Influence of the nature of the support on the catalytic properties of Pt-based catalysts for hydrogenolysis of glycerol

Journal of Molecular Catalysis A: Chemical 367 (2013) 89–98

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Influence of the nature of the support on the catalytic properties of Pt-based catalysts for hydrogenolysis of glycerol Séverine Noe Delgado, David Yap, Laurence Vivier, Catherine Especel ∗ IC2MP (Institut de Chimie des Milieux et des Matériaux de Poitiers), UMR 7285 CNRS, Université de Poitiers, 4 rue Michel Brunet, 82022 Poitiers Cedex, France

a r t i c l e

i n f o

Article history: Received 23 July 2012 Received in revised form 27 September 2012 Accepted 1 November 2012 Available online 9 November 2012 Keywords: Glycerol hydrogenolysis Propanediol Platinum catalysts Aqueous phase process Aqueous phase reforming

a b s t r a c t A series of platinum catalysts supported over various supports (Al2 O3 , Al2 O3 –SiO2 and TiO2 ) were prepared and characterized by X-ray diffraction, N2 sorption, H2 chemisorption, temperature programmed reduction, FTIR of adsorbed pyridine, 3,3-dimethyl-1-butene isomerization and cyclohexane dehydrogenation. These catalysts were evaluated for aqueous-phase process (APP) of glycerol at 210 ◦ C, under N2 or H2 atmosphere (60 bar as total pressure). Among the tested catalysts, Pt/TiO2 was the most active and the most selective toward C3 products (propanediols, propanol) which can be further valorized into chemicals. TiO2 was identified as the support leading to the most stable Pt metallic phase, catalytic phase on which the hydrogenation/dehydrogenation reactions take place. The presence of acidic sites brought by the oxide support is necessary for the dehydration reactions (i.e. for C O cleavages), but a too high quantity of these sites can promote the C C bond cleavages via an acidic cracking mechanism. Among the various supported Pt-based catalysts studied in this work, Pt/TiO2 sample appears to be the most promising system for the transformation of polyol in aqueous phase. © 2012 Elsevier B.V. All rights reserved.

1. Introduction At the time of the decrease in the fossil reserves of energy, the development of new processes for the transformation of the lignocellulosic biomass (resulting from the plants as wood, green residues, straw, etc.) in incorporable compounds to the fuel pool is a major orientation for the researchers. The use of the lignocellulosic biomass is attractive because of the low cost and the no competitiveness with the food plants. At the present time, research for the development of “second generation” biofuels is mainly directed toward the production of bioethanol, by saccharification of the carbohydrates resulting from the lignocellulosic part of the plants and fermentation of obtained sugars. Recently, a process consisting in transform sugars and polyols resulting from lignocellulosic biomass into aqueous phase (Aqueous Phase Process, APP) was developed [1–9]. Supported Pt catalysts have been often used in this process, although other metals were shown as also or even more active. The final products of the reaction are H2 , alkanes, CO and CO2 , and deoxygenated molecules (compared to the initial reactant) resulting of competitive C C and C O bond cleavages. C C cleavages will lead preferentially to the production of COx and H2 (route called aqueous phase reforming, APR), whereas selective C O cleavages allow obtaining saturated hydrocarbons. However, the nature of

∗ Corresponding author. Tel.: +33 05 49 45 39 94; fax: +33 05 49 45 37 41. E-mail address: [email protected] (C. Especel). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.11.001

hydrocarbons thus obtained is inherent to the structure of sugar or polyol used. The selectivity of the reaction can be directed by the experimental conditions and the nature of catalyst used (generally bifunctional system associating metallic and acidic functions). In this context, this work deals with the aqueous phase transformation of glycerol on supported Pt catalysts in order to identify, on this polyol of quite simple structure, selective systems toward C O cleavages to the detriment of the C C ones. Glycerol is one of the 12 molecules building blocks retained by the US Department of Energy to the investigations in the three important domains: fuels, chemicals with high added values and energy. The glycerol derived from biomass is interesting for its renewable nature and its abundance, due to biodiesel manufacture (100 kg of glycerol produced per ton of biodiesel each day) [10]. This last is continually increasing its production, thus the secondary products are generating in glut and it is necessary to find a valorization for those, notably for the glycerol. Among routes existing for glycerol transformation, the most known is the valorization in chemicals like propanediols, acrolein and lactic acid [11–14]. The propanediols are used in fabrication of unsaturated polyester resins, pharmaceuticals/cosmetics, paints, etc. Acrolein is the main feedstock for the production of 1,3propanediol and acrylic acid. Lactic acid is used in food industry or in biopolymer synthesis. In aqueous phase, in the presence of supported metallic catalysts, glycerol can be dehydrated to acetol or 3-hydroxypropanal, and then hydrogenated to propanediols. By the same mechanism, propanediols lead to 1- or 2-propanol and then to propane without

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O

+H2

HO

3-hydroxypropanal (3-HPA)

-H2O

OH

1,3-propanediol (1,3-PD)

OH

-H 2O

H2

-H2O

O

OH HO

OH

1-propanol (1P) 2-propanol (2P)

OH

H2

Glycerol

OH

Acetol

H2

H2 Propane -H2O

-H2O

-H 2

HO OH O

1,2-propanediol (1,2-PD)

OH

Glyceraldehyde (GLA) C-C cleavage and reforming Glycerol or intermediates

-Ethylene Glycol (EG) -Ethanol (EtOH) -Methanol (MeOH) -Ethane -Methane -H2 -CO2 -...

