The catalyst support effect on NiMoS of bio-hydrogenated diesel production was investigated via density functional theory. The activity, stability and selectivity of catalyst on three widely used catalyst supports (Al2O3, TiO2 and SiO2) were studied and compared.
Catalyst-support interaction and Brønsted acidity strength of different catalyst supports were characterized, which could potentially provide the prediction of catalyst stability in the hydrogenation reaction due to the correlation between catalyst stability and support property. The results showed catalyst-support interaction in the order (high to low): SiO2 > TiO2 > Al2O3, which indicates sintering order of the catalyst (high to low): Al2O3 > TiO2 > SiO2. Brønsted acidity strength of three different catalyst supports were assessed by NH3 binding energy on catalyst supports, which is in the order of (high binding energy to low binding energy): Al2O3 > TiO2 > SiO2. The results indicate coking rate on different supports are expected in the order (high to low): Al2O3 > TiO2 > SiO2. Considering both sintering and coking, the deactivation of the catalyst on
different supports are in the order (high to low): Al2O3 > TiO2 > SiO2. The modeling results are in a good agreement with the experimental observations.
Two main reaction mechanisms in bio-hydrogenated diesel production were studied using propanal as the study molecule: hydrodeoxygenation (HDO) and hydrodecarbonylation (HDC). Propanal is chosen as the study molecule because it is a key intermediate for HDO and HDC. Multiple reaction pathways were studied and calculated to identify the preferred reaction pathways in HDO and HDC. The preferred reaction pathway in HDO involves: 1) propanal protonation, 2) propanol formation, 3) hydrogen-assisted C-O bond cleavage and 4) protonation to propane. The preferred reaction pathway in HDC involves: 1) C-H bond cleavage in propanal, 2) C-C bond cleavage, 3a) protonation to ethane or 3b) deprotonation to ethylene. It is thermodynamically preferable to form ethane, since the energy of ethane is lower compared with ethylene and hydrogen. However, it is kinetically preferable to form ethylene, since the activation energy of ethylene formation is relatively lower compared with ethane formation. The most critical step in HDO is hydrogen-assisted C-O bond cleavage and the most critical step in HDC is C-H bond cleavage of propanal. These two steps were further studied on different catalyst supports to study the effect of the catalyst support. The results revealed C-O bond cleavage on three different supports require different activation energy, which is in the order (from high to low): Al2O3 > TiO2 > SiO2. The results revealed C-H bond cleavage on three different catalyst supports require different activation energy, which is in the order (from high to low): SiO2 > TiO2 > Al2O3. The difference in activation energies could possibly come from the different charge distribution of catalyst and reactant, caused by different catalyst support.
When comparing the activation energies of the critical steps in HDO and HDC on the three different catalyst supports, HDO activation energy was consistently lower than HDC activation energy. The activation energy difference between HDC and HDO on the three different supports are in the order (from large to small): SiO2 > TiO2 > Al2O3. As a result, the selectivity of HDO on the three different supports is expected in the order (from high to low): SiO2 > TiO2 > Al2O3, which is in a good agreement with experimental data reported in literature.