Effect of Metal-Acid Balance on Hydroprocessed Renewable Jet Fuel

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2018, 57, 1429−1440

Effect of Metal−Acid Balance on Hydroprocessed Renewable Jet Fuel Synthesis from Hydrocracking and Hydroisomerization of Biohydrogenated Diesel over Pt-Supported Catalysts Tepin Hengsawad,† Chayasari Srimingkwanchai,† Suchada Butnark,‡ Daniel E. Resasco,§ and Siriporn Jongpatiwut*,†,∥ †

The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand PTT Research and Technology Institute, PTT Public Company Limited, Ayutthaya 13170, Thailand § School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States ∥ Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand ‡

ABSTRACT: The development of Pt-supported catalysts for a selective hydrocracking and hydroisomerization of biohydrogenated diesel (BHD, n-C15−C18) to hydroprocessed renewable jet (HRJ) fuel (C9−C14) was investigated. The different acidic supports (i.e., HY zeolite with SiO2/Al2O3 ratios of 5.5 and 100 and amorphous silica−alumina supports) loaded with 0.5 wt % Pt content were prepared and tested for the catalytic conversion of BHD to HRJ fuel. The Pt supported on HY zeolite with SiO2/Al2O3 ratio of 100 denoted as Pt/HY(100) catalyst exhibited the highest jet fuel yield with high branched isomers due to its good balance between metal and acid function. The effects of the reaction temperature, reaction pressure, and liquid hourly space velocity (LHSV) on the hydrocracking and hydroisomerization performance were studied over Pt/HY(100) catalyst. The jet fuel yield was obtained at maximum value of 33 wt % at 310 °C, 450 psig, LHSV of 1.0 h−1, and H2/BHD molar ratio of 30. A stability test was also conducted over Pt/HY(100) catalyst for 160 h. No significant change in the catalytic activity and selectivity during the test was observed, indicating the high stability of the Pt/HY(100) catalyst.

1. INTRODUCTION The world consumption of conventional jet fuel is approximately 1.5−1.7 billion barrels per year, accounting for 10% of global transportation energy.1,2 According to aviation industry expansion in recent years, resulting from the growth in the tourism sector, the demand for jet fuel has been increasing and is forecast to gradually increase for the next 30 years, leading to more greenhouse gas (GHG) emissions.3 The International Air Transport Association (IATA) aims to reduce carbon emissions on a voluntary basis by 50% by 2050 through the use of alternative fuels.4 These have attracted increased attention for developing technologies to reduce GHG emissions, such as improvement of air traffic management, use of new technologies, and use of new aircraft, but these will only slow the emission process.5 In order to actually cut down the amount of GHG emission and provide a long-term sustainable alternative to petroleum jet fuel, the development of green aviation biofuels has been an important part of the aviation industry’s future.5−8 Jet fuels must meet very stringent international specifications (i.e., freezing point, viscosity, etc.), which makes it much more difficult to develop an alternative fuel for aviation than for automobile applications.9 Currently, several bioenergy conversion technologies from biomass to © 2018 American Chemical Society

hydrocarbons have been explored and developed, such as catalytic hydroprocessing of vegetable and animal oils,7,9−16 Fischer−Tropsch synthesis using biomass-derived syngas,6,17−20 and catalytic transformation of sugars to jet fuel.21−23 Among these technologies, the hydroprocessing comprising hydrogenation, deoxygenation, hydroisomerization, and hydrocracking is particularly promising. This process is at a relative high maturity level, commercially available, and recently used to produce jet fuel for military flights.1,21 Hydroprocessed renewable jet (HRJ) fuel can be used in commercial and military aviation fuels up to a 50/50 blend of HRJ with petroleum-derived jet fuels (Jet A-1, Jet A, and JP-8)6,7,17 because of its advantages, such as higher cetane number, lower aromatic content, lower sulfur content, and potentially lower GHG emissions.1,21 Triglycerides in biomass are hydrogenated and broken down into free fatty acids (FFAs). The fatty acid products are sent to deoxygenation steps, either through decarboxylation−decarbonylation route or hydrodeoxygenation Received: Revised: Accepted: Published: 1429

