Al2O3 catalysts. The role of interaction between palladium and copper on determining catalytic properties

Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

Hydrogenation of furfural over Pd–Cu/Al2 O3 catalysts. The role of interaction between palladium and copper on determining catalytic properties M. Lesiak a , M. Binczarski a , S. Karski a , W. Maniukiewicz a , J. Rogowski a , a,∗ ´ E. Szubiakiewicz a , J. Berlowska b , P. Dziugan b , I. Witonska a b

Institute of General and Ecological Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland Institute of Fermentation Technology and Microbiology, Lodz University of Technology, Wolczanska 171/173, 90-924 Lodz, Poland

a r t i c l e

i n f o

Article history: Received 23 June 2014 Received in revised form 16 August 2014 Accepted 26 August 2014 Available online 6 September 2014 Keywords: Biofuels Furfural reduction Pd–Cu/Al2 O3 Pd–Cu alloys

a b s t r a c t This paper studies the effect of copper on the activity and selectivity of home-made supported palladium catalysts during liquid phase hydrogenation of furfural. Bimetallic Pd–Cu/Al2 O3 catalysts containing 5 wt.% Pd and 1.5–6 wt.% Cu showed high activity. However, the incorporation of Cu limited the formation of tetrahydrofurfuryl alcohol in comparison to monometallic Pd/Al2 O3 catalysts. Activating the catalysts at 300 ◦ C in a hydrogen atmosphere led to the formation of Pd–Cu alloys, as confirmed by XRD, SIMS-ToF, SEM-EDS, FTIRS-CO and TPR-H2 . The composition of the alloys depended on the amount of copper incorporated into the catalysts. The incorporation of copper can therefore be assumed to modify the catalytic properties of bimetallic Pd–Cu/Al2 O3 systems used in the hydrogenation of furfural. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Furfural is a useful raw material from an industrial point of view [1,2]. Furfural production is based on the Quaker Oats process, which uses biomass such as corn cobs, wood, cottonseed hulls, or straw [1,3,4]. The hydrogenation of furfural yields valuable products, such as furfuryl alcohol (FA) and tetrahydrofurfuryl alcohol (THFA). These can be obtained through the reduction of furfural in the gas phase over catalysts such as: copper chromites [5,6], Raney Ni [7], Raney Co [8], Raney Cu [8], nickel amorphous alloy [9,10], Al, Fe or Mn-doped mixed copper–zinc oxides [11], or alternatively over homogeneous complexes of Ru, Rh or Pt [12,13]. These systems are often promoted using Na, Ca, Co, La or Ni [14–16]. Rao and Baker [2] used carbon-supported copper systems in the reduction of gas phase furfural into furfuryl alcohol, with 2methylfuran (2-MTHF) obtained as a second important reaction product. Bankmann et al. [8] have patented a copper catalyst containing an intimate mixture of copper (10–70 wt.%) and pyrogenic silica for the selective hydrogenation of furfural into FA. Their design specifies an average copper crystallite size of between 5 and 50 nm. In tests, the catalysts with the highest copper content

∗ Corresponding author. Tel.: +48 42 631 30 94; fax: +48 42 631 31 28. ´ E-mail address: [email protected] (I. Witonska). http://dx.doi.org/10.1016/j.molcata.2014.08.041 1381-1169/© 2014 Elsevier B.V. All rights reserved.

(70 wt.%) also exhibited the highest furfural conversion rate (99.4%). However, selectivity towards FA was considerably reduced in comparison to catalysts with lower copper contents. Nagaraja et al. [17,18] performed vapour phase reduction of furfural over Cu/MgO and Cu–MgO systems, and have shown that these catalysts demonstrate high furfural conversion (98%) and good selectivity towards FA (98%). Sitthisa et al. [19] used 10%Cu/SiO2 in gas phase hydrogenation of furfural. The main reaction product was FA with only a ´ et al. [20] describe the use small amount of 2-methylfuran. Kijenski of platinum catalysts in gas phase furfural reduction, with again the main product being furfuryl alcohol. Sitthisa and Resasco [21] have also compared the catalytic properties of supported copper catalysts with analogous palladium and nickel systems. In gas phase reactions on supported copper catalysts, the main product of furfural hydrogenation was furfuryl alcohol. However, furan was the main product when palladium or nickel systems were used. The authors conclude that the main route for reactions using supported palladium or nickel catalysts is decarbonylation. In a second study by Sitthisa et al. [22], the addition of copper to palladium supported catalysts caused a significant decrease in selectivity towards furan and increased selectivity towards furfuryl alcohol. The authors suggest this may have been due to the formation of palladium–copper alloys leading to more efficient hydrogenation of furfural, and limiting the decarbonylation of furfural to furan. In contrast, modifying nickel

