Article pubs.acs.org/IECR
Ionic Liquid Catalytic Rearrangement of Polycyclic Hydrocarbons: A Versatile Route to Alkyl-Diamondoid Fuels Tingting Ma, Ren Feng, Ji-Jun Zou,* Xiangwen Zhang, and Li Wang Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *
ABSTRACT: Rearrangement of three types of fused polycyclic hydrocarbons with carbon atoms 12−14 was conducted using chloroaluminate ionic liquid (IL) as acid catalyst. The hydrocarbons undergo quick configurational isomerization and then skeletal rearrangement toward alkyl-adamantanes. Although having different molecular structure, these hydrocarbons all lead to adamantanes substituted with 2−3 methyl and/or ethyl groups. The alkyl-adamantanes show high density, low freezing point and viscosity, and are very attractive as high-density fuels. Computation using density functional theory confirms that alkyladamantanes are thermodynamically favored and explains the experimental product distribution very well. On this basis, the reaction pathway of each hydrocarbon was illustrated. The reaction conditions including acidity (AlCl3 fraction), temperature, and IL dosage show significant effect on the rearrangement. Moreover, the distribution of alkyl-adamantanes can be tuned by adjusting the reaction conditions, allowing the fuels’ properties to be fine-tuned. This work provides a versatile route to synthesizing alkyl-diamondoid fuels using simple chemical feedstocks in an effective way.
1. INTRODUCTION As an important part of liquid propellants and/or fuels, highdensity fuels are particularly demanded in volume-limited aircraft, missiles, and rockets. Specifically, liquid hydrocarbons with high density are very attractive because they possess energy contents comparable to those of carcinogenic fuels (such as hydrazine) but are much safer.1−10 However, the suitability of a fuel for practical application depends on many properties that sometimes conflict with each other. One of them is the low temperature property, critical for ensuring proper operation in cold conditions. Unfortunately, with the increase of density, the freezing point and viscosity of hydrocarbons are generally increased. So, a trade-off between the density and low temperature property is necessary for highdensity fuels. For example, high-density fuels RJ-4 and RJ-5 were abolished because of their high freezing points.1 Diamondoids are hydrocarbons containing at least one adamantane unit in the diamond lattice.11−13 As a result of their compact molecular structure, diamondoids’ superior energy density makes them an attractive candidate for high-density fuels. In particular, alkyl-substituted diamondoids possess significantly lower freezing point and viscosity than the parent molecules. For example, the freezing point of trimethyldiamantane is less than −53.8 °C, and the mixture composed of alkyl-adamantanes, alkyl-diamantanes, and alkyl-triamantanes shows higher volumetric energy than JP-10 and RJ-4.1 However, naturally occurring diamondoids, especially alkyldiamondoids, are very limited and are found in trace in some crude oil and natural gas condensates.1,13 Therefore, the artificial synthesis of alkyl-substituted diamondoids is of great importance. Schleyer first reported the rearrangement of endotetrahydrodicyclopentadiene (endo-THDCPD) to adamantane, the simplest diamondoid, using Lewis acid-like AlCl3 as catalyst.11 Then, some substituted or unsubstituted diamond© 2013 American Chemical Society
