Vol.:(0123456789)1 3

Catal Surv Asia DOI 10.1007/s10563-017-9226-1

Sulfonic Acid-Functionalized Solid Acid Catalyst in Esterification and Transesterification Reactions

Shagufta1 · Irshad Ahmad1 · Rahul Dhar2 

© Springer Science+Business Media New York 2017

catalyst, such as equipment corrosion, release of toxic resi-dues causing environmental issues, purification and separa-tion of desired product and recovery of the catalyst. There-fore, in recent years’ continuous efforts have been made by investigators around the world to replace the conventional homogeneous acid catalysts by environmentally friendly and efficient heterogeneous catalyst [1, 2].

In 1992, the discovery of M41S mesoporous silica and later its application as a catalyst support in heterogene-ous catalysis due to a high specific surface area, ordered mesoporous structure, high thermal stability and large pore volume motivated researchers to develop more efficient mesoporous materials [3]. This leads to the discovery of SBA-15 which is a mesoporous silica material with well-ordered hexagonal structure, large pore size, high surface area, and high thermal stability [4]. The functional groups can easily be introduced into the mesopourous framework of SBA-15 by grafting or co-condensation and this makes it efficient solid catalysts with improved catalytic properties as compared to conventional homogeneous and heteroge-neous catalysts. Additionally, functioned SBA-15 materi-als are likewise used as adsorbents [5, 6], sensors [7, 8], support for chiral catalysts [9–11] and carrier materials in the drug delivery [12, 13]. The different organic func-tional groups such as amines, carboxylic acids, phosphoric acid, amino acids, and isocyanate etc. have been introduced successfully on the internal surface of SBA-15 for differ-ent applications [14–19]. Furthermore, in the recent years, researchers are focused on the development of solid acid catalysts that resulted in the development of various inor-ganic/organic materials, including ion-exchange resins hav-ing sulfonic acid groups [20], metal-containing molecular sieves [21], sulfated or mixed oxides [22], heteropoly acids [23], metal–organic frameworks-based solid acids [24, 25], acid functionalised silica/mesoporous silica [26, 27],

Abstract The recent development of an environmentally benign solid acid catalyst has been a relatively cutting-edge area of research in the synthesis of value added esters and biodiesel. Solid acid catalysts are economically viable, effective, and environmentally amicable than conventional homogeneous catalysts and reusability of the catalyst is another advantage of these catalysts. The applicability of sulfonic acid-functionalized solid acid catalysts in the well-known esterification and transesterification reactions for the synthesis of esters and biodiesel, respectively along with their reusability aspect are discussed in this review.

Keywords Sulfonic acid functionalized solid catalysts · Esterification · Heterogeneous catalysts · Transesterification

1 Introduction

The liquid phase of sulfuric acid is used as homogeneous catalyst in many chemical reaction processes for the syn-thesis of different chemical products and is of great indus-trial application. However, several drawbacks are associ-ated with the application of liquid phase sulfuric acid as a

* Shagufta shagufta.waseem@aurak.ac.ae

* Irshad Ahmad iahmad@aurak.ac.ae

1 Department of Mathematics and Natural Sciences, School of Arts and Sciences, American University of Ras Al Khaimah, Ras Al Khaimah, United Arab Emirates

2 Department of Chemical and Petroleum Engineering, School of Engineering, American University of Ras Al Khaimah, Ras Al Khaimah, United Arab Emirates

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carbonaceous acidic materials [28, 29], zeolites [30] and so on.

Environmentally benign sulfonic acid modified solid acid catalysts have been successfully applied for various organic transformations such as etherification, esterifica-tion, dehydration, oxidation, acetylation, acetoxylation and silylation reactions etc. Due to its higher acid strength, higher surface area, large pore space and increased thermal stability, which makes it an efficient catalyst for organic reactions [31]. In recent years a number of reviews have been published discussing the functionalized mesostruc-tured materials and their application in organic transforma-tions [32–35]. In this review, we have particularly focused on the synthesis and application of sulfonic acid function-alized solid acid catalysts applied in the esterification and transesterification reaction and the literature of the last two decades have been included.

2 Sulfonic Acid Functionalized Solid Acid Catlyst in Esterification Reaction

Esterification is an important class of organic reactions where an ester is prepared from organic acids and alcohols. Esters are commercially very essential organic compound which is used as fragrances, industrial solvents for phar-maceutical manipulations, lubricants and plasticizers [36, 37]. Furthermore, these days the esters are considered as an important substitute fuels (biodiesel) for the replace-ment of fossil fuels [38]. The esterification reaction is an acid catalyzed reaction and previously liquid sulfuric acid has been widely used as a catalyst in this chemical reaction. However, liquid sulfuric acid has several drawbacks, such as high toxicity, corrosion, and difficult to separate from the reaction mass [39, 40]. Therefore, design of environmen-tally friendly and efficient solid acid catalysts for replac-ing the liquid acid catalysts was the focus of the research-ers [41–43]. Thus, in the recent years solid sulfonic acid catalysts based on porous silicas, solid organic/inorganic composites and carbons have been reported in the literature for esterification reaction. To produce solid sulfonic acid catalysts through binding of sulfonic acid (−SO3H) groups, generally organic based solids are preferred because they are more hydrophobic than inorganic solids. In this section we have discussed the sulfonic acid functionalized solid acid catalysts which have been reported recently for esteri-fication reaction.

In 1999, T Salmi and his coworkers performed the com-parative study of polyvinylbenzene and polyolefin-sup-ported sulphonic acid catalysts for esterification of acetic acid with methanol.

