Chemical Engineering Journal 263 (2015) 257–267

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Chemical Engineering Journal

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Catalytic performance of kenyaite and magadiite lamellar silicatesfor the production of a,b-unsaturated esters

http://dx.doi.org/10.1016/j.cej.2014.11.0161385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +55 85 33 66 90 41/9982.E-mail address: alcineia@ufc.br (A.C. Oliveira).

1 Tel./fax: +55 85 3366 90 08.

Marília E.R. Oliveira a, Edson C. da Silva Filho a, Josue M. Filho b,1, Sebastião S. Ferreira c,Alcineia C. Oliveira c,⇑, Adriana F. Campos d

a Universidade Federal Piauí, Departamento de Química, Campus Universitário Ministro, Petrônio Portella, Bairro Ininga – Teresina, Piaui, Brazilb Universidade Federal do Ceará, Departamento de Física, Campus do Pici-Bloco 922, Fortaleza, Ceará, Brazilc Universidade Federal do Ceará, Campus do Pici-Bloco 940, Departamento de Química Analitica e Físico-Química, 60.000.000 Fortaleza, Ceará, Brazild Centro de Tecnologias Estrategicas do Nordeste-CETENE, Av. Prof. Luiz Freire, 1 – Cidade Universitária, 50740-540 Recife, Pernambuco, Brazil

h i g h l i g h t s

� Co3+, Al3+, Er3+-containing kenyaite or magadiite were synthesized.� The catalyst loading, type of solvent and temperature were investigated.� Knoevenagel condensation performance was due to the trivalent cations.� The affinity between Si and Er in kenyaite structure explained ErKen performance.

a r t i c l e i n f o

Article history:Received 9 August 2014Received in revised form 25 October 2014Accepted 1 November 2014Available online 13 November 2014

Keywords:Lamellar silicatesCatalystsFine chemistryCharacterizations

a b s t r a c t

Trivalent cation (Co3+, Al3+, Er3+) containing-kenyaite and magadiite lamellar silicates were synthesizedand tested as catalysts for the Knoevenagel condensation of butyraldehyde (BUT) with ethyl cyanoacetate(CAE). The as-synthesized catalysts were thoroughly characterized by X-ray powder diffraction (XRD),transmission electron microscopy (TEM), Raman spectroscopy, solid state nuclear magnetic resonanceof silicon (29Si NMR) and nitrogen physisorption measurements. Significant effects of the structuraland textural features of kenyaite and magadiite on catalyst activity in the Knoevenagel condensationwere observed. The high catalytic activity and selectivity were ascribed to the cations incorporated intosilicate layers, being Co3+ and Al3+ leached from their structures during the reaction. Er3+-containingkenyaite exhibited excellent catalytic activity in the Knoevenagel condensation, suggesting that thepresence of silanol groups along with trivalent cation was responsible for the exceptional activity ofthe catalyst at mild conditions. The effects of catalyst loading, type of solvent and temperature on thecatalytic properties demonstrated that a 100 mg of catalyst mass and BUT/CAE molar ratio of 1 inthe presence of toluene at 90 �C were the best conditions for testing Er3+-containing kenyaite andmagadiite catalysts. The affinity between Si and Er took place in kenyaite structure and this could explainthe better performance of Er3+-containing kenyaite, since Er remained in the framework during thereaction. On the contrary, leaching of the erbium in magadiite was responsible for the lesser performanceof the solid.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The liquid-phase Knoevenagel condensation reaction is ofgreat interest because it allows obtaining a,b-unsaturated esters

molecules from carbonyl compounds (e.g., aldehyde or ketone)and active methylene groups [1–5]:

258 M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267

The product of the reaction is a C@C double bond compound, whichcan be used as chemical intermediate for the synthesis of anti-hypertensive drugs and calcium channel blockers and especiallyfor ethylcyano acrylates (ECAs) production. These compounds arehighly desirable in polymer industries and fine chemistry due totheir use as end-products for fragrances and therapeutic drugs[6–8].

Utilizing suitable catalysts is one of the most promisingapproaches, since they allow total conversion of aldehyde orketone at relatively low temperatures. Also, a better selectivitytowards ECAs is achieved with dilute bases and acids catalysts[2–8]. Recent studies have shown that more than 1.5 million tonsof bulk chemicals are produced annually via processes catalyzedby alkaline bases such as NaOH, KOH and Ca(OH)2 [9]. However,the major challenge of using the homogeneous catalysts is the cor-rosiveness and disposal of spent acid and base materials, whichwill not only induce the deactivation of catalysts by leaching, butalso cause environmental drawbacks [7]. Thus, deactivation ofthe catalysts is the primary hindrance to the development materi-als for Knoevenagel condensation process.

For these reasons, studies on heterogeneous systems includingoxides, modified carbon nanotubes, heteropolyacids, molecularsieves functionalised with amino groups, anionic resins, clays, zeo-lites, metal organic framework based materials, hydroxyapatites,oxides immobilized in ionic liquids, as-synthesized molecularsieves, oxynitrides, aluminophosphates, organic polymers, hydro-talcites and nitrogen-containing carbon, among others [2–12] arevery highly regarded. An important advantage of carrying out theliquid-phase Knoevenagel condensation at mild conditions withheterogeneous catalysts is that self-condensation and oligomeri-sation reactions are suppressed with the facile separation andthe purification of the product formed.

Catalysts based on silicates and aluminosilicates are cheap andactive at low temperatures, being optimal to avoid production ofundesirable side-products [12–15]. However, the reaction needshigh catalyst mass, temperatures and use of solvents to be active.It is generally accepted that trivalent cations (Al3+, Co3+ and rareearth cations such as Ce3+) included on silicates are the activespecies for a,b-unsaturated esters production via isomerizations,Michael addition as well as aldol, Knoevenagel and Claisen–Schmidt condensations [16–18]. This implies that the aforesaidtrivalent cations, which are active centers present in the silicatesstructures, may promote the catalytic activity.

