*2.2. FTIR and Structural Parameters Analysis*

### 2.2.1. FTIR Results Analysis

Infrared spectroscopy is closely related to the chemical structure of the substance. It can be confirmed the aromatic structure, oxygen-containing structure and fat structure of coal by FTIR detection [11]. Infrared spectra of all samples are shown in Figure 1, with curves smoothed and baselines corrected.

**Figure 1.** FTIR spectra of lignite before and after modification.

It can be observed that selected coal samples contain similar functional groups, hence the absorption of sample to infrared spectrum occurs at same wavenumber positions [11]. These obtained spectra are comprehensive curves of many independent peaks, which have to be deconvoluted to achieve. All spectra were divided into 4 regions, namely 700–900 cm−<sup>1</sup> (aromatic structures), 1000–1800 cm−<sup>1</sup> (oxygen-containing structures), 2800–3000 cm−<sup>1</sup> (aliphatic structures) and 3000–3600 cm−<sup>1</sup> (hydroxyl

structure). Consequently, the area values of specific peak can be derived. The corresponding relationships between peak position and functional group are shown in Tables 2–5 [12,13].


**Table 2.** Aromatic structure of HLH before and after modification.

**Table 3.** Oxygen-containing functional group of HLH before and after modification.




**Table 5.** Hydroxyl structure of HLH before and after modification.


It can be found that there are 3 substitution modes of hydrogen atoms on benzene ring in HLH structure. The proportion of triple substituted aromatics (3H) is the largest among all the samples. The proportion of triple substituted aromatics of HLH raw coal is 86.498%, the MM sample is 86.427% in, the UM sample is 87.893%. The tetrasubstituted hydrocarbons (2H) of HLH lignite has also changed significantly. Tetrasubstituted hydrocarbons (2H) are not found in the infrared spectra of HLH lignite after both modification. In the process of modification, the substitution reaction of aromatic hydrocarbons may cause structural changes, which is due to the instability of other atoms and aliphatic side chains in benzene rings.

It is also found that the form and proportion of oxygen elements in HLH sample changed a lot after modification, however the carbonyl (C=O), alkyl ether (C-O-C), and phenol hydroxyl (-OH) groups are still the main existing forms of oxygen-containing functional group of HLH.
