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Pd, Pt and Pt-Cu catalysts supported on CNT for the selective oxidation of glycerol in alkaline and base-free conditions Lucília S. Ribeiro, Elodie G. Rodrigues, Juan Jose Delgado, Xiaowei Chen, Manuel Fernando R. Pereira, and José J.M. Órfão Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01732 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 21, 2016

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Industrial & Engineering Chemistry Research

Pd, Pt and Pt-Cu catalysts supported on CNT for the selective oxidation of glycerol in alkaline and base-free conditions

Lucília S. Ribeiro1, Elodie G. Rodrigues1, Juan J. Delgado2, Xiaowei Chen2, M. Fernando R. Pereira1, José J.M. Órfão1,*

1

Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and

Materials (LSRE-LCM), Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal 2

Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química

Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Campus Rio San Pedro, 11510 Puerto Real, Cádiz, Spain

[email protected], [email protected], [email protected], [email protected], [email protected], [email protected]*

*Corresponding author: José J.M. Órfão Tel. +351 225 081 665 Fax: +351 225 081 449 1 ACS Paragon Plus Environment


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Abstract The liquid-phase oxidation of glycerol was studied to obtain value-added oxidation products such as dihydroxyacetone or glyceric acid. The effect of the reaction conditions was studied for carbon nanotube supported Pd and Pt catalysts. Under strongly alkaline conditions, conversions close to 90% were reached after 5 h of reaction, with selectivities to glyceric acid around 60-70%. Carbon nanotube supported bimetallic Pt-Cu catalysts were prepared and tested in the reaction under base-free conditions, where, besides glyceric acid, there was the formation of dihydroxyacetone and glyceraldehyde. Bimetallic Pt-Cu/CNT was found to be more effective than monometallic Pt/CNT towards the selective glycerol oxidation.

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1

Introduction

Recently, the depletion of fossil fuels and environmental degradation has caused growing concerns of a world energy crisis 1. Biodiesel appears as an important renewable biofuel, and can be produced from the transesterification of animal fats and plant oils, thus helping to decrease the dependence on fossil energy 2. As a side product of biodiesel production results glycerol, which is a key platform chemical for the production of high-added value compounds2-5. Among various alternatives to convert glycerol, catalytic oxidation has been appointed as an attractive process, since oxygen is readily available and also inexpensive 2, 4-6. Catalytic liquid phase glycerol oxidation can be quite complex due to the great amount of products that can be produced (Figure 1) 79

, being a promising process if the catalyst used is sufficiently active and selective

towards the products of interest. Among the different products, glyceric acid (GLYA) and dihydroxyacetone (DHA) are the most important due to their potential use as intermediates in the fine chemicals industry, particularly in pharmaceutics 10. Oxidation of the terminal hydroxyl groups gives GLYA and glyceraldehyde (GLYDH) 9, while the oxidation of the secondary hydroxyl group of glycerol gives DHA (Figure 1). The selective oxidation of glycerol with heterogeneous noble metal catalysts (i.e. Pd, Pt, and Au based catalysts) combined with oxygen is a highly appealing sustainable system for glycerol oxidation 4, 11. Figure 1 The selective liquid phase glycerol oxidation over heterogeneous noble metal catalysts in oxygen or air was initially tested by Kimura 12 and Garcia’s groups. 13. Using a Bi-Pt catalyst, they found glyceric acid as main product in alkaline conditions and dihydroxyacetone in base-free conditions. From then, this process has gained much



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attention due to its potential economic and environmental benefits 14. Glycerol oxidation has already been tested using several metals as catalysts, such as Pt and Rh

2, 5, 14-15

, Au

10, 16-18

19

. Kimura et al. found that the selectivity to DHA could be greatly increased

(from 10 to 80%) by the addition of Bi to Pt catalysts

12

, and since then many efforts

have been dedicated to the optimization of the metal-promoted catalysts and corresponding reaction conditions. Promoters, such as Bi 11-12, 20 , Pb 21, Sb 8, Ag 6, Te 22 and Sn

23

catalysts

have shown an important impact on the activity and selectivity of these

24

. Likewise, for the electrocatalytic oxidation of glycerol such changes on

activity and selectivity have also been verified when modifying Pd and Pt catalysts with Bi25-26 and Sn27-28. The activity and the oxidation products distribution depend on the reaction conditions (such as temperature, pressure, pH) and the catalyst (metal, support). The main product of glycerol oxidation using activated carbon supported Pt is GLYA 13, but palladium catalysts in alkaline conditions can become more selective to GLYA than platinum catalysts 29, selectivities of 70% at 90% conversion being achieved 13. A high glyceric acid yield (83%) was attained over 1%Au/C in 0.6 M NaOH 30. Without base addition, Au based catalysts were not active for glycerol oxidation 31. Under base-free conditions, Pt catalysts supported on carbon nanotubes and carbon nanofibers were more active than Pt/AC due to easier accessibility of reactants to Pt on the outer walls of the tubes

