Abstract:
The present invention relates to a process for the preparation of dimethylcumenes comprising alkylating a substrate comprising of one or more xylene isomers with an alkylating agent in the presence of a solid acid zeolite catalyst selected from ultrastable zeolite Y (Si/Al=5 to 50) and Beta (Si/Al=10-120), and separating the products formed in vapour phase.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a process for the preparation of dimethylcumenes. More particularly, the present invention relates to a process for the preparation of dimethylcumenes by the catalytic isopropylation of xylenes using isopropylene as the alkylating agent over a solid catalyst.  
         BACKGROUND OF THE INVENTION  
         [0002]    Dimethylcumenes are useful raw materials for the production of xylenols, pesticides, pharmaceuticals, perfumery, heat transfer media, polymers and special solvents. Xylenols are an important class of phenolic substrates useful as raw materials for making phenolic resins having specific thermoelastic properties.  
           [0003]    The most common method for the preparation of dimethylcumenes is by the alkylation of xylene carried out in the presence of metal halides, AlCl 3 , ZnCl 2  and inorganic acids HCl, H 2 SO 4  [(i) Journal of Chemical Education, Vol. 70 (6), pages A 152-154, 1993; and (ii) U.S. Pat. No. 5,300,717, which disclose a process for the preparation of dixylylpropane wherein one step comprises the formation of 1,2-dimethyl-4-isopropylbenzene from o-xylene and propene in the presence of conventional Friedel Crafts catalysts such as BF 3 —H 3 PO 4 , AlCl 3 —CH 3 NO 2 ). This process suffers from the disadvantage that it is environmentally hazardous, giving rise to problems in terms of handling, safety, corrosion and waste disposal.  
           [0004]    Rare earth modified zeolites such as (Nd—Na—Y) are reported as catalyzing the alkylation, including the isopropylation of o-xylene (Dokl Akad. Nauk., Vol. 335 (3), 1994, p. 322-325&lt;CA: 121:182357). In this case, although the isopropylation of o-xylene resulted in the formation of a mixture of dimethylcumenes, alkylation could not be obtained when p-xylene was used as a substrate. The disadvantage of this catalyst system is its inactivity for other xylene isomers such p-xylene. The conversions and selectivity are also low for the desired dimethylcumene isomers. Additionally, this catalyst is less active for isopropylation as compared to tertiary butylation of o-xylene. Solid catalysts such as TiO 2 —SiO 2 —Al 2 O 3  (Tisial) and MoO 3 —SiO 2 —Al 2 O 3  (Mosial) are also reported for the catalytic alkylation of benzene, toluene and xylene isomers with C 2 , C 3 , and C 4  aliphatic alcohols (Indian Journal of Chemistry, Vol. 33B, pp. 1053-1061, 1994). However, the yield of desired mono dimethylcumenes is low (ca.50%).  
           [0005]    In view of the above-mentioned drawbacks and limitations of the above process, it was found desirable to develop an improved process for the production of dimethylcumene isomers with isopropanol using a solid acid catalyst, either in batch reactor or in fixed bed down flow reactor.  
         OBJECTS OF THE INVENTION  
         [0006]    The main object of the present invention is to provide an improved process for the preparation of dimethylcumenes overcoming the above-mentioned drawbacks by contacting xylene isomers with isopropanol using a solid acid catalyst with high yield and selectivity.  
           [0007]    It is another object of the present invention to sue solid heterogeneous catalyst which are reusable by simple thermal treatment in the presence of air.  
           [0008]    Yet another object of the invention is to provide a process for the preparation of dimethylcumenes in either batch reactor or in a continuous fixed bed reactor.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, the present invention relates to a process for the preparation of dimethylcumenes comprising alkylating a substrate comprising of one or more xylene isomers with an alkylating agent in the presence of a solid acid zeolite catalyst, and separating the products formed in vapour phase.  
           [0010]    In one embodiment of the invention, said substrate and alkylating agent are contacted with said solid acid zeolite catalyst at a temperature in the range of 80-250° C. and for a period of at least 1 hour.  
           [0011]    In another embodiment of the invention, the product is separated from the vapour phase by condensation at a temperature in the range of 0-3° C.  
           [0012]    In yet another embodiment of the invention, the substrate is selected from o-xylene, m-xylene, p-xylene and any mixture thereof.  
