Energy & Fuels 1988,2, 326-332
326
Determination of Hydroxyl Group Concentration in Coal Liquids by 31PNMR Eric J. Dadey,*f Stanford L. Smith,*#and Burtron H. Davis*+ Kentucky Energy Cabinet Laboratory, P.O.Box 13015, Lexington, Kentucky 40512, and Chemistry Department, University of Kentucky, Lexington, Kentucky 40506 Received May 11, 1987. Revised Manuscript Received December 28, 1987 An analytical technique has been developed for the analysis of OH groups in coal liquids. The label at the active site by derivatizing the hydroxyl group technique involves the introduction of a 31P with diphenylphosphinyl chloride, which is then quantitated by 31PNMR. The signals corresponding to phosphinate esters are integrated to produce a quantitative measure of the OH content of a sample. Results indicate a good correlation between the 31PNMR technique and established chemical methods. Coal liquids samples ranging from distillate overheads to vacuum bottoms can be analyzed without excessive sample preparation. A comparison of the 31PNMR results with more conventional derivatization techniques is made.
Introduction Although coal and coal liquids are composed mainly of carbon and hydrogen, identification of hydrogen-containing heteroatom functional groups, such as COOH,OH,SH, and NH,, is essential for a better understanding of the liquefaction and upgrading reactions. A number of analytical techniques have been utilized recently to determine the active hydrogen in complex organic mixtures. These methods involve reacting the functional group with reagents containing NMR active nuclei; the percent active hydrogen is related to the integral corresponding to the derivatized functional group. Because of the high sensitivity and large chemical shift range of '?F N M R spectra, a number of reagents have been used to introduce a '?F label at active hydrogen sites. Dorn et suggested trifluoroacetyl chloride, 2,2,2-trifluorodiazoethane, or p-fluorobenzoyl chloride for this purpose, while Manatt4 and Leader6 proposed trifluoroacetic anhydride and hexafluoroacetone respectively as other fluorine labels. Although the observed chemical shifts of these derivatives in the 'V NMR spectrum differentiate among acids, phenols, mercaptans, and amines and to some extent among individual compounds within each class, a number of problems are encountered with their use. Instability of some derivatives in hydrolytic conditions, incomplete reaction of specific compounds, difficult sample preparation conditions, and inability to derivatize high molecular weight material limit these reagents for quantitative analyzes. Reactions that introduce a 29Sitag at active hydrogen sites have been explored. Rose et al.edemonstrated that N,O-bis(trimethylsily1)trifluoroacetamidewas effective in labeling the hydroxyl groups present in coal liquids with 29Si. The trimethylsilyl derivatives are easily prepared, and the '%i spectra permit the calculation of quantitative values for the weight percent of OH in model compounds. Coleman' demonstrated the use of hexamethyldisilazane as a silylating reagent for hydroxyl groups. Dereppe and Parbhoo8 utilized l9Fand % ' i NMR spectrometry to determine the hydroxyl content of petroleum oil extracts. However, %i labels are not without problems. Although there is some differentiation among individual compounds in the phenol region, the amount of instrument time re+ Kentucky
Energy Cabinet Laboratory.
* University of Kentucky.
quired to obtain the required signal-to-noise ratio for quantitative integration ranges from 30 min for a high weight percent phenolic sample to 3-4 h for a low weight percent phenolic sample; this is a major limitation to ?3i tagging. Following the initiation of our work, Schraml et al.gdescribed a %Si NMR method for the analysis of polar functional groups where the INEPT technique is utilized to enhance the NMR signal and thereby lower the analysis time. Snape et al.'O suggested acetylation using acetic anhydride in pyridine and methylation using tetra-n-butylammonium hydroxide with methyl iodide in tetrahydrofuran as 13Clabels for phenols and carboxylic acids. The low sensitivity of '3c N M R and overlap between the signals from the label and similar signals from the coal liquid itself limits the usefulness and reliability of these reagents. They also suggested trimethylchlorosilane in pyridine as a 'H tag for active hydrogen compounds. Although the 'H nucleus is the most sensitive of all the nuclei for NMR analysis, they found overlap of the proton signal of methyl groups present in the coal liquid and the protons in the label; this makes quantitative integration difficult. In a recent paper,ll tri-n-butyltin oxide (TBTO) was suggested as a '19Sn label. The reaction involves refluxing a coal liquid and TBTO in toluene for 2 h. The solvent and, unavoidably, some light hydrocarbons are removed under vacuum by rotary evaporation. The '19Sn spectrum and integrals of the derivatized sample are recorded. Although quantitative derivatization is reported for model compounds, the lengthy derivatization procedure may limit the utility of this technique for the analysis of routine samples. 