Hee Chul Woo

Mixed alcohol synthesis from carbon monoxide and dihydrogen over potassium-promoted molybdenum carbide catalysts

Abstract Unsupported molybdenum carbides, β-Mo 2 C and α-MoC 1-x , promoted by K 2 CO 3 were studied as catalysts for carbon monoxide-hydrogen reactions at 573 K and 8.0 MPa. Unpromoted molybdenum carbides produce mainly hydrocarbons of C 1 -C 12 under these conditions. Addition of K 2 CO 3 as a promoter, however, greatly enhances the selectivity to alcohols composed of linear C 1 -C 7 . Compared with better known alkali-promoted MoS 2 catalysts, carbide catalysts show higher selectivities of C 2 -C 7 alcohols. The carbon number distributions of both hydrocarbon and alcohol are consistent with the Schulz-Flory equation. Unlike MoS 2 catalysts, the addition of hydrogen sulfide in the feed over molybdenum carbide catalysts causes a gradual decrease in activity and selectivity for alcohols with reaction time.

Role of alkali promoters in K/MoS2 catalysts for CO-H2 reactions

Abstract The effect of alkali promoters on selectivity of CO-H 2 reactions was studied for potassium-promoted MoS 2 employing different potassium salts and pretreatment conditions (oxidized vs. fresh samples). Promoters assisted either chain growth of hydrocarbon products or alcohol formation. A good correlation was observed between p K a of the conjugate acid of each promoter and its spacetime yield of alcohol formation. Alcohol selective promoters such as K 2 CO 3 , KOH and K 2 S readily removed their counter anions under the reaction conditions to form a new potassium complex and spread themselves uniformly over MoS 2 . This complex appears to serve as an active site which adsorbs carbon monoxide molecularly and, at the same time, cover the majority of the MoS 2 surface which is responsible for dissociative carbon monoxide adsorption and hydrogenation. Promoters for chain growth such as K 2 SO 4 and KCl maintained their initial chemical states throughout the reactions and showed highly nonuniform lateral distributions. Thus, the promoters have a limited coverage over MoS 2 , yet modify the electronic state of MoS 2 which interacts directly with carbon monoxide. Exposure of K 2 CO 3 - or KOH-promoted MoS 2 to atmosphere for an extended period oxidized the catalyst and caused segregation of potassium into the bulk of MoS 2 Thus, the most of MoS 2 surface is now exposed, yet modified by potassium located in the subsurface region of MoS 2 . These modified catalysts promoted hydrocarbon chain growth without forming alcohols. The results demonstrate that the distribution of promoter is one of the primary factors determining its role in catalytic CO-H 2 reactions.

Catalytic skeletal isomerization of n-butenes to isobutene over natural clinoptilolite zeolite

Abstract The proton form of a natural zeolite, clinoptilolite, was found to be an efficient catalyst for the skeletal isomerization of n-butenes to isobutene. The acidity and characteristic pore structure of this zeolite appeared to be responsible for the good performance. The effects of process variables were studied. When reaction conditions were chosen such that the dimerization reaction, the major side reaction, was suppressed, high selectivities to isobutene were obtained.

ROOM-TEMPERATURE OXIDATION OF K2CO3/MOS2 CATALYSTS AND ITS EFFECTS ON ALCOHOL SYNTHESIS FROM CO AND H2

Potassium-promoted MoS2 is used as a catalyst for mixed alcohol synthesis from CO and H2. This study investigates the room-temperature oxidation of the catalyst and its effect on the surface structure and catalytic activity in alcohol synthesis at 573 K and 1.5 MPa. Catalysts were stored in the atmosphere or in a vacuum oven for several weeks. The characterization of catalyst was performed using XRD, XPS, FT-IR, and TGA/DTA methods. The XPS data of K2CO3/MoS2 stored in the atmosphere for extended periods indicated the oxidations of Mo(IV) (as sulfide) to Mo(VI) (as oxide) as well as S2− (as sulfide) to S6+ (as sulfate) on the MoS2 surface. The IR results showed that sulfate species first produced by oxidation had Td symmetry, which was further transformed into C2ν (bidentate) upon a prolonged storage. The sulfate species formed on the catalyst surface were stable until they were decomposed above 1000 K. The oxidized K2CO3/MoS2 catalyst showed enhanced catalytic activity and high selectivity to C2+ hydrocarbons, rather than forming alcohols as did fresh K2CO3/MoS2. These modified catalytic properties were similar to those of fresh K2SO4/MoS2.

Chemical equilibria and catalytic reaction of gas-phase methanol synthesis from methyl formate

Abstract Gas-phase methanol synthesis from methyl formate was studied by thermodynamic evaluations of the hydrogenolysis reaction (HCOOCH 3 +2H 2 = 2CH 3 OH) and the decarbonylation reaction (HCOOCH 3 = CH 3 OH + CO) concurrently. Based upon the chemical equilibria being calculated from these thermodynamic results, the gas-phase methanol synthesis reaction over copper chromites was carried out and the catalytic data were collected. Copper chromite catalysts prepared by the decomposition of an homogeneous citrate complex were characterized to investigate the active site for the gas-phase reaction. Copper metallic (Cu ° ) and cuprous chromite spinel (CuCrO 2 ) phases were proved to play important roles in methanol production. The catalyst prepared from the citrate complex, especially, showed higher activity and selectivity to methanol in the hydrogenolysis reaction than the commercial catalysts. The maximum activity and selectivity for methanol formation were obtained when the catalysts with 50–70% copper content were calcined at 623 K.

Oxidized K2CO3/MoS2 as a novel sulfur-resistant catalyst for Fischer-Tropsch reaction

Potassium-promoted MoS2 is an active catalyst for mixed alcohol synthesis from CO and H2. When K2CO3/MoS2 is oxidized, however, it produces mostly hydrocarbons, of which 40–50 wt% are C2–C4 alkanes. The reaction rates also substantially increase. Since oxidized K2CO3/MoS2 can tolerate a high level (up to 300 ppm) of H2S in the CO-H2 feed, it could serve as a novel sulfur-resistant Fischer-Tropsch catalyst.

Surface species on the oxidized K2CO3/MoS2 and their effects on catalytic carbon monoxide hydrogenation

Abstract The properties of K2CO3/MoS2 oxidized at room temperature and the effects of thermal treatments of the sample on its catalytic activity and physicochemical properties were investigated by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy, infrared. X-ray dif- fraction and electron spin resonance. The TPD exhibited four peaks of SO2 evolution. Among them, three peaks originated from the decomposition of surface SO2−4 species withC2v and Td symmetries. The decomposition of sulfate by heat treatment resulted in the production of SO2 and oxidized molybdenum species such as MoO3+ which were converted to MoO2 by further heat treatment. The thermal treatment of oxidized catalyst above 1013 K caused a solid-state reaction between potassium and molybdenum, producing K2MoO4. The treated catalyst on which the surface SO2−4 species had been removed, recovered their selectivity to alcohol formation in CO-H2 reactions which had been lost when the catalysts were oxidized.

Kinetic modeling and simulation on the synthesis of mixed alcohols over K/MoS 2 catalyst

A mechanistic kinetic model on mixed alcohol synthesis from CO and H 2 over 17 wt% K 2 CO 3 promoted MoS 2 catalyst has been developed by using LHHW formalism and steady state approximation for reaction intermediates. The model successfully predicted the formation and distribution of the products within the range of experimental condition. By the simulation of the model, the effects of reaction temperature, pressure, space time and H 2 /CO feed ratio on catalytic activity were also examined.