Separating n-alkane mixtures by exploiting differences in the adsorption capacity within cages of CHA, AFX and ERI zeolites

The saturation capacity of n-alkanes in CHA, AFX and ERI zeolites, that consist of cages separated by windows, decreases with increasing carbon number. The major aim of the present communication is to demonstrate the possibility of separating n-alkane mixtures relying on differences in saturation capacities. To investigate this possibility, Configurational-Bias Monte Carlo simulations for adsorption of C3–nC6, nC4–nC6, and nC5–nC6 mixtures in CHA, AFX and ERI were carried out for equimolar bulk fluid phase. These mixture simulations show that for operation at fluid phase fugacities below about 1 MPa, the adsorbed phase in equilibrium with the bulk vapor phase is predominantly the alkane with the longer chain length, i.e. nC6. However, for operation at pressures in excess of 1 MPa, the adsorbed phase in equilibrium with the bulk liquid phase is richer in the component with the smaller chain length. In some cases, the nC6 is practically excluded from the zeolite.     

Synthesis and characterization of zeolite NaY from rice husk silica

Rice husk silica (RHS) in amorphous phase with 98% purity was prepared from a waste rice husk from rice milling by leaching with hydrochloric acid and calcination. The RHS was used effectively as a silica source for the synthesis of zeolite Y in sodium form (NaY). The zeolite in pure phase was obtained from a two-step synthetic route in which the starting gels were mixed, aged for 24 h at room temperature and crystallized for 24 h at 90 °C. The diameter of single crystal particles from a scanning electron microscope was approximately 1.0 μm, whereas the average particle diameter from laser diffraction particle size analyzer was approximately 10 μm because of the agglomeration of small crystals. Longer crystallization time in this route resulted in a mixed phase containing NaY and zeolite P in sodium form (NaP). In addition, a one-step synthetic route (no aging) was studied and the product was also a mixed phase zeolite.     

Direct synthesis of copper faujasite

Copper containing faujasite has been successfully prepared for the first time using a direct synthesis method. Faujasite type zeolite can be prepared in the presence of copper species by tuning the synthesis conditions. Ammonium hydroxide was used to form a copper complex that was later mixed with the reacting gel. Sodium is required to obtain copper faujasite. The complete elimination of sodium ions from the starting gel produces amorphous material. Crystallization took place at 358 K for 11 days. Crystallization temperature of 373 K produces ANA type zeolite as an impurity. Increasing by two times the amount of copper complex added to the reacting gel increases the crystallization time of Cu-FAU from 11 to 20 days (the crystallization rate decreases). The copper containing faujasite obtained was characterized by XRD, FESEM, EDX, EPR, FT-IR, TPR, and BET. According to the XRD pattern only FAU type zeolite was obtained. According to TPR experiments, the reduction temperature for Cu²⁺ ions present in Cu-FAU prepared by direct synthesis was 70 K more than for Cu-FAU prepared by ion-exchange. This difference can be due to the different location of the copper ions in the supercages or in the sodalite cages of the faujasite.     

Multistage Zeolite Drying for Energy-Efficient Drying

This work discusses the potential of three multistage zeolite drying systems (counter-, co-, and cross-current) with a varying number of stages. The evaluation showed that for 2-4 stages with heat recovery the efficiency of the systems ranges between 80 and 90%. Additionally, by introducing a compressor, the latent heat in the exhaust air from the regenerator is recovered and used to heat the inlet air for an additional drying stage. As a result, for the counter-current drying system and compressor pressure 1.5-2 bar, a maximum energy efficiency of 120% is achieved, which results in halving the energy consumption compared to conventional drying systems.     

沸石催化剂在甲苯甲基化中的择形催化形用

综述了影响甲苯甲基化择形催化的因素,讨论了沸石的内外表面酸中心及酸强度、沸石的孔道、反应物和产物的吸附等因素对提高对二甲苯选择性的影响。     

Evaporation induced self-assembly of zeolite A micropatterns due to the stick-slip dynamics of contact line

In this study, a new surfactant-solvent system was described for the preparation of periodic stripe patterns of zeolite A on solid substrates. The evaporation induced self-assembly of zeolite A particles was due to the stick-slip dynamics of the three-phase contact line of the colloid solutions in acetone containing 10% (v/v) poly(dimethylsiloxane) (PDMS) fluid (2 cst.). In order to investigate the possible effects of particle size and the particle concentration on the stick-slip dynamics, three types of zeolite A samples with different particle sizes (zeolite A-I: 250-500 nm, zeolite A-II: 100-250 nm and zeolite A-III: 0-100 nm) were utilized to prepare 0.007-0.06% (w/v) colloidal dispersions. Zeolite A micropatterns were self-assembled on the surface of glass, high density polyethylene (HDPE) and poly(tetrafluoroethylene) (PTFE) substrates, which were placed vertically inside the colloid solutions and held against the wall of the cylindrical vial during the evaporation of acetone. The stripe patterns of zeolite A particles were analyzed with field emission scanning electron microscope (FE-SEM) and optical microscope. The widths of microstripes and the distance between the stripes were found as 2-20 μm and 40-60 μm respectively depending on the particle concentration. By using the stick-slip dynamics of colloids, the linear micropatterns of zeolite A nanocrystals were prepared with low cost and low energy.     

