Chemical Engineering Journal 228 (2013) 300–307

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Chemical Engineering Journal

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Acetalization reaction between glycerol and n-butyraldehyde usingan acidic ion exchange resin. Kinetic modelling

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.04.107

⇑ Corresponding author. Tel.: +34 946014273; fax: +34 946014179.E-mail address: belen.guemez@ehu.es (M.B. Güemez).

M. Belen Güemez ⇑, Jesus Requies, Ion Agirre, Pedro L. Arias, V. Laura Barrio, Jose F. CambraDepartment of Chemical and Environmental Engineering, Engineering School of the University of the Basque Country (UPV/EHU), c/Alameda Urquijo s/n, Bilbao 48013, Spain

h i g h l i g h t s

� Reached results are useful for engineering process design for a sustainable glycerol valorization.� Mixtures of cyclic acetals are the main products obtained in acetalization reaction.� Amberlysts 47 acidic ion exchange resin is highly active and stable in acetals production.

a r t i c l e i n f o

Article history:Received 5 January 2013Received in revised form 25 April 2013Accepted 29 April 2013Available online 7 May 2013

Keywords:Cyclic acetalsGlyceroln-ButyraldehydeAcidic ionic exchange resinKinetic parameters

a b s t r a c t

The acetalization reaction between glycerol and n-butyraldehyde using Amberlyst 47, an acidic ionexchange resin catalyst, was studied. These acetals can be obtained from renewable sources and seemto be good candidates for different applications such as oxygenated diesel additives. Amberlyst 47 acidicion exchange showed good activity and stability after five consecutive cycles of reuse. Therefore, 100% ofselectivity towards the formation of acetals and butyraldehyde conversions between 92% and 98% wereobtained for different molar feed ratios of glycerol:aldehyde. For glycerol:aldehyde molar ratios lowerthan the stoichiometric one, 2,4,6-tripropyl-1,2,3-trioxane is also detected as product when glycerol isalmost totally consumed. Moreover, a pseudo-homogeneous kinetic model able to explain the reactionmechanism was adjusted and the corresponding overall reaction order was determined. The additionof around 15 wt% of water in the feed produces only a slight decrease in the butyraldehyde conversion,and it improves the mixing and transport properties of the reaction mixture.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Acetals have their main application in the chemical industryand fine chemicals – cosmetics, pharmaceutical – , as intermedi-ates or final compounds. However, as in recent years governmentshave increased restrictions on environmental pollution, the use ofsome types of acetals and ketals, derived from biomass, as poten-tial oxygenated additives to fuels is being researched. For example,nowadays acetals can be used to reduce particulate emissions [1,2],to increase octane number in gasoline [3], to improve the proper-ties of biodiesel (viscosity, oxidation stability, flash point, etc.)[4–7], or as antifreezing agents of biodiesel [6]. Acetals are ob-tained typically by the reaction of carbonyl compounds (aldehydes,ketones) and alcohols with or without solvent and in the presenceof strong mineral acids as catalysts like H2SO4, HF, HCl, H3PO4 orp-toluenesulphonic acids [2,8–11]. These catalysts also causecorrosion and environmental problems and make the separationand purification process of the products more difficult. These

disadvantages could be solved using solid catalysts, and in thissense, efforts are being made to develop adequate solid acids. Inthe literature several types of heterogeneous acid catalysts likeion exchange resins [7,9–17], zeolites [11,18,19], montmorillonites[11,14,18] and MoO3/SiO2 [20] are reported to be used for acetal-ization reactions. Capeletti et al. [11] concluded that Amberlystion exchange resins are the best ones.

The process for acetals synthesis could be considered a sustain-able process if raw materials from renewable sources are used, asbio-alcohols and aldehydes derived from bio-alcohols.

Not all acetals can be used as additives for diesel fuels. Some ofthem show low flash points and are not useful as additives [2,17].Those acetals derived from polyols and short chain aldehydes, suchas acetaldehyde and butyraldehyde, present flash points close tothe diesel specifications, while those derived from ethanol, requirelong chain aldehydes to produce acetals with an acceptable flashpoint. Among bioalcohols, glycerol, the main by-product of biodie-sel production (transesterification of vegetable oils or animal fats)is the alcohol that have gained more attention in recent years [21–24] due to its increasing production, associated to the increase ofbiodiesel world production. Therefore, to valorise its potential uses,

M.B. Güemez et al. / Chemical Engineering Journal 228 (2013) 300–307 301

and to improve the economic balance of the biodiesel productionprocess, new applications are being developed [21,23], includingthe use of its acetals as diesel fuel additives.

