3.2 Hydrogenation of cyclohexyl acetate to cyclohexanol
Since the hydrogenation of cyclohexyl acetate to cyclohexanol has also
not been explored to the best of our knowledge, we developed the
Cu/ZnO/SiO2 catalysts for this purpose. The number of
catalyst systems allowing the production of alcohols from ester
hydrogenation is limited. The catalysts of choice, which allow the
highly selective conversion of esters to alcohols, are mostly based on
Cu.27–29 Besides, the Cu-based catalysts are more
cost-effective than the noble metal-based catalysts for this
purpose,30 which is an important factor to be
considered for a reaction with significant industrial background. ZnO is
capable of dispersing and stabilizing Cu nanoparticles to replace the
conventional copper chromite catalysts that are hazardous to the
environment.27,31 SiO2 was adopted as
the support, since it is not only able to disperse
Cu,27 but also weakly acidic in nature and would not
lead to considerable undesired intramolecular/intermolecular dehydration
or isomerization of the hydrogenation products.
Firstly, we systematically examined the effects of the contents of Cu
and the Cu/Zn molar ratios on the catalytic performances of the
Cu/ZnO/SiO2 catalysts in the hydrogenation of cyclohexyl
acetate. As summarized in Table S3, over the
Cu0Zn1Si2 catalyst
without Cu, the hydrogenation reaction did not occur. When increasing
the contents of Cu while fixing the Zn: Si molar ratio at 1: 2, the
conversion of cyclohexyl acetate increased drastically, reaching a
maximum over the
Cu1Zn1Si2 catalyst,
manifesting that Cu is the active center for ester hydrogenation. When
further increasing the contents of Cu, the conversion of cyclohexyl
acetate declined monotonically, which is attributed to the agglomeration
of excessive Cu. A similar effect of the content of Cu on the
hydrogenation activity was observed on the
Cu/ZnO/Al2O3 catalysts for the
hydrogenation of ethyl acetate.32 When altering the
Cu/Zn molar ratio while fixing the (Cu + Zn): Si molar ratio at 2: 2,
the conversion of cyclohexyl acetate evolved in a volcanic shape. At low
Cu/Zn ratio, there was no sufficient active centers for ester
hydrogenation, while at high Cu/Zn ratio, there was no sufficient ZnO to
disperse Cu, thus an optimal Cu/Zn molar ratio occurred at 1: 1. An
optimal Cu/Zn ratio was also identified for the
CuZn–SiO2 bimetallic catalysts in the hydrogenation of
methyl acetate.33 Among the
Cu/ZnO/SiO2 catalysts investigated, the
Cu1Zn1Si2 catalyst
displayed the highest conversion of 99.5% and the highest cyclohexanol
selectivity of 98.3% under identical reaction conditions of 483 K, 5.0
MPa, H2/ester molar ratio of 30, and weight hourly space
velocity (WHSV) of 0.5 h–1. As anticipated, there
were only trace amounts of the acid-catalyzed by-products including
ethyl acetate, cyclohexane, and cyclohexyl ether.
To further improve the selectivity, we in situ modified the
Cu1Zn1Si2 catalyst
during the co-precipitation process by adding 0.1 molar fraction of La
relative to Cu (denoted as
Cu1Zn1Si2La0.1).
The modification effects of lanthanum oxide have been reported to be
originated from several aspects, such as the textural change in catalyst
to form basic active sites and the improvement of the surface area of
the catalyst.34,35 The basic physicochemical
properties of the
Cu1Zn1Si2La0.1and Cu1Zn1Si2 catalysts
are presented in Table S4. As anticipated, the basicity of lanthanum
oxide further suppressed the occurrence of the acid-catalyzed reactions
during the hydrogenation of cyclohexyl acetate, which improved the
cyclohexanol selectivity up to 99.7%, while did not impose an adverse
effect on the conversion (Table S3 and Figure 4). Moreover, increasing
the WHSV from 0.5 to 1.1 h–1 did not change
significantly the conversion and selectivity over the
Cu1Zn1Si2La0.1catalyst (Table S3).
According to Table S4, the specific surface area
(S BET) of the
Cu1Zn1Si2La0.1catalyst is larger than that of the
Cu1Zn1Si2 catalyst. The
XRD patterns of the
Cu1Zn1Si2 and
Cu1Zn1Si2La0.1catalysts in Figure 5A show that aside from the broad peak at 2θof ~25o from amorphous
SiO2, only the diffraction peaks due to metallic Cu
(fcc Cu, JCPDS 04-0836) were identified, indicating that ZnO is
in the amorphous state on both catalysts. The phase relating to La on
the
Cu1Zn1Si2La0.1catalyst was also not discerned, which is attributed to its low loading
and/or high dispersion. The crystallite sizes of Cu calculated by the
Scherrer formula in terms of X-ray line broadening are 18.0 nm and 14.8
nm for the Cu1Zn1Si2 and
Cu1Zn1Si2La0.1catalysts, respectively, which may account for the high conversion of
cyclohexyl acetate when increasing the WHSV on the
Cu1Zn1Si2La0.1catalyst. The HRTEM image of the
Cu1Zn1Si2La0.1catalyst in Figure 5B shows only the Cu(111) lattice fringes with the
interplanar spacing of 2.08 Å, while the lattice fringes of ZnO were not
identified, which is consistent with the XRD result. The TEM image and
particle size distribution histogram of the
Cu1Zn1Si2La0.1catalyst in Figure 5C shows that the average particle size of Cu is 15.8
nm, which agrees well with the crystallite size derived from XRD.
Figures 5D–5I present the scanning transmission electron
microscopic-energy dispersive spectroscopic (STEM–EDS) images of the
Cu1Zn1Si2La0.1catalyst. Figure 5D is the HAADF–STEM image, Figures 5E–5H are the
corresponding EDS mappings of Si, Cu, Zn, and La, and Figure 5I is the
overlapping of Figures 5E–5H. Comparison of Figure 5D with Figure 5F
readily leads to the conclusion that the bright particles in Figure 5D
are originated from Cu. Comparison of Figure 5G with Figures 5D and 5F
reveals that Zn is located preferentially on or in vicinity to Cu rather
than distributed randomly on SiO2, signifying that ZnO
interacts strongly with Cu. This conclusion is additionally
substantiated by Figure 5I. On the other hand, the distribution of La is
homogeneous and does not show such a preference as that of Zn.
From the experimental results demonstrated above, we figured out the
overall atom economy of 99.4% and the yield of cyclohexanol per pass of
34.6% for the novel cyclohexene esterification–hydrogenation process,
as also demonstrated in Figure 1. The former is comparable to that of
the most selective cyclohexene hydration process, whereas the latter is
more than twice of that of the most efficient phenol hydrogenation
process. It is evident that the cyclohexene
esterification–hydrogenation process is highly advantageous in
producing cyclohexanol in a safe, efficient, and green manner.