Adsorption and transport in hierarchical materials are investigated by means of adsorption and nuclear magnetic resonance experiments. Using micro/mesoporous zeolites with well-defined mesoporosity, ...we show that adsorption at a given pressure can be described as a simple linear combination of the adsorbed amounts taken at the same pressure for the pure microporous (zeolite FAU-Y) and mesoporous (Al-MCM-41) solids. Such a quantitative decomposition allows us to demonstrate the ability of diffusion measurements by Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) to probe interconnectivity in hierarchical solids. On the one hand, transport in the mechanical mixtures can be described as the superimposition of diffusion in pure microporous and mesoporous solids. On the other hand, PFG NMR for the hierarchical sample provides an effective diffusivity that is intermediate between those for the pure zeolite and mesoporous silica. Furthermore, this effective diffusivity is slower than the linear combination of the two diffusivities weighted by the number of molecules present in each phase (used in the independent domain and fast-exchange theories) clearly showing interconnectivities and transfer limitations between the microporous and mesoporous domains. We also discuss the ability of combining theories such as the fast exchange model and the effective medium theory to quantitatively predict diffusion in such microporous/mesoporous materials.
Adsorption and transport in hierarchical porous solids with micro- (∼1 nm) and mesoporosities (>2 nm) are investigated by molecular simulation. Two models of hierarchical solids are considered: ...microporous materials in which mesopores are carved out (model A) and mesoporous materials in which microporous nanoparticles are inserted (model B). Adsorption isotherms for model A can be described as a linear combination of the adsorption isotherms for pure mesoporous and microporous solids. In contrast, adsorption in model B departs from adsorption in pure microporous and mesoporous solids; the inserted microporous particles act as defects, which help nucleate the liquid phase within the mesopore and shift capillary condensation toward lower pressures. As far as transport under a pressure gradient is concerned, the flux in hierarchical materials consisting of microporous solids in which mesopores are carved out obeys the Navier–Stokes equation so that Darcy’s law is verified within the mesopore. Moreover, the flow in such materials is larger than in a single mesopore, due to the transfer between micropores and mesopores. This nonzero velocity at the mesopore surface implies that transport in such hierarchical materials involves slippage at the mesopore surface, although the adsorbate has a strong affinity for the surface. In contrast to model A, flux in model B is smaller than in a single mesopore, as the nanoparticles act as constrictions that hinder transport. By a subtle effect arising from fast transport in the mesopores, the presence of mesopores increases the number of molecules in the microporosity in hierarchical materials and, hence, decreases the flow in the micropores (due to mass conservation). As a result, we do not observe faster diffusion in the micropores of hierarchical materials upon flow but slower diffusion, which increases the contact time between the adsorbate and the surface of the microporosity.
In this study, we show that the Friedel–Crafts (FC) alkylation of aromatic hydrocarbons by thiols to form C–C bond is feasible. The gas-phase reaction between toluene and CH3SH catalyzed by HZSM-5 ...zeolite was chosen as model reaction. In the temperature range of 350–550 °C, the alkylation of toluene to produce xylenes was the main reaction involved in the process. The reaction between toluene and CH3SH was compared with the well-known reaction between toluene and CH3OH. Significant similarities exist, notably the ability of both CH3SH and CH3OH to generate methoxonium species on the zeolite surface and to methylate the aromatic ring in a typical FC process. The maximum alkylation yield of 41% for CH3OH was reached at 350 °C, while that of 67.3% for CH3SH was reached at 450 °C. This difference in temperature can be correlated with the energy barriers required for the formation of methoxonium species (i.e., 24.6 kcal/mol (CH3OH) and 26.4 kcal/mol (CH3SH)). The high performance in alkylation proved by CH3SH was attributed to its lower consumption in the side reactions (i.e., the formation of light hydrocarbons).
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•An hydrocracking catalyst has been prepared by alumination of a MCM-48 silica.•It shows a higher selectivity toward isomerization than Y-type catalysts.•Primary hydrocracking ...products result preferentially from central cracking.
A model hydrocracking catalyst with a narrow and homogeneous distribution of mesopores of 3.2nm of diameter has been prepared by alumination of the surface of a siliceous MCM-48 material, loaded with platinum and evaluated in the hydroconversion of n-hexadecane. Compared to zeolite catalysts featuring a similar acidity but containing both micropores and mesopores, the Pt/Al-MCM-48 catalyst leads to the same overall activity but with higher hexadecane isomer yields and a symmetrical repartition of the cracked products, typical for primary central cracking, even at very high conversion levels. It is suggested that the unique behavior of the model Pt/Al-MCM-48 mesoporous hydroconversion catalyst is due to optimal sorption energetics of the primary reaction products in the network of uniformly distributed mesopores.