November 14, 2017 January 9, 2018 January 16, 2018 January 16, 2018 DOI: 10.1021/acs.iecr.7b04711 Ind. Eng. Chem. Res. 2018, 57, 1429−1440

Article

Industrial & Engineering Chemistry Research

Figure 1. Classical bifunctional pathways of hydroisomerization and hydrocracking of an n-alkane on Pt-supported catalysts.

sites. In the other ways, the carbenium ions can be produced via addition of protons to alkanes on Brønsted acid sites and then dehydrogenation of carbonium ions on metal sites. The formation rate of carbenium ions via path A is much faster than that via path B. On Brønsted acid sites, alkenes and alkylcarbenium can undergo competitive adsorption−desorption. Alkylcarbenium ions can either go through skeletal rearrangements or undergo cracking through β-scission. It is well-known that if the rate-controlling elementary step is the rearrangement of carbenium ions on acid sites, the formed isomerized alkenes, iso-CiH2i, can be released and diffuse to metal sites where they are hydrogenated to isomerized alkanes, n-CiH2i+2.29,36,41,42 Therefore, the formation of isomerized products can be considered as an indirect measurement of the acid function activity. In the case of cracking by β-scission reactions, the fragments are a smaller alkylcarbenium ion, CjH2j+1+, and an alkene, C(i−j)H2(i−j), which is immediately hydrogenated. At Brønsted acid sites, monobranched isomers could convert to dibranched isomers in consecutive reactions, and then dibranched isomers could convert to tribranched isomers. Therefore, bifunctional catalysts for hydrocracking and hydroisomerization of BHD require an appropriate combination of metallic sites for hydrogenation−dehydrogenation and acidic sites of the support for isomerization−cracking for high jet fuel yield with good cold flow properties. In this work, the performances on the hydrocracking and hydroisomerization activity and the selectivity to jet fuel with branched isomer content (C9−C14) and selectivity to isomerized diesel (i-C15−C18) were investigated over Pt supported on different acidic supports, such as HY zeolite and amorphous silica−alumina. The performance of the hydrocracking and hydroisomerization of BHD to produce HRJ fuel depends on many factors that affect their activity, selectivity, and stability. Thus, the influence of metal−acid balance, acidity of supports, reaction temperature, reaction pressure, contact time, and long-term experiments were also studied.

route to remove oxygen content in the form of CO, CO2, or H2O. Subsequently, the n-alkanes in the diesel range from C15 to C18, called biohydrogenated diesel (BHD), are produced, which had poorer cold flow properties and lubricity compared to biodiesel.24 In order to improve cold flow properties, the BHD should have high branching isomers obtained from hydroisomerization reaction. To meet the jet fuel specification with good cold flow properties, the hydrocracking and hydroisomerization of BHD over bifunctional catalysts under elevated temperature and pressure are required to produce HRJ fuel product with carbon chains ranging from C9 to C14.1 The hydrocracking and hydroisomerization of long-chain paraffins have received increasing attention in the last two decades. Many studies have been conducted to investigate reaction mechanisms, kinetics, and the nature of bifunctional catalysts by using model compounds (e.g., n-heptane, n-octane, n-decane, n-hexadecane, n-heptadecane, and n-octadecane).4,10,25−33 However, there is less research investigating the hydrocracking and hydroisomererization of real feedstocks from industry. Hence, in this work, the BHD, composed of hydrocarbons in the range of C15 to C18, obtained from a local hydroprocessing plant utilizing jatropha oils as feedstocks, was used to investigate the reaction mechanism and develop a bifunctional catalyst giving a high jet fuel yield under mild conditions. Bifunctional catalysts for this process are composed of metal sites for hydrogenation−dehydrogenation function and acid sites for isomerization−cracking function. Various metals (e.g., Pt, Pd, Ni, Mo) and different acidic supports, such as zeolites, SAPO, sulfated zirconia, and amorphous silica− alumina, have been employed.19,29,34,35 Figure 1 illustrates the generalized reaction pathways of hydroisomerization and hydrocracking of BHD (n-C15−C18) over bifunctional catalysts.31,36−40 The reactant as n-alkanes, nCiH2i+2, is dehydrogenated on metal sites to the mixture of nalkenes, n-CiH2i, and followed by desorption from metal sites and diffusion to Brønsted acid sites. The formation of alkylcarbenium ions, n-CiH2i+1+, as key intermediates are generated through protonation of n-alkenes on Brønsted acid 1430

