- Email: [email protected]
Fish bone derived natural hydroxyapatite-supported copper acid catalyst: Taguchi optimization of semibatch oleic acid esterification
Chemical Engineering Journal 215–216 (2013) 491–499
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Fish bone derived natural hydroxyapatite-supported copper acid catalyst: Taguchi optimization of semibatch oleic acid esterification R. Chakraborty ⇑, D. RoyChowdhury Department of Chemical Engineering, Jadavpur University, Kolkata 700 032, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Natural hydroxyapatite (NHAp) was
"
"
"
"
derived from fish (Lates calcarifer) bone. NHAp supported efficient, costeffective, reusable copper Lewis acid catalyst. Catalyst preparation through Tungsten–halogen-irradiationassisted freeze-drying. Optimization of semibatch ethyl oleate synthesis through Taguchi robust design. Enhanced oleic acid conversion by in situ water removal employing adsorbent.
a r t i c l e
i n f o
Article history: Received 20 August 2012 Received in revised form 9 November 2012 Accepted 14 November 2012 Available online 23 November 2012 Keywords: Fish bone-natural HAp Tungsten–halogen-irradiation-assisted freeze-drying Cu–natural HAp catalyst Oleic acid esterification Taguchi optimization
a b s t r a c t Natural hydroxyapatite (NHAp) derived from waste fish (Lates calcarifer) bone has been effectively utilized as a support for preparation of low-cost, recyclable, heterogeneous copper acid catalyst. The novel catalyst has been prepared through wet-impregnation method involving tungsten–halogen-irradiation assisted freeze-drying. The catalyst was characterized through TGA, SEM, XRD, BET–BJH and FTIR analyses. The catalyst possessed 16.78 m2/g specific surface area, 0.0313 cc/g pore volume and 33.14 nm modal pore size with an acidity of 11.22 mmol KOH/g catalyst. The developed acid catalyst demonstrated excellent efficacy in the semibatch esterification of oleic acid with ethanol. The Taguchi robust design method (L9 orthogonal array) was applied to optimize process parameters governing oleic acid conversion. The maximum oleic acid conversion over a span of 1 h was 91.86% corresponding to the parametric values viz. 90 °C freeze-drying temperature, 1.0 weight ratio of copper nitrate to NHAp, 0.8 mL/min ethanol flow rate and 1000 rpm stirrer speed. Moreover, in situ water removal within the reactor through use of silicagel desiccators could significantly enhance oleic acid conversion. The innovative Cu–NHAp catalyst demonstrated excellent reusability and regeneration characteristics. Thus, the article explores an innovative and environmentally-benign utilization avenue of waste fish bone as a promising heterogeneous catalyst support. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Fatty acid esters generated through reversible esterification reactions form an industrially important class of substances [1]. ⇑ Corresponding author. Tel./fax: +91 3324146378. E-mail address: [email protected] (R. Chakraborty). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.11.064
Ethyl oleate derived from ethanol and oleic acid, can be used as industrial solvents for pharmaceutical manipulations, as lubricant or plasticizer, water resisting agent and hydraulic fluids [2]. Due to its excellent properties, synthesis of ethyl oleate had been studied by many researchers employing different heterogeneous catalyst viz. modified ZnO Nanoparticles [3], 12-tungstophosphoric acid supported on zirconia [4]. Among the possible sources of
492
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
Nomenclature
cOA XFDT X Cu L XFF
oleic acid conversion (%) freeze-drying temperature (°C) weight ratio of copper nitrate to NHAp ethanol feed flow rate (mL/min)
reagents economically available, ethanol has the desirable advantage of being a low toxicity alcohol when compared to methanol, propanol, butanol and some other larger chain alcohols. Development of effective methods for ethyl oleate production from oleic acid and ethanol by means of pervaporation-assisted esterification using heterogeneous catalysis was investigated owing to the usefulness of ethyl oleate as biodiesel blending stock [5]. More recently [6], esterification of oelic acid with ethanol over organophosphonic acid-functionalized silica has been reported. Owing to the desirable properties i.e. structural stability, ionic substitution ability, acid–base properties and adsorption capacity, hydroxyapatite (HAp) had been employed as a support for producing high performance heterogeneous catalyst. Mori and co-researchers [7] reported preparation of a highly efficient HAp supported Pd catalyst for oxidation of alcohol and Heck reaction. Successful application of HAp as a catalyst support for the Knoevenagel reaction without solvent was detailed by Sebti et al. [8]. Synthetic HAp supported ZnCl2, NiCl2 and CuCl2 Lewis acids could effectively catalyze the Friedel–Crafts alkylation of benzene, toluene and p-xylene by benzyl chloride [9]. However, for preparation of HAp, expensive analytical grade chemicals were used in all of the above-mentioned research works. Few researchers used fish bone for preparation of HAp as a cheaper source for HAp. HAp was derived from fish bone of Seir fish (Thynnus Thynnus) [10] for application as a coating material on 316L SS surface and also from fish harvested from Alaska [11] for removal of heavy metals. Besides, removal of aqueous chromium was demonstrated using fish bone based HAp [12]. However, to the best of our knowledge, no report is available in scientific literature on the application of fish bone derived-natural HAp as a catalyst support. Freeze-drying is a dehydration operation usually conducted under vacuum in which water is removed by sublimation of ice from pre-frozen materials. Drying is one of the crucial steps in preparation of solid grains/powders for various applications. In past, multicationic oxides La0.9Sr0.1Ga0.8Mg0.2O2.85 perovskite powders were prepared by a method involving freeze-drying of an aqueous solution of metallic nitrates containing hydroxypropylmethyl cellulose [13]. Freeze-drying was applied to produce extremely small grains of SnO2 for the purpose of CO gas sensing [14]. Besides, preparation of LaCoO3 samples were performed through thermal decomposition of La–Co citrate precursors obtained by freeze-drying of the corresponding solutions [15] and fiber-supported La0.65Sr0.35Ni0.29Co0.69Fe0.02O3 perovskite combustion catalyst was prepared through freeze-drying for methane and natural gas combustion [16]. Although use of freeze-drying could improve the specific surface area and morphological properties of the catalyst, and thus, could render superior catalytic activity; however, inadequate information is available in scientific literature on application of freeze-drying for preparation of HAp supported catalyst. The Taguchi method [17] involves analysis of experiments considering several process parameters with the smallest number of experiments using a design matrix, referred to as the orthogonal array. Over recent past, the Taguchi method was applied as a powerful tool for assessing parametric effects on process response and optimization of process conditions [18]. The salient advantage of
XSS S=N S/Nj
stirrer speed (rpm) S/N ratio signal to noise ratio mean of the S/N ratio the S/N ratio corresponding to the optimal process parameters, j and its level
Taguchi optimization method lies in the fact that much lower numbers of experimental runs are required for process optimization in comparison with response surface methodology (RSM) when several process factors are considered at a time. The present work primarily aims to prepare and characterize highly efficient, reusable and cost-effective heterogeneous copper acid catalyst using fish (Lates calcarifer) bone derived natural hydroxyapatite (NHAp) as support. The catalyst has been prepared through wet-impregnation of copper nitrate precursor on NHAp, followed by tungsten–halogen-irradiation assisted freeze-drying and air-calcination. The characterization of developed catalyst has been performed through TGA, XRD, BET, FTIR, SEM and EDX analyses. The catalytic activity was evaluated through oleic acid esterification with ethanol in a semibatch reactor configuration. Oleic acid conversion was maximized and the corresponding process conditions viz. freeze-drying temperature, weight ratio of copper nitrate to NHAp, ethanol flow rate and stirrer speed were evaluated employing Taguchi L9 orthogonal method. Under optimal condition, effect of incorporation of solid desiccant in the reactor for in situ water removal on oleic acid conversion was also investigated. 2. Experimental 2.1. Materials Waste Bekti fish (Lates calcarifer) bone was collected from local fish market. The chemicals viz. 25% aqueous ammonia solution, KOH, ethanol, oleic acid and Cu(NO3)25H2O were purchased from Merck, India. 2.2. Preparation of Cu–NHAp catalyst Waste fish (L. calcarifer) bone was subjected to rigorous washing with hot deionized water to remove oily and fleshy materials. The defatted fish bone was subsequently pulverized to paste to obtain natural hydroxyapatite (NHAp). Later, the prepared NHAp support was taken in a 1L three-necked glass flask equipped with two reflux condensers and a centrally mounted mechanical stirrer for wet-impregnation of Cu(NO3)25H2O precursor. In a representative C1.0 catalyst (i.e. 1:1 weight ratio of Cu(NO3)25H2O to NHAp) preparation process, a measured quantity (50 g) of Cu(NO3)25H2O was dissolved in 36.28 mL water, and subsequently added to 50 g NHAp as per wet-impregnation method [19] under vigorous mixing for 3 h, at 60 °C and 4 pH by using 25% ammonia solution in order to form a precipitate. Afterwards, the precipitate was ripened for 24 h, and eventually separated from mother liquor through vacuum filtration to obtain copper nitrate impregnated-NHAp (CNIN) mass. Consequently, the CNIN was freeze-dried using tungsten– halogen lamp (90 W) irradiation for 3 h at various temperatures of 70, 80 and 90 °C. The freeze-dried solid was finally subjected to calcination in air at pre-set 500 °C for 2 h to develop the Cu–NHAp catalyst. Identical procedure was applied for making catalysts with different copper nitrate to NHAp weight ratio in order to analyze the effects of copper nitrate loading on the properties of catalyst. Thus, according to the three weight ratio i.e. 0.5:1,
