POST-SYNTHESIS INCORPORATION

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 post-synthesis incorporation of aluminium into the lattice of pure siliceous zeolite-p was attempted using aluminium isopropoxide as aluminating agent in a non-aqueous environment. The XRD structural analysis of the Al-grafted materials showed an increase in the unit cell parameters which was associated with the insertion of aluminium into the framework. Quantitative multinuclear NMR investigation showed that the amount of framework aluminium incorporated into the zeolite lattice was related to the concentration of defect sites in the parent Si-p zeolite. This indicated that the alumination proceeds through a mechanism which involves the reaction between Al(OPr)3 and silanol groups at defect sites. Calcination after alumination led to the completion of the process, whereby octahedral-coordinated aluminium, (partially) attached to the framework, was transformed into tetrahedral-coordinated framework aluminium. 

The Ti was loaded using two methods direct incorporation into the synthesis mixture, and post-synthesis grafting. In all cases the Ti-loading was 1.5 -1.8 wt%. Selectivity towards the epoxide was always 100%. Table 41.1 summarizes the results comparing Ti-TUD-1 and Ti-MCM-41 for cyclohexene epoxidation (15). For the direct incorporation, Ti-TUD-1 is five times more active than Ti-MCM-41, even though they have equivalent surface area. However, the grafted MCM-41 is also more active than its as-synthesized counterpart. 

Purely siliceous SBA-15 was synthesized according to previously published method [2]. Al incorporation in SBA-15 lattice was performed either by direct synthesis for which we have applied an already known method, but improved in some details [3], or by post-synthesis according to the reported procedure [4, 5], Modified SBA-15 are denoted as Al-SBA-15. 

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. 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

A different approach to the substitution of metal atoms into the framework is the secondary synthesis or post-synthesis method. This is particularly effective in synthesizing metallosilicates that are difficult to crystallize from the gels containing other metal atoms or hardly incorporate metal atoms by the direct synthesis method. Substitution of Ti for A1 goes back to the 1980s. The reaction of zeolites with an aqueous solution of ammonium fluoride salts ofTi or Fe under relatively mild conditions yields materials that are dealuminated and contain substantial amounts of either iron or titanium and are essentially free of defects [58]. However, no sufficient evidence for the Ti incorporation has been provided. 

Post-synthesis gas-solid isomorphous substitution methods are also known [61]. Ti-beta essentially free of trivalent metals can be prepared from boron-beta. However, the gas-phase method is not efficient for Ti incorporation and could have some disadvantages such as the deposition of Ti02 [62],... 

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]. 

We have earlier addressed the problem of the post-synthesis insertion of aluminium in zeolites ZSM-5 (12) and Y (Hamdan, H. Sulikowski, B. Klinowski, J. T.Phvs.Chem.. (in press)). The substitution of gallium in silicalite-n has also been achieved (13). It was therefore of considerable interest to establish whether boron can also be incorporated into silicate frameworks after the completion of synthesis. We report isomorphous substitution of boron into zeolite ZSM-5 by mild hydrothermal treatment with borate species. 

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]. 

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. 

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]. 

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... 

Figure 8.47 Incorporation of organic functions in mesoporous silica (a) Surface grafting of organic functions on the mesopore walls by post-synthesis direct incorporation of organic functions by co-condensation of organosilanes (b) or bridging silasesquioxanes (c). Reproduced with permission from [51]. Copyright (2002) American Chemical Society...

---