The fibrous polymer-supported sulphonic acid catalysts (Smopex-101) showed second-order rate constant value

higher than the traditional polyvinylbenzene-supported cat-alyst (Amberlyst 15) [44]. Later in 2002, fibrous polymer-supported sulphonic acid catalyst activity for esterification reaction of the propanoic acid with methanol evaluated and advanced kinetic model was developed using Amberlyst 15 as reference [45]. The fibrous polymer-supported sulphonic acid catalyst, Smopex-101 was very active in all the experi-ments and showed a higher reaction rate comparison to conventional resin catalyst (Amberlyst 15). The esterifica-tion rate increases with the increase of the temperature and initial molar ratio of propanoic acid and methanol whereas no effect was observed with the change in the degree of cross-linking and fibre dimensions. The catalyst showed good durability and after four successive experiments the catalyst activity and capacity remains constant. Based on the adsorption of the acid and water a new rate equa-tion was suggested, which explained the recorded reaction kinetics correctly. Later, esterification of different acids with alcohols were performed in the presence of fibrous polymer supported sulfonic acid catalyst, Smopex-101 (Fig. 1) and comparative study was done with other cata-lysts such as ion-exchange resin, Amberlyst 15 and homo-geneous catalyst, liquid HCl [46]. In addition the effect of different molar ratios between the acid and alcohol on the reaction rate was investigated and structural relationships in kinetics were established. It was observed that the increase of the alcohol or carboxylic acid chain length and branch-ing of the alcohol chain have a detrimental effect on the rate of reaction. The rate constants of different acids with alcohols were dependent on the substitution effect of react-ing molecules according to the Taft equation. Among both reactants that is alcohol and acid, only substituent effects of different alcohols followed the Taft relationship. The sim-ple second-order model and a more advanced adsorption-based model were used to model the experimental results. The adsorption-based model was superior to the second-order model because it included the adsorption of carbox-ylic acid and water and was constant with structure–activity relationship.

Considering the effect of chemical properties and porosity of mesoporous-types of catalysts for the esterifi-cation reaction I. D´ıaz et al. in 2001 decided to improve the activity and selectivity of catalyst by increasing strong acid site density and a narrow pore size distribu-tion. Therefore the synthesis of thiol-MCM-41 materi-als from gels containing cationic surfactants with differ-ent chain length was reported [47]. The powder X-ray diffraction (PXRD) and transmission electron micros-copy (TEM) measurements suggested that the materi-als obtained from this modified gels present a higher order in the channels arrangement with higher sur-face area. Additionally, the pore size of these materi-als was decreased, possibly due to close packing of the

Sulfonic Acid-Functionalized Solid Acid Catalyst in Esterification and Transesterification…

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propylthiol chains protruding from the walls into the channels. Further the incorporated thio groups of these materials were oxidized with hydrogen peroxide to pro-duce corresponding sulfonic acid (SO3H-MCM-41) with strong acidity. The SO3H-MCM-41catalyst was evaluated for the esterification of glycerol with fatty acids such as oleic and lauric acids (Fig.  2). The results revealed that the SO3H-MCM-41 preparation with mixtures of sur-factants are more selective to the monoglycerides in the

esterification of glycerol with fatty acids compared with the standard materials, because of higher order in the channels packing of these catalysts.

To improve the physical and chemical properties of mesoporous materials several researchers incorporated functionalized organic groups either by grafting on the preformed mesopore surface or by co-condensation dur-ing synthesis. These sulfonic acid groups functionalized organic–inorganic hybrid mesoporous silicas showed

Fig. 1 Esterification of dif-ferent acids with alcohols in the presence of catalyst, Smopex-101 [46]

RC

O

O

H +R'

OH R

CO

O

R' + H2OCatalyst

SO3H

Smopex 101Styryl sulfonic acid grafted fibre

R = -CH3; -CH2CH3; -CH2CH2CH2CH3

R' = -CH3; -CH2CH3; -CH2CH2CH3; -CH(CH3)2; -CH2CH2CH2CH3; -CH(CH3)CH2CH3

Carboxylic acid Alcohol Ester

Fig. 2 Esterification of glycerol with fatty acids such as oleic and lauric acids in the presence of SO3H-MCM-41catalysts [47]

SO3H-MCM-41

SO

3 H

SO3 H

SO3H

SO 3H SO

3H

SO

3H

HC +

C

O

HO R

C

O

HO R

C

O

HO RH2C OH

OH

H2C OH

Catalyst

H2O

HC

H2C O

O

H2C O C

O

R

C

O

R

C

O

R

Glycerol Fatty acid Triacyl glycerol

Fatty acid = Oleic acid; lauric acid

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promising results shown successful results for acid-cata-lyzed reactions. Considering these reports I. K. Mbaraka et  al. in 2003 synthesized organosulfonic acid (propylsul-fonic and arenesulfonic groups) functionalized mesoporous silicas in a one-step by co-condensing inorganic–organic reagents in the presence of different surfactant templates with in situ oxidation of the thiol groups to the sulfonic acid groups [48]. The synthesized materials bearing propylsul-fonic groups and arenesulfonic groups were tested for their catalytic performance in the esterification of fatty acid with methanol to produce methyl esters (Fig.  3) and compared with several commercial catalysts such as sulfuric acid, p-toluene sulfonic acid, Nafion NR50 and Amberlyst-15. The organosulfonic acid-functionalized mesoporous silicas exhibited maximum reactivity compared to commercially available solid acid catalyst for fatty acid esterification reaction. The results demonstrated that the performance of the functionalized mesoporous material was dependent on two factors that is the median pore diameter of the catalyst and on the acidic strength of the organosulfonic acid group. The results showed the possibility of designing catalysts rationally using organic–inorganic mesoporous materials.

The reports of surfactant-templated mesoporous silicas functionalized with alkylsulfonic acid groups onto the pore surface as efficient solid acid catalysts for the esterifica-tion reaction motivated Q. Yang et al. in 2005 to increase the hydrophobicity by incorporating organic groups on the surface of sulfonic acid functionalized mesoporous silicas [49]. The sulfonic acid-functionalized benzene-silica was synthesized by co-condensation of 1,4-bis(triethoxysilyl)benzene [BTEB: (EtO)3Si-C6H4-Si(OEt)3] and 3-mercap-topropyltrimethoxysilane [MPTMS: (MeO)3Si-C3H6-SH] using a surfactant template followed by post-synthesis oxidation using HNO3. In the synthesized bifunctional mesoporous material the sulfonic groups were attached to the silica layers of the crystal-like periodic pore walls. Bearing in mind the benefits of periodic porous surface in the selectivity and activity of the catalyst, the sulfonic acid functionalized mesoporous benzene-silica with crystal-like wall structure were used in the esterification of acetic acid with ethanol (Fig. 4). The synthesized catalyst exhib-ited significant catalytic activity compared to Nafion and it was proposed that increased catalytic activity is due to the unique surface properties and crystal-like wall structure.

