catalyzed by mixture of Pd/C and HZSM-5, the obtained fuel blending improves the freezing point from 2.6 °C to−22 °C compared with pure bicyclohexane, meanwhile keep the high density of 0.880 g/mL unchanged. The improved low-temperature property is contributed to the formation of (cyclopentylmethyl)cyclohexane. Moreover, the pathways of isomerization and its competition with hydrodeoxygenation are also investigated.
1. Introduction Advanced jet fuel with high density and good low-temperature properties (such as low freezing point and low viscosity) is desired to extend the flight distance and increase the payload of aircrafts. However, it is still not easy to synthesize a fuel with both high density and good low-temperature performance [1]. So many strategies have been developed to improve the low-temperature properties of fuel meanwhile retain the high density. Typically, blending several fuel components can balance the density and low-temperature properties. For example, RJ-5 has high density of 1.08 g/mL but high freezing point above 0 °C [2], while as 20–25 % of RJ-5, 65–70 % of JP-10 and 10–12 % of methylcyclohexane are blended, a new fuel (i.e. JP-9) is obtained with high density of 0.94 g/mL and low freezing point of −65 °C, attributed to the extremely low freezing point of JP-10 and methylcyclohexane [2]. It is better to directly produce a mixture containing both high-density component and low-freezing point component. With this regard, modulating molecular structure by isomerization is an efficient way. For example, isomerization of tetrahyrodicyclopentadiene and tetrahyrotricyclopentadiene into exo-isomers and alkyl-adamantanes using acid catalyst have been investigated to reduce the freezing point and viscosity of high-density fuels [3–7]. The hydroisomerization of linear paraffin to branched isomers is also a key for oil refining to improve the low-temperature property of liquid fuels [8,9]. ⁎
Hydrocarbons with bicyclic structure, especially bicyclohexane, have been intensively investigated as component of jet fuel [10–12]. This is because bicyclohexane has promising density about 0.887 g/mL which is higher than the traditional jet fuel, such as RP-3 (0.783 g/mL, 15 °C) [13]. Additionally, the synthesizing process of bicyclohexane is easy. Typically, bicyclohexane is synthesized by the hydrogenation of biphenyl [14] or dibenzofuran [15] and cross-coupling of cycloalkanes [16], which are separated from fossil oil. Recently, bicyclohexane as important high-density biofuel has also been produced from lignocellulose derived platform compounds [17–19]. The synthesis of biobicyclohexane requires two steps, that is, the synthesis of bicyclohexane precursor and then hydrodeoxygenation of the precursor. Many literatures have reported the synthesis of bicyclohexane precursor like cyclohexylphenol from aldol condensation of cyclohexanone or alkylation of phenols with cyclohexanol [18,20,21]. It is worth noting that phenols are classical platform chemicals from the hydrolysis of lignin while cyclohexanol can be obtained by selective hydrogenation of phenols [22–25]. The purpose of hydrodeoxygenation is to remove the oxygen atom and saturate the C]C bonds. It has been reported that ring contraction can occur in the hydrodeoxygenation of phenols, which produces some methylcyclopentane [26,27]. Such isomerization is also expected to occur in the hydrodeoxygenation of bicyclohexane precursor. In fact, isomerization has been noticed in hydrodeoxygenation of phenolic dimers, where about 8.6 % of isomerized product
Corresponding author. E-mail address: [email protected] (J.-J. Zou).
https://doi.org/10.1016/j.cattod.2020.01.033 Received 1 November 2019; Received in revised form 9 January 2020; Accepted 28 January 2020 0920-5861/ © 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Genkuo Nie, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.01.033
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((cyclopentylmethyl)cyclohexane) along with 88.0 % of hydrodeoxygenation product are produced [28]. And (cyclopentylmethyl) cyclohexane has good low-temperature properties due to the asymmetry molecular structure [29]. The density of bicyclohexane is high, but the freezing point (2.6 °C) is needed to be improved. Therefore, if considerable hydroisomerization can take place during the hydrodeoxygenation of bicyclohexane precursor, a mixture of bicyclic hydrocarbons may be produced to achieve both high density and good low-temperature properties. With these considerations, here we report the synthesis of fuel blending containing bicyclohexane and (cyclopentylmethyl)cyclohexane by hydroisomerization of cyclohexylphenol. The isomerization is enhanced by inhibiting the quick hydrogenation of intermediates by using mixture of Pd/C and HZSM-5 as catalyst, with the selectivity of (cyclopentylmethyl)cyclohexane reaching 32.0 %. Compared with normal hydrodeoxygenation product, the freezing point of fuel blending is reduced to −22 °C from 2.6 °C, meanwhile keep the high density of 0.880 g/mL unchanged.
Fig. 1. Product distribution of hydroisomerization of cyclohexylphenol. Reaction condition: 50 mL water, 10 mmol cyclohexylphenol, 0.08 g Pd/C, 10 g HZSM-5, 150 °C, 4 MPa H2, 30 h.
2.3. Measurements of fuel properties 2. Experiment
The fuel density was measured by 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 (ASTM D445). Each measurement was performed for three times.
2.1. Catalysts and chemicals Methylcyclohexene (AR) and cyclohexanol (AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.. Phenol (AR) was purchased from Tianjin Guangfu Fine Chemical Research Institute. HZSM-5 (SiO2/Al2O3 = 25) and Hβ (SiO2/Al2O3 = 25) were purchased from Nankai Catalysts Co., Ltd. and calcined in air at 580 °C for 3 h before use. Pd/C (5 wt%) was purchased from Shanxi Rock New Materials Co., Ltd.. 0.08 %Pd/HZSM-5 and 0.08 %Pd/Hβ were prepared according to literature [28].
