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催化研究中的意外发现:用于烷烃氧化脱氢的硼基材料

Serendipity in Catalysis Research: Boron-Based Materials for Alkane Oxidative Dehydrogenation.

作者信息

Venegas Juan M, McDermott William P, Hermans Ive

机构信息

Department of Chemical and Biological Engineering , University of Wisconsin-Madison , 1415 Engineering Dr. , Madison , Wisconsin 53706 , United States.

Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States.

出版信息

Acc Chem Res. 2018 Oct 16;51(10):2556-2564. doi: 10.1021/acs.accounts.8b00330. Epub 2018 Oct 4.

Abstract

Light olefins such as ethylene and propylene form the foundation of the modern chemical industry, with yearly production volumes well into the hundreds of millions of metric tons. Currently, these light olefins are mainly produced via energy-intensive steam cracking. Alternatively, oxidative dehydrogenation (ODH) of light alkanes to produce olefins allows for lower operation temperatures and extended catalyst lifetimes, potentially leading to valuable process efficiencies. The potential benefits of this route have led to significant research interest due to the wide availability of natural gas from shale deposits. Advances in this area have still not yielded catalysts that are sufficiently selective to olefins for industrial implementation, and ODH still remains a holy grail of selective alkane oxidation research. The main challenge in selective oxidation lies in preventing the overoxidation of the desired product, such as propylene during propane oxidation, to CO and CO. Research into selective heterogeneous catalysts for the oxidative dehydrogenation of propane has led to the extensive use of vanadium oxide-based catalysts, and studies on the surface mechanism involved have been used to improve the catalytic activity of the material. Despite decades of research, however, selectivity toward propylene has not proven satisfactory at industrially relevant conversions. It is imperative for new catalytic systems that minimize product overoxidation to be developed for future applications of oxidative dehydrogenation processes. While rational catalyst design has been successful in developing homogeneous catalyst systems, its practical use in heterogeneous catalyst development remains modest. The complexity of surfaces with a variety of terminations and bulk structures, let alone their modification by the chemical potential of a reaction mixture, makes heterogeneous catalyst discovery serendipitous in many cases. The catalyst family presented in this Account is no exception. The importance of catalysis research lies in exploring the science behind serendipity. In this Account, we will first present our initial discovery of boron nitride (BN) as an unexpected catalyst for the oxidative dehydrogenation of light alkanes. Beyond its surprising activity, BN also drew interest due to its low selectivity to carbon oxides. This observation made BN distinct from previously studied metal oxide catalysts for selective alkane oxidation. We narrowed down its unique reactivity to the oxygen functionalization of the catalyst surface, particularly the formation of B-O species as probed by various spectroscopic techniques. In investigating the critical role of each of the structural elements during ODH, we discovered that not only BN but an entire class of boron-containing compounds are active and selective for the formation of propylene from propane. All these materials form a complex oxidized surface with a distribution of BO surface sites. This discovery opens the doors to a new field of boron-based oxidation chemistry that currently has more questions than answers. We aim to make this Account a starting point for the research community to explore these new materials to understand their surface mechanisms and the surface species that offer a unique selectivity toward olefinic products. Effective use of these materials may lead to novel processes for efficient use of abundant light alkane resources by oxidation chemistry.

摘要

乙烯和丙烯等轻质烯烃构成了现代化学工业的基础,年产量高达数亿吨。目前,这些轻质烯烃主要通过能源密集型的蒸汽裂解生产。另外,轻质烷烃的氧化脱氢(ODH)制烯烃可在较低的操作温度下进行,并延长催化剂寿命,有可能带来可观的工艺效率提升。由于页岩气储量丰富,这种路线的潜在优势引发了大量的研究兴趣。该领域的进展尚未产生对烯烃具有足够选择性以用于工业应用的催化剂,ODH仍然是选择性烷烃氧化研究的圣杯。选择性氧化的主要挑战在于防止所需产物的过度氧化,例如丙烷氧化过程中丙烯过度氧化为一氧化碳和二氧化碳。对丙烷氧化脱氢的选择性多相催化剂的研究导致了氧化钒基催化剂的广泛使用,并且对所涉及的表面机理的研究已用于提高材料的催化活性。然而,尽管经过了数十年的研究,在工业相关转化率下对丙烯的选择性仍不尽人意。开发能够将产物过度氧化降至最低的新型催化体系对于氧化脱氢工艺的未来应用至关重要。虽然合理的催化剂设计已成功开发出均相催化剂体系,但其在多相催化剂开发中的实际应用仍然有限。具有各种表面终止和体相结构的表面的复杂性,更不用说它们会因反应混合物的化学势而发生改性,这使得在许多情况下多相催化剂的发现具有偶然性。本综述中介绍的催化剂家族也不例外。催化研究的重要性在于探索偶然性背后的科学。在本综述中,我们将首先介绍我们最初发现的氮化硼(BN)作为轻质烷烃氧化脱氢的意外催化剂。除了其惊人的活性外,BN还因其对碳氧化物的低选择性而引起关注。这一观察结果使BN与先前研究的用于选择性烷烃氧化的金属氧化物催化剂有所不同。我们将其独特的反应性归因于催化剂表面的氧官能化,特别是通过各种光谱技术探测到的B-O物种的形成。在研究ODH过程中每个结构元素的关键作用时,我们发现不仅BN,而且一整类含硼化合物对于由丙烷形成丙烯都具有活性和选择性。所有这些材料都形成了具有BO表面位点分布的复杂氧化表面。这一发现为基于硼的氧化化学新领域打开了大门,目前该领域的问题多于答案。我们旨在使本综述成为研究界探索这些新材料以了解其表面机理和对烯烃产物具有独特选择性的表面物种的起点。有效利用这些材料可能会带来通过氧化化学高效利用丰富的轻质烷烃资源的新方法。

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