Membrane reactors for hydrogenation and dehydrogenation processes based on supported palladium

Optimized bimetallic ratios for durable membrane catalyst-film reactors in treating nitrate-polluted water

Nitrate contamination of surface and ground water is a significant global challenge. Most current treatment technologies separate nitrate from water, resulting in concentrated wastestreams that need to be managed. Membrane Catalyst-film Reactors (MCfR), which utilize in-situ produced nanocatalysts attached to hydrogen-gas-permeable hollow-fiber membranes, offer a promising alternative for denitrification without generating a concentrated wastestream. In hydrogen-based MCfRs, bimetallic nano-scale catalysts reduce nitrate to nitrite and then further to di-nitrogen or ammonium. This study first investigated how different molar ratios of indium-to-palladium (In:Pd) catalytic films influenced denitrification rates in batch-mode MCfRs. We evaluated eleven In-Pd bimetallic catalyst films, with In:Pd molar ratios from 0.0029 to 0.28. Nitrate-removal exhibited a volcano-shaped dependence on In content, with the highest nitrate removal (0.19 mgNO3--N-min-1 L-1) occurring at 0.045 mol In/mol Pd. Using MCfRs with the optimal In:Pd loading, we treated nitrate-spiked tap water in continuous-flow for >60 days. Nitrate removal and reduction occurred in three stages: substantial denitrification in the first stage, a decline in denitrification efficiency in the second stage, and stabilized denitrification in the third stage. Factors contributing to the slowdown of denitrification were: loss of Pd and In catalysts from the membrane surface and elevated pH due to hydroxide ion production. Sustained nitrate removal will require that these factors be mitigated.

Collectable Single Pure-Pd Metal Membrane with High Strength and Flexibility Prepared through Electroplating for Hydrogen Purification

Among the various film preparation methods, electroplating is one of the simplest and most economical methods. However, it is challenging to collect a dense single Pd film through plating, owing to the accumulation of stress in the film during the process. Therefore, the characteristics of a single plated film have not been clearly identified, although pure Pd is widely used in metallic-hydrogen-purification membranes. In this study, stress concentration in film during preparation was reduced by optimizing the plating process, and a dense single flat film was successfully collected. No impurities were detected. Thus, a high-purity Pd film was prepared. Its surface texture was found to be significantly different from that of the rolled film, and several approximately 5 μm sized aggregates were observed on the surface. The plated film is reported to have mechanical properties superior to those of the rolled film, with twice the displacement and four times the breaking point strength. The hydrogen permeabilities of the plated film (5.4 × 10−9–1.1 × 10−8 mol·m−1·s−1·Pa−1/2 at 250–450 °C) were comparable to those of the rolled and reported films, indicating that the surface texture does not have a strong effect on hydrogen permeability. The results of this study promote the practical use of Pd-based membranes through electroplating.

Flux-Reducing Tendency of Pd-Based Membranes Employed in Butane Dehydrogenation Processes

We report on the effect of butane and butylene on hydrogen permeation through thin state-of-the-art Pd-Ag alloy membranes. A wide range of operating conditions, such as temperature (200-450 °C) and H2/butylene (or butane) ratio (0.5-3), on the flux-reducing tendency were investigated. In addition, the behavior of membrane performance during prolonged exposure to butylene was evaluated. In the presence of butane, the flux-reducing tendency was found to be limited up to the maximum temperature investigated, 450 °C. Compared to butane, the flux-reducing tendency in the presence of butylene was severe. At 400 °C and 20% butylene, the flux decreases by ~85% after 3 h of exposure but depends on temperature and the H2/butylene ratio. In terms of operating temperature, an optimal performance was found at 250-300 °C with respect to obtaining the highest absolute hydrogen flux in the presence of butylene. At lower temperatures, the competitive adsorption of butylene over hydrogen accounts for a large initial flux penalty.

