Site titration with organic bases during catalysis: selectivity modifier and structural probe in methanol oxidation on Keggin clusters.
Site titration with organic bases during catalysis: selectivity modifier and structural probe in methanol oxidation on Keggin clusters.
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DOI：
10.1002/anie.200352393
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发表时间：
2003-10
影响因子：
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通讯作者：
Haichao Liu;N. Bayat;E. Iglesia
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作者：
Haichao Liu;N. Bayat;E. Iglesia
催化反应通常需要具有不同功能的几种类型的位点，并且这些位点的相对丰度影响所需反应的速率和选择性。具有Keggin结构的杂多酸簇合物，含有酸和氧化还原功能，2]最近成为涉及双功能途径的有机反应的有趣催化剂。我们最近发现了一个在低温（453-493 K）下甲醇在未负载和SiO2负载的H3+ nPVnMo 12 nO 40（n= 0- 4）Keggin簇上选择性氧化一步合成二甲氧基甲烷（CH 3 OCH 2 OCH 3，DMM，甲缩醛）的反应。DMM的产率和选择性（基于不存在二甲醚）类似于使用负载型ReOx催化剂获得的产率和选择性，负载型ReOx催化剂是能够以显著产率形成DMM的唯一催化剂。DMM合成需要氧化还原和布朗斯台德酸位点，该反应涉及CH 3OH氧化脱氢为甲醛（HCHO），CH 3OH/HCHO混合物的酸催化缩醛化，以及半缩醛或甲氧基甲醇中间体（在缩醛化反应中形成）与CH 3OH缩合形成DMM。Br 8 nsted酸度是完成DMM合成所必需的，但反应速率主要由氧化还原位点上的HCHO的初始形成控制，在我们的研究中，通过改变Keggin结构的分散度和V/Mo比来改变HCHO的密度和反应性，从而改变DMM合成速率。由于平衡电荷所需的化学计量，这些组成变化导致酸位点数量的同时变化。涉及CH 3OH脱水的副反应会降低DMM的选择性。这些反应由H3+ nPVnMo 12 nO 40中的强酸性质子催化，并导致不期望的二甲醚（DME）的形成。后一种产物最终转化为HCHO和DMM产物，甚至可以重新形成CH 3OH，但形成DMM的速度比CH 3OH慢。[3]因此，通过这些副反应形成DME需要更长的停留时间以实现从CH 3OH高产率地制备DMM。我们在这里报告的选择性滴定的质子与有机碱，以控制密度的酸网站在Keggin集群和测量其分散;在这两种情况下，我们这样做的催化反应过程中，所施加的动态变化的可访问性，所产生的反应的极性分子对Keggin集群的要求。以这种方式，我们能够测量周转率（每暴露Keggin单位; KU），并控制氧化还原和酸的性质独立为一个给定的组合物的Keggin集群。这种方法导致了前所未有的DMM选择性（> 80%）和一族稳定的有机-无机复合材料，其为广泛类别的氧化还原-酸双功能反应提供有效的双功能催化剂。Keggin结构的分散性是通过在CH 3OH和O2的混合物的催化反应期间用空间位阻吡啶（2，6-二叔丁基吡啶）滴定Brønsted酸位点来测量的。这种2，6-二叔丁基吡啶滴定剂可以质子化布朗斯台德酸中心，但由于N原子附近的空间限制，它不能与刘易斯酸中心相互作用。其本质上的疏水特性也防止其溶解和迁移到Keggin簇的二级结构中。这一结果与更极性的吡啶滴定剂形成鲜明对比，后者溶解并渗透到这些二级结构中。因此，在CH 3OH反应过程中吸收2，6-二叔丁基吡啶（每KU）反映了可获得质子的数量，并且对于给定的H3+ nPVnMo 12 nO 40化学计量，在支撑和未支撑的二级结构中，在外表面处可获得的Keggin结构的分数。我们注意到，这样的滴定必须在反应过程中进行，因为已知的各种反应物的能力，溶剂化和暴露的Keggin簇的二级包装结构内的内部区域在不同程度上。在H5 PV 2 Mo 10 O 40/SiO2（SiO2表面密度为0.28 KUnm 2）上，在453 K下CH 3OH和O2反应期间吸附的2，6-二叔丁基吡啶分子数量随时间增加，并在约12 ℃ 10 s后达到饱和，达到1.2 H/KU（图1）。该值对应于0.24的标称分数分散，基于预期的H/KU化学计量;然而，我们注意到，化学计量起始簇中的一些质子可能在溶剂化Keggin簇中的OH基团与硅烷醇锚定到SiO2的缩合反应期间被除去。当H5 PV 2 Mo 10 O 40/SiO2样品上的KU表面密度从0.1增加到0.65KUnm2时，H/KU比率从1.6/1降低到0.7/1，这对应于分数KU分散从0.32降低到0.15。对于散装H5 PV 2 Mo 10 O 40，该值为0.02 H/KU。随着表面密度从0.1增加到0.65 KUnm 2，DMM合成速率（每KU）与KU分散分数的降低平行降低（图2）。对于所有样品，包括该Keggin组合物的无载体版本，速率和滴定剂吸收之间的这种极好的相关性表明，2，6-二叔丁基吡啶主要滴定参与双官能DMM的那些Keggin结构[*] Prof. Dr. E.伊格莱西亚博士Liu，N.美国加州大学伯克利分校化学工程系，化学科学部，E.O.劳伦斯伯克利国家实验室Berkeley，CA 94720（USA）传真：（+1）510 -642-4778电子邮件：iglesia@cchem.berkeley.edu
Catalytic reactions often require several types of sites with distinct functions, and the relative abundance of these sites influences the rate and selectivity of desired reactions. Heteropolyacid clusters with Keggin structures, which contain acid and redox functions, 2] have recently emerged as interesting catalysts for organic reactions involving bifunctional pathways. We recently discovered a selective one-step synthesis of dimethoxymethane (CH3OCH2OCH3, DMM, methylal) by oxidation of methanol at low temperatures (453–493 K) on unsupported and SiO2-supported H3+nPVnMo12 nO40 (n= 0– 4) Keggin clusters. The yields and selectivities (based on the absence of dimethyl ether) of DMM resemble those obtained using supported ReOx catalysts, the only catalysts that enable the formation of DMM in substantial yields. Redox and Brønsted acid sites are required for DMM synthesis, and the reaction involves oxidative dehydrogenation of CH3OH to formaldehyde (HCHO), acid-catalyzed acetalization of CH3OH/HCHO mixtures, and condensation of hemiacetal or methoxymethanol intermediates (formed in acetalization reactions) with CH3OH to form DMM. Br8nsted acidity is required to complete the synthesis of DMM, but reaction rates are predominately controlled by the initial formation of HCHO on redox sites, the density and reactivity of which were varied in our studies by changing the dispersion and V/Mo ratio of the Keggin structures, with consequent changes in the rates of DMM synthesis. These