Kinetic Modeling of Aqueous-Phase Glycerol Hydrogenolysis in a

May 21, 2010 - As inferred from Table 1, typical material balance analysis for 1% Ru/C and 1% Ru−1% Re/C catalysts shows very good mass balance clos...
0 downloads 4 Views 1MB Size
Download PDF
10826

Ind. Eng. Chem. Res. 2010, 49, 10826–10835

Kinetic Modeling of Aqueous-Phase Glycerol Hydrogenolysis in a Batch Slurry Reactor Arely Torres, Debdut Roy, Bala Subramaniam, and Raghunath V. Chaudhari* Center for EnVironmentally Beneficial Catalysis, Department of Chemical and Petroleum Engineering, UniVersity of Kansas, 1501 Wakarusa Dr., Lawrence, Kansas 66047

The kinetics of the aqueous-phase hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO) was studied using a bimetallic Ru-Re/C catalyst in a slurry reactor in a temperature range of 493-513 K and a hydrogen pressure of 2.4-9.6 MPa. Glycerol hydrogenolysis to produce 1,2-PDO via C-O cleavage (i) proceeds with parallel C-C cleavage, reforming, water-gas shift, and Fischer-Tropsch reactions, (ii) results in a very complex reaction network with several gaseous- and liquid-phase products, and (iii) poses a challenge to design selective catalysts. It is observed that Ru-Re bimetallic catalyst shows higher hydrogenolysis activity (glycerol conversion of X ) 57.7%) and 1,2-PDO selectivity (S ) 36.6%), compared to the monometallic Ru catalyst (X ) 52.1%; S ) 18.9%) but the monometallic Re catalyst showed no catalytic activity for the reaction. Stirred-batch reactor data on the transient concentrations of reactants and products in both the gas and liquid phases were obtained using a bimetallic Ru-Re/C catalyst under different conditions to understand the reaction pathways, selectivity behavior, and intrinsic kinetics of the different reaction steps. For kinetic modeling, several experiments were performed at different initial pressures of hydrogen, catalyst concentration, and temperatures. The proposed rate equations, along with the regressed kinetic and activation energy parameters, were found to represent the experimental data for the multistep hydrogenolysis reaction very satisfactorily. Introduction In the search for renewable sources for fuels and chemicals, the conversions of lignocellulosic biomass and vegetable oils to such products have been receiving increased attention.1,2 In this context, biodiesel production via the transesterification of fatty acid methyl esters (FAME) has attained some success and the process is at an advanced stage of commercialization.3 However, the sustainability of this process and its commercial viability depend on utilization of the large quantities of glycerol byproduct produced (10% by weight) in this process.4 Among many possibilities,5 the hydrogenolysis of glycerol to 1,2propanediol (1,2-PDO) and ethylene glycol (EG) provides a practical solution to glycerol utilization, because this route is not only expected to be less expensive for the commodities (1,2PDO and EG), compared to the petroleum-based routes, but also uses abundantly available renewable feedstocks. The transformation of glycerol to 1,2-PDO involves catalytic C-O bond cleavage, commonly known as hydrogenolysis, and represents a class of reactions useful in conversion of many other polyhydroxy compounds to lower oxygenated industrial chemicals.6 However, the challenge is to achieve selective conversion of glycerol to 1,2-PDO and EG, and to understand the underlying kinetics and mechanism of the catalytic hydrogenolysis. In previous reports, several supported transition-metal catalysts consisting of Ru,7–9 Pt,10–12 Rh,13 Cu,14–16 and Ni17,18 have been investigated and the activity of different supported metal catalysts for glycerol hydrogenolysis generally follows the order19 Ru ≈ Cu ≈ Ni > Pt > Pd Although Ru is known to be one of the best catalysts for this reaction, it is also known to promote unwanted C-C cleavage to produce gaseous products (mainly methane).20 A few * To whom correspondence should be addressed. Tel.: +1 785 864 1634. Fax: +1 785 864 6051. E-mail address: [email protected].