338

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

catalysts by the incorporation of Fe leads to significantly lowerfurfural conversion and decreased formation of other C4 products and 2-methylfuran [4]. Because of their lower energy requirements and lower consumption of gas materials, the chemical industry is looking to find new liquid phase catalytic processes. Liquid phase hydrogenation of furfural is one area of particular interest. From the commercial point of view, the most important products of this process are furfuryl alcohol (FA) and tetrahydrofurfuryl alcohol (THFA). FA is mainly used for the preparation of resins (which accounts for more than 85% of world FA production), as a phenolic resin modifier and in the production of surface coatings, urea resins and solvents. FA is also used in the preparation of tetrahydrofurfuryl alcohol (THFA), lysine, vitamin C, lubricants, dispersing agents, plasticizers and rocket fuels [1,3,17]. THFA, meanwhile, is mainly used as a “green solvent” for fats and resins. An important use of THFA is in the production of crop sprayers and proprietary stripping formulations – for instance, to remove protective coatings, especially in the automotive industry [23]. THFA is also used as a precursor for other chemical compounds and as a binder in the catalysts of new pebble bed reactors [1]. Selective liquid phase hydrogenation of furfural to furfuryl alcohol has been performed over copper chromite (CuCr2 O4 ·CuO) catalysts [24–26], often promoted with oxides of alkali-earth metals (CaO, BaO, ZrO) [27,28]. Yan and Chen [29] produced Cu–Cr catalysts supported on hydrotalcite with Cu2+ /Cr3+ ratios of 0.5, 1 and 2. These systems, especially the catalyst with a Cu2+ /Cr3+ ratio of 2, showed good catalytic properties. Typically, reactions are conducted in organic solvents, at mild temperatures (<200 ◦ C) and with high hydrogen pressure (60–105 bar) [24,29,7,30,31]. Cu, Ni- or Co-based mono- or multimetallic catalysts, such as Raney Ni [7], Ni–B [32], Ni–Co–B [33], Co–B [34], have also been investigated for use in the hydrogenation of furfural. These catalysts, however, showed unsatisfactory selectivity towards furfuryl alcohol [7,32–34]. Among the monometallic heterogenous catalysts, copper catalysts have been shown to preferentially reduce C O bonds to produce FA, but these systems are easy deactivated [35]. Cu–Fe catalysts supported on hydrotalcite have been used successfully for the reduction of biomass-derived furfural into furfuryl alcohol [35]. A system described as CuO–CuFe2 O4 , in which the Cu2+ /Fe3+ ratio was 1 produced an FA yield of 75.5% with a furfural conversion of 92.8% [35]. Supported noble metal nanoparticles are often used at an industrial scale as the catalysts in hydrogen transfer reactions, for example in energy processing, fine chemical production and air pollution control. Catalysts based on nanoparticles of Pt and Pd [36,37], in some cases with the addition of other metals (Sn, Ge) [38,39] to improve activity and/or selectivity towards furfuryl alcohol, have also been investigated for use in liquid phase furfural hydrogenation. A nanoparticle palladium catalyst 5%Pd/Al2 O3 , prepared using the CO2 -assisted method, produced furfuryl alcohol with 84.9% selectivity at a mild temperature of 140 ◦ C [40]. Pd/Al2 O3 catalysts synthesized using the CO2 -assisted method also exhibited enhanced performance, compared with supported palladium catalysts prepared according to the traditional method. The addition of a second metal can improve the selectivity of palladium. In previous studies by the authors, the incorporation of metals such Ag, Bi, Cu, In, Te and Tl into supported palladium catalysts was shown to modify the activity and selectivity of these systems during hydrogen transfer reactions [41–46]. In light of this finding and the literature presented above, a subsequent investigation was conducted into the effects of incorporating copper into a Pd/Al2 O3 catalyst for use in the liquid phase hydrogenation of furfural. This paper presents the results of an analysis of the activity and selectivity of supported palladium catalysts modified with copper used in the liquid phase hydrogenation of furfural into

furfuryl alcohol. The phase composition of these bimetallic systems was also determined using TPR-H2 , XRD, SIMS-ToF, SEM-EDS and FTIRS measurements. 2. Experimental 2.1. Catalyst preparation A catalyst containing 5 wt.% palladium was prepared by the impregnation of Al2 O3 (Fluka, 143 m2 /g) with an aqueous solution of PdCl2 (POCH, anhydrous, pure p.a.), acidified to around pH 5 using HClaq (CHEMPUR, 35–38%, pure p.a.). The water was evaporated at an elevated temperature (T = 60 ◦ C) under a vacuum. Monometallic 5%Pd/Al2 O3 catalysts were dried in air at 110 ◦ C for 6 h, calcined at 500 ◦ C for 4 h in an oxygen atmosphere (O2 , Air Products, 99.5%, at a rate of 20 mL min−1 ), cooled in argon to room temperature (Ar, Linde 5.0, at a rate of 20 mL min−1 ), then reduced in an hydrogen atmosphere (H2 , Air Products, Premium Plus, 99.999%, at a rate of 20 mL min−1 ) for 2 h at 300 ◦ C before catalytic measurements were taken. The linear temperature increase rate was 20 ◦ C min−1 between each thermal processing step. A monometallic 5% Cu/Al2 O3 (wt.%) catalyst was prepared from a solution of Cu(NO3 )2 ·3H2 O (POCH, pure p.a.), according to the procedure described above. Bimetallic Pd–Cu/support catalysts containing 5 wt.% Pd and 1.5, 3, 6 wt.% Cu were prepared by co-impregnation of Al2 O3 with a water solution of Cu(NO3 )2 ·3H2 O (POCH, pure p.a.) and PdCl2 (POCH, anhydrous, pure p.a.) acidified to around pH 5 using hydrochloric acid (CHEMPUR, 35–38%, pure p.a.), according to the procedure described above. 2.2. Hydrogenation of furfural Hydrogenation of furfural in aqueous solution (0.1 M L−1 , 25 mL) was performed in a 50 mL autoclave (Parr Company) at a temperature of 90 ◦ C and under 20 bar of H2 pressure. The reactions were conducted with equal amounts of catalyst (mcat = 0.5 g) in each experiment. The mixture was stirred at 500 rpm. The autoclave was flushed with argon (Ar, Linde 5.0, at a rate of 20 mL min−1 , at 20 ◦ C, for 15 min) to remove the air. It was then flushed again with hydrogen (H2 , Air Products, Premium Plus, 99.999%, at 20 ◦ C, for 15 min). The autoclave was pressurized with hydrogen to 20 bar, and the temperature gradually raised to 90 ◦ C with a heating rate of 20 ◦ C min−1 . The reaction conditions were optimized for monoand bimetallic palladium catalysts. The reaction was sustained for 2 h. After the reaction, the autoclave was cooled to room temperature in a controlled manner using a water bath. The reaction mixture was filtered and analyzed using HPLC (LaChrome, Merck-Hitachi, column: Kromasil 100 C18, mobile phase: acetonitrile/phosphate buffer = 5:95 (v/v), pH = 4.5, Cphosphate = 0.01, UV:  = 210 nm) to determine the concentration of furfural. Products of furfural hydrogenation were also screened for using GC-FID analysis (Hewlett Packard 5890A; packed column 8% Carbowax 1540 on Chromosorb W; injection port temperature: 170 ◦ C, injection volume: 5 ␮l; FID detector temperature: 250 ◦ C; column oven temperature: 190 ◦ C; He (Linde, 99.999%): 30 mL min−1 ). The liquid products were in addition analyzed using a PerkinElmer GC–MS (model Clarus 580 with MS Clarus SQ 8 S) equipped with an Elite5MS capillary column (30 m length, 0.25 mm i.d. and 0.5 ␮m film thickness). The operating conditions of the GC–MS analysis were: Electron Impact at 70 eV; 35–350 m/z mass range; injection port temperature 250 ◦ C; interface temperature 300 ◦ C; column oven temperature programme: 35 ◦ C for 7 min, ramped at 3 ◦ C min−1 to 155 ◦ C, ramped at 20 ◦ C min−1 to 300 ◦ C with 3 min hold; helium

