Research articleEffect of operating parameters on the selective catalytic deoxygenation of palm oil to produce renewable diesel over Ni supported on Al2O3, ZrO2 and SiO2 catalysts
Introduction
Green Diesel is a mixture of C15–C18 normal and isomer paraffins which has significant advantages over fossil petroleum Diesel and fatty acid methyl esters (FAME or biodiesel), such as better combustion and lower emissions. Its chemical resemblance to fossil petroleum Diesel allows its use in neat or blend form without requirements for engine modifications and since it does not contain oxygen, it is non-corrosive and more stable than oxygenated FAME. At the same time, Green Diesel is a renewable product of domestically available biomass sources [[1], [2], [3], [4]].
Green Diesel may be produced by a large variety of first (vegetable oils), second (lignocellulosic raw matter) or third (microalgae) generation biomass sources via a number of different technologies such as the hydroprocessing of vegetable oils and animal fats, the upgrading of bio-oils and biocrudes obtained from pyrolysis and hydrothermal liquefaction of lingnocellulosic and algal biomass, the catalytic upgrading of sugars and finally, through Fischer-Tropsch synthesis from the products of biomass gasification [[5], [6], [7], [8], [9], [10]]. Among these technologies, hydroprocessing is currently the most established, exploiting the same raw sources used for the production of FAME. Hydroprocessing involves the reaction of the triglycerides with hydrogen (H2) at about 280–450 °C and 10–50 bar leading to the production of paraffins in the C15–C18 range [11,12]. This is accomplished by an initial hydrogenation process that saturates the double bonds of the fatty acid chains of the triglycerides and then by subsequent processes of oxygen removal, which are known as deoxygenation (DO) reactions and may be further classified as reactions of hydrodeoxygenation (HDO), decarbonylation (deCO) and decarboxylation (deCO2). HDO removes oxygen in the form of H2O molecules while deCO and deCO2 remove oxygen as CO and CO2 molecules respectively, as it is shown in the following reaction scheme:where R is a saturated alkyl group and R΄ is an unsaturated alkyl group.
At the same time, depending on the catalytic system and reaction conditions (temperature, pressure, LHSV, oil/H2 feed ratio, oil composition), various side reactions such as isomerization, cyclization, ketonization, dimerization and cracking may also take place [5,11]. The gaseous products, such as H2O, CO and CO2, may further react in the gas phase with reactions such as methanation (Eqs. (4), (5)) and water-gas-shift (Eq. (6)).
(1), (2a), (2b), (3) may attain different rates due to a selective promotion of the HDO, deCO or deCO2 paths (the latter two reactions are also known as deCOx). An increase of the temperature up to a critical value benefits the conversion of oil and the selectivity to C15–C18 paraffins. Above the critical temperature however, both these metrics reduce due to the hydrocracking of the paraffins into lighter hydrocarbons [13,14]. High H2 pressures promote the HDO reaction pathway and increase conversion and selectivity [15,16]. In addition, unsaturated oil feedstocks require a greater consumption of H2 to saturate the double bonds and, consequently, increase the cost of the overall process. Hydrogen consumption is also higher in the order HDO > deCO > deCO2 and therefore, in terms of cost, catalysts which promote deCOx are preferred to those promoting HDO [17]. On the other hand, deCO2 and deCO reactions remove carbon atoms from the initial fatty acid chains in the form of CO and CO2 leading to a paraffin blend with fewer carbon atoms than the oil feedstock. This is important for the production of the C15–C18 paraffin blend since most fatty acids (palmitic, palmitoleic, stearic, oleic, linoleic, and linolenic) of vegetable oils, animal plants and microalgae have chains in the range of C16–C18. The removal of carbon atoms corresponds also into an energy loss, which reduces the heating value of the Green Diesel blend. In contrast, HDO reactions do not remove carbon and give paraffin blends of higher energy content [18,19].
The hydroprocessing of the triglycerides is currently accomplished mainly by four groups of catalysts: (a) bimetallic sulfide catalysts such as NiMoS2/Al2O3, CoMoS2/Al2O3, and NiWS2/Al2O3, (b) metal phosphide and carbide catalysts such as Ni2P, W2C and Mo2C, (c) noble metal catalysts, such as Pd, Pt, Rh and Ru on various supports, and (d) transition metal catalysts on various supports [3,19].
The bimetallic sulfided catalysts are known refinery catalysts for the removal of sulfur (hydrodesulfurization, HDS) and nitrogen (hydrodenitrogenation, HDN) from petroleum distillates. Due to the similarity of the HDS and HDN processes with the hydroprocessing of triglycerides, these catalysts have been used extensively, showing high conversions and high selectivity towards C15–C18 paraffins [[20], [21], [22], [23], [24], [25], [26], [27], [28]]. A drawback of the sulfided catalysts is the need for constant resulfurization, which tends to contaminate the end paraffin mixture with sulfur traces. Like sulfidation, nitration and phosphatization have also been observed to create active sites on the catalyst surface and to improve the activity for deoxygenation. Research on this type of catalysts is still insufficient, but some works have shown promising results for Mo2N/Al2O3 [29], Ni2P/SiO2 [30] and Ni2P/SBA-14 zeolite catalysts [31].
