Influence of Ionic Additives on the Pyrolysis Behavior of Paper

In the course of this study the influence of ionic additives (sodium, potassium, lithium, magnesium, and manganese as cations; acetate, lactate, malate, malonate, succinate, and citrate as anions) on the pattern of volatile pyrolysis products of finished paper is investigated. The pyrolysis of paper causes a cascade of reaction products. As expected, the most abundant pyrolysis product is levoglucosan, however, along with other volatile products, such as hydroxyl and carbonyl compounds, furan and pyran derivatives, phenols, and other anhydrosugars, respectively. These compounds can easily be separated and characterized online using analytical pyrolysis in combination with gas chromatography (GC) and mass spectrometry (MS) detection. Both the composition and total amount of volatile pyrolysis products are significantly altered when the paper samples contain metal salt ions and salts of organic acids, respectively. Principal Component Analysis (PCA) was employed for the multivariate analysis of the obtained pyrolysis products. This allows for a qualitative interpretation on how the various ionic additives affect the formation of specific pyrolysis products. When organic acids are added onto the paper, the pyrolysis pattern mainly depends on the protic properties of the organic acids (mono/di/triprotic) and to a lesser extent on the type within a protic class (monoprotic acetate or lactate vs.diprotic malate or malonate or succinate vs. triprotic citrate). The pyrolysis pattern of the paper samples is more markedly influenced by the type of metal ions rather than by the type of organic acid. These effects significantly depend on both the valence and the concentration of the specific metal salt.


Introduction
The thermal degradation of wood and wood products has long been of interest not only towards the understanding of the underlying mechanisms of the combustion chemistry, but also with respect to the formation of the combustion products. On the contrary, finished paper has only been rarely investigated towards its pyrolysis products. Besides cellulose, finished paper often consists of a significant part of filler (e.g. ground and precipitated calcium carbonate, titanium oxide, kaolin, talcum). Depending on the type of paper, the filler contents can reach up to 40% -50% (w/w) of the paper substance. Filler is added to the paper for various purposes, such as reducing costs, increasing brightness and other optical properties, improvement of sheet formation and dimensional stability of the fiber sheet, and for improving printability.
However, literature on the pyrolysis of finished paper products is rare. Mainly thermogravimetric analysis has been described, reporting the formation of carbon monoxide, carbon dioxide, water, hydrocarbons, and char at different heating rates and atmospheres [1]. Gupta et al. [2] describe the dependence of degradation temperature on heating rate and atmosphere. Baldry et al. [3] report on the formation of carbon monoxide and char when cigarette paper, impregnated with organic and inorganic salts, is pyrolyzed.
On the contrary, reports on the pyrolysis of cellulose are numerous. In an early report, Shafizadeh [4] describes the reaction mechanisms of the cellulose backbone degradation and the formation of levoglucosan as an important product. Arseneau [5] reports specific degradation mechanisms of cellulose using thermogravimetric and differential-thermoanalytical methods. The further degradation of levoglucosan to carbon monoxide, carbon dioxide, and other volatile carbonyl compounds, as well as the existence of acid and base catalyzed reaction paths was investigated by Shafizadeh et al. [6].
Kinetic investigations have been reported by Bradbury et al. [7] and Agrawal [8] [9] assuming a 3-reaction-model where activated cellulose is involved in the further degradation. Diebold [10] published a model involving seven reaction steps. The investigations of Ball et al. [11] considered heating rate, gas flow and water vapour on the reaction cascade forming volatile products. Pouwels et al. [12] identified numerous pyrolysis products using Curie-point pyrolysis and GC/MS. Essig et al. [13] showed that the presence of NaCl reduces the amount of levoglucosan produced during hydrolysis while the amount of glycoaldehyde is increasing. They also investigated the presence of other salts, such as sodium carbonate, sodium sulfate, magnesium chloride, and cesium chloride, as well as of oxygen, methanol, and propanol in the pyroylsis atmosphere on the formation of levoglucosan.
The presence of ammonium sulfate and ammonium phosphate, respectively, causes a reduced formation rate of levoglucosan and an increase of water, carbon dioxide, and char, respectively [14]. Dobele et al. [15] report about the influence American Journal of Analytical Chemistry of phosphoric acid and iron (III) ions on the formation of levoglusocsan and levoglucosenone from cellulose. Low concentrations of phosphoric acid below 1% favour the formation of levoglucosan, whereas higher concentrations of phosphoric acid above 2% produce more levoglucosenone. The presence of iron (III) ions cause an increased level of levoglucosan compared to untreated cellulose.
In a recent review Dale [16] reports on cellulose pyrolysis kinetics and the existence of intermediate active cellulose. The influence of the chain length of cellulose and of end-group effects during isothermal pyrolysis is subject of a recent paper by Mettler et al. [17]. The conversion of cellulose to furans and small oxygenates is also described by the same group [18]. A comprehensive overview on the pyrolysis of organic molecules is given by Moldoveanu [19].  [24], only little has been reported on the pyrolysis of finished paper containing ionic additives or contaminants [25].
In this paper, the basic focus is primarily laid on receiving a general picture of the effects of anionic and cationic additives on volatile pyrolysis products of paper during online-pyrolysis. For this purpose, principal component analysis (PCA) is applied to interpret the pyrolysis data. By this means, the pyrolysis products can be correlated with respective samples preferably producing more or less of these specific compounds.

