Low temperature catalytic liquid phase water gas shift reaction for carbon monoxide free hydrogen generation

Abstract

In applications such as fuel cells which require high purity hydrogen, carbon monoxide impurity has to be removed completely. Hence water gas shift reaction (WGS) in which carbon monoxide and water catalytically react to produce hydrogen and carbon dioxide is often carried out in conjunction with hydrogen production units using carbon containing fuels. WGS is conventionally performed in gas phase at temperatures above 1200C. Most studies on direct liquid phase WGS have been conducted only at high temperature and pressure. Hydrogen produced from these high temperature reactions has some carbon monoxide present due to the unfavorable approach to equilibrium for the exothermic WGS reactions. This thesis investigated liquid phase WGS (LWGS) at ambient temperature and pressure as an effective means to produce carbon monoxide free hydrogen. The use of liquid phase as a reaction medium and as a separator for the reactant and product gas phases has been attempted for the first time. Two processes with different combinations of primary reactions have been studied and discussed in customized reactors with formic acid as a liquid phase intermediate. In the first LWGS process, (i) carbon monoxide hydration reaction catalyzed by homogeneous [Ru HEDTA (Cl)] K.2H2O producing formic acid is followed with (ii) formic acid dehydrogenation to hydrogen and carbon dioxide catalyzed by PtRuBixOy/C (PRB) pellets. A customized reactor with two vessels was used to study the process at temperatures between 20 and 50 0C and at moderate pressure up to 21 atm. The process has an Ea value of 52.3 kJ/mol with a hydrogen generation rate of 2*10-4 moles/hr at 400C, 5 mM [[Ru HEDTA (Cl)] K.2H2O], 0.1 g PRB and 11 atm pco in 150 ml solution. Hydrogen generated from the process was analyzed by gas chromatography and was free of carbon monoxide In the second LWGS process, (i) methanol carbonylation reaction producing methyl formate in the presence of a heterogeneous catalyst Amberlyst A 26-OMe, is followed with (ii) methyl formate hydrolysis to formic acid, which in the presence of PRB dehydrogenates to hydrogen and carbon dioxide. The process was studied in a two reactor assembly at 50-80 0C with provision for the feedback of fresh or recycled methanol. The process was found to have an Ea value of 56 kJ/mol and an initial hydrogen generation rate of 1.5*10-3 moles/hr at 800C and 0.1 g PRB in 80 ml solution. Hydrogen generated was analyzed to be free of carbon monoxide. In addition to producing carbon monoxide free hydrogen under ambient conditions, another highlight of LWGS processes described here is that they integrate formic acid producing reactions with formic acid dehydrogenating reaction, a concept that has not been experimentally demonstrated in the literature. Limitations of the processes with respect to solubility of CO or catalysts have been discussed and improvements needed for practical applications have been proposed.