Within this context, the search for lamellar silicates catalysts(kenyaite and magadiite) for Knoevenagel condensation that avoidleaching of the active sites is an open issue. The goal of the presentwork is to study the role of the trivalent cation insertion into thelamellar silicates structure on the catalytic performance of the sol-ids. The reaction parameters affecting the yield of a,b-unsaturatedesters over trivalent cations-containing lamellar silicates are alsoinvestigated. The lamellar silicates-containing trivalent cationsare expected to yield ethyl 2-cyano-3-propylacrylate (ECPA). Theparameters investigated include type of catalyst, temperature,use of solvents and catalyst loading as well. Despite the abovemen-tioned advances, to the best of our knowledge, the influence of thereaction conditions on the catalytic performance is not clear yet.Also, the nature of the active site for enhancing catalyst stabilityover the lamellar silicates is not reported in the literature.

2. Experimental

2.1. Kenyaite and magadiite preparation

Synthesis of kenyaite and magadiite lamellar silicates wasbased on the classical hydrothermal method [19,20], which wasmodified in the preparation of trivalent cations-containing lamel-lar silicates.

In a typical synthesis of magadiite, about 10 g of silica gel(0.16 mol, Aldrich, Particle size range: 35–75 lm/200–425 meshpossessing 98% of purity) was dissolved in a 1.0 mol L�1 of sodiumhydroxide solution (0.060 mol, Impex) under continuous stirringfor 40 min to form a white suspension. The obtained gel having acomposition of 10SiO2:1NaOH was then submitted to hydrother-mal treatment in a Teflon lined autoclave at 150 �C during 72 h.The product was filtered, washed thoroughly with Milli-Q waterto remove the excess of sodium hydroxide until pH 9.0 and finallydried at 50 �C for 24 h. The obtained solid was designed as NaMag.

Sodium-containing kenyaite (NaKen) was synthesized byfollowing the above mentioned procedure by using sodiumhydroxide (Impex) and sodium carbonate (Vetec) solutions witha SiO2/(NaOH + Na2CO3) molar ratio of 20. After being stirred atroom temperature for 40 min., the solution was transferred intoan autoclave and kept at 180 �C for 96 h. The final solid wasrecovered, washed and finally air-dried at 50 �C for 24 h.

The ion exchange of Na+ by H+ was carried out over NaMag andNaKen under room temperature by stirring 100 mg of each lamel-lar silicate with a 0.1 mol L�1 aqueous solution of hydrochloric acid(Impex) during 24 h. After washing and drying, the solids weredenoted as HMag and HKen.

2.2. Kenyaite and magadiite containing Al3+, Er3+ or Co3+ cations

Syntheses of kenyaite and magadiite containing Al3+, Er3+ orCo3+ cations were carried out by hydrothermal procedure, asabovementioned. Briefly, around 0.50 g (0.0025 mol) of aluminumisopropoxide (Al(OCH(CH3)2)3, Fluka Analytical) was added to theaforesaid alkaline silica solution under vigorous stirring for 5 h.The resultant solution mixture was transferred to the Teflon linedautoclave vessel by using the temperatures mentioned for alkalineNaMag and NaKen. The same procedure was used to prepare thesolids containing erbium and cobalt, respectively by using theaforesaid molar amount of (Er(NO3)3�5H2O, Aldrich) and (Co(NO3)2�6H2O, Aldrich). The obtained as-synthesized lamellar silicates werenamed as AlMag, ErMag, CoMag, AlKen, ErKen, CoKen.

2.3. Characterizations

The as-synthesized kenyaites and magadiites were character-ized by X-ray diffraction (XRD), nitrogen adsorption–desorptionmeasurements, chemical analyses, 29Si Nuclear Magnetic Reso-nance (29Si NMR), chemical analysis and Raman spectroscopy.

XRD analysis was performed in a Rigaku X-ray diffractometer(Rigaku model, 40 kV and 25 mA) using CuKa radiation. Crystalstructures were determined using wide-angle diffraction patternsin the 2h = 3–70� range.

0 10 20 30 40 50 60 70

CoMag

ErMag

AlMag

HMagInte

nsity

(u.a

.)

2θ (degree)

NaMag

0 10 20 30 40 50 60 70

CoKen

ErKen

AlKen

HKenIn

tens

ity (u

.a.)

2θ (degree)

NaKen

(a)

(b)

Fig. 1. XRD patterns of as-synthesized lamellar silicates: (a) magadiites and (b)kenyaites.

M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267 259

Nitrogen adsorption measurements were carried out on anBelsorp Mini II instrument. The surface areas (SBET) and pore vol-umes were estimated from the N2 adsorption isotherms at 196 �Cusing Brunauer–Emmett–Teller (BET) equation and the t-plotmethod, respectively. Average pore-size of mesoporous materialwas obtained from nitrogen adsorption branch, using the Barrett–Joyner–Halenda (BJH) method. Before the analysis, all sampleswere pretreated in a vacuum at 90 �C for 24 h.

Transmission electron microscopy (TEM) images were recordedusing a JEOL-JEM 2100 ARP and FEI-TECNAI G2S-Twin setup oper-ating with a 200 kV. The TEM samples were prepared by droppingan aqueous suspension of sample powder on a holey carbon-coatedcopper grid and letting the water evaporate at room temperature.

Chemical analyses were performed by inductively coupledplasma (ICP) optical emission spectroscopy was used to measurethe concentration of erbium, aluminum or cobalt in each of thesamples synthesized above. About 0.1 g sample of the catalystswas dissolved in 40 wt% hydrofluoric acid solution and heated at200 �C in a sand bath. The solution obtained was evaporated anddried, and a solution of 3.4 mL of nitric acid and 10 mL of waterwas added prior to the measurements. The obtained solutions werethen analyzed in a sequential ARL model Perkin Elmer equipment.

Raman spectra of the solids were obtained on a Witec Ramanspectrometer under ambient conditions. A 532 nm laser line wasused as the exciting source on the sample surface with a powerof 10 mW. The measurements were referenced to Si at 521 cm�1

with 16 data acquisitions in 120 s. The lens focus was 100 times.Solid-state 29Si NMR nuclear magnetic resonance measure-

ments were performed for selected samples in a Bruker AC 300/Pspectrometer using a zirconium rotor with 4.5 kHz of resolution.The spectra were referenced to tetramethylsilane (TMS).