In 2008, W. Zhao et  al. reported the synthesis of are-nesulfonic functionalized SBA-15 (SBA-15-Ph-SO3H) by direct condensation of tetraethyl orthosilicate (TEOS) and 2-(4-chlorosulfonylphenyl) ethyltrimethoxysilane (CSPTMS) under the acidic condition [50]. The successful incorporation of arenesulfonic groups into the channel of SBA-15 was confirmed through PXRD, nitrogen sorption, elemental analysis and thermal desorption. The SBA-15-Ph-SO3H showed good catalytic activity in the esterifica-tion of ethyl caprylate (Fig. 5) and the maximum conversion was obtained from the sample with the less arenesulfonic group and better channel structure. It was observed that all the samples have similar selectivity of ethyl caprylate and the reaction requires only catalytic amount of strong acid.

The organic–inorganic hybrid silicas have been reported with wide applications and specifically polymer/silica

RC

O

OH

+ CH3OH

RC

O

OCH3

catalyst+ H2O

Fatty acid

Fatty acid = palmitic acid;refined soyabean oil

SiO

2

OOO

Si

SO3H

SiO

2 OOO

Si SO3H

MethanolMethyl esters

Fig. 3 Esterification of fatty acid with methanol in the presence of organosulfonic acid functionalized mesoporous silicas [48]

Fig. 4 Esterification of acetic acid with ethanol in the pres-ence of sulfonic acid functional-ized mesoporous benzene-silica catalyst [49]

H3CC

OH

O

+CH2

OH

H3CC

O

O

H2C + H2O

OOO

Si SO3H

Catalyst

SiSi

Acetic acid

H3C

CH3

etatecalyhtElonahtE

Sulfonic Acid-Functionalized Solid Acid Catalyst in Esterification and Transesterification…

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nanocomposites are mostly explored due to dual benefits from both the mesoporous silica and polymer. C. Li et al. in 2009 synthesized sulfonic acid-functionalized mesoporous organic–inorganic hybrid materials through atom transfer radical polymerization (ATRP) method [51]. For the syn-thesis of poly-p-styrenesulfonic acid SBA-15 nanocompos-ites (PSS-SBA-15), SBA-15 material was first grafted with 4-(chloromethyl) phenyltrimethoxysilane followed by sur-face-initiated polymerization of sodium p-styrenesulfonate inside the pore wall of functionalized SBA-15 by copper-mediated ATRP technique via a silane initiator, [4-(chlo-romethyl)phenyl]trichlorosilane (CTS)]. With the help of FT-IR, nitrogen sorption isotherm, PXRD, TEM and ther-mal gravimetric analysis results it was proved that the PSS-SBA-15 materials have ordered mesoporous structure, open channels and styrenesulfonic acid functionalities. The cata-lytic activity of the PSS-SBA-15 synthesized material was evaluated for the esterification reaction of lauric acid with ethanol (Fig. 6) and the results revealed that it is having a higher catalytic activity compared to commercial Nafion resin and can be reused without noticeable loss in activity. It was also suggested that the presence of styrenesulfonic acid on the porous surface of SBA-15 possibly responsible for the availability of the active site to the reactants.

Acid catalyzed esterification reaction is applied to elimi-nate the organic acids present in bio-oil by treating them

with alcohol present in bio-oil or with added alcohol. In this process heterogeneous catalyst is more advantageous than homogeneous catalyst because it makes the bio-oil processing simple and lower the manufacturing cost. Con-sidering the potential benefits of heterogeneous catalyst, S. Miao et al. in 2009 synthesized propylsulfonic acid group functionalized mesoporous silicas with different loading by using one-step co-condensation procedure [52]. The resulting materials were examined for the esterification of acetic acid with methanol (Fig.  7), which was consid-ered as a model reaction for stabilization of bio-oil. Water is an important component in bio-oil and its presence reduce esterification activity over acid catalysts and water. Additionally, water is a product of esterification reaction and thus inhibits the reaction rate and negatively impacts the reaction equilibrium. Therefore, to study the effect of water on the esterification reaction the water tolerance properties of the materials were investigated in detail. The results revealed that the sulfonic acid groups functional-ized mesoporous materials have comparable site activi-ties for acetic acid esterification to sulfuric acid (H2SO4). The effect of propylsulfonic acid loading exhibited that the intrinsic per site activity increased with the number of acid sites, which suggested the presence of acidic site coopera-tivity. The results of the water tolerance experiments proved that SO3H-SBA-15 exhibited less reaction inhibition due to

Fig. 5 Esterification of caprylic acid with ethanol in the pres-ence of arenesulfonic function-alized SBA-15 catalyst [50] O

OO

Si

SO3H

C7H15

COH

O

+HO

H2C

CH3C7H15

CO

O

H2C

CH3

Catalyst

Mesoporous wall

+ H2O

Caprylic acid Ethanol Ethyl caprylate

Fig. 6 Esterification of lauric acid with ethanol in the pres-ence of catalyst PSS-SBA-15 [51]

H3C(H2C)9H2CC

OH

O

+H3C

H2C

OH H3C(H2C)9H2CC

O

O

H2C

CH3

Catalyst

H2O

O SiO

OCH2 CH2CH Cl

SO3H

n

Ethanol Ethyl laurateLauric acid

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the presence of water than H2SO4. Analysis of the result suggested that the hydrophobic propyl groups conjoint to the sulfonic acid groups might be responsible to retard pro-ton solvation by water compared with free protons in the liquid phase from H2SO4. The stability and reusability stud-ies of the catalyst revealed that the materials have multiple cycle stability for acetic acid esterification reaction without significant loss in activity. Later the kinetics of acetic acid esterification with methanol using a propylsulfonic acid-functionalized SBA-15 catalyst was examined [53]. To study the difference between heterogeneous and homoge-neous catalyzed esterification reaction the propane sulfonic acid, a homogeneous catalyst, having the same structure as the functional groups grafted on the silica was used. On the basis of results it was observed that the esterification of acetic acid with methanol over the functionalized SBA-15 catalyst required adsorption of both acetic acid and metha-nol and followed a dual-site Langmuir–Hinshelwood type reaction mechanism. In comparison esterification reaction with the homogeneous catalyst followed an Eley–Rideal mechanism. For the heterogeneous catalyst system the kinetic data successfully fit using an L-H model with the surface reaction as the rate-limiting step.

