Article pubs.acs.org/IECR
Synthesis of Jet-Fuel Range Cycloalkanes from the Mixtures of Cyclopentanone and Butanal Jinfan Yang,†,‡ Shanshan Li,†,‡ Ning Li,*,† Wentao Wang,† Aiqin Wang,† Tao Zhang,† Yu Cong,† Xiaodong Wang,† and George W. Huber*,§ †
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100049, China § Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States ‡
S Supporting Information *
ABSTRACT: High-density (0.82 g mL−1) jet-fuel range cycloalkanes were synthesized in high overall yields (∼80%) by the aldol condensation of cyclopentanone and butanal followed by hydrodeoxygenation (HDO). Among the investigated solid-base catalysts, magnesium−aluminum hydrotalcite (MgAl-HT) exhibited the highest activity for the solvent-free aldol condensation of cyclopentanone and butanal. The excellent performance of MgAl-HT was rationalized by the synergism between strong base and weak acid sites on the surface of this material. The aldol condensation products were further hydrodeoxygenated to butylcyclopentane and 1,3-dibutylcyclopentane over the Ni/SiO2, Pd/SiO2, and bimetallic Ni−Pd/SiO2 catalysts. Compared to the Ni/SiO2 and Pd/SiO2 catalysts, the bimetallic Ni−Pd/SiO2 catalyst exhibited higher activity for the solvent-free HDO reaction. Over the 4%Ni−1%Pd/SiO2 catalyst, high carbon yield (88.0%) of butylcyclopentane and 1,3-dibutylcyclopentane was achieved at 503 K. According to the characterization results, the excellent performance of Ni−Pd/SiO2 catalyst can be explained by the formation of Ni−Pd alloy.
1. INTRODUCTION As a near term solution to the environmental problems resulted from the utilization of fossil energy, the catalytic conversion of renewable biomass to liquid transportation fuels1−3 and chemicals4−8 is receiving more and more attention. Lignocellulose is the cheapest and the most abundant biomass. Diesel and jet fuel account for 46% of liquid transportation fuels worldwide.9 It has been predicted by ExxonMobil,9 British Petroleum,10 and the U.S. Energy Information Association11 that diesel and jet fuel demand will increase more than any other liquid fuels in the next 20−30 years. In recent years, substantial work has been carried out on the production of diesel and jet-fuel range straight or branched paraffins with lignocellulose-derived components (including furfural, 2methylfuran, 5-hydroxymethylfurfural, levulinic acid, etc.).12−29 These paraffins have high cetane numbers and good thermal stability. However, their densities (∼0.76 g mL−1) and volumetric heating values are lower than those of conventional jet fuels (∼0.8 g mL−1). As we know, conventional jet fuel is a complicated mixture of (straight and branched) paraffins (∼70%) and cyclic hydrocarbons (including cycloalkanes and aromatic hydrocarbons) (∼30%). Because © 2015 American Chemical Society
of the strong ring strain, cyclic hydrocarbons have higher densities and volumetric heating values than those of straight and branched paraffins.30 As a solution to this problem, it is necessary to develop an economically feasible route for the production of jet-fuel range cyclic hydrocarbons with lignocellulosic platform compounds.31−34 Cyclopentanone is a lignocellulosic platform chemical that has a cyclic carbon chain structure.35 It has been demonstrated that cyclopentanone can be produced in high carbon yields (60−96%) from the aqueous selective hydrogenation of lignocellulosic derived furfural with Pt/C,36 Ni−Cu/SBA15,32 or Ru catalyst-supported acidic MOF material.37 In a previous work of our group, it was found that cyclopentanone can be used as a potential building block for the synthesis of diesel and jet-fuel range cycloalkanes. For example, we produced a mixture of C9−C15 branched alkanes and cycloalkanes, with a density of 0.82 g mL−1, from the Received: Revised: Accepted: Published: 11825