31Plabels for active hydrogen have been investigated on a limited basis. Stadelhofer et a1.12derivatized phenols (1) Dorn, H. C.; Sleevi, P. S.; Koller, K.; Glass, T. Prepr.-Pap. Am. Chem. SOC.,Diu. Fuel Chem. 1979,24, 301. (2)Koller, K.;Dorn, H. C. Anal. Chem. 1982,54, 529. (3)Spratt, M. P.;Dorn, H. C. Anal. Chem. 1984,56, 2038. 1966,88, 1323. (4)Manatt, S.L.J. Am. Chem. SOC. (5)Leader, G.R. Anal. Chem. 1970, 42, 16. (6) Rose, K. D.; Scouten, C. G. personal communication, 1981. (7)Coleman, W.M.; Boyd, A. R. Anal. Chem. 1982,54, 133. (8)Dereppe, J.-M.; Parbhoo, B. Anal. Chem. 1984,56, 2740. (9)Schraml, J.; Blechta, V.; Kv%alevH,M.; Nondek, L.; Chvalovsky, V. Anal. Chem. 1986,58, 1892. (10) Snape, C. E.; Smith, C. A.; Bartle, K. D.; Matthews, R. S. Anal. Chem. 1982,54, 20. (11)M i, E.; Faure, R.; Lena, L.; Vincent, E.; Metzger, J. Anal. Chem. 1985,57, 2854.
0887-0624/88/2502-0326$01.50/00 1988 American Chemical Society
Hydroxyl Group Concentration i n Coal Liquids
by using phosphoric acid but, due to the tribasic nature of the acid, found complex fine structure in the 13P NMR resulting from the formation of mixed esters. Pomfret et al.13 prepared phosphate and thiophosphinic ester derivatives of coal-derived phenols; they found complete derivatization of model compounds for dithiophosphinic esters. Again, this method may prove informative, but extensive sample preparation may limit its usefulness on a routine basis. Schiff et al.14 utilized 1,3,2-dioxaphospholanyl chloride to prepare derivatives of alcohols, phenols, and carboxylic acids; they obtained quantitative results for model compounds. Although methods have been developed for the analysis of hydroxyl groups in coal liquids, problems still exist. There is need for a method that permits reliable and routine determination of the phenolic content of a wide range of samples (ie. from high-boiling vacuum bottoms, resids, or recycle solvents to low-boiling samples that contain a wide range of phenolic concentrations). Because of the relatively high sensitivity of 31PNMR, efforts were made to find a suitable 31P-containingreagent that would (1) react quantitativelywith active hydrogens, (2) require a short time to prepare derivatives, and (3) permit analysis of samples containing a wide range of sample types, including those containing high molecular weight components. Diphenylphosphinyl chloride (DPPC) reacts with alcohols in the presence of triethylamine to form the corresponding esters of diphenylphosphinic acid.15 It has also been reported to be a protecting group for amino functions in stereochemical synthesis of peptides.16 Consequently, the utility of DPPC as a 31Plabel for active hydrogen in coal liquids was investigated. The NMR results are compared to the weight percent phenolics obtained by base extraction. In addition, the 31Presults are also compared to those obtained for lSF and 29Sitagging. Experimental Section A. Analysis of Phenolic Compounds by Extraction. The weight percent phenolics reported herein were determined by extraction into a caustic aqueous solution. Approximately 10-20 g of a coal liquid was weighed and dissolved in 20-25 mL of benzene (or other solvent suitable for the compound class). The acidic compounds were then extracted from the organic layer into an aqueous layer containing 10% NaOH. The aqueous layer was separated from the organic layer and then neutralized with concentrated phosphoric acid. The phenolic compounds were extracted from the aqueous layer into diethyl ether. The ether was removed by rotary evaporation and weight percent of phenolics determined. A more detailed description of the procedure is in ref 17. B. Phenol Derivatization. 1. '9Labeling. '9 derivatives were prepared by a modification of the procedure described by Burke et al.lS Initial experiments were carried out by weighing approximately 0.50g of a coal liquid into a test tube fitted with a side arm. The sample was diluted with 3 mL of a stock solution (12) Stadelhofer, J. W.; Bartle, K. D.; Matthews, R. S. Proc.-Znt. Kohlenwiss. Tag., 1981 1981,792. (13) Pomfret, A.; Bartle, K. D.; Barrett, S.; Taylor, N.; Stadelhofer, J. W. Erd6l Kohle, Erdgas, Petrochem. 1984,37, 515. (14) Schiff, D. E.; Verkade, J. G.; Metzler, R. M.; Squires, T. G.; Veniur, C. G. Appl. Spectrosc. 1986, 40, 348. (15) Berlin, K. D.; Austin, T. H.; Nagabhushanam, M. J. Org. Chem. 1965,30, 1267. (16) Kenner, G. W.; Moore, G. A,; Ramage, R. Tetrahedron Lett. 1976 40,3623. (17) Davis, B.; Thomas,G.; Sagues, A,; Jewitt, C.; Baumert, K. In Annual Report 80181; A Kentucky Energy Resources Utilization Program; IMMR Publication Section: Lexington, KY, 1981; pp 22-15. (18) Burke, F. Pp.; Winschel, R. A,; Pochapsky, T. C. "Development of a Correlation Between Slurry Oil Composition and Process Performance", U.S.Topical Report FE-14053-1 on US. DOE Contract DE-AC05-79ET-14503,
