Furfural, pyrolysis

Manufacture. Furan is produced commercially by decarbonylation of furfural in the presence of a noble metal catalyst (97—100). Nickel or cobalt catalysts have also been reported (101—103) as weU as noncatalytic pyrolysis at high temperature. Furan can also be prepared by decarboxylation of 2-furoic acid this method is usually considered a laboratory procedure. 

Several cases of spontaneous ignition after exposure to air of fine coke particles removed from filter strainers on a petroleum refinery furfural extraction unit have been noted. This has been associated with the use of sodium hydrogen carbonate (bicarbonate) injected into the plant for pH control, which produced a pH of 10.5 locally. This would tend to resinify the aldehyde, but there is also the possibility of a Cannizzaro reaction causing conversion of the aldehyde to furfuryl alcohol and furoic acid. The latter, together with other acidic products of autoxidation of the aldehyde, would tend to resinily the furfuryl alcohol. Pyrolysis GLC showed the presence of a significant proportion of furfuryl alcohol-derived resins in the coke. The latter is now discarded into drums of water, immediately after discharge from the strainers, to prevent further incidents. 

The wood pyrolysis is attractive because forest and industrial wood residues can be readily converted into liqtrid products. These liqtrids, as erode bio-oil or slurry of charcoal of water or oil, have advantages in transport, storage, combustion, retrofitting and flexibility in production and marketing (Demirbas, 2007). In the first step of pyrolysis of carbohydrates dehydration occtrrs and at low temperatures dehydration predominates. Dehydration is also known as a char-forming reaction. Between 550 and 675 K volatile products, tar, and char are formed. The volatile products are CO, CO, H O, acetals, furfural, aldehydes and ketones. Levoglucosan is the principle component in tar. 

Fig. 5.2(A) presents the pyrolysis mass spectrum for the soil extract. In previous work (ref. 358,359,365) it was shown that complex organic materials like polysaccharides, proteins, lignins, and soil humic fractions have characteristic peaks yielding a typical pattern, which give preliminary information about the composition of the pyrolysis fragments. Thus, characteristic peaks for polysaccharides were observed at 60, 68, 82, 84, 96, 98, 110, 112, and 126 m/z, which were also present in the soil extract. They were shown to be related to acetic acid, furan, methylfuran, hydroxyfuran, furfural, furfuryl alcohol, methylfurfural, methoxy-methylfuran, and a typical pyrolysis fragment of polysaccharides with hexose and/or deoxyhexose units, respectively.

Considering the volatiles that evolve from hot pressing and heat-treating operations, it is concluded that the major chemical changes occurring in this stage are pyrolytic. Methanol, acetic acid, furfural, and ligneous tars are the common volatiles produced by the slow pyrolysis of wood practiced in destructive-distillation processes. The temperatures used in the board conversion operations approach pyrolysis temperatures of wood and the evidence indicates that pyrolysis is indeed active in board conversion. 

From the molecular beam MS of the pyrolysis products of the P/N fractions, a number of phenolic compounds were detected guaiacol (2-methoxyphenol) (m/z 124), catechols (m/z 110), isomers of substituted 2-methoxyphenols with alkyl groups such as methyl (m/z 138), vinyl (m/z 150), 3-hydroxy-propen(l)-yl (m/z 180), allyl (m/z 164), hydroxyethyl (m/z 168), and ethyl (152), most likely in the para position. In addition, a few carbohydrate-derived components are also present in this fraction such as furfuryl alcohol and other furfural derivatives. 

The furanic aldehydes 5-(hydroxy-methyl)furfural and 2-furaldehyde, systematically present in the toasted wood, can be formed by the thermal degradation of 3-deoxyosone during sugar pyrolysis or Maillard reactions (27). They could also be formed from glyceraldehyde, coming from degradation of DDMP, by condensation with subsequent elimination of water or formaldehyde (24). 

Dehydration Reactions. Detailed analysis of the pyrolysis tar as discussed previously (Figure 12 and Scheme 3) shows the presence of levoglucosan, its furanose isomer (1,6-anhydro-p-D-glucofuranose) and their transglycosylation products as the main components. In addition to these compounds, the pyrolyzate contains minor amounts of a variety of products formed from dehydration of the glucose units. The dehydration products detected include 3-deoxy-o-erythrohexo-sulose, 5-hydroxymethyl-2-furaldehyde, 2-furaldehyde (furfural), other furan derivatives, levoglucosenone (l,6-anhydro-3,4-dideoxy-P-D-glycerohex-3-enopyranos-2-ulose), l,5-anhydro-4-deoxy-D-hex-l-ene-3-ulose, and other pyran derivatives. The dehydration products are important as intermediate compounds in char formation. 

