Molecular Sieves Of MCM-41 Type Research Articles

Effect of nickel loading on hydrogen adsorption capacity of different mesoporous supports

Pure siliceous and aluminosilicate mesoporous molecular sieves of MCM-41 type have been used as support for nickel incorporation (2.5 wt%) by wet impregnation method. The hydrogen adsorption capacities at 77 K of these materials have been studied. Various techniques such as X-ray diffraction, N2 adsorption–desorption, X-ray photoelectron spectroscopy, Temperature-Programmed Reduction, UV–Vis diffuse reflectance spectroscopy and adsorption of pyridine coupled to infrared spectroscopy were employed to characterize the materials. In addition, Density Functional Theory calculations were used in order to interpret the results of hydrogen adsorption. The results obtained show that isolated metallic species are capable to promote hydrogen favorable sites. Isolated mononuclear Ni2+ species on the surface strengthen the interaction with the H2, enhancing the hydrogen adsorption capacity.

The role of niobium in MCM-41 supported with Pt and Au—A comparative study of physicochemical and catalytic properties

This paper reviews a series of results, partly published recently, on niobosilicate mesoporous molecular sieves of MCM-41 type modified with noble metals (Pt, Au). Platinum is introduced to MCM-41 by impregnation technique, whereas gold is incorporated by two methods: during the synthesis by co-precipitation (COP) and by impregnation (IMP). Three fundamental features of the prepared materials have been considered: (i) texture/structure, (ii) noble metal state and dispersion, and (iii) catalytic activity. The latter has been tested in selective reduction of NO by propene, methanol oxidation and WGS process. The role of niobium, as well as the effect of the nature of noble metals and the preparation method on the physicochemical and catalytic properties of the final materials based on NbMCM-41 (in comparison with siliceous MCM-41) is demonstrated.

Adsorption of polycyclic aromatic hydrocarbons (PAHs) from isooctane solutions by mesoporous molecular sieves: Influence of the surface acidity

Three different mesoporous molecular sieves of MCM-41 type (a pure silica and two aluminum doped silica with Si/Al molar ratios of 10 and 30) were evaluated for adsorption of polycyclic aromatic hydrocarbons (PAHs) from an organic solution. The aluminum containing samples were synthesized at room temperature using a sol–gel method with two reaction steps: acid hydrolysis of the metallic alkoxides (pH 2) and alkaline condensation of the hydrolysis products (pH 10). The removal of the organic template (dodecylamine hydrochloride) was performed by solvent extraction (ethanol/HNO 3 0.1 M) followed by calcination at 550 °C for 3 h. Surface chemistry of synthesized samples was analysed by X-ray photoelectronic spectroscopy (XPS). Acidity values (proton concentration) were obtained using thermogravimetric analysis of the adsorbed cyclohexylamine (CHA) at 250 °C on the calcined materials. Equilibrium adsorption properties of naphthalene, anthracene and pyrene (PAHs) in isooctane were measured at 25, 40 and 60 °C by the contact method (finite bath). The Langmuir model correlates well the equilibrium data, indicating decreasing adsorption maximum capacity ( q m) in the following order: Al-MCM-41(10) > Al-MCM-41(30) > MCM-41. For all samples, PHA adsorption capacity increased with the number of aromatic rings. The estimated heats of adsorption (−Δ H) show a regular increase with increasing number of π-electrons in the aromatic structure. These results indicate that electrostatic interactions between the π-electrons of aromatic rings and the acidic centers of the MCM-41 materials, caused by both isomorphic and extra-framework Al, may play a significant role in adsorption of PAHs in addition to van der Waals forces.

Immobilization of Chiral Ruthenium(II) Complex and Its Catalytic Application in Enantioselective Transfer Hydrogenation

Chiral Ru-BsDPEN, (1R,2R)-N,p-benzenesulfonyl-1,2-diphenylethylenediamine, catalyst has been immobilized on a mesoporous molecular sieve of MCM-41 type successfully. A hybrid mesoporous molecular sieve was synthesized using a precursor bearing benzene group, which in organosilica were sulfonylated and reacted with (1R,2R)-1,2-diphenylethylenediamine and [RuCl2(p-cymene)]2 successively to afford immobilized catalyst. The Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore size decreased after immobilization of catalyst onto the mesoporous material. Enantioselective transfer hydrogenation of ketones catalyzed by immobilized catalyst showed the highest yield of 22.36% and e.e. value of 31.47% by using acetophenone as substrate when reaction time was 48 and 16 h respectively.

