Microporous and Mesoporous Materials 168 (2013) 111–120

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Microporous and Mesoporous Materials

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Reactive H2S chemisorption on mesoporous silica molecular sieve-supportedCuO or ZnO

Diana Montes a, Edumaris Tocuyo a, Eduardo González a, Douglas Rodríguez a, Roger Solano a,Reinaldo Atencio b, Miguel A. Ramos b, Alexander Moronta a,⇑a Instituto de Superficies y Catálisis, Facultad de Ingeniería, Universidad del Zulia, Maracaibo 4003, Venezuelab Instituto Zuliano de Investigaciones Tecnológicas (INZIT), km 14 vía La Cañada de Urdaneta, Estado Zulia, Venezuela

a r t i c l e i n f o

Article history:Received 8 August 2012Received in revised form 26 September2012Accepted 26 September 2012Available online 4 October 2012

Keywords:H2S removalMesoporous silicaMetal oxide content

1387-1811/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.micromeso.2012.09.018

⇑ Corresponding author. Tel./fax: +58 261 4128791E-mail address: ajmoronta@gmail.com (A. Moront

a b s t r a c t

Natural gas, a valuable energy carrier, can be used as a fuel or as a raw material for the production of syn-thesis gas and hydrogen. However, significant quantities of undesirable contaminants, especially hydro-gen sulfide (H2S), generate harmful environmental emissions. The objective of this work was to develop ascavenger system for the H2S removal at room temperature. The potential of mesoporous silica molecularsieve (MSU-1) supported ZnO or CuO adsorbents has been studied at room temperature for H2S removalto develop a more effective adsorbent for this important application. Zn2+ or Cu2+ loadings of 10, 20 and30 wt.% were incorporated by the incipient wetness method. The obtained solids were characterized byX-ray fluorescence (XRF), X-ray diffraction (XRD), temperature programmed reduction (TPR), scanningelectron microscopy (SEM) and N2 adsorption/desorption, using the BET method, to investigate their var-ious characteristics. The MSU-1 support did not show activity for H2S removal, however the addition ofcopper or zinc increased the removal performance. The most active solids for H2S were 10Zn/MSU-1(42.3 mg g�1) and 20Cu/MSU-1 (19.2 mg g�1). A decrease in H2S removal was observed at a zinc loadinghigher than 10 wt.% (14.8 and 11.5 mg g�1 for 20 and 30Zn/MSU-1), while 10Cu/MSU-1 presented a lowadsorption capacity (10.9 mg g�1) and 30Cu/MSU-1 has a similar performance than that of 20Cu/MSU-1.The adsorption capacity of the obtained materials strongly depends on the pore system as well as thewell-proportioned distribution of the active phase inside the porous material and the size of metal oxidenanoparticles.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Hydrogen sulfide and other compounds containing sulfur can befound in natural gas and other industrially important hydrocarbonfeedstock [1,2]. Hydrogen sulfide (H2S) is considered as one of themost noxious industrial gases for the atmosphere [3,4]. Its removalfrom natural gas is particularly required for reasons of health (e.g.,sore throats, breathing difficulties, respiratory infections), odorproblems and safety; it also causes pipelines corrosion that limitsplant lifetime [1] and might poison many industrial catalysts[5,6]. Many methods have been developed over years for removalof H2S from gaseous emissions. H2S removal, with subsequent sul-fur recovery, is at present mostly preferred using the Claus process[7]. Some of the disadvantages of this method for gas treatment arethat it requires relatively large investment and operational costs(e.g., special chemicals, corrosion equipment, high pressures and

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.a).

temperatures) and they require special operational safety andhealth procedures.

