Fourier Transform IR Spectroscopy - ACS Symposium Series (ACS

3 Fourier Transform IR Spectroscopy

Downloaded by COLUMBIA UNIV on December 9, 2014 | http://pubs.acs.org Publication Date: November 12, 1982 | doi: 10.1021/bk-1982-0205.ch003

Application to the Quantitative Determination of Functional Groups in Coal PAUL C. PAINTER, RANDY W. SNYDER, MICHAEL STARSINIC, MICHAEL M. COLEMAN, DEBORAH W. KUEHN, and ALAN DAVIS Pennsylvania State University, College of Earth and Mineral Sciences, University Park, PA 16802 Fourier transform infrared (FTIR) spectroscopy is potentially a powerful tool for the characterization of coal. Although the optical advantages of these instrument compared to traditional dispersive devices are important, we believe that the most significant results can be obtained by applying sophisticated data analysis programs. However, if these programs are applied uncritically there is the real possibility of serious error, particularly in quantitative work. Consequently, in this paper we will attempt an assessment of the application of FTIR to the quantitative determination of functional groups, with particular emphasis on O-H and C-H groups. We will consider the use of programs for spectral subtraction, derivative spectra, curve resolving and factor analysis. 1

In the late 1950*s and early I960 s infrared spectroscopy was a widely used analytical tool for investigating the structure of coal. Unfortunately, this technique was limited in part by the problems associated with investigating highly absorbing systems but predominantly by the overlap and superposition of the vibrational modes characteristic of such complex multicomponent systems. In fact, by 1973 there was a general decline in the use of infrared spectroscopy for chemical studies as other techinques (eg NMR) came to the fore. This state of affairs prompted H.A. Laitinen, in an editorial in Analytical Chemistry (1), to draw an analogy between the seven ages of an analytical technique and Shakespeare's seven ages of man, using infrared spectroscopy as a prime example. Senescence and an early demise seemed assured. However, for analytical

CgWW^WmwWaiMiSRSiucal Society Society Library 1155 16th St. N. W. In Coal and Coal Products: Analytical Techniques; Fuller, E.; Washington, 0. C. Characterization 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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COAL AND COAL PRODUCTS

techniques there i s always the p o s s i b i l i t y of r e i n c a r n a t i o n and at the same time as L a i t i n e n ' s depressing prognosis appeared commerc i a l F o u r i e r transform i n f r a r e d instruments were being d e l i v e r e d to a number of l a b o r a t o r i e s . The subsequent resurgence of i n f r a red spectroscopy has been remarkable and has l e d to a renewed i n t e r e s t i n a p p l y i n g t h i s technique to the c h a r a c t e r i z a t i o n of c o a l structure. A d e s c r i p t i o n of F o u r i e r transform i n f r a r e d (FTIR) instruments together with an account of the u n d e r l y i n g theory and o p t i c a l advantages compared to conventional d i s p e r s i v e spectrometers has been given by a number of authors (2-4). Although the improvements i n the q u a l i t y of the s p e c t r a that can be obtained from m a t e r i a l s such as c o a l are u s e f u l , i n our o p i n i o n the most s i g n i f i c a n t advances have been made through the use of the dedicated o n - l i n e minicomputer that i s an i n t e g r a l p a r t of the system. Programs capable of a range of manipulations, from the simple such as s p e c t r a l s u b t r a c t i o n to the complex l i k e f a c t o r a n a l y s i s can now be r o u t i n e l y a p p l i e d . N a t u r a l l y , t h i s type of a n a l y s i s i s not unique to FTIR and many s o p h i s t i c a t e d techniques were developed i n the I960 s f o r use with d i g i t i z e d data obtained from d i s p e r s i v e instruments. Nevertheless, computer methods were not r o u t i n e l y a p p l i e d and the type of a n a l y s i s that depends on data manipulation has subsequently become a s s o c i a t e d with FTIR. Havelock E l l i s remarked that "What we c a l l progress i s the exchange of one nuisance f o r another nuisance". The advent of computerized instruments has c e r t a i n l y made r o u t i n e a number of d i f f i c u l t s p e c t r o s c o p i c measurements. U n f o r t u n a t e l y , the s o p h i s t i c a t i o n of the procedures now a v a i l a b l e can r e s u l t i n erroneous r e s u l t s or i n t e r p r e t a t i o n s i f they are not used with c a u t i o n . A c l a s s i c example i s the use of c u r v e - r e s o l v i n g techniques. There i s always a s u s p i c i o n that a good f i t between an observed s p e c t r a l p r o f i l e and a number of bands can be obtained p r o v i d i n g that a s u f f i c i e n t number of the l a t t e r are i n c l u d e d i n the a n a l y s i s . C l e a r l y , i n such cases a p r i o r knowledge of the number of bands and t h e i r frequency would s i g n i f i c a n t l y i n c r e a s e our confidence i n the r e s u l t s . In t h i s review we w i l l attempt to c r i t i c a l l y assess the a p p l i c a t i o n of FTIR procedures to the c h a r a c t e r i z a t i o n of the s t r u c t u r e of c o a l . To an extent much of what we have to say i s not original. D i f f i c u l t i e s , such as those mentioned above f o r curve r e s o l v i n g , were encountered and addressed a number of years ago. However, with the advent of any new instrumentation there i s a tendency to ignore segments of previous work that were obtained on i n f e r i o r machines and i n e f f e c t spend a c o n s i d e r a b l e amount of time and e f f o r t " r e i n v e n t i n g the wheel". A c c o r d i n g l y , we w i l l not simply review the r e s u l t s of the FTIR s t u d i e s of c o a l published to date, but f i r s t consider the use of c e r t a i n computer r o u t i n e s . We w i l l then consider how such r o u t i n e s can be a p p l i e d to the q u a n t i t a t i v e determination of s p e c i f i c f u n c t i o n a l groups i n c o a l . Published work concerning the a n a l y s i s of mineral matter i n c o a l 1

