Gas Phase Hydrogenation of Furaldehydes via Coupling with Alcohol Dehydrogenation over Ceria Supported Au-Cu.

We have investigated the synthesis and application of Au-Cu/CeO2 (Cu: Au = 2) in the continuous gas phase (P = 1 atm; T = 498 K) coupled hydrogenation of 5-hydroxymethyl-2-furaldehyde (HMF) with 2-butanol dehydrogenation. STEM-EDX analysis revealed a close surface proximity of both metals in Au-Cu/CeO2 post-TPR. XPS measurements suggest (support → metal) charge transfer to form Auδ- and strong metal-support interactions to generate Cu° and Cu⁺. Au-Cu/CeO2 promoted the sole formation of 2,5-dihydroxymethylfuran (DHMF) and 2-butanone in the HMF/2-butanol coupling with full hydrogen utilisation. Under the same reaction conditions, Au/CeO2 was fully selective to DHMF in standard HMF hydrogenation (using an external hydrogen supply), but delivered a lower production rate and utilised less than 0.2% of the hydrogen supplied. Exclusive -C=O hydrogenation and -OH dehydrogenation is also demonstrated for the coupling of a series of m-substituted (-CH3, -CH2CH3, -CH2OH, -CF3, -N(CH3)2, -H) furaldehydes with alcohol (1-propanol, 1-butanol, 2-propanol, 2-butanol, cyclohexanol) dehydrogenation over Au-Cu/CeO2, consistent with a nucleophilic mechanism. In each case, we observed a greater hydrogenation rate and hydrogen utilisation efficiency with a 3⁻15 times lower E-factor in the coupling process relative to standard hydrogenation. Our results demonstrate the feasibility of using hydrogen generated in situ through alcohol dehydrogenation for the selective hydrogenation of m-furaldehydes with important industrial applications.

Role of Support Oxygen Vacancies in the Gas Phase Hydrogenation of Furfural over Gold.

ABSTRACT: We have examined the role of support oxygen vacancies in the gas phase hydrogenation of furfural over Au/TiO2 and Au/CeO2 prepared by deposition-precipitation. Both catalysts exhibited a similar Au particle size distribution (1-6 nm) and mean (2.8-3.2 nm). Excess H2 consumption during TPR is indicative of partial support reduction, which was confirmed by O2 titration. Gold on CeO2 with a higher redox potential exhibited a greater oxygen vacancy density. A lower furfural turnover frequency (TOF) was recorded over Au/CeO2 than Au/TiO2 and is linked to suppressed H2 chemisorption capacity and strong -C=O interaction at oxygen vacancies that inhibited activity. Gold on non-reducible Al2O3 as benchmark exhibited greater H2 uptake and delivered the highest furfural TOF. Full selectivity to the target furfuryl alcohol was achieved over Au/TiO2 and Au/Al2O3 at 413 K and over Au/CeO2 at 473 K with hydrogenolysis to 2-methylfuran at higher reaction temperature (523 K). A surface reaction mechanism is proposed to account for the activity/selectivity response.

Palladium Promoted Production of Higher Amines from a Lower Amine Feedstock.

ABSTRACT: The catalytic (Pd/Al2O3 and Pd/C; mean Pd size 2.5-3.0 nm from (S)TEM analysis) synthesis of di-butylamine (DBA) and tri-butylamine (TBA) from mono-butylamine (MBA) and DBA, respectively, in continuous gas phase operation is demonstrated. Exclusive production of DBA (from MBA) has been established over both catalysts where 453 ≤ T ≤ 523 K (∆Ea = 79 kJ mol-1). Greater activity for Pd/C is associated with higher levels of surface acidity (from NH3 chemisorption/TPD) and spillover hydrogen (from H2 TPD). Reaction of DBA over both catalysts when configured in series delivered full selectivity to TBA. Our results establish a novel clean alternative route for the continuous production of higher (secondary and tertiary) amines.

Gas phase selective hydrogenation over oxide supported Ni-Au.

