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Effect of Pd/MCM-41 pore size on catalytic activity and cistrans selectivity for partial hydrogenation of canola biodiesel Plaifa Hongmanorom, Apanee Luengnaruemitchai, Nuwong Chollacoop, and Yuji Yoshimura Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00832 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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Fig. 1 XRD patterns of synthesized MCM-41 supports. 108x92mm (96 x 96 DPI)

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Fig. 2 (a) Nitrogen adsorption-desorption isotherms of MCM-41 supports 115x86mm (96 x 96 DPI)


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Fig. 2 (b) Pore size distribution calculated using the BJH method of MCM-41 supports. 116x89mm (96 x 96 DPI)


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Fig. 3 (a) Nitrogen adsorption-desorption isotherms of supported Pd catalysts 115x88mm (96 x 96 DPI)


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Fig. 3

(b) Pore size distribution calculated using the BJH method of supported Pd catalysts. 114x89mm (96 x 96 DPI)


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Fig. 4 SEM micrographs of (a) Pd/MCM-41-a8


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Fig. 4 (b) Pd/MCM-41-a5


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Fig. 4 (c) Pd/MCM-41-a3.


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Fig. 5 TEM micrographs of (a) MCM-41-a8


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Fig. 5 (b) MCM-41-a5


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Fig. 5 (c) MCM-41-a3


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Fig. 5

(d) Pd/MCM-41-a8


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Fig. 5

(e) Pd/MCM-41-a5


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Fig. 5

(f) Pd/MCM-41-a3. The scale bars for (a-c) are 50 nm, where the scale bars are 100 nm for (d-f).


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Effect of Pd/MCM-41 pore size on catalytic activity and cis-trans selectivity for partial hydrogenation of canola biodiesel Plaifa Hongmanorom a, Apanee Luengnaruemitchai a,*, Nuwong Chollacoop b, Yuji Yoshimura c a

The Petroleum and Petrochemical College, Chulalongkorn University,

Soi Chula12, Phayathai Rd., Pathumwan, Bangkok 10330, Thailand b

National Metal and Materials Technology Center, 114 Thailand Science Park,

Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120, Thailand c

National Institute of Advanced Industrial Science and Technology (AIST),

Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan KEYWORDS Partial hydrogenation, Biodiesel, Pd/MCM-41, Pore size, Oxidative stability, Cloud point 

Address author correspondence:

Tel. 662-218-4148; Fax. 662-611-7220; e-mail address: [email protected]

ABSTRACT

The effect of pore size of Pd/MCM-41 on catalytic activity and selectivity in partial hydrogenation of canola oil-derived biodiesel was studied under mild reaction conditions. The catalysts with different pore sizes were obtained by varying the amount of aqueous ammonia added during the synthesis: Pd/MCM-41-a8, Pd/MCM-41-a5, and Pd/MCM-41-a3 with average pore diameters of 3.72, 3.99, and 7.55 nm, respectively. The supports and supported Pd catalysts were characterized by BET surface area analyzer, X-ray Powder ACS Paragon Plus Environment

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Diffraction (XRD), Transmission electron microscopy (TEM), CO pulse chemisorption, and TemperatureProgrammed Desorption-NH3. The highest hydrogenation activity was found for the largest pore catalyst, Pd/MCM-41-a3, as presented in term of turnover frequency (TOF); whereas Pd/MCM41-a8 and Pd/MCM41-a5 provided higher selectivity toward cis-monounsaturated Fatty acid methyl ester, which was attributed to their limited pore dimension. The correlation between catalyst pore size and maximum diameter of major compounds (C18 FAMEs) was introduced and utilized to explain the results of TOF and selectivity.