In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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or chemical changes such as o x i d a t i o n w i l l only be considered i n p a s s i n g , t o the extent that they i l l u s t r a t e the usefulness o f a given procedure.


Sample P r e p a r a t i o n When commercial FTIR instruments were f i r s t introduced the w i d e l y touted o p t i c a l advantages of the instrument sometimes gave the impression that one could p l a c e a b r i c k i n the sample chamber and s t i l l o b t a i n a spectrum. The p r e p a r a t i o n o f samples was not emphasized, but i f anything the data manipulations that have s i n c e became r o u t i n e have made c a r e f u l and c o n s i s t e n t sample p r e p a r a t i o n even more important, p a r t i c u l a r l y f o r q u a n t i t a t i v e work. Coal samples are most c o n v e n i e n t l y prepared by d i s p e r s i o n i n an a l k a l i h a l i d e m a t r i x , u s u a l l y KBr o r C s l . Estep e t a l (5) examined the problems i n v o l v e d i n t h i s method o f p r e p a r a t i o n w i t h s p e c i f i c reference t o the a n a l y s i s o f m i n e r a l matter i n c o a l . These authors preground the m i n e r a l o r ash under i n v e s t i g a t i o n and then simply mixed a measured weight o f the m a t e r i a l w i t h d r i e d C s l . For optimum a b s o r p t i o n and the m i n i m i z a t i o n o f problems a s s o c i a t e d w i t h p a r t i c l e s c a t t e r i n g i t i s necessary that the s i z e of the sample p a r t i c l e s are l e s s than the wavelength o f the i n f r a r e d r a d i a t i o n and t h a t they are evenly dispersed w i t h i n the matrix. Estep e t a l (5) noted that t h i s method of sample p r e p a r a t i o n d i d not give maximum a b s o r p t i o n , g r i n d i n g o f the C s l was a l s o necessary. Consequently, i n work i n t h i s l a b o r a t o r y we a l s o g r i n d the a l k a l i h a l i d e ( u s u a l l y KBr) together w i t h the c o a l using a P e r k i n Elmer Wig-L-Bug. [For hard minerals and shale i t i s necessary t o a l s o p r e g r i n d the samples ( 6 , 7 ) ] . The g r i n d ing time necessary t o achieve optimum a b s o r p t i o n w i l l vary from l a b o r a t o r y t o l a b o r a t o r y according t o the equipment a v a i l a b l e . An i n i t i a l g r i n d i n g study should be conducted t o determine optimum c o n d i t i o n s . For example, F i g u r e 1 i s a p l o t o f the i n t e g r a t e d a b s o r p t i o n of the aromatic C-H out-of-plane bending modes of a v i t r i n i t e concentrate (1.3 mg o f maceral i n 300 mg KBr) p l o t t e d as a f u n c t i o n o f g r i n d i n g time. I n r e c e n t l y acquired equipment optimum a b s o r p t i o n was reached a f t e r g r i n d i n g f o r 30 seconds, although on o l d e r equipment approximately 20 minutes g r i n d i n g was r e q u i r e d . G r i n d i n g i n t o KBr i s a convenient and r e l a t i v e l y s t r a i g h t forward method o f sample p r e p a r a t i o n , but i t i s not without problems. I n terms of c o a l a n a l y s i s the most severe d i f f i c u l t y i s the u b i q u i t o u s presence of water, s i n c e the strong water a b s o r p t i o n near 3400 cm" overlaps the c o a l bands due to OH and NH groups. The e f f e c t of water can o f t e n be minimized by h e a t i n g the p e l l e t s under vacuum, but i n t h i s l a b o r a t o r y we have never succeeded i n completely e l i m i n a t i n g water a b s o r p t i o n by such procedures. This problem was addressed some years ago by F r i e d e l (8) who observed t h a t h e a t i n g p e l l e t s t o 175°C i s



