Selective oxidation of low-concentration coal mine gas to methanol in oleum over PbSO4 catalyst

Selective oxidation of low-concentration coal mine gas to methanol was carried out over PbSO4 catalyst in a concentration of 20% oleum. The effects of reaction temperature, reaction pressure, reaction time and catalyst dosage on the conversion of methane in gas as well as the yield of methanol were studied. What is more, the reaction mechanism of selective oxidation of low-concentration coal mine gas was also investigated. The results showed that the optimal reaction conditions are determined under the reaction temperature of 180°, the reaction pressure of 5MPa, the reaction time of 3h, and the PbSO4 dosage of 300μmol for selective oxidation of low-concentration coal mine gas to methanol by PbSO4 catalyst in oleum. Under this technological condition, the conversion ratio of methane in gas is 8.56% while the methanol selectivity is 78.59%. Mechanism research showed that methane in gas is transformed into methyl bisulfate firstly. Besides, the methyl bisulfate is hydrolyzed into methanol. The selective oxidation of low-concentration coal mine gas is following the mechanism of electrophilic substitution.


Introduction
Coal mine gas is a kind of unconventional natural gas. Methane, as the main component of coal mine gas, can be utilized to serve as chemical material [1,2]. On the situation of the energy shortage and the severity of work safety conditions, it is important to research preparation of methanol with coal mine gas, especially low-concentration coal mine gas as raw materials [3]. In 1993, by using concentrated sulfuric acid as both the solvent and the oxidant, Periana et al. [4]succeeded in converting methane to methanol with an HgSO 4 catalyst. In this system, methane in gas was transformed into methyl bisulfate (CH 3 OSO 3 H), which was immediately hydrolyzed to methanol: CH 4 + 2H 2 SO 4 → CH 3 OSO 3 H +2H 2 O + SO 2 (1) In 1998, a solvent oleum was proposed to replace concentrated sulfuric acid with the conversion of methane by Periana et al. [5], and the conversion of methane and selectivity of methanol was developed further. In this system, the oxidant is SO 3 . Since then, researchers have studied in depth on selective oxidation of methane in oleum. Moreover, gratifying progress has been made [6][7][8][9][10][11][12]. It has been shown that some catalysts, such as Pt(bpym)Cl 2 , Hg 2 SO 4 , Pd 2 SO 4 and iodine compounds, could achieve high methane conversion and yield of methanol [5,[13][14][15][16]. On the basis of the above references, this paper investigated the liquid-phase selective oxidation of low-concentration coal mine The catalytic oxidation of low-concentration coal mine gas was carried out in a high pressure reactor. The reactor was made by Dalian Sanling Power Electronic Instrument Factory. The effective volume of the reactor was 20 milliliters. A polytetrafluoroethylene liner was placed inside of the reactor in order to separate the oleum from the reactor wall.

Catalytic reactions
A certain amount of sulfate catalyst and oleum were added into the reactor. The reactor was closed and purged with experimental mine gas for three times. The reactor was thereafter sealed with experimental gas under the desired pressure. The contents of the reactor were heated at the reaction temperature, and reacted for a certain reaction time under agitation conditions. After the end of reaction, the reactor was cooled until the room temperature.
The gas mixture in the reactor was collected and analyzed by gas chromatograph. The liquid sample in the reactor was diluted for 20 times with distilled water, and the resulting solution was hydrolyzed for 3 h at 90℃. After the hydrolysis, the solution was analyzed by gas chromatograph.

Product analysis
The gas mixture in the reactor was analyzed by a GC9790Ⅱ gas chromatograph equipped with a packed column( TDX-01, 2m×4mm ). Carbon monoxide, carbon dioxide and methane were tested by FID detector, and oxygen and nitrogen were tested by TCD detector. Methanol, as the product of hydrolysis of the liquid sample in the reactor, was analyzed by a GC9790 gas chromatograph equipped with a packed column (GDX-102, 2m×3mm) and a FID detector. The external standard method was used for the analyses of the above products. The selectivity of carbon monoxide, carbon dioxide and methanol as well as the conversion of methane can be calculated after carbon balance. Moreover, these formulas are defined as follows: where P 1 and P 2 are gas pressure before and after the reactions respectively, Pa; V is the volume of gas phase inside the reactor, m 3 ; T 1 and T 2 are temperatures before and after the reactions respectively, K; R is universal gas constant, J· mol -1 ·K -1 ; x CH4 is mole fraction of CH 4 in gas before the reactions, %; x CO and x CO2 are mole fractions of CO and CO 2 in gas after the reactions, %; n CH3OH is the amount of substance of CH 3 OH, mol.