3. Results and discussion Hydroisomerization of cyclohexylphenol (i.e a) can produce bicyclohexane (i.e e) fromed by the direct hydrodeoxygenation and (cyclopentylmethyl)cyclohexane (i.e g) formed by the isomerization (Scheme 1 and Fig. S1). The product distribution of hydroisomerization in 30 h is shown in Fig. 1 in order to investigate the composition evolution in this process. The reactant a is converted very quickly in 1 h and disappear until 5 h. At the same time, bi(cyclohexan)-one (i.e b) is produced largely with a small amount of bi(cyclohexan)-ol (i.e c), bi (cyclohexan)-ene (i.e d), bi(cyclohexane) (i.e e), and (cyclopentylmethyl)cyclohexane (i.e g). Then, the amount of c, e and g increases continuously with the further conversion of b. With time going on, all of c is converted to e and g. Only a small amount of d is detected in the whole process, indicating it is a metastable intermediate that will be quick hydrogenated and/or isomerized. The most possible pathway for the formation of g is that, d is protonated, rearranged and finally hydrogenated to g [26,30,31]. In order to understand the role of catalyst, several element steps during in the hydroisomerization of a using different catalysts are investigated (Scheme 2). As shown in Scheme 2 and Fig. S2, with the presence of Pd/ C a is hydrogenated to b and c with small amount of e, which indicates that Pd/C cannot catalyze the isomerization (Reaction A). When using c as the reactant, e is also obtained as the only product (Reaction B). With the presence of only HZSM-5, c is catalyzed to d and (cyclopent-2en-1-ylmethyl)cyclohexane (i.e. f) (Reaction C). This result implies
2.2. Hydroisomerization reaction Cyclohexylphenol was synthesized according to our previous work [29]. The hydroisomerization of cyclohexylphenol (Scheme 1) was carried out with 100 mL autoclave (EasyChem E100). With 50 mL water/cyclohexane/ethanol as solvent, 10 mmoL substrate, mixture of 0.08 g Pd/C plus 0.5–15 g of zeolite, or 5 g 0.08 %Pd/zeolite were used as catalyst. The reaction was conducted at 2–6 MPa H2 and 120–200 °C for 10 h. Then the upper organic phase was collected and dewatered using anhydrous MgSO4. The liquid products were quantified by a gas chromatography (Agilent-7820A) equipped with an FID detector and a capillary column HP-1 capillary column (30 m ×0.53 mm), and determined qualitatively using an Agilent 6890/5975 gas chromatography–mass spectrometry (GC–MS) equipped with HP-5 capillary column (30 m ×0.5 mm). The temperature of GC–MS column was raised from 50 °C to 280 °C at ramped speed of 10 °C/min and kept at 280 °C for 10 min, for GC analysis the column temperature was increased from 40 °C to 280 °C at ramped speed of 20 °C/min and kept at 280 °C for 10 min. The products were also identified with 13C and 1H NMR spectra collected using Bruker Avance 400 M Spectrometer.
Scheme 1. Synthesis of bicyclic fuel blending by hydroisomerization of cyclohexyphenol. 2
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contrast, the g selectivity catalyzed by mixing catalyst of Pd/C-Hβ is better than loaded catalyst of Pd/Hβ, also that of Pd/C-HZSM-5 is better than Pd/HZSM-5. Especially, the hydroisomerization selectivity of mixed Pd/C-HZSM-5 is the best with g selectivity of 16 %. Over loaded catalyst, the intermediate d formed on acid zeolite will be quickly hydrogenated to e by nearby metal site, due to the very intimate contact of two kinds of sites (Fig. 3a). For mixed catalyst (Fig. 3b), d has more chance to be rearranged by acidic sites before trapped by a metal site, attributed to the spatially separation of zeolite and Pd. The acidity of HZSM-5 and Hβ are characterized by NH3-TPD spectra. As shown in Fig. S5a, the peaks below 300 °C and above 450 °C appear for HZSM-5 and Pd/HZSM-5, while peaks below 200 °C and above 310 °C are observed on Hβ and Pd/Hβ, ascribed to desorption of ammonia on the weak and strong acid sites, respectively [34]. This indicates that the acid strength of HZSM-5 is much higher than that of Hβ. The amount of acid sites with different strength is calculated as the ammonia uptake for each catalyst and the results are listed in Table 1. It shows that loaded Pd decreases the acid amount of zeolites and the total acid amount of HZSM-5 is higher than that of Hβ, especially the strong acid sites. This result is in agreement with that of Py-IR spectra shown in Fig. S5b. Two absorption peaks stated at 1450 and 1540 cm−1 ascribed to the Lewis and Brønsted acid sites respectively, whose intensities are used to calculate the concentrations of two types of acid sites (Table 1). Table 1 also shows the change of Brønsted and Lewis acid concentration with desorption temperature. Typically, desorption temperature of 200 °C and 300 °C are ascribed to the weak and strong acid. Evidently, both Brønsted and Lewis acid concentration decrease with increasing desorption temperature and the loading of Pd decreases the acid amount. Similar to the NH3-TPD result, the B/L ratio of HZSM5 is much higher than that of Hβ, especially the strong acid sites. Based on these results, one can conclude that strong Brønsted acid site is favorable for the isomerization, which is in consistent with the hydroisomerization of paraffin and methylcyclohexane [30,35,36]. The effect of Brønsted acid is also confirmed by the reaction catalyzed by phosphotungstic acid (with Brønsted acid stronger than H2SO4). As shown in Fig. S6, the selectivity of compound g catalyzed by phosphotungstic acid is higher than that by HZSM-5. However, phosphotungstic acid is easily dissolved in polar solvent, especially water to form homogeneous protonic acid solution, which is corrosive and recyclable hardly. In order to obtain more isomerized product, the dosage of HZSM-5 in the mixed catalyst is optimized (Fig. 4a). With the dosage of HZSM-5 increasing, the alkanes yield and selectivity of g increase. At low dosage of HZSM-5, when a is converted to c which has a lone electron pair (on oxygen atom) [37] and act as Lewis bases