Urea-Assisted Synthesis and Characterization of Saponite with Different Octahedral (Mg, Zn, Ni, Co) and Tetrahedral Metals (Al, Ga, B), a Review

Abstract

Clay minerals surfaces potentially play a role in prebiotic synthesis through adsorption of organic monomers that give rise to highly concentrated systems; facilitate condensation and polymerization reactions, protection of early biomolecules from hydrolysis and photolysis, and surface-templating for specific adsorption and synthesis of organic molecules. This review presents processes of clay formation using saponite as a model clay mineral, since it has been shown to catalyze organic reactions, is easy to synthesize in large and pure form, and has tunable properties. In particular, a method involving urea is presented as a reasonable analog of natural processes. The method involves a two-step process: (1) formation of the precursor aluminosilicate gel and (2) hydrolysis of a divalent metal (Mg, Ni, Co, and Zn) by the slow release of ammonia from urea decomposition. The aluminosilicate gels in the first step forms a 4-fold-coordinated Al³⁺ similar to what is found in nature such as in volcanic glass. The use of urea, a compound figuring in many prebiotic model reactions, circumvents the formation of undesirable brucite, Mg(OH)₂, in the final product, by slowly releasing ammonia thereby controlling the hydrolysis of magnesium. In addition, the substitution of B and Ga for Si and Al in saponite is also described. The saponite products from this urea-assisted synthesis were tested as catalysts for several organic reactions, including Friedel-Crafts alkylation, cracking, and isomerization reactions.

Keywords: clay minerals; heterogeneous catalysis; saponite; synthesis; urea.

Figure 1

A schematic representation of saponites showing the major elements occupying the tetrahedral, octahedral sheets and interlayer spaces.

Figure 2

Typical X-ray diffraction pattern of a synthetic Zn-saponite synthesized for 20 h with 72.1 g urea. Based on data from [27]. Note: no vertical axis was given in the original publication.

Figure 3

Typical infrared (IR) spectrum of a synthetic Mg-saponite synthesized in 20 h with 72.1 g urea. The IR spectrum was collected using a Perkin Elmer (1600 series) spectrometer in transmission mode using 256 scans at 4 cm⁻¹ resolution using a KBr tablet (5 mass% sample).

Figure 4

Magic-angle spinning-nuclear magnetic resonance (MAS-NMR) of Zn-saponite with varying Si/Al ratio and varying synthesis time. ²⁷Al MAS-NMR (left) were performed at 130.321 MHz with a pulse length of 1 µs and a pulse interval of 1 s and chemical shifts (δ) reported in ppm relative to [Al(H₂O)₆]³⁺. ²⁹Si MAS-NMR (right) was performed at 99.364 MHz with a pulse length of 6.5 µs and a pulse interval of 40s and chemical shifts (δ) reported in ppm relative to [(CH₃)₄Si]. Modified from [27].

Figure 5

⁷¹Ga MAS-NMR of gallium substituted Mg- and Zn-saponite performed at 152.531 MHz with a pulse length of 2.0 μs and a pulse interval of 0.5 s and chemical shifts (δ) reported in ppm relative to [Ga(H₂O)₆]³⁺. Modified from [45].

Figure 6

¹¹B MAS-NMR of Mg-saponite with different Si/Al ratios and after calcination at 300 °C. ¹¹B MAS-NMR were run at 160.466 MHz with a pulse length of 0.8 μs and a pulse interval of 0.25 s and chemical shifts in ppm relative to [BF₃(OEt)₂]. Modified from [41].

Figure 7

BET total and micropore surface area and pore volume of Mg- (left) and Zn-saponite (right) as function of synthesis time. These results are calculated based on Figure 5 in [27].

Figure 8

X-ray fluorescence (XRF) analyses of different M-saponites (M = Ni, Co, Mg, Zn) and Zn-saponite with different Si/Al ratios. Results based on Table 5 of [24].

Figure 9

Thermogravimetric (TGA) and differential thermal analysis (DTA) of Co-saponite (Al³⁺ exchanged). Modified from Figure 1 [44].

Figure 10

²⁷Al MAS-NMR of Mg- and Zn-saponite thermally treated for 4 h at 400 and 600 °C. Redrawn from Figure 2 in [44].

Figure 11

BET surface area and pore volume as function of calcination temperature of M-saponites (M = Mg, Ni, Co, Zn). Modified from Figure 3 in [44].

Figure 12

Temperature-programmed reduction profiles of Cu-, Ni-, and Co-containing saponites. Modified from Figure 6 in [44].

Figure 13

Selectivity expressed in product ratios for the cracking of n-dodecane. C = paraffin, i = iso, n = normal, total = total amount of olefins and paraffins. This figure is based on Table 2 of Vogels et al. [50].

Figure 14

The conversion of n-heptane at rising reaction temperatures of Co-saponite (Si/Al ratio 7.89) and Mg-saponite (Si/Al ratio 7.89). Modified from Figure 3 in [50].

Figure 15

The influence of the hydration state of 0.2 wt% Zn-saponite (Si/Al ratio 7.89) on the catalytic activity. H₂O means wet saponite sample. Reaction conditions: temperature 160 °C, duration 0.25 h, benzene/propylene molar ratio ~7. Modified from Figure 5 in [50].

Figure 16

The influence of the composition of the octahedral sheet (Ni²⁺, Co²⁺, Mg²⁺, and Zn²⁺) with (blue) H⁺- and (orange) Al³⁺-exchanged saponites (Si/Al ratio 7.89) on the catalytic performance. H⁺-saponites: 1.5 wt% catalyst at 190 °C and 2 h, Al³⁺-saponites: 0.2 wt% catalyst at 160 °C for 0.25 h. Modified from Figure 6 in [50].