Anal. Chem. 1983, 55, 1689-1694
1889
Analysis of Oil Fractions Derived from Hydrogenation of Aspen Wood D. G . B. Boocock Department of Chemical Engineering, University of Toronto, Toronto, Ontario M5S l A 4 , Canada
R. K. M. R. Kallury and Thomas T,, Tidwell* Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada
The carboxylic, phenolic, basic, and neutral fractions resulting from fractlonat#onof four oll samples derlved from wood hydrogenatlon were analyzed by IR, 'H and ''C NMR, VPC, HPLC, GC/MS, and CIMS. About 20% of the phenollc fraction Is comprised of dlstlllable alkyl phenols and catechols, the ratios of which could be determlned as 2:l and 1:l for Raney nlckel and nickel carbonate catalyzed oils, respectlvely, by VPC, CIMS, anid "C NMR technlques. One-thlrd of the neutral fractlon conslsted of alkyl cyclopentanones and cyclohexanones In a 1:2 ratlo as determined by "C carbonyl peak lntegratlons and by VPC. The composltlon of the carboxyllc acld fractlon was obtained by VPC and CIMS, the latter belng utlllzed to arrlve at the relatlve amounts of C,-C, allphatlc acids. Combhation of VPC and CIMS facllltated the identlflcatlon of alkyl imldazoles as the malor constltuents of the bask fractlon.
Research on the utility of biomass as an alternate source to petroleum for producing fuels and chemicals has gained momentum in recent years (1-15). Two major approaches are being pursued in this direction, viz., microbial and thermochemical. The latter method has the advantage that both lignin and cellulose, the major constituents of wood, undergo degradation into smaller molecules. In particular, hydrogenation of biomass a t 350 O C in the presence of a nickel catalyst was shown to result in the complete liquefaction of aspen wood (15). Analyses of the chemical composition of oils from such liquefactions, however, are limited to a few preliminary repoh3 (16-19). The goal of the current investigation was to devise a[ relatively simple scheme for the analysis of wood oils that combined fractionation of the oil on the basis of solubility anld acid-base properties with the general chemical analysis of each fraction based on chromatographic and spectroscopic techniques. The objective was not to identify a large number of individual componentn but rather to classify the oil quantitatively on the basis of classes of components. Emphasis has been placed on the 13CNMR and CIMS in view of the specific ritructural information that can be derived by these methods CIS has been demonstrated in recent publications (20-25).
EXPERIMENTAL SECTION
All solvents were purified by distillation. The oils were obtained by previously reported procedures (15). Acids, phenols, and cyclic ketones used for VPC comparisons were commercial samples. The alkyl imidazoleri used as standards for VPC were made by known methods (26). Fractionation of the Oils. The oil (10-15 g) was extracted with ether (5 X 100 mL) and the insoluble residue removed by gravity filtration. The ether solution was successively extracted with NaHC03 (5% aqueous, 100 mL), NaOH (5% aqueous, 200 mL), and HCl(1:l aqueous, 50 mL), the ether layer being washed with water after each treatment. The neutral fraction (fraction G) remaining in the ether after these extractions was recovered
after drying (MgS04). Acidification (pH 6) of the bicarbonate and sodium hydroxide layers yielded the strongly and weakly acidic fractions (fractions A and G, respectively) which were recovered by ether extraction. The basic fraction (fraction H) was obtained by neutralizing the HC1 extract with solid sodium carbonate (pH 8) and extracting with ether. The weakly acidic fraction (fraction B) was fractionated further by the following two methods: distillationunder reduced pressure (1mm) afforded a distillate, bp 40-120 "C which was subjected to chromatographic and spectroscopic analysis; alternately, treatment with dichloromethane gave insoluble (fraction C) and soluble (fraction D) fractions. Fraction C (comprising -35% of fraction B) was found to be a dark brown amorphous material, not melting up to 350 "C, and was not pursued further. Fraction D was subjected to column chromatography (silica gel) and separated into two major fractions, viz., fraction E and fraction F, respectively, which were screened by HPLC (conditions given under Instrumentation). Fraction G was distilled, initially at atmospheric pressure up to 150 "C and then under reduced pressure wherein two fractions (30-80 "C (7-5 mm) and 80-120 OC (2 mm)) were collected. The fractions did not show clean separation and were combined for further analysis. Methylation of Phenols for Making Standard Anisoles and Methyl Ethers of the Oil Phenols. The literature procedure (27) was slightly modified in that lower quantities of methyl iodide (20 mL) and sodium hydride (2 g) were utilized per 2 g of either