Ammonia Titration Methods To Quantify Bronsted Acid Sites in Zeolites Substituted with Aluminum and Boron Heteroatoms
Ammonia Titration Methods To Quantify Bronsted Acid Sites in Zeolites Substituted with Aluminum and Boron Heteroatoms
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DOI：
10.1021/acs.iecr.8b00933
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发表时间：
2018-05-16
影响因子：
4.2
通讯作者：
Gounder, Rajamani
中科院分区：
文献类型：
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作者：
Kester, Philip M.;Miller, Jeffrey T.;Gounder, Rajamani
氨滴定方法被开发来区分和量化不同强度的布朗斯台德酸位点，其补偿沸石骨架内并入的铝和硼杂原子。合成了具有不同Al含量和微晶尺寸的硼硅酸盐和MFI沸石（B-Al-MFI），这通常是水热合成的铝硅酸盐中相关的结构性质，但在此通过引入硼作为第二骨架杂原子和使用乙二胺作为结构导向剂而独立地变化。采用程序升温脱附法（TPD）对经液相NH（4）NO（3）离子交换饱和的B-Al-MFI样品中的氨进行了定量分析。相比之下，在流动氦气（433 K）中吹扫NH 4-形式B-Al-MFI样品之后或在气相NH 3吸附（433 K）到H-形式B-Al-MFI样品上之后进行的TPD仅定量质子电荷补偿骨架Al杂原子。甲醇脱水为二甲醚的周转率，当在主要对布朗斯台德酸强度敏感的零级动力学制度中测量时，仅取决于B-Al-MFI沸石中补偿骨架Al原子的质子数。这里开发的NH3滴定方法是有用的，在严格规范化的Bronsted酸催化反应在硼铝硅酸盐沸石，这已被公认为以前仅依赖于铝含量的周转率。将B杂原子结合到沸石骨架中，其产生基本上不反应的质子，提供了一种策略来影响与Al含量无关的微晶尺寸，特别是在催化行为受晶内传输现象影响的情况下。
Ammonia titration methods were developed to discriminate and quantify Bronsted acid sites of different strength that compensate aluminum and boron heteroatoms incorporated within zeolite frameworks. Borosilicate and MFI zeolites (B-Al-MFI) were synthesized with different Al contents and crystallite sizes, which are typically correlated structural properties in aluminosilicates synthesized hydrothermalfy, but independently varied here by incorporating boron as a second framework heteroatom and using ethylenediamine as a structure directing agent. Temperature-programmed desorption (TPD) of ammonia from B-Al-MFI samples saturated via liquid-phase NH(4)N0(3) ion exchange resOlted in quantifying the total number of Al and B heteroatoms. In contrast, TPD performed after NH4-form B-Al-MFI samples were purged in flowing helium (433 K), or after gas-phase NH3 adsorption (433 K) onto H-form B-Al-MFI samples, quantified only protons charge-compensating framework Al heteroatoms. Turnover rates for methanol dehydration to dimethyl ether, when measured in zero-order kinetic regimes that are sensitive predominantly to Bronsted acid strength, are dependent only on the number of protons compensating framework Al atoms in B-Al-MFI zeolites. The NH3 titration methods developed here are useful in rigorously normalizing turnover rates of Bronsted acid-catalyzed reactions in boroaluminosilicate zeolites, which have been recognized previously to be dependent solely on Al content. The incorporation of B heteroatoms into zeolite frameworks, which generate protons that are essentially unreactive, provides a strategy to influence crystallite sizes independently of Al content, especially relevant in cases where catalytic behavior is influenced by intracrystalline transport phenomena.