Al2O3 catalysts

Journal of Catalysis 290 (2012) 79–89

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Liquid-phase glycerol hydrogenolysis by formic acid over Ni–Cu/Al2O3 catalysts I. Gandarias a,⇑, J. Requies a, P.L. Arias a, U. Armbruster b, A. Martin b a b

School of Engineering (UPV/EHU), Alameda Urquijo s/n, 48013 Bilbao, Spain Leibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany

a r t i c l e

i n f o

Article history: Received 29 December 2011 Revised 2 March 2012 Accepted 3 March 2012 Available online 5 April 2012 Keywords: Biomass Hydrogenolysis 1,2-Propanediol Transfer hydrogenation Formic acid

a b s t r a c t Two series of Ni–Cu/Al2O3 catalysts with different Cu/Ni ratio and different total metal contents were prepared and tested in the glycerol hydrogenolysis to 1,2-propanediol under N2 pressure and using formic acid as hydrogen donor molecule. The results from the deep characterization of the catalysts by N2physisorption, transmission electron microscopy (TEM), temperature-programed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programed desorption of ammonia (TPD-NH3), together with the results from the activity tests allowed deeper understanding of the role played by Ni, Cu, and acid catalytic sites. The kinetic study performed with the optimized catalyst revealed that the OH groups of glycerol and of the target product, 1,2-propanediol, compete for adsorption on the acid sites of the Al2O3 support. 90% glycerol conversion and 82% 1,2-propanediol selectivity after 24 h reaction time were achieved when operating at 493 K. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction It is widely assumed that the dependence of human well-being on fossil fuels should be reduced, mainly due to geopolitical, natural resource scarcity, and environmental factors. Biomass appears as the only renewable source for liquid fuels and plastics [1]. Main biomass conversion processes that are being currently developed, like bio-oils from the pyrolysis or high-pressure liquefaction of biomass [2,3], sorbitol valorization to polyols [4], or glycerol hydrogenolysis to propanediols (PDO) [5–17], require oxygen removal reactions. In these hydrogenation/hydrogenolysis reactions, high hydrogen pressures are required to reach acceptable conversions and selectivities. Molecular hydrogen is easily ignited in contact with air and shows high diffusivity; therefore, it presents considerable hazards when working at high pressures. In addition, most of the nowadays available hydrogen gas is produced from fossil fuels by energy intensive processes [18]. In situ generation of the hydrogen can be a promising alternative to avoid the inherent drawbacks of working with molecular hydrogen. Nevertheless, the so far published results on liquid-phase glycerol hydrogenolysis generating the hydrogen by simultaneous glycerol reforming [19–21] are still far to the best results reported under H2 pressure [12,15] (see Table 1). Another interesting option that allows working under inert atmosphere is to use hydrogen donor molecules. However, catalytic transfer hydrogenation reactions have not been extensively reported in the glycerol hydrogenolysis process. Musolino et al. ⇑ Corresponding author. Address: Escuela Técnica Superior de Ingeniería, Alameda Urquijo s/n, P.C., 48013 Bilbao, Spain. Fax: +34 946 014 179. E-mail address: [email protected] (I. Gandarias). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.03.004

studied 2-propanol and ethanol as solvents and hydrogen donor molecules for the process under inert atmosphere [22] or low hydrogen pressure [23]. It was recently reported that the reaction mechanism is different regarding the origin of the active hydrogen is molecular hydrogen or a hydrogen donor molecule. Under H2 pressure, glycerol is first dehydrated to acetol, which subsequently undergoes a hydrogenation process to give 1,2-PDO [24] (Scheme 1A). On the other hand, when the hydrogen atoms are produced from 2-propanol dehydrogenation, glycerol is directly converted to 1,2-PDO through intermediate alkoxide formation (Scheme 1B). Moreover, it was also observed that the hydrogen donor and glycerol compete for the same active sites [25]. In a previous work, not shown here, a semi-continuous process was developed in which the hydrogen donor molecule (2-propanol, methanol, or formic acid) was continuously fed into the autoclave containing the glycerol aqueous solution and Ni–Cu/Al2O3 catalyst. It was observed that there was an optimum feeding rate for each donor that maximized 1,2-PDO production, and that the bests results in terms of glycerol conversion and 1,2-PDO selectivity were obtained using formic acid. An additional advantage of using formic acid as hydrogen donor is that it can be obtained from renewable resources through the Biofine Process. This process transforms non-food biomass feedstock by acid-catalyzed hydrolysis to give levulinic acid and formic acid, together with some furfural and a char residue [26]. Formic acid is also considered a promising H2 storage as it can be obtained through CO2 hydrogenation [27,28]. Formic acid can be catalytically dehydrogenated to give hydrogen and CO2. A parallel dehydration reaction gives water and CO, which can be further converted to CO2 and hydrogen through water–gas shift reaction

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Table 1 Selected examples of glycerol hydrogenolysis under H2 atmosphere and by in situ generation of hydrogen through simultaneous glycerol reforming. Catalyst

Atmos.

P (MPa)

Temp (K)

Time (h)

Conv. (%)

Selectivity to 1,2-PDO (%)

Refs.

2.7 Pt/NaY 5 Pt/Al2O3 + 5 Ru/Al2O3 0.6 Ir/C + NaOH (1 M) Copper-chromite Cu0.4/Mg5.6Al2O8.6

Inert Inert Inert H2 H2

4.2 4.1 3.0 2.0 3.0

503 493 453 473 453

15 6 8 24 20

85.4 50.1 76.0 65.3 80

64.0 47.2 9.2 89.6 98.2

[17] [18] [19] [12] [15]

Scheme 1. (A) Reaction scheme of glycerol hydrogenolysis and degradation under H2 pressure [24]. (B) Direct glycerol hydrogenolysis to 1,2-PDO using 2-PO as a hydrogen donor molecule [25].

