Anand, S

Byrappa, K

Kumar, Surcsh B V

IACS Institutional Repository

U P  A- an international journalHydrothermal crystallization and electrical conductivity of aluminophosphate zeolitesB V  S u rc s h  K u m a r* , K  B y fa p p aDepartment ot Geology, Manasagangotn, Uiivcrsity of Mysore,Mysore-570006, India ■fS A n a n d  a n d  K  M  L o k n a t ^  R a iDepartment of Chemistry, Manasagangotn. University of Mysore Mysore-570 006, IndiaE-mail . bvsureshkumar@yahO)o.com Received 13 June 20(H). accepted 4 July 2000vbslrac t Synthesis of AIPO^ zeolites has been carried out under hydrothermal conditions. The electrical conductivity has been determined lorA|}’0  ^ Zeolites, which contain mobile cations located in sites in cavities, on the channel walls, and free within the channels coordinated with water iih.lixules The electrical conductivity exhibited by zeolites is ionic and arises trom the migration of cations through the zeolite framework The ihirmodynaniic parameters have been evaluatedKi>words Aluminophosphatcs (AIPO^), zeolites, electrical conductivity Nos. 83 SO.Pc, 12 HO Tin1. IntroductionThe a lu m in o p h o s p h a le - b a s e d  m o le c u la r  s ie v e  z e o l i te s  a re  sy n th e s iz e d  b y  h y d r o t h e r m a l  c r y s t a l l i z a t i o n  o f  r e a c t i v e  :iIum inophosphate  g e ls  c o n ta in in g  th e  a d d it io n a l  f r a m e w o rk  t^^lcnicnts an d  a n  o rg a n ic  te m p la te s ,  a t te m p e ra tu re s  f ro m  100^’C  li) 25(T’C . T h e  o p tim u m  c ry s ta l l iz a t io n  te m p e ra tu re  d e p e n d s  o n  co m p o s itio n  a n d  s t r u c tu r e  b e lo w  1 5 0 ”C . I 'h e  p r e s s u r e  is  generally a u to g e n e o u s  p re s s u re  a p p ro x im a te ly  e q u iv a le n t  to  '^aiuraied v a p o u r  p r e s s u r e  ( s v p )  o f  w a te r  a t th e  te m p e ra tu re  signaled . T h e  c o n d u c t iv i ty  m e a s u re m e n t  h a s  b e e n  c a r r ie d  >^tii for th e  A lP O ^  -  z e o li te s .Experimental methodsHic g en e ra l r o u te  w h e re b y  z e o l i te s  c a n  b e  s y n th e s iz e d  in  a kboratory  o r  a  p la n t s c a le  a n d  s c h e m a tic  re p re se n ta t io n  o f  z e o lite  'Synthesis is  s h o w n  in  F ig u r e  1 [ 1 ]. T h e  s t ru c tu re  d ir e c tin g  a g e n ts  listed are  o n ly  e x a m p le s  o f  a  w id e  r a n g e  o f  m a te r ia ls  th a t  h a v e  I'ccn used . Z e o li te s  h a v e  b e e n  s y n th e s iz e d  o n ly  b y  h y d ro th e rm a l niethod. T h e  c o n d i t io n s  g e n e ra l ly  u se d  in  s y n th e s is  a re  [ 2 ,3 ] :Corresponding Author.ReagentsSi, Al, P, G a, B, etc.Source cationsGelling H^OAmorphous hydrogel Tem perature, subambient -  250^C Pressure, atmospheric -  autogeneous Time (hours), weeksDissolution Mineralization agent O H -,FStructural building unitsNucleation growth Structure -  directing agents e.g. i. Na\ K*. Ba"^ii. organic cations like TM A  Hi. polymeric species Iv. unchanged organic moleculesFigure 1. Schematic repre.<sentation of zeolite syntheses© 2 0 0 1  lA C S114 B V Suresh Kumar, K  Byrappa, S  Ananci and  K  M  Loknatha Rai(a) R e a c t iv e  s t a r t i n g  m a t e r i a l s  s u c h  a s  f r e s h ly  c o -  p re c ip ila tc d  g e ls , o r  a m o rp h o u s  so lid s ,(b ) re la tiv e  h ig h  p H  in tro d u c e d  in  th e  fo rm  o f  an  a lk a l i m e ta l h y d ro x id e  o r  o th e r  s tro n g  b a se ,(c) lo w  te m p e r a t u r e  h y d r o th e r m a l  c o n d i t i o n s  w ith  c o n c u r r e n t  lo w  a u to g e n e o u s  p re s s u re  a t s a tu ra te d  w a te r  v a p o r  p re s su re ,(d ) a h ig h  d e g re e  o f  s u p e r s a tu ra t io n  o f  th e  c o m p o n e n ts  o f  th e  g e l le a d in g  to  th e  n u c le a lio n  o f  a la rg e  n u m b e r  o f  c ry s ta ls .It in v o lv e s  s ilic a , a lu m in a  an d  /  o r  p h o sp h o ru s  sp e c ie s , m e ta l c a tio n s , o rg a n ic  m o le c u le s  a s  te m p la te s  a n d  w a te r , w h ic h  a rc  c o n v e r t e d  via a n  a lk a l in e  s u p e r s a tu r a t e d  s o l u t i o n  in to  a  m i c r o p o r o u s  c r y s t a l l i n e  z e o l i t e s .  