Al2O3

Article pubs.acs.org/IECR

Kinetic Modeling of Sorbitol Aqueous-Phase Reforming over Pt/Al2O3 Alexey Kirilin, Johan War̈ nå, Anton Tokarev, and Dmitry Yu. Murzin* Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Turku/Åbo, Finland S Supporting Information *

ABSTRACT: Aqueous-phase reforming of polyols was investigated in the current work by mathematical modeling using sorbitol, which represents a C6-polyol originating from biomass processing. The reaction was studied in the presence of Pt/Al2O3 catalyst at 498 K and 29.3 bar in a continuous fixed-bed reactor under kinetic control. The feasible scheme describing main pathways of sorbitol transformation was proposed considering experimental and literature data. The kinetic model was compared with experimental data through numerical data fitting showing good correspondence.

1. INTRODUCTION Constant increase in consumption of fossil fuels and huge energy demand lead to the task of development of new technologies for production of fuels and energy-rich materials from renewable resources. Biomass has recently attracted a lot of scientific and industrial attention1 as a renewable source of hydrogen and transportation fuels.2 Aqueous-phase reforming (APR) is regarded as a promising technology for both hydrogen and alkanes production from polyols (C2−C6).3 The process is typically performed at elevated temperatures (498−523 K) and pressures (29−45 bar), and water solutions of polyols are utilized.4,5 The system pressure during APR experiments is normally maintained 4 bar above the pressure of saturated water vapors at experimental temperature. Therefore, APR is a liquid phase reaction.5 Aqueous-phase reforming of polyols can be performed over various supported metal catalysts with Pt being the most effective in terms of activity and selectivity to desired products.3 Moreover, Pt-based catalysts (Pt/Al2O3) possess high stability in the APR reaction and have been successfully implemented in aqueous reforming of xylitol6 and sorbitol.7 Basically, the transformation of polyol in the APR process results in formation of H2 and CO. Subsequently carbon monoxide reacts with water via the water-gas shift (WGS) reaction to produce more hydrogen and CO2 as shown in Scheme 1 for

possible intermediates and products. There have been several studies targeted on identification of reaction products and intermediates in APR of glycerol,9 xylitol,6 and sorbitol.7,8,10 It must be noted that the number of products formed during APR of xylitol5 (C5) and sorbitol7 (C6) is much higher compared to glycerol.9 For instance, in the case of glycerol APR, formation of compounds with different functionality takes place including monoalchohols, diols, carboxylic acids, ketones, and hydroxyketones.9 As evidenced by Wawretz et al.,9 formation of hydroxyketone and carbonyl surface intermediates, such as glycerol aldehyde, occurs during APR. It has been stated, the approximately 20% of the total hydrogen formed during APR of glycerol reacts further in the liquid phase resulting in formation of oxygenated products. The apparent reaction order was estimated to be 0.8 and 0.6 for the formation of oxygenated products and gaseous products (H2, CO2, CO, and alkanes), respectively. In the systematic studies devoted to investigation of the reaction products and intermediates during APR of higher polyols originating form biomass, such as xylitol5 and sorbitol6 as well as during APD/H of sorbitol8 and mannitol,10 it has been shown that the number of reaction products is significantly higher compared to the case of glycerol. It is important to note, the composition of the liquid phase is similar for APR6 and APD/H8 of sorbitol. Molecules with different functionality can be formed during APR of sorbitol including alcohols, diols, triols, ketones, carboxyltic acids, furane derivatives, and isosorbide (the product of sorbitol dehydration).6 Li and Huber were able to detect 43 reaction products in the APD/H of sorbitol.8 Successful development of effective catalytic systems is not possible without understanding kinetic parameters of APR process. Aiouache et al.10 performed kinetic modeling of sorbitol APR11 to describe experiments carried out in a batch reactor. A lumped kinetic model was implemented to describe catalytic results obtained from APR of sorbitol over Ni and Ni− Pd catalysts supported on γ-Al2O3, ZrO2, and CeO2. However, in most of the cases APR is carried out in continuous reactors.

