Emil Dumitriu

Effects of channel structures and acid properties of large-pore zeolites in the liquid-phase tert-butylation of phenol

The liquid-phase alkylation of phenol with tert-butanol in the presence of the H+ form of the zeolites with FAU, BEA, and MOR topologies was investigated. Over dried catalysts, tert-butyl phenyl ether is the main product of alkylation. The accumulation of water molecules on the catalyst surface suppresses this reaction. The irreversible C-alkylation of phenol becomes the main reaction only after an induction period. The catalytic activity and selectivity are controlled by the porous structure and acid properties of the zeolite. The three-dimensional interconnecting pore system of Beta and USY zeolites showed a higher catalytic activity than the monodimensional system of pores of the MOR zeolites. As concerns the catalytic selectivity, the USY zeolites were selective for the ortho-butylation, whereas BEA zeolites were the most efficient catalysts for the para isomer forming.

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.

Non-hydrolytic synthesis of mesoporous silica-titania catalysts for the mild oxidation of sulfur compounds with hydrogen peroxide.

A SiO(2)-TiO(2) mesoporous xerogel prepared in one-step by a non-hydrolytic route shows excellent performance in the mild oxidation of sulfides, sulfoxides and thiophenes with aqueous solutions of H(2)O(2).

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.

Immobilization of Thermoanaerobium brockii alcohol dehydrogenase on SBA-15

The activity of Thermoanaerobium brockii alcohol dehydrogenase (TBADH) adsorbed on mesoporous silica SBA-15 was compared with that of the free enzyme in water and in biphasic system (water phase up to 50% v/v water). TBADH was active at a water concentration ≥10% v/v. In the reduction reaction of sulcatone to sulcatol carried out in biphasic systems, the yield obtained with SBA-15-adsorbed TBADH was up to 5.5-fold higher than that with the free enzyme, which suggests a higher stability of the immobilized enzyme toward the organic solvent. The nature of the organic solvent substantially influenced the degree of conversion that, for example, was 7.4% in toluene and 31.6% in petroleum ether.

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.