Acetic acid hydrogenation to ethanol over supported Pt-Sn catalyst: Effect of Bronsted acidity on product selectivity

Molecular Catalysis 448 (2018) 78–90

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Acetic acid hydrogenation to ethanol over supported Pt-Sn catalyst: Effect of Bronsted acidity on product selectivity Pranab Kumar Rakshita,b, Ravi Kumar Voolapallia, Sreedevi Upadhyayulab, a b

T

⁎

Bharat Petroleum Corporation Ltd., Plot No: 2A, Udyog Kendra, Surajpur Industrial Area, Greater Noida, Uttar Pradesh, 201306, India Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110016, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Acetic acid hydrogenation Pt-Sn catalysts Sn effect Acetic acid Ethanol Support acidity

Gas phase hydrogenation of acetic acid was investigated over a series of SiO2-Al2O3 supported platinum-tin (PtSn) catalysts. The active metals were impregnated over the support using incipient wetness technique and the resulting catalyst samples were characterized by Transmission electron microscopy, Hydrogen pulse chemisorption, BET surface area analyzer, Powder X-Ray diffraction, NH3-Temperature programmed desorption and H2-Temperature programmed reduction methods. Acetic acid hydrogenation reaction was carried out in an isothermal fixed bed catalyst testing unit. The results revealed that bimetallic Pt-Sn catalyst forms Pt-Sn alloy upon reduction which favors acetic acid hydrogenation to ethanol compared to competing side product CH4. The magnitude of Pt-Sn alloy formed per unit mass of catalyst depends upon the Pt/ Sn molar ratio in the calcined catalyst sample. 3 wt% Pt- 3 wt% Sn on SiO2-Al2O3 was found to be the optimum catalyst loading, resulting in 81% acetic acid conversion with 95% ethanol selectivity at 2 MPa and 270 °C. Further increase in ethanol selectivity would require prevention of esterification of acetic acid with ethanol, which leads to formation of ethyl acetate as by-product. The effect of catalyst acidity on acetic acid conversion and ethanol selectivity was studied and it was observed that proton donating capability of the support leads to the formation of ethyl acetate as byproduct which, in turn, reduces ethanol selectivity. The ethanol synthesis reaction and esterification reaction over Bronsted acid sites takes place in series. The rate of esterification reaction was found to be highly dependent on the Bronsted acid density of the catalysts. Other catalyst parameters have little role on ethyl acetate yield.

1. Introduction Increase in energy sustainability and growing concerns of environmental impact of fossil fuel combustion are prompting researchers across the world to investigate alternative routes for fuel production [1]. Along with few other transportation fuel alternatives such as biodiesel, green diesel, DME and Fischer-Tropsch product, ethanol has emerged as one of the major alternate drop-in fuel or blend stock to fossil fuels [2,3]. Presently, ethanol is commercially produced by fermentation of sugars derived from grains of plant crops [4]. However, thermo-chemical route for conversion of carbonaceous feedstock to syngas (CO + H2) through gasification and further conversion of syngas directly to alcohols has been the focus of researchers due to its direct nature and higher productivity levels compared to fermentation. However, direct conversion of syngas to ethanol produces high amounts of methane and other hydrocarbons as by-products which make commercialization of this process challenging [4–9]. Indirect synthesis of ethanol from syngas was, hence, explored to prevent carbon loss in terms of hydrocarbon formation and accordingly, few possible routes ⁎

were suggested in literature [4,10–14]. Conversion of syngas to methanol, subsequent conversion of methanol to acetic acid and acetic acid hydrogenation to ethanol is a promising option in spite of being multi-step because of the possibility of achieving high selectivity levels compared to other possible routes. Syngas conversion to methanol and subsequent CO carbonylation of methanol to acetic acid are highly selective processes and are well studied [15–19]. Development of efficient and highly selective catalyst system for acetic acid hydrogenation will enable commercialization of this route of ethanol production. Hydrogenation of acetic acid to ethanol has following reaction stoichiometry: CH3COOH (g) + 2H2 → C2H5OH (g) + H2O (g) ΔRH0 = −44.17 kJ/mol (1) Apart from this desired reaction, there are a few side reactions that are known to take place alongside. CH3COOH (g) + C2H5OH ΔRH0 = −18.37 kJ/mol

Corresponding author. E-mail address: [email protected] (S. Upadhyayula).

https://doi.org/10.1016/j.mcat.2018.01.030 Received 9 October 2017; Received in revised form 22 December 2017; Accepted 24 January 2018 Available online 21 February 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.

(g) → CH3COOC2H5

(g) + H2O

(g) (2)

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wetness technique over commercial silica-alumina support. Tetraamineplatinum (II) nitrate (Alfa Aesar) was used as the precursor for platinum and tin (II) acetate (Sigma Aldrich) as the precursor for tin. Commercially available ultra – pure Silica-alumina (SA1) extrudates containing 99% SiO2 and 1% alumina were procured from Alfa Aesar and used as support. The SA1 was crushed and sieved for the desired particle size of 0.4–0.6 mm. The Pt salt was dissolved in 1 mol/L HNO3 and tin salt was dissolved in 1:1 solution of glacial acetic acid and water. The resulting solutions were co-impregnated on the support by adding drop wise in a rotating round bottom flask. After impregnation, the catalyst was aged for 4 h at atmospheric condition and then vacuum dried in a Rota-vapor at 65 °C. The catalyst was further dried in an oven at 85 °C for 12 h. The dried catalyst was calcined at 500 °C for 5 h to obtain it in the final impregnated form. The catalysts prepared were labeled as xPt-ySn-SAz where x and y are the weight percentages of Pt and Sn respectively, with respect to weight of silica-alumina support and z = 1,2 or 3 depending upon the support chosen. Six catalysts were prepared namely, 1Sn-SA1, 1Pt-SA1, 3Pt-SA1, 3Pt-1.5Sn- SA1, 3Pt-3SnSA1 and 3Pt-4.5Sn-SA1 to study the effect of Pt and Sn content on catalyst performance. Reported work on similar catalysts was done with total metal loading of 1–2% weight [24–26]. At low concentration levels, the XRD profiles reported were not conclusive enough to identify the phases of the metals. Hence, in the present work higher concentration of metals was impregnated on the support. In order to study the effect of catalyst acidity on acetic acid hydrogenation, Pt-Sn bimetallic catalyst was also prepared using two more commercially available SiO2-Al2O3 supports: SA2 (containing 95% SiO2 and 5% Al2O3 sourced from Sigma Aldrich) and SA3(containing 85% SiO2, 15% Al2O3 sourced from Saint Gobain). Supports of varying alumina content were procured purposely to vary the total acidity of the catalyst and check their effect on conversion and selectivity.

