Potential of Supported Gold Bimetallic Catalysts for ...

Potential of Supported Gold Bimetallic Catalysts for Green Synthesis of Adipic Acid from Cyclohexane A. Alshammari, A. Köckritz, V. N. Kalevaru, A. Bagabas & A. Martin

Topics in Catalysis ISSN 1022-5528 Top Catal DOI 10.1007/s11244-015-0475-9

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Author's personal copy Top Catal DOI 10.1007/s11244-015-0475-9

ORIGINAL PAPER

Potential of Supported Gold Bimetallic Catalysts for Green Synthesis of Adipic Acid from Cyclohexane A. Alshammari1 • A. Ko¨ckritz2 • V. N. Kalevaru2 • A. Bagabas1 • A. Martin2

Ó Springer Science+Business Media New York 2015

Abstract Adipic acid (AA) is the main raw material for making nylon 6,6 fiber, which is further used in the production of different commercially important products (e.g. coatings, polymers etc.). Herein, we demonstrated a green chemistry route for the one-step oxidation of cyclohexane (CH) to AA using supported bimetallic catalysts containing gold and palladium or silver nanoparticles. The catalysts were prepared in two steps using sol–gel and impregnation techniques and were then characterized by different techniques such as ICP, BET surface area, XRD, TEM etc. Activity tests were carried out using Parr autoclave under the following optimum conditions (400 mg of catalysts, acetonitrile as solvent (5 ml), 0.1 g of TBHP as initiator, 10 bar of O2, 150 °C). XRD patterns revealed that no reflections corresponding to either Au or Pd are present in Au–Pd/TiO2 system, while a weak reflection belonging to metallic Au phase could be seen in Au–Ag/TiO2 catalyst. In terms of catalytic activity, Pd/TiO2 exhibited somewhat inferior performance (X–CH = 16 %, S–AA = 18 %) compared to Au/TiO2 solid (X–CH = 25 %, S– AA = 26 %). Interestingly, the combination of Au and Pd (Pd–Au/TiO2) markedly improved the selectivity of AA from 26 to ca. 34 %. In case of Au–Ag/TiO2, the combination of Ag and Au (i.e. 1 % Au–1 % Ag/TiO2) enhances the conversion of CH and selectivity of cyclohexanone

& A. Alshammari [email protected] 1

Materials Science Research Institute, King Abdulaziz City for Science and Technology, King Abdullah Road, P.O. Box 6086, Riyadh 11442, Saudi Arabia

2

Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany

(X–CH = 33 %, S–One = 41 %); compared to monometallic Ag/TiO2 solid. The results showed that the nature of second metal has shown a clear influence on the conversion of CH as well as the product distribution. Keywords Adipic acid Cyclohexane Bimetallic catalysts Nano-gold Titanium dioxide

1 Introduction The main industrial applications of adipic acid (AA) are for the production of nylon 6,6 fiber and resins, with much minor volumes going to AA ester and polyester polyols (e.g. polyurethanes), which work well in the field of plasticizers (e.g. PVC). Nylon 6,6 fiber is used in different important products such as carpet yarn, tyre cord, home furnishings, clothes, etc. [1–4]. Other application of AA can also be found in medicine and food industries [5, 6]. In 2011, the global consumption of AA was around 3 million metric tons. The worldwide growth rate for AA is *3 % per year, which is expected to reach 8–11 % by 2015 particularly in Asia–pacific region. In general, the production of AA involves a two-step reaction. The first step is synthesis of a mixture of cyclohexanone and cyclohexanol (i.e. KA oil) at approximately 150 °C and at 10–20 bar of air using a cobalt or a manganese catalyst [4, 7, 8]. The second step involves the oxidation of the mixture of cyclohexanone/cyclohexanol (Ketone/Alcohol, the socalled KA oil) to AA by nitric acid. However, this process is environmentally unfriendly because a large amount of nitrogen oxides (N2O and NOX) is formed [9]. Therefore, it is indeed very advantageous to explore different possibilities to develop one-step process for the direct synthesis of AA from CH without producing NOX.