2.4. Catalyst testing

Knoevenagel condensation reaction was studied in a 30.0 cmi.d. fully baffled mechanically agitated reactor of 300 cm3 totalcapacity, which was equipped with a thermostatic bath and areflux condenser. The reactor was kept in an isothermal bathwhose temperature could be maintained at the desired value byusing a temperature indicator controller. Initially, the experimentswere carried out at atmospheric pressure using an equimolar mix-ture of ethyl cyanoacetate and butyraldehyde (4.6 mmol each), and0.1 g of the as-synthesized catalyst based on total liquid volume at80 �C and 1200 rpm. Aliquots of the mixture were withdrawn atregular intervals. The mixture was then centrifuged and rapidlycooled and the amount of the product formed was determinedby gas chromatography using a CG, 17A model from Shimadzuchromatograph equipped with a FID detector. The productsobtained were also identified by gas chromatography coupled witha mass spectrometer (CG-MS) QP 2010 Plus model from Shimadzuusing a RTX-5MS column operating in the 80–280 �C range, underhelium flow (1.93 mL min�1).

The yields were defined as the percentage of the starting butyr-aldehyde converted into products. The conversion and yields arethe same for those reactions, in which a single product is formed.

The effects of the reaction conditions such as temperature, typeof solvent and loading of catalysts were studied in detail.

3. Results and discussion

3.1. Structural characterizations

3.1.1. XRDStructural properties of the lamellar silicates obtained are given

by XRD. The layered structure of as-synthesized kenyaite andmagadiite is confirmed by the patterns, as shown in Fig. 1.

Magadiites exhibit a well known diffractogram with the mainpeaks associated to the (001), (002), (003), (020) and (021)planes, which are characteristic of their monoclinic structures(Fig. 1a). As magadiite has the chemical composition of Na2Si14O29�9H2O [21], the presence of alkaline species present in the interlayerspace of NaMag is confirmed by the sharpness of the d00l and d021

peaks at 2h = 5.94 and 25�, respectively in close agreement withthe JCPDS card 42-1350 [21,22]. Chemical composition of theNaMag is close to that described in literature. Modeling studieshave shown that magadiite has a lamellar structure composed ofone or multiple negatively charged sheets of SiO4 tetrahedra withabundant silanol terminated surfaces [22], although the exactstructure of magadiite remains unknown. Indeed, the silicate lay-ers of magadiite are compensated by either Na+ or H+ ions in theinterlayer spaces. The intensity of the XRD peaks gradually weak-ened, when ion exchange of Na+ ions to obtain protonic magadiiteis performed and the other small peaks gradually disappeared.Accordingly, the basal spacings of NaMag from (001) reflection isobserved to be 1.58 nm, as suggested by JCPDS card 29-0668. Adecrease in ordering degree of the lamellar structure of HMagwhen compared to NaMag is observed due to lack of ions function-alities as well as loss of water upon replacement sodium by pro-tons in the interlayer spaces; thus, HMag has a basal spacing of1.10 nm for 2h = 7.81�. The basal spacings are in agreement withthose previously reported for magadiites (1.55 nm for Na+-magadi-ite, 1.32 nm for H+-magadiite) [19–31].

260 M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267

The decationization of magadiite by ion exchange processresults in a more or less pronounced decrease of the unit-cell peri-odicity in direction perpendicular to the layers, since the stronglinks between adjacent crystal sheets via coordinatively boundsodium cations are replaced by much weaker hydrogen bonds[23], in close agreement with our results. Magadiite containingAl3+, Er3+ or Co3+ cations have the trivalent metals chemical com-positions close to that obtained theoretically e.g. 2 wt%. XRD pat-terns of the trivalent cations-containing magadiites reveal thatall these solids displays intense diffraction peaks compared toHMag. The characteristic peak corresponding to (001) reflexionof magadiite is slightly shifted to 2h values of 5.85, 5.78 and5.93� in AlMag, ErMag, CoMag, respectively. As consequence, theinterlayer distances little decreased from 1.58 to 1.50 nm, 1.53and 1.48 nm, respectively. This indicates the hydrothermal synthe-sis of the solids can provide the trivalent cations insertion in thesilicate layer. Previous studies also have suggested that incorpora-tion of divalent and trivalent cations in the silicon network resultsin the following phenomena (i) stronger acidic sites and a strongerinteraction with the hydrated sodium cations to maintain thestructural neutrality is likely [24] and (ii) a little contraction ofthe interlayer distance due to the arrangements of cations withregard to the silica framework, which depends on the nature ofthe exchangeable cation [31]; consequently, the proximity of thesequence of the lamella should be possible. The aforesaid effectsdo not change the structure of the solid. Additional phase such astrydimite may appear at 2h = 20.7, 22.4 and 36.0� for AlMag andErMag due to the fact that the surroundings small crystallites actsas seeds in the reaction gel forming phases, which are thermody-namically more stable than magadiite [25]. Nevertheless, thesepeaks are superimposed with those of magadiite phase; therebyit is difficult to ascertain the presence of trydimite.