Chemicals
All chemicals used in this investigation were of analytical reagent grade.

Paper Samples
For the impregnation experiments a single grade low substance paper (23 g/m 2 ) was used. The paper consisted of 72% alpha cellulose and 28% precipitated calcium carbonate. Papers with a high amount of filler (calcium carbonate) have specific properties, such as a high opacity and brightness.
The finished paper was then impregnated with solutions containing the ions of interest. The molar on-paper concentrations of the cations (Li + , Na + , K + , Mg 2+ , and Mn 2+ , respectively) were adjusted to 0.05, 0.25, and 0.50 mmol/g, respec- tively. The on-paper concentrations of the anions (acetates, lactates, malates, malonates, succinates, and citrates, respectively) were adjusted to 0.25 mmol/g.
The on-paper concentrations of the chemicals were determined using capillary electrophoresis after extraction of the papers with ultra-pure water. For this purpose, 300 mg paper was suspended in 20 mL water and sonication for 15 minutes. An aliquot was then analyzed using the methods described in chapters 5.1 and 5.2 in reference [26].
After impregnation, paper samples were cut to pieces of uniform size (5 mm diameter) using an office puncher.

Pyrolysis Gas Chromatography Mass Spectrometry (Py-GC-MS)
The cut paper samples were pyrolyzed using a Thermo Desorption System (TDS) and an online pyrolysis module PM1 (

Data Analysis
Prior to data analysis, the peak areas of the paper samples were corrected in order to compensate for the steady decrease of peak areas due to the MS intensity decline over the project time. The correction factors for each set of papers were calculated by comparison with the repeatedly measured reference papers.
The data matrix consisting of sample ID (rows) and peak areas of the pyrolysis products (columns) was then analyzed by Principal Component Analysis (PCA) using the software package The Unscrambler X 10.1 64 bit (Camo, Oslo, Norway). Prior to PCA, all peak areas were normalized by their standard deviations in order to compensate for the huge variance in the absolute peak areas of the analytes.

Results and Discussions
The purpose of this study was to analyze the pyrolysis products of paper samples which have been impregnated with various salt solutions each containing either anionic (mono, di, and triprotic organic acids) or cationic (mono and diprotic inorganic cations) species at various concentrations. The specific goal was to find certain qualitative patterns of the pyrolysis products in dependence of the employed salt solutions.
In the present study the pyrolysis of paper leads to a variety of different reaction products. Table 1 lists the compounds which can be found in a typical pyrogram of paper at 500˚C. The compounds have been divided in various classes (aliphatic carbonyls, cyclic carbonyls, furans, pyrans, cyclic carbonyls, cyclic alcohol, acids, phenols, and anhydro sugars), however, considering that some of the analytes belong to more than one class of compounds. The extent to which certain pyrolysis products are formed not only depends on the respective pyrolysis conditions but also on type and concentration of the employed paper additives. For a qualitative analysis of the pyrolysis data multivariate data analysis was used. The primary scope of this investigation was to elucidate how the different cationic and ionic additives cause a predominant or suppressed formation of certain pyrolysis products.
K. Stadlmann et al.   Table 1. Concentrations of impregnation compounds were adjusted to 0.25 mmol/g paper.

