collisions MADE by the gas molecules with the walls of the container are  Inelastic collisions, then:  the kinetic energy, after BOUNCING from the walls, of the molecules will be less than  before colliding with the walls.   So there is less change of MOMENTUM of the molecules during the collision.  There is an energy loss and perhaps it is converted into some other form of energy.  Since change of momentum per unit  time is less, the force exerted on the walls is less.  So pressure on the walls is reduced.  Same is the case if there is a cohesive force between the walls and the molecules of gas.   However, if the walls are INSULATED, then the there is no loss of internal energy.  The temperature has to remain the same.  Then the AVERAGE kinetic energy of the gas molecules has to remain constant.   if the walls are not insulated, then there may be some loss of energy in the form of heat or sound or light or other radiation.  In that case the temperature of the gas may change until an equilibrium is established between the walls, surroundings, and the gas.  The ideal gas law or the Vander Waal's equation needs to be updated for the factor of adhesion force and inelastic collisions between the gas molecules and the walls of the container.