Most of the studies reported in the literature for acetalizationreactions are focused on analysing and comparing the activity ofthe solid catalysts. Moreover, some authors also explained the pos-sible reaction mechanism [9,10,25–30], but a few proposed a reac-tion kinetic model [9,10,12,16,17,26–29,31–33].

In the case of the synthesis of cyclic acetals obtained from thereaction between a polyol and an aldehyde or ketone, Sharmaand Chopade [10] described the mechanism of acetalization of eth-ylene glycol and formaldehyde as a two-step reversible process.The first stage involves the formation of an hemiacetal, an interme-diate compound formed by the addition of an alcohol molecule tothe carbonyl group, and in the second stage, the hemiacetal hydro-xyl group reacts with another OH group to give the 1,3-dioxolane,releasing a water molecule. For the reaction of glycerol and acetal-dehyde, Da Silva Ferreira et al. [28] showed in more detail thestages of possible re-organization of the hemiacetal that lead tothe formation of mixed cyclic acetals. Fig. 1 shows the reactionmechanism proposed by Sharma and Chopade [10] adapted tothe reaction of glycerol and alkyl-aldehydes. In this case, depend-ing on which hemiacetal OH reacts to form the acetal,4-hydroxy-2-alkyl-1,3-dioxolane (cis and trans) or 5-hydroxy-2-alkyl-1,3-dioxane (cis and trans) are formed. The four isomers tendtowards equilibrium between them through reorganization of thecorresponding hemiacetal structure [28].

In this work, the acetalization reaction of n-butyraldehyde andglycerol has been studied in a batch reaction system. The ion ex-change resin Amberlyst 47 (A47) has been used as heterogeneouscatalyst since its mechanical resistance makes this resin, a priori,the most suitable one for its use in slurry reactors or in structuredpackings. Moreover, Agirre et al. [16] compared the activity ofthree Amberlyst ion exchange resins (A15wet, A47 and A35wet)obtaining similar results in activity for the acetalization reactionof ethanol and n-butyraldehyde. The experimental data have beenextensively treated in order to propose a pseudo-homogeneous ki-netic model for the overall reaction. Kinetic parameters have alsobeen estimated. Catalyst reuse and the effect of water in the feedhave also been studied.

glycerol alkyl aldehyde

HO

OH

OH R C H

O

4-hydrox

5-hydro

HO

OH

O OH

R

HO

OH

O OH

R

Fig. 1. Generalized reaction mechanism of glycerol and alkyl–ad

2. Materials and methods

2.1. Materials and analysis

Glycerol (G) (Panreac, 99 wt%) and n-butyraldehyde (B) (Merck,99 wt%), both of quality for synthesis, were used as reagents.Amberlyst 47 (A47) catalyst was kindly supplied by Rohm & Haas.Both, reactants, glycerol and n-butyraldehyde, and reaction prod-ucts, 5-hydroxy-2-propyl-1,3-dioxane (cis and trans isomers)(AC1) and 4-methanol-2-propyl-1,3-dioxolane (cis and trans iso-mers) (AC2) were analyzed by gas chromatography (Agilent6890 N) using a flame ionization detector (FID) and the water(W) produced was analyzed using a thermal conductivity detector(TCD). An Agilent DB-1 (60 m � 0.53 mm � 5 lm) capillary columnwas used with helium as the carrier gas.

The elution order of these organic products reported in theliterature is [15,29,34]: cis-5-hydroxy-2-alkil-1,3-dioxane, cis-4-methanol-2-alkil-1,3-dioxolane, trans-4-methanol-2-alkil-1,3-dioxolane and trans-5-hydroxy-2-alkil-1,3-dioxane. In this study,reaction products were identified analyzing their mass spectra ob-tained by GC/MS and verifying that their MS-spectra included theexpected molecular fragments.

2.2. Activity test

The reaction tests were carried out in a 1 L glass batch stirredtank reactor (BSTR). The reaction temperature was controlled byan external thermostat (Lauda RE 304). This thermostat containsa thermocouple placed inside the reacting mixture that allows con-trolling the reaction temperature with an accuracy of ±0.05 K. Thereactor is also connected to a condenser in order to condense andreflux all the vapours keeping the reaction volume approximatelyconstant.

