Different Types Of Zeolites Research Articles

Tuning the Hydrothermal Synthesis of FAU-type and ANA-type Zeolites Derived from the Philippine Bentonite

The synthesis of zeolites from local resources has been an attractive route to address the demand for more economical means in the production of zeolites for commercial applications. In particular, Philippine bentonite – a local clay material –has been used as a raw ingredient to synthesize zeolites, faujasite-type (FAU), and analcime-type (ANA) via hydrothermal synthesis. The silicon-aluminum ratio (SAR) and crystallization temperature were varied to investigate the conditions in the synthesis of pure FAU and ANA-type zeolites. X-ray diffraction allowed the identification of zeolite phases upon comparison to reference patterns from the International Zeolite Association database. The sample with the highest degree of crystallinity of 98% (ANA-type) was prepared at a temperature of 150 °C and an initial SAR of 1.18. This sample also exhibited the largest particle size of 29.1 μm in diameter. Scanning electron micrographs showed octahedral, spherical, and cross morphologies of the products, with average particle size ranging from 1.24 (± 0.13) to 29.1 (± 1.66) μm. The transformation of Philippine bentonite to different types of zeolites demonstrates the viability of local clays as raw resource material for hydrothermal synthesis of a wide range of zeolite frameworks.

Preparation and thermal properties of zeolite/MgSO4 composite sorption material for heat storage

Thermochemical sorption heat storage based on salt hydrates offers the potential advantages such as high energy density, negligible heat loss, small volume change and flexible working modes. Magnesium sulphate (MgSO4) is a high energy density hydrated salt available for the thermochemical sorption heat storage. In this work, different types of zeolites (including 3A, 5A and 13X zeolites) have been selected as mass-transfer-enhancing and structural stabilizing materials, and a series of zeolite/MgSO4 composites have been fabricated. To investigate the thermal energy storage performance of zeolite/MgSO4 composites, the properties of the composites, such as micro-morphology, pore structure, chemical composition and sorption-desorption behavior, have been characterized and analyzed. Results indicated that the mass transfer performance of the composite materials is significantly better than that of the pure zeolites. As the mass fraction of MgSO4 in the composite sorbents increases, the specific surface area and porosity of the composite sorbent decreases, and the sorption performance has been enhanced. Compared with 3A zeolite-based and 5A zeolite-based composites, 13X zeolite/MgSO4 composite materials show the best heat storage performance. At the condition of 25 °C and 60% RH, the 13XM20 has the highest sorption capacity of 0.21 g/g, which is 24% higher than that of pure zeolite 13X. Accordingly, the maximum heat storage density is 438.4 kJ/kg, which exhibits the superior heat storage performance among of the hydrated salt sorbents. The potential of 13X zeolite/MgSO4 as sorption heat storage material in efficient utilization of solar heat and low-grade industrial waste heat has been revealed.

Zeolites Derived from Natural Kaolinite for CO2 Adsorption

This manuscript deals with the synthesis of different types of zeolites from natural kaolinite samples for CO2 adsorption. A zeolite A was prepared from kaolinite by means of an alkaline fusion process, followed by hydrothermal treatment, whereas a highly crystalline zeolite X was synthesized by optimizing the previously mentioned synthetic procedure. In detail, the SiO2/Al2O3 molar ratio in the preliminary mixture was modified with the addition of a secondary silicon source (sodium silicate) in order to obtain the one required for zeolites X. The physicochemical properties of the pristine clay and of the different zeolites were investigated by means of a multi-technique approach, including XRPD; SEM-EDX; 23Na, 27Al and 29Si MAS NMR spectroscopy; and N2 physisorption analysis at 77 K. Since the Si and Al molar ratios and reactivities are key parameters for the synthesis of zeolites, these aspects, primarily related to the use of a naturally occurring aluminosilicate as the raw material, have been investigated for their correlation with the physicochemical properties of the synthetic products. Moreover, by means of a custom-built volumetric apparatus, the CO2 adsorption capacity of the resulting zeolites at low gas pressures (<1 bar) and at 25 °C was assessed.