XRD patterns of kenyaites are shown in Fig. 1b. For sodium-con-taining kenyaite, the characteristic (001) reflection at 2h value of4.42� corresponds to the interlayer spacing of 1.98 nm, which issimilar to that reported in the literature for tetragonal kenyaitecrystal and in agreement with the findings [25]. Moreover, chemi-cal composition of kenyaite is given as Na2Si22O41(OH)8 6H2O andthe exact structure of kenyaite remains unknown because no suit-able single crystal for X-ray diffraction analysis has ever beenobtained. Both monoclinic trydimite (2h = 20.7 and 36.2�) and cris-tobalite (2h = 21.8 and 31.2�) phases appear in concomitance withkenyaites. The intensities of the peaks of HKen are little weakerthan those of alkaline form suggesting that during the ionexchange process, the relative crystallinity as well as the basaldistance (1.60 nm) slightly decreased, as compared with those ofNa-Ken. This was found to be very similar to observations reportedelsewhere [25,26]. The structure of kenyaite is maintained espe-cially when aluminum and erbium are incorporated with somemodifications in the positions and intensities of the peaks due tothe presence of the cations, as observed for kenyaites [27]. Thebasal spacing of ca. 1.25–1.37 nm between the layers is observedand these values can be explained due to the same factors relatedfor magadiites. In contrast, cobalt may have collapsed the crystalsurface of the layered silicate and a more significant decrease ofthe crystallinity is observed. Magadiite transformation intokenyaite can be likely due to alkali ions presence, temperature ofcrystallization, among other effects [25,27]. Moreover, thetrydimite and crystobalite phases appear in concomitant with thatof kenyaite suggesting the incorporation of the trivalent cationsinto the lamellar silicates.

3.1.2. Raman spectroscopyRaman spectra for as-synthesized magadiites are shown in

Fig. 2. From the spectroscopy point of view, magadiites belongsto the 2/m (C2h) space group. NaMag spectrum displays the

presence of Raman modes at around 1044 (mas(SiAOAH2OANa+)),1181(mas(SiAOASi)) and 1238 cm�1 (mas(SiAOASi)), which areassociated to the asymmetric stretching of the silanol groups, ingood agreement with the previously reported data [27–29]. Abroad band at around 1400 cm�1 is assigned to structural waterpresence in the layered silicate (Fig. 2a1). Additionally, modes ataround 750 and 952 cm�1 appear and they are attributed tostretching vibrations of (ms(SiAOASi)). At higher frequency region,modes at around 610, 455, 497 and 430 cm�1 are ascribed to bend-ing of SiAOASi linkages whereas those at around 398, 361, 327,153 and 117 cm�1 are associated to d(SiAO) of the lattice modes[28]. Modes at around 946 and 948 cm�1 can also be associatedto the OH groups vibrations. It is important to point out that thevery strong Raman mode at about 465 cm�1 is assigned to thesymmetrical stretching ms(SiAOASi) of the six-membered rings ofsilicon-oxygen tetrahedra [28–31]. Protonic magadiite has thesame spectral features of NaMag, except the fact that the frequen-cies are displaced to low wavenumbers regions, suggesting aweakening of the modes in reason of sodium exchange by protonsin the interlayer region. This result is in accordance with XRD thatindicate a perturbation of the lamellar silicate structure due toreplacement of water and sodium by protons in the interlayerspaces. For magadiites containing trivalent cations (Fig. 2a2), themodes are found at 1240, 1186, 1042 cm�1, which correspond tothe standard Na-Mag. These modes have an additional shift of10 cm�1 compared to NaMag and this is attributed to the weaken-ing of the SiAO bond relative to the Me3+AO bond, especially forErMag and AlMag. In case of CoMag, Raman modes intensitygreatly weakened confirming the same effects of cobalt incorpora-tion suggested by XRD analysis. Absence of modes of the oxides inthe Raman spectra of the trivalent cations-containing magadiiteindicates the purity of the material.

Raman spectra of as-synthesized kenyaites are shown inFig. 2b1. The unit cell symmetry of kenyaite is either C4h or D4h,in which the silicates have multilayer structures with five – andsix-membered rings [29]. At low wavenumber regions, both NaKenand HKen exhibit strong vibrational modes at 455 and 410 cm�1

which are associated with the breathing mode of the silicon tetra-hedron from six-membered rings kenyaites [29]. A broad and lessintense mode at around 770 cm�1 is due to bending of SiAOASigroups d(SiAOASi)). Below to 400 cm�1, the modes are owing tothe deformation of SiAOASi and OASiAO groups as well as thesymmetrical stretching of NaAO bonds ms(NaAO). The weak modeat around 1061 cm�1 is ascribed to the asymmetrical stretching ofSiAO� bonds. In addition, the bending vibrations of water mole-cules appear centered at 1400 cm�1, as aforesaid. Raman spectrumof NaKen is similar to that of HKen, with exception of the diminish-ing of the modes at 455 and 410 cm�1; indeed, appearance of fewother modes from (ms(SiAOASi)) at about 643, 696, 760, 790 and820 cm�1 can be noted. Modes associated with either trydimiteor cristobalite may appear in the same region of kenyaite phaseand no distinguishable features among these phases are visibleby Raman spectroscopy.

Raman spectra of the trivalent cations-containing kenyaites (Fig2b2) reveal that all these solids are hardly affected by the trivalentcations presence, since the Raman modes of Fig. 2b1 differs fromthose of Fig. 2b2. Consistently with presence of a Me3+ in the lay-ered silicate structure, the modes shifted to high frequenciesregions mainly at 407 and 468 cm�1 both of them related toms(SiAOASi) and those at around 315 and 1044 cm�1 due tom(SiAO�), (SiAOASi) and d(OASiAO) vibrations as well. Further,the mode at about is 765 cm�1 experience a sharpening, whichsuggest that the (ms(SiAOASi)) are largely affected or suggestingsome aluminum species segregation [31]. In case of ErKen, allmodes shifts to lower frequencies values than those of NaKenmainly that at about 1045 and 1065 cm�1., implying that an

200 400 600 800 1000 1200 1400

1181

123810

44

946

Ram

an in

tens

ity (a

.u.)

Wavenumber (cm-1)

NaMag

HMag

986

(a1)

200 400 600 800 1000 1200 1400 1600

CoMag

AlMag

Ram

an in

tens

ity (u

.a.)

Wavenumber (cm-1)

ErMag

(a2)

200 400 600 800 1000 1200 1400 1600

Ram

an in

tens

ity (a

.u.)

Wavenumber (cm-1)

Wavenumber (cm-1)

NaKen

HKen

(b1)

200 400 600 800 1000 1200 1400 1600

CoKen

Ram

an in

tens

ity (a

.u.)

