Sodium titanate as basic catalyst in transesterification reactions

Fuel 118 (2014) 48–54

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Sodium titanate as basic catalyst in transesterification reactions Letícia L. Marciniuk a,d,⇑, Peter Hammer b,1, Heloise O. Pastore c,2, Ulf Schuchardt c,2, Dilson Cardoso a a

Catalysis Laboratory, Federal University of São Carlos, Chemical Engineering Department, 13565-905 São Carlos, SP, Brazil State University of São Paulo (UNESP), Institute of Chemistry, 14800-900 Araraquara, SP, Brazil c University of Campinas, Institute of Chemistry, Cidade Universitária Zeferino Vaz, 13084-970 Campinas, SP, Brazil d Federal Technological University of Paraná (UTFPR), Institute of Chemistry, Via do Conhecimento, 85503-390 Pato Branco, PR, Brazil b

h i g h l i g h t s  The sodium titanate shows catalytic activity toward the transesterification reaction.  The material not-calcined was more stable and showed catalytic activity.  The 4th use was accompanied by the appearance of a new species of O and Na.  The new species of O and Na segregated shows the loss of activity of sodium titanate.

a r t i c l e

i n f o

Article history: Received 11 July 2013 Received in revised form 6 October 2013 Accepted 14 October 2013 Available online 6 November 2013 Keywords: Sodium titanate Sol–gel Basicity Transesterification Heterogeneous catalysis

a b s t r a c t Sodium titanate was synthesized by the sol–gel method and characterized using X-ray diffraction, thermogravimetry-mass spectrometry, atomic absorption spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis and nitrogen physisorption. The non-calcined material was active as a catalyst in transesterification reactions and showed high stability. An appreciable loss of activity on the fourth reuse was accompanied by the appearance of a new species of oxygen and segregated sodium, identified by X-ray photoelectron spectroscopy (XPS). The XPS spectrum showed that the basic nature of the framework oxygen was inferior to the original basicity, which explained the decline in catalytic activity. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Transesterification is a classical organic synthesis reaction in which one ester is transformed into another by interexchange of the alkoxy moiety (Eq. (1)). Since the reaction is an equilibrium process, the transformation can proceed by simply mixing the two components. However, it has long been known that the reaction is accelerated by acid or base catalysts [1]. Many homogeneous or heterogeneous catalysts are suitable for this purpose, and the reaction can be used as a simple tool for comparison of the basic strengths of these materials [2].

RCOOR0 þ R00 OH ! RCOOR00 þ R0 OH

ð1Þ

⇑ Corresponding author at: Catalysis Laboratory, Federal University of São Carlos, Chemical Engineering Department, 13565-905 São Carlos, SP, Brazil. Tel.: +55 (16) 3351 8693; fax: +55 (16) 3351 8266, tel.: +55 (46) 3220 2596; fax: +55 (46) 3220 2613. E-mail address: [email protected] (L.L. Marciniuk). 1 Tel.: +55 (16) 3301 9500; fax: +55 (16) 3322 2308. 2 Tel.: +55 (19) 3521 3000; fax: +55 (19) 3521 3023. 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.10.036

The most common route for the production of biodiesel involves the transesterification (or alcoholysis) reaction between an alcohol and a triacylglyceride ester [3]. Typical liquid-phase catalysts used are NaOH, KOH, HCl, H2SO4, and HNO3. Many heterogeneous catalysts may also be used: metals, transition metal compounds, organometallics and anchored metal complexes, as well as solid bases and acids (including inorganic oxides such as Al2O3, SiO2, SiO2Al2O3, zeolites, ion-exchange resins, TiO2, ZrO2, alkaline earth metal oxides, hydrotalcites, and carbonates, amongst others) [4–8]. The basic catalysts are most active in the reaction, favoring faster reaction rates under milder conditions [9]. This advantage, together with the inherent benefits of heterogeneous catalysis, such as the possibility of recovering and reusing the catalyst, has intensified the search for suitable basic heterogeneous catalysts [10]. The primary material used to produce biodiesel is triacylglyceride (glycerol ester), found in vegetable oils and animal fats. In addition to this compound, various other substances contribute

