SAPO-5and Esterification Reaction
Silicoaluminophosphates(SAPOs) are a class of materials formed through the introduction ofsilicon atoms into the neutral framework of Aluminophosphates (ALPOs)(Zokaie, 2012). The formation of SAPOs involves the addition of asilicon source to the synthesis batch (Zokaie, 2012). An importantproperty of SAPOs is the aggregation of silicon atoms to form siliconislands. When the silicon content in SAPOs reaches a certainthreshold which is specific for each topology, Silicon islands areformed (Zokaie, 2012). Silicoaluminophosphates are of differentstructures such as SAPO-5, SAPO-34, SAPO-41, SAPO-11, and SAPO-46,among others. SAPO-34, SAPO-41, SAPO-11, and SAPO-46 are of theframework structure of CHA (Xiao et al., 2016).
Thestructure of any Silicoaluminophosphate (SAPO) compound is majorlydictated by the thermal and hydrothermal stability states of thesubstance under consideration. For instance, the firmness of SAPO-37depends on the heating temperature and the immediately adsorbedlevels. On the contrary, the steadiness of SAPO-5 can never bechanged under such environmental conditions [ CITATION JWe14 l 1033 ].Exposingfree samples of SAPO-34 and SAPO- 37 to the ambient atmosphericconditions makes them lose some part of their crystalline nature.This change is temporary for SAPO-34 after undergoing dehydrationprocess. Besides, a solid state reaction can be seen in heatingSAPO-37 without any observable changes in its crystal alignment(Otera,2011).
Ina study to find out the effect of temperatures on different SAPOs,three samples were heated in a crucible for almost 20 hours andfinally rehydrated for one full day at room temperature and pressure.While some temperature changes were observed for SAPO-34 and SAPO-37,there were no changes in temperature for SAPO-5 (MacIntosh &Huang, 2013). However, the bands broadened and defined, indicatingsome loss of crystalline nature. The reason for such change wastwofold, that is, due to progressive template remove altered bondangles and lengths. Water re-adsorption may have opened up some bondsor at least disturbed some of them (Xiaoet al., 2016). Consequently,a general trend for stability within the three templates wasconfirmed. Thefocus of this paper is on SAPO-5 of the framework AFI structure
Howsubstitution of silicon ions into the framework can affect acidity inSAPOs
Theacidity of Silicoaluminophosphates is significantly influenced by thepresence of Silicon ions in their structure. Many studies have beendone so far to determine the effect of substituting Silicon ions inthe framework of Silicoaluminophosphates. Silicoaluminophosphateshave attracted attention for research due to its various applicationsin the industry. In any given SAPO, silicon ions can substitutephosphate ions. When this happens, charge imbalance in the SAPOmolecule occurs (Bhaumik & Dhepe, 2013). When hydrogen ionscompensate this imbalance, Bronsted acid sites are generated in themanner as in Zeolites when Aluminium ions substitute Silicon ions. Anexperiment by Roldánet al. (2011) indicates that Silicon ions can concurrently fill upboth Aluminium and Phosphate ions sites. This, in turn, leads to theformation of Silicon Islands whose PH is neutral. This kind ofmechanism of substitution is referred to as the SM2 (Si4+forP5+)(Cui,et al., 2013).Thisreplacement mechanism results in the formation of an isolatedBronsted acid sites and SM3 (2 Si4+forone P5+and one Al3+)(Roldán et al., 2011).Dueto the absence of Si-O-P bonds, the domains of zeolite that have beengenerated by the Substitution mechanism 3 (SM3) have to beaccompanied by Substitution mechanism two that delivers some Bronstedacidity (Si-(OH)-Al groups) in Silicon Islands borders. If suchhappens, strong acid sites are likely to be generated, and a varietyof acidity is covered due to acidic sites can be formed (Yadav &Sakthivel, 2014). A different way of making strong acidity is byintroducing Aluminium ions to the Silicon Island which gives azeolite-like acidity.