Synthesis and Characterization of Bifunctional Periodic Silica with Surface and Framework Benzene Functionality

Novel organic-inorganic hybrid mesoporous molecular sieve of MCM-41 type containing both bridge-bonded phenyl groups in the walls and terminally bonded phenyl groups protruding into the channel space was synthesized using TEOS, 1,4-bis(triethoxysilyl)benzene (BTESB) and (triethoxysilyl)benzene (TESB) as precursors, cetyl pyridine bromide (CPBr) as template under acidic conditions. This new material was characterized by means of FT-IR, PXRD and nitrogen adsorption-desorption isotherms. The mesoporous organosilica was in-situ sulfonylated and reacted with diamines.

Catalytic liquid-phase oxidation in heterogeneous system as green chemistry goal—advantages and disadvantages of MCM-41 used as catalyst

This review article is devoted to the discoveries and/or knowledge developed in the past 10 years in the area of catalytic liquid-phase oxidation with hydrogen peroxide on mesoporous molecular sieves of MCM-41 type modified with various transition metals. The following parameters influencing activity and selectivity of the catalysts are considered: (i) nature of matrix for metal species limiting diffusion effects; (ii) isolation of metals; (iii) hydrophobicity of catalysts; (iv) nature of solvent; (v) reducibility of metal species; (vi) leaching of metal from the solid. Advantages and disadvantages of MCM-41 materials are pointed out in relation to the properties of TS-1 and TS-2 zeolites.

Nickel containing MCM-41 and AlMCM-41 mesoporous molecular sieves: Characteristics and activity in the hydrogenation of benzene

Pure siliceous and aluminosilicate mesoporous molecular sieves of MCM-41 type have been used as matrices for nickel incorporation (1.7–5%, w/w) via wet impregnation. Moreover, the AlMCM-41-32 material was modified by the cation-exchange method. The properties of the prepared catalysts were studied by means of XRD, XPS, N 2-adsorption, H 2-TPR, H 2-adsorption and TPD. The reaction of gas-phase benzene hydrogenation was carried out on the prepared catalysts. The hydrogen spillover effect in this reaction was also studied using 3% Ni/MCM-41 and 3% Ni/AlMCM-41-32 catalysts diluted by MCM-41 and AlMCM-41-32, respectively. NiSiO 3 and Ni 2O 3 species in the calcined precursor were shown by H 2-TPR and XPS techniques. The presence of metallic aluminium is shown in the Ni/AlMCM-41-32 reduced materials. The reducibility of the catalyst is strongly influenced by the nature of the matrix and the method of preparation. The kinetic study of benzene hydrogenation on 1.7% Ni catalysts demonstrated various partial orders in hydrogen and benzene ascribed to the existence of several surface chemical processes. Investigations on the hydrogen spillover effect showed a maximum of activity for the diluted catalyst. The intensity of hydrogen spillover is strongly decreased in the presence of Al. The kinetic study with the mechanical mixtures suggested changes in the surface reactions due to the contribution of the support to the chemical processes. Moreover, the results strongly indicated that the migration of hydrogen atoms from the metal phase onto the support oxide is assisted by the aromatic hydrocarbon.

Preparation and characterization of novel CO 2 “molecular basket” adsorbents based on polymer-modified mesoporous molecular sieve MCM-41

Novel CO 2 “molecular basket” adsorbents were prepared by synthesizing and modifying the mesoporous molecular sieve of MCM-41 type with polyethylenimine (PEI). The MCM-41-PEI adsorbents were characterized by X-ray powder diffraction (XRD), N 2 adsorption/desorption, thermal gravimetric analysis (TGA) as well as the CO 2 adsorption/desorption performance. This paper reports on the effects of preparation conditions (PEI loadings, preparation methods, PEI loading procedures, types of solvents, solvent/MCM-41 ratios, addition of additive, and Si/Al ratios of MCM-41) on the CO 2 adsorption/desorption performance of MCM-41-PEI. With the increase in PEI loading, the surface area, pore size and pore volume of the PEI-loaded MCM-41 adsorbent decreased. When the PEI loading was higher than 30 wt.%, the mesoporous pores began to be filled with PEI and the mesoporous molecular sieve MCM-41 showed a synergetic effect on the adsorption of CO 2 by PEI. At PEI loading of 50 wt.% in MCM-41-PEI, the highest CO 2 adsorption capacity of 246 mg/g-PEI was obtained, which is 30 times higher than that of the MCM-41 and is about 2.3 times that of the pure PEI. Impregnation was found to be a better method for the preparation of MCM-41-PEI adsorbents than mechanical mixing method. The adsorbent prepared by a one-step impregnation method had a higher CO 2 adsorption capacity than that of prepared by a two-step impregnation method. The higher the Si/Al ratio of MCM-41 or the solvent/MCM-41 ratio, the higher the CO 2 adsorption capacity. Using polyethylene glycol as additive into the MCM-41-PEI adsorbent increased not only the CO 2 adsorption capacity, but also the rates of CO 2 adsorption/desorption. A simple model was proposed to account for the synergetic effect of MCM-41 on the adsorption of CO 2 by PEI.

