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High Silica Zeolite as Heterogeneous Catalyst
611 B. Drzaj, S. Hocevar and S. Pejovnik (Editors), Zeolites
© 1985 Elsevir Science Publishers B. V. Amsterdam - Printed in Yugoslavia
HIGH SILICA ZEOLITE AS HETEROGENEOUS CATALYST Liang Juan Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian, Liaoning, China
ABSTRACT The nucleation and crystallization processes and the role of inorganic and 0rgano-cations in the 6ynthesis of high silica ~o lites have been studis4. The resulting ZSM-5 and ZSM-ll preparations have been modified by incorporating Mg-, AI-, P- and other additives for catalytic app~cations. Improved catalysts with specific shape selectivity and high stability were obtained and some examples are described and discussed, particularly in relation to their catalytic performance for methanol conversion. INTRODUTION Zeolite based catalysts are now becoming the most important catalysts in the petroleum and petrochemical industries(ref.l). Their distinct structural and chemical properties make zeolites very attractive and worthwhile for stUdy from the different points of view of interdisciplinary sciences. Our knowledge about zeolite chemistry in the present stage is perhaps still in its infancy but will be rapidly developed by joint effort and COllaboration between scientific researchers in different fields. In regard to the catalysis area, the striking and continuous advances in zeolite synthesis are expected to provide a variety of new catalytic processes. Undoubtedly, the more experience we have in zeolite synthesis, the more exciting the future prospects will be. In this paper, a brief account of our work on zeolite synthesis, with high silica zeolites in particular, is described in connection with some applications in heterogeneous catalysis. ZEOLITE SYNTHESIS One of the most important problems in catalytic practice is the preparation of zeolite catalysts of satisfactory selectiVity and stability from starting materials of low toxicity and in plentiful supply. To approach this problem, better understanding of the crystallization process in relation to the resulting zeolite pre-
612
parations is highly desirable. Our experiences in the choice of synthesis conditions and investigations of the role of cations in the crystallization process are illustrated in the following text. Controlled growth of crystal grain Starting from an alumina-silica supersaturated gel ·of high basicity, crystal nuclei are gradually separated to form a new phase during nucleation stage in zeolite synthesis. Although it is very desirable to monitor and control the nucleation process in the very b.eginning stage, i t is still difficult to get detailed insight into the developing structures of crystal nuclei by means of the sophisticated instruments so far available. According to a general understanding 0 f the factors innuencing the formation of a new phase during precipitation, we can conceivably expect that the number of nuclei formed could be regulated by adjusting the wall effect and by the addition of suitable structure directing agents. As demonstrated in TABLE 1(ref.2), successful control of crystal sizes in the r~ge between ""'3 to 2O)(36)U by changing the aging history of the directing agent is manifested. The zeolite prepared is of good un::j.formity. The crystallization can be further shortened by increasing the stirring speed or/and adding NaCI which produce-s a zeolite of much smaller crystals up to O. 1"XO.07 )J. As expected, the crystal size greatly influences the shape selectiVity of zeolite catalysts. For toluene disproportionation the l~ge size ZSM-5 gives p-xylene in 30 wt.% exoess of eqUilibrium yield, showing the diff\lsion constraint for 0- and m-xylene in long channels. The shape: selectiVity for sorption of various molecul~s is clearly demonstrated in Table Z(ref.Z). TABLE 1 Changes in grain size of '~eolites directing agentS/lIIlple No. A-3 .\-5 ,A-7 A-9
•Temp.
Aging time (hr. ) 72 120 160 ZSM-5 seed
With aging time of s'tructure Av. crystal size
Vu)
20)(36 14)(.21 9x14 ""'3
1700C; Autoclave with special lining.
'XRD analysis ZSM-5 ZSM-5 ZSM-5 ZSM-5
613
TABLE 2 properties of ZSM-5 samples
P~ysico-chelllli..cal
Chemical Formula Sample Av. Cryst. Rramework Density(g/ml) No. Size CAl) 2.2 A-3 20X36 NaO• 52 H2• 35 A1 2• 87 Si 93• 13 °192 2.2 A-9 3 NaO• 47 H2• 16 A1 2• 63 Si 93• 37 °192 TABLE 2 Continued Adsorption Capacity(wt.%)· 2,2-DMB Benze~~ Toluene 8.7 6.5 9.6 non non 6.a 10.9 1.6 non 7.6 0C -Adsorption temp. 30
p-xylene 12.7 12.7
a-xylene
Improvement of Crystallinity During high silica zeolite synthesis it is important to avoid, as much as possible, the incorporation of .,(-Si0 2 or mordenite during nucleation stage. As an effective route to minimize the formation of (>(-8i0 2 phase, the nucleation and crystallization are carried out separately in diffS?rent environments(ref.3). The nucleation of the gel is initiated in an organo-cation medium. At a certain stage of nucleation, Na+ is added to speed up crystallization. Fig. 1 compares the results of the two-stage process with that of the conventional method, illustrating a facilitated crystallization in the presence of added Na+. The two-stage procedure provides a convenient way to prepare ZSM-5 and ZSM-1f 2 with Si/Al ratios in the range from 10 to ./ ./' 1700 with high XRD 1 /' crystallinity • /' /' A series of sam_r' ples have been prepared and studied 5 7 9 11 13 for the process 0 f t1ae (day) methanol conversion to low olefins(MTO). Fig. 1 The Crystallization Curve of Z8M-5 The effect of time 1.+0rganic amine medium without Na. 2. With of crystallization Na added into the medium aged for 5 days.
_--
....
.
614
is investigated in relation to the methanol catalysis and ammonia desorption behavior of the resulting zeolite samples(Fig. 2, 3). The crystallization is completed in a period of 90 hrs as monitored by XRD analysis. Zeolite samples resulting from 54 or 72 hrs
0"
......
~
;
.....
80
t ....
60
•
•
.
~
HZS....5(40)
o
40
II
III
_
20
',,-/
--> G~~~
~-.~ A_ _
~A,/
50
70
0
-6
_0
110 90 U.-e (hr)
I~
216
Changes in selectivity for methanol conversion with crystallization time of synthetic ZSM-5. A C~; 0 C C D C A Arom.
