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The First Liquefaction of High-Rank Bituminous Coals by Preceding Hydrogenation with Homogeneous Borane or Iodine Catalysts.

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Coal Liquefaction
DOI: 10.1002/anie.200502614
The First Liquefaction of High-Rank Bituminous
Coals by Preceding Hydrogenation with
Homogeneous Borane or Iodine Catalysts**
Matthias W. Haenel,* Janchig Narangerel,
Udo-Burckhard Richter, and Anna Rufińska
Dedicated to Professor G"nther Wilke
on the occasion of his 80th birthday
The prospect that the worlds petroleum reserves will be
depleted within a few decades has caused the price of oil to
escalate. Hence the more abundant coal might become
economically competitive as a source for liquid fuels and
[*] Prof. Dr. M. W. Haenel, Dr. J. Narangerel, Ing. grad. U.-B. Richter,
Dr. A. Rufińska
Max-Planck-Institut f.r Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 M.lheim an der Ruhr (Germany)
Fax: (+ 49) 208 306–2980
[**] The work was supported by the German Academic Exchange Service
and the German Research Foundation (scholarships to J.N.).
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. Int. Ed. 2006, 45, 1061 –1066
chemicals. The Bergius direct coal liquefaction and the
indirect liquefaction consisting of coal gasification and
Fischer–Tropsch synthesis are two fundamental Coal-ToLiquid technologies that have been available since the first
half of the last century.[1] Direct liquefaction is a hydrocracking process by which coal is converted at 450 8C under
hydrogen pressures of > 30 MPa in the presence of a processderived solvent and a heterogeneous catalyst (mainly iron
oxides and sulfides).[1, 2] For the thermal CC bond breaking
and the transfer of hydrogen to carbon radicals, the solvent
plays a main role (“hydrogen shuttling” and “hydrogen
donation”).[3] On the other hand, in the initial liquefaction
stage the efficiency of the catalyst is low, mainly because a
heterogeneous inorganic solid cannot penetrate the macromolecular network structure of insoluble coal.[4] The expectation that a homogeneous catalyst would be more suitable in
this regard[5] prompted us some time ago to investigate the
hydrogenation/hydrogenolysis of coals with borane catalysts.[6] This led to the first example of an effective homogeneous hydrogenation of coal and opened a route for the
subsequent liquefaction of high-rank bituminous coals, which
up to now can be processed only by combustion or gasification.
At T > 200 8C trialkylboranes and tetraalkyldiboranes can
be used as homogeneous catalysts for the hydrogenation of
olefins[7] and polycyclic aromatic hydrocarbons[8, 9] including
coal tar pitch[10] and for the hydrogenolytic cleavage of CC
bonds, in particular the CaromCaliph bonds of diarylmethanes
and 1,2-diarylethanes.[8, 11] But whereas a cross-linked insoluble polystyrene was degraded with hydrogen in presence of
triethylborane forming benzene and various alkylbenzenes,
no reaction occurred with bituminous coal.[8, 12] However, by
using iododi-n-propylborane (nPr2BI, DPIB)[13] we observed
the hydrogenation/hydrogenolysis of bituminous coals under
non-liquefaction conditions (T < 350 8C, no hydrogen-donor
solvent). The product obtained by stirring a suspension of a
medium-volatile bituminous coal [mvb coal, German Fettkohle (coking coal)] in toluene together with DPIB for 48 h at
280 8C under hydrogen (15 MPa initial pressure at RT) was
still a solid, but its solubility in pyridine, relative to that of the
original coal, had changed from 13 % to 91 % and the C/H
ratio from C100H63 to C100H99 (Table 1, entries 1 and 4). The
dramatically increased solubility is attributed to hydrogenolytic cleavage of CaromCaliph bonds disrupting the network
structure of coal as well as to partial hydrogenation of
polycyclic aromatic units. The latter leads to an increase of
aliphatic carbon atoms at the expense of aromatic carbon
atoms, as shown by the solid-state 13C CP/MAS NMR spectra
(Figure 1 a,d; CP = cross polarization, MAS = magic angle
spinning). The Caliph/Carom ratio increased from 20:80 in the
original mvb coal to 40:60 in the product.