[29]. The selectivity to CO2 and hydrogen depends on the operating condition and the catalyst used. This paper studies the role of supported Ni, Cu, and acid catalytic sites in the liquid-phase glycerol hydrogenolysis to 1,2-PDO when formic acid is used as the hydrogen donor molecule. 2. Experimental 2.1. Catalysts preparation Ni–Cu/Al2O3 catalysts were prepared by the sol–gel method. Aluminium isopropoxide (Aldrich) was dissolved in deionized water (9 mL of H2O per gram of aluminium isopropoxide) by vigorous stirring of the solution at 313 K. The pH was measured and kept between 3.8 and 4.2 adding the required amounts of HNO3 (1.0 M). Simultaneously, nickel (II) nitrate hexahydrate (Aldrich) and/or copper (II) nitrate hemi pentahydrate (Alfa Aesar) were dissolved in ethanol. The precursor solution was slowly added to the aluminium isopropoxide solution. The mixture was stirred for 30 min at 313 K and then introduced into an ultrasonic apparatus for another 30 min. The mixture was then rested for 48 h at 328 K and subsequently for another 12 h at 375 K. The product obtained was crushed (granule size between 300 and 500 lm) and calcined from room temperature to 723 K at a heating rate of 2 K/min. This final temperature was maintained for 4 h. Previous to the reaction test, the catalyst powder (0.9 g) was pre-reduced in a tubular oven from room temperature to 923 K at a heating rate of 10 K/min under a hydrogen flow of 75 mL/min. The final temperature was maintained for 1 h. After cooling down to room temperature, the reduced sample was transferred, avoiding the contact with air, to a glovebox and stored there under Ar atmosphere before use. 2.2. Catalyst characterization The catalysts were chemically analyzed using the ICP-OES spectrometer Optima 3000XL (Perkin Elmer). Before measurement,

samples were digested in a microwave oven (Anton Paar Multiwave, Perkin Elmer) with a mixture of aqua regia and hydrofluoric acid. Surface area, pore volume, and pore size distributions were determined by N2 physisorption at 77 K on a Micromeritics ASAP 2010 instrument. Prior to the analysis, all samples were degassed at 473 K at 0.1 mbar for 4 h. The surface area was calculated using the Brunauer, Emmett, and Teller (BET) method, while pore size distributions were calculated using the Barrett, Joyner, and Halenda (BJH) method applied to the desorption leg of the isotherms. The temperature-programed reduction of the samples was studied on a Micromeritics Autochem AC2920 apparatus. The reduction of previously calcined catalyst samples was carried out under a gaseous mixture of 5 vol.% of hydrogen in argon, at a heating rate of 10 K/min from room temperature to 1173 K and a gas flow rate of 50 mL/min. TPR was finished by an isothermal period of 1 h at 1173 K. The hydrogen consumption was monitored by a calibrated thermal conductivity detector. The XPS measurements were performed with an ESCALAB 220iXL (ThermoScientific) using monochromatic AlKa radiation. The binding energies were referred to the C1s peak at 284.8 eV. The spectra were fitted with Gaussian–Lorentzian curves to determine the peak maxima and areas. The concentrations of the elements in the near-surface region were obtained after division by the element-specific Scofield factors and the transmission function of the spectrometer. X-ray powder diffraction (XRD) measurements were carried out on a STADI P automated transmission diffractometer (STOE, Darmstadt) with CuKa1 radiation and Ge monochromator. The pattern was scanned in the 2h range of 5–60° (step width 0.5°, 100 s per step) and recorded with a STOE position sensitive detector. The samples were prepared for flat plates. Phase analysis was carried out with the Win Xpow software package, including the powder diffraction file (PDF). For TEM characterization, a Phillips CM200 microscope equipped with a LaB6 filament and a supertwin lens operating at 200 kV was used. Bright-field images were acquired using a CCD

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camera (TVIPS GmbH). Powders of the catalysts samples were dispersed in ethanol with ultrasound, and drops were deposited on copper grids coated with amorphous carbon films. The acidity of the calcined and reduced samples was determined by ammonia temperature-programed desorption (TPD) measurements. The sample was pre-treated in a He stream at 673 K for 0.5 h and then in situ reduced using a gas mixture of 5 vol.% of H2 in Ar from 323 to 723 K at a heating rate of 10 K/ min and a gas flow rate of 100 mL/min. Next, the sample was cooled down to 373 K and saturated with ammonia using a gas mixture of 5 vol.% NH3 in He (flow rate 50 mL/min) for 0.5 h. Following catalyst equilibration in a helium flow, NH3 was desorbed using a linear heating rate of 10 K/min up to 723 K. The area under the curve was integrated to determine the total acidity of the sample from its NH3 desorption profile. Total carbon content in used catalysts was determined using a Truespace CHNS analyzer (LECO instruments, Ltd.).

by means of a pneumatic 6-port valve with a loop size of 100 ll and injected via a splitter into two separate analytical lines (Line 1: Poraplot Q, 25 m  0.53 mm  0.20 lm, flame ionization detector (FID) for analysis of CO, CH4 and CO2 via a methanizer; line 2: HP PLOT molesieve, 25 m  0.53 mm, thermal conductivity detector (TCD) for analysis of N2, H2, CH4, and CO) using Ar as carrier gas. The liquid phase was analyzed in another gas chromatograph (Shimadzu 17A GC-FID with autosampler; Chrompack FFAP column, 25 m  0.32 mm  0.3 lm, carrier He) using 1,4-butanediol as internal standard. Glycerol conversion and selectivity values were calculated on carbon basis. The carbon balance was crosschecked by total organic carbon (TOC) analysis of the liquid sample after each activity test using the model TOC-VCPN (Shimadzu). 1 mL of the liquid sample from the reactor was diluted in 100 mL before the TOC analysis.

2.3. Activity test

3.1. Optimization of the Cu/Ni ratio

Fig. 1 shows a scheme of the activity test set-up utilized. Inside the glovebox, under Ar atmosphere, the reduced catalyst was introduced into a basket and then into the autoclave (Parr, 300 mL stainless steel). Next, the autoclave was covered with parafilm, taken out from the glovebox to the activity plant and tightened under N2 flow. After purging and leakage check, the reactor temperature was set to 473 or 493 K, and the N2 pressure was increased to 15 bar. A 130 mL of 4 wt.% glycerol aqueous solution were placed in a steel feed cylinder and the pressure inside the feed cylinder was increased to 45 bar. Next, the stirring speed was set constant at 550 rpm. The reaction start time (t0) was established when the line connecting the feed cylinder and the reactor was opened. N2 was directed through the feed cylinder to the reactor till the pressure of the system reached 45 bar, guaranteeing that all the feed went into the reactor. When the operating pressure was reached, the line connecting the feed cylinder and the reactor was closed. From the beginning to the end of the activity test, an aqueous solution of formic acid (7.0 or 12.5 wt.%) was pumped into the autoclave at a con1 1 stant rate of 0.02 mL/min. These feed rates of 2:1 mmol gcatal h of formic acid for the activity tests performed at 473 K and 1 1 3:6 mmol gcatal h for the activity tests performed at 493 K were optimized in a previous work. During the reaction, increments in the system pressure were observed due to the formation of CO2 and H2 from the decomposition of the pumped formic acid. Nevertheless, final pressure never exceeded 57 bar. After 16-h reaction time, the reactor was cooled down and the gas phase was analyzed by directly connecting the reactor to the gas chromatograph (Hewlett–Packard 5890). Samples were taken

