Modeling Hydrogen Sulfide Adsorption on Chromium-Based MIL-101 Metal Organic Framework

Modeling Hydrogen Sulfide

Adsorption on the Chromium-Based

MIL-101 Metal Organic Framework

Antonio Pelusoa, Nicola Gargiuloa, Paolo Apreaa, Francesco

Pepeb, Domenico Caputoa,*

a Dipartimento di Ingegneria Chimica, dei Materiali e della

Produzione Industriale, Università Federico II, P.le

Tecchio 80, 80125 Napoli, Italy

b Dipartimento di Ingegneria, Università del Sannio, P.zza

Roma 21, 82100 Benevento, Italy

*Corresponding author (e-mail: [email protected];

address: Dipartimento di Ingegneria Chimica, dei Materiali

e della Produzione Industriale, Università Federico II,

P.le Tecchio 80, 80125 Napoli, Italy; telephone:

+390817682396; fax: +390817682394)

Abstract

In this work, hydrogen sulfide (H2S) adsorption on a

laboratory-synthesized polymeric chromium terephthalate

(MIL-101) metal-organic framework was modeled by means of

the semiempirical Sips equation in order to obtain

parameters of engineering interest. MIL-101 (Cr) samples,

synthesized by a simple solvothermal process, were

characterized by means of X-ray diffraction, field emission

scanning electron microscopy, microporosimetric analysis

and Fourier transform infrared spectroscopy; high

crystallinity and very high specific surface area and pore

volume were found. H2S adsorption isotherms on MIL-101 (Cr)

were evaluated at four different temperatures (specifically

273, 298, 323 and 348 K) at pressures of up to

approximately 0.1 kPa by means of a gravimetric technique

using a McBain-type balance. The modeling and experimental

results showed that MIL-101 (Cr) showed a high H2S

adsorption capacity at near-ambient temperature and low

heat release during adsorption, suggesting a potential use

of the selected metal-organic framework for fixed-bed

adsorption operations.

Keywords: Metal organic frameworks; MIL-101; Hydrogen

sulfide; Adsorption; Modeling

1. Introduction

The search for new sources of energy as an alternative to

oil is one of the main challenges faced today and involves

a great many scientists from all over the world. Hydrogen,

in particular, because of its high energy content, has been

identified as one of the most efficient environmental-

friendly fuels to be used in fuel cells to power electric

motors or to be burnt in internal combustion engines.

Biogas, produced by the anaerobic digestion of

biodegradable materials such as biomass, is one of the

potential alternative media to provide hydrogen for fuel

cells operations;1 it may, however, contain significant

amounts of sulfur, mainly as H2S. Hydrogen sulfide (H2S) is

a colorless, extremely toxic, flammable gas; furthermore,

it also corrodes the very expensive, noble-metal-based

equipment required for catalytic conversion operations.2

For these reasons, the removal of H2S (with concentration

in biogas streams usually in the order of a magnitude of

103 ppm) is very important in the hydrogen production

process. H2S removal from gaseous streams has been

historically carried out by physical or chemical

absorption, using aqueous solvents such as moderately

concentrated amine solutions, or potassium carbonate

solutions. Adsorption is an important alternative

technology, usually carried out as pressure swing

adsorption (PSA) or sometimes as vacuum swing adsorption

(VSA). The materials most often considered for PSA/VSA

processes have been mesoporous or microporous adsorbents.

In particular, pioneering work carried out by Bandermann et

al.3 during the early 1980s had already considered the use

of PSA on zeolite or carbon molecular sieves to separate

hydrogen sulfide from hydrogen or gas mixtures such as

natural gas. More recently, Tomadakis et al.4 reported the

PSA separation of H2S from CO2 with 4A, 5A, and 13X

zeolitic molecular sieves, showing high selectivity of H2S

over CO2 for all the substrates considered. As regards H2S

adsorption on mesoporous substrates, literature gives

interesting insights related to polyethylenimine-

functionalized mesoporous silicas,5 that are also

particularly suitable in terms of achieving CO2 selective

adsorption at very low partial pressures.6, 7

Recently, considerable interest has also been shown for

metal organic framework-based adsorbents (MOFs). MOFs are

crystalline hybrid porous solids consisting of metal

clusters connected by organic linkers to form

tridimensional structures.8 When an appropriate choice is

made of metal groups and/or organic linkers, many different

chemical structures can be obtained,9 sometimes with very

large pores and surface areas,10 thereby improving their

adsorption affinity towards acidic compounds such as H2S.

