Direct Oxidation of Methane to Methanol Group 7 Joey Saah, Richard Graver, Shekhar shah, Josh Condon

Methanol Applications 21 million tons produced per year Solvent Gasoline additive Feedstock for many chemical processes 21 million tons produced per year Primary component of natural gas Currently an abundant fuel source Difficult and uneconomical to transport Simple and cost effective on-site process required wikimedia.org

Standard Methanol Synthesis Conversion of natural gas into syngas Steam reforming reactor with catalyst Creates carbon monoxide and hydrogen from methane Syngas to Methanol H2 and CO react in 2:1 ratio to form methanol Catalyst selectively forms methanol Haldor Topsoe

One-Step Methanol Process Advantages Steam in steam reformer is expensive to produce Less capital costs required to build one-step plant May be created near more remote methanol sources Remote methanol sources more profitable, attractive Challenges Past one-step reactions showed low yield or selectivity with homogeneous and heterogeneous catalysts Other methods did not produce methanol levels required for commercialization Liquid phase or Supercritical reactor methods

History of Methanol 1661 – Methanol discovered by Robert Boyle 1834 – The chemical structure and identity of methanol was identified by Dumas and Peligrot Destructive distillation of wood was the first method to produce methanol Pyrolysis in a distillation apparatus

History of Industrial Methanol Production First synthetic methanol production route discovered in 1905 BASF commercialized the process in 1934 Zinc-Chromium Oxide Catalyst 300 °C and 200 atm This process was used until 1966 when a lower pressure, higher efficiency method was discovered

History of Industrial Methanol Production cont. Imperial Chemical Industries, Ltd. discovered a new process in 1966 Copper-Zinc Oxide catalyst 250-300 °C 50-100 atm Poisoning of catalyst was a problem for this method Lifetime of catalyst is about 4 years Only if there is good control of temperature and feedstock purity

Thermodynamic Analysis   Reaction ΔG reaction (KJ/mol) 298 650 700 750 800 1000 Direct CH4+1/2O2 è CH3OH -111 -93 -91 -88 -86 -76 SR CH4 + 1/2O2 è CO + 2H2 -152 -162 -172 -182 -222 CH4 + H2O è CO + 3H2 142 60 48 36 23 -27

Optimal Conditions Traditional steam reforming requires 1000 K or higher Direct oxidation (direct synthesis) favorable at lower temperatures thermodynamically Theoretically, 33% equilibrium conversion at 298 K for direct synthesis Maximum of 5 % equilibrium conversion in direct synthesis actually obtained due to high activation energy

Thermodynamic Requirements Direct synthesis would be economically feasible (compared to traditional method) if 5.5% conversion and 80% selectivity to methanol is obtained in the reaction Low conversion requirement is indicative of the high cost of current methanol production CO2 and CO are thermodynamically the most favorable products of direct oxidation (selectivity to methanol is a challenge)

Why do we need a catalyst? Methane is a more stable molecule than methanol Reaction equilibrium favors methane at operating conditions Low conversion Large activation energy Complete oxidation to carbon monoxide and carbon dioxide is thermodynamically favored Low selectivity for the partially oxidized product Harsh operating conditions Requires 440 kJ/mol to break the first C—H bond 50-100 bar and 500-550 K Energy intensive step would be eliminated The first question you might ask is: why do we even need a catalyst? Is a catalyst absolutely necessary? YES! Methane is a very stable molecule and it is very difficult to abstract a hydrogen atom from methane (roughly … kJ/mol). This reaction will not proceed in the forward reaction due to thermodynamic restrictions on the equilibrium. In order to make this reaction happen we need to lower the activation energy that is necessary for the reaction to occur. The first step is the creation of a free radial methyl group and is followed by subsequent attack of a water molecule to get the remaining OH group attached.

Important Catalytic Parameters Temperature Pressure Oxygen concentration in the feed gas Gas flow rate Additives These parameters are critical to optimizing the conversion of methane and selectivity toward the methanol product. As shown in the thermodynamic table a couple slides ago, as temperature increases the change in gibbs free energy becomes increasingly negative for the complete oxidation, while, for the partial oxidation, the opposite is true. This indicates that the reaction should be run at lower temperatures, and we see from the table that the partial oxidation becomes the dominating reaction at 298 K. This is one of the biggest reasons why this process is of such interest to researchers, the energy intensive reforming to syngas can be completely bypassed!

Heterogeneous Catalytic Partial Oxidation Direct conversion of methane and oxygen to methanol Conversion of methane and selectivity to desired product are limiting factors Activation energy for the reaction is extremely large and limits the conversion of methane to products Involves the abstraction of a hydrogen from methane to create a methyl radical followed by subsequent reaction to form methoxide ions Solid catalysts containing metal oxide catalysts have been effective at removing hydrogen CH4 + ½ O2 CH3OH Direct Oxidation Reaction

References Khirsariya, P., Mewada, R. “Single Step Oxidation of Methane to Methanol – Towards Better Understanding”. Procedia Engineering 51 409-415. 2013.