Structure Of Molecular Sieves Research Articles

Liquid Hydrocarbon Fuels from Catalytic Cracking of Waste Cooking Oils Using Basic Mesoporous Molecular Sieves K2O/Ba-MCM-41 as Catalysts

Mesoporous molecular sieves K2O/Ba-MCM-41, which feature base sites, were prepared under hydrothermal conditions. The structure, base properties, and catalytic activity of the mesoporous molecular sieves as heterogeneous catalysts for the cracking of waste cooking oil (WCO) were then studied in detail. K2O/Ba-MCM-41 exhibited higher catalytic performance for the cracking of WCO than traditional base catalysts such as Na2CO3 and K2CO3. Moreover, the cracking of WCO generates fuels (main composition is C12∼C17 alkane or olefin) that have similar chemical compositions to diesel-based fuels, and K2O/Ba-MCM-41 is of excellent stability. The catalyst could be recycled and reused with negligible loss in activity for four cycles. K2O/Ba-MCM-41 is an environmentally benign heterogeneous basic catalyst for the production of liquid hydrocarbon fuels from low quality feed stocks.

The Study of Cooperation Solidification of Cs based on ZSM-5 Zeolite

In this work, using modified zeolite molecular sieve as a sorbent carrier, adjusting the content of cesium, adding Ca, Mg, Zr or Ba, different samples of solidification were prepared by soakage- absorption-calcination methods. The crystal characteristics of the samples were tested by XRD, the results showed that the higher the calcination temperature or the greater capacity of the Cs packages, the structure of ZSM-5 zeolite molecular sieve collapsed and pollucite emerged more easily. Furthermore, samples with the addition of Ba could accelerate the formation of pollucite. MCC-1 method was used to test the chemical durability of the solidification samples, the results show that when the content of Cs is lower than 30%, the leaching rate of Cs is at the level of 10-6cm•d-1,but when the content of Cs reached up to 35%, the leaching rate of Cs increased to 10-4 cm•d-1. After doped Ba, the Cs leaching rate of 3Ba32Cs-850 is much lower than the Cs leaching rate of 35Cs-850, the results indicated that the solidification of doped Ba could improve the chemical durability of pollucite.

Molecular simulation and experimental studies of a mesoporous ZSM-5 type molecular sieve

The mesoporous zeolite is a novel porous material possessing mesopores as well as the inherent micropores of zeolites. This material can exhibit the dual merits of two different pore structures and enable zeolites to have maximum structural functions. During the past few decades, various synthetic strategies have been well developed. However, up to now, there has only been a few attempts to model mesoporous zeolites. In this paper, the structural properties of a mesoporous ZSM-5 type molecular sieve, which has mesopore walls that are made up of ZSM-5 zeolite-like frameworks, were studied using an atomistic model. The full-atom model of the mesoporous ZSM-5 type molecular sieve was constructed using a molecular modeling technique. The structure model was characterized by estimating the nitrogen accessible solvent surface area, small-angle and wide-angle X-ray diffraction patterns, toluene and benzene adsorption. It was found that these simulated results match well with the experimental data. Furthermore, the present approach can be extended to construct other micro-mesoporous molecular sieve structure models in the future.

Synthesis of mesoporous MCM-48 molecular sieves of high quality in a fluorine-containing system

Mesoporous MCM-48 molecular sieves were synthesized in a fluorine-containing system using tetraethylorthosilicate (TEOS) as silica source and cetyltrimethylammonium bromide (CTAB) as template. To investigate the influence of various factors for synthesis on the structure and properties of the MCM-48 molecular sieves obtained, they were characterized by X-ray diffraction (XRD), N2 adsorption-desorption and transmission electron microscopy (TEM). The results showed that under the optimal conditions, i.e. a CTAB/Si ratio of 0.65 and crystallization at 393 K for 24 h, the as-synthesized MCM-48 exhibits high crystallinity with a specific surface area of 1305 m2/g and an average pore size of 2.416 nm.