Scheme 1. Aqueous Phase Reforming of Sorbitol Representing the Stoichiometry of H2 and CO2 Formed

APR of sorbitol. Generally, APR is utilized for production of hydrogen and alkanes; however, there is a modification of the process which is called aqueous-phase dehydration/hydrogenation (APD/H). In this process hydrogen is cofed together with the solution; therefore, thte reaction is steered toward hydrocarbons production.8 In fact, hydrogen which is formed during APR process may react with the initial substrate thus increasing the number of © 2014 American Chemical Society

Received: Revised: Accepted: Published: 4580

August 18, 2013 February 4, 2014 March 4, 2014 March 4, 2014 dx.doi.org/10.1021/ie403813y | Ind. Eng. Chem. Res. 2014, 53, 4580−4588

Industrial & Engineering Chemistry Research

Article

The model10 provided by Aiouache et al. describes only formation of gaseous products: the sorbitol profile along with profiles of gaseous species formed was considered for the development of the model. Meanwhile, in the APR of higher polyols9 and especially those originating from biomass (xylitol and sorbitol) liquid-phase products play a significant role.7,10 The model proposed by Moreno et al. based on automated network generation method described sorbitol hydrodeoxygenation (i.e., APD/H) in the presence of Pt/SiO2−Al2O3.12 The generated reaction network consisted of 4804 reactions giving a total of 1178 distinct chemical compounds. The authors were able to chemically identify 43 reaction components which were grouped according to the number of carbon atoms and functionality. Furthermore, the mass flow was modeled and compared to the experimental results showing good correspondence for the overall mass flow. Therefore, the present study was targeted to develop a mechanism-based kinetic model which is able to describe formation of gaseous and liquid products in the APR of sorbitol over Pt/Al2O3 in a continuous fixed-bed reactor.

loaded into the reactor. The catalyst was reduced prior to the measurements in H2 flow as described above. The reaction was carried out at 498 K and 29.3 bar, at a range of space velocities of 0.9−6.0 h−1. A mixture of 1 vol. % of He in N2 was used as a carrier gas (flow 30 mL min−1). Weight hourly space velocity (WHSV) is defined as mass of substrate fed per mass of the catalyst per hour [gsub gcat−1 h−1]. An aqueous solution (10 wt %) of sorbitol was used as the feedstock and fed in a continuous manner via HPLC pump. The reactor is coupled to the micro-GC system; therefore, gaseous products were taken periodically and analyzed with Agilent Micro GC 3000A. The GC is equipped with 4 columns: Plot U, OV-1, Alumina and Molsieve. The micro-GC was calibrated to perform quantitative analysis for the following gases: H2, CO2, CO, CH4, and linear hydrocarbons C1−C6 while 1 vol. % of He in N2 was used as an internal standard. Liquid samples were withdrawn periodically and analyzed by means of high-performance liquid chromatography (HPLC), applying an injection volume 2 μL, Aminex HPX-87H column, eluent 5 mM H2SO4, flow rate 0.6 mL min−1, 318 K, 70 min) using a refractive index (RI) detector to determine conversion of the substrate. The carbon balance was monitored by means of total organic carbon analysis (TOC instrument) and was confirmed to a degree of 95−100% for all of the measurements. Methods for calculation of substrate conversion, selectivity to H2, CO2, and CxHy in APR process are reported elsewhere.7

2. EXPERIMENTAL SECTION 2.1. Catalyst Description. For kinetic measurements the Pt/Al2O3 catalyst (Degussa F 214 XSP) was used. Before catalytic tests the sample was prereduced in a hydrogen flow in situ using the following temperature ramp: 298 to 523 K with a ramp rate of 5 K min−1, then dwell for 2 h at 523 K (hydrogen flow 30 mL min−1), followed by flushing for 60 min in He at 523 K (He flow 30 mL min−1). Prior to the catalytic measurements, the catalyst was characterized in order to determine its physicochemical properties. The temperature-programmed profile of Pt/Al2O3 exhibited two hydrogen consumption peaks related to reduction of Pt species: at 348 and 453 K, respectively, confirming that under the experimental conditions the catalyst was completely reduced (Figure S1, Supporting Information). The metal dispersion was determined by CO pulse chemisorption (Autochem 2900 Micromeritics). The CO uptake for Pt/ Al2O3 was 74.6 μmol gcat−1 which corresponds to an average Pt particle size of 3.0 nm. The calculated dispersion of the catalyst was found to be 29%. The catalyst exhibited a surface area of 110 m2 g−1 determined by low-temperature N2 adsorption using Micromeritics ASAP 2010. For determination of the specific surface area BET equation was applied, while the pore volume characteristics were determined from the desorption branch using the BJH method. The total pore volume and average pore diameter were 0.25 cm3 g−1 and 8.2 nm, respectively. The acidic properties of the catalyst were examined by NH3 desorption technique as described earlier.13 The total amount of surface acid sites was 317 μmol g−1 (Figure S1, Supporting Information). The energy of ammonia desorption Edes was found to be 52 kJ mol−1 (Figure S3, Supporting Information). 2.2. Catalytic Tests. For APR reported in the present study, a continuous fixed-bed reactor setup (stainless steel reactor, d = 4.8 mm (external diameter), l = 18 cm) equipped with a furnace was used. The reactor setup is shown in Figure 1S. The catalyst containing 5 wt % of Pt according to the manufacture specification was pressed into pellets, crushed, and then sieved to 125−250 μm fractions. In a standard experiment the catalyst (1 g) was mixed with ca. 3 g of quartz sand and

