Influence of sulphide Cu (I) promoting additives concentration on acid and catalytic properties of high-silica zeolites in straight-run gasoline conversion

In present article the influence of Cu2S promoting additives concentration on acid and catalytic properties of high silica MFI-type zeolites is investigated in the process of conversion of straight-run gasoline fractions of gas condensate into high octane components of motor fuels. It was shown that zeolite modified with 1% of Cu2S nanoscaled powder possesses the highest acid centers concentration and highest catalytic activity.


Introduction
Research and development of high-yield advanced stock processing is an challenge of oil and gas chemistry. Within catalytic thermal processes antiknock can sufficiently improve the properties of straight-run gasoline fractions of oil and gas condensates.
One of the modern trends of rational nature management is a conversion of light hydrocarbons into high octane motor fuels component by means of zeolite-containing catalysts [1][2][3]. MFI-type zeolite is widely used in zeolite-based catalytic processes. It was stated that zeolite can be used to catalyze lots of acid-base hydrocarbons conversion reactions [4][5][6][7][8][9][10]. Due to the unique structure that facilitates its molecular-sieve selectivity zeolite sorbs only size-defined substances. Also, zeolite possesses unique acid properties and is related to supersacids. All its features facilitate the application of zeolite in petrochemical industry. Moreover, zeolite is more active catalyst than convenient catalytic systems. In present article we compared acid properties and catalytic activity of MFI-type zeolite and zeolite modified with Cu 2 S.

Experimental procedure
High-silica zeolites (HSZ) were produced by hydrothermal synthesis from alkaline aluminosilica gels at 175-185 °C in the course of 2-4 days [11]. After the crystallization was complete, the resulting zeolites were washed with water, dried at 110°C, and calcined at 600°C for 6 h. The HSZ were converted to the hydrogen active form by treatment with a 25% NH 4 Cl solution at 90 °C for 2 h, with subsequent drying at 110 °C and calcination at 600 °C for 6 h (the content of Na 2 O in decationized zeolites was less than 0.01%). The HSZ catalysts obtained were identified by IR spectroscopy (Nicolet 5700 IR Fourier spectrometer) and X-ray phase analysis (DRON-3 X-ray installation, Mo anode, Ni filter). According to X-ray phase analysis data, all the HSZ samples obtained belong to zeolites of the MFI type.
A certain amount of Cu 2 S nanoscaled powder was added to the origin zeolite. Mechanochemical activation was carried out with vibratory ball mill KM-1 for 12 hours at 25 о С. Nanoscaled particles of Cu 2 S were obtained within very high-temperature synthesis. The described method was applied for the synthesis of HSZ modified by 0.5, 1 and 3 % (mass) of Cu 2 S. The acid properties of HSZ were studied in a thermal-desorption installation on the basis of ammonia adsorption in a flow of carrier-gas helium in the temperature range 50-650°C at a linear heating rate of 10 deg min -1 . To eliminate diffusion hindrances and the effect of the physical form of ammonia adsorption on, HSZ ammonia was adsorbed at a high flow rate of the carrier-gas (110 cm 3 min -1 ) at 100 °C in the course of 1 h. After that, the reactor with a sample under study was cooled to 50 °C and ammonia was desorbed, with a katharometer serving as detector. Helium of A brand (99.995 vol %) and ammonia of pure grade were used in the experiments. The concentration of acid centers (μmol per gram of catalyst) in the samples under study was determined from the amount of ammonia contained in desorption peaks (forms), with the determination accuracy of adsorbed ammonia by gas chromatography being ±2.5%.
Transformations of the straight-run gasoline fraction with a boiling onset point of 170 °C from the gas condensate were studied on zeolite-containing catalysts on a flow-through catalytic installation with a fixed catalyst bed (reactor volume 10 cm 3 ) in the temperature range 350-425 °C at a volumetric flow rate of the raw material of 2 h -1 , atmospheric pressure, and experiment duration of 1 h at each fixed process temperature.
The gas condensate from the Myl'dzhinskoe deposit has the following fraction composition: boiling onset point 31 °C, 82 vol % of the condensate boils away at 200 °C, end boiling point 297 °C, loss and residue 10 vol %. As regards the group hydrocarbon composition, the straight-run gasoline fraction with a boiling onset point of 170 °C from the gas condensate has the following composition (wt %): n-alkanes 22.7, isoalkanes 33.2, naphthenes 39.9, and arenes 4.2. The octane number of the straight-run gasoline fraction with boiling onset point of 170 °C from the gas condensate of the Myl'dzhinskoe deposit is 60 points by the research method (RM) scale.
Gaseous hydrocarbons were analyzed on a stainless steel column (length 3 m, inner diameter 3 mm) packed with 5% NaOH on Al 2 O 3 , and liquid hydrocarbons, on a quartz glass capillary column (100 m × 0.25 mm × 0.25 μm) with a supported fixed ZB-1 phase. Gaseous and liquid products formed in conversion of straight-run gasoline fractions of the gas condensate were quantitatively analyzed by gas chromatography on a hardware-software complex based on a Khromatek-Kristall 5000 v. 1 gas chromatograph, with Khromatek-Analitik processing software. The research octane numbers were calculated by using the Khromatek-Analitik processing software from results of a gas-chromatographic analysis of the hydrocarbon composition of the starting raw material and liquid products formed in conversion of straight-run gasoline on the zeolite-containing catalysts under study. The error of the gaschromatographic determination of gaseous and liquid hydrocarbons is ±2.5%.