PERFORMANCE OF Ce-INCORPORATED KIT-6 SUPPORTED COBALT CATALYSTS FOR FISCHER–TROPSCH SYNTHESIS

A series of Ce -incorporated structure of cubic large mesoporous molecular sieves, KIT-6, with different Ce contents were synthesized by a direct hydrothermal process. A sample of Ce -loaded KIT-6 material was synthesised by incipent wetness impregnation. Similarly, catalysts with 15 wt.% Co loading on the above supports were also synthesized by incipient wetness impregnation. The supports and catalysts were characterized by X-ray diffraction, diffuse reflectance UV-vis, solid-state 29Si magic-angle spinning nuclear magnetic resonance, H2 -temperature programmed reduction, H2 -temperature programmed desorption and oxygen titration. The structure of the KIT-6 support was well retained after Ce incorporation. Small amounts of Ce in the Co catalyst were found to improve the activity and increase the selectivity to C5+ hydrocarbons for Fischer–Tropsch synthesis, while larger amounts of Ce had the reverse effect. Meanwhile, methane selectivity shows an opposite trend as compared with that of C5+ selectivity. Ce -loaded KIT-6 supported Co catalyst showed lower activity than KIT-6 supported Co catalyst.

Synthesis and Characterization of the ZSM-5/MCM-41 Composite Molecular Sieves

The ZSM-5/MCM-41 composite molecular sieves were synthesized in the method of hydrothermal crystallization in the presence of cetyltrimethylammonium bromide (CTMABr) as template agent. The prepared composite molecular sieves were characterized by the means of X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and the thermogravimetric and differential thermal analysis (TG-DTA). The experimental results showed that as-prepared ZSM-5/MCM-41 composite molecular sieves kept both crystal structure and compositions of ZSM-5 zeolites and MCM-41 molecular sieves. The decomposition temperature of the template agent was 293°C, and the decomposition temperature of the molecular sieve framework was higher than 800 °C. We supposed that the obtained ZSM-5/MCM-41 composite molecular sieves had high thermal stability, framework rigidity and structural stability, and could combine the properties of ZSM-5 zeolites and MCM-41 molecular sieves successfully.

Iron Oxide Silica Derived from Sol-Gel Synthesis.

In this work we investigate the effect of iron oxide embedded in silica matrices as a function of Fe/Si molar ratio and sol pH. To achieve homogeneous dispersion of iron oxide particles, iron nitrate nonahydrate was dissolved in hydrogen peroxide and was mixed with tetraethyl orthosilicate and ethanol in a sol-gel synthesis method. Increasing the calcination temperature led to a reduction in surface area, although the average pore radius remained almost constant at about 10 Å, independent of the Fe/Si molar ratio or sol pH. Hence, the densification of the matrix was accompanied by similar reduction in pore volume. However, calcination at 700 °C resulted in samples with similar surface area though the iron oxide content increased from 5% to 50% Fe/Si molar ratio. As metal oxide particles have lower surface area than polymeric silica structures, these results strongly suggest that the iron oxides opposed the silica structure collapse. The effect of sol pH was found to be less significant than the Fe/Si molar ratio in the formation of molecular sieve structures derived from iron oxide silica.

Electricity generation in continuous flow microbial fuel cells (MFCs) with manganese dioxide (MnO 2) cathodes

A novel cost-effective cathode catalyst, manganese dioxide (MnO 2) featured with a cryptomelane-type octahedral molecular sieve (OMS-2) structure, was examined in continuous flow microbial fuel cells (MFCs). Power generation and organic substrate removal efficiency of two metal ions doped OMS-2 cathodes (cobalt (Co)-OMS-2 and copper (Cu)-OMS-2) were compared with platinum (Pt) cathodes under different hydraulic retention times (HRTs) and chemical oxygen demands (CODs). The 600-h continuous flow tests showed that Cu-OMS-2 MFCs and Co-OMS-2 MFCs achieved the stable power generation of 200 ± 8 mV and 190 ± 5 mV, and were 50–60 mV higher than that of Pt MFCs. The COD removal efficiencies of Cu-OMS-2 MFCs and Co-OMS-2 MFCs were 83–87%, which were 15–19% higher than that of Pt MFCs. The power generation and COD removal efficiency increased with longer HRTs. The Cu-OMS-2 exhibited the highest power density (201 mW/m 2) at the COD of 1000 mg/L. However, Co-OMS-2 cathodes had the better performance than Cu-OMs-2 at high COD concentrations of 2000–4000 mg/L, with the power density of 897 mW/m 2 and COD removal efficiency of 46%. The continuous flow MFC tests demonstrated that the fast reaction rate of OMS-2 cathodes enhanced power generation and COD removal efficiency, and had a great potential to be applied in real-world wastewater treatment processes.