3. RESULTS AND DISCUSSION 3.1. Mass Transfer. Prior to obtaining the experimental catalytic data which were further used for model validation the absence of external and internal diffusion limitations was shown in APR of sorbitol. In order to verify the absence of internal diffusion limitations the Weisz−Prater criterion was used.14 Due to this criterion no pore diffusion limitations occur, if the Weisz−Prater modulus Φ=

robsR2 cDeff

(1)

for the first order reaction is below unity (Φ < 1), for zero order reaction Φ < 6 and for the second order reaction the modulus is below 0.3. In eq 6, robs is the maximal initial reaction rate, R is the the mean radius of the catalyst particle, and c is the the substrate concentration. The largest radius of the Pt/Al2O3 catalyst particle is 1.25 × 10−4 m (125 μm). The effective diffusion coefficient (Deff) of substrate (sorbitol) in water is defined as Deff = D(ξ/χ), where D is the substrate diffusion coefficient in the liquid phase,ξ χ are catalyst porosity and tortuosity, respectively. Typical values of porosity are in the range 0.3−0.6, while values of tortuosity are varied from 2 to 5. Equation of Wilke−Chang was used for calculation of the molecular diffusion coefficient: Do AB =

7.4 × 10−8(ϕMB)1/2 T 0.6 ηBV b(A)

[cm 2/s] (2)

The dimensionless association factor ϕ is taken 2.6 for water, MB is the molecular weight of solvent, ηB = 0.11888 cP is solvent dynamic viscosity at reaction temperature T (K) and pressure (estimated at 498 K and 30 bar), Vb(A) = 122.15 cm3 mol−1 is the liquid molar volume at solute’s normal boiling 4581

dx.doi.org/10.1021/ie403813y | Ind. Eng. Chem. Res. 2014, 53, 4580−4588


Article

point. Assuming ξ/χ = 1/10 the diffusion coefficient of sorbitol is calculated to be Deff = 1.18 × 10−9 m2/s (498 K and 30 bar). The concentration of the substrate in the solvent is equal to 0.515 mol L−1. For the maximal sorbitol reforming rate6 (1.93 × 10−4 mol L−1 s−1 calculated at 2.7 h−1) obtained for the Pt/ Al2O3 catalyst the estimated Weisz-Prater modulus was amounted to Φ = 0.005. It indicates that substrate diffusion inside the catalyst pores does not affect the reaction rate. The results obtained are in a good correlation with literature date reported by Shabaker et. al.15 for ethylene glycol reforming. The authors utilized the Madon−Buduart method for determination of transport limitations in the case of 0.59% Pt/Al 2 O 3 and showed that Madon−Boudart test was satisfactory. Shabaker et al. observed, however, that transport limitation can be present for high loaded catalysts. In the present work we have shown using Weisz−Prater parameter that no transport limitations take place in reforming of xylitol over 5% Pt/Al2O3. It should be noted that one of the catalyst used by Shabaker et al. had a size of 3 mm and for precisely this catalyst the effectiveness factor was 0.8, while for smaller particles even al large loadings there were no intraparticle mass transfer limitations. For ethylene glycol reforming Shabaker et al. demonstrated that interphase mass transfer becomes limiting only after intraphase mass transfer resistance is significant. One can assume that the same holds for aqueous phase reforming of sorbitol. Evaluation of intraparticle heat transfer is less straightforward since it requires evaluation not only the reaction enthalpy but also activation energy. Note that in the current work as well as in many other studies on APR of sorbitol the temperature range is very limited preventing reliable assessment of activation energy and intraparticle heat transfer. 3.2. Catalyst Performance in APR of Sorbitol: Stability and Selectivity. 3.2.1. Stability Studies. The catalyst stability was studied at T = 498 K and P = 29.3 bar at weight hourly space velocity of 0.6 h−1. The Pt/Al2O3 catalyst demonstrated stable performance with time-on-stream (TOS) for 123 h: decrease in hydrogen and carbon dioxide production rates did not exceed 5% after 123 h TOS compared to values observed at 7 h TOS (Figure S4, Supporting Information). 3.2.2. Formation of the Main Products in the APR of Sorbitol. The main products which are formed during aqueousphase reforming of sorbitol are H2, CO2, and a mixture of alkanes in the gas phase as well as oxygenated products which are mainly present in the liquid phase (as shown in Scheme 1). Carbon monoxide formed is then converted via WGS reaction, to form CO2 and additional hydrogen molecules. Experimental results concerning formation of the main products, hydrogen and carbon dioxide, in the APR of sorbitol are shown in Figure 1. Conversion of sorbitol was 100%. However, hydrogen formed may be involved in side hydrogenation processes of CC or CO bonds as well as hydrogenolysis reactions leading to its consumption and formation of hydrogenated intermediates. The gaseous products were collected in bottles and further analyzed by means of micro-GC and GC-MS.7 Therefore, the analysis of gas-phase products revealed formation of C2−C6 alkanes in the gaseous phase. The concentrations of hexane and pentane, however, were very low. It is important to note that CO did not appear among the reaction product (which in fact means that its concentration is below the detection limit by micro-GC being ∼100 ppm). Detailed analysis of gaseous and liquid

