New catalysts for biodiesel additives production

Applied Catalysis B: Environmental 103 (2011) 404–412

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

New catalysts for biodiesel additives production Maciej Trejda ∗ , Katarzyna Stawicka, Maria Ziolek Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland

a r t i c l e

i n f o

Article history: Received 6 December 2010 Received in revised form 28 January 2011 Accepted 2 February 2011 Available online 2 March 2011 Keywords: Glycerol Esterification Niobium SBA-15 MPTMS

a b s t r a c t Mesoporous silicate and metallosilicate (Al or Nb) materials of SBA-15 type were prepared in the presence of MPTMS, i.e. (3-mercaptopropyl)trimethoxysilane, and hydrogen peroxide. The samples prepared were characterised by different techniques (N2 adsorption/desorption, XRD, XRF, elemental analysis, thermal analysis, FTIR, UV–Vis) and applied as catalysts in glycerol esterification with acetic acid. The impact of different factors such as temperature, glycerol to acetic acid molar ratio, and metal concentration on the course and yield of glycerol esterification was examined. The role of niobium in the formation of sulphonic species was considered and discussed. The most important finding is that niobium source present in the SBA-15 synthesis gel together with MPTMS and H2 O2 improves the efficiency of –SH oxidation towards sulphonic species and increases the stability of the modifier (oxidised MPTMS), which results in increased activity and selectivity of the catalysts to triacetylglycerol in esterification of glycerol. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Improvement of biodiesel composition is a huge challenge because of the importance of its positive impact on the environment [1–3]. Biodiesel is an alternative to fossil fuels allowing a reduction in CO2 emission [4,5]. Its production is mainly realised by transesterification of triglycerides with alcohols. This process leads to formation of a difficult side product, glycerol that needs to be utilized [6]. There are several routes leading to transform glycerol to valuable products, e.g. oxidation [7,8], hydrogenolysis [9,10], dehydration [11,12], carbonylation [13] or etherificaton [14,15]. Biodiesel obtained by over-mentioned method shows some drawbacks like poor oxidation stability and high boiling point. These features can be improved by introduction of some additives, e.g. triacetylglycerol [16,17], which is a product of glycerol esterification with acetic acid (Fig. 1). Triacetylglycerol can be applied to increase viscosity or to enhance cold resistance and anti-knocking properties [1]. Typically, glycerol esterification is catalysed by mineral acids [2,5,18], which due to the economical and environmental aspects are welcome to be replaced by heterogeneous catalysts having strong acidic properties. One of the best-known solids having acidic centres on the surface are zeolites. Moreover, zeolites possess a uniform system of pores and/or cages in the structure, which is very attractive from the catalytic point of view. Different kinds of such materials (e.g. H-ZSM-5, H-Y, H-mordenite) have been tested in the

∗ Corresponding author. Tel.: +48 61 8291243; fax: +48 61 8658008. E-mail address: [email protected] (M. Trejda). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.02.003

glycerol esterification processes [19–25]. However, it has not been possible to obtain high selectivity to di- and tri-substituted glycerol products because of a relatively small pore size of these solids. High selectivity to monoglycerides, which are also important for industrial applications, has not been possible because of the activity of the external surface towards formation of more bulky products. Another group of potential catalysts examined in esterification of glycerol included natural or synthetic sulphonic resins [26,27]. Similarly to zeolites, these materials have acidic properties but their structure is not so stable as that of zeolites. Resins can interact with alcohols, which leads to swelling, i.e. increase in volume. These materials allow obtaining much higher activity in esterification of glycerol with fatty acids, however they also show much lower selectivity to monoglycerides than zeolites. Moreover it was shown [27] that resins need a careful regeneration, in particular for a high degree of water removal. The end of the 20th century brought a new class of solids, i.e. the mesoporous silicalites [28,29]. Their uniform pore structure, usually in the range of 2–10 nm, was very promising for processes in which bulky molecules were used. However, mesoporous aluminosilicates did not show such strong acidity as zeolites [30]. For this reason these materials showed very low activity in glycerol esterification with acetic acid [31]. To improve their properties, mesoporous materials were modified by sulphonic groups [32–40] generated from different sulphur containing organo-silica compounds. The above-mentioned materials showed promising catalytic activity in glycerol esterification with acetic acid. The use of these catalysts allows reaching high conversion of glycerol (ca. 90%) and high selectivity towards di- and triacetylglycerol (ca. 85%) [35].

