Preparation and Characterization of Zeolite Supported

Article pubs.acs.org/IECR

Preparation and Characterization of Zeolite Supported Fluoropalladium Oxalate Catalyst for Hydrodeoxygenation of Oleic Acid into Paraffinic Fuel O. B. Ayodele,† Hazzim F. Abbas,‡ and Wan Mohd Ashri Wan Daud*,† †

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia Department of Chemical Engineering, Nizwa University, Nizwa, Oman

‡

S Supporting Information *

ABSTRACT: Oleic acid (OA) was hydrodeoxygenated in this study using zeolite-supported fluoropalladium oxalate (FPdOx/ Zeol) catalyst. The FPdOx/Zeol was prepared via a pH controlled simple dissolution method and characterized with thermal gravimetric analysis, energy dispersive X-ray, X-ray fluorescence, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and Brunauer−Emmett−Teller techniques. The results showed that the deposited Pd particle was highly dispersed on the zeolite (Zeol) because of the presence of oxalate ligands and proper calcination. This observation was corroborated by the transformation of the Zeol support from crystalline into amorphous in FPdOx/Zeol as seen in the XRD and scanning electron microscopy results. The best experimental condition for the hydrodeoxygenation (HDO) of 3.5 g of OA was 370 °C, 20 mg of FPdOx/Zeol, and 100 mL/min of reducing gas (5% H2/N2) flow rate. The FTIR spectra of the evolved products at these conditions showed that the HDO of OA proceeded via the formation of stearic acid as intermediate product. A mixture of highly purified paraffinic fuel (iso-octadecane, ∼18%, and n-ocatadecane, ∼69%) was obtained after 44 min of HDO. The production of iso-octadecane which is an excellent fuel additive because of its antifreezing quality was due to the presence of fluoride ion in the FPdOx/Zeol. The FPdOx/Zeol demonstrated excellent qualities, and the results are promising toward further research and industrialization.

1. INTRODUCTION Recently attention has drifted toward production of renewable and sustainable fuel (RSF) that can conveniently complement or possibly replace conventional fossil fuel (CFF) in the future, since CFF is gradually becoming unpopular due to its many negative effects on the environment such as climate change, in addition to its alarming depletion rate.1,2 Among the first classified RSF was bioethanol and its derivatives produced from carbohydrate through a series of biochemical conversion processes where varieties of enzymes have been developed, tested and commercialized.3 Similarly, in the past decades, second generation RSF popularly referred to as biodiesel (fatty acid methyl esters) was researched and is still currently being studied to find optimum operating conditions, catalyst formulation, and sustainable sources of feed stock.3−7 Even though biodiesel has good lubricity and a high cetane number, it equally has a number of shortcomings, which include cold filter plugging points that increase its potential to clog the fuel filter more than regular diesel fuel. Other drawbacks include poor storage stability and engine compatibility issues, and it naturally has a higher viscosity.7,8 These factors have limited the biodiesel potential to adapt to the existing machineries and automobiles without necessarily manipulating their original design and configurations.6−8 It is also worth mentioning that a large volume of glycerol is produced (10 wt %) in the process which has not found a matching consumption market.9 In view of the foregoing, the need to search for newer and cheaper raw materials as well as alternative cheaper production techniques for RSF becomes imperative.6,8 © XXXX American Chemical Society

Currently, biofuel (green diesel) obtained from catalytic deoxygenation of bio-oil, carboxylic acids, and triglycerides have been identified as a promising candidate for the production of RSF. This is due to its ability to reduce hazardous vehicular emissions and its constituents, and it has properties that are similar to that of the hydrocarbons found in conventional petroleum diesel such that there may be no need for serious engine modifications.4,7−9 It is emerging as one of the most strategically important sustainable fuel sources and can be considered an important approach toward limiting greenhouse gas emissions and improving air quality.10−13 Previous studies have shown that deoxygenation of bio-oil can proceed via decarboxylation (−CO2), decarbonylation (−CO), and hydrodeoxygenation (HDO) (−H2O) reactions.14−26 On the basis of these approaches, several researchers have recently developed different technologies and processes for the production of paraffinic fuels which closely resemble petroleum diesel in their properties.4,7,18−20 Bernas et al.18 reported that the major challenge inherent in the deoxygenation of fatty acids via dearboxylation or decarbonylation (Decarbs) processes is rapid catalyst deactivation which summarily leads to low reaction rates. Subsequent to this demerit, Mortensena et al.21 discussed the advantages and industrial possibilities of catalytic hydrotreatment of different bio-oils into upgraded biofuel as already Received: October 11, 2013 Revised: December 9, 2013 Accepted: December 19, 2013