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

carrier gas (flow rate of 30 cm3 min−1 ); 1 ␮l injection volume; 1:200 split ratio. 2.3. Powder X-ray diffraction (XRD)

339

of both mono and bimetallic systems at a temperature of 40 ◦ C with a flow rate of CO 10 mL min−1 . The spectra were collected every 5 min (between the 5th and 35th minutes). CO desorption was then performed in an Ar stream (30 mL min−1 ) at the same temperature. 2.7. Scanning electron microscopy with X-ray microanalysis (SEM-EDS)

Room temperature powder X-ray diffraction patterns were collected using a PANalytical X’Pert Pro MPD diffractometer in the Bragg–Brentano reflection geometry. Copper CuK␣ radiation was use from a sealed tube. Data were collected in the 2 range 5–90◦ with a step of 0.0167◦ and an exposure per step of 27 s. The samples were spun during data collection to minimize preferred orientation effects. A PANalytical X’Celerator detector based on Real Time Multiple Strip technology and capable of simultaneously measuring intensities in the 2 range of 2.122◦ was used. For qualitative analysis and to estimate the size of crystallite ions, the PANalytical High Score Plus software package was used combined with the International Centre for Diffraction Data’s (ICDD) powder diffraction file (PDF-2 ver. 2009) database of standard reference materials.

SEM measurements were taken using an S-4700 scanning electron microscope (HITACHI, Japan), equipped with an energy dispersive spectrometer (Thermo Noran, USA). Images were reordered at several magnifications using a secondary electron (SE) detector or back-scattered electron (BSE) YAG detector. Energy Dispersive Spectrometry (EDS) enabled qualitative analysis of the elements present in micro-areas of the sample surface layer. A map was made of the distribution of elements on each of the studied micro-areas. The accelerating voltage was 25 kV. The samples were coated with a carbon target using the Cressington 208 HR system.

2.4. Temperature programmed reduction (TPR-H2 )

2.8. Time of flight secondary ion mass spectrometry (ToF-SIMS)

TPR-H2 measurements were carried out in an automatic TPR system AMI-1 (Altamira) in a temperature range of 25–900 ◦ C with a linear heating rate of 10 ◦ C min−1 . Prior to these measurements, the samples (weight around 0.1 g) were calcined at 500 ◦ C for 2 h in a mixture of oxygen–argon (10% O2 –90%Ar) at a flow rate of 40 mL min−1 . TPR runs were performed in the temperature ranges: TPR(1): 20–300 ◦ C; TPR(2): 20–900 ◦ C, using a mixture of hydrogen–argon (5% H2 –95% Ar) with a gas volume velocity of 40 mL min−1 . Hydrogen consumption was monitored using a thermal conductivity detector.

Secondary ion mass spectra and images were recorded using a ToF-SIMS IV mass spectrometer (Ion-Tof GmbH, Germany). The instrument was equipped with a liquid metal Bi3 + primary ion gun and a high mass resolution time-of-flight (ToF) mass analyzer. Images were recorded in burst alignment mode or extreme crossover mode, to provide high lateral resolution. Secondary ion mass spectra were recorded on areas of the sample surface approximately 100 ␮m × 100 ␮m. During measurement, the area was irradiated with pulses of 25 keV ions at a repetition rate of 10 kHz and with an average ion current of 0.5 pA. The analysis time was 30 s giving an ion dose below the static limit of 1 × 1013 ions/cm2 . Secondary ions emitted from the bombarded surface were mass separated and counted using the ToF mass analyzer. Spectra were recorded with a high mass resolution (m/m) at a 29 m.u. typically greater than 7100 and with a primary ion pulse width of 840 ps.

2.5. Chemisorption of CO measurements Analysis of carbon monoxide (CO, Linde, 99.999%) chemisorption was carried out in a Micromeritics ASAP 2020 apparatus. The samples had previously been reduced, under the same conditions used to prepare the catalysts (see Section 2.1). After reduction, the samples were cooled to room temperature under He stream (He, Linde, 99.999%, flow rate 30 mL min−1 ). The chemisorbed carbon monoxide was analyzed at room temperature using the adsorption–backsorption isotherm method. 2.6. Infrared Fourier transform spectroscopy (FTIRS) The infrared spectra of chemisorbed CO (Linde, 99.999%) were recorded using a Thermo Scientific Nicolet 6700 FTIR spectrometer equipped with a liquid nitrogen cooled MCT detector. A resolution of 4.0 cm−1 was used throughout. Before analysis, the catalyst samples were compacted into pellets (20 mg) and reduced at 300 ◦ C under an H2 (99.999%) stream for 1 h to clean the surface. The measuring cell was then purged with argon (99.999%) to remove the hydrogen. CO sorption experiments were conducted on the surfaces

Fig. 1. Selectivities to furfuryl alcohol and tetrahydrofurfuryl alcohol in the hydrogenation of furfural as a function of Pd–Cu/Al2 O3 catalyst composition.

