Chapter 4.1 - Hydrogen-Production Technologies Using Amorphous Silica Membranes

Abstract

Hydrogen production technologies using membrane reactors with hydrogen-selective amorphous silica membranes are reviewed. A membrane reactor is a system that integrates “reaction” with catalysts and “separation” with membranes. When the hydrogen-selective membranes are applied to membrane reactors for the reactions to dehydrogenate organic hydrides, decompose hydrogen sulfide, or for steam reforming of hydrocarbons, equilibrium shifts can be achieved because of the extraction of the produced hydrogen from the reaction side to the permeate side. In other words, higher conversions are achieved at lower reaction temperatures using membrane reactors than using conventional reactors. In addition, hydrogen with higher purity can be obtained without any additional posttreatment because the hydrogen-selective membrane can also achieve hydrogen purification. To install adequate hydrogen-selective amorphous silica membranes into the membrane reactors, methods for pore-size control must be developed because the intended gas species to be separated from hydrogen differs according to the hydrogen production reaction systems, and the separation mechanism of the silica membranes is that of a molecular sieve, which means that gas species smaller than the pore can permeate and the others cannot. Therefore, a pore-size-control technique by utilizing the chemical structures of the silica precursors is introduced. Different silica precursors produce different transient intermediates during the formation of the silica layer, which determines the pore structures of the membranes. Examples of the development of membrane reactors with pore-size-controlled hydrogen-selective silica membranes are then reviewed. The membrane reactors presented in this review are for dehydrogenating cyclohexane or methylcyclohexane, for decomposing hydrogen sulfide, and for methane steam reforming. Because the reaction conditions differ in these three reaction systems, each membrane installed in the membrane reactor is required to have different properties. Therefore, these points are clarified first, and the performances of the membrane reactors are then introduced in each case. All of the membrane reactors successfully showed effective equilibrium shifts under various operating conditions because of the excellent hydrogen extraction and purification performances of the pore-size-controlled hydrogen-selective silica membranes. In particular, it should be noted that as high as 99.95% purity of hydrogen can be obtained in the membrane reactor that dehydrogenates methylcyclohexane, which means that this enables us to supply the produced hydrogen directly to fuel cells.