By way of example, Aprea et al. compared the CO2 adsorption

of a copper-bearing MOF (Cu-BTC) with that of a commercial

13X zeolite;11 the results revealed that the selected MOF

performed better in terms of adsorption capacity at ambient

temperature, in addition to releasing less heat during

adsorption.

However, when related to sulfur compounds and corrosive

gases like H2S, few studies have been conducted on the

adsorption capacity of MOFs, due to their poor chemical

stability towards this compound.12

In 2009, Hamon et al.12 carried out comparative studies on the

hydrogen sulfide adsorption of two types of MOFs, with

small or large pores. The first series included rigid and

flexible solids formulated as M(X){O2C-C6H4-CO2} with M = VIV

(MIL-47) or M = AlIII, CrIII, FeIII (MIL-53) (X= O for M = VIV,

OH for M = AlIII, CrIII, FeIII), while the second class

considered the rigid chromium-based solids

Cr3F(H2O)2O{C6H3(CO2)3}2 (MIL-100) and Cr3F(H2O)2O{O2C-C6H4-CO2}3

(MIL-101). The best results were achieved by MIL-101,

although adsorption appeared to be partially irreversible.

MIL-101 metal organic framework was first synthesized in

its chromium terephthalate based form by Férey et al.13 in

2005 at Institut Lavoisier (MIL is the acronym of Materiaux

Institut Lavoisier). Its zeotype cubic structure has a

“giant” cell volume (702 nm3) and large pores (3.0 to 3.4

nm). Furthermore, MIL-101 is acknowledged as being stable

under air atmosphere and does not alter when treated with

various organic solvents at ambient temperature or in

solvothermal conditions.13 These properties make MIL-101

one of the most attractive MOFs for studies on the

adsorption of various gas species.

The gas adsorption properties of MIL-100 and MIL-101 were

also studied by Llewellyn et al.14 , who observed that MIL-101

in particular was able to adsorb a huge quantity of various

gases such as CO2 and CH4 and proved to be stable at

relatively high pressure values (< 5 MPa). Furthermore, the

regeneration of these solids was shown to be possible in

mild conditions (<423 K or secondary vacuum).

This is why the main aim of this work is the modeling of

H2S adsorption isotherms on chromium-based MIL-101 metal

organic framework. The isotherms were obtained at four

different temperatures ranging between 273 and 348 K at

pressures of up to approximately 0.1 kPa, taking into

account the fact that the H2S concentration in gas streams

of engineering interest rarely leads to a partial pressure

significantly higher than this value. The experimental data

were processed by means of the Sips model to find the

values of the isosteric heat of adsorption (i.e., the ratio

of the infinitesimal change in the adsorbate enthalpy to

the infinitesimal change in the amount adsorbed) and of

other significant parameters such as H2S affinity towards

the selected adsorbent and the heterogeneity of the

adsorption process.

2. Experimental

MIL-101 (Cr) samples were obtained as powders using a

procedure derived from that described by Férey et al.:13 4.00

g of Cr(NO3)3∙9H2O (J.T. Baker) was dissolved in 43.20 g of

ultra-purified water, that was produced by means of a TKA

Smart2Pure device; 1.64 g of terephthalic acid (Aldrich)

and 5.0 ml of a 4 wt% hydrofluoric acid solution, obtained

by diluting a 37 wt% pristine solution (Carlo Erba), were

then added and kept stirring for about 5 min. The resulting

suspension was then put in a Teflon-lined autoclave bomb

and kept in an oven at 493 K for 8 h. After equilibration

at ambient temperature, the significant quantity of large

terephthalic acid crystals present in the batch was

eliminated by filtration using a large pore fritted glass

filter (n. 2); the water suspension of MIL-101 powders that

passed through the filter was then filtered again on

Whatman ashless grade 42 filtration paper. The retained

green product was finally dried at 333 K overnight.