The chemoselective continuous gas phase (T = 573 K; P = 1 atm) hydrogenation of nitroarenes (p-chloronitrobenzene (p-CNB) and m-dinitrobenzene (m-DNB)) has been investigated over a series of oxide (Al2O3 and TiO2) supported Au and Ni-Au (1 : 10 mol ratio; 0.1-1 mol% Au) catalysts. Monometallic supported Au with mean particle size 3-9 nm promoted exclusive formation of p-chloroaniline (p-CAN) and m-nitroaniline (m-NAN). Selective hydrogenation rate was higher over smaller Au particles and can be attributed to increased surface hydrogen (from TPD measurements) at higher metal dispersion. (S)TEM analysis has confirmed an equivalent metal particle size for the supported bimetallics at the same Au loading where TPR indicates Ni-Au interaction and EDX surface mapping established Ni in close proximity to Au on isolated nanoparticles with a composition (Au/Ni) close to the bulk value (= 10). Increased spillover hydrogen due to the incorporation of Ni in the bimetallics resulted in elevated -NO2 group reduction rate. Full selectivity to p-CAN was maintained over all the bimetallic catalysts. Conversion of m-DNB over the lower loaded Ni-Au/Al2O3 generated m-NAN as sole product. An increase in Ni content (0.01 → 0.1 mol%) or a switch from Al2O3 to TiO2 as support resulted in full -NO2 reduction (to m-phenylenediamine). Our results demonstrate the viability of Ni-promotion of Au in the continuous production of functionalised anilines.

Genomic organisation, activity and distribution analysis of the microbial putrescine oxidase degradation pathway.

The catalytic action of putrescine specific amine oxidases acting in tandem with 4-aminobutyraldehyde dehydrogenase is explored as a degradative pathway in Rhodococcus opacus. By limiting the nitrogen source, increased catalytic activity was induced leading to a coordinated response in the oxidative deamination of putrescine to 4-aminobutyraldehyde and subsequent dehydrogenation to 4-aminobutyrate. Isolating the dehydrogenase by ion exchange chromatography and gel filtration revealed that the enzyme acts principally on linear aliphatic aldehydes possessing an amino moiety. Michaelis-Menten kinetic analysis delivered a Michaelis constant (K(M)=0.014 mM) and maximum rate (Vmax=11.2 µmol/min/mg) for the conversion of 4-aminobutyraldehyde to 4-aminobutyrate. The dehydrogenase identified by MALDI-TOF mass spectrometric analysis (E value=0.031, 23% coverage) belongs to a functionally related genomic cluster that includes the amine oxidase, suggesting their association in a directed cell response. Key regulatory, stress and transport encoding genes have been identified, along with candidate dehydrogenases and transaminases for the further conversion of 4-aminobutyrate to succinate. Genomic analysis has revealed highly similar metabolic gene clustering among members of Actinobacteria, providing insight into putrescine degradation notably among Micrococcaceae, Rhodococci and Corynebacterium by a pathway that was previously uncharacterised in bacteria.

Role of amine oxidase expression to maintain putrescine homeostasis in Rhodococcus opacus.

While applications of amine oxidases are increasing, few have been characterised and our understanding of their biological role and strategies for bacteria exploitation are limited. By altering the nitrogen source (NH4Cl, putrescine and cadaverine (diamines) and butylamine (monoamine)) and concentration, we have identified a constitutive flavin dependent oxidase (EC 1.4.3.10) within Rhodococcus opacus. The activity of this oxidase can be increased by over two orders of magnitude in the presence of aliphatic diamines. In addition, the expression of a copper dependent diamine oxidase (EC 1.4.3.22) was observed at diamine concentrations>1mM or when cells were grown with butylamine, which acts to inhibit the flavin oxidase. A Michaelis-Menten kinetic treatment of the flavin oxidase delivered a Michaelis constant (KM)=190µM and maximum rate (kcat)=21.8s(-1) for the oxidative deamination of putrescine with a lower KM (=60µM) and comparable kcat (=18.2s(-1)) for the copper oxidase. MALDI-TOF and genomic analyses have indicated a metabolic clustering of functionally related genes. From a consideration of amine oxidase specificity and sequence homology, we propose a putrescine degradation pathway within Rhodococcus that utilises oxidases in tandem with subsequent dehydrogenase and transaminase enzymes. The implications of PUT homeostasis through the action of the two oxidases are discussed with respect to stressors, evolution and application in microbe-assisted phytoremediation or bio-augmentation.

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.

Catalytic hydrodechlorination of chloroaromatic gas streams promoted by Pd and Ni: the role of hydrogen spillover.

