In The Count of Monte Cristo, how does M. Noirtier intend to break the engagement of granddaughter Valentine?

Villeforte wants Valentine (his daughter from his first marriage) to marry the Baron Franz d'Epinay. This union is not motivated by love, but by Villefort's personal political gains and Valentine's "security" that would result from the marriage.

It is important to note that Valentine and her grandfather Noirtier (Villefort's father) are extremely close. Valentine is the only one who can communicate effectively with the elderly man (he is paralyzed from a stroke and cannot speak),...

Villeforte wants Valentine (his daughter from his first marriage) to marry the Baron Franz d'Epinay. This union is not motivated by love, but by Villefort's personal political gains and Valentine's "security" that would result from the marriage.

It is important to note that Valentine and her grandfather Noirtier (Villefort's father) are extremely close. Valentine is the only one who can communicate effectively with the elderly man (he is paralyzed from a stroke and cannot speak), and he is able to read his granddaughter's mood and emotions well. When Noirtier learns about this potential union, he immediately wants to put an end to the idea for two reasons: Valentine does not love Franz; she loves Max, and Noirtier played a role in Franz's father's death.

Initially, Noirtier threatens to change his will and remove Valentine as his sole heir. Valentine is amenable to this arrangement, because it means she will not have to marry Franz. Noirtier does this to threaten Villefort. He knows that Valentine's future and financial security is important to Villefort. 

If this plan fails (and it does), Noirtier will reveal his role in the Baron's father's death. This has a double edged-sword. It will effectively break the engagement of Valentine and Franz, and it will reveal to the public Noirtier's political affiliations with Napoleon.  Noirtier prepares a document to give to Franz detailing how Noirtier killed his father. Noirtier is hoping this will enrage Franz and cause the engagement to be dissolved. It does. This will have a devastating on Villefort's political and social standing.