Optimization of continuous esterification of oleic acid with ethanol over niobic acid

Optimization of continuous esterification of oleic acid with ethanol over niobic acid

Accepted Manuscript Optimization of continuous esterification of oleic acid with ethanol over niobic acid Letícia L. Rade, Caroline O.T. Lemos, Marcos...

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Accepted Manuscript Optimization of continuous esterification of oleic acid with ethanol over niobic acid Letícia L. Rade, Caroline O.T. Lemos, Marcos Antônio S. Barrozo, Rogério M. Ribas, Robson S. Monteiro, Carla E. Hori PII:

S0960-1481(17)30762-0

DOI:

10.1016/j.renene.2017.08.035

Reference:

RENE 9133

To appear in:

Renewable Energy

Received Date: 6 December 2016 Revised Date:

28 April 2017

Accepted Date: 4 August 2017

Please cite this article as: Rade LetíL, Lemos COT, Barrozo MarcosAntôS, Ribas RogéM, Monteiro RS, Hori CE, Optimization of continuous esterification of oleic acid with ethanol over niobic acid, Renewable Energy (2017), doi: 10.1016/j.renene.2017.08.035. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Optimization of continuous esterification of oleic acid with ethanol over niobic acid

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Letícia L. Radea, Caroline O. T. Lemosa, Marcos Antônio S. Barrozoa, Rogério M. Ribasb, Robson S. Monteirob,c, Carla E. Horia,* a

Universidade Federal de Uberlândia, Faculdade de Engenharia Química, 38408-144, Uberlândia, Brazil b Companhia Brasileira de Metalurgia e Mineração - CBMM, 38183-903, Araxá, Brazil c Catalysis Consultoria Ltda, 22793-081, Rio de Janeiro, Brazil *[email protected]

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ABSTRACT: The aim of this work was to evaluate the continuous production of

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biodiesel through the esterification reaction between oleic acid and ethanol using niobic

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acid as a solid acid catalyst. In this study, different calcination temperatures of niobic

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acid were tested. W/F tests were carried out, to avoid mass transfer problems and, then,

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the catalytic activities of all the samples were evaluated. Results showed that niobic acid

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calcined at 350 oC presented the highest catalytic activity. The experimental conditions

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of temperature, amount of catalyst and ethanol:oleic acid molar ratio were optimized by

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using design of experiments (DOE) and canonical analysis. All three single parameters

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were significant on the yield of esters. The esterification reaction of oleic acid led to

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yields of esters up to 70% and conversion up to 90%, at 249 oC, ethanol:oleic acid

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molar ratio of 10.83:1 and 0.7 g of niobic acid.

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Keywords: continuous esterification, oleic acid, niobium, niobic acid, design of

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experiments, canonical analysis.

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1. INTRODUCTION

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Most of the energy currently consumed in the world in the transportation sector

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comes from fossil fuel sources. However, due to environmental concerns, restrictions on

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the fuel combustion emissions are imposed through legislation in most developed

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countries [1]. Furthermore, the demand for energy is increasing continuously. In this

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context, the development of alternative energy sources has been stimulated [1].

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Biodiesel is a promising alternative to petroleum-based fuels because it is produced

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from renewable sources [2, 3], is biodegradable and nontoxic and emits less polluting

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gases during combustion [1]. Biodiesel can be produced by transesterification of

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triglycerides or esterification of free fatty acids (FFA) using short chain alcohols.

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Conventionally, it is synthesized by transesterification. However, the importance of

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esterification reaction studies is growing, once this reaction uses feedstocks that present

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high percentage of FFA, which could make the conventional transesterification

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unfeasible [4]. According to Nandiwale and Bokade [5], the esterification of long chain

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carboxylic acids such as oleic acid can represent well the biodiesel production process,

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because this compound is present in most of oil crops (jatropha curcas, soybean,

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sunflower, rapeseed, palm, etc).

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Esterification is the process of forming esters from carboxylic acids and alcohol.

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In the esterification reaction, in the presence of an acid catalyst and heat, the hydroxyl (-

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OH) from the carboxylic acid is removed and, after that, the hydrogen from the alcohol

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is removed. The hydroxyl and the hydrogen combine to form water as byproduct of the

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reaction and ester as a main product (biodiesel). The esterification reaction between free

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fatty acid and alcohol is shown in Equation 1:

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Among the esterifying agent options, methanol is the most used because it

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presents lower cost and is widely available. However, this alcohol is toxic and it is

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obtained from petroleum, which makes the final biodiesel product not a truly renewable

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energy source. On the other hand, ethanol is less toxic for humans, renewable, derived

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from agricultural products and produced in large scale in Brazil.

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Conventionally, mineral liquid acids (sulfuric acid, hydrochloric acid) and

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organic acids like p-toluenesulfonic are used to catalyze esterification reactions [1, 6].