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L.L. Marciniuk et al. / Fuel 118 (2014) 48–54

to the compositions of oils and fats, such as free fatty acids, phospholipids, and carotenes (which are responsible for the color of oils). It is therefore preferable to use model molecules, such as pure esters, in studies of the transesterification reaction chemistry and investigations of the influence of catalysis on the substrate [7]. The present work concerns the evaluation of new basic catalysts composed of sodium titanate. This anionic lamellar compound consists of octahedra of titanium and oxygen (TiO6). Alkaline metal cations, which compensate the charge of the titanate anions, are located in the interlamellar spaces [11] and can be exchanged for protons by washing with aqueous acid solutions [12]. The lamellar structure is maintained, but there is a shift from basic to acidic properties [13]. A further consideration is that the physical and chemical characteristics of alkaline metal titanates are dependent on the synthesis method and the type of precursors employed [14]. Morgado et al. patented the use of protonated lamellar titanates as acidic heterogeneous catalysts for the production of biodiesel [15]. However, there have been no previous reports concerning the basic properties of these materials and their application as basic catalysts. 2. Experimental 2.1. Titanate preparation The initial sodium titanate, TS-0, was synthesized by the sol–gel method according to the procedure of Ramírez-Salgado et al. [16], in which the precursor is titanium tetrabutoxide. Two solutions were prepared and heated to 55 °C: (A) 21 g of titanium tetrabutoxide in 50 mL of butan-1-ol, and (B) 1.6 g of sodium hydroxide in 100 mL of water. After mixing, these concentrations of solutions A and B resulted in a Ti/Na molar ratio of 1.5. The two solutions were added simultaneously to an excess of water (230 mL) at 55 °C, to achieve a rapid hydrolysis and condensation of the precursor. The mixture was agitated for 40 min at this temperature, and the excess alcohol was removed using a rotary evaporator. The solid was dried at 60 °C for 24 h. The influence of calcination temperature was then evaluated by calcining samples of the solid for 2 h at 200, 400, 600, and 800 °C.

ture range was 25–1000 °C, and the heating rate was 10 °C min1. The instrument was coupled to a ThermoStar GSD301 mass spectrometer, and the intensities of the signals corresponding to water, carbon dioxide, and butanol were monitored during the heating process. The atomic percentage of sodium was determined using a Varian Spectra AA-200 flame atomic absorption spectrometer, operated in emission mode at 589 nm. The specific areas of the catalysts were determined by the physisorption of N2 using the static volumetric method and a relative pressure interval of between 0.002 and 0.99 atm. The experiments employed a Micromeritics instrument (model ASAP 2010). The samples were first activated under vacuum at 200 °C for 2 h to remove adsorbed water and any other gases that might be present. They were then cooled to the temperature of liquid nitrogen (196 °C), followed by adsorption using a mixture of 10% N2 in He. The catalytic activity of the solids was evaluated using the transesterification reaction of ethyl acetate with methanol (Eq. (1)). For successful comparison of the influence of the catalysts in the conversion of the esters, it was important to use mild reaction conditions that resulted in low conversion rates (around 30%). This was achieved using an ester/alcohol molar ratio of 1:6, a quantity of catalyst equivalent to 4% of the total mass of the liquid phase, and a reaction time of 30 min at 50 °C. After the reaction, the liquid phase was analyzed by gas chromatography, using a Shimadzu GC 2010 instrument fitted with a flame ionization detector (FID) and a capillary column RT-X1, 100% dimethyl polysiloxane (30 m  0.25 mm  0.25 lm). In all the experiments, the reaction was 100% selective, with formation of only the products indicated in Eq. (2).

CH3 COOACH2 CH3 þ CH3 AOHƒƒƒ!CH3 COOACH3

þ CH3 CH2 AOH

ð2Þ

After each run, the catalyst was recovered by centrifuging, washed with methanol, and dried at 60 °C prior to the next run. The solids that had been used in the reactions were again characterized in order to identify any possible alterations in their composition, structure, morphology, and basicity.