1. INTRODUCTION Diesel fuel is considered as one of the petroleum products that contributes to higher economic growth since it is largely used, particularly in heavy duty vehicles for transport of industrial and agricultural goods. In consequence of continuously increasing diesel consumption, more greenhouse gases have been emitted to the atmosphere, causing local air pollution and also global warming problems1. To solve these problems, biodiesel has become one of the most promising alternatives for petroleum-based diesel because of its lower exhaust emissions, higher cetane number, non-toxicity, and renewability2– 3. Biodiesel or fatty acid methyl ester (FAME) can be produced via transesterification by chemically reacting lipids from renewable sources with a short chain alcohol4. Albeit biodiesel has several advantages over petroleum diesel as mentioned previously, there are some drawbacks related to important properties of biodiesel, which are oxidative stability and cold flow properties. These two properties are strongly influenced by structural features of fatty acid. More specifically biodiesel with high contents of polyunsaturated FAMEs exhibits low resistance to oxidative degradation; whereas biodiesel with high saturated FAME composition presents poor cold flow properties5–7. To reach a compromise between these two properties, biodiesel should be a monounsaturated FAME-rich fuel. Therefore, partial hydrogenation is employed to transform polyunsaturated FAMEs into monounsaturated FAME as much as possible with minimal incremental amounts of saturated FAME

8–10

. It is also important to note that cis-isomer is

meanwhile isomerized to more thermodynamically stable trans-isomer, which is less favorable due to its worse cold flow properties 11–12. For these reasons, selective catalyst is necessarily required. Supported Pd catalysts used in liquid phase hydrogenation have been reported for more than a decade. They can provide higher catalytic activity than other most commonly active catalysts such as Pt, Rh, and Ni ACS Paragon Plus Environment

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. In addition to effect of metal type, the effects of type of support, pore structure, and pore size on both

catalytic activity and selectivity have been extensively investigated as well 15–17. Among numerous supports, hexagonal mesoporous silica MCM-41 has attracted much attention since it has remarkably high surface area, large pore volume, uniform pore structure, and tailored pore size up to 12 nm18–19. Pore diameter of MCM-41 has a strong influence on the transfer rate of FAME molecules to active sites of the catalyst as well as controlling the selectivity toward the desired products20. Varying the amount of aqueous ammonia, the mineralizing agent, during the synthesis is one of the practical methods employed to obtain MCM-41 with distinguishable pore diameters. The more aqueous ammonia is added, the smaller pore size of MCM-41 can be acquired. The reason is that hydrophobic chain of the surfactant may shrink more under higher alkaline condition. Additionally, the micelle aggregation number of surfactant decreases with increasing the solution pH, resulting in smaller size of micelles and consequently smaller pore size of MCM-4121. Thus, this study has mainly focused on the partial hydrogenation for biodiesel upgrading using Pd supported on MCM-41 with a variety of pore diameter synthesized by adjusting the composition of aqueous ammonia.

2. MATERIALS AND METHODS

2.1. Materials. Cetyltrimethylammonium bromide (CTAB, ≥96%, Fluka) was used as the template that helps build up the mesoporous framework. Tetraethylorthosilicate (TEOS, ≥98%, Fluka) was used as the silica precursor. Aqueous ammonia (NH4OH, 28%, QRëC) was utilized as the mineralizing agent. Pd(NH3)4Cl2∙xH2O (N.E. Chemcat co.) was applied as the Pd precursor. Canola oil, purchased from Lam Soon (Thailand) Public Company Limited, was used as a starting material for transesterification.

2.2. Preparation of MCM-41 Supports. MCM-41 was prepared in the similar manner as that of Loganathan et al21. The mixture was first prepared from 2 g of CTAB added to 120 mL of water and stirred for 30 min. After the solution became homogeneous, 10 ml of TEOS was poured into the mixture with 5 min stirring. After that, 28% aqueous ammonia was added to the solution with subsequent stirring for 12 h at room temperature. The samples were named MCM-41-a3, MCM-41-a5, and MCM-41-a8, where a refers to aqueous ammonia and the numeric values represent the amount (in mL) of 28% aqueous ammonia added ACS Paragon Plus Environment

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during synthesis. The obtained solid material was washed until pH 7 and then dried at 100 °C overnight. The surfactant template was removed by calcination in air at 500 °C for 6 h with the heating rate of 0.5 °C/min.

2.3. Preparation of Supported Pd Catalysts. Supported Pd catalyst was prepared by an impregnation method. Firstly, synthesized MCM-41 was dried in an oven overnight at 80 °C to remove the adsorbed water. An aqueous solution containing Pd(NH3)4Cl2 was impregnated into the support under vacuum and kept for 24 h. The metal loading of Pd was 1 wt%. After impregnation, the catalyst was dried in 3 steps as follows: using a rotary evaporator at room temperature for 2 h, using a rotary evaporator at 60 °C for 2 h, and using a vacuum pump at 60 °C for 2 h. Next, the catalyst was calcined under the oxygen gas flow of 1 L/min at 400 °C for 2 h, and lastly reduced under the hydrogen gas flow of 100 mL/min at 200 °C for 2 h.

