A cartridge microreactor includes a tubular heater, a metal capillary reaction tubing and a brazing alloy. The tubular heater includes an inner tube surrounded by an outer heater sheath. The metal capillary reaction tubing has a high aspect ratio, an inlet port and an outlet port and is wrapped around the tubular heater so that it is in intimate thermal contact with the heater sheath. The brazing alloy permanently bonds the tubular heater with the metal capillary reaction tubing. The metal capillary reaction tubing is configured to be heated by the tubular heater and is configured to receive one or more reactants through the inlet port and to allow the reactants to react under a selected temperature and pressure and to produce one or more products exiting through the outlet port.