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Page 1 of 10 Pleasersc Do Not Advances Adjust Margins RSC Advances This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 10 PleaseRSC do not Advances adjust margins Journal Name ARTICLE Perovskite as nickel catalyst precursor – Impact on catalyst stability on xylose aqueous-phase hydrogenation Received 00th January 20xx, a a c b a Accepted 00th January 20xx Ruddy Morales , Cristian H. Campos , J.L.G Fierro , Marco A. Fraga *, Gina Pecchi *. DOI: 10.1039/x0xx00000x Precursors materials with formula mater La 1-xCe xAl 0.18 Ni 0.82 O3 (x=0.0, 0.1, 0.5, 0.7) were successfully used as precursors to prepare Ni nanoclusters to be used as catalysts in the hydrogenation of xylose to xylitol. For the Ce free and lower Ce www.rsc.org/ content (x=0.0; 0.1) the perovskite structure was obtained, whereas for higher Ce content (x=0.5; 0.7) an ordinary CeO 2- La 2O3 solid dissolution with no perovskite structure were obtained. Under reduction conditions, the perovskite structure lead to ~30% of metallic Ni without loss of the perovskite structure (x=0.0; 0.1) and the CeO 2-La 2O3 solid dissolution allows a Ni reduction of ~40% of Ni (x=0.5; 0.7). As expected, the similar reduced Ni content do not show large differences in the aqueous-phase xylose hydrogenation products distribution, meanwhile, the larger differences in the precursors; perovskite structure (x=0.0; 0.1) and solid dissolution (x=0.5; 0.7) point out the importance of the perovskite structure in the remarkable leaching-resistant showed by Ni perovskite-precursor (x = 0.0; 0.1) catalysts, showing no Ni leaching at all Manuscript over 6 h of reaction in aqueous medium. leading to poisoning of the active sites, and metal leaching. 5 Introduction These technical hurdles motivate the pursuit of alternative catalysts by changing their chemical formulation and/or their Liquid phase hydrogenation of aldoses on porous metal surface architecture through different active surface site catalyst is an important process in the synthesis of polyhydric assemblies. It can be accomplished by exploring different alcohols, which have long been used as natural sweetening agents in the food industry. Xylitol, obtained by the synthesis procedures and catalyst precursors with distinctive hydrogenation of xylose, is a sweetener molecule, which has topology or flexible crystalline structure. been increasingly commercialized 1 due to its attractive Perovskite-type oxides are crystalline structures, represented Accepted properties as high solubility in water, stability upon storage by the general formula ABO 3, where A is a large cation and B is and do not caramelize at elevated temperatures. Moreover, a small cation of the d-transition series. A multitude of the sweetening capacity of xylitol exceeds that of saccharose chemical compositions is possible since a large number of by 20-25% with no insulin requirement. 2,3 chemical elements may be accommodated at both A and B Xylitol is typically produced by an isothermal hydrogenation positions of such flexible structure. Furthermore these process of xylose aqueous solutions in a triphasic batch reactor crystalline structures have a large stability in oxidant on a metallic dispersed catalyst. Wisniak et al. 3,4 studied atmosphere; meanwhile they can be total or partially reduced different metal catalysts in the hydrogenation of glucose, in reductive atmosphere. Due to such interesting properties, they have been extensively exploited as catalyst precursors for fructose and xylose, reporting that the activity of the metals 6,7 2 decreases in the order Ru>Ni>Rh>Pd. The performance of Ni- many reactions. The main advantage of perovskite-like oxide Advances based catalysts stands out due to their lower price compared as catalyst precursor is the possible formation of small and to noble metals and that they can be efficiently used in batch uniform particle size catalyst, which could provide a reactors, with high activity and selectivity to polyalcohols. distinguished catalytic performance. Ni-containing perovskites have been widely investigated as Nevertheless, the major drawback of Raney Ni catalysts is their 8 catalyst precursor for a broad variety of reactions. Natural gas RSC large and fast deactivation due to accumulation of organic 9 10 impurities (from the starting material) on the catalyst surface, and alcohols reforming, water-gas shift reaction , methane combustion 11 and soot oxidation 12 can be listed just to mention a few processes. In the case of LaNiO 3 perovskite, the reducible element (Ni 3+ ) in the perovskite structure can be easily reduced to metallic Ni° dispersed on La 2O3 under 13 appropriate H 2 reduction condition. In hydrogenation reactions, one strategy to tailor catalyst selectivity is the employment of promoters, which can also be accommodated into perovskite catalyst precursor structure leading to a well- This journal is © The Royal Society of Chemistry 20xx J. Name ., 2013, 00 , 1-3 | 1 Please do not adjust margins PleaseRSC do not Advances adjust margins Page 2 of 10 ARTICLE Journal Name dispersed system. Ceria, a well-known promoter with The specific areas were calculated using the BET method from outstanding redox and acid-base properties, has been the nitrogen adsorption isotherms obtained on a exploited in this regard. 14,15 Micromeritics ASAP 2010 apparatus at -196°C. Samples were In this work, perovskite structures were investigated as a previously pretreated at 150°C under nitrogen atmosphere for strategy to stabilize Ni nanoclusters in metal catalysts for 2 h to dehydrate and clean catalysts surface from adsorbed aqueous-phase hydrogenation of xylose to xylitol. All catalyst gases, followed by vacuum. Isotherms were recorded taking 25 presented a nominal Ni loading of 20 wt%, which was ensured relative pressure points ranging from 0.0-1.0 for adsorption by adding Al alongside in B position. Different Ce loadings were and 23 for desorption process. considered to partially substitute La in A position. A full X-ray powder diffraction (XRD) patterns were obtained with structural and textural characterization of synthesized La 1- nickel-filtered CuK α1 radiation ( λ=1.5418 Å) using a Rigaku xCe xAl 0.18 Ni 0.82 O3 (x=0.0, 0.1, 0.5, 0.7) structures was carried diffractometer and collected in the 2 θ range of 20-70° in steps out to explain the activity and selectivity of the oxidized and of 2°C min -1. reduced catalysts. Temperature programmed reduction (TPR) experiments were performed in a TPR/TPD 2900 Micromeritics system with a thermal conductivity detector (TCD). Prior to the reduction Experimental experiments the samples (50 mg) were thermally treated under air stream at 700°C to remove any contaminants on Preparation of the catalysts catalysts surface. The reduction profiles were recorded under -1 -1 La 1-xCe xAl 0.18 Ni 0.82 O3 (x=0.0, 0.1, 0.5, 0.7) materials, all of them 10% H 2/N 2 flow at 25 mL min at a heating rate of 5°C min with 20 wt% of nominal Ni loading and different Ce and La from room temperature to 1000°C. 16 contents, were prepared by self-combustion method. Glycine NH 3-TPD experiments were carried out on the reduced (H 2NCH 2CO 2H), used as ignition promoter, was added to an samples (2 h at 500°C) on a TPR/TPD 2010 apparatus, aqueous solution of metal nitrates with the appropriate saturating the catalyst surface at 100°C with ammonia pulses. Manuscript - stoichiometry to get a NO 3 /NH 2 (molar ratio) = 1. The resulting Then the sample was cooled down to room temperature and, solution was slowly evaporated until a vitreous gel was once the baseline was restored, the temperature linearly obtained. The gel was heated up to over 265 °C, temperature increased up to 500°C. at which the ignition reaction occurs producing a powdered The XPS measurements were performed using a VG Escalab precursor. After the combustion process, the powders were 200R electron spectrometer equipped with a hemispherical ground and subjected to air thermal treatment calcination to electron analyzer and Mg Kα (1253.6 eV) X-ray source. Prior to eliminate the remaining carbon and obtain the perovskite analysis, the samples were reduced in situ under hydrogen at structure. Solids were crushed and sieved to obtain the 500°C for 1 h and transported to the analysis chamber without required particle size (<200 µm) and then finally calcined at a contact with air. Charging effects on the samples were -1 Accepted heating rate of 1°C min up to 700°C and maintained for 5 h. corrected by taking the C1s peak of adventitious carbon at Since hydrogen was used to activate the solids prior to 284.8 eV. The peaks were decomposed into several catalytic activity measurements, the calcined perovskites were components assuming a Gaussian/Lorentzian shape.
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