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Author's personal copy Top Catal

Moreover, the production of AA can also be achieved using various other routes as shown in Scheme 1 [9–13]. For instance, AA produced by direct oxidation of CH using hydrogen peroxide, by carbonylation of butadiene, by dimerization of methyl acrylate, or fermentation of glucose. In addition, AA can also be obtained from the oxidation of cyclohexene with 30 % hydrogen peroxide (H2O2) and sodium tungstate in the presence of phase transfer catalyst [14]. Another type of catalyst such as peroxy tungstate-organic complex was also used to catalyse oxidation of cyclohexene using 30 % H2O2 to produce AA with good yield [15]. In spite of these various routes, the direct oxidation of CH to AA in one step using O2, as an oxidant, is indeed an economic, environmentally friendly approach, which is the prime objective of this work. The application of heterogeneous catalysts in the direct oxidation of CH to AA is also known from the prior art. For instance, iron phthalocyanine encapsulated in Y-zeolite was used as a catalyst for the oxidation of CH to AA in one step reaction [16]. However, such method is unattractive for industry due to the fact that it suffers from much longer induction periods, i.e. the catalyst requires approximately 300 h to convert ca. 35 % of CH and needs 600 h to get higher amounts of AA in the product stream. Furthermore, efforts were also made by various researchers [e.g. 17] to

Scheme 1 Summary of the different pathways for AA production

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apply gold-based catalysts for the direct oxidation of CH to AA, but to the best of our knowledge, all such attempts went unsuccessful until now. For instance, various gold catalysts such as Au/MCM-41 [18] and SiO2 [19] supports were applied for the said reaction, which gave only cyclohexanol and cyclohexanone as major products without any AA in the product stream. Using such catalyst systems, the conversion of CH was varied in the range from 6 to 20 %, but virtually no formation of AA was obtained. Nevertheless, the selectivity of KA oil products together is observed to be between 17 and 90 % depending on the type of catalyst and the reaction conditions used. Interestingly, we have reported that gold metal nanoparticles (AuNPs) supported on different metal oxide carriers were good catalyst systems for the production of AA from CH [20, 21]. Herein, we made attempts to improve the catalytic activity for the direct oxidation of cyclohexane (CH) to AA using different gold-containing bimetallic catalysts.

2 Experimental 2.1 Materials Tetrachloroauric acid (99 %, Fluka), Palladium chloride (Aldrich, 99 %), Silver nitrate (Aldrich, 99.0 %), tri-


Sodium citrate dihydrate (99.5 %, Fluka) and tannic acid (98 %, Aldrich) are commercially available materials and were used as received. Commercially available TiO2 (anatase) was also used as a catalyst carrier. Deionized water (18.2 MX cm) was obtained from a Milli-Q water purification system (Millipore). 2.2 Catalyst Preparation Catalyst preparation of the three monometallic catalysts (M:Au/TiO2, Pd/TiO2 and Ag/TiO2) were prepared in two steps with varying the type of metal precursors. The first step deals with the preparation of the colloidal gold nanoparticles (MNPs) by the reduction of 1 mM M in aqueous solution using 1 % tannic acid and 1 % sodium citrate. The second step involves the impregnation of the colloidal MNPs with a suitable oxidic support to obtain slurry, which was stirred for 2 h at room temperature and then the excess solvent was removed by a rotary evaporator. The obtained solid was oven dried at 120 °C for 16 h and then calcined at 350 °C for 5 h in air. Catalyst preparation of bimetallic system (AuPd/TiO2 and AuAg/TiO2) also involves two steps. In the first step, 1.0 mM of HAuCl4 was dissolved in 60 mL distilled water. In the second step, 1.0 mM of PdCl2 solution (M) was dissolved in 10 mL of distilled water. This solution was heated to 50 °C for 10 min and few drops of HCl were added to completely dissolve the PdCl2 precursor. The solution was then added to the gold chloride solution that prepared in the first step followed by the reduction of bimetallic solution using a mixture of 1 % tannic acid and 1 % of sodium citrate under stirring at 60 °C. After that, the resulting materials from step-1 and -2 were deposited onto the chosen oxidic support (i.e. anatase TiO2). Afterwards, the resulting slurry was stirred for 2 h at room temperature and then the excess solvent was removed using rotary evaporator. The obtained solid was oven dried at 120 °C for 16 h and then calcined at 350 °C for 5 h in air. In a similar way, another metal such as Ag was doped separately as second metal using similar preparation method. 2.3 Catalyst Characterization 2.3.1 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) ICP-OES was used to determine the percentage of the metals component in the catalysts samples. ICP results were obtained by PERKIN ELMER instrument (model: optima 3000XL), using a microwave pressure digestion (MDS 200; CEM) with hydrofluoric and aquaregia.

