Selective Conversion of Glycerol into Propylene: Single-Step versus

Research Article pubs.acs.org/journal/ascecg

Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process Zhijie Wu,*,†,‡ Kaiqiang Zhao,† Shaohui Ge,§ Zhi Qiao,† Jinsen Gao,† Tao Dou,† Alex C. K. Yip,*,∥ and Minghui Zhang*,‡ †

State Key Laboratory of Heavy Oil Processing and Key Laboratory of Catalysis of CNPC, China University of Petroleum, 18 Fuxue Road, Changping, Beijing 102249, China ‡ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, 94 Weijin Road, Tianjin 300071, China § Petrochemical Research Institute, PetroChina Company Limited, E318, A42 Block, China Petroleum Innovation Base, Changping, Beijing 100195, China ∥ Department of Chemical and Process Engineering, University of Canterbury, Christchurch 8140, New Zealand S Supporting Information *

ABSTRACT: Dehydration and catalytic cracking reactions can be combined to convert glycerol into light olefins using solid acid catalysts. The combination is suitable for a singlestep process to convert glycerol into light olefins at high temperatures (26−36% selectivity at 873 K). However, large quantities of carbon oxides are produced (31−39% COx selectivity), and catalyst deactivation also occurs. High light olefin selectivity (62−65%) and a smaller quantity of carbon oxides (11−12% COx selectivity) can be obtained by using a tandem process involving the dehydration of glycerol and subsequent catalytic cracking of the dehydration products (mainly acetol and acrolein). Furthermore, the ratio of propylene to ethylene can be adjusted by changing the dehydration catalysts to favor the production of acetol or acrolein: Acetol forms propylene, and acrolein forms ethylene. To overcome the fast deactivation of acid catalysts in glycerol dehydration, the hydrogenolysis and catalytic cracking reactions can be synchronized to convert glycerol into hydrocarbons using a combination of metal and acid catalysts. The single-step conversion of glycerol over a metal or bifunctional catalyst formed alcohols and paraffin. The highest selectivity for propylene production (approximately 76%) was obtained in a tandem process via the selective hydrogenolysis of glycerol to propanols over Pt/ZSM-5 catalysts followed by the catalytic dehydration/cracking of propanols to propylene over ZSM-5 catalysts at low temperatures (523 K). The selectivity for propylene was improved by increasing the Si/Al ratio of the ZSM-5 catalysts and the reaction time. Under these conditions, economically competitive crude glycerol (mainly mixtures of glycerol and methanol) can be used to synthesize light olefins (approximately 61% selectivity) with a long lifetime (∼500 h) in single-route reactions by increasing the cracking temperature to 773 K, which is suitable for practical methanol to propylene process. KEYWORDS: Glycerol, Propylene, Zeolite, Hydrogenolysis, Metal catalyst

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INTRODUCTION

Currently, refined glycerol is available for 800−900 US$ ton−1, but crude glycerol can be purchased for approximately 150 US$ ton−1.6 In addition to the 65−85 wt % glycerol content in crude glycerol waste streams, the other primary components are methanol and soaps.13 Thus, the GTO process is useful for the simple utilization of crude glycerol (both glycerol and methanol together) by incorporating MTO or MTP technologies into the glycerol to hydrocarbon process. The diameter of the glycerol molecule is 0.44 nm, which is larger than the pore sizes of the industrial MTO catalysts (mainly SAPO-34 zeolite (0.38 nm aperture size)), but smaller than those of the industrial MTP catalysts (mainly ZSM-5 zeolite (0.55 nm aperture size)). Therefore, only the zeolites

The selective conversion of glycerol into higher value fuels and chemicals has attracted considerable interest because a surplus of glycerol has been created by the expansion of biodiesel production and because the traditional consumers of glycerol (i.e., cosmetics and tobacco industries, etc.) cannot absorb the large glycerol oversupply.1−10 In addition to the typical processes in which bioglycerol is used to synthesize epichlorohydrin and biomethanol, other applications, involving oxidation,1,2,8 dehydration,3,6 hydrogenolysis,4,5 and etherification,9 have been explored to increase the bioglycerol’s value. In light of the industrial success of methanol to gasoline (MTG), methanol to olefins (MTO), and methanol to propylene (MTP) processes,10−12 the conversion of glycerol to olefins (GTO) and glycerol to aromatics have been proposed in the past 10 years.7 © XXXX American Chemical Society

Received: April 6, 2016 Revised: June 14, 2016

A

DOI: 10.1021/acssuschemeng.6b00676 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Glycerol (99.9%), 1,2-propanediol (99.9%), 1,3-propanediol (99%), acetol (99.8%), 1-propanol (99.9%), 2-propanol (99.9%), and acrolein (99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Pt/Al2O3 (5 wt % Pt) was purchased from J&K Scientific, Ltd. (China). Cu(NO3)2·3H2O (99%) was purchased from Guanfu Chemical, Ltd. (Tianjin, China). γ-Al2O3 was obtained from CNOOC Tianjin Chemical Research and Design Institute (Tianjin, China). All chemicals were used as received. For catalyst preparation, γ-Al2O3 and SiO2 were dried at 393 K in an oven for 6 h and then calcined at 823 K for 4 h. Catalyst Preparation. To obtain the acidic zeolite catalysts, the ammonium-formed zeolites were calcined at 773 K in air for 6 h. For the preparation of the supported catalysts, Pt and Cu precursors were deposited onto the metal oxides or zeolites by incipient wetness impregnation using aqueous solutions of Pt(NH3)4(NO3)2 and Cu(NO3)2 in deionized water. All samples were heated at 383 K in an oven for 24 h. The dried samples were crushed into powders with sizes < 0.38 mm, heated to 673 K with a 5 K min−1 ramping rate under a dry air flow (100 cm3 gsolid−1 min−1), and held at this temperature for 3 h. The samples were cooled to ambient temperature, heated to 573 K with a 5 K min−1 ramping rate under a 10% H2/N2 flow (100 cm3 gsolid−1 min−1), and held at this temperature for 2 h. All samples were then cooled to ambient temperature and passivated under a 0.5% O2/ N2 flow (100 cm3 gsolid−1 min−1) for 3 h before exposure to ambient air. Characterizations. The compositions of the catalysts were analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES) on an IRIS Intrepid spectrometer. The dispersion of Pt metal crystallites was measured using H2 chemisorption (Micromeritics ASAP2020) at 313 K and 5−50 kPa. The samples were first treated in pure H2 for 1 h at 573 K and then in a dynamic vacuum for 1 h at 573 K. The Cu metal dispersion was measured by CO chemisorption (Micromeritics ASAP2020) at 313 K and 5−50 kPa. The metal dispersions were calculated by assuming stoichiometries of H/Pts = 1 and CO/Cus = 1. Nitrogen adsorption−desorption measurements of zeolites (at 623 K for 5 h under vacuum) were performed on a Micromeritics ASAP 2020 instrument at 77 K, and the specific surface areas were calculated by the Brunauer−Emmett−Teller (BET) equation. The total pore volumes were evaluated from the volume adsorbed at P/P0 = 0.99, whereas the micropore areas and the micropore volume were determined by the t-plot method. Temperature-programmed desorption of ammonia (NH3-TPD) was performed on a Huasi DAS-7000 chemisorption unit. Prior to the measurement, 100 mg samples (0.250−0.425 mm) were outgassed at 873 K for 1 h and then cooled to 373 K. Subsequently, the samples were treated with an anhydrous NH3 stream for 30 min at 373 K and then flushed with N2 for 30 min at the same temperature to remove excess NH3. Finally, the desorption of the NH3 was performed by heating the samples to 873 K at a rate of 10 K min−1 from 373 K, and the signal of the desorbed NH3 was recorded simultaneously by the thermal conductivity detector (TCD). Catalytic Reactions. The dehydration and single-step conversion of glycerol were performed in a fixed-bed quartz reactor with a diameter of 1.0 cm and a length of 60 cm, and the temperature was regulated within a three-zone resistively heated furnace. The bed temperature was measured using a type K thermocouple held within a 3 mm stainless steel sheath. Prior to the reaction, 1.0 g of a metal catalyst placed in the catalyst bed was reduced at 573 K for 1 h under a H2 flow (100 cm3 gsolid−1 min−1), whereas 1.0 g of a solid acid catalyst was treated at 673 K under a N2 flow (100 cm3 gsolid−1 min−1) for 1 h. Subsequently, 59 wt % (21.9 mol %) aqueous glycerol was fed through the top of the reactor at a liquid feed rate that produced the desired weight hour space velocity (WHSV), defined as (g of glycerol (g of catalyst)−1 h−1). Typically, WHSV = 1 h−1 (similar to that of the MTP reaction) was used throughout this study with H2 (molar ratio of H2 to glycerol =100) or without H2. The calculated C-balance was usually >95% after a time on stream (TOS) of 5 h. For the tandem conversion of glycerol, two typical processes were used for the reaction with a 59 wt % aqueous glycerol solution. (1)

