The demand for fuel is an ever-increasing phenomenon. Energy production in a cleaner way is a global trend. For long, fossil fuels are responsible to meet the 80% of world energy demand . But, the world is going to face a great problem with the energy crisis because of the depletion of fossil resources  . Besides this, global warming due to greenhouse gas emission via burning fossils becomes the alarming concern of the international community . As a result, the substitution of fossil fuels with sustainable and environmentally friendly energy sources is growing up with great interest. Fuels like H2 and biodiesel can be the alternative fuel from renewable sources  . Low cost and environment benign processes or the preparation techniques of these promising fuels draw much attention to the researchers throughout the world. The process involves the selection of proper feed materials, good catalysts, optimum reaction conditions, etc. Here a suitable catalyst choice for effective fuel processing through thermo-catalytic reforming techniques or reactions from different hydrocarbons or biomass is must  . Reaction conditions of these thermo-catalytic methods play a vital role in the final product. Catalyst is the most important controller or key to control the reaction conditions  - . HT derived materials or mixed oxides are well established as catalysts because of their unique properties. Different factors are strongly related to the catalytic performance of HT derived materials. The performance of HT derived catalyst mostly depends on its preparation procedure     , pore size , total surface area   , modification    , types of metal constituents   , composition    , promoters    , operating temperature . Different metallic oxides prepared by HT precursors have shown super catalytic performance during fuel processing         . Sometimes, it is necessary to design or tune the HT derived catalyst for superior performance  . This redesign can be done by being adjusted the Mi and Mii metal ions as well as their ratio      . Basąg et al.,  reported that the performance of HT derived catalyst obtained at higher temperatures. In the last decades, a number of efforts have been taken where the focuses on highly stable and active catalysts. Catalyst deactivation through: 1) coke deposition, 2) sintering and 3) active site oxidation are the common phenomena that occurred during the thermochemical reaction to produce hydrogen or bio-fuel. These drawbacks are mostly removed by using various types of promoters, catalyst supports, and methods of preparation of catalysts. Ali Awad et al.,  and Gennequin C. et al.,  took a successful attempt to increase the lifetime of HT derived catalysts that showed longer deactivation time due to carbon deposition. The development of a bimetallic catalyst was one of the effective solutions to this of type of process . Some scholars studied tri-metallic catalysts that exhibited outstanding performances over mono or bimetallic catalyst   .
Hydrotalcite has a significant number of effective usages such as anion exchangers, polymer stabilizers, neutralizers (antacids), catalysts and catalyst supports, anion scavengers, electro-active, filtration, adsorbents, pharmaceutics and photoactive materials  - . HT derived catalysts contain some unique properties over other materials due to its easier tunable properties, versatility, cost-effective materials, and simplicity. Hydrotalcite is considered a better precursor for developing user-friendly catalysts . The catalysts obtained from HT precursors are widely used in fuel processing. Awad et al., observed 75% of CH4 conversion using Ni cased catalyst . In spite of having superior performance (90% conversion yield) of Co-based catalysts, high cost, toxicity and the catalyst lifetime limit its application. Most of the cases 87% and 85% methane conversion were reported using tri-metallic (10%Zn-10%Cu-50%Ni) catalysts at 1023˚K temperature and bimetallic catalyst (40%Coe10%W) at 973˚K respectively. Lower carbon deposition for tri-metallic catalysts was reported . Reyna-villanueva et al.,  obtained 98.59% conversion of an ester using HT derived Mg-Al catalyst at 65˚C. The modification of HT derived catalyst is performing day by day to maximize its performance. The transesterification reactions are catalyzed by both heterogeneous and homogeneous catalysts, like alkoxides of various natures   . These catalysts can fasten the reaction remarkably. But some limitations like reactants purity, sensitive to fatty acid, toxicity and additional cost associated with purifying the final product decreases its applications   . Very recently, researchers are focusing on heterogeneous catalysts because of reusability, easy to recover, low purifying cost and simple process of final product separation, etc.  . Li et al.,  prepared HT derived Mg-Co-Al-La catalysts by co-precipitation method followed by calcination of 600˚C for six hours. The application of 2% of this catalyst showed 96% - 97% product yields at 200˚C. Zeng et al.,  found 90.5% ester conversion from 1.5% Mg/Al catalyst containing molar ratio 3 at 65˚C for four hours while Liu et al.,  achieved 93% conversion at 120˚C for eight(8) hours using Mg/Al catalyst having molar ratio 2.3 but found no leaching. Sikander U. et al.,  successfully tailored Mg-Ni-Al catalyst to understand the effect of Ni metal to produce hydrogen through thermal decomposition of CH4. They reported more than 80˚% of methane conversion using Mg-Ni-Al catalyst doped 40% Ni because of forming the spinel-like structure of carbon nanofiber formed during the course of a thermal decomposition reaction. Improved performance of HT derived catalysts in fuel processing was reported by various scholars   .
Hydrotalcite derived materials have been investigated as catalyst widely for different reactions or processes. The phases or structure of the hydrotalcite derived catalyst, reaction or process conditions and its performances for hydrogen production is a vital factor for better applications or industrialization of the processes and further improvement. An updated collective summary for hydrogen production via reforming of methane/methanol/ethanol, their efficiency, catalytic conditions of those processes using hydrotalcite derived catalyst is rarely found . This review highlights/finds the superior performance of the reforming or conversion reaction using hydrotalcite derived catalyst, catalytic conditions and how and why the catalyst performs best for hydrogen production.
2. Hydrotalcite Precursor to Final Fuel Processing Catalyst
HT like materials is the best choice as a catalyst due to several properties like large surface area, memory effect, basicity, etc.  . However, different techniques used to increase the catalytic activity and stability of HT before utilizing it in catalytic systems. Generally, the calcination treatment of HT was performed in air at a temperature ranging from 400˚C to 800˚C and time span between 4 and 6 hours  .
Impregnation of HT with promoter species was also found to enhance the performance of the HT catalyst very effectively. According to the literature reviewed here HT based catalysts impregnated with metal species like Ni, Ce, Zr, Co, etc. exhibited higher activity, stability, and product yield during different fuel processing reactions  .
Other than calcination and metal impregnation, reduction pretreatment of HT in hydrogen and/or nitrogen environment at elevated temperatures (between 400˚C and 800˚C) was also utilized by several researchers to achieve better performance from HT derived fuel processing catalysts .
