**4.1 Activity of ZSM-5 h catalysts without gas O2**

The first work carried out in this partial oxidation of methane to methanol was carried out using Co/NaZSM-5 h catalysts without the presence of gas O2 [12], based on work reported in [11]. It is shown that the partial oxidation of methane to methanol could occur without the presence of molecular oxygen, suggested that the superoxide (O2<sup>−</sup>) from cobalt oxide (Co3O4) or surface of ZSM5 framework could act as oxidation agent at 423 K. This phenomenon has been also studied computationally in [23]. **Table 2** summarizes the results in the reaction of various ZSM-5 h without additional molecular oxygen. It can be seen that using any ZSM-5 h catalysts, the percentage (%) yield of methanol increased when the reaction time was longer and Co/HZSM-5 h is more

*Partial Oxidation of Methane to Methanol on Cobalt Oxide-Modified Hierarchical ZSM-5 DOI: http://dx.doi.org/10.5772/intechopen.86133*


**Table 2.**

*Percentage (%) yield of methanol from partial oxidation of methane4 .*


### **Table 3.**

*Conversion of methane to methanol in the presence of oxygen7 .*

active than Co/NaZSM-5 h catalyst. This could be indicative of contribution of Bronsted sites (-Si-OH, silanol groups) existing more significantly in HZSM-5 h than in NaZSM-5 h, as shown by FTIR spectra. The silanol groups would interact with produced methanol through hydrogen bond.

Furthermore, the effect of certain amount of molecular oxygen in the reaction was also studied. ZSM-5 h modified with Co oxide is able to catalyze the partial oxidation of methane with O2 as the oxidant [24]. The reaction results were tabulated in **Table 3**. It shows that the presence of molecular oxygen in the reaction system could increase the yield of methanol, so that the reaction could occur in shorter reaction time. For example, when Co/NaZSM-5 h was used in reaction time of 30 min, the percentage (%) yield of methanol for condition with additional oxygen is 42%, which is 4.7 times more than the reaction without oxygen. Similar results were also obtained using Co/HZSM-5 h as catalyst in 60 minute reaction time; the percentage (%) yield of methanol is 1.5 times higher in the presence of molecular oxygen. The effect of oxygen is more apparent when using Co/NaZSM-5 in 30 minute reaction time. Moreover, using Co/HZSM-5 h as catalyst and in the presence of oxygen, the percentage (%) yield of methanol reached the highest when the reaction took place for 60 min and then decreased after 120 minutes of reaction. The percentage (%) yield of methanol even significantly decreased when Co/NaZSM-5 h was employed for 120 min. It is suggested that the additional oxygen, in short reaction time, could regenerate the Co oxide species after their superoxides (O2<sup>−</sup>) are being used to oxidize methane. However, in longer reaction time, the additional oxygen could also oxidize further the produced methanol to CO2. As a result, complete oxidation takes place.

### **4.2 Reaction mechanism**

Plausible reaction mechanism of partial oxidation of methane to methanol is proposed as shown in **Figure 12**. It is a modification from the mechanism reported in [23].

**Figure 12.**

*Schematic diagram of plausible mechanism reaction of partial oxidation of methane on Co/NaZSM-5.*