In order to optimize the Cu/Ni ratio, six different Ni–Cu/Al2O3 catalysts were prepared by sol–gel method with a constant total nominal metal content of 35 wt.% and different proportions of Ni and Cu. These catalysts were deeply characterized and tested in the glycerol hydrogenolysis reaction using formic acid as hydrogen donor molecule.

Fig. 1. Scheme of the activity test set-up.

3. Results and discussion

3.1.1. Elemental analysis and N2-physisorption Table 2 summarizes the theoretical and real metal contents together with the main textural properties of the calcined supported catalysts. Each catalyst is named with the rounded nominal content of each metal. As it can be observed, the real Ni and Cu contents measured by ICP-OES were lower than the theoretical ones in all the samples. This means that with the used sol–gel method, it was not possible to fix the desired metal amounts in the catalysts, which points at a systematic deviation in preparation procedure. Measured amounts ranged from 27.7 to 30.1 wt.%. However, it is interesting to observe that the theoretical and real Cu/Ni ratio values were quite similar. Concerning the N2 physisorption results, all the samples presented Type IV isotherms, with the typical hysteresis loops of mesoporous materials. Moreover, there is an interesting trend, as both BET area and total pore volume increased with increasing Ni proportion in the catalyst, slightly decreasing for the catalyst containing only Ni. 3.1.2. Temperature-programed reduction The active Ni and Cu sites in glycerol hydrogenolysis are reduced sites [25]. In order to set the temperature for the pre-reduction of the samples, the TPR analysis of 7Ni28Cu/Al2O3 and 28Ni7Cu/Al2O3 was carried out. These two catalysts were selected as they were the ones, among the catalyst having both Ni and Cu, with the highest and the lowest Cu/Ni ratio. The TPR spectra of the investigated samples are shown in Fig. 2. Calcined 7Ni28Cu/Al2O3 catalyst presented two reduction peaks for copper (a and b) at 503 and 549 K. The presence of more than one reduction signal in TPR profiles of Ni–Cu/Al2O3 catalysts was already observed and explained by different species [30–32]. Some authors hypothesized the presence of CuAl2O4 besides pure CuO [30,31], which undergoes reduction at higher temperatures than CuO [33]. Nevertheless, the formation of CuAl2O4 was not expected in the studied catalysts, as it is only formed at calcination temperatures above 973 K [30], and these catalysts were calcined at 723 K. Moreover, in the XPS analysis of 7Ni28Cu/Al2O3 and 28Ni8Cu/ Al2O3 (shown in Section 3.1.3), the presence of CuAl2O4 in the

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Table 2 Characteristics of the catalysts with constant total nominal metal content. Sample/ Al2O3

a

Ni content wt.% Theoretical (Real)a

Cu content wt.% Theoretical (Real)a

Ni + Cu wt.% Theoretical (Real)a

Cu/Ni ratio Theoretical (Real)a – –

SBET (m2/g)

Total pore volume (cm3/g)

Average pore size (nm)

103.5

0.074

4.53

0Ni35Cu

0.0 (0.0)

35.0 (30.0)

35.0 (30.0)

7Ni28Cu

6.6 (5.1)

28.4 (23.8)

35.0 (28.9)

4.30 (4.67)

157.9

0.167

3.83

13Ni22Cu

13.3 (10.4)

21.7 (17.3)

35.0 (27.7)

1.63 (1.66)

201.2

0.207

3.84

20Ni15Cu

20.3 (17.3)

14.7 (12.8)

35.0 (30.1)

0.72 (0.74)

229.6

0.247

3.66

28Ni7Cu

27.5 (22.7)

7.5 (6.2)

35.0 (28.9)

0.27 (0.27)

281.6

0.282

3.54

35Ni0Cu

35.0 (27.9)

0.0 (0.0)

35.0 (27.9)

0.00 (0.00)

240.2

0.213

3.33

Chemical composition determined by ICP.

Peak α

TCD Signal (a.u.)

Peak β

28Ni7Cu Peak β

Peak α

7Ni28Cu

300

400

500

600

700

800

900

1000 1100 1200

Temperature (K) Fig. 2. TPR profiles of calcined 7Ni28Cu/Al2O3 and 28Ni7Cu/Al2O3 catalysts.

near-surface region was not detected. Also, in the XRD analysis of calcined catalysts, diffraction reflections stemming from CuAl2O4 were not detected (see Section 3.1.4). Some other authors attributed the lower TPR signal to the reduction of highly dispersed CuO species (peak a), and the higher TPR signal to the reduction of bulk CuO (peak b) [32,34,35]. This second explanation fits better with the obtained TPR profiles. In the case of 7Ni28Cu/Al2O3 catalyst, the peak corresponding to bulk CuO (peak b) presents higher intensity as compared to the peak related to dispersed CuO (peak a). The significant formation of bulk CuO is coherent with the high Cu content of this catalyst. On the other side, in the case of 28Ni7Cu/Al2O3 catalyst, the peak at higher temperature corresponding to bulk CuO (peak b) shows lower intensity than the peak related to dispersed CuO (peak a). As the Cu content in this catalyst is comparatively smaller, the proportion of dispersed CuO is enhanced. Concerning the reduction of nickel, for both catalysts a broad peak reduction profile in the temperature range 650–1050 K was recorded. As expected, the intensity was higher for the catalyst with higher Ni content. This broad peak suggests the presence of a mixture of NiOx species having different interaction with the support [36]. Based on TPR results, the reduction temperature for catalyst activation was set to 923 K. 3.1.3. X-ray photoelectron spectra The same two calcined catalysts, 7Ni28Cu/Al2O3 and 28Ni7Cu/ Al2O3, analyzed by TPR, were analyzed by XPS. Atomic surface

