Over fifty years ago it was recognized that alkylbenzene sulfonates (ABS) were quite effective detergents superior to natural soaps in many respects. Because of their lower price, their price stability, and their effectiveness in a wide range of detergent formulations, ABS rapidly displaced soaps in household laundry and dishwashing applications and became the standard surfactants for the detergent industry.
The alkylbenzene sulfonates as initially prepared had substantial branching in the alkyl chain. This situation was maintained until the early 1960's when it became apparent that the branched alkyl-based detergents were contributing to the pollution of lakes and streams and forming relatively stable foams. Examination of the problem showed that the branched structure of the alkyl chains was not susceptible to rapid biodegradation and the surfactant properties of the detergent thus persisted for long periods of time. This was not the case earlier when natural soaps were used, because of the rapid biodegradation of the linear chains in natural soaps.
After recognizing the biodegradability of ABS based on alkylation by linear olefins, industry turned its attention to the production of these unbranched olefins and their subsequent use in the production of linear alkyl benzenes. Processes were developed for efficient alkylation of benzene by available feedstocks containing linear olefins, and the production of linear alkyl benzenes (LAB) became another reliable process broadly available to the petroleum and petrochemical industry. It gradually evolved that HF-catalyzed alkylation was particularly effective in LAB production, and an HF-based alkylation process became the industry standard.
With increasing environmental concern came increasing disenchantment with HF as a catalyst and a concomitant need to find a substitute equal or superior to it in all respects. As regards criteria in addition to the price, the extent of conversion effected by the catalyst, the selectivity of monoalkylbenzene formation, and the linearity of alkylbenzenes produced loomed large. At this point the definition of several terms are necessary to adequately understand and appreciate what follows.
Alkylation typically is performed using an excess of benzene relative to olefins. The ideal catalyst would show 100% conversion of olefins using an equal molar proportion of benzene and olefins, but since this has not been attainable one strives for maximum olefin conversion using a benzene to olefin molar ratio up to about 30. The better the catalyst, the lower will be the benzene:olefin ratio at a high conversion of, say, 98%. The degree of conversion at a constant value of benzene-olefin ratio is a measure of catalytic activity (subject to the caveat that the ratio must not be so high that the degree of conversion is invariant to small changes in this ratio). The degree of conversion may be expressed by the formula, EQU V=C/T.times.100,
where V equals percent conversion, C equals moles of olefin consumed, and T equals moles olefin initially present.
However active the catalyst may be, it is not valuable unless it also is selective. Selectivity is defined as the percentage of total olefin consumed under reaction conditions which appears as monoalkylbenzene and can be expressed by the equation, EQU S=M/C.times.100,
where S equals selectivity, M equals moles of monoalkylbenzenes produced, and C equals moles olefin consumed. The better the selectivity, the more desirable is the catalyst. An approximate measure of selectivity is given by the equation, ##EQU1## where "total products" includes monoalkylbenzenes, polyalkylbenzenes, and olefin oligomers. At high selectivity (S&gt;85%) the results calculated from the two equations are nearly identical. The latter of the foregoing two equations is routinely used in commercial practice because of the difficulty in distinguishing between oligomers and polyalkylbenzenes.
Finally, the reaction of linear olefins with benzene in principal proceeds according to the equation, EQU C.sub.6 H.sub.6 +R.sub.1 CH=CHR.sub.2 .fwdarw.C.sub.6 H.sub.5 CH(R.sub.1)CH.sub.2 R.sub.2 +C.sub.6 H.sub.5 CH(R.sub.2)CH.sub.2 R.sub.1.
Note that the side chain is branched solely at the benzylic carbon and contains only one branch in the chain. Although strictly speaking this is not a linear alkylbenzene, nonetheless the terminology which has grown up around the process and product in fact includes as linear alkylbenzenes those materials whose alkyl group chemically arises directly from linear olefins and therefore includes alpha-branched olefins. Because alkylation catalysts also may induce the rearrangement of olefins to give products which are not readily biodegradable (vide supra), for example, .alpha.,.alpha.-disubstituted olefins which subsequently react with benzene to afford an alkyl benzene with branching at other than the benzylic carbon, ##STR1## the degree to which the catalyst effects formation of linear alkyl benzenes is another important catalyst parameter. The degree of linearity can be expressed by the equation, EQU D=L/M.times.100,
where D equals degree of linearity, L equals moles of linear monoalkyl benzene produced, and M equals moles of monoalkyl benzene produced.
