Oxidation of cyclohexanol to adipic acid with molecular oxygen catalyzed by ZnO nanoparticles immobilized on hydroxyapatite

Article abstract of DOI:10.1039/c6ra12831a

One of the most important aliphatic diacids is adipic acid, which is produced industrially because of its application in the manufacture of nylon-6 and nylon-66. In the present work, a green methodology for the direct oxidation of cyclohexanol to adipic acid in a one-step reaction using ZnO nanoparticles supported on hydroxyapatite was developed. This work could be one of the very few articles on the use of ZnO nanoparticles for preparing adipic acid by using molecular oxygen as the greenest oxidant. The ZnO nanoparticles were prepared using different methods, such as reverse micelle, osmosis, alkaline hydrolysis, and impregnation, to compare their catalytic performances. We believe that oxygen deficiency in the ZnO nanocatalyst plays the main role in this oxidation reaction. The catalysts were characterized by different methods, such as FESEM, XRD, TEM, BET, and ICP. Among all the investigated catalysts, ZnO nanoparticles prepared by the reverse micelle method exhibited superior activity over all the other tested catalysts. To obtain the optimized conditions for the maximum conversion and selectivity, the reaction parameters, such as temperature, time, oxygen pressure, the amount of catalyst, amount of ZnO loaded on hydroxyapatite, and the method of preparation of the catalyst, were optimized. Under the found optimized conditions, a maximum conversion of 86% and selectivity of 96% was achieved for the production of adipic acid with ZnO/hydroxyapatite catalyst prepared by the reverse micelle method. The optimum conditions for the oxidation of cyclohexanol were 25 mg of catalyst, 120 °C, 15 bar oxygen pressure, and 8 h. Furthermore, this catalyst retained its catalytic activity with 84% conversion and 70% selectivity for adipic acid production after four cycles.

Full text of DOI:10.1039/c6ra12831a

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Oxidation of Cyclohexanol to Adipic Acid with Molecular Oxygen Catalyzed by
ZnO nanoparticles Immobilized on Hydroxyapatite
a,
a
a
b
M. Ghiaci *, S. M. Hosseini , A. Shahzeydi and M.V. Martínez-Huerta
a
Isfahan University of Technology, Department of Chemistry, Isfahan 8415683111, Iran
b
Institute of Catalysts and Petroleochemistry, CSIC, Marie Curie 2, 28049 Madrid, Spain
mghiaci@cc.iut.ac.ir*
One of the most important aliphatic diacids is adipic acid that produced industrially because of its application in the manufacture of
Nylon-6 and Nylon-66. In this work, a green methodology for direct oxidation of cyclohexanol to adipic acid in one-step reaction using ZnO
nanoparticles supported on hydroxyapatite has been developed. This work could be one of the rare reported articles that use ZnO
nanoparticles for preparing adipic acid by using molecular oxygen as the greenest oxidant. The ZnO nanoparticles were prepared using different
methods such as reverse micelle, osmosis, alkaline hydrolysis, and impregnationto compare their catalytic performances.We believe that
oxygen-deficiency in the ZnO nanocatalyst has the main role in this oxidation reaction. The catalysts were characterized by different methods
such as FESEM, XRD, TEM, BET, ICP. Among all investigated catalysts, ZnO nanoparticles prepared by reverse micelle exhibited the superior
activity compared to all other tested catalysts. To obtain optimized conditions for maximum conversion and selectivity, the reaction parameters
such as temperature, time, oxygen pressure, the amount of catalyst, amount of ZnO loaded on hydroxyapatite and the method of preparation
of catalyst were optimized. Under the optimized conditions, a maximum of 86% conversion and 96% selectivity for the production of adipic acid
was achieved with ZnO/hydroxyapatite catalyst prepared by the reverse micelle method. The optimum conditions for oxidation of cyclohexanol
were 25 mg of the catalyst, 120 °C, 15 bars of oxygen pressure, and 8 h. Furthermore, this catalyst kept its catalytic activity with 84% conversion
and 70% selectivity for adipic acid production after 4 cycles.
1
. Introduction
1
Adipic acid one of the most important dicarboxylic acid which is used mostly in the production of nylon 6,6. In
2
3
4
addition, it is utilized in the preparation of many other products such as foams, synthetic lubricants, paper additives,
5
6
7
8
9
polyurethanes, resins, adiponitrile and cyclopentanone. Adipic acid has a variety of uses such as increasing heat stability,
1
0
11
12
13
14
corrosion reduction, moisture resistance rise, glasses protection, air pollutants reduction, gas desulfurization,
1
5
16
17-18
detergent synthesis, foods protection, and increasing longevity and stability of food.
