Lan Yang

Facile synthesis and enhanced catalytic performance of graphene-supported Ni nanocatalyst from a layered double hydroxide-based composite precursor

In this paper, graphene-supported Ni nanocatalyst (Ni/G) was prepared via self-reduction of a hybrid Ni–Al layered double hydroxide/graphene (NiAl-LDH/G) composite precursor. NiAl-LDH/G nanocomposite was assembled via a facile one-step coprecipitation route, which involved the nucleation and growth of NiAl-LDH, simultaneously accompanied by the reduction of graphene oxide without the addition of any reducing agents. The characterization results demonstrated that NiAl-LDH nanoplatelets were homogeneously dispersed on both sides of an exfoliated, structurally flexible graphene The graphene component in the precursor, serving as reducing agent, could in situ reduce Ni2+ species to Ni0 on heating under an inert atmosphere, thus facilitating the formation of highly dispersed Ni nanoparticles with a uniform size. Compared with those prepared by conventional methods, as-formed graphene-supported Ni nanocatalyst exhibited superior catalytic performance in the liquid phase selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde owing to the much higher metal dispersion and smaller size of Ni nanoparticles in the catalyst. The present finding provides a simple approach to fabricate new types of graphene-supported, metal-based heterogeneous catalysts with advanced catalytic performance.

Formation of a mesoporous bioreactor based on SBA-15 and porcine pancreatic lipase by chemical modification following the uptake of enzymes

Taking advantages of the nano-sized pore diameter, large surface area, and high pore volume of SBA-15 as a support, porcine pancreatic lipase (PPL) was immobilized in the mesoporous channels of the support through physical adsorption. The chemical modification was then performed to reduce the pore openings of SBA-15 uptaking PPL, in order to prevent the leaching of PPL. Two procedures were applied to shrink the pore size of SBA-15 entrapping PPL. The first procedure involves the surface grafting of the SBA-15 entrapping enzyme with 3-(trimethoxysilyl)propyl methacrylate (H2CC(CH3)CO2(CH2)3Si (OCH3)3, abbreviated as PMA). The second procedure involves the in situ polymerization of the pendant vinyl groups with free PMA, following the silylation. The assay of enzyme activity shows that the reduction in pore size by chemical modification prevents the PPL leaching as expected. The formation and structure of the resulting inorganic–organic hybrid materials, which can be regarded and used as a mesoporous bioreactor, were investigated in this paper.

Solvent-free oxidation of ethylbenzene over hierarchical flower-like core–shell structured Co-based mixed metal oxides with significantly enhanced catalytic performance

Herein, we reported the development of new and cost-effective cobalt-based metal oxide catalysts for the oxidation of ethylbenzene, which is considered to be of much importance for the production of high value-added raw materials. The heterogeneous Co-based catalyst system, hierarchical flower-like core–shell structured Co–Zn–Al mixed metal oxides supported on alumina (CoZnAl-MMO/Al2O3), was reproducibly prepared by a two-step process, which involved in situ growth of a two-dimensional Co–Zn–Al layered double hydroxide precursor on amorphous alumina microspheres followed by calcination. The materials were characterized by XRD, SEM, TEM, HRTEM, TPR, XPS and nitrogen adsorption–desorption measurement. The results revealed that CoZnAl-MMO/Al2O3 catalysts exhibited high dispersion of cobalt species due to well-developed three-dimensional flower-like CoZnAl-MMO platelets as well as the separating effect of the resulting ZnO phase. As-synthesized CoZnAl-MMO/Al2O3 catalysts were studied in the oxidation of ethylbenzene without the addition of any solvent and additive using tert-butyl hydroperoxide as the oxygen source and showed much higher catalytic activity and selectivity for acetophenone compared with the conventional supported Co-based catalyst prepared by incipient impregnation. Furthermore, such cost-effective CoZnAl-MMO/Al2O3 catalysts possessed high stability and could be reused at least three times without remarkable loss of the catalytic activity.

