Utilizing ZSM-5 Zeolite, Synthesized from Kaolin Clay, as a Catalyst Presents an Efficient Approach for Reducing Emissions in Compression Ignition (CI) Engines ^†

Abstract

This investigation focuses on synthesizing ZSM-5 zeolite from kaolin clay and its application as a catalytic converter to reduce NOx emissions in CRDI diesel engines. By doping the synthesized zeolite with CuCl₂ and AgNO₃ and coating it on a ceramic monolith, this study demonstrated superior catalytic activity for NO_x reduction compared to conventional converters. A set of experimental trials conducted by using a diesel engine with an AVL DI-gas analyzer showed that CuCl₂-ZSM5 and AgNO₃-ZSM5 catalysts reduced the NO_x conversion efficiencies to 72% and 66%. Additionally, these catalysts effectively reduced CO and HC emissions. The results highlight the potential of kaolin-derived zeolites with copper and cobalt dopants as efficient catalysts for emission control in internal combustion engines, offering a promising, sustainable solution for improving air quality and environmental sustainability.

1. Introduction

Internal combustion (CI) engines have been instrumental in propelling the global economy forward, fueling diverse means of transportation and industrial equipment. Yet the extensive utilization of these engines has incurred substantial environmental consequences, chiefly through the release of pollutants like nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HCs), and particulate matter (PM) into the air [1]. These emissions foster air pollution, the creation of smog, and alterations in climate patterns, presenting grave health hazards to both human populations and ecosystems.

Before being released into the environment, hazardous pollutants are converted into less-damaging compounds by means of chemical reactions facilitated by catalysts in catalytic converters [2,3].

One catalyst that has attracted considerable interest for reducing emissions in compression ignition (CI) engine exhausts is ZSM-5 zeolite. Zeolites are crystalline alumina silicate materials characterized by well-defined microporous structures and high surface areas, making them excellent candidates for catalytic applications [4]. ZSM-5, in particular, is esteemed for its exceptional thermal stability, acidity, and shape selectivity, making it highly proficient in catalyzing various chemical reactions involved in emission reduction [5].

What makes ZSM-5 zeolite even more compelling is its potential to be derived from kaolin clay, a naturally abundant and economically viable source. China clay, or kaolin clay, is a soft white clay that is mostly made of kaolinite, a mineral that is high in silicon and aluminum. By utilizing kaolin clay as a precursor, researchers have developed a sustainable and cost-effective method for synthesizing ZSM-5 zeolite, thereby addressing concerns related to the availability and cost of traditional zeolite precursors [6].

Synthesizing ZSM-5 zeolite from kaolin clay involves purification, chemical treatment, and crystallization, resulting in a high surface area, uniform pore size, and strong acidity, ideal for catalytic applications in CI engine exhaust treatment. Additionally, the acidic nature of ZSM-5 zeolite enables it to selectively catalyze specific reactions, such as the conversion of NOx to nitrogen and oxygen, CO to CO₂, and hydrocarbons to water and carbon dioxide, while minimizing undesirable side reactions [7].

Moreover, ZSM-5 zeolite catalysts exhibit excellent thermal stability, ensuring sustained catalytic activity under the high temperatures typically encountered in CI engine exhaust systems [8,9]. Overall, the use of ZSM-5 zeolite catalysts derived from kaolin clay presents a promising strategy for reducing emissions in CI engine exhaust. These catalysts not only exhibit high catalytic activity and selectivity but also address concerns regarding sustainability, cost-effectiveness, and resource availability [10]. In the following sections, we will delve into the synthesis, characterization, and performance of ZSM-5 zeolite catalysts for emission reduction in CI engine exhausts, shedding light on notable advancements, existing challenges, and future prospects in this domain [10].

2. Methodology

Investigating the production of ZSM-5 zeolite from kaolin clay and its application as a catalytic converter to lower NO_x emissions in CRDI engines is the aim of this work [11]. In incorporating CuCl₂ and AgNO₃ into the zeolite structure and applying it onto a ceramic monolith, our aim is to enhance catalytic activity for NO_x reduction compared to conventional converters. Custom-made CuCl₂-ZSM5 and AgNO₃-ZSM5 catalysts are tested experimentally on a CRDI diesel engine equipped with an eddy current dynamometer and an AVL DI-gas analyzer for emission monitoring [12]. This study seeks to demonstrate significant improvements in NO_x conversion efficiency and reduction in CO and HC emissions, highlighting the potential of kaolin clay-derived zeolites as effective catalysts for emission control in internal combustion engines and contributing to the development of sustainable automotive emission control solutions [13].