Table 1 Activity and selectivity for hydrogenation of furfural over Pd/Al2 O3 , Cu/Al2 O3 and Pd–Cu/Al2 O3 catalysts after 2 h of reaction. Catalyst

Chemisorption of CO (␮mol/gkat )

Dispersion (%)

5%Pd/Al2 O3 5%Pd–1.5%Cu/Al2 O3 5%Pd–3%Cu/Al2 O3 5%Pd–6%Cu/Al2 O3 5%Cu/Al2 O3

109 43 27 30 21

23 9 6 6 4

d (nm) Chem. CO

XRD

5 – 20 19 –

7 9 25 21 –

X (%)

SFA (%)

STHFA (%)

TOF (h−1 )

100 100 99 94 81

28 41 48 56 100

72 59 52 44 0

23 58 92 78 96

Activation of catalysts: drying in air at 110 ◦ C, 6 h; oxidation in O2 at 500 ◦ C, 2 h; reduction in H2 at 300 ◦ C, 2 h. Reaction conditions: T = 90 ◦ C, mcat = 0.5 g, Vfurfural = 25 mL, Cfurfural = 0.1 M, pH2 = 20 bar.

340

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

Fig. 2. Chromatograms of reaction mixture after hydrogenation of furfural over mono- and bimetallic catalysts. Liquid products were analyzed with a PerkinElmer GC–MS (Clarus 580, MS Clarus SQ 8 S) equipped with a capillary column: Elite-5MS (30 m length, 0.25 mm i.d. and 0.5 ␮m film thickness).

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

2.9. Total organic carbon (TOC) Fresh as well as used 5%Pd/Al2 O3 , 5%Cu/Al2 O3 and 5%Pd–x%Cu/Al2 O3 (x = 1.5, 3, 6) catalysts were used in the activity tests and were tested for organic carbon, which would indicate the presence of degradation products of furfural. The total carbon deposited on the catalysts was determined using a Shimadzu-TOC 5000 combustion oxidation instrument with a solid sample module, where the carbon deposit is burnt at 900 ◦ C in oxygen. The results were negative, which suggests that the catalysts were stable under the conditions used in the experiment.

3. Results and discussion It is known from the literature that gas phase hydrogenation of furfural to furfuryl alcohol can be performed successfully over copper catalysts supported on SiO2 [19,21], MgO [17] or C [2]. However, little research has been conducted into the use in this process of supported palladium catalysts [21]. The conversion of furfural over Pd/SiO2 produces furan mainly by decarbonylation. Different product distributions can be explained in terms of the strength of the interaction between the furan ring and the metal surface [21]. Sitthisa et al. [22] show that the distribution of products is a strong function of the metals used and their interactions on the support surface. The incorporation of Cu into Pd/SiO2 catalyst, for instance, results in a lower decarbonylation rate and an increased rate of hydrogenation. Following previous studies by the authors on the influence of intermetallic interactions on the catalytic properties of palladium catalysts in hydrogen transfer reactions [41–46], this paper builds on the literature by investigating the effect of incorporating copper in 5%Pd/Al2 O3 catalysts on hydrogenation reactions with liquid phase furfural. The study focuses on the activity and selectivity of monometallic 5%Pd/Al2 O3 and 5%Cu/Al2 O3 catalysts and bimetallic 5%Pd–x%Cu/Al2 O3 (x = 1.5, 3, 6) systems during liquid phase hydrogenation of furfural. The reactions were conducted in aqueous solutions under mild conditions (90 ◦ C, 20 bar of H2 , 2 h). The aim of the research was to develop a stable bimetallic catalyst based on palladium that would show high selectivity towards FA.

341

As can be seen from Table 1, supported palladium catalysts showed particularly high activity, and the main product of furfural hydrogenation was tetrahydrofurfuryl alcohol (THFA). The incorporation of copper into the 5%Pd/Al2 O3 systems did not influence their activity significantly. However, their selectivity towards individual reaction products varied dramatically, depending on the percentage of added Cu (Fig. 1). From all tested mono- and bimetallic systems, only furfuryl alcohol and tetrahydrofurfuryl alcohol were detected as reaction products (Fig. 2). Selectivity to furfuryl alcohol (FA) rose in line with the Cu content in bimetallic Pd–Cu/Al2 O3 catalysts, whereas selectivity towards THFA reduced. The lowest conversion of furfural was measured over the monometallic 5%Cu/Al2 O3 catalyst. However, this system showed the highest selectivity towards furfuryl alcohol (FA). The results seem to be in good agreement with the literature on gas phase hydrogenation of furfural over Cu-, Pd- and bimetallic Pd–Cu supported catalysts, which suggests that the addition of copper to palladium systems should lead to increased selectivity towards FA [22]. Copper is known to be a good catalyst for selective C O bond hydrogenation. However, palladium catalysts are more efficient at unselective reducing of double bonds. Sitthisa et al. [22] show that incorporation of Cu into Pd/SiO2 catalyst results in the formation of Pd–Cu alloys, which may have different electronic structures from pure Pd. This electronic perturbation results in a lower level of electron back-donation from the catalyst surfaces to the ␲* system of furfural. In our study, the monometallic palladium catalyst was able to reduce both C O and C C bonds (Table 1). However, with increased Cu content in bimetallic Pd–Cu/Al2 O3 catalysts, increased concentrations of FA were detected in the reaction mixture. One of the causes may have been the formation of alloys on the surface of the catalysts, caused by strong interactions between the metals. The rate of C O hydrogenation may therefore increase over bimetallic Pd–Cu/Al2 O3 systems while that of C C hydrogenation may decrease. On the other hand, in our investigations of furfural hydrogenation over Pd/support systems, furan was not detected as a product, as had been postulated in another study [21,47–51]. Sitthisa and Resasco [21] investigated furfural hydrogenation in the gasphase over Pd catalysts supported on SiO2 , on a continuous-flow

Fig. 3. The XRD patterns (() Al2 O3 ; () Pd; X: Cux Pdy ) (a) 5%Cu/Al2 O3 , (b) 5%Pd/Al2 O3 , (c) 5%Pd–1.5%Cu/Al2 O3 , (d) 5%Pd–3%Cu/Al2 O3 , (e) 5%Pd–6%Cu/Al2 O3 .

