Scholarly article on topic 'Production of high-quality biodiesel fuels from various vegetable oils over Ti-incorporated SBA-15 mesoporous silica'

Production of high-quality biodiesel fuels from various vegetable oils over Ti-incorporated SBA-15 mesoporous silica Academic research paper on "Chemical sciences"

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{"Biodiesel fuels" / Transesterification / Ti-SBA-15 / "Lewis solid acid" / "international fuel standard" / "Water and free fatty acid"}

Abstract of research paper on Chemical sciences, author of scientific article — Shih-Yuan Chen, Takehisa Mochizuki, Yohko Abe, Makoto Toba, Yuji Yoshimura

Abstract Ti-incorporated SBA-15 mesoporous silica (shortly termed Ti-SBA-15) was a highly efficient and recyclable solid acid to synthesize high-quality biodiesel fuel (BDF) derived from various vegetable oils at moderate reaction condition, in comparison to siliceous SBA-15 and commercial TiO2 catalysts with different anatase sizes, where the catalytically active sites mainly related to the tetrahedral-coordinated Ti(IV) species with weak Lewis acid nature. The TOF values of Ti-SBA-15 catalysts were around 18–166h−1, an order of magnitude larger than those of commercial TiO2 catalysts. A high-quality BDF containing more than 98.4 mass% of fatty acid methyl ester (FAME), which met with international fuel standard, was obtained over 3Ti-SBA-15 catalyst at 200°C using a methanol/oil ratio of 108. Most importantly, the 3Ti-SBA-15 catalyst showed extremely high water and free fatty acid (FFA) tolerance levels, which were several ten times better than homogeneous and heterogeneous catalysts in conventional BDF production technology.

Academic research paper on topic "Production of high-quality biodiesel fuels from various vegetable oils over Ti-incorporated SBA-15 mesoporous silica"

Short Communication

Production of high-quality biodiesel fuels from various vegetable oils over Ti-incorporated SBA-15 mesoporous silica^

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Shih-Yuan Chen *, Takehisa Mochizuki, Yohko Abe, Makoto Toba, Yuji Yoshimura

Hydrotreating Catalysis Team, Research Center for New Fuels and Vehicle Technology (NFV), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

ARTICLE INFO

Article history:

Received 23 May 2013

Received in revised form 2 July 2013

Accepted 10 July 2013

Available online 19 July 2013

Keywords:

Biodiesel fuels

Transesterification

Ti-SBA-15

Lewis solid acid

international fuel standard

Water and free fatty acid

ABSTRACT

Ti-incorporated SBA-15 mesoporous silica (shortly termed Ti-SBA-15) was a highly efficient and recyclable solid acid to synthesize high-quality biodiesel fuel ( BDF) derived from various vegetable oils at moderate reaction condition, in comparison to siliceous SBA-15 and commercial TiO2 catalysts with different anatase sizes, where the catalytically active sites mainly related to the tetrahedral-coordinatedTi(IV) species with weak Lewis acid nature. The TOF values of Ti-SBA-15 catalysts were around 18-166 h-1, an order of magnitude larger than those of commercial TiO2 catalysts. A high-quality BDF containing more than 98.4 mass% of fatty acid methyl ester (FAME), which met with international fuel standard, was obtained over 3Ti-SBA-15 catalyst at 200 °C using a metha-nol/oil ratio of 108. Most importantly, the 3Ti-SBA-15 catalyst showed extremely high water and free fatty acid (FFA) tolerance levels, which were several ten times better than homogeneous and heterogeneous catalysts in conventional BDF production technology.

© 2013 The Authors. Published by Elsevier B.V. All rights reserved.

