The structure of the species arising from phosphoric acid impregnation on silica, alumina, and titania (mainly anatase) has been investigated by TG-DTA analyses, X-ray diffraction, and FT-IR spectroscopy also using CO as a probe molecule. It has been concluded that the interaction between supported phosphoric acid and silica is weak, producing mainly liquidlike species, together with covalently bonded phosphate and hydrogen phosphate species. The interaction with the ionic supports alumina and titania looks much stronger, producing well-spread ionically bonded hydrogen phosphate and phosphate species that poison the support's basic sites. The Lewis sites of the support seem instead to be only slightly affected by the supported species.

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The surface acidity of catalysts prepared by impregnation of silica, alumina, and titania with phosphoric acid (called PS, PA, and PT, respectively) has been studied by FT-IR spectroscopy of adsorbed n-butylamine, ammonia, pyridine, tetrahydrofuran, and acetonitrile and by adsorption microcalorimetry using n-butylamine as the probe. The adsorption of olefins such as 1-butene and isobutene has also been studied by FT-IR. Moreover, the activity in the catalytic conversion of 1-butene has been investigated. Both Lewis and Bransted acid sites are observed on PT and PA, while only Bransted sites are detected on PS. The Bransted acid strength of PS is the strongest, as deduced by FT-IR, while the strongest total acidity is detected on PT by microcalorimetry. The strongest activity of PT in polymerizing olefins in the IR cell has been related to the tendency to form carbonaceous deposits mainly on this catalyst, as well as with its higher catalytic activity in skeletal isomerization, cracking, and hydrogenation of 1-butene.

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