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Activity and Selectivity of Platinum−Copper Bimetallic Catalysts Supported on Mordenite for Glycerol Hydrogenolysis to 1,3Propanediol Samudrala Shanthi Priya,†,‡ Ponnala Bhanuchander,† Vanama Pavan Kumar,† Suresh K. Bhargava,‡ and Komandur V. R. Chary*,† †

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Hyderabad-500007, India Centre for Advanced Materials & Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, GPO BOX 2476, Melbourne-3001, Australia

‡

S Supporting Information *

ABSTRACT: Biomass derived glycerol is considered an ideal feedstock with a prospective to be converted into a number of valuable compounds. Catalytic glycerol hydrogenolysis to produce 1,3-propanediol is one of the pioneering biosustainable pathways. Bimetallic Pt−Cu catalysts supported on H-mordenite were synthesized with various copper loadings and applied in the selective glycerol hyrogenolysis to 1,3-propanediol in a continuous fixed bed reactor performed in vapor phase under atmospheric pressure. Several techniques such as XRD, ICPAES, NH3-TPD, Pyr FTIR, BET, TPR, HR-TEM, XPS, and solid state NMR were employed to characterize the physical and chemical properties of Pt−Cu/Mor catalysts. A detailed reaction parametric study has been carried out. The results designated that well dispersed Pt−Cu catalysts with small particle size, supported on a Brønsted acidic H-mordenite with a multiple pore system and strong bimetallic phase-support interaction, promote the selectivity to 1,3-propanediol. Over the Pt−Cu/Mor catalyst of optimum composition (2% Pt and 5% Cu by weight) and under the optimum reaction conditions (210 °C, H2 flow rate of 80 mL min−1, and gly concentration of 10 wt %), the glycerol conversion and 1,3-PD selectivity reached 90% and 58.5%, respectively. Structural characterizations and reusability of the Pt-5Cu/Mor catalyst were also performed. With evident advantages of selective C−O hydrogenolysis with low C−C cleavages, the bimetallic Pt−Cu/Mor catalysts hold great potential as high-performance catalysts for glycerol conversion to 1,3propanediol.

1. INTRODUCTION 1,3-Propanediol (1,3-PD) is a promising target for its versatile application as an important monomer in the manufacturing of polymethylene terephthalate (PTT).1 It is a valuable chemical intermediate widely used in the manufacturing of cosmetics, personal care, cleaning, lubricants, and medicines and in the production of heterocyclic compounds.2 The other applications include engine coolants, food and beverages, deicing fluids, water-based inks, heat transfer fluids, and unsaturated polyester resins.3 The expanding use of 1,3-PD is expected to register the highest growth and in turn has created an increasing demand. Conventionally, 1,3-PD was produced by fermentation of glucose and chemically by two methods: the hydration process of acrolein or hydroformylation reaction of ethylene oxide followed by hydrogenation.4 The chemical methods, however, have many disadvantages such as the use of high pressure, high temperature, and low selectivity. Nonetheless, the major limitation of the fermentation route is the relatively high cost of glucose feedstock. The economically attractive solution would be the development of a novel, cost-effective process that utilizes renewable resources as feed stocks for sustainable © XXXX American Chemical Society

chemical methods. Introduction of such processes would be able to provide considerable energy savings besides reducing the dependency on fossil fuels and avoiding greenhouse gas emissions. Glycerol is a renewable three-carbon triol available as a major byproduct of biodiesel production and has been recognized as an important unit for upcoming chemical refineries. For each gallon of biodiesel formed from the transesterification of triglycerides with methanol, one pound of glycerol (approx. 10 wt % of the whole product) is produced which is comparable or even greater than that obtained from the soap manufacturing process.5 Subsequently, the value of glycerol in the market has been falling sharply, and the utilization of glycerol as a new feedstock for producing green chemicals would partially compensate the profitability of biodiesel plants in the biodiesel industry. In this regard, the catalytic hydrogenolysis of low cost Received: January 14, 2016 Revised: March 25, 2016 Accepted: March 28, 2016

A

DOI: 10.1021/acs.iecr.6b00161 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Reaction of Glycerol Hydrogenolysis over Pt−Cu/Mor Catalysts

glycerol to high value 1,3-PD is considered to be a highly viable alternative route. Until now, substantial work has been carried out for converting glycerol to valuable compounds through various reactions and processes.6,7 The present work aims to provide more details about the catalytic reaction of selective glycerol hydrogenolysis to 1,3-PD. The hydrogenolysis of glycerol has been investigated by quite a few researchers using various heterogeneous catalysts such as Cu based,8,9 Raney Ni,10 and noble metal (Pt, Ru, Rh, Ir, etc.) catalysts.11−14Among the noble metals, Pt based catalysts are highly active and thermally stable toward glycerol hydrogenolysis.15,16 Moreover, several researchers have employed Pt based catalysts for selective production of 1,3-PD, but their yields are still not satisfying.17−21 The catalyst deactivation caused by coke formation constrains the catalytic efficiency of platinum catalysts.22 The modification of Pt catalysts is indeed necessary to further improve the catalytic performance in the glycerol hydrogenolysis. It has been reported that bimetallic catalysts often exhibit higher activity than either of their monometallic catalysts.23 It is clearly demonstrated that addition of a second metal significantly increases catalytic activity, improving the selectivity and catalyst stability.24 In view of this, the catalytic production of propanediols from hydrogenolysis of glycerol has been investigated in the previous years for which various bimetallic catalysts have been explored comprising supported Ru−Re,25,26 Pt−Re,27 Ru−Cu,28 and Pt−Ru29 catalysts. Zhou et al. explored the CuAg/Al2O3 catalyst for the selective hydrogenolysis of glycerol to propanediols at 200 °C, 1.5 MPa H2 pressure for 10 h and resulted in almost 100% selectivity to propanediols with 27% glycerol conversion.30 There are however only a few reports which illustrated the selective conversion of glycerol into 1,3-PD. Tomishige and group described that the Ir-ReOx/ SiO2 catalyst was highly active in the hydrogenolysis of glycerol to selectively produce 1,3-PD performed in liquid phase at 120 °C, 8 MPa H2.31,32 In spite of several research studies, the reaction was not commercially successful due to some common problems such as the use of high reaction pressure, reaction medium, and the catalytic system used. It is, therefore, certain that developing new chemical methods with the use of highly proficient and stable catalysts for conversion of glycerol to high valued 1,3-PD under mild reaction conditions is of great significance. The Pt catalysts demonstrate good activity for the glycerol hydrogenolysis reaction but often promote C−C bond cleavage giving rise to undesired products. Copper is a well-known active metal used for many hydrogenation reactions. Cu-based catalysts show high efficacy in C−O bond hydrodehydrogenation and are recognized for their selective C−O bond cleavage without C−C bond cleavage which formally occurs during the hydrogenolysis process. Therefore, copper-based catalysts have drawn much attention as they are relatively cheaper catalysts

