Vapor-phase dehydration of C4 unsaturated alcohols to 1,3-butadiene

Article abstract of DOI:10.1016/j.apcata.2016.11.035

Vapor-phase dehydration of C4 unsaturated alcohols such as 3-buten-1-ol, 2-buten-1-ol and 3-buten-2-ol were performed to produce 1,3-butadiene over various solid catalysts including ordinary solid acid catalysts such as Al₂O₃, SiO₂-Al₂O₃, and TiO₂ and basic rare earth metal oxides such as Yb₂O₃ and CeO₂. In the reaction of 3-buten-1-ol, 1,3-butadiene could not be selectively produced because the acid catalysts decomposed 3-buten-1-ol into propylene. In contrast to the acid catalysts, CeO₂ inhibited the decomposition of 3-buten-1-ol and showed a relatively high performance for the formation of 1,3-butadiene. In the dehydration of 2-buten-1-ol and 3-buten-2-ol, however, acid catalysts were effective for the formation of 1,3-butadiene. Among the tested catalysts, commercial SiO₂-Al₂O₃ showed a relatively high catalytic performance, although it deactivated rapidly. We prepared a series of SiO₂/Al₂O₃ catalysts by depositing SiO₂ species onto Al₂O₃ support, and the SiO₂/Al₂O₃ with a SiO₂ content of 10?wt.% showed more stable catalytic activity than the commercial SiO₂-Al₂O₃. Furthermore, modification of SiO₂/Al₂O₃ with Ag at a loading of 3–5?wt.% was found to be efficient for inhibiting the coke formation and improving the catalytic stability of SiO₂/Al₂O₃ in the dehydration of both 2-buten-1-ol and 3-buten-2-ol under hydrogen flow conditions.

Full text of DOI:10.1016/j.apcata.2016.11.035

Accepted Manuscript
Title: Vapor-phase dehydration of C4 unsaturated alcohols to
1
,3-butadiene
Author: Daolai Sun Shigenori Arai Hailing Duan Yasuhiro
Yamada Satoshi Sato
PII:
S0926-860X(16)30584-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.11.035
APCATA 16083
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
21-9-2016
24-11-2016
29-11-2016
Please cite this article as: Daolai Sun, Shigenori Arai, Hailing Duan, Yasuhiro Yamada,
Satoshi Sato, Vapor-phase dehydration of C4 unsaturated alcohols to 1,3-butadiene,
Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.11.035
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A revised manuscript fortheoriginalpapersubmittedtothe journalofAppl. Catal.A:Gen.
APCATA-D-16-01668
Vapor-phase dehydration of C4 unsaturatedalcohols to1,3-butadiene
DaolaiSun, ShigenoriArai, Hailing Duan, Yasuhiro Yamada, SatoshiSato*
GraduateSchoolofEngineering, ChibaUniversity, Chiba,263-8522, Japan
*
Correspondingauthor.Tel. +81432903376;Fax:+81432903401
E-mailaddress:satoshi@faculty.chiba-u.jp(S. Sato)
1

Graphic abstract
Highlights


Vapor-phasedehydrationofC4unsaturatedalcoholsto1,3-butadienewereperformed.
1,3-Butadieneselectivityof97.1and93.4%overSiO -Al wasachievedfrom2-buten-1-oland3-
buten-2-ol, respectively.
ModificationofSiO /Al
Acidcatalystspromoted3-buten-1-oldecompositionandgavelow1,3-butadieneselectivities.
CeO showedbetterperformancethanSiO -Al inthedehydrationof3-buten-1-olto1,3-butadiene.
2
2 3
O



2
2
O byAgwas effectiveforinhibitingthecatalyticdeactivation.
3
2
2
O
2 3
Abstract
Vapor-phase dehydration ofC4 unsaturated alcohols such as 3-buten-1-ol, 2-buten-1-oland
-buten-2-olwereperformedtoproduce1,3-butadieneover varioussolidcatalysts includingordinary
3
solid acid catalysts such as Al
2
O
3
, SiO
2
-Al
2
O
3
, and TiO and basic rare earth metal oxides such as
2
Yb and CeO . In the reaction of 3-buten-1-ol, 1,3-butadiene could not be selectively produced
2
O
3
2
because the acid catalysts decomposed 3-buten-1-ol into propylene. In contrast to the acid catalysts,
2

CeO inhibited the decomposition of3-buten-1-oland showed a relatively high performance for the
2
formation of 1,3-butadiene. In the dehydration of 2-buten-1-ol and 3-buten-2-ol, however, acid
catalysts were effective for the formation of 1,3-butadiene. Among the tested catalysts, commercial
SiO
prepared a series of SiO
SiO /Al with a SiO
commercialSiO -Al . Furthermore, modificationofSiO
2
-Al
2
O
3
showed a relatively high catalytic performance, although it deactivated rapidly. We
/Al catalysts by depositing SiO species onto Al support, and the
content of 10 wt.% showed more stable catalytic activity than the
/Al with Ag at a loading of3-5 wt.%
2
2
O
3
2
O
2 3
2
2
O
3
2
2
2
O
3
2
O
2 3
was found to be efficient for inhibiting the coke formation and improving the catalytic stability of
SiO /Al inthedehydrationofboth2-buten-1-oland3-buten-2-olunder hydrogenflow conditions.
2
O
2 3
Keywords:C4unsaturatedalcohols, 1,3-butadiene, Ag-modifiedSiO
2
/Al
2
O , catalyticdeactivation.
3
1. Introduction
1,3-Butadiene (BD) is an important bulk chemical used for producing rubber and polymers.
Currently, BD is primarily produced as a by-product in ethylene plant through the naphtha steam
cracking [1-3]. In recent years, the production and the supply of BD are influenced by the unstable
price of crude oil as well as the increasing production of shale gas. Thus, alternative routes for
producing BD are expected. The conversion of ethanol to BD, which has been attempted at the
industrial scale, is the most reported alternative route to replace fossil oils and gases, and ca. 80%
yield of BD could be achieved in some catalytic processes [2, 3]. The recent developments on the
fermentation technology for producing bio-derived ethanol further encourage the investigations for
BD production from ethanol. On the other hand, butanediols such as 1,3-butanediol, 1,4-butanediol
and 2,3-butanediol are derivable from biomass by either fermentation or catalytic conversion [4-6],
andtheyalso havepotentialasresources forproducingbiomass-derivedBD[3].
3

