Acid–base properties of niobium-zirconium mixed oxide catalysts for glycerol dehydration by calorimetric and catalytic investigation

Applied Catalysis B: Environmental 106 (2011) 94–102

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Acid–base properties of niobium-zirconium mixed oxide catalysts for glycerol dehydration by calorimetric and catalytic investigation P. Lauriol-Garbey a , G. Postole a , S. Loridant a , A. Auroux a , V. Belliere-Baca b , P. Rey c , J.M.M. Millet a,∗ a Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR5256 CNRS-Université Claude Bernard Lyon 1, 2 avenue A. Einstein, F-69626 Villeurbanne cedex, France b Rhodia, 52 rue de la Haie Coq, 93308 Aubervilliers cedex, France c Adisseo, Antony Parc 2, 10 Place du Général de Gaulle, 92160 Antony, France

a r t i c l e

i n f o

Article history: Received 14 March 2011 Received in revised form 3 May 2011 Accepted 10 May 2011 Available online 17 May 2011 Keywords: Glycerol Acrolein Dehydration Acid catalysis Nb2 O5 -ZrO2 mixed oxide Deactivation Adsorption microcalorimetry

a b s t r a c t The acid–base properties of fresh and used niobium-zirconium mixed oxide catalysts, used for the dehydration of glycerol to acrolein, have been characterized by various techniques. These techniques include ammonia thermo-programmed desorption (TPD), infrared spectroscopy of absorbed pyridine, and adsorption microcalorimetry of ammonia and sulfur dioxide. Relationships between the catalytic properties and the acid–base properties of fresh catalysts have been investigated. The most efficient catalysts were shown to be those for which the zirconia support had a better niobium oxide species covering, but no specific relationship could be established between the acid–base, and the selectivity to acrolein. Characterization of the used catalysts showed that the acidic properties of the catalysts had changed considerably with the time spent on stream. No further strong acid sites, and only weak or very weak acid sites were detected. A linear relationship between the total quantity of remaining acidic sites and the rate of glycerol conversion was determined, taking into account an intrinsic activity of the stronger sites, which is more than ten times that of the weaker ones. The deactivation of the catalysts as a function of time on stream has been related to the formation of cyclic molecules, produced by the reaction of acrolein with by-products resulting from the decomposition of hydroxyacetone and also possibly acrolein and glycerol. © 2011 Published by Elsevier B.V.

1. Introduction The catalytic dehydration of glycerol to acrolein has attracted great interest in recent years [1–9]. Numerous catalysts have already been proposed, all of which exhibit strong catalytic properties, with selectivity to acrolein ranging between 70 and 90% at total glycerol conversion. Comparative studies have shown that acrolein synthesis efficiency appears to be enhanced at medium acidity strengths (−8.2 < H0 < 3.0) [7], whereas more recent studies have concluded that its enhancement is optimal at weak [8], or strong acid [9] strengths. A better agreement was derived from the fact that Brønsted acid sites were more active and selective than Lewis acid sites [6–9]. It was proposed that the Lewis acid sites were more selective to hydroxyacetone, this being explained by a different pathway for glycerol activation. Whereas the reaction would be initiated through protonation of the secondary hydroxyl

∗ Corresponding author. Tel.: +33 0 472445317; fax: +33 0 472445399. E-mail address: [email protected] (J.M.M. Millet). 0926-3373/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.apcatb.2011.05.011

group of the glycerol molecule by a proton on a Brønsted acid site, it would start with a concerted transfer of a terminal hydroxyl group on a Lewis acid site and the migration of the secondary proton to the neighboring metal center [9]. The intermediate formation of an enol, which can easily tautomerize to hydroxyacetone, would explain the higher selectivity to the latter compound on a Lewis acid site. It has also been systematically observed by all authors that the catalysts underwent more or less rapid deactivation due to extensive coke deposition on their surfaces, which thus varies constantly with time on stream. Insufficient attention is paid to the latter feature when comparing the properties of catalysts. We have recently proposed new niobium-zirconium mixed oxide catalysts, which were efficient for the dehydration of glycerol into acrolein, and considerably more stable than those reported in the literature [10]. The best selectivity to acrolein at or near total conversion, obtained after 10–20 h on stream, was 72%. Although this is lower than the best efficiencies obtained in the literature, which exceed 80%, the catalysts appeared to be considerably more stable on stream, with a glycerol conversion rate lowered by only 10% after 200 h under the best conditions, with no loss in selectivity.

P. Lauriol-Garbey et al. / Applied Catalysis B: Environmental 106 (2011) 94–102

Characterization by means of various techniques has shown that the catalysts corresponded to polymeric niobium oxide species supported on zirconia. This work has also shown that niobium was incorporated into the zirconia, and this appeared to be a key factor in obtaining efficient and stable catalysts. Both covering of the support by the active species, and incorporation of niobium into zirconia, appeared to contribute to the neutralization of the Lewis Brønsted acid/base sites of uncovered zirconia, as shown by the study of NH3 -TPD and XPS surface characterization [10]. Basic zirconia sites were also suspected to be unselective, although no quantitative data were obtained to support this hypothesis. The absence of such unselective zirconia sites at the surface of efficient catalysts, which were presumably coke formation initiating sites, also explained their high stability as a function of time spent on stream. In the present study, efficient ZrNbO mixed oxide catalyst samples were prepared and studied by adsorption microcalorimetry experiments using specific probe molecules. The acidic characteristics of the solids were monitored by ammonia adsorption. Because the materials studied could have contained residual carbonates adsorbed onto the zirconia surface, the basic properties were analyzed using SO2 as an acidic probe, rather than the more commonly used CO2 . SO2 is able to titrate strong, medium and weak sites contrarily to CO2 , which titrates only strong basic sites [11]. Pyridine adsorption followed by infrared spectroscopy (FTIR) and temperature-programmed desorption of ammonia (NH3 -TPD) were respectively used to determine the nature of the adsorbed surface species, and to compare the acidic properties of a larger number of catalysts. Their catalytic behavior for the conversion of glycerol was tested in order to search for a correlation between acid–base properties and catalytic properties.