Powder X-ray diffraction (XRD) patterns of MIL-101 samples

were collected using a Philips PW1710 apparatus with Cu Kα1

radiation. The scanning range was 2-15° in 2θ, the scanning

step size was 0.02°, and the time for each step was 4 s.

The GSAS II software peak indexing module15 was then used

to calculate the framework symmetry and cell parameters.

Field emission scanning electron microscopy (FE-SEM)

micrographs were collected with a Zeiss Ultra Plus

instrument, while microporosimetric characterization was

carried out by N2 adsorption at 77 K, and the specific

surface area was evaluated by means of the Brunauer-Emmett-

Teller (BET) method. A Micromeritics ASAP 2020 volumetric

instrument was used for this purpose, and synthesized

samples were degassed at 373 K overnight prior to

characterization. Moreover, Fourier transform infrared

(FTIR) spectroscopy was performed using a Thermo Nicolet

Nexus spectrometer on KBr pressed disks containing 1 wt% of

MIL-101 samples: this analysis was carried out on pristine

powders in addition to on samples previously used for H2S

adsorption measurements.

The H2S adsorption isotherms on MIL-101 samples at four

different temperatures (namely 273, 298, 323 and 348 K)

were obtained using a gravimetric technique based on a

McBain-type balance, (see Caputo et al.16 for a detailed

description of the experimental apparatus). This device is

equipped with a quartz spring (Ruska Instrument Co.,

Houston, Texas), with a sensitivity of 5 mm/mg, and a small

quartz pan, containing between 10 and 15 mg of the

adsorbent material, hooked to the spring. The amount of

adsorbate was evaluated by measuring the spring elongation

with the help of a cathetometer, which enabled the reading

of the spring deflection down to 0.05 mm. Gas pressure in

the adsorption chamber was electronically measured by means

of a capacitive pressure transducer (Edwards Datametrics

1500). A Heto thermostating unit allowed temperature

control of the gas in the adsorption chamber within a range

of ±0.1 K. Before measurement, the samples were degassed in

situ at 423 K under high vacuum (p < 10−3 Pa) by means of an

Edwards turbomolecular pump for 12 h using a thoroidal

furnace. After cooling to the working temperature, aliquots

of the selected gas were allowed to enter the balance

chamber and sufficient time was awaited to attain

equilibrium. Adsorption isotherms were outlined by plotting

the amounts of adsorbate on dry basis versus gas partial

pressures at equilibrium.

3. Results and discussion

3.1. Characterization of synthesized MIL-101

The XRD pattern of synthesized MIL-101 is shown in Fig. 1

and is consistent with that reported by previous

investigators.13 The pattern refinement confirmed the cubic

symmetry of the framework (a = 8.88 nm, in excellent

agreement with the value of ~ 8.9 nm reported elsewhere)13

and the space group assignment (Fd-3m).

Fig. 2 shows FE-SEM micrographs of synthesized MIL-101:

sub-micron-sized crystallites are clearly visible. In

particular, inspection of Fig. 2 reveals well-defined

octahedral crystals, that are consistent with other SEM

investigations performed on MIL-101.17 It is interesting to

observe that it has been reported elsewhere18 that, as a

general rule, one possible way of increasing the size of

MOF crystals is to increase synthesis times; however, as

regards MIL-101, this approach would appear to be rather

unfeasible. Indeed, it is known that both reaction

temperature and time strongly influence the outcome of the

synthesis process for this MOF. More specifically, the

coordination of the metallic species, the nuclearity, and

the dimensionality of the inorganic subnetwork are known to

depend greatly on reaction temperature, which also

influences the crystallization and condensation rates of

inorganic chromium clusters. As a matter of fact, the MIL-

101 phase is favorably formed in the narrow temperature

range between 473 and 493 K.17 As regards synthesis time,

it is noted that when other process variables (temperature,

pH, reagent activities, etc.) are kept fixed at optimized

values, increasing reaction time up to 16 h would appear to

have a positive effect on product yield of MIL-101, whilst

prolonging the synthesis over 16 h results in the

occurrence of another phase (i.e., MIL-53).17

Fig. 3 shows the N2 adsorption isotherm on synthesized MIL-

101 measured using the ASAP 2020 apparatus at 77 K. The

curve reported in Fig. 3 shows both features of Type I and

Type IV isotherms:19 in actual fact, on the one hand, the N2

adsorbed amount is already high at low relative pressures

(i.e., almost 400 cm3 STP per gram of adsorbent at a relative

pressure of about 0.01), suggesting the micropore filling

mechanism to which Type I isotherms are usually related. On

the other hand, the steep increase of the N2 adsorbed

amount at relative pressures between 0.20 and 0.25 suggests

a capillary condensation phenomenon that is characteristic

of Type IV isotherms: in particular, the quite low relative

pressure values at which capillary condensation occurs can

be related to the well-known small sizes of the mesoporous

cages inside the MIL-101 framework.13 As is established

practice for mesoporous materials, total pore volume was

estimated from N2 adsorbed amounts at p/po = 0.99 and found

to be 1.49 cm3/g, while the specific surface area, as

estimated by applying the BET method, was 3028 m2/g. As

reported by Hong et al., MIL-101 powders can undergo

different purification processes in order to enhance their

textural properties.17 In this work, a purification attempt

by hydrothermal treatment in ethanol was performed but, as

already observed by Henschel et al.,20 no increase of the

surface area and/or the pore volume was detected.

Finally, FTIR studies were performed to preliminarily check

the structure preservation of MIL-101 after H2S adsorption

runs. MIL-101 samples, submitted to the H2S adsorption

procedure described in Section 2, were subsequently re-

degassed, backfilled with inert gas, taken away from the

adsorption chamber and finally transferred to the FTIR

spectrometer. FTIR spectra of MIL-101 before and after H2S

adsorption runs are presented in Fig. 4. The two spectra

look very similar, as well as closely resembling other MIL-

101 spectra found in literature.21 Indeed, strong bands at

1546, 1508 and 1402 cm-1 can be related to the stretching

(both asymmetric and symmetric) of COO groups and to that

of C-C bonds, clearly showing the presence of the

dicarboxylate linker inside the crystal. Moreover, the

quite significant and narrow bands at 1019 and 748 cm−1 can

be attributed to in-plane and out-of-plane bending modes,

respectively, of the C-H bonds belonging to the aromatic

rings of the terephthalate moieties from which the

framework is built up.21 The comparison of the two spectra

reported in Fig. 4 hints at the chemical stability of MIL-

101 towards H2S, making it a potentially good adsorbent for

such a chemically aggressive gas.

3.2. H2S adsorption isotherms on MIL-101

Fig. 5 reports the H2S adsorption isotherms on MIL-101 MOF

at 273, 298, 323 and 348 K for H2S pressure ranging between

0 and 0.1 kPa, together with fits to the Sips equation.

Fig. 5 shows that, in the pressure range considered,

isotherms are a significant way off asymptotizing to their

maximum level, with a strong dependence of the adsorbed

amount of H2S on pressure. As regards the dependence on

temperature, it clearly appears that the H2S adsorbed

amount decreases as temperature increases, indicating an

exothermic behavior for the adsorption process. In

particular, at p=0.1 kPa, the adsorbed amount q was

revealed to be about 5.3 mol/kg at T=273 K and about 1.7

mol/kg at T=348 K, with a ratio slightly higher than 3.