composition was computed from XPS peak intensities, and the results are presented in Table 3. These values are compared to the bulk content of Ni and Cu as obtained from ICP results with Al and O serving as stoichiometrical balance. In order to estimate the Al2O3 content, it was assumed that all the Ni was in the form of NiO and that all the Cu was in the form of CuO. As it can be observed in Table 3, for both catalysts, the Ni and Cu surface mass fractions are significantly lower than in the bulk. This seems to indicate that Ni and Cu are not well dispersed on the surface of the support, probably forming relatively big particles, which is coherent with the high metal content of the catalysts. Those Cu and Ni atoms far from the surface of these big particles are hard to detect by XPS measurements. It is interesting to point out that the Cu/Ni surface ratio for both catalysts is around two times smaller than the Cu/Ni bulk ratio. This suggests that Ni is better dispersed than Cu, forming smaller particles, as it was confirmed by the XRD analysis of the same calcined catalysts (see Section 3.1.4). The surface C content detected in the catalysts (7–12%) can be related to the adsorption of ambient hydrocarbons and CO2 by the high surface area Al2O3 support during calcination in air at 723 K. In fact, the 284.8 eV binding energy observed for the C 1s electrons (see Table 4) is characteristic of the adventious C that comes from CO2 and ambient hydrocarbons.

Table 3 Bulk atomic composition obtained from ICP results and surface atomic composition obtained from XPS results. Sample/Al2O3

O (%)

Al (%)

7Ni28Cu Bulka Surface

C (%)

Ni (%)

37.4 60.0

33.7 27.5

– 7.0

5.10 1.9

28Ni7Cu Bulka Surface

37.6 52.7

33.5 23.8

– 12.3

22.7 9.5

Cu (%)

Cu/Ni

23.8 3.6

4.7 1.9

6.2 1.6

0.27 0.16

a Atomic bulk composition obtained from ICP results for Ni and Cu, including stoichiometrical balance by O and Al.

Table 4 Binding energies (eV) and relative abundance (%) of core electrons of C 1s, Cu 2p and Ni 2p. Sample/Al2O3

C 1s (%)

Cu 2p3/2 (%)

Ni 2p3/2 (%)

7Ni28Cu 28Ni7Cu

284.8 (83) 284.7 (90)

933.5 (100) 934.0 (100)

855.2 (100) 854.8 (100)

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The Cu and Ni chemical species found on the catalysts surfaces, and their proportions were also evaluated. As observed in Table 4, in the near-surface region of the samples only NiO and CuO were found. 3.1.4. X-ray diffraction Fig. 3 shows the XRD patterns of the calcined catalysts before reduction. For high contents of Cu, sharp reflections that can be assigned to CuO (JPCDS; 41-0254) were detected. The intensity of these reflections declined with decreasing Cu content in the catalysts; the reflections were not visible when Cu content was lower than 7 wt.%. Diffraction lines due to the CuAl2O4, usually detected at 2h = 31.3° and 37.9° (JPCDS; 33-0448), were not observed, which agrees with the XPS results where only CuO was detected in the near-surface region. In the case of high Ni content, reflections at 2h = 37.0° and 43.3° attributed to (1 0 1) and (0 1 2) planes of NiO (JPCDS; 44-1159) could be observed. These reflections are smaller and broader as compared to the reflections related to CuO, indicating that NiO crystallite size was smaller than CuO crystallite size. When the Ni content was 13 wt.% or less, reflections related to NiO were not detected. NiO and CuO crystallite sizes were obtained by applying the Scherrer equation. For CuO, sizes between 38 and 53 nm were obtained, whereas for NiO, sizes between 5 and 12 nm were calculated. Therefore, the dispersion of Ni was significantly better than the dispersion of Cu, which agrees with the above shown XPS results. XRD analysis of reduced samples 13Ni22Cu/Al2O3 and 20Ni15Cu/Al2O3 was also carried out. The samples were reduced following the same procedure as described in Section 2.1. Inside the glovebox, part of the reduced catalyst powder was introduced into a glass capillary (diameter 0.7 mm; wall thickness 0.01 mm), which was afterwards sealed by melting in order to avoid contact with air. Next, the capillary was introduced into the XRD equipment for analysis. As observed in Fig. 4 for both reduced samples, metallic Cu, Ni, and a Cu–Ni alloy were detected. Obviously, the formation of the Cu–Ni alloy took place during the reduction under H2 flow at 923 K. Small proportions of the CuO and NiO phases that were initially present in the calcined samples were not reduced, as demonstrated by the reflections at about 37.5° and 61–62°. Finally, the broad reflections at 2h = 66.7° correspond to a well-dispersed c-Al2O3 phase. 3.1.5. TEM images The six calcined catalysts were also characterized by TEM. For all the samples, a heterogeneous size distribution of metal oxide

Fig. 3. XRD patterns of calcined Ni–Cu/Al2O3 catalysts with constant nominal metal content and different Cu/Ni metal ratio.

Fig. 4. XRD patterns of reduced 13Ni22Cu/Al2O3 and 20Ni15Cu/Al2O3 catalysts with constant nominal metal content and different Cu/Ni metal ratio.