Scheme 1 shows the production route of BD from butanediols. Butanediols are expected to
be converted to BD by two-step dehydration via unsaturated alcohols. However, among the
butanediols, 2,3-butanediol is the only one butanediol that can be directly converted into BD in the
dehydration over ThO
2
[7], Sc
2
O
3
[8] Na-P/SiO
2
[9], and CsH
2
PO
4
/SiO [10]. In addition, the
2
dehydration of butanediols to unsaturated alcohols, as the first step, is also difficult to be selectively
catalyzed by solid acid catalysts: 1,4-butanediol and 2,3-butanediol are primarily converted to
tetrahydrofuran and 2-butanone, respectively, over acid catalysts such as Al
2
O
3
[11-13]. In the case
of 1,3-butanediol, solid acids such as SiO -Al , Al , and TiO produce a wide variety of
2
2
O
3
2
O
3
2
products including 3-buten-1-ol, formaldehyde, acetaldehyde and acetone [14, 15]. We have
conductedaseriesofworksonthevapor-phasedehydrationofbutanediols, and foundthat somerare
o
earth metal oxides calcined at high temperatures such as 800 C were effective for the selective
formation of unsaturated alcohols from butanediols [16]. In particular, CeO gave ca. 90% total
2
selectivityto 3-buten-2-ol(3B2OL) and 2-buten-1-ol(2B1OL) in the dehydration of1,3-butanediol
o
at 325 C [17]; Yb
2
O
3
gave 87.6% selectivity to 3-buten-1-ol (3B1OL) in the dehydration of 1,4-
provided ca. 90% selectivity to 3B2OL in the dehydration of 2,3-
o
butanediol at 375 C [18]; Sc
O
2 3
o
butanediol at 325 C [19]. The achievements on the production of unsaturated alcohols from
butanediols provide an opportunity for producing BD from butanediols via two-step dehydration.
On the other hand, although such a novelroute for BD production has been paid much attention [2,
3], detailed studies on the second step, which is selective dehydration of C4 unsaturated alcohols to
BD, havenotbeenreportedexceptthepreliminaryreports[8, 15, 20, 21].
In this study, vapor-phase dehydration of C4 unsaturated alcohols such as 2B1OL, 3B1OL
and 3B2OLwas studied to produce BD over various solid catalysts. The suitable catalyst systemfor
each reaction was investigated, and the studies for the inhibition of the catalytic deactivation were
also conducted.
2. Experimental
2.1Samples
4

3B1OL and 2B1OL were purchased from Tokyo Chemical Industries Co., Ltd., Japan, and
2B1OL was a mixture of cis and trans isomers with a cis/trans ratio of ca. 1/17. 3B2OL was
purchased fromSigma-AldrichCo.,Ltd., Japan. 1-Hexanol, whichwasusedasthe internalstandard,
waspurchased fromWako PureChemicalIndustries, Ltd.,Japan.
2
-1
γ-Al
2
O
3
(N611N, specific surfacearea, SBET=210 m g )andSiO
m g ) with an Al content of ca. 15 wt.% were purchased from Nikki Chemical Co., Ltd.
Another Al (JRC-ALO-7, SBET= 180 m g ) and anatase TiO
were supplied by Catalyst Society of Japan.Yb was purchased from Kanto Kagaku Co. Ltd.,
2
-Al
2
O (N631L, SBET=440
3
2
-1
O
2 3
2
-1
2
-1
2
O
3
2
(JRC-TIO-4, SBET= 50 m g )
2 3
O
2
-1
2 -1
Japan. Monoclinic ZrO
byDaiichiKigenso Kagaku Kogyo Co. Ltd, Japan. Yb
before the use as catalysts, and the specific surface areas of Yb
2
(RSC-HP, SBET= 39 m g ) andCeO
2
(HS, SBET= 98 m g ) were supplied
o
2
O
3
andCeO
and CeO
, and Cu(NO
2
werecalcinedat 800 C for3h
2
O
3
2
after calcination were
·3H O were
2
-1
2
9 and 42 m g , respectively. Tetraethyl orthosilicate, AgNO
3
3
)
2
2
purchased fromWako PureChemicalIndustries, Ltd., Japan.
Self-made SiO
2
/Al
2
O
3
was prepared by an adsorption method described in the related
(JRC-ALO-7) was used as the support, and an ethanol solution of
procedure elsewhere [22]. Al
O
2 3
tetraethyl orthosilicate was used as the precursor ofSiO
2
. The Al
2
O
3
was immersed in the tetraethyl
was filtered, the resulted
/Al with a
) had a specific surface area of 162 m g . Fig. S1 and S2
isotherm and pore size distribution of 10SiO /Al , respectively. It is obvious that
isa mesoporous material.
Metal-modified SiO /Al catalysts were prepared by an incipient wetness impregnation
o
orthosilicate solution at 25 C for 168 h. After the Al
2
O
3
coated with SiO
2
o
o
catalysts were dried at 110 C for 12 h, and then calcined at 500 C for 3 h. The SiO
2
O
2 3
2
-1
SiO
show the N
0SiO /Al
2
loading of 10 wt.% (10SiO
2
/Al
2 3
O
2
2
O
2 3
1
2
O
2 3
2
O
2 3
method using a solution with a prescribed amount of metal nitrate dissolved in distilled water. Self-
made 10SiO /Al was used as the support. AgNO and Cu(NO ·3H O purchased from Wako
2
2
O
3
3
3
)
2
2
Pure Chemical Industries, Ltd., Japan were used as the precursor of the corresponding metal. After
o
the impregnation process, the samples were dried at 110 C for 12 h, and then calcined for 3 h. The
o
calcinationtemperatureofmetal-modifiedSiO
2
/Al
2
O
3
catalystswas500 C.
5