DOI: 10.1021/acs.iecr.7b04711 Ind. Eng. Chem. Res. 2018, 57, 1429−1440

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Table 1. Properties of Biohydrogenated Diesel Derived from Jatropha Oils (Jatropha-BHD) Compared to Specification of Conventional Diesel Limits property

unit

paraffinic diesel limit (EN 15940)

EU’s conventional diesel limit (EN 590)

Thailand’s limit (EN 14214)

Jatropha-BHD

density at 15 °C viscosity at 40 °C color cetane index flash point pour point lower heating value

kg/m3 mm2/s − − °C °C MJ/kg

765−800 2.0−4.5 − − >55 − −

800−845 2.0−4.5 − >46 >55 − −

810−870 1.8−4.1 red >50 (2012) >52 Pt/HY(100) > Pt/HY(5.5), indicating that the Pt/ASA had higher active metal site densities (higher capability for hydrogenation and dehydrogenation) than the Pt/HY(100) and Pt/HY(5.5) catalysts, respectively. Figure 4 shows the H2-TPR profiles for the calcined catalysts with different acidic supports. It can be observed that Pt-

Figure 3. NH3-TPD profiles of the unloaded HY(5.5), HY(100), and ASA supports and the corresponding Pt-supported catalysts.

Generally, the decomposition of isopropylamine (IPA) to ammonia (NH3) and propylene (C3H6) over Brønsted acid sites via Hofmann elimination reaction can be used to estimate the quantity of Brønsted acid sites.56 The results revealed that the number of Brønsted acid sites increased remarkably in the sequence ASA < HY(100) < HY(5.5). After the Pt impregnation on the supports, the Pt-supported catalysts had an acid feature similar to the corresponding supports, but the acid strength slightly decreased. However, the number of Brønsted acid sites significantly decreased, while the number of Lewis acid sites increased, leading to an increase in the total acidity of the Pt-supported catalysts. This was because the Pt species ([Pt(NH3)4)]2+ ions) had high affinity for strong acid sites and thus covered the Brønsted acid sites more than Lewis acid sites.19,43,51 In addition, the coordinately unsaturated platinum atoms could provide new Lewis acid centers, compensating for the original Lewis acid sites covered by metal.51,57 The Pt dispersion of the catalysts determined from H2 chemisorption are listed in Table 3. For the Pt-supported catalysts with the same Pt content (0.5 wt %), the Pt dispersion values were relatively dependent on the type of supports with different Brønsted acidity.19 The results showed that the Pt dispersion increased remarkably in the order of Pt/ASA < Pt/ HY(100) < Pt/HY(5.5), consistent with the order of the amount of Brønsted acid sites. The nPt/nA ratio, defined as the ratio of the total number of exposed Pt atoms (on the surface of

Figure 4. TPR profiles of Pt-supported catalysts.

supported catalysts exhibited two H2 consumption peaks, corresponding to the reduction of Pt oxides (Pt2+) to the metallic state (Pt0). The first reduction peaks at 100−300 °C were attributed to the reduction of large particle size of PtO species located on the external surface which interacted relatively weakly with the support.51,60 The second reduction peaks at 400−580 °C indicated the corresponding PtO species were more difficult to reduce because the highly dispersed PtO (small particle size) interacted strongly with the acidic supports and most possibly located in the internal pore.51,61 The H2 consumption of the second reduction peaks decreased in the following order: Pt/HY(5.5) > Pt/HY(100) > Pt/ASA, suggesting that the Pt/HY(5.5) had higher well-dispersed Pt (smaller Pt particles) and stronger metal−support interaction than Pt/HY(100) and Pt/ASA, respectively. 3.2. Catalytic Performance in Hydrocracking and Hydroisomerization of BHD over Pt-Supported Cata1433


