Unimodal Catalysts Research Articles

Effect of unimodality and bimodality of Pd nanoparticles on the catalytic activity of Pd/SiO2 in the removal of diclofenac from water

In this work, we investigated the catalytic performance of uni- and bimodal Pd/SiO2 catalysts in the aqueous phase hydrodechlorination (HDC) of diclofenac – a very popular nonsteroidal anti-inflammatory drug. Unimodality and bimodality was confirmed by temperature-programmed hydride decomposition (TPHD) and TEM. The highest activity and 100% efficiency in batch and flow mode were observed for bimodal Pd NPs. This outstanding efficiency could be attributed to the close coexistence of small and large metal nanoparticles with different affinity to hydrogen and chloroorganic compound. The obtained results encourage discussion on the necessity to strive for perfect unimodal catalysts for the HDC processes.

Smart designing of metal-support interface for imperishable dry reforming catalyst

A strategy has been made to design a more active and stable metal-support interface by controlled synthesis of Ni nanoparticle on bimodal alumina support for CO2 reforming with methane. A bimodal porous alumina support was prepared via the evaporation-induced self-assembly (EISA) method, and the nickel nanoparticles (5 wt.%) on bimodal alumina was synthesized using four different synthesis methods such as freeze drying, wet impregnation, urea deposition-precipitation, and chemical vapor deposition method. The reactivity of nickel nanoparticles was evaluated for CO2 reforming with methane, concerning the CO2 and CH4 conversion as well as the H2 to CO ratio of the produced syngas. The catalysts were thoroughly characterized before and after the reaction using different techniques such as X-ray diffraction (XRD), N2 sorption analysis, H2-temperature programmed reduction (H2-TPR), H2-temperature programmed desorption (H2-TPD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), inductive coupled plasma-mass spectrometry (ICP-MS) and transmission electron microscopy (TEM). Characterization of various catalysts revealed that variation of synthesis procedure affects the metal-support interface and the type of nickel species present on the γ-alumina support, as well as the textural properties of the catalysts. The catalytic behavior was entirely different for each of the as-developed metal-support interface, derived in by the use of different synthesis procedures. The catalyst prepared by urea deposition-precipitation method was found to be most active and stable at 700 °C and 1atm for a period of a 100h run of time on stream with diluted gas feed and even without the dilution of feed gas for next 25h of the run. Comparing with bimodal catalyst, a unimodal catalyst with same metal loading exhibited the inferior catalytic activity and stability. The results of the comparative study showed that the stable catalytic performance of the bimodal structured catalyst is due to the combined effect of the type of metal-support interface of the catalyst, smaller size of nickel particle and bimodal structure of support material. The bimodal pore character of catalyst support has shown to prevent, Ni particles from sintering during the reaction and hence, better catalytic performance and resistance to coking.

Trimodal pore catalyst preparation method by growth of zeolite inside macroporous matrices of silica

In this paper, a novel catalyst support with trimodal pore structure was hydrothermally synthesized by building up nano-particles of titanium silicalite-1 inside the macroporous system of silica gel using the tetrapropylammonium hydroxide (TPAOH) as template, titanium (IV) tetrabutoxide monomer (TBOT) as titanium source and the silica gel with macropores as in situ silicon source as well as matrices. The prepared support exhibited intrinsic pore of TS-1, intercrystalline mesopores and residual macropores of silica gel. When loaded with Co, the trimodal catalyst was employed to catalyze the methanation of carbon monoxide, showing higher CO conversion and CH 4 selectivity than the bimodal and unimodal catalysts due to the favored metallic Co surface area and dispersion.

A new and direct preparation method of iron-based bimodal catalyst and its application in Fischer–Tropsch synthesis

Abstract A bimodal iron-based catalyst was prepared by a new one-step impregnation method. The active components were used as the “brick” to directly build the small pores inside the large pores of support, which was quite different from the previous bimodal catalysts that were prepared once more on a bimodal support. Comparing with the unimodal catalysts and conventional co-precipitated catalyst, the prepared bimodal catalyst exhibited excellent activity in Fischer–Tropsch synthesis due to the improved active metal dispersion and fastened diffusion efficiency. This preparation method is much simpler than the previous methods and can be extended to prepare various bimodal catalysts with different chemical compositions.