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Figure 1. Plot of the integrated absorption in the region 900-700 cm in the spectrum of a coal sample vs. grinding time in KBr pellet preparation. 1



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r e q u i r e d to completely remove water bands, which nevertheless r e appear upon c o o l i n g . Breger and Chandler (9) a l s o reviewed t h i s problem and reported that they could not e l i m i n a t e the p r e s i s t e n t 3400 cm~l water absorption d e s p i t e numerous attempts at dehydration. I t was suggested that t h i s band may not only be due to water, but a l s o hydroxyl groups s u b s t i t u t e d i n the a l k a l i h a l i d e matrix, an i n t e r p r e t a t i o n a l s o proposed by Efcirie and Dzewczyk (10). The problems a s s o c i a t e d with water i n a l k a l i h a l i d e preparations suggest that other p r e p a r a t i v e techniques are worth pursuing. M u l l i n g techniques were o r i g i n a l l y used i n c o a l s t u d i e s but r e q u i r e long g r i n d i n g times. Furthermore, q u a n t i t a t i v e s t u d i e s are d i f f i c u l t i f not impossible on the r e s u l t i n g "smears" that are u s u a l l y used f o r a n a l y s i s . Recently, F u l l e r and G r i f f i t h s (11) have demonstrated that d i f f u s e r e f l e c t a n c e could be extremely u s e f u l . Photoacoustic spectroscopy a l s o has p o t e n t i a l (12=15)· However, at t h i s time the a p p l i c a t i o n of these techniques to the q u a n t i t a t i v e determination of f u n c t i o n a l groups i n c o a l remains to be demonstrated. Corrections Or Adjustments To The Spectra In order to obtain q u a n t i t a t i v e measurements of the f u n c t i o n a l groups present i n c o a l by FTIR i t i s necessary to f i r s t account f o r the minerals that may be present. Solomon (16,17) reported that c o a l s p e c t r a were c o r r e c t e d by s u b t r a c t i n g the cont r i b u t i o n s of k a o l i n i t e and i l l i t e and s c a l i n g the s p e c t r a to give the absorbance f o r 1 mg of c o a l dmmf. However, i t i s o f t e n the case that the s p e c t r a of mineral components are dominated by c l a y s such as k a o l i n i t e and i l l i t e (18,12), but these may c o n s t i t u t e only 20 to 30% by weight of the mineral matter present. Furthermore, p y r i t e does not have absorption bands i n the m i d - i n f r a r e d region used i n c o a l s t u d i e s , so that the cont r i b u t i o n of t h i s mineral would not be measured by simple subt r a c t i o n procedures. The most accurate method f o r determining mineral content and a d j u s t i n g the s p e c t r a to account f o r a l l the m a t e r i a l s that may be present i s to f i r s t o b t a i n the low temperature ash and then adjust the spectrum to the equivalent of 1 mg of organic m a t e r i a l using the f r a c t i o n a l amount of ash i n the c o a l so determined. The spectrum of t h i s ash can then be d i r e c t l y subtracted from that of the c o a l so as to e l i m i n a t e the bands of the mineral c o n s t i t u e n t s (19). O c c a s i o n a l l y , organic s u l f u r and n i t r o g e n can be f i x e d as inorganic s u l f a t e and n i t r a t e i n the ashing process, but these minerals can r e a d i l y be d e t e c t ed and q u a n t i t a t i v e l y measured by FTIR methods (19,20). The second adjustment to c o a l s p e c t r a that concerns us, s i n c e we b e l i e v e i t may lead to s p e c t r a l a r t i f a c t s , i s the c o r r e c t i o n of the b a s e l i n e f o r s c a t t e r i n g . In h i s seminal paper on the i n f r a r e d spectra of c o a l , Brown (21) discussed the o r i g i n of the s l o p i n g background and concluded that s c a t t e r i n g was not the only source. M i c r o s c o p i c examination of various KBr p e l l e t s


