Theoretical calculation study on the reaction mechanism of methanol/dimethyl ether carbonylation catalyzed by B/Al/Ga-MOR Zeolites
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摘要: 采用DFT理论计算详细比较了B、Al、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;473K条件下,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;对于羰基化反应和由三甲基氧鎓离子生成芳烃致催化剂失活的副反应,T3位点表现出更好的羰基化择形性而T4位点上更倾向于发生副反应。与Al-MOR相比,在T3位点引入B、Ga会导致羰基化反应能垒的升高,降低其催化性能;在T4位点引入B、Ga尤其是B则可大幅提升其生成三甲基氧鎓离子的能垒,进而抑制该过程进行,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 DFT theoretical calculations were used to comprehensively compare the commonalities and differences in the mechanism of methanol and dimethyl ether carbonylation reactions when B, Al and Ga respectively substitute for the eight-membered ring side pocket T3 and the twelve-membered ring channel T4 sites of the MOR zeolite. The results indicate that CO inserting into methoxy to form acetyl groups follows the SN2 mechanism and is a rate-determining step in the carbonylation reaction. Under the condition of 473K, when methanol or dimethyl ether is used as feedstock, the formed acetyl group prefers to interact with CH3O in methanol to form methyl acetate. For the carbonylation reaction and the side reaction of catalyst deactivation by generation of aromatics from trimethoxonium ions, the T3 sites show better carbonylation selectivity whereas T4 sites show better trimethoxonium ions selectivity. Comparing with Al-MOR, the introduction of B or Ga at the T3 site increases the energy barrier of carbonylation reaction and weakens catalytic activity, while the introduction of B or Ga especially B at the T4 site can substantially increase the energy barrier of generating trimethyloxyonium ions, which effectively suppresses the side reaction and improves the stability of the catalyst. This work contributes to understanding the differences in the carbonylation reaction mechanism when isomorphous replacement occurs at acidic sites in the different channels of the MOR zeolite and provides certain theoretical support for tailoring and designing efficient MOR zeolite catalysts.
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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:八元环侧袋 T3位点上insertion机理(a)和SN2机理(b)及十二元环孔道T4位点上insertion机理(c)和SN2机理(d)。其中红球代表O原子,黄球代表Si原子,紫球代表Al原子,白球代表H原子,灰球代表C原子;球棍模型代表反应中心原子,棍棒模型代表Brönsted酸位点,线条代表外层框架原子。
Figure 3 Structures of ISs, TSs and FSs for the reaction of methoxy group with CO: the insertion mechanism (a) and SN2 mechanism (b) on the T3 site of the 8-MR side pocket, and the insertion mechanism (c) and SN2 mechanism (d) on the T4 site of the 12-MR channel. Red spheres represent O atoms, yellow spheres represent Si atoms, purple spheres represent Al atoms, white spheres represent O atoms, gray spheres represent C atoms; 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:(a)、(b)、(c)依次为B/Al/Ga-MOR分子筛八元环侧袋 T3位点。其中红球代表O原子,黄球代表Si原子,粉球代表B原子,紫球代表Al原子,橙球代表Ga原子,白球代表H原子,灰球代表C原子;球棍模型代表反应中心原子,棍棒模型代表Brönsted酸位点,线条代表外层框架原子
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-MR side pocket of B/Al/Ga-MOR, respectively. Red spheres represent O atoms, yellow spheres represent Si atoms, pink spheres represent B atoms, purple spheres represent Al atoms, orange spheres represent Ga atoms, white spheres represent O atoms, gray spheres represent C atoms; 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 在反应形成乙酰基的本征吉布斯自由能垒(
$ {\Delta \mathrm{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $ )Table 1 Computed intrinsic Gibbs free energy barriers (
$ {\Delta \mathrm{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $ ) for the reaction of methoxy group with CO to form an acetyl group at 473 Kmechanism $ {\Delta \mathrm{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 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 \mathrm{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}_{R} $ )Table 2 Calculated kinetic results of free energy barriers (
$ {\Delta \mathrm{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}_{R} $ ) of each reaction step for MeOH and DME carbonylation in the 8-MR side pockets of B/Al/Ga-MOR at 473 KReaction step
8MR$ {\Delta \mathrm{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}_{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 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, kJ mol−1) and NH3 adsorption energies (
$ {\Delta E}_{ads-NH3} $ , kJ mol−1) in the 8-MR side pockets and 12-MR channels of B/Al/Ga-MORAcid sites PA
(kJ mol−1)$ {\Delta E}_{ads-NH3} $
(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 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 \mathrm{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}_{R} $ )Table 4 Calculated kinetic results of free energy barriers (
$ {\Delta \mathrm{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}_{R} $ ) of each reaction step for MeOH and DME carbonylation in the 12-MR channels of B/Al/Ga-MOR at 473 KReaction step
12MR$ {\Delta \mathrm{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}}^{\ne } $
(kJ mol−1)$ k $
(s−1)$ {\Delta H}_{int}^{\ne } $
(kJ mol−1)$ {-T\Delta S}_{int}^{\ne } $
(kJ mol−1)$ {\Delta G}_{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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