Figure 103. Pyrolysis Data for Furfural as Obtained by a Flow Test.

Noteworthily, at a contact time of 0.2 seconds, only 5 percent of the furfural decomposes at 660 °C. At such elevated temperatures, furfural decomposes to furan and carbon monoxide, in complete analogy to benzaldehyde which decomposes to benzene and carbon monoxide. The best conditions for the production of furan by pyrolysis of furfural were found to be 725 °C and a contact time of five seconds. Such a process gives a furan yield of 16.5 percent. 

As all pyrolysis reactions, the decomposition of furfural to furan and carbon monoxide is accompanied by other minor reactions. Of the gases formed, only 80 percent is carbon monoxide, other gases identified being 10 percent hydrogen as well as small quantities of carbon dioxide, butadiene, propadiene, ethylene, propylene, acetylene, methylacetylene, and cyclopropene. As the quantities of carbon dioxide and butadiene are roughly equal on a molar basis, it is believed that a part of the furfural reacts with pyrolytically liberated hydrogen ... 

Two pyrolysis products that are formed during pectin pyrolysis are furfural (2-furancarboxaldehyde, 2-furaldehyde) and 4-(hydroxymethyl)-1,4-butyrolactone. The proportion of the butyrolactone compared to that of furaldehyde in the pyrolysis products of pectin was found to correlate with the methylation degree of pectin [6]. The formation of 2-furaldehyde from the galacturonic unit probably takes place with the following mechanism (hydrogens are shown with shorter bonds) ... 

Several Py-MS studies were done on homoglycans. A Py-FI MS study on a (1- 4)-p-xylan [2] showed several characteristic ions such as m/z = 132 (associated with the monomer unit C5H8O4), the ion m/z = 96 (associated with furfural, C5H4O2), and the ion m/z = 114 (probably associated with 3-hydroxy-2-penteno-1,5-lactone, CsHsOs). The study showed that retro-aldolization between C-1 and C-2 atoms does not take place. Smaller ions seen in the spectrum were probably generated from further pyrolysis of the initially formed fragments. 

As can be seen from Figures 12.3.1 and 12.3.2 and from Table 12.3.1, the carbohydrate moiety is not detected because furfural, acetol, and other typical pyrolysis products from sugars are absent. However, in these pyrograms only the more volatile components can be seen. Several less volatile components can be seen in a GC analysis only after the pyrolysate is derivatized, for example, to generate trimethylsilyl derivatives (TMS). The chromatogram of the silylated pyrolysate (pyrolysis done at 510° C) from egg albumin is shown in Figure 12.3.3, and that from bovine serum albumin in Figure 12.3.4... 

Hydroxyacetic aldehyde, hydroxypropanone, furfural and 5-methyl-furfural are pyrolysis products of carbohydrates (JA), but the ab-... 

A new preparative synthesis of 2-arylquinazolin-4(3//)-ones, which was reported recently, involved the pyrolysis of Schiff bases derived from 3-amino-l,2,3-benzotriazin-4-one in paraffin oil at 300 C, or in boiling 1-methylnaphthalene. The yields were as high as 86-100% but failed entirely when p-HO-, P-O2N-, and p-Me2N-benzaldehyde, furfural, and pyridine-2-aldehyde were used. The mechanism in Eq. (5) was proposed although the... 

The main compounds yielded by flash pyrolysis are transformation or degradation products of biogenic precursors. Next to the amino acid glycine I most of the identified compounds are structurally related to carbohydrates, amino acids and condensed molecules of both components resulting from Maillard reactions. Examples include furfural 2, methylfurfural 3 and pyrrol-2-carboxaldehyde 4 (see Fig. 3). 

The study has concentrated on the biologically active substances produced by pyrolysis, in particular the Hoffmann analytes. These analytes are believed by regulatory authorities in Canada and U.S.A. to be relevant to smoking-related diseases. They are based on lists published by Hoffmann and co-workers of the American Health Foundation in New York. For the pyrolysis of many of the non-volatile ingredients, no Hoffmann analytes were detected amongst the products. When they were occasionally formed, they included phenols, benzene, toluene, styrene and furfural (furfural is biologically active but it does not appear on any of the Hoffmann or regulatory authority lists). 

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