compositional changes led to concurrent changes in the number of acid sites, because of the stoichiometry required to balance the charge. DMM selectivity is decreased by side reactions involving CH3OH dehydration. These reactions are catalyzed by strongly acidic protons in H3+nPVnMo12 nO40 and lead to the undesired formation of dimethyl ether (DME). The latter product ultimately converts into HCHO and DMM products, and can even re-form CH3OH, but forms DMM more slowly than CH3OH. [3] Thus, the formation of DME through these side reactions necessitates longer residence times to achieve high yields of DMM from CH3OH. We report here the selective titration of protons with organic bases to control the densities of acid sites in Keggin clusters and to measure their dispersion; in both cases we do this during the catalytic reaction, a requirement imposed by the dynamic changes in accessibility that arise from reactions of polar molecules on Keggin clusters. In this manner we are able to measure turnover rates (per exposed Keggin unit; KU) and to control the redox and acid properties independently for a given composition of Keggin cluster. This approach has led to unprecedented DMM selectivities (> 80%) and to a family of stable organic–inorganic composites that provide effective bifunctional catalysts for broad classes of redox–acid bifunctional reactions. The dispersion of Keggin structures was measured by titration of Brønsted acid sites with a sterically hindered pyridine (2,6-di-tert-butylpyridine) during catalytic reactions of mixtures of CH3OH and O2. This 2,6-di-tert-butylpyridine titrant can protonate Brønsted acid sites, but it cannot interact with Lewis acid sites because of steric constraints near the N atom. Its essentially hydrophobic character also prevents its dissolution and migration into secondary structures of Keggin clusters. This result is in contrast with more polar pyridine titrants, which dissolve and penetrate into these secondary structures. Thus, uptake of 2,6-di-tert-butylpyridine during CH3OH reactions (per KU) reflects the number of accessible protons, and for a given H3+nPVnMo12 nO40 stoichiometry, the fraction of the Keggin structures accessible at external surfaces in supported and unsupported secondary structures. We note that such titrations must be carried out during the reaction, because of the known ability of various reactants to solvate and expose internal regions within secondary packing structures of Keggin clusters to varying degrees. The number of 2,6-di-tert-butylpyridine molecules adsorbed during reactions of CH3OH and O2 at 453 K on H5PV2Mo10O40/SiO2 (0.28 KUnm 2 surface density on SiO2) increased with time and reached saturation at 1.2 H per KU after about 12 C 10 s (Figure 1). This value corresponds to a nominal fractional dispersion of 0.24, on the basis of the expected H/KU stoichiometry; we note, however, that some of the protons in the stoichiometric starting cluster may have been removed during condensation reactions of OH groups in solvated Keggin clusters with silanols on anchoring to SiO2. H/KU ratios decreased from 1.6/1 to 0.7/1, which corresponds to a decrease in fractional KU dispersion from 0.32 to 0.15, as the KU surface densities increased from 0.1 to 0.65 KUnm 2 on H5PV2Mo10O40/SiO2 samples. This value was 0.02 H per KU for bulk H5PV2Mo10O40. The rates of DMM synthesis (per KU) decreased in parallel with this decrease in fractional KU dispersion as the surface density increased from 0.1 to 0.65 KUnm 2 (Figure 2). This excellent correlation between rates and titrant uptake for all samples, including an unsupported version of this Keggin composition, indicates that 2,6-di-tert-butylpyridine predominately titrates those Keggin structures that participate in bifunctional DMM [*] Prof. Dr. E. Iglesia, Dr. H. Liu, N. Bayat Department of Chemical Engineering University of California at Berkeley, and Chemical Sciences Division, E.O. Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) Fax: (+1)510-642-4778 E-mail: iglesia@cchem.berkeley.edu