Fluidized-Bed Isobutane Dehydrogenation over Alumina-Supported

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Fluidized-Bed Isobutane Dehydrogenation over AluminaSupported Ga2O3 and Ga2O3–Cr2O3 Catalysts Anna N. Matveyeva, Nadezhda Zaitseva, Päivi Mäki-Arvela, Atte Aho, Anastasia Bachina, Sergey Petrovich Fedorov, Dmitry Yu. Murzin, and Nikolai Pakhomov Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04571 • Publication Date (Web): 24 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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Industrial & Engineering Chemistry Research

Fluidized-Bed Isobutane Dehydrogenation over Alumina-Supported Ga2O3 and Ga2O3–Cr2O3 Catalysts Anna N. Matveyeva1, Nadezhda A. Zaitseva2, Päivi Mäki-Arvela3, Atte Aho3, Anastasia K. Bachina4, Sergey P. Fedorov1, Dmitry Yu. Murzin1,3*, Nikolai A. Pakhomov1 1

Laboratory of Catalytic Technologies, St. Petersburg State Institute of Technology (Technical University), Moskovsky Ave. 26, St. Petersburg 190013, Russia 2

3

Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia

Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Biskopsgatan 8, Turku/Åbo 20500, Finland 4

Department of Physical Chemistry, St. Petersburg State Institute of Technology (Technical University), Moskovsky Ave. 26, St. Petersburg 190013, Russia *

e-mail: [email protected]

Abstract: Dehydrogenation of isobutane to isobutene over supported gallium oxide microspherical catalysts was investigated in a fluidized-bed reactor. A partially crystallized nanostructured aluminum hydroxide-oxide, which is a product of gibbsite centrifugal thermal activation (CTA) obtained using a CEFLAR technology, was used as a catalyst support. The structural and textural properties of Ga2O3/Al2O3 catalysts were characterized by a range of techniques including XRD, N2-physisorption, TPD of NH3 and CO2, IRS of adsorbed pyridine and selective adsorption of a series of acid-base indicators. Ga–Al oxide catalyst exhibited a stable performance close to activity of Cr–Al oxide catalysts not containing soluble hexavalent chromium. Upon addition of Cr2O3 (6 wt % of Cr), in amounts lower than in an industrial chromia/alumina catalyst (10.9% Cr), and 1% ZrO2 to 6%Ga/Al2O3 catalytic activity in isobutane dehydrogenation reaches the performance of the industrial KDM catalyst (“Sintez”, Russia).

Keywords: dehydrogenation, isobutene, gibbsite, thermal activation, Ga2O3, TPD.

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1. Introduction The catalytic dehydrogenation of alkanes is a process of considerable importance, because it represents a route for obtaining alkenes from poorly reactive and low-cost saturated feedstock. There are several industrial options for C3–C5 paraffin dehydrogenation different both in process parameters and catalytically active components.(1) Currently, Cr2O3/Al2O3 systems predominate half of the world market of commercial catalysts for light paraffin dehydrogenation and 100% of the market in Russia.(2) The following two types of chromia/alumina catalysts are used in industrial processes: –

for fixed-bed dehydrogenation: granulated catalysts at a reduced hydrocarbon

partial pressure (Catofin process(3) for isobutane and propane dehydrogenation and Catadiene process(4) for the single-step dehydrogenation of n-butane to butadiene); –

for fluidized-bed dehydrogenation: microspherical catalysts to obtain

isobutene, isopentene and propene (Yarsintez–Snamprogetti process(1)). Since a part of chromium ions in chromia/alumina catalyst is stabilized to carcinogenic Cr6+ species(5), these catalysts present a serious threat for the environment, and therefore there is a continuous demand for developing of novel alkane dehydrogenation catalysts with a lower content of chromium or preferably without it. In recent years, Ga2O3-based catalysts have attracted interest as potential candidates for alkane dehydrogenation in the presence or absence of oxidizing agents.(6)– (8)

This was prompted by the studies of Nakagawa et al. showing exceptionally high

activity of commercial Ga2O3 for dehydrogenation of ethane to ethylene in the presence of CO2.(9) Dehydrogenation activity of gallium oxide supported on ZSM-5 was also reported in the late 1980s–early 1990s in the studies(10)–(13) related to conversion of propane to aromatics (Fig. 1).