M. Trejda et al. / Applied Catalysis B: Environmental 103 (2011) 404–412

405

O

OH

CH3

O

CH3

O OH

OH

O OH OH OH

Monoacetylglycerols (MAG)

O +

H3C

OH

G lycerol

O

O

OH

A cec ac id O

O

CH3

O

CH3

CH3

O

CH3 O

O

CH3

OH O

OH

O

O

H3C

O

Diacetylglycerols (DAG)

O H3C

O

Triacetylglycerol (TAG)

Fig. 1. Scheme of glycerol esterification with acetic acid.

The aim of this study was to prepare the mesoporous silicate and metallosilicate materials of SBA-15 type modified by (3-mercaptopropyl)trimethoxysilane (MPTMS) and to examine the role of their chemical composition on the catalytic performance in glycerol esterification with acetic acid. The novelty in this study is the application of niobium source in the synthesis of the catalysts and identification of the role of Nb species in the formation and stabilisation of active sulphonic species.

as reference materials. The reaction mixture consisted of water, hydrochloric acid (P.O.Ch.), Pluronic P123 (BASF) and TEOS (Fluka), at the molar ratio: 1 SiO2 : 0.005 Pluronic P123: 1.45 HCl: 124 H2 O. After dissolving Pluronic P123 in hydrochloric acid solution, a source of silica was added. The mixture was stirred at 328 K for 8 h and then moved into a PP bottle and heated without stirring at 353 K for 12 h. The solid was filtered off, washed with water and finally dried at 333 K for 12 h. The template was removed by calcination at 823 K for 8 h in air in static conditions (temperature ramp 5 K min−1 ).

2. Experimental 2.1. Preparation of catalysts

2.2. Catalyst characterisation

The organo-silica catalysts were prepared via modified hydrothermal synthesis procedure described previously by Margolese [32]. The synthesis was performed in polypropylene bottles and Pluronic P123 (Poly(ethylene glycol)-blockPoly(ethylene glycol)-block-Poly(ethylene glycol)-block) was used as a surfactant. The reaction mixture consisted of water, hydrochloric acid (P.O.Ch.), Pluronic P123 (BASF), MPTMS (3mercaptopropyl)trimethoxysilane) (Aldrich), hydrogen peroxide (Merck) and TEOS (Fluka), at the molar ratio: 1 TEOS: 0.018 Pluronic P123: 0.1 MPTMS: 0.3 H2 O2 : 6.4 HCl: 182 H2 O. Ammonium niobate(V) oxalate (Aldrich) or aluminium sulphate (Aldrich) was also added to the gel keeping—TEOS/Nb (or Al) = 64 or 32 or 16 for the preparation of NbSBA-15 and AlSBA-15 respectively. Then the gel was stirred at 313 K for 20 h and heated at 383 K for 24 h without stirring. The product was separated and washed. The organic template was removed by constant extraction with ethanol for 24 h using the Soxhlet apparatus and finally dried at RT overnight. One sample (denoted as MP-SBA-15*) was prepared in the same procedure as described above, but hydrogen peroxide was not added into the synthesis gel. The oxidation of thiol groups with H2 O2 was performed after hydrothermal synthesis. To perform the oxidation the catalyst was immersed in hydrogen peroxide and stirred for 2 h at room temperature. After decantation the material was washed with a mixture of ethanol and water (1:1). Then the catalyst was immersed in 1 M H2 SO4 and stirred for 2 h at room temperature. Finally, the product was washed with a mixture of ethanol and water (1:1) and dried at room temperature. Samples without MPTMS were also synthesised according to the standard synthesis procedure reported by Stucky [41] and applied

XRD patterns were recorded at room temperature on a Bruker AXS D8 Advance apparatus using CuK␣ radiation ( = 0.154 nm), with a step of 0.02◦ and 0.05◦ in the small-angle range and in the high-angle range, respectively. N2 adsorption/desorption isotherms were obtained on a Micromeritics ASAP equipment, model 2010. The samples (200 mg) were pre-treated in situ under vacuum at 423 K overnight. The surface area was calculated using the BET method. Elemental analyses of the solids were carried out with Elementar Analyser Vario EL III. Infrared spectra were recorded with a Bruker Vector 22 FTIR spectrometer using an in situ cell. Samples were pressed under low pressure into a thin wafer of ca. 8 mg cm−2 and placed inside the cell. Catalysts were evacuated at different temperatures. Before pyridine adsorption catalysts were evacuated at 423 K during 6 h and pyridine was then admitted at 423 K. After saturation with pyridine the samples were degassed at 423 and 473 in vacuum for 30 min at each temperature. The spectrum without any sample (“background spectrum”) was subtracted from all recorded spectra. The IR spectra of the activated samples (after evacuation at 423 K) were subtracted from those recorded after the adsorption of pyridine followed by various treatments. The reported spectra are the results of this subtraction. The number of Brønsted acidic sites were calculated using extinction coefficient ε1550 = 1.8 ␮mol cm−1 [42]. UV–Vis spectra were recorded using a Varian-Cary 300 Scan UV–Visible Spectrophotometer. Catalyst powders, were placed into the cell equipped with a quartz window. The spectra were recorded in the range from 800 to 190 nm. Spectralon was used as a reference material.

