Density Functional Theory Investigation on the Effect of Alkali Metal Cations on the Catalytic Performance for Cu+Y Zeolites in Oxidative Carbonylation of Methanol†

doi:10.7503/cjcu20150327

Abstract

Abstract:

On the basis of density functional theory, influence of co-cations(M=Li⁺, Na⁺, K⁺, Rb⁺)adjacenting active center Cu⁺ on the electronic properties and catalytic activity of Cu⁺Y zeolites in oxidative carbonylation of methanol to dimethyl carbonate was investigated. CuMY stable configurations were constructed, CO insertion CH₃O species to CH₃OCO was used to evaluate the catalytic activity of CuY zeolites, due to the fact that it was the rate-limiting step of oxidative carbonylation of methanol. The calculation results indicate that Li⁺, Na⁺, K⁺ cations can locate site Ⅰ' in small cages of Y zeolites surrounding the active center Cu⁺, the adsorption energy of CH₃OH, CO, CH₃O and co-adsorbed CO/CH₃O on CuMY zeolite increases gradually with increase of cation size, the stability of transition states for CO insert into CH₃O reaction comes to decrease, increases the activation barriers and the corresponding catalytic performance declines. Whereas, Rb⁺ and Cs⁺cations located Ⅱ^* in the supercage as increase in cation size, decreases activation energy for CO insertion reaction, improves the stability of transition states and the catalytic activity of CuY zeolites. The order of catalytic activity of CuMY zeolites is CuLiY-Ⅰ'>CuCsY-Ⅱ^*>CuNaY-Ⅰ'>CuRbY-Ⅱ^*>CuKY-Ⅰ'>CuCuY-Ⅰ'. The CuLiY-Ⅰ' catalyst exhibits the best catalytic performance with a lowest rate-limiting reaction activation barrier of 52.74 kJ/mol.

Key words: Alkali metal cation, Density functional theory, CuY zeolite, Oxidative carbonylation, Dimethyl carbonate

CLC Number:

O643

TrendMD:

ZHANG Yanqing, ZHENG Huayan, ZHANG Riguang, LI Zhong, WANG Baojun, ZHAO Qiuyong. Density Functional Theory Investigation on the Effect of Alkali Metal Cations on the Catalytic Performance for Cu⁺Y Zeolites in Oxidative Carbonylation of Methanol^†[J]. Chem. J. Chinese Universities, 2015, 36(10): 1945.

Figures/Tables 9

Fig.1 Structure of the faujasite(A) Stereodiagram of the faujasite-framework with cation sites and different crystallographic oxygen positions; (B) the cluster model Y zeolite. Red, yellow, purple, white and orange balls stand for O, Si, Al, H atoms, and cation sites, respectively.

Fig.2 Stable structures of CuMY zeolitesOrange and green balls stand for Cu+ and alkali metal cations, respectively, others see Fig.1. (A) CuMY-Ⅰ'(M=Li, Na, K); (B) CuMY-Ⅱ*(M=Rb, Cs).

Table 1 Location of alkali metal cations after opti-mization, the net charge[q(M)] and the binding energies(Eb) of metal cations at different sites of Y zeolites

Metal cation	Location	E_b/(kJ·mol^-1)	q(M)/e
Li⁺	Ⅰ'	605.88	0.623
Na⁺	Ⅰ'	525.34	0.747
K⁺	Ⅰ'	444.98	0.804
Rb⁺	Ⅱ^*	387.96	0.784
Cs⁺	Ⅱ^*	355.92	0.860

Table 2 Binding energies(Eb), net charge [q(Cu/M)] and electronic configurations of Cu+ of CuMY zeolite

Catalyst	E_b/(kJ·mol^-1)	q(Cu)/e	q(M)/e	Electronic configuration
CuCuY-Ⅰ'^[9]	647.39	0.393	0.340	3d^9.9534s^0.4254p^0.229
CuLiY-Ⅰ'	658.63	0.385	0.617	3d^9.9524s^0.4334p^0.230
CuNaY-Ⅰ'	672.20	0.367	0.758	3d^9.9424s^0.4554p^0.235
CuKY-Ⅰ'	690.10	0.340	0.821	3d^9.9314s^0.4974p^0.233
CuRbY-Ⅱ^*	670.70	0.343	0.783	3d^9.9414s^0.4764p^0.239
CuCsY-Ⅱ^*[9]	673.21	0.345	0.801	3d^9.9434s^0.4774p^0.235

Table 3 Adsorption energies of single CH3OH(CO, CH3O), co-adsorbed CO/CH3O and transition state(TS) involving in CO insertion into CH3O to CH3OCO on CuMY zeolites

Catalyst	E_ads/(kJ·mol^-1)
Catalyst	Adsorbed CH₃OH	Adsorbed CO	Co-adsorbed CO	Adsorbed CH₃O	Co-adsorbed CH₃O	Co-adsorbed CO/CH₃O	TS
CuCuY-Ⅰ'	103.17	154.40	42.35	177.32	110.10	241.11	101.80
CuLiY-Ⅰ'	111.61	156.47	87.56	172.35	150.11	218.49	165.75
CuNaY-Ⅰ'	117.28	158.25	91.46	176.66	143.42	224.25	148.69
CuKY-Ⅰ'	124.29	162.37	95.15	179.79	113.17	234.61	141.98
CuRbY-Ⅱ^*	117.88	165.34	96.98	193.58	157.53	230.12	141.77
CuCsY-Ⅱ^*	103.17	153.66	95.89	175.11	145.62	222.59	167.19