with medium pores (10-member ring such as ZSM-5) and large pores (12-member ring such as Y and Beta, 0.74 and 0.7 nm aperture size, respectively) are considered suitable for the GTO process. Similar to the MTO and MTP reactions, a high selectivity for light olefin production has been observed on the ZSM-5 zeolites, whereas a large quantity of aromatics form on the Y and Beta zeolites.14−22 Considering the abundance of methanol in crude glycerol, these results show that ZSM-5 zeolites are potential catalysts for practical GTO reactions because of the lower price of crude glycerol than of methanol (250−300 US$ ton−1). For instance, a method for converting glycerol/alcohol mixtures to aromatics has been developed with similarity to the MTG process.23 Similar to the MTO or MTP reactions, a dehydration reaction over the acid catalyst occurs first in the GTO reaction. Rapid deactivation caused by the formation of carbonaceous deposits on the surfaces or in the channels of zeolite catalysts has been a serious limitation that must be overcome for these materials to be useful for industrial applications.1,3,6 Low selectivity for light olefins and a large amount of coke formation have been observed on the ZSM-5 catalysts in the GTO reaction at high temperatures (>773 K).7,14−22 Recently, the conversion of glycerol to light olefins (especially propylene) in the presence of H2 has been observed with good stability without obvious deactivation on metal catalysts supported on acidic oxides.24−28 In addition to the one-pot hydrodeoxygenation of glycerol to propylene in a batch reactor,26,27 singlestep24 and tandem processes25,28 of the GTO reaction in H2 have been developed by first converting the dehydration products (mainly acrolein and acetol) to propanols and then to propylene. Unlike the dehydration of glycerol and the direct GTO reaction at high temperatures (573−593 K and 673−923 K, respectively),14−22 a lower temperature (approximately 523 K)25−28 for the GTO reaction in H2 is sufficient for the high conversion with a low weight hour space velocity. The stability of solid acids in the dehydration of glycerol at low temperatures (approximately 523 K) remains unclear. In light of the industrial MTP process, which combines the dehydration of methanol into dimethyl ether over γ-Al2O3 catalysts at low temperature (approximately 503 K), and the conversion of the dimethyl ether and methanol mixture into light olefins (723− 773 K), it is important to study the feasibility of incorporating the MTP techniques into the GTO reaction.29 Herein, we systematically compare the single-step GTO process over ZSM5 and ZSM-5-supported metal catalysts and the tandem GTO processes with/without H2 over metal oxides, zeolites, and their resulting supported metal catalysts to establish a feasible industrial GTO process. To understand the tandem processes in the GTO reaction, studies of the dehydration or hydrodeoxygenation of glycerol over metal oxides, zeolites, and their resulting supported metal catalysts with/without H2 at low temperature (473−523 K) have been carried out. Cu metal, which is active only in the cleavage of C−O bonds, and Pt metal, which is active in the cleavage of both C−C and C−O bonds,30 were used to study the roles of the metal species in promoting propylene selectivity.

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EXPERIMENTAL SECTION

Chemicals. ZSM-5 zeolites containing ammonium (SiO2/Al2O3 = 30, 400 m2 g−1; and SiO2/Al2O3 = 200−400, 400 m2 g−1) and Beta zeolite containing ammonium (SiO2/Al2O3 = 25, 680 m2 g−1) were purchased from Alfa Aesar. Fumed SiO2 (0.014 μm, 200 ± 25 m2 g−1) and Pt(NH3)4(NO3)2 (99.9%) were purchased from Sigma-Aldrich. B


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ACS Sustainable Chemistry & Engineering Table 1. Composition and Textural Properties of Zeolites and Zeolite-Supported Metal Catalysts surface area (m2 g−1) sample

Si/Al

ZSM-5 Pt/ZSM-5 Cu/ZSM-5 ZSM-5 Beta zeolite

15 15 15 127 12

a

metal loading (wt %, metal dispersion)

a,b

c

BET

external surface

408 401 385 395 675

321 312 301 325 545

0.3 (0.85) 20.2 (0.15)

d

volume (cm3 g−1)

micropore 87 89 84 70 130

d

total

e

0.20 0.19 0.18 0.21 0.24

micropored

mesoporef

0.05 0.05 0.07 0.04 0.09

0.25 0.24 0.25 0.25 0.33

a Si/Al and metal loading were obtained by ICP. bMetal dispersion was measured by chemisorption. cValue determined by the BET method. dValue determine by the t-plot method. eValue estimated at P/P0 of 0.99. fValue determined by the BJH desorption cumulative.

Two-stage processes containing dehydration and catalytic cracking: Prior to the reaction, 1.0 g of dehydration catalyst (on top) and the cracking catalysts (on bottom) were placed in a double bed reactor and were treated at 673 K under a N2 flow for 1 h. (2) Two-stage processes containing hydrogenolysis and catalytic cracking with H2 (molar ratio of H2 to glycerol = 100): The process is similar to process 1 except for the cofeeding of the aqueous glycerol solution with H2 and the replacement of the dehydration catalysts (solid acids) with hydrogenation catalysts (supported metal catalysts). Prior to the reaction, the supported metal catalysts were treated at 573 K for 1 h under H2 flow (100 cm3 gsolid−1 min−1). The reactant and product concentrations were measured with an online gas chromatograph (GC 6890) using a methyl silicone capillary column (Agilent HP-1, 30 m × 0.32 mm × 1.0 μm) with a flame ionization detector, a capillary column (Agilent HP-PLQT-Q, 30 m × 0.32 mm × 20 μm) with a flame ionization detector, and a packed column (TDX-01, 6 m × 3 mm) with a thermal conductivity detector. The conversion and selectivity (based on carbon) were measured according to the work of Suprun et al.31

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NH3-TPD measurements (Figure 1) were performed to investigate the acidity of the samples. Each of the zeolite or