493
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
0.75:1 and 1:1 of Cu(NO3)25H2O to NHAp, the catalyst were designated as C0.5, C0.75, and C1.0 respectively. 2.3. Catalyst characterization Copper nitrate impregnated NHAp [CNIN] corresponding to C1.0 and unimpregnated-NHAp support were subjected to thermogravimetric analysis (TGA) in order to assess the effect of calcination temperature on thermal stability of the prepared catalyst. TGA was conducted using Perkin–Elmer TGA analyzer (Pyris Diamond TG/DTA) in a platinum crucible, under nitrogen atmosphere (150 mL/min) from 30 °C to 500 °C with an increasing temperature rate of 12 °C/min. The mass of the sample was 4 mg while alpha alumina was taken as reference powder. The specific surface area of the C1.0 catalyst that exhibited optimum performance was measured by BET method (Quantachrome make NOVA 4000e). X-ray diffraction (XRD) patterns of C0.5, C0.75 and C1.0 catalysts obtained at 90 °C freeze-drying temperature and at fixed 500 °C calcination temperature were analyzed (Rigaku Ultima III) using Cu Ka source outfitted with an Inel CPS 120 hemispherical detector. The analysis was performed at 2h Ranging from 20 to 80 °C at a scanning speed of 1 min1at 40 Kv and 30 mA. The infrared spectra of optimal catalyst were analyzed by FTIR-Shimadzu (alpha) from 400 to 4000 cm1. The morphology of the optimal catalyst was studied by a Scanning Electron Microscope (SEM) at 17 KV (JEOL Ltd., Japan, model JSM 6700F). The samples were placed on two sides adhesive tape attached to specimen stubs and were vacuum coated with a platinum layer with 80 s time of coating, pressure (5 N/m2), current (20 mA) and voltage (17 kV) respectively. In order to assess the effect of freeze-drying temperature on acidity of the prepared catalyst, all the CNIN samples developed by varying precursor loading were tested for acidity [20] after freeze-drying at 70, 80 and 90 °C (i.e. before calcination). The freeze-dried CNIN samples were subsequently calcined and the corresponding acidity of the developed catalysts were determined (Table 1). 2.4. Experimental design (Taguchi method) Table 2 displays the independent process factors and their levels examined in a L9 orthogonal array using Taguchi design. A standard orthogonal array L9 [18] was used and all experiments were conducted in triplicate using three levels of the four process factors (Table 2) viz. freeze drying temperature (XFDT), weight ratio of copper nitrate to NHAp (X C u L ), ethanol flow rate (XFF), and stirrer speed (XSS) in esterification of oleic acid with ethanol. The Taguchi method was employed to assess parametric interactions and to deter-
mine a set of optimal process factors corresponding to maximum oleic acid conversion through the S/N ratios and analysis of variance (ANOVA) using the software MINITAB-16 (Minitab Inc. USA for Windows7). Instead of conducting 27 (i.e. 34) experiments involving factorial experiment design, only nine experimental runs were required in the present study to optimize the parameter settings for maximum oleic acid conversion (cOA) to ethyl oleate. The experimental design matrix and mean of three experimental values for the cOA along with the calculated S/N ratio (Eq. (1)) are presented in (Table 3). The S/N ratio values corresponding to the (cOA) are calculated, using the ‘higher-the-better’ characteristics. The S/N ratio for any run was calculated according to the Eq. (1) [18] using the oleic acid conversion i.e. yi of the corresponding run. n 1X 1 S=N ¼ 10 log n i¼1 y2i
! ð1Þ
where i is the number of replicate, n is the number of trial experiments performed in any particular parametric combinations as per Table 3. The estimated S/N ratio can be expressed using following equation:
S=Nest ¼ S=N þ
k X ðS=Nj S=NÞ
ð2Þ
j¼1
where S=N is the mean of the ratio, S/N and S/Nj is the S/N ratio corresponding to the optimal process parameters, j and k is the number of the process parameters that significantly affected the optimal conditions of cOA. 2.5. Oleic acid esterification using Cu–NHAp catalyst The performance of the developed Cu–NHAp catalyst was assessed through oleic acid esterification with ethanol over a fixed reaction time of 1 h at 70 °C temperature while the other process factors were used as per Table 3 in all nine runs. The laboratory semibatch reactor consisted of a three-necked glass flask containing measured quantity of oleic acid; one neck of the flask was fitted with a reflux condenser and the second one was connected to the delivery pipe of a peristaltic pump for dosing ethanol at a regulated flow rate (varied from 0.40 mL/min to 0.80 mL/min). The central neck was equipped with a mechanical stirrer (REMI, RQ-121/D, AXIAL TURBINE, equipped with step-less electronic speed regulator and digital speed indicator in the range 200–2000 rpm). The reactor flask was placed on a heating mantle maintained at the required temperature using a PID temperature controller. After 1 h of operation, the reaction mixture was filtered by vacuum
Table 1 Acidity of the developed Cu–NHAp catalysts corresponding to different loading ratios of Cu(NO3)2 to NHAp and different freeze-drying temperatures. Acidity of C0.5 (mmol KOH/g)
Acidity of C0.75 (mmol KOH/g)
Acidity of C1.0 (mmol KOH/g)
After freeze-drying
After calcination
After freeze-drying
After calcination
After freeze-drying
After calcination
2.4 1.46 1.04
5.66 3.67 3.09
3.6 2.72 1.26
7.83 7.40 3.44
9.6 6.14 3.52
11.22 8.09 5.12
Freeze-drying temperature (90 °C) Freeze-drying temperature (80 °C) Freeze-drying temperature (70 °C)
Table 2 Experimental factors and levels for oleic acid esterification with ethanol in semibatch reactor employing the developed Cu–NHAp catalyst. Process factors
XFDT Freeze drying temperature (°C)
XCu L Ratio of copper nitrate to HAp
XFF Ethanol flow rate (mL/min)
XSS Stirrer speed (rpm)
Level 1 Level 2 Level 3
70 80 90
0.50 0.75 1.00
0.4 0.6 0.8
800 1000 1300
494
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
Table 3 L9 orthogonal array experimental matrix at different operating conditions and corresponding mean oleic acid conversion (QOA) and signal-to-noise (S/N) ratio. Trial no.
XFDT (°C)
X C u L (wt.%)
XFF (mL/min)
XSS (rpm)
QOA (%)
S/N ratio (db)
1 2 3 4 5 6 7 8 9
L1 L1 L1 L2 L2 L2 L3 L3 L3
L1 L2 L3 L1 L2 L3 L1 L2 L3
L1 L2 L3 L2 L3 L1 L3 L1 L2
L1 L2 L3 L3 L1 L2 L2 L3 L1
83.57 85.05 87.34 84.10 86.22 88.26 90.20 90.32 91.95
38.44 38.59 38.82 38.49 38.71 38.91 39.10 39.11 39.27
L1 (low level value), L2 (middle level value), L3 (upper level value).
filtration to separate the solid catalyst. Later, the filtrate was thoroughly washed with hot water and finally centrifuged at about 5000 rpm for 10 min. The operation separated the reaction mixture into an upper layer (ester phase) consisting of a mixture of unreacted oleic acid and product ethyl oelate, and a lower aqueous layer containing ethanol. 2.6. Conversion analysis The upper ester phase obtained upon centrifugation, was analyzed through titration [21,22] for determination of acid number (Eq. (3a)) in order to evaluate the oleic acid conversion (Eq. (3b)) as detailed below:
A¼
cOA
56:11 M V m AOLEICACID A ¼ 100 AOLEICACID
ð3aÞ
and a prime 27% weight losses occurred over 30–100 °C ascribing to evaporation of adsorbed water. Further weight losses of 8.6% and 3% took place for the samples between 100 °C and 300 °C because of loss of lattice water. On further heating over a temperature range between 300 °C and 500 °C, it was found that 31% and 18% weight losses occurred for NHAp and CNIN samples respectively owing to disintegration of macromolecules and other organic substances in addition to solid state reaction leading to formation of complex compound such as libethenite Cu2(OH)(PO4) as shown in XRD plot (Fig. 2). Relatively insignificant weight losses of 4% occurred for CNIN compared to 6% NHAp respectively between 500 °C and 700 °C. On further investigation, it was found that at temperatures above 700 °C very insignificant weight losses occurred for both NHAp (4%) and CNIN (1%). Thus, in order to minimize energy consumption in calcination, 500 °C was selected as calcination temperature since, all the active crystalline phase were found to be prominent at 500 °C temperature [23].
ð3bÞ
where A is the Concentration of oleic acid; M the Normality of potassium hydroxide solution; V the Volume of potassium hydroxide consumed from burette; AOLEICACID the Acid value of oleic acid = 200; m is the Weight of ester layer. 3. Result and discussion
3.1.2. Acidity of the developed catalyst The freeze-dried (at three freeze drying temperatures, i.e. 70, 80 and 90 °C) CNIN samples prepared at three different ratios of copper nitrate to NHAp viz. 0.5:1, 0.75:1 and 1:1 were tested for acidity. All the freeze-dried CNIN samples were subsequently calcined at 500 °C to prepare Cu–NHAp catalysts and tested for acidity (Table 1). It is evident from Table 1, that the acidity of the catalyst
3.1. Catalyst characterization 3.1.1. TGA Analyses TGA patterns (Fig. 1) of representative samples of NHAp and CNIN corresponding to C1.0 show the weight losses of samples on heating over a temperature range from 30 °C to 860 °C. Thermo gravimetric analyses of NHAp and CNIN manifest respective 6.4%
Fig. 1. TGA analyses of CNIN(C1.0) and NHAp samples from 30 to 860 °C.
Fig. 2. Powder XRD patterns of Cu–NHAp catalyst obtained by calcination at 500 °C at different precursor loadings: (a) C0.5, (b) C0.75 and (c) C1.0. [Characteristic peaks due to Hydroxyapatite ( ), b-Calcium phosphate ( ), copper oxide ( ), libethenite ( )].
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
495
Fig. 3b. Cumulative pore volume vs. pore diameter plot for Cu–NHAp catalyst (C1.0).
