Carbon Dioxide Assisted Conversion of Hydrolysis Lignin Catalyzed by Nickel Compounds

Abstract

In this work, hydrolysis lignin with nickel compounds deposited on the surface was prepared. The resulting material was introduced into the process of carbon dioxide assisted conversion and the catalytic activity of the deposited nickel compounds in this reaction was evaluated. Use of the obtained catalytic system increases CO₂ conversion by more than 30% in the temperature range 450–800 °C. After the conversion process, the material was subjected to a study using a variety of physico-chemical analysis methods (TEM, SEM-EDX, and X-ray phase analysis). Physico-chemical methods of analysis of a sample calcined at 300 °C to decompose nickel nitrate revealed NiO nanoparticles with an average particle size of 16.9 nm.

1. Introduction

Currently, an extremely important technological problem from the point of view of ecology is the development of ways to use waste from the woodworking industry, as well as low-grade coals, pitches, and spent carbon materials, including their efficient conversion into renewable energy sources and other demanded materials with high consumer properties [1,2]. With the current global trend of a constant growth in energy consumption, there is an increasing interest in the use of alternative sources of organic raw materials, including wastes from the woodworking industry [3,4,5], as well as coal [6,7,8,9] and low-quality carbon materials such as soot [10], spent carbon materials in the form of sorbents [11], and municipal waste [12,13].

One of these unclaimed carbon materials is hydrolytic lignin. Lignin is a mixture of aromatic polymers contained in the cells of woody plants [14]. Lignin is a large-tonnage product of the woodworking industry with ineffective usage due to its low reactivity and poor solubility [15]. The development of methods for the usage of lignin is promising based on the fact that almost 95% of lignin is utilized for low-value purposes, such as a low-grade fuel for thermal power engineering or as an additive to concrete (lignosulfonate) [16]. Unlike many other sources of biomass, such as corn, starch, etc., lignin has no nutritional value, and its use as a precursor for the chemical or fuel industry will have no effect on the food security of society.

Researchers are currently considering various methods of using lignin, such as gasification [16,17,18,19], pyrolysis [20,21,22], acid [23], and basic [24] hydrolysis, reductive [25], and oxidative [26] conversion. Using lignin as a basis for value-added functional materials is also being developed at the present time [27]. Lignin is an aromatic biopolymer, and one of the goals of researchers is to obtain industrially important industrial aromatic products, such as phenols, vanillin, aromatic acids, etc [28]. However, due to its heterogeneous structure, the direct preparation of well-defined compounds from lignin is a difficult task. To obtain well-defined products, it is necessary to pre-treat the material to reduce the complexity of intermediate products [16].

Thus, for all strategies other than gasification, the first stage of lignin conversion usually involves the selective breaking of the macromolecular polymer into small fragments and their subsequent separation. Furthermore, the structure and composition of lignin depends both on the type of biomass from which it was extracted and on the deposit of one or another type of biomass [29]. Thus, for each batch of lignin, it is necessary to select conditions for processing, which negatively affect the unification of the process and its introduction into industry.

The gasification process is devoid of the above disadvantages and makes it possible to obtain carbon monoxide, the main component of synthesis gas, which can be used to produce liquid fuels (the Fischer–Tropsch process) [30] and can be used as a gaseous fuel. An important goal at the moment is to develop methods for the reasonable use of this valuable renewable resource.

Gasification with carbon dioxide as a gasification agent allows one to obtain carbon monoxide [17,31,32,33] (reaction 1), which is an important industrial by-product of the chemical industry. For example, an industrial method for obtaining formic acid is the interaction of methanol with carbon monoxide catalyzed by cobalt compounds [26]. The modern trend is also the utilization of the greenhouse gas carbon dioxide [34,35,36,37,38,39,40].

$${\mathrm{CO}}_2+\mathrm{C}\rightleftarrows 2\mathrm{CO}\hspace{1em}\hspace{1em}\hspace{1em}\Delta \mathrm{H}=176\mathrm{kJ}/\mathrm{mol}$$

Due to the fact that carbon dioxide conversion is a highly endothermic process, a significant conversion of carbon dioxide to carbon monoxide occurs at high temperatures of about 1000 °C [41]. To maintain such high temperatures, additional energy is required, which negatively affects the economic performance of this process. Catalysts applied to the surface of the carbon material can reduce the process temperature by 150–200 °C [17,42]. Compounds of alkaline, alkaline earth, and transition metals exhibit catalytic activity in such processes [17,37,43,44,45]. Iron triad compounds are of particular interest to researchers due to their availability, high catalytic activity in various processes, and low environmental impact.