and exhibit superior performance in a glycerol hydrogenolysis reaction.33−35 However, Pt although highly efficient is a rare and expensive noble metal. Such a combination of two metals may produce a more efficient catalyst with certain advantages such as improved catalytic properties and enhanced catalytic activity. Mordenite is a protonic zeolite which finds wide applications in many industrial processes as solid acid catalyst and catalyst support.36 Mordenite is a shape selective well-defined crystalline structured zeolite and exhibits explicit properties such as simple regeneration and thermal stability. The strong Brønsted acidity of mordenite due to bridging hydroxyl groups in the Si−OH−Al units of the zeolite structure is responsible for its outstanding catalytic performance.37 In our previous work,38 we had successfully demonstrated the catalytic activity of the mordenite supported platinum catalyst in the vapor phase hydrogenolysis of glycerol and found that Brønsted acidity of mordenite was an important factor for the selective production of 1,3-propanediol. These findings prompted us to investigate Pt−Cu bimetallic catalysts over H-mordenite support in order to study the influence of the modified Pt catalyst which could presumably favor C−O hydrogenolysis of glycerol and facilitate the selective production of 1,3-PD (Scheme 1). To this end, we have synthesized a series of Pt−Cu/Mor catalysts by the coimpregnation method and employed them for hydrogenolysis of glycerol to 1,3-PD in a vapor phase continuous fixed bed reactor. Various characterization techniques, such as XRD, NH3-TPD, Pyr FTIR, CO chemisorption, TEM, TPR, and surface area measurements, were applied to study the physicochemical properties of the catalysts. Concurrently, the effect of reaction parameters, i.e., catalyst loading, reaction temperature, H2 pressure, feed flow rate, and reaction time, has been studied for the exploration of the optimized reaction conditions. More specifically, the important synergistic effect of Pt and Cu on improving the catalytic activity during glycerol hydrogenolysis has been revealed. To the best of our understanding, our report illustrates the first example of an efficient bimetallic Pt−Cu catalyst supported on H-mordenite for vapor phase glycerol hydrogenolysis to 1,3-PD under atmospheric pressure.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. H2PtCl6·6H2O and Cu(NO3)2· 3H2O (analytical grade, produced by Sigma-Aldrich Co., Ltd.) were used as metal precursors. The protonated form of the mordenite zeolite (hereafter designated as Mor) with a SiO2/ Al2O3 ratio of 20 was obtained from Conteka, The Netherlands (BET-surface area ∼510 m2 g−1). The monometallic catalysts (e.g., Pt/HM and Cu/HM) prepared by the wet impregnation method were dried at 120 °C overnight and calcined at 550 °C for 4 h in the muffle furnace. The bimetallic Pt−Cu catalysts B


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of the vapors released during the reduction. TCD monitors the hydrogen concentration in the effluent, and the GRAMS/32 software was used to integrate the peak areas. High resolution transmission electron microscopy (HRTEM) measurements were performed with a JEOL 2010 electron microscope operating at 200 kV. The catalyst powder samples were ultrasonicated in ethanol and dispersed on copper grids. Specimens were placed to the microscope column, at an evacuation of less than 1 × 10−6 Torr. X-ray photoelectron spectroscopy analysis was achieved on a Thermo K-5 Alpha XPS instrument employing a focused monochromatic Al Kα X-ray (1486.6 eV and 150W) source running at 1 × 10−9 Torr pressure to characterize the surface chemical states of catalysts. All spectra core levels were aligned with a C 1s binding energy of 285 eV. 2.3. Reaction Experiment. The vapor phase hydrogenolysis of glycerol was conducted in a vertical continuous fixed bed quartz reactor (i.d.9 mm, 40 cm length) under atmospheric pressure. The catalyst bed is prepared by charging 0.5 g of the catalyst in the constant temperature zone of the reactor, and a thermocouple was employed to control the temperature. Preceding the reaction, the catalysts (0.5 g) were reduced at 350 °C for 2 h in flowing H2 (60 mL min−1). After reduction, the reactor was cooled down to the desired reaction temperature (210 °C) and fed with a mixture of 10 wt % glycerol aqueous solution and hydrogen (80 mLmin−1) through a syringe feed pump. Finally, the reaction mixture was condensed and collected in an ice−water trap for analysis every hour on a GC (Shimadzu) equipped with a DB-wax 1237033 (Agilent) capillary column (0.32 mm i.d., 30 m long) and a FID detector. The conversion of glycerol was calculated as the ratio of amount of the glycerol used up in the reaction to the initial glycerol fed into the reactor. The selectivity of products (1) was quantified according to the following equation