2.2. Characterization of the catalysts

3. Results and discussion

The catalysts were characterized by X-ray diffraction, using a Rigaku Multiflex instrument, with Cu Ka radiation (k = 0.154056 nm). The results were compared with the diffractogram obtained for a standard (CAF 31-1329). The X-ray photoelectron spectroscopy (XPS) analyses were carried out at a pressure of less than 107 Pa, using a UNI-SPECS UHV spectrometer. The Mg Ka line was employed (hm = 1253.6 eV), and the analyzer pass energy for the recording of high-resolution spectra was set to 10 eV. The inelastic background of the Ti 2p, Na 1s, O 1s, and C 1s electron core-level spectra was subtracted using Shirley’s method. The composition of the near-surface region was determined, with an accuracy of ±10%, from the ratio of the relative peak areas, corrected by Scofield’s sensitivity factors for the corresponding elements. The spectra were fitted without placing constraints using multiple Voigt profiles. The width at half maximum (FWHM) varied between 1.2 and 2.1 eV, and the accuracy of the peak positions was ±0.1 eV. A Philips scanning electron microscope (model XL30-FEG), operated at 30 kV, was used for the micrographs and energy-dispersive X-ray analyses. The thermogravimetry-mass spectrometry analyses were performed using a Setaram Setsys Evolution 16/18 instrument, under an N2 atmosphere, with a gas flow of 16 mL min1. The tempera-

The X-ray diffraction pattern of the sodium titanate synthesized by the sol–gel method (prior to the calcination step) showed broad bands of low intensity, indicative of an amorphous material with a poor level of organization (Fig. 1a). The well-defined diffraction pattern of the sodium titanate was observed following thermal treatment at 800 °C (Fig. 1b). The catalytic activities of the materials in the transesterification reaction were evaluated before and after calcination. Fig. 2 displays the conversion of ethyl acetate in the presence of the titanate, assynthesized (TS-0) or calcined at 200, 400, 600, and 800 °C. The reaction was performed under mild conditions (50 °C), so that the different materials could be clearly distinguished. The as-synthesized titanate was found to be most active in the transesterification, while increasing the calcination temperature progressively decreased the catalytic activity of the solids. With increased temperature, there was either a reduction in the number of active sites that promoted the transesterification reaction, or inclusion of a fraction of these sites within the interlamellar space, where they were not accessible to the reagents. This behavior is discussed below, using the O 1s and Na 1s XPS spectra. As the titanate that had not been calcined was the most active catalyst, the evaluation of catalyst stability was only performed using the TS-0 material. Fig. 3 shows the conversion of ethyl

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L.L. Marciniuk et al. / Fuel 118 (2014) 48–54

35

500

(a) Ethyl acetate conversion, %

400

300

Intensity [cps]

200

100

500

(b)

30 25 20 15 10 5 0

400

1

2

300

100

10

20

30

40

50

2θ Fig. 1. X-ray diffractograms of the titanates, (a) before the calcination step, and (b) after calcination at 800 °C.

50

Ethyl acetate conversion (%)

4

5

6

7

8

Fig. 3. Conversion of ethyl acetate in the presence of the TS-0 catalyst during eight consecutive cycles. Reaction conditions: 4 wt.% of catalyst; ethyl acetate/methanol molar ratio = 1:6; 30 min at 50 °C.