Figure 1. Formation of H2 and CO2 in the APR of sorbitol as a function of WHSV.

products formed during APR of sorbitol was thoroughly investigated and reported earlier.6,7 Here the composition of the gas and liquid phase is briefly described. The corresponding quantitative data of the gaseous phase and liquid phase composition are provided in Tables 1 and 2 (gas-phase products) and 3 (liquid-phase products). On the basis of the experimental data obtained6,7 as well as on literature data reported for a similar process APD/H,8,10 the reaction network for transformation of sorbitol during APR explaining formation of the main products and intermediated was proposed.7 The main pathways of sorbitol transformation are illustrated in Scheme 2. Therefore, the main transformations of sorbitol can be presented including dehydrogenation of the substrate followed by a decarbonylation step (Scheme 2, path 1) and dehydration followed by a hydrogenation step (Scheme 2, path 2). On top of that, the carbonyl species formed might react with water to form carboxylic acids7 which further undergo decarboxylation (not shown in the scheme for clarity) leading to species containing one carbon atom less than the previous compound. Wawretz et.al.9 suggested that Tishchenko/ Cannizaro type disproportionation reactions (as well as crossed Cannizzaro reactions) might take place during APR of glycerol giving carboxyltic acids which further undergo decarboxylation. Secondary hydroxyl groups may liberate hydrogen on Pt surface to form ketones. Basically, path 1 leads to formation of polyol species containing one carbon atom less than the initial substrate, whereas path 2 results in formation of hydrocarbons. The molecules presented in Scheme 2 were detected among the reaction products in APR of sorbitol and detailed description of the analytical procedures involved for identification and quantification is provided earlier.7 The large number of products and reactions makes the development of the kinetic model a complicated task. The scheme proposed and based on experimental results is already too complicated for construction of an applicable model which will be able to describe available experimental data with a high degree of accuracy. Moreover, concentrations of some intermediates and products (such as carboxylic acids and ketones) are very low,6,7 therefore, the scheme of sorbitol transformations may be simplified (corresponding composition of the liquid phase products is reported in Tables S1 and S2, Supporting Information). Thus, the model would be based on those products and intermediates which are experimentally measurable and are present in the reaction mixture in reasonable concentrations. Additionally, all gaseous products (H2, CO, CO2, alkanes C2−C4) will also be considered. It is important to note, that methane was not detected among the reaction products thus showing that Pt nanoparticles have low activity in the methanation reaction. 4582



Article

Table 1. Formation of the Main Products: H2, CO2, CxHy, and Carbon Distribution between Phases in the APR of Sorbitol WHSV, h−1

H2, mol/min [×104]

CxHy, mol/min [×105]

CO, mol/min [×106]

CO2, mol/min [×104]

C at inlet, mol/min [×104]

