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Review

A Mini Review on Liquid Phase Catalytic Exchange for Hydrogen Isotope Separation: Current Status and Future Potential

by
Siti Munirah Mhd Yusof
1,2,
Serene Sow Mun Lock
1,2,*,
Nur Najwa Abdul Talib
1 and
Liew Chin Seng
1
1
Centre of Carbon Capture, Utilisation and Storage (CCCUS), Universiti Teknologi PETRONAS, Bandar Seri Iskandar 32610, Perak, Malaysia
2
Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar 32610, Perak, Malaysia
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(11), 4796; https://doi.org/10.3390/su16114796
Submission received: 16 April 2024 / Revised: 20 May 2024 / Accepted: 24 May 2024 / Published: 4 June 2024
(This article belongs to the Special Issue Nuclear Energy and Technology and Its Environmental Impact)
Browse Figures
Review Reports Versions Notes

Abstract

:
Liquid phase catalytic exchange (LPCE) appears a highly promising technology for separating hydrogen isotopes due to being less energy-intensive and having a high separation factor. This paper provides an overview of the current development of the hydrophobic catalysts used in the LPCE process, including the LPCE fundamentals, factors influencing its effectiveness, and proposals for future research areas. This paper specifically reviews the active metal catalysts, catalyst supports, operating temperatures, and molar feed ratio(gas-to-liquid,G/L). The addition of a second metal such as Ir, Fe, Ru, Ni, or Cr and modified catalyst supports showed enhancement of LPCE performance. Additionally, the validated optimized temperature of 60–80 °C and G/L of 1.5–2.5 provide an important basis for designing LPCE systems to improve separation efficiency. This paper concludes by highlighting potential research areas and challenges for future advancements in the sustainability of LPCE for hydrogen isotope separation, which include the optimization, scalability, techno-economic analysis, and life-cycle analysis of modified catalyst materials.

1. Introduction

Hydrogen isotopes are hydrogen variants with the same number of protons but different numbers of neutrons. The three main hydrogen isotopes are protium, deuterium, and tritium. These isotopes have different physical properties that can be exploited for separation. Deuterium and tritium are two isotopes of hydrogen that play a crucial role in numerous applications.
Deuterium and tritium have various applications in industries due to their unique properties, as shown in Figure 1 and Figure 2. Deuterium, also known as heavy hydrogen, is commonly used in nuclear reactors for moderating and sustaining nuclear fission. It is an essential component in heavy water, which is used as a moderator and coolant in certain types of nuclear reactors [1,2,3]. Additionally, deuterium is used in the production of deuterated compounds, which are important in pharmaceuticals, organic chemistry, and biochemical research [4,5,6]. Deuterated compounds are compounds in which one or more hydrogen atoms are replaced with deuterium atoms. For example, deuterated water (D2O) is commonly used as a solvent in NMR spectroscopy to study the structure of organic molecules. Deuterated compounds also have applications in the pharmaceutical industry, where they are used to study metabolic processes and the stability of drugs [6,7]. Another example is deuterated benzene, used as a solvent in certain NMR experiments [8]. Deuterated compounds are valuable in organic chemistry because they have slightly different chemical behaviors than their non-deuterated counterparts, making them useful in studying reaction mechanisms and pathways. In addition, deuterated compounds find applications in the field of environmental chemistry, where they are used as tracers to study the transport of chemicals in the environment [9]. The unique properties of deuterium make deuterated compounds essential in various scientific and industrial applications.
On the other hand, tritium is utilized in producing luminous paints for watch dials and instrument panels due to its radioactive properties which make it suitable for creating self-luminous sources that remain visible in the dark [10,11]. It is also used in nuclear fusion research and in manufacturing certain types of nuclear weapons. Moreover, tritium has also been used as a tracer for radioactive measurement [4]. The unique characteristics of deuterium and tritium make them valuable resources across various industrial applications, contributing to advancements in energy production, materials science, and other fields.
In the field of nuclear fusion, both deuterium and tritium are used as fuel for fusion reactors [12,13,14,15]. The potential for harnessing the energy released from nuclear fusion reactions involving deuterium and tritium has garnered significant interest as a clean and virtually limitless energy source for the future. Deuterium–deuterium and deuterium–tritium fusion reactions have the potential to generate substantial amounts of energy with minimal environmental impact and without producing long-lived radioactive waste [12].
The process of separating hydrogen isotopes, particularly deuterium, is a crucial step in the production of heavy water for nuclear reactors and in obtaining pure deuterium for various industrial and scientific applications. Efficient separation methods are essential for ensuring a reliable supply of deuterium, which is vital for advancing research and development in energy-related fields such as nuclear fusion and materials science.