The effect of the active site and substrate structure in preparation of substituted tetrahydropyrans via intramolecular cyclization

Substituted tetrahydropyrans (THPs) are important biologically active compounds and synthetic intermediates. Their prevalence and diverse applications make them desirable targets in synthetic organic chemistry. Current synthetic routes for THP synthesis involve homogeneous catalysts and usually multiple steps and the use of expensive heavy metal catalysts, which pose environmental concerns. Therefore, there is a need for alternative approaches that can enhance efficiency, eco-friendliness, and cost-effectiveness This study focuses on the acid-catalyzed synthesis of THPs through the intramolecular cyclization of substituted (E)− 5-phenylpent-4-en-1-ols over zeolites as heterogeneous catalysts. The study particularly focused on investigating the influence of functional groups, as well as substrate size, on the reaction outcome. The results demonstrated that electron-donating groups enhance the conversion of the reactants and increase selectivity towards THPs, while electron-withdrawing groups have the opposite effect. Furthermore, the cyclization reaction also occurred on substrates containing different aromatic cycles. However, the conversion of a bulky substrate containing naphthyl group decreased due to mass transfer limitations. The catalytic performance of different types of zeolites with diverse acid sites was also examined. Brønsted acidic zeolites such as Al-BEA and Ga-BEA exhibited promising catalytic performance and 94 % and 83 % selectivity towards THPs, respectively. In contrast, Lewis acidic zeolites such as Sn-BEA, Ti-BEA, and Zr-BEA showed conversions below 6.5 % and poor selectivity, showing that Brønsted acidic zeolites are more suitable for the cyclization reaction than Lewis acidic zeolites.

Low-temperature catalytic dechlorination of model plastic pyrolytic oil over zeolite catalysts

Catalytic dechlorination of model plastic pyrolytic oil was investigated over different types of zeolites and ion-exchanged 13X zeolites at the low temperatures of 20–120 °C. Among different types of parent zeolites (13X, β, MCM-41), 13X had the highest catalytic dechlorination performance. After ion exchange of 13X with Ag+, its catalytic dechlorination performance was significantly enhanced, and the chlorine removal over Ag-13X reached 97.2 % after reaction for 5 h at 120 °C. The characterization results indicated that the acidity of the zeolite governed its catalytic dechlorination performance, and higher acid amount and stronger acid strength are preferred. Different types of chlorinated compounds followed different dechlorination mechanisms. The primary chlorinated alkanes (e.g., 1-chlorooctane) underwent nucleophilic substitution by the surface –OH groups or H2O to form alcohols and HCl, or dehydrochlorination reaction to produce olefins and HCl, while higher chlorinated alkanes with adjacent hydrogen (e.g., chlorocyclohexane) mainly underwent dehydrochlorination reaction to produce olefins and HCl. Chlorinated aromatics (e.g., chlorobenzene) was removed mainly by physical adsorption. Ag-13X showed good stability, and the chlorine removal still retained 72.1 % after 102 h on stream. After four dechlorination-regeneration cycles, the chlorine removal had only a slight decrease due to the partial collapse of its structure and slight loss of its acid amount.

Recent Advances in the Catalytic Conversion of Methane to Methanol: From the Challenges of Traditional Catalysts to the Use of Nanomaterials and Metal-Organic Frameworks.

Methane and carbon dioxide are the main contributors to global warming, with the methane effect being 25 times more powerful than carbon dioxide. Although the sources of methane are diverse, it is a very volatile and explosive gas. One way to store the energy content of methane is through its conversion to methanol. Methanol is a liquid under ambient conditions, easy to transport, and, apart from its use as an energy source, it is a chemical platform that can serve as a starting material for the production of various higher-value products. Accordingly, the transformation of methane to methanol has been extensively studied in the literature, using traditional catalysts as different types of zeolites. However, in the last few years, a new generation of catalysts has emerged to carry out this transformation with higher conversion and selectivity, and more importantly, under mild temperature and pressure conditions. These new catalysts typically involve the use of a highly porous supporting material such as zeolite, or more recently, metal-organic frameworks (MOFs) and graphene, and metallic nanoparticles or a combination of different types of nanoparticles that are the core of the catalytic process. In this review, recent advances in the porous supports for nanoparticles used for methane oxidation to methanol under mild conditions are discussed.