Preparation and Characterization of Zeolite Beta with Low SiO_2/Al_2O_3 Ratio

Zeolite beta with a low SiO2/Al2O3 ratio was synthesized by a novel two-step process.The synthesized sample was characterized with XRD,SEM,FTIR,and N2 adsorption-desorption and solid-state MAS NMR.The results showed that aluminium species were inserted into the framework of zeolite beta.The BET surface area,volume,and particle size of the zeolite beta sample decreased with decreasing SiO2/Al2O3 ratio.Our process was proved an efficient route to synthesize zeolite beta with a low SiO2/Al2O3 ratio.     

Zeolite based films, membranes and membrane reactors: Progress and prospects

The integration of reaction and separation in catalytic membrane reactors has received increasing attention during the past 30 years. The combination promises to deliver more compact and less capital-intensive processes with substantial savings in energy consumption. With the advent of new inorganic materials and processing techniques, there has been renewed interest in exploiting the benefits of membranes in many industrial applications. Zeolite membranes, however, have only recently been considered for catalytic membrane reactor applications. Despite the significant recent interest in these types of membranes there are relatively few reports of the application of such membranes in high-temperature catalytic membrane reactor applications. This can be attributed to a number of limitations that still need to be addressed such as the relatively high price of membrane units, the difficulty of controlling the membrane thickness, permeance, high-temperature sealing, reproducibility and the dilemma of upscaling. A number of research efforts, with some degree of success have been directed to finding solutions to the remaining challenges. This review makes a critical assessment of what has been achieved in the past few years in terms of hurdles that still stand in the way of the successful implementation of zeolite membrane reactors in industry.     

Dehydrocyclization of Alkanes Over Zeolite-Supported Metal Catalysts: Monofunctional or Bifunctional Route

The direct catalytic conversion of alkanes into aromatics has found potentially important industrial applications. Initially only alkanes with 6 and more carbon atoms in the chain were concerned. Supported platinum catalysts were found active for the aromatization of alkanes; the drawbacks of these catalysts were their deactivation with time on stream and the existence of simultaneous parallel reactions. Much discussion has been published on the aromatization of C₆₊ alkanes. A bifunctional mechanism which involves both the metal and the acid sites of the support and a monofunctional mechanism involving only the metallic sites operate over, respectively, Pt supported on acidic support and Pt supported on nonacidic support. In the present review the mechanisms proposed for the aromatization of alkanes are described. Over monofunctional Pt catalysts two possible mechanisms prevail: 1,6 ring closure on the Pt surface involving primary and secondary C-H bond rupture, followed by dehydrogenation of the cycloalkanes into aromatics (1,5 ring closure to a lesser extent also contributes to aromatic production); or dehydrogenation of the alkanes into olefins, dienes, and trienes followed by thermal ring closure. Zeolites were found most suitable as support for preparing catalysts more active and more selective in the alkane aromatization. In addition catalysts based on noble metals supported on zeolite appeared more resistant against deactivation by coke. In this review the aromatization of hexane, heptane, and octane over Pt-zeolite catalysts is discussed in detail. Comparisons between different zeolite structures and different dehydrogenation sites are given. In particular a critical analysis of the results and interpretation concerning Pt-KL catalysts strongly suggests that the exceptional high selectivity towards aromatization of n-hexane exhibited by Pt-KL could not be explained by only the nest or constraint effect exerted by the channel dimension and morphology, not by only the terminal cracking properties, not by only the partial electron transfer from the zeolite support to the Pt particles, and not by only the Pt particle size. Zeolite structure also affects the aromatic product distribution, in particular when the alkane contains more than 7 carbon atoms. It is shown how Pt on medium-pore zeolites such as In-ZSM-5, silicalites will favor the aromatization of C₈ alkane isomers into ethylbenzene-styrene with respect to other C₈ aromatics. Aromatization of light alkanes, C₂-C₅, requires the increase of the hydrocarbon chain length up to 6 carbon atoms and higher, followed by cyclization reaction. Recently new processes to convert C₂-C₅ alkanes into aromatics have been disclosed, M2-forming from Mobil, Cyclar from BP-UOP, and Aroforming from IFP-Saluted. In general these processes use bifunctional catalysts possessing a dehydrogenating and an acid function. The catalysts consist of a metal ion or metal oxide supported on a microporous acid solid. In this review we analyze the results concerning mainly platinum supported on pentasil-type zeolite. It is shown that althoug Pt has better dehydrogenating properties as compared with gallium and zinc, the efficiency of catalysts based on Pt-ZSM-5 for light alkane aromatization is less because undersirable reactions such as hydrogenolysis and ethene (olefins) hydrogenation occur on the platinum surface, resulting in the production of unreactive alkanes, CH₂, C₂H₆. These drawbacks could be partially suppressed by alloying Pt and by increasing the reaction temperature.     

Selective O-Alkylation Reaction of Hydroquinone with Methanol over Cs Ion-Exchanged Zeolites

O-alkylation reaction of hydroquinone with excess methanol was performed by using alkali metal ion-exchanged zeolite catalysts in a slurry type reactor to substitute the solid zeolite catalysts for the homogeneous liquid phase catalysts. This was also done to produce selectively mono-alkylated 4-methoxyphenol, a valuable intermediate for the perfume, flavor, food and photo industries. The effects of the basicity of various zeolites and reaction conditions such as temperature, reaction time and the amount of catalyst on the catalytic activity and selectivity were tested to maximize the yield of 4-methoxyphenol. Thus far, 84% selectivity at 95% conversion of hydroquinone was obtained at the optimum reaction conditions (240 ‡C, reaction with 0.6 g catalyst for 16 h), which was thought to result from the strong basic property and shape selectivity of the Cs ion-exchanged NaX zeolite.