Energy Fuels 2010, 24, 2404–2409 Published on Web 03/10/2010
: DOI:10.1021/ef901607t
Production of Synthetic Diesel by Hydrotreatment of Jatropha Oils Using Pt-Re/H-ZSM-5 Catalyst Kazuhisa Murata,* Yanyong Liu, Megumu Inaba, and Isao Takahara Biomass Technology Research Center, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received December 29, 2009. Revised Manuscript Received February 27, 2010
Renewable green diesel-type alkanes can be produced by hydrotreating jatropha oil and vegetable oils at standard hydrotreating conditions (i.e., 543-573 K) with Pt/H-ZSM-5 catalysts, which are active under the weight ratio of jatropha or vegetable oil/catalyst of 1. The carbon molar yield of straight chain C15-C18 alkanes was ∼80% for hydrotreating pure jatropha oil. However, under the jatropha oil/catalyst weight ratio of 10, being important from a practical point of view, the alkanes yield falls to only 2.3%. Under a high jatropha oil/catalyst ratio of 10, rhenium-modified Pt/H-ZSM-5 catalyst is found to be effective for raising the C15-C18 alkanes yield. The yield of C15-C18 alkanes is 67% at an optimun Re/Al molar ratio of 0.8. Investigation of catalyst natures indicates that metallic Pt and Re are independently present on the surface, but synergism of these two metals could play an important role in the hydrotreating reaction, even at a high ratio of jatropha oil/catalyst of 10. The reaction pathway involves hydrogenation of the CdC bonds of the jatropha oils followed by mainly hydrodeoxygenation with decarbonylation and decarboxylation to form C15-C18 straight chain alkane mixtures.
Introduction
Experimental Procedures
Jatropha oils are one of a renewable feedstock suitable for production of biofuels from sustainable nonfood biomass resources. There are many benefits of biofuels including domestic production of fuels, decreased greenhouse gas emissions, decreased dependence on fossil fuels, improvement of rural economies, and increased national security.1 Biodiesel production from trans-esterification of triglycerides such as vegetable oils is currently the primary route for production of biofuels from vegetable oils. In this process, new biodiesel plants must be built and a large amount of glycerol is produced as a byproduct. Thus, another option of biofuel production is a direct hydrotreating of nonfood triglyceride resources to form C15-C18 hydrocarbons. These processes can be performed using existing petroleum refineries.2 Hydrotreating is used in the petroleum refinery to remove S, N, and metals from petroleum-derived feedstocks including heavy gas-oil or vacuum gas-oil. However, hydrotreating is also very useful to produce straight chain alkanes ranging from n-C15-C18, from a fatty acid fraction of various vegetable oils with triglyceride structures.3 The normal alkanes produced have a high cetane number (above 98), whereas typical diesel fuel has a cetane number around 50. In this paper, we report on a direct hydrotreating of jatropha oils using remodified Pt/H-ZSM-5 catalyst to produce C15-C18 hydrocarbons.