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These homogeneous catalysts are very effective. However, they can lead to serious

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contamination problems, since they are toxic, corrosive and form by-products, which

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are difficult to separate from the reaction medium. Besides that, homogeneous catalysts

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cannot be reused and generate high amounts of waste and effluents [3]. All these

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problems can result in higher production costs [1]. The use of heterogeneous catalysts

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shows easier catalyst separation, product purification, high activity and stability [5]. In

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addition, these catalysts can be reused, decreasing production costs.

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Amberlyst 15 [7], zeolites [8] and ZrO2 [9] are the most used solid acid catalysts

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for esterification of oleic acid with some promising results. However, niobia based

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materials, such as niobic acid and niobium phosphate, may present great potential as

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catalysts in the production of biodiesel [6], for many reasons: the acidity is maintained

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in aqueous reactions, they present large surface area and regular porosity. In some cases,

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these catalysts showed better catalytic performances than some zeolites [10]. However,

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the reports on the use of niobium compounds for esterification reaction are still scant.

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ACCEPTED MANUSCRIPT The vast majority of the esterification studies uses batch type reactors. One

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problem of using batch reactors is that the water is not removed from the reaction

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medium. The presence of water promotes a loss of active acid sites of the catalysts and

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shifts the reaction equilibrium to the reagents side. Moreover, in batch reactors, the

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energy costs are high and the catalysts must be separated from the product. On the other

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hand, continuous reactors present many advantages such as smaller reactors, better

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productivity, no separation and recycle of catalysts, among others.

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Response surface methodology (RSM) is an efficient statistical technique used

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for modeling experimental data and evaluation of the influence of parameters on the

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response process [5]. This methodology allows obtaining much information by

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performing a small number of experiments and permits the evaluation not just of the

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single experimental parameters but also of their interactions [5]. Design of experiments

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coupled with response surface method gives more information than unplanned

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approaches, reduces waste of materials and time and leads to better knowledge of the

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process studied [2].

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Accounting for the fact that there are not many studies in the literature devoted

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to esterification reaction using continuous reactor and niobium catalysts, the objective

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of this work is to investigate the continuous production of biodiesel from oleic acid and

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ethanol and niobic acid as a heterogeneous catalyst. The catalytic activity of a series of

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samples was evaluated and after selecting the best catalyst, the experimental conditions

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were optimized using design of experiments (DOE). The parameters investigated were:

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temperature (from 220-290 oC), mass of catalyst (from 0 to 0.8 g) and ethanol:oleic acid

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molar ratio (from 2:1 to 14:1). The effect of each parameter and their interactions on the

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yield of esters were studied using a central composite design (CCD) coupled with

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response surface method (RSM), developed by Statistica software and optimized by

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canonical analysis technique.

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2. MATERIAL AND METHODS

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2.1.

Materials and reagents

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The substrates used in the esterification reactions were oleic acid (Synth) and

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ethanol (Synth 99.5%). N-heptane (Química Moderna, 99.9%) was used as a solvent

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for the gas chromatography analysis. The catalysts were prepared by heating

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Nb2O5.nH2O (HY-340, supplied by Companhia Brasileira de Metalurgia e Mineração -

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CBMM) at different calcination temperatures, in air (20 mL/min) for 2 hours. The

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calcination temperatures varied from 300 to 600 °C.

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2.2.

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Catalysts characterization

BET surface areas were determined from nitrogen adsorption measurements at

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77 K on a Quantachrome Nova-1200 apparatus. Adsorption isotherms were used to

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determine the specific area of the catalysts. The specific pore volume and pore

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dimension were obtained by the desorption isotherm. In all samples, the validity of the

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BET method was based on the value of the constant C associated to the

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adsorbate/adsorbent interaction in which C> 0 was the adjustment region of the

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isotherms. Pore sizes were calculated from BJH distribution method.

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The combined thermogravimetric (TGA) and differential scanning calorimetric

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(DSC) analyses were performed using Q600 SDT differential analyzer (TA instruments)

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coupled to a thermo-balance. For the analyses, the niobic acid sample (12 mg) was

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heated from 25 °C to 800 °C at a rate of 20 °C/min and air flow rate of 100 mL/min. The ex-situ X-ray powder diffraction (XRD) patterns were obtained on a Rigaku

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diffractometer, using Cu Kα radiation (λ = 1.540 Å) over a 2θ range of 20° to 70°, at a

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scan rate of 0.05°/step and scan time of 2 s/step.

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X-ray diffraction in-situ was carried out at the XPD-D10B beamline, at Brazilian

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Synchrotron Light Laboratory (LNLS). For the analysis, samples were sieved (20

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mesh), heated at a rate of 10 oC/min and, then, maintained at the desired temperature of

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calcination for 2 hours, with a synthetic air flow rate of 20 mL/min. Scans were

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obtained from 20° to 40°, with a step size of 0.003 and a counting time of 1 s, using a

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wavelength of 1.54056 Å and a resolution of 8 keV.