A computational model for the catalytic hydrogel membrane reactor

The catalytic hydrogel membrane reactor (CHMR) is a promising new technology for hydrogenation of aqueous contaminants in drinking water. It offers numerous benefits over conventional three-phase reactors, including immobilization of nano-catalysts, high reactivity, and control over the hydrogen (H₂) supply concentration. In this study, a computational model of the CHMR was developed using AQUASIM and calibrated with 32 experimental datasets for a nitrite (NO₂^-)-reducing CHMR using palladium (Pd) nano-catalysts (~4.6 nm). The model was then used to identify key factors impacting the behavior of the CHMR, including hydrogel catalyst density, H₂ supply pressure, influent and bulk NO₂^- concentrations, and hydrogel thickness. Based on the model calibration, the reaction rate constants for the NO₂^- steady-state adsorption Hinshelwood reaction equation, k₁ and k₂, were 0.0039 m³ mole-Pd^-1 s^-1 and 0.027 (mole-H₂ m³)^1/2 mole-Pd^-1 s^-1, respectively. The reactant flux, which is the overall NO₂^- removal rate for the CHMR, is affected by the NO₂^- reduction rate at each catalyst site, which is in turn controlled by the available NO₂^- and H₂ concentrations that are regulated by their mass transport behavior. Reactant transport in the CHMR is counter-diffusional. So for thick hydrogels, the concurrent concentrations of NO₂^- and H₂ are limiting in the middle region along the x-y plane of the hydrogel, which results in a low overall NO₂^- removal rate (i.e., flux). Thinner hydrogels provide higher concurrent reactant concentrations throughout the hydrogel, resulting in higher fluxes. However, if the hydrogel is too thin, the flux becomes limited by the amount of Pd that can be loaded, and unused H₂ can diffuse into the bulk and promote biofilm growth. The hydrogel thickness that maximized the NO₂^- flux ranged between 30 and 150 μm for the conditions tested. The computational model is the first to describe CHMR behavior, and it is an important tool for the further development of the CHMR. It also can be adapted to assess CHMR behavior for other contaminants or catalysts or used for other types of interfacial catalytic membrane reactors.

Preparation of High Stability Pd/Ceramic/Ti-Al Alloy Composite Membranes by Electroless Plating

High stability Pd/ceramic/Ti-Al alloy composite membranes were prepared by electroless plating. Ceramic membranes fabricated by an in situ oxidation method were used as an inter-diffusion barrier between the Pd layer and the Ti-Al alloy support of the membranes to prevent intermetallic diffusion. The stabilities of the ceramic membranes at high temperatures in an H₂ atmosphere were investigated. The permeation performances and stabilities of the Pd/ceramic/Ti-Al alloy composite membranes were also studied. The results showed that the thickness, pore size, and microstructure of the ceramic membranes did not change significantly after the treatment in an H₂ atmosphere at high temperatures, indicating that the ceramic membranes prepared by the in situ oxidation method were stable in an H₂ atmosphere at high temperatures. The thickness of the Pd layer was ~13 μm. The hydrogen permeability and H₂/N₂ selectivity of the Pd composite membranes at 773 K were 2.13 × 10⁻³ mol m⁻² s⁻¹ Pa^−0.5 and 600, respectively. In addition, the H₂ flux, N₂ flux, and H₂/N₂ selectivity of the composite membranes remained nearly constant over three heat cycles (under the same conditions), indicating that the structures of the Pd/ceramic/Ti-Al alloy composite membranes were stable.

Activity and stability of the catalytic hydrogel membrane reactor for treating oxidized contaminants

The catalytic hydrogel membrane reactor (CHMR) is an interfacial membrane process that uses nano-sized catalysts for the hydrogenation of oxidized contaminants in drinking water. In this study, the CHMR was operated as a continuous-flow reactor using nitrite (NO₂^-) as a model contaminant and palladium (Pd) as a model catalyst. Using the overall bulk reaction rate for NO₂^- reduction as a metric for catalytic activity, we evaluated the effect of the hydrogen gas (H₂) delivery method to the CHMR, the initial H₂ and NO₂^- concentrations, Pd density in the hydrogel, and the presence of Pd-deactivating species. The chemical stability of the catalytic hydrogel was evaluated in the presence of aqueous cations (H⁺, Na⁺, Ca²⁺) and a mixture of ions in a hard groundwater. Delivering H₂ to the CHMR lumens using a vented operation mode, where the reactor is sealed and the lumens are periodically flushed to the atmosphere, allowed for a combination of a high H₂ consumption efficiency and catalytic activity. The overall reaction rate of NO₂^- was dependent on relative concentrations of H₂ and NO₂^- at catalytic sites, which was governed by both the chemical reaction and mass transport rates. The intrinsic catalytic reaction rate was combined with a counter-diffusional mass transport component in a 1-D computational model to describe the CHMR. Common Pd-deactivating species [sulfite, bisulfide, natural organic matter] hindered the reaction rate, but the hydrogel afforded some protection from deactivation compared to a batch suspension. No chemical degradation of the hydrogel structure was observed for a model water (pH > 4, Na⁺, Ca²⁺) and a hard groundwater after 21 days of exposure, attesting to its stability under natural water conditions.

