Platinum Supported on H-Mordenite: A Highly Efficient Catalyst for

Research Article pubs.acs.org/journal/ascecg

Platinum Supported on H‑Mordenite: A Highly Efficient Catalyst for Selective Hydrogenolysis of Glycerol to 1,3-Propanediol Samudrala Shanthi Priya,†,‡ Ponnala Bhanuchander,† Vanama Pavan Kumar,† Deepa K. Dumbre,‡ Selvakannan R. Periasamy,‡ Suresh K. Bhargava,‡ Mannepalli Lakshmi Kantam,§ 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 § Department of Chemical Sciences, Tezpur University, Napaam 784028, Assam, India ‡

S Supporting Information *

ABSTRACT: The selective production of 1,3-propanediol from glycerol under mild reaction conditions is of high interest. The current work describes the use of a highly selective catalyst consisting of platinum supported on mordenite zeolite employed for the first time for vapor phase hydrogenolysis of glycerol to 1,3-propanediol under atmospheric pressure. The catalysts with varying Pt content (0.5−3 wt %) were prepared and thoroughly characterized by X-ray diffraction, temperature-programmed desorption of ammonia, FT-IR of adsorbed pyridine, CO chemisorptions, transmission electron microscopy, X-ray photoelectron spectroscopy, and BET surface area. The influence of reaction parameters has been studied to unveil the optimized reaction conditions. A high 1,3-propanediol selectivity (48.6%) was obtained over a 2 wt % Pt/H−mordenite catalyst at 94.9% glycerol conversion. According to the results obtained, the selectivity to 1,3-propanediol is better influenced by Pt dispersion and Brønsted acidity of the support. A plausible reaction mechanism has been presented. The spent catalyst exhibited consistent activity and selectivity toward the desired product during the glycerol hydrogenolysis reaction. KEYWORDS: Glycerol, Vapor phase hydrogenolysis, 1,3-Propanediol, Brønsted acidity, Platinum, H-mordenite, X-ray photoelectron spectroscopy

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oxidation9 have been reported to convert glycerol to useful fine chemicals by several research groups. Among these, catalytic hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) has been known to be a more promising and economically viable route. Interestingly, the hydrogenolysis product 1,3-PDO is most valuable and finds versatile application as an important monomer for the production of polyester fibers.10 Considerable research has been focused on the catalytic hydrogenolysis of glycerol to 1,2-

INTRODUCTION During the past decade, the catalytic conversion of biomass to chemicals and fuels has been gaining growing importance due to environmental concerns as well as depletion of resources.1 Glycerol is one of the biomass-derived oxygenated hydrocarbons and an important renewable resource currently used in the pharmaceutical, food, and cosmetics industries. Because of the ample availability of glycerol as a byproduct of biodiesel production, new applications such as sustainable production of value-added products are commanding much attention and will inevitably benefit the economy of the biodiesel production industry.2,3 Many potential reactions such as acetalization,4 dehydration,5 esterification,6 etherification,7 aqueous phase reforming,8 and © XXXX American Chemical Society

Received: October 10, 2015 Revised: December 10, 2015

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DOI: 10.1021/acssuschemeng.5b01272 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Reaction Pathways of Glycerol Hydrogenolysis

Mordenite, with an ideal composition of Na8Al8Si40O96· nH2O, is one of the most important zeolites applied in catalysis and has been used for hydroisomerization, alkylation, and dewaxing processes.36,37As a Brønsted acidic zeolite with a multiple pore system, H-mordenite has received much attention in both industrial and academic research.38,39 Platinum is known for its high dispersion on zeolite supports. The interaction of metal particles with the acid sites of the mordenite provides promising possibilities for the design of bifunctional catalysts. Therefore, a bifunctional catalyst having Brønsted acidity and hydrogenation properties would be the ideal catalyst for the conversion of glycerol to 1,3-PDO. Indeed, there is also a need to develop new, innovative, and greener catalytic processes to transform glycerol into high-value 1,3PDO over a highly efficient catalyst. In an attempt to develop an environmentally benign and economically feasible route of glycerol hydrogenolysis, the present study was aimed at investigating the catalytic performance of various loadings of Pt/HM catalysts for vapor phase selective conversion of glycerol to 1,3-PDO in a continuous fixed bed reactor under atmospheric pressure. The product distributions as a function of different reaction variables, viz., metal loading, reaction temperature, hydrogen flow rate, weight hourly space velocity (WHSV), and contact time, are also presented to determine the favorable reaction conditions. In addition, coke deposition on a spent catalyst is also studied, which could give a deeper understanding of the changes in the catalytic performance and catalyst deactivation. To date, there is no study in the literature on hydrogenolysis of glycerol to 1,3-PDO over a Pt/HM catalyst, and the research work reported herein is the first of its kind in successful use of H-mordenite as a catalyst support for selective production of 1,3-PDO via glycerol hydrogenolysis.

