OXYGENATE SYNTHESIS DURING CO HYDROGENATION ON CoCuZnO- BASED CATALYSTS

The research of catalytic synthesis of methanol and other higher alcohols from CO hydrogenation has received great attention since 1980s. The focus of this research is to establish a better fundamental insight into heterogeneous metal catalysts for oxygenate (especially alcohol) synthesis by CO hydrogenation. Co-based catalysts have been reported widely as the high-performance Fischer-Tropsch Synthesis (FTS) catalysts. The solid base, hydrotalcite (HT), using as a support for Co catalysts resulted in higher activity for CO hydrogenation comparing to other supports (pre-calcined hydrotalcite, MgO and Al2O3). The activities of Co/HT reduced at different reduction temperatures (300-600oC) were also compared. Reduction at 500oC resulted in the highest activity. However, CH4 selectivity also enhanced. It was found that the thermal stability properties of hydrotalcite, BET surface area, particle size of Co, the interaction between Co and the support, and the reducibility of Co were all important in governing the catalytic performance of the Co catalysts for CO hydrogenation. A comparison of the relationship of H2 or CO chemisorption measurements at 25-100oC to similar results measured under CO hydrogenation conditions by steady-state isotopic transient kinetic analysis (SSITKA) is made for a wide variety of Group VIII metal catalysts. The ratio NT*/Nchem (amount of chemisorption by SSITKA vs. by static chemisorptions) was found to be close to unity for most Co catalysts. SSITKA can, thus, be applied as a complementary technique to static chemisorption, TEM and XRD for better understanding of metal dispersion and the availability of metal surface active sites for Co catalysts with wide variety of promoters/supports. However, the application of SSITKA for characterizing metal dispersion for the other metal is limited at this time. The effects of individual components and an Al2O3 support on CuZnO for methanol (MeOH) synthesis were investigated at a site level using SSITKA for the first time. Surface reaction parameters for MeOH and dimethyl-ether (DME) were corrected for readsorption effects. SSITKA results suggested that CuZnO-based catalysts exhibited higher MeOH formation rates due to both higher intrinsic site activities and higher concentrations of active surface intermediates. The presence of ZnO seems to decrease the hydrocarbon formation ability of Cu. The synergy between Cu and ZnO was surprisingly less than an order-of-magnitude improvement based on MeOH TOFITK (a measure of site activity for MeOH formaiton). The addition of Co into CuZnO has been investigated for the effect of component interaction on the synthesis of hydrocarbons and oxygenates during CO hydrogenation. The relationships between the surface kinetics of formation of the various products were investigated for the first time using multiproduct SSITKA. CO hydrogenation and SSITKA were carried out in a fixed-bed differential reactor at 250oC and 1.8 atm. The SSITKA results showed that Cu can decrease the activity for all products probably due to blockage by Cu of the Co surface. ZnO appears to serve as a support/dispersion agent for Co, keeping Co highly dispersed and active for hydrocarbon and higher oxygenate synthesis. However, the effects for Cu and ZnO with Co were not additive. The Co-Cu-ZnO combination resulted in a synergy that maintain the oxygenate synthesis ability of highly dispersed Co (such as Co/Al2O3) while decreasing the ability to make hydrocarbons by loss of hydrocarbon sites. Interestingly, the rate of synthesis for C2 oxygenates on Co/CuZnO was the essentially the same to that on Co/Al2O3- but without the high production/rate of hydrocarbons. Co/CuZnO is thus a selective but not an active catalyst for higher oxygenate synthesis