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showed no s i g n i f i c a n t v a r i a t i o n i n average c o a l p a r t i c l e s i z e , but there was a general trend to higher background absorption with i n c r e a s i n g rank. Reviewing the work of Brown and other s i m i l a r s t u d i e s , Dryden (22) concluded that at l e a s t i n higher rank coals part of the background can be a t t r i b u t e d to the 'wings of e l e c t r o n i c absorption bands extending i n t o the i n f r a red. The background absorption may t h e r e f o r e vary with frequency i n a non-linear f a s h i o n . In f a c t , i f we examine the spectrum of a v i t r i n i t e sample shown i n Figure 2, a s u b s t a n t i a l s l o p i n g b a s e l i n e i s apparent between 3800 and about 1850 cm~l, but the spectrum f l a t t e n s out and apparently slopes i n the opposite d i r e c t i o n between 1800 and 500 cm~l. This type of v a r i a t i o n i n background i s apparently not unique to c o a l . Maddam's (23) has pointed out that various workers have employed p a r a b o l i c functions to f i t b a s e l i n e s . C l e a r l y , i f we s t r a i g h t e n the basel i n e according to the slope i n one region (e.g. between 3800 and 1850 cm 1 , as shown i n Figure 3, there w i l l be a d i s t o r t i o n i n the remaining part of the spectrum. Solomon (16,17) used t h i s approach and obtained spectra s i m i l a r to those reproduced here, with the absorbance minima between 1000 and 500 cm" clearly r a i s e d above the new ( s t r a i g h t ) b a s e l i n e . In subsequent curve r e s o l v i n g an extremely broad band ( h a l f width approximately 200 cm ) centered near 700 cm~l was determined. This band was t e n t a t i v e l y assigned to a carbonyl group and was stronger than the c h a r a c t e r i s t i c C=0 s t r e t c h i n g mode near 1700 cnT^-. However, the c h a r a c t e r i s t i c s of the 700 cm~^ band do not correspond to any known carbonyl group frequency. We suggest that i t may be an a r t i f a c t of the method used to s t r a i g h t e n the b a s e l i n e . The bottom spectrum shown i n Figure 3 d i s p l a y s the r e s u l t of using two separate s l o p i n g s t r a i g h t b a s e l i n e s , i l l u s t r a t e d i n the top spectrum, to adjust the s p e c t r a between 3800 and 1850 cm"l and between 1850 and 500 cm"^, r e s p e c t i v e l y . In t h i s adjusted spectrum there i s c l e a r l y no broad (200 cm~l h a l f width) underl y i n g absorption centered near 700 cm"^.