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Figure 1. Reaction scheme for propane aromatization on H-ZSM5 catalysts. The brackets for C6–C8, non-aromatic compounds, mean that these compounds, which are highly thermodynamically unfavored, appear only as traces. (Reproduced with permission from ref. (11). Copyright 1988, Elsevier)

Similar to Al2O3, Ga2O3 has several polymorphs, designated as α-, β-, γ-, δ-, and ε-Ga2O3. All of these phases of gallium oxide except ε-Ga2O3 can be prepared under specific conditions.(14),(15) Among the polymorphs, β-Ga2O3 is the most stable and exhibits the highest dehydrogenation activity, both in the presence and absence of CO2. It is even more active than chromia in the presence of CO2.(7) The real nature of the active sites in Ga2O3-based dehydrogenation catalysts is still uncertain and under dispute. β-Ga2O3 has a monoclinic structure (mp 1740 °C) with the oxide ions having a distorted ccp arrangement and GaIII located in distorted tetrahedral and octahedral sites.(16) The structure owes its stability to these distortions. Lower coordination of a half of GaIII results in density just 10% lower than for α-alumina (corundum). Based on the analogy to alumina, gallium oxide acid sites should be Lewis acid sites formed via dehydration of OH-groups from gallium cations in tetrahedral positions.(7) Different supports (e.g. Al2O3, ZrO2, SiO2 etc.) were used in dehydrogenation catalysts containing gallium oxides. Inclusion of acid and basic sites on the surface of support is thought to be important to achieve a high conversion.(17)–(19) Xu et al. reported(18) that in the absence of an oxidizing agent, conversion levels of propane 3 ACS Paragon Plus Environment


obtained using 5 wt % β-Ga2O3 supported on ZrO2, Al2O3 and TiO2 were high, medium, and low, respectively, while SiO2 and MgO-supported Ga2O3 were almost inactive. Additionally, these authors proposed the activity to be dependent on the total amount of surface acid sites, in particularly medium to strong ones. CO2 having a promoting effect on dehydrogenation activity over Ga2O3/TiO2 influenced negatively in case of Ga2O3/ZrO2 and Ga2O3/Al2O3. Specific interactions between Ga2O3 and Al2O3 due to formation of spinel-type γGa2O3–Al2O3 solid solutions are suggested to play a key role in better distribution of surface gallium sites, which results in very active and stable to deactivation with time on stream of GaxAl10−xO15 complex (with x varying from 0 to 10) for dehydrogenation.(20) GaxAl10−xO15 samples were synthesized through a co-precipitation method of ethanolic solutions of gallium and aluminum nitrates.(21) Other known dehydrogenation catalysts were prepared by the same method using indium, gallium and aluminum salts resulting in solid solutions of mixed oxides.(22),(23) Comparison of C3H6 yield in oxidative propane dehydrogenation on freshly prepared GaOx/SiO2 catalysts was shown by Gaidai et al.(24) to be dependent on Ga content, which varied from 5 to 60 wt %. The highest initial yield was demonstrated by a catalyst with 15% Ga, while 60% of gallium displayed the lowest activity. According to our knowledge no data are available for and Ga2O3/Al2O3 catalysts in isobutane dehydrogenation. Some recent literature studies address catalytic properties of Ga containing mesoporous silica and SAPO-11 in dehydrogenation of n-butane and isobutane.(25),(26) Most gallium oxide-containing catalysts, considered, above were investigated in fixed-bed reactors. At the same time, it is of interest to evaluate a possibility of using these catalysts for isobutane dehydrogenation in a fluidized-bed reactor. It should be