SAPO-5belongs to the family of large pore category and thus it can processand produce bulkier molecules as compared to SAPO-34 which falls tothe family of small pore molecules (Zokaie, 2012). The SAPO-5 of theAFI structure is made up of a one-dimensional 12 membered-ringchannel system that has a hole of 0.73 mm in diameter (Terasaka,Imai, & Li, 2015)Synthesisof Silicoaluminophosphates has been done by several researchers. Theexperiments resulted in the identification of a considerable largenumber of structure-directing agents as organic amine that issuitable for the crystallization of the AFI structure (Roldán etal., 2011). As opposed to other SAPOs, the SAPO-5 of the AFIstructure has a substantial likelihood of incorporating Silicon ionsthrough the substitution mechanism 3 (SM3) and hence producing silicaislands (Jhung et al., 2003). Various researchers have studied theeffect of substituting silicon ions into the framework of SAPO-5. Inan experiment done by where samples were prepared at high pH, Roldánet al. (2011), the SAPO-5 AFI structure with a small amount ofsilicon ions content had the largest number of acid sites. This is incontrary to a previous study done by Sinha & Seelan whichrevealed an increase in acidity with an increase in silicon ionscontent (Seelan & Sinha, 2012).
Advantagesof SAPO-5 regarding Physical properties and Re-usability fordifferent reactions and how these catalysts solved a previous problem
SAPO-5has various industrial applications due to its catalytic nature. Itis used to catalyze different chemical reactions in the making ofseveral chemicals that are utilized in the industry. As previouslystated, SAPO-5 belongs to the family of large-pore molecules. It is aone-dimensional molecule whose acid sites are higher when siliconions are substituted into the framework. One particular applicationis in the making of Cumene. SAPO-5 can be used in the synthesis ofCumene which has broad industrial applications. Cumene is mainlyutilized for the manufacturing of phenol and acetone (Terasaka, Imai,& Li, 2015).Thedemand for cumene has been on the rise due to the increased demandfor bisphenol. It is estimated that its market will be growing at arate of 3.5% every year (Degnan, 2007). This creates the need for amore and efficient way of producing such a product. In the past, theproduction of cumene was done using Solid phosphoric acid (SPA) andFriedel catalysts (Weitkamp & Puppe, 2013). Several studies havesince then, been done to make the production more efficient. The useof SPA as a catalyst in the manufacture of cumene has severaldisadvantages. One of the shortcomings of using SPA is the fact thatthe catalytic reaction cannot be regenerated. This implies that onceit used, the catalyst can no longer be re-used, and therefore thewhole process becomes expensive. Another disadvantage is that theproductivity is low. A lot of time is spent in unloading the usedcatalyst from the reactor. The use of Friedel catalysts such as AlClrequires special materials and painting of its coating to preventcorrosion and rust. This implies that an additional cost is added tothe process and also its surface poses a threat to the environment(Nielsen, et al., 2012).Alternative ways of the production of cumene had to be brought, andthis led to the use of molecular sieves such as SAPO-5 which is moreadvantageous to the rest.
SAPO-5is regarded as a fascinating catalyst for various petrochemicalreactions (Silva,et al., 2015).Its bronsted acidity majorly determines the Alkylation activity ofSAPO-5. Apart from the concentration of the bronsted acidity, SAPO-5is also strong, and this makes it more desirable for industrialapplications (Højholt,et al., 2011). In an experiment to determine the catalytic suitability of SAPO-5and SAPO-11, a synthesis of the two was done using Pseudoboemite,dilute phosphoric acid, water, fumed silica and an organic amine. Theresults were that the cumene output was high with the use of SAPO-5as compared to the use of SAPO-10. There was also a considerableamount of either when SAPO-11 was used and thus adding impurities tothe final product (Silva, et al., 2015).
Anotherindustrial application of SAPO-5 is in the ring opening ofnaphthenes. Recently, ring opening of naphthenes has been known to bea major way of converting naphthenes into Alkylnaphthenes and alkanesfor the purpose of increasing the octane number of diesel fuels(Junzo,2012).In the study dealing tetralin and decalin, information on isomerismand ring opening was obtained (deKlerk & Furimsky, 2011).Itwas noted that ring opening of decalin takes place on acidic zeoliteswithout a metal function, but the yield of the desired product isquite small (Xiao,et al., 2016).SAPO-5 with AFI topology has also found its application in thehydroisomerization of alkanes which is governed by a bifunctionalmechanism (Degnan, 2014).