Fig. 3a. Pore volume vs. relative pressure (P/P0) of Cu–NHAp catalyst (C1.0) at optimal i.e. 90 °C freeze-drying temperature [inset: BJH differential pore size distribution].
samples increased with increase in copper nitrate loading as well as freeze-drying temperature and accordingly, highest acidity (11.22 mmol KOH/g catalyst) was detected for C1.0 catalyst developed at 90 °C freeze-drying temperature. 3.1.3. XRD Analysis XRD patterns for the (Cu–NHAp) catalyst obtained (at fixed freeze-drying temperature of 90 °C and 500 °C calcination temperature) at three different copper nitrate loadings C0.5, C0.75, and C1.0 respectively are exhibited in Fig. 2a–c respectively. The angles corresponding to the peaks representing crystalline phases of HAp (25.691°, 32.102°, 53.418°, 66.281°), calcium phosphate (20.14°, 21.26°, 72.315°), copper oxide (35.462°, 38.642°, 66.299°, 51.239°) and libethenite [Cu2(OH)(PO4)] (29.302°, 53.418°, 61.238°, 68.062°) could be detected at all three precursor loadings. Here, it should be noted that, copper oxide crystalline phases represent isolated CuO crystal lattice. Libethenite [Cu2(OH)(PO4)] was formed as the surface Ca2+ ions of the NHAp were exchanged with Cu2+ ions of the precursor, which were chemisorbed onto the surface of NHAp support. Diffraction patterns pertaining to libethenite crystalline phase corroborate well with Tounsi and co-workers [23]. It is worthy to note that, with gradual increase in copper nitrate dosage the peak intensity of copper oxide was found to amplify to a greater extent compared to other crystalline phases. The XRD patterns obtained in the present study matched closely with chemically synthesized HAp supported copper oxide catalyst [24–26]. 3.1.4. BET analysis The BET (Brunauer–Emmett–Teller) specific surface area of 16.78 m2/g was determined for the developed Cu–NHAp(C1.0) catalyst prepared at 90 °C freeze-drying temperature and 500 °C calcination temperature. The ‘convex to the (P/P0) axis’ shape of the isotherm (Fig. 3a) of the developed catalyst corroborates type III of the IUPAC classification [27]. An isotherm of this type signifies adsorbate–adsorbate interactions. In the present case, a thin layer of hysteresis occurred in the P/P0 range of 0.5–0.65 implying capillary condensation in meso-porous structures. The catalyst pore volume of 0.0313 cc/g and 31.34 nm modal pore size were estimated using BJH method (inset of Fig. 3a). Fig. 3b depicts the cumulative pore volume vs. pore diameter plot for C1.0 representative catalyst sample. It can be observed that, 65.25% of the total catalyst pore volume relates to meso-porous size (8.2–50 nm), while, the remaining 34.75% pore volume corresponds to macro
Fig. 4. FTIR analyses of the Cu–NHAp(C1.0) optimal catalyst.
porous size (51–100 nm). Thus, the catalyst morphology could assist in achieving high oleic acid conversion to ethyl oleate. 3.1.5. FTIR Analysis FTIR spectra transmittance vs. wave number bands of Cu–NHAp(C1.0) catalyst (developed at 90 °C freeze-drying temperature and 500 °C calcination temperature that possessed highest acidity) are shown in Fig. 4. The bands 755, 1050, 1395 and 1548 cm1 characterize the bending vibration of phosphate
Fig. 5. (a) SEM image of freeze-dried (90 °C) Cu-NHAp(C1.0) optimal sample before calcination. (b) SEM image of freeze-dried (90 °C) Cu–NHAp(C1.0) optimal catalyst after calcination.
496
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
spectra at 400, 475, 520, 600 and 750 cm1 denote the band of copper oxide. All these bands corroborate well with literature reported data [23–26]. 3.1.6. Microscopic analyses SEM images of representative C1.0 samples before calcination (after freeze-drying at 90 °C) and after calcination at 500 °C are depicted in Fig. 5a and b respectively. Fig. 5a displays irregular shaped particles with uniform void spaces in the freeze-dried CNIN(C1.0) sample. Fig. 5b shows well agglomerated, irregular structure with non uniform pore of the calcined catalyst. 3.2. Mechanism of ethyl oleate formation using Cu–NHAp catalyst
Fig. 5. (continued)
(PO3 4 , O–P). Existence of carbonate ion could be confirmed through spectra at 1605, 1715, 2016 and 2156 cm1. The peaks corresponding to absorbed hydrate at 2300, 2712, 2740, 3435 and 3600 cm1 correspond to the stretching vibration of lattice OH ions. FTIR
The existence of CuO crystal lattice on the surface of the Cu–NHAp framework indicates that Cu2+ species are present on catalyst surface which can act as Lewis acid in catalyzing the ethyl oleate formation. Besides, the presence of Cu2+ cations in the new solid phase-libethenite Cu2(OH)(PO4) [28] also reinforces catalytic activity. It is known that both Cu2+ and Cu1+ can act as Lewis acid. In the present case, highly dispersed Cu2+ species, are first reduced to Cu1+ and then to Cu0 (i.e. Cu2+ ? Cu1+ ? Cu0 in concurrence with Liu et al. [29]). Since, Cu1+ species can also act as Lewis acid, the proposed reaction mechanism (Fig. 6) has been presented in terms of Cu1+ species. Nevertheless, similar mechanism could have also been presented in terms of Cu2+ species. Oleic acid was adsorbed on catalyst surface that led to generation of carbocation owing to the interaction between carbonyl oxygen of oleic acid and Lewis acidic site (copper cation) of the catalyst. Consequently, ethanol
Fig. 6. Mechanism of oleic acid esterification with ethanol in presence of Cu–NHAp catalyst.
497
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499 Table 4 Analysis of variance (ANOVA) of factors affecting ethyl oleate synthesis process. Factor
DF
Sum of squares
Mean square
F value
P-value
XFDT X Cu L XFF XSS Error
1 1 1 1 4
45.375 15.596 0.437 0.008 8.786
45.375 15.596 0.437 0.008 2.196
20.66 7.10 0.20 0.03
0.010 0.056 0.679 0.955
Total
8
70.202
present in liquid phase formed a tetrahedral intermediate through a nucleophilic attack on the carbocation. Subsequently, elimination of one water molecule from the tetrahedral intermediate generated a molecule of ethyl oleate. Thus, the catalyst became reusable for the subsequent esterification. 3.3. Analysis of oleic acid conversion and prediction of optimal conditions Summary results of the ANOVA are shown in Table 4, which indicated that all of the selected factors were significant parameters at the 98.02% confidence level. The analysis of variance (ANOVA) based on the observed data states that freeze-drying temperature contributes 45.375, sum of square as well as mean square followed by weight ratio of copper nitrate to NHAp (15.596), ethanol flow rate (0.437) and stirrer speed (0.008) in the oleic acid esterification process. According to the ‘‘F’’ and ‘p’ criteria, (Table 4), stirrer speed (XSS) had least effect on ethyl oleate formation process among all process factors; whereas, the freezedrying temperature was observed to be the most significant process factor. As the target of the process was to maximize oleic acid conversion (cOA), the ‘‘larger is better’’ criterion was chosen during the calculation of S/N ratio. The highest S/N ratio in the each factor was desirable to maximize cOA using the developed catalyst in semibatch reactor. The higher the difference between the minimum and the maximum S/N ratios in each factor, the higher is its effect on oleic acid conversion (Table 5). Considering the four factors [i.e. freeze-drying (FD) temperature, ratio of Copper nitrate to NHAp, ethanol flow rate and stirrer speed], it can be concluded that 90 °C FD temperature, 1.0 weight ratio of Cu(NO3)25H2O to NHAp loading, 0.8 mL/min ethanol flow rate and 1000 rpm stirrer speed resulted highest S/N ratio corresponding to highest oleic acid conversion (Fig. 7). The optimal levels of the process factors were identified and the optimum value of the S/N ratio could be computed based on the selected levels of the significant factors [17]. The values of the optimal S/N ratio and oleic acid conversion were 39.2710 db and 91.95%, respectively. A confirmation run was conducted based on these optimal conditions and the response was 39.1245 db, and the corresponding conversion of oelic acid was 91.86%.
Fig. 7. S/N ratio plot for oleic acid esterification with ethanol employing Cu–NHAp catalyst.
between any two process factors are assessed at the optimal values of the remaining two factors. The interaction between A (freezedrying temperature) and B (copper nitrate to NHAp weight ratio) revealed that at all three levels of A, an increase in B could monotonically enhance cOA over the range of the parametric value selected in the present study. At lowest level of A, there was a monotonic increase in cOA with gradual increase in C (ethanol flow rate); and the maximum cOA was observed at highest levels of A and C, however, the minimum cOA was found at the intermediate A level. A more or less similar interaction was observed between A and D (stirrer speed) in affecting cOA. Thus, maximum cOA could be achieved only at highest A level for all different factorial values. These observations strongly advocate the pronounced effect of Tungsten halogen irradiation-assisted freeze-drying temperature. Whereas, when A and D were set at optimal levels, maximum cOA could be achieved at highest B level and central C level. At lowest B level, an increase in C level had a synergistic effect on conversion, in other words, at minimal copper nitrate loading, an increase in ethanol flow rate would enhance the cOA indicating ethanol acting as a limiting reactant and reaction is possibly surface reaction controlled. Interaction plot of B and D exhibited that, at highest B level, an increase in D would not enhance oleic acid conversion, suggesting that the reaction was not mass transfer limited; whereas, at intermediate B level (C0.75), D could positively influence the conversion, indicating a mass transfer controlled regime. Under optimal levels of A and B, at lowest ethanol flow rate (C), enhancement in stirring speed (D) could increase oleic acid conversion, signifying a diffusion controlled regime. At highest C (0.8 mL/ min), the conversion increased up to 1000 rpm, however, further increase in D resulted in reduced oleic acid conversion owing to decrease in global reaction rate in the multi-step heterogeneous catalytic reaction.
