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Introduction

Esterification is one of the several processes involved in organic chemistry, esterification refers to the reaction between alcohols and carboxylic acids to produce esters [1]. The study of esters has attracted researchers due to the applications of esters in various fields such as solvents, plasticizers, and intermediates [2].
Esters are compounds that are obtained from carboxylic acids. The structure of Carboxylic comprises of the –C00H group, and in an ester, the hydrogen molecule in is substituted by a hydrocarbon group of a particular type [3]. The reaction cannot proceed normally and must take place in the presence of a catalyst. Esterification reaction that results from the Lewis and Bronsted acid catalyzed the reaction of carboxylic acids with alcohols is illustrated below [4].
                                                                                                                                     (1)

The reagents in the equation above are at equilibrium; this implies that removing one product from the reactants side or using an excess of one of the reactants, the equilibrium can significantly be affected. Esterification reaction requires a lot of energy for the removal of the Hydroxyl ions from the carboxylic acid. It, therefore, requires a catalyst and heat for it to occur [5]. With the necessary catalyst and heat, the hydroxyl ions can be removed and the oxygen molecule linked to the carbon. Since oxygen was already present in the carbon, the connection of the other oxygen to the carbon makes the carbon have two oxygen molecules connected on both sides and this form an ester [6]. The general formula for an ester is as shown below.
 
Esterification reaction has a broad range of uses in the chemical industry. The main use of esterification is the making of ethyl acetate. Ester or Ethyl Acetate is characterized by a sweet smell and therefore it has a broad spectrum of uses. The group of compounds called esters all have a pleasant smell. Ester is used in the making of artificial fruit extracts and fragrance enhancers, and artificial flavors. Ethyl Acetate is also used as a solvent in various applications such as in the making of varnishes and paints. [7]
Another industrial application is the making of polyesters. There has been a development of polyesters since the 19th Century [8]. Esterification reaction is responsible for the preparation of polyesters which have a variety uses. One of the methods of preparing Polyesters is by the exchange reaction between ester and hydroxyl groups; a process often referred to as alcoholysis [9]. Without a catalyst, alcoholysis occurs at a slower pace even when the temperatures are very high [10]. Examples of catalysts used in the process are lead (Pb), Zinc(Zn), Magnesium(Mg), and Cobalt(Co). Polyesters find their uses in several fields such as in the making of textile fibers, bottles, plastics and resins [11].
Esterification is also used in modifying oils and fats to form better products. Oils and fats are classified as triglycerides and are obtained from plants and animal products. Esterification is used to modify their chemical composition through ester exchange and hydrogenation, and the result is an increase in utility of such products [12].
Esterification also finds its use in food emulsifiers. Mixtures of mono and diacylglycerols are made by a process of transesterification of glycerol and fatty acids. Esterification can also produce soaps. In making soaps, oils and fats are transesterified to produce methyl esters of fatty acids after which they are subjected to saponification to deliver alkali salts of fatty acids and menthol [7].
The demand for ethyl acetylene was estimated to be 60 tons per year in 2012 [8]. This need was forecasted to hit 95 tons by the year 2018. A recent report by Mary shows that INEOS, one of the companies in Switzerland is working on an expansion of its plant in Hull to boost production of Ethyl Acetate by 100, 000 tons per anum [12]. The decision was reached after a market analysis which estimated the demand for Ethyl Acetate to rise drastically. The cost of production of Ethyl Acetate is high due to power consumption and use of expensive materials and equipment. However, the output supersedes the production cost.
As noted earlier, esterification requires heat and a catalyst for it to occur. Examples of typical catalysts that are used include Concentrated sulphuric acid and hydrochloric acid. The previous studies were based on the chemical structure of the alcohol, acid and the acidic catalyst [7].  The rate of esterification is greatly influenced by the chemical structure of  the alchohol used. When simple alcohols are used, the reaction occurs faster since they are relatively small and do not contain carbon atom that may stop their reaction [7]. There are factors to be considered for the choice of catalysts to be used for esterification. In small scale laboratory production, sulphuric acid and hydrochloric acid are commonly used. The traditional homogeneous catalyzed reactions have been associated with various challenges such as the attendant problems of separation and reuse [13]. The other types of catalysts used are cation-exchange resins and zeolites [7].
The application of zeolites to replace the convention catalysts has unique advantages [14]. One of the main advantages of the zeolites is that they can be reused. Also, the zeolites are cost-effective and eco-friendly. In addition, zeolites can be modified for increased yield and better catalytic activity.
 
