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A sustainable process for the production of γ-valerolactone by hydrogenationof biomass-derived levulinic acid

Anna Maria Raspolli Galletti,* Claudia Antonetti, Valentina De Luise and Marco Martinelli

Received 19th July 2011, Accepted 13th December 2011DOI: 10.1039/c2gc15872h

A sustainable process for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) is reported.GVL can be easily obtained in high yield, adopting very mild reaction conditions, by the hydrogenationof an aqueous solution of levulinic acid using a commercial ruthenium supported catalyst in combinationwith a heterogeneous acid co-catalyst, such as the ion exchange resins Amberlyst A70 or A15, niobiumphosphate, or oxide. All the hydrogenations were carried out at 70–50 °C and at low hydrogen pressure(3–0.5 MPa). The most effective acid co-catalyst was the ion exchange resin Amberlyst A70, whichproduced a high yield of GVL (99 mol%) and an activity of 558 h−1 after 3 h of reaction, whilst workingat 0.5 MPa of hydrogen and 70 °C. The combined effect of acid and hydrogenating heterogeneouscomponents was also verified for the hydrogenation of aliphatic ketones to the corresponding alcohols,thus opening a new perspective for this process.

Introduction

Biomass represents the main source for the production of renew-able fuels and chemicals, as such the interest in the green cataly-tic conversion of renewable feedstocks into chemicals andbiofuels is booming.1–9 In addition, bio-based products haveunique properties compared to fossil ones, such as biodegrad-ability and bio-compatibility; as a consequence, much attentionis devoted to improve the use of biomass for energy, chemicalsand materials supply. The major component of plant biomass,carbohydrates, can be transformed into several chemicals, suchas n-butanol, ethanol, sorbitol,10 hydroxymethylfurfural,11

methylfurfural,12 γ-valerolactone (GVL).13 Very recently,Horváth has described the latter as an ideal, sustainable liquidwhich can be used for the production of both energy and carbon-based consumer products.14 GVL is renewable, safe to store andcan be employed as a liquid fuel, food additive, solvent and asan intermediate in the synthesis of many fine chemicals.15–19

Moreover, recent important papers focus on GVL as the centerof novel cascade processes for the production of liquid fuels,obtained adopting different smart approaches.20–23 The startingmaterials for obtaining GVL are levulinic acid and its esters, inturn obtained by acid hydrolysis of lignocellulosicbiomasses.24–25 The hydrogenation of levulinic acid (LA) toγ-valerolactone (reported in Scheme 1), has been investigated byseveral groups, adopting two different approaches: formic acidwas used as the hydrogen source (path a) or an external hydro-gen overpressure was employed (path b).26–47

Several authors investigated the first path20,31–35 and we canconclude that if it is possible to carry out the reaction directly onthe raw reaction solution, the formic acid-based process appearsinteresting and feasible, achieving high yields under mild reac-tion conditions without catalyst deactivation.

On the other hand, the second approach,13,22,36–44 the hydro-genation with molecular hydrogen, becomes sustainable if it ispossible to perform this process on the raw hydrolysis solutionor on a LA aqueous solution without a previous dehydrationstep, adopting mild reaction conditions, in particular low hydro-gen pressure. With regard to this, recently, Horváth13 studied themulti-step process for the conversion of sucrose to levulinic acidand subsequent hydrogenation to GVL, 1,4-pentandiol and 2-methyl-THF. Levulinic acid was produced from the degradationof sucrose dissolved in water and full conversion of this substrateto GVL was reached with the catalytic system Ru(acac)3/PBu3/NH4PF6 working at 14 MPa and 135 °C for 8 h. This novelapproach is very interesting but involves homogeneous catalysisunder drastic reaction conditions. Among heterogeneous cata-lysts, many metals have been tested, such as Pt, Ru, Re, Cu, Rh,

Scheme 1 Reduction of levulinic acid to γ-valerolactone.