Energy & Fuels, Vol. 2, No. 3, 1988 327 containing CDC13as the solvent, 0.035 g of lutidine, and a known amount of l,l,l-trifluoroacetophenoneas an internal standard. The reaction mixture was mld under N2to -23 "C in a CC&/dry ice bath. Nitrogen was bubbled through the solution during derivatization to minimize hydrolysis of the products, to mix the reactants, and to remove unreacted derivatizing reagent as the sample was subsequently warmed to room temperature. Trifluoroacetyl chloride, ((TFA)Cl, bp -18 "C), was condensed into the test tube until about a 60% volume increase was observed. After 10 min, the reaction was assumed complete and the products were slowly warmed to room temperature. 19FNMR spectra and integrals were obtained on a Varian FT-80A spectrometer operating at 74.844 MHz. Each spectrum consisted of 24 transients over a 500-Hz sweep width. Each transient consisted of a 5-bs pulse (35O flip), an acquisition time of 5 s, and no 'H decoupling. The trifluoroacetophenone 'q signal was used as reference, and peak areas in the frequency range from -2.95 to -3.90 ppm were considered to originate from phenol esters. The peak a t -3.90 ppm was identified as trifluoroacetic acid, formed by the reaction of (TFA)Cl with water and was not included in the calculations. A weight percent of OH was calculated by using the ratio of the total integral from the phenolic region to the integral of the internal standard, trifluoroacetophenone, and the sample weight. In this and the following NMR data, the number percent of OH is defined as the weight percent of the total sample that is present as the phenolic OH group. To obtain quantitative and reproducible results, our experiments demonstrated that it was necessary to add an amount of 2,g-lutidine that was a t least stoichiometric with the moles of phenolic compounds present; in the present study excess 2,6lutidine was added. Thus, enough lutidine to derivatize a 0.50-g sample of a coal liquid containing 60 wt % of the phenolic compound was added in our modification of the Burke et al. method.'* 2. 29SiLabeling. 29Siderivatives were prepared by using a modified version of the method outlined by Rose! Approximately 0.50 g of coal liquid was weighed into a 10-mm NMR tube. The sample was dissolved in 3 mL of a solvent (33:67 w/w mixture of pyridine and CDC13) containing a known amount of bis(trimethylsily1)methane for internal standard and chemical shift reference. To the above mixture was added 0.50 mL of N,Obis(trimethylsily1)trifluoroacetamide (BTSFA), and the contents were thoroughly mixed. Roses did not use pyridine; however, we found that it increased the solubility of high molecular weight coal liquids and did not react under our conditions. The 29Si spectra and integrals were recorded on a Varian XL-200 operating at 39.542 MHz. Each spectrum consisted of 180 transients over a 3000-Hz sweep width, with each transient consisting of a 5-s recycle time. To overcome the negative nuclear Overhauser effect and long relaxation times of the ?3i nucleus, a modification of the INEPT (insensitive nuclei enhanced by polarization transfer) pulse sequence described by SchramllDwas employed. The 29Sisignal resulting from bis(trimethylsily1)methane (BTSM) was assigned the value of 0.00 ppm and its integral recorded. Peaks in the 17.0-20.0 ppm region relative to BTSM were integrated as silylated phenols, and weight percent of OH was calculated by comparison to the area of the standard BTSM peak. Peaks at 6.8 and 10.5 ppm were identified as hexamethyldisiloxane(HMDS) and N(trimethylsily1)trifluoroacetamide (N-TMSTFA) respectively and were excluded from the calculations. HMDS results from the hydrolysis of BTSFA, and N-TMSTFA is a byproduct of the derivatization. 3. 31PLabeling. 31Pderivatization was initiated by weighing approximately 0.50 g of coal liquid into a 10-mm NMR tube. The sample was diluted with 3 mL of a stock solution containing a 5050 w/w mixture of pyridine and CDC13and a known amount of triphenyl phosphate as an internal standard and chemical shift reference. Approximately 0.25 g of diphenylphosphinylchloride was added and the reaction thoroughly mixed. To insure complete reaction of phenols, the sample was placed in a 50 "C water bath for 2 h prior to data acquisition. The 31Pspectra and integrals were recorded on a Varian XL-200 spectrometer operating at 80.983 MHz. Each spectrum consisted (19) Schraml, J. Collect. Czech. Chem. Commun. 1983 48, 3402.