were also prepared by wet impregnation of H-mordenite. The H-mordenite support was impregnated with a solution containing a mixture of Pt and Cu precursor salts by the coimpregnation method. The resulting solids were dried at 120 °C overnight and calcined at 550 °C for 4 h. A series of catalysts with different Cu loadings were synthesized and designated as Pt-xCu/Mor, where x represents the weight loading of Cu (2, 5, 8, and 10 wt %). The loading amount of Pt was set at 2 wt % for all the catalysts. 2.2. Catalyst Characterization. The structures of synthesized catalysts were characterized by powder X-ray diffraction (XRD) analysis over a Rigaku miniflex X-ray diffractometer using Ni filtered Cu Kα radiation (λ = 0.15406 nm). The measurements were obtained in the 2θ range of 2 to 65°, at a scan rate of 2° min−1 operated at a beam voltage of 30 kV, and a beam current of 15 mA respectively. The average crystallite size of metals was measured using the Scherrer equation from the line width of respective XRD peaks. Inductively coupled plasma atomic emission spectrometry (Agilent Technologies-4200 MP-AES) was used to analyze the Pt and Cu contents in all Pt-xCu/Mor catalysts. The samples were prepared by digesting approximately 10 mg of the catalyst in 2 mL of aqua regia and then diluting with Millipore water to the required concentrations. The resultant solutions were analyzed by ICP-AES. The textural characteristics including the Brunauer−Emmet− Teller (BET) surface area, average pore diameter, and pore volume of the catalysts were characterized by N2 adsorption− desorption isotherms at −197 °C using Quantachrome Autosorb 1 instruments. NH3-TPD experiments were performed on an AutoChem 2910 instrument (Micromeritics, USA) equipped with a TCD detector for continuous monitoring of desorbed NH3. Prior to TPD measurements, 0.1 g of sample was dried in flowing (99.995%) helium (50 mL min−1) at 200 °C for 1 h, then adsorbed with 10% NH3−He at 80 °C for 1 h until saturation, and later flushed with He (50 mL min−1) at the same temperature for 30 min. TPD analysis was conducted out from 100 to 800 °C with 10 °C min−1 heating rate, and the quantity of NH3 desorbed was measured using GRAMS/32 software. For FTIR analysis of adsorbed pyridine, the catalysts were pretreated in N2 flow at 300 °C for 1 h to take out the moisture content from the samples, followed by adsorption of pyridine at 120 °C until saturation. Then the samples are allowed to cool until room temperature. Each of the samples was grounded with KBr, pressed into the sample holder, and mounted in the cavity of the spectrometer. The FTIR spectra were then recorded on a GC-FT-IR Nicolet 670 spectrometer in the spectral range of 1400−1800 cm−1 using KBr background. 27 Al MAS NMR experiments were conducted on an Agilent Technologies DD2 Oxford Magnet AS-500 MHz spectrometer with aluminum oxide (Aldrich) as a reference material to study the skeleton framework composition. The chemical shifts (δ) are mentioned in ppm. Temperature-programmed reduction (TPR) experiments were conducted on an AutoChem 2910 instrument (Micromeritics, USA). Initially, 0.1 g of sample (dried at 110 °C for 12 h) was pretreated in a flow of argon gas (50 mL min−1) at 200 °C. The sample was then cooled to ambient temperature and is purged with the carrier gas consisting of 5% hydrogen balance argon (50 mL min−1) raising the temperature to 400 °C at a heating rate of 10 °C min−1. A cold trap submerged in a slurry of liquid nitrogen and isopropyl alcohol helps in condensation

Selectivity (%) =

moles of one product × 100 moles of all products

(1)

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. Physicochemical Properties of the Catalysts. Figure S1 (Supporting Information) illustrates the XRD patterns of mordenite supported platinum−copper bimetallic catalysts. The diffraction peaks observed at 2θ = 9.76°, 13.52°, 19.70°, 22.42°, 25.73°, 26.44°, and 27.64° are the characteristic diffraction peaks reported for mordenite. After metal impregnation, additional peaks have been observed. A small diffraction peak of platinum at 2θ = 39.76° assigned to the (1 1 1) reflection reveals the occurrence of small crystals of Pt on the support surface of Pt-xCu/Mor catalysts with almost the same intensity. The XRD pattern of 5Cu/Mor and Pt-xCu/Mor catalysts show the presence of both CuO phases (JCPDS File No. 80-1917) and Cu2O phases (JCPDS File No. 78-2076). The majority of peaks corresponds to the monoclinic phase of CuO (2θ = 35.54°, 48.52°, 53.48°), whereas a peak at 2θ = 36.39° is indexed to the cubic Cu2O phase.39 Meanwhile, the intensity of the CuO peak appears to increase as the amount of copper deposited on the catalyst increases. From the full width at half maxima (fwhm) of copper (1 1 1) plane diffraction, the average crystallite sizes of CuO and Cu2O phases on Pt-xCu/Mor were calculated using the Scherrer equation, and the dimensions are in the range of 6.1−13.7 nm C


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Industrial & Engineering Chemistry Research Table 1. Physicochemical Properties of Mor, Pt/Mor, and Pt-xCu/Mor Catalysts average crystallite sizeb (nm)

a

a

2 −1

a

a

−1

catalyst

BET SA (m g )

pore diameter (nm)

pore volume (cc g )

Mor 2Pt/Mor 5Cu/Mor Pt-2Cu/Mor Pt-5Cu/Mor Pt-8Cu/Mor Pt-10Cu/Mor

507 398 402 333 276 268 235

3.71 3.29 3.30 3.19 2.75 2.17 1.04

0.35 0.33 0.34 0.26 0.19 0.14 0.06

Pt

CuO

Cu2O

8.4 6.1 7.6 11.4 13.7

4.1 3.5 3.8 3.8 3.9

3.9 3.7 3.6 3.7 3.5

ICP-AESc (wt %) Pt

Cu

1. 1 0.7 0.6 0.4 0.4

3.4 1.6 2.9 4.7 5.3

Data obtained from BET adsorption−desorption isotherms. bDetermined from XRD. cMetal contents determined from ICP-AES analysis.

