Thermodynamically Favorable Conversion of Hydrogen Sulfide into Valuable Products through Reaction with Sodium Naphthalenide

Article abstract of DOI:10.1002/cplu.201500299

Hydrogen sulfide (H₂S) is an extremely hazardous chemical waste that is generated at large scale in many industries; its abatement has long been an energy-extensive and cost-ineffective liability due to the thermodynamic limitations of the selected approaches and low value of the final products, sulfur and water. Here we introduce an attractive method for H₂S abatement that yields value-added products via a thermodynamically favorable process. Specifically, sodium naphthalenide (Na-NAP) is used to capture H₂S to produce anhydrous Na₂S nanocrystals and 1,4-dihydronaphthalene, which are important materials for batteries and liquid fuels, respectively. This multipurpose process is driven by the acid/base neutralization reaction between hydrogen cations from H₂S and radical anions from naphthalenide. It is spontaneous and irreversible at ambient temperature and pressure, proceeding to completion very rapidly. Waste not, want not: The chemical waste hydrogen sulfide (H₂S) is notorious for its production in many industries, harming creatures, corroding equipment, poisoning catalysts, and requiring energy-intensive and cost-ineffective abatements. Now a thermodynamically favorable process can convert H₂S into value-added energy materials.

Full text of DOI:10.1002/cplu.201500299

DOI: 10.1002/cplu.201500299
Communications
Thermodynamically Favorable Conversion of Hydrogen
Sulfide into Valuable Products through Reaction with
Sodium Naphthalenide
Xuemin Li,[a] Rachel M. Morrish,[b] Yuan Yang,[a] Colin A. Wolden,*[b] and Yongan Yang*[a]
Hydrogen sulfide (H2S) is an extremely hazardous chemical
waste that is generated at large scale in many industries; its
abatement has long been an energy-extensive and cost-inef-
fective liability due to the thermodynamic limitations of the se-
lected approaches and low value of the final products, sulfur
and water. Here we introduce an attractive method for H2S
abatement that yields value-added products via a thermody-
namically favorable process. Specifically, sodium naphthalenide
(Na-NAP) is used to capture H2S to produce anhydrous Na2S
nanocrystals and 1,4-dihydronaphthalene, which are important
materials for batteries and liquid fuels, respectively. This multi-
purpose process is driven by the acid/base neutralization reac-
tion between hydrogen cations from H2S and radical anions
from naphthalenide. It is spontaneous and irreversible at ambi-
ent temperature and pressure, proceeding to completion very
rapidly.
and is sold at a loss;[9] and 4) the byproduct H2O is undesirable,
particularly for oil refinery industries where the hydrogen in
H2S originates from expensive H2 gas.[10] Other approaches that
employ scavengers (such as organoamines,[5] porous materi-
als,[11] metal oxides/salts,[12] biological oxidation,[13] or caustic
soda[14]) produce no valuable products and must be periodical-
ly regenerated, creating additional waste streams. Combustion
is not a practical option as it generates SO2, itself a highly
regulated chemical as the precursor to acid rain.
Perhaps the most aesthetically pleasing approach for H2S
abatement would be decomposition into its elemental constit-
uents [Eq. (2)].[7,8]
DGom¼334 kJ molÀ1
H S
2
H þ 1=8 S
ð2Þ
ƒƒƒƒƒƒƒƒ!
2
8
Numerous techniques have been pursued to conduct reac-
tion (2), such as thermal decomposition,[15] thermochemical
conversion,[16] plasmochemical decomposition,[17] photochemi-
cal decomposition,[18] electrochemical decomposition,[19] and
photoelectrochemical decomposition.[20] However, they all
suffer from the common challenge that reaction (2) is thermo-
dynamically unfavorable and significantly endothermic, which
limits the commercial viability. In summary, methods that can
convert H2S to value-added products via thermodynamically fa-
vorable pathways are highly desirable.