particles over the alumina support was observed. As an example, in Fig. 5, the TEM images of 35Ni0Cu/Al2O3, 28Ni7Cu/Al2O3, and 7Ni28Cu/Al2O3 are displayed. 3.1.6. Activity test results The six calcined and reduced Ni–Cu/Al2O3 catalysts with constant total nominal metal content and different Cu/Ni ratio were tested in glycerol hydrogenolysis with formic acid as hydrogen donor molecule. Fig. 6 illustrates the main test results, in terms of glycerol conversion, and 1,2-PDO selectivity and yield, achieved with each catalyst at 473 and 493 K. For both temperatures, there was a clear correlation between glycerol conversion and the Cu/Ni ratio of the catalysts: with increasing Ni proportion in the catalyst, the glycerol conversion increased. Therefore, it is clear that the Ni plays a key role in the transfer hydrogenation of glycerol utilizing the active hydrogen coming from formic acid. Nevertheless, there was a significant decrease in 1,2-PDO selectivity in the tests performed with the catalyst containing only Ni, namely 35Ni0Cu/ Al2O3. Hence, the presence of Cu seems to be necessary for obtaining high 1,2-PDO selectivities. At this point, a deeper analysis of the product distribution as a function of the Cu/Ni ratio of the catalysts was required. For this purpose, Table 5 presents the product distributions and carbon balances of the activity tests. For clarity, only the results obtained at 493 K are displayed, but the same discussion could be done with the ones obtained at 473 K. It is interesting to observe that there is an increase in the selectivity to products with less than 3 C atoms, which means products coming from C–C bond hydrogenolysis, with increasing Ni content on the catalyst. The overall selectivity for such cleavage products rises to 50.5% in the test performed with the catalyst containing only Ni. In the light of these results, it can be suggested that Ni is active in the hydrogenolysis of glycerol using the active hydrogen atoms coming from formic acid. In the absence of Cu, the hydrogenolysis of C–C and C–O bonds occurred in a similar extent. When Cu was present in the catalyst together with Ni, the selectivity to cleavage products significantly decreased (see Table 5). How does Cu reduce the activity of Ni in the C–C bond cleavage? It is well known that in bimetallic catalysts, the presence of a second metal may induce significant changes in both activity and selectivity for catalytic reactions. For instance, the selectivity effects induced by alloying Ni with Cu were observed in cyclopentane hydrogenolysis [37] and in ethane hydrogenolysis [38]. The variation in the catalytic performance is often explained by the ensemble theory, which considers that the addition of an inactive

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Fig. 5. TEM images of calcined Ni–Cu/Al2O3 samples. (A) 35Ni0Cu/Al2O3, (B) 28Ni7Cu/Al2O3, (C) 7Ni28Cu/Al2O3.

metal results in a dilution of the surface-active metal atoms, and then, in a decrease of the active ensemble size [39,40]. Cu is also active in the C–O bond hydrogenolysis but not in the C–C bond hydrogenolysis; therefore, it can be considered as an inert metal in the C–C bond cleavage reaction [41]. Above shown XRD results of reduced samples confirm the formation of a NiCu alloy in the bimetallic NiCu/Al2O3 catalysts. In the alloy, Ni and Cu atoms are intercalated. This causes a reduction in the size of the Ni ensembles available on the active surface. In order to check whether glycerol C–C bond cleavage reaction is sensitive to the size of the Ni ensemble, reduced samples of the six catalysts were analyzed by XPS and the surface atomic composition of Ni, Cu, Al, O, and C was obtained from XPS peaks intensities. The total mmol of C < 3 products (ethylene glycol, ethanol, methanol, and methane) measured at the end of each activity test were correlated to the relative proportion of Ni and Cu surface atoms. As it can be seen in Fig. 7, the total mmol of C < 3 products

formed at the end of the test exponentially decreased with reducing the Ni surface proportion (and therefore increasing the Cu proportion). This indicates that glycerol C–C bond cleavage requires a whole ensemble of contiguous Ni atoms. The formation of a Ni–Cu alloy reduces Ni ensemble size, and as a result, the C–C hydrogenolysis activity of the catalyst drastically diminishes. Hence, the presence of both metals is required for obtaining high yields to 1,2-PDO. Ni provides comparatively high hydrogenolysis activity of C–C and C–O bonds and the formation of Ni–Cu alloy limits C–C hydrogenolysis and promotes C–O hydrogenolysis. Indeed, there is an optimum in Cu/Ni ratio (0.72) that maximizes 1,2-PDO production, as for both reaction temperatures the maximum in 1,2-PDO yield was obtained with the 20Ni15Cu/Al2O3 catalyst (see Fig. 6C). The performance of formic acid during the reaction must be also discussed. In a previous work, not shown here, we observed that formic acid readily reacted to CO2 and hydrogen, with 100% con-

Fig. 6. Glycerol conversion, 1,2-PDO selectivity and yield obtained at 473 and 493 K for each Ni–Cu/Al2O3 catalyst: 16-h reaction time, 45 bar N2 pressure, 135 mL glycerol 1 1 aqueous solution (4 wt.%), 0.9 g of catalyst, 2.1 or 3:6 mmol gcatal h formic acid feed rate.

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Table 5 Product selectivities and carbon balances for the tests performed at 493 K. 16-h reaction time, 45 bar N2 pressure, 135 mL glycerol aqueous solution (4 wt.%), 0.9 g of catalyst, 1 1 3:6 mmol gcatal h formic acid feed rate. Catalyst/Al2O3

0Ni35Cu 7Ni28Cu 13Ni22Cu 20Ni15Cu 28Ni7Cu 35Ni0Cu

Conv. (%)

29.2 40.9 47.5 49.3 47.7 53.0

Selectivity (%)

Liquid finalb C (%)GC

Liquid finalc C (%)TOC

Solid finald C (%)CHN

Total finale C (%)

94.9 97.0 94.6 99.1 99.3 76.0

92.6 97.4 94.9 102.5 102.1 75.9

1.05 1.14 1.12 1.15 1.06 0.57

96.5 103.2 97.6 101.7 101.9 93.4

a

1,2-PDO

Acetol

1-PO

C<3

75.5 82.2 78.1 75.4 74.3 42.0

11.2 5.0 3.0 4.1 2.4 1.8

5.0 4.5 7.4 9.2 9.2 3.3

8.3 8.3 11.4 11.3 14.0 50.5

a

Ethylene glycol, ethanol, and methane. (Final mass of C in the liquid phase obtained by GC-FID analysis/initial mass of C in the reactor)  100. c (Final mass of C in the liquid phase obtained by TOC analysis/initial mass of C in the reactor)  100. d (Mass of C in the catalyst at the end of the test obtained by CHN analysis/initial mass of C in the reactor)  100. e (Final mass of C in the liquid and gas phase obtained by GC-FID analysis, plus mass of C in the catalyst obtained by CHN elemental analysis/initial mass of C in the reactor)  100. b

ble amounts of polyglycerols were formed during the reaction. These compounds are not detectable with capillary GC, but TOC is sensitive for any carbonaceous compound. If such compounds were formed, then they should be found in the solid or on the catalyst. Carbon content of used catalysts obtained by CHN analysis of solid residue gives an idea of the amount of coke (or polyglycerols) formed during the reaction. The carbon content measured in the catalysts represented only 1% of the initial glycerol carbon (see Table 5), and in terms of catalyst weight, the measured carbon represented less than 2.7 wt.% in all cases. Moreover, part of the carbon amount detected in spent samples was already present in the fresh catalysts from the preparation procedure (see XPS results in Table 3). Therefore, the obtained results indicate that negligible amounts of coke were formed during the reaction. Fig. 7. Total mmol of C < 3 products formed after 16-h reaction time as a function of the surface Ni/(surface Ni + surface Cu) ratio, which was obtained by XPS analysis of the six reduced catalysts. 45 bar N2 pressure, 135 mL glycerol aqueous solution 1 1 (4 wt.%), 0.9 g of catalyst, 3:6 mmol gcatal h formic acid feed rate, 493 K.

