Sol?gel synthesis of titania?alumina catalyst supports - santili

Sol?gel synthesis of titania?alumina catalyst supports - santili

(Parte 1 de 2)

Applied Catalysis A: General 235 (2002) 71–78

Sol–gel synthesis of titania–alumina catalyst supports a Instituto de Quımica, UNESP, P.O. Box 355, 14800-970 Araraquara, SP, Brazil b Área Interunidades, IFSC/USP, P.O. Box 365, 13560-0 Sao Carlos, SP, Brazil c Instituto Militar de Engenharia, Pça Gal Tiburcio 80, 22290-970 Rio de Janeiro, Brazil


Titaniumoxideisagoodcandidateasnewsupportforhydrotreating(HDT)catalysts,buthastheinconvenienceofpresenting small surface area and poor thermal stability. To overcome these handicaps TiO2–Al2O3 mixed oxides were proposed as catalyst support. Here, the results concerning the preparation, characterization and testing of molybdenum catalyst supported on titania–alumina are presented. The support was prepared by sol–gel route using titanium and aluminum isopropoxides, chelatedwithacetylacetone(acac)topromotesimilarhydrolysisratioforboththealcoxides.Theeffectofnominalcomplexing ratios [acac]/[Ti] and of sol aging temperature on the structural features of nanometric particles was analyzed by quasi-elastic light scattering (QELS) and N2 adsorption isotherm measurements. These characterizations have shown that the addition of acac and the increase of aging temperature favor the full dispersion of primary nanoparticles in mother acid solution. The dried powder presents a monomodal distribution of slit-shaped micropores, formed by irregular packing of platelet primary particles, surface area superior to 200m2 g−1 and mean pore size of about 1nm. These characteristics of porous texture are preserved after firing at 673K. The diffraction patterns of sample fired above 973K show only the presence of anatase crystalline phase. The crystalline structure of the support remained unaltered after molybdenum adsorption, but the surface area and the micropore volume were drastically reduced. © 2002 Published by Elsevier Science B.V.

Keywords: Sol–gel process; Molybdenum catalysts; Thiophene hydrodesulfurization

1. Introduction

Molybdenum supported on alumina promoted by nickel or cobalt constitutes the most important catalysts for petroleum hydrotreating (HDT) [1]. These catalysts are used mainly to saturate unsaturated hydrocarbons in order to remove sulfur (hydrodesulfurization—HDS), nitrogen (hydrodenitrogenation— HDN), oxygen (hydrodeoxygenation—HDO) and metals (hydrodemetallation—HDM) from different

∗ Corresponding author. Tel.: +5-16-201-6638; fax: +5-16-2-7932. E-mail address: (S.H. Pulcinelli).

petroleum streams [1]. The more stringent environmental regulations that have been recently enacted throughout the world has increased the need for more active HDT catalysts and this has led to the study of a variety of compositions that differ from standard sulfide catalysts. Basically the approach has been to explore: (a) new supports; (b) noble metal catalysts; (c) zeolite-containing combinations; (d) new compositions. Therefore, development of more active and more selective catalysts has become a challenge for refiners [2,3]. Among the new supports, titania [3] has attracted attention in view of the higher HDS activity displayed by molybdenum sulfide supported on this oxide [4]. It has been found, for example, that

0926-860X/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S0926-860X(02)00236-3 for thiophene HDS, molybdenum catalysts supported on titania are four times more active than those supported on alumina [4,5]. However, titania supports generally present not only low specific surface area (50m2 g−1) when compared to alumina (200m2 g−1) but also poor thermal stability of the anatase phase at high temperatures [5–7]. To overcome these handicaps and taking advantage of the good properties of titania (high activity) and alumina (excellent texture, mechanic and thermal properties), TiO2–Al2O3 mixed oxides have been proposed as HDT catalyst [5,8].

