Unit Mass Of Catalyst Research Articles

The effect of pH on structural and catalytic properties of MoVTeNbO catalysts

Mo-V-Te-Nb mixed oxide catalysts, prepared by a slurry method at various pHs (in the range of about 4.5–1.0), have been tested in the partial propane oxidation to acrylic acid. The best catalytic performances were achieved on catalysts prepared at a pH from 2.0 to 2.8. In this case, formation rates of acrylic acid per unit mass of catalyst and per unit time, STY AA, close to 50 g AA kg cat h −1 have been obtained at 653 K. The characterization of the catalyst precursors (uncalcined samples) suggests the presence of (NH 4) 6TeMo 6O 24· nH 2O in the sample prepared at a pH of 4.5. However, the crystallinity of the molybdotellurate decreases when the pH of the slurry decreases and amorphous materials are observed at pH of slurry lower than 3.0. In addition, the characterization results suggest that the pH of the slurry influences the partial incorporation of heteroatoms (V and Nb) and/or the presence of heteroatoms (V and Nb) highly dispersed on the (NH 4) 6TeMo 6O 24· nH 2O in catalyst precursors, which determines the formation of active and selective crystalline phases. The characterization of heat-treated (873 K in a N 2 stream) samples suggests the presence of TeMo 5O 16 crystalline phase for the sample prepared at the highest pH. However, other crystalline phases, i.e. Te 2M 20O 57 and Te 0.33MO 3.33 (M=Mo, V, Nb) and Mo 5− x (V/Nb) x O 14, can be proposed in heat-treated samples obtained from precursors prepared at lower pHs. So, three crystalline phases could be involved in the selective oxidation of propane to acrylic acid.

Effect of coke deposition upon pore structure and self-diffusion in deactivated industrial hydroprocessing catalysts

Pulsed field gradient-nuclear magnetic resonance (PFG-NMR) has been applied to probe molecular motion of hydrocarbon probe molecules in the pores of deactivated catalysts. It is demonstrated that the technique works very well for coked catalysts, which were operated in an industrial reactor for up to 4 years. Cross-polarisation-magic angle spinning (CP-MAS) 13 C NMR was used to characterise the chemical nature of the coke deposited. It was found that reductions of up to 16% in average BJH pore diameter, 40% in surface area of 48% in pore volume of the catalyst occurred in the most heavily coked sample compared with the fresh catalyst. The decrease in pore volume could not be explained by the reduction in pore diameter alone, implying that significant pore blockage also occurred. The self-diffusivities of pentane and heptane probe molecules were found to decrease linearly with increasing coke content. The catalyst effectiveness factor was estimated to decrease by 10% in the most heavily coked sample compared with the fresh catalyst. This reduction in effectiveness factor was considered to have less influence on the overall rate of reaction per unit mass of catalyst than the considerable loss of pore volume. The tortuosities experienced by heptane imbibed in pellets were in the range 2.37–3.71, which was higher than those for pentane in the range 1.6–1.91; for both molecules, tortuosity showed a general increase with coke deposition.

Vanadium Oxide Supported on Mesoporous MCM-41 as Selective Catalysts in the Oxidative Dehydrogenation of Alkanes

The application of different techniques (51V NMR, diffuse reflectance, and TPR-H2) in the characterization of vanadia supported on mesoporous MCM-41 catalysts shows that the nature of the vanadium species depends on the V loading. In dehydrated catalysts with V content lower than 7 wt%, vanadium is mainly in a tetrahedral environment. In the presence of moisture, most V5+, placed in accessible positions, binds to water molecules, becoming hexacoordinated, while some V5+ species, probably located within the pore walls, keep their tetrahedral coordination. Higher V contents in the catalyst leads to the formation of polymeric V2O5-like species. Both XRD and textural results indicate that the mesoporous structure of the support is mostly maintained after the vanadium incorporation, and therefore high surface areas were obtained on the final catalysts. MCM-41-suppported vanadia catalysts are active and selective in the oxidative dehydrogenation of propane and ethane, although the catalytic behavior depends on the V loading. A high rate of formation of propylene or ethylene per unit mass of catalyst per unit time have also been observed as a consequence of the high dispersion of V atoms achieved on the surface of the support.

Definition of the Intrinsic Rate of Photocatalytic Cleavage of Water over Pt-RuO 2/TiO 2 Catalysts

The photocatalytic cleavage of water over Pt-RuO 2/TiO 2 catalysts under near UV illumination is investigated in a semicontinuous flow photoreactor. The effects of operational variables on the intrinsic rate of H 2 production, expressed per unit mass of catalyst per unit "transparency" of the reaction slurry, are investigated. It is shown that the intrinsic rate is a linear function of the intensity of incident illumination, an exponential function of the reaction temperature, and an exponential function of the steady-state pH value of the slurry. An equation describing the intrinsic rate of H 2 production over Pt-RuO 2/TiO 2 slurries is developed and values of the kinetic parameters are defined.