Hydrogen sulfide (H2S) is a major chemical waste produced
predominantly through desulfurization of natural gas and pe-
troleum reserves.[1] It is extremely hazardous to human
health,[2] corrosive to equipment,[3] and poisonous to cata-
lysts.[4] Thus, removing H2S is an inevitable liability. For several
decades, the most prevailing and successful technique for H2S
abatement in the oil and natural gas industries has been the
Claus process, which utilizes air to partially oxidize H2S to S
and H2O in two steps with the overall reaction expressed by
Equation (1).[5,6]
Therefore, we aim to address H2S remediation through reac-
tion with alkali metals as described by Equation (3), where M=
Li, Na, K.
DGom¼204 kJ molÀ1
DGom<À300 kJ molÀ1
H2S þ 1=2 O2
1=8 S þ H O
ð1Þ
ƒƒƒƒƒƒƒƒ!
8
2
2 M þ H S
H þ M S
ð3Þ
ƒƒƒƒƒƒƒƒƒ!
2
2
2
While this reaction is thermodynamically favorable,[7] its prac-
tical operation remains energy intensive and cost ineffective,
because 1) it requires high temperature (ca. 1300 K); 2) the tail
gas still contains a few percent of unreacted H2S and SO2 and
needs further treatment;[8] 3) the S produced exceeds demand
The very negative Gibbs free energy (DGmo) implies that this
reaction is thermodynamically favorable and expected to be
spontaneous and irreversible.[7] A significant benefit of this
chemistry is the accompanying synthesis of valuable M2S, an
attractive cathode material for alkali metal/sulfur (M–S) batter-
ies.[21,22] Existing technologies for the production of anhydrous
M2S are complex, energy inefficient, and not environmentally
benign.[23,24] As M2S is many times much more valuable than
M, the proposed technique has the potential to be practically
attractive even without H2 generation. Thus, this new synthetic
approach deserves a systematic exploration.
[a] X. Li, Prof. Y. Yang, Prof. Y. Yang
Department of Chemistry and Geochemistry
Colorado School of Mines
1012 14th Street, Golden, CO 80401 (USA)
E-mail: yonyang@mines.edu
[b] Prof. R. M. Morrish, Prof. C. A. Wolden
Department of Chemical and Biological Engineering
Colorado School of Mines
Herein we introduce an efficient abatement of H2S through
scrubbing an H2S/Ar gas stream with a solution of sodium
naphthalenide (Na-NAP, NaC10H8). The reaction between Na-
NAP and H2S proceeds spontaneously to completion at ambi-
ent temperature and pressure, reducing H2S to below detec-
1613 Illinois Street, Golden, CO 80401 (USA)
E-mail: cwolden@mines.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500299.
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tion limit. Highly pure, crystalline Na2S nanoparticles are pro-
duced as well as a small amount of H2 in accordance with reac-
tion (3), but the majority of the hydrogen released from H2S
hydrogenates naphthalene to form 1,4-dihydronaphthalene,
a valuable byproduct commonly known as 1,4-dialin that is
used as an organic solvent and jet fuel.[25,26]
dynamics of the sampling apparatus, identical to the response
observed by closing a valve. The mass spectrum obtained
during the bypass mode (Figure 1B, black) clearly shows the
expected fragments of H2S across mass/charge (m/z) ratios of
32–34. In contrast, the spectrum recorded from the effluent
(Figure 1B, red) presents no signals from H2S but the expected
Ar signal. Additionally, the gasses from the head space in the
reactor were flushed through a 0.1m AgNO3 solution; the ob-
servation of no precipitate (Ag2S) indicated that the residual
H2S at the end of the reaction was below 2.2”10À40 ppb.[27]
This result was consistent with the equilibrium concentration
of H2S residue predicted by the extreme negative DGmo value.
Thus, we concluded that the demonstrated H2S removalwith
Na-NAP was spontaneous, complete, and nearly instantaneous.