Microporous materials such as H-ZSM-5 and USY were provided by Zeolyst and Tosoh Co. Ltds. Pt(NH3)4Cl2 3 H2O and NH4ReO4 were purchased from Soekawa Chemicals, Japan. The Pt/zeolite catalyst used in this study was prepared by impregnating zeolite support with Pt(NH3)4Cl2 3 H2O, followed by drying at 373 K and calcination for 5 h at 773 K. Pt-Re/zeolite was prepared by impregnation of zeolite with Pt(NH3)4Cl2 3 H2O and NH4ReO4 followed by drying at 373 K and calcination for 5 h at 773 K. The concentrations of platinum metal were 5 wt %. X-ray diffraction (XRD) patterns of the fresh catalysts were recorded on a Philips 1850 diffractometer (operated at 40 kV and 40 mA) using Cu radiation. The reducibility of the prepared Pt/ zeolite and Pt-Re/zeolite catalysts was studied by H2-temperature-programmed reduction (TPR). Before the analysis, 50 mg of the sample was placed in a quartz tube and purged with He at 473 K for 1 h in order to remove impurities from the catalyst surface. The sample was cooled to room temperature in flowing He; then, TPR measurement was carried out using a 3% H2/He (v/v) mixture (flow rate of 30 mL/min) at a heating rate of 5 K min-1 from room temperature to 900 K, and the H2 consumption was recorded with a thermal conductivity detector (TCD). NH3 temperature-programmed desorption (TPD) experiments were carried out for Pt/H-ZSM-5 and Pt-Re/H-ZSM-5 on a special NH3-TPD apparatus (BELCAT, Nippon Bell), which was connected to a Q-Mass analyzer (Pfeiffer Vacuum, Omnistar GCD301) for measuring the acidic properties of the catalysts. After pretreatment under He flow at 773 K for 60 min, adsorption of NH3 at 373 K for 90 min, and desorption of the weakly adsorbed NH3 under He flow at 373 K for 30 min, TPD profiles were recorded under a He flow between 373 and 1073 K (heating rate, 10 K min-1). CO chemisorption experiment for the determination of Pt particle size was carried out by using the same BELCAT apparatus. Before adsorption of CO, the catalysts were pretreated in He for 35 min and in O2 for 15 min and then reduced for 30 min in a 5% H2/Ar gas flow of 50 mL/min and in He for 15 min at 673 K in a reaction chamber. After this pretreatment,
*To whom correspondence should be addressed. E-mail: kazu-murata@ aist.go.jp. (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044– 4098. (2) Corma, A.; Huber, G. W.; Sauvanaud, L.; O’Connor, P. J. Catal. 2007, 247 (2), 307–327. (3) Stumborg, M.; Wong, A.; Hogan, E. Bioresour. Technol. 1996, 56 (1), 13–18. r 2010 American Chemical Society
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: DOI:10.1021/ef901607t
Murata et al. Table 1. Catalyst Propertiesa Pt
Re
catalystb
catalystb BET surface area/m2 g-1
dispersion/ % (nm)e
lattice constant (Lc)/nm (d)f
dispersion/ % (nm)e
lattice constant (Lc)/nm (d)f
Pt/H-ZSM5 (23) Pt-20Re/H-ZSM-5c Pt-10Re/H-ZSM-5d
245 174 188
3.115 (36.4) 5.444 (20.8) 4.29 26.4)
11.06 (0.200) 14.17 (0.200) 8.29 (0.200)
0.202 (1367) 0.185 (751.6)
10.35 (0.211) 6.59 (0.210)
pore volume (cm3/g)
peak temp (K) of NH3-TPD (mmol)g
0.0475 0.0371
651.6 (0.812) 601.9 (0.814)
a The catalysts were calcined at 873 K for 5 h. b The Si/Al2 ratio is 23. The amount of Pt is 1 wt %. c The amount of Re was 20 wt %, which corresponds to Re/Al = 0.80 (molar ratio). d The amount of Re was 10 wt % (Re/Al = 0.40 (molar ratio)). e The Pt and Re dispersion were estimated by CO adsorption. The numbers in parentheses denote Pt and Re particle sizes. f Lattice constants (Lc) can be estimated by XRD measurements at 2θ = 43° (Re) and 2θ = 46.3° (Pt). The number in parentheses denote the interplanar spacing (d). g These values exhibit the temperature and adsorption amount of the second peak in NH3-TPD.
Figure 1. Transmission electron microscope (TEM) images of Pt/H-ZSM-5 and Pt-Re/H-ZSM-5 catalysts after hydrogenation.
