Browse

You are looking at 101 - 110 of 392 items for :

  • Chemistry and Chemical Engineering x
  • Architecture and Architectonics x
  • All content x
Clear All
Reaction Kinetics, Mechanisms and Catalysis
Authors: Olga Yugay, Tatyana Mikhailovskaya, Lyudmila Saurambaeva, Dauren Sembaev, and Pavel Vorobiov

Abstract

The effect of ZrO2 and ZrV2O7 on the polymorphic transformation of anatase to rutile in the TiO2–V2O5, TiO2–ZrO2, TiO2–ZrV2O7, and V2O5–TiO2–ZrO2 systems has been investigated. It is shown that the temperature and duration of the heat treatment influence on the anatase stability in the investigated binary and ternary systems. It is established that V2O5 and ZrV2O7 accelerate the polymorphic transformation of anatase to rutile, but ZrO2 inhibits the phase change. Taking into account the ratio between V2O5, ZrO2, and ZrV2O7, formed in the ternary system, it is possible to maintain a definite quantity of anatase in a catalyst system. This permits one to influence the activity and selectivity of the modified vanadium oxide catalysts in the oxidation and ammoxidation of alkylbenzenes and alkylpyridines.

Restricted access

Abstract

12-Molybdophosphoric acid (MPA) supported on V2O5 dispersed γ-Al2O3 catalysts with different loadings were prepared and characterized by BET surface area, X-ray diffraction, FT-infrared, laser Raman, X-ray photoelectron spectroscopy and temperature programmed reduction techniques. The catalytic activities were evaluated for the aerobic oxidation of benzyl alcohol. The conversion of benzyl alcohol increased with the amount of MPA content and the catalyst with 15 wt% of MPA showed highest activity. The synergistic effect of V2O5 and MPA was observed for the oxidation of benzyl alcohol compared to MPA on alumina without V2O5. The XPS results suggest the participation of both Mo and V in the reaction as the used catalysts showed the reduced oxidation states of both Mo and V. The high selectivity of the catalysts is due to the presence of V2O5, which induces the redox nature to the catalyst and also preventing the decomposition of MPA on Al2O3.

Restricted access

Abstract

The properties of defects on the (100) MoS2 surface have been investigated by the perturbed cluster method. The perturbed cluster method provides an accurate description of the local defect properties while taking into account the interaction between the defect and the surrounding crystal. The surface energies, including correlation correction, of different defect structures of various sizes on the (100) MoS2 surface are reported and compared with the energy of a reference surface cluster. The results, in conjunction with calculations of the electronic properties and electrostatic potential of the different defect sites, show that the chemistry of the defects differs from that of the perfect (100) “as-cleaved” surface. The enhanced reactivity of the defects is ascribed to the anisotropy in the electrostatic potential. The presence of “nodes” in the surface electrostatic potential suggests that the adsorption of small polarizable molecules will preferentially take place in the vicinity of these defects.

Restricted access

Abstract

Various tungstated zirconia catalysts with a WO3 loading of about 16 wt% were characterized both in their acid and oxidation properties. The samples have been characterized in their micro-structural and surface properties by BET, X-ray diffraction, Raman spectroscopy, temperature programmed reduction, elemental chemical analysis. The surface acidity was determined by the techniques of NH3 adsorption microcalorimetry and pyridine infrared spectroscopy (FT-IR). Improved acidity has been detected upon addition of WO3 to zirconia by both techniques. The global acid strength and the total number of acid sites increased greatly with the formation of WOx clusters on the zirconia support. This acidity increase can be attributed to the creation of Br⊘nsted acid sites generated by the well dispersed WOx domains, as observed by FT-IR pyridine desorption.

Restricted access

Abstract

The cyclodimerization of crotonaldehyde was performed over acid or base catalysts in the gas phase. We first attempted the reaction over various acid and base catalysts using a pulse reactor. The typical bases, CaO and MgO, effectively promoted the reaction to form methylcyclohexadienecarbaldehydes (MCHC) and tolaldehydes. In contrast, no significant formation of the dimers was observed over the acid catalysts, such as SiO2–Al2O3 and H-mordenite. Aluminum oxide also promoted the dimerization, indicating that the dimerization proceeds on the weaker base sites. The main products in the formed dimers were 6-methylcyclohexa-1,3-dienecarbaldehyde and o-tolualdehyde. In addition, small amounts of 4-methylcyclohexa-1,5-dienecarbaldehyde and p-tolaldehyde were produced as dimers. When the dimerization was performed over CaO, MgO, and Al2O3 using a fixed-bed flow reactor, the catalytic activities of all the catalysts significantly decreased during the initial stage of the reaction. The TG–DTA analysis of the used catalysts clearly indicated that a large amount of the condensation products had adsorbed on the catalyst surface. The maximum selectivity to the dimers (MCHC and tolaldehydes) was 38% for an approximate 30% conversion, which was obtained during the initial stage of the reaction over the Al2O3 and CaO catalysts.

