Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes. Present day industrial use of methane as a chemical feedstock typically proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widely used process, or by dry reforming. Steam reforming proceeds according to Equation 1.CH4+H2O CO+3H2  (1)Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The partial oxidation of hydrocarbons, e.g., natural gas or methane is another process that has been employed to produce syngas. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to the steam reforming processes, which are endothermic. Partial oxidation of methane proceeds exothermically according to the following reaction stoichiometry:CH4+1/202 CO+2H2  (2)
In the catalytic partial oxidation processes, natural gas is mixed with air, oxygen or oxygen-enriched air, and is introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2. This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. Furthermore, oxidation reactions are typically much faster than reforming reactions. This makes possible the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by the existing catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
A number of process regimes have been described in the literature for the production of syngas via catalyzed partial oxidation reactions. The noble metals, which typically serve as the best catalysts for the partial oxidation of methane, are scarce and expensive. The more widely used, less expensive, catalysts have the disadvantage of promoting coke formation on the catalyst during the reaction, which results in loss of catalytic activity. Moreover, in order to obtain acceptable levels of conversion of gaseous hydrocarbon feedstock to CO and H2 it is typically necessary to operate the reactor at a relatively low flow rate, or space velocity, using a large quantity of catalyst. For successful operation at commercial scale, however, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance.
As a result, substantial effort has been devoted in the art to the development of economical catalysts allowing commercial performance without coke formation. Not only is the choice of the catalyst's chemical composition important, the physical structure of the catalyst and catalyst support structures must possess mechanical strength, in order to function under operating conditions of high pressure and high flow rate of the reactant and product gasses.
Of the methods that employ catalysts for oxidative conversion of methane to syngas, typically catalytic metals are dispersed throughout a porous support. The porous support provides longitudinal channels or passageways that permit high space velocities with a minimal pressure drop. In ideal conditions, the catalytic metals are dispersed throughout the porous channels and promote further conversion.
There is a continuing need for better, more economical processes and catalysts for the catalytic partial oxidation of hydrocarbons, particularly methane, or methane containing feeds, in which the catalyst retains a high level of activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity and elevated pressure.
As is known, in a diffusion-limited heterogeneous process, the contribution of internal active surface areas is defined with an effectiveness factor (η) that has a value ranging from 0 to 1. The effectiveness factor (η) is determined by the intrinsic reaction kinetics and mass transfer coefficients, and can be expressed as a function of the Thiele modulus (Φ), which in turn is given by Equation 3:Φ=L√{square root over (r/Deff)}  (3)where L is the diffusion distance (e.g. radius for sphere); r is the intrinsic reaction rate, and Deff is the effective diffusion coefficient of the reactants. The effectiveness factor (η) is related to the Thiele modulus as shown in FIG. 1.
As indicated in FIG. 1, the effectiveness factor (η) can be increased by decreasing the Thiele modulus (Φ). In porous materials, the effective diffusion coefficient (Deff) is a function of gas property and material porosity (pore volume and pore size distribution). This relationship is qualitatively illustrated in FIG. 2.
As indicated in FIG. 2, the effective diffusion coefficient (Deff) can be increased by increasing pore size. For example, for pores in the range of 1-100 nm, increasing pore size by 10-fold can roughly increase Deff by an order of 10, which decreases Φ value 3-fold. As the result, the effectiveness factor (η) can be increased substantially.