2.4. Catalyst Characterization. The X-ray diffraction measurement was applied to characterize the crystallinity of catalysts. The patterns of the samples were obtained on a Rigaku TTRAX III diffractometer using nickel-filtered CuKα radiation (λ = 1.5405 Å) and operated at 50 kV and 300 mA. The diffraction data was measured from 1° to 7° for small-angel mode with the scanning speed of 2°/sec. Quantachrome Autosorb-1MP surface area analyzer was used to determine the BET surface area, pore volume, average pore diameter, and pore size distribution of catalysts. Before analyzing, the volatile species adsorbed on the catalyst surface were eliminated by heating the catalyst under vacuum atmosphere at 250 C overnight. Surface texture and morphology of MCM-41 after incorporation of Pd were observed using scanning electron microscopy (SEM), Hitachi Model S4800. The electrons were backscattered or emitted from the specimen surface to the samples that were placed on a stub and coated with gold. Micro-texture and microstructure of electron transparent samples were characterized by Transmission electron microscopy (TEM). Both MCM-41 and Pd/MCM-41 were dispersed in volatile liquid, ethanol, and then sonicated for 10 min. Then, the solution was dropped on carbon coated copper grids and allowed to dry. TEM was performed on a JEOL JEM-2100 microscope operating at 120 kV at the Scientific and Technological Research Equipment Center, Chulalongkorn University (STREC).


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Pulse CO chemisorption was used to measure palladium dispersion using Temperature-programmed desorption/reduction/oxidation (TPD/R/O), Ohkura R6015. Prior to chemisorption, the catalyst was reduced in a flow of hydrogen for 1 h and followed by purging with helium for 10 min. Pd dispersion was calculated based on the assumption that the stoichiometry of CO/Pd was equal to1. The average size of Pd particles was given by d = 5/ρS, where d is Pd particle size, ρ density of Pd, and S surface area of Pd particle, while cross-section area of Pd = 0.151 nm2 22. Total acidity and acid strength of the supports were determined by the Temperature-Programmed Desorption of Ammonia (Thermo Finnigan TPDRO 1100 series). The sample was pretreated at 300 °C for 30 min with He and then cooled to 100 °C. Next, it was adsorbed with NH3 by using 1.13% NH3/N2 for 1.5 h and cooled to 40 °C. The system started analyzing the Temperature-Programmed Desorption from 40 °C to 800 °C with the heating rate of 10 °C/min.

2.5. Transesterification of Canola Oil. Canola oil was added to a three-neck round-bottom flask, and then was placed on the hot plate which was set at 60 °C. Methanol (9:1 methanol to oil molar ratio) and KOH catalyst (1 wt% compared to the starting canola oil) were vigorously mixed together and added into the flask. After 1 h of transesterification, the solution was poured into a separatory funnel to allow a phase separation for 1 night. The lower layer of glycerine was removed and the upper layer of biodiesel was washed with distilled water at 60 °C several times. Finally, the canola oil-derived FAME was kept in the glass bottle containing anhydrous Na2SO4 in order to remove leftover washed water.

2.6. Partial Hydrogenation of Canola Biodiesel. Partial hydrogenation of canola oil-derived FAME was operated at 100 °C and 4 bar in a stainless steel semi-batch reactor. About 1.304 g of reduced catalyst was put in the reactor containing 130.4 g of biodiesel (1 wt% as compared to biodiesel). Then, the reactor was purged with nitrogen for air removal, followed by hydrogen with the flow rate of 150 mL/min. During partial hydrogenation step, the stirrer was used at 500 rpm to mix biodiesel, catalyst, and hydrogen thoroughly. When the temperature and pressure reached desired points, the sample was collected every 30 min and this was done for 4 h. The product was analyzed by Hewlett Packard gas chromatograph 5890 Series II equipped with a FID detector. GC-FID analysis conditions were as follows: the carrier gas was ACS Paragon Plus Environment

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helium with a flow rate of 70 ml/min, the injector temperature was at 200 °C with a split ratio of 75:1, and the temperature of detector was at 230 C. First, 0.2 µl of sample was injected at an oven temperature of 130 C. An isothermal period was held for 2 min. Then, the GC oven was ramped to 220 C with a heating rate of 2 C/min, and held for 15 min. One cycle run-time was 62 min.

