Investigation of bifunctional zeolites for the adsorptive desulfurization of fuels.
Kevin X. Lee and Julia A. Valla
Department of Chemical & Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Unit 3222, Storrs, CT 06269-3222, USA,
Phone: +1-860-486 4602, e-mail: ioulia.valla@.uconn.edu
Naturally occurring sulfur in transportation fuels has been considered a threat to environment and health safety. The combustion of sulfur produces toxic sulfur oxides (SOx), which contributes to acid rain and catalyst deactivation. Furthermore, the slightest amount of sulfur can poison an entire fuel-cell system. Due to this global issue, many desulfurization methods have been developed to meet the stringent emission standards of 10ppm by 2017.1 A promising method to achieve this goal is desulfurization by adsorption using zeolites because it is a cost effective and environment friendly process.2 Zeolites are microporous aluminosilicates minerals that are widely used in catalysis, adsorption, and other energy applications due to their unique pore structure and large surface area. Unlike the energy intensive hydrodesulfurization method, zeolites can effectively remove refractory sulfur compounds such as alkyl-benzothiophenes and their derivatives at ambient conditions and without the consumption of hydrogen.
Further modification to the microporous zeolite can be done either by introducing mesoporosity and/or incorporating d-block metal cations.3 Metal-incorporated zeolites, in combination with the ordered and open pore structures, were shown to be promising due to the sulfur-metal bond and the π-complexation interactions.4,5 The crystallinity of the zeolite is also preserved to study the effect of acid sites and microposority on desulfurization.6 Hierarchical zeolite Y and USY were prepared via desilication and surfactant methods.7 The crystal structure and pore size were identified using x-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET), respectively. Brönsted and Lewis acid sites were quantified using pyridine adsorption by diffuse reflectance infrared spectroscopy (DRIFT-FTIR). DRIFTS-FTIR experiments were also used to study diffusion limitations of thiophenic compounds through the zeolites. Ce, Ni and Cu metals were introduced onto the zeolites via either ion-exchange or wet incipient impregnation method. The metal loadings on the zeolite were determined using ICP.
The adsorption of a model fuel (eg. benzothiophene in octane) was studied via dynamic fixed-bed chromatography experiments. The sulfur concentrations in the zeolite and in the effluent after reaching equilibrium were evaluated using elemental analysis and the gas-chromatograph-sulfur chemiluminescence detector (GC-SCD), respectively. Breakthrough curves of final benzothiophene concentrations were generated and compared to evaluate the performance of each type of zeolite. The most promising zeolite for sulfur removal appears to be the Ni-impregnated on zeolite Y. Sigma bond connecting S in benzothiophene and Ni metal yields a strong S-M interaction and longer sorbent lifetime. This convincing result will allow for further investigation on other transition metals. Our results also demonstrate that zeolite Y is a more effective sorbent than USY, which suggests that the number of acid sites could play a more significant role than the pore size for benzothiophene. Precise and careful modification of zeolites is essential to optimize diffusivity, selectivity and adsorption rate.
The study has been extended to diesel fuels to examine the feasibility of sulfur adsorption with zeolites in industrial applications. The diesel fuel was spiked with one or several sulfur model compounds (eg. thiophene, benzothiophene and dibenzothiophene) as well as other aromatic hydrocarbons (eg. benzene, toluene, and pyridine). Dynamics experiments have been conducted to assess the capacity and the lifetime of each zeolite. This study explores wide applications of hierarchical pore structured zeolites as promising adsorbents of thiophenic compounds by investigating adsorption kinetics, selectivity, and capacity.
To simulate the adsorption behavior and predict sorbent performances, a mathematical model was developed based with the following assumptions: (1) isothermal conditions, (2) constant flow rate, (3) transport of diluted species, (4) constant particle and bed porosities, (5) internal mass transfer described by Linear Driving Force model, (6) adsorption based on Langmuir isotherm.8 The overall differential-algebraic mass balance equation of the bulk (liquid) phase is given by:
Where c is the concentration of sulfur, D is the axial dispersion coefficient, u is the interstitial velocity, ε is the bed porosity, q ia the adsorbed sulfur concentration, k is the mass transfer coefficient, KL is the Langmuir constant, and qm is the sorbent capacity. The boundary conditions are given by:
The study on temperature effects allowed for thermodynamic evaluation where the Gibbs free energy change is a function of the Langmuir constant.
The negative value of ΔG suggested that the adsorption of benzothiophene is a spontaneous process. The changes in entropy and enthalpy of the adsorption process were evaluated by the following equation:
The analysis of thermodynamics indicated that the adsorption of benzothiophene is both spontaneous and exothermic. Metal-modified zeolite Y has shown to be a promising adsorbent for the desulfurization of fuels due to strong S-M or π-complexation interactions. Microporosity of the zeolite also plays an important role in providing acid sites for the sulfur compounds. For larger refractory thiophenic compounds where diffusion limitations may be an issue, the introduction of mesoporosity could enhance the adsorption of these molecules. The selectivity, capacity, lifetime, and kinetics of the adsorbents are dictated by the type of zeolite. The overall performance of each zeolite can be determined via the breakthrough curves. The mathematical model has shown to be a good representation of the breakthrough curves described by the non-linear Langmuir isotherm.
1. “EPA Proposes Tier 3 Motor Vehicle Emission and Fuel Standards,” 2013.
2. Song, C.; Ma, X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl. Catal. B Environ 2003, 41, 207 – 238.
3. Tian, F.; Yang, X.; Shi, Y.; Jia, C.; Chen, Y. Adsorptive desulfurization over hierarchical beta zeolite by alkaline treatment. J. Nat. Gas Chem. 2012, 56, 111 – 126.
4. Hernandez-Maldonado, A. J.; Yang, F. H.; Qi, G.S.; Yang, R. T. Desulfurization of transportation fuels by π-complexation sorbents: Cu(I)-, Ni(II)-, and Zn(II)-zeolites. Appl. Catal. B Environ. 2005, 56, 111 – 126.
5. Velu, S.; Ma, X.; Song, C. Selective adsorption for removing sulfur jet fuel over zeolite-based adsorbents. Ind. Eng. Chem. Res. 2003, 42, 5293 – 5304.
6. Sun, H.; Sun, L.; Li, F.; Zhang, L. Adsorption of benzothiophene from fuels on modified NaY zeolites. Fuel Processing Technology 2015. 134, 284 – 289.
7. Li, K.; Valla, J.; Garcia-Martinez, J. Realizing the commercial potential of hierarchical zeolites: new opportunities in catalytic cracking. Chem. Cat. Chem. 2014, 6, 46 – 66.
8. Raghavan, N. S.; Hassan, M. M.; Ruthven, D. M. Numerical simulation of a PSA system. Part I: Isothermal trace component system with linear equilibrium and finite mass transfer resistance. AIChE J. 1985, 31, 385 – 392.
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