Mesoporous crystalline acid composition of a diphosphonate-phosphite of a tetravalent metal which can be used as a catalyst

A solid acid catalyst based on a diphosphonate-phosphite of a tetravalent metal which is obtained by a process, which comprises contacting a diphosphonate-phosphite of a tetravalent metal having the formula:

• M((O.sub.3 P--R--PO.sub.3).sub.1-x-y (HPO.sub.3).sub.2x (O.sub.3 P--R--PO.sub.3 H.sub.2).sub.2y)
• wherein:
• M is a tetravalent metal,
• R is a bivalent organic radical,
• x varies from 0.3 to 0.6 and
• y varies from 0.05 to 0.3.
• having a lamellar structure of the .alpha.-type with an interstratum distance of between 7.4 and 20 .ANG., a B.E.T. surface area of between 250 and 400 m.sup.2 /g, and a porosity in the range of mesopores, with at least 50% of the pores having a diameter greater than 30 .ANG. and less than or equal to 50 .ANG.,
• with a solution, in an aqueous or organic solvent or both, of
• i) a bifunctional phosphonic-sulfonic acid having the formula H.sub.2 O.sub.3 P--R'--SO.sub.2 X, wherein R' is a bivalent organic radical, and X is --OH or --Cl;
• or
• ii) an aryl phosphonic acid containing one or more aromatic rings;
• for a time sufficient to at least partially exchange diphosphonate and phosphite surface groups of the diphosphonate-phosphite of the tetravalent metal with phosphonic-sulfonic groups or, respectively, with aryl phosphonic groups;
• and, only in the case of the product exchanged with arylphosphonic acid, further subjecting the solid to sulfonation with a sulfonating agent.