Ind. Eng. Chem. Res. 2008, 47, 1443-1452
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Synthesis of Biogasoline from Ethanol over Hydroxyapatite Catalyst Takashi Tsuchida,*,† Tetsuya Yoshioka,† Shuji Sakuma,† Tatsuya Takeguchi,‡ and Wataru Ueda‡ Central Research Center, Sangi Co., Ltd., Fudoinno 2745-1, Kasukabe-shi, Saitama 344-0001, Japan, Catalysis Research Center, Hokkaido UniVersity, Kita 21 Nishi 10, Kita-ku, Sapporo 001-0021, Japan
Plant-derived, dehydrated ethanol was effectively converted to biogasoline in a one-step process on a highly active nonstoichiometric hydroxyapatite (HAP) catalyst. The biogasoline had a research octane number of 99 and comprised chiefly hydrocarbons from C6 to C10 as well as oxygenates not generated in the methanol-to-gasoline (MTG) process using zeolite. To confirm a proposed scheme for the synthesis of gasoline from ethanol over HAP, alcohol conversions were carried out using paired combinations of five different alcohols with ethanol. Results indicated that: (1) normal and branched alcohols were obtained by the Guerbet reaction from normal alcohols, (2) dienes were synthesized by the Lebedev reaction, and (3) aldehydes and olefins were mainly synthesized by dehydrogenation and dehydration of branched alcohols respectively. Introduction Problems and Recent Technologies in Automotive Fuel. Gasoline is the volatile liquid petroleum fraction whose boiling point ranges from around 30 to 200 °C, comprising a mixture of hydrocarbons from C4 to C12.1 It is the world’s major automotive fuel and influences each country’s economy and energy strategy. The United States is the largest consumer, accounting for more than 40% of total world gasoline consumption,2 at about 387 million gallons per day in 2006.3 Gasoline is currently produced from crude oil, and the future depletion of the world’s oil resources is a serious threat. In addition, the spread of the motor car as civilization advances has resulted in mass consumption of fossil fuel, discharging a huge amount of carbon dioxide into the atmosphere - considered to be one of the main causes of global warming. A serious reduction in carbon dioxide emissions is urgently required, and in response, car makers worldwide have developed the hybrid car, the flexible fuel vehicle (FFV), the fuel-cell car, the battery car, and the combined hybrid-FFV, all of which are slightly highpriced but beginning to penetrate markets. For fuel syntheses, various technologies such as methanolto-gasoline technology (MTG) using a zeolite catalyst developed by Mobil Corp. (now Exxon Mobil Corp.) and gas to liquid technology (GTL) using syngas are already known.4-17 However, the methanol and syngas used as raw materials for MTG and GTL are synthesized mainly from natural gas, that is, fossil fuel resources. GTL produces a light oil alternative fuel, but it cannot be used as a power source in gasoline-consuming vehicles. On the other hand, ethanol, butanol, MTBE, and ETBE are used as car fuel additives.18-25 The addition of up to 10% ethanol is believed not to affect present gasoline-based vehicle engines, and 10% ethanol-containing gasoline (gasohol) is sold as E10 in the United States, while in Brazil 22-26% ethanol is already included in gasoline sold for general use. For gasoline containing 85% ethanol (E85) and for 100% ethanol fuel, however, a specially designed car engine is necessary. However, for both E10 and E85, mileage per liter is not as good as that for regular gasoline, because the combustion heat of ethanol is lower than that of gasoline. Moreover, the accidental addition of even a † ‡
Central Research Center, Sangi Co., Ltd.. Hokkaido University.
small percentage of water can cause stratum separation into gasohol, resulting in engine trouble. In the United States, MTBE was used in the 1980s as a NOx reduction agent in exhaust gas and an octane booster replacing tetraethyl lead, but its use is being increasingly avoided because it pollutes subsurface water. In Europe, ETBE is now being used as a gasoline additive in place of MTBE.24 ETBE, synthesized from isobutylene and bioethanol, is a partially plant-derived fuel, but problems of production cost and safety remain, and ETBE makes little contribution to the resolution of global warming because the origin of isobutylene is oil. Though increased production of newstyle cars and new fuels can be expected in the future, present gasoline-based