A DFT study of the structural units in SBA-15 mesoporous molecular sieve

Geometries of silica rings in SBA-15 mesoporous molecular sieve were investigated by the density functional theory (DFT). The stable conformations of model clusters with monocyclic and bicyclic silica rings were obtained. By comparing the calculated values of Si–O bond length and O–Si–O, Si–O–Si bond angles with the experimental data of amorphous silica, we found that all monocyclic rings are not the reasonable model to represent the structure of SBA-15 mesoporous molecular sieve. After having investigated the bicyclic model clusters theoretically, we inferred that 5-, 6- and 7-membered rings should be the main structural elements in the SBA-15 framework. Among the model clusters employed in this study the model 5–6-s was proved to be the best one for the SBA-15 molecular sieve. A comparison between the calculated IR frequencies of the model clusters and the measured values of SBA-15 further verified that the bicyclic model clusters employed here are reasonable and the model 5–6-s is the most suitable model cluster for the SBA-15 mesoporous molecular sieve.

Microwave synthesis and textural property of europium substituted mesoporous molecular sieves

Using cetyltrimethyl ammonium bromide (CTAB) as the template and sodium silicate as the silicon source, the MCM-41 mesoporous molecular sieves with Eu incorporated in the framework were synthesized under microwave irradiation condition and the influence of the Si/Eu molar ratio on the crystalline structure, textural properties and the long-range ordering of the resulting sample was investigated by various physicochemical techniques such as X-ray diffraction (XRD), transmission electron microscope (TEM), diffuse reflectance ultraviolet–visible spectroscopy (UV–vis), thermal gravimetric–differential scanning calorimeter (TG–DSC) and N 2 physical adsorption. The results of N 2 adsorption and XRD reveal that the synthesized sample has the ordered hexagonal mesoporous structure. UV–vis spectra provide the strong evidences that most of europium ions were incorporated into the framework of the MCM-41 sample. The crystalline structure, textural properties and mesoporous ordering of the resultant mesoporous materials are related to the amount of europium incorporation. Small amount europium incorporated into the silica-based MCM-41 does not strongly modify the structure of mesoporous molecular sieve. An increase of the Eu content in sample led to reduction of the specific surface area and the deterioration of the long-range ordering.

Ionothermal synthesis of FeAlPO-16 molecular sieve by microwave irradiation in eutectic mixture

FeAlPO-16 molecular sieve with AST topology has been ionothermally prepared under microwave irradiation in succinic acid-choline chloride eutectic mixture for the first time. The structure and morphology of FeAlPO-16 molecular sieve are characterized in detail by powder X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance UV–visible (UV–vis) spectroscopy, thermogravimetry–differential scanning calorimetry analysis (TG-DSC) and scanning electron microscopy (SEM). UV–vis results suggest that iron species may partially enter the framework of FeAlPO-16 molecular sieve. SEM images indicate that large spherical particles observed in the images are agglomerates of FeAlPO-16 molecular sieve nanocrystalline (ca. 100 nm in size). Effects of synthesis conditions, such as P2O5/Al2O3 ratio, pyridine/Al2O3 ratio, reaction temperature and crystallization time, as well as iron and aluminum source on the crystallization of FeAlPO-16 molecular sieve are systematically investigated. Experiments show that FeAlPO-16 molecular sieve can be prepared over a wide molar ratio of Al2O3:xP2O5:yPyridine:0.4Fe2O3:58EU (x = 1.0–3.5, y = 0.8–7.8) using aluminum isopropoxide and ferric ammonium citrate as iron and aluminum source, respectively. The synthesis of FeAlPO-16 molecular sieve is highly sensitive to iron source, but not to aluminum source, crystallization temperature and time.

ChemInform Abstract: Mechanism of n-Propyltoluene Formation in C3 Alkylation of Toluene: The Effect of Zeolite Structural Type.

Abstract The mechanism of n -propyltoluene formation in toluene alkylation with propanols has been investigated over molecular sieves of H-ZSM-5, H-Y, and H-mordenite structures. It has been shown that isopropyltoluenes are formed in the first alkylation step followed by a bimolecular reaction with toluene molecules leading to n -propyltoluenes. This mechanism is evidenced by the reaction of p -isopropyltoluene with benzene and isopropylbenzene with toluene leading to n -propylbenzene and n -propyltoluene, respectively. The bimolecular isomerization reaction takes place only in the three-dimensional channel system of H-ZSM-5 and H-Y, enabling the transition complex formation, which is enhanced by the channel geometry of H-ZSM-5. Therefore, structure-directed transition state selectivity, controlled by the molecular sieve structure, operates in this isomerization reaction.