-Fig.2
3;. c:3;. 4; 5;
!
0
400 600 teap. °c
Fig. 3 The effect of crystallization time on TPD diagram of adsorbed NH 1. 54; 3• hrs. 2. 72; 3. 90; 4. 216
crystallization exhibit similar features of acidity in TPD diagram and similar product distribution with high yield of low olefins in methanol conversion. However, after complete crystallization, the zeolite gives different TPD diagram with two sharp peaks at about 270 0C and at 430 0C Or higher Samples of complete c;ystallization convert methanol with high yields of aromatics and propane. It seems eVident that the product distribution or selectivity for methanol conversion is closely related with the number and strength of acid sites. The strong acid site(with TPD peak at 500 oC,or higher) tends to further the chain growth reaction and the hydrogen transfer reaction. Therefore, the ZSM-5 samples with crystallization period longer than 90 hrs produce more aromatics and propane, in comparison with the shorter period samples. The obvious increase in C~ in the product of the long period samples is likely due to the p~esence af medium strength acid sites which crack C; to a great extent. It is interesting that when the crystallization period is
615
prolonged for more than 200 hrs and the resulting zeolite sample exhibits no detectable changes in crystal features, a further increase in surface acidity and a corresponding change in product distribution for methanol conversion can still be observed. It seems that some final adjustment of surface states is possible even after crystallization reaches completion. The role of cation Since Barrer(ref. 4) and Kerr(ref. 5) discovered the synthesis of zeolite with organic bases, the role of organic cation has been one of the most important problems in zeolite chemistry. It was postulated that organic cation Lncr-eases the solubility 0 f silicate and aluminate. Some results in our laboratory(ref. 6) repeated that the presence of organic cation favors the nucleation of high SilAl ratio zeolite from a variety of gels of different compoesitions. However, it is difficult to use NH 0H to prepare zeo4 lite of Si/Al ratio higher than 50, though zeolite of Si/Al ratio in the range of 20-30 can be easily synthesized in an ammonium hydroxide medium. If we try to increase the concentration of silicate in an inorganic alkaline solution, the mixing-precipitation of ~-Si02 will become a serious problem in addition to the long induction period encountered. Fig. 4 illustrAtes the effect of cation on the crystallization process. With NHhOH, the silicate concentration keeps an invariant of low Yalue(ahout 4.8 ~/l). However , by using e~nylamine the initial concentration of silicate can be as 1.0 ~10 Iiigh as 14.6 gIl ana the crystalli-~--_., zation 0 f a high / / Si/Al zeolite is ,/ completed in a ./ ~ • .-.'4' short period. It 2 """-----..,--6 8 8 is found that both - ........ ---...... --- - ... ----.- -- .. the readiness 0 f 40 80 120 160 time (hr) crystallization and the solubility of silicate can be Fig~ 4 Concentration changes gf liqUid phase further increased dur~ng crystallization at 170 C. - with ethylamine; --- with NH ~ Basicity; when using tetra40H; o Con.c.o f Si02J 11 Ads. n-C6.
.
..
.. .
.
616
propylamine. Ia conformity with the (Si/ll) conclusion reached by DerouSample Si/Al ratio (Si/ll)s ane(ref. 7), the distributibulk surface !fo. 0.86 b on gradient of Si/Al i. rouA 31 36 0.80 nd to be highly dependent on B 85 34 c t80 135 g:~f the conditioD!l of synthelBi•• D 193 340 TABLE 3 illustrates the trend of Al-enrichlllent on the surface of high Si/Al preparations. Work in our laboratory showed 80me general results concerning the effect of synthesis conditions such as that the crystallization takes place faster during the synthesis of higher Si/Al samples, the crystallinity increases with Si/Al ratio etc. These facts are similar to the observations made by Sand(ref. 8). Other things being equal, the increasing trend of crystallization rate with increasingSi/Al concentration ratio in the starting liquid implies that eome Si-rich complex must be developed to facilitate the nucleation process. If this is true, the separation Of~-Si02 phase might occur simultaneously and could be the main problem during BY.thesis of high Si/Al zeolites. We feel that the role or organic cation is, among others, to ayoid the simUltaneous crystallization of 0(.-Si0 2 by accomplishing the 'templating' effect. For instance, the tetrapropylamine can prevent 0<."Si0 from precipitating out for a 2 crystallization period as long as 216 hrs, while in NH medium, 40H the precipitation of~-Si02 occurs in a much shorter time. Tetrapropylamine is also very '.1-1,------=-& characteristic in hastening _ - - -.......---..-b the crystallization rate. Fig. ~----+c d 1D8 5 shows that by the addition 0 f t> I only 5% of' a TPA-c
•
617
zeQlitEB with JI1.dely different Si/Al ratios, with the mat.nt enance of high uniformity and crystallinity. Although all these exper1.mental observations are less contributory to the mechanistic details of templating effect, a conceivable and conformable interpretation of the structural effects of tetrapropylamine in zeolite synthesis is due to its affinity to coordinate with the silica-microstructures and to develop replications through H-bonding interact~on (ref. 9). The fact thatthe coadintiol\bEl:ween NH and the silica-micro40B structures is less extensive and not strong enough makes the templating effect hardly possible to induce. As a consequence, the silica concentration in the liqUid phase maintains a constant low value(as shown in Fig. 4). Continuous examination of solid phase structures with electronmicroscope reveals that the number of crystal particles increases monotonically without any indication of auto-catalytic growth of particles. MODIFICATION OF ZEOLITE AlIOng other, the versatile adaptability of zeolite to structure and cheltieal modification is very important aJld benificial to catalytic practice. Owing to these character-tcttcs, the stahility and selectivity of z.eolite catalysts can be adjusted and IIOdified by incorporating cations through impregnation or ion-exchange, housing 0 f guest -molecule, isotropic substi tutioD, etc. Joimt effect of physical and chemical modifications Additives incorporated into zeolite may be kinetically effective or not according to how the reaction pathway interacts With pore structures. An example is giv-en in Fig. 6 which shows the improvement on ~atalytic selectivity of large crystal zeolite for prowlene aromatization gained by adding Sb 20 • 100% p-xyleDe 3 selectivity can be obtained over Sb20 zeolite catalyst 3-lIlOdified of large crystal sizes at 500°C. For zeolite of small crystal sizes the sam~ additive is less effective. It seems obvious that aaape selecti vity could be actuated onl,. when the reactant itself(and its reaction) is confined to a certain structural barrier. Fig. 7a and 7b demonstrate how the selectivity for methanol conversion could be changed by impregnating magnesium and aluminum additives onto ZSM-5(ref. 10). A significant increment of ~ and yi:eJ.ds is achieved with Mg-modified zeolite catalyst of small crystal: sizes. Increases of 51'% in C 47% in and 6.8 times in
03.