Due to chemical affinity of H2S for metallic cations, the mostfrequently used technique to purify natural gas from its hydrogensulfide constituents is chemical reaction over a solid surface. Theremoval of H2S molecules from natural gas, to obtain low concen-trations, is mainly conducted utilizing solid adsorbents at hightemperatures, ranging from 200 to 800 �C. Copper and zinc, withtheir superior properties, are classified as the most favorable met-als for the removal of H2S. The pure metal oxides used as sorbents,however, suffer from evaporation, loss in the surface area andporosity, sintering and mechanical disintegration that affect theirperformance and life time adversely. With the purpose of overcom-ing this problem and to improve their performance, metal oxidesare loaded to the inert support materials with high surface area.In the literature it has been widely reported the use of metal oxi-des, such as copper, iron, aluminum, zinc, cobalt and manganesesupported over, alumina [8], clays [9], carbons [10], and zeolites[11,12] among others. Additionally, mixed metal oxides, such asM–Zn–Ti–O (M = Mn, Cu, Mo) [13], Fe–Mn–Zn–Ti–O [14], Co–Zn–Al–O [15], Fe–Sb–O and Fe–Sn–O [16] have been studied as H2S

Table 1Chemical composition (wt.%) of the starting mesoporous silica and its derived copperor zinc samples.

Sample SiO2 CuO (Cu) ZnO (Zn) Cl

MSU-1 100 – – –10Cu/MSU-1 87.7 12.3 (9.8) – –20Cu/MSU-1 74.4 25.6 (20.4) – –30Cu/MSU-1 63.5 36.5 (29.2) – –10Zn/MSU-1 88.2 – 11.8 (9.5) –20Zn/MSU-1 70.4 – 24.0 (19.3) 5.630Zn/MSU-1 57.5 – 36.7 (29.5) 5.8

Fig. 1. XRD pattern at low angle for MSU-1 silica molecular sieve obtained fromtetraethylorhtosilicate and Brij 56, calcined at 600 �C.

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adsorbent materials. The adsorption capacity is enhanced whendeposited on a suitable support and changed significantly withtheir composition.

The sorbents are composite materials that consist of an activemetal oxide and an inert support. The main properties requiredfor support materials are inertness, high surface area, large poresand good mechanical strength. Mesoporous silica molecular sieveis an interesting support material for metal oxides and metals,due to a combination of good accessibility, uniform pore size and

Fig. 2. XRD patterns for (a) calcined MSU-1, (b) 10Cu/MSU-1

high surface area [17,18]. This material is obtained through tem-plate synthesis, using a mechanism similar to that employed forthe synthesis of M41S materials [19]. Therefore, the objective ofthis work was to test the performance of mesoporous silica(MSU-1) impregnated with high loadings of ZnO or CuO as scav-engers of hydrogen sulfide at room temperature.

2. Experimental

2.1. Materials

Zinc chloride (Merck, 98%), brij 56 (Aldrich Chemical Company),tetraethylorhtosilicate (TEOS, Acros Organic), cupric oxide (J.T. Ba-ker, 99.98%) and zinc oxide (Merck) are of analytical grade, whichwere purchased and used without further purification. One hun-dred milliliters of cupric nitrate 0.48 M was prepared from 6.02 gof cupric carbonate (Baker Analyzed Reagent) and 6.7 ml of nitricacid (65%, Fluka) to obtain the precursor solution.

2.2. Synthesis of mesoporous silica molecular sieve (MSU-1)

The mesoporous silica was prepared by mixing 20.4 g of surfac-tant brij 56 with 300 ml of de-ionized water. The suspension wasvigorously stirred for 4 h to homogenize the solution; afterwards67.8 ml of TEOS was added at room temperature, under stirringfor 24 h. The assembly process is carried out at a molar ratioTEOS/surfactant of 10. Then, the suspension was separated by fil-tration, washed with abundant de-ionized water to remove any ex-cess of surfactant. The obtained solid was dried at 75 �C for 12 hand calcined at 600 �C (1 �C min�1) for 20 h, to remove the surfac-tant and produce the mesoporous material.

2.3. Scavengers preparation

CuO and ZnO supported on MSU-1 were prepared by wetimpregnation of aqueous solutions of Cu(NO3)2 or ZnCl2. The cop-per or zinc contents were 10, 20 and 30 wt.%. The prepared solidswere ground, dried at 120 �C overnight and calcined at 450 �C for4 h. The solids derived from the starting MSU-1 were identifiedaccording to the Cu or Zn content, for example, the solid 10Zn/MSU-1 indicates that the mesoporous silica support was

, (c) 20Cu/MSU-1, (d) 30Cu/MSU-1 and (e) copper oxide.