CH3COOH (g) + H2 → CH3CHO (g) + H2O (g) ΔRH0 = 24.5 kJ/mol (3) CH3COOH (g) + 4H2 → 2CH4 + 2H2O (g) ΔRH = −20 kJ/mol 0

(4)

Catalyst for acetic acid hydrogenation to ethanol is commonly prepared by wet impregnation of noble metals on support materials [20]. The support adsorbs the acetic acid molecules and hydrogenates it with the dissociated hydrogen made available by the impregnated metals [20,21]. Hence, the conversion of acetic acid to ethanol would strongly depend on the ability of the metal to dissociate hydrogen, ability of support to adsorb acetic acid, spillover of hydrogen to the adsorbed acetic acid, acidity of support, and metal dispersion. Side reactions (reactions (2)–(4)) to produce ethyl acetate, acetaldehyde and methane are also reported to be feasible [20–23]. The selectivity to these side products tend to increase with increase in temperature and decrease in pressure [26]. Therefore, the catalyst must have the ability to suppress these side reactions and improve ethanol selectivity at lowest possible operating temperature and pressure. Conversion of carboxylic acids to alcohols using noble metal based heterogeneous catalyst system is known since long period of time [20]. Pt on oxide supports dissociates hydrogen and transfers it to adjacent support sites via spillover mechanism, wherein, adsorbed acetic acid molecule gets hydrogenated to produce aldehydes or alcohol [20,21]. Accordingly, noble metal catalysts, mainly Pt, emerged as an active metal for acetic acid hydrogenation because of their ability to dissociate and transfer hydrogen by spillover mechanism. Subsequently, literature reported that bimetallic Pt-Sn catalysts tested at 270–350 °C temperature, 2–2.6 MPa pressure, WHSV = 0.6–3 h−1 and H2/acetic acid molar ratio of 10–20 gave improved activity and selectivity for acetic acid hydrogenation reaction as compared to monometallic catalysts owing to improved dispersion of Pt and formation of Pt-Sn alloy. However, even then, the activity and ethanol selectivity varied significantly with variation in support material [24,26]. Catalytic performance of Pt-Sn catalyst supported on carbon nanotubes (CNT), silicon carbide (SiC) and few other oxide supports such as SiO2 and Al2O3 showed variable yields [24–26]. It is evident that the support plays a key role in the activity and selectivity of ethanol formation. Hence, the mechanism of chemical interaction between support and reactants needs to be thoroughly examined. It has been reported that oxide supports with proton donation ability can contribute towards esterification reaction of acetic acid and ethanol leading to ethyl acetate production via reaction (2) [27]. It is also reported that the lewis acid sites of the support can lead to ethanol or ethyl acetate production competitively [25]. Hence, the effect of support acidity on activity of acetic acid hydrogenation and selectivity of ethanol needs to be studied. SiO2-Al2O3 is a well-accepted support in industrial catalytic systems with tunable surface properties for different applications and to our knowledge SiO2-Al2O3 supported bimetallic Pt-Sn catalyst has not yet been studied for acetic acid hydrogenation. Hence, for the present work, SiO2-Al2O3 has been chosen as the preferred support to study the performance of bimetallic Pt-Sn catalysts for conversion of acetic acid to ethanol. The Pt-Sn catalysts were impregnated on SiO2-Al2O3 support with varying silica and alumina content, characterized and tested for activity and selectivity. A systematic study was carried out to study the effect of catalyst acidity and reduction temperature on acetic acid conversion and ethanol selectivity. Based on the results, an effort was made to establish the structure – activity relationship to achieve high ethanol yield over supported Pt-Sn catalysts.

2.2. Catalyst characterization 2.2.1. Powder X-ray diffraction (XRD) The calcined forms of catalysts were characterized by powder X-ray diffraction (XRD) analysis using a Rigaku miniflex X-ray diffractometer using Ni filtered Cu Kα radiation (λ = 0.15406 nm) from 2θ = 6 to 85°, at a scan rate of 2 min−1 with the beam voltage and a beam current of 30 kV and 15 mA respectively. Sample preparation for the X-ray analysis involved packing of approximately 0.3–0.5 g of powder into the sample holder with light compression to make it flat and tight. 2.2.2. Temperature programmed desorption NH3-TPD measurement of the catalyst samples was performed using Thermo Fischer Scientific TPDRO (1100 series) analyzer to determine the surface acidity. Approximately 150 mg of the sample was loaded in a quartz cell and initially flushed with a He gas flow at 400 °C for 2 h, cooled to 150 °C and then saturated with ammonia at the same temperature for 30 min at a flow rate of 20 mL/min. After exposure to ammonia, the catalyst samples were subsequently degassed with the He flow and the temperature was raised up to 800 °C at a linear heating rate of 10 °C/ min for measuring desorbed NH3 by using a thermal conductivity detector (TCD). For TPD experiments with Isopropylamine (IPA), degassed samples were first saturated with IPA using IPA pulses coming from vapor generator kit operating at ambient temperature. After saturation with IPA, the catalyst samples were subsequently degassed with the He flow and the temperature was raised up to 800 °C at a linear heating rate of 10 °C/ min for measuring the adsorbed IPA decompose as propylene using thermal conductivity detector (TCD).

2. Experimental section

2.2.3. Temperature programmed reduction study (TPR) Reduction studies of the catalyst samples were performed on the same instrument used for TPD. In this analysis, 80 mg of the sample was loaded in an annular quartz reactor and was flushed with Ar gas at a flow rate of 20 mL/min and at 200 °C for 2 h, following which the

2.1. Catalyst preparation Supported bimetallic Pt–Sn catalysts were prepared using incipient 79

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velocities reported in literature for similar work [23–25]. The catalyst was loaded in the middle of the reactor and it occupied 10–12 cm length of the reactor tube. The remaining length was filled with inert glass beads from both sides to keep the catalyst bed fixed in the middle. The glass beads act as a preheating zone for the feed gases. As shown in Scheme 1, the test reactor set up is equipped with mass flow controllers (MFCs), tubular furnace, pressure control valve, condensers, liquid collection pot and wet gas meter for effluent gas flow measurement. The loaded catalyst was subjected to a two stage activation process inside the reactor. Firstly, nitrogen was passed through the catalyst bed maintained at 150 °C for 8 h, to remove any traces of moisture or volatile matter present in the catalyst. After this, the catalyst was reduced at 400 °C for 8 h with 25% H2 in N2. In a typical activity test procedure, acetic acid was pumped using a high precision HPLC pump followed by in-line vaporizer into the reactor. Gaseous reactant H2 was mixed with acetic acid prior to the vaporizer and the combined stream enters the reactor from the top. The products leaving the reactor are then condensed and collected in the high pressure liquid collection vessel downstream of the reactor. The non-condensable gases leave from the top of the collection pot through an online wet gas meter which quantifies the flowrate. The gaseous products were analyzed using Agilent 7890A gas chromatograph equipped with 1) Haysep Q and Molsieve 5X column and thermal conductivity detector (TCD) for estimation of hydrogen, 2) Haysep N and Molsieve 13 x columns followed by TCD for estimation of CO and CO2 and 3) HP-PLOT column with Flame Ionization detector (FID) for the determination of methane, ethane and other hydrocarbons. Liquid products containing ethanol, ethyl acetate, acetaldehyde, acetic acid in water were analyzed using Agilent 7890A gas chromatograph having DB-Wax column and FID. The reaction conditions employed for the study are 270 °C temperature, 2 MPa pressure, WHSV 3 h−1, H2/acetic acid molar ratio 10 mol/mol. Under these conditions, the turn over frequency (TOF) of the hydrogenation reaction was calculated based on the exposed moles of platinum calculated by hydrogen pulse chemisorption. The TOF is defined as the moles of acetic acid converted per second per mole of exposed Pt. Acetic acid (AA) conversion and the selectivity of the products were calculated in mol% by using the following equations.