and 3.5−3.9 nm, respectively. The average size of platinum crystallites calculated from the line width of the (111) crystal plane remained almost constant, since the loading amount of Pt (2 wt %) was fixed in all the catalysts. The results are presented in Table 1. The average size of copper crystallites on Pt-xCu/ HM (x = 2, 5) is small compared with the other catalysts (x = 8, 10), suggestive of a strong interaction between copper and platinum particles on the support, which brought about the copper particles dispersed well on the support. Nonetheless, this should be the reason that at higher Cu loading, the interaction between copper and platinum particles in turn with Mor support was weak; as a result, big copper particles could be formed on Mor. However, it can be observed that X-ray diffraction profiles did not show any significant change in the crystallinity of zeolite structure after deposition of Pt and Cu. Total Pt and Cu metal contents in all Pt-xCu/Mor catalysts were determined by an Inductive Coupled Plasma-Atomic Emission Spectrometer (ICP-AES), and the results are depicted in Table 1. The textural properties, such as BET surface area, pore diameter, and pore volume obtained for all the catalysts from the physisorption measurements, are shown in Table 1. The N2 adsorption−desorption isotherms and the pore size distributions of Mor, 2Pt/Mor, 5Cu/Mor, and Pt-xCu/Mor catalysts with different Cu contents are shown in Figure S2 (A,B) of the Supporting Information. The catalysts exhibited Type IV isotherms with a featured hysteresis loop type H, indicating the mesoporous nature of the catalysts. The surface area of pure H-mordenite (Mor) is 507 m2 g−1, whereas after metal impregnation the surface areas of the catalysts decreased and are in the range of 398−235 m2 g−1. The pore volume of all the catalysts exists in the range of 0.31−0.06 cm3 g−1, which is lower than Mor. The measured pore diameter of all catalysts is in the range of 3.2−1.0 nm. The smooth decline in both surface area and pore volume coupled with the diameter of the pore channels is ascribed to the pore blockage as well as the structural collapse of precursors during calcination of the catalyst after metal impregnation.40 3.1.2. Evaluation of Catalyst Acidity. 3.1.2.1. NH3-TPD Analysis. NH3-TPD experiments were performed to investigate the acidity and acidic strength distribution of Mor, Pt/Mor, and various Pt-xCu/Mor catalysts (Figure 1). The quantitative estimation of acid strength distribution at different regions as per the amount of ammonia desorbed is briefed in Table 2.The NH3-TPD profiles confirm that the acid sites in Mor and Pt/ Mor are found in two different regions i.e., at 100−250 °C and above 500 °C which attributes to weak and strong acid sites. The acidic sites in the 5Cu/Mor catalyst are distributed in two different regions 100−250 °C and 250−500 °C which correspond to weak acid sites and moderate acid sites

Figure 1. NH3-TPD profiles of Mor, 2Pt/Mor, and various Pt-xCu/ Mor catalysts.

Table 2. Acidities of Mor, Pt/Mor, and Pt-xCu/Mor Catalysts from NH3-TPD Analysis NH3 uptake (μmol g−1) catalyst

weak

Mor 2Pt/Mor 5Cu/Mor Pt-2Cu/Mor Pt-5Cu/Mor Pt-8Cu/Mor Pt-10Cu/Mor

289.7 207.3 237.0 173.3 94.2 95.0 95.8

moderate

strong 721.6 438.28

335.0 212.2 669.0 504.9 481.8

330.7 102.0 90.7 96.52

total NH3 uptake (μmol g−1) 1011.3 645.5 572.0 716.2 865.2 690.6 674.12

respectively, while more than two desorption peaks can be seen on Pt-xCu/Mor catalysts in three different regions i.e., at 100−250 °C, 250−500 °C, and above 500 °C. In general, the D


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Industrial & Engineering Chemistry Research desorption of ammonia from the first region is attributed to weak acidic sites, the second region denotes to desorption of ammonia from moderate strength acidic sites, and the third region refers to strong acidic sites.41 Taking into account the results of NH3-TPD from Table 2, the Mor displays the highest acidic strength, and the acidic strength of catalysts is decreased significantly after metal impregnation. The Pt/Mor catalyst displayed weak and strong acidic sites, whereas the Cu/Mor catalyst showed the presence of weak and moderate acidic sites. However, it is quite interesting to note that the Pt-xCu/Mor catalysts displayed three types of acid sites (weak, moderate, and strong) distributed in three different regions. It is observed that addition of Cu induced new moderate strength acidic sites and platinum facilitated the formation of strong acidic sites in the bimetallic Pt-xCu/Mor catalysts thereby increasing the total acidity of bimetallic catalysts compared to monometallic ones. The total acidity of the bimetallic catalysts was higher due to the synergetic interaction of platinum, copper, and Mor support. The concentration of acidic sites was found to increase with copper loading up to 5 wt % and beyond which it decreased. The decrease in total acidity at higher copper loadings could be due to the agglomeration of copper crystallites as evident from XRD studies. This behavior is well correlated with the catalytic activity. It is also observed that with an increase in the copper loading, there is loss of weak acidic sites and a substantial increase in the moderate and strong acid sites. This clearly indicates that a proper balance of moderate to strong acidic sites was responsible for hydrogenolysis of glycerol to propanediols. The concentration of acidic sites is decreased in the following sequence: Mor > Pt-5Cu/Mor > Pt-2Cu/Mor > Pt-8Cu/Mor > Pt-10Cu/Mor > 2Pt/Mor > 5Cu/Mor. Therefore, the TPD results suggest that the acidic strength of catalysts plays an important role in defining the catalytic activity for hydrogenolysis of glycerol. 3.1.2.2. Pyr Adsorption−Desorption Experiments Followed by FTIR (Pyr FTIR). For the acid-catalyzed hydrogenolysis of glycerol, besides the surface acid strength, the nature of acid sites indeed play a main role in determining the catalytic performance. Therefore, FT-IR spectra of pyridine absorption were recorded to probe accessible surface acid sites, which is a powerful tool for identifying the nature of acid sites. As displayed in Figure 2, all the samples presented typical bands corresponding to strong Brønsted bound pyridine, at around 1540 cm−1. The bands at around 1450 cm−1 were assigned to the Lewis acid sites by adsorbed pyridine, while the band at 1489 cm−1 originated from the combination of pyridine on both Brønsted and Lewis acid sites.42 Based on the results from Pyr IR, it is noteworthy that Brønsted acid sites are more prevalent in Pt-xCu/Mor catalysts (x = 2, 5 wt %) with lower Cu loadings; while at higher loadings (x = 8, 10 wt %), the PtxCu/Mor samples showed an increase in the Lewis acid strength along with a reduced intensity band for Brønsted acidity. 3.1.3. Solid State 27Al NMR. Solid state 27Al NMR spectroscopy is an important experimental technique used for the characterization of aluminosilicate zeolites and provides an insight about the coordination of aluminum sites with the metal nanoparticles. 27Al NMR spectra of all the catalysts are presented in Figure 3. The 27Al NMR spectrum of pure Hmordenite (Mor) exhibits a peak at 53.7 ppm which is ascribed to the tetrahedral coordinated aluminum sites in the zeolite framework.43 The presence of this peak in the pure mordenite