to compete with d to react with acid sites and decreases the isomerization of d by HZSM-5 but increases the direct hydrogenation of d by Pd/C. As HZSM-5 dosage increases, c is dehydrated quickly with more chance for isomerization of d. With the dosage of HZSM-5 increasing to 10 g, the alkanes yield is close to 100 %, the selectivity of g (18 %) does not change any more. Temperature is also an important influence factor to the hydroisomerization (Fig. 4b). As temperature increases, compound d is accumulated with more chance for isomerization. However, as the temperature rises to 180 °C, alkanes yield is up to 93 % while the selectivity of g begins to drop, suggesting hydrogenation is more sensitive than isomerization at high temperature. Hydrogenation pressure is also investigated as shown in Fig. 4c. Definitely, high hydrogenation pressure favors the hydrogenation. With H2 pressure increases, more compound d is produced from the conversion of compound a, which increases the chance for isomerization. However, as the pressure is more than 4 MPa the g selectivity decreases, which means the direct hydrogenation of compound d increases more quick under this condition. Solvent effect is also optimized as shown in Fig. 4d. As can be seen, a is converted completely using water as solvent with the highest isomerization selectivity of g (30 %), because water is favorable for stabilizing carbonium ions and proton transformation for subsequent rearrangement. When cyclohexane is used as the solvent, direct
Scheme 2. Element steps in hydroisomerization of cyclohexylphenol.
that acid site is the activity site for ring contraction. So with the mixture of Pd/C and HZSM-5 as catalyst, c is converted to either e or g (Reaction D). The rearrangement of methylcyclohexene to ethylcyclopentane by HZSM-5 (Fig. S3) also confirms that acidic catalyst can realize the ring contraction. Lastly, with mixture of Pd/C and HZSM-5 (Reaction E) or sole HZSM-5 (Reaction F) as catalyst, e does not take place any reaction, which suggests that e cannot be isomerized to g under H2. Based on the above analysis, sequence elementary steps for the formation of e and g are shown in Scheme 1. a is first hydrogenated to c (step 1 and 2) and then dehydrated to d (step 3). d is converted to bi (cyclohexane)oxides by acid catalyst (step 4). The bi(cyclohexane)oxide is isomerized to (cyclopentylmethyl)cyclohexaneoxide via ring contraction (step 5). (Cyclopentylmethyl)cyclohexaneoxide deprotonate via β-hydrogen transfer to acid site in quasi-equilibrated step to form f (step 6) and is hydrogenated to g (step 7) finally. This reaction is similar to the conversion of cyclohexene to methylcyclopentane where carbocationic isomerization is the key step [32,33]. At the same time, d can be directly hydrogenated to e (step 8). Acid sites favor step 4 and metal site favors step 8, and these two reactions compete with each other. Then series of catalysts including metal-acid mixture and metal loaded acid catalyst are investigated in order to enhance step 4 and suppress step 8 and finally improve the selectivity of isomerization (Fig. 2). a can be converted completely by all the catalysts, except Pd/ HZSM-5, because the pore size of HZSM-5 is too small (0.68 nm) to let a (0.49 nm) diffuse freely (Fig. S4). Consequently, the activity of mixed Pd/C-HZSM-5 is also poor. The yield of alkanes catalyzed by Pd/Hβ is better than mixed Pd/C-Hβ, because the hydrogenation and deoxygenation are accelerated by the synergetic effect of Pd and Hβ. In
Fig. 2. Hydroisomerization of cyclohexylphenol catalyzed by different cataCyclohexylphenol coversion, yield of e and g, the selectivity of lysts. g. Reaction condition: 50 mL water, 10 mmol cyclohexylphenol, 0.08 g Pd/C with 5 g zeolite, or 5 g 0.08 % Pd/zeolite, 180 °C, 6 MPa H2, 10 h. 3
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Fig. 3. Reaction pathways on (a) loaded Pd/HZSM-5 vs (b) mixture of Pd/C and HZSM-5. Table 1 Acid properties of catalysts derived from NH3-TPD and Py-IR. Catalysts
HZSM-5 Pd/HZSM-5 Hβ Pd/Hβ
Acidity by strength(mmol/g)
Brønsted acid sites(mmol/g)
Lewis acid sites(mmol/g)
B/L
Weak
Strong
Total
S/(S + W)
200 °C
350 °C
200 °C
350 °C
200 °C
350 °C
1.467 1.323 1.413 1.389
0.478 0.435 0.321 0.308
1.945 1.758 1.734 1.697
0.246 0.247 0.185 0.181
0.168 0.139 0.083 0.027
0.154 0.134 0.052 0.024
0.009 0.027 0.015 0.014
0.005 0.006 0.014 0.010
19.389 5.148 5.533 1.929
28.179 22.333 3.714 2.400
this reaction was checked. After each use, HZSM-5 is separated, washed with water and used in next reaction directly. As shown in Fig. S7, HZSM-5 is relatively stable under the investigated conditions. With the increase of recycling cycle, the conversion of compound a and alkanes yield keep constant while the selectivity of compound g decreases slightly, which may be related to the formed coke on HZSM-5. Under optimal conditions, a mixture of 62.0 % of e and 32.0 % of g
hydrodeoxygenation of compound a is more favored with only 3 % selectivity of g, because carbonium ions is not stable enough in cyclohexane. Ethanol is the worst solvent for both hydrogenation and isomerization, owing to the lone electron pair (on oxygen atom) as Lewis base competing with reactants to be absorbed by HZSM-5. When water is used as solvent with prolonging reaction time to 30 h, about 62.0 % of e and 32.0 % of g are obtained. The reusability of HZSM-5 zeolite for
Fig. 4. Effects of (a) dosages of HZSM5, (b) temperature, (c) H2 pressure and (d) solvents on hydroisomerization of cyclohexylphenol using mixing Pd/C and HZSM-5 as catalyst. Cyclohexylphenol coversion, yield of the selectivity of g. e and g, Reaction conditions: (a) 50 mL water, 10 mmol cyclohexylphenol, 0.08 g Pd/ C, 180 °C, 6 MPa H2, 10 h; (b) 50 mLwater, 10 mmol cyclohexylphenol, 0.08 g Pd/C with 10 g HZSM5, 6 MPa H2, 10 h; (c) 50 mLwater, 10 mmol cyclohexylphenol, 0.08 g Pd/ C with 10 g HZSM-5, 150 °C, 10 h; (d) 50 mL solvents, 10 mmol cyclohexylphenol, 0.08 g Pd/C with 10 g HZSM5, 150 °C, 4 MPa H2, 10 h.