Noble metal catalysts have been studied as non-sulfided alternatives and exhibited high catalytic activity and promotion of the deCO2 and deCO paths [[32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]] however, their high cost makes their use on large scale applications prohibitive. Therefore, research focuses on the study of transition metals with Ni-based catalysts attracting interest due to their ability to adequately promote the deoxygenation of triglycerides, as discussed in several recent works and reviews [19,[45], [46], [47],[49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64]]. Ni catalysts have been based on a variety of supporting materials such as carbon [63], metal oxides [50,[56], [57], [58],60,61,63] and zeolites (HSZM-5, SAPO-11) [56,57].
In general, the production of Green Diesel through deoxygenation of triglycerides requires catalysts of mild acidity, since strong acidic catalysts are known to promote hydrocracking and are more suitable for lighter fuel fractions [19,51]. High acidity also correlates to increased carbon deposition on the surface of the catalyst, which leads to gradual deactivation. Kumar et al. [50] compared the deoxygenation of stearic acid with Ni (10 wt% loading) over γAl2O3, SiO2 and HSZM-5 zeolite supports and showed that, at 270 °C and 8 bar, conversion followed the order HZSM-5 > γ-Αl2O3 > SiO2. This was attributed to the high acidity of the HSZM-5 catalyst compared to the mild acidity of γAl2O3 and SiO2. They also observed that the value of rate constants increases with temperature as expected. They also found that similar to apparent activation energy (205.2 kJ/mol), high activation energy were observed for all rate constants. Specifically, the highest activation energy was observed for k4 and k5 indicating that formation of n-pentadecane and n-hexadecane dominated at higher temperatures. Peng et al. [40] compared the deoxygenation of palmitic acid with Ni (5 wt% loading) over γAl2O3, SiO2 and ZrO2 supports and showed that at 260 °C and 12 bar conversion followed the order ZrO2 (100%) > γ-Αl2O3 (51%) > SiO2 (41%) and selectivity towards n-C15H32 was ZrO2 (90%) > γ-Αl2O3 (74%) > SiO2 (59%). Again, this was attributed to the higher acidity of ZrO2.
The present work investigated the production of Green Diesel through the deoxygenation of palm oil over Ni catalysts supported on γ-Αl2O3, ZrO2 and SiO2, prepared through the wet impregnation method at a constant metal loading of 8 wt%. Although these are commonly used supports due to their mild acidity, this is the first time that their effect on the performance of Ni catalysts is comparatively assessed, at the same reaction conditions, for a continuous flow fixed bed reactor. For this reason, a comprehensive experimental study was carried out in order to examine the effects of temperature, pressure, LHSV and H2/oil feed ratio on catalytic activity during short (6 h) and long (20 h) time-on-stream experiments. The catalysts prior to reaction were exhaustively characterized by N2 adsorption/desorption, XRD, NH3-TPD, CO2-TPD, H2-TPD, H2-TPR, XPS and TEM in an effort to shed light on the specific properties that affect performance. Moreover, the spent catalysts were also examined in depth (TEM/HR-TEM and Raman) in order to determine the effect of sintering and carbon deposition on catalytic stability.
Section snippets
Reagents & feedstock
The chemical reagents used herein included palmitic acid (≥99%), linoleic acid (≥99%), stearic acid (≥95%), oleic acid (≥99%), tridecane (≥99%), cyclohexanone (≥99.5%) and a mixture of 37 fatty acid methyl esters (supelco, C4–C24 FAMEs); these were purchased from Sigma-Aldrich. Heptane (≥99%), dodecane (≥99%), cyclohexane (≥99.5%) and chloroform (≥99.8%) were purchased from Honeywell. A standard mixture of n-C8 to n-C18 alkanes (Boiling Point Calibration Sample Kit #2) was acquired from Agilent
Catalysts characterization
Fig. 1 shows the N2 adsorption-desorption at 77 K. The isotherms of the Ni/Al2O3 and Ni/SiO2 catalysts correspond to type III according to the IUPAC classification, typical for mesoporous materials and their BJH pore size distribution exhibits a bimodal pore size in the 10–75 nm range. The hysteresis loop, indicative of the pore structure, is of type H3 corroborating the presence of non-uniform size or shape. The Ni/ZrO2 catalyst shows a IV-type isotherm with a H4-type hysteresis, which is
Conclusions
Herein, the production of Green Diesel through the deoxygenation of palm oil over Ni catalysts supported on γ-Αl2O3, ZrO2 and SiO2. This is the first time that the effect that these supports have on the performance of Ni catalysts is comparatively assessed for a continuous flow fixed bed reactor and a comprehensive experimental study was carried out in order to examine the effects of temperature, pressure, LHSV and H2/oil feed ratio on catalytic activity during short (6 h) and long (20 h)
Declaration of competing interest
None.
Acknowledgments
KNP is grateful for the support of the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under the HFRI PhD Fellowship grant (GA. no. 359). MAG and NDC gratefully acknowledge that this researched was co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme “Human Resources Development, Education and Lifelong Learning” (MIS-5050170). SD is thankful for financial assistance
Credit author statement
Kyriakos N. Papageridis: Conceptualization, Methodology, Validation, Investigation, Writing - Original Draft, Funding acquisition; Nikolaos D. Charisiou: Investigation, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration; Savvas Douvartzides: Writing - Original Draft;
Victor Sebastian: Investigation, Writing - Review & Editing; Steven J. Hinder: Investigation; Mark A. Baker: Investigation, Writing - Review & Editing; Sara AlKhoori: Investigation; Kyriaki
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