It could also be used as stability
1
9
multiplier. Over the years, the global production of adipic acid has been increased so that since 2010 the worldwide capacity
2
0
of its production has reached around 2.6 million metric tons per year, and it is expected to reach 3.3 million ton/y in 2016.
Therefore, designing of the processes with high yield and selectivity to this valuable product has always been considered. To
this end, it is necessary to design and construct appropriate catalysts and/or nanocatalysts to reach better yield.
Heterogeneous solid catalysts that are being increasingly used today are the best option to achieve this goal. Industrially, adipic
acid can be prepared using two methods (1) ring cleavage of the cyclohexanol/cyclohexanone mixture, and (2) cyclohexane
oxidation. Economically, adipic acid is produced from cyclohexane through a two-step process. In the first step, cyclohexane is
oxidized to cyclohexanol/cyclohexanone mixture (KA=alcohol/ketone) at 125-165 °C and 8-15 bar oxygen pressure in the
presence of Mn- or Co-salts, with 10-12% conversion and 80-85% selectivity to KA. Here, acetic acid as solvent is
environmentally unfavorable because of its corrosion effects. In the second step, KA mixture undergoes further oxidation to
2
0-22
give adipic acid using ring cleavage method.
In this stage, air or nitric acid can act as oxidants, and the reaction is catalyzed
using Cu-salts or Cu-Mn-acetate. Today, nitric acid is mostly utilized in the oxidation of KA mixture to adipic acid in industrial
1

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scale. However, HNO , as an oxidant emits NO , which makes it an inappropriate reagent to be used in large scales due to the
3
x
2
3
environmental problems and concerns such as global warming, corrosion effects, ozone depletion, and costliness. To
overcome these problems, today scientists have used green oxidants such as molecular oxygen and compressed air which are
not only compatible with green chemistry but are also considered as economically-suitable reagents with high activity and
24
efficiency without any chemical waste and pollutant.
During the last decades, efforts have been made to prepare adipic acid in a one-pot process because of the
economical and operational needs. The major focus has been on the oxidation reaction of cyclohexane, which is more desirable
25-31
industrially.
However; this process lacks high conversion because of the absence of any active functional groups on the
cyclohexane ring, which finally leads to the reduction of adipic acid yield, especially in the cases where molecular oxygen or
compressed air are used as oxidants. In addition, choosing cyclohexane as a starting material for preparing adipic acid through
oxidation has more byproducts in comparison with other starting materials like cyclohexanol or cyclohexanone, and as a result
selectivity of adipic acid is reduced. On the other hand, catalytic oxidation of cyclohexanol to adipic acid has always been with
3
2
32
33
34
low selectivity to adipic acid. In this respect, the catalytic systems such as Mn-MCM-41, Cr(Salten), WO3, V O -MoO -
2
5
3
3
5
36
37
2 3,
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39
M O (M = Na, K, Cs), Au.NP@PION, ZrO -Y O CoO/SBA-15, CeO2, oxovanadium(V) or (V)–complex, heteropoly acid H5-
2
2
4
0-42
[
PMo V O ], vanadium-containing heteropoly anions (with kegging structure) and Copper-complex,
have been utilized
10 2 40
that altogether have resulted in cyclohexanone with high conversion and selectivity. All of these studies, which have been done
in the presence of acetic acid as solvent or co-solvent, and promoter reagents like HMPA have cyclohexanone as the major
product. Furthermore, the mentioned catalysts contain hazardous species and pollutant materials even in small amounts so
that using them in large scales is not advised. Cyclohexanol oxidation to cyclohexanone is mostly performed through
3
5,43
dehydrogenation. This process is performed over Zn or Cu catalyst at 400-500 °C and atmospheric pressure.
Current
industrial production of cyclohexanone from cyclohexanol dehydrogenation is performed using ZnO and Cu/MgO catalysts. This
reaction that results in 75% and 99% conversion and selectivity to cyclohexanone needs 200-450 °C temperature. In addition,
37
selectivity to adipic acid in these systems is very low, and thus the purification stage for obtaining a pure product is essential.
Therefore, designing an appropriate catalyst for selective oxidation of cyclohexanol and/or cyclohexanone to adipic acid is a
37
favorable option. Hence, adipic acid and caprolactam production has been the topic of many studies on the oxidation of
cyclohexanol in large scale. In addition, increasing the catalyst life-time, decreasing the operational temperatures and
4
4
improving adipic acid selectivity should be considered. Furthermore, in the studies on cyclohexanol oxidation have often
utilized hydrogen peroxide as oxidizing agent. Although this oxidizing agent has high oxygen content and low price, but its usage
45-46
in large scale is not appropriate environmentally and economically.
