Pure and Al-substituted MgO catalysts are studied to identify the contributions of acid–base sites in the formation of two valuable xylene analogs, ortho- and para-tolualdehydes, from an ethanol derivative, acetaldehyde. The catalyst properties are characterized through XRD, ²⁷Al MAS NMR, ICP-AES, N₂ physisorption, TPD-MS, and DRIFTS experiments. Reactivity comparisons of untreated and CO₂-titrated catalysts at 250 °C, coupled with CO₂ DRIFTS studies on fresh and spent samples, indicate the formation of tolualdehydes from intermediates is initiated through deprotonation by a medium-strength basic site in a specific, metal–oxygen (M–O)-type coordination environment. Analyses of the catalytic surface properties and reactivity, pathways of formation, and natural bond orbital (NBO) charge distribution suggest C₄ + C₄ (rather than C₂ + C₆) mechanistic steps dominate tolualdehyde production over these catalysts under the investigated reaction conditions. Isomeric selectivity to ortho-tolualdehyde is 92 and 81 mol% over pure and Al-substituted MgO catalysts, respectively. We propose that the shift in isomeric selectivity towards para- upon introduction of a proximal Lewis acidic functionality (Al³⁺/MgO) to the catalyst is caused by electron redistribution in the conjugated enolate from the γ-C (forming ortho-) towards the α-C (forming para-) due to the carbonyl-O/Lewis acid coordination. This insight provides a framework for the development of next generation catalysts that give improved reactivity in cascade reactions of C₂ feedstocks to aromatics.

中文翻译：

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识别酸碱位在基于MgO的催化剂上从乙醛到甲苯甲醛形成途径中的作用

对纯和铝取代的MgO催化剂进行了研究，以鉴定酸碱部位在由乙醇衍生物乙醛形成的两种有价值的二甲苯类似物，邻-和对-甲苯醛中的作用。催化剂的性能通过XRD，²⁷ Al MAS NMR，ICP-AES，N ₂物理吸附，TPD-MS和DRIFTS实验表征。未处理的催化剂和CO ₂滴定的催化剂在250°C下与CO _2的反应性比较DRIFTS对新鲜和用过的样品进行的研究表明，中间体的甲苯甲醛形成是通过在特定的金属-氧气（M-O）型配位环境中，由中等强度的碱性位点通过去质子化而引发的。对催化表面性质和反应性，形成途径和天然键轨道（NBO）电荷分布的分析表明，在研究的反应条件下，C ₄ + C ₄（而不是C ₂ + C ₆）机理步骤主导了这些催化剂上甲苯的生产。与纯和铝取代的MgO催化剂相比，对邻甲苯甲醛的异构选择性分别为92和81 mol％。我们建议异构体选择性向对-在将近端Lewis酸性官能团（Al ³⁺ / MgO）引入催化剂后，是由于共轭烯醇化物中电子从γ-C（形成邻位-）向α-C（形成对位）的重新分布引起的羰基-O /路易斯酸配位。该见解为开发下一代催化剂提供了框架，该催化剂可改善C ₂原料与芳烃的级联反应的反应性。