Post-synthesis modifications

11-0-01 - Gold-based mono- and bimetallic nanoparticles on HY zeolites 

Guillemot, M. Polisset-Thfoin, D. Bonnin and J. Fraissard Laboratoire SIEN, Universite Paris VI, Paris, polisset(a).ccr. iussieu. fr. France fax 01.44.27.55.36 

11-0-02 - Unravelled from the back kinetics of alkoxysilane CVD on zeolites and evidence for pore mouth plugging determined from model conversion over stepwise silanised samples 

Chemical vapor deposition (CVD) of tetraethoxysilane on HZSM5 was performed stepwise under well-controlled, mild conditions. Several test reactions were performed over the series of modified samples. Under mild conditions, CVD follows first order kinetics with respect to uncovered external sites on the zeolite crystals. The external surface is homogeneous with regard to both CVD and catalytic activity. Reactions, which are controlled by strong internal mass transfer restrictions, do respond in a way, which indicates that CVD causes pore mouth plugging rather than pore mouth narrowing. 

11-0-03 - Templating role of F towards D4R units study of the transformation of the fluorogallophosphate Mu-3 into Mu-2 

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Karge FI G 1997 Post-synthesis modification of microporous materials by solid-state reactions Stud. Surf. Sol. Catal. 105 1901-48... 

Karge, H.G. and Weitkamp J. (eds) (2002) Molecular Sieves 3 - Post-synthesis Modification I, Springer-Verlag, Heidelberg, pp. 203-255. 

This indicates that post-synthesis modification of MOFs by coordinating hydrophobic ligands to unsaturated metal sites may be a powerful method to generate new sorbents for gas separation under humid conditions. Amine functionalization to target the water stability of MOFs will be further discussed in the next section. 

Research efforts have thus been made to improve the hydrothermal stability of PSM by either changing the synthesis procedure or post synthesis modification. The addition of various sodium salts into the gel mixtures was reported to form MCM-41 having disordered structures, making MCM-41 relatively stable in boiling water for 12 h [5]. The hydrothermal stability of MCM-41 was also found to be improved by adding cations such as tetraalkylammonium or sodium to the synthesis gel [6] or by modification with organic molecules in the synthesis [7], MCM-41 could be stabilized... 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

Post-synthesis modifications have been successful in preparing titanium-containing molecular sieves active in oxidation. The method employed for the post-synthesis incorporation of Ti to the zeolite beta was also applied to the incorporation of Ti into MOR and FAU [60]. 

The incorporation of Ti into the MOR structure was confirmed by the appearance of the specific absorption band in the IR spectra [20]. Very recently, Kubota et al. have synthesized Ti-YNU-2 (MSE) [25] by the post-synthesis modification ofYNU-2 (P) that has a large number of defect sites [87]. Ti-YNU-2 has proved to be a very active catalyst in liquid-phase oxidation using H202 as oxidant [25]. 

Direct synthesis of metal-substituted zeolites has long been sought. However, since the post-synthesis modifications can be made under wide-ranging conditions (temperature, solvent, atmosphere, pH, etc.) far from those for the zeolite synthesis, the modifications of zeolites present us with powerful indirect methods for manipulating the properties of zeolites. Therefore, the fine-tuning of the properties of zeolites will continue to be achieved by developing various post-synthesis modification procedures as well as direct synthetic techniques. 

This paper demonstrates, how the post-synthesis modification of the zeolite crystals by silytation affects the concentration and composition of products in the pores and enhances the selectivity to p-xylene. It will also be shown that the (post-synthesis) induced deactivation of the catalyst in the course of the reaction is caused by formation of polymethylated aromatic molecules that may block the active sites and /or the pores of HZSM5. 

The calcined OMSs used in post-synthesis modification possessed a hexagonal (MCM-41 and SBA-15) or cubic (MCM-48) arrangement of mesopores. Benzoyl isothiocyanate was reacted with aminopropyl-modified surfaces of MCM-41 and MCM-48 to give the OMM-1 and OMM-2 samples, respectively.15,44... 

Post-synthesis methods (pore-size engineering) allow an existing shape-selectivity effect to be intensified, and also a new one to be established. However, normally not only the pore size will be influenced by most of these methods, but also the catalytic activity. Vansant [104] gives a classification of post-synthesis modification methods which covers the entire range of zeolite applications (gas separation, gas purification, encapsulation of gases and catalysis). 

At present, the oil refining industry is faced with important challenges, such as the processing of heavier and more contaminated crudes, the increasing demand for higher quality transportation fuels with reduced emissions of contaminants, and the need for more petrochemical feedstocks (e.g. olefins, aromatics). In this context, there is no doubt that zeolites (and related molecular sieves) can help refiners to achieve the new goals. Recent advances in zeolite synthesis and post-synthesis modifications are expected to contribute to the development of improved catalysts and processes. 

