Novel cobalt-based catalysts for the intensification of the Fischer-Tropsch synthesis

The development of the optimal cobalt based FT catalyst, with improved activity and selectivity to desired products, has been the goal of this PhD thesis work. This goal has been pursued by a combination of experimental activities at different scales, starting from the preparation of the catalysts, to their characterization and eventually the assessment of their catalytic performances in the FTS. In the first part of this thesis work the development of a novel preparation procedure for an eggshell-type Co/-Al2O3 catalyst is reported. The main characteristics of the obtained material are a diameter smaller than 1 mm which is the most common value in the scientific literature, and a diffusive length ensuring the absence of mass transfer limitations. Interestingly, the prepared eggshell catalyst shows great catalytic performances if compared with those of a powdered catalyst (obtained by grinding and sieving the eggshell catalyst) working in a kinetically controlled regime. This result has been explained by considering the onset of weak mass transport limitations in the case of the eggshell catalyst, which boost both the CO conversion kinetics and the catalyst hydrogenating ability. These results show that engineered eggshell catalysts with an optimal thickness of the active shell represent an optimal solution for applications purpose. In fact, it is possible to achieve high C15+ yields per mass of cobalt in the reactor, while limiting the ∆P in compact packed-bed reactors for the FTS. The second part of this thesis work focuses in developing a highly active Co/-Al2O3 catalyst as uniformly impregnated with a pellet size of 300 m in diameter. In compact FT reactors, such diameter represents a compromise to keep under control mass transfer and pressure drop issues. In this regard, two strategies have been adopted in this thesis work to increase the number of cobalt metal surface sites available for the FTS. The decrease of the cobalt metal particle size through the improvement of the preparation methods commonly applied at the industrial scale (in view of the necessity not to complicate the catalyst synthesis) and the enhancement of the catalyst reducibility with the addition of a small amount of noble metals (in view of the catalyst cost). Concerning the first approach, a highly active Co-based catalyst has been synthetized by diluting the Co-nitrates impregnating solution with an organic compound (i.e. diethylen glycol, DEG). It has been found that thanks to the occurrence of combustion phenomena between Co nitrates, which act as oxidizers, and DEG, which act as fuel, the decomposition from Co nitrates to Co oxides species during calcination becomes exothermic and fast. This generates highly dispersed Co oxides particles, which, however, are difficult to reduce by a standard reduction treatment in H2 at 400°C. Despite the lower reducibility, when compared with a catalyst with the same formulation but prepared without using DEG, the catalyst prepared with DEG shows very interesting catalytic performances, thanks to the fact that the small Co0 particles are intrinsically very active. In line with the decrease of the Co0 crystallite size, the catalyst prepared with DEG shows slightly higher hydrogenating activity. Nevertheless, the increase of the CO conversion overcompensates the decrease of the C5+ selectivity, thus resulting in a raised C5+ yield. Concerning the second approach, the possibility to increase the number of active Co0 centers through the addition of small amount of platinum in the catalyst formulation has been investigated. In particular, the effect of 0.1wt.% of Pt has been studied on the properties and catalytic performances of a catalyst prepared with (small crystallites) and without (big crystallites) DEG, in the Co-impregnating solution. The Pt-promoted catalysts have been prepared by varying the impregnation order of Pt and Co: (i) Pt after Co (sequential deposition order, SDO) and (ii) Pt before Co (reverse sequential deposition order, RSDO). Regardless of the initial Co3O4 crystallites size, all the Pt-promoted catalysts show a strong enhancement of the catalyst reducibility and Co0 dispersion with respect to the corresponding unpromoted catalysts. This indicates the ability of Pt in favoring the reduction of cobalt oxides species, even if present in small quantity. Moreover, it has been found that the catalytic activity trend reflects that of the Co0 dispersion with the catalysts prepared without DEG, thus justifying why the RSDO catalyst is more active than SDO, which in turn is more active than the unpromoted sample. Furthermore, the selectivity to the main FTS products is almost unvaried with respect to the unpromoted sample, thanks to the small amount of Pt used. On the contrary, the promising results obtained with the Pt-promoted catalysts prepared with DEG in terms of catalyst characterization (good reducibility and increase of the number of Co0 sites) do not reflect the catalyst activity, which results (in the bad case) almost halved with respect to the unpromoted catalyst. This result has been explained with the fact that the intrinsic activity of the Co0 sites (turnover frequency, TOF) of the Pt-promoted catalysts is significantly decreased. This has been attributed to the fact that these catalysts have a Co0 particle size distribution shifted to lower values than that of the unpromoted sample, thus entering in the TOF-size range where the FTS has been reported to be structure sensitive. In the final part of this thesis work, the effect of water on the catalytic performances of Co-based catalyst is investigated. This is of interest in view of the fact that commercial FTS practices require that Co-based catalysts withstand long-term use at high CO conversion, and hence at high water