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Thermocatalytic deoxygenation of methyl laurate over potassium FAU zeolites
Microporous and Mesoporous Materials 284 (2019) 122–127
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Thermocatalytic deoxygenation of methyl laurate over potassium FAU zeolites
T
José María Gómez∗, Eduardo Díez, Araceli Rodríguez, Rodrigo Palanca Catálisis y Operaciones de Separación (CyPS), Department of Chemical and Materials Engineering, Faculty of Chemistry, Complutense University of Madrid, 28040, Madrid, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Deoxygenation X zeolite Potassium Methyl laurate
Deoxygenation of methyl laurate was carried out over potassium FAU zeolites. The influence of the silicon/ aluminum molar ratio was studied using K-USY (3.2), KNaY (2.6) and KNaX (1.1). Decreasing the Si/Al ratio increases the basicity of the zeolite, which improved the deoxygenation capacity by decarboxylation/decarbonylation of the methyl laurate. The use of methanol as solvent it is necessary to avoid the transesterification or the hydrolysis of the methyl ester, as well as to produce hydrogen in situ increasing the formation of dodecane. Potassium X zeolite (K-KNaX) showed a steady catalytic activity with the time on stream over 40% with yield to bio-hydrocarbons C10 to C12 of 30% and reaching selectivity of 70%. The biofuel produced reduced the viscosity 25% and improved the heat value 2500 kJ/kg respect to the initial methyl laurate. Therefore, it is possible to upgrade of the biodiesel by thermocatalytic processes using potassium X zeolites.
1. Introduction Nowadays, the interest in renewable fuels as replacement for fossil fuels has rapidly increased due to the growing of the world population and energy consumption. Fossil fuels are dwindling rapidly, so the development of clean and renewable alternative energy resources is very urgent. Researches are paying special attention to biomass to produce biofuels, as a renewable and clean environment friendly energy source. There are different types of biofuels according to the states of matter: solid as wood wastes, gas like biomethane, or liquid like biodiesel. Biodiesel is obtained from vegetable oil or animal fat and it is a mixture of fatty acid alkyl ester, which are produced by catalytic transesterification with methanol. The application of biodiesel presents some problems compared with the petroleum-based diesel fuel, which are the bottleneck in the development of this renewable source. Biodiesel has a large number of oxygenated groups that reduce the quality of the biofuel: it decreases the heating value, the chemical and thermal stability and it increases the viscosity and the corrosiveness. These disadvantages of the biodiesel can be overcome by deoxygenation, the removal of oxygen atoms from a molecule. Deoxygenation would allow to obtain an economic biofuel, which could be used in transport, reducing the toxic emissions into the atmosphere from engines running [1,2]. The deoxygenation reaction of fatty acid ester can be carried out by
∗
direct hydrodeoxygenation (HDO) or by decarboxylation/decarbonylation. The HDO involves several consecutive reactions through which oxygen is eliminated as water and hydrocarbons, with the same number of carbons. Nevertheless, the consumption of considerable amounts of hydrogen added to the high pressures (10–50 bars) are important disadvantages to the widespread use of the HDO process. Supported metallic catalysts such as noble metals (Pt, Pd or Re) have been used [3–5], as well as, supported metal sulphides (Ni, Mo …) [6–8] and, more recently, metal phosphides (Ni, Co, Mo, W, P) [9–12] or metal phosphide on mesostructured silica support as Ni2P/SBA15 [13]. On the other hand, in the decarboxylation/decarbonylation reactions the oxygen is directly released in form of CO2 (decarboxylation) and/or CO (decarbonylation). In this case hydrocarbons with one carbon atom less than the corresponding fatty acid are produced [10]. These reactions were also studied over supported metal catalyst such as Pd/C at high pressure (15–27 bar) and 573–633 °C [14,15] or Pt/Al2O3, previously reduced under flowing H2, at atmospheric pressure and 573623 °C [16]. FAU zeolites are an alternative to these metallic catalysts since they are able to produce decarboxylation and decarbonylation reactions without hydrogen consumption, lower pressure and temperature and less coke formation. The use of zeolites, as heterogeneous catalysts, in industrial processes is well known because they are inexpensive and environmentally benign. However, in spite of this, few
Corresponding author. E-mail address: [email protected] (J.M. Gómez).
https://doi.org/10.1016/j.micromeso.2019.04.025 Received 11 March 2019; Received in revised form 11 April 2019; Accepted 14 April 2019 Available online 17 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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2.4. Deoxygenation reaction
works have been developed using X zeolite as deoxygenation catalyst. Deoxygenation of aromatic compounds, as benzaldehyde [17], m-toluic acid [18] or benzyl acetate [19] has been studied over cesium and potassium ion-exchanged X zeolite without direct hydrogen consumption obtaining benzene and toluene. Methyl octanoate deoxygenation over FAU zeolites has been studied as a model reaction for the production of deoxygenated liquid hydrocarbons from biodiesel [20,21]. However, the composition of biodiesel implies chain length of saturated FAME (Fatty acid methyl esters) from C12 to C24 depending on the raw material. The aim of this work is to study the use of FAU zeolites exchanged with potassium as catalysts in the deoxygenation of methyl laurate (C12) as model compound of a biodiesel. Potassium exchanged FAU zeolites, with different Si/Al molar ratios, were employed as catalysts.
2. Materials and methods
Deoxygenation of methyl laurate was carried out at 698 K in a fixed bed (Dinternal = 0.14”) with continuous nitrogen flow (20 mL min−1) at atmospheric pressure. Previously, the zeolites (2 g) were calcined in situ for 1 h at the reaction temperature, 698 K, in nitrogen stream. The reactants (methyl laurate and solvent/co-reactant in different concentrations) were continuously fed to the reactor at the desired flow rate (0.1 and 0.3 ml/min) by a HPLC pump (Lab Alliance Serie III Digital). Samples from the reaction (each 15 or 30 min) were analyzed by Gas Chromatography in a Varian CP-3800 equipped with a capillary column (30 m) and flame ionization detector (FID). Desirable products such as bio-hydrocarbons like C12, C11, C10, etc. are the result of methyl laurate deoxygenation. All this products are named as DO products. The values of conversion of methyl laurate, yield and selectivity and turnover frequency (TOF) to each product were obtained using the following equations (Ec. (2) to Ec. (5):
2.1. Materials
XMethyl
Sodium silicate (Na2SiO3), sodium hydroxide (NaOH) and potassium hydroxide (KOH) were supplied by Panreac and sodium aluminate (NaAlO2) by Carlo Erba. Chemicals used in the catalytic reaction were methyl laurate (CH3(CH2)10COOCH3), methanol (CH3OH), tetrahydrofurane (C4H8O) and ethanol (CH3 CH2OH) from Sigma-Aldrich.