2.7. Product Analysis. The oxidative stability corresponds to the induction period for the production of volatile organic acids, which were byproducts of fatty acid ester degradation. The oxidative stability of partially hydrogenated biodiesel was tested by Metrohm 743 Rancimat according to EN 14112 method. Air stream with flow rate of 10 L/h was blown through a heated sample at 110 °C, whose volatile oxidation products were transferred into a conductivity-measuring vessel containing deionized water. The conductivity was plotted as a function of time exhibiting the inflection point, which was known as Induction Period, IP (h). Cloud point, which is the temperature at which a cloud of crystals first appears in a fuel sample, was investigated in accordance with ASTM D2500 using CPP 5Gs Automated Cloud and Pour Point Analyzer. Biodiesel was poured into test jar and cooled by built-in cooling system. The cloud point was reported on screen at the interval of 0.1 °C or rounded.

3. RESULTS AND DISCUSSION 3.1. Support and Catalyst Characterization. As presented in Fig. 1, low-angle XRD patterns revealed very sharp peaks (100), (110), and (200) that can be indexed to P6mm hexagonal structure for MCM-41-a8 and MCM-41-a5, while the XRD spectra of MCM-41-a3 exhibited only a single broad line, indicating loss of structural regularity of MCM-41 23. The main peak (100) slightly shifted from right to left by decreasing amount of aqueous ammonia added during synthesis. Likewise, when the amount of aqueous ammonia was lower, interplanar spacing (d100), pore center distance (a0), and primary mesopore size (dp) showed the increasing trend (see Table 1), which in turn indicated the enlargement of pore size24. From these results, it was apparent that pore size was increased. A IV type nitrogen isotherm, which is the characteristic feature of the mesoporous material, was obtained for all supports (see Fig. 2a). The isotherm depicted a H1 hysteresis loop and reversible adsorption and ACS Paragon Plus Environment

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desorption branches25. The sharp inflection because of capillary condensation occurred at relative pressure (P/P0) between 0.25 and 0.40. This part of the curve pointed out the uniformity of pore size distribution of synthesized MCM-4126. In Fig. 2b, all of MCM-41s had the same pattern of pore size distribution, exhibiting BJH pore diameter around 2.9 nm. Their average pore diameters in Table 2 were different though. Their trend was in good agreement with the unit cell parameters obtained from XRD, which confirmed that pore size of MCM-41 was increased significantly under condition with small amount of aqueous ammonia. All synthesized MCM-41s had exceptionally large pore volumes and high specific surface areas, as shown in Table 2. After Pd impregnation, palladium loading did not appreciably change the BJH pore diameters of all catalysts. Particularly, BJH pore diameter of Pd/MCM-41-a3 was equal to 2.91 nm even after the incorporation of Pd particles. Both BJH surface area and total pore volume of all Pd/MCM-41 materials were gradually decreased compared to the parent supports, corroborating that the pore channels were partially blocked by Pd particles. This partial blockage of pores was also the reason for the change in shape of isotherm27–28. The isotherm shape was altered especially in Pd/MCM-41-a8, as shown in Fig. 3a. Moreover, there was a bimodal pore size distributions in Fig. 3b, meaning that Pd/MCM-41-a8 lost its uniform pore distribution. In case of Pd/MCM-41-a5 and Pd/MCM-41-a3, their height of mesopore condensation was slightly decreased; however, the sharpness of inflection still appeared. SEM images recorded for Pd/MCM-41-a8, Pd/MCM-41-a5, and Pd/MCM-41-a3 are displayed in Figs. 4(a-c), respectively. The micrographs showed agglomerates of particles with some Pd clusters on the outer surface. More interestingly, it was observed that quantities of aqueous ammonia added as mineralizing agent clearly had an effect on morphology and particle size of MCM-41. In other words, pH of the synthesis gel demonstrated the significance in controlling the way particle morphology of MCM-41 was formed29–30. Pd/MCM41-a8 was in disordered worm-like morphology with roughly measured particle diameter of 400 nm and length varying from 800 to 1600 nm. Pd/MCM-41-a5 consisted of irregular particle shapes with undefined form and smaller particle size compared to Pd/MCM-41-a8. The other sample, Pd/MCM-41-a3, exhibited the mixture of irregular and spherical morphology with smallest particle size, approximately ranging from 80 to 300 nm. These results evinced the correlation that particle size was larger as the amount of aqueous ammonia was higher (higher pH). By way of explanation, hydrolysis rate of silica was increased due to higher concentration of mineralizing agent, thereby generating more silicon oxy-anions. Higher ACS Paragon Plus Environment