PDO.11−14 However, selective formation of 1,3-PDO is difficult, and the reported systems have been limited. In general, glycerol hydrogenolysis to propanediols proceeds via a dehydration−hydrogenation route over a typical bifunctional system formed by a metal for hydrogenation and an acid catalyst for dehydration.15,16 Moreover, the type of propanediol produced depends on the selective cleavage of the primary or secondary hydroxyl group of glycerol. As mentioned, there are two possible pathways in hydroxyl group selective removal, namely, the hydroxyacetone (HA) pathway and 3-hydroxypropionaldehyde (3-HPA) pathway, of which the 3-HPA pathway has been the most challenging. HA could be produced by the coordination of Lewis acid to the primary hydroxyl group of glycerol, and further hydrogenation gives 1,2-PDO. According to the theory of the 3-HPA pathway, the secondary hydroxyl group of glycerol is dehydrated by Brønsted acid to form 3HPA, followed by catalytic hydrogenation to 1,3-PDO. Therefore, the presence of a Brønsted acid in the system is required to selectively produce 1,3-PDO.17 As a matter of fact, the acid strength and texture properties significantly affect the selectivity to 1,3-PDO through the hydrogenolysis of glycerol. A number of homogeneous and heterogeneous bifunctional metal−acid catalysts, combining acid sites for dehydration and metal sites for a hydrogenation reaction, have been attempted for the conversion of glycerol to 1,3-PDO either in the liquid phase or gas phase.18−22 While metals including Cu, Pt, Ru, Ir, Re, and Rh have been extensively used as the active components for hydrogenolysis of glycerol,23−26 platinum stands in the forefront of the metals presenting a high activity for glycerol hydrogenolysis. Most effective catalysts for 1,3-PDO production contained a noble metal with tungsten or rhenium as the acid additive.27−31 Nevertheless, the inherent disadvantage of the existing processes ascends from the usage of liquid acids and organic solvents accompanied by high reaction pressure, which violates environmental protection. Accordingly, careful design of an alternative heterogeneous solid acid catalyst and its usage under mild reaction conditions are highly desirable for practical application. Protonic zeolites are one of the most widely used acidic catalysts in industry.32 They are microporous crystalline materials that exhibit an outstanding catalytic performance because of their specific properties such as high Brønsted acidity, thermal stability, facile regeneration, and shape selectivity.33,34Most of the researches in gas phase dehydration of glycerol to acrolein focused on the use of zeolites due to their versatility. HY zeolite-supported Ru catalysts employed for the hydrogenolysis of glycerol exhibited the best catalytic performance with 60.1% glycerol conversion and 81.3% selectivity to 1,2-PDO at 220 °C and 3 MPa H2 pressure.35

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EXPERIMENTAL SECTION

Preparation of Catalysts. The protonated form of the mordenite zeolite with a SiO2/Al2O3 ratio of 20 was obtained from Conteka, The Netherlands (BET surface area ∼540 m2/g). Chloroplatinic acid hexahydrate (H2PtCl6·6H2O) (analytical grade, produced by SigmaAldrich Co., Ltd.) was used as a precursor on the support. A series of platinum catalysts with varying platinum loading from 0.5 to 3.0 wt % was prepared by the wet impregnation method. The prepared catalysts were dried overnight at 110 °C and subsequently calcined at 550 °C for 4 h in air. The prepared catalysts were designated as XPt/HM, where X refers to Pt loading. Characterization of Catalysts. The crystallinity of the zeolite structure was verified by powder X-ray diffraction (XRD) analysis using a Rigakuminiflex X-ray diffractometer instrument operating with Ni-filtered Cu Kα radiation (λ = 0.15406 nm) generated at 30 kV and 15 mA. The diffraction scans were measured from 2θ = 2° to 65° at a scan rate of 2° min−1. The acidity of the catalysts was measured by TPD of ammonia on AutoChem 2910 (Micromeritics, U.S.A.) instrument. A total of 100 B


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ACS Sustainable Chemistry & Engineering mg of each sample was flushed with high purity (99.995%) helium (50 mL min−1) at 200 °C for 1 h after which the sample was saturated with a mixture of 10% NH3−He (50 mL min−1) at 80 °C for 1 h. The physisorbed ammonia was subsequently removed from the sample in a He flow (50 mL min−1) at 80 °C for 30 min. During the TPD analysis, temperature was raised to 700 °C at a heating rate of 10 °C min−1, and the amount of desorbed NH3 was measured using GRAMS/32 software. A FT-IR study of pyridine-adsorbed samples was also carried out to analyze the nature of acidic sites. In a typical test, the catalysts were first activated in N2 flow at 300 °C for 1 h to remove moisture from the samples followed by adsorption of pyridine on the activated catalysts at 120 °C until saturation, after which the samples were cooled to room temperature. FT-IR spectra of the catalysts were recorded on a GC-FT-IR Nicolet 670 spectrometer by the KBr disc method under ambient conditions. Nitrogen adsorption−desorption experiments were performed at −196 °C using Autosorb 1 (Quantachrome Instruments, U.S.A.) by the multipoint BET method with 0.0162 nm2 as its cross-sectional area in order to determine the textural properties. CO-chemisorption measurements were conducted in an AutoChem 2910 (Micromeritics, U.S.A.) instrument equipped with a TCD detector. Prior to adsorption, 0.1 g of the sample was reduced in flowing hydrogen (50 mL/min) at 300 °C for 3 h, flushed with helium gas at 300 °C for 1 h, and then cooled to ambient temperature. CO uptake was determined by injecting several pulses of 9.96% CObalanced helium over the reduced samples at 300 °C in regular intervals. Platinum dispersion and average particle size were calculated assuming the stoichiometric factor (CO/Pt) as 1. The morphologies of Pt as well as mordenite were evaluated by transmission electronic microscopy (TEM) on a high resolution JEOL 2010 microscope for which 1 mg of the reduced sample is allowed to disperse in 5 mL of methanol for 10 min by sonication. A drop of suspension was then mounted onto to a holey copper grid coated with a carbon film. X-ray photoelectron spectroscopy was employed to study the surface chemical composition of catalysts. XPS measurements were obtained with a Thermo K-5 Alpha XPS instrument at a pressure greater than 1 × 10−9 Torr with core levels aligned with C 1s binding energy of 285 eV. Reaction Procedure. Glycerol hydrogenolysis was performed in a vertical fixed bed quartz reactor (40 cm length, 9 mm i.d.) in vapor phase under atmospheric pressure. In a typical test, 0.5 g of catalyst was loaded in the constant temperature zone of the reactor. Prior to the reaction, the catalysts were reduced in flowing H2 (40 mL min−1) at 350 °C for 2 h. After cooling to the reaction temperature (225 °C), an aqueous solution of 10 wt % glycerol together with a flow of hydrogen (60 mL/min) was continuously fed into the reactor through a syringe perfusor. The condensed reaction products collected in an ice water trap were analyzed every hour on a gas chromatograph GC2014 (Shimadzu) equipped with a DB-wax 123-7033 (Agilent) capillary column (0.32 mm i.d., 30 m long) and FID. The carbon content in the used catalysts was determined by a CHNS analyzer Elementar vario micro cube model (results shown in Table 3). Generally, the carbon mass balance was found to be >98% during the steady state. The conversion of glycerol and selectivity of products were calculated by the following expressions:

samples were used to study the bulk crystallinity and framework structure of mordenite zeolite. The XRD patterns of the Pt/ HM samples with various platinum contents (0.5−3 wt %) are shown in Figure 1. The typical diffraction peaks of mordenite-

Figure 1. XRD patterns of various Pt/HM catalysts.

type zeolites mainly located at 2θ = 9.76°, 13.52°, 19.70°, 22.42°, 25.73°, 26.44°, and 27.64° were clearly observed in all the samples, according to the diffraction patterns found in the literature40,41 and which agree to the respective mordenite pattern quoted in JCPDS File Card No. 80-0645. In addition, the Pt [111] reflection appeared at 2θ = 39.76°, and the intensity of the Pt peak increased with loading. However, in Figure 1, one can observe that there was no change in the crystallinity of zeolite structure in spite of the introduction of different percents of Pt loading. Platinum deposition did not alter the basic structure of the HM zeolite, which is a clear indication that textural properties of the zeolite remain the same even after the platinum deposition. The average crystallite sizes are calculated from the Scherer equation using the X-ray line broadening method, and the results are presented in Table 2. The (111) crystallite of platinum nanoparticles increases marginally with platinum loadings up to 2 wt % and increased significantly in the case of 3 wt %Pt/HM. Acidity Measurements. Surface acidity and its strength is considered to be a key factor for the hydrogenolysis of glycerol to 1,3-propanediol, as this reaction occurs on the surface acid sites of catalysts. NH3−TPD and PyFT-IR are the two techniques commonly employed to evaluate the acidity of the catalysts: the former to determine the acidic strength and amount of acid sites and the latter to find the nature of acidic sites in the catalysts. NH3−TPD Experiments. The temperature-programmed desorption of ammonia (NH3-TPD) profiles of H-mordenite and various loadings of Pt/H-mordenite catalysts are presented in Figure 2a. The amount of acid sites (Table 1) can be calculated from the desorption peak area, and acid strength was estimated from the maximum TPD peak position. In general, the desorption maxima peaks of ammonia could be distinguished at three temperature regions to denote the type of acidic sites as weak acid sites centered at 150−300 °C, moderate at 300−450 °C, and strong acid sites in the range 450−650 °C.42 The NH3−TPD results of HM and Pt/HM catalysts showed well-resolved desorption peaks in two distinct temperature regions, viz., low temperature region (157 °C) and high temperature region (470 °C), which were attributed to weak acid sites and strong acid sites, respectively. It indicates that the low temperature TPD peak resulted due to nonframework Al

Conversion (%) moles of glycerol (in) − moles of glycerol (out) = × 100 moles of glycerol (in)

Selectivity (%) =

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moles of one product × 100 moles of all products

RESULTS AND DISCUSSION Characterization Techniques. Structural Characterizations of Catalysts. XRD analyses of the calcined Pt/HM C


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ACS Sustainable Chemistry & Engineering