1044

460

207

158

422

AlKen

ErKen

(b2)

Fig. 2. Raman spectra of the solids: (a) magadiites and (b) kenyaites.

M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267 261

important modification of the silicate lamella has occurred due tosilicon replacement inside the original structure by the trivalentcation. Also, the presence of erbium promotes luminescence insome compounds [32], but this is not observed in this work dueto the low amount of the trivalent cation in the solid. At frequen-cies below 422 cm�1, vibrations are ascribed to silicate layers andcharge balancing cations are suggested. Additionally, the hypothe-sis of trydimite and cristobalite modes superimposition in case ofAlMag and ErMag can be not neglected, as suggested by XRD. Onthe other hand, CoKen has their Raman modes intensity greatlydecreased even the intense feature at 430 cm�1 almost disappearsand this denotes the SiO4 perturbations due to cobalt presence inthe interlayer region. Raman results for kenyaites thus corroboratewith the observations made from XRD analyses.

3.1.3. TEM micrographsFig. 3 gives TEM images of selected samples. The clear rectangu-

lar plate like particle is observed in the micrographs of NaMag [33].Moreover, the lattice fringes indicate that magadiites are highlycrystallized. The self-aggregation of the tiny particles results fur-ther in pore textures corresponding to the aggregation of the plate-lets. The trivalent cation incorporation in magadiite such as inErMag gives a layered structure similar to NaMag, although its lay-ers are curved and arranged disorderly. It is hard to find the inter-planar distance because at the edge of ErMag crystal lie severalpieces of curved layers, which are supposed to be other phasessuch as trydimite and crystobalite (suggested by XRD) deposited

on its external surface. Tentatively, estimation of the d spacing is2.6 nm which is close to the value measured by XRD e.g.,2.82 nm. TEM images of magadiites are almost identical to thatof kenyaites (not shown).

For ErKen, TEM images display a tangle of shapeless crystalstogether with crumpled silicate layer, which is consistent withthe formation of isolated silicate layers, as suggested by XRD thatshows tridymite and cristobalite phases formation. Ordered nano-particles with uniform spacing of 2–3 nm between the layers areobtained from the TEM images of ErKen, in line with the findings[34,35].

3.1.4. 29Si NMR29Si NMR spectra of selected samples are shown in Fig. 4.

NaMag gives a strong resonance at �98.9 ppm along with a muchweaker resonances at �110.6 and 113.5 ppm. The findings statesthat the signal at around �99.1 ppm is assignable to siliconatoms in a Q3 tetrahedral environment, as for HOSi(OSi)3 orNa+(OSi(OSi)3) [31,36]. The weaker signals are associated with thesilicon in a Q4 configuration such as Si(OSi)4 [37,38].The spectrumof ErMag indicates that the three characteristic peaks of magadiitedo not shift after erbium incorporation; that is to say, the modifi-cations in the SiAO bond lengths and SiAOASi angles upon substi-tution of Na+ by Er3+are not visible. Both magadiites have a Q3/Q4

intensity ratio of ca. 0.33, which is consistent with that depictedpreviously [31,37]. Pinnavaia et al. proposed that the layers of sil-icon could be viewed as arising from double sheets of Q4 tetrahedra

Fig. 3. TEM images of ErMag and ErKen lamellar silicates.

Fig. 4. 29Si NMR spectra of as-synthesized lamellar silicates.

262 M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267

Table 1Textural properties of the as-synthesized lamellar silicates. Surface area (Sg), porevolumes (Vp) and diameter (D) of the solids studied. Total acidity measured by NH3-TPD.

Catalyst Sg (m2 g�1) Vpa (cm3 g�1) Da (Å) NH3 (uptake) lmol gcat

�1

NaMag 39 0.11 270 1.5HMag 54 0.15 124 128.2AlMag 15 0.09 116 185.2ErMag 41 0.18 179 21.7CoMag 47 0.19 137 12.6NaKen 27 0.07 124 1.1HKen 30 0.05 62 120.2AlKen 84 0.07 86 163.2ErKen 12 0.03 103 11.3CoKen 77 0.15 104 14.4

M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267 263

with 25% of the tetrahedra ‘‘inverted’’ to form Q3 units [38]. Theseresults can be read as a layered structure with a constant amountof Q3 and Q4 environments, where the layers of silicon would con-sist of five planes of atomic oxygen, in reasonable agreement withthe observed basal spacing obtained by XRD for NaMag and ErMagand Raman observations. However, XRD and Raman results havepredicted a slight ordering loss in the structural network of theErMag magadiite layers. Unexpectedly, no evidence for this phe-nomenon is suggested by NMR and it may be interpreted as localrearrangement in the layers without affecting the magadiitestructure.

The spectra of kenyaites (Fig 4) exhibit four signals 98.7 (Q4),107.3 and 110.3 (Q3) and 112.5 ppm as well. These values arealmost same as those of sodium kenyaites [31]. The Q3/Q4 ratioof these signals is 0.33. The resonance at �98.7 ppm is due to sili-con atoms in Q3 [Si(OSi)3(OR)], in which R is either H or Na species.The very weak resonances are assigned to Q4 (Si(OSi)4) species, asobserved for other silicates [39]. Again, the nature of the cationsdoes not appear to provoke any displacement of the Q3 and Q4 sig-nals and thus, any significant influence the stacking of elementarylayers on the polarizability of the interlayer species is observed.

3.2. Textural properties of the solids

The nitrogen adsorption–desorption isotherms and the pore-size distributions of as-synthesized kenyaites and magadiites areshown in Fig. 5.

Isotherms of magadiites exhibit considerably sharp nitrogenuptakes at very low pressure similar to that of type III isotherm.The intermediate P/Po region corresponds to gradual increase innitrogen uptake, together with H3-type hysteresis desorption loopsat high relative pressure, characteristic of split shaped poresbetween the plaques, in accordance with IUPAC classification[40]. NaMag has BET surface area of 39 m2 g�1, pore volume ofca. 0.11 cm3 g�1and pore size of ca. 27 Å. These values are closeto that of alkaline magadiite found in the literature [41–43]. Sur-face areas of magadiite cannot be totally ascribed to the externalsurface because of their layered structure, in which the nitrogenhas access. Hence the low surface area of NaMag is a consequenceof the adsorption nonporous external surfaces.