Among the studies reported on the preparation of
adipic acid through cyclohexanol oxidation in liquid phase, carbon-supported platinum (Pt/C) as heterogeneous catalyst for
4
7
oxidation by molecular oxygen led to 50% conversion and 50% selectivity to adipic acid. Regardless of the relatively high
content of expensive platinum metal, low conversion and selectivity observed in this study is not convincing. In addition, the
very low solubility of cyclohexanol in water solvent remains as a major challenge.
Sato and Usuai (2003) investigated one-step oxidation of cyclohexanol and cyclohexanone to adipic acid separately
using H O as the oxidant in the presence of H WO as catalyst. They finally obtained adipic acid as the main product with 87 %
2
2
2
4
48
yield for cyclohexanol and 91% yield for cyclohexanone as starting materials during 20 h reaction. Reported conversions were
obtained in relatively harsh conditions, and adipic acid was produced along with other dicarboxylic acid by-products. Therefore,
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the purification step of the final product could be questionable. In addition, despite the fact that small amount of tungestic acid
has been used but use of high amount of H O might create special environmental problems.
2
2
Herein, we have prepared ZnO nanoparticles by different techniques such as microemulsion (reverse micelle) and
studied their catalytic activity in the oxidation of cyclohexanol. The microemulsion phenomenon used as media for synthesis of
the catalyst applied in this study is a very useful strategy in preparing nanoparticles with a good distribution on the support. In
the other words, by constructing droplets in nanometer scale, agglomeration of nanoparticles could be avoided. However, it is
necessary to use appropriate amounts of surfactant, organic solvent, and aqueous solution of the metal salt for creation of the
49-50
desired droplet with appropriate size.
Because of high cost of using surfactant in large scale, efforts have been made to use
cheaper pathways instead of reverse micelle in preparing the catalysts. To this end, osmosis, impregnation and simple alkaline
hydrolysis methods were also used in this study. All of these methods are cheap, simple, and are done in a short period of time.
The hydroxyapatite used as support for the catalysts has advantages including very low price, and ease of
5
1
52
preparation. These properties have converted it to a suitable support for immobilization of catalysts. Hydroxyapatite is a
53-54
natural material with the formula of Ca (PO ) (OH), that researchers in different fields use it as adsorbent and catalyst.
5
4 3
ZnO is mostly known as an important semiconductor with a wide direct band gap of 3.37 eV. This material exists in
55-57
various nanostructural forms
which has obtained significant applications in optoelectronic, energy harvesting,
5
8-59
60-61
electrochemical, electromechanical devices,
and catalysis.
Increasing the surface area of zinc oxide in its nanoparticles
undoubtedly can improve its atomic surface and catalytic properties, although densities of active sites could also influence on
catalytic activity. As a general understanding, in a heterogeneous catalyst the solid state defects could be proposed as the
61-62
active sites of the catalyst.
We believe that oxygen-deficiency of the ZnO has the main role in this oxidation reaction but the
analysis of the oxygen deficiency of the catalysts is beyond the scope of this article. Our results show that a uniform size
distribution of ZnO nanoparticles results in decreasing dehydrogenation temperature and reaction time as well as very high
amounts of cyclohexanol conversion and adipic acid selectivity in the presence of molecular oxygen as oxidant. The result
obtained in this work, in comparison with other studies conducted on the production of adipic acid from cyclohexanol
oxidation, is very high and has the potential for using in large scales. Moreover, among all catalysts prepared with different
techniques in this work, microemulsion catalyst has yielded the best result for related reaction. On the other hand, high
conversion of cyclohexanol with very low selectivity to cyclohexanone and other diacids by-products including succinic acid and
glutaric acid, leads to approximately pure adipic acid after being directly cooled, and finally eliminates purification step in our
new proposed catalytic system.
2
. Experimental
2
.1. Materials
All chemicals, reagents and solvents were synthetic grade and were purchased from Merck Company, and used
without any extra purification. Compounds such as cyclohexane and n-hexane were just dried before use. Deionized water with
conductivity about 0.07 µs was used for dissolving metal salts and preparing related solutions. Fresh bone was obtained from
rib bone of cow andwas washed by dilute hydrochloric acid and deionized water, respectively before calcination.
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2
.2. Equipments and characterization
The oxidation reaction was performed in a batch system in a reactor with 200 mL volume made of titanium that was
equipped with a digital thermometer. Furthermore, this reactor had inlet and outlet ports for importing and vacating oxygen
gas before and after reaction, respectively. This system was heated by a mantel containing paraffin oil. The exact amount of
63
required oxygen for reaction and accurate pressure of this reactor were provided by a pressure gauge.