oids with two, three, and even four adamantane units have been synthesized.12−16 Theoretically, diamondoids are favored in thermodynamics, and many polycyclic hydrocarbons containing ten or more carbon atoms can be converted to diamondoid via skeletal rearrangement. However, the reaction kinetics seems very difficult, probably because of the high reaction barrier. In fact, the diamondoids reported are synthesized by treating some complex polycyclic hydrocarbons under harsh conditions, which are not practical for fuel synthesis due to the high cost and difficulty in scale-up. Ionic liquid (IL) has been widely used as green catalyst and solvent due to the advantages such as negligible vapor pressure and noflammability.17−21 Specifically, chloroaluminate IL has been used as acid catalyst in many reactions, such as isomerization, alkylation, and Friedel−Crafts reaction.22−31 The phase separation from nonpolar hydrocarbons and recycling ability makes IL-based operations suitable for continuous flow reaction systems. There are already some pilots or industrial process involving IL publicly announced, and new continuous reactors are developed specifically to facilitate the mixing of different phases, separation of product, and recycling of catalyst inside the reactor system.19,32,33 Chloroaluminate IL shows considerably high activity in the isomerization of endo-THDCPD to exo-THDCPD, which is the major component of synthetic fuel JP-10.22,23 Furthermore, by increasing the dosage of IL, both endo- and exo-THDCPD can be converted to adamantane via skeletal rearrangement.23,24 Interestingly, our recent work shows that this IL can facilitate the skeletal rearrangement of tetrahydrotricyclopentadiene Received: Revised: Accepted: Published: 2486
November 22, 2012 January 19, 2013 January 25, 2013 January 25, 2013 dx.doi.org/10.1021/ie303227g | Ind. Eng. Chem. Res. 2013, 52, 2486−2492
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Scheme 1. Isomerization and Rearrangement Pathways of MTCDs
Scheme 2. Isomerization and Rearrangement Pathways of TCDD
hydrochloride (Et3NHCl, >99 wt %) and anhydrous AlCl3 (>99 wt %) were purchased from J&K Scientific and Tianjin Suzhuang Chemical Reagent Factory, respectively. All the chemicals were used as received. 2.2. Synthesis of Polycyclic Hydrocarbons and Ionic Liquid. The synthesis details of polycyclic hydrocarbons were described in the Supporting Information. Typically, methyltricyclo[5.2.1.02,6]decane (MTCDs) composed of dimethyland trimethyl- compounds (DMTCD 74 wt % and TMTCD 23 wt %) was obtained by hydrogenating the mixture of methyldicyclopentadiene (see Scheme 1). It is worth noting that DMTCD is also the primary component of high-density fuel RJ-4.1 Tetracyclo[6.2.1.13,6.02,7]dodecane (TCDD, >99 wt %) and tetracyclo[9.2.1.02,10.03,8]tetradecane (TCTD, >99 wt %) were synthesized through a Diels−Alder addition followed with hydrogenation, as shown in Schemes 2 and 3, respectively. These polycyclic hydrocarbons containing many stereoisomers were used without separation in the rearrangement reaction. IL was prepared using Et3NHCl and anhydrous AlCl3 in a three-neck flask under flowing nitrogen. The chemicals were mixed and stirred at room temperature for 1 h and heated to 60−100 °C for several hours, according to the molar fraction of AlCl3, to ensure the formation of ionic liquid. The symbol x was used to represent the molar fraction of AlCl3 in IL, that is, x = n(AlCl3)/(n(AlCl3) + n(Et3NHCl)).
(THTCPD) to alkyl-diamondoids with high yield. In comparison, no diamondoids are formed when AlCl3 is used as catalyst, and only trace diamondoids are formed in case of CF3SO3H catalytic reaction.5,25 The mixture of alkyl-diamondoids shows both high density and good low-temperature property, suggesting that the IL-catalytic rearrangement of polycyclic hydrocarbons is a facile and simple way to synthesize alkyl-diamondoid fuels. To extend the synthetic strategy of alkyl-diamondoid fuels, here we explored the IL-catalytic rearrangement of several polycyclic hydrocarbons with 12−14 carbon atoms that can be synthesized simply using common chemicals. It was found that alkyl-diamondoids suitable as liquid fuels are the principal products. The reaction pathways were illustrated based on both experimental and computational results. The effect of reaction conditions was also investigated in order to optimize the yield of alkyl-diamondoids.