carbon in gas phase, %

carbon in liquid phase, %

carbon balance, %

0.6 0.9 1.2 1.5

1.2 1.6 2.2 2.5

6.9 7.8 9.6 5.9

0 0 0 9.3

1.7 2.2 2.8 2.9

3.3 5.0 6.7 8.3

73 60 57 43

24 36 39 52

97 96 96 95

A set of stoichiometric numbers of steps is defined as a reaction route. Routes are essentially different, and it is impossible to obtain one route through multiplication of another route by a number, even in cases when their respective overall equations are identical. The number of basic routes, P, is determined by P = S + W − I, where S is the number of steps, W is the number of balance (link) equations, and I is the number of intermediates.16 Such balance (link) equations can correspond to the total coverage of sites equal to unity. On the right-hand side of equations for the steps stoichiometric numbers along the four routes N(1) - N(8) are given. These numbers are selected in a way, that the overall chemical equations do not contain intermediates. In the Scheme 3 there are 17 steps (fast steps are not included), two balance equations (for each type of sites) and 11 intermediates (*, *′, C6O6H14*, C5O5H12*, C4O4H10*, C3O3H8*, C2O2H6*, C4O4H10*′, C3O3H8*′, C2O2H6*′, O*) leading thus to 8 independent routes. Other intermediates, such as for example C6O6H12*are short-lived and are transformed in fast steps, thus their coverage on the surface can be neglected. Based on quasi-equilibrium adsorption of polyols on metal (steps 1, 3, 5, 7, and 9) and support sites (12, 14, and 16) the following expressions can be written respectively for metallic sites

Table 2. Selectivity to Gas-Phase Carbon-Containing Products in the APR of Sorbitol WHSV, h−1

carbon in gas phase, %

selectivity to CxHy, %

selectivity to CO2, %

0.6 0.9 1.2 1.5

73 60 57 43

29 26 25 17

71 74 75 81

3.2.3. Development of the Kinetic Model. Two main reaction transformations paths for a particular sugar alcohol have been selected for the present kinetic model development (Scheme 2). The main pathways of sorbitol transformation during APR for model construction were chosen on the basis of experimental data which we obtained earlier6,7 as well as on results reported in the literature.9 On acidic sites sorbitol undergoes dehydration to a ketone (Path 2 in Scheme 3) with subsequent hydrogenation to a C6 alcohol with one hydroxyl group less than the starting sorbitol. This first initial step of dehydration is considered to the rate determining one while the subsequent fast steps of hydrogenation − dehydration-hydrogenation, etc were lumped together. A similar concept was applied for all alcohols of CnOnH2n+2 type. Since C6 and C5 alkanes (i.e., hexane and pentane) as well as methane were observed in the reaction products in inferior quantities7 it is sufficient to consider formation of only C2−C4 alkanes. The path to alcohols of CnOnH2n+2 type with n < 6 (path 1 in Scheme 3) starts with dehydrogenation on the metal sites leading to a corresponding aldehyde. These dehydration steps (for different alcohols) were assumed to be slow (rate limiting), while decarbonylation steps were considered to be fast ones. This hypothesis is supported by data published earlier by Shabaker et al.15 It has been stated that hydrogen formation rates for APR of methanol and ethylene glycol were similar thus indicating that C−C bond cleavage cannot be a rate determining step. In addition to these two main paths water gas shift reaction (route N(5)) was included in the mechanism, which comprised eight reaction routes: The elementary steps above can be described by eight reaction routes, i.e., sets of stoichiometric numbers of steps. Elementary reactions are grouped in steps, and chemical equations of steps contain reactants and intermediate products.

θC6O6H14 = K1CC6O6H14θ V ; θC5O5H12 = K3CC5O5H12θ V ; θC4O4 H10 = K5CC4O4 H10θ V ; θC3O3H8 = K 7CC3O3H8θ V , θC2O2H6 = K 9CC2O2H6θ V

(3)

and acidic sites θC4O4 H10′ = K12CC4O4 H10θ V′; θC3O3H8′ = K14CC3O3H8θ V′, θC2O2H6′ = K16CC2O2H6θ V′

(4)

where K1, etc. are the equilibrium constant of respective steps, CC6O6H14, etc. are concentrations, and θV and θV′ are the coverage of the vacant sites on the metal and the support. Coverage of adsorbed water is determined by the quasi equilibrium step 10 leading to θH2O = K10C H2Oθ V

(5)

where K10 is the equilibrium constant of step 10.

Table 3. Selectivity to Liquid-Phase Carbon-Containing Products in the APR of Sorbitol selectivity, % WHSV

carbon in liquid phase, %

C2

C3

C4

C5

C6

other products

total carbon detected, %

0.6 0.9 1.2 1.5 2.1

24 36 43 52 83

2.3 7.4 4.9 4.8 6.7

8.9 7.9 12.0 24.9 13.4

38.4 41.7 23.3 28.4 9.3

8.0 12 6.8 − 3.6

19.5 21.6 31.7 25.6 35.5

22.9 9.3 21.3 16.3 31.5

77.1 90.6 78.7 83.7 68.5

4583



Article

Scheme 2. Aqueous Phase Reforming of Sorbitol: the Main Transformation Pathways Selected for Development of the Mathematical Model

The rate along the route N(5) can be defined through either of the steps 10 and 11, thus