The 13C CP/MAS NMR spectra of the products from
control experiments without any catalyst or with iodine
(50 mol % compared to DPIB) as catalyst showed only a very
low or modest increase of the Caliph/Carom ratio from 20:80 to
22:78 or 27:73, respectively (Figure 1 b,c and Table 1, entries 2
and 3). It has been known for a long time that iodine in
various forms promotes coal liquefaction,[14] but comparison
of spectra (c) and (d) clearly shows that the activity of DPIB
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Table 1: Conditions and results for the hydrogenation/hydrogenolysis of bituminous coals.[a]
Cat. (mmol)
H2 [MPa]
T [8C]
t [h]
Sol.[d] [%]
C/H[f ]
mvb coal[g]
mvb coal
mvb coal
mvb coal
mvb coal
mvb coal
lvb coal[g]
lvb coal
lvb coal
lvb coal
lvb coal
lvb coal
lvb coal
lvb coal
lvb coal
I2 (15)
nPr2BI (29.6)
NaBH4/I2 (30/30)
NaBH4/I2 (30/30)
NaBH4/I2 (30/30)
NaBH4/I2 (30/30)
NaBH4/I2 (30/30)
NaBH4/I2 (30/30)
I2 (30)
I2/TiI4 (30/5)
BI3 (20)
NaBH4/I2 (30/30)
NaBH4/I2 (30/30)
I2 (30)
I2/TiI4 (30/5)
BI3 (20)
< 5:95
[a] Suspension of 40 g of coal in 150 mL of toluene, stirring autoclave with a volume of 500 mL. [b] Medium-volatile bituminous coal (mvb coal,
Fettkohle), Westerholt Mine, Robert Seam: 23.6 % volatile matter (daf), 89.4 % C (daf), 4.5 % ash (wf). Low-volatile bituminous coal (lvb coal,
Magerkohle), Niederberg Mine: 11.2 % volatile matter (daf), 91.2 % C (daf), 7.1 % ash (wf). Anthracite, Sophia Jacoba Mine: 7.4 % volatile matter,
91.9 % C (daf), 6.5 % ash (wf). daf: dry and ash-free; wf: water-free. [c] Pressure of hydrogen at room temperature. [d] Solubility in pyridine as
determined by Soxhlet extraction. [e] Ratio of aliphatic to aromatic carbon atoms from 13C CP/MAS NMR solid-state spectra. [f ] Carbon/hydrogen ratio
from elemental analysis. [g] Untreated original coal.
Figure 1. 13C CP/MAS NMR spectra (solid state, 75 MHz) and solubility in pyridine of a) the original mvb coal (German Fettkohle, Westerholt Mine, Robert Seam, Table 1) and b–f) the products obtained by
hydrogenation (15 MPa H2, 280 8C, toluene): b) without a catalyst,
48 h; c) with I2, 48 h; d) with DPIB, 48 h; e) with NaBH4/I2, 12 h;
f) with NaBH4/I2, 48 h. The spectra were calibrated to equal intensities
of the signals for Carom.
cannot be attributed solely to its iodine.[15] An equimolar
mixture of sodium borohydride and iodine can be used
conveniently to generate an iodoborane catalyst in situ, which
was found to be as active as DPIB (Figure 1 e,f and Table 1,
entries 5 and 6).[16]
The IR spectrum (KBr) of the coal product obtained by
hydrogenation with DPIB showed, in comparison to the
original mvb coal, the relative decrease of the aromatic CH
stretching (3040 cm1) vibration and the increase of the
symmetrical and asymmetrical aliphatic CH stretching
vibrations (2950–2850 cm1). But there was also a notable
decrease of the broad, weakly structured bands of CO
stretching vibrations between 1300 and 1000 cm1.[17] In the
course of the hydrogenation apparently some oxygen-containing functional groups in the coal, mainly phenolic ether
links,[4, 15a] were removed by stoichiometric reactions with the
borane catalyst, which is thus consumed and converted into
borates.[18] Since during geochemical coalification, the
number of oxygen-containing functional groups in coals
decreases, high-rank bituminous coals have the lowest
oxygen content.[4] Hence one could assume that the highrank bituminous coals in particular, even though they are
generally considered to be less reactive, might be the most
appropriate for the borane-catalyzed hydrogenation.
Indeed, a low-volatile bituminous coal (lvb coal, German
Magerkohle) suspended in toluene was hydrogenated under
more drastic conditions (350 8C, 25 MPa H2). Whereas the
control reaction (no catalyst, 6 h) resulted in only small
changes of Caliph/Carom ratio and the solubility (Figure 2 a,b,
Table 1, entries 7 and 8), extensive hydrogenation/hydrogenolysis was observed in the presence of equimolar NaBH4/
I2. With increasing reaction times (3, 6, 12, and 24 h) the Caliph/
Carom ratio increased from 11:89 in the original lvb coal to
60:40 in the product obtained from the hydrogenation after
24 h, and concomitantly the solubility in pyridine increased
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1061 –1066
Figure 2. 13C CP/MAS NMR spectra (solid state, 75 MHz) and solubility in pyridine of a) the original lvb coal (German Magerkohle,
Niederberg Mine, Table 1) and b–f) the products obtained by hydrogenation (25 MPa H2, 350 8C, toluene): b) without a catalyst, 6 h; c–
f) with NaBH4/I2 for 3, 6, 12, and 24 h, respectively. The spectra were
calibrated to equal intensities of the signals for Carom.