2.3.2 BET Surface Areas and Pore Volumes Brunauer, Emmett, and Teller surface areas (BET-SA) and pore size distribution of the catalysts were obtained on Micrometrics Gemini III-2375 (Norcross, GA, USA) instrument by N2 physisorption at -196 °C. Prior to the measurements, the known amount of the catalyst was evacuated for 2 h at 150 °C to remove physically adsorbed water. 2.3.3 X-Ray Diffraction (XRD) XRD patterns were recorded for phase analysis and crystallite size measurement on a STADI P instrument with transmission geometry and equipped with a Ge primary monochromator with CuKa radiation in the 2h ranges from 5 to 60° and a position-sensitive detector. The diffraction patterns of the crystalline phases were compared with those of standard compounds reported in the JCPDS Date files. 2.3.4 Transmission Electron Microscopy (TEM) The size and morphology of catalysts were investigated using a JEM-2100F high-resolution electron microscopy at a voltage of 200 kV. Sample preparation of supported AuNPs samples were dispersed in water/methanol and treated with ultrasound for 5 min, and then deposited on a carbon coated grid. 2.4 Catalytic Tests Catalytic tests were carried out under pressure using a Parr autoclave (100 ml) according the procedure described below. In a typical experiment, the reaction mixture consists of 0.4 g of supported metal catalyst, 5 mL of CH, 25 mL of acetonitrile as solvent, and 0.1 g of tert-butyl hydroperoxide (TBHP). These components were taken in an autoclave and flushed three times with O2 before setting the initial reaction pressure of O2 to 10 bar. Concerning the start-up procedure, the tests were performed by opening the O2 line to the autoclave, and the set up works in such a way that once the O2 released is consumed, it will be automatically replaced from the cylinder in order to maintain the overall pressure constant. The stirring speed of reaction mixture was set to 1500 rpm in general and the reaction was performed at 150 °C for 4 h unless otherwise stated. At the end of the reaction, the solid catalyst was separated by centrifugation. The identification of the reaction products from the oxidation of CH was confirmed by off-line gas chromatograph (Agilent 6890 N) fitted with a HP-5 column and a flame ionization detector (FID).