Hereafter, the commercially supplied SiO
The self-made SiO /Al catalysts are named as xSiO
percentage of SiO . Metal-modified SiO /Al catalysts are denominated as yM-xSiO
2
-Al
2
O
3
(N631L) is named as SiO
/Al , where x indicates the weight
/Al
2
-Al
2
O
3
-CS.
2
2
O
3
2
O
2 3
2
2
2
O
3
2
2
O ,
3
where yindicatesthe weight percentageofthe additive metal;M indicatesthe speciesofthe additive
metal. Forexample, 3Ag-10SiO /Al indicates3wt.%Ag loadedonaself-madeSiO /Al with
2
2
O
3
2
O
2 3
aSiO of10wt.%.
2
2
.2Catalyticreaction
The dehydrationofunsaturated alcohols was performed ina fixed-bed down-flow glass reactorwith
an inner diameter of 17 mm at an ambient pressure of N . Prior to the reaction, either 0.5 or 0.2 g
catalyst was preheated at the reactiontemperature for 1 h. After that, an unsaturated alcoholwas fed
2
3
-1
3
-1
to the reactor at a liquid feed rate of 1.75 cm h together with N flow of 30 cm min . The liquid
2
o
effluents collected in a trap at 0 C every hour were analyzed by a FID-GC (GC-2014, Shimadzu,
Japan) witha30-mcapillarycolumnofTC-WAX (GL-Science, Japan). The gaseous productswere
identified by on-line gas chromatography (GC-8A, Shimadzu, Japan) with a 6-m packed column
(VZ-7, GL-Science, Japan). A GC-MS (QP5050A, Shimadzu, Japan) was used for the
identificationofthe collected liquid products. 1-Hexanolwas used as an internal standard substance.
The mass balance could be achieved at a range of 70-99%. The conversion and selectivity were
calculatedas follows:
푚표푙푒 표푓 푟푒푐표푣푒푟푒푑 푟푒푎푐푡푎푛푡
Conversion (%) = (1 −
) × 100
(Equation1)
(Equation2)
푚표푙푒 표푓 푡ℎ푒 푟푒푎푐푡푎푛푡
푚표푙푒 표푓 푐푎푟푏표푛 ꢀ푛 푠푝푒푐ꢀ푓ꢀ푐 푝푟표푑푢푐푡
푚표푙푒 표푓 푐푎푟푏표푛 ꢀ푛 푐표푛푣푒푟푡푒푑 푟푒푎푐푡푎푛푡
Selectivity (%) =
× 100
2.3Characterizationofcatalysts
The specific surface area of the catalysts was measured by the BET method using N
2
o
isotherm at -196 C. The temperature-programmed desorption (TPD) of adsorbed NH
measured by neutralization titration using an electric conductivity cell immersed in an aqueous
solution of H SO . The amount of desorbed NH , which was bubbled into the solution, was
3
was
2
4
3
6

monitored bythe change in the conductivityofthe solution. Acumulative amount ofdesorbed NH
3
was obtained as a function oftemperature and then differentiated to obtain a TPD profile as an acid
strengthdistribution. TheTPDwasperformedusingself-madeapparatuswithasampleweight ofca.
o
-1
3
0 mg, atemperature increaseof10 C min , and a heating range fromthe roomtemperatureto 850
oC. The systemimage ofthe experiment forthe measurement ofNH
3
-TPD is shown in Fig. S3, and
the details ofTPD experiment are described inthe supporting information as well as in our previous
studies [23, 24]. The thermogravimetry analysis (TG) was conducted using Thermoplus 8120E2
(Rigaku) under the following conditions: sample weight, ca. 10 mg; the rate of the temperature
o
-1
o
increase, 5 C min ;heatingrange, fromtheroomtemperatureto 900 C.
3
. Resultsanddiscussion
.1. Dehydrationof2B1OL, 3B2OLand 3B1OLovervarioussolidcatalysts.
Firstly, some commercial catalysts were tested in the dehydration of 2B1OL, 3B2OL and
B1OL. The catalysts can be divided to two groups: one group includes the ordinary solid acid
3
3
catalysts such as Al
2
O
3
, SiO
2
-Al
2
O
3
-CS, TiO
2
and ZrO ; the other group includes some rare earth
2
metal oxides such as CeO
2
2 3
and Yb O , which show specifically high catalytic performance for the
selective formation of unsaturated alcohols in the dehydration of 1,3-butanediol [17] and 1,4-
butanediol [18], respectively. The rare earth metal oxides have weak basicity, which has been
confirmed inourprevious study[25].
Table 1 shows the reaction results in the dehydration of 2B1OL over various catalysts. The
reactions were performed at different temperatures according to the different activityofthe catalysts.
3
-1
The catalyst weight and the flow rate of N carrier gas were 0.5 g and 30 cm min , respectively.
2
Besides BD, butenes, 1-butanol, butanal, and ethers were generated with different selectivities
depending on the catalysts. Here, the butenes include 1-butene, cis- and trans-2-butene, but do not
include isobutene. The ethersrepresentthe linerunsaturated ethers, which havea formulaofC
8
H14O,
produced by the intermolecular dehydration of 2B1OL. SiO -Al -CS showed a high activity for
2
O
2 3
the formationofBD, and97.1%BD selectivitywith99.3%2B1OLconversionwasachievedat 260
7