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Since various difficulties were encountered in reproducing these synthetic samples, an alternative method was optimized [10]. This was similar to the first method, but was carried out in water using ammonium oxalato-niobate, and led to a considerably less exothermic reaction. 24 mmol of ammonium oxalato-niobate Nb(NH4 )(C2 O4 )2 NbO·xH2 O (Aldrich, 99.99%) were dissolved into 65 mL of de-ionized water, acidified to pH < 0.5 by the addition of concentrated nitric acid. The solution was heated to 45 ◦ C for about 40 min in order to dissolve the niobium compound, and then cooled to room temperature before adding the hydrated zirconia, assuming a Zr/Nb molar ratio equal to 3. The reaction medium was maintained at room temperature for 24 h under stirring and the solid was separated by filtration. The solid was calcined at 600 ◦ C in airflow using the method described above. Samples with different compositions were prepared using this method, and the solids obtained were denoted by ZrNbO-x, with x representing the Zr/Nb ratio. For comparison a zirconia supported Nb2 O5 catalyst was prepared by impregnation. The support corresponded to a mixture of tetragonal and monoclinic phases, with a specific surface area of 95 m2 g−1 . It was obtained by calcining zirconium hydroxide at 600 ◦ C for 2 h. 10 g of the support was added to a 50 mL aqueous solution containing respectively 2.5 mmol of ammonium oxalatoniobate (corresponding to a Nb surface density of 3.3 atom nm−2 , or 30% of a theoretical Nb2 O5 monolayer). The solution was aged for 1 h 30 min at room temperature under stirring, and evaporated under vacuum and calcined at 600 ◦ C for 2 h (heating rate 5 ◦ C min−1 ). The solid and the support alone are referred to as: NbOx /ZrO2 and (m + t)ZrO2 .

2.2. Characterization of the catalysts 2. Experimental 2.1. Preparation of the catalysts Two methods have been developed for the synthesis of ZrNbO catalysts. The first of these consists in impregnating hydrated zirconium ZrOx (OH)4–2x oxide with a peroxo-niobate solution. The amorphous hydrated zirconia was prepared by co-precipitation, at a constant pH of 8.8, from a 0.4 M solution of zirconyl nitrate (ZrO(NO3 )2 ·xH2 O Aldrich, 99%), by addition of an ammonia aqueous solution (28%). After precipitation, the solid was maintained in the solution for 1 h, separated by centrifugation, and washed several times with de-ionized water. The solid was then dried at 110 ◦ C overnight and ground into a powder. The water content of the final solid was determined using thermogravimetric analysis. The peroxo-niobate solution was prepared by solubilizing 24 mmol of Nb(NH4 )(C2 O4 )2 NbO·xH2 O (Aldrich, 99.99%) into 65 mL of a 35 wt% solution of H2 O2 (Sigma–Aldrich), acidified to pH ≈0.5 with concentrated nitric acid, and by heating the solution to 50 ◦ C for 1 h. The hydrated zirconia was slowly added to the peroxo-niobate solution at room temperature, ensuring a molar ratio of ZrO2 :Nb2 O2 = 6:1. The reaction was strongly exothermic and an ice bath was needed to cool the reaction medium. After the addition of hydrated zirconia, the reaction mixture was maintained at room temperature for 24 h. The final solution was evaporated under vacuum and the resulting solid was calcined in air at 600 ◦ C for 2 h (heating rate 5 ◦ C min−1 ). The heating protocol involved two steps at 350 and 450 ◦ C to eliminate the remaining nitrates. The samples obtained were denoted by ZrNbO-HP. Because the samples synthesized in the same manner exhibited different catalytic properties, efficient and less efficient catalysts were characterized to gain a better understanding of the origin of this discrepancy. The efficient, and less efficient catalysts were respectively denoted ZrNbO-HP(a) and ZrNbO-HP(b).

The metal content of the mixed oxides was determined by atomic absorption (ICP), and their specific surface areas were determined using the BET method with nitrogen adsorption. Powder X-ray diffraction (XRD) patterns were obtained using a BRUKER D5005 diffractometer. Microcalorimetric measurements of ammonia and sulfur dioxide adsorption were carried out at 150 ◦ C using a Tian-Calvet-type apparatus (C80 from Setaram). This device was connected to a volumetric line equipped with a Barocel capacitance manometer (Datametrics) for pressure measurements, allowing the introduction of reactive gaseous probes. Prior to NH3 /SO2 adsorption, the catalyst (around 0.120 g) was pretreated overnight, under vacuum at 300 ◦ C. The adsorption was performed at 150 ◦ C, to limit physisorption. Successive doses of NH3 /SO2 were pulsed onto the sample until a final equilibrium pressure of 67 Pa was obtained. The sample was then out-gassed for 30 min at the same temperature, and a second adsorption was performed (still at 150 ◦ C) until an equilibrium pressure of approximately 27 Pa was attained, thus making it possible to compute the irreversible amount adsorbed at this pressure. The difference between the amounts of gas adsorbed at 27 Pa during the two adsorption runs corresponded to the number of strong adsorption sites. The surface acidity of the catalysts was investigated by means of a Fourier transform infrared (FTIR) spectroscopic study of pyridine adsorption. Pyridine FTIR spectra were recorded with an IFS110 BRUKER spectrometer (DTGS detector). The samples were pressed into self-supporting pellets (30–40 mg, 18 mm diameter), placed in an IR cell, and treated at 300 ◦ C under vacuum (0.0013 Pa) for 2 h. After cooling to room temperature, they were exposed to pyridine vapor for 5 min (vapor pressure 3.3 kPa). Their spectra (200 scans, resolution: 1 cm−1 , range of acquisition: 1000–4000 cm−1 ) were then recorded after evacuation (0.0013 Pa) for 1 h at 100, 200 and 300 ◦ C.