Continuous adsorption process of [formula omitted]/[formula omitted]/[formula omitted] from [formula omitted] flow using type A zeolite adsorbents in the presence of ultrasonic waves

In this study, the adsorption capacity of different types of zeolite (3A, 4A, and 5A) for adsorbing CO2 from a mixture of CH4, N2, and water vapor was investigated with and without the use of ultrasonic waves. In this study, functional groups of O–H, Si–O–Al and Si–O–Si bands were identified in Fourier transform infrared (FTIR) spectroscopy of these adsorbents. X-ray diffraction patterns (XRD) for zeolite 3A and 5A showed the structure of chabazite and for zeolite 4A the structure of sodalite. In the Brunauer-Emmett-Teller (BET) test, the specific surface area of zeolites was measured as 16, 11.96 and 437 m2g−1, respectively. Ultrasonic waves increased the adsorption capacity of zeolites to adsorb CO2 at lower temperatures 80.64 to 175.44 mgg−1. Zeolite 5A has a higher affinity for CO2 than CH4 or N2, the selectivity of CO2/ N2 and CO2/ CH4 were 19.55 and 24.17, respectively. Data from adsorption experiments were used to learn an artificial neural network (ANN), and the ANN provided acceptable results for predicting the adsorption process.

Insight into the stability and thermal decomposition behavior of nitrocellulose supplemented with different types of zeolites

AbstractNitrocellulose is commonly used as a main ingredient in propellant formulations, but its thermal instability necessitates the employment of stabilizers as additives. In this study, the potential of four adsorbents, namely, zeolites MOR, BEA, MFI, and FAU as stabilizing agents for nitrocellulose was evaluated. Their performance was compared to that of the traditional stabilizer, DPA. The thermal stability of nitrocellulose (NC) that contains each stabilizer was assessed through VST, B&J test, and TGA analysis. The compatibility of NC with each stabilizer was also scrutinized using FTIR, heat of combustion, and DSC. The computation of the kinetic parameters for NC that contains each stabilizer was carried out using non‐isothermal DSC experiments. The obtained results demonstrated that the different zeolites exhibited promising stabilizing efficiency toward NC. The zeolites revealed good compatibility with NC as well, as confirmed by FTIR and DSC analyses. This work contributes to the ongoing efforts to develop environmentally friendly stabilizers for NC.

Zeolites as Selective Adsorbents for CO2 Separation

Zeolites are a very versatile class of materials that can display selective CO2 adsorption behavior and thus find applications in carbon capture, storage, and utilization (CCSU). In this contribution, the properties of zeolites as CO2 adsorbents are reviewed by critically presenting and discussing their assets and limitations. For this purpose, we first provide an overview of the CO2 adsorption mechanisms on different types of zeolites. Then, we systematically discuss the relationship between the physicochemical properties of zeolites (framework type, Si/Al ratio, and extra-framework cations) and their performance as CO2 adsorbents for the separation of CO2/CH4 (biogas) and CO2/N2 (flue gas) mixtures. Based on the trends and properties identified, we provide a comparison of the different zeolites and highlight their advantages and drawbacks for applicability in CO2 adsorption. Finally, we present the state of the art in the shaping of zeolites in macroscopic format, which is a key step toward their industrial utilization as adsorbents.

Review on the Catalytic Conversion of Naphtha to Aromatics: Advances and Outlook

Naphtha is the main feedstock used in the production of gasoline and benzene, toluene, and xylene (BTX) aromatics, which are widely applied as chemical intermediates in the fields of packaging, synthetic fibers, and solvents. This article reviews the recent literature regarding the catalytic conversion of naphtha to higher-value BTX in terms of catalysts used, catalyst properties, reaction mechanisms, kinetic modeling, operating conditions, and product selectivity. Most studies focused on ZSM-5 and ZSM-5 modified using Ga, Zn, Ni, Co, Cu, Fe, Mo, W, Sn, etc. The promoting effects of metals and catalyst properties on naphtha conversion and the selectivity toward aromatics were critically analyzed. Pt-promoted zeolite L exhibited an excellent catalytic performance in naphtha aromatization for the selective production of benzene using n-hexane. Other studies investigated different types of zeolites, such as ZSM-11, BEA, X, Y, MOR, ZRP, and TsVM high-silica zeolites, with distinct properties. Nonzeolitic catalysts included Pt, Re, and Sn supported on Al2O3; activated carbon; and mesoporous silicates (MCM-41 and SBA-15). Finally, the challenges and perspectives of naphtha aromatization related to emerging naphtha types, cofeeding, and improved catalysts are highlighted.

