Asian Journal of Chemistry; Vol. 26, No. 3 (2014), 795-798 http://dx.doi.org/10.14233/ajchem.2014.15546 Low-Temperature Oxidation of Methylene Bridge Bond in Coal by Model Compound 1,2,* 1 2 3 1 YIBO TANG , ZENGHUA LI , JERRY C. TIEN , WENXUAN YIN and DONGJUAN MA 1School of Safety Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, P.R. China 2Mining & Nuclear Engineering, Missouri University of Science and Technology, Rolla, MO 65401, USA 3School of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, P.R. China *Corresponding author: E-mail: [email protected] Received: 26 March 2013; Accepted: 27 July 2013; Published online: 30 January 2014; AJC-14639 In order to find out characteristics of active structures in coal at early phase of oxidation, diphenylmethane was adopted as the model compound to study the oxidation properties of bridge bond (methylene) in coal with low-temperature experimental condition (293 K-423 K), according to structure of molecular coal. Mean while, the generated oxidation products under different reaction conditions were analyzed qualitatively via FTIR spectrometer and GC-MS. The results show that the new structures which involve C-O, C=O, Ar-O and Ar-OH are constantly generated with temperature rise in the process of low-temperature oxidation of diphenylmethane, especially after the 393 K. Besides, the phenols, benzaldehyde, diphenylmethanone, phenyl benzoate, 2-benzyl phenols and 2-phenolic-diphenylmethanone were detected in oxidation products, which demonstrated this oxidation is a multi-step reaction. Meantime, the substitution between O and H is more easily occurs in -CH2- than in benzene ring. The temperature and time are major factors which affecting low-temperature oxidation of diphenylmethane. In conclusion, this study provides reference for explaining and controlling the low-temperature oxidation of coal from chemical perspective. Keywords: Low-temperature oxidation, Methylene, Diphenylmethane, Model compound, Spontaneous combustion. INTRODUCTION inducing spontaneous combustion of coal. As a result, well targeted study of oxidation properties of bridged bond and The spontaneous combustion of coal is a worldwide active group in coal in the process of low-temperature oxidation 1 disaster , which often occurs in the coal-related industries. Coal has become the top priority of study concerning spontaneous fire raised by self-heating of coal also responsible for the combustion of coal. environmental problems such as surface subsidence, toxic gas The low-temperature oxidation of coal is the most impor- 2-3 th emissions and water pollution . Since the 17 century, tant initial part19-21 for the whole process on self-ignite. Meth- 4-7 numerous reasons were considered to be inducing factors ylene is typical bond in coal molecule21,22. Therefore, studying which effect on spon-combustion of coal. Although many low-temperature oxidation properties of methylene is of certain scholars attempted to reveal variations of various coal mole- guiding significance to reveal spontaneous combustion of coal. cular structure in the process of self-ignite with multiple testing Thus, model compound comprised by methylene and its 8-11 means , its mechanism is still unknown. At present, making coupled aromatic ring was adopted as experimental subject in use of model compound to study complicated chemical system this paper. is a effective research approach which widely applied in proce- 12-13 14 sses such as coal liquefaction , coal desulfuration and EXPERIMENTAL petroleum refining15. In line with molecular structure of coal, the main cons- Experimental set-up: The schematic diagram of experi- truction units of coal are aromatic rings which interrelated via mental apparatus was shown in Fig. 1. In experimental system, bridged bond (aliphatic hydrocarbons bond, thioether bond dry air uniformly passes through pipeline. The temperature and ether bond etc.) and active group16. Besides, chemical control system assures that the sample can enables to be conduc- characteristics of aromatic ring are relatively stable. Bridged ted by temperature programming and stabilized at the setting bonds and active groups in coal molecular structure are firstly temperature. oxidized17-18. In other words, the oxidation of such structures Experimental procedure: The diphenylmenthane (50 g) as bridged bonds and active groups play an important role in was placed into glass bottle and the air (2 L/m) passed into it 796 Tang et al. Asian J. Chem. stretching vibration in alkane, at 1601 cm-1 representing C=C skeletal vibration on aromatic ring, at 1492 and 1444 cm-1 representing C-H flexural vibration in alkane, at 729 and 681 cm-1 representing C-H out-of-plane deformation vibration of aromatic hydrocarbon where H was replaced. The activity structure in dipenylmethane was oxidized