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density of anions attracted more positive charges of micelles as a result of electrostatic interaction. This possibly induced the assembly of several micelles, which were subsequently shaped into worm-like silicate micelles and eventually resulted in larger particle size of mesoporous silica. On the contrary, lower aqueous ammonia concentration produced lower density of silicate anions, resulting in less chance of silicate micelles joining together. Hence, smaller particle size with spherical morphology was obtained 31–33. The micro-texture and microstructure of MCM-41 supports and supported Pd catalysts were displayed in Figs. 5(a-c) and 5(d-f), respectively. Well-ordered hexagonal long-range array of the channels was evidently seen for all MCM-41 supports, even MCM-41-a3 which possessed only peak for 100 plane in XRD pattern. After the Pd incorporation into mesoporous supports, the appearance of these supports was considerably changed. Surface area and pore volume of supports were reduced by the presence of metal particles34. More obviously, less ordered pore structure was detected, that was in good agreement with the nitrogen adsorption isotherm. Nevertheless, the existence of array of pores for both Pd/MCM-41-a5 and Pd/MCM-41-a3 was still noticeable. It also could be seen that Pd particles with diameter smaller than pore size of every MCM-41 were incorporated within MCM-41 frameworks. On the other hand, some bigger Pd particles were formed, especially on the outer surface of MCM-41-a5. This observation of Pd location was consistent with many literatures35–37. In case of Pd supported on MCM-41-a8, non-uniform particle sizes of Pd were found. The contrast between Pd and support was not clear enough to measure Pd particle sizes from TEM micrographs, so the Pd dispersions and Pd particle sizes were determined from CO chemisorption

38–39

. Pd

dispersions in Table 2 did not have the same trend as BJH surface area of the parent supports. Instead, all catalysts showed similar Pd dispersions around 16–19%, which were further calculated and then gave Pd particle sizes in the range of 2.52–3.18 nm. By comparing size of Pd nanoparticles with pore size of support, it was found that Pd particles were probably predominantly dispersed inside the channels.

3.2. Pore Size Effect on Partial Hydrogenation. Catalytic activity and cis-trans selectivity of each catalyst were identified by FAME compositions during partial hydrogenation. Here, the catalytic performance was compared under two typical depths of hydrogenation, i.e., deep hydrogenation where the conversion of total amount of polyunsaturated FAMEs was around 97%, and light hydrogenation where the conversion of total amount of polyunsaturated FAMEs was around 20%. Former reaction condition was ACS Paragon Plus Environment

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selected to decrease the total amount of polyunsaturated FAMEs less than 1% because polyunsaturated FAMEs, such as triunsaturated FAME and diunsaturated FAME, are determining components for oxidative stability of FAMEs. For deep hydrogenation (Table 3), the results from GC-FID led to explicit observations that Pd/MCM-41 provided great catalytic activity since triunsaturated FAME and diunsaturated FAME were greatly hydrogenated. To be precise, triunsaturated FAME lastly reached 0% and diunsaturated FAME nearly approached 0% within 4 h. At the same time, monounsaturated FAME was formed and approximately raised by 20%. About 4-10% of saturated FAME was eventually produced within 4 h of reaction. Hydrogenation activity of each catalyst was represented in term of turnover frequency or TOF (see Table 3) using Eq. (1) in the same manner as Thunyaratchatanon et al40. -1

