Metal Complexes in Fossil Fuels - American Chemical Society

Chapter 4 Rationalization for the Predominance of Nickel and Vanadium Porphyrins in the Geosphere

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J. M. E. Quirke Department of Chemistry, Florida International University,TamiamiTrail, Miami, FL 33199

The predominance of nickel and vanadium porphyrins, over other metalloporphyrins in the geosphere may be explained by taking into account four factors: the abundances of the metals, and their environment within the sediments and/or the water column, the stability of the nitrogen-metal bonds, and the lability of the bridge positions of the metal porphyrin complexes.

Since the o r i g i n a l isolation of porphyrins from a variety of sedimentary environments, two t y p e s o f metal complex -the n i c k e l ( I I ) and v a n a d y l (V=0)- have dominated g e o p o r p h y r i n c h e m i s t r y . (J_,2) The reason f o r the s e l e c t i o n o f these two metals i s not immediately o b v i o u s i n view o f t h e a r r a y o f p o t e n t i a l m e t a l c a n d i d a t e s , some o f which, e.g. chromium ( I I I ) , can form more s t a b l e complexes than either nickel o r vanadium. ( F i g . 1, .3,4) problem i s further c o m p l i c a t e d because the t i m i n g o f the metal insertion i n the d e g r a d a t i v e pathway from the c h l o r o p h y l l s to the geoporphyrins i s not w e l l understood. Thus i t i s u n c l e a r whether m e t a l insertion occurs i n t h e water column, o r i n t h e water column-sediment boundary, o r w i t h i n t h e sediments. One p o s s i b l e e x p l a n a t i o n i s based on t h e p r o b a b i l i t y t h a t t h e metals i n o r g a n i c r i c h sediments o c c u r as t h e i r s u l p h i d e s because such sediments have h i g h c o n c e n t r a t i o n s o f hydrogen s u l p h i d e . F o r metal s u l p h i d e s w i t h low s o l u b i l i t y products e.g. copper ( I I ) s u l p h i d e t h e metal i o n would be p r e s e n t i n v e r y low c o n c e n t r a t i o n s , and t h e r e f o r e would be l e s s l i k e l y to chelate with the porphyrins than those s u l p h i d e s w i t h h i g h e r s o l u b i l i t y p r o d u c t s . T h i s argument i s n o t n e c e s s a r i l y sound. The m e t a l i o n s i n s o l u t i o n , a l b e i t i n v e r y low c o n c e n t r a t i o n , c o u l d chelate with porphyrins and c h l o r i n s , f o r c i n g more m e t a l ion into solution u n t i l the porphyrins were T t l e

0097-6156/87/0344-0074$06.00/0 © 1987 American Chemical Society

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


*

Mg

Ca

Sr

Ba

Na

K

Rb

Cs

Lanthanides

Be

Li

La

Fig.

*

Y

Sc

Ce

Pr

Ta

Nb

V

Nd

W

Mo

Cr

Re

Mn

Sm

Os

Ru

Fe

Eu

Ir

Rh

Co

Gd

Pt

Pd

Tb

Au

Ag

Cu

with

Ni

1 E l e m e n t s known t o Form C h e l a t e s

Hf

Zr

Ti

Ho

Tl

In

Ga

Porphyrins.

Dy

Hg

Cd

Zn

Al

B

Er

Pb

Sn

Ge

Si


Tm

Bi

Sb

As

Yb

Lu

—i

S3

o c

76

M E T A L C O M P L E X E S IN FOSSIL FUELS


c o m p l e t e l y c h e l a t e d . In a d d i t i o n , the s o l u b i l i t y p r o d u c t argument cannot r e a d i l y account f o r the p o s s i b i l i t y that the porphyrins undergo c h e l a t i o n on t h e m i n e r a l s u r f a c e . A d i f f e r e n t approach t o r a t i o n a l i s i n g the s e l e c t i v i t y of the c h e l a t i o n o f the geoporphyrins w i l l be p r e s e n t e d . The hypothesis encompasses both the n a t u r e o f t h e metal p r i o r t o c h e l a t i o n and t h e s t a b i l i t y o f the m e t a l l o p o r p h y r i n s a f t e r c h e l a t i o n . I t i s assumed that the d i f f e r e n t s k e l e t a l types o f p o r p h y r i n (I) will behave s i m i l a r l y i n the c h e l a t i o n process, although the absolute rates of c h e l a t i o n may d i f f e r f o r each s k e l e t a l type of geoporphyrin. The f a c t o r s which i n f l u e n c e c h e l a t i o n a r e as f o l l o w s : a) The a v a i l a b i l i t y o f e f f i c i e n t metal i o n c a r r i e r s . b) The abundance o f the elements. c ) The s t a b i l i t y o f the metal-nitrogen bond i n the porphyrin complex. d) The s t a b i l i t y o f the m e t a l - n i t r o g e n bond i n p o r p h y r i n s i n t h e g e o l o g i c a l environment. e) The l a b i l i t y o f t h e p o r p h y r i n meso ( b r i d g e ) p o s i t i o n s . T h i s h y p o t h e s i s has the v i r t u e o f being sufficiently flexible so t h a t i t can be used t o examine the p o s s i b i l i t y o f m e t a l i n s e r t i o n e i t h e r a t the water column-sediment i n t e r f a c e o r deeper down i n the sediment. The h y p o t h e s i s may be f u r t h e r r e f i n e d by as more d a t a a r e o b t a i n e d on t h e n a t u r e o f t h e p o r p h y r i n s bound on m i n e r a l m a t r i c e s .