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Industrial & Engineering Chemistry Research lysts. Table 4 shows the hydrocracking and hydroisomerization behavior of BHD (n-C15−C18) over the unloaded HY(5.5), Table 4. Catalytic Performance in Hydrocracking and Hydroisomerization of BHD over the Monofunctional Acid Catalysts and Bifunctional Pt-Supported Catalystsa selectivity (wt %) catalyst

conversion (%)

cracked products

isomerized products

iso/normal ratio in cracked products

HY(5.5) Pt/HY(5.5) HY(100) Pt/HY(100) ASA Pt/ASA

9.6 93.2 5.1 90.6 13.3 69.9

72.2 75.6 75.7 62.8 87.7 52.0

27.8 24.4 24.3 37.2 12.3 48.0

1.14 2.03 0.54 2.34 0.52 0.90

Figure 5. Scheme of hydrocracking and hydroisomerization of BHD over Pt-supported catalysts.

Reaction conditions: T = 310 °C, P = 500 psig, LHSV= 1.0 h−1, H2/ BHD = 30, and TOS = 8 h.

a

balance between metal and acid functions (nPt/nA) had an important influence on the isomerization selectivity.33,41,51,59,70 At the higher nPt/nA value of the Pt/ASA catalyst with the higher hydrogenation−dehydrogenation ability (Pt sites) or lower isomerization/cracking ability (Brønsted acid sites), each carbenium ion formed at Pt dehydrogenating site came into contact with very few Brønsted acid sites between two Pt sites (as seen in Figure 5, path I). Thus, the hydrogenation and desorption of isomerized diesel fuel (i-C15−C18) were favored, and the consecutive cracking reaction of the carbenium ions on the acid sites was suppressed, resulting in higher isomerized products.25,29,31,36 Another difference between these Ptsupported catalysts was the degree of branching in the cracked products influenced by the balance between metal and acid functions (nPt/nA). As seen in Table 4, the ratio between branched and linear hydrocarbons in cracked products reached values of 2.34 and 2.03 for the Pt/HY(100) and Pt/HY(5.5) catalysts, respectively, and was three times lower for the Pt/ ASA catalyst. According to the bifunctional mechanism, as the weaker hydrogenation−dehydrogenation function, carbenium ions went through successive skeletal rearrangements and spent longer time on the acid sites before being hydrogenated, leading to an increase in the degree of branching. However, if the Brønsted acid sites were excessive, the secondary cracking would increase, resulting in an increase in light gases (C1−C4) and gasoline (C5−C8). This indicated that the Pt/HY(100) catalyst possessed the well-balanced metal and acid function, resulting in high branching in cracked products, high selectivity to jet fuel (C9−C14), and low selectivity to gasoline (C5−C8) and light gases (C1−C4) as compared to other Pt-supported catalysts. The catalytic behavior could also be described by considering the carbon number distribution of the cracked products. If the catalysts followed the bifunctional mechanism of ideal hydrocracking catalyst (i.e., Pt/CaY), the carbon number distribution would be symmetrically centered at C8 (hydrocracking of C16); no C1, C2, C14, or C15 should be present;38 and only primary cracking products (C4−C12) as branched and linear fragments were obtained, with a content of branched isomers amounting to ca. 50%.71 Figure 6 displays the cracked product distribution per carbon atom for Pt-supported catalysts. For all Pt-supported catalysts, no C1 and C2 were formed; C3 was formed only as a linear fragment; and each higher cracked product between C4 and C14 was formed as two types, which were linear and branched fragments in line with the cracking