A

dx.doi.org/10.1021/ie4034158 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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commercialized by Neste Oil.22 Since the catalytic HDO process does not suffer catalyst deactivation problems, and also is capable of generating both n-paraffins and iso-paraffins with minimal or no nonhydrocarbons as byproducts, it can be considered a suitable alternative to Decarbs.23−25 Furthermore the existing HDO facilities in the present conventional petroleum refineries could be adapted to reduce capital expenditures.26 Among the catalyst considered for biofuel production in both Decarbs and HDO, palladium appear to be the most suitable.8,20,21,26 In efforts toward enhancing the quality (cold flow property) of biofuel, a catalyst with isomerization ability is an advantage, and this could be achieved via acidifying the catalyst.23,25 Simacek et al.27 sulfided NiMo/zeolite catalyst to enhance its acidity and tested it within a temperature and pressure range of 260−360 °C and 70−150 bar, respectively, on rapeseed oil to obtain some quantities of isoparaffin. Similarly, sulfided NiMo/ SiO2-Al2O3 and sulfided NiMo/Al2O3 catalysts were investigated on the HDO of gas oil24 and rapeseed oil,28 respectively, and the results showed an increase in the iso-paraffins when temperature was increased and pressure decreased as compared with the nonacidified version. Since the presence of sulfur could be of environmental concern, there is need to find a suitable substitute acidifying agent. In this present study, fluoro-palladium oxalate catalyst supported on zeolite was developed by functionalization of a freshly prepared palladium(II) oxalate complex with fluoride ion to enhance its acidity. The catalyst was characterized for physical and chemical properties, and its activity was monitored on the HDO of oleic acid as a model compound for the production of biofuel. The chemistry of the HDO process was carefully monitored on the Fourier transformed infrared (FTIR) spectroscopy of the evolved products at different HDO time intervals. The catalyst showed a minimal number of byproducts and shorter reaction time due to the presence of the Pd-oxalate ligands.

each sample was heated in a silica crucible at a constant heating rate of 10 °C/min operating in a stream of N2 atmosphere with a flow rate of 40 mL/min from 30 to 700 °C and weight loss per time, weight loss per temperature increment, and temperature increment versus time were recorded. X-ray fluorescence (XRF) analyses of the samples were done using a μXay μEDX 100 Schmadzu, NY and X-ray tube of rhodium anode and scintillation detector operating on a 40 mA current and 40 mV voltage to determine the chemical composition of the sample. Energy dispersive X-ray (EDX) was performed to determine the elemental composition variation in the FPdOx/ Zeol compared with the Zeol using EDX microanalysis system (Oxford INCA 400, Germany) connected to the FESEM machine. The EDX analysis used Mn Kα as the energy source operated at 15 kV of accelerating voltage, 155 eV resolutions, and 22.4° take off angle. Scanning electron microscopy (SEM) was used to study the surface morphology of all the samples. The analysis was carried out using a scanning electron microscope (model EMJEOL-JSM6301-F) with an Oxford INCA/ENERGY-350 microanalysis system. The samples were evenly distributed on a black double-sided carbon tape attached to the aluminum stub and vacuumed for about 10 min prior to analysis. The samples were scanned at various magnifications. The X-ray diffraction (XRD) patterns of the samples were measured with a Philip PW 1820 diffractometer to determine the crystal phase and structure of the metal oxides earlier detected by XRF analysis. Diffraction patterns of the samples were recorded with Cu Kα radiation and recorded in the range of 5−90° (2 theta) with a scanning rate of 2°/min and a step size of 0.01°. The X-ray tube was operated at 40 kV and 120 mA. Fourier transformed infrared spectroscopy (FTIR) analyses were performed on the samples using Perkin-Elmer Spectrum GX infrared spectrometer with a resolution of 4 cm−1 operating in the range of 4000−400 cm−1. The samples and analytical grade KBr were dried at 100 °C overnight prior to the FTIR analysis. The spectra were automatically plotted by the software installed on the computer attached to the FTIR instrument. The Raman spectra were obtained with a Spex Triplemate spectrograph coupled to a Tracor Northern 1024 large area intensified diode array detector. The excitation source was a 488 nm line (Lexel Model No. 95 Ar+) laser with a grating monochromator used to reject any spurious lines and background from the laser before the radiation entered the spectrometer. The spectra were taken with 1 cm−1 resolution. Nitrogen adsorption−desorption measurements (Brunauer− Emmett−Teller (BET) method) were performed at liquid nitrogen temperature (−196 °C) with an autosorb BET apparatus, Micromeritics ASAP 2020, surface area and porosity analyzer to determine the surface area, pore size, and structure, and the pore volume. Before each measurement, the samples were first degassed at 300 °C for 2 h and thereafter kept at liquid nitrogen temperature to adsorb nitrogen. The adsorption−desorption isotherm were obtained by measuring the equilibrium pressure during the adsorption and desorption of known volume of liquid nitrogen. 2.4. Catalyst Testing and Process Description. A thermogravimetric analyzer (STA 449 F1/F3 with Automatic Sample Changer (ASC), Netzsch, Germany) was the reactor chamber, and it was coupled to Fourier transform infrared spectroscopy (OPUS v7.0, Bruker Optics) to investigate the mass loss rate (rate of reaction) of oleic acid (OA) in the presence of FPdOx/Zeol and the qualitative analysis of the evolved reaction products concurrently. The quatitative analysis

