Conversion of straight-run gas-condensate benzenes into high- octane gasolines based on modified ZSM-5 zeolites

This paper describes the conversion of straight-run benzene of gas condensate into high-octane gasoline based on zeolite catalyst ZSM-5, modified in binary system oxide- based Sn (III) and Bi (III). It was defined that the introduction of the binary system oxide-based Sn(III) and Bi (III) into the basic zeolite results in the 2-fold increase of its catalytic activity.High-octane gasoline converted from straight-run benzene is characterized by a low benzol content in comparison to the high-octane benzenes produced during the catalytic reforming.


Introduction
Today one of the basic processs in petroleum refining is catalytic reforming , i.e. a technological procees where high-octanes benzenes are produced from straight-run gasoline.The catalytic reforming proceeds at 450-500 °С in hydrogen-containing gas environment based on alumino-platinum catalysts. Alumino-platinum catalyst reforming is rather sensitive to different micro-impurities, therefore, raw hydrocarbons should be preliminarily subjected to high purification from sulphur-, oxygen and nitrogen compounds. Produced high-octane benzene in catalytic reforming based on aluminoplatinum catalysts includes up to 50-70 % aromatic hydrocarbon and up to 7-15% benzol, which , in this case, requires additional separation of arene and especially benzol so as to produce finished motor gasoline from liquid reforming products [1][2].
In this respect, the most practical process is zeoforming [3] on zeolite-containing catalysts-ZSM-5 type for production of high-octane benzenes with low aromatic hydrocarbon content. Zeolites have a unique property [4]: they have high activity and selectivity in such reactions as dehydration, cracking isomerization, oligomerization and dehydrocyclization of various hydrocarbon source groups due to their unique microporous structure (micropore size-0.5-0.8 nm) and molecular and size analysis properties.The application of zeolite-containing catalysts excludes the preliminarily raw material hydrofining.
The conversion process of straight-run gasoline based on zeolite catalysts is quite different from that of catalyst reforming as the first produces high-octane benzene with low benzol content (not more than 1-3%), total aromatic hydrocarbon content-not more than 30-35 % and sulphur-not more than 0.05-0.10 mass %. It should also be noted that preliminarily raw hydrocarbon hydrofiningis excluded due to the application of different modified zeolite-containg catalysts [5][6][7][8][9].
The target of this research is to investigate the conversion process of straight-run benzene-gas condensate into high-octane gasoline based on zeolite catalysts, modified by binary systems on complex tin and bismuth oxides.

Experimental procedure amd applied methods
High-silica zeolites (HSZ-G) with high silic-module 50 was produced from alkaline alumosilicic gel at 175-180°С during 3-4 days applying organic structure-forming additive hexamethylenediamine as template. After crystallization synthezied zeolites are flushed out in distilled water several times, dried at 110°С and then calcinated at 550°С for 6 hours. Reactive HSZ-G zeolites were converted in 1m NH 4 NO 3 water solution at 90°С for 2 hours and further drying-out at 110°С and calcinating at 550°С for 6 hours (Na 2 0 content in decationated zeolites is less than 0.01%). Modified HSZ-G is produced by liquid infiltration of 0. 1 М НCl solution, containing tin and bismuth oxides in the following element ratio: Sn/Bi = 10:1, to zeolite saturation capacity [10]. Complex tin and bismuth oxides are produced by coprecipitated hydroxide calcination. After this, zeolites with compound Sn and Bi oxide coating ( 1-5mass%)are dried at 110 0 С and calcinated at 550 0 С for 6 hours.
To identify the produced zeolite catalysts HSZ-G, infrared spectroscopy (infrared Fourier spectroscopy Nicolet 5700) and X-ray phase analysis (X-ray unit DRON-3, Mo-anode; Ni-filter) were applied. Infrared-spectrum was conducted within the range of 450-2000 cm -1 ). The X-ray of synthesized samples showed lines with the following interplanar spacing (d, Å): 11.05, 10.19, 4.26, 4.07, 3.87, 3.83, 3.73, 3.66, typical for high-silica zeolites ZSM-5 type. Produced zeolite infrared -spectra showed absorption bands at 1000-1200, 795-800, 451 and 541 cm -1 , as in this range, the absorption bands (a.b.) of the major oscillations of AlO 4 , SiO 4 tetrahedron skeleton are found. Intense absorption band of 1000-1200 cm -1 is conditioned by antisymmetric stretch vibrations of ТО 4 tetrahedron; band of 794 cm -1 is related to the valence vibration in which SiO 4 tetrahedron is the basic one, whereas, the position of this band influences the silicate module (SiO 2 /Al 2 O 3 ) in zeolite scaffold. Absorption band of 541 cm -1 is stipulated by five-membered rings in the zeolite skeleton and indicates the fact that this zeolite is of the ZSM group. All synthezied HSZ-G samples according to infrared spectroscopy and X-ray phase analysis could be related to zeolite group.
Conversion of straight-run benzene fraction of gas condensate at 70-170°С based on modified zeolite catalysts was performed in a circulating catalytic unit with a fixed-bed catalyst (reactor volume 8 cm 3 ) within the temperature interval of 350-425°С at feed space velocity of raw material2 hrs. -1 , atmosphere pressure and exposure for each fixed temperature process was 1 hour.
The analysis of gaseous hydrocarbons was performed in the stainless steel packed column (length-3m; inner diameter-3mm), filled up with 5% NaOH to Аl 2 О 3 (fraction-0.25-0.50 mm), liquid hydrocarbons-in the quartz-glass capillary column (100 m. х 0.25 mm. х 0.25 mkm) with a fixed-bed of stationary phased ZB-1. Quantitative analysis of gaseous and liquid conversion products of straightrun benzene was performed by the following method: gas chromatography via hardware and software package of gas chromatographer "Chromatek-kristall 500", error-correcting by "Chromatek Analitik" program.Positional error of gaseous and liquid hydrocarbons by chromatography method is ±2.5%.
According to the hydrocarbon composition group, straight-run benzene fraction of 70 -170°С gas condensate includes: 35 mass% n-paraffine,40 % isoalkane, 20 % naphthenes and 4 % arenes. The octane number of straight-run benzene fraction of 70 -170°С gas condensate is 65 points in investigated method (IM). The catalytic activity unit was the number of produced arenes from straightrun benzene.

