The hydroisomerization of n-hexadecane over Pd/SAPOs bifunctional catalysts with different opening size: Features of the diffusion properties in pore channels and the metal-acid synergistic catalysis

In this work, SAPO-11, SAPO-31 and SAPO-41 with one-dimensional channels are synthesized hydrothermally by employing a di-n-butylamine (DBA) template, industrial silicon and aluminum sources, and the corresponding Pd/SAPOs bifunctional catalysts are prepared by loading 0.5 wt% Pd via the wet impregnation method. The diffusivity of n-hexadecane and 2-methylpentadecane in the micropores of different SAPOs is determined by molecular dynamics (MD) simulation, which indicates that the opening size has strong effects on the diffusion property of reactants and intermediates in the micropore channels. The calculated self-diffusion coefficients (Ds) of both n-hexadecane and 2-methylpentadecane based on MD simulation of three SAPOs decrease as SAPO-31 > SAPO-41 > SAPO-11, which is consistent with their minor axis size of micropores. The results of n-hexadecane hydroisomerization test indicate that the Pd/SAPO-31 catalyst shows the highest activity and iso-hexadecane yield of 85.0% due to the significantly improved diffusion and favorable metal-acid balance. Different from Pd/ SAPO-11 and Pd/SAPO-41, hydroisomerization over Pd/SAPO-31 occurs via the key lock mechanism, which leads to an increased multi-branched iso-hexadecane proportion of 66.3% at the n-hexadecane conversion of 95.3%. The Pd/SAPO-31 catalyst is proven to be a highly effective bifunctional catalyst for the industrial production of bio-diesel with excellent low-temperature fluidity.