September 10, 2015 November 11, 2015 November 16, 2015 November 16, 2015 DOI: 10.1021/acs.iecr.5b03379 Ind. Eng. Chem. Res. 2015, 54, 11825−11837
Article
Industrial & Engineering Chemistry Research
an OminiStar mass spectrometer (MS) equipped with the software quadstar 32-bit. The acidity of the solid-base catalysts was measured with NH3 temperature-programmed desorption (NH3-TPD) on a Micrometeritics AutoChem II 2920 Automated Catalyst Characterization System. For each test, 0.1 g of sample was used. Before the measurement, the sample was purged with He flow at 393 K for 2 h. After the saturated adsorption of NH3 at 373 K, the sample was kept at 373 K under He flow for 45 min to remove the physically adsorbed ammonia. The desorption of NH3 was conducted under He flow from 373 to 1073 K at a heating rate of 10 K min−1. The desorbed NH3 molecules were detected by an OminiStar mass spectrometry (MS) equipped with the software quadstar 32-bit. The types of acid sites on the surfaces of the solid-base catalysts were investigated by Fourier transform infrared (FTIR) spectra that were collected with a Bruker Equinox 55 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector and an extended KBr beam splitter. The spectrometer was operated in the absorbance mode at a resolution of 4 cm−1. Before the test, the samples were pressed into self-supporting tablets and loaded into a IR cell with CaF2 windows. The samples were heated at 723 K for 2 h (at a heating rate of 10 K min−1) and then outgassed at this temperature for 30 min. When the samples were cooled to room temperature (303 K), the spectra of the samples were recorded as background. Subsequently, ammonia (99.999% purity) was introduced to the samples at room temperature. After evacuation at 303, 373, 473, and 573 K for 15 min, the FT-IR spectra of the surface species arising from NH3 adsorption (NH3−FT-IR spectra) were recorded by subtracting the sample spectra with the background. The X-ray diffraction (XRD) patterns of the 5%Ni/SiO2, 5% Pd/SiO2, and the bimetallic Ni−Pd/SiO2 catalysts were recorded with a PANanalytical X’Pert-Pro powder X-ray diffractometer, using Cu Kα monochromatic radiation (λ = 0.1541 nm) at a scan speed of 5° min−1. Before the tests, the catalysts were reduced by hydrogen flow at 673 K for 2 h. The average sizes of metal particles on the catalysts were estimated by the Debye−Scherrer equation. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images of the catalysts were obtained with a JEM-2100F field-emission electronic microscopy operated at 200 kV. Scanning transmission electron microscopy with energy-dispersive X-ray spectrometry (STEMEDS) elemental analysis was conducted using an energy dispersive spectroscopy analyzer attached to JEM-2100F equipped with a STEM dark-field detector under the accelerating voltage of 200 kV. The reduced catalysts were suspended on a copper grid before the analysis. The in situ X-ray photoelectron spectra (in situ XPS) of the 5%Ni/SiO2, 5%Pd/SiO2, and bimetallic Ni−Pd/SiO2 catalysts were acquired by a VG ESCALAB MK2 apparatus utilizing an Al Kα (1486.6 eV) radiation source. Before the tests, the catalysts were reduced in situ by H2 flow at 673 K for 2 h. After the cooling down of samples to room temperature under H2 flow and evacuation, the XPS spectra of reduced catalyst samples were recorded at a pressure of 5 × 10−6 Pa. The binding energies (BE) were justified taking the Si 2p peak at 103.4 eV as reference. The amounts of metallic sites on the surfaces of HDO catalysts were measured by H2 chemisorption that was carried out with a Micromeritics Autochem II 2920 automated