2. EXPERIMENTAL SECTION 2.1. Materials. Mixture of dimethyl- and trimethyldicyclopentadiene (>97 wt %) was purchased from Puyang Shenghuade Chemical Co., Ltd. Norbornene (>99 wt %) and Indene (>95 wt %) were purchased from Jilin Dayu Chemical Co., Ltd. Dicyclopentadiene (>95 wt %) was purchased from Hangzhou Yangli Petrochemical Co., Ltd. Triethylamine 2487
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Scheme 3. Isomerization and Rearrangement Pathways of TCTD
2.3. Rearrangement Reaction and Analysis Method. The rearrangement reaction was conducted as follows: 0.05 mol of hydrocarbons as reactant and a defined amount of preprepared IL as catalyst were added in a 50 mL three-neck flask fitted with a condenser and heated in an oil bath. The reaction was ignited by magnetic stirring. After the reaction was completed, the mixture was allowed to separate into two layers due to the different polarity of hydrocarbons and IL. The upper hydrocarbon layer was recovered by decantation. For sample analysis during the reaction, the stirring was stopped temporarily for phase separation and the upper organic layer was drawn. Before the analysis, the sample was washed with NaOH solution to remove the possible existence of trace IL. The structure of products was determined using a gas chromatography mass spectroscopy (GC-MS) instrument (Agilent 6890/5975) equipped with a capillary column (HP5, 30 m × 0.5 mm). The products were identified according to the library (NIST05a.L) embedded in the MS operation software (see Figure S1, Supporting Information). Their concentration was analyzed using a GC instrument (Agilent 7820) equipped with a capillary column (AT-SE-54, 50 m × 0.32 mm) and a hydrogen flame ionization detector. Hexane was used as the internal standard sample. The detailed GC spectra of typical samples are shown in Figures S2−4 in the Supporting Information. The conversion and selectivity were calculated as follows: Conversion (%) = Selectivity (%) =
Figure 1. Product distribution in IL-catalytic rearrangement of MTCDs (reaction conditions: temperature, 80 °C; x(AlCl3), 0.67; IL/MTCDs, 0.5).
distribution of products varying with the reaction time. It can be seen that almost all the starting materials are converted in 20 min. During this period, configurational isomers are the major product and their summed concentration reaches the maximum value. This indicates that the configurational isomerization happens very quickly in the presence of IL. After that, the amount of stereoisomers begins to decline, and some alkyladamantanes appear with a tendency of increase, indicating the occurrence of skeletal rearrangement from stereoisomers to alkyl-adamantanes. After a reaction time of 5 h, alkyladamantanes account for 80% of the products, along with some undefined products formed via fragmentation, disproportionation, and polymerization reactions. Six alkyl-adamantanes are determined by GC-MS analysis, including 1,4-DMAM, 1,3-DMAM, 1-EAM, 1,3,4-TMAM, 1,3,5-TMAM, and 1-M-3-EAM (see Scheme 1 for the structures). The former three are generated from DMTCD, whereas the later three are from TMTCD. As shown in Figure1, at the beginning (1 h), 1,4-DMAM and 1,3,4-TMAM are the major alkyl-adamantanes in the two parallel reactions, respectively. However, they decrease to a very small quantity in the later stage. On the contrary, 1,3-DMAM and 1,3,5-TMAM increase with the prolonging of reaction time and finally become the major products. This indicates that 1,4-DMAM and 1,3,4-TMAM are the intermediate products in the skeletal rearrangement, which are respectively converted to 1,3-DMAM and 1,3,5-TMAM with prolonged reaction time. After 4 h, the amount of products keeps stable, except the continuous increase of 1,3-DMAM, suggesting a direct route from DMTCD to 1,3-DMAM. As stated in the introduction, the driving force for the formation of diamondoids is the thermodynamic preference. Theoretic calculations were conducted to assess the total energy of the related compounds. The energy gaps (also the gaps of enthalpies of formation) of products relative to the corresponding reactants were shown in Table 1. Endo-3,9DMTCD and endo-3,4,9-TMTCD were selected as the benchmarks of DMTCDs and TMTCDs because they are the most abundant. It can be seen that the starting compounds have the highest energy and the alkyl-adamantanes have lower energy, suggesting that these diamondoids are favored in thermodynamics. Based on the calculated energy gaps, the thermodynamic preference for the rearrangement of DMTCD and TMTCD are 1,3-DMAM > 1,4-DMAM > 1-EAM > 2EAM > DMTCD and 1,3,5-TMAM > 1-M-3-EAM >1,3,4-