5, 32

. Many efforts have been dedicated to the development of efficient

catalysts for base-free glycerol oxidation 2, 4-5. As reported by Zope et al., in the absence of a base, aldehyde and ketone are the main products detected over Pt- and Pd-based catalysts, rather than acids 33. Among the noble metals tested so far, Pt showed to be the most active for glycerol oxidation. Also, alloying metals with platinum allowed reaching higher activities and resistances to deactivation than the corresponding monometallic catalysts, as well as tuning the selectivity to specific oxidation products 4,


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29, 34

. For example, Pt-Au catalyst enhanced the selectivity towards DHA when

compared to the respective monometallic Pt catalyst (from 13 to 21%)

35

. Despite the

interesting result obtained, Au is quite an expensive metal for alloying with Pt, and so the search for optimal and less expensive metals to alloy is of great importance. Liang et al. reported a Pt-Cu catalyst supported on activated carbon that yielded 62% of glyceric acid in base-free conditions36. As far as we are concerned, Pt-Cu/CNT has not yet been reported for glycerol oxidation in base-free conditions. So, in this work, carbon nanotube supported Pt-Cu catalysts were prepared by incipient wetness impregnation and directly compared in glycerol oxidation under highly alkaline and base-free conditions. Different reaction conditions (temperature, pressure, pH), as well as the catalysts stability, were studied for this system.

2 2.1

Experimental Materials and methods

2.1.1 Materials Unless stated otherwise, chemicals were purchased from commercial sources and used as received. The metal precursors PdCl2 (99% ACS, Pd min 59.5%), H2PtCl6·6H2O (99.9% ACS, Pt min 37.5%) and Cu(NO3)2·3H2O (>98%) were supplied by Alfa Aesar. NaOH (>97%), glycerol (99.5%), oxalic acid (99%) and glyceraldehyde (99%) were provided by Sigma-Aldrich. Glycolic acid (99%) and formic acid (98%) were obtained from Fluka and glyceric and tartronic acids (98%) from Alfa Aesar. Dihydroxyacetone (98%) and sulfuric acid (96%) were obtained from Merck and Panreac, respectively. Carbon nanotubes Nanocyl-3100 were supplied by Nanocyl. 5 ACS Paragon Plus Environment


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2.1.2 Preparation procedures Commercial Nanocyl-3100 carbon nanotubes were used as support for this work. According to the supplier, this material has an average length and external diameter of 1.5 µm and 9.5 nm, respectively, and a carbon purity of at least 95%. Pd, Pt and Pt-Cu catalysts were prepared by incipient wetness impregnation of carbon nanotubes with solutions of the corresponding metallic precursors. The amount of metal was determined in order to attain a nominal metal loading of 1% wt. of Pd or Pt. For the preparation of bimetallic catalyst a Cu loading of 0.65% was selected. The support (CNT) was primarily introduced in an ultrasonic bath for 30 min. Secondly, the precursor solution was added dropwise, with a peristaltic pump (50 mL·h-1), until all the support was wet. After more 90 min in the ultrasonic bath, the catalyst was dried in an oven at 110 ºC for 24 h and subsequently stored in a desiccator for posterior use. After heat treatment under nitrogen flow for 3 h (50 cm3·min-1), the catalysts were activated by reduction under hydrogen flow for 3 h (50 cm3·min-1). The appropriate reduction temperature (250 and 350 ºC for palladium and platinum catalysts, respectively) was determined by temperature programmed reduction (TPR) (see Section 3.1). The heat treatment under N2 flow was performed at the same temperature of the reduction. The prepared catalysts were denoted as 1%Pd/CNT, 1%Pt/CNT and 1%Pt-0.65%Cu/CNT.

