Application of Molybdenum Carbide Catalysts for the CO2-assisted Oxidative Dehydrogenation of Ethane

The rising demand for light olefins is at present mainly met via catalytic/thermal dehydrogenation of alkanes at temperatures of up to 900 °C. Under these severe process conditions, competing side reactions and catalyst deactivation via coking are the major challenges. Co-feeding an oxidant significantly decreases the reaction temperature. The oxidative dehydrogenation of ethane to ethylene, using CO2 as the oxidant (CO2-ODH), has earned a lot of interest in the past decade. The use of CO2, a soft oxidant in comparison to O2, prevents the overoxidation reaction of the paraffin to CO2 and allows for improved heat control. Besides that, the coking effect, which is believed to be the main catalyst deactivation pathway during these high temperature processes, could be significantly lowered due to the reverse Boudouard reaction. The most common catalytic materials reported are reducible metal oxides (MOx) due to their redox properties; a key concept to activate the C-H bond of the alkane and subsequently activate CO2. Besides metal oxides, transition metal carbides have also shown to be active for the CO2-ODH, reaching high yields of ethylene. Specifically, molybdenum carbide (MoxCy) has shown to be a highly efficient catalyst for CO2 activation and alkane dehydrogenation, demonstrating its ability to cleave C-H bonds. These characteristics are important in making a MoxCy-based catalyst a serious candidate for the CO2- ODH of light alkanes. This work entails the design of novel MoxCy-based catalysts for application in the CO2-ODH of C2H6. Previous work on MoxCy-based catalysts found that the bulk material has limited activity and selectivity towards producing C2H4 but is significantly improved once dispersed on a support material. The type of support material dictates whether the CO2-ODH reaction takes place, or if one of the major side reactions, the dry-reforming of C2H6 to synthesis gas, is preferred. In this study, MoxCy nanoparticles were prepared via various (novel) synthesis techniques, dispersed on a variety of MOx support materials as well as modified with various promoters. Besides the exploratory nature of this study, gaining knowledge on the activity of the various formulations of MoxCy-based catalysts, the preparation conditions of the carbide materials were investigated. To prepare MoxCy, the precursor samples (in the molybdate or oxide phase) are exposed to a temperature programmed treatment (carburization) in the presence of a carbonaceous and reductive gas mixture. The carbide formation, in terms of crystallite structure, surface composition as well as potential fouling mechanisms is highly dependent on the heating rate, gas mixture, final temperature and precursor composition. Various experiments utilizing in situ characterization techniques, such as in situ X-ray diffraction, X-ray adsorption and Raman spectroscopy as well as online product analysis techniques were employed to gain knowledge on the carburization process, the structural and chemical properties and their effect on the activity of the various prepared catalysts in the CO2-ODH as well as the reverse water-gas-shift reaction. The use of MoxCy-based catalysts in the CO2-ODH reaction has not been thoroughly investigated in literature before and is still a very new topic to the scientific community. The presented research can contribute on various aspects of the use and viability of MoxCy-based catalysts in CO2 utilizing reactions and can be extended to dry-reforming or CO2 hydrogenation to fuels. In terms of catalyst synthesis, the extensive characterization exposing the various possible crystal structures of MoxCy nanoparticles and application of surface sensitive techniques, allowed for a better understanding of the possible active phases responsible for CO2 and alkane activation. Besides the identification of the active phase, the deactivation mechanism for MoxCy-based catalysts in the CO2-ODH reaction is studied in more detail by focusing on the crystal structure and the presence of carbon on the catalyst surface. By varying catalyst compositions as well as reaction conditions, including the use of various co-feeding experiments, an increase in catalytic stability, while maintaining high yields of the desired product from CO2-ODH (ethylene), was achieved

Oxygenate formation over K/β-Mo2C catalysts in the Fischer-Tropsch synthesis

The Fischer–Tropsch (FT) process, producing long chained waxes and transportation fuels, is competing with fuels derived from crude oils and its profitability is therefore dependent on the global oil price. However, increasing the value of synthesized products could render the profitability of the FTS independent of the common fluctuations in the commodity price (which are mostly due to global political trends and only to a lesser extent due to market requirements). One way to achieve this, is to target the more valuable products of the Fischer–Tropsch spectrum, for example oxygenates. This study investigates the effect of synthesis protocols on the surface characteristics of molybdenum carbide and the use of potassium promoted Mo2C as a catalyst for higher oxygenate (C2+ oxygenates) synthesis in CO hydrogenation. A graphitic surface layer was observed with TEM, XPS and Raman analysis for Mo2C samples carburized at ≥760 °C. The graphitic carbon, blocking active sites and therefore significantly lowering catalytic activity, could be partially removed by means of a temperature programmed hydrogenation, forming methane. An unpromoted β-Mo2C catalyst, carburized at 630 °C, reached CO conversions up to ±40% at the conditions applied. Initial 6.2 wt% K/Mo promotion of the catalyst with potassium showed a significant drop in catalyst activity, however, an increase in potassium content did not further decrease catalyst activity. The selectivity towards oxygenates was enhanced, yet it has a certain optimum with regards to promotor concentration. Simultaneously, the oxygenate distribution shifted towards higher alcohols. The initial methanol content in the total oxygenate product was around 60 C% and decreased to approximately 20 C% upon potassium promotion

