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Transcript of (SICI)1097-4660(199812)73-4-414--AID-JCTB965-3.0
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J.Chem.T echnol.Biotechnol. 1998,73, 414420
Catalyst Prepared byElectroless MethodCu/SiO2Ching-Yeh Shiau*& J. C. Tsai
Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei,Taiwan
(Received 26 January 1998; accepted 5 August 1998)
Abstract: catalysts prepared by an electroless deposition method wereCu/SiO2
investigated and compared with those by an impregnation method. Copper con-tents varied from 5% to 15% and was used as support. All catalysts wereSiO
2characterized by BET, DSC, SEM and TPR and tested by ann-butanol dehydro-genation reaction for activities and stabilities. BET analysis showed that thecatalysts prepared by the two methods present larger average pore size and lesssurface area than those of the fresh indicating that smaller pores may getSiO
2,
blocked during the course of preparation. This blockage is more severe in theimpregnation method. SEM photos showed that the electroless method producessmaller copper crystals than the impregnated method. The reaction activity wasfound to be in the order of the calcined electroless copper catalyst [ the freshelectroless copper catalyst[ impregnated copper catalyst. 1998 Society of(Chemical Industry
J.Chem.T echnol.Biotechnol.73, 414420 (1998)
Key words: copper catalyst; electroless plating; impregnation; n-butanolSiO2
;dehydrogenation
INTRODUCTION
Copper-based catalysts are widely used in many indus-
trial reactions, especially for dehydrogenation reactions.
Church and Joshi1and Franckaerts and Froment2indi-cated that a CuCoCr catalyst had good activity for
ethanol dehydrogenation. Pepe et al.3 studied the dehy-drogenation of isopropanol and found that the reaction
rate increased linearly with the exposed copper surface
area of the supported catalysts. Doca and Segal4h7usedCr-, Mn-, Fe- and Ni-promoted copper catalysts to
investigate the kinetics of low aliphatic alcoholic
dehydrogenation in the temperature range of(C2C
4)
200280C by a chromatographic pulse technique. They
found that the fresh copper catalyst was very active, but
its activity decreased rapidly with pulse number. If the
promoters Cr and Mn were added into the catalysts, the
activity of the catalysts remained fairly constant for a
* To whom correspondence should be addressed.Contract/grant sponsor : National Science Council of the
Republic of China; Contract/grant number: NSC84-2214-E011-005.
longer time due to the formation of a Cu-oxide junc-
tion. Prasad et al.8 and Tu et al.9 studied the role ofchromia in a copper catalyst for the dehydrogenation of
ethanol; the addition of chromia improved dispersion of
copper on the support and helped to prevent the cata-
lyst from sintering. Sivarajet al.10 found that CuZnOAl catalysts prepared by a depositionprecipitation
method had high selectivity for the dehydrogenation of
cyclohexanol to cyclohexanone. Lin et al .11 used a
CuZnO catalyst to investigate the oxidative dehydro-genation of cyclohexanol to cyclohexanone. Sivaraj et
al.12 examined the selectivity dependence on the acidityof supported copper catalysts prepared using the urea
hydrolysis procedure in the hydrogenation of cyclo-
hexanol. Shiau and Liaw13 found that a CuBa catalystwas potentially attractive for the dehydrogenation ofn-
butanol to butyraldehyde. Shiau and Chen14 also foundthat a CuCr catalyst was quite eective for the ethanol
dehydrogenation reaction. Grift et al .15 studied theeect of preparation procedure on the reduction behav-
ior of silica-supported copper catalysts. They found that
the dispersion of copper strongly depended on the prep-
aration methods and the conditions used.
4141998 Society of Chemical Industry. J.Chem.T echnol.Biotechnol. 0268-2575/98/$17.50. Printed in Great Britain(
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Cu/SiO2
catalyst prepared by electroless method 415
Traditionally, copper catalysts are prepared by either
impregnation methods or precipitation methods.
