Adrian Ungureanu

Composition-Dependent Morphostructural Properties of Ni–Cu Oxide Nanoparticles Confined within the Channels of Ordered Mesoporous SBA-15 Silica

NiO and NiO-CuO polycrystalline rodlike nanoparticles were confined and stabilized within the channels of ordered mesoporous SBA-15 silica by a simple and viable approach consisting in incipient wetness impregnation of the calcined support with aqueous solutions of metal nitrates followed by a mild drying step at 25 °C and calcination. As revealed by low- and high-angle XRD, N2 adsorption/desorption, HRTEM/EDXS and H2 TPR analyses, the morphostructural properties of NiO-CuO nanoparticles can be controlled by adjusting their chemical composition, creating the prerequisites to obtain high performance bimetallic catalysts. Experimental evidence by in situ XRD monitoring during the thermoprogrammed reduction indicates that the confined NiO-CuO nanoparticles evolve into thermostable and well-dispersed Ni-Cu heterostructures. The strong Cu-Ni and Ni-support interactions demonstrated by TPR and XPS were put forward to explain the formation of these new bimetallic structures. The optimal Ni-Cu/SBA-15 catalyst (i.e., Cu/(Cu+Ni) atomic ratio of 0.2) proved a greatly enhanced reducibility and H2 chemisorption capacity, and an improved activity in the hydrogenation of cinnamaldehyde, as compared with the monometallic Ni/SBA-15 or Cu/SBA-15 counterparts, which can be associated with the synergism between nickel and copper and high dispersion of active components on the SBA-15 host. The unique structure and controllable properties of both oxidic and metallic forms of Ni-Cu/SBA-15 materials make them very attractive for both fundamental research and practical catalytic applications.

Synthesis of highly thermostable copper-nickel nanoparticles confined in the channels of ordered mesoporous SBA-15 silica

CuNi nanoparticles were effectively confined in the mesopores of SBA-15 silica by a simple incipient wetness impregnation method. After impregnation, the samples were dried at room temperature, which is considered as the key preparation step to obtain high and stable dispersions of the supported CuNi nanoparticles, irrespective of the thermal conditions during the calcination and reduction steps. The catalysts were systematically characterized by powder X-ray diffraction at low and high angles, transmission electron microscopy and nitrogen physisorption, hydrogen temperature-programmed reduction, in situXRD after temperature-programmed reduction, hydrogen chemisorption as well as by catalytic tests for the hydrogenation of cinnamaldehyde in liquid phase. Characterization revealed a strong interaction between Cu and Ni, resulting in improved reducibility as compared to either Cu or Ni monometallic materials. Moreover, the complete interdiffusion of Cu and Ni atoms to form continuous solid alloy solutions is prevented due to the stabilization of Ni by 1 : 1 nickel phyllosilicate. As a result, the bimetallic CuNi/SBA-15 materials present two distinct metallic phases, one rich in copper and another rich in nickel. At the calcination temperature of 500 °C, the materials displayed the highest chemisorption capacity and highest catalytic activity, as well. Nevertheless, the chemoselectivity of supported CuNi/SBA-15 to cinnamyl alcohol or hydrocinnamaldehyde does not depend on the calcination conditions.

Selective Hydrogenation of Furfural to Furfuryl Alcohol in the Presence of a Recyclable Cobalt/SBA‐15 Catalyst

The hydrogenation of furfural to furfuryl alcohol was performed in the presence of a Co/SBA-15 catalyst. High selectivity (96 %) at a conversion higher than 95 % is reported over this catalytic system. As the conversion of furfural to furfuryl alcohol occurs over metallic Co sites, the effect of reduction temperature, H2 pressure, and reaction temperature were studied. Optimum reaction conditions were: 150 °C, 1.5 h, 2.0 MPa of H2 . The catalyst was recyclable, and furfuryl alcohol was recovered with a purity higher than 90 %. The effect of the solvent concentration was also studied. With a minimum of 50 wt % of solvent, the selectivity to furfuryl alcohol and the conversion of furfural remained high (both over 80 %). Likewise, the activity of the catalyst is maintained even in pure furfural, which confirms the real potential of the proposed catalytic system. This catalyst was also used in the hydrogenation of levulinic acid to produce γ-valerolactone selectively.