Results and discussion
The straight-run benzene conversion temperature effect on virgin zeolite catalyst HSZ-G activity showed that with a rise in process temperature from 375 to 425°С and straight-run benzene feed space velocity of 2 hrs. -1 high-octane gasoline yield from straight-run benzene decreases from 63.2 to 54.8 % due to the increasing conversion intensity of raw hydrocarbons (table1, figure1). Firstly, gaseous product yield increases from 34. 8  С 6 -С 9 content in liquid products increases from 23.4 to 26.7 %. Toluene and xylene are prodominate in the arene group, where, with a rise in process temperature from 1.5 and 1.2 at 375°С to 2.0 and 1.3 % at 425°С benzol and olefine С 5+ content increases, respectively; whereas, naphthene and n/isoparaffin hydrocarbon C 5+ content decreases with a rise in temperature (Table 1). Among the gaseous products of the straight-run benzene conversion process the prodominate products are propane and butan, where with a rise in temperature from 375 to 425°С, the propane concentration among other gaseous products increases from 57.6 to 59.3 %; while the total content of propane and butane among gaseous products is 93-97 % and methane and ethane content-not more than 3-7 %. Introducing tin and bismuth oxides into HSZ-G (samples-1-3 % (Sn:Bi = 10:1)/99-97 % HSZ-G) significantly increases the content of arenes up to 32.9-43.8 % in straight-run benzene conversion process liquid products and the decreases the content of iso-paraffin and naphthene hydrocarbons from 27.4-34.9 % and 18.0-21.8 % ,respectively. High-octane benzene yield in the straight-run benzene conversion process with a rise in temperature from 375 to 425 0 С and feed space velocity of 2 hrs.  It should be noted that the produced high-octane gasoline from straight-run benzene based on zeolite catalysts modified 1-3 % (Sn:Bi = 10:1)/99-97 % HSZ-G fully complies with the corresponding motor gasoline "Evro-4 and 5". Further increase of tin and bismuth content (catalyst 5 % (Sn: Bi = 10:1)/95 % HSZ-G ) significantly decreases the arene content from 12.6-15.0 % in liquid products of the straight-run benzene conversion, while arene formation selectivity is not more than 30 %. Temperature C 0 aromatic hydrocarbons HSZ-G Thus, introducing tin and bismuth oxide mixture as 1-3 % (catalysts: 1-3 % (Sn:Bi = 10:1)/99-97 % HSZ-G) into zeolite ZSM-5 significantly increases the catalyst activity in the conversion process of straight-run benzene into high-octane gasoline. In this case, produced zeolites modified by complex tin and bismuth oxide mixture already show a high aromatic activity at the temperature from 375-400 о С. at the temperature reaction of 375 о С for sample 1 -1 % (Sn:Bi = 10:1)/99 % HSZ-G , the aromatic hydrocarbon yield is 35 %, which is 1.5 times higher than in the case of catalyst HSZ-G at the same temperature. This proves the perspective application of zeolite catalysts, modified by complex tin and bismuth oxide mixture in the conversion process of straight-run benzene into highoctane gasoline.

Conclusion
Thus, introducing modified complex additive based on tin and bismuth oxide mixture into zeolite ZSM-5 significantly increases the catalyst activity in the conversion process of straight-run benzene into high-octane gasoline, in comparison to HSZ-G feed. The most perspective are HSZ-G catalysts, modified 1-3 % complex tin and bismuth oxide mixture.