From the balance equations for different types of sites, coverage of vacant sites can be easily computed

r (5) = k11C H 2OCCO/⎡⎣1 + K1CC6O6H14 + K3CC5O5H12

θ V = 1/⎡⎣1 + K1CC6O6H14 + K3CC5O5H12 + K5CC4O4 H10 + K 7CC3O3H8 + K 9CC2O2H6 + K10C H2O⎤⎦

θ V′ =

1 + K12CC4O4 H10

1 + K14CC3O3H8 + K16CC2O2H6

+ K5CC4O4 H10 + K 7CC3O3H8 + K 9CC2O2H6 + K10C H2O⎤⎦

(6)

(12)

Similarly for the reactions routes occurring on the support sites the reaction rates could be formulated

(7)

The reaction rate r(1) along the first route N(1) is defined through step 2 which can be considered as the one limiting the rate in this route

r (6) =

r (1) = k 2θC6O6H14

r (7) =

= k 2K1CC6O6H14θ V = k 2K1CC6O6H14 /⎡⎣1 + K1CC6O6H14 + K3CC5O5H12

r (8) =


The rates along the second, third and fourth routes are given in a similar fashion r

−

= k4K3CC5O5H12 /⎡⎣1 + K1CC6O6H14 + K3CC5O5H12

1 + K12CC4O4 H10 + K14CC3O3H8 + K16CC2O2H6

(14)

k17K16CC2O2H6 1 + K12CC4O4 H10 + K14CC3O3H8 + K16CC2O2H6

dCC6O6H14 dτ

− r (7);

(9)

r

(13)

k15K14CC3O3H8

= r (1) ,

dCC5O5H12

= r (2) − r (3) − r (6);

+ K5CC4O4 H10 + K 7CC3O3H8 + K 9CC2O2H6 + K10C H2O⎤⎦ (3)

1 + K12CC4O4 H10 + K14CC3O3H8 + K16CC2O2H6

(15)

Finally, the generation rates of compounds can be written as

(8)

(2)

k13K12CC4O4 H10

= k6K5CC4O4 H10/⎡⎣1 + K1CC6O6H14 + K3CC5O5H12

dCC2O2H6 dτ

= r (7);

(10)

r (4) = k 8K 7CC3O3H8/⎡⎣1 + K1CC6O6H14 + K3CC5O5H12

dC C 2 H 6 dτ

dτ dCC3O3H8 dτ

dCC4O4 H10 dτ

= r (3) − r (4)

dCCO = r (1) + r (2) dτ dCC3H8 = r (6); dτ

= r (4) − r (8);

+ r (3) + r (4) − r (5);


= r (1) − r (2);

dCC4H10

= r (8);

dτ dC H 2 dτ

+ r (5) − 4r (6) − 3r (7) − 2r (8);

= r (1) + r (2) + r (3) + r (4) (16)

where τ is the residence time. The kinetic modeling was done for all reaction rates. For the calculation of parameters, a set of differential equations

+ K5CC4O4 H10 + K 7CC3O3H8 + K 9CC2O2H6 + K10C H2O⎤⎦ (11) 4584



Article

Scheme 3. Elementary Step and Basic Routes Proposed for Sorbitol APRa

where * is the active sites on the metal surface, *′ is active sites on the support, compounds of CnOnH2n structure are aldehydes (C6C6H12 is glucose), CnOnH2n+2 are alcohols (i.e., C6O6H10 is sorbitol), CnOn−1H2n are ketones (for example C4O3H8), CnH2n+2 are alkanes. a

describing the changes in the concentrations profiles of the reagents and products along the reactor length taking into account changes in the volume was solved by means of ModEst software.17

In fact, the model based on eqs 8-15, was overparameterized. The presence of K1, K3, K5, K12, K14, and K16 in the denominators resulted in too large errors during parameters estimation. Hence, the simplified version of the model was 4585



Article

Table 4. Values of the Estimated Kinetic Parametersa

developed. For that purpose K1, K3. K5, K12, K14, and K16 were removed from the model. Therefore, the modified model which comprised of eqs 17-24 was obtained:

r (1) = k 2′CC6O6H14

(17)

The other rates were also rewritten: r (2) = k4′CC5O5H12

(18)

r (3) = k6′CC4O4 H10

(19)

r (4) = k′8 CC3O3H8

(20)

r (5) = k′11CCO

(21)

r (6) = k′13 CC4O4 H10

(22)

r (7) = k′15 CC3O3H8

(23)

r (8) = k′17 CC2O2H6

(24)

a

(25)

This simplified model did not affect the generation rates of compounds written in eq 16 except for the substrate consumption rate −

dCC6O6H14 dτ

= r (1) + r (9)

(26)