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Temperature-programmed desorption of ammonia (NH3-TPD) was performed

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using a ChemBET-3000 equipment. The samples (0.18 g) were treated under nitrogen

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flow rate of 20 mL/min at 300 oC for 1 hour, followed by adsorption of ammonia at 100

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o

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under nitrogen flow rate of 20 mL/min, at 100 oC for 2 hours. Finally, the NH3-TPD

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was performed between 100 and 700 oC with a heating rate of 10 oC /min under

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nitrogen flow.

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C for 30 minutes. After that, the removal of physically adsorbed ammonia was carried

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2.3.

Esterification reaction

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Reactions were carried out in a fixed bed stainless steel tubular reactor (o.d., 3/8

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in; i.d. 12.5 mm; length, 29.5 cm). The substrate (oleic acid and ethanol) was

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continuously fed into the system using a high-pressure liquid pump (LabAlliance, Series

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III), at a volumetric flow rate of 0.30 mL/min. After filling the system with substrate,

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ACCEPTED MANUSCRIPT the reactor was heated by two electric furnaces (heating power of 2000 W) connected to

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temperature controllers (Novus N1100), with a heating rate of 10 °C/min and precision

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of ± 2 °C. Samples were collected after waiting at least two residence times at the

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desired condition. A schematic diagram of the experimental apparatus used for the

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synthesis of esters is illustrated in Figure 1.

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Figure 1- Schematic diagram of the experimental apparatus. S- stirrer, M- substrate

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mixture, P- high-pressure liquid pump, F1- pre-heating zone furnace, F2- furnace of the

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fixed bed reactor, B- water bath, C- product collector.

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2.4.

Sample analysis

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The samples obtained were dried in incubator at 80oC to constant weight, to ensure the total evaporation of the ethanol. The oleic acid conversion was calculated as shown in Equation 2: 7

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(2)

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Where

and

are free fatty acid content at the initial substrate and at the

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samples collected, respectively (mg of FFA/100 mg of sample).

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The percentage of free fatty acid of the samples was determined according to

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method Ca 5a-40, as presented by Abdala et al. [4]. This method is based on acid-base

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titration using as titrant methanol solution of potassium hydroxide (KOH) previously

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standardized. The free fatty acid content was defined as Equation 3:

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(3)

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Where ‫ܯ‬C is the molar concentration of KOH, ܸ is the volume of KOH used in the

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titration and w is the sample weight.

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The esters content of the samples were analyzed by gas chromatography (GC). The analytical method was the same used by Rade et al. [11].

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2.5.

Design of experiments

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The design of experiments (DOE) was used to optimize the operational

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conditions for the yield of esters in the esterification reaction using niobic acid as a solid

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acid catalyst. The experiments were carried out according to a central composite design

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ACCEPTED MANUSCRIPT with four center points, resulting in 18 experiments, using alpha for orthogonality (α) of

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1.414. The process parameters were defined as reaction temperature (220-290 oC), mass

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of catalysts (0-0.8 g) and ethanol-to-oleic acid molar ratio (2:1–14:1). The selection of

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the levels was defined according to results obtained in preliminary tests. The effect of

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each parameter and their interactions on the yield of esters were studied using a central

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composite design (CCD) coupled with response surface method (RSM), developed by

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Statistica software. The actual and coded values of the variables are given in Table 1.

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Yields of esters obtained were fitted in a quadratic model using regression analysis and

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p-value less than 0.05 were considered statistically significant. The coded variables

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were defined as follow:

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205 206

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(5)

(6)

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(4)

Where XT, XMC, XMR are coded variables of temperature, mass of catalyst and ethanol-

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to-oleic acid molar ratio, respectively.

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Table 1- Coded and uncoded levels of variables used for central composite design Coded levels Parameters

Symbol -1

0

+1

+1.414

XT

220

230

255

280

290

Mass of catalyst (g)

XMC

0

0.1

0.4

0.7

0.8

Ethanol:oleic acid molar ratio

XMR

2:1

4:1

8:1

12:1

14:1

Optimization

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Temperature (oC)

220 221

-1.414

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Canonical analysis technique was employed to determine the optimal conditions

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of temperature (XT,), mass of catalyst (XMC) and ethanol-to-oleic acid molar ratio (XMR)

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for the continuous esterification reaction. According to Leite et al. [12], the response

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function can be expressed in terms of new variables w1, w2, … wK, whose axes

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correspond to the principal axes of the contour system. Thus, the canonical form

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consists in the function in terms of these new variables and is given according to

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Equation 7:

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(7)

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The nature of the fitted surface stationary point (x0) can be determined by the

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sign and magnitude of the characteristic roots (λi). If λi<0, a move in any direction from

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the stationary results in a decrease in , which means that the stationary point is a point

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of maximum. In contrast, if λi>0, the stationary point is a point of minimum. If λ’s

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differ in sign, the stationary point represents a saddle point [12]. In this work, the

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canonical analysis was implemented using the software Maple 7. 10

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3. RESULTS AND DISCUSSION

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3.1.