2. Current Status of Hydrogen Isotope Separation Technologies

Various hydrogen isotope separation techniques have been developed and studied extensively to meet the increasing demand for deuterium and tritium. Isotope separation involves the process of separating isotopes of a particular element based on their differing masses. This is achieved by exploiting the physical and chemical properties of the isotopes, allowing their separation from a mixture. Several methods have been proposed and utilized for hydrogen isotope separation. These methods include cryogenic distillation, quantum sieving, electrolysis, and liquid phase catalytic exchange (LPCE).
Table 1 shows the summary of the advantages, disadvantages, and challenges of cryogenic distillation, quantum sieving, electrolysis, and LPCE technologies used for hydrogen isotope separation. Most of the technologies can separate high-purity hydrogen isotopes [16,17,18]. Well-established methods like cryogenic distillation, electrolysis, and LPCE are widely used in the industry for large-scale hydrogen isotope separation. Cryogenic distillation was utilized for tritium separation in the Savannah River Site Tritium Facilities and the National R&D Institute for Cryogenics and Isotopes Technologies—ICSI Rm. Valcea [17,19,20]. Meanwhile, electrolysis is predominantly combined with LPCE through Combined Electrolytic Catalytic Exchange, used at facilities like the Canada Deuterium Uranium nuclear power plant [21]. One of the advantages of these technologies is that all of them can be operated at ambient pressure, which eliminates the need for high-pressure equipment and reduces the complexity of the separation process. Moreover, technologies such as cryogenic distillation and LPCE are capable of processing large quantities of feed [22]. This capability is crucial to ensuring a reliable supply of high-purity deuterium and tritium for ongoing research and development in the field of nuclear fusion and other related industries.
However, each technology has its limitations and challenges. Cryogenic distillation, though efficient, requires a significant amount of energy [16]. In cryogenic distillation, the isotopes are separated based on their different boiling points at low temperatures. The process takes advantage of the small mass difference between isotopes, causing them to condense at different temperatures. The mixture is cooled to very low temperatures around 20–24 K, causing the isotopes to liquefy and then vaporize at different points, allowing for their separation [20,23]. The process is energy-intensive due to the extremely low temperatures required and the need for continuous distillation. Moreover, cryogenic distillation also requires expensive equipment such as cryogenic distillation columns, cold boxes, and heat exchangers, which further increases the cost of implementation. The same limitation is faced by electrolysis technology, where a significant amount of electricity is required to split the water molecules into hydrogen and oxygen. This can make water electrolysis for hydrogen isotope separation costly and energy-intensive [24]. Consequently, continuous research efforts have focused on enhancing electrolysis performance by improving electrode effectiveness and other related areas [25,26,27,28,29].
Consequently, quantum sieving has been proposed as an alternative technology for hydrogen isotope separation due to its lower energy consumption. This method utilizes preferential adsorption at low temperatures in a solid microporous material, offering a potential solution to the challenges associated with conventional separation methods. The materials used for quantum sieving, such as metal–organic frameworks (MOFs), zeolites, and carbon-based materials like activated carbon and graphene exhibit exceptional characteristics that make them suitable for this process [30,31,32]. Unlike cryogenic distillation, which requires extremely low temperatures and continuous distillation, quantum sieving operates at a lower energy consumption with an operating temperature of around 20–100 K by utilizing the quantum effect and restricted rotation of isotopic molecules within the solid material’s pores [30,33,34]. This not only reduces the energy-intensive nature of the separation process but also offers the potential for cost savings in large-scale applications. However, this technology is still in the early stage of development and further research is required to overcome the limitations of quantum sieving technology, especially on the limited scalability of quantum sieving materials.
LPCE is another method that has been proposed for hydrogen isotope separation. As compared to other separation methods, such as cryogenic distillation and quantum sieving, LPCE stands out due to its simplicity and cost-effectiveness [35,36]. This is because LPCE can be operated at ambient conditions, eliminating the need for extreme temperature and pressure. Most of the LPCE process was conducted at ambient pressure and temperatures ranging from ambient temperature to 363 K, with the optimum temperature identified in the range of 333–353 K [37,38,39,40]. Additionally, LPCE also has the potential for high separation efficiency and the ability to handle large volumes of feed since it uses a simple trickle bed reactor, making it more scalable and cost-effective for industrial applications. Thus, researchers have been exploring various aspects of LPCE to enhance its efficiency and applicability in different industries. This review paper aims to discuss the current research and developments in LPCE for hydrogen isotope separation. This review will cover three aspects, which are the fundamentals of LPCE for hydrogen isotope separation, factors affecting LPCE performance, and the potential future direction and challenges of LPCE technology.

