We developed nanoporous adsorbent exhibiting unprecedented performance in separation of toxic carbon monoxide (CO). The adsorbent was prepared by dispersing CuCl on mesoporous boehmite via thermal monolayer dispersion route. A key point of the present synthesis is dispersing optimized amount of CuCl on the boehmite at a moderate temperature to maintain the characteristics of the boehmite. We performed a systematic study to reveal that a CuCl/boehmite composite (30 wt% CuCl in total) thermally treated at 573 K was the best optimized sample for CO separation. The CuCl/boehmite had a high capacity of CO adsorption (1.56 mmol g(-1)) and an exceedingly low capacity of CO2 adsorption (0.13 mmol g(-1)) under 100 kPa of each gas at 293 K. The CO/CO2 separation factor was 12.4. To the best of our knowledge, this value is the best on record. The achievement of this work is attributed to finding a new type of suitable supporting material: boehmite. The boehmite has a high affinity to CuCl, exhibits excellent dispersion of the CuCl, and achieves a superior CO adsorption capacity. However, it has a weak interaction with CO2. The CuCl/boehmite composite is a promising adsorbent for selective separation of CO from combustion exhaust and industrial off-gas streams. (C) 2017 Elsevier B.V. All rights reserved.