vehicles are certain to remain in the market for at least the next 20 years. Economic and Environmental Merits of Bioethanol. Bioethanol, derived from plant material, contains carbon recycled from the atmosphere by photosynthesis, and in its dehydrated form is used mainly as a gasoline additive. It is classified as carbon neutral in the Kyoto Protocol adopted by the United Nations Framework Convention on Climate Change, COP3 (1997), that is, its consumption does not count as a source of new carbon dioxide emissions. World production of bioethanol amounted to 51 × 106 kL in 2006, an 11% increase from the previous year. Chief producers were the United States, Brazil, China, and India, in that order.26 Brazil is the most advanced country in terms of bioethanol business, with a full infrastructure - from sugar cane cultivation to logistics and commercial use of ethanol - already in place, and the world’s lowest production cost. However, production has grown especially rapidly in the United States, where under national policy production from waste cellulose materials such as wheat straw, corn stalks, paper, wood, and garbage has been developed.27-32 In the near future, the yield of bioethanol per unit of area is expected to more than double.33,34 HAP Catalysts. Stoichiometric hydroxyapatite (HAP) (Ca10(PO4)6(OH)2) has a Ca/P molar ratio (Ca/P ratio) of 1.67 and a H2O content of 1.79 wt %, but in nonstoichiometric forms, its Ca/P ratio can range from 1.50 to1.70, with the loss of calcium ions. As a catalyst, HAP has the unusual property of containing both acid sites and base sites in a single-crystal lattice.35,36 Nonstoichiometric HAP is represented by the formula Ca10 - Z(HPO4)Z(PO4)6 - Z(OH)2 - Z‚nH2O; 0 < Z e 1,
10.1021/ie0711731 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/17/2008
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Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008
Table 1. List of Main Reaction Products Shown in Figure 1a number
name
1 2 3 5 6 7 8 9 12 13 14 16 17 18 19 20 21 24 25 26 28 29 30 31 33 34 35 36 37 39 40 41 42 43 44 45 48 50 53 54 55 56 58 59 61 62 63 64 65 66 69 70 73 78 82 83 84 86 90 92 107 110 112 113
methane ethylene + ethane propylene acetaldehyde 1,3-butadiene + 1-butene trans-2-butene cis-2-butene ethanol acetone oxygenate 1-pentene diethylether C5H10 C5H10 C5H8 C5H8 oxygenate butyraldehyde 2-butanone 1-hexene C6H12 C6H10 oxygenate + C6H12 C6H10 + 3-buten-1-ol C6H8 1-butanol + C6H10 benzene C6H10 2-penetanone + C6H8 + C6H10 2-pentanol n-butylethylether C7H14 C7H14 C7H14 C7H12 2-methyl-1-butanol 2-ethylbutyraldehyde toluene+ 3-hexanone hexaldehyde C8H16 C8H14 + C8H16 C8H16 + C8H14 C6 oxygenate + C8H14 2-ethyl-1-butanol C8H14 + 4-heptanone 1-hexanol C8H14 ethylbenzene m-xylene p-xylene stylene o-xylene oxygenate 2-ethylhexaldehyde C9H18 C9H18 C9H18 2-ethyl-1-hexanol 1-octanol butyl benzene C10-alcohol C10-alcohol 1-decanol hexylbenzene
a We used chemical reagents to confirm the identification of main components synthesized from ethanol. Other products were identified by searching the GCMS library (NIST-62 & NIST-12).
n ) 0 to 2.5, with the stipulation that OH- sites in this formula can be partially substituted with H2O.37 Many studies on the catalytic properties of HAP have been reported.38-65 However, there has been no report describing direct biogasoline synthesis efficiently from ethanol over HAP. In a previous article, the authors reported that higher alcohols
were synthesized by successive propagation of ethanol over a nonstoichiometric HAP catalyst at low temperature in a onestep process.61 In the present study, we found that a gasolinelike composition was synthesized from ethanol over the same HAP catalyst61 at high temperature. The properties of this biogasoline were examined and compared with the present gasoline to assess its potential as an alternative fuel. In our previous study, the authors presented a scheme simulating the synthesis of 1-butanol from ethanol over HAP catalyst and confirmed the propriety of the scheme.61 In the present study, we present a similar scheme for the synthesis over HAP of biogasoline, which includes C5 to C10 hydrocarbons heavier in composition than 1-butanol. Experimental Section Synthesis and Characterization of HAP Catalysts. HAP catalysts were prepared by the precipitation method. All of the chemicals used throughout the study were purchased from Wako Pure Chemical