Using Levenberg−Marquardt simplex method, the target function, which was defined as incompliance between the experimental and calculated values of concentrations was used to solve the system. The sum of the residual squares between the model and the experimental data was minimized using the following objective function Q = || xexp − xest ||2 =

∑ ∑ (xexp ,it − xest,it)2 t

i

(27)

where xexp is the experimental value and xest denotes the predictions given by the model, i is the component index and t is the time value. The quality of the fit and accuracy of the model description was defined by the degree of explanation R2; which reflects comparison between the residuals given by the model to the residuals of the simpliest model one may think of, i.e., the average value of all of the data points. The R2 value is given by the expression R2 = 100

relative standard error (%) 12.7 large 36.3 large 31.9 37.4 24.8 Large 9.1

Residual sum of squares 0.1595 × 10−6.

4. CONCLUSIONS The APR of sorbitol over Pt/Al2O3 catalyst was studied at 498 K and 29.3 bar at different weight hourly space velocities under kinetic control in a continuous fixed-bed reactor. The reaction kinetics was modeled based on the mechanistic considerations for sorbitol transformation during APR proposed on the basis of experimental data and literature data. The calculated results were compared with experimental data for formation of the main gaseous (H2, CO2) and the liquid products through numerical data fitting. For several components (i.e., C3 and C6 in the liquid phase) a very good correspondence was obtained (with the degree of explanation for C6 above 97%), while description of minor components in the gas phase (alkanes) and other components in the liquid phase was mediocre. The developed kinetic model can be still improved and further

(ymodel − yexperiment )2 (ymodel − yexperiment )2 ̅

value 0.851 × 10−1 1.18 × 102 0.537 0.742 × 10−2 1.85 0.284 0.187 0.996 0.327

Figure 2 displays as an example a comparison between the experimental obtained data (shown in Figure 1) and model predictions for CO2 and hydrogen formation in the APR of sorbitol at different conditions (WHSV). Conversion of sorbitol was 100% for all experimental points. As can be seen from Figure 2a, hydrogen formation increases with an increase in WHSV in agreement with experiments. In accordance with the results reported earlier, more hydrogen was observed at higher space velocities, in other words, at lower contact times. The results are understandable since at higher contact times hydrogen is more consumed in various hydrogenation reactions. Formation of CO2 predicted by the model as a function of WHSV is shown on Figure 2b. The results are in a good correlation with the experimental data obtained and confirm the ability of the model to describe at least the main trends in the formation of hydrogen and CO2 as a function of WHSV. Figure 3 displays a comparison between predicted corresponding carbon flows for C3 and C6 compounds present in the liquid phase during APR of sorbitol and the experimental data (Table 3). As can be seen from Figure 3 the model is able to predict rather well the carbon flows for major liquid products, C3 and C6 (with the degree of explanation above 97%) which are present in the liquid phase in higher concentration compared to other products. Liquid products containing two, four and five carbon atoms in the structures were also included in the model; however, the error of description was rather large. It should be noted that deviations from the experimental data were also observed for calculated values of hydrocarbons. Compounds CxHy were included in the model, but due to significantly lower concentration of each hydrocarbon component compared to CO2 and hydrogen the error was large.

These modified constants contain also the adsorption coefficients. Additionally in order to satisfy the mass balance one more reaction was added to the model accounting for formation of other components from the reactant, for example furans: r (9) = k′18 CC3O3H8

parameter k′2 (min −1) k′4 (min −1) k′6 (min −1) k′8 (min −1) k′11 (min −1) k′13 (min −1) k′15 (min −1) k′17 (min −1) k′18 (min−1)

(28)

3.2.4. Validation of the Kinetic Model. The values of the calculated frequency factors, and as well as the estimated relative standard errors (in %) of the tested reaction mechanism based on eqs 16−26 are presented in Table 4. The overall residual sum of squares for all the products was 0.1595 × 10−6. 4586



Article

Figure 2. Fit of the model to the experimental data: (a) formation of hydrogen and (b) carbon dioxide as a function of WHSV in the APR of sorbitol. Conditions: 1 g of catalyst, 498 K, 29.3 bar, N2 flow 30 mL min−1, 10 wt % sorbitol solution, conversion 100%.

Figure 3. Fit of the model to the experimental data: (a) formation of C3 (carbon flow) and (b) C6 products (carbon flow) in the liquid phase a function of WHSV in the APR of sorbitol. Conditions: 1 g of catalyst, 498 K, 29.3 bar, N2 flow 30 mL min−1, 10 wt % sorbitol solution, conversion 100%.

gratefully acknowledged for financial support. In addition the COST−Action CM0903 (UbioChem) is acknowledged.

advanced by taking into account other intermediates and pathways of sorbitol transformation. The proposed kinetic model has a potential to be applied for description of experimental data available in the literature on APR9 of C3 and APD/H8,10 of C6 polyols in a continuous fixed-bed reactors.