3.4. Interactions among process factors
3.5. Effect of use of solid-desiccator on oleic acid conversion
The interactions among the process factors governing the oleic acid conversion have been demonstrated in Fig. 8. The interactions
The effect of incorporation of adsorbent (silica gel) within the reactor on oleic acid conversion exposed interesting observations.
Table 5 S/N ratios at different levels of the process factors and delta values (difference of the S/N values between highest and lowest levels of process factors). Level
Freeze-drying temperature (°C)
Ratio of copper nitrate to NHAp (wt.%)
Ethanol flow rate (mL/min)
Stirrer speed (rpm)
1 2 3 Delta Rank
38.62 38.71 39.16 0.54 1
38.68 38.81 39.00 0.32 2
38.82 38.79 38.88 0.09 3
38.81 38.87 38.81 0.06 4
498
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
Fig. 8. Interaction plot for oleic acid esterification with ethanol employing Cu–NHAp catalyst [A: freeze-drying temperature; B: copper nitrate to NHAp weight ratio; C: ethanol flow rate; D: stirrer speed].
Final oleic acid conversion after 1 h semibatch reactor operation at the determined optimal conditions could be enhanced to 92.4%, 94.3%, 95.8% and 96.5% corresponding to use of 22.34, 44.68, 67.02 and 89.36 wt.% of adsorbent as compared to 91.86% conversion without adsorbent. This can be ascribed to the significant decrease in extent of backward reaction in the reversible esterification of oleic acid owing to the removal of water by the solid desiccant; thus, rendering appreciably higher conversion of oleic acid. After each reaction cycle the silica gel could be easily recovered and regenerated through air drying in hot air oven at 105 °C for subsequent reuse.
adsorbent (silica gel) within the reactor for water removal could significantly enhance the oleic acid conversion. The developed catalyst demonstrated appreciable reusability and easy regeneration characteristics. A novel and eco-friendly waste utilization path could, thus, be explored for preparation of cost-effective solid catalyst to synthesize biodiesel blending stock leading to a tidier and healthier globe. Acknowledgment Authors are grateful to University Grants Commission, New Delhi, India for financial support [(F. No. 36-99/2008 (SR)].
3.6. Reusability References Reusability of the Cu–NHAp catalyst was tested by carrying out 1 h semibatch esterification run for three consecutive reaction cycles at the optimal process conditions. Before each reuse, the recovered catalyst was washed with ethanol to remove the adsorbed stains and then air dried at 105 °C for 1 h. The oleic acid conversion remained unchanged after two consecutive runs, however, it decreased to 90.67% from 91.86% on third reaction recycle and on further recycle conversion remained constant for three more subsequent runs. The small decrease in catalytic activity could be ascribed to very minute extent of leaching of the active catalyst species into the reaction medium (copper and calcium ion content of the final ethyl oleate product obtained after six consecutive catalyst usage was found to be 0.02 and 0.03 ppm). On the other hand, in addition to ethanol washing and air drying, if the catalyst could be regenerated through air calcination at 500 °C for 2 h, the oleic acid conversion was observed to remain unaltered for eight successive runs. This indicates excellent stability and reusability characteristics of the developed catalyst upon simple air-calcination. 4. Conclusion Natural hydroxyapatite (derived from waste bekti fish bone) supported, cost-effective, efficient copper solid acid catalyst was prepared by wet-impregnation method involving tungsten–halogen-irradiation assisted freeze-drying protocol. The catalyst showed high catalytic efficacy in esterification of oleic acid with ethanol in a semibatch reactor. Incorporation of recyclable
[1] Ulmann’s Encyclopaedia of Industrial Chemistry, vol. 12, VCH, Weinheim, 2001. [2] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresour. Technol. 70 (1999) 1–15. [3] K.J. Hun, G.Y. Peng, C. Yue, W.Z. Chen, Preparation and UV-light absorption property of oleic acid surface modified ZnO nanoparticles, Chem. Res. Chin. Univ. 27 (2011) 500–502. [4] C.F. Oliveira, L.M. Dezaneti, F.A.C. Garcia, J.L.D. Macedo, J.A. Dias, S.C.L. Dias, K.S.P. Alvim, Esterification of oleic acid with ethanol by 12-tungstophosphoric acid supported on zirconia, Appl. Catal. A 372 (2010) 153–161. [5] K.C.S. Figueiredo, V.M.M. Salim, C.P. Borges, Ethyl oleate production by means of pervaporation-assisted esterification using heterogeneous catalysis, Braz. J. Chem. Eng. 27 (2010) 609–617. [6] P. Yin, L. Chen, Z. Wang, R. Qu, X. Liu, S. Ren, Production of biodiesel by esterification of oleic acid with ethanol over organophosphonic acidfunctionalized silica, Bioresour. Technol. 110 (2012) 258–263. [7] K. Mori, K. Yamaguchi, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, Controlled synthesis of hydroxyapatite-supported palladium complexes as highly efficient heterogeneous catalysts, J. Am. Chem. Soc. 124 (2002) 11572–11573. [8] S. Sebti, R. Tahir, R. Nazih, A. Saber, S. Boulaajaj, Hydroxyapatite as a new solid support for the Knoevenagel reaction in heterogeneous media without solvent, Appl. Catal. A 228 (2002) 155–159. [9] S. Sebti, R. Tahir, R. Nazih, S. Boulaajaj, Comparison of different Lewis acid supported on hydroxyapatite as new catalysts of Friedel–Crafts alkylation, Appl. Catal., A 218 (2001) 25–30. [10] K. Prabakaran, S. Rajeswari, Development of hydroxyapatite from natural fish bone through heat treatment, Trends Biomater. Artif. Organs 20 (2006) 20–23. [11] W. Admassu, T. Breese, Feasibility of using natural fishbone apatite as a substitute for hydroxyapatite in remediating aqueous heavy metals, J. Hazard. Mater. 69 (1999) 187–196. [12] M. Ozawa, K. Satake, R. Suzuki, Removal of aqueous manganese using fish bone hydroxyapatite, J. Mater. Sci. Lett. 22 (2003) 513–514. [13] K. Traina, M.C. Steil, J.P. Pirard, C. Henrist, A. Rulmont, R. Cloots, B. Vertruyen, Synthesis of La0.9Sr0.1Ga0.8Mg0.2O2.85 by successive freeze-drying and selfignition of a hydroxypropylmethyl cellulose solution, J. Eur. Ceram. Soc. 27 (2007) 3469–3474.
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499 [14] O. Dos Santos, M.L. Weiller, D.Q. Junior, A.N. Medina, CO gas-sensing characteristics of SnO2 ceramics obtained by chemical precipitation and freeze-drying, Sens. Actuators, B 75 (2001) 83–87. [15] S. Ivanova, E. Zhecheva, R. Stoyanova, Microstructure of LaCoO3 prepared by freeze-drying of metal–citrate precursors revealed by EPR, J. Phys. Chem. Solids. 68 (2007) 168–174. [16] D. Klvana, J. Delval, J. Kirchnerova, J. Chaouki, Deactivation of fiber-supported La0.65Sr0.35Ni0.29Co0.69Fe0.02O3 catalyst by mercaptan during combustion of methane or natural gas, Appl. Catal. A 165 (1997) 171–182. [17] G. Taguchi, Introduction to quality engineering, Asian Productivity Organization, Tokyo. [18] R.K. Roy, Design of Experiments Using the Taguchi Approach: 16 Steps to Product and Process Improvement, John Wiley & Sons, Inc., US. [19] R. Chakraborty, S. Bepari, A. Banerjee, Transesterification of soybean oil catalyzed by fly ash and egg shell derived solid catalysts, Chem. Eng. J. 165 (2010) 798–805. [20] A.K. Singh, S.D. Fernando, Transesterification of soybean oil using heterogeneous catalysts, Energy Fuels 22 (2008) 2067–2069. [21] Y. Wang, S. Ou, P. Liu, F. Xue, S. Tang, Comparison of two different processes to synthesize biodiesel by waste cooking oil, J. Mol. Catal. A: Chem. 252 (2006) 107–112.
499
[22] R. Tesser, L. Casale, D. Verde, M. Di Serio, E. Santacesaria, Kinetics of free fatty acids esterification: batch and loop reactor modelling, Chem. Eng. J. 154 (2009) 25–33. [23] H. Tounsi, S. Djemal, C. Petitto, Copper loaded HAp catalyst for selective catalytic reduction of nitric oxide with ammonia, Appl. Catal. B107 (2011) 158–163. [24] M.R. Johan, M. Shahadan, M. Suan, N.L. Hawari, H.A. Ching, Annealing Effects on the properties of copper oxide thin films prepared by chemical deposition, Int. J. Electrochem. Sci. 6 (2011) 6094–6104. [25] E.N. Muhamad, R. Irmawati, A.H. Abdullah, Y.H. Taufiq-Yap, S.B. Abdul Hamid, Effect of number of washing on the characteristics of copper oxide nanopowders, Malaysian J Anal. Sci. 11 (2007) 294–301. [26] M. Kooti, L. Matouri, Fabrication of nanosized cuprous oxide using fehling’s solution, Trans. F: Nanotechnol. 17 (2010) 73–78. [27] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 57 (1985) 603–619. [28] J. Jemal, H. Tounsi, K. Chaari, C. Petitto, G. Delahayc, S. Djemel, A. Ghorbel, NO reduction with NH3 under oxidizing atmosphere on copper loaded hydroxyapatite, Appl. Catal. B: Environ. 113–114 (2011) 255–260. [29] Z. Liu, M. Amiridis, Y. Chen, Characterization of CuO supported on tetragonalZrO2, J. Phys. Chem. B 109 (2005) 1251–1255.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Fish bone derived natural hydroxyapatite-supported copper acid catalyst: Taguchi optimization of semibatch oleic acid esterification R. Chakraborty ⇑, D. RoyChowdhury Department of Chemical Engineering, Jadavpur University, Kolkata 700 032, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Natural hydroxyapatite (NHAp) was
"
"
"
"
derived from fish (Lates calcarifer) bone. NHAp supported efficient, costeffective, reusable copper Lewis acid catalyst. Catalyst preparation through Tungsten–halogen-irradiationassisted freeze-drying. Optimization of semibatch ethyl oleate synthesis through Taguchi robust design. Enhanced oleic acid conversion by in situ water removal employing adsorbent.