(F0Methyl
Laurate
– FMethyl
Laurate)
YProduct = FProduct /
F0Methyl Laurate
100
SProduct = FProduct /
(F0Methyl Laurate
– FMethyl
TOF = ΣFDO
Products
/ mass of zeolite 100
/ F0Methyl
Laurate
100
(2) (3)
Laurate)
100
(4) (5)
−1
Where (mol·s ) is the molar flow rate of methyl laurate at the inlet, FMethyl Laurate (mol·s−1) and Fproduct (mol·s−1) are the molar flow rate of methyl laurate and the molar flow rate of the different products at the outlet of the reactor, respectively. Finally, ΣFDOproducts (mol·min−1) is the sum of the molar flow rate of deoxygenation products: saturated and unsaturated hydrocarbons. F0Methyl Laurate
2.2. Catalyst preparation X and Y zeolite syntheses with different silicon/aluminum molar ratio were carried out via the hydrothermal method according to previous studies [22]. In this method, sodium aluminate was dissolved in water and then a mixture of sodium hydroxide and potassium hydroxide was added, for the synthesis of KNaX zeolite. Finally, sodium silicate was slowly added to the sodium aluminate-alkaline metal hydroxide solution. All the solids obtained were washed, with 0.01 M sodium hydroxide or potassium hydroxide solution to avoid protonation, dried at 373 K overnight and calcined at 773 K for 3 h (heating rate 10 °C/min). Commercial NaX and USY zeolites were supplied by SigmaAldrich and Grace Davison, respectively. Potassium FAU zeolites (K-USY, K-NaY, K-NaX and K-KNaX) were obtained by potassium ion-exchange of the as-synthesised and commercial zeolites. Five ionic-exchange was performed at room temperature for 10 min using 0.5 M KCl solution with a solid-liquid ratio of 50 mL g−1. After each ionic-exchange, the zeolites were filtered and a fresh solution was added. Finally, the samples were washed until absence of chloride with 0.01 M KOH solution to avoid the protonation and dried at 323 K overnight. Ionic exchange degree was calculated according to the equation (1): IE(%) = (Na0–Naf) / Na0 100
Laurate =
3. Results and discussion Acid properties of zeolites has been widely studied since its catalytic properties are implied in important industrial reactions. However, basic properties has been less studied. In addition, basicity measurements of the zeolites is under discussion, being accepted that the basic properties cannot be measured by any single method. Therefore, the better proof of the basicity is the catalytic activity. Specifically for the zeolites is assumed that their basic properties are improved by decreasing the Si/ Al molar ratio and with the ionic-exchange for less electronegative cations [23,24]. In previous works it was clearly shown that the enhancement of the basic properties of FAU zeolites improves its catalytic activity in methyl esters deoxygenation of [16,17]. For this reason, this work is focused on potassium FAU zeolites (K-USY, K-NaY, K-NaX and K-KNaX).
3.1. Structural properties of the catalysts
(1)
XRD patterns of the potassium zeolites (Fig. 1) correspond on the FAU framework with the characteristic peaks of this structure. As it can be expected, it can be observed a peak intensity reduction as compared with the initial FAU zeolite (sodium form). This was due to the effect of the potassium cations (larger than the sodium cations) on the framework rather than the collapse of the crystalline structure [25]. In addition, the presence of amorphous halo which would indicate the collapse of the framework was not detected. The nitrogen adsorption/desorption isotherms were measured at 77 K (not shown) for all potassium zeolites exhibiting the typical shape of the microporous materials, without significant changes between them. The isotherms belong to type I in the IUPAC classification with H4 hysteresis loop, attributed to the aggregation of particles. The characteristics of all zeolites are presented in Table 1.
Where Na0 is the initial content of sodium in the zeolite and Naf is the sodium content after the ion-exchange.
2.3. Characterization of zeolites N2 adsorption–desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2020. X-ray diffraction (XRD) patterns were recorded on a PHILIPS diffractometer (X’PERT MPD) with CuKα radiation and Ni filter. The scanning range of 2θ was set between 5° and 50° with a step size of 0.1. Infrared spectra were recorded on a Thermo Nicolet Avatar 380 FTIR spectrometer in the range of 400–4000 cm−1. The samples were examined in the form of self-supporting thin KBr wafers (1 wt%). Chemical composition was determined by X-ray fluorescence (XRF) using a PHILIPS PW-1480 instrument. 123
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Fig. 1. XRD patterns of the FAU zeolites before and after the ion exchange with potassium.
Fig. 2. Catalytic activity of the K-NaY zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
Table 1 Physical properties of the zeolites.
Si/Ala (molar) %Naa wt%u.c. %Ka wt%u.c. (Na + K)/Al (molar) Ion exchangeb (%) SBETc (m2/g)
K-USY
K-NaY
K-NaX
KNaX
K-KNaX
3.2 0 14 0.5 100 735
2.6 2.9 26 1.1 79 675
1.4 2.9 37 1.2 100 695
1.1 16 15 0.9 /d 775
1.1 3.7 38 1.2 70 520
wt% u.c.: weight percentage per unit cell (Si + Al = 192 atoms). a Composition determined from XRF data. b Calculated by Ec. 1. c Specific surface calculated from N2 adsorption data applying BET method. d KNaX as-synthesized with potassium.
