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Introduction to Solid Base Catalyst

Indu Sindhu1, Ravi Tomar2, and Anshul Singh1

1Baba Mastnath University, Department of Chemistry, Asthal Bohar, Rohtak, 124021, India

2SRM Institute of Science & Technology, Department of Chemistry, Delhi-NCR Campus, Modinagar, Ghaziabad 201204, India

1.1 Introduction

With the expeditious growth of material chemistry and catalysis chemistry, the sophisticated arrangement and manufacture of solid base catalysts have sparked stupendous interest in the heterogeneous catalysis field. Moreover, there is an array of shortcomings for the homogeneous catalysts: they are nonrecyclable, produce low-quality side products, and generate a large amount of waste that leads to energy and chemical waste that still needs to be addressed. The principal supremacy of heterogeneous catalysts over homogeneous catalysts is their harmless nature and highly basic nature. In the context of catalyst regeneration and the potential to be reused in ongoing processes, heterogeneous catalysts reduce the drawbacks of homogeneous catalysis. The heterogeneous catalytic process has been reported to have greater economic potential than the homogeneous one, as reported in the literature. For example, the solvent role is very different in solid base catalysis in comparison to homogeneous catalysis because in solid catalysis the reaction can be executed without the use of a solvent [1]. As a consequence, when the reaction occurs entirely, there is no incentive to separate the solvents. Evidently, no solvent waste is emitted and solid catalysts could potentially be used in additional reactions [2, 3]. For instance, heterogeneous catalysts have 4-20% lower refinery expenses as compared with homogeneous catalysts [4, 5]. Overall, solid base catalysts have a number of incentives encompassing effectiveness in separating their components from the reaction mixture, the potential to be reutilized, stability in challenging reaction conditions and the ability to proceed in a shorter time [6-8]. They are often involved in a variety of industrial processes, notably hydrogenation, aldol condensation, transesterification, Henry reaction, Knoevenagel, Wittig reaction, Michael addition and Cannizzaro reaction. These catalysts facilitate the reaction pathways by having basic sites on their surfaces that can receive protons (H+) or donate lone pairs of electrons to reactants [9]. Solid base catalysts are present in a different phase, generally a solid, while the reactants are in a different phase, typically a liquid or a gas. This is in contrast to conventional homogeneous catalysts, which are present in the similar phase as the reactants [10]. In essence, solid base catalysts ought to replace the role of homogeneous base catalysts in industrial processes for the purpose of streamlining procedures and sustaining the environment. For the purpose of elucidating their catalytic properties in solid base catalysts, their basic properties must be scrutinized. It depends on the locality of basic sites, the definite count of basic sites and their basic dominance factors.

The fundamental constituents of solid base catalysts are the poisoning of the active site by acidic molecules such as HCl, CO2 and water, which persuades the firmness of the basic sites. Characterization methods and several kinds of other techniques, such as the variation in color of acid-base indicators and the adsorption of acidic molecules, signify the presence of basic sites on surfaces [11]. The reaction pathway includes the inclusion of anionic intermediates, as evidenced by spectroscopic analyses and the reaction proceedings are extremely comparable to base-catalyzed reactions that are prominent in homogeneous systems. Since the beginning of the twentieth century, it has been discovered that plenty of reactions are possible using various solid base catalysts. Numerous studies on solid base catalysts suggest that their catalytic behavior was principally determined by structure, property and the existence of active basic sites on surface [12-14].