Fig. 1. Reaction scheme for aqueous phase transformation of glycerol.

cleavage of C C bonds [15]. These consecutive reactions correspond to selective hydrogenolysis to C3 products, compared to the C C bond cleavage and reforming. Indeed, in the same conditions, glycerol or intermediates can produce C2 and C1 compounds, such as ethylene glycol, ethanol, methanol, ethane, methane, as well as H2 and CO2 (Fig. 1). In latest years, much attention has been paid to develop supported metallic catalysts for the transformation of glycerol, in order to obtain high selectivity toward C3 products and low selectivity toward degradation products, as well as a good stability of the catalytic systems under the operating conditions. The catalysts the most largely studied involve monometallic systems based on Ni [16], Cu [16–18], Ru [19–21], Rh [22], Ir or Pt [15,23], as well as bimetallic systems combining these metals [24–26] or using Re as modifier [27–32]. The effects of the nature of the support were also reported in the literature, based mainly on the use of C, Al2 O3 , Al2 O3 –SiO2 and SiO2 . The purpose of this study is to prepare and characterize Ptbased catalysts supported over various oxides, and to examine their performances for the aqueous phase transformation of glycerol in a batch reactor, either in the APR conditions (under N2 atmosphere) or under H2 atmosphere. Three oxides are studied: two supports commonly used for this kind of reaction, i.e. alumina and silica–alumina, and another one few studied until now for the glycerol hydrogenolysis, i.e. titania. 2. Experimental

as precursor salt and water as solvent. According to this procedure, the metal solution was added to the support previously hydrated and acidified to pH 1 with HCl (36 wt.%). The water was further evaporated at moderate temperature (e.g. 40 ◦ C). The catalysts were dried overnight in oven at 120 ◦ C. After this step, the supported catalysts were calcined under an artificial air flow (80% N2 + 20% O2 ) at 450 ◦ C for 2 h. Finally, they were reduced for 2 h in flowing H2 at 500 ◦ C or 300 ◦ C (the last temperature was exclusively used in the case of TiO2 support). 2.2. X-ray diffraction Powder X-ray diffraction (XRD) patterns were recorded on a Panalytical Empyrean diffractometer. The diffractograms were per˚ in a 2 range from 10◦ formed with Cu K␣ radiation ( = 1.5404 A) to 90◦ with a 0.04◦ step size. Crystalline phases were identified by comparison with the reference data from International Center for Diffraction Data (ICDD) files. 2.3. N2 sorption The BET surface areas were deduced from N2 adsorption performed at −196 ◦ C with a Micromeritics apparatus. Prior to the measurement, the samples were treated at 250 ◦ C under vacuum for 12 h in order to eliminate the adsorbed species.

2.1. Catalyst preparation

2.4. H2 chemisorption

TiO2 (Degussa P25, surface area = 55 m2 g−1 ), ␥-Al2 O3 (Puralox NGA 150, surface area = 140 m2 g−1 ), Al2 O3 –SiO2 SIRAL 20, labeled S20, containing 20 wt.% SiO2 (SASOL, surface area = 420 m2 g−1 ) and Al2 O3 –SiO2 SIRAL 40, labeled S40, containing 40 wt.% SiO2 (SASOL, surface area = 522 m2 g−1 ) were used as supports. They were calcined in flowing air for 4 h at 500 ◦ C. Prior to use, they were ground (except SIRAL) and sieved to retain particles with sizes comprised between 0.04 and 0.10 mm. Catalysts containing 3.0 wt.% Pt were prepared by ion exchange method using H2 PtCl6 ·6H2 O (Alfa Aesar)

Pt dispersion was determined by H2 chemisorption. The catalysts were first reduced under H2 at 500 ◦ C or 300 ◦ C (for TiO2 support) for 1 h, then evacuated at the same temperature under Ar for 2 h and cooled down to room temperature. Pulses of H2 were injected at room temperature, every minute up to saturation (HC1 ). A new series of pulses was injected over the sample, after 30 min of purge under pure Ar, in order to eliminate the reversible part of the chemisorbed hydrogen (HC2 ). The irreversible part was taken as HC = HC1 − HC2 and allows to estimate the metallic accessibility

S.N. Delgado et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 89–98

considering the stoichiometry between hydrogen and a surface Pt atom (H/Pts ) equal to 1. The average particle size (d) was determined from the metal accessibility value (%D) according to Hugues et al. [33], who assimilated particles to cubes with one face in contact with the support. The equation is the following: d=

5 × 106 M DS

with M the atomic weight (g mol−1 ), S the surface by mol of Pt metal (53 626 m2 mol−1 ) and  the metal volumic weight (g cm−3 ). Analysis was carried out by using a thermal conductivity detector. 2.5. Temperature programmed reduction experiments (TPR) Prior to the TPR, the monometallic platinum catalysts were first pretreated in situ under O2 for 30 min at 450 ◦ C, and cooled down to room temperature. The TPR experiments were performed with a 1.0 vol.% H2 /Ar gas mixture. The temperature range was 25–500 ◦ C with a ramp of 5 ◦ C min−1 and was then maintained at 500 ◦ C for 1 h. The measurements of the H2 consumption were made in an AutoChem II/Micromeritics apparatus, using a thermal conductivity detector. 2.6. FTIR of adsorbed pyridine The Fourier transformed infrared spectroscopy (FTIR) of adsorbed pyridine was used to determine the number of Lewis acid sites on the metallic catalysts [34]. The samples, pressed into thin wafers (20–25 mg), were pretreated under vacuum at 450 ◦ C for 12 h. The background spectrum, recorded under identical operating conditions without sample, was systematically subtracted. The samples were exposed to pyridine vapors at a pressure of 2 mbar for 5 min at 150 ◦ C. Afterwards, spectra were recorded using a Nicolet-Magna IR 750 spectrometer, after evacuation for 1 h at various temperatures (150 ◦ C, 250 ◦ C and 350 ◦ C). Normalized band areas were calculated by fitting the spectral profile with a Gaussian–Lorentzian function using IR OMNIC software. The concentration of Lewis acid sites was calculated from the integrated area of the band at 1450 cm−1 using the value of the molar extinction coefficient of this band (1.28 cm ␮mol−1 ) determined by Guisnet et al. [35].

