催化学报 ›› 2022, Vol. 43 ›› Issue (8): 1964-1990.DOI: 10.1016/S1872-2067(21)64032-9
杨旭港a,†, 刘宗辉b,†, 魏国良a, 顾宇a,#, 施慧a,*()
收稿日期:
2021-12-17
接受日期:
2022-02-14
出版日期:
2022-08-18
发布日期:
2022-06-20
通讯作者:
顾宇,施慧
作者简介:
第一联系人:†共同第一作者
基金资助:
Xugang Yanga,†, Zonghui Liub,†, Guoliang Weia, Yu Gua,#, Hui Shia,*()
Received:
2021-12-17
Accepted:
2022-02-14
Online:
2022-08-18
Published:
2022-06-20
Contact:
Yu Gu, Hui Shi
About author:
First author contact:†Contributed equally to this work.
Supported by:
摘要:
固体和水所形成的界面在各类化学和生物体系中非常常见, 围绕相关物化现象的研究也一直是界面科学的前沿热点. 然而, 多相催化研究中对固-液界面发生的催化转化过程背后的微观机制的认识依旧十分有限, 再加上水的诸多特殊理化性质, 理解固-水界面的多相催化反应极具挑战性.
本综述针对三类代表性的酸碱催化反应(醇类脱水、羟醛缩合和糖类异构), 总结了一系列水(包括水分子本身、溶于其中的离子和由水衍生而来的其他物种)在这些体系中对表界面催化行为、反应机理和构效关系的常见影响方式, 并批判性地归纳了业已提出的分子层面的观点和解释. 当水的化学势较高(液态水或者水分压较大)时, 其通常会抑制固体酸碱表面的催化反应, 原因可以归结为: 水分子在表面活性位上的竞争吸附、对活性位酸碱强度的削弱和对中间物种的溶剂化稳定作用(从而提高活化自由能能垒). 水的存在也可造成活性位性质发生变化(例如活性较低的Lewis酸向活性更高的Brønsted酸转化), 或直接/间接开辟新的反应路径, 从而提高催化反应速率. 此外, 最新研究还揭示了活性位和表面反应物种(包括过渡态)溶剂化过程中许多重要的微观现象, 包括: 水在限域孔道内形成团簇结构和横跨活性位的溶剂链、表面酸性质子发生转移和迁移、高(水合氢)离子浓度的纳米级限域介电环境等. 这些复杂的化学过程都会改变反应中关键中间体和过渡态的能量稳定性, 从而显著影响催化剂的催化活性、选择性和稳定性. 虽然从现有研究中已能挖掘出一些普适性的规律和原则, 但有关固-水界面催化中不少现象和效应的解释仍局限于特定体系(水的物相、反应类型、条件和化学微环境等). 在各种错综复杂的因素中, 着重关注氢键相互作用和界面水合离子在水相和富水液相体系中的酸碱催化反应里的独特角色, 同时深入剖析水对酸碱催化剂表面活性中心的溶剂化效应, 并基于已有的光谱证据和理论框架, 探讨其物理化学本质. 本文最后还提出了该前沿研究领域的一些展望, 及该领域面向未来的几项重要研究任务.
杨旭港, 刘宗辉, 魏国良, 顾宇, 施慧. 固-水界面的酸碱催化反应中水分子和溶剂化离子的角色[J]. 催化学报, 2022, 43(8): 1964-1990.
Xugang Yang, Zonghui Liu, Guoliang Wei, Yu Gu, Hui Shi. A critical assessment of the roles of water molecules and solvated ions in acid-base-catalyzed reactions at solid-water interfaces[J]. Chinese Journal of Catalysis, 2022, 43(8): 1964-1990.
Fig. 1. (a) Bimolecular ethanol dehydration turnover rate (per H+, 373 K) on H-Al-Beta-F (synthesized in HF medium) as a function of the C2H5OH/H2O pressure ratio, in different ranges of H2O pressures. (b) Apparent first-order bimolecular ethanol dehydration rate constant (per H+, 373 K) as a function of H2O pressure on H-Al-Beta-F (■), H-Al-TON (▲), H-Al-FAU (◆), H-Al-MFI (●), H-Al-AEI (□), H-Al-CHA (○), and HPW/Si-MCM-41 (). (c) Free energy diagram illustrating the effects of solvation by extended hydrogen-bonded H2O networks. Adapted with permission from Ref. [31]. Copyright 2020, Royal Society of Chemistry.
Fig. 2. (a) Effects of calcination temperature of MoO3-ZrO2 catalysts (Mo/Zr = 0.1) on the conversion of 2-butanol dehydration in the absence of water (○) and in the presence of water at 44 kPa (●) at 423 K. (b) Reversible influence of 44 kPa water on the MoO3-ZrO2 catalyst calcined at 1073 K. (c) Effects of partial pressures of water on the 2-butanol dehydration and esterification over the MoO3-ZrO2 catalyst calcined at 1073 K (○) and SiO2-Al2O3 (●). All reactions were performed in the gas phase with He as balance. Adapted with permission from Ref. [28]. Copyright 2002, Springer Nature.
