Rational Catalyst Design for Selective Hydrogenations:Nitroarenes and Alkynes as Case Studies

Catalyst design for selective hydrogenations is of major importance for the manufacturing of fine chemicals. Catalytic procedure which uses scarce and expensive noble metals is very challenging in terms of exclusive attack of a single functionality or substituent. The approach taken in this thesis is based on rational catalyst design that calls on a combination of catalyst synthesis and characterization. This has been applied for the design of catalysts suitable for industrially relevant reactions, i.e. the partial catalytic reduction of substituted nitroaromatic compounds and alkynes. Molybdenum nitrides were selected as they represent a more sustainable alternative to noble metals, less expensive and easy to prepare. First, beta-Mo2N was used to catalyze the liquid phase selective hydrogenation of a series of para-substituted nitroarenes to give the corresponding aromatic amine. Incorporation of low amounts (0.25% wt.) of Au nanoparticles on beta-Mo2N enhanced hydrogen uptake and catalytic activity while delivering an ultraselective response. In a second step, the influence on the catalytic response of nitride crystallographic phase (beta- vs. gamma-Mo2N) and surface area (7-66 m2 g-1) was examined. Both phases promoted the exclusive hydrogenation of p-chloronitrobenzene to p-chloroaniline where the beta-form delivered a higher specific (per m2) rate; the one for gamma-Mo2N was independent of surface area. The inclusion of Au on both nitrides served to enhance p-chloroaniline production. Finally, it was shown that incorporation of N in Mo structure can increase nitrobenzene hydrogenation rates on Mo2N samples with higher nitrogen content. In contrast, -C=O hydrogenolysis was favored with benzaldehyde as a result of lower N content. As a powerful tool for tailoring metal nanoparticle morphology, colloidal methods were used to design structured catalysts effective for the selective alkyne and âNO2 group reduction. The development of a catalyst based on polyvinylpyridine modified structured carbon nanofibers on sintered metal fibers supported (4 nm) polyvinylpyrrolidone (PVP) stabilized Pd was shown to be an optimum formulation for acetylene semi-hydrogenation where the catalyst shows high selectivity (93%) and stability with time-on-stream. We have established that reducing agent does not play a critical role in catalytic response while steric (vs. electronic) stabilizers and larger colloidal metal crystallites are more efficient. PVP-stabilized Ni nanocrystals were also prepared via colloidal method and tested in the m-dinitrobenzene hydrogenation were an antipathetic structure sensitivity was recorded. TOF and selectivity to m-nitroaniline exhibited a marked dependency on the presence of the stabilizer. It was attributed to preferential adsorption of PVP on selective edge and vertex atoms, leaving the plane atoms free. Supporting the Ni nanoparticles on activated carbon fibers and removing traces of PVP by a UV-ozone treatment resulted in a dramatic increase in selectivity to target m-NAN even at high conversions. Two-sites Langmuir-Hinshelwood kinetic model has been applied to rationalize the results. In summary, this thesis demonstrates that the product distribution can be controlled on a nano-level by tuning the properties of the active phase through modifications of the structural parameters (allotropic form, composition), modifications on the nanoparticle microenvironment (organic ligand shell) and optimization of particle size

Effect of Crystallographic Phase (β vs. γ) and Surface Area on Gas Phase Nitroarene Hydrogenation Over Mo2N and Au/Mo2N