To analyze the quantity and kind of products produced during the reaction, an Agilent gas chromatograph equipped
with a FID detector and OV-17 capillary column with 60 m length, and a gas chromatograph coupled with a mass spectrometer
(
Fisons Instruments 8060, USA) was used. The initial temperature for gas chromatograph analysis was 50 ˚C for 1 min and by
ramp of 15 °C/min, this was increased to 150°C and maintained at this temperature for 1 min, then was ramped at 25 °C/min to
20°Cand a Hewlett-Packard 1090-II liquid chromatograph (Agilent, Palo Alto, CA, USA) equipped with a UV-Vis DAD (210 nm
2
wavelength) was used for determination of amount of products and unconsumed material during oxidation reaction of
cyclohexanol. Field emission scanning electron micrographs were obtained using the Cambridge Oxford 7060 Scanning Electron
Microscope (SEM) connected to a four-quadrant backscattered electron detector with resolution of 1.38 eV in order to analyze
the morphology and particle size of the synthesized catalysts. In fact, it is necessary to dust sample on a double-sided carbon
tape that was placed on a metal stub and coated with a layer of gold to prevent charging effect.X-ray diffraction patterns of
powders were taken on a Phillips diffract meter with Cu Kα radiation (40 kV, 30 mA) over a 2θ range from 10 to 80° connected
to a DACO-MP microprocessor using the Diffract-AT software. To measure thespecific surface area and pores size distribution, a
micrometric digisorb 2600 system at 196 °C with nitrogen gas was used.
2
.3. Preparation of hydroxyapatite supported ZnO nanocatalyst by reverse micelle method (Mic-ZnO/HAP)
In the first step, in order to prepare hydroxyapatite as support for the catalysts, pieces of cow bone were cleaned so
that all carbonic and fats portions were removed from their surface. Then, the sample was burned on the flame and calcined at
00 °C for 48 h. The resultant powder was now used as support for immobilization of ZnO nanoparticles in reverse micelle and
alkaline hydrolysis methods. To prepare Mic-ZnO/HAP nanocatalyst, a microemulsion solution was prepared as follows: 4.63 g
10.4 mmol) of AOT (Dioctyl sodium sulfosuccinate) was dissolved in 100 mL dry n-hexane and stirred for 1 h. This solution was
named solution A. On the other hand, the solution B was prepared through dissolving 0.28 g of Zn(NO ) 6H O in 5 mL of
7
(
3
2
2
deionized water. In this step, the solution B was added drop wise to the solution A during 1 h. Then, it was necessary to let the
resultant solution be stirred for 1 h so that the zinc nitrate was trapped in the droplets of microemulsion solution. In the next
2+
step, to hydrolyze Zn cations, an aqueous solution of ammonia (25% w/w) was added to reverse micelle solution during 2 h;
the molar ratio of ammonia/metal was 4. Then, the microemulsion solution was stirred for 12 h. In order to immobilize the
prepared Zn(OH) over hydroxyapatite support, 2.16 g hydroxyapatite was added to microemulsion solution by stirring. The
2
mixture sonicated for 30 min, and stirred overnight. The precipitate was centrifuged and washed by absolute ethanol, dried at
1
20 °C, and finally calcinated for 4 h at 500 °C under vacuum.
2
.4. Preparation of hydroxyapatite supported ZnO nanocatalyst by osmosis method (Os-ZnO/HAP)
To prepare Os-ZnO/HAP nanocatalyst, 100 mL aqueous solution of Zn(NO ) 6H O (0.01 M) was prepared, 2.16 g
3
2
2
2+
cleaned and piecemealed cow bone was immersed into the solution for a period of one week. During this time, Zn cations
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could penetrate into the pores of cow bone. After one week, the bone pieces were extracted from solution and after washing
with deionized water,they were burned under flame. In the next step, the bone pieces were calcinated at 700 °C for 48 h.
2
.5. Preparation of ZnO/hydroxyapatite by impregnation method (Imp-ZnO/HAP)
In the first step, a 100 mL aqueous solution of Zn(NO ) .6H O (0.01 M) was prepared and 2.16 g hydroxyapatite
3
2
2
(
prepared from calcination of bone) was added to this solution. This solution was heated to dryness, and the bone pieces were
burned. In the next step, the pieces were calcinated at 700 °C for 48 h.
