hydrocarbon steam reforming on highly stable nickel nanoparticules encapsulated inside hollow silicalite-1 crystal

SSCI-VIDE+ING+FRM:ATU:DFA:DLPNational audienceSteam reforming of hydrocarbons is a cornerstone process of the petrochemical industries producing syngas that is used as base feedstock for the manufacturing of a wide range of chemicals. Steam reforming catalysts operate under severe conditions that raise stability issues. The high temperatures used lead to an irreversible sintering and the presence of tars and sulfur-containing compounds leads to carbon laydown and poisoning. Nickel-based catalysts are typically used for cost reasons, and thus focused a lot of interests. However, relationships between catalyst structural features and catalytic performances and durability during steam reforming are still unclear. We report in here a method1,2 for synthesizing nickel nanoparticles of controlled size distri-bution, which are encapsulated in microporous zeolite membrane that can be used as well-defined model catalysts to unravel steam reforming structure-activity-stability relationships. 5%Ni@silicalite-1 catalyst was obtained by the dissolution/ recrystallization synthesis method. The encapsulation prevents the particles to sinter under harsh steam reforming conditions leading to a high and stable nickel dispersion. Ni@silicalite-1 catalyst was evaluated in the catalytic hydrogenation of substituted aromatics, used as model reactions to assess the sieving properties of the zeolite shell. Mesitylene was essentially not converted, while toluene was efficiently hydrogenated, indicating a sharp diffusional cut-off limit and a promising potential against coking by tars2.Traditional Ni-based catalysts are known to be prone to deactivation during the steam reforming of methane at 700°C. Yet, our 5%Ni@silicalite-1 catalyst that was exposed to a simulated biomass-derived reformate stream demonstrated an excellent activity/stability. A commercial Ni-based catalyst (10% Ni/CaAl2O3 used in steam reforming processes) was also tested under similar reforming conditions. The 5%Ni@silicalite-1 catalyst exhibited a stability that was similar to that of the commercial steam reforming catalysts, while exhibiting a two-fold higher reaction rate and a higher hydrogen production. Furthermore, after 36 hours, the obtained H2/CO ratio was much more appropriate in case of 5%Ni@silicalite-1 for an optimal operation of the Fischer-Tropsch unit located downstream of the steam reforming process (Table 1).Further analysis of this catalyst shall provide insights of the deactivation mechanisms, paving the way to a rational design of highly stable catalysts for the steam reforming of biomass by-products. This work is carried out in the frame of FASTCARD and supported by the EC under the Grant Agreement 60427

Selective removal of external Ni nanoparticles on Ni@silicalite-1 single crystal nanoboxes: Application to size-selective arene hydrogenation

SSCI-VIDE+ING+DLP:ATU:DFA:FRMInternational audienceUndesired metal nanoparticles located outside zeolite nanoboxes (hollow zeolites) can be formed during the preparation of zeolite-embedded metal nanoparticles. The present work demonstrates that it is possible to use citric acid to selectively leach outmost of the external Ni nanoparticles from a Ni@silicalite-1 material. The leached sample exhibited an improved selectivity in the hydrogenation of toluene as compared to that of the bulkier mesitylene. (C) 2017 Elsevier B.V. All rights reserved

hydrocarbon steam reforming on highly stable nickel nanoparticules encapsulated inside hollow silicalite-1 crystal

