A comparative study on H2S removal using Mg–Al spinel(MgAl2O4)and MgO/Al2O3 nanocomposites☆

S.M.Latifi*,J.Bakhshi Azghandi,A.Salehirad M.Parvini

1 Department of Chemical Technology,Iranian Research Organization for Science&Technology,Tehran,Iran

2 Chemical Engineering,Oil&Gas,Semnan University,Semnan,Iran

1.Introduction

Hydrogen sul fide(H2S)is found in such industries as natural gas processing,petrochemicals,coal derived gaseous production,biogas and wastewater treatment[1–4].H2S is highly toxic and corrosive[5–8]and its allowable concentration for combined cycle power plants and reducing gas are 500 and 50 μl·L-1,respectively[9].Moreover,even trace amountofH2S can be poisoning for the chemicalprocess catalysts that are mostly composed of precious metals[10].Alumina(Al2O3),as a known adsorbent,and its composites with other metal oxides have been used for the removal of H2S.Tajizadeganet al.[11]precipitated ZnO nanosheets on alumina heterogeneously,using bayerite seed particles and used the obtained composite for H2S removal of a stream with about 1 wt%H2S at 150°C.Atakulet al.[9]prepared MnO2/γAl2O3viawet impregnation for H2S uptake from fuel gas at high temperatures.Also Lianget al.synthesized Mn/γAl2O3through wet impregnation and investigated its capability and regenerability for H2S removal in high temperatures[12].Koet al.[6]synthesized various weight percents of MO/Al2O3sorbents,in which MO included Co3O4,Fe2O3,CuO,Mn2O3,CeO2and ZnO,through incipient wet impregnation route and used them for hydrogen sul fide removal from syngas at high temperatures(500–700 °C).Their results revealed that among the used metal oxides,manganese oxide presented the best performance in hydrogen sul fide sorption.Tahmankaret al.[13]fabricated mixed oxides including CuO–Al2O3(1:1 molar ratio)and CuO–Fe2O3–Al2O3(2:1:1 molar ratio)for H2S removal at high temperatures.The synthesized porous sorbents with very high pore volume showed improved performance compared to conventional H2S sorbents.Chytilet al.[14]fabricated MnxOy/γAl2O3,within 15–30 wt%Mn loading range,by wet impregnation method as sorbent for H2S uptake from a simulated dry producer gas stream(including 0.4 vol%H2S in 40 vol%H2).They used 10 vol%O2in N2for regeneration of consumed sorbents at 450°C.Chenetal.[15]prepared P-promoted aluminaviaintroducing phosphorous species by using gel-coprecipitation technique for catalyzing H2S reaction with dimethyl sul fide.Kanget al.[16]investigated the Cu–Zn–Al2O3sorbents prepared through precipitation of Cu and Zn on phosphorous-modified and unmodified alumina and found that the modified sorbents exhibited better capability in H2S removal.Junget al.[17]synthesized ZnO/Al2O3sorbent by using coprecipitation method for simultaneous removal of H2S and NH3from coal based synthesis gas and found out that the addition of such metals as cobalt,iron and nickel can promote the sorbent performance in H2S uptake.Dobrovolszkyet al.[18]prepared MoOx/Al2O3catalysts promoted by Co,Pt,Ir,Ru and Pd for the removal of radioactive H2S from H2stream.Their results revealed that only the catalysts promoted by Co and Ir exhibited more cumulative H2S uptake than MoOx/Al2O3alone.

Literature survey indicates that few works have been done on the use of MgO/Al2O3composites or spinel for hydrogen sul fide sorption.In this work the H2S removal potential of the Mg–Al spinel in comparison with the MgO/Al2O3nanocomposites containing various Mg additions was investigated.

2.Experimental

2.1.Materials

Chemical materials used in the fabrication of alumina and MgO/Al2O3composites including Al(NO3)3·9H2O,Mg(NO3)2·6H2O,NH4OH,(NH4)2CO3,Na2CO3,CH4NO2(urea)and C6H8O7(citric acid)were provided from LOBA Chem and used without further purification.

2.2.Synthesis

Alumina sorbents were prepared through three routes namely,sol–gel,precipitation by sodium carbonate and ammonium carbonate.In the precipitation by ammonium/sodium carbonate,at first,an aqueous solution of ammonium/sodium carbonate was added to an aqueous solution of Al(NO3)3·9H2O at pH fixed around 9.The obtained mixture was stirred at 70°C for 3 h and then filtered,washed with deionized water and alcohol and dried at 120°C for 3 h.At last,the obtained powder was calcined at 550 °C for 3 h with the rate of 10 °C·min?1.In sol–gel route Al(NO3)3·9H2O was added to a citrate solution drop-wisely at 90 °C and stirred for 8 h.The product was dried at 200 °C for 3 h and finally calcined at 800°C for 5 h.