Fig. 3. XRD patterns for (a) zinc oxide, (b) 10Zn/MSU-1, (c) 20Zn/MSU-1 and (d) 30Zn/MSU-1.

Fig. 4. Nitrogen adsorption–desorption isotherms of MSU-1 and its derived coppersamples.

Fig. 5. Nitrogen adsorption–desorption isotherms of MSU-1 and its derived zincsamples.

Table 2Structural parameters calculated from nitrogen adsorption isotherms.

Sample SBET (m2 g�1) Vt (cm3 g�1)

100MSU-1 818 0.4910Cu/MSU-1 467 0.3120Cu/MSU-1 323 0.2330Cu/MSU-1 179 0.1410Zn/MSU-1 263 0.3120Zn/MSU-1 18 0.1230Zn/MSU-1 11 0.05

SBET, specific surface area.Vt, total pore volume.

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impregnated with 10 wt.% of zinc. Pure CuO and ZnO were used asreference materials.

2.4. X-ray fluorescence (XRF)

The copper, zinc and silica contents of the prepared sampleswere measured by XRF, utilizing a Shimadzu EDX-700 HS, operat-ing at 50 kV and 30 mA. Each sample was ground, placed into asample crucible using polypropylene paper of 6 lm, and analyzedby the powder method in vaccum.

2.5. X-ray diffraction (XRD)

The crystalline structure of the starting MSU-1 solid and its der-ivate impregnated with copper and zinc was investigated by XRD,utilizing a Bruker D8 Focus diffractometer, operating at 40 kW and40 mA at a scan rate of 2 min�1, using Cu Ka radiation

(k = 1.5418 Å). The crystalline phases were identified employingthe tabulated powder diffraction files of the International Centerof Diffraction Data (ICDD) d spacing files.

Fig. 6. Pore size distribution of MSU-1 derived (a) copper samples and (b) zinc samples.

Fig. 7. TPR profiles of MSU-1, it derived copper samples and copper oxide.

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2.6. Nitrogen adsorption

The textural characterization of the metal impregnated poroussilica was based on the nitrogen adsorption isotherms at�196 �C, conducted on a Micromeritics ASAP 2010 apparatus. Sam-ples were outgassed at 300 �C and 0.13 Pa for 2 h before the mea-surements. BET surface area (SBET) was calculated from the linearportion of the BET plots. The total pore volume (Vt) was determinedby the single point method at a relative pressure close to unity andthe average pore size distribution was calculated by BJH method inthe adsorption branch.

2.7. Temperature programmed reduction (TPR)

The TPR measurements were carried out in a stainless steelreaction line coupled to a TCD detector. Fifty milligrams of unre-duced sample were placed on a U-shaped quartz reactor andheated up to 120 �C in Ar at a flow rate of 30 cm3 min�1

(99.9999%) for 1 h and then cooled down to room temperature.Then, the flow rate was changed to 30 cm3 min�1 of H2 (5.225%)/Ar. The temperature was increased from room temperature up to700 �C, using a heating rate of 10 �C min�1. The water desorbedduring the reduction treatment was collected in a molecular sievetrap at the reactor exit.

2.8. Scanning electron microscopy (SEM)

The morphology and particle size of ZnO or CuO over preparedsamples were observed using a microscope FEI, model Quanta200F. The instrument utilizes an electron gun by field emissionthat reaches 30,000 V. Samples were analyzed in vacuum(130 Pa) and 30 kV. Micrographs were obtained at 2000 and16,000� of magnification using backscattered electrons.