2.2

Characterization of materials

Both support and catalysts were characterized by N2 adsorption at -196 ºC. The catalysts were also characterized by TPR, H2 chemisorption and transmission electron microscopy (TEM). 6 ACS Paragon Plus Environment

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Support and catalysts were characterized by N2 adsorption at -196 ºC using a Quantachrome NOVA Surface Area and Pore Size analyser. Surface area calculations were made using the Brunauer-Emmett-Teller (BET) equation, and the micropore volumes (Vmicro) and mesopore surface areas (Smeso) were obtained by the t-method. The most frequent mesopore diameter (dBJH) and the pore volume (VBJH) were determined from the isotherm desorption branch using the Barrett, Joyner and Halenda (BJH) method. TPR allows to find the most suitable reduction temperature for each material. TPR profiles were acquired using a fully automated AMI-200 equipment (Altamira Instruments). About 150 mg of the catalyst were introduced into a U-shaped quartz reactor and heated to 700 ºC at 5 ºC·min-1 heating under 5% (v/v) H2 flow diluted in air (total flow rate = 30 cm3·min-1). The reactor effluent composition was followed by a thermal conductivity detector. H2 consumption peaks directly show the range of temperature in which the reduction occurs. H2 chemisorption tests were performed using the AMI-200 equipment mentioned above, according to the pulse method and considering a stoichiometric relation of metal atoms/H2 =2. Pulses of 58 µdm3 (from a calibrated loop) were injected into the carrier gas (25 cm3·min-1 of Ar), passing through the catalyst bed (150 mg) until its saturation. The amount of gas adsorbed was obtained by calculating the peak area corresponding to each injection and subtracting the sum of these values from the total H2 amount injected. TEM experiments were performed on a JEOL2010 field emission gun instrument equipped with an EDXS spectrometer Oxford INCA Energy 2000 system. Particle size distributions were determined by measuring at least 100 particles, and the average



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diameter was determined by = ∑ / ∑ ( is the number of particles with

diameter ).

2.3

Catalyst evaluation

Glycerol oxidation was studied in a 350 mL stainless steel reactor equipped with a nitrogen and oxygen supply system, a manometer, a thermocouple, a magnetic stirrer and a sample outlet. In standard tests, 150 mL glycerol solution (0.3 mol·L-1), 45 mL NaOH (2 mol·L-1) and 700 mg of catalyst were placed inside the reactor under stirring at 1000 rpm. NaOH/glycerol molar ratio of 2 allowed maintaining a pH of about 13. The reactor was then sealed and pressurized with N2 at 3 bar. Once the desired temperature (60 ºC) was achieved, the reaction was started by pressurizing the system with oxygen (3 bar). The pressure was kept constant by feeding O2 continuously. Samples (0.6 mL) were periodically taken for analysis. After reaction (7 h), the catalyst was filtered and the reaction solution was analysed by atomic absorption spectroscopy (UNICAM 939/959) to test for metal leaching to the solution. Different reaction conditions were evaluated (temperature, pressure, pH). The quantification of the reaction products was performed by high performance liquid chromatography (HPLC). The chromatograph (Elite LaChrom HITACHI) was equipped with an ultraviolet (210 nm) and a refractive index (RI) detectors and the products were separated using an ion exclusion column (Alltech OA-1000). A 5 mM H2SO4 solution mobile phase was used as eluent with a 0.5 mL·min-1 flow rate. An injection volume of 23 µL and a measuring time of 20 min were selected. Dihydroxyacetone (DHA), glyceraldehyde (GLYDH) and oxalic (OXA), formic (FORM), glyceric (GLYA), glycolic (GLYCOA) and tartronic (TART) acids were the products detected. The 8 ACS Paragon Plus Environment

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selectivities (Si) into the different products i at time t were determined using the equation: =

(1)

∙ ∙

where Ci is the concentration of the product i (mol·L-1), C0 is the initial glycerol concentration (mol·L-1), X is the glycerol conversion and νi corresponds to the moles of product i produced per mol of glycerol consumed, according to the stoichiometry. The yields (Yi) were obtained as: = 3 3.1

= ∙ (2)

∙

Results and discussion Characterization of materials

Table 1 summarizes the textural properties of the support (CNT) and the 1%Pd/CNT catalyst, determined by the corresponding N2 adsorption isotherms. The materials present surface areas close to 300 m2·g-1, and are non-microporous. Most of the porosity observed corresponds to large mesopores that result from the free space in carbon nanotube bundles. As expected, no significant differences in the materials textural properties were detected. Only a small decrease in the surface area was noticed with the introduction of the metallic phase, and then it was considered that the catalysts textural properties did not significantly differ from those of the support. Table 1 The reducibility of the prepared catalysts was studied by TPR. Figure 2 shows that the reduction temperatures of carbon nanotubes supported Pd and Cu monometallic catalysts are around 150 and 250 ºC, respectively. According to the TPR profile it is not 9 ACS Paragon Plus Environment


