Isotope Labeling to Study the Hydrogen Transfer Route during Lignite Modification in a Subcritical D₂O–CO System

Abstract

The hydrogen transfer reaction is one of the key chemical processes of coal conversion. The elucidation of the hydrogen transfer mechanism is essential for rational control of the properties and distribution of products. This paper is focused on the hydrogen transfer mechanism of lignite modification in a subcritical D2O–CO system. Using Soxhlet extraction, isotope ratio mass spectrometry, 1H nuclear magnetic resonance, and pyrolysis–gas chromatography–mass spectrometry (Py-GCMS), the deuterium transfer routes in deuterated products and their representative structures were studied. The existing form of deuterium in deuterated products was also proposed. The results showed that the subcritical D2O–CO system can generate active deuterium, which combines with the free radical fragments resulting from coal pyrolysis to form three kinds of deuterated products: n-hexane solute (NS-D), benzene solute (BS-D), and tetrahydrofuran solute (TS-D). Deuterium in the three solutes migrates in the order TS-D → BS-D → NS-D. It was observed that the active deuterium is not transferred to the β sites of the solutes but rather to their ar, α,2, α, and γ sites. Py-GCMS detection results showed that the solutes mainly consist of six representative structures, that is, monocyclic and polycyclic aromatics, alkenes, alkanes, alcohols, and esters. In the modification process, deuterium is incorporated into the monocyclic aromatic structures in the aliphatic side chains first and then in the aromatic ring. For the polycyclic aromatic structures, the active deuterium enters the side chain first and then the aromatic ring having that side chain. A small amount of active deuterium can be incorporated into the alkene and alkane structures directly. However, it mainly transfers to such structures indirectly through thermal cracking of deuterium-containing macromolecular aromatic structures. Active deuterium reduces oxygen in the oxygen-containing structures to form OD or DHO, yet it cannot easily enter the aliphatic and aromatic structures linked to the oxygen-containing structures. This research provides a new perspective for the study of hydrogen transfer routes and can serve as a reference for discussing the molecular mechanisms involved in various coal conversion processes.

Role of FeS Catalyst in the Hydromodification of Lignite in a Subcritical Water–CO System

“…The larger value of X CO indicates a more abundant generation of active hydrogen in the reaction system, which provides the preconditions for the generation of caking components. 28 Indeed, there are two reasons for the improvement of X CO . First, FeS catalyzes the WGSR and accelerates the kinetic reaction rate of WGS.…”

“…This phenomenon suggests that the conversion of CO is closely related to the development of caking properties in a subcritical water–CO system. The larger value of X CO indicates a more abundant generation of active hydrogen in the reaction system, which provides the preconditions for the generation of caking components . Indeed, there are two reasons for the improvement of X CO .…”

Influence of H2O and O2 on the Homogeneous Conversion of Toluene and the Underlying Reaction Mechanisms

“…Stable isotope tracing is a widely used method in the study of chemical reaction processes. It relies on stable isotopes, such as D 2 O, D 2 , and 18 O 2 , as tracers to track the transfer and transformation of atoms or molecules during chemical reactions. − By analysis of the distribution of stable isotopes in the reaction products, the nature of reaction intermediates and transition states can be inferred, revealing the detailed steps of the reaction. This method is essential to understanding the specific mechanisms of complex reactions.…”

Solvent Extraction of Superfine Pulverized Coal. Part 2. Free-Radical Characteristics

“…As an effective method, solvent extraction is now widely used in the study of the composition and structural characteristics of coal. − At the same time, coal pyrolysis is one of the important steps of coal utilization, but the coal pyrolysis process is affected by many factors, including the influence of small molecular components, the cracking of macromolecular component bonds, and the synergy between them. In the research of small molecules, organic solvent extraction is an effective method. , What is more, the solvent extraction is one of the most effective ways to study the element-transfer paths during the coal modification. , The radical concentration of coal changed significantly during the extraction process − due to the combined effects of the physical actions and bond breaking. Diverse extraction reagents have been used based on the existing research, including single-solvent, polar solvent, and high-boiling-solvent extractions.…”

“…9,10 What is more, the solvent extraction is one of the most effective ways to study the element-transfer paths during the coal modification. 11,12 The radical concentration of coal changed significantly during the extraction process 13−15 due to the combined effects of the physical actions and bond breaking. Diverse extraction reagents have been used based on the existing research, including singlesolvent, polar solvent, and high-boiling-solvent extractions.…”