sample was cooled down to 50 °C at a rate of 10 °C/ min. The sample was then heated up to 800 °C at a heating rate of 10 °C/ min in the presence of a mixture of 5% H2 in Ar. 2.2.4. H2 pulse-chemisorption The number of active catalyst sites used for TOF calculation was measured by H2-pulse chemisorption study carried out in the TPR instrument. 150 mg of sample was loaded in the annular quartz reactor and then degassed at 150 °C for 2 h using Argon. The sample was next reduced at 400 °C using 5% H2 in Argon for 2 h and then further degassed at 400 °C for 2 h using Argon. The degassed sample was cooled down to 40 °C. Pulse chemisorption experiment was next carried out by injecting ten calibrated H2 pulse in the system at an interval of 15 min and recording the volume loss on account of chemisorption. The chemisorption data was used to calculate chemisorbed monolayer volume which, along with 0.5 stoichiometric factor (H2/Pt), was used to estimate the Pt dispersion as per literature [24]. 2.2.5. Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) images were taken on a JEOL 1010 and 2010 microscope operated at 100 kV and 200 kV, respectively. Samples for TEM analyses were prepared by applying one drop dispersed solution of the sample onto the carbon coated Cu grid and allowing the solvent to slowly evaporate at room temperature. ImageJ software was used to process these images and estimate mean particle size. 2.2.6. Surface area measurement Brunauer-Emmett-Teller (BET) surface area of the catalyst samples was estimated by multipoint N2 adsorption-desorption method using Micromeritics ASAP 2020 analyzer. In a typical measurement, approximately 100 mg of calcined catalyst sample was taken in a glass reactor and degassed under helium at 300 °C for 3 h at a heating rate of 10 °C/ min.k 2.3. Catalyst evaluation The catalytic performance of all the catalysts was tested in a high pressure fixed bed tubular reactor (Scheme 1). Typically, 15 g of catalyst (mean particle size 500 μm) was used for the activity runs. Higher quantity of catalyst was used because the reactor setup is large, however, WHSV of 3 h−1 was maintained which is higher than the space

Scheme 1. Schematic diagram of a tubular reactor for acetic acid hydrogenation.

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Table 1 Physicochemical properties of Pt/Sn catalysts. Catalyst

1Sn-SA1 1Pt-SA1 3Pt- SA1 3Pt-1.5Sn- SA1 3Pt-3Sn- SA1 3Pt-4.5Sn- SA1 a b c

Observed Metal contenta, % wt

Surface area

Mean particle sizeb

Pt dispersion, %

Pt crystallite size in calcined catalystc

Pt

Sn

m2/g

nm

TEM

H2 Chemisorption

nm

0 0.95 2.87 2.91 2.98 2.94

0.86 0 0 1.11 2.37 4.13

161 159 146 136 133 128

– 2.11 3.34 3.22 2.24 2.84

– 53.6 33.9 35.1 50.5 39.8

– 44.2 25.8 34.0 42.4 33.8

– 8.73 12.61 11.24 9.63 10.02

Observed metal content is obtained from calcined catalyst. Mean particle size is calculated from number averaging of particle size distribution. Crystallite size determined using Scherrer equation on maximum intensity peak.

Conversion ofAcetic acid =

(inlet moles ofAA − outlet moles ofAA) × 100 inlet moles ofAA

structure. All the catalysts except 1Sn-SA1 show- four distinct peaks at 39.7°, 46.2°, 67.4° and 81.2° corresponding to the diffractions of (111), (200), (220) and (311) planes of fcc Pt (PDF 03-065-2858) [28]. Four additional shoulder peaks at 39.4°, 45.9°, 66.9° and 80.5° are also obtained along with peaks of metallic Pt, which correspond to planes of fcc Pt2O (PDF 03-065-5066) [30]. Therefore, it can be concluded that calcination of mono-metallic Pt catalyst produces partially oxidized Pt and metallic Pt sites. On the other hand, XRD patterns of the three bimetallic Pt-Sn catalysts and 1Sn-SA1 catalyst show three peaks at 26.5°,33.9° and 51.7° corresponding to SnO2 phase (PDF 01-088-0287) [29]. Bimetallic Pt-Sn catalysts produces pure metallic Pt phase, Pt2O and SnO2 upon calcination with no traces of Pt-Sn alloy formation. This observation is in line with the results of elemental analysis shown in Table 1. This might be due to the fact that the SA1 support offers very little or weak interaction with the Pt and Sn molecules, which upon calcination segregates from each other leading to distinct XRD patterns [30]. The maximum intensity peaks of the XRD patterns corresponding to Pt were also used to estimate the crystallite size of the particles and are tabulated in Table 1. Results show similar trends as that of mean particle diameter estimated using TEM. Monometallic Pt catalysts showed increase in crystallite size from 8.73 to 12.61 with increase in Pt loading. 1Sn-SA1 catalyst crystallite size could not be determined as its size is too small. In case of bimetallic catalysts, incorporation of Sn decreased the crystallite size to 9.63 for 3Pt-3Sn-SA1 catalyst and further increase in Sn content increased the size to 10.02 nm. Fig. 2 shows the diffraction pattern of 3Pt-3Sn-SA1 catalyst in reduced phases. The XRD pattern of calcined catalyst samples showed two well resolved phases of Pt and SnO2. However, upon reduction with hydrogen at 200 and 400 °C, many additional peaks appear in the diffraction pattern. When the catalyst sample is reduced at 200 °C, the diffraction pattern shows three different phases; peaks at 2θ = 41.7° and 44.1° corresponds to Pt-Sn alloy (PDF 03-065-0959), peaks at 2θ = 26.6 and 33.9 corresponds to SnO2 phase and peaks corresponding to metallic Pt were also observed [29,32]. The same catalyst, on reduction at 400 °C, shows three phases, that of metallic Pt, PtSn alloy and SnO (PDF 01-085-0712). Pt-Sn is the only alloy phase that was formed and other forms of PtxSny alloy could not be identified from the XRD profile. As per the binary phase diagram of Pt and Sn, Pt/Sn weight ratio of around 1.6 leads to Pt-Sn alloy [31]. However, in presence of oxide support and at low concentration levels of Pt and Sn, PtSn alloy is reported to be formed [30,31]. Hence, based on the XRD results in Fig. 1 and 2, it can be inferred that in the calcined form of bimetallic catalysts both Pt and Sn remains in oxide form and only upon reduction, the oxides reduce to form Pt-Sn alloy and metallic Pt. SnO2 partially reduces to form Pt-Sn alloy or SnO. The XRD diffraction pattern of the catalyst reduced at 400 °C appears to be much sharper and intensified compared to the one reduced at 200 °C. Interpreting this result in accordance with Scherrer equation conveys that the catalyst reduced at 400 °C has higher mean particle diameter compared to the

(5)

Selectivity (mol%) =

moles of produced component × 100 Total moles of all products excluding water (6)

