Loker Hydrocarbon Research Institute and Department of Chemistry , University of Southern California , University Park , Los Angeles , California 90089-1661 , United States.
Acc Chem Res. 2019 Oct 15;52(10):2892-2903. doi: 10.1021/acs.accounts.9b00324. Epub 2019 Sep 5.
The capture of CO from concentrated emission sources as well as from air represents a process of paramount importance in view of the increasing CO concentration in the atmosphere and its associated negative consequences on the biosphere. Once captured using various technologies, CO is desorbed and compressed for either storage (carbon capture and storage (CCS)) or production of value-added products (carbon capture and utilization (CCU)). Among various products that can be synthesized from CO, methanol and formic acid are of high interest because they can be used directly as fuels or to generate H on demand at low temperatures (<100 °C), making them attractive hydrogen carriers (12.6 and 4.4 wt % H in methanol and formic acid, respectively). Methanol is already produced in huge quantities worldwide (100 billion liters annually) and is also a raw material for many chemicals and products, including formaldehyde, dimethyl ether, light olefins, and gasoline. The production of methanol through chemical recycling of captured CO is at the heart of the so-called "methanol economy" that we have proposed with the late Prof. George Olah at our Institute. Recently, there has been significant progress in the low-temperature synthesis of formic acid (or formate salts) and methanol from CO and H using homogeneous catalysts. Importantly, several studies have combined CO capture and hydrogenation, where captured CO (including from air) was directly utilized to produce formate and CHOH without requiring energy intensive desorption and compression steps. This Account centers on that topic. A key feature in the combined CO capture and conversion studies reported to date for the synthesis of formic acid and methanol is the use of an amine or alkali-metal hydroxide base for capturing CO, which can assist the homogeneous catalysts in the hydrogenation step. We start this Account by examining the combined processes where CO is captured in amine solutions and converted to alkylammonium formate salts. The effect of amine basicity on the reaction rate is discussed along with catalyst recycling schemes. Next, methanol synthesis by this combined process, with amines as capturing agents, is explored. We also examine the system developments for effective catalyst and amine recycling in this process. We next go through the effect of catalyst molecular structure on methanol production while elucidating the main deactivating pathway involving carbonylation of the metal center. The recent advances in first-row transition metal catalysts for this process are also mentioned. Subsequently, we discuss the capture of CO using hydroxide bases and conversion to formate salts. The regeneration of the hydroxide base (NaOH or KOH) at low temperatures (80 °C) in cation-conducting direct formate fuel cells is presented. Finally, we review the challenges in the yet unreported integrated CO capture by hydroxide bases and conversion to methanol process.
从浓缩排放源以及空气中捕获 CO 是一个极其重要的过程，因为大气中 CO 浓度的不断增加及其对生物圈的负面影响。使用各种技术捕获 CO 后，将其解吸并压缩，用于储存（碳捕集与封存 (CCS)）或生产增值产品（碳捕集与利用 (CCU)）。从 CO 合成的各种产品中，甲醇和甲酸具有很高的应用价值，因为它们可以直接用作燃料，或在低温（<100°C）下按需生成 H，从而成为有吸引力的氢载体（甲醇和甲酸中分别含有 4.4 wt%和 12.6 wt%的 H）。甲醇在全球范围内的产量巨大（每年 1000 亿升），也是许多化学品和产品的原材料，包括甲醛、二甲醚、轻烯烃和汽油。通过对捕获的 CO 进行化学回收来生产甲醇是我们在研究所与已故的 George Olah 教授提出的所谓“甲醇经济”的核心。最近，在使用均相催化剂从 CO 和 H 低温合成甲酸（或甲酸盐）和甲醇方面取得了重大进展。重要的是，几项研究将 CO 捕集与加氢相结合，其中捕获的 CO（包括空气中的 CO）无需进行能量密集的解吸和压缩步骤，即可直接用于生产甲酸盐和 CHOH。本报告重点讨论了这一主题。迄今为止，在用于合成甲酸和甲醇的 CO 捕集和转化研究中，一个关键特征是使用胺或碱金属氢氧化物作为 CO 的捕获剂，这可以在加氢步骤中帮助均相催化剂。本报告首先考察了 CO 在胺溶液中捕获并转化为烷基铵甲酸盐的组合工艺。讨论了胺碱性对反应速率的影响以及催化剂的回收方案。接下来，探讨了用胺作为捕获剂的组合工艺合成甲醇。还考察了该过程中有效催化剂和胺回收的系统发展。然后，我们研究了催化剂分子结构对甲醇生产的影响，同时阐明了涉及金属中心羰基化的主要失活途径。还提到了该过程中第一过渡金属催化剂的最新进展。随后，我们讨论了使用氢氧化物碱捕获 CO 并将其转化为甲酸盐盐的情况。介绍了在阳离子导电直接甲酸燃料电池中，在低温（80°C）下再生氢氧化物碱（NaOH 或 KOH）的情况。最后，我们回顾了尚未报道的用氢氧化物碱集成 CO 捕集并转化为甲醇过程中存在的挑战。