3. Hydrotalcite (HT) Based Catalysts for Hydrogen Production Reactions
Hydrotalcite compound and its derivatives have been reported as active catalysts for various chemical processes like dry reforming of hydrocarbon, steam reforming of hydrocarbon, transesterification reaction, NOX & CO2 capture, etc.    . Properties like large surface area, porosity, memory effect, small crystal size, thermal stability, and basic properties make HT and their derivatives are promising catalyst precursors for sustainable and eco-friendly fuel production. Results achieved by researchers reveal that catalysts derived from hydrotalcite precursors are potential candidates for application in industrial fuel processing systems  . Effective utilization of mixed oxide catalyst synthesized from HT precursor for environmentally friendly fuel (hydrogen, alcohol, and bio-diesel) has been discussed below:
3.1. HT Based Catalysts for CO2 or Dry Reforming of Methane (DRM)
CO2 or dry reforming of methane (DRM) is an influential process for hydrogen (H2) production and an effective way of the utilization of carbon dioxide (CO2) and methane (CH4) for environmentally friendly fuel production. DRM process has the potentiality of decreasing greenhouse gases like CH4 and CO2. The reaction for dry reforming of CH4 is given below :
According to Equation (1): strong C-H (439 kj·mol−1) bonds present in CH4 leads to an endothermic change in the DRM process that requires high temperature . Researchers have investigated the DRM reaction mechanism, but some issues remained unsolved. For example, there are different views about rate-determining steps (RDS)  . DRM process is mainly affected by temperature, activity, H2/CO ratio and carbon deposition . Where higher temperatures ensure better productivity but make the DRM method less feasible for industrial applications. The introduction of the catalyst has the potentiality to solve the problem as catalysts make higher activities achievable at lower temperatures. For 50% CO2 conversion in the DMR process temperature as high as 1035˚C is needed if no catalyst was used . According to recent literature reviews and experiments where catalysts used, especially hydrotalcite based catalysts ensure higher CO2 and CH4 conversions at temperatures lower than 800˚C. It is investigated that, methane can be activated and absorbed by the active sites of transition metals present in group 9 and 10 of the periodic table, while Carbon dioxide can be activated and absorbed by oxides . This makes transition metal promoted hydrotalcite derived catalysts promising for the DRM process. Table 1 presents recent works by different researchers on hydrotalcite derived catalysts for DRM.
Accordingly, Oscar et al.,  synthesized Pr promoted Ni-Mg-Al based hydrotalcite catalyst by the self-combustion method with microwave-assistance. They found that the promotion of Praseodymium (Pr) in Ni-Mg-Al improved catalyst stability and reduced the carbon deposits on the catalyst surface. However, the incorporation of Pr did not increase the performance of the developed catalyst. Then again, Tanios et al., synthesized Co, Ni, Mg, and Al-based catalyst by hydrotalcite route. The synthesized Co2Ni2Mg2Al2800 catalyst showed better conversions than commercial Ni (50%)/Al2O3 catalyst. The difference in catalytic performance between Co2Ni2Mg2Al2800 and commercial Ni (50%)/Al2O3 catalyst was higher in temperatures 500 ˚C and 700˚C. The developed catalyst contained less Ni than the commercial catalyst. Moreover, the new catalyst showed good stability at 800˚C. Pellet and powder form of the catalyst showed similar methane conversions. The researchers also found that the presence of toluene decreases CH4 conversion by an average of 15%. Meanwhile, CO2 conversion remains almost unaffected by toluene. On the other hand, cerium (Ce) and yttrium (Y) promoted double layered hydroxide (hydrotalcite) catalyst was synthesized by Swirk et al.  to study the effect of Y on DRM. They promoted hydrotalcite (containing Ni2+, Mg2+ & Al3+ ions) with Ce by co-precipitation method and promoted hydrotalcite with Y by impregnation method. Their study revealed that incorporation of Y and Ce decreased Ni reducibility, improved Ni dispersion, reduced Ni crystal size and increased basicity. Ce Promotion increased both CH4 and CO2 conversions at the temperature range of 600˚C - 750˚C and 0.2 wt% loading of yttrium increased conversions during isothermal DRM tests. Elsewhere, the catalytic performance of layered double hydroxide (hydrotalcite) containing a certain amount of Ni and promoted with Iron (Fe) was evaluated by Wierzbicki et al.,  for the DRM process. The tests revealed that, a small amount of Fe incorporation affected CO2 adsorption, increased moderate basic sites and at a low temperature of 250˚C increased catalytic activity. However, decreased catalytic performance was observed during higher Fe promotions.
Table 1. Review report of HT based catalyst for CO2 or Dry Reforming of Methane (DRM).
The catalytic activity of cobalt (Co) supported hydrotalcite (Mg, Al) catalyst on DRM was studied by Guo et al., . They synthesized Co/MgAl catalyst having a hydrotalcite-like structure of MgAl. The developed catalyst has a significant amount of medium basic sites, impressive initial activity, comparatively higher stability and better resistance to sintering and cooks formation. Co supported hydrotalcite catalyst Co/Mg3Al was also studied by Li et al.,  to investigate its performance on low-temperature DRM. They also compared the performance of Co/Mg3Al with nickel-based catalyst Ni/Mg3Al. According to their comparative study revealed Co/Mg3Al demonstrated better catalytic activity than Ni/Mg3Al at low temperate. Co catalysts showed high cook resistance and stable catalytic activity at both low and high temperatures. Hydrotalcite based nickel (Ni) catalyst containing little amount of CeO2, ZrO2 and ZnO oxides as promoters have been studied by Niu et al., . According to their study CeO2-Ni-Hydrotalcite has better stability, highest H2/CO2 ratio and CO2 activation among the developed catalysts. They also observed that oxide promotion slightly decreased the performance of the Ni-catalyst. Then again, Świrk et al.,  promoted Ni-based hydrotalcite catalyst with yttrium (Y). The addition of 1.5 wt% of Y increased the catalytic performance of the Ni-(Mg/Al) hydrotalcite catalyst. They also reported increased specific surface area, a higher fraction of medium basic sites and reduced Ni crystallite size. Aider et al.,  developed Co-Al-hydrotalcite and Co-Fe- hydrotalcite catalysts to enhance the catalyst stability and resistance to carbon deposition during the DRM process. Their study revealed that, Co-Al-hydrotalcite has higher catalytic activity due to higher specific surface area, the smaller size of Co and presence of homogeneous Co particles, but due to the active phase re-oxidation of by water, Co-Fe-hydrotalcite has an inferior catalytic performance. Świrk et al.,  studied DRM over Zr (zirconium) and (Y) yttrium promoted hydrotalcite (Ni/Mg/Al) catalyst. The Zr and Y impregnated Ni/Mg/Al-HT improved catalytic activity and the modified catalyst was stable in DRM at 700˚C. Kalai et al.,  developed Ni(Mg)-Al Htlc catalyst by conventional incipient wetness impregnation method for Ni promotion and co-precipitation method for hydrotalcite development. Their synthesized 20Ni-Mg-Al catalyst exhibited high CH4 conversion and a low deactivation rate during DRM. The stability of the developed catalyst was also remarkable.