3. Results and discussion 3.1. Physical properties The physical properties of the six synthesized catalysts are listed in Table 1. Elemental analysis of the calcined catalyst samples were estimated by means of inductively coupled plasma (ICP) technique (Perkin Elmer, Model: Optima 2000). Results indicate that the observed Pt content in the samples is close to the targeted Pt content of the samples. Pt-Sn catalysts are known to form Pt-Sn alloy on support surface and once formed, the participating metals can’t be quantified using ICP elemental analysis [28]. The present results do not indicate Pt loss compared to the introduced quantity. However, estimated Sn content was observed to be lower than the introduced Sn content. This was because the precursor tin acetate tends to precipitate during impregnation and fall off from the support prior to calcination. Hence, it can be inferred that all the Pt and Sn atoms present on the calcined catalyst surface exist without any alloy formation. The surface area of the synthesized catalysts decreased with increase in total metal loading which is probably due to the fact that few pores were inaccessible after loading the metal. Results of mono-metallic Pt samples show that with increase in Pt content from 1%wt to 3%wt, mean particle size of Pt particle increases from 2.11 nm to 3.34 nm and Pt dispersion decreases from 44% to 25%. This is attributed to the fact that with higher metal loading, the metal atoms tend to agglomerate leading to higher particle size and lower dispersion. However, with addition of Sn, dispersion of Pt estimated using H2 chemisorption follows a volcano type profile, with the maximum dispersion of 42% achieved for 3%wt Sn content. Further increase of Sn content to 4.5% wt reduced Pt dispersion to 33%. Pt dispersion estimated using TEM analysis also shows similar trend. Mean particle size follows an inverse volcano trend w.r.t increase in Sn content. Lowest mean particle size of 2.24 nm was achieved for 3Pt3Sn-SA1. The superior dispersion of 3Pt-3Sn-SA1 catalyst over other PtSn catalyst samples can be attributed to the fact that at this optimum Pt and Sn content, Sn atoms prevent Pt atoms to agglomerate, leading to low mean particle size and high Pt dispersion. 3.2. XRD and TEM analysis Fig. 1 shows the XRD diffraction pattern of calcined Pt-Sn catalysts supported on SiO2-Al2O3 support SA1. The diffraction patterns show that the catalyst samples are obtained in their distinct crystalline 81

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Fig. 1. XRD diffraction pattern of SA1 supported Pt-Sn catalysts in calcined state.

and is shown in Table 1. Results revealed that for monometallic catalyst samples, increase in platinum loading from 1%wt to 3%wt increases mean particle size from 2.11 nm to 3.34 nm. However, for constant Pt content, the particle size exhibits an inverse volcano type curve with increase in Sn content. With 1.5%wt Sn addition the mean particle size reduced to 3.22 nm. In case of 3%wt Sn, the mean particle size decreased further to 2.24 nm. Further increase in Sn content to 4.5%wt, increased the mean particle size to 2.84 nm. This confirms that with optimum content of Pt and Sn (3%wt each), mean particle size of Pt can be reduced significantly, which in-turn improves dispersion. HRTEM image of 3Pt-3Sn-SA1 catalyst sample reduced at 400 °C is

one reduced at 200 °C. This is probably because of the fact that at elevated temperatures and under reducing conditions Pt-Sn molecules tend to form alloy and the resulting ensembles have higher particle size [33]. The mean particle diameter of the catalyst samples were not estimated using the XRD patterns because below 10 nm size, XRD patterns does not predict the mean particle size as accurately as TEM would predict. TEM images and corresponding particle size distribution of 3Pt-3SnSA1 and 3Pt-SA1 calcined catalyst are given in Fig. 3A and B respectively. The average particle size of five out of six synthesized catalyst samples are estimated from TEM analysis using more than 120 particles

Fig. 2. XRD diffraction patterns of 3Pt-3Sn-SA1 catalyst sample in reduced state.

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Fig. 3. TEM images and corresponding particle size distribution of (A) 3Pt-3Sn-SA1 and (B) 3Pt-SA1.

This corresponds to the reduction of SnO2 to SnO [34]. The reduction pattern of monometallic Pt catalyst samples shows only one peak at 126 °C which corresponds to the reduction behaviour of Pt oxides to Pt (0) [35,36]. The intensity of the peaks was also observed to be much less compared to the bimetallic catalysts. This is due to the fact that Pt atoms are partially in metallic phase after calcination and only the Pt oxide molecules take part in the reduction process. This inference agrees well with the XRD patterns (Fig. 1) wherein, prominent peaks of metallic Pt were observed along with peaks of Pt oxides. The TPR profile of the bimetallic Pt-Sn catalysts shows four distinct zones of reduction: first at 126 °C, second, a well designated region around 244 °C, third, a distinct peak around 457 °C and the fourth at 570 °C. As reported in the literature, the peak at 126 °C corresponds to the unalloyed Pt molecules having weak interaction with the support [34,35]. This is also confirmed by the reduction profiles of monometallic 1Pt-SA1 and 3Pt-SA1 catalysts. The subsequent zone having peaks around 244 °C also corresponds to the unalloyed Pt molecules but having stronger interaction with the support [37]. The third zone with peaks around 457 °C corresponds to the hydrogen consumption associated with Pt-Sn alloy formation [34]. And the final zone of peaks around 570 °C corresponds to the reduction of Sn4+ to Sn2+ and Sn2+ to Sn0 as obtained in 1Sn-SA1 catalyst [34,37]. Comparison of the TPR profiles of the three bimetallic catalysts highlights that with 1.5% wt Sn loading and 3%wt Sn loading, a distinct Pt-Sn peak is observed around 457 °C. However, at higher loading of tin (4.5%wt), the peaks of unalloyed Pt and Pt-Sn get suppressed and the reduction of Sn oxides dominates. This can be due to the fact that higher Sn content covers the active sites of Pt and Pt-Sn alloy and only reduction of Sn oxides are visible in the TPR profile. The TPR profiles of the catalyst samples were also used to estimate the hydrogen consumption pattern of each catalyst sample (Table 2). 1Sn-SA1 and 1Pt-SA1 catalyst showed minimum consumption of

Fig. 4. HRTEM image of 3Pt-3Sn-SA1 catalyst sample reduced at 400 °C.

shown in Fig. 4. The catalyst sample on reduction showed three well resolved lattice fringes, with inter-planar spacing of 0.22 nm, 0.21 nm and 0.17 nm. These lattice fringes corresponds to the (1 0 2) and (1 1 0) plane of Pt-Sn alloy (PDF 03-065-0959) and (2 0 0) plane of SnO (PDF 01-085-0712), respectively. These are also evident in the XRD patterns of reduced catalyst (Fig. 2) and hence, confirm the formation of Pt-Sn alloy phase upon catalyst reduction.

3.3. Reduction behaviour The reduction behaviour of the calcined catalysts is illustrated in Fig. 5. 1Sn-SA1 catalyst on reduction showed only one peak at 570 °C. 83

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Fig. 5. Temperature reduction profile of Pt-Sn catalysts.

of the catalyst. With further increase in Sn content, the hydrogen consumption at 240 °C and 450 °C reduces significantly and maximum consumption is observed at 570 °C. This substantiates that at higher Sn loading, an envelope is formed around the Pt and Pt-Sn molecules which reduces their access to hydrogen. Using the hydrogen consumption data, the percentage of hydrogen consumption on account of Pt-Sn alloy formation (Pt-Sn reduction%) was estimated by considering the ratio of hydrogen consumed at 450 °C to the total hydrogen consumed. Results showed that for 3Pt-3Sn-SA1, Pt-Sn reduction% is 65.6% which is much more than the results of the other two bimetallic catalysts. The effect of reduction temperature on hydrogen uptake was studied over monometallic 3Pt-SA1 catalyst and bimetallic 3Pt-3Sn-SA1 catalyst. The catalyst samples were reduced at 300 °C and 400 °C temperature levels and then H2 pulse chemisorption was carried out to estimate the hydrogen uptake and H2/Pt ratio of the two catalyst samples. The results showed that with the increase in reduction temperature, hydrogen uptake and H2/Pt ratio of the catalyst samples decreases (Table 3). Monometallic 3 Pt-SA1 catalyst sample upon reduction at 300 °C and 400 °C chemisorbed 17.83 and 16.76 μmol g−1 of H2, respectively. On the other hand, 3Pt-3Sn-SA1 had hydrogen uptake of 20.94 and 17.45 μmol g−1 on reduction at 300 °C and 400 °C respectively. Increase in reduction temperature increases the Pt-Sn alloy content of the catalyst sample, which is also confirmed by the XRD

