The 98,000 cu metre-ships will be delivered in 2026 and 2027 to the UK-based firm and will feature dual-fuel ethane engines.
The vessels will also be fitted with optimised cargo handling systems, lower CO2 and SOx emissions and additional energy-saving devices to further contribute to emission reductions.
Both vessels will be delivered directly into long-term time charters, said Julian Proctor, of Purus.
The company has been involved in over 60 deals in various shipping segments, including ammonia, LNG, offshore wind, carbon capture and scrubber-fitted containerships and electric ferries.
The company has previously ordered four 45,000 cu metre dual-fuel ammonia-ready medium-sized gas carriers at Hyundai Mipo Dockyard, expected to primarily carry ammonia.