acidity would be required for a one-pot hydrotreating of triglyceride than for sorbitol, because triglyceride is less reactive than sorbitol. So, we used zeolite families with a low Si/Al2 ratio as supports of Pt. Table 1 shows properties of 1 wt % Pt-based catalysts. The BET surface area of rhenium-modified-Pt/H-ZSM-5 catalyst is lower than that of Pt/H-ZSM-5. CO adsorption measurements of the catalysts make it possible to estimate Pt dispersions and particle sizes. As shown in Table 1, for 1 wt % Pt-20 wt % Re/ H-ZSM-5, the value of Pt dispersion is 5.44%, higher than that of Pt/H-ZSM-5 (3.11%), and the Pt particle size is 20.8 nm. The particle size of Re is estimated as roughly 1367 nm, due to Re content as high as 20 wt %. Figure 1a,b shows TEM data of 1 wt % Pt/H-ZSM-5 and 1 wt % Pt-20 wt % Re/H-ZSM-5 catalyst after reduction. For reunmodified Pt/H-ZSM-5, the Pt particle size of ∼57 nm could be consistent with that of the value from CO adsorption. For remodified Pt/H-ZSM-5, however, two kinds of images are observed: One is very huge particles of 200-400 nm, which would be reaggregation, and the other is 5-50 nm Pt particles, the particle sizes of which are not uniform. As shown in Table 1, both Pt particle sizes and Re aggregation from TEM measurements are roughly consistent with CO adsorption data, although these Pt particle sizes slightly vary. This variation would be, at least in part, due to the presence of a large amount of Re content of 20 wt %. Figure 2 shows the XRD patterns of the Pt-Re/H-ZSM-5 catalysts, one is after calcination (part a) and the other is after hydrogenation (part b). The calcined catalysts exhibit patterns characteristic of Re2O7 at 2θ = 23.3° as well as patterns of PtO2 at 2θ = 34.8° and 54.0°, while reduced catalysts show diffractions peaks corresponding to metallic rhenium (2θ = 43.0°) and platinum (2θ = 39.7°, 46.3°, and 67.3°), indicating that each metal would be independent on the zeolite surface with no alloy formation.
the samples were cooled down at 323 K under He gas flow and CO pulse measurements were carried out using a 5% CO/He gas flow of 50 mL/min. Finally, the particle size of Pt was determined from the CO pulse data. For comparison, a particle size of Re is estimated, based on the assumption that Pt and Re are independently present on the zeolite surface. Transmission electron microscope (TEM) images for determination of particles size were obtained on the TOPCON EM-002B operated at 120 kV. The catalysts were reduced by H2 pretreatment at 473 K for 5 h under 2 MPa bar. After this reduction, the TEM samples were deposited as a dry powder on a microgrid mounted on a copper grid. The catalyst (0.1 g) was introduced into a 100 cm3 autoclavetype reactor and pretreated by reduction with 2 MPa of H2 at 473 K for 5 h. After the reduction, 1 g of jatropha oil was introduced with 9 g of H2O and the autoclave was pressurized with 6.5 MPa of H2/N2 gas mixture (H2/N2 = 91/9 vol %). The reaction was carried out with vigorous stirring at 543 K for 12 h. After reaction, the gaseous and liquid products were collected and analyzed by offline flame gas chromatography-flame ionization detector (FID) and gas chromatography-thermal conductivity detector (TCD), which were equipped with a SE-30 column for FID and a Porapak Q and MS 5A columns for the TCD detector. TCD analysis was performed using an N2 internal standard, while for FID analysis, hexacosane(C26) was used as an internal standard. The each carbon selectivity is defined as the moles of carbon in each product divided by the total carbon in the feed. The amount of total carbon soluble in the water liquid phase was estimated by elemental analysis, and the amount of carbon deposit over the catalyst surface was analyzed by thermogravimetric analysis under air conditions.
Results and Discussion The Pt/SiO2-Al2O3 catalyst is active for an aqueous reforming of sorbitol into C1-C6 hydrocarbons.4 Stronger (4) Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Angew. Chem., Int. Ed. 2004, 43 (12), 1549–1551.
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Figure 2. XRD patterns of fresh (A) and reduced (B) Pt-Re/H-ZSM-5 catalysts (Pt, 1 wt %): (a) Pt/H-ZSM-5, (b) 20 wt % Re/H-ZSM-5 (Re/ Al = 0.80), (c) Pt-1 wt % Re/H-ZSM-5 (Re/Al = 0.04), (d) Pt-5 wt % Re/H-ZSM-5 (Re/Al = 0.20), (e) Pt-10 wt % Re/H-ZSM-5 (Re/Al = 0.40), (f) Pt-20 wt % Re/H-ZSM-5 (Re/Al = 0.80), (g) Pt-25 wt % Re/H-ZSM-5 (Re/Al = 1.0).
Figure 4. TPR plots of Pt-Re/H-ZSM-5; Re/Al = 0, 0.04, 0.20, 0.40, 0.80, and 1.0.
Figure 3. NH3-TPD profiles of Pt/H-ZSM-5 and Pt-Re/H-ZSM-5 (Pt, 1 wt %; Re, 20 wt %).
