Ammonia IRMS-TPD study on the distribution of acid sites in mordenite
Ammonia IRMS-TPD study on the distribution of acid sites in mordenite
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DOI：
10.1021/jp051304g
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发表时间：
2005-10-13
影响因子：
3.3
通讯作者：
Atoguchi, T
中科院分区：
文献类型：
--
作者：
Niwa, M;Suzuki, K;Atoguchi, T
利用氨的IRMS-TPD（程序升温脱附），我们研究了丝光沸石的性质、强度、晶体学位置和酸中心的分布。在这种方法中，红外光谱（IR）和质谱（MS）一起工作，分别跟踪吸附和解吸的氨的热行为;因此，吸附的物种被识别，并且它们的热行为与温度升高过程中氨的解吸直接相关。IR-测量的NH 4+阳离子的TPD与MS-测量的TPD相似，因此显示了布朗斯台德酸性的性质。从OH带的行为，发现质子酸中心由高、低波数的两种OH带组成，分别归属于丝光沸石结构的12元环和8元环上的OH带。这些布朗斯台德羟基的量和强度使用曲线拟合法基于理论方程定量测量。多达约交换度为30%时，NH 4+在12-MR上与Na+交换达到饱和;因此，在该区域中，布朗斯台德酸中心位于12-MR的大孔上。然后NH 4+阳离子与8-MR上的Na+交换，并最终超过12-MR上的量。在99%NH 4-丝光沸石中，Bronsted酸中心主要位于8-MR上，而不是12-MR上。无论NH 4+交换程度如何，Bronsted OH的强度Δ H在12-和8-MR上分别为145和153 W mol（-1）;即，8-MR上的布朗斯台德酸中心的强度大于12-MR上的布朗斯台德酸中心的强度。密度泛函理论（DFT）计算支持酸中心强度的差异。8-MR上B酸中心的催化裂化活性迅速下降，而12-MR上B酸中心的催化裂化活性则保持不变。强度和/或空间容量的差异可能导致催化剂寿命的这种差异。
Using an IRMS-TPD (temperature programmed desorption) of ammonia, we studied the nature, strength, crystallographic location, and distribution of acid sites of mordenite. In this method, infrared spectroscopy (IR) and mass spectroscopy (MS) work together to follow the thermal behavior of adsorbed and desorbed ammonia, respectively; therefore, adsorbed species were identified, and their thermal behavior was directly connected with the desorption of ammonia during an elevation of temperature. IR-measured TPD of the NH4+ cation was similar to MS-measured TPD, thus showing the nature of Bronsted acidity. From the behavior of OH bands, it was found that the Bronsted acid sites consisted of two kinds of OH bands at high and low wavenumbers, ascribable to OH bands situated on 12- and 8-member rings (MR) of mordenite structure, respectively. The amount and strength of these Bronsted hydroxyls were measured quantitatively based on a theoretical equation using a curve fitting method. Up to ca. 30% of the exchange degree, NH4+ was exchanged with Na+ on the 12-MR to arrive at saturation; therefore, in this region, the Bronsted acid site was situated on the large pore of 12-MR. The NH4+ cation was then exchanged with Na+ on 8-MR, and finally exceeded the amount on 12-MR. In the 99% NH4-mordenite, Bronsted acid sites were located predominantly on the 8-MR more than on the 12-MR. Irrespective of the NH4+ exchange degree, the strengths Delta H of Bronsted OH were 145 and 153 W mol(-1) on the 12- and 8-MR, respectively; that is, the strength of Bronsted acid site on the 8-MR was larger than that on the 12-MR. A density functional theory (DFT) calculation supported the difference in the strengths of the acid sites. Catalytic cracking activity of the Bronsted acid sites on the 8-MR declined rapidly, while that on the 12-MR was remarkably kept. The difference in strength and/or steric capacity may cause such a difference in the life of a catalyst.