Empowering Catalyst Supports: A New Concept for Catalyst Design Demonstrated in the Fischer–Tropsch Synthesis

The Fischer–Tropsch (FT) synthesis is traditionally associated with fossil fuel consumption, but recently this technology has emerged as a keystone that enables the conversion of captured CO2 with sustainable hydrogen to energy-dense fuels and chemicals for sectors which are challenging to be electrified. Iron-based FT catalysts are promoted with alkali and transition metals to improve reducibility, activity, and selectivity. Due to their low concentration and the metastable state under reaction conditions, the exact speciation and location of these promoters remain poorly understood. We now show that the selectivity promoters such as potassium and manganese, locked into an oxidic matrix doubling as a catalyst support, surpass conventional promoting effects. La1–xKxAl1–yMnyO3−δ (x = 0 or 0.1; y = 0, 0.2, 0.6, or 1) perovskite supports yield a 60% increase in CO conversion comparable to conventional promotion but show reduced CO2 and overall C1 selectivity. The presented approach to promotion seems to decouple the enhancement of the FT and the water–gas shift reaction. We introduce a general catalyst design principle that can be extended to other key catalytic processes relying on alkali and transition metal promotion

Effect of ammonia co-feeding on oxygenates over K-Mo2C in the Fischer-Tropsch synthesis

The Fischer-Tropsch (FT) process, producing long chained waxes and transportation fuels, is competing with fuels derived from crude oils and its profitability is therefore dependent on the global oil price. However, increasing the value of synthesized products could render the profitability of the FTS independent of fluctuations in the oil price (which are mostly due to global political trends). One way to achieve this, is to target fine chemicals instead of fuels. At the Catalysis Institute, this has been investigated by adding ammonia to the feed gas stream and obtaining highly valuable amines, amides and nitriles. It has been shown that the so-called nitrogen containing compounds are formed instead of the Fischer-Tropsch typical albeit minor products alcohols, aldehydes and carboxylic acids, i.e. oxygenates. Increasing the oxygenate selectivity was investigated in numerous studies as no commercial FT based process exists which produces oxygenates at a significant yield. Typically, transition metals such as Fe, Co, Rh and Ni are active for the FT synthesis. Based on reaction conditions employed, commercial Fe and Co based catalysts have been shown to produce between 6 and 12 C% oxygenates. Rh has been shown to have a high oxygenate selectivity, but the associated high raw material cost becomes prohibitive for use as a commercial FT catalyst. Catalysts other than the traditionally known FT active transition metals have shown promising results in terms of oxygenate selectivity. Transition metal carbides such as Mo2C, have been investigated under Fischer-Tropsch conditions. While the bare catalyst produces mainly methane and other hydrocarbons, upon promotion with potassium the selectivity shows a significant shift towards oxygenates. This project investigates the use of potassium promoted molybdenum carbide as a catalyst for high oxygenate selectivity in the Fischer-Tropsch synthesis. β-Mo2C was synthesized and subsequently promoted with different levels of potassium and its Fischer-Tropsch synthesis performance was evaluated in a stainless steel fixed bed reactor. The influence of catalyst synthesis protocols, reactor pressure and temperature, feed gas space velocity, and K/Mo wt.% promotion on catalyst activity and selectivity were studied. At a stable CO conversion (±10%) and its related oxygenate selectivity (±35 C%) ammonia was co-fed to the catalyst to study the conversion of oxygenates to nitrogen containing compounds. In summary, an unpromoted β-Mo2C catalyst reached CO conversions to ±40% at the conditions applied. Initial promotion of the catalyst with potassium showed a significant drop in catalyst activity, however, an increase in potassium content did not further decrease catalyst activity. The selectivity towards oxygenates was greatly enhanced from 10 C% up to 42 C% (CO2-free) at similar reaction conditions. Simultaneously, the oxygenate distribution shifted towards higher alcohols. The initial methanol content in the total oxygenate slate was around 60 C%, decreasing to about 20 C% upon potassium promotion. During co-feeding of ammonia, N-containing compounds were observed in the form of nitriles (±9 C%, CO2-free) and small traces of amides (±0.1 C%, CO2-free). Acetonitrile was the most dominating formed N-containing compound (≥58 C%). Upon the co-feeding of ammonia, the oxygenate selectivity decreased by roughly 10 C% points (CO2-free) but did not reach zero. Catalyst activity was slightly affected but recovered with time on stream. A slowly building up blockage appeared after 1-3 hours TOS simultaneously with a decreasing CO2 selectivity, suggesting the reaction with NH3 forming ammonium carbonate. This could however not be confirmed. The benefits of producing N-containing compounds using a potassium promoted β-Mo2C needs to be further investigated, trying to avoid the blockage by suppressing the WGS-activity of the catalyst. It is promising that the activity is hardly affected and that in the short period of time on stream N-containing compounds were observed

