Comparison of the heterogeneous enantioselective hydrogenation of 2,3-butanedione over cinchona modified platinum catalysts in three different reactors

The enantioselective hydrogenation of 2,3-butanedione has been studied in the liquid and gas phase over cinchonidine modified Pt/silica catalysts. Reactions were conducted in three different reactors a stirred autoclave, a fixed-bed trickle-bed reactor and a micro-flow gas phase reactor. Screening of eight powder and four granular (0.5-1.0 mm) 2.5 % Pt/silica catalysts was conducted in the autoclave to identify ideal support characteristics. The rate of 2,3- butanedione enantioselective hydrogenation was used as the means to determine these characteristics. The support characteristics used for the manufacture of extruded silica gel supports were: PV > 0.40 ml/g, PD > 50 A and a SA > 300 m2/g (< 600 m2/g). Optimum values of rate and enantiomeric excess were identified with (i) pre-reduction at 200 C for 2 h of catalyst samples, (ii) modifier concentration of 1.89 mM, (iii) toluene with 0.1 M acetic acid additive, (iv) hydrogen pressure was 30 bar and (v) stirring speed of 1000 rpm. Under these conditions the rate achieved 1104 mmol/h/gcat in DCM was comparable to the reference catalyst EUROPT-1. The granular catalysts were used in the trickle-bed reactor to identify optimum reaction conditions and support characteristics. Ultraviolet spectroscopy of cinchonidine modification protocols and cyclohexene hydrogenation were employed to facilitate optimisation. In situ and ex situ pre-modification of the bed with cinchonidine were investigated. Conversion of dione was ca. 10 % for catalysts which possessed an un- restrictive pore structure. The ee of (K)-hydroxybutanone achieved was ca. 12 %, however, was at its maximum initially after in situ pre-modification. Whereas, the enantiomeric excess tended to increase over an ex situ modified catalyst bed. Continual replacement of modifier was necessary to ensure an ee was maintained. The reaction conditions used for the study of the catalyst with extruded silica gel support were, (i) ex situ pre-modification with 16.98 mM cinchonidine in DCM, (ii) 1 ml/min of feedstock with 0.1 M reactant and 8.49 mM CD in DCM, (iii) hydrogen pressure of 0.5 barg (4800 /h), (iv) catalyst bed consisting of 2g of catalyst and SiC fines. Hydrogenation of methyl pyruvate under these conditions yielded an initial conversion of 80 % and an ee of 52%. Hydrogenation of 2,3-butanedione and methyl pyruvate over an extrudate catalyst bed under the optimised conditions achieved an ee of 12 % and 36 % respectively at low conversion. Bed pre-modification was determined to be essential to achieve an enantiomeric excess. Hydrogenation of 2,3-butanedione was attempted at the gas/solid interface over pre-modified platinum catalysts. The strength of adsorption of the dione prevented a sustained reaction. An ee of 15 % at 35 C was achieved with a saturator temperature of 0 C. The addition of cinchonidine to the catalyst proved beneficial for greater reaction times. The lower strength of adsorption pyruvate esters on platinum facilitated the identification of optimum reaction conditions. The highest values of enantiomeric excess of (ft)-lactate ester were achieved when (i) catalyst samples were pre-modified ex situ with 3.4 mM CD in DCM, (ii) reaction temperature of 40 C, (v) helium then helium/hydrogen passed over bed prior to reaction, (iii) hydrogen concentration of 25 % in helium with a total flow rate of 80 ml/min, (iv) methyl pyruvate at 20 C (0.066 g/h). Under these reaction conditions an ee of ca. 36 % CK)-lactate ester was achieved over a 2.5 % Pt/SiC>2 catalyst (0.025 g) at 100 % pyruvate conversion for 5 h. This was enhanced to ca. 43 % by pre-treatment of the bed with the ()-lactate for 100 minutes prior to the reaction. Methyl and ethyl pyruvate were used to investigate the enantioselective site on the Pt surface. The outcome indicated that the site is substrate specific after 100 minutes on-line. The enantioselective hydrogenation of 2,3-butanedione has been studied in the liquid and gas phase over cinchonidine modified Pt/silica catalysts. Reactions were conducted in three different reactors a stirred autoclave, a fixed-bed trickle-bed reactor and a micro-flow gas phase reactor. The rate of 2,3-butanedione hydrogenation in the autoclave was used to compared a number of platinum catalysts. The outcome of this screening program resulted in an ideal set of support characteristics for the manufacture of an extruded silica gel support for use in the trickle-bed reacto

Silver palladium catalysts for the direct synthesis of hydrogen peroxide

A series of bimetallic silver–palladium catalysts supported on titania were prepared by wet impregnation and assessed for the direct synthesis of hydrogen peroxide, and its subsequent side reactions. The addition of silver to a palladium catalyst was found to significantly decrease hydrogen peroxide productivity and hydrogenation, but crucially increase the rate of decomposition. The decomposition product, which is predominantly hydroxyl radicals, can be used to decrease bacterial colonies. The interaction between silver and palladium was characterized using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR). The results of the TPR and XPS indicated the formation of a silver–palladium alloy. The optimal 1% Ag–4% Pd/TiO 2 bimetallic catalyst was able to produce approximately 200 ppm of H 2 O 2 in 30 min. The findings demonstrate that AgPd/TiO 2 catalysts are active for the synthesis of hydrogen peroxide and its subsequent decomposition to reactive oxygen species. The catalysts are promising for use in wastewater treatment as they combine the disinfectant properties of silver, hydrogen peroxide production and subsequent decomposition. This article is part of a discussion meeting issue ‘Providing sustainable catalytic solutions for a rapidly changing world’.</jats:p