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were purchased from Sigma Aldrich except hydrofluoric acid (HF) and oxalic acid that were purchased from Spectrum Chemicals. 2.2. Catalyst Synthesis. The catalyst synthesis was achieved by reacting 2.36 g of H2PdCl4 with aqueous oxalic acid in stoichiometric ratios to yield the polynuclear palladium(II) oxalate complexes in a 500 mL conical flask that was wrapped with aluminum foil, since metal oxalate complexes have a high photosensitivity index expecially under UV light.29 As the reaction proceeded, drops of 2 M HF acid were added to lower the pH from in situ 5.4 to 2.5, and the solution was ripened for 1 h; the resulting fluoropalladium oxalate (FPdOx) complex (Scheme S1, Supporting Information) was carefully added to the already prepared 50 g of Zeol dispersion under intense stirring at 50 °C to ensure successful incorporation of the FPdOx into the Zeol matrix. The resulting product was filtered, washed with deionized water to remove chloride ions, and dried in the oven at 110 °C. The synthesized fluoropalladium oxalate supported zeolite catalyst (FPdOx/Zeol) was finally ground to powder and calcined at 400 °C for 3 h. 2.3. Catalyst Characterization. Thermal gravimetric analysis (TGA) analyses were carried out with a SHIMADZU DTG-60/60H instrument to determine the weight loss on the samples with temperature increment in order to determine the heat treatment required during calcination. A 2 g portion of B



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of the evoled products at the end of the experiment was achieved in the associated STAultrafast mass spectrometry (UFMS). On the basis of a series of preliminary experimental runs, 3.5 g of OA and 20 mg of FPdOx/Zeol were premixed thoroughly and placed in the TGA crucible made of high thermal resistant alumina which serves as the reactor. The mixture was heated from 27 to 370 °C at a high heating rate of 30 °C/min in order to minimize the “lag time”. A mixture of high purity nitrogen (95%) and hydrogen (5%) at a flow rate of 100 mL/min was continuously bubbled into the reactor to provide the reducing environment. Another 20 mL/min of nitrogen was supplied as protective gas for the crucible reactor to maintain balance. Prior to the experimental study, a blank test for the thermogravimetric behavior of the FPdOx/Zeol catalyst was conducted with the same experimental conditions to ascertain if the mass loss during the experiment was solely due to evolved products or in combination with the moisture and volatile matter loss from the catalyst. The vapor of the reaction products was conveyed into the FTIR spectrometer through a transfer tube which was maintained at 200 °C to prevent the condensation of reaction products on the carrier tube wall. The products spectra were analyzed in the range of 4500−600 cm−1 using spectrum resolution of 1 cm−1 with a temporal resolution of about 7.03 s.

