Tailoring of the Pore-Size Distribution

Recently, a novel type of binder for catalyst shaping has been identified that allows for tailoring of the pore-size distribution in a different way. The material is an amorphous aluminum phosphate hydrate (APH) available as a powder, which is employed like conventional binders, for instance, in extrusion. In contrast to conventional particulate binders, however, calcination at 500 °C transforms APH via water evaporation, viscous sintering, and a phase transition into a dense, crystalline phase with the structure of tridymite. 

Mercury-intrusion plots of calcined alumina extrudates from the top to the bottom, the specific mechanical energy input during mixing of the pastes was increased L/S is the ratio liquid to solid in the extrusion paste. 

If zeolite crystals or other refractory particles are present, a pore structure evolves, formed by the embedded particles and the sinter necks of the AIPO4 matrix in between. Here, the pore-size distribution can be tailored through choice of both, the size of the embedded particles and their relative amount in the shaped bodies. Other than with particulate binders, the coformation of small mesopores can be avoided, and macropores in a narrow, monomodal size distribution can be realized [7]. 

For many catalytic applications, it may be desirable to furnish catalysts or carriers with a high portion of macropores to enhance diffusional transport of reactants. Unfortunately, the mechanical strength of the bodies decreases with increasing porosity and with increasing size of the powder particles employed [5, 7]. For an agglomerated body composed 

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Porosity can be an advantage or a disadvantage in ceramic powders, depending upon the processing and final application. Tailoring of the pore size distribution is very important for catalytic substrates, because access to the catalytic sites depends on these diflusional pathways. 

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