Mixed-Metal Oxide Catalyst for Liquid Phase Benzene Alkylation

Development of cheaper, active and more ecofriendly heterogeneous acid catalyst is a challenge mitigating the petrochemical industries. CuO-MoO3/ZrO2 solid catalyst was prepared by impregnation using suitable precursor materials supported over zirconia. Upon calcination at 450°C for 2 h and activation (by soaking in 2M H2SO4 for 30 minutes), available techniques were employed for the characterization. The available oxides and minerals in the catalyst were revealed by the XRF and XRD profiles respectively. The catalyst crystallite size (131.6nm) was obtained using the Bragg’s equation from the latter. Thermal analysis showed three weight loss stages between (49.25-152.06°C), (152.06-559.47°C) and (559.47-752.0°C ) while presence of sulphate and zirconia oxides was revealed by the FTIR analysis due to appearance of absorption bands around 1225-980cm-1 and 700-600cm-1 respectively. The catalyst (1wt%) was tested for alkylation in a continuous stirred reactor at 80°C using variable (2:1, 4:1 and 10:1) benzene to 1-decene molar ratios. The effects of reaction time and molar ratios on the selectivity, conversion and yield were determined. The alkylation results showed that the catalyst is highly selective to 1-decylbenzene as low amount of side products was obtained. The product yield and conversion increased with reaction time and benzene /1-decene molar ratio while selectivity decreased with increase in benzene /1-decene molar ratio with time.


Introduction
The dependence of petrochemical industries on active, selective and cheap heterogeneous catalysts cannot be overemphasized. A catalyst is a chemical substance which, even in a minute quantity, affects the rate of a chemical reaction either positively or negatively without being appreciably consumed in the reaction (Petrov et al. [16], Galadima and Muraza [10]).
Homogeneous and heterogeneous catalysis are the broad categories which catalysis is divided into, and benzene alkylation has been carried out by both methods. While homogeneous catalysis deals with study of catalysts and reactants in the same phase, heterogeneous catalysis deals with reactants and catalysts in different phases (Mann and Saunders [15]). Homogeneous catalysts exist in the same (homogeneous phase) virtually almost always liquid phase. In heterogeneous catalysis, a solid catalyst is used for vapour or liquid phase reactions.
Depending on their uses, heterogeneous catalysts may be in the form of metals (skeletal metals), metal oxides, sulphides, heteropolyacids, nitrides, carbides, borides, alloys, molecular sieves, ceramics, fibres, wires, salts and mineral acids etc. (Smith and Notheisz [20], Galadima and Muraza [10], Gushchin et al. [11]). The use of inorganic or mineral acid along with heterogeneous catalysts for liquid phase industrial reaction has become a focus. This is possible because there is ease in handling and product separation, catalyst re-use and minimization of wastes attributed to them. In addition, bimetallic supported catalysts are reported to be good catalysts for alkylation reactions (Wilson and Clark [22], Bolognini et al. [2]).
Alkylation is the reaction in which an alkyl (R-) group is added by means of substitution to an aromatic hydrocarbon. The unsaturated (or substituted) aryl or alkyl group usually added to the benzene ring (by substitution), could range from C 1 -C 14 .
Alkylation of benzene with olefins proceeds with a two-step mechanism via formation of a carbonium ion followed by attack on the benzene ring to form an alkyl benzene. The first step, involves the reaction of the olefin with the acid site to form an  [13]).

Catalyst preparation
The CuO-MoO 3 /ZrO 2 catalyst was prepared by the impregnation method as described by Haber et al. [12].

Characterization
The XRD analysis was carried out using 5g of the catalyst on a PANalytical 2830 ZT XRD analyser. Two-Theta starting position was 4 degrees to 75 degrees with a twotheta step of 0.026261 at 8.67 seconds per step. The tube current was 40mA and the tension was 45VA. Values of theta, d-spacing, 2 theta and peaks corresponding to ejected electrons are obtained. The crystallite size (t) of the catalyst was established based on the XRD patterns using the relation: where t is crystallite size, K is constant with values from 0.92 to 1.0, λ is wave length of the incident x-ray (usually taken as 0.1542nm), B is full width at half maximum (FWHM) for the highest peak and must be converted to radians, using equation (2.2) ( ) , 360 where Π = 3.142, B is full width at half maximum (FWHM) for the highest peak.