hydroxyalkylation/alkylation (HAA) of cyclopentanone with 2methylfuran followed by hydrodeoxygenation (HDO).38 Moreover, we also developed another route to selectively produce a bicyclic C10 hydrocarbon (i.e., bi(cyclo-pentane)) by the solvent-free self-aldol condensation of cyclopentanone followed by HDO.39 The bi(cyclo-pentane) obtained has a high density of 0.866 g mL−1.40,41 Therefore, it can be used as an additive to improve the volumetric heat value of conventional bio jet fuel. Butanol can be produced industrially by the acetone− butanol−ethanol (ABE) fermentation of lignocellulosic-derived sugars.42,43 Butanal is a platform chemical that can be produced in high yields from partial oxidation or dehydrogenation of butanol. This compound can be used as a feedstock in the production of jet-fuel range branched alkanes.17,44 To the best of our knowledge, there is no report about the synthesis of diesel or jet-fuel range cycloalkanes with butanal and cyclopentanone as the feedstocks. The objective of this paper is to study the viability of producing diesel and jet-fuel range cycloalkanes from the cross aldol condensation of cyclopentanone and butanal, followed by HDO.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The KF/Al2O3 catalyst was obtained according to literature45 by the wetness impregnation of γ-Al2O3 with an aqueous solution of KF followed by drying at 353 K overnight and calcination under nitrogen flow at 873 K for 2 h. The weight percentage of KF on the KF/Al2O3 catalyst was controlled at 30%. The nickel−aluminum hydrotalcite, cobalt−aluminum hydrotalcite, lithium−aluminum hydrotalcite, and magnesium−aluminum hydrotalcite (denoted as NiAl-HT, CoAl-HT, LiAl-HT, and MgAl-HT) were prepared according to the literature.46−49 Before being used in the aldol condensation of cyclopentanone and butanal, these solid-base catalysts were calcined under N2 flow at their preparation temperatures (723 K for NiAl-HT, LiAl-HT, and MgAl-HT; 573 K for CoAl-HT) for 2 h. The 5%Pd/SiO2 and 5%Ni/SiO2 catalysts used in the HDO process were prepared by the incipient wetness impregnation of SiO2 with aqueous solution of PdCl2 or Ni(NO3)3·6H2O. After being dried at 353 K for 4 h, the catalysts were reduced by hydrogen flow at 673 K for 2 h, cooled down to room temperature under hydrogen flow, and passivated with 1% O2 in nitrogen. Analogously, the bimetallic Ni−Pd/SiO2 catalysts were prepared by the coimpregnation method. To facilitate comparison, the theoretical total weight percentages of Ni and Pd in the bimetallic Ni−Pd/SiO2 catalysts were fixed at 5%. 2.2. Catalyst Characterization. The specific BET surface areas (SBET) of the catalysts were measured by nitrogen adsorption at 77 K using an ASAP 2010 apparatus. Before each measurement, the sample was evacuated at 573 K for 3 h. The basicity of the solid-base catalysts used in aldol condensation was characterized by CO2 temperature-programmed desorption (CO2-TPD) experiments on a Micromeritics AutoChem II 2920 Automated Catalyst Characterization System. For each test, 0.1 g of sample was used. The catalyst was placed in a quartz reactor, pretreated under He flow at its preparation temperature for 1 h and cooled down under He flow to 353 K. After the saturated adsorption of CO2, the sample was purged with He at 353 K for 45 min to remove the physically adsorbed CO2. The desorption of CO2 was carried out under He flow from 353 to 1073 K at a heating rate of 10 K min−1. The desorbed CO2 molecules were detected by 11826