342

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

reactor under hydrogen at atmospheric pressure in the temperature range of 210–290 ◦ C. The conversion of furfural over Pd/SiO2 was found to lead mainly the production of furan by decarbonylation. Other products from this reaction are terahydrofuran (THF) and tetrahydro-furfuryl alcohol (THFA). The authors to conclude that Pd can readily adsorb the furan ring due to the strong interaction between the metal and the p bonds in the molecule. Zhang et al. [47] performed gas-phase decarbonylation of furfural to furan on K-doped Pd/Al2 O3 catalysts in a fixed-bed reactor. In this case, the decrease in C O adsorption of furfural by K doping was the main factor suppressing the hydrogenation of furfural.

There is increased industry interest in the decarbonylation of furfural to furan over palladium catalysts [48–51], since furan is a useful starting material for industrial chemicals such as pharmaceuticals, herbicides, stabilizers, and polymers. However, in our studies over 5%Pd/Al2 O3 we obtained THFA as the main product of the reaction and FA in smaller amounts. The formation of THFA, which is a hydrogenated ring product, clearly indicates that Pd favours interaction with the furan ring. It is likely that the mild conditions (90 ◦ C, 20 bar of H2 ) used in our study of furfural hydrogenation over Pd/support systems were not favourable in terms of decarbonylation, while hydrogenation of the double bond in the furan ring took place. On the other hand, the addition of even a

Fig. 4. ToF-SIMS image of positive secondary ions in (a) 5%Pd–3%Cu/Al2 O3 and (b) 5%Pd–6%Cu/Al2 O3 catalyst surfaces after oxidation (O2 , 4 h, 500 ◦ C) and reduction (H2 , 2 h, 300 ◦ C). Surface area: 100 ␮m × 100 ␮m. During measurement the analyzed samples were irradiated with pulses of 25 keV Bi3 + ions at a repetition rate of 10 kHz and with an average ion current of 0.5 pA. Secondary ions emitted from the bombarded surface were mass separated and counted in a time of flight (ToF) analyzer.

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

small amount of copper to the 5%Pd/Al2 O3 catalyst reduced the formation of THFA, probably due to electronic effects which occur during alloy formation. X-ray diffraction (XRD) was used to monitor the formation of alloys in all the studied bimetallic catalysts. Crystalline phases were identified by comparison with the corresponding ICDD files. Fig. 3 shows the results for 5%Pd/Al2 O3 , 5%Cu/Al2 O3 and 5%Pd–x%Cu/Al2 O3 (x = 1.5, 3, 6) systems. In the sample 5 wt.% Pd–1.5 wt.% Cu/Al2 O3 the main broadened peak was observed at 2 = 40.24◦ which indicates the existence of Pd (Pd (1 1 1)). As the content of copper in the samples grew, Pd–Cu alloys formed. The main Pd–Cu (1 1 1) alloy peaks in Fig. 3b and c are between the Cu(1 1 1) and Pd(1 1 1) peaks. In terms of thermodynamics, the Cu–Pd system can be described as a disordered continuous solid solution with a face-centred cubic (f.c.c.) structure and lattice spacing ranging from 3.615 A˚ (pure copper) to 3.890 A˚ (pure palladium) (Table 2). At lower temperatures, ordering of Cu and Pd atoms is

343

energetically favoured and the cubic CuPd alloy adopts the b.c.c.based structure (CsCl-type). The alloying of Pd and Cu in the systems studied can therefore be related to the presence of the CuPd disordered alloy. The broadening due to small crystallite size may be expressed according to the Scherrer equation: Crystallite size(average) =

K Bs cos 

Table 2 Lattice parameters in Pd–Cu/Al2 O3 systems estimated using Vegard’s law. Catalyst

aCu (Å) PDF 004-0836

aPd (Å) PDF 046-1043

aCuPd (Å) exp

XPd (%)

5%Pd–3%Cu/Al2 O3 5%Pd–6%Cu/Al2 O3

3.615 3.615

3.890 3.890

3.648 3.640

12 9

Fig. 5. The ToF-SIMS (+) spectra of (a) 5%Pd–3%Cu/Al2 O3 and (b) 5%Pd–6%Cu/Al2 O3 catalysts after oxidation (O2 , 4 h, 500 ◦ C) and reduction (H2 , 2 h, 300 ◦ C).

344

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

Fig. 6. Results of SEM-EDS measurements for 5%Pd–3%Cu/Al2 O3 catalyst after oxidation (O2 , 4 h, 500 ◦ C) and reduction (H2 , 2 h, 300 ◦ C).

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

where Bs is the broadening due solely to crystallite size, K a constant which depends on particle size (taken as 0.9),  is the Brag’s angle and  the wavelength of the incident X-ray beam. The crystallite sizes were determined from the broadening of the PdCu (1 1 1) line, which approximates the Cauchy function of the experimental and instrumental profiles, on the assumption that the lattice strain is negligibly small (Table 1). Vegard’s law is an approximate empirical rule which holds that at constant temperature a linear relation exists between the crystal lattice constant of an alloy and the concentrations of its constituent elements. CuPd alloy is a solid solution with a cubic structure. A linear combination of the lattice spacing of its individual metal components and their mole fraction is given by equation: aCuPd = xPd aPd + (1 − xPd )aCu where a is the lattice constant and x is the mole fraction. Using the lattice constants of pure Cu and the alloying atom (Pd), the lattice constant of the remainder of the composition of the alloy can be estimated as a number between 0 and 100. The results of this calculation are shown in Table 2. ToF-SIMS was used to observe the interactions between palladium and copper through changes on the catalyst surface, which are not visible using XRD (e.g. surface enrichment or surface segregation in bimetallic catalysts). Fig. 4 shows an ionic view of a microarea of a palladium catalyst supported on alumina and modified with copper after oxidation in an oxygen atmosphere for 4 h at 500 ◦ C and reduction in a hydrogen atmosphere for 2 h at 300 ◦ C.

