Zeolite — Adsorbent Families, Numbers & the Low-Regeneration Pivot

Zeolite is one of the core solid sorbents of Adsorption Cooling — a crystalline porous solid that binds a refrigerant vapour (almost always water) on its internal surface, then releases it when heated. It offers higher uptake, specific cooling power, and stability than silica gel, but classic grades pay for it with a high regeneration temperature that locks out low-grade heat. This article is the sorbent-side companion to Performance & Numbers and Composite Salt Sorbents — what zeolite is, the families, the numbers, and why the action has moved to aluminophosphates.

Zeolite belongs unambiguously to the adsorption side of the sorption split: a solid surface sorbent, never a liquid bulk one — see Adsorption vs Absorption. The refrigerant is almost always water (R718); ammonia and methanol pairs exist but are niche.

Material families

Family	Examples	Water uptake (kg·kg⁻¹)	Regen / desorption temp	Role
Classic aluminosilicate	13X (NaX, FAU), 4A (LTA), NaY	0.22–0.30	high, ~150–250 °C	High uptake & stability; the historic workhorse, but too hot for waste heat
Natural zeolites	mordenite, clinoptilolite	0.11–0.12	high	Cheap, low capacity; mostly of academic interest
Silico-/alumino-phosphates	SAPO-34, AlPO, AQSOA Z01/Z02/Z05, EMM-8	~0.22–0.28	low, 60–90 °C	The low-regeneration pivot — high COP on low-grade solar/waste heat
Other zeotypes studied	ZSM-5 (MFI)	low–moderate	high	Studied for solar adsorption; underperforms SAPO-34

The distinction that matters is classic aluminosilicate vs aluminophosphate, and it is mechanistic, not cosmetic (see below).

The numbers

Property	Value	Notes
Water uptake, 13X	0.30 kg·kg⁻¹	4A: 0.22; mordenite/clinoptilolite: 0.11–0.12 (Demir)
Heat of adsorption	3300–4400 kJ·kg⁻¹	High — the root of the high regen temperature and lower COP (Elsheniti, Demir)
Energy density, 13X	~1290 kJ·kg⁻¹	Working-pair table, 30–350 °C range (Demir)
Desorption range	70–250 °C	Classic grades effectively need ~150–200 °C (Elsheniti, Kumar)
Max cooling COP (classic zeolite/water)	~0.30	Anyanwu & Ogueke theoretical max; best of the classic pairs for A/C duty (Al-Yasiri)
Single-bed experimental	COP ~0.25, SCP up to 200 W·kg⁻¹	5 kW, evap 6.5 °C, 1320 s cycle (Kumar)
Mobile double-bed zeolite/water	COP 0.4, SCP 600 W·kg⁻¹	High SCP, compact (Kumar; [[references/adsorption-performance
Thermal-wave two-bed	COP ≈ 1.0	Internal heat recovery (Elsheniti)
Cascade (zeolite/water → AC/methanol)	COP 1.06	Bridges a large temperature gradient (Dous & Meunier, via Demir)
Stratisorp, water–zeolite Li-Y	heating COP 1.98	200 °C drive, stratified-storage internal recovery ([[references/adsorption-limitations-and-mitigations

The central drawback — and the aluminophosphate pivot

Classic zeolites bind water by strong electrostatic interaction with framework cations. That gives a high heat of adsorption (3300–4400 kJ·kg⁻¹) and therefore a high regeneration temperature (~150–250 °C) — which disqualifies low-grade waste heat and low-temperature solar, the very niche adsorption cooling exists to serve (Kumar: “zeolite-water needs ~200 °C”).