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The reaction of glycerol and acetic acid was performed in a liquid phase in batch reactor without any solvents. The reaction was carried out under nitrogen atmosphere at 373, 398 and 423 K for 4 h using 100 mg of catalyst. Different molar ratios of glycerol to acetic acid were applied (1:3, 1:6, 1:9). For the most active catalyst the reuse test was performed. Prior this process the catalyst after first reaction was separated from reactant mixture by centrifugation and than dried overnight at 373 K. The same mass of catalyst was applied for the second and third run. Products were analysed by a gas chromatograph (Varian CP 3800) equipped with 60 m VF-5ms capillary column and FID detector.

ters. The pores of the samples are also ordered in the long range as evidenced from the appearance of additional one or two peaks. However, the ordering of the pores is different depending on the number and nature of metal species (Al or Nb) incorporated together with MPTMS. This ordering is distorted (the peaks at 2 theta > 1◦ disappear or are very weak) when niobium is incorporated into the synthesis gel together with the other compounds. As such a distortion is not observed for NbSBA-15-64 (Fig. 2i), in whose synthesis Nb species is a component of the synthesis gel but MPTMS is absent, the perturbation in the long range ordering can be supposed to be caused by the interaction of Nb species with MPTMS during the synthesis. In addition, for MP-SBA-15* sample the presence of MPTMS species in the synthesis gel allows obtaining the best ordering of the final material. This fact also confirms that MPTMS must have to interact with Nb species during the preparation of niobiosilicate samples. However, it is not the case for MP-AlSBA-15 samples, in which the long range ordering is lower only for the highest amount of aluminium (MP-AlSBA-15-16; Si/Al = 16). The high angle XRD patterns (not shown here) do not show the presence of crystalline phases in the samples. Only an increase in the intensity of XRD line at 2 theta ca. 20–30◦ is observed. Some authors assign this feature to the presence of amorphous silica [43]. It should be stressed that although the channels of SBA-15 sample are uniform, their walls consist of amorphous silica. The mesoporosity of the solids prepared is also supported by the character of the N2 adsorption/desorption isotherms (selected isotherms in Fig. 3). According to IUPAC classification they can be classified as type IV, which is characteristic of mesoporous materials [44]. For all samples the hysteresis loop is observed and it is typical of SBA-15 type materials. The presence of this loop is related to the condensation of nitrogen inside the mesopores. However, the shape of these hysteresis loops is different for the niobium containing samples. This observation can be explained by a broader mesopore size distribution observed for the niobium containing samples (not shown here).

3. Results and discussion

3.2. Efficiency of the metal and MPTMS incorporation

3.1. Texture/structure characterisation

The data on metal concentration in the samples (expressed in Si/M ratio) as well as the amount of MPTMS incorporated (expressed as mmol of sulphur per one gram of material) are shown in Table 2. The results of XRF analysis show that niobium is easily introduced into the SBA-15 structure. The assumed Si/Nb ratio (indicated by the last number in the catalyst symbol) is very close to that estimated by XRF analysis. Only for the lowest Si/Nb ratio (the highest concentration of niobium in the synthesis gel) less Nb is incorporated, which is not the case for aluminium. The conditions (especially very low pH during the synthesis) used for sample preparation allow incorporation of ca. 10% (or less) of the amount assumed in the final material. The highest amount of MPTMS species was incorporated into mesoporous silicate material when hydrogen peroxide was added during the synthesis (MP-SBA-15). The presence of niobium in the synthesis gel decreases the molar concentration of MPTMS species in the final material, however the amount of sulphur detected is independent of the niobium concentration. Such a dependence is observed for the aluminium containing samples. The less aluminium in the synthesis gel the higher the concentration of MPTMS is found.

5

10

Intensity, a.u.