3.2. Catalytic activity The main aim of the deoxygenation of methyl esters, as methyl laurate, over basic zeolites, is to obtain hydrocarbons, avoiding the formation of oxygenated compounds (aldehydes, alcohols and/or acids). Basic zeolites are able to produce deoxygenation reactions through the decarbonylation (elimination of CO) and/or the decarboxylation (elimination of CO2) of the methyl esters. In addition, hydrogenation/dehydration, especially when methanol is used as solvent, could be produced [17]. Therefore, a wide product distribution is expected in the deoxygenation of methyl laurate, including hydrocarbons as C12, C11 and C10 (alkanes and alkenes), lauryl alcohol, lauraldehyde and lauric acid, light gases (CO, CO2, etc.) and even the formation of condensation products as lauric anhydride (C24). Firstly, the reaction without catalyst was carried out as blank of reaction. The thermal conversion of methyl laurate reached a steady value of around 5% at 3 h time on stream, without production of desirable products (long chain hydrocarbons, C10, C11 or C12) due to the thermal cracking. In addition, a protonic zeolites (USY) was used as acid catalyst to compare with the basic zeolites. This zeolite showed high catalytic activity but with very low selectivity to long chain hydrocarbons due to the undesirable cracking reactions. Figs. 2–4 display the results in the deoxygenation of methyl laurate as yield to deoxygenation products (C8 to C12) and conversion for the different catalysts prepared. These reactions were carried out with methanol as solvent at 10%wt methyl laurate concentration. Reaction over zeolite with the higher silicon/aluminum molar ratio (not shown), K-USY zeolite (Si/Al = 3.2), produced as reaction product two immiscible liquid phases, one polar, with water and several esters,
Fig. 3. Catalytic activity of the K-NaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
and a non-polar phase, with hydrocarbons. This was due to the acid character of this zeolite with (Na + K)/Al molar ratio less than 1.0, which means that the zeolite is protonated. Acid mechanism leads to cracking products with a high water and methyl esters (pentanoate, hexanoate, heptanoate etc …) content. Therefore, acid mechanism is not desirable in order to upgrade the biodiesel. When the silicon/aluminium molar ratio was decreased to 2.6 (K-NaY zeolite, Fig. 2) only one phase was produced. However, the yield to hydrocarbons was low, still maintaining a high formation of undesirable products as hydrocarbons cracking products and the complementary ester, for example, methyl pentanoate and heptane/heptene. With this zeolite, the oxygen content was not reduced in the reaction products as compared to the initial corresponding to the methyl laurate. When the silicon/aluminium molar ratio was decreased till 1.4 (K-NaX, Fig. 3) higher yield to hydrocarbons was obtained. Conversion of methyl laurate for the K-NaX was steady, around 60–65%, with the time on stream, indicating very low deactivation of the catalysts. However, the selectivity was around 60% due to the condensation of lauric acid to lauric anhydride, which was detected qualitatively in an analysis on GC mass (Agilent 5973 124
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Fig. 4. Catalytic activity of the KNaX zeolite (as-synthesised) in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
Fig. 5. Catalytic activity of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
inert GC/MS System). The formation of this product was not detected in the reaction without zeolite, where the conversion was only 5%. The KNaX zeolite (Fig. 4), with potassium incorporated in the synthesis, showed an important decrease in the conversion of methyl laurate, from 80% at the beginning of the reaction to 40% at 4 h. However, the yield to hydrocarbons was steady with the time on stream, reaching a constant value of 20%, while selectivity to hydrocarbons was around 60% at the end of the time on stream. This difference in the behaviour of the conversion respect to the yield was due to the deactivation of the acid sites of the zeolites, while the basic sites, responsible of the hydrocarbons production, remained active. Acid sites are intrinsic to zeolites containing aluminium in the framework since this element can accept electrons; being Lewis acid sites. On the other hand, the rest of reaction products were methyl esters as methyl acetate, methyl propionate, methyl butanoate, etc., others hydrocarbons as heptane, heptenes, hexane, hexenes, etc. and condensation products. However, this zeolite only presented a potassium content of 15 wt% u.c. versus 37% of the KNaX. Therefore, it was possible to increase their deoxygenation capacity by potassium ionic-exchange. The K-KNaX zeolite was prepared by ionexchange of the as-synthesized KNaX zeolite. The potassium content was increased to 38 wt% u.c., similar to the other potassium-exchanged X zeolite. Fig. 5 shows the results of methyl laurate deoxygenation of with the K-KNaX zeolite. As it can be seen in Fig. 6, the K-KNaX zeolite showed a steady catalytic activity, both conversion (≈43%) and yields to deoxygenation products (≈30%), reaching a selectivity value around 70%. Among the main products are C11 (undecene and undecane) and C12 (dodecane), which clearly reduce the oxygen content respect to the initial ester. These products were 80% of the deoxygenation products whereas for the K-NaX were 60%. The main difference introduced by the post synthesis potassium ion exchange was the amount of C12 obtained, higher over the K-KNaX (43% of the desirable products) than over the K-NaX (28% of the desirable products). The formation of C12 is linked to the hydrogenation reaction of the methyl laurate. The presence of hydrogen in the reaction media would be due to the methanol, which can produce hydrogen and formaldehyde/dimethyl ether over basic zeolites [26]. K-KNaX zeolite (Si/Al: 1.1) presents more basic sites than K-NaX zeolite (Si/Al: 1.4) because the silicon/aluminium molar ratio was lower. When the silicon/aluminium molar ratio is decreased, the negative charge density over the oxygen atoms is increased. Therefore, the formation of hydrogen would be favoured over K-KNaX leading to the formation of dodecane. K-KNaX zeolite (520 m2/g) presented good results in spite of the decrease in the surface area (25%) respect to the
Fig. 6. Catalytic activity of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 7.4 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 5 g zeolite·h/mol feed.
K-NaX zeolite (700 m2/g). Therefore, the effect of the increase of the active sites number is more important than the surface area decrease over the deoxygenation catalytic performance of the zeolite.
