Light Alkane Dehydrogenation to Light Olefin Technologies a Comprehensive Review

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Open Access Article

Modification by SiO_two of Alumina Support for Light Alkane Dehydrogenation Catalysts

A. Butlerov Institute of Chemistry, Kazan Federal University, Kazan 420008, Russia

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Author to whom correspondence should be addressed.

Bookish Editor: Keith Hohn

Received: two September 2016 / Revised: 11 October 2016 / Accepted: thirteen October 2016 / Published: 20 Oct 2016

Abstract

Due to the continuously rise demand for C₃–C_v olefins information technology is important to amend the performance of catalysts for dehydrogenation of calorie-free alkanes. In this work the event of modification by SiO₂ on the properties of the alumina support and the chromia-alumina catalyst was studied. SiO₂ was introduced by impregnation of the support with a silica sol. To characterize the supports and the catalysts the post-obit techniques were used: low-temperature nitrogen adsorption; IR-spectroscopy; magic bending spinning ²⁹Si nuclear magnetic resonance; temperature programmed desorption and reduction; UV-Vis-, Raman- and electron paramagnetic resonance (EPR)-spectroscopy. It was shown that the modifier in amounts of 2.5–seven.5 wt % distributed on the support surface in the form of SiO _ten -islands diminishes the interaction between the alumina support and the chromate ions (precursor of the active component). As a result, polychromates are the compounds predominantly stabilized on the surface of the modified support; under thermal activation of the catalyst and are reduced to the amorphous Cr₂O₃. This in turn leads to an increase in the activity of the catalyst in the dehydrogenation of isobutane.

one. Introduction

Catalysts with surface chromium species as an active component have large practical importance and are widely used in industrial organic synthesis. Microspherical chromia-alumina catalysts for paraffin dehydrogenation in terms of consumption hold the leading position in the petrochemical manufacture of Russian federation [1]. This process is intended for production of C₃–C₅-olefin which is the raw fabric for synthesis of rubbers, plastics, synthetic films and filaments, loftier-octane components of fuel, etc. [2].

There are 2 methods of microspherical chromia-alumina catalysts preparation: the mixing of precursors of the support and agile component followed past spray drying of the slurry obtained [3]; and the impregnation of the back up by the solutions of the active component and the promoter. The catalysts obtained by the commencement method accept a low mechanical strength under the conditions of a fluidized bed and the continuous apportionment betwixt the reactor and regenerator [4]. The method of support impregnation seems the most promising. Before a method of a gibbsite transformation to boehmite (precursor of γ-Al₂O_iii) in the book of the microgranule by a sequential thermal and hydrothermal treatment was adult [five]. The back up obtained that way has a loftier mechanical strength, middle porosity, and a depression surface acerbity.

Continuously rising need for light olefins [2,6] likewise equally the increase in the cost of natural energy resource requires high active and selective catalysts. It is possible to ameliorate the catalyst performance by introduction of the modifiers. The modifiers of chromia dehydrogenation catalyst SiO_two [7], ZrO₂ [8,9,ten], lanthanum [ten,11], tin [vii,12] and cerium [13] are usually used. Among these modifiers SiO_ii is the near attainable and inexpensive; its use is broadly described in the patent literature [14]. However, the mechanism of its positive effect on the functioning of the dehydrogenation goad has not been described in detail. Therefore, in this article nosotros studied the effect of modification of the structure and properties of the novel alumina support and chromia-alumina catalyst by SiO₂.

2. Results and Discussion

ii.1. Alumina Back up

2.1.ane. Thermal Treatment of Boehmite Precursor

The microspherical back up was used equally a precursor of alumina (Figure 1). It was obtained according to the Scheme one [5]:

Co-ordinate to the 10-ray diffraction (Effigy 2), the alumina forerunner is a well-crystallized boehmite with crystallite size D ₍₀₂₀₎ = 44 nm and D ₍₁₂₀₎ = 47 nm. A loftier degree of boehmite crystallinity causes its small specific surface expanse (Southward _BET) and pore volume (5 _p) = 27 k²·thousand^−one and 0.07 cmⁱⁱⁱ·chiliad^−ane respectively (Tabular array i).

The selection of the boehmite thermal treatment temperature was made based on changes in the specific surface area and the number of strong acid sites (with the energy of ammonia desorption E _des.NH3 more than 150 kJ·mol⁻¹) obtained in alumina (Table 1). Information technology is known that potent acid sites are active in hydrocarbon cracking reactions [fifteen,16].

Thermal treatment of the precursor at 750 °C leads to the germination of mesopores (Figure iii), an increase in the specific surface surface area upwards to 92 m^two·thousand⁻¹ and in the pore volume upward to 0.27 cm³·g⁻¹ every bit a result of dehydration and the phase transition of boehmite to γ-Al_iiO_iii (Figure 2). An increase of treatment temperature to 850 °C resulted in the decrease of the specific area to 62 g^two·g⁻¹ due to pore enlargement (Table 1).