Dahdah et al.,  experimented on the effect of lanthanum (La) doping on hydrotalcite (NixMg6-xAl2) catalyst for CO2 reforming of methane. Ni2Mg4Al1.8La0.2 catalyst synthesized by Dahdah and co-workers  exhibited better catalytic performance by means of CH4 and CO2 conversions, increment of basic sites and catalyst stability. They also mentioned that La helped to remove deactivating carbon by forming La2O2CO3. Kalai et al.,  also evaluated La promotion on hydrotalcite derived Ni catalyst for DRM. They concluded that La promotion is beneficial to develop hydrotalcite derived Ni catalyst for DRM. They also mentioned about the formation of lanthanum carbonate hydroxide phase due to La addition. Debek et al., , examined the performance of hydrotalcite (Ni/Mg/Al and Ni/Al) derived catalyst promoted with cerium (Ce) on DRM. They promoted hydrotalcite precursor Ni/Mg/Al and Ni/Al with Ce by adsorption of Ce species from [Ce(EDTA)]− solution. The Ce promoted catalyst found to increase CH4 conversion in DRM, but an increased amount of CH4 and CO2 in the feed adversely affected both CH4 and CO2 conversions. In another study Debek et al.,  evaluated zirconia promoted Mg(Ni,Al)O Htlcs catalyst in CO2 reforming of CH4. Here, Zr species was introduced at the co-precipitation stage. Their investigative study revealed that the introduction of Zr strongly enhanced selectivity and stability of the catalyst, but reduced activity. Ni containing hydrotalcite (Mg-Al) catalyst was studied by Wierzbicki et al., . According to the researchers, the presence of Ni increased the reducibility and CO2 adsorption capacity of the catalyst.
Zhu et al.,  examined the effect of Mg/Al ratio in Ni-Mg-Al hydrotalcite catalyst during DRM. They found that a higher ratio of Mg and Al in the catalyst leads to higher activity and stability. Where 1 is found to be the best ratio between Mg and Al. Then again, the effect of Ni/Al ratio in the hydrotalcite catalyst for DRM was studied by F. Touahra and co-workers . According to their study, catalytic activity of the Ni-Mg/Al hydrotalcite catalyst was proportional to the molar ratio of Ni2+/Al3+ in the catalyst and higher calcination temperature during catalyst synthesis was found to enhance activity. Ce-Zr, Zr, and Ce promoted Ni-Mg-Al (hydrotalcite) catalysts were synthesized by Debek et al., to evaluate its performance over low-temperature dry methane reforming. They reported reduced conversion of both CH4 and CO2 but the incorporation of Zr in the developed catalyst helped to determine both selectivity and activity of the catalyst. Likewise, CeO2 to promote hydrotalcite (Ni-Mg/Al) catalyst for pressurized CO2 reforming of CH4 was studied by Ren and co-workers . Both the impregnation and co-precipitation method was utilized by the researchers to add CeO2 in the Ni-Mg-Al precursor. CeO2 incorporation increased activity but decreased stability and surface area of the catalyst. Finally, the influence of Ni loading on Ni-Mg/Al hydrotalcite catalysts for CO2 reforming of CH4 was studied by Lin et al., . According to their work hydrotalcite derived Ni/Mg-Al catalysts were mentioned to be highly active for CO2 reforming of CH4. They also observed that at higher temperatures sintering of Ni particles increases and coke deposition decreases with higher Ni loadings in the catalyst.
Mg(Al)O hydrotalcite type catalysts are promising for CO2 reforming of CH4(DRM). Especially, Ni-based hydrotalcite (Ni-Mg-Al) material promoted with metals like Zr, Y, Ce, Co, and Pr showed potential development in catalytic properties during DRM such as improved activity, stability, coke resistance, and sintering resistance.
3.2. HT Based Catalysts for Steam Reforming of Methane (SRM)
Steam reforming of methane (SRM) or wet reforming of CH4 is a well-established and industrially applied process for H2 production. Generally, methane is used for hydrogen production in steam reforming technology but other light hydrocarbons can also be used. In this process, hydrogen (H2) and carbon monoxide (CO) is produced by reacting methane (CH4) and steam (H2O). CO generation in the reaction is subjected to the WGS (water gas shift) reaction to increasing hydrogen productivity. Equation (2) and (3) illustrates the reactions for the process.
The temperature of the reactions is usually above 700˚C and the catalyst is used to enhance H2 yield. Nickel (Ni) based catalyst is generally used in SRM for their cost-effectiveness and availability. However, Ni as a compound is less activated than other metals and during SRM Ni catalysts become deactivated due to sintering of active phase and coke formation in the surface . Catalytic supports are very effective in reducing coke deposition and sintering . Hydrotalcite catalyst has a large surface area, uniform active metal distribution on the surface and basicity which makes hydrotalcite based catalyst very promising for SRM . This is why hydrotalcite based catalysts are potential for SRM. Hydrotalcite catalysts promoted with metals like Ni, Ce, Zr, Rh, etc. are studied for SRM by researchers and promising results were found. The recent development of hydrotalcite based catalyst for SRM is presented in Table 2.