1. Introduction

The current technology for making BDF, which mainly consists of FAME, is through transesterification of vegetable oils with methanol catalyzed by NaOH at mild condition (60 °C and 1 bar) or by heterogeneous ZnAl mixed oxides in Esterfip-H process at severe condition (230 °C and 60 bar) [1,2]. To prevent saponification and catalyst deactivation, the refined and edible vegetable oils with FFA content lower than 0.5 wt% can be only fed, making high production cost and the shortage of food supply. Although FFAs in crude vegetable oils can be removed by sulfuric acid-catalyzed pre-esterification, the homogeneous acids and bases are corrosive and difficult to be recycled, causing serious environmental pollutions [3]. This two-step method also makes the BDF production more complicatedly [4]. Numerous solid acid catalysts have recently applied in direct transformation of crude vegetable oils with high acid values into BDF [5-8]. Due to low reaction rate, the acid-catalyzed BDF synthesis is generally carried out at high temperature and pressure using excess amounts of catalyst and methanol. To overcome these problems, the development of innovative solid catalysts for transformation of a great diversity of crude vegetable oils into high-quality BDF products at moderate condition has been an emerging field of research.

☆ This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author. Tel.: +81 298612680; fax: +81 298614532. E-mail address: sy-chen@aist.go.jp (S.-Y. Chen).

Ti-containing porous catalysts, such as TS-1 and Ti-MWW, have shown remarkable activities in selective oxidation and ammoximation [9-12]. However, a few studies have dealt with BDF synthesis because the Ti-containing catalysts have been regarded to be poor in acid-catalyzed reactions [13-15]. Siano et al. indicated that only 64% of FAME yield was obtained by transesterification of soybean oil over Ti-grafted amorphous silica at 180 °C [14,15]. In addition to serious Ti leaching, the FAME yields were significantly dropped around 20-27% in the presence of a few amounts of FFA and water. In this communication, we reported that the Ti-SBA-15 catalyst was highly active in high-quality BDF synthesis, which met with international fuel standards, such as EN14214:2009, through transesterification of various vegetable oils with methanol at moderate reaction condition, in comparison to siliceous SBA-15 and commercial TiO2 catalysts with different anatase sizes as reference catalysts. Effects of structural and acidic properties on catalytic activity and tolerance levels to water and FFA poisons were particularly explored. To the best of our knowledge, there is no report on acidic nature of Ti-incorporated mesoporous materials for the BDF production.

2. Experimental

2.1. Preparation of Ti-SBA-15 catalyst

The Ti-SBA-15 catalysts with different Ti loadings were prepared by a modified co-condensation method [11]. Typically, 8.4 g of tetraethyl orthosilicate (TEOS, Strem) was pre-hydrolyzed in 160 g of synthesis

1566-7367/$ - see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.07.021

Table 1

Textural and catalytic properties of various Ti-SBA-15 and reference catalysts.3

Catalysts Ti (mol%) TiO2 (nm)b SBET(m2g-1) VTotal (cm3 g-1) ® (nm) Acid capacity (mmol g-cat 1) TOF (h-1) FAME (mass%) Gj (mass%)

Blankc - - - - - - - 23.2 9.0

SBA-15 0 - 914 1.1 7.5 1.65 - 23.9 9.4

TiO2-L 100 84.7 11 0.10 30 0.115 0.18 33.9 7.8

TiO2-M 100 25.5 128 0.32 8.7 0.586 0.24 39.8 7.9

TiO2-S 100 15.0 164 0.38 9.2 0.704 0.73 54.9 5.9

1Ti-SBA-15 0.808 n.d. 890 0.96 7.9 1.84 166 83.9 2.8

3Ti-SBA-15 2.46 n.d. 840 0.91 7.6 2.05 66 90.9 1.6

5Ti-SBA-15 3.76 n.d. 810 0.86 7.1 2.09 39 89.5 1.7

7Ti-SBA-15 5.83 n.d. 770 0.85 7.3 2.09 30 85.8 2.3

10Ti-SBA-15 6.78 6.3 766 0.79 7.3 1.78 18 74.6 4.3

a Reaction condition: 200 °C for 3 h under autogenous pressure, 2.5 g CJO (2.89 mmol), 2.5 g methanol (78.1 mmol), 0.375 g catalysts. b Estimated by Scherrer equation, where "n.d." indicates "not detectable." c The blank test.