4
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balance of (cyclopentylmethyl)cyclohexane, the fuel blending synthesized by hydroisomerization shows improved low-temperature properties compared with that synthesized by normal hydrodeoxygenation, meanwhile still keeps high density.
Table 2 Bicyclic alkanes and the corresponding fuel properties. Entry
Compounds
Density (g/mL, 20 °C)
Freezing point (oC)
Viscosity (mm2/s, 20 °C)
Reference
1
71.4 %
0.880
−22
3.2
This work
CRediT authorship contribution statement Genkuo Nie: Conceptualization, Methodology, Investigation, Writing - original draft, Funding acquisition. Yiying Dai: Validation. Junjian Xie: Data curation. Xiangwen Zhang: Supervision. Lun Pan: Supervision. Ji-Jun Zou: Methodology, Supervision, Resources, Project administration.
＋28.6 % 2
0.887
2.6
4.5
[12,18]
3
0.867
−38.0
1.6 (25 °C)
[18]
4
0.880
< −75
2.0
[29]
5
0.876
−20
5.0
[41]
6
0.871
−74
–
[39,40]
7
0.870
< −80
2.9
This work
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
with small amount of CeC coupling compounds is obtained. After vacuum distillation, a fuel blending contains 71.4 % of e and 28.6 % of g is finally obtained. As shown in entry 1 of Table 2, this fuel has a density of 0.880 g/mL, a freezing point of −22 °C and a viscosity of 3.2 mm2/s (20 °C) which is much better than e (density of 0.887 g/mL, freezing point of 2.6 °C and viscosity of 4.5 mm2/s at 20 °C) (entry 2 of Table 2). The much-improved low-temperature properties are attributed to the presence of g. In order to investigate the properties of g, a vacuum distillation is conducted to get 96.0 % purity of g. As shown in entry 7 of Table 2, this compound has density about 0.870 g/mL, freezing point of lower than −80 °C and viscosity of 2.9 mm2/s at 20 °C. In order to understand the relationship between structure and fuel properties of compound g, some bicyclic alkanes are compared in Table 2. All these bicyclic alkanes have density about 0.867∼0.887 g/ mL, while their low-temperature properties vary greatly. The freezing point of bi(cyclohexane) (2.6 °C, entry 2) [12,18] is much higher than bi(cyclopentane) (−38 °C, entry 3) [18], and the freezing point of cyclopentylcyclohexane (−75 °C, entry 4) [29] is much lower than that of bi(cyclohexane) and bi(cyclopentane), suggesting cyclopenty ring and the asymmetry structure are favorable to improve the low temperature properties. Actually, the low-temperature properties improved by breaking the center of molecule symmetry is in consist with previous report [38] that the absence of symmetry center decreases the freezing point. Besides, the freezing point of dicyclopentyl-methane (−74 °C, entry 6) [39,40] is much lower than bi(cyclopentane) (−38 °C, entry 3) [18] and that of dicyclohexylmethane (-20 °C, entry 5) [41] is much lower than bi(cyclohexane) (2.6 °C, entry 2) [12,18]. This result is consistent with the report that the freezing point of diphenylmethane (23−24 °C) [42] is much lower than that of biphenyl (70−72 °C) [43], which suggests that bicyclic-methane is more favorable than bicyclic alkanes from the aspect of low-temperature properties. Based on the above analysis, the good low-temperature properties of (cyclopentylmethyl)cyclohexane is attributed to the bicyclic-methane structure and the break of the center of symmetry of molecule by fivemember ring.
