Ordered mesoporous materials occupy a critical space in heterogeneous catalysis, separation science, and drug delivery research. Their structural properties — pore diameter, pore ordering, unit cell parameters, and wall thickness — determine how effectively they perform as catalyst supports, adsorbents, and carrier matrices. Characterizing these structures accurately requires access to diffraction data at very low angles, which has traditionally been regarded as the domain of specialized small-angle X-ray scattering (SAXS) instruments available only at synchrotron facilities or major research institutions.
This article demonstrates that small-angle XRD mesoporous materials — including the full resolution of (100), (110), and (200) diffraction peaks characteristic of hexagonally ordered SBA-15 — can be achieved reliably on the AMI Lattice Series benchtop diffractometer. Applied to SBA-15 samples synthesized at two crystallization temperatures, the Lattice Pro resolves peak positions, unit cell parameters, and structural ordering differences that directly correlate with measured pore properties from complementary nitrogen physisorption analysis using the AMI Matrix 1000.
Metal oxide catalyst supports are the backbone of heterogeneous catalysis. While the support material is not itself catalytically active in most systems, it governs the stability, specific surface area, and accessibility of the catalytic active sites it carries. Among the most studied support materials — SiO₂, Al₂O₃, TiO₂, and carbon-based materials — silica occupies a privileged position because of raw material availability, low cost, and chemical stability across a wide range of reaction conditions.
Among the many silica structures explored for catalysis and separation applications, ordered mesoporous silicates stand out for their precisely controlled pore geometry and large accessible surface areas. Two materials dominate the literature:
Discovered by Beck et al. in 1992 using ionic surfactant (CTAB) template synthesis, MCM-41 was the first ordered mesoporous silica to be widely studied. It adopts a hexagonal crystal structure in the P6mm space group, with one-dimensional cylindrical pores arranged in a hexagonal array. Pore diameters range from 2 to 7 nm (Figure 1a; alt text: MCM-41 hexagonal mesoporous silica structure showing parallel cylindrical pores in P6mm symmetry). MCM-41’s uniform pore size and very high surface area make it valuable for molecular sieving of macromolecules and as a controlled-release carrier.
SBA-15 was developed by Zhao et al. using triblock polymer (Pluronic P123) template agents. It shares MCM-41’s P6mm hexagonal symmetry but differs in three critical structural respects (Figure 1b; alt text: SBA-15 hexagonally ordered mesoporous silica with larger 5–15 nm pores, thicker walls, and interconnected micropores):
SBA-15’s combination of large mesopores, thick walls, and high stability has made it the standard catalyst support for a wide range of heterogeneous catalysis applications — from acid-catalyzed reactions to supported metal hydrogenation and oxidation catalysts.
The long-range ordering of mesoporous materials like SBA-15 occurs at a length scale of several nanometers — corresponding to diffraction at very low 2θ angles. For SBA-15 with a unit cell parameter around 10 nm, the characteristic (100) reflection appears at approximately 2θ = 0.9°. The (110) and (200) reflections fall near 1.6° and 1.8° respectively.
Bragg’s law relates the diffraction angle θ to the interplanar spacing d and X-ray wavelength λ:
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Bragg’s Law: nλ = 2d sin θ where n = order of reflection, λ = X-ray wavelength, d = interplanar spacing, θ = angle of incidence |
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At these very low angles — below 4° 2θ — standard XRD measurements face three significant challenges:
These challenges have historically pushed researchers toward dedicated small-angle X-ray scattering (SAXS) instruments for mesoporous material characterization. However, SAXS instruments require specialized optics, detectors, and often synchrotron access — representing significant cost and accessibility barriers for routine laboratory characterization. The Lattice Pro addresses these challenges within an affordable benchtop configuration, as demonstrated in this study.
Two SBA-15 samples were synthesized using the standard triblock polymer (Pluronic P123) template procedure, differing only in crystallization temperature:
Crystallization temperature is a known synthesis variable for SBA-15 that affects micelle formation, silica condensation kinetics, and ultimately the pore size and ordering of the final material. Comparing samples at two temperatures provides a direct test of the instrument’s ability to resolve structural differences between closely related mesoporous materials.