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The IR spectra of the solid catalyst was obtained by grinding the catalyst into fine powder with a particle size about 1-2 microns. Seven milligram (7mg) of finely ground catalyst sample was placed on a KBr plate and a small drop of Nujol (mineral oil) was added. The sample was evenly distributed with the second KBr plate and placed in the sample holder which was scanned between 4000-400cm −1 using an MB3000 IR analyzer and the spectra was generated using a high-tech Thermo Scientific Nicolet software.
Two milligrams (2mg) of the ground samples of pulverized catalyst were placed into a sample cup. Elemental composition determination was carried out on a current of 14kv for major oxides and 20kv for the trace elements/rare earth metals. Selected filters were "kapton" for major oxides, Ag/Al-thin for the trace elements/rare earth metals. The spectra was developed using Horizon MB ® XRF software. Each sample was measured for 100 seconds and the air medium was used throughout.
Thermal analysis was carried out using a DTG-60AH Thermogravimetric Analyser with the heating rate set at 20°C/min and nitrogen flow rate 40 ml/min while the final heating temperature was 900°C. The spectra was developed using a versatile Proteus ® software while maintaining a cooling time of approximately 12 minutes.

Catalyst evaluation
Alkylation of benzene is an important process in the petrochemical industry. One of the most useful alkyl benzenes, ethylbenzene (EB), is used as feedstock in the production of styrene, polystyrene etc. with higher olefins/substituted alkanes (C 10 -C 14 ), linear alkyl benzenes (LABs), which are the primary raw materials for the synthesis of lab sulfonates and surfactant intermediates used in producing detergents, are produced (Devassy et al. [6], Hernandez-Cortez et al. [13], Zhang et al. [24]).
The set up was maintained and refluxed for 2 h reaction time, taking a fraction of the products every 40 minutes. Agilent 7890A GC coupled with 5977MS was used for the analysis of the alkylation products in the variable fractions. 1µl of the sample was punctured through the inlet using an automatic sampling device. As the sample pass through the column (located in a temperature-controlled oven), equipped with an HP-5 capillary column (30m×0.25mm

Product analyses and selectivity properties determination
i.d) with a coating of 0.25µm thick to the detector, the GC was temperature-programmed from 50°C to 300°C at 5 °C/ min held at final temperature for 20 minutes with hydrogen as the carrier gas with a flow rate of 1ml/min, pressure of 50 kPa. Representative peaks matching the available compounds were generated by Agilent ChemStation software.

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The moles of limiting reactant and that of the target product of the reaction were used to determine the yield, as presented in equation (3)  The percentage conversion was obtained by using the equation (4) The selectivity is the ability of the catalyst to cause, direct or favor the formation of one product in a process where several others are possible. It was obtained using the equation (5)

Results
The results obtained from catalyst characterization and testing are presented in the following Tables 1-6

