Post-synthesis alumination

Post-synthesis Alumination of MCM-41 by A1(N03)3 (I) Improvement in Acidity for Purely Siliceous MCM-41... 

The effects of post-synthesis alumination on purely siliceous MCM-41 material with A1(NC>3)3 on acidity have been studied by FTIR, NH3-TPD, and IPA decomposition reaction. The FTIR results of pyridine absorption show that both Lewis and Bronsted acid sites are increased by the post-modification. The amount of NH3 adsorbed on the alumina-modified MCM-41 samples increases with the loading of Al onto the surface of MCM-41. Due to the improved acidity, the alumina-modified MCM-41 materials show considerably higher catalytic activity for dehydration of isopropanol than purely siliceous MCM-41. In addition, XRD and N2 adsorption results show that all MCM-41 samples maintained their uniform hexagonal mesoporous structure well after they have been subjected to post-synthesis alumination with the loading of Al species on Si-MCM-41 varied from 0.1 wt. % up to 10 wt. % (calculated based on AI2O3). 

Table 1. The surface areas and pore properties of post-synthesis aluminated MCM-41...

The surface areas and pore parameters of AMM samples are listed in Table 1. It is found that, after post-synthesis alumination, the surface area of the resulting AMM material decreases. For PSM, the surface area is 1311 m2/g. With the increase of A1 content, the surface area of AMM samples drops gradually from 1211 to 997 m2/g. Meanwhile, the pore diameter and total pore volume decrease with increasing A1 content. Since the dioo spacing is not affected by post-synthesis alumination, the decrease of pore diameter indicates that the pore wall becomes thicker. The results... 

Although the surface area, pore diameter and total volume of AMM samples decreased (as a result of post-synthesis alumination), their pore size distributions are still very narrow. For example, Figure 2 shows that, even having an AI2O3 loading of 10 wt.%, AMM-10 has N2 adsorption-desorption isotherms similar to that of PSM material. The capillary condensation for mesopores is in a very narrow range of P/Po = 0.2-0.35. A sharp pore size distribution peak (25-30 A) is obtained from the isotherm. The results indicate that the uniform mesoporous structure of MCM-41 is still well maintained after post-synthesis alumination. 

The strength of the acid sites generated on the AMM samples is characterized by NH3-TPD. As shown in Figure 4, the amount of NH3 adsorbed on PSM is very low. After alumination, the amount of adsorbed NH3 increases with the amount of A1 species on the AMM samples. This increased amount of NH3 adsorbed on AMM samples shows that more acid sites have been created upon post-synthesis alumination. This result is in good agreement with that of FT study, where both the amount of Lewis... 

Post-synthesis alumination using A1(N03)3 as the precursor improves the acidity of siliceous MCM-41 materials significantly. FTIR results show that both Bronsted and Lewis acid sites are increased upon alumination. The number of acid sites increases with the Al content on MCM-41. NH3-TPD reveals the mild strength of these created acid sites. Due to the improved acidity, the catalytic activity for dehydration of isopropanol to propylene over these alumina-modified MCM-41 materials is considerably promoted by post-synthesis alumination. The results of XRD and N2 adsorption show that the enhancement of acidity for siliceous MCM-41 by postsynthesis alumination does not cause any serious structural deformation of the resulting material. 

The above results show that post synthesis alumination of PSM with AlfNOjfi improves the hydrothermal stability of the resulting AMM material. Similar effect has been observed by Mokaya et al. [12], who reported that the hydrothermal stability of MCM-41 could be enhanced by reaction with chlorohydrate of aluminium. Moreover, from the study of high Si/Al ratio of Y zeolite, Lutz et al. [13] reported that the hydrothermal stability of Y zeolite was enhanced by an external introduction of non-structural aluminum species onto the surface of Y zeolite. The surface layer of Al-rich aluminosilicate or aluminum oxide was suggested to block the terminal OH groups and energy-rich =Si-0-Si= bonds on the surface of Y zeolite, hence minimizing the attack of water molecules on the framework. Due to these properties, the non-structural... 

