Activation of n-hexane using vanadium-exchanged zeolites.

Where the work of others has been used, this is duly acknowledged in the text. I thank the technical staff of the University of KwaZulu-Natal, School of Chemistry who helped me along the way.

2 Partially oxidized hexane (Scheme 1.1) is of economic interest, as most partially oxidized hydrocarbons are used as important building blocks for the production of special and everyday chemicals and are also used in the production of plastics and synthetic fibers [3].

C 6 Olefinic type compounds

Commercial processes used in the manufacture of C 6 olefins

The reactions lead to the release of the desired long-chain α-olefins with the simultaneous formation of short-chain trialkylaluminum compounds. These α-olefins are used to produce α-olefin sulfonate which is used in the wetting agent market in India.

Uses of C 6 olefins

In the final step, the mixture is passed over an aluminum supported molybdate metathesis catalyst. Another area where C6 α-olefins are useful is the production of hindered esters which are used as lubricants for jet engines and other high performance applications [6].

C 6 oxygenated type compounds

Manufacture of C 6 oxygenated compounds

7 Another process leading to the formation of alcohols is the hydrogenation of aldehydes, carboxylic acids and esters in the presence of homogeneous or heterogeneous catalysts [13]. In the Zeigler process, ethylene is added to triethylaluminum to form a mixture of trialkylaluminum products which are then oxidized with air to the corresponding aluminum alkoxides.

Uses of C 6 oxygenated compounds

Other useful C6-oxygenated compounds are C6-aldehydes (hexanal, 2-methylpentanal, 2-ethylbutanal and 3-methylpentanal) which are prepared almost exclusively by hydroformylation or aldol condensation. The C6-aldehydes are used as starting materials for sedatives and in the production of agrochemicals, perfumes and catalysts for cross-linking of polyesters [16].

Oxidation of hydrocarbons

Electrophilic oxidation

On the solid surface, dioxygen is adsorbed and is either present as a superoxide ion or a peroxide ion. Under heterogeneous catalytic conditions, peroxy and epoxy complexes formed as a result of an electrophilic attack of O2 or O˙ species on the π bonds of the hydrocarbon molecules at the surface of an oxide lead to the decomposition of the carbon skeleton [18].

Nucleophilic oxidation

Catalyst cations act as oxidizing agents in several of the successive steps forming activated hydrocarbon species. This redox mechanism was postulated by Mars and van Krevelen to explain the kinetics of the oxidation of aromatic substances over V2O5.

Oxidation of n-hexane

The results implied that benzene production below 400 °C was catalytically driven, while production above 400 °C was thermodynamically driven [31].

Introduction

18 Table 2.1 also shows the pore sizes of zeolites which are of the order of molecular dimensions so that only a few molecules fit into the pores. However, the packing of molecules in the pores depends on the architecture (dimensionality) of the zeolite.

Table 2.1 Properties of some industrially important synthetic zeolites [2-5]

ZSM-5

Structure of ZSM-5
Impregnation
Direct synthesis
Chemical vapour deposition (CVD)
Ion-exchange

The size and shape of zeolite crystals or aggregates can be investigated using electron microscopy [3]. In ZSM-5 zeolite, the tetrahedrally coordinated Al3+ positions are widely spaced.

Figure 2.3 Illustraton showing the possible location of metal cation in zeolites [19]

Applications of transition metal modified zeolites

Transition metal modified zeolites and silicalites

Vanadium and chromium silicates were tested in the liquid phase using TBHP (tert-butyl hydrogen peroxide) as an oxidant at 10°C. The oxidation was carried out in the liquid phase using a glass reactor and the included products.

Rationale for choice of transition metal modified zeolite for this study

Silanisation

Objectives of this study

Further studies have been conducted on the use of different methods to eliminate surface acidity and also on the kinetic rates and diffusion limitations [73-77]. The overall conclusions from these studies show that the elimination of external acidity by deposition of alkosilanes causes some pore constriction and results in an increase in shape selectivity for zeolites.

Reactor Design

The reactor tube and catalyst packing

First the thermocouple was clamped to the reactor tube and glass wool was added. Glass wool was used to seal the carborundum and a fixed volume of catalyst was added to the reactor tube.

Gas chromatographs

First the thermocouple fitting was added to the tube, then the glass wool and carborundum. After the glass wool glass beads (3 mm Soda glass, Promark Chemicals) were added up to the end of the tube and glass wool was used as a stopper.

Product quantification and analysis

Four consecutive runs were performed at each temperature and only results obtained with a relative standard deviation of one were used. When in-flow time experiments were performed, the experiments were repeated and again only results obtained with a relative standard deviation of one were used.