z'
c'3
618
ratio can be obtained when the 0.25,.... ...... 0.5)l HZSM-5 is aodified '#.. ~ 60 400°C ) by magnesium. It is inOO~....... ....... teresting that zeolites \. ~~ of different crystal sizes 140 400°C It differ in the way of .edi............. ~ 50 ~ fication to improve their -.............. -.~ ~ catalytic selectivity; For )20 Al-modit1ed samples, a maI} }4 26 }O 1 5 9 increase in selectiximum Pulse No• vity is reached on the 3-5 ~ crystal size sample. Fig. 6 Aromatization of propene over Sb203-ZSM-:-5 Moreover, it has been fo.. A-3 20)(361\1; • A-9 ..... 3"u; -Arom. und that the opti.um amoin product; --- PX in xylene; Propene unt of Al-additive also cony. 80%. Pulse Microreactor. depends on the crystal size, equal to 3% for the 0.25-0.5.A1 sample and 4% for the 3-5.A1 sample respectively. This means that the Al-additive and the pore structure are two interac~ng factors which affect the kinetics of methanol conversion jointly. C~+C3/C~+C3
.
. .
~_
.",
_...
~
...
A#"-
I
~
~'
.
.J /')-\{j
is-s
}-5
20)(40 q25-Q5}-5
l4x20
20X}CJ
Crystal size (p)
Fig. 7 Grain size effect 01 Mg-ZSM-5(a) and Al-ZSM-5(b) on methanol conversion at 5000C. Pulse Microreactor. --- HZSM-5.
619
Zeolite modifications with placement of guest molecules Shape selectivity of zeolite catalysts can be adjusted ~y guest molecules which will locate on the external surface or enter into the pore structures t according to the relative magnitude of molecule and pore sizes. For instance(ref. 11)t large molecules like tetramethylquinoline can be possesed only by outer surface and will affect the catalytic behavior of zeolite having extended external surface to a great extent. TAB~E 4 shows the resultant increase in selectivity on a tetramethylquinoline poisoned HSW which is a novel preparation of erionite-offer1tite structure. On the contrary, the high Si/Al HZSM-5 consists of only a limited num?er of external acid sites and its selectivity for methanol conversion is, therefore, 'almost unaffected when accommcdating guest molecules of tetramethylquinoline. When small molecules with easy accession to pore channels, such as pyridine or ammonia, are used as additive, profound changes in selectivity for methanol conversion may happen tor small-pore zeoliteswith high strength of surface acidity such as HSW(see fig. 8) • For HZSM-5, the selectivity for C~ and C production steadily inereases with added pyridine up ta 1.6~1. After that, the catalytic activity drops readily and approaches zero when 2.0MI pyridine(corresponding to 2.98Xl0 20 sites/g) is added. The existence of a sharp threshold at which the total conversion drops drastically is an indication of the uniformity of active sites over the pore structures of our HZSM-5 preparation. It is interesting that as the conversion declines to a very low value, C~ yield is four times as much as A plausible interpretation for tlUs ob---".'--~.c:::::::-'-'-4-'~-4-'---"-" 100 servation is that the """ ",."" ........ -. ........ formatibn 0 s C2 is ~'t" mechanistically more . r' - " .",,' -.... ~ . _ . - .-.-. .-' -0-'" 50 ......... feasi bl e in the -ini~ tial stage 0 f metha!-::-8 nol conversion. o 1 2 ~OA
3
d3.
~
~
.""..
..... pyridine (iUl)---' NH
3
(ml)
Fig. 8 Product distribution over pyridine and ammonia poisoned HZSM-5 and HSW (400 o C)
HZSM-5; ---HSW x conversion ~ in Hcs
. c:
in Hes pufse reactor
620
Modifications of isotropic substitution Great attent~on has been focused recently on the investigation of isotropic substitution in zeolite synthesis chemistry. A variety of elements including B, Ge, In, Tl, Fe and Sn have been used in our laboratory TABLE 4 Effect of poisoning on methanol for replacement of laconversion* ttice alumina or silica 5 catalysts HSW of ZSM- or ZSM-ll to TMQ in~cted( 1Il~ 0 2.6 improve the selectivity prod.str.Cwt )** 4.4 4.2 for methanol convez-sf.oa , (CO + CH 4 + CO 2) 14.1 8.1 15.3 15.3 TABLE 5 shows some insc~ 19.6 31.1 tances. The B-Si-HZSM-5 ~ 12.5 13.2 16.9 18.4
C3
yields 4.0 .times of in comparison with its HZSM-5 basis. The TPD measurements(Fig. 9) suggest a signiftcant
ez~2CZ
4~:t
o
6~.6
o
59.0 57.9 C=~c7/ev-Co 1.5 2.3 1.6 1.5 2 ~ 2 4 C2/C3 1.6 2.4 0.9 0.8 *temperature of pulsemicro-reactor:400 u C methanol cony. 100% **dry basis
reduction in acid sites on the surface of B-Si-HZSM-5 catalyst. A narrow distribution of acid sites is created by boron replacement which is expected to be kinetically favorable for formation of propylene. Some zeolite modifications with satisfactory selectivity to low olefins are listed in TABLE 6. Improvement on selectivity can be substantiated by subtle adjustment of surface acidity with reference to the pore structures of zeolites(ref. 12, 13, 14,15, 16). TABLE 5: Methaaol conversion over ~Si-HZSM-5 and Al-Si-ZSM-5 Catalysts Si~
B-Si-HZSM-5 Al-Si-HZSM-5 32 25 2.0 11.5 60.0 8.2 87.• 9 46.2
1.0 1.0 11.7 14.8 34.4 35.2 1.2
0.2
0.8
*methanol feed cont~g 70% water, -1 reaction temp. 450 C; WHSV, 5 hr ; conversion 100%. ** dry basis wt%
(',
'I
:
;
200
I
a,'''''
I \
/
/
I
\/ b
460
teap.
