Zeolites medium pore size

Among the medium-pore sized zeolites, perhaps the most studied are the pentasil zeolites, ZSM-5 and ZSM-11 (Figure 13). These zeolites also have three-dimensional pore structures a major difference between the pentasil pore structures and the faujasites described above is that the pentasil pores do not link cage structures as such. Instead, the pentasils are composed of two intersecting channel systems. For ZSM-5, one system consists of straight channels with a free diameter of about 5.4 x 5.6 A and the other consists of sinusoidal channels with a free diameter of about 5.1 x 5.5 A. For ZSM-11, both are straight channels with dimensions of about 5.3 x 5.4 A. The volume at the intersections of these channels is estimated to be 370 A3 for a free... 

We are interested in the role of organic ions and organic molecules in zeolite synthesis. The use of organic molecules in zeolite synthesis mixtures in addition to inorganic bases is known to result in the medium-pore-size low-aluminum-content zeolites, such as ZSM-5, that revolutionized several processes of importance to the oil-refining and petrochemical industry. 

Molecular sieve catalysts that have been used for the conversion of methanol to hydrocarbons fall into two general classifications. Most of the initial research was done using ZSM-5 (MFI), a medium-pore size zeolite with a three dimensional pore system consisting of straight (5.6 x 5.3 A) and sinusoidal channels (5.5 x 5.1 A). While most of this work was directed at the conversion of methanol to liquid hydrocarbons for addition to gasoline, it was found that the product slate could be shifted toward light olefins by the use of low pressure and short contact times. 

Among all varieties of zeolites, the zeolites with 10-membered oxygen rings in their structure and medium pore size, ZSM-5 and ZSM-11 are remarkably stable as acidic catalysts. 

A gas phase process for thymol manufacture using medium pore-sized zeolites (erionite, mordenite, or ZSM-23) as heterogeneous catalyst was developed in 1988. It was reported that reaction temperatures were lower (230-270°C) than in the older liquid phase process. Reaction pressure was reported to be normal or slightly elevated. 

Conversion over Unmodified H-ZSM-5 and H-US-Y - The influence of the pore structure of the zeolites on the product distribution of m- and p-cymene isomers has been studied in the presence of an unmodified shape-selective medium pore-sized H-ZSM-5-(55) and the unmodified H-US-Y zeolite at temperatures between 200 and 300 °C. In contrast to the H-US-Y, the p-cymene formation in the presence of H-ZSM-5 is favoured due to the shape selectivity. ... 

Large As for medium- pore-sized zeolites Common Mordenite... 

Diffusion in most zeolites cannot be accurately described as Knudsen diffusion since the number of collisions between molecules is large compared with that with the micropore wall. Only in the wide-pore zeolite faujasite does the diffusion constant show Knudsen-type behavior which follows an dependence. In the narrow and medium pore size... 

The results reported indicate that cyclohexanol appears to be a good test molecule for probing acidity, due to the relatively low deactivation and the possibility of tracing acid sites with different strength. Unfortunately, the molecule seems to be too large to enter the pores of small and maybe even medium pore size zeolites and, therefore, diffusional limitations might affect the kinetic rates measured. 

Dewaxing is the process aimed at the conversion of linear long-chain hydrocarbons while leaving intact the branched isomers. The different options of the technology were developed by Mobil, Chevron, and Akzo and are based on the shape-selectivity effects of ZSM-5 zeolite. Other types of medium-pore size zeolites (ZSM-23, B-ZSM-5) have been also used. 

Studied [269,272,273], Both ZSM-5 catalysts emerge as the best catalysts with the highest yields of hydrocarbon products and lowest coke formation [269], The aromatics yield tends to decrease in the order ZSM-5 >H-beta>H-mordenite>H-ferrierite/HY [273], Stefanidis et al. demonstrated that, in comparison to a range of amorphous catalysts such as alumina, zirconia/ titania, and magnesium oxide, ZSM-5 is more suitable for the reduction of undesirable compounds and production of aromatics in the upgrading of pyrolysis vapors from beech wood [274], The excellent performance of ZSM-5 is attributed to the important role of its medium pore size [269], Besides, Park et al. pointed out that ZSM-5 is more efficient than Y zeolites due to the proper distribution of strong acid sites [275],... 

According to the pore size, zeolites are classified into small- (pore size up to 5 A), medium-(pore size 5-6 A), and large-pore types (pore size 7-8 A). Typical representatives of the different types are zeolite A, ZSM-5, and the faujasites (X and Y), respectively [5], According to the International Zeolite Association Website, about 190 different framework types of zeolites, zeolitic silicates and phosphates with precisely estimated structures are known to date (2010) [6],... 

Small to medium pore size zeolites, such as H-clinoptilolite, H-of6etite or H-eri-onite, are efficient for the hydroamination of ethylene [51-54]. Ethylene and NH3 react at 360°C and 50 bar over H-clinoptilolite to give EtNHi only (11.4% conversion). There is a clear shape selectivity since propene and 1-butene as well as higher amines give rise to extremely low conversions [52]. In contrast to H-cIinoptilolite or H-erionite, H-offretite is effective for proprene hydroamination with NH, (7.2% conversion, 90% i-PrNH -i- 8% i-Pr2NH) [55]. Small pore size H-erionite is the best catalyst in terms of lifetime, conversion and selectivity for the synthesis of ethyl-amine [56]. The efficiency of H-clinoptilolite can be improved by acid or base plus acid treatment of natural clinoptilolite (18% conversion, EtNH2/Et2NH>20) [57]. 

The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

The silicoaluminophosphate (SAPO) family [30] includes over 16 microporous structures, eight of which were never before observed in zeolites. The SAPO family includes a silicon analog of the 18-ring VPI-5, Si-VPI-5 [31], a number of large-pore 12-ring structures including the important SAPO-37 (FAU), medium-pore structures with pore sizes of 0.6-0.65 nm and small-pore structures with pore sizes of 0.4-0.43 nm, including SAPO-34 (CHA). The SAPOs exhibit both structural and compositional diversity. 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

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