Department of Chemistry and Industrial Chemistry, University of Pisa,via Risorgimento 35, 56126 Pisa, Italy. Fax: +39 50 2219260; Tel: +3950 2219290

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Ir and Pd.36–44 Recently, Manzer40,41 and Poliakoff42 re-investi-gated this reaction adopting different metal catalysts, in particularruthenium systems, but severe reaction conditions (140–160 °C,3–6 MPa of hydrogen, 1,4-dioxane as solvent) are claimed.Manzer also patented the continuous hydrogenation of levulinicacid to GVL in supercritical CO2,

43 pumping LA dissolved in1,4-dioxane. Poliakoff replaced 1,4-dioxane with water in super-critical CO2 working at 10 MPa, ascertaining almost quantitativeyield.

A recent work44 studies the effect of different catalysts on thehydrogenation of methanol solutions of LA and compares theperformances of Ru/C, Pd/C, RANEY® nickel and Urushibaranickel. Ru/C catalyst resulted particularly active and selective forthis reaction, leading, under the optimized conditions, (130 °C,1.2 MPa of hydrogen), to a 91 mol% yield to GVL, although agreen replacement solvent is necessary, instead of methanol.

Particularly significant are the results reported by Lange,45

who patented LA hydrogenation in the presence of hydrogenat-ing components chosen among Ni, Rh, Pd, Pt, Re, Ru or a com-bination of two or more of them. The reaction was studied attemperature in the range of 150–250 °C and at hydrogen pressureof 5–8 MPa, in the presence of a zeolitic cocatalyst. However,this process still involves drastic conditions. More interestingly,this author also studied22 the hydrogenation of LA obtained byhydrolysis of biomass in the presence of sulphuric acid adoptingdrastic conditions (200 °C and 4 MPa of hydrogen) working in acontinuous tubular reactor in order to evaluate the catalysts lifeafter long reaction times (>100 h). The evaluation of about 50catalysts evidenced the best performances of Pt/TiO2 or ofPt/ZrO2.

Finally, the importance of GVL is underlined in the funda-mental works of Dumesic who chose this intermediate as a plat-form for “valeric biofuels” in an integrated bio-refining strategy,adopting different approaches.1,2,8,20,21,23,46,48 Working with aone-pot process based on cellulose acid deconstruction, an acid

aqueous solution of levulinic acid was obtained which was cata-lytically processed up to an organic liquid stream enriched inpentanoic acid, through the intermediate formation of GVL.Different metal catalysts were tested in the hydrogenation of LA:using 5% Ru/C nearly quantitative GVL yield was achievedworking on 15.2 wt% aqueous solution of LA at 150 °C and 3.5MPa of hydrogen, notwithstanding the presence of 0.5 M sul-phuric acid. This approach is particularly valuable in the futureperspective of direct conversion of raw biomass, instead of purecellulose, to GVL.20

In this context, we have studied the hydrogenation of watersolutions of LA to GVL with the aim of setting up as mild aspossible reaction conditions in the perspective of a sustainableprocess, carried out on aqueous LA, a less expensive substratethan pure LA, due to the high cost of its dehydration.

Results and discussion

LA aqueous solution (5 wt%) was hydrogenated on commercialruthenium catalysts, 5% Ru/Al2O3 and 5% Ru/C. These werechosen for their large scale availability, easy recovery andbecause ruthenium has been demonstrated to be the most activecatalyst for the hydrogenation of aliphatic carbonyl compoundsto give the corresponding alcohols.49 In some preliminary runs,those catalysts were employed in reference experiments carriedout at 70 °C and 3 MPa of hydrogen, (see Table 1, runs 1 and 8and Fig. 1 and 2).

The catalytic performances resulted moderate under thesemild conditions and Ru/C more active (run 8, Table 1), leadingto a levulinic acid conversion after 3 h of 48 mol% compared to24 mol% achieved with Ru/Al2O3 under identical catalytic con-ditions. These results can be related to the very high surface areaof Ru/C, which is about tenfold higher than that of Ru/Al2O3. Itis notable that under the adopted conditions, the selectivity to

Table 1 Hydrogenation of LA to GVL in the presence of 5% Ru/Al2O3 and 5% Ru/C and acid co-catalysts. Reaction conditions: H2O: 40 ml, LA:1.97 g, Ru: 1 meq

Run Catalyst Acid co-catalyst T (° C) P H2 (MPa)LA Conv. (mol%)after 3 h

GVL Sel. (mol%)after 3 h Aa (h−1)

