There is a grammatical mismatch here, of course: "it" is the subject of the main clause, and "it" can't be limited by a theoretical explanation. In fact, "it", as an anticipatory subject, IS the theoretical explanation given later.

However, I think the problem everyone seems to be having here is with the meaning of "limited". I think "limited" here means "limited", in the usual legal sense. The applicant does not want to be limited by the theoretical explanation. That is to say, whoever it is does not want the scope or applicability of the patent to be restricted by the validity or otherwise of the theoretical explanation. So if the theoretical explanation turns out to be wrong, they don't want the patent to be invalidated by this. Similarly, they don't want some smartarse using the same combination of solvents, but with a different theoretical explanation of why it works to get around the patent.

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Note added at 18 hrs (2006-11-19 16:29:45 GMT)
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Hello Andrea. If you put a participial phrase at the beginning of the sentence, it modifies the subject of the sentence. So, for example, "Walking home, I was hit on the head by a coconut." This means that I was walking home when the coconut hit me. If I said "Walking home, a coconut hit me on the head", this is nonsense, because it implies the coconut was walking home. (I got this example from one of my university teachers, who used to teach schoolkids in tropical Africa--hence the reference to cocunuts!)

The same goes if you put "without" in front of the phrase. So "Without limiting myself by any theoretical explanation, I postulate that..." is fine, because I am the one not being limited. Similarly, if I say, "Without being limited by anty theoretical explanation, I postulate that...", that is fine too for the same reason.

If I say, however, "Without being limited by any theoretical explanation, it is believed that....", what does the phrase "Without being limited by" apply to? Logically, it can only apply to "it", which doesn't make any sense.

In answer to your question, the "it" is impersonal, but it is a special type of impersonal "it", namely an anticipatory subject. The "real" subject is the clause. In English, where the subject is a clause, particularly if it is a long clause, we use the dummy subject "it" and defer the actual subject to the end. We could say, quite grammatically, "That the combination of two polar solvents helps dissolve the asphaltenes is believed", but this is a very clumsy sentence and would never be used by a native speaker of English (although the parallel construction is used A LOT in academic German!).

Hope that helps.