∑ reactant converted × 100 ∑ original reactant
alkyl‐adamantanes formed × 100 ∑ reactant converted
It was found that trace organics also exist in the IL layer, but the composition is the same to the upper organic layer (Figure S2, Supporting Information). Therefore, the composition of the upper layer can reflect the actual conversion and selectivity of hydrocarbons. 2.4. Theoretical Computation. The optimized geometries (Figure S5, Supporting Information) as well as the sum of electronic and zero-point energies of the related reactants and products were obtained using density functional theory at B3LYP/6-31+g(d,p) level. Analytical frequency calculations at the same level confirm that the structures are minima without imaginary frequency. All calculations were carried out in the Gaussian 09 software package.34
3. RESULTS AND DISCUSSION 3.1. Rearrangement of MTCDs. Both DMTCD and TMTCD have many stereoisomers because the methyl group may be anchored on different carbon atoms. Figure 1 shows the 2488
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Table 1. Energy Gaps (ΔE, kJ/mol) between Reactants and the Corresponding Products abbrev. exo-endo-TCDD 1,3-DMAM 1,4-DMAM 1-EAM 2-EAM 1,3,5-TMAM 1-M-3-EAM 1,3,4-TMAM 1,2-TMAM
ΔEa
ΔEa
ΔEa
ΔEa
−159.3 −3402.5 −3390.8 −3380.7 −3363.2
−236.3 −224.6 −214.5 −197.0 −165.6 −146.2 −145.1
−102.8
ΔE relative to endo-3,9-DMTCD, endo-3,4,9-TMTCD, exoexoTCDD, and TCTD, respectively; see Schemes 1−3 for structures and Figure S5 (Supporting Information) for geometries. a
TMAM > TMTCD, respectively. This confirms that 1,3DMAM and 1,3,5-TMAM are two most favored products in the rearrangement. Also, 1,4-DMAM and 1,3,4-TMAM are prone to transfer to more stable 1,3-DMAM and 1,3,5-TMAM via methyl transfer, respectively. For the rearrangement of DMTCD, only a few 1-EAM are formed because they are not preferred in thermodynamics when compared with dimethyl-adamantanes. As for the rearrangement of TMTCD, considerable 1-M-3-EAM is generated because it is more preferred than 1,3,4-TMAM. The order of abundance of product is in good agreement with the computed result, showing that the rearrangement is thermodynamically controlled. The reaction pathway based on the experimental and computational results is shown in Scheme 1. Figure 2 shows the effect of acid strength (AlCl3 fraction), temperature, and IL dosage on the rearrangement. The molar fraction of AlCl3 in IL determines the type of anions that show different acid strength. [Et3NH]Cl + AlCl3 → [Et3NH]+ + [AlCl4]− [AlCl4]− + AlCl3 → [Al 2Cl 7]−
[Al 2Cl 7]− + AlCl3 → [Al3Cl10]−
The acidity order of anions is acidic [Al3Cl10]− > acidic [Al2Cl7]− > neutral [AlCl4]−. A conversion of 80% is obtained with x = 0.64, and it is increased to about 100% at x = 0.7. In the range of x = 0.64−0.67, the selectivity of thermodynamically favored products (1,3-DMAM and 1,3,5-TMAM) increases quickly whereas that of other alkyl-adamantanes decreases. The distribution of product does not change much when x is beyond 0.67, indicating that the acidity of [Al2Cl7]− ions is strong enough to catalyze the skeletal rearrangement. Also the temperature shows a significant effect on the reaction. There is an obvious increase in the conversion from 71% to 99% when the temperature rises from 60 to 80 °C. The total selectivity (SAMs) of alkyl-adamantanes reaches the highest value of 77% at 80 °C, but it shows a downside trend with the further increase of temperature. Specifically, the selectivity of 1,3-DMAM declines considerably. As to the effect of IL dosage, enough IL is necessary to ensure considerable yield of alkyladamantanes in acceptable reaction period. When the IL/ MTCDs ratio goes up from 0.25 to 0.5, the conversion and SAMs reach 99% and 78%, respectively. Overall, at mild reaction conditions, the selectivity of 1,3-DMAM and 1,3,5-TMAM increases but that of 1,4-DMAM and 1,3,4-TMAM is reduced
Figure 2. Effect of reaction conditions on the IL-catalytic rearrangement of MTCDs (■ = conversion; ● = S1,3‑DMAM; ▲ = S1‑EAM; Δ = S1,4‑DMAM; □ = S1,3,5‑TMAM; ○ = S1,3,4‑TMAM; ★ = S1‑M‑3‑EAM; ☆ = SAMs. Reaction conditions: time, 5 h; x(AlCl3), 0.7; temperature, 80 °C; IL/ MTCDs, 0.5).