Characterization

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3.1.1. Nitrogen adsorption

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Table 2 shows the BET surface area, pore volume and the average pore size for

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niobic acid calcined from 300 °C to 600 °C. According to the results, it is possible to

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observe that increasing calcination temperature causes a decrease in the surface area,

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from 111 to 15 m2/g. The same effect was observed for pore volume of the samples

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(from 0.114 to 0.015 cm3/g). However, the average pore sizes did not varied

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significantly with the increase of calcination temperature. These results were consistent

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with data reported by Florentino et al. [13].

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Table 2- Textural properties of niobic acid calcined between 300 and 600 °C Surface area

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Calcination

Total pore volume

Pore size

(m2/g)

(cm3/g)

(Å)

300

111

0.114

20.5

350

95

0.097

20.3

400

86

0.083

19.3

450

66

0.060

18.0

500

34

0.034

20.2

600

15

0.015

20.3

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3.1.2. Thermogravimetric analysis

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The results of TGA and DSC analysis for fresh niobic acid are presented in

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Figure 2. According to the TGA, niobic acid presents a weight loss of 11% between 40

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and 350 °C. This weight loss is an endothermic process, which can be attributed to the

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elimination of adsorbed and structural water [14]. For temperatures higher than 350 °C,

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no significantly weight loss was observed. These results are in good agreement with the

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ones obtained by Iizuka et al. [15] and Lebarbier et al. [14].

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3.1.3. Ex-situ X-ray diffraction (ex-situ XRD)

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Figure 2- TGA and DSC analysis of fresh niobic acid

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Figure 3 shows the XRD pattern of niobic acid calcined at 300, 400, 500 and 600

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°C. Samples calcined at 300 and 400 °C showed an amorphous structure. However, 12

ACCEPTED MANUSCRIPT when the calcination temperatures were 500 or 600 °C, the samples changed their

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structure from amorphous to crystalline. The analysis of the XRD pattern of the niobic

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acid calcined at 500 °C indicates the presence of Nb2O5 with hexagonal structure,

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whereas analysis of samples calcined at 600 °C indicates the presence of essentially

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orthorhombic Nb2O5. These results are coherent with the ones reported by Paulis et al.

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[16] for niobic acid samples calcined at 400, 500, 700 and 900 °C. In their work, the

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behavior of phase changes of this material while studying the aldol condensation of

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acetone was investigated. The surface area of niobium oxide decreases from 144 to

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4 m2/g with the increase of calcination temperature from 400 to 900 °C, while the

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structure of Nb2O5 also changes from amorphous to crystalline in different phases.

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Figure 3- XRD pattern of niobic acid calcined between 300 and 600 °C 13

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3.1.4. In-situ X-ray diffraction (in-situ XRD)

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Figure 4 presents the results obtained for the in-situ X-ray diffraction technique

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for niobic acid calcined from room temperature to 500 °C. As it was observed in ex-situ

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XRD analyses, samples calcined at temperatures higher than 500 °C changed their

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structure from amorphous to crystalline. As indicated by Figure 4, the sample changed

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its structure with a calcination time of only five minutes at 500 °C. This result proves

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that a calcination time of 2 hours is more than enough to fully change the structure from

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amorphous to crystalline for calcination temperatures equal or above 500 oC. For lower

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temperatures, this calcination time is also adequate to ensure an amorphous structure.

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Figure 4- In-situ XRD pattern of niobic acid calcined from room temperature to 500 °C

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3.1.5. Temperature-programmed desorption of ammonia (NH3-TPD)

300

The total acidity of the niobic acid calcined at different temperatures was

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evaluated by NH3-TPD and the results are shown in Table 3. The concentration of the

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acid sites decreases from 0.10 to 0.03 mmol NH3/g with the increase of calcination

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temperature, showing that the acid properties are affected by the thermal treatment.

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According to Iizuka et al. [15], niobic acid has acidic OH groups, which are Bronsted

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acid sites, even after heat treatment at 100 °C. After a pretreatment at 300 °C, the water

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present at the surface of the catalyst is almost removed and the Lewis sites increase with

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the decrease of Bronsted sites. However, according to their results, after a heat treatment

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at 500 °C, niobic acid starts to show weak acidic properties and becomes inactive due to

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the phase transformation of amorphous Nb2O5.nH2O to crystalline, that happens around

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500-600 °C.

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Table 3- Total acidity of niobic acid calcined at different temperatures Calcination Temperature (°C)

Total acidity (mmol NH3/g)

300 oC

0.10129

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350 oC

0.08207

400 oC

0.07426

450 oC

0.06053

500 oC

0.03471

600 oC

-*

* Total acidity present in the sample was at the noise level

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3.2.

W/F tests and catalytic activity in the esterification reaction

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Initially, tests with niobic acid calcined at 300 °C were performed, in order to

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define the conditions of kinetic regime, without external diffusion limitations [17]. For

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these tests, experiments were carried out using a fixed feed flow rate and varying the

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catalyst mass, evaluating the conversion of the reaction. W/F value was defined as the

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ratio of catalyst mass (g) to organic feed flow rate (mol/min). For this purpose, reactions

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were performed at 250 oC, ethanol:oleic acid molar ratio of 6:1 and flow rate of 0.3

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mL/min. The mass of catalysts varied from 0 to 0.8 g. The results obtained are

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presented in Figure 5 and Table 4.