Catalytic Pyrolysis of Plastic Waste and Molecular Symmetry Effects: A Review

The present review addresses the latest findings and limitations in catalytic pyrolysis for the processing of plastic waste into valuable fuels. Compared to thermal degradation of plastics, catalytic pyrolysis provides better results in regards to the quality of the obtained liquid hydrocarbon fuel. Different types of catalysts can be used in order to improve the thermal degradation of plastics. Some of the most used catalysts are different types of zeolites (HUSY, HZSM-5, Hβ), Fluid Catalytic Cracking (FCC), silica-alumina catalysts, or natural clays. There is a need to find affordable and effective catalysts in the aim of achieving commercialization of catalytic pyrolysis of plastic waste. Therefore, this study summarizes and presents the most significant results found in the literature in regards to catalytic pyrolysis. This paper also investigates the symmetry effects of molecules on the pyrolysis process.

Catalytic Properties Study of Mixed MFI-MORD Type Zeolite in Bioethanol Transformation

The decrease in stocks of traditional fossil fuels contributes to the widespread growth of interest in renewable sources of raw materials and energy. Bioethanol can become a serious alternative to traditional types of fossil raw materials and fuels due to the possibility of its widespread production from agricultural waste and wood processing. Bioethanol can be used directly as a fuel, or after transformation into hydrocarbons. The transformation of bioethanol into hydrocarbons is carried out using zeolites and zeotypes of various types, while the main problem encountered for these systems is deactivation during the catalytic transformation. In this case, one of the possible solutions to this problem is the regulation of the acidic and diffusion properties of the catalytic surface of zeolites. Changing the acidic properties can contribute to a significant increase in the stability and activity of zeolites. In this case, the variation of acidic properties is possible by combining different types of zeolites. The article presents the results of a study of a mixed zeolite of the MFI type and mordenite in the reaction of the transformation of ethanol into hydrocarbons. Zeolite synthesis was carried out by a sequential method using zeolite seed grains for the synthesis of MFI structures and n-butylamine for the synthesis of a mordenite layer. The synthesized sample was tested on a flow-type setup with a tubular reactor. The effect of temperature, specific ethanol feed rate, and total pressure in the system was investigated. An increase in the reaction temperature from 350℃ to 370℃ contributed to an increase in the rate of accumulation of liquid hydrocarbons from 0.52 to 0.64 g(HC)/(g(Cat)*h), while a further increase in temperature to 430℃ contributed to a decrease in the rate of formation of liquid hydrocarbons to 0.32 g(HC)/(g(Cat)*h). An increase in the specific feed rate of ethanol from 0.5 to 2 g(EtOH)/(g(Cat)*h) contributes to a decrease in the yield of liquid hydrocarbons. An increase in the total pressure in the system from 1 atm to 15 atm promotes an increase in the rate of accumulation of liquid hydrocarbons from 0.34 to 0.83 g(HC)/(g(Cat)*h).

Adsorption and thermal transformation of lignin model compound (ferulic acid) over HY zeolite surface studied by temperature programmed desorption mass-spectrometry, FTIR and UV–Vis spectroscopy

The adsorption of an aqueous solution of ferulic acid (FA) on protonic zeolites was studied. Three different types of zeolites and adsorption parameters (time, solid/liquid ratio, temperature and pH) were analyzed and optimized. The HY zeolite (SiO2/Al2O3 = 12) showed an adsorption percentage of 90% at 30 °C, 5 min, 10 g L−1 and pH ≤ 3. The adsorption complexes structures on the molecular adsorbates were determined by UV–Vis DRS and FTIR spectroscopies. The FA molecule interacted through phenol and carboxyl group forming monodentate and bidentate surfaces complexes (bridge and chelate). Temperature programmed desorption mass-spectrometry (TPD MS) has been used to study the decarboxylation, demethoxylation, and transalkylation processes during pyrolysis of FA surface complexes with the formation of carbon dioxide, 4-vinylguaiacol, methanol, methane, polyaromatic hydrocarbons, phenols and others.

Preparation and properties of zeolite-fly ash-slag composite porous materials: CO2 adsorption performance and mechanical property.

