An IR, FR, and TPD study on the acidity of H-ZSM-5, sulfated zirconia, and sulfated zirconia-titania using ammonia as the probe molecule
An IR, FR, and TPD study on the acidity of H-ZSM-5, sulfated zirconia, and sulfated zirconia-titania using ammonia as the probe molecule
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DOI：
10.1021/jp000937m
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发表时间：
2000-08-10
影响因子：
3.3
通讯作者：
Valyon, J
中科院分区：
文献类型：
--
作者：
Barthos, R;Lónyi, F;Valyon, J
在293-673K温度范围内，在133PaNH(3)压力下记录了H-ZSM-5沸石、ZrO(2)/SO(4)(2-)和ZrO(2)-TiO(2)/SO(4)(2-)分子筛的红外(IR)和频率响应(FR)光谱，结果表明，离子对络合物由固体B酸的I-He共轭碱位和[NH(4)]键组成。NNH(3)](+)缔合形成。温度越高，NH(3)覆盖度越低，生成的缔合物越小，即NH(4)(+)离子的酸强度越高。NH(3)的脱附伴随着质子从阳离子向硫酸盐氧化物的反向转移，表明这种缔合对离子对的能量稳定做出了重要贡献。在NH(3)/H-ZSM-5体系中，直到673K，几乎所有的质子都以缔合或NH(4)(+)离子的形式存在，因此，NH(4)(+)离子与沸石骨架的相互作用比NH(4)(+)离子与硫酸化氧化锆的相互作用更能有效地稳定离子对。酸中心的去质子化能和稳定(介质)效应决定了酸对碱进行质子化的效率，即酸强度。结果表明，仅用去质子化能，或任何反映O-H键强度的光谱参数，都不足以比较化学和结构上与硫化氧化锆和沸石完全不同的固体的酸度。在酸碱相互作用中，H-ZSM-5表现出比ZrO(2)/SO(4)(2-)更强的酸性，这是因为H-ZSM-5在沸石孔道中对吸附的碱或离子对的稳定性好于在氧化锆表面的稳定性。IR、FR和程序升温脱附(TPD)测试结果表明，硫酸化的ZrO(2)含有两种酸强度明显不同的Lewis酸中心和具有广泛酸强度分布的Bronsted酸中心。
Infrared (IR) and frequency-response (FR) spectra of zeolite H-ZSM-5, ZrO(2)/SO(4)(2-), and ZrO(2)-TiO(2)/SO(4)(2-) were recorded under 133 Pa of NH(3) pressure in the temperature range 293-673 K. It was shown that ion-pair complexes comprising i-he conjugated base sites of the solid Bronsted acid and H-bonded [NH(4). nNH(3)](+) associations were formed. Smaller associations or NH(4)(+) ions of higher acid strengths were obtained as NH(3) coverage decreased at higher temperatures. Desorption of NH(3) was accompanied by proton back-transfer from the cations to the sulfated oxides, indicating that the association contributed significantly to the energy stabilizing the ion pair. In the NH(3)/H-ZSM-5 system, virtually all the protons remained localized in associations or NH(4)(+) ions up to 673 K. Thus, the NH(4)(+) ion-zeolite framework interaction stabilizes the ion pair more effectively than the interaction of the NH(4)(+) ion with the sulfated zirconia. The deprotonation energy of the acid sites and also the stabilization (media) effect determine the efficiency of the acid in protonating a base, i.e., the acid strength. Results suggest that deprotonation energy alone, or any spectroscopic parameter reflecting the strength of the O-H bond, is not sufficient for comparing the acidities of solids that are chemically and structurally as different as sulfated zirconia and zeolite. In acid-base interactions, H-ZSM-5 exhibits stronger acidity than ZrO(2)/SO(4)(2-) due to the better stabilization of the adsorbed base or the ion pair in the zeolite channels than on the zirconia surface. Results of IR, FR, and temperature-programmed desorption (TPD) examinations suggest that sulfated ZrO(2) contains two kinds of Lewis acid sites of distinctly different acid strengths and Bronsted sites with a broad acid strength distribution.