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3 Results and Discussion 3.1 Catalysts Characterization 3.1.1 BET Surface Areas and Pore Volumes Gold-(Pd or Ag) bimetallic catalysts loaded on TiO2 are studied with the same metal contents, i.e. 1 wt% Au and 1 wt% second metal. BET surface area, pore volumes and ICP values of these catalysts are presented in Table 1. As shown in this Table, ICP results confirmed that nominal and actual loading of bimetallic catalysts were comparable values (0.9–1.2 wt% Au, Pd, and Ag). In addition, it is clear from the Table 1 that the surface areas and pore volumes are observed to change significantly by changing the nature of metal. Between the two, AuPd/TiO2 showed relatively higher surface area and pore volume than those for AuAg/TiO2. 3.1.2 X-ray Diffraction (XRD) Analysis The XRD patterns of the bimetallic catalysts are shown in Fig. 1a, b. As shown in the figure, Au–Pd catalysts supported on TiO2 did not show any reflections belonging to either Au or Pd metallic phases due to their low concentrations. This result suggests that the metal particle size of the prepared catalysts seemed to be too small to be detected by XRD, probably they are X-ray amorphous. This observation also lent reasonable support to the observations made by TEM that the particle size of metals is found to be relatively small. In addition, Au–Pd catalysts revealed only reflections that correspond to TiO2. In contrast, TiO2 supported 1 % Au–1 % Ag catalyst, showed very weak reflection corresponding to Au metallic phase besides the typical TiO2 (anatase) reflections. 3.1.3 Transmission Electron Microscopy (TEM) Study TEM studies were used to explore the properties of supported bimetallic catalysts since the catalytic activity of noble metals catalyst (e.g. Au, Pd, etc.) is strongly dependent on its particle size [22, 23]. Electron micrographs of the Au–Pd and Au–Ag catalysts supported on Table 1 BET surface areas and ICP results of Au-bimetallic (1 wt% each) catalysts over TiO2 (anatase)

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Entry

Catalyst

Fig. 1 XRD patterns of different bimetallic catalysts (a AuPd/TiO2, b AuAg/TiO2)

TiO2 are depicted in Fig. 2a, b, respectively. It should be noted that the size distribution of both bimetallic system (inset of Fig. 2a, b) were calculated by measuring almost hundred individual metal particles (i.e. M = Au, Pd, Ag) using ImageJ software on digitized micrographs. The metal diameter was calculated using the following formulae: dM = Rni di/Rni, where ni is the number of metal particles of diameter di. All these bimetallic systems showed a narrow size particle distribution (inset of Fig. 2a, b) but varying size of the noble metal particles, which again depended on the type of metal used. TEM image in the case of Au–Pd catalyst (Fig. 2a) indicates that the particles supported on TiO2 are nearly spherical and the average particle size ranges between 1 and 5 nm with the maximum close to 2.5 nm. The (111) lattice spacing of Au and Pd are also determined to be 0.234 and 0.223 nm, respectively, as shown in Fig. 2c. Bulk Au and Pd have lattice spacing of 0.236 and 0.225 nm, respectively. On the other hand, the size of Au–Ag bimetallic particle supported again on TiO2 exhibited somewhat bigger particles than that for Au–Pd catalyst, which vary between 2 and 9 nm, as shown in Fig. 2b. Additionally, the lattice spacing calculated from the HRTEM for Au–Ag/TiO2 solid (Fig. 2d) has shown the value of about 0.230 nm, which lies between the Ag (111) plane (0.226 nm) and the Au (111) plane (0.235 nm). Such

Catalyst composition (ICP) Au (wt%)

Pd/Ag (wt%)

BET-SA (m2/g)

Pore volume (cm3/g)

1

Au/TiO2

0.9

–

43

0.12

2

Pd/TiO2

–

1.0

55

0.80

3

Ag/TiO2

–

1.0

39

0.90

4

AuPd/TiO2

1.1

1.2/–

38

0.07

5

AuAg/TiO2

1.2

–/1.2

31

0.05


Fig. 2 TEM (a Au–Pd and b Au–Ag) and HR-TEM (c Au–Pd and d Au–Ag) images of Au–Pd and Au–Ag catalysts supported on TiO2 (anatase). EDX spectrum of AuPd/TiO2 (inset a) and AuAg (inset

b) corresponding catalysts. Histogram showing particle size of AuPd (inset a) and AuAg (inset b) supported on TiO2

distortions in the value of lattice fringes provide some indirect hints for a probable alloy state, which is more likely amorphous in nature. However, such alloying is not confirmed from XRD patterns of the present study probably due to the presence of considerably small amounts of metal contents and their possible amorphous nature. Nonetheless, such alloying possibility is more likely in case of Pd–Au sample. Furthermore, the corresponding EDX patterns for some representative particles are shown in the insert images of Fig. 2a, b of the Au–Pd and Au–Ag catalysts supported on TiO2.