version in all the tests. Formic acid was not detectable with the GCFID equipment used for liquid-phase analysis. Therefore, in order to ensure that in all the performed tests formic acid was completely converted, five liquid samples stemming from different activity tests were selected and analyzed by GC–MS (Shimadzu GC–MS QP2010 SE; CP-Sil 5 CB column suited for formaldehyde, 60 m  0.32 mm  0.45 mm) as well as one sample of the solution pumped. Formic acid was only detected in the solution pumped. Concerning the gas phase, negligible CO amounts were detected, indicating that if CO was formed via formic acid dehydration, it was readily converted to CO2 by water–gas shift reaction. Molecular hydrogen was detected in the gas phase. Those hydrogen atoms, coming from formic acid decomposition, that were not transferred to glycerol combined to form H2. The hydrogen balance (mol of H coming from formic acid = mol of H in the gas phase + mol of H transferred to liquid products) closed within a 10% error in all the experiments. Table 5 also shows the carbon balances for the different activity tests. For the liquid phase, it was assumed that all the carbon detected in the TOC measurements came from glycerol, as all the formic acid was converted to CO2. For the gas phase, it was considered that all the CO2 detected came from formic acid and that all the CH4 detected came from glycerol decomposition (no Fischer–Tropsch reaction). As observed in Table 5, there is a quite good agreement between final liquid carbon amounts obtained by GC-FID and TOC analysis (used as a cross-check). This proves that negligi-

3.2. Optimization of the total metal content Another series of five different Ni–Cu/Al2O3 catalysts was prepared by sol–gel method with the optimized Cu/Ni ratio of 0.72, derived from the results reported in the previous section, and different total metal content. These catalysts were also characterized and tested in glycerol hydrogenolysis using formic acid as hydrogen donor molecule. 3.2.1. Elemental analysis and N2 physisorption Table 6 summarizes the theoretical and real metal content together with the main textural properties of the calcined catalysts. As seen for the above-described first catalyst series, the real Ni and Cu contents were lower than the theoretical ones in all the samples, whereas the real Cu/Ni ratio values were closer to the theoretical ones in those catalysts with high metal content. Concerning the N2 physisorption results, all the samples presented Type IV isotherms, with hysteresis loops typical for mesoporous materials. Within this series, there is no clear trend in BET area and total pore volume as a function of total metal content. Nevertheless, as expected, the catalysts with higher metal contents, and therefore lower alumina contents, presented lower BET areas. This is explained by the surface coverage and plugging of the pores. 3.2.2. X-ray diffraction Fig. 8 illustrates the XRD patterns of the calcined Ni–Cu/Al2O3 catalysts having similar Cu/Ni ratio and different total metal content. The pattern of the calcined 20Ni15Cu/Al2O3 does not appear in Fig. 8 because it was already shown in Fig. 3. The two catalysts with less metal content, 3Ni2Cu/Al2O3 and 12Ni8Cu/Al2O3 showed mainly the reflections related to the Al2O3 support. This indicates a

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Table 6 Characteristics of the catalysts with constant nominal Cu/Ni ratio and different metal content. Name/Al2O3

a

Ni content (wt.%)

Cu Content (wt.%)

Ni + Cu (wt.%)

Ratio Cu/Ni

Theoretical (Real)

Theoretical (Real)

Theoretical (Real)

Theoretical (Real)

SBET (m2/g)

Total pore volume (cm3/g)

Average pore size (nm)

3Ni2Cu

2.9 (2.6)

2.1 (2.2)

5.0 (4.8)

0.72 (0.85)

271.9

0.201

2.72

12Ni8Cu

11.6 (9.0)

8.4 (7.5)

20.0 (16.5)

0.72 (0.83)

207.9

0.194

3.40

20Ni15Cu

20.3 (17.3)

14.7 (12.8)

35.0 (30.1)

0.72 (0.74)

229.6

0.247

3.66

29Ni21Cu

29.0 (26.8)

21.0 (20.8)

50.0 (47.6)

0.72 (0.78)

142.0

0.217

5.43

38Ni27Cu

37.8 (37.6)

27.3 (26.7)

65.1 (64.3)

0.72 (0.71)

95.1

0.112

5.79

Chemical composition determined by ICP.

Fig. 8. XRD patterns of calcined Ni–Cu/Al2O3 catalysts with constant nominal Cu/Ni metal ratio and different total metal content.

high dispersion of the NiO and CuO phases on the support. In the patterns of the two catalysts with the highest metal content (47.7 wt.% and 64.3 wt.%, respectively), diffraction peaks related to the metal oxide phases were detected. Apart from CuO and NiO phases, some mixed Cu–Ni oxides with different Cu/Ni ratio were detected (Ni0.75Cu0.25O, Ni0.95Cu0.05O). Ni–Cu oxide phases were not recorded in the XPS analysis of the catalysts with constant nominal metal content of 35 wt.% (see Fig. 3). This seems to indicate that for a significant formation of Cu–Ni oxide crystallite phases high Ni and Cu contents are required.