The researchers presented a number of works [46,47] in which the biomass is subjected to a two-stage process. The first being pyrolysis with the formation of gaseous products, and then the conversion of these gaseous products using various catalysts to obtain synthesis gas, hydrocarbons, etc. This approach has a drawback due to the fact that the catalyst in the second stage must be regenerated in the same way because as coking of the oligomeric pyrolysis products occurs, the catalytic activity decreases. In the case of gasification using a supported catalyst, there is no such problem due to the fact that solid residues remain after the process, the content of the metal catalyst compound (usually a metal oxide) can be dissolved in nitric acid, and a new portion of the carbon-containing material can be impregnated with the resulting solution. Thus, the catalytic activity of the system is always at the optimum level. Another disadvantage of the processes is that during two-stage pyrolysis processes followed by conversion, a mixture of gaseous products is always obtained, which is in contrast to gasification in carbon dioxide at an atmospheric pressure where the only by-product of the process is methane, with the maximum selectivity being less than 5% at temperatures of 300–400 °C, and where the water gas shift process constant passes through a maximum. At higher temperatures (700–800 °C), the methane selectivity is less than 0.5%.

It was previously shown that iron and cobalt compounds demonstrate catalytic activity in the process of carbon dioxide assisted conversion of hydrolysis lignin [17]. Nickel as a catalyst in CO₂ reactions is widely studied in such processes such as low temperature dry reforming of methane (DRM) [48,49,50], CO₂ methanation, [51] etc. The activity of supported catalysts in the gasification process has been widely discussed for various grades and types of coals [52]; however, such data are not presented for such an important unclaimed carbon material as hydrolytic lignin. The aim of this work is to show the catalytic activity of nickel compounds deposited on the surface of hydrolytic lignin in the process of carbon dioxide conversion.

2. Materials and Methods

The hydrolysis lignin taken for the study (Table 1) has an internal surface area according to BET (S(lignin) = 163.8 m²/g). Elemental analysis showed a high carbon content in the material (52.8%) [23]. Four samples of hydrolysis lignin weighing 3 g each were impregnated with an aqueous solution of nickel (II) nitrate so that the mass fraction of Ni in the resulting material after complete drying was 1, 3, 5, and 7% by weight, respectively. All obtained samples were dried at room temperature for 24 h.

After deposition and preparation, the materials were subjected to SEM-EPMA analysis with a Leo Supra 50VP scanning electron microscope at a reduced pressure in a nitrogen atmosphere with detectors of secondary electrons (VPSE), reflected electrons (QBSD), and an INCA Energy+ X-ray spectrometer (Oxford Instruments, X-Max- 80, Abingdon, UK). To assess the distribution of elements, the samples were mapped. The surface compositions of the obtained materials were determined by the XMA method at various points.

The powder XRD diffraction pattern was obtained using a Rigaku IV Ultra device using CuKα radiation. The samples were examined in the region 2θ = 5–60 o at a sweep rate of 1 degree per minute.

The study by transmission electron microscopy was performed with a transmission electron microscope JEM-2100 (JEOL, Tokyo, Kanto, Japan).

The obtained and initial materials were introduced into the interaction with carbon dioxide at atmospheric pressure. Thus, the evaluation of the catalytic activity of the obtained materials in the process of carbon dioxide assisted conversion relative to the source material was carried out.

