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Kinetics of Sodium Borohydride Hydrolysis in Aqueous-Basic Solutions

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Kinetics of Sodium Borohydride Hydrolysis in Aqueous-Basic Solutions View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Repositório do LNEG IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries. Estoril, Portugal, June 26-28 2013 Kinetics of Sodium Borohydride Hydrolysis in Aqueous-Basic Solutions Rajasree Retnamma1*, C. M. Rangel1, A. Q. Novais1, Michael A. Matthews2 1National Laboratory of Energy and Geology, Estrada do Paço do Lumiar 22, 1649-038, Lisbon, Portugal 2 Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, 301 South Main Street, Columbia, SC 29208, USA *[email protected] ______________________________________________________________________________________ Abstract Liquid-phase catalytic hydrolysis of sodium borohydride (NaBH4) for hydrogen production necessitates long-term sta- bility of base-stabilized NaBH4 solutions at higher temperatures. The present paper reports the kinetics of aqueous-basic 0 solutions containing 20 wt% NaBH4 with 1-15 wt% sodium hydroxide (NaOH) at 80 C. The established kinetic model employs a modified isoconversional method assuming single-step kinetics. The estimation of kinetic parameters is per- formed by gPROMS (general PRocess Modeling System) parameter estimation tool. The reaction kinetics differs from low to highly-concentrated NaOH solutions. In highly-basic (≥10 wt% NaOH), aqueous solutions of NaBH4, the rate is in- dependent of NaOH concentration, while for lower-basic (< 10 wt% NaOH) solutions, the dependence is -0.57, confirm- ing the inhibition of hydrolysis kinetics by NaOH. ______________________________________________________________________________________ Keywords: Sodium borohydride, Hydrogen production, Isoconversional method, gPROMS, Kinetics 1 Introduction Catalytic hydrolysis of aqueous sodium Churikov et al. [3] studied the influence of medi- 0 borohydride (NaBH4) solutions has been widely um to high temperatures (50-100 C) and high ini- accepted as a potential technology for delivering H2 tial pH (12.5-14.5) on the hydrolysis rate of dilute gas (eq.1) to fuel cells due to its high theoretical NaBH4 solutions (0.15 wt% to 3 wt%). The estab- hydrogen storage capacity (10.9 wt%) and safe lished kinetic model for conversions up to 30% operation. Aqueous solutions of NaBH4 undergo comprises two parts. The first part represents the self-hydrolysis even at low temperature. An in- high pH range (>12), where the hydrolysis rate is crease in solution pH inhibits the hydrolysis reac- independent of pH, and the second part represents tion rate and the conventional catalytic hydrolysis is the low pH range (<12) where the hydrolysis rate generally carried out on sodium hydroxide (NaOH) increases with decrease in pH. This model besides stabilized solutions. being developed for diluted NaBH4 solutions, exhib- its some limitations deriving from: (1) assumption of NaBH + (2+×)H O catalyst NaBO .×H O + 4H + heat (1) 4 2 ⎯⎯⎯⎯→ 2 2 2 first-order dependence on borohydride concentra- The stability of aqueous borohydride solution tion throughout the reaction in all the studied tem- increases with base concentration. However, ele- peratures; and (2) reaction order greater than unity vated temperatures and long-term storage de- (1.79) regarding hydrogen ion concentration. Most crease the stability, and this should be carefully recently, Bartkus et al. [4] established two semi- considered during the handling and storage of the- global rate equations describing the consumption se solutions [1-2]. The rate dependence of pH and rate of NaBH4 and hydrogen ion based on exten- mechanistic details were extensively studied in sive experimental kinetic data (0.0 wt% to 25 wt% 0 1960s and 1970s in controlled buffer solutions [2]. NaBH4, 0.0 wt% to 4.0 wt% NaOH and 25 C to 75 The hydrolysis kinetics differ from low pH (<12) to 0C). The rate expression for hydrogen ion, which is high pH. In the pH range of 7.7-9.5, there is first- a major contribution of this work is empirical and order dependence on both borohydride and hydro- does not allow, therefore, gaining further theoretical gen ion concentration. However, the rate becomes insight on the influence of pH on the self-hydrolysis less sensitive to hydrogen ion concentration with kinetics. an increase in pH and the order decreases to about The present work reports the kinetic study of 0.4 at very high pH (>12). These models were es- aqueous concentrated NaBH4 solutions (20 wt %) tablished for very dilute buffered NaBH4 solutions containing 1-15% NaOH at higher temperature (80 (< 0.5 wt %) at low to medium temperatures (15-35 0C) by a modified isoconversional method. The 0C), which are very restrictive conditions for practi- objective is to determine the hydrogen loss during cal applications. the handling and storage of aqueous-basic solu- tions of NaBH4 under practical applications. IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries. Estoril, Portugal, June 26-28 2013 2 Experimental hydrolysis. The present work attempts to address this issue by using a model-based isoconversional The experimental kinetic data used in the pre- method, which has the potential to capture the sent study are those reported by Yu and Matthews change in mechanism of the process during the [5]. The evolution of metaborate formation and progress of reaction, as well as with temperature or NaBH consumption were characterized in-situ by 4 temperature regime. 