the oil phenol or a commercial phenol (1g in the case of catechol). Instrumentation. The IR spectra were recorded on a Pye Unicam SP3-200 infrared spectrometer as a neat liquid film or in chloroform with polystyrene as standard. Measurements of 'H NMR were carried out on a Varian T-60 NMR spectrometer in CDC13 with Me4Si as internal reference while I3CNMR were obtained on a Bruker WP-80 and/or Varian CFT-20 NMR spectrometers in the FT mode in CDC13with Me4Si as standard. A pulse delay of 2 s was utilized for the latter spectra. Electron ionization mass spectra were recorded on a Du Pont 21-490 or an AEI MS-30 mass spectrometer for low- and highresolution spectra with an electron beam energy of 70 eV and source temperatures of 100-120 OC. Samples were introduced through heated inlet systems. Exact mass measurements were made by means of the DS-50 data system with PFK as reference. Chemical ionization mass spectra were taken on a Du Pont 21-490B mass spectrometer equipped with a high-pressure source using methane or isobutane reagent gases (0.24.3 torr) at the same source temperatures as for EIMS. GC/MS experiments were performed on an AEI MS-30 mass spectrometer linked to the DS-50 data system and interfaced with a Shimatzu GC-Mini 2 gas chromatograph provided with a temperature programmer on a 5% QF-1 column. Analytical GCs were carried out on a temperature programmed Varian 2700 aerograph equipped with dual flame ionization detectors and a CDS-111 computer system on either 3% OV-17 or 5% QF-1 columns with helium carrier gas flow rates of 20-25 mL/min. Prep GCs were made on a Varian 900 gas chromatograph equipped with a thermal conductivity detector. Temperatures ranging from ambient to 200 OC (2 OC/min) were utilized.
0003-2700/83/0355-1689$01.50/0@ 1983 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
Table I. Conditions for Wood Liquefaction ( 1 5 ) oil
catalyst
i..
reaction conditions
I Raney nickel (20 g )
Aspen wood (150 g), water (750 mL), cold hydrogen pressure 3.4 MPa, temperature 350 'C, time 2 h same as for I, except cold I1 Raney nickel (20 g) hydrogen pressure 1.7 MPa I11 Raney nickel (5 g) same as for I IV nickel carbonate (30 g) same as for I , except cold hydrogen pressure 6.9 MPa High-performance liquid chromatography experiments were performed on a Water Associates liquid chromatography apparatus on a PRP-1column (28)with acetonitrile-water (1:l)solvent mixture and flow rates of 1.0-1.5 mL/min for each of the phenolic fractions. The individual phenolic fractions were subsequently extracted with ether and the residues from the concentrations were screened by E1 and CIMS. Analyses for water content in the oils were done on the Karl Fischer apparatus in chloroform solution. L
RESULTS AND DISCUSSION Table I lists the conditions under which the four oil samples currently examined were produced while Table I1 indicates the relative percentage yields of the individual fractions along with the amounts of distillable material from the phenolic and neutral fractions. Analysis of Whole Oils. The IR spectra of the ethersoluble portion from each oil revealed strong hydroxyl and carbonyl absorptions together with bands due to aromatic and C-0 linkages. The lH NMR spectra contained two major regions of signals around 6 1.0-3.0 ppm and 6.3-7.6 ppm. Absorption in the 6 3.0-4.0 region in the lH NMR and 6 50-60 ppm region in the 13C NMR was weak indicating a low concentration of methoxyls or other alkyl ethers. The 13CNMR ratio of 3:2 in addition to the showed a Cdiphatic:Caromatic presence of carboxylic (6 175-185 ppm) and aliphatic/alicyclic ketonic (6 200-220 ppm) carbons. The strong singlet at 6 -30 is suggestive of tertiary methyl carbons (C(CH&) (29) and/or methylene carbons belonging to long chain fatty acids (30). Analysis of the Carboxylic Acid Fraction A. The Hdiphatic:Haromatic ratio of 14:l obtained from lH NMR measurements (with signals in the regions 6 0.8-2.6, 6.6-7.4, and 9.0-9.2 ppm) along with the Csaturated:Cunsaturatedratio of 5:l in the 13C NMR spectrum of this fraction (see Figure 1 for a representative spectrum from oil I) suggested that about 80% of this fraction is made up of aliphatic acids. The CIMS of this material (Figure 2) indicated strong peaks at mlz 89,103, 117, and 131 corresponding to the MH' of C4-C7 acids which together constituted 70% of the total ion current. In addition, the peaks at m/z 129,143, and 157 were attributable to olefinic or cyclic Cs-Cs acids and these represented about 10% of the total ion current. The correspondence of the GC retention times with those of cyclohexane carboxylic acid and its monoand dimethyl homologues and the absence of vinylic proton absorption in the NMR strongly suggested the presence of
i
l
L
l
l
i
L
l
l
~
l
i
~~l~~~~ l
~
100
200
Figure 1. 'C NMR spectrum of the NaHCO, solubles (fraction A) from I.