5

e d b a 2

4

6

8

i h g f

c 2

4

6

2 theta,

8

0

2

4

6

8

o

Fig. 2. XRD patterns of: a) MP-SBA-15; b) MP-SBA-15*; c) MP-AlSBA-15-64; d) MPAlSBA-15-32; e) MP-AlSBA-15-16; f) MP-NbSBA-15-64; g) MP-NbSBA-15-32; h) MPNbSBA-15-16; i) NbSBA-15-64.

Thermogravimetry measurements were carried out in air atmosphere using SETARAM SETSYS-12 apparatus with temperature ramp 5 K min−1 . 2.3. Glycerol esterification with acetic acid

The textural/structural parameters of materials modified by MPTMS species as well as those of the pristine supports are collected in Table 1 (the samples modified by MPTMS are indexed with MP-). The surface areas of samples prepared are relatively high and reach values from 620 to 900 m2 g−1 . The highest surface areas are observed for the samples obtained without MPTMS addition during their synthesis. The modification of the mesoporous materials by the MPTMS species leads to decrease in the surface area. Moreover, the addition of hydrogen peroxide (applied for the oxidation of thiol species) decreases also this textural parameter in the final material. This decrease is higher when H2 O2 is added during the synthesis (MP-SBA-15* vs. MP-SBA-15). The modification of mesoporous silicates or metallosilicates by MPTMS increases the other two textural parameters. In the above-mentioned materials the volume of pores and their diameter are higher than those found in the samples prepared without MPTMS. This fact suggests that MPTMS added into the synthesis gel acts also as a co-surfactant resulting in enlargement of the material pores. Consequently, the pore volume and pore diameters increase. The low angle XRD patterns of all materials prepared are characteristic of ordered mesoporous solids (Fig. 2). They show a typical peak at 2 theta at ca. 1◦ . The origin of this peak is assigned to the presence of regular interspace distance between the walls of mesoporous channels. For the samples prepared with the MPTMS addition, this peak shifts to lower 2 theta values because of the increase in the interspace distance, i.e. the increased pore diame-

3.3. State of the elements incorporated into SBA-15 The FTIR spectroscopy is a very efficient tool used for characterisation of materials surfaces. This technique was applied to estimate the content of organic species present in modified SBA-15 materials. MPTMS molecule posses a characteristic functional group (–SH)

M. Trejda et al. / Applied Catalysis B: Environmental 103 (2011) 404–412

407

Table 1 Textural/structural characterisation. Surface area, m2 g−1

Catalysta

Pore volume, cm3 g−1

Pore diameter, nm Average

Mesopore** (from PSD)

SBA-15 MP-SBA-15* MP-SBA-15

815 750 670

0.76 0.85 0.86

3.8 4.5 5.1

5.8 5.9 6.8

NbSBA-15-64 MP-NbSBA-15-64 MP-NbSBA-15-32 MP-NbSBA-15-16

900 805 645 720

0.89 1.00 0.82 1.16

3.9 5.0 5.1 6.4

5.3 5.5 5.9 9.2

AlSBA-15-64 MP-AlSBA-15-64 MP-AlSBA-15-32 MP-AlSBA-15-16

850 710 620 650

0.73 0.81 0.80 0.77

3.4 4.6 5.1 4.8

5.9 5.9 7.6 5.9

a * **

MP—stands for MPTMS; the last number in the symbol means Si/M molar ratio in the synthesis gel. H2 O2 was not added during the synthesis. Maximum in the pore size distribution plot.

100

3

Volume adsorbed, cm /g

100

MP-TMS-SBA-15*

MP-TMS-SBA-15 0,2

0,4

0,6

0,8

1,0

0,2

100

0,4

0,6

0,8

1,0

100

MP-TMS-NbSBA-15-64

MP-TMS-AlSBA-15-64 0,2

0,4

0,6

0,8

1,0

0,2

0,4

0,6

0,8

1,0

Relative pressure, p/p0 Fig. 3. N2 adsorption/desorption isotherms.

that gives a low intense FTIR band at ca. 2580 cm−1 [32]. This band is only hardly seen for MP-SBA-15 sample prepared with hydrogen peroxide addition during the hydrothermal synthesis. For the other samples this band is not detected. This feature suggests the trans-

formation of thiol groups both, during the synthesis of mesoporous materials and via oxidation procedure for MP-SBA-15* sample. The FTIR spectra of MPTMS containing mesoporous silicate and metallosilicate samples are shown in Fig. 4. The bands assigned to

Table 2 Results of elemental analysis, XRF, and pyridine adsorption. Catalyst

Si/M molar ratio in the synthesis gel

MP-SBA-15* MP-SBA-15

S, mmol/g

H+ , mmol/g

real (from XRF)

– –

– –

1.0 1.1

0.023 –

MP-NbSBA-15-64 MP-NbSBA-15-32 MP-NbSBA-15-16

64 32 16

65 28 30

0.8 0.8 0.7

0.030 0.040 0.024

MP-AlSBA-15-64 MP-AlSBA-15-32 MP-AlSBA-15-16

64 32 16

606 451 255

1.0 0.9 0.6

– – –

M—Nb or Al. * H2 O2 was not added during the synthesis.

