Zeolite theta

High-resolution EM also showed that the synthetic zeolitic catalyst ZSM-23 (MTT) is a recurrently twinned version of the synthetic zeolite theta-1 (TON) (51). It is noteworthy that the elucidation of the structures of zeolite beta, for a long time an enigma and problematic for X-ray crystallographers, came only through the application of HRTEM (50). 

Fig. 26. J9Si MAS NMR spectra (471) at 79.80 MHz of the novel zeolite theta-1 (439) (top) and its hydrothermally dealuminated form (bottom).

Translation (T) This operator describes the vertical translation of the sheet along the stacking direction. Zeolite theta-1 is constructed in this manner, which is illustrated in Figure 7.5. 

Figure 2.10 Comparison of the structures of the lOMR zeolites Theta-1 and ZSM-23 shows that ZSM-23 is related to Theta-1 by unit cell twinning. The photograph illustrates the unit cells of the two related structures.

Figure 11.2 Structures of four selected zeolites (from top to bottom faujasite or zeolites X, Y zeolite ZSM-12 zeolite ZSM-5 or silicaUte-1 zeolite Theta-1 or ZSM-22) and their micropore systems and dimensions [9].

Alumina is an amphoteric catalyst, which can difficult to characterize via chemical and physic methods. The transformation of cyclopentanol/cyclohexanone mixture allows us to estimate at the same time the acid-base properties of aluminas. From this transformation, it was shown that aluminas can be classified into two families only basic aluminas, such as theta, which were more basic than MgO, and acido-basic aluminas, eta, gamma and delta, which possess an acidic character less pronounced than dealuminated HMOR zeolite... 

Bell et al. (81) presented forced diffusion calculations of butene isomers in the zeolite DAF-1. DAF-1 (82) is a MeALPO comprising two different channel systems, both bounded by 12-rings. The first of these is unidimensional with periodic supercages, while the other is three-dimensional and linked by double 10-rings. The two channel systems are linked together by small 8-ring pores. It is a particularly useful catalyst for the isomerization of but-l-ene to isobutylene (S3) its activity and selectivity are greater than those of ferrierite, theta-1, or ZSM-5. 

Possibly the earliest theoretical study of diffusion of aromatics in zeolites was published in 1987 by Nowak et al. (89), who considered diffusion of benzene and toluene in the pores of silicalite and theta-1. Theta-1 (90) has a unidimensional medium-sized pore opening bounded by 10-rings. In this study, only the straight channel of silicalite was considered, making the... 

Zeolite Nu-10 was demonstrated to possess the identical framework topology than zeolites ZSM-22 (55), Theta-1 (57), KZ-2 (38) and ISI-1 (59). The complete crystal structure of Theta-1, ZSM-22 and of its Si end-member was elucidated (Table m). 

Preservation of the zeolite structure was verified by X-ray powder diffraction (XRD) patterns recorded on a CGR Theta 60 instrument using Cu Kcq filtered radiation. The chemical composition of solids was determined at the Service Central d Analyse CNRS (Solaize, France). Copper in the zeolite was characterised by DR-UV-visible spectroscopy using a Perkin-Elmer Lambda 14 apparatus, equipped with a reflectance sphere, and by temperature programmed reduction (TPR), using a Micromeritics Autochem 2910, equipped with a katharometer (3% H2/Ar gas mixture at 30 mL.min1 and 10 K.min 1). 

The main objective of the present work was to investigate the possibilities of direct (and selective) n-butane dehydroisomerisation into isobutene over Ga-containing zeolites. Another objective was to evaluate the role played by Ga and acid sites in this reaction. For this work such medium pore zeolites, as ferrierite (FER) and theta-1, were chosen because of their superior performance in n-butene isomerisation reaction.3,7 The modifying metal, Ga, was chosen due to the known high dehydrogenation activity of Ga-ZSM-5 catalysts in propane and n-butane conversions. 10 However, Ga-ZSM-5 catalysts were not used in this study because of their high aromatisation activity,8,9 which would not allow to stop the reaction at the stage of formation and isomerisation of butenes. 

Two medium-pore zeolites, namely, ferrierite (Si/Al = 6.3) and theta-1 (Si/Al = 30) were used in this work. They were used as catalysts in the H-form or were modified by gallium before catalytic experiments. In the latter case, Ga was introduced into the zeolites by an incipient wetness impregnation method, using aqueous solutions of Ga(N03)3. In this work, catalysts with a Ga content of 2.2 wt.% were investigated. 