           [0013]    In another embodiment of the invention, the solid acid zeolite catalyst comprises a solid heterogeneous solid acid catalyst selected from ultrastable zeolite Y (Si/Al=5 to 50) and Beta (Si/Al=10-120), preferably, the Si/Al ratio in said catalyst is between 5 to 20.  
           [0014]    In another embodiment of the invention, the alkylating agent is selected from propylene and propyl alcohols such as isopropanol and n-propanol.  
           [0015]    In another embodiment of the invention, the temperature of the reaction is in the range of 100-200° C., preferably 120-180° C.  
           [0016]    In yet another embodiment of the invention, the molar ratio of xylene substrate to the alkylating agent in the feed is in the range of from 1:2 to 20:1, preferably 1:1 to 10:1, more preferably, in the range of 1:2 to 5:1.  
           [0017]    In another embodiment of the invention, the weight hourly space velocity (WHSV) of the feed is in the range of 0.5 to 30 h −1 , preferably 1 to 20 h −1 , more preferably 2 to 10 h −1 .  
           [0018]    In a further embodiment of the invention, the alkylation reaction is carried out in a fixed bed reactor or a batch reactor.  
           [0019]    In yet another embodiment of the invention, p-xylene is alkylated using isopropanol in the presence of zeolite beta catalyst.  
           [0020]    In another embodiment of the invention, m-xylene and o-xylene are alkylated using isopropanol as the alkylating agent in the presence of ultrastable zeolite Y (USY) as catalyst.  
           [0021]    In yet another embodiment of the invention, a mixture of p-xylene and isopropyl alcohol in a molar ratio of 4:1 is reacted in a fixed bed reactor in the presence of ultrastable zeolite Y (hereinafter USY) catalyst.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0022]    The present invention provides a process for the preparation of dimethylcumenes by reacting a xylene isomer or a mixture of xylene isomers with an alkylating agent in presence of a solid acid zeolite catalyst. The reaction can be carried out either in a fixed bed reactor or a batch reactor. Preferably, the temperature of the alkylation reaction is in the range of 80-250° C., and the molar ratio of the xylene isomer substrate to the alkylating agent in the feed is in the range of 1:2 to 20:1. Reaction is carried out at a WHSV of 0.5 to 30 h −1 . Propylene or propyl alcohols such as isopropanol or n-propanol can be used as alkylating agents.  
           [0023]    The novelty and inventive step in this invention resides in the use of solid acid zeolite catalysts such as zeolite USY or zeolite beta as the catalysts for alkylation reaction. Solid acid zeolite catalysts show very high catalytic activity and selectivity in alkylation of all xylene isomers, unlike in the prior art wherein only o-xylene and not m- or p-xylenes could be alkylated using Nd—Na—Y zeolite catalyst. While both zeolite USY and zeoltie Beta are suitable for propylation of xylene isomers, it is observed that zeolite Beta is more suitable for isopropylation of p-xylene and zeolite USY is preferred for isopropylation of m- and p-xylene. Without wishing to be bound by any theory, it is believed that this could be due to the slightly small pore openings of zeolite Beta as compared to zeolite USY. p-xylene being the smallest among all the xylenes, has faster ingress into zeolite Beta when compared to ortho- or meta-isomers. This diffusional limitation is not present in the case of zeolite USY. H-Mord. and MFI are not as good catalysts as zeolite Beta or zeolite USY for the alkylation reaction of this invention, due to their smaller pore size. p-xylene yields only one dimethylcumene isomer, 2,5 dimethylcumene since all regio positions are the same. o-xylene yields 2,3 dimethylecumene and 3,4 dimethylcumene isomers as the primary products. While in principle m-xylene can yield three dimethylcumene isomers, 2,4 dimethylcumene, 2,6 dimethylcumene and 3,5 dimethylcumene, only two isomers, 2,4 dimethylcumene, 2,6 dimethylcumene are formed as primary products since the alkylation is an ortho-para directing electrophilic substitution reaction and the regio position 3,5 in m-xylene is deactivated due to the meta position from both the methyl groups in m-xylene.  
           [0024]    The following examples are provided to illustrate the invention and should not be construed as limiting the scope of the invention. 