Principal Component Analysis (PCA)
46 different pyrolysis products as listed in Table 1 were investigated in the present study. With the experimental setup (111 pyrolysis experiments) these compounds would form a 46 dimensional data space under the assumption that they were completely independent from each other without any correlation whatsoever. However, the probability that all 46 compounds formed during pyrolysis of the paper samples are completely orthogonal and independent from each other is very low. On the contrary, it is also highly unlikely that the pyroly- In the scores plot, the individual data points represent different pyrolysis experiments and also indicate regions of similar properties. Paper samples with their respective additives which cause similar pyrolysis patterns are located closer together than samples which produce different pyrolysis products.

Effect of Anionic Additives
The graphs in Figure 2 show the scores plot (above) and loadings plot (below) of a PCA of the pyrolysis data of the anion impregnation series. More than three quarters (76%) of the variance of the original data space (46 variables, 48 samples) are explained by only two dimensions (PC1 59%, PC2 17%). This means that many of the pyrolysis products are at least partly correlated and the most important effects can be explained with a high amount of statistical probability using a projection of the data on only two dimensions.
At first sight, the scores plot of the anion series (top of Figure 2  The reference samples are located in the left hand side of the scores plot and separate mainly along the PC 1 axis from the impregnated papers which are located in the middle region of the PC 1 axis close to the origin. This separation of reference papers and impregnated papers, respectively, along the PC 1 axis is mainly caused by the different pyrolysis products and accounts for 59% of the variance of the original data set. The subsequent separation within the paper samples along the PC 2 axis is significantly less marked (17%) and is obviously due to the variation caused by the different classes of organic acids used for impregnation.
The lower graph in Figure 2  In addition, other effects become obvious in the scores plot of Figure 2. At first, the pyrograms of papers treated with the sodium salt of the same acid differ slightly from the potassium salts. Potassium salt containing papers are located slightly above the respective sodium salts in Figure 2. This means that during pyrolysis they produce a slightly higher concentration of specific furans (3-vinyl furan, furfurylalcohol, 2-acetyl furan), cyclic carbonyls (2-hydroxy-γ-butyrolactone, hydroxyl benzaldehyde, hydroquinone), and propionic acid.
At second, the pyrograms of papers with the mono sodium (or mono potassium) salt of citrate and malate, respectively, differ from the pyrograms of the respective fully saturated salts (trisodium and tripotassium citrate, disodium and dipotassium malate, respectively). This can be illustrated in more detail after a PCA calculation leaving out the reference samples. In the scores plot of Figure 3 the impregnated the paper samples separate according to their protic character, with papers containing monoprotic acids (acetate and lactate) at the left hand side of the PC 1 and papers impregnated with a triprotic acid (citrate) at the right hand side. Although the explained variance in this PCA (47% and 18%,  Figure 3. Scores (above) and Correlation Loadings (below) Plots of the pyrolysis data from papers samples impregnated with various anionic salt solutions without reference samples. Color assignments of Scores and Loadings Plots do not correspond! respectively), is lower for the first two PC's compared to Figure 2 it is sufficiently high for the required purpose of explaining the effects. The mono sodium and mono potassium salts of citrate and malate, respectively, can be clearly distinguished from the respective fully saturated salts.
Most of the pyrolysis products are located in the left half (negative PC 1) of the correlation loadings plot in Figure 3. When compared to the scores plot, this region generally corresponds to the papers impregnated with single or divalent anions. Separated into the classes of compounds, cyclic alcohols and aldehydes, as well as acids are mainly produced by the acetate and lactate samples (both sodium and potassium), respectively, which are located in the left hand side of the loadings plot. Anhydrosugars (except for levoglucosan) come to lie in the upper left quadrant and correspond to the potassium salts of the monovalent acids. Phenols are clustered in the right lower quadrant which matches the region where the monosodium salts of citrate and malate are found in the scores plot.
Papers impregnated with the salts of higher charged carboyxylic acids (far positive side of PC1) seem to reduce the complexity of the pyrograms leading to a considerably lower variety of volatile pyrolysis products. The concentration of levoglucosan (#46), however, is not significantly influenced by one or the other impregnation salt as indicated by its position in the loadings plot of Figure 3 (near the PC 2 axis within the inner circle of correlation).
This behavior indicates that the degree to which the acid anions are saturated by metal cations mainly determine the pattern of the pyrolysis products rather than the formal protic character of the acid anions.
It is known from the literature that the presence of alkaline cations significantly affects the mechanisms of thermal degradation of lignocellulosic materials. The cations cause a fragmentation of the cellulosic backbone rather than a depolymerization [27]- [32]. In addition, gas yields increase while liquid yields decrease. Thus it can be assumed that the catalytic effect from additives is due to the involved cations rather than the type of acid anion.

