Mesoporous catalytic membranes: Synthetic control of pore size and wall composition

Abstract

Atomic layer deposition (ALD) on anodic aluminium oxide (AAO) is shown to be a facile, flexible route to the synthesis of catalytic membranes with precise control of pore wall composition and diameters. The oxidative dehydrogenation of cyclohexane was shown to depend strongly on pore diameter and to be more specific than similarly active alumina powder catalysts.Micro and mesoporous catalytic materials, predominantly in the form of zeolites, have gained wide acceptance as industrial catalysts for oil refining, petrochemistry, and organic synthesis, particularly for molecules with kinetic diameters below 1 nm. Here we report the first demonstration that ultra-uniform inorganic catalytic membranes, synthesized using a combination of anodic aluminum oxidation (AAO) [1-4] and atomic layer deposition (ALD) (a facile, flexible functionalization route) [5][6][7], have novel catalytic properties. Atomic level control of both the pore diameter and the pore wall composition along the entire pore length offers catalyst environments that include larger pores than conventional mesoporous materials (for efficient in-diffusion of large/elaborate molecular precursors or feedstock molecules, and for out-diffusion of large/ elaborate product molecules), tailored channel sizes and wall compositions (including tailored channel surfaces ranging from hydrophobic to hydrophilic in nature), catalyst mobility constraints to hinder agglomeration, and flow control of reagents in and out of the catalyst.Mesoporous and microporous catalytic material syntheses are often approached in a ''one step '' process (e.g.,). In this paper, we present an approach based on two steps: production of a stable mesoporous scaffold and then carefully controlled modification of the scaffold, where each step can be optimized independently. A related two step synthesis method utilizing a wet chemical modification method has been reported [12]. The modification method described here is based on gas phase deposition allowing directed control of the pore wall composition and diameter.AAO membranes are an appealing scaffold with highly-aligned, parallel pores and narrow pore diameter distributions. Electrochemical conditions can be arranged to produce most probable pore diameters in the range 20-400 nm [1-4, 13, 14] and membrane thicknesses in the range 0.5 to >250 lm. Prior catalytic studies using AAO showed interesting yield enhancements, but were limited to unreleased (pore blocked) films [15][16][17][18]. Moreover, as-grown AAO membranes suffer from poorly defined pore wall morphology and composition with significant (5%) amounts of incorporated electrolyte anions [19,20].For this work, highly-ordered, 70 lm thick AAO scaffolds produced in oxalic acid [13,14] are used. A typical plan view Secondary Electron Micrograph (SEM) of a scaffold is displayed in figure 1a. The hexagonal arrangement of pores (dark features) is clearly evident on this length scale; over longer distances the registry is less well defined. ALD was used to precisely control the chemical com...

Principles of Heterogeneous Catalysis

“…6 shows that essentially all studies of heterogeneous catalysis begin at the Materials Level. Recent advances in nanotechnology offer new routes for catalyst synthesis (e.g., atomic layer deposition, self-assembly methods) [93][94][95][96] and, importantly, also for catalyst characterization (e.g., techniques such as scanning tunneling microscopy that allow atomic-scale imaging of materials at elevated temperatures and pressures) [73,97]. However, as an as increasing number of research groups become involved in nanotechnology, it is possible that an ''applications gap'' will be created in heterogeneous catalysis, where new materials are formed without clear applications for catalytic processes.…”

Multiscale modeling of transport and residence times in nanostructured membranes

“…3 This article focuses on new, ultrauniform, inorganic, nanostructured membranes, fabricated by a combination of anodic aluminum oxidation (AAO) and atomic layer deposition (ALD). 4 The combination of these two techniques provides a flexible synthesis route to control the membrane pore diameter and the structure and composition of the pore walls. 4 AAO allows the electrochemical production of membranes with uniform cylindrical pores having diameters that range from 20 to 200 nm and lengths between 0.5 and 200 m. 5,6 The pore diameter and wall composition can later be tailored by placing layers of oxides (such as Al 2 O 3 , SiO 2 , and TiO 2 ) or other materials using ALD.…”

“…4 The combination of these two techniques provides a flexible synthesis route to control the membrane pore diameter and the structure and composition of the pore walls. 4 AAO allows the electrochemical production of membranes with uniform cylindrical pores having diameters that range from 20 to 200 nm and lengths between 0.5 and 200 m. 5,6 The pore diameter and wall composition can later be tailored by placing layers of oxides (such as Al 2 O 3 , SiO 2 , and TiO 2 ) or other materials using ALD. 7,8 Experimental results using AAO/ALD membranes fabricated with different diameters show improved selectivity toward the desired products in the oxidative dehydrogenation of cyclohexane compared to a conventional alumina powder catalyst at the same conversion.…”

Pt–Al2O3dual layer atomic layer deposition coating in high aspect ratio nanopores

“…AAO membrane, especially, represents an interesting substrate for the fabrication of functional nanoporous material. AAO structures have been functionalized using atomic layer deposition (ALD) [41][42][43][44][45], among other techniques, as recently reported by Banerjee et al [46], who made use of AAO defined nanopatterns, functionalized with a metal-oxide-metal multilayer, to form a regular array of nanocapacitors and Gu et al [47] who formed nested semiconductor/metal nanotubes using ALD. ALD is a technique that allows for the deposition of extremely conformal coatings on complex 3D nanostructures [48][49][50][51][52].…”