Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 110−118
pubs.acs.org/journal/ascecg
Selective Hydrogenolysis of Glycerol over Acid-Modified Co−Al Catalysts in a Fixed-Bed Flow Reactor Fufeng Cai,*,†,‡ Xianghai Song,‡ Yuanfeng Wu,‡ Jun Zhang,*,† and Guomin Xiao*,‡ †
CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, No. 99 Haike Road, Pudong District, Shanghai 201210, China ‡ School of Chemistry and Chemical Engineering, Southeast University, No. 2 Southeast University Road, Jiangning District, Nanjing 211189, China S Supporting Information *
ABSTRACT: In this study, different acid-modified Co−Al catalysts were prepared and employed for glycerol hydrogenolysis by the addition of B, Ce, Zr, and heteropolyacids (HSiW, HPW, HPMo) to Co−Al catalysts. The catalysts prepared in this work were thoroughly examined by various characterization methods such as BET, ICP, SEM, H2 chemisorption, TEM, XRD, H2-TPR, NH3-TPD, XPS, and FTIR. The results showed an increase in the acid strength and Co dispersion on the catalytic surface for the modified Co−Al catalysts. This facilitated the conversion of glycerol. When ethanol was used as a solvent, the selectivity of 1,2-propanediol (1,2-PDO) by the acid-modified Co−Al catalysts decreased slightly, attributable to the enhanced etherification activity of glycerol with ethanol. However, when water was used as a solvent, the modified Co−Al catalyst with the B, Ce, and Zr species increased the selectivity of 1,2-PDO. Addition of heteropolyacids to the Co−Al catalyst enhanced the selectivity of 1,3-propanediol (1,3-PDO) as compared to 1,2PDO selectivity which was relatively low due to its association with Brønsted acid sites on the modified Co−Al catalysts. The optimal HSiW/Co−Al catalyst (in terms of both 1,2- and 1,3-PDO selectivity) showed 76.3% glycerol conversion and 18.3% 1,3PDO selectivity with a good stability. This could be attributed to the existence of well-dispersed Co particles with strong interaction between Co and W species. KEYWORDS: Glycerol, Hydrogenolysis, Propanediols, Acid-modified Co−Al catalysts, Heteropolyacids
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INTRODUCTION In recent years, large-scale biodiesel production has yielded an excess amount of glycerol as a byproduct. This has resulted in a rapid drop in the market price of glycerol.1 In this regard, how to utilize the oversupply of glycerol has received much attention in both industrial and academic communities. Currently, the application of several catalytic technologies, including transesterification, reforming, etherification, dehydration, oxidation, hydrogenolysis, and carboxyalation, in converting the cheap glycerol into other high-margin chemicals has been explored.2−4 Specifically, selective conversion of glycerol into propanediols (i.e., 1,2-PDO and 1,3-PDO) has received significant research attention. In industry, 1,2-PDO can be employed for produce cosmetics, pharmaceuticals, polyesters resins, and so on. 1,3-PDO is used primarily as a monomer for the manufacture of polytrimethylene terephthalate (PTT) with a higher flexibility and elasticity.5 Therefore, selective hydrogenolysis of glycerol can be viewed as a better alternative process to produce propanediols compared to the conventional method derived from the petroleum derivates. In general, the conversion of glycerol into propanediols catalyzed by a solid acid catalyst follows the dehydration− hydrogenation reaction pathway as reported in the literature.6,7 First, the glycerol conversion intermediates that are acetol or 3© 2017 American Chemical Society
