As-synthesized zeolites

In zeolite synthesis (ref. 2) an aqueous mixture containing a silicon source, an aluminum source, an alkali source (usually NaOH) is autoclaved and subjected to hydrothermal treatment. Hydrated Na-ions are then filling the pore system in the as-synthesized zeolite. In the case of relatively high Si/Al zeolites an organic template is required which is usually a tetraalkylammonium compound, applied as the bromide or the hydroxide. 

The aluminum content of the as-synthesized zeolites also influences their X-ray powder diffraction pattern. The height of the main peak in the patterns decreases with decreasing Si/Al ratio in the zeolite, but their width increases simultaneously so that the area remains practically constant for all samples. On the other hand, the dhki distance corresponding to the diffraction peak at 43 of 20 correlated linearly with the aluminum content of the zeolite (Figure 4). However, the lack of knowledge of the crystal structure of Beta zeolite makes it impossible to correlate the Al content and unit cell parameters. 

Si spin-lattice relaxation in zeolite ZSM-39 (97) and a range of zeolites ZSM-5 (134) have been studied. As-synthesized zeolite ZSM-5 with TPA+ as template has very long Tj (up to 145.1 sec for Si/Al = 5000) in a 300 MHz magnet. When 1,6-hexanediol is used as template, Tl was as short as 2.6 sec. Calcination causes a marked reduction of T (to several seconds) in all samples. Further reduction of was observed when cyclohexane or benzene were adsorbed. 

The hydrogen forms of the ZSM-5 and ZSM-11 samples were prepared from the parent as-synthesized zeolites ( ) by drying at 125°C, calcining in nitrogen... 

Y-type zeolite prepared by steaming a, Y-type zeolite dealuminated with ammonium hexafluorosilicate , after La3+ exchange to level of maximum activity , ZSM-20 g, after La3+ exchange to level of maximum activity v, as synthesized zeolite Y , after exchange to level of maximum activity , HLa-X. Reproduced with permission from Ref. 20. 

After the synthesis procedure, the zeolite should be calcined to remove the organic compounds that are blocking the pores. Calcination entails heating the as-synthesized zeolite in an air flow to a temperature normally ranging from 350°C to 400°C. 

Powder x-ray diffraction (Siemens D-500 diffractometer) and scanning electron microscopy (JEOL, JSM-840) methods were used for characterization of zeolite products. Infrared spectra (Perkin Elmer 783 infrared spectrophotometer) were used for investigation of nucleation gel. The amount of Si, Al, Na in zeolite products was analysed by conventional analytical method, i.e. gravimetric for Si, gravimetric and volumetric methods (titration by EDTA) for Al and flame photometric method for Na. The WL value in Table 1 was used to estimate the amount of water and organic species in an as-synthesized zeolite product. 

Figure 4.45 SEM images of an as-synthesized zeolite coating on AI-2024-T3 a) top view b) cross-sectional view. Reproduced with permission from [155], Copyright (2001) Electrochemical Society Inc.

ZSM-5 zeolite used in this work was synthesized according to a method described in the patent [16]. The structural, morphological and acidity features were characterized by XRD, Al MAS-NMR, SEM, and temperature programmed desorption of ammonia respectively. The Si/Al ratio was determined by a combination of wet chemical analysis and atomic absorption spectrophotometry. The as-synthesized zeolite was converted to active proton form as reported in our earlier work [12]. 

The crystallization time was 166 h. The zeolite T crystals obtained showed an euhedral morphology with the largest dimension varying from 1 to 4 nm. As-synthesized zeolite T was ion exchanged with 1 N NH4CI solution under reflux conditions. 

ZSM-22 (TON) can be produced from potassium based 1,6 hexamethylenediamine synthesis mixture, while, ZSM-34 (an OFF/ERI intergrowth) is synthesized from the same synthesis mixture with the addition of sodium. NMR analyses of the as-synthesized zeolite samples indicated the presence of a carbonyl species in all of the ZSM-34 samples, but not in any of the preparations that produced ZSM-22. The carbonyl species was present only after the synthesis mixture was heated above ambient temperature. Molecular modeling studies calculated a favorable fit of the carbamic species within the pore system of the Erionite suggesting a reason for the formation of the OFF/ERI intergrowth. 

Examination of the as synthesized zeolite by scanning microscopy enables one to distinguish crystalline and amorphoous materials. This is particularly usefull to monitor the presence of oxide species of the modifying element. This technique is particularly effective when associated with chemical analysis. This would help discriminate the modified zeolite and possibly amorphous and sometimes crystallines associations of the starting elements (oxides, silicates, etc...)... 

Solid tate 2D refocused INADEQUATE Si Si double quantum and 2D Si H) HETCOR (dipolar-mediated) NMR spectra have been used by Chmelka et al to characterize two as-synthesized zeolites, ITW and MTT. The obtained results, in combination with synchrotron X-ray diffraction analyses, allowed the authors to establish the Si-O- Si site eonneetivities providing insight on the complicated order and disorder within the silieeous zeolite framework structures. 

The diffuse reflectance spectroscopy (DRS) has been used to investigate structural modifications of mazzite zeolite subject to calcination, acid leaching, and acetylacetone treatments [OlZl] - Fig. 63. The diffuse reflectance spectra of as-synthesized sample consists of a single band at = 220 run, which arises from Al-0 charge transfer of foitr coordinated framework Al, characteristic of as-synthesized zeolite [82G1]. Upon... 

H-Beta, H-mordenite and H-MFI zeolites have been prepared from as-synthesized zeolites using classical modification procedures ionic exchanges in NH4NO3 solutions followed by calcination under air at 823K. The main physicochemical characterists of the three solids are given in table I. 

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