1 Ru/Al2O3 — 70 3 24 96.0 136.72 Ru/Al2O3 A15, 2.35 meq 70 3 50 98.2 284.83 Ru/Al2O3 NBO, 0.25 meq 70 3 32 97.5 182.34 Ru/Al2O3 NBP 0.25 meq 70 3 36 97.8 205.05 Ru/Al2O3 A70, 2.35 meq 70 3 56 99.0 319.06 Ru/Al2O3 A70, 2.8 meq 70 3 57 98.6 324.77 Ru/Al2O3 A70, 5.6 meq 70 3 57 98.1 324.78 Ru/C — 70 3 48 97.5 273.49 Ru/C A15, 2.35 meq 70 3 91 98.5 518.310 Ru/C NBO, 0.25 meq 70 3 75 98.1 427.211 Ru/C NBP, 0.25 meq 70 3 82 98.4 467.112 Ru/C A70, 2.35 meq 70 3 100 99.9 569.613 Ru/C — 50 3 23 98.8 131.014 Ru/C A70, 2.35 meq 50 3 93 99.1 529.715 Ru/C — 70 0.5 13 99.8 74.016 Ru/C A70, 2.35 meq 70 0.5 98 99.5 558.217 Ru/C NBP, 0.25 meq 70 0.5 78 98.3 444.318 Ru/C NBO, 0.25 meq 70 0.5 72 98.6 410.119 Ru/C A15, 2.35 meq 70 0.5 91 98.9 518.320 Ru/C A70, 2.35 meq 50 0.5 85 99.1 484.2

aActivity measured at 3 h of reaction = mol LA converted/mol Ru·h.

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GVL is about 96 mol%, only very low amounts of γ-hydroxy-valeric acid being detected.

In order to achieve higher activities maintaining the totalselectivity to GVL, we studied the combination of a hetero-geneous acid, such as cationic exchange resins, niobium phos-phate (NBP) or oxide (NBO), and the already mentionedcommercial ruthenium supported catalysts. NBP and NBO arewell-known for their acid properties, which can be preservedalso in polar media.50–52 They are amorphous solids with strongBrønsted acid sites and medium–strong Lewis acid sites at thesurface, due to coordinatively unsatured Nb+5 species.53 Theacidic, textural, and catalytic properties of NBP are usuallysuperior to those of NBO and are retained also at higher temp-eratures.52 In the field of the catalytic conversion of renewablefeedstocks, these Nb derivatives resulted in being effective alsofor the selective dehydration of saccharides to 5-hydroxymethyl-2-furaldeide (HMF).53–54

On the other hand, Amberlyst resins are present in many cata-lytic processes involving heterogeneous acids, the benchmark ofthe industry due to their very high activity in the areas of esterifi-cation, etherification and oligomerization.55 Very recently,several strongly acidic resins were studied in the conversion offurfuryl alcohol into ethyl levulinate, obtaining interestingresults.56 In our study, the two commercial sulfonated Amberlystresins, A70 and A15, were employed, their main characteristicsbeing reported in Table 2. A70 is a strongly acid resin resistantto very high temperatures, up to 190 °C, due to the presence ofchlorine atoms in its structure.

The catalytic performances of 5% Ru/Al2O3 combined withdifferent acid co-catalysts are reported in Table 1 (Table 1, runs2–7 and Fig. 1). The employed amount of the two Amberlyst

resins, characterized by high concentration of acid sites, werechosen in order to achieve similar levels of acidity for the tworesins. In the case of niobium catalysts, characterized by a sig-nificantly lower concentration of effective acid sites, theemployed amount of the NBO and NBP allowed to achievelevels of acidity about a tenth of those reached with the acidresins (0.25 meq for niobium derivatives vs. 2.35 meq for acidresins). The amount of Ru remained constant at 1 meq for everyrun.

The effect of the addition of the acid co-catalyst on the reac-tion rate appears immediately remarkable (compare run 1 withruns 2–7) and the ion exchange resin A70 demonstrated to bethe most effective co-catalyst. This result can be related to thestrength of acid sites on A70 which is higher than A15, due tothe activating halogen substitution of the benzene rings55

A large excess of resins is not necessary for obtaining thehighest activity (compare run 5 with 6 and 7) and, on the con-trary, with too much resin the selectivity resulted slightlydecreased by the formation of the isomerization product (pseudolevulinic acid).