The catalytic tests were carried out in a quartz flow reactor with an internal diameter of 8 mm. The loading weight was 1 g. The CO₂ flow rate was 30 mL/min at a total pressure of 1 atm. A Bronkhorst EL-FLOW SELECT F-111B gas flow controller was used to determine the gas flow rate. The temperature was increased at a constant rate: 600 °C/h in the range from 100 to 850 °C. During the reaction, the reaction gas products were analyzed using a Chromatek Crystal 5000 gas chromatograph with thermal conductivity detectors, M ss316 3 m * 2 mm columns, Hayesep Q 80/100 mesh, and CaA molecular sieves. The ratios of the amounts of substances were determined from the data of gas chromatography by the method of absolute calibration. The efficiency of the catalyst was evaluated by the conversion of carbon dioxide in the temperature range from 100 to 850 °C. The following reactions took place in the reaction zone for lignin (denoted as C):

$${\mathrm{CO}}_2+\mathrm{C}\to 2\mathrm{CO}$$

(1)

$$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2$$

(2)

$$2{\mathrm{H}}_2+\mathrm{C}\to {\mathrm{CH}}_4$$

(3)

The conversion of carbon dioxide during the tests with lignin was calculated by the formula:

$$\mathrm{Conv}{.}_{{\mathrm{CO}}_2}=\frac{0.5\mathrm{n}\left(\mathrm{CO}\right)+\mathrm{n}\left({\mathrm{CH}}_4\right)}{\mathrm{n}\left({\mathrm{CO}}_2\right)+0.5\mathrm{n}\left(\mathrm{CO}\right)-2\mathrm{n}\left({\mathrm{CH}}_4\right)}$$

(4)

The main parameters of the gasification process for the samples of lignin and activated carbon are given in the table below:

Table 1. The main parameters of the gasification process.

Parameter	Description
CO2 pressure	1 bar
Test protocol	Temperature ramp up from 100 °C up to 850 °C with the rate 600°/h
CO2 WHSV at the inlet	500.94 h−1
WHSV for 1% wt. of metal	353.6 h−1
WHSV for 3% wt. of metal	117.9 h−1
WHSV for 5% wt. of metal	70.7 h−1
WHSV for 7% wt. of metal	50.5 h−1
Analytical methods	Gas chromatography for gaseous products. SEM-EDX, TEM and XRD analysis for solid residue.

Table 1. The main parameters of the gasification process.

Parameter	Description
CO2 pressure	1 bar
Test protocol	Temperature ramp up from 100 °C up to 850 °C with the rate 600°/h
CO2 WHSV at the inlet	500.94 h−1
WHSV for 1% wt. of metal	353.6 h−1
WHSV for 3% wt. of metal	117.9 h−1
WHSV for 5% wt. of metal	70.7 h−1
WHSV for 7% wt. of metal	50.5 h−1
Analytical methods	Gas chromatography for gaseous products. SEM-EDX, TEM and XRD analysis for solid residue.

3. Results and Discussions

3.1. Study of Catalytic Activity

Figure 1 shows the dependence of the carbon dioxide conversion on temperature for the initial sample of hydrolysis lignin and for lignin samples with deposited nickel compounds with a metal mass fraction of 1, 3, 5, and 7%. The use of supported catalysts makes it possible to transfer the curve to a lower temperature region, and the use of 7% wt. nickel makes it possible to increase the conversion of carbon dioxide by about 30% in the range 450–800 °C.

The material balance during this process converges by more than 85%. Such significant losses are explained by the fact that in the course of reaching the temperature regime, thermal destruction of hydrolysis lignin occurs, during which fragmented monomers are formed. These monomers condense in the trap and in the cold parts of the pipelines of the installation, and it is not possible to collect them and to determine the mass.

At temperatures of 350 to 500 °C, a noticeable conversion of carbon dioxide was observed not into carbon monoxide, but into methane. This is explained by the fact that at these temperatures the water gas shift reaction (WGSR) occurs (reaction 2). In the course of this reaction, hydrogen is formed, which interacts with carbon from the carbon material to form methane (reaction 3). However, as the temperature rises above 650–700 °C, the proportion of methane in the products decreases significantly and tends to zero due to the fact that the water gas shift reaction constant decreases with increasing temperature.