oC. Al
because ofthe formation of ethers and undetected products. TiO
formation ofBD from2B1OL, whereas the selectivities to BD were much lower than SiO
CS. Yb showed78.9%selectivityto BDwitharelativelylow conversionof42.4%. CeO
O
gave a 2B1OL conversion of 90.3%, while the selectivity to BD was as low as 27.2%
and ZrO were also active for the
-Al
gavea
2 3
2
2
2
2
O -
3
2
O
3
2
high 2B1OL conversion of 93.4%, whereas the selectivity to BD was as low as 21.3% because of
the by-productionofbutanalandundetectedproducts.
Table 2 shows the reaction results in the dehydration of 3B2OL. BD, butenes, 2-butanone,
and ethers werethe main products. SiO
2
-Al
2
O -CS was also effective forthe dehydrationof3B2OL
3
to BD, and provided 93.4% selectivity to BD at a 3B2OL conversion of74.4%. The selectivities to
BD were 52.1%, 58.9and 48.3%over Al
2
O
3
, TiO
2
2
and ZrO , respectively. These were much lower
than SiO -Al -CS. Al gave a relatively high selectivity to ethers, and ZrO
2
2
O
3
2
O
3
2
gave a relatively
high selectivity to 2-butanone. Al
etherificationofalcohols suchas methanolto dimethyl ether [26-28], which canexplain the fact that
Al gave high selectivities to ethers in the dehydration of either 2B1OL or 3B2OL. In contrast to
2
O
3
has been widely used as an effective catalyst for the
O
2 3
the ordinary acid catalysts, the rare earth metal oxides showed low catalytic activities for the
formation of BD from 3B2OL. Yb gave a relatively high selectivity to 2-butanone, while CeO
2
O
3
2
gavethe highestselectivitytobutenes.
Based on the above reaction results, we found that acid catalysts were effective for the
dehydration of2B1OL and 3B2OL to BD. Compared with Al
2
O
3
, TiO
2
and ZrO
2
, SiO
2
-Al
2
O
3
-CS
was more effective for the dehydration of both 2B1OL and 3B2OL to BD. In Section 3.2, we will
give a further study on the dehydration of 2B1OL and 3B2OL to BD over self-made SiO
2
/Al
2
O -
3
basedcatalysts.
8

Table 3 shows the reaction results in the dehydration of 3B1OL, and Scheme 2 shows the
main products. The formation of BD, propylene, formaldehyde, methanol, 1-butanol, butanal,
2B1OL, and ethers were confirmed byGC-MS, while formaldehyde was unable to be detected and
quantified byFID-GC. The formationofpropylene, methanol, and formaldehyde indicates that C-C
cleavage of 3B1OL occurred in all the tested reactions, which is different from the case in the
reactions of 3B2OL and 2B1OL. In a similar way, it is known that 3-methyl-1,3-butanediol
decomposes to isobutene, formaldehyde and methanol via 3-methyl-3-buten-1-ol [29]. The acid
catalysts gave BD selectivities lower than 44% and propylene selectivities higher than 17%,
indicating that C-C cleavage of 3B1OL was easily catalyzed by acid catalysts. Among the tested
catalysts, SiO
selectivity of 53.1%. In the case of rare earth metal oxides, Yb
5.9%andaBDselectivityof33.8%, while CeO gavethe lowest propyleneselectivityof9.3%and
inhibits the C-C cleavage of
2
-Al
2
O
3
-CS gave the lowest BD selectivity of 12.0% and the highest propylene
O
2 3
gave a propylene selectivity of
2
2
the highest BD selectivity of 54.5%. This result indicates that CeO
B1OLand is relativelyeffective forthe formationofBD from3B1OL.
Inthe case ofacid catalysts, the order of the reactivity ofthe unsaturated C4 alcohols under
the same conditions is 2B1OL> 3B2OL> 3B1OL (Tables 1-3), which means 3B1OL is less
reactive over the acids. However, in the case of CeO , the reactivity of 3B1OL is similar to that of
B1OL, and is higher than that of 3B2OL. This result suggests that the catalytic mechanism over
acid catalysts and rare earth metaloxides such as CeO is different inthe dehydrationofunsaturated
C4 alcohols. We propose that the different positions ofthe hydroxyl group and the double bond in
B1OL, 3B2OL, and 3B1OL lead to different adsorption modes on catalyst surface, which causes
thedifferent reactivityandtheproduct distribution. Inthedehydrationof1,3-butanediol, CeO witha
cubic fluorite structureshowsspecificallyhighactivityforthe formationof2B1OLand3B2OL[16].
2
3
2
2
2
2
2
4+
3+
The specific activity of CeO
2
was supposed to be assigned to the Ce -Ce redox nature of CeO ,
2
which activated the H atom at position 2 of 1,3-butanediol firstly, followed by the dehydration of
,3-butanediol over CeO through a radical mechanism: an H atom located at position 2 was
1
2
abstracted first, followed by the elimination of an OH radical at position 1 [16, 20]. The proposed
9

