Research Article pubs.acs.org/journal/ascecg
Kinetic Modeling of Sorbitol Hydrogenolysis over Bimetallic RuRe/C Catalyst Xin Jin,†,§ Prem S. Thapa,‡ Bala Subramaniam,† and Raghunath V. Chaudhari*,† †
Center for Environmentally Beneficial Catalysis, Department of Chemical & Petroleum Engineering, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States ‡ Microscopy and Analytical Imaging Laboratory, Haworth Hall, University of Kansas, 1200 Sunnyside Avenue, Lawrence, Kansas 66045, United States S Supporting Information *
ABSTRACT: Sorbitol hydrogenolysis kinetics using bimetallic RuRe catalyst is reported based on multiple experiments in parallel batch slurry reactors (H2 pressure: 1.0−6.5 MPa, temperature: 473−503 K) to obtain concentration−time profiles. It is observed that RuRe/C bimetallic catalysts with Ca(OH)2 as a base promoter show significantly higher activity and selectivity toward liquid phase products such as 1,2-propanediol, lactic acid, ethylene glycol, and linear alcohols compared with monometallic Ru/C catalysts and other base promoters. It is further found that sorbitol hydrogenolysis initiates with dehydrogenation and subsequent C−C cleavage via retro-aldolization to form smaller molecules (C2−C4). Those smaller intermediates undergo dehydration, reorganization, and C−O cleavage to form C2−C3 acids, glycols, and linear alcohols as products, which are very similar to glycerol conversion chemistry. For the kinetic modeling, experimental data on concentration−time profiles were obtained using RuRe/C catalysts with Ca(OH)2 promoter in which H2 pressure, catalyst loading, and temperature were varied. The analysis of kinetic models employed a batch slurry reactor model with which several rate equations based on different complex multistep reaction mechanisms were fit to the experimental data in order to gain insights into the reaction pathways and mechanisms. Activation energies for sorbitol hydrogenolysis to glycols and further conversion of glycols to corresponding alcohols are found to be in the range 38 kJ/mol to 125+ kJ/mol. The kinetic model from this work provides the framework for developing rational multiphase reactor engineering strategies for upgrading polyol mixtures (e.g., glycerol, xylitol, sorbitol, and mannitol) to value-added glycols and alcohols. KEYWORDS: Kinetic modeling, Polyol hydrogenolysis, Bimetallic catalysts, Reaction pathways
■
INTRODUCTION Cellulosic biomass is an abundantly available renewable material that is considered to be an important alternate feedstock for the sustainable production of fuels and chemicals amidst the changing feedstock landscape.1 Cellulose can be converted to sugar polyols (e.g., sorbitol, xylitol, etc) via hydrolysis-hydrogenation routes.2 Currently, sorbitol and xylitol are utilized as additives in the food industry. However, they can also be converted to fuels and high value chemicals, including furans, isosorbide,3 hydrogen,2 1,2-propanediol (PDO),4 ethylene glycol (EG),5 1,6-hexanediol, and lactic acid (LA), which are currently produced from petroleum-based routes. Hydrogenolysis of sugar polyols by metal catalysts to PDO and EG involves a complex reaction network following catalytic C−C and C−O bond cleavage, which also represents a class of useful reactions in converting several renewable feedstocks to lower oxygenated commodity chemicals. A major challenge in this regard is the understanding of the underlying reaction mechanism and kinetics of the hydrogenolysis reactions as well as the C−C and C−O bond cleavage reactions in order to maximize the selectivity toward the liquid products.6−8 © XXXX American Chemical Society
In a previous report, Sohounloue and co-workers studied supported Ru catalysts for sorbitol hydrogenolysis and proposed retro-aldolization as a key step.9 However, experimental work by Montassier et al. debated that hydrogenolysis follows C−C and C−O bond breakage by a retro-Michael reaction on Ru catalysts.10 Zhao and co-workers reported sorbitol hydrogenolysis over a Ru/carbon nanofiber catalyst with 68% conversion and 53% combined selectivity to glycerol (Gly), PDO, and EG.11 They found that the presence of base promoters (NaOH, CaO) facilitates selective retro-aldolization of sorbitol rather than retro-Michael reaction; thus, C2 and C3 products (e.g., Gly, PDO, and EG) selectivity increases with increasing base promoters concentration.12 With regard to the effects of cations on reaction pathways, Banu and Venuvanalingam4 found that the presence of Ca2+ and Na+ in Ni/NaY Special Issue: Building on 25 Years of Green Chemistry and Engineering for a Sustainable Future Received: June 15, 2016 Revised: September 26, 2016
