Production of biodiesel by esterification of palmitic acid over mesoporous aluminosilicate Al-MCM-41

Biodiesel has been obtained by esterification of palmitic acid with methanol, ethanol and isopropanol in the presence of Al-MCM-41 mesoporous molecular sieves with Si/Al ratios of 8, 16 and 32. The catalytic acids were synthesized at room temperature and characterized by atomic absorption spectrometry (AAS), thermal analysis (TG/DTA), X-ray diffraction (XRD), nitrogen absorption (BET/BJH), infrared spectroscopy (IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The reaction was carried out at 130 °C whilst stirring at 500 rpm, with an alcohol/acid molar ratio of 60 and 0.6 wt% catalyst for 2 h. The alcohol reactivity follows the order methanol > ethanol > isopropanol. The catalyst Al-MCM-41 with ratio Si/Al = 8 produced the largest conversion values for the alcohols studied. The data followed a rather satisfactory approximation to first-order kinetics.     

Fabrication and morphological control of three-dimensional cage type mesoporous titanosilicate with extremely high Ti content

TiSBA-1 materials with extremely high Ti content, up to silicon to titanium ratio (n_Si/n_Ti) of 2.4, have been successfully prepared through direct synthesis method by controlling the molar ratio of hydrochloric acid to silicon (n_HCl/n_Si). It has been found that the amount of Ti content and the structure of the TiSBA-1 can easily be controlled by the simple adjustment of n_HCl/n_Si ratio. X-ray diffraction pattern (XRD) of the obtained materials revealed that the materials are highly ordered and possess cubic three-dimensional cage type structure with open windows. N₂ adsorption–desorption measurement confirmed the narrow pore size distribution, high specific surface area (1317–1491 m² g⁻¹), and large specific pore volume (0.68–0.75 cm³ g⁻¹) for all the samples. UV–vis DR spectra of the prepared materials confirmed that Ti atoms are exclusively incorporated within silica framework and occupy the tetrahedral position while the presence of isolated bulk titania could be negligible. Morphologies of the TiSBA-1 materials have been also controlled by simply adjusting the n_Si/n_Ti ratio. With the appropriate Ti content, TiSBA-1 materials can be obtained as regular fine spheres. Moreover, the detailed mechanism on the morphological and phase transition control, and the incorporation of high amount of Ti in the framework of TiSBA-1 materials has been also discussed in detail.     

Basic catalytic properties of as-synthesized molecular sieves

A series of as-synthesized molecular sieves have been tested and revealed useful as basic catalysts for Knoevenagel condensation under mild conditions. In order to understand the role of the Si/Al ratio, the structure, porosity and template agent, fifteen structures were synthesized and studied in this reaction. Siliceous and large pore molecular sieves showed high conversion levels (97%), while aluminum zeolites, even when containing large amount of organic material, showed distinct activity behavior, which was mainly dependent on the amount of framework silicon. This result, in addition to literature data, indicated that most likely siloxy anions (SiO⁻) are the basic active sites. XPS measurements of these hybrid organic–inorganic catalysts showed that O1s binding energy is influenced by the presence of organic template, leading to the conclusion that they are interacting with framework oxygen, possibly balancing framework anions. Calcined siliceous molecular sieves, which had only silanol groups (SiOH), showed negligible catalytic activity (lower than 2%). As-synthesized siliceous molecular sieves have proven to be useful for basic heterogeneous catalysis, particularly for the synthesis of fine chemicals or for catalytic processes demanding strong basic sites.     

Controlling the textural parameters of mesoporous carbon materials

The mesoporous carbon materials prepared by inorganic templating technique using mesoporous silica, SBA-15 as a template and sucrose as a carbon source, have been systematically investigated as a function of sucrose to mesoporous silica composition, with a special focus on controlling the mesoporous structure, surface morphology and the textural parameters such as specific surface area, specific pore volume and pore size distribution. All the materials have been unambiguously characterized by XRD, N₂ adsorption–desorption isotherms, high-resolution transmission electron microscopy, high-resolution field emission scanning electron microscopy, and Raman spectroscopy. It has been found that the porous structure, morphology and the textural parameters of the mesoporous carbons materials, CMK-3-x where x represent the sucrose to silica weight ratio, can be easily controlled by the simple adjustment of concentration of sucrose molecules. It has also been found that the specific surface area of the mesoporous carbon materials systematically increases with decreasing the sucrose to silica weight ratio. Moreover, the specific pore volume of the materials increases from 0.57 to 1.31 cm³/g with decreasing the sucrose to silica weight ratio from 5 to 1.25 and then decreases to 1.23 cm³/g for CMK-3-0.8. HRTEM and HR-FESEM also show a highly ordered pore structure and better surface morphology for CMK-3-1.25 as compared to other materials prepared in this study. Thus, it can be concluded that the sucrose to silica weight ratio of 1.25 is the best condition to prepare well ordered mesoporous carbon materials with good textural parameters, pore structure and narrow pore size distribution.     

Rational design of shape selective separation and catalysis—II: Mathematical model and computational studies

In our previous work [Gounaris, C.E., Floudas, C.A., Wei, J., 2006. Rational design of shape selective catalysis and separation: I. Concepts and analysis. Chemical Engineering Science, in press, doi: 10.1016/j.ces.2006.09.012], we introduced the concept of molecule projections, referred to as footprints, and briefly described rigorous criteria that define them. We also introduced the concept of strain index and showed how it can be used to identify portals that have the potential of being highly selective between two molecules. In this paper, we present in detail the mathematical formulations for the calculation of footprints and the complete mathematical model and algorithmic framework developed for the calculation of the strain index of any given molecule/portal pair. Thirty-eight molecules of interest, as well as a collection of 123 zeolite structures, involving 217 different windows, have been considered and selectivity results at ambient temperature are presented. A number of commercially interesting examples are studied in further detail, across a wide range of temperatures. It should be noted that the methodology is generic and can be applied for any pair of molecule and portal. The results indicate that there is a great potential in systems involving non-regular portals that are usually not considered in the selection process of a catalyst or molecular sieve.     