Following the information obtained on the chemical

stability of MIL-101 from the FTIR characterization, a

single MOF sample was used to perform different H2S

adsorption isotherms at different temperatures: after a

single run, the used adsorbent sample was re-activated

under vacuum at 423 K and its mass was re-evaluated,

finding no significant differences with the pristine

measure. This leads to the consideration that H2S

adsorption on MIL-101 is fully reversible: a result that is

apparently in contrast with that stated by Hamon et al.,12 ,

but which can be explained by the fact that, in the latter

work, the H2S activity reaches levels far higher than those

reported in Fig. 5. Bearing in mind that the H2S

concentration in gas streams of engineering interest rarely

leads to a partial pressure significantly higher than 0.1

kPa, MIL-101 can thus be considered an H2S adsorbent with

complete regeneration capabilities.

In order to have a clearer understanding of the adsorption

phenomena examined, a modeling effort was undertaken using

the semiempirical three parameter Sips isotherm.22 The Sips

isotherm (sometimes called the “Langmuir–Freundlich

isotherm”) is a semiempirical model which contains

mathematical aspects of both the Langmuir and Freundlich

isotherms: even if its thermodynamical consistency shows

limits in the very low pressure region (it does not reduce

to Henry’s law), it has already proven to be useful in

describing the adsorption of different small molecules,

such as CO211 and water vapor18 on metal organic frameworks.

According to this equation, the pressure dependence of the

adsorbed amount takes the following form:

(1)

where qmax, n and b are model parameters: qmax represents the

maximum adsorption capacity, which (under subcritical

conditions) depends on both the pore volume of the

adsorbent and the liquid molar density of the adsorbate; b

is the affinity constant, and n is the heterogeneity

coefficient (in particular, for n=1, the Sips isotherm

reduces to the Langmuir isotherm, which applies to

homogeneous adsorbent-adsorbate systems). Sips parameters

are in general dependent on temperature, as reported by

Do.23 In this case, n is assumed to be independent on

temperature, while the maximum adsorption capacity qmax

(whose temperature dependence form, as reported by Do,23

can be considered arbitrary) was estimated using the

“Gurvitch rule”,24 which assumes that qmax can be evaluated

as the product between the total pore volume of the

adsorbent, Vpore, and the adsorbate liquid molar volume, ρmol:

(2)

In Eq. (2), Vpore assumes the experimental value reported in

Section 3.1; using the correlation proposed by Perry and

Green,25 it can be verified that the values of ρmol at the

temperatures relative to the reported isotherms can be very

satisfactorily fitted by a linear function:

(3)

In Eq. (3), ρ0 is the value of ρmol at a reference

temperature T0 and χ is a parameter: this equation was used

to fit the values of ρmol estimated, accordingly to the

literature,25 at different temperatures between 273 and 348

K. Using T0=273 K as the reference temperature, a value of

χ=0.874 was obtained, with a value of the regression

coefficient R2=0.988. As a consequence, the following

temperature dependence form of qmax can be obtained:

(4)

As regards the expression for the dependence of the

affinity coefficient b on temperature, it was chosen

accordingly to Do: 23

(5)

in which bo is the value of b at To and Q is a parameter

related to the adsorption heat.23

An attempt to describe H2S adsorption on MIL-101 (Cr) was

thus performed by coupling Eq. (1) with Eqs (4, 5). In

particular, the experimental data of H2S adsorption on MIL-

101 (Cr) were submitted to non-linear regression using the

MATLAB Surface Fitting Toolbox to simultaneously calculate

the optimal values of the parameters that appear in Eqs.

(1, 5), i.e., bo, Q and n. The calculated values of the

parameters, obtained using To=273 K as the reference

temperature, are reported in Table 1, and the comparison

between model and experimental results is reported in Fig.

5, in which the symbols refer to experimental data and the

continuous curves refer to the best fitting Sips

theoretical isotherms. Inspection of Fig. 5 clearly

indicates a good correlation between model curves and

experimental points: this is also confirmed by value of the

regression coefficient R2 reported in Table 1.