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It can be observed that when an amount of niobic acid up to 0.4 g (W/F of 888

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g.min/mol of oleic acid) is used, there are no mass transfer limitations. After this point,

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the conversion is not proportional to the amount of catalyst, which is an indication of

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mass transfer limitations. Moreover, this means that the evaluation of the catalytic

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activity of niobic acid calcined at different temperatures can be performed with 0.3 g of

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catalyst. Therefore, the catalytic activity of niobic acid calcined at 300, 350, 400, 450,

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500 and 600 oC were tested at 250 oC, ethanol:oleic acid molar ratio of 6:1, flow rate of

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0.3 mL/min and 0.3 g of catalyst.

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ACCEPTED MANUSCRIPT Table 4- Data of W/F tests performed at 250 °C, ethanol:oleic acid molar ratio of 6:1,

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flow rate of 0.3 mL/min (0.00045 mol OA/min) and mass of catalyst from 0 to 0.8 g Mass of niobic acid

W/F

Conversion

(g)

(g min/mol AO)

(%)

0

0

0.1

222

0.3

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0.4

888

0.6

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8.8

20.7

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0.8

1776

81.5 84.8

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Figure 5- Conversion versus W/F, for esterification reaction of oleic acid and ethanol, at

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250 °C, ethanol:oleic acid molar ratio of 6:1 and flow rate of 0.3 mL/min. Mass of

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niobic acid varied from 0 to 0.8 g.

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Figure 6 shows the catalytic tests performed with niobic acid calcined at

346

different temperatures. There is a progressive decrease of the catalytic activity with

347

increasing calcination temperature. The conversions obtained using niobic acid calcined

348

at 300, 350 and 400 oC were 44%, 50% and 48%, respectively. These results were

349

analyzed using analysis of variance (ANOVA). The analysis showed that there is no

350

significant difference between the conversions obtained for these three temperatures.

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However, lower conversions were obtained for samples calcined at 450, 500 and 600 oC

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(24%, 16% and 7%, respectively).

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Figure 6- Conversion of the continuous esterification reaction versus niobic acid

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calcination temperature. Reaction conditions: 250 oC, ethanol:oleic acid molar ratio of

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6:1, flow rate of 0.3 mL/min and 0.3 g of catalyst.

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niobic acid. According to the characterizations performed with samples calcined at

371

different temperatures, it was possible to conclude that the textural and acid properties

372

were affected by the thermal treatment. BET area, pore volume and acidity of niobic

373

acid decreased with increasing the calcination temperature from 111 to 15 m2/g, 0.11 to

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0.02 cm3/g and 0.10 to 0.03 mmol NH3/g, respectively. Consequently, the conversion of

375

esterification reaction also decreased with the increase of calcination temperatures from

376

300 to 600 °C. According to X-ray diffraction data (Fig. 4), with the increase of

377

calcination temperatures, niobic acid starts to change the structure from amorphous

378

(non-crystalline) to crystalline and these changes on the structure make Nb2O5.nH2O

379

inactive with very weak acidity [15]. Lebarbier et al. [14] obtained similar results. In

380

their work, they tested samples of niobium oxide calcined at different temperatures

381

(from 150 to 550 °C) and investigated the relationship between the structure obtained,

382

acidity and the catalytic performance for the propan-2-ol dehydration. Results showed

383

that the catalytic activity of niobium oxide samples decreases with increasing

384

calcination temperatures. The authors suggest that the decrease in the catalytic activity

385

is result of a decrease in the surface area and in the abundance of the Brønsted acid

386

sites, which were monitored by lutidine adsorption followed by desorption at 250 °C.

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M AN U

TE D

EP

Summarizing, these results show that the conversion of esterification reaction

AC C

387

RI PT

369

388

decreases with increasing the calcination temperature. Knowing that surface area and

389

total acidity of niobic acid decrease with increasing calcination temperatures, it is

390

possible to say that there is a direct relationship between the amount of acid sites, the

391

surface area and the esterification activity.

392 393

19

ACCEPTED MANUSCRIPT 394

3.3.

Effects of process parameters for central composite design

395

Based on the fact that niobic acid calcined at 300, 350 and 400 °C showed

397

similar conversions on the esterification reaction (with no significant difference), the

398

sample calcined at 350 °C, which presented the highest value of conversion (50%), was

399

selected to the next step of this work. In this step, the experimental conditions of

400

temperature, mass of catalyst and ethanol-to-oleic acid molar ratio were evaluated using

401

design of experiments (DOE). As already mentioned, the experiments were carried out

402

according to a central composite design (CCD) in the following ranges: temperature

403

from 220 to 290 oC, mass of catalysts from 0 to 0.8 g and ethanol-to-oleic acid molar

404

ratio from 2:1 to 14:1.