Discussion
The result of XRD analysis of the prepared catalyst presented in Table 1  The crystallite size (t) of the catalyst was determined to be 131.6 nm which was much greater than size of < 2 nm (diameter) reported by Devassy and co-workers [6] for zirconia-supported 12-tungstophosphoric acid utilized as solid catalyst for the synthesis of linear alkyl benzenes. However, Zhang et al. [24] reported that upon calcination, diffractions shifts to higher angles with reduced intensities leading to an increased crystallinity. The large crystallite size could possibly be due to the absence of molybdenum.  Figure 5) indicated a significant weight loss between room temperature to 100 or 125°C within which all absorbed water would evaporate.
Hernandez-Cortez and co-workers [13] outlined that in solid samples; the first weight loss stage occurring from room temperature to about 152°C was due to physisorbed water and accounted for 55 weight loss (%). The mass loss between 152°C to 752°C was attributed to loss of water molecules due to crystallization and accounted for 11.8%.
Wan and Davis [21] reported 1.9wt% water loss for a heterogeneous catalyst used for naproxen asymmetric synthesis. and CeO 2 that appeared in the XRF result could be attributed to impurities in the zirconia. Table 5 depicts the variation with time, of percentage decyl benzene and other side products realized from the 2:1, 4:1 and 10:1 benzene to 1-decene molar ratio. There is an observable increase in the quantity of decyl benzene realized with time. The % yield for decyl benzene increased from 13.19% to 51.22% at 40 and 120 minutes respectively.
There is an observable increase as well, in the available side products from 40 minutes to 80 minutes. This clearly indicated the effect of residence time as reported by Yuan et al. [23] that at reaction time, an increase in yield and selectivity is generally observed.
However, the final conversion decreases with increase in temperature. Saxena et al. [18] reported the highest conversion of 86.6% on alkylating benzene with zeolite-based catalyst and concluded that increase in reaction temperature could not enhance product conversion.
The percentage decyl benzene obtained at 40 minutes (18.77%) increased to 37.91% at 80 minutes which increased slightly to 39.81% at 120 minutes. As compared to the percentage products realized using 2:1 ( Zhang et al. [24] and Khlebnikova et al. [14]. It could similarly be observed that, there was an increase in the product (decylbenzene) yield with time. Gushchin et al. [11] reported a yield of 88.11% at 30 minutes, which decreased to 60.06% at 5 h reaction time in the alkylation of benzene using dimethyldichlorosilane.
The calculated percentage selectivity, yield and conversion obtained for the various At 4:1 benzene/1-decene molar ratios, the selectivity shifted from 55.99% (40 minutes) to 75.09% (80 minutes). However, at 120 minutes, it decreased to 64.56%. The increased selectivity from 55.99% to 75.09% at 40 and 80 minutes respectively, could be attributed to the difference in the residence time. On the other hand, the selectivity in 4:1 benzene /1-decene molar ratio decreased from 75.09% (80 minutes) to 64.56% (120 minutes) shows a decrease in selectivity at high molar ratios with time. Yuan and coworkers [23] reported a decrease in selectivity (26.0% -22.6%) at high molar ratios.
The product selectivity at 40 minutes increased from 52.5% (2:1) to 55.99% (4:1) and further to 60.09% (10:1). This shows the effect of increased selectivity with increase in benzene/1-decene molar ratios. The selectivity for decyl benzene in all molar ratios increased with increase in benzene / 1-decene molar ratios but decreased with time at higher molar ratios. The selectivity for decylbenzene in all the molar ratios increased with increase in benzene-alkene molar ratios except in 4:1, 80 minutes (75.09%) which decreased to (64.56%) at 120 minutes and 10:1, further at 80 minutes (84.04%) which also decreased to (76.69%) at 120 minutes. An effect of benzene-alkene molar ratio plays a significant role in alkylation reaction. The highest selectivity observed for 10:1 (80 minutes) is 84.04% which is much higher than 26.0% (selectivity) reported for USY zeolite for alkylation of benzene with 1-Dodecene by Yuan et al. [23]. The difference in residence time and reactant molar ratios could account for this, as reported by Yuan et al. [23], Faghihian and Mohammadi [8] and Gushchin et al. [11].

Conclusion
The prepared solid catalyst (CuO/ZrO 2 ) was found active, selective and gave desired products along with some by-products. In addition, benzene to alkene molar ratios has positive effects on product yield, conversion and selectivity for alkyl benzene. Highest conversion of 89.37%, selectivity of 84.04% and yield of 68.54% were obtained.
Furthermore, reaction residence time increases the product yield, conversion and selectivity of alkylation reaction. Reaction is still on going to explore new parameters such as catalyst lifetime, optimal performance and deactivation properties for process upgrade.