To solve the environmental problems caused by greenhouse gas emissions, porous materials based on alkaline solid wastes (fly ash and slag) were prepared in this study. Besides, the preparation scheme of the porous materials was optimized through an orthogonal test by taking the compressive strength and static CO2 adsorption capacity as the target parameters. The two target parameters of the porous materials prepared under the optimal ratio are 15.395N and 0.83mmol/g. On the basis of the optimal ratio, zeolite composite porous materials were prepared by adding different types of zeolite, which further improves the CO2 adsorption capacity of the porous materials. Furthermore, the micro-morphologies, CO2 adsorption performances, and mechanical properties of the porous materials were analyzed by a scanning electron microscope (SEM), the Brunauer-Emmett-Teller (BET) method, and a universal testing machine. It is discovered that the addition of different types of zeolite decreases the compressive strength but significantly improves the CO2 adsorption performance as it can promote the specific surface area and the pore volume. Moreover, the best performance is achieved with the addition of 13X zeolite. In this case, the adsorption capacity reaches 2.68mmol/g, about 3.2 times greater than that of the porous materials without zeolite. The CO2 adsorbent proposed in this study, which boasts excellent mechanical property and CO2 adsorption performance, is applicable to goafs and has a bright prospect in the comprehensive utilization of solid wastes and the control of global warming.

Possibilities of Using Zeolites Synthesized from Fly Ash in Adsorption Chillers

Adsorption chillers produce cold energy, using heat instead of electricity, thus reducing electrical energy consumption. A major industrial waste, fly ash, can be converted to zeolite and used in adsorption chillers as an adsorbent. In this research, three different types of zeolites were synthesised from fly ash via a hydrothermal reaction in an alkaline solution (NaOH). The obtained samples (Na-A zeolites) were modified with K2CO3 to increase the water adsorption capacity of these samples. Phase and morphology analyses shows that desired zeolites formed properly but other crystalline phases also exist along with nonporous amorphous phases. The determined specific surface areas for Na-A zeolite (12 h) and Na-A zeolite (24 h) are 45 m2/g and 185 m2/g respectively, while the specific surface area for synthesized 13X zeolite is almost negligible. Water-isotherm for each of these samples was measured. Considering the application of adsorption chillers, average adsorption capacity was very low, 1.73% and 1.27%, respectively, for the two most probable operating conditions for synthesized 13X zeolite, whereas no water was available for the evaporation from Na-A zeolite (12 h) and Na-A zeolite (24 h). This analysis implies that among the synthesized materials only 13X zeolite has a potential as an adsorber in sorption chillers.

Characteristics of cytotoxicity of native zeolite against cattle epithelial cells

Relevance. Special attention of researchers is attracted by zeolites — minerals of the family of aluminosilicates of alkaline and alkaline earth metals. Considering that the composition of zeolites is extremely variable, including the content of trace elements, most zeolite tuffs of Russian Federationare not evaluated in terms of possible cytotoxicity. The aim of this study was to characterize the cytotoxicity of native clinoptilolite against epithelial cells of cattle.Methods. To characterize cytotoxicity, 5 fractions of native clinoptilolite (mining quarry — Drozhzhanovsky district of the Republic of Tatarstan) with particle sizes were selected: No. 1 — 0.2 mm, No. 2 — 0.24 mm, No. 3 — 0.8 mm, No. 4 — 0,8–2,5 mm, No. 5 — 2,5–5,0 mm. Two transferable cell lines were selected as models for determining the toxicity of zeolites: LEK (epithelium of the lung of a cow embryo) and TR (tracheal epithelium of a cow embryo) obtained from a Collection of vertebrate cell cultures (ICiG RAS, Russia). MTTtestwas carried out. Cell viability was assessed by the activity of mitochondrial dehydrogenase according to a standard method. Statistical data processing was performed using “GraphPad Prism 6.0” software (USA). The experiments were carried out in 5 repetitions, p ≤ 0.05 was taken as a statistically reliable level.Results. It was found that concentrations up to 300 micrograms/mlof all fractions of zeolites do not have a cytotoxic effect on the studied cells. The cytopathogenic effect manifests itself in the concentration range of 400-1000 micrograms/ml and is in direct correlation with the size of zeolite particles — the proportion of cells in the state of apoptosis is up to 35.23±2.3%, which is probably due to the mechanical effect of large particles of the rocks in question on cellular structures. Information about the cytotoxic effects of different types of zeolites on the cells of the body of productive animals will later form the basis of their toxicological assessment and will allow us to develop a strategy for their safe use.