Synthesis, characterisation and water-gas shift activity of nano-particulate mixed-metal (Al, Ti) cobalt oxides

The formation of mixed-metal cobalt oxides, representing potential metal-support compounds for cobalt-based catalysts, has been observed at high conversion levels in the Fischer-Tropsch synthesis over metal oxide-supported cobalt catalysts. An often observed increase in the carbon dioxide selectivity at Fischer-Tropsch conversion levels above 80% has been suggested to be associated to the formation of water-gas shift active oxidic cobalt species. Mixed-metal cobalt oxides, namely cobalt aluminate and cobalt titanate, were therefore synthesised and tested for potential catalytic activity in the water-gas shift reaction. We present a preparation route for amorphous mixed-metal oxides via thermal treatment of metal precursors in benzyl alcohol. Calcination of the as prepared nanoparticles results in highly crystalline phases. The nano-particulate mixed-metal cobalt oxides were thoroughly analysed by means of X-ray diffraction, Raman spectroscopy, temperature-programmed reduction, X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure, and high-resolution scanning transmission electron microscopy. This complementary characterisation of the synthesised materials allows for a distinct identification of the phases and their properties. The cobalt aluminate prepared has a cobalt-rich composition (Co1+xAl2-xO4) with a homogeneous atomic distribution throughout the nano-particulate structures, while the perovskite-type cobalt titanate (CoTiO3) features cobalt-lean smaller particles associated with larger ones with an increased concentration of cobalt. The cobalt aluminate prepared showed no water-gas shift activity in the medium-shift temperature range, while the cobalt titanate sample catalysed the conversion of water and carbon monoxide to hydrogen and carbon dioxide after an extended activation period. However, this perovskite underwent vast restructuring forming metallic cobalt, a known catalyst for the water-gas shift reaction at temperatures exceeding typical conditions for the cobalt-based Fischer-Tropsch synthesis, and anatase-TiO2. The partial reduction of the mixed-metal oxide and segregation was identified by means of post-run characterisation using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy energy-dispersive spectrometry

Supplementary information files for Empowering catalyst supports: A new concept for catalyst design demonstrated in the Fischer–Tropsch synthesis

Supplementary files for article Empowering catalyst supports: A new concept for catalyst design demonstrated in the Fischer–Tropsch synthesis The Fischer–Tropsch (FT) synthesis is traditionally associated with fossil fuel consumption, but recently this technology has emerged as a keystone that enables the conversion of captured CO2 with sustainable hydrogen to energy-dense fuels and chemicals for sectors which are challenging to be electrified. Iron-based FT catalysts are promoted with alkali and transition metals to improve reducibility, activity, and selectivity. Due to their low concentration and the metastable state under reaction conditions, the exact speciation and location of these promoters remain poorly understood. We now show that the selectivity promoters such as potassium and manganese, locked into an oxidic matrix doubling as a catalyst support, surpass conventional promoting effects. La1–xKxAl1–yMnyO3−δ (x = 0 or 0.1; y = 0, 0.2, 0.6, or 1) perovskite supports yield a 60% increase in CO conversion comparable to conventional promotion but show reduced CO2 and overall C1 selectivity. The presented approach to promotion seems to decouple the enhancement of the FT and the water–gas shift reaction. We introduce a general catalyst design principle that can be extended to other key catalytic processes relying on alkali and transition metal promotion.  </p

Empowering catalyst supports: A new concept for catalyst design demonstrated in the Fischer–Tropsch synthesis

The Fischer–Tropsch (FT) synthesis is traditionally associated with fossil fuel consumption, but recently this technology has emerged as a keystone that enables the conversion of captured CO2 with sustainable hydrogen to energy-dense fuels and chemicals for sectors which are challenging to be electrified. Iron-based FT catalysts are promoted with alkali and transition metals to improve reducibility, activity, and selectivity. Due to their low concentration and the metastable state under reaction conditions, the exact speciation and location of these promoters remain poorly understood. We now show that the selectivity promoters such as potassium and manganese, locked into an oxidic matrix doubling as a catalyst support, surpass conventional promoting effects. La1–xKxAl1–yMnyO3−δ (x = 0 or 0.1; y = 0, 0.2, 0.6, or 1) perovskite supports yield a 60% increase in CO conversion comparable to conventional promotion but show reduced CO2 and overall C1 selectivity. The presented approach to promotion seems to decouple the enhancement of the FT and the water–gas shift reaction. We introduce a general catalyst design principle that can be extended to other key catalytic processes relying on alkali and transition metal promotion. </p