011
./.
Figure 2. Isobutane CIMS of fractlon A from oil I.
these acids. The other identifiable acids were benzoic, phenylacetic, and phenylpropionic acids corresponding to the CIMS peaks at m / z 123, 137, and 151, respectively. Again, comparative GC with standards and exact mass measurements on the base peak ions at m / z 105 (C7H50+= benzoyl cation) and 91 (C7H7+= tropylium cation) as well as the parent ions at m/z 122,136, and 150 respectively in the E1 spectra of these acids support the above structural assignments. In view of the considerable overlap observed between isomeric/homologous C5-C7 acids in the GC, the percentages of these homologous acids in the mixture were derived from the fractions of the total ion current of the respective MH' ions in the isobutane chemical ionization mass spectra semiquantitatively (see Table 111). Considerable amounts of acetic and propionic acids were detected in the aqueous phase floating on top of the oil in the reactor during liquefaction, and the compositional analysis of this phase will be reported separately.
Table 11. Fractionation of the Wood Oils-Percent of Fractions' oil
fraction A (NaHCO, solubles)
fraction B (NaOH solubles)
fraction H (HCl solubles) 1.o
% distillable from
fraction G (neutrals)
residue e
45.0 35.0 40.0
8.5 17.0
fr.
BC
fr. G d
33 30 17.0 30 1.0 50.0 7.0 20 IV HPLC used to separate individual Karl Fischer titration of the oils indicates on an average about 12% water content. 30-150 "C, 2 torr 40-120 'C, 1 torr (figures under this column represent % of fraction B). phenolic components. (figures under this column represent % of fraction G). e Represents ether insolubles from the original oil. I
I1 I11
2.5 2.5 2.5 2.0
32.5 30.0 27.5 25.0
1.0 1 .o
22 b 18 12
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
____-
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-
Table 111, Analytical GC and CIMS Data on Fraction A (Oil I )
component CH,COOH CH,CH,COOH CH,CH,CH ,COO11 (CH,),CHCOOH CH,CH,CH ,CH,COOH CH ,(CH,),COOH CH,(CH,),COOH branched C,-C, acids cy clohe xariecarboxylic acid C, , C ,-cyclohexanecarboxylic acids benzoic acid phenylacetic acid phenylprolpionic acid unidentifietd
% in mixture by GC analysis a
2.0 2.0 5.5 3.5 12.OC 15.0' 9.5c 23.5b 4.5 6.5
3.0 3.5 4.0 10.0
CIMS data MH' ( m / z ) C,d % 61 2.0 75 2.0 12.0 89 25.0 103 117 25.0 131 1.0 129
5.0
143 157 123 137
3.0 2.0 5 .O 2.5 2.5
151 145
159 161 173 175
ID0
100
0
NMR spectrum of the NaOH-soluble fraction B distillate
Flgure 3. from oil I.