328 Energy & Fuels, Vol. 2, No. 3, 1988
k DERIVATIZED PHENOLS-
DIPHENYL PHOSPHlNlC JACID
DIPHENYL PHOSPHINYL A CHLORIDE
TRIPHENYL PHOSPHATE
io
60 DERlVATlZED PHENOLS
WTMSTFA
BTSM HMDS
BTSFA
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,
,
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l
26
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C
1.2
0
-1
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I
I
.2
-3
-4
I
I
-5.5
Chemical Shift, ppm
Figure.1. NMR spectra for a sample collected at the Wilsonville, AL,pilot plant on June 6,1980, from the atmospheric distillation tower tray 9 that has been derivatized by (A) diphenylphosphinyl chloride, (B) bis(trimethylsilyl)methane, and (C)trifluoroacetic acid. Scheme I A. Phosphorous Derivatization
B. Silicon Derivatization
C. Fluorine Derivatization \
Dadey et al. Table I. Extent of *lPDerivatization of Model Compounds Representative of Those in Coal Liquids compound mol % converted phenol 102, 98.3 o-cresol 96.6 m-cresol 98.4 p-cresol 99.2 2,6-dimethylphenol 105, 91.3, 86.5 a-naphthol 91.8, 93.6, 99.1 2,6-diisopropylphenol 93.8, 75.2 2-octanol 101 tert-butyl alcohol 96,62 benzoic acid a, 1.3, a p-methoxybenzoicacid a, 2.1, a p-ethylaniline 102 aniline 102 2,6-dimethylaniline 65, 84, 81 piperidine 0, 0 benzyl mercaptan 99, 41, 69 B-ethylphenyl mercaptan 108 a
Not detectable. Table 11. Phenol and OH Content of a Range of Coal Liquids Samples content, wt % phenolics by sample extraction OH by 31PNMR T-105 ovh 7/28/80 54.3 9.50 "-105 tray 9 6/6/80 51.3 9.66 T-104 tray 15 6/15/80 32.3 5.50 T-104 btm 8/21/80 28.0 4.54 P-171 1/23/80 27.9 5.10 T-102 tray 3 8/6/80 27.7 5.10 T-104 btm 8/13/80 22.4 3.12 V-160 1/23/80 20.7 2.80 V-160 6/15/80 20.6 3.01 V-131B 7/23/80 20.4 2.83 T-102 tray 8 8/12/80 17.3 2.42 Ky. No. 9 380-650a 17.0 2.43 Ky. No. 9 660+O 14.0 1.29 T-104 ovh 8/21/80 12.2 3.10 V-1078 1/11/85 10.9 2.05 Ky. No. 9 180-380a 9.1 2.00 V-1018 6/5/84 3.9 0.63 V-1078 10/25/84 1.4 0.33 V-1078 4/28/85 0.9
a These samples are from the H-Coal pilot plant at Catlettsburg, KY, coal run 11, while all others are samples from the SRC plant at Wilsonville, AL.