Influence of Cu 2 S promoting additives on acid properties of zeolite-containing catalysts
Acid properties of catalysts were estimated by means of temperature-programmed ammonia desorption. Temperature desorption profiles of all catalysts have two peaks. Low-temperature peak (120-250 ºС) of initial zeolite with Т max = 185 ºС is mainly related with ammonia desorption from weak acid centers (form I) that are coordinative unsaturated aluminium ions. High temperature peak (300-550 ºС) with Т max = 400 ºС is mainly related with ammonia desorption from strong acid centers (form II) that are hydrogen ions of hydroxylic bridge groups. From the data presented in table 1 it can be seen that initial high-silica zeolite possesses 383 µmol/g of weak acid centers and 128 µmol/g of strong acid centers. Within Cu 2 S modification the increase of total acid centers concentration takes place due to the increase in weak acid centers concentration while the concentration of strong acid centers remains approximately the same. It should be noted that samples modified by 0.5 % and 1 % of sulphide Cu(I) nanoscale powder possess the highest acid centers concentration (weak acid centers concentration is respectively 415 µmol/g and 410 µmol/g). Also, Cu 2 S additions lead to the rise of ammonia desorption temperature. For example, for weak acid centers it increases at 5-12 ºС and at 10-20 ºС for strong centers. Firstly the yield of gas products, primarily due to С 3 -С 4 paraffines, increases from 9.7 % (350 ºС) to 33.7 % (425 ºС). The yield of С 6 -С 9 liquid aromatic hydrocarbons increases from 9.3 % to 24.8 %. Toluol and xylol take the main part of aromatics. With the increasing temperature the yield of benzene and С 5+ oleffines increases from 0.4 % and 1.6 % (350 ºС) to 1.5 and 2.9 % (425 ºС), respectively. But the yield of naphthenic, n-and isoparaffinic С 5+ hydrocarbons decreases with the increasing temperature (table 2). Gas products are mainly presented by propane and butane.
The dependence of obtained liquid products octane number on temperature is presented in figure 1  (a). а) б)   With this figure we can conclude that the promoting addition of copper sulphide increases the octane number of obtained catalysate in comparison with the catalysate obtained with the origin HSZ. The highest octane number was obtained with samples 2 (0.5 % Cu 2 S / 99.5 % HSZ) and 3 (1 % Cu 2 S / 99 % HSZ). It varies from 82 (350 ºС) to 91 (425 ºС) points of RM.
The influence of temperature of straight-run gasoline conversion with zeolite-containing catalyst on catalysate yield is shown in figure 1 (b). Also, we can see that the highest gas product yield takes place with the origin zeolite. Promoting additions increase the activity of raw hydrocarbons conversion. This facilitates the increasing of liquid catalysate yield if compare to the origin zeolite. It should be pointed out that at all process temperature the sample with 1 % of Cu 2 S and 99 % of HSZ performs better yield than the sample with 0.5 % of Cu 2 S and 99.5 % of HSZ. So the yield of catalysate increases at 2-4 %.

Conclusion
Acid properties of initial high-silica zeolite and zeolite-containing catalysts modified with nanoscale Cu 2 S powder were investigated. Introduction of 0.5 % mass and 1 % mass of Cu 2 S promoting addition facilitated the increase of weak centers concentration at 9 %. Catalytic properties of zeolite-containing catalysts in straight-run gasoline fractions conversion into high octane components of motor fuels were investigated. It is stated that the yield of aromatic hydrocarbons increases with increasing temperature for all investigated catalysts. Also, increasing temperature leads to the decrease of main product yield due to the increase in degree of stock conversion. High octane catalysate mostly consists of isoparaffins, arenes and naphthens. Gas products mostly consist of propane and butane.
From all the proposed catalysts the most active samples contain 0.5 % Cu2S / 99.5 % HSZ and 1 % Cu2S / 99 % HSZ. These catalysts facilitate the obtaining of products with maximum octane numbers: 82 points at 350 ºС and 91 points at 425 ºС (research method). However, in the temperature range 350-425 ºС catalysate yield with the sample, modified with 1 % of nanoscaled powder of Cu 2 S, is higher than the one obtained with the sample, modified with 0.5 % of nanoscaled powder, at 2-4 %, respectively. Thus, it was shown that the sample containing 1 % Cu 2 S / 99 % HSZ performs the best activity and desired reaction product ratio.