Table 2 Hydrogen consumption of different catalyst samples during reduction. Catalyst

1Sn-SA1 1Pt-SA1 3Pt-SA1 3Pt-1.5Sn-SA1 3Pt-3Sn-SA1 3Pt-4.5Sn-SA1

Hydrogen consumption μmol g−1

Pt-Sn Reduction%

130 °C

240 °C

450 °C

570° C

Total

– 19.3 96.7 12.1 12.6 0.0

– 11.7 35.3 84.5 85.0 38.9

– 0.0 0.0 104.3 285.5 181.1

26 0.0 0.0 132.6 52.0 471.1

26 31.0 132.0 333.6 435.0 691.0

– – – 31.2 65.6 26.2

hydrogen owing to the fact that Pt and Sn loading is less and significant portion of those remain in metallic and oxide state, respectively. 3PtSA1 consumed a total of 132 μmol/g of H2 distributed over the temperature ranges of 130–270 °C. The hydrogen consumption of 3Pt-SA1 is more than 4 times than that of 1Pt-SA1 though the metal loading is only 3 times higher. This is because of higher Pt/PtOx molar ratio in 1Pt-SA1 compared to 3Pt-SA1 which is also confirmed by the XRD patterns of the calcined catalysts (Fig. 1). The total hydrogen consumption of 3%wt Pt catalysts increased with increase in Sn loading. The XRD patterns of the catalyst samples in Fig. 1 highlight that the Sn atoms are present as Sn-oxides on the catalyst surface. In the presence of Pt, these SnOx molecules reduces to form Pt-Sn alloy or SnOy, where x > y [34,36]. Hence, with increase in total Sn loading, the total hydrogen consumption also increased. 3Pt-1.5Sn-SA1 and 3Pt-3Sn-SA1 catalyst samples have similar hydrogen consumption in the temperature range of 100–250 °C. This means that the unalloyed metallic Pt content in both the samples is nearly same. However, the hydrogen consumption at 450 °C is higher for 3Pt-3Sn-SA1. This means that formation of PtSn alloy depends on the ratio of Pt and Sn oxides present in the vicinity. In case of 3Pt-3Sn-SA1 catalyst, XRD pattern in Fig. 2 and TPR patterns in Fig. 3 confirms the formation of Pt-Sn alloy upon reduction

Table 3 Hydrogen uptake and H2/Pt atomic ratio of 3Pt-SA1 and 3Pt-3Sn-SA1 catalyst. Catalyst Sample

Reduction temperature °C

H2 uptake (μmol g−1)

H2/Pt

Pt-Sn site (μmol g−1)

3Pt-SA1

300 400 300 400

17.83 16.76 20.94 17.45

0.12 0.11 0.14 0.12

– – 98.2 95.4

3Pt-3Sn-SA1

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the other hand, bimetallic Pt-Sn catalysts showed exemplary levels of selectivity towards ethanol at high conversion levels. XRD profile of bimetallic catalyst in Fig. 2, TPR profiles in Fig. 5 and hydrogen consumption in Table 2 when compared with activity result confirms the fact that PtSn alloy formation directly contributes towards ethanol production. Comparing the results of Table 2 with yield of ethanol shows it has a direct relationship with hydrogen consumption at 450 °C. Higher the hydrogen consumption at 450 °C, higher is the conversion and ethanol selectivity. At an optimum Pt and Sn loading, highest amount of Pt-Sn alloy formation takes place, which leads to higher conversion and ethanol selectivity. Pt-Sn alloy formation decreases the size of the Pt ensembles [26]. This was also observed in the Pt mean particle size shown in Table 1 wherein, 3Pt-3Sn-SA1 has the lowest mean particle size of 2.24 nm among all bimetallic catalyst samples. Hence, it can be inferred that, optimum Pt-Sn alloy content in the catalysts improves the conversion as well as ethanol selectivity. However, in case of 3Pt-4.5Sn-SA1 catalyst sample, the SnO2 molecules cover the active sites of the catalyst thereby, increasing Pt particle size and decreasing conversion and selectivity [24]. TOF values calculated for the hydrogenation reaction (Table 4) follow volcano like profile with the maxima for 3Pt-3Sn-SA1 catalyst. With increase in Sn content from 0 to 3%, TOF improved significantly. TOF of 0.317 s−1 was obtained for 3Pt-1.5Sn-SA1 catalyst and further increase in Sn content to 4.5%wt reduced the TOF to 0.290 s−1. The TOF of 3Pt-3Sn-SA1 is 0.325 s−1 as against the TOF of around 0.43 s−1 obtained for similar catalyst tested at 270 °C, 2.6 MPa and WHSV 2 h−1. The TOF values can be considered to be comparable considering the fact that the WHSV are different for both cases. At WHSV of 3 h‐1, 3Pt3Sn-SA1 gave 81% conversion with more than 95% ethanol selectivity as against 82% conversion and around 90% ethanol selectivity [25]. In order to confirm the fact that, upon reduction, bimetallic Pt-Sn catalysts forms alloy which in turn, favours higher yields of ethanol and other side products, experiments were carried out with calcined and reduced form of 3Pt-3Sn-SA1 catalyst sample. These runs were carried out at 270 °C, 1 MPa, H2/acetic acid molar ratio of 10 and WHSV 5 h−1. The variation of reaction effluent composition for both calcined and reduced form of the catalyst sample is shown in Fig. 6 and S1 respectively. Results revealed that for the in-situ reduced 3Pt-3Sn-SA1 catalyst, the effluent composition almost remains constant for the studied time on stream (TOS) of 8 h. However, for the calcined catalyst, the composition varied with time. Acetic acid weight fraction decreased from 98% to 68% in 8 h TOS and ethanol weight percentage increased from 1.4% to 16.2% (Fig. 6A) due to the increase in acetic acid conversion. Ethyl acetate and acetaldehyde weight fraction also increased slowly with increase in TOS. Fig. 6B shows the amount of CH4 in gaseous effluent leaving the reactor. It was observed that CH4 concentration decreased from 400 ppm to nil with increase in TOS. It can be inferred from the activity results of 1Pt-SA1 and 3Pt-SA1 catalysts that at the early stages of reaction over the calcined catalyst, hydrogenation of acetic acid over metallic Pt leads to the formation of methane. However, as the TOS increases, the catalyst is reduced further in the presence of H2 and the dispersed Pt phase interacts with the adjacent SnO2 molecules to form Pt-Sn alloy which is also confirmed from the XRD patterns of reduced catalyst in Fig. 2. Formation of this Pt-Sn alloy, in turn, alters the selectivity for CH4 formation to ethanol and accordingly CH4 content in gaseous effluent stream decreased and ethanol weight fraction increased in the liquid stream. Overall, it was observed that the optimum catalyst formulation 3Pt3Sn-SA1 achieves superior conversion and ethanol selectivity at a moderate temperature of 270 °C, pressure of 2 MPa, WHSV 3 h−1 and H2/acetic acid molar ratio of 10. Inspite of this superior conversion, there is still scope for further improvement in the selectivity to ethanol. Ethanol selectivity can be further improved by reducing the formation of ethyl acetate, which is the primary by-product of the reaction. Carboxylic acid and alcohol can undergo esterification reaction in the presence of acid catalysts leading to the formation of esters [39–41]. As