Catalyst synthesis and characterisation

Synthesis of the solid state ion-exchanged Na-V-ZSM-5
Silanisation procedure
Structural and chemical characterization of precursors and Na-V-ZSM-5

Inductively coupled plasma-optical emission spectroscopy (ICP-OES)
Fourier transform infrared (FT-IR) spectroscopy
X-ray diffraction (XRD)
Braunauer Emmet Teller (BET surface area) and porosity measurements
Ammonia - temperature programmed desorption (NH 3 -TPD)
Hydrogen - temperature programmed reduction (H 2 -TPR)
Scanning electron microscopy (SEM)

Chemicals and reagents

The deposition procedure was performed in the liquid phase and was achieved by first drying 2.5 g of the catalyst at 70°C for 3 h. In TPR measurements a mixture of 5% H2 in argon was used as a reducing agent at a flow rate of 10 ml/min with a heating rate of 10°C/min [8].

The structure of the precursors and the structural changes that occurred before the replacement and after the replacement were investigated using a combination of techniques and are described in this chapter. Finally, the vanadium-substituted zeolite underwent the silanization procedure as described in Chapter Three, Section 3.3.2.

Precursor zeolites and vanadium exchanged catalysts

Braunauer Emmet Teller (BET) surface area and porosity measurements
Ammonia-temperature programmed desorption (NH 3 -TPD)
Hydrogen-temperature programmed reduction (H 2 -TPR)
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) 60

The IR spectra of the vanadium-exchanged zeolites are similar to those of the Na-ZSM-5 precursor (Figure 4.2). This suggests that water molecules are adsorbed on the surface of precursors and catalysts.

Table 4.1 Composition of precursors and vanadium exchanged zeolites

Silanised catalyst

Braunauer Emmet Teller (BET) surface area and porosity measurements
Ammonia-temperature programmed desorption (NH 3 -TPD)
Hydrogen-temperature programmed reduction (H 2 -TPR)
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) 67

The 27Al spectrum for the silanized catalyst has a lower intensity than that of the parent catalyst. This evidence correlates with the results of the XRD analysis, which showed a severe decrease in the relative crystallinity of the silanized catalyst.

Table 4.7 Composition of silanised Na-V-ZSM-5 and Na-V-ZSM-5 (0.9%)

The investigation of n-hexane using a carborundum packed reactor

Non-oxidative conditions

The oxidation of n-hexane was investigated using a vanadium-exchanged zeolite synthesized by a solid-state ion exchange method. This chapter details an investigation into the optimization of the conditions for the formation of the desired end products using vanadium-exchanged zeolite.

Oxidative conditions

73 A comparative assessment of the product profile with a previous study using a carborundum packed reactor for n-hexane oxidation provided a contrast in results [1]. The mechanism proposed from this study shows that initial hydrogen abstraction is not preferred as it led to low product selectivity.

Evaluation of precursor zeolites under non-oxidative and oxidative conditions

Non-oxidative conditions

The number and strength of acidic sites present in the catalyst, dictated by the distribution and number of aluminum ions in the ZSM-5 crystal lattice, are important parameters affecting the activity and selectivity of the product. As the aluminum content increases, the selectivities and yields of the cyclic compounds increase, indicating increased participation of the oligomerization reaction and hydrogen transfer.

Oxidative conditions

Compared to the results of the non-oxidation study, it was seen that a significant change in the catalytic activity was noticed in the presence of gaseous oxygen. The results obtained from the non-oxidative and oxidative study of the precursor zeolites, together with the reports from the literature, led to the decision to investigate vanadium-exchanged Na-ZSM-5 zeolites for the oxidation of n-hexane.

Table 5.6 Selectivity to products from the oxidation reaction of n-hexane over H-ZSM-5 (100)

Evaluation of vanadium exchanged zeolites under oxidative conditions

Effect of temperature on n-hexane activation

The formation of 1-hexane could follow direct oxidative dehydrogenation of n-hexane or dehydration of 1-hexanol pathways. The selectivity for 1-hexanol decreases as the formation of 1-hexene increases, indicating that 1-hexene is a secondary product.

Figure 5.4 Selectivity to terminal products in n-hexane oxidation over Na-V-ZSM-5 (0.9%) at 350°C and 0.8 s contact time

Effect of contact time on n-hexane activation

Therefore, it is assumed that the conversion of n-hexane during this contact time can be mainly attributed to the reactions that take place at the active sites on the outer surface of the zeolite. BET surface area and porosity measurements of the spent catalysts are shown in Table 5.11 below.

Effect of fuel-air ratio on n-hexane activation

Effect of vanadium loading on the activation of n-hexane

XRD crystallinity and BET surface area and micropore volume measurements for the catalysts with the higher loadings showed that the vanadium species affected the structure of the catalysts. Further conclusions suggested that the electronic promoting effect of the alkali metals (regardless of the alkali used) was significant for the propylene selectivity [15].

Figure 5.14 Effect of vanadium loading on the conversion of n-hexane 0%

Regeneration

After this, the initial experimental conditions were resumed and the activity of the catalyst was monitored until stable activity was achieved after 7 h on-stream. The results show that the catalyst activity and selectivity to products were the same as those obtained with the fresh catalyst within experimental error.