3. Hydrogen Isotope Separation via Liquid Phase Catalytic Exchange

LPCE is a process used for hydrogen isotope separation, specifically the exchange of hydrogen isotopes between water and hydrogen gas. This process involves the use of catalytic packing in an isotopic exchange column, where the catalyst plays a big role in the exchange of hydrogen isotopes between the liquid water and the gaseous hydrogen. LPCE was first used at Chalk River Tritium Extraction Plant in Canada for large-scale hydrogen isotope separation [41,42,43]. This technology is used at the Canada Deuterium Uranium (CANDU) nuclear power reactor plant for heavy water production in combination with electrolysis, which is called the Combined Electrolytic Catalytic Exchange—Heavy Water Process (CECE-HWP) [21]. This technology is also being used in Petersburg Nuclear Physics Institute at the industrial plant scale for heavy water detritiation [44]. Figure 3 shows the general process flow diagram for hydrogen isotope separation using LPCE technology. This process flow is characterized by three main steps, which are feed preparation, isotopic exchange, and further purification and recovery. During feed preparation, the starting material (tritiated water or tritiated hydrogen) undergoes treatment to meet the required specifications for the LPCE process. The liquid feed is then heated to the desired temperature using a pre-heater before being introduced into the LPCE column. Mass flow controllers are used to control the flow rate of the gas and liquid. Then, in the LPCE column, the isotopic exchange process occurs, where the catalytic packing facilitates the transfer of hydrogen isotopes between the liquid and gaseous phases. Finally, the purification and recovery step involve the separation and collection of the desired hydrogen isotope.

3.1. Fundamentals of Hydrogen Isotope Separation Using LPCE

In the LPCE process, the transfer of hydrogen isotopes happens between two phases, the gas and liquid phase, over a catalytic packing material. The gas and liquid are contacted by flowing through an exchange column such as a packed column in a co-current or counter-current condition. Figure 4 shows an example of a counter-current LPCE process using a packed column, where the gas is introduced from the bottom of the column and liquid is fed from the top. Inside the column, gas and liquid flow in a counter-current, passing through catalytic packing arranged in either a random or layered configuration. The volume ratio of packing to catalyst varies based on the column design, catalyst efficiency, and operational parameters. Huang et al. (2018) varied this ratio from 2 to 6, while Li et al. (2019) identified the optimal ratio as 3 using a random mode catalytic packing configuration and operating temperature of 70 °C [37,40]. Figure 5 shows the illustration of the catalytic exchange process that occurs over the mixed catalytic packing. The catalytic exchange process takes place over two consecutive steps as follows:
H D O   l + H 2 O   v ↔ H D O   v + H 2 O   l
H D O   v + H 2 g ↔ H D   g + H 2 O   v
H D O   l + H 2   g ↔ H D   g + H 2 O   l
where g ,   l , and v are the gas, liquid, and vapor phases, respectively. The reaction in Equation (1) takes place at any gas–liquid interface, while the reaction in Equation (2) occurs as a catalytic process on the hydrophobic catalyst’s surface. The overall hydrogen isotope transfer is shown in Equation (3). For the first reaction, the transfer depends on the gas–liquid equilibrium and the interfacial area. It is essential to have a large interface between the gas and liquid phases to facilitate effective mass transfer. Therefore, using packing in a packed column significantly enhances the gas–liquid mass transfer by increasing the contact area between the two phases [45,46]. In the case of the reaction in Equation (2), the reaction takes place between the hydrogen gas and the water vapor on the active catalyst surface. It is important to prevent liquid water from blocking the catalyst micropores, thus exposing the active catalyst surface to the gaseous reactant. This explains the use of a hydrophobic catalyst for the LPCE process. One of the most common catalysts used for the LPCE process is Pt/C/Polytetrafluoroethylene (Pt/C/PTFE) [40,46,47]. An effective hydrophobic catalyst can significantly improve the process by repelling liquid water and facilitating the transport of gaseous reactants and reaction products to and from catalytic active centers. The catalytic reaction occurs at the catalyst sites consisting of two routes, which are Route 1 and Route 2. In Route 1, the reaction happens due to the dissociation of hydrogen molecules to hydrogen atoms on the active metal sites and an isotope exchange reaction with water vapor. In this case, the water molecules cannot be dissociated and exist as intact molecules on the metal surfaces. Equations (4)–(8) show the reaction of deuterium exchange between hydrogen and water molecules on the metal surfaces. In this case, the process reaction employs the common metal used in the LPCE process, which is platinum, which will be further explained in the next sections.
H D + 2 P t → H   a d s + D ( a d s )
D a d s + n H 2 O → D H 2 O n + a d s + e −   ( n > 1 )
D H 2 O n + a d s → H H D O H 2 O n − 1 + a d s   ( n > 1 )
H H D O H 2 O n − 1 + a d s + e − → H a d s + H D O + n − 1 H 2 O ( n > 1 )
H   a d s + H a d s → 2 P t + H 2
Additionally, water molecules may undergo dissociation to -H and -OH on the surface of doped elements and their oxides, potentially leading to an alternative reaction pathway known as Route 2. In this pathway, hydrogen is expected to adsorb in a dissociative manner on the first metal sites (Pt), while water vapor molecules may adsorb dissociatively on the second metals and their oxide sites, as shown in Equations (9)–(13).
H D + 2 M → H   a d s + D ( a d s )
H 2 O + 2 M → H   a d s + O H ( a d s )
D a d s + O H a d s → H D O   a d s + M
H D O   a d s → H D O + M
H   a d s + H a d s → 2 M + H 2
where M is the doped metal and oxides. After the reaction is complete, the product desorbs from the catalyst surface and is carried away by the gas or liquid stream.

