Description of the Prior Art Alkylation processes are used industrially to convert light refinery gases such as C4-C5 isoparaffins into more valuable branched chain gasoline-range C7-Cg alkylate compounds. Particularly valuable alkylates are trimethylpentanes (TMPs) and 2,2- dimethylbutane (neohexane) which are used as high-octane blending components for aviation and civilian gasolines. It is estimated that about 13% ofthe U.S. gasoline pool is made up of alkylates.

Conventional alkylation processes utilize HF or H2SO4. These acidic processes are undesirable for a number of reasons including environmental and transportation hazards, and the difficulty of economical acid disposal or regeneration. Alkylation processes are also known where the isoparaffin and olefin reactants contact a zeolite, sulfated zirconia, wide-pore MCM-type or other solid alkylation catalysts at elevated temperature and pressure reaction conditions.

A significant problem with solid-catalyzed alkylation processes is the tendency for the catalysts to rapidly deactivate by virtue of coke laydown on the catalyst which tends to plug the catalyst pores. This problem is compounded in that most solid alkylation catalysts cannot be regenerated without causing irreversible degradation of the catalysts. Certain reactor operating strategies have been proposed to mitigate the problem of coke laydown, such as the use of a slurry reactor or supercritical operation.

However, these efforts have not been truly successful; for example, at the high

temperatures required to achieve supercritical reaction conditions when not using a co- solvent or diluent such as carbon dioxide, undesirable side reactions such as oligomerization and cracking are favored. As a consequence, while catalytic alkylation processes have shown promise, a number of intractable problems exist which limit the practical utility thereof.

SUMMARY OF THE INVENTION The present invention overcomes the problems outlined above and provides improved alkylation processes which ameliorate catalyst deactivation stemming from coke laydown and which give enhanced yield of preferred branched chain, high octane number alkylates. Broadly speaking, the processes of the invention involve providing a reaction mixture comprising an isoparaffin, an olefin and a molar excess of an inert co-solvent or diluent such as carbon dioxide, methane or hydrogen (relative to the moles of isoparaffin and olefin); the co-solvent or diluent has a critical temperature that is lower than the critical temperatures of both the isoparaffin and olefin starting reactants.

This reactant mixture is contacted with a solid porous alkylation catalyst to produce a reaction mixture containing alkylate, and the critical temperature (Tc) of the reaction mixture is significantly lowered owing to the presence of excess inert co-solvent(s).

The contacting step is generally carried out in a reactor at near-critical or supercritical conditions for the reaction mixture, typically at temperatures of from about 0.9-1.3 Tc of the reaction mixture and a pressure of from about 300-3500 psi. In preferred forms, the contacting step is carried out at supercritical conditions for the reaction mixture (1.01 - 1.3 Tc of the reaction mixture, preferably 1.01 - 1.2 Tc), typically from about -50- 2000C, and most preferably 5-1 00~C, with the pressure being in excess of the critical pressure (Pc) for the reaction mixture.

The alkylation processes of the invention can be carried out using a variety of reactants and operating conditions. For example, the isoparaffin and olefin reactants can be selected from the C4-C10 isoparaffins and olefins, while the CO2: (isoparaffin+ole fin) mole ratio can be within the range of from about 5:1-300:1. Any suitable alkylation catalyst can be used, such as a zeolite, sulfated zirconia or other solid catalysts which favor alkylate formation. The catalyst surface area can generally range from about 5- 1000 m2/g while the catalyst pore volume can range from about 0.01-0.5 cc/g. The fluid density of the reaction mixture generally ranges from about 0.05-0.65, but can be greater if desired; it will be understood that this density is significantly affected by the reaction pressure. It is particularly preferred that the isoparaffin and olefin reactants be

pretreated to remove moisture, peroxide and oxygenate impurities or other free radical producing species which catalyze oligomer formation in the bulk fluid phase, which have been found to adversely affect catalyst performance. As used herein, oxygenates refer to free-radical or free-radical providing oxygenate compounds. The total content of peroxides in the isoparaffin and olefin reactants should be no more than about 200 ppm, while the oxygenate concentration should be no more than about 200 ppm.

It has been found that the use of a co-solvent or diluent such as CO2 allows lower temperature alkylation resulting in reduced catalyst coking rates and increased production of valuable TMPs. Moreover, CO2 is an inexpensive component which is environmentally friendly and fully recyclable. In addition, operation at supercritical reaction conditions relative to the alkylate reaction mixture where CO2 or other suitable diluent is used as a co-solvent gives an optimum combination of gas-like transport properties and liquid-like densities. This serves to extract coke precursors in situ from the solid porous catalysts. Consequently, catalyst deactivation rates are minimized, and the rates of the desired alkylation reactions to give a narrower alkylate spectrum are maximized.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph of butene conversion versus USHY catalyst age in a series of phase comparison alkylation (isoparaffin-butene) reactions, using a zeolite catalyst; Fig. 2 is a graph oftotal trimethlypentanes/C5+ versus USHY catalyst age for the phase comparison alkylation reactions of Fig. 1; Fig. 3 is a graph of butene conversion versus catalyst age in a series of phase comparison alkylation (isoparaffin-butene) reactions, using a sulfated zirconia catalyst; and Fig. 4 is a graph oftrimethlypentane/C5+ versus sulfated zirconia catalyst age for the phase comparison alkylation reactions of Fig. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following Example illustrates various alkylation reactions carried out using liquid, gas, near-critical and supercritical reaction conditions, with and without carbon dioxide as a co-solvent. It is to be understood that the example is provided by way of demonstration only and nothing therein should be taken as a limitation upon the overall scope of the invention.