mechanism was also supported by density functional theory (DFT) and paired interacting orbital
PIO) calculations [30, 31]. Based on the previous findings, we supposed a reaction mechanism for
the dehydration of 3B1OL to BD over CeO as shown in Scheme 3. It is reasonable that the
(
2
dehydrationof1,3-butanediolover CeO
2
also undergoes aradicalmechanism, which is quite similar
above-mentioned.
to thedehydrationof1,3-butanedioloverCeO
2
3
.2. Catalytic activity of self-made SiO
2
/Al
2
O modified by metals in the dehydration of 2B1OL and
3
3B2OL.
In the dehydration of 2B1OL and 3B2OL, although SiO
2
-Al
2
O -CS showed a relatively
3
high activity for 1,3-butadiene formation among the acid catalysts, it deactivated rapidly under low
contact time conditions. Deactivation of catalysts is a critical problem in catalytic conversion under
continuous flow conditions especially in a practical industrial process. Thus, further studies were
conducted inordertodevelopstablecatalysts forthedehydrationof2B1OLand3B2OL.
Firstly, we prepared a series of SiO
contrast to SiO -Al -CS with a low Al
prepared with low SiO contents. Fig. 1 shows the dependence of the catalytic activity on the SiO
2
/Al
2
O
3
-based catalysts to regulate the acid strength. In
2
2
O
3
2
O
3
content of ca. 15 wt.%, the SiO
2
/Al catalysts were
2 3
O
2
2
content inthe dehydrationof2B1OL, andthe product distribution ineachreactionandthe change in
the catalytic activity with time on stream are shown in Table S1 and Fig. S4. The conversion of
2
B1OL increased with increasing the SiO
at SiO contentsof5-10 wt.%. The selectivityto BD increased with increasing the SiO
was maximized at 97.5% at a SiO content of 10 wt.%. On the other hand, the selectivity to ethers
was higherthan20% over the Al support without SiO , whereas it decreased with increasing the
SiO content and decreased to 0% at SiO contents higher than 5 wt.%. These results demonstrate
that the self-made SiO /Al catalysts are effective for the dehydration of 2B1OL to BD, and the
suitableSiO content inSiO /Al is10wt.%.
2
content, and 100% conversion of2B1OL was achieved
2
2
content, and
2
2
O
3
2
2
2
2
2 3
O
2
2
2 3
O
1
0

In order to study the catalytic stability, the dehydration of 2B1OL was performed at a low
W/Fof0.14 h(Fig. 2), where Wand Farecatalyst weightand flowrateofreactant, respectively. Fig.
2
shows the changes in activity ofSiO
of 0.14 h. SiO -Al -CS significantly deactivated with time on stream. In contrast, the activity of
0SiO
2
-Al
2
O
3
-CS and 10SiO
2
/Al
2
O with time on streamat a W/F
3
2
O
2 3
1
2
/Al
In our group, an effective operation for stabilizing the catalytic activity ofacid catalysts has
been reported [24, 32-35]. For example, vapor-phase aldol condensation of butanal to 2-ethyl-2-
2
O
3
was morestable, while slight decrease intheconversionof2B1OLwas stillobserved.
hexenal can be catalyzed byTiO
2
, whereas TiO deactivates rapidly because ofthe accumulationof
2
coke on the catalyst surface. We have developed an Ag-modified TiO
stablecatalyticactivityforthe formationof2-ethyl-2-hexenalfrombutanal[35]. Inasimilarway,the
loading of Ag onto SiO -Al , Cu onto Al , and Ag onto Al are found to be effective for
2
catalyst, which exhibits
2
2
O
3
2
O
3
O
2 3
stabilizing the vapor-phase dehydration of 1,2-propanediol into propanal [24], dehydration of
tetrahydrofurfurylalcoholinto 3,4-2H-dihydropyran[33],and cyclodehydrationofdiethylene glycol
to 1,4-dioxane [34], respectively. In these processes, H
catalytic deactivation. Beforethereaction, the loaded metaloxide isreduced to metal, and it removes
theproductonthecatalyst surfacetogetherwithH to suppressthe formationofcoke.
2
carrier gas is required for inhibiting the
2
In the present study, Ag- and Cu-modified 10SiO
2
/Al
2
O catalysts were prepared and tested
3
in the dehydrationof2B1OL. Table 4 summarizes the reactionresults in the dehydrationof2B1OL
over 10SiO
Compared with 10SiO
modification of 10SiO
8.5%. On the other hand, although 3Cu-10SiO
2
/Al
2
O
3
, 3 wt.% Ag- and Cu-modified 10SiO
/Al , both the conversion and the selectivity to BD were improved by the
/Al with 3 wt.% Ag: the selectivity to BD was 94.1% at conversion of
/Al gave a high conversion of 98.6%, the
2
/Al
2
O
3
in H
2
flow at a W/F of 0.34 h.
2
2 3
O
2
O
2 3
9
2
O
2 3
selectivity to BD was only 0.6%, which was attributed to the production of butenes and 1-butanol
with selectivities of 51.8 and 21.3%, respectively. Since Cu is widely used as a hydrogenation
catalyst [36, 37], it is reasonable that the loaded Cu metal catalyzes the hydrogenation of 2B1OL to
1-butanol and the hydrogenation of the produced BD to butenes. Thus, the modification of
1
0SiO /Al by Cu was not effective for the dehydration of 2B1OL to BD. The reactions over
2
2 3
O
1
1