Figure 2. Pyr FT-IR spectra of Mor and various Pt-xCu/Mor catalysts.

Figure 3. 27Al NMR spectra of Mor and various Pt-xCu/Mor catalysts.

and all the metal impregnated catalysts clearly showed that metal impregnation did not change the Al tetrahedral sites or the basic framework. However, metal impregnation resulted in a shift and significant broadening of the peak, which became prominent at higher metal loadings. This is perhaps due to the metal interaction caused by the expulsion of Al atoms from the framework sites due to pore blockage caused by metal impregnation. This is in line with the results of N 2 adsorption−desorption studies. As reported, the Brønsted sites are associated with the extra framework protons, while tetrahedral aluminum sites act as Lewis acid sites; all of the synthesized samples possess both Brønsted (B) and Lewis (L) acid sites.44 It is noteworthy, that the intensity of the peak decreases with metal loading and infers that the pure mordenite possesses a greater number of acid sites than for Pt-xCu/Mor samples. These 27Al NMR results are well correlated with those obtained from NH3-TPD and Pyr FTIR. E


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Industrial & Engineering Chemistry Research 3.1.4. Temperature-Programmed Reduction (TPR). To study the reducibility and the degree of interaction between metal particles with support, temperature-programmed reduction experiments on the monometallic and bimetallic calatysts were performed. The TPR profile of the Pt/Mor catalyst (Figure S3 of the Supporting Information) is characterized by a single reduction peak at Tmax = 370 °C, which was likely the reduction of PtO2 species and some Pt2+ or Pt4+ ions, whereas that of Cu/Mor displayed a peak centered at Tmax = 252 oC, which refers to the reduction of CuO.45 Notably, the reduction peaks are shifted much toward the lower temperature in the bimetallic samples compared to the monometallic one. It may be noted that the addition of copper in the catalyst system has significantly promoted the reduction, for the presence of copper is facilitating the reduction of platinum species, decreasing the reduction temperature. The TPR profile of bimetallic Pt-xCu/Mor catalysts gives rise to two broad reduction peaks between 150 and 300 °C corresponding to the reduction of CuO and PtO species in different interactions with the support. The reduction temperature of bimetallic catalysts was found to be in between the reduction temperatures exhibited by platinum and copper monometallic catalysts which clearly suggest that there exists an intimate interaction between copper and platinum particles as well as with the support.46 However, at higher copper loadings, a shift in the second reduction peak toward lower temperature region was observed. This decrease in the reduction temperature clearly designates that a close interaction between copper and platinum species was attained in Pt-xCu/Mor bimetallic catalysts. An additional peak observed at low temperatures for the Pt-10Cu/Mor catalyst could be ascribed to mutually interacting mixed Pt−Cu oxidized species.47 The additional shoulder peak observed could be due to the large sized copper particles dispersed on the surface of support and weakly interacting with the support. As far as the TPR of the Pt-5Cu/ Mor catalyst is concerned, one can observe important hydrogen consumption and the most prominent maximum around 272 °C corresponding to the reduction of copper species strongly interacting with the Pt and support. 3.1.5. Transmission Electron Microscopy (TEM). The morphology and distribution of Pt and Cu particles on the mordenite zeolite framework is revealed by transmission electron microscopy. TEM images of various Pt-xCu/Mor catalysts with different Cu loadings presented in Figure 4 (a, b, c, d) display that the samples contain small crystallites with an average size of about 5−15 nm. The metal particles are homogeneously distributed and well dispersed on the surface of mordenite. Apparently, the Cu loading affects the size of metal particles. The lattice fringes characteristic of the mordenite zeolite is revealed from the high resolution (HR) TEM image shown in Figure 4 (e). TEM images clearly describe that the metal particles are fairly evenly distributed on the zeolite framework. 3.1.6. X-ray Photoelectron Spectroscopy (XPS). XPS studies of the catalysts were executed to investigate the oxidation and surface chemical states of copper, platinum, and aluminum. The XPS core level spectra of Cu 2p, Al 2p, and O 1s core levels are presented in Figure 5, and the binding energies of different components are given in Table S1 (Supporting Information). XPS analysis of Pt-xCu/Mor catalysts revealed the presence of two chemically distinct copper species (Cu 2p3/2) on the surface of support which is well correlated with the results obtained from XRD. The first kind of copper species exhibits its

Figure 4. TEM images (a,b,c,d) of various Pt-xCu/Mor catalysts; HRTEM (e, f) images of Mor and Pt-5Cu/Mor catalysts.