Similarly, zeotype molecular sieves are synthesized by mixing the basic ingredients with the organic template, e.g. aluminophosphates are prepared from alumina and phosphoric acid. Other main group or transition elements can be incorporated into the framework by adding them to the initial sol-gel. Alternatively, different elements can be introduced by post-synthesis modification (see later), e.g. by dealumination followed by insertion of the new elements into the framework position [31]. 

The common means of introducing redox catalytic activity in zeolites is by the substitution of framework atoms such as Si, A1 or P with redox-active metal cations. This has been accomplished by two different methods (1) hydrothermal synthesis and (2) post-synthesis modification. Irrespective of the method of preparation, with the notable exception of titanium silicalites, these redox metals in the framework are susceptible to leaching due to the solvolysis of M-O bonds [77]. Even the Ti silicalites suffer from leaching under basic conditions [76a]. 

Post-synthesis modification involves isomorphous substitution of framework atoms with the desired redox metals either in aqueous media with soluble metal salts or in the gas phase with volatile chlorides. Incorporation of Ti into the framework of faujasite, zeolite-) and ZSM-5 has been accomplished by treating the zeolite with ammonium titanyl oxalate, TiCU or Ti(0/-Pr)4. Substitution of V for framework atoms has been reported with VOCI3 vapor. A more generalized method involving the reoccupation of the silanol nests created by the deboronation of bor-osilicates (ZSM-5 and zeolite- ) shows considerable promise for the incorporation of redox metals into the framework [79]. 

Another consequence of the rather limited number of molecular sieves that are used, is a high emphasis is being placed on post synthesis modification of molecular sieves. Especially, the introduction of a secondary pore structure (such as achieved for mordenites [263]), the deactivation of outer crystal surface and the adjustment of the acid strength by selected ion exchange procedures are examples for that approach. 

A phosphoramidite monomer derived from (71) has been introduced into ODNs for post-synthesis modification. The ketone was treated with a number of aminooxy derivatives, and it was shown that the modification had little effect on duplex stability. This may be used to decorate DNA for a number of DNA applications. A series of modified dUTP analogues (72) which can incorporate a variety of amine derivatives, including biotinylated and fluorescent derivatives, has been prepared for study in PCR reactions. It was found that KOD Dash DNA polymerase was able to incorporate the modified triphosphates during PCR whilst other conventional DNA polymerases would not. 

Halo-dU derivatives have been used in DNA duplexes as they undergo photochemical crosslinking reactions. 5-Iodo-dU has been used in crosslinking reactions in Z-form DNA, and 4-thio-5-bromo-dU has also been prepared and incorporated into ODNs where it was demonstrated that cells containing it became sensitive to UVA light. The formation of a crosslink between Br-dC and dG has also been reported. 5-Iodo pyrimidines as well as 7-iodotuberci-din have been used for post-synthesis modification whilst on solid support for Pd-catalysed substitution by an alkynylated spin label. ... 

In addition to hydrothermal procedures, incorporation of Ti into zeolites framework may be achieved by different post-synthesis modifications such as the treatment of alumino- or... 

Titanium-containing pure-silica ZSM-48 (e.g., [71, 72]), a unidimensional medium pore zeolite, and titano-aluminosilicates with the structure of zeolite Beta [72-74] are materials which are currently scrutinized in catalytic oxidation reactions [75]. In the latter case, however, residual acidity created by framework aluminum leads to undesired side reactions. Since, so far, the direct synthesis of Al-free pure titaniumsilicate Beta was not successful, van Bekkum et al. [76] developed a special post-synthesis modification technique. The three-step procedure... 

Non-contact atomic force microscope (AFM) and N2 absorption measurements on beta zeolites reveal the extreme irregularity of the external crystal surface which can make up a considerable proportion of the total surface area. A catalytic test, the acylation of 2-methoxynaphthalene, shows that active sites on the outer surface play an important role in the catalytic activity of the zeolite. Attempts to influence the external surface area and its catalytic activity through synthesis or post-synthesis modification such as dealumination show that the principle influence on the external surface comes from the synthesis procedure. 

The use of organometallic molecules as SDAs is promising due to several other interesting properties. Bein [40] has recently reviewed the chemistry and applications of zeotype/organometallic composites that were prepared by conventional post-synthesis modifications. In most cases, the results apply to directly synthesized composite structures as well. For example, organometallic molecules can serve as valuable precursors for catalytic systems that are immobilized inside the zeotype framework and thus profit from the shape-selectivity that makes zeolites themselves important catalytic systems. 

J.P.M. Niederer and W.F. Hoelderich, Isomorphous Substituted Early Transition Metal Containing BEA Vin Post-Synthesis Modification of H-[B]-BEA. Proceedings of the 12th International Zeolite Conference, Part III, ed. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, and... 

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