Pyrolysis Catalyst

Fluid catalytic cracking (FCC) has been used since the 1950s to turn heavy distillates (vacuum gas oil) into a series of light and dense fractions. The FCC catalysts can also be used as pyrolysis catalysts. 

Ruddy DA, Schaidle JA, FerreU JRI, Wang J, Moens L, Hensley JE. Recent advances in heterogeneous catalysts for bio-oU upgrading via ex situ catalytic fast pyrolysis catalyst development through the study of model compounds. Green Chem 2014 16 454-90. 

The yield of products from catalytic pyrolysis depends on catalyst type and catalyst to feed ratios besides pyrolysis temperature and residence time (Ojha and Vinu, 2015). Different types of catalysts have different properties such as surface acidity, specific surface area, pore size, and pore size distributions which also determine the yield and selectivity of various products. The range of different functionalities of the catalysts should be matched to the various pyrolysis feedstocks as each feedstock may have a preferred pyrolysis catalyst. Therefore research has been precise in developing particular catalysts for specific raw material (depending on pyrolysis reactor). The section below describes the properties and effect specific catalysts have on pyrolysis vapor upgrading. 

Many of the reactions listed at the beginning of this section are acid catalyzed, although a number of basic catalysts are also employed. Esterifications are equilibrium reactions, and the reactions are often carried out at elevated temperatures for favorable rate and equilibrium constants and to shift the equilibrium in favor of the polymer by volatilization of the by-product molecules. An undesired feature of higher polymerization temperatures is the increased probability of side reactions such as the dehydration of the diol or the pyrolysis of the ester. Basic catalysts produce less of the undesirable side reactions. 

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. 

Vlayl fluoride [75-02-5] (VF) (fluoroethene) is a colorless gas at ambient conditions. It was first prepared by reaction of l,l-difluoro-2-bromoethane [359-07-9] with ziac (1). Most approaches to vinyl fluoride synthesis have employed reactions of acetylene [74-86-2] with hydrogen fluoride (HF) either directly (2—5) or utilizing catalysts (3,6—10). Other routes have iavolved ethylene [74-85-1] and HF (11), pyrolysis of 1,1-difluoroethane [624-72-6] (12,13) and fluorochloroethanes (14—18), reaction of 1,1-difluoroethane with acetylene (19,20), and halogen exchange of vinyl chloride [75-01-4] with HF (21—23). Physical properties of vinyl fluoride are given ia Table 1. 

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic mbbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high i i -polyisoprene which dupHcates the stmcture of natural mbber. In one method (3,4) a Ziegler-type catalyst of tri alkyl aluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all i7j -l,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

Propylene Dimer. The synthesis of isoprene from propjiene (109,110) is a three-step process. The propjiene is dimeri2ed to 2-methyl-1-pentene, which is then isomeri2ed to 2-methyl-2-pentene in the vapor phase over siUca alumina catalyst. The last step is the pyrolysis of 2-methyi-2-pentene in a cracking furnace in the presence of (NH 2 (111,112). Isoprene is recovered from the resulting mixture by conventional distillation. 

The production of ketene by this method has no significant environmental impact. The off-gases from the ketene furnace are either circulated to the furnace and burned to save energy or led to a flare system. The reaction can also be carried out at 350—550°C in the presence of alkaH-exchanged zeoHte catalysts (54). Small quantities of ketene are prepared by pyrolysis of acetone [67-64-1] at 500—700°C in a commercially available ketene lamp (55,56). 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

An excess of crotonaldehyde or aUphatic, ahcyhc, and aromatic hydrocarbons and their derivatives is used as a solvent to produce compounds of molecular weights of 1000—5000 (25—28). After removal of unreacted components and solvent, the adduct referred to as polyester is decomposed in acidic media or by pyrolysis (29—36). Proper operation of acidic decomposition can give high yields of pure /n j ,/n7 j -2,4-hexadienoic acid, whereas the pyrolysis gives a mixture of isomers that must be converted to the pure trans,trans form. The thermal decomposition is carried out in the presence of alkaU or amine catalysts. A simultaneous codistillation of the sorbic acid as it forms and the component used as the solvent can simplify the process scheme. The catalyst remains in the reaction batch. Suitable solvents and entraining agents include most inert Hquids that bod at 200—300°C, eg, aUphatic hydrocarbons. When the polyester is spHt thermally at 170—180°C and the sorbic acid is distilled direcdy with the solvent, production and purification can be combined in a single step. The solvent can be reused after removal of the sorbic acid (34). The isomeric mixture can be converted to the thermodynamically more stable trans,trans form in the presence of iodine, alkaU, or sulfuric or hydrochloric acid (37,38). 