4.

QUIRKE

Predominance of Nickel and Vanadium Porphyrins

11

R e s u l t s and D i s c u s s i o n


Each o f t h e f i v e parameters i n t h e model w i l l be d i s c u s s e d i n t u r n . The p o s t - c h e l a t i o n parameters a r e i n t e r r e l a t e d , and t h e r e f o r e t h e s e f a c t o r s are discussed together. The Importance o f t h e Ion C a r r i e r . The n a t u r e o f t h e i o n c a r r i e r i n the s e d i m e n t a r y environment i s o f p r i m a r y importance i n d e t e r m i n i n g the p o s s i b i l i t y o f c h e l a t i o n because t h e f i r s t s t a g e i n t h e m e t a l i n s e r t i o n i s the deconvolution o f t h e m e t a l i o n from the i o n carrier. Very s t a b l e i o n c a r r i e r s a r e v e r y i n e f f i c i e n t r e a g e n t s f o r the m e t a l l a t i o n o f the p o r p h y r i n s . In t h e l a b o r a t o r y , h i g h l y charged m e t a l i o n s a r e more d i f f i c u l t t o i n s e r t than lower valence species because t h e n e g a t i v e l y charged anions, and/or the n e g a t i v e l y polarised ligands a r e more tightly bound t o t h e metal. T h i s i s particularly significant for the insertion of tetraand p e n t a - v a l e n t m e t a l s , many o f which form s t a b l e metalloporphyrins. The l a b o r a t o r y s y n t h e s e s o f such compounds u s u a l l y require vigorous c o n d i t i o n s , and o f t e n use water s e n s i t i v e m e t a l l a t i n g agents (e.g. T i C l ^ , S i C l ^ , A 1 C 1 , R^AIH). (3) Thus, s i l i c o n p o r p h y r i n s a r e among the most s t a b l e complexes known, b u t they a r e n o t found i n nature because t h e s i l i c o n o c c u r s i n s i l i c a t e s , which a r e too s t a b l e to p a r t a k e i n t h e m e t a l l a t i o n p r o c e d u r e . Germanium, and aluminium a r e u n l i k e l y to chelate with porphyrins i n t h e sediments because t h e ions will n o t be r e a d i l y available for complexation i n the s e d i m e n t a r y environment. S i m i l a r l y , chromium ( I I I ) porphyrins are e x c e p t i o n a l l y s t a b l e ; however, chromium ( I I I ) i o n c a r r i e r s a r e v e r y u n r e a c t i v e . ( 3 ) Gold ( I I I ) porphyrins a r e a l s o s t a b l e , but they a r e n o t p r e v a l e n t i n n a t u r e because g o l d o c c u r s p r i m a r i l y as t h e n a t i v e m e t a l , which i s q u i t e i n e r t . 3

F o r many t r a n s i t i o n e l e m e n t s , t h e n a t u r e o f t h e i o n s i n aqueous s o l u t i o n i s complex. C o n s i d e r t h e group IVB e l e m e n t s . T i t a n i u m would appear t o be a good candidate f o r c h e l a t i o n i n the g e o l o g i c a l environment. I t i s abundant, ( T a b l e I ) and forms stable porphyrin complexes. However, t h e n a t u r e o f t h e t i t a n i u m i n aqueous s o l u t i o n p r e c l u d e s c h e l a t i o n . There i s no s i m p l e s o l v a t e d T i i o n , because of Jjljie h i g h charge t o r a d i u s r a t i o , n o r i s t h e r e e v i d e n c e o f s i m p l e TiO i o n s . ( 5 ) The major m i n e r a l form o f t i t a n i u m i s t i t a n i u m o x i d e , a s t a b l e compound, which i s a poor i o n c a r r i e r . S i m i l a r problems a r ^ encountered w i t h z i r c o n i u m . There i s no i n d i c a t i o n o f Zr o r ZrO i n s o l u t i o n . Instead, polymeric forms o f ZrO seem dominant. Such species w i l l further h i n d e r the m e t a l l a t i o n p r o c e s s . Hafnium i s l i k e l y t o p r e s e n t t h e same d i f f i c u l t y i n t h e l i g h t o f the v e r y g r e a t s i m i l a r i t i e s i n the c h e m i s t r y o f t h e s e two e l e m e n t s . ( 6 ) In aqueous s o l u t i o n , molybdenum and t u n g s t e n can o c c u r ina number o f forms i n c l u d i n g t h e molybdate and t u n g s t a t e a n i o n s . Such s p e c i e ^ w i l l n o ^ l e n d t h e m s e l v e s e a s i l y t o c h e l a t i o n . ( 3 ) The c a t i o n s of Mo and Mo are bimolecular.(7) Vanadium i s the e a s i e s t o f the t e t r a v a l e n t elements t o i n s e r t i n the l a b o r a t o r y using a t e t r a v a l e n t c h e l a t i n g agent. (_3,4_) The metallation i s effected rapidly i n refluxing aqueous a c e t i c acid using vanadyl sulphate as t h e c h e l a t i n g agent. In a d d i t i o n i t i s abundant both i n the water column and i n t h e s e d i m e n t a r y rocks


M E T A L C O M P L E X E S IN FOSSIL F U E L S

78

Table

I.

Average

Abundances

of

Metalloporphyrins


Element

Al Si Ti V Cr Mn Fe Co Ni Cu Ga Ge As Y Zr Nb Mo Ru Rh Pd Ag In Sn Sb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Tl Bi

Seawater Concentration (ppm,

<

Metal Complexes in Fossil Fuels - American Chemical Society

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