SSCI-VIDE+ING+FRM:ATU:DFA:DLPNational audienceSteam reforming of hydrocarbons is a cornerstone process of the petrochemical industries producing syngas that is used as base feedstock for the manufacturing of a wide range of chemicals. Steam reforming catalysts operate under severe conditions that raise stability issues. The high temperatures used lead to an irreversible sintering and the presence of tars and sulfur-containing compounds leads to carbon laydown and poisoning. Nickel-based catalysts are typically used for cost reasons, and thus focused a lot of interests. However, relationships between catalyst structural features and catalytic performances and durability during steam reforming are still unclear. We report in here a method1,2 for synthesizing nickel nanoparticles of controlled size distri-bution, which are encapsulated in microporous zeolite membrane that can be used as well-defined model catalysts to unravel steam reforming structure-activity-stability relationships. 5%Ni@silicalite-1 catalyst was obtained by the dissolution/ recrystallization synthesis method. The encapsulation prevents the particles to sinter under harsh steam reforming conditions leading to a high and stable nickel dispersion. Ni@silicalite-1 catalyst was evaluated in the catalytic hydrogenation of substituted aromatics, used as model reactions to assess the sieving properties of the zeolite shell. Mesitylene was essentially not converted, while toluene was efficiently hydrogenated, indicating a sharp diffusional cut-off limit and a promising potential against coking by tars2.Traditional Ni-based catalysts are known to be prone to deactivation during the steam reforming of methane at 700°C. Yet, our 5%Ni@silicalite-1 catalyst that was exposed to a simulated biomass-derived reformate stream demonstrated an excellent activity/stability. A commercial Ni-based catalyst (10% Ni/CaAl2O3 used in steam reforming processes) was also tested under similar reforming conditions. The 5%Ni@silicalite-1 catalyst exhibited a stability that was similar to that of the commercial steam reforming catalysts, while exhibiting a two-fold higher reaction rate and a higher hydrogen production. Furthermore, after 36 hours, the obtained H2/CO ratio was much more appropriate in case of 5%Ni@silicalite-1 for an optimal operation of the Fischer-Tropsch unit located downstream of the steam reforming process (Table 1).Further analysis of this catalyst shall provide insights of the deactivation mechanisms, paving the way to a rational design of highly stable catalysts for the steam reforming of biomass by-products. This work is carried out in the frame of FASTCARD and supported by the EC under the Grant Agreement 60427

Highly Dispersed Nickel Particles Encapsulated in Multi-hollow Silicalite-1 Single Crystal Nanoboxes: Effects of Siliceous Deposits and Phosphorous Species on the Catalytic Performances

SSCI-VIDE+ING+DLP:ATU:DFA:FRMInternational audienceMulti-hollow silicalite-1 single crystals (MH) were prepared for the first time by an original synthesis pathway by using tetrabutylphosphonium hydroxide (TBPOH) as a mild desilicating agent. This new generation of hierarchical zeolite allowed the encapsulation of nanoparticles (NPs) featuring an enhanced confinement of the metallic guest and a thin wall thickness. The MH catalyst exhibited a better stability for methane steam reforming at 700 degrees C than a single-hollow counterpart (SH). Ni average particle size could be kept lower than 4 nm after 20 h on stream for the MH sample. However, a detailed analysis of kinetic data of the structure-insensitive CO methanation used as a model reaction revealed that the sample activity was adversely affected by two main factors deriving from the preparation steps. First, a siliceous over-layer derived from the decomposition of intermediate Ni phyllosilicates, which partly covered the resulting Ni nanoparticles. Second, phosphorus from the templates remained in the samples, probably forming a Ni-P compound upon reduction. The overall catalytic activities observed here were therefore a complex interplay of improved dispersion and poisonous effects

Influence of crystal size and mesopores connectivity on adsorption/diffusion properties of silicalite-1 crystals

SSCI-VIDE+ING+CPG:DLP:ATU:FRM:DFANational audienceIt is well know that the auxiliary network in hierarchical zeolites alleviates diffusional transport limitation. However, the study of connectivity between pores in the secondary network, is not straight forward. Here, we report the influence of the crystal size and the type of additional porosity on the adsorption and diffusion properties of silicalite-1 crystals. We have prepared a series of five model porous silicalite-1 with well controlled mesopore connectivity.The scanning isotherm method by using N2 adsorption/desorption isotherms has been carried out for the determination of possible connections between mesoporous cavities. In addition, the effect of mesopores connections on transport has been measured by DRIFTS and ZLC methods. For instance, we have shown that the transport was mostly limited by surface effects in the case of small crystals, while it was not the case for much larger crystals. The presentation will show correlations between transport and the pore network / size of the crystals

Transition-Metal Nanoparticles in Hollow Zeolite Single Crystals as Bifunctional and Size-Selective Hydrogenation Catalysts