Fasolini et al., synthesized Rh-Mg-Al hydrotalcite catalyst for SRM and they found that a lower Mg/Al ratio and higher rhodium (Rh) amount (from 1% to 2%) in the catalyst increased catalytic activity. Moreover, they developed a thermal treatment method for the catalyst that enhanced catalytic activity by increasing reduced Rh at the surface. They also mentioned that pellet size and amount of catalyst at constant contact time have no influence on catalytic activity. On the other hand, cerium (Ce) promoted catalyst containing nickel, cobalt, and hydrotalcite was developed by Ghungrud et al., for sorption enhanced H2O reforming of CH4. Ni and Co containing hydrotalcite were synthesized by the co-precipitation method for HT precursor and then the incipient wetness impregnation technique was applied for Ce promotion. Ce incorporation found to raise thermal stability and basicity. The developed catalyst enhanced H2 production and inhibited coke formation with good stability. Thus, the researchers concluded that Ce promoted Ni-Co-hydrotalcite is a potential catalyst for high purity H2 production.
Table 2. Review report of HT derived catalyst for Steam/wet Reforming of Methane (SRM).
Wang et al., studied the role of bimetallic alloys in SRM reaction. They synthesized hydrotalcite catalyst containing Ni and Ag to study the effect of Ni/Ag alloying on SRM. According to their research, a silver (Ag) atom can influence seven nearby sites and those sites are found to be inactive to C-H bonds of CH4 and Ag blocked the more active sites in the edges and steps of Ni nano-particles. Though, Ni/Ag hydrotalcite catalyst has better surface alloying than other bimetallic catalysts. Elsewhere cerium (Ce) and zirconium (Zr) was used to promote Ni/hydrotalcite catalyst to evaluate its performance in SRM by Dewoolkar et al., . High basicity and large surface area of the doped catalyst restricted unwanted reactions and coke formation. The developed catalysts, especially Ce doped hydrotalcite catalyst exhibited improved H2 production and stability. According to their findings, the researchers concluded that Ce and Zr are the potential promoters for Ni/hydrotalcite catalysts to produce pure H2.
Hydrotalcite type Ni/Mg-Al catalyst for wet reforming of methane was investigated by Qi et al., . Their study reviled that Ni/Mg-Al catalyst has good stability and higher activity than Ni/γ-Al2O3 and Ni/α-Al2O3 catalysts. Where, Ni/Mg-Al catalyst containing Ni/Al/Mg molar ratio of 0.5:1.0:2.5 showed the best activity. They also observed that raising temperature favors CH4 conversion, but CO2 selectivity enhanced at low temperatures. Ni-based hydrotalcite catalyst (Ni-HT) was also studied by Dewoolkar et al., . Their hybrid catalytic material comprised of Ni-HT (Ni-hydrotalcite) catalyst and Ni-CaO/Al2O3 sorbent. Their developed catalytic mix was able to produce high purity H2 with good stability. Thus, a conclusion was brought that, the multifunctional hybrid material (Ni-Htlc and Ni-CaO/Al2O3) is a viable catalyst for pure hydrogen production from sorption enhanced steam/wet reforming of methane. Kim and co-researchers  also studied Ni-hydrotalcite based mixed catalyst containing Ni/MgAl and Cr/Fe3O4. By using the catalyst in SRM process high H2 and CO selectivity coupled with better CH4 conversion were reported. It was also reported that Cr/Fe3O4 as a catalyst enhanced WGS reaction. On the other hand, Hydrotalcite catalyst promoted with nickel (Ni) and ruthenium (Ru) was studied by Velasco and co-workers . According to the study, hydrotalcite derived Ni/Rh bimetallic catalyst was more resistant to carbon deposition then only Ni containing catalysts. They also reported higher CH4 conversion that was close to values assumed by thermodynamic equilibrium. Ru supported Ni-Mg-Al (hydrotalcite) catalyst for the SRM process was also studied by Nawfal et al., . Ru impregnation was found to increase the reactivity of oxides and CH4 conversion during SRM. Rh doped Ni-Mg/Al catalyst was reported to exhibit better stability and catalytic activity.
Nickel hydrotalcite coupled with calcium aluminate catalytic system for high purity H2 production from sorption enhanced SRM was examined by Broda et al., . Ni-hydrotalcite catalyst and calcium aluminate sorbent were able to produce highly pure hydrogen (99 vol %). The calcium aluminate sorbent demonstrated high cyclic CaO conversion and good stability which enhanced H2 productivity. For high H2 productivity from sorption enhanced SRM Chanburanasiri et al.,  also developed hydrotalcite based catalyst containing Ni and CaO. At steam, to methane ratio 3, temperature 600˚C and atmospheric pressure the catalytic system offered a high H2 concentration of 80%. It was also mentioned that CO2 absorption can improve reaction conditions only if the catalyst is active enough. Finally, commercial hydrotalcite (Mg-Al) promoted with nickel (Ni) and platinum (Pt) as an intelligent catalyst was evaluated by Zhan et al.,  for SRM. The developed catalyst exhibited self-activation and self-regeneration properties. Well crystallized alloying between Pt–Ni and Mg(Al)O on the catalyst surface enhanced self-activation and self-regeneration. According to the above-mentioned discussion, Ni containing hydrotalcite (Ni-HT) is a promising catalyst for H2 via steam reforming of methane. It was tried to further enhance the catalytic activity by doping Ni-Htlc with promoters like Rh, Ce, Zr, Ag, Pt, Ru, etc. Among the promoted catalysts: Ce doped Ni-hydrotalcite (Ce/Ni-HT) catalyst excellent better results and the researcher referred that the catalyst has the potentiality to be used in the SRM process for a large scale H2 production. In conformity with Table 2 hydrogen productivity from SRM when HT derived catalysts were used is presented below in Figure 1.
3.3. HT Derived Catalysts for Methanol Steam Reforming
Methanol is a promising fuel and it is considered as an attractive hydrogen storage medium as the gravimetric hydrogen density of CH3OH is much higher than that of compressed or liquid hydrogen . The use of methanol for hydrogen storage is advantageous as the necessity of cryogenic or pressurized containers can be avoided. The liberation of hydrogen for methanol can be done by several catalytic reactions such as methanol decomposition, WGS (water gas shift) reaction, methanol steam reforming and partial oxidation of CH3OH. Among these reactions, methanol steam reforming reaction is more convenient for H2 production due to low operational temperature (300˚C - 400˚C), no direct CO production and highest H2 concentration in the product stream . The reaction for CH3OH steam reforming is presented in Equation (4) below.