concentration levels. The obtained data indicate that water addition leads to a remarkable catalyst deactivation already at low water concentration. This phenomenon is a rather slow process, whose rate depends on the feed concentration of water and whose extent depends on the duration of water co-feeding. The presence of water in the feed also affects the process selectivity, and both reversible and irreversible effects have been observed. In particular, the exposure of the catalyst to water results in an irreversible increase of the olefin to paraffin ratio (mostly due to the increased olefin selectivity), of the 1-olefins in the alkenes pool (due to the decreased double bond shift activity) and of the CO2 selectivity (due to increased WGS activity). These effects have been explained considering that the active sites evolve during the water co-feeding in those cobalt oxides species which are formed as a result of the oxidation of a fraction of the metallic cobalt sites initially present on the catalyst. Reversible effects, which mostly consist in the increased chain growth probability and CO2 selectivity and in the decreased CH4 and alcohol selectivities, have been explained by assuming that the water inhibits the hydrogenation reactions and acts as reactant in the WGS reaction. The results obtained in this PhD thesis work pave the way for new future searches. In particular, a further improvement of the preparation method adopted for the eggshell catalysts is recommended in order to decrease the large number of steps of impregnation and calcinations currently adopted. A detailed investigation of the effect of diffusive length in a wider range may be also useful to gain more insights on the effect of mass transfer limitations in the FTS. Furthermore, the preparation method used for the catalyst prepared with DEG should be improved with the goal to obtain a more homogeneous Co oxides particles size distribution without the presence of too much small cobalt particles on the catalyst. To the scope, it may be useful to act either on the amount of DEG used, or on the type of impregnation. In this regard, the slurry impregnation method is suggested. Moreover, the effect of Pt should be investigated also using a smaller amount of noble metal than that used on this work (<0.1wt.%). Finally, since the effect of water strongly depends on the structure and on the properties of the catalyst, additional experiments should be done with both the catalysts prepared with DEG and catalysts promoted with Pt.

Lo sviluppo del catalizzatore FT a base di cobalto, con una migliore attività e selettività ai prodotti desiderati, è stato l'obiettivo di questo lavoro di tesi di dottorato. Questo obiettivo è stato perseguito tramite una combinazione di attività sperimentali su scale diverse, a partire dalla preparazione dei catalizzatori, alla loro caratterizzazione ed infine alla valutazione delle prestazioni catalitiche nelle FTS. Nella prima parte di questa tesi è stato sviluppata una procedura di preparazione per un catalizzatore di tipo eggshell. Tale catalizzatore è considerato un giusto compromesso tra le perdite di carico in un reattore a letto fisso e le limitazioni diffusive intraparticellari. È interessante notare che il catalizzatore preparato ha ottime prestazioni catalitiche se confrontati con quelli di un catalizzatore in polvere (ottenuta dalla macinazione e setacciatura del catalizzatore eggshell) che lavora in regime chimico. La seconda parte di questa tesi si è focalizzata nello sviluppo di un catalizzatore altamente attivo come uniformemente impregnato con una dimensione pellet di 300 microns di diametro. In reattori FT compatti, tale diametro rappresenta un giusto compromesso per tenere sotto controllo i problemi di trasferimento di massa e le perdita di carico. Due strategie sono state adottate in questo lavoro di tesi per aumentare il numero di siti superficiali cobalto disponibili per la FTS. La diminuzione della dimensione delle particelle di cobalto metallico attraverso il miglioramento dei metodi di preparazione comunemente applicati su scala industriale (in vista della necessità di non complicare la sintesi catalizzatore) e la valorizzazione della riducibilità del catalizzatore con l'aggiunta di una piccola quantità di metallo nobile (in vista del costo del catalizzatore). Nella parte finale di questa tesi, l'effetto dell'acqua sulle prestazioni catalitiche di un catalizzatore FT è stato indagato. Ciò è di interesse in considerazione del fatto che le applicazioni commerciali richiedono che i catalizzatori resistano a lungo termine a conversione elevate e quindi elevate concentrazioni di acqua. I dati ottenuti indicano che l'aggiunta di acqua porta ad un notevole disattivazione del catalizzatore già alle concentrazioni di acqua più basse. Questo fenomeno è un processo piuttosto lento, la cui entità dipende dalla concentrazione di acqua in alimentazione e dalla durata della coalimentazione. La presenza di acqua influisce anche sulla selettività del processo, e sono stati osservati effetti sia reversibili che irreversibili. In particolare, l'esposizione del catalizzatore ad acqua si traduce in un aumento irreversibile del rapporto olefina su paraffina (principalmente a causa della maggiore selettività ad olefina) e della selettività a CO2 (a causa di una maggiore attività WGS). Questi effetti sono stati spiegati considerando che i siti attivi evolvono, durante la coalimentazione di acqua, in quelle specie di ossidi di cobalto che si formano a causa della ossidazione di una frazione dei siti cobalto metallico inizialmente presenti sul catalizzatore. Effetti reversibili, che consistono principalmente nella maggiore probabilità di crescita della catena e le selettività a CH4 e alcool. Questi sono stati spiegati assumendo che l'acqua inibisce le reazioni di idrogenazione e agisce come reagente nella reazione WGS.