when increasing the temperature, acid strength, or IL dosage, as a result of thermodynamic control. 3.2. Rearrangement of TCDD. TCDD is a mixture of two stereoisomers (exo-exo 96.7 wt % and exo-endo 2.4 wt %) (see Scheme 2). Similar to the case of MTCDs, TCDD first undergoes configurational isomerization from exo-exo- to exoendo-TCDD and then rearrangement to dimethyl-adamantanes (1,3- and 1,4-DMAM) and ethyl-adamantanes (1- and 2-EAM), see Figure 3 and Scheme 2. For dimethyl-adamantanes, 1,3DMAM always increases during the reaction, with small amount of 1,4-DMAM formed. For ethyl-adamantanes, 2EAM is formed in large amount but then disappears, along with the increase of 1-EAM. When the reaction time exceeds 3 h, the 2489
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harsh conditions. This suggests that the barrier energy for thermodynamically most feasible reaction channels is very high. This result also hints at the possibility to tune the distribution of diamondoids by adjusting the reaction conditions. 3.3. Rearrangement of TCTD. Treatment of TCTD with AlCl3 IL gives 1,2-TMAM as the only diamondoid, along with minor configurational stereoisomers. As shown in Figure 4 and
Figure 3. Product distribution in IL-catalytic rearrangement of TCDD (reaction conditions: temperature, 40 °C; x(AlCl3), 0.67; IL/TCDD, 2).
amount of 2-EAM decreased is equal to that of 1-EAM formed. This clearly shows the ethyl transfers from 2-EAM to 1-EAM. It is worth noting that although DMTCD and TCDD have distinctly different molecular structures, they give the same alkyl-adamantanes in the reaction. This testifies that the rearrangement of polycyclic hydrocarbons will finally lead to thermodynamically favored diamondoids, although via different reaction channels with different barriers. Therefore, some simple polycyclic hydrocarbons can be used to synthesize diamondoids. It is interesting that the distribution of the four alkyladamantanes changes significantly with reaction conditions, see Table 2 and Table S1, Supporting Information. At mild conditions, considerable ethyl-adamantanes are formed, especially when the IL/TCDD ratio is below 2. In this range, the summed selectivity of 1-EAM and 2-EAM keeps stable (48%∼59%) with the increase of temperature or AlCl3 fraction, and much more 2-EAM is formed at low temperature and low AlCl3 fraction. For example, two extreme cases appear under conditions of 50 °C, IL/TCDD = 0.5, x = 0.67, and 60 °C, IL/ TCDD = 1.5, x = 0.67, with the selectivity of 2-EAM and 1EAM being 53.6% and 0.9%, and 3.1% and 47.4%, respectively. When IL/TCDD > 2, dimethyl-adamantanes increase quickly with the increase of temperature and AlCl3 fraction. The selectivity of 1,3-DMAM is 65.7% under conditions of 60 °C, IL/TCDD = 4, x = 0.67; meanwhile, the summed selectivity of ethyl-adamantanes is just 1.3%. It is also noted that considerable amount of 1,4-DMAM is formed under mild conditions, but much less is formed under harsh conditions. So, diamondoids less preferred in thermodynamics are predominant under mild conditions, whereas the thermodynamically most preferred compounds become major products only under
Figure 4. Product distribution in IL-catalytic rearrangement of TCTD (reaction conditions: temperature, 60 °C; x(AlCl3), 0.67; IL/TCTD, 0.125).
