Fernando Cárdenas-Lizana

Exclusive production of chloroaniline from chloronitrobenzene over Au/TiO2 and Au/Al2O3.

The gas-phase continuous hydrogenation of p-chloronitrobenzene (p-CNB) over 1 mol% Au/TiO2 and Au/Al2O3 was compared for the first time. Both catalysts exhibit 100% selectivity in terms of -NO2 group reduction, resulting in the sole formation of p-chloroaniline (p-CAN). Au/TiO2 exhibited a narrower particle size (1-10 nm) distribution than Au/Al2O3 (1-20 nm) and a smaller surface-area-weighted mean Au size (6 nm versus 9 nm). Au/TiO2 delivered a higher specific hydrogenation rate (by a factor of up to four), a response that is discussed in terms of Au particle size and a possible contribution of the support to p-CNB activation. A CNB isomer reactivity sequence was established, that is, o> p> m, which is attributed to resonance stabilisation effects. The results presented establish a basis for the development of a sustainable alternative route for the production of haloamines.

The development of gold catalysts for use in hydrogenation reactions

With increasing emphasis placed on cleaner chemical synthesis, energy efficiency and waste minimisation, the manufacture of pharmaceuticals and fine chemicals is undergoing a progressive shift from conventional stoichiometric organic processes to a harnessing of catalytic selectivity. In hydrogenation processes, gold catalysts have untapped potential in terms of selectivity in the reduction of a target functionality in multifunctional reactants. This Review provides a comprehensive evaluation of the catalytic applications of Au in hydrogenation, assessing the benefits relative to conventional transition metal (e.g. Pt, Pd and Ni) catalytic systems. Hydrogenation activity requires the formation of nanoscale Au particles that are (typically) anchored to oxide supports. The crucial catalyst structural and surface properties required to achieve enhanced hydrogenation performance in terms of rate, selectivity and stability are discussed. The synthesis procedures and characterisation methodologies directed at catalyst optimisation are evaluated. The practical application of Au catalysts is illustrated taking, as a case study, the hydrogenation of nitroaromatics, where critical features such as hydrogen adsorption/activation, structure sensitivity, metal–support interactions and active site characteristics are discussed. Commonality with the catalytic action of supported Ag is flagged with a consideration of the future outlook and direction for selective hydrogenation using Au catalysts.

Support effects in the selective gas phase hydrogenation ofp-chloronitrobenzene over gold

The catalytic continuous gas phase hydrogenation of p-chloronitrobenzene (P=1 atm;T=423 K) has been investigated over a series of oxide (Al2O3, TiO2, Fe2O3 and CeO2) supported Au (1 mol %) catalysts. The application of two catalyst synthesis routes,i.e. impregnation (IMP) and deposition-precipitation (DP), has been considered where the DP route generated smaller mean Au particle sizes (1.5-2.8 nm) compared with the IMP preparation (3.5-9.0 nm). The catalysts have been characterised in terms H2 chemisorption and BET area measurements where the formation of metallic Au post-activation has been verified by diffuse reflectance UV-Vis, XRD and HRTEM analyses.p-Chloroaniline was generated as the sole reaction product over all the Au catalysts with no evidence of C-Cl and/or C-NO2 bond scission and/or aromatic ring reduction. The specific hydrogenation rate increased with decreasing Au particle size (from 9 to 3 nm), regardless of the nature of the support. This response extends to a reference Au/TiO2 catalyst provided by the World Gold Council. A decrease in specific rate is in evidence for smaller particles (< 2 nm) and can be attributed to a quantum size effect. The results presented establish the basis for the design and development of a versatile catalytic system for the clean continuous production of high value amino compounds under mild reaction conditions.

