Aluminum distribution and properties of faujasites. Basis of models and zeolite acidity

Abstract

N a content. In the NaH-Ms with higher N a contents, the Na cation prefers position IV, which directly confronts the sorbed molecule in the channel. This produces a strong electrostatic field similar to that of pure Na-M and, therefore, results in a qst value close to that of pure Na-M as shown in Figure 3. On the other hand, Na may prefer position 11 in the NaH-Ms with its lower Na content. This produces a relatively weak electrostatic field as in pure H-M, resulting in as low a qst value as that of pure To solve question 3, we calculated the (ED + ER) values for three kinds of hypothetical zeolites which possess straight, cylindrical channels of 8-, lo-, or 12-membered rings stretching along the c axis as shown in Figure 6. Assuming that the oxygenoxygen distance and aOx value are 0.3 nm and 1.65 X m3, respectively, we summed up the contributions from -8000 oxygen atoms surrounding the origin contained in a cube of 6 X 6 X 6 nm3. The obtained ED + ER value at the positions on the line X (or Y) at Y = 2 = 0 (or X = 2 = 0) (see Figure 6) is shown in Figure 7. Figure 7 shows that the magnitude of the (ED + ER) value is a function of the size and shape of the zeolite channel. The l(ED + E R )~ value increases with a decrease in the channel size. This accords well with the result obtained for H-M and H-ZSM-5 as shown in Figure 5. The difference in the magnetic H-M.of the ED + ER value apparently observed between H-M and the hypothetical zeolite having the 12-membered-ring channel, or between H-ZSM-5 and the hypothetical zeolite having the 10-membered-ring channel, arises from the assumption that the oxygen-oxygen distance in the hypothetical zeolite is 0.3 nm. If we use the value of 0.25 nm for the distance which is close to the distance in H-M and H-ZSM-5, we can obtain for the hypothetical zeolites the same magnitude of (ED + ER) values as those for H-M and H-ZSM-5. Now it becomes quite certain that the channel size effects the magnitude of the (ED + ER) value. To obtain a I(ED + ER)I value smaller than the qst value = 3.5 kcal mol-' for Ar sorbed by H-ZSM-5, we should conclude that its a,, value is not 1.65 X m3 as already mentioned. This means that the electric charge of oxygen in H-ZSM-5 is smaller than that in H-M. The conclusion obtained above is fairly reaonsable if one notices the fact that H-ZSM-5 is more siliceous than H-M. Since the siliceous zeolite contains fewer aluminum atoms, the bonds between oxygen atoms and T (Le,, Si or Al) atoms in it become less ionic. m3 but smaller than 1.2 X Registry No. Ar, 7440-37-1; 02, 7782-44-7; N2, 7727-37-9; NO, 10102-43-9; CO, 630-08-0.Computational procedures are used to simulate aluminum distributions in faujasite frameworks assuming Loewenstein's rule is obeyed. Several models are used as a basis for these computations, and in each case the average occupancy, by aluminum, of "T" atoms second neighbor to each aluminum is calculated. Results so obtained are used in correlation of properties which depend upon zeolite acidity. IntroductionIn 1956 Loewenstein' propose...

The influence of extraframework aluminum on H-FAU catalyzed cracking of light alkanes

“…The catalytic activity and selectivity of these zeolites depend upon the type, the strength and the concentration of these sites. The individual role of these three parameters for rate and selectivity in hydrocarbon conversion has been vividly debated [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…The acid sites in H-ZSM5 (and other protonic forms of high silica zeolites), however, are unique as they 120 A: General 146 (1996) [119][120][121][122][123][124][125][126][127][128][129] are so remote from each other that mutual influences are minimal. However, zeolites with higher aluminum concentration in the lattice, such as H-FAU and H-MOR, have aluminum lattice atoms as next nearest neighbours, which supposedly decrease the strength of the individual sites [10][11][12]14].…”

Stabilizing Low‐Silica Zeolites through Aluminum Sulfate Assisted Cannibalistic Dealumination

“…During the dealumination, the next nearest neighbor aluminum atoms in four‐membered rings (NNN‐Al) are more easily removed than isolated aluminum atoms with no neighbor aluminum atoms (0‐NNN‐Al) . The removal of NNN‐Al affirmatively results in a decreased amount of Si( n Al) ( n ≥2) units, whereas removal of 0‐NNN‐Al has a much lower chance of reducing these Al‐rich units . If different dealumination methods remove the same amount of FAL, the one with the higher selectivity for removal of NNN‐Al may contain fewer Si( n Al) ( n ≥2) units .…”

“…The removal of NNN‐Al affirmatively results in a decreased amount of Si( n Al) ( n ≥2) units, whereas removal of 0‐NNN‐Al has a much lower chance of reducing these Al‐rich units . If different dealumination methods remove the same amount of FAL, the one with the higher selectivity for removal of NNN‐Al may contain fewer Si( n Al) ( n ≥2) units . Here, the proportion of Si(2Al) in CDY‐wash is lower than that in USY, the reason for which may lie in the more selective removal of NNN‐Al by the aluminum sulfate assisted dealumination approach.…”

“…The samples possess both Brønsted acid sites and Lewis acid sites. HCDY‐wash and HUSY have more strong acid sites than HY, probably because of extensive dealumination and removal of NNN‐Al . On the other hand, HCDY‐wash possesses 22 % more strong acid sites than HUSY, though they have comparable FSARs, and HUSY has a higher microspore volume.…”