hydroxypropionaldehyde are generated from the dehydration of either the primary or secondary hydroxyl group of glycerol on the acid sites of the catalyst. Afterward, the hydrogenation of the intermediates on the metal sites of the catalyst produces 1.2-PDO for acetol and 1,3-PDO for 3-hydroxypropionaldehyde. From the above analysis, it can be seen that both catalytic metal and acid sites have a significant influence in the production of propanediols from glycerol.8,9 Hydrogenolysis of glycerol involving different bifunctional metal-acid catalysts has been widely applied. Nakagawa et al.10 examined the catalytic performance for glycerol hydrogenolysis on modified Ir-ReOx/SiO2 catalysts with different solid acid cocatalysts, for example, ZrO2, TiO2, HZSM-5, Amberlyst 70, and so forth. They noted that the catalytic activity significantly increased with the addition of solid acid co-catalysts. Among them, HZSM-5 was the best solid acid co-catalyst in view of the catalyst stability and activity. Slightly different from the report of Nakagawa et al, Li et al.11 claimed that the modification of Ru/Al 2 O 3 and Ru/SiO 2 with HZSM-5 promoted the conversion of glycerol, while the selectivity of propanediols Received: April 20, 2017 Revised: October 24, 2017 Published: December 1, 2017 110
DOI: 10.1021/acssuschemeng.7b01233 ACS Sustainable Chem. Eng. 2018, 6, 110−118
Research Article
ACS Sustainable Chemistry & Engineering
Catalyst Preparation. Preparation of the Co−Al catalyst was based on the co-precipitation method as in the literature.17 Following the general procedure, an aqueous solution of aluminum nitrate nonahydrate (36.80 g) and cobaltous nitrate hexahydrate (4.94 g) was calculated and dispersed for 30 min at 50 °C with ultrasonic dispersion and stirred vigorously for 120 min at room temperature. Precipitation of the liquid mixture was done by adding a certain amount of concentrated solution of K2CO3 (0.2 g·mL−1) dropwise until a pH of 9.0 was obtained. The suspension was then stirred (800 rpm) with a mechanical stirrer continuously for 48 h at 25 °C. Then, the precipitate was filtered by a Büchner funnel and repeated washing done five times (1000 mL × 5) with deionized water. Finally, the sample was dried at 100 °C in an oven for 24 h and calcined at 400 °C in a muffle furnace for 4 h to achieve the product. The acid-modified Co−Al catalysts were prepared by impregnating uncalcined Co−Al powder (>120 mesh) after the drying procedure with a calculated amount of aqueous solution of H3BO3, H4[Si(W3O10)4]·xH2O, Ce(NO3)3·6H2O, H3[P(W3O10)4]·xH2O, ZrO(NO3)2·xH2O, or H3[P(Mo3O10)4]·xH2O. After impregnation, the water was removed by heating and stirring at 90 °C under vacuum using a rotary evaporator. Subsequently, the sample underwent drying and calcination at 100 °C in the oven for 24 h and at 400 °C in the muffle furnace for 4 h, respectively. The catalysts produced were ground with a mortar and then pressed by a tablet machine at 10 MPa to form granules with sizes of 20 to 40 meshes. In the prepared catalysts, the nominal weight loadings for Co and promoters (i.e., B, Ce, HSiW, HPW, HPMo or Zr) were fixed at 20 and 8 wt %, respectively, on the Al2O3 support. Catalyst Characterization. The actual loadings of the Co and promoters in the catalysts were examined by inductively coupled plasma optical emission spectroscopy (Optima 7300 DV, PerkinElmer). The physical properties of modified Co−Al catalysts such as BET specific surface areas and total pore volume were measured at −196 °C by means of a Beishide 3H-2000PS1 instrument. Degassing the catalyst at 200 °C for 12 h using a vacuum was done prior to the characterization. The surface morphologies of the modified Co−Al catalysts were observed with a Philips XL-30 ESEM. During the measurements, the operating voltage of the instrument was 15 kV. Meanwhile, the morphologies of the modified Co−Al catalysts were also examined by using a FEI Tecnai G2 transmission electron microscope coupled with an energy dispersive spectrometer (EDS). The operating voltage of the instrument was 200 kV in the measurements. Using ultrasonic dispersion, the analyzed catalyst was mixed with ethanol for 30 min, and then, drops of the solution were mounted on a holey copper grid coated with an amorphous carbon film. Furthermore, hydrogen chemisorption was conducted to measure the Co dispersion of the catalysts. The 100 mg of modified Co−Al catalysts was first heated at 400 °C for 60 min in a flow of He (30 mL·min−1) and then decreased to 30 °C. After the pretreatment, the modified Co−Al catalysts were in situ reduced at 400 °C for 120 min in flowing a 5% H2/N2 mixture gas at a flow velocity of 30 mL·min−1 and subsequently purified with flowing He for 30 min