FT-IR of Adsorbed Pyridine. To explore further the acidity of catalysts, FT-IR spectroscopy is applied to detect adsorbed pyridine on the catalyst to investigate the nature of acid sites present on the catalyst and further to quantify them. Pyridine is a useful basic probe molecule to distinguish accessible Brønsted and Lewis acid sites. Figure 2b shows the FT-IR spectra of pyridine adsorbed on HM and Pt/HM catalysts in the region of 1400−1600 cm−1. As shown in Figure 2b all of the samples exhibit an absorption band at 1448 cm−1 when pyridine coordinates Lewis acid sites, whereas the absorption band at 1547 cm−1 corresponds to the pyridinium ion adsorbed on Brønsted acid sites. On the other hand, a characteristic band appeared at 1490 cm−1, which corresponds to pyridine adsorbed on both Brønstedand Lewis acid sites.44,45 Furthermore, the absorption band at 1547 cm−1 was found to be more intense than the band located at 1452 cm−1. Therefore, these results permit us to conclude that the Brønsted acid sites are more predominant in Pt/HM catalysts than the Lewis ones, as evidenced by Py adsorption and IR spectroscopy.The amount of Brønsted and Lewis acid sites were calculated from the absorbance intensities of the corresponding spectral bands by using the molar extinction coefficients.46 The results are shown in Table S1 of the Supporting Information. From the results, it is observed that there is a decrease in the total acidity of the Pt/HM samples as compared to that of pure Hmordenite. This could be attributed to a partial pore blockage of acidic sites of support by Pt particles, which prevents the interaction of pyridine molecules with the acidic sites. This result is well correlated with the results of surface area measurements and NH3−TPD studies (Table 1). Furthermore, it is interesting to observe the changes in acid site distribution in various (0.5−3 wt %) Pt/HM catalysts, which are expected to be due to interactions between platinum crystallites and acid sites. This is in accordance with the previous literature, which demonstrates that acidity of a zeolite support can be modified by incorporating Pt particles.47 Physicochemical Properties of Catalysts. The BET surface areas and pore volumes of pure HM and various loadings of Pt/ HM catalysts are summarized in Table 1. The pure Hmordenite sample (HM) had a total surface area of 527 m2/g with a pore diameter of 3.29 nm. It is observed that the surface area of all the Pt/HM catalysts was reduced when compared to the pure support as the Pt content increased. This could be possibly attributed to the blocking or filling of H-mordenite pores by extra-framework species or by the incorporation of Pt species. The reduction in pore volume of the catalysts in comparison with the pure support can be explained through the same assumption. CO-Chemisorption. The Pt dispersion, metal surface area, and average particle size were measured from the irreversible CO-chemisorption on various Pt/HM catalysts, and the results are illustrated in Table 2. The results reveal that the CO uptake value increases with an increase in Pt loading on the HM support up to 2 wt % and decreased slightly in the case of 3 wt % Pt/HM. This is probably due to the aggregation of platinum particles at higher loadings. From Table 2, it is evident that by increasing the platinum loading the metal size notably increased from 2.9 to 6.7 nm, with a resultant decline in platinum dispersion (from 48.2% to 20.7%). It is for this reason that at higher loadings Pt particles agglomerate on the support due to increased deposition of platinum on the external surface of HM. TEM Analysis. The size and morphology of various HMsupported platinum catalysts (0.5, 1, 2, and 3 wt %) are

Figure 2. (a) TPD of ammonia profiles of various Pt/HM catalysts. (b) PyrFT-IR spectra of various Pt/HM catalysts.

Table 1. Physicochemical Properties and Acidities of Pure HM and Pt/HM Catalysts catalyst

SA (m2/g)

DBJH (nm)

Vp (cc/g)

total acidity (μmol NH3/g)

pure HM 0.5Pt/HM 1Pt/HM 2Pt/HM 3Pt/HM

527 496 445 398 312

3.29 3.21 3.08 2.71 2.01

0.35 0.33 0.29 0.24 0.19

1011.3 612.3 535.9 645.5 442.6

SA, BET surface area; DBJH, average pore diameter; Vp, total pore volume

sites or possibly the terminal silanol groups on the external surface of the zeolite structure, whereas the high temperature TPD peak is due to framework Al sites.43 Peak position did not vary substantially with an increase in metal loading; however, the amount of NH3 adsorbed on the catalyst decreased with an increase in metal loading. D


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varies from 3 to 10 nm with highly dispersed spherical particles of platinum. It can be inferred that the platinum particles were mainly located on the external surface of mordenite, and a small fraction of platinum might have been located within the zeolite channels.48 This result indicates good dispersion of the platinum particles on the zeolite and is in line with the results of CO-chemisorption and XRD. Surface Chemical States Analysis by XPS. XPS analysis of mordenite and mordenite-supported platinum catalysts were carried out to understand the chemical states of platinum and its interaction with the zeolite support. Figure 4 shows the XPS core level spectra of Al 2p, Si 2p, and O 1s core levels. Since Al 2p (present in the mordenite support) and Pt 4f core levels appear (71−76 eV) in the same region, it is difficult to decouple the spectra of both levels. However, the corresponding Pt 4d5/2 (BE 314−315.3 eV) of Pt metal has been detected, which clearly shows an increase in the peak intensity with an increase in Pt loading (Figure S1, Supporting Information).49 In the case of mordenite support, an Al 2p binding energy was observed around 74.4 eV, characteristic of aluminosilicate materials (Figure 4A). However, the binding energy of this Al 2p level shifted to higher binding energies (75.1 eV) with significant broadening as the loading of platinum was increased. The formation of zero-valent platinum was clearly observed in the case of 2Pt/HM and 3Pt/HM, and their 4f7/2 binding energies were centered on 71.2 eV. In contrast, 0.5Pt/HM and 1Pt/HM did not exhibit the formation of metallic platinum. In addition, the shift in Al 2p binding energies after platinum impregnation was a clear indication of the metal−support interaction. Figure 4B shows the Si 2p core level spectra, and it was deconvoluted into three chemically distinct silicon oxide species including aluminosilicate, silica, and surface silanol groups. Similar to the Al 2p spectra, the Si 2p spectra showed significant broadening and a shift in binding energies after platinum loading, and this may be the direct consequence of platinum−mordenite interaction. The O 1s core level spectra of the mordenite support was deconvoluted into three chemically distinct components, and platinum loading exhibited significant broadening toward the lower binding energy region (Figure 4C). This is a characteristic feature of metal−oxygen bond formation and becomes

