Effect of NaOH content for the properties of HZSM-5 zeolites and its catalytic performance on gas phase carbonylation of dimethoxymethane
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摘要: 通过调变合成过程中NaOH的含量，制备了一系列HZSM-5分子筛，并对其催化甲缩醛（DMM）气相羰基化合成甲氧基乙酸甲脂（MMAc）反应性能进行了详细考察。结果表明，本合成体系中，NaOH含量为0.8%（质量分数）时，ZSM-5分子筛表现出最佳的催化活性。BET、27Al NMR、NH3-TPD、Py-FTIR等多种表征结果证实NaOH含量的改变可有效调变分子筛孔道中介孔孔容及中强Brønsted酸（B酸）位点的分布，两者是影响分子筛催化活性的主要因素。中强B酸位点增加，原料DMM反应加剧，转化率提高；介孔孔容增大，产物扩散途径缩短，孔道限域效应减弱，副反应被抑制，MMAc选择性增加。进一步采用密度泛函理论对DMM与HZSM-5分子筛作用过程进行了初步探索，发现反应过程中将首先形成甲氧基甲基（ZOCH2OCH3）中间物种，在此基础上，提出了DMM羰基化生成MMAc的可能机制。Abstract: A series of ZSM-5 zeolites were prepared by changing the NaOH content, and their catalytic performance on the vapor-phase carbonylation of dimethoxymethane (DMM) to synthesize methyl methoxyacetate (MMAc) was investigated detailedly. The results indicate that the ZSM-5 zeolite prepared under the 0.81% NaOH content shows the best catalytic performance. Various characterization results, including BET, 27Al NMR, NH3-TPD and Py-FTIR, illustrate that medium-strong Brønsted acid sites and mesoporous volume are the chief factors in promoting carbonylation of DMM over ZSM-5 zeolite, which can be effectively regulated by changing NaOH content. The increase of medium-strong Brønsted acid sites can improve DMM conversion by providing more active acid sites; the introduction of mesoporous can increase MMAc selectivity by shortening the product diffusion path, weakening steric constraint of pore walls and suppressing parts of side reactions. Density functional theory was further carried out to study the interaction between DMM and ZSM-5 zeolite. The calculated results find that intermediate species ZOCH2OCH3 is formed firstly during DMM decomposition. Based on this, a possible formation mechanism of MMAc was then proposed.
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Key words:
- dimethoxymethane /
- carbonylation /
- methyl methoxyacetate /
- NaOH /
- HZSM-5
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图 1 不同NaOH含量合成HZSM-5分子筛的XRD谱图
Figure 1 XRD patterns of HZSM-5 zeolites prepared under different NaOH contents
图 2 不同NaOH含量合成HZSM-5分子筛的SEM照片
Figure 2 SEM images of HZSM-5 zeolites prepared under different NaOH contents
图 3 不同NaOH含量合成的HZSM-5分子筛的氮气吸附-脱附等温曲线
Figure 3 Nitrogen sorption isotherms of HZSM-5 zeolites prepared under different NaOH contents
图 4 不同NaOH含量合成HZSM-5分子筛的27Al MAS NMR谱图
Figure 4 27Al MAS NMR spectra of HZSM-5 catalysts prepared under different NaOH contents
图 5 不同NaOH含量合成HZSM-5分子筛的NH3-TPD谱图
Figure 5 NH3-TPD profiles of HZSM-5 zeolites prepared under different NaOH contents
图 6 不同NaOH含量合成HZSM-5分子筛的Py-FTIR谱图
Figure 6 Py-FTIR spectra of HZSM-5 zeolites prepared under different NaOH contents
图 7 DFT理论预测DMM在ZSM-5分子筛上的反应路径示意图
Figure 7 Theoretically projected reaction path for DMM over ZSM-5
图 8 ZSM-5分子筛上甲缩醛降解势能面示意图
Figure 8 Potential energy surface of DMM decomposition over ZSM-5 zeolite
图 9 DMM羰基化过程的可能反应机制
Figure 9 Proposed mechanism of gas phase carbonylation of dimethoxymethan
表 1 不同NaOH含量合成HZSM-5分子筛的组成及织构性质
Table 1 Composition and textural properties of HZSM-5 catalysts prepared under different NaOH contents
Sample Si/Al molar ratioa ABETb/(m2·g-1) vmicroc/(cm3·g-1) vmeso/(cm3·g-1) Z-0.14 27 328 0.15 0.10 Z-0.27 25 351 0.13 0.16 Z-0.54 23 357 0.13 0.17 Z-0.81 20 362 0.12 0.21 Z-1.1 16 359 0.12 0.17 Z-1.3 15 344 0.12 0.12 note: a: determined by XRF; b: determined by the BET method; c: determined by the t-plot method 
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表 2 不同NaOH含量合成HZSM-5分子筛的酸性特征
Table 2 Acidic properties of HZSM-5 zeolites prepared under different NaOH contents
Sample Acidity by strengthd /(mmol·g-1) Acidity by typee /(mmol·g-1) weak medium-strong Brønsted Lewis Z-0.14 0.28 0.26 0.43 0.43 Z-0.27 0.33 0.42 0.52 0.29 Z-0.54 0.40 0.48 0.53 0.24 Z-0.81 0.44 0.48 0.59 0.27 Z-1.1 0.68 0.64 0.58 0.20 Z-1.3 0.82 0.66 0.56 0.24
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表 3 不同NaOH含量合成ZSM-5分子筛的催化性能
Table 3 Catalytic performance of H-ZSM-5 zeolites prepared under different NaOH contents
Sample DMM conversion x/% Products selectivity s/% MMAc MF MeOH DME Z-0.14 2.5 35.3 35.9 3.1 25.7 Z-0.27 4.6 40.3 31.9 2.1 25.6 Z-0.54 9.9 61.3 18.6 1.4 18.6 Z-0.81 11.9 69.3 17.2 1.2 12.2 Z-1.1 9.3 56.8 24.6 1.5 17.2 Z-1.3 7.1 59.6 21.0 1.9 17.5 reaction conditions: 110 ℃, 0.6 MPa, DMM and CO rate of flow is 2.4×10-2 mol/h and 0.27 mol/h, 0.5 g catalysts
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