Smectite clay minerals [45] are layer aluminosilicates with a broad variety of properties which are beneficial for catalyzing an organic reaction, and especially montmorillonites are used as acid catalyst [39]. Therefore, the acidity and surface area of this smectites are often modified by different methods. Furthermore, they can be altered by impregnation with metal salts or intercalation of heteropolyacids (HPA) or Lewis acids [46]. One method to modify clays is the treatment with inorganic acids such as hydrochloric acid and sulfuric acid. That results in a leaching of Al3+ and creates therefore mesopores in the clay structure, contributing to a higher surface area. In addition, interlamellar cations are replaced with protons of the applied acid. Furthermore, the leaching of Al3+ results in a decrease of strong Brønsted acid sites, but the overall concentration of weak Brønsted acid sites and the population of Lewis acid sites increases [47,48,49]. Bentonite treated with hydrochloric acid of different concentrations show a significant difference in Brønsted acid sites and according performance in a reaction system with CHP [50]. Treatment with low concentrated hydrochloric acid seems to increase the activity of the catalyst as more Brønsted acid sites are created, but treatment with highly concentrated acids leads to an elution of strong Brønsted acid sites. As strong Brønsted acid sites are required for the decomposition of CHP [51], the elutriation due to higher concentrated acids results in a deactivation.

Knifton and Sandserson used acid treated montmorillonites clays with different acidity, ranging from 10–16 mg KOH/g, in batch and continuous experiments. The former showed a yield of 98 mol % for the clays with high acidity. The experiments in a continuous plug-flow reactor revealed that acid treated montmorillonites clays are capable of performing for at least 1000 h in continuous conditions. The reaction parameters were set between 60 to 80 °C, 6 to 65 bar and a LHSV from 1 to 10 Lh⁻¹. Although the reactor was cooled, the temperature profile in the reactor raised up to 25 °C with LHSV 1 and 45 °C at LHSV 10. With a set temperature of 80 °C this leads to a drop of the phenol yield to 94% and shows the significant problem with heat removal operating this highly exothermic reaction in a fixed bed reactor [52]. Han et al. activated montmorillonite powder with 30 wt % sulfuric acid and used cordierite as support. The loading of montmorillonite on the support varied between 13.0 wt % and 38.5 wt %. Phenol yields ranged from about 65% with 19.5 wt % montmorillonite loading and 75% with 38.5 wt % montmorillonite loading at 60 °C and about 85% phenol yield with 19.5 wt % montmorillonite loading and 95% with 38.5 wt % montmorillonite loading at a reaction temperature of 90 °C. The highest selectivity was achieved at 60 °C with >99%, slightly dropping to about 99.5% at 90 °C. In their experiments HAN et al. also used a plug-flow reactor and found the optimum WHSV at 90 h⁻¹. Overall, the optimal parameters with montmorillonite on cordierite wherewith a loading of 32.5 wt % acid per clay, reaction temperature of 80 °C, WHSV of 90 h⁻¹, and mass ratio of CHP to acetone of 1:3 [53].

Heteropolyacids (HPAs) are complex proton acids which are formed by condensation of transition metal oxoacids. The octahedral structure is built by heteroatoms (group I–VII possible) connected to a transition metal (addenda atoms, mostly Mo, W, and V) via oxygen atoms. The most researched HPAs are those with the Keggin structure, as these HPAs are simply prepared and show the highest Brønsted acidity [54,55,56,57,58,59]. Immobilized on supports like clays or titania, the downsides that normally occur if used as homogeneous catalyst plus the low surface area can be eluded. Experiments with dodecatungstophosphoric acid (DTP) and similar HPAs with Keggin structure show phenol yields up to 99 mol % at quantitative conversion of CHP if impregnated on a montmorillonite clay, even with high feed rates like LHSV at 10 h⁻¹ [52]. To achieve a high stability of the HPAs in solution and to enhance the surface area and thermal stability, protons can be partial substituted with Cs+-Ions. The modified salts of HPAs und montmorillonite clays show high stability and reusability with high conversion rates of CHP [18].

