Post-synthesis treatment

Preparation variables are very important in determining the physical properties of LDHs in some cases, poorly crystallised materials are obtained and thus post-synthesis treatments need to be applied. 

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Modulation of zeolite acidity by post-synthesis treatments in Mo/HZSM-5 catalysts for methane dehydroaromatization... 

Post-Synthesis Treatment of Zeolites and Modification of Zeolites.105... 

Understanding the adsorption, diffusivities and transport limitations of hydrocarbons inside zeolites is important for tailoring zeolites for desired applications. Knowledge about diffusion coefficients of hydrocarbons inside the micropores of zeolites is important in discriminating whether the transport process is micropore or macropore controlled. For example, if the diffusion rate is slow inside zeolite micropores, one can modify the post-synthesis treatment of zeolites such as calcination, steaming or acid leaching to create mesopores to enhance intracrystalline diffusion rates [223]. The connectivity of micro- and mesopores then becomes an... 

Corma, A., Martinez, A., Arroyo, P.A., Monteiro, J.L.F., and Sousa-Aguiar, E.F. (1996) Isobutane/2-butene alkylation on zeolite beta influence of post-synthesis treatments. Appl. Catal. A, 142,... 

To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po 0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). 

Ti-silicate materials were prepared utilising automated robotic equipment. The preparation required three steps (1) gel synthesis, (2) material crystallization under hydrothermal conditions, and (3) post-synthesis treatment that includes at least one of calcination, organic extraction and silylation. 

MF1 type zeolites are known to show enhanced selectivity to p-substituted products in alkylation and isomerization of aromatic molecules [e.g. 1,2,3,4]. This shape selectivity is more pronounced with larger zeolite crystals and can be further enhanced by modification of the parent zeolites through post-synthesis treatments like impregnation with basic oxides, metal salts or the deposition of silica or coke [3,5,6,7]. The gain in selectivity is, however, usually accompanied with loss in catalytic activity and in some cases more rapid deactivation [8,9]. Despite the large number of patents and reports in open literature, the reasons for the enhancement of shape selectivity of MFI zeolites by post-synthesis treatment and the limits of the severity of this treatment are not unequivocally explained to date. 

Post-synthesis treatment of SBA-15 by silanation with [NIt R3]+Cl-(R/ = silane) gives a cationic function, which has allowed adsorption of Au(OH)nClJ n anions at various values of pH.147,148 These methods give usefully small particle sizes (<2.5 nm) and sometimes quite high metal loading (16wt.%), but they exhibit poor catalytic properties in oxidation of carbon monoxide148 (Section 6.3.3.4). 

Afterwards, the synthesis of various new zeolites, especially ZSM5 (MFI, 1967), the discovery of new shape selective transformations such as the (accidental) discovery of the remarkably stable and selective conversion of methanol into gasoline range hydrocarbons over HZSM5 (7), the development of post-synthesis treatments of zeolites,. .. combined to make them the single most important family of catalysts used all other the world. 

Each zeolite type can be easily obtained over a wide range of compositions directly by synthesis and/or after various post synthesis treatments. Moreover, various compounds can be introduced or even synthesized within the zeolite pores (ship in a bottle synthesis). This explains why zeolites can be used as acid, base, acid-base, redox and bifunctional catalysts, most of the applications being however in acid and in bifunctional catalysis. 

Direct Synthesis the crystallization is done in the presence of organic templates (2) such as TBAOH, crown ethers,. ..These syntheses unfortunately suffer from two economic drawbacks, i.e. the long synthesis times (between 3 to 8 days) compared to the classical synthesis (typically 16 hours) and the use of costly, sometimes toxic, and not always recyclable organic molecules Hydrothermal Treatment a post-synthesis treatment widely practiced in the industry because it also modifies the texture (creation of desired mesoporosity) of the zeolite. Such a treatment also takes place in the regenerator of every FCC unit... 

Several articles on synthesis and characterization of Mo/V/M/O (M = Te, Sb, Al) catalysts have been published recently [28]. The effects of metal oxide precursor sources, synthesis conditions and post-synthesis treatments are subjects of current studies. Asahi [29] has modified the composition of the Mitsubishi catalyst, by incorporation of Sb in place of Te in the Ml phase this catalyst is more stable than the original one, and hence provides longer catalyst lifetime. 

EXAFS. Au(CH3)2(acac)/MgO. X-ray absorption fine structure spectroscopy (EXAFS) was used to study the local structure of MgO-supported gold samples prepared by decorating the MgO with Au(CH3)2(acac). These studies were completed for the sample without any post-synthesis treatment ( initial structure ) and for samples developed upon heating this sample in either He or H2 atmospheres. 

Zeolite beta samples with different framework and extraframework composition have been prepared by submitting the acid form of a commercial TEA-beta sample to different post-synthesis treatments, i.e. steam calcination, acid (HCl) leaching, and ammonium hexafluorosilicate (HFS) treatment. The samples were characterized by XRD, adsorption of N. at 77 K. i.r. spectroscopy with adsorbed pyridine, Si and Al MAS-NMR and XPS. Bifunctional catalysts were obtained by impregnation with 0.3 wt% Pt, and the catalytic activity for the isomerization of a simulated LSR feed (n-Cj/n-Cj, 60/40 wt%) was measured under different reaction conditions. 

Recently, bifunctional Pt/beta catalysts have shown a good performance for LSR isomerization (9). Its catalytic activity was seen to depend on the synthesis condition of the zeolite (10). In this work we have studied the influence of post-synthesis treatments, i.e. framework dealumination and EFAL extraction, on the activity and selectivity of Pt/beta catalysts for LSR isomerization. 

The fact of transformation of SBA-8 synthesized at room temperature readily into MCM-41 through post-synthesis treatment confirms the intermediate nature of this phase. SBA-8 (c2mm) can be induced to transform into MCM-41 (p6mm) in the mother synthesis solution at 70-100 °C. When the as-made SBA-8 is treated in water at 100 °C (pH 7-10), the phase transformation to MCM-41 is completed within 2 h. 

In fact, since the mesophase synthesis or reaction is a kinetically control process and the solid formed is not a thermally stable phase, the phase transformation is very common during the synthesis of mesoporous materials. The phase transformations include the transition from one structure or symmetry into another structure or symmetry, or the transition from a disordered phase to an ordered phase, or from an ordered phase to a disordered phase. The intermediate phase can be isolated as a product or be observed by analysis techniques. The phase transformation can occur during the synthesis process or in a post-synthesis treatment. The early famous example is the transition of lamellar mesophase of silicate into hexagonal mesophase in basic or near neutral media.[5]... 

Some synthesis methods can be considered to be modifications of the general EISA synthesis. For example, the direct calcination of wet surfactant-silicate gel and some post-synthesis treatments (phase transformation). 

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