3.3. Variation of W/F The reaction was carried out at a higher value of the feed flow, maintaining the zeolite mass (2 g), in order to decrease the W/F ratio and to study the influence of this variable. Fig. 6 displays the catalytic activity of the K-KNaX zeolite at W/F of 5 g zeolite·h/mol feed. There was a slight decrease in the conversion with the time on stream, from 40% at 1 h to 30% at 4 h (about 25%). The yield to deoxygenation products presented similar behaviour, decreasing from the first hour to the fourth hour. The main products were C12 and C11. The drop in the catalytic activity, compare to Fig. 5 (W/F = 15 g zeolite·h/mol feed), was due to the decrease of the contact time of the reactants with the catalyst since the feed flow was higher. However, in order to be able to draw conclusions on the suitability of certain catalysts with substrates for deoxygenation (in the form of 125
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Fig. 7. TOF of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min.
Fig. 9. Catalytic activity of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 7.4 g/h, [Methyl Laurate] = 10%wt, solvent: Ethanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 5 g zeolite·h/mol feed.
decarboxylation and decarbonylation) Dawes et al. [27] have proposed the turnover frequency (TOF) as a suitable indicator for reactor productivity, regardless of type (plug flow, batch, etc.). Turnover frequency was defined as deoxygenation product moles per zeolite mass per time on stream (minutes) since the interest products are hydrocarbons. (Ec. 5). Fig. 7 displays the evolution of the TOF with the time on stream for both W/F ratio. Production of deoxygenation compounds was higher when the W/F was decreased, around twice. In spite of the reactants-catalysts contact time was shorter, the zeolite was able to process more molecules of reactants (methyl laurate), leading to higher bio-hydrocarbons production per gram of zeolite, with a better efficiency of the catalyst. Variation of the TOF in the deoxygenation of methyl laurate at higher initial concentration of methyl laurate (30%wt) is displayed in Fig. 8. As it can be seen in the figure the zeolite was not able to produce more deoxygenation products.
The reaction with THF as solvent (not shown) did not produce interest products (bio-hydrocarbons). The main product was lauric acid with a yield about 8%. This was due to the hydrolysis of the methyl laurate. Fig. 9 displays the reaction results when ethanol was used as solvent. The main reaction was the transesterification of the methyl laurate to ethyl laurate with a yield about 40%, steady with the time on stream. The conversion was between 50 and 60% for all the time on stream, with barely deactivation. However, the yield to deoxygenation products was lower, under 5%. Therefore, the use of methanol as solvent is a key factor to avoid the transesterification reaction and promote the hydrogen formation in situ, which favoured the deoxygenation of the methyl laurate.
3.5. Improving the biofuel? With the aim of confirming the feasibility of the deoxygenation process as a way of improving the quality of biofuels, the viscosity and the gross heating value of a theoretical biofuel were calculated with the property-set module of ASPEN PLUS® simulator. The composition of the theoretical biofuel was established in accordance with the obtained results in the methyl laurate deoxygenation reactions, and is shown in Table 2. A biofuel with only methyl laurate presents a heat value of 37970 kJ/kg with a viscosity of 2.8 cP at 25 °C. However, the upgraded biofuel after the deoxygenation reaction with the K-KNaX zeolite would increase the heat value in 2500 kJ/kg (6.5%) decreasing the viscosity 24%, until 2.1 cP.
3.4. Influence of solvent The deoxygenation of methyl laurate was carried out using ethanol and tetrahydrofuran (THF) as solvents with 10%wt of methyl laurate.
Table 2 Composition simulated of the biofuel obtained with the K-KNaX zeolite. wt% Methyl Laurate C8 (n-octane) C9 (n-nonane) C10 (n-decane) C11 (n-undecane) C12 (n-dodecane) Lauric acid
Fig. 8. TOF of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 5 g zeolite·h/mol feed (■) 6 g zeolite·h/mol feed (▲). 126
73 1.2 0.8 1.6 10 12.4 1
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4. Conclusions
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Upgrading biofuel, as bio-diesel, can be carried out by deoxygenation over potassium X zeolite. The decrease of the silicon/aluminum molar ratio in the zeolite leads to the deoxygenation of methyl laurate, reaching a steady yield with the time on stream around 30% to deoxygenation products (C10 to C12), with a selectivity about 70%. The new biofuel increased the heat value (2500 kJ/kg) and decreased the viscosity (24%) respect to the methyl laurate initial. The oxygen content respect to the initial bio-fuel was reduced. The use of methanol as solvent is a key factor to avoid the transesterification or the hydrolysis reactions, which lead to oxygenated compounds. Moreover, the use of methanol promotes the hydrogen formation in situ, which favoured the hydrogenation of the methyl laurate to produce dodecane. Decarbonylation (undecene) and decarboxylation (undecane) were also produced over these zeolites. Therefore, the increase of the basicity in the zeolites (low silicon/aluminium ratio and potassium ion-exchange) favours the deoxygenation reaction by the different mechanisms increasing the quality of the biofuel. Acknowledgements This work was supported by the financial support of the SantanderUCM 2018 project (PR75718). References [1] G. Knothe, Fuel Process. Technol. 86 (2005) 1059 https://doi.org/10.1016/j. fuproc.2004.11.002. [2] M.A. Peralta, T. Sooknoi, T. Danuthai, D.E. Resasco, J. Mol. Catal. A Chem. 312 (2009) 78–86 https://doi.org/10.1016/j.molcata.2009.07.008. [3] V.P. Vladimir, M. Nathan, A. Kwangjin, A. Selim, A.S. Gabor, Nano Lett. 12 (2012) 5196–5201 https://dx.doi.org/10.1021/nl3023127. [4] H. Lee, H. Kim, M.J. Yu, C.H. Ko, J. Jeon, J. Jae, S.H. Park, S. Jung, Y. Park, Sci. Rep. 6 (2016) 28765 https://doi.org/10.1038/srep28765. [5] L. Boda, G. Onyestya, H. Solt, F. Lonyi, J. Valyon, A. Thernesz, Appl. Catal. Gen. 374 (2010) 158–169 https://doi.org/10.1016/j.apcata.2009.12.005. [6] D. Kubička, J. Horáček, M. Setnička, R. Bulánek, A. Zukal, I. Kubičková, Appl. Catal. B Environ. 145 (2014) 101–107 https://doi.org/10.1016/j.apcatb.2013.01.012.
