Friedrichs' early contributions are discussed in On the Stone-von Neumann Uniqueness Theorem and Its Ramifications by S.J. Summers:
In the early 1950's, K.O. Friedrichs undertook an influential attempt to reduce the hand-waving typical of quantum field theory up to that time [1,2]. The result of greatest interest was his construction of some representations of the canonical commutation relations for infinitely many degrees of freedom which were not unitarily equivalent to the Fock representation.
[1] K.O. Friedrichs, Mathematical Aspects of the Quantum Theory of Fields, 1953.
[2] Commun. Pure Appl. Math., 4, 161–224 (1951);
5, 1–56 (1952); 5, 349–494 (1952); 6, 1–72 (1953).