Hydrodeoxygenation of Stearic Acid into Normal and Iso-Octadecane

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Hydrodeoxygenation of Stearic Acid into Normal and IsoOctadecane Biofuel with Zeolite Supported Palladium-Oxalate Catalyst O. B. Ayodele,*,† Hazzim F. Abbas,‡ and Wan Mohd Ashri Wan Daud*,† †

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

‡

ABSTRACT: This study reports the hydrodeoxygenation (HDO) of stearic acid (SA) into paraffinic biofuel with synthesized palladium-oxalate zeolite supported catalyst (PdOx/Zeol). The PdOx/Zeol was synthesized via the functionalization of dihydrogen tetrachloropalladate (II) with aqueous oxalic acid (OxA) to form the polynuclear palladium(II) oxalate (PdOx), which was supported on zeolite. The SEM and XRD characterization results showed that the zeolite support is highly crystalline but loss some degree of crystallinity in the PdOx/Zeol sample after PdOx incorporation. The activity of the PdOx/Zeol tested on the HDO of SA showed that temperature, pressure, gas flow rate, and PdOx/Zeol loading have significant effects on the HDO process, and their best observed conditions were 360 °C, 20 bar, 100 mL/min, and 25 mg, respectively to achieve 92% biofuel production from 35 g SA. The biofuel product distribution showed 71% n-C18H38, 18% iso-C18H38, and 3% C17H36. The presence of iso-C18H38, which is an excellent biofuel value-added-component due to its low freezing point, was ascribed to the functionalization of Pd with OxA, which increases PdOx/Zeol acidity. The results showed that PdOx/Zeol is a prospective catalyst toward further research and commercialization of HDO process of SA.

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INTRODUCTION The recent shift of attention on energy generation from fossils toward renewable sources has led to different funded research studies, such as sourcing the most reliable renewable feed stocks and the most economical and safe operating conditions.1−3 Other studies also focused on sustainable and efficient catalyst development, process equipment design, and engineering.4−6 The most fundamental in all these studies is the catalyst development because it affects the rate of reaction, which is a major determining factor in reactor design and engineering. It also determines the types and qualities of product(s) to be obtained, which in turn has a strong bearing on the product separation, purification, and standardization. A good catalyst for the production of biofuel is expected to produce both normal and iso-paraffin in order to have a reasonably high cetane number as well as improve the biofuel cold flow properties. The presence of iso-paraffin enhances the cold flow properties of biofuel by lowering the freezing point of the fuel; for example, the freezing point of the C16−C18 nparaffin lies between 18 and 28 °C, while with the presence of 20% iso-paraffin it can be reduced to about 10−12 °C.7−9 Different catalysts have recently been researched and reported in the literature while concerted efforts at improving both the catalyst synthesis procedure and operating conditions are still ongoing. Among the best reported active metals used in the hydrodeoxygenation (HDO) process to remove oxygen molecules from feed stocks is palladium. Madsen et al.5 reported the deoxygenation of dilute and concentrated stearic acid over 2% Pd/C beads in a continuous reactor at 300 °C, 20 bar and achieved 95% conversion, but the production of isoparafins were not reported. Similarly, in the work of Arend et al.,10 in a continuous gas phase deoxygenation of oleic acid using granular 2 wt % Pd/C catalyst, a maximum selectivity of © 2014 American Chemical Society

28.5 mol % for the formation of heptadecane and heptadecenes was found at a temperatures of 380 °C using 5 g 2 wt % Pd/C in a 4 h reaction. Lestari et al.11 reported deoxygenation of stearic acid into n-pentadecane and n-heptadecane with selectivity up to 67% after 5 h reaction using palladium supported on mesoporous silica SBA15 and MCM-41. Bernas et al.12 developed egg-shell type palladium catalyst supported on mesoporous carbon and reported the production of undecane and undecene from a feed stock of dodecanoic acid over temperature and pressure range of 300−360 °C and 5−20 bar, respectively. Simakova et al.13 studied four synthesized Pd/ C catalysts over the temperature range 260−300 °C on the catalytic deoxygenation of palmitic and stearic acids in a semibatch reactor, their main liquid phase products were nheptadecane and n-pentadecane. Other available reports14−16 on the use of supported Pd for the production of biofuel also do not report the presence of iso-paraffins. One of the promising methods of enhancing both the catalytic and isomerization activity of a catalyst is to increase its acidity; for example, Simacek et al.17 sulfided NiMo/Al2O3 catalysts and obtained some quantities of isoparaffin over the temperature range 260−360 °C and pressure of 70−150 bar from rapeseed oil diluted with isooctane. Krar et al.18 and Kubicka et al.19 also investigated the ability of sulfided NiMo/ SiO2−Al2O3 and sulfided NiMo/Al2O3 catalysts in the HDO of gas oil and rapeseed oil and reported increase in the iso-paraffin contents as temperature was increased and pressure decreased. In view of the environmental concerns on the use of sulfide functionalization, Kovacs et al.20 replaced the sulfide functionReceived: June 13, 2014 Revised: August 4, 2014 Published: August 5, 2014 5872