M AN U

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396

The results obtained for the experiments based on the CCD matrix using niobic

406

acid calcined at 350 °C are presented in Table 5. The yield of esters obtained varied

407

from 6 to 62%, depending on the experimental conditions. On the other hand, the yield

408

of esters of center points (runs 15, 16, 17 and 18) range from 38 % to 42% with an

409

average of 41%. Therefore, the experimental error obtained was smaller than 2%.

EP

411

AC C

410

TE D

405

20

ACCEPTED MANUSCRIPT 412 413

Table 5- Experimental conditions studied in CCD matrix, with uncoded values of

414

parameters and results obtained Temperature

Mass of

Ethanol:oleic

Yield of esters

number

(oC)

catalyst (g)

acid molar ratio

(%)

1

230

0.1

4:1

2

280

0.1

4:1

3

230

0.7

4:1

4

280

0.7

5

230

0.1

6

280

7

230

8

280

9

220

10

290

11 12

SC

19

33 51

12:1

17

M AN U

4:1

29

0.7

12:1

55

0.7

12:1

60

0.4

8:1

27

0.4

8:1

51

255

0

8:1

6

255

0.8

8:1

62

255

0.4

2:1

29

255

0.4

14:1

59

TE D

12:1

AC C

14

8

0.1

EP

13

RI PT

Run

15

255

0.4

8:1

40

16

255

0.4

8:1

42

17

255

0.4

8:1

38

18

255

0.4

8:1

42

415 416

21

ACCEPTED MANUSCRIPT 417 418

Table 6- Analysis of variance (ANOVA) for response surface quadratic model for the

419

yield of ester Significance (at Sum of

Mean DF

Prob> F

square

95% confidence

RI PT

squares

F value

5088.90

9

565.43

28.25

0.00

Significant

XT

551.37

1

551.37

23.73

0.01238

Significant

XT2

32.94

1

32.94

1.42

0.267934

Not Significant

XMC

3591.63

1

3591.63

154.56

0.000002

Significant

XMC2

167.57

1

167.57

7.21

0.027695

Significant

XMR

711.58

1

711.58

30.62

0.000551

Significant

XMR2

1.18

1

1.18

0.05

0.827677

Not Significant

XT.XMC

0.004

1

0.004

0.0002

0.989790

Not Significant

XT.XMR

14.15

XMC.XMR

18.48

TE D

M AN U

Model

SC

interval)

1

14.15

0.61

0.457635

Not Significant

1

18.48

0.80

0.398499

Not Significant

XT: variable for temperature, XMC: variable for mass of catalyst and XMR: variable for

421

ethanol:oleic acid molar ratio.

423

AC C

422

EP

420

The esters content (y) was defined as the response of the process. These values

424

were used to develop a quadratic polynomial equation that correlates the yield of esters

425

as a function of the independent parameters and their interactions. The multiple

426

regression was carried out, using Statistica software, by eliminating the insignificant

427

parameters, which presented p-values were higher than 0.05 (Table 6). The equation

428

obtained, expressed in coded factors, is given by Equation 8.

429 22

ACCEPTED MANUSCRIPT (8)

430

431

Where XT: coded variable for temperature, XMC: coded variable for mass of catalyst and

433

XMR: coded variable for ethanol:oleic acid molar ratio.

RI PT

432

434

The quality of developed model was determined by the value of correlation (R2).

436

Equation 8 had a correlation coefficient (R2) of 0.95, which is very close to unity. This

437

value means that 95% of the variation in the results was attributed to the independent

438

variables investigated [18]. As shown in Figure 7, there are no tendencies in the

439

regression fit and the points are close to the regression line. Therefore, it is possible to

440

affirm that Equation 8 is statistically consistent and describes very accurately the

441

experimental data.

M AN U

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435

445 446 447 448 449 450

EP

444

AC C

443

TE D

442

451 452

Figure 7- Predicted values versus observed values obtained in the CCD experiments

453

23

ACCEPTED MANUSCRIPT According to the regression coefficients obtained in Eq. 8 and the ANOVA

455

(Table 6), the three single parameters temperature (XT), mass of catalyst (XMC) and

456

ethanol:oleic acid molar ratio (XMR) are significant and affect the yield of esters, as well

457

as the quadratic term of the mass of catalyst (XMC.XMC). No interaction between

458

variables was significant. Analyzing the regression coefficient obtained, it can be

459

noticed that all three parameters showed a positive effect. The mass of catalysts is the

460

most significant variable, with a coefficient value of 17.30, followed by ethanol:oleic

461

acid molar ratio (7.70) and temperature (6.78). The quadratic term obtained for the mass

462

of catalysts (XMC.XMC) indicates a non-linear behavior of the system. This means that at

463

higher amounts of catalyst, this parameter will not increase the yield of esters anymore,

464

probably due to mass transfer limitations.

M AN U

SC

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454

The positive effects of the parameters can also be observed in the Table 5.