2
.6. Synthesis of ZnO/Hydroxyapatite by the alkaline hydrolysis method (Ah-ZnO/HAP)
An aqueous solution of Zn(NO ) .6H O (0.01 M) was prepared by dissolving 0.28 g of Zn(NO ) .6H O salt in 100 mL
3
2
2
3 2
2
deionized water and added 2.16 g of hydroxyapatite (prepared from calcination of bone) to the above solution on stirring. The
solution sonicated for 30 min and then hydrolyzed by adding a 25% w/w ammonia solution drop wise during 24 h under
continuous stirring. The reaction mixture was maintained at room temperature until a gel was formed, and then the precipitate
was centrifuged. After drying at 120 °C for 12 h, it was calcinated at 500°C for 5 h.
2
.7. Catalytic reaction
In the optimized conditions, an appropriate amount of the catalyst, cyclohexanol, and solvent were added toa
titanium batch reactor that was described previously. The correct pressure was provided by oxygen and the temperature of
reaction flask was settled on a desired amount. After completion ofthereaction, in the mixture of reaction, two phases were
observed.Anorganic phase contained solvent, unreacted cyclohexanol, cyclohexanone, and a solid phase containing adipic acid,
and the catalyst.The solid phase was dissolved in ethanol, and after separating the catalyst, the sample was analyzed by high
performance liquid chromatography (HPLC). In the case of organic phase, the components were determined by GC and
identified with GC-MS.
3
. Results and discussion
3
3
.1. Characterization of catalysts
.1.1. XRD data
XRD patterns of the prepared catalysts and the hydroxyapatite from bone after calcination are presented in Fig. 1. In
the XRD patterns of the catalysts the diffraction lines corresponding to crystalline ZnO, which are not hidden under the lines of
hydroxyapatite, are marked. According to reference data (JCPDS card, No- 780-0075), main peaks of the crystalline zinc oxide
appear at 31.7°, 34.2°, 36.3°, 46.7°, 66.4°, 65.5° and 75.9° related to (100), (002), (101), (102), (110), and (202) diffraction levels,
6
4
respectively. All diffraction peaks correspond to the characteristic hexagonal Wurtzite structure of zinc oxide particles (a =
0
.315 nm and c = 0.529 nm). Since the significant lines of crystalline zinc oxide overlap with diffraction peaks of the
hydroxyapatite, and the amount of ZnO is very low in the catalysts, therefore all of the lines corresponding to zinc oxide could
not exactly be identified.
3
.1.2. FE-SEM and TEM images
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FE-SEM images of the ZnO catalysts synthesized by different methods are shown in Fig. 2. As observed, morphology of
the catalyst prepared by alkaline hydrolysis of zinc aqueous solution in the presence of hydroxyapatite, could be imagined as
hydroxyapatite particles that are covered by ZnO particles Fig. 2(g,h). On the other hand, in the SEM images of the catalysts
prepared by the osmosis, [Fig. 2(e,f)], and reverse micelle [Fig. 2(a,b)] techniques, one observes an aggregation of
hydroxyapatite particles with the size of less than 1 µ, without roughness that clearly can be seen in the catalyst prepared by
alkaline hydrolysis method. Also for any reason, in these two catalysts the hydroxyapatite particles are very smaller than the
particles in the other two catalysts. This point in regard to the particle size shows itself in the Fig. 2(c,d). As a matter of fact, in
the catalysts prepared by impregnation and osmosis methods we just used zinc nitrate as the source of ZnO, but in the other
two catalysts we have used aqueous ammonia or sodium hydroxide which might cause breaking of the crystalline size of the
hydroxyapatite.
Since high catalytic activity of the optimized catalyst depends on the particle size of the ZnO, and porosity of the
catalyst, TEM images of the best catalyst, i.e., Mic-ZnO/HAP were studied. The images in different magnifications are shown in
Fig. 3. In these images as observed, nanoparticles of ZnO have spherical shapes, with size lower than 50 nm with narrow size
distribution; the data of elemental analysis are given in the supplementary data.
3
.1.3. BET and ICP
The respective N adsorption/desorption isotherm of the Mic-ZnO/HAP (Figure 4), shows an isotherm similar to type
2
II with typical H hysteresis loop and demonstrates simultaneously the properties of typical non-porous and macro-porous
3
materials. Hysteresis loop between adsorption and desorption isotherms is associated with differences in the rates of capillary
5
2,65
condensation and evaporation.
The textural parameters of the corresponding catalyst such as the specific surface area
(
SBET), total pore volume (V ), average pore size (daverage) and Barrett–Joyner–Halenda (dBJH) desorption average pore diameter
tot
of the catalyst are summarized in Table 1. Even having a low surface area, oxidative activity of this catalyst was significant.