All XRD measurements were performed using the AMI Lattice Series X-ray diffractometer in the Lattice Pro configuration. The Lattice Pro uses a theta-theta goniometer geometry (radius 170 mm), which is essential for reliable small-angle measurements: in theta-theta geometry, both the X-ray source and detector move symmetrically while the sample remains stationary, eliminating the sample displacement and height errors that distort peak positions at low angles in theta-2theta instruments.
The 1600 W X-ray tube with Cu target (λ = 0.1548 nm) provides the high source intensity required to generate adequate signal-to-noise at the weak diffraction peaks characteristic of mesoporous materials below 4° 2θ. The 2D photon direct-read array detector captures the full diffraction pattern simultaneously, further improving counting statistics for low-intensity peaks. The measurement range was set from 2θ = 0.5° to 4.0°.
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Why the Lattice Pro for small-angle XRD: (1) Theta-theta goniometer eliminates sample displacement errors critical at low angles. (2) 1600 W X-ray source improves signal-to-noise for weak mesoporous diffraction peaks. (3) ±0.01° 2θ step size resolves the closely spaced (100), (110), and (200) reflections of SBA-15 below 2° 2θ. (4) 2D photon direct-read detector maximizes counting efficiency without sacrificing resolution. |
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Complementary pore structure analysis was performed using the AMI Matrix 1000 gas sorption analyzer. N₂ adsorption-desorption isotherms were measured at 77 K, providing specific surface area by BET analysis, total pore volume, and pore size distribution by BJH analysis. The Matrix 1000 is a modular multi-station analyzer designed for both micropore and mesopore characterization — directly suited to SBA-15’s hierarchical pore structure.
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Related reading: For a detailed discussion of how different gas adsorbate probes (N₂, Ar, CO₂) affect pore characterization accuracy in nanoporous and mesoporous materials, including zeolites and activated carbons, see our article on comparing gas adsorbates for pore-structure characterization of nanoporous materials. The Matrix 1000 used in this study supports all three adsorbate probes. |
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The small-angle XRD spectra for both SBA-15 samples (Figure 2; alt text: small-angle XRD spectra from 0.5°–4° showing characteristic (100), (110), (200) peaks for SBA-15 crystallized at 100°C and 120°C) exhibit three clearly resolved characteristic peaks at approximately 2θ = 0.93°, 1.60°, and 1.82°. These reflections are indexed as the (100), (110), and (200) planes of the P6mm hexagonal space group — the definitive signature of SBA-15’s two-dimensional hexagonal mesopore ordering. The observation of three well-resolved reflections confirms high-quality long-range ordering in both samples.
Comparing the two spectra reveals two systematic effects of increasing crystallization temperature from 100°C to 120°C:
The unit cell parameter a of the hexagonal SBA-15 structure is calculated from the (100) d-spacing using the geometric relationship for a hexagonal system:
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a = 2d(100) / √3 where d(100) is calculated from Bragg’s law: d = nλ / (2 sin θ) |
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The combined XRD and N₂ adsorption results are summarized in Table 1 below (alt text: results table comparing SBA-15 samples crystallized at 100°C and 120°C across d(100), unit cell parameter a, BET surface area, pore size, pore volume, and wall thickness).
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Parameter |
SBA-15 (100°C) |
SBA-15 (120°C) |
Change |
|---|---|---|---|
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d(100) (Å) |
92.87 |
94.54 |
+1.67 (+1.8%) |
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Unit cell parameter a (Å) |
107.24 |
109.17 |
+1.93 (+1.8%) |
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BET specific surface area (m²/g) |
717.086 |
852.896 |
+136 (+19%) |
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Pore size (Å) |
60.263 |
63.695 |
+3.4 (+5.6%) |
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Pore volume (cm³/g) |
0.966 |
1.169 |
+0.20 (+21%) |
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Wall thickness (Å) |
46.97 |
45.48 |
−1.49 (−3.2%) |
The wall thickness is calculated as the difference between the unit cell parameter a and the pore size — the portion of the unit cell not occupied by the pore void. The slight decrease in wall thickness with increased crystallization temperature (46.97 → 45.48 Å) despite the larger unit cell confirms that the pore growth outpaces wall growth: a larger fraction of each unit cell is occupied by pore void at 120°C than at 100°C.
