Jointly published by Akadémiai Kiadó, Budapest and Kluwer Academic Publishers, Dordrecht
React.Kinet.Catal.Lett. Vol. 80, No. 2, 311-317 (2003)
RKCL4210 CHARACTERIZATION OF ZrO2 - Al2O3 MIXED OXIDES SUPPORT PREPARED BY UREA HYDROLYSIS Folorunsho Aberuagbaa*, Manoj Kumarb, Jai Krishna Guptab, Gudimella Muralidharb and Lakshmi Datt Sharmab a
Department of Chemical Engineering, Federal University of Technology, P.M.B. 65 , Minna, Nigeria b Catalyst Laboratory, Indian Institute of Petroleum, Dehradun 248 005, India Received September 24, 2003 In revised form May 29, 2003 Accepted June 11, 2003
Abstract The characteristics of Al2O3 , ZrO2 and three binary mixtures of ZrO2 -Al2O3 were studied by determining their BET surface areas, micropore surface area, total pore volume, adsorption-desorption isotherms, the X-ray diffractogram, surface acidity and catalytic functionality for cumene cracking. The XRD results show that the incorporation of alumina into the zirconia from 50% and beyond renders it amorphous. Furthermore, the mixed oxide containing 50% alumina and 50% zirconia had the highest BET surface area of 199.9 m2/g whilst pure zirconia had the lowest BET surface area of 37.19 m2/g. The pores for all the mixed oxides were found to be monomodal and zirconia pores were more open. The results of the acidity measurements and cumene cracking functionality indicates that whilst pure zirconia has low total acidity, the incorporation of alumina increases its acidity through a synergistic effect. Keywords: ZrO2- Al2O3 , support
INTRODUCTION Petroleum products hydrotreatment is undertaken for the removal of sulfur compounds that are sources of environmental polluting emissions and poisons of precious metal catalysts used in reforming processes. The reactions have been widely undertaken on supported molybdenum and tungsten catalysts. ______________________________________________ * Author to whom correspondence should be addressed 0133-1736/2003/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.
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Although alumina is the most widely used support material [1-3], the use of other materials such as TiO2 [4], ZrO2 [5-6], MgO [7-10] for the production of catalysts of improved activity and selectivity have been proposed. In recent times promising results have been obtained with catalysts supported on mixed oxides [11-12 ]. One mixed oxide that has recently attracted attention is zirconia based mixed oxide. This is because it gives rise to a substantially different interaction between the active phase and the support, altering the activity and selectivity of the system [13-14]. The use of zirconia as a support has been limited in the past because it can exist in three crystallographic forms, namely, monoclinic, tetragonal and cubic, depending on the temperature. It’s tetragonal structure has however been reported to produce a catalyst of higher hydrodesulfurization activity than alumina supported catalyst [13]. It has also been demonstrated that this tetragonal structure can be stabilized by constraining its particles into an alumina matrix which has a higher elastic modulus than zirconia [15]. In this regard it should be possible to take advantage of tuning the mixed ratio of zirconia to alumina to produce a support of higher activity and selectivity than either pure zirconia or pure alumina. There are a number of methods for the production of mixed oxides, however, the urea hydrolysis method of precipitation has been reported to be cheap and permits the formation of a precipitate at a nearly ideal rate sought which are rarely attained by conventional methods [16]. In this communication we present the results of an investigation of the characterization of zirconia-alumina mixed oxides prepared by urea hydrolysis method. The mixed oxides were characterized by BET surface area, micropore surface area, pore size distribution, adsorption-desorption isotherm, X-ray diffraction, surface acidity and catalytic functionality for cumene cracking. EXPERIMENTAL Materials Zirconia oxychloride (ZrOCl2.8H2O) of Analar grade, a product of Loba Chem., India was used as a precursor for zirconia. Extra pure aluminium nitrate (Al(NO3)3.9H2O) a product of S.D. Chemicals Ltd. India., was used as a precursor of alumina. Extra pure urea ((NH2)2CO) a product of E. Merck (India) Ltd., was used for in-situ ammonia production.
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Support preparation Five samples of ZrO2-Al2O3 mixed oxides were prepared with different molar ratios [x = ZrO2/(ZrO2+Al2O3)] ranging from x=0.0 to 1.0 namely alumina (x=0), S25ZA (x = 0.25), S50ZA (x = 0.5), S75ZA (x = 0.75) and zirconia (x = 1.0). 1 molar solutions each of aluminium nitrate [Al(NO3)3.9H2O] and zirconium oxychloride (ZrOCl2.8H2O) and a solution of 60% (w/v) urea ((NH2)2CO) were prepared. In all cases solutions were prepared in water and urea was used as hydrolyzing agent. Appropriate volumes of the solutions were poured into a 20 L flask and heated to 90oC under total reflux for 4 hours. During heating the pH of the mixture was maintained at 7.2. The precipitate formed was filtered and washed repeatedly with distilled water until no chloride ions was detectable from the filtrate. Presence of chloride ions was detected by adding a few drops of silver nitrate solution to some of the filtrate, and the formation of a white precipitate signifies the presence of chloride ions. After washing, the precipitate was oven dried at 110oC for 48 hours followed by calcination at 550oC for 6 hours. Support characterization BET surface area, pore size distribution and adsorption/desorption isotherm The five-point BET surface area and pore size distribution were determined by nitrogen (99.9998% pure) physisorption at 77 K using micromeritics (U.S.A) ASAP 2010 unit. The support was dried under vacuum at 523 K in situ prior to analysis for surface area, pore size distribution and adsorption- desorption isotherms. Adsorption-desorption isotherms were recorded taking sixty points. The adsorbed volume for relative pressure (P/Po) from 0.02 to 0.99 for adsorption and from 0.99 to 0.25 for desorption were recorded to obtain the complete isotherm. Total pore volumes were obtained by amount of physisorbed N2 for P/Po=0.99. The BJH method [17] was applied to calculate pore size distribution. X-ray diffraction XRD patterns for powder samples were obtained from G.E-XRD-6 diffractometer fitted with XRD-9000 detector and XSPEX software ver. 5.40, using Ni-filtered Cu-Kalpha radiation (λ =1.541838 Å) at a scan rate of 1.0o per min.