2.9. H2S breakthrough capacity

Dynamic experiments were carried out in a stainless steelreaction line, at room temperature to evaluate the capacity of theprepared materials for H2S removal. The reactor consisted of aU-shaped Pyrex tube of 6 mm o.d. and 4 mm i.d. The reactor wasloaded with 200 mg of sample previously calcined. A mixture ofH2S/CH4 (5% H2S) was passed through the sample at a flow rateof 5 cm3 min�1, and at outlet of the reactor the gas mixture was di-luted (100�) with air before entering to the H2S sensor. Thus, thespace velocity (GHSV) of the test was equal to 586 h�1. The break-through concentration of H2S was monitored using an Interscan

(continuous monitoring system) LD-17 sensor. The experimentwas stopped at a breakthrough concentration of 100 ppm. The re-moval capacity of each solid, based on mg of H2S per gram of mate-rial (X/M), was calculated by integration of the area above thecurve, and from the H2S concentration in the inlet gas, the flowrate, the breakthrough time and the mass of solid, using Eq. (1)

X=M ¼ ðQ �MW=W � VMÞ C0 � TS �Z TS

0CðtÞdt

� �ð1Þ

where Q is the inlet flow (m3 s�1), MW is the molecular weight ofH2S (34.06 g mol�1), W is the mass of solid (g), VM is the molargas volume (22.4 cm3 mmol�1), C0 is the inlet concentration ofH2S (ppm), C(t), is the outlet concentration and TS is the saturationtime at 100 ppm of H2S.

For each sample, the H2S test was repeated at least twice. Thedetermined capacities agreed to within 5%.

3. Results and discussion

3.1. X-ray fluorescence

The chemical compositions of the prepared support (MSU-1)and its derived samples impregnated with different contents ofcopper or zinc, determined by XRF, are given in Table 1. The start-ing MSU-1 is mainly composed of SiO2, and after Cu or Zn incor-poration, it is confirmed that the metal contents correlate well

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with the expected values. Samples impregnated with high load-ings of ZnCl2 show chlorine values around 5.6%, which denotesthat during calcination treatment a complete removal of this ele-ment was not achieved. pH measurements indicated that residualchlorine did not affect the surface pH values of the starting mate-rial after zinc impregnation; these values were: 6.0 for MSU-1,and 6.1, 6.2 and 6.3 for 10, 20 and 30 wt.% Zn supported onMSU-1.

3.2. X-ray diffraction

Fig. 1 illustrates the powder diffraction pattern, in the small-an-gle range, of calcined MSU-1. This sample displays an intensereflection at low 2h corresponding to a pore–pore correlation dis-tance of 4.9 nm, typically associated to disordered wormhole-likepore structures, similar to that reported for MSU-X, prepared withthe same surfactant but with a sodium silicate solution as silicaprecursor [17]. The sample, presented in the wide-angle range abroad band in the region of 15–30� (Fig. 2a) attributed to amor-phous silica [20]. The XRD pattern of pure cupric oxide (Fig. 2e)

Fig. 8. Low magnification SEM images of (a) 10Cu/MSU-1, (b) 10Zn/MSU-1,

shows diffraction peaks at 35�, 39�, 49�, 53�, 58�, 61�, 66� and68�, which correspond to crystallographic planes (�111), (111),(�202), (020), (202), (�113), (022) and (220) of CuO specieswith a monoclinic crystalline structure, whose cell parametersare: a = 4,653 Å, b = 3,410 Å, c = 5,108 Å and a = 90�, b = 99,48�and c = 90�. Additionally, in the diffraction pattern others peaksat 29�, 36� and 42� are observed, indexed as (110), (111) and(200) of Cu2O species. The XRD patterns of MSU-1 impregnatedwith 10, 20 and 30 wt.% of copper (Fig. 2b–d) present an increasein peaks intensity as metal loading is intensified, showing evidenceof a particle size growth over MSU-1 support. The XRD pattern ofpure ZnO (Fig. 3a) shows diffraction peaks at 32�, 35�, 36�, 48�,57�, 63�, 66�, 67� and 69�, that are associated to the crystallo-graphic planes (100), (002), (101), (102), (110), (103), (200),(112) and (201) of ZnO with a hexagonal crystal system and cellparameters: a = b = 3,242, c = 5,176, and a = b = 90 and c = 120.Samples 10Zn/MSU-1 and 20Zn/MSU-1 do not present significantdiffraction signals of ZnO (Fig. 3b and c). This result suggests thatthe metal oxide is in amorphous state, in harmony with theresults reported by Wang et al. [21]. The diffraction pattern of

(c) 20Cu/MSU-1, (d) 20Zn/MSU-1, (e) 30Cu/MSU-1 and (f) 30Zn/MSU-1.