Effect of processing on carbon molecular sieve structure and performance

Sub-micron sized carbon molecular sieve (CMS) materials were produced via ball milling for subsequent use in hybrid material formation. A detailed analysis of the effects of the milling process in the presence of different milling environments is reported. The milling process apparently alters the molecular scale structure and properties of the carbon material. Three cases: unmilled, air milled and nitrogen milled, were analyzed in this work. The property changes were probed using equilibrium sorption experiments with different gases. Furthermore, WAXD and BET results also showed differences between milling processes. Finally in order to improve the interfacial polymer-sieve region of hybrid membranes, the CMS surface was chemically modified with a linkage unit capable of covalently bonding the polymer to the sieve. A published single-wall carbon nanotube (SWCNTs) modification method was adopted to attach a primary aromatic amine to the surface. Several aspects including rigidity, chemical composition, bulky groups and length were considered in selecting the preferred linkage unit. Fortunately kinetic and equilibrium sorption properties of the modified sieves showed very little difference from unmodified samples, suggesting that the linkage unit is not excessively filling or obstructing access to the pores of the CMSs during the modification process.

Thermal and hydrothermal stability of ZrMCM-41 mesoporous molecular sieves obtained by microwave irradiation

ZrMCM-41 mesoporous molecular sieves were synthesized by using the zirconium sulfate as zirconium source and using cetyltrimethyl ammonium bromide as a template under microwave irradiation condition. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductive coupled plasma (ICP) technique, Fourier transform infrared spectroscopy (FT-IR) and N2 physical adsorption, respectively. The effect of the different initial ZrO2: SiO2 molar ratio, the different thermal treatment temperature and hydrothermal treatment time on textural property was investigated. The results show that the obtained products possess a typical mesoporous structure of MCM-41 and have specific surface areas in the range of 598·1∼971·4 m2/g and average pore sizes in the range of ca. 2·46∼3·43 nm. On the other hand, the BET specific surface area and pore volume of the synthesized ZrMCM-41 mesoporous molecular sieve decrease with the increased amount of zirconium incorporated in the starting material, the rise of thermal treatment temperature and the prolonging of hydrothermal treatment time, the mesoporous ordering deteriorates. The mesoporous structure of the ZrMCM-41 mesoporous molecular sieve still retains after calcination at 750°C for 3 h or hydrothermal treatment at 100°C for 6 days, however, the mesoporous ordering is poor.

Manganese dioxide as a new cathode catalyst in microbial fuel cells

This study focused on manganese oxides with a cryptomelane-type octahedral molecular sieve (OMS-2) structure to replace platinum as a cathode catalyst in microbial fuel cells (MFCs). Undoped (ud-OSM-2) and three catalysts doped with cobalt (Co-OMS-2), copper (Cu-OMS-2), and cerium (Ce-OMS-2) to enhance their catalytic performances were investigated. The novel OMS-2 cathodes were examined in granular activated carbon MFC (GACMFC) with sodium acetate as the anode reagent and oxygen in air as the cathode reagent. The results showed that after 400 h of operation, the Co-OMS-2 and Cu-OMS-2 exhibited good catalytic performance in an oxygen reduction reaction (ORR). The voltage of the Co-OMS-2 GACMFC was 217 mV, and the power density was 180 mW m −2. The voltage of the Cu-OMS-2 GACMFC was 214 mV and the power density was 165 mW m −2. The internal resistance ( R in) of the OMS-2 GACMFCs (18 ± 1 Ω) was similar to that of the platinum GACMFCs (17 Ω). Furthermore, the degradation rates of organic substrates in the OMS-2 GACMFCs were twice those in the platinum GACMFCs, which enhance their wastewater treatment efficiencies. This study indicated that using OMS-2 manganese oxides to replace platinum as a cathodic catalyst enhances power generation, increases contaminant removal, and substantially reduces the cost of MFCs.

Acylation Reactions over Zeolites and Mesoporous Catalysts

Molecular sieves are highly active and selective catalysts with industrial potential for acylation reactions. Zeolites are the catalysts of choice when shape selectivity influences the preferential formation of some products, while high conversions are achieved over mesoporous catalysts with enhanced diffusion rates of reactants and products. In this Minireview, we focus on the understanding of the relationship among the structure of molecular sieve, type and concentration of acid sites and activity/selectivity in various acylations of aromatic and olefinic hydrocarbons. The products of these acylation reactions are important compounds for the pharmaceutical industry, fragrance and flavor materials, dyes, polymers, agrochemicals, and other applications.