Catalytic hydrodechlorination (HDC) is an effective means of detoxifying chlorinated waste. Involvement of spillover hydrogen is examined in gas phase dechlorination of chlorobenzene (CB) and 1,3-dichlorobenzene (1,3-DCB) over Pd and Ni. The catalytic action of single component Pd and Ni, Pd/Al(2)O(3), Ni/Al(2)O(3) and physical mixtures with Al(2)O(3) has been considered. Catalyst activation is characterized in terms of temperature programmed reduction, the supported nano-scale metal phase by transmission electron microscopy and hydrogen/surface interactions by chemisorption/temperature programmed desorption. Pd/Al(2)O(3) generated significantly greater amounts of spillover hydrogen (by a factor of over 40) compared with Ni/Al(2)O(3). Hydrogen spillover on Pd/Al(2)O(3) far exceeded the chemisorbed component, whereas chemisorbed and spillover content was equivalent for Ni/Al(2)O(3). Inclusion of Al(2)O(3) with Ni and Ni/Al(2)O(3) increased spillover with an associated increase in specific HDC rate (up to a factor of 10) and enhanced selectivity to benzene from 1,3-DCB. HDC rate delivered by Pd and Pd/Al(2)O(3) was largely unaffected by the addition of Al(2)O(3). This can be attributed to the higher intrinsic HDC performance of Pd that results in appreciable HDC activity under conditions where Ni/Al(2)O(3) was inactive. Spillover was partially recovered (post TPD) for the Ni samples but the loss was irreversible in the case of Pd.

Catalytic transformation of waste polymers to fuel oil.

Waste not, want not: The increase in waste polymer generation, which continues to exceed recycle, represents a critical environmental burden. However, plastic waste may be viewed as a potential resource and, with the correct treatment, can serve as hydrocarbon raw material or as fuel oil, as described in this Minireview.Effective waste management must address waste reduction, reuse, recovery, and recycle. The consumption of plastics continues to grow, and, while plastic recycle has seen a significant increase since the early 1990s, consumption still far exceeds recycle. However, waste plastic can be viewed as a potential resource and can serve, with the correct treatment, as hydrocarbon raw material or as fuel oil. This Minireview considers the role of catalysis in waste polymer reprocessing and provides a critical overview of the existing waste plastic treatment technologies. Thermal pyrolysis results in a random scissioning of the polymer chains, generating products with varying molecular weights. Catalytic degradation provides control over the product composition/distribution and serves to lower significantly the degradation temperature. Incineration of waste PVC is very energy demanding and can result in the formation of toxic chloro emissions. The efficacy of a catalytic transformation of PVC is also discussed.

Syntheses and structures of alkaline Earth-transition metal bimetallic complexes as heterogeneous hydrodechlorination catalyst precursors.

A series of alkaline earth-transition metal heterobimetallic complexes {(DMF)(x)M(mu-CN)(y)M*(CN)(4-y)}(infinity) [M = Ba: M* = Ni 1, M* = Pd 2, M* = Pt 3; M = Sr: M* = Ni 4, M* = Pd 5, M* = Pt 6; x = 3 or 4; y = 3 or 4] was prepared. Single-crystal X-ray diffraction analysis revealed that {(DMF)(4)Ba(mu-CN)(3)Ni(CN)}(infinity) 1, {(DMF)(4)Sr(mu-CN)(3)Ni(CN)}(infinity) 4, {(DMF)(4)Sr(mu-CN)(3)Pd(CN)}(infinity) 5, and {(DMF)(4)Sr(mu-CN)(3)Pt(CN)}(infinity) 6 form isostructural one-dimensional "ladder" arrays through isocyanide linkages (M-NC-M*), while {(DMF)(3)Ba(mu-CN)(4)Pd}(infinity) 2, and {(DMF)(3)Ba(mu-CN)(4)Pt}(infinity) 3 are two-dimensional puckered sheet like arrays. These bimetallic complexes can serve as effective precursors for the synthesis of supported bimetallic catalysts. It has been established that Ba-Pd/SiO(2) and Sr-Pd/SiO(2) catalysts, prepared from 2 and 5 loaded onto a silica support, delivered specific reaction rates in the hydrodechlorination of mono- and dichloro benzene that were over an order of magnitude greater than that achieved with conventional Pd/SiO(2).

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.

Palladium supported on structured and nonstructured carbon: a consideration of Pd particle size and the nature of reactive hydrogen.