       
    
    
     EXAMPLE 1  
       [0025]    This example illustrates effect of time on stream (TOS) on conversion and product selectivity in isopropylation of p-xylene. USY catalyst (0.5 g) was loaded into the reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of p-xylene and isopropyl alcohol in a 4:1 molar ratio was introduced into fixed bed reactor by syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in presence of inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 6.48 h −1  and a temperature of 140° C. for a period of 8 hours. Product samples were collected periodically (Table I below), chilled at 0° C. and analyzed by gas chromatograph (Shimadzu GC-14B) using flame ionization detector and 3 m×⅛″ packed column with 5% bentone and 5% DIDP on chromosorb WHP, with mesh size of 801100. Results of the reaction are given in Table I below.  
                                                                     TABLE I                           Effect of TOS on conversion and product selectivity       in the isopropylation of p-xylene over zeolite USY                Time on stream            Conversion or selectivity (mole %)   1 h   3 h   5 h   8 h                    Conversion of p-xylene   16.2   16.5   14.6   10.0       Selectivity of dimethylcumenes in total   94.2   88.8   87.8   84.0       products a         Selectivity of 2,5 dimethylcumene among   100   100   100   100       DMC&#39;s       Selectivity of other products in total   5.8   11.2   12.2   16.0       products       % yield of DMC in total products w.r.t.   61.2   58.8   51.2   33.6       limiting reagent, i.e. isopropylating       agent                          
 
       EXAMPLE 2  
       [0026]    This example illustrates effect of temperature on conversion and product selectivity in isopropylation ofp-xylene. USY catalyst (0.5 g) was loaded into reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of p-xylene and isopropyl alcohol in 4:1 molar ratio was introduced into fixed bed reactor by syringe pump (Sage Instruments, Model 352, USA) in continuous manner in presence of inert carrier gas (nitrogen, flow=35 ml/min) at WHSV of 6.48 h −1  and at different temperatures (Table II) for a period of 1 hour. Products were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of reaction are given in Table II.  
                                                                     TABLE II                           Effect of temperature on conversion and product selectivity       in the isopropylation of p-xylene over zeolite USY                Temperature, ° C.            Conversion or selectivity (mole %)   120   140   160   180                    Conversion of p-xylene   6.9   16.2   22.6   28.0       Selectivity of dimethylcumenes in total   96.5   94.2   78.6   56.1       products a         Selectivity of 2,5 dimethylcumene among   100   100   97.0   89.0       DMC&#39;s       Selectivity of other products in total   3.5   5.8   21.4   43.9       products       % yield of DMC in total products w.r.t.   26.8   61.2   71.2   62.8       limiting reagent, i.e. isopropylating       agent                          
 
       EXAMPLE 3  
       [0027]    USY catalyst (0.5 g) was loaded into the reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of p-xylene and isopropyl alcohol in a 4:1 molar ratio was introduced into fixed bed reactor by syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in the presence of inert carrier gas (nitrogen, flow=35 ml/min) at different WHSV (See Table III) and a temperature of 140° C. for 1 hour. Product was chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of reaction are given in Table III.  
                                                                     TABLE III                           Effect of space velocity on conversion and product selectivity       in the isopropylation of p-xylene over zeolite USY                WHSV, h −1                  Conversion or selectivity (mole %)   3.2   6.5   12.9                            Conversion of p-xylene   24.3   16.2   11.8           Selectivity of dimethylcumenes in   90.5   94.2   96.2           total products a             Selectivity of 2,5 dimethylcumene   98   100   100           among DMC&#39;s           Selectivity of other products in   9.5   5.8   3.8           total products           % yield of DMC in total products   88.0   61.2   45.6           w.r.t. limiting reagent, i.e.           isopropylating agent                                  
 
       EXAMPLE 4  
       [0028]    This example illustrates effect of molar ratio of p-xylene to isopropanol on conversion and product selectivity in isopropylation of p-xylene. USY catalyst (0.5 g) was loaded into the reactor such that catalyst bed was sandwiched between inert porcelain beads. A mixture of p-xylene and isopropyl alcohol with different molar ratio (Table IV) was introduced into fixed bed reactor by syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in the presence of inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 6.48 h −1  and a temperature of 140° C. for a period of 8 hours. Products were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of reaction are given in Table IV.  