The sol–gel process has been used for the last 20 years to prepare different alumina [9,10] and titania [1] powders with uniform particles size and high surface area. The possibility of fine tuning the texture of these supports by careful control of the sol–gel process parameters (hydrolysis ratio, complexing ratio, aging temperature and acidity) has been demonstrated [9–1]. However, the sol–gel routes to prepare titania–alumina mixed oxides as HDT supports have not been developed yet [8]. Within the material science area, parallel work has been in progress directed at development of sol–gel strategy to produce titania–alumina membranes [12], however, the texture of these materials is inadequate for using them as catalyst support.

The work described in the present article combines the strategies frequently used to controlling the texture of sol–gel-derived transition metal oxides, to prepare titania–alumina mixed oxides for application as HDT catalyst support. The effect of the modification of the reactivity of precursor (titanium tetraisopropoxide) by acetylacetone (acac) as complexing ligand, of the temperature of sol aging and of the powder firing on the texture of titania–alumina powder was investigated. The performance of this new support in the thiophene HDS reaction is reported.

2. Experimental 2.1. Samples preparation

All experiments were performed under atmospheric conditions and all chemicals were used without further purification. A solution of titanium tetraisopropoxide (Strem Chemical, 98%) in isopropanol (1moldm−3) was added slowly to acac (Fluka, 9%) and the mixture was stirred at 298K for 15min. An exothermic reaction of complexation occurred leading to a yellow solution. Different amounts of acac were used to adjust the nominal complexing ratios [acac]/[Ti] to 0, 0.5, 1 and 2. Aluminum di(isopropoxide) acetoacetic ester chelate (Strem Chemical, 98%) was mixed to the solution containing the complexed titanium precursor to yield the molar ratio Ti/(Ti + Al) equal to 0.5. Hydrolysis of the clear solution was then performed by dropwise addition of aqueous para-toluene sulfonic acid (PTSH) (Vetec, 9.9%) solution (0.1moldm−3). The resulting solution exhibited a pH of 2.4, nominal hydrolysis ratio (h = stirring and then aged under reflux at different temperatures (3, 343, 353 and 363K) for 16h. The aged suspensions were cooled down to room temperature and dialyzed, i.e. aliquots of 15 ml of so-prepared suspensions were put inside acetylcellulose membranes tubing (12–14000MW) and then submitted to static dialysis against 100 ml of bi-distilled water for 4 days, until pH reached 7, to eliminate as much as possible the soluble species. The water of dialysis was changed each 8h. The colloidal particles were isolated from suspension by freeze-drying at 268K and pressure of ∼1 mHg. The powders were heat-treated at different temperatures (473, 673, 773 or 973K) for 2h.

The molybdenum catalysts were prepared by the adsorption method. Succinctly, ammonium heptamolybdate aqueous solution (0.004moldm−3) was contacted with the support for 36 h. After this adsorption period, the solution was evaporated, the obtained solid was dried at 383K for 18h and then fired at 673K for 3h. The volume of ammonium heptamolybdate solution and the mass of titania–alumina powder in the mixture was calculated in such a way to obtain a catalyst containing four molybdenum atomsnm−2 .

2.2. Characterization

The hydrodynamic diameter of particles in the studied sols were determined from quasi-elastic light scattering (QELS) measurements, performed with a solid state laser of 25mW (λ = 532nm) and a BI-1000AT Brookhaven photocorrelator. The X-ray powder diffraction (XRPD) measurements have been performed on dried and fired samples with a Siemens

D5000 diffractometer, using the graphite monochromated Cu K radiation. The BET surface area and the porous texture of catalyst supports have been evalu- ated by N2 adsorption isotherms obtained at 77K in a relative pressure range between 0.001 and 0.998, us- ing an ASAP 2010 (Micrometrics) equipment. Before each measurement, samples were degassed at 353K under vacuum (≈1 m Hg) for time enough (12 <t < 18h) to observe the absence of significant change in vacuum stability. Micropore size was calculated from the adsorbed volume (V) versus the statistical thickness of the adsorbed layer (t) curves, also called t-plot [13]. The thickness of this layer was calculated for each relative pressure from the Harkins and Jura [14] equations, following the algorithm proposed in [15]. Before catalytic testing, the samples were sulfided