3.2.3. Activity test results Fig. 9 shows the main activity test results obtained at 473 and 493 K, with the series of Ni–Cu/Al2O3 catalysts with nearly constant Cu/Ni ratio and different total metal content. For both temperatures, glycerol conversion increased with total metal loading, until a maximum was reached for the catalyst having a nominal metal content of 35 wt.%. For higher nominal metal contents, glycerol conversion decreased with additional increases of the total amount of metals in the catalyst. Hence, there is an optimum metal-Al2O3 proportion that enhances glycerol conversion. This result is coherent with the fact that – as it will be discussed below – both metal and acid sites of the Al2O3 support are supposed to play a role in the glycerol hydrogenolysis reaction. Regarding the effect of total metal content of the catalyst on the selectivity to 1,2-PDO, it can be observed in Fig. 9A and B that there was no significant influence. The selectivities in the tests

performed at 473 K (around 85%) were higher as compared to the values recorded in the tests at 493 K (around 75%). This is in accordance with the results from the tests with the first series of catalysts. Therefore, the selectivity to 1,2-PDO depends more on the Cu/Ni ratio of the catalysts and on the reaction temperature than on the total metal content. Fig. 9C illustrates 1,2-PDO yield obtained with each catalyst at 473 and 493 K. For both temperatures, the maximum was observed when the catalyst having a nominal metal content of 35 wt.% was used. The acidity of 12Ni8Cu/Al2O3, 20Ni15Cu/Al2O3, and 38Ni27Cu/ Al2O3 was evaluated by TPD-NH3. The results are displayed in Table 7. As expected, the acidity of the samples decreased as the nominal metal content of the catalysts increased. This decrement on acidity can be related to the Al2O3 content per gram of catalyst of the samples. In order to determine whether the decrement was also caused by Ni and Cu particles occupying the acid sites of the Al2O3 support, ammonia desorption results were normalized by mass of Al2O3 (see Table 7 last column). It can be observed that the increase in the metal content reduced the support acidity, indicating that Ni and Cu metal particles occupied acid sites of the Al2O3 [42]. The role of acid sites in direct glycerol conversion mechanism to 1,2-PDO through the intermediate formation of 1,3-dihydroxyisopropoxide was firstly suggested for a system under H2 pressure and using Rh–Re/SiO2 [32] or Ir–Re/SiO2 [43] catalysts. The alkoxide was formed by adsorption on the ReOx cluster. Next, the hydride coming from molecular hydrogen and activated on Ir or Rh sites attacked the C–O bond. Chia et al. [44] and King et al. [45] have recently suggested the formation of acid sites associated with the presence of ReOx clusters. Therefore, Re acting as an acid site is responsible for the alkoxide adsorption, and Ir or Rh sites of the hydride attack. In our system, acid sites are provided by Al2O3 support instead of ReOx, while hydrides are formed from formic acid decomposition and not from dissolved molecular hydrogen dissociation. Therefore, not only the glycerol hydrogenolysis mechanism, but also the formic acid decomposition mechanism needs to be taken into account. Concerning alkoxide formation, 1,3-dihydroxyisopropoxide is formed by glycerol adsorption on an acid site of the alumina support. Alkoxide formation on the acid sites of an alumina support has already been reported [46]. Regarding hydride formation, it is likely that formic acid adsorbs dissociatively to give a formate species and an adsorbed hydrogen atom, and that the formate species then dissociate to give gaseous CO2 and another adsorbed hydrogen atom [47,48]. If 1,3-dihydroxyisopropoxide is adsorbed in an acid site near a metal site having an adsorbed hydrogen atom, the hydride attacks the CO bond of the alkoxide. Finally, the hydrolysis of the reduced alkoxide gives 1,2-PDO (see Fig. 10).

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Fig. 9. Glycerol conversion, 1,2-PDO selectivity and yield obtained at 473 and 493 K reaction temperature for each Ni–Cu/Al2O3 catalyst with constant Cu/Ni ratio and 1 1 different metal loading. 16-h reaction time, 45 bar N2 pressure, 135 mL glycerol aqueous solution (4 wt.%), 0.9 g of catalyst, 2.1 or 3:6 mmol gcatal h formic acid feed rate.

Table 7 Acidity of the reduced samples from temperature-programed desorption of ammonia. Sample/Al2O3

NH3 desorption mmol NH3/g catal

NH3 desorption mmol NH3/g Al2O3

12Ni8Cu 20Ni15Cu 38Ni27Cu

0.33 0.16 0.06

0.39 0.23 0.17

This mechanism allows explaining why there must be an optimum proportion of acid and metal sites. For the catalysts presenting low metal content, 1,3-dihydroxyisopropoxide is formed on the highly available acid sites. However, due to the low relative amount of metal sites, only a low proportion of these alkoxides are adsorbed near a metal site having an adsorbed hydrogen atom and are therefore desorbed without reacting. In the opposite case, which means a catalyst with high metal content, there are not enough acid sites available for glycerol adsorption. For the Ni–Cu/ Al2O3 catalytic system, the optimum was obtained for a catalyst having a nominal metal content of 35 wt.%. However, the formation of 1,3-dihydroxyisopropoxide also on metal sites cannot be excluded. A test was run using Raney nickel,

493 K and the same operating conditions. The yield to 1,2-PDO obtained (10.1%) was lower than that obtained with the catalyst having the higher metal content, 38Ni27Cu/Al2O3 (22.3%), and 3.6 times lower than the one obtained with the optimized 20Ni15Cu/ Al2O3 catalyst (36.4%). 3.3. Kinetic study So far, no information about the evolution of glycerol conversion with the reaction time has been shown, as in the above presented test results, liquid and gas phase were only analyzed at the end on the experiments after 16 h. In order to gain a deeper knowledge on the process, further tests were carried out with the optimized catalyst, 20Ni15Cu/Al2O3, but taking five liquid samples in regular time intervals during the activity test. As observed in Fig. 11A, the activity of the 20Ni15Cu/Al2O3 catalyst decreased markedly during reaction time. In the first 2 h, an 1 1 average glycerol TON of 4:7  10 3 molglyc gcatal h was obtained, while for the last 16 h the average TON was only 1 1 0:5  10 3 molglyc gcatal h . In principle, complete conversion should be possible if hydrogen donor was fed in excess. Three main

Fig. 10. Model structure of the transition states of the hydride attack to the adsorbed 1,3-dihydroxypropoxide, under inert atmosphere, with formic acid as hydrogen donor molecule and over Ni–Cu/Al2O3 catalyst.