The catalytic action of Mo2N and Au/Mo2N has been assessed in the selective gas phase hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN). The nitrides were synthesised via temperature programmed treatment of MoO3 in H2+N2 and Au introduced by deposition-precipitation with urea. We have examined the influence of nitride crystallographic phase (tetragonal β-Mo2N vs. cubic γ-Mo2N) and surface area (7-66m2g−1) on the catalytic response. Catalyst activation by temperature programmed reduction has been monitored and the reduced catalysts characterised in terms of BET area/pore volume, H2 chemisorption/temperature programmed desorption (TPD), powder X-ray diffraction (XRD), elemental analysis, scanning (SEM) and transmission (TEM) electron microscopy and X-ray photoelectron spectroscopy (XPS) measurements. The formation of β- and γ-Mo2N was confirmed by XRD and TEM. γ-Mo2N exhibits a platelet morphology whereas β-Mo2N is characterised by an aggregation of small crystallites. Hydrogen chemisorption and TPD analysis have established a greater hydrogen uptake capacity (per unit area) for β-Mo2N relative to γ-Mo2N, which is associated with surface nitrogen deficiency, i.e. higher surface Mo/N for β-Mo2N. Incorporation of Au on both nitrides resulted in an increase in surface hydrogen. The Au phase takes the form of nano-scale particles with a mean size of 7 and 4nm on β-Mo2N and γ-Mo2N, respectively. Both β-Mo2N and γ-Mo2N promoted the exclusive hydrogenation of p-CNB to p-CAN where the β-form delivered a higher specific (per m2) rate; the specific rate for γ-Mo2N was independent of surface area. The inclusion of Au on both nitrides served to enhance p-CAN productio

Pax genes in embryogenesis and oncogenesis

The paired box genes are a family of nine developmental control genes, which in human beings (PAX) and mice (Pax) encode nuclear transcription factors. The temporal and spatial expressions of these highly conserved genes are tightly regulated during foetal development including organogenesis. PAY/Paxgenes are switched off during the terminal differentiation of most structures. Specific mutations within a number of PAX/Pax genes lead to developmental abnormalities in both human beings and mice. Mutation in PAX3 causes Waardenburg syndrome, and craniofacial-deafness-hand syndrome. The Splotch phenotype in mouse exhibits defects in neural crest derivatives such as, pigment cells, sympathetic ganglia and cardiac neural crest-derived structures. The PAX family also plays key roles in several human malignancies. In particular, PAX3 is involved in rhabdomyosarcoma and tumours of neural crest origin, including melanoma and neuroblastoma. This review critically evaluates the roles of PAX/Pax in oncogenesis. It especially highlights recent advances in knowledge of how their genetic alterations directly interfere in the transcriptional networks that regulate cell differentiation, proliferation, migration and survival and may contribute to oncogenesis

Vertex-Colored Graphs, Bicycle Spaces and Mahler Measure

The space C of conservative vertex colorings (over a field F) of a countable, locally finite graph G is introduced. When G is connected, the subspace C-0 of based colorings is shown to be isomorphic to the bicycle space of the graph. For graphs G with a cofinite free Z(d)-action by automorphisms, C is dual to a finitely generated module over the polynomial ring F[x(1)(+/- 1),..., x(d)(+/- 1)]. Polynomial invariants for this module, the Laplacian polynomials Delta k, k \u3e= 0, are defined, and their properties are discussed. The logarithmic Mahler measure of Delta(0) is characterized in terms of the growth of spanning trees

Impact of organic-ligand shell on catalytic performance of colloidal Pd nanoparticles for alkyne gas-phase hydrogenation

Monodispersed Pd nanoparticles (NPs) have been prepared by colloidal technique and deposited on a structured support consisting of carbon nanofibers (CNF) grown on sintered metal fibres (SMF). The surface properties of Pd NPs have been fine-tuned by (i) changing the nature of stabilizing agent (electrostatic vs. steric), (ii) controlling Pd NPs size (2-10 nm) and (iii) grafting N-containing ligands onto the CNF/SMF surface. In the semi-hydrogenation of acetylene (T= 393 K; 13= 1 bar) catalytic response was insensitive to the nature of the reducing agent where equivalent activity/selectivity were obtained over Pd NPs with similar dispersion, prepared with the same stabilizer. A similar product distribution was recorded over Pd NPs with similar crystal size irrespective of the colloidal stabilizer (electrostatic vs. steric). In contrast, a stronger inhibiting effect on hydrogenation rate has been found with electrostatic stabilizer (sodium di-2-ethylhexylsulfosuccinate) as compared to the steric ones (polyvinylpyrrolidone or polyvinylalcohol) and assigned to geometric and electronic effects. Decrease (from 8 2 nm) in Pd NPs size results in a concomitant decrease in activity (antipathetic size-sensitivity), but higher selectivity to target ethylene product. Grafting of nitrogen-containing modifiers (polyvinylpyridine or polyethylenimine) on the CNF/SMF support results in a significant increase in olefin selectivity (up to 93%) where the catalyst shows remarkable stability during 120 h on-stream. This is explained by the electronic modifications promoted by interactions between the Pd NPs and the grafted ligands as confirmed by XPS analysis. In comparison, stabilizer-free Pd/CNF/SMF has low selectivity to ethylene (65%). In summary, controlled size Pd (core) nanoparticles with organic ligands ( shell) demonstrated increased selectivity and remarkable stability in catalytic gas-phase alkyne semi-hydrogenation opening new tools for rational catalyst design. (C) 2014 Elsevier B.V. All rights reserved