In a typical experiment, the reactor was first charged with 0.5 Lof the reaction mixture, then the system was closed, the agitationspeed was fixed and, after the system stabilization at the desiredtemperature, the catalyst was added. This moment was consideredas the initial time for the reaction. Before adding the catalyst sam-ple, its moisture was modified waiting until it reached equilibriumwith the room one. Therefore the weight could be controlled. All

1,2-propanediol-3-(1-hydroxy-alkoxy)(hemiacetal)

(1)

(2b)

HR

O

H2C OH

O + H2O

ymethyl-2-alkyl-1,3-dioxolane

HR

OO

OH

+ H2O

xy-2-alkyl-1,3-dioxane

HO

OH

O OH

R

(2a)

ehyde adapted from proposed by Chopade and Sharma [10].

0,0

0,5

1,0

1,5

2,0

2,5

3,0

Ace

tals

(mol

/L)

0%

20%

40%

60%

80%

100%

Conversion

150 rpm 500 rpm 1000 rpm 1250 rpm

302 M.B. Güemez et al. / Chemical Engineering Journal 228 (2013) 300–307

experiments were carried out using 0.5 wt% of A47 with respect tototal weight of reactants feed. Moreover, some glass wool wasplaced in the sampling output in order to keep the catalystsamount constant in the reactor. At specific time intervals, smallsamples (1.5 mL of volume and a maximum of 15 samples byrun) were withdrawn in order to analyze them by GC.

To establish the temperature range of operation, an analysis ofthe physical properties of fed mixture using the UNIFAC-LL ther-modynamic method was carried out. Table 1 shows the mixtureboiling temperature for different G:B (glycerol to n-butyraldehyde)molar ratios and the mixture viscosity at those temperatures.

0 90 180 270 360 450 540

Time (min)

Fig. 2. Effect of the stirring rate on n-butyraldehyde conversion and acetalconcentration (AC1 - empty, AC2-fulled) (3:1 G:B molar ratio, 353 K and 0.5 wt%catalyst).

3. Results and discussion

In order to study the catalytic activity of the ion exchange resinAmberlyst 47 in the acetalization reaction of glycerol and n-butyr-aldehyde, different parameters were studied: agitation speed, reac-tion temperature, G:B molar ratio fed, presence of water in the feedmixture and catalyst reuse. When the G:B molar ratio used wasgreater than 1, activity tests showed that the products obtainedwere just a mixture of cyclic acetals and water.

3.1. Effect of stirring speed

High temperatures, low stirring speeds and viscous liquid mix-tures are the most unfavorable conditions in which resistance toexternal mass transport may occur. Therefore, the effect of the stir-ring rate on n-butyraldehyde conversion and acetal concentrationswas studied within a wide range of agitation speeds, 150 rpm(minimum speed that the available stirring system is able to pro-vide) and 1250 rpm, at 353 K and 3:1 G:B molar ratio. The resultsare shown in Fig. 2. No external mass transfer limitations were ob-served even at the lowest stirring speed, (150 rpm). However, itwas observed that initial vigorous stirring was necessary to achievea homogeneous mixture of the reagents due to difficult mixing be-tween glycerol and n-butyraldehyde. Because of this reason, therest of the experiments were carried out at 1000 rpm.

3.2. Effect of reaction temperature

The temperature effect on the reaction of glycerol and n-butyr-aldehyde was carried out in the temperature range of 323–353 Kusing the stoichiometric feed ratio. As expected, an increase in

Table 1Physical properties of the mixture fed at the operating conditions used for the kineticstudy (the physical properties were estimated using the UNIFAC-LL thermodynamicmethod).

Molarratio G:B

Boilingpoint (K)

Temperature (K) Density(g/cc)

Viscosity(cPo)

0.2 351 338 0.87 0.70346 0.86 0.62353 – –

0.5 358 338 0.95 1.66346 0.94 1.39353 0.93 1.21

1 366 338 1.03 3.93346 1.02 3.12353 1.01 2.60

2 376 338 1.10 9.27346 1.10 7.00353 1.09 5.70

3 383 338 1.14 14.2346 1.14 10.5353 1.13 8.16

temperature increased the overall reaction rate (see Fig. 3a). Inthe presence of catalyst and after 100 min of reaction time the con-version of n-butyraldehyde increased from 69% to 90% when thetemperature was increased from 323 K to 353 K. In the absenceof catalyst and at the highest temperature used, the conversiondid not exceed 62% after 3 h of reaction time. It was also observedthat over the range studied, a temperature increase had practicallyno influence on the final equilibrium conversion. This can be re-lated to a near zero overall heat of reaction involved in this process(Fig. 3b). A similar behaviour in the overall heat of reaction wasalso observed in other studies of open-chain acetal synthesis reac-tions carried out in batch systems with solid catalysts and molarfeed ratios alcohol:aldehyde higher than the stoichiometric ones[9,25,27].