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alization with fluoride and reported a reasonable level of isomerization with the same operational variable trend earlier reported by Krar et al.18 and Kubicka et al.19 Results from a recent study21 on the catalytic deoxygenation of triglycerides and fatty acids to hydrocarbons in a semibatch autoclave over 20 wt % Ni/C and 5 wt % Pd/C catalysts showed that 20 wt % Ni/C yielded satisfactory blendable products range than 5 wt % Pd/C. According to the authors, the difference in the performance of these two catalysts was attributed to the higher acidity of the Ni-based formulation. Another expedient method of increasing catalyst acidity is via functionalization with oxalic acid (OxA) to develop metal−oxalate catalysts having organometallic abilities that have been reported to be more reactive than the metal oxide counterparts22,23 due to the metal−oxalate ligand’s ability to minimize the formation of intermediate or side products22−25 from reactions such as hydrocracking, methanization, cyclization, and water−gas-shift reaction. Consequently, they have the ability to maintain their catalytic activity over a wide range of process conditions because of the active metal stability after proper calcination.23−25 In view of the above and the fact that Pd is a very expensive metal (for example, its cost is about 1000 times more expensive than the cost of Ni),21 it is imperative to synthesize acidified supported Pd catalyst that can achieve both HDO and isomerization in one single processing step to enhance the overall process economics. Therefore, in this study, we functionalized palladium with OxA to develop a novel palladium oxalate catalyst supported on zeolite A (PdOx/ Zeol) with increased acidity. Zeolite A was selected because of its thermal and structural stability, and in addition, it is currently being cheaply produced from coal fly ash (CFA), which hitherto is the waste product of combustion of coal in coal-fired power stations with about 800 million tons per annum CFA production.26 The catalyst was chacterized for the physical and chemical properties and its HDO and isomerization activities were tested on stearic acid.

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detector operating on a 40 mA current and 40 mV voltage. Scanning electron microscopy (SEM) was employed to study the surface morphology of both Zeol support and the PdOx/Zeol catalyst. 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 scanned at various magnifications. The X-ray diffraction (XRD) patterns of the PdOx/Zeol catalyst and the Zeol support were measured with SIEMENS XRD D5000 equipped with Cu Kα radiation and recorded in the range 5−90° with a scanning rate of 2°/min. Fourier Transformed Infrared Spectroscopy (FTIR) analysis was performed on the Zeol support and the PdOx/Zeol catalyst using a PerkinElmer Spectrum GX Infrared Spectrometer with resolution of 4 cm−1, in the range 4000− 400 cm−1 to determine their surface functional groups. The Raman spectra for the Zeol support and the PdOx/Zeol catalyst were obtained to study the Pd−zeolite interaction using a Spex Triplemate spectrograph connected to a large area intensified diode array detector (Tracor Northern 1024) with a 488 nm line (Lexel Model No. 95 Ar+) laser as the excitation source with a grating monochromator for rejecting any spurious lines and background. Both Zeol support and the PdOx/Zeol catalyst spectra were taken from 200 to 1600 cm−1 with resolution of 1 cm−1. Nitrogen adsorption−desorption measurements with Brunauer−Emmett−Teller (BET) method was performed at liquid nitrogen temperature (−196 °C) with an autosorb BET apparatus, Micromeritics ASAP 2020, to determine the samples textural properties (surface area, pore volume and diameter). Stearic Acid Hydrodeoxygenation Experiments. Hydrodeoxygenation of stearic acid (SA) was conducted using a 100 mL high pressure batch reactor, the reaction temperature and pressure were varied within 300−380 °C and 10−60 bar, respectively. The reaction pressure and flow of carrier gas inlet and outlet were controlled by a pressure controller (Brooks 5866) and flow (Brooks 58505 S). In a typically experiment, 35 g (∼40 mL) of SA was placed in the reactor followed by 25 mg of PdOx/Zeol (except during the study of effect of catalyst loading) and the catalyst was reduced in situ under flowing H2 at 200 °C for 1 h, after which the reactor was purged with He. The operating temperature was established and monitored by a type-K Omega thermocouple located within the reactor. Before the reaction started, 50 mL of 90 vol % N2 and 10 vol % H2 were passed through the reactor until the desired reaction pressure was reached and the reaction commences by turning on the stirrer. Based on preliminary studies, all experiments were performed under 60 min and the reactor set up was cooled by forced air and dismantled for product analysis. Withdrawn liquid samples from the reactor were dissolved in pyridine and followed by silylation with (100 wt % excess of) N,Obis(trimethyl)-trifluoroacetamide, BSTFA, in an oven at 60 °C for 1 h prior to GC analysis. The internal standard eicosane (C20H42) was added for quantitative calculations. The analysis of the withdrawn samples was achieved with a gas chromatograph (GC, HP 6890) equipped with DB-5 column (60 m × 0.32 mm × 0.5 mm) and a flame ionization detector. A sample (1 mL) was injected into the GC with split ratio of 50:1, and helium was used as the carrier gas. The chromatographic pressure program was carefully adjusted to obtain a satisfactory separation of the desired identified products which were validated with a gas chromatograph mass spectrometer (GCMS). Since there were technical limitations and difficulty in the online quantification and analysis of the evolved gases (Egas) during the study, they were calculated according to eq 1, and the liquid product distribution was evaluated using eq 2.