TOF (h ) = [

𝐶18:2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛(%) 1 × 𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑏𝑖𝑜𝑑𝑖𝑒𝑠𝑒𝑙 (𝑔) × 100 𝑀𝑊 𝑜𝑓 𝐶18:2 (294𝑔⁄𝑚𝑜𝑙) %(𝑤⁄𝑤)𝑚𝑒𝑡𝑎𝑙 %𝑚𝑒𝑡𝑎𝑙 𝑑𝑖𝑠𝑝𝑒𝑟𝑠𝑖𝑜𝑛 1 × × 𝑊𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (𝑔) × 100 100 𝑀𝑊 𝑜𝑓 𝑚𝑒𝑡𝑎𝑙

] ÷ 𝑡𝑖𝑚𝑒

(1)

The conversion of C18:2 was gained within 0.5 h of reaction time. The results revealed that pore size of catalyst was important factor that contributed to the effective diffusion of FAMEs into the catalyst pore, and consequently controlled hydrogenation activity. Besides, the amount of unsaturated FAMEs becoming saturated FAME through deep hydrogenation and the amount of cis-isomer turning into trans-isomer were also dependent on MCM-41 pore size. To lucidly explain the effect of catalyst pore size on TOF and cistrans C18:1 selectivity, the maximum diameters of major compounds (C18 FAMEs) in canola oil basedbiodiesel were roughly estimated (using published data from ChemSpider). It is defined as the longest part of a molecule, hence it can ensure that all molecules can pass through the catalyst pore41. The maximum diameters of predominant compositions were listed as follows: methyl linolenate (C18:3, 1.352 nm), methyl linoleate (C18:2, 1.350 nm), cis-methyl oleate (cis-C18:1, 1.603 nm), methyl-trans elaidate (trans-C18:1, 2.022 nm), and methyl stearate (C18:0, 2.381 nm). Pore diameter of Pd/MCM-41-a3 was five times as large as polyunsaturated FAMEs. This may facilitate the diffusion of those polyunsaturated C18 FAMEs into the catalyst pores to react with active sites and transform to monounsaturated FAME. Accordingly, TOF of Pd/MCM-41-a3 was much higher when compared with smaller pore Pd/MCM-41-a5 and Pd/MCM-41-a8. It was noteworthy that pore size of Pd/MCM-41-a3 was even three times larger than the maximum diameter of ACS Paragon Plus Environment

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C18:0, so there was enough space for unsaturated FAMEs to convert to saturated FAME, which is less desirable. Even under the light hydrogenation condition, as seen in Table 4, ratio of cis-18:1 to cis- and trans-C18:1 was lowest for Pd/MCM-41-a3 when the polyunsaturated FAMEs conversion was around 20%. The performance of respective catalysts was quite similar in TOF and selectivity, irrespective of depth of the conversion of polyunsaturated FAMEs. The explanation regarding pore diameter of catalyst and size of molecules could be applied to cis-trans selectivity as well. Large amount of cis-C18:1 was isomerized to less favorable trans-C18:1 for Pd/MCM-41-a3. Unlike the biggest pore size catalyst, limited pore dimension of Pd/MCM-41-a8 and Pd/MCM-41-a5 may retard cis-isomer so as not to isomerize to trans-isomer. Apart from pore size of support, acidity was reported as the other factor that came into play in catalytic activity and cis-C18:1 selectivity42. Total acidity and acid strength of the supports were determined with TPD-NH3. The desorption peaks at 100-400 °C in TPD-NH3 profile are assigned to weak acidic sites, while medium acidic sites belong to peaks between 400–550 °C and strong acidic sites correspond to peaks at higher than 550 °C. In this work, the influence of acidic property of support was not evident; that is to say, catalytic activity and cis-C18:1 selectivity were inconsistent with the total acidity trend, as presented in Table 5. The reason may be that surface acidity of non-substituted MCM-41 materials was too weak to govern the interaction between basic double bonds of unsaturated FAMEs and acid sites. It was conceivable due to the fact that Si-MCM-41 structure was constructed from association of pure silica, and thus it acidic nature must be contributed from the silanol groups only43.