Figure 1. X-ray fluorescence of Zeol and FPdOx/Zeol samples.

to the effect of proper calcination that reduced the strongly bonded water as seen the TGA profile. 3.1.3. Energy Dispersive X-ray. The results in Table 1 show the elemental composition of Zeol and FPdOx/Zeol samples. The Zeol composition is basically silica and alumina with some amount of sodium and calcium oxides, the Zeol Si/Al ratio is 1.02, typical of zeolite NaA. After the incorporation of the catalyst precursor and calcination, there is slight increment in the Si/Al ratio to 1.50. This is in consonance with an earlier report on zeolite NaA which showed that there is an inverse relationship between the amount of zeolitic water and the Si/Al ratio.30 As observed in the TGA profile (Supporting Information, Figure S1), calcination reduced both the physisorbed and the strongly bonded water, and this is further supported by the reduction in the amount of oxygen as shown in Table 1. The EDX spectrum shown in Figure 2 also corroborated the presence of the Pd particle between 2.4 and 3.8 keV as well as the successful functionalization of fluoride ion. 3.1.4. Scanning Electron Microscopy (SEM). The changes in the morphology of zeolite and even clays were considered to have significant effects on their catalytic properties.32 The SEM images in Figure 2a revealed the morphology of Zeol which consists of agglomerates of microsized cubical symmetry crystal structure. The crystalline nature of the Zeol sample was seen to have changed into amorphous-like material in FPdOx/Zeol with considerable reduction in particle size as seen in Table 1 (textural properties of the samples as studied by the N2 adsorption/desorption of BET). This denaturation of the crystalline structure can be ascribed to the effect of oxalic acid and probably drops of HF at the catalyst synthesis stage and the effect of calcination.33 Aluminosilicates (zeolites and clays) have been reported to transform from crystalline into amorphous under acid treatments,29 and this transformation is always followed by an increase in specific surface area29,34 as seen in the textural properties of FPdOx/Zeol in Table 1. 3.1.5. X-ray Diffraction (XRD). X-ray diffraction of the Zeol and FPdOx/Zeol samples are presented in Figure 3. The Zeol sample showed characteristics peaks of Zeolite A according to the JCPDS card 43-0142.35 The Zeol sample is seen to be highly crystalline but becomes amorphous after the FPdOx/ Zeol synthesis, and this further corroborates the earlier observation on the morphology of the samples (Figure 2). Previous studies have shown that transformation of the catalyst sample after thermal and/or chemical treatment from crystalline into amorphous also assures a high dispersion of the active metal into the matrix of the support, which inevitably guarantees proper anchoring of the metal onto the support.29,32,36 The presence of a Pd particle is seen at 2θ = 18° and 35.7°, the latter being close to the 33.9° earlier

3. RESULTS AND DISCUSSION 3.1. Catalyst characterization. 3.1.1. Thermal Gravimetric Analysis (TGA). Figure S1 (Supporting Information) shows the result of thermal gravimetric analysis of Zeol and FPdOx/ Zeol (before and after calcination) with three characteristics regions. For both Zeol and FPdOx/Zeol (before calcination) there exists a rapid weight loss up to 110−140 °C which was undoubtedly due to the release of physisorbed water.29,30 The value of the physisorbed water appeared higher in the latter than in the former due to the high degree of hydration at the catalyst synthesis stage. The second region is due to the presence of strongly bonded water molecules that are present in the first coordination sphere and require heat treatment up to about 650 °C for their removal, and finally in the third region was the structural hydroxyl groups that will condense and dehydrate at temperatures above 650 °C.29 It can be seen that the catalyst synthesis protocol was able to increase the strongly bonded water even more than the physisorbed water. This implied that the hydrated FPdOx precursor was able to penetrate into the supercage structure of the Zeol support. The effect of calcination showed a drastic reduction in both the physisorbed and strongly bonded water as seen in the FPdOx/ Zeol (after calcination). Ohtsuka and Tabata31 reported that the presence of water in the synthesized Pd-catalyst leads to the agglomeration of Pd particles, while proper calcination guarantees a high dispersion of Pd particles which is a requirement for reduction processes such as HDO, although agglomerated Pd particles are very active for oxidation processes such as methane combustion.31 3.1.2. X-ray Fluorescence. The XRF spectra of both Zeol and calcined FPdOx/Zeol are shown in Figure 1. The incorporation of Pd into the matrix of the Zeol is seen in the XRF spectrum of FPdOx/Zeol at Lα1 of 2.99 keV, and it is in accordance with the standard card of peak identification (EDXRF-EPSILON 3 XL, PANalytical). Another Pd peak is observed at Lβ1 of 3.20 keV which possibly suggests a high dispersion of Pd particles from palladium oxalate (PdOx) due C



