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Y. Nie et al. / Journal of Catalysis 246 (2007) 223–231
Scheme 1. Designed cascade reaction for the conversion of 4-alkylphenols to cis-4-alkylcyclohexanols.
HCl, H2SO4, HClO4, or HBr as co-catalyst [12–15]. The
highest stereoselectivity (100%) was claimed by Yadav and
Goel [16] using Rh/C in the presence of methanesulfonic acid.
They postulated that the formation of the cis- or trans-alcohol
was site-specific and that the presence of the acid blocked
the sites leading to the trans-alcohol. Compared with liquid
phase, the vapor-phase hydrogenation of 4-tert-butylphenol
over Pt/SiO2 formed mainly 4-tert-butylcyclohexanol and 4-
tert-butylcyclohexanone with deoxygenated byproducts such as
tert-butylbenzene and tert-butylcyclohexane [17]. More trans-
than cis-4-tert-butylcyclohexanol was formed under steady-
state conditions with a cis:trans ratio of 0.88:1.
the alcohol via the MPV reaction, and (iii) minimize the com-
peting reaction of hydrogenating the ketone to alcohol. The
direct formation of cis-4-alkylcyclohexanol (alkyl = tert-butyl,
methyl) from corresponding alkylphenols was investigated over
bifunctional Rh/Zr-beta catalysts. For comparison, zirconia,
Zr-silica, γ -alumina, Al-zeolite beta, and Zr-impregnated Al-
zeolite beta were used as supports. Along with Rh, Pd also
was investigated for its hydrogenation activity in the reaction,
because Pd is known to be a good catalyst for the phenol-to-
cyclohexanone conversion.
2
. Experimental
A number of studies also have been carried out on the hy-
drogenation of cresols. Palladium catalysts are highly selec-
tive for the partial hydrogenation to the corresponding cyclo-
hexanones [18,19]. A silica-supported chitosan-palladium com-
2
.1. Catalyst preparation
◦
The synthesis of aluminium-free Zr-zeolite beta (Si/Zr 100)
plex catalyzed the hydrogenation of phenol and cresols at 70 C
has been described previously [23]. The Zr-beta-supported Rh
catalysts (Rh/Zr-beta) were prepared by incipient-wetness im-
pregnation at room temperature with an aqueous solution of
RhCl3·3H2O (Pressure Chemicals). The impregnated samples
and 1 bar, forming high yields of the corresponding cyclo-
hexanones [20]. A silica-supported cross-linked poly(maleic
acid-co-styrene) Pt complex catalyzed the hydrogenation of
◦
para-cresol in water under mild conditions (30–50 C, 1 bar)
◦
were dried overnight at 100 C. After calcination in air at
to give 4-methylcyclohexanol via 4-methylcyclohexanone as an
intermediate [21]. Xia et al. [22] reported that over a zeolite-
supported complex catalyst composed of Pt, polyvinyl alco-
hol, and amino acids, the initial rate of hydrogenation de-
creased from m-cresol to o-cresol to p-cresol, with trans-
methylcyclohexanol preferentially formed.
The mechanism of formation of the cis- and trans-isomers
has not been elucidated. Many studies have proposed a reaction
scheme in which the reaction proceeds via an intermediate enol
◦
5
50 C for 4 h, they were reduced in a 50/50 H2/He mixture
◦
for 4 h at 300 C. Samples with Rh loadings of 0.25, 0.5, 1,
and 2 wt% were prepared. A commercial 5% Rh/C (Degussa)
was similarly reduced and its hydrogenation activity evaluated.
A 2 wt% Pd sample was prepared by impregnating Zr-beta with
an aqueous solution of palladium(II) chloride dissolved in am-
◦
monia. The sample was calcined in air at 550 C for 4 h before
◦
being reduced in H2 for 2 h at 150 C.
(
4-tert-butyl-tetrahydrophenol), which can undergo tautomer-
Hydrous zirconia was prepared by the hydrolysis of zirco-
nium chloride in excess 5 M NaOH [24]. A 10 wt% ZrCl4 so-
lution was added via a peristaltic pump to the 5 M NaOH solu-
ization to 4-tert-butylcyclohexanone. Continued hydrogenation
during residence on the surface could lead to the cis-isomer.
When the ketone desorbed and subsequently readsorbed, per-
haps more trans-isomer would be formed due to preferred ad-
sorption with the tert-butyl group directed away from the sur-
face. Another possible tetrahydro-intermediate, 4-tert-butyl-3-
cyclohexenol, when desorbed and readsorbed, also can give rise
to both cis- and trans-isomers in difficult-to-predict ratios. We
previously found that Zr-zeolite beta is a highly stereoselective
catalyst for the Meerwein–Ponndorf–Verley reduction of 4-tert-
butylcyclohexanone to cis-4-tert-butylcyclohexanol, with a se-
lectivity of 99% [23]. Hence, in the present study we combined
the hydrogenation activity of Rh with the high stereoselectiv-
ity of Zr-zeolita beta for cis-4-tert-alkylcyclohexanol to form
a bifunctional catalyst (Scheme 1). This bifunctional catalyst
should have the ability to (i) promote the hydrogenation of the
phenol to the intermediate ketone, (ii) transform the ketone to
◦
tion. After precipitation, the gel solution was digested at 100 C
for 4 days. The filtrate was washed with dilute ammonium ni-
◦
trate until free of chloride, dried at 100 C overnight, and then
◦
subjected to calcination at 500 C for 12 h. Zr-silica (Si/Zr
100) was prepared by mixing tetraethoxysilane and ZrCl4 in
tetrapropylammonium hydroxide, followed by hydrothermal
◦
treatment at 160 C for 2 days. The sample was filtered, washed,
◦
and calcined at 550 C for 12 h. A sample of Al-zeolite beta
(Zeolyst, Si/Al 12.5) was added to a solution of ZrOCl2 to give
a Si/Zr ratio of 100 and stirred for 4 h at room temperature,
followed by evaporation of water. This sample, designated Zr-
◦
◦
Al-beta, was dried at 100 C and calcined at 550 C for 4 h.
The thus-obtained zirconia, Al-zeolite beta, and Zr-Al-zeolite
beta were impregnated with 0.5 wt% Rh. A 3% Rh/γ -alumina
sample also was prepared.