M. L. Testa et al. in 2010 prepared propyl sulfonic func-tionalized silica catalysts by implementing three different procedures and that is grafting, co-condensation and in situ oxidation [54]. During the grafting and co-condensation method the oxidation of the precursor thiol groups (–SH) were performed by hydrogen peroxide whereas in situ oxi-dation method includes the oxidation by hydrogen perox-ide during the condensation reaction. The acid-functional-ized silica materials were characterized by N2 adsorption/desorption measurements and by X-ray photoelectron spectroscopy and evaluated for the esterification reaction

(Fig. 8). The catalytic result revealed that the sample syn-thesized by the in situ oxidation method is the best catalyst for the esterification of acetic acid with 1-butanol because the material synthesized by in  situ oxidation were with higher surface area and higher acid capacity compared to those obtained with post-oxidation methods. The recycling experiments showed the amount of sulfonic group on the catalyst decreases, however the activity was quite stable. Furthermore, the same year J. A. Posada et al. synthesized four mesophases MCM-41, MCM-48 and SBA-15 with a high grade of structural ordering [55]. The functionaliza-tion of mesostructured silicas with propylsulfonic acid groups was carried out by using grafting and co-condensa-tion techniques in a sol–gel process. The synthesized mate-rials were characterized through PXRD, TGA, FT-IR, and SEM analysis. The catalytic activity of the materials was evaluated for the esterification reaction of acetic acid with 1-butanol. The catalytic performance of the supported cata-lysts prepared from the materials was dependent on meso-phase and functionalization technique. It was observed that the SBA-15 functionalized by grafting route was the post suitable catalyst to produce 1-butylacetate in reactive distil-lation (RD) column at atmospheric pressure.

X. Tian et  al. in 2011 were interested to evaluate the activity and recyclability of benzene phenol polymers and correlative carbons and thus synthesized sulfonated ben-zene polymer and sulfonated porous carbons and tested them in the esterification reaction of methanol with acetic acid (Fig. 9) [56]. The porous polymer was synthesized in the presence of Pluronic P123 template. Next carbonization of polymer at elevated temperature provided porous carbon and further sulfonation of the porous polymer and carbons produced sulfonated solids with surface –SO3H groups. The synthesized materials composition, pore structures and

Fig. 7 Esterification of acetic acid with methanol in the presence of propylsulfonic acid-functionalized SBA-15 catalyst [52, 53]

H3CC

OH

O

+H3C

OH

H3CC

O

O

CH3 + H2OCatalyst

SiO

2

OOO

Si SO3H

Acetic acid Methanol Methyl acetate

Fig. 8 Esterification of acetic acid with 1-butanol in the presence of propylsulfonic acid-functionalized SBA-15 catalyst [54]

H3CC

OH

O

+CH2

OH

H3CC

O

O

H2C

H2O

CatalystH2C

CH2

H3C

CH2

H2C

CH3Acetic acid 1- Butanol

SiO

2

OOO

Si SO3H

Butyl acetate

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surface functional groups were characterized by using ele-mental analysis, thermogravimetric analysis, and physical adsorption of nitrogen, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The char-acterization results revealed that during the carbonization process the aromatic rings in the polymer bonded together to produce carbon sheets and increase in carbonization temperature increase the size of carbon sheets and thus the polymer framework changes to carbon. In comparison to carbon framework the polymer framework observed to be higher in the density and the stability of attached –SO3H group and therefore sulfonated benzene polymer showed better catalytic performed than the sulfonated porous car-bons in the reaction of esterification of methanol with ace-tic acid.

F. Alrouh et  al. in 2012 reported an environmentally friendly direct esterification of olive–pomace oil with

glycerol in the presence of acid homogeneous and hetero-geneous catalysts [57]. By following the literature proce-dure propyl sulfonic acid group functionalized mesoporous silica MCM-41 and SBA-15 were synthesized and char-acterized by powder X-ray diffraction, N2 adsorption and the H+ exchange capacities of the sulfonic acid groups were titrated. The synthesized functionalized mesoporous silicas were used as catalyst in the esterification reaction of glycerol with olive–pomace oil (Fig.  10). The reaction was monitored through the GC, the two fatty acids exam-ple palmitic and oleic acids are considered as reactants in olive–pomace oil and their related monoacylglycerols that is glycerol monopalmitate (GMP) and monooleate (GMO) as the reaction product. The comparative study of the cata-lytic activities of the functionalized mesoporous silica and commercially available homogeneous and heterogeneous catalysts, for example p-toluenesulfonic acid (PTSA) and

Fig. 9 Esterification reaction of acetic acid with methanol in the presence of catalysts sulfonated benzene polymer and sulfonated porous carbons [56]

H3CC

OH

O

+H3C

OH

H3CC

O

O

CH3 + H2OCatalyst

H2C

H2C

H2C

OHH2C

HO

SO3H

OH H2C

OH

SO3H

SO3H

CH2

OH

HO3S

HOH2C OH

OH

SO3H SO3HOH

SO3H

OH

SO3H

OHSO3HHO

OH

HO3S

HO

HO

OH

sulfonated benzene polymersulfonated porous carbons

Acetic acidMethanol Methyl acetate

Fig. 10 Esterification reaction of glycerol with olive–pomace oil in the presence of catalysts MCM-41-SO3H and SBA-15-SO3H [57]

OOO

Si

SO3H

MCM-41 or SBA-15

HC + C

O

HO R

H2C OH

OH

H2C OH

Catalyst

H2O

HC

H2C OH

OH

H2C O C

O

R

GlycerolMonoacyl glycerol=monopalmitate and monooleate

Olive pomace oil = Fatty acid = Oleic acid and palmitic acid

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Amberlyst-15 were completed and the results revealed that acid mesoporous silica MCM-41-SO3H was more selective with the total yield of monoacylglycerols (GMO+GMP) nearly 40% and reusable at least three times without any loss of activity.

