THE TRANSFORMATION OF ISOBUTANOL AFTER PLASMA-CHEMICAL TREATMENT

The catalytic properties of plasma-chemically and thermally treated Bi4V2-2xCu2xO11–δ with х= 0.00–0.16 (BICUVOX) in the transformation of isobutanol are studied. Plasma treatments lead to the total deactivation of the active sites of dehydration reaction, and the alcohol is converted with 100% selectivity into aldehyde. Activity of the dehydrogenation of alcohol after PCT increases in ~ 1.7 times on samples with xCu=0.04-0.06 (. phases) compared to the initial samples. Similar values which were found for the samples with xCu=0.0-0.06, including the double vanadate, suggests that PCT affects the ceramic matrix with Bi, O and V elements. Addition of small amounts of copper does not change this effect.


Introduction
Layered materials, with their structure consisting of stacked sheets, represent an interesting opportunity for developing new materials with a tailored nano-design, controlled accessibility to the sites and properties, tuneable pore size and volume, and high surface area.The use of layered materials (layered perovskite, anionic clays, pillared clays) in catalytic reactions is reviewed with emphasis on the possibilities offered from these catalysts to develop new processes for environmental protection, selective oxidation and refinery/biorefinery [1].Of a great interest are bismuth vanadates belonging to the BIMEVOX family, for which oxygen-ionic conductivity is typical.In the crystal structure of bismuth vanadate Bi4V2O11-δ, cations Bi 3+ are in tetrahedral coordination; V 5+ cations, in octahedral coordination.Bismuth vanadates exist in several polymorphic modifications (α-, or monoclinic; β-, or orthorhombic; and γ-, or tetragonal).The tetragonal modification occupies a special place among bismuth vanadate polymorphic modifications due to its high transport characteristics.It is the γ-phase that has the highest conductivity and the lowest activation energy value [2,3], since all oxygen atoms in vanadium octahedra are involved in the diffusion processes.In polymorphic perovskites, α ↔ β and β ↔ γ phase transitions are observed as abrupt changes in the total conductivity with rising temperature.The catalytic activity of polymorphic copper-containing BICUVOX perovskites with the composition Bi4V22xCu2xO11-δ (х =0.00-0.16) in isobutanol transformations was studied in [4].Plasma-chemical treatment (PCT) allows us to obtain completely new materials with widely varied surface properties, while preserving most properties of the original materials.This advantage is used in the activation of catalysts and adsorbents.In some cases, plasma-chemical treatment (PCT) leads to a multifold increase in the activity of catalysts, due to changes in the nature of their active sites [5].The aim of this work was to study the influence of PCT in oxygen on the catalytic activity of polymorphic copper-containing BICUVOX perovskites with the composition Bi4V2-2xCu2xO11-δ (х=0.00-0.16) in isobutanol reactions.
Thermal pre-treatment of the BICUVOX samples included their heat-treated in a helium flow at 703 K for 1 h, with an air flow of 1.1 l/h.The samples were cooled to the temperature of the catalyst activity experiment (523 K) in a flow of helium as well.
PCT was conducted in a glow discharge of oxygen in an evacuated flow apparatus using 50 Hz alternating current.The reactor design allowed temperature to be measured simultaneously with the treatment of the samples.Hollow water-cooled electrodes were placed outside the reaction zone.The samples were placed on a quartz pod, forming a 1 mm thick layer.The reactor's flow mode promoted the removal of the reaction products, which were concentrated in a trap cooled with liquid nitrogen.The reactor was pumped down to 10 -4 mm Hg residual pressure, the discharge current was 200 mA, and the voltage between the electrodes was 1.4 kV.Treatment was performed for 45 min at 433 K.A violet-rosy glow was observed inside the discharge.After conducting each experiment, the samples were cooled to ambient temperature in vacuum.
The composition of the surface layers of the samples was analyzed by means of X-ray photoelectronspectroscopy (XPS).The photoelectron spectra were recorded on a XSAM-800 instrument, using AlKα1, 2 radiation with an energy of 1486.6 eV; the С1s line (Ebond= 285 eV) of each sample was used as our internal standard.The accuracy of determining the Ebond was ±0.2 eV.The alcohol conversion in vapour phase was performed in a U-shaped continuous-flow microreactor at atmospheric pressure and 473-673 K with the use of 30 mg of the catalyst.Isobutanol diluted with He was fed to the reactor at a partial pressure of 760 Pa; the total flow rate was 1.1 l -1 h -1 .The reaction mixture was analyzed chromatographically (FID, helium as a carrier gas).Oxide catalysts exhibit catalytic activity due to their acidic properties, both Lewis and Bronsted type [7].Adsorption of a weak basepyridinewas used in order to determine the acidic centers [8] on the surface of Bi4V2--2xCu2xO11-δ.Pyridine adsorption was studied by spectrophotometry on a Specord, UV-VIS, SF-103 instrument.Sample weights of 10 mg were added to 0.3 μmol/L solutions of pyridine in octane.Spectra were recorded every 15 min for the first incubation hour, then after 24 h total incubation time and the equilibrium concentration of pyridine was determined by the spectral data.