SSCI-VIDE+ING:+SLI:ATU:DLP:FRM:DFAInternational audienceTransition-metal nanoparticles (Co, Ni, and Cu) encapsulated in hollow zeolite single crystals were prepared by recrystallization of impregnated bulk MFI crystals in the presence of tetrapropylammonium (TPAOH) solutions. The size and number of particles in hollow MFI depended mainly on the aluminum content. The encapsulation of the nanoparticles prevented them from growing, thus enabling the control of particle size even after high temperature treatments. For low metal loadings (<3 wt %), the mean particle sizes for Co, Ni, and Cu in hollow silicalite-1 were 3.5 +/- 0.3, 3.1 +/- 0.5, and 1.5 +/- 0.2 nm, respectively. In the case of hollow ZSM-5, higher loadings (similar to 8 wt %) could be obtained with mean particle sizes of 17 +/- 2 nm, 13 +/- 2 nm, and 15 +/- 2 nm for Co, Ni, and Cu systems. The mechanism of transition metal nanoparticle formation was markedly different from that of noble metals. At high pH values, transition-metal cations first reacted with dissolved silica species yielding fibrous metal phyllosilicates that were located inside the crystal cavities. The metal phyllosilicates were then converted into nanoparticles upon reduction under H-2 at high temperature (500-750 degrees C). Silicalite-1 encapsulated Ni particles were used in the catalytic hydrogenation of substituted benzenes and showed an outstanding size-selectivity effect. Ni particles were accessible to toluene but not to mesitylene, confirming that the activity is directly related to the diffusion properties of molecules through the zeolite membrane

Effect of polyaromatic tars on the activity for methane steam reforming of nickel particles embedded in silicalite-1

SSCI-VIDE+ING+DLP:CTH:ATU:DFA:FRMInternational audienceThe steam reforming of methane was studied over Rh and Ni-based catalysts exposed to naphthalene, which was used as a representative of polyaromatic tars found in biomass-derived biogas. In particular, two Ni-based samples in which part of the metal was encapsulated within silicalite-1 nanoboxes were tested. The reforming reaction was carried at 700, 800 and 900 degrees C using a model feed and high space velocities to limit methane full conversion and better evidence any deactivation. A strong decrease of methane conversion was observed in the presence of 1400 ppm of naphthalene, stressing the marked deleterious effect of this molecule in the present conditions. The effect of naphthalene was partly reversible, especially at higher temperatures. The silicalite-1 membrane could not prevent the deactivation of embedded nickel particles, probably because naphthalene (kinetic diameter = 0.62 nm) could diffuse throughout the MFI-type (pore diameter ca. 0.56 nm) porous layer at the high reaction temperatures used. The effect of 5 ppm of the bulkier pyrene (kinetic diameter = 0.74 nm) was investigated at 700 degrees C and also led to a rapid deactivation of the Ni@silicalite-1, likely because pyrene was cracked into naphthalene, which could then enter the silicalite-1 nanoboxes. The poisoning effect of toluene on the Ni-based catalysts was minor in comparison to that induced by the polyaromatics. A marked sintering of the embedded Ni was also observed. (C) 2016 Elsevier B.V. All rights reserved

Influence of crystal size and mesopores connectivity on adsorption/diffusion properties of silicalite-1 crystals

SSCI-VIDE+ING+CPG:DLP:ATU:FRM:DFANational audienceIt is well know that the auxiliary network in hierarchical zeolites alleviates diffusional transport limitation. However, the study of connectivity between pores in the secondary network, is not straight forward. Here, we report the influence of the crystal size and the type of additional porosity on the adsorption and diffusion properties of silicalite-1 crystals. We have prepared a series of five model porous silicalite-1 with well controlled mesopore connectivity.The scanning isotherm method by using N2 adsorption/desorption isotherms has been carried out for the determination of possible connections between mesoporous cavities. In addition, the effect of mesopores connections on transport has been measured by DRIFTS and ZLC methods. For instance, we have shown that the transport was mostly limited by surface effects in the case of small crystals, while it was not the case for much larger crystals. The presentation will show correlations between transport and the pore network / size of the crystals

Effects of H2S and phenanthrene on the activity of Ni and Rh-based catalysts for the reforming of a simulated biomass-derived producer gas

SSCI-VIDE+ING+DFA:YSC:FRMInternational audienceMethane steam reformin