In the open literature about the synthesis of open-chain acetalswith heterogeneous catalysis, published results indicate that thereaction rate is strongly limited by thermodynamics. Therefore,alcohol:aldehyde molar ratios higher than the stoichiometric onesare required in order to achieve high conversions. Higher catalystloadings than the ones used in this study were also reported in or-der to get high reaction rates [9,11,16,25,26]. For example, Agirreet al. studied the reaction between ethanol and n-butyraldehydeusing a 0.5 wt% of Amberlyst 47 [16]. They obtained an equilibriumconversion of 48% at 313 K for a stoichiometric feed molar ratio(2:1 of ethanol to butyraldehyde). When this molar ratio was mul-tiplied by a factor of two an equilibrium conversion of 72% was re-ported. The main difference in the reaction rate and in theexistence or not of thermodynamic limitations between open chainacetal synthesis (using monoalcohols) and cyclic acetal synthesis(using polyalcohols) may be related to different reaction mecha-nisms. In the first case two alcohol molecules and one aldehydemolecule are required in order to get the acetal. The reaction ofthe hemiacetal, which is formed when one alcohol molecule reactswith one aldehyde molecule, with the second alcohol molecule,uses to be the rate limiting step and accounts for the larger ther-modynamic limitations. In the second case, the synthesis of cyclicacetals, the polyalcohol (e.g. glycerol) reacts with the aldehyde giv-ing the hemiacetal and finally this intermediate molecule rear-ranges itself through the reaction of two of its hydroxyl groupsforming the cyclic acetal. According to the results that can be seenbelow, it seems that this second step in the formation of cyclic ace-tals is easier than the second step when open chain acetals areformed.

Regarding the type of acetal formed (see Fig. 3b), the resultsindicate that theAC2 mixture of isomers (dioxolane) was formedmore rapidly than AC1 (dioxane). When the concentration of AC2in the reaction mixture was high enough, the isomerisation reac-tion converting AC2–AC1 acetals became more important. Agirre

0%

20%

40%

60%

80%

100%

Time (min)

Con

vers

ion

323 K338 K346 K353 K353 K, without catalyst

0

1

2

3

4

5

0 90 180 270 360 450 540

0 90 180 270 360 450 540

Time (min)

Ace

tals

(mol

/L)

353 K 346 K 338 K 323 K 353 K, without catalyst

(a)

(b)

Fig. 3. Effect of the temperature on n-butyraldehyde conversion (a) and acetalconcentration (b) (AC1 - empty, � and AC2-filled, +) (1:1 G:B molar ratio, 1000 rpmand 0.5 wt% of catalyst).

M.B. Güemez et al. / Chemical Engineering Journal 228 (2013) 300–307 303

et al. observed a similar behaviour in the reaction of glycerol andacetaldehyde using Amberlyst 47 as catalyst [35]. Previously,Aksnes et al. (1965) made the same observations in their studyof the equilibrium mixture of acetals obtained from the reactionof glycerol and acetaldehyde [34]. In this same reaction, Da Silvaet al. studied the evolution of the different isomers in aging winesof Porto and Madeira [28]. They described a similar behaviour tothat observed in this study for wines under two years of age, butin older wines, the proportion of 1,3-dioxane was greater than

0%

20%

40%

60%

80%

100%

0 360 720 1080 1440 1800

Time (min)

Con

vers

ion

0

1

2

3

4

5

Concentration (m

ol/L)

Conversion4-methanol-2-propyl-1,3-dioxolane

5-hydroxy-2-propyl-1,3-dioxane

7h 24h

Fig. 4. Evolution of the concentration of mixed acetals and n-butyraldehydeconversion to long reaction times (1:1 G:B molar ratio, 338 K, 1000 rpm and 0.5 wt%of catalyst).

