As of 2021, here are ten notable real estate investors specializing in sustainable properties:

1. Blackstone Group: One of the largest real estate investment firms globally, they have made significant investments in sustainable and energy-efficient properties.

2. Hines: Known for their sustainable development projects, Hines focuses on creating environmentally friendly spaces and has a strong commitment to sustainability.

3. GreenOak Real Estate: They specialize in investing in sustainable properties and have a focus on environmentally responsible development and operations.

4. Grosvenor Group: This privately-owned international property company prioritizes sustainability and environmental considerations in their investments and developments.

5. LaSalle Investment Management: LaSalle has a dedicated sustainability program and invests in properties with energy-efficient features and environmentally friendly practices.

6. Morgan Stanley Real Estate Investing: Morgan Stanley has implemented a sustainability strategy that includes investing in green buildings and sustainable development projects.

7. Starwood Capital Group: They have a strong focus on sustainability and aim to bring sustainable solutions to their real estate investments through energy-efficient designs and environmentally conscious operations.

8. Brookfield Asset Management: Brookfield has been actively investing in renewable energy, green infrastructure, and sustainable developments, particularly through their renewable power subsidiary.

9. Macquarie Infrastructure and Real Assets: With a focus on renewable energy and sustainable assets, they invest in projects that promote environmental sustainability.

10. PGIM Real Estate: Their approach to sustainability includes investing in energy-efficient properties, resource conservation, and sustainable development practices.

Please note that rankings may vary, and this list represents a sample of prominent real estate investors specializing in sustainable properties.