It is also accompanied past a decrease in the number of strong acid sites due to the reduction of the support surface, its dehydroxylation, as well as the transformation of the crystal structure of the support.

Nosotros believe the optimal temperature of boehmite precursor treatment is 800 °C. γ-Al_twoO₃ support obtained under those atmospheric condition has a low concentration of stiff acid sites (x.0 μmol·thousand^−one) and a medium surface area (Due south _BET = 83 mⁱⁱ·g^−one). A decrease of the temperature to 750 °C is accompanied past a one.half-dozen-fold increase in the concentration of strong acrid sites (upwardly to 16.2 μmol·g⁻¹) and an increase of surface surface area by merely ix k²·thou⁻¹ (upwardly to 92 m²·one thousand⁻¹). An increase of the temperature to 850 °C leads to a decrease in the specific surface expanse past 21 one thousand²·yard^−ane and a decrease in the concentration of stiff acrid sites by but 1.five μmol·g⁻ⁱ (Tabular array one).

two.1.2. SiO_ii-Modification of the Support

SiO₂-modification of the support was carried out by its impregnation with an aqueous SiO_ii-sol and subsequent thermal handling at 800 °C. Introduction of silica in an amount of 2.five–vii.5 wt % does not bear upon the phase limerick of the support. In the X-ray diffraction patterns of all the samples (not shown in the current paper) the characteristic peaks of only γ-Al_iiO_iii were identified. The absence of changes in the crystalline structure of the supports leads to the conclusion that the SiO₂ is localized on the back up's surface. In the IR-spectrum of the SiO₂-modified support the absorption band of Si–OH at 3741 cm^−one [17,eighteen] appears with a decrease of the intensity of the bands at 3751 and 3771 cm⁻ⁱ (Al–OH bond vibrations [19,20]) (Figure 4). This likewise indicates that the SiO₂ is distributed on the support surface.

On the surface of the modified supports silica is distributed in the course of SiO _x -fragments Si(OSi)₃O and Si(OSi)₄. This is indicated by the occurrence of the signals at −101 and −125 ppm on the ²⁹Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectra (Figure v) [18,21,22,23,24].

The distribution of the modifier on the surface leads to changes in the porous system of the supports. The volume of the pores with diameter less than x nm decreases from 0.08 to 0.06 cm³·g⁻ⁱ. SiO _x -islands form additional porosity in the range of diameters ten–30 nm; the volume of these pores increases from 0.13 to 0.xvi cmⁱⁱⁱ·g⁻¹ (Figure half dozen). At the same fourth dimension the specific surface area does non modify significantly (Table 2).

SiO_two-modification of the support causes changes in the surface acerbity (Effigy 7, Table 3). It increases from 102.3 to 109.0–125.3 μmol·g⁻¹ due to the germination of weak (E _des.NHiii < 100 kJ·mol^−one) and medium (E _des.NH3 = 100–150 kJ·mol^−ane) acid sites (Table 3). This is indicated by the growth of the NH₃-TPD (temperature programmed desorption) profiles in the range 160–350 °C (Figure 7). Along with the germination of an additional amount of the weak and medium sites the concentration of stiff acid sites (with E _des.NH3 > 150 kJ·mol^−one) decreases (Table 3). This is indicated by the decrease in intensity of the high temperature component (400–500 °C) of the NH_three-TPD profile. The maximal temperature of ammonia desorption shifts to lower values (Figure 7) indicating a decrease in forcefulness of the acid sites.

It is likely that the acid sites formed are hydroxyl groups bonded to the silicon atoms and the Lewis acid sites of various structures [21,24]. For example, Iengo et al. [21] suggested, that the new acid sites are formed at the border of the SiO ₁₀ -islands, where aluminum and silicon are arranged relative to each other in such a fashion, that the aluminum atoms are positively charged and partially neutralized by the negative charges of the silicates. The decrease in number of the stiff acid sites upon modification is probably due to them being covered by SiO ₁₀ -islands.

2.2. Chromia-Alumina Catalyst

2.2.ane. Composition, Crystal and Pore Construction, Acerbity of Catalysts

On the ground of the initial alumina support it was established that surface concentration 10 atoms·nm^−two provides both loftier activity and the absence of crystalline α-Cr_twoO_iii in the catalyst (Figure 8), which accelerate catalyst sintering and decreases its thermal stability (Table 4).