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4. Conclusion A jet-fuel ranged hydrocarbon blending containing (cyclopentylmethyl)cyclohexane and bi(cyclohexane) was synthesized by hydroisomerization of cyclohexylphenol. The selectivity of isomerized product, (cyclopentylmethyl)cyclohexane is improved to 32.0 % by using a mixed catalyst of Pd/C and HZSM-5. The high surface area, strong Brønsted acid and spatially separated of acid site and metal site benefit to the isomerization. (Cyclopentylmethyl)cyclohexane has good low-temperature properties attributed to the bicyclic-methane structure and breaking of symmetry center by five-member ring. Due to the 5
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Improving low-temperature properties of lignin-derived jet-fuel-ranged hydrocarbons via hydroisomerization Genkuo Nie, Yiying Dai, Junjian Xie, Xiangwen Zhang, Lun Pan, Ji-Jun Zou* 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
A R T I C LE I N FO
A B S T R A C T
Keywords: Biofuel Hydroisomerization (Cyclopentylmethyl)cyclohexane Bi(cyclohexane) Cyclohexylphenol
Fuel blending generally exhibits better low-temperature flow properties (i.e. low viscosity, low freezing point) than pure component fuel. Here, a fuel bending containing bicyclohexane and (cyclopentylmethyl)cyclohexane was synthesized directly by hydroisomerization of cyclohexylphenol to reduce the freezing point. The selectivity of (cyclopentylmethyl)cyclohexane formed by isomerization is enhanced through inhibiting the quick hydrogenation of intermediates using mixing acid catalyst and metal catalyst. Zeolite with strong Brønsted acid site and big surface area, such as HZSM-5 is better for the isomerization. Catalyzed by mixture of Pd/C and HZSM-5, the obtained fuel blending improves the freezing point from 2.6 °C to−22 °C compared with pure bicyclohexane, meanwhile keep the high density of 0.880 g/mL unchanged. The improved low-temperature property is contributed to the formation of (cyclopentylmethyl)cyclohexane. Moreover, the pathways of isomerization and its competition with hydrodeoxygenation are also investigated.
1. Introduction Advanced jet fuel with high density and good low-temperature properties (such as low freezing point and low viscosity) is desired to extend the flight distance and increase the payload of aircrafts. However, it is still not easy to synthesize a fuel with both high density and good low-temperature performance [1]. So many strategies have been developed to improve the low-temperature properties of fuel meanwhile retain the high density. Typically, blending several fuel components can balance the density and low-temperature properties. For example, RJ-5 has high density of 1.08 g/mL but high freezing point above 0 °C [2], while as 20–25 % of RJ-5, 65–70 % of JP-10 and 10–12 % of methylcyclohexane are blended, a new fuel (i.e. JP-9) is obtained with high density of 0.94 g/mL and low freezing point of −65 °C, attributed to the extremely low freezing point of JP-10 and methylcyclohexane [2]. It is better to directly produce a mixture containing both high-density component and low-freezing point component. With this regard, modulating molecular structure by isomerization is an efficient way. For example, isomerization of tetrahyrodicyclopentadiene and tetrahyrotricyclopentadiene into exo-isomers and alkyl-adamantanes using acid catalyst have been investigated to reduce the freezing point and viscosity of high-density fuels [3–7]. The hydroisomerization of linear paraffin to branched isomers is also a key for oil refining to improve the low-temperature property of liquid fuels [8,9]. ⁎
Hydrocarbons with bicyclic structure, especially bicyclohexane, have been intensively investigated as component of jet fuel [10–12]. This is because bicyclohexane has promising density about 0.887 g/mL which is higher than the traditional jet fuel, such as RP-3 (0.783 g/mL, 15 °C) [13]. Additionally, the synthesizing process of bicyclohexane is easy. Typically, bicyclohexane is synthesized by the hydrogenation of biphenyl [14] or dibenzofuran [15] and cross-coupling of cycloalkanes [16], which are separated from fossil oil. Recently, bicyclohexane as important high-density biofuel has also been produced from lignocellulose derived platform compounds [17–19]. The synthesis of biobicyclohexane requires two steps, that is, the synthesis of bicyclohexane precursor and then hydrodeoxygenation of the precursor. Many literatures have reported the synthesis of bicyclohexane precursor like cyclohexylphenol from aldol condensation of cyclohexanone or alkylation of phenols with cyclohexanol [18,20,21]. It is worth noting that phenols are classical platform chemicals from the hydrolysis of lignin while cyclohexanol can be obtained by selective hydrogenation of phenols [22–25]. The purpose of hydrodeoxygenation is to remove the oxygen atom and saturate the C]C bonds. It has been reported that ring contraction can occur in the hydrodeoxygenation of phenols, which produces some methylcyclopentane [26,27]. Such isomerization is also expected to occur in the hydrodeoxygenation of bicyclohexane precursor. In fact, isomerization has been noticed in hydrodeoxygenation of phenolic dimers, where about 8.6 % of isomerized product
Corresponding author. E-mail address: [email protected] (J.-J. Zou).
https://doi.org/10.1016/j.cattod.2020.01.033 Received 1 November 2019; Received in revised form 9 January 2020; Accepted 28 January 2020 0920-5861/ © 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Genkuo Nie, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.01.033
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((cyclopentylmethyl)cyclohexane) along with 88.0 % of hydrodeoxygenation product are produced [28]. And (cyclopentylmethyl) cyclohexane has good low-temperature properties due to the asymmetry molecular structure [29]. The density of bicyclohexane is high, but the freezing point (2.6 °C) is needed to be improved. Therefore, if considerable hydroisomerization can take place during the hydrodeoxygenation of bicyclohexane precursor, a mixture of bicyclic hydrocarbons may be produced to achieve both high density and good low-temperature properties. With these considerations, here we report the synthesis of fuel blending containing bicyclohexane and (cyclopentylmethyl)cyclohexane by hydroisomerization of cyclohexylphenol. The isomerization is enhanced by inhibiting the quick hydrogenation of intermediates by using mixture of Pd/C and HZSM-5 as catalyst, with the selectivity of (cyclopentylmethyl)cyclohexane reaching 32.0 %. Compared with normal hydrodeoxygenation product, the freezing point of fuel blending is reduced to −22 °C from 2.6 °C, meanwhile keep the high density of 0.880 g/mL unchanged.
Fig. 1. Product distribution of hydroisomerization of cyclohexylphenol. Reaction condition: 50 mL water, 10 mmol cyclohexylphenol, 0.08 g Pd/C, 10 g HZSM-5, 150 °C, 4 MPa H2, 30 h.
