Alkylation of Benzene with Technical Fraction Propylene-Propane Over Modified B-(Al)-HZSM-5 Catalysts

IULIEAN VASILE ASAFTEI1, MARIA IGNAT1*, CATALIN NECULAI LUNGU1, ION SANDU2,3*, ELVIRA MAHU1,3 1 Alexandru I. Cuza University of Iasi, Faculty of Chemistry, Laboratory of Materials Chemistry, 11 Carol I Blvd, 700506, Iasi, Romania 2 Alexandru I. Cuza University of Iasi, ARHEOINVEST Interdisciplinary Platform, 22 Carol I Blvd, 700506, Iasi, Romania 3 Romanian Inventors Forum, 3 Sf. Petru Movila, Bl. L11, Sc. A, Et. III, Ap. 3, 700089, Iasi, Romania 4 Petru Poni Institute of Macromolecular Chemistry, Laboratory of Inorganic Polymers, 41A Grigore Ghica Voda Alley, 700487, Iasi, Romania

In the last decades, the pentasil -type zeolites, containing B, Ga, Zn, Fe, Ti, etc., were found very active catalysts in many reactions: aromatization, alkylation, oxidation, etc. Ga-HZSM-5 and /or pentasil-type gallosilicates, and Zn-HZSM-5 proved to be ver y active catalysts for the aromatization of light hydrocarbons (alkanes and alkenes) .
The MFI-titanosilicates are industrial catalyst for the oxidation of alkenes or aromatics hydrocarbons with hydrogen peroxide as oxidant [34].
The conventional catalysts for benzene alkylation with alkenes (C 2 = , C 3 = ) are supported phosphoric acid in a fixed bed reactor with reactants in liquid phase (UOP) or Friedel-Crafts AlCl 3 (Monsanto). Environment protection, process safety and avoidance of corrosion request for replacing these catalysts by regenerable solid acids.
In this work we are presenting our results on alkylation of benzene with C 3 = /C 3 technical fraction on the B-HZSM-5 catalyst.

Experimental part
Synthesis of NaZSM-5 The NaZSM-5 zeolite was synthesized by hydrothermal crystallization at 180±5 o C for 24 h under autogenously pressure from a mixture containing sodium silicate, aluminum sulphate, ethylene glycol (EG), sulphuric acid, ammonia solution and distilled water. The procedure of synthesizing NaZSM-5 in our laborator y from the amorphous hydrogel with molar ratio SiO 2 /Al 2 O 3 = 58.92, HOavailable/SiO 2 = 0.22, Na + total/SiO 2 = 0.72, EG/SiO 2 = 0.40 and H 2 O/SiO 2 = 31.10 and pH = 11.50 is based on method described in [60]. The first synthesis of ZSM-5 zeolite using tetrapropylammonium bromide as template belongs to Argauer and Landolt [61].The crystalline product was filtered, washed with distilled water, dried at 110 o C for 6 hours and calcined at 550 o C for 6 h to remove the organic material and to obtain sodium form, Na-ZSM-5.

Zeolite modification
The sodium form of the Na-ZSM-5 (SiO 2 /Al 2 O 3 = 34.65) was converted into the NH 4 + form by ion exchange (three consecutive times) with a solution of 1M NH 4 NO 3 (ratio solid: liquid = 1: 5) at 80 o C for 6 h under mild stirring. The solid was then filtered, washed, dried over night at 110 o C and calcined in air at 550 o C for 6h when the protonic form HZSM-5 with acid properties was obtained.
By treating HZSM-5 sample with 1M solution of H 3 BO 3 (0.09 g/g B 2 O 3 /HZSM-5) at 80 o C for 10 h a nonskeletal boroncontaining catalyst was prepared. The suspension was dried over night at 110 o C and calcined at 400 o C for 6 h when nonskeletal and skeletal boron catalyst was prepared. The solid encoded B-(Al)-HZSM-5 contain 8.26 wt% as B 2 O 3 (2.56 wt% B).
The final catalyst was prepared by mechanically mixing the B-(Al)-HZSM-5 powders with 20 wt% γ-Al 2 O 3 as binder and a little distilled water until a soft paste was obtained and extruded into pellets. The pellets were dried at 110 o C overnight and activated in N 2 at 450 o C for 6 h.
The acidity and strength distribution of H-ZSM-5 and B-(Al)-HZSM-5 were evaluated by using temperature programmed desorption (TPD) of ammonia technique. The samples were activated in a flow of dry N 2 at 500 o C for 4 h and after cooling to 80 o C ammonia adsorption take places. Physically adsorbed ammonia was removed by degassing in flowing N 2 at 100 o C for 3 h. The amount of ammonia desorbed from 100 to 800 o C (at the rate of 10 o C/min) was quantitatively monitored volumetrically (absorption in 1 M HCl). The total ammonia desorbed corresponds to number of acid sites and the desorption temperature to the strength of acid sites (weak and strong). Si and Al contents were determined using the ordinary wet chemical methods and the content of Na was measured flamphotometrically at 589 nm.