Comparisons to calibrated mixtures revealed that the maxi-
mum amount of H2 generated (Figure 1A) did not exceed
1.5% of the amount expected, if H2S was completely convert-
ed to H2 as indicated by Equation (4).[7]
The proof-of-concept experiment was completed as shown
in Figure SI-1. A dimethoxyethane (DME) solution of 0.04m Na-
NAP in a Parr reactor was supplied with a premixed gas con-
sisting of 10 atom% H2S/90 atom% Ar. The effluent was sam-
pled in line using a differentially pumped quadrupole mass
spectrometer (QMS). When the molar ratio between Na in the
initial solution and H2S totally supplied reached 2.0, the inlet
and outlet were closed simultaneously.
Figure 1A plots the relative concentrations of H2S (black)
and H2 (red) recorded by the QMS. Upon diversion of the H2S
stream from the bypass into the reactor (marked by the
arrow), the H2S signal immediately began to fall and eventually
dropped below the detection limit of the instrument, indicat-
ing that at least >99.9% of H2S supplied was consumed. The
characteristic timescale of H2S depletion herein reflects the gas
2 NaC10H8 þ H2S ! H2 þ Na2S þ 2 C10H8
ð4Þ
This suggested that the hydrogen was captured in the solution
as expected (vide infra). The X-ray diffraction (XRD) pattern of
the solid product shown in Figure 1C (blue), matches that of
the Na2S standard (black), indicating the production of anhy-
drous, phase-pure, crystalline Na2S. The scanning electron mi-
croscope (SEM) image (inset) shows that Na2S existed as nano-
particles less than 100 nm in diameter.
We anticipated that hydrogen was retained in the solution
in the form of hydrogenated NAP (i.e., 1,4-dihydronaphthalene
C10H10) as expressed in Equation (5).
2 NaC10H8 þ H2S ! Na2S þ C10H10 þ C10H8
ð5Þ
To confirm this, the liquid phase after reaction was character-
1
ized by FTIR spectroscopy and H NMR spectroscopy (Figure 2).
The crude solution was concentrated through rotary vaporiza-
tion, by taking advantage of the low boiling point (858C) of
the DME solvent, the tendency of NAP to sublime, and the
high boiling points (>2008C) of NAP derivatives.[26] The FTIR
spectrum was taken on the solid residue left after a droplet of
the solution applied to the attenuated total reflection (ATR)
crystal had dried. The spectrum (Figure 2A) has characteristic
features of 1,4-dihydronaphthalene (the dihydrogenated
1
NAP).[28] The H NMR spectrum (Figure 2B) was taken by dis-
persing liquid product in C6D6. In addition to the expected
DME, benzene, and NAP, three weak peaks at 6.91–5.66 pm
can be assigned to the hydrogen atoms at positions (a) and (b)
of 1,4-dihydronaphthalene.[28] The signal from the hydrogen
atoms at position (c), expected to be at 3.3–2.9 ppm, are be-
lieved to be buried within the intense DME signal.
To collect more convincing evidence for the production of
1,4-dihydronaphthalene, the liquid phase was further analyzed
by gas chromatography/mass spectroscopy (GC–MS) using
DME as the solvent. The chromatogram in Figure 3A shows
two peaks. The mass spectrum for peak 1 is well consistent
with that of 1,4-dihydronaphthalene in the literature.[29] The
Figure 1. Demonstration of the proposed concept of removingH2S using Na-
NAP. A) Real-time QMS measurements of the relative changes in H2S and H2
upon introduction of H2S to Na-NAP. B) Mass spectra showing that the 10%
of H2S in Ar (black) is completely removed after treatment (red). C) XRD pat-
tern (blue) and SEM image (inset) of the solid product and XRD stick pat-
tern (black) of the Na2S standard.
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Figure 2. Characterization of the organic compounds in the liquid phase.
A) FTIR spectrum of the solid residue after removal of NAP and DME. B)
1H NMR spectrum of the liquid phase directly from the reaction system.
mass spectrum for peak 2 is well consistent with that of
NAP.[29] Thus, the characterization results discussed above all
together confirm that the Na-NAP strategy of removing H2S
follows the overall transformation described by Equation (5)
with 1,4-dihydronaphthalene being the principle NAP deriva-
tive.