the one of 1.3 dioxolane proving that these isomerisations takeplace at very slow rates (at room temperature in the absence ofany added catalysts). In the study of the glycerol reaction with dif-ferent alkylaldehydes (C4–C12), with dimethylsulphoxide (DMSO)as solvent and Amberlyst 15 as heterogeneous catalyst, Silva et al.obtained a higher selectivity towards the formation of 1,3-dioxo-lane [7]. However, in the case of the reaction with decanal, the ratioof dioxane to dioxolane was practically one, especially for longreaction times. This was attributed to the low conversion obtainedand the higher stability of six-member ring acetals. In the reactionof glycerol and trioxane, as formaldehyde source, and using bothhomogeneous and heterogeneous catalysts, Ruiz et al. indicatedthat the isomerisation reaction proceeds toward the formation ace-tals of the five-member ring [30], while the ratio dioxane:dioxo-lane was less than unity for the reaction of glycerol, n-heptanaland trioxane (solvent) with homogeneous catalysts. Agirre et al.in the reaction of glycerol and formaldehyde using Amberlyst 47as catalyst found that the ratio dioxane:dioxolane was initially lessthan unity, but it increased with reaction time [17].

In order to check the different observations reported in the lit-erature about the evolution of the distribution of five/six-memberacetals, an experiment of 30 h was performed. Under the experi-mental conditions used in this work, Fig. 4 shows that the diox-ane:dioxolane ratio is 0.80 after 7 h of reaction and a valueslightly higher than 1.02 after 24 h of reaction. If the reaction timehad been longer, higher values of the dioxane:dioxolane ratiowould have been measured. Thus, these results support the generalmechanism shown in Fig. 1 (simplified diagram) for the acetaliza-tion reaction between glycerol and alkylaldehydes and, publishedin detail by Da Silva Ferreira et al. for the reaction of glycerol andacetaldehyde [28]. The reaction of formation of cyclic acetals pro-ceeds preferentially through two parallel pathways formation of1,3-dioxolane (reaction 2a, Fig. 1) and 1,3-dioxane formation (reac-tion 2b, Fig. 1), being the first faster than the second. In addition, anisomerisation reaction occurs towards the more stable acetal struc-ture (six-member ring) through the formation of the correspondinghemiacetal. The results of this work and others reported in the lit-erature suggest that the alkylaldehyde type used in the process, theoperating conditions and the catalyst used may influence the isom-erisation reaction rate and therefore, the ratio dioxane:dioxalanefinally obtained is a function of time unless equilibrium is reached.

3.3. Effect of the G:B molar ratio

The effect of the initial G:B molar ratio on the n-butyraldehydeconversion was studied at a temperature of 353 K, except for thecase of the molar ratio of 0.2 where a temperature of 338 K wasused to ensure that the reaction was carried out in liquid phase.(The bubble point of this mixture is estimated to be 351 K, seeTable 1). Fig. 5 shows that when an excess of alcohol was used –molar ratio greater than the stoichiometric (1:1) – the reactionshifted towards the products formation increasing the conversionof n-butyraldehyde, as expected. Thus, for a reaction time of100 min, the conversion of n-butyraldehyde increased from 88%to 98% when the initial G:B molar ratio increased from the stoichi-ometric ratio (1:1) to the highest molar ratio studied (3:1). Theconversion of glycerol (only indicated in Fig. 5) was 100% after40 min of reaction time when the G:B molar ratio is 0.2, whereasin the studied reaction time period, it varies between 93% and98% when the G:B molar ratio is 0.5. Experiments showed thatwhen glycerol is the limiting reagent and it is almost totally ex-hausted in the reaction medium, small amounts of another com-pound are detected. This compound was identified by GC/MS asthe 2,4,6-tripropyl-1,3,5-trioxane and it is formed by cyclotrimer-ization of n-butyraldehyde:

0%

20%

40%

60%

80%

100%

0 90 180 270 360 450 540

Time (min)

Con

vers

ion

Ratio* 0.2:1 Ratio 0.5:1 Ratio 1:1 Ratio 2:1 Ratio 3:1

[93-98] % glycerol conversion

100% glycerol conversion

Fig. 5. Effect of molar ratio G:B on n-butyraldehyde conversion at 353 K (338 K toratio molar G:B 0.2:1), 1000 rpm and 0.5 wt% of catalyst.

