Off-line thermochemolysis versus flash pyrolysis for the in situ methylation of lignin: Is pyrolysis necessary?

JOURNAL OI ANALYTICAL and APPLIED PYROLYSIS

Journal of Analytical and Applied Pyrolysis 34(1995)41-46

ELSEVIER

Off-line thermochemolysis versus flash pyrolysis for the in situ methylation of lignin: is pyrolysis necessary? Daniel aFuel

E. McKinney ‘, Daniel M. Carson a, David J. Clifford “. Robert D. Minard ‘, Patrick G. Hatcher ‘.*

Science Program,

’ Deportment

The Pennsyhania

of Chemistry.

State

The Pennsvlranicr

University,

Stule

Received 20 July 1994; accepted

Unicersitj,

Unitiersiry

Park,

Uniwrsit~~

in final form

I2 October

PA

Park,

PA

16802,

L’SA

16802. (!.SA

1994

Abstract

Lignin, a major biopolymer in vascular plants, is shown to undergo thermochemolysis reactions when subjected to flash pyrolysis with in situ methylation using tetramethylammonium hydroxide. The product distribution, composed of methylated lignin monomers, implies that the thermochemolysis involves cleavage of the p-O-4 ether bonds in the lignin. Because thermochemolysis occurs with equal effectiveness at sub-pyrolysis temperatures of 300°C. we conclude that the analytical method does not require use of specialized pyrolysis equipment, and can be implemented in a batch mode in which internal standards can be added and the products quantified. Keywords:

Flash

pyrolysis;

Lignin;

Pyrolysis;

Tetramethylammonium

hydroxide:

Thermo-

chemolysis

I, Introduction

A new technique

recently

reported

for flash

pyrolysis

with

in situ derivatization

(TMAH) [l-3] has been shown to be mainly a thermally assisted chemolysis rather than pyrolysis [4-71. As initially introduced [l], the technique involved flash pyrolysis of phenolic polymers al temperatures of 610°C in the presence of TMAH. The products formed were methylated and thereby more amenable to gas chromatographic analysis than their polar pyrolysis more polar unmethylated analogs. TMAH not only methylates

using

tetramethylammonium

* Corresponding

hydroxide

author.

0165-2370/95/$09.50

0

1995 - Elsevier

SSDI 0165-2370(94)00865-5

Science

B.V. All rights reserved

42

D.E. McKinney

et al. 1 J. Anal. Appt. Pyrolysis

34 (1995) 41-46

products but also assists in bond cleavage; de Leeuw and Bass [4] demonstrated its potential for hydrolytic reactions, especially the hydrolysis/methylation of esters in cuticles of tomato plant leaves. Hatcher and Clifford [5] discovered that TMAH is as effective at 300°C as it is at 700°C for the production of volatile products (fatty acid methyl esters and diesters, aromatic carboxylic acid methyl esters, and methoxybenzenes) from humic acids, suggesting that a significant amount of bond breakage occurs at sub-pyrolysis temperatures. Applying this technique to wood samples at both pyrolysis and sub-pyrolysis temperatures in a flash pyrolysis-gas chromatographic/mass spectrometric system, we and others [8-lo] have discovered that the procedure effectively ruptures the p-O-4 ether bonds in lignin at sub-pyrolysis temperatures, producing a series of methoxybenzenes characteristic of the different monomers in various wood types. The main products from guaiacyl type lignins are 3,4_dimethoxybenzaldehyde, 3,4-dimethoxyacetophenone, and the cis/trans isomers of B,3,4_trimethoxystyrene. Various methoxylated propane and propylene derivatives of the dimethoxybenzenes are also observed. The syringyl type lignins produce a similar series of compounds having trimethoxybenzenes as the aromatic nuclei. It is clear from this study that the TMAH procedure, hereafter called thermochemolysis, at temperatures of 3OO”C,can induce ether/ester cleavage or elimination reactions with either concomitant or subsequent methylation of oxygen functionalities. The volatile methylated products can be used for the characterization of lignin in various plant types. At these lower temperatures, the method can be performed in a sealed tube instead of the pyrolysis system used in prior studies. This allows for the addition of internal standards for quantitative analysis. We report here results of preliminary studies involving TMAH thermochemolysis at 300°C in a sealed tube which provides quantitative characterization of lignin in wood samples, and compare this procedure with TMAH treatment at pyrolysis temperatures using a resistively-heated platinum filament pyrolysis probe.

