Michela Martinelli

Fischer–Tropsch synthesis: effect of ammonia on product selectivities for a Pt promoted Co/alumina catalyst

The effects of co-fed ammonia in synthesis gas on the activity and product selectivities of a typical cobalt catalyst (0.5% Pt–25% Co/Al2O3) were investigated during the Fischer–Tropsch synthesis using a continuously stirred tank reactor (CSTR). The product selectivities were compared at a similar CO conversion level for various concentrations (10–1000 ppmv) of ammonia, as well as clean (un-poisoned) conditions. The addition of 10–1000 ppmv ammonia (concentration of ammonia with respect to the syngas feed) significantly decreased activity; the percentage of deactivation was similar (∼40%) for the various concentrations of ammonia used. At similar CO conversions, the addition of ammonia caused an increase in olefin selectivity and the corresponding paraffin and alcohol selectivities were decreased compared to the ammonia free synthesis conditions. Olefin selectivity increased with increasing concentration of ammonia, and the paraffin and alcohol selectivities were decreased with increasing ammonia concentration. At similar CO conversions, ammonia addition exhibited a positive effect on hydrocarbon selectivity (i.e., lower light gas products and higher C5+) and also light gas product selectivities (C1–C4) were decreased and C5+ selectivity increased with increasing concentration of ammonia compared to ammonia free conditions.

Dehydration of 1,5‐Pentanediol over Na‐Doped CeO2 Catalysts

The effects of CeO2 doped with Na on the dehydration of 1,5‐pentanediol were studied by using a fixed‐bed reactor at two different temperatures (350 and 400 °C) and atmospheric pressure. For characterization, BET surface area, hydrogen temperature‐programmed reduction, CO2 temperature‐programmed desorption, and diffuse reflectance infrared Fourier transform spectroscopy techniques were utilized. The conversion of the diol on CeO2 was found to depend on Na loading. The selectivity to the desired product (i.e., unsaturated alcohol) increased and the selectivity to undesired products (i.e., tetrahydropyran, tetrahydropyran‐2‐one, cyclopentanol and cylopentanone) decreased with increasing Na content on CeO2. The basicity of hydroxyl groups or surface oxygen on CeO2 was altered with the addition of Na, and controlled the dehydration reaction pathway.

Fischer–Tropsch Synthesis: XANES Spectra of Potassium in Promoted Precipitated Iron Catalysts as a Function of Time On-stream

XANES K-edge spectra of potassium promoter in precipitated Fe catalysts were acquired following activation by carburization in CO and as a function of time on-stream during the course of a Fischer–Tropsch synthesis run for a 100Fe:2K catalyst by withdrawing catalysts, sealed in wax product, for analysis. CO-activated and end-of-run spectra of the catalyst were also obtained for a 100Fe:5K catalyst. Peaks representing electronic transitions and multiple scattering were observed and resembled reference spectra for potassium carbonate or potassium formate. The shift in the multiple scattering peak to higher energy was consistent with sintering of potassium promoter during the course of the reaction test. The catalyst, however, retained its carbidic state, as demonstrated by XANES and EXAFS spectra at the iron K-edge, suggesting that sintering of potassium did not adversely affect the carburization rate, which is important for preventing iron carbides from oxidizing. The method serves a starting point for developing better understanding of the chemical state and changes in structure occurring with alkali promoter.Graphical Abstract

Dehydration of 1,5-Pentanediol over CeO2-MeOx Catalysts

The dehydration reaction of 1,5‐pentanediol was performed over CeO2 and modified CeO2 (CeO2−MnOx, CeO2−ZnO, CeO2−MgO, CeO2−CaO, CeO2−Na2O) catalysts in a fixed‐bed tubular reactor at 350 °C and an atmospheric pressure. The undoped CeO2 produced a mixture of the products containing mainly 4‐penten‐1‐ol, 1‐pentanol, cyclopentanol, cyclopentanone and tetrahydropyran‐2‐one from 1,5‐pentanediol, while additions of MgO, MnOx, or ZnO to CeO2 was found to enhance the overall production rate of unsaturated alcohol. On the other hand, more basic metals like CaO or Na2O tend to decline the dehydration activity of CeO2. The porous structure of CeO2 did not change appreciably with the addition of metal oxides. Temperature programmed desorption of adsorbed CO2 on an activated catalyst suggest more CO2 remain on the catalyst surface, particularly CeO2−CaO and CeO2−Na2O indicating that fewer defect sites are only available for reaction. The defect sites or oxygen vacancy on CeO2 controls both activity and selectivity for the dehydration of 1,5‐pentanediol.

Effect of Phosphorus on the Activity and Stability of Supported Cobalt Catalysts for Fischer-Tropsch Synthesis

Phosphorus promotion on Fischer‐Tropsch (FT) synthesis was investigated for Co/Al2O3 and Co/SiO2 catalysts having the same Co/P ratio. When P is added to Co/Al2O3, CO conversion on a per g catalyst basis decreased, while methane selectivity increased. Catalyst stability was higher for the sample containing both P and Pt. The main cause for lower initial conversion is Co site blocking, while the lower extent of cobalt reduction for the P‐promoted Co/Al2O3 sample played a lesser role. When SiO2 is used to support cobalt particles, an initial induction period for the P‐promoted catalyst was observed, where CO conversion increased. Higher CO conversion at steady state, as well as improved catalyst stability during FT testing, suggest that P hindered sintering. Over the same period, a decline and leveling off of conversion were observed for the unpromoted catalyst. Completely different effects were observed depending on support type. P only acted as a poison for Co/Al2O3, whereas beneficial effects on steady state CO conversion and stability occurred with Co/SiO2. The different effects of P for Al2O3 and SiO2 supported Co catalysts can be explained by differences in Co‐support interactions. With alumina, Co clusters are already stabilized by strong interactions with the support. P has no benefit, as it mainly interacts with alumina instead of Co; pore blocking by P also occurred. In contrast, SiO2 has weak interactions with Co and less Co cluster stabilization. With P promotion, P anchors Co to the support, improving Co dispersion, stability and steady‐state conversion.

Effect of Gallium Additions on Reduction, Carburization and Fischer–Tropsch Activity of Iron Catalysts

Undoped and gallium-doped iron oxide catalysts (100Fe, 100Fe:2Ga, and 100Fe:5Ga) were prepared by following a continuous co-precipitation technique using ammonium hydroxide as precipitant. The catalysts were characterized by BET surface area, X-ray diffraction, H2-temperature programmed reduction, 57Fe Mössbauer spectroscopy, and temperature programmed decarburization techniques. The addition of gallium affects both reduction as well as carburization of iron oxide. The CO conversion decreases with an increase of gallium content relative to iron. The gallium-doped iron catalyst (100Fe:2Ga) exhibits initially a lower CO conversion after H2 activation than an undoped iron catalyst; however, the activity of the doped catalyst kept increasing with time. A strong interaction between iron oxide and gallium could explain the suppressed formation of χ-Fe5C2 and metallic iron during the carburization and reduction of iron oxide, respectively. The relative percentage of iron in χ-Fe5C2 was found to correlate with the initial rates of FT and WGS activity indicating that iron carbide is the main active component for both FT and WGS reactions.Graphical Abstract