Investigation of n-butane conversion over H-forms of the ferrierite and theta-1 zeolites demonstrated that the isobutene selectivities were similar (and low) for these catalysts. The maximum selectivities (7-8 %) were obtained at low n-butane conversions (5-10 %) and decreased with increasing conversion of n-butane due to olefin interconversion and aromatisation reactions. Isobutene was in equilibrium with the other butene isomers due to the high isomerisation activity of the parent zeolites. The maximum selectivity to butenes, which was observed at low conversions, was around 20 %. This value reflects a moderate contribution of the dehydrogenation steps in n-butane transformation over H-forms of the ferrierite and theta-1 zeolites and indicates an important role of the n-butane protolytic cracking steps over these two catalysts. 

In order to check this conclusion, we collected the IR spectra of the H-theta-1 and Ga-theta-1 zeolites after their calcination in vacuum. These spectra are shown in Figure 4 (spectra a and b), and demonstrate that there is practically no difference in the number of the acid sites in these two catalysts. However, this result does not prove that the number of the acid sites in these catalysts should be similar under reaction conditions. Indeed,... 

Conversion of n-butane into isobutene over theta-1 and ferrierite zeolites was studied in a continuous flow microreactor at 530°C and 100% n-butane as a feed. The zeolites were used as catalysts in the H- and Ga-forms. Insertion of Ga into the zeolites resulted in improved isobutene selectivities due (i) to an increase in the dehydrogenation activities and (ii) to a decrease in the cracking activities of the catalysts. The highest selectivities to isobutene (-27%) and butenes (-70%) were obtained with the Ga-theta-1 catalyst at n-butane conversions around 10%. These selectivities decreased with increasing conversion due to olefin aromatisation, which was enhanced considerably by the Ga species present in the catalysts. 

Despite many advances in analytical methods in recent years, the structural characterization of materials that only occur as microcrystals less than about 30 l in diameter remains difficult and laborious. High resolution electron microscopy in the lattice imaging mode is by far the most powerful tool in giving the direct evidence of structural details essential for modelling clues, as has been demonstrated in the cases of recent zeolite structure solutions of theta-l/ZSM-23 (26) and beta (27), in addition to ECR-1. X-ray diffraction methods provide the essential confirmatory data, and sorption molecular probing and various well established spectroscopic methods are useful ancillary tools. 

O Malley, P. J., and J. Dwyer (1988). Ah initio molecular orbital calculations on the siting of aluminum in the Theta-1 framework some general guidelines governing the site preferences of aluminum in zeolites. Zeolites 8, 317-21. 

Although a wide variety of catalysts have been employed to crack PE, zeolites have proven particularly effective. For example, Garforth et al. reported that activation energies (La) measured when PE was catalytically cracked by HZSM-5, HY, and MCM-41 were much lower than when no catalyst was present. [66] They concluded that HZSM-5 and HY have similar activities and that both of these zeolites were more effective than MCM-41. Manos and co-workers found that catalytic cracking of PE by HZSM-5 and HY was effective in producing gasoline size hydrocarbons in a laboratory semibatch reactor [67, 68]. Mordi and co-workers reported that H-Theta-1 and H-Mordenite... 

Mordi et al. [17, 18] Sealed reactor LDPE, PP HZSM-5, H-mordenite, H-theta-1 zeolite... 

With time on stream the activity of the zeolites for butene conversion decreases and the selectivity for isobutoie increases [150,157,159,160,165]. Ferrierite initially produces cracking and oligomerisation products [165] but with time on stream the rate of formation of these by-products decreases Pereas the rate of formation of isobutene first increases before showing a slow decrease. The still active and selective catafyst can contain up to 8 - 10 wt.-% coke wUch is aromatic in character (H/C =1) and wiiich reduces the accestible pore volume dramatically [160, 165]. In the case of Theta-1 it was observed by uting lower calcination tenq)eratures (325°C instead of 500°C) that the isobutene yield increased agnificantly from 4.6 mol-% to 25.5 mol-% [157]. This has been ascribed to residual tenq>late inside the pores [161]. 

F was used for the first time as mineralizer in the synthesis of zeolites under neutral or acidic conditions by Flanigen and Patton1941 Subsequently, Guth and Kessler further studied this new synthetic method.[95] Many high- or all-silica zeolites such as ZSM-5, ZSM-23, Theta-1, ferrierite, / , MTN, AST, UTD-1, ITQ-3, ITQ-4, and heteroatom (B, Al, Fe, Ga, and Ti)-substituted high-silica zeolites could be synthesized from an F synthetic system. In addition, most of the open-framework gallophosphates could be hydrothermally synthesized under the influence of F. ... 

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