HY(100), and ASA and the corresponding Pt-supported catalysts carried out at 310 °C, 500 psig, and H2/BHD ratio of 30. The main products of hydrocracking and hydroisomerization of BHD (n-C15−C18) over Pt-supported catalysts were (1) isomerized diesel fuel (i-C15−C18) as a consequence of the isomerization reaction controlled by the acid function and (2) cracked products consisting of normal and branched hydrocarbons in the range of light gases (C1− C4), gasoline (C5−C8), and jet fuel (C9−C14), resulting from cracking reaction partly controlled by the metal and acid functions. It was found that all bare acid support catalysts exhibited relatively low conversion of BHD. This could be explained by noting that the carbenium ions were slowly formed in the absence of platinum that had no hydrogenation− dehydrogenation function to facilitate the formation of carbenium ions (as followed path B in Figure 1). In contrast, the Pt-supported catalysts exhibited significantly high conversion of BHD which could be because the presence of Pt catalyzed the dehydrogenation of the alkanes to alkenes, thus rapidly transforming into carbenium ions. The carbenium ions were ready to form isomers via skeletal rearrangements before cracking on acid sites.62−64 Moreover, a possible explanation of platinum’s superior performance was its efficiency in hydrogen spillover phenomena, resulting in the generation of protonic acid sites.65−69 For the Pt-supported catalysts, the BHD conversion and selectivity to cracked products tended to decrease in the following order: Pt/HY(5.5) > Pt/HY(100) > Pt/ASA, consistent with the sequence of Brønsted acidity and Pt dispersion. This indicated that the activities of the catalysts mainly depended on the Brønsted acid sites and Pt dispersion. The larger number of Brønsted acid sites and higher Pt dispersion might cause closer interaction between acid and metal sites, and then more carbenium ions were rapidly generated. With the higher acid density, each carbenium ion came into contact with more acid sites between two Pt sites (as seen in Figure 5, path II), leading to more cracked products formation. As a result, the Pt/HY(5.5) catalyst with larger number of Brønsted acid sites and higher Pt dispersion showed higher activity and selectivity to cracked products than the other Pt-supported catalysts. On the other hand, the selectivity to isomerized products tended to increase in the order Pt/ HY(5.5) < Pt/HY(100) < Pt/ASA, and the balance between metal and acid functions (nPt/nA) showed the same sequence over these catalysts. According to the reaction pathways, the 1434


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Figure 7. Classification of β-scission reaction of alkylcarbenium ions.

Figure 6. Cracked products distribution per carbon number of Ptsupported catalysts: (a) Pt/HY(5.5), (b) Pt/HY(100), and (c) Pt/ ASA. Reaction conditions: 310 °C, 500 psig, LHSV of 1.0 h−1, H2/ BHD molar ratio of 30, and TOS of 8 h.

type A and type B β-scission. However, with increasing SiO2/ Al2O3 ratio (decreasing Brønsted acidity), the cracked products distribution per carbon atom of Pt/HY(100) catalyst was a flatter bell-shaped distribution and skewed toward the higher carbon number products, which was close to an ideal hydrocracking catalyst, implying good balance between metal and acid function.38,73,74 The vast majority of hydrocarbon products were in the range from C6 to C13, indicating that the secondary cracking of the primary fragments could be occurring slightly. In addition, a high degree of branching of cracked products was obtained, suggesting that the hydrocracking reaction tended to favor type A and type B β-scission. The cracked product distribution per carbon atom of Pt/ASA catalyst exhibited a lack of symmetry with a maximum at C5 and was skewed to the lower carbon number products, in which the formation of cracked products in the range of C4−C8 was much more pronounced because of the presence of secondary cracking. The amount of linear fragments was higher than the amount of branched fragments, implying that type C β-scission could occur to some extent.41 Figure 8 shows the products yield consisting of cracked products, i.e., light gases (C1−C4), gasoline (C5−C8), jet fuel (C9−C14) with both linear and branched hydrocarbons, and