The textural properties slightly increased, when ion exchange ofsodium by protons is performed on HMag possibly owing to thethicker silica walls and the removal of residual sodium and this

0.0 0.2 0.4 0.6 0.8 1.0

Ads

orbe

dvo

lum

e (c

m3 .g

-1)

P/Po

ErKenErMag

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0.02

0.04

0.06

0.08

dV/d

log(

D) P

ore

volu

me

(cm

³g-1

)

Pore Diameter (Å)

ErKen

ErMag

Fig. 5. Nitrogen adsorption–desorption curves of ErKen and ErMag. The includedfigures are the corresponding pore size distribution of the solids.

is in accordance with the decrease of the lamellar spacing. It wasnoticed that the surface areas gradually decrease compared toNaMag with trivalent ions incorporation (Table 1). However, bothsurface areas and pore volumes increase with either erbium orcobalt incorporation and these values are almost close to HMag,which may be due to internal surface enlargement indicating thecations incorporation mainly at the interlayer region. AlMag hasvery small textural parameters, probably due to the trydimitephases presence on solid surface, as suggested by the XRD results.The pore size distribution (figure included) has a feature indicatingthe presence of micropores (70 Å) and majoritary a broad pore sizeone in the meso–macropore range (270 Å), which corresponds to afull accessibility of nitrogen to the basal planes of thick individuallayers. This agrees with the observations previously suggested byTEM, XRD and Raman measurements. HMag shows a gradualdecrease of its pore size while a development of the microporosityis observed. The trivalent cations-containing magadiites aremicro–mesoporous materials with about 70% of their surface inpores larger than 116 Å.

Nitrogen adsorption–desorption and pore sizes distributionsprofiles of kenyaites are similar to that of magadiites (Fig. 5). Theisotherms are well described by high nitrogen uptake at low pres-sure due to micropores meanwhile a progressive nitrogen uptakewithout a sharp capillary condensation step at intermediate pres-sure giving read either a high external surface area or a broad poresize distribution [33]. The textural properties given in Table 1 sug-gest the surface area and pore volume increase, when cationsincorporation is performed. In addition, the micropore and rela-tively large mesopore volumes for ErMag and CoMag are observed.NLDFT studies of N2-sorption isotherms reveal the microporosityoriginates from the formation of 8-membered ring channels bycondensation of the layered silicates [33]. The corresponding poresizes distribution of the as-synthesized kenyaites show featuressimilar to that of magadiites (inset of Fig. 5). It can be observed thatthe sizes of the mesopores and macropores are in a broad area.Accordingly, the pore diameters show much less variation forCoKen and AlKen, whereas pore diameter of ErKen is very low(Table 1). Drying and washing process of layered silicates are inad-visable because of undesired crosslinking of the silica sheets, espe-cially in case of some ionic exchange of sodium by cations canresult in low surface areas and this could prevent the intercalationof the cations in the silicates, as for ErKen.

3.3. Acidity by NH3-TPD measurements

Alkali-magadiite and kenyaite possess similar NH3-TPD spectra(not shown), thus revealing a lack of acid sites due to the presenceof a relatively large amount of sodium. In contrast, the NH3-TPDresults (Table 1) reveal that the relatively medium acidity ofprotonic HMag and HKen, (e.g., 120.5 and 128.2 lmol gcat

�1,

100

Without solventEtOAcCH2Cl2Selectivity

264 M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267

respectively) is ascribed to a large number of hydroxyl groups cre-ated by ion exchange of Na+ by H+. Though 1H NMR studies men-tion the importance of the structure of lamellar magadiite andkenyaite on providing the existence of acid sites. The structuralmodel from NMR and IR studies consists of a multilayer structureof six-members rings of tetrahedra and blocks containing five-member rings attached to both sides [28,31]. Other authors arguedthe presence of additional SiAOASi linkages would provide verylarge bond angles near 180� and the presence of structural water[31]. Based on this information, the presence of both OH groupsinvolved in relatively strong hydrogen bonds between adjacentlayers and ‘‘free’’ OH groups, probably pointing to holes of the nextlayers is likely. On this base, the acidity of the silicates upon protonexchange can be taken into account.

Aluminum incorporation on both magadiite and kenyaiteincrease the number of relatively medium acid sites (Table 2). Itwas found that aluminum plays a very important role in enhancingthe acidity of lamellar silicates due to greater acid strength ofBrønsted- and Lewis-acid sites associated with framework andextra-framework aluminum species [19,23].

The Co-containing lamellar silicates exhibit very few changescompared to that of AlMag. The similarity between CoMag andCoKen indicates that these solids have close acidity, however, les-ser than that of Al-based lamellar silicates. Cobalt is known to haveacid sites of low strength in silicates [44], in agreement with theobtained results.

NH3-TPD studies on Er2O3 suggested that the oxide have noacidic sites [45]. Keeping this result in mind, the low number ofacid sites in both ErKen and ErMag can be reasonably explainedby the OH groups from silicates. In addition, no differencesbetween the magadiites and kenyaite are observed, when metal-ion exchanges are performed.

3.4. Catalytic performance of the as-synthesized solids

Catalytic results in Knoevenagel condensation between butyral-dehyde and ethyl cyanoacetate reaction are shown in Table 2. As afirst attempt in this study lead the as-synthesized catalysts, thereaction is carried out in mild conditions using toluene as solvent.Alkali NaKen and NaMag are inactives in the reaction due to theirlow textural properties. In addition, because the reaction requiresthe presence of either Lewis acid–base pairs or strong Brønstedcenters, these results are similar to that obtained when alkali-exchanged Y zeolites are used as catalysts for reaction [3].

