Selective isomerization of alkanes on supported tungsten oxide acids

Conference · Tue Dec 31 04:00:00 EST 1996

OSTI ID:467994

Iglesia, E; Barton, D G ^[1]; Miseo, S ^[2]

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1. Univ. of California, Berkeley, CA (United States)
2. Exxon Research and Engineering Co., Annandale, NJ (United States); and others

Acid catalysis is a critical requirement in octane enhancement processes such as isomerization and alkylation. Currently, these processes rely on liquid acids and on metal oxides which pose corrosion problems and environmental challenges. Tungsten oxide species form strong acid sites on ZrO{sub 2} supports. After calcination at 1000-1100 K and promotion with Pt, these solids catalyze C{sub 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 bimolecular reactions involving hydrogen transfer from alkanes or H{sub 2}, which cause the desorption of isomeric carbocations before P-scission occurs. On Pt/SO{sub x}-ZrO{sub 2}, carbocation desorption is slow, leading to long surface residence times and extensive cracking. On Pt/WO{sub x}-ZrO{sub 2}, carbocation desorption is rapid and surface isomerization steps limit n-heptane isomerization turnover rates. Saturation coverage by WO{sub x} surface species inhibits ZrO{sub 2} sintering and its tetragonal to monoclinic structural transformation. High isomerization turnover rates appear to require the presence of WO{sub x} clusters on ZrO{sub 2} surfaces. X-ray absorption at the W-L{sub I} and W-L{sub III} edges suggests the predominant presence of distorted octahedral species, even after dehydration at 673 K, in all WO{sub x}-ZrO{sub 2} samples calcined at 1073 K. Tetrahedral species, which lead to a strong pre-edge feature in the W-L{sub I} absorption edge, are not detectable in these samples. UV-visible spectra suggest an increase in WO{sub x} domain size with increasing loading. These distorted octahedral WO{sub x} domains on ZrO{sub 2} differ markedly in structure, reduction rates, and alkane isomerization turnover rates and selectivities from tetrahedral WO{sub x} species on Al{sub 2}O{sub 3}.

• OSTI ID: 467994
• Report Number(s): CONF-960652--; CNN: Grant CTS-9510575
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
ALKANES
ANTIKNOCK RATINGS
CATALYTIC EFFECTS
CRACKING
DESORPTION
GASOLINE
HEPTANE
HYDROGEN TRANSFER
ISOMERIZATION
PLATINUM
TUNGSTEN OXIDES
VISIBLE SPECTRA
ZIRCONIUM OXIDES