reaction through β-scission (Figure 7). Type A β-scission required a tribranched carbenium ion and produced two branched fragments, whereas type B β-scission gave one branched and one linear fragment obtained from dibranched carbenium ion. Type C β-scission occurred on the monobranched ion, which gave two linear fragments. Type D βscission, which would start from dibranched carbenium ion to monobranched carbenium ion, probably did not play a role for hydrocracking because of the high energy content of the monobranched carbenium ions involved.38 Thus, it is worth noting that the hydrocracking reaction via type D β-scission did not take place for all Pt-supported catalysts, leading to no C1 and C2 formation.28,38 For the Pt/HY(5.5) catalyst, the cracked products distribution per carbon atom was similar to a bellshaped distribution with a maximum at C8 and skewed toward the lower carbon number products. The cracked products in the range of C5−C12 were remarkably formed as a consequence of the secondary cracking.30,72 The amount of branched fragments was higher than the amount of linear fragments, suggesting that cracking reaction was favored via

Figure 8. Products distribution in fuel range hydrocarbons, comprising light gases (C1−C4), gasoline (C5−C8), and jet fuel (C9−C14) over Pt-supported catalysts. Reaction conditions: 310 °C, 500 psig, LHSV of 1.0 h−1, H2/BHD molar ratio of 30, and TOS of 8 h. 1435


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Figure 10 shows the effect of temperature on the hydrocracking and hydroisomerization behavior of BHD over

isomerized diesel (i-C15−C18). As a result, the yields of jet fuel and gasoline tended to decrease in the following order: Pt/ HY(5.5) > Pt/HY(100) > Pt/ASA, and the yields of light gases slightly decreased in the sequence of Pt/ASA > Pt/HY(5.5) > Pt/HY(100). On the other hand, the yields of isomerized diesel significantly decreased in the order of Pt/HY(100) > Pt/ HY(5.5) > Pt/ASA. Thus, it is worth noting that the Pt/ HY(100) catalyst was the most suitable for hydrocracking and hydroisomerization of BHD, as it exhibited of high jet fuel yield (ca. 33 wt %) with large amount of branched isomers, leading to good cold flow properties (freezing point = −47 °C), while the yields of gasoline and light gases were relatively low. A high yield of isomerized diesel was also obtained (ca. 34 wt %). To determine the formation and the amount of carbonaceous deposits, the spent catalysts obtained after reaction (TOS of 8 h) were characterized by temperature-programmed oxidation (TPO). The TPO profiles and amount of coke deposits over the Pt-supported catalysts are illustrated in Figure 9 and listed

Figure 10. Effect of temperature on BHD conversion and products distribution in fuel range hydrocarbons over Pt/HY(100) catalyst. Reaction conditions: 500 psig, LHSV of 1.0 h−1, H2/BHD molar ratio of 30, and TOS of 8 h.

Pt/HY(100) catalyst under the same reaction conditions (H2 pressure = 500 psig, LHSV = 0.5 h−1, and H2/BHD molar ratio = 30). With increasing reaction temperature, the BHD conversion apparently increased up to 97% at 320 °C and the selectivity to total cracked products also increased from 10 to 86 wt %, but the selectivity to isomerized diesel drastically decreased from 90 to 14 wt %. In terms of cracked products distribution, when the temperature increased from 290 to 320 °C, the selectivity to light gases slightly increased from 0.1 to 3 wt %, while the selectivity to gasoline and jet fuel remarkably increased from 4 to 39 wt % and from 5 to 44 wt %, respectively. This behavior could be explained by the thermodynamic equilibrium of the hydroisomerization reaction. At lower temperature, the equilibrium reaction might shift to the side of isomerized diesel formation; on the other hand, at higher temperature, the isomerized alkylcarbenium ions might shift toward the consecutive cracking reaction.27,63 Additionally, the degree of branching in the cracked products (iso/normal ratio) increased from 1 to 2.3 by increasing the temperature from 290 to 310 °C and then decreased to 1.9 at 320 °C. In line with the work Weitkamp et al.,32,38 the proposed transformation of alkylcarbenium ions through β-scission reaction by the increase in iso/normal ratio at the higher temperature could be the consequence of more enhancements in the cracking rate through type A and type B β-scission reactions (Figure 7). However, as the temperature increased up to 320 °C, the secondary and primary carbenium ions were increasingly cracked through type C and type D β-scission reactions (Figure 7), leading to the decrease in iso/normal ratio. However, despite the highest selectivity to jet fuel (C9−C14) obtained at 320 °C, the highest selectivity to light gases (C1− C4) and gasoline (C5−C8) were also obtained. In order to suppress the excess of light hydrocarbon products and achieve the desired products, jet fuel and isomerized diesel, the reaction temperature of 310 °C was determined to be the optimum temperature for further investigation. Because the optimum reaction temperature for the high BHD conversion and selectivity to jet fuel was found at 310 °C, the effect of pressure was studied from 400 to 600 psig at this temperature. As depicted in Figure 11, with increasing hydrogen pressure, the BHD conversion was slightly decreased