New insights into the effect of nitrogen incorporation in Mo: catalytic hydrogenation vs. hydrogenolysis

The catalytic effect of nitrogen incorporation into Mo on hydrogenation (of -NO2 to -NH2 in nitrobenzene to aniline) and hydrogenolysis (of -C?O in benzaldehyde to toluene) processes has been assessed. Bulk Mo was prepared by temperature programmed reduction of MoO3 (in H-2 to 933 K) and -Mo2N (confirmed by powder XRD) subsequently synthesised by Mo nitridation in N-2/H-2. Two intermediate samples (MoN-1 and MoN-2) with different Mo/N ratio were prepared by altering the duration (1 and 2 h) of the nitridation step. XPS analysis revealed a nitrogen surface enrichment (Mo/N = 2.2 0.9 from MoN-1 to -Mo2N) relative to the bulk (Mo/N = 5.1 2.5). Incorporation of N did not affect morphology and each sample exhibited (by SEM analysis) aggregates (<5 m) of crystals (27-36 nm) with unchanged specific surface area (ca. 4 m(2) g(-1)). Hydrogen chemisorption and release (by TPD) increased with decreasing Mo/N (Mo < MoN-1 < MoN-2 < -Mo2N). Gas phase hydrogenation of nitrobenzene to aniline exhibited increasing rate from Mo -Mo2N, attributed to higher availability of surface heterolytic hydrogen (on Mo-N). In contrast, conversion of benzaldehyde to toluene was favoured by increasing Mo/N (from -Mo2N Mo) where hydrogenolytic -C?O scission is favoured by homolytic hydrogen chemisorption (on Mo). Our results provide the first evidence that N incorporation in Mo structure can control catalytic hydrogenation vs. hydrogenolysis performance

Effect of crystallographic phase (beta vs. gamma) and surface area on gas phase nitroarene hydrogenation over of Mo₂N and Au/Mo₂N

The catalytic action of Mo2N and Au/Mo2N has been assessed in the selective gas phase hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN). The nitrides were synthesised via temperature programmed treatment of MoO3 in H-2 + N-2 and Au introduced by deposition-precipitation with urea. We have examined the influence of nitride crystallographic phase (tetragonal beta-Mo2N vs. cubic gamma-Mo2N) and surface area (7-66 m(2) g(-1)) on the catalytic response. Catalyst activation by temperature programmed reduction has been monitored and the reduced catalysts characterised in terms of BET area/pore volume, H-2 chemisorption/temperature programmed desorption (TPD), powder X-ray diffraction (XRD), elemental analysis, scanning (SEM) and transmission (TEM) electron microscopy and X-ray photoelectron spectroscopy (XPS) measurements. The formation of beta- and gamma-Mo2N was confirmed by XRD and TEM. gamma-Mo2N exhibits a platelet morphology whereas beta-Mo2N is characterised by an aggregation of small crystallites. Hydrogen chemisorption and TPD analysis have established a greater hydrogen uptake capacity (per unit area) for beta-Mo2N relative to gamma-Mo2N, which is associated with surface nitrogen deficiency, i.e. higher surface Mo/N for beta-Mo2N. Incorporation of Au on both nitrides resulted in an increase in surface hydrogen. The Au phase takes the form of nano-scale particles with a mean size of 7 and 4 nm on beta-Mo2N and gamma-Mo2N, respectively. Both beta-Mo2N and gamma-Mo2N promoted the exclusive hydrogenation of p-CNB to p-CAN where the beta-form delivered a higher specific (per m(2)) rate; the specific rate for gamma-Mo2N was independent of surface area. The inclusion of Au on both nitrides served to enhance p-CAN production

