2C Catalysts Research Articles

K/Ni/β-Mo 2C: A highly active and selective catalyst for higher alcohols synthesis from CO hydrogenation

Nickel and potassium promoted β-Mo 2C catalysts were prepared for CO hydrogenation to higher alcohols synthesis. The results revealed that β-Mo 2C produced mainly hydrocarbons, but the addition of potassium resulted in a remarkable selectivity shift from hydrocarbons to alcohols over β-Mo 2C. Moreover, it was found that potassium enhanced the ability of chain propagation of β-Mo 2C catalyst and led to a higher selectivity to C 2 +OH. The addition of nickel further enhanced higher alcohols synthesis, which showed the optimum at 1/8–1/6 of Ni/Mo molar ratios. The characterization suggested that there might be a synergistic effect of potassium and nickel on β-Mo 2C, which favored the alcohols synthesis. The production of alcohols appeared to be relevant to the presence of Mo 4+ species, whereas the formation of hydrocarbons was closely associated with Mo 2+ and/or Mo 0 species on the surface of β-Mo 2C-based catalysts.

Synthesis of higher alcohols from syngas over Fischer–Tropsch elements modified K/β-Mo 2C catalysts

Fischer–Tropsch elements (Fe, Co and Ni) modified K/β-Mo 2C catalysts were prepared and investigated for their performance in CO hydrogenation. The results revealed that Ni or Co modified K/β-Mo 2C catalysts were shown to be highly active and selective towards the synthesis of mixed alcohols, especially for the C 2 + OH production, but Fe caused a negative effect. Ni or Co led to a decrease in apparent activation energies for C 1–C 4 alcohols, whereas Fe caused a slight increase for methanol and the decrease for C 2–C 4 alcohols. As a result, the promotion effects of Fischer–Tropsch elements were different and followed the sequence: Ni ≫ Co > Fe for the activity and Ni > Co > Fe for the C 2 + OH alcohol selectivity.

Mixed alcohols synthesis from carbon monoxide hydrogenation over potassium promoted β-Mo 2C catalysts

Potassium-promoted β-Mo 2C catalysts were prepared and their performances in CO hydrogenation were investigated. The main products over β-Mo 2C catalyst were C 1–C 4 hydrocarbons, only ∼4 C-atom% alcohols were obtained. The products of hydrocarbons and alcohols obeyed traditional linear Anderson–Schultz–Flory (A–S–F) distribution. However, modification with K 2CO 3 resulted in a remarkable selectivity shift from hydrocarbons to alcohols. Moreover, it was found that potassium promoter enhanced the ability of chain propagation of β-Mo 2C catalysts and resulted in a higher selectivity to C 2+OH. For K/β-Mo 2C catalysts, the hydrocarbon products also obeyed traditional linear A–S–F plots, whereas alcohols gave a unique linear A–S–F distribution with remarkable deviation of methanol compared with that on β-Mo 2C catalyst. It could be concluded that potassium promoter might exert a prominent function on the whole chain propagation to produce alcohols. A surface phase on the K/β-Mo 2C catalysts such as the “K–Mo–C” explained the higher value for C 2+OH, especially could promote the step of C 1OH to C 2OH, or could have a role in producing directly C 2OH, but again this would be speculative. At the same time, the influence of the loadings of K 2CO 3 on the performances of β-Mo 2C catalyst was investigated and the results revealed that the maximum yield of alcohol was obtained at K/Mo molar ratio of 0.2.

Hydrodenitrogenation of indole over Mo 2C catalyst: Insights into mechanistic events through DFT modeling

The molecular reaction mechanism of hydrodenitrogenation of indole was studied using density-functional theory calculations of the adsorbed o-ethylaniline surrounded by mobile hydrogen atoms. It was found that the hydrogenation of o-ethylaniline occurs through two steps: consisting in redistribution of the π electron density to form multiple partial Mo C(ring) bonds with the surface upon adsorption, and a subsequent hydrogen attack directed on the aromatic ring or the amine group. The direction of the hydrogen attack and the associated energy barriers determine the rate constants of the early (DDN, direct denitrogenation) and late (HYD, hydrogenation) nitrogen removal steps, and thus rules the selectivity of indole hydrodenitrogenation to ethylcyclohexane or ethylbenzene.

