Introduction
Ammonia is a crucial synthetic product and ranks as the second-largest chemical compound manufactured globally, with an annual production of 150/160 million tonnes.
Around 80% of ammonia is used in the fertilizer industry to synthesize nitrogen compounds, like urea. Ammonia is converted into nitric acid which, upon being mixed with ammonia, results in ammonium nitrate. (1)
With a hydrogen content of 17.6% by weight and no carbon, ammonia has been recognised as a promising medium for energy storage.
Ammonia is commonly produced via the Haber-Bosch process, reliant on iron-based catalysts, where hydrogen and nitrogen (in a 3:1 ratio) react in the gas phase under elevated temperatures (300-500 °C) and pressures (100-250 bar).
The reaction to consider is the following:
3 H2 + N2 ↔ 2 NH3
Hydrogen is generated in methane steam-reforming plants, whilst nitrogen is obtained from secondary reforming units.
Despite being an exothermic reaction (∆H ° = -91.8 kJ/mol), the dissociation of the triple bond in nitrogen requires high energy (941 kJ / mol).
From a thermodynamic perspective, achieving acceptable yields in this reaction necessitates a low temperature and high pressure, whereas from a kinetic perspective, high temperatures are necessary.
To produce ammonia on an industrial scale, iron-based catalysts are typically preferred for ammonia synthesis because due to their lower cost.
Specifically, magnetite (Fe3O4) is commonly used, while aluminium, potassium, calcium, and metal oxides (Al2O3, K2O) serve as promoters to increase the thermal stability of iron and prevent sintering. Alternative catalysts to magnetite include ruthenium, supported on graphite (Ru/C), MgO, γ-Al2O3, γ-Al2O3 -KOH (with barium and caesium as promoters), wustite, cobalt or nickel. These have a conversion rate of 10-15%.
Due to the expensive price of ruthenium and its unstable nature (where the carbonaceous support experiences methanation reaction in the presence of hydrogen), iron-based catalysts are favoured for industrial usage. (2)
Recently, electrides, a type of ionic compounds O2- where electrons act as anions (including C12A7:e-, Ca2N:e-1, Y5Si3, LaScSi, LaRuSi, and CeRuSi) have been tested as potential supports for ruthenium. However, their restricted surface area renders them impractical. (3)
Materials and methods
To obtain a more precise representation of industrial advancements in ammonia production, researchers can consult patent literature.
An analysis of these patents can be carried out using keywords, classification codes, or semantic search.
However, the most precise method is conducting searches via classification codes.
There are two primary classification systems for patent information: IPC and CPC.
Both systems have a common hierarchical structure, which includes a main part (section, class, subclass) and a secondary part that provides more detailed information on the subject matter or technological application.(4)
The International Patent Classification (IPC) was established by the Strasbourg Agreement of 1971. Although only 62 countries have signed the agreement, the IPC has been adopted by over 100 national offices, the four regional offices (EPO, EAPO, OAPI and ARIPO), and WIPO, which manages it.
The IPC divides inventions into eight sections, with approximately 76,000 subdivisions.
The IPC classification is updated on an annual basis, with a new edition published every 1st of January. The current version in force is 2024.01. (5, 6)
Both the EPO and the USPTO apply the CPC classification system (7), which became effective on 1 January 2013. This system is based on the IPC system but is more detailed and subject to greater revisions.
The CPC has more than 260,000 entries (8) and divides patentable technologies into nine sections (A – Y), which are further divided into increasingly detailed levels (subsections, classes, subclasses, groups, and subgroups).
The variations between the two systems are based on the quantity of subgroups and documents that are classified.
The use of either the IPC or the CPC code yields different results.
Restricting the search to only CPC or IPC classes may hinder a comprehensive analysis of patent information.
When searching by classification codes, both systems (IPC and CPC) should be used to avoid losing important data.
An initial exploratory search using specific keywords was conducted on Espacenet ((https://worldwide.espacenet.com) to identify the main classification codes (IPC and CPC). These codes are displayed in Table 1.
The most effective search string turned out to be the following:
(ftxt=(“ammonia” prox/distance<3 “synthesis”) AND ftxt=(“industrial” prox/distance<3 “production”)) OR (ctxt = “ammonia” AND ftxt=(“industrial” prox/distance<3 “synthesis”))
This search yielded 3,272 outcomes (database accessed on 4th November 2023).
The top five IPC classification symbols obtained through an exploratory search are presented in Figure 1.
The list of classification symbols and search queries is fully displayed in Appendix 1.
The main classification (IPC and CPC) codes are the following: C01C 1/02 (IPC/CPC – Preparation of ammonia), C01C 1/04 (IPC/CPC – Preparation of ammonia by gas phase synthesis), C01C 1/0405 (CPC only- Preparation of ammonia by gas phase synthesis from nitrogen and hydrogen in the presence of a catalyst) and C25B 1/27 (IPC/CPC – Electrolytic production of ammonia).
The complete search (as shown in Table 2) was carried out on Orbit Intelligence database (https://www.orbit.com), using FamPat database (Accessed on 6th November 2023).
Results and discussion
According to patent analysis, it seems that thermocatalytic methods, specifically the Haber-Bosch process, are the most frequently claimed. Electrocatalytic methods are next in line, while photocatalytic and chemical looping methods are on a decline (refer to Figure 2).
The number of patent families related to thermocatalytic processes that are currently in force (patent applications/granted patents) has increased over the last decade (refer to Figure 3 for a visual representation). Casale, a Swiss company, has filed the largest number of patent families.
As far as thermocatalytic processes are concerned, fixed-bed reactors are the most frequently claimed in patents, with only a small number of patents relating to fluidized-bed reactors (refer to Table 3).
