Alcohol dehydration using zeolite

Alcohol Dehydration using New Shape Selective Zeolites. - The shape selectivity of zeolites has been referred to and exploited in numerous ways. Nevertheless, within the last ten years a new class of zeolite (Type ZSM-5) has been reported, which possesses pore openings, intermediate in size between the large and small pore zeolites its sieving properties in respect of alkyl aromatics and various aliphatic hydrocarbons have been reported. 

Tungstated zirconia (WZ) catalysts have been proposed as viable candidates for alkane isomerizations, especially those heavier than C4 [1]. In this work, alcohol dehydrations were used to rank the acidity of WZ with respect to that of HY zeolite, and to evaluate the ability of these catalysts to resist coking during a reaction that yields an olefin molecule as the primary product. The results from temperature-programmed reaction and infrared spectroscopy studies allowed us to gain some insight into the relative stability of WZ. 

Figure 2.3 Dehydration of alcohols using zeolites dehydration of (1) butan-2-ol on zeolite-X (2) n-butan-l-ol on zeolite-X (3) butan-l-ol on zeolite-A (4) butan-2-ol on zeolite-A...

Numerous studies, including mechanistic and kinetic investigations mostly with simple alcohols, have been performed with molecular sieves as dehydration catalysts [8,32-34]. Although highly active these are rarely used for converting alcohols with complicated structures to alkenes. The reason is that these catalysts are not selective-a prevalent El mechanism, i. e. the involvement of carbocationic intermediates, and parallel inter- and intramolecular processes result in the formation of isomeric alkenes and ethers. Alcohols with specific structure, however, can be selectively transformed to alkenes. For example, 1-phenyl-1-ethanol is transformed to styrene in 95 % yield over HZSM-5 zeolite at 493 K [34]. Ether formation, however, was shown to be significant when a-(p-tolyl)ethanol was reacted over zeolite HY [35]. A low concentration of the reactant alcohol inside the zeolite is required to prevent such dimerization-type reaction a suitable competing solvent should be selected. 

After all this, it seems at this point that still there are no truly useful probes for distinguishing materials with respect to acid strength, and one should rely on reactivity. I think that one can even question the adequacy of using basic probe molecules, especially in those cases in which the last objective is to correlate those observations with the catalytic activity for different hydrocarbon reactions. I keep asking myself, why use a reaction between a basic molecule (pyridine, NHj, amines etc.) and the acid zeolite to predict the reactivity of other, very different molecules in reactions, such as alcohol dehydration, double bond isomerization, chain isomerization, alkylations, cracking, and so fotth. Since it is obvious that the acidity measured with a probe molecule will always correlate with the particular basic molecule used for characterization, how... 

Humphries et al. [92,93] gave an overview of different test reactions used to characterize the acidity of zeolites. The reactions included cracking, isomerization, disproportionation, and alcohol dehydration as well as hydride transfer reactions involving cyclohexene conversion. In the following sections, we will discuss each class of these test reactions for their ability to give information about the nature, concentration, and strength of the active sites involved in acid-catalyzed reactions. 

Although there are some important differences between what we describe as 3-connected aluminium sites in our bb-matrices and what the active sites are thought to be in zeolites, we have begun a preliminary study of the activities of the Al, Ti and V-containing bb-catalysts as solid acid catalysts in the dehydration of alcohols. For this type of bench marking reaction, there are two parameters that can be used as preliminary indicators of catalytic activity lightoff temperatures and product selectivity. A plot of conversion versus temperature produces what is known as a lightoff curve. The temperature at which 50% of the maximum... 

The catalytic hydroxylation of methane [39, 95], ethane [136, 137], propane [138-141], cyclohexane [68] and cyclododecane [142] has been tested using the Fe-contain-ing zeolites. In most cases the selectivity was low or even zero. With ethane and propane, instead of their hydroxylation to alcohols, oxidehydrogenation (ODH) takes place, yielding ethylene and propylene, respectively, in yields of up to 25-30%. According to Bulanek et al. [140] the ODH of propane proceeds via intermediate formation of propanol, which further is subjected to dehydration. 

Microporous superbasic catalysts based on zeolites suitable for alkene isomerization were developed by Martens et al. [28-30]. They have high activity in the 1-butene isomerization and side-chain alkenylation of xylene [31], and are prepared by impregnating a dehydrated zeolite (NaY) with NaN3 in alcoholic solution. Subsequent decomposition of the azide inside the zeolite pores produces metallic sodium particles Nax°and cationic sodium clusters Na43+. The high catalytic activity was attributed to the ionic sodium clusters as they could be detected using ESR spectroscopy. 

For the dehydration step, molecular sieves made of zeolite can be used. The alcohol is... 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

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). 

The commercial process for the production of nylon 6 starts with the oxidation of cyclohexane with oxygen at 160°C to a mixture of cyclohexanol and cyclohexanone with a cobalt(II) catalyst. The reaction is taken to only 4% conversion to obtain 85% selectivity. Barton and co-workers have called this the least efficient major industrial chemical process.240 They have oxidized cyclohexane to the same products using tort bu(ylhydroperoxide with an iron(III) catalyst under air (70°C for 24 h) with 89% efficiency based on the hydroperoxide. The oxidation of cyclohexanol to cyclohexanone was carried out in the same way with 99% efficiency. A cobalt catalyst in MCM-41 zeolite gave 38% conversion with 95% selectivity in 4 days at 70 C.241 These produce ferf-butyl alcohol as a coproduct. It can be dehydrated to isobutene, which can be hydro-... 

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 Y zeolites have been identified as active catalysts for various reactions such as isomerisation, amination, alkylation and deamination [1-4]. Cu containing Y zeolites have been found particularly useful to catalyse Diels - Alder cycloaddition [5-6]. Dehydration of t-BuOH has been suggested as a model reaction for estimating the zeolite acidity [7]. The dehydration of t-BuOH has received particular attention as tertiary species gives the most stable carbonium ion. The dehydration of alcohols by zeolites has been extensively studied... 

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