It is found that in the first 1 h of the reaction period, the butyr-aldehyde conversions decay to zero over protonic HMag and HKen.This could be attributed to the presence of magadiite and kenyaitephases possessing acid sites of low strength [23], which are unableto promote the Knoevenagel condensation. Additionally, selectivityto ECPA is negligible due to the low conversions achieved over theprotonic solids. Even having higher textural properties, these solids

Table 2Knoevenagel condensation of butyraldehyde and ethyl cyanoacetate catalyzed by as-synthesized silicates. Reaction conditions: temperature of 90 �C; solvent: toluene;reaction time 4 h and catalyst mass of 100 mg.

Catalyst Conversion ofbutyraldehyde (%)

Selectivity toECPA (%)

HMag 0.1 –AlMag 1.0 –ErMag 12.3 95CoMag 4.8 –HKen 0.5 –AlKen 2.2 –Er Ken 90.7 97CoKen 6.1 –

deactivate along of the reaction because surface Brønsted sites maybe reduced, which seems to be responsible for lessening activity.This is contrary to what is expected upon using AlMag, AlKen,CoMag and CoKen metal incorporated catalysts; but the conversionof butyraldehyde is found to be less over the solids possessingtrydimite and cristobalite phases. The selectivity to ECPA productwould not be simply dependent on the accessibility of the sub-strates to the acid sites of the as-synthesized AlMag, AlKen, CoMagand CoKen catalysts. Hence, the incorporation of Al3+ and Co3+ ionsinto the lamellar silicates would impede the accessibility of thereactants to these Lewis acid centers resulting in small conver-sions. Another factor to be considered is the fact that the contribu-tion of leaching of the trivalent Co3+ and Al3+ species would beresponsible for deteriorating the catalytic performance of the as-synthesized solids, as shown by chemical analyses of the solutionsafter the catalytic tests. In addition, the trydimite and cristobalitesilicates phases present in kenyaites are inactive phases for thereaction.

The NH3-TPD results reveal that the amount of acid sites pos-sessing strong or medium acidity is not relevant for Knoevenagelcondensation, since a poor catalytic performance of protoniclamellar silicates is observed. In case of the trivalent cations-con-taining silicates, the large number of hydroxyl groups of the cata-lyst and their relatively weak acidity is not favorable for highcatalytic efficiency in this catalyst system. Particularly, ErMagand ErKen samples demonstrate significant performance due tothe efficiency of their weak acid sites in the catalytic reaction.However, other factors besides the acidity should be taking intoaccount.

Moreover, the conversions overcame 12% in case of ErKen andErMag after 4 h of the reaction. Elevated selectivities are reachedvery fast with using these catalysts, thereby giving rise to up 95%of Knoevenagel product selectivity after 4 h of the reactionbetween butyraldehyde and ethyl cyanoacetate. Again, the state-ment concerning the structure, texture and chemical interactionbetween trivalent cations and silanol species is assumed by thisphenomenon. Therefore, ErKen and ErMag are chosen to the fur-ther study of the condensation of Knoevenagel towards the influ-ence of the temperature, catalyst loading and use of solvents.

3.4.1. The effect of solvent and temperature dependenceThe reaction is carried our using BUT/CAE molar ratio of 1 at

90 �C in the presence of dichloromethane (CH2Cl2), ethyl acetate(EtOAc) or toluene (C7H8) as well as in the absence of solvent.Fig. 6a summarizes the effects of solvent in function of the time.

The condensation reaction between butyraldehyde and ethylcyanoacetate under solvent-free conditions is somewhat fast with

2 4 6

20

40

60

80

Buty

rald

ehyd

e co

nver

sion

(%)

Time (h)

Without solventEtOAcCH2Cl2Selectivity

0

Fig. 6. (a) Evolution of the butyraldehyde conversion at T = 90 �C in function of thetime for various solvents tested.

M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267 265

96% conversion over ErKen; however, selectivity experiences decaydue to ECPA oligomerisation byproducts formed. Moreover, previ-ous studies have shown that Knoevenagel condensation is effi-ciently promoted in the absence of any solvent [2,10].Nevertheless, the high exothermicity of the solvent-free reactionmakes these conditions undesirable. It should be noted that con-versions over NaMag are lower than those of ErKen, independentlyof the solvent used.

The comparison of the reactivities of BUT and CAE with thethree solvents selected viz., toluene, dichloromethane and ethylacetate indicates that the most polar solvent is less reactive in allcases. Similarly, Shanks et al. suggested a decreased yield of theproducts in case of using polar solvents due to the interfaces ofthe solvent in the catalyst-substrate interactions [43], in line withour results. In addition, hydrophilic solvents could preactivate thecarbonyl group of BUT, which now has enhanced nucleophilicitygiving rise to condensation reaction with ECPA molecules [5].Among the three solvents studied, high conversions and selectivi-ties to the product are obtained with toluene for ErKen (Table 2),since nonpolar aprotic solvents do not affect the mass transfer orsteric hindrance of the active sites.

Moreover, toluene improves ECPA production as compared withpolar solvents. It could be explained by the fact that the bulkinessof toluene imposes serious restriction on the formation and/or

2 40

20

40

60

80

100

ErKen 150 mg ErKen 50 mg ErMag 50 mg

Buth

yral

dehy

de c

onve

rsio

n(%

)

Time

ErKen 100 mg ErMag 100 mg ErMag 150 mg

30 60

20

40

60

80

100

Conversion over ErKen Selectivity over ErKen Conversion over ErMag

Buty

rald

ehyd

e co

nver

sion

(%)

Temperatur

Selectivity over ErMag

(a)

(b)

Fig. 7. (a) Effect of the temperature on the conversion and selectivity to E

diffusion for occurrence of possible side reactions, such as thecross-aldol condensation, self-condensation of butyraldehyde [5].

The results indicate that a detailed study with suitable temper-ature and mass of catalyst to optimize the reaction may provewhatever ErKen or ErMag can be considered as potential catalystfor Knoevenagel condensation.