Example In these series of alkylation tests, a conventional plug flow alkylation reactor was employed for the alkylation reaction between isobutane (Tc = 135 0C, Pc = 529 psi) with butene (Tc = 146"C, Pc = 583 psi) and in certain cases using an excess of CO2 (Tc = 31 ~C, Pc = 1071 psi) , using two separate alkylation catalysts. In the first series, 5 g of a USHY zeolite catalyst was employed having a Si:Al ratio of 2.8, a surface area of 560 m2/g, a pore volume of 0.33 cc/g; the catalyst was pretreated at 4500C for 3 hrs.

In the second series, 10 g of sulfated zirconium hydroxide catalyst was employed, having 4% SO4, a surface area of 100 m2/g, and a pore volume of 0.07 cc/g; this catalyst was pretreated at 6000 C for 1 hr. to transform it into the oxide form. The butane and butene reactants were pretreated by contact with activated alumina to remove peroxides.

For the runs using the zirconia catalysts, the feed was also treated with sodium sulfate to remove moisture traces and oxygenates.

The reaction operating conditions were: temperature, 50-176 "C; pressure 500- 2250 psi; olefin WHSV (weight hourly space velocity), 0.25 hi-'; isoparaffin:olefin molar ratio, 9: 1; CO2:(isoparaffin:olefin) molar ratio, 90:9:1 (95"C), 180:9:1 (50"C).

The extent of butene conversion was monitored along with the ratio of the preferred trimethylpentanes versus Cs and above alkylates.

Figs. 1 and 2 illustrate the results obtained (i.e., butene conversion and TMP/C5+) using the zeolite catalyst for four different reaction phases, namely liquid phase at 50"C and 500 psi, gas phase at 140"C, 500 psi, near-critical phase at 140"C, 880 psi, and supercritical phase with CO2 as co-solvent at 500C, 2250 psi. Figs. 3 and 4 provide similar data for the sulfated zirconia catalyst.

The results of these tests demonstrated that butene conversion reached a steady state in all reaction phases, using both the zeolite and zirconia catalysts. However, gas phase high temperature operation resulted in cracked products (C5-C7) which are undesirable owing to their low octane number. Moreover, the data of Figs. 2 and 4 confirmed that extended activity for TMP alkylate production is obtained only during supercritical operation with CO2 as a co-solvent. Further, analysis of the catalyst samples after completion of the reactions established that supercritical phase operation with CO2 as a co-solvent gave more catalyst surface area and pore volume, as compared with the other tests. The following table 1 sets forth a comparison of fresh and after- reaction catalyst properties observed in these tests.

Table 1 I:O:C02 Surface Area Pore Volume Observed Catalyst Condition molar ratio (m2)/g (cm3/g) Color USY Fresh -- 560 0.33 white USY Liquid phase, 50"C, 500 9:1:0 350 0.22 yellow-beige psi USY Gas phase, 140"C, 500 psi 9:1:0 130 0.09 brown USY Near-critical, 140"C, 1000 9:1:0 190 0.13 brown psi USY Supercritical, 50"C, 2250 9:1:90 430 0.25 off-white psi USY Supercritical, 95"C, 2250 9:1:180 420 0.25 beige psi SZ Fresh -- 100 0.07 white SZ Liquid phase, 50"C, 500 9:1:9 10 0.01 yellow psi SZ Supercritical, 50"C, 2250 9:1:90 45 0.06 off-white psi SZ Supercritical, 95 2250 9:1:180 35 n l brown psi I = iso-butane O = butene CO2 = carbon dioxide USY = ultrastable "Y" zeolite SZ = sulfated zirconia USY runs: feed pretreatment with activated alumina unless otherwise indicated SZ runs: feed pretreated with activated alumina and sodium sulfate It will be appreciated that the alkylation reactions ofthe invention can be carried out using a number of different alkylation catalysts and reaction conditions. The following Table 2 summarizes these variables in terms of approximate broad and preferred ranges.

Table 2 Reaction Parameter Broad Range Preferred Range Isoparaffin Type (carbon number) C4-C,0 C4-C5 Olefin Type (carbon number) C2-C,0 C2-C6 Co-Solvent/Diluent Type CO2, CH4 CO2 and/or H2 Reaction Temperature (relative to Tc of reac- 0.9-1.3 Tc 1.01-1.2 tion mixture) Tc of Reaction Mixture (~C) -50-200 5-100 Reaction Pressure (psi) 14-3500 500-3000 Isoparaffin:Olefin mole ratio 4:1-20:1 7:1-12:1 Co-SolventlDiluent:Isoparaffin mole ratio 0.01:1-50:1 2:1-20:1 CO2:Isoparaffin mole ratio 5:1-25:1 10:1-20:1 Co-Solvent/Diluent:Olefin mole ratio 1:1-300:1 20:1-200:1 CO2:Olefin mole ratio 50:1-300:1 70:1-200:1 Co-Solvent/Diluent:Isoparaffin+Olefin mole 0.01:1-325:1 1:1-200:1 ratio CO2:(Isoparaffin+Olefin) mole ratio 2:1-300:1 10:1-80:1 Reactant Peroxide Content (ppm) up to 200 up to 50 Reactant Oxygenate Content (ppm) up to 200 up to 50 Reactant Moisture Content (ppm) up to 100 up to 50 Catalyst Surface Area (BET, m2/g) 5-1000 10-600 Catalyst Pore Volume (cc/g) 0.01-0.5 0.05-0.4 Reaction Mixture Fluid Density (g/cc) 0.05-0.65 0.2-0.5