bimetallic catalysts consisting of Ru-Cu,21 Pt-Ru, Au-Ru,22 Ni-Re,23 and Ru-Re24 are also reported. In a recent study, different Ru-Cu catalyst compositions were tested for glycerol hydrogenolysis, and it was observed that a Ru:Cu ratio of 3:1 gave the optimum conversion (70%) of glycerol and 1,2-PDO (71% of the liquid-phase products).21 However, the study reflects that, more than the synergy of the two components in the bimetallic catalyst, the metals work independently. Maris et al. studied the bimetallic effect using Pt-Ru and Au-Ru catalysts and reported that, although the individual metals showed different activities, the bimetallic catalysts showed the activity similar to that with only an Ru catalyst.22 Marincean et al. studied the pH and solvent effects on 1,2-PDO selectivity behavior using a Ni-Re catalyst.23 However, the bimetallic catalyst was not compared with the monometallic Ni or Re catalysts under identical conditions to evaluate the advantage of the bimetallic catalyst. In another study with Ru-Re bimetallic catalysts, it was reported that the conversion of glycerol increased, compared to monometallic Ru, without changing the 1,2-PDO and propanol selectivity.24 Lahr et al. studied the kinetics of glycerol hydrogenolysis using 5% Ru/C under basic conditions (at pH 8 and 11.7).25 This work proposed a Langmuir-Hinshelwood-type rate model, considering the formations of EG and 1,2-PDO, as well as the effect of competitive adsorption of these products and glycerol on the catalyst surface. In a later study, Lahr et al. investigated temperature effects on glycerol hydrogenolysis with sulfurpromoted Ru catalyst.26 However, a limitation of these works is that the authors considered only 1,2-PDO and EG as products for model predictions and the side reactions that lead to the formation of gaseous products are not considered. Furthermore, there have been no other published reports on the kinetics of glycerol hydrogenolysis, on either mono or bimetallic catalysts that employ a comprehensive reaction network that accounts for the formation of both liquid- and gas-phase products. Therefore, the objective of this work is a detailed understanding of the reaction pathways associated with the hydrogenolysis of

10.1021/ie100553b  2010 American Chemical Society Published on Web 05/21/2010

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

10827

Figure 1. Possible reactions during glycerol hydrogenolysis.

glycerol to 1,2-PDO and the development of appropriate kinetic models that will aid in catalyst development and process optimization. Based on the literature reports and experiments in our laboratory, the possible reactions involved in glycerol hydrogenolysis are shown in Figure 1.27 It is evident that a series of reactions occur during glycerol hydrogenolysis, which lead to several gas- and liquid-phase products. This clearly poses a significant challenge to achieve high selectivity to the desired products such as 1,2-PDO. In our recent report,28 we have shown that a supported Ru-Re bimetallic catalyst gives improved activity (glycerol conversion of X ) 57.7%) and 1,2-PDO selectivity (S ) 36.6%) over the supported Ru catalyst (X ) 52.1%; S ) 18.9%) for glycerol hydrogenolysis. In particular, the Ru-Re catalyst reduced the gaseous product formation (methane, ethane, etc.) and improved the liquid-phase product selectivity, compared to the monometallic Ru catalyst. Therefore, it was thought worthwhile to investigate the intrinsic kinetics of glycerol hydrogenolysis using the Ru-Re bimetallic catalyst. For the kinetic study, the effects of H2 partial pressure, catalyst concentration, and temperature on concentration-versus-time profiles were determined experimentally in a batch-slurry reactor. The integral batch-reactor data were used to determine rate and activation energy parameters. The results from this study will be useful in understanding the kinetics of the competing reactions and selectivity behavior during hydrogenolysis of glycerol and guide the rational design of catalysts as well as the modeling of slurry- and fixed-bed reactors. Experimental Section Materials. Glycerol (g99.5%, spectrophotometric grade), activated carbon, ruthenium trichloride hydrate (RuCl3 · xH2O), and perrhenic acid (HReO4) were purchased from Sigma-Aldrich and used without further purification. Hydrogen (>99.5% pure,