solution containing 3 g of Pluronic P123 (Aldrich, Mn = 5800) and 2.36 g of sodium chloride (NaCl, Wako) at 35 °C for 4 h before adding TiOCl2 precursor. The gel compositions could be varied in 0.0087-0.22 P123: 1 TEOS: 0.010-0.10 titanium tetraisoproxide (TTIP): 0.015-3.0 HCl: 0-6 NaCl: 220 H2O. The mixtures sealed in the polypropylene bottles were stirred at 35 °C for 24 h and hydrothermally treated at 100 °C for another 24 h without stirring. The as-made samples collected by filtering, washing and drying were calcined at 500 °C for 12 h. The resulting samples were designed as xTi-SBA-15 catalysts, where x is the Ti/Si molar percentages in the gels.

2.2. Characterization

X-ray diffraction patterns (XRD) were recorded by a Bruker AXS D8 advance diffractrometer using Cu Ka at 45 kV and 40 mA. N2 physisorption was measured by a BELSORP 28SA instrument (BEL Japan) at 77 K. Inductively coupled plasma-optic emission spectroscopy (ICP-OES) was analyzed by a Thermo Scientific iCAP 6300 ICP spectrometer. The acid capacity and strength were determined by differential heat of NH3 adsorption using a CSA-450G micro-calorimeter (Tokyo Riko, Japan) at 50 °C. The acidic property was studied by diffuse reflectance infrared flourier transform (DRIFT) spectroscopy of pyridine adsorption using a Thermo Nicolet Nexus 870 FT-IR instrument equipped with a smart collector.

23. Synthesis and analysis ofBDF

Transesterification of various vegetable oils with methanol was performed in a high-pressure batch-type stainless steel reactor lined with glass tube at 200 °C for 3 h in N2. Typically, 5 g of crude Jatropha oil (CJO) with an acid value of 17.8 mg KOH/g, 5 g of methanol and 0.75 g of dried catalyst sealed in batch-type stainless steel reactor with N2 atmosphere were quickly inserted into a pre-heated electric furnace with a constant vibrational frequency. The methanol/CJO molar ratio was 27. After reaction, the reactor was quickly air-cooled. The BDF products obtained by removals of catalyst and methanol were analyzed by Agilent 6890 N GC-FID instruments using EN

Fig. 1. DRIFT spectra of the 500 °C dehydrated 3Ti-SBA-15 catalyst in comparison to those of TS-1 and SBA-15 as reference catalysts.

Fig. 2. DRIFT spectra of pyridine adsorbed on 3Ti-SBA-15 catalyst after desorption at different temperatures, where L and B represent Lewis and Bronsted acids, respectively.

Fig. 3. Relationship of acidic strengths and FAME yields of various Ti-SBA-15 and three TiO2 catalysts.

14103:2009 and 14105:2009 methods (SI). The used catalyst was regenerated by calcining at 500 °C for 3 h.

3. Results and discussion

Small-angle XRD patterns and corresponded N2 physisorption isotherms exhibit that all the Ti-SBA-15 catalysts possess well-ordered p6mm structure and narrowly distributed mesopores, akin to that of siliceous SBA-15 (Figs. S1(a) and S2, SI) [16]. In wide-angle region, the Ti-SBA-15 catalysts with Ti loadings of 0.808-5.83 mol% have amorphous frameworks (Fig. S1 (b), SI). Above this loading, a weak diffraction peak appears at 20 = 25.5°, corresponding to the (101) plane of anatase TiO2 nanocrystallites with a size of 6.3 nm estimated by Scherrer equation [11]. This result suggests that the maximum Ti loading to retain Ti in X-ray amorphous species and homogeneously incorporated in Ti-SBA-15 is around 5.83 mol%.