345

Places of particular brightness in the images correspond to the intensity of secondary ion emission. On the basis of the images of bimetallic palladium–copper catalysts (Fig. 4), it can be seen that Pd and Cu atoms are not distributed homogenously on the surface of the alumina. However, these images also show that the secondary ions Pd+ , Cu+ and CuPd+ are emitted from the same regions on the surface. In particular, the emission of CuPd+ ions from the surface of the catalysts confirms a mutual interaction between the palladium and copper atoms. Moreover, the intensity of Cu+ and CuPd+ ion emission from the 5%Pd–6%Cu/Al2 O3 catalyst was around double that observed from catalysts with half the amount of copper (Fig. 5). The alloying of active phase metal components in Pd–Cu/Al2 O3 catalysts can influence their selectivity towards FA and THFA during furfural reduction. Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) with a magnification of 10,000× (Fig. 6) was used to confirm the occurrence of interaction between palladium and copper in the 5%Pd–3%Cu/Al2 O3 catalyst. Fig. 6a shows the various particle sizes in the 5%Pd–3%Cu/Al2 O3 system. According to the results of SEM analysis, the particle sizes were in the range of 0.1–50 ␮m. The distributions of aluminium, palladium and copper are shown in Fig. 6b. EDS analysis confirmed that the distribution of Pd and Cu on the alumina surface was inhomogeneous. However, linear X-ray analysis (Fig. 6c) as well as point X-ray analysis (Fig. 6d) of the chosen grain of bimetallic catalyst revealed the presence of both Pd and Cu in the same regions on the surface. This fact points to the alloying of Pd and Cu which had also

Fig. 7. Temperature programmed profiles (a) TPR-H2(1) , (b) TPR-H2(2) of Pd and Pd–Cu catalysts.

346

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

been observed using XRD and ToF-SIMS (see Fig. 4 and discussion above). The catalytic samples, having undergone preliminary calcinations in O2 at 500 ◦ C, were treated with hydrogen in a TPR-H2(1) process (Fig. 7a). Comparing the TPR-H2(1) profiles for 5%Pd/Al2 O3 and 5%Pd–x%Cu/Al2 O3 (x = 3, 6), it is clear that palladium is easily reduced at room temperature and at around 75 ◦ C shows hydrogen desorption peaks only, associated with the decomposition of ␤hydride of palladium formed during the first contact of palladium catalyst with hydrogen [43–46,52–54]. The addition of copper to the 5%Pd/Al2 O3 catalyst influenced the intensity of the hydrogen evolution peak and the negative peak, both of which became less intense – suggesting that the incorporation of Cu inhibited ␤-PdH formation. Moreover, in the case of bimetallic palladium–copper catalysts, new maxima of reduction appeared in the TPR-H2(1) profiles, and shifted in the direction of higher temperatures as the amount of added copper increased. The presence of these additional peaks is likely to be connected with the interaction between different amounts of palladium and copper oxides, which probably results in the formation of Pd–Cu alloys, as discussed above and suggested by other works [52–56]. Following preliminary oxidation in O2 at 500 ◦ C and treatment with H2 in a TPR-H2(1) process up to 300 ◦ C (Fig. 7a), the

catalysts were reoxidized at 500 ◦ C for 2 h. TPR-H2(2) measurements were then taken. The results are presented in Fig. 7b. For catalyst 5%Pd/Al2 O3 , desorption peaks were observed at around 75 ◦ C, which corresponds to ␤-PdH decomposition, and a small adsorption peak appeared at around 100 ◦ C, connected with the reduction of the oxide form of palladium, which interacts more strongly with the support. It had been postulated that the incorporation of a second metal such as Ag, Cu, Sn or Pb in supported bimetallic catalysts would lead to the suppression of the ␤-PdH phase [41,57,58]. This was confirmed in our study, where the addition of copper (5%Pd–3%Cu/Al2 O3 and 5%Pd–6%Cu/Al2 O3 systems) modified the intensity of the hydrogen evolution peak and the negative peak disappeared, suggesting that ␤-PdH formation had been inhibited. In the TPR-H2(2 ) profile of the reoxidized bimetallic 5%Pd–3%Cu/Al2 O3 catalysts, only one reduction peak, which appeared at 150 ◦ C, was observed. In the system with the highest amount of copper (5%Pd–6%Cu/Al2 O3 ), the position of this peak is shifted in the direction of higher temperatures (near 180 ◦ C). It is worth noticing that with the bimetallic Pd–Cu/Al2 O3 system, the position of the peak rate of maximum H2 consumption is located between the peaks of PdO reduction (≈75 ◦ C for 5%Pd/Al2 O3 ) and CuO reduction (≈325 ◦ C for 5%Cu/Al2 O3 ) in monometallic systems.

Fig. 8. FT-IR spectra of carbon monoxide adsorbed on bimetallic Pd–Cu/Al2 O3 and monometallic 5%Pd/Al2 O3 , 5%Cu/Al2 O3 catalysts, after reduction at 300 ◦ C in H2 for 1 h, purging with Ar at 40 ◦ C for 0.5 h, CO sorption at 40 ◦ C with flow 10 mL min−1 . The IR spectra were collected at the same temperature after CO desorption in Ar stream (30 mL min−1 ).