On the other hand the results obtained with niobium co-cata-lysts are remarkable, taking into account that the amount ofinvolved acid sites is about a tenth of the acid resins. NBPresulted more efficient than NBO, as expected52 (compare run 3with run 4, Table 1).

The effect of the acid co-catalyst was then studied for 5% Ru/C (Table 1, runs 8–12, Fig. 2).

The obtained results show that the presence of the acid co-cat-alyst has again a remarkable positive effect on the catalytic per-formances (compare run 8 with run 9–12, Table 1). Once again,the best results are achieved employing the two cationicexchange resins. A70 achieves the best performances: after only30 min, 90% of substrate was converted to GVL with total selec-tivity. The positive effect of the combination of these two

Fig. 1 Effect of different heterogeneous acid co-catalysts on LA Con-version (mol%) vs. time (h), employing 5% Ru/Al2O3 (runs 1–5,Table 1).

Fig. 2 Effect of different heterogeneous acid co-catalysts on LA Con-version (mol%) vs. time (h), employing 5% Ru/C. (runs 8–12, Table 1).

Table 2 Main characteristics of Amberlyst A15 and A70 ion-exchange resins

Resin Physical formMoisturecontent (wt%)

Concentration ofacid sites (meqH+ g−1)

Surface Area(m2 g−1)

Average porediameter (Å) T max (°C)

A15 Opaque beads Dry 4.70 53 300 120A70 Dark brown, spherical beads 55% 2.55 36 220 190

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heterogeneous components, Ru/C and A70, has been recentlyclaimed even in the acid assisted glycerol hydrogenolysis to 1,2propandiol.57

It is well-known that in the perspective of industrial appli-cations, as mild as possible reaction conditions should beadopted: therefore, it was investigated whether it is possible tocarry out the hydrogenation under milder experimental con-ditions. Due to the better performances of 5% Ru/C, only thissystem was employed in the subsequent experiments. The reac-tion conditions and the achieved results are summarized inTable 1, runs 13–20 and Fig. 3.

The first two experiments, 13 and 14, were carried at 50 °Cwithout and with A70 respectively: the acid component signifi-cantly enhanced the yields, allowing a molar yield of GVLupper than 75 mol% after only 30 min. On the other hand, amore critical parameter is hydrogen pressure: in order to findmore sustainable conditions, runs 15–19 were carried out atlower hydrogen pressure, 0.5 MPa. Once again, A70 resulted themost effective co-catalyst, leading to an almost complete yield toGVL, only after 3 h of reaction (run 16, Table 1). It is interestingto note that even employing A15 (run 19, Table 1), promisingresults were obtained, although lower with respect to A70(89.9% GVL molar yield in run 19 against 97.5% in run 16,after 3 h of reaction). Finally, in run 20 low pressure was com-bined with low temperature (50 °C): despite these mild con-ditions the GVL yield reached after 3 h was 84.2 mol%.

In conclusion, with this combined catalytic system allows toreach interesting performances also under mild reaction con-ditions (0.5 MPa and 50 °C), involving heterogeneous systemscommercially available, no hazardous and easily recovered byfiltration.

The combined catalyst used in run 16 was recovered by fil-tration and reused 5 times, every time adding new substrate, asshown in Fig. 4.

It is evident that no activity loss was ascertained after 30 h oftotal reaction time.

On the other hand, the repetition of the hydrogenation reactionon the supernatant liquid without the addition of fresh solid cata-lyst evidenced that the reaction does not proceed. This result

allowed to exclude the presence of any active species in thehomogeneous phase. The negligible ruthenium leaching wasalso confirmed by the ICP spectrometry controls performed onthe reaction mixtures.

In order to better clarify the role of the acid co-catalyst, thereaction mechanism was considered. In agreement with theliterature27–28,58–59 the conversion of levulinic acid to GVL mayproceed according to two different mechanisms shown in theScheme 2: via hydrogenation of 4-hydroxypentanoic acid fol-lowed by esterification to gamma valerolactone (path I) or viaesterification of the enol form of levulinic acid to angelicalactone (α-AL) followed by hydrogenation to the desired productGVL (path II).