Industries, Ltd. (Japan). A mixed aqueous solution containing 560 mL of 0.60-mol/L Ca(NO3)2‚4H2O and 500 mL of 0.40-mol/L (NH4)2HPO4 was titrated with aqueous ammonia to a pH of ∼9-10. The solution was then stirred for 24 h and filtered, and the filtrate washed with deionized water and dried at 140 °C. The powder obtained was mixed with deionized water so that the resulting slurry contained 10 wt% powder. The gel obtained from this process was aged and dried at 140 °C, and the resulting substance was ground in a mortar and baked for 2 h at 600 °C to produce the final HAP powder catalyst. The catalyst was characterized by powder X-ray diffraction (XRD) and measurement of its BET surface area, and the Ca/P ratio was measured by X-ray fluorescence (XRF) analysis. XRD and XRF confirmed that the catalyst synthesized was composed of crystalline HAP with a Ca/P ratio of 1.64, and its BET surface area was 33 m2/g. Nonstoichiometric HAP powder was placed in a molding machine to form pellets, and the pellets were then lightly crushed to a particle size of 14 to 26 mesh for reaction with ethanol. Reaction Method. Three series of experiments were carried out in the present study. All of the reactions including gas liquefaction and product distillation took place at atmospheric pressure, and products were analyzed by gas chromatograph with a mass spectrometer (GCMS) in the range M/Z ) 10400 and by gas chromatograph with a flame ionization detector (GC-FID). In both cases, columns supplied by J&W Scientific Corporation (liquid phase, DB-1; film thickness, 5.00 µm; column dimension, 30 m × 0.323 mm) were used. We used chemical reagents to confirm the identification by GC-FID of main components produced (acetaldehyde, butyraldehyde, crotonaldehyde, 1-butanol, 1-hexanol, 2-ethyl-1-butanol, 1-octanol, 2-ethyl-1-hexanol, 1-decanol, 1-hexene, etc.) Other products shown as small peaks were identified by searching the GCMS library (NIST-62 & NIST-12). (1). Biogasoline Synthesis on HAP. Only ethanol was used as the raw material for biogasoline synthesis. Using 2 g of catalyst, the reaction products from ethanol conversion at different reaction temperatures were analyzed. In each experiment, 15.8 vol % ethanol gas diluted with helium was passed at 116 mL/min through a fixed-bed silica tubular reactor with an interior diameter of 5 mm. Using 10.5 g of catalyst, a larger volume of biogasoline was produced to assess its fuel properties. Ethanol gas (100%) was fed at 1700 mL/min through a fixed-bed SUS-316 tubular reactor with an interior diameter of 10 mm at 490 °C. The biogasoline
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1445
Figure 1. Product distribution by GC-FID from ethanol conversion on nonstoichiometric HAP (Ca/P ratio ) 1.64). Reaction conditions; contact time, 1.78 s; temperature, 450 °C; atmosphere. Table 2. Gas Composition after Ethanol Conversion over HAP (Ca/P Ratio ) 1.64) at 400 °C (unreacted ethanol included) components/ C wt % hydrocarbons paraffins olefins dienes aromatics others (subtotal) oxygenates alcohols aldehydes ethers ketones others (subtotal)
carbon number 1
2
3
4
5
6
7
8
2.3
0.3
1.5 3.8
0.3 0.1
0.7 1.8 0.5
0.1 0.0 0.1
0.2 1.1 1.1
0.1
2.5
9
10
more
0.0
0.0
2.3
0.3
36.0 1.5
5.2
0.3
3.0
28.9 0.9 0.3
0.2
9.6 0.6 0.1
0.0
0.1
0.2
0.1
0.2
0.0 5.3 6.8 2.0 0.0 14.2
0.0
37.5
0.0
30.0
0.3
10.4
0.5
3.0
0.0
0.7
0.0
78.0 3.4 0.4 0.6 0.0 82.3
0.0
39.8
0.3
35.3
0.6
13.3
0.6
5.5
0.0
0.8
0.2
100.0
0.0
0.1
2.6 0.4
0.7
0.5
unidentified total
total
3.5
produced was liquefied in a cold trap at 0 °C, then distilled from 2 to 220 °C. (2). MTG and ETG on Zeolite. Methanol-to-gasoline (MTG) and ethanol-to-gasoline (ETG) conversions were carried out on synthesized zeolite (MFI-40, SiO2/Al2O3 ) 40), a known MTG catalyst, for comparison with ETG over HAP. Reaction conditions were as follows. For MTG, 20 vol % methanol gas diluted with helium was passed through a fixed-bed silica tubular reactor with an interior diameter of 5 mm on 0.21 g of catalyst at 380 °C and a gas space velocity (GHSV) of 10 000 h-1. For ETG, 19 vol % ethanol gas diluted with helium was passed through an identical reactor on 0.23 g of catalyst at 410 °C and GHSV of 11 600 h-1. (3). Assessment of the Fuel Properties of Biogasoline. Assessment of the fuel properties of biogasoline was carried out by Sanseki Techno Co., Ltd. (now Nippon Oil Co.), which examined each product’s density (at 15 °C), research octane