2.1. Synthesis of Kaolin Clay

The synthesis of zeolite from kaolin clay involves several key steps to enhance its suitability for catalytic applications. First, kaolin clay powder is oven-dried at 120 °C overnight to remove moisture and form metakaolin. Next, 20 g of metakaolin is added to and stirred with 250 mL of 2 M sulfuric acid at 80 °C for 4 h, resulting in dealumination and achieving the desired silica–alumina ratio. The product is then filtered and dried at 80 °C overnight. The dried material is fused with sodium hydroxide at a 1:1.2 ratio, ball-milled at 300 rpm for 30 min, and calcined at 550 °C for 20 min to form a crystalline structure. The calcined material is mixed with 150 mL of distilled water, cured at room temperature for 24 h, and refluxed at 100 °C for 3 h to promote crystallization. Finally, the solid is filtered, washed with distilled water, and oven-dried at 80 °C overnight to produce the final zeolite-like material product. This synthesized zeolite, derived from kaolin clay, is well suited for catalytic applications, particularly in reducing emissions from internal combustion engines.

2.2. Zeolite

Zeolites are crystalline hydrated alumina silicates known for their high porosity, featuring an Al+Si ratio of 2. They exhibit a three-dimensional framework with extra-framework charge-balancing alkali and alkaline earth ions. Similarly, to feldspar and felspathoids, zeolites possess pores within their structure capable of accommodating molecules as small as 1 nanometer in diameter. These pores consist of interconnected cages and channels, spanning one, two, or three dimensions.

The Greek words “zeo”, which means “to boil”, and “lithos”, which means “stone”, are the origin of the word “zeolite”, reflecting their historical use as “boiling stones”. Zeolites have diverse applications as adsorbents, catalysts, and ion exchangers.

2.3. Properties of Zeolite

A thorough explanation of zeolites’ diverse properties must take into account a number of factors, including their physical, chemical, ion exchange, and adsorption capabilities, as well as their mineralogical and morphological features, thermal behavior, acid resistance, crystal structure, and structural makeup.

2.4. Cordierite Monolith

Recent commercial orders for oxidation and blank monoliths have been placed by Bocent Advanced Ceramics Co., Ltd, Zibo, China. The dimensions of these monoliths are 400 cells per inch (cpi) in length and 90 mm in diameter. Notably, both the oxidation monoliths and blank monoliths share a uniform cell density of 0.17 mm. For a visual reference, you can view the design of the empty monoliths in the provided Figure 1.

2.5. Preparation of Catalyst

High-activity zeolites are used in the catalyst’s formulation, as several researchers have advised. A zeolite sample, ZSM-5, was purchased from Zeolites International USA for the experimental procedure. This zeolite serves as the base material, with the addition of AgNO₃ and Cucl₂ as transition metals. In the laboratory, two distinct catalyst types, namely, AgNO₃-ZSM-5 and Cucl₂-ZSM-5, were prepared using the procedure described below.

2.5.1. Na⁺ Ion Exchange Method

The catalyst preparation initiates with the utilization of the Na⁺ ion exchange technique, chosen for its efficiency and straightforwardness [14]. The protocol commences by blending 100 g of ZSM-5 zeolite powder with 100 milliliters of 0.5 M AgNO₃ solution to form a solution. This mixture is then transferred to a round-bottom flask and stirred continuously for 24 h at room temperature, facilitating the ion exchange of Ag and Ni. Following this, the resultant slurry is moved to an oven and gradually heated to 500 °C over a span of 6 h [15]. An identical procedure is replicated for the preparation of the zeolite catalyst. The composition of ZSM5, and AgNO₃-ZSM5 catalyst showed in

Table 1. ZSM5 and AgNo₃-ZSM5 catalyst.

Composition	ZSM5	Cucl2-ZSM5	AgNO3-ZSM5
Sio2	70.71	70.07	67.633
Al2O3	6.32	7.26	8.722
Fe2O3	0.93	0.00	0.75
caO	0.91	0.00	0.25
MgO	1.11	0.00	0.62
SO2	1.17	0.00	0.01
Na2O	14.35	18.33	19.28
K2O	0.87	0.00	1.92
CuO	1.12	10.24	0.00
P2O5	1.27	0.00	0.612
TiO2	0.29	0.00	0.10
AgO	0.00	0.00	10.23

.

