Fossil raw materials such as petrol, natural gas, and coal are important sources for fuel production and the chemical industry [1
]. In recent years, complex mining methods such as the MultiFrac method (fracking) have been increasingly used [3
]. This has an impact on the availability and price of one of the most important raw materials in the rubber industry, namely 1,3-butadiene [4
The large number of polymers synthesized from 1,3-butadiene provides a diverse range of products, with more than half of the end products found in the tire and seal industries [4
]. The consumption of synthetically produced rubber in 2018 was about 15.4 million tons [5
]. An overview of the processing of 1,3-butadiene and the application of the end products is given in the Appendix A
(see Table A1
Around 95% of the 1,3-butadiene demand is currently covered by steam cracking for the production of short-chain olefins [4
]. However, the amount of 1,3-butadiene recovered depends strongly on the petrochemical raw material used. Utilizing the petroleum fractions naphtha or gas oil, the product stream contains about 9 vol% C4
hydrocarbons with a 1,3-butadiene content of 30–60% [6
]. However, the use of shale gas with a high ethane content as a feedstock for steam cracking results in a lower 1,3-butadiene content in the product stream [8
]. A production based on renewable raw materials and independent of fossil raw materials is therefore desirable [9
An interesting alternative to the petrochemical production of 1,3-butadiene is the Lebedev process, as it is based on the use of ethanol obtained from fermentation as a biobased raw material [10
]. This process has been known since the 1930s [11
], but lost importance due to the development of petrochemistry and the relatively inexpensive production by steam cracking.
The total reaction equation of the conversion of ethanol to 1,3-butadiene is shown in Scheme 1
. The generally accepted scheme of the reaction process is shown in Scheme 2
In the first step, ethanol is dehydrated to acetaldehyde. This requires a catalyst with active basic sites. The intermediate product acetaldol is then formed by the aldol addition of two acetaldehyde molecules. It is known from homogeneously catalyzed liquid-phase reactions that this reaction can take place with both basic and acidic catalysts [17
]. Therefore, the same is assumed for the heterogeneous catalyzed gas-phase reaction [18
] presented in this paper. The subsequent dehydration to crotonaldehyde takes place at acid sites. The mono-unsaturated crotonaldehyde is reduced to alcohol in the next step. In this case, however, hydrogenation is not carried out directly with hydrogen, but via a Meerwein-Ponndorf-Verley reduction with simultaneous dehydrogenation of ethanol to acetaldehyde at the adjacent strongly basic and weakly acid surface sites [19
]. In the last step, the desired product 1,3-butadiene is formed from crotyl alcohol by further dehydration at acid sites of the catalyst.
In the literature, different statements can be found regarding the rate-determining step in this multi-step reaction scheme to the formation of 1,3-butadiene. On the one hand, the dehydrogenation of ethanol to acetaldehyde is mentioned as the limiting reaction step [21
] and, on the other hand, the aldol reaction to build up the C4
In view of the large number of basic and acidic catalyzed reactions during the conversion of ethanol to 1,3-butadiene, the necessity of using a multifunctional catalyst with very defined properties in this case becomes clear. The Lebedev process uses heterogeneous mixed oxide catalysts with the basic and acidic surface properties required for the conversion of ethanol [14
]. The quantity and strength of the basic and acid sites can be controlled by selecting the oxidic compounds. The ratio of these active sites can be adjusted by the proportions of the respective components in the mixed oxide [16
According to Lebedev, a mixed oxide catalyst consisting of 75 mol% dehydrogenation (basic) and 25 mol% dehydration (acidic) components should be used for the selective formation of 1,3-butadiene [11
]. In the early 1930s, Lebedev patented the combination of magnesium oxide and silicon dioxide as a catalyst system for the production of 1,3-butadiene in a single-step process [22
]. To date, MgO/SiO2
is the most promising binary mixed oxide for 1,3-butadiene synthesis from ethanol [12
]. The yield of 1,3-butadiene depends strongly on the chosen reaction conditions. The most frequent by-product is ethene formed at the acid sites. With regard to the yield of 1,3-butadiene, different data can be found on the optimum Mg/Si ratio of the binary catalyst. However, a high MgO content of 75 to 85 mol% is often recommended [21
], even though lower MgO contents of 65 mol% have also been reported [30
]. In recent times, the role of the multifunctionality of the catalyst systems used separately has also been studied more closely [31
In the present study, the molar composition of the binary MgO/SiO2 mixed oxides was investigated and confirmed or optimized to achieve high 1,3-butadiene yields. New mixed oxide systems with an optimal MgO/SiO2 ratio were produced with mesoporous MgO. Special emphasis was placed on an extensive variation of the precipitation reagents and the duration of hydrothermal treatment during the production of mesoporous magnesium oxides. The obtained systems were subjected to systematic textural, structural, and surface chemical characterization. Their suitability as effective catalysts for the heterogeneous catalyzed conversion of ethanol to 1,3-butadiene was demonstrated.
