Synthesis of sulfated-zirconia aerogel: effect of the chemical modification of precursor on catalyst porosity
Introduction
Inorganic oxide aerogels exhibit very high porosities and specific surface areas; moreover, their rigid porous framework also retards the sintering and crystallization of the material upon heating [1]. These properties make them interesting both as catalysts and as catalyst supports [1], [2].
A renewed interest for zirconia has been induced by the acidic properties of sulfated-zirconia catalysts (SZ) that are able to isomerize paraffins under mild conditions [3], [4].
A common practice in the preparation of SZ catalysts is a stepwise procedure consisting of the impregnation of aqueous H2SO4 or (NH4)2SO4 on previously precipitated Zr(OH)4, followed by drying and calcination treatments [5]. This set of operations may possibly lead to irreproducibility of the physical properties of the samples. In order to overcome some of these problems we have synthesized sulfated-zirconia catalysts through a one-pot preparation [6], [7], [8], which is based essentially on sol–gel technology. Subsequent supercritical drying of the gel leads to aerogel catalysts. In order to obtain a sol–gel material with the required physical properties, a high degree of control of the hydrolysis and condensation reaction rates, normally very fast with Zr alkoxide precursor, is necessary.
The addition to the alkoxide precursor of a nucleophilic reagent such as a glycol, an organic acid or acetyl acetone and its exchange with the alkoxy groups is known to control the degree and the rate of hydrolysis [9]. The degree of hydrolysis is generally reduced as these reagents partially occupy the co-ordination sites of alkoxides and have higher resistance to substitution by hydroxyl groups.
The aim of this work is to explore the possibility of preparing SZ catalysts with different morphologies through the modification of the zirconia propoxide precursor. Three different modifiers were added to control the hydrolysis reaction: acetic acid (AcOH), acetyl acetone (acacH), 2-methylpentane-2,4-diol (HG). When AcOH [10] is added, the acid reacts with the solvent (i-PrOH) by esterification giving H2O. This reaction is in competition with alkoxide modification: either gels or precipitates can be obtained depending on the value of the hydrolysis ratio. Instead the addition of acacH, [11] a strong chelating ligand, gives a fast reaction with the metal alkoxide stabilizing the precursor toward hydrolysis. The 2-methylpentane-2,4-diol can change the hydrolysis rate of the starting alkoxides by complexing the precursor [12].
In all cases, supercritical evaporation was applied in the drying step. We chose this method because in the case of conventional drying (in an oven, at atmospheric pressure or in a vacuum), various detrimental forces are active causing differential macroscopic and microscopic shrinkage, thus causing the sol–gel structure to collapse.
Section snippets
Sample preparation
Zirconium propoxide [Zr(OPr)4] was used as the starting material with three different modifiers added to control the hydrolysis reaction: (i) acetic acid (AcOH), (ii) acetyl acetone (acacH), (iii) 2-methylpentane-2,4-diol (HG).
i-PrOH, H2SO4, and H2O were added to each solution. The i-PrOH concentrations are given in Table 1. The amount of H2SO4 in all samples was 8 wt% SO42− with respect to the initial Zr(OPr)4 weight. A ratio H2O/Zr(OPr)4=4 was used for all samples. Three different final
Surface area and pore structure
Comparison of the adsorption–desorption isotherms (Fig. 1, Fig. 2, Fig. 3, Fig. 4) shows that the samples prepared with the lowest and the largest concentrations behave quite differently, independently of the modifier used.
For all samples, the surface area decreases as the concentration of the precursor increases (Table 2). In particular, at the lowest concentration, the surface area of the modified materials appears to be the same as the reference sample. The values
Conclusion
The present study has shown that an appropriate choice of the modifier can deeply affect the porosity of zirconia sulfate catalyst, in particular at low concentration. This discloses the way to exert a significant degree of control in the morphology of these materials.
The weight loss of sulfur observed after the reaction cannot be associated to the loss of catalytic activity observed in SZ catalyst. In fact the initial activity of the catalyst is completely restored after the thermal
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