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M. Trejda et al. / Applied Catalysis B: Environmental 103 (2011) 404–412

1380

0.2

d

Absorbance

1455

c b a

1720

Fig. 6. Schema of MPTMS immobilisation in the presence of H2 O2 on silicate and niobiosilicate materials.

1800

1600

1400 -1

Wavenumber, cm

Fig. 4. FTIR spectra of: a) MP-AlSBA-15-16; b) MP-SBA-15; c) MP-SBA-15*; d) MPNbSBA-15-16.

ı(CH3 ) vibrational mode in methoxy groups at 1455 cm−1 [45–47] are detected for all materials. For silicate materials the intensity of these bands is much lower for sample prepared with the addition of hydrogen peroxide to the synthesis gel. It should be reminded that the concentration of organic species containing sulphur (expressed in mmol of sulphur) is similar for both samples (Table 2). This difference can be explained by assuming the transformation of –OCH3 species, e.g. via oxidation. Indeed, for MP-SBA-15 sample the band at 1720 cm−1 (Fig. 4b), assigned to C O vibrations [45–47], is very well seen. This means that at least part of hydrogen peroxide is used for the oxidative dyhydrogenation of methoxy species present in MP-SBA-15, whereas the application of post-synthesis oxidation procedure does not cause the transformation of methoxy species (Fig. 4c). However, for the niobiosilicate samples (Figs. 4d and 5c,d) the band assigned to ı(CH3 ) in methoxy species is not intense although the band at 1720 cm−1 is not observed. It suggests, that more than one of methoxy group are involved in bonding of MPTMS on the material surface. It is postulated that maximally two methoxy species could be bounded to the silicate surface [48]. Moreover, it is known that –OH group connected with niobium located in the skeleton of niobiosilicate has almost the same properties as –OH associated with siliceous [49]. Therefore, according to FTIR data one can propose a scheme of MPTMS species bounded onto silicate and niobiosilicate samples (Fig. 6).

0.2

1380

1455

d

Fig. 5 shows the FTIR spectra of niobium and aluminium containing materials with the highest and the lowest metal concentration (Si/M = 16 and 64). The preparation procedure applied for aluminosilicate materials leads also to the oxidation of methoxy species as demonstrated by the presence of 1720 cm−1 band. However, the presence of niobium protects the oxidation of residual methoxy species. As discussed later, this feature is a result of the preferential oxidation of thiol species by hydrogen peroxide, which takes place in the presence of niobium. For both types of materials shown in Fig. 5, the presence of band at 1380 cm−1 is observed. This band, assigned to sulphonic species, is more intensive for niobiosilicate materials. Therefore, the highest concentration of these species in niobiosilicate materials can be postulated on the basis of FTIR measurements. Very useful information concerning sulphur-containing species can be deduced from UV–Vis measurements. The UV–Vis spectra of all materials prepared are shown in Fig. 7. The spectrum of MP-SBA-15 sample shows only one band at 220 nm and a shoulder at ca. 255 nm (Fig. 7a). The spectra of the other materials show two bands at 220 and 255 nm. The band at 220 nm is characteristic of electron charge transfer form the sulphur present in mercaptopropyl group [50]. Thus, almost only this species is observed in MP-SBA-15 sample. The domination of –SH species in this sample estimated by UV–Vis technique is in line with the FTIR measurements. Thiol species are observed by FTIR spectroscopy only for MP-SBA-15 material. The band at 255 nm can be assigned to electron charge transfer in sulphonic groups formed via oxidation of mercaptopropyl groups. This band is well seen for MP-SBA-15* and niobium containing samples. Moreover, the intensity of this band increases with increasing of niobium content. This band (the shoulder) is less intense for aluminosilicate materials. These results are again in line with the observations from FTIR spectroscopy. As shown above, part of hydrogen peroxide is involved in the oxidation of methoxy species. As a

