13898
J. Phys. Chem. C 2007, 111, 13898-13904
Fractal Dimension of MoP-Al2O3 Catalysts and Their Activity in Hydrodesulfurization of Dibenzothiophene A. Montesinos-Castellanos, E. Lima,* J. A. de los Reyes H, and Vı´ctor Lara UniVersidad Auto´ noma Metropolitana, Iztapalapa, A. P. 55-532, AV. San Rafael Atlixco No. 186, 09340 Me´ xico D.F., Me´ xico ReceiVed: April 26, 2007; In Final Form: July 4, 2007
Alumina-supported molybdenum phosphide catalysts were prepared by temperature-programmed reduction at 823 and 1123 K under a H2 flow. The catalysts were characterized by physicochemical and spectroscopy techniques. Molybdenum and phosphorous concentration, as well as reduction temperature, emerged as two parameters which influence directly on the morphological and structural aspects of MoP particles at the alumina surface. 95Mo NMR evidences the formation of metallic molybdenum and molybdenum phosphide where molybdenum has a metallic character. The fractal dimension of the catalyst was correlated linearly with the performance in the catalysis of hydrodesulfurization of dibenzothiophene.
Introduction An excess of pollutants emerged from fossil fuel combustion and industrial activities may cause damage to human health.1 Then, more severe regulations have been stated concerning the emission of some compounds to the environment. Among these restrained compounds, nitrogen oxides and sulfur oxides have been focused as targets to reduce their levels of emission. Over the last years, these regulations motivated refineries and research centers to develop new technologies. The most common refinery process for reducing sulfur content is hydrodesulfurization (HDS).2-4 Indeed, many materials have been proposed as catalysts for this reaction, alumina being the most common solid to support active phases of molybdenum or tungsten. Most papers concerning HDS using alumina-supported catalysts are focused to discussing the structure of catalysts and mass transfer phenomena in the catalytic performance. However, in heterogeneous catalysis, the activity of a catalyst is determined, certainly, by the intrinsic activity of the active component and, of course, by effects of mass transport.5,6 The latter are related to the intra- or intercrystalline diffusion of reactants and products. The intrinsic activity depends, of course, on the chemical and physical surface properties of the involved species. Transition metal nitrites, carbides, and phosphides have been used to catalyze reactions of hydrodesulfurization and hydrodenitrogenation (HDN), emerging as promising catalysts based on Ni2P or MoP dispersed on alumina or silica because of their high thermal stability. Stinner et al.7 reported that bulk MoP is 6 times more active than bulk MoS2 in the HDN of o-propilaniline. Oyama et al.8 showed that MoP/Al2O3 catalyst exhibits higher reaction rates than a NiMoS/Al2O3 catalyst in the HDS of a model liquid mixture with DBT, quinoline, benzofuran, tetralin, and tetradecane. Other works support the higher activity of the MoP phase, compared with MoS2.9,10 Concerning the structural properties of MoP supported on alumina catalysts, several studies11,12 agree on a structural model for MoP catalysts supported on alumina with a relatively low surface area (60-100 m2/g). Variations on the size of the MoP particle as a function of metal content were reported. * Corresponding author. E-mail:
[email protected]. Fax: (525) 58044666. Phone: (525) 58044667.
