Extra-framework aluminum

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Dealuminated samples were obtained by hydrothermal treatment of calcined MCM-22 (Si/Al = 15) at different temperatures (673, 773, 873 K) for 2-24 h under a saturated flow of a nitrogen/steam mixture (flow rate of 200 ml min"1). These steamed samples were further treated with 6N HNO3 solution at 353 K for 4 h in order to remove the extra-framework aluminum species. 

III.B.3. Lewis Acid Sites and Extra-Framework Aluminum. 260... 

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

For weak acceptors like charge balancing cations (Na, K, etc.), the interaction with the lone pair on the nitrogen is weak and results in a absorbance band around 1440cm . For stronger acceptors like extra-framework aluminum atoms (Lewis... 

Al MAS NMR has been demonstrated to be an invaluable tool for the zeoHte sdentist It provides a simple and direct way to quantify the proportions of A1 in four [Al(4)j, five [Al(5)j and six [Al(6)j coordinations. Quantitative determination of these species is an important issue in catalysis, and major effort is devoted on this topic. As mentioned already, for A1 only the central transition (-i-half to —half selective exdtation ) is detected. The central transition is unaffected by first order quadmpolar interaction, but the presence of second order effects causes broadening and complicates the quantitation of the A1 species. Usually hydrated samples and short radiofrequency pulses are employed for quantitative determination of framework and extra framework aluminum species. It is uncertain whether hydration changes the coordination of A1 species. Certain extra framework A1 can have very large quadmpolar interactions resulting in very broad lines ( NMR invisible ) [155, 202]. Unlike Si NMR, Al has a short relaxation time due to its quadmpolar nature, and the Al NMR spectrum with good signal to noise can be obtained in a relatively short time. 

In addition to the thermal and ion exchange treatments described above, another rich area in the study of zeolite stabilizahon and acidity modification involves a broad class of post stabilization chemical treatments to remove extra-framework aluminum, and silicon, species parhally or completely after steaming. In actual commercial practice, such treatments primarily involve use of mineral acids such... 

It is generally accepted that Lewis acidity in zeolites is due mainly to extraframework aluminum (16,17,18). Consequently, Lewis acid sites measured by pyridine adsorption must correlate with extra-framework aluminum. In Table I, the amount of pyridine coordinated to Lewis sites decreases for samples with the lowest Si/Al ratio, showing that, after thermal treatment, the amount of extraframework aluminum decreases with Si/Al ratio of the Beta zeolite. 

When the thermokinetic parameter was plotted versus the amount of NH3 adsorbed for samples of H-ZSM5 (Si/Al = 10.3) pretreated at 400 and 800°C it was found that the maximum time constant is higher for the sample pretreated at 800°C than for that pretreated at 400°C [103]. In fact, the increase of the pretreatment temperature caused dealumination extra-framework aluminum species were created that restricted the access to the channels and created diffusional limitations. 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

When zeolite NH4-Na-Y was treated at 400°C under DB conditions a decrease in the number of observable Al atoms was found as the degree of ammonium exchange increased from 0 to 90 %. In the latter case, only ca. of Al present in the zeolite is observed by 27A1 NMR (see Table XIV). The authors estimate vQ > 1.2 MHz for the unobservable Al. However, extralattice Al can be detected by contacting the zeolite with a 38% solution of acetylacetone (Hacac) in ethanol, whereupon mobile Al(acac)3 complexes are formed, and a very narrow 27A1 NMR line results the solution does not affect framework aluminum. It was found that the amount of six-coordinated (i.e., extra-framework) Al increases from 5 % in 84 De Na-Y 300 SB zeolite to 50% in 84 De Na-Y 500 DB zeolite (in this notation the first number refers to the... 

Numbers of OH Groups per Cavity (4 of Unit Celt), Framework Aluminum Atoms (Al, ), Extra-Framework Aluminum (A/A — -4/gS J, and Aluminum in the Form of Mobile Hydrated Complexes... 

Al(acac)a is the number of extra-framework aluminum atoms per cavity which can be extended using an acetylacetone/ethanol solution. 

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