In the synthesized catalysts an optimum mass ratio of chromium to potassium was used (Cr/K = 8) which was established by Kataev [25] for alumina supports obtained by the sequential thermal-hydrothermal treatment of gibbsite. Potassium every bit a promoter performs several functions in chromia-alumina catalysts. It neutralizes the stiff acid sites [15] and thus decreases the activity of the catalyst in hydrocarbon smashing reactions (Tabular array 4). Potassium in catalyst forms potassium chromates at the expense of the low-active crystalline Cr_iiO₃ phase. These chromates are reduced to catalytically agile Cr(Three) phase in dehydrogenation conditions [xv,26]. However, an excessive amount of potassium poisons the catalyst due to the coverage of the active Cr(III) sites [26] and neutralization of the weak and medium acrid sites which adsorb alkane molecules [15]. Optimal potassium content depends non only on the surface area and acidity of the back up but also on the concentration of chromium because potassium as aluminate and chromates is distributed both on the open areas of alumina and on the Cr_twoO_three particles [26].

The chromia-alumina catalysts with a surface concentration of chromium 10 atoms·nm⁻² were synthesized on the SiO₂-modified supports (Table five).

As a outcome of the chromium and potassium distribution in the pores of initial and SiO₂-modified support (Effigy ix), specific surface surface area and pore volume subtract slightly-by 6–17 thousand²·g⁻¹ and 0.04–0.06 cmⁱⁱⁱ·thou⁻¹ respectively (Tabular array ane and Table five).

Introduction of 6.5 wt % chromium and 0.8 wt % potassium to supports decreases the number of acrid sites by fifteen.7–29.9 μmol·g⁻ⁱ (Table 3 and Table 6). SiO₂ affects the acidity of the catalysts like to those of the supports: introduction of 2.5–seven.5 wt % SiO_ii increases the full acidity of goad due to the formation of an additional weak and medium acid sites; it too decreases the number of strong acid sites.

2.2.2. Active Component of Catalysts

Formation on the support surface of SiO _ten -islands results in changes in the distribution of the active component. Modification of the support by two.5–vii.5 wt % SiO₂ leads to a decrease of Cr(VI) content in the catalysts based on it.

Past diffuse reflectance UV-Vis spectroscopy it was found that chromium deposition on the SiO_ii-modified supports results in an increment of Cr(Three) content in comparing to the initial support. It tin be seen by the increase in intensity of the betoken at ~17,000 cm⁻ⁱ (Figure x) which corresponds to an electronic transition ⁴A_2g→⁴T_2g in the Cr(III)_oct ion [26,27]. Increasing the Cr(3) content is consistent with the information of the iodometric titration, showing that the concentration of Cr(VI) (calculated as CrO_iii) decreases from 2.v to 1.4 wt % with an increment of SiO₂ content from 0 to vii.five wt % (Table 5). UV-Vis spectra also demonstrate a shift of the Cr(VI) signal at ~27,000 cm⁻¹ [26,28,29] to the short-wavelength region and an increment in intensity of the complex Cr(III)-Cr(VI) indicate at ~22,000 cm⁻¹ [26,27,28,29] (Figure 10). In combination these indicate a subtract in symmetry of surface chromates due to oligomerization of Cr(Vi) compounds [26].

The increment in Cr(III) content observed past UV-Vis spectroscopy and the oligomerization of surface Cr(Half dozen) compounds are consistent with the Raman-spectroscopy data. Here, the increase in SiO₂ content results in the growth in intensity of the signal at ~550 cm^−one, respective to the vibrations of the Cr(III)_oct–O bond [26,30,31]; the intensity of hydrated dichromate signals at 906 and 948 cm⁻ⁱ [28,30] decreases; the typical signals of dehydrated polychromates at 850–900 and m cm⁻¹ [28,30,31,32] too appear (Figure xi).

Decreasing Cr(Vi) content and the simultaneous oligomerization of chromates indicates a subtract of stabilization of the chromates by the support surface through X–O–Cr (10 = Al, Si) bonds. This is also supported past the temperature-programmed reduction of catalysts. All H₂-TPR profiles are decomposed into 3–4 Gaussian components which correspond to the chromates with dissimilar degrees of bounden to the surface. The higher the temperature of chromate reduction the greater is the number of X–O–Cr bonds per 1 chromium atom [26,33]. Therefore, the low-temperature components (at 300–450 °C and 400–450 °C) most probable represent to polychromates, and the loftier-temperature components (at 450–600 °C and 500–650 °C) stand for to mono- and dichromates. The introduction of 2.5–7.v wt % SiO₂ leads to the redistribution of the hydrogen consumption temperature: the depression temperature component of the H₂-TPR profile (in the region 300–400 °C) increases and the high temperature component (in the range 500–600 °C) decreases. The Gaussian component of the H₂-TPR profile in the region 550–600 °C, corresponding to monochromates with the highest degree of interaction with the back up surface, disappears. The maximum on the H₂-TPR profiles sequentially shifts from 436 to 419 °C (Effigy 12).