binding energy, ranging from 933.4 to 933.8 eV, and it was found to be more intense than the other copper species. The second copper species binding energy appeared around 936 eV, close to copper hydroxide. These results revealed that copper exists on the surface of mordenite in two chemically distinct forms at higher loadings, and these two components were attributed to the copper present in the bimetallic phase (replacing the extra-framework brønsted protons) and the zeolite framework by replacing the aluminum from its sites.48 The shift in the Cu 2p3/2 binding energy present in the bimetallic phase is conclusive evidence of the strong metal support and metal−metal interaction. In general, since Al 2p and Pt 4f binding energies (71−76 eV) appear in the same region, it is difficult to decouple the spectra of both elements, and it appears to overlap with each other. The similar findings are reported in the previous literature.49 At lower copper loading (Figure 5B), the presence of platinum was clearly evident from the appearance of the Pt 4f7/2 component around 72 eV, while its intensity decreased considerably at higher loadings. This observation agrees with the ICP-AES results that at higher copper loadings, the weight ratio of copper versus platinum was different from their precursor’s ratio. Since it is a coimpregnation method, copper preferentially adsorbs over platinum, probably that accounts for the reduction of platinum at higher copper loadings. In addition, the Al 2p core level spectra (Figure 5B) showed a significant change as the copper loading increased. This is a clear indication of copper interaction with the aluminum sites in the zeolite framework. Figure 5C shows the O 1s core-level spectra that shows only two kinds of chemically distinct oxygen species in the case of Pt−Cu/Mor at the lowest loading. These F


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Figure 5. XPS of (A) Cu 2p, (B) Al 2p, and (C) O 1s of Pt-xCu/Mor catalysts.

two species correspond to the framework oxygen and surface hydroxyl groups. After successive copper loadings, the surface hydroxyl oxygen shows a significant shift as a result of their interaction with the metal particles. Interestingly, the third kind of oxygen component is observed at higher copper loadings in the region of 531 eV, close to the metal oxide species. The XPS results clearly reveal that there is a strong bimetallic phase interaction with the zeolite support and that metal oxide species becomes prominent at higher copper loadings. The results are in corroboration with 27Al NMR and TPR studies. 3.2. Activity Test for Hydrogenolysis of Glycerol. 3.2.1. Monometallic and Bimetallic Catalytic Performance. Catalytic activity and product selectivity have been investigated over Pt, Cu monometallic, and Pt-xCu/Mor bimetallic catalysts in a vapor phase continuous fixed-bed reactor at 210 °C and atmospheric pressure. The catalytic results are studied in comparison to those obtained from monometallic catalysts as shown in Table 3. The catalysts exhibit an appreciable activity

for the glycerol hydrogenolysis reaction. In all cases, 1,3-PD was the major product along with other reaction products such as 1,2-PD and ethylene glycol (EG). The major gaseous product identified in the glycerol hydrogenolysis reaction over Pt-xCu/ Mor catalysts was propane with some other unidentified products in negligible amounts. The other products such as propanols (PO) (1-propanol + 2-propanol), ethanol, acetone, and methanol were also detected in minor amounts. However, Cu deposited on Mor (Cu/Mor) appears to be particularly effective for 1,2-PD formation, with selectivities of up to 62.6% at 77.6% glycerol conversion. It is noteworthy that Pt−Cu/Mor bimetallic catalysts exhibited superior performance to facilitate glycerol conversion and excellent selectivity with respect to 1,3PD formation comparative to monometallic (Pt/Mor and Cu/ Mor) catalysts under the existing reaction conditions. The results of conversion of glycerol and selectivity to liquid products are shown in Table 3. The results of selectivity including both liquid and gaseous products are depicted in Table S2 of the Supporting Information. The enhanced catalytic activity of bimetallic Pt-xCu/Mor catalysts for glycerol hydrogenolysis is due to the presence of bifunctionality and high acidic strength which clearly reflects the intimate interaction of Pt−Cu species with the underlying Mor support. Transformation of glycerol to propanediols can proceed via two possible reaction routes (Scheme 2). The first route (A) involves dehydration of glycerol over Brønsted acidic sites to form 3-hydroxypropionaldehyde (3-HPA) as a reaction intermediate. The hydrogenation of the intermediate on metal sites forms 1,3-PD.18,50,51 The second route (B) proceeds through acetol formed by the dehydration of glycerol over Lewis acidic sites, and subsequent hydrogenation of acetol gives 1,2-PD. Product distribution obtained in this study suggests that the reaction proceeds through the first reaction route. Reaction Parameter Studies-Catalyst Screening. To optimize glycerol conversion and 1,3-PD selectivity, the effect of metal loading, hydrogen pressure, reaction temperature, WHSV, reaction time, and W/F on the hydrogenolysis of glycerol is examined over Pt-xCu/Mor catalysts. 3.2.2. Effect of Cu Loading. In our earlier study,22 the prominent effect of 2 wt % Pt on the activity of supported Pt catalysts was revealed in glycerol hydrogenolysis to 1,3-PD. Therefore, 2 wt % Pt was preferred as the standard loading,

Table 3. Effect of Cu Loading on Glycerol Hydrogenolysis to 1,3-PDa selectivity to liquid products (%) catalyst 2Pt/ Mor 5Cu/ Mor Pt-2Cu/ Mor Pt-5Cu/ Mor Pt-8Cu/ Mor Pt10Cu/ Mor

PO

others

carbonb (%)

25

8.7

5.7

1.47

14.2

8.0

6.5

1.61

8.4

1.58

conversion (%)

1,3PD

1,2PD

EG

94.9

48.6

12

77.6

8.7

62.6

92

53.6

16

9.0

13

90

58.5

18

8.4

9.6

5.5

1.51

86

50.8

20

9.8

9.2

10.2

1.65

82

49.0

25

10.6

7.5

7.9

1.62

Reaction conditions: 0.5 g catalyst; reaction temperature: 215 °C, 0.1 MPa H2; H2 flow rate: 80 mL min−1; WHSV = 1.02 h−1; 1,3-PD: 1,3propanediol, 1,2-PD: 1,2-propanediol, EG: ethylene glycol, PO: propanols, others include hydroxyacetone, ethanol, methanol, acetone. b Carbon content on the used catalysts estimated from CHNS analysis. a