1

I f c o a l s p e c t r a are to be curve resolved or i f spectra of m a t e r i a l s having d i f f e r e n t backgrounds are to be a c c u r a t e l y compared, i t may prove necessary to adjust the b a s e l i n e . However, the s p e c t r a reproduced i n Figure 3 i n d i c a t e that such procedures are more dependable i f a p p l i e d s e p a r a t e l y to s p e c i f i c l o c a l regions of the spectrum. Computer A p p l i c a t i o n s The primary f u n c t i o n of the computer i n FTIR instruments i s to perform the F o u r i e r transformation that converts an i n t e r ferogram to a recognizable spectrum. However, the a v a i l a b i l i t y of an o n - l i n e mini-computer has opened the door to r o u t i n e data manipulations. In t h i s s e c t i o n we w i l l review procedures that have been or promise to be u s e f u l i n c o a l c h a r a c t e r i z a t i o n . C e r t a i n data a n a l y s i s operations, such as numerical i n t e g r a t i o n


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Coal OH groups

—•· — I — · — ι — · — \ — ι 3Κ00 3200

•—«—ι—·—ι—•—ι—•—•—»—ι—·—ι—·—ι—«—•—ι ι • 38(30 2M00 2000 1600 1200

Figure 2.

•—·—ι—«— N-H groups, r e s p e c t i v e l y . A c c o r d i n g l y , t h i s p r e l i m i n a r y work i n d i c a t e s that FTIR should not only be u s e f u l i n determining t o t a l OH, through measurements of the i n t e n s i t y of the CH^ and f

-1


1


68


Figure 11. Difference spectrum between 1900 and 1550 cm obtained from acety lated PSOC 272 coal (bottom) and sec ond derivation of spectrum (top). 1

1S00

1800

1700


1600

«™* Ί



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1773

I

Figure 14. The resolution of the differ ence spectrum obtained from the acetyl— ated lignite into six bands. IS 00

ι 1800

i 1700


i 1600

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C-0 bands, but a l s o has the c l e a r p o t e n t i a l f o r d i s c r i m i n a t i n g between types of OH groups and a l s o measuring N-H groups through an a n a l y s i s of the C=0 s t r e t c h i n g r e g i o n , p r o v i d i n g that curve r e s o l v i n g methods are a p p l i e d with circumspection.


The Determination

Of C-H

Groups

In a number of s t u d i e s attempts have been made to o b t a i n s t r u c t u r a l information by measuring the peak heights or i n t e g r a t e d i n t e n s i t i e s of bands assigned to aromatic and a l i p h a t i c C-H modes. As we mentioned above, there i s some v a r i a t i o n i n values of ext i n c t i o n c o e f f i c i e n t s determined by d i f f e r e n t groups (16, 21, .22-41). We have suggested that at l e a s t i n part t h i s v a r i a b i l i t y i s due to the methods used to obtain, data, namely measuring i n t e g r a t i n g i n t e n s i t i e s over an e n t i r e s p e c t r a l region thus assuming that the e x t i n c t i o n c o e f f i c i e n t s of each band i s the same (or that the bands maintain a constant r e l a t i v e i n t e n s i t y from c o a l to c o a l ) . At the beginning of t h i s s e c t i o n we used the aromatic C-H out-of-plane bending modes to i l l u s t r a t e t h i s p o i n t . The same argument a p p l i e s to measurements i n the a l i p h a t i c C-H s t r e t c h i n g region where i t has become conventional_Jo measure the i n t e g r a t e d absorption between 3000 and 2800 cm as a measure of t o t a l a l i p h a t i c C-H c o n c e n t r a t i o n . This region of the spectrum has to be considered as the sum of c o n t r i b u t i o n s from three components, CH, CH^ and CH^. I f the s p e c t r a l c o n t r i b u t i o n of these groups could be separated, then not only would we have obtained a greater i n s i g h t i n t o the s t r u c t u r e of c o a l and i t s v a r i a t i o n with rank, but more c o n s i s t e n t a l i p h a t i c to aromatic C-H r a t i o s might be obtained by t a k i n g i n t o account d i f f e r e n c e s i n the e x t i n c t i o n c o e f f i c i e n t s of these groups. In order to o b t a i n a meaningful r e s o l u t i o n of the bands i n the C-H s t r e t c h i n g region we followed the procedures o u t l i n e d above f o r a n a l y s i s of the carbonyl region of the d i f f e r e n c e s p e c t r a of a c e t y l a t e d c o a l s . Figure 15 shows a s c a l e expanded p l o t of the a l i p h a t i c C-H s t r e t c h i n g region of the spectrum of the v i t r i n i t e concentrate considered i n Figure 2. A l s o d i s p l a y e d i n Figure 15 i s the second d e r i v a t i v e p l o t , c l e a r l y i n d i c a t i n g the presence of f i v e bands. With an appropriate i n i t i a l choice of band p o s i t i o n and width at h a l f height obtained from t h i s p l o t , the C-H s t r e t c h i n g region can be r e s o l v e d i n t o the f i v e components shown i n F i g u r e 16. At t h i s p o i n t i t i s tempting to make the obvious assignment of the 2956 and 2864 cm"" modes to asymmetric and symmetric CH s t r e t c h , the 2923 and 2849 cm" barujls to the asymmetric and symmetric CH^ s t r e t c h and the 2891 cm band to lone C-H groups. However, we have to consider one more l i m i t a t i o n to curve r e s o l v i n g methods. The i d e n t i f i c a t i o n of peaks i n a p r o f i l e by second d e r i v a t i v e techniques w i l l obviously depend upon the r e l a t i v e i n t e n s i t i e s of the peaks i n v o l v e d and t h e i r s e p a r a t i o n r e l a t i v e to t h e i r h a l f widths, as discussed previously. I f we c a r e f u l l y consider e s t a b l i s h e d group 1