The alkyl levulinates, a non-edible biomass considered as an excellent candidate for the formulation of gasoline, diesel and biodiesel. One of the effective alternative route to produce alkyl levulinates is esterification of levulinic acid, commercially obtained from lignocellulosic biomass, with alcohols under moderate reaction conditions. J.A. Melero et  al. in 2013 screened organosulfonic acid-modi-fied mesoporous materials for the esterification of levulinic acid with different alcohols (Fig. 11). The results revealed that propyl- and arene-SO3H-modified mesostructured SBA-15 are active catalysts in the esterification of levulinic acid with ethanol, better than commercial sulfonic-acid-based homogeneous and heterogeneous acid and with reus-ability [58]. The propyl-SO3H acid with lower acid strength showed maximum activity due to lower hydrophilicity of the sulfonic-acid site microenvironment responsible for decreasing the poisoning effect of water, released from the esterification reaction. In the presence of propyl-SO3H SBA-15 catalyst the lower alcohols with less steric inhibi-tion showed a better conversion of levulinic acid compared to higher alcohols such as iso-propanol and 2-butanol. The catalyst was reusable up to three times and showed high catalytic activity without any regeneration treatment.

S. Jeenpadiphat et  al. in 2015, functionalized the mesoporous silica material with propylsulfonic acid (Pr-SO3H) group by using post synthesis grafting with

3-mercaptopropyltrimethoxysilane as a propyl-thiol pre-cursor [59]. The analytical techniques such as PXRD, nitrogen sorption and titration were used to substantiate the presence of Pr-SO3H functional groups into and on the mesoporous silica material. After the Pr-SO3H func-tionalization, no alteration was observed in a well ordered hexagonal mesoporous structure of all the materials. The catalytic properties of Pr-SO3H-functionalized mesoporous silicas were evaluated in the esterification of oleic acid with methanol and glycerol (Fig.  12). The catalytic effect of the pore size of three different types of mesoporous sil-ica SBA-15 (rope, rod and fiber) on the substrate conver-sion and product yield were studied and compared with commercially available Amberlyst-15 and the small pore sized MCM-41. The results revealed that the synthesized heterogeneous acidic catalysts were extremely active due to the presence of the sulfonic acid groups and the prod-uct composition could be adjusted by selective choice of the mesopore size. The conversion of substrate and product yield is governed by different factors such as acid strength, specific surface area, catalyst pore size and product molec-ular size. The highest catalytic activity for the esterification of oleic acid with either methanol or glycerol was observed with Pr-SO3H-functionalized rope-shaped SBA-15 and all the synthesized mesoporous supports were highly effective in comparison to commercial Amberlyst-15 catalyst and non-porous silica.

Considering the significance of sulfonic acid-function-alized solid silicas in several catalytic reactions, Z. Hasan et al. in 2015 synthesized sulfonated silica catalyst by sim-ple, inexpensive and one-step in situ method from tetraethyl

Fig. 11 Esterification of levulinic acid with different alcohols in the presence of catalysts propyl- and arene-SO3H-modified mesostructured SBA-15 [58]

CH2

COH

O

H2C

CO

CH3

+R

OH + H2O

CatalystCH2

CO

O

H2C

CO

CH3

R

or

Levulinic acid

R = -CH3; -CH3CH2; -CH(CH3)2; -CH(CH3)CH2CH3

Ester

SiO

2 OOO

Si SO3H

SiO

2 OOO

Si

SO3H

Alcohol

Fig. 12 Esterification of oleic acid with methanol and glycerol in the presence of catalyst pro-pyl sulfonic acid functionalized SBA-15 [59]

R OH

O

+R'

OH

R O

O

R' + H2OCatalyst

Oleic acid Methanol; Glycerol Ester

SiO2

OOO

Si SO3H

Sulfonic Acid-Functionalized Solid Acid Catalyst in Esterification and Transesterification…

1 3

orthosilicate and chlorosulfuric acid [60]. The synthesized catalyst was applied for the liquid phase esterification of oleic acid with methanol under both conventional oil bath (COB) and microwave (MW) heating conditions (Fig. 13). The ester yield was over 93% in both COB and MW heat-ing, condition however in comparison to COB heating the MW heating showed around 20 times enhanced kinetics. The catalyst exhibited stability under harsh condition and was effective after hydrothermal treatment at temperatures up to 125 °C. Additionally, the catalyst was reusable after the third run of the esterification of oleic acid with insig-nificant loss in activity. Furthermore the study included effects of different reaction parameters such as reaction time, reaction temperature, the amount of catalyst, and type of alcohol. To prove the versatility of the synthesized cata-lyst, the acetylation of salicylic acid was performed and the obtained yield of aspirin was competitive to those of the other solid catalysts.

In view of the catalytic activity of sulfonic acid-func-tionalized metal organic frameworks (MOFs) for the esteri-fication of butanol [61], Z. Hasan et al. in 2015 synthesized porous and acidic, metal–organic framework (MOF), MIL-101(Cr)-SO3H hydrothermally in one step using sulfonic acid-functionalized ligands [62]. The synthesized mate-rial was successfully used in the liquid phase esterifica-tion of oleic acid with methanol (Fig. 14) as well as in the vapor-phase dehydration of 2-butanol. After the filtration of the catalyst there was no increase in the esterification yield, suggesting that esterification is catalyzed in the het-erogeneous mode. The catalyst for esterification was reus-able, which is confirmed by very less decrease in yield up to third run. Additionally, esterification reaction was also performed under microwave (MW) irradiation condition, showing decrease in reaction time for the similar yield.

One of the simplest and effective methods to improve the bio-oil stability is to remove the acetic acid, a major impurity from the bio-oil through esterification reaction. J.