             Aluminophosphate of AFI structure (AlPO4-5) being one of the molecular sieves, can be functionalized and used to catalyze esterification reaction. The AFI structure of AlPO4-5 belongs to the AFI family of Aluminophosphate. It structure is composed of alternating PO4 tetrahedrons and AlO4 which create one-dimensional cylindrical pores extending across the framework. The cylindrical pores are electrically neutral having a uniform diameter of 7.3 Å that stretches in the direction parallel to the c axis of the crystal [15] .
  For the designing of novel catalyst, it is necessary to incorporate transition metal ions into the molecular sieves of aluminophosphate framework sites. Unsubstituted AlPO4-5 is neutral, largely non-catalytic material; where redox metals such as iron, cobalt, manganese, magnesium, nickel and zinc can all be used to replace aluminium sites to produce a Brönsted acid site through the framework [16]. Brönsted acid sites also can be produced using silicon, and titanium as replacements for phosphorus (V) [16].
Successful incorporation of metals via isomorphous substitution have been reported for a variety of cases including FeAPO-5, CoAPO-5, MgAPO-5, and SAPO-5[17, 18] . Application of these materials as catalysts was reported for a variety of reactions such as catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone [19], Isopropylation of benzene with 2-propanol to produce cumene [20] and conversion of p-xylene to tetraphthalic acid [21].
Metal substituted aluminophosphates MeAPO-5 have numerous acidic sites [16] which make it suitable for catalyzing acetic acid esterification reaction. For esterification to occur, hydroxyl ions are supposed to be removed from the alcohol. The esterification process using an acid catalyst is summarized below.
(2)
 