G


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Scheme 2. Course of Glycerol Hydrogenolysis over (A) Brønsted Acidic Sites and (B) Lewis Acidic Sites of Pt−Cu/Mor Catalysts

and, in the present work, a series of H-mordenite (Mor) supported Pt−xCu bimetallic catalysts (x = 2, 5, 8, and 10 wt %) was synthesized to investigate the catalytic activities in glycerol hydrogenolysis (Table 3). With 2 wt % copper loaded, 92% glycerol was converted, and 1,3-PD selectivity was 53.6%. When Cu loading is increased to 5 wt %, a significant increase in 1,3-PD selectivity (58.5%) was observed with a small decrease in the glycerol conversion (90%). An increase in Cu loading to 8 and 10 wt % shows a tempered influence on both glycerol conversion and 1,3-PD selectivity. The gradual decrease in glycerol conversion with an increase in copper loading might be probably due to a decrease in the total acidity or the concentration of acidic sites in 2Pt-xCu/Mor catalysts with an increase in Cu loading, as evident from the results of NH3-TPD (Table 2). In addition a substantial increase in the selectivity of 1,2-PD and EG was noticed with an increase in Cu loading. The enhanced Lewis acidity at higher copper loadings led to the increased formation of 1,2-PD. Therefore, we can conclude that the Pt-5Cu/Mor catalyst possessed appropriate Brønsted acidity and the proper number of active metal sites required for hydrogenation of the reaction intermediate to 1,3PD during glycerol hydrogenolysis. This result indicates that 5 wt % Cu on the Pt-xCu/Mor catalyst was sufficient for maximum conversion of glycerol and favors 1,3-PD selectivity which could be due to the synergistic effect of metal species and Brønsted acidic sites of Mor. Hence, further experiments were performed with the optimal 2Pt-5Cu/Mor catalyst. 3.2.3. Effect of Reaction Temperature. To explore the influence of reaction temperature on the glycerol hydrogenolysis, reactions were performed at 150, 180, 210, and 240 °C respectively under atmospheric pressure over the Pt-5Cu/ Mor catalyst. Figure 6 shows the obtained results. As the temperature increases from 150 to 240 °C, a constant increase

Figure 6. Effect of reaction temperature on hydrogenolysis of glycerol to 1,3-PD reaction conditions: reaction temperature = 150−210 °C, reduction temperature = 350 °C ; H2 flow rate = 80 mL min−1; WHSV = 1.02 h−1.

in the glycerol conversion from 64 to 86% was observed.52 The maximal 1,3-PD selectivity of 58.5% was obtained at 210 °C reaction temperature. However, the selectivity of 1,3-PD decreased with the further increase in temperature. A similar observation in the selectivity of 1,2-PD and a gradual increase in the selectivity of propanols and EG was noticed. This indicated that 1,3-PDO undergoes sequential hydrogenolysis at higher temperature (>210 °C), leading to C−C bond cleavage and producing lower alcohols like methanol and ethanol. 3.2.4. Effect of Hydrogen Flow Rate. The influence of hydrogen flow rate was considered in the range from 40 to 100 mL min−1 at reaction temperature 210 °C. The results are shown in Figure 7. It can be found that glycerol conversion accelerated with the increasing hydrogen flow rate. The more noticeable effect of the hydrogen flow rate was on the H


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literature.14 Overall selectivity of 1,3-PD decreases from 58.5% to 39.6% with increasing WHSV from 1.02 to 4.08 h−1. In contrast, an increase in the selectivities of 1,2-PD and EG was observed with the increase of WHSV. Therefore, the result suggests that a WHSV 1.02 h−1 might allow activation of a secondary hydroxyl group of glycerol to make the hydrogenolysis reaction more selective to 1,3-PD. 3.2.6. Effect of Contact Time (W/F). The amount of the PtxCu/Mor catalyst was varied over a fixed feed rate of glycerol to evaluate the effect of contact time (W/F) on glycerol conversion and 1,3-PD selectivity. The results are depicted in Figure 9. As expected, glycerol conversion dramatically boosted Figure 7. Effect of hydrogen flow rate on hydrogenolysis of glycerol to 1,3-PD reaction conditions: reaction temperature = 210 °C; H2 flow rate = 40, 60, 80, and 100 mL min−1; WHSV = 1.02 h−1.

selectivity of propanediols. At low hydrogen pressure, 40 mL min−1, about 74% of glycerol is converted to a mixture of 1,3PD, 1,2-PD, EG, and PO. Under these conditions 1,3-PD was the major product with a selectivity value of 36.2%. By increasing the hydrogen flow rate to 80 mL min−1, a sharp increase in the selectivity of 1,3-PD from 36.2 to 58.5% was achieved associated with a decrease in 1,2-PD selectivity. It infers that the rate of hydrogenation of 3-HPA to 1,3-PD is being enhanced at a higher hydrogen flow rate where the active hydrogen species absorption increases on the metal surface. Further increase of hydrogen pressure resulted in a decrease in 1,3-PD selectivity, as it is likely to promote excessive hydrogenolysis to form PO. Such a trend is in close agreement with the reported results.53 This result reveals that the hydrogen flow rate of 80 mL min−1 seems to be sufficient for the hydrogenation of the 3-HPA intermediate to the desired 1,3-PD. 3.2.5. Effect of Weight Hourly Space Velocity (WHSV). The conversion of glycerol decreases with increasing WHSV when reactions were performed over the Pt-5Cu/Mor catalyst at 210 °C with the varying 10 wt % glycerol feed flow from 0.5 to 2.0 mL h−1 (corresponding WHSV = 1.02−4.08 h−1) (Figure 8). The limited number of active sites available for conversion of glycerol to 1,3-PD would be the probable reason for the decrease in glycerol conversion. The decrease in conversion of glycerol with increasing WHSV is consistent with the

Figure 9. Effect of contact time (W/F) on hydrogenolysis of glycerol to 1,3-PD reaction conditions: reaction temperature = 210 °C; H2 flow rate = 80 mL min−1; glycerol feed: 0.5 mL h−1; W/F = 0.4, 0.6, 0.8, 1.0 g mL−1 h.