Rational design of shape selective separation and catalysis—I: Concepts and analysis

A molecule can enter a zeolite channel when it fits the size and shape of the opening. Zeolites have been used commercially for their ability to admit selectively molecule A but not molecule B. The search and design of zeolites for separation has been based on finding a channel with a diameter that is larger than molecule A, but smaller than molecule B. This method is not sufficiently accurate when the molecule is not a sphere or the channel is not a circle. We present here first a more accurate screening method based on comparing both the major and minor diameters of the molecule projection, called the “footprint”, with the diameters of the channel. Then we present an even better method that is based on the activation energy required to strain and distort a molecule to fit a given channel, thus leading to a lowering of its Boltzmann concentration in this channel. This paper compiles the activation energies encountered among thirty-eight (38) molecules and two hundred and seventeen (217) zeolite channels into a database, and shows how one can use this database to identify the most promising zeolites for the separation of a set of molecules of interest. Such a screening method would help to identify a zeolite that has much lower activation energy for molecule A than for molecule B. The appropriate temperature range for separation would be centered around the optimal temperature T^*, where the degree of selectivity is at a maximum. The separation of the three pentane molecules was selected as an illustration.     

OPTIMIZATION OF HETEROGENEOUS CATALYTIC PROCESSES IN FLUIDIZED BED

A number of large-scale chemical processes is carried out in fluidized-bed reactors (FBR). Optimization of processes in FBR is thus of great importance both for the increase of efficiency of the existing plants and for the design of new ones. Selection of optimal conditions for exothermic processes should recognize conditions for heat removal and multiplicity and stability of stationary states rather than just the reaction kinetics and fluidized bed hydrodynamics. Mathematically this problem is rather cumbersome. A process in FUR is described by the boundary problem for differential equations while statement of an optimization problem requires some additional limitations to be imposed on concentrations at the input and output of the reactor, on temperature, design parameters etc. One should take special notice of the limitation on the process stability which (owing to great reverse thermal agitation) is much more important in FBRs than in packed-bed reactors.     

OPTIMIZATION OF HETEROGENEOUS CATALYTIC PROCESSES IN FLUIDIZED BED

A number of large-scale chemical processes is carried out in fluidized-bed reactors (FBR). Optimization of processes in FBR is thus of great importance both for the increase of efficiency of the existing plants and for the design of new ones. Selection of optimal conditions for exothermic processes should recognize conditions for heat removal and multiplicity and stability of stationary states rather than just the reaction kinetics and fluidized bed hydrodynamics. Mathematically this problem is rather cumbersome. A process in FUR is described by the boundary problem for differential equations while statement of an optimization problem requires some additional limitations to be imposed on concentrations at the input and output of the reactor, on temperature, design parameters etc. One should take special notice of the limitation on the process stability which (owing to great reverse thermal agitation) is much more important in FBRs than in packed-bed reactors.     

Fractal characteristics of gas-solids flow in a circulating fluidized bed

A fractal approach is adopted to describe the dynamic behavior of a circulating fluidized bed. Two times series, differential pressure fluctuations along the riser height and solids momentum fluctuations along the radial direction, are measured and analyzed in terms of fractal dimensions. The influences of operating conditions and axial/radial positions on the fractal dimension are discussed. Attempts are also made to interpret the flow structure in the bed in terms of the fractal dimension. It is found that fractal analysis can provide a useful tool for understanding the characteristics of gas-solids flow in circulating fluidized beds.     

微孔-中孔复合分子筛的合成研究进展

概述了最近几年引起人们广泛关注的微孔-中孔复合分子筛的合成情况,重点介绍了不同复合模式的微孔-中孔复合分子筛的合成方法,包括单模板剂法、双模板剂法、附晶生长法、孔壁晶化法、碱处理法、微孔沸石硅源法等一系列方法,分析了各种合成方法的优点。从目前已取得的研究结果看,附晶生长法和碱处理法值得进一步关注。     

Zeolite-based materials for novel catalytic applications: Opportunities, perspectives and open problems

Zeolites and related materials (including a wide range of microporous and mesoporous materials with ordered pore structure) have been one of the areas in the field of materials and catalysis with the largest impact on science, technology and industrial processes. We discuss here some recent developments in this field, with particular references how to tailor and design zeolite and related material properties to control/enhance the catalytic performances. Four main topics have been addressed. (i) The recent progress and perspectives in the field of tailored syntheses, with selected examples showing the trend and prospects to develop new structures, control the location of active sites, and the crystal size and morphology, including nanoarchitecture of the final catalysts. (ii) The development and prospects of two-dimensional zeolites presenting an extended view/concept of zeolite structures integrating the classical 3D frameworks and the various lamellar forms. (iii) The progresses in the design and synthesis of hierarchical zeolites, with discussion on the still existing challenges related to the synthesis, characterization and catalytic application. (iv) Novel opportunities and needs in terms of zeolite multifunctional design for catalytic applications, with a discussion of the critical issues related to the use in the field of fine chemicals, organic industrial syntheses and biorefinery, and the prospects for the use in two novel challenging areas of the direct conversion of CO₂ to light olefins and methane to methanol.