\
\
\
\
60'0
°c
Fig. 9. TPD diagram 0 r NH 3 on Al-Si-HZSM-5(a) and B-Si-HZSM-5(b)
621
TABLE 6. Methanol conversion over zeolite catalysts of improved selectivity and stability* Catalysts HZSM-5 MgZSM-5 PZSM-5** PZSM-l1 Zn-Pd-Mg ZSM-5 React. Temp. °c 550 500 550 550 331 WHSV Hr- 1 5.0 7.6 4.4 5.7 4.3 Prod. distr.(wt.%) (dry basis) o o 0.5 o 76.2 M,e 20 H-Gs. 97.5 96.1 99.2 98.9 23.3 HCs. distr.(wt.%) C1 3.3 1.7 1.2 3.3 1.5 C~ 25.7 17.7 38.7 36.2 47.1 C~ 32.3 61.8 42.8 37.9 47.3 C _ 14.0 1.4 1.6 3.2 0 o2 67.2 94.9 92.7 84.5 98 ..5 C2-C; Cg-C;/C~-C~ 4.3 47.5 20.6 9.3 0 C2/C} 0.8 0.3 0.9 0.9 1.0
SVi-2*** 401 4.9 1.3 95.5 4.4 60.9 19.7 0 81.6 10.8 3.1
*methanol conversion 100%; **This catalyst is stable for a duration of >200 hrs reaction period and its select1.vity for ethylene production is maintained at 35%. ***A modified sample of HSW. For zeolite of moderate pore size such as PZSM-5, selectivity is relative to the Bronsted acid sites of medium strength, as is shown. by our TPD and IR measurements. According to thermodynamic equilibrium considerations, high yield of ethylene is expected as the reaction temperature is increased so as to suppress the trend of formation of heavy fractions. Moreover, since propylene rather than ethylene would be the main cracking product according to the Jl-fission rule of carbonium ion, high yield of propylene is obtainable on zeolites of relatively weak acid strength at moderate reaction temperatures. This is probably the mechanistic basis for the high selectivity of Mg-ZSM-5 to produce propylene. For zeolites of small pore sizes(including the Zn-Pd-MgZSM-5) the formation of molecules larger than C is greatly 4-isomers prohibited owing to the strikingly limited diffusion through the narrow channel. The in~orporated effect between the pore constraint and BrenRted acid sites of medium strength creates a selectivity as high as 60% of ethylene in the effluent products. However, the accumulation of carbonaceous products occurs readily and rapid deactivition becomes inevitable for the small pore zeolites. On the basis of the above reasoning, we tend to conclude that im-
622 provement on selectivity for low olefina can be achieved for different zeolites by means of proper modification. The key problem in MTO process turns out t o be tbe development of zeolite catalyst of high selectiVity with good performance in stability, i.e to increase its hydrothermal stability in particular against the dealumination process which occures on catalyst 9uroface under high temperoature treatment with the water vapor in the reaction stream. Our achievement in this respect is examplified in TABLE 6 by the result obtained on a PZSM~ catalyst. Further work along this direction will be continued in our laboratory. REFERENCES 1. T.E. Whyte, JR.~ and R.A. Dalla Betta, CATAL. REV. SCI ENG., 24(4), 567(1982). 2. Liang Juan, Liu Baoxiang, Zhao Suqin, Ying Muliang, and Li Hongyuan, J. of Fuel Chem. and Tech.(China) 11,1,64(1982). 3. Li Hongyuan, Liang Juan, Ying MUliang, Liu Baoxing, J. of Cabl. (China) 4, 3, 244( 1983). 4. R.M. Barrer and P.J. Denny, J. Chem. Soc., p.971(1961). 5. G.T. Kerr, J. Inorg. Chem. 5, 1537(1966). 6. Li Hongyuan, Liang Juan, Crystallization of ZSM-5 and ZSM-11 in Various Cation Systems (1982), unpublished. 7. E.G. Derouane, J.P. Gilson, Z. Gabelica etal., J. Catal. ?1, 447{198l) • a. V. Leoluze, L.B. Sand, Recent Progr. Rep. 5th Int. Conf. Zeol. (Sersale, ED.), 4144, ~iannini, Naples,(1981). 9. E.M. Flanigen, Proc. 5th Int. cenr. Zeol. (L. V.C. Rees ed, ) Heyden London(1980), p.760. 10. Li Bei~u, Liang Juan, $un Jinfeng, Li Hongyuan, and Liu Baoxiang, Petrochem. Tech.(China) 13, 4, 242Z1984). 11. Chen Guoquan and Liang Juan, Proc. 6th Int. Conf. Zeol. Reno, U.S.A., July(1983), Investigation o~ Methanol Conversion to Lower Olefins over Small Pore Zeolitee( post. Paper). 12. Liang Juan, Zhao Suqin, Li Hongyuan. Yin~ Muliang, and Liu Baoxiang , Petrochem. Tech. (China) 12, 9, 531 (1983)· 13. Chen Guoquan find Liang Juan, China-Japan-U.S.A. Symp. on HeterOgeneous Catalysis, 1982, Dalian, China, paper AOIC. 14. Li Beilu, ~Iapg\Juan, Sun Jinfen, Li Hongyuan, and Liu Baoxiang J. of Catal.(China) 4, 3, 248(1983). 15. Guo Wengui, Liang Juan, Ying Muliang, and Zhao Suqin, The IR StUdy of Methanol Conversion on HSW and PZSM-5(1983), to be published. 16. Guo Wengui, Liang Juan, Hu Jiehan, Song Yongzhe, and Li Beilu, The Surface Character of AI-ZSM-5 and Mg-ZSM-5 and Its Adsorptivity to Olefins(1983), unpublished.