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NH3 -TPD was performed on a MicromeriticsAutochem 2910 apparatus, with 100 mg of catalysts. The samples were pretreated for 30 min at 300 ◦ C under helium (30 mL min−1 ) and then successively treated in a flux of 5% ammonia in helium (30 mL min−1 ) for 15 min at room temperature, and then in helium for 1 h at 100 ◦ C. The temperature of the calorimeter furnace was then programmed with a heating rate of 10 ◦ C min−1 , with a helium flow rate of 30 mL min−1 . The concentration of ammonia desorbing from the sample was continuously monitored by gas chromatography. The output was then plotted against temperature, to determine the TPD profile. The dehydration of glycerol was carried out in a fixed bed reactor operating at atmospheric pressure, under conditions described elsewhere [10]. The catalytic properties were determined at 300 ◦ C in a conventional downflow reactor, with a catalyst volume of 4.5 mL. The feedstock composition was glycerol/water/inert gas of 2.3/46.3/51.4 with GHSV = 1930 h−1 . The gas products were analyzed by on-line gas chromatography with a Carboxen 1000 column. The organic substrates were condensed at a low temperature during the reaction, and analyzed off-line using Nukol and ZBwax plus columns. Transformation of hydroxyacetone was also studied over ZrNbO-12 at 300 ◦ C using a feedstock composition hydroxyacetone/water/inert gas of 2.3/46.3/51.4 with GHSV = 1930 h−1 . The conversion (conv) was calculated using

 conv =

1−

Nf



Ni

× 100%

where Ni and Nf were respectively the number of mole of glycerol in the gas feed before and after catalytic reaction. The selectivity for a certain product p (Sp ) was calculated on the basis of carbon efficiency using

 Sp =

ap Np 3(Nf − Ni )

 × 100%

where Np was the number of mole of the p product with ap carbons. 3. Results Catalyst samples were prepared using different methods, and with different niobium contents. A summary of their physicochemical properties is provided in Table 1. Further details can be found in the original paper [10], which is cited throughout the

Table 1 Specific surface area measured using the BET method, chemical composition determined from chemical analysis and phases identified by XRD for the different catalysts studied. Catalyst

SSA (m2 g−1 )

Chem. comp. Zr/Nb

XRD phases

ZrNbO-HP(a) ZrNbO-Hp(b) ZrNbO-12 ZrNbO-31 NbOx /ZrO2

45 49 55 74 88

3.5 3.5 12 31 35

m-ZrO2 , t-ZrO2 , Nb2 O5 m-ZrO2 , t-ZrO2 , Nb2 O5 m-ZrO2 , t-ZrO2 m-ZrO2 , t-ZrO2 m-ZrO2 , t-ZrO2

SSA = specific surface area, Chem. phases = phases identified by XRD.

comp. = chemical

composition,

XRD

present study. The various techniques used to characterize the materials are also described in detail in the same paper. All of the prepared samples contained a mixture of monoclinic and tetragonal zirconia only, with the exception of the samples with the highest niobium content (those prepared using peroxo-niobate (ZrNbO-HP(a) and -HP(b)), which also exhibited a niobium pentoxide phase. For all samples, Raman spectroscopy indicated spreading of the polymeric niobium oxide species over the surface of the zirconia support, and with the exception of the sample prepared by grafting niobium oxide onto the surface of zirconia (NbOx /ZrO2 ), the formation of a solid solution of Nb into ZrO2 . Finally it should be noted that all the physico-chemical properties of the ZrNbOHP(a) and HP(b) were the same, and that no phase transformation was detected in any of the tested samples. 3.1. Catalytic testing The catalytic properties of the studied solids, in terms of the dehydration of glycerol, are provided in Table 2. Since the catalysts became progressively deactivated on stream, the time spent on stream at which the catalytic properties were recorded is also indicated in this table. In the case of the catalysts, which exhibited an increase in selectivity during the first hours of testing, the catalytic properties correspond to those found at their maximum level of selectivity in acrolein. The results show that acrolein was always the main product. The best selectivity to acrolein, at or near total conversion, was equal to 72% and was obtained on the ZrNbO-HP(a) and ZrNbO-12 catalysts. In the case of the other catalysts, the low selectivity to acrolein was compensated for by a higher selectivity to hydroxyacetone and acetaldehyde, and in