in situ using a 10% (v/v) H2S/H2 gas mixture (AGA, 9.99%) at 673K during 2h. Immediately after the sulfiding step, the reaction was started by passing a 10/1 molar ratio of H2/C4H4S reactant mixture through the reactor. This ratio was obtained by flowing pure H2 (1.2lh−1) through two saturators connected in series maintained at room temperature by a water bath and filled with thiophene (Merck, 9%). The reaction conditions were adjusted to maintain low conversions of thiophene (<10%) and absence of diffusional effects. Products were analyzed on-line by gas chromatography using a HP 2900 gas chromatograph.

3. Results

QELS experiments were performed on synthesized sols obtained after aging in PTSH aqueous solution (pH = 2.4). The hydrodynamic size distribution was calculated supposing a set of particles with a multimodal distribution. The effects of the aging temperature and complexing ratio on the particles size distribution are shown in Fig. 1a and b, respectively. The analysis of these data shows that sols are made of monomodal distribution of nanometric colloidal particles, the mean diameter of which increases when [acac]/[Ti] increases up to 0.5 or the synthesis temperature increases up to 343K.

The effect of the aging temperature and of complexing ratio on the powder texture of titania–alumina dried powders is verified by changes in the N2

Fig. 1. Evolution of the hydrodynamic diameter distribution of particles with the (a) aging temperature ([acac]/[Ti] = 1) and (b) complexing ratio (T = 343K).

adsorption–desorption isotherms shown in Fig. 2a and b, respectively. The plateau at high p/p0 range, the high volume of gas adsorbed at low relative pres- sure (p/p0) and the absence of hysteresis, in the curve corresponding to the sample synthesized at 343K with different complexing ratio, is typical of materials presenting micropores (type I isotherm, according to IUPAC classification [13]). Above this temperature and for [acac]/[Ti] = 1( Fig. 2b) a considerable increase in the adsorption occurs at high p/p0 range, indicative of the mesoporosity and isotherms present a hysteresis loop types H3 and H2 for samples synthesized at 353 and 363K, respectively. This hysteresis loop evolution indicates that the regularity of cross section along the longitudinal direction of pores increases by increasing the temperature. In an idealized description, this is equivalent to the transformation

Fig. 2. Evolution of N2 adsorption–desorption isotherms with the (a) aging temperature ([acac]/[Ti] = 1) and (b) complexing ratio (T = 343K).

from slit-shaped towards inkbottle-shaped mesopores [13], that may correspond to the changes of plate-like to corpuscular particles.

Quantitative information about the micro and mesopore contribution on the overall texture of titania–alumina powders was obtained from t-plot. Non-porous solids are characterized by a linear relationship between V and t, in which the specific surface area (St) is proportional to the slope of the line (for N2 adsorption, St = 15.47V/t). The capillary condensation of adsorbate in the mesopores leads to a positive deviation of V–t curves. On the other hand, for solids containing micropores the thickness of adsorbed layer is limited by the core radius of the pores and the corresponding t-plot presents a negative deviation from linearity. An example of this negative deviation is observed in the t-plot corresponding to samples aged at 343 and 363K, shown in Fig. 3. Two straight lines are obtained for sample prepared at 343K and the t-values corresponding to intersection of them (ti) allows to calculate the mean micropore size (d = 2ti). The volume of micropore (V )w as estimated by extrapolating the less step line above the intersection to the ordinate. The external volume

(Ve) or the volume of mesopore (Vm) can be calculated by subtracting the total pore volume (Vp) from

V . The total (St = SBET) and the external (Se) areas were calculated from the slope of straight lines, and

Fig. 3. The t-plot corresponding to samples prepared with [acac]/[Ti] = 1 and aged at 343 and 363K, illustrating the proce-

the subtraction of them gave the micropore area (S ). The average size of micropores was calculated by the following equation [13]:

Table 1 Textural parameters of dried titania–alumina powders and supports fired at 673K prepared from powders aged at different temperaturesa

Experimental parameter Surface area (m2 g−1) Pore volume (cm3 g−1) Mean pore size (nm) Average pore size (nm)

a The values in parenthesis correspond to the molybdenum-impregnated supports. b Supports fired at 673K.

where the constant f depends on the geometric form of the pores, assuming values of 4 and 2 for cylindrical and slit-shaped pores, respectively.