C. Manayil et  al. in 2016 synthesized propylsulfonic acid functionalized SBA-15 (PrSO3H/SBA-15) by following the literature procedure i.e. post modification of SBA-15 with mercaptopropyltrimethoxysilane (MPTMS) [63].The mate-rial showed excellent water tolerance and was investigated for acetic acid esterification with benzyl alcohols in toluene as a simulated bio-oil medium (Fig. 15). The rate of benzyl acetate production was maximum for a saturation adlayer, proportional to the PrSO3H surface coverage, works effi-ciently under acid-rich conditions with aromatic alcohols and active even in the high concentration of water and well-suited to the upgrading of pyrolysis bio-oils.

Organic sulfonic acid functionalized hexagonal mesoporous silicas (HMSs) are considered as a promis-ing catalyst in the esterification reaction because of their wormhole-like porous structure with high surface area and strong sulfonic acid groups. However it was observed that the residual silanol groups left on the surface of the mate-rial after oxidation process were responsible for.

increasing hydrophilicity and re-adsorption of H2O on the acid sites and thus retarded the reaction [64].

Fig. 13 Esterification of oleic acid with methanol in the pres-ence of sulfonated silica catalyst [60]

RC

OH

O

+R'

OH

RC

O

O

R' + H2OCatalyst

Oleic acid

R' = -CH3; -CH2CH3

SiOO

O

O SO3HSi

Si

Si

COB and MWAlcohol Ester

RC

OH

O

+HO

CH3

RC

O

O

CH3 + H2OCatalyst

Oleic acid

SO3H

S

O

O

OH

Cr Cr

Cr Cr

Methanol Ester

Fig. 14 Esterification of oleic acid with methanol in the presence of catalyst MIL-101(Cr)-SO3H [62]

Shagufta et al.

1 3

Considering the above facts in 2016, S Nuntang et  al. increased the hydrophobicity by dispersing natural rubber (NR) in the wormhole-like mesostructure of HMS (NR/HMS) and synthesized propylsulfonic acid-functionalized natural rubber (NR)/hexagonal mesoporous silica (HMS) nanocomposites (NR/HMS-SO3H) with different acid con-tents using in  situ sol gel process [65, 66]. Additionally, the catalytic activity of NR/HMS-SO3H in the esterifica-tion of model carboxylic acids and palm fatty acid distillate (PFAD) with ethanol (Fig. 16) was explored and the results compared with SAC-13 and HMS-SO3H [67]. The NR/HMS-SO3H composites showed a wormhole-like frame-work with high mesoporosity and enhanced wall thick-ness and hydrophobicity. Therefore, when compared with the HMS-SO3H materials the acid sites of these catalysts are expected to be less poisoned by H2O. Additionally, the increased hydrophobicity of NR/HMS-SO3H catalysts pro-moted the initial rate and the acid conversion, especially in the esterification with long hydrocarbon chains such as octanoic acid and lauric acid. In comparison to com-mercial Nafion/silica composite solid acid catalyst (SAC-13) and conventional propylsulfonic acid-functionalized HMS (HMS-SO3H) the catalytic activity results of NR/

HMS-SO3H composites were promising and the NR/HMS-SO3H catalyst can be successfully regenerated at least four times.

3 Sulfonic Acid Functionalized Solid Acid Catalyst in Transesterification Reaction

Biodiesel is a mixture of mono-alkyl esters of vegetable oils or animal fats and is a suitable alternative to conven-tional diesel fuel. It is classified as renewable and biode-gradable fuel, which causes less air pollution and thus helps us in our goal to be independent of the fossil fuels [68, 69]. The trans esterification is a most common method used for the production of biodiesel. The reaction of tri-glyceride molecules present in vegetable oils or animal fats with short chain alcohols such as methanol and ethanol in the presence of catalyst produces biodiesel. The properties of biodiesel fuel make it a promising substitute for diesel fuel; however the production of biofuel is not economically feasible due to high raw material and production costs [70, 71]. An alternative solution is waste oil and grease which is significantly less expensive than food-grade oil [72–74].

Fig. 15 Esterification of acetic acid with benzyl alcohols in the presence of catalyst propyl sulfonic acid functionalized SBA-15 [63]

H3C OH

O

+ H3C O

O

Catalyst

HO H2O

Acetic acid Benzyl alcohol Ester

SiO

2 OOO

Si SO3H

Fig. 16 Esterification of dif-ferent acids with ethanol in the presence of catalyst NR/HMS-SO3H catalyst [65, 66] OO O

Si

HO3S

OOOSiHO

OOOSi

SO3H

Natural rubber

Mesosilica wall

RC

OH

O

+ CH2

OH

RC

O

O

H2C + H2O

Catalyst

Carboxylic acid = acetic acid;octanoic acid; lauric acid

H3C

CH3Ethanol

Ester

Sulfonic Acid-Functionalized Solid Acid Catalyst in Esterification and Transesterification…

1 3

The transesterification reaction is mainly catalyzed by alka-line catalyst such as sodium hydroxide and sodium meth-oxide however, due to the homogeneous nature of catalyst numerous drawbacks are associated such as acid neutrali-zation, water washes and separations. Furthermore the free fatty acid and water present in oils, greases and low-cost feedstock or produced from the hydrolysis of oils and fats generally reacts with a catalyst to produce a soap that decreases the reaction rate and product yield and compli-cate the purification and therefore increases the purification costs [75, 76]. To overcome these disadvantages the hetero-geneous acid catalysts are an attractive alternative because they simplify catalyst removal, minimize the amount of waste formed and reduce the catalyst cost. Additionally, heterogeneous acid catalyst can simultaneously catalyze esterification of free fatty acids (FFAs) and transesterifica-tion of triglycerides [77, 78]. For the transeserification a large number of solid catalysts, including cation exchange resins [79–82], supported heteropolyacids [83–86] sul-fated or tungstated zirconia [87–90], and solid acids such as sulfonic acid functionalized mesoporous silica have been reported in the literature [81, 91]. Special attention has been devoted to sulfonic groups containing catalysts in par-ticular organosulfonic-silica solids that have been proved to be active at moderate temperatures and pressure [34, 92]. In this section we summarized the sulfonic acid functional-ized solid acid catalysts reported recently in the literature for the transesterification reaction.