91

2.8. Dehydrogenation of cyclohexane Cyclohexane dehydrogenation is a structure insensitive reaction used to characterize the metallic function [40]. The reaction was carried out under atmospheric pressure in a continuous flow reactor at 270 ◦ C. Injection of cyclohexane was made using a calibrated motor-driven syringe. The partial pressures were 97 and 3 kPa for H2 and cyclohexane, respectively. All measurements were performed with a total flow rate of 100 mL min−1 . Analysis of the reaction products was performed by gas chromatography with a flame ionization detector (Varian 3400X) on a HP-PLOT Al2 O3 “KCl” column. The only detected product was benzene. 2.9. Aqueous-phase transformation of glycerol The aqueous phase reaction was carried out under N2 or H2 atmosphere, at a total pressure of 60 bar, and temperature of 210 ◦ C for 360 min in a 300 mL stirred batch autoclave (Autoclave Engineers) fitted with systems for liquid and gas sampling. In a typical experiment, glycerol (4.5 wt.%) was loaded with 10 mL of ultra-pure water into a feeder and the catalyst (1 g) was loaded with 140 mL of ultra-pure water into the reactor. The catalysts were pre-reduced and immersed in the solvent (ultra-pure water) without exposure to air before introduction into the autoclave. The autoclave was flushed with N2 and after with H2 (for experiments under H2 atmosphere). The temperature of the reactor was raised to 210 ◦ C under 20 bar. Then, diluted glycerol (10 mL) was loaded into the autoclave and the pressure was adjusted to 60 bar. Zero time was taken when stirring was switched on. At different times, liquid samples were manually collected during the run and analyzed by HPLC. The mobile phase in HPLC was sulfuric acid in water (0.004 mol L−1 ) with a flow rate of 0.7 mL min−1 . The column was BIO-RAD Aminex HPX 87H and it was used at 30 ◦ C with both refractive index (RI) and ultraviolet (UV) detectors. Gaseous products were analyzed online by a CP-3800 RGA system with 3 channels (2 TCD and 1 FID). Conversion of the glycerol and selectivity were calculated on the basis of the following equations: moles of glycerol consumed × 100 moles of glycerol initially charged

Conversion(%) =

Selectivity(%) =

moles in specific products × 100 moles in all detected products

2.7. Isomerization of 3,3-dimethyl-1-butene

3. Results and discussion

The model reaction of skeletal isomerization of 3,3-dimethyl-1butene (33DMB1) was used to characterize the Brønsted acid sites. Indeed, as demonstrated by Kemball et al. [36,37], the Lewis centers are not involved in this reaction and the 33DMB1 isomerization is likely to occur through a pure protonic mechanism [38,39]. The sample (100 mg), loaded in a glass reactor, was calcined in situ for 1 h at 450 ◦ C (air flow = 60 mL min−1 ), then purged under N2 for 15 min. Afterwards, the flow was switched to H2 and the catalyst was reduced for 1 h (H2 flow = 60 mL min−1 ). After treatment, the catalyst was cooled down to 150 ◦ C or 300 ◦ C (reaction temperature) under N2 (N2 flow = 30 mL min−1 ). The reaction was performed under atmospheric pressure with a reactant flow rate of 15.2 mmol h−1 and a reactant partial pressure of 20 kPa. Analysis of the reaction products was performed by gas chromatography with a flame ionization detector (AlphaMos PR2100) on a Rtx-1 (Restek) column (105 m × 0.53 mm × 3.00 ␮m). The detected products were 2,3-dimethyl-1-butene and 2,3-dimethyl-2-butene.

3.1. XRD analysis The diffractograms of the supports were measured before and after the glycerol transformation test performed under N2 atmosphere (Fig. 2). For the TiO2 support (Fig. 2a and e), peaks of anatase phase were located at 25.3◦ , 37.8◦ , 48.1◦ and 54.0◦ . Small peaks of rutile phase were observed at 27.4◦ , 36.1◦ and 54.3◦ . On both diffractograms, the results show a mix of rutile and anatase phases with small differences in respective intensities between the fresh and used supports. These results are in agreement with the stability in aqueous phase of this support [21]. Before catalytic test, the ␥-Al2 O3 (Fig. 2b) presents a typical diffractogram with two main peaks located at 45.8◦ and 67.0◦ corresponding to the (4 0 0) and (4 4 0) crystal planes of ␥-Al2 O3 structure, respectively [41]. After catalytic test, the main peaks observed on Al2 O3 support correspond to a boehmite phase (Fig. 2f), i.e. 14.5◦ , 28.2◦ , 38.3◦ and 49.0◦ associating to the (0 2 0), (1 2 0),

S.N. Delgado et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 89–98

Intensity (a.u.)

(A)

10

(B)

1 a.u.

(a)

*

*

(b)

20

30

40

*

*

(c)

*

*

(d)

50

60

70

80

1 a.u.

(e)

Intensity (a.u.)

92

*

*

(f) (g) (h)

90

10

20

30

2

40

50

60

70

80

90

2

Fig. 2. X-ray diffraction patterns of (A) fresh and (B) used supports (used samples obtained after glycerol transformation under N2 atmosphere): (a and e) TiO2 ; (b and f) ␥-Al2 O3 ; (c and g) Al2 O3 –SiO2 S20; (d and h) Al2 O3 –SiO2 S40. Peak assignment: () TiO2 rutile, () TiO2 anatase, (*) ␥-Al2 O3 , (♦) AlO(OH).

(1 4 0, 0 3 1), and (0 5 1) crystal planes, respectively [41]. The formation of AlO(OH) in the hydrothermal conditions on this type of supports has been reported in the literature [42–44]. Contrary to a recent study [44], no evident formation of boehmite was observed after catalytic test in alumina–silica supports (SIRAL 20 and 40, Fig. 2c–g and d–h respectively), evidencing that SiO2 stabilizes the Al2 O3 phase in these supports. Reactants constituting the reaction medium during glycerol transformation play a significant role in the transformation of ␥Al2 O3 . Indeed, it was showed that this support reacts differently in aqueous solution of glycerol or sorbitol and in pure water [41]. Finally, this support was more affected by the aqueous medium reaction than the others.