To synthesize MgO, firstly,an aqueous solution of urea(2.53 g in 100 ml deionized water)was stirred at 80°C for 15 min and then,an aqueous solution of Mg(NO3)2·6H2O(6.36 g in 100 ml deionized water)was added dropwise to the urea solution.The resulted mixture was stirred at 80°C for 8 h and after that,the formed precipitant was filtered and washed with water and ethanol and then dried at 120 °C for 3 h.Finally,the obtained powder was calcined at 450 °C for 5 h.

MgO/Al2O3nanocomposites with various mass ratios(5 wt%MgO/95 wt%Al2O3,10 wt%MgO/90 wt%Al2O3and 25 wt%MgO/75 wt%Al2O3(presented by 5-Mg–Al,10-Mg–Al and 25-Mg–Al,respectively)were synthesizedviaprecipitation–impregnation route.Determined amounts of Al(NO3)3·9H2O were dissolved in deionized water and then NH4OH aqueous solution was added to the solutions drop-wisely untilthe precipitantstarted to form.The obtained mixtures were stirred at 70°C for 30 min and after that,appropriate amounts of the synthesized MgO were added to the solutions and the resultants were stirred for 3 h.The resulted mixtures were filtered,washed with deionized water and ethanol,dried at 120°C for 24 h and finally calcined at 550 °C for 5 h with the rate of 10 °C·min?1.

The Mg–Al spinel was fabricated by ion-pair complex precursor route described in detail elsewhere[19].

2.3.Characterization

X-ray diffraction(XRD)was used for studying the crystalline structures of the Al2O3,MgO and MgO/Al2O3nanocomposites.XRD patterns were conducted on an Inel-3000 diffractometer using CuKα(λ =0.15418 nm)as incident radiation.The crystalline phases were identified by employing the tabulated powder diffraction files of the International Center of Diffraction Data(ICDD).To observe particle size distribution and textural morphology field emission scanning electron microscopy(FESEM,Model:Mira 3-XMU)was used.The surface area(BET)was measured by using an automated gas adsorption analyzer(Tristar 3000,Micromeritics).Temperature Programmed Desorption(CO2-TPD)was conducted by using an automatic apparatus(ChemBET-3000 TPR/TPD,Quantachrome)with thermal conductivity detector.In CO2-TPD experiments,after pretreatment at 300 C for 3 h in a He atmosphere,the samples were cooled down to ambient temperature,and then exposed in CO2for 30 min.CO2-TPD was performed with the rate of 10 °C·min?1from ambient temperature under He stream.

Fig.1.Experimental setup for H2S removal investigation.

2.4.H2S removal System

H2S removal took place in a thermo well-equipped stainless steel column with an inner diameter of 2 mm.Heating power was supplied by an electrical heater coiled around the outer surface of the column and the inside temperature was controlled using a PID controller.Feed streams including 200 μl·L-1H2S in N2and pure isobutane with the flowrates of 105 ml·min?1,adjusted by a rotameter,were sent to a static mixer to become homogeneous before entering sorption column.Each run was performed for nanocomposite,spinel and alumina samples by using 0.25 g sorbents,respectively.The temperature and pressure atsorption column were keptat313 Kand 0.1 MPa,respectively.H2S concentration atthe column outletwas measured by an electrochemical sensor-based device.Fig.1 demonstrates a scheme for H2S removal system.

3.Results and Discussion

3.1.Samples characterization

(at2θ=43°,62°and 79°according to JCPDS No.2-1092)are observable for 10-Mg–Al and 25-Mg–Al in Fig.2.It is obvious in Fig.2 that by increasing MgO content in the nanocomposites the intensity of MgO-related peaks increases.From line broadening of re flections and using the Scherrer equation the average crystallite size of MgO in 10-Mg–Al and 25-Mg–Al was calculated as 36.12 and 48.17 nm,respectively.

The XRD pattern for the spinel is depicted in Fig.3[19].As can be seen in Fig.3,all the peaks are related to single phase MgAl2O4spinel with the crystallite size of 8.6 nm.

Fig.4 shows the SEM images of MgO/Al2O3nanocomposites.As can be seen in Fig.4 the particle size distributionsin 5-Mg–Al,10-Mg–Aland 25-Mg–Al are in the ranges of 20–50,30–60,50–70 nm,respectively.This observing indicates the fact that with increasing the MgO content in the nanocomposites,the particle size and agglomeration degree increase.These findings are in agreement with XRD results that showed MgO crystallite size for 10-Mg–Al is lower than 25-Mg–Al.The SEM images exhibits the presence alumina spherical nanoparticle in the composite samples and also con firms the existence of MgO nano flakes in 10-Mg–Al and 25-Mg–Al that has not been observed in 5-Mg–Al due to low content of MgO.It should be noted that synthesis of MgO in the form of nano flakes has been reported in other works[20–22].

Comparison of the SEM results for Mg–Al spinel[19]and the nanocomposite samples indicates narrower particle size distribution,smaller particle size and lower degree of agglomeration for the spinel sample.