EXPERIMENTAL SECTION

Materials. All chemicals and the zeolite support were purchased from Sigma-Aldrich except oxalic acid (OxA), which was purchased from Spectrum Chemicals. Catalyst Development. Prior to the synthesis of the palladium oxalate zeolite supported catalyst (PdOx/Zeol), the zeolite (Zeol) support was oven-dried at 150 °C for 3 h to remove free and bounded moisture as well as any partial occluded organic matter. The PdOx/ Zeol was synthesized via functionalization of 2.36 g of H2PdCl4 with stoichiometric ratio of aqueous OxA and was ripened for 1 h to develop the polynuclear palladium(II) oxalate complexes (PdOx) in an aluminum foil wrapped 500 mL conical flask because of the metal− oxalate complex photosensitivity.23 The precursor was then added to already prepared 50 g Zeol dispersion and left stirring for 4 h at 50 °C for the deposition of PdOx particles on the Zeol support and the pH was observed to stabilize at 5.3 ± 0.3 as the stirring progressed. The synthesized PdOx/Zeol was filtered and washed to remove the chloride ions, followed by drying at 100 °C for 12 h, and then, it was calcined at 400 °C for 3 h on the basis of a few preliminary studies. Catalyst Characterization. Thermal Gravimetric Analysis (TGA) analysis was carried out with a SHIMADZU DTG-60/60H instrument. A known weight of the samples were 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 25 to 700 °C and weight loss per time and temperature increment were recorded. X-ray fluorescence spectroscopy (XRF) analysis of the samples were done to determine their chemical composition using a μXay μEDX 100 Schmadzu, NY, and X-ray tube of rhodium anode and scintillation

Egas = [Mb + M H2 − Ma] ωi(%) =

ni 100 ∑ ni

(1)

(2)

where Mb is the mass of reactor with the OA and catalyst before reaction, MH2 is the mass of total H2 gas required during the experiment evaluated from the H2 flow rate and its density. Ma is the mass of the reactor with the liquid product and used catalyst after 5873

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reaction. Similarly, ωi(%) is the mass fraction of the components in the liquid product, and n refers to the total number of liquid components. Stearic Acid Hydrodeoxygenation Process Chemistry. Prior to the studies of the factors that influence the hydro deoxygenation of stearic acid (SA), a preliminary analysis was done to investigate the deoxygenation process chemistry with reaction time. A high thermal resistant alumina crucible (HTRAC) located in the thermogravimetric analyzer (STA 449 F1/F3 with Automatic Sample Changer, Netzsch, Germany) was used as the reactor chamber. The products exit port was coupled to a FTIR spectroscopy instrument (OPUS v7.0, Bruker Optics) to concomitantly monitor the rate of SA disappearance (i.e., rate of reaction) in the presence of PdOx/Zeol catalyst and the qualitative analysis of the evolved products of HDO of SA. In a typical catalytic experiment based on preliminary studies, 3.5 g SA and 2.5 mg of PdOx/Zeol were carefully premixed in the HTRAC. The mixture was heated at the heating rate of 30 °C/min from 30 to 360 °C, with 20 mL/min of nitrogen flow as protective gas for the HTRAC. After about 10 min, when traces of products were noticed from the FTIR display, a mixture of high purity H2/N2/ (5%) at a flow rate of 100 mL/min was continuously bubbled into the reactor. The vapors of the reaction product were conducted into the FTIR instrument via a transfer tube which was kept at 200 °C to maintain the reaction products in the vapor phase for analysis. The analysis of the products spectra was done every 7.03 s at a spectra resolution of 1 cm−1 in the wavenumber range 4000−600 cm−1. In order to verify that the mass loss recorded by the TG analyzer was strictly due to the HDO of SA and not in conjuction with moisture and volatile matter from PdOx/ Zeol, a blank experiment was earlier conducted solely for the thermal gravimetric behavior of PdOx/Zeol catalyst.

structural hydroxyl group that will condense and dehydrate at temperatures above 500 °C.23,27 The profile of the PdOx/Zeol (precalcination) showed that the amount of the strongly bonded water in the second weight loss region is higher than in the Zeol sample even unto the third weigh loss region. This implied that PdOx/Zeol synthesis procedure was able to guarantee the penetration of the PdOx precursor into the lattice of Zeol. After calcination at 400 °C, both the strongly bonded water and physisorbed water were seen to have been drastically reduced in the PdOx/Zeol sample leaving behind the PdOx deposit on the support. Proper calcination around 400 °C has been reported to ensure large surface area and pore volume hence increasing the number of active sites,29,30 which in turn guarantee a high dispersion of Pd particles since the presence of water molecules leads to its agglomeration, which essentially is not very effective in reduction processes such as HDO, although very effective for oxidation processes.31 X-ray Fluorescence. Figure 2 showed the XRF spectrum of Zeol and the calcined PdOx/Zeol samples, the characteristics

