Selective dehydration, butanol

Selective lertiarj-Butanol Dehydration to Isobutylene via Reactive Distillation and Solid Acid Catalysis... 

Another property of zeolites is the high conversion rates in the channel system. It was also observed that with different spatial configurations of channels, cavities, windows, etc, the catalytic properties are changed and the selectivity orientates toward less bulky molecules due to limitation in void volume near the active sites or to resistance to diffusivity. This feature termed shape-selectivity, was first proposed by McBain (20) demonstrated experimentaly by Weisz et al (21) and reviewed recently (22). For instance CaA zeolite was observed to give selective dehydration of n-butanol in the presence of more bulky i-butanol (23) while CaX non selective zeolite converted both alcohols. In a mixture of linear and branched paraffins, the combustion of the linear ones was selectively observed on Pt/CaA zeolite (24). Moreover, selective cracking of linear paraffins was obtained from petroleum reformate streams resulting in an improvement of the octane number known to be higher for branched paraffins and for aromatics than for linear paraffins. Shape selectivity usually combines acidic sites within... 

Zeolites are also excellent catalysts in the selective dehydration of tertiary alcohols. Shape selectivity, i. e. selective dehydration of 1-butanol to 1-butene over CaA zeolite in the presence of 2-butanol has also been reported [36]. The bulky 2-methylcyclohexanol, in turn, undergoes dehydration over zeolite Y, although resulting in a mixture of alkenes [37]. 

An example of a specialty olefin from an amyl alcohol is Phillips Petroleum s new process for 3-methyl-1-butene (used in the synthesis of pyrethroids) from the catalytic dehydration of 3-methyl-1-butanol (21,22). The process affords 94% product selectivity and 94% alcohol conversion at 310°C and 276 kPa (40 psig). 

Reactant shape-selective catalysis is demonstrated in the dehydration of butanols. If butan-l-ol (rt-butanol) and butan-2-ol (rso-butanol) are dehydrated... 

The sulfate anion radical is not a very strong hydrogen acceptor. It acquires an atomic hydrogen from organic substrates at significantly smaller rates a compared with the rates for one-electron oxidations. For instance, dehydration rate constants are 107, 106 and 105 I.-mole -sec 1 for methanol, tert-butanol, and acetic acid, respectively (Goldstein Mc-Nelis 1984 Zapol skikh et al. 2001). Such a peculiarity is very important for the selectivity of ion radical syntheses with the participation of SOT. 

The set of catalysts selected for the dehydration of 2-butanol was also tested for the Friedel-Crafts acylation of anisole [69, 70]. The catalytic test was performed in the liquid phase due to the high boiling points of the reactants and products of this reaction. Anisole was reacted with acetic anhydride at 120 °C in the absence of solvent. In principle, acylation can occur on both the ortho and para positions of anisole. The main product (>99%) over all catalysts in this study was para-methoxyacetophenone, indicating that the reaction predominantly takes place inside the zeolite micropores. The same trend in catalytic activity as in the 2-buta-nol dehydration reaction is observed the conversion of anisole into para-nicihoxy-acetophenone increases upon increasing Ge content of the catalyst (Fig. 9.17) [67]. The main cause of deactivation for this reaction is accumulation of the reaction products inside the micropores of the zeolite. The different behavior of Ge-ZSM-5, compared with ZSM-5, may therefore be due to improved diffusional properties of the former, as the presence of additional meso- and macropores allows for... 

In substrate selectivity, access to the catalytically active site is restricted to one or more substrates present in a mixture, e.g. dehydration of a mixture of n-buta-nol and isobutanol over the small pore zeolite, CaA, results in dehydration of only the n-butanol [38] while the bulkier isobutanol remains unreacted. Product... 

If the charge balancing cation in a zeolite is then the material is a solid acid that can reveal shape selective properties due to the confinement of the acidic proton within the zeolite pore architecture. An example of shape selective acid catalysis is provided in Figure 5.3.7. In this case, normal butanol and isobutanol were dehydrated over CaX and CaA zeolites that contained protons in the pore structure. Both the primary and secondary alcohols were dehydrated on the X zeolite whereas only the primary one reacted on the A zeolite. Since the secondary alcohol is too large to diffuse through the pores of CaA, it cannot reach the active sites within the CaA crystals. 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

Reactant selectivity is operative when there are reagents of different size present in the reaction medium but access to the active site is restricted for one or more of them. In the narrow pore zeolite, CaA, n-butanol is dehydrated but the more bulky isobutanol is not (Eqn. 10.20). In the wider pore, CaX, isobutanol is dehydrated more readily than n-butanol as anticipated for normal, imrestricted acid catalyzed dehydration reactions. 2... 

Dehydration of 1-butanol Ta-M Ti-M Ta-M showed 100% selectivity for butene. With Ti-M the major products were butenes, with butenal and dibutyl ether (minor). Among the alkoxides Ti-(OEt)4, -(OPr)4, -iso(OPr)4, -(OPr)4, the Ti-(OEt)4 gave uniform density distribution of pillars and gave high conversion. 60 61... 