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Thermocatalytic deoxygenation of methyl laurate over potassium FAU zeolites
T
José María Gómez∗, Eduardo Díez, Araceli Rodríguez, Rodrigo Palanca Catálisis y Operaciones de Separación (CyPS), Department of Chemical and Materials Engineering, Faculty of Chemistry, Complutense University of Madrid, 28040, Madrid, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Deoxygenation X zeolite Potassium Methyl laurate
Deoxygenation of methyl laurate was carried out over potassium FAU zeolites. The influence of the silicon/ aluminum molar ratio was studied using K-USY (3.2), KNaY (2.6) and KNaX (1.1). Decreasing the Si/Al ratio increases the basicity of the zeolite, which improved the deoxygenation capacity by decarboxylation/decarbonylation of the methyl laurate. The use of methanol as solvent it is necessary to avoid the transesterification or the hydrolysis of the methyl ester, as well as to produce hydrogen in situ increasing the formation of dodecane. Potassium X zeolite (K-KNaX) showed a steady catalytic activity with the time on stream over 40% with yield to bio-hydrocarbons C10 to C12 of 30% and reaching selectivity of 70%. The biofuel produced reduced the viscosity 25% and improved the heat value 2500 kJ/kg respect to the initial methyl laurate. Therefore, it is possible to upgrade of the biodiesel by thermocatalytic processes using potassium X zeolites.
1. Introduction Nowadays, the interest in renewable fuels as replacement for fossil fuels has rapidly increased due to the growing of the world population and energy consumption. Fossil fuels are dwindling rapidly, so the development of clean and renewable alternative energy resources is very urgent. Researches are paying special attention to biomass to produce biofuels, as a renewable and clean environment friendly energy source. There are different types of biofuels according to the states of matter: solid as wood wastes, gas like biomethane, or liquid like biodiesel. Biodiesel is obtained from vegetable oil or animal fat and it is a mixture of fatty acid alkyl ester, which are produced by catalytic transesterification with methanol. The application of biodiesel presents some problems compared with the petroleum-based diesel fuel, which are the bottleneck in the development of this renewable source. Biodiesel has a large number of oxygenated groups that reduce the quality of the biofuel: it decreases the heating value, the chemical and thermal stability and it increases the viscosity and the corrosiveness. These disadvantages of the biodiesel can be overcome by deoxygenation, the removal of oxygen atoms from a molecule. Deoxygenation would allow to obtain an economic biofuel, which could be used in transport, reducing the toxic emissions into the atmosphere from engines running [1,2]. The deoxygenation reaction of fatty acid ester can be carried out by
∗
direct hydrodeoxygenation (HDO) or by decarboxylation/decarbonylation. The HDO involves several consecutive reactions through which oxygen is eliminated as water and hydrocarbons, with the same number of carbons. Nevertheless, the consumption of considerable amounts of hydrogen added to the high pressures (10–50 bars) are important disadvantages to the widespread use of the HDO process. Supported metallic catalysts such as noble metals (Pt, Pd or Re) have been used [3–5], as well as, supported metal sulphides (Ni, Mo …) [6–8] and, more recently, metal phosphides (Ni, Co, Mo, W, P) [9–12] or metal phosphide on mesostructured silica support as Ni2P/SBA15 [13]. On the other hand, in the decarboxylation/decarbonylation reactions the oxygen is directly released in form of CO2 (decarboxylation) and/or CO (decarbonylation). In this case hydrocarbons with one carbon atom less than the corresponding fatty acid are produced [10]. These reactions were also studied over supported metal catalyst such as Pd/C at high pressure (15–27 bar) and 573–633 °C [14,15] or Pt/Al2O3, previously reduced under flowing H2, at atmospheric pressure and 573623 °C [16]. FAU zeolites are an alternative to these metallic catalysts since they are able to produce decarboxylation and decarbonylation reactions without hydrogen consumption, lower pressure and temperature and less coke formation. The use of zeolites, as heterogeneous catalysts, in industrial processes is well known because they are inexpensive and environmentally benign. However, in spite of this, few
Corresponding author. E-mail address: [email protected] (J.M. Gómez).
https://doi.org/10.1016/j.micromeso.2019.04.025 Received 11 March 2019; Received in revised form 11 April 2019; Accepted 14 April 2019 Available online 17 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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2.4. Deoxygenation reaction
works have been developed using X zeolite as deoxygenation catalyst. Deoxygenation of aromatic compounds, as benzaldehyde [17], m-toluic acid [18] or benzyl acetate [19] has been studied over cesium and potassium ion-exchanged X zeolite without direct hydrogen consumption obtaining benzene and toluene. Methyl octanoate deoxygenation over FAU zeolites has been studied as a model reaction for the production of deoxygenated liquid hydrocarbons from biodiesel [20,21]. However, the composition of biodiesel implies chain length of saturated FAME (Fatty acid methyl esters) from C12 to C24 depending on the raw material. The aim of this work is to study the use of FAU zeolites exchanged with potassium as catalysts in the deoxygenation of methyl laurate (C12) as model compound of a biodiesel. Potassium exchanged FAU zeolites, with different Si/Al molar ratios, were employed as catalysts.
2. Materials and methods
Deoxygenation of methyl laurate was carried out at 698 K in a fixed bed (Dinternal = 0.14”) with continuous nitrogen flow (20 mL min−1) at atmospheric pressure. Previously, the zeolites (2 g) were calcined in situ for 1 h at the reaction temperature, 698 K, in nitrogen stream. The reactants (methyl laurate and solvent/co-reactant in different concentrations) were continuously fed to the reactor at the desired flow rate (0.1 and 0.3 ml/min) by a HPLC pump (Lab Alliance Serie III Digital). Samples from the reaction (each 15 or 30 min) were analyzed by Gas Chromatography in a Varian CP-3800 equipped with a capillary column (30 m) and flame ionization detector (FID). Desirable products such as bio-hydrocarbons like C12, C11, C10, etc. are the result of methyl laurate deoxygenation. All this products are named as DO products. The values of conversion of methyl laurate, yield and selectivity and turnover frequency (TOF) to each product were obtained using the following equations (Ec. (2) to Ec. (5):
2.1. Materials
XMethyl
Sodium silicate (Na2SiO3), sodium hydroxide (NaOH) and potassium hydroxide (KOH) were supplied by Panreac and sodium aluminate (NaAlO2) by Carlo Erba. Chemicals used in the catalytic reaction were methyl laurate (CH3(CH2)10COOCH3), methanol (CH3OH), tetrahydrofurane (C4H8O) and ethanol (CH3 CH2OH) from Sigma-Aldrich.