Concerning patents for catalytic systems, the most common ones use iron (Fe3O4, Fe1-xO, Fe2O3) with promoters like K2O, BaO and Al2O3. These are followed by patents for ruthenium, cobalt, nickel, and metal nitrides (refer to Figure 4).
Magnetite is the most commonly occurring iron oxide.
The patent search on Espacenet yielded limited results for the application of electrides, as outlined in Table 4. The classification code (IPC/CPC) C01C 1/02 pertains to methods of producing ammonia.
Conclusions
The low-temperature synthesis of ammonia remains a key challenge for catalysis, owing to the inert nature of nitrogen.
The most used catalytic systems in industry are based on iron. However, they cannot operate at the same temperatures as ruthenium, resulting in an expensive process.
To achieve temperatures below 300 °C nickel-based systems supported on LaN or BaH2 or cobalt-based systems can be used. (9)
Supports capable of increasing the catalytic activity are rare earth oxides, such as CeO2, La2O3 and Pr2O3, in the form of nanoparticles. (10)
Patent analysis indicates a shift towards developing alternative catalytic systems instead of those using iron and ruthenium.
The number of patents related to electrocatalytic processes pertaining to ammonia production has risen, alongside thermocatalytic processes.
Table 1. Results of the exploratory patent search.
Table 2. Complete list of search queries used in FamPat database.
Table 3. Number of patents claiming the fixed or fluidised bed reactors in HB processes.
Table 4. Number of patents claiming the use of electrides in ammonia production.
Figure 1. Top five IPC classification symbols retrieved by conducting an exploratory search.
Figure 2. Filing trend for ammonia production methods (source Orbit, own calculations)
Figure 3. Number of in force patent families filed in the last ten years on thermocatalytic methods (source Orbit).
Figure 4. Number of patent families vs. types of catalysts (source Orbit, own calculations).
REFERENCES AND NOTES
- A. E. Yüzbaşıoğlu et al., Curr. Res in Green and Sus. Chem., 5, 100307 (2022)
- P. Puspitasari, N. Yahya, 2011 National Postgraduate Conference (2011) doi: 10.1109/NatPC.2011.6136449
- J.A. Faria, Curr. Opin. Green Sustain. Chem., 29, 100466 (2021)
- B. Degroote, P. Held, World Pat. Inf., 54(S), S78-S84 (2018)
- International Patent Classification https://ipcpub.wipo.int/
- Guide to the International Patent Classification (2023) https://www.wipo.int/publications/en/details.jsp?id=4656&plang=EN
- Cooperative Patent Classification: https://www.cooperativepatentclassification.org/home
- E. Marttin, A-C. Derrien, World Pat. Inf. 54(S), S33-S43 (2018)
- J. Humphrey et al., Adv. Energy Sustainability Res., 2, 2000043 (2021)
- J. Feng et al., Catal. Sci. Technol., 11, 6330-6343 (2021).
Appendix 1 – Complete list of (IPC/CPC) classification codes for ammonia synthesis
(IPC/CPC) C01C 1/02 • Preparation or separation of ammonia
(IPC/CPC) C01C 1/04 •• Preparation of ammonia by synthesis in the gas phase
(CPC) C01C 1/0405 ••• from N2 and H2 in presence of a catalyst
(CPC) C01C 1/0411 •••• characterized by the catalyst.
(CPC) C01C 1/0417 •••• characterized by the synthesis reactor.
(CPC) C01C 1/0423 ••••• Cold wall reactors.
(CPC) C01C 1/0429 ••••• Fluidized or moving bed reactors.
(CPC) C01C 1/0435 ••••• Horizontal reactors
(CPC) C01C 1/0441 ••••• Reactors with the catalyst arranged in tubes.
(CPC) C01C 1/0447 •••• Apparatus other than synthesis reactors.
(CPC) C01C 1/0452 ••••• Heat exchangers.
(CPC) C01B 3/025 Production of hydrogen •• Preparation or purification of gas mixtures for ammonia synthesis
(CPC) C01B2203/068 Integrated processes for the production of hydrogen or synthesis gas •• ammonia synthesis
(IPC/CPC) C07C 273/10 – Preparation of urea •• combined with the synthesis of ammonia.
(IPC/CPC) C07C 273/04 – Preparation of urea •• from carbon dioxide and ammonia.
(CPC) C01C 1/0488 •••• Processes integrated with preparations of other compounds, e.g., methanol, urea or with processes for power generation.
Catalysts
(IPC/CPC) B01J 23/745 •• Iron
(IPC/CPC) B01J 23/78 ••• with alkali- or alkaline earth metals
(IPC/CPC) B01J 23/46 ••• Ruthenium, rhodium, osmium, or iridium
(CPC) B01J 23/462 •••• Ruthenium
(IPC/CPC) B01J 23/75 ••• Cobalt
(IPC/CPC) B01J 23/755 ••• Nickel
(IPC/CPC) B01J 23/10 • of rare earths
(IPC/CPC) C01B 21/06 • Binary compounds of nitrogen with metals
(IPC/CPC) C01G 49/04 •• Ferrous oxides (FeO)
(IPC/CPC) C01G 49/06 •• Ferric oxide (Fe2O3)
(IPC/CPC) C01G 49/08 •• Ferroso-ferric oxides (Fe3O4)
Electrocatalytic ammonia synthesis
(IPC/CPC) C25B 1/27 Electrolytic production •• of ammonia
cl = “C25B1/27/low” OR (cl = “C25B1/00” AND ctxt = “ammonia”)
Chemical looping
ctxt all “chemical looping” AND (ctxt=(“ammonia “ prox/ordered “synthesis”) OR ctxt=(“ammonia “ prox/ordered “production”))
ctxt=(“chemical “ prox/ordered “looping”) AND (cl any “C01C1/02” OR cl any “C01C1/04”)
