**1. Introduction**

Methane is a greenhouse gas considered as the second largest amount after carbon dioxide. Methane is released during the production and transport of coal, natural gas, and petroleum. In addition, methane is also produced by the farms and from the decay of organic waste in landfills [1].

Methane is a principal component of natural gas that has the potential to be converted into valuable oxygenated products such as formaldehyde, formic acid, and especially methanol [2–4]. Methanol is one of bulk chemical that has numerous applications in chemical industries. It is used as solvent, gasoline additive, or chemical feedstock for the production of biodiesel and hundreds of other chemicals [5]. Thus, direct oxidation of methane to methanol has gained interest since conventional methods consume more energy as high temperature or pressure is required [6].

Many heterogeneous catalysts have been investigated for direct selective oxidation of methane to methanol. The main types of metal catalysts active for methane oxidation to C1-oxygenates have a degree of oxidation exceeding 3+. They include Pd, Mn, Co, Fe, V, Mo, and Ga [7]. Fe/ZSM-5 has been reported to be active for this conversion with the use of expensive N2O as oxidant [4, 8, 9]. Therefore, the use of O2 or air as oxidant is more attractive. In 2005, selective oxidation of methane into methanol using Cu/ZSM-5 was reported [10]. In 2010, Co/ZSM-5, prepared from cobalt impregnation on alkali-treated ZSM-5 zeolite aggregates, was reported to have catalytic activity in partial oxidation of methane, and selectivity and activity of catalyst depend on cobalt speciation [11].

Since 2012, our research group has conducted work on partial oxidation of methane to methanol using Co/ZSM-5 zeolite as heterogeneous catalyst. Three types of Co/ZSM-5 zeolite catalysts, i.e., cobalt-impregnated microporous ZSM-5, cobalt-impregnated hierarchical ZSM-5, and cobalt ion-exchanged hierarchical ZSM-5, have been prepared, and their catalytic activities were tested [12]. It is shown that both mesoporous properties and the type of Co species play an important role in the use of Co/ZSM-5 as heterogeneous catalyst in this reaction.

It is interesting to study further on the role of hierarchical ZSM-5 as catalyst support, especially its acidity, both Brønsted and Lewis sites. Therefore, further work has been carried out to compare the activity of hierarchical Co/ZSM-5 catalysts using the as-synthesized NaZSM-5 and H-exchanged ZSM-5 (HZSM-5) as support. The work on Co/HZSM-5 h has been explained previously, including the effect of additional trace of oxygen in the gas mixture to the yield of methanol [13]. This chapter will discuss the results from the above work, followed by exploration on the future prospect of utilize natural resources as alumina and silica precursors in ZSM-5 synthesis as well as bio-methane conversion.
