I cannot be a materialist because there is nothing I know with more certainty than my free will. Really, nothing could be more obvious than I have freedom and I make choices about how to exercise it. In fact, I just freely chose to delete this sentence and rewrite it in a very different way than how I initially (and freely) put it down. It seems equally obvious to me that you have free will as well.
The axiom that human beings are free is completely incompatible with the idea that human beings are no more than material, merely automatons, just objects (albeit highly complex objects) enslaved to the law of physics and enmeshed in its web of causes and effects.
And, if my freedom is completely illusory---what does it matter? I find myself making illusory choices about how to exercise my illusory freedom, and this "insight" saves nothing save the attachment of the unnecessary and uninformative adjective "illusory".