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Deactivation of Zeolite Catalysts in Methanol Conversion to Hydrocarbons
C.H. Bartholomewand J.B.Butt (Editors),Catalyst Deactivation 1991 0 1991 Elsevier Science PublishersB.V., Amsterdam
815
DEACTIVATION OF ZEOLITE CATALYSTS IN METHANOL CONVERSION TO HYDROCARBONS Zhongmin Liu, Guangyu Cai
Guoquan Chen. Juan
Liang.
Qingxia
Wang
and
Dalian Institute of Chemical Physics, Chinese Academy of Science. Dal ian, China SUMMARY zeolites The deactivation of HY. HT. HMd. especially HZSM-5. has been studied in methanol conversion reaction. The strong acidic sites of zeolites may be the active sites of coking reaction, and the pore structure and pore size of zeolites are the major factors affecting the coking rate and coke content of the catalysts. For HZSM-5 catalyst, the coking reaction not onl, takes place on the outer surface but also inside the zeolite channels. The carbonaceous materials formed inside the HZSV-5 are hydrogen-rich substance such as paraffins or cscloparaffins, which will eventually block up the pores and the channels of the zeolite. The additional deactivation due to the hydrotherrta; effect may also play a rather inportant role in deactivating Lhe HZSM-5 catalyst. 1NTRODUCTION Coke formation and hydrothermal effects are the main probleu!> encountered in the MTO or MTG process for the maintenance of catalyst efficiency during the on-stream period. Studies hu\c been reported on deactivation bs coke formation. The niech.sisul
of coking is, however. not set f u l l y understood. As to i h e combined effect of coking and hydrothermal on deacti\ation of zeolite catalyst. few results are available in the literature. In the present work, the deactiL-ation of El’, HT. EVci, especially HZSM-5 zeolite catalysts has been studied. F o l HZSX-5 zeolite. the hydrothermal effect on deactivation in reaction h a 5 been investigated as we1 1 .
EXPERIMENTAL
Methanol conversion reaction was carried o u t ~ i t ilh f i r . r d 'Jed continuous microreactor (0.8x25cm quartz t u b e ) ~ith 0.5~ catalyst. The coke content of the coked catalyst was eva!uaLei b!, a Temperature-Programmed-Oxidat ion (TPO) apparatus i+ h i c 11 incorporting with a chromatograph t o analsze the C , H : a . t : o or
816
coke on the catalsst and the reaction products on-line. The
NH3of adsorption determined
number of strong acid sites was determined from the
TPD peak at temperatures higher than 300 OC. The surface area
the catalyst was calculated from the liquid nitrogen amount with BET equation and the 13C-NMR spectra was on a XL-200 superconductive apparatus.
RESULTS AND DISCUSSION It has been found that the acidity of zeolite and the Lield of
aromatics decrease simultaneously with increasing coke content on catalyst. The linear relationship between the number of strong acidic sites and the initial coking rate (Fig. 1 ) shows that the strong acidic sites OR the zeolites are the main Ltctite sites for the coking reaction in methanol conversion There are peculiar differences between HZSM-5 and other coke content (Fig. 2'1, zeolites in regard to the coking rate the nature of coke (Table 1) and methanol conversion activits These differences indicate that the pore structure and pore size
.
.
.
of the zeolite are the decisive factors affecting the coke formation in the reaction. The HZSM-5 zeolite. owing to its
.
unique pore structure possesses high stability and exhibits ;I coking resistant character. With gradualiy increasing coke accumulation,however, its pores and channels are eventual l;
blocked u p and the activity and the specific surface area decreases sharply (Fig. 3 ) . Table 1 also shows that the carbonaceous materials formed on different zeolites are different in nature. which ma> hclve close relation with the mechanism of coke formation. From the C.H ratio and 13C-NNIR result (Fig. 4, of cokes formed on H E M - 5 zeolite, it seems that the coking reaction takes place not only on the outer surface, but also inside the pores and channels ~ i t h o r cycloparaff ins and some monoc5cl i c the species of + C H Z t n aromatics as the probable coke precursors. Hydrothermal effect leads to appreciable decrease in t h e number of acidic sites and the catalytic activits of the EiZS'U-5 catalyst for methanol conversion. Table 2 shoT,\5 that t h e Luke formation i s more important than hydrothermal effect in but deactivating HZSM-5 catalyst at lower temperature r 3 5 O 0 C > at higher temperatureC500 C). the large amount of hater ~ ' 8 p o : i n the product may have significant effect on the deactivation.
.
817
-
1.2
d
P-
F-
.rl
E ap
\
/
0.8
3
v
a
m
O-
EY
HT
4.J
Ll
0.4
M
r: .A
2
al 24
HZSM-5
0
V
0
0
10 20
0
30
40
A c i d i c site6 (x
C
d7)
iieac ; i o n time (min.