                                                                     TABLE IV                           Effect of molar ratio of p-xylene and isopropanol       on conversion and product selectivity in the isopropylation       of p-xylene over zeolite USY                p-xylene/isopropanol (molar)            Conversion or selectivity (mole %)   2:1   4:1   8:1   10:1                    Theoretical maximum conversion   50   25   12.5   10       Conversion of p-xylene   35.4   16.2   9.7   7.1       Selectivity of dimethylcumenes in   76.8   94.2   95.6   100       total products a         Selectivity of 2,5 dimethylcumene   97.3   100   100   100       among DMC&#39;s       Selectivity of other products in   23.2   5.8   4.4   0       total products       % yield of DMC in total products   54.4   61.2   74.4   71.0       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 5  
       [0029]    This example illustrates effect of time on stream (TOS) on conversion and product selectivity in isopropylation of m-xylene. USY catalyst (0.5 g) was loaded into a reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of m-xylene and isopropyl alcohol in a 4:1 molar ratio was introduced into fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in presence of inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 6.48 h −1  and a temperature of 140° C. for a period of 1-8 hours. Products were chilled at 0° C., collected every hour up to 8 hours and analyzed by gas chromatograph (Shimadzu GC-14B) using flame ionization detector and 3 m×⅛″ packed column with 5% bentone and 5% DIDP on chromosorb WHP, with mesh size of 801100. Results of the reaction are given in Table V.  
                                                                     TABLE V                           Effect of TOS on conversion and product selectivity       in the isopropylation of m-xylene over zeolite USY                Time on stream            Conversion or selectivity (mole %)   1 h   3 h   5 h   8 h                    Conversion of m-xylene   16.6   19.1   15.7   14.2       Selectivity of dimethylcumenes in total   91.0   93.0   91.7   87.0       products a         Selectivity of 2,4 dimethylcumene among   78.9   56.1   52.8   29.6       DMC&#39;s       Selectivity of 2,6 dimethylcumene among   21.1   43.9   47.2   70.4       DMC&#39;s       Selectivity of other products in total   9.0   7.0   8.3   13.0       products       2.4 DMC/2.6 DMC   3.7   1.3   1.1   0.4       % yield of DMC in total products w.r.t.   60.0   70.8   57.6   49.6       limiting reagent, i.e ispropylating       agent                          
 
       EXAMPLE 6  
       [0030]    This example illustrates effect of temperature on conversion and product selectivity in isopropylation of m-xylene. USY catalyst (0.5 g) was loaded into reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of m-xylene and isopropyl alcohol in 4:1 molar ratio was introduced into fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in continuous manner in presence of an inert carrier gas (nitrogen, flow=35 ml/min) at WHSV of 6.48 h −1  and at different temperatures (Table VI) for a period of 1 hour. Products were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of reaction are given in Table VI.  
                                                                     TABLE VI                           Effect of temperature on conversion and product selectivity       in the isopropylation of m-xylene over zeolite USY                Temperature, ° C.            Conversion or selectivity (mole %)   120   140   160   180                    Conversion of m-xylene   12.3   16.5   27.3   29.3       Selectivity of dimethylcumenes in total   93.3   91.0   88.0   64.1       products a         Selectivity of 2,4 dimethylcumene among   58.8   78.9   75.3   67.2       DMC&#39;s       Selectivity of 2,6 dimethylcumene among   41.2   21.1   24.7   32.8       DMC&#39;s       Selectivity of other products in total   6.7   9.0   12.0   35.9       products       2,4 DMC/2,6 DMC   1.4   3.7   3.1   2.0       % yield of DMC in total products w.r.t.   46.0   60.0   96.1   75.2       limiting reagent, i.e. isopropylating       agent                          
 
       EXAMPLE 7  
       [0031]    USY catalyst (0.5 g) was loaded into the reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of m-xylene and isopropyl alcohol in a 4:1 molar ratio was introduced into the fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in the presence of an inert carrier gas (nitrogen, flow=35 ml/min) at different WHSV (Table VII) and a temperature of 140° C. for 1 hour.  
         [0032]    Product was chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table VII below.  
