M.J. Wulfers, F.C. Jentoft / Journal of Catalysis 307 (2013) 204–213
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acidic OH groups (Fig. 8) indicates that proton transfer to a hydro-
carbon species takes place at some point of olefin conversion. The
obtained surface cations can be neutralized, provided a sufficiently
strong base is used. For example, the proton affinities of water,
ammonia, and pyridine are 696, 853, and 930 kJ molÀ1, respec-
tively [58]. These values apply for the free molecules and ions in
the gas phase, and it is conceivable that the influence of the zeolitic
surroundings makes possible the neutralization of alkyl-substi-
tuted cyclopentenyl species by ammonia. In light of the combined
experimental evidence and information from the literature, the
bands at 292–296 nm and 1505 cmÀ1 are assigned to alkyl-substi-
tuted cyclopentenyl species.
inaccuracy of the temperature measurement in the in situ cell
and the low time resolution (about 10 min per spectrum).
There are four pieces of information regarding the identity of
the deposits formed above ꢀ550 K. GC–MS and UV–vis analysis
of the dichloromethane extract after digestion of the zeolite in
hydrofluoric acid (Figs. S6 and 10) and the UV–vis spectrum of
the catalyst after exposure to water vapor (Figs. 9 and 10) provide
information on the neutral species. The in situ UV–vis and infrared
spectra provide information on the species as they exist on the zeo-
lite, probably as cations (Figs. 3 and 9, and [15]). The discussion of
results will proceed in this order.
The GC–MS chromatogram of the extracted deposits revealed
mostly naphthalenes and anthracenes, and a few non-chromo-
phoric impurities leached from the polyethylene vials. The transi-
tion away from ‘‘low temperature coke’’ is said to occur around
473 K, with polyaromatic coke dominating at 623 K [6]. On H-
mordenite, phenanthrene was identified at 613 K [5]. Polynuclear
aromatics formed on H-mordenite from ethylene at and above
550 K [65]. Stepanov et al. [53] found polycyclic aromatics to form
on H-ferrierite already at 523 K. Thus, the formation of polycyclic
aromatics at ꢀ550 K is in agreement with previous reports.
The UV–vis spectra of the catalyst taken in situ after exposure to
water vapor or ex situ (after exposure to atmospheric water vapor)
are representative of the deposits in their neutral state. The neu-
tralizing effect of water vapor has been reported previously by Lef-
tin and Hobson [33] who used it to neutralize the trityl cation on
silica–alumina. Subsequent analysis showed that water did not
cause hydroxylation of the cationic species with formation of tri-
phenylmethanol, but instead triphenylmethane was recovered. In
this work, no hydroxylated aromatics were found in the dichloro-
methane extract, confirming that neutralization occurs by proton
abstraction and not by hydroxylation. According to the GC–MS
analysis, the most prevalent naphthalene was 2-methylnaphtha-
lene, which has a band with partial Lorentzian shape at 224 nm
(‘‘E’’ band) and a broader benzenoid band (‘‘B’’ band) characterized
by a fine structure with maxima at 266, 275, and 286 nm [66]. The
most prevalent anthracenes had two methyl groups, but the loca-
tion of these groups could not be determined exactly. A represen-
tative example would be 1,3-dimethylanthracene which has an E
band at 257 nm and a B band characterized by fine structure bands
at 343, 360, and 380 nm [67]. Thus, the band at 228 nm in Fig. 10 is
assigned to methyl-substituted naphthalenes and the band at
257 nm is assigned to methyl-substituted anthracenes. In the
UV–vis spectrum of the extract, the band at 228 nm is partially
cut off through the absorption of the dichloromethane solvent in
the UV range. The bands at 344, 359, and 381 nm are part of the
fine structure of the anthracene B band. The bands at 447 and
473 nm in the spectrum of the solid zeolite are consistent with a
methyl-substituted tetracene; tetracene has an intense E band at
278 nm and the most intense components of the B band are at
443 and 473 nm [68]. Tetracenes could not be detected in the ex-
tract, but they are known to have limited solubility in dichloro-
methane [69]. It is notable that the aromatic deposits are all
members of the acene family, which suggests that the one-dimen-
sional pore structure of H-mordenite plays a prominent role in
determining the type of species that can form.