During the last three decades, zeolites have become more and more relevant catalysts as they are cheap, readily available, and—due to their strong acidity—capable of phasing-out several mineral acids. Zeolites are shape-selective, often improving the processes yield and selectivity, and consequentially products quality. For the cleavage of CHP in the cumene process, several types of zeolites have been tested, including X-, USY-, Beta-, and ZSM-5-type zeolites [51,60,61,62,63,64,65,66]. Zeolites are mostly used in a fixed bed reactor and the reaction parameters strongly depend on the used zeolite. Beta and ZSM-5 zeolites and corresponding metallosilicates containing trivalent elements in the framework (B, Fe, Ga) are active at room-temperature and a reaction time of 5 min are enough for 100% conversion of CHP [51]. The selectivity to phenol varies from 86% to 92%, with a slightly better selectivity if Beta zeolites are used. Over 10 h under fixed-bed conditions the H-Al-Beta zeolite can hold the conversion rate of 100%, but the selectivity towards phenol decreases from 95% to 90%. ZSM-12, NCL-1, ZSM-22, EU-1, MCM-22, and ZSM 48 need elevated temperatures (40–60 °C) to reach high conversion rates [51]. With medium-pore unidimensional zeolites like ZSM-22 and ZSM-48 the CHP conversion is significantly lower than over ZSM 5 or Beta zeolites, 65% for ZSM-22, and 45% for ZSM-48. This can be explained with diffusion restrictions due to the channel system of these zeolites. The performance of unidimensional zeolites with larger pores and thus better accessible channel system (ZSM-12, NCL-1, and MCM-22) show 85–90% conversion. The selectivity is only slightly inferior to ZSM-5 and Beta zeolites. KUMAR showed that lowering the particle size of ZSM-5 crystals from 90 nm to nanocrystals with a particle size of 18.3 nm results in a rise in CHP conversion from 60.1% to 94% [66] due to lower diffusion restrictions and therefore an enhanced accessibility to the acid sites. Besides good accessibility of the catalytic active sites of the catalyst, the conversion of CHP strongly depends on the number and strength of Brønsted acid sites of the zeolites [51,65]. The lack or only small amount of acid sites in molecular sieves like AIPO-5 and SAPO 5 results in low conversions of 10% for the aluminophosphate and 25% for the silico-aluminophosphate, respectively [51,67,68]. The maximum number of protonic sites is equal to the number of framework aluminum atoms [69,70], but the strength of the acid site is higher if isolated by Si in next nearest neighbor (NNN)-positions. The strength of the protic sites is therefore inversely proportional to the Al content [65,69,71]. For ultra-stable Y zeolites in H-form (HUSY) an additional property has been found: partially dealuminated HUSY can mediate reactions only expectable to occur in superacids. The protonation inside the zeolite pores follow a certain mechanism, resulting in high kinetic acidity. Over HUSY with Si/Al ratios of 15–40 CHP converts up to 100% with good selectivity (0.1% w/w HUSY, 60 °C, 20 min reaction time). The high hydrophilicity of the HUSY zeolite require to be activated at 500 °C. That pre-activation diminishes poisoning by water, preventing loss of activity over several runs [66,72,73,74].

Another class that can be used as catalysts for CHP cleavage is ion exchange resins. The resins used consist of a macroporous polystyrene backbone and sulfonic acid groups as ion-exchange groups. Ye et al. used AMBERLYST 35WET and AMBERLYST 35DRY with the polystyrene-backbone cross-linked with divinylbenzene [74]. Huang et al. selected a resin entitled TH-02 with additional not otherwise specified functional groups to obtain higher exchange capacity and large internal pores [75]. The larger internal pores result from less crosslinking, making the resins mechanically less stable. Therefore, Huang et al. used a special reactor consisting of a riser, a main reactor, a liquid–solid separator and a loop, called TPCFB reactor [75]. At a WHSV of 30 h⁻¹ the conversion of CHP varies between 99.5% at 60 °C and 100% at 90 °C and higher. The selectivity is >99.9% at 60 °C and drops slightly to 97% at 98 °C. YE et al. used a lab-scale RD column to test the AMBERLYST 35 resins for the decomposition of CHP. Their experiments showed that a dry resin has superior catalytic activity. This is explained by the negative influence of the high water content on the intermediate dehydration step in the CHP decomposition [74].

The different catalysts and their performance regarding the decomposition of CHP are listed in Table 1.