a r t i c l e
i n f o
Article history: Received 20 August 2012 Received in revised form 9 November 2012 Accepted 14 November 2012 Available online 23 November 2012 Keywords: Fish bone-natural HAp Tungsten–halogen-irradiation-assisted freeze-drying Cu–natural HAp catalyst Oleic acid esterification Taguchi optimization
a b s t r a c t Natural hydroxyapatite (NHAp) derived from waste fish (Lates calcarifer) bone has been effectively utilized as a support for preparation of low-cost, recyclable, heterogeneous copper acid catalyst. The novel catalyst has been prepared through wet-impregnation method involving tungsten–halogen-irradiation assisted freeze-drying. The catalyst was characterized through TGA, SEM, XRD, BET–BJH and FTIR analyses. The catalyst possessed 16.78 m2/g specific surface area, 0.0313 cc/g pore volume and 33.14 nm modal pore size with an acidity of 11.22 mmol KOH/g catalyst. The developed acid catalyst demonstrated excellent efficacy in the semibatch esterification of oleic acid with ethanol. The Taguchi robust design method (L9 orthogonal array) was applied to optimize process parameters governing oleic acid conversion. The maximum oleic acid conversion over a span of 1 h was 91.86% corresponding to the parametric values viz. 90 °C freeze-drying temperature, 1.0 weight ratio of copper nitrate to NHAp, 0.8 mL/min ethanol flow rate and 1000 rpm stirrer speed. Moreover, in situ water removal within the reactor through use of silicagel desiccators could significantly enhance oleic acid conversion. The innovative Cu–NHAp catalyst demonstrated excellent reusability and regeneration characteristics. Thus, the article explores an innovative and environmentally-benign utilization avenue of waste fish bone as a promising heterogeneous catalyst support. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Fatty acid esters generated through reversible esterification reactions form an industrially important class of substances [1]. ⇑ Corresponding author. Tel./fax: +91 3324146378. E-mail address: [email protected] (R. Chakraborty). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.11.064
Ethyl oleate derived from ethanol and oleic acid, can be used as industrial solvents for pharmaceutical manipulations, as lubricant or plasticizer, water resisting agent and hydraulic fluids [2]. Due to its excellent properties, synthesis of ethyl oleate had been studied by many researchers employing different heterogeneous catalyst viz. modified ZnO Nanoparticles [3], 12-tungstophosphoric acid supported on zirconia [4]. Among the possible sources of
492
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
Nomenclature
cOA XFDT X Cu L XFF
oleic acid conversion (%) freeze-drying temperature (°C) weight ratio of copper nitrate to NHAp ethanol feed flow rate (mL/min)
reagents economically available, ethanol has the desirable advantage of being a low toxicity alcohol when compared to methanol, propanol, butanol and some other larger chain alcohols. Development of effective methods for ethyl oleate production from oleic acid and ethanol by means of pervaporation-assisted esterification using heterogeneous catalysis was investigated owing to the usefulness of ethyl oleate as biodiesel blending stock [5]. More recently [6], esterification of oelic acid with ethanol over organophosphonic acid-functionalized silica has been reported. Owing to the desirable properties i.e. structural stability, ionic substitution ability, acid–base properties and adsorption capacity, hydroxyapatite (HAp) had been employed as a support for producing high performance heterogeneous catalyst. Mori and co-researchers [7] reported preparation of a highly efficient HAp supported Pd catalyst for oxidation of alcohol and Heck reaction. Successful application of HAp as a catalyst support for the Knoevenagel reaction without solvent was detailed by Sebti et al. [8]. Synthetic HAp supported ZnCl2, NiCl2 and CuCl2 Lewis acids could effectively catalyze the Friedel–Crafts alkylation of benzene, toluene and p-xylene by benzyl chloride [9]. However, for preparation of HAp, expensive analytical grade chemicals were used in all of the above-mentioned research works. Few researchers used fish bone for preparation of HAp as a cheaper source for HAp. HAp was derived from fish bone of Seir fish (Thynnus Thynnus) [10] for application as a coating material on 316L SS surface and also from fish harvested from Alaska [11] for removal of heavy metals. Besides, removal of aqueous chromium was demonstrated using fish bone based HAp [12]. However, to the best of our knowledge, no report is available in scientific literature on the application of fish bone derived-natural HAp as a catalyst support. Freeze-drying is a dehydration operation usually conducted under vacuum in which water is removed by sublimation of ice from pre-frozen materials. Drying is one of the crucial steps in preparation of solid grains/powders for various applications. In past, multicationic oxides La0.9Sr0.1Ga0.8Mg0.2O2.85 perovskite powders were prepared by a method involving freeze-drying of an aqueous solution of metallic nitrates containing hydroxypropylmethyl cellulose [13]. Freeze-drying was applied to produce extremely small grains of SnO2 for the purpose of CO gas sensing [14]. Besides, preparation of LaCoO3 samples were performed through thermal decomposition of La–Co citrate precursors obtained by freeze-drying of the corresponding solutions [15] and fiber-supported La0.65Sr0.35Ni0.29Co0.69Fe0.02O3 perovskite combustion catalyst was prepared through freeze-drying for methane and natural gas combustion [16]. Although use of freeze-drying could improve the specific surface area and morphological properties of the catalyst, and thus, could render superior catalytic activity; however, inadequate information is available in scientific literature on application of freeze-drying for preparation of HAp supported catalyst. The Taguchi method [17] involves analysis of experiments considering several process parameters with the smallest number of experiments using a design matrix, referred to as the orthogonal array. Over recent past, the Taguchi method was applied as a powerful tool for assessing parametric effects on process response and optimization of process conditions [18]. The salient advantage of
XSS S=N S/Nj
stirrer speed (rpm) S/N ratio signal to noise ratio mean of the S/N ratio the S/N ratio corresponding to the optimal process parameters, j and its level
Taguchi optimization method lies in the fact that much lower numbers of experimental runs are required for process optimization in comparison with response surface methodology (RSM) when several process factors are considered at a time. The present work primarily aims to prepare and characterize highly efficient, reusable and cost-effective heterogeneous copper acid catalyst using fish (Lates calcarifer) bone derived natural hydroxyapatite (NHAp) as support. The catalyst has been prepared through wet-impregnation of copper nitrate precursor on NHAp, followed by tungsten–halogen-irradiation assisted freeze-drying and air-calcination. The characterization of developed catalyst has been performed through TGA, XRD, BET, FTIR, SEM and EDX analyses. The catalytic activity was evaluated through oleic acid esterification with ethanol in a semibatch reactor configuration. Oleic acid conversion was maximized and the corresponding process conditions viz. freeze-drying temperature, weight ratio of copper nitrate to NHAp, ethanol flow rate and stirrer speed were evaluated employing Taguchi L9 orthogonal method. Under optimal condition, effect of incorporation of solid desiccant in the reactor for in situ water removal on oleic acid conversion was also investigated. 2. Experimental 2.1. Materials Waste Bekti fish (Lates calcarifer) bone was collected from local fish market. The chemicals viz. 25% aqueous ammonia solution, KOH, ethanol, oleic acid and Cu(NO3)25H2O were purchased from Merck, India. 2.2. Preparation of Cu–NHAp catalyst Waste fish (L. calcarifer) bone was subjected to rigorous washing with hot deionized water to remove oily and fleshy materials. The defatted fish bone was subsequently pulverized to paste to obtain natural hydroxyapatite (NHAp). Later, the prepared NHAp support was taken in a 1L three-necked glass flask equipped with two reflux condensers and a centrally mounted mechanical stirrer for wet-impregnation of Cu(NO3)25H2O precursor. In a representative C1.0 catalyst (i.e. 1:1 weight ratio of Cu(NO3)25H2O to NHAp) preparation process, a measured quantity (50 g) of Cu(NO3)25H2O was dissolved in 36.28 mL water, and subsequently added to 50 g NHAp as per wet-impregnation method [19] under vigorous mixing for 3 h, at 60 °C and 4 pH by using 25% ammonia solution in order to form a precipitate. Afterwards, the precipitate was ripened for 24 h, and eventually separated from mother liquor through vacuum filtration to obtain copper nitrate impregnated-NHAp (CNIN) mass. Consequently, the CNIN was freeze-dried using tungsten– halogen lamp (90 W) irradiation for 3 h at various temperatures of 70, 80 and 90 °C. The freeze-dried solid was finally subjected to calcination in air at pre-set 500 °C for 2 h to develop the Cu–NHAp catalyst. Identical procedure was applied for making catalysts with different copper nitrate to NHAp weight ratio in order to analyze the effects of copper nitrate loading on the properties of catalyst. Thus, according to the three weight ratio i.e. 0.5:1,
493
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
