Equilibrium shift, poisoning prevention, and selectivity enhancement in catalysis via dehydration of polymeric membranes

Many industrially important and environmentally friendly chemical reactions produce water as a byproduct, which can significantly reduce the yield of the desired product. Traditional methods for water removal, such as using absorbents or inorganic membranes, often suffer from limitations in stability, selectivity, or operational complexity. The need for a stable and simple system capable of selectively removing water across a wide range of reaction temperatures is therefore significant. This research aims to address this challenge by developing a novel polymeric membrane for in-situ water removal in catalytic reactors. The key research question revolves around whether a newly developed thermally rearranged poly-benzoxazole (TR-PBO) membrane can effectively remove water from various chemical reactions at high temperatures, thus improving reaction yield and catalyst stability. The context of this study lies in the significant economic and environmental benefits of improving the efficiency of various chemical processes. The purpose is to design, synthesize, and characterize a membrane reactor capable of enhancing the efficiency and sustainability of industrially relevant chemical reactions by selectively removing water. The importance lies in addressing crucial challenges in chemical reaction engineering, including equilibrium limitations, catalyst poisoning, and unwanted side reactions that are frequently linked to water production.

Previous attempts to selectively remove water from chemical reactions have involved the use of sorbents within the reactor and the incorporation of additional reactants that consume water. The use of water-permeable membranes has also been explored as a promising approach due to its simplicity and energy efficiency. However, existing inorganic membranes (zeolites, silica, metal oxides) suffer from reduced water permselectivity at elevated temperatures due to the diminished pore-blocking effect. Metal-organic framework (MOF)-based membranes face challenges in thermal and chemical stability, and poor processability. While a modified polyimide membrane has shown moderate durability at 300 °C, its operational temperature is limited. This study builds upon the existing literature by focusing on a novel polymeric membrane, TR-PBO, designed for superior thermal stability and water permselectivity, exceeding the capabilities of previously reported membranes.

The TR-PBO membrane was synthesized in two steps: first, hydroxyl polyimide (HPI) hollow fibers were fabricated using a dry-jet/wet-quench method. Second, the HPI fibers underwent thermal rearrangement at 425 °C under a nitrogen atmosphere, converting them into TR-PBO. This process creates a unique porosity with a bimodal microcavity distribution, enhancing both selectivity and permeability. The membrane's structural properties were characterized using techniques such as attenuated total reflectance infrared (ATR-IR) spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Gas permeance and permselectivity were evaluated at various temperatures using pure and mixed gases. The performance of the TR-PBO membrane reactor was then tested in three different reactions: reverse water-gas shift (RWGS), methane combustion, and Fischer-Tropsch olefin (FTO) synthesis. Each reaction was carried out both with and without the membrane under identical conditions to compare the impact of in-situ water removal. Detailed descriptions of the experimental setups and reaction conditions are provided in the Supplementary Information. Catalyst characterization techniques such as X-ray photoelectron spectroscopy (XPS) were used to assess the effect of water removal on catalyst stability. Data analysis involved comparing reaction yields, conversion rates, and product selectivity with and without the membrane, and correlating these observations with the amount of water removed.

The TR-PBO membrane exhibited high H₂O permselectivity up to 440 °C, significantly exceeding the performance of previously reported polymeric and inorganic membranes. The membrane reactor demonstrated effectiveness in three different catalytic reactions: 1. **RWGS reaction:** The TR-PBO membrane reactor successfully shifted the thermodynamic equilibrium of the RWGS reaction, leading to a significant increase in CO₂ conversion beyond the equilibrium limit. The increased CO₂ conversion was directly proportional to the amount of water removed. 2. **Methane combustion:** In methane combustion, the membrane prevented water-induced deactivation of the Pd catalyst, maintaining high CH₄ conversion over a 150-hour period, while the catalyst without the membrane exhibited significant activity loss. XPS analysis showed that the membrane reactor prevented the irreversible phase transition of PdO, which is responsible for catalytic activity. 3. **FTO synthesis:** In the FTO reaction, the membrane suppressed the water-gas shift (WGS) side reaction, leading to an increase in CO conversion and a significant enhancement in olefin selectivity. By removing water, the unwanted WGS reaction was reduced from 47.9% to 15.1%, enhancing the olefin-to-paraffin ratio. The TR-PBO membrane demonstrated exceptional thermal, chemical, and mechanical stability throughout the long-term tests.

These findings demonstrate the significant potential of the TR-PBO membrane reactor for improving the efficiency and sustainability of a wide range of industrial chemical processes. The ability to overcome thermodynamic limitations, prevent catalyst poisoning, and suppress unwanted side reactions is a major advancement in reaction engineering. The results highlight the advantages of using a polymeric membrane compared to traditional methods for water removal. The superior thermal stability and processability of the TR-PBO membrane make it particularly attractive for high-temperature applications. The successful application of the membrane reactor in three distinct chemical reactions further underscores its versatility and wide applicability.

This study successfully demonstrated a novel TR-PBO membrane reactor for highly selective water removal in various high-temperature catalytic reactions. The membrane enabled significant improvements in reaction yields through equilibrium shifts, catalyst protection, and side reaction suppression. Future research could focus on optimizing the membrane reactor design, investigating the effects of membrane area and flow configurations, and upscaling the process for industrial applications.

The study was conducted using lab-scale reactors, and the scalability of the membrane reactor to industrial settings needs further investigation. While the effects of reactant crossover were mitigated by using identical feed and sweep gas compositions, optimizing the reactor design to fully minimize this effect is crucial for industrial applications. The performance was evaluated under specific reaction conditions and may vary with different catalyst compositions, reaction parameters, or gas mixtures.