■

■

(1) Thermochemical Conversion of Biomass to Liquid Fuels and Chemicals; Crocker, M., Ed.; RSC Publishing: London, 2010. (2) Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Renewable Alkanes by Aqueous-Phase Reforming of Biomass-Derived Oxygenates. Angew. Chem., Int. Ed. 2004, 43, 1549. (3) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. A Review of Catalytic Issues and Process Conditions for Renewable Hydrogen and Alkanes by Aqueous-Phase Reforming of Oxygenated Hydrocarbons over Supported Metal Catalysts. Appl. Catal. B: Environ. 2005, 56, 171. (4) Tanksale, A.; Beltramini, J. N.; Qing, G.; Lu, M. A Review of Catalytic Hydrogen Production Processes from Biomass. Renew. Sust. Energy Rev. 2010, 14, 166. (5) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. A Review of Catalytic Issues and Process Conditions for Renewable Hydrogen and Alkanes by Aqueous-Phase Reforming of Oxygenated Hydrocarbons over Supported Metal Catalysts. Appl. Catal. B: Environ. 2005, 56, 171. (6) Kirilin, A. V.; Tokarev, A. V.; Kustov, L. M.; Salmi, T.; Mikkola, J.-P.; Murzin, D.Yu. Aqueous Phase Reforming of Xylitol and Sorbitol: Comparison and Influence of Substrate Structure. Appl. Catal. A. Gen. 2012, 435− 436, 172.

ASSOCIATED CONTENT

S Supporting Information *

Figures describing catalyst characterization with TPR, TPD; time on stream performance and tables with composition of the liquid phase. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: dmurzin@abo.fi. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Academy of Finland in collaboration with the North European Innovative Energy Research Programme (N-Inner) and the Graduate School in Chemical Engineering (GSCE) are 4587



Article

(7) Kirilin, A. V.; Tokarev, A. V.; Murzina, E. V.; Kustov, L. M.; Mikkola, J.-P.; Murzin, D.Yu. Reaction Products and Transformations of Intermediates in the Aqueous-Phase Reforming of Sorbitol. ChemSusChem 2010, 3, 708−718. (8) Li, N.; Huber, H. Aqueous-Phase Hydrodeoxygenation of Sorbitol with Pt/SiO2−Al2O3: Identification of Reaction Intermediates. J. Catal. 2010, 270, 48. (9) Wawrzetz, A.; Peng, B.; Hrabar, A.; Jentys, A.; Lemonidou, A. A.; Lercher, J. A. Towards Understanding the Bifunctional Hydrodeoxygenation and Aqueous Phase Reforming of Glycerol. J. Catal. 2010, 269, 411. (10) Kim, Y. T.; Dumesic, J. A.; Huber, G. W. Aqueous-phase Hydrodeoxygenation of Sorbitol: A Comparative Study of Pt/Zr Phosphate and Pt/ReOx/C. J. Catal. 2013, 304, 72. (11) Aiouache, F.; McAleer, L.; Gan, Q.; Al-Muhtaseb, A. H.; Ahmad, M. N. Path Lumping Kinetic Model for Aqueous Phase Reforming of Sorbitol. Appl. Catal. A. Gen. 2013, 466, 240. (12) Moreno, B. M.; Li, N.; Lee, J.; Huber, G. W.; Klein, M. T. Modeling Aqueous-Phase Hydrodeoxygenation of Sorbitol over Pt/ SiO2−Al2O3. RSC Adv. 2013, DOI: 10.1039/c3ra45179h. (13) Kirilin, A. V.; Tokarev, A. V.; Manyar, H.; Hardacre, C.; Salmi, T.; Mikkola, J.-P.; Murzin, D. Yu. Aqueous-Phase Reforming of Xylitol over Pt-Re Bimetallic Catalyst. Catal. Today 2014, 223, 97. (14) Murzin, D. Yu. Engineering Catalysis; Walter De Gruyter: Berlin/ Boston, 2013. (15) Shabaker, J. W.; Davda, R. R.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Aqueous-Phase Reforming of Methanol and Ethylene Glycol over Alumina-Supported Platinum Catalysts. J. Catal. 2003, 215, 344. (16) Murzin, D.; Salmi, T. Catalytic Kinetics; Elsevier: Amsterdam, 2005. (17) Haario, H. ModEst 6.0, User Guide; Helsinki, 2010.

4588


Al2O3

Recommend Documents