Muthu Kumaran Gnanamani

Fischer–Tropsch Synthesis: Effect of Potassium on Activity and Selectivity for Oxide and Carbide Fe Catalysts

The effect of potassium on oxides and carbides of iron for Fischer–Tropsch synthesis (FTS) was investigated by pretreating Fe3O4 and K-promoted Fe catalysts with different gases (H2/H2O and CO). A freshly activated sample and catalysts that were recovered from the CSTR before, during and after FT synthesis were characterized ex situ using Mössbauer spectroscopy. Iron carbide is found to be active for both FT and water gas shift (WGS) reactions. After H2/H2O activation, all three catalysts (Fe3O4, low α-Fe, and high α-Fe) exhibit a steady but low FT activity for a period of FT synthesis. However, both FT and WGS activity for Fe3O4 and low α-Fe catalysts were greatly improved after CO activation. In contrast, the high potassium containing catalyst (high α-Fe) did not show any further improvement in activity after CO activation. The difference in FT and WGS activity observed after pretreatment conditions using these catalysts may be associated to the amount of potassium and conversely the iron carbide present in the catalysts.Graphical Abstract

Fischer–Tropsch Synthesis: Studies on the Effect of Support Doping with Si, Mn and Cr on the Selectivity to Alcohols in Ceria Supported Cobalt Catalysts

Mn and Cr doped CeSi mixed oxides were used as supports for Co and tested for CO hydrogenation. Co/CeSi was found to be more active and significantly more selective to n-alcohols/olefins. An increasing selectivity to n-alcohols and decreasing selectivity to olefins as a function of time on stream was also observed, suggesting a trade-off between those two products. Addition of Mn led to similar behavior, although at slightly lower conversions. Addition of Cr, however, considerably suppressed n-alcohol formation, while it kept selectivities to olefins within a 20–30 % range over more than 250 h of testing, indicating either higher alcohol dehydration activity, or that the presence of Cr ions lowered the hydrogenating activity of Co. The present work indicates that enhanced contact area between Co and the reducible support is likely a key factor for enhancing selectivity to alcohols.

Hydrogenation of carbon dioxide over K-Promoted FeCo bimetallic catalysts prepared from mixed metal oxalates

The hydrogenation of carbon dioxide over K‐promoted FeCo bimetallic catalysts prepared by sequential oxalate decomposition and carburization of FeCo with CO was studied in a fixed‐bed reactor at 240 °C and 1.2 MPa. The initial CO2 conversion was found to be dependent on K loading, whereas both unpromoted and K‐promoted FeCo catalysts (except 90Fe10Co3.0K) exhibited similar levels of CO2 conversion after a few hours of time on stream. A decarburization study on freshly activated and used FeCo suggests that potassium increases the stability of iron carbides and graphitic carbon under a reducing atmosphere. Also, K addition tends to decrease the hydrogenation function of FeCo bimetallic catalysts and, thus, controls product selectivity. Under similar CO2 conversions, potassium enhanced acetic acid formation while suppressing ethanol production, which indicates that a common intermediate might be responsible for the changes observed with C2 oxygenates.

Deuterium Exchange Study for Hydrogenation of D5-1-Pentene (4,4,5,5,5) Over Conventional Cobalt Fischer–Tropsch Catalyst

The hydrogen–deuterium exchange reaction was performed for hydrogenation of D5-1-pentene (4,4,5,5,5) under realistic cobalt Fischer–Tropsch synthesis conditions. In the presence of CO, the added deutero-1-pentene did not show any significant H/D exchange but a step-wise H/D exchange occurs when CO was replaced with N2. The inhibition effects of CO and other FT products on H/D exchange of D5-1-pentene and a pressure dependency effect on H/D exchange are observed.Graphical Abstract

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.

Dehydration of 2-octanol over CeO2-CaO mixed oxides

Vapor‐phase catalytic dehydration of 2‐octanol was investigated over Ca‐doped CeO2 at 375 °C and atmospheric pressure. Ca doping up to 0.15 wt % was found to increase the dehydration activity for 2‐octanol, whereas further increases in Ca content (0.50 and 1.25 wt %) detrimentally affected the conversion. Catalyst surface area and pore volume increased with increasing Ca content in Ca‐doped CeO2. Hydrogen temperature‐programmed reduction (TPR) profiles indicate that the partially reduced state of surface ceria (i.e., Ce3+), which increases with increasing Ca loading up to 0.15 wt %, might play an important role in promoting dehydration.

Fischer–Tropsch synthesis: effect of Cu, Mn and Zn addition on activity and product selectivity of cobalt ferrite

The effect of Cu, Mn and Zn addition on cobalt ferrite was investigated for Fischer–Tropsch synthesis (FTS). Oxalate co-precipitation followed by decomposition under inert conditions was used to obtain various metal containing cobalt ferrites (Co0.7M0.3Fe2O4). The carburization of cobalt ferrite in flowing CO at 270 °C and 175 psig yielded iron carbides (χ-Fe5C2 and e′-Fe2.2C) along with a bimetallic FeCo alloy. The extent of carburization was compared among Cu, Mn, and Zn doped catalysts with undoped cobalt ferrites under similar conditions. XRD and Mossbauer spectroscopy analysis of the freshly carburized samples followed by passivation revealed that carburization of cobalt ferrite did not change appreciably with addition of Cu or Mn. On the other hand, Zn was found to retard the carburization of cobalt ferrite. Analysis of the used FT catalysts suggests that Cu is less efficient over Mn and Zn in stabilizing the iron carbides (i.e., active form of iron) during FT synthesis. The FT activity remains more or less the same for the undoped, Cu and Zn containing cobalt ferrites at higher temperatures. The CO conversion of Co0.7Mn0.3Fe2.0 catalyst was much lower than the other catalysts tested. Addition of Zn or Mn to cobalt ferrite was found to promote alcohol formation, particularly at higher reaction temperatures. The water–gas shift activity of the catalysts was found to decrease in the following order, Co1.0Fe2.0 > Co0.7Mn0.3Fe2.0 > Co0.7Zn0.3Fe2.0 > Co0.7Cu0.3Fe2.0.

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.