Figure 9. TPO profiles of the spent catalysts after TOS of 8 h.

in Table 3, respectively. The TPO profiles of the unloaded acid catalysts gave the higher oxidation temperature than the bifunctional Pt-supported catalysts, indicating the larger amount of hard-to-remove carbon (polyaromatic compounds) in the absence of Pt. For the Pt supported on HY(5.5) and HY(100) catalysts, both TPO profiles showed the same trend of a pronounced a larger peak at 320 °C with a shoulder at 250 °C, indicating that the soluble coke was formed by rearrangement and condensation products and located on the metal, or metal and zeolite interface,75,76 and subsequently oxidized at low temperature (320 °C). In contrast, the TPO profile of the Pt/ASA catalyst exhibited a small peak at 260 °C, implying that a small amount of soluble coke formed in mesopores of ASA support could be completely removed at 260 °C.77 As shown in Table 3, the amount of coke tended to decrease in the following order: Pt/HY(5.5) > Pt/HY(100) > Pt/ASA, corresponding to the activity and selectivity to cracked products in hydrocracking and hydroisomerization of BHD. 3.3. Influence of Reaction Condition Parameters. To investigate the effect of reaction conditions (i.e., temperature, pressure, and liquid hourly space velocity) on the hydrocracking and hydroisomerization performance and to determine the optimal reaction conditions for achieving high jet fuel yield, the Pt/HY(100) catalyst was selected to use in a series of experiments under different operating conditions. 1436


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the selectivity to isomerized products increased. Similar results were also reported by Chica and Corma,80 when the hydroisomerization of n-C5, n-C6, and n-C7 over platinumloaded different zeolite supports was investigated. In terms of product distribution, it was found that the selectivity to jet fuel, gasoline, and light gases significantly increased as LHSV decreased. The highest degree of branching in cracked products (iso/normal ratio) was obtained at LHSV of 1 h−1. Therefore, LHSV of 1.0 h−1 was chosen for stability testing. 3.4. Stability Testing of Pt/HY(100) Catalyst. The catalytic stability testing over the Pt/HY(100) catalyst were conducted in a long-term operation of 160 h. The reaction was conducted under the optimal operation conditions of 310 °C, 450 psig, H2/BHD molar ratio of 30, and LHSV of 1.0 and 4.0 h−1. Figure 13 illustrates the BHD conversion and the products

Figure 11. Effect of pressure on BHD conversion and products distribution in fuel range hydrocarbons over Pt/HY(100) catalyst. Reaction conditions: 310 °C, LHSV of 1.0 h−1, H2/BHD molar ratio of 30, and TOS of 8 h.