© 1985 Elsevir Science Publishers B. V. Amsterdam - Printed in Yugoslavia
HIGH SILICA ZEOLITE AS HETEROGENEOUS CATALYST Liang Juan Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian, Liaoning, China
ABSTRACT The nucleation and crystallization processes and the role of inorganic and 0rgano-cations in the 6ynthesis of high silica ~o lites have been studis4. The resulting ZSM-5 and ZSM-ll preparations have been modified by incorporating Mg-, AI-, P- and other additives for catalytic app~cations. Improved catalysts with specific shape selectivity and high stability were obtained and some examples are described and discussed, particularly in relation to their catalytic performance for methanol conversion. INTRODUTION Zeolite based catalysts are now becoming the most important catalysts in the petroleum and petrochemical industries(ref.l). Their distinct structural and chemical properties make zeolites very attractive and worthwhile for stUdy from the different points of view of interdisciplinary sciences. Our knowledge about zeolite chemistry in the present stage is perhaps still in its infancy but will be rapidly developed by joint effort and COllaboration between scientific researchers in different fields. In regard to the catalysis area, the striking and continuous advances in zeolite synthesis are expected to provide a variety of new catalytic processes. Undoubtedly, the more experience we have in zeolite synthesis, the more exciting the future prospects will be. In this paper, a brief account of our work on zeolite synthesis, with high silica zeolites in particular, is described in connection with some applications in heterogeneous catalysis. ZEOLITE SYNTHESIS One of the most important problems in catalytic practice is the preparation of zeolite catalysts of satisfactory selectiVity and stability from starting materials of low toxicity and in plentiful supply. To approach this problem, better understanding of the crystallization process in relation to the resulting zeolite pre-
612
parations is highly desirable. Our experiences in the choice of synthesis conditions and investigations of the role of cations in the crystallization process are illustrated in the following text. Controlled growth of crystal grain Starting from an alumina-silica supersaturated gel ·of high basicity, crystal nuclei are gradually separated to form a new phase during nucleation stage in zeolite synthesis. Although it is very desirable to monitor and control the nucleation process in the very b.eginning stage, i t is still difficult to get detailed insight into the developing structures of crystal nuclei by means of the sophisticated instruments so far available. According to a general understanding 0 f the factors innuencing the formation of a new phase during precipitation, we can conceivably expect that the number of nuclei formed could be regulated by adjusting the wall effect and by the addition of suitable structure directing agents. As demonstrated in TABLE 1(ref.2), successful control of crystal sizes in the r~ge between ""'3 to 2O)(36)U by changing the aging history of the directing agent is manifested. The zeolite prepared is of good un::j.formity. The crystallization can be further shortened by increasing the stirring speed or/and adding NaCI which produce-s a zeolite of much smaller crystals up to O. 1"XO.07 )J. As expected, the crystal size greatly influences the shape selectiVity of zeolite catalysts. For toluene disproportionation the l~ge size ZSM-5 gives p-xylene in 30 wt.% exoess of eqUilibrium yield, showing the diff\lsion constraint for 0- and m-xylene in long channels. The shape: selectiVity for sorption of various molecul~s is clearly demonstrated in Table Z(ref.Z). TABLE 1 Changes in grain size of '~eolites directing agentS/lIIlple No. A-3 .\-5 ,A-7 A-9
•Temp.
Aging time (hr. ) 72 120 160 ZSM-5 seed
With aging time of s'tructure Av. crystal size
Vu)
20)(36 14)(.21 9x14 ""'3
1700C; Autoclave with special lining.
'XRD analysis ZSM-5 ZSM-5 ZSM-5 ZSM-5
613
TABLE 2 properties of ZSM-5 samples
P~ysico-chelllli..cal
Chemical Formula Sample Av. Cryst. Rramework Density(g/ml) No. Size CAl) 2.2 A-3 20X36 NaO• 52 H2• 35 A1 2• 87 Si 93• 13 °192 2.2 A-9 3 NaO• 47 H2• 16 A1 2• 63 Si 93• 37 °192 TABLE 2 Continued Adsorption Capacity(wt.%)· 2,2-DMB Benze~~ Toluene 8.7 6.5 9.6 non non 6.a 10.9 1.6 non 7.6 0C -Adsorption temp. 30
p-xylene 12.7 12.7
a-xylene
Improvement of Crystallinity During high silica zeolite synthesis it is important to avoid, as much as possible, the incorporation of .,(-Si0 2 or mordenite during nucleation stage. As an effective route to minimize the formation of (>(-8i0 2 phase, the nucleation and crystallization are carried out separately in diffS?rent environments(ref.3). The nucleation of the gel is initiated in an organo-cation medium. At a certain stage of nucleation, Na+ is added to speed up crystallization. Fig. 1 compares the results of the two-stage process with that of the conventional method, illustrating a facilitated crystallization in the presence of added Na+. The two-stage procedure provides a convenient way to prepare ZSM-5 and ZSM-1f 2 with Si/Al ratios in the range from 10 to ./ ./' 1700 with high XRD 1 /' crystallinity • /' /' A series of sam_r' ples have been prepared and studied 5 7 9 11 13 for the process 0 f t1ae (day) methanol conversion to low olefins(MTO). Fig. 1 The Crystallization Curve of Z8M-5 The effect of time 1.+0rganic amine medium without Na. 2. With of crystallization Na added into the medium aged for 5 days.
_--
....
.
614
is investigated in relation to the methanol catalysis and ammonia desorption behavior of the resulting zeolite samples(Fig. 2, 3). The crystallization is completed in a period of 90 hrs as monitored by XRD analysis. Zeolite samples resulting from 54 or 72 hrs
0"
......
~
;
.....
80
t ....
60
•
•
.
~
HZS....5(40)
o
40
II
III
_
20
',,-/
--> G~~~
~-.~ A_ _
~A,/
50
70
0
-6
_0
110 90 U.-e (hr)
I~
216
Changes in selectivity for methanol conversion with crystallization time of synthetic ZSM-5. A C~; 0 C C D C A Arom.