2.3. Measurements of fuel properties 2. Experiment
The fuel density was measured by 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 (ASTM D445). Each measurement was performed for three times.
2.1. Catalysts and chemicals Methylcyclohexene (AR) and cyclohexanol (AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.. Phenol (AR) was purchased from Tianjin Guangfu Fine Chemical Research Institute. HZSM-5 (SiO2/Al2O3 = 25) and Hβ (SiO2/Al2O3 = 25) were purchased from Nankai Catalysts Co., Ltd. and calcined in air at 580 °C for 3 h before use. Pd/C (5 wt%) was purchased from Shanxi Rock New Materials Co., Ltd.. 0.08 %Pd/HZSM-5 and 0.08 %Pd/Hβ were prepared according to literature [28].
3. Results and discussion Hydroisomerization of cyclohexylphenol (i.e a) can produce bicyclohexane (i.e e) fromed by the direct hydrodeoxygenation and (cyclopentylmethyl)cyclohexane (i.e g) formed by the isomerization (Scheme 1 and Fig. S1). The product distribution of hydroisomerization in 30 h is shown in Fig. 1 in order to investigate the composition evolution in this process. The reactant a is converted very quickly in 1 h and disappear until 5 h. At the same time, bi(cyclohexan)-one (i.e b) is produced largely with a small amount of bi(cyclohexan)-ol (i.e c), bi (cyclohexan)-ene (i.e d), bi(cyclohexane) (i.e e), and (cyclopentylmethyl)cyclohexane (i.e g). Then, the amount of c, e and g increases continuously with the further conversion of b. With time going on, all of c is converted to e and g. Only a small amount of d is detected in the whole process, indicating it is a metastable intermediate that will be quick hydrogenated and/or isomerized. The most possible pathway for the formation of g is that, d is protonated, rearranged and finally hydrogenated to g [26,30,31]. In order to understand the role of catalyst, several element steps during in the hydroisomerization of a using different catalysts are investigated (Scheme 2). As shown in Scheme 2 and Fig. S2, with the presence of Pd/ C a is hydrogenated to b and c with small amount of e, which indicates that Pd/C cannot catalyze the isomerization (Reaction A). When using c as the reactant, e is also obtained as the only product (Reaction B). With the presence of only HZSM-5, c is catalyzed to d and (cyclopent-2en-1-ylmethyl)cyclohexane (i.e. f) (Reaction C). This result implies
2.2. Hydroisomerization reaction Cyclohexylphenol was synthesized according to our previous work [29]. The hydroisomerization of cyclohexylphenol (Scheme 1) was carried out with 100 mL autoclave (EasyChem E100). With 50 mL water/cyclohexane/ethanol as solvent, 10 mmoL substrate, mixture of 0.08 g Pd/C plus 0.5–15 g of zeolite, or 5 g 0.08 %Pd/zeolite were used as catalyst. The reaction was conducted at 2–6 MPa H2 and 120–200 °C for 10 h. Then the upper organic phase was collected and dewatered using anhydrous MgSO4. The liquid products were quantified by a gas chromatography (Agilent-7820A) equipped with an FID detector and a capillary column HP-1 capillary column (30 m ×0.53 mm), and determined qualitatively using an Agilent 6890/5975 gas chromatography–mass spectrometry (GC–MS) equipped with HP-5 capillary column (30 m ×0.5 mm). The temperature of GC–MS column was raised from 50 °C to 280 °C at ramped speed of 10 °C/min and kept at 280 °C for 10 min, for GC analysis the column temperature was increased from 40 °C to 280 °C at ramped speed of 20 °C/min and kept at 280 °C for 10 min. The products were also identified with 13C and 1H NMR spectra collected using Bruker Avance 400 M Spectrometer.
Scheme 1. Synthesis of bicyclic fuel blending by hydroisomerization of cyclohexyphenol. 2
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contrast, the g selectivity catalyzed by mixing catalyst of Pd/C-Hβ is better than loaded catalyst of Pd/Hβ, also that of Pd/C-HZSM-5 is better than Pd/HZSM-5. Especially, the hydroisomerization selectivity of mixed Pd/C-HZSM-5 is the best with g selectivity of 16 %. Over loaded catalyst, the intermediate d formed on acid zeolite will be quickly hydrogenated to e by nearby metal site, due to the very intimate contact of two kinds of sites (Fig. 3a). For mixed catalyst (Fig. 3b), d has more chance to be rearranged by acidic sites before trapped by a metal site, attributed to the spatially separation of zeolite and Pd. The acidity of HZSM-5 and Hβ are characterized by NH3-TPD spectra. As shown in Fig. S5a, the peaks below 300 °C and above 450 °C appear for HZSM-5 and Pd/HZSM-5, while peaks below 200 °C and above 310 °C are observed on Hβ and Pd/Hβ, ascribed to desorption of ammonia on the weak and strong acid sites, respectively [34]. This indicates that the acid strength of HZSM-5 is much higher than that of Hβ. The amount of acid sites with different strength is calculated as the ammonia uptake for each catalyst and the results are listed in Table 1. It shows that loaded Pd decreases the acid amount of zeolites and the total acid amount of HZSM-5 is higher than that of Hβ, especially the strong acid sites. This result is in agreement with that of Py-IR spectra shown in Fig. S5b. Two absorption peaks stated at 1450 and 1540 cm−1 ascribed to the Lewis and Brønsted acid sites respectively, whose intensities are used to calculate the concentrations of two types of acid sites (Table 1). Table 1 also shows the change of Brønsted and Lewis acid concentration with