90 -
8.0
These a Identified by comparative GC with standard. acids were tentatively identified by comparative GC with available standards and by CIMS of corresponding prep GC fractions. ' GC peaks corresponding to these compounds were observed, but overlap with peaks from other acids cannot be excluded. Figures shown for m/z 89, 103, 117, and l 3 1 represent straight chain and branched C,, C,, C,, and C, aliphatic acids taken together. -
40
I
109
m/z
Analysis of the Phenolic Fraction B. Approximately 12-22% of this fraction in oils I, 111,and IV could be distilled between 40 and 120 " C a t 1torr (Table 11). Examination of the distilled material by GC/MS permitted assignment of molecular formulas corresponding to 75-85% of the total distillables as alkyl phenols and alkyl catechols (Table IV). In many of these cases, the GC retention times of authentic samples were compared and were found to be consistent with these assignments. Alternately, the sodium hydroxide soluble material was further fractionated into fractions E and F (see Experimental Section) and analyzed by HPLC. The mass spectra of the individual fractions obtained from fraction E by HPLC indicated the same alkyl phenols/catechols as from the distillates while those from fraction F showed the presence of polyhydroxystilbenes and benzofurans. Monitoring of the total ion current (TIC) as a function of time/mass scans during the GC/MS of the distillates from the phenolic fraction yielded a cross-scan report which paralleled the analytical GC of the same material with respect to the order of elution. Munson (31) has reported such similarity between GC and CIMS with simple organic mixtures. Further, since the relative sensitivities of organic molecules in general and a homologous series in particular (e.g., alkyl benzenes) are nearly the same during chemical ionization with nonselective reagent ions such as CH5+ and CPHS+,a semiquantitative analysis of a mixture of closely related components is possibde by this technique (31, 32). An added advantage of the methane CIMS with the phenols is the fact that only MH+ ions are foTmed with virtually no fragmentation. Therefore, the fraction of the total ion current carried by a particular ion represents semiquantitatively the percentage of the related component in the mixture. Table IV shows the CIMS data thus obtained. Comparison with the VPC results (Table V) shows a reasonable correspondence between the two methods, in particular between the alkyl phenokalkyl catechol ratios. The saine phenol/catechol ratio could be derived by
Flgure 4. CH, CIMS of methylated phenols from fraction B, oil I . r
2E0
Flgure 5.
110
0
NMR spectrum of the neutral distillate from oil I.
integration of the peaks corresponding to the carbons attached to the phenolic hydroxyls in their 13C NMR spectra (Figure 3) on the basis of the observations that such carbons in monophenols absorb in the region 6 150-155 ppm (33-35) while similar catechol carbons do so at 6 142-145 ppm (36, 37). The VPC and GC/MS of the methylated phenols lend further support to the above structural information. Most of these anisoles were isolated by preparative GC and their structures were verified by IH and 13C NMR. The methane CIMS of the anisoles from the phenolic distillates is presented in Figure 4. The methylation products from the residues of the distillations were also screened by VPC and CIMS and besides the methyl ethers of dibutylphenol (MH+, m / z 221) and dibutylcresol (MH", m / z 235), the methyl ethers of methylnaphthol (MH+, m / z 173) and ethyl-/dimethylnaphthol (MH+, m / z 187) could be tentatively identified. Analysis of the Neutral Fraction G. The neutral fractions from oils I-IV showed little difference in their compositions and the data presented in Tables VI and VI1 represent a genral picture, as does the following discussion. The IR
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
Table IV. Mass Spectral Data on the Phenolic Distillate (Oil I ) molecular ion M + (m/z) (from GC/MS)
MH' (from CIMS) ( m/z 1
94 108
95 109
110
111
122 124 134 136 138 150 152
123 125 135 137 139 151 153 149 163 177 179 193 207 221
148
162 176 178 192 206 220 a
compositiona of M + and M in MH'
c,% (in CIMS) 2.5 7.5 8.5 18.0
5.0 5.5 8.0 3.0 5.0 7.0 2.5 2.0 1.5 1.0
0.5 0.5 10.5
structure phenol cresols catechol xylenols and ethylphenols methylcatechols C ,H,-phenols C ,H,-phenols C,H,-catechols C,H,-phenols C ,H,-catechols C,H,-phenols C ,H, -phenol C6H,,-phenol C6H14-phenol C ,H,,-phenol C,H,,-phenol di butylcresol
Compositions based on computer high-resolution data for M' (EI) and by isotopic ratio measurements for MH' (CI).