-Fig.2
3;. c:3;. 4; 5;
!
0
400 600 teap. °c
Fig. 3 The effect of crystallization time on TPD diagram of adsorbed NH 1. 54; 3• hrs. 2. 72; 3. 90; 4. 216
crystallization exhibit similar features of acidity in TPD diagram and similar product distribution with high yield of low olefins in methanol conversion. However, after complete crystallization, the zeolite gives different TPD diagram with two sharp peaks at about 270 0C and at 430 0C Or higher Samples of complete c;ystallization convert methanol with high yields of aromatics and propane. It seems eVident that the product distribution or selectivity for methanol conversion is closely related with the number and strength of acid sites. The strong acid site(with TPD peak at 500 oC,or higher) tends to further the chain growth reaction and the hydrogen transfer reaction. Therefore, the ZSM-5 samples with crystallization period longer than 90 hrs produce more aromatics and propane, in comparison with the shorter period samples. The obvious increase in C~ in the product of the long period samples is likely due to the p~esence af medium strength acid sites which crack C; to a great extent. It is interesting that when the crystallization period is
615
prolonged for more than 200 hrs and the resulting zeolite sample exhibits no detectable changes in crystal features, a further increase in surface acidity and a corresponding change in product distribution for methanol conversion can still be observed. It seems that some final adjustment of surface states is possible even after crystallization reaches completion. The role of cation Since Barrer(ref. 4) and Kerr(ref. 5) discovered the synthesis of zeolite with organic bases, the role of organic cation has been one of the most important problems in zeolite chemistry. It was postulated that organic cation Lncr-eases the solubility 0 f silicate and aluminate. Some results in our laboratory(ref. 6) repeated that the presence of organic cation favors the nucleation of high SilAl ratio zeolite from a variety of gels of different compoesitions. However, it is difficult to use NH 0H to prepare zeo4 lite of Si/Al ratio higher than 50, though zeolite of Si/Al ratio in the range of 20-30 can be easily synthesized in an ammonium hydroxide medium. If we try to increase the concentration of silicate in an inorganic alkaline solution, the mixing-precipitation of ~-Si02 will become a serious problem in addition to the long induction period encountered. Fig. 4 illustrAtes the effect of cation on the crystallization process. With NHhOH, the silicate concentration keeps an invariant of low Yalue(ahout 4.8 ~/l). However , by using e~nylamine the initial concentration of silicate can be as 1.0 ~10 Iiigh as 14.6 gIl ana the crystalli-~--_., zation 0 f a high / / Si/Al zeolite is ,/ completed in a ./ ~ • .-.'4' short period. It 2 """-----..,--6 8 8 is found that both - ........ ---...... --- - ... ----.- -- .. the readiness 0 f 40 80 120 160 time (hr) crystallization and the solubility of silicate can be Fig~ 4 Concentration changes gf liqUid phase further increased dur~ng crystallization at 170 C. - with ethylamine; --- with NH ~ Basicity; when using tetra40H; o Con.c.o f Si02J 11 Ads. n-C6.
.
..
.. .
.
616
propylamine. Ia conformity with the (Si/ll) conclusion reached by DerouSample Si/Al ratio (Si/ll)s ane(ref. 7), the distributibulk surface !fo. 0.86 b on gradient of Si/Al i. rouA 31 36 0.80 nd to be highly dependent on B 85 34 c t80 135 g:~f the conditioD!l of synthelBi•• D 193 340 TABLE 3 illustrates the trend of Al-enrichlllent on the surface of high Si/Al preparations. Work in our laboratory showed 80me general results concerning the effect of synthesis conditions such as that the crystallization takes place faster during the synthesis of higher Si/Al samples, the crystallinity increases with Si/Al ratio etc. These facts are similar to the observations made by Sand(ref. 8). Other things being equal, the increasing trend of crystallization rate with increasingSi/Al concentration ratio in the starting liquid implies that eome Si-rich complex must be developed to facilitate the nucleation process. If this is true, the separation Of~-Si02 phase might occur simultaneously and could be the main problem during BY.thesis of high Si/Al zeolites. We feel that the role or organic cation is, among others, to ayoid the simUltaneous crystallization of 0(.-Si0 2 by accomplishing the 'templating' effect. For instance, the tetrapropylamine can prevent 0<."Si0 from precipitating out for a 2 crystallization period as long as 216 hrs, while in NH medium, 40H the precipitation of~-Si02 occurs in a much shorter time. Tetrapropylamine is also very '.1-1,------=-& characteristic in hastening _ - - -.......---..-b the crystallization rate. Fig. ~----+c d 1D8 5 shows that by the addition 0 f t> I only 5% of' a TPA-c
•
617
zeQlitEB with JI1.dely different Si/Al ratios, with the mat.nt enance of high uniformity and crystallinity. Although all these exper1.mental observations are less contributory to the mechanistic details of templating effect, a conceivable and conformable interpretation of the structural effects of tetrapropylamine in zeolite synthesis is due to its affinity to coordinate with the silica-microstructures and to develop replications through H-bonding interact~on (ref. 9). The fact thatthe coadintiol\bEl:ween NH and the silica-micro40B structures is less extensive and not strong enough makes the templating effect hardly possible to induce. As a consequence, the silica concentration in the liqUid phase maintains a constant low value(as shown in Fig. 4). Continuous examination of solid phase structures with electronmicroscope reveals that the number of crystal particles increases monotonically without any indication of auto-catalytic growth of particles. MODIFICATION OF ZEOLITE AlIOng other, the versatile adaptability of zeolite to structure and cheltieal modification is very important aJld benificial to catalytic practice. Owing to these character-tcttcs, the stahility and selectivity of z.eolite catalysts can be adjusted and IIOdified by incorporating cations through impregnation or ion-exchange, housing 0 f guest -molecule, isotropic substi tutioD, etc. Joimt effect of physical and chemical modifications Additives incorporated into zeolite may be kinetically effective or not according to how the reaction pathway interacts With pore structures. An example is giv-en in Fig. 6 which shows the improvement on ~atalytic selectivity of large crystal zeolite for prowlene aromatization gained by adding Sb 20 • 100% p-xyleDe 3 selectivity can be obtained over Sb20 zeolite catalyst 3-lIlOdified of large crystal sizes at 500°C. For zeolite of small crystal sizes the sam~ additive is less effective. It seems obvious that aaape selecti vity could be actuated onl,. when the reactant itself(and its reaction) is confined to a certain structural barrier. Fig. 7a and 7b demonstrate how the selectivity for methanol conversion could be changed by impregnating magnesium and aluminum additives onto ZSM-5(ref. 10). A significant increment of ~ and yi:eJ.ds is achieved with Mg-modified zeolite catalyst of small crystal: sizes. Increases of 51'% in C 47% in and 6.8 times in
03.