Table 2. Results of CO Uptake, Dispersion, Metal Area, and Average Particle Size of Pt/HM Catalysts catalyst 0.5Pt/ HM 1Pt/ HM 2 Pt/ HM 3 Pt/ HM a

dispersion (%)

CO uptake (μmol/g)

metal area (m2/gcat)

particle sizea (nm)

avg. size of crystallitesb (nm)

48.2

12.34

0.48

2.9

3.1

36.9

18.86

0.73

3.8

3.4

31.4

32.18

1.26

4.4

3.9

20.7

31.8

1.24

6.7

6.2

Determined from CO uptake values. bDetermined from XRD.

determined by transmission electron microscopy and are presented in Figure 3. The average particle size of platinum

Figure 3. TEM images of various Pt/HM catalysts.

Figure 4. XPS of HM and various Pt/HM catalysts. E


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ACS Sustainable Chemistry & Engineering Table 3. Effect of Pt Loading on Glycerol Hydrogenolysis to 1,3-PDOa selectivity (%) Pt loading

conversion (%)

1,3-PDO

1,2-PDO

EG

HA

others

0.5 1.0 2.0 3.0

80.5 90.4 94.9 77.2

32.4 40.1 48.6 29.9

14 11 12 16

29 22 25 33

9.8 8.3 7.5 5.8

14.8 18.6 6.9 15.3

b

carbon (%) 1.66 1.53 1.47 1.93

Reaction conditions: 0.5 g of catalyst; reaction temperature, 225 °C; 0.1 MPa H2; H2 flow rate, 60 mL/min; WHSV, 1.02 h−1; 1,3-PDO, 1,3propanediol; 1,2-PDO, 1,2-propanediol, EG, ethylene glycol; HA, hydroxyacetone. Others include methanol, acetone, ethanol, 1-propanol, and 2propanol; bCarbon estimated from CHNS analysis after 8 h continuous operation on the used catalysts. a

prominent at higher Pt loading, which may be formed at the interface of platinum nanoparticles and zeolite oxygen. Reaction Results: Evaluation of Catalytic Activity. Catalyst Screening. The catalytic performance of glycerol hydrogenolysis over a series of Pt/HM (0.5−3 wt %) catalysts was performed in a vapor phase continuous fixed bed reactor at 225 °C and atmospheric pressure. The primary products were 1,3-propanediol (1,3-PDO), 1,2-propanediol (1,2-PDO), ethylene glycol (EG), and hydroxyacetone (HA). In addition, propanols and a few degradation products such as ethanol, acetone, and methanol were also detected in small amounts. All the Pt/HM catalysts presented higher conversion and 1,3-PDO selectivity during glycerol hydrogenolysis. In order to selectively produce the desired products and minimize the negative effects of hydrogenolysis, the reaction conditions have to be subtly adjusted. Therefore, the present research was aimed at systematic optimization of the reaction parameters in order to achieve good conversion and 1,3-PDO selectivity. Effect of Pt Loading. In order to determine the optimum catalyst required to obtain maximum selectivity to 1,3-PDO, glycerol hydrogenolysis was studied using Pt/HM catalysts with varying Pt loading (0.5−3 wt %). The results obtained from the effect of Pt loading on glycerol conversion and 1,3-PDO selectivity are presented in Table 3. With 0.5 wt % platinum loaded on H-mordenite, 80.5% glycerol was converted, and 1,3PDO selectivity was 32.4%. With an increase in Pt loading to 2 wt %, a linear increase in glycerol conversion and 1,3-PDO selectivity was observed, but a further increase in Pt loading to 3 wt % shows a tempered influence on both glycerol conversion and 1,3-PDO selectivity. In addition, a substantial increase in the selectivity of 1,2-PDO and EG was noticed with an increase in Pt loading. Therefore, we can conclude that the 2 wt % Pt/ HM catalyst possessed the proper number of Pt metal sites required for hydrogenation of the reaction intermediate to 1,3PDO during glycerol hydrogenolysis. Thereby, the optimal catalyst dosage was chosen to be 2 wt % for further tests. Effect of Reaction Temperature. The reaction was investigated at different reaction temperatures ranging from 150 to 250 °C to evaluate the influence of reaction temperature on glycerol conversion and 1,3-PDO selectivity, and the results are shown in Figure 5. Previous studies suggested that increasing the reaction temperature has a positive influence on glycerol conversion.16 As expected, glycerol conversion increased monotonically from 46.2% to 94.9% with an increase in temperature from 150 to 225 °C. The maximal 1,3-PDO selectivity of 48.6% was obtained at a reaction temperature of 225 °C. Noteworthy, the product distribution varied considerably as temperature increased from 150 to 225 °C, but a further increase in temperature to 250 °C caused a decrease in glycerol conversion and 1,3-PDO selectivity. This is because the increase in reaction temperature will accelerate the coking rate of the catalysts, which results in the availability of fewer