                                                             TABLE VII                           Effect of space velocity on conversion and product selectivity       in the isopropylation of m-xylene over zeolite USY                WHSV, h −1              Conversion or selectivity (mole %)   3.2   6.5   12.9                    Conversion of m-xylene   20.7   16.5   10.7       Selectivity of dimethylcumenes in   90.0   91.0   93.8       total products a         Selectivity of 2,4 dimethylcumene   75.8   78.9   53.3       among DMC&#39;s       Selectivity of 2,6 dimethylcumene   24.2   21.1   46.5       among DMC&#39;s       Selectivity of other products in   10.0   9.0   6.2       total products       2,4 DMC/2,6 DMC   3.1   3.7   1.2       % yield of DMC in total products w.r.t.   74.4   60.0   40.0       limiting reagent, i.e. isopropylating       agent                          
 
       EXAMPLE 8  
       [0033]    This example illustrates the effect of molar ratio of m-xylene to isopropanol on conversion and product selectivity in isopropylation of m-xylene. USY catalyst (0.5 g) was loaded into the reactor in such a way that the catalyst bed was sandwiched between inert porcelain beads. A mixture of m-xylene and isopropyl alcohol with different molar ratio (Table VIII) was introduced into fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in the presence of an inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 6.48 h −1  and a temperature of 140° C. for a period of 8 hours. The product were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table VIII.  
                                                                     TABLE VIII                           Effect of molar ratio of m-xylene and isopropanol       on conversion and product selectivity in the isopropylation       of m-xylene over zeolite USY                m-xylene/isopropanol (molar)            Conversion or selectivity (mole %)   2:1   4:1   8:1   10:1                    Theoretical maximum conversion   50   25   12.5   10       Conversion of m-xylene   19.1   16.5   11.3   6.7       Selectivity of dimethylcumenes in   74.6   91.0   98.0   99.0       total products a         Selectivity of 2,4 dimethylcumene   54.2   78.9   71.1   72.0       among DMC&#39;s       Selectivity of 2,6 dimethylcumene   45.8   21.1   28.9   28.0       among DMC&#39;s       Selectivity of other products in   25.4   9.0   2.0   1.0       total products       2,4 DMC/2,6 DMC   1.2   3.7   2.5   2.6       % yield of DMC in total products   28.6   60.0   88.8   66.0       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 9  
       [0034]    This example illustrates effect of time on stream (TOS) on conversion and product selectivity in isopropylation of o-xylene. USY catalyst (0.5 g) was loaded into the reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of o-xylene and isopropyl alcohol in a 4:1 molar ratio was introduced into fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in the presence of inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 6.48 h −1  and a temperature of 140° C. for a period of 1-8 hours. Products were chilled at 0° C., collected every hour up to 8 hours and analyzed by gas chromatograph (Shimadzu GC-14B) using flame ionization detector and 3 m×⅛″ packed column with 5% bentone and 5% DIDP on chromosorb WHP, with mesh size of 801100. Results of reaction are given in Table IX.  
                                                                     TABLE IX                           Effect of TOS on conversion and product selectivity       in the isopropylation of o-xylene over zeolite USY                Time on stream            Conversion or selectivity (mole %)   1 h   3 h   5 h   8 h                    Conversion of o-xylene   15.6   18.6   16.9   15.1       Selectivity of dimethylcumenes in total   85.1   85.0   83.8   80.0       products a         Selectivity of 3,4 dimethylcumene among   92.6   90.9   89.7   85.4       DMC&#39;s       Selectivity of 2,3 dimethylcumene among   6.4   8.4   9.7   14.1       DMC&#39;s       Selectivity of other products in total   14.9   15.0   16.2   20.0       products       3,4 DMC/2,3 DMC   14.5   10.8   9.2   6.1       % yield of DMC in total products w.r.t.   53.2   63.2   56.8   48.4       limiting reagent, i.e. isopropylating       agent                          
 
       EXAMPLE 10  
       [0035]    This example illustrates effect of temperature on conversion and product selectivity in isopropylation of o-xylene. USY catalyst (0.5 g) was loaded into reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of o-xylene and isopropyl alcohol in 4:1 molar ratio was introduced into fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in continuous manner in presence of inert carrier gas (nitrogen, flow=35 ml/min) at WHSV of 6.48 h −1  and at different temperatures (Table X) for a period of 1 hour. Products were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of reaction are given in Table X.  