3.2. Physico-Chemical Analysis of Obtained Materials

The obtained materials were examined by a number of physico-chemical analysis methods: XRD and SEM-EDX. Nickel (Ka1) mapping was also carried out for a sample of lignin with a mass fraction of nickel of 7% (Figure 2). Judging by the mapping, nickel compounds were evenly distributed over the surface of the hydrolysis lignin. According to the EDX data from the 7% Ni/lignin sample, the metal content by weight was 6.9%. The proximity of the average value of the nickel fraction on the surface and a small standard deviation (1.92) indicates the uniformity of the distribution of nickel over the surface of the sample.

Figure 3 shows the XRD data for the initial lignin sample (a), a sample of hydrolysis lignin impregnated with nickel (II) nitrate (7 wt. % Ni) after calcination in a CO₂ atmosphere for 1 h at a temperature of 300 °C (b), and residues of a sample of hydrolysis lignin with supported nickel compounds (7 wt. % Ni) after the carbon dioxide conversion process (c). The diffraction patterns obtained on all samples contain reflections of silicon dioxide on the diffraction patterns (a and b) SiO₂ (100) at 20.90 and (011) at 26.69° (JCPDS card No. 01-085-1054); and on the diffraction pattern (c) SiO₂ (100) at 20.83°, (011) at 26.62°, (200) at 42.4°, and (112) at 50.11° (JCPDS card No. 01-078-1252). Diffraction patterns (a–c) contain a reflection corresponding to graphite: C (002) at 26.43° (JCPDS card No. 00-008-0415). The diffraction pattern (b) contains a reflection corresponding to the compound (H₃O)₂NiO₂: (003) at 13.34° (JCPDS card No. 01-078-1252). The diffraction pattern (c) also contains reflections corresponding to nickel (II) oxide: NiO (003) at 37.25° and (012) at 43.30° (JCPDS card No. 00-022-1189) and pure nickel: Ni (111) at 44.6° and (200) at 51.91° (JCPDS card No. 00-001-1260). It is widely known that in a similar process of gasification of activated carbon and graphite catalyzed by metal nanoparticles, waste is dissolved in metal particles with the formation of carbides [52]. Thus, diffusion of carbon atoms occurs through the metal particle to the surface, where the interaction of carbides occurs; however, XRD of the sample after conversion does not show the presence of reflections related to nickel carbides. Apparently, nickel carbides decompose and transform into the oxide form when the system is cooled to room temperature. The same phenomenon was observed during the gasification of activated carbon earlier [41].

A sample of the initial hydrolysis lignin after gasification, as well as one with deposited nickel compounds (7% wt.) after calcination in a CO₂ atmosphere at 300 °C after gasification were examined by transmission electron microscopy. According to the TEM data of the Ni/lignin sample after calcination, the average nickel particle size was determined to be 16.9 ± 4.4 nm (Figure 4 and Figure 5). It is also possible to note the uniform distribution of nickel nanoparticles on the surface of the material (Figure 4). After the conversion process, nickel oxide forms spherical nanoparticles with an average particle size of 127 ± 52.3 nm shown on the TEM microphotographs of the lignin sample with Ni compounds (Figure 6); however, there are no such spherical nanoparticles on the TEM images of pure lignin after gasification (Figure 7). Elemental analysis carried out during the study using transmission electron microscopy showed the presence of nickel in materials with supported nickel compounds after calcination in a CO₂ atmosphere and after the gasification process (Figure 8). Elemental analysis also showed the presence of calcium compounds, which also have catalytic activity in the process of carbon dioxide conversion of carbon materials [53].

Previously presented works devoted to the catalytic carbon dioxide conversion of hydrolytic lignin demonstrate a rather high activity of iron triad metals in this process. The papers consider supported catalysts containing iron and cobalt [17,31,32], with heating occurring both in a furnace and under the action of microwave radiation [31]. Scanning electron microscopy studies of residues after carbon dioxide conversion showed similar spherical structures, as in the case obtained in this work. This additionally confirms the mechanism of this reaction, which occurs through the dissolution of carbon in a metal nanoparticle with subsequent interaction with CO₂ on the surface of the particle [52].

Thus, systems containing nickel are promising for the utilization of carbon dioxide and lignin with the production of carbon monoxide as a precursor for further synthesis of compounds with a higher added value.