and decreased to 100 °C. Hydrogen chemisorption was carried out by injecting numbers of H2 pulses until saturation with H2 for the sample. The Co dispersion of the catalyst was calculated as in published literature.18,19 The phase composition of the modified Co−Al catalysts was verified by using a Bruker D8-Discover X-ray diffractometer with Cu Kα radiation. The operating voltage and electricity of the instrument were 40 kV and 30 mA, respectively. During the measurements, the scanning range was set from 10° to 90° at a scanning speed of 6°· min−1. The reducibility of modified Co−Al catalysts was studied by the H2-TPR method. After pretreating at 400 °C for 60 min in a flow of He, the TPR was conducted by raising the temperature from 50 to 780 °C at a heating rate of 10 °C·min−1 with a flow of 5% H2/N2 mixture gas. Measurement of the hydrogen consumption was done by means of the TCD. The NH3-TPD method was employed to analyze the acidity of the modified Co−Al catalysts. The 100 mg of modified Co− Al catalysts was first pretreated at 400 °C for 60 min in a flow of He and subsequently saturated at 100 °C for 30 min with pure NH3 stream. The modified Co−Al catalysts were purified at 100 °C for 60
decreased dramatically, which was attributed to the fact that the acidity of HZSM-5 enhanced the activity of Ru to break C−C bonds of glycerol. Zhu and co-workers12,13 reported that incorporation of the heteropolyacids (HSiW, HPW, HPMo) into a Pt/ZrO2 catalyst exhibited a significant promoting effect on the production of 1,3-PDO from glycerol, attributable to the high levels of Brønsted acid sites and exceptional thermal stability. In order to raise the catalytic activity and stability of a Cu/SiO2 catalyst, Zhu et al.14 prepared an admixture of B2O3 and Cu/SiO2 catalysts in different B/Cu weight ratios for glycerol hydrogenolysis. They found that addition of the B2O3 to Cu/SiO2 catalyst remarkably enhanced the acid strength and Cu metal dispersion, which led to the increased catalytic activity and stability. Likewise, Jiménez-Morales et al.15 in their study established that the Ce-modified Ni/SBA-15 catalysts possessed very high catalytic activity during the conversion of glycerol attributing this result to the strong acid sites and high Ni metal dispersion on the catalyst. However, the hydrogenolysis of glycerol involving acid-modified Co−Al catalysts has not been published in literature. Based on the literature, generally, a cobalt catalyst exhibited relatively poor catalytic activity for glycerol hydrogenolysis as compared to some non-noble metal catalysts such as copper and nickel catalysts.14−17 According to the study of Guo et al.,16 a MgO-supported cobalt catalyst for 1,2-PDO production from glycerol only exhibited 44.8% glycerol conversion and 42.2% selectivity of 1,2-PDO using an autoclave reactor at 200 °C and 2.0 MPa initial hydrogen pressure. Rekha et al.17 systematically investigated the activity of glycerol conversion on Co−ZnO catalysts with different mass ratios of Co and Zn at 180 °C and 4.0 MPa hydrogen pressure with a high catalyst loading in a batch reactor. Among the catalysts examined, a mass ratio of Co and Zn of 1:1 possessed the best catalytic activity giving rise to 70% of glycerol conversion and 80% of 2-PDO selectivity, but large quantities of byproducts such as ethylene glycol were observed. Several researches involving the dehydration−hydrogenation mechanism for propanediols production from glycerol has shown that modifying the catalyst by adding a promoter with appropriate acidity significantly enhanced propanediols selectivity and also glycerol conversion.8,9 Therefore, in this work, different acid species such as B, Ce, Zr, HSiW, HPW, and HPMo were introduced into the Co−Al catalyst to investigate their catalytic activities focusing more on the structure−activity relationship of modified Co−Al catalysts for glycerol hydrogenolysis. The influences of reaction variables, for example, solvent, reaction temperature, and feeding rate, on glycerol hydrogenolysis were examined. Finally, the catalyst stability and reaction pathway for propanediols production from glycerol were also studied.