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. 

The disparity in size of the aluminate and the silicate tetrahedra must be the reason why, at least for some frameworks, the range of Si/Al ratios, and therefore the extent of the post-synthesis isomorphous substitution of Al for Si is limited (27). For boron, with the ionic radius of 0.23 A as compared with 0.51 A for aluminium, the disparity in size is even greater (2). Quantum chemical calculations predict that the tetrahedral coordination of aluminium is favoured in comparison with BO4 groupings (32.33). An attempt to insert boron into the framework of ferrierite (34). a structurally related zeolite, was unsuccessful. 

Titanium Silicalite-2 (TS-2), structurally similar to TS-1, could be prepared likewise using tetrabutylammonium hydroxide as the template [13, 14]. Titanium aluminum Beta (Ti,Al-[3) was prepared by hydrothermal synthesis from amorphous silica, sodium aluminate, tetraethyltitanate and tetraethylammonium hydroxide [15]. The presence of A1 was necessary for the crystallization of the product. Al-free Titanium Beta (Ti-[3) could be obtained in the presence of particular templates, such as dibenzyldimethylammonium hydroxide [16]. Titanium Mordenite (Ti-MOR), conversely, was obtained by post-synthesis insertion of Ti to dealuminated Mordenite [17]. Ti-MWW (Ti-MCM-22) was obtained by the synthesis of the lamellar precursor of Ti,B-MCM-22 followed by acid treatment to remove most of the boron and extra-framework Ti and finally calcination to burn out the template and bring about the condensation of lamellae into the three-dimensional MWW structure [18]. Ti is present in a number of different environ-... 

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. 

Post-synthesis alumination of siliceous forms of MCM-41 and MCM-48 can be readily achieved by reaction of parent materials at low temperatine in nonpolar solvents with various aluminum sources. Among the latter, trimethyl aluminum [113], aluminum trichloride [114], aluminum chlorohydrate [115] aluminum isopropoxide [116,117] and aluminum acetylacetonate [118,119] have been used After calcination, the resulting materials preserve the typical narrow pore distribution of the parent mesophase and Al NMR measurements reveal the presence of aluminum atoms in both tetrahedral and octahedral configurations generating Bronsted and Lewis acid sites with a strength comparable to that of mesostructures of similar composition prepared by direct synthesis. 

As a promising step in food analysis, post-synthesis aluminated mezoporous zeolite molecular sieve, AlMCM-41, was used as neutral ionophore for the preparation of aluminum ISEs by Arvand and coworkers [78]. They found that the best performing measuring membrane contained 30% PVC, 67% aminophen as a plasticizer, and 3% ionophore. The slope of the linear range of the electrode response was 19.5mV/decade, while the dynamic range reached from 10 to 10 M. Methods were worked out and tested for measuring the Al " content of rice, mushroom, potato, and Mg-Al syrup. 

Chen LY, Ping Z, Chuah GK, Jaenicke S, Simon G. A comparison of post-synthesis alumination and sol-gel synthesis of MCM-41 with high framework aluminum content. Microporous Mesoporous Mater 1999 27 231-42. 

Although materials prepared by post-aliunination procedures did not feature acid sites stronger than those obtained by direct synthesis, the catalytic activity of the former (evaluated for ciunene cracking [115,116], isopropanol dehydration [120,121] or aldehyde condensation [117]) was found systematically higher than that of the latter. Such a result must be attributed to a better accessibility of the acid sites in the case of post-aluminated materials. This fact was actually shown by CoUart et al. [118,119] in the case of [Si]-MCM-48s treated with Al(acac)3. The munber of acid sites titrated by ammonia amounted to 100% and 68% of the total number of aliuninum atoms deposited in samples containing 1.2wt% (Si/Al = 37) and 5.1 wt% (Si/Al = 17) Al, respectively. 

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