Figure 1. Performance of different HT based catalysts during SMR.
However, obstacles like high endothermicity and rapid speed variation of methanol steam reforming reaction affect H2 production. To overcome these problems partial oxidation methods and effective catalysts are used . Broadly applied catalysts for CH3OH steam reforming are zinc oxide, copper, and alumina containing materials which are usually synthesized via co-precipitation method from malachite-boehmite precursor . Hydrotalcite derived materials having copper (Cu), zinc (Zn) and aluminum (Al) are found to have potential applications as a catalyst for methanol steam reforming . In recent years, researchers experimented with hydrotalcite (Htlc) based catalysts containing different Zn, Cu, and Al ratios and metal promoted Zn, Cu, Al-Htlc catalysts for enhancing productivity during methanol steam reforming. Table 3 presents the catalytic activity and overall performance study of hydrotalcite derived catalysts for CH3OH steam reforming.
According to Table 3 Yang et al.,  studied the influence of rare earth metals (Ce, La, Sm, Y, and Gb) on the performance of Cu/Zn-Al-Htlc catalyst for CH3OH steam reforming. They utilized the in-situ method on γ-Al2O3 for hydrotalcite precursor (Cu/ZnAl-Htlc) synthesis and then modified the precursor with rare earth metals (Ce, La, Sm, Y, and Gb) by wet impregnation technique. The researchers found that the performance of Cu/Zn-Al-Htlc catalyst is dependent on the reducibility and surface area of Cu. Moreover, the addition of Ce, Sm and Gb further enhanced the catalytic activity of Cu/Zn-Al-Htlc catalyst. Among their developed catalysts, Ce-Cu/ZnAl-Htlc exhibited better results with high activity, 100% methanol conversion and very low CO concentration (0.39%) in the product. He et al.,  also used the in-situ and wet impregnation method to synthesized Cu supported ZnAl-Htlc catalyst for CH3OH steam reforming. The developed copper supported Zn-Al-Htlc catalyst was able to convert 99.98% CH4 at 300˚C with the H2 production rate of 981 cm3kg−1s−1. Then again, Kim et al.,  utilized the co-precipitation process to develop Cu-Al based catalysts from hydrotalcite for utilization in methanol steam reforming. The developed hydrotalcite derived catalyst containing Cu, Ni, Zn, and Al was able to convert 76% CH3OH. They mentioned that Ni incorporation slightly increased the catalytic performance of the developed catalyst.
The mixed catalyst containing CuO/ZnO/Al2O3 and K-HT (potassium promoted hydrotalcite) was evaluated by Xiang Wu and Sufang Wu  for the production of highly pure H2 from sorption-enhanced methanol steam reforming. It was able to obtain 98.36% H2 in the product stream. Also found that the introduction of K-HT reduced the Me-OH (methanol) conversion temperature to nearly 50˚C and increased the hydrogen concentration over 20% in the product stream. Copper supported zinc-aluminum hydrotalcite (Cu/Zn-Al-Htlc) catalyst for CH3OH steam reforming was evaluated by Hammoud and co-workers . Co-precipitation (for HT precursor) and wet impregnation (for Cu support) technique was applied for the catalyst development. Among the developed
Table 3. Review report of HT derived catalysts for CH3OH Steam Reforming.
Cu/Zn-Al-HT catalysts, 10% Cu containing HT showed the best result and it exhibited an H2 yield of 75.44% at 250˚C with 51.87% methanol conversion. Moreover, it was able to convert 99.78% Me-OH at 350˚C. It was also found that catalytic activity and reaction efficiency were dependent on the amount Cu2O that was present in the enhanced catalyst. Cu, Zn, Al-HT catalyst for CH3OH steam reforming was also studied by Kuhl et al., . They used the micro- emulsion technique for catalyst synthesis. The researchers were able to reduce the embedding of the copper particles and increase the copper surface area by applying the micro-emulsion technique. Thus, higher catalytic activity was achieved during CH3OH steam reforming by the developed catalyst. Finally, Pd2Ga and PdZn intermetallic nanoparticles supported hydrotalcite catalyst for CH3OH steam reforming were synthesized by Ota et al., . The researchers found that the Ga presence in the catalyst increased selectivity towards Me-OH and CO2. However, the intermetallic nano-particles supported hydrotalcite catalyst was less active to Me-OH steam reforming then Cu-ZnO-Hydrotalcite based catalyst.
Hydrotalcite derived catalysts showed high Me-OH conversion and hydrogen productivity in methanol steam reforming. Especially, copper-containing hydrotalcite catalysts showed excellent performance. Methanol conversion by catalysts mentioned in Table 3 is presented in Figure 2.
3.4. HT Based Catalysts for Ethanol Steam Reforming
Nowadays, ethanol (C2H5OH/Et-OH) has become a potential source of hydrogen (H2). Ethanol for hydrogen production is advantageous than natural gas and liquefied petroleum as it is renewable (bio-ethanol), non-toxic, easy to handle and high hydrogen yield . Hydrogen from ethanol can be produced by partial oxidation, steam reforming or oxidative reforming method. Among these methods, ethanol steam reforming has a higher H2 yield as part of it comes from Steam. Moreover, ethanol steam reforming is easy to apply process. Equations (5) and (6) present the prime reactions of ethanol steam reforming.
However, C2H5OH steam reforming is a complicated process, where the number of reactions such as dehydrogenation, dehydration, coking, and decomposition can take place successively or even simultaneously . Temperature management during C2H5OH steam reforming is a significant issue due to the endothermic nature of ethanol steam reforming reaction and the necessity of continuous heat supply . Moreover, methane formation during ethanol steam reforming Equation (7) decreases H2 yield.
To overcome such obstacles, catalysts can play an important role. Catalysts can effectively reduce operating temperature and prevent methane formation  . Previous studies revealed that noble metal-derived catalysts exhibited good results for ethanol H2O reforming . However, noble metal-based catalysts become deactivated over time during ethanol steam reforming due to the carbonaceous material deposition on the surface of the catalyst. The incorporation of catalytic support that can neutralize the acidic sites of the catalyst can solve the problem  . Hydrotalcite with basic sites in its structure is a potential catalyst precursor for ethanol steam reforming  . Metallic elements (Ni, Co, K, La, Cu, Rh, and Fe, etc.) used in HT to ease the conditions of ethanol steam reforming and enhance product yield that tabulated in Table 4.