number (RON), vapor pressure, distillation characteristics, residue formation, copper corrosion, oxidation stability, and gum content, unwashed. (4). Assessment of Proposed Scheme of Ethanol Conversion Reaction. Five kinds of equimolar alcohol mixtures, ethanol and 1-butanol, ethanol and 1-hexanol, ethanol and 2-ethyl-1-butanol, ethanol and 1-octanol, and ethanol and 2-ethyl-1-hexanol, were used to assess a proposed scheme for the ethanol conversion reaction. For this testing, 1.2 g of catalyst was used. To obtain data used in this assessment, 120 mL/min of each respective 20 vol % equimolar alcohol mixture, diluted with helium, was passed through a fixed-bed silica tubular reactor with an interior diameter of 5 mm. Calculation Formulas. The alcohol conversion rate, yield, and selectivity to products were calculated as follows: 〈In the case of only ethanol as raw material〉:
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Table 3. Gas Composition after Ethanol Conversion over HAP (Ca/P Ratio ) 1.64) at 500 °C (Unreacted Ethanol Included) components/ C wt % hydrocarbons paraffins olefins dienes aromatics others (subtotal) oxygenates alcohols aldehydes ethers ketones others (subtotal)
carbon number 1
2
3
4
5
6
7
8
1.1
1.7 5.8
4.3
6.4 12.6
3.0 1.2
7.1 6.1 7.8 1.0 22.0
0.9 1.2
2.9 2.6 7.7
0.5
2.2
1.5
2.1
13.1
0.5
2.2
1.5
2.8 30.2 22.5 20.9 1.0 77.3
1.1
7.5
4.3
18.9
4.2
0.1 0.7
0.1
0.3 0.9 0.4
0.5
more
0.4
0.8
0.6
1.6
2.3
3.0
1.0
1.9
0.0
0.4
0.0
1.1
8.3
4.9
20.5
6.6
25.0
3.1
15.0
0.5
2.6
1.5
100.0
9
10
others
total
1.0
unidentified total
total
0.0
1.9
0.6 1.3
10
2.5 4.8 1.0 3.3 0.4 12.0
0.5
0.5 1.9 0.6
9
10.7
Table 4. Gas Composition after Methanol Conversion over MFI-40 (Si/Al Ratio ) 80) at 380 °C components/ C wt % hydrocarbons paraffins olefins dienes aromatics others total
carbon number 1
2
3
4
5
6
7
0.4
0.2 7.2
0.4 18.7
14.4 12.6
7.0 4.4
4.1 1.9 0.4 0.5
0.5 0.5 1.0 3.9
0.7 10.1
4.7
1.0 5.6
26.8 45.2 2.2 20.2 5.6
6.9
5.9
10.8
4.7
1.0
5.6
100.0
9
10
others
total
0.7
11.9 76.7 2.8 7.9 0.7
0.7
100.0
0.4
7.3
19.1
26.9
11.4
8
Table 5. Gas Composition after Ethanol Conversion over MFI-40 (Si/Al Ratio ) 80) at 410 °C components/ C wt % hydrocarbons paraffins olefins dienes aromatics others total
carbon number 1
2
3
4
5
6
7
8
0.0
0.3 25.9
0.3 23.4
7.2 16.7 0.7
2.4 6.7 0.0
1.5 2.5 0.4 0.4
0.1 1.5 0.9 2.0
0.0 0.7 3.5
0.1 1.7
0.3
4.7
4.5
4.2
1.8
0.3
0.0
26.2
23.8
24.6
9.1
Table 6. Results of Fuel Property Test and Comparison with Japanese Industrial Standard (JIS) property research octane number (RON) vapor pressure distillation characteristics initial boiling point 5 vol % 10 vol % 20 vol % 30 vol % 40 vol % 50 vol % 60 vol % 70 vol % 80 vol % 90 vol % 95 vol % 97 vol % final boiling point residue copper corrosion oxidation stability gum content, unwashed
units kPa °C °C °C °C °C °C °C °C °C °C °C °C °C °C vol % (50 °C, 3h) min mg/100 mL
results 99.2 17.5 70.5 76.5 79.5 81.0 84.0 86.5 90.5 98.0 112.0 124.5 147.5 170.5 185.0 224.0 0.5 1a 60 381
(reference) JIS no. 2 min 89.0
max 70
75-110
max 180
ConVersion for each alcohol (%) ) ((C moles of the alcohol before reaction - C moles of the unreacted alcohol)/C moles of the alcohol before reaction) × 100 Product yield (%) ) (C moles of product/C moles of total outlet gases) × 100 Product selectiVity (C wt%) ) (C moles of product/C moles of reacted ethanol or reacted alcohols) × 100 Note that GCMS showed the total amount of CO and CO2 in the reaction product throughout the experiments was less than 1.0%, and carbon deposition on the catalyst was minimal, indicating that there was almost no decomposition of ethanol on HAP catalysts over the range of temperatures used in this study. We confirmed that the match between in/out carbon mass balance of the reactor was more than 94% in all of the reaction conditions. The repeatability of these tests was good. Results and Discussion
max 220 max 2.0 max 1 min 240 max 20
Ethanol conVersion (%) ) (1 - C moles of unreacted ethanol/C moles of total outlet gases) × 100 〈In the case of a mixture of ethanol and higher alcohol as the raw material〉:
Biogasoline Synthesis from Ethanol over HAP Catalyst. Figure 1 shows the GC-FID chart for ethanol conversion at 450 °C over nonstoichiometric HAP with a Ca/P ratio of 1.64. Table 1 lists the main reaction products comprising the chart. The reaction products consisted of olefins, dienes, alcohols, aldehydes, and aromatics from carbon C2 to around C12, and the chromatograph resembled that of gasoline. The obtained liquid contained oxygenates such as alcohols, aldehydes, and