3.3. Biodiesel Properties. Oxidative stability and cloud point of feed biodiesel and partially hydrogenated biodiesel after 4 h of reaction are shown in Table 3. Polyunsaturated FAMEs contents in biodiesel were discovered to be a key factor for determining oxidative stability. Oxidative stability of feed biodiesel containing relatively high polyunsaturated FAMEs compositions was only 3 h. It could not pass EN 14214:2012 standard whereby biodiesel must resist oxidation for at least 8 h. As the total contents of polyunsaturated FAMEs decreased through partial hydrogenation, oxidative stability of partially hydrogenated biodiesel increased. Although largest pore size catalyst satisfyingly improved oxidative stability of biodiesel, cloud point also increased and exceeded a maximum of 16 °C (EN 14214:2012) due to ACS Paragon Plus Environment

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large amount of saturated FAME and trans-C18:1. This may cause problem if used in the engine in cold climate countries or in Thailand during cool climate. Two smaller pore size catalysts were still able to maintain the level of saturated composition within 4 h and their cis-C18:1 levels were higher than transC18:1. Therefore, both catalysts not only enhanced oxidative stability greatly but provided acceptable cloud point as well.

4. CONCLUSIONS Three catalysts with different pore sizes were employed in partial hydrogenation for upgrading canola oilderived biodiesel. An improvement in oxidative stability could be achieved with all of them, making the addition of antioxidant superfluous. More importantly, it was found that hydrogenation of FAMEs was chiefly driven by pore size of catalyst. Despite the fact that all catalysts exhibited similar Pd dispersions, largest pore size Pd/MCM-41-a3 catalyst presented highest TOF, followed by Pd/MCM-41-a5 and Pd/MCM-41-a8, respectively. Nevertheless, deep hydrogenation was observed for Pd/MCM-41-a3, resulting in worse cloud point. Unavoidable cis-trans isomerization occurring along with hydrogenation reaction was also influenced by catalyst pore size. The selectivity toward desirable cis-C18:1 was in this order: Pd/MCM41-a8 ≈ Pd/MCM-41-a5 > Pd/MCM-41-a3. All in all, the catalyst with average pore size of 4 nm was optimal for both effective transport of FAMEs inside porous supported catalyst and good restraint of saturated FAME and trans-C18:1 formation. Its partially hydrogenated biodiesel, therefore, showed considerably high oxidative stability and acceptable cloud point remaining within the required limit.

Corresponding Author *Email : [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources ACS Paragon Plus Environment

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Ratchadapisek Sompote Endowment Fund, Chulalongkorn University [CU-58-058-EN, 2015] and the Center of Excellence on Petrochemical and Materials Technology. ACKNOWLEDGEMENTS The authors would like to thank for the funding supported by the Ratchadapisek Sompote Endowment Fund, Chulalongkorn University [CU-58-058-EN, 2015] and the Center of Excellence on Petrochemical and Materials Technology for scholarship funding of Ms.Plaifa Hongmanorom. We are sincerely thankful to the National Metal and Materials Technology Center (MTEC), Thailand for instrumental analysis support. The authors also gratefully acknowledge Dr. Shih-Yuan Chen, AIST, Japan, for his help on instrumental analysis. REFERENCES

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(40) Thunyaratchatanon, C.; Jitjamnong, J.; Luengnaruemitchai, A.; Numwong, N.; Chollacoop, N. ; and Yoshimura, Y. Influence of Mg modifier on cis-trans selectivity in partial hydrogenation of biodiesel using different metal types. Applied Catalysis A: General. 2016, 520, 170–177. (41) Jae, J.; Tompsett, G.A.; Foster, A.J.; Hammond, K.D.; Auerbach, S.M.; Lobo, R.F.; and Huber, G.W. Investigation into the shape selectivity of zeolite catalysts for biomass conversion. Journal of Catalysis. 2011, 279(2), 257–268. (42) Numwong, N.; Luengnaruemitchai, A.; Chollacoop, N.; and Yoshimura, Y. Effect of SiO2 pore size on partial hydrogenation of rapeseed oil-derived FAMEs. Applied Catalysis A: General. 2012, 441–442, 72– 78. (43) Choundary, V.R.; and Mantri, K. Adsorption of aromatic hydrocarbons on highly siliceous MCM-41. Langmuir, 2000, 16(17), 7031–7037.