The XRDpatterns forthe MgO(5–25 wt%)/Al2O3nanocomposites are shown in Fig.2.As expected,due to low content of MgO in 5-Mg–Al the re flections attributed to MgO are notdetectable,while these diffractions

Fig.2.XRD patterns of MgO/Al2O3 nanocomposites.

3.2.H2S removal

Fig.5 presents the breakthrough curves of H2S removal,as an important measure of sorbent performance,for the synthesized alumina sorbents.Breakthrough curve presents the ratio of outlet H2S concentration to inlet concentration versus time and the time outlet concentration started to rise is called breakthrough time that has a direct relationship with the sorbent capacity.As can be seen in Fig.5 the alumina sorbent fabricated from precipitation with ammonium carbonate exhibited slightly better performance in H2S removal than the other alumina samples.

Fig.3.XRD pattern of Mg–Al spinel[20].

As Fig.6 depicts,the capability of the MgO/Al2O3nanocomposites in H2S removal is significantly greater than the alumina sorbents as according to the breakthrough times(Figs.5 and 6)after MgO addition to the alumina,the H2S sorption capacity increases more than 10 times.This substantial augment in H2S removal is due to MgO basic property that provides more efficient active sites for interaction with acidic H2S[23].Also it can be seen in Fig.6 that by increasing the MgO addition from 5 to 10 wt%the breakthrough time enhances about 20%,while more increasing in MgO content(to 25 wt%)results in almost the same H2S uptake.The lowest sorption capacity of 5-Mg–Al is explained by the lowest MgO content in this nanocomposite.However,unchanging the sorption capacity with increasing MgO addition from 10 to 25 wt%can be justified by XRDand SEMresults.As XRD and SEMresults revealed before,increasing the MgO content from 10 wt%to 25 wt%led to increase in crystallite size and degree of agglomeration that may reduce the accessibility to the MgO sorption active sites.

The results of H2S removal on Mg–Al spinel sorbent shown in Fig.6 indicate the superior capability of the Mg–Alspinel relative to the MgO/Al2O3nanocomposite samples,as approximately the H2S sorption ofthe Mg–Al spinel is more than severaltimes greater than those of the nanocomposites.To explain this observation,it should be mentioned that Mg–Al is a single phase in which Mg2+and Al3+coexist in one crystal lattice that supplies much more available active sites for interaction with H2S.Furthermore,the crystallite size and degree of agglomeration in the Mg–Alspinelare very low[19]thatgenerate large surface area for the H2S sorption.To investigate the basic sites in spinel and nanocomposite the TPD pro file of CO2by the spinel and 25-Mg–Al which has the closest Mg contentto the spinelamong the experimented nanocomposites,was shown in Fig.7.The appearance of desorption peaks in the experimented temperature range reveals the presence of strong basic sites for both the nanocomposite and the spinel,though desorption of CO2at higher temperatures indicates stronger basic sites[24]for the nanocomposite.On the other hand,the number of basic sites(or basic site density)has a direct relationship with the peak area of CO2desorption[25]presented in Table 1 for both samples.It can be observed from Fig.7 and Table 1 that the peak area of CO2desorption for the spinel is higher than the nanocomposite that can be attributed to the nature of spinel discussed above.This superiority in the amount of basic sites along with the higher specific surface area(Table 1)can explain the higher capability of the spinel in H2S adsorption.

Fig.4.SEM images for(a)5-Mg–Al(b)10-Mg–Al(c)25-Mg–Al.

The shape of breakthrough curve is in fluenced by equilibrium and kinetics(mainly mass transfer)of the process and therefore,analyzing the breakthrough curve may give some ideas about the equilibrium and kinetic behavior of sorption process[26,27].It can be seen from Fig.6 that the breakthrough curves for Mg–Al spinel and MgO/Al2O3nanocomposites(especially 10-Mg–Al and 25-Mg–Al)are steep,suggesting non-controlling mass transfer and strong sorption affinity[8,28,29]between sorbent,resulted from the MgO existence,and H2S.

Fig.5.H2S breakthrough curves for the alumina samples at 40°C.

Fig.6.H2S breakthrough curves of the MgO/Al2O3 nanocomposites and Mg–Al spinel at 40 °C.

Fig.7.CO2-TPD pro files of 25-Mg–Al nanocomposite and Mg–Al spinel.

Table 1Basic and structural properties of spinel and 25-Mg–Al composite

4.Conclusions

The comparison between breakthrough curves obtained from H2S sorbents including,Mg–Al spinel,MgO/Al2O3nanocomposites and alumina indicated that MgO addition plays key role in H2S removal.Furthermore,the results showed that in MgO/Al2O3nanocomposites there is an optimum addition for MgO over which the capability of the nanocomposite sorbents decreases as a resultofnanoparticle agglomeration and increasing crystallite size.Distinct superiority on H2S uptake observed for Mg–Al spinel in comparison with the other studied sorbents was ascribed to more basic site density and also favorable structural characteristics.

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