γ-Valerolactone (GVL) has been identified as a potentially sustainable platform molecule for the production of renewable fuels and fine chemicals [1-6], and its production is mainly based on the hydrogenation of levulinic acid (LA), which can be directly obtained from cellulosic materials by acidic hydrolysis [1]. A cascade reaction involving hydrogenation and dehydration steps takes place (Scheme 1) under acidic conditions in the presence of metallic catalysts. On this topic a lot of studies have been carried out [7, 8] in particular on Ru [9] or Cu [10] supported on activated carbon (AC) or oxidic supports in different solvents, such as water or alcohols [4-6] and dioxane [11, 12]. The addition of acidic co-catalyst, such as niobium phosphate or oxide, to Ru/AC [13] or the use of acidic carbons [14] has been shown to speed up the reaction rate under mild reaction conditions (70 ℃, 0.3 MPa of H2), keeping a selectivity > 98% towards the GVL product. This result has been attributed to the promotional effect of the acidic sites on the dehydration step (Scheme 1).
The major limitation of most of the heterogeneous catalysts applied in this reaction is the low stability and in this respect the addition of Au to Ru/AC with the formation of a bimetallic phase appeared essential from both activity and durability point of view [15]. The synergistic effect between Au and Ru has been addressed to an alloyed phase due to the Au atom diffusion within the Ru particles. However, it was not possible to disentangle any electronic effects or improved metal-support interaction could not be excluded.
Ir-based catalysts have been poorly investigated in this reaction and only a few reports deals with heterogeneous systems showing that Ir/SiO2 is poorly active and also poorly selective to GVL [16]. It became active (but not selective) only when Mo is added as modifier. Soluble Ir complexes, however, have been reported to show a good activity in levulic hydrogenation as reported for half-sandwich [17] or pincer complexes [18]. A computational study of this latter case evidenced two possible pathways: the first where the limiting rate step is the hydride transfer to give La-H-and LIrH2+, the second where a trihydride iridium complex is formed [19].
Based on this finding we thought that a reducible oxide as TiO2 would have helped the activity of supported Ir particles as in the case of MoOx. Therefore herein we investigate in the reduction of levulinic acid to GVL, Ir/TiO2 catalyst and investigated the effect of Au addition through sequential deposition-precipitation technique. We have previously fully characterized these catalysts and DFT calculations are also available providing us useful information about the interaction with the support (TiO2) [20].
Titania Evonik P25 was used as a support (45 m2/g, nonporous 70% anatase, 30% rutile, purity 99.5%). Commercial HAuCl4·3H2O, IrCl4·4H2O and RuCl3 from Aldrich were used as gold, iridium and ruthenium precursors, respectively. Before preparation, TiO2 was dried in air at 100 ℃ for at least 24 h. NaBH4 of purity > 96% from Fluka was used. Gaseous hydrogen from SIAD was 99.99% pure.
The preparation of the 1 wt% Au/TiO2 and Ir/TiO2 sample was performed by deposition-precipitation with urea (DPU) in the absence of light, following a previously reported procedure [21-24].
The gold precursor HAuCl4 or the iridium precursor IrCl4 (4.2 mmol), and urea (0.42 mol) were dissolved in 50 mL of distilled water. Then, 1 g of titania was added to this solution. The suspension temperature was then increased to 80 ℃ and kept constant for 16 h under stirring. The samples were then washed with water and centrifuged four times, and dried under vacuum at 80 ℃ for 2 h and then calcined at 400 ℃ under H2 atmosphere for 1 h. The samples were stored at room temperature away from light. The metal loading of 1 wt% was obtained.
Monometallic 1% Ru/TiO2 has been prepared by incipient wetness impregnation. Solid RuCl3 (equivalent to Ru 10 mg) was dissolved in water (1.5 mL) and added to 1 g of support (final ruthenium loading of 1 wt%). The catalyst was then dried at 80 ℃ for 2 h and reduced in H2 at 200 ℃ for 2 h.
A sequential deposition method was used to prepare the bimetallic catalyst. Iridium was first deposited on TiO2 by deposition-precipitation with urea as described above. After drying at 80 ℃ for 2 h, the Ir/TiO2 sample was calcined in air at 500 ℃ for 2 h at a heating rate of 2 ℃/min, before gold was deposited by deposition-precipitation. The same procedure of washing, drying and calcination as above was applied. The total loading of the metals was 1 wt% and the molar ratio Au/Ir=1/1. Chemical analysis of Au and Ir in the samples to determine the actual loadings was performed by X-ray fluorescence (XRF) using a spectrometer XEPOS HE (AMETEK). The Au and Ir mass loadings were expressed in grams of each metal per gram of the sample (Table 1).
The Au-Ru was prepared following a sequential impregnation. A solution of NaAuCl4 was added to 0.27 wt% Ru/TiO2 prepared as above reported. After 3 h the catalyst was filtered, dried and then calcined as for the monometallic sample. The total metal loading was 1 wt% and the molar ratio Au/Ru=8/2. The metal content was checked by ICP analysis of the filtrate using a Jobin Yvon JY24 instrument. Morphology of the catalysts was characterized in a Philips CM200 FEG electron microscope, operating at 200 kV and equipped with a Gatan imaging filter, GIF Tridiem.
Levulinic acid (LA) hydrogenation was performed at 70 ℃, using a stainless steel reactor (50 mL capacity), equipped with heater, mechanical stirrer, gas supply system and thermometer. The LA solution (30 mL; 0.3 mol/L) was added into the reactor and the desired amount of catalyst (LA/metal molar ratio=1000) was suspended in the solution. The autoclave was then purged three times with nitrogen before charging 0.7 MPa of H2. The mixture was heated to the reaction temperature, 70 ℃, and mechanically stirred (1250 r/min).