As shown in Table 1, lattice constants (Lc) of Pt are ∼814 nm, being in good agreement with values from CO adsorption and TEM. On the other hand, the Lc values of Re are 6-10 nm, which is different from those from TEM and CO adsorption. However, in general, the lattice constant is not the same as particle size. Probably, the size of Re aggregation is much larger than the value of the lattice constant. At this point, further study is required to elucidate the morphology of Pt and Re. Ammonia is usually used as a probe molecule to explore acid properties on the catalyst surface using temperatureprogrammed desorption. Thus, NH3-TPD measurements were carried out at 373-1063 K by using Pt/H-ZSM-5 and Pt-Re/H-ZSM-5. As shown in Figure 3, there are mainly two kinds of major peaks: The first peak is 373-550 K and the second peak is 550-900 K. The second peak, in particular, could be closely related to the surface acid properties.5 As shown in Table 1 (last column), the temperature of the second NH3-TPD peak for Pt-Re/H-ZSM-5 is lower than that for Pt/H-ZSM-5 (23) and the amounts of NH3 adsorption are almost the same. Thus, there could be a little difference between remodified and unmodified catalysts. Figure 4 shows the TPR profiles of 1 wt % Pt-xRe/HZSM-5 (23) (Re/Al ratio = 0, 0.04, 0.2, 0.4, 0.8, 1.0). The reduction profile of Pt-Re/H-ZSM-5 (23) shows two uniform reduction peaks at ∼436 K and 640 K. Intensities of both peaks increase with the increase in Re/Al ratio, indicating reduction of Re7þ to Re0,6 while Pt reduction peak (Pt4þ to
Figure 5. Gas chromatograph data of C15-C18 hydrocarbon products after hydrotreating of vegetable oil at 573 K.
Pt0) would not be clear, probably due to a small content of Pt of 1 wt %. As shown in Figure 3, the addition of Re to Pt/HZSM-5 (23) caused the Re reduction peak at 436 K to affect the position of the maximum and, at a Re/Al ratio = 0.2, the position is shifted to 444 K. In order to investigate the hydrotreating activity of the Ptbased catalyst with different supports, hydotreating reactions of vegetable oil were carried out over 1 wt % Pt/support catalyst at 573 K for 12 h under 6.5 MPa of H2/N2 (85/15, vol %). Figure 5 shows the GC-FID charts of liquid products from the hydrotreatment of vegetable oil over various catalysts. The main products were n-C18H38, n-C17H36, n-C16H34, and n-C15H32 over each catalyst. Small amounts of C5-C14 hydrocarbons were also detected by GC. As shown in Table 2, the conversion into hydrocarbons, in which hydrocarbons and CO/CO2 are included, was above between 5.33% and 100%, depending on the support, where metal oxides such as H-ZSM-5, H-MOR, γ-Al2O3, USY, Beta, and SiO2 were used. Under these employed conditions, the C10-C20 yield was in the order USY (6.3) > USY (5.1) >
(5) Matsuhashi, H.; Arata, K. Phys. Chem. Chem. Phys. 2004, 6, 2529–2533. (6) Simonetti, D. A.; Kunkes, E. L.; Dumesic, J. A. J. Catal. 2007, 247 (2), 298–306.
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Table 2. Catalytic Conversion of Vegetable Oil into Gaseous Hydrocarbonsa carbon selectivity/%d
run no.
catalyst supportb
conversion into hydro carbons/%c
C10-C20 yield/%
CH4
C2-C4
C5-C9
C10-C20
C21þ
COx
1 2 3 4 5 6 7 8 9 10 11
SiO2 SiO2-Al2O3 H-ZSM5 (23) H-ZSM-5 (30) USY (6.3) USY(5.1) Beta (5.1) Y(5.1) H-MOR(18.3) FER(20) L (6.0)
13.2 9.53 83.8 65.0 100 83.9 37.0 52.1 7.00 59.7 5.33
2.42 5.63 67.6 43.1 90.0 75.4 24.9 29.6 2.27 51.7 1.00
11.6 8.76 2.53 6.02 1.18 1.18 4.52 1.73 12.9 3.68 15.7
58.5 20.1 5.15 9.98 2.64 3.07 4.54 1.71 46.8 5.07 35.9
21.0 0 7.13 12.0 0.20 0.07 18.9 0.20 0.22 0.19 0.51
1.83 59.1 80.7 66.3 90.0 89.9 67.4 56.8 32.4 86.7 18.7
5.03 0 1.40 1.49 3.00 1.78 3.38 33.4 3.20 1.26 7.69
2.02 12.0 3.06 4.20 2.95 4.03 1.28 6.11 4.46 3.14 21.5
a Reaction conditions: 1 wt % Pt/support 1.0 g, vegetable oil 1 g, H2O 9 g, T = 573 K, 12 h, H2/N2 = 85/15 (vol %), 6.5 MPa. Pretreatment: reduction at 473 K under 2 MPa of hydrogen for 4 h. b The numbers in parentheses denote Si/Al2 ratio. c Conversion to hydrocarbon = [(C atoms in gas-phase product)/(total C atoms introduced)] 100. d Carbon selectivity = (moles alkane number of carbon atoms in alkane)/(total moles of carbon atoms in alkane products) 100.