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possible to discern the decomposition of Pd β-hydride, which presents a hydrogen desorption peak at about 60 ºC 37, indicating that Pd is well dispersed on the support. Pt mono- and bimetallic catalysts profiles show wide reduction peaks between about 125 and 250 ºC. A reduction peak around 240 ºC is expected for pure CuO; depending on the particle size and its interaction with the support, supported CuO can present higher reduction temperatures 37. For the bimetallic catalyst, the reduction happens around 200 ºC, which can be a result of the reduction of Cu oxides promoted by the presence of Pt 37

. The decrease in the reduction range of supported Cu in the bimetallic catalyst reveals

the achievement of a close proximity between copper and platinum species. Accordingly, Pd and Pt catalysts were reduced at 250 and 350 ºC, respectively, to assure total reduction of the metal. Figure 2 The metallic dispersion and average particle diameter of the catalysts were determined by H2 selective chemisorption (pulse method). The volume of H2 adsorbed by the catalyst was determined from the number of pulses injected and the area of the resulting peak: = −

$

! % (3) !"#

being the number of the pulses injected, Vpulse the H2 pulse volume, Ai the peak area for each pulse and Amax that for non-adsorbed pulses. Pd and Pt monometallic catalysts presented similar metal dispersions of about 35% and particle diameters around 3 nm (Table 2). Table 2


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TEM micrographs (Figure 3) were obtained to get information about the metal particle size distributions. Small spherical particles could be observed for all the catalysts. The average crystallite sizes of the catalysts are presented in Table 2 and do not differ much from those obtained by H2 chemisorption. As can be seen in the corresponding histogram (Figure 3c), the addition of Cu to the Pt catalyst causes some slight widening of the particle size distribution and increase of the average particle size to 5.4 nm in comparison to 4.6 nm of the Pt monometallic catalyst (Table 2). Figure 3

3.2

Catalytic experiments

3.2.1 Comparison and stability of monometallic catalysts To study the influence of several parameters in the oxidation of glycerol and compare the performance of different catalysts, two different metals (Pd and Pt) were supported on carbon nanotubes using the same method. Moreover, the same reaction conditions were kept constant in all tests. Glycerol conversion was studied using typical conditions: 3 bar of O2, 60 ºC, 150 mL of glycerol 0.3 mol·L-1, NaOH/glycerol = 2 mol/mol and 700 mg of catalyst. To test the catalyst deactivation, four successive tests were performed at similar conditions, after washing and drying the used catalyst. Because of some catalyst losses during filtration at the end of the reaction, a minor amount of fresh catalyst (< 5% wt.) was added to the reactor before each run. After each run (7 h reaction), the catalyst was separated and reused without any further treatment by adding a fresh glycerol solution. The reaction solution obtained after filtration was analysed by atomic absorption spectroscopy to test for metal leaching to the solution. Both palladium and platinum were detected (leaching 11 ACS Paragon Plus Environment


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< 5% after the four cycles), which means that the metal particles do not interact strongly with the support. Figure 4 shows that, under these conditions, both catalysts are very stable and reusable, at least up to the 4th cycle. It also appears that 1%Pd/CNT has a better stability and resistance to deactivation than 1%Pt/CNT. The conversion of glycerol attained after 7 h of reaction varied from 90-93% and 87-89%, with selectivities to glyceric acid (GLYA) around 65 and 60%, for palladium and platinum catalysts, respectively. Figure 4 No major differences were observed both in terms of catalyst activity and products distribution for each catalyst. Glycerol oxidation originates several products such as glyceric acid, glyceraldehyde, dihydroxyacetone, tartronic acid, hydroxypyruvic acid or glycolic acid (Figure 1), depending on the oxidation degree. GLYA was the main product in all the tests performed under strongly alkaline conditions. For 1%Pd/CNT, there was a slight increase in glyceric acid selectivity (62 to 67% within the 4 cycles) corresponding to a decrease in the selectivities to tartronic (TART) and formic (FORM) acids. This variation was more pronounced from the first to the second cycle. On the opposite, 1%Pt/CNT seems to favour the production of formic acid over tartronic acid. The selectivity to glyceric acid decreased about 5-10%, relatively to the palladium catalyst, while there was an increase of about 7-10% in the selectivity to glycolic acid (GLYCOA). Thus, it can be concluded that in strongly alkaline medium 1%Pd/CNT is slightly more favourable to the formation of glyceric acid than 1%Pt/CNT.


