A DFT study on the formation mechanism of side product 1,2-propanediol in the hydrogenation of dimethyl oxalate over copper catalyst
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摘要: 利用密度泛函理论对Cu(111)及Cu2O(111)表面上草酸二甲酯加氢副产物1,2-丙二醇（1,2-PDO）的生成机理进行了探究，计算了两种表面上1,2-PDO生成的不同反应路径基元步骤的热力学数据以及所涉及物种的吸附行为，进行了局域态密度以及差分电荷密度分析，阐明了铜催化剂的主要活性位点及1,2-PDO生成的主要路径。结果表明，1,2-PDO主要由乙二醇和甲醇于Cu2O(111)表面通过Guerbet醇缩合反应生成，具体包括醇脱氢、羟醛缩合以及不饱和醛加氢三个过程。Cu2O(111)表面${\rm{Cu}}_{{\rm{us}}}^{+} $及${\rm{O}}_{{\rm{suf}}}^-$位点形成的Lewis酸碱对能够促进反应物、产物及反应中间体的吸附且对于1,2-PDO生成过程的整体催化活性更高。Cu2O(111)表面的${\rm{O}}_{{\rm{suf}}}^- $位点是醇类脱氢生成醛、羟醛缩合过程中生成烯醇物种以及不饱和醛类中间体加氢的主要活性中心，而C−C偶联反应则发生在${\rm{Cu}}_{{\rm{us}}}^{+} $金属位点上。论文研究结果可为铜催化剂设计和改性以及草酸酯加氢工艺的优化提供理论指导。Abstract: The costly separation of 1,2-propanediol (1,2-PDO), an unavoidable byproduct in the hydrogenation of dimethyl oxalate (DMO), significantly hampers the economic viability of coal-to-ethylene glycol (EG) technology. To address this challenge, the formation mechanism of the side product 1,2-PDO on the Cu(111) and Cu2O(111) surfaces during DMO hydrogenation was investigated, which focused on the active sites of copper catalyst and the dominant pathway through density functional theory calculation. The thermodynamics of each elementary step and the adsorption behavior of various species involved in the reaction network along with the local density of states and charge density difference were systematically analyzed. The results indicate that 1,2-PDO is generated more favorably on the Cu2O(111) surface than that on the Cu(111) surface, owing to the Lewis acid-base pairs, i.e. ${\rm{Cu}}_{{\rm{us}}}^{+} $ and ${\rm{O}}_{{\rm{suf}}}^- $ sites, present on the Cu2O(111) surface, which strengthens the binding of reactants, products, and reaction intermediates to the substrate. EG reacts primarily with methanol (MeOH) to form 1,2-PDO through Guerbet alcohol condensation reaction through three consecutive steps: alcohol dehydrogenation, aldol condensation, and unsaturated aldehyde hydrogenation. The ${\rm{O}}_{{\rm{suf}}}^- $ sites promote the dehydrogenation of alcohols into aldehydes, the generation of enolates during aldol condensation and the hydrogenation of unsaturated aldehydes, while the ${\rm{Cu}}_{{\rm{us}}}^{+} $ sites are responsible for the C–C coupling reaction. These findings may shed light on the mechanism of 1,2-PDO formation over Cu catalyst and provide fundamental knowledge for the development of more efficient catalysts and process optimization.
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图 1 Cu(111)及Cu2O(111)表面模型侧视图及活性位点
(a): Cu(111)表面模型；(b): Cu(111)表面活性位点；(c): Cu2O(111)表面模型；(d): Cu2O(111)表面活性位点
Figure 1 Side views and active sites of Cu(111) and Cu2O(111) surfaces
(a): side view of Cu(111) surface; (b): possible active sites of Cu(111) surface; (c): side view of Cu2O(111) surface; (d): possible active sites of Cu2O(111) surface.