3. Materials and Methods
3.1. Chemicals Used
lists the chemicals used to produce the mixed oxides. The MgO precursors synthesized in different ways are named M1, M2, and M3.
The deionized water, which was used both in the production of the MgO precursors or mixed oxides for the suspension and in the educt mixture for catalytic testing, was provided by the GENO-OSMO WK 1–50 reverse osmosis plant from Grünbeck (Hoechstaedt a. d. Donau, Germany). Non-denatured ethanol (purity: >99.5 vol%) from Berkel AHK (Ludwigshafen, Germany), was used for the catalytic tests.
3.2. Preparation of Binary MgO/SiO2 Mixed Oxides from Commercially Available Precursors
Initially, binary mixtures of MgO and SiO2 with various molar compositions were produced. The MgO precursor M1 was obtained by calcination for 4 h at 900 °C from Mg(OH)2. Highly disperse silica (Wacker Silicones, HDK N20) was used as SiO2 precursor.
A total mass of 10 g of the oxidic precursors was weighed into a 500 mL round flask and suspended with deionized water at 2.5 times the volume of the bulk volume of the solids. The suspension was homogenized at 70 °C and 800 mbar for 30 min in a rotary evaporator and then concentrated at 170 mbar. After drying overnight at 70 °C in the drying chamber, calcination was carried out at 500 °C for 6 h in an air stream (heating rate 1.2 K/min). The powder obtained was pressed into tablets under 15–20 bar, then mortarized and classified. For the catalytic tests, the particle fraction of 200 to 400 µm was used.
3.3. Preparation of Binary MgO/SiO2 Mixed Oxides with Mesoporous MgO
The mesoporous MgO precursors M2 and M3 were produced according to the method presented by Cui et al. [35
]. First, 500 mL of a Mg(NO3
solution (0.624 mol/L) were placed in a 1 L round flask. Then, 250 mL of a carbonate solution (Na2
, each 1.248 mol/L) were added via a dropping funnel at a rate of two drops per second using a magnetic stirrer. Precipitation and subsequent homogenization for 30 min took place at room temperature. The precipitation suspension was then hydrothermally treated at 180 °C in an autoclave (2 L capacity). The duration of the autoclave treatment was varied (0 h, 5 h, 9 h, 12 h, 24 h, and 48 h) and is indicated in brackets behind the precursor name in Section 2.2
. The aged precipitated suspension was filtered, washed with deionized water, and dried overnight at 70 °C. The precipitated MgCO3
was converted to MgO by calcination at 600 °C for 2 h (heating rate 5 K/min). The MgO precursor M2 was prepared with Na2
, the precursor M3 with K2
To produce the mixed oxide, the respective mesoporous MgO precursor was mixed with SiO2
using the mechanical stirrer FL-300 MS (FLUID Misch- und Dispergiertechnik GmbH, Loerrach, Germany). For this purpose, 4.282 g M2 or M3 and 0.718 g SiO2
(molar Mg/Si ratio of 90:10) were suspended with 200 mL deionized water. The aqueous suspension was homogenized at 400 rpm and 70 °C for 30 min. The temperature was then raised to 95 °C, the stirring speed was reduced to 300 rpm, and the suspension was concentrated for about 30 min. The further preparation steps of drying, calcination, pressing, and classification were carried out in the same way as described in Section 3.2
3.4. Characterization of MgO/SiO2 Mixed Oxides and Precursors
The prepared mixed oxides and the respective precursors were characterized texturally by the specific BET surface areas and pore volumes calculated from nitrogen physisorption at 77 K (Sorptomatik 1990, Carlo Erba Instruments, Egelsbach, Germany). Prior to the adsorption experiment, the solid samples were treated under standard conditions by heating to 250 °C under high vacuum with a heating rate of 1 K/min and maintaining at this temperature for 8 h.