Figure 5.18 Selectivity to terminal products in n-hexane oxidation over regenerated Na-V-ZSM- Na-V-ZSM-5 (2.Na-V-ZSM-5%) at 400°C, 1.1 s contact time and fuel-air ratio of 1.3

Silanisation

The higher selectivity to aromatics and cyclics seen over time in the stream may be due to the higher formation of olefins (1-hexene) as these are known precursors to aromatics. The aim was to increase the selectivity to 1-hexanol, but by reducing the surface acidity, the vanadium content, surface area and pore volumes were reduced.

Possible mechanism of n-hexane partial oxidation

It is widely agreed that lattice oxygen plays an important role in selective oxidation. To mechanistically show the formation of hexene occurs via a Mars and van Krevelen redox mechanism, the first step uses lattice oxygen atoms to abstract hydrogen atoms from C6H14, in an irreversible C-H bond activation step [24].

Other products

The detection of water as part of the product spectrum as a byproduct refers to the fact that the above-mentioned mechanisms for the formation of 1-hexanol and 1-hexene are predominant. Under certain conditions, an increase in the production of cracking products was observed and this could be due to the modification of the catalyst due to the presence of water.

Comparison to literature

The highest combined yields achieved in this study were ~8% under conditions that are also the optimal conditions for C6 terminal yields. C6 monoolefins were also part of the products formed and again the highest yield (3.6%) was achieved under optimal conditions for C6.

Some important derivatives from n-hexane

Thermal cracking of waxes [1]

3 However, the use of a catalyst facilitates linear olefins with the same number of carbon atoms as linear paraffin (Scheme 1.3). The Pacol process (paraffin catalyst olefin) is the most important industrial process using this technology.

Catalytic dehydrogenation of paraffins [1]

Two step Ziegler process [1]

Both processes require less triethylaluminum, with the former requiring only catalytic amounts of triethylaluminum, with the growth stage and elimination reactions occurring simultaneously in the same reactor. The process results in higher ethylene conversion but has the disadvantage of lower quality long-chain α-olefins [8].

Step one in the SHOP process [8]

C4 are subjected to transalkylation with long-chain aluminum alkyls, while the longer chain can be used directly. In the first step, ethylene is oligomerized in a polar solvent (1,4-butanediol) to give a mixture of linear even-numbered α-olefins ranging from C4 to C40.

Oxo-synthesis [10, 11]

The dominant sources of 1-hexanol are either the Zeigler process using ethylene (Conoco or Ethyl Corporation) or its synthesis from natural products derived from coconut or palm oils (Henkel).

Oxidation of trialkylaluminum compounds [13]

8 This process is a variant of the Ziegler process that was used to synthesize linear olefins as described in Section 1.2.1. The processes differ in chain length distribution and the linearity of the alcohols produced, as well as the processes for generating and controlling distribution.

Intramolecular (a) and intermolecular (b) oxidative dehydrogenation [18]

Most of the catalysts used correspond to metal oxides with vanadium and molybdenum as one of the key elements, as well as cations of variable oxidation states such as Fe3+/Fe2+, V5+/V3+, Mo6+/Mo5+, Cr6+/Cr3+, Cu2+/Cu+, etc. The incorporation of oxygen from the gas phase into the oxide surface may not take place at the same site from which the surface oxygen is inserted into the hydrocarbon molecule, but may occur at a different site, meaning that oxygen ions are transported through the lattice.

Oxidation-reduction cycles postulated by Mars and van Krevelen [18, 24, 26]

The adsorption capacity of the zeolite plays an important role because different molecules entering the channels can be adsorbed differently. ZSM-5 where crossing channels occur, parts of the zeolite become inaccessible to long or bulky molecules [8].

Formation of Brønsted and Lewis sites [7]

In the case of 2.5% filled zeolite (Figure 4.3 (g)), the intensity of the peak at ~60 ppm decreased, but the resonance signal at ~4 ppm greatly increased. Scanning electron microscopy can be used to gather information about the surface morphology of a compound.

Suggested homogenous reaction pathways for n-hexane partial oxidation

This alludes to the possibility that the primary and secondary products react further to eventually form CO2. As mentioned in section 5.2.2 above, the formation of coke or carbonaceous deposits in the pores or on the surface of the catalyst results in deactivation of the catalyst over time [10].

Generation of active oxygen species

98 Activation of oxygen on the catalyst surface is a necessary step for the heterogeneous partial oxidation reaction. It is assumed that during absorption on the reduced surface, oxygen can accept electrons one at a time going continuously up to the fully reduced form, i.e.

Possible interaction of vanadium species with the zeolite support

Redox cycle for a metal containing complex

Water is a "by-product" of this reaction and may be responsible for the modification of the catalyst. XRD analysis of the spent catalyst showed an unchanged structure, indicating that the effect of dealumination was not dominant in this part.

Suggested mechanism for n-hexane cracking [27]

Suggested mechanism for other products formed [27]

These results were consistent with those obtained by non-oxidative testing of the precursor zeolites. C6 mono-olefins were also part of the products formed, and again the highest yield (3.6%) was achieved under optimal conditions for C6 end products.