3.2. Factors Affecting LPCE Performance

Several factors, including gas–liquid equilibrium, the interfacial area between the gas and liquid, and the catalyst itself, can influence the efficiency and effectiveness of the LPCE process. Research has been focused on improving these factors to enhance the overall LPCE performance.

3.2.1. Active Metal of Hydrophobic Catalyst

The catalyst is crucial in the isotopic exchange process as it allows the exchange reaction to occur, facilitating the formation of deuterium oxide (D2O) or heavy water [37]. The choice of catalyst, consisting of active metal, support, and hydrophobic coating, plays a key role in the LPCE process. Monometallic catalysts such as platinum, palladium, nickel, and rhodium have been thoroughly researched for LPCE application and proven to be efficient [48,49,50,51,52]. In a study conducted by Burwell et al. (1956), nickel–silica was used as the catalyst for the hydrogen isotopic exchange, while Rae et al. (1978) used three catalysts, Pt/C, potassium methylamide, and potassium amide, for three different feedstocks [51,52]. Among all, platinum has emerged as the most effective catalyst for LPCE due to its high catalytic activity. Since their introduction by Steven in 1968, platinum-based catalysts have gained wide usage in research and industry [53]. Researchers have developed various strategies to further enhance the catalytic activity and stability of platinum-based catalysts. One of the effective methods is the incorporation of a second metal, known as a bimetallic catalyst, such as chromium, titanium, and iridium into the catalyst to improve the catalytic activity and anti-poisoning properties. Additionally, incorporating a non-noble metal such as iron or nickel has the potential to reduce the cost of the catalyst. This review paper specifically focuses on the current development of bimetallic catalysts for the LPCE process.
In recent years, there have been significant advancements in the development of bimetallic catalysts for LPCE in hydrogen isotope separation. These advancements have focused on improving catalyst performance, reducing costs, and increasing stability through the incorporation of different dopants and modifications in the catalyst structure. Table 2 shows the summary of studies conducted on the performance of bimetallic catalysts for the LPCE process. The studies show that the addition of second metals such as Ir, Fe, Ru, Ni, and Cr enhances the performance of the LPCE process as compared to the commercial catalyst Pt/C/PTFE. All the studies measured the Pt particle size using transmission electron microscopy (TEM) and concluded that the addition of the second metal did not significantly change the metal particle size. On the other hand, the addition of the second metal changes the crystalline structure of the catalyst and creates more active sites for hydrogen isotope adsorption and exchange [54,55]. A study conducted by Hu et al. (2012) shows that the addition of Fe in the catalyst forms the Fe oxides. The presence of oxides leads to the rapid dissociation of water molecules on the surface of oxides, thus accelerating the exchange reaction by Route 2 (Equations (9)–(13)) [56]. This finding is supported by the studies conducted by Hu et al. (2010) and Ye et al. (2014), where the addition of a second metal significantly enhanced catalytic performance. On the surface of metallic Pt, water molecules exist as intact molecules, whereas the water molecules can be dissociated into -H and -OH on the doped metals and their oxides, which makes it possible for the reaction in Route 2 to occur [54,55].
These findings show that the development of bimetallic catalysts has shown great promise in improving the performance and efficiency of the LPCE process for hydrogen isotope separation. These advancements in bimetallic catalyst development have the potential to address the high cost of platinum, a commonly used active metal in LPCE, without compromising performance. As research in this field continues, the exploration of bimetallic catalysts holds significant promise for further improving the efficiency and effectiveness of hydrogen isotope separation in the LPCE process. Continued efforts in developing cost-effective and high-performance catalysts will be crucial for the advancement of hydrogen isotope separation technologies.