Comparison of three different structures of zeolites prepared by template-free hydrothermal method and its CO2 adsorption properties

In this study, zeolite sodalite SOD (50NaO2:Al2O3:5SiO2), zeolite LTA (2NaO2:Al2O3:1.926SiO2) and zeolite FAU (16NaO2:Al2O3:4SiO2) of different structures were synthesized successfully through simple conventional hydrothermal crystallization technique without using any template agent. Morphological analysis of three different types of zeolites revealed that the samples exhibit three different shapes such as the “Raspberry-like”, “Dice” cube like and “Octahedral” shaped morphology respectively. The thermal stability was found to be about 4.8%, 14.6% and 20.5% for the synthesized zeolites SOD, LTA and FAU respectively. From the N2 adsorption-desorption studies, it was observed that adsorption types IV and I correspond to the synthesized samples. CO2 adsorption by the synthesized zeolite SOD, LTA and FAU were examined in the pressure range from 0 to 101.325 kPa at a constant temperature of 297.15 K. The highest adsorption capacity of 3.7 mmol/g was obtained for zeolite FAU. The synthesized zeolite was studied using a nonlinear regression curve fit to determine the adsorption isotherm model using Langmuir and Freundlich isotherm model. It has been found that the synthesized zeolites have a large electric field gradient due to which they can strongly adsorb quadrupole of CO2 molecules.

Comparative study on potassium poisoning of Cu-CHA catalysts for NH3-SCR: Stability and transformation of Cu2+ ions

Cu-chabazite based catalysts (Cu-SSZ-13 and Cu-SAPO-34) may suffer from chemical deactivation due to the deposition of alkali impurities on the surface of wall-flow particulate filters. In this work, the resistance of two Cu-CHA catalysts to potassium poisoning was compared, and the key factors affecting preservation and evolution of Cu2+ ions were ascertained based on the experimental characterizations and DFT calculations. The feasibility of atom (Cu and H) at various sites (6MR and 8MR) in different types of zeolites (SSZ-13 and SAPO-34) upon K+ attacking was compared. The stability of isolated Cu2+ ions in the zeolite framework and the aggregates of detached copper species determined the catalytic performance of the poisoned catalysts.

Removal of copper ions from alembic cachaça using agro-industrial residues as biosorbents

Cachaça is a typical Brazilian distilled beverage made from fermented sugarcane juice. The copper levels in alembic cachaça are of great concern among producers, and the removal of this contaminant is important for the quality of the product. The present study aimed to remove copper ions from alembic cachaça by adsorption. Sugarcane bagasse, okara, Moringa oleifera Lam., three different types of zeolites, and two types of commercial charcoal were tested as biosorbents. The heat-treated sugarcane bagasse removed 100% of the copper present in a cachaça sample, while other low-cost natural adsorbents had close to 50% removal as was observed for M. oleifera seeds and okara. All adsorbents had porous and fibrous structures, favorable to adsorption. A kinetic study showed that a pseudo-second-order model was appropriate, with equilibrium times of 15 h for heat-treated sugarcane bagasse, okara, and M. oleifera seeds used as adsorbents. The Langmuir model better fitted the experimental data, indicating monolayer adsorption. The maximum adsorption capacity was 17.00, 0.77, and 5.33 mg of Cu g−1 for the heat-treated sugarcane bagasse, M. oleifera seeds, and okara, respectively. The results presented here are promising indicating three agro-industrial residues were favorable to the adsorption of copper ions from alembic cachaça.

Solid Acid Catalysts for the Hock Cleavage of Hydroperoxides

The oxidation of cumene and following cleavage of cumene hydroperoxide (CHP) with sulfuric acid (Hock rearrangement) is still, by far, the dominant synthetic route to produce phenol. In 2020, the global phenol market reached a value of 23.3 billion US$ with a projected compound annual growth rate of 3.4% for 2020–2025. From ecological and economical viewpoints, the key step of this process is the cleavage of CHP. One sought-after way to likewise reduce energy consumption and waste production of the process is to substitute sulfuric acid with heterogeneous catalysts. Different types of zeolites, silicon-based clays, heteropoly acids, and ion exchange resins have been investigated and tested in various studies. For every type of these solid acid catalysts, several materials were found that show high yield and selectivity to phenol. In this mini-review, first a brief introduction and overview on the Hock process is given. Next, the mechanism, kinetics, and safety aspects are summarized and discussed. Following, the different types of heterogeneous catalysts and their performance as catalyst in the Hock process are illustrated. Finally, the different approaches to substitute sulfuric acid in the synthetic route to produce phenol are briefly concluded and a short outlook is given.