CATALYST EFFECTIVENESS FACTORS UNDER GAS-TO-LIQUID MASS TRANSFER LIMITING REGIME IN LIQUID PHASE METHANOL SYNTHESIS PROCESS

ABSTRACT The effectiveness factors of methanol synthesis catalyst were experimentally measured under condition of gas-to-liquid mass transfer limiting regime in the liquid phase methanol synthesis process, where the synthesis catalyst is slurried in an inert liquid phase. The experimental measurements of effectiveness factors were based on an intrinsic methanol synthesis rate per unit mass of catalyst (gmol/kg cat.h) which is not limited by external mass transfer. The experiments were carried out under well-defined conditions of temperature, pressure, syngas feed flow rate, and impeller speed. The experiments were carried out in a 1-L stirred autoclave. The catalyst slurry ratios were varied from 10 g in 550 mL of Witco-40 oil (corresponding to a slurry ratio of 2.2%) to 150 g in 550 mLof Witco-40 oil (corresponding to a slurry ratio of 25.1%). The experimental measurements have been summarized in one single plot as generalized catalyst effectiveness factor as a function of catalyst slurry ratio. The resu...

MASS TRANSFER CCMPARISON BEIWEEN MECHANICALLY AGITATED SLURRY REACTCR AND LIQUID ENTRAINED REACTOR IN THE METHANOL SYNTHESIS PROCESS

ABSTRACT The commercial reactor proposed for the liquid phase methanol synthesis process (LPMeCHTM) is a liquid entrained reactor (LER), since it possesses several operational advantages over a mechanically agitated slurry reactor (MASR). This paper discusses in detail the develocment of a correlation for the prediction of the overall gas-liquid mass transfer coefficient (KLiaB) in a LER. The overall mass transfer coefficient experimentally determined for the LER, is compared with the corresponding value obtained for the MASR under nominally identical operating conditions. It was observed that the overall gas-liquid mass transfer coefficient and hence the overall rate of methanol production per unit mass of catalyst, is significantly higher in the LER compared to the MASR at identical operating conditions. In the LER mode, the limitation posed on the system by chemical equilibrium, is alleviated by selective renewal of products thus making the process ideal for the once-through methanol (CIM) This justifi...

Dynamic studies of catalyst poisoning: Effect on adsorption and surface reaction rates for hydrogenation of α‐methyl styrene by Pd/Al2O3

AbstractPulse‐response data for the hydrogenation of α‐methyl styrene were obtained as a function of poisoning of the Pd/Al2O3 catalyst. The reaction is a first‐order, reversible adsorption of hydrogen followed by a first‐order, irreversible surface reaction. The experiments, at 298 to 323 K and atmospheric pressure, were carried out in a three‐phase slurry reactor for which mass transport effects could be accurately accounted.Independent adsorption equilibrium data in cumene slurries showed that adsorption occurred to the same extent on both poisoned and unpoisoned sites with a heat of adsorption, ΔHc = −16.3 kJ/mol. Converted to styrene as the slurry liquid, ΔHs = −20.9 kJ/mol.The zero and first moments of the response curves indicated that the rate constant for the surface reaction was independent of the extent of poisoning. The analysis required developing a new theory for interpreting dynamic data where reactant adsorption occurred on both poisoned and unpoisoned sites. The rate constant (per unit mass of catalyst) for adsorption decreased continuously to zero as the extent of poisoning increased.The results are consistent with the concept that poisoning reduced the number of sites active for reaction, but that the activity per site for the surface reaction was constant. However, the activity per site for adsorption, while constant at poison levels up to 40%, appeared to decrease sharply at higher levels.

Crystallite size effects during the catalytic oxidation of propylene on [formula omitted

This work reports results on the kinetics and crystallite size effects during the catalytic oxidation of propylene on Pt γ- Al 2 O 3 . The experiments were carried out in a reactor-chemisorption apparatus that can be used to study the reaction kinetics and in situ characterization of the catalyst by measuring the chemisorption of H 2. It was found that reaction rate decreased during reaction and that oxygen pretreatment restored the catalyst activity. Kinetic studies were carried out using catalysts with 1% Pt and average crystallite sizes of 11, 60, and 144 Å. The kinetic results revealed a complex dependence between reaction rate and C 3H 6 concentration. At low propylene concentration, the reaction rate increased with concentration until it reached a maximum. A further increase in reactant concentration produced a decrease in reaction rate, and at high reactant concentration the reaction rate became zero order. The reaction rates per unit mass of catalyst were quantitatively similar for catalysts with different crystallite sizes; consequently, the reaction rates per unit area of catalyst (specific reaction rates) were different since there was a tenfold difference in average crystallite size. The results indicated that the reaction exhibits demanding behavior and that the specific reaction rate increases significantly with crystallite size. This finding indicates that larger crystallites catalyze the oxidation of propylene more effectively which has also been observed in the case of CO oxidation on Pt α- Al 2 O 3 .