Effect of solubilization in bitemplate synthesis of a homologous series of organosilica mesophases and mesoporous molecular sieves

Peculiarities of bitemplate synthesis of a homologous series of organosilica mesophases (precursors of mesoporous molecular sieves of MCM-41 type) were studied, and an influence of a nature of templating substances and a solubilization on a perfection of their spatial structure was found. n-alkyl(C8–C18)pyridinium halides as solubilizers (micelle-forming surfactants) and monoethanolamides of saturated n-(C10–C16)aliphatic acids as solubilizate were used. Some physicochemical factors promoting a spatial ordering of organosilica mesophases and mesoporous molecular sieves were considered.

Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture

The objective of the work presented here is to develop a nanoporous solid adsorbent which can serve as a “molecular basket” for CO2 in the condensed form. Polyethylenimine (PEI)-modified mesoporous molecular sieve of MCM-41 type (MCM-41-PEI) has been prepared and tested as a CO2 adsorbent. The physical properties of the adsorbents were characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption, and thermogravimetric analysis (TGA). The characterizations indicated that the structure of the MCM-41 was preserved after loading the PEI, and the PEI was uniformly dispersed into the channels of the molecular sieve. The CO2 adsorption/desorption performance was tested in a flow system using a microbalance to track the weight change. The mesoporous molecular sieve had a synergetic effect on the adsorption of CO2 by PEI. A CO2 adsorption capacity as high as 215 mg-CO2/g-PEI was obtained with MCM-41-PEI-50 at 75 °C, which is 24 times higher than that of the MCM-41 and is even 2 times that of the pure ...

MCM-41 mesoporous molecular sieves supported nickel—physico-chemical properties and catalytic activity in hydrogenation of benzene

Siliceous, aluminosilicate and niobosilicate mesoporous molecular sieves of MCM-41 type have been used as matrices for nickel incorporated via the wet impregnation. The N 2 adsorption, XRD, H 2-TPR and Ni dispersion study were applied for the characterization of the prepared catalysts. The metal–support interaction (MSI) was the highest in Ni/NbMCM-41 material as indicated from H 2-TPR results, in which the highest dispersion of nickel occurs. Hydrogen chemisorption strength and the reducibility of Ni-species are inversely proportional to MSI strength. Both parameters determine the catalytic activity of the mesoporous materials in the hydrogenation of benzene at higher temperatures (423–473 K), whereas their activity at lower temperatures increases with increasing Ni dispersion. The kinetic study allows the determination of the activation energy in the benzene hydrogenation on Ni/MCM-41 which is very low, 36.5 kJ mol −1.

Alkylation and disproportionation of aromatic hydrocarbons over mesoporous molecular sieves

Toluene alkylation with propylene and trimethylbenzene (TMB) disproportionation were investigated over mesoporous molecular sieves of MCM-41 type of different pore dimensions, possessing various Si/Al ratios or modified with heteropoly acids (HPA) of Keggin type. It was shown that the acid strength of (Al)MCM-41 was sufficient to activate propylene in the alkylation of toluene providing cymene selectivity over 96% while the rate of consecutive reactions was negligible. In contrast, TMB disproportionation proceeded at a significantly lower extent than over zeolites. Siliceous MCM-41 modified by HPA (HPA-MCM-41) exhibited a lower rate of cymene isomerization compared to (Al)MCM-41. Moreover, m-cymene was mainly formed at the expense of o-cymene. In TMB disproportionation over HPA-MCM-41 (HPA=H6PV3Mo9O12) significantly higher concentrations of xylenes were reached in comparison with (Al)MCM-41. m- and o-xylenes as primary products of TMBs disproportionation were preferentially formed.