NiAl and CoAl materials derived from takovite-like LDHs and related structures as efficient chemoselective hydrogenation catalysts

The catalytic performance of metallic catalysts derived from layered double hydroxide (LDH) precursors with nickel or cobalt incorporated in the brucite-like layers besides aluminium (i.e., NiAl takovite and related CoAl) was investigated for the first time in the chemoselective hydrogenation of cinnamaldehyde. The precursors in the as-synthesized and calcined forms were thoroughly analysed by ICP, XRD, nitrogen physisorption, DR UV-vis spectroscopy, TPR and in situ XRD after temperature-programmed reduction. According to the XRD results, both as-synthesized samples contained purely LDH phases with degrees of crystallization depending on the nature of incorporated metal cations (i.e., the CoAl sample presents larger crystallites than the NiAl takovite-like sample). For the calcined NiAl and CoAl samples, well-crystallized oxide phases of NiO and Co3O4, respectively, besides amorphous alumina and corresponding spinels were evidenced by XRD. The TPR and in situ XRD results for the calcined samples indicated strong metal–alumina interactions, which resulted in depressed sintering of evolved metal nanoparticles in the high-temperature range of 600–800 °C, in well agreement with TEM analysis. Materials derived from the studied LDH systems are found to be efficient catalysts for the hydrogenation of cinnamaldehyde, showing enhanced control over the activity or selectivity, depending on the nature of active metals and the thermal regime of the catalyst activation.

Nanosized transition metals in controlled environments of phyllosilicate–mesoporous silica composites as highly thermostable and active catalysts

Stabilization of transition metals in nano-phyllosilicate phases generated by digestion of mesoporous silica is reported as an efficient route for the formation of highly dispersed metallic nanoparticles with outstanding catalytic activity.

Enhancing the performance of SBA-15-supported copper catalysts by chromium addition for the chemoselective hydrogenation of trans-cinnamaldehyde

SBA-15-supported copper–chromium mixed oxide nanoparticles (CuCr/SBA-15) were prepared by incipient wetness impregnation followed by mild drying at 25 °C and calcination. The Cu:Cr weight ratios were 1:1, 5:1, and 10:1, at a constant total loading of 5 wt%. Monocomponent SBA-15-supported Cu–oxide (Cu/SBA-15) and Cr–oxide (Cr/SBA-15) were prepared as reference samples. The materials were systematically characterized by XRD at low and high angles, N2 physisorption, DR UV-vis, FT-IR, and XPS spectroscopies, and TPR. XRD at low angles and N2 physisorption confirmed the preservation of the mesoporous structure of the SBA-15 support after impregnation and calcination. In the case of monocomponent samples, CuO appeared poorly dispersed, while the Cr species (Cr2O3, mono- and polychromates) appeared highly dispersed on the surface of the SBA-15 support. The progressive addition of chromium to copper had positive effects on the average crystallite size of CuO, which decreased from ∼28 nm (Cu/SBA-15) to ∼3 nm (CuCr/SBA-15; Cu:Cr = 1:1), and on reducibility, as well. Metallic active phases were obtained by reducing of the oxide phases under a hydrogen flow at 350 °C. By comparison to the monometallic catalysts, the reduced CuCr/SBA-15 materials were active in the hydrogenation of cinnamaldehyde and chemoselective towards cinnammyl alcohol (>50 mol%). The activity can be correlated with the particle size of copper, whereas the high selectivity to unsaturated alcohol can be associated with the presence of dual Cu0–Crn+ sites.

