No abstract

The effect of the chemical nature of the oxide support on the performance of cobalt Fischer–Tropsch catalysts is investigated. A series of supports is synthesized via monolayer coverage of porous γ-Al2O3 with various oxides representative of a wide range of Lewis acid–base character, as quantified by UV–vis spectroscopy coupled to alizarin adsorption. Incorporation of cobalt (20 wt %) results in model catalysts with identical porosities and similar Co particle sizes (>10 nm), allowing the study of support effects without overlap from diffusional or particle size factors. Under realistic reaction conditions, the initial TOF scales with the acidity of the oxide support, whereas the cobalt time yield and selectivity to industrially relevant C13+ hydrocarbons show a volcano dependence, with a maximum at an intermediate acid–base character. As inferred from in situ CO-FTIR, “selective” blockage of a few cobalt sites, though crucial for CO hydrogenation, by atoms from basic oxides and “unselective” site blockage via decoration of Co nanoparticles (strong metal–support interaction) with acidic, reducible oxides cause a decrease in reaction rate for supports with pronounced alkaline and acidic character, respectively. The extent of secondary isomerization reactions of α-olefin products, of relevance for chain reinsertion processes and product selectivity, also correlates with the support acidity. For a TiO x /Al2O3 as support, a remarkable C13+ productivity exceeding 0.09 molC gCo –1 h–1 is achieved, owing to the combination of optimal activity and selectivity. These results provide a unifying view of the support effects over a considerably broad study space and delineate a blueprint toward advanced Fischer–Tropsch catalysts.

Some fundamental and practical aspects of a Hf-Zn catalyst, with a nominal composition of 3.0 wt% Hf and 9.3 wt% Zn and prepared as a physical mixture of Hf/SiO2 (85 wt%) and the zincsilicate hemimorphite (15 wt%), have been studied for the one-step conversion of ethanol to 1,3butadiene. The elucidation of the main reactions leading to 1,3-butadiene and by-products was made by means of kinetic curves and catalytic tests where intermediates were individually fed. In addition, the convenience of by-product separation from unreacted ethanol in an industrial process was studied by performing experiments where ethanol was co-fed with intermediates.The causes of catalyst deactivation and the impact on catalyst structure and performance of regeneration by calcination were also investigated. According to our results, the pathway to 1,3butadiene over the Hf-Zn catalyst includes ethanol dehydrogenation, acetaldehyde aldol condensation, crotonaldehyde reduction with ethanol, and crotyl alcohol dehydration. The recycling of the by-products butanal, acetone and 1-butanol into the reactor should be avoided, as the first two are converted to heavy compounds by aldol condensation reactions, while 1-butanol dehydration leads to butenes, which are difficult to separate from 1,3-butadiene. The suppression of diethyl ether formation from ethanol by recycling to extinction is possible. It has been found that catalyst deactivation is mainly caused by the retention of oxygenated aromatic-type coke species, preferentially formed on the dehydrogenating Zn 2+ sites associated with the hemimorphite component of the catalyst, and probably also by a loss in Zn 2+ sites due to the reduction to Zn 0 during catalysis. This reduction induces an imbalance between Hf 4+ and Zn 2+ sites, which changes catalyst selectivity. Regeneration by calcination with air removes coke and re-oxidizes a fraction of Zn 0 back to Zn 2+ , but it does not fully re-establish the original Zn 2+ /Hf 4+ balance.