The decrease in interaction betwixt the back up and the active component is due to the changes in hydroxyl comprehend of the support. The isoelectronic indicate (IEP) of alumina is vii.2–8.vi, and the IEP of silica is 2.0–3.9 [34,35,36]. This is caused by the predominance of the following reactions on the surface of the hydrated supports [34,36]:

–Al–OH + H⁺ ↔ –AlOH_ii ⁺

(ane)

Due to a larger number of basic sites on the alumina surface during deposition of the active component from an aqueous solution of chromic acid and subsequent heat treatment, a greater interaction of chromate ions with Al₂O_iii rather than with SiO_ii and SiO_ii–Al_iiO₃ is observed [37]. The interaction of chromate ions with the alumina surface is complex [35] and consists of:

(1) acid-base reaction between the neutral surface hydroxyl groups and chromate anions (Scheme two):

(2) electrostatic attraction (Scheme 3) between the chromate ions and the positive-charged surface sites (in the inner Helmholtz airplane of the electrical double layer) which is formed by protonation of the surface hydroxyl groups (Reaction (1)) [35]:

Therefore, on the surface of alumina predominantly mono- and dichromate anions are stabilized (Scheme iv).

As is shown the formation of SiO₁₀-islands on the alumina surface leads to diminution in interaction between Cr(VI) compounds and the support. It may occur due to the following reasons:

–subtract in the number of surface basic hydroxyls capable of acid-base of operations reaction (Scheme ii) with chromate ions;

–SiO_ii introduction results in a drop of the pH of the electric double layer. This in plow leads to a shift of the equilibrium of Reactions (3)–(five) to the correct, towards formation of polynuclear chromate ions [31].

2CrO_iv ^two− + 2H⁺ ↔ Cr_iiO_seven ²⁻ + H₂O

(three)

3Cr_iiO_vii ²⁻ + 2H⁺ ↔ 2Cr₃O_ten ⁱⁱ⁻ + H_iiO

(four)

4Cr₃O₁₀ ^two− + 2H⁺ ↔ 3Cr₄O₁₃ ^two− + H₂O

(5)

–according to Reaction (2) the function of the inner Helmholtz plane becomes negatively charged which results in the electrostatic allure between support surface and chromate ions diminishing.

A decrease in the number of Cr–O–10 (10 = Al, Si) bonds leads to the stabilization of the polychromate ions and polynuclear [–Cr(III)–O–Cr(Three)–] _northward ions (also chosen amorphous Cr₂O_three [26] or Cr₂O₃-clusters [28]) (Scheme 4) by the SiO_two-modified support surface afterward thermal activation of the goad. A similar issue of SiO₂ has too been reported by Weckhuysen et al. [29]; at the transition from Al₂O₃ to aluminosilicate (with 40 wt % SiO₂) the chromate/dichromate band intensity ratio on the UV-Vis spectra of the samples calcined at 550 °C decreased from ∞ to 2.xviii. Weckhuysen et al. [38] reported that in SiO₂ with 0.two wt % Cr calcined at 720 °C, Cr(III) ions were identified past UV-Vis spectroscopy, whereas for the same amount of chromium on alumina, Cr(III) ions were not observed.

Increasing interaction between Cr(Three) ions was also confirmed by EPR-spectroscopy. Upon introduction of SiO₂ the intensity of the δ-signal component at g = 3.8–4.0, corresponding to isolated Cr(Iii) ions [39], decreases while the intensity of β-indicate with chiliad = 1.98 and ΔH = 1400–1600 G, respective to magnetically concentrated Cr(3) ions [28,39], increases (Effigy 13).

2.ii.3. Isobutane Dehydrogenation Performance of Catalysts

Chromia-alumina catalysts were tested in isobutane dehydrogenation at 570 °C. The curves of dehydrogenation and cracking rate versus time on stream are shown on Figure 14a,b, respectively. All the curves on Figure 14a show the maxima as a function of fourth dimension on stream. In all the cases at the initial phase of the dehydrogenation cycle (for the outset 30–55 min) an increment of catalyst dehydrogenation activity by 8%–9.5% is observed (Effigy 14a); this is the goad development period. Equally we know [40,41] during catalyst development the surface stabilized Cr(Six) compounds are reduced by hydrocarbons to additional catalytically agile Cr(3) compounds. Decrease of the dehydrogenation rate at the last stage of the cycle is due to the goad coking. The subtract of the neat rate during the starting time 30 min (Figure 14b) is also caused by covering of the acid sites by coke deposits [xv].

The increment in the content of amorphous Cr₂O₃ phase, which is the well-nigh active in the dehydrogenation of calorie-free alkanes [26], and the formation of polychromates which too reduce to amorphous Cr_iiO_iii, causes an increment in dehydrogenation activity of catalysts based on SiO₂-modified supports (Figure 14a). The average dehydrogenation activity increases past 10%–14%—from 2.9 to iii.2–3.3 μmol C_fourH₁₀·g_{true cat} ⁻¹·southward^−one. At the same time the catalyst development period diminishes due to a subtract in the Cr(VI) content. The ascension in dehydrogenation activity with the increase in the polynuclear Cr(III) ions content is consequent with the results of the kinetic studies by Airaksinen et al. [42] and Carra et al [43]. Co-ordinate to these works the dehydrogenation of one molecule of butane proceeds at two active sites.