The nitrogen adsorption-desorption isotherms (Figure 3a; alt text: Type IV N₂ isotherms with H1 hysteresis loops for SBA-15 at 100°C and 120°C) confirm the mesoporous character of both samples. Both exhibit Type IV isotherms with H1-type hysteresis loops — the IUPAC classification for ordered mesoporous materials with uniform, cylindrical pore geometry and narrow pore size distribution. The capillary condensation step at P/P₀ = 0.6–0.8 confirms the mesopore size range and shifts slightly to higher relative pressure for the 120°C sample, consistent with its larger pore diameter.
The pore size distribution curves (Figure 3b; alt text: narrow pore size distribution peaks for SBA-15 at 100°C and 120°C showing 60.3 and 63.7 Å peaks respectively) show sharp, narrow peaks for both samples — confirming the high uniformity of pore size that is characteristic of well-ordered SBA-15. The 120°C sample’s distribution peak at 63.7 Å is clearly resolved from the 100°C sample’s peak at 60.3 Å, demonstrating that the synthesis temperature difference of only 20°C produces a statistically meaningful and practically relevant change in pore size.
The large increase in BET specific surface area from 717 to 853 m²/g (+19%) at higher crystallization temperature reflects both the larger pore size and the improved pore ordering. More complete silicate condensation at 120°C produces fewer structural defects and more accessible pore surface — translating directly into higher specific surface area for the same mass of material.
This study demonstrates an integrated approach to mesoporous material characterization combining two complementary AMI instruments. Each instrument provides information that the other cannot, and together they deliver a complete structural and textural picture of the material.
The Lattice Series provides the crystal structure and long-range ordering information. For mesoporous materials, XRD confirms the space group and hexagonal ordering, yields the unit cell parameter a from the (100) d-spacing, resolves relative peak intensities that indicate degree of ordering, and detects shifts in peak position that reflect changes in pore size with synthesis conditions. Three model configurations are available:
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Model |
X-Ray Power |
Goniometer |
Best For |
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Lattice Mini |
600 W |
Theta/2-theta, 158 mm radius |
Phase identification and routine characterization; adequate for well-ordered samples with strong diffraction peaks |
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Lattice Basic |
1600 W |
Theta/2-theta, 170 mm radius |
High-throughput characterization with improved signal-to-noise for weaker diffraction |
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Lattice Pro |
1600 W |
Theta/theta, 170 mm radius — stationary sample |
⭐ RECOMMENDED for small-angle XRD: theta-theta geometry eliminates sample displacement errors critical at low 2θ angles; 1600 W source maximizes signal at weak mesoporous peaks |
The Matrix 1000 provides the textural characterization that XRD cannot — specific surface area, total pore volume, and pore size distribution from adsorption isotherm analysis. Its modular multi-station design supports up to 12 simultaneous analysis stations and can be configured for both micropore (for SBA-15’s interconnected micropore network) and mesopore (for the main cylindrical channels) characterization in the same instrument. The integrated high-temperature degassing furnace prepares samples without contamination from ambient moisture or residual surfactant template.
Together, XRD and N₂ physisorption provide the full structural characterization package required for mesoporous catalyst support evaluation: the former establishes crystallographic ordering and unit cell geometry; the latter measures accessible surface area and pore volume that determine catalytic performance. For a full comparison of gas probe selection for different pore size regimes, see our article on comparing gas adsorbates for nanoporous material characterization.