466

Comparing runs 1 and 5, one can observe that the temperature and mass of catalysts are

467

the same (230 °C and 0.1 g, respectively), but the ethanol:oleic acid molar ratio is

468

increased from 4:1 to 12:1. This increase of the ethanol:oleic acid molar ratio causes an

469

increase in the yields of esters from 8 to 17%. The same behavior can be observed

470

comparing runs 2 and 6 (from 19 to 29%), 3 and 7 (from 33 to 55%), 4 and 8 (from 51

471

to 60%) and 13, center points and 14 (29, 41 and 59%, respectively). Therefore,

472

consistently with the literature, these results show that an increase in the ethanol:oleic

473

acid molar ratio positively affects the response of the process. According to Alinezi et

474

al. [19], the stoichiometric ratio for the esterification reaction requires an ethanol:oleic

475

acid molar ratio equals to 1. Thus, higher molar ratios result in greater production of

476

esters, because the excess of alcohol provides a better contact between the reactants and

477

causes the equilibrium to shift the reaction to esters formation.

AC C

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TE D

465

24

ACCEPTED MANUSCRIPT The effect of mass of catalyst on the yield of esters, keeping temperature and

479

ethanol:oleic acid molar ratio constants, can be observed by comparing runs 1 and 3

480

(from 8 to 33%), 2 and 4 (from 19 to 51%), 5 and 7 (from 17 to 55%), 6 and 8 (from 29

481

to 60%) and 11, point centers and 12 (6, 41 and 62%, respectively). As expected, an

482

increase in the mass of niobic acid promotes an increase on the number of acid sites

483

and, consequently, favors the formation of esters.

RI PT

478

Maintaining the mass of catalyst and ethanol:oleic acid molar ratio constant and

485

raising the temperature of the process, promotes an increase in the esters content. This

486

can be observed comparing runs 1 and 2 (from 8 to 19%), 3 and 4 (from 33 to 51%), 5

487

and 6 (from 17 to 29%), 7 and 8 (from 55% to 60%) and 9, point centers and 10 (27, 41

488

and 51%, respectively). This result is consistent with others reported in literature [6, 19,

489

20, 21]. At higher temperatures, the solubility of fatty acid in alcohol increases,

490

benefiting the conversion of the reaction.

M AN U

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484

Response surfaces were constructed using the multiple regression of CCD. The

492

response surface plot of yield of esters against coded ethanol:oleic acid molar ratio and

493

coded mass of catalysts is shown in Figure 8. The response surface plot of yield of

494

esters against coded mass of catalysts and coded temperature is shown in Figure 9.

495

Analyzing Fig. 8 and 9, it is possible to observe a convex behavior, caused by the

496

quadratic term of the mass of catalysts. As already mentioned, the yield of esters will

497

start to stabilize even with higher amount of catalyst, probably due to the mass transfer

498

limitations.

AC C

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TE D

491

25

SC

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ACCEPTED MANUSCRIPT

M AN U

499 500

Figure 8- CCD Response surface plot of FAEE yield versus coded ethanol:oleic acid

501

molar ratio and coded amount of catalyst (temperature at level zero)

AC C

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502

503 504

Figure 9- CCD response surface plot of FAEE yield versus coded amount of catalyst

505

and coded temperature (ethanol:oleic acid molar ratio at level zero) 26

ACCEPTED MANUSCRIPT 506 5073.4.

Optimization

508

The highest yield of esters obtained in the central composite design (CCD) was

510

62%. The optimization using canonical analysis was used to determine the maximum

511

yield of esters that could be obtained in this reaction system. The empirical model was

512

expressed in the canonical form and the model obtained was:

SC

513

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509

514

M AN U

515

(9)

Equation 9 shows two characteristic roots (λ1 and λ2) with a negative sign (-4.58

517

and -2.23, respectively) and that the last one (λ3) with a positive sign (+0.20). This fact

518

means that the stationary point is a saddle point. Thus, to maximize Equation 9, w1 and

519

w2 shall be zero, for different values of w3.

TE D

516

Testing different values of w3, the optimum conditions of temperature, mass of

521

catalyst and ethanol:oleic acid molar ratio optimized were defined. These conditions are

522

presented in Table 7.

524 525 526

AC C

523

EP

520

527 528 529

27

ACCEPTED MANUSCRIPT 530

Table 7- Temperature, ethanol:oleic acid molar ratio and mass of catalyst optimized

531

using canonical analysis and predicted yield of esters for each test

(oC)

catalysts (g)

Ethanol:oleic

Predicted

Yield of

acid molar

yield of

esters

ratio

esters (%)

obtained (%)

73

67

79

69

86

68

Run

19

292

0.960

15:1

20

285

0.965

18.9:1

21

278

0.965

22.8:1

22

271

0.966

26.6:1

93

69

M AN U

532

RI PT

Mass of

SC

Temperature

With the experimental results presented in Table 7, it is possible to conclude that

534

yield of esters higher than 62% can be obtained. However, the values of predicted yield

535

of esters and the experimental yield of esters were not the same. This fact means that

536

Eq. 9 did not represent with accurate the extrapolated ranges of the parameters. The

537

reactions presented in Table 7 were coupled at the CCD matrix and the optimization

538

was carried out again, also using the canonical analysis technique. The new model

539

obtained was:

EP

AC C

540

TE D

533

541 542

(10)

Equation 10 shows that all three characteristic roots (λ1, λ2 and λ3) presented

543 544

negative signs (-14.45, -12.17 and -2.26, respectively). This means that any

545

displacement from the stationary point causes a decrease of the response and that the

546

stationary point is a maximum point. Then, the optimal conditions were defined at: 249

547

o

C, ethanol:oleic acid molar ratio of 10.83:1 and 0.7 g of niobic acid. Under these

28

ACCEPTED MANUSCRIPT 548

operational conditions, the canonical model predicts a yield of esters of 72%.