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30Zn/MSU-1 (Fig. 3d) exhibits very low intensity peaks at 32�, 35�and 36�, corresponding to planes (100), (002), (101) of ZnO.

3.3. Nitrogen adsorption isotherms

The textural characteristics of the silica based materials and itscopper or zinc impregnated samples were determined based onnitrogen adsorption and desorption measurements, as shown inFigs. 4 and 5. The adsorption/desorption isotherm for the MSU-1material before CuO or ZnO impregnation, showed the typical typeIV isotherm with a negligible hysteresis (H2 type) at lower relativepressure (0.35–0.50). This type IV isotherm is characteristic ofmesoporous material defined in IUPAC [22] and it provides usefulinformation about the mesopore structure [22,23]. According tothe IUPAC classification of adsorption hysteresis, type H2 is ob-served in the case of mesoporous solids consisting of particlescrossed by nearly cylindrical channels or made by aggregates (con-solidated) or agglomerates (unconsolidated) of spherical particlesof nonuniform size and shape that have certain geometry, forexample, (a) a tubular capillary opening at both ends, (b) tubular

Fig. 9. High magnification SEM images of (a) 10Cu/MSU-1, (b) 10Zn/MSU-1,

capillaries with cross sections of two different main dimensions,and (c) a narrow necked ink bottle. A dramatic decrease in theadsorption–desorption isotherms of MSU-1 was observed whenCuO or ZnO loadings are increased. After copper or zinc impregna-tion the isotherms change in form, which might be due to a partialblockage of the pores by CuO or ZnO [24], or by a partial collapse ofthe mesoporous structure [25]. However for low loading of Zn aclear increase in the hysteresis loop is observed and can be as-cribed to the open up of pores.

From the adsorption isotherms of the mesoporous silica and theCuO and ZnO supported adsorbents, the specific surface area andpore volumes were obtained, as illustrated in Table 2. The surfacearea of MSU-1 support decreased considerably after copper or zincimpregnation (42%, 60% and 78% for 10, 20 and 30 wt.% of copperand 67%, 98% and 99% for 10, 20 and 30 wt.% of zinc), due to thereason previously mentioned. This indicates that at 20 wt.% of cop-per or 10 wt.% of zinc loading, MSU-1 almost collapses due to anincrease in nanoparticle size that plugs (partially or totally) thepores entrance, a result that it is in agreement with the informa-tion obtained by X-ray diffraction.

(c) 20Cu/MSU-1, (d) 20Zn/MSU-1, (e) 30Cu/MSU-1 and (f) 30Zn/MSU-1.

Fig. 10. H2S breakthrough curved curves for (a) MSU-1 copper samples and (b) MSU-1 zinc samples.

D. Montes et al. / Microporous and Mesoporous Materials 168 (2013) 111–120 117

The pore size distribution (PSD) profiles of MSU-1 and its de-rived copper or zinc samples are illustrated in Fig. 6. MSU-1 sup-port shows its characteristic shape with both micropores andmesopores system; after copper incorporation, the PSD does notchange significantly at low metal loading, however the microporesand mesopores are reduced as a consequence of pore blocking. Forthe case of zinc impregnated samples, a slightly large pore width isobserved in 10Zn/MSU, which might be due to partial collapse ofthe wormhole framework pore structure of MSU support by metalloading [25]. A straight line was obtained in the pore size distribu-tion for samples impregnated with 20 and 30 wt.% of zinc that asmentioned earlier, there is an increase in the nanoparticle size thatplug the pore mouth.