A decrease in the number of strong acrid sites upon SiO₂-modification of support leads to a decrease in cracking rate on catalysts with two.v–7.5 wt % SiO₂ (Figure 14b). The average groovy rate decreases by xviii%–34%—from 0.29 to 0.xix–0.24 μmol (C_one–C_three)·m_{true cat} ^−one·s⁻¹.

3. Materials and Methods

3.one. Preparation of Supports and Catalysts

Microspherical (40–200 μm) boehmite support prepared by consecutive thermal and hydrothermal treatment of aluminum trihydroxide (GD00 grade, produced by Bogoslovsk Aluminum Smelter, Krasnoturyinsk city, Russian federation) under industrial atmospheric condition [11] in an autoclave for 3 h at 195 °C (Chemical Found Karpov, Mendeleyevsk city, Russia), was used for the synthesis of alumina supports and chromia-alumina catalysts. According to thermogravimetry the phase composition of the support precursor is the following: boehmite—96.0 wt %, gibbsite—3.0 wt %, Al₂O_three—ane.0 wt %.

SiO_two was introduced into the support by incipient wetness impregnation with a water silica sol ("Leiksil" brand produced by "Compas" Scientific and Technical Eye, Kazan city, Russia).

The goad was prepared by incipient wetness impregnation of the support with an aqueous solution of chromic acrid and potassium carbonate, followed by drying under vacuum. Then the catalyst was activated by thermal treatment in a muffle furnace at 750 °C for 4 h.

3.2. Characterization of Supports and Catalysts

Thermogravimetric analysis was performed with a STA-449C (Netzsch, Selb, Germany) combined thermogravimetric and differential scanning calorimetric (DSC) analyzer coupled with an Aeolos QMS 403 quadruple mass spectrometer (Netzsch, Selb, Germany) in a temperature range of thirty–1000 °C at a heating rate of 10 °C. min⁻¹ in a flow of argon. The concentrations of aluminum hydroxide phases were calculated from the corporeality of water released in their dehydration.

Scanning electron microscopy was performed on an EVO l XVP (Carl Zeiss, Oberkochen, Germany) electron microscope.

The elemental composition of the catalysts was determined by Ten-ray fluorescence spectroscopy on a Clever C31 musical instrument (ELERAN, Elektrostal, Russia).

Powder 10-ray diffraction measurements were carried out using a DRON-2 diffractometer (Burevestnik, Petrograd, Russia). The patterns were obtained using CuKα radiations and graphite monochromator (λ = 1.54187 Å) at 30 kV and 15 mA. The identification of unlike crystalline phases in the samples was performed by comparing the data with the Joint Committee for Pulverization Diffraction Standards (JCPDS) files. The crystallite size of the boehmite stage was calculated using the Selyakov–Scherrer Equation. The error in determining the crystallite size was 10%.

Specific surface (S _sp) and pore volume (5 _p) of samples were determined from the Northward₂ physisorption measurements at 196 °C using an universal Autosorb-iQ analyzer (Quantachrome, Boynton Embankment, FL, U.s.). Prior to measurement, the sample was outgassed for 1 h at 150 °C (for boehmite precursor) or for iii h at 300 °C (for alumina supports and catalysts). S _sp and V _p were calculated according to the Brunauer-Emmett-Teller method. The pore diameter distribution was calculated by the desorption branch of isotherm using the standard Barrett–Joyner–Halenda method.

The ²⁹Si MAS NMR spectra of supports were recorded at room temperature on an Avance 400 spectrometer (Bruker, Ettlingen, Germany) operating at frequencies of 79.5 MHz with spectral resolution 48.83 Hz. The sample rotation frequency was 5 kHz.

The IR spectra of supports were measured on a VERTEX 70 (Bruker, Ettlingen, Deutschland) instrument fitted with a mercury−cadmium−telluride detector. The measurements were done in transmission mode using a Harrick high temperature cell. A background spectrum and the spectra were measured at 480 °C and a residual pressure of less than x⁻³ mbar with a resolution of 1 cm⁻¹ and averaged past 128 scans. For the IR analysis, samples were prepared in a tablet-shape of twenty mg; optical density was 20 mg·cm⁻².

Hexavalent chromium concentration in the catalyst was determined by dissolution of Cr(VI) in sulfuric acid and subsequent volumetric titration with the iodometric method.