On the fresh catalysts, the Pt dispersion values are function of the support, in the 60–70% range for the alumina and alumina–silica (i.e. an average Pt particle size d around 1.3 nm) and equal to 35% for TiO2 support (i.e. d around 2.4 nm). After the glycerol transformation performed under N2 or H2 atmosphere (used catalysts), the BET surfaces decrease in a little extend (< 15%) on the Al2 O3 –SiO2 and TiO2 supports. A more remarkable decrease is obtained with Pt catalyst supported on Al2 O3 whatever the atmosphere used during the test (N2 or H2 ), since a loss of around 45% of the initial BET surface is observed. This could be attributed to the destruction of the initial structure of alumina, and to the hydration phenomenon leading to the boehmite formation (as observed by XRD). With alumina support, the decrease in BET surface during the aqueous phase transformation of glycerol is associated with a marked decrease of the pore volume. Concerning the Pt dispersion, whatever the nature of the support, a noticeable decrease (superior to 65%) occurs during the catalytic test performed under N2 atmosphere. This loss of dispersion is linked to a sintering phenomenon of the Pt particles during the catalytic test in aqueous phase under 210 ◦ C and 60 bar, as already observed in previous study [45]. Recently published papers devoted to the stability of Pt catalysts during aqueous phase hydrogenolysis of polyol clearly describe this phenomenon [41,46,47]. These authors observed that platinum sintering occurs in liquid water even at room temperature. The experimental

3.2. Specific surface and metal accessibility measurements The physicochemical properties (specific area and Pt dispersion) were measured for each catalyst before (fresh) and after the glycerol transformation (used) (Table 1). Before Pt impregnation, the BET surfaces and pore volumes can be classified as follows: TiO2 < Al2 O3 < SIRAL 20 < SIRAL 40. For the alumina–silica supports, the BET surface increases with the SiO2 content. After Pt impregnation, the specific surfaces of the fresh samples decrease in a more or less extend according to the nature of the support.

Table 1 Properties of Pt-based catalysts before (fresh) and after (used) aqueous phase transformation of glycerol. Catalysts

Ssp a (m2 g−1 ) Fresh

Pt/TiO2 Pt/Al2 O3 Pt/S20e Pt/S40e a b c d e

52 140 314 373

VP a (cm3 g−1 ) Usedd

Fresh

N2

H2

50 78 338 335

44 72 336 326

0.32 0.42 0.46 0.69

Dispersionb (%) Usedd

Fresh

N2

H2

0.25 0.24 0.56 0.76

0.23 0.23 0.56 0.76

Specific surface (Ssp ) and pore volume (Vp ) determined by BET measurement. Determined by H2 chemisorption. Determined by TPR experiment. N2 or H2 : atmosphere used during glycerol transformation in batch reactor. Al2 O3 –SiO2 support (S20: SIRAL 20 and S40: SIRAL 40).

35 69 68 61

H2 consumptionc (␮mol g−1 catal )

Usedd

H2 /Ptc

Fresh

N2

H2

12 21 21 19

38 32 21 45

86 287 206 190

0.6 1.9 1.3 1.2

S.N. Delgado et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 89–98

93

0.05 a.u.

Absorbance

Pt/TiO2

Pt/Al2O3

Pt/S20 Pt/S40 1700

1650

1600

1550

1500

1450

1400

Wavenumber (cm-1) Fig. 3. TPR profiles of Pt-based catalysts as a function of the support nature (protocol: 1 h O2 450 ◦ C – 15 min Ar Tamb – 1% H2 /Ar 500 ◦ C 5 ◦ C min−1 – 1 h at 500 ◦ C).

Fig. 4. IR spectra of adsorbed pyridine at 150 ◦ C on the supported Pt catalysts.

3.4. FTIR of adsorbed pyridine parameters used for the polyol transformation (T, P) as well as the composition of the aqueous phase (polyol and derivates) further contribute to accentuate the sintering phenomenon generated mainly by the presence of water. Under H2 atmosphere, the Pt dispersion does not evolve compared to the fresh catalyst in the case of TiO2 support, and decreases for Al2 O3 and SIRAL 40 supports but in a lower extend than under N2 atmosphere. For SIRAL 20, the same evolution between the fresh and used samples is obtained for both atmospheres. The different behaviors according to the atmosphere used during the glycerol transformation must be linked to various compositions of the reaction mixture. Moreover, a pH decrease is observed after tests from 7 (glycerol aqueous solution) to approximately a value of 3, whatever the catalyst and the atmosphere.

3.3. Temperature programmed reduction experiments Fig. 3 presents TPR profiles of the various Pt-based catalysts (experiments performed on fresh samples). The reduction of the metallic phase occurs in a temperature range comprised between 50 and 300 ◦ C for the Pt catalysts supported on alumina and alumina–silica, the maximum of the consumption peak being located at a slight higher temperature for Pt/S20. A reduction profile less large and at lower temperature is obtained for the Pt/TiO2 sample. The evolutions according to the support nature are in agreement with the different values of Pt dispersion (D%: Pt/Al2 O3 ≈ Pt/Al2 O3 –SiO2 > Pt/TiO2 ) knowing that Pt particles are all the most easily reduced than their size is important, i.e. in weaker interaction with the support. For all the catalysts, the H2 consumptions gathered in Table 1 are lower than the quantity necessary to reduce all the metal atoms (153 ␮molPt g−1 catal ), if this one is considered to be in the PtO2 oxidized form before the TPR experiments (in that case, 2 mol of H2 would be necessary to reduce 1 mol of PtO2 , involving a H2 /Pt ratio equal to 2). This observation means that the pre-oxidation treatment is probably not sufficient to oxidize until the core the Pt particles in a PtO2 form and this notably for catalysts supported on both Al2 O3 –SiO2 and TiO2 (0.6 < H2 /Pt < 1.3, Table 1). On the other hand, the formation of another oxide phase of lower stoichiometry (PtO) could not be excluded on these systems. Finally, the TPR experiments allow confirming that the metallic phase of the Pt catalysts is totally reduced after the activation treatment performed at the end of their preparation (reduction at 500 ◦ C for Pt deposited on Al2 O3 and Al2 O3 –SiO2 supports, and at 300 ◦ C on TiO2 support).

