First observation of highly efficient dehydrogenation of methanol to
anhydrous formaldehyde over novel Ag–SiO –MgO–Al O catalyst†
2
2
3
a
a
a
b
a
Li-Ping Ren, Wei-Lin Dai,* Yong Cao, Hexing Li and Kangnian Fan*
a
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry,
Fudan University, Shanghai 200433, P. R. China. E-mail: wldai@fudan.edu.cn; Fax: 86-21-65642978
Department of Chemistry, Shanghai Normal University, Shanghai 200234, P. R. China
b
Received (in Cambridge, UK) 27th August 2003, Accepted 22nd October 2003
First published as an Advance Article on the web 11th November 2003
Novel Ag–SiO
2
–MgO–Al
2
O
3
catalyst prepared by sol–gel
The preparation of the C catalyst was as follows: 32.4 ml of
Si(OC (TEOS) was mixed with 50 ml of ethanol. Then 3
mol·dm HNO was added dropwise until pH = 5.0. The
solution was refluxed by agitating with a magnetic stirrer in an
0 °C oil bath for 1.0 h to obtain a silica sol solution. After that,
to the refluxing solution was added a desired amount of aqueous
solution containing AgNO Al(NO ·9H and
Mg(NO ·6H O, followed by 50 ml of distilled water. The
resulting mixed sol was stirred at 75 °C until the gel was formed
( ~ 12 h). Then the gel was dried at 120 °C over night followed
by calcination at 800 °C in air for 12 h and finally ground to
grain size of 40–60 meshes for catalytic test.
method showed extremely high activity and selectivity (both
equal to 100%) in the direct dehydrogenation of methanol to
anhydrous formaldehyde.
2 5 4
H )
2
3
3
7
Beyond the memory of catalysis, one of the ultimate goals of all
the chemists in this field is to find or develop the highly efficient
catalyst. That is, the raw material will be converted completely
to the product without any by-products. Therefore, the separa-
tion of the product from the product mixture can be omitted due
to the absence of any unconverted raw materials and/or by-
products and the process will be easily industrialized because of
its simpleness and low-cost. In recent years, as a widely-used
and commodity chemical, anhydrous formaldehyde (HCHO)
has been paid more and more attention due to its comprehensive
application in many fields, such as the preparation of many new
types of agricultural chemicals and medical intermediates, the
3
,
3
)
3
2
O
3
)
2
2
The direct dehydrogenation of methanol was carried out in a
fixed-bed flow-type quartz reactor (i.d. = 4.5 mm) at a
temperature range of 350–700 °C with the space velocity
4
21
(GHSV) at 1 3 10 h . Before feeding methanol into the
reactor, the catalyst was treated in situ with high purity
Ar( > 99.999%) flowing at 600 °C for 3 h to get rid of any
influence of the adsorbed oxygen in the catalyst. Methanol was
evaporated and then fed into the reactor by Ar flow. The weight
ratio of methanol in Ar was determined as 19.3%. The products
were analyzed by on line gas chromatography-mass spectros-
1
synthesis of polyoxymethylene and solid HCHO etc. Gen-
erally, anhydrous HCHO is now obtained by the dehydration of
aqueous solution of HCHO, which mainly produced from
partial oxidation of methanol by air in the presence of a silver
catalyst or an iron-molybdenum catalyst.2 However, the
removal of water and methanol from the aqueous solution of
HCHO is very cumbersome and expensive, leading to the high
price of anhydrous HCHO. Thus a one-step process to produce
water-free HCHO is a desirable route. It is well known that
methanol can be converted to anhydrous HCHO and hydrogen
,3
copy (GC-MS). CH
water were determined by TCD with a Propak-N column, while
, CO, CO were analysed also by TCD with a TDX column.
3
OH, HCHO, di-methyl ether (DME) and
H
2
2
Carbon balance was tested as 0.98 ~ 1.02.
From Table 1, it is very interesting to find that, besides Ag
and SiO , the catalyst with Al O leads to higher selectivity
2 2 3
4
without any catalyst at a very high temperature. So the direct
dehydrogenation of methanol is a very promising way to
produce anhydrous HCHO and the selection or development of
a highly active and selective catalyst is pivotal.
Many materials, such as: transition metal based oxides,
sodium carbonate, and zeolites, showed certain activity in the
title reaction, however, the results of those catalysts reported
were not satisfactory due to their low conversion of methanol,
while the other with MgO shows much higher methanol
conversion under similar reaction conditions. Thus, one can
naturally get an idea to try for the composite catalyst consisting
of both Al
mass ratio of Ag : SiO
2
O
3
and MgO. To our great surprise, C with a relative
: MgO : Al at 20 : 55.2 : 8.3 : 16.5
2
2 3
O
shows a highly efficient catalytic performance, that is, 100%
conversion and 100% selectivity. Under the optimal conditions,
low selectivity toward HCHO and/or the high reaction tem-
H
2
and HCHO were almost in equal mount as detected by GC
and the consumed methanol was also equal to the produced
HCHO, while CO and CO were undetectable. In addition, no
5
perature. As we know, besides the main reaction (CH
3
OH ?
), there are two side-reactions: the decomposition of
methanol (CH OH ? CO + 2H ) and the dehydration reaction
2CH OH ? (CH O + H O), respectively. To the best of our
HCHO + H
2
2
3
2
deposited carbon was observed on the surface of the catalyst,
indicating that there were no other carbon-containing products
formed under the reaction conditions and methanol was all
converted to HCHO and H
novel catalyst will declined to 10% CH
(
3
3
)
2
2
knowledge, there was no any catalysts reported, which showed
yield of HCHO exceeding 70%, restricting its industrial
application. Our previous results showed that silver-based
2
. However, the performance of the
OH conversion after a
3
catalysts were considered to be promising candidates in the
10 h reaction due to the accumulation of silver on the surface as
described below. But the selectivity toward HCHO still kept at
100%, indication that there were no any other impurities present
6
above reaction. The Ag–SiO
2
–Al
2
O
3
(A) showed much higher
–MgO(B) showed
selectivity toward HCHO, while Ag–SiO
2
higher methanol conversion. Thus, we believe that one silver-
based catalyst that possesses the perfect performance in the
direct dehydrogenation could be obtained through careful
tuning of the surface properties of the catalyst. Herein we report
for the first time the preparation and characterization of the
novel Ag–SiO –MgO–Al O (C) catalyst, which shows a highly
2 2 3
efficient performance toward the direct dehydrogenation of
methanol.
in the product except for the unconverted CH
reaction time.
3
OH for a long
In addition, the content of silver influenced little on its
activity when the silver content exceeded 20%. However, much
higher silver content ( > 50%) will do harm to its catalytic
performance. Lower silver content only leads to lower conver-
sion of methanol, the selectivity still keeps at 100%. The
relative composition of the novel C catalyst shows big influence
on its catalytic performance. As can be seen from Table 1, there
is an optimal composition among the composite oxides. The
optimal content of MgO was first determined as 15 wt% as
†
Electronic supplementary information (ESI) available: XRD and SEM.
See http://www.rsc.org/suppdata/cc/b3/b310316a/
2
compared to SiO according to a series of “trial and error”
3
030
CHEM. COMMUN., 2003, 3030–3031
This journal is © The Royal Society of Chemistry 2003
Ren, Li-Ping