Table 2 Catalytic performances of the ZrNbO and NbOx /ZrO2 catalysts. Reaction temperature 300 ◦ C, glycerol aqueous solution (20 wt%) flow rate 3.8 g h−1 , inert gas flow rate 75 mL min−1 . Catalyst TOS (h) Conv (%) Selectivity (%) Acrolein Hydroxyacetone Acetaldehyde Propanal Acetone Allyl alcohol 2,3-Butanediol 2-Cyclopentene-1-one2-Methylcyclopentene-1-one 3-Methylcyclopentenolone Phenol COx Carbon balance (%) TOF-20% (h)

ZrNbO-HP(a) 20 98 72 12.4 3.2 1.7 1.7 0.7 0.7 – 0.1 0.3 0.9 1.0 95 130a

ZrNbO-HP(b)

ZrNbO-12

ZrNbO-31

NbOx /ZrO2

6 100

48 100

48 100

8 99

71 11.3 3.8 2.1 1.1 1.1 0.7 – – – 0.9 1.5 95 215

51 24 7.2 3.9 2.8 3.5 0.8 – – – 0.5 2.1 97 248a

35 5.7 7.9 5.1 1.1 1.9 1.6 0.6 0.5 1.3 4.0 7.0 73 25

51 6.8 6.5 5.1 2.4 0.6 0.9 0.3 1.0 0.6 2.8 3.0 87 22

TOS = time on stream, Conv = conversion of glycerol, TOF-20% = time on stream after which the conversion decreased by 20%. a This value has been extrapolated because the testing was stopped before.

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(a)

97

100°C 200°C

Absorbance (a.u.)

300°C

1700

1650

1600

1550

1500

1450

1400

1450

1400

Wavenumber (cm‐1)

(b)

100°C

Fig. 1. NH3 -TPD thermograms of ammonia on different solids: (a) (m + t) ZrO2 , (b) NbOx /ZrO2 , (c) ZrNbO-12, (d) ZrNbO-HP(b) and (e) ZrNbO-HP (a).

some case to the other by-products. Although the main by-products were hydroxyacetone and acetaldehyde, numerous other compounds such as propanal, acetone, allyl alcohol, 2,3-butanedione and cyclic molecules were detected in small amounts. These cyclic molecules correspond to phenol and several cyclopentones, which are described in detail in Table 2. CO was also systematically detected. We have previously shown that all the catalysts were progressively deactivated as a function of their time on stream, and that this deactivation was related to the deposition of coke onto the surface of the catalysts [10]. In order to evaluate and compare the stability of the various catalysts, we measured the time spent on stream, after which the conversion decreased by 20%. This time is also shown in Table 2. Strong differences were found between the catalysts: the most stable of these were ZrNbO-HP(a), ZrNbO-12 and ZrNbO-31. Although the glycerol conversion clearly decreased with time on stream, their selectivity to acrolein remained stable. Furthermore, no strongly significant change was observed in their by-product distributions. 3.2. Acidic properties of the fresh catalysts: NH3 -TPD Several of the catalysts were characterized by ammonia TPD, as shown by the results in Fig. 1. All of the recorded thermograms had the same profile with two broad maxima, centered around 170 ◦ C, and between 350 and 400 ◦ C. The pure zirconia (m + t)ZrO2 sample, studied for the purpose of comparison, had the most intense second peak. This can be attributed to its stronger acid sites. The density of the strong acid sites on the NbOx /ZrO2 sample was also slightly lower, but markedly higher than that observed on the ZrNbO catalysts. The lowest density of strong acid sites was observed on ZrNbO-HP(a), which is one of the most selective and stable catalysts. This outcome appeared to indicate that such sites should have a negative effect on both parameters. However, Fig. 1 shows that the density of strong acid sites, as well as the entire desorption profile of the ZrNbO-HP(b) and ZrNbO-12 catalysts, which did not exhibit the same catalytic properties, was comparable. This leads to the intermediate conclusion that the acidic properties of a fresh catalyst may not be the only key parameter affecting its catalytic efficiency.

Absorbance (a.u.)

200°C

1700

300°C

1650

1600

1550

1500

Wavenumber (cm‐1) Fig. 2. Infrared spectra of (a) ZrNbO-HP(a) and (b) ZrNbO-HP(b) after desorption under vacuum at different temperatures of adsorbed pyridine.

3.3. Acidic properties of the fresh catalysts: pyridine adsorption followed by FTIR The nature of acid sites (Lewis and Brønsted) on the surface of the ZrNbO-HP(a) and -HP(b) catalysts was studied using FTIR spectroscopy of pyridine adsorption. The spectra arising from pyridine adsorption at room temperature, followed by evacuation at 100, 200 and 300 ◦ C are shown in Fig. 2. The bands at 1610, 1575, 1485, and 1445 cm−1 were assigned to the vibrational modes of Lewis site coordinated pyridine [12]. Similarly, the bands at 1638, 1608, 1535 and 1485 cm−1 correspond to vibrations of a pyridinium ion bonded to a Brønsted site. The bands around 1485 and 1535 cm−1 are simultaneously associated with both Brønsted and Lewis acid sites. All the bands reported above were observed in the spectra recorded at room temperature. The Brønsted and Lewis relative acidity of the ZrNbO-HP(a) and -HP(b) catalysts, expressed as areas under the 1535 cm−1 (PyrH+ ) and 1445 cm−1 (PyrL) peaks, normalized to spectral surface units, were compared at 100 ◦ C. The results show that these catalysts have comparable proportions of Brønsted and Lewis acid sites (A(PyrH+ )/A(PyrL) = 0.4). Following out-gassing at increasing temperatures, the intensities of all the bands decreased. Both the Lewis and the Brønsted acid sites could still be detected at 300 ◦ C on the catalysts, indicating significant Brønsted and Lewis acidities. It is important to note that the degree of protonation of a base such as pyridine is an indication of the relevant strength of the Brønsted sites. The stability of the pyridinium cations, even after evacuation at 300 ◦ C, is a further indication that some of the sites were rather strong. At 300 ◦ C, ZrNbO-HP(a) lost a slightly higher proportion of acid sites (76%) than ZrNbO-HP(b) (65%). This was in agreement with the NH3 -TPD data obtained on both catalysts