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3.2.2 Influence of the reaction temperature The influence of the reaction temperature was tested for values between 50 and 80 ºC. The evolution of the conversion of glycerol at different temperatures is shown in Figure 5. An increase in glycerol conversion was detected with temperature increase. Also, glycerol conversion increased rapidly in the first two hours, and then increased slowly and gradually during the remaining time. However, the increase in the reaction temperature appears to be unfavourable to the selectivity of the product of interest, glyceric acid, which decreased due to C-C cleavage, as the selectivity to GLYCOA rose (Figure 6). These results are consistent with those obtained by Rodrigues et al.

18

and

Zhang et al. 2. Figure 5 Figure 6 The catalyst deactivation caused by oxygen poisoning at temperatures of 50 and 60 ºC inhibited the achievement of total conversion within the duration of the experiment. As there is higher oxygen dissolution at lower temperatures, it seems that the dissolved oxygen had an effect on the metal leaching, since this increased with the decrease of temperature and, therefore, with the increase of the dissolved O2. The variation of metal leaching with the temperature was more pronounced for 1%Pd/CNT (from 1% at 80 ºC to 10% at 50 ºC) than for 1%Pt/CNT (around 7% at all temperatures tested).

3.2.3 Influence of the initial NaOH/glycerol ratio The influence of the initial ratio between NaOH and glycerol concentrations, denoted as R, was examined varying it from 1 to 4 (Figure 7). There was an important reduction in



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glycerol conversion with the decrease in the initial NaOH/glycerol ratio (e.g. for 1%Pd/CNT there was a decrease from 98% at a ratio of 4 to 64% at a ratio of 1). Figure 7 From the analysis of the results presented in Figure 7, it appears that the distribution of products does not depend significantly on the NaOH/glycerol initial ratio when using 1%Pd/CNT as catalyst. For 1%Pt/CNT, the results show an important decrease in glyceric and tartronic acids selectivities and an increase in glycolic acid selectivity (C2 favoured relatively C3 products – see Figure 1) with a molar NaOH/glycerol initial ratio of 4.

3.2.4 Influence of oxygen pressure Tests were performed at different oxygen pressures between 0.5 and 6 bar. These tests were carried out with a catalyst amount of 400 mg. In the tests carried out at an oxygen pressure of 0.5 bar, a low initial rate was noticed; moreover, the rate remained practically constant during the experiment (Figure 8). At an oxygen pressure of 6 bar, the opposite was observed for both catalysts (i.e. high initial activity and progressive rate decrease). These results are in accordance with those obtained by Rodrigues et al.

19

on activated carbon supported Rh catalysts, and may be

explained by the easier catalyst deactivation by metal over-oxidation at higher oxygen pressures29. Figure 8 The results obtained for metal leaching seem to confirm the influence of the dissolved O2 in leaching, as described before with the temperature variation. In fact, metal leaching to the solution increases with the increase of oxygen pressure (from 1-10% and 14 ACS Paragon Plus Environment

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11-15% for palladium and platinum catalysts, respectively). This effect of the oxygen partial pressure in leaching is conforming with the data obtained by Jun et al. 38 for tin. Glyceric acid selectivity considerably decreases with the decrease in the pressure of oxygen, due to the formation of glycolic acid (Figure 9). This difference in the products distribution is much more significant for 1%Pt/CNT. Although the initial reaction rate is higher when using an oxygen pressure of 6 bar, it may be more advantageous to carry out the reaction at lower oxygen pressures. In this case, the glycerol conversions attained for longer reaction times are higher, since there is less catalyst deactivation and metal leaching to the solution. The glyceric acid selectivity is also important to consider in the optimization of the operating conditions. An oxygen pressure of 3 bar seems to be the one that best matches the high initial activity with the reasonable resistance to catalyst deactivation by metal over-oxidation. Figure 9

3.2.5 Influence of the solution pH and bimetallic catalysts The palladium catalyst was not active for glycerol oxidation without the presence of base. This behaviour could be assigned to the fact that Pd cannot promote the initial dehydrogenation (H-abstraction) step in glycerol oxidation. As reported in literature, the oxidation mechanism proceeds via dehydrogenation in aqueous solution, followed by hydride abstraction and successive oxidation by O2 34. On the other hand, Pt supported catalysts are still active for the base-free glycerol oxidation, but the reaction is much slower than under highly alkaline medium (Figure 10). When the reaction was performed without the addition of base, glycerol conversions of only 30% were achieved after 30 h of reaction, while in highly alkaline 15 ACS Paragon Plus Environment


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medium conversions close to 90% were easily achieved in 7 h. During base-free glycerol oxidation, the three main products quantified were glyceric acid, dihydroxyacetone (DHA) and glyceraldehyde (GLYDH) (Figure 11), but glycolic and tartronic acids were also obtained in less significant amounts (

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