Copy paste link below for details:
MQ2 - http://www.seeedstudio.com/wiki/Grove_-_Gas_Sensor(MQ2)
MQ3 - http://www.seeedstudio.com/wiki/Grove_-_Gas_Sensor(MQ3)
MQ4 - https://www.pololu.com/file/0J311/MQ4.pdf
MQ5 - http://www.seeedstudio.com/wiki/Grove_-_Gas_Sensor(MQ5)
MQ6 - https://www.sparkfun.com/datasheets/Sensors/Biometric/MQ-6.pdf
MQ7 - https://www.sparkfun.com/datasheets/Sensors/Biometric/MQ-7.pdf
MQ8 - http://www.elecdesignworks.com/index.php/shop-layout/hydrogen-gas-sensor-detail?tmpl=component&format=pdf&iframe=true&width=800&height=550
MQ9 - http://www.seeedstudio.com/wiki/Grove_-_Gas_Sensor(MQ9)
MQ135 - http://lib.chipdip.ru/184/DOC001184451.pdf