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EXPERIMENTAL SECTION
Chemicals. The purities of 1,2-PDO, 1,4-butanediol, Ce(NO3)3· 6H2O, 1,3-PDO, ZrO(NO3)2·xH2O, and acetol supplied from Aladdin Industrial Corporation were 99.5, 99.5, 99.0, 99.5, 99.5, and 90.0 wt %, respectively. Other chemicals such as glycerol, cobaltous nitrate hexahydrate, ethylene glycol, H3BO3, methanol, aluminum nitrate nonahydrate, ethanol, H4[Si(W3O10)4]·xH2O, 1-propanol, H3[P(W3O10)4]·xH2O, 2-propanol, potassium carbonate, and H3[P(Mo3O10)4]·xH2O, all of analytical grade, were acquired from Sinopharm Chemical Reagent Co., Ltd. The aforementioned chemicals were employed for the experiments without undergoing further purification. In addition, deionized water self-made in the lab was used throughout the experiment. 111
DOI: 10.1021/acssuschemeng.7b01233 ACS Sustainable Chem. Eng. 2018, 6, 110−118
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ACS Sustainable Chemistry & Engineering min with a flow of He and then increased up to 750 °C at a heating rate of 10 °C·min−1. The signal detection of the NH3 desorption was done using the TCD. The surface chemical composition of the modified Co−Al catalysts was probed by an X-ray photoelectron spectrometer with a Mg Kα radiation source. The obtained binding energies of all elements were calibrated by the C 1s binding energy of 284.6 eV. Fourier transform infrared spectra of adsorbed pyridine (Py-IR) on the modified Co−Al catalysts were analyzed by a Nicolet 5700 FTIR spectrometer. The 30 mg of modified Co−Al catalysts was first heated at 300 °C for 60 min in a vacuum and subsequently decreased to 30 °C. Then, a FTIR spectrum was determined and expressed as spectrum I. After adsorption of pyridine for 30 min, the modified Co−Al catalysts were purified at 150 °C for 30 min, and then, spectrum II was measured. The final spectrum of the modified Co−Al catalysts was achieved by subtracting spectrum I from spectrum II. Glycerol Hydrogenolysis Experiments. In this study, the experiments were conducted in a vertical fixed-bed flow reactor with inner diameter of 11 mm and length of 950 mm as shown in Figure S1. Following the general procedure, 4.0 g of modified Co−Al catalysts with particles sizes from 20 to 40 meshes was fixed in the middle region of the reaction tube, which was filled with quartz sand (20−40 mesh) at both ends. Prior to the experiment, the modified Co−Al catalysts were reduced in situ in the reactor at 400 °C for 120 min with a pure H2 flow at a flow velocity of 100 mL·min−1. After the reduction process, the temperature of the reactor was decreased to 230 °C, and then, the system pressure was slowly increased to 3.5 MPa with a back pressure regulator. Subsequently, a mixture of 20 wt % glycerol in ethanol preheated to 230 °C was pumped into the reaction system at a flow velocity of 27.8 mL·h−1 by means of a metering pump combined with pure hydrogen (100 mL·min−1), which corresponded to WHSV of 1.21 h−1. After certain time of running, the reaction products were well cooled by ethylene glycol in a condenser and then stored in a gas− liquid separator. At regular intervals of time (30 min), the liquid samples in the gas−liquid separator were taken and analyzed by a gas chromatography (GC). The GC (Shimadzu GC-2014) was equipped with a FID, and the chromatographic column was Rtx-WAX (30 m × 0.25 mm). The GC analysis was done using 1,4-butanediol as an internal standard substance and also incorporated into the liquid sample. In addition, the liquid samples in the gas−liquid separator were also analyzed by means of GC-MS (Shimadzu GCMS-QP2010). The glycerol conversion, product selectivity, and yield were calculated by means of the following equations:
displayed in Table S1, the actual loadings of Co and promoters measured by ICP-OES were very close to the nominal values, although there were small differences, most probably due to the metals loss in the preparation process. The specific surface areas and pore volumes of the acid-modified Co−Al catalysts were slightly lower than those of the Co−Al catalyst, which was likely attributable to the blockage of some pores by the promoters. As shown in Figure S3, all the acid-modified Co−Al catalysts had blocky texture with few particles deposited on the surface. It was also clear to see that more small particles existed in the Ce/Co−Al, HSiW/Co−Al, and Zr/Co−Al catalysts than in the Co−Al catalyst, indicating their superior structure. Together with the TEM test, the EDS analysis was performed to estimate the chemical compositions of the catalysts. As shown in Figure S4, the results of the EDS measurement indicated that the heteropolyacids (HSiW, HPW, HPMo) were