3.2.2. Catalyst Support

Another important factor influencing LPCE performance is the catalyst support. Carbon is commonly used as the support material due to its high surface area and good electrical conductivity. Different types of carbon supports, such as carbon black, activated carbon, and carbon nanotubes, have been studied for their potential to enhance the catalytic activity and stability of the catalyst by providing a stable and conductive platform for active metal deposition. Additionally, two-dimensional materials like graphene and carbon nitride are gaining attention for their unique properties that make them promising candidates for improving the efficiency and stability of LPCE catalysts. Table 3 summarizes the studies conducted on various types of catalyst support for the LPCE process.
By modifying the catalyst support, the LPCE performance can be significantly enhanced due to various reasons. First, the modification of the support material can improve the dispersion of Pt particles, which leads to smaller Pt particle size as shown in Table 3. The Pt particle size of the catalysts is measured using TEM analysis. In a study conducted by Fu et al. (2021), the catalyst was modified by introducing carbon nitride as the support material. Carbon nitride binds Pt through a nitrogen-containing group on its surface, preventing the aggregation of Pt nanoparticles, and effectively reduces the Pt particle size to about 0.82 nm. This leads to higher LPCE performance when compared to the traditional Pt/AC catalyst [48]. Vasut et al. (2019) also managed to reduce the particle size by using graphene oxide as the support material due to better dispersion of Pt on the support material [58]. By having smaller Pt particle size, the catalyst can increase the active surface area available for catalytic reactions, leading to enhanced reaction kinetics and higher efficiency in the LPCE process.
Another way to enhance the LPCE performance is by increasing the hydrophobicity of the catalyst. Previous studies reported that Pt/C/PFTE and Pt/SDB catalysts commonly used for the LPCE process have a water contact angle of around 110° [49,59,60,61]. However, by modifying the catalyst support, the hydrophobicity of the catalyst can be enhanced. Lu et al. (2023) reported a contact angle of 158° for the Pt/SBA-15-tetramethyldisilazane/PVDF catalyst, while Fu et al. (2019) achieved a high contact angle of 156° using MIL-101 as the catalyst support [59,62]. A contact angle above 150° is considered superhydrophobic. A high hydrophobicity of the catalyst is necessary for the LPCE process to prevent water from covering the active sites on the catalyst surface, ensuring better accessibility for hydrogen isotopes, and improving the overall efficiency of the LPCE process.
In addition to improving catalytic activity and hydrophobicity, modifying the catalyst support can also enhance the stability of LPCE catalysts. In a study conducted by Fu et al. (2021), a Pt/s-C3N4-7 catalyst demonstrated excellent stability during long-term LPCE reactions, showing no significant loss in catalytic activity even after 30 h of continuous operation, while the performance of Pt/AC dropped about 20% [48]. Other studies have also shown that the modified catalyst performance remains stable over extended periods of operation of up to 30 days, indicating the potential for improved catalyst durability in the LPCE processes [59,62].
Furthermore, the porosity of the catalyst support plays a crucial role in influencing LPCE performance. Fu et al. (2019) reported that Pt/MIL-101/PVDF can adsorb a higher amount of hydrogen as compared to Pt/SDB, mainly due to the porosity properties of the catalyst [59]. Catalysts with high porosity enhance the exchange process by improving the contact between reacting phases. However, this finding contradicts with the finding reported by Hu et al. (2011), where catalysts with lower porosity exhibited superior performance. This discrepancy arises from the high surface area and porosity of the catalyst support, causing most of the Pt particles to be deposited in the micropores of the support [56]. Hence, the reactants access the active sites through a longer reaction channel, which leads to lower performance. This shows that the surface area of the catalyst is not the only determining factor that affects LPCE performance. Instead, LPCE performance also depends on synergistic factors such as the catalyst design and operating parameters.
Table 3. Summary of studies conducted on various types of catalyst supports for the LPCE process.
Table 3. Summary of studies conducted on various types of catalyst supports for the LPCE process.
MetalSupportCoatingPt Particle Size (nm)Contact AngleSpecific Surface Area, SBET (m2/g)StabilityPerformanceReferences
PtModified carbon nitride (C3N4)PDMSPt/AC: 3.01 ± 0.36 nm
Pt/s-C3N4-7: 0.82 ± 0.08 nm		Pt/s-C3N4-7: 159.3°Pt/s-C3N4-7: 65.7 m2/g1.25 dayPt/AC: ~98%
Pt/C3N4-7: 98%
at T = 80 °C; G = 12 mL/min; L = 2 mL/h		[48]
Pt1. Multiwalled carbon nanotubes (MWNTs)
2. Activated carbon (AC)
3. Vulcan XC-72 carbon black (XC)		PTFEPt/MWNT: 2.36 ± 0.8 nm
Pt/AC: 1.98 ± 0.5 nm
Pt/XC:2 ± 0.52 nm		N/APt/MWNT: 233 m2/g
Pt/AC: 950 m2/g
Pt/XC: 230 m2/g		N/AColumn efficiency:
Pt/AC: ~75%
Pt/MWNTs: ~88%
Pt/XC: ~85%
at T = 50 °C; G = 1 L/min		[56]
PtChromium-based metal–organic frameworks (MIL-101)PVDFMIL-101:pt particle size: 2.9 nm
styrene divinylbenzene copolymer (SDB): pt particle size 3.3 nm		Pt/SDB: 132°
Pt/MIL-101: 156°		Pt/SDB: 103 m2/g
Pt/MIL-101: 289 m2/g		30 daysColumn efficiency:
Pt/SDB: ~90%
Pt/MIL-101/PVDF: ~98%
at T = 60 °C; G = 0.5 L/min		[59]
PtSanta Barbara 15 (SBA-15)-tetramethyldisilazanePolyvinylidene fluoride (PVDF)2.03 nm158°N/A30 daysColumn efficiency:
Pt/SBA-15-tetramethyldisilazane: ~72%
Pt/SDB: ~60%
at T = 70 °C; G = 300 mL/min; L = 0.25 mL/min		[62]
PtDual modified graphene x-S-NH2-GR
(x is amount of trimethoxyoctylsilane)		Poly(dimethylsiloxane)
PDMS		1. Pt/NH2-GR: 1.95 ± 0.29 nm
2. Pt/200-S-NH2-GR: 1.85 ± 0.35 nm		Pt/NH2-GR: 135°
Pt/200-S-NH2-GR: 150°		N/A600 minColumn efficiency:
5 wt% Pt/C: ~87.5%
3.2 wt% Pt/200-S-NH2-Gr: ~90%
at T = 80 °C		[63]
Overall, the use of modified catalyst supports in LPCE processes has shown promising results in terms of improving catalytic activity, hydrophobicity, and overall stability. The studies show that the use of materials such as graphene oxide, SBA-15-tetramethyldisilazane, and MIL-101 as the catalyst support successfully enhanced the LPCE column efficiency. However, LPCE performance comparisons between those studies are difficult to conduct due to the difference in LPCE reactor design and experimental conditions, such as temperature, flow rates, and catalyst loading. Nevertheless, modifying the catalyst support plays a crucial role in optimizing the LPCE process and achieving higher separation efficiency. Further research and development in this field are crucial for the continued improvement of hydrogen isotope separation technologies and the development of cost-effective, high-performance catalysts.