The adsorption of pyridine followed by FTIR was performed in order to quantify, on the fresh catalysts, the amount of the Lewis acid sites and the distribution of their strength (weak, medium and strong). The spectral range studied was 1400–1700 cm−1 . The different attributions of bands are reported in the literature [48,49]. Fig. 4 shows the spectra of the Pt-based catalysts after pyridine adsorption at 150 ◦ C. The commonly accepted procedure for FTIR spectral interpretation of adsorbed pyridine attributes adsorption at 1624 and 1455 cm−1 to strong sites, 1617, 1576 and 1451 cm−1 to weak Lewis acid sites, 1540 cm−1 to Brønsted acid sites, and 1494 cm−1 to both Brønsted and Lewis sites [34,48,50]. On the four spectra, the very low intensity of the band at about 1540 cm−1 indicates that there are no Brønsted acid sites on the surface strong enough to form pyridinium ions (PyH+ ). The total number of Lewis acid sites was calculated from the integrated area of the band at 1455 cm−1 on the spectra after evacuation for 1 h at 150 ◦ C. The strength of Lewis acid sites was determined by difference between the integrated areas obtained at various temperatures (250–150 ◦ C: weak sites/350–250 ◦ C: medium sites/area remaining at 350 ◦ C: strong sites). The results are presented in Fig. 5 for the four Pt-based catalysts. The total number of Lewis sites is more important on Al2 O3 and Al2 O3 –SiO2 (around 300 ␮mol g−1 ) than on TiO2 (150 ␮mol g−1 ). The number of weak acid sites predominates on all samples. Al2 O3 –SiO2 supports (S20 and S40) exhibit a noticeable quantity of medium acid sites compared to the other supports. Finally, strong acid sites are in greatest quantity on Pt/Al2 O3 and Pt/S40 catalysts. 3.5. Isomerization of 3,3-dimethyl-1-butene (33DMB1) At a temperature below 300 ◦ C, the isomerization of 3,3dimethyl-1-butene leads to 2,3-dimethyl-1-butene and 2,3dimethyl-2-butene and allows the characterization of the Brønsted acid centers with a medium strength. Methylpentenes appear at higher temperatures (>300 ◦ C) and their formation requires relatively strong Brønsted acid sites [38]. The reaction was performed at 150 ◦ C for Pt/Al2 O3 –SiO2 (S20 and S40) and 300 ◦ C for Pt/Al2 O3 and Pt/TiO2 so as to measure correctly the conversion. Table 2 gives the temperature of reaction and the activities measured per ␮mol m−2 h−1 and calculated by considering the BET surface area reported in Table 1 for each fresh catalyst. The activation energy was close to 95 kJ mol−1 [39], which allows establishing a classification of the Brønsted acidity based on an extrapolation of the activity at 300 ◦ C (Fig. 6). Although the

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Fig. 5. Lewis acid sites distribution determined by FTIR of pyridine on the Pt-based catalysts.

Pt/TiO2

Pt/Al2O3

Pt/S20

Pt/S40 0

2

4

6

8

10

12

14

Ln (A) Fig. 6. Classification of the Brønsted acidity of the supported Pt catalysts, based on the extrapolation of their activity A for 33DMBl isomerization at 300 ◦ C (A per ␮mol m−2 h−1 ).

IR spectra of adsorbed pyridine show that there are few Brønsted acid sites (at 1540 cm−1 ), the Pt/S20 and Pt/S40 catalysts show a good isomerization activity even at 150 ◦ C. The Brønsted acidities of Pt/TiO2 and Pt/Al2 O3 samples are quite similar and around twice lower than these of Pt/Al2 O3 –SiO2 (S20 and S40) (Fig. 6). 3.6. Dehydrogenation of cyclohexane The different catalysts after preparation (fresh samples) were tested for cyclohexane dehydrogenation into benzene at 270 ◦ C and under atmospheric pressure. The performances of Pt-based catalysts are reported in Table 3. This model reaction is known to be catalyzed only by the metallic phase and to be insensitive to the catalyst structure [40,51,52]. In function of the support nature (Al2 O3 , Al2 O3 –SiO2 , TiO2 ), the obtained specific activities (As) are comprised between 3.1 and 4.5 mol of cyclohexane converted per hour and per gram of metal. These values are quite comparable for the samples supported on Al2 O3 –SiO2 (S20 and S40), assuming an error of around 10% on the

determination of As value, a little more important for Pt/Al2 O3 , and the highest one was obtained for Pt/TiO2 . As all the catalysts do not have the same number of accessible platinum atoms, the intrinsic activities or TOF (turn over frequency) are more appropriated data to compare the properties of the different systems. TOF values are estimated taking into account the number of accessible Pt atoms per gram deduced from the H2 -chemisorption experiments performed on fresh samples. The TOF values present the following order: Pt/S20 < Pt/Al2 O3 < Pt/S40  Pt/TiO2 . So, a Pt surface atom is more active for this reaction when deposited on the TiO2 support compared to the Al2 O3 and Al2 O3 –SiO2 ones. This behavior can result from different metal–support interactions between the samples, leading to different electronic enrichments of the metallic particles, and then to various adsorption strengths of the reactant (cyclohexane) and product (benzene) onto the Pt surface. Indeed, TPR experiments have shown previously that the preoxidized Pt particles are reduced at lower temperature when deposited on TiO2 compared to other supports (Fig. 3). Thus, the highest metal–support interaction obtained with Al2 O3 and Al2 O3 –SiO2 may induce an electronic transfer from metallic particles toward oxide supports, involving modifications of the metallic phase properties. These modifications, corresponding to an electronic deficiency, lead to a higher interaction between the Pt particles and the reaction product (benzene) and consequently to lower TOF values for Pt/Al2 O3 and Pt/Al2 O3 –SiO2 systems. Moreover, it can be noticed that among these three samples, the Pt/S20 catalyst showed a TPR profile slightly shifted to higher temperatures, suggesting a metal–support interaction slightly higher in this case. This behavior can explain that the TOF value of this last sample during cyclohexane dehydrogenation is the lowest of the series. In conclusion, from the catalytic performances obtained during the probe reaction of cyclohexane dehydrogenation, we can assume that the four studied Pt-based catalysts present different metallic phases in terms of electronic properties.