Catalytic tests
Performances of the B-Al-HZSM-5 catalyst for benzene alkylation with C 3 = /C 3 technical fraction were established in temperature at 250 o C, under 40 atm. pressure, WHSV = 2,35 h -1 and benzene : propylene molar ratio of 7:1, in a fixed bed continuous flow stainless-steel reactor (Twin Reactor System Naky, Metrimpex). The operating conditions (temperature, WHSV, pressure and benzene: propylene molar ratio) were in advance selected to obtain the high yield of IPB during the catalytic tests. Control of temperature, pressure, as well as gaseous feed was done through automatic devices. The reaction products were separated into gaseous and liquid fractions through an icetrap. The gases resulted from catalytic test were analyzed using a Carlo Erba G.C. (Model C, TCD) equipped with a 6 m column filled with squalane and dimethylsulfolane on Chromosorb P. The collected liquid corresponding to each catalytic test was analyzed with a Carlo Erba Vega G.C. (FID) equipped with a 25 m capillary column filled with SE-52 stationary phase.

Results and discussions
Structure, morphology and specific surface area Figure 1 shows the XRD patterns of parent NaZSM-5 sample after calcinations and of B-HZSM-5 composite after the heat treatment. The pattern confirms that the synthesized zeolite has the structure identical to MFI-type zeolite [62]. The parent NaZSM-5 has a high crystallinity derived from the high intensities of the XRD reflections in the range of 22.5 -25 o (2θ). No other diffraction lines were found in the XRD pattern. The XRD pattern of B(Al)-HZSM-5 composite obtained at 400 o C shows that the structure of host ZSM-5 was retained and in addition exhibit reflection at 14.61 and 28.10 (2θ) which are characteristic for the B 2 O 3 crystalline particles that cover the external and internal surface of HZSM-5 zeolite. The intensities of these peaks  Figure 2 presents the SEM images of parent NaZSM-5 and of HZSM-5 and the elemental composition by EDX spectra. It reveals the well-defined morphology of crystals indicating highly crystalline material. The EDX spectrum of HZSM-5 zeolite proves the absence of Na + ions after the ammonium exchange of NaZSM-5 zeolite.
The chemical oxidic composition of the calcined NaZSM  Propylbenzenes (PB) selectivity was calculated as: The reaction scheme for benzene alkylation with propylene by zeolite catalyst can be illustrated by the following typical equations: Table 1 TOTAL ACID SITES AND ACID STRENGTH DISTRIBUTION formation of secondary propenium ions The formation of NPB during the alkylation catalyzed by zeolites has been reported as isomerization of IPB by intramolecular and by intermolecular transalkylation with benzene [44,46]: Acid -catalysed alkylation's reactions of benzene with propylene are commonly considered to take place via carbenium-ion type mechanism. Propylene is first protonated on Bronsted acid site forming the active carbocations (secondar y propenium ions). By the electrophilic attack on secondary propenium ions on the benzene ring cumene (isopropylbenzene, IPB) is formed. Primary IPB can be dialkylated to o-, p-, and m-DIPB.
Depending on reaction conditions and catalysts, the DIPB isomers can by transalkylate with benzene to cumene. NPB is a by-product that heavily affects the quality of cumene because of the difficulty to separate it from cumene. The active species (the isopropyl cations) can react with another propylene molecule producing C 6 species transformed through oligomerization, cracking, isomerization and alkylation into mainly propylene oligomers and alkylbenzenes.

Conclusions
B-(Al)-HZSM-5 catalyst presented good activities and selectivity's in reaction of benzene alkylation with a C 3 /C 3 = technical fraction. IPB selectivity vs. of benzene and propylene consumed in reaction is upper 83 wt. %, in all effectuated tests.
The n-PB, an undesirable by-product, is obtained in small quantity. The di-isopropylbenzenes (1,3-and 1,4-DIPB) are the important by-products because they are separated in the liquid product and by transalkylation reaction with benzene in excess producing a supplementary quantity of cumene. Benzene alkylation with propane -propylene mixture on the B-(Al)-HZSM-5catalyst has been demonstrated to be prospective routes for synthesis of isopropyl benzene.
The unique advantages of zeolites as replacements for conventional acids and bases catalysts will serve as driving force to continue the expansion of practical uses of zeolites.