UV-Vis diffuse reflectance spectra of the catalysts were recorded using a V-650 spectrophotometer (Jasco, Tokyo, Japan) equipped with an integrating sphere ISV-722 (Jasco, Tokyo, Nihon). A BaSO₄ plate was used as the reference. Spectra were recorded in the wavenumber range 12,500–50,000 cm⁻¹ with the spectral resolution 2 nm. UV-Vis-spectra were deconvoluted into Gaussian bands to decide the positions and intensities of the bands' maximums.

Raman spectra of catalysts were recorded using a dispersion Raman-microspectrophotometer Nicolet Almega XR (Thermo Fisher Scientific, Waltham, MA, U.s.a.). The 532 nm line of a Nd-YAG laser was used as an excitation. The spectra were recorded in the wavenumber range 100–1100 cm⁻ⁱ with the spectral resolution two cm⁻¹. Each spectrum was received by averaging 10 exposures on 10 south.

EPR measurements were fabricated at the temperature of −196 °C on a RE-1306 EPR-spectrometer (Institute of Belittling Instrumentation of Russian University of Sciences, St. petersburg, Russian federation) with a working frequency nine.37 GHz and 100 kHz magnetic field modulation. Diphenylpicrylhydrazyl (yard = 2.0036) was used as reference for 1000-value determination.

NH₃-TPD and H₂-TPR measurements were carried out on the ChemBET Pulsar TPR/TPD (Quantachrome, Boynton Beach, FL, The states). Earlier NH₃-TPD measurement the sample was degassed at 600 °C for 2 h in a helium flow. The adsorption step is carried out in an ammonia flow at 100 °C for thirty min. So the physically sorbed ammonia was removed with helium at 100 °C for 30 min and the sample was cooled to a room temperature in the helium flow. Temperature programmed desorption was performed from room temperature to 700 °C at a heating rate of ten °C·min⁻¹. The forcefulness of the acid sites was evaluated by the temperature of ammonia desorption [44]. A temperature of 175 °C corresponds to ammonia desorption energy of 100 kJ∙mol⁻ⁱ and a temperature of 380 °C to the desorption energy 150 kJ∙mol⁻¹. Acid sites with ammonia desorption energy lower than 100 kJ∙mol^−one were attributed to weak ones, while the sites with desorption energies of 100–150 kJ∙mol⁻¹ and higher than 150 kJ∙mol⁻ⁱ were attributed to medium and strong sites respectively. The number of weak, medium and potent acid sites was calculated from the area under the NH₃-TPD profiles in the temperature ranges <175 °C, 175–380 °C and >380 °C respectively.

Before H₂-TPR measurement the catalyst was heated to 650 °C and held at this temperature for 60 min in a flow of a gas mixture (v vol% O_two + 95 vol% N_two). Then the goad was cooled downwards to room temperature in the helium menstruation. Temperature programmed reduction was performed from room temperature to 700 °C at a heating charge per unit of 10 °C·min⁻¹. NH₃-TPD and H_two-TPR profiles were Gauss fitted using the TPRWin software (version 3.52, Quantachrome, Boynton Beach, FL, U.s.a., 2012).

3.three. Catalyst Testing

The catalysts were tested in the reaction of isobutane dehydrogenation in a steel stock-still bed reactor of 10 mm internal diameter under atmospheric pressure. An amount of 2 grand of fresh catalyst (sieve fraction 40–200 μm) was filled into the reactor. The catalyst was heated at 5 °C·min^−one to 650 °C in an air period (60 mL·min⁻¹) followed by flushing with air for 30 min at the aforementioned temperature. The goad was cooled in air to 570 °C and flushed with argon for 15 min at that temperature. Then a mixture of thirty vol% C₄H_ten in Ar was fed at a rate of threescore mL·min⁻¹ at the same reaction temperature. The reaction was run for 130 min followed by goad regeneration in an air flow for threescore min at 650 °C. The regeneration/dehydrogenation cycles were repeated three times.

The hydrocarbon composition of feed and reaction products were analyzed by gas chromatography on a GH-thousand instrument (Chromos, Dzerzhinsk, Russia) with a flame-ionization detector and a capillary VP-Alumina/KCl column (VICI Valco, Houston, TX, Usa). The concentrations of H₂, CH₄, and CO were determined with the use of a cavalcade filled by 13× molecular sieves on a GH-g appliance with a thermal conductivity detector.