DOI: 10.1021/acs.iecr.5b03379 Ind. Eng. Chem. Res. 2015, 54, 11825−11837
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Industrial & Engineering Chemistry Research
catalyst, the base strength of the CoAl-HT catalyst is relatively higher because an additional CO2 desorption peak at 635 K is observed. For KF/Al2O3 catalyst, a CO2 desorption peak occurred at 780 K. In the CO2-TPD profiles of the LiAl-HT and MgAl-HT catalysts, evident CO2 desorption peaks at higher temperatures (>860 K) were observed, indicating that these materials have base strengths higher than those of other catalysts. According to the desorption temperatures and the amounts of CO2 desorbed from the solid-base catalysts, the base strength of different solid-base catalysts decreases in the order of MgAl-HT ≈ LiAl-HT > KF/Al2O3 > CoAl-HT > NiAlHT. The total concentration of base sites (Table 1) decreases in the order of MgAl-HT > LiAl-HT > CoAl-HT ≈ NiAl-HT > KF/Al2O3. From Figure 1b, we can see that the NH3-TPD profiles of the MgAl-HT, LiAl-HT, CoAl-HT, and NiAl-HT catalysts only have low NH3 desorption peaks centered at 450 K. Compared with those of the hydrotalcite catalysts, KF/Al2O3 has higher acid strength because an NH3 desorption peak appeared at 850 K. However, the intensity of this peak is much lower than those of hydrotalcite catalysts. According to the areas of NH3 desorption peaks, the amount of acid sites decreases in the order of CoAl-HT > MgAl-HT > LiAl-HT ≈ NiAl-HT > KF/ Al2O3 (Table 1). The NH3−FT-IR spectra of the solid-base catalysts are illustrated in Figure 2. According to literature,52 the bands at 1454, 1489, and 1685 cm−1 can be attributed to the ammonia that is adsorbed on Brønsted acid sites, whereas the bands at 1597 and 1050−1300 cm−1 can be assigned as the ammonia that is adsorbed on Lewis acid sites. From Figure 2, we can see that all of the solid-base catalysts used in this work contain both Brønsted acid sites and Lewis acid sites. With the increasing of evacuation temperature, the intensities of these ammonia adsorption peaks peaks decrease, which can be explained by the desorption of NH3 from the surfaces of the catalysts. 3.1.2. HDO Catalysts. The SiO2-loaded Ni, Pd, and Ni−Pd bimetallic catalysts used in the HDO step were characterized by XRD, N2 adsorption, TEM, STEM-EDS, and XPS. In the XRD patterns of the 5%Ni/SiO2 and 5%Pd/SiO2 catalysts (Figures 3 and S1), we can only see the peaks of SiO2, Ni, and Pd. No peaks from Ni oxide or Pd oxide were observed. The XRD patterns of NiPd bimetallic catalysts only contain the peaks of SiO2 support and Ni−Pd alloy phase.53 No peaks of Ni or Pd were observed. According to literature,54,55 palladium and nickel can form alloy over the whole range of composition, which has been explained by the very low mixing enthalpy. With the increasing content of Pd in the Ni−Pd/SiO2 catalyst, the peaks corresponding to Ni(111) and Ni(200) became broader and gradually shifted to the lower angle. This can be explained by the formation of NiPd alloy with smaller particle size (Table 2) and larger lattice spacing on the surfaces of bimetallic Ni−Pd/SiO2 catalysts.53,56,57 As shown in Table 2, the BET surface areas of the 5%Ni/ SiO2, 5%Pd/SiO2, and bimetallic Ni−Pd/SiO2 catalysts are very close to that of SiO2 support (401 m2 g−1), which means that the loading of Ni and Pd does not have a significant effect on the specific BET surface area of catalyst. Figure 4 shows the TEM images and the metal particle size distributions of 5%Ni/SiO2, 5%Pd/SiO2, and bimetallic Ni− Pd/SiO2 catalysts. According to the statistical results listed in Table 2, the average diameters of metal particles of the 5%Ni/ SiO2 and 5%Pd/SiO2 catalysts are estimated as 12.7 ± 2.6 and 6.7 ± 1.5 nm, respectively. Compared with those of the 5%Ni/