Effect of Cationic Additives
In order to investigate the effect of various types of cations on the pyrolysis pattern in more detail, papers impregnated with solutions of acetates of monovalent (Li + , Na + , and K + ) and divalent (Mg 2+ , Mn 2+ ) cations, respectively. Similar to acid anions of different formal valence, the pyrolysis pattern of the paper samples  generating 46 pyrolysis products can be reduced to a two dimensional plot still explaining almost 80% of the total variance of the whole experimental space.
The biggest influence in the cationic scores plot, however, can be attributed to the different concentrations of the cations on the papers rather than to the type of additive. With increasing concentrations, the paper samples clear away from the reference samples moving towards the right hand side of the plot along the PC 1. Obviously, monovalent cations have a much stronger influence on the py-rolysis behavior of papers compared to divalent cations. The lowest concentration of the series of monovalent cations is located in the middle of PC 1, at roughly the same position where the highest concentration of the series of the divalent cations ends. This depicts the stronger influence of monovalent cations. In addition, mono and divalent cations separate along the PC 2 axis and can be distinguished by specific pyrolysis products (lower graph in Figure 4).
Pyrans and furans are produced at higher concentrations by paper samples impregnated with low and medium concentrations of the divalent cations (left hand side of the lower left quadrant). Sugars and acids (except for propionic acid) cluster in the outer left positive region of PC 1, corresponding to the reference samples. On the opposite side, high concentrations of monovalent cations cause significantly higher concentrations of phenols and specific cyclic carbonyls (2-hydroxyγ-butyrolactone, hydroxy benzaldehyde, hydroquinone, 2-ethyl-hydroquinone, 2-hydroxy-3-methyl-2-cyclopentenone, 2-hydroxy-3-ethyl-2-cyclopentenone, 2-hydroxy-3-ethyl-2-cyclopentenone) during pyrolysis. In contrast to the samples impregnated with anionic additives, the concentration of levoglucosan (left upper quadrant in the loadings plot of Figure 4) is reduced by all types and concentration levels of cationic additives. This is in well accordance with the literature where alkaline and earth alkaline ions in wood induce low levels of levoglucosan [32]. These cations also induce high char and low tar yields.

Combined Anionic and Cationic Effects
In order to distinguish between the effects of anions and cations on the pyrolysis patterns, the pyrolysis products of both series are analyzed together and discussed using one model.
The upper graph of Figure 5 depicts the combined experiments of both anionic and cationic additives. The first two PC's explain 76% of the total variance of the chromatographic data (PC 1: 63%; PC 2: 13%). For the cationic additives the scores plot looks almost identical to the scores plot in Figure 4 (without anionic additives). When compared to the anionic data set in Figure 2, the area with the anionic additives looks almost similar, however, only slightly shifted towards the positive side of the PC 1 axis.
This indicates that the pyrolysis pattern is predominantly determined by the type and concentration of the cationic additives and to a much lesser extent by the anionic additive.
The higher the on-paper concentration of an additive is chosen (especially for the acid anion and the monovalent cations) the lower concentrations of the compounds are produced during pyrolysis. These compounds are located on the left-hand side of the loadings plot in Figure 5 (acids, sugars, pyrans, furans).

Conclusions
The results of the pyrolysis of a series of papers impregnated with certain concentrations of mono, di, and triprotic anions and of mono and divalent cations