(F0Methyl
Laurate
– FMethyl
Laurate)
YProduct = FProduct /
F0Methyl Laurate
100
SProduct = FProduct /
(F0Methyl Laurate
– FMethyl
TOF = ΣFDO
Products
/ mass of zeolite 100
/ F0Methyl
Laurate
100
(2) (3)
Laurate)
100
(4) (5)
−1
Where (mol·s ) is the molar flow rate of methyl laurate at the inlet, FMethyl Laurate (mol·s−1) and Fproduct (mol·s−1) are the molar flow rate of methyl laurate and the molar flow rate of the different products at the outlet of the reactor, respectively. Finally, ΣFDOproducts (mol·min−1) is the sum of the molar flow rate of deoxygenation products: saturated and unsaturated hydrocarbons. F0Methyl Laurate
2.2. Catalyst preparation X and Y zeolite syntheses with different silicon/aluminum molar ratio were carried out via the hydrothermal method according to previous studies [22]. In this method, sodium aluminate was dissolved in water and then a mixture of sodium hydroxide and potassium hydroxide was added, for the synthesis of KNaX zeolite. Finally, sodium silicate was slowly added to the sodium aluminate-alkaline metal hydroxide solution. All the solids obtained were washed, with 0.01 M sodium hydroxide or potassium hydroxide solution to avoid protonation, dried at 373 K overnight and calcined at 773 K for 3 h (heating rate 10 °C/min). Commercial NaX and USY zeolites were supplied by SigmaAldrich and Grace Davison, respectively. Potassium FAU zeolites (K-USY, K-NaY, K-NaX and K-KNaX) were obtained by potassium ion-exchange of the as-synthesised and commercial zeolites. Five ionic-exchange was performed at room temperature for 10 min using 0.5 M KCl solution with a solid-liquid ratio of 50 mL g−1. After each ionic-exchange, the zeolites were filtered and a fresh solution was added. Finally, the samples were washed until absence of chloride with 0.01 M KOH solution to avoid the protonation and dried at 323 K overnight. Ionic exchange degree was calculated according to the equation (1): IE(%) = (Na0–Naf) / Na0 100
Laurate =
3. Results and discussion Acid properties of zeolites has been widely studied since its catalytic properties are implied in important industrial reactions. However, basic properties has been less studied. In addition, basicity measurements of the zeolites is under discussion, being accepted that the basic properties cannot be measured by any single method. Therefore, the better proof of the basicity is the catalytic activity. Specifically for the zeolites is assumed that their basic properties are improved by decreasing the Si/ Al molar ratio and with the ionic-exchange for less electronegative cations [23,24]. In previous works it was clearly shown that the enhancement of the basic properties of FAU zeolites improves its catalytic activity in methyl esters deoxygenation of [16,17]. For this reason, this work is focused on potassium FAU zeolites (K-USY, K-NaY, K-NaX and K-KNaX).
3.1. Structural properties of the catalysts
(1)
XRD patterns of the potassium zeolites (Fig. 1) correspond on the FAU framework with the characteristic peaks of this structure. As it can be expected, it can be observed a peak intensity reduction as compared with the initial FAU zeolite (sodium form). This was due to the effect of the potassium cations (larger than the sodium cations) on the framework rather than the collapse of the crystalline structure [25]. In addition, the presence of amorphous halo which would indicate the collapse of the framework was not detected. The nitrogen adsorption/desorption isotherms were measured at 77 K (not shown) for all potassium zeolites exhibiting the typical shape of the microporous materials, without significant changes between them. The isotherms belong to type I in the IUPAC classification with H4 hysteresis loop, attributed to the aggregation of particles. The characteristics of all zeolites are presented in Table 1.
Where Na0 is the initial content of sodium in the zeolite and Naf is the sodium content after the ion-exchange.
2.3. Characterization of zeolites N2 adsorption–desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2020. X-ray diffraction (XRD) patterns were recorded on a PHILIPS diffractometer (X’PERT MPD) with CuKα radiation and Ni filter. The scanning range of 2θ was set between 5° and 50° with a step size of 0.1. Infrared spectra were recorded on a Thermo Nicolet Avatar 380 FTIR spectrometer in the range of 400–4000 cm−1. The samples were examined in the form of self-supporting thin KBr wafers (1 wt%). Chemical composition was determined by X-ray fluorescence (XRF) using a PHILIPS PW-1480 instrument. 123
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Fig. 1. XRD patterns of the FAU zeolites before and after the ion exchange with potassium.
Fig. 2. Catalytic activity of the K-NaY zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
Table 1 Physical properties of the zeolites.
Si/Ala (molar) %Naa wt%u.c. %Ka wt%u.c. (Na + K)/Al (molar) Ion exchangeb (%) SBETc (m2/g)
K-USY
K-NaY
K-NaX
KNaX
K-KNaX
3.2 0 14 0.5 100 735
2.6 2.9 26 1.1 79 675
1.4 2.9 37 1.2 100 695
1.1 16 15 0.9 /d 775
1.1 3.7 38 1.2 70 520
wt% u.c.: weight percentage per unit cell (Si + Al = 192 atoms). a Composition determined from XRF data. b Calculated by Ec. 1. c Specific surface calculated from N2 adsorption data applying BET method. d KNaX as-synthesized with potassium.
