Article pubs.acs.org/EF
Metal−Organic Framework Materials for Desulfurization by Adsorption David Peralta,†,‡ Gérald Chaplais,† Angélique Simon-Masseron,† Karin Barthelet,‡ and Gerhard D. Pirngruber*,‡ †
Equipe Matériaux à Porosité Contrôlée (MPC), Institut de Science des Matériaux de Mulhouse (IS2M), LRC CNRS 7228, UHA, ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France ‡ IFP Energies Nouvelles, Rond Point échangeur de Solaize, BP3, 69360 Solaize, France S Supporting Information *
ABSTRACT: Current European regulations limit the sulfur content of gasoline to 10 ppmw. Such deep desulfurization levels can be achieved by catalytic hydrodesulfurization processes, but they are accompanied by excessive H2 consumption for unwanted side reactions, in particular, for the hydrogenation of olefins. Selective adsorption constitutes an attractive alternative to catalytic desulfurization. The main challenge is to find adsorbents able to remove the sulfur compounds with very high selectivity from a complex mixture of paraffins, naphthenes, olefins, and aromatic compounds. In the present contribution we present the screening of a large number of metal−organic frameworks (MOFs) for this purpose, using batch adsorption experiments. For the two most promising structures (HKUST-1 and CPO-27-Ni, two cus-MOFs, that is, with coordinatively unsaturated sites), the dynamic behavior, the impact of a model nitrogen-containing compound (pyridine) on the adsorption properties, as well as the regenerability were also evaluated by breakthrough experiments. The good results obtained in purification of our model feeds incited us to perform measurements with a real gasoline feed using batch measurements. The feasibility of adsorptive desulfurization of gasoline using MOFs is discussed on the basis of these results.
1. INTRODUCTION Fluid catalytic cracking (FCC) naphtha constitutes roughly 1/3 of the gasoline pool in the United States and Europe. The sulfur content of raw FCC naphtha is in the order of 200−7000 ppmw, and a large majority of the sulfur content of the gasoline blend originates from FCC naphtha. Generally, FCC naphtha contains hydrogen sulfide, thiols, disulfide, thiophene, and its alkyl derivatives, the two latest representing 60−70 wt % of the total sulfur compounds.1,2 The European directive 2009/30/ CE imposes a limit of 10 ppmw on the concentration of sulfur compounds in gasoline, whereas in the U.S. the current limits are between 30 and 80 ppmw. In order to meet these stringent regulations, FCC naphtha is subjected to catalytic hydrodesulfurization (HDS). The HDS reaction transforms organic sulfur compounds to H2S, thereby consuming H2. However, the HDS reaction competes with the hydrogenation of olefins in the FCC naphtha. Hydrogenation of olefins is undesired because it means a loss of octane number (octane numbers of paraffins are lower than those of the corresponding olefins) and a superfluous consumption of H2. For the deep desulfurization levels that are currently required, it is very difficult to limit the parasitic hydrogenation of olefins with a catalytic process. Much research has therefore aimed at completing or replacing the catalytic desulfurization by an adsorption process whose task is to remove the last traces of thiophene and alkylthiophenes in FCC naphtha.3,4 The adsorbent of a such process has to be highly selective for adsorption of thiophenic molecules versus the major components of FCC naphtha, that is, paraffins (20−40%), naphthenes (5−15%), olefins (20−40%), and aromatics (20− 40%). Moreover, it should be easily regenerable by heating or © 2012 American Chemical Society
by purging with a solvent. Activated carbons have good adsorption capacities in model fuels,5 but the performances of these adsorbents decreased by a factor of 10 times in real diesel fuel,6 because of the insufficient selectivity for the adsorption of sulfur containing aromatic compounds compared to polyaromatics, and nitrogen and oxygen containing compounds.7 Also, in zeolites NaY and NaX, the adsorption of aromatic molecules such as benzene or toluene is in strong competition with the adsorption of thiophene.8−10 However, it was previously shown that the thiophene/toluene selectivity can be tuned via the choice of the extraframework cation.11 The best results were obtained with cations that are able to form πcomplexes with the aromatic sulfur compounds, that is, Cu+, Ag+, and Ni2+.12−17 ZSM-5 and the HY are highly selective for the thiophene (versus toluene), but strong interactions do not allow a total regeneration of the adsorbents.18−21 More recently, the behavior of MOF materials for adsorption of sulfur compounds was examined.22−26 Some MOFs are able to adsorb thiophenic molecules in large quantity, even in the presence of aromatics.6,27,28 Among the tested materials, UMCM-150, a Cu-based MOF with coordinatively unsaturated sites, performed best, but the reasons for its superior capacity remain unknown. Unfortunately, the majority of the cited literature studies has focused on sulfur compounds in the jet fuel or diesel range, that is, benzo- and dibenzothiophenes. Much less is known about the adsorption behavior of thiophenic molecules in the gasoline Received: November 2, 2011 Revised: July 4, 2012 Published: July 6, 2012 4953
dx.doi.org/10.1021/ef300762z | Energy Fuels 2012, 26, 4953−4960
Energy & Fuels
Article
range, that is, thiophene itself and alkyl-thiophenes. To evaluate the potential of MOFs as sulfur-selective adsorbents for the purification of naphtha, we screened several structures (HKUST-1,29 CPO-27-Ni,30 RHO-ZMOF,31 ZIF-8,32 and ZIF-7633 (Figure 1)) with different topologies and polarities
Table 1. Textural Properties and Thermal Stability of the Samples under Study
a
sample
SBET (m2/g)
Vmicro (mL/g)
Decomposition temp. (K)a
HKUST-1 CPO-27-Ni ZIF-8 ZIF-76 RHO-ZMOF NaY DAY
1840 1423 1813 1561 563 842 642
0.74 0.54 0.65 0.60 0.22 0.31 0.28
563 633 688 673