As mentioned above, the structure of HDS catalysts, which is in part responsible for physicochemical properties, has been largely studied. In contrast, despite the case of solid-supported catalysts, the morphology of the dispersed phase is a crucial parameter for catalytic activity, but little attention has been paid to find the relation between the surface properties and the atomic arrangement in the bulk. The bulk arrangement is highly ordered, and impurities or defects can be negligible. Often the most active surfaces correspond to materials containing a less ordered bulk structure. Thus, in this report we present a study concerning the morphological and structural properties of MoP phase supported on industrial alumina with a high surface specific area and the activity of such catalysts in the HDS of dibenzothiophene as currently agreed on for use as a model sulfur compound.3 Experimental Methods Catalyst Preparation. The support was an industrial alumina provided by the Instituto Mexicano del Petro´leo (IMP). MoP/ Al2O3 catalysts were prepared by incipient wetness impregnation of support with aqueous solutions of (NH4)6Mo7O24‚4H2O and (NH4)2HPO4, in the amount required to reach the content reported in Table 1. Note that content of phosphorus and molybdenum varies in both catalysts but, in order to reach the stoichiometry of the MoP phase, the Mo/P molar ratio remains close to 1. After impregnation, catalysts were dried at 393 K and then calcined at 823 K under an air flow. The calcined materials were reduced either at 823 or 1123 K for 2 h at a rate of 5 K/min under a hydrogen flow (1500 NTP cm3/g min). Last, reduced materials were quenched in helium flow. Characterization. X-ray Diffraction (XRD). X-ray diffraction patterns samples were obtained on a Siemens D500 diffractometer with a molybdenum X-ray anode tube. The KR radiation (wavelength of 0.70930 Å) was selected with a diffracted beam monochromator. The radial distribution functions were calculated from the full diffraction patterns as shown by Magini and Cabrini.13 In order to obtain high values of the angular parameter h ) 4π sin θ/λ, the diffractogram was measured by step scanning at angular intervals of 0.08°. Small-Angle X-ray Scattering (SAXS). A Kratky camera coupled to a copper anode tube was used to measure the SAXS
10.1021/jp073232u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2007
Fractal Dimension of MoP-Al2O3 Catalysts curves. The distance between the sample and the linear proportional counter was 25 cm; a Ni filter selected the Cu KR radiation. The sample was introduced into a capillary tube. Intensity I(h) was measured for 9 min in order to obtain good quality statistics. The SAXS data were processed with the ITP program,14-18 where the angular parameter (h) is defined as h ) 2π sin θ/λ, where θ and λ are the X-ray scattering angle and the wavelength, respectively. The Kratky plot, that is, h2I(h) versus h, provided the shape of the scattering heterogeneities. For instance, if heterogeneities are globular, the Kratky plot presents a peak. Last, from the slope of the curve log I(h) versus log(h), the fractal dimension of the scattering objects can be obtained.19 Multinuclear Solid-State Nuclear Magnetic Resonance. The solid-state one-pulse nuclear magnetic resonance (NMR) spectra were obtained under magic angle spinning (MAS) conditions using a Bruker Avance-300 spectrometer with a magnetic field strength of 7.05 T, corresponding to a 31P Larmor frequency of 121.4 MHz. The spectra were obtained with a sample spinning rate of 5 kHz. The chemical shifts were referenced to a H3PO4 solution (85%). The 27Al MAS NMR spectra were achieved by operating the spectrometer at 78.3 MHz. The calibrated π/2 pulse was 2 µs, and the recycle time used was 0.5 s. The 27Al chemical shifts were referenced using an aqueous solution of Al(NO3)3 as the external reference. In order to collect the NMR data on the reduced samples, they were placed in a glovebox under argon and were packed in ZrO2 rotors (4 mm o.d.) immediately after preparation. The time for recording one NMR (27Al or 31P) spectrum was not longer than 10 min. 95Mo MAS NMR spectra were obtained using a 7 mm probe, special for low frequencies, operating at 19.5 MHz. The π/2 pulse was 20 µs, and a delay repetition time of 1 s was used. Because of the low relative receptivity of this nucleus (0.001741 relative to 13C), spectra were acquired with scan numbers ranging from 17 500 to 107 000. The signals were referenced to an aqueous solution of molybdate ammonium. N2 Adsorption. Nitrogen adsorption-desorption isotherms were obtained on an Autosorb Quantachrome apparatus. Prior to nitrogen adsorption, the samples were outgassed at 473 K for 4 h. Transmission Electron Microscopy. Digital images of several catalysts were obtained using two electronic microscopes. Conventional transmission electron microscopy (TEM) images were obtained using a JEOL-100CX electron microscope with a point to point resolution of 0.35 nm. Catalytic Test. The HDS of DBT (Aldrich; 99.9%) was carried out in a continuous-flow microreactor in the vapor phase working with a total flow of 6-10 l h-1, pH2 ) 3.4 MPa, pDBT ) 15 kPa, at 553 K. The catalyst mass was approximately 50 mg. The reaction evolution was followed by chromatography in a gas chromatograph equipped with a flame ionization detector and a capillary column of 5% phenyl-95% methylpolysiloxane (EC5), purchased from Alltech. The specific reaction rates were calculated according to the next expression:
specific reaction rate )
molar flow rate of DBT catalyst mass (conversion of DBT)
All rates were estimated at low conversion (