A
DOI: 10.1021/acssuschemeng.6b01346 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
ACS Sustainable Chemistry & Engineering
■
catalysts enhanced the selectivity (S = 81%) to C3 products (Gly and PDO). Ni−Ce/Al2O3 catalysts prepared by a coprecipitation method exhibit an excellent stability after 18 runs during sorbitol hydrogenolysis.13 Characterization of fresh and spent catalysts reveals that Ce4+ improved the dispersion of Ni particles, resulting in enhanced activity and stability of Ni catalysts. Cu catalysts were also tested for sorbitol hydrogenolysis under similar conditions (T = 453−503 K, PH2 = 7− 13 MPa), but they exhibited low C−C cleavage activity compared with Ni and Ru catalysts.14,15 Sun and co-workers studied xylitol conversion and found that retro-aldolization leads to the formation of smaller molecules such as PDO, LA, and EG in the presence of a CaO promoter.16 We recently reported a series of solid base supported Cu catalysts for sorbitol hydrogenolysis to PDO (S = 46%) and EG (S = 15%), but the overall activity of Cu catalysts is very low.17 According to previous reports, the activity for C−O and C− C during sorbitol and xylitol conversion follows the following order: Ru > Pt > Ni > Cu. Ru is found to exhibit superior activity in hydrogenolysis reactions, which is also known to promote unwanted side reactions, such as methanation.18 The addition of a second metal, such as Pt and Re, was previously found to enhance the C−O cleavage of polyols and restrain side reactions such as water gas shift (WGS) and methanation reactions.19,20 However, to the best of our knowledge, no published experimental data are available to understand the role of Re in the rational control of C−C cleavage during sorbitol and xylitol hydrogenolysis. Although glycerol (C3) hydrogenolysis to PDO, EG, and LA in the presence of various metal catalysts has been studied previously, the kinetics and reaction pathways of hydrogenolysis of sugar polyols (e.g., sorbitol, xylitol) are less studied.21 Quite different from glycerol conversion, hydrogenolysis of sorbitol involves simultaneous consecutive/parallel reactions resulting in the formation of linear alcohols and other carboxylic acids besides PDO and EG as the major products. Clearly, the following two fundamental challenges need to be addressed: (1) Detailed description of possible reaction pathways of sorbitol hydrogenolysis involving C−C and C−O cleavage. (2) Quantitative assessment of several possible reactions on a solid catalyst surface which leads to C−C cleavage of sorbitol to smaller molecules. The development of appropriate kinetic models is essential to unambiguously address these challenges and provide the essential insights for rational catalyst design and process optimization for upgrading a biopolyol mixture to value-added products. In our recent report, we have shown that bimetallic RuRe catalysts (conversion = 84%, selectivity to glycols = 52%) show remarkable performance for sorbitol hydrogenolysis compared with monometallic Ru catalyst (conversion = 61%, selectivity to glycols = 21%).22 Therefore, it was the objective of this work to investigate the intrinsic kinetics of sorbitol hydrogenolysis in the presence of RuRe catalyst. In particular, the effects of parameters such as reaction temperature (453−503 K), hydrogen pressure (P H2 = 7−13 MPa), and catalyst concentration (3.3−13.4 kg/m3) on the concentration−time profiles were investigated. Based on the experimental concentration−time profiles, kinetic models and rate parameters were developed to facilitate green reactor engineering; i.e., the rational design and scale-up of multiphase catalytic reactors for maximizing the yield of the renewable liquid chemical products.