present in the modified Co−Al catalysts and had a good stability. In addition, there were certain amounts of Cu and K in the catalysts, which were derived from a copper grid used for the TEM test and the residual precipitant (i.e., potassium carbonate), respectively. The TEM bright field images and dark field images of the catalysts are displayed in Figures S5 and S6, respectively. In Figure S5, the Co particles of the catalysts were seen to distribute evenly on the support surface, while the TEM dark field images (Figure S6) showed that the Co particles of acidmodified Co−Al catalysts were more dispersed than those of the Co−Al catalyst. In order to clarify this point further, the Co dispersions of the acid-modified Co−Al catalysts measured by hydrogen chemisorption are given in Table S1. It is clear from these data in Table S1 that the addition of acid modifiers increased the Co dispersion of the catalysts. One possible explanation for this was that the introduction of promoters helped to restrain the aggregation of Co particles. Among the modified Co−Al catalysts prepared, the HSiW-modified Co−Al catalyst possessed the highest Co dispersion, which might be attributed to the existence of the strong interaction between the Co and W species (see below). As indicated in Figure S7, it is apparent that the presence of Co3O4 (PDF#42-1467)17 gave rise to diffraction peaks for calcined Co−Al catalyst at 36.8° and 44.8°. Also, in all the acidmodified Co−Al catalysts, the Co3O4 phase was noted. However, no obvious characteristic peaks corresponding to the promoters were observed in acid-modified Co−Al catalysts except for Ce/Co−Al catalyst. After the reduction process, no noticeable diffraction peak of CeO2 (PDF#43-1002, 2θ = 28.5°)20 in the Ce/Co−Al catalyst was observed. However, the Co3O4 phase was still evident in all the acid-modified Co−Al. It was worthy to mention that the patterns of both Co (PDF#150806, 2θ = 44.2°)17 and Co3O4 phases were not evidently distinguished by XRD analysis. But for the Co3O4 phase, it was clear that the strength and width of diffraction peak were, respectively, weakened and broadened after the reduction process, indicating that some Co3O4 species in the catalysts were reduced to CoO and/or Co.21 Furthermore, the peak intensity at 44.2° for the acid-modified Co−Al catalysts was weaker than that of the Co−Al catalyst, suggesting the presence of higher dispersive Co species in the acid-modified Co−Al catalysts, in line with the measurement results of H 2 chemisorption. The reducibility of the acid-modified Co−Al catalyst was examined by using the H2-TPR method. As displayed in Figure S8, all the acid-modified Co−Al catalysts displayed two clear hydrogen consumption peaks α and β in the order of 150−500
Conversion (%) moles of glycerol (in) − moles of glycerol (out) = × 100 moles of glycerol (in) (1) Selectivity (%) =
moles of carbon in a specific product × 100 moles of carbon in glycerol consumed (2)
Yield (%) =
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Conversion (%) × Selectivity (%) 100
(3)
RESULTS AND DISCUSSION Catalyst Characterization. As depicted in Figure S2a, it was evident that the N2 adsorption−desorption isotherm of the Co−Al catalyst corresponded to the type IV isotherm and showed an obvious hysteresis loop of H1 type in the range of P/P0 = 0.5−0.95. The isotherm shapes of the catalysts remained unchanged after the introduction of promoters, but the pore diameter distribution for the acid-modified Co−Al catalysts decreased (Figure S2b), indicating that the promoters were small enough to enter the channels of the Co−Al catalyst. As 112
DOI: 10.1021/acssuschemeng.7b01233 ACS Sustainable Chem. Eng. 2018, 6, 110−118
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Table 1. Activity Results for Experiment Performed on Acid-Modified Co−Al Catalysts When Using Water as a Solventa Selectivity (%) Catalyst
Conversion (%)
1,2-PDO
1,3-PDO
1-Propanol
Ethylene glycol
Others
Co−Al Co−Alb B/Co−Al Ce/Co−Al HSiW/Co−Al HSiW/Co−Alb HPW/Co−Al HPMo/Co−Al Zr/Co−Al
48.6 2.0 61.9 68.3 76.3 2.8 65.8 57.4 72.5
77.2 68.9 81.2 79.5 60.2 64.6 65.1 74.3 85.0
0.6 0.2 1.1 1.3 18.3 3.7 15.1 3.4 1.7
0.4 0.4 0.9 0.8 9.5 2.1 7.8 2.9 1.0
11.9 15.7 6.4 7.6 4.2 10.0 4.8 6.9 5.1
9.9 14.8 10.4 10.8 7.8 19.6 7.2 12.5 7.2
a Process parameters: solvent; water; glycerol concentration, 10 wt %; feed rate of glycerol−water mixture, 9.7 mL·h−1; flow velocity of pure hydrogen, 100 mL·min−1; mass of catalyst, 4.0 g; temperature of preheater, 230 °C; temperature of reactor, 230 °C; pressure of reactor, 3.5 MPa; data collection after 180 min running. Others include 2-propanol, acetol, methanol, etc. bWithout the reduction process.