Alumina-Supported Ni-Au: Surface Synergistic Effects in Catalytic Hydrodechlorination

Catalytic gas‐phase hydrodechlorination (HDC) of 2,4‐dichlorophenol (2,4‐DCP) has been investigated over Ni/Al2O3 and Au/Al2O3 prepared by impregnation, and Au–Ni/Al2O3 prepared by reductive deposition of Au onto Ni. Catalyst activation by temperature‐programmed reduction is examined and the activated catalysts are characterized in terms of H2 chemisorption, XRD and TEM‐energy dispersive X‐ray (EDX) measurements. Ni/Al2O3 (<1–10 nm) and Au/Al2O3 (<1–15 nm) exhibit a relatively narrow metal size distribution while Au–Ni/Al2O3 bore larger particles (1–30 nm) with variable surface Ni/Au ratios. Au/Al2O3 exhibits low H2 uptake and low HDC activity to generate 2‐chlorophenol (2‐CP) as the sole product. H2 chemisorption on Au–Ni/Al2O3 was approximately five times lower than that recorded for Ni/Al2O3 but both catalysts delivered equivalent initial HDC activities. Ni/Al2O3 exhibits an irreversible temporal deactivation where partial dechlorination to 2‐CP is increasingly favored over full dechlorination to phenol. In contrast, thermal treatment of Au–Ni/Al2O3 in H2 after reaction elevates HDC activity with a preferential full HDC to phenol. This response is linked to a surface reconstruction resulting in a more homogeneous combination of Ni and Au. This result was also achieved by a direct treatment of Au–Ni/Al2O3 with HCl. A parallel/ consecutive kinetic model is used to quantify the catalytic HDC response.

Gold nano–particles supported on hematite and magnetite as highly selective catalysts for the hydrogenation of nitro–aromatics

The catalytic action of nano–sized Au particles supported on hematite (Fe2O3) and magnetite (Fe3O4) is compared in the continuous gas phase hydrogenation of p–chloronitrobenzene and m–dinitrobenzene. The catalysts were prepared by deposition–precipitation and have been characterised in terms of BET/pore volume, powder X–ray diffraction (XRD), temperature programmed reduction (TPR), H2 chemisorption, high–resolution transmission electron microscopy (HRTEM) and X–ray photoelectron spectroscopy (XPS) measurements. XRD confirmed the formation Fe2O3, which was transformed into Fe3O4 during TPR to 673 K with a concomitant decrease in BET area and pore volume. Post–TPR to 423 K, Au/Fe2O3 exhibited well dispersed pseudo–spherical Au particles with mean diameter = 2.0 nm. HRTEM and XPS demonstrate the encapsulation of Au in the Fe3O4 matrix after TPR to 423 K, which inhibited hydrogenation rate. Thermal treatment to 673 K resulted in the segregation of Au on the Fe3O4 surface and the formation of nano–scale particles with mean diameter = 4.0 nm. Similar activities were recorded over both Au/Fe2O3 and Au/Fe3O4 with exclusive nitro–group reduction to yield p–chloroaniline and m–nitroaniline, a response that is discussed in terms of Au electronic character.

Nano-scale Au supported on Fe3O4: characterization and application in the catalytic treatment of 2,4-dichlorophenol

Catalytic hydrodechlorination (HDC) is an effective means of detoxifying chlorinated waste. Gold nanoparticles supported on Fe(3)O(4) have been tested in the gas phase (1 atm, 423 K) HDC of 2,4-dichlorophenol. Two 1% w/w supported gold catalysts have been prepared by: (i) stepwise deposition of Au on α-Fe(2)O(3) with subsequent temperature-programmed reduction at 673 K (Au/Fe(3)O(4)-step); (ii) direct deposition of Au on Fe(3)O(4) (Au/Fe(3)O(4)-dir). TEM analysis has established the presence of Au at the nano-scale with a greater mean diameter (7.6 nm) on Au/Fe(3)O(4)-dir relative to Au/Fe(3)O(4)-step (4.5 nm). We account for this difference in terms of stronger (electrostatic) precursor/support interactions in the latter that can be associated with the lower pH point of zero charge (with respect to the final deposition pH) for Fe(2)O(3). Both catalysts promoted the preferential removal of the ortho-Cl substituent in 2,4-dichlorophenol, generating 4-chlorophenol and phenol as products of partial and total HDC, respectively, where Au/Fe(3)O(4)-step delivered a two-fold higher rate (2 × 10(-4) mol(Cl) h(-1) m(Au)(-2)) when compared with Au/Fe(3)O(4)-dir. This unprecedented selectivity response is attributed to activation of the ortho-C-Cl bond via interaction with electron-deficient Au nanoparticles. The results demonstrate the feasibility of a controlled recovery/recycling of chlorophenol waste using nano-structured Au catalysts.