The values of porous texture parameters are summarized in Table 1. It is found that the average width (d ) of slit-shaped pore is in good agreement with the mean size of micropores obtained from the intersection of the two straight lines of the t-plot. This result indicates clearly that the micropore are slit-shaped. Furthermore, it is of interest to note that the majority of parameters characteristic of porous texture increases by increasing the synthesis temperature or by decreasing the complexing ratio. As the more pronounced change in texture is induced by synthesis temperature, we have choice to study the effect of this experimental parameter on the catalytic activity of titania–alumina-supported molybdenum catalyst.

The effect of firing at 673K and subsequent adsorption with four molybdenum atomsnm−2 on the surface area, pore volume and average pore size of titania–alumina support are presented in Table 1.A dramatic reduction of surface area and porous volume is verified after molybdenum impregnation on the titania–alumina support indicating that molybdenum

Fig. 4. Effect of firing temperature (curves a–d) and aging (curves e–f) on the XRPD patterns of titania–alumina powder prepared with [acac]/[Ti] = 1.

is preferentially being deposited at the external surface, thus blocking the micropores. The effect of impregnation on the porous texture increases as the synthesis temperature decreases and results in the significant rise in the micropores contribution to the total porosity. Furthermore, the comparison of dried and fired samples reveals that the initial characteristics of porous texture are not considerably affected by thermal treatment. These results evidence the good thermal stability of titania–alumina support.

Further information about the effect of aging and firing temperatures on the thermal stability of titania–alumina support is shown in XRPD patterns presented in Fig. 4. Below 673K, the XRPD patterns present broad peak profiles, typical of materials containing nanocrystalline domains. The peak intensity increases, while their width decreases as the firing temperature increases indicating crystallite growth. The diffraction pattern of the sample fired at 973K shows only the presence of anatase crystalline phase. The XRPD patterns of impregnated supports (not presented) show similar behavior. Furthermore, a small increase in crystallinity is observed when the aging temperature is increased from 298 to 343K.

The catalytic activities of the titania–aluminasupported molybdenum catalyst and that of commercial alumina support were evaluated in the thiophene HDS reaction carried out at different temperatures. Fig. 5a and b show the dependence of the mean selectivity to butane formation with the temperature and the Arrhenius plot of reaction rate, respectively. The different curves correspond to titania–alumina supports prepared at different aging temperature. The apparent activation energy, calculated from the slopes of the curves, is essentially the same either for the supports obtained at different synthesis temperatures (81kJmol−1 ± 5kJmol−1) or for the molybdenum supported on commercial alumina sample (74kJmol−1 ± 5kJmol−1).

4. Discussion

The presented experimental data show that the aging temperature and the complexing ratio of titanium precursor affect the process of formation and aggregation of titania–alumina powders in different ways. The drastic decrease of the mean hydrodynamic diameter of particles from 70 to 4nm, when the ratio [acac]/[Ti] increases from 0 to 0.5, is not followed by a considerable reduction of the surface area of dried powder (Table 1). This feature suggests the presence of aggregates in the sol prepared with [acac]/[Ti] = 0 and that the addition of a small amount of acac promotes the desaggregation of particles.