Scheme 3, almost all the TCTD molecules are converted and the concentration of 1,2-TMAM reaches 85% in 1 h. Both the configurational isomerization from TCTD to stereoisomers and further skeletal rearrangement to 1,2-TMAM are fast and easy in the presence of IL. Calculation data in Table 1 also show that the energy of 1,2-TMAM is lower than that of TCTD so it is preferred in thermodynamics. Figure 5 shows that the reaction conditions have significant effects on the conversion, but the selectivity is almost unchanged. When x increases from 0.6 to 0.67, there is a quickly rise in the conversion from 73% to 98%, so the acidity of [Al2Cl7]− is strong enough to catalyze the skeletal rearrangement of TCTD. Increasing the temperature or IL dosage can also improve the reaction. As a result, the conversion can be as high as 99%, with the selectivity of 1,2TMAM greater than 90%. The recyclability of IL was also explored in two ways for the rearrangement of TCTD. In one operation, a new reactant was directly added in the reaction mixture of a previous run. As shown in Figure 6, the conversion and selectivity decrease slowly, because the accumulated polymerized byproducts and water deactivate the IL. In another operation, the upper hydrocarbon layer was removed and the bottom IL layer was used for subsequent run. It can be seen that the conversion and selectivity decrease very quickly, due to the loss of IL dissolved
Table 2. Effect of Reaction Conditions on the IL-Catalytic Rearrangement of TCDDa
a
temp. (°C)
x
IL/TCDD molar ratio
C (%)
S1,3‑DMAM (%)
S1‑EAM (%)
S1,4‑DMAM (%)
S2‑EAM (%)
SAMs (%)
50 40 60 40 40 60 60
0.67 0.67 0.67 0.6 0.67 0.67 0.67
0.5 1 1.5 2 2 2 4
21.8 30.6 98.2 17.8 98.2 98.6 99.2
27.5 24.3 38.8 23.4 30.2 41.1 65.7
0.9 4.1 47.4 2.9 39.0 39.4 0.6
5.1 6.3 1.8 4.6 2.2 4.1 0
53.6 50.5 3.1 49.3 21.8 2.8 0.7
87.9 85.0 91.1 92.2 93.2 87.5 67.0
Reaction time: 7 h. 2490
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TMAM, are very attractive as high-density fuels due to their high density, low freezing points, and low viscosity. Theoretical computation shows that the formation of alkyl-adamantanes is a result of thermodynamic preference and hydrocarbons with different molecular structures give similar and even the same products. The distribution of alkyl-adamantanes can be tuned by adjusting the reaction conditions, such as acidity (AlCl3 fraction), temperature, and IL dosage, allowing the fuels’ properties to be fine-tuned. This work provides a versatile way to synthesize alkyl-diamondoid fuels using simple chemical feedstocks in an effective and environmentally friendly way.
■
ASSOCIATED CONTENT
S Supporting Information *
Synthesis details, optimized geometries of related reactants and products, GC and MS spectra of typical samples, supplementary data of Table 2. This information is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciate the supports from the Natural Science Foundation of China (21222607), the Foundation for the Author of National Excellent Doctoral Dissertation of China (200955), and the Program for New Century Excellent Talents in University (NCET-09-0594).
Figure 5. Effect of reaction conditions on the IL-catalytic rearrangement of TCTD (reaction conditions: time, 3 h; x(AlCl3), 0.67; temperature, 60 °C; IL/TCTD, 0.125).
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REFERENCES
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Figure 6. Reuse of IL in catalytic rearrangement of TCTD. (Solid line, the upper layer not removed; dashed line, the upper layer was removed after each run. Reaction conditions: time, 3 h; x(AlCl3), 0.67; temperature, 60 °C; IL/TCTD, 0.125.)
in the hydrocarbon layer. This result suggests that the incomplete phase separation leading to the loss of IL is a major problem for recycling.
4. CONCLUSIONS Fused polycyclic hydrocarbons with carbon atoms of 12−14, including MTCDs, TCDD, and TCTD, are easily converted to alkyl-adamantanes in the presence of chloroaluminate IL. The adamantanes substituted with 2−3 methyl and/or ethyl groups, such as 1,3-DMAM, 1,3,5-TMAM, 1-M-3-EAM, and 1,22491
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dx.doi.org/10.1021/ie303227g | Ind. Eng. Chem. Res. 2013, 52, 2486−2492