0.75:1 and 1:1 of Cu(NO3)25H2O to NHAp, the catalyst were designated as C0.5, C0.75, and C1.0 respectively. 2.3. Catalyst characterization Copper nitrate impregnated NHAp [CNIN] corresponding to C1.0 and unimpregnated-NHAp support were subjected to thermogravimetric analysis (TGA) in order to assess the effect of calcination temperature on thermal stability of the prepared catalyst. TGA was conducted using Perkin–Elmer TGA analyzer (Pyris Diamond TG/DTA) in a platinum crucible, under nitrogen atmosphere (150 mL/min) from 30 °C to 500 °C with an increasing temperature rate of 12 °C/min. The mass of the sample was 4 mg while alpha alumina was taken as reference powder. The specific surface area of the C1.0 catalyst that exhibited optimum performance was measured by BET method (Quantachrome make NOVA 4000e). X-ray diffraction (XRD) patterns of C0.5, C0.75 and C1.0 catalysts obtained at 90 °C freeze-drying temperature and at fixed 500 °C calcination temperature were analyzed (Rigaku Ultima III) using Cu Ka source outfitted with an Inel CPS 120 hemispherical detector. The analysis was performed at 2h Ranging from 20 to 80 °C at a scanning speed of 1 min1at 40 Kv and 30 mA. The infrared spectra of optimal catalyst were analyzed by FTIR-Shimadzu (alpha) from 400 to 4000 cm1. The morphology of the optimal catalyst was studied by a Scanning Electron Microscope (SEM) at 17 KV (JEOL Ltd., Japan, model JSM 6700F). The samples were placed on two sides adhesive tape attached to specimen stubs and were vacuum coated with a platinum layer with 80 s time of coating, pressure (5 N/m2), current (20 mA) and voltage (17 kV) respectively. In order to assess the effect of freeze-drying temperature on acidity of the prepared catalyst, all the CNIN samples developed by varying precursor loading were tested for acidity [20] after freeze-drying at 70, 80 and 90 °C (i.e. before calcination). The freeze-dried CNIN samples were subsequently calcined and the corresponding acidity of the developed catalysts were determined (Table 1). 2.4. Experimental design (Taguchi method) Table 2 displays the independent process factors and their levels examined in a L9 orthogonal array using Taguchi design. A standard orthogonal array L9 [18] was used and all experiments were conducted in triplicate using three levels of the four process factors (Table 2) viz. freeze drying temperature (XFDT), weight ratio of copper nitrate to NHAp (X C u L ), ethanol flow rate (XFF), and stirrer speed (XSS) in esterification of oleic acid with ethanol. The Taguchi method was employed to assess parametric interactions and to deter-
mine a set of optimal process factors corresponding to maximum oleic acid conversion through the S/N ratios and analysis of variance (ANOVA) using the software MINITAB-16 (Minitab Inc. USA for Windows7). Instead of conducting 27 (i.e. 34) experiments involving factorial experiment design, only nine experimental runs were required in the present study to optimize the parameter settings for maximum oleic acid conversion (cOA) to ethyl oleate. The experimental design matrix and mean of three experimental values for the cOA along with the calculated S/N ratio (Eq. (1)) are presented in (Table 3). The S/N ratio values corresponding to the (cOA) are calculated, using the ‘higher-the-better’ characteristics. The S/N ratio for any run was calculated according to the Eq. (1) [18] using the oleic acid conversion i.e. yi of the corresponding run. n 1X 1 S=N ¼ 10 log n i¼1 y2i
! ð1Þ
where i is the number of replicate, n is the number of trial experiments performed in any particular parametric combinations as per Table 3. The estimated S/N ratio can be expressed using following equation:
S=Nest ¼ S=N þ
k X ðS=Nj S=NÞ
ð2Þ
j¼1
where S=N is the mean of the ratio, S/N and S/Nj is the S/N ratio corresponding to the optimal process parameters, j and k is the number of the process parameters that significantly affected the optimal conditions of cOA. 2.5. Oleic acid esterification using Cu–NHAp catalyst The performance of the developed Cu–NHAp catalyst was assessed through oleic acid esterification with ethanol over a fixed reaction time of 1 h at 70 °C temperature while the other process factors were used as per Table 3 in all nine runs. The laboratory semibatch reactor consisted of a three-necked glass flask containing measured quantity of oleic acid; one neck of the flask was fitted with a reflux condenser and the second one was connected to the delivery pipe of a peristaltic pump for dosing ethanol at a regulated flow rate (varied from 0.40 mL/min to 0.80 mL/min). The central neck was equipped with a mechanical stirrer (REMI, RQ-121/D, AXIAL TURBINE, equipped with step-less electronic speed regulator and digital speed indicator in the range 200–2000 rpm). The reactor flask was placed on a heating mantle maintained at the required temperature using a PID temperature controller. After 1 h of operation, the reaction mixture was filtered by vacuum
Table 1 Acidity of the developed Cu–NHAp catalysts corresponding to different loading ratios of Cu(NO3)2 to NHAp and different freeze-drying temperatures. Acidity of C0.5 (mmol KOH/g)
Acidity of C0.75 (mmol KOH/g)
Acidity of C1.0 (mmol KOH/g)
After freeze-drying
After calcination
After freeze-drying
After calcination
After freeze-drying
After calcination
2.4 1.46 1.04
5.66 3.67 3.09
3.6 2.72 1.26
7.83 7.40 3.44
9.6 6.14 3.52
11.22 8.09 5.12
Freeze-drying temperature (90 °C) Freeze-drying temperature (80 °C) Freeze-drying temperature (70 °C)
Table 2 Experimental factors and levels for oleic acid esterification with ethanol in semibatch reactor employing the developed Cu–NHAp catalyst. Process factors
XFDT Freeze drying temperature (°C)
XCu L Ratio of copper nitrate to HAp
XFF Ethanol flow rate (mL/min)
XSS Stirrer speed (rpm)
Level 1 Level 2 Level 3
70 80 90
0.50 0.75 1.00
0.4 0.6 0.8
800 1000 1300
494
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
Table 3 L9 orthogonal array experimental matrix at different operating conditions and corresponding mean oleic acid conversion (QOA) and signal-to-noise (S/N) ratio. Trial no.
XFDT (°C)
X C u L (wt.%)
XFF (mL/min)
XSS (rpm)
QOA (%)
S/N ratio (db)
1 2 3 4 5 6 7 8 9
L1 L1 L1 L2 L2 L2 L3 L3 L3
L1 L2 L3 L1 L2 L3 L1 L2 L3
L1 L2 L3 L2 L3 L1 L3 L1 L2
L1 L2 L3 L3 L1 L2 L2 L3 L1
83.57 85.05 87.34 84.10 86.22 88.26 90.20 90.32 91.95
38.44 38.59 38.82 38.49 38.71 38.91 39.10 39.11 39.27
L1 (low level value), L2 (middle level value), L3 (upper level value).
filtration to separate the solid catalyst. Later, the filtrate was thoroughly washed with hot water and finally centrifuged at about 5000 rpm for 10 min. The operation separated the reaction mixture into an upper layer (ester phase) consisting of a mixture of unreacted oleic acid and product ethyl oelate, and a lower aqueous layer containing ethanol. 2.6. Conversion analysis The upper ester phase obtained upon centrifugation, was analyzed through titration [21,22] for determination of acid number (Eq. (3a)) in order to evaluate the oleic acid conversion (Eq. (3b)) as detailed below:
A¼
cOA
56:11 M V m AOLEICACID A ¼ 100 AOLEICACID
ð3aÞ
and a prime 27% weight losses occurred over 30–100 °C ascribing to evaporation of adsorbed water. Further weight losses of 8.6% and 3% took place for the samples between 100 °C and 300 °C because of loss of lattice water. On further heating over a temperature range between 300 °C and 500 °C, it was found that 31% and 18% weight losses occurred for NHAp and CNIN samples respectively owing to disintegration of macromolecules and other organic substances in addition to solid state reaction leading to formation of complex compound such as libethenite Cu2(OH)(PO4) as shown in XRD plot (Fig. 2). Relatively insignificant weight losses of 4% occurred for CNIN compared to 6% NHAp respectively between 500 °C and 700 °C. On further investigation, it was found that at temperatures above 700 °C very insignificant weight losses occurred for both NHAp (4%) and CNIN (1%). Thus, in order to minimize energy consumption in calcination, 500 °C was selected as calcination temperature since, all the active crystalline phase were found to be prominent at 500 °C temperature [23].
ð3bÞ
where A is the Concentration of oleic acid; M the Normality of potassium hydroxide solution; V the Volume of potassium hydroxide consumed from burette; AOLEICACID the Acid value of oleic acid = 200; m is the Weight of ester layer. 3. Result and discussion
3.1.2. Acidity of the developed catalyst The freeze-dried (at three freeze drying temperatures, i.e. 70, 80 and 90 °C) CNIN samples prepared at three different ratios of copper nitrate to NHAp viz. 0.5:1, 0.75:1 and 1:1 were tested for acidity. All the freeze-dried CNIN samples were subsequently calcined at 500 °C to prepare Cu–NHAp catalysts and tested for acidity (Table 1). It is evident from Table 1, that the acidity of the catalyst
3.1. Catalyst characterization 3.1.1. TGA Analyses TGA patterns (Fig. 1) of representative samples of NHAp and CNIN corresponding to C1.0 show the weight losses of samples on heating over a temperature range from 30 °C to 860 °C. Thermo gravimetric analyses of NHAp and CNIN manifest respective 6.4%
Fig. 1. TGA analyses of CNIN(C1.0) and NHAp samples from 30 to 860 °C.
Fig. 2. Powder XRD patterns of Cu–NHAp catalyst obtained by calcination at 500 °C at different precursor loadings: (a) C0.5, (b) C0.75 and (c) C1.0. [Characteristic peaks due to Hydroxyapatite ( ), b-Calcium phosphate ( ), copper oxide ( ), libethenite ( )].
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
495
Fig. 3b. Cumulative pore volume vs. pore diameter plot for Cu–NHAp catalyst (C1.0).