patterns of the reduced catalysts (Fig. 2). This probably reduces the availability of unalloyed Pt content of the catalyst samples which is reflected on the total hydrogen uptake. Higher hydrogen uptake of 3Pt3Sn-SA1 compared to 3Pt-SA1 at both the reduction temperatures is due to higher metal dispersion offered by 3Pt-3Sn-SA1. In the absence of Sn atoms, Pt atoms tend to agglomerate leading to lower hydrogen uptake and dispersion. This is also evident in the particle size of the catalysts shown in Table 1. An effort was made to quantify the active sites attributed to Pt-Sn alloy formation in 3Pt-3Sn-SA1 catalyst. This was done by multiplying the hydrogen uptake in Table 3 with Pt-Sn Reduction% (Table 2) and dividing the result with hydrogen to Pt ratio. Results showed that 3Pt-3Sn catalyst provides around 98 μmol per gram of catalyst. 3.4. Hydrogenation activity of the catalysts Preliminary activity runs were conducted with catalyst samples of varying mean particle size (0.3 mm–0.6 mm) to quantify the influence of intra-particle mass transfer resistances. The conversion and selectivity variation w.r.t to different particle sizes was observed to be insignificant in the range studied (Fig. S4). Accordingly, 0.5 mm particle size was used for further runs. The effect of possible external mass transfer resistances were checked as per procedure given in literature [38] and accordingly, Mear’s criterion for external mass transfer resistance were found to be within range (details in supplementary information). Heat transfer limitations were observed to be negligible as the temperature difference between internal catalyst bed and reactor external wall was less than 2 °C. The performance of the catalyst samples for acetic acid hydrogenation to ethanol was evaluated at 270 °C, 2 MPa, WHSV of 3 h−1 and H2/acetic acid molar ratio of 10. Results of the same are shown in Table 4. Acetic acid hydrogenation reaction over blank support SA1 and 1Sn-SA1 catalyst showed no conversion. Monometallic Pt has the tendency to produce significant amount of hydrocarbon but at a very low conversion of 8–12%. This is attributed to the fact that mono-metallic Pt favours CeC bond cleavage, resulting in formation of hydrocarbon [21,23]. However, the bimetallic Pt-Sn catalysts show very high selectivity towards ethanol at a reasonable level of conversion. The introduction of 1.5%wt Sn on 3%wt Pt resulted in 67.8% conversion with ethanol selectivity of 82%. Further increase in Sn content to 3%wt improves the conversion of acetic acid to 81% with ethanol selectivity of 95.4%. However, when the Sn content is further increased to 4.5% wt, both conversion and selectivity drops to 63.4% and 88.2% respectively. It can be inferred from the results that monometallic Pt or Sn impregnated over SA1 support are incapable of hydrogenating acetic acid selectively to produce ethanol. Correlating these results with the XRD profile shown in Fig. 2, one can deduce that metallic Pt, PtO or SnO atoms present in the reduced form of catalysts does not contribute towards ethanol production. On Table 4 Catalyst activity and product selectivity of Pt/Sn catalysts. Catalyst

SA1 1Sn-SA1 1Pt-SA1 3Pt-SA1 3Pt-1.5Sn-SA1 3Pt-3Sn-SA1 3Pt-4.5Sn-SA1

TOF sec−1

0.0 0.0 0.134 0.102 0.317 0.325 0.290

Conversion, % Selectivity, %

0.0 0.0 8.4 11.9 67.8 81.0 63.4

Ethanol Ethyl acetate

Acetaldehyde Methane

0.0 0.0 55.7 62.6 84.2 95.4 88.2

0.0 0.0 0.0 0.0 0.9 0.4 1.1

0.0 0.0 0.0 1.5 14.9 4.2 10.7

0.0 0.0 44.3 35.9 0.0 0.0 0.0

Reaction conditions: Temp. 270 °C; press.2 MPa; WHSV 3 h−1; H2/acetic acid mole ratio 10.

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Fig. 6. Product analysis of over calcined 3Pt-3Sn-SA1 catalyst sample. (A) Liquid products and (B) Gaseous products. Reaction conditions: Temp. 270 °C; Press. 1 MPa; WHSV 5 h−1; H2/ acetic acid ratio ∼ 10.

final catalysts decreased by 20–25% as compared to the pure support samples. This confirms that the impregnation of Pt and Sn on the surface has not altered the pore sizes significantly. Platinum dispersion of the three catalysts 3Pt-3Sn-SA1, 3Pt-3Sn-SA2 and 3Pt-3Sn-SA3 was estimated using H2 pulse chemisorption technique and results revealed that Pt dispersion of all the three catalyst samples are in the range of 40–42% confirming uniform metal loading in all the samples. NH3 molecule was used as a molecular probe for measuring total acid density of the support and the final catalyst samples because of its selective interaction with both Brӧnsted and Lewis acid sites. NH3 was first allowed to absorb on the surface of the sample at constant temperature and then desorbed from the surface with increase in temperature at constant rate. Desorption of NH3 at lower and higher temperatures are correlated with weaker and stronger acidic sites of the sample [42]. The NH3-TPD profile of the as-synthesized 3Pt-3Sn-SiO2Al2O3 catalysts are shown in Fig. 7. From these profiles, it can be seen that there are two prominent peaks at 150 °C and 580 °C corresponding to weak and strong acidic sites respectively [42]. The corresponding acid densities of all the samples under each peak are listed in Table 5. Results show that the strong acid sites are the major contributor to the total acidity of the support or the final catalyst formulations. The estimated acid densities also reveal that SA1 has the lowest acidity, followed by SA2 and SA3. The impregnated catalyst samples follow similar trend, with total acidity increasing in the order of 3Pt-3SnSA1 < 3Pt-3Sn-SA2 < 3Pt-3Sn-SA3. Since the alumina content of the support also varies with similar trend, it can be inferred that the total acidity of the catalyst is directly proportional to the Al2O3 content in the support, which is known to offer acidic sites. Isopropylamine TPD of the three Pt-Sn catalysts impregnated on SA1, SA2 and SA3 supports were carried out to estimate the number of Bronsted acid sites on the catalysts. The acid density results are shown in Table 6 and the TPD profiles are shown in Fig. S3. Results showed that the acid density increases in the order of 3Pt-3Sn-SA1 < 3Pt-3SnSA2 < 3Pt-3Sn-SA3. So it can be inferred that the Bronsted acid density increased with increasing alumina content. Similar trends were also observed for total acidity. Hence, it can be concluded from the results shown in Tables 5 and 6, that the Bronsted acid sites contributed majorly towards the total acidity of the silica-alumina supported catalysts.

per the reaction mechanisms elaborated in the literature for esterification of acetic acid with ethanol, it is the acidity of the catalyst that promotes the reaction [25,39]. Presence of lewis acid sites promotes both ethanol and ethyl acetate formation and accordingly, both products form competitively through adsorbed ethoxy species [25]. On the other hand, presence of Bronsted acid sites initiates esterification reaction of acetic acid and ethanol to form ethyl acetate [39]. Hence, in order to minimize the formation of ethyl acetate, the influence of support acidity on ethanol selectivity was studied in detail.