Catalytic Hydrogel Membrane Reactor for Treatment of Aqueous Contaminants

Heterogeneous hydrogenation catalysis is a promising approach for treating oxidized contaminants in drinking water, but scale-up has been limited by the challenge of immobilization of the catalyst while maintaining efficient mass transport and reaction kinetics. We describe a new process that addresses this issue: the catalytic hydrogel membrane (CHM) reactor. The CHM consists of a gas-permeable hollow-fiber membrane coated with an alginate-based hydrogel containing catalyst nanoparticles. The CHM benefits from counter-diffusional transport within the hydrogel, where H₂ diffuses from the interior of the membrane and contaminant species (e.g., NO₂^-, O₂) diffuse from the bulk aqueous solution. The reduction of O₂ and NO₂^- were investigated using CHMs with varying palladium catalyst densities, and mass transport of reactive species in the catalytic hydrogel was characterized using microsensors. The thickness of the "reactive zone" within the hydrogel affected the reaction rate and byproduct selectivity, and it was dependent on catalyst density. In a continuously mixed flow reactor test using groundwater, the CHM activity was stable for a 3 day period. Outcomes of this study illustrate the potential of the CHM as a scalable process in the treatment of aqueous contaminants.

New Insight to the Effects of Heat Treatment in Air on the Permeation Properties of Thin Pd77%Ag23% Membranes

Sputtered Pd77%Ag23% membranes of thickness 2.2–8.5 µm were subjected to a three-step heat treatment in air (HTA) to investigate the relation between thickness and the reported beneficial effects of HTA on hydrogen transport. The permeability experiments were complimented by volumetric hydrogen sorption measurements and atomic force microscopy (AFM) imaging in order to relate the observed effects to changes in hydrogen solubility and/or structure. The results show that the HTA—essentially an oxidation-reduction cycle—mainly affects the thinner membranes, with the hydrogen flux increasing stepwise upon HTA of each membrane side. The hydrogen solubility is found to remain constant upon HTA, and the change must therefore be attributed to improved transport kinetics. The HTA procedure appears to shift the transition from the surface to bulk-limited transport to lower thickness, roughly from ~5 to ≤2.2 µm under the conditions applied here. Although the surface topography results indicate that HTA influences the surface roughness and increases the effective membrane surface area, this cannot be the sole explanation for the observed hydrogen flux increase. This is because considerable surface roughening occurs during hydrogen permeation (no HTA) as well, but not accompanied by the same hydrogen flux enhancement. The latter effect is particularly pronounced for thinner membranes, implying that the structural changes may be dependent on the magnitude of the hydrogen flux.

Effect of capillary condensation on gas transport properties in porous media

We investigate the effect of capillary condensation on gas diffusivity in porous media composed of randomly packed spheres with moderate wettability. To simulate capillary phenomena at the pore scale while retaining complex pore networks of the porous media, we employ density functional theory (DFT) for coarse-grained lattice gas models. The lattice DFT simulations reveal that capillary condensations preferentially occur at confined pores surrounded by solid walls, leading to the occlusion of narrow pores. Consequently, the characteristic lengths of the partially wet structures are larger than those of the corresponding dry structures with the same porosities. Subsequent gas diffusion simulations exploiting the mean-square displacement method indicate that while the effective diffusion coefficients significantly decrease in the presence of partially condensed liquids, they are larger than those in the dry structures with the same porosities. Moreover, we find that the ratio of the porosity to the tortuosity factor, which is a crucial parameter that determines an effective diffusion coefficient, can be reasonably related to the porosity even for the partially wet porous media.

Catalytic Layer Optimization for Hydrogen Permeation Membranes Based on La5.5WO11.25-δ/La0.87Sr0.13CrO3-δ Composites

(LWO/LSC) composite is one of the most promising mixed ionic-electronic conducting materials for hydrogen separation at high temperature. However, these materials present limited catalytic surface activity toward H₂ activation and water splitting, which determines the overall H₂ separation rate. For the implementation of these materials as catalytic membrane reactors, effective catalytic layers have to be developed that are compatible and stable under the reaction conditions. This contribution presents the development of catalytic layers based on sputtered metals (Cu and Pd), electrochemical characterization by impendace spectroscopy, and the study of the H₂ flow obtained by coating them on 60/40-LWO/LSC membranes. Stability of the catalytic layers is also evaluated under H₂ permeation conditions and CH₄-containing atmospheres.

Pd-Cu-M (M = Y, Ti, Zr, V, Nb, and Ni) Alloys for the Hydrogen Separation Membrane

