Tailoring sulfated zirconia catalysts for isomerization-cracking of heavy paraffins through gel aging

M. BUSTO; C.R. VERA; J.M. GRAU

Madrid, España

Congreso; International Symposium on “Catalysis for Clean Energy and Sustainable Chemistry” CCESC; 2008

Instituto de Catálisis y Petroleoquímica de Madrid

The performance of heterogeneous catalysts depends both on the nature of the catalytic surface and on the way this surface is organized inside the catalyst particle.  The activity per unit mass (r) can be expressed as r=ks Sg x, in which ks, the activity per unit surface is directly related to the active sites per unit surface and is usually a constant related to the chemical nature of the surface.  Sg is the surface area per unit mass; x is the effectiveness and is directly related to the ease of diffusion of molecules in the pore network.  For materials of constant ks optimization of textural parameters influences Sg and x and hence r.  Optimization of the textural parameters is specially important in systems with diffusional problems in which not all the catalyst surface is accesible due to the difficulty of bulky molecules to diffuse inside the pores.  In this case special techniques are necessary for tailoring the texture of the catalyst.  Sulfated-zirconia (SZ) is a known commercial acid catalyst mainly used for the isomerization of short paraffins.  Active catalysts are fairly crystalline and have a dominant tetragonal structure.  Their sulfur surface density approaches half a monolayer after calcination at 600 ºC.  This is a micro-mesoporous material that performs well in the gas phase reaction of short n-alkanes at relatively low temperatures, showing a remarkable yield of branched isoparaffins, important compounds for the refnery gasoline pool.  However standard sulfated zirconia (SZ) is fairly inefficient when used “as is” in the reaction of bulky molecules such as the transesterification of vegetable oils, isomerization of fatty acid methyl esters or isomerization-cracking of long paraffins [1,2].  Attempts of producing higher surface area materials usually produce more microporous materials and fail to optimize the zirconia support for these systems.  Dispersion of zirconia over mesoporous silica increases both Sg and the mean pore radius but decreases the activity per unit reactor volume due to the low density of silica [3].  An attempt to increase both Sg and the mesoporosity of SZ by reflux aging (110 ºC, 12 h, pH=10) of the freshly precipitated zirconia gel (from ZrOCl2, pH=10, dropwise addition of NH4OH) is done in this paper.  The thus prepared zirconia supports were impregnated by immersion in SO4H2 1 N (10 ml g-1) and calcined at 600 ºC in air.  Then they were impregnated with chloroplatinic acid by incipient wetness.  The catalysts (0.5% w/w Pt) were tested in the reaction of long paraffins isomerization-cracking using n-C8 as a model compound (WHSV=1 h-1, H2/n-C8=6, 300 ºC, 1 atm).  A comparison is made between aged (Pt/SZAG), non-aged materials (Pt/SZlab) and a commercial sample (Pt/SZcom, MEL Chem.).