RESULTS AND DISCUSSION

Structural, Textural, and Acidity Characterization. The X-ray diffraction patterns of these samples (Figure S1) show that all ZSM-5 and Pt/ZSM-5 samples possess a series of characteristic diffraction peaks at 2θ of 7.9, 8.8, 14.8, 23.2, 23.9, and 24.4°, which correspond to an MFI zeolite topology (Powder Diffraction File (PDF) No. 49-0657, Joint Committee on Powder Diffraction Standards (JCPDS), [2004]), and the Beta samples show typical diffraction peaks corresponding to the BEA structure (PDF No. 47-0183, JCPDS, [2004]). No other impurity can be observed in these patterns. No diffraction peaks ascribed to Pt were observed, which is likely caused by high metal dispersion (0.85) and low metal loading (0.3 wt %). Figure S2 shows SEM images of ZSM-5 and Beta zeolite samples. ZSM-5 (Si/Al = 15) possesses particle sizes (500 nm to 1 μm) smaller than those of ZSM-5 (Si/Al = 127, 1−1.5 μm). Beta zeolite shows the smallest size (300−600 nm). The textural properties and chemical composition of the zeolite and zeolite-supported metal samples are listed in Table 1. Beta zeolite exhibited the lowest Si/Al ratio and the highest surface area and pore volume, resulting from its having the smallest particle size. Both ZSM-5 samples showed similar surface areas and pore volumes, even when different Si/Al ratios were present. After the deposition of Pt on the ZSM-5 samples, these values changed minimally. The high metal dispersion and low metal loading indicate the homogeneous distribution of Pt in the channel of the ZSM-5 (Si/Al = 15) zeolites. However, the Cu/ZSM-5 sample had a slightly smaller surface area but a larger mesoporous volume. This finding suggests that Cu nanoparticles formed on the external surface of zeolite as a result of high metal loading (20.2 wt %) and low metal dispersion (0.15).

Figure 1. NH3-TPD profiles of samples.

zeolite-supported metal sample profiles is characterized by two desorption peaks, in the temperature regions at 460−505 K and 650−740 K. The desorption peak at low temperature is ascribed to the chemisorption of ammonia molecules on weak acid sites; the high-temperature peak is attributed to the desorption on strong Brønsted and Lewis acid sites. Table 2 shows the amount of acid sites calculated from the TPD measurements. The bulk ZSM-5 (Si/Al = 15) samples possess higher amounts of weak acid, strong acid, and total acid sites and a higher Si/Al ratio than does ZSM-5 (Si/Al = 127). However, the ratio of strong acid sites to weak acid sites is similar (1.2). The total amount of ZSM-5 (Si/Al = 15) increased from 0.240 mmol g−1 to 0.246 and 0.262 mmol g−1 after depositing 0.3 wt % Pt and 20.2 wt % Cu, respectively, on ZSM-5. Unlike the bulk ZSM-5 samples, the total amount of acid sites primarily arose from the weak acid sites for the ZSM5-supported metal catalyst, decreasing the ratio of strong acid sites to weak acid sites from 1.2 to 0.9 on Pt/ZSM-5 and to 0.2 on Cu/ZSM-5, respectively. This result suggests that the strength of the acid sites decreases with the deposition of metal on the ZSM-5 zeolite. For the metal oxide samples, the γ-Al2O3 and Cu/γ-Al2O3 samples only showed one peak from 455 to 490 K corresponding to weak acid sites. Notably, after the deposition of ∼20 wt % Cu on γ-Al2O3, the amount of acid sites increased sharply from 0.017 to 0.032 mmol g−1, indicating the formation of new Lewis acid sites in the presence of Cu. Thus, an increased number of weak acid sites and a decreased number C


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ACS Sustainable Chemistry & Engineering Table 2. Acid Properties of Samples from NH3-TPD Spectra weak acid sites

strong acid sites

sample

Si/Al

temperature (K)

acid amount (mmol g−1)

temperature (K)

acid amount (mmol g−1)

total acid amount (mmol g−1)

ZSM-5 ZSM-5 Pt/ZSM-5 Cu/ZSM-5 Al2O3 Cu/Al2O3

127 15 15 15

465 505 500 467 455 465

0.021 0.109 0.132 0.225 0.017 0.032

671 690 685 640

0.026 0.131 0.114 0.037

0.047 0.240 0.246 0.262

Figure 2. Effect of reaction temperature on the product distribution in the single-step conversion of glycerol over (a) ZSM-5, Si/Al = 15 and (b) ZSM-5, Si/Al = 127 (21.9 kPa glycerol, 78.1 kPa H2O, WHSV = 1 h−1, 30 min).

Scheme 1. Proposed Reaction Pathways of Glycerol Dehydration on Brønsted Acid Sites (I and II), Lewis Acid Sites (II), and Basic Sites/Metal Sites (III) Reported in the Literature1−10,23−30

dehydration, cracking, and hydrogen transfer reactions is required and can be accomplished using an appropriate catalyst and reaction conditions for the selective production of olefins.7,14 Figure 2 shows the selectivities of olefins in the single-step conversion of glycerol over ZSM-5 zeolites at a high temperature (673−873 K). Similar to the work of Corma et al.13,14 and Amin et al.,16,18,20 ethylene and propylene

of strong acid sites were observed on the Cu/ZSM-5 compared to the ZSM-5 zeolite. Single-Step Conversion of Glycerol to Propylene over ZSM-5 Zeolites. The biggest challenge with glycerol conversion to olefins is the efficient removal of oxygen from the glycerol molecule, which is hydrogen-deficient relative to petroleum-based feedstocks.32 Specifically, a proper balance of D


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Figure 3. Effect of reaction temperature on reactant conversion and product distribution in the transformation of glycerol over (a) γ-Al2O3, (b) Cu/ γ-Al2O3, (c) beta zeolite, (d) ZSM-5 zeolite, (e) Cu/ZSM-5, and (f) Pt/ZSM-5 (21.9 kPa glycerol, 78.1 kPa H2O, WHSV = 1 h−1, 30 min; other product: glycerol polycondensation products, cyclic glycerol ethers, acids, aromatics, etc.). (g) Effect of time on stream on the conversion of glycerol at 543 K. E


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Figure 4. Effect of reaction temperature on the product distribution of (a) acetol and (b) acrolein over ZSM-5 catalysts with ∼100% conversion (21.9 kPa reactant, 78.1 kPa H2O, WHSV = 1 h−1, 30 min; other products: CO, CO2, butylenes, acetaldehyde, acids, etc.).

methanol, and COx).1−10 The C−C bond cleavage can be promoted by increasing the number of acid sites and the acid strength. Thus, ZSM-5 with a high Si/Al ratio of 127, which has a very low total number of acid sites (both weak and strong acid sites) (Figure 1 and Table 2), shows a higher selectivity for light olefin production with a lower COx selectivity, as shown in Figure 2. In addition to acidic and basic catalysts, metal catalysts can also catalyze the dehydration of glycerol to acetol (routes II and III in Scheme 1).33−35 Here, metal oxide (γ-Al2O3 with Lewis acid), zeolite (HZSM-5 and H-Beta with Brønsted acid), and supported metal (Cu/γ-Al2O3, Cu/ZSM-5, and Pt/ZSM-5) catalysts were used to catalyze the dehydration reaction and alter the product distribution. The altered product distribution for glycerol dehydration at different reaction temperatures is shown in Figure 3. Generally, dehydration over acid catalysts was performed at a lower temperature prior to the thermal decomposition of glycerol (∼623 K) to obtain oxygenated compounds without obvious cracking.1−10 Here, in addition to acetol and acrolein, only a few cracking products (i.e., acetaldehyde, methanol, ethanol, acetic acid, COx, and hydrogen in Figure 3) were obtained, and the main byproduct of the dehydration was polyglycerol. The results of Figure 3 were obtained after 30 min of dehydration, and the total duration of each experiment was 10 h. For the conversion of glycerol, the catalysts containing zeolites show much higher values (>70% from 503 to 573 K) than the γAl2O3 catalyst (8−21% from 503 to 573 K) because of its higher total amount of acid sites and higher amount of strong acid sites, as shown in Figure 1 and Table 2. This result suggests that activity is enhanced by promoting the acid strength and the amount of acid sites using Brønsted acid catalysts. For the γ-Al2O3 catalyst, the glycerol molecules coordinate to the Lewis acid sites, enabling the concerted cleavage of the terminal C−OH to form prop-2-ene-1,2-diol, which then isomerizes to acetol (route II, Scheme 1). In addition to the formation of acetol, the dehydration reaction leaves a hydrated Lewis site on the surface of the catalyst that can further act as a pseudo-Brønsted site and catalyze the dehydration of glycerol to acrolein.6,36 Thus, the main

