Multiple entry into Beijing

Hi I hold a Singapore passport. I am flying Singapore to Beijing and two days later Beijing to London on a separate ticket. Two weeks later I am flying back London to Beijing and three days later Beijing to Singapore. ( two different return flights on two separate airlines).

As I can enter China visa free for 15 days, do I need to apply for the 144 hr transit visa?

Secondly, if I enter China visa free on my Singapore passport can I enter again two weeks later visa free?

Thank you in advance!