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Table 3 Calorimetric and volumetric data of NH3 and SO2 adsorption at 150 ◦ C on fresh catalysts. Catalysts

ZrNbO-HP(a) ZrNbO-HP(b) ZrNbO-12 ZrNbO-31 NbOx /ZrO2

NH3 adsorption on fresh samples

SO2 adsorption on fresh samples

Qdiff (kJ mol−1 )

nirr (␮mol g−1 )

(␮mol m−2 )

Qdiff (kJ mol−1 )

nirr (␮mol g−1 )

(␮mol m−2 )

137 155 158 170 170

45.0 49.1 60.6 96.2 96.8

1.00 1.00 1.10 1.30 1.10

153 167

10.8 13.2

0.24 0.27

188

126.15

1.68

Qdif = initial heat of adsorption, nirr = amount irreversibly adsorbed under an equilibrium pressure of 27 Pa.

(Fig. 1). Furthermore it exhibited at this temperature a higher proportion of Lewis acid sites (A(PyrH+ )/A(PyrL) = 0.5 instead of 1.3 for ZrNbO-HP(b)). 3.4. Acidic properties of the fresh catalysts: NH3 adsorption followed by calorimetry The chemisorption data are presented in Table 3, whereas the variations in differential heat of ammonia adsorption on the mixed oxides, as a function of NH3 uptake, are shown in Fig. 3. For all of the solids, the initial heats of approximately 150 kJ mol−1 , revealing the presence of strong acid sites, decreased to 70 kJ mol−1 (chemisorption–physisorption limit) without any plateau thus indicating the heterogeneity of the acid site strengths. This distribution was in agreement with that highlighted by NH3 -TPD. In Fig. 4, the strength distribution of the acid sites (in terms of quantities of chemisorbed NH3 associated with a range of adsorption heat values) is shown. The sites are separated into four categories, according to their force as defined by the value of their differential heats of adsorption: very strong (Q > 160 kJ mol−1 ), strong (Q between 160 and 120 kJ mol−1 ), weak (Q between 120 and 80 kJ mol−1 ) and very weak (Q < 80 kJ mol−1 ). All the catalysts had comparable surface acid site densities. The most selective catalysts (ZrNbO-HP(a) and ZrNbO-12) exhibited comparable site strength distributions, without very strong acid sites. Only ZrNbO-31 and NbOx /ZrO2 have some very strong acid sites, however their main site distributions were also comparable. This outcome is in good agreement with the results obtained by NH3 -TPD, showing that these two catalysts have stronger acid sites. No major differences were detected between the ZrNbO-HP(a) and -HP(b) samples. This result is again in agreement with the NH3 -TPD and pyridine adsorption results. The acidic properties of the latter two catalysts

Fig. 3. Differential heats of adsorption of NH3 as a function of coverage for different fresh catalysts: ZrNbO-HP(a) (squares), ZrNbO-HP(b) (rhombus), ZrNbO-31 (circles), ZrNbO-12 (triangles) and NbOx /ZrO2 (crosses).

Fig. 4. Acid strength distribution determined from adsorption of NH3 monitored by differential scanning calorimetry for different fresh catalysts. Four categories were distinguished: very strong (Q > 160 kJ mol−1 ), strong (Q between 160 and 120 kJ mol−1 ), weak (Q between 120 and 80 kJ mol−1 ) and very weak (Q < 80 kJ mol−1 ).

thus did not appear to provide a suitable explanation for the strong differences observed between them, during catalytic testing. 3.5. Basic properties of the fresh catalysts: SO2 adsorption followed by calorimetry The basic characteristics of the ZrNbO-HP(a), ZrNbO-HP(b) and ZrNbO-31 catalysts were monitored using SO2 adsorption as an acidic probe. The results are shown in Figs. 5 and 6 and in Table 3. The quantity of SO2 adsorbed, and thus the density of the basic sites, was very high on the ZrNbO-31 sample as compared to the other two. The initial differential heat of adsorption of the ZrNbO-31

Fig. 5. Differential heats of adsorption of SO2 as a function of coverage for different fresh catalysts: ZrNbO-HP(a) (squares), ZrNbO-HP(b) (rhombus) and ZrNbO-31 (circles).