range of 2–4 % after 4 h of reaction and interestingly no AA was detected in the product stream. This result clearly indicates that (i) no significant reaction takes place under the conditions applied and (ii) the nature of catalyst plays a key role on the catalytic activity. To investigate the influence of the second-metallic element (i.e. bimetallic system) on the catalytic performance in the direct oxidation reaction of CH, a set of experiments were also performed under identical reaction conditions. For these tests, four different catalysts; pure TiO2 support, monometallic systems (e.g. 1 % Au/TiO2 and 1 % Pd/TiO2) and bimetallic system (1 % Au–1 % Pd/ TiO2) were used. The results of these investigations are portrayed in Fig. 3. As expected, the pure TiO2 was found to show the poorest performance, while the Au/TiO2 displayed considerably improved catalytic activity (i.e. X–CH = ca. 25 %, and S–AA = 26 %). In addition, the monometallic Pd catalyst (i.e. Pd/TiO2) exhibit somewhat inferior performance (X–CH = 16 %, S–AA = 18 %) compared to Au/ TiO2 solid. Interestingly, the combination of Au and Pd (PdAu/TiO2) markedly improved the selectivity of AA from 26 to ca. 34 %, which is almost double to the S–AA obtained on

3.2 Catalytic Tests 3.2.1 Au–Pd Bimetallic System Prior to perform the catalytic tests, some blank tests were carried out initially under similar reaction conditions as that of real tests to determine whether the oxidation of CH can take place in the absence of catalyst and/or TBHP/O2. Such blank tests showed almost negligible CH conversion in the

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Fig. 3 Comparison of the catalytic activity of mono-metallic (Au, Pd) and bi-metallic (Au–Pd) catalysts on the oxidation of cyclohexane. (X conversion, S selectivity, Y yield, X–CH conversion of cyclohexane, S–AA selectivity of adipic acid, S–One selectivity of cyclohexanone, S–Ol selectivity of cyclohexanol, Y–AA yield of adipic acid)

Pd/TiO2 and also remarkably higher even when compared to monometallic Au/TiO2 catalyst. Furthermore, the conversion of CH (X = 21 %) obtained on bimetallic Pd-Au/TiO2 was significantly higher than monometallic Pd/TiO2 but slightly lesser than monometallic Au/TiO2 sample. Moreover, the highest amount of undesired side-products such as CO and CO2 were observed mainly on pure TiO2 support where the sum up selectivity has gone up to 35 % on TiO2 and considerably less on all other metal catalysts. Considering all these effects, it can be claimed that the addition of second metallic component has a clear promotional effect on the selectivity of AA, which might be due to expected synergistic effects between Pd and Au and also the formation of small Au nanoparticles (AuNPs). However, the precise role of the second metal is still a matter of discussion, and hence, further studies are necessary for better understanding of the properties of bimetallic catalysts. 3.2.2 Au–Ag Bimetallic System Besides the application of Pd as a second metal, subsequent studies were focused on the usage of another potential noble metal such as Ag and checked its influence on the catalytic performance. Keeping this aspect in mind, some additional tests were also performed using Ag as a second metal, i.e. Au–Ag/TiO2 bimetallic catalyst. Similar to the above approach, the influence of monometallic (here it is 1 % Ag) and bi-metallic catalysts (Au–Ag/TiO2) in the CH oxidation were investigated and compared in Fig. 4. The catalytic activity of both pure support and Au/TiO2 were already discussed above; nonetheless, for the sake of better