desorption temperature. Typically, desorption temperature of 200 °C and 300 °C are ascribed to the weak and strong acid. Evidently, both Brønsted and Lewis acid concentration decrease with increasing desorption temperature and the loading of Pd decreases the acid amount. Similar to the NH3-TPD result, the B/L ratio of HZSM5 is much higher than that of Hβ, especially the strong acid sites. Based on these results, one can conclude that strong Brønsted acid site is favorable for the isomerization, which is in consistent with the hydroisomerization of paraffin and methylcyclohexane [30,35,36]. The effect of Brønsted acid is also confirmed by the reaction catalyzed by phosphotungstic acid (with Brønsted acid stronger than H2SO4). As shown in Fig. S6, the selectivity of compound g catalyzed by phosphotungstic acid is higher than that by HZSM-5. However, phosphotungstic acid is easily dissolved in polar solvent, especially water to form homogeneous protonic acid solution, which is corrosive and recyclable hardly. In order to obtain more isomerized product, the dosage of HZSM-5 in the mixed catalyst is optimized (Fig. 4a). With the dosage of HZSM-5 increasing, the alkanes yield and selectivity of g increase. At low dosage of HZSM-5, when a is converted to c which has a lone electron pair (on oxygen atom) [37] and act as Lewis bases to compete with d to react with acid sites and decreases the isomerization of d by HZSM-5 but increases the direct hydrogenation of d by Pd/C. As HZSM-5 dosage increases, c is dehydrated quickly with more chance for isomerization of d. With the dosage of HZSM-5 increasing to 10 g, the alkanes yield is close to 100 %, the selectivity of g (18 %) does not change any more. Temperature is also an important influence factor to the hydroisomerization (Fig. 4b). As temperature increases, compound d is accumulated with more chance for isomerization. However, as the temperature rises to 180 °C, alkanes yield is up to 93 % while the selectivity of g begins to drop, suggesting hydrogenation is more sensitive than isomerization at high temperature. Hydrogenation pressure is also investigated as shown in Fig. 4c. Definitely, high hydrogenation pressure favors the hydrogenation. With H2 pressure increases, more compound d is produced from the conversion of compound a, which increases the chance for isomerization. However, as the pressure is more than 4 MPa the g selectivity decreases, which means the direct hydrogenation of compound d increases more quick under this condition. Solvent effect is also optimized as shown in Fig. 4d. As can be seen, a is converted completely using water as solvent with the highest isomerization selectivity of g (30 %), because water is favorable for stabilizing carbonium ions and proton transformation for subsequent rearrangement. When cyclohexane is used as the solvent, direct
Scheme 2. Element steps in hydroisomerization of cyclohexylphenol.
that acid site is the activity site for ring contraction. So with the mixture of Pd/C and HZSM-5 as catalyst, c is converted to either e or g (Reaction D). The rearrangement of methylcyclohexene to ethylcyclopentane by HZSM-5 (Fig. S3) also confirms that acidic catalyst can realize the ring contraction. Lastly, with mixture of Pd/C and HZSM-5 (Reaction E) or sole HZSM-5 (Reaction F) as catalyst, e does not take place any reaction, which suggests that e cannot be isomerized to g under H2. Based on the above analysis, sequence elementary steps for the formation of e and g are shown in Scheme 1. a is first hydrogenated to c (step 1 and 2) and then dehydrated to d (step 3). d is converted to bi (cyclohexane)oxides by acid catalyst (step 4). The bi(cyclohexane)oxide is isomerized to (cyclopentylmethyl)cyclohexaneoxide via ring contraction (step 5). (Cyclopentylmethyl)cyclohexaneoxide deprotonate via β-hydrogen transfer to acid site in quasi-equilibrated step to form f (step 6) and is hydrogenated to g (step 7) finally. This reaction is similar to the conversion of cyclohexene to methylcyclopentane where carbocationic isomerization is the key step [32,33]. At the same time, d can be directly hydrogenated to e (step 8). Acid sites favor step 4 and metal site favors step 8, and these two reactions compete with each other. Then series of catalysts including metal-acid mixture and metal loaded acid catalyst are investigated in order to enhance step 4 and suppress step 8 and finally improve the selectivity of isomerization (Fig. 2). a can be converted completely by all the catalysts, except Pd/ HZSM-5, because the pore size of HZSM-5 is too small (0.68 nm) to let a (0.49 nm) diffuse freely (Fig. S4). Consequently, the activity of mixed Pd/C-HZSM-5 is also poor. The yield of alkanes catalyzed by Pd/Hβ is better than mixed Pd/C-Hβ, because the hydrogenation and deoxygenation are accelerated by the synergetic effect of Pd and Hβ. In
Fig. 2. Hydroisomerization of cyclohexylphenol catalyzed by different cataCyclohexylphenol coversion, yield of e and g, the selectivity of lysts. g. Reaction condition: 50 mL water, 10 mmol cyclohexylphenol, 0.08 g Pd/C with 5 g zeolite, or 5 g 0.08 % Pd/zeolite, 180 °C, 6 MPa H2, 10 h. 3
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Fig. 3. Reaction pathways on (a) loaded Pd/HZSM-5 vs (b) mixture of Pd/C and HZSM-5. Table 1 Acid properties of catalysts derived from NH3-TPD and Py-IR. Catalysts
