Brønsted Acidity of Isomorphously Substituted ZSM-5 by B, Al, Ga, and Fe. Density Functional Investigations

Abstract

The Brønsted acidity of isomorphously substituted ZSM-5 by B, Al, Ga, and Fe has been studied at the B3LYP level of density functional theory. On the basis of the calculated proton affinity, natural charge on the acidic proton, and the adsorption energy of NH3, the Brønsted acidity increases in the order: B−(OH)−Si < Fe−(OH)−Si < Ga−(OH)−Si < Al−(OH)−Si, in agreement with the experiment. In both Al and Ga modified clusters, the adsorbed NH3 becomes ammonium (NH4 +) stabilized by two N−H···O hydrogen bonds, while the physisorbed NH3 is stabilized by one N···H−O hydrogen bond in Fe and B substituted clusters. It is also found that NH3 adsorption changes the B coordination sphere.

The Effect of Ge Incorporation on the Brønsted Acidity of ZSM‐5

“…When comparing the ZSM-5 cluster geometry with the results reported by Yuan et al, [9] it is clear that the bond lengths and the SiÀO 1 ÀAl angle are comparable, while small differences probably originate from differences in fixed terminal OÀH bond length (0.98 instead of 1.0 ) and the different basis sets employed. Upon Ge substitution all clusters show slightly different behaviour.…”

“…While the two Ge-ZSM-5 clusters based on the potential-relaxed (see Computational Details section) Ge-containing unit cell (entries 2 and 3 in Table 1) both have a slightly lower deprotonation energy than that of the ZSM-5 cluster (À0.24 and À0.82 %, respectively), the Ge-ZSM-5 clusters based on the experimental unit cell (entries 4 and 5 in Table 1) have a higher deprotonation energy than that of the ZSM-5 cluster (the differences being 1.68 % and 1.75 %, respectively). These results may be compared to calculations by Chatterjee et al [8] and by Yuan et al [9] on isomorphously Al, Ga, Fe, Table 1, entries 6-13). Clusters based on the experimental unit cell are expected to be representative of an acid site in Ge-ZSM-5 with a low Ge content, as global structural effects can be expected to be negligible in this case.…”

“…This method of cluster termination has been used widely and successfully in the modelling of silicas and zeolites, [25,29] and owes much of its utility to the electronegativity of hydrogen, which lies between that of silicon and oxygen. [30] Furthermore, it has been shown in numerous previous studies, [8][9][10] that such 8 T-atom clusters with suitably fixed terminating hydrogen atoms are very good models for calculating the acidity of zeolites, as experimentally observed trends in zeolite acidity can be reproduced. All electron calculations were performed for four different clusters (two neutral clusters with T = Si and T = Ge, and the corresponding deprotonated clusters) employing density functional theory (DFT).…”

“…For the calculations a fragment of 8 T atoms [((HO) 3 SiO) 3 -Al-O(H)-T-(OSi(OH) 3 ) 3 , T = Si or Ge, see Figure 1 A] was used, based on the atomic positions in the crystal structure of ZSM-5 (Si/Al = 320), reported by Van Koningsveld et al [27] ("experimental unit cell"). Of the 12 distinct tetrahedral sites present in this structure, the T 12 position was chosen as the site of Al substitution, consistent with previous studies, [8][9][10] regarding the acidity of ZSM-5. To saturate the dangling oxygen bonds of the cluster, hydrogen atoms were placed at an appropriate bond length of 0.98 [28] in the direction of the missing OÀSi bonds from the periodic structure.…”

“…[4] Generally speaking, zeolite framework acidity can be investigated in several ways: i) IR spectroscopy, the stretching vibrational band of the acidic bridging hydroxy groups shifts to higher frequencies upon decreasing acidic strength. Additionally, FTIR studies can also be used to analyse the frequency shift of this vibration upon adsorption of (basic) probe molecules, such as CO or pyridine, [5,6] ii) X-ray photoelectron spectroscopy (XPS), [7] iii) computational techniques, [8][9][10] iv) solid-state NMR spectroscopy ( 29 Si NMR, [11] or 31 P/ 15 N NMR when adsorbed phosphine or amine probe molecules are investigated), v) temperature-programmed desorption (TPD) of basic probe molecules such as ammonia and pyridine, and vi) catalytic test reactions (the acidic strength of a series of zeolites may be assessed in a suitable test reaction by comparing the catalytic activities and selectivities for the reaction).…”