selectivities increased with temperature (i.e., from approximately 7 to 20% for ethylene and from 2 to 6% for propylene among 673−873 K). However, the COx selectivity (approximately 39%, mainly CO) was very high at 873 K. To promote selectivity for light olefins, the ZSM-5 catalyst with a higher Si/ Al ratio of 127 was used and found to inhibit the formation of COx. Specifically, ethylene and propylene selectivities of approximately 22 and 14%, respectively, were obtained, whereas the COx selectivity was reduced to approximately 31%. Thus, it is feasible to obtain light olefins and CO by the single-step conversion of glycerol over ZSM-5 zeolites at a high temperature, and the CO could be used to produce additional hydrogen via the water−gas shift (WGS) reaction.13,14 Tandem Conversion of Glycerol to Propylene over Acid Catalysts. As shown in Figure 2, the catalytic cracking of glycerol over a zeolite is very effective at removing oxygen. However, oxygen was not released via the optimal pathway, and excess COx was produced. The process also leads to the formation of hydrogen through the steam-reforming reaction, dehydrogenation, or the WGS reaction. To establish an efficient direct conversion of glycerol to olefins, a two-stage route involving glycerol dehydration and cracking over acid catalysts was developed. For the MTP process, the conversion of methanol into hydrocarbons involves the dehydration of methanol to dimethyl ether (DME) and the subsequent conversion of this DME/ methanol mixture into hydrocarbons. Similar to this process, the dehydration of glycerol occurs first in the conversion of glycerol into hydrocarbons. Unlike the production of a single product (i.e., DME) in the dehydration of methanol, the dehydration of glycerol produces different products depending on the catalyst. The formation of acrolein is generally accepted for Brønsted acids (route I in Scheme 1), and the formation of acetol is catalyzed by both Lewis acids (route II in Scheme 1) and basic catalysts (route III in Scheme 1).3,6 Notably, the selectivity of the dehydration product distribution is changed by not only the type of catalyst but also the properties of the catalyst, especially the acid density and strength. The formation of acrolein or acetol is accompanied by the formation of some C−C bond cleavage compounds (i.e., ethylene glycol, ethanol, F


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Figure 5. Effect of reaction temperature on the product distribution in the two-stage conversion of glycerol via dehydration and catalytic cracking over (a) Cu/γ-Al2O3 (for dehydration) and ZSM-5 (Si/Al = 127, for cracking) and (b) ZSM-5 (Si/Al = 15, for dehydration) and ZSM-5 (Si/Al = 127, for cracking) (21.9 kPa glycerol, 78.1 kPa H2O, WHSV = 1 h−1, 30 min).

dehydration products on the γ-Al2O3 catalyst were acetol and acrolein, and the selectivity decreased gradually with increasing reaction temperature (Figure 3a). After the deposition of Cu metal on the γ-Al2O3 catalyst, the conversion of glycerol increased, and the selectivity for acetol increased from 40 to approximately 54% at 503 K, whereas the selectivity for acrolein decreased from 33 to 23% (Figure 3b). This result confirms the enhancing effect of the metal sites on the formation of acetol (route III, Scheme 1) and on the number of acid sites. When the hydration of glycerol was performed on the zeolite samples, acrolein became the main product. As shown in Figure 3c,d, the acrolein selectivity decreased with increasing temperature. Acrolein selectivities of 54−62% were obtained over ZSM-5 zeolite catalysts, whereas higher values of 75−88% were observed on Beta zeolite catalysts. Moreover, Beta zeolite catalysts provided better conversions of glycerol than did ZSM5 catalysts (Figure 3c,d), but a drastic conversion decrease was observed with longer TOS (Figure 3g), suggesting a fast deactivation process. After the deposition of 0.3 wt % Pt and ∼20 wt % Cu on ZSM-5 zeolites, respectively, the resulting catalysts showed improved glycerol conversion and acetol selectivity but decreased acrolein selectivity. However, a higher selectivity for acrolein was still observed on Cu/ZSM-5 and Pt/ ZSM-5 catalysts, suggesting that the dehydration predominantly occurred over the Brønsted acid sites. Figure S3 shows the selectivities of acrolein and acetol over the ZSM-5, Cu/ ZSM-5, and Pt/ZSM-5 catalysts with a similar conversion of glycerol at ∼80%. Cu/ZSM-5 with a smaller amount of Brønsted acid sites but a larger amount of Lewis acid sites showed significantly higher selectivity for acetol production. These data show that the hydrocarbons in the conversion of glycerol should primarily come from acrolein over catalysts containing Brønsted acid sites and from acetol over Lewis acid catalysts, regardless of the presence of metal sites in the catalysts. As reported by Corma et al.,14,15 acrolein and acetol are the primary reaction products from the glycerol dehydration. Thus, it is possible to produce different light olefin distributions from glycerol using different glycerol dehydration catalysts to obtain acrolein or acetol.

The main oxygenates produced by glycerol dehydration were acetol or acrolein (Figure 3). Figure 4 shows the conversion and product distribution of acetol and acrolein to propylene over ZSM-5 (Si/Al = 15) catalysts. Generally, the cracking of the CO bond should be more difficult than that of the C− OH bond. In fact, the conversion of acetone or propanal into hydrocarbons over acid catalysts is accompanied by the cracking of isobutylene, which is produced from an aldol condensation of acetone or propanal, and the main product of the reaction is aromatics.24,37−40 Here, aromatics were also primary products with the ZSM-5 catalyst in the conversion of acetol and acrolein. The selectivity for light olefins (ethylene and propylene) increased with the reaction temperature. Specifically, ZSM-5 showed higher selectivity for propylene than for ethylene production in the acetol conversion, whereas an opposite trend was observed in the conversion of acrolein. This result suggests that it is feasible to control the ratio of propylene to ethylene by controlling the product distribution of glycerol dehydration by using different types of catalysts, as shown in Figure 2. However, it should be noted that the light olefin selectivity is low (37% in acetol conversion, 36% in acrolein conversion at 773 K). Taken together, these data indicate that the conversion of the glycerol dehydration product over zeolite catalysts is more suitable for the preparation of aromatic components or gasoline. Figure 5a shows the selectivities of olefins and COx in the dehydration of glycerol over Cu/γ-Al2O3 at 573 K with subsequent cracking on ZSM-5 (Si/Al = 127) at different temperatures. Clearly, the selectivity for light olefins increased with reaction temperature. Cu/γ-Al2O3 leads to high selectivity for acetol in the glycerol dehydration (Figure 3b), which should show a higher selectivity for propylene than for ethylene (Figure 4a). In the two-stage process, the selectivities for propylene and ethylene increased from 14.3 to 35.2% and from 22.2 to 27.4%, respectively, at 873 K compared to the singlestep process (Figure 2); the COx selectivity was reduced from 31 to 11% (Figure 2), and the hydrogen yield was reduced from 2.5 to 1.2%. Moreover, the ratio of propylene to ethylene increased from 0.6 to 1.3. When the dehydration catalyst was G