2.5.2. Catalyst Characterization—SEM

SEM characterization of ZSM-5, CuCl₂-ZSM-5, and AgNO₃-ZSM-5 revealed distinct surface morphologies, indicating successful metal impregnation and changes in particle aggregation and surface texture [16]. The SEM characterization of ZSM5, CuCl₂-ZSM5 and AgNO₃-ZSM5 are shown in Figure 2, Figure 3 and Figure 4.

2.5.3. Catalyst Characterization—XRD

XRD characterization of ZSM5, CuCl₂-ZSM-5, and AgNO₃-ZSM-5 confirmed the preservation of the crystalline structure of ZSM-5 after metal impregnation, with slight shifts and intensity changes indicating successful incorporation of Cu and Ag species [17]. The XRD characterization of ZSM5, CuCl₂-ZSM5 and AgNO₃-ZSM5 are shown in Figure 5, Figure 6 and Figure 7.

3. Experimentation

A four-stroke twin-cylinder diesel engine was employed in the experimental study to measure emissions in two phases. The engine details are given in

Table 2. Engine details.

Particulars	Details
Make/Model	Mahindra Maximo
Bore	83 mm
Stroke	84 mm
Type	Common Rail Direct Injection
Cooling Type	Water
Displacement (Swept Volume)	909 cc
Fuel	Diesel
Speed	2000 rpm
Torque	50 Nm
Maximum Load in Dynamometer Load cell	18 kgs
Starting Type	Electric Start

and experimental setup shown in Figure 8. Emissions, including CO, CO₂, HC, and NO_x, were directed into a catalytic converter containing CuCl₂-ZSM5 and AgNO₃-ZSM5 catalysts and measured with an AVL DI-gas analyzer [18,19]. A comparative analysis with and without the catalytic converter identified optimal results. Despite higher NOx emissions, a Selective Catalytic Reduction (SCR) technique was employed for partial NO_x reduction. A monolith-coated converter with AgNO₃-ZSM-5 and CuCl₂-ZSM5 catalysts was designed and constructed, effectively reducing NOx and other emissions.

4. Emission Characteristics

The Figure 9 shows that load and biodiesel volume affect CO emissions. The CuCl₂-ZSM-5 catalytic converter exhibits lower CO emissions compared to AgNO₃-ZSM-5, especially under peak load conditions [20], due to its higher ion exchange conversion efficiency, demonstrating superior performance in reducing CO emissions.

From Figure 10 illustrates the relationship between brake power (BP) and hydrocarbon (HC) emissions for various catalysts, measured at varying speeds of 1300, 1500, and 1700 rpm and an injection pressure (IP) of 300, 600, and 900 bar. Overall, HC emissions decrease as BP increases. Among the catalysts tested, Cu-ZSM5 consistently exhibits the lowest HC emissions across all BP levels. Cu-ZSM5 (commercial) and Cu-ZSM5 (kaolin) show similar emission patterns, yielding slightly lower emissions at higher BP levels. Conversely, Ag-ZSM5 produces higher HC emissions compared to the other catalysts, especially at lower BP levels.

The graph illustrates the correlation between engine load and emissions of CO₂, unburnt hydrocarbons (HCs), and NOx. Higher CO₂ emissions from diesel fuel, observed with both AgNO₃-ZSM-5 and CuCl₂-ZSM-5 catalysts, indicate more complete combustion due to higher oxygen-to-carbon ratios. Conversely, diesel exhibits higher HC values, reflecting incomplete combustion, whereas a speed of 1500/IP of 600 (CuCl₂-ZSM-5) shows lower HC emissions due to effective oxidation. Additionally, zeolite-based catalytic converters significantly reduce NO_x emissions by 30–40%, utilizing numerous active sites for conversion [21,22]. These results underscore the effectiveness of advanced catalysts in improving combustion efficiency and reducing harmful emissions.

5. Conclusions

The homemade developed ZSM5-coated catalytic converter outperforms the OEM counterpart across a wide range of exhaust gas temperatures. Under a 4 kW load condition, CuCl₂-ZSM5 and AgNO₃-ZSM5 zeolite catalysts achieved NO_x conversion efficiencies of 72% and 66%, respectively, while the OEM converter managed only 30%. Moreover, the zeolite-based catalyst exhibited remarkable CO conversion efficiencies of 93.5% and 91% at 4 kW and 16 kW load, respectively, alongside HC conversion efficiencies ranging from 89% to 92% under various load conditions. Importantly, even after 100 h of experimental testing, no discernible deactivation of the converter was found, and the back pressure produced stays within allowable bounds throughout the catalytic bed.