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noted that attempts have been already made to use this active component for the fluidized bed with promoting additives Pt(27)–(30) and Ni(31). As a support in this work the product of gibbsite (Al(OH)3) centrifugal thermal activation (CTA-product) was used. This product obtained using CEFLAR technology is an amorphous nanostructured aluminum hydroxide–oxide.(32) Details of this technology and the properties of the resulting support are described in detail in ref. (32)–(35). The aim of the current work was to develop a supported gallium oxide catalyst allowing to achieve catalytic performance comparable with the industrial KDM catalyst. In particular, the paper focuses on investigation of Ga and Ga–Cr oxides supported on CTA-product for nonoxidative isobutane dehydrogenation in a fluidized-bed reactor. The catalytic activity of a range of gallium containing catalysts was compared with an alumina-supported chromium oxide catalyst.

2. Experimental 2.1. Catalyst preparation The catalyst support comprised a hydroxide-oxide of aluminum (referred below as “CTA-product”) was obtained from gibbsite via the CEFLAR technology. The centrifugal thermal activation of gibbsite was performed at 550 °C. The gibbsite for further application in CEFLAR technology was derived from bauxite using the Bayer process.(36) A fraction of the CTA-product with the particle sizes ranging from 40 to 100 µm is 83 wt %. The support after thermal activation did not experience any calcination prior to impregnation. Supported Ga and Ga–Cr oxide catalysts were prepared by an incipient-wetness impregnation method of the CTA-product with appropriate amounts of aqueous solutions of Ga(NO3)3·8H2O and CrO3. Impregnation was carried out in a flask under stirring by dropping an aqueous solution of nitrate gallium. The second active component Cr2O3 was 5 ACS Paragon Plus Environment


deposited by re-impregnation after completion of gallium oxide formation. The gallium content of the final Ga2O3/CTA-product catalyst varied from 1.5 to 9 wt % (will be referred below as e.g. “9Ga/CTA”). Atomic ratios of Ga/Cr in supported gallia–chromia catalysts were 0.5, 1 and 2 at total amounts = 9, 12 and 9 wt %, respectively (will be referred below as e.g. “3Ga6Cr/CTA”). After drying at 90–110 °C for 2 h, the samples were gradually calcined in air up to 650–900 °C for 8–10 h (heating rate of 10 °C/min). The determination of the heat treatment conditions was carried out in accordance with the data of preliminary performed DTA and TGA analysis of the active component precursors on support. Zirconium oxide containing catalyst (1 wt %) was prepared by co-impregnation with Zr(CO3)2, CrO3 and Ga(NO3)3·8H2O (calcination temperature of 750 °C). Chromia–alumina systems not containing soluble hexavalent chromium were prepared by impregnation of CTA-product with (i) Cr(OAc)3 or (ii) CrO3. In the former case after drying the catalyst was calcined in an inert atmosphere at 750 °C, whereas for latter catalyst heat treatment at 750 °C in an oxidizing atmosphere (air) was followed by washing with boiled water to remove chemically non-bonded hexavalent chromium, and subsequent calcination in an inert atmosphere at 600 °C.

2.2. Characterization The samples were characterized using several techniques. The phase composition of the samples was determined by the powder X-ray diffraction (XRD) on a SmartLab III intelligent X-ray diffraction system (Rigaku, USA) equipped with 1D detector (DteX250) at the following conditions: CuKα λ=1.54056 Å radiation, 40 kV voltage, 30 mA current, 2°/min scan speed, 0.01° step width in the range 10–80° 2θ. The patterns of four samples (CTA-650, CTA-850, 6Ga-850 and 9Ga-850) were collected in multiple scan mode at 1°/min scan speed. Obtained 8–10 scans for each 6 ACS Paragon Plus Environment

