Sabato Miseo

Selective isomerization of alkanes on supported tungsten oxide acids

Abstract Tungsten oxide species form strong acid sites on ZrO 2 supports. After calcination at 1000–1100 K and promotion with Pt, these solids catalyze C 7+ alkane isomerization at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. Alkane isomerization proceeds via biomolecular reactions involving hydrogen transfer from alkanes or H 2 , which cause the desorption of isomeric carbocations before β-scission occurs. On Pt/SO x -ZrO 2 , carbocation desorption is slow, leading to long surface residence times and extensive cracking. On Pt/WO x -ZrO 2 , carbocation desorption is rapid and surface isomerization steps limit n-heptane isomerization turnover rates. Saturation coverage by WO x surface species inhibits ZrO 2 sintering and its tetragonal to monoclinic structural transformation. High isomerization turnover rates appear to require the presence of WO x clusters on ZrO 2 surfaces. X-ray absorption at the W-L 1 and W-L III edges suggests the predominant presence of distorted octahedral species, even after dehydration at 673 K, in all WO x -ZrO 2 samples calcined at 1073 K. Tetrahedral species, which lead to a strong pre-edge feature in the W-L 1 absorption edge, are not detectable in these samples. UV-visible spectra suggest an increase in WO x domain size with increasing loading. These distorted octahedral WO x domains on ZrO 2 differ markedly in structure, reduction rates, and alkane isomerization turnover rates and selectivities from tetrahedral WO x species on Al 2 O 3 .

Soft Chemical Synthesis of Mixed Metal Molybdate Oxidation Catalysts and Their Structural Relationship to Hydrotalcite

Low temperature soft chemical syntheses, which allow preparation of metastable materials, have been commonly used in preparing zeolites, but have not been studied extensively for other mixed oxide catalyst systems. We have found a low temperature solution synthesis route to convert some hydrotalcite-related phases into ammonium mixed-metal molybdates that crystallize with a hydrotalcite-related superstructure and which are useful precursors for high surface area mixed metal oxides.