To determine the content of zinc in the Mic-ZnO/HAP and Os-ZnO/HAP as well as in pure hydroxyapatite, ICP-AES technique
was applied and the data are shown in Table 2. As observed, the amount of zinc in the selected catalyst for oxidation of
cyclohexanol was 0.177 mmol/g, or in other words this catalyst has 1.11 wt% zinc oxide.
3
.2. Catalytic performance
The catalytic performance of the Mic-ZnO/HAP was tested in one-step oxidation reaction of cyclohexanol to adipic
acid in the presence of molecular oxygen without any other oxidant. The results are shown in Table 3. In order to reach
optimized condition factors such as temperature (°C), time (hours), amount of catalyst (mg), oxygen pressure (bar), amount of
zinc oxide immobilized on hydroxyapatite (wt% of ZnO) and the method used for preparation of the ZnO/HAP catalyst were
investigated. The transformation of cyclohexanol to adipic acid could be achieved through multistep oxidation involving
oxidation of cyclohexanol to cyclohexanone, a Baeyer-Villiger type oxidation, hydrolysis, and oxidation of the probable
intermediate, 6-hydroxy-hexanoic acid to adipic acid (Scheme 1). Oxidation of C-2 carbon adjacent to carboxyl group will end up
6
6,67
to glutaric acid, and other by-products.
The amount of products, conversion of cyclohexanol and selectivity towards each of
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the products were followed by gas chromatography and liquid chromatography and each product was identified by GC-Mass
and LC-Mass analysis.
3
.2.1. Effect of temperature
Oxidation reaction was performed in 100, 120 and 140 °C. The results are shown in Table 3. As shown, the results
indicate that a temperature higher than 100 °C was necessary for the formation of adipic acid. As the temperature increased,
the conversion rate of cyclohexane increased gradually while the selectivity of adipic acid first increased and then decreased. It
was really interesting that the yield of adipic acid and formation of tarry materials was highly sensitive to reaction temperature.
At higher temperatures after formation of a certain amount of adipic acid its degradation increases which decreases the
selectivity of the adipic acid sharply. Therefore, temperature probably is the most important reaction parameter for having
desire selectivity to the adipic acid. Thus, the optimized temperature for this reaction was chosen at 120 °C.
3
.2.2. Effect of time
Table 3 represents the results of reactions which were performed at 120 °C temperature and different times (h). As
the time (h) of reaction was increased, although conversion did not change appreciably, but the selectivity for adipic acid
production increased until 9 h, and after this time, selectivity reduced. Reduction in selectivity to adipic acid with increasing
time is related to catalytic conversion of adipic acid to other diacids such as succinic acid and glutaric acid by decarboxylation,
and tarry materials. Therefore, 9 h was chosen for other experiments in this work.
3
.2.3. Effect of amount of catalyst
To examine the effect of the catalyst amount on the adipic acid production, we carried out the oxidative reaction of
cyclohexanol at 120 °C, and 15 bars oxygen pressure for 9 h with various amounts of the catalyst. As shown in Table 3, the
conversion of cyclohexanol and selectivity of adipic acid increased, and then decreased when the amount of catalyst increased.
This decrease in conversion and selectivity of the reaction by increasing the amount of catalyst might be solved by using a more
efficient mixing device. Since the media is hydrophobic and the catalyst has a high tendency to coagulate in the reaction
condition one would think of losing active site of the catalyst because of coagulation. Based on these results, the amount of 25
mg of the Mic-ZnO/HAP catalyst (containing 0.0044 mmol zinc oxide) was chosen as an optimized amount of the catalyst in
these reactions which leads to 86% conversion and 96% selectivity to adipic acid.
3
.2.4. Effect of pressure
Table 3 also shows the results of optimization of pressure in oxidation reaction of cyclohexanol by the ZnO/HAP
catalyst at 120°C. An increase in pressure from 5 to 15 bar leads to an increase in both reaction conversion and adipic acid
selectivity because of the presence of more oxidant molecules in the reactor. By increasing the oxygen pressure to 20 bars,
there was not any noticeable change in conversion and selectivity, therefore the optimized pressure for this reaction was
chosen at 15 bars.
3
.2.5. Effect of wt% ZnO immobilized on the Mic-ZnO/HAP catalyst
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In this step of the work, different amounts of ZnO was immobilized on hydroxyapatite through reverse micelle
method and tested in oxidation reaction of cyclohexanol to adipic acid. Among these catalysts, as observed, the amount of 4
wt% ZnO immobilized on HAP was enough to increasethe conversion and selectivity to 86 % and 96 %, respectively. Probably by
enhancing the load of zinc oxide on hydroxyapatite aggregation of the catalyst particles causes to lose some of its active sites
and lower the conversion considerably (Table 4).