3.5. Pt-Sn catalysts on supports of varying acidity The optimum catalyst formulation, 3Pt-3Sn was prepared over three different supports with varying SiO2-Al2O3 content. These supports are termed as SA1, SA2 and SA3 having SiO2 purity of 99%, 95% and 85% respectively and remaining part being Al2O3. The supports were tested for possible impurities using FTIR and the results are shown in Fig. S2. Peaks at 803 cm−1, 1090 cm−1, 1635 cm−1 and 3440 cm−1 were observed which are characteristic of silica-alumina material inferring that impurities are absent in the support materials. Physical properties of the support and the final impregnated catalysts are shown in Table 5. The surface area of all the samples was estimated using N2 adsorptiondesorption isotherms from multi-point BET instrument. Results revealed that all the selected supports have high surface area in the range of 185–262 m2/g. Upon impregnation of metals, the surface area of the Table 5 Physical properties of the silica-alumina supported 3Pt-3Sn catalysts. Catalyst

SA1 SA2 SA3 3Pt-3Sn-SA1 3Pt-3Sn-SA2 3Pt-3Sn-SA3

Acid density (mmol of NH3/g of catalyst)

Surface Area

Metal Dispersion

Weak (T1)

Strong (T2)

Total

m2/g

%

0.040 0.122 0.347 0.029 0.094 0.260

0.100 0.277 0.542 0.083 0.230 0.450

0.140 0.399 0.889 0.113 0.325 0.713

185 256 262 133 206 211

– – – 42.4 41.5 40.3

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Fig. 7. NH3-TPD profile of 3Pt-3Sn-SiO2-Al2O3 catalyst over different supports.

per reaction (2). The performance of 3Pt-3Sn catalyst impregnated over different supports were evaluated for acetic acid hydrogenation to ethanol at 270 °C temperature, 2 MPa pressure, WHSV of 3 h−1 and H2/acetic acid molar ratio of 10. Results are summarized in Table 7. Acetic acid conversion was observed to be decreasing in the order of 3Pt-3SnSA1 > 3Pt-3Sn-SA2 > 3Pt-3Sn-SA3. Correlating this observation with the acid densities of the catalyst listed in Table 5 shows that with increase in total acid site density from 0.113 to 0.713 mmol NH3/g catalyst the conversion of acetic acid decreases from 81% to 76.5%. Acid density can also be correlated to the ethanol selectivity, wherein, with increase in total acid density from 0.113 to 0.713 mmol NH3/g catalyst, ethanol selectivity decreases from 95% to 52% and ethyl acetate selectivity increases from 3.9% to 44.6%. However, the effect of total acidity on acetaldehyde formation was observed to be insignificant. The TOFEA of ethyl acetate formation for the catalysts listed in Table 7 were estimated using the rate of ethyl acetate formation and total acid site density values given in Table 5. Results show that TOFEA

Table 6 Isopropylamine (IPA) TPD of the SiO2-Al2O3 supported 3Pt-3Sn catalysts. Catalyst

Acid density (μmol of IPA per gram of catalyst)

3Pt-3Sn-SA1 3Pt-3Sn-SA2 3Pt-3Sn-SA3

98 203 594

3.6. Effect of catalyst acidity on ethanol selectivity In order to validate the ability of the synthesized catalysts to carryout esterification reaction, experiments were carried out with 1:1 mixture of acetic acid and ethanol as feed over SA1 support, 3 Pt-3SnSA1 catalyst and blank inert glass beads. Results revealed that inert glass beads gave no signs of ethyl acetate formation. However, in the presence of SA1 support and 3 Pt-3Sn-SA1 catalyst noticeable amount of conversion could be observed (Fig. 8). This confirms the fact that the oxide supported Pt-Sn catalyst facilitates the esterification reaction as

Fig. 8. Ethanol conversion w.r.t Time on-stream (TOS) for esterification reaction of 1:1 Acetic acid and ethanol over SA1 support, 3 Pt3Sn-SA1 catalyst and inert beads carried out at 270 °C, 10 bar pressure and WHSV of 3 h−1.

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Table 7 Catalytic performance of SiO2-Al2O3 supported 3Pt-3Sn catalysts. Catalyst

3Pt-3Sn-SA1 3Pt-3Sn-SA2 3Pt-3Sn-SA3

Conversion%

81.0 79.4 76.5

TOFEA sec−1

Selectivity, % Ethanol

Ethyl acetate

Acetaldehyde

Methane

95.4 80.2 51.9

3.9 17.5 44.6

0.4 2.2 3.5

0.0 0.0 0.0

containing well distributed Pt-Sn ensembles obtained by reduction using hydrogen to give surface acyl species and a hydroxyl group (reaction (8)). The gaseous hydrogen molecule splits into H atom on the metallic sites M* (reaction (7)) and further attacks the hydroxyl group via spillover mechanism to form H2O (reaction (9)). On removal of water, the adsorbed acetaldehyde molecules can either desorb to form acetaldehyde (reaction (10)) or further hydrogenate to form ethanol (reactions (11) and (12)). Results show that with increased formation of Pt-Sn alloy, the extent of reactions (9) and (11) increases significantly leading to enhanced selectivity of ethanol. On the other hand, in presence of proton donating catalyst (Bronsted acid site), acetic acid molecule adsorbs associatively to form di-hydroxyl radical which, in turn, reacts with the ethanol molecule to form ethyl acetate. The following reaction scheme elaborates the process.

Table 8 Catalytic performance of bimetallic Pt-Sn catalysts. Catalyst

Total acid site density mmol of NH3/g catalyst

TOFEA sec −1

3Pt-1.5Sn-SA1 3Pt-3Sn-SA1 3Pt-4.5Sn-SA1

0.125 0.113 0.105

0.0034 0.0031 0.0033

of ethyl acetate formation remains almost constant (0.003 s−1) and does not vary with varying acid densities. In order to ascertain the effect of metal loading on the rates of ethyl acetate formation, TOF of ethyl acetate formation was calculated based on the activity runs of bimetallic catalysts listed in Table 4. The TOF of ethyl acetate formation and along with their acid densities are shown in Table 8. From the values obtained, it is clear that even with the difference in metal content of the catalyst samples, the TOF of ethyl acetate formation remains constant around 0.0034 s −1. This means that the rate of esterification reaction is purely governed by the number of Bronsted acid sites present in the catalyst and the metal content or composition has little implication on ethyl acetate yield. It is mentioned in the literature that presence of Lewis acid in silica supported Pt-Sn catalyst favours both ethanol and ethyl acetate. These two products form in parallel by virtue of hydrogenation of ethoxy group and reaction of acetyl and ethoxy group, respectively [25]. However, in case of silica-alumina supports, Bronsted acid sites are much more in quantity than Lewis acid sites. Hence, the effect of Lewis acid sites is overshadowed by the presence of Bronsted acid sites, which contribute towards the formation of ethyl acetate. This is further substantiated by the results shown in Fig. 8, wherein, experiment was conducted only with acetic acid and ethanol without addition of hydrogen. As per the mechanism suggested in literature, conversion of acetyl group to ethoxy group takes place in the presence of disassociated hydrogen [25]. Since in this experiment, hydrogen was not added, ethoxy group could not have formed. Accordingly, further condensation of ethoxy and acetyl group to ethyl acetate is not feasible under those circumstances. Yet, there was evidence of ethyl acetate formation, which is only possible by proton donation from Bronsted acid sites. A more detailed study on this aspect will be done in future work. The entire results of the current work can be explained using the mechanisms of acetic acid hydrogenation and acetic acid esterification proposed in literature [23,41]. Dissociative adsorption of acetic acid leads to hydrogenation reaction and thereby, produces ethanol or acetaldehyde as per the following reaction scheme. H2 + 2M* ⟵→ 2H − M*