Table V. Percent Composition of Alkyl Phenols/Catechols in the Phenolic Distillates as Determined by VPC phenol phenol o-cresol m-/p-cresol xylenols ethyl phenols catechol methylcatechols ethylcatechols prop ylphenols allyl phenols propylcatechols di-tert-butylcresol others pheno1s:catechols
oil I
oil I11
oil IV
3.5
3.0
3.0
1.0
1.0
1.0
8.5 5.0
6.5 4.0
18.0
18.0
8.0 3.5 4.0 6.5 6.5
10.5 3.5 4.5 5.0 5.0 7.0 6.5 26.0 2:l
6.5 1.0 13.5 14.5
8.0
1 3.O 15.0 2:l
4.0
6.0 13.0 2.0 16.0 3.0 17.0 1:l
showed strong bands a t 1720-1760 cm-l consistent with the presence of ketone functions. The 13C NMR of this material (Figure 5) exhibited no signals attributable to the carbonyl carbons of aldehyde, ester, or lactone functions and aldehydic hydrogens were also absent in the lH NMR. The isobutane CIMS (Table VII) indicated major peaks corresponding to the formula C,H2,-20 (n = 5-10) suggesting the presence of cyclopentanone and cyclohexanone derivatives. The absence of a,@-unsaturatedcarbonyl bands in the IR and vinyl absorptions in the lH NMR argue against the presence of significant amounts of open chain unsaturated isomers. Cycloheptanone and cyclooctanone IR absorptions a t 1670-1690 cm-l were also absent. The isobutane CIMS of the 2,4-DNl" derivatives of these carbonyls confirmed their molecular
composition (Table VII). A cyclohexanones/cyclopentanones ratio of 2:l could be derived from the relative integrals of the 13C peaks a t 6 210-215 and 220-223 ppm, corresponding to the respective absorption positions of the carbonyl carbons of these derivatives (38). The ketonic fractions were estimated as about 35% of the total distillable portion of fraction G by GC (Table VI). Other materials were tentatively identified as alkanols CnH2n+20, cycloalkanols CnH2,0,benzyl alcohols, and methylbenzofurans on the basis of their molecular compositions (Table VII) and E1 fragmentation patterns. Minor peaks corresponding to the formulas C,Hzn40 and the corresponding DNPH derivatives were also observed. The residue from the distillation of the neutrals was found to have a molecular weight above 10000 by osmometric measurements and was not studied further. Analysis of the Basic Fraction H. This dark brown fraction had a pyridine-like odor and exhibited two major peaks a t m / z 111 (C6H10N2+ H') and 125 (C,H12N2 + H ' ) in addition to a less intense ion at m/z 97 (C6H8N2+ H') and several very low intensity peaks in the mass range 100-200. GC/MS gave the mass spectrum of the most abundant component (30% of the total fraction H) as m/z 110 (M', C6H1&, 100%) with fragment ions a t m / z 109 (25%), 95 (40%), 56 ( E % ) , 42 (24%), 41 (15%), and 39 (25%). The IH NMR of fraction H showed strong peaks in the regions 6 2.0-2.2 and 2.4-2.6 ppm with less intense signals a t 6 1.8-2.0 ppm and extremely weak aromatic proton absorption. In the I3C NMR, clusters of peaks were observed at 6 15-40 ppm and 110-150 ppm in a ratio 3:2. These NMR data are suggestive of alkyl-substituted heteroaromatic systems and comparison of GC retention times with authentic 4,5-dimethyl- and 3,4,5-tri-
Table VI. Composition of the Distillate from the Neutral Fraction G" % in method of identification class of compounds distillate 13CNMR, GC/MS, CIMS (isobutane) comparative GC with standards 35.5 alkyl cyclopentanones and cyclohexanones 11.5 GC/MS and CIMS (isobutane) benzyl alcohol and its homologues 9.0 GC/MS and CIMS (isobutane) alkyl benzofurans comparative GC with standards, GC/MS and CIMS (isobutane) 8.0 cyclopentanols and cyclohexanols 4.0 CIMS (isobutane) unsaturated cyclic ketones 1.5 comparative GC with standards open chain C, and C, alcohols 1.0 comparative GC with standards open chain C, and C, ketones 25.0 unidentified a
Oils I-IV display the same values approximately.
