Hydrogen-selective silica membranes for use in membrane steam reforming (1999)

R(COOH) x + R'(OH) y o RCOOR' + H 2 OClearly, in such a reaction the removal of the produced water will lead to an enhanced conversion.Commercially available polymer membranes cannot withstand the severe operation and cleaning conditions for this process (150-300C) and microporous silica membranes again come into the picture. Hydrogen selective membranesWhen considering membrane reactors for dehydrogenation and reforming reactions, three types of membrane are of most interest: dense palladium or palladium composite membranes, silica compound, such as silane [36][37][38] or Tetra Ethyl Ortho Silicate [39][40][41][42] and the oxidising agent is usually pure oxygen, air or steam.The porous materials used for CVI are commonly -alumina supports [39], -alumina membranes [40] or porous Vycor glass [36,37].In theory CVI membranes are very promising.Especially membrane stability is expected to be very good, because the separative layer is located inside the support, where it is protected against mechanical damage and chemical attack.In addition, measured permselectivities of CVI-silica membranes are very high: for H 2 :N 2 values as high as 3000 have been measured [36,37].However, large-scale commercial production of CVI-membranes is hindered by the complicated equipment needed for their synthesis and the associated high costs.Furthermore the permeance of CVI membranes (1*10 -7 mol/m 2 sPa or lower) are still too low.Maybe these low permeances can be increased using different precursors or other reaction conditions, but a large improvement is not to be expected. Sol-gel silica membranesA third type of membrane is the sol-gel microporous silica membrane.This type of membrane is of major importance in this thesis.Below, a short overview will be provided of state-of-theart silica membranes at the start of the project (1995).This has been the starting point from which the new membranes described in this thesis were developed. SynthesisAt the start of the project microporous sol-gel silica membranes were under investigation in various research groups.An extensive literature review is provided in the introduction of the thesis of De Vos [43].Common supports for sol-gel silica membranes are mesoporous -alumina membranes [44][45][46][47].These mesoporous membranes are either home made [43][44][45] or obtained from commercial sources [46,47].A standard membrane as prepared by de Lange [45] consisted of a die-pressed -alumina support, fired at 1360C with a pore diameter of 160 nm on which a -alumina membrane was coated with a home-prepared boehmite sol.The coated -alumina layer was calcined at 600C, had a thickness of 7 m with a pore diameter of ~5 nm.On top of this mesoporous membrane, essing.A literature review of the basic concepts of the preparation of colloidal suspensions is provided in chapter 3, whereas the actual preparation is dicussed in chapter 4.On top of the newly developed supports a steam-stable intermediate layer was coated.The preparation of these layers is treated in detail in chapter 5.After this, the permselective silica layer was applied, which should be resistant against high temperature and steam-containing environments as well.The experimental procedure together with some transport and Rutherford BackScattering (RBS) studies are described in chapter 6.Apart from the silica membranes prepared by dipcoating, also Chemical Vapour Infiltration (CVI)-type membranes have been prepared.Chapter 7 is dedicated to this type of membrane.In chapter 8 a new project has been formulated for the use of membrane reactors for the thermal dehydrogenation of H 2 S. Compared to the conventional Claus process, the application of a membrane reactor in the thermal H 2 S might have some large advantages.Finally, in chapter 9, conclusions are drawn and suggestions made for further research on (steam-stable) molecular sieving silica membranes or mesoporous -alumina membranes.Though not all of the project objectives were obtained, progress was made in the synthesis of micro-and mesoporous membranes.Especially the development of steam stable membranes may be a large step forward in the development of ceramic membranes. References