Catalyst selection
The catalytic activity of several sulfate compounds for activation of low-concentration coal mine gas in oleum was researched, and the results of catalytic tests are given in Table 1. It can be seen that the NiSO 4 , CdSO 4 and ZrSO 4 have no catalytic activity in the oxidation of low-concentration coal mine gas to methanol. However, PbSO 4 has the capacity to catalytically convert low-concentration coal mine gas to methanol. Table1 shows that the concentration of oxygen in mine gas has not changed after the reactions. It means that oxygen has not participated in selective oxidation of methane. For the selective oxidation of low-concentration coal mine gas in oleum, the oxidant is SO 3 .

Effect of reaction temperature
The products of the selective oxidation of low-concentration coal mine gas were mainly CH 3 OH, CO and CO 2 . The typical distribution of products as a function of temperature is shown in Figure1. When the reaction temperature was raised to above 140℃, the selective oxidation of mine gas started in oleum(20% SO 3 ) in the presence of PbSO 4 . At a temperature range of 160-180℃, the conversion ratio of methane in gas rose with the increasing of reaction temperature. When the reaction temperature was reached to 200℃, the conversion ratio of methane decreased. This shows that the reaction temperature is a key factor influencing the selective oxidation of mine gas. At a temperature range of 160-200℃, the increase in the temperature resulted in a slight decrease in CH 3 OH selectivity. Because the methanol is more active than the methane, the deep oxidation to carbon dioxide is easily occurred. Based on the methane conversion and the selectivity of CH 3 OH, it was concluded that the temperature of 180℃ should be the optimum operating temperature for low-concentration coal mine gas transformation into methanol.

Effect of reaction time
Figure3 shows the effects of reaction time on the selective oxidation of low-concentration coal mine gas. When the reaction time increased from 1h to 2h, the methane conversion sharply rose from 3.52% to 6.63%. Once the time is above 2h, the conversion of methane began to slow down. The methane conversion gradually increased from 6.63% to 7.23% if the reaction time rose from 2h to 4h. Such results might indicate that the reaction of selective oxidation of low-concentration coal mine gas is basically completed 2h later. Methane is deeply oxidized to CO and CO 2 with a further increase in the reaction time. The increase in the reaction time resulted in a slight decrease in CH 3 OH selectivity.

Effect of reaction pressure
The reaction pressure plays a major role in the selective oxidation of low-concentration coal mine gas. In order to observe the effects of the reaction pressure on the methane conversion and the selectivity of products formed from mine gas oxidation, the pressure of mine gas was varied in the range of 2-5MPa and the corresponding results are given in Figure4. The conversion of methane was found to be faster when the reaction pressure was below 5MPa. With the increase of pressure, selectivity of CH 3 OH increased. At the pressure of 5MPa, the selectivity of CH 3 OH reached to the maximum of 78.59%. Generally, in the liquid-phase reaction, the product formation is highly dependent on the solubility of reactive gas in the solvent. The solubility of mine gas in solvent rose with the increasing of the pressure of gas in the reactor. Thus, more mine gas would get to the PbSO 4 catalyst, consequently increasing methane conversion and methanol selectivity. On the basis of these studies, it was found that the reaction pressure of 5MPa should be the optimum operating pressure for low-concentration coal mine gas transformation into methanol.

Reaction mechanism
The mechanism of electrophilic substitution is most popular for selective oxidation of methane in Oleum [17,18]. Therefore, it is concluded that the mechanism of electrophilic substitution is also followed by the selective oxidation of low-concentration coal mine gas in Oleum. In the liquid phase oxidation of low-concentration coal mine gas, Pb 2+ , as a electrophilic reagent, makes an electrophilic attack on methane in gas, generating the metal complex of CH 3  Figure5. Reaction mechanism for the partial oxidation of low-concentration coal mine gas catalyzed by PbSO 4

Conclusions
The catalytic activity of several sulfate compounds for activation of low-concentration coal mine gas in oleum medium was studied in this paper. The results show that PbSO 4 has the ability to catalytically convert low-concentration coal mine gas into methanol. However, NiSO 4 , CdSO 4 and ZrSO 4 have no catalytic activity in the oxidation of low-concentration coal mine gas to methanol. The influence of reaction conditions (including reaction temperature, reaction pressure, reaction time and catalyst dosage) on selective oxidation of low-concentration coal mine gas was investigated in a concentration of 20% oleum medium with PbSO 4 catalyst. These results indicate that the optimal reaction conditions are determined under the reaction temperature of 180℃, the reaction pressure of 5MPa, the reaction time of 3h, and the PbSO 4 dosage of 300μmol for selective oxidation of low-concentration coal mine gas to methanol by PbSO 4 catalyst in oleum. Under this technological condition, the conversion ratio of methane in gas is 8.56% while the methanol selectivity is 78.59%. It is concluded that the mechanism of electrophilic substitution is followed by the selective oxidation of low-concentration coal mine gas in Oleum.