Acatalytic synthesis of SAPO-5 and VPI-5 Zeolite was carried todetermine the suitability of the two in catalyzing decalin. Thechemical reagents utilized were pseudo-boehmite, orthophosphoricacid, di-n-propylamine, and distilled water (Junzo,2012).After the synthesis was done, an autoclave was quenched, and thecrystalline product was then filtered and cleaned with the distilledwater. The sample prepared was then allowed to cool for 12 hours atambient temperatures and then dried for 15 hours at temperatures of318 k (Xiao,et al., 2016).It was discovered that SAPO-5 yielded more decalin than VPI-5 due tothe presence of Bronsted acid sites (Liu,et al., 2012).Whentemperatures are increased, the activity of SAPO-5 increases rapidlyand hence resulting in more yield whereas, the activity of VPI-5remained constant with an increase in temperatures (Liu,et al., 2012).Thismakes SAPO-5 more advantageous over VPI-5. The catalytic activity ofSAPO-5 is mainly accredited to the hydrogenation and dehydrogenationfunction of Platinum (Pt) and Iridium (Ir) that expedite theisomerization of decalin (Hoke,et al., 2016)mainlythrough the bifunctional mechanism. The conclusion of the experimentwas that SAPO-5 catalysts are more suitable for decalin isomerizationand thus more appropriate for increasing the octane number of dieselfuels (Moliner,2012).SAPO-5 catalyst displayed a high ring opening activity and thusincreasing its suitability in the process of increasing octane numberof diesel fuels (Zhang,et al., 2013).
2.Esterification Reaction (Ethanol+Acetic Acid = Ethyl Acetate)
Ina simple definition, esterification refers to the reaction betweenalcohols and carboxylic acids to produce esters (Zhang, et al.,2013). Esters are compounds that are obtained from carboxylic acids.The structure of Carboxylic comprises of the –C00H group, and in anester, the hydrogen molecule in is substituted by a hydrocarbon groupof a particular type (Swathi& Sebastian, 2013).Esterification reaction that results from the Lewis and Bronsted acidcatalyzed the reaction of carboxylic acids with alcohols, andequilibrium was created (Lee, et al., 2015). Thereagents in the equation above are at equilibrium this implies thatremoving one product from the reactants side or using an excess ofone of the reactants, the equilibrium can significantly be affected.Esterification reaction requires a lot of energy for the removal ofthe Hydroxyl ions from the carboxylic acid. It, therefore, requires acatalyst and heat for it to occur (Liu, et al., 2012).Withthe necessary catalyst and heat, the hydroxyl ions can be removed andthe oxygen molecule linked to the carbon. Since oxygen was alreadypresent in the carbon, the connection of the other oxygen to thecarbon makes the carbon have two oxygen molecules connected on bothsides and this form an ester (Singh,et al., 2016). The general formula for an ester is as shown below.
IndustrialApplication of Esterification
Esterificationreaction has a broad range of uses in the chemical industry.Themain useof esterification is the making of ethyl acetate (Hoke, et al.,2015). Ester or Ethyl Acetate is characterized by a sweet smell andtherefore it has a broad spectrum of uses. The group of compoundscalled esters all have a pleasant smell. Ester is used in the makingof artificial fruit extracts and fragrance enhancers, and artificialflavors (Hoke, et al., 2015).EthylAcetate is also used as a solvent in various applications such as inthe making of varnishes and paints.Anotherindustrial application is the making of polyesters. There has been adevelopment of polyesters since the 19thCentury (Profile on the production of Ethyl Acetate, 2012).Esterification reaction is responsible for the preparation ofpolyesters which have a variety uses. One of the methods of preparingPolyesters is by the exchange reaction between ester and hydroxylgroups a process often referred to as alcoholysis (Mascia, et al.,2013). Without a catalyst, alcoholysis occurs at a slower pace evenwhen the temperatures are very high (Nielsen,2012). Examplesof catalysts used in the process are lead (Pb), Zinc(Zn),Magnesium(Mg), and Cobalt(Co). Recently, SAPO-5 have also been usedto catalyze the reaction. Polyesters find their uses in severalfields such as in the making of textile fibers, bottles, plastics andresins (Laura,2014).Polyesters have the characteristic nature of biodegradable polymerswhich have already been commercialized (Research and Markets, 2016).These have a variety of uses and the most popular being in thehealthcare industry. Polymers are used in the manufacturing ofprosthetic devices wich include bone plates and orthopedic pins andscrews, intravascular stents, artificial blood vessels and drugdelivery systems for controlled release among others. Thebiodegradable property of these materials has led to an increase indemand for eco-materials (Laura,2014). Esterificationis also used in modifying oils and fats to form better products. Oilsand fats are classified as triglycerides and are obtained from plantsand animal products. Esterification is used to modify their chemicalcomposition through ester exchange and