Table 3. Catalytic Conversion of Jatropha Oil into Hydrocarbonsa carbon selectivity/%i jat/cat ratiob 1 1 1 2c 10d
catalyst e
f
Pt /H-ZSM-5 (23) Pt/USY (6.3)f Pt/Tube g Pte/ H-ZSM-5 (23)f Pte/H-ZSM-5 (23)f
conversion into hydrocarbons /%h
CH4
C2-C4
C5-C9
C10-C13
C15
C16
C17
C18
C19þ
COx
103.4 31.2 13.6 21.6 14.2
1.74 1.37 5.43 5.51 1.23
5.98 4.19 0 25.3 38.8
7.13 0 0 0.64 10.9
3.48 1.22 12.6 0.74 3.61
0.47 4.88 19.2 3.67 3.23
11.7 6.17 0.47 9.66 1.72
19.2 19.3 1.66 5.03 0.83
47.6 37.8 2.40 44.5 10.2
1.74 19.1 58.2 1.16 26.8
0.96 5.97 0 3.75 2.69
a Reaction conditions: catalyst 1 g, jatropha oil 1 g, H2O 9 g, T = 543 K, 12 h, H2/N2 = 85/15 (vol %), 6.5 MPa. Pretreatment: reduction at 473 K under 2 MPa of hydrogen for 4 h. b Weight ratio. c Catalyst: 0.2 g. d Catalyst: 1 g. e Pt content is 1 wt %. f Si/Al2 ratio. g Catalyst 0.5 g/jatropha oil 0.5 g. h Conversion into hydrocarbon = [(C atoms in hydrocarbons)/(total C atoms introduced)] 100. i Carbon selectivity = (moles hydrocarbon number of carbon atoms in hydrocarbon)/(total moles of carbon atoms in hydrocarbons) 100.
selectivity to C15-C18 hydrocarbons of 79.0%. In this case, the conversion of 103.4%, which is a little larger than the total C atoms introduced, would be errors in GC calculations, because the conversion is a sum of many hydrocarbon products. The conversions were in the order: Pt/H-ZSM-5 (23) > Pt/USY (6.3) > Pt/Tube, indicating that the strength of the Br€ onsted acid could be a key factor for hydrotreating activity. Catalytic activity into hydrocarbons was found to be greatly affected by the jatropha oil/catalyst weight ratio (abbreviated as jat/cat ratio). At the jat/cat weight ratios of 2 and 10, the conversions into hydrocarbons were only 21.6% and 14.2%. In these cases, white solid products of hydrogenated triglycerides (see Figure 9) were detected, ascertained by GC analysis. This indicates that at a high jat/cat ratio, unsaturated triglycerides would only convert to saturated structure without any decarboxylation (eq 1), decarbonylation (eq 2), and hydrodeoxygenation (eq 3).9 R-CH2 -COOH f R-CH3 þ CO2 ð1Þ
H-ZSM-5 (23) > FER (20) > H-ZSM5 (30) > Y (5.1) > Beta (5.1) > SiO2-Al2O3 > H-MOR (18.3) ≈ SiO2 > L (6.0), where the number in parentheses denotes the Si/Al2 ratio. In Pt/USY (Si/Al2 = 6.3), in particular, conversion into hydrocarbons was 100% with selectivity to C10-C20 of 90.0%. Other products were lower hydrocarbons (C1-C9) and COx with small amounts of C21þ hydrocarbons. In this case, free fatty