Dependence of n-butane activation on active site of vanadium phosphate catalysts

The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi–Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi–Fe mixture dopants led to formation of αII-VOPO4 phase together with (VO)2P2O7 as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi–Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V5+ species as compared to the undoped catalyst, which gave the reduction of V4+ as the major feature. An excess of the oxygen species (O2−) associated with V5+ in Bi–Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V4+-O− was shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O−) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction

Investigating the influence of Fe speciation on N2O decomposition over Fe–ZSM-5 catalysts

The influence of Fe speciation on the decomposition rates of N2O over Fe–ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe–ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N2O decomposition at 600 °C improved to 3.99 × 103 s−1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 103 s−1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N2O decomposition when propane is present. When only N2O is present, low metal loading Fe–ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe3+ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as FexOy, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of FexOy nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF

The formation of methanol from glycerol bio-waste over doped ceria based catalysts

A series of ceria-based solid-solution metal oxides were prepared by co-precipitation and evaluated as catalysts for glycerol cleavage, principally to methanol. The catalyst activity and selectivity to methanol were investigated with respect to the reducibility of the catalysts. Oxides comprising of Ce-Pr and Ce-Zr were prepared, calcined and compared to CeO2, Pr6O11 and ZrO2. The oxygen storage capacity of the catalysts was examined with analysis of Raman spectroscopic measurements and a temperature programmed reduction, oxidation and reduction cycle. The incorporation of Pr resulted in significant defects, as evidenced by Raman spectroscopy. The materials were evaluated as catalysts for the glycerol to methanol reaction and it was found that an increased defect density or reducibility was beneficial. The space time yield of methanol normalised to surface area over CeO2 was found to be 0.052 mmolMeOH m-2 h-1 and over CeZrO2 and CePrO2 this was to 0.029 and 0.076 mmolMeOH m-2 h-1 respectively. The inclusion of Pr reduced the surface area, however, the carbon mole selectivity to methanol and ethylene glycol remained relatively high, suggesting a shift in the reaction pathway compared to that over ceria. This article is part of a discussion meeting issue “Science to enable the circular economy”

Synthesis of crystalline microporous Mo−V−Bi oxide for selective (Amm)oxidation of light alkanes

Bismuth (Bi) was successfully introduced into the crystalline orthorhombic Mo3VOx (MoVO) structure for the first time by using the ethylammonium cation (EtNH3+) as a structure-directing agent in hydrothermal synthesis, and the catalytic activities of MoVO-containing Bi (MoVBiO) for selective oxidation of ethane and ammoxidation of propane were compared with those of ternary MoVO. Bi and EtNH3+ were located in hexagonal and heptagonal channels in the MoVO structure, respectively. EtNH3+ could be removed without collapse of the crystal structure by appropriate heat treatment, leaving the heptagonal channels empty. The introduction of Bi had only a little effect on the catalytic activity for selective oxidation of ethane. On the other hand, the conversion of propane was significantly enhanced in propane ammoxidation. Acrylonitrile selectivity was also enhanced by the introduction of Bi, especially at high temperatures (>440 °C)

Ammonia decomposition enhancement by Cs-Promoted Fe/Al2O3 catalysts

A range of Cs-doped Fe/Al2O3 catalysts were prepared for the ammonia decomposition reaction. Through time on-line studies it was shown that at all loadings of Cs investigated the activity of the Fe/Al2O3 catalysts was enhanced, with the optimum Cs:Fe being ca. 1. Initially, the rate of NH3 decomposition was low, typically < 10% equilibrium conversion (99.7%@500°C) recorded after 1 h. All catalysts exhibited an induction period (typically ca. 10 h) with the conversion reaching a high of 67% equilibrium conversion for Cs:Fe = 0.5 and 1. The highest rate of decomposition observed was attributed to the balance between increasing the concentration of Cs without blocking the active site. Analysis of H2-TPR and XPS measurements indicated that Cs acts as an electronic promoter. Previously, Cs has been shown to act as a promoter for Ru, where Cs alters the electron density of the active site, thereby facilitating the recombination of N2 which is considered the rate determining step. In addition, XRD and N2 adsorption measurements suggest that with higher Cs loadings deactivation of the catalytic activity is due to a layer of CsOH that forms on the surface and blocks active sites

Investigating the preparation of Cu3Mo2O9 as a photocatalyst

The structural formation of copper molybdate was investigated using a surfactant added during the preparation. The calcined materials were confirmed by XRD to be Cu3Mo2O9 and the absorbance band-gap was calculated to be ca. 2 eV. The addition of surfactant resulted in a platelet morphology, in contrast to the agglomerated particles of the standard preparation. Photocatalytic degradation over the catalysts was investigated with indigo carmine. The Cu3Mo2O9 catalysts were able to degrade the dye under irradiated conditions. However, the catalyst prepared with surfactant were not as active, although, surface decoration by Cu species was greatly reduced

Can gold be an effective catalyst for the Deacon reaction?

The Deacon reaction is an important industrial process for the oxidation of hydrogen chloride, thereby enabling chlorine to be recycled. As gold is an efficient catalyst for reactions involving hydrogen chloride and oxygen, we have studied the use of gold as a potential catalyst for the Deacon reaction. Unfortunately, gold displays only limited activity; however, this is markedly increased if hydrogen is cofed as a reactant