Fig. 3a. Pore volume vs. relative pressure (P/P0) of Cu–NHAp catalyst (C1.0) at optimal i.e. 90 °C freeze-drying temperature [inset: BJH differential pore size distribution].
samples increased with increase in copper nitrate loading as well as freeze-drying temperature and accordingly, highest acidity (11.22 mmol KOH/g catalyst) was detected for C1.0 catalyst developed at 90 °C freeze-drying temperature. 3.1.3. XRD Analysis XRD patterns for the (Cu–NHAp) catalyst obtained (at fixed freeze-drying temperature of 90 °C and 500 °C calcination temperature) at three different copper nitrate loadings C0.5, C0.75, and C1.0 respectively are exhibited in Fig. 2a–c respectively. The angles corresponding to the peaks representing crystalline phases of HAp (25.691°, 32.102°, 53.418°, 66.281°), calcium phosphate (20.14°, 21.26°, 72.315°), copper oxide (35.462°, 38.642°, 66.299°, 51.239°) and libethenite [Cu2(OH)(PO4)] (29.302°, 53.418°, 61.238°, 68.062°) could be detected at all three precursor loadings. Here, it should be noted that, copper oxide crystalline phases represent isolated CuO crystal lattice. Libethenite [Cu2(OH)(PO4)] was formed as the surface Ca2+ ions of the NHAp were exchanged with Cu2+ ions of the precursor, which were chemisorbed onto the surface of NHAp support. Diffraction patterns pertaining to libethenite crystalline phase corroborate well with Tounsi and co-workers [23]. It is worthy to note that, with gradual increase in copper nitrate dosage the peak intensity of copper oxide was found to amplify to a greater extent compared to other crystalline phases. The XRD patterns obtained in the present study matched closely with chemically synthesized HAp supported copper oxide catalyst [24–26]. 3.1.4. BET analysis The BET (Brunauer–Emmett–Teller) specific surface area of 16.78 m2/g was determined for the developed Cu–NHAp(C1.0) catalyst prepared at 90 °C freeze-drying temperature and 500 °C calcination temperature. The ‘convex to the (P/P0) axis’ shape of the isotherm (Fig. 3a) of the developed catalyst corroborates type III of the IUPAC classification [27]. An isotherm of this type signifies adsorbate–adsorbate interactions. In the present case, a thin layer of hysteresis occurred in the P/P0 range of 0.5–0.65 implying capillary condensation in meso-porous structures. The catalyst pore volume of 0.0313 cc/g and 31.34 nm modal pore size were estimated using BJH method (inset of Fig. 3a). Fig. 3b depicts the cumulative pore volume vs. pore diameter plot for C1.0 representative catalyst sample. It can be observed that, 65.25% of the total catalyst pore volume relates to meso-porous size (8.2–50 nm), while, the remaining 34.75% pore volume corresponds to macro
Fig. 4. FTIR analyses of the Cu–NHAp(C1.0) optimal catalyst.
porous size (51–100 nm). Thus, the catalyst morphology could assist in achieving high oleic acid conversion to ethyl oleate. 3.1.5. FTIR Analysis FTIR spectra transmittance vs. wave number bands of Cu–NHAp(C1.0) catalyst (developed at 90 °C freeze-drying temperature and 500 °C calcination temperature that possessed highest acidity) are shown in Fig. 4. The bands 755, 1050, 1395 and 1548 cm1 characterize the bending vibration of phosphate
Fig. 5. (a) SEM image of freeze-dried (90 °C) Cu-NHAp(C1.0) optimal sample before calcination. (b) SEM image of freeze-dried (90 °C) Cu–NHAp(C1.0) optimal catalyst after calcination.
496
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
spectra at 400, 475, 520, 600 and 750 cm1 denote the band of copper oxide. All these bands corroborate well with literature reported data [23–26]. 3.1.6. Microscopic analyses SEM images of representative C1.0 samples before calcination (after freeze-drying at 90 °C) and after calcination at 500 °C are depicted in Fig. 5a and b respectively. Fig. 5a displays irregular shaped particles with uniform void spaces in the freeze-dried CNIN(C1.0) sample. Fig. 5b shows well agglomerated, irregular structure with non uniform pore of the calcined catalyst. 3.2. Mechanism of ethyl oleate formation using Cu–NHAp catalyst
Fig. 5. (continued)
(PO3 4 , O–P). Existence of carbonate ion could be confirmed through spectra at 1605, 1715, 2016 and 2156 cm1. The peaks corresponding to absorbed hydrate at 2300, 2712, 2740, 3435 and 3600 cm1 correspond to the stretching vibration of lattice OH ions. FTIR
The existence of CuO crystal lattice on the surface of the Cu–NHAp framework indicates that Cu2+ species are present on catalyst surface which can act as Lewis acid in catalyzing the ethyl oleate formation. Besides, the presence of Cu2+ cations in the new solid phase-libethenite Cu2(OH)(PO4) [28] also reinforces catalytic activity. It is known that both Cu2+ and Cu1+ can act as Lewis acid. In the present case, highly dispersed Cu2+ species, are first reduced to Cu1+ and then to Cu0 (i.e. Cu2+ ? Cu1+ ? Cu0 in concurrence with Liu et al. [29]). Since, Cu1+ species can also act as Lewis acid, the proposed reaction mechanism (Fig. 6) has been presented in terms of Cu1+ species. Nevertheless, similar mechanism could have also been presented in terms of Cu2+ species. Oleic acid was adsorbed on catalyst surface that led to generation of carbocation owing to the interaction between carbonyl oxygen of oleic acid and Lewis acidic site (copper cation) of the catalyst. Consequently, ethanol
Fig. 6. Mechanism of oleic acid esterification with ethanol in presence of Cu–NHAp catalyst.
497
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499 Table 4 Analysis of variance (ANOVA) of factors affecting ethyl oleate synthesis process. Factor
DF
Sum of squares
Mean square
F value
P-value
XFDT X Cu L XFF XSS Error
1 1 1 1 4
45.375 15.596 0.437 0.008 8.786
45.375 15.596 0.437 0.008 2.196
20.66 7.10 0.20 0.03
0.010 0.056 0.679 0.955
Total
8
70.202
present in liquid phase formed a tetrahedral intermediate through a nucleophilic attack on the carbocation. Subsequently, elimination of one water molecule from the tetrahedral intermediate generated a molecule of ethyl oleate. Thus, the catalyst became reusable for the subsequent esterification. 3.3. Analysis of oleic acid conversion and prediction of optimal conditions Summary results of the ANOVA are shown in Table 4, which indicated that all of the selected factors were significant parameters at the 98.02% confidence level. The analysis of variance (ANOVA) based on the observed data states that freeze-drying temperature contributes 45.375, sum of square as well as mean square followed by weight ratio of copper nitrate to NHAp (15.596), ethanol flow rate (0.437) and stirrer speed (0.008) in the oleic acid esterification process. According to the ‘‘F’’ and ‘p’ criteria, (Table 4), stirrer speed (XSS) had least effect on ethyl oleate formation process among all process factors; whereas, the freezedrying temperature was observed to be the most significant process factor. As the target of the process was to maximize oleic acid conversion (cOA), the ‘‘larger is better’’ criterion was chosen during the calculation of S/N ratio. The highest S/N ratio in the each factor was desirable to maximize cOA using the developed catalyst in semibatch reactor. The higher the difference between the minimum and the maximum S/N ratios in each factor, the higher is its effect on oleic acid conversion (Table 5). Considering the four factors [i.e. freeze-drying (FD) temperature, ratio of Copper nitrate to NHAp, ethanol flow rate and stirrer speed], it can be concluded that 90 °C FD temperature, 1.0 weight ratio of Cu(NO3)25H2O to NHAp loading, 0.8 mL/min ethanol flow rate and 1000 rpm stirrer speed resulted highest S/N ratio corresponding to highest oleic acid conversion (Fig. 7). The optimal levels of the process factors were identified and the optimum value of the S/N ratio could be computed based on the selected levels of the significant factors [17]. The values of the optimal S/N ratio and oleic acid conversion were 39.2710 db and 91.95%, respectively. A confirmation run was conducted based on these optimal conditions and the response was 39.1245 db, and the corresponding conversion of oelic acid was 91.86%.
Fig. 7. S/N ratio plot for oleic acid esterification with ethanol employing Cu–NHAp catalyst.
between any two process factors are assessed at the optimal values of the remaining two factors. The interaction between A (freezedrying temperature) and B (copper nitrate to NHAp weight ratio) revealed that at all three levels of A, an increase in B could monotonically enhance cOA over the range of the parametric value selected in the present study. At lowest level of A, there was a monotonic increase in cOA with gradual increase in C (ethanol flow rate); and the maximum cOA was observed at highest levels of A and C, however, the minimum cOA was found at the intermediate A level. A more or less similar interaction was observed between A and D (stirrer speed) in affecting cOA. Thus, maximum cOA could be achieved only at highest A level for all different factorial values. These observations strongly advocate the pronounced effect of Tungsten halogen irradiation-assisted freeze-drying temperature. Whereas, when A and D were set at optimal levels, maximum cOA could be achieved at highest B level and central C level. At lowest B level, an increase in C level had a synergistic effect on conversion, in other words, at minimal copper nitrate loading, an increase in ethanol flow rate would enhance the cOA indicating ethanol acting as a limiting reactant and reaction is possibly surface reaction controlled. Interaction plot of B and D exhibited that, at highest B level, an increase in D would not enhance oleic acid conversion, suggesting that the reaction was not mass transfer limited; whereas, at intermediate B level (C0.75), D could positively influence the conversion, indicating a mass transfer controlled regime. Under optimal levels of A and B, at lowest ethanol flow rate (C), enhancement in stirring speed (D) could increase oleic acid conversion, signifying a diffusion controlled regime. At highest C (0.8 mL/ min), the conversion increased up to 1000 rpm, however, further increase in D resulted in reduced oleic acid conversion owing to decrease in global reaction rate in the multi-step heterogeneous catalytic reaction.