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Figure 6. Effect of reaction temperature on reactant conversion and product distribution in the transformations of glycerol under H2 after 30 min of reaction over (a) Cu/SiO2, (b) Cu/γ-Al2O3, (c) Cu/ZSM-5, (d) Pt/SiO2, (e) Pt/γ-Al2O3, and (f) Pt/ZSM-5 (1.0 kPa glycerol, 3.4 kPa H2O, 95.6 kPa H2, WHSV = 0.1 h−1, 30 min.). (g) Effect of time on stream on the conversion of glycerol at 523 K. H


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Figure 7. Effect of reaction temperature on the reactant conversion and product distribution of 1,2-propanediol over (a) γ-Al2O3 and (b) ZSM-5 and effect on the product distribution of (c) propanal, (d) 1-propanol, and (e) 2-propanol over ZSM-5 catalysts with ∼100% conversion (21.9 kPa reactant, 78.1 kPa H2O, WHSV = 1 h−1, 30 min; other products: CO, CO2, butylenes, acetaldehyde, acids, etc.). (f) Butylane to butylene ratio (C4/ C4) in the conversion of acetol, acrolein, propanal, propanols, and 1,2-propanediol over ZSM-5 catalysts.

should be suitable for MTP technology when the deactivation of the dehydration catalysts by coking is overcome.3,6 Tandem Conversion of Glycerol to Propylene over Metal and Acid Catalysts with the Aid of Hydrogen. In addition to the high selectivity for COx and the relatively low yield of light olefins via the conversion of glycerol over acid catalysts,7,16,20,21 a fast deactivation of zeolite by coking is observed.3,6,13,14 As shown in Figure 3, the dehydration of glycerol at 503−573 K mostly leads to oxygenated compounds containing CO bonds, increasing the difficulty of removing oxygen for further conversion into hydrocarbons.29 Moreover, the deactivation of the acid catalyst by coking causes a fast decrease in conversion with longer reaction time (Figure

replaced by ZSM-5 (Si/Al = 15), as shown in Figure 5b, an increase in the propylene (26.8%) and ethylene (38.2%) selectivities was observed, and the ratio of propylene to ethylene changed to 0.7. Again, the COx selectivity was reduced to approximately 12%, and the hydrogen yield was reduced from 2.5 to 1.3%. Together with the results of the single-step process (Figure 2), these data suggest that the two-stage route could provide the proper balancing of dehydration, cracking, and hydrogen transfer reactions in the conversion of glycerol to olefins. Moreover, the product distribution and the ratio of propylene to ethylene could be adjusted by changing the dehydration catalysts and conditions. The two-stage process I


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ACS Sustainable Chemistry & Engineering 3g).3,6,15 Thus, tandem processes involving hydrodeoxygenation/hydrogenolysis and cracking reactions have been reported to promote the production of aromatics or olefins.17,24,28 In addition to the dehydration of glycerol to acrolein, the hydrogenolysis of glycerol to 1,2-propanediol is another effective route for the utilization of glycerol.30,33 Acetol is usually regarded as the precursor of 1,2-propanediol, and further hydrogenolysis of 1,2-propanediol can produce propanols (1-propanol and 2-propanol) as the main byproducts. The net result makes acetol an ideal precursor in propylene production.24,28,41,42 Figure 6 shows the gas-phase conversion of glycerol over acid and metal catalysts in the presence of hydrogen. Cu metal catalysts, which are active only in the cleavage of C−O bonds, and Pt metal catalysts, which are active in the cleavage of both C−C and C−O bonds,30 were used to change the product distributions. All catalysts showed approximately 100% conversion of glycerol in the hydrogenolysis reactions from 453 to 573 K. Cu/SiO2 catalysts showed very high selectivities (>80%) for 1,2-propanediol (Figure 6a). Acetol and propanols were the main byproducts of the reaction with trace amounts of 1,3-propanediol. At low reaction temperature (453 K), approximately 6% selectivity for acetol was observed, and this value decreased to approximately 1% by increasing the temperature to 573 K. However, selectivity for 1,2-propanediol also decreased with increased temperature, and the propanol selectivity increased. This result suggests that the hydrogenolysis of 1,2-propanediol to propanols is dominant over the hydrogenation of acetol to 1,2-propanediol. When the acid support (γ-Al2O3) was used to deposit Cu metal, the selectivity of the product distribution changes slightly (Figure 6b). The selectivity for 1,2-propanediol over the Cu/γ-Al2O3 catalyst (60−86% at 453−573 K) is much lower than that over the Cu/SiO2 catalyst (83−93% at 453− 573 K). A new byproduct from C−C bond cracking, ethylene glycol, was observed, and 1,3-propanediol and acetol were also produced. Specifically, the selectivity for 1,3-propanediol appears to increase in the presence of acid catalysts. The amount of C−C cracking product (typical for ethyl glycol) increased over catalysts combining metal sites and acid sites. Thus, the formation of ethylene glycol likely occurs by the synchronization of the metal sites and acid sites. As shown in Figure 3, ZSM-5 possessed a higher activity in the dehydration of glycerol because of its stronger acidity compared to that of γ-Al2O3. The Cu/ZSM-5 catalyst showed much higher 1,3-propanediol, ethylene glycol, and propanol selectivities than did Cu/γ-Al2O3. Specifically, the 1,2-propanediol selectivity decreased from 45 to 12% upon increasing the hydrogenolysis temperature from 453 to 573 K, and ethylene glycol and propanol selectivities increased from 8 to 22% and from 27 to 50%, respectively. Pt/SiO2 shows a similar trend in selectivity as Cu/ZSM-5 but has a higher 1,2-propanediol selectivity and lower ethylene glycol or propanol selectivity. This indicates that Pt metal catalyzes the C−O and C−C bond cracking but that the combination of Cu metal and ZSM-5 exhibits better performance than Pt metal alone (Figure 6c,d). Alternatively, propanol formation is promoted in the presence of acid catalysts in the hydrogenolysis of glycerol over metal catalysts.43,44 Figure 6e,f shows the product distribution in glycerol hydrogenolysis over Pt/γ-Al2O3 and Pt/ZSM-5 catalysts, respectively. A high selectivity for propanols (>50%) was observed on the Pt/γ-Al2O3 catalyst at 453 K, and this increased to 72% by increasing the reaction temperature to 523 K. Similarly, the propanol and ethylene glycol selectivities over