continuously with the ceaseless temperature increase. After oxidation, there lies in larger variation of activity structure in Fig. 1. Experimental set-up dipenylmethane. To be specific, when the reaction temperature was 333 K, absorption peak representing C-O stretching continuously for oxidation. Keep oxidizing 24 h. under diffe- vibration occurred at 1072 and 1034 cm-1 after low-temperature rent setting temperatures (333 K-393 K) and collect the sample oxidation for 48 h, which indicated that C-O bond was gradually of 1 mL at intervals of 4 h. Afterwards, products constituents generated. Meanwhile, a small adsorption peak representing were qualitatively analyzed through GC-MS (split ratio of C=O (ketone) also emerged at 1076 cm-1. And there were fewer 30:1). The concentration variation of its oxidation products changes among other adsorption peaks. When reaction tempe- under different reaction conditions was analyzed quantitatively rature rose to 353 K, the adsorption peak representing ether via gas chromatograph. Meanwhile, the variation features of bond gradually appeared at 1319 and 1277 cm-1, which showed its functional group under different reaction conditions were that stretching vibration of aliphatic ether structure and aromatic tested by FTIR. ether structure started to appear along with temperature rise. -1 RESULTS AND DISCUSSION Besides, at 1706 and 1746 cm signified that adsorption peak of C=O gradually enhanced. When the temperature reached -1 Infrared spectroscopy is a kind of test method which 373 K, at 2907 and 2843 cm representing CH2 antisymmetric could effectively reflect the changes of functional groups in stretching vibration in naphthene or aliphatic series involved organics23. As shown in Fig. 2, the main characteristic peaks slight enhancement aside from adsorption peaks represented of infrared spectrogram of diphenylene include absorption by previously generated structures. peak at 3023 cm-1 representing C-H stretching vibration on Finally, when reaction temperature reached 393 K, the aromatic ring, at 2907 cm-1 and 2843 cm-1 representing C-H adsorption peak of Ar-OH was relatively obvious showing at 3487 cm-1. Yet, this adsorption peak under reaction temperature 3.5 of 333 K exhibited in the spectrogram. Its intensity gradually increased along with temperature rise, which indicated that 3.0 phenolic compounds were generated in low-temperature 2.5 oxidation process of diphenylmethane. e 2.0 c n a O b (c) r 1.5 o 2500000 s b OH A 1.0 (d) (a) 0.5 20 00000 e O c (e) 0.0 n a O d O n 15 00000 -0.5 u 500 1000 1500 3000 3500 o OH b Wavenumber (cm–1 ) A (b) (a) Diphenyl methane 10 00000 (f) OH O 500000 3.2 Undefined (g) 393K 2.4 e 4.00 6.00 8.00 10.00 12.00 14.00 c n Time a 373K b r Fig. 3. Chromatograms for composition of oxidation products from o s 1.6 b diphenylmethane at 393 K A 353K 0.8 The liquid oxidation products of diphenylmethane were qualitatively analyzed via GC-MS. It can be clearly shown 0 333K from Fig. 3 that (d) diphenylketone was generated by 500 1000 1500 2000 2500 3000 3500 4000 diphenylmethane oxidation. Meanwhile, such substances as Wavenumber (cm–1 ) (a) phenol, (b) benzaldehyde, (d) diphenylketone, (e) phenyl (b) Diphenyl methane after oxidized 24 h benzoate, (f) 2-benzylphenol, (e) 2-phenolic -diphenylketone Fig. 2. Infrared spectrum of diphenyl methane and its oxidation were tested. As we all know, the active group included in Vol. 26, No. 3 (2014) Low-Temperature Oxidation of Methylene Bridge Bond in Coal by Model Compound 797 α diphenylmethane is methylene on which C atom and bilateral 1.4 benzene rings involve δ-π hyperconjugation. As a result, the electron cloud between αC atom and αH atom and conjugated 1.2 system of aromatic ring could be integrated into one. In this 1.0 way, electron delocalization extent enlarged where free radical ) 0.8 can be easily formed. And, bridged bond among benzene rings % ( d was easier than benzene ring itself to make adsorption with l e i 0.6 oxygen. Combined with oxidation products detected by experi- Y ment, it can be inferred that the reaction course of diphenyl- 0.4 methane and O2 is as follows (Fig. 4): 0.2 0 0 4 8 12 16 20 24 ••• Time (h) O OH O O••• (a) Phenol O O O••• 12 ••• • O O•• 10 O••• O••• OH O ) 8 O % OH ( d l e i 6 Y and 4 Fig. 4. Simple oxidation reaction pathway of diphenyl methane This pathway suggested that the supplant primarily hap- 2 pened between oxygen atom and hydrogen atom on bridge bond of methylene during the oxidation of diphenylmethane. 0 5 10 15 20 25 Time (h) Meantime, some oxygen atoms replaced the hydrogen atom (b) Diphenyl ketone on the benzene ring. In the process of reaction, the generation of carbon monoxide and carbon dioxide along with the breaking Fig. 5. Effect of temperature and time on the oxidation of diphenyl methane of bonds.