The surface acidity of the mixed oxides and the corresponding precursors was determined using temperature-programmed ammonia desorption (NH3-TPD) (TPDRO 1100, Thermo Scientific, Milan, Italy). During the analysis, the samples were heated to 450 °C at 10 K/min in a helium stream. The desorbed ammonia was detected with a thermal conductivity detector. Assuming that the ammonia load corresponds to the number of acid sites on the solid surface, the acid-site density follows from the quotient of the specific ammonia load and the specific surface area .
The morphology of the solid samples was examined by scanning electron microscopy (DSM 982 GEMINI, Carl Zeiss, Jena, Germany).
3.5. Catalytic Testing
The activity and selectivity during the conversion of ethanol were determined for the prepared MgO/SiO2
mixed oxides. A multiple reactor plant consisting of six identically constructed reactor lines was used [36
]. The sample was taken from one of the six product streams via a multi-position valve and then automatically injected into the gas chromatograph. The flow diagram of one of the six reactor lines is shown in Figure A4
The liquid educt mixture of 94 wt% ethanol and 6 wt% deionized water was placed in a reservoir. The volume flows of the liquid educt mixture and the carrier gas were controlled by two mass flow controllers and fed through a flow tube reactor (Bergmann RST, Dresden, Germany, stainless steel, 14 mm inner diameter) filled with 500 mg of the mixed oxide catalyst (particle size 200–400 μm) between two layers of quartz glass wool. The catalyst load (weight hourly space velocity (WHSV)) during the test results from the ratio of the educt mass flow to the catalyst mass.
An electric heating element was used to heat the reactor, whereby the temperature was measured directly above the catalyst sample. Temperature programs of different durations were used for catalytic testing. After 8.75 h at 400 °C, the reactor was heated up to 450 °C within 15 min and then maintained at this temperature for 8 to 17 h. Finally, a temperature of 400 °C was set again for 7 h (see Figure A5
). Each catalyst was only tested once. All lines upstream of the reactor were heated to 120 °C, and those between the reactor outlet and the gas chromatograph to 200 °C.
The qualitative and quantitative analysis of the reaction products was performed by means of a 7820A gas chromatograph (Agilent, Santa Clara, CA, USA) with a flame ionization detector and a non-polar J&W HP-1 19091Z-530 separation column (Agilent, Santa Clara, CA, USA). The gas chromatographic analyses of the product flow at 450 °C reactor temperature were carried out between 1 and 17 h after reaching 450 °C. The obtained conversions and selectivities represent average values from several gas chromatographic measurements.
In the present work, binary MgO/SiO2 mixed oxides were prepared in different ways and characterized by means of N2-adsorption, SEM, and NH3-TPD. Their catalytic activity during the conversion of ethanol to 1,3-butadiene was evaluated. The molar composition and the carbonate source as well as the duration of the hydrothermal treatment during the preparation of the MgO precursors for the synthesis of the mixed oxides were varied.
It was shown that binary MgO/SiO2 mixed oxides from commercially available precursors exhibit an optimum yield of 1,3-butadiene at a MgO content of 85–95 mol%. This confirms qualitatively the necessary excess of the basic component in the mixture described in the literature.
The pure oxides MgO and SiO2 showed only a very small number of active acid sites and thus only a low catalytic activity. The number of active sites increased only when both components were mixed in aqueous suspension, while at the same time the surface texture was changed. More acid sites were formed as the specific surface area of the structure-determining magnesium oxide increased. A maximum value of the acid-site density was obtained with equimolar to magnesium-rich MgO/SiO2 mixed oxides.
The precipitation of MgCO3 and subsequent hydrothermal treatment of the precipitated suspension successfully produced mesoporous magnesium oxide with comparatively large specific BET surface areas. The MgCO3 platelets grew with increasing treatment time in the autoclave and formed a secondary pore system, as confirmed by SEM images. Different carbonate sources influenced the textural and surface chemical properties of the MgO precursor and the resulting mixed oxide.
The temperature influence of the hydrothermal treatment on the MgCO3 structure and thus the catalytic activity of the resulting mixed oxide during the conversion of ethanol to 1,3-butadiene is the subject of later investigations.