T h e  c o m p le x  c h e m i c a l  p ro c e s se s  in v o lv e d  in  th is  t r a n s fo rm a t io n  c a n  b e  d e n o te d  as  z e o li iiz a iio n . T h e  p ro c e s s  o f  z c o li t iz a tio n  is th e rm a lly  a c tiv a te d  a n d  u s u a l ly  ta k e s  p la c e  a t e le v a te d  te m p e ra tu re  in o rd e r  to  a c h ie v e  h ig h  y ie ld  o f  c ry s ta ls  in  an  a c c e p ta b le  p e r io d  o f  tim e .It is w e ll k n o w n  th a t th e  c h o ic e  o f  s o u rc e  m a te r ia ls  p la y s  a c r u c i a l  r o l e  in  t h e  s y n t h e s i s  a n d  p h a s e  p u r i t y  o f  a lu m in o p h o s p h a te  z e o li te s .  S y n th e s is  o f  a lu m in o p h o s p h a te  z e o li te s  n o rm a lly  ta k e s  p la c e  by th e  fo l lo w in g  s te p s  :i) N e u t r a l i z a t io n  o f  th e  AI -  s o u r c e  s u s p e n d e d  in  w a te r  w i th  n e a r ly  e q u i m o l a r  a m o u n t  o f  d i l u t eTo vacuum systemp h o s p h o r i c  a c i d  t o  o b t a i n  th e  r c a c tu .  a lu m in o p h o s p h a te  g e l.ii) A g in g  o f  th e  re a c t iv e  g e l.iii) A d d itio n  o f  a  p a r tic u la r  o rg a n ic  ad d ili ve to  the rcaai\^. g e l. T h is  is  r e f e r re d  to  a s  p re c u r s o r  g e l.iv) A g in g  o f  th e  p re c u r s o r  g e l, if  n e c e s s a ry  an d  ImalKV) C lo s e d  h y d ro th e rm a l tr e a tm e n t o f  th e  p recu rso r lh !T h e  tim e  re q u ire d  fo r c ry s ta l l iz a t io n  v a r ie s  fro m  a few  hnur  ^to  s e v e ra l d a y s . A  c ry s ta l l in e  p ro d u c t is  fo rm e d  w h ic h  is knoy n a s  a lu m in o p h o s p h a te  z e o li te s . T h e  e le c tr ic a l p ro p e r tie s  fia\t b e e n  s tu d ie d  fo r  th e s e  a lu m in o p h o s p h a te  z e o li te s .T h e  e le c tr ic a l c o n d u c tiv i ty  o l v a r io u s  fo rm s  o f  th e  symhcliv z e o lite s  w a s  m e a s u re d  u s in g  p o ly c ry s ta l l in c  c o m p a c ts  coniamcd in a  c o n d u c tiv i ty  c e ll a s  i l lu s tra te d  in  F ig u re  2 [4 ].T h e  c o n d u c tiv i ty  d a ta  w e re  o b ta in e d  on  alum inophospluitv z e o li te s  a s  a fu n c tio n  o f  te m p e ra tu re  at d if fe re n t conccniiaiioii o f  Na"^ io n s . T y p ic a l c o n d u c tiv i ty  d a ta  fo r  a lu m in o p h o sp lu n  z e o li te s  a rc  g iv e n  in F ig u re  3, a s  a r rh c n iu s  p lo ts , c o n d u c tiv ii\ o v e rs u s  l / 'T  T h e  a c t iv a t io n  e n e rg y  fo r c o n d u c tio n , A H  is shovoi in  F ig u re  4 . a s  a  fu n c tio n  o f  th e  N a^ io n  c o n c c n tra iio n . ilu10Eoo5E8  2(a)(b)(c)(d)(e) (02.9 3 0 3 1 3 21/T(x 10- )^3 3 3 4Figure 3. Arrhenius plots of elcttncal coiuluctivity • (a) in 69t NaOH. (b) in 5% NaOH. (c) in 4% NaOlf (d) in 3% NaOH, (e) m 2% NaOH arul (0  in 1% NaOHFigure 2. Cell for measuring conductivity of compacted, zeolite powder specimen.Figure 4. Variation of the activation energy with cation density in “ zeolites.H ydrotherm al crystallization and  electrical conductivity etc 115j^pical c o n d u c t i v i t y  d a t a  f o r  d i f f e r e n t  c o n c e n t r a t i o n  o f  ilu n n n o p h o sp h a te  z e o l i t e  a s  a  f u n c t io n  o f  t e m p e r a tu r e  a t on^tanl n u m b e r  o f  Na"*" io n s  w e re  d e te r m in e d  a n d  a c tiv a tio n  i:y, AH is o b ta in e d  f ro m  a r r h e n iu s  p lo ts  (F ig u re  5 ).0.05 0 1 015AlPO, (gms)0.25l<idlin' 5. Variation o< the activation energy with AlPO^ ~ zeolites (a) in (,^ i N'aOH. (b) m NaOH and (c) in 1% NaOHX Results and discussion7hc a b o v e  r e s u lts  in d ic a te  th a t th e  c o n d u c tiv i ty  w a s  fo u n d  to  tlcpnuJ s tro n g ly  o n  th e  c a tio n  m o b iliz a tio n  in  a lu rn in o p h o sp h a te  /c o llie s  T h e  in c r e a s e  o f  n u m b e r  o f  N a ^  io n s  a t c o n s t a n t  a lu r n in o p h o s p h a te  z e o l i t e  c o n c e n t r a t i o n  o r  d e c r e a s e  o f  voik o n tra tio n  o f  a lu rn in o p h o s p h a te  z e o li te  at c o n s ta n t Na"^ io n s , d iv rcases th e  c a tio n  d e n s i ty  in  th e  a lu r n in o p h o s p h a te  z e o lite . Hr’ a c tiv a tio n  e n e rg y , A H  in c r e a s e s  r a p id ly  s ta r tin g  fo r  0 ,0 5  cnis ()1 a lu r n in o p h o s p h a te  z e o li te  o r  A l / P  r a t io  o f  1,5. T h is  im licaics tw o  ty p e s  o f  c a tio n  s ite s . F u rth e r , th e  a c tiv a tio n  e n e rg y  AH i n c r e a s e s  a s  t h e  c a t i o n  d e n s i t y  d e c r e a s e s  in  a lu rn in o p h o sp h a te  z e o l i te  w h ic h  is a  c h a r a c te r i s t ic  o f  m o re  M iongly b o u n d  c a t io n .T h e  c o n d u c t iv i ty  w a s  fo u n d  to  d e p e n d  s t r o n g ly  o n  th e  c a t io n  s iz e  a n d  th e  s iz e  o f  th e  c h a n n e l s  w ith in  th e  z e o li te  s t r u c tu r e .  T h e  a c t i v a t i o n  e n e r g y  A H  d e c r e a s e s  d u e  to  a d im in is liin g  C o u lo m b ic  a ttra c t io n  b e tw e e n  th e  c a tio n  an d  c a tio n  s ite . T h e  a d d it io n  o f  O H ' m o le c u le s  to  th e  a lu r n in o p h o s p h a te  z e o li te  s t ru c tu re  p ro d u c e  a  p ro n o u n c e d  c h a n g e  in th e  e le c tr ic a l c o n d u c tiv ity . T h e  c o n d u c tiv i ty  in crea .ses lin e a rly . T h is  re f le c ts  th e  p re fe re n tia l  h y d ra t io n  o f  o n e  ty p e  o f  c a tio n  in  th e  s tru c tu re . T h e  c o n d u c tiv ity  in c re a s e s  as  th e  c ry s ta ls  b e c o m e  s a tu ra te d , in d ic a tin g  th a t th e  c h a n n e l sy s te m s  a re  Id le d . S o m e  o f  th e  Na*  ^io n s  aril fre e  to  m o v e  ra n d o m ly  w ilh in  th e  c h a n n e ls .4. ConclusionsT h e  c c |id u c t io n  m e c h a n is m  in c lu d e s  a  Ire q u c n c y  d e p e n d e n t p ro c e s s  a sc r ib e d  to  m o tio n  o f  c a tio n s  in th e  s u p e r  c a g e s  o v e r  a ra n g e  o f  e n e rg y  b a rr ie rs . A ssu m in g  th a t th e  c o n d u c tio n  p ro c e s s  invo lv i^s o n ly  c a tio n  lo c a te d  m th e  s u p e r  c a g e s , io n s lo c a te d  on s ite  II W ould  h a v e  to  m ig ra te  o v e r  a  s e q u e n c e  o f  th re e  4* rin g s to  re a c h  a n o th e r  s u r fa c e  s i te  II. T h e  d if fe re n c e  in th e  a c tiv a tio n  e n e rg y  fo r  c o n d u c tio n  h a s  b e e n  a t tr ib u te d  to  th e  p re s e n c e  o f  c a t io n s  in  th e  w e a k ly  b o u n d  s i te s  o r  to  d i f l e r e n c c  in th e  e le c tro s ta tic  in te ra c t io n  b e tw e e n  c a tio n s  an d  th e  o x y g e n  fram e  w ork.Referencc.s11 j A Dyer Zeolite Surfat e,s and Reai tmi\, in Mineral Siolai es ted) D J Vaughan and R A [) Palinck (London The Mincialogical Society Senes) (199S)f2J D W Hreck and E M Flanigen M o le m h o  S i e \ e \  (London Society of Chemical Industry) p47 (1968)(3) R M t i i ir r c r  M o l t ( u l a i  S ie v e s  (London Society of Chemical Industry) p39 (1968)[4) D C Freeman (Jr) and 1> N Stamivcs / C lie n t PInw 35 799 (1961)

Hydrothermal Crystallization and Electrical Conductivity of Aluminophosphate Zeolites -

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Hydrothermal Crystallization and Electrical Conductivity of Aluminophosphate Zeolites -

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