3.2.3. Operating Temperature

In the reaction shown in Equation (1), the transfer of deuterium between vapor and liquid phases occurs at any gas–liquid interface. Thus, the gas–liquid equilibrium and the interfacial area significantly affect the transfer efficiency. Few factors affect the gas–liquid equilibrium such as the operating temperature. The operating temperature is an important parameter in the LPCE process as it directly affects the separation efficiency and energy consumption. Several studies have explored the impact of operational temperature on the LPCE process and identified the most effective temperature for the reaction. Table 4 shows the summary of the parametric studies conducted for hydrogen isotope separation using the LPCE process. Most experiments investigating hydrogen isotope separation through LPCE methods have studied the effect of temperatures ranging from room temperature to 90 °C. For instance, Li et al. (2019)’s research revealed that the optimal reaction temperature for the LPCE process utilizing a Pt/C/PTFE catalyst falls within the range of 60–80 °C [37]. This discovery is consistent with other studies indicating that the ideal temperature for an LPCE process employing this catalyst also lies within this interval [40,64,65]. At lower temperatures, thermodynamic analysis corresponds well with experimental results, indicating that the separation efficiencies are primarily governed by equilibrium factors. As the temperature increases, the suppression of kinetic factors becomes more important in determining the separation efficiencies [66]. At elevated temperatures, the rate of water vapor and hydrogen gas velocity increases, reducing the duration for which the reactant interacts with the catalyst. This ultimately results in reduced catalytic efficiency. Therefore, it is important to carefully consider the operating temperature in LPCE processes to achieve optimal separation efficiencies.

3.2.4. Molar Feed Ratio (G/L)

In addition to the operating temperature, the molar feed ratio also plays a crucial role in determining the efficiency of the LPCE process. The G/L ratio represents the ratio of hydrogen to water. According to Li et al. (2019)’s study, it was found that the optimal G/L ratio falls within 1.5–2.5 for operating temperatures ranging from 30 °C to 80 °C, as shown in Table 4. A higher G/L ratio of more than 2 leads to a significant increase in gas flow rate, which reduces contact time between gas and catalyst [37,62]. Moreover, the fast-flowing gas carries water vapor and droplets out from the reactor, further decreasing contact time between gas and liquid and ultimately leading to decreased separation efficiency. At elevated operating temperatures, the impact is more significant due to the increase in the water vapor formation. This observation aligns with findings from other studies, which also show that the ideal temperature for LPCE varies depending on the G/L ratio. According to the results, as the G/L ratio increases, the optimal temperature decreases [37,64]. It is essential to conduct thorough experimental investigations considering factors such as catalyst type, operating temperature, and reactor design to determine the ideal G/L ratios for the specific LPCE systems.