Effect of chemical composition of SBA-15 on the adsorption and catalytic activity of α-chymotrypsin

The adsorption of α-chymotrypsin (α-CT) on Al-containing mesoporous materials based on SBA-15 topology was investigated. The insertion of various amounts of aluminium was achieved using the two-step (pH-adjusting) method and the physico-chemical properties of these materials were characterized by small angle X-ray diffraction, nitrogen adsorption/desorption isotherm curves and 27Al MAS NMR spectroscopy, ammonia adsorption microcalorimetry and chemical analyses. The amount of adsorbed enzyme was found to be dependent on the Si/Al ratio of solid surfaces, pH of adsorption and, in the case of Al-containing material, on the ionic strength. Various mechanisms of adsorption are discussed. The activity of immobilized α-CT was examined using the transesterification of N-acetyl-L-phenylalanine ethyl ester with 1-propanol as test-reaction. The catalytic efficiency is strongly dependent on the nature of the support. In order to reduce the leaching of enzyme from the support surface and improve its reusability, cross-linking of the adsorbed enzyme was carried out by glutardialdehyde. In general, these mesoporous silica materials are attractive candidates for enzyme immobilization and biomedical applications due to their high surface area, tunable surface properties and pore size, large pore volume and biocompatibility.

An efficient route to prepare highly dispersed metallic copper nanoparticles on ordered mesoporous silica with outstanding activity for hydrogenation reactions

Copper nanoparticles at relatively high metal loading of 10 wt.% were successfully synthesized via incipient wetness impregnation by using polyether-functionalized ordered mesoporous silica as organic–inorganic hybrid supports. The effect of functionalized triblock copolymer Pluronic P123 (0, 25 and 50 wt.% of total triblock copolymer in gel) on the structural and catalytic properties of copper nanocomposite materials was specifically investigated. The oxide forms of materials were systematically characterized by nitrogen physisorption, SAXS, WAXS, TEM, EDXS, DR UV–vis and TPR, while the metallic forms were analysed by N2O chemisorption, WAXS and TEM. The results indicated that the use of mesoporous silica hybrids leads to highly dispersed supported copper nanoparticles (dispersions in the range of 43–57%) displaying excellent activity in the hydrogenation of cinnamaldehyde. The average particle size was shown to decrease from 2.3 to 1.7 nm with the increase in the amount of functionalized triblock copolymer from 0 to 50 wt.%. Under standard test conditions, all the nanosized copper catalysts showed high catalytic activity but selectivity towards the saturated aldehyde. Further improvements in activity and selectivity towards the unsaturated alcohol were achieved by increasing the hydrogen pressure and applying a two-step reduction–oxidation pre-treatment, respectively. The structure and controllable properties of nanoscale copper materials developed herein, both oxidic and metallic forms, make them very attractive candidates for both fundamental research and practical catalytic applications.

Selective conversion of styrene oxide to 2-phenylethanol in cascade reactions over non-noble metal catalysts

The catalytic hydrogenation of styrene oxide (SO) and phenylacetaldehyde (PAA) to 2-phenylethanol (2-PEA), with H2 in a tri-phase system, was investigated under various reaction conditions using supported Co and Ni metal catalysts. The catalysts were produced from tri-component layered double hydroxides (i.e., hydrotalcite-like compounds (Ni)CoMgAl with 1 : 1 : 1 molar ratios), after calcination and reduction of the layered double hydroxides precursors. Among the tested solids, the metallic Co-based catalyst has proven to be an efficient material for the selective hydrogenation of PAA towards 2-PEA. Taking into account the high activity and selectivity of Al-SBA-15 for SO isomerization to PAA, a two-step process was proposed as an original and highly selective route for producing 2-PEA from SO. This process involves cheap catalysts and it includes a first step of SO isomerization reaction over a mild acid strength mesoporous solid followed by hydrogenation of the resulted PAA over a redox metallic catalyst.

Improved dispersion of transition metals in mesoporous materials through a polymer-assisted melt infiltration method

Melt infiltration (MI) has been described as an effective way to disperse transition metals (TM) in ordered mesoporous supports. The alternative method described here is based on the infiltration of molten precursors into the support pores in the presence of a support polymer porogen. The resulting materials contain metal oxide particles (metallic particles after reduction) smaller than 2 nm, dispersed within the support micropores, which prove to be stable upon reduction up to 900 °C and to exhibit improved catalytic performances for the hydrogenation of cinnamaldehyde due to the high fraction of the active metal exposed.