P. Lauriol-Garbey et al. / Applied Catalysis B: Environmental 106 (2011) 94–102

Fig. 6. Basic strength distribution determined from adsorption of SO2 monitored by differential scanning calorimetry for different fresh catalysts. Four categories were distinguished: very strong (Q > 160 kJ mol−1 ), strong (Q between 160 and 120 kJ mol−1 ), weak (Q between 120 and 80 kJ mol−1 ) and very weak (Q < 80 kJ mol−1 ).

catalysts was also higher, reaching 190–200 kJ mol−1 , which corresponds to that found in the literature [13] for pure ZrO2 , that contains strong basic sites. Comparison of ZrNbO-HP(a) and ZrNbOHP(b) revealed a higher basic site density and initial differential heat of adsorption in the case of ZrNbO-HP(b). These results tend to show that the distribution of the NbOx species at the surface of ZrNbO-HP(b) is not optimum and that the zirconia support remains uncovered in this case. 3.6. Acidic properties of the used catalysts: NH3 adsorption followed by calorimetry Three samples were characterized after various times on stream: ZrNbO-12 and ZrNbO-31 were used for 7 days, and ZrNbO-HP(b), which was much less stable, for 1 day. The results are shown in Figs. 7 and 8 and in Table 4. In the same table, the catalytic properties of the catalysts at the time when the testing was stopped, together with their specific surface area after testing, have been recorded. It can be seen that the catalysts were still very active, at least for the ZrNbO-12 and ZrNbO-31 catalysts, when the testing was stopped, and that their selectivity to acrolein was that observed when the catalysts were the most efficient. A significant decrease in specific surface area occurred during testing, and could be attributed to the deposit of coke at the surface of the solids.

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Fig. 8. Acid strength distribution determined from adsorption of NH3 monitored by differential scanning calorimetry for different used catalysts. Four categories were distinguished: very strong (Q > 160 kJ mol−1 ), strong (Q between 160 and 120 kJ mol−1 ), weak (Q between 120 and 80 kJ mol−1 ) and very weak (Q < 80 kJ mol−1 ). Table 4 Catalytic performances of the ZrNbO catalysts (reaction temperature 300 ◦ C, glycerol aqueous solution (20 wt%) flow rate 3.8 g h−1 , inert gas flow rate 75 mL min−1 ) and calorimetric and volumetric data of NH3 adsorption at 150 ◦ C on the corresponding used catalysts. Catalyst

ZrNbO-HP(b)

ZrNbO-12

ZrNbO-31

TOS (h) Conv (%) SACR (%) SSA (m2 g−1 ) Qdiff (kJ mol−1 ) nirr (␮mol s−1 )

24 69 50 22.5 73 1.55

168 90 70 20.0 115 1.25

168 88 50 31.5 110 0.67

TOS = time on steam, Conv = conversion of glycerol, SACR = selectivity to acrolein, SSA = specific surface area, Qdif = initial heat of NH3 adsorption, nirr = amount irreversibly adsorbed under an equilibrium pressure of 27 Pa.

Concerning the acid properties of the catalysts, it appeared that both the amount and the distribution of the sites, in terms of acid strength, were strongly modified. Only weak (Q between 120 and 80 kJ mol−1 ) and very weak (Q < 80 kJ mol−1 ) sites remained. This observation is logical, since it is known that coke precursors are preferentially adsorbed onto stronger acid sites [14]. From a comparison of the acid site strength distributions on the same catalysts, before and after testing (Figs. 4 and 8), it can be seen that if strong and very strong sites disappear, the number of weaker acid sites increases. This increase can be explained by changes in the acid strength of some sites, but also by the formation of new weak acid sites that could be attributed to the coke itself covering the catalyst. 4. Discussion

Fig. 7. Differential heats of adsorption of NH3 as a function of coverage for different used catalysts: ZrNbO-12 used for 7 days (triangles), ZrNbO-31 used for 7 days (circles) and ZrNbO-HP(b) used for 1 day (rhombus).

Before entering into a full discussion of the results obtained during this study, it is important to recall some of the data already determined for the studied catalysts [10]. Three types of sample were studied: samples prepared using peroxo-niobate (ZrNbOHP), samples prepared using a water-soluble niobium precursor (ZrNbO), and a sample prepared by grafting niobium oxide onto the surface of a pre-prepared zirconia support (NbOx /ZrO2 ). These three different samples were comparable in terms of their phase composition, with the exception of the ZrNbO-HP samples, which contained a niobium oxide phase. However this phase did not appear to have a strong influence on the acidic, nor on the catalytic properties of the catalysts. The catalysts did not all have comparable catalytic properties. Discrepancies were even observed