Recently, electroless deposition method has been
adopted in the preparation of copper catalysts. Chang
and Saleque16h19 used this method to prepare a coppercatalyst on for the dehydrogenation reactionsAl
2O3
and found that the catalysts prepared by the electrolessmethod were better than those prepared by the conven-
tional methods. Shiau and Tsai20 also found similarresults for the dehydrogenation reaction of n-butanol
when using catalysts prepared by the electro-Cu/Al2
O3
less method. In this study, the electroless deposition
method was used to prepare copper catalysts for the
dehydrogenation of n-butanol. was used asSiO2
support. All the catalysts prepared by the electroless
method were compared with the catalysts prepared by
the impregnation method.
EXPERIMENTAL
Catalyst preparation
Electroless method
The electroless method is an oxidationreduction
chemical deposition reaction, which can deposit certain
metals on a substrate without an external electrical
source. In this study was used as substrate. BeforeSiO2
electroless deposition, was rstly treated withSiO2
dilute acid solution to remove any fats and oils. The
clean was then treated with palladium chlorideSiO
2solution for activation. At this stage, palladium servedas seeds for catalytic nucleating centers. The activated
was nally contacted with copper solution forSiO2
copper plating. In the copper solution, formaldehyde
was added as reducing agent for the oxidation
reduction reaction. The plating bath was maintained at
70C and the pH was varied from 11 to 13. The plated
was then ltered and washed with distilled water,SiO2
then dried at 110C for 24 h. The copper content was
controlled by the plating solution concentration as well
as the plating time. The plated copper catalysts can be
directly used as the plated copper is present as copper
atoms instead of copper oxide. However, some freshelectroless copper catalysts were also calcined at 450C
for 3 h to study the calcination eect.
Impregnation method
Catalysts were prepared by putting support into aSiO2
copper nitrate solution container for 30 min impregna-
tion. The container was then placed in an isothermal
water bath to vaporize most of water. The bath tem-
perature was maintained at 70C. The soaked paste was
then dried at 110C and calcined, if necessary, at 450C
for 3 h.
Six catalysts were prepared, namely, I05, I10, I15,
E05, E10 and E15 in which I denotes impregnation
method, E denotes electroless method and the numbers
represent the copper content in the catalyst as a per-
centage. The copper content was determined by atomic
absorption spectrometry. The catalysts were character-
ized by BET, DSC, SEM and TPR analyses.
n-Butanol reaction
The catalyst activity data were obtained by carrying out
the dehydrogenation reaction ofn-butanol in a contin-
uous ow type reactor with a xed catalyst bed under
atmospheric pressure. The reactor was made of a Pyrex
tube with 136 mm inner diameter, equipped with a
thermocouple well along the central axis. A known
amount of catalyst was loaded in the reactor. Pure n-
butanol was fed into the reactor by a metering pump.
The reaction temperature was set at 270C. All connect-
ing gas lines and valves were wrapped with heating tape
to prevent possible condensation. The compositions ofinlet and outlet streams were determined chromato-
graphically using a column loaded with Porapak Q.
The experiments were conducted under conditions free
of external mass transfer and internal diusion resist-
ances.
RESULTS AND DISCUSSION
Catalyst characterization
Table 1 shows the specic surface areas and the meanpore sizes for the six types of catalysts measured by the
BET method. Fresh was also included for refer-SiO2
ence. As can be seen from this table, all the catalysts
have less surface area and larger mean pore size than
the fresh and the catalysts prepared by the elec-SiO2
,
troless method exhibit a larger surface area and smaller
mean pore size than all the impregnated catalysts.
Moreover, the surface area increases and the mean pore
size decreases with increasing copper loading for the
electroless catalysts, but vice versa for the impregnated
catalysts. All this information indicates that the impreg-
nation method will block some small pores and this
TABLE 1
BET Surface Analysis
Catalyst Surface area Pore size
(m2 g~1) (A )
SiO2 2988 2113
I05 2552 2430
I10 2415 2562
I15 2347 2664
E05 2644 2385
E10 2712 2324
E15 2785 2253
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416 C.-Y. Shiau, J. C . T sai
Fig. 1. Pore distribution for impregnated catalysts.