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RESULTS AND DISCUSSION Catalyst Characterization. Thermal Gravimetric Analysis (TGA). The profiles of thermal gravimetric analysis of Zeol and PdOx/Zeol (before and after calcination) are shown in Figure 1

Figure 2. X-ray fluorescence of Zeol and PdOx/Zeol samples.

peak of Si dominance (Si/Al > 1) in the Zeol sample were seen at 1.739 keV, and the presence of K is also seen at 3.58 keV. The successful intercalation of Pd into the framework of Zeol support is seen in the XRF plot of PdOx/Zeol at Lβ1 of 3.17 keV, which is in accordance with the standard card of peak identification (EDXRF-EPSILON 3 XL, PANalytical). Another growing peak is observed at 21.2 keV, which possibly confirm successful dispersion of Pd from PdOx as a result of adequate calcination, since Pd(Kα) is only metal that has such peak at 21.2 keV according to the EDXRF-EPSILON 3 XL, PANalytical standard card. Energy Dispersive X-ray. Table 1 shows the elemental composition of the Zeol and PdOx/Zeol samples. Zeol composition comprises of silica and alumina with some oxides of calcium and sodium, its Si/Al ratio was found to be 1.02, which suggests a Zeolite A type. The ratio increased to 2.24 after successful incorporation of acidic PdOx into Zeol and calcination. Pal-Borbely28 reported that reduction in the amount of zeolitic water (as seen in the TGA profile) is usually accompanied by an increase in the Si/Al ratio. Consequently, the increment in the Si/Al ratio of the PdOx/ Zeol can be ascribed to the effect of proper calcination at 400 °C and the functionalization with OxA, which removed framework and extra-framework alumina, respectively. The increment in the amount of O2 can be ascribed to the presence of oxalate functional group of the PdOx; similarly, the incorporation of Pd is seen in Table 1 and further corroborated in Figure 3 between the peaks 2.2 and 3.8 keV in the EDX spectra. The observed 1.89% Pd (which is an average of three analysis from different spot) in Table 1 is slightly lower than

Figure 1. Thermal gravimetric analysis of Zeol and PdOx/Zeol (before and after calcination).

with three characteristics weight loss regions (WLR) of the alumino-silicates (the post-calcination PdOx/Zeol will hence forth be referred to simply as PdOx/Zeol). The first WLR is ascribed to loosely held moisture and physisorbed water that can be rapidly removed at a temperature around 150 °C.27,28 Both Zeol and PdOx/Zeol (precalcination) samples showed similar weght loss in the first WLR, which implied that the drying stage (at 100 °C) during the synthesis of PdOx/Zeol was able to remove the physisorbed and loosely bonded water molecules from the hydration effect of the PdOx precursor incorporation into Zeol during the catalyst synthesis. The second WLR between 200 and 500 °C can be ascribed to the existence of strongly bonded molecules of water that are localized in the first coordination sphere27,29 and also probably due to the partial occlusion of organic matter at the catalyst synthesis stage.27 Lastly, in the third WLR region is the 5874

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

a

BETb 2

3

sample

Si

Al

O2

Na

Ca

Pd

Si/Al

surface area (m /g)

pore vol. (cm /g)

pore diam. (nm)

particle size (nm)

Zeol PdOx/Zeol

16.88 19.78

16.48 8.83

49.58 56.97

12.41 9.72

4.2 2.8

0.00 1.89

1.02 2.24

202 371

0.13 0.23

10.41 24.34

10404 2655

Determined by elemental dispersive X-ray. bDetermined by the nitrogen adsorption−desorption measurements (BET method).

Figure 3. Energy dispersive X-ray and scanning electron microscopy of Zeol and PdOx/Zeol samples.

the calculated 2% and this could be due to the high hydration degree of PdOx/Zeol sample at the synthesis stages.23 Scanning Electron Microscopy (SEM). The scanning electron micrographs of the Zeol and PdOx/Zeol samples shown Figure 3 (inset) revealed some level of morphological variation in PdOx/Zeol which are expected to have a significant role on its catalytic activity. The morphology of Zeol Figure 3a revealed agglomerates of microsized cubical symmetry sharp crystal structure,28,31 which was seen to have reduced in crystallinity in the PdOx/Zeol sample (Figure 3b), and it was also occasioned by increase in surface area and reduction in particle size as shown in Table 1 (BET analysis). This loss of crystallinity as seen in the reduction of the crystal size can be ascribed to the effect of OxA functionalization and proper calcination at the PdOx/Zeol synthesis stage resulting in dealumination from the lattice structure. Aluminosilicates have been reported to experience loss of crystallinity under acid influence and thermal treatment, which is usually accompanied by increase in the specific surface area, and such loss is a function of the acid type/molarity and degree of thermal treatment.32−34 Nitrogen Adsorption/Desorption Isotherm. As earlier noted in the morphology results (Figure 3) that reduction in crystallinity of PdOx/Zeol due to the effect of OxA and proper calcination at the catalyst synthesis stage has effect on its textural properties, it can be seen from Table 1 that there is an increase in surface area and pore volume of PdOx/Zeol compared to the parent material (Zeol). The decrease in the average particle size also confirmed the increase in the specific surface area, while increase in the amount of N2 adsorbed shown in the isotherm of PdOx/Zeol compared to Zeol in Figure 4 confirmed the increase in the pore volume. Both samples exhibited Type II isotherms at lower pressure, which is a characteristic of the formation of monolayer followed by multilayer.27 However, at high relative pressures, they eventually conformed to Type IV isotherm with the steep uptake of N2 emphasizing the possible presence of interparticle voids formed by agglomeration of the micro- or nanosized and plate-like particles containing slit-shaped pores, which are