Thus, to gain insight into the operating pathway, the reactionmixture composition was monitored via GC-MS. The detectedspecies were found to be GVL, pseudo LA, γ-hydroxyvalericacid, and LA. The most relevant by-product was γ-hydroxyvale-ric acid, product of the direct hydrogenation of LA, thus indicat-ing that the first proposed mechanism contributes to the catalyticprocess. The amount of the isomer of LA gradually decreased asthe reaction progressed. The positive effect of the acid co-cata-lyst can be played on both steps, keto group hydrogenation andsuccessive intramolecular lactonization. Thus, in order to demon-strate this last assumption, a series of experiments about thehydrogenation of simple ketones was carried out. To the best ofour knowledge, the effect of the heterogeneous acid co-catalyston ketones hydrogenation has never been highlighted. Parallelruns, with and without the acid components, were carried out onthe two ketones more similar to levulinic acid: 2-butanone(methyl-ethyl-ketone) and 2-pentanone (methyl-propyl-ketone).We made a comparison between the catalytic performances ofRu/C catalyst and its combination with resin A70 and niobiumphosphate (NBP), respectively the most active organic and inor-ganic tested acid co-catalysts. The hydrogenations were carriedout under the condition already applied for LA (70 °C and 0.5MPa of hydrogen pressure): the results are reported in Table 3and Fig. 5.

The achieved yields clearly show that the co-catalytic effect ofthe heterogeneous acid is effective for both ketones: high yieldsunder mild conditions were reached, with the almost complete

Fig. 3 Effect of different heterogeneous acid co-catalysts on LA Con-version (mol%) vs. time (h), employing 5% Ru/C under mild conditions.(runs 15–19, Table 1).

Fig. 4 LA Conversion and Selectivity to GVL (mol%) after 2 h ofreaction in the presence of the catalyst 5% Ru/C and Amberlyst A70(run 16 and successive recycles of the solid catalyst).

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selectivity to the alcohol. The most effective co-catalyst alwaysresulted to be the resin A70.

As far as we know, this result, has never been mentioned yetin the literature. This confirms the importance of the presence of

the acid co-catalyst in the first step of LA hydrogenation, andcould be very important for further applications of carbonylderivatives hydrogenation, even in the field of fine chemicalssynthesis. Further research devoted to better clarify the reactionmechanism is in progress.

Experimental

5% Ru/C (surface area 880 m2 g−1) was purchased from Engel-hard and used as received. 5% Ru/Al2O3 (surface area 77 m2

g−1) was purchased from Fluka and used as received.Levulinic acid, 2-butanone and 2-pentanone (Sigma Aldrich)

were of analytical purity and were used without any furtherpurification. Amberlyst A15 dry and Amberlyst A70 wet werekindly supplied by Rohm and Haas: they were stored at roomtemperature and were employed as received. Niobium phosphate(NbOPO4, NBP) and niobium oxide (Nb2O5·nH2O, NBO, watercontent 20 wt%) were provided from CBMM (Companhia Brasi-leira Metalurgia e Mineracao). NBO was used as received: it hasan effective acidity of 0.21 meq g−1 determined by acid–base

Table 3 Hydrogenation of 2-butanone and of 2-pentanone in thepresence of 5% Ru/C and acid co-catalysts. Reaction conditions: T:70 °C, P H2: 0.5 MPa, H2O: 40 ml, Ru: 1 meq

Run Substrate (g) Acid co-catalyst

Alcohol yield(mol%) after 1 hof reactiona

21 2-Butanone (1.23) — 4022 2-Butanone (1.23) Amberlyst 70, 2.8 meq 9223 2-Butanone (1.23) NBP, 0.25 meq 7424 2-Pentanone (1.46) — 5425 2-Pentanone (1.46) Amberlyst 70, 2.8 meq 9126 2-Pentanone (1.46) NBP, 0.25 meq 84

a The selectivities to the target alcohols are > 99.9%, only traces of thecorresponding ethers being detected.

Scheme 2 Mechanisms for LA hydrogenation.

Fig. 5 Yield (mol%) of 2-butanol (runs 21–23, Table 3) and of 2-pentanol (runs 24–26, Table 3) vs. time (min): effect of the addition of the acidcomponent.