200

0

3

Number of uses of TS-0

40

30

organic and inorganic phases were analyzed by thermogravimetrymass spectroscopy, EDX, and flame atomic absorption spectroscopy. The surface area was measured by nitrogen physisorption, and the chemical bonding structure by XPS. Fig. 4 shows the X-ray diffractograms obtained for TS-0, before use and after four and eight reactions. After four cycles, the amorphous halos became slightly more intense and then remained constant during further runs. Apart from this effect, the catalyst remained essentially unchanged up to the eighth run. Table 1 lists the atomic contents of sodium, titanium, and oxygen, derived from the EDX measurements. There was no leaching of the compensation cations, as shown by the sodium atomic content, which remained at around 10% in all cases. Fig. 5 shows micrographs of the samples before use in the catalysis experiments and after the first and fourth cycles. The morphology of the solids did not show any significant alterations, and

20

600

10

400

(a) 200

0 0

200

400

600

800

Calcination temperature (°C)

acetate in the presence of this catalyst during eight consecutive cycles. Due to the unusual behavior of the conversion efficiency, showing a steep drop in catalytic activity after the third run, the measurements were performed in triplicate to confirm that the results were reproducible. The conversion efficiency of the ester was found to be around 30% for up to three catalyst test cycles, and then showed an appreciable decline to approximately 15% followed by a further gradual decrease in conversion efficiency up to the eighth run. These results were confirmed in three independent experiments. The used catalyst was characterized by several techniques in order to identify factors that could have contributed to the partial loss of activity. The structure and morphology of the catalyst were analyzed using XRD and SEM, while the compositions of the adsorbed

Intensity [cps]

Fig. 2. Conversion of ethyl acetate (measured in triplicate) in the presence of TS-0 and the solids calcined at 200, 400, 600, and 800 °C. Reaction conditions: 4 wt.% of catalyst; ethyl acetate/methanol molar ratio = 1:6; 30 min at 50 °C.

600

(b)

400

200

600

(c) 400

200

0

10

20

30

40

50

2 θ (degrees) Fig. 4. X-ray diffractograms of the TS solids, (a) as-synthesized, (b) after four runs, and (c) after eight runs as catalyst in the transesterification reaction.

L.L. Marciniuk et al. / Fuel 118 (2014) 48–54 Table 1 Atomic contents of sodium, titanium, and oxygen, as measured by EDX (average of five determinations). Sample

Na (%)

Ti (%)

O (%)

TS-0 TS-1 TS-4 TS-8

10.9 10.6 10.6 10.2

18.6 18.1 19.4 22.4

70.5 70.3 69.9 67.4

tion reaction, with values lower than 1 m2 g1. This indicates that the specific area of the materials did not have any important influence on their catalytic activity. Fig. 6 presents the results of the thermogravimetric analysis of the as-synthesized sample (TS-0) and the TS-1 and TS-3 catalysts after use in the transesterification reaction. As shown by the derivatives of the curves, the weight losses occurred in six stages (indicated by the letters A–F). Table 2 presents the percentage weight losses corresponding to each temperature interval. The interpretation of this behavior is discussed below, using the results of the mass spectrometry analyses. Fig. 7 shows the intensities of the signals corresponding to water and CO2, obtained for samples TS-0, TS-1, and TS-3 using mass spectrometry. The mass losses in the regions 50–300 °C and 300–500 °C coincide with the peaks corresponding to the signal for water. The mass losses in these regions were therefore due to the removal of water formed by different processes. According to Sauvet et al. [17], the loss of mass at around 100 °C involves the removal of physisorbed water, while the loss in the region 150–300 °C could be due to the elimination of water chemisorbed on active sites. Kanta et al. [18] reported that the loss of water at around 500 °C resulted from dehydroxylation of titanol groups (TiAOH), forming bridges between titanium atoms (ATiAOATiA). This probably also occurs in the case of alkaline titanates exposed to the same temperature. The dehydroxylation proceeds via a condensation reaction, leading to the formation of a molecule of water, as shown in Eq. (3) [18]. The temperatures at which this reaction occurs are very mild, so the titanols may be in the form of neighbors or pairs, or directed towards each other in the interlamellar space.