Figure 4. Nitrogen adsorption/desorption isotherm of the Zeol and PdOx/Zeol samples. Inset: pore size distribution of PdOx/Zeol sample.

typical of H3 hysteresis loop.27,34 This observation is generally an attribute of solids having mesopores,34 and this was further corroborated by Figure 4 (inset) for PdOx/Zeol showing average pore size of 11.14 nm which falls with the 2−50 nm range for mesoporous materials. This average pore size is considered adequate for this study since the SA effective size at the HDO temperature would be less than 46.6 Å (4.66 nm);35 hence, SA can have unhindered diffusion through the PdOx/ Zeol pores. X-ray Diffraction (XRD). The X-ray diffraction pattern for the Zeol support and PdOx/Zeol catalyst are shown in Figure 5. The Zeol support exhibited characteristics peaks at 2θ value of 7.2°, 10.0°, 12.4°, 24.0°, and 30.0°, which according to the JCPDS card 43-0142 are typical of Zeolite A.26 The Zeol sample appeared to be highly crystalline but loss some degree of crystallinity in the PdOx/Zeol sample as observed in the reduced peaks, this is similar to the earlier observation in the samples morphology and it is due to different treatments during catalyst synthesis. This reduction in crystallinity affirms adequate and not excessive thermal treatment during 5875

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mostly in the region 900−600 cm−1,42 a comparison of the two spectra (neglecting the effect of water vibrations between 3700−2900 and 1650 cm−1) showed a perfect match until around 900 cm−1 beyond, which PdOx/Zeol showed a deviation from the Zeol and of importance is the absorption effects at 680 and 639 cm−1. This deviation and absorption are undoubtedly due to the successful intercalation of Pd particles in the Zeol lattice. Raman Spectroscopy. The spectra in Figure 7 show the characteristic bands of Zeol and the variations due to PdOx

Figure 5. X-ray diffraction of the Zeol and PdOx/Zeol samples.

calcination stage,36,37 since excessive calcination temperature leads to increased sharp peaks, which indicate increase in the degree of crystallization resulting from crystal growth.37 Jia et al.38 also reported a linear relationship between increase in calcination temperature and crystal growth (crystallinity) but an inverse correlation with catalytic activity; hence, the reduction in the crystallinity of PdOx/Zeol is an indication to excellent catalytic activity via enhanced dispersion of active metal into the matrix of the support.39 The presence of Pd, which is usually seen at around 2θ = 18° and 34°,40,41 is not very visible, probably because they were not present as bulk PdO and this confirmed that the PdOx species are highly dispersed in the Zeol due to the OxA functionalization and adequate calcination.29 In addition, since a small amount of Pd (about 2%) was employed during synthesis, it is also possible that the PdOx species are partially covered by the Zeol particles; however, the lowering of the diffraction between 2θ of 2.3 and 5.4 was ascribed to expansion of the Zeol lattice structure due to intimate contact of the incorporated PdOx. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra of the Zeol support and PdOx/Zeol catalyst are shown in Figure 6. Zeol sample exhibited vibrations with 3715−2970

Figure 7. Raman spectroscopy of the Zeol and PdOx/Zeol samples.

incorporation and other synthesis effects on the PdOx/Zeol sample. The bands 280, 330, 405, 490, 700, 977, 1040, and 1150 cm−1 are characteristics of zeolite A having 4-, 6-, and, probably, 8-membered rings.45 The band at 280 cm−1 in the Zeol sample can be accredited to the bending mode of rings higher than 4- and 6-membered rings, probably of the 8membered rings of zeolite A more so that earlier study46 had shown that higher rings give bands at lower wavenumbers and vice versa. The bands at 330 and 405 cm−1 are reflection to the bending mode of 6-membered Si−O−Al rings and the strongest band at 490 cm−1 is assigned to the bending mode of 4-membered Si−O−Al rings.46 The bands at 977, 1030, and 1150 cm−1 are ascribed to asymmetric T-O stretching motions.41,46 The effect of the incorporation of PdOx on the Zeol support was first observed by the multiple stretches of vibrations that appear like background noise which are reflections of the presence of organics, that is, oxalate.29 This confirmed that the Pd particles in the PdOx/Zeol retained their oxalate ligand structure even after calcination.29 After the catalyst synthesis, the bands at 490, 1150, and 1457 cm−1 in the Zeol were seen to have slightly denatured and shifted to 480, 1120, and 1440 cm−1, respectively, in the PdOx/Zeol, while the band at 800 cm−1 disappeared completely probably due to the loss in crystallinity earlier observed in the SEM and XRD results. The new band at 445 cm−1 and the bands at 650, 740, 928, and 1290 cm−1 confirmed the presence of dispersed palladium particles in the PdOx/Zeol catalyst. Ohtsuka and Tabata31 reported 648 cm−1 for the Raman active B 1g vibrational mode of the PdO phase for single crystals or PdO foils and between 626 and 640 cm−1 for oxidized Pd dispersion on alumina or zirconia.41 Similarly, Pommier and Gelin40