The surface area (SBET), pore volume (VTotal) and pore diameter (0) of Ti-SBA-15 catalysts are 766-890 m2 g-1, 0.79-0.96 cm3 g-1 and 7.1-7.9 nm, respectively, slightly decreased in increasing the Ti loading due to the formation of extra-framework Ti clusters or anatase nanocrystallites (Table 1). Siliceous SBA-15 has slightly higher SBET and VTotai values whereas three TiO2 catalysts have much lower SBET and VTotal but larger 0 values. The acid capacity measured by pulsed NH3 chemisorption decreases in the order of Ti-SBA-15 > SBA-15 > TiO2-S > TiO2-M > TiO2-L. The higher acid capacities of Ti-SBA-15 catalysts than siliceous SBA-15 should have to do with

the incorporated Ti species. Three TiO2 catalysts have low acid capacities (0.115-0.704 mmol g-cat-1), proportional to surface area and disproportion to anatase size. It indicates that the acidities of Ti-containing catalysts are mainly related to the coordinately unsatu-rated Ti species, which are rich in amorphous Ti-SBA-15 but poor in crystalline TiO2.

The DRIFT spectra in Fig. 1 show that the 500 °C dehydrated Ti-SBA-15 catalysts contain two distinct bands centered at 946 and 980 cm-1, assigned to the stretching of tetrahedral Ti(IV) species in amorphous SBA-15 framework and surface silanol groups, respectively [17]. After adsorbed pyridine, the tetrahedral Ti(IV) species show weak Lewis acid nature (Fig. 2) [18]. Noteworthily, the DRIFT bands of tetrahedral Ti(IV) species increase to a maximum intensity for the Ti loadings of 2.46-5.83 mol% and then decrease by higher Ti loading. It suggests that the tetrahedral Ti(IV) species with Lewis acid nature are uniformly incorporated in Ti-SBA-15. The octahedral Ti(IV) species are likely present while the Ti loadings are around 2.46-5.83 mol%. As higher Ti loading, the extra-framework Ti clusters or anatase nanocrystallites are formed.

For transesterification of CJO with methanol to Jatropha BDF over Ti-containing catalysts, the FAME yields increase to 33.9-90.9 mass% with diminishing totally glycerides (Gr) and FFA values in the order of Ti-SBA-15 > TiO2-S > TiO2-M > TiO2-L, which follows the trend of acid capacity (Table 1), whereas siliceous SBA-15 and blank test only give 23-24 mass% of FAME yields. The 3Ti-SBA-15 catalyst gives a highest FAME yield with lowest GT and FFA values. Turnover frequency (TOF, h-1) is calculated by dividing the molar yield of FAME by per mole of Ti in one hour. The TOF values of Ti-SBA-15 catalysts are around 18166 h-1, decreased in increasing the Ti loading due to the formation of aggregated Ti species and much higher than those of three TiO2 catalysts. Evidently, the isolated Ti(IV) species are superior to extraframework Ti(IV) clusters or anatase nanocrystallites in catalyzing BDF synthesis.

The differential heat of NH3 adsorption shows that the siliceous SBA-15 and 3Ti-SBA-15 catalysts with tetrahedral-coordinated Ti(IV) species mainly belongs to very weak and weak acids, respectively (Table S2 and Fig. S4, SI). The acid strengths ofTiO2-M and TiO2-S are uniformly distributed from strong to very weak acid whereas TiO2-L has a few very weak acids. Fig. 3 indicates that the amount of weak acid in Ti-containing catalysts positively relates to the FAME yields but the strong and medium acids of TiO2-S and TiO2-M catalysts, derived from the surface coordinately unsaturated Ti(IV) species, have no activity in BDF synthesis. The tetrahedral-coordinated Ti(IV) species with weak Lewis acid nature in Ti-SBA-15 catalyst clearly contribute to the BDF production.