ATiAOH þ HOATiA ! ATiAOATiA þ H2 O

Fig. 5. Micrographs of the titanate, (a) before use as catalyst in the transesterification reaction, (b) after the first cycle, and (c) after the fourth cycle.

always consisted of agglomerates. Similarly, the specific areas of the materials remained unchanged after use in the transesterifica-

51

ð3Þ

Sauvet et al. [17] attributed the mass loss in the temperature range 700–900 °C to the elimination of hydroxyl groups present on the surface of the material and formation of the sodium titanate crystalline phase. This was confirmed from the diffraction pattern of the crystalline phase (shown in Fig. 1b). The mass loss in this region was very small, suggesting that concentrations of species that form CO2 and H2O should be low. The mass loss profiles of the catalysts used once and three times were similar to that of the original material (Fig. 6a), however the intensity of peak B, corresponding to chemisorbed water, was very low, so that the peak appeared as a shoulder on peak A (Fig. 6b and c). Peak C was more intense after three catalytic runs, compared to the initial material or the catalyst after the first run. This could be indicative of the presence of more TiAOH groups after the catalytic runs, compared to the original material, since the release of water resulting from their condensation occurs at the temperature corresponding to peak C. This observation suggests the breaking of bonds during the catalytic reaction, with an increase in the concentration of TiAOH. Fig. 7b shows the intensity of the signal corresponding to the formation of CO2, for samples TS-0, TS-1, and TS-3. It can be seen that there was no significant loss of CO2 during use of the catalyst. The stability of the sodium titanate catalyst was evaluated using the XPS spectra recorded for the as-synthesized material (TS-0), and after the first and fourth cycle of reuse in the transesterification reaction (TS-1 and TS-4), as well as for the titanate calcined at 800 °C. This technique could also be used to compare the basicity of the materials. Martins et al. [19] reported that an increase in the binding energy of the O 1s electrons led to a decrease in the basicity of the Si-MCM-41 catalyst structure. In the presence of CTA+ cations, the binding energy of the O 1s corresponding to SiO2 shifted to lower values (from 532.8 to 532.3 eV), indicating that the basicity

L.L. Marciniuk et al. / Fuel 118 (2014) 48–54

TS-0 D

E

C

dW/dT (mg/°C)

(%) Weight

-0.05

96

weight (%) o dW/dT(mg/ C)

92

-0.10

88 -0.15

B

84

A

(a)

80

TS - 1 D

E F

C

92

weight (%) o dW/dT(mg/ C)

-0.05 -0.10

88 -0.15 84

(b)

A

-0.20 80

TS - 3

100

0.00

0.00 D

96

E F

weight (%) o dW/dT(mg/ C)

92 C

-0.05 -0.10

88 -0.15 84

-0.20

(c)

A

80

100 200 300 400 500 600 700 800

100 200 300 400 500 600 700 800

100 200 300 400 500 600 700 800

Temperature (°C)

Temperature (°C)

Temperature (°C)

dW/dT (mg/°C)

100

dW/dT (mg/°C)

96

0.00 F

Weight (%)

100

Weight (%)

52

-0.20

Fig. 6. Thermograms and their derivatives obtained under an inert atmosphere for (a) the TS-0 sample, and the catalysts after (b) one and (c) three catalytic cycles.

Table 2 Loss of mass of samples TS-0, TS-1, and TS-3. Temperature range (°C)

Mass loss (%)

50–300 (A and B) 300–500 (C) 500–900 (D–F)

TS-0

TS-1

TS-3

12.2 2.5 3.1

9.7 2.8 2.4

9.8 2.8 2.5

of the material increased in the presence of these cations [19]. Here the formation of different oxygen species with higher binding energies accounts for significant changes taking place on the catalyst surface which, as will be shown below, are related to the partial loss of basicity that occurred with successive transesterification test cycles. The deconvoluted O 1s spectra of the TS-0, TS-1, TS-4, and TS800 °C materials are shown in Fig. 8. The binding energies of the peaks corresponding to the different chemical environments of the oxygen atoms are presented in Table 3, together with the respective peak area percentages.