acids (FFAs) were not detected as byproduct.7 In other supports, besides alkanes products, the rest of products may be accounted for by organic carbons present in the aqueous phase due to oxygenates formation as well as carbon deposit over the catalyst. Thus, Pt/zeolite catalysts, which could be effective for hydrocracking of glycerol into propane,8 were found to be active for hydrotreating of triglyceride into C10-C20 hydrocarbons, mainly C15-C18. Typically, hydotreating reactions of jatropha oil (oil/cat weight ratio = 1) were carried out over 1 wt % Pt/support at 543 K for 12 h under 6.5 MPa of H2/N2 (85/15, vol %), where H-ZSM-5 (23), USY (6.3), and carbon-nanotube (Tube) were used as supports. Table 3 summarizes a detail of hydrocarbons and other gaseous products. Selectivities to liquid alkane products were similar to those from vegetable oil. Other byproducts were gaseous hydrocarbons (C1-C4) and CO2/ CO with small amounts of C19þ. At the temperature as low as 543 K, Pt/H-ZSM-5 (Si/Al2 = 23) was found to be more active than Pt/USY (6.3) for hydrotreating of jatropha oil and conversion into hydrocarbons was approximately 100% with
R-CH2-COOH þ H2 f R-CH3 þ CO þ H2 O
ð2Þ
R-CH2-COOH þ 3H2 f R-CH2 - CH3 þ 2H2 O
ð3Þ
Thus, from a practical point of view, a modified catalyst active even at the jat/cat weight ratios of 10 has to be explored. In order to improve the Pt/H-ZSM-5 catalyst, rheniummodified catalysts were prepared, because Pt-Re-based catalysts such as Pt-Re/Al2O3 are practically effective for naphthareforming and other industrialized processes.10 Figure 6 shows
(7) Berchmans, H. J.; Hirata, S. Bioresour. Technol. 2008, 99 (6), 1716–1721. (8) Murata, K.; Takahara, I.; Inaba, M. React. Kinet. Catal. Lett. 2008, 93 (1), 59–66.
(9) Huber, G. W.; O’Connor, P.; Corma, A. Appl. Catal., A 2007, 329 (1), 120–129.
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Figure 6. Effect of weight ratio of jatropha oil to catalyst, conditions: catalyst, Pt-Re/H-ZSM-5 (Re/Al = 0, 0.04, 0.40, 0.80); jatropha oil 1 g; H2O 9 g; T=543 K; 12 h; H2/N2 =91/9 (vol %); 6.0 MPa. Pretreatment: reduction at 473 K under 2 MPa of hydrogen for 4 h.
Figure 8. Effect of metallic component: catalyst, metal-Re/HZSM-5 (metal, 1 wt %; Re/Al = 0.80); jat/cat weight ratio = 10; other conditions, see Figure 6.
Figure 7. Effect of Re/Al molar ratio: catalyst, Pt-Re/H-ZSM-5 (Re/Al = 0, 0.04, 0.40, 0.80, 1.0); jat/cat weight ratio = 10; other conditions, see Figure 6. Figure 9. Postulated reaction pathway.