Pt/ZSM-5 catalysts increased from 69 to 76% by increasing the reaction temperature from 453 to 523 K. The amount of 1,2propanediol and 1,3-propandiol decreased with increasing reaction temperature. Notably, the trend for ethylene glycol is similar to that of Cu/ZSM-5, Pt/SiO2, Pt/γ-Al2O3, and Pt/ ZSM-5 catalysts, and the value is also similar at the highest hydrogenolysis temperature (approximately 20%). These data show that the conversion of glycerol to hydrocarbons using metal catalysts in H2 should mainly involve the transformation of propanediols or propanols into hydrocarbons in the presence of acetol and ethylene glycol. It is worth noting that the conversion of glycerol over metal catalysts in H2 shows a much higher and more stable conversion than the dehydration of glycerol over acid catalysts at low temperature (Figures 3g and 6g). Moreover, the Pt−acid bifunctional catalyst is a good catalyst for the highly selective conversion of glycerol into propanols, which is consistent with other reports.43,44 Therefore, combining the glycerol hydrogenolysis and cracking process using bifunctional catalysts under H2 flow may enable the stable and highly selective transformation of glycerol into light olefins (i.e., propylene) via feasible conversion of propanols.24,28 Figure 7 shows the reactant conversion and product distribution for hydrogenolysis/hydrodeoxygenation over acid catalysts from 473 to 773 K. The conversion of 1,2-propanediol over ZSM-5 catalyst was higher than over γ-Al2O3 and increased with reaction temperature (Figure 7a,b). The major product was propanal, and the selectivity was higher than 50% on ZSM-5 and higher than 20% on γ-Al2O3. Other products over γ-Al2O3 identified in large quantities were propanols and acetone. However, the selectivity for propylene is lower than that for propanal. A propylene selectivity of 0.7% was obtained on ZSM-5 catalyst at 573 K. The results of the 1,2-propanediol conversion show the difficulty of forming large amounts of propylene via catalytic cracking at low temperature (473−773 K). Although propylene selectivity is expected to increase at temperatures higher than 773 K, thermal cracking and the deactivation of catalysts remain problematic for the reaction at this high temperature.14−18,20,21 As shown in Figure 6e,f, main products of glycerol hydrogenolysis were propanols (1-propanol and 2-propanol). Figure 7d,e shows the propylene selectivity in the conversion of 1-propanol and 2-propanol, respectively, over ZSM-5. Propylene can be simply obtained via dehydration over acid catalysts,40,41 but the simultaneous oligomerization of propylene leads to C4+ olefins and aromatics.45,46 Here, propylene selectivities as high as 41% were obtained at 473 K (Figure 7d), and only trace amounts of ethylene (roughly 1% selectivity) were measured. Such a high ratio of propylene to ethylene suggests a feasible route for the preparation of propylene from propanols. However, the selectivity for propylene decreased with reaction temperature as did C4+ olefin selectivity. Thus, to develop an effective route for the conversion of glycerol to propylene, the selectivity of propylene production from propanols should be improved. For the MTP process over ZSM-5 catalyst, a high selectivity for light olefins can be promoted by increasing the Si/Al ratio.47 In light of this effect, a ZSM-5 catalyst with a high Si/Al ratio of 127 was used to catalyze the dehydration of propanols to propylene (Figure 8). The selectivity for propylene was increased to approximately 95% at 473 K in the conversion of propanols, but the conversion of propanols to propylene decreased to 72%. Although the conversion of propanols can be J


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increased to approximately 100% by increasing the reaction temperature to 673 K, the selectivity for propylene decreased to approximately 60%. In Figure 8c, the effects of residence time on the conversion and propylene selectivity were shown for feeds of 1-propanol and 2-propanol at 673 K. The reaction conversion decreased, and the propylene selectivity increased with residence time. Figure 8c shows that the best propanol WHSV is 5 h−1 with a propylene selectivity higher than 90% and approximately 100% conversion. Tandem processes involving hydrodeoxygenation/hydrogenolysis and cracking reactions have been reported to promote the production of aromatics or olefins.17,24,28 The introduction of hydrogen to the hydrogen-deficient glycerol molecule promotes the formation of oligomerized hydrocarbons and thus enhances the yield of aromatics or olefins over acid catalysts. Recently, Sun et al. obtained approximately 85% selectivity for propylene in the conversion of glycerol over double-bed catalysts (9.3%WO3 in Cu/Al2O3 + SiO2−Al2O3) using 20 wt % (4.7 mol %) glycerol aqueous solution with WHSV = 0.3 h−1 and a H2/glycerol ratio = 150 (approximately 0.67 kPa glycerol, 11.7 kPa H2O, and 87.6 kPa H2) at 523 K.28 Here, a similar two-stage process of glycerol conversion using Cu/ZSM-5 or Pt/ZSM-5 as hydrodeoxygenation catalysts and ZSM-5 as cracking catalysts was used to selectively convert glycerol to propylene (Figure 9, 1.0 kPa glycerol, 3.4 kPa H2O, 95.6 kPa H2). The selectivity for propylene decreased with reaction temperature, similar to the results for the conversion of propanols to propylene shown in Figures 7 and 8. The oligomerization of propylene was found in the tandem dehydration of propanols to propylene. For the oligomerization, the effects of temperature and pressure on oligomerization activity have been investigated and can be adjusted to tailor the oligomerization products of propylene. 46 For example, increased pressure favors higher molecular weight oligomers, and higher temperatures favor the formation of aromatics. Here, the highest propylene selectivity of 76% was observed in the two-stage process using Pt/ZSM-5 and ZSM-5 catalysts at 523 K (Figure 9b), which is slightly lower than the values (85− 88%) reported by Sun et al. and Yu et al.25,28 This discrepancy may be caused by the high partial pressure of glycerol, the high weight hour space velocity, a low hydrogen-to-reactant ratio, and different hydrogenolysis product distributions. Together with the literature data, our results show that the tandem process consisting of the formation of propanols as intermediates through the hydrogenolysis of glycerol and of a subsequent dehydration to propylene is highly efficient for the conversion of glycerol to propylene. Tandem Conversion of Glycerol/Methanol Mixture to Propylene. Here, we show an advance in the conversion of glycerol to propylene via the tandem process. At present, refined glycerol is available for 900−1000 US$ ton−1, but propylene costs 700−800 US$ ton−1. The price inversion between the reactant and product appears to limit the desirability of this process. However, crude glycerol (approximately 80 wt % glycerol and 20 wt % methanol; molar ratio of glycerol to methanol = 1/1.28) can be purchased for approximately 150 US$ ton−1, which is even cheaper than methanol at 250−300 US$ ton−1. Thus, the conversion of crude glycerol to propylene should be more economical than the MTP process without considering other technological and/ or instrumentation concerns. Thus, it is important to investigate the feasibility of the conversion of crude glycerol (glycerol/methanol mixture) to propylene with reaction

Figure 8. Effects of reaction temperature on the product distribution of (a) 1-propaonol and (b) 2-propanol on ZSM-5 (Si/Al = 127). (c) Effects of residence time on the conversion and propylene selectivity in propanol conversion at 673 K (21.9 kPa propanols, 78.1 kPa H2O, WHSV = 1 h−1, 30 min). K


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Figure 9. Effect of reaction temperature on the product distribution in the two-stage conversion of glycerol via hydrogenolysis and catalytic cracking over (a) Cu/ZSM-5 (Si/Al = 15, for hydrogenolysis) and ZSM-5 (Si/Al = 127, for cracking) and (b) Pt/ZSM-5 (Si/Al = 15, for hydrogenolysis) and ZSM-5 (Si/Al = 127, for cracking) (1.0 kPa glycerol, 3.4 kPa H2O, 95.6 kPa H2, glycerol WHSV = 0.1 h−1, 30 min).

