Energy dispersive X-ray (EDX) and inductively coupled plasma-atomic emission spectroscopic (ICP-AES) techniques were used to determine the elemental composition on the surface and in the bulk of the catalyst, respectively. The average vanadium oxidation state (AV) in the bulk of the catalyst was determined by means of a titrimetric method.
EXPERIMENTAL
MA selectivity versus percent n-butane conversion for four different promoted catalysts, from VPO-I to VPO-4. MA yield versus % n-butane conversion for four different accelerated catalysts, VPO-1 to VPO-4.
INTRODUCTION
Background into reaction chemistry
There is much inconsistency in the open literature regarding the nature of the active phase. The role of dioxygen is to regenerate or maintain the oxidized state of the catalyst.
Catalyst models
Both transformations can occur simultaneously or sequentially, depending on the nature of the precursor phase and the heat treatment (Cavani, F. They can operate during the catalytic work with the majority of V in the +4 oxidation state, even in oxygen-rich sources.
An idealized model for the orthorhombic structure ofvanadyl pyrophosphate
The close-packed pattern for the basal plane and the relative positions of the V and P sites in the octahedral and tetrahedral interstices are shown in the figure. There is a common misconception that the bulk structure of vanadyl pyrophosphate is characterized as a compact solid. oxide.
Influence of reducing agent and solvent
Influence ofP:V ratio
However, the reactivity of the bulk of these catalysts was found to be strongly dependent on the P:Y ratio, and a slight excess of P strongly inhibited the mobility of lattice oxygen through the bulk. Upon calcination, the surface area of the calcined catalyst is greater than that of the precursor.
Promoted vanadium-phosphorous-oxide catalysts
- Conclusion
Addition of dopant beyond this optimal loading results in a decrease in the surface area of the catalyst. This decrease in selectivity was attributed to the loss of P from the surface zone of the catalyst.
Surface acidity of the VPO catalyst
The medium Lewis acidity is attributed to coordinatively unsaturated 4+ ions exposed on the surface in y=double bonds. The formation of the very strong Lewis sites can be attributed to the two-tactic mechanism of formation of vanadyl pyrophosphate from YOHP04 ·J/2H20.
Relationship between catalyst preparation, phase composition and selectivity
Extremely high initial selectivities in MA, on the order of 80%, were observed when n-butane was contacted with a calcined catalyst in the absence of gas-phase oxygen (Hodnett, B.K. The reduced core could be produced by hydrogen pretreatment or by ion structural stabilization y4+ with excess P (Hodnett, B.K.
Proposed mechanism for n-butane oxidation to MA over a VPO catalyst
Through a more systematic and detailed study of all the factors that influence the preparation of the industrial VPO catalysts. The final challenge will be the identification, on a molecular scale, of the nature of the surface sites responsible for the individual steps of the transformation.
INDUSTRIAL AND LABORATORY PROCESSES
Introduction
- Fixed-bed reactors in relation to fluidized-bed and transport-bed reactors
In a realistic model of the fixed-bed reactor, a wide range of length scales must be run through. In addition to the requirement for a sharp distribution of residence time, the ideal fixed bed reactor must also ensure that all parts of the catalyst bed can fully participate in the overall conversion. Therefore, the requirement for an ideal reactor is better expressed as "even irrigation" of all parts of the catalyst bed (Sie, T.
In an isothermal reactor, the temperature of the reactant stream is constant in the axial direction. Heated transfer lines between the sampling valve and the gas chromatograph prevent condensation of products from the reactor. The diameter of the particles should be less than one-tenth the diameter of the reactor and about one-hundredth the length of the catalyst layer.
Conclusion
Most of these measures will increase the pressure drop along the catalyst bed, but pressure changes generally have much less influence on reaction rates than temperature changes. Due to the beneficial effect of radial diffusion in gas phase processes in counteracting transverse velocity profiles, caused for example by the disruption of packing homogeneity near the reactor wall, and by taking advantage of the improvements in axial dispersion and catalyst irrigation resulting from dilution of the catalyst bed with fine inert material it is possible to obtain reproducible and meaningful results from tests in microreactors. From the analysis of limiting factors discussed so far, it follows that even smaller reactors than microreactors can be used, as the bed diameter of the latter can be further reduced without adverse effects, as long as the reactor tube still accommodates the catalyst particles.
Catalyst testing can thus be performed in reactors with diameters in the millimeter range that contain less than one gram of catalyst. Due to the exothermic nature of the reaction, it is important to ensure that the reactor is isothermal. If the temperature in the catalyst bed is not properly controlled, the reaction can easily become unstable and the temperature will rise in both an industrial and laboratory reactor setup.
