Benzene Alkylation by Ethanol over Catalysts on the Base of Modified Natural Zeolites

Alkylation of aromatic compounds with ethanol was researched on modified natural zeolites of Kazakhstan deposits. These natural zeolites were identified as Clinoptilolite-Heulandite type. Zeolites were treated by hydrochloric acid, which leads to removal of some of the cations and to dealumination. They were modified by metals of II-III groups and mixed with synthetic zeolites Y and ZSM-5. The catalysts were characterized by elemental analysis, scanning electron microscopy, thermal analysis, BET and temperature-programmed desorption of ammonia. It is shown that natural zeolites, treated by acids and modified by metals, as well as synthetic zeolites can be used as catalysts for process of alkylation of benzene by ethanol. They demonstrate high activity and 71% selectivity in synthesis of ethylbenzene.


Introduction
Alkylation of benzene is one of the important methods of processing of hydrocarbon raw materials because products of this reactionethylbenzene and styrene -are the basic products of petrochemical synthesis. Ethylbenzene is a component for synthesis of styrene -a monomer of polystyrene production, which output in the world is continuously increasing. In industrial process the alkylation is carried out over ecologically dangerous catalysts such as HF, H 2 SO 4 , AlCl 3 , ВF 3 . Carrying out of this process environmentally friendly, corrosion-resistant and over solid acid catalysts is an important and urgent task. Now synthetic zeolites are widely used as ecologically clean and efficient catalysts for many petrochemical processes, including alkylation. Recently much attention is paid to the development and implementation of zeolite catalysts for the production of ethylbenzene [1][2]. Zeolite catalysts commonly replace traditional catalysts in oil processing and in petrochemistry, due to their unique crystal and adsorption properties, providing a high catalytic activity [3].
According to the Resolution of the Government of the Republic of Kazakhstan on October 13, 2006 №989 diversification of Aktau plant of plastics will be held on the base of complete technological scheme of polystyrene production based on benzene alkylation to ethylbenzene, production of styrene and its polymerization. Production of ethylbenzene will reach 127 thousand tons per year on petrochemical complex on the basis of benzene and para-xylene. In this aspect it is very important to develop highly effective and eco-friendly catalysts for studied process.
Kazakhstan possesses rich deposits of natural zeolites. Synthetic zeolites are usually expensive, so the purpose of this article is an attempt to engage local raw materials -natural zeolites of Kazakhstan for use in the alkylation process. In the present work we investigated zeolites of Kazakhstan deposits, studied their physicochemical modification.

Experimental
The process of alkylation of benzene with alcohols over zeolite catalysts was carried out in a flow type system under atmospheric pressure. Natural zeolites of deposits of Kazakhstan XRD analysis of these zeolites before and after acidic treatment showed that their crystalline structure is preserved. Acid activation of natural zeolites in essence does not change the crystalline surface of the zeolite particles and shows that this process involves the release of more effective crystal surface by the removal of foreign substances and the substitution rates of metal ions on the hydrogen ion.