the effect of the oil/catalyst weight ratio on conversion into hydrocarbons at different Re/Al ratios. At Re/Al = 0.04 molar ratio (1 wt % Re), the conversion into hydrocarbons was lower than that of Pt/H-ZSM-5 over the range of the jat/cat ratio of 1-10. At Re/Al = 0.4 (10 wt % Re), catalyst activity was much more improved than the unmodified catalyst. At Re/Al = 0.8 (20 wt % Re), the conversion into hydrocarbons was found to be constant at ∼80% at any jat/ catlyst ratio employed. Total carbons in the aqueous phase, estimated by elemental analysis of carbon, was 3.8%, due to the small amounts of oxygenate formation such as glycerol and 1,2-propylene glycol, while saturated fatty acids were not detected. Carbon deposit over the catalyst was approximately 10%. Thus, the sum of these three kinds of carbons could account for ∼94% of the substrate reacted. Figure 7 shows the effect of the Re/Al molar ratio on the conversion into hydrocarbons and selectivities at a jat/cat weight ratio of 10. The conversion into hydrocarbons increased with the increase in the Re/Al ratio from 0.04 to 0.8, and it decreased with a further increase up to a Re/Al ratio of 1.0. The similar trends are true for selectivity to C18 hydrocarbons: The selectivity increased from 10.2% to 68.8% with increasing up to a Re/Al ratio of 0.8 and thereafter decreased
to 59.0%. The selectivities to C15-C17 hydrocarbons were also analogous. Thus, the highest conversion into hydrocarbons was achieved using 1 wt % Pt-20 wt % Re (Re/ Al = 0.8)/H-ZSM-5 (23) even at a high jat/cat weight ratio of 10. Figure 8 summarizes the effect of the metallic component on the hydrocarbon conversion and product selectivities, where the Jat/catlyst weight ratio was 10 and the metal content was 1 wt %. Under these conditions, Pd-Re/H-ZSM-5 catalyst was also found to be effective, and the total selectivity to C18 and C17 hydrocarbons was 89.6%, much higher than Pt-based catalyst (69.8%), indicating that Pd-based catalyst could be active for hydrodeoxygenation of carboxyl group (eq 3) as well as decarboxylation (eq 1) and decarbonylation (eq 2) with a poor C-C bond cleavage ability. On the contrary, the Ru-based catalyst possesses excellent C-C bond cleavage ability, which could lead to the increased formations of the C15 anf C16 hydrocarbons and CH4. The conversion into hydrocarbons is in the order: Pt-Re-Z > Pd-Re-Z > Re-Z > Ru-Re-Z > Co-Re-Z > Ni-Re-Z > Pt-Z, where Z denotes H-ZSM-5. Thus, Pt or Pd-modified Re/H-ZSM-5 catalysts are excellent candidates for triglyceride conversion. As already mentioned, Tables 2 and 3 show the product selectivities for n-C15, n-C16, n-C17, n-C18, CO, CO2, and other hydrocarbons. Octadecane is the most abundant
(10) D’Ippolito, S. A.; Vera, C. R.; Epron, F.; Samoila, P.; Especel, C.; Marecot, P.; Gutierrez, L. B.; Pieck, C. L. Ind. Eng. Chem. Res. 2009, 48, 3771–3778.
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liquid alkane, which is produced from the hydrodeoxygenation of the triglycerides. The yield of C18 alkane goes through a maximum at a Re/Al molar ratio of 0.8. A reaction pathway for conversion of triglycerides into alkanes is shown in Figure 9. In the first step of this reaction pathway, the CdC double bonds in the triglyceride are hydrogenated on the Pt-Re active sites and then broken down into various intermediates which could be monoglycerides, diglycerides, carboxylic acids, and waxes. These intermediates are then deoxygenated on Pt-Re sites into C15-C18 alkanes with concomitant formations of CO, CO2, and H2O by, probably, three different pathways: decarboxylation, decarbonylation, and hydrodexogenation, as mentioned above (eqs 1-3).9 Successive hydrocracking produced light alkanes (C1-C14) on acid sites of H-ZSM-5. When rhenium-unmodified Pt/H-ZSM-5 was used at a jat/cat weight ratio of 10, hydrogenated triglycerides waxes were formed predominantly over liquid alkanes, indicating that Pt/HZSM-5 catalysts without Re do not have a sufficient deoxygenation ability.
Conclusions Pt/H-ZSM5 catalysts, which were active for glycerol reforming into propane, were found to be active for hydrotreating of triglycerides such as vegetable oil and jatropha oil to form C15-C18 hydrocarbons directly. For vegetable oil on Pt/USY (6.3) at 573 K, the highest conversion into hydrocarbons was 100%, with selectivity to C10-C20 alkanes of 90%. Other major products were C1-C10 hydrocarbons and CO2. For hydrotreatment of jatropha oil at 543 K, where the jat/cat weight ratio is 1, the highest conversion was achieved on Pt/H-ZSM-5 (23) with the C15-C18 alkane selectivities is 79%. However, at high jat/cat ratios of 2 and 10, the C15-C18 alkane yields fell to 13.5 and 2.7%. Rhenium-modified Pt/H-ZSM-5 catalysts were found to be much more effective for hydrotreating jatropha oil even at a high jat/cat ratio of 10, and 80% conversion and 70% C18 selectivity were achived. The reaction pathway involves hydrogenation of the CdC bonds of the these triglycerides followed by mainly C15-C18 alkane production through hydrodeoxygenation with decarbonylation and decarboxylation.
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