同晶置换改性FAU分子筛结构及吸附性能的密度泛函理论研究

doi:10.7503/cjcu20160449

摘要/Abstract

摘要：

采用密度泛函理论(DFT)方法, 考察了八面沸石(FAU)型分子筛β笼孔道结构内含氧化合物(甲醇、二甲醚、丙醛)的吸附, 并进一步计算研究了Zn, Ca同晶置换改性的作用机理. 研究结果表明, β笼孔道结构内, Al原子为甲醇、二甲醚和丙醛的吸附活性位, Si原子无吸附活性. Zn, Ca掺杂的β笼结构内, 正2价的Zn和Ca掺杂替换正3价的Al, 导致邻近的Si原子位置形成缺电子空穴, 增强了甲醇、二甲醚和丙醛的吸附, 而杂原子Zn和Ca本身并没有吸附活性.

关键词: 密度泛函理论, FAU分子筛, 掺杂, 吸附

Abstract:

The adsorption of oxygenated chemicals(methanol, dimethyl ether, propionaldehyde) in channel structure of FAU type zeolite β cage, as well as the modification mechanism of Zn/Ca isomorphous substitution were investigated by means of density functional theory(DFT) calculations. The results indicate that methanol, dimethyl ether and propionaldehyde can chemisorb at the top site of the Al atom, while they can barely adsorb at the Si atom. For Zn-/Ca-doped β cage, Al³⁺ was substituted by Zn²⁺/Ca²⁺, a hole was formed on the adjacent Si atom due to the loss of one electron, therefore the adsorption of methanol, dimethyl ether and propionaldehyde were enhanced. However, the dopant atom itself doesn’t serve as adsorption site.

Key words: Density functional theory(DFT), FAU zeolite, Doping, Adsorption

TrendMD:

杨丙星, 叶丽萍, 顾慧劼, 徐华胜, 罗勇, 李会英. 同晶置换改性FAU分子筛结构及吸附性能的密度泛函理论研究. 高等学校化学学报, 2016, 37(11): 2018.

YANG Bingxing, YE Liping, GU Huijie, XU Huasheng, LUO Yong, LI Huiying. Theoretical Studies on the Structure and Adsorption Properties of Isomorphously Substituted FAU Zeolite^†. Chem. J. Chinese Universities, 2016, 37(11): 2018.

图/表 11

Fig.1 Calculated structure of FAU zeolite(A), β cage(B) and the isosurface(50 e/nm3) of calculated spin charge densities(C) The Si, Al and O atoms are represented by balls in yellow, green and red, respectively.

Fig.2 Calculated structures of Al(A, B), Si(C, D), Al'(E, F) and Si'(G, H) atoms substituted by Zn(A, C, E, G) and Ca(B, D, F, H) atoms The Zn and Ca atoms are represented by balls in purple and blue, respectively.

Table 1 Calculated lengths of Al/Si/Zn/Ca-O bonds, rRMS of different substituted atoms and substitution energies of Zn/Ca doped β cage

Defect	Substitution atom	d(Al/Si/Zn/Ca—O)/nm	r_RMS/nm	E_sub/eV	Figure
Clean	Al	0.1701, 0.1698, 0.1701			1(B)
	Si	0.1641, 0.1647, 0.1643			1(B)
	Al'	0.1685, 0.1699, 0.1696			1(B)
	Si'	0.1646, 0.1629, 0.1641			1(B)
Zn doped	Al	0.1842, 0.1999, 0.1863	0.0213	4.89	2(A)
	Si	0.1888, 0.1860, 0.1929	0.0250	5.67	2(C)
	Al'	0.1975, 0.1852, 0.1859	0.0211	5.05	2(E)
	Si'	0.1900, 0.1833, 0.1909	0.0244	5.72	2(G)
Ca doped	Al	0.2133, 0.2112, 0.2254	0.0470	0.92	2(B)
	Si	0.2147, 0.2129, 0.2169	0.0505	1.86	2(D)
	Al'	0.2103, 0.2141, 0.2254	0.0477	1.00	2(F)
	Si'	0.2181, 0.2096, 0.2162	0.0509	1.90	2(H)

Fig.3 Calculated structures of methanol(A), dimethyl ether(B) and propionaldehyde adsorption(C) at Al site of β cage and isosurfaces(6 e/nm3)(D—F) of charge redistribution of (A—C), respectively The yellow and blue isosurfaces denote charge gain and miss, repectively. The C and H atoms are represented by balls in black and white, respectively.

Table 2 Calculated lengths of Al/Si—O bonds, rRMS of methanol, dimethyl ether and propionaldehyde adsorption at Al/Si site of β cage

Defect	Adsorption molecular	Before adsorption Al/Si—O/nm	After adsorption Al/Si—O/nm	r_RMS/nm	Figure
Clean	Ethanol	0.1701, 0.1698, 0.1701	0.1726, 0.1736, 0.1718	0.0028	3(A)
	Dimethylether		0.1727, 0.1728, 0.1707	0.0023	3(B)
	Propionaldehyde		0.1733, 0.1737, 0.1727	0.0033	3(C)
Zn doped	Ethanol	0.1550, 0.1590, 0.1589	0.1614, 0.1618, 0.1640	0.005	4(G)
	Dimethyl ether		0.1592, 0.1612, 0.1631	0.0037	4(H)
	Propionaldehyde		0.1619, 0.1625, 0.1665	0.0063	4(I)
Ca doped	Ethanol	0.1595, 0.1602, 0.1552	0.1620, 0.1622, 0.1557	0.0019	4(D)
	Dimethyl ether		0.1625, 0.1626, 0.1559	0.0023	4(E)
	Propionaldehyde		0.1629, 0.1630, 0.1579	0.003	4(F)

Table 3 Calculated adsorption energies of ethanol, dimethyl ether and propionaldehyde at Al and Si sites

Site	E_ads/eV
Site	Ethanol	Dimethyl ether	Propionaldehyde
Al	0.83	0.97	0.76
Si	0.54

Fig.4 Calculated structures of ethanol(A, D, G), dimethyl ether(B, E, H) and propionaldehyde(C, F, I) adsorption in Ca-doped(D—F) and Zn-doped(G—I) β cageThe insets denote the isosurfaces(6 e/nm3) of charge redistribution.

Site	E_ads/eV
Site	Ethanol	Dimethyl ether	Propionaldehyde
Zn
Si(Zn)	2.28	1.08	3.93
Ca	0.45	0.25	0.15
Si(Ca)	0.77	0.68	2.29

Fig.5 Local structures of clean(A), Ca-doped(B), Zn-doped(C) β cage

Table 5 Calculated Bader charge of different Si sites

Si number	Charge/e
Si number	Clean	Ca-doped	Zn-doped
1	0.83	0.76	0.00
2	0.82	0.00	0.76
3	0.82	0.78	0.78

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