of 180 transients over a 6000-Hz sweep width. Each transient consisted of a 8.5-c~~ pulse ( 3 5 O flip), an acquisition time of 1.6 s, gated lH decouplii to suppress NOE effects, and a delay time of 3.4 s for a total recycle time of 5 s. The 31Psignal from triphenyl phosphate (TPP) was used as an internal standard and a chemical shift reference. Peaks in the region 45.8-47.4 ppm, referenced to TPP, were considered to be derivatized phenols, and weight percent of OH was calculated by comparison of the area in this region to the area of the TPP internal standard. The peak centered at 45.6 ppm was identified as diphenylphosphinicacid that is formed by the reaction of DPPC with water; thus, it was not included in the calculations. (Triphenylphosphine was initially chosen as the internal standard, but preliminary results indicated that it reacted under these conditions and was therefore replaced by triphenyl phosphate.) In this technique, the pyridine has two functions: it acts as a solvent for high molecular weight material, and it serves as HCl scavenger to ensure complete derivatization. The three derivatizing reactions are outlihed in Scheme I, and a representative spectrum representing each reaction is shown in Figure 1. C. Materials. Samples were collected from several process points a t the Wilsonville, AL, pilot plant during thermal liquefaction runs. Three distillations were effected during thermal
process operation. An atmospheiic tower (T-104) fractionated the initial naphtha-range material (initial boiling point (IBP) to ca. 450 OF) to produce an overhead and bottoms sample. The naphtha-free thermal products were fractionated in a vacuum distillation tower (T-102) into an overhead fraction (P-171) as well as distillate fractions that were withdrawn from tray 3 (bp range ca. 300-500 OF)and tray 8 (bp range ca.500-950 OF) sample valves and a nondistillate fraction. An atmospheric distillation tower was utilized to separate the combined T-104 bottoms and T-102 tray 3 plus tray 8 materials into an overhead fraction (IBP to ca. 500 OF) and a bottoms sample; in addition, samples could be withdrawn from tray 1 (top tray), tray 9, or tray 15 sample ports. Samples from two feed vessels were also analyzed: V-160, the feed for the atmospheric distillation tower, and V-131B, the process solvent used to prepare the feed coal slurry. A more detailed description of these process samples can be found in ref 20. Samples were also obtained from the Catlettsburgh, KY, H-coal pilot plant. These were blended according to the mass balance ratio and distillated at the KECL to yield three boiling fractions: (20) Davis, B. H.; Sagues, A. A.; Thomas G.; Baumert, K. L. Fuel Process. Technol. 1985 11, 183.
Energy & Fuels, Vol. 2, No. 3, 1988 329
Hydroxyl Group Concentration in Coal Liquids
“-1
I
I
I
1
2
3
I
1
MMoles of Lutidlne Added
Figure 2. Millimoles of OH in samples collected at the Wilsonville, AL,pilot plant on July 23,1980, corresponding to the atmospheric distillation column overhead (T-105overhead) (A) and vacuum distillation tower overhead (P-171)(0) calculated from NMR area for samples with increasing amounts of 2,6lutidine. 180-380,380-650, and 660 O F + (these samples are identified by Ky. No. 9 in Table 11). Samples were also obtained from the Wilsonville, AL, facility when it was operated in the two-stage configuration. These samples (V-1078)are the IPB 450 OF overhead fraction from the distillation of the catalytic hydrotreater produd For more details of the Wilsonville samples, monthly or run reports should be consulted.21
BTSM
-.,
HMDS
Results A T-105 tray 9 sample from the SRC plant at Wilsonville was derivatized by using each of the three methods described above. Spectra from each technique are shown in Figure 1; these results are representative for the coal liquid samples used in this study. The role 2,6-lutidine played in derivatization was examined in two experiments. In one case, a series of NMR samples were prepared by using two samples by weighing a portion of each sample, specifically a T-105 overhead sample collected on July 28, 1980, and a P-171 sample collected on July 23, 1980, into separate NMR tubes. A different amount of 2,&lutidine was added to each sample, and the phenols were derivatized with trifluoroacetyl chloride. The ‘OF integrals clearly show that the molar amount of 2,6-lutidine must be at least equal to the moles of phenols to ensure complete derivatization of the phenols (Figure 2). The data further indicate that an excess of base does not affect the derivatization stoichiometry; thus, an excess should be used. The impact of the quantity of 2,6-lutidine upon the phenol determination is also demonstrated in experiments utilizing 10 coal liquids (by chemical extraction) ranging i of phenolics. Each liquid was defrom 1.4 to 46.8 w t 9 rivatized with trifluoroacetyl chloride. Five samples contained 2,6-lutidine in the “catalytic” concentration as described in ref 18, while the second five contained a molar excess of this base. The weight percent of OH obtained from the leF spectra corresponds to that expected for the weight percent phenols from chemical extraction data when a molar excess of base was used. However, this was not the case for the “catalytic” concentration; rather, it appears that the % OH determined by NMR attains a limiting value that is dependent upon the base concentration (Figure 3). Quantitative values for the weight (21) Technical Progress Report, DOE/PC/50041-75, October 1986, and earlier reports.
(22) Oradaz, F. E. Masters Thesis, University of Kentucky, 1982.
Dadey et al.