(7)

CH3COOH + S* ⟵→ CH3CO − S* − OH

(8)

CH3CO − S* − OH + 2H-M* ⟵→ CH3CHO − S* + H2O + 2M*

(9)

CH3CHO − S* ⟵→ CH3CHO + S*

(10)

CH3CHO − S* + 2 H − M* ⟵→ CH3CH2OH − S* + 2M*

(11)

CH3CH2OH − S* ⟵→ CH3CH2OH + S*

(12)

0.0034 0.0032 0.0031

CH3COOH + H − S* ⟵→ CH3C(OH)2 − S*

(13)

CH3C(OH)2 − S* + CH3CH2-OH ⟵→ CH3C(OH)C2H5 − S* + H2O (14) CH3C(OH)C2H5 − S* ⟵→ CH3COOC2H5 + H − S*

(15)

Upon proton transfer to the acetic acid from the catalyst surface H-S (reaction (13)), the protonated acetic acid is accessible for nucleophilic attack by the hydroxyl group of the ethanol. This leads to the elimination of water molecule (reaction (14)) and formation of ethyl acetate (reaction (15)) [39]. Thus, the conversion of acetic acid to ethanol and subsequent esterification with ethanol to ethyl acetate is majorly governed by the concentration of sites containing oxide supported Pt-Sn alloy ensemble [S*] and the concentration of proton donating oxide support [H-S*] on the catalyst surface. With high acidity of the catalyst, concentration of [H-S*] increases on the catalyst surface. The acetic acid molecules adsorbed associatively on these [H-S*] sites cannot further take part in hydrogenation reaction. Hence, the total acetic acid molecule available for hydrogenation per unit mass of catalyst reduces with increasing acidity, which in turn, reduces the total acetic acid conversion. Accordingly, 3Pt-3Sn-SA3 having the highest acidity gives the lowest conversion and 3 Pt-3Sn-SA1 having the lowest acidity gives highest conversion of acetic acid (Table 6). On the other hand, these protonated acetic acid molecules can undergo esterification reaction with the hydrogenated product ethanol. Henceforth, increase in acidity increases the extent of esterification reaction and accordingly reduces the ethanol selectivity and increases ethyl acetate selectivity. The TOF estimated for ethyl acetate formation almost remains constant highlighting the fact that the rate of ethyl acetate formation is a function of acidic sites present per unit mass of catalyst. The total acid density of the catalysts studied in this work is much less as compared to standard acidic materials such as γ-alumina or zeolites and hence, the concentration of [H-S*] sites is much lesser than concentration of [S*] sites. This means, the rate of acetic acid esterification reaction would be much lesser compared to the rate of acetic acid hydrogenation reaction. In order to summarize the present work, Scheme 2 has been proposed. Platinum was first impregnated on the oxide support using suitable precursor and upon calcination produced metallic Pt impregnated

Acetic acid molecule dissociates appropriately on oxide support S* 88

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Scheme 2. Hydrogenation of acetic acid over Pt-Sn-SiO2 catalyst.

compared to monometallic Pt catalysts. The addition of Sn to Pt catalyst forms Pt-Sn alloy upon reduction at elevated temperatures. The results of characterization and catalytic performance reveal that at optimal Pt and Sn content (3Pt-3Sn-SA), maximum Pt-Sn alloy formation takes place which results in highest ethanol productivity. The effect of reduction temperature was studied at two temperatures and it was found that in order to maximize Pt-Sn alloy formation a minimum reduction temperature is essential which will ensure all the Pt and Sn atoms are alloyed to provide maximum active sites per unit weight of catalyst. The effect of catalyst acidity on acetic acid conversion and ethanol selectivity was studied and it was found that proton donating capability of the support leads to the formation of ethyl acetate as by-product which in turn reduces ethanol selectivity. Overall, it can be summarized that superior catalyst preparation technique and appropriate Pt/Sn ratio will lead to higher ethanol formation but presence of Bronsted acid sites will consume these ethanol molecules to form ethyl acetate. Hence, superior catalyst support needs to be identified which will lead to suppression of ethyl acetate and maximise ethanol production.

on SiO2-Al2O3 support (Step 1). Monometallic Pt under hydrogen environment with acetic acid as reactant produced large amount of hydrocarbon due to high CeC bond cleavage favoured by metallic Pt atoms (Step 2). However, impregnation of Sn using suitable precursor and further calcination produced metallic Pt and SnO2 impregnated catalyst (Step 3). This catalyst sample was then reduced using H2 at elevated temperatures (400 °C) to produce Pt-Sn alloy (Step 4), which is the active site for acetic acid conversion to ethanol. Hydrogen and acetic acid reacts in the presence of this catalyst to produce ethanol and acetaldehyde as products (Step 5). In presence of excess hydrogen maintained inside the reactor, ethanol yield is much higher compared to acetaldehyde. The ethanol produced can either desorb from the catalyst surface (Step 6) or react with the reactant acetic acid to produce ethyl acetate (Step 7). The extent of step 7 is governed by the ability of the catalyst to donate H+. Hence, the catalysts with higher acidity produced more ethyl acetate compared to the catalysts having lower acidity. In the light of this, further work needs to be carried out on modifying the support to reduce the acidic sites for further improvement of ethanol selectivity. Recent literature reports the improved performance of K and Ca modified Pt-Sn catalysts [43,44], but more work needs to be carried out to arrive at the best possible solution to mitigate ethyl acetate formation.

Acknowledgements We acknowledge the support provided by Bharat Petroleum Corporation Ltd. (BPCL) for pursuing this work. The authors acknowledge the support provided by Dr. Selvakannan Periasamy, Dr. Deepa Dumbre and Dr. Suresh Bhargava of Royal Melbourne Institute of Technology for facilitating catalyst characterization. Special thanks to Dr. Chanchal Samanta, Mr. Pintu Maity and Ms Sonal Asthana for providing assistance while carrying out the present work.

4. Conclusions Pt-Sn bimetallic catalysts were co-impregnated over SiO2-Al2O3 support using incipient wetness technique. The resulting catalyst samples were characterized by Transmission electron microscopy, H2-pulse chemisorption, BET surface area analyzer, Powder X-Ray diffraction, NH3-TPD and H2- TPR methods. Catalyst samples were evaluated using isothermal fixed bed reactor. The bimetallic catalyst samples showed improved performance for acetic acid conversion to ethanol, in terms of moderate conversion (∼81%) and high ethanol selectivity (95%), as

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2018.01.030. 89

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