Chemical phase separation and pseudomorphic transformation are very powerful tools for fine tuning the porous architecture of inorganic oxides. In this short review the basic principles of the ...preparation of centimetric bodies made of zeolites is introduced. The synthesis of monoliths with homogeneous distributions of interconnected macropores with skeleton made of SOD, LTA and FAU crystals is described. Their unique hierarchical porous texture featuring the structural micropores, intercrystalline mesopores and flow-through macropores leads to remarkable hydrodynamic behavior in separation, catalysis and ion exchange processes operated in continuous-flow mode. In the base-catalyzed carbon-carbon bond formation, productivities twice those achievable with fixed-beds of packed particles are observed. In the capture of strontium present in radioactive effluents their efficiency is three orders of magnitude that of traditional powder in batch.
C,NTiO2 monoliths with homogeneous interconnected macro-/mesoporous hierarchical porosity, consisting in 83% anatase phase, exhibiting high visible light absorption were prepared in one pot ...synthesis. The hierarchical porosity was controlled by coupling a sol-gel method with a spinodal decomposition and the improved visible light absorption was obtained by self C,N-grafting during thermal treatment. Titanium isopropoxide, N-methylformamide, poly(ethylene oxide), and hydrochloric acid were used as reagents to form a sol, which was then treated at 40 and 60 °C, followed by a solvothermal treatment in autoclave at 200 °C in isopropanol. The monoliths were further heated at different temperatures from 250 to 500 °C under air. The best compromise between the structural and textural properties (TiO2 phase, surface, volume, pore diameter), the visible light absorbance and the mechanical properties was obtained for a calcination at 350 °C for 5 h. In batch mode, in glass containers, the monoliths demonstrated remarkable efficiency as photocatalysts under natural sunlight and artificial visible light with the total discoloration of the azo dye Orange G aqueous solution in 1 h compared to benchmark TiO2 P25 nanoparticles, which proved inefficient under these conditions. More interestingly, the monoliths used as reactors in flow mode in a recirculating system proved very efficient for the total discoloration of Orange G dye solution revealing the high potential of these TiO2 monoliths for continuous flow wastewater treatment under visible light.
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•Easy C,N-codoping of TiO2 monoliths with hierarchical macro-/mesoporosity.•High decrease of band gap of TiO2 photocatalyst from 3.2 to 1.5 eV.•Higher activity under sunlight than TiO2 P25 benchmark photocatalyst and undoped TiO2 monoliths.•Fast elimination of dyes pollutants contained in water in batch and in flow modes.•A new family of photocatalysts highly efficient under visible light and easy to handle.
LTA zeolite monoliths (6 mm diameter, 3 cm length) featuring a hierarchical trimodal network of micro-/meso- and macropores (obtained by either controlling the nucleation step of LTA crystallization ...into nanocrystals or by creating mesopores into micronic crystals by using organosilane surfactant) were used for strontium capture in aqueous medium. LTA monoliths were compared to other LTA zeolite architectures: LTA microcrystals, commercial LTA beads and bimodal micro-/macroporous LTA zeolite monoliths. In batch mode, the presence of mesopores allowed to increase remarkably by a factor 15 the diffusion of ions, whereas macropores had no influence on ions transport. In flow mode, only LTA monoliths featuring flow-through macropores proved suitable as microreactors. The trimodal LTA monoliths were 1000 times more efficient than packed-beds of LTA beads, and 4 times than bimodal (micro-/macroporous) LTA monoliths due to higher rates of diffusion. Trimodal LTA monoliths were able to treat efficiently 4 L of Sr2+ solution (10 mg L−1) with 1 mL of material at a flow rate of 0.5 mL min−1 (or 1 m h−1); ie. 4200 bed volumes (BV) were efficiently treated at a flow rate of 34 BV h−1, with no Sr2+ detectable by ionic-chromatography in the effluent. This result highlights the fact that the multiscale pore architecture engineering of an adsorbent is crucial for process intensification: macropores allow uniform mass transport of solutions with low pressure drop while the generation of mesopores in zeolites leads to faster ionic transport and more efficient crystal use in cation-exchange processes, both in batch and flow modes.
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•Zeolite A monoliths micro-/meso-/macroporous are highly efficient for Sr2+ capture.•Mesopores increases remarkably by a factor 15 the rate of diffusion of ions.•Macropores allows uniform mass transport of solutions with low pressure drop.•LTA monoliths treat efficiently 4 L of Sr2+ solutions with 1 mL of material.•Multiscale pore architecture of an adsorbent is crucial for process intensification.