Figure 6. FTIR spectroscopy of the Zeol and PdOx/Zeol samples.

cm−1 with minimal around 3320 cm−1, which reflect the existence of zeolitic water,42,43 while the band at 1655 cm−1 is a reflection of water vibration.44 Since the presence of water (freely bonded, physisorbed, and strongly bonded) in supported Pd catalyst has been reported to cause agglomeration of Pd particles31 which does not favor HDO process as earlier discussed. The disappearance of these water bands confirmed that the thermal treatment (Figure 1) during calcination was adequate in removing all unwanted water molecules thereby guaranteeing high Pd dispersion.31 Generally, the presence of exchangeable cations is normally seen in the lower wavenumber 5876

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biofuel molecules occurs at temperatures beyond 360 °C. Another possible cause for this reduction could be what Krar et al.21 referred to as secondary reactions such as polymerization, water−gas-shift reaction, methanization and cyclization. Unfotunately the monitoring and quantification of these process was not feasible at the time we conduct this study due to technical limitations. The effect of PdOx/Zeol loading at different temperature showed that its increase also increased the HDO efficiency as shown in Figure 8a because more Pd active sites are made available for the reaction. Furthermore, the result showed that the effect of PdOx/Zeol increment is not very significant at lower temperature as compared to high temperature. Considering the 20 and 25 mg loadings, for example, it can be seen that there is no appreciable difference between 300 and 320 °C, but a distinctive improvement is seen at 360 °C. This implied that PdOx/Zeol increment cannot enhance the HDO process efficiency beyond the thermodynamically feasible extent in those tested temperatures. Therefore, the result showed that both temperature and catalyst are strong process parameters to be considered in HDO process. PdOx/Zeol seem to show more prospects compared to some earlier reports in the literature on the application of Pd10−15 where higher catalyst loading, temperature, and time were required to achieve the HDO of SA and other related feed stocks. For example, Arend et al.,10 reported a temperature and catalyst loading of 380 °C and 3 g, respectively, which are well above the values in this study and they recorded high yield of C17H36 since high temperature according to Kovacs et al.20 usually favors decarboxylation process, which is considered unwanted in this present study. Effect of Pressure and Gas Flow on the Deoxygenation of Stearic Acid. Figure 8b shows the effect of pressure on the HDO of SA at different gas flow rate based on the earlier established best temperature of 360 °C and PdOx/Zeol loading of 25 mg. In the three gas flow rates tested, low pressure of 10 bar does not favor the HDO process, but increases to 20 bar showed improvement in the process; however, further increment beyond 20 up to 60 bar does not also favor the HDO process. For example, at the best observed gas flow rate of 100 mL, C18 production first increased from about 70% at 10 bar to 92% at 20 bar, further increment in pressure from 20 to 60 bar resulted in drastic reduction of C18 production from 92 to about 46%. The increment in the efficiency when the pressure was increased from 10−20 bar could be ascribed to increase in the H2 solubility in the reaction mixtures due to its increased partial pressure leading to high impact of H2 on the SA.6 However, the decrease in C18 production could be due to the inability of the produced C18 to easily desorb from the catalyst sites under high pressure thereby undergoing series of secondary reaction such as cracking and WGS reactions earlier discussed. Kovacs et al.20 also reported reduction in the C18 production with increase in pressure and attributed it to the formation of intermediate oxygenated compounds; therefore, moderately reduced pressure is favorable to the desorption of the C18 to avoid the formation of oxygenated compounds that can deactivate the catalyst active site.11 The best observed pressure of 20 bar in this study is same with the works of Kovacas et al.20 and is within the range reported in previous studies.8−12 However, it is in clear variance with the works of Monniera et al.47 that reported high value of 71.5 bar probably due to poor bonding arrangement between the reacting species and their catalyst surface. The effect of gas flow at different

confirmed successful incorporation of palladium particles into zeolite support at 928 cm−1 forming stretching vibrations with the T−O bonds. Hydrodeoxygenation of Stearic AcidEffect Of Operational Variables. A systematic approach was adopted for the HDO of SA, first effect of temperature was studied as a thermodynamic state function by conducting the HDO at different temperatures while keeping other variables constant. Second, the kinetics of the process was studied by varing the PdOx/Zeol loading over the tested temperature range. Lastly, the effect of pressure and gas flow were investigated at the best temperature and PdOx/Zeol loading, and all studies were conducted within 60 min reaction time. Effect of Temperature and PdOX/Zeol Loading on the Hydrodeoxygenation of Stearic Acid. The profile of effect of temperature on the HDO of SA at different PdOx/Zeol loading is shown in Figure 8a. The results showed that irrespective of