Based on of the results of chromatographic analysis the rates of i-C₄H₁₀ dehydrogenation and cracking of hydrocarbons were calculated using Equations (6) and (seven), respectively.

$$\text{Dehydrogenation Rate}=\frac{X({\text{iC}}_4{\mathrm{H}}_{\mathrm{x}})\times F}{3600\times 100\times {\mathrm{yard}}_{\text{cat}}}$$

(6)

$$\text{Cracking Rate}=\frac{(C_{\mathrm{C}{\mathrm{H}}_4}+C_{{\mathrm{C}}_2{\mathrm{H}}_6}+C_{{\mathrm{C}}_{\mathrm{two}}{\mathrm{H}}_4}+C_{{\mathrm{C}}_3{\mathrm{H}}_{\mathrm{eight}}}+C_{{\mathrm{C}}_3{H}_{\mathrm{6}}})\cdot V^{\text{outlet}}}{22400\cdot 3600\cdot 100\cdot m_{\text{true cat}}}$$

(7)

where 10(iC₄H₁₀) is the isobutane conversion, %; F is the feed rate of isobutane, mole·h⁻¹; grand _cat is the weight of goad, k; C _CH4 , C _C2H6 , C _CtwoH4 , C _CiiiH8 , C _CiiiH6 are the concentrations of methane, ethane, ethylene, propane, propylene respectively in the reaction products, vol%; Five ^outlet is the volumetric flow of the reaction products, mL·h⁻¹.

iv. Conclusions

The distribution of SiO₂ and its effect on the construction and acidity of the alumina back up, as well as the effect of SiO_two on the active component and the operation in isobutane dehydrogenation of chromia-alumina catalyst were investigated. It was shown that SiO_two in an amount of 2.5–7.five wt % is distributed on the surface of alumina in the course of Si(OSi)₄ and Si(OSi)₃(O–) compounds. These SiO _x -islands on the support produce additional porosity with a pore range x–30 nm, also equally boosted weak and medium acrid sites. At the same time SiO₂-modification causes a decrease in the number of strong acrid sites.

Upon introduction of the agile component, the SiO _x -islands diminish the interaction of chromate ions with the back up surface; as a consequence polychromates are the compounds predominantly stabilized on the support. During thermal activation of the catalyst these polychromates are reduced to a phase of amorphous Cr₂O_iii. This results in the increase of the goad action in the isobutane dehydrogenation. At the same time, the subtract in the number of strong acrid sites in SiO₂-modified catalysts leads to a diminution of its activity in hydrocarbon cracking reactions.

Acknowledgments

The work was performed co-ordinate to the Russian Government Program of Competitive Growth of Kazan Federal Academy.

Author Contributions

Southward.R.E and A.A.L conceived and designed the experiments; G.East.B. and A.N.M. performed the experiments. All authors participated in the analysis of the information and writing of the paper. G.East.B., A.N.1000. and S.R.E. proofread the manuscript.

Conflicts of Involvement

The authors declare no conflict of interest.

References

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Scheme i. Production of alumina precursor.

Scheme 1. Production of alumina precursor.

Figure i. Scanning electron microscopy (SEM) image of the microspherical support.

Figure 1. Scanning electron microscopy (SEM) image of the microspherical back up.

Figure 2. X-ray diffraction patterns of precursor of alumina and alumina support, obtained at 800 °C.

Figure 2. Ten-ray diffraction patterns of precursor of alumina and alumina support, obtained at 800 °C.

Figure 3. Pore size distribution for precursor of alumina and alumina supports, obtained at 750 °C and 850 °C.

Effigy 3. Pore size distribution for precursor of alumina and alumina supports, obtained at 750 °C and 850 °C.

Figure iv. IR spectra of initial alumina support and a back up containing 4.5 wt % SiO_two.

Figure 4. IR spectra of initial alumina back up and a support containing 4.five wt % SiO₂.

Effigy 5. ²⁹Si Magic Bending Spinning Nuclear Magnetic Resonance (MAS NMR) spectra of SiO₂-modified supports.

Figure 5. ²⁹Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectra of SiO₂-modified supports.

Figure 6. Pore size distribution for initial and SiO₂-modified supports.

Figure 6. Pore size distribution for initial and SiO₂-modified supports.

Figure 7. NH_iii-TPD (temperature programmed desorption) patterns of initial and SiO_two-modified supports.

Figure 7. NH_three-TPD (temperature programmed desorption) patterns of initial and SiO₂-modified supports.

Effigy 8. X-ray diffraction patterns of catalysts.

Figure 8. X-ray diffraction patterns of catalysts.

Figure 9. Pore size distribution for support with 4.five wt % SiO₂ and for catalyst based on it.

Effigy 9. Pore size distribution for support with four.v wt % SiO₂ and for catalyst based on it.

Effigy 10. Lengthened reflectance UV-Vis-spectra of catalysts (6.five wt % Cr; 0.viii wt % M).

Figure 10. Lengthened reflectance UV-Vis-spectra of catalysts (6.5 wt % Cr; 0.8 wt % K).

Figure 11. Raman-spectra of catalysts.

Figure 11. Raman-spectra of catalysts.

Figure 12. H₂-temperature programmed reduction (TPR) profiles of catalysts.

Effigy 12. H_two-temperature programmed reduction (TPR) profiles of catalysts.

Scheme two. Reaction between chromate ion and surface hydroxyl groups.

Scheme 2. Reaction between chromate ion and surface hydroxyl groups.

