Fuel 110 (2013) 76–82

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Influence of toluene on the catalytic activity of NiPdCe catalystfor selective hydrogenation of 1,3-butadiene

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2012.11.024

⇑ Corresponding author. Tel.: +58 2124422980; fax: +58 2129063743.E-mail address: scurbelo@usb.ve (S. Curbelo).

Luís Lozano a, Joaquín L. Brito b, Claudio Olivera a, Julia Guerra a, Susana Curbelo a,⇑a TADiP Group, Thermodynamics and Transport Phenomena Department, Simón Bolívar University, AP 89000, Caracas 1080, Venezuelab Physicochemical Surfaces Lab, Venezuelan Institute of Scientific Research (IVIC), AP 20632, Caracas 1020-A, Venezuela

h i g h l i g h t s

" Conversion of BD increases with toluene addition." Conversion of BD increases with higher pressure." Toluene addition enhances 2BE/1BE selectivity and favors the trans-isomer." 1BE hydrogenation losses are reduced by toluene addition and pressure lowering.

a r t i c l e i n f o

Article history:Received 18 June 2012Received in revised form 12 October 2012Accepted 5 November 2012Available online xxxx

Keywords:Selective hydrogenation1,3-ButadieneNiPdCe/c-Al2O3

Egg-shell catalystToluene

a b s t r a c t

The selective hydrogenation of 1,3-butadiene to 1-butene with toluene as a co solvent is studied exper-imentally over a NiPdCe/c-Al2O3 catalyst. To reduce mass transfer limitations, an egg-shell catalyst isused in a fixed-bed reactor for liquid-phase hydrogenation of 1,3-butadiene at 40 �C and two pressurelevels, 90 and 210 psig. Experimental results indicate that 1,3-butadiene conversion and selectivity ofthe hydrogenation process are improved when toluene is added. Conversion of 1,3-butadiene increasesfrom 65% without toluene to 75% for 20% toluene concentration at 90 psig, while for 210 psig theobserved increase is from 66% to 81%. The 1,3-butadiene selectivity toward 2-butenes is favored against1-butene, always with preference for the thermodynamically stable trans-isomer. The 1-butene hydroge-nation is controlled, decreasing the loss from 25% to 10% in the presence of toluene a 210 psig. It can beconcluded that the toluene presence enhances the 1,3-butadiene selective hydrogenation and controls1-butene consumption.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

One of the most important reactions in petroleum refining andin heterogeneous catalysis studies is the selective hydrogenation ofunsaturated compounds. These compounds can be obtained as theoutcome of the FCC unit; specifically, the C4 cut undergoes severalextractions and hydrogenation processes. Their products are usedeither in alkylation reactions for the production of high octane gas-oline or in polymerization reactions for the petrochemical indus-try. Highly unsaturated compounds are undesirable in thesecuts, as they lead to gum and other residua formation byoligomerization/polymerization reactions. Selective hydrogenationof 1,3-butadiene (BD) in the presence of 1-butene (1BE) is thearchetype of these reactions. The key for this process is to selec-tively hydrogenate BD to 1BE or 2-butene (2BE) (cis and trans)while avoiding the formation of butane (BA). A scheme for the gen-

eral mechanism of selective hydrogenation of BD is shown in Fig. 1as proposed by Ardiaca et al. [1].

Transition metals such as nickel, cobalt, copper, and noble met-als, e.g., palladium or platinum are generally used for their hydro-genating properties; however, Pd catalysts supported over aluminaor silica are preferred due to their higher activity and selectivity tothe desired products [1–4]. Pd nanocrystals have been proposedshowing a highly active, selective, and stable catalyst system[5,6]. Recent studies have shown that mixed phase Ni–Pd precur-sor leads to a metal alloy that exhibits synergistic effects on theactivity and selectivity of hydrogenation reactions [7,8]. The sup-port is often a high surface area alumina [9,10]. Problems due tothe acidity of the support and the dispersion of Pd are controlledby incorporating a lanthanide metal such as cerium [6,7,11]. Sincethis reaction is exothermic, the reactants can easily polymerize un-der normal conditions, leading to a variety of problems such asselectivity loss, hot spot generation, rubber formation and com-plete saturation of the reactants. The solution to these problemsis achieved by incorporating the metal phase only at the external

Fig. 2. Experimental setup.

Fig. 1. Reaction mechanism for selective hydrogenation of BD (Ardiaca et al. [1]).

Table 1Reaction conditions.

Parameter Value

Temperature (�C) 40Catalyst mass (g) 0.11LHSV (h�1) 1060Solvent/unsaturated nominal ratio 75:25Olefins/diolefins nominal ratio 20:1

L. Lozano et al. / Fuel 110 (2013) 76–82 77

surface of the solid particles in order to avoid the heat and masstransfer resistances into the center of the porous solid [12].