11B nuclear magnetic resonance (NMR) technique The NaBH hydrolysis process exhibits its for NaOH-stabilized NaBH solutions. The details of 4 4 maximum rate at the beginning of the reaction, the experimental runs in terms of NaOH concentra- which decreases continuously with the extent of tion, duration of the experiments and conversion at the end of the experiments are given in Table 1. As conversion. The most appropriate form of f (α) can be seen, the final conversion decreases with for such decelerating type process is the ‘reaction- increase in NaOH concentration, confirming that order model’ which is of the form [6]: NaOH inhibits the hydrolysis reaction. f (α) = (1 - α)m (2) Table 1. Details of Hydrolysis Experiments for 20 wt% 0 where m is the apparent reaction order with respect NaBH4 solutions at 80 C. to NaBH4 concentration. Assuming the above ‘reaction-order model’, the NaOH Duration Final NaBH4 (wt%) (hr) Conversion rate expression for NaOH-stabilized solutions can (%) be written as: 0 24.0 98.00 dα m-1 m n = kap (T) C0,NaBH (1-α) CNaOH (3) 1 25.0 81.00 dt 4 5 18.5 41.30 where C is the initial concentration of 0,NaBH4 10 24.5 30.70 NaBH4, CNaOH the initial concentration of NaOH, 15 23.0 5.40 and n the apparent reaction order with respect to NaOH concentration. 3 Modelling The parameter estimation objective function is based on maximum likelihood estimation (MLE), a The kinetics of complex heterogeneous pro- popular statistical method used in a large variety of cesses that are stimulated by change in tempera- applications. The optimization of the parameter ture is generally described by isoconversional estimation objective function is performed by the method assuming single-step [6]. This method pro- parameter estimation tool of gPROMS 3.1.3. (gen- vides deconvolution of multi-step reactions and eral PRocess Modeling System), an advanced pro- enables modelling the reaction kinetics without a cess modeling software by Process System Enter- deeper insight into its mechanism. This model-free prise (PSE) for simulation, optimization and pa- isoconversional method is considered to produce rameter estimation (both steady-state and dynam- consistent kinetic results from isothermal and non- ic) of highly complex processes. The MXLKHD isothermal experiments. The present study em- solver based on maximum likelihood approach is ploys this approach for the complex homogeneous used, where the global optimum is found by apply- NaBH4 hydrolysis kinetics. ing a sequential quadratic programming (SQP) The isoconversional method by single-step ki- method. The MXLKHD solver calls the DASOLV netic approximation is expressed as [6]: solver for the solution of the kinetic model compris- dα ing the differential algebraic equations. Besides = kap (T) f (α) (1) rapid estimation of the parameters, the powerful dt numerical method provided by gPROMS allows where α is the extent of conversion, t the time, rigorous statistical testing of the estimated parame- ters for uncertainty in the values and the model kap (T ) the apparent rate constant dependent on validation, i.e. how effectively the model represents temperature T, and f (α) the conversion function the system. representing the process mechanism. Equation (1) intrinsically assumes constant re- 4 Results and Discussion action mechanisms throughout the process and the 4.1. Parameter Estimation mechanism of the process is solely a function of conversion and independent of any temperature or The goal was to estimate kap, m and n for a pre- temperature regime [6]. This may not hold true in defined conversion and investigate the effect of the the case of highly complex reaction such as NaBH4 extent of conversion on the estimated parame- IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries. Estoril, Portugal, June 26-28 2013 0 Table 2. Estimated Parameters for 20 wt% NaBH4 solutions containing various concentrations of NaOH at 80 C. NaOH Conversion (%) (wt%) Parameter 5 10 15 20 40 Mean kap 0.51±0.004 0.46±0.001 0.41±0.008 0.32±0.009 0.11±0.04 - 0 m 0.27±0.001 0.25±0.030 0.26±0.003 0.28±0.006 0.28±0.01 0.268±0.01 kap 0.043±0.003 0.046±0.001 0.043±0.01 0.045±0.02 0.038±0.03 0.043±0.02 1, 5 m 1.010±0.150 0.990±0.150 1.020±0.11 0.990±0.10 1.110±0.09 1.020±0.12 n -0.59±0.040 -0.56±0.060 -0.55±0.07 -0.580±0.04 -0.57±0.05 -0.57±0.05 kap 0.012±0.008 0.013±0.001 0.009±0.002 0.010±0.005 - 0.011±0.004 10 m 0.980±0.050 0.970±0.170 1.080±0.002 1.040±0.001 - 1.018±0.060 n 0.031±0.001 0.019±0.008 0.003±0.002 0.002±0.001 - 0.014±0.003 kap 0.004±1.6E-6 - - - - - 15 m 0.960±0.080 - - - - - n 0.008±0.0002 - - - - - ters.
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