1. T h e linear relationship between the number of acidic sites of the zeo!ites and the initia! coking :-ate MeOH LHSV=ll.0 4 ) Fig.
.
1
ST,I'O;L~~
<5OO0C,
Fig. 2. Variation in c o k e content with reaction time <500°C. HMd. MeOH LHSV=1.76; for HY. HT. HZSM-5. tle0tl LHSV=ll.Oi~.
i,
E
id
Li
m
1 Lo-,
300-
v
al
"O*
foA
200
-
100
-
a V
m
0 0
b I
I
I
1
I
Coke on HZSM-5 (Wt.%) Fig.
3. Variation in specific surface area with c o k e c v n L r n t ~ ~ 5 0 O o C
4. "C-NMR spectra of Loke formed on HZSV-5 a : , e : reaction at 500OC.
Fig.
12
.iv!:i~
818 Table 1 The C/H ratio of coke formed on zeolites
<
5OOOC) .
~
Zeolites
HZSM-5
HY
HT
nMd
Reaction time (h) C/H
22 0.73
0.5 1.56
0.89
0.33
0.25 3.37
”For HZSM-5. HY and HT, LHSV=l1.04, f o r HPd. LHSV=1.76.
Table 2 Comparison between the specific activity of used HZSM-5 in methanol conversion and after steamtreatmenta After steam treatment 35OoC 5OO0C
After methanol conversion 35OoC 5OO0C
5h.
12h.
5h.
1Zh.
5h.
12h.
5h.
12h.
0.98
0.90
0.44
0.37
0.95
0.70
0.30
0.32
aThe specific activity was measured by pulse reaction at 35OoC, with helium carrier gas 30 mllmin., fresh HZSM-5 as standard. CONCLUSION The strong acidic sites of zeolite may be the active sites of coking reaction, and the pore size and pore structure of the zeolite are decisive factors affecting the coke formation. F o r HZSM-5 zeolite, the coking reaction not only takes place on the with outer surface, but also inside the zeolite channels hydrogen-rich substance as the probable coke precursors. The additional deactivation due to the hydrothermal effect may also play a rather important role in deactivating the HZSM-5 catalyst.
REFERENCES 1 B.E. Langner, , A p p l . Catal.. 2, 289<1982). 2 E.G. Derouane. Proc. of 7th Int. Cong. Catal., 1982, p332. 3 R.F. Howe. J. eatal.. 99, 466(1966). 4 J.C. Vedrine. J. Catal., 70. 123C1981). 5 B. Meicier, Stud. Surf. Sci. Catal.. 34, 589(1967).
815
DEACTIVATION OF ZEOLITE CATALYSTS IN METHANOL CONVERSION TO HYDROCARBONS Zhongmin Liu, Guangyu Cai
Guoquan Chen. Juan
Liang.
Qingxia
Wang
and
Dalian Institute of Chemical Physics, Chinese Academy of Science. Dal ian, China SUMMARY zeolites The deactivation of HY. HT. HMd. especially HZSM-5. has been studied in methanol conversion reaction. The strong acidic sites of zeolites may be the active sites of coking reaction, and the pore structure and pore size of zeolites are the major factors affecting the coking rate and coke content of the catalysts. For HZSM-5 catalyst, the coking reaction not onl, takes place on the outer surface but also inside the zeolite channels. The carbonaceous materials formed inside the HZSV-5 are hydrogen-rich substance such as paraffins or cscloparaffins, which will eventually block up the pores and the channels of the zeolite. The additional deactivation due to the hydrotherrta; effect may also play a rather inportant role in deactivating Lhe HZSM-5 catalyst. 1NTRODUCTION Coke formation and hydrothermal effects are the main probleu!> encountered in the MTO or MTG process for the maintenance of catalyst efficiency during the on-stream period. Studies hu\c been reported on deactivation bs coke formation. The niech.sisul
of coking is, however. not set f u l l y understood. As to i h e combined effect of coking and hydrothermal on deacti\ation of zeolite catalyst. few results are available in the literature. In the present work, the deactiL-ation of El’, HT. EVci, especially HZSM-5 zeolite catalysts has been studied. F o l HZSX-5 zeolite. the hydrothermal effect on deactivation in reaction h a 5 been investigated as we1 1 .
EXPERIMENTAL
Methanol conversion reaction was carried o u t ~ i t ilh f i r . r d 'Jed continuous microreactor (0.8x25cm quartz t u b e ) ~ith 0.5~ catalyst. The coke content of the coked catalyst was eva!uaLei b!, a Temperature-Programmed-Oxidat ion (TPO) apparatus i+ h i c 11 incorporting with a chromatograph t o analsze the C , H : a . t : o or
816
coke on the catalsst and the reaction products on-line. The
NH3of adsorption determined
number of strong acid sites was determined from the
TPD peak at temperatures higher than 300 OC. The surface area
the catalyst was calculated from the liquid nitrogen amount with BET equation and the 13C-NMR spectra was on a XL-200 superconductive apparatus.