Figure 5. Effect of reaction temperature on hydrogenolysis of glycerol to 1,3-PDO. Reaction conditions: reaction temperature = 150−250 °C; reduction temperature = 350 °C; H2 flow rate = 60 mL/min; WHSV = 1.02 h−1.

catalytically active sites to allow for efficient conversion of glycerol.50 Also, higher reaction temperatures would promote excessive C−O and C−C bond scission resulting in other byproducts with decreased 1,3-PDO selectivity. Therefore, we suggest that a reaction temperature of 225 oC was sufficient enough to promote glycerol hydrogenolysis, which might allow a secondary hydroxyl group of glycerol to be activated and make the hydrogenolysis reaction highly selective to 1,3-PDO with maximum glycerol conversion. Effect of Hydrogen Flow Rate. The influence of the hydrogen flow rate on glycerol hydrogenolysis was investigated by varying the hydrogen flow rate at 20, 40, 60, and 80 mL/min over 2 wt % of Pt/HM catalyst at 225 °C under atmospheric pressure. It is shown in Figure 6 that glycerol conversion and selectivity of 1,3-PDO increased with hydrogen flow rate up to 60 mL/min. It is obvious that hydrogen is one of the reactants in glycerol hydrogenolysis, and an increase in the hydrogen flow rate results in an increase in glycerol conversion. This is because an increased amount of protons and hydride ions formed from the hydrogen function as active sites that enable hydrogenation of the dehydrated intermediate from glycerol. However, when the hydrogen flow rate was further increased to 80 mL/min, 1,3-PDO selectivity slightly dropped in spite of a distinct increment in glycerol conversion to 96%. This is probably due to the enhancement of consecutive C−O hydrogenolysis of propanediols over hydrogenolysis products such as propanols by the excess concentration of protons and hydride ions at higher hydrogen flow rates. Therefore, the results suggest that the selectivity of propanediols is suppressed F


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On the contrary, an increase in W/F resulted in a decrease in 1,2-PDO and HA selectivity. No noticeable change in the EG selectivity was observed during the reaction. This suggests that higher contact time tends to increase selectivity of 1,3-PDO, which might be caused by the enhancement of secondary hydroxyl removal from glycerol with subsequent hydrogenation, which decreases the selectivity of undesired products such as methanol, ethanol, and acetone. Effect of Weight Hourly Space Velocity (WHSV). To study the influence of WHSV on catalytic performance, reactions over the 2Pt/HM catalyst sample were done at 225 °C with a flow rate of 10 wt % glycerol solution varied from 0.5 to 2.0 mL/h (corresponding WHSV = 1.02−4.08 h−1) by using a 60 mL/ min hydrogen flow rate (Figure 8). When the WHSV was

Figure 6. Effect of hydrogen flow rate on hydrogenolysis of glycerol to 1,3-PDO. Reaction conditions: reaction temperature = 225 °C; H2 flow rate = 20, 40, 60, and 80 mL/min; WHSV = 1.02 h−1.

with the increased formation of propanols at a hydrogen flow rate of 80 mL/min (Figure 6). A similar tendency of glycerol conversion with hydrogen flow rate has been reported in previous studies.30 A maximum 1,3-PDO selectivity of 48.6% at 94.9% glycerol conversion was obtained at a 60 mL/min hydrogen flow rate. Thus, a 60 mL/min hydrogen flow rate was considered to be optimum for further studies. Effect of Contact Time (W/F). The effect of contact time (W/F) on the hydrogenolysis of glycerol was examined over a fixed feed rate of glycerol by changing the weight of 2Pt/HM catalyst, and the results are depicted in Figure 7. It is obvious

Figure 8. Effect of WHSV on hydrogenolysis of glycerol to 1,3-PDO. Reaction conditions: reaction temperature = 225 °C; H2 flow rate = 60 mL/min; catalyst wt = 0.5 g; WHSV = 1.02, 2.04, 3.06, and 4.08 h−1.

increased from 1.02 to 4.08 h−1, the glycerol conversion decreased from 94.9% to 75.2%. Also, the selectivity to 1,3PDO decreased rapidly from 48.6% to 19.4%. It can be inferred that at higher WHSV, the excess amount of glycerol would not possibly find an adequate number of active sites on the catalyst surface to interact, and hence, the contact time of glycerol with the catalyst was reduced, resulting in low glycerol less selectivity to 1,3-PDO.52 The results therefore suggest that the lowest WHSV, 1.02 h−1, resulted in maximum 1,3-PDO selectivity and glycerol conversion. Catalyst Stability. To gain further insight into the catalytic performance, the sustainability of activity of the catalyst was studied by conducting a time-on-stream experiment over the best catalyst, 2Pt/HM, under the optimum reaction conditions (Figure 9). The reaction was carried out at 225 °C under atmospheric pressure for a duration of 8 h in order to study the stability of the catalyst. Initially, 84%−86% conversion of glycerol was obtained, reaching a maximum (94.9%) at 3 h and subsequently dropping with time, although remaining stable until 5 h. In addition, selectivity to 1,3-PDO also maximized at 3 h, showing a drop as reaction time proceeded. The time-onstream studies demonstrate that a slight decrease in the activity of the catalyst is observed as reaction time was extended, which might be ascribed to the fact that the solid acid catalyst undergoes a gradual deactivation due to many factors such as coke formation, decreased BET surface area, total acidity, and sintering of Pt during the reaction, which is in accordance with