0%

20%

40%

60%

80%

100%

Time (min)

Con

vers

ion

ratio 3:1, with catalyst ratio 1:1, with catalystratio 3:1, without catalyst ratio 1:1, without catalyst

0,0

0,2

0,4

0,6

0,8

1,0

0 90 180 270 360 450

0 90 180 270 360 450

Time (min)

Ace

tal/f

ed b

utyr

alde

hyde

mol

ar ra

tio

ratio 3:1, with catalyst ratio 3:1, without catalystratio 1:1, with catalyst ratio 1:1, without catalyst

(a)

(b)

Fig. 6. Comparison between non-catalytic homogeneous reaction and heteroge-neous catalytic reaction (0.5 wt% of catalyst): 353 K, 1000 rpm and G:B molar ratio1:1 and 3:1.

304 M.B. Güemez et al. / Chemical Engineering Journal 228 (2013) 300–307

The formation of this trioxane molecule may explain the slightincrease of the n-butyraldehyde conversion observed for reactiontimes exceeding 40 min and at a G:B molar ratio fed of 0.2. Inthe case of 0.5 feed molar ratio, the overall reaction system reachedequilibrium for a glycerol conversion of 98%. However, for reactiontimes longer than 30 min, part of the butyraldehyde could be con-sumed due to the cyclotrimerization reaction. For higher reactiontimes, the peak abundance of trioxane was appreciable. When feedmolar ratios greater than the stoichiometric ratio (G:B molar ratio1:1) were used the trioxane peak was not identified or its contribu-tion was negligible.

Fig. 6a shows the n-butyraldehyde conversion when the reac-tion is carried out with or without solid catalyst for two differentG:B molar feed ratios, 1:1 and 3:1, respectively. It can be observedthat for the G:B 1:1 feed molar ratio (stoichiometric molar ratio)reaction progresses slowly. After 7 h n-butyraldehyde conversionwas 66% versus 93% (approximately equilibrium conversion) ob-tained when the catalyst was present. However, in the absence ofcatalyst and when the reaction was carried out with excess ofone reactant, in this case glycerol (molar ratio G:B 3:1), the equilib-rium conversion was almost reached after 4 h of reaction. Note thatin the scientific literature available no published data on non-cat-alytic acetalization reactions could be found.

Regarding the acetal isomer formed, major differences areobserved when the catalyst is present in the reaction medium(see Fig. 6b). The results shown were normalized to the initial con-centration of aldehyde. The behaviour observed in the presence ofcatalyst is as described in section 3.2 for both G:B molar ratios.However, an excess of alcohol (molar ratio G:B 3:1) seems to pro-mote the formation of 4-methanol-2-propyl-1,3-dioxolane (AC2),while the initial G:B molar ratio practically does not influencethe rate of formation of 5-hydroxy-2-propyl-1,3-dioxane. In theabsence of catalyst, the most affected reaction rate is the isomeri-sation one. As shown in Fig. 6b, the AC2 concentration increasedquite rapidly but the formation of AC1, both from direct glycerolacetalization and from AC2 isomerisation took place quite slowly.

3.4. Modelling of the kinetic data

For the kinetic study of the overall acetalization reaction ofglycerol and n-butyraldehyde (Eq. (1)) the data obtained using feedmolar ratios from 1 to 3 at a temperature of 353 K were used in or-der to establish the order of reaction. The results obtained in theoperating temperature range of 338–353 K allowed estimation ofthe kinetic constants of the rate law. In all the above mentionedoperating conditions, the selectivity to the cyclic acetals mixturewas 100%.

ð1Þ

A pseudo-homogeneous model reaction was used for interpret-ing the experimental data. The order of the reaction for each com-pound was determined from the experimental data varying theconcentration. The best result was achieved for order of one foreach compound according to the following equation:

d½Ac�dt¼ wk1½G�½B� �wk2½Ac�½W� ð2Þ

where [] is the concentration of compound ‘‘i’’ in mol L�1; w theweight of catalyst per unit volume of reaction in gcat L�1; and ki isthe apparent rate constant velocity in L2 mol�1 min�1 g�1

cat.A fourth-order Runge–Kutta integration method was used to

solve the differential equation. Thus, minimizing the sum ofsquares of the difference between the experimental data and

Table 2Kinetic constants at different temperatures and Arrhenius’ parameters estimated forglycerol and n-butyraldehyde reaction.