The esterificaton reaction has been studied by differet researchers. In an experiment to analyze the effect of Sulphuric acid on the esterification of acetic acid and ethanol, the two chemical components were studied at 45-75 degrees Celsius with different molar ratio of the two elements [22]. The experiment was conducted at various temperature ranges of 318.15K, 328.15K, 338.15K, 348.15K, and 358.15K. It was found that the mole ratio, temperature, and the catalyst had significant effects on the reaction. From the reaction, it emerged that the rate increases as the temperature increase up to a certain level where the rate of reaction remains constant (65 degrees Celsius). Further analysis of the reaction showed that increasing the molar ratio of the acetic acid to ethanol resulted in increasing reaction rate, due to the reaction was catalyzed by sulphuric acid. Maximum conversion, about 83% was achieved at the temperature of 650C with a molar ratio of 1:5 for acetic acid.
In another experiment, the reaction kinetics of acetic acid and butanol was assessed in the presence of Dowex 50 catalyst [23] . The aim of the research was to assess the effect of various parameters such as temperature, catalyst loading, and the feed ratio of the chemical reactants. The research was carried out in a batch reactor where the temperature was varied from 343 to 363k. During the experiment, the conversion rate of n-butanol was recorded at three different temperatures of 343, 353, and 363K. From the results, it emerged that with an increase in temperature, the rate of reaction also increased significantly. Focusing on the catalyst, it was evident that before the introduction of the Dowex 50, the reaction rate was slow and conversion rate was 27%. However, when Dowex 50 was introduced in the reaction process, the conversion rate rose to 67%. The Dowex 50 increased the reaction sites thus speeding up the rate of reaction and conversion of n-butanol to form ethyl acetate acid. Catalyst load was also varied with loads of 12, 24, and 36 grams. It was evident that the catalyst load increased the rate of reaction; however, there was no significant effect on the conversion of n-butanol. The conversion rate was also assessed at different acetic to n-butanol feed ratios. There were three different feed ratios (1:1, 1:2, and 2:1). From the experiments at different feed rates, it was observed that using utilizing 100% excess quantities of acetic acid led to an increase in n-butanol conversion from 67% to 86%[23]. On the other hand, using 100% excess of n-butanol results in a 27% decrease of n-butanol conversion from 67% to 40%. From the experiment, it was concluded that the optimum temperature for the highest n-butanol conversion was 90oC.
Another experiment on the esterification of acetic acid with ethanol using acidic ion-exchange resin as a catalyst was performed using a batch reactor [24]. Temperature was varied during the experiment from 323K to 353K to ascertain its impact on the reaction process. The reaction mixture was stirred magnetically at roughly 900 rpm.  The test was observed under different molar ratios of acetic acid and ethyl alcohol while varying the temperatures from 323K to 353K. The catalyst was loaded at 1.0, 3.0, 5.0, 7.0 and 10.0 grams. In the beginning, the results of the experiment were taken at an interval of 20 minutes, however towards the end; the results were obtained after one hour. The results of the investigation indicated that increasing the temperature led to an increased rate in the reaction process. The maximum conversion of 78% occurred at a temperature of 353k. The mole ratio was also assessed, and the results showed that as the mole ratio of acetic acid increased, the rate of reaction also rose. A mole ratio of acetic acid to ethyl alcohol of 1:2 gave the maximum conversion. Finally, catalyst loading was examined experimentally. With varied quantities of the acidic ion-exchange resins, the maximum conversion was obtained using 5.4 grams of the catalyst.
In another set of experiment, the esterification of acetic acid and ethanol using Nanoporous MgAPO-5 was evaluated [25]. The set up comprised of a batch reactor, reflux condenser, and a thermometer. In the experiment, the effect of the reaction time, temperature, mole ratio, and catalyst dosage was analyzed. It was observed that at 100oC and 150oC, the conversion rate was at its maximum while remaining almost constant at 200oC. With reaction time, maximum conversion of 87% was achieved at 3 hours. The mole ratio was also varied to ascertain if it had any significant effect on the reaction. The ratio of acetic acid to ethanol was changed from 1:2 to 5:1. The results showed a corresponding increase in the rate of reaction from 1:1 to 3:1. Increasing the mole ratio beyond 3:1 had a diminishing effect on the reaction. Finally, catalyst dosage was carefully studied. The dosage was varied from 0.1grams to 0.5 grams. It was observed that with increase in catalyst dosage, the reaction rate of the mixture increased, due to the number of active sites of the catalyst increased.
An experiment to evaluate the effect of zeolites on the catalytic esterification of alcohol with acetic acid was set up where different forms of zeolites were used [26]. Zeolites Y, β, and ZSM-5 were used in the reaction. In the experiment, 0.5 grams of the zeolites were used in the reaction. The feed ratio of acetic acid to benzyl alcohol was varied from 1:1, 1:2, and 2:1. The total volume was kept at 16 cubic centimeters. The temperature was maintained at 403K all through the reaction process. The results were obtained for the three forms of zeolites after1, 2, 4, 5, 6, 8, and 15 hours and recorded. From the onset of the reaction, it was inferred that all the three forms of zeolites catalyzed the reaction. It was observed that β zeolites had a high surface acidity and high surface area compared to ZSM-5 and Y. However, the yield of its reaction was small. The analysis of feed ratio indicated that the reaction rate increased when one of the reactants was in excess. When time was analyzed, it was observed that there was no increase in the yield of ethyl acetate from 8 hours to 15 hours. Concerning the effect of the catalysts, it was observed that the yield was less in the absence of the catalysts. Also, it was found that even after reuse, the zeolites remain active several times.
From the above reviews, it emerges that different catalysts have been used to catalyze esterification reaction, but no work has been done using different metal-substituted aluminophosphates (MeAPO-5) as catalysts. This research paper is, therefore, the first work to incorporate such catalysts in the acetic-acid esterification.
 