Restricted access

Abstract

The Friedel–Crafts benzoylation of biphenyl (BP) was carried out under liquid-phase batch conditions using raw and modified clinoptilolite zeolites as catalysts. Solventless benzoylation of BP with benzoyl chloride preferentially gave p-monoacylated product, 4-phenyl-benzophenone (4-PBP), as the main product for all catalysts used and only small amount of diacylated product, 4,4′-dibenzoylbiphenyl (4,4′-DBBP) was observed. The highest yield of 4-PBP was achieved by using HT-823 zeolite as catalyst. Calcined zeolites, except the clinoptilolite calcined at 1,023 K, are more active than their raw counterparts. Raw clinoptilolite gave 4-PBP yield of 48.5% while the HT-823 proves to be a highly efficient catalyst giving a very high selectivity to 4-PBP (~98.5%) with the conversion of BP ~86.9% at 453 K. The results revealed that zeolite HT-823 is the best catalyst for the benzoylation of biphenyl to 4-PBP. Also, increasing the reaction temperature appeared to play a significant role on the 4-PBP yield.

Restricted access

Abstract

A study of the interaction between some simple molecules (dihydrogen, acetylene, and ethylene) and Pd–Pb catalysts has been performed using the B3LYP hybrid density functional. The reaction paths for the H2 molecule reacting with the PdPb dimer are reported for the singlet and triplet spin states. The C2H2 and C2H4 molecules were adsorbed in a few characteristic sites on the Pd(100) surface doped with Pb. This surface was modeled using Pd13Pb clusters. The results of the calculations indicate that the Pd–Pb catalysts interact with the H2, C2H2, and C2H4 molecules more weakly than the corresponding monometallic Pd catalysts do, and thus the bimetallic catalysts exhibit the reduced activity toward these simple molecules.

Open access

Abstract

The role of V and Mn incorporated mesoporous molecular sieves was investigated for the vapor phase oxidation of o-xylene. Mesoporous monometallic V-MCM-41 (Si/V = 25, 50, 75 and 100), Mn-MCM-41 (Si/Mn = 50) and bimetallic V-Mn-MCM-41 (Si/(V + Mn) = 100) molecular sieves were synthesized by a direct hydrothermal (DHT) process and characterized by various techniques such as X-ray diffraction, DRUV-Vis spectroscopy, EPR, and transmission electron microscopy (TEM). From the DRUV-Vis and EPR spectral study, it was found that most of the V species are present as vanadyl ions (VO2+) in the as-synthesized catalysts and as highly dispersed V5+ ions in tetrahedral coordination in the calcined catalysts. The activity of the catalysts was measured and compared with each other for the gas phase oxidation of o-xylene in the presence of atmospheric air as an oxidant at 573 K. Among the various catalysts, V-MCM-41 with Si/V = 50 exhibited high activity towards production of phthalic anhydride under the experimental condition. The correlation between the phthalic anhydride selectivity and the physico-chemical characteristics of the catalyst was found. It is concluded that V5+ species present in the MCM-41 silica matrix are the active sites responsible for the selective formation of phthalic anhydride during the vapor phase oxidation of o-xylene.

Restricted access

Abstract

The selectivity obtained in the aromatization of propane over Zn-ZSM-5-based catalysts (Si/Al nominal ratio = 35) increased from about 69% (carbon basis) to 74–91% when a second transition metal ion is introduced into the catalyst. The effect, previously reported for the case of Fe3+ ions, has now been shown to be more general and is seen also for the ions of chromium, cobalt or manganese. It is likely that these ions are present under aromatization conditions in a partially reduced state. H2/D2 isotope exchange rates show large enhancements on the reduced Fe-Zn/ZSM-5 and Co-Zn/ZSM-5 samples. This aspect of aromatization catalysis has not been reported before.

Restricted access

Abstract

The hydrogenation of m-dinitrobenzene to m-phenylenediamine was carried out as a model hydrogenation reaction of importance to the pharmaceutical and fine chemicals industries with the aim of investigating the kinetics of the reaction. The effect of different conditions: hydrogen pressure, m-dinitrobenzene concentration, reaction temperature, and weight of catalyst on the conversion of m-dinitrobenzene and the yield of m-phenylenediamine were studied using Pt/TiO2 catalyst. During the kinetic study, the intermediate m-nitroaniline was detected. Therefore, the overall reaction was treated as consecutive reactions: first the reduction of m-dinitrobenzene to m-nitroaniline and then, the reduction of m-nitroaniline to m-phenylenediamine. The apparent activation energies of the reaction were determined in each step, to be 33.4 ± 0.4 and 39.8 ± 0.6 kJ/mol. Those results indicated that the hydrogenation of m-nitroaniline toward m-phenylenediamine is the rate determining step in the hydrogenation of m-dinitrobenzene. Two rate equations assuming Langmuir–Hinshelwood mechanism provided the best fit to the experimental data.

Restricted access