Desulfurization of Vacuum Gasoil by MCM-41 Supported Molybdenum−Nickel Catalysts

Hydrodesulfurization (HDS) of vacuum gas oil (VGO) was carried out in a trickle bed reactor at 400 °C and 7.2 MPa with a weight hourly space velocity (WHSV) = 2 h−1 over a series of molybdenum−nickel catalysts, which were prepared by loading the same amounts of MoO3 (12 wt %) and NiO (3 wt %) on supports including: tubular shaped MCM-41, nontubular shaped MCM-41, alumina, and silica. The HDS activity of the MoNi catalysts with different supports was investigated. The tubular shaped MCM-41, which possesses a regularly ordered molecular sieve structure, was shown to give significant improvements in the diffusion of large molecules. The results indicated that the tubular shaped MCM-41 supported catalyst has a higher activity than the other catalysts when measured over a 4−5 day running period. The effect of Si/Al ratios in the framework is also discussed. Various methods and techniques were used to characterize the catalysts such as: scanning electron microscopy (SEM), transmission electronic microscopy (TEM), temperature programming reduction (TPR), X-ray diffraction (XRD), mercury-porosity, and the generation of nitrogen adsorption−desorption isotherms.

A novel hollow sphere mesoporous material synthesized by using DADD template and embedding Nd into framework simultaneously

A hollow sphere neodymium embedded mesoporous silica material (NEMS) has been synthesized successfully by using neutral 1,12-diaminododecane as template and embedding Nd into the framework simultaneously. It is interesting that a lamellar structured material with MSU-V structure was obtained if only DADD was used as template without embedding of Nd. The synthesized materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), BET method, thermal gravimetry/differential thermal analysis (TG/DTA), and X-ray photoelectron spectroscopy (XPS). The hollow sphere morphology was observed by TEM images and clearly revealed the structure of NEMS molecular sieves. Both XRD and XPS results confirmed that the Nd atoms were incorporated into the framework of mesoporous materials. The TG/DTA results verified the good thermal stability of the synthesized NEMS material. The surface area of samples with Si/Nd molar ratio from 10 to 60 reached from 543 to 1103 m 2 g −1, respectively. It also found that the Nd embedded mesoporous silica material showed good catalytic activity and selectivity towards the oxidation of styrene, proportional to the neodymium content in the catalysts.

Effect of Ni-doping on the pore structure of pure silica MCM-41 mesoporous molecular sieve under microwave irradiation

Long-range and ordered Ni-MCM-41 mesoporous molecular sieves with different amount of nickel were successfully synthesized using sodium silicate and nickel chloride as raw materials and cetyltrimethyl ammonium bromide (CTAB) as a template by microwave irradiation method. The structure, morphology, crystalline size, specific surface area and pore size distribution of the as-synthesized samples were characterized by means of small-angle X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), temperature programmed reduction (TPR), transmission electron microscopy (TEM) and N2 physical adsorption, respectively. The experimental results show that Ni-MCM-41 mesoporous molecular sieves were successfully synthesized under microwave irradiation, and the ordering of the pore channels of the mesostructure was improved after incorporation of nickel into MCM-41 mesoporous molecular sieve framework. The synthesized samples were calcined at 550 °C for 10 h, the template was effectively removed. A large specific surface area and a more uniform pore distribution were obtained for Ni-MCM-41 mesoporous molecular sieve.

Characterization of pore structure of carbon molecular sieves using DFT analysis of Ar and H 2 adsorption data

A combination of hydrogen and argon adsorption isotherms at cryogenic temperatures (77 and 87 K) were used to analyze microporosity of a series of carbon molecular sieves (CMS). The pore size distribution (PSD) for a carbon sample was calculated by fitting the non local density functional theory (NLDFT) model isotherms to a set of experimental data consisting of both Ar and H 2 adsorption isotherms. Thus, the calculated PSD is consistent with both isotherms. The range of pore size analysis in this method is extended to smaller pore sizes compared to the standard nitrogen or argon adsorption analysis. In addition, it is shown that the data of the two isotherms are partially complementary so that if the low pressure part of the Ar isotherm is truncated, the calculated PSD remains almost unchanged. The results of the analysis of a series of CMSs show that the presented approach is very sensitive to actual carbon pore structure and allows differentiating between seemingly similar samples. It makes the method a useful tool for a detailed characterization of CMSs.