Self-supported fcc Pd-Cu-M (M = Y, Ti, Zr, V, Nb, and Ni) alloys were studied as potential hydrogen purification membranes. The effects of small additions (1-2.6 at. %) of these elements on the structure, hydrogen solubility, diffusivity, and permeability were examined. Structural analyses by X-ray diffraction (XRD) showed the fcc phase for all alloys with induced textures from cold rolling. Heat treatment at 650 °C for 96 h led to the reorientation in all alloys except the Pd-Cu-Zr alloy, exhibiting the possibility to enhance the structural stability by Zr addition. Hydrogen solubility was almost doubled in the ternary alloys containing Y and Zr compared to Pd_65.1Cu_34.9 alloy at 300 °C. It was noted that hydrogen diffusivity is decreased upon additions of these elements compared to the Pd_65.1Cu_34.9 alloy, with the Pd-Cu-Zr alloy showing the lowest hydrogen diffusivity. However, the comparable hydrogen permeability of the Pd-Cu-Zr alloy with the corresponding binary alloy, as well as its highest hydrogen permeability among the studied ternary alloys at temperatures higher than 300 °C, suggested that hydrogen permeation of these alloys within the fcc phase is mainly dominated by hydrogen solubility. Hydrogen flux variations of all ternary alloys were studied and compared with the Pd_65.1Cu_34.9 alloy under 1000 ppm of H₂S + H₂ feed gas. Pd-Cu-Zr alloy showed superior resistance to the sulfur poisoning probably due to the less favorable H₂S-surface interaction and more importantly slower rate of bulk sulfidation as a result of improved structural stability upon Zr addition. Therefore, Pd-Cu-Zr alloys may offer new potential hydrogen purification membranes with improved chemical stability and hydrogen permeation compared to the binary fcc Pd-Cu alloys.

Development of Flow-Through Polymeric Membrane Reactor for Liquid Phase Reactions: Experimental Investigation and Mathematical Modeling

Incorporating metal nanoparticles into polymer membranes can endow the membranes with additional functions. This work explores the development of catalytic polymer membrane through synthesis of palladium nanoparticles based on the approaches of intermatrix synthesis (IMS) inside surface functionalized polyethersulfone (PES) membrane and its application to liquid phase reactions. Flat sheet PES membranes have been successfully modified via UV-induced graft polymerization of acrylic acid monomer. Palladium nanoparticles have been synthesized by chemical reduction of palladium precursor loaded on surface modified membranes, an approach to the design of membranes modified with nanomaterials. The catalytic performances of the nanoparticle incorporated membranes have been evaluated by the liquid phase reduction of p-nitrophenol using NaBH4 as a reductant in flow-through membrane reactor configuration. The nanocomposite membranes containing palladium nanoparticles were catalytically efficient in achieving a nearly 100% conversion and the conversion was found to be dependent on the flux, amount of catalyst, and initial concentration of nitrophenol. The proposed mathematical model equation represents satisfactorily the reaction and transport phenomena in flow-through catalytic membrane reactor.

Conceptual Approach in Multi-Objective Optimization of Packed Bed Membrane Reactor for Ethylene Epoxidation Using Real-coded Non-Dominating Sorting Genetic Algorithm NSGA-II

An isothermal plug flow reactor model with extended Fick diffusion model for transport through the porous membrane is utilized for simulation of ethylene oxide formation in a packed bed membrane reactor (PBMR). The model was verified and validated using published experimental data from an existing lab-scale unit. Sensitivity analysis was performed to determine robustness of the model. A conceptual approach on operation and design stage multi-objective optimization study is discussed. Real-coded NSGA-II is used and effect of its parameters on optimization of reactor performance is also studied. The results of three two-objective operation-stage (with 4 decision variables) and one two-objective design-stage (with 6 decision variables) optimization case studies are presented. Good convergence to a Pareto optimal solution is achieved for all cases. Significant improvement over current experimental operation is observed in terms of increase in conversion of ethylene, selectivity to ethylene oxide and ethylene oxide product flow rate.

Evaluation of the Parameters and Conditions of Process in the Ethylbenzene Dehydrogenation with Application of Permselective Membranes to Enhance Styrene Yield

Styrene is an important monomer in the manufacture of thermoplastic. Most of it is produced by the catalytic dehydrogenation of ethylbenzene. In this process that depends on reversible reactions, the yield is usually limited by the establishment of thermodynamic equilibrium in the reactor. The styrene yield can be increased by using a hybrid process, with reaction and separation simultaneously. It is proposed using permselective composite membrane to remove hydrogen and thus suppress the reverse and secondary reactions. This paper describes the simulation of a dehydrogenation process carried out in a tubular fixed-bed reactor wrapped in a permselective composite membrane. A mathematical model was developed, incorporating the various mass transport mechanisms found in each of the membrane layers and in the catalytic fixed bed. The effects of the reactor feed conditions (temperature, steam-to-oil ratio, and the weight hourly space velocity), the fixed-bed geometry (length, diameter, and volume), and the membrane geometry (thickness of the layers) on the styrene yield were analyzed. These variables were used to determine experimental conditions that favour the production of styrene. The simulation showed that an increase of 40.98% in the styrene yield, compared to a conventional fixed-bed process, could be obtained by wrapping the reactor in a permselective composite membrane.