КИНЕТИКА И КАТАЛИЗ, 2010, том 51, № 3, с. 406-413
УДК 542.973:546.821-31:541.031
HETEROATOM EFFECT ON HYDRODESULPHURIZATION ACTIVITY OF TiO2-SUPPORTED MOLYBDENUM HETEROPOLYOXOMETALATES
OF ANDERSON TYPE © 2010 A. A. Spojakina, E. Y. Kraleva, K. Jiratova*
Institute of Catalysis, Bulgarian Academy of Sciences, Sofia, Bulgaria * Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Prague, Czech Republic
E-mail: kiyky@ic.bas.bg; mariamaleva@yahoo.ie Received 14.01.2009
The properties of the Mo-containing samples (6 wt. % Mo) prepared by mechanochemical mixing of Fe, Co or Ni heteropolymolybdates of Anderson type and the TiO2 support are reported. A detailed characterization of the surface properties has been performed by BET, scanning electron microscopy, IR, XPS and thermo programmed reaction methods and by testing of catalysts in the thiophene conversion. The effects of the mixing time and the nature of heteroatom on the properties of the samples are shown. The nanosized particles are formed when mechanochemical synthesis is running 30 min. The Ni-containing catalyst reveals the highest and the most stable activity compared to Co- and Fe-containing catalysts. Their hydrodesulphurization activity is compared to those of alumina and titania catalysts synthesized by the conventional impregnation method.
Nowadays, the need to improve the removal of sulfur from gasoline and diesel oil by means of deep hydrodesulphurization (HDS) is driven by the new environmental legislation regarding fuel specifications. Since 1960's, cobalt and molybdenum on alumina or silica are the most studied systems in HDS of petroleum fractions [1]. Many efforts are aimed to design more active HDS catalysts by either using new catalytic supports or changing the active phase [2—5]. Supported MoO3 and other oxides of the first transition series have been widely studied as catalysts for hy-drotreating processes. Recently industrial and scientific interest has focused on heteropolyoxometalates.
To date, there are few reports on the application of heteropolyacids (HPAc) in preparation of HDS catalysts [6, 7], including heteropolycompounds ofAnder-son type [8—10]. Heteropolycompounds can serve as a source of active components of hydrotreating catalysts. They contain both fundamental and promoting elements (heteropolyanion and countercation) in a single compound of defined structure, and therefore, their use in a study of HDS catalysts is valuable and can give useful information for further improvement of the catalysts. Initial Anderson type heteropolyoxomolybates of general formula [XM6O24H6f-, where M = Mo, W and X = Co, Cr, Rh, Al, Te, Ni, are characterized by the presence of central octahedral X atom surrounded by 6 edge-sharing M06 octahedra showing planar hexagonal configuration [11]. The planar structure provides a close contact with the support surface.
Earlier we showed that the cobalt and nickel het-eropolyoxomolybdates ofAnderson type having atomic ratio Co(Ni)/Mo = 0.17, are suitable precursors of
active sites for hydrotreating catalysts [8, 9]. Lately we received promising results with CoMo/Al2O3 catalyst, synthesized using CoMo heteropolyoxomolybdate of Anderson type, in the HDS of gas oil in the pilot plant catalytic unit [12].
The nature of the active phase in the prepared catalyst is determined not only by the structure of the oxide precursor species but it is strictly dependent on the preparation methods [5].
One of the important factors that affect catalyst activity is the interaction between the active components and support. The metal—support interaction plays an important role in the catalyst activity by influencing both dispersion of active ingredients and their ability to be reduced and sulfided [13, 14].
Different supports have been used for loading of HPAc. In case that HPAc are deposited on alumina their characteristic heteropolyanion structure is destroyed, and catalytic behavior of resulting catalysts is similar to that of the conventional Mo(W)-containing catalysts [7]. TiO2 support has been reported to provide higher dispersion of the active phase [15] and increase the activity of CoMoS catalyst in HDS of ben-zothiophene [16] because heteropolycompound is kept on the support (or a newly formed compound) in a greater extent.
Method of catalyst preparation plays an important role in achieving of the optimal catalyst activity. Usage of different methods is known: impregnation, co-precipitation, sol-gel method, mechanochemical treatment [17]. Co-precipitation of the initial compounds from water solutions or impregnation of the support with the initial compounds is used to synthesize the catalysts most often. Mechanochemical treatment is
less often applied in catalyst manufacture but it offers some advantages. First of all, it is simple and moreover it does not produce large quantities ofwaste waters etc. The positive role of the mechanochemical treatment on the HDS catalyst activity was shown earlier [18, 19]. It was also pointed out that the effect of this preparation method on the HDS activity depends on the support used [20]. The method gives especially good results when a strong acid and a base as reagents realize the interaction [21].