Adsorption hysteresis in nanopores

Capillary condensation hysteresis in nanopores is studied by Monte Carlo simulations and the nonlocal density functional theory. Comparing the theoretical results with the experimental data on low temperature sorption of nitrogen and argon in cylindrical channels of mesoporous siliceous molecular sieves of MCM-41 type, we have revealed four qualitatively different sorption regimes depending on the temperature and pore size. As the pore size increases at a given temperature, or as the temperature decreases at a given pore size, the following regimes are consequently observed: volume filling without phase separation, reversible stepwise capillary condensation, irreversible capillary condensation with developing hysteresis, and capillary condensation with developed hysteresis. We show that, in the regime of developed hysteresis (pores wider than 5 nm in the case of nitrogen sorption at 77 K), condensation occurs spontaneously at the vaporlike spinodal while desorption takes place at the equilibrium. A quantitative agreement is found between the modeling results and the experimental hysteresis loops formed by the adsorption-desorption isotherms. The results obtained provide a better understanding of the general behavior of confined fluids and the specifics of sorption and phase transitions in nanomaterials.

Effects of cation composition in MCM-41 mesoporous molecular sieves on the structures of encapsulated 4-pentyl-4′-cyanobiphenyl liquid crystals

The proportions of the various states are determined by the cation composition of the matrix for liquid-crystal material encapsulated in the pore spaces of mesoporous molecular sieves of MCM-41 type having high aluminum contents. The liquid-crystal properties are retained in the encapsulated state.

Effect of encapsulation in MCM-41 type molecular sieves on vibrational spectra of liquid crystalline state

The paper deals with the investigation of IR absorption spectra of a pure 4- n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal in nematic liquid crystalline (LC) state, pure aluminosilicate molecular sieves of MCM-41 type (with channels about 40 Å in diameter) and their heterogeneous binary systems. It was shown that 5CB molecules are able to interact with active centers in the MCM-41 channels forming sufficiently strong hydrogen bonds of the —CN⋯H—O— type. This conclusion was confirmed by the data obtained from DSC measurements. Relative amount of the 5CB molecules interacting with the channel walls and that of the molecules retaining the LC state when encapsulated in molecular sieves MCM-41 and CuMCM-41 were estimated.

Formation of Transition Metal Cyanoferrates in Mesoporous Molecular Sieves of the MCM-41 Type

It is shown that formation of fragments of the structures of massive transition metal cyanoferrates in mesoporous molecular sieves of MCM-41 type containing high aluminum content does not cause important changes in the porous structures of the inclusion compounds.

Influence of Confinement on The Phase Transition And Spectral Characteristics of Nematic Liquid Crystals

The results of nematic liquid crystal - isotropic liquid phase transition study by the method of differential scanning calorimetry for inclusion compounds - mesoporous aluminosilicate molecular sieves of MCM-41 type (including Cu-exchanged samples) with encapsulated in inner-crystalline space nematic liquid crystal (5CB), as well as IR spectroscopic data for such compounds were represented. It was shown, that the 5CB molecules are able to interact with the active centers in the MCM-41 channels forming strong enough bonds of -C≡N⋅⋅⋅H-O- type. The relative amount of 5CB molecules interacting with the walls of channels and those retaining ‘liquid crystalline’ state in binary systems of molecular sieves MCM-41 and CuMCM-41 was estimated. This conclusion was confirmed by the data obtained from differential scanning calorimetry measurements.

Mesoporous molecular sieve MCM-41 supported Co–Mo catalyst for hydrodesulfurization of petroleum resids

In this work, we explored the potential of mesoporous zeolite-supported Co–Mo catalyst for hydrodesulfurization of petroleum resids, atmospheric and vacuum resids at 350–450°C under 6.9 MPa of H 2 pressure. A mesoporous molecular sieve of MCM-41 type was synthesized; which has SiO 2/Al 2O 3 ratio of about 41. MCM-41 supported Co–Mo catalyst was prepared by co-impregnation of Co(NO 3) 2·6H 2O and (NH 4) 6Mo 7O 24 followed by calcination and sulfidation. Commercial Al 2O 3 supported Co–Mo (criterion 344TL) and dispersed ammonium tetrathiomolybdate (ATTM) were also tested for comparison purposes. The results indicated that Co–Mo/MCM-41(H) is active for HDS, but is not as good as commercial Co–Mo/Al 2O 3 for desulfurization of petroleum resids. It appears that the pore size of the synthesized MCM-41 (28 Å) is not large enough to convert large-sized molecules such as asphaltene present in the petroleum resids. Removing asphaltene from the resid prior to HDS has been found to improve the catalytic activity of Co–Mo/MCM-41(H). The use of ATTM is not as effective as that of Co–Mo catalysts, but is better for conversions of >540°C fraction as compared to noncatalytic runs at 400–450°C.