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

Such TPR-H2(2) spectra suggest the presence in these catalytic systems of intermetallic oxides (Pdx Cuy Oz ) containing various proportions of metals. Reduction of these mixed-oxides might lead to the formation of alloys with different compositions. This result is in good agreement with our X-ray studies. The formation of Pd–Cu alloys had been previously confirmed using X-ray absorption spectroscopy [53] and by X-ray photoelectron spectroscopy [22,53] with Pd–Cu/support (SiO2 , Al2 O3 ) catalysts. A very efficient way to investigate the alloying of palladium with other metals (Ag, Cu, etc.) in bimetallic systems is to monitor the sorption of CO using IR spectroscopy. The IR spectrum of CO adsorbed on Pd surfaces exhibits two intense bands, which appear in the ranges 2100–2050 and 2000–1800 cm−1 , and are associated with linearly bound and bridge-bonded CO respectively [54]. It is also known from the literature that the addition of Cu to Pd/support catalysts leads to lower intensity bands corresponding to bridged species or/and shifts of CO to lower values [22,59]. Fig. 8 shows the IR spectra of CO adsorbed at low temperature (40 ◦ C) over bimetallic Pd–Cu/Al2 O3 and monometallic 5%Pd/Al2 O3 , 5%Cu/Al2 O3 catalysts, after reduction in H2 (300 ◦ C, 1 h). In the case of reduced monometallic 5%Pd/Al2 O3 catalyst, a strong band composed of a main maximum at 1865 cm−1 and a pronounced shoulder at 1964 cm−1 , and a band with maximum at 2085 cm−1 can be observed. The first band is connected with the presence of the bridge and multi-bonded CO species on the surface of the Pd catalyst. The second is connected with linear adsorbed CO on Pd surface atoms. The bridge and multi-bonded CO species on the Pd surface are dominant. In accordance with previous reports [22,60–62], the bridge and multi-bonded CO species were greatly reduced when even a small amount of Cu was incorporated into the catalyst, which is typically explained in the literature as being due to the dilution of Pd ensembles [63,64]. It is worth noting that in the spectrum of bimetallic Pd–Cu catalysts rich in Cu, the band connected with the presence of the bridge and multi-bonded CO species practically disappears. In addition, in the spectrum of the 5%Pd–6%Cu/Al2 O3 catalyst, a new band can be observed growing at around 2175 cm−1 . The position of this maximum is shifted into higher wavenumber values in comparison to that of the band at 2097 cm−1 , which is connected with the linear absorption of CO by Cu atoms in the monometallic 5%Cu/Al2 O3 system. This band has been associated in the literature with CO adsorbed on bimetallic Pd–Cu sites [60–62]. The alloying of Pd and Cu in bimetallic catalysts also confirms the expected shift of the peak at higher wavenumbers (from 2085 in 5%Pd/Al2 O3 to 2118 in 5%Pd–6%Cu/Al2 O3 ), which is connected with linear adsorption of CO on Pd atoms. 4. Conclusions

(c) Alloying of palladium and copper, confirmed with XRD, TPRH2 and FTIRS techniques, may be the cause of changes in the selectivity of bimetallic catalysts Pd–Cu/Al2 O3 towards furfuryl alcohol in furfural hydrogenation.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

From this study of liquid phase furfural hydrogenation in water solution on Pd, Cu and Pd–Cu catalysts, the following conclusions can be made: (a) On pure 5%Pd/Al2 O3 catalyst, furfuryl alcohol is produced and further reduced to tetrahydrofurfuryl alcohol. This is probably connected with the activation of H2 on the surface of palladium and the production of hydrogen species active in the reduction of double C C bonds in the furfuryl alcohol ring. Contrary to gas phase furfural hydrogenation, the formation of furan was not observed. (b) Whereas the addition of copper influences the activity of palladium catalysts only slightly, it significantly modifies their selectivity. As the amount of copper introduced into palladium catalysts rises, their selectivity towards furfuryl alcohol also increases.

347

[40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54]