3.3. Potential Future Direction of Hydrogen Isotope Separation via LPCE

Hydrogen isotope separation using LPCE has shown promising results in achieving the efficient separation of heavy water and the production of deuterium-depleted potable water. However, there are still several areas that require further research and development. One area of focus is the development of more efficient catalyst materials. Currently, platinum catalysts supported on carbon are commonly used in LPCE processes. However, there is room for improvement in terms of catalytic activity and stability. Efforts should be directed toward exploring alternative catalyst materials that can enhance the performance and longevity of the LPCE process.
Additionally, current research on improving the catalyst by the addition of a second metal as well as modified catalyst support has shown promising results. However, most of these studies have concentrated on characterizing and understanding how the alterations in catalyst structure and properties impact catalytic effectiveness. Further studies on the LPCE process optimization using the modified catalyst materials are necessary to understand their impact on separation efficiency fully and to optimize the process parameters accordingly.
As the research and development of hydrogen isotope separation via LPCE continues to progress, it is essential to consider the environmental impact and sustainability of the process. An efficient method for assessing the environmental efficiency of the LPCE process involves conducting a life-cycle analysis (LCA). This approach investigates the environmental effects linked to every stage of a product’s life cycle, from extracting raw materials to disposing of them at the end of its life [67,68,69]. For example, it is crucial to assess the sustainability implications of the modified catalysts. Through the implementation of LCA, it becomes possible to pinpoint the potential environmental advantages and drawbacks linked to various catalyst materials. Consequently, initiatives aimed at minimizing the environmental impact of the process can be pursued, thereby advancing sustainable development.
In addition to environmental considerations, conducting a techno-economic analysis (TEA) of the LPCE process is essential to evaluate the economic viability and scalability of the process. By considering aspects like capital expenditure (CAPEX), operating expenditure (OPEX), production yield, and sensitivity analysis, TEA can provide insights into the cost-effectiveness of implementing the LPCE process with the modified catalyst [70]. Material scalability should also be considered, as the LPCE process may require large-scale production to meet industrial demands.

4. Conclusions

In conclusion, hydrogen isotope separation using LPCE is a promising method for the detritiation of heavy water and the production of deuterium-depleted potable water. Significant progress has been made in developing hydrophobic catalysts, particularly platinum with a second metal as well as platinum with a modified support. These catalysts have shown improved activity and stability, leading to higher separation efficiencies in the LPCE process. Moreover, the optimization of process parameters such as reaction temperature and feed ratio has been shown to further enhance the separation efficiency. The findings from the process optimization will be beneficial for the future development of LPCE reactor design and implementation of the LPCE process on a larger scale. However, further investigation into alternative catalyst materials for the LPCE process is necessary to enhance catalytic activity and stability. Additionally, integrating sustainability considerations, such as environmental and cost impacts via LCA and TEA, into the ongoing research and development of the LPCE process is crucial. By doing so, more sustainable and efficient pathways for hydrogen isotope separation can be achieved, leading the industry toward a greener and more resource-efficient future.