z'
c'3
618
ratio can be obtained when the 0.25,.... ...... 0.5)l HZSM-5 is aodified '#.. ~ 60 400°C ) by magnesium. It is inOO~....... ....... teresting that zeolites \. ~~ of different crystal sizes 140 400°C It differ in the way of .edi............. ~ 50 ~ fication to improve their -.............. -.~ ~ catalytic selectivity; For )20 Al-modit1ed samples, a maI} }4 26 }O 1 5 9 increase in selectiximum Pulse No• vity is reached on the 3-5 ~ crystal size sample. Fig. 6 Aromatization of propene over Sb203-ZSM-:-5 Moreover, it has been fo.. A-3 20)(361\1; • A-9 ..... 3"u; -Arom. und that the opti.um amoin product; --- PX in xylene; Propene unt of Al-additive also cony. 80%. Pulse Microreactor. depends on the crystal size, equal to 3% for the 0.25-0.5.A1 sample and 4% for the 3-5.A1 sample respectively. This means that the Al-additive and the pore structure are two interac~ng factors which affect the kinetics of methanol conversion jointly. C~+C3/C~+C3
.
. .
~_
.",
_...
~
...
A#"-
I
~
~'
.
.J /')-\{j
is-s
}-5
20)(40 q25-Q5}-5
l4x20
20X}CJ
Crystal size (p)
Fig. 7 Grain size effect 01 Mg-ZSM-5(a) and Al-ZSM-5(b) on methanol conversion at 5000C. Pulse Microreactor. --- HZSM-5.
619
Zeolite modifications with placement of guest molecules Shape selectivity of zeolite catalysts can be adjusted ~y guest molecules which will locate on the external surface or enter into the pore structures t according to the relative magnitude of molecule and pore sizes. For instance(ref. 11)t large molecules like tetramethylquinoline can be possesed only by outer surface and will affect the catalytic behavior of zeolite having extended external surface to a great extent. TAB~E 4 shows the resultant increase in selectivity on a tetramethylquinoline poisoned HSW which is a novel preparation of erionite-offer1tite structure. On the contrary, the high Si/Al HZSM-5 consists of only a limited num?er of external acid sites and its selectivity for methanol conversion is, therefore, 'almost unaffected when accommcdating guest molecules of tetramethylquinoline. When small molecules with easy accession to pore channels, such as pyridine or ammonia, are used as additive, profound changes in selectivity for methanol conversion may happen tor small-pore zeoliteswith high strength of surface acidity such as HSW(see fig. 8) • For HZSM-5, the selectivity for C~ and C production steadily inereases with added pyridine up ta 1.6~1. After that, the catalytic activity drops readily and approaches zero when 2.0MI pyridine(corresponding to 2.98Xl0 20 sites/g) is added. The existence of a sharp threshold at which the total conversion drops drastically is an indication of the uniformity of active sites over the pore structures of our HZSM-5 preparation. It is interesting that as the conversion declines to a very low value, C~ yield is four times as much as A plausible interpretation for tlUs ob---".'--~.c:::::::-'-'-4-'~-4-'---"-" 100 servation is that the """ ",."" ........ -. ........ formatibn 0 s C2 is ~'t" mechanistically more . r' - " .",,' -.... ~ . _ . - .-.-. .-' -0-'" 50 ......... feasi bl e in the -ini~ tial stage 0 f metha!-::-8 nol conversion. o 1 2 ~OA
3
d3.
~
~
.""..
..... pyridine (iUl)---' NH
3
(ml)
Fig. 8 Product distribution over pyridine and ammonia poisoned HZSM-5 and HSW (400 o C)
HZSM-5; ---HSW x conversion ~ in Hcs
. c:
in Hes pufse reactor
620
Modifications of isotropic substitution Great attent~on has been focused recently on the investigation of isotropic substitution in zeolite synthesis chemistry. A variety of elements including B, Ge, In, Tl, Fe and Sn have been used in our laboratory TABLE 4 Effect of poisoning on methanol for replacement of laconversion* ttice alumina or silica 5 catalysts HSW of ZSM- or ZSM-ll to TMQ in~cted( 1Il~ 0 2.6 improve the selectivity prod.str.Cwt )** 4.4 4.2 for methanol convez-sf.oa , (CO + CH 4 + CO 2) 14.1 8.1 15.3 15.3 TABLE 5 shows some insc~ 19.6 31.1 tances. The B-Si-HZSM-5 ~ 12.5 13.2 16.9 18.4
C3
yields 4.0 .times of in comparison with its HZSM-5 basis. The TPD measurements(Fig. 9) suggest a signiftcant
ez~2CZ
4~:t
o
6~.6
o
59.0 57.9 C=~c7/ev-Co 1.5 2.3 1.6 1.5 2 ~ 2 4 C2/C3 1.6 2.4 0.9 0.8 *temperature of pulsemicro-reactor:400 u C methanol cony. 100% **dry basis
reduction in acid sites on the surface of B-Si-HZSM-5 catalyst. A narrow distribution of acid sites is created by boron replacement which is expected to be kinetically favorable for formation of propylene. Some zeolite modifications with satisfactory selectivity to low olefins are listed in TABLE 6. Improvement on selectivity can be substantiated by subtle adjustment of surface acidity with reference to the pore structures of zeolites(ref. 12, 13, 14,15, 16). TABLE 5: Methaaol conversion over ~Si-HZSM-5 and Al-Si-ZSM-5 Catalysts Si~
B-Si-HZSM-5 Al-Si-HZSM-5 32 25 2.0 11.5 60.0 8.2 87.• 9 46.2
1.0 1.0 11.7 14.8 34.4 35.2 1.2
0.2
0.8
*methanol feed cont~g 70% water, -1 reaction temp. 450 C; WHSV, 5 hr ; conversion 100%. ** dry basis wt%
(',
'I
:
;
200
I
a,'''''
I \
/
/
I
\/ b
460
teap.