Figure 2. Methanol conversion by HT based catalysts in methanol steam reforming.
Table 4. Review report of HT derived catalyst for Ethanol Steam Reforming.
Cerda-Moreno et al.,  synthesized Zinc-hydrotalcite (Zn-HT) catalyst containing different amounts of cobalt (Co) via co-precipitation method and subjected the catalysts to ethanol steam reforming. They found that Zn-HT catalyst with 20 wt% Co exhibited the best result in the means of total ethanol conversion and high H2 yield. A high presence of reduced cobalt and small crystallite size of metallic cobalt in the 20% Co supported Zn-HT was found to be the cause behind the enhanced catalytic activity. Furthermore, 20% Co supported Zn-HT catalyst with 1 wt% of lanthanum (La) improved catalytic stability. Thus, C. Cerda-Moreno and co-workers concluded that Co supported Zn-HT catalyst promoted with La is a potential catalyst for hydrogen production. Developed catalyst offers enhanced Et-OH conversion, good stability and, high H2 selectivity during Et-OH steam reforming. Munoz et al.,  synthesized mixed oxide NiCo-MgAl catalyst from HT precursor for oxidative steam reforming of ethanol. They utilized microwave or ultrasound-assisted co-precipitation method for catalyst development and found that microwave or ultrasound assistance reduced catalyst preparation time and improved both reducibility and basicity of the catalyst. However, ultrasound treatment caused surface defects and high basicity of the catalyst favored side reaction that might reduce H2 yield. Ultrasound-assisted co-precipitation method was also utilized by Espitia-Sibaja and co-workers  to prepare the Co-Mg/Al catalyst from HT-like precursor for oxidative ethanol steam reforming. The Co-Mg/Al catalyst with 10% to 15% Co content exhibited the best performance. Whereas, Co content less than 10% promoted the production of undesirable by-products and Co content more than 15% in the catalyst increased particle agglomeration. They also reported that the percentage of active Co phase has an effect on the properties and structure of the catalyst. Cobalt-5HT catalyst (Co/Mg/Al-Htlc) for Et-OH steam reforming was also evaluated by Espinal et al., . They doped the (Co/Mg/Al-Htlc) in catalyst with potassium (K) and used a catalytic membrane reactor for the Et-OH steam reforming and staged membrane reactor. Catalytic membrane reactor at 600˚C temperature and 12 bar pressure showed 3 times higher H2 yield than staged membrane reactor. Thus, the researchers were able to demonstrate long term ethanol steam reforming at moderate temperatures using Co-HT catalyst. Besides, hydrotalcite derived NiXMg2AlOY catalyst was developed by Fang et al.,  for the production of hydrogen from bio-ethanol. They studied the influence of Ni content in the performance of the NiXMg2AlOY catalyst. Ni content and H2 pretreatment of the catalyst were found to affect the catalytic performances. Their NiXMg2AlOY catalyst prepared via co-precipitation and thermal treatment method had particle size around 4 nm and showed high H2 yield with total Et-OH conversion.
Zeng et al.,  explored a slightly different path, where they synthesized La-Ni(Mg-Al) from perovskite and hydrotalcite precursor for Et-OH steam reforming. The shell core type perovskite at HT catalyst exhibited high H2 yield, excellent catalytic activity, and improved stabilities during ethanol steam reforming. They mentioned that LaNi(Mg, Al)O3 perovskite with shell-type configuration present in the catalyst could be linked to the enhanced performance of the catalyst. Zeng and co-workers  in another study evaluated the catalytic performance of shell-core Ni-Mg/Al catalyst derived from hydrotalcite type material for ethanol steam reforming. Comparing shell core Ni-Mg/Al catalyst with bulk Ni-Mg/Al catalyst the researchers found that shell core catalysts having lower nickel content exhibited improved H2 yield at 700˚C. On the other hand, Abelló et al.,  studied Ni-Fe-HT catalyst for H2 production by Et-OH steam reforming. They synthesized Ni-Fe catalyst from hydrotalcite-like material (reevesite) and examined the effect of the calcination temperature and Fe content of the catalyst on the catalytic performances. According to their findings, catalyst calcined at 500˚C with nickel/iron ratio of 1exhibited the best catalytic performances. They concluded that Fe addition positively affects Ni-based HT catalysts by increasing H2 selectivity, improving Ni dispersion and decreasing carbon deposition. Ni-Fe-HT catalyst was also studied by Bolshak et al.,  for Et-OH steam reforming. They found a similar result that, the catalyst containing Ni/Fe ratio of 1:1 exhibited higher catalytic activities. However, they also mentioned that increasing temperature during Et-OH steam reforming with Ni-Fe-HT catalyst promoted ethanol dehydrogenation and decreased carbon deposition.
Cunha et al.,  prepared Cu-Mg/Al HT catalyst for sorption enhanced ethanol steam reforming by impregnating active copper particles to a commercial hydrotalcite compound containing magnesium and aluminum. The developed Cu-Mg-Al catalyst showed good stability and improved hydrogen production during sorption enhanced ethanol steam reforming. But, high CO content in the product stream due to limited catalytic activity for WGS reaction is a drawback of the catalyst. On the other hand, Espinal et al.,  synthesized cobalt hydrotalcite (Co-Mg/Al-Htlc) catalyst for Et-OH steam reforming and observed that, Co/Mg/Al-Htlc catalyst having Co:Mg:Al molar ratio of 1:2:1 exhibited the best catalytic performance and selectivity towers H2 at moderate temperatures. In situ experiments conducted by the researchers indicated that Co2+ species are beneficial for ethanol steam reforming and spinel Co-Al and CoO particles interact with MgO strongly. Elsewhere, Moura et al.,  conducted Et-OH steam reforming over Rh (rhodium) and/or Co (cobalt) promoted HT catalyst and examined the effect of the support on the catalytic activity. The catalytic performance evaluation was done in a fixed bed tubular quartz reactor containing 0.150 g catalyst under atmospheric pressure and 500˚C temperature. Among the developed catalysts, Mg-Al HT catalyst containing both Rh and Co exhibited the best performance during Et-OH steam reforming with a H2 yield of 40% and selectivity of 57%.
