The present invention is a gas separator for separating a gas mixture into a product gas. The gas separator has an adsorbent bed including a separation chamber with first and second ports and a molecular sieve material contained in the separation chamber. A first pumping chamber is connected to the first port. A first valve regulates flow of the gas mixture between the first port and first pumping chamber. A first piston is located in the first pumping chamber. A second pumping chamber is connected to the second port. A second valve regulates flow of the product gas between the second port and second pumping chamber. A second piston is located in the second pumping chamber. A drive system coordinates operation of the first and second pistons and first and second valves in a cycle including pressurization gas shift and depressurization stages.