1.2 History and Main Facts on Solid Base Catalysts

Pines and Haag conducted the foremost work on the heterogeneous base catalyst, demonstrating that sodium metal dissolved in alumina acted as a potent catalyst for the double-bond isomerization of alkenes [15]. "Solid Acids and Bases" written by Professor Kozo Tanabe was an innovative work that popularized the concept of "solid base" in the catalysis world in 1970 [2]. The work addressed significant research on solid acid and base catalysis carried out throughout the 1950s and 1960s. As a sequel to "Solid Acids and Bases," Tanabe, Misono, and other authors published "New Solid Acids and Bases" in 1989, providing an overview of the advances in the area during the 1970s and 1980s [16]. It described contemporary advances in the field throughout the 1970s and 1980s. Both solid acids and solid bases were covered in the two publications, but the focus was primarily on the former, which was typical for those decades. Significant strides have been made in solid base-catalyzed processes and catalytic materials since the early 1990s. This is because solid base catalysts has been proven to be environmentally benign. Consequently, there is an expanding utilization of solid base catalysts in organic reactions.

Until that time, research on solid bases has advanced significantly in terms of catalyst materials and catalytic processes, but with greater slack than research on solid acids. When the catalysts were prepared under vacuum, Tanabe and colleagues reported in 1972 that magnesium oxide and calcium oxide demonstrated extraordinarily high catalytic activity for 1-butene isomerization [4, 17]. This research elucidated the significance of the base catalyst pretreatment and preparation techniques.

Tanabe and his colleagues conducted a thorough investigation of the fundamental characteristics of different metal oxides, combined metal oxides, and the wide range of processes that these substances catalyzed in the 1970s. A family of solid bases for new materials entered the market in the interim period. Different approaches to identifying the reaction intermediates and describing the fundamental characteristics of solid surfaces were also developed [18].

1.3 Literary Perspective of Solid Base Catalyst

The coprecipitation approach was adopted to fabricate heterogeneous solid catalysts made of mixed oxides of CaO and ZrO2, containing various Ca-to-Zr ratios. Temperature-programmed desorption, Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy(XPS) were employed for the characterization by Dehkordi and Ghasemi. The increase in catalyst activity with an uptick in the Ca-to-Zr ratio is corroborated by the results of the experiment. These chemically synthesized mixed oxides were tested as catalysts in the transesterification of residual cooking oil and methanol at 65°C to yield biodiesel fuel [19].

Wang et al. designed Mg-Al oxides, which were utilized as catalysts for the transesterification of ethanol and dimethyl carbonate. Mg-Al-layered double hydroxide was crushed without the assistance of a solvent. Fourier transform infrared (FTIR), scanning electron microscopy (SEM), XPS, and XRD were employed to validate the surface properties. With moderate basic sites and high BET surface areas, a molar ratio of 2.0 was shown to yield satisfactory catalytic activity [20].

The solid base catalysts was fabricated by Wei et al. by heating a porous hydroxyapatite doped with Sr(NO3)2 at 873 K. The thermogravimetric analysis (TGA), XRD, FTIR, SEM, BET, and indicator techniques were implemented for the characterization. With an 85% conversion rate, these innovative solid base catalysts demonstrate use in the transesterification of soybean oil [21].

Magnesium oxide/carbon mesoporous composites were fabricated utilizing potassium chloride as the salt template and alkali lignin as the carbon source, resulting in a range of Mg-doping ratios by Wang and research team. For characterization, the BET, FTIR, SEM, and transmission electron microscopy (TEM) were employed. To optimize the fructose yield during glucose isomerization, these chemicals were employed as base catalysts [22].

Na-modified graphite carbon nitrides were synthesized by Kim et al. and used to transesterify soybean oil and methanol. Melamine and sodium hydroxide were polymerized co-thermally to produce the catalyst. Carbon dioxide diffuse reflectance, SEM, FTIR, and XRD were employed for the characterization. The density functional theory (DFT) theory and CO2 diffuse reflectance infrared Fourier transform spectroscopy verified the basicity produced by the electron transfer from the sodium to nitrogen atoms [23].

Lithium-doped Li/NaY zeolite materials were designed using microemulsion-assisted impregnation with varying Li2CO3 to NaY molar ratios by Li and research group. Several alkali species were added to the zeolites, which increased the strength and catalytic property of Li/NaY catalysts in the ethanol-castor oil transesterification reactions [24].

The TiO2-based Na-SiO2...