Research Article
EXPERIMENTAL SECTION
Sorbitol (98%), activated carbon, RuCl3·xH2O, HReO4 (65−70 wt %), Ca(OH)2, NaOH, MgO, and Ba(OH)2 were purchased from Sigma. Ammonia hydroxide (NH3·H2O) was purchased from Fisher. Hydrogen (>99.5%, Air Gas) and nitrogen (>99%, Linweld) gases were used as purchased without further treatment. PTFE materials were purchased from McMaster to fabricate the reactor inserts used in hydrogenolysis experiments. Catalyst Synthesis. Monometallic Ru/C and bimetallic RuRe/C catalysts were prepared via a coprecipitation method.18 Specifically, the calculated amount of activated carbon was first charged into a 1000 cm3 three neck flask before introducing 700 cm3 of deionized (DI) water. The slurry was stirred at 1000 rpm at 363 K for 2 h. Then required amounts of RuCl3·xH2O and HReO4 were dissolved in approximately 50 cm3 of DI water added dropwise to the stirred slurry. After stirring for another 2 h, a dilute aqueous solution of NH3·H2O was added dropwise to the suspension until the solution reached pH ∼ 10. The slurry was then stirred for 3 h before it was filtered, washed with 500 cm3 of hot DI water, and dried in an oven overnight (T = 363 K). The catalyst sample was then activated in a tube furnace under 573 K in the presence of hydrogen gas flow. In particular, the tube was first purged with nitrogen for 0.5 h at room temperature. The tube was heated at a rate of 5 K/min until 423 K, at which point the gas flow was switched to hydrogen and the temperature was maintained at 423 K for 0.5 h. Finally, the tube was heated at a rate of 5 K/min until 573 K and maintained for 5 h. After the activation process was completed, the tube furnace was cooled down naturally and the gas was switched back to nitrogen at 423 K. The catalyst sample was taken out at room temperature and stored under dry conditions for hydrogenolysis experiments. The monometallic Ru/C and bimetallic RuRe/C catalysts were characterized by inductively coupled plasma (ICP) and transmission electron microscopy (TEM) for metal content and surface morphology. The procedure is briefly described as follows: 0.1 g of solid catalysts was digested using 2.0 g of hydrofluoric acid (47−51%, Fisher), 1 g of sulfuric acid (98%, Sigma), and 7.0 g of H2O. The slurry was sealed in a steel autoclave and kept in a drying oven at 393 K for 10 h. The resultant sample was then diluted further and stored for 2 days before ICP analysis. The ICP measurement was conducted in JY2000 (HORIBA, Jobin Yvon Inc., flow rates of plasma gas: 12 L/min, aux: 0 L/min and gainage: 0.2 L/min). The actual compositions of Ru(1 wt %)/C and Ru(1 wt %)Re(1 wt %)/C catalysts were Ru(1.08 wt %)/C and Ru(0.99 wt %)Re(0.91 wt %)/C. In this manuscript, mono- and bimetallic Ru-based catalysts are denoted as Ru/C and RuRe/C for the sake of conciseness. The measured value of the metal content (ppm) in the solution was divided by a dilution ratio to calculate the actual metal content in carbon supports. Transmission Electron Microscopy (TEM). Samples were prepared by suspending the catalyst in ethanol and agitating in an ultrasonic bath for 15 min. Ten microliters of catalyst sample was placed onto a copper mesh grid with a lacey carbon film. The wet grids were allowed to air-dry for several minutes prior to being examined under TEM. The catalyst particle size and morphology were examined by bright-field and dark-field transmission electron microscopy (TEM) using an FEI Technai G2 transmission electron microscope at an electron acceleration voltage of 200 kV. High resolution images were captured using a standardized, normative electron dose and a constant defocus value from the carbon-coated surfaces. Energy dispersive X-ray spectroscopy (EDS) was carried out using an EDAX detector. Concentration−Time Profiles. The hydrogenolysis experiments were carried out in a high pressure, high temperature multiphase reactor system supplied by Parr Instrument.23 Each reactor is equipped with a thermowell, a pressure transducer, a gas inlet−outlet, and a rupture disk, and it can be operated under different pressures and temperatures. A magnetic stirrer with controlled agitation speed provides mixing in each reactor. The pressure and temperature in each individual reactor are controlled and monitored by a computer-aided user interface. The parameters of the reactor system, including B
DOI: 10.1021/acssuschemeng.6b01346 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