Table 4 Net charge of single CH3OH(CO, CH3O) and co-adsorbed CO/CH3O on CuMY zeolites

Catalyst	q/e
Catalyst	CH₃OH	CO	co-adsorbed CO	CH₃O	co-adsorbed CH₃O
CuCuY-Ⅰ'^[9]	0.247	0.426	0.395	-0.116	-0.124
CuLiY-Ⅰ'	0.173	0.404	0.354	-0.065	-0.158
CuNaY-Ⅰ'	0.241	0.417	0.364	-0.066	-0.121
CuKY-Ⅰ'	0.248	0.420	0.365	-0.078	-0.072
CuRbY-Ⅱ^*	0.258	0.414	0.373	-0.102	-0.174
CuCsY-Ⅱ^*[9]	0.243	0.409	0.378	-0.098	-0.161

Fig.3 Stable configurations of CO, CH3OH and CH3O adsorbed on CuY and CuMY zeolitesBond lengths are in nm.(A) CO on CuCuY-Ⅰ', dC—O=0.1148, dCu—C=0.1788; (B) CO on CuLiY-Ⅰ' , dC—O=0.1148, dCu—C=0.1787; (C) CO on CuNaY-Ⅰ' , dC—O=0.1154, dCu—C=0.1784; (D) CO on CuKY-Ⅰ' , dC—O=0.1148, dCu—C=0.1788; (E) CO on CuRbY-Ⅱ* , dC—O=0.1149, dCu—C=0.1786; (F) CH3OH on CuCuY-Ⅰ' , dCu—OCH4=0.1910; (G) CH3OH on CuLiY-Ⅰ' , dCu—OCH4=0.1929; (H) CH3OH on CuNaY-Ⅰ' , dCu—OCH4=0.1919; (I) CH3OH on CuKY-Ⅰ' , dCu—OCH4=0.2098; (J) CH3OH on CuRbY-Ⅱ*, dCu—OCH4=0.1932; (K) CH3O on CuCuY-Ⅰ' , dCu—OCH3=0.1810; (L) CH3O on CuLiY-Ⅰ' , dCu—OCH3=0.1804; (M) CH3O on CuNaY-Ⅰ' , dCu—OCH3=0.1801; (N) CH3O on CuKY-Ⅰ', dCu—OCH3=0.1799; (O) CH3O on CuRbY-Ⅱ* , dCu—OCH3=0.1808.

Fig.4 Stable configurations of co-adsorbed CO and CH3O on CuMY zeolitesBond lengths are in nm.(A) CO/CH3O on CuCuY-Ⅰ' , dC—O=0.1145, dCu—O=0.1844, dC—OCH3=0.3312; (B) CO/CH3O on CuLiY-Ⅰ', dC—O=0.1142, dCu—O=0.1882, dC—OCH3=0.2488; (C) CO/CH3O on CuNaY-Ⅰ', dC—O=0.1140, dCu—O=0.1879, dC—OCH3=0.2670; (D) CO/CH3O on CuKY-Ⅰ' , dC—O=0.1142, dCu—O=0.1845, dC—OCH3=0.3123; (E) CO/CH3O on CuRbY-Ⅱ* , dC—O=0.1140, dCu—O=0.1889, dC—OCH3=0.2539; (F) TS on CuCuY-Ⅰ', dC—O=0.1150, dCu—O=0.2356, dC—OCH3=0.2041; (G) TS on CuLiY-Ⅰ', dC—O=0.1179, dCu—O=0.2298, dC—OCH3=0.1845; (H) TS on CuNaY-Ⅰ', dC—O=0.1150, dCu—O=0.2353, dC—OCH3=0.1889; (I) TS on CuKY-Ⅰ', dC—O=0.1193, dCu—O=0.2414, dC—OCH3=0.2012; (J) TS on CuRbY-Ⅱ* , dC—O=0.1180, dCu—O=0.2275, dC—OCH3=0.1858; (K) CH3OCO on CuCuY-Ⅰ' , dC—O=0.1225, dCu—O=0.2817, dC—OCH3=0.1336; (L) CH3OCO on CuLiY-Ⅰ', dC—O=0.1224, dCu—O=0.2824, dC—OCH3=0.1332; (M) CH3OCO on CuNaY-Ⅰ' , dC—O=0.1224, dCu—O=0.2805, dC—OCH3=0.1332; (N) CH3OCO on CuKY-Ⅰ', dC—O=0.1223, dCu—O=0.2816, dC—OCH3=0.1331; (O) CH3OCO on CuRbY-Ⅱ* , dC—O=0.1220, dCu—O=0.2726, dC—OCH3=0.1339.

Fig.5 Potential energy profiles for the reaction of CO insertion into CH3O species to CH3OCO over CuMY zeolites

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