A. Miyagawa, et al.
MolecularCatalysis471(2019)60–70
Fig. 2. Effect of reaction temperature on the reaction of 2-methoxycyclohexanone over Pt/C.
Reaction conditions: 2-methoxycyclohexanone 5 mmol, water 20 mL, Pt/C 0.1 g, PAr 1 MPa. Detailed data are shown in Table S1.
demethoxylation products, and we selected Pt/C as catalyst in the fol-
lowing study.
was used, further deoxygenation of target demethoxylation products to
cyclic hydrocarbons (i.e. over-reaction) significantly proceeded and
decreased the yield of target products. In short, the reaction stopped
before total conversion of intermediates with too small amount of
catalyst, and too much catalyst promotes over-reaction of target pro-
ducts. Deactivation of the catalyst seems to be a key factor for this
phenomenon: smaller the amount of catalyst is, faster deactivation
occurs and reaction stops. Deactivation was investigated in the next
section. Anyway, 0.1 g of Pt/C was found to be the best catalyst amount
for this reaction. To summarize this section, the best catalyst is Pt/C and
the best conditions are 493 K, water solvent and 0.1 g of catalyst (with
5 mmol 2-methoxycyclohexanone substrate.)
The effect of reaction temperature was investigated in the range of
463–513 K using Pt/C catalyst (Fig. 2). The detailed data are shown in
Table S1. At low temperature such as 463 K, the main product is 2-
methoxycyclohexanol, where H2 can be supplied by the formation of
guaiacol or phenol. At 483 or 493 K, the yield of target products become
maximum (ca. 50%) at 24 h and the yield of 2-methoxycyclohexanol is
rather small. At higher temperatures (503 and 513 K), the formation of
total deoxygenation products was significant and the yield of the target
demethoxylation products was lower than that at 493 K. The yield of
“others” including undetected products by GC increased with higher
reaction temperature and longer reaction time. Produced catechol and
phenol, highly dehydrogenated compounds, may polymerize at high
temperature [8,29–31]. As a result, we selected 493 K as the reaction
temperature.
3.2. Performance of Pt/C
Fig. 4 shows the time course of reaction of 2-methoxycyclohexanone
over 0.1 g of Pt/C at 493 K in water solvent. The detailed data are
shown in Table S4. Conversion at 0 h means the reaction during the
heating from room temperature to the target one which took about
30 min. The conversion increased rapidly to 99% and selectivity of
target demethoxylation products also increased rapidly during 3 h.
From 3 h to 24 h, the selectivity increased gradually, and highest yield
of 48% (phenol 13%, cyclohexanone 19%, cyclohexanol 16%) was
obtained at 24 h. Deoxygenation of target products and methanol to
hydrocarbons proceeded slowly after 24 h.
The effect of solvents was tested by using water, ethanol, n-dode-
cane and toluene as well as none (solventless condition) (Table S2).
Water solvent is by far better solvent to obtain target demethoxylation
products. In the case of ethanol solvent, large amount of C1 products
and H2 were produced from ethanol and the main identified product
from substrate was 2-methoxycyclohexanol, probably because of the
large supply of H2 from the solvent. In the cases of n-dodecane solvent,
toluene solvent and solventless condition, the main products were 2-
methoxycyclohexanol and guaiacol, and the produced H2 was smaller
than the case of water solvent. As mentioned before, H2 was supplied to
the system via dehydrogenation (Eqs. (7) and (8)) and APR of methanol
Because H2 was needed for target demethoxylation, the smaller amount
of H2 in the non-water solvent systems except ethanol can limit the
formation of the target demethoxylation products. Carbon monoxide
which is detected in non-water solvent, especially ethanol, is another
factor for suppressing the formation of the target products because
carbon monoxide easily covers to the Pt metal surface. In water solvent,
however, active Pt site can be regenerated by the conversion of ad-
sorbed carbon monoxide and water to carbon dioxide and H2, i.e. water
gas shift reaction.
Next, Pt/C catalyst was applied to related substrates (2-methox-
ycyclohexanol and guaiacol) and the results are shown in Table 3 and
Table S5. In the case of guaiacol, small amount of H2 was added to the
gas phase because it is difficult to supply H2, which is needed for hy-
drogenolysis, by dehydrogenation of guaiacol. The target demethox-
ylation products were obtained from both substrates in similar total
yields: phenol 1%, cyclohexanone 15% and cyclohexanol 33% from 2-
methoxycyclohexanol (Entry 4); phenol 41%, cyclohexanone 4% and
cyclohexanol 1% from guaiacol (Entry 6). Pt/C is effective to de-
methoxylation of both guaiacol and hydrogenated guaiacol. The highest
yield of target demethoxylation products from 2-methoxycyclohexanol
was obtained at shorter time than the case of 2-methoxycyclohexanone
(Entries 2 and 4). This trend suggests that 2-methoxycyclohexanol is an
intermediate of demethoxylation. The reaction route will be discussed
in detail in later section.
Fig. 3 shows the reaction results using different amount of Pt/C
(0.02-0.20 g). The detailed data are shown in Table S3. When we used
smaller amount of Pt/C, 2-methoxycyclohexanone reacted very slowly,
and the yield of the target demethoxylation products did not increase
even in 48 h reaction. On the other hand, when a large amount of Pt/C
The reuse experiments were conducted to check the stability of Pt/C
(Fig. 5 and Table S6). The used Pt/C was collected, washed with water
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