H. L. Wan

Preparation and characterization of rare earth orthovanadates for propane oxidative dehydrogenation

High purity rare earth orthovanadates (REVO4), YVO4, LaVO4, CeVO4, NdVO4, SmVO4 and EuVO4, were prepared by the citrate method. XRD, FT-IR, LRS and TPR techniques were employed to characterize these orthovanadates. The catalytic performance of SmVO4, LaVO4 and YVO4 in the oxidative dehydrogenation of propane can compete with that of Mg3V2O8. The selectivity of propene over CeVO4, NdVO4 and EuVO4 was relatively lower. The correlation between the reducibility and the selectivity of the catalysts implied that the V4+/V3+ couple might be involved in the dehydrogenation process.

Pulse studies of CH4 interaction with NiO/Al2O3 catalysts

Pulse studies of the interaction of CH4 and NiO/Al2O3 catalysts at 500°C indicate that CH4 adsorption on reduced nickel sites is a key step for CH4 oxidative conversion. On an oxygen-rich surface, CH4 conversion is low and the selectivity of CO2 is higher than that of CO. With the consumption of surface oxygen, CO selectivity increases while the CO2 selectivity falls. The conversion of CH4 is small at 500°C when a pulse of CH4/O2 (CH4∶O2=2∶1) is introduced to the partially reduced catalyst, indicating that CH4 and O2 adsorption are competitive steps and the adsorption of O2 is more favorable than CH4 adsorption

The activation of O2 and the oxidative dehydrogenation of C2H6 over SmOF catalyst

The activation of O2 over SmOF was studied by in situ laser Raman spectrometry and temperature programmed desorption (TPD). When the hydrogen- and helium-treated (1 h for each gas at 973 K) SmOF sample was cooled to 303 K in oxygen, Raman bands which correspond to the existence of O22−, O2n− (2 >n > 1), O2− and O2δ-(1 >δ > 0) species were observed. From 303 to 973 K, there was no O2 desorption but the Raman bands observed at 303 K reduced in intensity and vanished completely at 973 K, even though the sample was under an atmosphere of oxygen. We suggest that as the sample temperature increased, dioxygen species were converted to mono-oxygen species such as O− which were undetectable by Raman spectrometry. O2 desorption occurred above 973 K, giving a TPD-peak at 1095 K. When C2 H6 was pulsed over the sample pretreated with oxygen and helium at 973 K, C2H4 selectivity was 91.8%. We conclude that the mono-oxygen species is responsible for the oxidative dehydrogenation of ethane to ethene.

Oxidative dehydrogenation of ethane over LaF3CeO2 catalysts

A new series of very selective rare earth oxyfluoride LaF3CeO2 catalysts was prepared and used for the oxidative dehydrogenation of ethane (ODE). At 953 K and C2H6 : O2 : N2=2 : 1 : 7, C2H4 selectivity of 94.5% with C2H6 conversion of 23.4% were achieved over LaF3CeO2 (1 : 1 in mole) catalyst. XRD and BET measurements showed that LaF3 dissolved in CeO2 and acted as an isolation material as well as partial anions (F−, O2−) and/or cations (La3+, Ce4+) exchange between LaF3 and CeO2 phases might be responsible for the high selectivity.

THE OXIDATIVE COUPLING OF METHANE AND THE ACTIVATION OF MOLECULAR O2 ON CEO2/BAF2

CeO2/BaF2 was used as the catalyst for the oxidative coupling of methane (OCM). At 800°C and CH4∶O2=2.7∶1,CH4 conversion of 34% with C2 hydrocarbon selectivity of 54.3% was obtained. XRD measurement showed that partial anion (O2−,F−) and/or cation (Ce4+,Ba2+) exchange between CeO2 and BaF2 lattices occurred. ESR study showed that O− species existed on degassed catalyst. XPS study revealed that, when BaF2 was added to CeO2, the binding energy of Be 3d5/2 was 2.2 eV lower than that in CeO2, and the “electron-enriched lattice oxygen” species was detected. XPS, ESR and Raman study showed that, under O2 adsorbing conditions, O22− and O−2 species were detected on CeO2/BaF2.