Pore diffusion simulation model of bimodal catalyst for Fischer‐Tropsch synthesis

AbstractThe Fischer‐Tropsch (FT) synthesis reaction was conducted using bimodal catalyst in a slurry phase reactor. The bimodal catalyst was prepared from a bimodal support which was tailor‐made by introducing SiO2 sol directly into the large pores of a SiO2 gel pellet The bimodal catalyst exhibited significantly higher activity in the slurry‐phase Fischer‐Tropsch synthesis than in other unimodal catalysts. The support with a large surface area allowed a highly dispersed cobalt particle, and its large pore size improved the diffusion of reactants and products. The diffusion of reactants inside the catalyst pores was simulated, and the effects of pore structure and pore size on catalytic performances were consistent with the simulation results. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Analyses of Illinois No. 6 Coal Liquefaction Results Generated in the Wilsonville, Alabama Unit

A database was set up to correlate the coal liquefaction results generated at the Department of Energy (DOE) Advanced Two-Stage Coal Liquefaction Facility in Wilsonville, AL. Published information available in the public domain was used, centering on runs made with Illinois No. 6 seam bituminous coal with two reactors in a close-coupled mode. A linear regression analysis was performed to determine the effects of process variables on conversions and product yields. Bimodal catalysts were more effective than a unimodal catalyst, as indicated by 10 wt % higher resid + unconverted coal conversion, 1 wt % greater hydrogen consumption, 19 wt % greater C4−1000 °F liquid production, and 14 wt % lower resid yield. Another significant result was a lower coal conversion, hydrogen consumption, C1−C3 yield, light (IBP−350 °F) distillate yield, and C1−C3 selectivity, when using half-volume reactors rather than full-volume reactors under similar conditions, including space velocities. This was apparently due to flatter ...

Hydrodenitrogenation of vacuum residue with a large-pore catalyst

The improved conversion (e.g. nitrogen conversion) achieved with laboratory-prepared macropore (MAP) catalysts could be attributed to either geometrical effects (diffusion or access to surface area) or chemical effects (surface acidity or more effective reaction sites). The MAP catalysts had much larger macropores than a commercial reference catalyst. Because these pores provided access to the catalysts' interior, the 15% MAP catalyst had a larger surface area than the commercial catalyst, when the catalysts were in their working conditions. Infrared spectroscopic measurements of ammonia and pyridine adsorption were made on several of the catalysts to determine whether or not the acidic properties of the MAP alumina were different from those of γ-alumina. A unimodal catalyst containing fluoride ions that enhanced its surface acidity and thereby enhanced conversions was compared with the MAP catalyst. Both the F-containing catalyst and the MAP catalyst produced greater nitrogen conversions than the unimodal catalyst that did not contain F ions. When the infrared spectra of the MAP catalyst were compared with those of the F catalyst, it was seen that at 400°C the F catalyst retained Brönsted acidity that was not observed in the MAP catalyst. Therefore, Brönsted acidity was not considered to be the cause of the conversion enhancement achieved with the MAP catalyst. On this basis, the enhanced conversion by the MAP catalysts was attributed to geometrical effects (pore diffusion).

Effect of catalyst pore structure on effective diffusivity

The effective diffusivity of the catalyst was experimentally measured in both octanedecane and polystyrene-chloroform systems for comparison with effects identified in the coal liquefaction model work. The diffusivity data were in good agreement with the theoretical solution and their reproducibility was satisfactory. In both experimental systems, the effective diffusivity was strongly dependent upon the ratio of diffusing species to catalyst pore size. When the steric exclusionhydrodynamic drag theory was applied, the tortuosity significantly varied with the ratio of diffusing molecule to catalyst pore size. Using the diffusivity data of unimodal catalysts, an empirical equation relating the size ratio to the restrictive factor was derived to evaluate the diffusivity data of bimodal catalysts.

Modeling of coal liquefaction with hindered diffusion

A simple model taking into account most important aspects of catalyst properties and reaction was developed to interpret a maximal point in catalyst activity observed using unimodal catalysts in coal-tetralin reactions. The model was found to be highly applicable to the prediction of catalyst activity and the maximal point in catalyst activity could be explained in terms of the trade-off between the hindered diffusion and surface area.

Large pore nickel/silica—alumina catalysts for hydrogenation of synthetic distillates. Effects of composition and structure: Effects of composition and structure

X-ray photoelectron spectroscopy measurements, scanning electron microscopy analysis, X-ray diffraction and porosimetry were used to investigate the influence of support composition and structure on the performance of large pore Ni/SiO 2-Al 2O 3 catalysts for the hydrogenation of aromatic compounds in synthetic distillates. The activity and deactivation by sulfur of a bimodal open-pore catalyst of high nickel loading were compared with those of a large-pore unimodal catalyst of similar metal content. Differences in the reducibility of the catalysts in 5% hydrogen-argon were related to differences in alumina content. Comparisons of catalytic activity were made with a conventional hydrotreating catalyst which was found to be significantly less active requiring a higher temperature range. Differences in operating pressures required to obtain corresponding conversions were linked to thermodynamic equilibrium effects.

A unified theory of coking deactivation in a catalyst pellet

The optimal performance of a reforming process is a direct function of the coking deactivation of its catalyst; hence the need to establish a fundamental relationship between the macroscopic (measurable) catalyst activity and the coke content. A general theoretical deactivation model of a unimodal pellet has been derived. This model incorporates the effects of coke deposition on decreased active surface area, diffusion resistance of reactants and pore blockage. Previous models are shown to be inadequate and design criteria are given for a minimum acceptable pore size to prevent pore plugging. It is clearly shown that high surface area catalysts (of small pore size) are not always efficient under deactivating conditions owing to pore plugging. Transient catalyst coke uptake curves are shown to have two stages: an initial kinetics-controlled period followed by an apparently diffusion-controlled period. These principles have been extended to a bidispersed pellet, showing that a single-pore effective diffusivity model for a reforming catalyst is inaccurate for deactivation modelling. It is also shown that a bimodal catalyst has much higher efficiencies than unimodal catalysts from a process viewpoint.