Another important use of a-pinene is the hydrogenation to i j -pinane (21). One use of the i j -pinane is based on oxidation to cis- and /n j -pinane hydroperoxide and their subsequent catalytic reduction to cis- and /n j -pinanol (22 and 23) in about an 80 20 ratio (53,54). Pyrolysis of the i j -pinanol is an important route to linalool overall the yield of linalool (3) from a-pinene is about 30%. Linalool can be readily isomerized to nerol and geraniol using an ortho vanadate catalyst (55). Because the isomerization is an equiUbrium process, use of borate esters in the process improves the yield of nerol and geraniol to as high as 90% (56). 

Isoprene (2-methyl-1,3-butadiene) can be telomerized in diethylamine with / -butyUithium as the catalyst to a mixture of A/,N-diethylneryl- and geranylamines. Oxidation of the amines with hydrogen peroxide gives the amine oxides, which, by the Meisenheimer rearrangement and subsequent pyrolysis, produce linalool in an overall yield of about 70% (127—129). 

SoHd by-products include sludge from wastewater treatment, spent catalyst, and coke from the EDC pyrolysis process. These need to be disposed of in an environmentally sound manner, eg, by sludge digestion, incineration, landfill, etc. 

Outside the realm of typical hydrocarbon pyrolysis is the high temperature pyrolysis of methane. In one variant of this process, which has only been commercialized to produce acetylene (with some BTX), methane reacts in an electric arc at about 1500°C (17) with very short contact times. At higher temperatures or with a catalyst and added hydrogen, BTX is produced with fairly high selectivity (18). 

Catalytic Pyrolysis. This should not be confused with fluid catalytic cracking, which is used in petroleum refining (see Catalysts, regeneration). Catalytic pyrolysis is aimed at producing primarily ethylene. There are many patents and research articles covering the last 20 years (84—89). Catalytic research until 1988 has been summarized (86). Almost all catalysts produce higher amounts of CO and CO2 than normally obtained with conventional pyrolysis. This indicates that the water gas reaction is also very active with these catalysts, and usually this leads to some deterioration of the olefin yield. Significant amounts of coke have been found in these catalysts, and thus there is a further reduction in olefin yield with on-stream time. Most of these catalysts are based on low surface area alumina catalysts (86). A notable exception is the catalyst developed in the former USSR (89). This catalyst primarily contains vanadium as the active material on pumice (89), and is claimed to produce low levels of carbon oxides. 

Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect the initiation of pyrolysis reactions and increase the overall reaction rate of feed decomposition (85). AppHcabiUty of this process to ethane cracking is questionable since equiUbrium of ethane to ethylene and hydrogen is not altered by a catalyst, and hence selectivity to olefins at lower catalyst temperatures may be inferior to that of conventional thermal cracking. SuitabiUty of this process for heavy feeds like condensates and gas oils has yet to be demonstrated. 

It may also be prepared by pyrolysis of 1,1-difluoroethane at 725°C over a chromium fluoride catalyst in a platinum tube or by the action of zinc dust on bromodifluoroethane at 50°C. 

Vapor grown carbon fiber (VGCF) is the descriptive name of a class of carbon fiber which is distinctively different from other types of carbon fiber in its method of production, its unique physical characteristics, and the prospect of low cost fabrication. Simply stated, this type of carbon fiber is synthesized from the pyrolysis of hydrocarbons or carbon monoxide in the gaseous state, in the presence of a catalyst in contrast to a melt-spinning process common to other types of carbon fiber. 

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