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Fig. 4 Comparison of the catalytic activity of the mono-metallic (Au, Ag) and bi-metallic (Au–Ag) catalysts in the oxidation of cyclohexane, (X conversion, S selectivity; Y yield). (For all other denotations one can refer to Fig. 3)

comparison such results are also shown here again in Fig. 4. The results illustrate that the reaction performed using 1 % Ag/TiO2 catalyst has shown considerable influence on the activity and selectivity behavior compared to Au/TiO2 (Fig. 3). In fact, Ag/TiO2 displayed poor performance in terms of conversion of CH and selectivity of AA and cyclohexanone. Nevertheless, it has given comparable selectivity of cyclohexanol. On the other hand, the combination of Ag and Au (i.e. 1 % Au–1 % Ag/TiO2) enhances the conversion of CH and selectivity of AA and cyclohexanone compared to monometallic Ag/TiO2 solid. The activity and selectivity properties of bimetallic Au– Ag/TiO2 suggests that the presence of Ag has a certain influence on the catalytic performance but in a different direction compared to Pd as a second metallic component. Using Ag as second metal, not only the selectivity of KA oil (S = 82 %) is improved compared to all mono-metallics (i.e. Ag/TiO2, Pd/TiO2 and Au/TiO2), but also the conversion of CH increased. Nevertheless, the influence of Ag addition on the selectivity of AA is not that bad (S– AA = 14.3 %), which accounts a yield of ca. 4 % AA. The missing selectivity on mono-metallic Ag/TiO2 catalyst is mainly due to CO and CO2 formation. Considering the effects on the whole, it can be stated that both these metallic components (Pd, Ag) have shown different influences on the catalytic performance, and thereby, different distribution of products. 3.2.3 Stability of Bimetallic System Recovery and reuse of the catalyst is an interesting issue in the industrial and environment point of view. To study the


might be due to either leaching of metal components or deactivation by coke or due to marginal loss of catalyst weight during the process of recovery of the catalyst. The third proposition is more likely to occur because there will be certainly a handling loss of catalyst recovery from first cycle to last cycle. This is also evidenced from observed weight loss in the recovered catalyst, i.e. there is a slight decrease in the weight of recovered catalyst sample from run to run.

4 Conclusions In summary, bimetallic system containing AuNPs supported on TiO2 (anatase) displayed reasonably good activity in the oxidation of CH to AA. It can also be stated that the product distribution certainly depended on the type of noble metal used in the catalyst composition. Furthermore, bimetallic catalysts (in particular Pd containing Au catalyst) improved selectivity towards AA compared to its mono-metallic parent sample. The combination of Au and Pd (Au–Pd/TiO2) markedly improved the selectivity of AA from 26 to ca. 34 %, which was almost double to the S–AA obtained on Pd/TiO2 and also remarkably higher than that of monometallic Au/TiO2 catalyst. On the whole, the presence of Pd (as a second metal) is crucial for enhancing the selectivity of AA, while Ag is important for improving the selectivity of KA-oil. Fig. 5 Recycling results of AuPd/TiO2 (a) and AuAg/TiO2 (b) catalysts for the oxidation reaction of cyclohexane, key as for Fig. 3

reusability and stability of the evaluated bimetallic catalysts, recycling tests and catalyst washing experiments were carried out and the results are portrayed in Fig. 5. After the first run, both bi-metallic catalysts were filtered in order to separate the solid catalysts, washed, dried at 120 °C and then subjected to the second run under the same reaction conditions. In a similar way, the catalysts were used for four such cycles. It is observed in case of AuPd/TiO2 catalyst that a slight decrease in the conversion of CH and the selectivity of products is observed after first run. However, as shown in Fig. 4, this catalyst could be used frequently for at least 3 times without significant decrease in the activity and selectivity. Interestingly, the selectivity to ‘‘–Ol’’ remarkably decreased from first run to second run and then remained relatively constant. On another hand, we could clearly observe in case of AuAg/ TiO2 catalyst that a remarkable decrease in the conversion of CH and the selectivity of products after four runs. This result indicates that AuAg/TiO2 catalyst is less stable compared to AuPd/TiO2. We can conclude that the catalyst activity decreased to some extent after four runs, which

Acknowledgments The authors gratefully acknowledge King Abdulaziz City for Science and Technology (KACST) for financing this work. The authors also thank Alromaeh (KACST) for TEM analysis and analytical staff of LIKAT for various solid-state characterisations.

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