Encouraged with the reports of utilizing sulfonic acid-functionalized carbohydrate-derived catalysts in the trans-esterification of vegetable oils, J. A. Melero et al. in 2009 applied sulfonic acid-functionalized SBA-15-type materials

in biodiesel production [93]. The sulfonic acid-functional-ized SBA-15-type materials were used as catalysts in the transesterification of various vegetable oils, both refined and nonrefined, and their reusability was also studied. The results showed that sulfonic acid functionalized SBA-15 was capable of producing biodiesel in substantial amount under relatively mild conditions with refined and crude vegetable oils as feedstock. This excellent catalytic perfor-mance was mainly due to the large surface area and pore diameter of the mesoporous support as well as the mod-erate acid strength of acid sites. Next, to demonstrate the importance of sulfonic acid-functionalized SBA-15-type materials to the one-step production of biodiesel from FFA-containing vegetable oils, the catalytic activity of pro-pylsulfonic acid-functionalized SBA-15 (Pr-SBA-15), are-nesulfonic acid-functionalized SBA-15 (Ar-SBA-15) and perfluorosulfonic acid-functionalized SBA-15 (F-SBA-15) were analyzed in the simultaneous esterification of FFA and transesterification of triglycerides of crude palm oil with methanol (Fig. 17) [94, 95]. The catalysis results revealed that for the esterification of free fatty acids and transesterifi-cation of triglycerides with methanol the most active mate-rial were sulfonic acid-modified SBA-15, active than con-ventional ion-exchange sulfonic resins (Amberlyst-36 and SAC-13). The yield of fatty acid methyl esters (FAMEs) was improved by using the mesostructured support and by increasing the acid strength of the catalytic site. The propyl -SO3H and arene-SO3H-modified mesostructured materi-als having Si-C bond showed promising catalytic activ-ity, stability and reusability compared to perfluorosulfonic acid-functionalized SBA-15 bearing Si–O–C bond. The arenesulfonic acid-functionalized mesostructured SBA-15

Fig. 17 Transesterification of triglycerides with methanol in the presence of sulfonic acid-modified SBA-15 catalysts [94, 95]

H2C

HC

H2C

O

O

O

CO

CO

CO

R1

R2

R3

+ 3CH3OH

H3C

H3C

H3C

O

O

O

CO

CO

CO

R1

R2

R3

+

H2C

HC

H2C

OH

OH

OH

Catalyst

Triglyceride(Vegetable oil)

Methanol Mixture of fatty esters "Biodiesel"

Glycerol

SiO2

O O O

Si

SO3H

SiO2

O O O

Si

SO3H

SiO2

O O O

Si

O

SO3H

FFF3C

F

Shagufta et al.

1 3

catalyst bearing combination of the textural properties of mesostructured SBA-15 silica with relatively strong arene-sulfonic acid sites has shown maximum catalytic activity in the simultaneous esterification of FFA and transesteri-fication of triglycerides to yield FAME using crude palm oil as feedstock. The catalyst was reusable after two con-secutive runs and retains its catalytic activity. Additionally, for improving the catalytic performance different factors such as temperature, catalyst loading and methanol to oil molar ratio were studied in details. The additionally cata-lytic performance of the arene-SO3H SBA-15 catalyst was increased by introducing hydrophobic trimethylsilyl groups which effectively increased the reaction rate while retaining the stability at least for a second catalytic run. Addition-ally, various lipids wastes and low-grade oils and fats have been characterized and screened as feedstocks for the are-nesulfonic acid-functionalized SBA-15 silica (Ar-SBA-15) catalyzed production of FAME [96]. The characterization result showed that these materials have significant contents of free fatty acids, Na, K, Ca, Mg, P, unsaponifiables mat-ter and humidity. The catalytic results of Ar-SBA-15 were promising and the yield of FAME was around 805 in the acid-functionalized SBA-15 silica catalyst has provided yields to FAME close to 80% in the simultaneous esteri-fication–transesterification of the different feedstocks. The yield was independent of the nature and properties of feed stock when using methanol under the following reaction conditions: 160 °C, 2 h, methanol to oil molar ratio of 30, 8 wt% catalyst loading, and 2000 rpm stirring rate. The pres-ence of a number of impurities such as unsaponifiable mat-ter, water, phosphorous and metals in low grade oils and fats have a negative effect on the yield of FAME and effect the reutilization of arenesulfonic acid-functionalized SBA-15 catalyst. The strong interaction of unsaponifiable mat-ter with the acid sites was the main reason for the catalyst

deactivation. However the conditioning of these materials by aqueous washing in the presence of cationic-exchange resin Amberlyst-15, followed by a drying step, resulted in a lower deactivation of the catalyst and remarkably increased the FAME yield in the second use of the catalyst.

Later, sulfonic acid-functionalized mesostructured SBA-15 silicas were utilized for the catalytic transesterification of glycerol with methyl acetate to produce diacetylglycer-ols (DAG) and triacetylglycerols (TAG) (Fig. 18) [97]. The results revealed that highest molar ratio and the highest catalyst loading were required to increase the conversion of pharmaceutical grade glycerol, the combined selectiv-ity towards DAG and TAG and to minimize the formation of undesired byproduct. The catalytic activity exhibited by arenesulfonic acid-functionalized SBA-15 was equivalent to that displayed by commercial catalysts such as the resin Amberlyst-70 or the composite Nafion-SAC-13. The most important parameters for the reaction were an acid strength of the catalytic sites, and their surface density. With tech-nical-grade glycerol arenesulfonic SBA-15 provided suit-able conversions and selectivity, whereas with crude glyc-erol the results were not satisfactory due to the deactivating effect of salts.