from < 1 % to 86 % (Figure 2 a,c–f, Table 1, entries 7 and 9–
12). Surprisingly, under the more drastic conditions (25 MPa
H2, 350 8C) iodine alone showed almost the same catalytic
activity as equimolar NaBH4/I2 in the hydrogenation of lvb
coal for 6 h (Table 1, entries 13 and 10).[19] The addition of
titanium tetraiodide (17 mol % relative to I2) raised the
activity of iodine to such an extent that this boron-free
catalyst became more active than equimolar NaBH4/I2 (cf.
entries 14 and 12 keeping the reaction times of 6 and 24 h in
mind). However, the best catalyst so far was found to be
boron triiodide, which after a hydrogenation time of 6 h gave
a product that was highly aliphatic (Caliph/Carom = 59:41) and
89 % soluble in pyridine (entry 16).[19]
A special challenge is the hydrogenation of anthracites,
which represent the high end of the rank range and are
distinguished by having the highest C/H and lowest Caliph/Carom
ratios of all coals.[4, 20] In comparison to the lvb coal the
hydrogenation of a German anthracite (toluene, 25 MPa H2,
350 8C) required considerably longer reaction times but still
resulted in the strong increase of aliphatic structures. With
equimolar NaBH4/I2 as the catalyst, the Caliph/Carom ratio
increased from < 5:95 in the original anthracite to 39:61 and
45:55 after reaction times of 12 h and 24 h, respectively
(Figure 3 a–c, Table 1, entries 17–19). For the hydrogenation
of anthracite, iodine alone was even somewhat more active
than equimolar NaBH4/I2 (24 h: Caliph/Carom = 51:49; entry 20,
Figure 3 d), and the activity also increased further by addition
of TiI4 (Caliph/Carom = 58:42; entry 21, Figure 3 e). However, as
in the case of the lvb coal, BI3 was the best catalyst and led to
Caliph/Carom = 62:38 in 24 h (entry 22, Figure 3 f).[19] This product, despite of the high aliphatic portion, was only 52 %
soluble in pyridine, and generally the solubility of the
hydrogenated products of anthracite was lower than those
of the lvb coal.
The number of catalytic cycles for the hydrogen uptake of
the coals can be calculated from the change in the C/H ratios
Angew. Chem. Int. Ed. 2006, 45, 1061 –1066
Figure 3. 13C CP/MAS NMR spectra (solid state, 75 MHz) and solubility in pyridine of a) the original German anthracite (Sophia Jacoba
Mine, Table 1) and b–f) the products obtained by hydrogenation
(25 MPa H2, 350 8C, toluene): b) with NaBH4/I2, 12 h; c) with NaBH4/
I2, 24 h; d) with I2, 24 h; e) with I2/TiI4, 24 h; f) with BI3, 24 h. The
spectra were calibrated to equal intensities of the signals for Carom.
determined by elemental analysis (Table 1, last column) and
the amount of catalyst. The number of catalytic cycles is about
20 in the case mvb coal and 28–43 in the case of lvb coal or
anthracite. The pressure drop of 4–6 MPa observed during the
coal hydrogenations corresponds surprisingly well to the
amount of hydrogen calculated from the elemental analysis
for the hydrogen uptake of the coals. This indicated a
sufficient stability of the solvent, and accordingly the toluene
recovered from the hydrogenations also contained only traces
of methylcyclohexane, benzene, and cyclohexane.
In investigations of model compounds, the three catalysts
iodine, equimolar NaBH4/I2, and boron triiodide showed
remarkable differences in yields and product distributions
(Scheme 1). The catalytic reactions of 1,2-di(1-naphthyl)ethane with hydrogen (15 MPa, cyclohexane,[17] 350 8C, 12 h)
resulted in hydrogenolytic CC bond cleavages at the
aliphatic link and hydrogenation of naphthalene units. With
iodine or equimolar NaBH4/I2 as the catalyst, naphthalene
was hydrogenated only partially to tetralin leaving one intact
benzene ring.[8, 9] In contrast, the reactions with the boron
triiodide catalyst led not only to a higher conversion and
extent of cleavage but also to the hydrogenation and hydrogenolytic degradation of tetralin units forming decalins and
alkylbenzenes, respectively. This difference was even more
pronounced in the case of the polycyclic aromatic hydrocarbon pyrene. The hydrogenation with iodine or equimolar
NaBH4/I2 gave mixtures of di-, tetra-, hexa-, and decahydropyrenes, but with boron triiodide the pyrene was completely
converted into aliphatic compounds, mainly perhydropyrenes
and compounds formed by subsequent hydrogenolytic degradation (Scheme 1).[17]
The Bergius process for direct coal liquefaction and all
new related processes give liquids in satisfactory yields only
for lignites (brown coals), subbituminous coals, and low-rank
bituminous coals such as high-volatile bituminous (hvb) coals.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 1. Hydrogenation/hydrogenolysis of 1,2-di(1-naphthyl)ethane and pyrene using I2, NaBH4/I2, and BI3 as the catalysts (15 MPa H2,
cyclohexane, 350 8C, 12 h). Product analysis by GC and GC/MS. If two or three products are listed in a column, the sum of the yields are given.
n.d.: not determined.