from Air Gas, Inc.) and nitrogen (>99% pure, from Linweld) were purchased and used as received. Catalyst Synthesis. The bimetallic Ru-Re and the monometallic Ru and Re catalysts were prepared in the laboratory via precipitation techniques.29 The required amount of activated carbon (Sigma-Aldrich) was charged in distilled water and stirred for 2 h in a round-bottomed flask, using a magnetic stirrer and a condenser in an oil bath (∼368 K). Required amounts of ruthenium trichloride hydrate (Sigma-Aldrich) and perrhenic acid (Sigma-Aldrich) then were dissolved in water and added dropwise to the stirred suspension. After stirring for 3 h, a dilute aqueous ammonia solution was added dropwise until the solution became strongly alkaline (pH ∼10). The solution was then stirred for 3 h, after which point the contents were filtered, washed using hot distilled water, and dried in an oven at 363 K under vacuum overnight. The catalyst was reduced for 6 h at 493 K in a slurry reactor under 80 bar of H2 pressure using water as a solvent. The resulting activated catalyst was filtered, dried, and stored under dry conditions. The supported Ru and Ru-Re catalysts were characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and surface area analysis. The surface areas of the activated carbon and 1% Ru/C, as well as fresh and spent 1% Ru-1% Re/C catalysts, were measured using a Gemini Model 2360 surface area analyzer (Micromeritics) and the values were 983.2, 987.1, 978.8, and 794.5 m2/g. The materials were degassed for 2 h under N2 flow at 363 K before analysis. Other specifications of the powdered 1% Ru-1% Re/C catalyst are as follows: Ru content, 0.97% (w/w); Re content, 0.98% (w/ w); average particle size, 125 µm; particle density, 413.5 kg/ m3; and surface area, 978.8 m2/g. Reactor Setup and Procedures. The kinetic experiments of hydrogenolysis of glycerol were conducted in a high-pressure, high-temperature multiple slurry reactor system supplied by Parr Instrument Co. (Moline, AL).27 The reactor system consists of a parallel array of six autoclave reactors that can be operated

10828

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

Table 1. Typical Material Balance Calculations for Glycerol Hydrogenolysisa 1% Ru-1% Re/C

1% Ru/C

concentration (kmol/m3) C (× 103 mol) conversion, X (%) concentration (kmol/m3) C (× 103 mol) conversion, X (%) Initial Concentration glycerol

1.104

99.36

1.086

97.74

Final Sample Concentration concentration (kmol/m3) C (× 103 mol) conversion, X (%) concentration (kmol/m3) C (× 103 mol) conversion, X (%) Liquid-phase components glycerol

0.467

42.03 3

3

concentration (kmol/m ) C (× 10 mol) Liquid-phase components ethylene glycol 1,2-propanediol lactic acid methanol ethanol 1-propanol 2-propanol Gas-phase components methane ethane propane n-butane n-pentane CO CO2 totals Carbon deficit

57.70 selectivity, X (%)

0.521

46.87 3

3

concentration (kmol/m ) C (× 10 mol)

52.05 selectivity, X (%)

0.070 0.233 0.000 0.013 0.034 0.011 0.060

4.20 20.97 0.00 0.390 2.04 0.990 5.40

7.33 36.58 0.00 0.68 3.56 1.73 9.42

0.156 0.107 0.000 0.019 0.011 0.000 0.000

9.39 9.62 0.00 0.56 0.65 0.00 0.00

18.45 18.92 0.00 1.11 1.28 0.00 0.00

0.005 0.001 0.001 0.000 0.000 0.000 6.4 × 10-4

10.62 4.92 5.22 0.00 0.00 0.00 0.15

18.52 8.58 9.11 0.00 0.00 0.00 0.26

0.011 0.001 1.4 × 10-4 0.00 0.00 0.00 6.6 × 10-4

26.27 3.01 1.06 0.00 0.00 0.00 0.16

51.63 5.91 2.09 0.00 0.00 0.00 0.32

96.93

95.77

97.59

99.71

2.4%

0.15%

Reaction conditions: glycerol concentration, 1.1 kmol/m (10 wt %); catalyst concentration, 16.66 kg/m3; temperature, 493 K; PH2 ) 6.9 MPa; solvent, H2O; initial liquid volume, 30 mL; and reaction time, 6 h. Gas-phase concentrations in this table represent values under atmospheric conditions. a