From the previous discussion, it follows that the intracrystalline volume in zeolites is accessible only to those molecules whose size and shape permits sorption through the entry pores thus, a highly selective form of catalysis, based on sieving effects, is possible. Weisz and coworkers 7) have conclusively established that the locus of catalytic activity is within the intracrystalline pores when Linde 5A sieve ( 5 A pore diameter) was used, selective cracking of linear paraffins, but not branched paraffins, was observed. Furthermore, isoparaffin products were essentially absent. With the same catalyst, -butanol, but not isobutanol, was smoothly dehydrated at 230-260°. At very high temperatures, slight conversion of the excluded branched alcohol was observed, suggesting catalysis by a small number of active sites located at the exterior surface. Similar selectivity between adsorption of n-paraffins and branched-chain or aromatic hydrocarbons is shown by chabazite and erionite (18). 

By making a comparison of the rates of dehydration of sec-butanol over Linde lOX and 5A zeolites at relatively high temperature and low conversion, Weisz (7) also found that the rate constant per unit volume of 5A was between two and three orders of magnitude smaller than that of lOX. These relative magnitudes were consistent with the ratio of available surface areas (0.6-3.5 m /gm for the external area of l-5/i sized crystals of shape-selective 5A and 500-700 m /gm for lOX, where the internal surface was accessible to the sorbate. 

Selectivity in the dehydration of olefins is improved with pillared clays. Clays with aluminum oxide or mixed aluminum and iron oxide pillars converted isopropyl alcohol to propylene with more than 90% selectivity.256 A small amount of isopropyl ether was formed. When zeolite Y is used, the two products are formed in roughly equal amounts. A tantalum-pillared montmorillonite converted 1-butanol to butenes at 500°C with 100% selectivity at 41% conversion.257 The product contained a 17 20 16 mixture of 1 -butene/c/s-2-butene/fra/ s-2-butene. No butyraldehyde or butyl ether was formed. A pillared clay has been used for the alkylation of benzene with 1-dodecene without formation of dialkylated products.258 The carbonylation of styrene proceeded in 100% yield (6.50).259... 

Water can be removed from methanol by a membrane of polyvinyl alcohol cross-linked with polyacrylic acid, with a separation factor of 465.204 A polymeric hydrazone of 2,6-pyridinedialdehyde has been used to dehydrate azeotropes of water with n- and /-propyl alcohol, s- and tort butyl alcohol, and tetrahydrofuran.205 The Clostridium acetobutylicum which is used to produce 1-butanol, is inhibited by it. Pervaporation through a poly(dimethyl-siloxane) membrane filled with cyclodextrins, zeolites, or oleyl alcohol kept the concentration in the broth lower than 1% and removed the inhibition.206 Acetic acid can be dehydrated with separation factors of 807 for poly(4-methyl-l-pentene) grafted with 4-vinylpyridine,207 150 for polyvinyl alcohol cross-linked with glutaraldehyde,208 more than 1300 for a doped polyaniline film (4.1 g/m2h),209 125 for a nylon-polyacrylic acid membrane (5400 g/m2h), and 72 for a polysulfone.210 Pyridine can be dehydrated with a membrane of a copolymer of acrylonitrile and 4-styrenesulfonic acid to give more than 99% pyridine.211 A hydrophobic silicone rubber membrane removes acetone selectively from water. A hydrophilic cross-linked polyvinyl alcohol membrane removes water selectively from acetone. Both are more selective than distillation.212... 

The type of by-products formed depends on the alcohol and the catalyst. 1-Butanol and 2-butanol dehydration overall the catalysts produces 1-butene, 2-butene, isobutene, and dibutylether. The ether formation is favoured at the lowest temperatures, thus the selectivity towards butenes decreases with decreasing temperature. An exception is found in the dehydration of /-butanol, as selectivity is higher at higher temperatures. This is due to the great numbers of by-products formed during the first stages of the reaction at temperatures as low as 55 C, this being especially true for the Dawson-type acid. 

As side-products diminishing the reaction selectivity butenes were detected which were originated in the consecutive dehydration of 2-butanol. 

Another example of reactant selectivity is the dehydration of butanols. On CaA zeolites, the straight-chain alcohol, which fits in the zeolite pores, is much more rapidly dehydrated than isobutanol, which has a larger molecular diameter [T24]. In spite of the considerable molecular sieve effect, 100% selectivity is often not at-... 

The selectivities of rare earth oxide catalysts for dehydration of 2-propanol and butanols have been examined recently by Bernal and Trillo (1980) as a function of reaction temperature. No definite variations of selectivity have been observed along the lanthanide series, but the highest percentage of 1-butene was found on H02O3 and LU2O3 and the lowest on La20j. This is opposite to what was expected on the basis of the respective basicities of these oxides. Bernal and Trillo (1980) have noted that the reaction temperature influences product distributions, and that care has to be exerted in establishing trends of a series of related catalysts because of these temperature effects. 

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