Figure 8. (a) Effect of temperature on the HDO of SA at different PdOx/Zeol loading, 100 mL/min carrier gas flow rate (5%H2/N2) at 40 bar. (b) Effect of pressure on the HDO of SA at different carrier gas (5%H2/N2) flow, T = 360 °C, PdOx/Zeol loading =25 mg.

the PdOx/Zeol loading, increment in temperature greatly enhanced the C18 (n- and iso-octadecane), for example, with 25 mg catalyst loading, temperature increment from 300 to 360 °C enhanced the HDO from 53.8 to about 85%. This confirmed that the HDO process of SA conforms to the Arrhenius theory of temperature dependence of reaction rates (i.e., Γi = k(T)), such that the molecules of the SA gained more kinetic energy to react with the H2 gas at the PdOx/Zeol active sites. In addition, the temperature increment is believed to have lowered the viscosity of the SA thereby enhancing the H2 propensity to penetrate the bulk of the SA, which in turn increases the H2 solubility in the reaction mixture. A reduction in the HDO efficiency was observed at 380 °C probably due to cracking of the already deoxygenated paraffin into smaller molecules. Previous studies8−21 have shown that cracking of 5877

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3820−3731 cm−1 are due to O−H stretching,48 while the peak at 3580 cm−1 as well as the band between 3400 and ∼2000 cm−1 are absolute signature of the O−H stretch of the carboxylic acids because no other functional groups had been reported to have such broad and intense band at high wavenumber.49 In fact, the widen band at 3400 and ∼2000 cm−1 is the major distinguishing factor between SA and other C18 carboxylic acid such oleic acid whose broad band span between 3000 and 2800 cm−1.49−52 Two sharp peaks at 2950 and 2856 cm−1, which are superimposed on the O−H stretch, are characteristics of asymmetric CH2 stretch and symmetric CH2 stretch, respectively, and the peak at 2377 is due to CO stretching of CO2.46 The intense peak at 1779 cm−1 is due to CO stretch of carboxylic acid functional group.49,50 Similarly, the band at 1462 cm−1 is assigned to O−H in plane band, while the peak corresponding to 1285 cm−1 is ascribed to the presence of the C−O stretch.48 The so-called progressional bands that arise from the CH wagging and twisting vibrations are seen in the region from 1150 to 1350 cm−1;49 finally, the band at 937 cm−1 is assigned to the O−H out-of-plane.47 As the HDO process progresses the characteristics features of the SA begins to diminish, for example, there is reduction in the peak at 3580 cm−1 earlier ascribed to the O−H stretch of carboxylic acids. The broad band, which is an attribute of SA, drastically narrowed from its initial range 3900∼2000 cm−1 to about 3490−2300 cm−1 within the first 25 min. Similarly, the CO stretching at 1779 and 2377 cm−1 were seen to be disappearing with time, while another peak in the region of the former at 1680 cm−1 is synonymous to −CH2 and −CH3 deformation. The reaction appeared to slow down between 25 and 45 min, as suggested by the superimposition of the spectra in those period, and there is a new peak at 3014 cm−1 on all the spectra in that period, which is due to the formation of unsaturation CH of alkane.48,52 This signature disappears with reaction time due to uptake of H2, an indication that herald the formation of the paraffinic biofuel (i.e., octadecane (C18H38)). After about 45 min of HDO reaction time, virtually all the characteristics of SA has disappeared both at the high and low wavenumber. The product spectrum data after 50 min showed excellent match with the standard spectrum of pure C18H38 previously reported by You et al.53 The products distribution after 60 min is shown in Figure 10. The presence of iso-C18H38 in the product is definitely due to the functionalization of the synthesized PdOx/Zeol with OxA at the catalyst

pressure is also shown in Figure 8b with 100 mL/min having the highest C18 production. It can be concluded that the amount of H2 present is sufficient for the HDO process and the gas sweeping rate is also adequate. Both 50 and 150 mL/min flow rate showed comparable reduction in the C18 production, the lower C18 production in the former can be attributed to both insufficient H2 required for the HDO and inadequate product removal rate, which can also lead to unwanted secondary reactions. On the other hand, the reduced C18 production at 150 mL/min flow rate is due to shorter reaction time due to quick transportation of premature products from the PdOx/Zeol active sites. Hydrodeoxygenation of Stearic AcidQualitative Study. The three dimension (3D) absorbance of the hydrodeoxygenation of SA into C18 biofuel using PdOx/Zeol being monitored on the FTIR spectra of the evolved products in the range 4000−600 cm−1 over the reaction time is shown in Figure 9a. It can be observed that it took about 10 min for any