Hydrotalcite (Mg-Al) based catalysts containing Co, Ni and Cu were evaluated by Guil-Lo’pez et al.,  for hydrogen production by oxidative C2H5OH steam reforming. They utilized the urea hydrolysis method for catalyst synthesis. Among the developed catalysts CoZnAl-HT showed the best catalytic performance (H2 selectivity of 85%) and CuMgAl-HT appeared to have the lowest catalytic performance (H2 selectivity of 21%) at a lower temperature (575˚C). Their experiment revealed that Co catalysts have batter H2 selectivity and yield during oxidative ethanol steam reforming than Ni catalysts. Ni loaded Mg-Zn-Al catalyst prepared from hydrotalcite precursor via co-precipitation method was investigated by Zeng and co-workers  to study the synergy effect of MgO and ZnO during C2H5OH steam reforming for hydrogen production . They found that at 700˚C Ni-Mg-Zn-Al Htlc catalyst with Mg/Zn ratio of 4 showed excellent stability and high hydrogen yield. Busca et al.,  synthesized a slightly different Ni/Co-Zn-Al catalyst from hydrotalcite precursor via the urea hydrolysis process for C2H5OH steam reforming. With the developed Ni-Co-Zn-Al catalyst Guido Busca and co-workers were able to achieve 90% H2 yield at 540˚C and water to ethanol feed ratio of 6. They mentioned that the reason behind the high selectivity of the catalyst to H2 might be related to the stability of partially reduced catalyst surface. Finally, Lucrédio et al.,  evaluated the influence of lanthanum (La) and cerium (Ce) addition on the catalytic performance of HT type Ni-Mg/Al catalyst during Et-OH steam reforming. Their analysis revealed that, Ni as NiO strongly and La moderately interacts with the Mg-Al support. But, Ce species has almost no interaction with the Mg-Al support. Both La and Ce incorporation resulted in High Et-OH conversion, improved H2 production, and low carbon deposition. As mentioned by the researchers, the improved catalytic performance of Ce-Ni-Mg-Al catalyst is linked to the less interaction of the support with Ce species, which increased H2O adsorption on CeO2 and thus helped ethanol and acetaldehyde conversions.
Hydrotalcite derived materials exhibited impressive catalytic performances in ethanol steam reforming. The hydrogen yield of HT based catalysts during ethanol steam reforming that are discussed in Table 4 is presented in Figure 3 below.
Figure 3. Performance of different Htlc based catalysts during ESR.
4. Discussion (Special Properties of Hydrotalcite Derived Catalyst to Reforming, Transesterification and Hydrogenation Reaction)
Methane reforming to hydrogen depends on a couple of factors that indicate performances of the system. The system requires the presence of steam and high temperature. Due to the interaction between support and catalyst and high temperature causes of catalyst sintering and coke formation. Both cases are fundamental problems of catalyst deactivation. This carbonaceous deactivation of the catalyst is a major limitation of methane reforming to hydrogen. There are couple of studies that have been cited on the pros and cons of DRM and SMR where corresponds to deposition problems and their remedies. Other than catalyst deactivation, the optimum composition of feed gas and steam ratio, operating pressure and temperature are also very important. Catalytic activity, selectivity and stability of a catalyst depends on material structure, active site and surface morphology, molar ratios of the metals, preparation methods, incorporation of promoters and mainly the elements that acts as promoters . As example, MSR has been widely investigated and demonstrated that Cu/ZnO/Al2O catalyst commonly used at 240˚C - 260˚C   . Since, copper leading catalyst are very effective, low cost and it produces significant amount of carbon monoxide but shows less stable and pyrophoric nature. Conant et al.,  showed catalyst deactivation using water molar fraction above the stoichiometric amount. Hydrotalcite derived Ni/Mg-Al or Fe/Mg-Al catalysts were mentioned to be highly active for CO2 reforming of CH4. They also observed that at higher temperature sintering of Ni particles increases and coke deposition decreases with higher Ni loadings in the catalyst. A less activation energy of the reaction is due to the catalyst performances that help to reform methane and prolong the deactivation of the catalyst. Activity, sulfur poisoning, carbon formation, sintering are four challenges of steam methane reforming . Group VIII metals are active selection for the steam reforming of methane. Alkalis’ make obstacle the activities of group VIII metals. The support should be able to withstand such conditions without losing strength . For steam methane reforming, Ni containing hydrotalcite and calcium aluminates catalyst work best (99%) at 550˚C that can overcome the limitation of steam methane reforming with the special properties of hydrotalcite derived catalysts.
Superior MSR (Methanol steam reforming) catalyst should more active, stable and cause of producing less CO2 in the reforming system. Moreover, moderate temperature and pressure also desired. The performance of MSR is signiﬁcantly dependent on the reforming catalyst. Copper-based and palladium-based catalysts for methanol steam reforming had studied well . Several schemes have been proposed regarding the reaction mechanism of methanol steam reforming. The methanol steam reforming-methanol decomposition-reverse water gas shift reaction scheme is generally accepted for methanol steam reforming reactions occurring over the copper-based or palladium-based catalysts. Three reactions of the methanol decomposition, water gas shift reaction and methanol steam reforming are assumed to occur in parallel. since, hydrotalcite derived catalysts are well stable, active surface that prolong the catalyst life and complete the all steps of reaction . Cu (copper) supported Zn, Al-containing catalyst performs best (99.98%) for steam reforming of methanol at 300˚C whereas Cu impregnated Mg-Al containing hydrotalcite is best (99%) for steam reforming of ethanol at 200˚C - 600˚C. Dehydration and dehydrogenation are main two reaction path of steam reforming of ethanol. Carbon deposition occur via the Boudouard reaction that encapsulates carbon and blocks the active sites of the catalysts results in catalyst deactivation. A particular ethanol and water ratio oppose the carbon deposition. Hydrotalcite derived catalyst act as a relatively good carbondioxide sorbent at 400˚C.