It may be useful to compare the calculated values of

parameters b and n with those obtained in the case of the

adsorption of other acid-behaving molecules (e.g., CO2) on

other MOFs (e.g., Cu-BTC)11, 26 that, similar to MIL-101,17

are known to adsorb by the interaction of their

coordinatively unsaturated sites with the target gas

species. Table 2 shows the values of b (calculated at T =

293 K), n and Q for the two systems now mentioned. The

difference with respect to b is quite striking, since the

value for H2S adsorption on MIL–101 (4.4∙10-1 kPa-1) is about

two orders of magnitude higher than the value calculated at

the same temperature for CO2 adsorption on Cu-BTC.11 In

regard to the heterogeneity parameter n, its value for H2S

adsorption on MIL-101 is slightly higher than 1.5, against

a value of about 1 relative to CO2 adsorption on Cu-BTC.11

Bearing in mind that these comparisons are made referring

to substrates whose adsorption features rely on active

sites sharing the same chemical nature, it may seem odd to

find such differences. One possible explanation to this

issue may be found in the different geometries of the

adsorptive molecules considered in the aforementioned

comparisons: indeed, while in this paper the interaction

between MIL-101 and a molecule, such as H2S, with a

permanent dipole, was studied, the adsorption experiments

mentioned above related to the interaction of Cu-BTC and

another acidic molecule, such as CO2, which, however is

strictly symmetrical and can thus explicate its interaction

with coordinatively unsaturated sites just by means of its

quadrupole moment.11

Using the Sips isotherm, it was possible to develop an

expression for the isosteric heat of adsorption, i.e. the

ratio of the infinitesimal change in the adsorbate enthalpy

to the infinitesimal change in the amount adsorbed, as a

function of the fractional coverage of the adsorbent

θ=q/qmax. According to Do,23 the isosteric heat of adsorption

can be calculated from the van’t Hoff equation:

(6)

After rewriting Eq. (1) in terms of p versus q,

substituting Eqs. (4, 5) and then taking the derivative of

its natural logarithm with respect to T, the following

expression for the isosteric heat of adsorption is

obtained:

(7)

Writing Eq. (6) in terms of the fractional coverage θ=q/qmax

leads to

(8)

Eq. (8) shows that (-H) is a function of both and T, as

already pointed out by Do.23 In particular, Fig. 6 reports,

for the four different temperatures considered (273, 298,

323 and 348 K), a plot of (–H) vs. . Inspection of Fig. 6

indicates that the isosteric heat of adsorption increases

as the fractional coverage of the adsorbent increases,

approaching an asymptote per θ→1: this trend is similar to

that of other expressions of the isosteric heat obtained

for the adsorption of polar molecules.27 The value of the

isosteric heat at zero coverage that can be extrapolated

from Fig. 6 is 16.7 kJ/mol at 298 K: this is about three

times lower than the values of (-ΔH) reported, for example

for H2S adsorption on nitrogen-containing activated

carbons28 Since fixed-bed adsorption is an essentially

adiabatic operation, the isosteric heat of adsorption is

responsible for the temperature rise during the process

and, since adsorption is an exothermic process, an increase

in temperature leads to a decrease in adsorption capacity.

For this reason, a lower isosteric heat of adsorption is

preferable when selecting adsorbents and MIL-101 could

represent a more efficient alternative adsorbent in fixed-

bed H2S adsorption processes.

4. Conclusions

The results reported in this work gave novel insight about

the use of metal organic frameworks for adsorption. More

specifically, H2S adsorption on MIL-101 (Cr) MOF was

modeled with the aim of both evaluating the performance of

the selected adsorbent and achieving a better understanding

of the mechanisms to which its interaction with the target

gas is subjected.

Laboratory-synthesized MIL-101 samples were characterized

by means of X-ray diffraction, field emission scanning

electron microscopy, microporosimetric analysis and Fourier

transform infrared spectroscopy. The characterization

indicated that the samples possess high crystallinity and

high specific surface area and pore volume.