Temperature (K) k1 (L2 mol�1 min�1 g�1cat) k2 (L2 mol�1 min�1 g�1

cat)

338 (1.42 ± 0.01) � 10�03 (4.18 ± 1.20) � 10�06

346 (2.06 ± 0.36) � 10�03 (9.86 ± 0.28) � 10�06

353 (3.31 ± 0.43) � 10�03 (24.0 ± 4.75) � 10�06

Ea (kJ mol�1) 55.6 115A (L2 mol�1 min�1 g�1

cat) 5.33 � 1005 2.67 � 1012

r2 0.986 0.996

0%

20%

40%

60%

80%

100%

30 60 120 300 420

Time (min)

Con

vers

ion

Run 1 Run 2 Run 3 Run 4 Run 5

Fig. 8. Effect of re-using the catalyst on n-butyraldehyde conversion at differentreaction times (1:1 G:B molar ratio, 353 K, 1000 rpm and 0.5 wt% of catalyst).

M.B. Güemez et al. / Chemical Engineering Journal 228 (2013) 300–307 305

predicted concentration, the pseudo rate constants, k1 and k2 wereestimated. Table 2 shows the apparent kinetic constants obtainedas a function of temperature. The corresponding parameters ofArrhenius equation, activation energy (Ea) and pre-exponentialfactor (A), were determined by lineal regression of lnk versus theinverse of temperature. In order to establish the goodness of theestimation of the kinetic parameters (activation energy and pre-exponential factor), the apparent kinetic constants, and the overallconcentration of acetals were estimated. Fig. 7 shows the agree-ment between experimental and estimated acetal concentrationwithin the temperature range studied.

3.5. Recycling of the catalysts

In order to study the stability of the resin A47 in the reaction ofn-butyraldehyde and glycerol, the catalyst was reused five times.Each operating cycle was carried out at the temperature of 353 Kand a G:B molar ratio of 1:1 and 120 min of reaction. The catalystused was recovered by simple filtration, then washed with 150 mLof distilled water and, finally, dried at the laboratory temperature,thus being ready to be reloaded into the reactor. Fig. 8 shows theresults of n-butyraldehyde conversion at different reaction timesand for each reaction cycle. It is observed that there was no signif-icant deactivation of the catalyst under the conditions studied. Thehighest fluctuations in the catalytic activity, although no signifi-cant (±1.9% standard deviation), were produced for the initial reac-tion times (30–60 min). This could be due to increased competitionby the acid centres of the catalyst when the concentration of reac-tants in the reaction medium is high. Also, as expected, when theequilibrium conversion was reached, the variations between eachrun (standard deviation ranges between ±0.7% and ±1.0% for reac-tion times between 120 and 420 min) decreased.

In order to study the possible loss of sulphonated groups, totalsulphur was determined for the fresh catalyst and after five runs.The analysis was performed on a total sulphur analyzer TruSpec.

0

1

2

3

4

5

6

0 1 2 3 4 5 6

Estimate data (mol/L)

Expe

rimen

tal d

ata

(mol

/L)

353 K346 K338 K

Fig. 7. Total concentration of acetals. Agreement between experimental andestimated data obtained applying Eq. (1).

The fresh catalyst showed a total sulphur content of12.29 ± 0.26 wt%, while the one for the used catalyst used was11.39 ± 0.13 wt%. These results show a decrease in total sulphurcontent slightly above 7% after 600 min of reaction. However, thisreduction of the total sulphur content did not seem to affect signif-icantly the activity of the catalyst after five reaction batches underthe operating conditions studied.

3.6. Effect of water in the feed mixture

The presence of water, one of the acetalization reaction prod-ucts, in the feed mixture may play a negative role. If water is ini-tially present in the reaction system, the viscosity of the mixturedecreases (see Table 3), and as a result both mixture mixing andtransportation ends being easier. However the bubble point ofthe mixture is lowered, so that to carry out the reaction in liquidphase, the operating temperature must be decreased at atmo-spheric pressure. Therefore, in order to study the effect of initialwater concentration on the acetalization process of glycerol andn-butyraldehyde, two mixtures of glycerol and water were pre-pared: (a) 90 wt% of glycerol and 10 wt% of water and (b) 80 wt%of glycerol and 20 wt% of water. Then, the required amount of n-butyraldehyde was added to obtain B:G molar ratios equal to 2:1,resulting in 15 wt% and 7 wt% of water concentration in the initialmixtures (see Table 3). This molar ratio was selected as an interme-diate one within the range studied in this work. The mass compo-sition and physical properties of the final mixtures initially fed tothe reactor are shown in Table 3. The maximum percentage ofwater in the mixture fed was chosen based on the work reportedby Agirre et al. [35] for the acetalization reaction of glycerol andacetaldehyde. In this work, the final weight percentage of waterin the feed mixture was varied between 12% and 25% dependingon the molar ratio alcohol:aldehyde employed. The results re-ported by them (acetaldehyde conversion of almost 100%) suggest

Table 3Water effect on the physical properties of the feed mixture to the molar ratio G:B 2:1.