549

Experimentally, this reaction got a yield of esters of 71%, confirming the result

550

predicted by the canonical analysis.

551

4. CONCLUSIONS

RI PT

552 553

In this work, niobic acid was calcined at temperatures between 300 and 600 °C.

555

As the calcination temperatures of niobic acid were increased, there was a decrease on

556

the surface area, total acidity and activity for esterification reaction. In the catalytic tests

557

performed at 250 oC, ethanol:oleic acid molar ratio of 6:1, flow rate of 0.3 mL/min, the

558

samples calcined at 300, 350 and 400 °C presented higher catalytic activity on the

559

esterification reaction, with no significant difference at the conversions obtained. For

560

niobic acid calcined at 350 °C, which presented higher catalytic activity, all three

561

parameters: temperature, mass of catalyst and ethanol:oleic acid molar ratio

562

significantly affected the yield of esters, in the experimental ranges studied. The highest

563

yield of ester obtained in the central composite design (CCD) was 62%, at 255 °C, 0.8 g

564

of niobic acid and 8:1 ethanol:oleic acid molar ratio. With the optimization, a yield of

565

esters of 71% was obtained, at 249 oC, 0.7 g of niobic acid and ethanol:oleic acid molar

566

ratio of 10.83:1. Finally, niobic acid calcined at 350 °C was an effective catalyst for the

567

continuous esterification with ethanol and oleic acid.

569

M AN U

TE D

EP

AC C

568

SC

554

ACKNOWLEDGEMENTS

570 571

The authors are grateful to CBMM for the financial support and for providing

572

the niobic acid (HY-340) used in this work, to CNPq, FAPEMIG, CAPES and Vale

29

ACCEPTED MANUSCRIPT 573

S.A. for the financial support and to LNLS (Laboratório Nacional de Luz Síncotron –

574

Campinas – Brazil) for the use of the XPD beamline.

575 576

REFERENCES

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[1] M.E. Borges, L. Díaz, Recent developments on heterogeneous catalysts for biodiesel

579

production by oil esterification and transesterification reactions: A review, Renew. and

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esters from vegetable oil as a biodiesel fuel optimization by factorial design, Chem.

583

Eng. J. 134 (2007) 93-99.

584

[3] Navjot Kaur, Amjad Ali, Preparation and application of Ce/ZrO2-TiO2/SO-24 as solid

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catalyst for the esterification of fatty acids, Renew. Energ. 81 (2015) 421-431.

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[4] A.C.A. Abdala, V.A.S. Garcia, C.P. Trentini, L. Cardozo Filho, E.A. da Silva, C.

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of Che. Eng. (2014) Article ID 803783, 1-5.

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Methodology and Kinetic Modeling for Synthesis of Methyl Oleate Biodiesel over

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H3PW12O40 Anchored Montmorillonite K10, Ind. Eng. Chem. Res. 53 (2014)

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Amberlyst as a heterogeneous catalyst, Bio. Technol. 101 (2010) 62-65.

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esterification of oleic acid over zeolite Y prepared from kaolin, Renew. Energ. 97

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catalytic biodiesel production, J. Supercrit. Fluids. 105 (2015) 21-28.

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properties of niobium phosphate and of niobium oxide, Appl. Catal., A, 89 (1992) 143-

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Bronsted acidity of niobic acid, Catal. Today, 192 (2012), 123-129.

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[15] T. Iizuka, K. Ogasawara, K. Tanabe, Acidic and catalytic properties of niobium

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[16] M. Paulis, M. Martín, D.B. Soria, A. Díaz, J.A. Odriozola, M. Montes, Preparation

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[18] K.T. Tan, M.M. Gui, K.T. Lee, A.R. Mohamed, An optimized study of methanol

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[19] R. Alenezi, G.A. Leeke, J.M. Winterbottom, R.C.D. Santos, A.R. Khan,

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636

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ACCEPTED MANUSCRIPT Highlights



AC C

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Niobic acid was used as heterogeneous acid catalysts for esterification reaction. Different calcination temperatures of niobic acid were evaluated. Temperature of 350°C presented higher surface area, acidity and catalytic activity. Temperature, mass of catalyst and alcohol:FFA molar ratio were study and optimized. In the best conditions tested, a yield of ester up to 70% was obtained.

RI PT

• • •