3


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PSMC 52

I

3000

8900

2800

Figure 15. Scale expanded aliphatic C—H stretching region of the spectrum of a vitrinite concentrate (bottom); and second derivative of the spectrum (top). PSMC 52

2956

3000 Figure 16.

2900

2800

The resolution of the aliphatic C—H stretching region intofivebands.


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frequencies i n conjunction w i t h what i s known of the s t r u c t u r e of c o a l we have to conclude that the 2923 cm" band i s a composite of two c o n t r i b u t i o n s , the asymmetric CH s t r e t c h i n g mode and the asymmetric CH~ s t r e t c h i n g mode of methyl groups attached d i r e c t l y to aromatic r i n g s . However, the p o s i t i o n of the symmetric CH^ s t r e t c h i n g m o d e appears to be l e s s s e n s i t i v e to l o c a l environment, appearing near 2865-2875 cm" f o r methyl groups attached to a l k y l chains or aromatic r i n g s . Consequently, i f appropriate e x t i n c t i o n c o e f f i c i e n t s can be determined i t should prove p o s s i b l e to use the symmetric CH« and CH^ modes to determine these groups and there_Js then the f u r t h e r p o s s i b i l i t y of using the 2956 cm" and 2923 cm bands to determine the d i s t r i b u t i o n of methyl groups. (The i n t e n s i t y of 2923 cm" band would have to be considered as the sum of the c o n t r i b u t i o n s from CH^ and CH^ groups). F i n a l l y , the 2891 cm could be a p p l i e d to the defermination of t e r t i a r y hydrogen. These i n i t i a l r e s u l t s i n d i c a t e that good curve r e s o l v i n g methods are capable of a l l o w i n g a greater i n s i g h t i n t o c o a l s t r u c t u r e through a more d e t a i l e d a n a l y s i s of the i n f r a r e d spect rum. However, there remains the d i f f i c u l t task of o b t a i n i n g ex t i n c t i o n c o e f f i c i e n t s . Methods used i n previous s t u d i e s , mainly the use of model compounds and c a l i b r a t i o n by means of s o l u b l e e x t r a c t s c h a r a c t e r i z e d by proton magnetic resonance, should prove u s e f u l , but we wish to suggest a t h i r d method. E s s e n t i a l l y , t h i s b u i l d s on the approach used by Von Tschamler and de R u i t e r (41) and more r e c e n t l y by Solomon (16,17). We propose to equate the experimentally determined elemental hydrogen to the sum of con t r i b u t i o n s from hydrogen c o n t a i n i n g f u n c t i o n a l gorups, as measured by the i n t e n s i t i e s of appropriate i n f r a r e d bands m u l t i p l i e d by a conversion f a c t o r (equivalent to an e x t i n c t i o n co e f f i c i e n t converted to weight u n i t s ) ; ?