Reactor setup
A rubber o-ring sealed the connection at both the inlet and outlet points of the reactor (Fig. 2, Appendix 1). A thermocouple was placed on the outer wall of the reactor tube to monitor the temperature of the catalyst. A three-way valve after the reactor controlled flow from either the bypass or the reactor to the gas sampling box, which was the initial step in the analytical process.
The lines entering the gas sampling box were the reactor product flow line and the return line from the pre-column of the Varian 3700 Gc. The lines exiting the gas sampling box were the sample feed to the pre-column on the Varian 3700 GC, the feed line to the analytical column on the Varian 3700 GC, and the feed line to the Perkin Elmer Gc. These rotary valves were attached to a hot plate which maintained them at a temperature of 160°C to prevent condensation of the products from the reactor in the valves.
2 -"OETECTO
Experimental procedure for catalyst testing
1 g (about 2 cm3) of pure calcined catalyst was tightly packed in the center of a quartz reactor tube with an inner diameter of 7 mm (outer diameter 10 mm). The length of the catalyst packing was about 50 mm, which varied according to the size of the catalyst particles. Carborundum, which is an inert SiC, was filled at the feed stream end of the reactor tube.
A glass wool plug was placed immediately behind the catalyst bed to prevent particles from entering the on-line sampling valves and consequently the gas chromatographs. Hourly gas space velocities (GHSVs) and temperatures were varied to study the ability of the Co-promoted catalyst to selectively convert n-butane to MA. A description of the columns used in the analysis of reactor products is given in Appendix I.
RESULTS AND DISCUSSION
Catalyst synthesis
- VPO-2
- VPO-3
- VPO-4
The reflux mixture from step I was cooled to room temperature before the addition of ortho-H 3 PO 4 . Addition of ortho-H3PO4 and further refluxing for 3 hours completed the reduction of any unreduced y5+ to y4+ from step I. XRD spectra of catalyst precursors (VPO-1p, VPO-2p, VPO-3p and VPO-4p) (Appendix 2). Only the (VO)2P207 phase was present in the catalyst used in this study, as at this stage the catalyst was exposed to a temperature higher than 400 °C during calcination and testing. 1997) claim that the only detectable phase in aged industrial catalysts is vanadyl pyrophosphate, (VO)2P207, which contains V4+ cations.
The XRD pattern of the catalyst used in this study (VPO-3u) (Appendix 2) showed the presence of (VO)2P20 7, which is known as an active catalyst for the oxidation of n-butane to maleic anhydride (Appendix 2). The catalyst surface contained the (VO)2P207 crystalline phase as revealed by the XRD spectrum of the used VPO-3u catalyst (Appendix 2). The P:V molar ratio on the catalyst surface was lower than that in the bulk.
CHAPTERS CONCLUSION
Recommendations
Using a recycle loop for the optimal VPO-3 catalyst that extends from the product stream and meets the feed line to the reactor. Byproducts can be trapped before unreacted n-butane enters the reactor feed stream. The composite feed in the reactor will be diluted with excess air entering the feed stream from the by-pass line.
Online characterization of the catalyst via XPS could elucidate the phases present during the oxidation of n-butane to MA. A precolumn in this gas chromatograph (GC) collects the products that condense at room temperature and allows the gaseous products to pass to the TCD. The Perkin Elmer GC is equipped with a flame ionization detector (Fill) for n-butane and MA analyses, among other things.
Swagelok :fitting with rubber o-ring
Product stream Glass wool
Carborundum
Glass wool Feed stream
Isothermal Varian 3700 Detector temperature 130°C
Perkin Elmer XL Autosystem Detector temperature 130°C
It may not cause immediate skin burns, but prolonged contact with wet skin causes redness and blisters or burns. In case of contact, immediately wash skin with soap and plenty of water for at least 15 minutes while removing contaminated clothing and shoes. Immediately flush the eyes with plenty of water for at least 15 minutes, occasionally lifting the lower and upper eyelids.
DO NOT use dry chemical, multi-purpose dry chemical, or charged media due to the potential for explosion due to the reactivity of the base ingredients in these extinguishing agents.
APPEND/X 2
IVWJV
Inductively coupled plasma - atomic emission spectroscopy (ICP-AES)l
PLASMA
SOFTWARE
X-ray spectrometry
The result of this electronic transition is the emission of a characteristic X-ray spectral line whose energy, hv, is equal to the difference between the binding energies of the two electrons participating in the transition. The energies or wavelengths of X-ray spectral lines are the basis for qualitative analysis. As the electron beam of a scanning electron microscope (SEM) scans across the surface of a sample, it creates X-ray fluorescence from atoms in its path.
A solid state detector consisting of Si and Li responds to the energies of the characteristic X-rays. The magnification produced by the scanning microscope is the ratio of the dimensions of the final image display to the field scanned on the specimen. The Bragg equation relates the d-spacings on the powdered sample to the angle of rotation of the analyzer crystal.
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