Table 3 Activity of Pt-based catalysts obtained for cyclohexane dehydrogenation at 270 ◦ C.

Table 2 Activity of the catalysts in 33DMBl isomerization. Catalysts

Reaction temperature T (◦ C)

Activity at T ◦ C (␮mol m−2 h−1 )

Pt/TiO2 Pt/Al2 O3 Pt/S20 Pt/S40

300 300 150 150

589 333 106 271

Catalysts

As (mol h−1 g−1 metal )

TOF (s−1 )

Number of accessible atoms g−1 metal determined by H2 chemisorption

Pt/TiO2 Pt/Al2 O3 Pt/S20 Pt/S40

4.5 3.9 3.1 3.4

0.70 0.31 0.25 0.38

1.07 × 1021 2.12 × 1021 2.09 × 1021 1.88 × 1021

S.N. Delgado et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 89–98

45

45

(a)

40

(b)

40

35

35

30

30

conversion %

conversion %

95

25 20 15 10 5

25 20 15 10 5

0

0 0

100

200

300

Time (min)

0

100

200

300

Time (min)

Fig. 7. Glycerol conversion in function of reaction time on Pt-based catalysts under (a) N2 and (b) H2 atmosphere (*: Pt/TiO2 ; : Pt/Al2 O3 ; : Pt/S20; : Pt/S40).

3.7. Aqueous phase transformation of glycerol The reaction pathway of the glycerol transformation is summarized in Fig. 1. Generally, the nature of the obtained products depends on several parameters like the nature of the metallic phase and of the support, the reaction conditions (temperature, pressure). Generally, the dehydration steps are associated to catalysis via an acidic function, whereas the hydrogenation/dehydrogenation reactions need a metallic function. In the present work, the effects of the nature of the support (Al2 O3 , Al2 O3 –SiO2 and TiO2 ) and of the reaction atmosphere (N2 or H2 ) have been studied. In the experimental conditions (4.5 wt.% glycerol, 210 ◦ C, 60 bar total pressure), the main products observed in liquid phase (detected by HPLC) were: – C3 products resulting from C O bond selective cleavages, i.e. the first dehydration or dehydrogenation products (acetol, glyceraldehyde (GLA), respectively), further hydrogenation products (1,2- and 1,3-propanediol (1,2-PD, 1,3-PD)), and further hydrogenolysis products (1- and 2-propanol (1P + 2P)). – C2 and C1 products (degradation products) resulting from one or two C C bond cleavages, respectively, i.e. ethylene glycol (EG), ethanol (EtOH) and methanol (MeOH). We can notice that the 3-hydroxypropanal (3-HPA), presented in Fig. 1 as a potential intermediary product, was not detected in any of the experiments, and no reference in the literature does not mention the detection of this compound. The stabilities of 3-HPA and acetol are different: stable acetol can be detected in the reaction while unstable 3-HPA is nearly instantaneously converted in subsequent reactions [15]. Hydrocarbons (propane, propene, ethane and methane) as well as CO2 and H2 were detected in the gas phase (CPG analysis). First, it is not detailed here but tests with only supports (before platinum impregnation) were made, the conversion of glycerol was inferior to 10% and the major product was acetol, product of dehydration. This behavior confirms on one hand, that the acid sites on the support are involved in the initial dehydration reaction [53], and on the other hand, that the hydrogenation reactions leading further to diols do not occur in the absence of metallic sites. The conversion of glycerol was determined in the presence of fresh Pt-based catalysts under N2 (Fig. 7a) and H2 (Fig. 7b) atmosphere. For all supported samples, the conversion is more important under N2 atmosphere than under H2 , suggesting an inhibiting effect of hydrogen. This phenomenon has already been reported to be caused by the blocking of surface sites by adsorbed hydrogen atoms, decreasing then the surface concentrations of reactive intermediates [4,13]. Whatever the atmosphere used (N2 or H2 ),

the Pt catalyst supported on TiO2 is by far more active than the others, highlighting the interesting behavior of this support for the conversion of glycerol in aqueous phase, compared to Al2 O3 and Al2 O3 –SiO2 supports more classically used in the studies dedicated to APP of polyols on metallic catalysts [13,44]. For the selective hydrogenolysis of glycerol in liquid phase, a lot of monometallic catalytic systems were studied before. Among then, platinum-based catalysts were largely used, supported on various supports (active carbon, SiO2 , Al2 O3 , Al2 O3 –SiO2 ), and tested under varied experimental conditions (type of reactor: batch or fixedbed one, temperature, total pressure, concentration of reactant, catalyst mass) [15,23–25,28]. The different experimental conditions performed in these studies make complex the comparison of the obtained catalytic performances. On 1 wt.% Pt/Al2 O3 –SiO2 catalysts tested in autoclave with 20 wt.% glycerol initial concentration, 45 bar pressure, 166 mg catalyst/mg glycerol, Gandarias et al. obtained 20% of conversion at 220 ◦ C after 24 h reaction time [15]. Under different conditions (1 wt.% glycerol solution, 40 bar pressure, 200 ◦ C, glycerol/surface platinum ratio of approximately 700), 3 wt.% Pt/C catalysts led to a conversion of 13% after 5 h reaction time [24]. To our knowledge, no Pt-based catalyst supported on TiO2 was studied until now for the glycerol transformation, apart Gong et al. who studied recently a complex system constituted of Pt/WO3 /TiO2 deposited on silica, in which the presence of TiO2 species was observed to favor the dispersion of platinum [54]. In the present study, the higher conversions obtained for the Pt/TiO2 catalyst can be linked to the Pt particle size more important on TiO2 support than on the others, as observed by H2 chemisorption on the fresh catalysts (Table 1). In fact, hydrogenolysis reactions are known to be structure sensitive, and favored by the presence of large particles [55,56]. Nevertheless, particle size evolves during the test since sintering of the metallic Pt phase was observed on each catalyst, but the initial state of the fresh catalyst can be crucial. Moreover, the Pt/TiO2 catalyst seems more stable than the others overall the test, since a more important deactivation phenomenon is observed on the Pt systems supported on Al2 O3 and Al2 O3 –SiO2 . The amounts of carboneous deposit on the catalysts after tests have been measured and these amounts (<0.2 wt.%) are not significant enough to explain this deactivation. Maris et al. evaluated the properties of carbon-supported monometallic and bimetallic systems constituted of Pt, Ru and Au, for the glycerol hydrogenolysis (solution of 1 wt.%) performed at 200 ◦ C, 40 bar H2 during 300 min [24]. On Pt/C catalyst, these authors observed the same trend than on the present Pt systems, i.e. a significant deactivation of the sample after around 20 min under reaction conditions. According to these authors, this behavior comes from a sintering of the Pt particles caused by the aqueousphase processing, sintering they confirm besides by TEM and EXAFS analyses.