图 2 DMO加氢过程中1,2-PDO的生成反应网络示意图
Figure 2 Plausible reaction network for the generation of 1,2-PDO during the DMO hydrogenation
图 3 Cu(111)及Cu2O(111)表面Stage I中MeOH*、H*、MeO*、EG*、Alko*、MG*、Acyl*物种最稳定吸附构型及对应脱附能
Figure 3 Most stable adsorption configurations and corresponding desorption energies of MeOH*, H*, MeO*, EG*, Alko*, MG*, Acyl* species on the Cu(111) and Cu2O(111) surfaces during Stage I for the generation of 1,2-PDO
图 4 Cu(111)及Cu2O(111)表面Stage II中GcoH*、Enl*、PA*、Int1*、GceH*、Int2*、OH*、H2O*物种最稳定吸附构型及对应脱附能
Figure 4 Most stable adsorption configurations and the corresponding desorption energies of the GcoH*, Enl*, PA*, Int1*, GceH*, Int2*, OH*, H2O* species on the Cu(111) and Cu2O(111) surfaces during Stage II for the generation of 1,2-PDO
图 5 Cu(111)及Cu2O(111)表面Stage III中H2*、2HAcH*、Int4*、2HPrL*、Int8*、1,2-PDO*物种最稳定吸附构型及对应脱附能
Figure 5 Most stable adsorption configurations and corresponding desorption energies of the H2*, 2HAcH*, Int4*, 2HPrL*, Int8*, 1,2-PDO* species on the Cu(111) and Cu2O(111) surfaces during Stage III for the generation of 1,2-PDO
图 6 MeOH*及2HAcH*物种在Cu2O(111)及Cu(111)表面最稳定吸附构型的局域态密度及片段差分电荷密度
(a): MeOH*物种中羟基O原子与Cu原子轨道的局域态密度；(b): 2HAcH*物种中酰基O原子与Cu原子轨道的局域态密；(c): MeOH*物种中羟基H原子与O原子及Cu原子轨道的局域态密度；(d): MeOH*物种最稳定吸附构型的片段差分电荷密度；(e): 2HAcH*物种最稳定吸附构型的片段差分电荷密度
Figure 6 LDOSs and the charge density differences of the most stable adsorption configurations of MeOH* and 2HAcH* on the Cu2O(111) and Cu(111) surfaces
(a): LDOSs for the orbitals of O atom in the hydroxy group of MeOH* with Cu atom; (b): LDOSs for the orbitals of O atom in the acyl group of 2HAcH* with Cu atom; (c): LDOSs for the orbitals of H atom in the hydroxy group of MeOH* with O atom and Cu atom; (d): charge density differences of the most stable adsorption configurations of MeOH*; (e): charge density differences of the most stable adsorption configurations of 2HAcH*
图 7 Cu(111)及Cu2O(111)表面1,2-PDO生成反应在473.15 K条件下的标准吉布斯自由能图
Figure 7 Standard Gibbs free energy diagram of the 1,2-PDO formation reaction on the Cu(111) and Cu2O(111) surfaces at 473.15 K
表 1 473.15 K条件下Cu(111)及Cu2O(111)表面Stage I各基元步的标准吉布斯自由能垒(Ea)及反应热(ΔE)
Table 1 Standard Gibbs free energy barriers ( Ea ) and reaction energies ( ΔE ) for each elementary step of Stage I on the Cu(111) and Cu2O(111) surfaces at 473.15 K
No. Elementary step Cu(111) Cu2O(111) Ea/eV ΔE/eV Ea/eV ΔE/eV r1 MeOH* → MeO*+H* 1.00 −0.05 0.32 0.27 r2 MeO* → PA*+H* 1.24 0.79 0.75 0.20 r3 EG* → Alko*+H* 0.88 −0.25 0.30 0.28 r4 Alko* → GcoH*+H* 1.06 0.67 0.76 0.05 r5 MG* → Acyl*+MeO* 0.94 0.40 2.47 2.00 r6 Acyl*+H* → GcoH* 0.54 −0.25 0.77 −0.58 
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表 2 473.15 K条件下Cu(111)及Cu2O(111)表面Stage II各基元步的标准吉布斯自由能垒(Ea)及反应热(ΔE)
Table 2 Standard Gibbs free energy barriers (Ea) and reaction energies (ΔE) for each elementary step of Stage II on the Cu(111) and Cu2O(111) surfaces at 473.15 K
No. Elementary step Cu(111) Cu2O(111) Ea/eV ΔE/eV Ea/eV ΔE/eV r7 GcoH* → Enl*+H* 0.90 0.46 0.57 0.42 r8 Enl*+PA* → Int1* 0.21 −0.48 0.59 0.06 r9 Int1*+H* → GceH* 1.01 0.21 −0.02 −0.30 r10 GceH* → Int2*+H* 0.57 0 0.36 0.02 r11 GceH* → Int3*+OH* 1.49 0.30 2.18 −0.03 r12 Int2* → 2HAcH*+OH* 0.51 −0.31 0.98 0.52 r14 H*+OH* → H2O* 1.13 −0.19 −0.01 −0.19
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表 3 473.15 K条件下Cu(111)及Cu2O(111)表面Stage III各基元步的标准吉布斯自由能垒(Ea)及反应热(ΔE)
Table 3 Standard Gibbs free energy barriers ( Ea ) and reaction energies ( ΔE ) for each elementary step of Stage III on the Cu(111) and Cu2O(111) surfaces at 473.15 K
No. Elementary step Cu(111) Cu2O(111) Ea/eV ΔE/eV Ea/eV ΔE/eV r15 H2* → H*+H* 1.01 0.08 0.57 0.19 r16 2HAcH*+H* → Int4* 0.39 −0.46 0.38 −0.17 r17 2HAcH*+H* → Int5* 0.90 0.17 1.42 0.47 r18 2HAcH*+H* → Int6* 0.79 −0.03 0.64 0.70 r19 2HAcH*+H* → Int7* 0.89 −0.43 0.49 −0.15 r20 Int4*+H* → 2HPrL* 0.56 −0.14 0.17 −0.21 r24 2HPrL*+H* → Int8* 0.45 −0.74 0.67 −0.15 r25 2HPrL*+H* → Int9* 1.05 0.63 1.01 1.06 r28 Int8*+H* → 1,2-PDO* 1.03 0.15 0 −0.14
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