2. Samples and methods

The sample used for this investigation was a degraded Douglas fir wood described previously by NMR spectroscopy [ 111. The wood sample was devoid of cellulosic material (denoted by NMR) due to microbiological degradation [ 111. The lyophilyzed sample was ground to a powder using a mortar and pestle and used directly. In situ methylation under pyrolysis conditions was carried out following the procedure described by Challinor [ 11. The degraded wood sample, approximately 200 ng, was placed in a quartz pyrolysis boat and covered with 4 ~1 of TMAH (35% in water). The quartz boat was then placed between the coils of a Chemical Data System Pyroprobe 1000 and inserted into the injection port of a Varian 2700 gas chromatograph. The injection port temperature was maintained at 280°C. A 15 s mixing time, as suggested by Anderson and Winans [ 121, was allowed to limit nitrogen-containing products. The pyroprobe temperature was then ramped at

D.E. McKinney

el ul.

1J. And. Appl. Pyrolysis 34 (1995) 4lL46

33

S”C/ms to 610°C and held for 10 s. To distinguish products obtained at pyrolysis temperatures of 610°C from possible thermochemolysis products, the degraded Douglas fir sample was also analyzed at 300°C by the same system. Volatiles were swept into a 25 m x 0.25 mm i.d. J&W DB-17 capillary column, whose front end was immersed in liquid nitrogen to cryofocus pyrolysis products, and were chromatographed from an initial temperature of 30 to 280, C. Mass spectra were collected on a DuPont 21-490B mass spectrometer directly coupled to the gas chromatograph. Data acquisition and analysis were obtained using a Teknivent Vector/One data system interfaced to the mass spectrometer. Structure assignments for the various peaks were based on literature reported mass spectra and/or NBS/Wiley library mass spectra. Tentative assignments were based on mass fragmentation interpretation. Batch thermochemolyses were conducted in a sealed Pyrex tube which contained the sample of degraded Douglas fir wood (3 mg). an internal standard for quantitation ( 150 ~41of o&o-hydroxycinnamic acid in methanol at a concentration of 192 ng//cl) and an excess of TMAH pentahydrate ( 12 mg). The methanol was removed under vacuum and the glass tube sealed under vacuum. The mixture was heated at 3OOC in a gas chromatographic oven for 10 min. After cooling, the tube was cracked open and all inside surfaces washed out with methylene chloride (3 x I ml). and the extracts were combined and reduced to dryness under a stream of N,. The sample was then diluted with a known volume of methylene chloride (500 ill). The diluted sample (1 ~1) was analyzed by capillary gas chromatography/mass spectrometry (GC/MS) on a Kratos MS-80 RFA high-resolution gas chromatograph/mass spectrometer system. The column used for GC separation was a 30 m x 0.25 mm i.d. fused silica capillary column (Restek Rtx-5). The diluted sample was injected onto a split/splitless injector operating in the splitless mode, and the column temperature was programmed from 60 to 280°C at a heating rate of 4”C/min after a 5 min isothermal period. The ionization mode on the mass spectrometer was electron impact (ET, 70 eV). Data acquisition and analysis were accomplished using a Kratos DS90 system coupled to a Kratos Mach 3 system.

3. Results and discussion Figs. 1 and 2 show the chromatograms obtained from a degraded Douglas fir sample using three variations of the TMAH/thermochemolysis procedure. Fig. l(a) is the traditional flash pyrolysis at 610°C with in situ methylation in the presence of TMAH. The dominant peaks are identified as methylated derivatives, with methylation occurring at the oxygens in the four-position of the aromatic ring and oxygens on the three-carbon side chain of the 4-hydroxy-3-methoxycinnamyl sub-units of the lignin. The major peaks are those of 3,4_dimethoxybenzaldehyde (G4). 3.4dimethoxyacetophenone (G5), 3,4-dimethoxybenzoic acid methyl ester (G6), and the cis and trans isomers of /I,3,4_trimethoxystyrene (G7 and G8). Less intense peaks are observed for 3,4_dimethoxystyrene (G3), the Z and E isomers of x, 3,4-trimethoxy-/I-methylstyrene (GlO and Gl l), 3,4-dimethoxyhydrocinnamic acid

44

D.E. McKinney

el al. / J. Anal. Appl. Pyrolysis

34 (1995) 41-46

G6

b)

q

20 Time (min)

Fig. 1. Gas chromatograms of a degraded Douglas at 610°C and (b) pyrolysis+GC/MS at 300°C.