Author Contributions

Conceptualization, S.M.M.Y. and S.S.M.L.; Writing—original draft preparation, S.M.M.Y.; Writing—review and editing, N.N.A.T., L.C.S., and S.S.M.L.; Supervision, S.S.M.L. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support was obtained from the MRA-FP project (Grant Number: 015MD0-131).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to acknowledge the Universiti Teknologi PETRONAS and the MRA-FP project.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Deuterium industrial applications.
Figure 1. Deuterium industrial applications.
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Figure 2. Tritium industrial applications.
Figure 2. Tritium industrial applications.
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Figure 3. General process flow diagram for hydrogen isotope separation using LPCE technology.
Figure 3. General process flow diagram for hydrogen isotope separation using LPCE technology.
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Figure 4. Counter-current LPCE process using a packed column.
Figure 4. Counter-current LPCE process using a packed column.
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Figure 5. Illustration of the exchange process over the mixed catalytic packing.
Figure 5. Illustration of the exchange process over the mixed catalytic packing.
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Table 1. Summary of the advantages, disadvantages, and challenges of cryogenic distillation, quantum sieving, and electrolysis.
Table 1. Summary of the advantages, disadvantages, and challenges of cryogenic distillation, quantum sieving, and electrolysis.
TechnologiesCryogenic DistillationQuantum SievingElectrolysisLiquid Phase Catalytic Exchange
Advantages1. Produces high-purity isotopes.
2. Capable of processing large quantities of feed.
3. Can be performed at ambient pressure. 		1. Can be performed at ambient pressure.
2. More energy-efficient as compared to cryogenic distillation.		1. Easy to scale up since it is a simple process.
2. High precision of separation.		1. High separation efficiency with high purity.
2. Can be performed at ambient pressure and low temperature, resulting in low energy consumption.
3. Capable of processing large quantities of feed.
Disadvantages and Challenges1. Energy-intensive.
2. Requires expensive equipment such as cryogenic distillation columns and cold boxes.		1. Complex and expensive synthesis methodologies of quantum sieving materials, which limits its scalability.
2. The selectivity of quantum sieving is temperature-dependent.
3. The practical application of quantum sieving is still in the early stage of development.		1. High energy consumption.
2. Complex and expensive equipment.
3. Highly dependent on the availability and cost of electricity.		1. The separation efficiency is highly affected by the performance of the catalyst.
2. The cost of the commercial catalyst used, which is platinum, is quite expensive.
Table 2. Summary of studies conducted on the performance of bimetallic catalysts for the LPCE process.
Table 2. Summary of studies conducted on the performance of bimetallic catalysts for the LPCE process.
First MetalSecond MetalSupportHydrophobic CoatingPt Particle Size (nm)PerformanceMain FindingsReferences
PtRuCPTFEPt/C: 1.9 ± 0.4 nm
Pt0.5Ru0.5/C: 1.9 ± 0.5 nm
Pt/RuO2/C: 2.5 ± 0.6 nm		Column efficiency:
Pt/C: ~77%
Pt0.5Ru0.5/C and Pt/RuO2/C: ~85%
at G = 2 L/min
  • Pt0.5Ru0.5/C and Pt/RuO2/C show higher performance than pure Pt.
  • The presence of Ru, either in alloy or hydrous oxide, enhances the catalytic activity for the LPCE process.
  • Pt/C and Pt0.5/Ru0.5 catalysts had similar particle size. This shows that the addition of Ru does not change the particle size but changes the crystalline structure.
[54]
Pt1. Fe
2. Co
3. Ni
4. Cr		CTeflon1. Pt/Fe: 2.3 nm
2. Pt/Co: 2.6 nm
3. Pt/Ni: 2.2 nm
4. Pt/Cr: 2.4 nm
5. Pt/C: 2.5 nm		Column efficiency:
1. Pt/Fe: ~59%
2. Pt/Co: ~52%
3. Pt/Ni: ~53%
4. Pt/Cr: ~67%
5. Pt/C: ~41%
at G = 2 L/min
  • The hydrogen isotope exchange activity increased in the order of Pt/C < Pt3Ni/CzPt3Co < Pt3Fe/C < Pt3Cr/C.
  • The enhanced activities of Pt3M bimetals over Pt are attributed to crystal structural factors and effective active sites.
[55]
PtFeCPTFEPt/C: 1.9 nm
Pt3Fe/C: 2.0 nm		Column efficiency:
Pt/C: ~58%
Pt3Fe/C: ~75%
at G = 2 L/min
  • Pt3Fe/C shows the highest performance as compared to Pt/C and other Fe/Pt molar ratios.
  • The catalytic activity of Pt3Fe-MI-H (hydrophobic Pt3Fe catalyst synthesized using microwave-irradiated ethyl glycol reduction method) is highest due to the formation of Fe oxides. Water is easily dissociated on the surface of Fe oxides, which promotes the catalyst activity (Route 2).
[56]
PtIrCPTFEPt/C: 2.1 nm
Pt4Ir1/C: 2.2 nm		Column efficiency:
Pt/C: ~73%
Pt4Ir1/: ~87%
at G = 2 L/min
  • Pt4Ir1/C/FN shows the highest performance as compared to Pt/C/Fn and other Ir/Pt molar ratios.
  • The addition of elemental Ir did not improve the dispersion of Pt particles but improved the synergistic effect of metal Pt and Ir.
[57]
Table 4. Parametric studies on hydrogen isotope separation using the LPCE process.
Table 4. Parametric studies on hydrogen isotope separation using the LPCE process.
CatalystOperating PressureOperating TemperatureG/LOptimized Condition IdentifiedMain FindingsReferences
0.8 wt% Pt/C/PTFE1 atm30–90 °C0.8–3.5Temperature: 60–80 °C
G/L: 1.5–2.5
  • Highest conversion at 70 °C under G/L of 1.4 with column efficiency of around 70%.
  • At temperature > 80 °C and G/L > 1.5, the kinetic factors hinder the LPCE performance.
[37]
0.8 wt% Pt/PTFEN/A30–90 °CN/ATemperature: 80 °C
  • As the temperature increases from 35 °C to 80 °C, the conversion rate increases from 17% to 34.5%.
[40]
10 wt% Pt/PTFE1 atm20–80 °C0.5–4Temperature: 70 °C
  • The increase in G/L from 0.5–4 resulted to reducing the optimum temperature from 80 °C to 45 °C.
[64]
0.37 wt% Pt/C/Teflon1 atm25–60 °CN/ATemperature: 60 °C
  • The increase in temperature from 25 °C to 60 °C increases the mass transfer coefficient to about 2.4-fold.
[65]
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Mhd Yusof, S.M.; Lock, S.S.M.; Abdul Talib, N.N.; Seng, L.C. A Mini Review on Liquid Phase Catalytic Exchange for Hydrogen Isotope Separation: Current Status and Future Potential. Sustainability 2024, 16, 4796. https://doi.org/10.3390/su16114796

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Mhd Yusof SM, Lock SSM, Abdul Talib NN, Seng LC. A Mini Review on Liquid Phase Catalytic Exchange for Hydrogen Isotope Separation: Current Status and Future Potential. Sustainability. 2024; 16(11):4796. https://doi.org/10.3390/su16114796

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Mhd Yusof, Siti Munirah, Serene Sow Mun Lock, Nur Najwa Abdul Talib, and Liew Chin Seng. 2024. "A Mini Review on Liquid Phase Catalytic Exchange for Hydrogen Isotope Separation: Current Status and Future Potential" Sustainability 16, no. 11: 4796. https://doi.org/10.3390/su16114796

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