When blending with petro-diesel or direct feeding in diesel engine, the quality of BDF is strictly required to meet with international fuel

Influences of methanol/CJO molar ratio, water, FFA and regeneration on quality of Jatropha BDF over 3Ti-SBA-15 catalyst in comparison to commercial TiO2-S catalyst.a

Catalysts MeOH/CJO (Molar Ratio) LA/CJO (wt%)b H2O/CJO (wt%) FAME (mass%) GT (mass%) TG (mass%) DG (mass%)c MG (mass%)d G (mass%

3Ti-SBA-15 14 - - 62.1 5.0 18 13 4.3 0.16

27 - - 90.9 1.6 1.5 1.5 4.3 0.17

54 - - 96.0 0.65 0.24 0.31 2.0 0.070

81 - - 97.7 0.25 0.046 0.060 0.89 0.010

108 - - 98.4 0.19 0.054 0.045 0.66 0.010

108e - - 98.5 0.20 0.055 0.052 0.71 0.0061

108 30 - 97.9 0.21 0.069 0.082 0.68 0.020

108 - 5 98.0 0.20 0.026 0.066 0.69 0.0070

TiO2-S 108 - - 85.3 2.71 3.2 3.5 6.6 0.19

108 30 - 76.8 3.7 7.1 7.5 6.6 0.18

108 - 5 51.2 7.2 20.3 17.5 8.8 0.32

EN 14214:2009 standard - - - >96.5 <0.25 <0.20 <0.20 <0.80 <0.02

a Reaction condition: 200 °C for 3 h under autogenous pressure, 2.5 g CJO (2.89 mmol), 2.5-10 g MeOH (78.1-313 mmol), 0.375 g 3Ti-SBA-15 catalyst. b The molar ratio between lauric acid (LA) and CJO. c DC is the abbreviation of diglycerides. d MG is the abbreviation of monoglycerides. e After the second regeneration.

standards, such as EN 14214:2009, in order to avoid the damages of vehicle's engine and exhaust systems (Table 2). With a methanol/oil ratio of 108, the quality ofJatropha BDF synthesized by 3Ti-SBA-15 catalyst meets the EN 14214:2009 standard whereas low-quality BDF is only obtained over TiO2-S catalyst. The ICP-OES analysis shows that no Ti species are leached from 3Ti-SBA-15 catalyst into BDF products. The regenerated 3Ti-SBA-15 catalyst maintains high activity in BDF synthesis even after the second recycling test. Most importantly, high-quality BDF is obtained over 3Ti-SBA-15 catalyst even though the 5 wt% of water or 30 wt% of FFA is present. In contrast, the activities of TiO2-S reference catalyst significantly drop ca. 10-40% by adding FFA and water. On the other hand, Macario and Giordano [7] lately reported that Amberlyst-15 acidic resin was able to produce BDF from FFA feed but it has very poor activity in conversion of TG. The results indicate that the tetrahedral-coordinated Ti(IV) species of weak Lewis acid property are highly tolerances to water and FFA poisons in addition to the well-ordered mesostructure with largely hydrophilic surfaces as a good medium for water adsorption. A great diversity of vegetable oils with various acid values in 1.47-190 mg KOH g-1 can be transformed into high-quality BDF products over the 3Ti-SBA-15 catalyst, except that made of low-grade palm fatty acid distillates (PFAD) with an extremely high acid value of 190 mg KOH g-1 (Table S3, SI). Nevertheless, the quality of PFAD BDF can meet with European standard when the catalyst amount and reaction period are increased to 30 wt% of PFAD and 5 h, respectively.

4. Conclusions

The Ti-SBA-15 catalyst with tetrahedral-coordinated Ti(IV) species was an efficient and reusable Lewis solid acid for high-quality BDF production, which met with international fuel standard. A great diversity of edible and non-edible vegetable oils with various acid values was used as oil feedstocks without any pre-esterification or pre-treatment. Particularly, the water and FFA tolerance levels of Ti-SBA-15 catalyst were several ten times better than the upper limits of homogeneous and heterogeneous catalysts in the conventional BDF production technology.

Acknowledgements

This research was supported by JST-JICA's SATREPS project.

Acknowledgements are extended to Dr. A. Endo and Dr. A. Kawai of

Research Institute for Innovation in Sustainable Chemistry, AIST, for

XRD measurement, and Mr. M. Kaitsuka and Dr. M. Oguma of NFV,

AIST, for ICP-OES analysis.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.

doi.org/10.1016/j.catcom.2013.07.021.

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