The H2S adsorption isotherms on MIL-101 samples at four

different temperatures (namely 273, 298, 323 and 348 K)

were obtained using a gravimetric technique based on a

McBain-type balance. The experimental data showed that, in

the pressure range considered, MIL-101 had high adsorption

capacity and a high affinity towards H2S; moreover, the

adsorption process was found to be fully reversible and not

damaging for the selected substrate. The semiempirical Sips

model was used to describe the obtained H2S adsorption

data, and substantial agreement between the model and the

experimental results was obtained. The model and the

experimental results indicated that the isosteric heat of

H2S adsorption on MIL-101 (Cr) was noticeably lower than

other adsorbents reported in the literature, suggesting

that this MOF may be particularly suitable for fixed-bed

H2S adsorption applications.

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[27] D Caputo, F. Iucolano, F. Pepe, C. Colella,

Microporous Mesoporous Mater.105, 260 (2007)

[28] F. Adib, A. Bagreev, T. J. Bandosz, Langmuir 16, 1980

(2000)

Tables

Table 1. Sips parameters for H2S adsorption on MIL-101

Parameter

95%

confidence

interval

lower limit

Best fitting

value

95%

confidence

interval

upper limit

b0 (kPa-1) 0.47 0.65 0.83

n 1.40 1.53 1.66

Q (kJ∙mol-1) 11.21 12.79 14.37

Regression coefficient R2=0.984

Table 2. Comparison of Sips parameters between MIL-101/H2S

and Cu-BTC/CO2 adsorbent/adsorbate systems. Values for Cu-

BTC/CO2 system are extracted from the literature11

Parameter MIL-101/H2S Cu-BTC/CO2

ba (kPa-1) 4.4∙10-1 5.2∙10-3

n 1.53 0.94

Q (kJ∙mol-1) 12.8 25.9

a Values calculated at 293 K

Figure captions

Figure 1. X-ray diffraction (XRD) pattern of synthesized

MIL-101 (Cr).

Figure 2. Field emission scanning electron microscopy (FE-

SEM) images of synthesized MIL-101 (Cr) crystals.

Figure 3. N2 adsorption isotherm at 77 K on synthesized

MIL-101 (Cr).

Figure 4. Fourier transform infrared (FTIR) spectra of MIL-

101 (a) before and (b) after H2S adsorption runs.

Figure 5. H2S adsorption isotherms on MIL-101 (Cr) at T =

273 K (circles),

298 K (squares), 323 K (diamonds), and 348 K (triangles).

Continuous lines: best

fitting Sips theoretical isotherms.

Figure 6. Isosteric heat of H2S adsorption on MIL-101 as a

function of the fractional coverage of the adsorbent.

Antonio Peluso, Nicola Gargiulo, Paolo Aprea, Francesco

Pepe, Domenico Caputo, Modeling Hydrogen Sulfide Adsorption

on the Chromium-Based MIL-101 Metal Organic Framework,

Figure 1




Figure 2




Figure 3




Figure 4




Figure 5




Figure 6

Graphical Table of Content Abstract

Hydrogen sulfide (H2S) adsorption on a laboratory-

synthesized polymeric chromium terephthalate (MIL-101)

metal-organic framework was modeled by means of the

semiempirical Sips equation. The experimental data showed

that, in the pressure range considered, MIL-101 had high

adsorption capacity and a high affinity towards H2S;

moreover, the adsorption process was found to be fully

reversible and not damaging for the selected substrate. The

model and the experimental results indicated that the

isosteric heat of H2S adsorption on MIL-101 (Cr) was

noticeably lower than other adsorbents reported in the

literature, suggesting that this MOF may be particularly

suitable for fixed-bed H2S adsorption applications.

Modeling Hydrogen Sulfide Adsorption on Chromium-Based MIL-101 Metal Organic Framework

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