The reaction is performed in the presence of toluene varying thetemperature. From Fig. 7a, low temperatures such as 30 �C disfa-vors the reaction meanwhile temperatures superior to 60 �C showsa level of conversion that is obviously lower than the standard assynthesized MCM-41 tested at room temperature in Knoevenagelcondensation [10]. Conversion increases linearly up to 5% withincreasing the temperature over both ErKen and ErMag due tothermodynamic reasons for an endothermic reaction. A compari-son of the performance of catalysts shows that the differencesbetween ErMag and ErKen appear at 60 �C, where BUT conversionover ErMag is below the minimum value reached for ErKen, whichis indeed 100% at 120 �C. An important aspect of this analysis is theconfirmation that the trivalent cation Er introduced into the sili-cate lattice of the layered solids has changed the silicate properties,since the profiles of the curves are not the same. It can be reason-able to suppose that affinity between Si and Er takes place inkenyaite structure and could explain the better performance ofErKen, since Er remains in the framework during the reaction. On

6 8 10 (h)

90 120

Sele

ctiv

ity to

EC

PA (%

)

e (oC)

20

40

60

80

100

CPA over ErKen and ErMag. (b) Catalyst mass in function of the time.

266 M.E.R. Oliveira et al. / Chemical Engineering Journal 263 (2015) 257–267

the contrary, leaching of the erbium in magadiite would be respon-sible for the lesser performance of ErMag.

Carefully compare the selectivities to ECPA, a drop of ECPA pro-duction is observed at elevated temperatures. This observationindicates that the oligomers formation has occurred together witha greater stacking disordered the layers due to oligomers deposi-tion over both catalysts. No isomers of the products are identifiedby GC–MS. At 90 �C, the ECPA is the sole product formed with usingErKen catalyst with a conversion of 100% whereas only a negligibleconversion (20%) is noted over ErMag. Thus, each catalyst studiedon its own is ability to promote the ECPA formation. This is an illus-tration of the structural features and the coexistent acid–base andredox pair in ErKen may influence in the reaction. The findingsstates that the Knoevenagel condensation reaction is constrainedby an unfavorable thermodynamic equilibrium [46]. Assuming thatthe reaction is a second-order reversible kinetic reaction [43], it islikely that the addition step of the enolate anion to the butyralde-hyde in the imine mechanism is the rate-limiting step of the reac-tion, which is hindered in ErMag due to the cited oligomersformation. Additionally, the cooperativity between the silanolacidic sites and the Er cation may not be disturbed by elevatedtemperatures, which indicates that the reactants have access tothe silica surface.

3.4.2. On the catalyst loading effectCatalyst mass is varied to increase the amount of the active sites

for the reaction. The performance of ErMag is not as good as ErKen,it only gives 10% conversion (Fig. 7b), implying that the accessibil-ity of the active sites to the reactants is on surface or in the poreschannels of ErMag is hindered. Thus, butyraldehyde conversionswith different catalysts loadings in any case much lower those ofErKen. It can be explained by the fact that the increased catalystloading impedes the accessibility of the reactants to the activesites. The catalyst loadings studies carried out with ErKen showsthat the number of active catalyst sites increase (i.e., the mass ofcatalyst) influences on the catalytic performance. Butyraldehydeconversion decrease drastically very slow at high catalyst loadings,as shown in Fig. 7b; this is due to the availability of the active sitesis kept constant, but no improvement in the catalytic performanceis observed. The performance of ErKen is evident as the amount theactive sites increases, since the catalyst weight is not crucial forenabling the Knoevenagel reaction occurring selectively to thedesired ethyl 2-cyano-3-propylacrylate product. Too high catalystamount can decrease the activation of reagent and therefore,100 mg was found to be the best catalyst mass to attain a highyield of the ECPA product. This is in line with the recent papers[2,5] where the authors indicated that there is an optimum numberof active sites for the effective activation of BUT and CAE.

Er3+containing kenyaite is easily synthesized and tested inKnoevenagel reaction without significant degradation in activityby comparing with the mesoporous MCM-41 [10,15], as typicalporous silicate catalyst that represent a current benchmark in mildreaction conditions. Activity is found to be strongly dependent onthe nature of the trivalent cation and the structural features ofkenyaites and magadiites. The effects of catalyst loading, type ofsolvent and temperature on the catalytic properties demonstratedthat a 100 mg of catalyst mass, a BUTCAE molar ratio of 1 in thepresence of toluene at 90 �C are the best conditions for testingas-synthesized catalysts. As-synthesized ErKen tested in theabovementioned conditions exhibit good catalytic activities whichare higher than those observed for Ce3+-containing nanotube tita-nate type catalyst reported earlier for the same reaction [5]. Basedon the structural characterizations and influence of reactionparameters studies, the high catalytic activity and selectivity ofEr3+containing kenyaite is ascribed to the accessible lamellae witherbium incorporated into silicate layers.

4. Conclusions

Influence of reaction parameters on the catalytic performanceof kenyaite and magadiite lamellar silicates for the production ofa,b-unsaturated esters was studied. The performance of the lamel-lar silicate catalysts possessing different trivalent cations e.g., Er3+,Co3+ and Al3+ in the Knoevenagel reaction is directly related to thesilicate structure. Leaching of Co3+ and Al3+ species is detected andcontributed to deteriorate the catalytic performance of the as-syn-thesized catalysts. The role played by kenyaites and magadiites canbe also considered taking into account the catalysts loading, tem-perature of reaction and type of solvent. High activity is observedfor Er3+-containing kenyaite and magadiite catalysts comparedwith the aforesaid trivalent cations due to its elevated texturalproperties, accessibility of the reactants to the active sites as wellas its stability during the catalytic runs. Moreover, Er3+-containingkenyaite catalyst is tested with no appreciable change of activityduring the search of the best reaction parameters. The accessibleEr3+ catalytic sites may facilitate the diffusion and thus, promotedthe activation of BUT and CAE molecules, leading to the enhancedactivity and selectivity.

Acknowledgements

The authors gratefully acknowledge CETENE-INT and Universid-ade Estadual de Campinas-Unicamp for TEM and 29Si NMR mea-surements as well as CNPq sponsoring this work through theProject 490162/2011-8 and Funcap with areas estrategicas 2011program.

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