These are two different things. 802.11x is related to network security and 802.11n refers to the communications between wireless devices. In other words, you can implement 802.11x on an 802.11n network, but you do not have to use 802.11x to use an 802.11n network.
802.11N IEEE 802.11n is an amendment to IEEE 802.11-2007 as amended by IEEE 802.11k-2008, IEEE 802.11r-2008, IEEE 802.11y-2008, and IEEE 802.11w-2009, and builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO) and 40 MHz channels to the PHY (physical layer), and frame aggregation to the MAC layer.
802.11x refers to a group of evolving wireless local area network (WLAN) standards that are under development as elements of the IEEE 802.11 family of specifications, but that have not yet been formally approved or deployed.