Density Functional Theory Study of Chemical Composition Influence on the Acidity of H−MCM-22 Zeolite

Abstract

In this work, the influence of the chemical composition (species and number of trivalent substitution atoms) on the acidity of the isomorphously substituted MCM-22 zeolite was investigated in detail by density functional theory. By calculating the proton affinity and NH 3 adsorption energy, it was found that the acidity of the MCM-22 zeolite decreased with increasing number of trivalent substitution atoms introduced into the zeolite framework. Moreover, the acidity of the substituted MCM-22 increased in the sequence of B-MCM-22 < Fe-MCM-22 < Al-MCM-22, independent of the number of trivalent substitution atoms. The calculation results also showed that when Fe or B elements were incorporated into the framework of the aluminosilicate MCM-22 zeolite, the proton affinity of MCM-22 zeolite decreased in the order of B-MCM-22 > [Al,B]-MCM-22 > Fe-MCM-22 > [Al,Fe]-MCM-22 > Al-MCM-22. All these results were in good agreement with the experimental results.

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].…”

“…Especially the M O3 distances are 0.353-0.520Å larger than the average values of the other three; however, the M O3 distances are dramatically reduced in the deprotonated states (BA − ), where the protons are deprived by the incoming probe molecules.The Brönsted acidity strengths of the M 4+ -and M 3+ -doped zeolites can be evaluated with the aid of proton affinity (PA)[5,[7][8][9][10]27,66,67],PA = E(BA − ) − E(BA)(8)where E(BA) and E(BA − ) represent the energies of the Brönsted acidic sites (BA) and corresponding deprotonated states (BA − ), respectively. The proton affinities (PA) of the various M 4+ -and M 3+ -doped zeolites decrease in the order Silicalite-1 (324.9) Ti…”

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

“…[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.…”