from 66% to 90% with an increasing contact time of 0.4 to 1.0 g mL−1 h. Also a significant increase in the 1,3-PD selectivity from 38.2% to 58.5% was observed. On the contrary, an increase in W/F resulted in a decrease of EG and PO selectivity. No noticeable change in the 1,2-PD selectivity was observed during the reaction. This suggests that longer contact time tends to increase the active metallic sites and allow the hydogenolysis reaction to be more feasible toward the formation of 1,3-PD. Similar findings have indeed been reported elsewhere.54 3.2.7. Effect of Reaction Time. Glycerol conversion was found to increase with increasing reaction time and reaches 90% after 3 h of reaction (Figure 10). Thereafter, it remains

Figure 8. Effect of WHSV on hydrogenolysis of glycerol to 1,3-PD reaction conditions: reaction temperature = 210 °C; H2 flow rate = 80 mL min−1; catalyst wt = 0.5 g; WHSV = 1.02 h−1, 2.04 h−1, 3.06 h−1, 4.08 h−1.

Figure 10. Effect of reaction time on hydrogenolysis of glycerol to 1,3PD over 2Pt/Mor catalyst reaction conditions: reaction temperature = 210 °C; H2 flow rate = 80 mL min−1; WHSV = 1.02 h−1. I


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Industrial & Engineering Chemistry Research Table 4. Studies on the Spent Pt-5Cu/Mor Catalyst

ICP-AESc (wt %) catalyst

conversion (%)

selectivity of 1,3PDO

average size of CuOcrystallitesa (nm)

average size of Pt crystallitesa (nm)

BET SA

acidityb (μmol g−1)

Pt

Cu

fresh spent

90 84

58.5 49.6

7.6 9.8

3.6 4.9

276 201

865.2 690.0

0.6 0.6

2.9 2.8

a

Determined from XRD. bMeasured from NH3-TPD studies; cMetal contents determined from ICP-AES analysis.

selectivity of 58.5% at 210 °C, 0.1 MPa hydrogen pressure, and WHSV of 1.021 h−1. Strong metal support interaction, small Pt−Cu particle size, and appropriate acidity of the mordenite support, evident from various characterization studies, enhanced the catalytic performance for the selective production of 1,3-PD from glycerol via dehydration-hydrogenation pathway. Reusability and structural stability of the 2Pt-5Cu/Mor catalyst was examined and revealed that there was a considerable change in catalyst morphology which led to a slight decline in the catalytic activity.

constant up to 5 h and finally it decreases to 76% after 8 h. Initially, 48% selectivity of 1,3-PD is achieved in the first hour, reached a maximum (58.5%) after 3 h of reaction, and decreased to 46% after 8 h. The decrease in 1,3-PD selectivity after a certain period of time might be due to over hydrogenolysis of 1,3-PD to produce propanols. This is also ascribed to the formation of byproducts as a result of excessive C−C bond scission. The time on stream results reveal that such deactivation of the catalyst could be attributed to the blockage of the active acid sites by carbon deposition as revealed by CHNS analysis (Table 3).55 3.3. Studies on the Spent Catalyst. To evaluate the reusability of the bimetallic Pt-xCu/Mor catalyst, the spent catalyst was first treated in air at 300 °C for 3 h followed by reduction in H2 flow at 300 °C for 3 h in order to reconstruct the chemical and structural features of the catalyst which is quite essential to complete a second reaction cycle under the same reaction conditions (210 °C, 0.1 MPa). The spent catalyst was shown to be active in terms of glycerol conversion (84%); however, contrary to what was observed in the first reaction, 1,3-PD selectivity dropped slightly from 58.5% to 49.6% over the spent catalyst. This indicates that some changes in the distribution of active metal and surface acid sites of bifunctional catalysts might have occurred during the course of the reaction. The cause of catalyst deactivation was investigated by ICP-AES, XRD, NH3-TPD analysis, BET surface area, and CHNS analysis. The results in Table 4 are compared with that obtained from the fresh sample. After the reaction, the Pt and Cu contents of the spent Pt-5Cu/Mor catalyst have been analyzed by ICP-AES in order to check any possible metal loss during the reaction. The results suggest that there is no appreciable leaching or loss in the amount of platinum or copper from the catalyst surface observed after the reaction. However, the acidity and BET surface area of the spent catalyst was found to be decreased, while the XRD pattern showed comparatively large metal crystallites with increased intensity (Figure S1). These results imply that Pt and Cu species agglomerate, and pore blockage of support would subsequently make bimetallic sites inaccessible for the reaction. This behavior can indeed be expected due to the possible coke deposition, evident from CHNS analysis, during the reaction and a consequent decline in the catalytic activity.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00161. Table S1, binding energy (eV) values of Cu 2p, Al 2p, O 1s, Si 2p, and Pt 4f of Pt-xCu/Mor catalysts as determined from XPS measurements; Table S2, selectivity of liquid and gas products obtained from glycerol hydrogenolysis; Figure S1, XRD patterns of Mor, 5Cu/Mor, and Pt-xCu/Mor catalysts; Figure S2, N2 adsorption−desorption isotherms (A) and pore size distribution curves (B); Figure S3, TPR profiles of PtxCu/Mor catalysts (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.S.P. gratefully acknowledges CSIR-IICT & RMIT for the award of Research Fellowship. The authors thank RMIT University for helping in the catalyst characterization work.

■

REFERENCES

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4. CONCLUSIONS In summary, Pt−Cu bimetallic catalysts supported on Hmordenite have been successfully employed as competent catalysts for the vapor phase hydogenolysis of glycerol. The bimetallic Pt and Cu exhibit excellent combination and manifested much higher activity than monometallic catalysts. The addition of Cu metal to a Pt-based catalyst distinctly improved the selectivity toward 1,3-PD. A comprehensive study of reaction variables suggested that the 2Pt-5Cu/Mor catalyst displayed maximum glycerol conversion of 90% and 1,3-PD J


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