                                                                     TABLE X                           Effect of temperature on conversion and product selectivity       in the isopropylation of o-xylene over zeolite USY                Temperature, ° C.            Conversion or selectivity (mole %)   120   140   160   180                    Conversion of o-xylene   4.6   15.6   20.7   25.0       Selectivity of dimethylcumenes in total   86.0   85.1   83.0   75.7       products a         Selectivity of 3,4 dimethylcumene among   72.3   92.6   95.0   95.6       DMC&#39;s       Selectivity of 2,3 dimethylcumene among   26.5   6.4   2.9   2.6       DMC&#39;s       Selectivity of other products in total   14.0   14.9   17.0   24.3       products       3,4 DMC/2,3 DMC   2.7   14.5   32.8   36.8       % yield of DMC in total products w.r.t.   16.0   53.2   68.8   75.6       limiting reagent, i.e. isopropylating       agent                          
 
       EXAMPLE 11  
       [0036]    USY catalyst (0.5 g) was loaded into the reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of o-xylene and isopropyl alcohol in a 4:1 molar ratio was introduced into fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in the presence of inert carrier gas (nitrogen, flow=35 ml/min) at different WHSV (Table XI) and a temperature of 140° C. for 1 hour. Product was chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of reaction are given in Table XI.  
                                                                     TABLE XI                           Effect of space velocity on conversion and product selectivity       in the isopropylation of o-xylene over zeolite USY                WHSV, h −1                  Conversion or selectivity (mole %)   3.2   6.5   12.9                            Conversion of o-xylene   24.3   15.6   6.4           Selectivity of dimethylcumenes in   82.0   85.1   86.5           total products a             Selectivity of 3,4 dimethylcumene   94.6   92.6   81.3           among DMC&#39;s           Selectivity of 2,3 dimethylcumene   4.2   6.4   18.0           among DMC&#39;s           Selectivity of other products in   18.0   14.9   13.5           total products           3,4 DMC/2,3 DMC   22.5   14.5   4.5           % yield of DMC in total products   79.6   53.2   22.0           w.r.t. limiting reagent, i.e.           isopropylating agent                                  
 
       EXAMPLE 12  
       [0037]    This example illustrates the effect of molar ratio of o-xylene to isopropanol on conversion and product selectivity in isopropylation of o-xylene. USY catalyst (0.5 g) was loaded into the reactor in such a way that the catalyst bed was sandwiched between inert porcelain beads. A mixture of o-xylene and isopropyl alcohol with different molar ratio (See Table XII) was introduced into the fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in the presence of an inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 6.48 h −1  and a temperature of 140° C. for a period of 8 hours.  
         [0038]    The product were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table XII below.  
                                                                     TABLE XII                           Effect of molar ratio of o-xylene and isopropanol       on conversion and product selectivity in the isopropylation       of o-xylene over zeolite USY                o-xylene/isopropanol (molar)            Conversion or selectivity (mole %)   2:1   4:1   8:1   10:1                    Theoretical maximum conversion   50   25   12.5   10       Conversion of o-xylene   21.4   15.6   10.7   9.4       Selectivity of dimethylcumenes in   82.0   85.1   86.0   89.3       total products a         Selectivity of 3,4 dimethylcumene   86.5   92.6   93.4   94.6       among DMC&#39;s       Selectivity of 2,3 dimethylcumene   13.5   6.4   6.6   5.4       among DMC&#39;s       Selectivity of other products in   18.0   14.9   14.0   10.7       total products       3,4 DMC/2,3 DMC   6.4   14.5   14.2   17.5       % yield of DMC in total products   35.0   53.2   73.6   84.0       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 13  
       [0039]    This example illustrates effect of zeolite structure on conversion of p-xylene and the yield of dimethylcumene in the total products. Different catalysts (0.5 g) (Table XIII) were loaded into the reactor such that respective catalyst beds were sandwiched between inert porcelain beads. A mixture of p-xylene and isopropyl alcohol with molar ratio 4:1 was introduced into the fixed bed reactor by a syringe pump (Sage Instruments, Model 352, USA) in continuous manner in presence of inert carrier gas (nitrogen, flow=35 ml/min) at WHSV of 3.24 h −1  and temperature of 140° C. for 1 hour.  
         [0040]    The products were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table XIII below.  