blockage will become more severe with increasing
copper loading, but the blockage phenomenon in the
electroless method is much less. These blockage pheno-
mena can also be seen from the pore distribution
curves, as shown in Figs 1 and 2. Figure 1 is the pore
distribution for the impregnation method and Fig. 2 for
the electroless method. It is clearly seen from Fig. 1 that
the specic pore volume was sharply decreased when
loading copper metal onto the support by the impreg-
nation method and the curves are slightly right-shiftingwhen the copper loading increases, indicating that more
smaller pores are blocked when increasing copper load-
ings. In contrast, the curves for the electroless method,
as shown in Fig. 2, are slightly left-shifting when the
Fig. 2. Pore distribution for electroless catalysts.
copper loading increases. This is due to the fact that
copper was more uniformly plated on the wall of the
pores by the electroless method. Expect for those ne
pores which became blocked, the pore size of the non-
blocked pores therefore becomes smaller when copper
loading increases.
Apart from the BET surface area, the copper particlesize also plays an important role in the catalysts activ-
ity. Figures 3 and 4 are the SEM photos of the I15 cata-
lyst and the fresh E15 catalyst, respectively. It is seen
from these two gures that the electroless method pro-
duces quite small ball-like copper catalysts whereas the
impregnated methods forms much larger needle-like
ones. Thus, the crystal type is quite dierent in the two
preparation methods and the copper crystal size pre-
pared by the electroless method is much smaller than
that prepared by the impregnation method. Figure 5 is
the SEM photo of calcined E15 catalyst. Comparing
Figs 3 and 5 shows that even the calcined E15 catalystgives smaller crystals than the I15 catalyst.
Figure 6 and 7 are the TPR proles of the six cata-
lysts. The TPR experiments were conducted in a xed
bed reactor from room temperature to 500C at a con-
Fig. 3. SEM photo of I15 catalyst.
Fig. 4. SEM photo of E15 catalyst.
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Cu/SiO2
catalyst prepared by electroless method 417
Fig. 5. SEM photo of E15 catalyst with calcination.
stant rate of 10C min~1. A mixture stream of H2
/Ar
(25%/75%) was introduced at 30 cm3 min~1 to reduce
the calcined catalyst sample. The effluent stream wasanalyzed for hydrogen by a thermal conductivity detec-
tor after removal of moisture by a molecular sieve trap.
For both preparation methods; the TPR of each cata-
lyst exhibited only one peak. This peak is attributed to
the reaction:
CuO]H2]Cu]H
2O
Both gures show that larger copper loadings have
larger hydrogen consumption. However, comparing the
two gures indicates that the reduction pattern is slight-
ly dierent for the two types of catalysts. Firstly, the
peak maxima of the impregnated catalysts are about
50C higher than those of the electroless catalysts. Sec-
Fig. 6. TPR spectra for impregnated catalysts.
Fig. 7. TPR spectra for electroless catalysts.
ondly, the TPR curves of the electroless catalysts are
much sharper and narrower than those of the impreg-
nated catalysts. Thirdly, as can be seen from Fig. 6,
there are no real peak maxima for the I05 and I10 cata-
lysts. These three points imply that the interaction
between copper and the support for the two prep-
aration methods are dierent and the electroless cata-
lysts are easier to get reduction.The DSC analysis traces, recorded in the temperature
range of room temperature up to 450C at a rate of
10C min~1 for the impregnated and the electrolesscatalysts are given in Figs 8 and 9 respectively. In Fig.
8, an endothermic peak was observed around 100C,
indicating the dehydration of the impregnated catalysts.
Starting from 230C to about 290C, a larger endo-
thermic peak was observed. This peak represents the
decomposition of copper salts and some kind of
bonding formed between copper and support. Higher
copper loadings give larger peaks. Above 300C, no
peaks were observed, indicating that the impregnatedcatalysts are quite stable from 300 to 450C. For the
electroless catalysts, as shown in Fig. 9, a similar endo-
thermic peak was observed around 100C followed by a
wide exothermic peak. The endothermic peak is the
vaporization of water inside the catalyst and the exo-
thermic peak represents the phase change of fresh elec-
troless copper to copper oxides. Again, higher copper
loadings present larger peaks. No endothermic peak at
around 250C was observed for the electroless catalysts,
indicating that the interaction between copper and
support for the electroless catalysts may be dierent
from that for the impregnation catalysts. This point
seems quite similar to the TPR result.