chemisorption analyzer. Before each test, the sample was reduced in 10% H2/Ar flow at 673 K for 1 h, purged with Ar flow at 683 K for 0.5 h and cooled down under Ar flow to 323 K. After the stabilization of baseline, the H2 adsorption was carried out by the pulse adsorption of 10% H2/Ar at 323 K. 2.3. Activity Test. 2.3.1. Aldol Condensation. The solventfree aldol condensation of cyclopentanone and butanal was conducted by a batch reactor. For each reaction, 5.16 g of cyclopentanone, 1.44 g of butanal, and 0.14 g of solid-base catalyst were used. The reaction was carried out at 413 K for 4 h. After the reaction, the reactor was quenched to room temperature by cold water. The catalyst was separated from liquid products by centrifugation. The liquid products were diluted with methanol and analyzed by a gas chromatograph equipped with a HP-5 capillary column and a hydrogen flameionization detector (FID). The methods for the calculation of butanal conversion and the carbon yields of different products are described in Supporting Information. 2.3.2. Hydrodeoxygenation. The HDO of aldol condensation products of cyclopentanone and butanal was carried out at 503 K with a stainless-steel, tubular fixed-bed reactor described in our previous work.38,39,50,51 In each test, 1.8 g of catalyst was used. Before the HDO reaction, the catalysts were reduced by hydrogen flow at 673 K for 2 h. The aldol condensation products (prepurified by the vacuum distillation) were fed into the reactor (at a rate of 0.04 mL min−1) from the bottom of the reactor by an HPLC pump, together with a hydrogen flow (at the rate of 120 mL min−1). The system pressure was controlled at 6 MPa by a back-pressure regulator. After going through the tubular reactor, the HDO products were cooled down to room temperature and became two phases in a gas−liquid separator. The gaseous products were analyzed online by an Agilent 6890N gas chromatograph. The liquid products were drained from the gas−liquid separator after the reaction was carried out for 5 h, diluted with tetrahydrofuran, and analyzed by another Agilent 7890A gas chromatograph. Detailed information about the calculation of carbon yields of different products in the HDO step are described in the Supporting Information.
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. Aldol Condensation Catalysts. The solid-base catalysts used in the solvent-free aldol condensation of cyclopentanone and butanal were characterized by N2 adsorption, CO2-TPD, and NH3-TPD. According to the results shown in Table 1, the KF/Al2O3, MgAl-HT, LiAl-HT, CoAl-HT, and NiAl-HT catalysts used in this work have similar BET surface areas (200−300 m2 g−1). In the CO2-TPD profiles of the NiAl-HT and CoAl-HT catalysts (Figure 1a), broad CO2 desorption peaks centered at 410 K were observed. Compared with that of the NiAl-HT Table 1. Specific Surface Area (SBET) and Concentration of Base and Acid Sites over Solid Base Catalysts
a
catalyst
SBET (m2 g−1)
base sites concentration (mmol g−1)a
acid sites concentration (mmol g−1)b
KF/Al2O3 NiAl-HT CoAl-HT LiAl-HT MgAl-HT
215 220 238 242 241
0.08 0.12 0.13 0.18 0.21
0.05 0.23 0.45 0.26 0.38
Measured by CO2-TPD. bMeasured by NH3-TPD. 11827
DOI: 10.1021/acs.iecr.5b03379 Ind. Eng. Chem. Res. 2015, 54, 11825−11837
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Figure 1. (a) CO2-TPD and (b) NH3-TPD profiles of solid-base catalysts.
Figure 2. FT-IR spectra of solid-base catalysts after NH3 adsorption at 303 K and evacuation at (a) 303 K, (b) 373 K, (c) 473 K, and (d) 573 K.