Figure 3. GC–MS analysis of the liquid phase of the reaction system: (A) the
chromatogram; (B) the MS spectrum for peak 1 in (A); and (C) the MS spec-
trum for peak 2 in (A).
The elementary reactions for this process are shown in
Scheme 1, with details described in the Supporting Information
(SI). This mechanism, the basis for us to devise the multipur-
pose process herein, was derived from two sets of literature re-
sults. First, a similar mechanism has been determined for pro-
ducing 1,4-dihydronaphthalene from alcoholysis of Na-NAP in
ammonia (or alkylamines such as ethylenediamine).[30] Second,
hydrolysis of Na-NAP/DME produces 1,4-dihydronaphthalene
and NaOH, without evolution of H2.[31] H2S, like alcohols and
water, can also provide dissociated protons.[30–32] To the best of
To better understand the mechanism of the formation of
Na2S, we conducted three more reactions in which the Na-
NAP/H2S molar ratio was controlled at 1.0, 1.1, and 3.4.
Figure 4 shows the XRD data of the solid products in these
three cases. In the case of Na-NAP/H2S=1.0 (Figure 4A), the
product is pure, anhydrous, crystalline NaHS. In the case of Na-
NAP/H2S=1.1 (Figure 4B), a tiny amount of Na2S is also detect-
able (as marked by the arrow) in addition to the predominate
NaHS. In the case of Na-NAP/H2S=3.4 (Figure 4C), the product
is pure, anhydrous, crystalline Na2S.
From the XRD results in Figure 1C and Figure 4 we surmise
that under conditions where the Na-NAP/H2S ratio is ꢀ2, Na2S
is the only solid product, and this appears to occur directly
and instantly, supporting the reaction mechanism in Scheme 1.
When additional H2S is added to the system beyond the stoi-
chiometric ratio, NaHS is formed according to Equation (6),
as reported in the literature for high-temperature
reactions.[33]
Scheme 1. The proposed reaction mechanism of removing H2S with Na-NAP
to produce 1,4-dihydronaphthalene through a radical hydrogenation reac-
tion. Intermediates: An NAP molecule (1) receives an electron from Na; an
NAPÀ· radical (2), a hydrogenated NAP· radical (3), a hydrogenated NAPÀ
anion (4), and a 1,4-dihydronaphthalene molecule (5).
Na2S þ H2S ! 2 NaHS
ð6Þ
our knowledge, there have not been studies of hydrogenating
NAP using a chemical waste like H2S. The minor amount of H2
production in competition with the production of the NAP de-
rivative is ascribed to the reversed electron transfer from naph-
thalenide radicals to H+ ions, producing H· radicals that dimer-
ize to H2.
All reactions appear to be very fast since the product distri-
bution appears to reflect the stoichiometric ratio of the two re-
actants, as otherwise it would be difficult to observe the tiny
amount of Na2S in the case of Na-NAP/H2S=1.1. While Na2S is
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Work is underway to assess battery performance of Na2S, gen-
eralize this technique to Li and K, explore other reducing
agents for obtaining more H2 gas, and study the sensitivity of
the process to impurities.
Experimental Section
Materials
Sodium metal (Na, ACS reagent grade, stick dry), naphthalene
(NAP, C10H8, 99%), 1,2-dimethoxyethane (DME, anhydrous, 99.5%),
and hexane (ACS grade) were purchased from Aldrich. [D6]-Ben-
zene (d-99.5%) was purchased from Cambridge Isotope Lab. Silver
nitrate (AgNO3, 99.9+%) was purchased from Alfa Aesar. All chemi-
cals were used as received.