Oxidative dehydrogenation of ethane over BaF2 promoted SmO3LaF3 catalysts

Abstract Oxidative dehydrogenation of ethane (ODE) was investigated on Sm 2 O 3 LaF 3 and BaF 2 promoted Sm 2 O 3 LaF 3 catalysts. Under the reaction conditions: 973 K, reactant mixture (C 2 H 6 : O 2 : N 2 =2 : 1 : 7) flow rate 40 ml/min, 89.7% ethene selectivity with 22.1% ethane (C 2 H 6 ) conversion was obtained on Sm 2 O 3 LaF 3 catalyst (molar ratio 1 : 1). The addition of BaF 2 significantly improved the catalytic performance. On 16.7 mol%Ba/F 2 Sm 2 O 3 LaF 3 (molar ratio 1 : 1), 83.9% ethene selectivity with 42.0% ethane conversion was obtained. XRD measurement revealed that partial cation or anion substitution occurred between Sm 2 O 3 and LaF 3 phases (for Sm 2 O 3 LaF 3 catalysts) as well as Sm 2 O 3 LaF 3 and BaF 2 phases (forBa/F 2 Sm 2 O 3 LaF 3 catalysts). Structure defects such as anionic vacancies, O − and F-center might be formed by the substitution. Rhombohedral Sm 2/3 La 1/3 OF was the main component in catalysts of Sm 2 O 3 LaF 3 (molar ratio 1 : 1) and 16.7 mol%Ba/F 2 Sm 2 O 3 LaF 3 (molar ratio 1 : 1). The formation of this kind of binary rare earth oxyfluoride might be one of the factors that are responsible for the high C 2 H 4 selectivity. XPS results that the binding energies of La3d 5/2 and Sm3d 5/2 in catalysts of Sm 2 O 3 LaF 3 are 0.5-1.5 eV negative shifts compared with the standard binding energies of La3d 5/2 and Sm3d 5/2 in La 2 O 3 , LaF 3 and Sm 2 O 3 show that the electron donating ability of Sm 2 O 3 LaF 3 catalysts is higher than pure La 2 O 3 , LaF 3 and Sm 2 O 3 . Adding 16.7 mol% BaF 2 to Sm 2 O 3 LaF 3 catalysts, although the binding energy of La3d 5/2 varied little compared with that in Sm 2 O 3 LaF 3 , the difference between the binding energies of La3d 5/2 and La3d 3/2 decreased slightly. The contraction of cubic BaF 2 and the decrease of the difference of the binding energy of La3d 5/2 and La3d 5/2 suggested that the isomorphous substitution between BaF 2 and LnOF (Ln La and/or Sm) lattice took place and more structure defects were formed. These might be the cause of the improvement of ethane conversion.

Oxidative coupling of methane overSrF2/Y2O3 catalyst

Abstract The use of SrF 2 as a dopant of Y 2 O 3 catalyst for oxidative coupling of methane apparently increased C 2 selectivity and yield under the same conditions. However, it simultaneously decreased both the acidity and basicity of the catalyst, indicating that there was no direct relationship between catalytic performance and acidity/basicity of the catalyst. The interaction between SrF 2 and Y 2 O 3 phase took place more or less in the course of the catalyst preparations, as proved by XRD results of the fresh and used catalysts. The intrinsic anionic vacancies in the catalyst would be favorable to the activation of molecular oxygen under the reaction conditions. ESR and in situ Raman spectral results showed that superoxide ion (O 2 − ) adspecies was observed on the surface of O 2 -pretreated 67 mol%SrF 2 /Y 2 O 3 catalyst at 973 K, which may be the active oxygen species for oxidative coupling of methane.

OXIDATIVE COUPLING OF METHANE OVER LAF3/LA2O3 CATALYSTS

We studied the oxidative coupling of methane over the LaF3/La2O3 (50∶50) catalyst. The catalyst was found active even at 873 K. At 1023 K, the C2 yield was 12.7% at 26.0% CH4 conversion and 49.1% C2 selectivity. It was found to be stable and had a lifetime not less than 50 h at 1023 K. The catalyst was effective in C2H6 conversion to C2H4. XRD results indicated that the catalyst was mainly rhombohedral LaOF. It is suggested that the catalyst has ample stoichiometric defects and generates active oxygen sites suitable for methane dehydrogenation.

Methane and Light Alkane (C 2 -C 4 ) Conversion Over Metal Fluoride-Metal Oxide Catalyst System in Presence of Oxygen

A novel series of metal fluoride-metal oxide catalysts with good to excellent catalytic performance for oxidative coupling of methane and oxidative dehydrogenation of ethane, propane and iso-butane were developed. XRD analysis suggested that, with the addition of metal fluorides to metal oxides, partial substitution of cations and/or anions happened between fluorides and oxides, leading to the formation of new phases ( e.g. LaOF ) and expansion/contraction of lattices ( e.g. in CeO2-BaF2 ). O2 n- (1 ≤ n ≤ 2) species were detected by the Raman spectroscopy over the catalysts after O2 adsorption. O2- species was observed by in situ FTIR spectroscopies at 650°C over SrF2-La2Q3 catalyst.