Additional confirmation of this assignment was sought through
digestion and extraction of H-mordenite that had reacted with 1-
butene or 1-pentene at 323 K. The extraction procedure was vali-
dated by the fact that the GC–MS data revealed a diverse mixture
of paraffins and olefins of varying chain length. This result is con-
sistent with the work of Guisnet and co-workers [4], who adsorbed
propene on USHY at 393 K and found the extract to contain poly-
mers with the number of carbon atoms not equal to multiples of
three, olefins, and naphthenes. However, the UV–vis spectrum of
the dichloromethane extract did not show a band at 295 nm
(Fig. S5), suggesting that the species responsible for this absorption
was either transformed or was not recovered. As the alkyl-substi-
tuted cyclopentadienes have high proton affinities, it is possible
that the species remained protonated and stayed in the hydroflu-
oric acid during the liquid–liquid extraction with dichloromethane.
Sorensen [45] used a combination of neutralization with potassium
hydroxide and extraction with hexane to recover protonated poly-
enes from sulfuric acid with less than 50% yields. It is also worth
noting that digestion and extraction of used methanol-to-hydro-
carbons catalysts has also not resulted in recovery of neutral forms
of alkyl-substituted cyclopentenyl species [59–61], even though
the ionic forms are known, or at least suspected, to be important
intermediates for conversion of methanol-to-hydrocarbons on zeo-
lite and zeotype catalysts [62–64]. While the extraction could not
confirm the cycopentadiene-derived monoenylic species, the find-
ings are explainable and do not contradict the interpretation.
For a monoenylic species to be formed from an olefin, abstrac-
tion of a hydride must occur. Thus, a Lewis acid is necessary. The
Lewis acidity can be provided by either coordinatively unsaturated
extra framework aluminum in the zeolite or from carbenium ions.
Another pathway leading to the formation of monoenylic species is
protonation of a diene; however, hydride abstraction is also a nec-
essary step in the acid-catalyzed formation of a diene from a
monoene. The band at 320 nm formed immediately after admis-
sion of 1-pentene (Fig. 6) could be representative of an initial acy-
clic monoenylic species formed either directly from 1-pentene or
after oligomerization, deprotonation, and hydride abstraction. In
order for a cyclic species to form, a 1,5 cyclization reaction must
occur, requiring formation of an intermediate dienylic cation.
Dienylic species absorb at longer wavelengths than monoenylic
species; for example, Sorensen [45] reported that the dienylic spe-
cies formed by protonation of 2,6-dimethyl-1,3,5-heptatriene ab-
sorbs at 396 nm. On zeolites, dienylic species have been reported
to absorb between 350–380 nm [20]. In fact, a small band at
385 nm grew during heating and then shrunk (Fig. 6, inset). Thus,
the band at 385 nm is assigned to an acyclic dienylic cation.
During conversion of alkanes, or after heating the initial surface
products of olefin adsorption in the presence of inert gas, the poly-
cyclic aromatic species exist in the zeolite as cations. The bands in
the UV–vis spectra that were recorded in situ can be grouped into
three main ranges of absorption, <250 nm, 250–310 nm, and 370–
500 nm. The bands at the longest wavelengths are central to the
interpretation, because these bands can only be explained by neu-
tral chromophores of a size and type that are unlikely to form un-
der these conditions and for which no other evidence exists, or by
the cationic forms of the aromatic species detected in the extract.
4.4. Identification of species formed at temperatures above ꢀ550 K
Bands formed at 350–500 nm after heating a zeolite containing
species absorbing at 292–296 nm to temperatures greater than
ꢀ550 K [15]. It is important to note that 550 K is an estimate of
the actual temperature at which the formation of bands at wave-
lengths longer than 350 nm begins to take place, because of the
Wulfers, Matthew J.