3

Figure 2. Conversion and selectivity of supported monometallic Ru and bimetallic Ru-Re catalysts. Reaction conditions: glycerol concentration, 1.1 kmol/m3; catalyst concentration, 16.66 kg/m3; PH2 ) 6.9 MPa; temperature, 493 K; solvent, H2O; initial liquid volume, 30 mL; and reaction time, 6 h. Re/C displays no activity. Table 2. Range of Experimental Conditions for the Kinetic Study of Glycerol Hydrogenolysis Using the 1% Ru-1% Re/C Catalyst parameter

value

glycerol concentration catalyst concentration temperature hydrogen partial pressure solvent (water) reaction time

1.1 kmol/m3 8.33-66.67 kg/m3 473-503 K 2.4-9.7 MPa 30 × 10-6 m3 1-6 h

simultaneously at different temperatures and pressures. Each autoclave reactor is equipped with a thermowell, pressure transducer, gas inlet, gas outlet, and a rupture disk. A magnetic stirrer with maximum agitation speed of 30 Hz provides mixing in each reactor. The temperatures and pressures in the individual reactors are independently controlled and monitored with a

computer interfaced with the control module of the reactor system. The agitation speeds of the reactors are controlled from the computer interface or with the manual controller in the reactor setup itself. The temperature and pressure of the reactors are logged every 10 s through SpecView data acquisition software. In a typical reaction experiment, an aqueous glycerol solution was charged in the autoclave with a measured amount of catalyst. The reactor was sealed and loaded into the multireactor setup with well-defined initial conditions. Subsequently, the reaction mixture was purged with N2 gas several times and then with H2 gas while stirring gently. The reactor was then heated to the desired operating temperature, and then the reactant gas (H2) was charged to the desired pressure. Following the fixed-time batch run, the reactor was cooled to room temperature. After noting the reactor pressure at room temperature, and the reactor was depressurized by releasing the gas-phase products to fill two external sampling loops for offline gas chromatography (GC) analysis (Shimadzu, Model GC-2014). In one sampling loop, C2-C5 alkanes were separated with a RT-QPLOT column (15 m long, inner diameter of ID ) 0.53 mm) and analyzed via flame ionization detection (FID). In a second sampling loop, methane, ethane, CO, CO2 were separated with a 60/80 Carboxane 1000 column (4.57 m long, ID ) 3.175 mm) and analyzed using a thermal conductivity detection (TCD) device. After the gas-phase analysis, the reactor was opened to extract a liquid sample to identify the remaining products in that phase. Glycerol, EG, 1,2-PDO, methanol (MeOH), ethanol (EtOH), 1-propanol, and 2-propanol were analyzed by high-performance liquid chromatography (HPLC) (Shimadzu Model LC-10A) using a Razex ROA-Organic Acid H+ (8%) column (300 mm × 7.8 mm), 0.005 N aqueous H2SO4 as mobile phase at a flow rate of 0.5 mL/min and a

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

10829

Figure 3. Effect of H2 partial pressure on glycerol conversion and 1,2-PDO and propanol selectivities in glycerol hydrogenolysis using 1% Ru-1% Re/C catalyst. Reaction conditions: glycerol concentration, 1.1 kmol/m3, 1% Ru-1% Re/C concentration, 16.66 kg/m3; temperature, 493 K; solvent, H2O; and initial liquid volume, 30 mL.

Figure 4. Effect of H2 partial pressure on gas-phase product selectivity in glycerol hydrogenolysis using the 1% Ru-1% Re/C catalyst. Reaction conditions are as described in Figure 3.

Figure 5. Effect of temperature on glycerol conversion and product selectivities in glycerol hydrogenolysis using 1% Ru-1% Re/C catalyst. Reaction conditions: glycerol concentration, 1.1 kmol/m3; 1% Ru-1% Re/C concentration, 16.66 kg/m3; PH2 ) 6.9 MPa; solvent, H2O; initial liquid volume, 30 mL; and reaction time, 6 h.

refractive index detector (RID). Based on calibrations with standards, glycerol and all the products in the hydrogenolysis reactions were quantified by GC and HPLC analyses to estimate a material balance for the reaction. It is important to mention that, as the glycerol hydrogenolysis reactions produced gas-phase as well as liquid-phase products, intermediate sampling was not done in any of the batch runs. To obtain concentration-time profiles, reactions under identical conditions were followed in multiple reactors and the