3.4. Temperature programmed reduction

The TPR profile of reference CuO and MSU-1 samples impreg-nated with copper are shown in Fig. 7. Two peaks are observed,the first one at 250 �C corresponds to the Cu2+ to Cu+ reduction,and the second one at 350 �C indicates the complete reduction ofCu+ to Cu0 [26]. The reduction profiles demonstrate that copperis totally reduced at a temperature of 380 �C for all Cu/MSU-1samples. Additionally, an increment in the reduction signal is no-ticed as the metal loading is intensified, in line with XRD resultspreviously discussed. For Zn/MSU-1 samples, no reduction peakswere detected (not shown) due to the fact that ZnO cannot bereduced below 630 �C [27]. The absence of peaks at tempera-tures higher than 450 �C indicates a lack of metal-supportinteractions.

3.5. Scanning electron microscopy (SEM)

The morphology and size of CuO or ZnO nanoparticles overMSU-1 support are shown in Figs. 8 and 9, at low and high magni-fication. From the micrographs, it is clear that nanoparticle size in-creases as the offered CuO or ZnO content is raised, in line withresults obtained by XRD and TPR. Well-dispersed nanoparticles ofCuO and ZnO are observed at low loading of 10 wt.% (Fig. 8a andb). For this loading, ZnO nanoparticles are scarcely noted in themicrograph, thus indicating that they are better dispersed thanCuO counterparts, therefore 10Zn/MSU-1 sample offers more ex-posed active metal oxide area than that of 10Cu/MSU-1. The dis-persion is reduced as the metal oxide content becomes higher,showing large clusters that had formed by agglomeration of theindividual nanoparticles (Fig. 8c–f). From high magnificationimages (Fig. 9), it is noticed that well-defined monoclinic crystalsare formed in CuO supported samples, while in the case of ZnOsupported ones no crystalline species are detected, suggesting thatZnO is in amorphous state, in accordance with XRD results.

3.6. Desulphurization activity

Both pore structure and surface chemistry contribute to theadsorption of hydrogen sulfide [9,25]. A mesoporous silica molec-ular sieve (MSU-1) and its derived samples impregnated with 10,20 and 30 wt.% of copper or zinc were used to establish and opti-mize values for this application. The breakthrough curves obtainedin our experiments (at room temperature, 1 atm and a GHSV of586 h�1), are presented in Fig. 10. The H2S breakthrough capacitiesare summarized in Table 3. No removal of H2S was detected for the

Table 3H2S breakthrough capacity results.

Sample bth time (min) bth capacity (mg g�1)

10Cu/MSU-1 5.7 10.920Cu/MSU-1 10.1 19.230Cu/MSU-1 10.2 19.4100CuO 8.7 16.510Zn/MSU-1 22.3 42.320Zn/MSU-1 7.8 14.830Zn/MSU-1 6.0 11.5100ZnO 11.1 21.1

bth, Breakthrough.

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mesoporous silica material, which obviously indicates the lack ofany chemical or catalytic effect of the surface. However, CuO orZnO incorporation over MSU-1 support considerably improvesthe capacity for H2S adsorption. The addition of copper or zinc in-creases the removal performance and the most active adsorbentswere 10Zn/MSU-1 and 20Cu/MSU-1. This is due to a good disper-sion of zinc or copper nanoparticles over MSU-1 support [24,25].A decrease in H2S removal was observed at a zinc loading higherthan 10 wt.% and a copper loading higher than 20 wt.%. This is as-cribed to the formation of a comparatively less reactive aggregateform of copper or zinc, which decreases both the uniform disper-sion of these metal oxides on the support and the reduction inthe specific area and pore volume caused by different factors.Among these can be cited the following: pore blockage, damageand possible collapse of the pores, as well as the formation of largeagglomerates of ZnO or CuO. These aspects were discussed in theprevious section and shown in Table 2 and XRD, TPR and SEM re-sults. These data are in accord with those reported by Hussainet al. [25] who prepared samples with different ZnO loadings (10,15 and 20 wt.%) over MCM-41, KIT-6, SBA-15S and SBA-15F andfound that samples with 15 wt.% of ZnO were the most active forthe sulfurization process and attributed this behavior to the smal-ler size of the ZnO nanoparticles in dispersed form; samples withhigher ZnO loading showed a reduction in adsorption capacity be-cause of the increased nanoparticle size.