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Table 1. Elemental Composition and Textural Properties of Zeol and FPdOx/Zeol Samples elemental composition (%) sample Zeol FPdOx/ Zeol

Si

Al

O2

Na + Ca

16.88 18.73

16.48 12.5

49.58 46.80

16.61 11.72

BET

F

Cu

Pd

0.0 8.2

0.45 0.12

0.00 1.86

Si/Al

surface area (m2/g)

pore volume (cm3/g)

pore diameter (nm)

particle size (nm)

1.02 1.20

301.84 385.68

0.13 0.27

19.41 17.11

10404 2534.1

Figure 2. Energy dispersive X-ray of (a) Zeol and (b) FPdOx/Zeol samples.

Figure 3. X-ray diffraction of the Zeol and FPdOx/Zeol samples.

Figure 4. FTIR spectroscopy of the Zeol and FPdOx/Zeol samples.

reported by Pommier and Gelin37 on H-ZSM-5 zeolite type and 34° reported by Gannouni et al.;38 others were seen at 38.4°, 42°, 58.7° and 61.8°.39 The successful incorporation of Pd is also seen to have elevated the baseline of the Zeolite A supercage structure in FPdOx/Zeol progressively toward to the low angle after transformation into an amorphous material. 3.1.6. Fourier’s Transform Infrared Spectroscopy (FTIR). The FTIR spectroscopy of the Zeol and FPdOx/Zeol samples are shown in Figure 4. The vibrations in the region 3700−2900 cm−1 with a minimum at 3317 cm−1 in the Zeol sample can be assigned to the presence of zeolitic water,40 while the band at 1650 cm−1 is due to water vibration. These bands disappeared in the FPdOx/Zeol sample because of the effect of proper calcination. It has been reported that the presence of zeolitic water or other forms of water vibrations causes transformation of dispersed Pd particles into agglomerated Pd which does not favor reduction processes31 such as the HDO of carboxylic acid being considered in this study. The disappearance of these bands indicates the possibility of highly dispersed Pd which is required for the HDO process. Pd siting in zeolite frameworks is not yet properly understood, but Bell et al.31 was quoted to have proposed models for Pd siting in the form of Z−-Pd2+ (OH)− or Z−H+(PdO), while the standing form takes Z−H+

(PdO)H + Z − for the aluminum site of zeolite. The smoothening between the band 1582 and 1306 cm−1 could be due to the presence of the oxalate ligand because it is in close agreement with the spectrum of C2O42− (aq) (νC−O at 1569 and νC−O at 1308 cm−1, both asymmetry) reported by Person and Axe41 in a study on metal-bonded surface complexes. It can be observed that immediately after the 1306 cm−1 band the FPdOx/Zeol spectrum showed a clear deviation, shifting from the spectrum of the Zeol. This occurs because the presence of metals (ions or oxides) on aluminosilicates is usually observed in the lower wavenumbers. Hence, the slight shift in 940 cm−1 to ∼918 cm−1 upon incorporation of the catalyst precursor and calcination can be considered as a good indicator for the presence of Pd oxalate ligands on the zeolite supercage structure which indicated that Pd particle dispersion was sustained after calcination.37 The lowering of the band at 675 cm−1 and absorption band at 576 cm−1 could also be ascribed to the interference effect of Pd incorporation. 3.1.7. Raman Spectroscopy. Raman spectroscopy bands of different silica mesoporous supports have been well reported such that the νsym(SiOSi) bands and the νasym(SiOSi) vibrations D



