Solid acid catalysts, isobutane

This contribution is an in-depth review of chemical and technological aspects of the alkylation of isobutane with lightalkenes, focused on the mechanisms operative with both liquid and solid acid catalysts. The differences in importance of the individual mechanistic steps are discussed in terms of the physical-chemical properties of specific catalysts. The impact of important process parameters on alkylation performance is deduced from the mechanism. The established industrial processes based on the application of liquid acids and recent process developments involving solid acid catalysts are described briefly. 2004 Elsevier Inc. 

M.C. Clark and B. Subramaniam. Extended alkylate production activity during fixed-bed supercritical 1-butene/isobutane alkylation on solid-acid catalyst using carbon dioxide as a diluent. Ind. Eng. Chem,. Res., 37(4) 1243-1250, 1998. 

Sarsani, V.R. and Subramaniam, B. (2009) Isobutane/butene alkylation on microporous and mesoporous solid acid catalysts probing the pore transport effeds with liquid and near critical reaction media. Green Chem., 11, 102-108. 

Carboxylic acids can also be formed by a reaction of small alkanes, carbon monoxide, and water on solid acid catalysts (93,94). By in situ C MAS NMR spectroscopy (93), the activation of propane and isobutane on acidic zeolite HZSM-5 was investigated in the presence of carbon monoxide and water. Propane was converted to isobutyric acid at 373 73 K, while isobutane was transformed into pivalic acid with a simultaneous production of hydrogen. On SZA, methyl isopropyl ketone was observed as evidence for the carbonylation of isobutane with carbon monoxide after the sample was held at 343 K for 1 h (94). When the reaction of isobutane and carbon monoxide was carried out in the presence of water, pivalic acid was identified as the main reaction product (94). These observations are rationalized by the existence of a small number of sites capable of generating carbenium ions, which can be further trapped by carbon monoxide (93). 

Fig. 17. Schematic representation of surface chain reaction scheme for isobutane conversion on solid acid catalysts (99).

Isobutane Alkylation with C4 Olefins Low Temperature Regeneration of Solid Acid Catalysts with Ozone... 

The alkylation of isobutane with C4 olefins using solid acid catalysts has become a growing research field during recent years. The main reason is that the currently used processes in industry have the HF or H2SO4 acids as catalysts, both of them being very difficult of being handled or disposed of they also present severe problems for the environment. However, in spite of an important research effort carried out both by industrial (1-4) and academic (5) laboratories, it has been very difficult to solve the major problem that the solid acid catalysts present, which is the fast deactivation due to the coke deposition. 

Zeolites (6,7), heteropolyacids (8,9), sulfated zirconia (10,11), and other materials (12) have already been explored. All of these materials deactivate in a rather short time, ranged in the order of minutes to hours, and therefore any process involving solid acid catalysts for isobutane alkylation would require frequent regenerations. 

In this work, the regeneration with ozone of Y-zeolite catalysts, exchanged with lanthanum, is studied. The objective is to find a low temperature regeneration procedure for the solid acid catalysts used in the isobutane alkylation reaction. 

The low temperature regeneration procedure using ozone could be an option to be considered for a process of isobutane alkylation with solid acid catalysts. 

Note, however, that liquid acids are still largely used in refinery and petrochemical processes. For example, HF alkylation (for isobutane alkylation with light olefins) is still among the top-ten refining processes licensed by UOP, with over 100 units installed worldwide. However, UOP introduced from 2002 the Alkylene process, which uses a liquid phase riser reactor with a solid acid catalyst for the isobutane alkylation. However, HF alkylation remains the best economic choice [223], notwithstanding environmental and corrosion problems. Also in this case, the conventional process has been improved, for example by HF aerosol vapor suppression. Other aspects of isobutane alkylation have been reviewed by Hommeltoft [224]. 

In aromatic alkylation with olefins, the solid acid catalyst based process has instead largely substituted the homogeneous acid catalysis process. This evidences that the change of substrate (isobutane vs. aromatic) could change completely the applicability of one technology with respect to another. 

Multiple supercritical isobutane regenerations of a partially deactivated USY solid acid catalyst also was tested utilizing a refinery alkylation feed blend Error Reference source not found,). The catalyst activity recovery was compared with the results of experiments that utilized a synthetic feed blend. 