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Table 4 Selectivity to products at isoconversion (10%) and yield to H2 at 360 min reaction time obtained during glycerol transformation with Pt-based catalysts. Catalysts

Atmospherea

Pt/TiO2

N2

Pt/Al2 O3 Pt/S20 Pt/S40

N2 N2 N2

Pt/TiO2

H2

Pt/Al2 O3 Pt/S20 Pt/S40

H2 H2 H2

a b c d e

Selectivity (%)

H2 yield (%mol)

Acetol

GLA

1,2-PD

32.3 8.8d 13.8 10.0 21.1

0.3 – 13.7 2.2 4.1

25.0 49.4d 21.0 20.0 22.9 68.0 61.8e 33.8 21.5 30.2

9.0 1.7e 2.4 1.6 2.2

0.9 3.9e 2.6 2.3 4.1

1,3-PD

1P + 2P

EG

EtOH

MeOH

– – – – –

– 2.4d – 0.6 2.1

31.1 11.9d 32.3 8.0 21.9

3.9 9.9d 5.6 3.8 14.4

– – – 43.5 4.4

– – 12.1 5.5 5.5

1.0 6.2e 2.6 4.0 5.9

14.5 14.7e 36.0 23.3 25.4

1.5 6.5e 4.3 5.5 8.4

– – – 29.4 8.5

HCb

Othersc

0.4 5.4d 7.8 2.8 1.5

1.0 8.7d 5.8 6.3 3.1

6.0 3.5d – 2.8 4.5

16.9 31.4 16.2 19.5

0.5 1.5e 3.1 1.6 2.2

1.6 3.0e 2.0 1.4 3.7

3.0 0.7e 1.1 3.9 3.9

– – – –

CO2

N2 or H2 gaseous atmosphere introduced in the autoclave. HC: gaseous saturated hydrocarbons. Others: ethylene, propene, acrolein, propanal, acetic acid. After 360 min reaction time (43% conversion). After 360 min reaction time (24% conversion).

Under H2 atmosphere, the performances of the four tested catalysts in terms of glycerol conversion can be classified as follow: Pt/TiO2  Pt/Al2 O3 ≈ Pt/S40 > Pt/S20. This classification was quite similar for the results obtained from the model reaction of cyclohexane dehydrogenation, with a turnover frequency for the Pt/TiO2 system more than three times higher compared to that of Pt/S20 catalyst, and around twice higher compared to these of Pt/Al2 O3 and Pt/S40 samples. Nevertheless, the comparison between the catalytic properties observed for that model reaction and for the glycerol transformation has to be considered cautiously, since the operating conditions are very different (gas phase reaction performed in a fixed-bed reactor for the first one, and aqueous phase reaction carried out in an autoclave for the second one). Table 4 presents the selectivity in products obtained on each Pt catalyst at isoconversion (10%). For Pt/TiO2 sample, the selectivity after 360 min reaction time is also reported. For the Pt/Al2 O3 , Pt/S20 and Pt/S40 catalysts, the results obtained at the final reaction time (360 min) are not added since quite similar to data already presented in Table 4, the glycerol conversion hardly exceeding 10% on these samples. Acetol, primary product of dehydration, is formed in a higher proportion under N2 atmosphere. Under H2 , this unsaturated product is easily hydrogenated toward C3 diol and mono-alcohols (1,2-PD, 1,3-PD, 1P + 2P). With the increase in the reaction time, this first intermediate is further transformed, and then its proportion decreases (as seen on the Pt/TiO2 catalyst). 1,3-Propanediol (1,3-PD) is formed only under H2 and should be issue from 3-hydroxypropanal hydrogenation, that aldehyde being easily hydrogenated and never observed in our experimental conditions. It is important to mention that a noticeable formation of H2 occurred under N2 atmosphere, providing by the Aqueous Phase Reforming (APR) of glycerol on Pt sites [6]. The H2 molecules as formed can react further easily with intermediates adsorbed on neighboring Pt sites or, after spill over, with intermediates adsorbed on acidic sites of the support. During glycerol transformation carried out under H2 atmosphere, it is not possible to precisely quantify the produced H2 but the APR route should not be thermodynamically favored in the presence of H2 into the reaction medium. Glyceraldehyde (GAL), product issue from one glycerol dehydrogenation, is formed in a significant amount in the presence of Pt/Al2 O3 under N2 atmosphere, that catalyst leading also to the highest H2 yield. The selectivities toward C3 diols and mono-alcohols (1,2-PD, 1,3-PD, 1P, 2P) are systematically higher under H2 atmosphere for all the catalysts. Nevertheless, after 360 min reaction time, the selectivity to 1,2-PD in the case of the Pt/TiO2 catalyst tested under N2 atmosphere is finally higher than this obtained on Pt/Al2 O3 ,