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have been assigned to the range between 800 and 850 cm−1 and between 1000 and 1200 cm−1, respectively.38,42 Similarly, the bands around 550−600 cm−1 and 270−425 cm−1 have been assigned to ν(SiO) + δ(OSiO) and δ (SiOSi), respectively.38 The characteristic bands of Zeol in Figure 5 appear at 280, 330,

which shows an increment in the adsorbed quantities of N2 at STP from 565 cm3 g−1 in Zeol to 677 cm3 g−1 in FPdOx/Zeol. Both samples showed Type IV isotherms which according to the IUPAC classification are generally observed for mesoporous solids.36 3.2. Hydrodeoxygenation of Oleic Acid Using FPdOx/ Zeol. 3.2.1. Preliminary Studies. On the basis of some preliminary studies, Figure S3 (Supporting Information) shows the best observed experimental condition with a temperature of 370 °C (DTG curve), reducing gas (5%H2/N2) flow rate of 100 mL/min, and 20 mg of FPdOx/Zeol loading. This value of temperature is slightly lower than 380 °C earlier reported by Arend et al.4 on the deoxygenation of OA using commercial 2% Pd supported on carbon supplied by Johnson Matthey. It is therefore logical to conclude that the FPdOx/Zeol synthesized in this study seems to be more reactive by lowering the activation energy required for the HDO due to the presence of the palladuim oxalate ligand (section 3.1). This perhaps explains why the gas flow rate of 100 mL/min required in this study doubles the value reported by the authors since the products turnover rate in this study is higher and there is need to prevent excessive secondary reactions such as polymerization and cracking.23 Generally, it has been investigated that catalysts containing metal oxalate ligands are more reactive than their metal oxide counterparts.29 This is due to their abilities to minimize the formation of intermediate compounds and maintain their catalytic activity over a wide range of process conditions including changes in the pH of the reaction medium.29,45,46 3.2.2. HDO Product Identification. Figure 6 shows the 3D absorbance of the evolved products of HDO of OA using

Figure 5. Raman spectroscopy of the Zeol and FPdOx/Zeol samples.

405, 490, 700, 977, 1040, and 1150 cm−1, which are typical of zeolite A which contains four- and six-membered rings.43 The strongest band at 490 cm−1 is assigned to the bending mode of four-membered Si−O−Al rings while the bands at 330 and 405 cm−1 are due to the bending mode of six-membered Si−O−Al rings.44 The bands at 977, 1040, and 1100 cm−1 are ascribed to asymmetric T−O stretching motions.38,44 The Zeol spectrum shows an unusual band at 800 cm−1 which is not a signature of Zeolite A; this band is similar to that found in ZSM-5 and it is ascribed to symmetric stretching.44 It is very remarkable that Zeol exhibits a band at 280 cm−1, which is attributed to the bending mode of higher rings than four- and six-membered rings, possibly of the eight-membered rings of zeolite A since previous studies had shown that smaller rings give bands at higher frequencies and vice versa.44 The presence of Pd particle on the FPdOx/Zeol is first observed around 640 cm−1 which is close to 648 cm−1 reported by Ohtsuka and Tabata,31 the band is ascribable to the Raman active B1g vibrational mode of the PdO phase which is commonly seen at 651 cm−1 for single crystals or PdO foils, or between 626 and 640 cm−1 for oxidized Pd deposited on alumina or zirconia.38 Previous studies have also shown that successful incorporation of Pd particles into zeolite support induces an absorption band around 928 cm−1 as seen in Figure 5, and it is attributed to the stretching vibrations of the T-O bonds.37 3.1.8. Nitrogen Adsorption/Desorption Isotherm (BET). The textural properties of Zeol and FPdOx/Zeol samples are shown in Table 1. As earlier noted that transformation of crystalline material into amorphous is usually associated with an increment in specific surface area, the results in Table 1 showed that there is increment in the specific surface area and pore volume of FPdOx/Zeol as compared to the parent Zeol. This increment is more evident by the reduction in the average particle size of the FPdOx/Zeol sample probably due to the effect of oxalic acid and calcination at the catalyst synthesis stage as earlier noted. The enhancement in the textural properties can be further corroborated by the N2 adsorption/ desorption isotherm in Figure S2 (Supporting Information)

Figure 6. 3D absorbance of the evolved products of HDO process of 3.5 g of OA into paraffinic fuel using 20 mg of FPdOx/Zeol catalyst.