Gasoline obtained by alkylation of isobutane with C3-C5 olefins is an ideal blending component for reformulated gasoline, since alkylate has a high octane number with a low octane sensitivity (difference between RON and MON), and is mainly formed by multibranched paraffins. If replacement of the environmentally hazardous sulfuric and hydrofluoric acids used as commercial alkylation catalysts by more friendly solid-acid catalysts becomes technically and economically feasible, this would greatly enhance alkylation capacity (Corma and Martinez 1993). 

Isobutane Alkylation. The deactivation of solid acid catalysts due to coke deposition is the cause of not having as yet, a commercially available process for isobutane alkylation with C4 olefins, using solid acid catalysts. The coke on these catalysts have been characterized with TPO analyses . The TPO profiles on zeolites used in this reaction, displayed two well defined burning zones. One peak below 300°C, and the other at high temperatures. The relative size of these peaks depends on the zeolite and the reaction temperature. In the case of the mordenite, the first peak was the most important, and in the case of the Y-zeolite, at 50°C or... 

It has been shown in the literature that isobutane (Pc = 36.5 bar, = 408 K)/butene (Pc= 40.2 bar, Tcf= 420 K) alkylation on solid acid catalysts at supercritical temperatures suffers from increased butene oligomerization and cracking reactions at these temperatures, increasing the catalyst deactivation potential [16-18]. Lower temperatures tend to favor the alkylation reaction. Supercritical operation at 95°C can be facilitated by diluting the isopar-affin/olefin feed with suitable amounts of a low inert solvent such as CO2 (Pc = 73.8 bar. Pc = 304 K), and has been shown to give rise to steady alkylation activity on USY and beta zeolites [19]. However, the alkylate yields are very low (< 10%) on these catalysts, attributed to severe pore diffusion limitations on these catalysts. 

The isomerization of n-butane to isobutane is of substantial importance because isobutane reacts under mild acidic conditions with olefins to give highly branched hydrocarbons in the gasoline range. A variety of useful products can be obtained from isobutane isobutylene, t-butyl alcohol, methyl t-butyl ether and t-butyl hydroperoxide. A number of methods involving solution as well as solid acid catalysts have been developed to achieve isomerization of n-butane as well as other linear higher alkanes to branched isomers. 

In 2002 a process for the direct carbonylation of saturated hydrocarbons has been patented (83). The process involves contacting the saturated hydrocarbons, which contain at least one primary, secondary or tertiary carbon atom, with carbon monoxide in the presence of a strong solid acid catalyst to produce an oxygenated saturated hydrocarbon. However, the observed conversions were small. For example, 119 g of isobutane, reacted at 100° C for 12 h with carbon monoxide (68 atm) using sulfated zirconia as the catalyst, produced only 0.14 g of pivalic acid and 0.007 g of methylisopropyl ketone. 

Several applications of the in situ decoking concept have appeared in the literature. One such application involves stabilizing the activity of solid acid catalysts such as in alkylation reactions. As reviewed elsewhere (55), numerous efforts aimed at developing solid acid alkylation catalysts and solid acid-based isobutane-olefin alkylation processes have been reported for more than three decades. However, to date, none of the solid alkylation catalysts has gained acceptance in industry for one or more of the following drawbacks rapid catalyst... 

MTBE is commercially produced by the reaction of isobutylene with methanol in the presence of an acidic ion-exchange resin as catalyst, usually in the liquid phase and at temperatures below 100°C. A typical catalyst is sulfonated styrene/divinylbenzene resin catalyst. Other solid acid catalysts such as bentonites are also effective and other novel catalysts have recently been discovered. Isobutylene is obtained from field butane by initial isomerization of n-butane to isobutane, followed by dehydrogenation to isobutylene. Commercial preparations of MTBE are 95.03 to 98.93% pure. Impurities are methanol (<0.43%), t-butyl alcohol (<0.80%), and diisobutylene (<0.25%). 

Several solid acid catalysts were tested in the laboratory for the alkylation of ethylene with isobutane following the introduction of the sulfuric acid alkylation process." These were mostly derived from aluminum chloride or boron trifluoride and were never used in the full-scale production of alkylates. 

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