                                                                     TABLE XIII                           Effect of different catalysts on conversion and product       selectivity in the isopropylation of p-xylene                Catalysts            Conversion or selectivity (mole %)   USY   H-Beta   H-Mord   H-ZSM-5                    Conversion of p-xylene   24.3   21.9   11.2   0.2       Selectivity of dimethylcumenes in   90.5   97.7   97.0   —       total products a         Selectivity of 2,5 dimethylcumene   98.0   94.4   95.6   —       among DMC&#39;s       Selectivity of other products in   9.5   2.3   3.0   —       total products       % yield of DMC in total products   88.0   85.6   10.9   —       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 14  
       [0041]    This example illustrates effect of zeolite structure on conversion of m-xylene and the yield of dimethylcumenes. Different catalysts (0.5 g) (Table XIV) were loaded into reactor such that the respective catalyst beds were sandwiched between inert porcelain beads. A mixture of m-xylene and isopropyl alcohol with molar ratio 4:1 was introduced into the fixed bed reactors by a syringe pump (Sage Instruments, Model 352, USA) in continuous manner in presence of an inert carrier gas (nitrogen, flow=35 ml/min) at WHSV of 3.24 h −1  and temperature of 140° C. for 1 hour.  
         [0042]    The products were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table XIV.  
                                                                     TABLE XIV                           Effect of different catalysts on conversion and product       selectivity in the isopropylation of m-xylene                Catalysts            Conversion or selectivity (mole %)   USY   H-Beta   H-Mord   H-ZSM-5                    Conversion of m-xylene   20.7   16.5   6.0   0.1       Selectivity of dimethylcumenes in   90.0   97.5   91.4   —       total products a         Selectivity of 2,4 dimethylcumene   75.8   44.1   6.5   —       among DMC&#39;s       Selectivity of 2,6 dimethylcumene   24.2   55.1   93.5   —       among DMC&#39;s       Selectivity of other products in   10.0   2.5   8.6   —       total products       2,4 DMC/2,6 DMC   3.1   0.8   0.07   —       % yield of DMC in total products   74.4   64.4   54.8   —       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 15  
       [0043]    This example illustrates effect of zeolite structure on conversion of o-xylene and the yield of dimethylcumenes. Different catalysts (0.5 g) (Table XV) were loaded into reactor such that the respective catalyst beds were sandwiched between inert porcelain beads. A mixture of o-xylene and isopropyl alcohol with molar ratio 4:1 was introduced into the fixed bed reactors by a syringe pump (Sage Instruments, Model 352, USA) in continuous manner in presence of an inert carrier gas (nitrogen, flow=35 ml/min) at WHSV of 3.24 h −1  and temperature of 140° C. for 1 hour.  
         [0044]    The products were chilled at 0° C. and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table XV.  
                                                                     TABLE XV                           Effect of different catalysts on conversion and product       selectivity in the isopropylation of o-xylene                Catalysts            Conversion or selectivity (mole %)   USY   H-Beta   H-Mord   H-ZSM-5                    Conversion of o-xylene   24.2   15.5   7.5   0.0       Selectivity of dimethylcumenes in   82.0   95.9   90.5   —       total products a         Selectivity of 3,4 dimethylcumene   94.6   88.5   97.2   —       among DMC&#39;s       Selectivity of 2,3 dimethylcumene   4.2   9.7   2.8   —       among DMC&#39;s       Selectivity of other products in   18.0   4.1   9.5   —       total products       3,4 DMC/2,3 DMC   22.5   9.1   34.7   —       % yield of DMC in total products   79.2   59.6   27.2   —       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 16  
       [0045]    This example illustrates use of batch reactor on conversion and product selectivity in isopropylation of p-xylene under autogenous pressure in presence of cyclohexane as solvent using H-Beta catalyst. The reaction was carried out in 100 ml Teflon lined autoclave. A mixture of p-xylene (80 mmol) and isopropyl alcohol (20 mmol) in molar ratio 4:1 was added to 50 g cyclohexane in a beaker. The mixture was then transferred to an autoclave and 0.5 g H-Beta (previously activated at 300° C. in presence of air) was added to the reaction mixture. The autoclave was then sealed and heated for 25 hours at 190° C. oven under self generated pressure. After given reaction time (Table XVI) the autoclave was allowed to cool to room temperature (ca. 2 hours). The solid was extracted with acetone and the mother liquor concentrated in rotavapour.  
         [0046]    The concentrated product was analyzed by gas chromatograph (Shimadzu GC-14B) using flame ionization detector and 3 m×⅛″ packed column with 5% bentone and 5% DIDP on chromosorb WHP, with mesh size of 801100. Results of reaction are given in Table XVI.  