Figure 6. Effect of catalyst concentration on glycerol conversion and product selectivities in glycerol hydrogenolysis using the 1% Ru-1% Re/C catalyst. Reaction conditions: glycerol concentration, 1.1 kmol/m3; PH2 ) 6.9 MPa; temperature, 493 K; solvent, H2O; initial liquid volume, 30 mL; and reaction time, 1 h.

reactions in the various reactors were stopped at various predetermined times to conduct analyses of gas- and liquidphase products. The glycerol conversion, selectivity to a particular product, and carbon deficit (to determine massbalance closure) are defined as follows: conversion (%) )

Cmolinitial - Cmolunreacted glycerol Cmolinitial

× 100

10830

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

Figure 7. Typical concentration-time profile for glycerol hydrogenolysis, using the 1% Ru-1% Re/C catalyst. Reaction conditions: glycerol concentration, 1.1 kmol/m3; 1% Ru-1% Re/C concentration, 16.66 kg/m3; temperature, 493 K; PH2 ) 6.9 MPa; solvent, H2O; and initial liquid volume, 30 mL.

in Table 2, in which concentration-time profiles, as well as data regarding hydrogen consumption versus time, were obtained. Results and Discussion

Figure 8. Glycerol hydrogenolysis reaction scheme considered for the kinetic study.

selectivity (%) )

carbon deficit (%) )

Cmolproduct Cmolinitial - Cmolunreacted glycerol

× 100

Cmolinitial - Cmolunreacted glycerol + products Cmolinitial

× 100

where Cmol represents the number of moles of carbon. In other words, the selectivity to a product is defined on a carbon basis. The logic behind the definition was discussed in our previous publication.27 As inferred from Table 1, typical material balance analysis for 1% Ru/C and 1% Ru-1% Re/C catalysts shows very good mass balance closure. In all the reactions, the material balance of glycerol reacted and the products formed was >95%. It is important to mention that the gas-phase hydrogen concentration could not be measured directly, because of the similar thermal conductivities of H2 and He, the carrier gas in the TCD analysis; however, the hydrogen concentration was calculated by difference from the gas-phase analysis. For the kinetic study, several experiments were performed over the range of conditions given

Experiments To Delineate Reaction Pathways. In a few preliminary experiments, carbon-supported catalysts consisting of either Ru or Re were evaluated against the bimetallic Ru-Re supported catalyst for hydrogenolysis of glycerol and the results are shown in Figure 2. Re was found to have a prominent effect as a promoter for the selectivity to 1,2-PDO (18.9% to 36.6%), as well as the liquid-phase products, such as ethylene glycol (EG), propanol, and ethanol (39.8% to 52.3%). This may be due to improved dispersion of Ru in the presence of Re, as reported earlier.24 Another important finding is that, by itself, Re has no activity toward the hydrogenolysis of glycerol. For a detailed comparison of the 1% Ru/C and 1% Ru-1% Re/C catalysts for the hydrogenolysis of glycerol, the material balance analyses are presented in Table 1. Note that 1-propanol and 2-propanol (S ) 9.4% and 1.7%, respectively) are the other major liquid-phase products with the Ru-Re bimetallic catalyst, along with 1,2-PDO and ethylene glycol (S ) 7.3%). The Ru/C catalyst showed relatively higher selectivity to EG and methane with respect to Ru-Re bimetallic catalyst indicating its higher activity for C-C bond breaking, compared to hydrogenolysis. Based on its distinct edge in hydrogenolysis efficiency over the monometallic catalyst, the Ru-Re bimetallic catalyst was selected for further studies. Glycerol conversion and 1,2-PDO and propanol selectivity profiles with the bimetallic catalyst at 493 K under different partial pressures of hydrogen are shown in Figure 3. Glycerol conversion increases as the initial hydrogen pressure increases, but the selectivity to 1,2-PDO follows an opposite trend. However, the 1,2-PDO selectivity decreased only marginally with time, except at a hydrogen partial pressure of 4.8 MPa. Second, there is a substantial difference in selectivity of 1,2PDO at 2.4 MPa and at higher hydrogen partial pressures. This is partly due to the higher hydrogenolysis activity of 1,2-PDO to propanol (Figure 3b) and a possible increase in the formation of gaseous products via the reforming of glycerol, as also indicated by the results in Figure 4. The effects of temperature on glycerol conversion and product selectivity pattern were studied over a temperature range of 473-503 K, and the results are shown in Figure 5. Glycerol conversion increased significantly with temperature, as expected, but the 1,2-PDO and propanol selectivities decreased marginally. The decreases in 1,2-PDO and propanol selectivities are