Modification of Natural Zeolites by Metals
Metal ions have significant effect on the activity and selectivity of zeolites in the acid-base reactions. Therefore zeolites are subjected to modification by adding metals. At the beginning we have used a variety of metals-modifiers and their combinations ranked on their catalytic properties. Secondly, for each added metal we analyzed and compared types of active positions of metals in the zeolite matrix to their acidic and catalytic properties.
It was established that the introduction of Group II metals (alkaline earth elements) leads to a change in the distribution of acid sites on the surface of zeolites [8]. The authors of [9] showed that the addition of Ca as a promoter to zeolite reduces coking of catalysts during catalytic reactions. It was established that the modification of zeolites by lanthanide group metals, in particular, La and Ce, increases the thermal stability of catalysts [10]. Introduction of La cations to zeolite also contributes to the selectivity of the process of alkylation of aromatic hydrocarbons. Following metals: La, Ce, Mg have been investigated as modifiers for our catalysts.
We studied the correlation of zeolites acidity with the nature of the metal-modifier. Acidity of zeolites was measured by TPD of ammonia. With changing of the metal the number of weak, moderate and strong acid centers decreases as well as total acidity was also decreased ( Table 2). This fact is explained by the removal of aluminum from the zeolite -aluminum as we know provides a surface acidity of the zeolite. However, the number of Lewis centers slightly increased with the introduction of transition metals.
Thermo-acidically treated and modified zeolites were tested in the alkylation of benzene with ethanol (Table 3). Initially thermo-acidically treated zeolite showed low catalytic activity. It is shown that the modification by Mg, La, Ce metals decreases the total acidity as well as the number of acid sites of medium strength. The surface of catalysts modified by magnesium and lanthanum contains of number of centers of high acidity and the total acidity is 1.91 and 1.84 mmol/g, respectively. In the case of Ce the total acidity totals to 1.61 mmol/g and the number of centers is significantly reduced, which probably leads to the Benzene Alkylation by Ethanol over Catalysts on the Base of Modified Natural Zeolites Eurasian ChemTech Journal 14 (2012) 211-217 reduced activity of the catalyst modified with the metal (Table 3).
According to [11] in the zeolite catalysts lanthanides are in the form of oxides and they are uniformly distributed on the surface. La and Ce form new active centers on the surface of the zeolite for the alkylation process. It is also known that the modification of zeolites by metals of Group II changes the distribution of acid sites in zeolites. The authors of the study [12] showed that zeolite catalysts exhibit the maximum activity if there are no strong Bronsted and Lewis acid sites. The most active catalysts have mainly acid sites of medium strength. Table 3 presents data for the alkylation of benzene with ethanol over the treated natural zeolite modified by metals. By itself, the natural zeolite has a very low activity. Modification of natural zeolite Shankanay increases conversion of benzene from 23 to 64%, the yield of ethylbenzene from 15 to 60% and selectivity to ethylbenzene from 65 to 94%. The ranking of modifiers by their influence on catalytic activity is: We studied the influence of the nature of the precursor -metal salt: nitrates, acetates, chlorides and carbonates on the activity of the modified zeolites. The most active catalysts prepared from magnesium nitrate and lanthanum nitrate increase the maximum conversion of benzene to 64%, the yield of ethylbenzene to 60% and the selectivity to ethylbenzene to 94%. According to these indicators magnesium precursors are arranged in the following: nitrate > carbonate > acetate > chloride. The lanthanum precursors are located in the following series by activity: nitrate > carbonate > chloride > acetate.

Modification of Natural Zeolites by Synthetic Zeolites
In addition to the modification of natural zeolites by metals, synthetic zeolites Y and ZSM-5 were introduced to natural zeolites in the amount of 15-20%.
Catalytic systems prepared three ways: 1mechanical mixing of zeolite, 2 -application of synthetic zeolite on natural zeolite, 3 -mixing aqueous suspensions of zeolites. The application of synthetic zeolite on natural zeolites proved to be optimal.
Modification of zeolites leads to higher conversion of benzene from 23 to 60%, yield of ethylbenzene from 15 to 55%, selectivity for the formation of ethylbenzene from 65 to 92% (Table  4). As an additive ZSM-5 was better than Y zeolite. Among the metals La was the best modifier, which increased the conversion of benzene to 64%, yield of ethylbenzene to 60% and selectivity for ethylbenzene to 94%. Twice modified zeolite (with La and by the addition of ZSM-5) showed the maximum benzene conversion 72%, yield of ethylbenzene to 69% and selectivity of 96% (Table  4). The combination of magnesium and lanthanum as modifiers to the Shankanay+ZSM-5 catalyst led to a better results: for benzene conversion 75%, ethylbenzene yield 73% and selectivity on ethylbenzene 97% (Table 4). In industry the synthesis of ethylbenzene by alkylation of benzene with ethylene is conducted on the AlCl 3 , ВF 3 catalysts. The yield is up to 75%. But the selectivity of industrial process is not high because there are a lot of by-products like toluene, isomeric xylenes etc.
The use of ethanol as an alkylating agent is a new and unexplored process. Chosen catalysts must be more active than usual because they must provide the dehydration of ethanol as the first step of this process. The reaction proceeds by following mechanism: The first stage of the process is the interaction of benzene with protonated surface active center forming intermediate surface complex H + -С 6 H 6 . In the next step ethanol molecule reacts with different active catalytic centers and dehydration of ethanol takes place to form ethylene. On the third stage protonated complex interacts with gas phase molecule of ethylene to form the final product ethylbenzene. Table 5 shows the data of the surface area measurements of the catalysts samples by BET method. The surface area of analyst Shankanay + NZSM-5 is reduced by the modification with magnesium and increases with the modification by La and Ce. Pore radius decreases, which is reflected in the decrease in pore volume. It is known that synthetic zeolites have a low mechanical strength. This parameter is very important for industrial processes. As a result of modifying the strength of the synthetic zeolites increases as shown in Table 6. Crushing strength of zeolites was measured by standard methods on the device PC-1. In the case of two zeolites Y and ZSM-5 (clean and with the addition of natural zeolite Shankanay) the mechanical strength increases from 0.001 to 0.08 kg/mm 2 ( Table 6).