Figure 10. Effect of reaction temperature on the product distribution in the two-stage conversion of glycerol/methanol mixture via hydrogenolysis over Pt/ZSM-5 and catalytic cracking over ZSM-5 (a) without H2 (21.9 kPa glycerol, 28.1 kPa methanol, 50 kPa glycerol, WHSV = 1, 30 min) and (b) with H2 (1.0 kPa glycerol, 1.2 kPa methanol, 2.2 kPa H2O, 95.6 kPa H2, WHSV = 0.5 h−1, 30 min).

conditions (50 kPa methanol and 50 kPa H2O, methanol WHSV = 1 h−1) similar to those of the MTP process. Figure 10 shows the results of the conversion of aqueous glycerol/methanol solution (molar ratio of glycerol to methanol = 1/1.28, molar ratio of oxygenated compound to H2O = 1) in a two-stage process with/without H2. For the MTP reaction over ZSM-5 catalysts, a low reaction temperature leads to the formation of gasoline components (especially for aromatics and C5+ olefins), and a high propylene selectivity (approximately 45%) can be obtained at high reaction temperatures (743−773 K). Figure 10a shows a similar trend by cofeeding methanol and glycerol, which is in agreement with the work of Jang et al.22 Here, a propylene selectivity (approximately 38%) lower than that of the MTP process could be obtained at 873 K. Unlike using glycerol as the raw material (Figure 9b), the cofeeding of methanol greatly changed the product distribution.

A higher selectivity was obtained for propylene than for ethylene, and a high propylene/ethylene ratio of 2.2 was achieved. Such a high ratio implies the formation of propylene via the catalytic cracking of long-chain olefins from the alkylation of olefins by methanol, which could be explained by the dual-cycle mechanism.48 These results show that the presence of methanol promoted selectivity for light olefins in the conversion of glycerol. As shown in Figure 9b, the combination of the hydrogenolysis of glycerol to propanols and the dehydration of propanols to propylene leads to high selectivity for propylene (approximately 76%) at a low temperature of 523 K. It has been confirmed that the highly selective formation of propylene from glycerol relies on the generation of propanols from acrolein or acetol (Scheme 1).10,25,28 This conversion could be achieved by controlling the hydrogenation of glycerol to propanols. Sun et L


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ACS Sustainable Chemistry & Engineering al. found only ∼22% selectivity for 1-propanol over Cu/Al2O3 catalysts by hydrogenation, but the direct incorporation of WO3 led to 84.8% selectivity for propylene. The selectivity for 1propanol was promoted, but it was continuously dehydrated to propylene.28 Yu et al. developed an efficient Ir/ZrO2 catalyst to obtain ∼99% selectivity for 1-propanol in the hydrogenolysis of glycerol and ∼94% selectivity from a glycerol/methanol mixture using a high hydrogen pressure of 5 MPa.25 Thus, the combination of Ir/ZrO2 and ZSM-5 (Si/Al = 30) led to ∼88% selectivity for propylene in the conversion of glycerol to propylene. However, it is difficult to obtain stable catalysts with this strategy (∼50 h single-route lifetime) because the acid catalysts with high acid site density (low Si/Al ratio) deactivate quickly in the dehydration reaction.1−10 In particular, we found much lower propylene selectivity (33%) at low reaction temperature (523 K) using reaction conditions identical to those of Yu et al. when converting the glycerol/methanol mixture to olefins. Many groups have found that the alkylation of light olefins (i.e., propylene) with methanol to C4+ olefins and aromatics is predominant in the conversion of methanol over zeolite catalysts.45,49−51 To obtain a high propylene yield, a high reaction temperature is required to catalyze the cracking of long-chain olefins into propylene. Thus, the highest propylene selectivity (approximately 61%) was observed at a high reaction temperature (773 K), which is similar to the best reaction temperature of MTP and MTO. The selectivity remains lower than that of the two-stage conversion of glycerol (which is approximately 76%), but it is much better than that of the MTP reaction. Moreover, the total amount of light olefins (propylene and ethylene, approximately 71%) is similar to that of the MTP reaction. Taken together, these data show that our proposed two-stage strategy is an effective protocol for the conversion of the glycerol/methanol mixture to propylene, which should enable the use of crude glycerol as starting material for the production of light olefins. Application of GTO in Crude Glycerol Conversion. Glycerol conversion decreased quickly with the time on stream over all of the zeolite catalysts.1−10 The dehydration of glycerol to acrolein is usually accompanied by side reactions that produce byproducts such as aromatic compounds, glycerol polycondensation products, and cyclic glycerol ethers. Here, the tandem conversion of glycerol by dehydration at low temperature and subsequent cracking dehydration at high temperature (Figure 5) increased the selectivity for light olefins (propylene and ethylene) 2-fold and markedly decreased COx selectivity compared to the single-step conversion of glycerol at high temperature (Figure 2). However, the glycerol dehydration could be reduced by half within 5 h (Figure 3g), and the selectivity for light olefins decreased quickly to a value similar to that of the single-step process after 6 h. Thus, a stable dehydration catalyst is required for the tandem process using only acid catalysts. The issue may also be resolved by a fluid bed reactor using dual reactors. Catalyst deactivation in the hydration of glycerol can be prevented by the tandem process combining the hydrogenolysis/hydrodeoxygenation and catalytic cracking reaction over metal and acid catalysts (Figure 3g, 6g). The introduction of hydrogen and the high molar ratio of hydrogen to glycerol (i.e., >100)24−28 will render engineering difficulty when applying GTO in the MTP unit. However, the high selectivity for propylene in the conversion of glycerol and the glycerol/ methanol mixture (∼76 and ∼61%, respectively) is good for the subsequent product separation. Thus, the key feature of the

tandem glycerol conversion process is the stability of the catalyst in the subsequent cracking reaction. Ideally, an industrial MTP catalyst can be used directly. The catalytic activities for the MTP reaction over a typical ZSM-5 zeolite catalyst52 were examined in a fixed-bed reactor under industrial conditions (753 K, 50 kPa methanol, 50 kPa water, WHSV = 1 h−1), and the product distribution measured at steady state is listed in Table 3. For comparison, the Table 3. Product Distribution of the MTP Reaction and the Tandem GTO Reaction with Hydrogena MTP

GTO

99.9

99.9

0.9 0.5 0.7 5.9 47.2 4.9 23.8 13.1 3.0 8.0 0.21 18.5

1.1 0.6 1.0 8.3 63.7 3.1 13.0 6.0 3.2 7.6 0.26 19.8

conversion (%) selectivity (C %) methane ethane propane ethylene propylene butane butylenes C5+b aromatics P/E ratio C4/C4 coke deposit carbon content (wt %)c a

MTP reaction: 753 K, 50 kPa methanol, 50 kPa water, WHSV = 1 h−1. GTO reaction: 1.0 kPa glycerol, 1.2 kPa methanol, 2.2 kPa H2O, and 95.6 kPa H2, WHSV = 1 h−1. bC5 and higher hydrocarbons excluding aromatics cAfter reaction for 500 h, the coke content was evaluated by thermogravimetric (TG) analysis.

conversion and selectivities of the tandem GTO process with the methanol/glycerol mixture via hydrogenolysis (523 K) over Pt/ZSM-5 and cracking (753 K) over the typical ZSM-5 zeolite catalyst were performed (1.0 kPa glycerol, 1.2 kPa methanol, 2.2 kPa water, 95.6 kPa H2, WHSV = 1 h−1). It is abundantly clear that compared with the MTP process, the GTO process shows noticeably higher selectivities for propylene (63.7 vs 47.9%), ethylene (8.3 vs 5.9%), C1−C3 alkanes, and aromatics, whereas its selectivities for butylene and C5+ hydrocarbon production are lower. Moreover, the GTO process exhibits a similar propylene/ethylene ratio. The variation of conversion and light olefin selectivity as a function of TOS was compared for the MTP and GTO processes and is presented in Figure 11. The reaction reached steady state and persisted for more than 550 h in the MTP reaction, whereas the one-route lifetime of the catalyst is somewhat shorter (∼500 h) in the GTO process. This difference may be caused by the faster coking rate over a catalyst in the GTO process, because it has a higher C4 hydrogen transfer index (HTI, denoted as the ratio of butanes to butylenes, C4/C4). As shown in Table 3, the C4−HTI for the GTO process is approximately 0.26, which is slightly higher than that in the MTP process. This finding implies that the hydrogen transfer activity of the acid catalyst is higher and that the secondary reactions such as cyclization and aromatization proceed more easily. As a result, the formation of heavy aromatics such as polymethylbenzenes, which will eventually lead to coking, is increased in the GTO process, creating a large M