The performances of binderless FAU-X monoliths (0.5 × 3 cm) build of nanocrystals featuring hierarchical trimodal porosity (micro-/meso-/macroporosity) in decontamination Cs-containing effluents have ...been evaluated. They have been compared to those obtained with newly synthesized FAU-X particles (1 mm) build with nanocrystals. Data have been confronted to those recently reported for a benchmark reference Cs+ adsorbent Sorbmatech®, based on copper hexacyanoferrate nanoparticles (15 nm) immobilized into mesoporous silica particles (250–500 μm) (Cu-HCF@SiO2). FAU-X monoliths show high rate of adsorption in batch in less than 2 min, much faster than Cs+ adsorption in FAU-X particles (60 min). This result highlights the importance of a homogeneous macropore network in adsorbents to enhance mass transport and access to the zeolite active sites. FAU-X monoliths with 20 μm macropore diameter have been used in continuous flow experiments for sequestering Cs+ (0.5 mmol/L) in mineral drinking water containing competing cations (Ca2+, Mg2+, Na+, K+) with flow rates of 0.5–1 mL/min, corresponding to Darcy rates of 1.5–3 m/h. FAU-X monoliths are very efficient for Cs+ removal and show ideal steep breakthrough curves characteristic of fast diffusion. FAU-X monoliths are as excellent as Cu-HCF@SiO2 and could represent an alternative adsorbent for safer processes, avoiding the handling of powders or particles. Above all, this study reveals the unique hydrodynamic behavior of FAU-X monoliths and opens the route for process intensification using FAU-X in continuous flow.
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•FAU-X monoliths built of nanocrystals featuring hierarchical trimodal porosity.•FAU-X particles (1 mm) synthesis without binders.•FAU-X monoliths for efficient water decontamination under continuous flow.•Perfect steep breakthrough curves for Cs+ sequestration from mineral water.•FAU-X monoliths with fast transport properties for process intensification.
The texture of mesoporous FAU-Y (FAUmes) prepared by surfactant-templating in basic media is a subject of debate. It is proposed that mesoporous FAU-Y consists of: (1) ordered mesoporous zeolite ...networks formed by a surfactant-assisted zeolite rearrangement process involving local dissolution and reconstruction of the crystalline framework, and (2) ordered mesoporous amorphous phases as Al-MCM-41, which coexist with zeolite nanodomains obtained by a dissolution-reassembly process. By the present systematic study, performed with FAU-Y (Si/Al = 15) in the presence of octadecyltrimethylammonium bromide and 0 < NaOH/Si ratio < 0.25 at 115 °C for 20 h, we demonstrate that mesoporous FAU zeolites consist, in fact, of a complex family of materials with textural features strongly impacted by the experimental conditions. Two main families have been disclosed: (1) for 0.0625 < NaOH/Si < 0.10, FAUmes are ordered mesoporous materials with zeolite walls, which coexist with zeolite nanodomains (100–200 nm) and (2) for 0.125 < NaOH/Si < 0.25, FAUmes are ordered mesoporous materials with amorphous walls as Al-MCM-41, which coexist with zeolite nanodomains (5–100 nm). The zeolite nanodomains decrease in size with the increase of NaOH/Si ratio. Increasing NaOH/Si ratio leads to an increase of mesopore volume, while the total surface area remains constant, and to a decrease of strong acidity in line with the decrease of micropore volume. The ordered mesoporous materials with zeolite walls feature the highest acidity strength. The ordered mesoporous materials with amorphous walls present additional large pores (50–200 nm), which increase in size and amount with the increase of NaOH/Si ratio. This alkaline treatment of FAU-Y represents a way to obtain ordered mesoporous materials with zeolite walls with high mesopore volume for NaOH/Si = 0.10 and a new way to synthesize mesoporous Al-MCM-41 materials containing extralarge pores (50–200 nm) ideal for optimal diffusion (NaOH/Si = 0.25).
The t-plot method is a well-known technique which allows determining the micro- and/or mesoporous volumes and the specific surface area of a sample by comparison with a reference adsorption isotherm ...of a nonporous material having the same surface chemistry. In this paper, the validity of the t-plot method is discussed in the case of hierarchical porous materials exhibiting both micro- and mesoporosities. Different hierarchical zeolites with MCM-41 type ordered mesoporosity are prepared using pseudomorphic transformation. For comparison, we also consider simple mechanical mixtures of microporous and mesoporous materials. We first show an intrinsic failure of the t-plot method; this method does not describe the fact that, for a given surface chemistry and pressure, the thickness of the film adsorbed in micropores or small mesopores (< 10σ, σ being the diameter of the adsorbate) increases with decreasing the pore size (curvature effect). We further show that such an effect, which arises from the fact that the surface area and, hence, the free energy of the curved gas/liquid interface decreases with increasing the film thickness, is captured using the simple thermodynamical model by Derjaguin. The effect of such a drawback on the ability of the t-plot method to estimate the micro- and mesoporous volumes of hierarchical samples is then discussed, and an abacus is given to correct the underestimated microporous volume by the t-plot method.
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