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Total acidity and acid sites distribution The total acidity and the acid strength distribution were determined by microcalorimetric technique using a Titan-Calvet type heat flux microcalorimeter (C80 model Setaram, France) connected to a volumetric vacuum adsorption unit for sample treatment and probe molecule delivery. A Val dyne low-pressure transducer (U.S.A.) has been attached with vacuum system for precision pressure measurements. Before microcalorimetric measurements, samples (≈ 0.1g) were outgassed under high vacuum at 450oC for 4 h and the ammonia adsorption studies were carried out at 175oC. Differential heats of NH3 adsorption vs adsorbate coverage were obtained by measuring the heats evolved from sequential doses of small quantities (µmol) of ammonia onto the sample until the surface became saturated by adsorbed species. The heat of adsorption generated for each dose was calculated from the resulting thermograms and amounts of ammonia adsorbed from the initial and final pressures. Sequential doses give the differential heat of ammonia adsorption as a function of coverage (i.e. differential heat curve). It provides the information about the number and strength of acid sites on catalyst surface.
Catalytic cracking functionality The catalytic functionality of the supports for cumene cracking was determined in a differential microreactor at one atmosphere. In a typical experimental run, 0.2 g of the support of particle size 18-40 mesh mixed with glass beads of the same size range in the ratio 1:1 to ensure uniform heating were secured between two plugs of quartz wool inside the glass reactor (pyrex glass tube, 0.8 cm, i.d.) and sulfided at 400oC for 2 h. Sulfiding was carried out using a mixture of CS2 and H2 at a flow of 40 mL/ml. After sulfidation, the support was maintained at the desired reaction temperature of 400oC and flushed in H2 flow until no CS2 could be detected in the effluent gas. Cumene was then introduced through the gas bubblers. An on-line GC was used for products analysis.
RESULTS AND DISCUSSION The textural properties of pure alumina, zirconia and three binary mixtures as determined by the BET surface areas (BETSA), micropore area, total pore volume and pore sizes are illustrated in Table 1.
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Table 1 Textural properties Support
BETSA (m2/g)
Micropore area (m2/g)
Total pore vol (cm3/g)
Dp(oA) Monomodal
Alumina S25ZA S50ZA S75ZA Zirconia
192.8 190.4 199.9 173.7 37.19
0.0 0.0 0.0 0.0 6.9738
0.347 0.384 0.354 0.224 0.116
58.6 80.0 57.5 45.0 162.12
Results show that the BET surface area increases slowly with zirconia content up to 50% zirconia (x=0.5) after which there was a sharp drop. A similar trend has been reported for ZrO2-Al2O3 mixed oxide prepared by sol-gel method [18]. The total pore volume of the support rose slightly with zirconia content up to 25% after which there was a decline with a further increase. The pore size distribution of the samples represented by the BJH adsorption volume indicate that pure alumina (x=0), S25ZA, S50ZA, S75ZA and pure zirconia were all monomodal with the maximum pores size distribution centered around 58.6, 80.0, 57.50, 45.0 and 162.12 Å, respectively. This shows that the incorporation of zirconia in alumina results in a shift from higher diameter pores at 25% zirconia content, to smaller pores at 75% zirconia content, after which there was a shift to much higher diameter pores for pure zirconia. The adsorption-desorption isotherms obtained showed that the hysteresis loops occurred at a relative pressure between 0.45 and 1.0 for all the samples investigated and the analysis of the isotherm profiles showed that they are of the Type iv in the BDDT system [19]. The X-ray diffraction pattern of the pure zirconia indicate a prominent peak of monoclinic phase with slightly low intensity peak for tetragonal phase. The mixed oxide show a broad peak in the vicinity of 30o (2θ) indicating the presence of both monoclinic and tetragonal phases. These peaks however disappear in the S50ZA samples, indicating that the incorporation of alumina up to 50% renders the zirconia-alumina mixed oxide more amorphous.. The total acidity and acid strength distribution by microcalorimetric method is shown in Table 2. From the above results it can be seen that whilst pure zirconia has low total acidity, the incorporation of zirconia into alumina up to 75% increases its acidity through a synergistic effect.
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Table 2 Total acidity and acid strength distribution Support
Total acidity (mmol/g sup)
Strong acid sites (mmol/g sup)
Medium acid sites (mmol/g sup)
Weak acid sites (mmol/g sup)
Alumina S25ZA S50ZA S75ZA Zirconia
0.336 0.377 0.441 0.458 0.206
0.085 0.075 0.115 0.125 0.095
0.105 0.15 0.145 0.16 0.065
0.146 0.152 0.181 0.173 0.046
The catalytic functionality of the support for hydrocracking as determined from cumene cracking at 400o C and at a reactant mixture flow rate of 40 mL/min is shown in Table 3. Table 3 Catalytic functionality for hydrocracking Support
rHYC x 103 (mol/h g support)
Alumina S25ZA S50ZA S75ZA Zirconia
45.06 45.27 45.5 46.05 15.5
From the above results, the cracking functionality of the mixed oxides appears to rise with zirconia content up-to 75%, after which a decrease is observed. This trend is somewhat similar to the trend of total acidity.
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