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1447 Table 7. Reaction Product Selectivity from Equimolar Mixed Solutions of Two Alcohols over HAP (Ca/P Ratio ) 1.64). Reaction Temperature in each case is that at which Ethanol Conversion is about 20% (T20) alcohol - 1 alcohol - 2 reaction temp (°C) conversion (%) alcohol - 1 alcohol - 2
ethanol 1-butanol 356
ethanol 1-hexanol 334
ethanol 2-ethyl-1-butanol 358
ethanol 1-octanol 329
ethanol 2-ethyl-1-hexanol 365
24.1 13.3
21.9 11.6
26.4 6.2
14.5 14.5
24.8 6.5
0.0 0.4 2.8 0.0 1.0
0.0 0.1 1.3 0.0 0.0
0.0 0.6 3.0 0.0 0.1
0.0 0.1 0.6 0.0 0.0
0.0 0.5 2.8 0.0 0.1
CH2dCHsCHdCH2 C2H5OC2H5 C3H7CHO C4dOH 1-C4H9OH
0.7 0.1 6.4 1.5
0.1 0.0 0.1 0.6 7.0
0.7 0.2 0.6 2.6 27.3
0.0 0.0 0.0 0.3 2.5
0.7 0.1 0.4 1.9 21.0
C5H10 C6H10 C6H12 C4H9OC2H5 C5H11CHO
0.1 1.1 0.1 0.3 0.6
0.7
0.1 0.4 3.1
selectivity (C wt %) CH4 C2H4 CH3CHO C3H6 C4H8
4.3
C2H5CH(C2H5)CHO C6dOH C2H5CH(C2H5)CH2OH 1-C6H13OH C8H14
0.4 4.4 15.2 30.9 0.7
1.1
C8H16 C8-ether branched C8-aldehyde C4H9CH(C2H5)CHO C7H15CHO
0.4 0.6
0.0 0.4
0.4 1.5
0.2 0.2
C8dOH branched C8-alcohols C4H9CH(C2H5)CH2OH 1-C8H17OH C10H18
0.4
0.1
0.0
18.3 0.4 0.4
4.4
2.3 0.5
0.1 0.1
1.1 1.4
0.4
5.5
1.1 0.3 24.0 2.8
14.7 2.8 0.2
11.1 30.0 0.1
0.7 22.6 0.3 0.3 0.1
C10H20 C10-aldehydes C10dalcohol branched C10-alcohols C4H9CH(C2H5)(CH2)2CH2OH
0.2 0.1
0.4 0.1
1.2 0.1
2.0
6.4
C6H13CH(C2H5)CH2OH 1-C10H21OH C12H22 C12-aldehydes branched C12-alcohols
1.8 0.4
0.1 0.1 0.1 0.8
2.6
0.2 0.4 0.4 23.1
1.5 0.6
0.1
3.0
1.9
6.3 18.5
0.5 0.0 0.0
0.3 0.9
2-butyl-1-octanol 2-ethyl-1-decanol 1-dodecanol C14-alcohol C16H30 C16H32 C16-ether 2-hexyl-1-decanol aromatics others
0.0 0.1
5.7
26.1 1.1 1.3
0.3
1.2 0.5
0.2 6.6
0.1 3.8
ketones, which are not found in commercial gasoline, and at the same time, aromatics such as benzene, toluene, and xylene, in smaller quantities than in commercial gasoline. Tables 2 and 3 show gas compositions (including unreacted ethanol) for each carbon number after ethanol conversion at 400 and 500 °C respectively over the HAP catalyst under the following conditions: feed gas, 15.8 vol % ethanol/He; GHSV, 2000 h-1; catalyst volume, 2 g. Table 2 shows the results of analysis at 400 °C. Selectivities to olefins from C2 to C8, dienes from C4 to C8, and aromatics, based on carbon weight, were 5.3%, 6.8%, and 2.0%, respectively. Selectivity to total hydrocarbons was 14.2%. In the case of oxygenates, selectivities to
0.2 4.6
0.6 0.7 35.8 0.1 26.6
0.2 8.5
alcohols from C2 to C10 (mainly unreacted ethanol, butanol, hexanol and octanol), aldehydes, ethers, and ketones were 78.0% (including unreacted ethanol 36.0%), 3.4%, 0.4%, and 0.6% respectively. At 400 °C, selectivity to C5+ products, including unidentified products, was 24.5%, and selectivity to total C6 to C9 products, which are the major components of gasoline, was 19.4%. Table 3 shows the results of analysis at 500 °C. Selectivities to paraffins C1 and C2, olefins from C2 to C8, dienes from C4 to C8, and aromatics were 2.8%, 30.2%, 22.5%, and 20.9%, respectively, bringing selectivity to total hydrocarbons to 77.3%. In the case of oxygenates, selectivities to alcohols from C2 to C8, aldehydes, ethers, and ketones were 2.5%
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(including unreacted ethanol 0.1%), 4.8%, 1.0%, and 3.3%, respectively. At 500 °C, selectivity to C5+ products, including unidentified products, was as high as 65.2% and selectivity to total C6 to C9 products was 43.6%. Results showed that product gas composition depends strongly on reaction temperature. A small amount of C4 in the biogasoline could also increase the vapor pressure for the improvement of cold-start car engine performance. MTG and ETG over Zeolite (MFI-40) Catalyst. Table 4 shows the gas composition after methanol conversion over MFI40 at 380 °C. Methanol was converted entirely into hydrocarbons. Selectivities to paraffins from C1 to C7, olefins from C2 to C7, dienes from C6 to C8, aromatics from C6 to C10, and others were 26.8%, 45.2%, 2.2%, 20.2%, and 5.6%, respectively. In this reaction, selectivity to C5+ products was 46.3% and to total C6 to C9 hydrocarbons 28.3%. The composition of the product of MTG was close to that of gasoline and very different from that obtained from ETG on HAP catalyst (Tables 2 and 3). Selectivity to C5+ products was also much lower for MTG on the zeolite catalyst than for ETG on the HAP catalyst at 500 °C (Table 3). Table 5 shows the gas composition after ethanol conversion over MFI-40 at 410 °C. Ethanol was converted entirely into hydrocarbons. Selectivities to paraffins from C1 to C7, olefins from C2 to C8, dienes from C4 to C9, aromatics from C6 to C10, and others were 11.9%, 76.7%, 2.8%, 7.9% and 0.7% respectively. In this reaction, selectivity to C5+ products was 25.4%, and selectivity to total C6 to C9 hydrocarbons was 15.3%. The main products of ETG conversion over zeolite were olefins. Product composition was very different from that of ETG over the HAP catalyst (Tables 2 and 3), and selectivity to C5+ products was much lower for ETG on the zeolite catalyst than for ETG on HAP at 500 °C (Table 3). Results of the above tests suggested that ETG over HAP is a promising way to synthesize biogasoline and that the scheme for ETG over HAP is different from that for ETG over a zeolite catalyst. This will be examined later. Fuel Properties of Biogasoline. Table 3 suggests that ethanol-derived biogasoline could have potential as an alternative fuel because its composition is similar to that of gasoline. Table 6 shows the results of fuel property testing of the biogasoline, after liquefaction and distillation as previously shown. The results were compared with Japan Industrial Standard (JIS) values for gasoline. The research octane number (RON) of the biogasoline was as high as 99.2. Most values were equivalent to JIS, except for oxidation stability and gum content unwashed, as there was a high proportion of unsaturated hydrocarbons such as dienes in the biogasoline. To satisfy fuel property standards, these would need to be stabilized. A driving test was also carried out using the biogasoline to power a motorbike with an AA01E air-cooled 49 cm3 fourstroke OHC single-cylinder engine. The motorbike ran smoothly, confirming the potential of the biogasoline as an alternate fuel and suggesting the possibility of 100% plant-derived gasoline in the near future. Conversion Reactions Using Equimolar Alcohol Mixtures over HAP. We postulated, from our analysis of the product composition shown in Tables 2 and 3, that the key mechanism for the synthesis of biogasoline from ethanol consists of the successive propagation of ethanol (to produce Guerbet alcohols) followed by dehydration/dehydrogenation of the generated alcohols. To further examine this postulated mechanism for the synthesis of biogasoline from ethanol, we focused on five Guerbet alcohols - 1-butanol, 1-hexanol, 2-ethyl-1-butanol,
1-octanol, and 2-ethyl-1-hexanol - which are the main products of ethanol conversion over HAP at around 350 °C and investigated reactions involving equimolar mixtures of each of these alcohols and ethanol. Table 7 shows the main products of these mixed-alcohol reactions at about 20% ethanol conversion (T20) over HAP catalyst. (Note however that in an actual situation of ethanol conversion to biogasoline over HAP, the concentration of each respective higher alcohol would be lower than that of ethanol, resulting in a much lower level of reaction product selectivity than occurred in the equimolar mixture reactions shown in Table 7). First, we compared the alcohols’ conversion rates at T20. (The rate for ethanol, for comparison, is shown in parenthesis). The conversion rates of 1-butanol, 1-hexanol, and 1-octanol, which are all normal alcohols, were 13.3% (24.1%) at 356 °C, 11.6% (21.9%) at 334 °C, and 14.5% (14.5%) at 329 °C, respectively. The respective rates of branched alcohols 2-ethyl1-butanol and 2-ethyl-1-hexanol were 6.2% (26.4%) at 358 °C and 6.5% (24.8%) at 365 °C, respectively. These results indicate that ethanol is more reactive than higher alcohols and that a normal alcohol is more reactive than a branched alcohol of the same carbon number. On the basis of our analysis of the reaction products of these equimolar mixtures of ethanol and higher alcohols over HAP, we then proposed the following detailed scheme for the synthesis of biogasoline from ethanol. Looking first at the normal alcohols, main reaction products from the mixture of ethanol and 1-butanol, in order of selectivity (C wt%), were 1-hexanol (30.9%), 2-ethyl-1-butanol (15.2%), 2-ethyl-1-hexanol (14.7%), and butyraldehyde (6.4%) at T20. The overall reaction equations for these products are as follows:
C2H5OH + 1-C4H9OH f 1-C6H13OH + H2O