Highly selective immobilized bimetallic Ni-Au nanoparticle catalyst for the partial hydrogenation of m-dinitrobenzene

Transition metal nanoparticles (NPs) are extensively used as catalysts for a wide and diverse range of organic transformations when immobilized on appropriate solid supports. We describe the development of a highly active and highly selective heterogeneous catalyst based on Ni-Au NPs supported on activated carbon fibers (ACFs) for the partial reduction of m-dinitrobenzene (m-DNB) to m-nitroaniline (m-NAN), an important platform chemical used in the synthesis of dyes and polymers. Initially, Ni NPs with narrow size distribution and ranging from 2 to 14 nm were prepared with poly-N-vinyl-2-pyrrolidone (PVP) as a stabilizer. Evaluation of the NPs as catalysts in the liquid-phase hydrogenation of m-dinitrobenzene led to the establishment of an antipathetic structure sensitivity, i.e. the larger NPs displayed a 6-fold higher turnover frequency than the smaller NPs. The selectivity to the target m-NAN product is independent of the size of the Ni NPs, possibly due to preferential PVP absorption of the NP edges and vertices. Consequently, Ni NPs of 2 nm were supported on ACFs and residual PVP was removed by a ultra-violet ozone (UVO) treatment, rendering a highly selective structured catalyst that affords m-NAN in almost 96% yield. A two-site (plane vs. edge Ni-atoms) Langmuir-Hinshelwood kinetic model is consistent with the experimental kinetic data confirming that low-coordination atoms (edges and vertices) are responsible for selective reaction. Consequently, we prepared bimetallic Ni-Au NPs (Ni:Au = 1:1) aiming to generate Ni surface sites mimicking the properties of edge and vertex atoms. The resulting UVO-treated Ni-Au NPs of 3 nm immobilized on ACFs afford m-NAN with a yield exceeding 98%. Such a high yield appears to be unprecedented and shows how careful nanocatalyst design, guided by detailed structural characterization and mechanistic studied, can lead to highly selective catalysts of industrial relevance

An examination of catalyst deactivation in p-chloronitrobenzene hydrogenation over supported gold

The stability of Au/Al2O3 in the continuous gas phase (423 K) hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) has been investigated over an inlet H-2/p-CNB = 4-390, i.e. from close to stoichiometry to H2 far in excess. The catalyst (activated unused and spent) has been characterised with respect to specific surface area (SSA)/porosity, temperature programmed reduction (TPR), powder XRD, H-2 chemisorption, STEM, XPS, elemental analysis and TGA-DSC measurements. Activation of Au/Al2O3 by TPR in hydrogen generated a narrow Au size distribution (1-8 nm, mean = 3.6 nm) with evidence (from XPS) of (support -> metal) charge transfer to generate surface Au delta-. Exclusive p-CAN production was achieved under conditions of kinetic control, which were established by parameter estimation and experimental variation of contact time, catalyst particle size and p-CNB/catalyst ratio. A temporal decline in activity was observed that was more pronounced at H-2/p-CNB <= 39. The spent catalyst exhibited equivalent SSA/porosity, Au particle size (from STEM) and electronic character (from XPS) relative to activated unused Au/Al2O3. A significant carbon content (6.3% w/w) was determined from elemental analysis and confirmed by XPS and TGA-DSC. This carbon deposit hindered H-2 chemisorption under reaction conditions, leading to suppressed hydrogenation activity. Catalyst regeneration by oxidative/reductive treatment resulted in a restoration of the initial hydrogenation activity, retaining exclusive selectivity to p-CAN. (C) 2014 Elsevier B.V. All rights reserved