The performance of the catalyst deteriorates with on-streamtime due to a variety of reasons. One of the reasons is the slowbuild-up of poisonous carbonaceous materials on the catalyst

Fig. 3. Effect of toluene on the TO

surface. To increase the catalyst life or service time, a solventmay be fed to the reactor with the feed streams [13,14]. The sol-vent is recovered from the reactor effluent streams in order to berecycled. A way to improve the conversion and selectivity of thisreaction is introducing an additive in the process stream [15–17].Several patented processes consists of selectively hydrogenatinga C4 cut in presence of a palladium-on-alumina catalyst in a liquidphase containing at least one aromatic hydrocarbon and one aminecompound in solution. Solvents include C4–C10 paraffinic hydrocar-bons, cyclohexane, methyl-cyclohexane, benzene, toluene, andethers such as tetrahydrofuran, ethylbenzene [18,19]. Aromaticcompounds are stable at reaction conditions for these catalysts.However, it is well known that an aromatic compound can be ad-sorbed on a porous media, and also, that its presence favors hydro-gen solubility [20]. The objective of this work is to evaluate theinfluence of an aromatic compound, specifically, toluene in theselective hydrogenation of BD using an egg-shell NiPdCe/c-Al2O3

catalyst.

2. Materials and methods

2.1. Egg-shell catalyst synthesis

Nickel nitrate hexahydrate (Ni(NO3)2�6H2O); tetraamine palla-dium (II) chloride hydrate, (Pd(NH3)4Cl2�H2O) and ammonium cer-ium (IV) nitrate ((NH4)2Ce(NO3)6) are used in the impregnatingsolution to place Ni, Pd and Ce respectively on the support, a com-mercial high surface area gamma alumina (c-Al2O3 Alfa Aesar). Theactive phase is incorporated by the incipient wetness impregnationmethod, to prevent long-range migration of the metal particlesaccording to a methodology described elsewhere [7]. The final cat-alyst particles are 1/800 cylindrical pellets with nominal mass con-tents of 0.572% Ni, 0.518% Pd and 0.1% Ce. For comparisonpurposes, a promoted palladium-on-alumina commercial catalystis tested.

F number for BD conversion.

Fig. 4. Comparison between commercial Pd and this work NiPdCe catalyst conversions at 210 psig.

Fig. 5. Effect of toluene on BD and 2BEs compositions at (a) 90 psi and (b) 210 psig.

78 L. Lozano et al. / Fuel 110 (2013) 76–82

2.2. Catalyst characterization

The synthesized catalyst is characterized by scanning electronmicroscopy including energy dispersive X-ray spectroscopy

(SEM–EDX) to determine the type of profile that is obtained inthe pellets. The results indicated that the largest proportion of me-tal is embedded in the surface and verified an egg-shell-like config-uration. The metal dispersion of the solid was observed by H2

Fig. 6. Effect of toluene on 1BE production at 90 psig and 210 psig.

L. Lozano et al. / Fuel 110 (2013) 76–82 79

adsorption in order to estimate the Turn Over Frequency (TOF) ofthe catalyst at several toluene compositions, exhibiting a metallicdispersion of 8.64%. Thermal gravimetric analysis is used in areductive atmosphere to determine synergistic interactions be-tween Ni–Pd metals. The surface area and the pore texture arestudied by N2 adsorption. Finally, Temperature ProgrammedDesorption of Ammonia is used to find out the amount andstrength (weak, medium and strong) of acid sites of Brønsted andLewis type acid sites. The characterization results have been re-ported elsewhere [7].

2.3. Reaction conditions

The reaction is carried out in a fixed bed differential reactorwith recirculation, at 40 �C for 150 min. The stirred vessel is fedwith hydrogen, BD, 1BE as reactants, n-hexane (HX) as solventand toluene as co solvent. The stirred vessel is pressurized andthe temperature is controlled to ensure the desired liquid phasehydrogenation. The condensed solution is fed to the trickle bedreactor, as shown in Fig. 2. The influence of toluene is determinedby increasing the aromatic content from zero up to 20% in solventloading, for two pressure conditions typical in industrial processes,90 psig and 210 psig. The effluents of the reactor were analyzed byFID gas chromatography. Other operating conditions are summa-rized in Table 1.