Consequently, the ideal catalyst is one where V equals 100, S equals 100, and D equals 100. The minimum requirement is that linearity be at least 90% at a selectivity of at least 85% and an initial conversion of at least 98%. These are minimum requirements; that is, if a catalyst fails to meet all of the foregoing requirements simultaneously the catalyst is commercially unacceptable.
The linearity requirement is assuming added importance and significance in view of the expectation in some areas of minimum standards for linearity in detergents of 92-95% near-term, increasing to 95-98% by about the year 2000. Since the olefinic feedstock used for alkylation generally contains a small percentage of non-linear olefins--a non-liner olefin content of about 2% is common to many processes--the requisite linearity in the detergent alkylate places even more stringent requirements on catalytic performance; the inherent linearity of the alkylation process must increase by the amount of non-linear olefins present in the feedstock. For example, with a feedstock containing 2% non-linear olefins the catalyst must effect alkylation with 92% linearity in order to afford a product with 90% linearity, and with a feedstock containing 4% non-linear olefins the catalyst must effect alkylation with 94% linearity to achieve the same result.
Clays of diverse type are known catalysts for detergent alkylation. We have observed for some time that clays as a group afford detergent alkylates with significantly higher linearity under comparable reaction conditions than do other detergent alkylation catalysts such as silica-aluminas and zeolites. However, commercial development of clays as alkylation catalysts has been hampered by their resistance to regeneration. Thus, in detergent alkylation quite typically one employs reaction conditions which affords 99-100% conversion of olefin with fresh catalyst. Deactivation of catalyst occurs invariably leading to a decrease in olefin conversion, and when olefin conversion is reduced by some predetermined amount relative to its initial conversion the catalyst is removed from service and treated so as to restore its activity to that approximately equal to fresh catalyst. This process is referred to as catalyst regeneration, and the preferred mode of catalyst regeneration is a simple benzene wash at a temperature at least equal to, but usually in substantial excess of, the alkylation temperature. Catalyst reaction cycles frequently may be on the order of 24 hours with the regeneration cycles on the order of 24 hours. That is, the material is used as a catalyst for about 24 hours, and then its activity is restored with a benzene wash over a like period of time. Whereas the foregoing regeneration procedure works admirably with, for example, silica-alumina, which we use as our reference catalyst, we have consistently found over many years of experimental observation that clays fail to respond to the foregoing method of regeneration.
Additionally, it is a more-or-less standard observation that after some number of benzene wash cycles the lifetime of a catalyst is considerably shortened, i.e., even though catalyst activity is restored the time over which it becomes deactivated gradually decreases until the reaction cycle time of the catalyst is so short as to make catalyst use impractical. At this point the deactivated catalyst usually is subjected to the more severe regeneration procedure of a carbon burn, i.e., a high temperature treatment with oxygen to remove all organic materials and coke from the catalyst surface. Although materials such as silica-aluminas respond well to a carbon burn, with complete restoration of activity and catalyst lifetime, we have observed that clays generally do not so respond. Evidently the condition under which a carbon burn occurs destroys the clay structure, often deactivating the clay entirely.
Another generic disadvantage of clays relative to the silica-aluminas is that generally they are less active and less stable. That is, at the same alkylation conditions clays generally are substantially less active than other alkylation catalysts, and generally have shorter lifetimes (i.e., lower stability) before regeneration becomes necessary.
Particularly in view of our consistent observations of relative non-regenerability of clays, it came as quite a surprise to find that tetrahedrally charged clays, whether delaminated or pillared, form a class of clays which are readily regenerated by a benzene solvent wash. The class of tetrahedrally charged clays is represented by saponite and beidellite as its most important and well-known examples. We were further surprised to find that the tetrahedral clays, such as pillared saponite and beidellites, exacted no activity penalty relative to, for example, the silica-aluminas. In particular, pillared saponites and beidellites are substantially more active than other clays, especially when calcined at relatively low temperatures, showing an activity comparable to silica-aluminas and exhibiting a like linearity. Summarizing, the pillared saponites and beidellites show behavior more similar to silica-aluminas as a detergent alkylation catalyst than to other clays.
We mention in passing that clays of diverse type are known catalysts for detergent alkylation. In U.S. Pat. No. 5,034,564 the patentee noted the combination of a pillared clay and a binder, including pillared clays based on saponite and beidellite, was useful in detergent alkylation. However, even here there was no teaching, nor even a suggestion, of regenerability. In fact, there was no distinction made between the non-regenerable octahedrally-charged pillared montmorillonite and the regenerable, tetrahedrally-charged pillared saponite and beidellite of this invention. Consequently it can be fairly stated that the prior art was devoid of any hint or suggestion of our invention.