This study sets out a comprehensive characterization of bulk Pd and Pd (ca. 8% w/w) supported on activated carbon (AC), graphite and graphitic nanofibers (GNF). Catalyst activation has been examined by temperature programmed reduction (TPR) analysis and the activated catalysts analyzed in terms of BET area, TEM, H2 chemisorption/TPD, and XRD measurements. While H2 chemisorption and TEM delivered the same sequence of increasing (surface area weighted) average Pd particle sizes, a significant difference (by up to a factor of 3) in the values obtained from both techniques has been recorded and is attributed to an unwarranted (but widely adopted) assumption of an exclusive H2/Pd adsorption stoichiometry=1/2. It is demonstrated that TEM analysis provides a valid mean particle size once it is established that the associated standard deviation is small and insensitive to additional particle counting. XRD line broadening yielded an essentially equivalent Pd size (20-25 nm) for each supported catalyst. The nature of the hydrogen associated with the supported catalysts has been probed and is shown to comprise of chemisorbed (on Pd), spillover (on the carbon support), and hydride (associated with Pd) species. Physical mixtures of bulk Pd + support (AC, graphite, and GNF) were also considered in order to assess hydrogen spillover by H2 TPD analysis. Generation of spillover hydrogen at room temperature is established where temperatures in excess of 740 K are required for effective desorption from the supported Pd catalysts, i.e., 280 K higher than that required for the desorption of chemisorbed hydrogen. Pd hydride formation (at room temperature) is shown to be reversible with decomposition occurring at ca. 380 K. Taking the hydrodechlorination of chlorobenzene as a test reaction, the capability of Pd hydride to promote a hydrogen scission of C-Cl in the absence of an external supply of H2 is demonstrated with a consequent consumption of the hydride. This catalytic response was entirely recoverable once the Pd hydride was replenished during a subsequent reactivation step.

Advantages of synthesizing trans-1,2-cyclohexanediol in a continuous flow microreactor over a standard glass apparatus.

Comparison is made between the preparation of trans-1,2-cyclohexanediol in standard glassware (conventional batch production) and in a microreactor (continuous flow production). The reaction sequence involved two exothermic steps where the standard procedure demands slow reagent addition and careful temperature control. In the microreactor, the reaction could be carried out safely with up to 3 times higher reagent concentration. Synthetic benefits were a faster reaction rate and a higher purity product free of colored impurities (a feature of the batch procedure).

Ni/SiO2 promoted growth of carbon nanofibers from chlorobenzene: characterization of the active metal sites.

The temporal changes to supported Ni sites during the growth of graphitic carbon nanofibers (GCNs) via the decomposition of chlorobenzene over Ni/SiO2 at 873 K have been investigated. The reaction of chlorobenzene with hydrogen also generated benzene, via catalytic hydrodechlorination, as the principal competing reaction. Reaction selectivity was found to be time dependent with a switch from a preferential hydrodechlorination to a predominant decomposition that generated an increasingly more structured carbon product over prolonged time-on-stream. These findings are discussed in terms of Cl/catalyst interaction(s) leading to metal site restructuring, the latter manifest in a sintering and faceting of the Ni metal particles. The pressure exerted on the metal/support interface due to fiber formation was of sufficient magnitude to extract the Ni particle from the support; the occurrence of an entrapped Ni particle at the fiber tip is a feature common to the majority of GCNs with the incorporation of Ni fragments along the length of the GCN. Metal site restructuring has been probed by temperature-programmed reduction of the passivated samples, H2 chemisorption/temperature-programmed desorption (TPD) and XANES/EXAFS analyses. This restructuring serves to enhance destructive chemisorption and/or facilitate carbon diffusion to generate the resultant GCN. The nature of the carbonaceous product has been characterized by a combination of TEM-EDX, SEM, XRD and temperature-programmed oxidation (TPO).

Catalyst support effects: gas-phase hydrogenation of phenol over palladium.

The catalytic action of 10% w/w Pd supported on two forms of graphitic carbon nanofibers (GCN) has been assessed and compared with the performance of 10% w/w Pd on SiO(2), Ta(2)O(5), activated carbon (AC), and graphite. Palladium nitrate served as metal precursor in each case but the role of the starting metal salt was also considered by examining the action of palladium acetate impregnated SiO(2). The activated catalysts have been characterized by hydrogen chemisorption, high-resolution transmission electron microscopy, and scanning electron microscopy. Phenol hydrogenation served as the test reaction, which proceeds in a stepwise fashion involving the partially hydrogenated cyclohexanone as a reactive intermediate. The occurrence and ramifications of Pd/support interaction(s) are related to hydrogenation activity and selectivity. The effects of contact time and reaction temperature (398-448 K) are reported and discussed in terms of phenol/catalyst interaction(s). Hydrogenation kinetics have been adequately represented by a standard pseudo-first-order approximation. The specific activities exhibited the following sequence of increasing values: Pd/AC

Growth of filamentous carbon from the surface of Ni/SiO2 doped with alkali metal bromides.

The growth of ordered filamentous carbon, catalytically generated from the decomposition of ethylene, has been studied over the temperature range 673-898 K using an 11% w/w Ni/SiO2 catalyst doped to varying degrees (0.1-9.3% w/w) with a range of alkali metal bromides. The effect of these alkali metal/halogen adatoms in promoting/inhibiting carbon growth has been assessed and variations in the associated carbon structural characteristics have been examined. The introduction of Li consistently promoted filamentous carbon growth (where 723 K