and 600−780 °C, respectively. According to the literature, the peak α in the range of 150−500 °C is considered to be the reduction of Co3O4 to CoO, whereas the peak β in the range of 600−780 °C describes the reduction of CoO to Co.21,22 In comparison to the Co−Al catalyst, the strength of the low temperature peak α for the acid-modified Co−Al catalysts decreased, whereas that of the high temperature peak β increased, despite almost the same Co content in the acidmodified Co−Al catalysts. This indicated that more CoO and/ or Co species were formed because the promoters were introduced. Table S2 gives the amounts of consumed hydrogen during TPR measurement. It should be noted that the amount of consumed hydrogen of B/Co−Al catalyst was relatively low. In Figure S8, the reduction peaks of B/Co−Al catalyst slightly shifted to higher temperature, showing the existence of a strong interaction between B and Co species, which might have led to the significant decrease in the amount of consumed hydrogen.23 In contrast, a high amount of consumed hydrogen at high temperature (>600 °C) was seen on the HPMo/Co−Al catalyst, which might be due to the fact that molybdena could be partially reduced at high temperature.24,25 This notwithstanding, the results of H2-TPR showed that only parts of the Co3O4 species were reduced to CoO and/or Co0 in this study, which was probably attributed to the existence of strong interaction between Co3O4 species and promoters and/or support.26 Figure S9 presents the results of NH3-TPD analysis. For the Co−Al catalyst, it was clear that a weak NH3 desorption peak existed in the range of 420−750 °C, reflecting the presence of some strong acid sites.27 When different promoters were introduced into Co−Al catalyst, evidently, NH3 desorption peaks appeared in the range of 150−370 °C, and also the strength of peaks in the range of 420−750 °C notably increased. Table S1 shows an analysis of the comparison of the acidities of the catalysts. It is evident that the addition of promoters into the Co−Al catalyst enhanced both the acid quantity and strength. Among the prepared catalysts, the HSiW/Co−Al catalyst possessed the highest total acidity. Regarding the XPS measurement, Co 2p XPS profiles of the acid-modified Co−Al catalysts are illustrated in Figure S10. In Figure S10a, only two peaks centered at approximately 780 and 795 eV, respectively, assigned to Co 2p3/2 and Co 2p1/2 were evident in the calcined catalysts. This result suggested that the Co species in the modified Co−Al catalysts were principally in a trivalent state.17 In Figure S10b, the binding energy (B.E.) values of Co 2p3/2 and Co 2p1/2 for all modified Co−Al
catalysts were increased to about 780.4 and 796.2 eV, respectively. Furthermore, two quite distinct satellite peaks at approximately 786.3 and 803.2 eV were noted. This could mean the presence of Co2+ and/or Co species in the reduced catalysts.21 Figure S10b again showed a slight shift to lower B.E. triggered by the Co 2 peak of the acid-modified Co−Al catalysts. This phenomenon could be attributed to the existence of a strong interaction between the promoters and cobalt species.14 According to existing literatures,17,28,29 the peak of Co 2p3/2 for the reduced catalysts can be deconvoluted into three components, i.e., Co3+, Co2+, and Co0 species, and the results are displayed in Figure S10c. As illustrated, the cobalt species in the reduced-modified Co−Al catalysts were mainly presented in a state of Co2+. Meanwhile, the Co2+ species in the acid-modified Co−Al catalysts can be partially reduced to Co0. To identify the chemical valence of promoters on the acidmodified Co−Al catalysts, Figure S11 shows the B 1s, Ce 3d, W 4f, Zr 3d, and Mo 3d XPS profiles of the reduced-modified Co−Al catalysts. It was found that the B 1s peak centered at 192.3 eV was associated with the presence of the B3+ state.30 The peaks of Ce 3d5/2 and Ce 3d3/2 for the reduced Ce/Co−Al catalyst suggested that the cerium species were primarily in the state of Ce4+.15 The two peaks at about 35.1 and 37.2 eV for the reduced HSiW/Co−Al and HPW/Co−Al catalysts were, respectively, assigned to the W 4f7/2 and W 4f5/2 peaks of W6+.13 The binding energies of Mo 3d (232.4 and 235.5 eV, respectively, corresponding to Mo 3d5/2 and Mo 3d3/2) and Zr 3d (182.3 and 184.6 eV, respectively, corresponding to Zr 3d5/2 and Zr 3d3/2) were in accordance with the characteristic values of MoO3 and ZrO2, respectively.31,32