1
0SiO
the reaction atmosphere. Compared with a high conversion of 92.5% and a low BD selectivity of
5.0% over 10SiO /Al in H atmosphere (entry 1), a low conversion of 85.9% and a high BD
2
/Al
2
O
3
and 3Ag-10SiO
2
/Al
2
O
3
inN flow were also performed inorderto studythe effect of
2
7
2
2
O
3
2
selectivity of 86.7% were obtained in N
2
atmosphere (entry 2), indicating that H
2
took part in the
reactionevenwithoutthe presence ofa metal. Onthe other hand, compared withthe reactionresults
obtained over 3Ag-10SiO
2
/Al
2
O
3
in an H
2
flow (entry4), both the conversion and the selectivity to
could efficientlypromotethe formationofBD inthe
BD were low inN (entry5), indicatingthat H
2
2
presenceofAg.
Table 5 summarizes the reaction results of the dehydration of 2B1OL over Ag-modified
/Al with different Ag loadings. In the reactions with Ag loadings lower than 3%, the
1
0SiO
2
O
2 3
selectivitytoBDtrendedto decrease, while theselectivityto undetectedproductstrendedto increase.
The catalysts with Ag loadings of 3 and 5 wt.% gave both high conversions of 2B1OL and high
selectivities to BD, indicating that the suitable loading amount of Ag for BD formation was 3-5
wt.%.
Fig. 2 compares the changes in 2B1OL conversion over SiO
Ag-10SiO /Al . It is obvious that the order of the catalytic stability is 3Ag-10SiO
/Al > SiO
2
-Al
2
O
3
-CS, 10SiO
2
/Al
2
O
3
and
3
2
2
O
3
2
/Al
2
3
O >
10SiO
2
2
O
3
2
-Al
2
O
3
-CS, indicating that the modificationof10SiO
2
/Al
2
O
3
by Ag is effective
for the inhibition of catalytic deactivation in the dehydration of 2B1OL. We further performed the
dehydration of 2B1OL over 10SiO
2
/Al
2
O
3
and 5Ag-10SiO
2
/Al
2
O
3
with a long run of 10 h. As
shown in Fig. 3, the conversion of 2B1OL over 10SiO
2
/Al
2 3
O
decreased from 97.5 to 76.8% with
BDselectivitydecreased from86.9to 63.3%during10h. Incontrast,theconversionof2B1OLover
Ag-10SiO /Al slightly decreased from99.4 to 96.2% with BD selectivity decreased from95.0
5
2
O
2 3
to 87.1%. The decrease in BD selectivity with time on stream was due to the increase in the
formation of undetected products. It seems that the formation of BD favors high conversions. It is
proposed that the reactant of unsaturated alcohols would take part in the formation of undetected
productsespeciallyat lowconversions, while there is no evidence.
1
2

The dehydration of 3B2OL was also performed over SiO
Ag-10SiO /Al ata W/Fof0.34h. Fig. 4showsthe changes in catalytic activityofthese catalysts.
It is clear that the order of the catalytic stability is 5Ag-10SiO /Al > 10SiO /Al > SiO -Al
CS, indicating that 5Ag-10SiO /Al is an effective catalyst for the dehydration of 3B2OL to BD.
On the other hand, SiO -Al -CS was stable in the dehydration of 2B1OL within 5 h at a W/F of
2
-Al
2
O
3
-CS, 10SiO
2
/Al
2
O
3
, and
5
2
O
2 3
2
2
O
3
2
2
O
3
2
2
O -
3
2
O
2 3
2
O
2 3
0.34 h, whereas it significantly deactivated in the dehydration of 3B2OL at the same W/F (Fig. 4a),
indicatingthat 3B2OLleadedto moreseverecatalyst deactivationthan2B1OL.
3.3. Characterizationof catalystsamples
In vapor-phase reactions over either acid or base catalysts, it is generally considered that the
accumulationofcarbonaceous compoundsonthe catalyst surface results inthe catalytic deactivation
[
35]. In order to evaluate the coke formation, TG analyses of the used SiO
0SiO /Al and 3Ag-10SiO /Al were performed in air. As shown in Fig. 5, a weight loss
below 200 C is considered to be attributed to the desorption of H O, and a weight loss can be
2
-Al
2
O
3
-CS,
1
2
2
O
3
2
O
2 3
o
2
o
observed at a temperature range from 200 to 600 C, which is considered to be caused by the
decomposition of coke [34]. The order of the amount of coke in the used catalysts was 3Ag-
1
0SiO
2
/Al
2
O
3
< 10SiO
2
/Al
2
O
3
< SiO
2
-Al
2
O
3
-CS, which was reverse to the order of the catalytic
stability (Figs. 2-4). This indicates that the formation of coke possibly affects the catalytic
deactivation, and the modification of 10SiO
2
/Al
2
O by Ag is effective for inhibiting the coke
3
formation. However, because there is no significant difference inthe amount ofcoke, different coke
component mayaffectthedeactivationbehavior.
In order to study the effect of the acidity on the dehydration of 3B2OL and 2B1OL, NH
TPD analyses offreshSiO -Al -CS, 10SiO /Al and 5Ag-10SiO /Al were performed (Fig.
). The number of acid sites in SiO -Al -CS, 10SiO /Al , and 5Ag-10SiO /Al was 0.52,
-Al
3
-
2
2
O
3
2
2
O
3
2
O
2 3
6
2
2
O
3
2
2
O
3
2
O
2 3
-1
0
.20, and 0.18 mmolg , respectively. SiO
2
2
O -CS showed two desorptionpeaks at 283 and 327
3
oC. 10SiO
/Al and 5Ag-10SiO /Al showed desorptionpeaks at 195 and 260 C, respectively.
O
O
o
2
2
3
2
2 3
Theseresults indicatethattheorderoftheaveragedacidstrengthisSiO
2
-Al
2
O
3
-CS>10SiO
2
/Al
2
O >
3
1
3