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titration in water using 2-phenyl-ethylamine as the basic probe,which was in good agreement with literature data.52

NbOPO4 (NBP) was treated at 255 °C for 6 h under highvacuum (5 Pa). Its effective acidity, determined by acid–basetitration in water by using 2-phenyl-ethylamine as the basicprobe, resulted 0.33 meq g−1.

Quantitative analyses of the reaction products were performedwith a Perkin Elmer Autosystem gas-chromatograph equippedwith a flame ionization detector and a Zebron ZB-WAX capillarycolumn (30 m × 0.32 mm × 0.5 μm) with a stationary phase ofpolyethylene glycol (PEG). The following temperature programwas used in the analysis: 60 °C (3 min) then increased at a rateof 8 °C min−1 to 200 °C. The detected reaction products werefound to be GVL (retention time: 9.3 min), pseudo LA (retentiontime: 12.1 min), γ-hydroxyvaleric acid (retention time:13.7 min). The sample analysis was confirmed by gas-chromato-graph-mass spectrometer GC-MS carried out using the instru-ment Hewlett–Packard HP 6890 with a MSD HP 5973 detectorusing a G.C. column Phenonex Zebron with a stationary phaseof 100% dimetylpolysiloxane (length of the column: 30 m, innerdiameter: 0.25 mm and thickness of the stationary phase:0.25 μm). The transport gas was helium 5.5 and the flow was1 ml min−1.

The BET surface area was determined by nitrogen adsorption,using a single point ThermoQuest Surface Area Analizer QsurfS1.

The hydrogenation reactions were carried out in a 300 mlmechanically stirred Parr 4560 autoclave equipped with a P.I.D.controller 4843. Before the catalytic test, the catalysts were acti-vated at atmospheric pressure under flowing hydrogen gas,increasing gradually the temperature until 300 °C, maintainingthat value for 3 h. In a typical procedure, the proper amount ofthe chosen ruthenium catalyst and the eventual heterogeneousacid co-catalyst were introduced in the autoclave under inertatmosphere. Then, the autoclave was closed, evacuated up to0.5 mm Hg and a solution containing the proper substrate(1.97 g of levulinic acid or 1.23 g of 2-butanone or 1.46 g of 2-pentanone) in 40 ml of water was introduced inside by suction,followed by the pressurization with hydrogen to 0.2 MPa. After-wards, the reactor was heated up to the chosen temperature and,once it was reached, the autoclave was pressurized with hydro-gen to the desired pressure, maintaining the stirring speed ascer-tained to assure the absence of any mass transfer limitations. Thepressure value was manually held constant at the chosen valueby repeated hydrogen feeds. The course of the reaction was mon-itored by periodically sampling the liquid from a sampling valveand analyzing it by gas-chromatography and GC-MS. When thereaction was complete, the autoclave was rapidly cooled, the gaswas discharged and the liquid mixture immediately analyzedafter dilution with acetone. Recycling experiments of the solidcatalyst were carried out in a similar manner but after removingthrough the sample valve the liquid reaction mixture, the auto-clave containing the solid catalysts was evacuated and againcharged with the fresh levulinic acid solution for a subsequentcatalytic cycle.

Ruthenium leaching was established on the reaction solutionby inductively coupled plasma-optical emission spectrometry(ICP-OES) employing a Spectro-Genesis instrument using asoftware Smart Analyzed Vision.

Conclusions

A sustainable process for the hydrogenation of LA to GVL hasbeen investigated: this new approach involves the combinedeffect of a hydrogenating and an acid heterogeneous componentand enables the adoption of very mild reaction conditionsworking in a green solvent like water.

It has been demonstrated that the presence of the hetero-geneous acid co-catalyst increases the reaction rate, not onlyfavouring the esterification reaction in the final step, but alsothrough the initial activation of the carbonyl group towards thehydrogenation to the intermediate γ-hydroxyvaleric acid. Thesecombined systems resulted significantly efficient also for thehydrogenation of model aliphatic ketones, thus suggesting theirpotential applications for fine chemicals production.

Acknowledgements

The authors thank the project PRIN 2008 Prot. 2008SXASBCfor financial support.

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