fir treated

with TMAH

using: (a) pyrolysis-CC/MS

methyl ester (G12), l-(3,4-dimethoxyphenyl)-3-methoxy-1-propene (G13), two diastereoisomers of l-( 3,4-dimethoxyphenyl)-1,2,3-trimethoxypropane (G14 and G15), and cis-1-(4-methoxyphenyl)-1,3-dimethoxy-1-propene (G16). From this distribution of products, it is clear that the TMAH procedure induces fragmentation of the ether cross links (such as the b-0-4 bonds) in lignin and methylation of phenolic, alcoholic, or carboxylic acid groups produced, resulting in the formation of a series of dimethoxybenzenes having a one-, two-, or three-carbon side chain. The side-chain carbons are either methoxylated ethylenes or propenes and propanes. The presence of the trimethoxypropane derivatives is indicative of either hydrolysis with subsequent methylation or methanolysis of carbocation intermediates. Methoxypropene and styrene derivatives indicate thermochemolysis

Fig. 2. Gas chromatogram of a degraded Douglas fir treated with TMAH ysis in an evacuated and sealed tube heated at 300°C for 10 min.

using off-line thermochemol-

D.E. McKinney et al. 1 J. Anal. Appl. Pyroiwis 34 (1995) 41 46

3s

resulting from elimination reactions. Formation of the aldehyde, and methyl ketone derivatives is entirely consistent with cleavage of the b-O-4 bond followed by elimination reactions involving the side-chain unit. The presence of the benzoic acid methyl ester derivative is probably indicative of lignin which has been partially oxidized at the r carbon. In comparing the distribution of products from conventional flash pyrolysis for this sample [ 131 with the distribution obtained from thermochemolysis at 610°C one can observe that they are vastly different due to the difference in reactions occuring with the two different techniques. The major products from conventional flash pyrolysis are guaiacol, 4-methylguaiacol. 4-vinylguaiacol, and the two isoeugenol isomers. It is difficult to compare these two techniques without proper knowledge of the chemistry involved in the thermochemolysis procedure. It is clear that the chemistry involved in formation of the various products from lignin is complex and is yet to be defined using model compounds. Thermochemolysis of the degraded Douglas fir at a reduced temperature of 300 C in the CDS pyroprobe, a temperature too low for significant pyrolysis, produces a nearly identical chromatogram to that obtained at 610 C (Fig. 1(b)). A similar result was obtained by Clifford et al. [lo] with a sample of fresh alder wood. Clearly, the reaction is largely a heat-induced chemolysis causing cleavage or elimination of the D-O-4 bonds in lignin. Considering that chemolysis occurs at this reduced temperature in the pyroprobe system, we investigated the possibility that the pyroprobe system was unnecessary. Fig. 2 shows a gas chromatogram of degraded Douglas fir treated with TMAH in an evacuated and sealed tube heated at 300°C for 10 min. The products resulting from the “off-line” thermochemolysis yielded a chromatogram nearly identical to that obtained by pyroprobe analysis. The only significant difference is the presence of a peak for 2-methoxycinnamic acid methyl ester derived from the o-methoxycinnamic acid added to the sample mixture as an internal standard. We can therefore conclude that the flash pyrolysis system is unnecessary for obtaining TMAH thermochemolysis products from lignin. Approximate yields of products were determined for each different thermochemolysis procedure to evaluate the relative ease with which methylated lignin analogs are produced. Because it is difficult to carry out quantitative pyrolysis and authentic standards are as yet unavailable for these methylated lignin analogs. we found it necessary to perform our calculations using a unit response factor for each compound identified and an external calibration was used for the experiments conducted in the pyroprobe. However, we feel at this time that it would be unwarranted for us to publish any calculated concentrations of lignin derivatives until correct standards and response factors can be assigned. Nonetheless. our approximate yields of methylated lignin analogs for the three different procedures indicate less than a factor of two difference in concentrations of methylated lignin analogs between the methods. The ability to perform the TMAH thermochemolysis in a batch system with addition of internal standards provides some significant advantages over the more traditional method involving flash pyrolysis apparatus. First, the addition of

46

D.E. McKinney et al. 1 J. Anal. Appl. Pyrolysis 34 (1995) 41-46

standards allows for quantitative characterization of the thermochemolysis products, a capability not readily attainable with flash pyrolysis systems. Second, the batch operation allows for large numbers of analyses to be conducted simultaneously without the need for flash pyrolysis equipment. Only gas chromatographic equipment is required.

Acknowledgments

We appreciate Bortiatynski.

the technical assistance of Scott D. Dible and Jacqueline

M.

References [I] [2] [3] [4] [S] [6] [7] [8] [9] [lo] [ll] [12] [13]

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