attributed to the lower surface tension of NiPd alloy.53 As we know, metals with lower surface tension are more resistant to sintering during the reduction process.58 According to Figure 4f, the lattice spacing of the metallic particles on the 4%Ni−1% Pd/SiO2 catalyst was estimated as 2.18 Å. This spacing is
SiO2 catalyst, the average diameters of metal particles on bimetallic Ni−Pd/SiO2 catalysts (Table 2) are 1.2−4 times smaller (especially at high Pd content). From the XRD and TEM results, we can see that the introduction of Pd decreases the average size of Ni particles. This phenomenon can be 11828
DOI: 10.1021/acs.iecr.5b03379 Ind. Eng. Chem. Res. 2015, 54, 11825−11837
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Industrial & Engineering Chemistry Research
concentration of metallic sites on the surface of the bimetallic Ni−Pd/SiO2 catalysts is lower than that on the monometallic 5%Pd/SiO2 (or 5%Ni/SiO2) catalyst. This phenomenon may be rationalized by higher activation energy for the dissociative adsorption of hydrogen on Ni−Pd alloy.62 3.2. Activity Tests. 3.2.1. Aldol Condensation. The solvent-free aldol condensation of cyclopentanone and butanal at a initial molar ratio of 3:1 was conducted with a series of solid-base catalysts (Figure 7). According to our GC and NMR analysis (Figures S3−S5), 2-butylidene-cyclopentanone and 2,5-dibutylidenecyclopentanone (i.e., compounds 1 and 2 in Scheme 1) were identified as the main products. Both compounds 1 and 2 can serve as precursors for jet-fuel range cycloalkanes. Moreover, it is worth mentioning that both compounds 1 and 2 exist in the liquid state at room temperature, which makes it possible for them to be hydrodeoxygenated under solvent-free conditions. Besides compounds 1 and 2, a small amount (carbon yield less than 10%) of 2-ethylhex-2-enal (i.e., compound 3 in Scheme 1) was also detected as a byproduct. This compound was generated by the self-aldol condensation of butanal. The cross aldol condensation between cyclopentanone and butanal has a higher reaction rate than the self-aldol condensation of butanal under the investigated reaction conditions. The rate difference between the cross and self-aldol condensation reactions may be attributed to the steric effect since the five-carbon ring makes the α carbons in cyclopentanone more accessible than the one in butanal. No self-aldol condensation product of cyclopentanone (i.e., compound 4 in Scheme 1) was observed. This result can be attributed to steric hindrance because the carbon anion on the cyclopentanone can more easily react with the carbonyl group on butanal than the carbonyl group on another cyclopentanone molecule. The highest butanal conversion (96.2%) and total carbon yield (87.0%) of compounds 1 and 2 were obtained with the MgAl-HT catalyst. The activity sequence for different solid-base catalysts is MgAl-HT > LiAl-HT > KF/Al2O3 > CoAl-HT > NiAl-HT. The butanal conversions are slightly higher than the total carbon yields of compounds 1−3. This could be explained either by the adsorption of butanal on the solid catalyst or by the Cannizzaro reaction of butanal (producing butanol and butyric acid).63 Small amounts of butyl butyrate (carbon yield less than 10%) were also identified in some liquid products. According to the results of characterization, there is no clear relationship between the activity of solid bases and their surface areas, the concentration of base (or acid) sites, or the types of acid sites on the surfaces of these catalysts. However, we noticed that there is a correlation between the desorption temperatures of CO2 and the activities of these catalysts for the solvent-free aldol condensation of cyclopentanone and butanal (Figure S6). On the basis of this phenomenon, we can see that the strong base site may be the active center for the aldol condensation of cyclopentanone and butanal. This is consistent with what we found in the self-aldol condensation of cyclopentanone39 or the solvent-free aldol condensation of furfural and lignocellulosic ketones.50,51 The slightly higher activity of MgAl-HT than LiAl-HT can be explained by the different concentrations of base sites on these two catalysts (0.21 vs 0.18 mol g−1). Another possible reason for the excellent performance of MgAl-HT may be the synergy effect of strong base and weak acid sites. In some recent literature, the beneficial effect of weak acid sites on the activity, selectivity, and stability of solid-base catalysts for the self-aldol
Figure 3. XRD patterns of the 5%Ni/SiO2, 5%Pd/SiO2, and bimetallic Ni−Pd/SiO2 catalysts with a Cu Kα source.
Table 2. Specific Surface Areas (SBET) and Average Metal Particle Sizes over Different HDO Catalysts catalyst
SBET (m2 g−1)
dpore (nm)a
dmetal,XRD (nm)b
5%Ni/SiO2 4.5%Ni−0.5%Pd/SiO2 4%Ni−1%Pd/SiO2 2.5%Ni−2.5%Pd/SiO2 5%Pd/SiO2 SiO2
395 361 363 398 433 401
6.5 6.4 6.4 5.6 6.5 7.8
13.6 10.3 9.3