330 Energy & Fuels, Vol. 2, No. 3, 1988
the method of choice, especially for samples containing low levels of phenols. The potential of a number of phosphorous-containing compounds as tagging reagents for compounds containing active hydrogen was determined. Chlorodiphenylphosphine [Ph2PC1],diphenylphosphinyl chloride (DPPC) [Php(O)Cl],diphenyl chlorophosphonate [(Ph0)2P(0)Cl], and methylphenylphosphinyl chloride [Ph(Me)P(O)Cl] were utilized in combination with a base: triethylamine, tributylamine, or pyridine. The combination of Ph2P(0)C1 in pyridine produced derivatives that were stable, and furthermore, phosphorus salts were not formed from these reagents. Ph2PCl derivatives were not stable. Considering the above reagents, the combination of DPPC and pyridine appeared to be the best choice. Therefore, a number of preliminary experiments were conducted to establish the standard conditions necessary for quantitative derivatization; these are summarized below. a. Effect of Pyridine in the Reaction. A known amount of pure phenol, 1.82 mM, was weighed into a 10mm NMR tube. The sample was diluted with 2 mL of CDC13 and enough DPPC to react with all of the OH present. Pyridine was added in 0.371 mmol aliquots until greater than a stoichiometric amount was added. 31P spectra were recorded after each addition of pyridine. The amount of unconverted phenol, calculated from the integral of the derivatized compound, was linearly related to the amount of pyridine added, and the phenols were completely derivatized when the moles of pyridine were equal to the moles of OH. It was also established that 20-30 min was required to completely derivatize the phenols. b. Stability of the 31PDerivatives. Since one of the problems with the 'gF reagents was the slow hydrolysis of the products, the stability of 31Pderivatives toward hydrolysis and/or decomposition over an extended time period was established. Known amounts of two model compounds, a-naphthol and 2,6-dimethylphenol, were placed into separate NMR tubes and derivatized with DPPC. During a 16-day period, small increments of water were added to each sample, which was kept at room temperature, and the amount of phosphorus in reactants and products was monitored by NMR throughout the period. Four phosphorus signals corresponding to the derivatized phenolic compound, the derivatizing reagent (DPPC), diphenylphosphinic acid and an unidentified peak at 28.1 ppm were observed. The amount of phosphorus in these compounds was calculated, and the results indicate that the derivatives are stable to hydrolysis at room temperature and do not undergo decomposition. During this period, all of the excess DPPC reacted with the added water to form diphenylphosphinic acid. With excess water present, the phosphinic acid peak decreased and a peak at 28.1 ppm appeared. It has been suggested that the 28.1 ppm peak may be the pyridine salt of diphenylphosphinic acid, which is stabilized by the presence of the additional water. c. Model Compound Derivatization. To evaluate the reliability for the weight percent of OH determination, a number of model compounds were derivatized. A mole percent converted for each compound was calculated, and the results are summarized in Table I. The data indicate the conversion of phenols, even the sterically hindered 2,6-diisopropylphenol, is essentially complete so that the NMR method provides a quantitative measure of the phenols. Organic acids were not derivatized, presumably because they form salts with pyridine. Anilines appear to be derivatized by the reagent but perhaps not as com-
I"
g
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1
2
3
4
5
6
7
8
9
10
Weight % -OH By NMR
Figure 5. Amount of OH determined by 31PN M R and the weight percent of phenols determined by chemical extraction (see text for definition of symbols A and B).