G (wt%) + W (wt%) Mass composition(wt%)

Mixture physical properties

G B W Tboiling (K) q (g/cc)a l (cPo)a

100 + 0 71.9 28.1 0.0 376 1.09 5.790 + 10 66.6 26.0 7.4 368 1.09 2.980 + 20 60.9 23.9 15.2 362 1.08 1.770 + 30 54.9 21.5 23.6 358 1.07 1.2

a Physical properties at 353 K.

0,0

0,2

0,4

0,6

0,8

1,0

0 90 180 270 360 450 540

Time (min)

Ace

tal/f

ed b

utyr

alde

hyde

mol

ar ra

tio

0%

20%

40%

60%

80%

100%

Conversion

AC2 without water

AC2 (90wt% of G + 10wt% of W)

AC2 (80wt% of G + 20wt% of W)

Fig. 9. Effect of the water presence in the mixture fed to the acetalization reaction system (60.9 wt% of G, 23.9 wt% B and 15.2 wt% W, 2:1 G:B molar ratio, 353 K, 1000 rpmand 0.5 wt% of catalyst).

306 M.B. Güemez et al. / Chemical Engineering Journal 228 (2013) 300–307

that the presence of a certain water concentration does not signif-icantly affect the overall rate of reaction.

The activity tests were carried out at 353 K. For the selectedoperating conditions, the dynamic viscosity of the mixture is re-duced between 49% and 70% from the viscosity measured whenwater is not present en the feed. Fig. 9 shows the molar ratio ofacetal:fed butyraldehyde and the conversion of n-butyraldehydeas a function of the reaction time.

It was observed that the introduction of 15 wt% water in thefeed mixture caused a slight decrease in the final conversion ofn-butyraldehyde, from 98% (without water) to 94% (with water)after 8 h reaction. However, the presence of water in the initialmixture fed seems to affect the isomerisation reaction rate (dioxo-lane (AC2) to dioxane (AC1)), although the differences observed be-tween using a 7 wt% or 15 wt% of water concentrations were notimportant, particularly at high reaction times. Furthermore, thedifferences observed along the first 90 min of reaction are practi-cally negligible. In the acetalization reaction of glycerol and ace-tone, Da Silva and Mota [36] studied the effect of waterconcentration in the feed. After one hour of reaction and whenwater was used, the conversion of glycerol decreased from 59%to 10%, depending on the initial water concentration and type ofheterogeneous acid catalyst used (Amberlyst 15 or zeolite H-Beta).In the acetalization reaction of ethanol and acetaldehyde, Capelettiet al. also found greater differences in the conversion of ethanolwhen the solid catalyst (Amberlyst 15) was used dry (at 373 K)or pre-wet standard [11]. Based on the results obtained in thisstudy and those mentioned above, it can be concluded that an ini-tial incorporation of water in the feed mixture may have small ef-fects on the reaction rate of glycerol and short-chain aldehydes,and contributes to improve the mixing and transport propertiesreducing the viscosity of the reaction mixture.

4. Conclusions

In the acetalization reaction of glycerol and n-butyraldehyde,Amberlyst 47 was found to be highly active and stable after fiveconsecutive cycles. Mixtures of cyclic acetals are the main productsobtained when initial glycerol:butyraldehyde molar ratio fed isgreater than the stoichiometric one. Moreover, water addition im-proves handling and transport characteristics of the reactant mix-ture without any significant effect on the process performance. The

studied reaction system can be described using a pseudo-homoge-neous kinetic model. The reported results could be of interest forfuture developments dealing with process engineering for bio-glycerol valorisation through the production of cyclic acetals. Thegreat advantage of the reaction under study is that, contrary toother acetals, its synthesis presents small thermodynamic limita-tions and, in principle, the industrial production of these acetalsfor their use as biodiesel additives could be carried out in conven-tional reaction systems with a simple downstream separationtrain.

Acknowledgements

This work was supported by funds from the Spanish Ministry ofScience and Innovation (Ref. ENE2009-12743-C04-04), the BasqueGovernment and the University of the Basque Country (UPV/EHU).The authors also gratefully acknowledge Rohm & Haas for kindlysupplying Amberlyst catalysts.

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