(1)
1-C4H9OH + C2H5OH f C2H5CH(C2H5)CH2OH + H2O (2) 1-C4H9OH + 1-C4H9OH f C4H9CH(C2H5)CH2OH + H2O (3) 1-C4H9OH f C3H7CHO + H2
(4)
The results obtained indicate that in this situation, the carbon propagating Guerbet reaction,66-72 which generates higher alcohols from lower alcohols, occurs more easily than the dehydrogenation of alcohol, which generates aldehydes. Also, in a Guerbet reaction between ethanol and 1-butanol, reactions (1) and (2) proceed via the activation of the β carbons of each alcohol respectively. The fact that product selectivity to 1-hexanol was higher than that to C2H5CH(C2H5)CH2OH suggests that the β carbon of ethanol is more easily activated than that of l-butanol. For the mixture of ethanol and 1-hexanol, main products in order of selectivity at T20 were 1-octanol (30.0%), 2-butyl-1octanol (26.1%), 2-ethyl-1-hexanol (11.1%), and 1-butanol (7.0%). The overall reaction equations for these products are as follows:
C2H5OH + 1-C6H13OH f 1-C8H17OH + H2O
(5)
1-C6H13OH + 1-C6H13OH f C6H13CH(C4H9)CH2OH + H2O (6) 1-C6H13OH + C2H5OH f C4H9CH(C2H5)CH2OH + H2O (7) C2H5OH + C2H5OH f 1-C4H9OH + H2O
(8)
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Similarly, for the mixture of ethanol and 1-octanol, main products in order of selectivity were 2-hexyl-1-decanol (35.8%), 1-decanol (18.5%), 2-ethyl-1-octanol (6.3%), 1-octanal (2.8%), and 1-butanol (2.5%). The overall reaction equations are as follows:
1-C8H17OH + 1-C8H17OH f C8H17CH(C6H13)CH2OH + H2O (9) C2H5OH + 1-C8H17OH f 1-C10H21OH + H2O
(10)
1-C8H17OH + C2H5OH f C6H13CH(C2H5)CH2OH + H2O (11) 1-C8H17OH f C7H15CHO + H2
(12)
C2H5OH + C2H5OH f 1-C4H9OH + H2O
(8)
Results for the above mixtures indicate that reaction (5) proceeded more easily than reaction (7), and that reaction (10) proceeded more easily than reaction (11), suggesting that the β carbon of ethanol was more easily activated than those of l-hexanol and 1-octanol, resulting in the formation of 1-octanol and 1-decanol, respectively. Next, we considered mixtures combining ethanol with a branched alcohol. In the case of ethanol and 2-ethyl-1-butanol, the main reaction products at T20 in order of selectivity (C wt%) were 1-butanol (27.3%), branched C8 alcohol (22.6%), 2-ethyl1-butanal (18.3%), branched C10 alcohol (6.4%), and C6 olefins (3.1%). Overall reaction equations are as follows:
C2H5OH + C2H5OH f 1-C4H9OH + H2O
Figure 2. Simulation curves vs actual data showing the effect of contact time on yields of 1-butanol, O; 1-hexanol, 4; 2-ethyl-1-butanol, 2; 1-octanol, 0; 2-ethyl-1-hexanol, 9; and C10 alcohols, ×, from ethanol on HAP catalyst (Ca/P ratio ) 1.64) at 400 °C. Table 8. Reaction Rate Constants for Biogasoline Synthesis from Ethanol on HAP Catalyst (Ca/P Ratio ) 1.64) at 400 °Ca reaction step S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18
(8)
C2H5CH(C2H5)CH2OH + C2H5OH f branched C8 alcohol + H2O (13) C2H5CH(C2H5)CH2OH f C2H5CH(C2H5)CHO + H2 (14) branched C8H17OH + C2H5OH f branched C10 alcohol + H2O (15) C2H5CH(C2H5)CH2OH f C6H12 + H2O
(16)
In the case of ethanol and 2-ethyl-1-hexanol, main products in order of selectivity (C wt%) were 2-ethyl-1-hexanal (24.0%), 4-ethyl-1-octanol (23.1%), 1-butanol (21.0%), branched C12 alcohol (5.7%), and C8 olefins (5.5%). Overall reaction equations are as follows:
C4H9CH(C2H5)CH2OH f C4H9CH(C2H5)CHO + H2 (17) C2H5OH + C4H9CH(C2H5)CH2OH f C4H9CH(C2H5)(CH2)2CH2OH + H2O (18) C2H5OH + C2H5OH f 1-C4H9OH + H2O
(8)
C4H9CH(C2H5)(CH2)2CH2OH + C2H5OH f branched C12 alcohol + H2O (19) C4H9CH(C2H5)CH2OH f C8H16 + H2O
(20)
These results indicated that, in the case of reactions involving ethanol and a branched alcohol, the reactivity of the branched alcohol with ethanol was lower than that of normal alcohol at low temperature. The results also suggested that, for branched alcohols, in contrast with normal alcohols, intramolecular reactions such as dehydrogenation and dehydration occur more easily than intermolecular reactions such as the Guerbet reaction.
a
reaction rate constant (kn/Unit) k1 k2 k3 k4 k5 k6 k7 k8 k9 k10 k11 k12 k13 k14 k15 k16 k17 k18
4.4 × 101 4.5 × 101 2.6 × 101 1.0 × 102 1.1 × 102 4.0 × 102 2.2 × 10-2 6.5 × 10-2 1.6 × 10-1 2.2 × 10-1 5.1 × 10-1 3.0 × 10-1 1.1 1.2 × 10-1 6.2 × 10-2 1.9 × 102 5.1 × 10-1 5.1 × 10-3
unit L mol-1 s-1 L mol-1 s-1 L mol-1 s-1 L mol-1 s-1 L mol-1 s-1 L mol-1 s-1 s-1 s-1 s-1 s-1 s-1 s-1 s-1 s-1 s-1 L mol-1 s-1 s-1 s-1
[C2H5OH]o ) 7.05 mmol/L.
