催化学报 ›› 2016, Vol. 37 ›› Issue (9): 1530-1538.DOI: 10.1016/S1872-2067(16)62484-1

• 论文 • 上一篇    下一篇

丝光沸石的酸性质调控及在二甲醚羰基化反应中的催化性能

王美霞, 黄守莹, 吕静, 程载哲, 李媖, 王胜平, 马新宾   

  1. 天津大学化工学院绿色合成与转化教育部重点实验室;天津化学化工协同创新中心, 天津 300072
  • 收稿日期:2016-04-13 修回日期:2016-05-30 出版日期:2016-09-21 发布日期:2016-09-21
  • 通讯作者: Shouying Huang, Xinbin Ma
  • 基金资助:

    国家自然科学基金(21325626,21406120);中国博士后科学基金(2014M560181,2015T80214).

Modifying the acidity of H-MOR and its catalytic carbonylation of dimethyl ether

Meixia Wang, Shouying Huang, Jing Lü, Zaizhe Cheng, Ying Li, Shengping Wang, Xinbin Ma   

  1. Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University;Collaborative Innovation Center of Chemical Science and Engineering(Tianjin), Tianjin 300072, China
  • Received:2016-04-13 Revised:2016-05-30 Online:2016-09-21 Published:2016-09-21
  • Contact: Shouying Huang, Xinbin Ma
  • Supported by:

    This work was supported by the National Natural Science Foundation of China(21325626, 21406120) and the Postdoctoral Science Foundation of China(2014M560181, 2015T80214).

摘要:

二甲醚(DME)羰基化制乙酸甲酯(MA)及MA加氢制乙醇,是一条新兴的合成气间接法制乙醇工艺.其中,DME羰基化合成MA反应原子经济性高,反应条件温和.特别是以丝光沸石(MOR)为催化剂时,反应在较低温度(473 K)下MA选择性即可达到99%,这使得该工艺具有良好的竞争力和工业化前景.在已有的文献报道中,学者们将MA的高选择性归结于分子筛八元环(8-MR)结构的限域效应,红外和DFT计算表明八元环的Brönsted酸位上可以专一性催化CO插入吸附态的甲基形成乙酰基中间体.为了建立八元环Brönsted酸位点与活性的关系,研究者一般采用化学脱铝和离子交换等方法减少八元环Brönsted酸位点,以观察反应活性降低的趋势.事实上,采用直接合成手段选择性地调控H-MOR的酸性,以此提高其羰基化活性的研究报道十分有限.因此,我们通过多种方法来调变MOR的骨架Al元素分布,包括改变水热合成中溶胶的组成、选择适宜的模板剂以及对样品进行酸处理等,获得了一系列酸量和酸分布不同的H-MOR分子筛.通过考察DME羰基化制MA的反应性能,进一步明确酸性调控对分子筛催化剂结构和性能的影响.首先通过XRD和N2物理吸附对所有样品的织构性质进行表征,结果发现各样品均为纯的MOR晶相,相对结晶度也较为相近.比表面积和孔分布计算结果显示,各样品的孔结构也大致相同.特殊地,以环己亚胺(HMI)为模板时得到的样品结晶度较低,比表面积和孔体积较小.这是因为HMI的碱性较弱,因此在水热合成过程中的结构导向力较弱,无法高效地促进分子筛成核.酸处理会造成分子筛骨架T原子少量脱除,导致结构轻微破坏,但总的来说影响不大.在排除了晶相与结构方面的差异后,我们基于不同环内酸性位点对不同尺寸碱性分子的可接近性存在差异这一特点,结合ICP-OES,27Al MAS NMR,NH3-TPD和Py-FTIR等多种表征方法对MOR分子筛八元环和十二元环的Brönsted酸位进行定量分析.通过计算骨架Al含量及NaM样品的对照,验证了NH3-TPD的峰位归属,认定高温处的NH3脱附峰能够准确反映H-MOR骨架中Brönsted的酸总量.通过Py-FTIR获得了H-MOR样品中十二元环的Brönsted酸量.由此发现通过不同方法得到的H-MOR催化剂上八元环内的Brönsted酸比例存在较大差异(55.5%-72.7%),并且普遍高于商业H-MOR样品(54.5%),说明我们通过控制合成条件或者酸处理可以有效地调控骨架Al元素的分布,进而导致Brönsted酸在不同环内的分布比例发生变化.尤其是当HMI作模板剂时,由于其分子尺寸较大,仅可以进入分子筛的十二元环,促使Al原子向八元环富集,因而在样品结晶度较低的情况下,八元环内的Brönsted酸仍显著增加.反应测试结果表明,不同催化剂虽然均具有较高的选择性,但反应初始活性差别明显.将MA的生成速率与八元环的Brönsted酸量相关联,发现二者呈线性正相关关系,由此验证了文献中八元环内Brönsted酸位是DME羰基化制MA反应的活性位点这一学术观点,同时更加肯定了我们对分子筛酸性的调控是有效且成功的.我们的实验结果为分子筛的酸量和酸分布调控提供了有益信息,也为后期DME羰基化反应催化剂的优化和设计提供了参考.

关键词: 沸石, 丝光沸石, 酸性质调控, 水热合成, 结构导向剂, 后处理, 二甲醚羰基化

Abstract:

Among the reactions catalyzed by zeolites there are some that exhibit high selectivity due to the spatial confinement effect of the zeolite framework. Tailoring the acidity, particularly the distribution and location of the Brönsted acid sites in the zeolite is effective for making it a better catalyst for these reactions. We prepared a series of H-mordenite(H-MOR) samples by varying the composition of the sol-gel, using different structure directing agents and post-treatment. NH3-TPD and IR characterization of adsorbed pyridine were employed to determine the amount of Brönsted acid sites in the 8-membered ring and 12-membered ring channels. It was shown that controlled synthesis was a promising approach to improve the concentration of Brönsted acid sites in MOR, even with a low Al content. Using an appropriate composition of Si and Al in the sol-gel favored a higher proportion of Brönsted acid sites in the 8-membered ring channels. HMI as a structure-direct agent gave an obvious enrichment of Brönsted acid sites in the 8-membered ring. Carbonylation of dimethyl ether was used as a probe reaction to examine the modification of the acid properties, especially the Brönsted acid sites in the 8-membered ring channels. There was a linear relationship between methyl acetate formation and the number of Brönsted acid sites in the 8-membered ring channels, demonstrating the successful modification of acid properties. Our results provide information for the rational design and modification of zeolites with spatial constraints.

Key words: Zeolite, H-mordenite, Modification of acidity, Hydrothermal synthesis, Structure-direct agent, Post treatment, Carbonylation of dimethyl ether