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GLYCEROL HYDROGENOLYSIS STUDY Catalytic Activity Test. Table S3 gives the activity results for the experiment conducted on acid-modified Co−Al catalysts at 230 °C of reaction temperature and 3.5 MPa of hydrogen pressure in using ethanol as a solvent. As illustrated, the conversion of glycerol and selectivity of 1,2-PDO, respectively, were 70.2% and 60.5% when employing Co−Al as a catalyst. Meanwhile, there were still large numbers of byproducts such as glyceryl ethers and ethylene glycol in the liquid phase. When different promoters were added to the Co−Al catalyst, the conversion of glycerol notably increased, but the selectivity of 1,2-PDO slightly decreased. High concentrations of glyceryl ethers were detected (Table S4). This catalytic activity result indicated that the addition of the acid modifiers raised the etherification activity of glycerol with ethanol. It is well 113
DOI: 10.1021/acssuschemeng.7b01233 ACS Sustainable Chem. Eng. 2018, 6, 110−118
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intensity band of Lewis acid sites (expressed as L) at ca. 1440 cm−1. In contrast, an obvious characteristic band corresponding to Brønsted acid sites at ca. 1540 cm−1 was observed on the Co−Al catalysts modified with HSiW and HPW, suggesting the presence of some Brønsted acid sites, which may have been derived from well-dispersed W species on the catalysts surface.13 In this study, the Co−Al catalysts modified with HSiW and HPW exhibited higher selectivity to 1,3-PDO, most probably attributed to the existence of Brønsted acid sites on the catalysts surface. This is in concordance with the literature report.38,39 Because of its higher 1,3-PDO yield, the HSiW/ Co−Al catalyst was selected for further investigation. Influence of Solvent. At present, different alcohols, e.g., methanol or ethanol, are often used as the solvents for the hydrogenolysis of glycerol except water. In general, hydrogen can dissolve more in alcohols than in water, which favors the increased glycerol conversion.40 On the other hand, H2 can be generated by the dehydrogenation of alcohols under suitable conditions.41 Therefore, the catalytic hydrogen transformation from the hydrogen donors such as methanol and 2-propanol to glycerol was also suitable for the conversion of glycerol.42 Moreover, several researchers suggested that the use of some organic reagents like sulfolane as solvents was helpful for the generation of 1,3-PDO from glycerol hydrogenolysis.43 By way of comparison, methanol and sulfolane were used to determine the influence of solvent on glycerol hydrogenolysis over HSiW/ Co−Al catalyst. It is shown in Figure 1 that the use of alcohols,
documented that the etherification activity of glycerol with ethanol can be improved in the presence of appropriate acid catalysts.33−35 In this study, the introduction of promoters into the Co−Al catalyst significantly enhanced the acidities on the catalyst surface, as confirmed by NH3-TPD, which might have led to the increase in etherification activity. To minimize the effect of the glycerol etherification, the catalytic activity tests were also carried out at 230 °C of reaction temperature, 3.5 MPa of hydrogen pressure, and 10 wt % glycerol aqueous solution with 9.7 mL·h−1 liquid flow rate. The