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sample were averaged and analyzed. Phase identification was conducted by comparison of the measured patterns with the ISCD data base. Average crystallite sizes were calculated with the Scherrer equation. Textural and structural properties were determined by performing N2 physisorption at −196 °C using an Autosorb-6iSA (Quantachrome Instruments, USA). Prior to the measurements the samples (0.1–0.2 g) were degassed at 250 °C under vacuum up to residual pressure 12 Pa for 1 h. The specific surface area was calculated by the MultiPoint BET (Brunauer, Emmett, Teller) method.(37) The presence of micropores was determined using the Alpha-S (αs) method.(38) The pore size distribution was calculated by DFT (Density Functional Theory) method.(39) It is now becoming more and more evident(37) that the pore size analysis of narrow mesopores cannot be reliably achieved by procedures based on the Kelvin equation, such as the Barrett–Joyner– Halenda (BJH) method.(40) In fact such approaches significantly underestimate the narrow mesopore size (for a pore diameter lower than ca. 10 nm the pore size is underestimated by ca. 20–30%).(39),(41),(42) The Brønsted and Lewis acid sites were measured by infrared spectroscopy (ATI Mattson FTIR, LabX) using pyridine as a probe molecule. A thin self-supported wafer of a sample was pressed (about 15–25 mg at 2 tons pressure for 5 min) and then placed into the FTIR-cell. The cell was evacuated and the temperature was raised to 450 °C and kept for 1 h. Thereafter, the temperature was decreased to 100 °C and the background spectra of a sample were recorded. Pyridine was adsorbed first for 30 min at 100 °C followed by desorption at 250, 350 and 450 °C for 1 h for determination of weak, medium and strong acid sites.(43)–(45) The spectra of the sample were recorded in between every temperature ramp. Scanning was performed under vacuum at 100 °C. Spectral bands at 1545 cm–1 and at 1450 cm–1 were used to identify Brønsted (BAS) and Lewis (LAS) acid sites,



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respectively. The quantitative amount of BAS and LAS was calculated with the constants of Emeis.(46) The functionality of samples was studied using selective adsorption from their aqueous solutions of a series of acid-base indicators with different intrinsic pKa values(47) in the range from −4.4 to 14.2 on the surface sites with the corresponding pKa values resulting in the change of the indicator solution optical density. The experimental procedure was reported in detail by Mjakin and co-authors.(48)–(50) Another factor contributing to the optical density change is the change of the aqueous medium acidity as a result of water interactions with the surface sites. For this reason, the performed analysis involved spectrophotometric measurements (using a spectrophotometer LEKI SS2109UV (Russia)) of the following optical density (D) values: D0 – for the initial aqueous solution of the indicator of a certain concentration; D1 – for the same solution containing a sample of the analyzed compound of a certain weight where both the indicator adsorption and pH change affect the optical density; D2 – for the same solution added to the solvent (water) decanted after the contact with a sample of the same weight. In this case, the optical density is affected only by pH value changed due to solvent (water)–surface interactions. This factor can be eliminated using the following approach. The measurements described above allow evaluation of adsorption sites with a certain pK on the studied surface according to the following equation: |

|

=

±

| |

∙ ∙ / ,

(1),

where Cind is the concentration of the indicator solution; Vind is the volume of the indicator solution used in the analysis; m1 and m2 are weights of the samples for measuring D1 and D2 correspondingly; “+” sign corresponds to the case where D1 and D2 are oppositely changed related to D0 (D1D0, i.e. the changes in optical density caused by adsorption and water-surface interactions are opposite and the decrease of optical density due to the indicator adsorption is larger than the increase due to the 8 ACS Paragon Plus Environment

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water-surface interactions); “–“ sign corresponds to unidirectional optical density changes caused by both adsorption and water-surface interactions (D1

Fluidized-Bed Isobutane Dehydrogenation over Alumina-Supported

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