D. Zuo et al. in 2013 synthesized different sulfonic acid group functionalized SBA-15 catalysts and evaluated in the transesterification of soybean oil with 1-butanol under microwave condition to produce low freezing point bio-diesel (Fig.  19) [98]. The different sulfonic acid groups involved in the study were propyl-SO3H, arene-SO3H and perfluoro-SO3H. The synthesized mesoporous materi-als found to have a high surface area and uniform poros-ity, confirmed through powdered X-rays diffraction and nitrogen adsorption analysis. It was observed that cata-lytic activity was independent of number of acid site, but largely dependent on acid strength. In the microwave

Fig. 18 Transesterification of glycerol with methyl acetate in the presence of sulfonic acid-modified SBA-15 catalysts [97]

SiO

2 OOO

Si

SO3H

SiO

2 OOO

Si SO3H

OH

H2CCH

OH

CH2

HO

+

H3CC

OCH3

O

Catalyst

O

H2CCH

O

CH2

O

CCH3

O

CCH3

O

CO CH3

O

H2CCH

O

CH2

HO

CCH3

O

CCH3

O

+

Glycerol Methyl acetate

Diacetylglycerol Triacetylglycerol

Sulfonic Acid-Functionalized Solid Acid Catalyst in Esterification and Transesterification…

1 3

assisted transesterification of soybean oil and 1-butanol the propyl-SO3H and arene-SO3H functionalized SBA-15 cata-lysts showed maximum activity. In contrast complete loss of activity was found after recycling with perfluoro-SO3H functionalized SBA-15 catalyst due to complete leaching of perfluoro-SO3H groups in the reaction medium. Addition-ally, surface modification of Ar-SO3H SBA-15with hydro-phobic groups exhibited no significant improvement in the catalytic activity. However, Ar-SO3H SBA-15 catalyst was found to be a promising candidate for processing low qual-ity feedstock containing high fraction of free fatty acids because it increases the reaction rate for combined transes-terification and esterification of FFA feedstock.

M. L. Testa et  al. in 2014 reported the synthesis of sulfonic acid-functionalized hybrid silicas with different structure i.e. amorphous, HMS and SBA-15 with variable amount of organic moieties [99]. These materials were syn-thesized using grafting and in-situ oxidation methodologies and characterized by X-Ray photoelectron spectroscopy, low angle X-Ray diffraction, N2 adsorption and acid capac-ity measurements. The synthesized materials were tested in the transesterification reaction of short chain esters example hexanoic to lauric ethyl ester (Fig. 20). The study showed

that the catalytic activity of sulfonic acid hybrid silica for transesterification reaction depends on the morphology of silica and the preparation procedure. The propyl-sulfonic SBA-15 catalyst synthesized through in situ oxidation pro-cess exhibited maximum activity and structural stability with no leaching of the sulfonic groups.

S. Y. Chen et  al. in 2014 prepared the propylsulfonic acid-functionalized SBA-15 mesoporous silica with platelet morphology and short channeling pores (15SA-SBA-15-p) in one pot using co-condensation method [100]. The synthesized 15SA-SBA-15-p material was used as solid acid catalyst for transesterification of crude Jatropha oil (CJO) with methanol (Fig. 21) in both batch-type and fixed bed reaction systems and results were compared to commercial ion exchange sulfonic resins of Amberlyst-15 and SAC-13. It was observed that for the synthesis of Jatropha biodiesel fuel (BDF), the 15SA-SBA-15-p catalyst showed higher activity and resistance to water and free fatty acid (FFA) than commercial sul-fonic resins of Amberlyst-15 and SAC-13. The longer durability of the production of high-quality Jatropha BDF with suppressing of the side reactions and coke forma-tion was perceived. In comparison, the Jatropha BDF

Fig. 19 Transesterification of soybean oil with 1-butanol in the presence of sulfonic acid-modified SBA-15 catalysts [98]

H2C

HC

H2C

O

O

O

CO

CO

CO

R1

R2

R3

+ 3CH3(CH2)3OH

H3C(H2C)3

H3C(H2C)3

H3C(H2C)3

O

O

O

CO

CO

CO

R1

R2

R3

+

H2C

HC

H2C

OH

OH

OH

Catalyst

Triglyceride(Soybean oil)

1-Butanol Mixture of fatty esters "Biodiesel"

Glycerol

SiO2

O O O

Si

SO3H

SiO2

O O O

Si

SO3H

SiO2

O O O

Si

O

SO3H

FFF3C

F

Fig. 20 Transesterification short chain esters using sulfonic acid-functionalized hybrid silicas as. [99]

SiO

2 OOO

Si SO3H

RC

O

H2C

O

CH3

+

RC

O

OH

+ CH3OHCatalyst

RC

OCH3

O

H2OH3C

H2C

OH+

R = hexanoic C6; caprylic C8; capric C10; lauric C12

Ester Ethanol

Shagufta et al.

1 3

synthesized by 15SA-SBA-15-p catalyst was better in quality, stability and cold flow characteristic than synthe-sized by Amberlyst-15 and SAC-13 catalysts.

4 Conclusion

From the above discussion in this review article, it is strong evidence that the various recyclable sulfonic acid-functionalized solid acid catalysts as an active and environmentally friendly heterogeneous esterification catalysts plays a vital role in the field of organic trans-formation, especially esterification and transesterification of alcohols with carboxylic acids for the production of value added commercial chemicals like esters and bio-diesel respectively. The catalytic activity correlated well with the number of strong acid sites which increased by increasing the surface area and sulfonic acid content. A good reusability of the sulfonic acid-functionalized solid acid catalysts was observed. Application of these sul-fonic acid-functionalized solid acid catalysts make the industrial processes easier, cleaner, and less complicated. These reported catalysts are environmentally friendly and cleaner than conventional homogeneous catalysts.

It is expected that the information compiled in this review article would be helpful to apprise researchers with the recent progress in the field of sulfonic acid-func-tionalized solid acid catalysts and pave the way to design the most efficient heterogeneous catalysts for the esterifi-cation and transesterification reactions.

Acknowledgements The author Dr. Shagufta appreciate the support provided by the School of Graduate Studies and Research, American University of Ras Al Khaimah, Ras Al Khaimah, UAE through seed grant project No. AAS/002/16.

Compliance with Ethical Standards

Conflict of interest On behalf of all authors, the corresponding au-thor states that there is no conflict of interest.

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