Fig. 3. Increasing hydration of the BAS leads to a shift in the identity of the rate-determining step (RDS) in E1-type elimination of an alkanol (ROH) to an alkene ([R-H]=). The left figure illustrates a plausible E1-type elimination path for alcohol dehydration on a dry BAS (H+Z-), for which the C-O scission step is the RDS, while the right figure shows a plausible E1-type path for alcohol dehydration over hydrated hydronium ions (H3O+hydr.), for which the C-H scission step becomes the RDS. Note the energy levels of TS2, TS3 and that of the intermediate prior to C-H cleavage (hydrated carbenium ion or surface alkoxide), relative to the respective initial state in either case, are most dramatically impacted by the solvation of water, causing such a shift in the identity of the rate-determining step and the kinetically relevant transition state.
Fig. 4. Effects of intrapore ionic strength on aqueous-phase dehydration of cyclohexanol over zeolites. (a) Comparison of TOFs of aqueous-phase cyclohexanol dehydration catalyzed by hydrated hydronium ions (H3O+hydr.) in H-MFI and H-BEA pores at 423 K. (b) Reaction free-energy barriers and excess chemical potential of the ground state (GS) and transition state (TS) under the ideal condition and under an ionic strength. (c) Schematic illustration of H3O+hydr. and cyclohexanol in H-MFI micropore channels and the mean distance dh-h between two neighboring H3O+hydr. and the mean distance db-b and volume Vb-b between the boundaries of neighboring H3O+hydr.. (d) Enthalpy of the ground and transition states as a function of db-b and Vb-b. (e) Gibbs free energy landscape of aqueous-phase cyclohexanol dehydration catalyzed by intrapore H3O+hydr. under ideal and nonideal (with non-negligible ionic strength) conditions. Adapted with permission from Ref. [65]. Copyright 2021, American Association for the Advancement of Science.
Fig. 5. Solvation effects on acid-catalyzed reactions in monophasic mixtures of water and polar aprotic solvent. (a) Gibbs free energy surface in H2O and polar aprotic organic solvents of the conversion of reactant R into product P catalyzed by a Brønsted acid. (b) Ratio of the turnover frequencies (TOF) for xylose conversion in gamma-valerolactone (GVL) and H2O versus the pKa value for homogeneous Brønsted acid catalysts. Experimental (?) and theoretical (—) TOF ratios are given. (c) TOFs for the dehydration of xylose to furfural in purely aqueous phase and in the GVL(90 wt%)-H2O (10 wt%) solvent mixture for heterogeneous acid catalysts (silicotungstate dissolves in both liquids). Note the TOFs of aqueous-phase dehydration of xylose has been multiplied by a factor of 5. Adapted wither permission from Ref. [71]. Copyright 2014, Wiley-VCH.
Fig. 6. (a) DFT calculations of H2O molecule adsorption on an amorphous SiO2 surface functionalized with SO3H groups and (b) an illustration of the concept of water-extended remote bond polarization, where the color codes indicate: Si (blue); O (red); C (silver); H (white); S (cream); H (green) of SO3H group; transferred H (cerulean). Adapted with permission from Ref. [87]. Copyright 2021, Elsevier.
Fig. 7. Adsorbed water fragments (OH groups) affect the adsorption strength and deformation severity of DAA on the Zn1Z11O12 surface: (upper panel) side views of diacetone alcohol adsorption at four sites in the vicinity of an adsorbed OH group on a hydrated surface type; (lower panel) side views of diacetone alcohol adsorption at four sites on the dehydrated surface. Zr, Zn, C, O, and H atoms or ions are represented by green, gray, black, red, and white spheres, respectively. Partial charge densities are presented by yellow isosurfaces for electron gains and by blue isosurfaces for electron losses. Adapted with permission from Ref. [90]. Copyright 2021, American Chemical Society.
Fig. 8. Baseline-corrected difference IR spectra of adsorbed water on a representative hydrophobic sample Ti-Beta-F-155 with 1.93 × 10-4 molSiOH g-1 (top, magnified for clarity) and a representative hydrophilic sample Ti-Beta-OH-46 with 7.02 × 10-4 molSiOH g-1 (bottom) at 298 K for (a) the δ(HOH) scissoring modes in the water bending region and (b) the ν(O-H) water-stretching region. Difference spectra reflect the subtraction of the spectrum measured on the sample under vacuum prior to water flow and corrected for background water adsorption onto the IR cell. Spectra for each sample displayed from bottom to top correspond to P/P0 = 0.1, 0.2, 0.5, and 0.75. The insets display the change in (a) the water bending peak area and (b) the water-stretching peak maximum with increasing water concentration for Ti-Beta-F-155 (●) and Ti-Beta-OH-46 (▲). Reproduced with permission from Ref. [102]. Copyright 2018, American Chemical Society.