The main peak in the O 1s spectrum, at around 530 eV, corresponds to the O2 oxidation state of the titanium–oxygen bonds [20]. Comparison of the spectra (Fig. 8a–c) revealed that after the fourth use of the catalyst, the signals at around 531.5, 532.6, and 534.2 eV increased in intensity, while a new component emerged at about 535.0 eV. As shown in Table 3, three effects were observed after the fourth test: an increase of hydroxyl and/or C@O groups (531.5 eV), a greater prevalence of CAO groups (532.6 eV) and OAC@O groups and/or Na2O (534.2 eV), and the presence of physisorbed water (535.0 eV). The presence of OAC@O and C@O groups, related to adventitious hydrocarbon species was confirmed by the C 1s spectra (not shown) at 289.2 and 287.8 eV, respectively. Fig. 8d shows the fitted O 1s spectra after calcination of the material at 800 °C. Analogously to the TS-4 catalyst, after calcination there was an increase of C@O species and OAC@O groups and/or Na2O, detected at about 534.0 eV [20,21]. It was therefore apparent that after use of the catalysts in the transesterification reaction, or after a calcination step at 800 °C,

-8

3,0x10

-8

3.0x10

(a)

TS-0 TS-1 TS-3

-8

2,5x10

TS-0 TS-1 TS-3

2.5x10

-8

-8

2,0x10

2.0x10

w/z (CO2)

w/z (H2O)

(b)

-8

-8

1,5x10

-8

1.5x10

-8

-8

1.0x10

-9

5.0x10

1,0x10

-9

5,0x10

0.0

0,0 100

200

300

400

500

600

700

800

100

200

300

Temperature (°C)

400

500

600

700

800

Temperature (°C)

Fig. 7. Intensities of the signals for (a) water and (b) CO2, obtained by mass spectrometry for the TS-0, TS-1, and TS-3 catalysts.

538 536 534 532 530 528

Binding energy (eV)

538 536 534 532 530 528

Binding energy (eV)

(c)

Intensity [a.u.]

(b)

TS-800 (O1s)

TS-4 (O1s)

Intensity [a.u.]

(a)

TS-1 (O1s)

Intensity [a.u.]

Intensity [a.u.]

TS-0 (O1s)

538 536 534 532 530 528

Binding energy (eV)

Fig. 8. O 1s XPS spectra obtained for (a) TS-0, (b) TS-1, (c) TS-4, and (d) TS-800 °C.

(d)

538 536 534 532 530 528

Binding energy (eV)

53

L.L. Marciniuk et al. / Fuel 118 (2014) 48–54 Table 3 Binding energies (EB) of the O 1s electrons, and the area percentage of each peak. TiAO

OAC@O/Na2O2

Adsorbed H2O

EB (eV)

Area (%)

EB (eV)

Area (%)

EB (eV)

Area (%)

EB (eV)

Area (%)

EB (eV)

Area (%)

530.2 530.2 530.1 529.8

75.8 71.1 45.1 23.4

531.6 531.4 531.5 530.9

14.4 14.5 16.3 15.6

532.6 532.4 533.0 532.2

8.3 10.9 18.4 17.8

533.9 533.6 534.0 533.5

1.5 3.5 13.2 23.4

– – 535.0 –

– – 6.9 –

TS-4 (Na 1s)

TS-1 (Na1s)

Intensity [a.u.]

TS-0 (Na1s)

Intensity [a.u.]

CAO

(a)

1078 1076 1074 1072 1070 1068

(b)

(c)

1078 1076 1074 1072 1070 1068

1078 1076 1074 1072 1070 1068

Binding energy (eV)

Binding energy (eV)

Binding energy (eV)

TS-800 (Na1s)

Intensity [a.u.]

TS-0 TS-1 TS-4 TS-800 °C

OAH

Intensity [a.u.]

Titanate

(d)

1078 1076 1074 1072 1070 1068

Binding energy (eV)

Fig. 9. Na 1s XPS spectra for (a) TS-0, (b) TS-1, (c) TS-4, and (d) TS-800 °C.