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Fig. 11. Evolution of glycerol conversion with reaction time. 20Ni15Cu/Al2O3 catalyst, 493 K, 24-h reaction time, 45 bar N2 pressure, 135 mL glycerol aqueous solution 1 1 (4 wt.%), 3:6 mmol gcatal h formic acid feed rate. (A) Effect of reusing the catalyst. (B) Effect of the presence of 1,2-PDO in the feed (0.5 mol of 1,2-PDO/mol of glycerol). (C) Effect of the amount of catalyst placed in the reactor. (D) Evolution of 1,2-PDO and 1-propanol selectivity in the test with high catalyst loading.

factors were selected as the possible causes for this decrease of the activity: (i) deactivation of the catalyst, (ii) insufficient hydrogen supply, and (iii) competitive adsorption for active sites between glycerol and 1,2-PDO. Thermodynamic equilibrium between glycerol and formic acid was not considered as a possible factor, because under similar operating conditions but using hydrogen pressure higher glycerol conversion and 1,2-PDO yields have been reported. In order to check whether the catalyst was deactivated during the reaction, it was decided to reuse the 20Ni15Cu/Al2O3 catalyst in a second run, under the same experimental conditions. It can be noticed from Fig. 11A that similar glycerol conversions with reaction time were achieved in the first and second run. Therefore, in the short term, the catalyst was not deactivated, as it was expected from the low carbon content detected in the spent catalysts after the activity tests, and hence another factor must be the reason of the decrease in the catalyst activity. Formic acid was continuously fed during 24 h at a constant rate 1 1 of 3:6 mmol h gcatal . As written above, it can be assumed that formic acid reacts instantaneously when it enters the reactor, releasing CO2 and two active hydrogen species. As a consequence, the supply of active hydrogen was constant during the entire activity test: the same active hydrogen supply rate when high TON (after 2 h) were registered and when low TON (in last 16 h) were registered. In other words, hydrogen donor and therefore active hydrogen species were always present in excess, and therefore reaction rate was not limited by the concentration of those. Hence, the hydrogen supply is not the main factor behind the decrease in the activity of the catalyst with the reaction time.

In order to determine whether there is competition for active sites between 1,2-PDO and glycerol, 1,2-PDO was introduced together with glycerol in the aqueous feed solution (0.5 mol of 1,2PDO/mol of glycerol). As Fig. 11B shows, glycerol conversion was significantly affected by the presence of 1,2-PDO in the feed. Indeed, final glycerol conversion obtained after 24 h decreased from 52.7% to 35.4%. It is therefore clear that 1,2-PDO affects glycerol reaction rate; there is product inhibition. Another activity test was carried out without 1,2-PDO in the feed (only glycerol aqueous solution), keeping the operating conditions as in the previous tests, and using three times more of the 20Ni15Cu/Al2O3 catalyst (2.7 g). Hence, there were three times more available active sites in the reactor for the same molar amount of glycerol. It can be noticed in Fig. 11C that glycerol conversion significantly increased when a higher amount of catalyst was used. Indeed, after 24 h, a glycerol conversion of 89.9% and a selectivity to 1,2-PDO of 81.6% were measured. These results clearly indicate that the competition for active sites between the OH groups of glycerol and 1,2-PDO is the main factor that limits the activity of the catalyst during the reaction time. The competition for active sites between the OH groups of 2propanol and glycerol in Ni–Cu/Al2O3 catalyst was previously reported [29]. 1,2-PDO competes with glycerol to be adsorbed in the acid sites of the support to form 2-hydroxypropoxide. Part of this adsorbed alkoxide reacts with the active hydrogen atoms coming from formic acid, being activated in the Ni–Cu metal sites, to yield 1-propanol. In Fig. 11D, it can be observed that after 10-h reaction time, there is a decrease in the selectivity to 1,2-PDO and an increase in the selectivity to 1-propanol. The fact that high

I. Gandarias et al. / Journal of Catalysis 290 (2012) 79–89

1,2-PDO selectivity was obtained after 24 h indicates that the Ni– Cu/Al2O3 catalytic system is more active in the glycerol hydrogenolysis to yield 1,2-PDO than in the 1,2-PDO hydrogenolysis to yield 1-propanol. It is worth to highlight the high glycerol conversion, 90%, and selectivity values, 82%, obtained in a system with in situ generation of hydrogen. Actually, these results are comparable to the best results reported under the traditional liquid phase process using hydrogen pressure (see Table 1). Further research should be accomplished to determine the effect of the pressure in glycerol hydrogenolysis through transfer hydrogenation, as less severe conditions can make the process even more attractive. Moreover, the performance of formic acid as hydrogen donor should be compared to molecular hydrogen under the same operating conditions. 4. Conclusions In the present work, the performance of Ni–Cu/Al2O3 bimetallic catalysts on the glycerol hydrogenolysis to 1,2-PDO under inert atmosphere and using formic acid as the source of hydrogen was studied. It was observed that there is an optimum Cu/Ni ratio and also an optimum in the ratio and distribution of metal and acid sites that maximize the yield of 1,2-PDO. The glycerol hydrogenolysis occurs when glycerol is adsorbed on an acid site of Al2O3 to form a secondary alkoxide, and when this alkoxide interacts with an adsorbed hydrogen atom coming from formic acid decomposition and being activated on a metal site. The optimum balance between the alumina acid sites and the metal sites was obtained for the catalyst with a nominal metal content of 35 wt.%. Regarding the role of the metallic sites, Ni provides comparatively high activity in the C–C and C–O bond hydrogenolysis, whereas Cu provides some activity in the C–O bond hydrogenolysis but not in the C–C bond hydrogenolysis. The formation of a Cu–Ni alloy during the reduction of the Ni–Cu/Al2O3 catalysts reduces the active Ni ensemble size. As a result, the activity of the bimetallic catalyst for C–C bond cleavage is diminished while the activity for C–O bond cleavage is promoted. The optimum Cu/Ni ratio that maximized the yield to 1,2-PDO was 0.72. The kinetic study revealed that the OH groups of glycerol and of 1,2-PDO competed for the same acid sites to form alkoxides. In order to minimize this factor, high catalyst loading was placed in the reactor, and a glycerol conversion of 90% and a 1,2-PDO selectivity of 82% were achieved after 24 h operating at 493 K. Acknowledgments This work was supported by funds from the Spanish Ministry of Science and Innovation ENE2009-12743-C04-04, and from the Basque Government (Researcher Training Programme of the Department of Education, Universities and Research). The authors greatly acknowledge Drs Schneider, Pohl, Radnik, Mr. Eckelt and Ms Evert for the work done in the characterization of the catalysts, and the Inorganic Chemistry Department at the University of Malaga for their technical support. References [1] T.E. Bull, Science 285 (1999) 1209.

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