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Figure 11. Conversion and selectivities for propylene, ethylene, and butylenes as a function of time (a) on stream over typical ZSM-5-based MTP catalysts51 in the MTP reaction (753 K, 50 kPa methanol, 50 kPa water, WHSV = 1 h−1) and (b) in the two-stage conversion of a glycerol/methanol mixture via hydrogenolysis (523 K) over Pt/ZSM-5 and catalytic cracking (753 K) over typical ZSM-5 based MTP catalysts51 (1.0 kPa glycerol, 1.2 kPa methanol, 2.2 kPa H2O, 95.6 kPa H2, WHSV = 1 h−1).

glycerol/methanol mixture to propylene may be caused by coke formation on the dehydration/cracking catalyst (ZSM-5), which is identical to the deactivation that occurs in the MTP reaction. Thus, the modification strategy for the MTP catalysts should also be suitable for the dehydration/cracking catalysts in our two-stage route to convert crude glycerol to propylene. The C4−HIT values (Figure 7f), the stability of the catalyst in the GTO process (Figure 11), and the coke composition analysis (Figure S4) indicate that the hydrogen transfer reaction, which results in coking, cannot be avoided, even with the incorporation of hydrogen during the acid catalytic process. However, the hydrogen transfer reaction rates can be reduced, as evidenced by the much lower aromatic selectivity of the two-stage route with hydrogen than that without hydrogen (Figure 10). In particular, the aromatic selectivity could be reduced from ∼25 to ∼12% in the conversion of glycerol/ methanol by introducing hydrogen at 823 K (Figure 10). Alternatively, the role of hydrogen is thought to be in promoting the stability of the catalyst and the selectivity for propylene. The metal or bifunctional catalyst with hydrogen forms 1-propanol by the hydrogenolysis of glycerol to 1propanol25 or the hydrogenation of the glycerol dehydration products (acrolein or acetol) to 1-propanol (Scheme 1).1−10 Again, the fast deactivation in the hydrogenolysis via coking was greatly promoted compared to the acid-catalyzed glycerol dehydration.1−10 The high yield of 1-propanol during the hydrogenolysis of glycerol leads to the formation of propylene via a simple dehydration.41 However, the conversion of acrolein and acetol over acid catalysts results in a large production of aromatics (Figure 4). Thus, the synchronization of hydrogen and bifunctional catalysts is required for the highly selective formation of propylene with superior stability. Here, we developed an efficient strategy to incorporate crude glycerol containing methanol into the MTP process for the production of propylene by combining the hydrogenolysis of glycerol to 1-propanol and the catalytic dehydration/cracking of 1-propanol and methanol to propylene. The combined use of these protocols is necessary to promote selectivity for

coke content (19.8 vs 18.5% in MTP) in the ZSM-5 catalyst after 500 h. The selectivity for butylene in the GTO process was much lower than in the MTP process (13.0 vs 23.8%, respectively). However, a higher C4−HTI value and higher aromatic selectivity were found for the GTO process. Figure 7f shows the effect of reaction temperature on the C4−HTI value in the conversion of acetol, acrolein, propanal, propanols, and 1,2propanediol over ZSM-5 (Si/Al = 15). The ZSM-5 catalyst with a lower Si/Al ratio possesses a much higher hydrogen transfer capability. For propanal, the aromatic is thought to originate from aldol condensation.23,36−39 For the propanol conversion (Figure 7d−f), the C4−HTI values increased with the formation of aromatics and ethylene but decreased with the growth of propylene. This finding indicates that the aromatics are primarily formed by the cyclization of the dehydration product of propanols, i.e., propylene. The carbon deposit was extracted using the method described by Guisnet,53 but not all of the occluded material could be retrieved. GC/MS analysis was used to identify the different compounds in the coke compositions. The insoluble material was attributed to graphitic carbon. Notably, the same aromatic compounds were detected (Figure S4) over the ZSM5-based MTP catalysts during the MTP and GTO reactions, as shown in Figure 11. The chromatogram obtained for the soluble coke can be divided into two regions: highly alkylated mono aromatics, which were also detected in the MTP and GTO reaction products, and polycyclic aromatic compounds. The former compounds are usually regarded as the active species for the conversion of methanol to light olefins,11 whereas the latter compounds are responsible for the blockage of the zeolite micropores, preventing the access of the reactants to the interior of the channel. Here, the coked ZSM-5-based MTP catalyst was refreshed by combustion with air. We found that we could nearly recover the conversion and selectivity for propylene during the GTO reaction without changing the hydrogenolysis catalysts. This result suggests that the deactivation of the catalyst in the two-stage conversion of the N


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Advanced Energy Materials Chemistry (Nankai University) (KLAEMC-OP201201).

propylene and to provide catalyst stability comparable to the MTP catalysts. However, the potential problems caused by the incorporation of hydrogen must be carefully considered, and the MTP process and technologies must be modified accordingly.

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CONCLUSIONS We systematically proposed a general strategy for changing the product selectivity in glycerol conversion by transforming the tandem process into a single-step process. The combination of dehydration and catalytic cracking reactions for glycerol conversion is possible with acid catalysts, and a single-step process over acid catalysts leads to low selectivity for light olefins (26−36% at 873 K) and high selectivity for carbon oxides and hydrogen but fast deactivation. A tandem process provides higher selectivity for light olefins (62−65%) and lower selectivity for carbon oxides, and it allows the control of the propylene/ethylene ratio by changing the dehydration catalysts. The synchronization of hydrogenolysis and catalytic cracking reactions to convert glycerol into hydrocarbons is a more feasible protocol with much better stability. However, the single-step process leads primarily to the formation of alcohols and paraffin. A tandem process consisting of the selective hydrogenolysis of glycerol to propanols over Pt/ZSM-5 catalysts followed by catalytic dehydration/cracking of propanols over ZSM-5 catalysts at low temperature (523 K) can greatly improve the selectivity to approximately 76%. This strategy can be extended to the conversion of inexpensive crude glycerol (mainly mixtures of glycerol and methanol) by increasing the cracking temperature to 773 K, which creates high selectivity (63.7%) for propylene and a long lifetime (∼500 h). Under these conditions, MTP processes and technologies may be suitable for the tandem conversion of crude glycerol to propylene.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00676. Brief description of XRD, SEM, TG, and GC-MS characterization methods; results of XRD and SEM imaging of different zeolites; selectivities for acrolein and acetol in glycerol dehydration over different catalysts; and GC spectra of the carbonaceous compounds formed in the deactivated MTP catalysts (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 10 89733235. Fax: +86 10 89734979. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the Natural Science of Foundation China (21206192 and 21576140), the Science Foundation of China University of Petroleum-Beijing (YJRC2013-38), the Doctoral Program of Higher Education of China (20120007120010), and the Open Project of Key Lab O


Research Article

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Selective Conversion of Glycerol into Propylene: Single-Step versus

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