pletely as phenols. DPPC does not react with piperidine;
this suggests that only those amines that are less basic than pyridine will react under our experimental conditions. Piperidine, being more basic than pyridine, probably acts as an HC1 scavenger so that it is present as a salt. Mercaptans and alcohols appear to react; however, these compounds have not been investigated in the depth needed to reach a reliable conclusion. Approximately 20 coal liquids, ranging from 54.3 to 0.9 wt % in phenols, were chosen for 31Pderivatization. The weight percent of OH was determined by 31PNMR for each sample, and these results are compared to the weight percent of phenolics determined by extraction in Figure 5 and Table 11. Considering the complex nature of the coal liquid samples and the experimental errors associated with the chemical extractions, the two sets of data agree extremely well. Differences in the average molecular weights of the phenolic compounds present in these samples probably account for a large fraction of the deviations from linearity in Figure 5. To illustrate this, consider two coal liquids that have the same weight percent of OH, but have this OH distributed among a group of phenols so that different average molecular weights are obtained. Since the chemical extraction is on a weight percent of phenol basis, the sample with the higher average molecular weight phenols will be heavier and should be above the solid line; on the other hand, the sample with the same weight percent of phenols but with lower average molecular weight phenols should be below the line. This effect is illustrated with two samples, one containing 14.0 (point A, Figure 5) and the other 12.2 (point B, Figure 5) wt ?& phenols as determined by chemical extraction. The 12.2 wt % (Sample B) sample b o w range (room temperature to 230 "C) is much lower than the sample containing 14.0% phenols (340-510 OC boiling range). Thus, sample B, with the much lower average molecular weight, has a larger % OH than the "average" sample determined by the solid line while sample A, with a higher average molecular weight, has a lower % OH. These two samples (A and B, Figure 5) were also analyzed by using the =Si NMR technique. For sample A the % OH calculated from the 29Sidata was 2.9% (vs. 3.1% by 31P)and for sample B the 29Siderived % OH was 1.40 (vs. 1.29 by 31P).Thus, the two derivitization agents lead to similar % OH for both samples A and B; presumably, similar agreement between the two methods would be obtained for other samples. However, these two methods, and other methods that measure phenolic OH by derivitization, acid titrations, phenolic oxygen, etc., suffer the
Energy & Fuels, Vol. 2, No. 3, 1988 331
Hydroxyl Group Concentration in Coal Liquids
.s C
30
c
3.04
1
V
f
,,1 2.5
wX
0 I
Weight % -OH By 31P NMR
Figure 6. Demonstration of linearity of the NMR % OH and the weight percent of phenols for a series of samples prepared by diluting a sample originally containing 32.3 wt % phenols.
/ II I I
DPPC Added, g.
Figure 8. Weight percent of OH (0) and weight percent of water (A), determined by NMr for 1.0-g "a of a solution containing 0.50 wt % OH,as the amount of DPPC was varied. Table 111. NMR Analytical Data for Analysis of Coal Liquid Comprised of Approximately 50% Distillate and 50% Nondistillate Materials anal., wt %
total sample
benzene soluble benzene insoluble
" I
Weight % -OH By 31P NMR
Figure 7. Plot showing expansion of the region of Figure 6 enclosed in the rectangle that includes the origin (full solid line represents line in Figure 6;the other line is determined by low OH data points).
disadvantage that the determined value cannot be related to a weight or volume yield without a determination of average molecular weight. A series of measurements was carried out to establish the lower limit for this N M R phenol analysis. A coal liquid sample containing 32.3 wt % phenols was diluted with different amounts of tetralin to produce samples ranging from 0.10 to 32.3 wt % phenols. The data in Figure 6 show good correlation between the 31PNMR % OH and the weight percent of phenols from extraction. However, at the lower OH levels the NMR method overestimates the 90OH because impurities in the derivatizig agent begin to contribute a significant fraction to the peak area in the phenolic region of the 31PNMR spectra. The impact of the impurity is illustrated in Figure 7 by plotting, on an expanded scale, the data enclosed by the broken line rectangle in Figure 6. At low weight percent of OH the NMR overestimates the amount of OH present. This effect is illustrated differently in Figure 8. To obtain the data in Figure 8, a portion of the coal liquid sample was diluted with toluene to provide a sample containing 0.50 w t % OH. One-gram aliquota of this sample were treated
OH
V-1064 collcd 12/8/84 sample 1 sample 2 1.48 1.20 1.15 1.10 0.20 0.21
as above with the exception that the amount of DPPC was varied over the range shown in Figure 8. The reagenta were not dried since the chemical shift of the hydrolysis product falls outside the derivatized phenol region. However, the competitive reaction of DPPC with phenols and water becomes a problem with low concentrations of DPPC. A sufficient amount of DPPC must be added to react with the water present. However, when this is done the impurity contributes to the signal in the phenolic region so that the amount of phenols measured depends upon the amount of DPPC added. These two problems must be solved if the method is to provide reliable data for samples containing below about 0.5 wt 90OH. Since the identity of the impurity is not known, this would require a considerable effort. In spite of these problems, the extrapolated weight percent of OH agrees closely with the correct value so that the method could be utilized for samples with low concentrations of phenols. The application of the chemical extraction method to high molecular weight coal liquefaction samples is, at best, difficult because, among other things, of sample insolubility and hydrocarbon contaminations. The utility of the NMR method for these samples is illustrated by the data in Table 111. The total sample (V-1064, containing a significant fraction of nondistillate SRC) contained about 1.35 wt % OH. Soxhlet extraction of this sample was effected with benzene for 4 days to produce an insoluble fraction and a benzene-soluble fraction. The NMR analysis of these two fractions shows that about 20% of the OH remains in the insoluble fraction and the chemical extraction would not account for this phenol content. In addition, our experience is that the actual fraction of OH unaccounted for by using chemical extraction with this type of sample is XHO% rather than the 20% due to solubility alone. Thus, the 31PNMR method is ,superior to chemical extraction for this type of sample. d. Comparison of t h e Three Derivatization Techniques. As a comparison of the three NMR techniques, five coal liquid samples that contain 51.3-1.4 w t % phenols
Dadey et al.