D.T. Win, Assumpt. Univ. J. Technol. 8 (4) (2005) 185–190. R.S. Rao, R.T.K. Baker, Catal. Lett. 60 (1999) 51–57. ´ A. Malinowski, D. Wardzinska, Chemik 66 (9) (2012) 982–990. D.M. Alonso, S.G. Wettstein, J.A. Dumesic, Chem. Soc. Rev. 41 (2012) 8075–8098. R. Rao, A. Dandekar, R.T.K. Baker, M.A. Vannice, J. Catal. 171 (2) (1997) 406–419. G. Seo, H. Chon, J. Catal. 67 (2) (1981) 424–429. B.H. Wojcik, J. Ind. Eng. Chem. 40 (1948) 210–216. M. Bankman, J., Ohmer, T. Tacke, U.S. Patent No 5,591,873, 1997. S.P. Lee, Y.W. Chen, Ind. Eng. Chem. Res. 38 (1999) 2548–2556. H. Lu, H. Li, L. Zahuang, Chem. Lett. 5 (2001) 404–405. J. Nowicki, Z. Maciejewski, Przem. Chem. 76 (1997) 53–54. M.J. Burk, T. Harper, P. Gregory, J.R. Lee, Ch. Kalberg, Tetrahedron Lett. 35 (28) (1994) 4963–4966. E.A. Karakhanov, E.B. Neimerovets, A.G. Dedor, Vestn. Mosk. Univ. Ser. 2: Khimiya 26 (1985) 429. X.Y. Hao, W. Zahou, J.W. Wang, Y.Q. Zhang, S.X. Liuo, Chem. Lett. 34 (2005) 1000–1001. B.M. Reddy, G.K. Reddy, K.N. Rao, A. Khan, I. Garesh, J. Mol. Catal. A: Chem. 265 (2007) 276–282. J. Wu, Y.M. Shem, C.H. Liu, H.B. Wang, C. Geng, Z.X. Zhang, Catal. Commun. 6 (2005) 633–637. B.M. Nagaraja, V. Siva Kumar, V. Shasikala, A.H. Padmasri, B. Sreedhar, B. David Raju, K.S. Rama Rao, Catal. Commun. 4 (2003) 287–293. B.M. Nagaraja, A.H. Padmasri, B. David Raju, K.S. Rama Rao, J. Mol. Catal. A: Chem. 265 (2007) 90–97. S. Sitthisa, T. Sooknoi, Y. Ma, P.B. Balbuena, D.E. Resasco, J. Catal. 277 (2011) 1–13. ´ J. Kjenski, P. Winiarek, T. Paryjczak, A. Lewicki, A. Mikołajska, Appl. Catal. A 233 (2002) 171–182. S. Sitthisa, D.E. Resasco, Catal. Lett. 141 (2011) 784–791. S. Sitthisa, T. Pham, T. Prasomsri, T. Soonknoi, R.G. Mallinson, D.E. Resasco, J. Catal. 280 (2011) 17–27. M.A. Tike, V.V. Mahajani, Ind. Eng. Chem. Res. 46 (2007) 3275–3282. H. Adkins, R. Connor, M. Wis, U.S. Patent No 2,094,975, 1937. D. Liu, D. Zemlyanov, T. Wu, R.J. Lobo-Lapidus, J.A. Dumesic, J.T. Miller, Ch.L. Marshall, J. Catal. 299 (2013) 336–345. M.M. Villaverde, N.M. Bertero, T.F. Garetto, A.J. Marchi, Catal. Today 213 (2013) 87–92. G. De Witt Graves, U.S. Patent No 2,077,409, 1937. R.V. Sharma, U. Das, R. Sammynaiken, A.K. Dalai, Appl. Catal. A 454 (2013) 127–136. K. Yan, A. Chen, Energy 58 (2013) 357–363. L.J. Frainier, H. Fineberg, U.S. Patent No 4,302,397, 1981. E.A. Preobrazhenskaya, J. Mamatov, I.P. Polyakov, U.S. Patent No 4,261,905, 1981. S.J. Chiang, B.J. Liaw, Appl. Catal. A 319 (2007) 144–152. B.J. Liaw, S.J. Chiang, S.W. Chen, Y.Z. Chen, Appl. Catal. A 346 (2008) 179–188. X. Chen, H. Li, H. Luo, M. Qiao, Appl. Catal. A 233 (2002) 13–20. K. Yan, A. Chen, Fuel 115 (2014) 101–108. P.D. Vaidya, V.V. Mahaiani, Ind. Eng. Chem. Res. 42 (2003) 3881–3885. A.B. Merlo, V.V. Vetere, J.F. Ruggera, M.L. Casella, Catal. Commun. 10 (2009) 1665–1669. A.B. Merlo, V. Vetere, J.M. Ramallo-Lopez, F.G. Requejo, M.L. Casella, React. Kinet. Mech. Catal. 104 (2011) 467–482. A.B. Merlo, V. Vetere, J.F. Ruggera, M.L. Casella, Catal. Commun. 10 (2009) 1665–1669. K. Yan, C. Jarvis, T. Lafleur, Y. Qiao, X. Xie, RSC Adv. 3 (2013) 25865–25871. ´ S. Karski, I. Witonska, J. Rogowski, J. Gołuchowska, J. Mol. Catal. A: Chem. 240 (1–2) (2005) 155–163. ´ S. Karski, I. Witonska, J. Mol. Catal. A: Chem. 191 (2003) 87–92. ´ I. Witonska, S. Karski, J. Rogowski, N. Krawczyk, J. Mol. Catal. A: Chem. 287 (2008) 87–94. ´ I. Witonska, A. Królak, S. Karski, J. Mol. Catal. A: Chem. 331 (2010) 21–28. ´ I. Witonska, M. Frajtak, S. Karski, Appl. Catal. A 401 (2011) 73–82. ´ I.A. Witonska, M.J. Walock, P. Dziugan, S. Karski, A.V. Stanishevsky, Appl. Surf. Sci. 273 (2013) 330–342. W. Zhang, Y. Zhu, S. Niu, Y. Li, J. Mol. Catal. A: Chem. 335 (1–2) (2011) 71–81. H.B. Copelin, D.I. Garnett, U.S. Patent No. 3,007,941 (1959). A.P. Dunlop, G.W. Huffman, U. S. Patent No. 3,257,417 (1963). D.G. Manly, J.P. O’halloran, U.S. Patent No. 3,223,714 (1963). R. Ozer, WO 2011026059 A1 (2009). A. Benedetti, G. Fagherazzi, F. Pinna, G. Rampazzo, M. Selva, G. Strukul, Catal. Lett. 10 (1991) 215–224. J. Batista, A. Pintar, D. Mandrino, M. Jenko, V. Martin, Appl. Catal. A 201 (2001) 113–124. B. Coq, F. Figueras, J. Mol. Catal. A: Chem. 173 (2001) 117–134.

348

M. Lesiak et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 337–348

[55] M. Fernandez-Garcia, J.A. Anderson, G.L. Haller, J. Phys. Chem. 100 (1996) 16247–16254. [56] A.M. Molenbroek, S. Haukka, B.S. Clausen, J. Phys. Chem. B 102 (1998) 10680–10689. [57] H.R. Aduriz, P. Bondariuk, B. Coq, F. Figueras, J. Catal. 129 (1991) 47–57. [58] Q. Zhang, J. Li, X. Liu, Q. Zhu, Appl. Catal. A 197 (2000) 221–228. [59] V. Sanchez-Escribano, L. Arrighi, P. Riani, R. Marazza, G. Busca, Langmuir 22 (2006) 9214–9219.

[60] F. Skoda, M.P. Astier, M. Primet, Catal. Lett. 29 (1994) 159–168. [61] S.S. Ashour, J.E. Bailie, C.H. Rochester, G.J. Hutchings, J. Mol. Catal. A: Chem. 123 (1997) 65–74. [62] F.B. Noronha, M. Schmal, M. Primet, R. Frety, Appl. Catal. 78 (1991) 125–139. [63] M. Fernandez–Garcia, J.A. Anderson, G.L. Haller, J. Phys. Chem. 100 (1996) 16247–16254. [64] V. Ponec, Appl. Catal. A 222 (2001) 31–45.