3.2. Catalytic activity The main aim of the deoxygenation of methyl esters, as methyl laurate, over basic zeolites, is to obtain hydrocarbons, avoiding the formation of oxygenated compounds (aldehydes, alcohols and/or acids). Basic zeolites are able to produce deoxygenation reactions through the decarbonylation (elimination of CO) and/or the decarboxylation (elimination of CO2) of the methyl esters. In addition, hydrogenation/dehydration, especially when methanol is used as solvent, could be produced [17]. Therefore, a wide product distribution is expected in the deoxygenation of methyl laurate, including hydrocarbons as C12, C11 and C10 (alkanes and alkenes), lauryl alcohol, lauraldehyde and lauric acid, light gases (CO, CO2, etc.) and even the formation of condensation products as lauric anhydride (C24). Firstly, the reaction without catalyst was carried out as blank of reaction. The thermal conversion of methyl laurate reached a steady value of around 5% at 3 h time on stream, without production of desirable products (long chain hydrocarbons, C10, C11 or C12) due to the thermal cracking. In addition, a protonic zeolites (USY) was used as acid catalyst to compare with the basic zeolites. This zeolite showed high catalytic activity but with very low selectivity to long chain hydrocarbons due to the undesirable cracking reactions. Figs. 2–4 display the results in the deoxygenation of methyl laurate as yield to deoxygenation products (C8 to C12) and conversion for the different catalysts prepared. These reactions were carried out with methanol as solvent at 10%wt methyl laurate concentration. Reaction over zeolite with the higher silicon/aluminum molar ratio (not shown), K-USY zeolite (Si/Al = 3.2), produced as reaction product two immiscible liquid phases, one polar, with water and several esters,
Fig. 3. Catalytic activity of the K-NaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
and a non-polar phase, with hydrocarbons. This was due to the acid character of this zeolite with (Na + K)/Al molar ratio less than 1.0, which means that the zeolite is protonated. Acid mechanism leads to cracking products with a high water and methyl esters (pentanoate, hexanoate, heptanoate etc …) content. Therefore, acid mechanism is not desirable in order to upgrade the biodiesel. When the silicon/aluminium molar ratio was decreased to 2.6 (K-NaY zeolite, Fig. 2) only one phase was produced. However, the yield to hydrocarbons was low, still maintaining a high formation of undesirable products as hydrocarbons cracking products and the complementary ester, for example, methyl pentanoate and heptane/heptene. With this zeolite, the oxygen content was not reduced in the reaction products as compared to the initial corresponding to the methyl laurate. When the silicon/aluminium molar ratio was decreased till 1.4 (K-NaX, Fig. 3) higher yield to hydrocarbons was obtained. Conversion of methyl laurate for the K-NaX was steady, around 60–65%, with the time on stream, indicating very low deactivation of the catalysts. However, the selectivity was around 60% due to the condensation of lauric acid to lauric anhydride, which was detected qualitatively in an analysis on GC mass (Agilent 5973 124
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Fig. 4. Catalytic activity of the KNaX zeolite (as-synthesised) in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
Fig. 5. Catalytic activity of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 4.7 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 15 g zeolite·h/mol feed.
inert GC/MS System). The formation of this product was not detected in the reaction without zeolite, where the conversion was only 5%. The KNaX zeolite (Fig. 4), with potassium incorporated in the synthesis, showed an important decrease in the conversion of methyl laurate, from 80% at the beginning of the reaction to 40% at 4 h. However, the yield to hydrocarbons was steady with the time on stream, reaching a constant value of 20%, while selectivity to hydrocarbons was around 60% at the end of the time on stream. This difference in the behaviour of the conversion respect to the yield was due to the deactivation of the acid sites of the zeolites, while the basic sites, responsible of the hydrocarbons production, remained active. Acid sites are intrinsic to zeolites containing aluminium in the framework since this element can accept electrons; being Lewis acid sites. On the other hand, the rest of reaction products were methyl esters as methyl acetate, methyl propionate, methyl butanoate, etc., others hydrocarbons as heptane, heptenes, hexane, hexenes, etc. and condensation products. However, this zeolite only presented a potassium content of 15 wt% u.c. versus 37% of the KNaX. Therefore, it was possible to increase their deoxygenation capacity by potassium ionic-exchange. The K-KNaX zeolite was prepared by ionexchange of the as-synthesized KNaX zeolite. The potassium content was increased to 38 wt% u.c., similar to the other potassium-exchanged X zeolite. Fig. 5 shows the results of methyl laurate deoxygenation of with the K-KNaX zeolite. As it can be seen in Fig. 6, the K-KNaX zeolite showed a steady catalytic activity, both conversion (≈43%) and yields to deoxygenation products (≈30%), reaching a selectivity value around 70%. Among the main products are C11 (undecene and undecane) and C12 (dodecane), which clearly reduce the oxygen content respect to the initial ester. These products were 80% of the deoxygenation products whereas for the K-NaX were 60%. The main difference introduced by the post synthesis potassium ion exchange was the amount of C12 obtained, higher over the K-KNaX (43% of the desirable products) than over the K-NaX (28% of the desirable products). The formation of C12 is linked to the hydrogenation reaction of the methyl laurate. The presence of hydrogen in the reaction media would be due to the methanol, which can produce hydrogen and formaldehyde/dimethyl ether over basic zeolites [26]. K-KNaX zeolite (Si/Al: 1.1) presents more basic sites than K-NaX zeolite (Si/Al: 1.4) because the silicon/aluminium molar ratio was lower. When the silicon/aluminium molar ratio is decreased, the negative charge density over the oxygen atoms is increased. Therefore, the formation of hydrogen would be favoured over K-KNaX leading to the formation of dodecane. K-KNaX zeolite (520 m2/g) presented good results in spite of the decrease in the surface area (25%) respect to the
Fig. 6. Catalytic activity of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 7.4 g/h, [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 5 g zeolite·h/mol feed.
K-NaX zeolite (700 m2/g). Therefore, the effect of the increase of the active sites number is more important than the surface area decrease over the deoxygenation catalytic performance of the zeolite.
3.3. Variation of W/F The reaction was carried out at a higher value of the feed flow, maintaining the zeolite mass (2 g), in order to decrease the W/F ratio and to study the influence of this variable. Fig. 6 displays the catalytic activity of the K-KNaX zeolite at W/F of 5 g zeolite·h/mol feed. There was a slight decrease in the conversion with the time on stream, from 40% at 1 h to 30% at 4 h (about 25%). The yield to deoxygenation products presented similar behaviour, decreasing from the first hour to the fourth hour. The main products were C12 and C11. The drop in the catalytic activity, compare to Fig. 5 (W/F = 15 g zeolite·h/mol feed), was due to the decrease of the contact time of the reactants with the catalyst since the feed flow was higher. However, in order to be able to draw conclusions on the suitability of certain catalysts with substrates for deoxygenation (in the form of 125
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Fig. 7. TOF of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: [Methyl Laurate] = 10%wt, solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min.