Effect of Pore-Size Distribution on the Catalytic Performance for Coal Liquefaction. II. Carbonaceous and Metallic Deposition on the Catalyst

Abstract The behavior of carbonaceous and metallic depositions on catalysts during hydroliquefaction of coal was investigated by using several kinds of catalyst supports with different pore-size distribution values in order to obtain the fundamental data for designing long-life catalysts. Higher concentration of carbonaceous deposition was observed on large-pore and bimodal catalysts than was observed on the comparative small pore and unimodal catalysts, respectively. However, the carbonaceous deposit was distributed more evenly inside large pores than it was inside small pores. The micropores of the bimodal catalysts were more effectively used than were those of the unimodal catalysts. Thus, the unimodal catalysts with small pores suffered more severe pore-mouth poisoning than the bimodal or the large-pore catalysts. The penetration behavior of the metallic components was found to be different for each metal. Discrete minerals such as clay could not penetrate into the catalyst pores and were deposited on the external surface of the catalyst particle. While, the metallic components coordinated with organic compounds such as phenolates or chelates deposited inside the macro- and mesopores. Easy access of the large asphaltenic molecules to the active sites resulted in high coal-liquefaction activity for the catalyst. This was accompanied by large amounts of both carbonaceous and metallic depositions. It is concluded that large-pore or bimodal pore structure is required for both high liquefaction activity and suppression of pore-mouth poisoning.

Hydroprocessing of nondeashed coal-derived residuum: Evaluation of catalyst performance

Four commercially available catalysts were evaluated for 100 hours in a continuous-flow unit to determine their effectiveness in the hydrotreating of nondeashed coal-derived residuum. These included Shell 324M and Shell 424 (both unimodal), and Shell 317 and Amocat 1C (both bimodal). All catalysts contained nickel and molybdenum supported on alumina; compositions were, however, different. The results showed that all catalysts performed equally well, except that the desulfurization activity of Shell 424 was substantially higher than the activities of the other catalysts. The bimodal catalysts accumulated more carbonaceous material and trace metals than the unimodal catalysts. Also, their catalytic activity was similar to the activities of the unimodal catalysts.

Large-pore [formula omitted] catalysts for coal-liquids upgrading

A novel method that can produce large-pore unimodal or bimodal alumina extradates was developed. Since the method used to produce the larger-diameter pores does not involve a steaming or sintering process, the resultant supports have high surface areas, ~350 m 2/g. Also, the technique allows catalysts with widely varying pore structures to be prepared while holding all preparation variables constant except for the extent of mixing used during preparation of the extrusion batch. These experimental supports were used to prepare NiMo catalysts for upgrading coal liquids. The results of an initial batch-screening test suggest that the resulting bimodal catalysts are more effective for upgrading coal liquids than the corresponding unimodal catalysts.

Intra-particle convection: Its measurement and effect on catalyst activity and selectivity

The influence of intraparticle forced convection on activity and selectivity to desired intermediate within a consecutive-parallel reaction pathway is analysed theoretically for individual catalyst pellets and for an isothermal packed bed. Conditions within which the effect of convection on activity and selectivity are maximised are identified. An experimental study of simultaneous intrapellet diffusion and convection for a range of bi-modal and uni-modal catalyst supports is presented. This study shows that for certain pore structures, convection may enhance the flux by a factor of 2–4 over that obtained for diffusion alone under quite modest total pressure gradients. The relation of convection to pore structure is examined in a rudimentary way.

Effects of Pore Size Distribution on the Catalytic Performance for Coal Liquefaction. I. The Activity and Selectivity of the Catalyst

Abstract Coal liquefaction experiments were carried out by using seven catalysts with different pore structures. As for the unimodal catalysts, the total conversion increased with the pore radius of the catalyst because of the lesser pore-diffusional limitation. However, the ratio of oil to oil-plus-asphaltene decreased with the pore radius. In the case of a large-pore catalyst, heavy components with a large molecular size and a low reactivity can approach the intra-particle active sites and occupy them to prevent the hydrocracking of asphaltene to oil. Bimodal catalysts with micropores (r<10 nm) and macropores (r>300 nm) gave higher values of both the coal liquefaction and the oil yields than did the unimodal catalysts. The presence of macropores seems to enhance the pore diffusion and increase the efficiency of the micropore use, while mesopores with radii between 10 nm and 300 nm did not make such a high contribution to the pore diffusion as was shown for the macropores. For the removal of heteroatoms, there was little difference in the performance among the catalysts with different pore structures. It is suggested that the active sites for the heteroatom removal are deactivated easily by the heavy components.