5
Ag-10SiO
2
/Al
2
O
3
. In the dehydration of 3B2OL, SiO
2
-Al
2
O -CS with the strongest acid strength
3
and the largest amount of acid sites gives both the highest initial conversion and the highest initial
selectivity to BD (Fig. 4), indicating that the large amount of strong acid sites is effective for BD
formation. However, it is generally considered that the catalyst with strong acidity promotes the
formation of coke and easily deactivates [38]. Thus, SiO
2
-Al
2
O -CS significantly deactivated in the
3
dehydration of 3B2OL (Fig. 4a) and 2B1OL (Fig. 2). It is clear that the loading of Ag on
0SiO /Al reduces the acid strength without significant effect on the total number of acid sites.
1
2
O
2 3
Similar results have been also obtained in our previous study using different kinds of SiO
2
-Al
2
O
3
[
24]. We propose that the additive Ag particles are locatedonthe surface of10SiO /Al , and they
2
2 3
O
poison the strong acid sites on 10SiO
2
/Al
2
O to form new active sites for BD formation. The
3
reduced acid strength bythe additionofAg is supposedto contributetothe relatively stable catalytic
activity ofthe Ag-modified 10SiO
2
/Al
2
O catalysts. The XRD profiles ofthe reduced Ag-modified
3
1
0SiO /Al catalysts are shown in Fig. S5. The peaks assigned to Ag metalwere observed at Ag
2
2 3
O
loadings higher than 5%, indicating the additive Ag was reduced to metal before the reaction, and
workedat a metalstateduringthereaction. It ispossible that Agalso plays another important rolefor
the inhibitionofcatalytic deactivation, as has beendescribed in our previous studies [24, 32, 34, 35].
It is supposed that H
effect, the generated H atoms can shift from Ag to SiO
SiO /Al is proposed to be effective for suppressing the formation of coke and inhibiting the
2
can be cleaved to produce H atoms by Ag metal, and through the spillover
+
+
2
/Al
2
O surface to form H . The H on
3
2
O
2 3
catalytic deactivation, thus, the selectivity to BD is increased. Compared with Cu, Ag has low
hydrogenation ability and does not catalyze the hydrogenation of unsaturated alcohols to saturated
alcohols as well as the further hydrogenation of BD to butenes. Therefore, the low hydrogenation
abilityofAg isanother important factor makingthe modificationofAgto beeffective.
3.4. FutureperspectiveofBDproduction
The production of BD from biomass-derived ethanol is a well-known process, and many
research works have been reported in vapor-phase condensation of ethanol to produce BD [2, 3].
1
4

Another possible route for producing BD is proposed via the dehydration of intermediate
unsaturated alcohols from renewable butanediols [3]. The butanediols-to-BD process have
advantage to give high BD selectivity although the biobutanediol is currently more expensive than
bioethanol. Both the two processes suffer from the catalytic deactivation, and the development of
stable catalysts and technology for inhibiting the catalytic deactivation would significantly affect the
industrialapplicationsoftheprocesses in future.
4
. Conclusions
Vapor-phase dehydration of C4 unsaturated alcohols, such as 2B1OL, 3B2OL and 3B1OL,
to BDwere investigatedover various solidcatalysts. Acidcatalystswere foundto beeffective forthe
dehydration of 2B1OL and 3B2OL to BD, and SiO -Al -CS showed a relatively high catalytic
2
O
2 3
performance. In the dehydration of 3B1OL, however, acid catalysts promoted the C-C cleavage of
B1OLto propylene, while rare earth metaloxides suchas CeO were effective forthe formationof
BD from 3B1OL. Compared with the dehydration of 2B1OL and 3B2OL, the dehydration of
B1OL undergoes different mechanism. The high performance of CeO in the dehydration of
B1OL into BD is considered to be attributed to its weak basicity as well as the Ce -Ce redox
nature of CeO . The dehydration of 2B1OL and 3B2OL was further studied using self-made
3
2
3
2
4+
3+
3
2
SiO
0SiO
the catalytic deactivation, metal-modified SiO
2
/Al
2
O
3
catalysts. The suitable SiO
catalyst showed a bettercatalytic performancethanSiO
/Al was prepared and tested. The SiO
2
content in SiO
2
/Al
2
O
3
was found to be 10 wt.%, and the
-Al -CS. Inorderto inhibit
/Al
1
2
/Al
2
O
3
2
O
2 3
2
2
O
3
2
O
2 3
modified by Ag with a loading of 3-5 wt.% was found to be efficient for inhibiting the catalytic
deactivation and improving the conversion and the selectivity to BD in the dehydration of 2B1OL
and 3B2OL. Inthe dehydrationof2B1OLover 5Ag-SiO
2
/Al
2
O the selectivityto BD was 94.6% at
3
conversionof99.1%.
1
5

Acknowledgement
This research was partly supported by JST, Strategic International Collaborative Research
Program, SICORP.
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8

Figurecaptions
100
80
60
40
20
Conv. of 2B1OL
Select. to BD
Select. to ethers
0
5
2
10
15
SiO content/ wt.%
Fig. 1Dehydrationof2-buten-1-oloverself-madeSiO
2
/Al
2
O
3
catalystswithdifferent SiO contents.
2
o
3
-1
Reaction conditions: temperature, 260 C; N
2
flow rate, 30 cm min ; catalyst weight, 0.5 g; W/F,
0.34 h. Theconversionsandselectivitieswereaveraged between1-5h.
1
9

100
80
60
40
20
3
Ag-10SiO/Al O
2
2
3
10SiO /Al O
2 3
2
SiO -Al O -CS
2
2
3
0
1
2
3
4
5
Time on stream (h)
Fig. 2 Changes in the conversion of 2-buten-1-ol with time on stream over SiO
2
-Al
2
O -CS,
3
1
0SiO /Al and3Ag-10SiO /Al
2
2
O
3
2
2
O .
3
o
3
-1
Reaction conditions: temperature, 260 C; H
2
flow rate, 30 cm min ; catalyst weight, 0.2 g; W/F,
0.14 h.
2
0