Fig. 9. Catalytic activity of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: Feed: 7.4 g/h, [Methyl Laurate] = 10%wt, solvent: Ethanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 5 g zeolite·h/mol feed.
decarboxylation and decarbonylation) Dawes et al. [27] have proposed the turnover frequency (TOF) as a suitable indicator for reactor productivity, regardless of type (plug flow, batch, etc.). Turnover frequency was defined as deoxygenation product moles per zeolite mass per time on stream (minutes) since the interest products are hydrocarbons. (Ec. 5). Fig. 7 displays the evolution of the TOF with the time on stream for both W/F ratio. Production of deoxygenation compounds was higher when the W/F was decreased, around twice. In spite of the reactants-catalysts contact time was shorter, the zeolite was able to process more molecules of reactants (methyl laurate), leading to higher bio-hydrocarbons production per gram of zeolite, with a better efficiency of the catalyst. Variation of the TOF in the deoxygenation of methyl laurate at higher initial concentration of methyl laurate (30%wt) is displayed in Fig. 8. As it can be seen in the figure the zeolite was not able to produce more deoxygenation products.
The reaction with THF as solvent (not shown) did not produce interest products (bio-hydrocarbons). The main product was lauric acid with a yield about 8%. This was due to the hydrolysis of the methyl laurate. Fig. 9 displays the reaction results when ethanol was used as solvent. The main reaction was the transesterification of the methyl laurate to ethyl laurate with a yield about 40%, steady with the time on stream. The conversion was between 50 and 60% for all the time on stream, with barely deactivation. However, the yield to deoxygenation products was lower, under 5%. Therefore, the use of methanol as solvent is a key factor to avoid the transesterification reaction and promote the hydrogen formation in situ, which favoured the deoxygenation of the methyl laurate.
3.5. Improving the biofuel? With the aim of confirming the feasibility of the deoxygenation process as a way of improving the quality of biofuels, the viscosity and the gross heating value of a theoretical biofuel were calculated with the property-set module of ASPEN PLUS® simulator. The composition of the theoretical biofuel was established in accordance with the obtained results in the methyl laurate deoxygenation reactions, and is shown in Table 2. A biofuel with only methyl laurate presents a heat value of 37970 kJ/kg with a viscosity of 2.8 cP at 25 °C. However, the upgraded biofuel after the deoxygenation reaction with the K-KNaX zeolite would increase the heat value in 2500 kJ/kg (6.5%) decreasing the viscosity 24%, until 2.1 cP.
3.4. Influence of solvent The deoxygenation of methyl laurate was carried out using ethanol and tetrahydrofuran (THF) as solvents with 10%wt of methyl laurate.
Table 2 Composition simulated of the biofuel obtained with the K-KNaX zeolite. wt% Methyl Laurate C8 (n-octane) C9 (n-nonane) C10 (n-decane) C11 (n-undecane) C12 (n-dodecane) Lauric acid
Fig. 8. TOF of the K-KNaX zeolite in the deoxygenation of methyl laurate. Reactions conditions: solvent: Methanol. T = 698 K, catalyst mass: 2 g, N2 flow: 20 ml/min, W/F = 5 g zeolite·h/mol feed (■) 6 g zeolite·h/mol feed (▲). 126
73 1.2 0.8 1.6 10 12.4 1
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4. Conclusions
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Upgrading biofuel, as bio-diesel, can be carried out by deoxygenation over potassium X zeolite. The decrease of the silicon/aluminum molar ratio in the zeolite leads to the deoxygenation of methyl laurate, reaching a steady yield with the time on stream around 30% to deoxygenation products (C10 to C12), with a selectivity about 70%. The new biofuel increased the heat value (2500 kJ/kg) and decreased the viscosity (24%) respect to the methyl laurate initial. The oxygen content respect to the initial bio-fuel was reduced. The use of methanol as solvent is a key factor to avoid the transesterification or the hydrolysis reactions, which lead to oxygenated compounds. Moreover, the use of methanol promotes the hydrogen formation in situ, which favoured the hydrogenation of the methyl laurate to produce dodecane. Decarbonylation (undecene) and decarboxylation (undecane) were also produced over these zeolites. Therefore, the increase of the basicity in the zeolites (low silicon/aluminium ratio and potassium ion-exchange) favours the deoxygenation reaction by the different mechanisms increasing the quality of the biofuel. Acknowledgements This work was supported by the financial support of the SantanderUCM 2018 project (PR75718). References [1] G. Knothe, Fuel Process. Technol. 86 (2005) 1059 https://doi.org/10.1016/j. fuproc.2004.11.002. [2] M.A. Peralta, T. Sooknoi, T. Danuthai, D.E. Resasco, J. Mol. Catal. A Chem. 312 (2009) 78–86 https://doi.org/10.1016/j.molcata.2009.07.008. [3] V.P. Vladimir, M. Nathan, A. Kwangjin, A. Selim, A.S. Gabor, Nano Lett. 12 (2012) 5196–5201 https://dx.doi.org/10.1021/nl3023127. [4] H. Lee, H. Kim, M.J. Yu, C.H. Ko, J. Jeon, J. Jae, S.H. Park, S. Jung, Y. Park, Sci. Rep. 6 (2016) 28765 https://doi.org/10.1038/srep28765. [5] L. Boda, G. Onyestya, H. Solt, F. Lonyi, J. Valyon, A. Thernesz, Appl. Catal. Gen. 374 (2010) 158–169 https://doi.org/10.1016/j.apcata.2009.12.005. [6] D. Kubička, J. Horáček, M. Setnička, R. Bulánek, A. Zukal, I. Kubičková, Appl. Catal. B Environ. 145 (2014) 101–107 https://doi.org/10.1016/j.apcatb.2013.01.012.
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