Table 4 Na 1s binding energies (EB), and the area percentages of each peak. Titanate

TS-0 TS-1 TS-4 TS-800 °C

NaAO

Na2O2

EB (eV)

Area (%)

EB (eV)

Area (%)

1071.8 1071.9 1071.7 1071.7

98.0 95.2 54.0 52.9

1074.1 1074.3 1073.7 1073.7

1.9 4.8 45.9 47.1

Table 5 Chemical composition of the surfaces of the solids, obtained from XPS measurements. Titanate

Na (at.%)

Ti (at.%)

O (at.%)

TS-0 TS-1 TS-4 TS-800 °C

17.6 ± 1.8 15.6 ± 1.6 27.7 ± 2.8 29.0 ± 2.9

23.8 ± 2.4 23.9 ± 2.4 22.1 ± 2.2 22.3 ± 2.2

58.6 ± 5.8 60.5 ± 6.1 50.2 ± 5.0 48.7 ± 4.9

as the formation of new catalytically inactive sodium species, whose high diffusivity resulted in segregation on the catalyst surface. As these species are bonded to oxygen the catalyst looses basic oxygen-containing sites. Hence, during the course of use of the catalyst, as well as after calcination, sodium peroxide was formed and fewer sodium cations were available for the active TiAOANa+ sites of the catalyst. This effect was correlated with the increasing presence of oxygenate species, observed at higher binding energies in the O 1s spectra. The consequence was a sharp decline in the catalytic activity of the material (Figs. 2 and 3). Finally, after the fourth cycle, as well as after calcination, the Ti 2p spectra (not shown) revealed the presence of a second component above 459 eV, which was attributed to the formation of Ti4+ oxygen vacancies induced by deactivation of the catalyst [22]. This finding is coherent with the formation of a new phase on the surface, resulting in a reduced number of available catalytic sites.

4. Conclusions sodium peroxide was formed on the catalyst surface. This was confirmed by the Na 1s spectra of the materials, as shown in Fig. 9. A peak at around 1071.1 eV can be attributed to the sodium cation of the titanate structure [20]. After the fourth catalyst cycle, as well as after calcination of the material (Fig. 9c and d), a strong increase of a second peak at about 1074 eV was detected. According to Schily and Heitbaum [21], this peak corresponds to the formation of sodium peroxide following segregation of sodium cations on the surface [21]. The binding energies of the two peaks corresponding to the different chemical environments of the sodium atoms, together with their respective area percentages, are provided in Table 4. The chemical composition of the catalyst surfaces, obtained from the XPS data, is shown in Table 5. The data for samples TS4 and TS-800 °C reveal the segregation of sodium cations in the surface layer (<5 nm), in contrast to unchanged sodium in the bulk of the material, as also observed using EDX (Table 1). Higher intensity of the second sodium peak (at 1074 eV) was accompanied by a strong increase of the sodium concentration on the catalyst surface, from about 17 at.% to 28 at.% and 29 at.% for samples TS-4 and TS-800 °C, respectively. Since the overall Na content of the material remained constant, as shown by the EDX data (Table 1), this finding can be interpreted

Sodium titanate prepared at low temperatures is an effective catalyst for the transesterification reaction under mild conditions, and achieved an ethyl acetate conversion efficiency of around 30%. The stability of the material was investigated in experiments involving consecutive catalytic cycles, which showed that the conversion rate remained almost constant for up to three runs. However, during the fourth cycle there was a sharp loss of activity, which then remained almost constant up to the eighth run. Analyses using EDX and flame atomic absorption spectrometry showed no changes in the atomic content of sodium in the catalyst, indicating that there was no leaching during the reactions. This confirmed that the reaction proceeded via heterogeneous catalysis. The loss of activity on the fourth run was accompanied by the appearance of a new species of oxygen (Na2O2), identified using XPS analyses. The XPS spectrum showed that the basic character of this oxygen species was inferior to that of the original species, which could explain the loss of catalytic activity. The deactivation was accompanied by the intensification of a second Na 1s component at higher binding energy, which confirmed the presence of sodium peroxide. The migration of these cations to the catalyst surface reduced their availability for active TiAOANa+ sites, which explained the loss of catalytic activity after the fourth test cycle or after calcination of the material.

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