What are the requirements to charge a Macbook Air?
Apple publishes technical specifications for its products and are available at its website.

Have a look at: MacBook Air (13-inch, 2017) - Technical Specifications

Under Battery and Power2 it shows:


45W MagSafe 2 Power Adapter with cable management; MagSafe 2 power port


As your power bank only has an output of 5V/2.4A, which would be 12 watts, it's little more the 1/4 the output of the 45W MagSafe 2 Power Adapter that comes with the MacBook Air (13-inch, 2017). It will charge it, but it will be very slow compared to using the 45W MagSafe 2 Power Adapter that comes with the MacBook Air (13-inch, 2017).

Personally, I'd use the 45W MagSafe 2 Power Adapter that comes with the MacBook Air (13-inch, 2017), not your power bank.