propane, and butane, and CO2,25,26 and the liquid products ranging from C1−C6, including sorbitol (C6), butanediol (C4), Gly (C3), PDO (C3), LA (C3), 2-propanol (2-PrOH, C3), EG (C2), ethanol (EtOH, C2), glycolic acid (Glo-acid, C2), and methanol (MeOH, C1), were quantitatively analyzed. Concentration−time profiles were obtained from experiments carried out at different batch times, from which conversion and selectivity profiles were calculated. These data were further used for the development of kinetic models and evaluation of rate parameters. Conversion, selectivity, and carbon balance are defined as follows:
pressure, temperature, and agitation rate, can be recorded every 10 s through SpecView data acquisition software. The ranges of various experimental parameters are summarized in Table 1. In a typical run of a hydrogenolysis experiment, a known
Table 1. Experimental Parameters in the Kinetic Study of Sorbitol Hydrogenolysis Sorbitol concentration (kmol/m3) Catalyst concentration (kg/m3) T (K) Hydrogen pressure (MPa) Solvent Volume of initial liquid mixture (m3) Reaction time (h)
0.272 3.33−13.3 473−503 2.0−6.5 Water 15 × 10−6 0.25−6.0
Conversion =
Selectivity =
amount of sorbitol (powders) was first charged into a calculated amount of DI water and mixed completely at room temperature. Then, a measured amount of catalysts (e.g., Ru/C and RuRe/C) together with a certain amount of a base promoters (e.g., MgO or NaOH or Ca(OH)2) was introduced to the PTFE lined reactor. The as-prepared sorbitol aqueous solution was introduced into the reactor and premixed with solid powders at 200 rpm for 1 min at room temperature. The reactor was sealed and loaded into one of the reactor wells in the multireactor system. Subsequently, the reactors were purged with nitrogen thrice at 2.0 MPa. The reactor was then heated to the desired temperature, during which the rotation rate of the magnetic stirrer was maintained at 50 rpm to achieve thermal equilibrium but ensure that negligible reaction occurs. Hydrogen was charged to a predetermined pressure at the desired reaction temperature, and the agitation speed was then increased to 800 rpm to eliminate external mass transfer (gas−liquid and liquid−solid) limitations during hydrogenolysis experiments. The experiments were carried out at constant pressure by replenishing the hydrogen from an external hydrogen reservoir. After a specific batch time, the reactor was cooled down to room temperature and depressurized by releasing the gas phase products to an offline gas chromatography (GC) system, after which the reactor was opened and the liquid mixture was analyzed using high performance liquid chromatography (HPLC). The details of the analysis procedures were described previously.17,24 The gaseous products including C1−C4 gas alkanes, including methane, ethane,
final initial Cmoles , sorbitol − Cmoles , sorbitol initial Cmoles , sorbitol final Cmoles , products final initial Cmoles , sorbitol − Cmoles , sorbitol
Carbon (%) =
TOF =
(1)
(2)
final Cmoles , sorbitol + products initial Cmoles , sorbitol
(3)
final initial Nmoles , sorbitol − Nmoles , sorbitol
Nmoles , surfaceRu·time
(4)
The carbon % above accounts for all the carbon molecules in the final gas and liquid mixture. TOF is defined as the mole of sorbitol converted per surface Ru atom (determined based on TEM characterization) based on differential conversion during a certain period of reaction time.
■
RESULTS AND DISCUSSION Catalyst Characterization. Characterization data of Ru/C and RuRe/C are shown in Figure 1. It is found that monometallic Ru particles display a size distribution of 12.4 ± 7.9 nm on carbon support as seen in Figures 1(a)−(b). In sharp contrast, bimetallic RuRe particles show much smaller size and narrow distribution (in the range of 5.4 ± 1.2 nm). Surface mapping of Ru and Re elements [Figure 1(f)] further confirms the existence of bimetallic nanoparticles on the surface of the carbon support. It is clear that Ru shows better
Figure 1. TEM data of monometallic Ru/C [(a) and (b)] and bimetallic RuRe/C catalysts [(c) and (d)] and EDX mapping of Ru and Re elements (white bars indicate 20 nm). C
DOI: 10.1021/acssuschemeng.6b01346 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Among all three investigated bases, Ca(OH)2 (entry #4) shows the highest promotional effect in RuRe/C (2006.4 h−1). In addition, RuRe/C gives the highest selectivity to alcoholic products in the presence of Ca(OH)2, compared with other base promoters. The improvement of C−O cleavage activity on RuRe/C over Ru/C was also reported previously.20 Similar to glycerol hydrogenolysis, Re/C does not show any activity for sorbitol conversion. The addition of Re to Ru increases the selectivity to glycols (from 8% to 23%, entries#1 and #2 in Table 2). Re is known to be in various ReOx forms on catalyst supports.20 The presence of ReOx is known to promote hydrogenation rather than dehydration.18 Further, characterization data clearly demonstrate that Re addition to Ru not