ANALYTICAL CHEMISTRY, VOL.
Table VII. Isobiutane CIMS of the Total Neutral Distillate and the 2,4-Dinitrophenylhydrazones Derived Therefrom distillate 2,4-DNPH MH+ composition MH' composition (m/z) ( o f M ) E, % (m/z) (of M) ~
~~
products in reactants
~
C,H,Ol C6H100
C7H1Z0
C,HI4O 9
16()
C,,H,,O C,,H,,O
2.0 3.0 8.0 10.0 5.5 3.5
2.0
265 279 293 307 321 335 349
C11H12N404
C12H14N404 C13H16N404 C14H18N404
1,H,0N404 C16H22N404 C17H24N404
z, % 2.5 7.0 20.0 24.5 17.0 10.0 5.0
1Jnsaturated Cyclic Ketones 111 125 139 153 167
0.5 1.0 1.0 0.5 0.5
291 305 319 333 347
C,,H14N404 Cl4Hl6N4O, C,,H,,N,04 Cl,H~oN,O, C,,HZ2N4O,
0.5 3.0 5.0 3.0 1.5
Cyclic Alcohols 101 115 129 143 157 109 123 137
0.5
1 .o
1.0 1 .o 0.5 Benzyl Alcohols 1.5 5.5 4.0
Benzofurans 133 147 161
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Table VIII. Results of Alkylation Studies on Phenol/Veratrole with Alcohols under Wood Liquefaction Conditions
Saturated Cyclic Ketones 85 99 113 127 141 155 169
55, NO. 11, SEPTEMBER 1983
3.0 4.0 4.0
methylimidazole samples (26) showed peaks with the same retention times and the CIMS peaks at m / z 27 and 111 can be correlated with these structures. By analogy, the peak at m / z 125 can represent a C4-substituted imidazole. As can be seen from the data in Table 11, the phenolic and neutral fractions (fractions B and G, respectively) constitute the bulk (65-75%) of the wood-derived oils, while the carboxylic and basic fractions (A and H, respectively) together make up less than 4% of the total. Both of the larger fractions B and G contain considerable volatile material and can be further fractionated by distillation. Fraction A consists predominantly of aliphatic carboxylic acids, fraction B of alkyl phenols and catechols together with more complex phenolic heterocycles, fraction 13 of heterocyclic bases, and fraction G of neutral compounds including many cyclic ketones and alcohols. Insoluble residues make up 7-17% of the total. Comparison of oil I to oil I11 shows that the former, which utilized four times as much Raney nickel, gave half as much residue (Table [I). Comparison of oil I (using Raney nickel) to oil IV (using nickel carbonate) shows a somewhat lower sodium hydroxide soluble (phenolic)fraction in the latter case which also contains a considerably greater proportion of catechols relative to phenols (Table V). Variation of hydrogen pressure does not seem to produce any appreciable change in the composition of the oil. Most of these phenolic products are presumably derived from thermal degradation of the lignin, but the possibility of alkylation of the simpler phenols to more complex structures under the reaction conditions also exists. To examine this point, model experiments were carried out (see Table VIII) from which it was found that nickel carbonate promotes some alkylation. The contribution of cellulose toward the phenolic
conditions
phenol (0.1 mol) and 350 'C, 2 h , 50 g of water, H, methanol (5 mol) (300 psi) phenol (0.1 mol) and 350 'C, 2 h, 50 g of methanol (5 mol) water, NiCO, ( 5 g), hydrogen (300 psi) PhOH (0.1 mol) and 2-methyl-1-propanol phenol (0.1 mol) and tert-butyl alcohol (2 mol) veratrole (0.1 mol) and tert-butyl alcohol (2 mol)
liquid phase
unreacted PhOH (95%) PhOH (75%) p-cresol (20%) o-cresol (2%) PhOH (80%) p-tert-butylphenol (10%) PhOH (75%) p-tert-butylphenol (15%) catechol (50%) guaiacol (15%) anisole (15%)
fraction under these conditions and the effect of Raney nickel as a catalyst for alkylation of the phenol ring are under investigation. It is also possible that some of these phenols occur as such (e.g., di-tert-butylphenol in essential oils (39)). The source of the alkyl imidazoles is another interesting question. These are known to be formed from carbohydrates and ammonia under mild conditions (40,41),but it is not yet known if the heterocyclic rings of these materials are present in the wood before liquefaction or whether they are formed during the hydrogenation. The characterization scheme introduced provides a straightforward and informative method for defining the general chemical composition of wood oils. This scheme should prove particularly useful for comparative analysis of such materials prepared by different processes. The identification of specific components of each of the different fractions can be refined to include many additional individual compounds if this is