Analysis of Local Structure, Acidic Property and Activity of Solid Acids by Solid-State Nuclear Magnetic Resonance Spectroscopy

doi:10.11938/cjmr20212913

Abstract

Abstract:

Solid acid is one of the most widely applied heterogeneous catalysts for industrial hydrocarbon conversion and biomass refining. It is crucial to understand the nature of solid acid such as its local structure and acidic properties. Such knowledge renders effective design of solid acid with better activity and stability for the desired reaction. Recently, solid-state nuclear magnetic resonance (SSNMR) spectroscopy has been applied as a powerful standard method for characterizing the local structure and acidic nature of solid acid in both qualitative and quantitative manners. Additionally, the applications of advanced two-dimensional SSNMR methodologies further reveal the symmetry of surface sites, spatial relationship of different sites, and thereby unmasking the structure-activity relationship. In this review, we summarize the general methods and SSNMR techniques for the routine characterization, focusing on the progresses in the understanding of local structure and acidic properties of solid acids via the application of SSNMR with or without probe molecules.

Key words: Brønsted acid sites, Lewis acid sites, solid acid, acid site structure, solid-state nuclear magnetic resonance (SSNMR) spectroscopy

CLC Number:

O482.53

Wen-jie YANG,Jun HUANG. Analysis of Local Structure, Acidic Property and Activity of Solid Acids by Solid-State Nuclear Magnetic Resonance Spectroscopy[J]. Chinese Journal of Magnetic Resonance, 2021, 38(4): 460-473.

Figures/Tables 8

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Table 1

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Table 2

Table 3

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References 78

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Solid acid	Hydroxyl group	δ_H	Assignment	Reference
Zeolite	Si(OH)	1.2~2.3	晶体表面或缺陷位的硅烷醇羟基	[25]
	Al(OH)	2.1~2.8	金属修饰分子筛上骨架外的桥接羟基	[26]
	Si(OH)Al	3.3~4.4	超笼中桥接羟基	[27-29]
	Si(OH)Al	4.6~8.0	方纳石笼中具有氢键或通过静电相互作用被骨架干扰的通道中桥接羟基	[34]
	Si(OH)Si	10.0~16.0	探针分子吸附后的探测到的双硅烷醇基或以氢键相连的内部硅烷醇基	[35]
Amorphous	Si(OH)	1.7~2.3	骨架上硅烷醇基	[36, 37]
Silica-alumina	Si(OH)Al/Al(OH)	2.5~4.0	酸性羟基	[36, 38]
Heteropoly acid	Keggin units H⁺	6.7~9.3	杂多酸的凯金(Keggin)结构中的酸性羟基	[39]
Metal-organic framework	Al(OH)Al	1.7~2.7	骨架上桥接羟基	[40]
Mixed oxides	M(OH)	2.0~6.7	末端金属羟基和桥连金属羟基	[41]
	M(OH)..H₂O..H₂O	9.5~10.3	通过氢键与水分子通过氢键相互作用两次的桥接金属羟基	[42]

Probe molecules	Strength of BAS	Strength of LAS	Reference
Deuterated acetonitrile (CD₃CN)	δ_H: 3.6 $ \to $ 9.6 Weak $ \to $ Strong	/	[66]
Pyridine-d5 (C₅D₅N)	δ_H: 12 $ \to $ 20 Strong $ \to $ Weak	/	[69, 70]
Acetone-2-¹³C ((CH₃)₂¹³CO)	δ_C: 216 $ \to $ 235 Weak $ \to $ Strong	δ_C: 233 $ \to $ 250 Weak $ \to $ Strong Super acid^* with δ_C≥ 245	[31, 71, 72]
¹⁵N-pyridine	δ_N: 212 $ \to $ 288 Strong $ \to $ Weak	δ_N: 260 $ \to $ 285 Strong $ \to $ Weak	[73]
Trimethylphosphine oxide (TMPO)	δ_P: 55 $ \to $ 86 Weak $ \to $ Strong	δ_P: 37 $ \to $ 55 Weak $ \to $ Strong	[64, 74]
Trimethylphosphine (TMP)	δ_P: -2 $ \to $ -5 Larger J-coupling constant between ³¹P and ¹H $ \to $ Stronger acid	δ_P: -13 ~ -52 δ_P depends on Lewis centers, for identical Lewis centers, smaller δ_P $ \to $ Stronger acid	[61, 65, 74]

Probe molecule	MAS NMR technique	Type		Strength		Density		Accessibility
Probe molecule	MAS NMR technique	B	L	B	L	B	L	Accessibility
Trimethylphosphine (TMP)	³¹P	√	√	√	√	√	√	√
Acetone-2-¹³C ((CH₃)₂¹³CO)	¹³C	√	√	√		❌	❌
Ammonia (NH₃)	¹H	√	√	❌	❌	√	√
Pyridine-d5 (C₅D₅N)	¹H	√	❌	√	❌		❌	√
¹⁵N-pyridine	¹⁵N	√	√	√	√	√	√	√
Trialkylphosphine oxides (R₃PO)	³¹P	√	√	√		√	√	√
Deuterated acetonitrile (CD₃CN)	¹H	√	❌	√	❌	√	❌
Perfluorotributylamine	¹H	√	❌	❌	❌		❌	√

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