\
\
\
\
60'0
°c
Fig. 9. TPD diagram 0 r NH 3 on Al-Si-HZSM-5(a) and B-Si-HZSM-5(b)
621
TABLE 6. Methanol conversion over zeolite catalysts of improved selectivity and stability* Catalysts HZSM-5 MgZSM-5 PZSM-5** PZSM-l1 Zn-Pd-Mg ZSM-5 React. Temp. °c 550 500 550 550 331 WHSV Hr- 1 5.0 7.6 4.4 5.7 4.3 Prod. distr.(wt.%) (dry basis) o o 0.5 o 76.2 M,e 20 H-Gs. 97.5 96.1 99.2 98.9 23.3 HCs. distr.(wt.%) C1 3.3 1.7 1.2 3.3 1.5 C~ 25.7 17.7 38.7 36.2 47.1 C~ 32.3 61.8 42.8 37.9 47.3 C _ 14.0 1.4 1.6 3.2 0 o2 67.2 94.9 92.7 84.5 98 ..5 C2-C; Cg-C;/C~-C~ 4.3 47.5 20.6 9.3 0 C2/C} 0.8 0.3 0.9 0.9 1.0
SVi-2*** 401 4.9 1.3 95.5 4.4 60.9 19.7 0 81.6 10.8 3.1
*methanol conversion 100%; **This catalyst is stable for a duration of >200 hrs reaction period and its select1.vity for ethylene production is maintained at 35%. ***A modified sample of HSW. For zeolite of moderate pore size such as PZSM-5, selectivity is relative to the Bronsted acid sites of medium strength, as is shown. by our TPD and IR measurements. According to thermodynamic equilibrium considerations, high yield of ethylene is expected as the reaction temperature is increased so as to suppress the trend of formation of heavy fractions. Moreover, since propylene rather than ethylene would be the main cracking product according to the Jl-fission rule of carbonium ion, high yield of propylene is obtainable on zeolites of relatively weak acid strength at moderate reaction temperatures. This is probably the mechanistic basis for the high selectivity of Mg-ZSM-5 to produce propylene. For zeolites of small pore sizes(including the Zn-Pd-MgZSM-5) the formation of molecules larger than C is greatly 4-isomers prohibited owing to the strikingly limited diffusion through the narrow channel. The in~orporated effect between the pore constraint and BrenRted acid sites of medium strength creates a selectivity as high as 60% of ethylene in the effluent products. However, the accumulation of carbonaceous products occurs readily and rapid deactivition becomes inevitable for the small pore zeolites. On the basis of the above reasoning, we tend to conclude that im-
622 provement on selectivity for low olefina can be achieved for different zeolites by means of proper modification. The key problem in MTO process turns out t o be tbe development of zeolite catalyst of high selectiVity with good performance in stability, i.e to increase its hydrothermal stability in particular against the dealumination process which occures on catalyst 9uroface under high temperoature treatment with the water vapor in the reaction stream. Our achievement in this respect is examplified in TABLE 6 by the result obtained on a PZSM~ catalyst. Further work along this direction will be continued in our laboratory. REFERENCES 1. T.E. Whyte, JR.~ and R.A. Dalla Betta, CATAL. REV. SCI ENG., 24(4), 567(1982). 2. Liang Juan, Liu Baoxiang, Zhao Suqin, Ying Muliang, and Li Hongyuan, J. of Fuel Chem. and Tech.(China) 11,1,64(1982). 3. Li Hongyuan, Liang Juan, Ying MUliang, Liu Baoxing, J. of Cabl. (China) 4, 3, 244( 1983). 4. R.M. Barrer and P.J. Denny, J. Chem. Soc., p.971(1961). 5. G.T. Kerr, J. Inorg. Chem. 5, 1537(1966). 6. Li Hongyuan, Liang Juan, Crystallization of ZSM-5 and ZSM-11 in Various Cation Systems (1982), unpublished. 7. E.G. Derouane, J.P. Gilson, Z. Gabelica etal., J. Catal. ?1, 447{198l) • a. V. Leoluze, L.B. Sand, Recent Progr. Rep. 5th Int. Conf. Zeol. (Sersale, ED.), 4144, ~iannini, Naples,(1981). 9. E.M. Flanigen, Proc. 5th Int. cenr. Zeol. (L. V.C. Rees ed, ) Heyden London(1980), p.760. 10. Li Bei~u, Liang Juan, $un Jinfeng, Li Hongyuan, and Liu Baoxiang, Petrochem. Tech.(China) 13, 4, 242Z1984). 11. Chen Guoquan and Liang Juan, Proc. 6th Int. Conf. Zeol. Reno, U.S.A., July(1983), Investigation o~ Methanol Conversion to Lower Olefins over Small Pore Zeolitee( post. Paper). 12. Liang Juan, Zhao Suqin, Li Hongyuan. Yin~ Muliang, and Liu Baoxiang , Petrochem. Tech. (China) 12, 9, 531 (1983)· 13. Chen Guoquan find Liang Juan, China-Japan-U.S.A. Symp. on HeterOgeneous Catalysis, 1982, Dalian, China, paper AOIC. 14. Li Beilu, ~Iapg\Juan, Sun Jinfen, Li Hongyuan, and Liu Baoxiang J. of Catal.(China) 4, 3, 248(1983). 15. Guo Wengui, Liang Juan, Ying Muliang, and Zhao Suqin, The IR StUdy of Methanol Conversion on HSW and PZSM-5(1983), to be published. 16. Guo Wengui, Liang Juan, Hu Jiehan, Song Yongzhe, and Li Beilu, The Surface Character of AI-ZSM-5 and Mg-ZSM-5 and Its Adsorptivity to Olefins(1983), unpublished.