Figure 9. (a) Three dimension (3D) absorbance of HDO process of stearic acid to produce C18H38 with reaction time using 2.0 mg of PdOx/Zeol and 20 mL/min (5%H2/N2) gas flow rate at 360 °C. (b) FTIR spectra of HDO process of stearic acid to produce C18H38 with reaction time using 2.0 mg of PdOx/Zeol and 100 mL/min (5%H2/ N2) gas flow rate at 360 °C.

traces of species to be detected because the molecules of SA has not gained sufficient thermal energy to commence breaking and/or formation of new bonds because the temprature was still ramping up and had not the reached the value where HDO can take place. The first traces of peaks were seen around 2900, 1779, and 1463 cm−1. For easy identification and interpretation, the 3D absorbance was resolved into two dimensions (2D), that is, wavenumber as a function of reaction time, as shown in Figure 9b. Analyzing the spectra in Figure 9b from the high wavenumber toward the low wavenumber showed that the peaks at

Figure 10. Products distribution of the HDO process of stearic acid using 2.0 mg of PdOx/Zeol and 100 mL/min (5%H2/N2) gas flow rate at 360 °C. 5878

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showed that increase in their initial values enhanced the HDO process efficiency up to a certain extent beyond which further increment leads to lower HDO efficiency. The best observed operating condition was 360 °C, 25 mg of PdOx/Zeol, 100 mL/min of gas flow under a reaction pressure of 20 bar to achieve about 92% biofuel production from 35 g SA. The process chemistry showed the composition of the biofuel to be 71% n-C18H38, 18% iso-C18H38, and 3% C17H36. The presence of the isomerized product was ascribed to the functionalization of the catalyst with OxA which increases its acidity. This study is very promising toward the advancement of biofuel research and commercialization.

development stage, which increased the acidity. Previous studies2,18−20 have shown that acidic catalysts are promising toward the production of iso-paraffins, which are known to be good biofuel additive due to their low freezing points. The presence of water confirmed that the O2 extraction proceeds via HDO process (since CO was not observed) while that of CO2 is likely due to decarboxylation process which justifies the traces of C17H36 observed. Part of the CO2 perhaps had undergone methanization reaction with the H2 in the carrier gas stream to form the traces of CH4 observed. However, since no other lower and higher hydrocarbons were observed, it implied that the operating conditions were not at extremes that could favor cracking and polymerization reactions and also the PdOx/Zeol has the ability to minimize the formation of other side product(s) due to the presence of the metal−oxalate ligands.23−26 In view of the foregoing, a comparison of the PdOx/Zeol activity with other related studies4−6,9−15 showed that PdOx/Zeol is comparably highly promising especially considering their feed/catalyst dosage ratio, higher reaction temperature, longer reaction time, and product purity. For example, Arend et al.10 used 3 g of 2% Pd/C and could only achieve deoxygenation at 380−450 °C; hence, the catalyst suffered deactivation due to carbon deposition. Similarly, Lestari et al.11 achieved catalytic deoxygenation of SA over Pd supported on acid modified mesoporous silica SBA15 and MCM-41 with 67% liquid n-pentadecane selectivity after 5 h reaction compared to over 90% combined n- and isooctadecane recorded in this study in 1 h HDO time. In all those studies,4−6,9−15 the authors did not claim any formation of isomerized products, which are value added components for biofuel to enhanced its cold flow properties. Catalyst Reusability. The catalyst reusability was studied at 360 °C, 100 mL/min gas flow rate and 20 bar in 60 min reaction time. After three consecutive experiments, the reusability results were consistent with 92% biofuel production comprising 71% n-C18H38, 18% iso-C18H38, and 3% C17H36 from 35 g SA. This ability was ascribed to the PdOx/Zeol synthesis protocol that employs the functionalization of palladium with OxA to develop organometallic palladium II oxalate complex catalyst precursor with increased acidity. Generally metal−oxalate catalysts have been reported to be more reactive than the metal oxide catalysts and are highly resistance to leaching of the active metal due to the presence of the strong Mn+−oxalate ligand, which also minimizes the tendencies of multiple side reactions.22,24,54 However, about 2% reduction in isomerization efficiency was observed probably due to slight reduction in the PdOx/Zeol acidity. Kovacs et al.20 in their study on hydrotreating of triglycerides using fluorinated NiMo/Al2O3 catalyst also reported some degree of acidity loss which resulted into reduced iso-paraffin production.

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AUTHOR INFORMATION

Corresponding Authors

*Tel +60164955453. Email: [email protected]. *Email: [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, Faculty of Engineering, University of Malaya.

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REFERENCES

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CONCLUSION Zeolite supported palladium oxalate (PdOx/Zeol) catalyst was synthesized via functionalization of palladium with aqueous oxalic acid (OxA) to form the polynuclear palladium(II) oxalate complex, which was supported on zeolite support (Zeol). The PdOx/Zeol characterization showed enhancement in the textural properties due to the catalyst synthesis protocol. The presence of highly dispersed Pd particle was confirmed by EDX, XRF, FTIR, and Raman spectroscopy. The activity of PdOx/ Zeol and effect of process variables were studied on hydrodeoxygenation (HDO) of stearic acid (SA). The effect of temperature, pressure and gas flow rate on the HDO process 5879

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