Theoretical calculation study on the reaction mechanism of methanol/dimethyl ether carbonylation catalyzed by the B/Al/Ga-MOR zeolites
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摘要: 采用DFT计算比较分析了B、Al和Ga分别同晶取代MOR分子筛八元环侧袋T3位点及十二元环孔道T4位点时甲醇及二甲醚羰基化反应机制的共性及差异。研究发现,CO插入甲氧基生成乙酰基的反应遵循SN2机制,且为羰基化反应过程中的决速步;473 K下,无论甲醇或二甲醚为原料,生成的乙酰基更倾向于与甲醇中的CH3O作用生成乙酸甲酯;T3位点具有更好的羰基化择形性,而T4位点上更倾向于发生由三甲基氧鎓离子生成芳烃导致催化剂失活的副反应。与Al-MOR相比,在T3位点引入B和Ga会导致羰基化反应能垒的升高,降低其催化性能;而在T4位点引入B和Ga(尤其是B)则可大幅提升其生成三甲基氧鎓离子的能垒,抑制芳烃生成过程,提升催化剂稳定性。本工作有助于认识MOR分子筛不同孔道内酸性位点发生同晶取代时催化羰基化反应机制的差异,为调控设计高效MOR沸石催化剂提供一定的理论支撑。Abstract: The reaction mechanism of methanol/dimethyl ether (DME) carbonylation catalyzed by isomorphously substituted B-, Al-, and Ga-MOR zeolites (B/Al/Ga-MOR) was comparatively investigated by the density functional theory (DFT) calculations. The commonalities and differences between methanol and dimethyl ether as the reactant as well as among various MOR zeolites in the catalytic reaction pathways were disclosed, where one Si atom was substituted by B, Al or Ga at the 8-ring side pockets T3 sites or the 12-ring channels T4 sites of MOR. The results indicate that the insertion of CO into methoxy group to form acetyl groups follows the SN2 mechanism and is the rate-determining step in the carbonylation reactions. Under 473 K, either methanol or dimethyl ether is used as feedstock, the formed acetyl group prefers to interact with CH3O in methanol to form methyl acetate. The T3 sites show better carbonylation selectivity, whereas T4 sites display better trimethoxonium ions selectivity which favors the generation of aromatics and leads to the catalyst deactivation. Comparing with Al-MOR, the introduction of Ga and B at the T3 sites increases the free energy barriers of carbonylation, whereas the introduction of Ga and B in particular at the T4 sites can substantially increase the energy barriers of generating trimethyloxonium ions, which can effectively suppress the side reaction and improve the catalyst stability. This work contributes to the understanding of the catalytic roles of various acidic sites in different channels of the MOR zeolites and provides certain theoretical support for tailoring and designing efficient MOR zeolite catalysts for methanol/dimethyl ether carbonylation.
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Key words:
- carbonylation /
- methanol /
- dimethyl ether /
- B/Al/Ga-MOR zeolites /
- DFT calculation /
- reaction mechanism
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图 3 甲氧基与CO反应的反应物、过渡态和产物的结构
Figure 3 Structures of ISs, TSs and FSs for the reaction of methoxy group with CO: the insertion mechanism (a) and SN2 mechanism (b) at T3 sites of the 8-ring side pockets, and the insertion mechanism (c) and SN2 mechanism (d) at T4 sites of the 12-ring channels
(Red, yellow, purple, white and gray spheres represent O, Si, Al, H and C atoms, respectively; ball-and-stick parts represent the reaction center atoms, stick parts represent Brönsted acid sites, and the lines represent the outer frame atoms)
图 6 乙酰基物种与MeOH反应(TS3(M2))的反应物、过渡态和产物的结构
Figure 6 Structures of ISs, TSs and FSs for the reaction of acetyl species with MeOH (TS3(M2)): (a), (b) and (c) are located at the T3 sites of the 8-ring side pockets of B/Al/Ga-MOR, respectively
(Red, yellow, pink, purple, orange, white and gray spheres represent O, Si, B, Al, Ga, H and C atoms, respectively; ball-and-stick parts represent the reaction center atoms, stick parts represent Brönsted acid sites, and the lines represent the outer frame atoms)
表 1 473 K下甲氧基与 CO 反应形成乙酰基的本征吉布斯自由能垒($ {\Delta {G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $)
Table 1 Calculated intrinsic Gibbs free energy barriers ($ {\Delta {G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $) for the reaction of methoxy group with CO to form an acetyl group at 473 K
