Round trip from Amsterdam to Hong Kong vía Beijing

You itinerary is fine and you will be covered by 24 hours visa free transit in both ways of your journey. Different airlines and also separate tickets are fine too. In order to exit the airport, you will apply, listen now, for 24 hours free entry permit and not for any kind of visa! To apply for the mentioned permit, beside your flight tickets, you will have to book an overnight and have a confirmation of your booking in hard copy. You will be almost certainly allowed to exit the airport, but don't forget your hotel booking because this is very important detail.

Thanks a lot for your answer. Just a final question, do I need to book a hotel even though my layovers are during the day? In both ways is only in the day.

Actually, you really do need any hotel booking, but this bookings usually empower the application for entry permit. I'm almost convinced that the Chinese immigration will let you exit the airport because of the length of your layover even without the hotel booking. The reason why we mention hotel bookings is primarily because some people were denied entry permit (those with shorter layovers), so they had to spend all their layover in the transit zone of the airport. You will decide for yourself.