The fix is the aluminophosphate (AlPO/SAPO) class, which binds water through a hydrogen-bonded water-cluster mechanism instead of strong electrostatics. The result is an S-shaped (Type V) isotherm — a sharp uptake step over a narrow humidity/temperature window — and a much lower isosteric heat, so the material desorbs at low temperature yet still delivers high uptake:

EMM-8 (low-cost SFO-topology zeolite-like aluminophosphate, 12-ring channels): uptake 0.28 g·g⁻¹, isosteric heat ~46.76 kJ·mol⁻¹ (barely above water’s 44 kJ·mol⁻¹ heat of vaporisation → easy desorption), regeneration at 65 °C, and a record cooling COP of 0.85 at a 63 °C driving temperature (Liu et al. 2022, Nature Communications). This brings high COP into the waste-heat / low-temp-solar band where silica gel is weak.
Mitsubishi AQSOA (Z01/Z02/Z05, aluminophosphate zeolite): the commercial OEM adsorbent enabling low-regeneration (<90 °C; Z01 best ~60 °C) machines. Supplied into chillers, not sold as one.
SAPO-34 beats ZSM-5 in a sun-tracking parabolic-trough rig (max COP 0.169, SCP 169.74 W·kg⁻¹, outperforming ZSM-5 at all adsorption times; Yuan); an SAPO-34/water evacuated-tube unit reached COP 0.575 / EER 5.8 at a 75 °C drive (Al-Yasiri). Recent work engineers directed mass-transfer channels into SAPO-34 to fix intracrystalline diffusion limits (Ammann 2026) — see Fractal / Hierarchical Bed Geometry.

Material selection is not obvious: Tatlier’s “Selection of a favorable zeolite for solar adsorption cooling: how straightforward is it?” argues that the best sorbent depends on the exact drive/sink/chilled temperatures — isotherm shape relative to the operating window matters more than headline uptake.

Engineering constraints

Poor heat and mass transfer. Zeolite beds have low thermal conductivity, the dominant kinetic bottleneck. Mitigations: coated vs packed beds (zeolite coatings on heat-exchanger fins cut thermal mass and improve kinetics — InvenSor’s direct-coated beds are the commercial example), a 3-D graphene binder for zeolite 13X (enhanced conductivity, vapour transport, and uptake kinetics; Gildernew 2022), hydrothermally grown zeolite supports, and copper foams. See Adsorbent Bed Engineering.

Vacuum operation. Water/zeolite runs under high vacuum (water boils at low pressure for the evaporator), an engineering and maintenance burden. NH₃/zeolite avoids the vacuum but runs at 4–11× ambient pressure instead; binary NH₃+H₂O fluids can run near ambient (Demir).

Cycle / hydrothermal stability. Repeated hydration–dehydration can degrade frameworks. A Fraunhofer ISE study profiles cycle stability across silica gels, zeolites, AlPO/SAPO, and MOFs under continuous water-vapour cycling — zeolites and aluminophosphates are generally the hydrothermally robust end relative to many MOFs.

What actually ships

Every commercial adsorption chiller still runs on silica gel or zeolite with a water refrigerant — not lab MOFs (the ~USD 215 M market analysis has zero MOF mentions). See Commercial Adsorption Chillers.

InvenSor GmbH — direct-coated zeolite + water, the highest catalog COP up to 0.75, plus an HTC line tuned for high-ambient (>40 °C) operation.
Mayekawa — >70 kW zeolite/water units; locomotive zeolite 13X/water coolers historically (Wolak).
Mitsubishi Chemical AQSOA — OEM aluminophosphate adsorbent (not a chiller).
OxiCool Inc. (US Patent 9,765,998 B2) — a zeolite/water two-bed vehicle cooler whose “winterization” configuration stores the water inside the crystalline structure to prevent freeze damage, desorbed by engine exhaust.
Façade integration: CoolSkin embeds a vacuum zeolite/water adsorber as a switchable solar collector (~54 W·m⁻², up to 150 W·m⁻² best case; Greiner).
Historically several firms (Zeopower, SunMark, BLM, Solaref) failed to commercialise intermittent zeolite/water units (Wolak).

Sources