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between the samples prepared using the same peroxo-niobate based protocol. The catalytic properties of the catalysts were related to the presence of polymeric species distributed over the surface of the zirconia support, and it was proposed that the distribution and these species and the presence of an uncovered zirconia support could have been at the origin of these discrepancies [10]. In that respect, it should be noticed that niobium doping of zirconia was observed for all catalysts, with the exception of NbOx /ZrO2 [10]. Finally, it was observed that after catalytic testing, coke formed at the surface of all the samples. The adsorption of pyridine, followed by infrared spectroscopy, has shown that the ZrNbO-HP catalysts have both Lewis and Brønsted acid sites. It is noteworthy that niobium based oxides have both types of acid sites on their surface [15]. Although both types of site can be active in a dehydration reaction, the Brønsted acid sites are generally reported to be the most active of the two. This is what has been reported for the dehydration of glycerol on various catalysts [9], and we propose that the same trend is observed on ZrNbO catalysts. In that respect, a study of isopropanol dehydration carried out on NbOx /ZrO2 catalysts has shown that an almost direct relationship could be observed between the rate of propene formation and the abundance of Brønsted acid sites [16]. The same article confirmed that the catalytic activity in a dehydration reaction and the Brønsted acidity were associated with polymeric NbOx species. Supported niobium oxide catalysts have also been investigated by Wachs and co-workers [17,18] by means of pyridine adsorption. These authors revealed a decrease in the number of Lewis acid sites on the support, resulting from the addition of Nb, and the formation of new acid sites attributed to NbOx species. In the present study, the characterization of the acidic properties of the fresh catalysts, using different techniques, led to quite similar results in terms of site density and strength distributions. Only the NbOx /ZrO2 and ZrNbO-31 catalysts were found to have some very strong acid sites. The two catalysts contain smaller quantities of niobium, and these strong acid sites likely correspond to sites of the uncovered and undoped zirconia support. This hypothesis is corroborated by the fact that the initial heat of adsorption of ammonia on these catalysts was comparable to that recorded on pure zirconia. Furthermore, the ZrNbO-31 catalyst exhibited numerous strong basic sites, which for similar reasons could be attributed to uncovered and undoped zirconia. With the above two exceptions, no significant differences were observed between the catalysts, and when the acidic or basic properties, and catalytic characteristics were compared, no clear relationship could be found. This appeared to be particularly true when the results obtained on ZrNbO-HP(a) and ZrNbO-HP(b) were compared. Both catalyst samples had similar physico-chemical characteristics, but different acrolein selectivity and stability. This is the discrepancy, which led us to make a specific study of their acid–base properties. Clearly, the results described here did not reveal highly significant differences between the two solids. The quantity of NH3 irreversibly adsorbed was the same on both solids. A higher initial heat of adsorption was observed on the ZrNbO-HP(b) sample, but this was comparable to that of ZrNbO-12, which was as active and selective as the ZrNbO-HP(a) sample. This outcome led to the additional conclusion that Nb2 O5 , present in both catalysts, does not have a marked effect on the acidic, nor on the catalytic properties. The ZrNbO-HP(b) sample has stronger basic sites than the ZrNbO-HP(a), which were however much weaker than those of ZrNbO-31, which exhibited a comparable activity and selectivity to acrolein. Although it was difficult to draw definitive conclusions, the results presented here tend to show that the distributions of the NbOx species on the surface of the zirconia support were different for these two catalysts, and that a larger area of uncovered support was present on the ZrNbO-HP(b) sample. This situation would correlate with the catalyst’s lower selectivity to acrolein, and its

stability with time on stream. A better distribution of NbOx species at the surface of ZrNb-HP(a) would imply a lower number of Lewis acid sites [17,18]. This would explain the lower A(PyrH+ )/A(PyrL) ratio observed at 300 ◦ C for ZrNb-HP(a) compared to ZrNb-HP(b). At 100 ◦ C, the presence of Brønsted acid sites related to the NbOx species would led to comparable A(PyrH+ )/A(PyrL) ratios. As mentioned in Section 1 different mechanisms of dehydration of alcohol can be proposed which are not based upon the formation of carbocations. These mechanisms known for the dehydration of other alcohol are concerted mechanisms occurring on acid–base site couples. They are generally reported to be less selective and one may postulate that they could be involved in our case and responsible for the selective formation of hydroxyacetone and degradation reactions of glycerol. They would further explain the high selectivity to hydroxyacetone obtained on ZrNb-31 presenting numerous basic sites and the relation between the presence of basic sites in WO3 /ZrO2 catalyst and the selectivity to hydroxyacetone already published [19]. A very recent study of sodium-doped metal oxide seemed to confirm the role of basic sites and the existence of an optimum number of acidic and basic sites to obtain catalysts selective to hydroxyacetone [20]. The study of the catalysts described here has highlighted that if some trends could be observed between the acidic properties of fresh catalysts and the catalytic properties, no direct correlation could be established. It shows that the strong acid sites disappear totally from the surface of the catalysts after a few hours on stream, and that only the weak and very weak acid sites remained. However, the catalysts remained very efficient, with a glycerol conversion efficiency of approximately 90% and selectivity to acrolein comparable to the initial value. It can thus be concluded that the active and selective catalytic sites correspond to the weak and very weak acid sites. These weak and very weak sites were respectively characterized by differential heats of NH3 adsorption ranging between 120 and 80 kJ mol−1 , and lower than 80 kJ mol−1 . A detailed analysis of the results showed that catalysts remaining the most active had fewer sites, but a proportionally greater number of weak acid sites than very weak acid sites, and it may be concluded that the former sites were more active. We reached this conclusion by calculating the rate of transformation of glycerol on the three catalysts, by taking the test data and plotting it as a function of that calculated from the acid site density, by attributing specific turnover numbers of 0.015 and 0.11 s−1 respectively to the very weak and weak sites. A good agreement was obtained between the two rates, thus confirming this hypothesis (Fig. 9). The weak acid sites were about ten times more active than the very weak ones. The variation of selectivity to acrolein on the same catalysts was more difficult to explain, and does not correlate well with the acid strength distribution of the active sites, although catalysts with less very weak sites were more selective to acrolein. Presumably, other parameters such as site distribution, presence of basic sites, or particles and pore sizes could also play a role. We have shown that all the catalysts became deactivated with time on stream, and that this deactivation was related to the deposition of coke onto the surface of the catalysts [10]. Similar conclusions were reached by other teams, with other efficient catalysts designed for the dehydration of glycerol [1–9]. When compared to the latter catalysts, the ZrNbO catalysts were shown to be characterized by a high degree of stability on stream. It was thus of interest to determine whether acidic or basic properties could explain this high stability. However, again, no direct correlations could be found between the stability on stream and the catalysts’ acidic properties. Coking is induced by the formation of coke precursor products such as aromatic or cyclic molecules, which remain adsorbed onto the surface for a longer time, and are then further transformed. The selectivity of the catalysts to several reaction products was plotted as a function of the time after which