3
.2.6. Effect of method of catalyst preparation
Catalytic activity of Os-ZnO/HAP, Imp-ZnO/HAP and Ah-ZnO/HAP was also investigated. Hence, reaction with each of
the prepared catalysts was performed in 120 °C, 15 bars oxygen pressure, 25 mg of the catalyst, and 9 h time. The results are
shown in Table 5. As observed, the Mic-ZnO/HAP was the one which showed excellent conversion and selectivity for this
reaction.
3
.2.7. Reusability of catalyst
For this purpose, catalytic activity of Mic-ZnO/HAP nanocatalyst was studied in oxidation reaction of cyclohexanol for
cycles and the stability of this catalyst was examined. The results are shown in Fig. 5. Because of the heterogeneity of ZnO
4
catalyst, it could be easily separated from the reaction mixture and after being washed with chloroform and dried at 100 °C
could be used for the next cycle. According to these data, both selectivity and conversion remain approximately steady after
four runs. Decreasing catalytic activity in this reaction is related to deactivation of active sites in the catalyst, and as this
reaction is carried in free solvent conditions, probably the leaching cannot be the reason for activity reduction. However, the
tarry materials produced during the reaction might be the main reason for deactivation of the catalyst.
3
.3. Kinetic analysis
The Langmiur-Hinshelwood kinetic model was used to calculate the rate constant of the catalytic oxidation of
cyclohexanol. In this regard, we assumed that cyclohexanol and molecular oxygen react with each other at the surface of the
68-70
catalyst.
According to this kinetic model, the rate constant is proportional to the fraction of the active sites covered by the
substrates. Time profile investigations of cyclohexanol oxidation in the presence of solvent are shown in Figure 6. Rate
constants and adsorption coefficient calculated at various temperatures are listed in Table 6 (supplementary data).
4
. Conclusion
In summary, a one-step green synthesis of adipic acid from cyclohexanol by using zinc oxide nanoparticles as the
catalyst and molecular oxygen as a green oxidant under different temperatures and oxygen pressure were reported here. On
one hand, we have shown that the ZnO nanoparticles prepared by reverse micelle method had a better performance than the
ZnO nanoparticles prepared by other methods. Probably the oxygen deficiency of the catalyst play an important role in this
reaction which can be a new platform for our research group and other researchers to do some in-depth studies on defects that
can be produced on the ZnO nanoparticles. Finally, in the optimized conditions, the best catalyst (4wt% ZnO/HAP) that was
prepared by reverse micelle method showed 86% conversion and 96% selectivity to adipic acid in 120 °C, 15 bars oxygen
pressure, 25 mg catalyst, and 9 h time.
8

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Acknowledgments
Thanks are due to the Research Council of Isfahan University of Technology and Center of Excellency in the Chemistry
Department of Isfahan University of Technology for supporting of this work.
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Table 1
Textural properties of the Mic-ZnO/HAP nanocatalyst (The actual wt.% of ZnO is 1.11%)
2
-1 a
b
3
-1 c
d
Sample
SBET(m g )
dBJH(nm)
V (cm g )
daverage(nm)
tot
Mic-ZnO/HAP catalyst
2.3
2.7
3.6
6.2
a
b
c
Brunauer–Emmet–Teller (BET) surface area.
Pore diameter calculated by the Barrett–Joyner–Halenda (BJH) method utilizing the adsorption branches.
Total pore volume calculated as the amount of nitrogen adsorbed at a relative pressure of 0.99.
Average pore diameter.
d
Table 2. Analytical data of the nanocatalysts and support
Analysis (mmol)
Sample (1 g)
Ca
Na
Mg
Li
K
Sr
Fe
Se
B
Zn
0
Calcined bone
Mic-ZnO/HAP
Os-ZnO/HAP
Imp-ZnO/HAP
Ah-ZnO/HAP
7.77
0.26
0.24
0.068
0.021
0.015
0.013
0.01 0.007
0.177
0.066
2.92
2.92
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Table 3.