FPdOx/Zeol catalyst as monitored with FTIR spectroscopy in the range of 4500−600 cm−1 at different reaction time. Similarly, Figure S4 (Supporting Information) shows an insight into the progress of the HDO process, especially the orthonormalized vector form of the FTIR analysis of the evolved products measured and plotted as a function of time using the Gram Schmidt (GS) process based on the 3D FTIR absorbance in Figure 6. It can be seen that as the HDO process progressed from t = 0 until t = 14 min, no material loss was observerved in both the TGA and the DTA plots (Figure S4, Supporting Information). This was further corroborated by the GS plot which showed a smooth curve until about 14 min, signaling no evolution of reaction products. This phenomenon E



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is also consistent with the observation on the time scale of Figure 6, and it implied that OA was still thermodynamically stable in those time regions because the thermal energy gained is not sufficient to initiate breaking and/or formation of new bonds. The FTIR spectra of the evolved species observed in those period is plotted in Figure S5 (Supporting Information) which suggests that water vapor and instrument noise are prodominant until around 10−14 min when traces of OA are gradually seen. The GS plot showed four regions between 15 and 40 min signifying that there are different stages of intermediate compounds formation until the final product was formed. Between 15 and 19 min which can be considered the initiation stage, only about 2% mass loss was observed as calculated from the TGA plot as the temperature rose to stabilize at 370 °C, this loss could be due to expulsion of moisture and some volatile contents in order to prepare the OA for reaction. The absorbance of the FTIR spectra of the evolved products within this time frame is plotted in Figure 7. All the spectra showed

Figure 8. FTIR spectra of evolved products between 20 and 25 min on the HDO of 3.5 g of OA using 20 mg of FPdOx/Zeol catalyst at 370 °C.

suggest the gradual formation of SA. Similarly, the intense peak seen at 1779 cm−1 which was ascribed to the existence of the CO stretch in OA is seen to be fast reducing, while another peak in its neighborhood at 1718 cm−1 confirmed the formation of SA in addition to those growing bands between 1463 and 1145 cm−1 which are characteristics of SA.49 One important feature that distinguished SA from OA is the increase in the bandwidth for the band between 3000 and 2800 cm−1 in OA to about 3400−2400 cm−1 in SA. Finally, there is a formation of a new peak at ∼3010 cm−1 at about 23−25 min, and it due to the formation of the C−H stretch of alkane;49,50 this observation heralds the gradual formation of the paraffinic biofuel, that is, octadecane (C18H38). C17H33COOH + H 2 → C17H35COOH

(1)

C17H35COOH + 2H 2 → C18H38 + H 2O

(2)

Successful formation of SA was clearly evident in Figure 9 especially between 26 and 30 min. The fast reducing band


distinct characteristics of OA. The band between 3900 and 3500 cm−1 as well as that between 3000 and 2800 cm−1 are absolutely due to the O−H stretch of the carboxylic acid since no other functional group had such a broad and intense band at high wavenumber.47 Two sharp peaks at 2954 and 2860 cm−1, which are superimposed on the O−H stretch are attributes of asymmetric CH 2 stretch and symmetric CH2 stretch, respectively.47,48 The intense peak at 1779 cm−1 is due to the existence of the CO stretch, and the band at 1462 cm−1 is assigned to CH2 bending modes, while the peak corresponding to 1285 cm−1 is due to the presence of the C−O stretch.47 The observed infrared spectra of OA conform to the OA standard spectrum in the NIST Chemistry Webbook.49 The second region observed in the GS plot is between 20 and 25 min and the spectra analysis showed that stearic acid (SA) was gradually being formed according to the FTIR spectra shown in Figure 8. Arend et al.4 and Immer et al.20 reported that the HDO of unsaturated carboxylic acids such as OA proceeded in two stages; first sequential hydrogenation and subsequent deoxygenation. In this case, OA is observed to be first hydrogenated into saturated SA (eq 1), followed by deoxygenation to obtain the paraffinic fuel (eq 2). From Figure 8, the drastic reduction of the O−H stretch of the carboxylic acid band between 3900 and 3500 cm−1 earlier observed in the OA spectra and the growth of two peaks around 2370 cm−1


between 3900 and 3500 cm−1 earlier observed in the OA spectra (Figure 8) has almost completely disappeared except for a little sprout at 3577 cm−1 which is a signature that the SA was formed from OA, since SA does not have such characteristics peak. The intense peak at 1779 cm−1 earlier ascribed to the existence of the CO stretch of carboxylic acid in OA (Figures 7 and 8) has also nearly disappeared at 32 min; this and the superimposition of the spectrum at 34 min on the F