results are tabulated in Table 1. Furthermore, in order to identify the active sites for the production of propanediols from glycerol, Table 1 shows the activity results of glycerol hydrogenolysis on unreduced Co−Al and HSiW/Co−Al catalysts in using water as a solvent. It can be observed from Table 1 that the catalytic activities of unreduced Co−Al and HSiW/Co−Al catalysts were very low. In contrast, the catalysts activity after reduction notably increased, suggesting that the reduced cobalt species were effective for glycerol hydrogenolysis.21 On the other hand, it was found that no glyceryl ethers existed in glycerol hydrogenolysis conducted on acidmodified Co−Al catalysts in an aqueous solution. For a Co−Al catalyst, the conversion of glycerol and selectivity of 1,2-PDO, respectively, were 48.6% and 77.2%. When the promoters were introduced into the Co−Al catalyst, glycerol conversion was enhanced to some extent, but the selectivity to ethylene glycol slightly decreased. This could be attributed to the excellent properties in the acid-modified Co−Al catalysts, such as large amounts of acid sites and more highly dispersed Co species, as verified by NH3-TPD and H2 chemisorption, respectively. There are two cases concerning the selectivity of 1,2-PDO for glycerol hydrogenolysis conducted on acid-modified Co−Al catalysts. The modification of Co−Al with B, Ce, or Zr species raised the selectivity of 1,2-PDO. In contrast, relatively lower concentration of 1,2-PDO was achieved on heteropolyacidsmodified Co−Al catalysts. Nevertheless, addition of the heteropolyacids raised the selectivity of 1,3-PDO, which was barely detected for glycerol hydrogenolysis conducted on Co− Al catalysts modified with B, Ce, or Zr species. Among the employed catalysts, the HSiW/Co−Al catalyst exhibited the highest 1,3-PDO yield, attaining 76.3% of glycerol conversion with 18.3% of 1,3-PDO selectivity. Documented evidence indicates that the production of 1,2and 1,3-PDO from glycerol catalyzed by solid acid catalysts involves the dehydration of glycerol to obtain intermediates and their hydrogenation processes.36,37 According to the related literature, the type of acid sites on the catalyst surface has much influence on the distribution and yield of hydrogenolysis product. 3-Hydroxypropionaldehyde, an intermediate of glycerol hydrogenolysis, can be generated by the coordination of Brønsted acid sites on the catalyst surface to the secondary hydroxyl group of glycerol, and further hydrogenation obtains 1,3-PDO, whereas Lewis acid sites on the catalyst surface preferentially produces 1,2-PDO by acetol hydrogenation pathway.38,39 To identify the nature of acid sites on the acidmodified Co−Al catalysts, in situ Py-IR spectra measurements were carried out. As displayed in Figure S12, the bands centered at about 1440 and 1540 cm−1 were considered to be pyridine ions formed by interaction with Lewis acid sites and Brønsted acid sites on the catalytic surface, respectively.39 As clearly shown in Figure S12, almost no Brønsted acid sites (expressed as B) existed on the Co−Al catalysts modified with B, Ce, or Zr species, though these catalysts showed the high
Figure 1. Activity results for the experiment performed on HSiW/ Co−Al catalyst in the presence of different solvents. Process parameters: glycerol concentration, 10 wt %; feed rate of liquid mixture, 9.7 mL·h−1; flow velocity of pure hydrogen, 100 mL·min−1; mass of catalyst, 4.0 g; temperature of preheater, 230 °C; temperature of reactor, 230 °C; pressure of reactor, 3.5 MPa; data collection after 180 min running.
i.e., methanol or ethanol, as solvents can obtain very high glycerol conversion (100%), which may be attributed to the presence of high concentration of hydrogen in the alcoholic solutions.44 However, there were high amounts of byproducts such as glyceryl ethers, and the selectivity of 1,3-PDO was very low (