1

1

1

b a

F(R)

Absorbance

c

h

1720

200

1800

1600 1400 -1 Wavenumber, cm

Fig. 5. FTIR spectra of: a) MP-AlSBA-15-16; b) MP-AlSBA-15-64; c) MP-NbSBA-1516; d) MP-NbSBA-15-64.

e

b a 400

g

d c

600 200

400

600 200

f 400

600

Wavelength, nm Fig. 7. UV–Vis spectra of: a) MP-SBA-15; b) MP-SBA-15*; c) MP-AlSBA-15-16; d) MP-AlSBA-15-32; e) MP-AlSBA-15-64; f) MP-NbSBA-15-64; g) MP-NbSBA-15-32; h) MP-NbSBA-15-16.

M. Trejda et al. / Applied Catalysis B: Environmental 103 (2011) 404–412

a

634

b

60

614

-1 -2

40

769

TG

DTA, µV

TG, mg

0

409

20

-3 0

DTA

c

637

d 40

-1

DTA, µV

TG, mg

0

599 20

785

725

-2 -3

0 400

600

800

1000

1200

Temperature, K

400

600

800

1000

1200

Temperature, K

Fig. 8. DTA–TG profiles of: a) MP-SBA-15; b) MP-SBA-15*; c) MP-AlSBA-15-64; d) MP-NbSBA-15-64. The results obtained in air atmosphere. Solid line—DTA curve, dotted line—TG curve.

3.4. Glycerol esterification with acetic acid The esterification of glycerol with acetic acid was performed in liquid phase. To exclude the glycerol oxidation by molecular oxygen all reactions were carried out under nitrogen atmosphere. This allowed obtaining only products of glycerol esterification (monoacetyloglycerols, diacetologlycerols and triacetyloglycerol) and carbon balance close to 100%. Different factors which could have an impact on catalytic behaviour were examined, i.e. type of catalyst, reaction temperature, glycerol to acetic acid ratio or metal concentration in the sample. The reuse tests were also performed.

30

808

25 20

DTA, µV

consequence less amount of H2 O2 can be used for the thiol oxidation. The information about both, the nature and the amount of organic species can also be obtained from thermal analysis of the samples. The thiol and sulphonic species present on the surface of silicate materials decompose at different temperatures, i.e. at 600–650 K and at ca. 800 K, respectively [32]. The TG and DTA curves of silicate materials and metallosilicate samples showing the lowest metal concentration are presented in Fig. 8. For all samples one endothermic effect at ca. 400–420 K attributed to water desorption is observed (DTA curve) and it is accompanied by a mass loss (TG curve). The other one or two effects are exothermic. Their appearance represents the decomposition of thiol and sulphonic species, respectively. Therefore, it can be postulated that mainly thiol species decomposes in MP-SBA-15 sample. This species also dominates on aluminosilicate materials. For MP-SBA-15* (Fig. 8b) and niobiosilicates samples (Figs. 8d and 9) sulphonic species are present at high concentrations. It is strong evidence of the role of niobium in formation of sulphonic species. The presence of niobium in the synthesis gel can play two different roles. One is the activation of H2 O2 by formation of peroxo species. Niobium is well known as an active centre in different oxidation processes carried out in the liquid phase and using hydrogen peroxide as oxidant [51,52]. The activity of these catalysts is related to the ability of formation of peroxy complexes in the presence of niobium [52]. Such activated hydrogen peroxide species are able to selectively oxidate different molecules, e.g. thiols [53,54]. However, the role of niobium is not only related to the formation of sulphonic species. The other role is stabilisation of the modifier (oxidised MPTMS). The stability of sulphonic species (Figs. 8 and 9) is higher for niobium containing samples (MPSBA-15*—769 K; MP-NbSBA-15-64—785 K; MP-NbSBA-15-32 and MP-NbSBA-15-16—808 K). Moreover, this stability is related to the real concentration of niobium in the sample (see Table 2). It points out the interaction between niobium and the silanopropyl chain ended by a sulphonic group.

15

c

10

b

5 0

a -5 -10 -15 400

600

800

1000 1200

Temperature, K Fig. 9. DTA profiles of: a) MP-NbSBA-15-16; b) MP-NbSBA-15-32; c) MP-NbSBA15-64.