Results and discussion
As shown by X-ray diffraction (Fig. 1), single-phase BICUVOX solid solutions over the entire range of the х values studied were obtained after annealing at 800С.The unit cell parameters calculated for the solid solutions are listed in Table 1.The change in the crystal lattice symmetry of BICUVOX with x growing is due to the entrance of aСu 2+ cation, which has a greater radius than a V 5+ or V 4+ cation: RCN6(V 5+ ) = 0.68 Å, RCN6(V 4+ ) = 0.72 Å, and RCN6(Cu 2+ ) = 0.87 Å (Table 1) [9][10][11].The stabilization of the α phase in the series of the solid solutions studied occurs in the concentration range of х ≤ 0.04.The crystal structure of the х = 0.06 solid solution sample is characterized by orthorhombic symmetry and belongs to the β phase.BICUVOX solid solutions having a tetragonal crystal lattice are formed in the range of 0.10 ≤ х ≤ 0.16.The IR spectra (Fig. 2) of all synthesized samples contain abroad absorption band in the region of ~650-930 cm -1 associated with the valence vibrations of V-O bonds inVO4 tetrahedra.The V-O valence vibrations inVO6 octahedra also appear in the spectra of all of the synthesized solid solutions in the low-frequency region of ~550-450 cm -1 .In addition, the IR spectra of 0.00 ≤ х ≤ 0.04 samples contain a low-intensity absorption band in the region of ~580-630 cm -1 , which corresponds to bond vibrations in VO5 polyhedra.The presence of V 5+ cations with CN = 5 in the vanadate layers of the BICUVOX crystal structure is characteristic of the α phase of the solid solutions, thereby verifying the stabilization of this phase in this concentration range at room temperature [12].The disappearance of the absorption band in the region of~580-630 cm-1 and broadening of absorption bands at ~650-930 and ~450-550 cm -1 in the х ≥ 0.06 sample are due to the increased concentration of Сu 2+ cations, which have a lower oxidation number and a greater ionic radius than V 5+ cations have.As х rises, the oxygen deficit increases in the vanadium sublattice of BICUVOX solid solutions.The consequence of these changes is the dominance of vanadium polyhedra with CN = 4 in the vanadate layers of Bi4V2-2xCu2xO11-δ and the ordering of oxygen vacancies, which accompanies the formation of the oxygen-deficient tetragonal γ polymorph.According to XPS spectroscopy, the surface of Bi4V1.88Cu2×0.16O11-has excess oxygen compared to the stoichiometry of Bi4V2O11-δ.The Bi/V atomic ratio on the surfaces is 5, i.e. higher compared to (Bi/V)stoich = 2, while the (O/V) ratio is 5 times higher.After PCT the abundance of the elements on the surface is increased, but their ratios become (Bi/V) = 2, which is in accord with stoichiometry, and (O/V) = 2, which I still elevated (table 1).The value of ЕbondBi4f = 158.7159.0eV corresponds tobismuth oxidation state +3.However, the spectrum for the sample subjected to PCT contains a clear "shoulder" with a maximum at 156 eV, which corresponds to the reduced form Bi 0 .The value of ЕbondV2p=517.4eV of the starting sample is typical for V +5 .Its decrease after PCT to ЕbondV2p=516.8eV demonstrates the decrease of the charge of vanadium, i.e. the presence of V +4 [13].All the spectra of O1s contain a peak with ЕbondO1s=529.8530.1 eV, corresponding to oxygen atoms in the oxide and to chemically adsorbed atomic oxygen.Below are the results of experiments on the study of catalytic activity BICUVOX.On all samples with initial surface, where xCu=0.04(1) 0.06 (2), and 0.16 (3), the main reaction is the dehydrogenation of an alcohol with a selectivity of isobutanal formation 83-100%.Isobutanol dehydration proceeds in parallel with the dehydrogenation reaction only on the sample with xCu= 0.16.Catalytic activity BICUVOX-perovskites increases with the copper increasing content and the highest catalytic activity is observed on superconductive tetragonal γ-phase (Table 2) [5].