The power of dispersion of such complexing agent can be evaluated by comparing the mean hydrodynamic diameter measured in the sol with the aver- age size (lvs) of primary particles in the powder given

Fig. 5. HDS reaction temperature dependence of the (a) mean selectivity to butane formation and (b) rate of reaction conversion of thiophene (Arrhenius plot) for supports prepared with [acac]/[Ti] = 1 at different aging temperatures.

lvs = f ρSBET where ρ is the density of powder measured by helium picnometry (ρ = 2.7 ± 0.1gc m−3) and f the shape factor, assuming values of 6, 4 and 2 for spherical (or cubic), equiaxial road and platelet geometry, respectively. Because the non-negligible contribution of adsorbed liquid layer to the size of particle measured by QELS, it may be higher than the true size of primary particles. This condition was only observed by using f = 2i n Eq. (2), that results in lvs = 2.3nm for the length of plate-like particles in the sample prepared with [acac]/[Ti] = 0.5. Using f = 6 or 4 results in lvs higher or similar to the mean hydrodynamic size, what is an absurd. Furthermore, the plate-like geometry of primary particles is consistent with the presence of slit-shaped micropores revealed from the N2 adsorption analyses (Table 1) and with the morphology of titania–alumina sols previously observed by electron microscopy [8]. Otherwise, the similarity between the average length of platelets and the hydrodynamic diameter indicates that particles are fully dispersed down the primary particle size. These findings indicate also that the acac ligand has a role of capping agent [1], that modifies the surface properties and prevents the aggregation of primary particles.

The overall effect of aging temperature on the characteristic of titania–alumina sol is in agreement with the results found by Yoldas [9] for bohemite sol [10]. The sol aged at room temperature has a collapsed structure of folded bohemite particles and increasing the aging up to 323K favors the formation of an expanded structure of unfolded bohemite particles [1]. This transformation results from peptization, that leads to the rupture of interlamellae hydrogen bonds [1] and irreversible fracture of particles into smaller platelets (Fig. 1a) increasing considerably the surface area (Table 1). However, there are some interesting differences between our results and the Yoldas [9] finding. The first is the decrease in the tendency to peptization, indicated by the growth of hydrodynamic size (Fig. 1a) as the aging temperature increases up to 343K, and the second is the absence of crystalline pseudo-bohemite phase in isolated powder (Fig. 4). The former can results from the well-known [1] decrease of the complexing power of the capping agent (acac) as the temperature increases. The later effect confirms the improvement of the thermal stability of amorphous pseudo-bohemite achieved in titania–alumina system [8].

Otherwise, the presence of alumina hinders the anatase–rutile transformation (Fig. 4), increasing the thermal stability of the porous texture. In fact, an increase of the surface area and porosity is observed after firing the sample aged at 333K (Table 1). This behavior can result from stress arising during dehydration process, leading to break down of particles and aggregates [6]. On the other hand, for titania–alumina aged at 343, 353 and 363K a slight decrease of the surface area and porosity and an increase of the mean pore size are observed after firing at 673K (Table 1). This behavior results from the sintering process. This general feature is in agreement with previous finding [12] showing that the presence of alumina inhibits the crystallite growth, increasing the stability of the anatase phase.

It is noteworthy that despite of the employed support, i.e. titania–alumina or alumina, the observed activation energy (Fig. 5a) is practically the same meaning that in all of the studied catalysts the active phase is alike and is constituted basically by MoS2. This result implies that the expected synergic effects associated to the titanium species formed during sul- fiding (e.g. TiSx) [5] either did not occur or, if so, the amount of such TiSx species is too low to improve the catalytic activity.

The performance of the titania–alumina-supported molybdenum catalysts is somewhat inferior to that pre- sented by the Mo/Al2O3 sample (Fig. 5b), meaning that the MoS2 dispersion on the former is lower than on the later. Since all of the catalysts were synthe- sized using the same methodology and were sulfided using the same temperature condition, the explanation for different dispersions has to lie on the nature of the supports. In fact, while the titania–alumina supports are essentially microporous and present a pronounced decrease in surface area after molybdenum incorporation as shown in Table 1, the alumina is basically mesoporous and did not present such a decrease in surface area after molybdenum adsorption. The decrease in surface area observed for the titania–alumina samples suggests that during metal adsorption there is a blockage of the microporous, resulting in large MoS2 particles, which present low catalytic activity. In fact, the butane selectivity results in Fig. 5a indicate that the titania–alumina samples indeed have larger MoS2 particles due to the lower value [16]. Nevertheless,

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