According to these results, micropores, mesopores, good disper-sion and small metal oxide nanoparticles are key factors that di-rectly influence the H2S adsorption process. Moreover, due tothese large agglomerates, most of the ZnO or CuO, at high loadingswas blocked in the pore system or trapped on the surface. Anotherfactor to be considered is that in large metal oxide nanoparticles

Fig. 11. Schematic representation of the reaction process. (left) lo

only the external surface is available to react with H2S, leavingthe inner part of the nanoparticle intact and there are also somemetal oxide nanoparticles inside the pore structure that are notaccessible to H2S molecules due to the blockage of pore mouthsby big metal oxide nanoparticles (Fig. 11). Therefore, the reactionis controlled by the diffusion of H2S into the support pore struc-ture. To further support this statement and to understand theadsorption mechanism of H2S over MSU-1 copper or zinc impreg-nated samples, the percentage of metal (from elemental analysis)was compared with the amount of H2S adsorbed on each sample(obtained from the breakthrough capacity). The results are pre-sented in Table 4. It is interesting to emphasize that for 10Zn/MSU-1 the number of moles of metal and hydrogen sulfide are al-most identical, which suggests a direct chemical reaction betweenzinc and H2S, and the amount of hydrogen sulfide decreases as thezinc content is increased. For the case of Cu/MSU-1 samples, thebreakthrough capacity increases as the copper content is higherand reaches a constant value at 20 wt.% of copper. Fig. 12 showssulfurization degree, expressed as molar percentage of active metaloxide converted to metal sulfur. Sample 10Zn/MSU-1 has the high-est percentage of metal content that reacts with hydrogen sulfide(85.8%) following the order: 10Zn/MSU-1� 20Zn/MSU-1� 30Zn/MSU-1� ZnO, while the order for copper samples followed thesame trend as those of zinc. This observation is in line with the dis-cussion mentioned above, which indicates that as the metal con-tent increases the active metal surface decreases, leaving onlyavailable the external surface of the nanoparticle, with the con-comitant drop in the sulfurization degree.

The reaction mechanism can be explained based on acid–basereaction, both Cu2+ and Zn2+ are moderate acids (which means thatthey are halfway between hard and soft acid) [28] while sulfur is asoft base [29], therefore the formation of CuS and ZnS is very favor-able. Moreover, ZnO has a slow diffusion from pore closure andforms a dense sulfide layer [30], while copper based sorbents havefavorable equilibrium between copper oxides and H2S; copperoxide is able to achieve low levels of H2S in the clean fuel gas pro-vided that the active phase is not reduced to elemental copper [31].The obtained results indicate that for the lowest loading of 10 wt.%,ZnO was more active than CuO. This finding can be explained bySEM results that evidenced a better dispersion of ZnO nanoparti-cles than those of CuO, allowing a higher exposed surface area ofZnO to react with H2S.

Different sulfur capacities are reported for copper or zincsupported on various sorbents in the literature. For comparative

w metal oxide content and (right) high metal oxide content.

Table 4Moles of metals impregnated on the surface and moles of H2S adsorbed.

Sample Me (mmol g�1) H2S (mmol g�1)

10Cu/MSU-1 1.55 0.3220Cu/MSU-1 3.23 0.5630Cu/MSU-1 4.59 0.5710Zn/MSU-1 1.45 1.2420Zn/MSU-1 2.95 0.4430Zn/MSU-1 4.51 0.34

Fig. 12. Sulfurization degree, expressed as molar percentage of active metal oxideconverted to metal sulfur. (a) MSU-1 copper samples and (b) MSU-1 zinc samples.