desirable. The detailed consideration of the chemical transformations involved in the wood liquefaction must await the outcome of specific experiments designed to test the different possibilities. However, a few comments can be made. The almost total absence of alkyl ethers in the current products, as indicated by the NMR and GC/MS analyses, is remarkable as the original wood lignin contains such function groups in large amounts. Such extensive cleavage of ether groupings has not been reported in previous studies. The material which is either insoluble in ether or which cannot be distilled from fractions B and G comprises 30-50% of the total oil. It is not yet clear whether this material is inert to the reaction conditions or whether it results from insufficient time of reaction to effect complete conversion.
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RECEIVED for review March 14,1983. Accepted June 1, 1983. A Strategic Grant from NSERC, Canada, is gratefully acknowledged. The authors are also grateful to A. G. Harrison, Department of Chemistry, University of Toronto, for permission to record the CI spectra.
Nonlinear Multicomponent Analysis by Infrared Spectrophotometry Mark A. Maris and Chris W. Brown*
Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881 Donald S. Lavery
Donald Lauery & Associates, Inc., 4225 Ruskin Street, Houston, Texas 77005
Matrix calculations are evaluated for least-squares regression analyses of multlcomponent spectrophotometrlc callbratlon data uslng absorbance as the Independent varlable. Transformations of the absorbance data are Investigated as means for modellng data exhibiting severe devlatlons from the Beer-Lambert law. Regresslon analyses have been performed on several sets of data from low-resolutlon vaporphase Infrared spectra of llght alkanes. Accuracles of the analyses were compared after varying the following regression parameters: number of standard samples, number and spacing of analytical frequencies and mathematical transformations applled to the raw absorbance matrix. I n general, the results showed that for these data a power series of the absorbance data glves slgnlflcant Improvements In accuracy over a slmple linear model. Addition of analytical frequencies Improves the results of the two-component analyses. Overdetermlnatlon of the regression by uslng addltlonal standard mlxtures adds greatly to the accuracy of most analyses. I f the correct equation Is found for fmlng the observed data, the optlmum number of standard mlxtures can be predlcted for any lndlvldual analysis.
Quantitative spectrophotometric analysis is based on the Beer-Lambert law. Chemical systems obeying this linear relationship between concentration and absorbance have been quantified accurately for many years. Problems in quantitative spectrometry arise when dealing with chemical data which show deviations from Beer's law. Fortunately, methods
now exist which, in many cases, can easily deal with such problems. The simplest application of the Beer-Lambert law is graphical. A calibration plot can be created for a single component by graphing chemical concentration vs. absorbance a t a single analytical frequency. For multiple components measured at several frequencies this technique rapidly becomes cumbersome, especially if interferences exist between components. For this reason multicomponent quantitative spectrometry was not truly practical until the advent of modern computer hardware. Most investigations have retained the traditional formulation of data with absorbance represented as a linear function of concentration. For many multicomponent systems, linear calibrations restrict the analyst to a narrow region of concentration of one or more of the chemical components. In addition, solving the problem using absorbance as the independent variable may offer definite advantages when dealing with data showing severe deviations from Beer's law.
THEORY In its simplest form, the Beer-Lambert law for one component is A = abc (1) where A is the absorbance, a the absorptivity, b the pathlength, and c the concentration. When the pathlength is constant, the terms a and b are usually combined to give a single proportionality constant. A modification of the Beer-Lambert law can be written as A = klc ko (2)
0003-2700/83/0355-1894$01.50/00 1983 American Chemlcal Society
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