 
 
 
 
 
 
 
 
 
 
Experimental
 Synthesis of MeAPO-5 crystals
AFI molecular sieves were synthesized by the hydrothermal reaction of a gel solution with the following molar composition: Al: 1.3 P: x Me: 100H2O: 1.2TEA, where Me is a divalent or trivalent metal ions, Si is a tetravalent ion for the synthesis of SAPO-5 and TEA is the organic template triethylamine. X is in the range (0.025-0.8). The precursors for alumina, phosphorus and template were aluminum isoproxide (98 wt%), phosphoric acid (H3PO4 85 wt%), and TEA (99 wt%) respectively. The metal ions were obtained from different sources including: iron(III) nitrate, cobalt(II) acetate tetrahydrate, magnesium chloride anhydrous and tetraethyl orthosilicate. all chemicals were purchased from Aldrich.
The AFI catalysts were prepared in accordance with the following procedures. First, aluminium isoproxide was dissolved in water and stirred for 3hrs, then phosphoric acid was added and stirred until the solution become homogenous, followed by drop wise addition of the organic template TEA. The desired metal was dissolved in a small amount of distilled water and then added to the solution. The gel was stirred with a magnetic stirrer for 24hrs and the pH was measured. The pH was adjusted through the addition of phosphoric acid to achieve a pH in the range of (5-6). 30 ml of the final precursor solution was subsequently transferred to a Teflon-lined stainless steel autoclave, and the autoclave was placed into an oven for 24hr with desired temperature of 1500 C.  Washing/centrifugation were followed and the powder was dried at 1000 C for 3 hr. At the end the powder was transferred to the furnace for calcination at 600o C for 6hr.
 
 
 
 
Reaction procedures
The effects of metal ions substituted AlPO4-5 (MeAPO-5) and metal content of MeAPO-5 were investigated. The reaction was studied over the synthesized samples (SAPO-5, CoAPO-5, FeAPO-5, and MgAPO-5) at 75oC with a catalyst mass of 1.5g and molar composition of (Al: 1.3 P: xMe: 1.2TEA: 100H2O), where Me is the inorganic metal ion, and x represents the molar ratio.
The esterification reaction of acetic acid with ethanol was carried out in a stirred batch system.  Reactions were carried out in the presence and in the absence of MeAPO-5 catalysts. The volume of the reaction mixture (24 ml) and the mole ratio of acetic acid to ethanol (20:1) remained constant during all experiments.
A 50 ml glass 3-way flask was used and charged with acetic acid, ethanol and MeAPO-5 catalysts. Then, the system was heated up to the reaction temperature and the reaction was carried out for 3 h. The product was analyzed by gas chromatography (GC) with the following method:
Column: DB-WAXETR 30m, 0.32mm, 1.OO, carrier gas: helium, 403 mL/min, split ratio: 200:1, 200, Oven: 60-120 hold for 1 minute. 120-200 at 120 hold for 5 minutes, Detector: FID, 250.
 
 
 
 
 
 
Material Characterizations
Different characterization techniques were applied for analysis. X-Ray diffraction (XRD) measurements were carried out on analytical X’Pert Pro powder diffractometer with 2θ range of 2-50°, step size of 0.03°/sec, voltage of 40 KV, intensity of 30 mA using Cu kα radiation with a wavelength of 1.5406 Ǻ. Scanning electron microscopy (SEM) images were collected using a FEI Quanta 200 ESEM with XL30 microscope. Prior to test, the samples were covered with carbon coating to minimize the charging of particles. Surface area was evaluated by means of a QUANTACHROM 1-C analyzer utilizing multipoint BET (Brunauer Emmet Teller) method.
 
 
 
 
 
 
 
 
 
 
 
 

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