Synthesis of higher alcohols from syngas over K/Co/β-Mo2C catalysts

K/Co/β-Mo2C catalysts were prepared and tested for higher alcohols (C2+OH) synthesis (HAS). The catalysts exhibited high catalytic activity and selectivity to C2+OH. The effect of Co/Mo molar ratio in the catalysts upon the catalytic performance of HAS was investigated and the best one was at Co/Mo molar ratio of 1/8–1/6. It could be concluded that the Co promoter exerted strong promotion for carbon chain growth, especially for the stage of C1OH to C2OH. The XPS spectra revealed that there was a strong synergistic interaction between Co and Mo, and it depended strongly on the Co/Mo molar ratio. The new phases “Co3Mo3C” and “Co2C” formed over Co promoted K/β-Mo2C catalysts, which might be responsible for the high activity of higher alcohols and hydrocarbons synthesis, respectively.

Performances of mixed alcohols synthesis over potassium promoted molybdenum carbides

Molybdenum carbides, β-Mo 2C and α-MoC 1− X , promoted by K 2CO 3 were studied as catalysts for CO hydrogenation reactions at T = 573 K, P = 8.0 MPa, GHSV = 2000 h −1, H 2/CO = 1.0. Unpromoted molybdenum carbides produced mainly light hydrocarbons under these conditions. Addition of K 2CO 3 as a promoter, however, both β-Mo 2C and α-MoC 1− X resulted in remarkable selectivity shift from hydrocarbons to alcohols. Moreover, the promoter of potassium enhanced the ability of chain propagation of β-Mo 2C and α-MoC 1− X with higher selectivity to C 2+OH. Meanwhile, the investigation of the loadings of K 2CO 3 in β-Mo 2C and α-MoC 1− X revealed that the maximum of yield of alcohol was obtained at K/Mo (molar ratio) of 0.2 and 0.1, respectively. On K/β-Mo 2C catalysts, the distribution of hydrocarbons obeyed traditional linear A–S–F plot, while alcohols gave a unique linear A–S–F distribution with remarkable deviation of methanol compared with that of on β-Mo 2C catalyst. However, there was no significant difference between α-MoC 1− X and K/α-MoC 1− X catalysts about the distributions of alcohols and hydrocarbons; they both have similar linear A–S–F plots. Thus, the potassium promoter played a different role over β-Mo 2C and α-MoC 1− X catalysts upon the catalytic performance of mixed alcohols synthesis.

K/Fe/β-Mo 2C: A novel catalyst for mixed alcohols synthesis from carbon monoxide hydrogenation

A novel K/Fe/β-Mo 2C catalyst was prepared by a temperature programmed reaction method, it exhibited high yield of alcohol, especially high C 2+OH selectivity for mixed alcohol synthesis from CO hydrogenation.

Stability of supported and promoted-molybdenum carbide catalysts in dry-methane reforming

The stability of molybdenum carbide (Mo 2C) in dry-methane reforming (DMR) was studied over Al 2O 3, ZrO 2 and MgO supports and in the presence of Ce, K and Zr promoters at 900 °C, 1 bar, a GHSV of 3800 h −1 and a stoichiometric feed. All catalysts deactivated via moving front oxidation under these conditions. The γ-alumina appeared to be superior compared to other supports due to its high surface area and high thermal stability, and Mo 2C/γ-Al 2O 3 had a much higher activity than a bulk Mo 2C catalyst. Over this support, a high Mo loading was found to be crucial for maintaining the stability of the catalyst due to the higher concentration of more reducible Mo oxides and the higher CH 4 conversion over the carbide. The impregnation order was found to be an important factor for Ce, and the 3 wt.% Ce-Mo 2C/Al 2O 3 was found to be the most stable catalyst due to the redox properties of the Ce promoter. On the other hand, the addition of CO in the feed, while preventing oxidation of all catalysts, resulted in slow deactivation due to coking over the Ce-promoted catalyst.

Hydrogen generation from 2,2,4-trimethyl pentane reforming over molybdenum carbide at low steam-to-carbon ratios

Because of the need for an efficient and inexpensive reforming catalyst, the objective of this work is to determine the feasibility of employing Mo 2C catalyst for the steam reforming and oxy-steam reforming of the higher hydrocarbons typical of transportation fuels such as gasoline. It is shown that bulk Mo 2C catalysts can successfully reform 2,2,4-trimethyl pentane (isooctane) to generate H 2, CO and CO 2 at very low steam/carbon ratios, without coke formation, eliminating the need for pre-reforming. Maximum hydrogen generation was observed at a S/C ratio of 1.3 and 1000 °C during SR reactions and S/C of 0.71, O 2/C of 0.12 at 900 °C during oxidative steam reforming reactions.