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Fig. 9. Intrinsic rate of glycerol transformation calculated from testing data as a function of the rate of transformation of glycerol calculated from acid sites intrinsic activities.

the activity was reduced by 20% (Fig. 10). It can be seen that the most acrolein selective catalysts are not necessarily the most stable ones. The most stable samples have a high selectivity to hydroxyacetone. A study of the reactivity of hydroxyacetone on the ZrNbO-12 catalyst at the catalytic reaction temperature showed that hydroxyacetone can easily be transformed to acetone acetaldehyde and CO (Table 5). Acetaldehyde can also be formed by the decomposition of acrolein or even directly from glycerol [5]. Decomposition of hydroxyacetone also occurs but more severely on the less stable catalysts (Table 2). In these cases, high selectivity to phenol or cyclic compounds (2-cyclopentene-1-one, 2-methylcyclopentene1-one, 3-methylcyclopentenolone) is observed. This observation is in agreement with the fact that less stable catalysts favor hydroxyacetone transformation since these cyclic compounds are formed from consecutive reactions involving acrolein and either hydroxyacetone directly, or products formed from hydroxyacetone, i.e. acetone, propanal and acetaldehyde (Fig. 11). These cyclic compounds are known to be coke precursors, and it is not surprising that strong coking occurred when these products were detected. Thus, although the formation of hydroxyacetone is detrimental to acrolein selectivity, its further decomposition into products, which react with acrolein, appears to be strongly detrimental to the catalyst’s stability. The direct decomposition of glycerol or that of acrolein should also be taken into account in the formation of products, which can react with acrolein. The decomposition of hydroxyacetone glycerol and acrolein should depend not only on the acid base properties of the catalysts, but also on the textural properties of the catalysts. It is known that smaller pores and larger particles increase the probability of decomposition of products formed in the pores. In that respect, it is noticeable that the most stable catalyst (ZrNbO-31) has the highest specific surface area. Furthermore, this also explains the dependence of efficiency on pore size of the support, as shown for several other supported

Fig. 10. Variation of the selectivity in acrolein (full square) and hydroxyacetone (full triangle) and in acetaldehyde (blue square), COx (blue triangle) and aromatic molecules (phenol, 2-cyclopentene-1-one, 2-methylcyclopentene-1-one, 3-methylcyclopentenol-one) (blue circle) as a function of the time on stream after 20% loss of conversion; NbOx /ZrO2 (1), ZrNbO-HP(b) (2), ZrNbO-HP(a) (3), ZrNbO-12 (4), ZrNbO-31 (6). Catalytic data corresponding to ZrNbO-19 (5) described in another paper have been added for comparison [10]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

catalysts [21,22]. The decomposition reactions of hydroxyacetone, glycerol and acrolein involved not only dehydration, but also C–C bond breaking and hydrogenation–dehydrogenation reactions. In general, the latter type of reaction on acid catalysts occurs with a concerted mechanism on certain types of acid/base centers [23]. The presence of both types of sites within close proximity may promote these reactions. Although there are no data to support this hypothesis, it may be noted that the less stable catalysts were among those exhibiting a larger proportion of uncovered or

Table 5 Catalytic results obtained on the ZrNbO-12 catalyst at 28% conversion of hydroxyacetone. Reaction temperature: 300 ◦ C. Product

Acetone

Acetaldehyde

Propanal

Acetic acid

Propionic acid

CO

Unknownsa

Selectivity (%)

39

20

8

18

2

11

<2

a

Coke is included in unknowns.

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The stability of the catalysts has been related to the formation of cyclic molecules formed by the reaction of acrolein with products arising from the decomposition of hydroxyacetone, glycerol or acrolein. These aromatic molecules could be strong coke precursors, leading to the formation of coke at the surface of the deactivated catalyst. The occurrence of the incriminated decomposition reactions should naturally depend on the texture of the catalysts, and be promoted by narrow pores and large particle sizes, and by the presence of catalytic sites other than weak acid sites. These sites could eventually be coupled acidic and basic sites acting in a concerted manner. Other studies will be needed in order to gain better insight into those sites which are responsible for the degradation of hydroxyacetone, glycerol or acrolein, and into the degradation reactions and reactions with acrolein, which are the key to coke formation and deactivation of the catalysts. Acknowledgments

Fig. 11. Proposed formation scheme of the main aromatic side-products.

The authors would like to thank S. Prakash who provided help and support in the recording of infrared spectra. The authors also gratefully acknowledge ADISSEO for financial support. References

undoped zirconia support surface area. The surface of the latter exhibits both acidic and basic sites, which could potentially be situated side by side. 5. Conclusions In the present study, the catalytic and acid–base properties of several ZrNbO catalysts, which are efficient and stable catalysts for the dehydration of glycerol, have been investigated. The most efficient catalysts were those for which the zirconia support has a better niobium oxide species coverage, and are doped with niobium. The presence of Nb2 O5 in a separate phase did not appear to be a determinant factor. Our comparison of the properties of the fresh and used catalysts clearly shows that the acid properties of the used catalysts have nothing in common with those of the fresh ones. The used catalysts, which were still very efficient at the on stream time after which they were recovered for characterization by microcalorimetric measurements of ammonia adsorption, had only weak (Q between 120 and 80 kJ mol−1 ) and very weak (Q < 80 kJ mol−1 ) acid sites. A linear relationship has been established between the surface density of these sites and the intrinsic activity of the catalysts, taking into account a higher activity for the most acid sites.

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