Small-angle XRD mesoporous materials — previously accessible only through specialized SAXS instrumentation or synchrotron beam time — can be performed reliably on the AMI Lattice Pro benchtop diffractometer. The theta-theta goniometer geometry eliminates sample displacement errors that distort peak positions at low angles, while the 1600 W X-ray source and 2D photon direct-read detector deliver the signal intensity and counting efficiency needed to resolve the weak reflections characteristic of ordered mesoporous structures below 4° 2θ.
Applied to SBA-15 mesoporous silica, the Lattice Pro resolves all three characteristic hexagonal diffraction peaks — (100), (110), and (200) — for both samples, and detects the systematic shift in peak position and intensity that corresponds to a 20°C increase in crystallization temperature. Combined with N₂ physisorption analysis from the Matrix 1000, the complete structural picture confirms that higher crystallization temperature produces larger pores (60.3 → 63.7 Å), higher surface area (717 → 853 m²/g), and greater pore volume (0.966 → 1.169 cm³/g) — with slightly thinner walls as a structural consequence of pore expansion.
Explore AMI’s full range of X-ray diffraction instruments and gas adsorption analyzers for mesoporous material characterization, or visit the AMI Technical Library for further application notes on XRD, BET surface area analysis, and pore structure characterization.
X-ray diffraction (XRD) of mesoporous materials characterizes the long-range periodic ordering of the pore structure by measuring diffraction peaks that arise from the regular spacing between pore channels. For hexagonally ordered materials like MCM-41 and SBA-15, the (100), (110), and (200) reflections in the small-angle region (below 5° 2θ) identify the P6mm space group and the interplanar spacings from which the unit cell parameter — directly related to pore-to-pore distance — is calculated. The number, position, and relative intensity of these peaks report on both the type of ordering and its quality.
Both MCM-41 and SBA-15 are hexagonally ordered mesoporous silicates with the same P6mm space group. The key structural differences are pore size, wall thickness, and stability. MCM-41 has smaller mesopores (2–7 nm) and thinner walls, making it highly surface-active but less stable under hydrothermal conditions. SBA-15 has larger pores (5–15 nm), thicker walls (3.1–6.4 nm), and interconnected micropores — giving it significantly higher thermal and hydrothermal stability. These properties make SBA-15 the preferred catalyst support for applications at elevated temperature or in aqueous environments.
Bragg’s law (nλ = 2d sin θ) relates the diffraction angle θ to the interplanar spacing d. Mesoporous materials like SBA-15 have periodicities at the nanometer scale — the repeat distance between adjacent pore channels is approximately 10 nm, giving a d(100) spacing of about 9.3 nm. At Cu Kα wavelength (0.154 nm), this corresponds to a diffraction angle of approximately 0.93° 2θ. The larger the structural periodicity, the smaller the diffraction angle — which is why SAXS or small-angle XRD capability below 5° 2θ is required for mesoporous material characterization.
Yes. The Lattice Pro — with its theta-theta goniometer, 1600 W X-ray source, and ±0.01° minimum step size — resolves the characteristic diffraction peaks of ordered mesoporous materials including SBA-15 from 2θ = 0.5° upward, without requiring dedicated SAXS optics or synchrotron access. The theta-theta geometry keeps the sample stationary during measurement, eliminating the sample displacement errors that distort peak positions at low angles in conventional theta-2theta instruments. The high-power X-ray source compensates for the inherently weak diffracted intensities from mesoporous structures at low angles.
Higher crystallization temperature during SBA-15 synthesis increases the effectiveness of micelle formation — the hydrophobic polypropylene oxide blocks of the Pluronic P123 template form larger, more stable micelles at higher temperature. This produces larger mesopores and improved pore channel ordering in the final calcined material. In this study, increasing crystallization temperature from 100°C to 120°C increased the unit cell parameter a from 107.24 to 109.17 Å, the pore size from 60.3 to 63.7 Å, the BET surface area from 717 to 853 m²/g, and the pore volume from 0.966 to 1.169 cm³/g. The XRD evidence for this structural change is the shift of all three diffraction peaks to slightly lower angles and an increase in all three peak intensities.
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