Mechanism $ {\Delta {G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $/ (kJ·mol−1) T3 T4 DFT DFT-D DFT DFT-D Insertion 220 174 220 214 SN2 132 130 128 139 表 3 B/Al/Ga-MOR分子筛八元环侧袋和十二元环孔道内的质子亲和势和氨气吸附能
Table 3 Proton affinities (PA) and NH3 adsorption energies (${\Delta E}_{{\rm{ads\cdot NH_3}}}$) in the 8-ring side pockets and 12-ring channels of B/Al/Ga-MOR
Acid sites PA/
(kJ·mol−1)${\Delta E}_{ {\rm{ads\cdot NH_3} } }$/
(kJ·mol−1)T3 B-MOR 6.17 −1.09 Al-MOR 5.89 −1.74 Ga-MOR 6.12 −1.59 T4 B-MOR 6.28 −1.09 Al-MOR 5.90 −1.81 Ga-MOR 6.13 −1.75 表 2 473 K下B/Al/Ga-MOR分子筛八元环侧袋中甲醇及二甲醚羰基化反应的自由能垒($ {\Delta{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $)、速率常数($ k $)、焓势垒($ {\Delta H}_{{\rm{int}}}^{\ne } $)、熵损失($ {-T\Delta S}_{{\rm{int}}}^{\ne } $)和反应自由能($ {\Delta G}_{{\rm{R}}} $)
Table 2 Calculated kinetic results of free energy barriers ($ {\Delta{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $), relative rate constants ($ k $), enthalpy barriers ($ {\Delta H}_{{\rm{int}}}^{\ne } $) and entropy losses ($ {-T\Delta S}_{{\rm{int}}}^{\ne } $), and thermodynamic results of reaction free energies ($ {\Delta G}_{{\rm{R}}} $) of each reaction step for MeOH and DME carbonylation in the 8-ring side pockets of B/Al/Ga-MOR at 473 K
Reaction step
8MR$ {\Delta{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $
/(kJ·mol−1)$ k $
/s−1$ {\Delta H}_{{\rm{int}}}^{\ne } $
/(kJ·mol−1)$ {-T\Delta S}_{{\rm{int}}}^{\ne } $
/(kJ·mol−1)${\Delta G}_{{\rm{R}}}$
/(kJ·mol−1)B-MOR TS1(M) 103 4.16×101 97 6 −50 TS1(D) 115 1.97×100 116 −1 −33 TS2 146 7.41×10−4 136 10 21 TS3(M1) 171 1.29×10−6 157 14 −35 TS3(M2) 133 2.02×10−2 114 19 −6 TS3(D) 306 1.59×10−21 271 35 −31 TS4 242 1.85×10−14 220 22 96 Al-MOR TS1(M) 113 3.27×100 114 −1 37 TS1(D) 87 2.43×103 87 0 −7 TS2 130 4.33×10−2 113 17 −1 TS3(M1) 97 1.91×102 86 11 −80 TS3(M2) 65 6.54×105 41 24 −14 TS3(D) 195 2.87×10−9 186 9 −55 TS4 212 3.81×10−11 187 25 35 Ga-MOR TS1(M) 107 1.50×101 108 −1 33 TS1(D) 80 1.44×104 81 −1 −16 TS2 137 7.31×10−3 117 20 5 TS3(M1) 123 2.57×10−1 123 0 −69 TS3(M2) 83 6.72×103 50 33 −13 TS3(D) 187 2.20×10−8 157 30 −54 TS4 207 1.36×10−10 183 24 45 表 4 473 K下B/Al/Ga-MOR分子筛十二元环孔道内甲醇及二甲醚羰基化反应的自由能垒($ {\Delta{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $ )、速率常数($ k $)、焓势垒($ {\Delta H}_{{\rm{int}}}^{\ne } $)、熵损失($ {-T\Delta S}_{{\rm{int}}}^{\ne } $ )和反应自由能(${\Delta G}_{{\rm{R}}}$)
Table 4 Calculated kinetic results of free energy barriers ($ {\Delta{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $), relative rate constants ($ k $), enthalpy barriers ($ {\Delta H}_{{\rm{int}}}^{\ne } $) and entropy losses ($ {-T\Delta S}_{{\rm{int}}}^{\ne } $), and thermodynamic results of reaction free energies (${\Delta G}_{{\rm{R}}}$) of each reaction step for MeOH and DME carbonylation in the 12-ring channels of B/Al/Ga-MOR at 473 K
Reaction step 12MR $ {\Delta{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $ (kJ·mol−1) $ k $
/s−1$ {\Delta H}_{{\rm{int}}}^{\ne } $
/(kJ·mol−1)$ {-T\Delta S}_{{\rm{int}}}^{\ne } $
/(kJ·mol−1)$ {\Delta G}_{{\rm{R}}} $
/(kJ·mol−1)B-MOR TS1(M) 77 3.09×104 75 2 −53 TS1(D) 94 4.10×102 86 8 −35 TS2 188 1.70×10−8 147 41 −14 TS3(M1) 128 7.21×10−2 112 16 −37 TS3(M2) 96 2.46×102 85 11 −67 TS3(D) 136 9.43×10−3 124 12 −45 TS4 134 1.57×10−2 115 19 86 Al-MOR TS1(M) 93 5.29×102 89 4 22 TS1(D) 91 8.79×102 95 −4 10 TS2 139 4.40×10−3 106 33 −38 TS3(M1) 39 4.86×108 30 9 −53 TS3(M2) 35 1.34×109 10 25 −93 TS3(D) 68 3.05×105 45 23 −51 TS4 73 8.55×104 69 4 −34 Ga-MOR TS1(M) 126 1.20×10−1 121 5 51 TS1(D) 92 6.82×102 93 −1 6 TS2 139 4.40×10−3 107 32 −33 TS3(M1) 57 5.00×106 27 30 −40 TS3(M2) 17 1.31×1011 8 9 −94 TS3(D) 80 1.44×104 75 5 −46 TS4 84 5.21×103 69 15 37 -
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