Surface synergistic effect in well-dispersed Cu/MgO catalysts for highly efficient vapor-phase hydrogenation of carbonyl compounds

The highly efficient vapor-phase selective hydrogenation of carbonyl compounds (e.g. furfural (FAL) and dimethyl 1,4-cyclohexane dicarboxylate (DMCD)) to corresponding alcohols was achieved excellently over well-dispersed MgO-supported copper catalysts (Cu/MgO), which were prepared by an alternative separate nucleation and aging step method. The characterization results revealed that the structure and catalytic performance of the as-formed Cu/MgO catalysts were profoundly affected by Cu loading. Especially, the results confirmed that the decrease in the Cu loading could lead to the improvement of metal dispersion and the formation of more surface strong Lewis basic sites. In the vapor-phase selective hydrogenation of FAL to furfuryl alcohol (FOL) and DMCD to 1,4-cyclohexane dimethanol (CHDM), two Cu/MgO catalysts with Cu loadings of 27.6 wt% and 70.9 wt% exhibited superior catalytic performance with higher conversions (>97.3%) and selectivities to alcohols (>96.0%) compared to the other supported ones. The high efficiency of the as-formed Cu/MgO catalysts was mainly attributed to the surface synergistic catalytic effect between the catalytically active metallic copper species and the Lewis basic sites, which held the key to the hydrogenation reaction related to the hydrogen dissociation and the activation of the carbonyl groups.

Enhanced visible-light-induced photocatalytic performance of a novel ternary semiconductor coupling system based on hybrid Zn–In mixed metal oxide/g-C3N4 composites

Hybrid composites of Zn–In mixed metal oxides (ZnIn-MMO) and g-C3N4 were synthesized by a facile thermal decomposition of Zn–In layered double hydroxide (ZnIn-LDH) and melamine mixture precursors. The structural and optical properties of the ZnIn-MMO/g-C3N4 composites were characterized by powder X-ray diffraction, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectra, photoluminescence spectra, electron spin resonance and transient absorption spectra. The results indicated that ZnIn-MMO nanoparticles were well distributed over the surface of the g-C3N4 sheets formed in situ. Compared with pristine ZnIn-MMO, the as-synthesized ZnIn-MMO/g-C3N4 nanohybrids showed stronger absorption in the visible light region. Furthermore, the ZnIn-MMO/g-C3N4 composite with a g-C3N4 amount of 36 wt% exhibited significantly enhanced photodegradation activity for Rhodamine B under visible light irradiation, in comparison with pure g-C3N4 and ZnIn-MMO, which was attributable to the unique heterostructure of the ternary semiconductor coupling system composed of g-C3N4, In2O3 and ZnO in the composites, facilitating efficient transportation and separation of the photogenerated electron–hole pairs and thus the continuous generation of reactive oxygen species. The present finding provides a simple approach for fabricating new types of visible-light-induced g-C3N4-based semiconductor composite photocatalysts for pollutant degradation in advanced oxidation processes.

The promotional effect of ZnO addition to supported Ni nanocatalysts from layered double hydroxide precursors on selective hydrogenation of citral

A series of highly-dispersed, ZnO-modified supported nickel nanocatalysts (Ni–ZnO/C) were prepared via in situ self-reduction of hybrid Ni–Zn–Al layered double hydroxide/carbon (NiZnAl–LDH/C) nanocomposite precursors. The materials were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), scanning transmission electron microscopy (STEM), ammonia temperature-programmed desorption (NH3-TPD), and X-ray photoelectron spectroscopy (XPS). The effect of ZnO addition on the catalytic properties of as-synthesized Ni–ZnO/C catalysts for liquid phase selective hydrogenation of citral to citronellol was examined. It was found that ZnO addition significantly modified their catalytic hydrogenation properties for citral, inducing an improved selectivity toward citronellol. A maximum yield of citronellol (~92%) was achieved when the bulk Zn/Ni atomic ratio was 0.25 in the catalysts. This promotional effect was mainly related to the existence of a ZnO–metal interaction, which was proposed to be responsible for enhanced adsorption of the CO bond in the citral molecule on the surface of catalysts and thus for activation of the CO bond.

Synthesis of highly dispersed boron-promoted nickel nanocatalysts and significantly enhanced catalytic performance in hydrodechlorination of chlorobenzene

Control of the dispersion and size of metallic nanoparticles, as well as metal–support interaction, is of vital importance to enhance the catalytic performance of supported metal nanocatalysts. In this work, carbon-supported boron-promoted Ni nanocatalysts (B–Ni) were synthesized via an in situ self-reduction process of hybrid borate-intercalated NiAl-layered double hydroxide/carbon nanocomposites, and the promotional effect of boron on the catalytic performance of Ni nanocatalysts formed in liquid phase hydrodechlorination of chlorobenzene was studied. A series of XRD, TEM, STEM, XPS, low temperature N2 adsorption, and H2 chemisorption results revealed that the resulting spherical B-modified Ni nanoparticles were homogeneously dispersed and anchored tightly on the surface of the carbon support. A suitable amount of boron was essential for the formation of highly dispersed and uniform nanoparticles and pronounced surface Ni–B interaction, as well as strong Ni–B–support interactions, accounting for the significantly enhanced hydrodechlorination activity, in comparison with a B-free Ni catalyst. Moreover, as-synthesized B–Ni nanocatalysts exhibited good stability, without obvious aggregation and loss of active species after five recycles.