For product analysis, the reaction mixture, after separation from the catalysts by filtration, was analyzed using high performance liquid chromatography (HPLC). Samples were removed periodically (0.5 mL) under stirring and analyzed by HPLC using a column (Alltech OA-10308, 300 mm x 7.8 mm) with UV and refractive index (RI) detection in order to analyze the product mixtures. H3PO4 solution (0.1 wt%) was used as the eluent. The identification of the possible products was done by comparison with the original samples.
In some cases an activation step was performed prior to the reaction and the catalyst was pre-reduced in the autoclave for 1 h at 150 ℃ under 0.3 MPa of H2.
The catalytic results of the monometallic sample are reported in Table 2. Monometallic Ir appeared very active, more than Ru. However, it presented a slight lower selectivity to GVL. The reductive pretreatment carried out in situ seems to have a beneficial effect restoring an almost full selectivity to GVL. The same pretreatment on Ru/TiO2 showed a negligible effect, maintaining very similar activity and selectivity as without the pretreatment. Monometallic gold did not show any activity.
We already demonstrated that when depositing Au on Ru catalyst, a migration of Ru on Au occurred forming Au-core particles with enriched Ru-surface [15]. On the contrary, when depositing Au on Ir catalysts, a segregation of Ir atoms toward the TiO2 surface occurred. Au-Ir bimetallic particles are formed on top of Ir slags [20]. The two catalysts showed a very different catalytic behavior (Table 2). In particular the addition of Au to Ir/TiO2 led to a drastic drop of the activity despite the small size of the metal particles (2.65±1.07 nm [20]) and only 12% of levulinic acid conversion could be obtained after 6 h reaction. Pretreating the catalyst with H2 a slight increase of the conversion was observed (38%) but still lower than the activity of Ir/TiO2 ( > 99%; Table 2). The presence of bimetallic Au-Ir species, however, promoted the selective formation of GVL as shown by the increasing of selectivity ( > 99). On the contrary, Au-Ru/TiO2 showed an enhancement of the activity (conversion after 3 h=86%; Table 2) with respect to monometallic, also maintaining the full selectivity to GVL. A representative TEM image with particle distribution is reported in Figure 1.
The real presence of bimetallic particles and the presence of synergism between Au and Ir (negative effect) or Ru (positive effect) was also studied by carrying out catalytic tests in the presence of physical mixture of the two catalysts: Au/TiO2+Ir/TiO2 or, alternatively, Au/TiO2+Ru/TiO2. In the first case the conversion reflected the composition of the mixture, i.e., consistent with the reduced amount of the active metal Ir. Moreover, the selectivity is basically the same as for monometallic iridium thus supporting the idea that no bimetallic species are formed within the reaction medium. On the contrary, in the case of Au/TiO2+Ru/TiO2 systems the conversion appeared higher than expected on the basis of the amount of the actually present Ru metal (i.e., calculated around 50% instead of 73%, Table 2). This finding can be explained by the in situ formation of alloyed species due probably, to the detachment of Ru particles from the TiO2 surface and the migration onto Au ones. This sort of migration, in the case of Ir, is probably neglected by the strong metal-support interaction of Ir with TiO2 established also by DFT calculations [20].
As reported in [24, 25] Au-Ir/TiO2 showed a higher activity than monometallic ones in the case of CO oxidation as well as in the total oxidation of propene [26]. The synergistic effect was addressed to a hindered reoxidation of Ir and the establishment of Au0-Ir0 interaction. Indeed XPS spectra in bimetallic sample showed the absence of the typical contribution of Ir4+ peak due to the reoxidation of Ir0 exposed to air [26]. In the monometallic sample this latter peak constitutes the main one of Ir. Under reducing conditions the Au0-Ir0 interactions are obviously still present, and therefore we have to address the negative synergistic effect to the Au interference into the redox mechanism of Ir atoms. Under reducing conditions the redox cycle of the metal could be even more influenced by the difficulty of the Ir to be oxidized representing the first elemental step of the reaction (Scheme 1). Previous study on CO chemisorption followed by DRIFT spectra supported the finding that alloying gold to iridium would result in the disappearance of contiguous Ir sites [26]. We thus might conclude that the interference in the redox cycle at the base of the catalytic activity is mostly due to this isolation of Ir sites.
We have tested and compared the activity of Ir/TiO2, Ru/TiO2 and bimetallic catalysts Au-Ir/TiO2 and Au-Ru/TiO2 in the levulinic acid hydrogenation. The bimetallic catalysts were prepared following a two steps procedure. Both catalysts presented a bimetallic structure but Ir provided a stronger metal-support interaction with TiO2 compared to Ru. This will be beneficial for the stabilization of the metals onto the support. However, in the case of iridium, the addition of gold lowered the activity of the catalyst even if it enhanced the selectivity to GVL. A possible reason of the detrimental effect of Au on Ir catalyst is the electronic interaction between the two metals, which apparently affected the redox cycle of Ir. The possible presence of isolated Ir site could be at the origin of this effect.
R. Zanella acknowledge the financial support granted by project UNAM-PAPIIT IN105416. We acknowledge V. Maturano for Au-Ir samples preparation.
2016,
Vol. 37 Issue (10): 1771-1775 
PDF
2016,
Vol. 37 Issue (10): 1771-1775 PDF