from 95 to 84% and the selectivity to jet fuel, gasoline, and light gases was also decreased. In contrast, the increase of hydrogen pressure led to reduced selectivity to isomerized diesel due to a decrease in the hydroisomerization reaction rate.78 This behavior could be plausibly explained by considering the bifunctional mechanism, as shown in Figure 1. At high hydrogen pressure, the iso-alkenes (i-CiH2i+1+) could be rapidly hydrogenated to iso-alkanes (i-CiH2i+2) on the Pt sites, which could be due to a shorter residence time for rearrangement and cracking of alkylcarbenium ion on the acid sites, thus suppressing the cracking activity.4,19,79 Although the highest selectivity to jet fuel was obtained at 400 psig, the lowest degree of branching in cracked products (iso/normal ratio) was attained, leading to poor cold flow properties of the liquid hydrocarbon products. However, the choice of a high operating pressure offers disadvantages for industrial operation in terms of higher cost of equipment and pumping, compared with lowpressure processes. Therefore, the optimum reaction pressure at 450 psig was selected for further investigation. The effect of varying the LHSV from 0.5 to 2.0 h−1 on product distribution and conversion was investigated. The hydrocracking and hydroisomerization of BHD (n-C15−18) were carried out at 310 °C, 450 psig, and H2/BHD molar ratio of 30. As observed in Figure 12, with increasing LHSV, the BHD conversion and selectivity to cracked products decreased while

Figure 13. BHD conversion and the distribution of fuel range hydrocarbon products as a function of the time on stream (TOS) over Pt/HY(100) catalyst: (a) high conversion at LHSV of 1.0 h−1 and (b) low conversion at LHSV of 4.0 h−1. Reaction conditions: 310 °C, 450 psig, and H2/BHD molar ratio of 30.

distribution as a function of time on stream (TOS). These results in both high and low conversions show that the BHD conversion and selectivity to jet fuel did not significantly change over the 160 h of the experiments, indicating the high stability of the Pt/HY(100) catalyst. In terms of coke formation, the amount of coke deposited as a function of TOS was investigated. The amounts of coke after 1, 3, 8, and 160 h were 8.8, 9.1, 9.3, and 12.8%, respectively. It can be noted that the coke deposit rapidly formed at the first 1 h because of the high activity of the fresh catalyst. However, after 1 h, the

Figure 12. Effect of LHSV on BHD conversion and products distribution in fuel range hydrocarbons over Pt/HY(100) catalyst. Reaction conditions: 310 °C, 450 psig, H2/BHD molar ratio of 30, and TOS of 8 h. 1437


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amount of coke slightly increased, which correlated well with the catalytic activity.

4. CONCLUSIONS To produce the hydroprocessed renewable jet fuel with good cold flow properties, the hydrocracking and hydroisomerization of the biohydrogenated diesel derived from jatropha oils was performed over Pt-supported different acid supports (i.e., HY zeolite and amorphous silica−alumina). The Pt/HY(100) catalyst exhibited well-dispersed Pt particles, a large amount of Brønsted acid sites, and a good balance between metal and acid functions, leading to the high activity in hydrocracking and hydroisomerization of BHD and high yield of jet fuel consisting of high branched isomers, resulting in good cold flow properties (freezing point = −47 °C). In addition, the effects of temperature, pressure, and liquid hourly space velocity on the hydrocracking and hydroisomerization behavior over the Pt/ HY(100) catalyst were investigated. The maximum yield of jet fuel was 33 wt % achieved at 310 °C, 450 psig, and LHSV of 1.0 h−1. The Pt/HY(100) catalyst was found to be stable for hydrocracking and hydroisomerization of BHD during the reaction time of 160 h, which is suitable for commercial scale applications.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +66-2-218-4139. Fax: +66-2-218-4459. E-mail: siriporn. [email protected]. ORCID

Siriporn Jongpatiwut: 0000-0002-1289-7826 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the contributions and financial support of the following organizations: the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0262/2553); the Center of Excellence on Petrochemical and Materials Technology; and the Petroleum and Petrochemical College, Chulalongkorn University. Moreover, the authors thank PTT Public Company Limited, Thailand for partial research funding.

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Effect of Metal-Acid Balance on Hydroprocessed Renewable Jet Fuel

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