RESULTS AND DISCUSSION It has been found that the acidity of zeolite and the Lield of
aromatics decrease simultaneously with increasing coke content on catalyst. The linear relationship between the number of strong acidic sites and the initial coking rate (Fig. 1 ) shows that the strong acidic sites OR the zeolites are the main Ltctite sites for the coking reaction in methanol conversion There are peculiar differences between HZSM-5 and other coke content (Fig. 2'1, zeolites in regard to the coking rate the nature of coke (Table 1) and methanol conversion activits These differences indicate that the pore structure and pore size
.
.
.
of the zeolite are the decisive factors affecting the coke formation in the reaction. The HZSM-5 zeolite. owing to its
.
unique pore structure possesses high stability and exhibits ;I coking resistant character. With gradualiy increasing coke accumulation,however, its pores and channels are eventual l;
blocked u p and the activity and the specific surface area decreases sharply (Fig. 3 ) . Table 1 also shows that the carbonaceous materials formed on different zeolites are different in nature. which ma> hclve close relation with the mechanism of coke formation. From the C.H ratio and 13C-NNIR result (Fig. 4, of cokes formed on H E M - 5 zeolite, it seems that the coking reaction takes place not only on the outer surface, but also inside the pores and channels ~ i t h o r cycloparaff ins and some monoc5cl i c the species of + C H Z t n aromatics as the probable coke precursors. Hydrothermal effect leads to appreciable decrease in t h e number of acidic sites and the catalytic activits of the EiZS'U-5 catalyst for methanol conversion. Table 2 shoT,\5 that t h e Luke formation i s more important than hydrothermal effect in but deactivating HZSM-5 catalyst at lower temperature r 3 5 O 0 C > at higher temperatureC500 C). the large amount of hater ~ ' 8 p o : i n the product may have significant effect on the deactivation.
.
817
-
1.2
d
P-
F-
.rl
E ap
\
/
0.8
3
v
a
m
O-
EY
HT
4.J
Ll
0.4
M
r: .A
2
al 24
HZSM-5
0
V
0
0
10 20
0
30
40
A c i d i c site6 (x
C
d7)
iieac ; i o n time (min.
1. T h e linear relationship between the number of acidic sites of the zeo!ites and the initia! coking :-ate MeOH LHSV=ll.0 4 ) Fig.
.
1
ST,I'O;L~~
<5OO0C,
Fig. 2. Variation in c o k e content with reaction time <500°C. HMd. MeOH LHSV=1.76; for HY. HT. HZSM-5. tle0tl LHSV=ll.Oi~.
i,
E
id
Li
m
1 Lo-,
300-
v
al
"O*
foA
200
-
100
-
a V
m
0 0
b I
I
I
1
I
Coke on HZSM-5 (Wt.%) Fig.
3. Variation in specific surface area with c o k e c v n L r n t ~ ~ 5 0 O o C
4. "C-NMR spectra of Loke formed on HZSV-5 a : , e : reaction at 500OC.
Fig.
12
.iv!:i~
818 Table 1 The C/H ratio of coke formed on zeolites
<
5OOOC) .
~
Zeolites
HZSM-5
HY
HT
nMd
Reaction time (h) C/H
22 0.73
0.5 1.56
0.89
0.33
0.25 3.37
”For HZSM-5. HY and HT, LHSV=l1.04, f o r HPd. LHSV=1.76.
Table 2 Comparison between the specific activity of used HZSM-5 in methanol conversion and after steamtreatmenta After steam treatment 35OoC 5OO0C
After methanol conversion 35OoC 5OO0C
5h.
12h.
5h.
1Zh.
5h.
12h.
5h.
12h.
0.98
0.90
0.44
0.37
0.95
0.70
0.30
0.32
aThe specific activity was measured by pulse reaction at 35OoC, with helium carrier gas 30 mllmin., fresh HZSM-5 as standard. CONCLUSION The strong acidic sites of zeolite may be the active sites of coking reaction, and the pore size and pore structure of the zeolite are decisive factors affecting the coke formation. F o r HZSM-5 zeolite, the coking reaction not only takes place on the with outer surface, but also inside the zeolite channels hydrogen-rich substance as the probable coke precursors. The additional deactivation due to the hydrothermal effect may also play a rather important role in deactivating the HZSM-5 catalyst.
REFERENCES 1 B.E. Langner, , A p p l . Catal.. 2, 289<1982). 2 E.G. Derouane. Proc. of 7th Int. Cong. Catal., 1982, p332. 3 R.F. Howe. J. eatal.. 99, 466(1966). 4 J.C. Vedrine. J. Catal., 70. 123C1981). 5 B. Meicier, Stud. Surf. Sci. Catal.. 34, 589(1967).