3.4. Interactions among process factors
3.5. Effect of use of solid-desiccator on oleic acid conversion
The interactions among the process factors governing the oleic acid conversion have been demonstrated in Fig. 8. The interactions
The effect of incorporation of adsorbent (silica gel) within the reactor on oleic acid conversion exposed interesting observations.
Table 5 S/N ratios at different levels of the process factors and delta values (difference of the S/N values between highest and lowest levels of process factors). Level
Freeze-drying temperature (°C)
Ratio of copper nitrate to NHAp (wt.%)
Ethanol flow rate (mL/min)
Stirrer speed (rpm)
1 2 3 Delta Rank
38.62 38.71 39.16 0.54 1
38.68 38.81 39.00 0.32 2
38.82 38.79 38.88 0.09 3
38.81 38.87 38.81 0.06 4
498
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499
Fig. 8. Interaction plot for oleic acid esterification with ethanol employing Cu–NHAp catalyst [A: freeze-drying temperature; B: copper nitrate to NHAp weight ratio; C: ethanol flow rate; D: stirrer speed].
Final oleic acid conversion after 1 h semibatch reactor operation at the determined optimal conditions could be enhanced to 92.4%, 94.3%, 95.8% and 96.5% corresponding to use of 22.34, 44.68, 67.02 and 89.36 wt.% of adsorbent as compared to 91.86% conversion without adsorbent. This can be ascribed to the significant decrease in extent of backward reaction in the reversible esterification of oleic acid owing to the removal of water by the solid desiccant; thus, rendering appreciably higher conversion of oleic acid. After each reaction cycle the silica gel could be easily recovered and regenerated through air drying in hot air oven at 105 °C for subsequent reuse.
adsorbent (silica gel) within the reactor for water removal could significantly enhance the oleic acid conversion. The developed catalyst demonstrated appreciable reusability and easy regeneration characteristics. A novel and eco-friendly waste utilization path could, thus, be explored for preparation of cost-effective solid catalyst to synthesize biodiesel blending stock leading to a tidier and healthier globe. Acknowledgment Authors are grateful to University Grants Commission, New Delhi, India for financial support [(F. No. 36-99/2008 (SR)].
3.6. Reusability References Reusability of the Cu–NHAp catalyst was tested by carrying out 1 h semibatch esterification run for three consecutive reaction cycles at the optimal process conditions. Before each reuse, the recovered catalyst was washed with ethanol to remove the adsorbed stains and then air dried at 105 °C for 1 h. The oleic acid conversion remained unchanged after two consecutive runs, however, it decreased to 90.67% from 91.86% on third reaction recycle and on further recycle conversion remained constant for three more subsequent runs. The small decrease in catalytic activity could be ascribed to very minute extent of leaching of the active catalyst species into the reaction medium (copper and calcium ion content of the final ethyl oleate product obtained after six consecutive catalyst usage was found to be 0.02 and 0.03 ppm). On the other hand, in addition to ethanol washing and air drying, if the catalyst could be regenerated through air calcination at 500 °C for 2 h, the oleic acid conversion was observed to remain unaltered for eight successive runs. This indicates excellent stability and reusability characteristics of the developed catalyst upon simple air-calcination. 4. Conclusion Natural hydroxyapatite (derived from waste bekti fish bone) supported, cost-effective, efficient copper solid acid catalyst was prepared by wet-impregnation method involving tungsten–halogen-irradiation assisted freeze-drying protocol. The catalyst showed high catalytic efficacy in esterification of oleic acid with ethanol in a semibatch reactor. Incorporation of recyclable
[1] Ulmann’s Encyclopaedia of Industrial Chemistry, vol. 12, VCH, Weinheim, 2001. [2] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresour. Technol. 70 (1999) 1–15. [3] K.J. Hun, G.Y. Peng, C. Yue, W.Z. Chen, Preparation and UV-light absorption property of oleic acid surface modified ZnO nanoparticles, Chem. Res. Chin. Univ. 27 (2011) 500–502. [4] C.F. Oliveira, L.M. Dezaneti, F.A.C. Garcia, J.L.D. Macedo, J.A. Dias, S.C.L. Dias, K.S.P. Alvim, Esterification of oleic acid with ethanol by 12-tungstophosphoric acid supported on zirconia, Appl. Catal. A 372 (2010) 153–161. [5] K.C.S. Figueiredo, V.M.M. Salim, C.P. Borges, Ethyl oleate production by means of pervaporation-assisted esterification using heterogeneous catalysis, Braz. J. Chem. Eng. 27 (2010) 609–617. [6] P. Yin, L. Chen, Z. Wang, R. Qu, X. Liu, S. Ren, Production of biodiesel by esterification of oleic acid with ethanol over organophosphonic acidfunctionalized silica, Bioresour. Technol. 110 (2012) 258–263. [7] K. Mori, K. Yamaguchi, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, Controlled synthesis of hydroxyapatite-supported palladium complexes as highly efficient heterogeneous catalysts, J. Am. Chem. Soc. 124 (2002) 11572–11573. [8] S. Sebti, R. Tahir, R. Nazih, A. Saber, S. Boulaajaj, Hydroxyapatite as a new solid support for the Knoevenagel reaction in heterogeneous media without solvent, Appl. Catal. A 228 (2002) 155–159. [9] S. Sebti, R. Tahir, R. Nazih, S. Boulaajaj, Comparison of different Lewis acid supported on hydroxyapatite as new catalysts of Friedel–Crafts alkylation, Appl. Catal., A 218 (2001) 25–30. [10] K. Prabakaran, S. Rajeswari, Development of hydroxyapatite from natural fish bone through heat treatment, Trends Biomater. Artif. Organs 20 (2006) 20–23. [11] W. Admassu, T. Breese, Feasibility of using natural fishbone apatite as a substitute for hydroxyapatite in remediating aqueous heavy metals, J. Hazard. Mater. 69 (1999) 187–196. [12] M. Ozawa, K. Satake, R. Suzuki, Removal of aqueous manganese using fish bone hydroxyapatite, J. Mater. Sci. Lett. 22 (2003) 513–514. [13] K. Traina, M.C. Steil, J.P. Pirard, C. Henrist, A. Rulmont, R. Cloots, B. Vertruyen, Synthesis of La0.9Sr0.1Ga0.8Mg0.2O2.85 by successive freeze-drying and selfignition of a hydroxypropylmethyl cellulose solution, J. Eur. Ceram. Soc. 27 (2007) 3469–3474.
R. Chakraborty, D. RoyChowdhury / Chemical Engineering Journal 215–216 (2013) 491–499 [14] O. Dos Santos, M.L. Weiller, D.Q. Junior, A.N. Medina, CO gas-sensing characteristics of SnO2 ceramics obtained by chemical precipitation and freeze-drying, Sens. Actuators, B 75 (2001) 83–87. [15] S. Ivanova, E. Zhecheva, R. Stoyanova, Microstructure of LaCoO3 prepared by freeze-drying of metal–citrate precursors revealed by EPR, J. Phys. Chem. Solids. 68 (2007) 168–174. [16] D. Klvana, J. Delval, J. Kirchnerova, J. Chaouki, Deactivation of fiber-supported La0.65Sr0.35Ni0.29Co0.69Fe0.02O3 catalyst by mercaptan during combustion of methane or natural gas, Appl. Catal. A 165 (1997) 171–182. [17] G. Taguchi, Introduction to quality engineering, Asian Productivity Organization, Tokyo. [18] R.K. Roy, Design of Experiments Using the Taguchi Approach: 16 Steps to Product and Process Improvement, John Wiley & Sons, Inc., US. [19] R. Chakraborty, S. Bepari, A. Banerjee, Transesterification of soybean oil catalyzed by fly ash and egg shell derived solid catalysts, Chem. Eng. J. 165 (2010) 798–805. [20] A.K. Singh, S.D. Fernando, Transesterification of soybean oil using heterogeneous catalysts, Energy Fuels 22 (2008) 2067–2069. [21] Y. Wang, S. Ou, P. Liu, F. Xue, S. Tang, Comparison of two different processes to synthesize biodiesel by waste cooking oil, J. Mol. Catal. A: Chem. 252 (2006) 107–112.
499
[22] R. Tesser, L. Casale, D. Verde, M. Di Serio, E. Santacesaria, Kinetics of free fatty acids esterification: batch and loop reactor modelling, Chem. Eng. J. 154 (2009) 25–33. [23] H. Tounsi, S. Djemal, C. Petitto, Copper loaded HAp catalyst for selective catalytic reduction of nitric oxide with ammonia, Appl. Catal. B107 (2011) 158–163. [24] M.R. Johan, M. Shahadan, M. Suan, N.L. Hawari, H.A. Ching, Annealing Effects on the properties of copper oxide thin films prepared by chemical deposition, Int. J. Electrochem. Sci. 6 (2011) 6094–6104. [25] E.N. Muhamad, R. Irmawati, A.H. Abdullah, Y.H. Taufiq-Yap, S.B. Abdul Hamid, Effect of number of washing on the characteristics of copper oxide nanopowders, Malaysian J Anal. Sci. 11 (2007) 294–301. [26] M. Kooti, L. Matouri, Fabrication of nanosized cuprous oxide using fehling’s solution, Trans. F: Nanotechnol. 17 (2010) 73–78. [27] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 57 (1985) 603–619. [28] J. Jemal, H. Tounsi, K. Chaari, C. Petitto, G. Delahayc, S. Djemel, A. Ghorbel, NO reduction with NH3 under oxidizing atmosphere on copper loaded hydroxyapatite, Appl. Catal. B: Environ. 113–114 (2011) 255–260. [29] Z. Liu, M. Amiridis, Y. Chen, Characterization of CuO supported on tetragonalZrO2, J. Phys. Chem. B 109 (2005) 1251–1255.