Figure 7. Effect of contact time (W/F) on hydrogenolysis of glycerol to 1,3-PDO. Reaction conditions: reaction temperature = 225 °C ; H2 flow rate = 60 mL/min; glycerol feed = 0.5 mL/h; W/F = 0.4, 0.6, 0.8, 1.0 g mL−1 h.

that glycerol conversion increases with increasing contact time. As the value of W/F increased from 0.4 to 1.0 g mL−1 h, glycerol conversion increased from 78% to 94.9%. Longer contact time allows for more active sites to be available for glycerol to react with, and hence, glycerol conversion is increased. Also, there was a gradual increase in 1,3-PDO selectivity from 21.4% to 48.6% with increasing contact time.51 G


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of the spent 2Pt/HM catalyst reveals a sharp diffraction line at 2θ = 39.7°, indicating the presence of large Pt particles on the exterior of the support (Figure 10). It is obvious that the Pt species agglomerate during the reaction, and pore blockage of H-mordenite channels by large Pt clusters could have made the Pt sites inaccessible to the reaction. The carbon content and decrease in surface area of the spent catalyst provides useful information on catalyst deactivation. As has been noted, the surface area and total acidity (NH3μmol/g) of the spent catalyst were found to decrease as the fresh catalyst decreased. CHNS analysis of the used catalysts reveals the amount of coke deposited during the catalytic reaction, and the results are shown in Table 3. Therefore, these results suggest that the deactivation of catalyst could be mainly attributed to the loss of acidic sites in the catalyst due to the blockage of pores by carbon deposition, which eventually resulted in a decrease in catalytic activity. Mechanism of Glycerol Hydrogenolysis to 1,3-PDO. Hydrogenolysis of glycerol occurs via acid-catalyzed dehydration to form intermediates (acetol and 3-hydroxypropaldehyde (3-HPA)) and subsequent hydrogenation to form propanediols (1,2-PDO and 1,3-PDO) on metal sites.16,17,54 Therefore, it is well established that metal and acid sites along with active hydrogen species reach outstanding catalytic performance of glycerol hydrogenolysis. Besides acid strength, the nature of the acidic sites, i.e., Brønsted and Lewis acidic sites, play a predominant role in determining product formation. It is highly evident that Brønsted acid sites are responsible for 1,3-PDO formation, while Lewis acid sites allow formation of 1,2-PDO.55 In this reaction mechanism, the secondary carbocation intermediate for 3-HPA is more stable than that of primary carbocation for acetol. Therefore, 3-HPA formation is kinetically more favorable over acetol production, although thermodynamically unstable. The possible reaction mechanism of glycerol hydrogenolysis to 1,3-PDO over Pt/HM is proposed based on our experimental investigations (Scheme 2). The pathway describes the protonation and dehydration of the secondary hydroxyl group of adsorbed glycerol (1) on the Brønsted acid sites of the zeolite by interacting with bridging OH groups of zeolite. The alkoxy species (2) formed as a result of dehydration undergoes desorption followed by readsorption onto the zeolite with the primary hydroxyl groups of glycerol to generate 3-hydroxypropene (3). Consequently 3-hydroxy propionaldehyde (3-HPA) was originated from keto-enoltautomerization (4) and undergoes quick hydrogenation on active Pt sites under a hydrogen atmosphere to yield 1,3-PDO (5). The fast hydrogenation of 3-HPA is important to prevent further dehydration of 3-HPA to produce acrolein.

Figure 9. Time-on-stream studies of hydrogenolysis of glycerol to 1,3PDO over 2Pt/HM catalyst. Reaction conditions: reaction temperature = 225 °C; H2 flow rate = 60 mL/min; WHSV = 1.02 h−1.

the literature report.53 This is further supported by the results of the analysis of the spent catalyst. Reusability of Catalyst. Glycerol conversion and selectivity of 1,3-PDO were examined over the used catalyst of 2 wt % Pt/HM under the same reaction conditions (225 °C, 0.1 MPa) to further verify the stability of the catalyst. As a process of regeneration or recovery of the spent catalyst, the catalyst was initially treated in air at 300 °C for 3 h followed by a reduction in H2 flow at 300 °C for 3 h.51 The catalytic activity of the regenerated catalyst was then evaluated in the hydrogenolysis of glycerol under optimized reaction conditions. The spent catalyst was also characterized by XRD, BET surface area, and acidity measurements. The results are presented in Table 4 Table 4. Studies of Spent Catalyst 2Pt/HM catalyst

conversion (%)

selectivity of 1,3-PDO

BET SA

acidity (μmol/g)a

fresh spent

94.9 89

48.6 42

398 307

645.5 430.0

a

Measured from NH3−TPD studies;

and Figure 10. As shown in Table 4, glycerol conversion and 1,3-PDO selectivity slightly dropped over the used catalyst when compared to that of the fresh catalyst. The XRD pattern