Table 5Sulfur capacity sorbents investigated for H2S from gases cited in the literature.

Sorbent H2Sin

(ppmv)H2Sout

(ppmv)T(�C)

P(atm)

Capacity(mg g�1)

Reference

20Cu/MSU-1 50000 10000 25 1 19.2 Montes et al.10Zn/MSU-1 50000 10000 25 1 42.3 Montes et al.Zn/Al-M 3000 1000 25 1 20.9 [9]Cu/Al-M 3000 1000 25 1 48.8 [9]9Zn/SBA-15 1000 50 25 1 38.9 [32]15Zn/STxM 3000 1000 25 1 17.8 [33]15Cu/ST3M 3000 1000 25 1 6.0 [34]15Zn/MCM-41 200 20 25 1 10.2 [25]15Zn/SBA-15S 200 20 25 1 22.4 [25]15Zn/SBA-15F 200 20 25 1 23.2 [25]20Zn/KIT-6 200 20 25 1 9.66 [25]

Al-M, Aluminum pillared montmotillonite; STxM, surfactant modified clay (STx-1);surfactant modified synthetic clay (ST-3); (MCM-41, SBA-15S, SBA-15F and KIT-6)mesoporous silica support materials.

D. Montes et al. / Microporous and Mesoporous Materials 168 (2013) 111–120 119

purposes, the data cited in the literature on investigated sorbentsfor the removal of hydrogen sulfide at 25 �C and 1 atm of pressureare briefly compiled and compared with the results for the mostactive solids found in this study (Table 5). This table contains onlythe most active samples for the adsorption of H2S reported in thecited works. As can be seen in the table, the adsorption capacityof 10Zn/MSU-1 and 20Cu/MSU-1 samples are comparable andsuperior to those samples reported in the literature for similarexperimental conditions. Nevertheless, Polychronopoulou et al.[13,14] studied the efficiency of various M–Zn–Ti–O (M = Mn, Cu,Mo) and Fe–Mn–Zn–Ti–O mixed metal oxides prepared by thesol–gel method towards H2S adsorption from a gas mixture con-taining 0.06 vol.% H2S, 25 vol.% H2, 7.5 vol.% O2 and 1 vol.% H2O inthe temperature range of 25–100 �C. They found that samples10Mn–45Zn–45Ti–O and 5Fe–15Mn–40Zn–40Ti–O were the mostactive for the adsorption process (41 and 75 mg H2S g�1 at25 �C). The former value is slightly similar to that found for themost active sample in our work, but the latter is superior. Theyattributed this performance to the specific surface area of the sol-ids. Additionally, it has been reported that MnO does not decom-pose to elemental manganese readily in reducing gasenvironment as ZnO [31], therefore, one would expect that

Mn–Zn–Ti–O mixed oxide to be more active for the H2S adsorptionthan ZnO/MSU-1. The adsorption performance of the best samplesreported in this work can be attributed to the high surface area ofthe prepared MSU-1 support that allow a good dispersion of metaloxide nanoparticles at low content of zinc and intermediate con-tent of copper, increasing the percentage of active sites to reactwith H2S.

4. Conclusions

The synthesized mesoporous silica material (MSU-1) presenteda high surface area and pore volume. The incorporation of CuO orZnO over the mesoporous silica support resulted in a reductionof the textural properties and in an enhancement in the capacityto remove H2S from natural gas. The best performing materialswere those impregnated with 10 wt.% of Zn and 20 wt.% of Cu,which is due to best dispersion on the surface and lower poreblockage. The adsorption capacity of the prepared materialsstrongly depends on the micropores/mesopores system as well asthe good distribution of the active phase inside the porous materialand the size of metal oxide nanoparticle.

Acknowledgments

This research was supported by the Fondo Nacional de Ciencia yTecnología (FONACIT, Venezuela), Project G-2005000433. Authorswish to thank professor Milagros Alburguez for English assistanceand to MSc. Ricardo Morales for SEM micrographs. Helpful sugges-tions from professor Jorge Sanchéz are greatly appreciated.

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