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TABLE CAPTIONS

Table 1

Unit cell parameters of MCM-41 materials synthesized using different amount of aqueous ammonia

Table 2

Characteristics of supports and supported Pd catalysts

Table 3

FAMEs composition and ratio of cis-/cis-&trans-C18:1 when the polyunsaturated FAMEs conversion was around 96-98%, TOF of C18:2 within 0.5 h of partial hydrogenation, and fuel properties of canola oil biodiesel and partially hydrogenated biodiesel after 4 h of reaction

Table 4

FAMEs composition and ratio of cis-/cis-&trans-C18:1 when the polyunsaturated FAMEs conversion was around 20%, and TOF of C18:2 within 0.5 h of partial hydrogenation

Table 5

Acidity of mesoporous MCM-41 supports


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Table 1 Catalyst

Interplanar spacing

Pore center distancea

Primary mesopore sizeb dp (nm)

Pore wall thicknessc w (nm)

d100 (nm)

a0 (nm)

MCM-41-a8

3.91

4.51

3.88

0.63

MCM-41-a5

4.07

4.70

4.13

0.57

MCM-41-a3

4.41

5.10

4.75

0.34

a

a0 = 2d100/√3, where the d spacing for MCM-41 appears at plane (100).

b

dp = cd100(ρsVp/1+ ρsVp)1/2, where c is a constant, ρs is the pore wall density, and

Vp is the specific primary mesopore volume. w = a0 – dp.

c


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Table 2 Catalyst

BJH surface

Total

area (m2/g)

pore volume (cc/g)

a

BJH

Average

Pore diameter

Pore diameter

(nm)

(nm)

Pd dispersiona

Pd particle sizea

(%)

(nm)

MCM-41-a8

1460

0.93

2.91

3.62

˗

˗

MCM-41-a5

1480

1.07

2.91

4.28

˗

˗

MCM-41-a3

1400

1.70

2.91

7.13

˗

˗

Pd/MCM-41-a8

1144

0.84

2.19

3.72

19.45

2.93

Pd/MCM-41-a5

1233

0.96

2.45

3.99

15.40

3.18

Pd/MCM-41-a3

1126

1.55

2.91

7.55

16.75

2.52

Calculated by pulse CO chemisorption.


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Table 3 Feed Biodiesel

Pd/MCM-41-a8

Pd/MCM-41-a5 Pd/MCM-41-a3

Triunsaturated FAME

5.66

0

0

0

Diunsaturated FAME

19.19

0.84

0.84

0.28

Monounsaturated FAME

63.27

84.59

84.71

83.64

Saturated FAME

9.31

13.19

13.04

14.62

Ratio of cis-/cis-&trans-C18:1a

1

0.69

0.69

0.64

TOF (h-1)

˗

1151.71

2704.53

5791.37

Oxidative stability (h)

3.04

41.38

74.61

93.39

Cloud point (°C)

0.5

11.2

16.2

23.5

FAME composition (%)a

Biodiesel properties

a

When the polyunsaturated FAMEs conversion was around 96-98%

20 ACS Paragon Plus Environment

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Table 4 Feed Biodiesel

Pd/MCM-41-a8

Pd/MCM-41-a5 Pd/MCM-41-a3

Triunsaturated FAME

5.66

2.93

2.96

2.13

Diunsaturated FAME

19.19

14.40

14.55

12.72

Monounsaturated FAME

63.27

69.63

69.51

71.62

Saturated FAME

9.31

10.50

10.72

11.18

Ratio of cis-/cis-&trans-C18:1a

1

0.97

0.97

0.96

TOF (h-1)

˗

1151.71

2704.53

5791.37

FAME composition (%)a

a

When the polyunsaturated FAMEs conversion was around 20%


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Table 5 Catalyst

Weak

Medium

Strong

Total

acidic site

acidic site

acidic site

Acidity

(mmol/g)

(mmol/g)

(mmol/g)

(mmol/g)

MCM-41-a8

0.213

˗

0.175

0.388

MCM-41-a5

0.169

˗

0.093

0.262

MCM-41-a3

0.186

0.009

0.089

0.284


MCM-41 Pore Size on the Catalytic ... - ACS Publications

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