In contrast, high-rank bituminous coals (from mvb or
coking coals to lvb coals), semi-anthracite, and
anthracite are not reactive enough for such hydrocracking processes.[1, 21] On the other hand, exactly
these high-rank bituminous coals are, as shown, most
suitable for the hydrogenation with homogeneous
borane and iodine catalysts. As a result, the aliphatic
portion of these coals is increased to such an extent
that the subsequent conventional hydrocracking into
liquid products might become possible. The feasibility
of such a two-stage liquefaction process for high-rank
coals was demonstrated on the example of the lvb
coal (Magerkohle). For this purpose the original lvb
coal having 11 % Caliph and its hydrogenation product
Scheme 2. Comparison of the products obtained from the one-stage and the
having 60 % Caliph (obtained from the hydrogenation
two-stage liquefaction of the lvb coal (German Magerkohle, Niederberg Mine,
with equimolar NaBH4/I2, Table 1, entry 12) were
Table 1). “Gas”: gas and volatile compounds that co-distilled with tetralin; “oil”:
compared with respect their reactivity towards a
soluble in pentane; “asphaltenes”: insoluble in pentane but soluble in toluene;
hydrocracking process (Scheme 2). The suspension of
“asphaltols”(preasphaltenes): insoluble in toluene but soluble in pyridine;
the coal or the hydrogenation product in tetralin,
residue: insoluble in pyridine.[17] The 13C CP/MAS NMR signals indicate Caliph/
which is generally used to simulate a hydroaromatic
recycling oil as the hydrogen-donor solvent in coalliquefaction processes, was stirred without a catalyst
pentane, toluene, and pyridine (Scheme 2).[22] Whereas the
under hydrogen (15 MPa initial pressure at RT) at 430 8C for
12 h. After the tetralin had been distilled off, the product 1
one-stage liquefaction of the lvb coal in tetralin gave oil, that
is, compounds soluble in pentane, in only 20 % yield, the twoof the original coal showed an increase in mass of 3 %, but
stage process produced gas and oil in 66 % yield (17 % gas and
product 2 of the hydrogenated coal showed a loss in mass of
49 % oil).[17]
17 %. Whereas the former is attributed to tetralin being
adsorbed and/or bonded chemically as a result of radical
In summary, it was shown that iodoboranes and iodine can
reactions, in the latter case a conversion of coal into lowbe used as homogeneous catalysts to hydrogenate high-rank
molecular-weight compounds, gas and volatile compounds,
bituminous coals, including anthracite, resulting in a strong
has taken place. Both products 1 and 2 were characterized by
increase of the aliphatic at the expense of the aromatic
solvent fractionation, that is, consecutive extractions with
structures of these coals. In our view this represents a first
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1061 –1066
case of an extensive homogeneous and “real” coal hydrogenation in the sense of hydrogen addition to unsaturates of
coal, and hence contrasts with coal liquefaction by hydrocracking. An iodoborane catalyst could simply be generated
in situ from the equimolar mixture of sodium borohydride
and iodine. Iodine itself, which is known for a long time to
promote the liquefaction of low-rank coals,[14] was shown to
be also useful as a homogeneous catalyst for hydrogenating
high-rank bituminous coals under non-liquefaction conditions. By adding titanium tetraiodide the catalytic activity of
iodine could be increased, but remained still lower than that
of boron triiodide, the best catalyst so far. The hydrogenation
of simple model compounds with different catalysts revealed
remarkable differences in yields and product distributions,
which to these details are unobtainable with coal. In view of
the progress recently made in the transition and rare earth
metal catalyzed hydroboration of alkenes[23] and borylation of
CH bonds,[24] combinations of boranes, iodine and various
metal compounds appear to have good prospects to develop
catalysts having activities high enough for applications. The
homogeneous coal hydrogenation presented for the first time
also enables high-rank bituminous coals to be liquefied in
subsequent conventional hydrocracking processes, and thus
might help to strengthen our potentialities of Coal-To-Liquid
Received: July 26, 2005
Published online: January 3, 2006
Keywords: boranes · coal liquefaction ·
homogeneous catalysis · hydrogenation · iodine
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mainly in phenolic ether links. b) The increase of the solubility in
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1061 –1066
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liquefaction, first, coal, homogeneous, high, preceding, iodine, hydrogenation, boranes, rank, catalyst, bituminous
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