only results in the formation of a bimetallic alloy but also promotes the dispersion of Ru particles. The alloy formation might be an important reason for the improvement of Ru catalyst activity during sorbitol hydrogenolysis. The RuRe/C + Ca(OH)2 system, which exhibits the best performance in terms of activity and selectivity for sorbitol hydrogenolysis, was chosen for further kinetic study. Based on the bench mark experimental data (Table 2), a tentative reaction scheme is proposed in Figure 2. Specifically, there exist two major reaction pathways, one involving the formation of hex-2-ketose and another one with generation of hex-aldose. In the first reaction path, hex-2-ketose formed during dehydrogenation instantaneously undergoes C−C cleavage and forms two C3 intermediates, which can be further converted to Gly and PDO via hydrogenation and hydrogenolysis, or transformed to LA in the presence of base promoters (e.g., MgO, NaOH). Secondary reactions of these hydrogenated polyols can potentially give C1−C3 alcohols. In the second path, formation of hex-aldose leads to C−C cleavage, resulting in the formation of C2 and C4 intermediates. C 4 intermediates can eventually form BDO while C 2 intermediates might form EG and Glo-acid as final products. The tentative reaction pathways discussed here are the basis for proposing the reaction scheme for kinetic modeling. The significance of each reaction pathway will be evaluated quantitatively under various reaction conditions aimed at further refining the reaction network. Effects of Temperature, Pressure, and Catalyst Loading. Reaction parameters such as temperature, H2 pressure, and catalyst loading are found to strongly affect product distribution. As seen from Figure 3, an increase in temperature results in a decrease in LA selectivity (from 42% to 35%), while those to PDO, EG, and alcohols increase marginally. This implies that higher reaction temperatures favor hydrodeoxygenation reactions, leading to further conversion of PDO and EG to propanol (PtOH) and EtOH, which also agrees well with previous work. 28 Typical concentration−time profiles of sorbitol conversion in the presence of RuRe/C catalyst are shown in Figures 4−6. It is seen that the concentrations of LA, PDO, and EG increase at higher sorbitol conversion. As regards the effect of reaction temperature, the hydrodeoxygenation rate increases significantly from 473 to 488 K, with a corresponding increase in GLY, PDO, and EG in the final product (from 0.20 kmol/m3 to 0.25 kmol/m3 at the sorbitol conversion levels of 65−75%). Furthermore, it is seen that increasing the hydrogen pressure (from 2.0 to 5.0 MPa) results in an increase in the overall hydrogenation rate (Figure 7). It is found that the concentrations of PtOH and EtOH increase (from 0.002
dispersion in the presence of Re than as monometallic Ru particles [Figure 1(c)]. In addition, lattice parameters measured from high resolution TEM images [Figures 1(b) and (e)] show that RuRe has 0.22 and 0.34 nm distance between lattices, which is the characteristic value for the Ru [101] surface plane. These results agree well with the previous findings in X-ray diffraction (XRD) analysis.27 Therefore, it is clearly shown that the addition of Re to the Ru system enhances the dispersion of the Ru element on the carbon surface, which is believed to influence the overall activity of Ru catalysts for sorbitol hydrogenolysis (as discussed in the next section). Performance of Ru/C and RuRe/C Catalysts in Sorbitol Hydrogenolysis. The performances of Ru/C and RuRe/C catalysts in sorbitol hydrogenolysis are compared at 473 K and 3.5 MPa hydrogen pressure in the presence of various base promoters. The major products detected during sorbitol hydrogenolysis are as follows. The liquid components in the product mixture range from C1 to C6, including unreacted sorbitol (C6), 1,2-butanediol (BDO, C4), Gly (C3), PDO (C3), LA (C3), 2-propanol (C3), EG (C2), ethanol (EtOH, C2), glycolic acid (Glo-acid, C2) and methanol (MeOH, C1), while the gaseous products included C1−4 gaseous alkanes (methane, ethane, propane, and butane) and CO2. The carbon balances in all experiments are in the range 92−100%. As seen from entries#1 and #2 of Table 2, RuRe/C displays improved Table 2. Comparison of Ru/C and RuRe/C in Sorbitol Hydrogenolysisa Entry#
1
Catalyst Ru/C Base MgO TOF (h−1) 250.1 Selectivity to major products (%) Gly 8.7 LA 38.0 Glycols 8.0 Alcohols 0.1 Gas alkanes 10.2 CO2 10.1
2
3
4
RuRe/C MgO 480.0
RuRe/C NaOH 1619.4
RuRe/C Ca(OH)2 2006.4
7.3 25.6 18.3 0.1 6.0 4.0
7.6 50.4 18.5 14.3 2.0
8.6 37.6 23.2 5.0 0.6 1.8
a Conditions: T, 473 K; PH2, 3.5 MPa; sorbitol, 0.55 kmol/m3; solvent, H2O; catalyst, 0.05 g; base, 0.10−0.15 g; conversion,
