Zeolites are versatile catalysts owing to their tunable acidity where the active site(s) can be placed within the crystal framework or as extra-framework species in confined pores. A common approach to tailor Brønsted acidity is heteroatom exchange involving the replacement of framework aluminum with alternative elements. Introduction of heteroatoms often results in less acidic catalysts that can be used in tandem reactions for targeted reactive intermediates. In this study, we prepare a series of three different zeolites with MWW, CHA, and MFI frameworks as aluminosilicates and gallosilicates to demonstrate their performance in alcohol dehydration reactions. The two reactions examined in this study are methanol conversion to dimethyl ether and ethanol conversion to ethylene. Our findings reveal that Ga-zeolites exhibit superior performance with Ga-MCM-22 (MWW) achieving nearly 100 % alcohol conversion and selectivity to desired products at contact times that are significantly less than most dehydration catalysts reported in literature. The unique properties of Ga-zeolites are attributed to their reduced acid site strength via a direct (one-pot) synthesis that avoids conventional time-intensive, multi-step post-synthesis modifications of Al-zeolites. Here we demonstrate the use of Ga-zeolites as tandem catalysts when paired with a downstream Al-zeolite in a dual-bed reactor configuration to convert cheaper reagents (alcohols) upstream to more desired intermediates as feeds for downstream catalysts to produce light olefins. We demonstrate that dual beds using Ga-zeolite – Al-ZSM-5 (upstream–downstream) pairings outperform single bed configurations with Al-ZSM-5 or a physical mixture (Ga-/Al-zeolite) for methanol to hydrocarbons reactions, with notable improvements in catalyst lifetime and increased light olefins selectivity. Similarly, dual beds using Ga-zeolite – Al-SSZ-13 (upstream–downstream) pairings result in improved performance for ethanol to propylene reactions with significant increases in propylene selectivity (>20 %) compared to a conventional single bed configuration of Al-SSZ-13. Overall, this study offers a new perspective on the use of heteroatom-exchanged zeolite catalysts in tandem reactions as a means of capitalizing on their reduced acid site strength and suggests differences in the intrinsic acidity of zeolite crystal structures.

中文翻译：

---

将具有定制酸度的 Ga/Al 沸石配对作为将醇转化为烯烃的串联催化剂

沸石是多功能催化剂，因为它们的酸度可调，其中活性位点可以放置在晶体框架内或作为有限孔中的框架外物质。调整布朗斯台德酸度的常用方法是杂原子交换，涉及用替代元素替换骨架铝。杂原子的引入通常会产生酸性较低的催化剂，可用于目标反应中间体的串联反应。在本研究中，我们制备了一系列具有 MWW、CHA 和 MFI 骨架的三种不同的沸石作为铝硅酸盐和没食子硅酸盐，以证明它们在醇脱水反应中的性能。本研究检查的两个反应是甲醇转化为二甲醚和乙醇转化为乙烯。我们的研究结果表明，Ga-沸石表现出优异的性能，Ga-MCM-22 (MWW) 可实现近 100% 的醇转化率和所需产物的选择性，接触时间明显少于文献报道的大多数脱水催化剂。 Ga-沸石的独特性能归因于其通过直接（一锅）合成降低了酸位点强度，避免了传统的耗时、多步骤的 Al-沸石合成后修饰。在这里，我们展示了使用Ga-沸石作为串联催化剂，当与双床反应器配置中的下游Al-沸石配对时，将上游更便宜的试剂（醇）转化为更理想的中间体，作为下游催化剂​​生产轻质烯烃的原料。我们证明，对于甲醇到碳氢化合物的反应，使用 Ga-沸石 – Al-ZSM-5（上游-下游）配对的双床优于使用 Al-ZSM-5 或物理混合物（Ga-/Al-沸石）的单床配置，催化剂寿命显着改善并提高了轻质烯烃的选择性。类似地，与 Al-沸石的传统单床配置相比，使用 Ga-沸石 - Al-SSZ-13（上游 - 下游）配对的双床可改善乙醇制丙烯反应的性能，并显着提高丙烯选择性 (>20%)。 SSZ-13。总体而言，这项研究为在串联反应中使用杂原子交换沸石催化剂作为利用其降低的酸位强度的手段提供了新的视角，并表明沸石晶体结构的固有酸性存在差异。