Plasma-chemical treatment (PCT) of all BICUVOX samples in oxygen, leads to the total deactivation of the active sites of dehydration reaction, and the alcohol is converted with 100% selectivity into aldehyde.Activity (conversion W, %) of the dehydrogenation of alcohol after PCT increases ~ 1.7 times on -phase samples compared to the initial samples (Fig. 3, Table 2).
For high-conductivity γ-phase there is no activating plasma effect, on the contrary, the activity decreases after treatment.During the catalyst in the repeated test the activity of γ-phase (sample 3*) is decreased by 15% by reducing the number of active sites with high adsorption heat, indicating instability of plasma effect and reaction medium effect on copper containing sites.
As opposed to initial samples with step-wise character of the Arrhenius dependences, the Arrhenius dependences on samples after PCT are linear (Fig. 4).Plasma treatment of BICUVOX in O2 changes the experimental activation energy Ea of aldehyde formation which depends on a copper content (Table 2).As opposed to samples without PCT, values Еаincrease with increasing of xCu, therefore, the condition of copper sites after PCT changes, resulting a decrease in heat of adsorption of alcohol with the copper increasing content, where as before PCT Qa value increases (Ea decreases).A change in the energy of the alcohol dehydrogenation reaction after PCT ΔEa = EaPCT Eainit increases in the series of samples xCu = 0.04 (24) → 0.06 (8) → 0.16 (+54 kJ/mol) indicating the weakening of bond strength between the reagent and the active site on BICUVOX surface, with the greatest effect at the α-phase.Activation of theα-phase of the catalyst is caused by the appearance of centers with increased alcohol heat of adsorption (Q).The similarity of parameters measured for the α-phase samples including the double vanadate, shows that PCT mainly influences on the state of the structuring matrix containing bismuth, oxygen and vanadium.The presence of small amounts of copper does not alter this effect.The total number of the acidic centers of the surface, determined by pyridine adsorption is decreased in 1.5 times after PCT compared to the initial surface.When the copper content in the samples is increased, Аpy tends to grow, which is in accord with the increase of the preexponential factor (Table 3).Compared to the starting BICUVOX samples, samples after PCT in oxygen have more acidic centers on the surface, which adsorb pyridine at a high rate W0 and their activity does not depend on xCu.DOI   Similar values of isobutanol conversion which were found for the α,-phase samples, including the double vanadate, suggests that PCT affects the ceramic matrix with Bi, O and V elements.Addition of small amounts of copper does not change this effect.For γ-phase, where reaction goes on copper sites with strong bond energy with isobutanol.Their role of such active centres is weakened some after PCT due to enrichment of -BICUVOX surface by Bi, O and V. Thus, PCT in O2 of the α-phase sample forms sites strongly attached to the surface of perovskite, therefore we observed activating effect of plasma.

Table 1
Unit cell parameters for Bi4V2-2xСuxO11δ solid solutions

Table 3
Surface acidity parameters for BICUVOX tested by pyridine adsorption (equilibrium absorption АPy, mol/g and adsorption rates for two acidic center types W, μmol/g×min) for the initial surface (I) and after PCT (II) in comparing with preexponential factor logarithm for isobutanol conversion to isobutanal Plasma treatment of BICUVOX activates such perovskite catalysts only in case of . phases.