                                                                             TABLE XVI                           Effect of time on stream on conversion and product selectivity in the       isopropylation of p-xylene using zeolite H-Beta in batch reactor                Reaction time, hours            Conversion or selectivity (mole %)   3   7   10   20   25                    Conversion of p-xylene   7.3   9.3   9.9   10.5   10.6       Selectivity of dimethylcumenes   100   100   100   100   100       in total products a         Selectivity of 2,5 dimethylcumene   100   100   100   100   100       among DMC&#39;s       Selectivity of other products in   0.0   0.0   0.0   0.0   0.0       total products       % yield of DMC in total products   29.2   37.2   39.6   42.0   42.4       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 17  
       [0047]    This example illustrates effect of time on stream (TOS) on conversion and product selectivity in alkylation of p-xylene using n-propanol as alkylating agent. USY catalyst (0.5 g) was loaded into the reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of p-xylene and n-propanol in a 4:1 molar ratio (Table XVII) was introduced into fixed bed reactor by syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in presence of inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 3.24 h −1  and a temperature of 140° C. for 1 hour. Product were chilled at 0° C., collected and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table XVII below.  
                                                                     TABLE XVII                           Effect of TOS on conversion and product selectivity       in the isopropylation of p-xylene over zeolite       USY using n-propanol as alkylating agent                Time on stream            Conversion or selectivity (mole %)   1 h   3 h   5 h   8 h                    Conversion of p-xylene   6.5   5.7   4.8   3.4       Selectivity of dimethylcumenes in   88.6   90.6   96.0   98.8       total products a         Selectivity of 2,5 dimethylcumene   80.5   61.0   62.0   63.0       among DMC&#39;s       Selectivity of other products in   11.4   9.4   4.0   1.2       total products       % yield of DMC in total products   23.0   20.7   18.4   3.4       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
       EXAMPLE 18  
       [0048]    This example illustrates effect of time on stream (TOS) on conversion and product selectivity in alkylation of p-xylene using propylene as alkylating agent. USY catalyst (0.5 g) was loaded into the reactor such that the catalyst bed was sandwiched between inert porcelain beads. A mixture of p-xylene and propylene in a 4:1 molar ratio (Table XVIII) was introduced into fixed bed reactor by syringe pump (Sage Instruments, Model 352, USA) in a continuous manner in presence of inert carrier gas (nitrogen, flow=35 ml/min) at a WHSV of 6.48 h −1  and a temperature of 140° C. for a period of 8 hours. Product were chilled at 0° C., collected and analyzed by gas chromatograph using flame ionization detector. Results of the reaction are given in Table XVIII.  
                                                                     TABLE XVIII                           Effect of TOS on conversion and product selectivity       in the isopropylation of p-xylene over zeolite       USY using propylene as alkylating agent                Time on stream            Conversion or selectivity (mole %)   1 h   3 h   5 h   8 h                    Conversion of p-xylene   22.3   17.2   13.8   12.5       Selectivity of dimethylcumenes in   95.4   91.8   89.6   86.4       total products a         Selectivity of 2,5 dimethylcumene   98.5   98.0   99.6   100       among DMC&#39;s       Selectivity of other products in   4.6   8.2   10.4   13.6       total products       % yield of DMC in total products   70.0   63.2   49.6   43.2       w.r.t. limiting reagent, i.e.       isopropylating agent                          
 
         [0049]    The salient features of the invention are that the reaction is carried out in a single step by direct contacting a mixture of xylene isomers and alkylating agent in continuous or batch processes with a solid acid zeolite catalyst (preferably USY or Beta). It is observed that the solid acid zeolite catalyst used show high activity and selectivity in the alkylation of all xylene isomers, unlike in the prior art where only o- and not p- or m-xylene could be alkylated.  
         [0050]    Advantages of the Invention  
         [0051]    1. The catalyst used is eco-friendly, easy to handle, easy to recover and has no or minimal corrosion. As a result disposal of waste is not a problem.  
         [0052]    2. The catalysts exhibit high activity and selectivity in alkylation of all xylene isomers and are easily regenerated by thermal treatment in the presence of air.  
         [0053]    3. Dimethylcumene isomers are formed with high selectivity.