Scheme iii. Electrostatic interaction between chromate ion and positive-charged surface sites of alumina.

Scheme iii. Electrostatic interaction between chromate ion and positive-charged surface sites of alumina.

Scheme 4. Hydration of alumina and silica surface, their interaction with chromate ions.

Scheme 4. Hydration of alumina and silica surface, their interaction with chromate ions.

Figure xiii. Electron paramagnetic resonance (EPR) spectra of catalysts.

Effigy 13. Electron paramagnetic resonance (EPR) spectra of catalysts.

Effigy 14. Dehydrogenation rate (a) and cracking rate (b) versus fourth dimension on stream.

Figure 14. Dehydrogenation rate (a) and dandy rate (b) versus time on stream.

Table 1. Integral parameters of the porous system and the acidity of alumina supports.

Table ane. Integral parameters of the porous system and the acerbity of alumina supports.

Sample	Temperature of Treatment	Crystalline Phase	Brunauer-Emmett-Teller (BET) Area (m2·g−1)	Pore Volume (cm3·g−one)	Total Number of Acid Sites (μmol·g−i)	Number of Acid Sites with Due east des.NHthree >150 kJ·mol−1 (μmol·g−1)
Precursor of Alumina	-	γ-AlOOH	27	0.07	-	-
Alumina support	750	γ-Al2O3	92	0.27	125.eight	xvi.ii
	800		83	0.26	102.iii	10.0
	850		62	0.26	91.8	8.v

Tabular array 2. Porous system parameters of supports.

Table 2. Porous system parameters of supports.

SiOtwo Content (wt %)	BET Surface Area (mtwo·one thousand−1)	Pore Volume (cm3·g−i)	Distribution of Pore Volume (cm3·g−i) over Pore Diameters
			<x nm	10–xxx nm	>thirty nm
0	83	0.26	0.08	0.13	0.05
2.five	80	0.27	0.06	0.fifteen	0.06
iv.5	82	0.27	0.06	0.xv	0.06
7.5	87	0.28	0.06	0.sixteen	0.06

Table 3. Results from NH₃-TPD (temperature programmed desorption) measurement of supports.

Table 3. Results from NH₃-TPD (temperature programmed desorption) measurement of supports.

SiO2 Content (wt %)	Total Number of Acid Sites (μmol·chiliad−i)	Distribution of Acid Sites (μmol·g−1) on the Energy of Ammonia Desorption
		<100 kJ·mol−i	100–150 kJ·mol−1	>150 kJ·mol−1
0	102.3	25.6	66.seven	10.0
two.5	115.2	29.0	81.eight	four.five
four.five	119.iv	37.5	78.seven	iii.1
seven.5	125.3	45.0	78.0	ii.4

Table four. The composition of catalysts and their functioning in isobutane dehydrogenation.

Tabular array 4. The composition of catalysts and their performance in isobutane dehydrogenation.

Chromium Content (wt %)	Potassium Content (wt %)	Surface Concentration of Chromium (atoms·nm−2)	Dehydrogenation Rate (μmolC4H10 ·yardcat −i·southward−1)		Cracking Rate (μmol[C1–C3]·gcat −1·south−one) Initial
			Initial	Afterwards Treatment at thousand °C
4.5	0.6	half-dozen.5	2.seven	2.4	0.32
6.five	0	x.0	ii.8	-	0.47
half-dozen.5	0.8	10.0	3.0	three.1	0.29
8.five	i.i	xiii.5	3.1	2.5	0.27

Tabular array v. Limerick, Cr(VI) content, specific surface area and pore volume of the initial and SiO₂-modified catalysts.

Table five. Limerick, Cr(6) content, specific area and pore volume of the initial and SiO₂-modified catalysts.

Chromium Content (wt %)	Potassium Content (wt %)	SiOtwo Content (wt %)	Cr(Vi) Content i (wt %)	BET Area (gii·thou−ane)	Pore Volume (cmthree·g−1)
half-dozen.five	0.8	0	ii.v	77	0.22
6.5	0.8	2.5	ane.9	73	0.22
6.5	0.8	4.v	1.6	74	0.22
6.5	0.eight	7.v	1.four	seventy	0.22

Tabular array 6. Results from NH₃-TPD measurement of catalysts.

Table 6. Results from NH_iii-TPD measurement of catalysts.

SiO2 Content (wt %)	Full Number of Acid Sites (μmol·chiliad−1)	Distribution of Acid Sites (μmol·thousand−ane) on the Energy of Ammonia Desorption
		<100 kJ·mol−i	100–150 kJ·mol−1	>150 kJ·mol−1
0	72.iv	9.3	53.four	9.seven
2.5	93.seven	12.9	78.0	2.8
4.v	102.8	xv.seven	85.v	1.half-dozen
vii.5	109.six	13.8	95.six	0.two

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