Promotional Effect of Ni in the Selective Gas-Phase Hydrogenation of Chloronitrobenzene over Cu-based Catalysts

We present 100 % selectivity to p‐chloroaniline through the continuous gas‐phase (atmospheric pressure; T=393–523 K) catalytic hydrogenation of p‐chloronitrobenzene over an activated CuAl (molar Cu/Al ratio 3:1) hydrotalcite. The synthesis by continuous co‐precipitation and application of a CuNiAl (atomic Cu/Ni/Al ratio 2.75:0.25:1) catalyst is also described. Temperature‐programmed reduction in hydrogen and XPS analyses of the ternary catalyst suggest CuNi interaction with no evidence of (bulk) alloy formation (based on XRD). Under the same reaction conditions, CuNiAl exhibited a higher (by up to a two‐fold) hydrogenation rate when compared to CuAl, while the exclusive nitro‐group reduction was maintained.

Selective Alkyne Hydrogenation over Nano-metal Systems: Closing the Gap Between Model and Real Catalysts for Industrial Applications

The relationship between catalytic response and properties of the active phase is difficult to establish in classical heterogeneous catalysis due to the number of variables that can affect catalytic performance. Ultrahigh-vacuum surface methods applied to model catalyst surfaces are useful tools to assess fundamental issues related to catalytic processes but they are limited by the significant differences with catalysts in the working state. In an attempt to overcome this issue, (unsupported) nano-metal systems with controlled size and shape have been synthesized and tested in selective alkyne hydrogenation. The results revealed a dependency of nano-particles (NPs) morphology (size and shape) and allowed the identification of the active sites for this type of reaction. The nature of the stabilizer (steric and electrostatic stabilization) used in the NPs preparation has been shown to influence catalytic performance. The tailored active phase was subsequently immobilized on suitable nano- and micro-structured inorganic (e.g. 3D sintered metal fibers) supports with controlled surface properties in order to corroborate if the results obtained on the optimized nano-metal systems could be extrapolated to real catalysts. This article highlights the advantages and limitations of the analysis of selective alkyne hydrogenation over nano-metal systems that close the gap between model and real catalysts where the main challenges that lie ahead are summarized.

Production of Butylamine in the Gas Phase Hydrogenation of Butyronitrile over Pd/SiO2 and Ba-Pd/SiO2

Abstract The gas phase (1 atm, 473-563 K) hydrogenation of butyronitrile has been studied over Pd/SiO2 and Ba-Pd/SiO2. Catalysts characterisation involved TPR, H2/NH3 chemisorption/TPD, XRD and TEM measurements. The incorporation of Ba with Pd resulted in the formation of smaller metal nano-particles (7 nm vs. 28 nm) with a resultant (seven-fold) higher H2 chemisorption and decreased total surface acidity (from NH3 chemisorption/TPD). Temperature related activity maxima were observed for both catalysts and are associated with thermal desorption of the nitrile reactant. Exclusivity to the target butylamine was achieved at T ≥ 543 K where Ba-Pd/SiO2 delivered higher selective hydrogenation rate (91 mol h −1 mol Pd -1 ) than Pd/SiO2 (54 mol h −1 mol Pd −1 ), attributed to greater availability of surface reactive hydrogen. Lower surface acidity served to minimise condensation to higher amines. The rate and selectivity to butylamine exceed those previously reported for gas phase operation.