The performance of transesterification process depends on mode of reaction, molar ratio of alcohol to oil, type of alcohol, reaction time, temperature and amount of catalyst. Catalyst one of the key parameters to influence the whole reforming process. Acid-catalyst transesterification and base catalyst transesterification are two best method of transesterification. Hydrotalcite derived catalyst can be designed into two both such as acid and base nature catalyst. The rate of transesterification is actively correlated with inter-layer electron density of HT. The conversion to methyl esters over the catalysts is in the following order: KI/Al2O3 > KF/Al2O3 > KOH/Al2O3 > KNO3/Al2O3 > K2CO3/Al2O3 > KBr/Al2O3·KI/Al2O3 demonstrated superior catalytic activity compared to the other catalysts . The most favorable environment (99.99%) for the transesterification of methanol is at 100˚C - 120˚C & 4.8 - 5.0 atm over Zr-doped Mg-Al containing hydrotalcite, whereas Ca-Al LDHs best for transesterification of ethanol. The structure of the HT are very suitable to re-structure as required for better catalysis of the system.
Hydrogenation of ethyl acetate to ethanol shows plausible reaction pathway. The most probable cause of higher conversion of ethyl acetate is carbonyl activation that depends on the active sites of the metal in catalysts . The weak acyl structure role is to desorbs resultant products from surface of the catalyst. Ethoxy group are formed through the cleavage of C-O bond of the hemiacetal group and later hydrogenated to ethanol. There are some side products during the hydrogenation reaction. Hydrotalcite derived catalyst could be design considering to prevent the sight reactions, to have enough active sites, formation of the weak acyle structure. Some are very economic metals that ensure those properties in hydrotalcite derived catalyst and happen a better hydrogenation to ethanol. The hydrogenation process can be performed with even small amount of catalyst and in the presence of lower quantities or without solvent. Y2O3-modified Cu/ZnO/Al2O hydrotalcite performs best (89.7%) for hydrogenation to different fuels at a temperature of 230˚C & 3 - 9 atm.
Multi-component hydrotalcite is an excellent precursor for the preparation of potential mixed oxides with a homogenous distribution of metal . Partial substitution of metal can be used to refine catalytic properties of the material. The anions and water are randomly placed in the interlayer and are loosely bounded. Suitable anion can be used as per the requirement of the functional system. The strength of the hydrogen bonding between anions and the hydroxyl groups of the layers, size and orientation influence to the thickness of the interlayer. Calcined hydrotalcite or mixed oxides are highly active and selective, and can play an important role in many catalyzed based reaction. Controlled thermal decomposition of hydrotalcite produced high specific surface area of mixed oxides that have various applications. Hydrotalcite show an important property “memory effect” by which the material recover its original layer structure if they contact with water. Regeneration of these materials can be done efficiently. Another important property of this material is acid-base pairs that can be created and offer active site for many reactions or gas adsorption. Added alkali solution with a fixed pH in such a rate as to maintain the pH at a certain range which help to careful control of charge density of (M(II)/M(III) ratio) of the hydroxide layers to co-precipitate of the two or more metallic salts . These conditions give rise to precipitate higher crystalline particle than those obtained under high super saturation conditions due to the rate of the crystal growth is higher than the rate of nucleation.
Special physical & chemical properties that hydrotalcite shows: 1) Homogenous distribution of metals, 2) Cations exchange materials, 3) High specific surface area and surface energy, 4) Tuneable surface morphology, 5) Cheap, Abundant and environment friendly, 6) High thermal stability make them very active catalyst especially as highly stable, less CO2 producing nano materials . Due to the multi-layered materials, it can control water ratio and exchange electrons between homogeneously distributed metals on the surface and reforming agent that stipulated kinetics and thermodynamics.
The reaction environment generated by the hydrotalcite derived catalyst is most favorable for steam reforming, transesterification and hydrogenation is because of modification and design flexibility for the desired favorable catalytic activities, large surface area, basic properties, high amount homogeneous dispersion of metals, stability against sintering  .
The aforementioned literature review intends to present an overview of the latest advancement achieved by the researchers to develop hydrotalcite (HT) derived catalyst for hydrogen and bio-fuel production. Hydrotalcite has favorable properties such as memory effect, option to choose between different cations (M2+ and M3+) and compositional ratios, basic properties and large surface area with an almost homogeneous distribution of active sites. This is why HT are favorable as catalyst precursors for different chemical processes. The comprehensive survey of literature articles reveals that the selectivity, catalytic performance, and stability of HT derived materials depend on structural property, chemical content, and phase composition. Factors like catalyst synthesis method, calcination, and reduction of catalyst, catalytic test conditions, the addition of promoters and the molar ratio of cations (M3+, M2+, and/or promoter ions) in the catalyst affect to the catalytic efficiency. Especially, chemical composition (ratio of metallic components) and the interaction of the metal with hydrotalcite precursor have important roles in catalytic performances. Moreover, the promotion of HT precursor with metals like Zr, Ce, Ni, Co, Rh has the potentiality to improve the selectivity, stability, catalytic activity, and product yield as mentioned by various investigations. However, catalyst deactivation due to sintering of active metal ions and coke formation in the catalyst surface after application for a certain period of time has been reported widely. Efforts have been made to overcome the disadvantage by utilizing a modified catalyst synthesis method, pre-treatment and incorporation of metals such as Ru, Pt, Cu, and Ti, etc. But, agreement on a reliable method that may ensure good catalytic stability has not been found.
HT derived materials as fuel processing catalyst has received impressive progress. According to the literature reviewed above researchers were able to synthesize HT based mix oxide catalyst with improved catalytic activity and those catalysts were viable for industrial use. However, to achieve the desired stability with excellent catalytic activity from HT derived catalysts, there are several areas need to provide much attention. Most of the published scholarly articles on the catalytic performance of HT based catalyst deal with one or two active components. But, comprehensive studies about synergic interactions between different active components present in the catalytic system and their effect on activity are hardly found. Generally, HT derived catalysts are used in fixed bed reactors. HT based catalysts might be used in fluidized bed reactors by manipulating the physical properties. This will give the option to add continuous catalyst regeneration to unite in the system. Few conventional metallic promoters to enhance the performance of HT catalysts have been investigated. A wide variety of other inorganic and organic compounds should be studied to evaluate their promotional effect on HT derived catalysts.
Declaration of Interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
We greatly appreciate the financial support by the annual development project, Ministry of science and technology, Bangladesh (Project Code. 1265-244250200), and the Strategic Priority Research Program of the BCSIR (Bangladesh Council of Scientific & Industrial Research (BCSIR).
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