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Table 3 Conversion and selectivity in glycerol esterification with acetic acid (glycerol/acetic acid = 1:3). Catalyst

Conversion, % 373 K

398 K

Selectivity, % 423 K

MAG 373 K

DAG 398 K

423 K

373 K

TAG 398 K

423 K

373 K

398 K

423 K

Blank test

49

59

47

73

50

47

25

44

46

2

6

7

SBA-15 MP-SBA-15 MP-SBA-15*

49 44 64

61 59 69

56 52 69

75 71 33

52 48 29

45 49 28

24 27 55

42 45 54

47 44 53

1 2 12

6 7 17

8 7 19

NbSBA-15-64 MP-NbSBA-15-64

50 67

71 69

55 74

73 34

52 33

46 18

25 54

43 51

46 54

2 12

5 16

8 28

AlSBA-15-64 MP-AlSBA-15-64

48 57

60 65

57 75

72 46

50 36

44 29

26 49

44 53

48 56

2 5

6 11

8 15

MAG—monoacetylglycerols, DAG—diacetylglycerols, TAG—triacetylglycerol. 373 K; 398 K; 423 K—reaction temperature.

3.4.1. Influence of the chemical composition of catalysts The results of glycerol esterification with acetic acid carried out at 373 K (glycerol to acetic acid ratio = 1:3) for catalysts with the lowest metal concentration are collected in Table 3. The reactions performed with the catalysts prepared without the presence of MPTMS and H2 O2 or without the catalyst addition (blank test) were also examined. Non-modified materials (SBA-15, AlSBA-15-64 and NbSBA-15-64) exhibit conversion and selectivity almost the same as the results obtained in blank test. It shows that only temperature drives the reaction. The dominant products, i.e. monoacetylglycerols, are formed with a selectivity higher than 70%. The application of MP-SBA-15 catalyst does not have an impact on the glycerol esterification process. Even, a slightly lower conversion is observed. Interestingly, for the MP-SBA-15*, MP-AlSBA-15-64 and MP-NbSBA-15-64 samples the conversion increases and reaches the highest value (67%) for niobiosilicate material. The lack of activity of MP-SBA-15 sample as well as the order of activity for the other catalysts containing organic species incorporated onto the material surface strongly documents the role of sulphonic species in this process. This role is even better visible when the selectivity to mono-, di- and tri-substituted glycerol compounds is regarded. The highest selectivity to triacetylglycerol (at 373 K and glycerol to acetic acid ratio 1:3) is reached for MP-SBA-15* and MP-NbSBA-15-64 materials.

3.4.2. Influence of the reaction temperature To examine the impact of temperature on the glycerol esterification process the reactions at 373, 398 and 423 K were performed (glycerol/acetic acid = 1:3). It can be observed (Table 3) that for non-active materials (samples without MPTMS and MP-SBA-15) as well as in the blank test, the conversion goes through a maximum with temperature increasing from 373 to 423 K. However, the higher the temperature of the process the higher the selectivity to di- and tri-substituted glycerol compounds is observed. Nevertheless, the selectivity to triacetylglycerol (ca. 8%) at 423 K reached in the presence of non-active samples is much lower that that observed for the most active catalysts (MP-SBA-15* and MPNbSBA-15-64) applied at the lowest temperature (373 K—12%). For the latter materials the conversion increases with rising temperature. The highest conversions are obtained on MP-AlSBA-15-64 and MP-NbSBA-15-64. Interestingly, the order of conversion values does not match the number of sulphonic groups present on the material surface. However, the amount of sulphonic species is in line with the order of selectivity to triacetylglycerol (the highest value observed for MP-NbSBA-15-64—28%). This feature suggests that the sulphonic species on the catalyst surface have much higher impact on selectivity than on the activity in the esterification of glycerol process.