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CONCLUSIONS Vapor phase hydrogenolysis of glycerol was investigated over a series of 0.5−3 wt % Pt/HM catalysts for the first time under atmospheric pressure. Of the catalysts examined, the 2 wt % Pt/ HM catalyst was proven be highly active and selective in the hydrogenolysis of glycerol with 48.6% 1,3-PDO selectivity at 94.9% glycerol conversion. From the results of the present study, it can be concluded that selective hydrogenolysis of glycerol to 1,3-PDO through a dehydration−hydrogenation route was highly efficient using a bifunctional catalyst consisting of a well-crystallized zeolite such as H-mordenite, whose acid function is constituted mainly by Brønsted acidity and a hydrogenating function such as dispersed platinum. On the other hand, a Pt/HM catalyst with proper metal/acid balance

Figure 10. XRD and NH3−TPD patterns of spent 2Pt/HM catalyst. H


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ACS Sustainable Chemistry & Engineering Scheme 2. Reaction Mechanism of Glycerol Hydrogenolysis to 1,3-PDO over Pt/HM Catalyst

(2) Zhou, C. H. C.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. M. Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals. Chem. Soc. Rev. 2008, 37, 527−549. (3) Werpy, T.; Petersen, G.; Aden, A.;Bozell, J.; Holladay, J.; White, J.; Manheim, A.; Eliot, D.;Lasure, L.; Jones, S. Top Value Added Chemicals from Biomass. Volume 1Results of Screening for Potential Candidates from Sugars and Synthesis Gas; U.S. Department of Energy: Washington, DC, 2004. (4) Umbarkar, S. B.; Kotbagi, T. V.; Biradar, A. V.; Pasricha, R.; Chanale, J.; Dongare, M. K.; Mamede, A. S.; Lancelot, C.; Payen, E. Acetalization of glycerol using mesoporous MoO3/SiO2 solid acid catalyst. J. Mol. Catal. A: Chem. 2009, 310, 150−158. (5) Kim, Y. T.; Jung, K. D.; Park, E. D. Gas-phase dehydration of glycerol over ZSM-5 catalysts. Microporous Mesoporous Mater. 2010, 131, 28−36. (6) Bossaert, W. D.; De Vos, D. E.; Van Rhijn, W. M.; Bullen, J.; Grobet, P. J.; Jacobs, P. A. Mesoporous Sulfonic Acids as Selective Heterogeneous Catalysts for the Synthesis of Monoglycerides. J. Catal. 1999, 182, 156−164. (7) Karinen, R. S.; Krause, A. O. I. New biocomponents from glycerol. Appl. Catal., A 2006, 306, 128−133. (8) King, D. L.; Zhang, L.; Xia, G.; Karim, A. M.; Heldebrant, D. J.; Wang, X.; Peterson, T.; Wang, Y. Aqueous phase reforming of glycerol for hydrogen production over Pt−Re supported on carbon. Appl. Catal., B 2010, 99, 206−213. (9) Demirel, S.; Lucas, M.; Warna, J.; Salmi, T.; Murzin, D.; Claus, P. Reaction kinetics and modeling of the gold catalysed glycerol oxidation. Top. Catal. 2007, 44, 299−305. (10) Zhu, S.; Zhu, Y.; Hao, S.; Chen, L.; Zhang, B.; Li, Y. A highly efficient and robust Cu/SiO2 catalyst prepared by the ammonia evaporation hydrothermal method for glycerol hydrogenolysis to 1,2propanediol. Catal. Lett. 2012, 142, 267−274. (11) Sun, D.; Yamada, Y.; Sato, S. Effect of Ag loading on Cu/Al2O3 catalyst in the production of 1,2-propanediol from glycerol. Appl. Catal., A 2014, 475, 63−68. (12) Delgado, S. N.; Yap, D.; Vivier, L.; Especel, C. Influence of the nature of the support on the catalytic properties of Pt-based catalysts for hydrogenolysis of glycerol. J. Mol. Catal. A: Chem. 2013, 367, 89− 98. (13) Salazar, J. B.; Falcone, D. D.; Pham, H. N.; Datye, A. K.; Passos, F. B.; Davis, R. J. Selective production of 1,2-propanediol by hydrogenolysis of glycerol over bimetallic Ru−Cu nanoparticles supported on TiO2. Appl. Catal., A 2014, 482, 137−144.

was highly desirable in improving catalytic activity The Brønsted acids on the surface of H-mordenite possibly play essential roles in accelerating the 1,3-PDO selectivity. The parametric study (effect of reaction temperature, hydrogen flow rate, contact time, and WHSV) showed that the activity and 1,3-PDO selectivity was favored by the increase in temperature and hydrogen flow rate. In addition, longer contact time resulted in maximum conversion and 1,3-PDO selectivity. Further, the structural aspects of the spent catalyst allowed evaluating the stability of the catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

TThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01272. Table illustrating Pyr-FT-IR results of pure HM and various Pt/HM catalysts. Figure related to Pt 4d XPS. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S.P. duly acknowledges RMIT-IICT joint research centre for providing the research facilities and fellowship. The authors thank the RMIT Microscopy and Microanalysis facility staff members for their scientific and technical assistance. M.L.K. thanks the Department of Science & Technology (Ministry of Science & Technology), Government of India, New Delhi, for the award of the J. C. Bose Fellowship.

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K


Platinum Supported on H-Mordenite: A Highly Efficient Catalyst for

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