Pt/S20 and Pt/S40 systems under H2 atmosphere. At 10% conversion under N2 atmosphere, the alcohols formed in aqueous phase are mainly C2 compounds, as well as methanol in the case of both alumina–silica supports. That peculiar formation of methanol can be due to the presence of a higher quantity of acidic sites on the Al2 O3 –SiO2 oxides compared to the other ones. Indeed, the previous characterizations have shown that Al2 O3 –SiO2 supports possess both the highest Lewis acidity (comparable to that of Al2 O3 ) and the highest Brønsted acidity (twice higher that on the other oxides). The presence of such concentration of acidic sites may promote the C C bond cleavages, via an acidic cracking mechanism. Under both atmospheres, the formation of CO2 is also observed issue from C C bond cleavages via decarbonylation and/or decarboxylation reactions, and/or from reforming process. It must be noticed that a part of CO2 can dissolve in the aqueous phase, and this quantity cannot be accounted since a significant amount is removed during processing of the liquid sample analysis (decrease of temperature, filtration). Moreover, a production of hydrocarbon (HC) is obtained on each catalyst in more or less extends according to the nature of the support and of the atmosphere used during glycerol transformation. At 10% glycerol conversion, the HC selectivity is maximum on the Pt/Al2 O3 and Pt/S20 samples tested under N2 atmosphere, but this value remains rather low (SHC ≈6%). After 360 min reaction time, the production of HC is finally the highest on the Pt/TiO2 system under N2 atmosphere (SHC ≈9% for 43% of converted glycerol). It must be noted that some other products are detected in rather limited quantities (listed as “Others” in Table 4). The catalytic performances of all the systems obtained at the end of the reaction time (360 min) are also gathered in Fig. 8. After 360 min reaction time, the Pt/TiO2 catalyst is by far the highest selective system toward C3 products, since the selectivity reaches 62% and 75% under N2 and H2 atmosphere, respectively. These C3 compounds are mainly composed of 1,2-PD (selectivity equal to 49% and 62% according to N2 or H2 atmosphere). The propanol (1P + 2P) selectivity remains low (<7%), as on the other Pt catalysts, as well as the propane one (equal to 1%). In fact, the limited quantity of hydrocarbon produced during the reaction is not only constituted of propane, but also of ethane and methane. Fig. 9 shows the distribution in hydrocarbon obtained with the various catalysts at 360 min of reaction time. Under N2 atmosphere, methane is the mainly formed hydrocarbon whatever the nature of the catalyst, and propane remains hugely limited suggesting probably that under this atmosphere, the CO produced by APR of glycerol may be transformed by methanation reaction. Under H2 atmosphere, the proportion of methane decreases in favor of propane

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Fig. 8. Catalytic performances of the supported Pt-based catalysts obtained at 360 min reaction time during glycerol transformation under N2 and H2 atmosphere.

Fig. 9. Distribution (mol%) in hydrocarbon at 360 min of reaction time obtained on Pt-based catalysts during glycerol transformation under N2 or H2 atmosphere.

and ethane, notably with the Pt/S40 catalyst, and above all with the Pt/TiO2 system. Finally, the performances of the Pt-based catalysts during glycerol transformation are linked to the nature of the support used, which modulates the dispersion of the metallic phase, the metal–support interaction, the electronic properties of the metallic particles and the acidic properties of the catalyst. Among the studied supports, TiO2 leads to the highest selective Pt catalyst toward propanediol and propanol, with the highest capacity to maintain the carboneous chain until propane formation without C C bond cleavage by working under H2 atmosphere. Consequently, successive dehydration/hydrogenation reactions occur selectively on Pt/TiO2 system, which could be a promising catalyst for further investigations concerning aqueous phase process of polyol transformation.

4. Conclusion Pt catalysts supported on various oxides (alumina, alumina–silica, titania) were prepared and characterized, and their performances were evaluated for the glycerol transformation performed in aqueous phase, at 210 ◦ C, under 60 bar as total pressure, in a batch reactor.

During aqueous phase reaction, the catalysts undergone structural and textural modifications, in a less or more extend according to the nature of the support, as well as to the nature of the gaseous atmosphere (N2 or H2 ) introduced into the batch reactor. XRD patterns showed the formation of boehmite from Al2 O3 support, whereas no change of crystalline phases was observed with the other supports (Al2 O3 –SiO2 and TiO2 ). A sintering of the Pt particles occurred during the test, mainly under N2 atmosphere. However, TiO2 was identified as the support leading to the most stable Pt metallic phase in the reaction conditions. A higher Lewis and Brønsted acidity was obtained on the catalysts supported on Al2 O3 –SiO2 . During glycerol transformation, the acidic sites are involved in the dehydration reactions, but unfortunately they can also promote the C C bond cleavages via an acidic cracking mechanism. Among the various supported Pt-based catalysts studied in this work, Pt/TiO2 sample appeared to be the most efficient system, leading to the highest glycerol conversion and to the highest selectivity toward C3 products (propanediols, propanol) that can be further valorized into chemicals. This catalyst was also the most efficient to keep intact the carboneous chain until propane formation without C C bond cleavage. In the field of the aqueous phase processes of C5 C6 polyols with the aim to produce C5 C6 hydrocarbons and incorporate them to fuel pool, Pt/TiO2 catalyst may then appear as promising. Acknowledgement We are grateful to Region Poitou-Charentes for financial support of this project. References [1] R.R. Davda, J.A. Dumesic, Angew. Chem. Int. Ed. 42 (2003) 4068. [2] R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B 43 (2003) 13. [3] G.W. Huber, J.W. Shabaker, J.A. Dumesic, Science 300 (2003) 2075. [4] J.W. Shabaker, R.R. Davda, G.W. Huber, R.D. Cortright, J.A. Dumesic, J. Catal. 215 (2003) 344. [5] J.W. Shabaker, G.W. Huber, R.R. Davda, R.D. Cortright, J.A. Dumesic, Catal. Lett. 88 (2003) 1. [6] G.W. Huber, R.D. Cortright, J.A. Dumesic, Angew. Chem. Int. Ed. 43 (2004) 1549. [7] J.W. Shabaker, J.A. Dumesic, Ind. Eng. Chem. Res. 43 (2004) 3105. [8] J.W. Shabaker, G.W. Huber, J.A. Dumesic, J. Catal. 222 (2004) 180. [9] J.W. Shabaker, D.A. Simonetti, R.D. Cortright, J.A. Dumesic, J. Catal. 231 (2005) 67. [10] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. Della Pina, Angew. Chem. Int. Ed. 46 (2007) 4434. [11] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411.

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