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quantitative analysis of the paraffinic fuel obtained showed about 18% iso-octadecane and ∼69% n-ocatadecane; the production of iso-octadecane which is an excellent biofuel additive because of its low freezing point is undoubtedly due to the presence of fluoride ion in the FPdOx/Zeol. The minimization of byproducts and side reactions is due to the presence of oxalate ligands in the FPdOx/Zeol. The study is promising for further research toward industrial application.

spectrum at 32 min confirmed gradual HDO of OA into C18H38. The bands between 1463 and 1145 cm−1 have nearly disappeared after 38 min in Figure 10, and the superimposed

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Scheme S1, formation of the fluoro-palladium oxalate catalyst precursor; Figure S1, thermal gravimetric analysis of Zeol and FPdOx/Zeol (before and after calcination); Figure S2, nitrogen adsorption/desorption isotherm of the Zeol and FPdOx/Zeol samples; Figure S3, result of best experimental condition for the HDO process of 3.5 g of OA into paraffinic fuel using 20 mg of FPdOx/Zeol catalyst; Figure S4, progress of the HDO process of OA using 20 mg of FPdOx/Zeol catalyst at 370 °C; Figure S5, FTIR spectra of evolved products between 0 and 14 min on the HDO of 3.5 g of OA using 20 mg of FPdOx/Zeol catalyst at 370 °C; Figure S6, composition of evolved product in the HDO process of 3.5 g of OA into paraffinic fuel using 20 mg of FPdOx/Zeol catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 10. FTIR spectra of biofuel formed from the HDO of 3.5 g of OA between 36 and 44 min using 20 mg of FPdOx/Zeol catalyst at 370 °C.

spectra within 38−44 min showed a high degree of correlation with the standard C18H38 spectrum available at the NIST Chemistry Webbook.49 The superimposed of spectra in that time range also confirmed the thermal stability of the C18H38 biofuel produced. This implied that the fuel can be conveniently stored without trepidation of deterioration due to any unwanted reaction; it is worth noting that saturated alkanes are not very reactive. 3.3. Product Quantification. The mass spectra quantitative analysis of the product after 44 min of HDO process using the Spectrum Search facility of OPUS software components library in conjunction with STA-UFMS showed that both nC18H38 and iso-C18H38 were formed with some accompanying heptadecane, methane, water, carbon dioxide, and carbon monoxide molecules as shown in Figure S6 (Supporting Information). The presence of C17H36 suggests the possibility of decarbonylation/decarboxylation reaction which was confirmed with the traces of CO/CO2. However, the formation of iso-C18H38 was due to the functionalization of the catalyst with fluoride ion at the synthesis stage which lowered the pH to 2.5. Acidic catalysts have been reported to have the ability to produce both normal and isomerized hydrocarbon during the HDO process.23 The iso-paraffins have the advantage of improving the cold flow properties of the produced biofuel, since the iso-C18H38 has a lower freezing point.14,23

Corresponding Author

*Tel.: +60164955453. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge the financial support from Higher Impact Research−Ministry of Higher Education Project No D000011-16001 of the Faculty of Engineering, University of Malaya.

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REFERENCES

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4. CONCLUSION The functionalization of Zeol with freshly prepared palladium oxalate complex modified with fluoride ion (FPdOx/Zeol) as a hydrodeoxygenation reaction catalyst has demonstrated excellent results in the production of a mixture of n-ocatadecane and iso-octadecane paraffinic fuel from oleic acid (OA). The catalyst characterization showed that the palladium particle incorporated into the Zeol support is highly dispersed due to the presence of oxalate ligands and proper calcination as shown by the TGA, XRD, FTIR, and Raman spectroscopy results. Similarly, the BET surface area result showed that the FPdOx/ Zeol has sufficient pore size to guarantee the diffusion of isomerized product from the catalyst. The catalytic activity of FPdOx/Zeol tested on the HDO of OA being monitored based on the FTIR spectra changes of the evolved product with time showed that the deoxygenation of OA into paraffinic fuel proceeded via the formation of stearic acid as intermediate. The G



Article

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Preparation and Characterization of Zeolite Supported

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