(a)
(b)
1
00
100
80
60
40
20
8
6
4
2
0
0
0
0
Conv. of 2B1OL
Select. to BD
Conv. of 2B1OL
Selec. to BD
0
2
4
6
8
10
0
2
4
6
8
10
Time on stream (h)
Time on stream (h)
Fig. 3 Changes in catalytic activity of 10SiO
2
/Al
2
O
3
(a) and 5Ag-10SiO /Al
2
2
O (b) in the
3
dehydrationof2-buten-1-olwithtimeonstream.
o
3
-1
Reaction conditions: temperature, 260 C; H
2
flow rate, 30 cm min ; catalyst weight, 0.2 g; W/F,
0.14 h.
2
1

(a)
(b)
(c)
100
100
100
8
6
4
0
0
0
80
80
60
60
4
0
40
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
Time on stream (h)
Time on stream (h)
Time on stream (h)
Fig. 4Changes inactivityofSiO
2
-Al
2
O
3
-CS(a), 10SiO
2
/Al
2
O
3
(b), and5Ag-10SiO
2
/Al
2
O (c) inthe
3
dehydration of 3-buten-2-ol with time on stream. The solid cycle and the open cycle represent the
conversionof3B2OLandtheselectivityto BD, respectively.
o
3
-1
Reactionconditions: temperature, 260 C; flow rateofcarrier gas, 30 cm min ; catalyst weight, 0.5
g;W/F, 0.34h.
2
2

0
-
10
20
30
40
-
3
1
Ag-10SiO /Al O
2
2
3
-
0SiO /Al O
2
2 3
SiO -Al O -CS
2
2
3
-
0
200 400 600 800
Temperature/ °C
Fig. 5TGprofiles oftheusedSiO
2
-Al
2
O
3
-CS, 10SiO
2
/Al
2
O
3
and3Ag-10SiO
2
/Al
2
O
3
.
o
3
-1
Reaction conditions for the used catalysts: temperature, 260 C; H
2
flow rate, 30 cm min ; catalyst
weight, 0.2g. W/F, 0.14h.
2
3

5
Ag-10SiO /Al O
2 2 3
1
0SiO /Al O
2 2 3
SiO -Al O -CS
2
2
3
0
200
400
600
800
Temperature/ °C
Fig. 6NH
3
-TPDprofiles ofSiO
2
-Al
2
O
3
-CS, 10SiO
2
/Al
2
O
3
and5Ag-10SiO
2
/Al
2
O .
3
2
4

OH
HO
OH
2
,3-Butanediol
3-Buten-2-ol
OH
-
H O
2
HO
HO
1
Cat.
,3-Butanediol
2-Buten-1-ol
1,3-Butadiene
OH
HO
HO
1
,4-Butanediol
3-Buten-1-ol
Scheme1Productionof1,3-butadiene frombutanediols viaunsaturatedalcohols
2
5

Dehydration
Decomposition
Hydrogenation
+
HCHO, CH OH
3
HO
HO
Dehydrogenation
Isomerization
O
HO
Etherification
O
Scheme2Products inthedehydrationof3B1OL
2
6

H
H
H
H O
2
.
OH
OH
+
+
4+
4+
4
+
4+
H
H
Ce
Ce
Ce
Ce
.
4
Ce
+
3
+
3+
OH
4+
Ce
Ce
Ce
4
+
4+
4
+
4+
Ce
Ce
Ce
Ce
Adsorption
Dissociation
Desorption
Regeneration
Scheme3Proposedreactionmechanisminthedehydrationof3B1OLoverCeO
2
2
7

Table1Dehydrationof2-buten-1-olovervarioussolidcatalystsa.
Temp. Conv.b Selectivity/%b
Catalyst
o
Butenesc
2.0
Ethersd
0.0
Otherse
0.7
/
C
/%
99.3
90.3
99.4
78.4
42.4
BD
97.1
27.2
71.6
57.0
78.9
21.3
1-Butanol Butanal
SiO
2
-Al
Al
TiO
ZrO
Yb
CeO
Reactionconditions:N
2
O
3
-CS
260
260
300
400
400
400
0.2
4.5
1.7
5.0
2.9
5.3
0.0
4.3
0.9
2.0
2.2
27.6
O
3
3.7
25.4
1.4
34.9
20.3
30.1
13.4
2
2
4.1
2
2.6
3.3
2
O
3
0.0
2.6
39.3
2
93.4
2.7
3.8
a
b
c
d
3
-1
2
flowrate,30cm min ;catalystweight,0.5g.W/F,0.34h.
Conversionandselectivityareaveragedbetween1hand5h.
Butenesinclude1-buteneand2-butene.
8
EthersrepresentthelinerunsaturatedetherswithachemicalformulaofC H14O.
eOthers representundetectableproducts.
2
8

Table2Dehydrationof3-buten-2-olovervarioussolidcatalystsa.
Selectivity/%b
Temp. Conv.b
Catalyst
o
Butenesc 2-Butanone
Ethersd
0.6
Otherse
4.1
/
C
/%
74.4
70.2
90.0
67.4
4.3
BD
93.4
52.1
58.9
48.3
17.7
26.5
SiO
2
-Al
Al
TiO
ZrO
Yb
CeO
Reactionconditions:N
2
O
3
-CS
260
260
300
400
400
400
1.5
1.4
8.1
4.7
3.6
28.5
0.4
11.1
7.1
O
3
9.0
28.4
24.8
25.7
62.0
2
2
1.1
2
18.2
13.4
5.9
3.1
2
O
3
3.9
26.8
2
61.7
2.3
a
b
c
d
3
-1
2
flowrate,30cm min ;catalystweight,0.5g.W/F,0.34h.
Conversionandselectivityareaveragedbetween1hand5h.
Butenesinclude1-buteneand2-butene.
8
EthersrepresentthelinerunsaturatedetherswithachemicalformulaofC H14O.
eOthersrepresentundetectableproducts
2
9