The goal of this paper is to study the effect of the synthesis method and the nature of heteroatom (Fe, Co, Ni) of molybdenum heteropolyoxometalates as the promoters in the TiO2-supported HDS catalysts on their phase composition and catalytic activity in thiophene hydrodesulphurization. The catalysts were synthesized by mechanochemical mixing of the support with FeMo, CoMo or NiMo heteropolycom-pounds ofAnderson type. The influence of the milling time on the properties of catalysts was also examined. The results are compared with activity of conventional alumina and titania supported catalysts, synthesized by impregnation.
EXPERIMENTAL
Catalyst preparation
The catalysts were prepared by mechanochemical mixing of the TiO2 support and the Fe, Co or Ni heteropolymolybdates of Anderson type in an agate mortar for 30 and 90 min. The laboratory synthe-
Table 1. Metal content in the HPA salts
sized TiO2 has anatase structure.
¿bet = 72 m2/g,
Sample Fe, Co or Ni, wt. % Mo, wt. % Mo/(Fe, Co or Ni), mol/mol
FeMo6 4.1 43.3 6.2
CoMo6 4.2 44.2 6.1
NiMo6 4.6 45.8 6.1
and pore volume 0.198 cm3/g. Commercial product [(NH4)3Fe(OH)6Mo6O18 salt ("Chimreactiv", Russia, pure) and [(NH4)3Co(OH)6Mo6O18] and [(NH4)4Ni(OH)6Mo6O18] salts, synthesized in our laboratory according to [22, 23] were used for the catalyst preparation. The salts are denoted as FeMo6, CoMo6 and NiMo6, respectively. The composition of all salts is presented in Table 1. The composition of the synthesized heteropolycompounds is close to the theoretical one. Amounts of the loaded salt and the support were chosen intentionally in order the amount of molybdenum in all catalysts was 6 wt. %. The milled catalysts were dried 4 hrs at 105oC and calcined 2 hrs at 350oC. Denotion of the catalysts includes both the abbreviation of the used salt (FeMo6, CoMo6, NiMo6) and the time of treatment (30, 90), e.g. FeMo6-30 means that the (NH4)3Fe(OH)6Mo6O18 salt is mixed with theTiO2 support for 30 min in the mortar.
Catalysts characterization
Specific surface area. The surface area of the catalysts was determined from the nitrogen adsorption— desorption isotherms at —195°C (BET method).
Scanning electron microscopy (SEM). Scanning electron microscope JSM-5300 ("Jeol") was used to study the morphology of the deposited layers in oxidic form. The photographs were taken in the regime of secondary electrons (SEI).
Thermoprogrammed reduction (TPR). TPR measurements were carried out in an apparatus described earlier [24]. Hydrogen/nitrogen mixture (10 mol. % H2) was used to reduce catalysts at a flow rate of 17 cm3/min. The temperature was linearly raised at a rate of 20°C/min up to 850°C.
IR spectra. IR spectra (400—1200 cm-1) were recorded at room temperature on a "Bruker IFS-25 Fourier transform" IR spectrometer. The catalysts were pressed with KBr in ratio 1 : 150. Alumina and titania absorption in the 400-1200 cm-1 range was compensated by subtraction of a normalized spectrum of the equivalent amount of support from the catalyst spectrum [25].
X-ray photoelectron spectroscopy (XPS). XPS measurements were performed with "ESCALAB-Mk II" ("VG Scientific") electron spectrometer at a base pressure 1 x 10-8 Pa. Samples were excited with Mg Ka radiation (hv = 1253.6 eV). The total energy resolution of the instrument was 1.2 eV as measured by half width of the Ag3d5/2 photoelectron peak. Powdered samples were pressed into 12 mm diameter stainless-steel sample holders to obtain a thickness of 0.7-1 mm. After introduction into the preparation chamber of the electron spectrometer, samples were evacuated to 10-6-10-7 Pa, and transferred to the analysis chamber for XPS measurements. The glass reactor with sulfided sample was opened in a glove box connected to the fast entry lock of the XPS instrument. The sample was transferred to its holder without exposure to air. The following photoelectrons were recorded: C 1s, O 1s, Mo 3d, Ti 2p, Fe 2p, Co 2p, Ni 2p and S 2p.
The binding energies (BE) of C 1s, O1s, Mo 3d, Ti 2p, Fe 2p, Co 2p, Ni 2p and S2p core electron levels were determined by computer fitting the measured spectra and were referenced to the C 1s XPS signal at 284.5 eV. The binding energy was taken at the Ti 2p at 458.5 eV and estimated error of 0.1 eV can be assumed for all the measurements. In order to obtain information on the surface structure and the dispersion of the active phases the surface atomic ratios were calculated as the ratio of the corresponding peak intensities, corrected with theoretical sensitivity factors based on Scofield's photoionisation cross-sections [26].
Table 2. Catalysts characteristics
Parameter*
Catalyst
FeMo6-30 FeMo6-90 CoMo6-30 CoMo6-90 NiMo6-30 NiMo6-90
¿BET
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