Methods
A typical reaction was run as follows. Appropriate amounts of NAP
and Na at the molar ratio of NAP/Na=1.3 were sequentially added
to DME in an argon-filled glove box to make a DME solution con-
taining 0.04m of sodium naphthalenide (Na-NAP). A dark green
color is immediately observed upon addition of the Na, and a ho-
mogenous solution was obtained after two hours of stirring (Fig-
ure SI-1). Then, the Na-NAP solution (50 mL) was placed in a Parr
reactor (model 4793). After that, the reactor was connected to the
gas line containing 10 atom% H2S/90 atom% Ar. A schematic dia-
gram and a photograph of the apparatus are shown in Figure SI-
1B,C, respectively. Initially, the H2S/Ar mixture was passed through
a bypass line at a flow rate of 40 sccm to establish a baseline read-
ing. Then, the bypass was closed and the H2S/Ar flow was diverted
through the Na-NAP solution, which was kept at ambient tempera-
ture and pressure. The effluent was evacuated using vacuum
pumps and sampled in line using a differentially pumped quadru-
pole mass spectrometer (QMS). The overall molar ratio of Na-NAP
to H2S supplied was controlled by controlling the duration of the
H2S flow.
Figure 4. XRD data of the solid products in three reactions using different
Na-NAP/H2S molar ratios: 1.0 (A), 1.1 (B), and 3.4 (C). The standard XRD pat-
terns (sticks) of the predominate chemicals are shown as references.
the preferred product for battery applications, the second reac-
tion doubles the capacity of the Na-NAP solution for H2S
abatement and provides the flexibility to tune the reaction
products. For instance, NaHS is a valuable chemical for many
industries, such as paper, pulp, and leather manufacturing and
mining.[34,35]
After the reaction, the reactor was connected to an Ar line to flush
the gas phase out of the reactor into a 0.1m AgNO3 solution
(1 mL). The purpose was to observe if any precipitate would be
generated, indicating the formation of Ag2S and in turn the exis-
tence of unreacted H2S in the reactor. Based on the extremely low
solubility of Ag2S (KSP =10À50.1),[27] the detection limit of H2S in the
reactor was as low as 2.2”10À40 ppb.
In conclusion, we have reported an innovative process for
efficiently removing H2S and producing value-added products
through a thermodynamically favorable reaction. The concept
is demonstrated by using sodium naphthalenide (Na-NAP) to
convert H2S into nanocrystalline Na2S (a battery material) and
1,4-dihydronaphthalene (a liquid fuel). Moreover, this irreversi-
ble reaction spontaneously proceeds to completion at ambient
temperature and pressure. This is an attractive technique for
M2S synthesis compared with the existing approaches,[23,24]
which either need H2S-derived S and M as precursors or re-
quire extensive procedures to remove impurities when cheap
precursors (such as Na2SO4 and carbon) are used. The introduc-
tion of this technique would be expected to complement but
not replace the Claus process. In addition, it is expected that
reactive species commonly found in industrial H2S waste
streams (such as H2O, O2, and CO2) should be removed before
employing this technique.[11] While this pretreatment adds
extra costs and alkali metals are expensive, the operation costs
could be offset by high-value M2S and 1,4-dihydronaphthalene.
Then, the reactor was transferred into an Ar-filled glove box. The
reaction solution was centrifuged to separate the solid phase and
the liquid phase. The solid phase was washed with DME several
times and dried for further characterization. The liquid phase was
removed from the glove box and concentratee via rotary vapora-
tion to 1/5 of the original volume. After that, the solution was
stored in the glove box for further characterization.
Sample Analysis and Characterization
The in line QMS measurements were obtained using a Stanford Re-
search Systems RGA300 by sampling the reactor effluent through
a differentially pumped orifice. X-ray diffraction (XRD) patterns
were collected on a Philips X’Pert X-ray diffractometer using Cu Ka
radiation (l=0.15405 nm). The samples were prepared in an
argon-filled glove box by pouring solid products onto glass sub-
strates. To prevent detrimental reactions due to the inevitable air
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Keywords: hydrogen sulfide
sodium · sodium sulfide
·
naphthalene
·
radicals
·
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Manuscript received: July 6, 2015
Final Article published: July 23, 2015
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