Surface Lewis acid-promoted copper-based nanocatalysts for highly efficient and chemoselective hydrogenation of citral to unsaturated allylic alcohols

Chemoselective hydrogenation of α,β-unsaturated aldehydes or ketones to unsaturated alcohols (UAs) is one of the key processes for the production of various important intermediate chemicals. In the present work, well-dispersed ZnO-promoted supported copper nanocatalysts were generated from Cu–Zn–Al layered double hydroxide (CuZnAl-LDH) precursors for liquid-phase chemoselective hydrogenation of citral to allylic alcohols (geraniol and nerol isomers). A series of characterizations including XRD, TEM, STEM, XPS, H2-TPR, and Py-IR demonstrated that the microstructure and catalytic performance of as-formed Cu-based nanocatalysts were significantly affected by the incorporation of Zn into catalyst precursors. It was found that the addition of more ZnO to catalysts could result in better metal dispersion and an increase in the surface Cu+/(Cu+ + Cu0) ratio and surface Lewis acid sites. In liquid-phase chemoselective hydrogenation of citral, a high selectivity toward allylic alcohols (>75%) at complete citral conversion was achieved successfully on as-formed non-noble-metal Cu-based nanocatalysts with a Cu/Zn molar ratio of 2u2006:u20061 under mild reaction conditions (e.g. 80 °C, 1.0 MPa). The high efficiency of the catalysts was attributed mainly to both the synergism between Cu0 and Cu+ species and the promotion of surface Lewis acid sites, thereby improving the dissociation of hydrogen and facilitating the adsorption of the citral molecule and the following activation of the carbonyl group during the citral hydrogenation.

Direct synthesis of hybrid layered double hydroxide–carbon composites supported Pd nanocatalysts efficient in selective hydrogenation of citral

This present study reports a facile one-pot strategy for the direct synthesis of hybrid layered double hydroxide (LDH)–carbon composites supported palladium nanocatalysts by the in situ reduction of PdCl42−-intercalated MgAl–LDH combined with amorphous carbon under mild hydrothermal conditions. The results demonstrated that most of the Pd(II) species intercalated in the interlameller space of MgAl–LDH could be reduced in situ to metallic Pd0 species, and simultaneously, the hybrid structure of the LDH–C composites facilitated the formation of uniform Pd nanoparticles with small diameter, as well as the strong metal–support interactions. Furthermore, with the decreasing proportion of the LDH component in LDH–C composites, the average diameter of Pd nanoparticles decreased progressively and the metal–support interactions were weakened. The as-formed supported Pd nanocatalyst with Pd loading of 5.5 wt% was found to show a superior catalytic activity in the liquid-phase selective hydrogenation of citral than other supported Pd nanocatalysts, while the one with the Pd loading of 2.7 wt% yielded a much higher yield of citronellal (∼80.0%) at 100% conversion. The catalytic performance of Pd nanocatalysts was proposed to be mainly related to both the metal–support interactions and the compositions of hybrid LDH–C composite supports.

Acid–base sites synergistic catalysis over Mg–Zr–Al mixed metal oxide toward synthesis of diethyl carbonate

In heterogeneous catalysis processes, development of high-performance acid–base sites synergistic catalysis has drawn increasing attention. In this work, we prepared Mg/Zr/Al mixed metal oxides (denoted as Mg2ZrxAl1−x–MMO) derived from Mg–Zr–Al layered double hydroxides (LDHs) precursors. Their catalytic performance toward the synthesis of diethyl carbonate (DEC) from urea and ethanol was studied in detail, and the highest catalytic activity was obtained over the Mg2Zr0.53Al0.47MMO catalyst (DEC yield: 37.6%). By establishing correlation between the catalytic performance and Lewis acid–base sites measured by NH3-TPD and CO2-TPD, it is found that both weak acid site and medium strength base site contribute to the overall yield of DEC, which demonstrates an acid–base synergistic catalysis in this reaction. In addition, in situ Fourier transform infrared spectroscopy (in situ FTIR) measurements reveal that the Lewis base site activates ethanol to give ethoxide species; while Lewis acid site facilitates the activated adsorption of urea and the intermediate ethyl carbamate (EC). Therefore, this work provides an effective method for the preparation of tunable acid–base catalysts based on LDHs precursor approach, which can be potentially used in cooperative acid–base catalysis reaction.