HZSM-5 Pd/HZSM-5 Hβ Pd/Hβ
Acidity by strength(mmol/g)
Brønsted acid sites(mmol/g)
Lewis acid sites(mmol/g)
B/L
Weak
Strong
Total
S/(S + W)
200 °C
350 °C
200 °C
350 °C
200 °C
350 °C
1.467 1.323 1.413 1.389
0.478 0.435 0.321 0.308
1.945 1.758 1.734 1.697
0.246 0.247 0.185 0.181
0.168 0.139 0.083 0.027
0.154 0.134 0.052 0.024
0.009 0.027 0.015 0.014
0.005 0.006 0.014 0.010
19.389 5.148 5.533 1.929
28.179 22.333 3.714 2.400
this reaction was checked. After each use, HZSM-5 is separated, washed with water and used in next reaction directly. As shown in Fig. S7, HZSM-5 is relatively stable under the investigated conditions. With the increase of recycling cycle, the conversion of compound a and alkanes yield keep constant while the selectivity of compound g decreases slightly, which may be related to the formed coke on HZSM-5. Under optimal conditions, a mixture of 62.0 % of e and 32.0 % of g
hydrodeoxygenation of compound a is more favored with only 3 % selectivity of g, because carbonium ions is not stable enough in cyclohexane. Ethanol is the worst solvent for both hydrogenation and isomerization, owing to the lone electron pair (on oxygen atom) as Lewis base competing with reactants to be absorbed by HZSM-5. When water is used as solvent with prolonging reaction time to 30 h, about 62.0 % of e and 32.0 % of g are obtained. The reusability of HZSM-5 zeolite for
Fig. 4. Effects of (a) dosages of HZSM5, (b) temperature, (c) H2 pressure and (d) solvents on hydroisomerization of cyclohexylphenol using mixing Pd/C and HZSM-5 as catalyst. Cyclohexylphenol coversion, yield of the selectivity of g. e and g, Reaction conditions: (a) 50 mL water, 10 mmol cyclohexylphenol, 0.08 g Pd/ C, 180 °C, 6 MPa H2, 10 h; (b) 50 mLwater, 10 mmol cyclohexylphenol, 0.08 g Pd/C with 10 g HZSM5, 6 MPa H2, 10 h; (c) 50 mLwater, 10 mmol cyclohexylphenol, 0.08 g Pd/ C with 10 g HZSM-5, 150 °C, 10 h; (d) 50 mL solvents, 10 mmol cyclohexylphenol, 0.08 g Pd/C with 10 g HZSM5, 150 °C, 4 MPa H2, 10 h.
4
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balance of (cyclopentylmethyl)cyclohexane, the fuel blending synthesized by hydroisomerization shows improved low-temperature properties compared with that synthesized by normal hydrodeoxygenation, meanwhile still keeps high density.
Table 2 Bicyclic alkanes and the corresponding fuel properties. Entry
Compounds
Density (g/mL, 20 °C)
Freezing point (oC)
Viscosity (mm2/s, 20 °C)
Reference
1
71.4 %
0.880
−22
3.2
This work
CRediT authorship contribution statement Genkuo Nie: Conceptualization, Methodology, Investigation, Writing - original draft, Funding acquisition. Yiying Dai: Validation. Junjian Xie: Data curation. Xiangwen Zhang: Supervision. Lun Pan: Supervision. Ji-Jun Zou: Methodology, Supervision, Resources, Project administration.
＋28.6 % 2
0.887
2.6
4.5
[12,18]
3
0.867
−38.0
1.6 (25 °C)
[18]
4
0.880
< −75
2.0
[29]
5
0.876
−20
5.0
[41]
6
0.871
−74
–
[39,40]
7
0.870
< −80
2.9
This work
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
with small amount of CeC coupling compounds is obtained. After vacuum distillation, a fuel blending contains 71.4 % of e and 28.6 % of g is finally obtained. As shown in entry 1 of Table 2, this fuel has a density of 0.880 g/mL, a freezing point of −22 °C and a viscosity of 3.2 mm2/s (20 °C) which is much better than e (density of 0.887 g/mL, freezing point of 2.6 °C and viscosity of 4.5 mm2/s at 20 °C) (entry 2 of Table 2). The much-improved low-temperature properties are attributed to the presence of g. In order to investigate the properties of g, a vacuum distillation is conducted to get 96.0 % purity of g. As shown in entry 7 of Table 2, this compound has density about 0.870 g/mL, freezing point of lower than −80 °C and viscosity of 2.9 mm2/s at 20 °C. In order to understand the relationship between structure and fuel properties of compound g, some bicyclic alkanes are compared in Table 2. All these bicyclic alkanes have density about 0.867∼0.887 g/ mL, while their low-temperature properties vary greatly. The freezing point of bi(cyclohexane) (2.6 °C, entry 2) [12,18] is much higher than bi(cyclopentane) (−38 °C, entry 3) [18], and the freezing point of cyclopentylcyclohexane (−75 °C, entry 4) [29] is much lower than that of bi(cyclohexane) and bi(cyclopentane), suggesting cyclopenty ring and the asymmetry structure are favorable to improve the low temperature properties. Actually, the low-temperature properties improved by breaking the center of molecule symmetry is in consist with previous report [38] that the absence of symmetry center decreases the freezing point. Besides, the freezing point of dicyclopentyl-methane (−74 °C, entry 6) [39,40] is much lower than bi(cyclopentane) (−38 °C, entry 3) [18] and that of dicyclohexylmethane (-20 °C, entry 5) [41] is much lower than bi(cyclohexane) (2.6 °C, entry 2) [12,18]. This result is consistent with the report that the freezing point of diphenylmethane (23−24 °C) [42] is much lower than that of biphenyl (70−72 °C) [43], which suggests that bicyclic-methane is more favorable than bicyclic alkanes from the aspect of low-temperature properties. Based on the above analysis, the good low-temperature properties of (cyclopentylmethyl)cyclohexane is attributed to the bicyclic-methane structure and the break of the center of symmetry of molecule by fivemember ring.
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