Fig. 9. Stability of pristine and silylated H-Beta zeolites during cyclohexanol dehydration: (a) Correlation between the lifetime of a BEA catalyst and the concentrations of Brønsted acid sites during catalysis, measured at 443 K; (b) Estimated water concentration in the zeolite micropores as determined from measuring the uptake of cyclohexanol in an aqueous solution (0.33 mol/L) at room temperature. Note the linear correlation between Brønsted acid concentration and water uptake; any upward deviation is linked to adsorption on defect sites, shown by green arrows. Adapted with permission from Ref. [106]. Copyright 2017, American Chemical Society.
Fig. 10. Effects of dynamic reorganization of water clusters stabilized at hydrophilic silanol defects in Ti-zeolites, shown for olefin epoxidation in acetonitrile (in the presence of H2O2 and some water) as an example: (a) Free energy landscape with a set of elementary steps to form epoxidation transition states over (left) hydrophobic Ti zeolites that contain few (SiOH)x defects and H2O molecules near the Ti-active sites and (right) hydrophilic materials with many (SiOH)x defects near the active sites that entrain more H2O molecules proximate to the reaction centers. (b) Enthalpy (ΔHexcess) and entropy (ΔSexcess) compensation relationships for disruption of confined H2O structures within Ti-FAU (orange), Ti-BEA (blue), and Ti-MFI (purple) zeolites during 1-hexene (■), 1-octene (●) and 1-decene (▲) epoxidation reactions. The shaded region is intended to represent the span of enthalpy-entropy compensation expected within these different pore environments due to the differences in (SiOH)x for a given zeolite topology. Adapted with permission from Ref. [110]. Copyright 2021, Springer Nature.
Fig. 11. Impact of zeolite Si/Al and hydronium ion concentrations on the adsorption of cyclohexanol in water. (a) Schematic illustration of positively charged hydronium ions and negatively charged framework Al sites in H-MFI zeolite crystal in water. (b) Adsorption isotherm of cyclohexanol on H-MFI zeolite in aqueous phase at 298 K. (c) experimentally determined standard adsorption constants (Ka,exp○) and the associated heat of adsorption (Qa,exp) of cyclohexanol on H-MFI as a function of BAS concentration in H-MFI at 298 K. (d) Ka,exp○ of cyclohexanol on H-MFI as a function of the activity coefficient (or its inverse) of the intrapore phase of cyclohexanol. Adapted with permission from Ref. [48]. Copyright 2019, Wiley-VCH.
Fig. 12. Effects of pore condensation on catalysis at the interface between a porous catalyst and a vapor phase. (a) An illustration of a porous catalyst which is in contact with gas-phase water but forms inside its pore systems varying degrees of liquid-like water (the extent of pore filling depends on the pore size at a given pressure). (b) Pore volume fraction for small-pore Ni-Al-MCM-41 (pore diameter 1.7 ± 0.5 nm) filled with N2 at 77 K (continuous line) or alkenes at 248 K (dashed lines), and for large-pore Ni-Al-MCM-41 (pore diameter 3.5 ± 2.5 nm) filled with N2 at 77 K (dotted line) as a function of the corresponding relative saturation pressure. (c) First-order deactivation constants during olefin dimerization on small-pore Ni-Al-MCM-41 catalyst as a function of relative saturation pressures for olefin reactants. The (b) and (c) are reproduced with permission from Ref. [114]. Copyright 2017, Elsevier.
Fig. 13. In situ titration during liquid-phase reactions allow more rigorous comparisons of site-specific activities. (a) In situ titration of SO3H-functionalized MCM-41 catalysts with pyridine during the cyclopentanone aldol condensation. (b) TOF (turnover frequency) of MCM-41-SO3H catalysts with varying acid density [87]. (c) In situ titration of active sites using pyridine (which potentially binds to both BAS and LAS; blue filled diamonds) and 2,6-dimethylpyridine (which binds more selectively to BAS; red filled squares) in H-MFI and H-Beta zeolites during cyclohexanol dehydration in water [24]. In (a) and (b), the TOFpair and TOFsingle correspond to the site-specific activities of spatially close acid sites and isolated acid sites, respectively; in (c), the identical trends of rate decrease as a function of the cumulative titrant uptake for both base titrants and on both fresh and reused catalysts (green filled circles indicate a fresh catalyst prior to titration; open triangles indicate experimental data collected on catalysts recycled after its first service in this reaction) demonstrate that BAS (hydronium ions) are exclusive active sites for the alcohol dehydration reaction and that these zeolite catalysts did not deactivate after the aqueous-phase reaction. Adapted with permission from Refs. [24,87]. Copyright 2017, Springer Nature; Copyright 2021, Elsevier.
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