1

Η = Σ η

I e η η

where Η i s the weight percent hydrogen content, I i s the i n t e n s i t y of the η band (e.g. a band assigned to CH^ groups) and e i s a conversion f a c t o r r e l a t i n g band i n t e n s i t y to the weight percent hydrogen percent at t h i s s p e c i f i c f u n c t i o n a l group. I f we consider the curve r e s o l v i n g r e s u l t s discussed above, i t i s p o s s i b l e to i d e n t i f y three bands a s s o c i a t e d with aromatic hydrogen, three bands a s s o c i a t e d with v a r i o u s a l i p h a t i c groups (CH, CH^ and CH«) and bands i n the s p e c t r a of a c e t y l a t e d samples that can be assigned to NH, a l k y l OH and p h e n o l i c OH. In low rank c o a l s bands near 1700 cm" can be r e a d i l y assigned to COOH groups. C l e a r l y , i f we have s u f f i c i e n t c o a l (or maceral) samples chosen so as to cover a range of rank and hence hydrogen content, then t h e o r e t i c a l l y the problem could be solved as a s e t of simultaneous equations. In f a c t , we should be able to "over-determine the problem by a n a l y z i n g a s u f f i c i e n t number of samples. T h i s approach has a number of advantages. Model compounds w i l l n

1

11


74


not be r e q u i r e d t o o b t a i n e x t i n c t i o n c o e f f i c i e n t s . If a s u f f i c i e n t l y l a r g e data base i s used, any systematic v a r i a t i o n s i n e x t i n c t i o n c o e f f i c i e n t s with rank and hence chemical s t r u c t u r e should be apparent p r o v i d i n g that a method f o r determining v a r i a t i o n s i n measured and c a l c u l a t e d (by FTIR) hydrogen i s i n cluded i n the procedure.


Summary And Conclusions One of the major advantages of FTIR i n the a n a l y s i s of complex, systems i s the ready a p p l i c a t i o n of s o p h i s t i c a t e d programs made p o s s i b l e by the n e c e s s i t y o f an o n - l i n e mini-computer i n these instruments. We have considered the a p p l i c a t i o n of s e l e c t e d programs and pointed out that they have t o be a p p l i e d with c o n s i d e r able c a u t i o n . In p a r t i c u l a r , we have attempted t o c r i t i c a l l y assess the problems i n v o l v e d i n determining f u n c t i o n a l groups by curve r e s o l v i n g methods. There i s a l a r g e body of work i n the l i t e r a t u r e concerning t h i s problem and by a p p l y i n g the r e s u l t s o f these s t u d i e s we can conclude t h a t ; a) E m p i r i c a l l y determined bandshapes should be c a l c u l a t e d using l e a s t squares curve f i t t i n g programs. b) In order to have confidence i n the r e s u l t s i t i s necessary to have a good i n i t i a l estimate of the number o f bands i n a prof i l e , t h e i r frequency and width a t h a l f h e i g h t . c) These methods should be a p p l i e d s e p a r a t e l y to s p e c i f i c regions o f the spectrum. We have considered the a p p l i c a t i o n of these c r i t e r i a to the determination of OH and CH groups i n c o a l . By combining FTIR measurements with a c e t y l a t i o n procedures i t appears f e a s i b l e not only to measure t o t a l ( r e a c t i v e ) OH and NH content, but t o d i s t i n g u i s h between p h e n o l i c and a l k y l OH groups by curve r e s o l v i n g methods. F i n a l l y , we have argued that the determination o f cons i s t e n t e x t i n c t i o n c o e f f i c i e n t s f o r a l i p h a t i c and aromatic C-H bands f o r v a r i o u s c o a l samples would be f a c i l i t a t e d by the use of curve r e s o l v i n g procedures and a determination of the r e l a t i o n ship between the e x t i n c t i o n c o e f f i c i e n t s o f i n d i v i d u a l components. In previous s t u d i e s i t has been general p r a c t i c e to sum the i n tegrated i n t e n s i t y o f a number of bands and hence i m p l i c i t l y assume that each mode has the same e x t i n c t i o n c o e f f i c i e n t . Acknowledgements The authors g r a t e f u l l y acknowledge the f i n a n c i a l support of the Department of Energy under c o n t r a c t No. De-AC22-OP30013 and the Penn State Cooperative Program i n Coal Research.

Literature Cited 1. Laitinen, H.A. Analytical Chemistry, 1973, 45(14), 2305. 2. Griffiths, P.R. Chemical Infrared Fourier Transform Spectroscopy. John Wiley and Sons, New York (1975).


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