3846
Ind. Eng. Chem. Res. 1998, 37, 3846-3852
The Influence of Calcination Treatment over Bifunctional Ni/ HZSM-5 Catalysts Marı´a D. Romero,* Jose´ A. Calles, Araceli Rodrı´guez, and Juan C. Cabanelas Departamento de Ingenierı´a Quı´mica, Facultad de Quı´micas, Universidad Complutense de Madrid, 28040 Madrid, Spain
Effects of the temperature and heating rate in the calcination step over some properties of Ni/ HZSM-5 catalysts have been studied and discussed. The catalysts have been characterized by temperature-programmed reduction, ammonia temperature-programmed desorption, X-ray diffraction line broadening, and X-ray photoelectron spectroscopy (XPS) techniques, and n-decane hydroconversion has been employed as a test reaction. Changes in the strength acidity distribution of the catalysts with the calcination temperature were observed. XPS measurements revealed that the nickel precursor compound Ni(NO3)2 decomposes totally in the catalysts calcined above 573 K, whereas a small proportion of the precursor was detected in the catalysts calcined at 573 K. This fact as well as the reduction of the different existing metallic species determines the minimum particle size for the catalysts calcined at 723 K. The reaction parameters in n-decane hydroconversion also showed the effect of the calcination temperature. 1. Introduction Zeolite-supported nickel catalysts find wide application in many important industrial hydrogenation processes, such as hydrocracking and hydroisomerization of normal paraffins. These processes have become increasingly important and have been employed in the current petroleum refining industry to enhance the octane number of light paraffins1-3 and to improve the low-temperature properties such as the pour point of heavy petroleum fractions.4-6 The catalysts used in these processes contain two types of active centers:7 hydrogenation sites (metal phase) to form olefinic intermediates and acidic sites (zeolite phase) to form carbenium ions to proceed with isomerization and cracking. The goal in such bifunctional catalysts preparation is to maximize the metal surface area and consequently the catalytic activity in the hydroconversion reaction. In this respect, the first condition to obtain high isomerization yield is an adequate contact between both functions.8,9 The required intimacy of the two catalytic functions, ensuring maximum synergy, has been quantified and is known as the Weisz intimacy criterion.10 It determines the critical distance of hydrogenating-dehydrogenating sites and acid sites, so that the diffusivity of a given alkane in the pores of the zeolite exceeds that of the intrinsic reaction rate for the alkane conversion on the acid sites. Another necessary condition is that hydrogenating and acidic activities of the catalysts be in balance.11-13 When this balance deviates from the optimum, the hydrogenation function is generally weakened and cracking is favored. For all that is mentioned above, an understanding of the elementary steps involved in particle formation is a prerequisite for the determination of the most effective preparation conditions. The common bifunctional catalyst preparation methods are multistep processes consisting mainly of the following: (i) deposition of a metal precursor compound * Author to whom correspondence should be addressed. E-mail:
[email protected].
over the support surface either by impregnation, ion exchange, precipitation, coprecipitation, mechanical mixing, or vapor-phase deposition, (ii) drying and calcining of the catalysts, and (iii) transformation of the precursor compound into the active metallic phase by reduction. It is well-established in these catalysts that the metal dispersion and thus the metal/acid ratio depends markedly on the way the metal is introduced and particularly on the conditions under which the calcination of the metal precursor compound and zeolite was carried out.14-16 Calcination brings about the following transformations: decomposition of the precursor compound and formation of oxide species. Reaction of the formed oxide with the support and sintering of the precursor compound or the formed oxide species are also expected. Accordingly, calcination may have a marked effect on parameters such as reducibility of the metal dispersion and distribution of the metal in the final products. Metal ions migrate during calcination to locations which provide greater electrostatic stabilization and higher coordination to lattice oxygen ions. The extent of migration and the location of metal ions after calcination is determined by size-to-charge ratios and the degree of complexation of the metal ion, the latter being controlled by the calcination temperature.17 In previous works, the influence of the preparation method and precursor compound was studied.18 Incipient wetness impregnation with an aqueous solution of nickel nitrate was selected. The aim of this work is to study the effect of the calcination treatment, temperature, and heating rate over Ni/HZSM-5 catalysts, specificially, on one hand, the state of the nickel and how it is influenced by the presence of HZSM-5 as the support, and on the other hand, how the nickel species can influence the properties of HZSM-5, especially its acidity. A number of techniques including temperatureprogrammed desorption (TPD) of ammonia, temperatureprogrammed reduction (TPR), X-ray diffraction (XRD) line broadening, and X-ray photoelectron spectroscopy (XPS) were applied to study the effect of the calcination
S0888-5885(98)00143-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/10/1998
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3847 Table 1. Calcination Conditions of the Catalysts catalysts
temp (K)
heating rate (K/min)
C0 C1 C2 C3 C4 C1R C2R
573 723 823 923 573 723
sudden sudden sudden sudden 2 2
time (h) 5 5 5 5 5 5