hydrogenation, and the resultis an increase in utility of such products (Mary,2016).Esterificationalso finds its use in food emulsifiers. Mixtures of mono anddiacylglycerols are made by a process of transesterification ofglycerol and fatty acids. Esterification can also produce soaps. Inmaking soaps, oils and fats are transesterified to produce methylesters of fatty acids after which they are subjected tosaponification to deliver alkali salts of fatty acids and menthol(Wang, et al., 2013). Thedemand for ethyl acetylene was estimated to be 60 tons per year in2012 (Profile on the production of Ethyl Acetate, 2012). This needwas forecasted to hit 95 tons by the year 2018. A recent report byMary shows that INEOS, one of the companies in Switzerland is workingon an expansion of its plant in Hull to boost production of EthylAcetate by 100, 000 tons per anum (Mary, 2016). The decision wasreached after a market analysis which estimated the demand for EthylAcetate to rise drastically. The cost of production of Ethyl Acetateis high due to power consumption and use of expensive materials andequipment. However, the output supersedes the production cost.Previous work on the use ofMolecular Sieves to Catalyze Esterification Asnoted earlier, esterification requires heat and a catalyst for it tooccur. Examples of typical catalysts that are used includeConcentrated sulphuric acid and hydrochloric acid. The previousstudies were based on the chemical structure of the alcohol, acid andthe acidic catalyst (Wang, et al., 2013). Therateof esterification is greatly influenced by the catalyst used. Whensimple alcohols are used, the reaction occurs faster since they arerelatively small and do not contain carbon atom that may stop theirreaction (Otera, 2011).Thereare factors to be considered for the choice of catalysts to be usedfor esterification. Apart from the typical catalysts, Lewis, andbronsted acids have been used as catalysts. In small scale laboratoryproduction, sulphuric acid and hydrochloric acid are commonly used.The other types of catalysts used are cation-exchange resins andzeolites (Otera, 2011).Theapplication of acid-regenerated cation resin exchangers to replacethe convention catalysts has unique advantages (Duan, et al., 2012).Zeolite can be used as an acid regenerated catalyst. Although suchcatalysts are more costly compared to the conventional ones, thecation exchangers have several characteristics that make their usemore economical (Moliner, 2012). Inan experiment to find out the effect of molecular sieves onlipase-catalyzed esterification of rutin with stearic acid, the rutinwas mixed with stearic acid in the absence of molecular sieves (Duanet al., 2012). The structure of rutin stearate was subjected to somemethods to check its suitability in the reaction. It was passedthrough a series of spectral methods of Fourier transform infraredand UV, and the results showed that regioselectivity of thelipase-catalyzed esterification of rutin occurred specifically at theC (4III)position of the rhamnose functional groups (Duan et al., 2012). Tocheck the effect of molecular sieves on the reaction, a small amountwas added and it was found that adding molecular sieves led to anincrease in both the rate of reaction and the yield (Mascia, et al.,2013).
HowSilicoaluminophosphates (SAPO-5) can catalyze Esterification
Silicoaluminophosphate(SAPO-5) being one of the molecular sieves, can be used to catalyzeesterification reaction. SAPO-5 has numerous acidic sites which makeit suitable for catalyzing the reaction (Jhung,et al., 2011).Accordingto Terasakaet al., (2015), the acidity of SAPO-5 increases as more zeolite ionsare substituted into its framework. For esterification to occur,hydroxyl ions are supposed to be removed from the alcohol. SAPO-5catalyzes esterification with the help of cations exchange (Pathak,2015). As Yadav & Sakthivel (2014) states, SAPOs can be regardedas the substitution of silicon ions into the framework ofAluminophosphates (ALPOs). The silicon incorporation to the frameworkof SAPO-5 gives it bronsted acidity which is responsible for thecatalytic property (Yadav & Sakthivel, 2014). The esterificationprocess using an acid catalyst is summarized below.
SAPO-5is an example of a solid acid catalyst and therefore it does notdissolve in the solution (Swathi,& Sebastian, 2013).When a solid acid catalyst is employed, the rate of acceleration isaided by the catalyst only. This happens by shifting the chemicalequilibrium between two the solvent in use and the AH to form the SH+species (Degnan, 2014).
S+ AH → SH++ A−
Silicoaluminophosphatesare important molecular sieves that are of enormous significance inthe chemical industry. Studies have been done to determine theirsuitability for various industrial applications and, although therehas been a milestone, there need for more studies to be carried out.One of the controversies is the effect of substitution of siliconions to the framework of SAPO-5 where two studies discussed in thepaper show differences in results. This calls for more experiments soas to validate the results.
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