A DFT Study on Isomorphously Substituted MCM-22 Zeolite

Abstract

The density functional theory has been used to study the isomorphously substituted MCM-22 zeolite for the first time. The effect of the basis sets on the calculation results is discussed in details. Data of several index properties for characterizing the relative acidity of T-MCM-22 (T ) B, Al, Ga, and Fe), including proton affinity, bond length and bond angle, OH stretching frequency, and charge on the acidic proton, show that the acidity of T-MCM-22 increases in the sequence of B-MCM-22 < Fe-MCM-22 < Ga-MCM-22 < Al-MCM-22. After making a correction, the calculated OH stretching frequencies for Al-MCM-22 and Fe-MCM-22 show a reasonable agreement with the experimental data. On the basis of an equilibrium structure of the B-MCM-22 zeolite, the effect of the B element in the synthesis of the Ti-MCM-22 is also discussed. The adding of the B element during the synthesis of the Ti-MCM-22 can decrease greatly the Ti substitution energy because of the forming of a structure quite similar to the terminal silanol group. The results can provide some constructively information for zeolite synthesis.

Density Functional Calculations on the Distribution, Acidity, and Catalysis of TiIV and TiIII Ions in MCM‐22 Zeolite

“…T3 is the most probable site for Ti IV incorporation; T7 is the next most preferred site but is not accessible to adsorbents. The distribution of Ti IV in MCM-22 zeolite is quite different from that of Al, [28][29] a situation similar to that of Ti IV and Al substitutions in MFItype zeolites. [16-17, 23-24, 26-27] Distribution of the Ti III ions: The local structures of the eight Ti III sites, taken from the optimized 49-T cluster model (Figure 1b), are shown in Figure 3.…”

“…[16][17][18][19][20][21][22][23][24][25][26][27] Recently, the Al locations in MCM-22 zeolite were studied by Corma et al and our group. [28][29][30] It was found that these ions are not equally distributed among the various T sites. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Herein, density functional calculations were employed to determine the Ti IV distribution in MCM-22 zeolite.…”

“…[28][29][30] It was found that these ions are not equally distributed among the various T sites. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Herein, density functional calculations were employed to determine the Ti IV distribution in MCM-22 zeolite. The design of isolated titanium dioxide (TiO 2 ) within zeolites is currently a research focus, [31] owing to the unique photocatalytic reactivity of the Ti III sites for a variety Abstract: Isolated Ti species in zeolites show unique catalytic activities for a variety of chemical reactions.…”

Lewis and Brönsted acidic sites in M4+-doped zeolites (M=Ti, Zr, Ge, Sn, Pb) as well as interactions with probe molecules: A DFT study

“…A number of experimental and theoretical efforts have devoted to clarifying the acidities of M 3+ -doped zeolites [4][5][6][7][8][9][10]. Calculations of various cluster models and theoretical levels indicated that the Brönsted acidities increase in the order B < Fe < Ga < Al-doped zeolites [7][8][9][10].…”

“…Calculations of various cluster models and theoretical levels indicated that the Brönsted acidities increase in the order B < Fe < Ga < Al-doped zeolites [7][8][9][10]. Instead, the studies with incorporation of tetravalent ions (M 4+ ) lag far behind [11][12][13][14][15][16][17].…”