Mechanistic study on the methane activation over various supported molybdenum carbide catalysts with isotopic tracer methods

The mechanism of methane activation in CH 4 CO 2 reaction over various unsupported and supported Mo 2C catalysts was studied by means of an isotopic tracer technique employing 13CH 4 and CD 4 as tracers. Incorporation of carbide carbons into CO took place in the case of CH 4 CO 2 reaction over unsupported Mo 2C, which was retarded significantly by dispersing Mo 2C onto ZrO 2 support. Methane dissociation step was also retarded significantly over supported Mo 2C catalysts, and the durability of the catalysts was improved by preventing inactive carbon accumulation. By dispersing Mo 2C on Al 2O 3, novel active acidic sites were produced, where CH 4 CD 4 exchange reaction took place even below room temperature through an associative mechanism.

Mo 2C and Mo 2C/Al 2O 3 catalysts for NO direct decomposition

A Mo 2C and an alumina-supported Mo 2C catalysts were prepared and their catalytic performance for the direct decomposition of NO to N 2 and O 2 was evaluated in a continuous-flow microreactor. The structural properties of the catalysts have been studied using X-ray diffraction as well as BET surface area measurement. The results show that the bulk molybdenum carbide and the Mo 2C/Al 2O 3 catalysts exhibit an excellent catalytic activity for NO direct decomposition. There is a stronger interaction between the β-Mo 2C phase and γ-Al 2O 3 support in the Mo 2C/Al 2O 3 catalysts when Mo content is higher. It is suggested that the base sites of Mo 2C and Mo 2C/Al 2O 3 catalysts may have played an important role in catalyzing the decomposition of NO.

The effects of sulfur and oxygen on the catalytic activity of molybdenum carbide during dry methane reforming

The sulfur tolerance of bulk Mo 2C catalyst has been evaluated for dry methane reforming (DMR) at 1050 °C, in the presence of dimethethylsulfide (DMS) at sulfur concentrations as high as 500 ppm. In addition, the effect of coupling the exothermic partial oxidation of methane with the highly endothermic reforming reaction was also investigated. It was determined that both DMS decomposition and partial oxidation take place in the gas phase, upstream of the catalyst bed, with CS 2 as the primary DMS decomposition product. The CS 2 was found to reversibly adsorbs on the catalyst surface, resulting in a stable but decreased activity. This conclusion was corroborated by examining the effects of DMS concentration, temperature, and in situ regeneration, and all are consistent with the occurrence of reversible sulfur chemisorption. Evidence is also presented to show that sulfur adsorption during DMR changes the redox chemistry of the catalyst, resulting in a slow oxidation of the surface. On the other hand, stable activity was maintained with oxygen in the feed and is attributed to lower CO 2 concentrations under these conditions. Comparison experiments with a supported Rh catalyst gave superior results with respect to sulfur tolerance during DMR, suggesting that sulfur poisoning of noble metal catalysts may not be a serious problem at the high temperatures used in methane reforming.

The influence of mass transfer conditions on the stability of molybdenum carbide for dry methane reforming

The effect of mass transfer conditions on the stability of a low surface area Mo 2C catalyst for dry (CO 2) methane reforming has been studied in a packed bed reactor at temperatures up to 1000 °C and pressures up to 8.3 bar using stoichiometric feeds (CO 2:CH 4 = 1). Because stoichiometric feeds can be net oxidizing at typical reforming temperatures, the carbide is oxidized to MoO 2, forming a moving oxidation front, which will eventually deactivate the entire bed. Based on experiments which isolated the effects of space velocity and molar feed velocity, it is concluded that these catalysts are stable under conditions where gas–solid mass transfer coefficients are low; that is, at high pressures and low mass flow rates. In these situations CO and H 2 product gases remain near the catalyst surface, preventing oxidation. Experimental results are also presented which show the temperature above which the carburizing rates are greater than oxidation rates, and this temperature is dependent on molar feed velocity and pressure. These results explain the observations made by previous researchers who observed that stability was greater at higher pressures and that the catalyst oxidized when space velocities were raised. Finally, it is concluded that Mo 2C catalysts can be stabilized with respect to oxidation by either operating at low mass velocities, higher temperatures, or by recycling product gases.