over the acid and metal functions. Hydroconversion of n-decane was used as a test reaction. 2. Experimental Section 2.1. Catalyst Preparation Procedures. All the catalysts were prepared from ZSM-5 as an acid function and nickel as a metallic function. The zeolite was synthesized in sodium form by a procedure developed in our laboratory,19 with an atomic ratio Si/Al ) 29, crystallinity of 100%, and an 8-µm medium particle size. HZSM-5 was prepared by ion exchange with 50 mL of 0.6 N HCl solution/g of catalyst in agitation for 5 h at room temperature of the Na form. The supported catalysts were prepared by the incipient wetness impregnation technique. The required amount of an aqueous solution of nickel nitrate was slowly added to the support at room temperature. This amount was calculated by using the water pore volume of the support. The concentration of the solution was adjusted to obtain catalysts of 5% metal loading. All the samples were dried at 383 K for 5 h. Calcination was carried out in an oxidizing static atmosphere for 5 h, at temperatures ranging from 573 to 923 K, and with a controlled heating rate of 2 K/min or a sharp sudden heating rate, as Table 1 shows. Reduction of all the samples was carried out in situ with a hydrogen stream at 723 K for 3 h and a hydrogen flow of 50 NmL/min per gram of catalyst in the same tubular fixed bed reactor employed for n-decane hydroconversion. 2.2. Characterization. To determine nickel content (wt %) by atomic absorption, catalysts were broken up in hydrofluoric acid and diluted to the interval of measurement with water. A ThermoJarrel ASH Corporation Smith Hiefje/11 spectrophotometer with a simple beam and Smith Hiejfe background correction was used to measure the quantity of nickel. X-ray diffraction (XRD) was used to measure the crystallinity of the synthesized zeolites. The diffractograms were collected with a Siemens Kristalloflex D-500 diffractometer having a CuKR radiation and a Ni filter. Relative crystallinity was decided from the peak height at 2θ ) 23.1 using a 100% crystalline zeolite pattern as reference. A Philips X′PERT MPD diffractometer with CuKR radiation and a Ni filter was used to measure the nickel oxide particle size on the calcined catalysts by X-ray diffraction line broadening, using the Debye-Scherrer method20 as described in our earlier paper. The acidity of catalyst samples was measured by the ammonia temperature-programmed desorption (ATPD) technique using a dynamic Micromeritics TPD/TPR 2900 analyzer characterization system with a thermal conductivity detector. The detailed measurement procedure and acidity calculation has been previously described elsewhere.21 Any assignment to specific acid
centers (Bronsted or Lewis) can be established by this method, but instead, it was considered helpful to compare the acidity of these catalysts.22 The average relative error in the total acidity determination was encountered by repeated measurements of similar samples and found to be below 3%. TPR was carried out with the same apparatus described for ATPD. After loading, samples were outgassed by heating in argon to 723 K and afterward cooled at room temperature. Next, they were stabilized in an Ar/H2 (85/15 volumetric ratio) flow. The temperature and detector signals were then continuously recorded while heating at 10 K/min to 723 K. The XPS measurements were carried out with a Perkin-Elmer Phi 5400 Sca System electron spectrometer using an aluminum anode operating at 400 W and a pass energy of 89.45 eV in the general scanning and 35.6 eV in the high-resolution scanning. The XPS spectra of catalyst samples were scanned after they had received calcination treatment. 2.3. Hydroconversion of n-Decane. The hydroconversion of n-decane over bifunctional zeolite catalysts has been widely used as a model to characterize bifunctional catalysts,23-26 since the balancing of the two catalytic functions, acid and metal, depending on the preparation method, determine the reaction mechanisms of skeletal hydroisomerization and hydrocracking of alkanes. The conversion of n-decane was carried out in a fixed-bed downflow stainless steel reactor under a total pressure of 5055 kPa at 573 K, continuously fed with hydrogen and n-decane with a contact time W/F of 26.6 g‚h/mol and a hydrogen-to-n-decane ratio of 10.9. The reaction products, obtained after a stabilization period of about 3 h, mainly consisted of alkanes C1C10. These were analyzed using gas chromatography, in a Hewlett-Packard chomatograph using a Porapack Q 80/100 ASTM column and TCD detector for the gas phase and fused silica capillary column and FID detector for the liquid phase. 3. Results and Discussion Table 1 shows the calcination conditions of the catalysts prepared. Sample C0 corresponds to a catalyst prepared without calcination prior to reduction. Samples C1, C2, C3, and C4 allow the study of the effect of the calcination temperature, since all of them were calcined with sharp sudden heating and for 5 h, at different temperatures. On the other hand, the two series of samples C1, C1R, C2, and C2R permit the study of the influence of the heating rate in the calcination stage. 3.1. Effect of the Calcination Temperature. 3.1.1. Characterization. Characterization parameters of the catalysts calcined at different temperatures, C1, C2, C3, and C4 are listed in Table 2. Nickel metallic species and their relative ratio as well as the presence of nitrogen species measured by means of the XPS technique are shown. The numbers between brackets represent the atomic percentage of each metallic species related to the total amount of nickel. Even though XPS is not a quantitative technique to determine this amount, since the measurement is performed only over a thin layer of the catalysts but not over the bulk, these numbers are very helpful in following the tendency of the metallic species with calcination temperature. The mean average crystallite size of nickel oxide, dXRD, in the calcined sample, overall acidity of the catalysts, AC, and the total acidity of four zeolite samples, AH, without
3848 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 2. Characterization of the Catalysts Calcined at Different Temperatures catalyst
C1
C2
C3
C4
nickel speciesa Ni(OH)2 NiO Ni2O3 (NOx)2Niyb
X [33%] X [67%] _ X [