Consequently, we found that the Guerbet reaction, which synthesizes higher alcohols from lower alcohols, occurred more easily between normal alcohols than between normal alcohol and branched alcohol. We found that the β carbon of ethanol was activated more readily than that of higher alcohols, so that reactions between ethanol and higher alcohols took precedence over other reactions between alcohols. We also found that branched alcohols generated aldehydes and olefins more readily than did normal alcohols. At high temperature, 1,3-butadiene, C6 and C8-dienes, aldehydes, and olefins were synthesized, as shown in Table 3. We postulate that these dienes are synthesized from ethanol by the Lebedev reaction,73-77 which is known to synthesize 1,3butadiene from ethanol and in which aldehyde functions as the catalyst. Aldehydes and olefins, which are generally branched, are mainly synthesized from branched alcohols by dehydrogenation and dehydration. As a result, a variety of reaction products from ethanol were synthesized over HAP catalyst. Proposed Mechanism of Biogasoline Synthesis over HAP. In a previous study, the authors presented a scheme simulating the synthesis of 1-butanol from ethanol over HAP catalyst using 13 reactions relating to 1-butanol.61 In the present study, we further develop this to a scheme for synthesis of biogasoline from ethanol over HAP, using the results of a series of model reactions carried out between ethanol and five respective higher alcohols in a more complex set of 18 equations involving heavier products than those in the previous article, such as C10 alcohol and hydrocarbons. On the basis of this simulation and the results
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obtained in the present research, we propose a mechanism for the synthesis of biogasoline from ethanol according to the following reaction steps. Aldehyde is known to function as a catalyst in the Lebedev reaction. We therefore postulate that the formation rate of 1,3-butadiene in eq S16 below occurs in proportion to the product of multiplying the ethanol concentration by the acetaldehyde concentration.61 k1
C2H5OH + C2H5OH 98 n-C4H9OH + H2O
(S1)
k2
C2H5OH + 1-C4H9OH 98 1-C6H13OH + H2O (S2) k3
1-C4H9OH + C2H5OH 98 C2H5CH(C2H5)CH2OH + H2O (S3) k4
C2H5OH + 1-C6H13OH 98 1-C8H17OH + H2O
(S4)
k5
1-C6H13OH + C2H5OH 98 C4H9CH(C2H5)CH2OH + H2O (S5) k6
1-C8H17OH + C2H5OH 98 (normal+branched) C10alcohols + H2O (S6) k7
C2H5OH 98 CH2dCH2 + H2O k8
1-C4H9OH 98 1-C4H8 + H2O k9
1-C6H13OH 98 1-C6H12 + H2O S9 k10
C2H5CH(C2H5)CH2OH 98 C6H12 + H2O k11
1-C8H17OH 98 1-C8H16 + H2O k12
C4H9CH(C2H5)CH2OH 98 C8H16 + H2O k13
(normal+branched) C10alcohols 98 C10H20 + H2O k14
C2H5OH 98 CH3CHO + H2 k15
branched alcohols 98 branched aldehydes + H2 k16
2 C2H5OH 98 CH2dCHCHdCH2 + 2H2O + H2 k17
Aldehydes 98 Others k18
C2H5OH 98 Aromatics
(S7) (S8) (S9) (S10) (S11) (S12) (S13) (S14) (S15) (S16) (S17) (S18)
Reaction constants k1-k18, shown in Table 8, are those for reaction steps S1-S18, respectively. Reaction constant values for the 13 equations included in our earlier study differ somewhat from those for the same equations in the present study,61 but this is a refinement due largely to the addition of five new reaction steps involving consumption of the respective substances produced. To clarify the reaction mechanism, the effect of contact time on yields of products from ethanol on HAP catalyst was examined at 400 °C. Figure 2 shows the relationship between simulation curves and actual yields of butanol, hexanol, octanol, and C10 alcohols over a range of contact times, and Figure 3 shows the relationship between
Figure 3. Simulation curves vs actual data showing the effect of contact time on the yields of ethylene, b; 1-butene, O; 1-hexene, 4; 1-octene, 0; aldehydes, 2; 1,3-butadiene, 9; and aromatics, ×, from ethanol on HAP catalyst (Ca/P ratio ) 1.64) at 400 °C.
simulation curves and actual yields of olefins, aldehydes, 1,3butadiene, and aromatics, using the parameters shown in Table 8. As the simulation curves closely fit the experimental data in both figures, we concluded that branched C6 to C10 hydrocarbons are synthesized by the Guerbet reaction from ethanol on HAP catalyst. Conclusion The synthesis of alternative gasoline from methanol (MTG), using a zeolite catalyst developed by the former Mobil Corp. is well known. However, the synthesis of alternative gasoline from ethanol (ETG) has not been reported. We developed a process for synthesizing biogasoline from plant-derived ethanol (bioethanol) in one step, with high selectivity, over a highly active nonstoichiometric HAP catalyst. The biogasoline produced has a research octane number of 99 and contains dienes and oxygenates almost entirely absent from the product of MTG or ETG over a zeolite catalyst. We found that the ratio of oxygenates in the biogasoline could be adjusted by changing the reaction conditions (mainly reaction temperature). On the basis of a set of model conversion reactions on the HAP catalyst using equimolar mixtures of ethanol and higher alcohols, and schematic simulation of the reactions involved, we proposed a mechanism for the synthesis of biogasoline from ethanol over HAP. We concluded that reactions to propagate carbon number occurred by the Guerbet reaction between mainly normal alcohols, forming normal and branched alcohols, whereas branched aldehydes and olefins were formed by dehydrogenation and dehydration, respectively, of mainly branched alcohols. We believe that ethanol conversion over HAP represents a new form of C2 chemistry involving different reaction processes from C1 chemistry or C2 chemistry on acid catalysts such as zeolite. Acknowledgment We thank the New Energy and Industrial Technology Development Organization (NEDO) of Japan, which provided financial assistance for this research. We also thank Dr. Roslyn Hayman for advice. Literature Cited (1) Gasoline Blending Streams Test Plan, Submitted to the US EPA by The American Petroleum Institute Petroleum HPV Testing Group, AR20113409A, 2001. (2) Motor Fuels: Understanding the Factors That Influence the Retail Price of Gasoline, United States Government Accountability Office, GAO05-525SP, May, 2005.
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ReceiVed for reView August 29, 2007 ReVised manuscript receiVed November 8, 2007 Accepted November 20, 2007 IE0711731