Acetone condensation over molybdenum nitride and carbide catalysts

Acetone condensation was carried out over high surface area molybdenum nitride and carbide catalysts. Rates and selectivities for these materials were compared to those for MgO (a solid base catalyst), USY (a solid acid catalyst) and 1% Pt/SiO 2 (a supported noble metal catalyst). The Mo 2N catalyst as well as MgO and USY catalyzed the condensation of acetone to products including mesityl oxide (MO), mesitylene and isophorone. Areal rates for the Mo 2C and Mo 2N catalysts were higher than those for the MgO and USY catalysts. The Mo 2N catalyst was most active for mesityl oxide production. The formation of mesitylene and isophorone over this catalyst indicated that Mo 2N possessed strong acid and base sites, respectively. Results from the thermal desorption of CO 2 and NH 3 confirmed the presence of these sites. The hydrogenation of acetone to isopropanol (IPA) and its subsequent dehydration to propylene were predominant reactions for the Mo 2C and 1% Pt/SiO 2 catalysts. Differences between the Mo 2N and Mo 2C catalysts are explained based on their base, acid and metallic properties.

Aromatization of n-butane and 1-butene over supported Mo 2C catalyst

The reaction pathways of n-butane were investigated on Mo 2C deposited on ZSM-5 and SiO 2. Particular attention was paid to the effects of the composition of ZSM-5, to the influence of the Mo 2C loading, and to the nature of supports. ZSM-5 itself catalyzed the reaction of n-butane well above 800 K. Its efficiency sensitively depended on the composition of zeolite. Whereas the conversion of butane was ∼90% on ZSM-5 with SiO 2/Al 2O 3=30 at 823 K, this value was only ∼24% on the sample with SiO 2/Al 2O 3=280. The dominant reaction was the cracking of butane yielding several C 1–C 3 compounds. Deposition of Mo 2C markedly changed the catalytic performance of ZSM-5, and the dehydrogenation and the aromatization processes came into prominence. This is particularly true in the case of less effective ZSM-5 (SiO 2/Al 2O 3=280). From the extrapolation of selectivities to zero conversion we obtained that methane, ethane, ethylene, propylene, butene, and hydrogen are the primary products on pure and Mo 2C-containing ZSM-5. Aromatics are formed in a secondary process, in the oligomerization and aromatization of butenes. The favorable effect of Mo 2C is well exhibited in the case of SiO 2, which was practically inactive. For 2% Mo 2C/SiO 2 at 823 K the selectivity of aromatics was 16–17% at a butane conversion of 26%. On this sample the main reaction was the dehydrogenation process. As the starting compound in the formation of aromatics is very likely butene, detailed measurements were performed on its reaction on the previously studied catalysts. 1-Butene exhibited a very high reactivity on pure ZSM-5 samples even at 723 K. The presence of 2% Mo 2C on the zeolites resulted only in a slight change in the conversion and product selectivities. The possible mechanism of the reactions and the role of Mo 2C are discussed, taking into account our surface science studies on the reaction of butyl species on a Mo 2C/Mo(100) surface.

Infrared study of the adsorption of CO and CH 3 on silica-supported MoO 3 and Mo 2C catalysts

It was demonstrated that polymeric carbon formed during the preparation of Mo 2C/SiO 2 from MoO 3/SiO 2, which is located on the support. The adsorptions of CO and CH 3 on silica-supported MoO 3 and Mo 2C catalysts was investigated by FTIR. While CO did not adsorb strongly on MoO 3/SiO 2, stable bands at 2089 and 2032 cm −1 on Mo 2C/SiO 2 were detected in CO adsorption, which can be regarded as an indication for noble metal-like character of supported Mo 2C. CH 3(a) on MoO 3/SiO 2 (2920 cm −1) proved to be more stable, than CH 3(a) on Mo 2C/SiO 2 (2924 cm −1). In the surface reaction of CH 3(a) on Mo 2C supported on silica ethylene formation was detected by mass spectrometric analysis. Ethylene did not form on SiO 2 and on MoO 3/SiO 2.