332 Energy & Fuels, Vol. 2, No. 3, 1988
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Figure 9. Comparison of the OH data from three NMR methods to the weight percent of phenols obtained by chemical extraction.
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Figure 10. Illustration of the expanded NMR region corresponding to derivatized phenols in a process solvent (V-1078 samde collected at the Wilsonville. AL. Dilot Dlant on Julv 11. 1988) for each of the three reagents used in figure 9: (Aj31P; (B) (C) 19F.
were analyzed by using IsF, 29Si,or 31Pderivatization reaction. A weight percent of OH for each sample was determined by each method and compared to the weight percent of phenolics by extraction (Figure 9). The data from each method correlate with the chemical extraction data equally well. Consequently, the sensitivity and ease of operation should determine which method is to be employed. The expanded chemical shift regions corresponding to the derivatization atom for each technique are shown in Figure 10 for a high-boilingsample of Wilsonville, AL,pilot plant process solvent. The 31Pis more sensitive to the chemical environment that the other two procedures and, hence, produces the better chemical shift dispersion so that a large number of peaks are observed. However, at the present time this should be an advantage only for lower boiling fractions where chemical structures can be assigned to the individual peaks.
Summary
31Plabeling with diphenylphosphinyl chloride appears to provide a new technique for the analysis of the hydroxyl group concentration in coal liquids. The technique introduces a 31Ptag at the active site. A weight percent of OH is then calculated from the corresponding integral in the 31Pspectra. These derivatives combine the sensitivity of l9F, the chemical shift dispersion of 29Si,and the reli-
ability of extraction data. It also overcomes a number of disadvantages of each of the three methods mentioned above. High molecular weight materials that are difficult, if not impossible, to analyze for phenolic OH by conventional methods can be analyzed in as little as 2 h with the 31Pmethod. This technique also allows for an accurate analysis of samples that contain as little as 0.5 w t % OH, and with further refinements to eliminate interferrences from water and impurities in the derivatizing agent, a limit quite a bit lower than this should be attainable. It appears, on the basis of limited data, that alcohols and weak N-H nitrogen bases are also derivatized. Organic acids do not appear to form 31Pderivatives, and N-H nitrogen compounds with a base strength greater than the pyridine solvent are not derivatized. For %Silabeling a number of improvements have been made to the original procedure described by Rose.6 A significant amount of signal enhancement has been obtained by using a 29SiINEPT pulse sequence. This enhancement decreases the amount of instrument time required for signal acquisition from 2-3 to about 0.5 h. Ekperimenta verified that pyridine can be used as a solvent without interfering with the derivatization reaction. This permits analysis of higher boiling pyridine-soluble materials (i.e., essentially all higher molecular weight petroleum and synfuels materials: preasphaltenes, recycle solvents, vacuum bottoms, etc.). The l9F method has been employed to determine hydroxyl group concentrations in a variety of coal liquids; this includes detailed studies of the phenol content of process solvents and how their concentrations vary with on-stream time.18 However, the role of 2,6-lutidine is not catalytic, as initially suggested; rather, it acts as an HC1 scavenger that drives the reaction to completion. The results from this study indicate that an equilibrium, eq 1, OH
+
CF3COCI f
is established before the addition of base. Additional 2,6-lutidine shifts the equilibrium toward products. Quantitative and reproducible results are obtained with the 19F NMR method only if a stoichiometric or greater amount of base is added. As a method to determine the amount of single compound or phenol subgroups, the region of the lgFspectrum resulting from the derivatized phenols is not as well resolved as the 31Pregion. Of the derivatization seems to be the three methods studied, '@F least sensitive to ita chemical environment. In summary, for optimal techniques, similar results are obtained by NMR analysis of OH for each of three deFurthermore, the rivatization elements: l9F,%Si,or 31P. results are in good agreement with data obtained by the conventional chemical extraction technique, and furthermore, much of the disagreement is a result of average molecular weight differences among samples.
Acknowledgment. This work was performed at the Kentucky Energy Cabinet Laboratory (KECL) with funding from the Kentucky Energy Cabinet, Commonwealth of Kentucky. The KECL is administered under contract by the University of Louisville.