3. Results and discussion

The effect of toluene concentration on the turnover frequency(TOF) over PdNiCe catalyst is determined by gradually increasingits concentration from one experiment to the next in the stirredvessel. The TOF was calculated based on the 8.64% of metallic dis-persion. Results are shown in Fig. 3. A 20 vol.% is the upper limitthat could be reached for the described experimental setup. Thislimit is attributed to an increase in solution viscosity that inhibitsadequate pump functioning. There is a positive correlation be-tween toluene concentration and BD conversion which increasesfrom 65% without TO to 75% with 20% concentration at 90 psig,while for 210 psig the observed increase is from 66% to 81%, as

shown in Fig. 4. The different trends at different pressures maybe ascribed to higher hydrogen concentration in the liquid phaseat 210 psig; these observed results are consistent with results ob-tained from PROII� estimation of the liquid–vapor equilibrium,which predicts a significant increase in the H2 dilution in the liquidphase at higher pressures.

For comparison purposes, similar experiments were carried outwith a Pd commercial catalyst. The same trends were obtained, asillustrated also in Fig. 4, with conversions of BD progressively en-hanced when up to 20 vol.% toluene is added.

The effect of toluene on the selectivity toward 1BE is illustratedin Fig. 5 for the two extremes of the concentration range investi-gated, 0% and 20 vol.% toluene. In both cases, progress of the hydro-genation reactions leads as expected to an increase in 2BEs and BA,with a decrease in BD. In the presence of toluene, catalyst activityis enhanced for 2BEs production. A trans/cis ratio always greaterthan one is reached toward the end of the reaction, and it is consis-tent with the concentrations predicted by thermodynamic equilib-rium calculations. It is worth noting that at the higher pressure(210 psig) the thermodynamic equilibrium is reached fasterincreasing the BD conversion rate by 44%; as before, this is attrib-uted to a higher hydrogen concentration in the liquid phase. Agreater gap in concentration between trans-2-butene (tBE) andcis-2-butene (cBE) concentrations is observed (Fig. 5b), favoringthe tBE isomer. At high pressure, when higher hydrogen concentra-tion in the liquid phase is attained, the system activity is increasedfor all products, both desired and not desired.

At lower pressure there is not observable effect of toluene onthe consumption of BE, because there is insufficient hydrogenavailable to drive the conversion; however, at higher pressurethe BD selectivity is increased by adding toluene, and the desiredresult of controlling the 1BE conversion is achieved. At these con-ditions, the solvent is possibly hindering 1BE consumption, andincreasing BD conversion toward the thermodynamically stabletBE isomer (Fig. 6).

The fluctuations in 1BE concentrations observed in Fig. 6 can beattributed to the fast formation of 2BE and BA, and are reflected ina decrease in the 1BE selectivity (Figs. 7 and 8). The commercialcatalyst provides similar selectivity under the same workingconditions.

Fig. 7. Effect of toluene on the 1BE/2BE selectivity at (a) 90 psig and (b) 210 psig.

80 L. Lozano et al. / Fuel 110 (2013) 76–82

The results indicated that PdNiCe catalyst and the presence oftoluene as additive, enhance the selective hydrogenation of 1,3-butadiene and help control 1BE consumption. Similar results havebeen reported by Alves et al. [16] using isoprene as an additive, andhave been explained in terms of competitive adsorption with 1,3-butadiene and 1-butene, attributing to isoprene an intermediateadsorption capacity between the diolefin and olefin. The isoprenethus allows controlling the consumption of 1BE through saturationof active sites. So, in this case, the toluene should be adsorbed less

strongly than BD but more strongly than 1BE. Competitive adsorp-tion tests with toluene should be made in order to verify the valid-ity of saturation of active sites hypothesis.

The 2BE isomers also reach thermodynamic equilibrium fasterat higher pressure, and as a consequence the previously observedgap in tBE/cBE concentration is further highlighted (Fig. 9), favor-ing the thermodynamically stable isomer tBE. This indicates thatan optimization of the operating pressure and co solvent concen-tration would help to move toward better selectivity. We have

Fig. 8. Effect of toluene on the 1BE/BA selectivity at 210 psig.

Fig. 9. Influence of toluene on the trans/cis selectivity 210 psig.

L. Lozano et al. / Fuel 110 (2013) 76–82 81

carried out preliminary experimental tests and found that an inter-mediate pressure of 150 psig provides the same conversion andimproves selectivities.

4. Conclusions

A positive correlation between toluene concentration and BDconversion is observed; different trends at different pressures

may be ascribed to higher hydrogen concentration in the liquidphase, as is predicted by the liquid–vapor equilibrium. It can beconcluded that PdNiCe catalyst and the presence of toluene as cosolvent enhance the selective hydrogenation of 1,3-butadiene, fa-vor the selectivity toward 2BE against 1BE, and help control 1BEconsumption. Competitive adsorption tests with toluene shouldbe made in order to verify the validity of saturation of active siteshypothesis in the present case.

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Acknowledgements

The authors acknowledge support from the National Founda-tion for Science, Technology and Innovation (FONACIT), throughProject No G-2005000437, and from the Dean’s Office for Researchand Development (DID) at Simon Bolivar University.

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