Effect of the reaction parameters
Selectivity (%)
Temperature
˚C)
Pressure
(bar)
Catalyst
(mg)
Conversion
Entry
Time (h)
Adipic
Other
(
(%)
Cyclohexanone
Acid
0
s
0
14
35
0
0
10
0
1
2
3
4
5
6
7
8
9
100
120
140
120
120
120
120
120
120
120
120
120
120
15
15
15
15
15
15
15
15
15
5
5
5
5
7
9
11
9
9
9
9
50
50
50
50
50
50
15
25
50
25
25
25
25
14
71
75
76
72
73
60
86
71
77
85
86
86
100
43
40
24
13
22
20
4
14
70
67
4
43
25
76
87
68
80
96
86
30
33
96
68
0
0
1
1
1
1
0
1
2
3
0
0
0
0
10
15
20
9
9
9
32
Reaction conditions: amount of cyclohexanol, 9.6 mmol (1 mL) and amount of solvent, 5 mL cyclohexane.
Table 4.
Effect of type of the nanocatalyst
a
Selectivity (%)
Cyclohexanone
Entry
Catalyst
Conversion (%)
Adipic acid
Others
b
1
2
3
4
4% Mic-ZnO/HAP
6%Mic-ZnO/HAP
8% Mic-ZnO/HAP
10% Mic-ZnO/HAP
86
76
59
52
4
2
7
4
96
94
88
86
0
0
0
0
Reaction conditions: temperature, 120 ˚C ; time, 9 h; oxygen pressure, 15 bar; amount of the catalyst, 25 mg;
amount of cyclohexanol, 9.6 mmol (1 mL) and amount of solvent, 5 mL cyclohexane.
a
All of catalysts in these experiments are prepared by reverse micelle method.
b
The amount of zinc in this catalyst is 0.177 mmol (based on ICP analysis ), and so is 1.11 wt.% zinc oxide rather to support.
Table 5.
Effect of method of nanocatalyst preparation
Selectivity (%)
Entry
Catalyst
Method of preparation
Conversion (%)
Other
s
0
2
3
Cyclohexanone
Adipic acid
1
2
3
4
5
1.11 % ZnO/HAP
0.08% ZnO/HAP
4 % ZnO/HAP
4 % ZnO/HAP
HAP
Reverse micelle
Osmosis
Impregnation
Alkaline hydrolysis
---
86
75
70
71
N.R
4
96
88
84
87
---
10
13
12
---
>1
---
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Reaction conditions: temperature, 120 ˚C; time, 9 h; pressure, 15 bar; amount of the catalyst, 25 mg; amount of
cyclohexanol, 9.6 mmol (1 mL) and amount of solvent, 5 mL cyclohexane.
Table 6.
Rate constants and adsorption coefficient calculated by fitting Eqn (1) to time profile data from Figure 6.
Temperature (K)
3
73
383
393
403
-
1
K (L mol )
0.066
0.00094
0.081
0.005
0.049
0.0159
0.017
0.055
-
1
k (min )
Reaction conditions: cyclohexanol 9.6 mmol (1 mL); oxygen pressure, 15 bar and solvent, 5 mL cyclohexane.
K is the adsorption coefficient, k is the rate constant
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4

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Fig. 1. X-ray diffraction of ZnO/HAP nanocatalyst prepared by different methods: Mic-ZnO/HAP (a,b), Imp-
ZnO/HAP (c), Os-ZnO/HAP (d) and X-ray diffraction of calcined bone (e)
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Fig. 2. FE-SEM images of Mic-ZnO/HAP (a,b), Imp-ZnO/HAP (c,d), Os-ZnO/HAP (e,f) and Ah-ZnO/HAP(g,h).
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Fig. 3. TEM images of Mic-ZnO/HAP in different magnifications (A,B).
Scheme 1. Oxidation reaction of cyclohexanol to adipic acid catalyzed by ZnO/HAP nanocatalyst.
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Adsorptive N2
Adsorption temperature 77 [K]
Fig. 4. Adsorption/desorption isotherm of the Mic-ZnO/HAP catalyst.
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100
96
9
5
90
80
70
86
86
8
5
7
9
79
7
0
6
5
4
0
0
0
Conversion
Adipic acid
selectivity
3
2
1
0
0
0
0
1
2
3
4
Run number
Fig.5. Reusability of the nanocatalyst at optimized conditions: temperature, 120˚C; pressure, 15 bar;
amount of catalyst, 25 mg; Time, 9 h and kind of nanocatalyst, 4% Mic-ZnO/HAP nanocatalyst.
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
4
3
3
3
03 K
93 K
83 K
73 K
0
100
200
300
400
500
600
Time (min)
Fig. 6. Time profile investigation for oxidation reaction of cyclohexanol in solvent, reaction conditions:
cyclohexanol 9.6 mmol (1 mL); oxygen pressure, 15 bar; amount of catalyst, 25 mg.
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2
54x190mm (96 x 96 DPI)