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

10831

Figure 9. Initial global rate of hydrogenation, as a function of temperature and pressure. Table 3. Mass Balance Equationsa

a

dCgly ) rgly ) -r1 - r2 - r5 dt

dCPDO ) rPDO ) r1 - r3 dt

dCEG ) rEG ) r2 - r4 dt

dCPrOH ) rPrOH ) r3 dt

dCEtOH ) rEtOH ) r4 dt

dCMeOH ) rMeOH ) r2 - r6 dt

Vg d(CCO)g ) 3r5 - r7 - r8 - 2r9 - 3r10 Vl dt

Vg d(CCO2)g ) r7 Vl dt

Vg d(CCH4)g ) r6 + r8 Vl dt

Vg d(CC2H6)g ) r9 Vl dt

Vg d(CC3H8)g ) r10 Vl dt

Vg d(CH2)g ) -r1 - r2 - 2r3 - r4 + 4r5 - r6 + r7 - 3r8 - 5r9 - 7r10 Vl dt

The initial conditions are as follows: At t ) 0, Cgly ) Cgly,0; (CH2)g ) (CH2)g,0. The concentrations of the other components are zero.

Table 4. Rate Terms for Reaction Steps Shown in Figure 8 r1 ) wk1Cgly

(CH2)g HH2

r3 ) wk3CPG1/2

(CH2)g HH2

r5 ) wk5Cgly

r7 ) wk7

r9 ) wk9

(CCO)g HCO (CH2)3/2 g HH2

r2 ) wk2Cgly

r4 ) wk4CEG

HH2

component

at 473 K

at 493 K

at 503 K

(CH2)g

H2 CH4 C2H6 C3H8 CO CO2

24.757 25.637 17.007 14.634 29.030 3.717

20.488 19.906 11.827 9.138 22.489 3.277

18.612 17.408 9.758 7.128 19.663 3.050

HH2 (CH2)g HH2

(CH2)g3/2

r10 ) wk10

Henry’s Constant, Hi (m3(liq)/m3(gas))

(CH2)g

r6 ) wk6CMeOH

r8 ) wk8

Table 5. Henry’s Constants for Different Gases in Water

HH2 (CH2)g3/2 HH2

attributed to higher reforming activity at higher temperatures, which leads to more gaseous products such as methane, ethane, and propane (see Figure 5). EG selectivity was almost constant (S ) 6%-7%) at all temperatures. Similar conversion and selectivity trends were observed with catalyst concentration variation, as shown in Figure 6. The glycerol conversion and product selectivity profiles shown in Figures 3-6 indicate parallel and consecutive reaction networks in glycerol hydrogenolysis, as shown in Figure 1. Furthermore, higher temperature, H2 pressure, and catalyst concentration each lead to more gaseous products, which adversely affects the 1,2PDO selectivity.

In summary, the various likely reactions in glycerol hydrogenolysis are shown in Figure 1, while the product distribution and material balance data for a specific case with a Ru-Re bimetallic catalyst are presented in Table 2. A typical concentration-time profile for the reaction is also shown in Figure 7 for 493 K and 6.9 MPa. The main liquid-phase products were 1,2-PDO, EG, propanol, ethanol, and methanol, whereas the gas-phase products included methane, ethane, propane, and traces of carbon dioxide. Based on these experimental observations, the reactions shown in Figure 8 were considered for the kinetic modeling. Analysis of Mass-Transfer Effects. To ensure that the batch experiments were performed in a kinetic regime, the initial global rates of hydrogenation were calculated for a wide range of conditions (Table 2) from the hydrogen consumption-time profiles observed in the batch reactions under conditions such that glycerol conversion was