The effect of Mo 2C synthesis and pretreatment on catalytic stability in oxidative reforming environments

The role of catalyst pretreatment on the stability of Mo 2C catalysts in oxidative reforming environments has been studied. Catalysts were produced by both the temperature programmed reaction (TPR) and a solution-derived (SD) synthesis method, and compared to a low surface area commercial catalyst. Using a variety of techniques, including in situ dynamic X-ray diffraction (DXRD), the effects of various hydrogen pretreatment protocols were evaluated, including catalyst thermal stability, oxidation resistance and susceptibility to coking. The high surface areas produced by the SD synthesis is attributed to the presence of excess synthesis carbon and, whereas the presence of excess synthesis carbon enhances thermal stability, it also appears to accelerate coking. It is pointed out that the lowered oxidation resistance of the high surface area catalysts is due to a combination of smaller crystallite sizes and competitive oxidation of the excess synthesis carbon, which alters the oxidation mechanism. In addition, it was also found that incomplete carburization during TPR synthesis, forms an oxycarbide and its acidity also promotes coking. Hydrogen pretreatment at 700 °C not only removes all excess synthesis carbon, but it also reduces the oxycarbide to Mo, which is easily carburized under reforming conditions. Pretreatment at 600 °C, was largely ineffective and it is concluded that high temperature pretreatment is necessary to form the stoichiometric carbide, which is required for stability during reforming. Both the TPR and SD catalysts pretreated at 700 °C, were found to be stable over a 72 h period, whereas the commercial carbide had almost identical activity but slowly deactivated over the same period, probably because of its low surface area. Finally, labeled isotope experiments revealed that carbon exchange occurs readily with bulk Mo 2C at temperatures above 550 °C, lending credence to a reforming redox mechanism.

Oxidation stability of Mo 2C catalysts under fuel reforming conditions

The oxidation stability of a low surface area Mo 2C catalyst has been studied in the presence of gases associated with the steam and dry (CO 2) reforming of methane, at temperatures up to 850 °C and pressures to 8 bar. The oxidation onset temperatures were found to be about 600 °C when the carbide was exposed to either steam or CO 2. There appears to be two distinct mechanisms for Mo 2C oxidation: direct oxidation at temperatures below 750 °C and thermal decomposition followed by oxidation of the Mo metal at temperatures above 750 °C. Although onset temperatures were similar, CO 2 was a stronger oxidant than steam at the higher temperatures. Both H 2 and CO were found to inhibit oxidation and the effect can be explained by their influence on the reactions governing carburization and oxidation. The water gas shift reaction readily occurred over the catalyst and it was found that a carburizing ratio, defined as the ratio of carburizing gases to oxidizing gases, was able to predict stability, with oxidation occurring at ratios of 0.8 or lower. The effect of pressure on the onset temperature of CO 2 oxidation of the carbide was found to be negligible, even when inhibited by CO.

Acid and base characteristics of molybdenum carbide catalysts

The acid and base properties of a high surface area Mo 2C catalyst were characterized using the temperature programmed desorption of CO 2 and NH 3, the decomposition of isopropyl alcohol (IPA) as a test reaction and monitoring changes in the associated rates and product selectivities on the addition of acid and base site poisons. The Mo 2C catalyst was prepared using the temperature programmed reaction method and passivated prior to exposure to air. Prior to carrying out the temperature programmed desorption experiments and reaction rate measurements, the Mo 2C catalyst was reduced in H 2 at 400 °C. Results obtained for the reduced Mo 2C catalyst were compared with those for MgO, HZSM-5 and 1% Pt/SiO 2 catalysts. The study provided evidence for the presence of both acid and base sites on Mo 2C. The base and acid sites on the Mo 2C catalyst were weaker than those on the MgO and HZSM-5 catalysts, respectively. The base and acid sites were likely created as a consequence of charge transfer from molybdenum to carbon.

Simultaneous hydrodesulfurization, hydrodeoxygenation, and hydrogenation with molybdenum carbide

Simultaneous hydrodesulfurization (HDS), hydrodeoxygenation (HDO), and hydrogenation (HYD) of model compounds were studied under high pressure conditions with a β-Mo 2C catalyst. The β-Mo 2C (hexagonal close packed) was synthesized by the temperature programmed reaction and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO chemisorption, and BET surface area measurements. Simultaneous HDS, HDO, and HYD were observed with minimal deactivation of Mo 2C for up to 30 ppm of sulfur, 2000 ppm of oxygen, and 5 wt % cumene for more than 80 h. A noble metal catalyst (Pt/γ-Al 2O 3) deactivated immediately upon addition of sulfur. The reactivity results on β-Mo 2C indicate there are two types of sites on the surface when simultaneous HDS, HYD and HDO are occurring. The first type of site is a molybdenum carbide site which is active for HYD and HDO. The second site is a molybdenum carbosulfide site which is active for HYD, HDS, and HDO. The results of this study may be applied not only to the refining of petroleum feedstocks, but also to the upgrading of synthetic fuels such as coal-derived liquids.