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418 C.-Y. Shiau, J. C . T sai
Fig. 8. DSC spectra for impregnated catalysts.
Catalyst activities
Dehydrogenation of n-butanol is used as the test reac-
tion for the catalysts activity. The catalysts loading was
04 g and time factor was 47 g-cat h gmole~1. Experi-mental results show that all the catalysts dehydrogenate
n-butanol into n-butyraldehyde and hydrogen. Only a
Fig. 9. DSC spectra for electroless catalysts.
Fig. 10. Reaction activity of 5% Cu catalysts.
trace amount of heavy compounds was detected.
Figures 1012 show the catalysts stabilities and activ-
ities for the n-butanol dehydrogenation reaction. The
calcined electroless copper catalysts are also included.
Comparing these three gures reveals three interesting
points. Firstly, higher copper loadings have better reac-
tion activities and the electroless copper catalysts
Fig. 11. Reaction activity of 10% Cu catalysts.
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Cu/SiO2
catalyst prepared by electroless method 419
Fig. 12. Reaction activity of 15% Cu catalysts.
present better performance for all copper loadings. Sec-
ondly, all the impregnated catalysts are deactivated sig-
nicantly, especially the I05 catalyst. In contrast, both
E10 and E15 are quite stable during an 8 h reaction
course. Thirdly, the calcined electroless copper catalysts
show much better reaction activities than the fresh elec-
troless copper catalysts. The higher activities of the cal-
TABLE 2
Carbon Content after Reaction
Catalyst Carbon content (wt%)
I05 632
I10 545
I15 498
E05 242
E10 114
E15 081
TABLE 3
Surface Area Change after Reaction
Catalyst BET surface area (m2 g~1)
Fresh Used % change
I05 2552 2222 [ 129I10 2415 2244 [ 71I15 2347 2274 [ 31E05 2644 2581 [ 24E10 2712 2685 [ 10
E15 2785 2734 [ 18
cined electroless copper catalysts may be attributed to
the interaction between copper and support during cal-
cination.
Catalyst deactivation
Coking and sintering problems which cause catalyst
deactivation were also investigated in this study. For
the coking problem, we used elemental analysis to
determine the weight % of carbon present in the cata-
lyst after an 8 h reaction at 270C, and for the sintering
problem we used the BET method to determine the
surface area change. Table 2 shows the result of elemen-
tal analysis which indicates that all the catalysts have
carbon deposition after undergoing the dehydroge-
nation reaction. The weight percentage of carbon on the
copper surface ranges from 081% to 632%. Table 2
also shows that after the dehydrogenation reaction thecatalysts prepared by the electroless method have much
less carbon deposition than the impregnated catalysts.
The amount of carbon deposition for the electroless
catalysts is much less than 40% of that for impregnated
ones. Table 3 compares the BET surface area between
the fresh and used catalysts. The fourth column in the
table is the percentage change of the surface area after
reaction. As can be seen from this result, the impreg-
nated catalysts have a signicant sintering phenomenon
while the degree of sintering is slight for the electroless
catalysts. The percentage decrease in surface area for
the electroless catalysts is about one-fth of that for the
impregnated catalysts. Tables 2 and 3 clearly indicatewhy the electroless catalysts show better stability and
activity than the impregnated catalysts do, as aforemen-
tioned in the reaction performance results.
CONCLUSIONS
(1) All the characterization results show that the elec-
troless deposition method provides more uniform
copper distribution than the impregnation method.
(2) Although the calcined electroless copper catalysts
have larger copper particles, they exhibit better reac-tion activities than the fresh electroless catalysts.
(3) The reaction activities are in the order of the cal-
cined electroless copper catalyst[ the fresh electro-
less copper catalyst[ the impregnated copper
catalysts.
ACKNOWLEDGEMENT
This work was supported by the National Science
Council of the Republic of China (NSC84-2214-E011-
005).
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420 C.-Y. Shiau, J. C . T sai
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