3.4.3. Influence of the molar ratio of glycerol to acetic acid The catalysts applied in the processes carried out at 423 K showed the best performance. Therefore the influence of glycerol to acetic acid molar ratio was examined at this temperature. The results are presented in Table 4. Contrary to the impact of temperature observed in the esterification of glycerol, different volumes of acetic acid applied in the reaction do not change the conversion of glycerol in a systematic way. However, higher amount of acetic acid used in the reaction favours the formation of triacetylglycerol. For MP-SBA-15* and MP-NbSBA-15-64 samples the selectivity to triacetylglycerol reaches 35% and 36%, respectively. That is 24% of yield to this compound. 3.4.4. Influence of the niobium concentration in the sample The influence of niobium concentration in the sample on the catalytic behaviour in the glycerol esterification process was examined in the optimised conditions, i.e. at 423 K and for glycerol to acetic acid ratio 1:9. It can be observed that both conversion and selectivity increase with the real niobium concentration in the sample (Table 5), therefore the influence of niobium on the catalyst performance can be considered, however as not direct. Firstly, as proved in the previous sections, niobium species enhance the oxidation of thiol species. This favours the formation of sulphonic species on the material surface, which are the active centres involved in the glycerol esterification process. The highest conversion is observed for the highest real concentration of niobium, which is also in line with the high concentration of sulphonic groups estimated for these samples (Table 2). The selectivity to triacetylglycerol reaches ca. 40% for these two samples (MP-NbSBA-15-32 and MP-NbSBA15-16), which gives 30% of yield. Moreover, niobium can be also considered to act as stabiliser for sulphonic species. It is shown that the highest stability of sulphonic species is reached in the presence of niobium at the highest concentration (Fig. 9). The higher stability of surface species is usually due to the stronger interactions of these groups with the material surface. It can lead to lower strength of proton bonding by the sulphonic group. This also implies a higher acidity of this proton. Mentioned feature could explain why MPNbSBA-15-16 sample is more active than MP-NbSBA-64 in spite the fact that the number of Brønsted acidic sites is higher for the later sample (Table 2). Due to the different interaction of niobium with sulphonic species it is difficult to find a straight relationship between the amount of protons present on the material surface and activity in glycerol esterification process. However, the most active catalyst (MP-NBSBA-32) shows the highest number of Brønsted acidic sites. It should be pointed out that there is no evidence of niobium acting as an active centre in the esterification process. However, niobium species is considered as promoters responsible for the stabilisation of the organic modifier.

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Table 4 The influence of glycerol to acetic acid molar ratio (1:3; 1:6; 1:9) on conversion and selectivity in glycerol esterification with acetic acid carried out at 423 K. Catalyst

Conversion, % 1:3

Selectivity, %

1:6

1:9

MAG

DAG

TAG

1:3

1:6

1:9

1:3

1:6

1:9

1:3

1:6

1:9

Blank test

47

60

63

47

33

24

46

54

58

7

13

18

MP-SBA-15 MP-SBA-15* MP-NbSBA-15-64 MP-AlSBA-15-64

52 69 74 75

63 76 74 69

54 69 66 60

49 28 18 29

31 16 17 20

26 11 11 17

44 53 54 56

55 54 54 60

58 54 53 60

7 19 28 15

14 30 29 20

16 35 36 23

MAG—monoacetylglycerols, DAG—diacetylglycerols, TAG—triacetylglycerol. 1:3; 1:6; 1:9—glycerol to acetic acid ratio.

Table 5 The influence of niobium concentration on conversion and selectivity in glycerol esterification with acetic acid carried out at 423 K and glycerol to acetic acid ratio = 1:9. Catalyst

Conversion, %

MP-NbSBA-15-64 MP-NbSBA-15-32 MP-NbSBA-15-16

66 73 72

Selectivity, % MAG

DAG

TAG

11 10 10

53 49 50

36 41 40

MAG—monoacetylglycerols, DAG—diacetylglycerols, TAG—triacetylglycerol.

Conversion/Selectivity, %

100 Conversion monoacetyloglycerols diacetyloglycerols triacetyloglycerol

80 60 40 20 0 1

2 Run

3

Fig. 10. Conversion and selectivity in esterification of glycerol with acetic acid on MPNb-SBA-15-32—the reuse test.

3.4.5. Reuse of catalysts To examine if the materials prepared can be reused after the esterification process, the most active catalyst was tested at 423 K (glycerol/acetic acid = 1:9). After the reaction, the catalyst was separated from the reaction mixture by centrifugation and dried at 373 K before application in the next run. The conversion and selectivity for the three runs of the reaction are shown in Fig. 10. The results presented show that the catalyst is stable in the reuse procedure. Moreover, the conversion after the first reaction increases and later it is stable in the third run. This feature is not unusual and was observed before [55,56]. This fact can be explained by the slightly modification of the catalyst surface. It can be also mentioned that the selectivity does not change and remains at a similar level. It proves that the active phase is not leached out to the reaction mixture. 4. Conclusions The one-pot synthesis route applied in this study allowed incorporation of (3-mercaptopropyl)trimethoxysilane species into the SBA-15 structure. The addition of niobium into the synthesis gel enhanced the transformation of thiols to sulphonic species via oxidation by hydrogen peroxide in comparison with silicate or aluminosilicate materials prepared in the same way. Generation of sulphonic species was also more effective via post-synthesis oxidation of silicate sample, however the sulphonic groups were less

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